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Atomic-Scale Imaging and Spectroscopy of Electroluminescence at Molecular Interfaces Klaus Kuhnke,*,† Christoph Große,*,†,§ Pablo Merino,*,†,∥ and Klaus Kern†,‡ †

Max-Planck-Institut für Festkörperforschung, Stuttgart 70569, Germany Institut de Physique, Ecole Polytechnique Fédérale de Lausanne, Lausanne 1015, Switzerland

‡

ABSTRACT: The conversion of electric power to light is an important scientific and technological challenge. Advanced experimental methods have provided access to explore the relevant microscopic processes at the nanometer scale. Here, we review state-of-the-art studies of electroluminescence induced on the molecular scale by scanning tunneling microscopy. We discuss the generation of excited electronic states and electron−hole pairs (excitons) at molecular interfaces and address interactions between electronic states, local electromagnetic fields (tip-induced plasmons), and molecular vibrations. The combination of electronic and optical spectroscopies with atomic-scale spatial resolution is able to provide a comprehensive picture of energy conversion at the molecular level. A recently developed aspect is the characterization of electroluminescence emitters as quantum light sources, which can be studied with high time resolution, thus providing access to picosecond dynamics at the atomic scale.

CONTENTS 1. Introduction 1.1. Optics at the Nanoscale 1.2. Optical Scanning Probes 1.3. Scanning Tunneling Microscopy (STM) 1.4. STM-Induced Luminescence (STML) 1.5. Properties of STML 1.6. Extending STM Functionality by Light Detection 1.7. The Origin of Spatial Resolution in STML 1.8. Organization of the Review 2. Instrumental 2.1. Local Access to Electroluminescence in the STM 2.2. Light Detectors for STML 2.3. Time-Resolved Measurements 3. Plasmon Excitations in the STM Junction 3.1. Plasmon Excitation on Flat Noble-Metal Surfaces 3.1.1. Localized Plasmon Modes in the STM Junction (Tip-Induced Plasmons) 3.1.2. Electrical Excitation of Tip-Induced Plasmons 3.2. Modification of the Plasmonic Emission of Noble-Metal Surfaces by Dielectric Films 3.2.1. Modification of the Plasmon Modes 3.2.2. Modification of the Plasmon Excitation Efficiency 3.3. Plasmon Excitation and Mapping of Individual Metallic Nanoparticles 4. Light Emission by Electron−Hole Pair Recombination 4.1. Electron−Hole Pairs (Excitons) in Condensed Matter © XXXX American Chemical Society

4.2. Excitons at the Nanoscale 4.3. Exploring Excitons with STML: Experimental Challenges and Strategies 4.4. Excitons in Semiconductors 4.4.1. Inorganic Semiconductor Surfaces 4.4.2. Organic Films: Porphyrins, Phthalocyanines, and Fullerenes 4.4.3. Absorption by Molecules Adsorbed near the Tip Apex 4.5. Excitons in Single Molecules and Nanostructures 4.5.1. Individual Molecules on Decoupling Layers 4.5.2. Molecules on Inorganic Semiconductor Surfaces 4.5.3. Self-Decoupled Molecules 4.5.4. Chromophores Integrated within Molecular Wires 4.5.5. Inorganic Semiconductor Quantum Dots and Nanowires 4.5.6. Heterogeneous Molecular Dimers on Decoupling Layers 4.5.7. Molecular Nanocrystals on Decoupling Layers 5. Plasmon−Exciton Interaction in Tunnel Junctions 5.1. Strong and Weak Plasmon−Exciton Coupling 5.2. Exciton Coupling to the Far-Field 5.3. Plasmon-Induced Generation of Excitons

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Special Issue: Vibrational Nanoscopy Received: September 19, 2016

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DOI: 10.1021/acs.chemrev.6b00645 Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews 5.4. Distinguishing Emission from Plasmons and Excitons in STML 6. Vibrational Spectroscopy in STML: Electron−Hole Pairs and Molecular Vibrations 6.1. Coupling between Electronic and Vibrational Transitions 6.1.1. Franck−Condon Principle: Coupling to Allowed Electronic Transitions 6.1.2. Herzberg−Teller Coupling: VibrationalInduced Electronic Transitions 6.1.3. Jahn−Teller Coupling: Vibrational Coupling between Electronic Degenerate States 6.2. Vibrational Spectroscopy of Single Porphyrin Molecules 6.2.1. Emission Characteristics of Porphyrins 6.2.2. Single Metalloporphyrin Molecules 6.2.3. Single Free Base Porphyrin Molecules 6.3. Vibrational Spectroscopy of Single Phthalocyanine Molecules 6.4. Spectral Modification by Plasmon Modes in the STM Junction 6.5. Intermolecular Interactions in Molecular Aggregates 6.5.1. Exciton Coherence Length in J-Aggregates 6.5.2. C60 Films and X Traps 7. Time-Resolved STML 7.1. Photon−Photon Time Correlations (Hanbury−Brown Twiss Interferometry) 7.2. Electroluminescence Transients 8. Conclusion and Outlook Author Information Corresponding Authors ORCID Present Addresses Notes Biographies Acknowledgments Abbreviations Substances and Compounds Others References

Review

Chemistry in 2014 for single molecular spectroscopy and subwavelength resolution imaging awarded to W. Moerner, E. Betzig, and S. Hell. Their research pioneered the study of single molecules within ensembles by using the wavelength selectivity, the near-field, or the time-dependent optical manipulation of their excited states to select them individually. However, it still is a challenge accessing processes at the nanometer scale in close proximity to where they occur. This challenge is due to the fact that there remains a fundamental mismatch between the extension of even tightly focused light fields, several hundred nanometers in the visible range, and the extension of electron wave functions in atoms or molecules, with features measuring between a fraction of a nanometer and a few nanometers. Thus, finding ways of reconciling these length scales is a crucial step for a better understanding of their interaction and for the development of densely packed functional devices. Over almost three decades innovative strategies to address and manipulate light below the diffraction limit have been put forward leading to the exciting field of nanophotonics. Confinement of light to dimensions below this limit has allowed one to characterize the interaction between light and matter at this reduced scale. The local interaction allows accessing optical properties and processes in the optical nearfield. The ultimate goal of closely “watching” how a single atom or molecule emits a photon upon electrical charge injection and analyzing the spatial distribution of the electronic states responsible for such emission remains elusive. Yet with scanning probe microscopies, this long-awaited dream can be coming true. Individual molecules can be investigated by charge injection, and their electronic and optical spectroscopies reveal structural details at angstrom resolution. The spatial distribution of charges responsible for photon generation can be traced down to the atomic scale, and electroluminescence from different positions on a molecule can be accessed spectroscopically. This Review addresses recent developments in using scanning tunneling microscopy (STM) as a tool for optically characterizing solids on the atomic scale. Recent experimental studies by STM-induced luminescence (STML) have shown continuous and fast progress. They demonstrated the real-space mapping of molecular vibrational modes2,3 and electronic orbitals4 by light emission. Optical emission and absorption spectroscopy can be carried out in the optical near-field. Single molecules and defects have been characterized with respect to their performance as nanometer light sources, creating the basis for the design of emitters at the single-molecule and quantum level. The advances in this field of research have the potential to promote both fundamental in-depth studies at the molecular level as well as the development of novel molecular light sources with tailored properties. Indeed, the interaction between charge carriers and charge-neutral electronic excitations (excitons), which is relevant in optoelectronic devices, can be accessed with unprecedented resolution. The use of timeresolved light detectors enables accessing their dynamics down to the picosecond range. Historically, spatially resolved electroluminescence spectroscopy became accessible in the 19th century when a free propagating electron beam was first directed toward a luminescent material. The spatial resolution, which is determined by the ability to focus the electron beam, has since then improved and reaches today values on the order of 10 nm. Electroluminescence induced by a scanning electrode in

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1. INTRODUCTION The interaction between electromagnetic radiation and electric charges is instrumental in many areas of science and technology. Photon absorption and charge separation is at the origin of biological processes like light-harvesting and photosynthesis. The reverse process, photon emission excited by recombining charges, is at the core of light-emitting diodes (LEDs), stimulated (LASER) sources, and many other devices that form valuable scientific tools and everyday commercial devices. A better understanding of the interaction between charges and photons on the molecular scale is crucial, not only for augmenting the efficiency of harvesting and emission devices, but also as a possible avenue toward new disruptive technologies. Photon excitation of electronic systems has been studied on the atomic scale in solid-state environments, revealing a variety of single-molecule properties not observable in ensembles.1 The scientific importance of the field led to the Nobel Prizes in B


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Figure 1. Experimental schemes probing optical near-fields by a scanning probe. (a) Scattering of the locally enhanced optical field from the object. Parts (b) and (c) combine this principle with a measurement of the current through a tunnel barrier. This Review focuses on electroluminescence induced by local charge injection in a scanning tunnel microscope (c). Acronyms: Scanning near-field optical microscope (SNOM or NSOM), apertureless SNOM (a-SNOM), tip-enhanced Raman scattering (TERS), terahertz scanning tunneling microscopy (THz-STM), shaken pulse pair excited STM (SPPX-STM), and scanning tunneling microscopy-induced luminescence (STML).

proximity to a sample was first proposed in the 1970s5 even before the advent of STM.6 Light emission from a tunnel junction was first reported from ultrathin insulator layers between planar metal electrodes.7,8 Pioneering studies of STMinduced light emission began in the late 1980s9,10 and early 1990s with sharp metal tips on single-crystalline metal11 and semiconductor12 substrates and organic adsorbate layers.13 A boost came from the growing interest in charge-carrier-induced light emission of inorganic and organic semiconductors and optical spectroscopy on individual fluorescent molecules in 2003.2 On the basis of the controlled charge injection possible by STM, the methodology has well proven its ability to study excitons. In the past, excellent reviews on STM-induced luminescence have appeared, which we also recommend to the reader.14−16

most efficient for metals because their plasmonic modes can provide an additional resonance enhancement. Exciting publications have emerged from experiments probing the near-field, from the development of optical antennas, and from the application of plasmonic structures.22−25 1.2. Optical Scanning Probes

There are several methods that combine light interaction with local scanning probes. When a nanometer-sized optical antenna is moved in a controlled manner over a sample, optical spectroscopy can be realized with very high spatial resolution. The interactions lead to three basic combinations (see Figure 1) characterized by optical excitation−optical readout (Figure 1a), optical excitation−electrical readout (Figure 1b), and electric excitation−optical readout (Figure 1c). One prominent method of local optical spectroscopy belonging to the case in Figure 1a is scanning near-field optical microscopy (SNOM), which is able to break through the optical diffraction limit by employing an optical near-field leaking through a subwavelength aperture.26−28 The light is guided inside a tapered glass fiber to a submicrometer aperture at its end. Light interacting with objects close to the aperture can be detected by radiated light or reflected light measured on its way back through the fiber.29 The glass fiber is in general covered with a metallic layer except for its very apex. In some cases, a nanometer-sized optical antenna was attached next to the aperture,30 allowing for detailed studies of polarization effects on scales below 100 nm. While subwavelength apertures greatly reduce the optical field intensity, enhanced intensities are obtained by virtually turning the setup inside out and focusing free-propagating light onto the tip of a solid metal needle. This geometry provides a pronounced field enhancement at the tip apex by the so-called lightning rod effect,18 which is based on the increased electromagnetic field at sharp edges. On the basis of this principle, the apertureless-SNOM (a-SNOM) can provide a wavelength-independent optical resolution. The enhanced optical field at the tip is basically determined by the geometry of the apex and the optical properties of its material. For coinage metal tips, the lightning rod effect is strongly enhanced due to the excitation of localized plasmon polaritons, which are oscillations of the charge carriers in the metal coupled to surface-confined electromagnetic fields. An interesting process proposed by Stockman guides plasmons efficiently along the surface of a conical tip to its very apex and became known as adiabatic nanofocusing.31 Combined with an optical grating carved into the tip shaft, it can provide a coupling to the

1.1. Optics at the Nanoscale

For a long time, it had appeared that the resolution of optical microscopy is fundamentally limited to about one-half of the employed wavelength. This borderline, often referred to as the Abbe diffraction limit,17 is determined by the fact that subwavelength structural information on an object is not retained at macroscopic distances from the object. However, it can be shown by simple calculations18,19 that the information about subwavelength structures is only gradually lost in the transition from the electromagnetic near-field, in close proximity to the source, toward the far-field, several wavelengths away. The limit of spatial resolution may thus be circumvented by methods that access the optical wave near the object to be imaged. In recent decades, three fundamentally different approaches have been developed. One is the optical manipulation of excitation on the level of single fluorescent molecules,20 for example, by optical excitation and stimulated depletion. On the basis of a significant number of techniques that use the manipulation of molecular excitation, the new field of superresolution imaging emerged.21 The second approach is the use of subwavelength orifices to emit or detect light in the optical near-field of an object. Finally, metallic structures of subwavelength dimensions can be employed, which allow an efficient coupling between electromagnetic near-fields and farfield radiation. The near-field is probed locally by the subwavelength structure and scattered into the far-field. The diffraction limit can be overcome because a single particle (or a sharp tip apex) relaxes the momentum conservation when its dielectric function is markedly different from that of its environment. This principle works for many dielectrics but is C


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electromagnetic far-field.32 Another development of the probe geometry has been presented with the “Campanile” design.33 An overview of the principles of near-field optics can be found in ref 18. Optical tip enhancement has been explored to utilize the processes behind surface-enhanced Raman scattering (SERS) as an efficient vibrational probe in a scanning mode. Tip-enhanced Raman spectroscopy (TERS) was first demonstrated using metal-coated AFM34,35 and solid metallic STM36 tips. The tunnel current in STM, which increases exponentially with decreasing tip−sample distance, can here be employed for improved distance control, and the combined operation with a tunneling microscope enables additional in situ characterization of the local topography.37

varying the delay between two light pulses (pump and probe) incident on the tunnel junction. An extension to a two-color correlation experiment has also been demonstrated, in which the two incident pulses have different wavelengths. This scheme allows the separation of the autocorrelation and mixed pulse correlation signals by their different emission wavelength.45 The time response is obtained by scanning the delay between the two incident pulses in the picosecond to nanosecond range. The read-out of this fast experiment is possible by the low-bandwidth measurement of the tunnel current, with a readout interval typically in the millisecond to microsecond range. The incident electromagnetic pulses at terahertz frequencies can be made short enough to basically degenerate to a single oscillation. Its electric field can be superposed on a constant (dc) voltage applied across the tunnel junction, thus generating a defined voltage trigger.46 Terahertz radiation can thus electrically monitor local dynamics driven, for example, by synchronized short IR pulses with pulse lengths in the femtosecond range.47 On the opposite extreme of time scales and very slow dynamics, the indirect coupling between continuous wave (cw) IR radiation and the tunnel current has also been explored. The wavelength of an infrared light beam can be tuned slowly over the vibrational absorption band of adsorbate molecules. The individual IR absorption lines can then be detected as an increased tunnel current invoked by the thermal expansion of the substrate due to heating by the absorbed infrared beam.48 Apart from the excitation of molecular vibrations, it has been established that incident light of higher energy (visible to UV) can induce reversible or irreversible conformational changes of individual molecules. While these morphology changes have been studied by STM with submolecular resolution,49 the STM tip is typically not close enough to the sample to locally increase the energy density of the incident electromagnetic field by the lightningrod effect. Moreover, the femtosecond to picosecond dynamics of the induced conformational changes have not yet been monitored by STM.

1.3. Scanning Tunneling Microscopy (STM)

Experimental techniques based on STM are well-known for their ability to record atomically resolved surface topography and map the local electronic density of states on conducting substrates such as metals, semiconductors, and doped or ultrathin insulators. By ramping the voltage applied across the tunnel junction, electronic spectroscopy can be performed, the energies of occupied and unoccupied electronic states can be determined, and their spatial pattern can be imaged. The basics and virtues of this method can be found in a number of text books.38,39 On single molecules and molecular layers, STM permits one to obtain high-resolution maps of molecular electronic orbitals.40,41 Such maps can be obtained in constant height or constant current mode. In the first case, the tip moves along a plane above the sample, and the information on the scanned structure is obtained through the varying tunnel current. In the second case, an electronic feed-back loop is switched on to continuously adjust the tip height to keep the tunnel current constant. The tip then follows a surface contour of constant current, and the recorded height information is displayed as a topography map. The vacuum tunnel barrier in the STM may be partially filled by an ultrathin insulating adlayer on which adsorbates can be placed. The insulator can be tailored to provide a well-balanced decoupling of molecular electronic states of an adsorbate and avoids hybridization with the electronic states of a metallic substrate.42 While optical near-field probes allow one to concentrate the electromagnetic field around nanometer-sized structures, their combination with a metallic electric contact permits one to study the near-surface region of conducting samples. The tunnel junction of an STM becomes the crossroads between the electronic structure of solids and a spatially confined electromagnetic field. It provides a defined nanometer region in which charged and neutral excitations of a solid can couple to electromagnetic fields and where both may exchange energy, allowing a detailed characterization of both components and their mutual interaction. Studies that formerly had to be carried out by diffraction-limited optical methods can now be zoomed in to the level of individual quantum systems, such as atoms, molecules, defects, or artificial quantum structures. The tunnel current can be employed to monitor charge carrier dynamics when a pulsed external light source is coupled into the STM cavity (Figure 1b). With a laser setup, femtosecond pulses can be achieved, and the generation of hot electrons charging a molecule within the tunnel junction has been observed by one- and two-photon excitation.43 An advanced method is shaken-pulse-pair-excited-STM (SPPXSTM),44 which is able to access the charge carrier dynamics by

1.4. STM-Induced Luminescence (STML)

In contrast to the studies of light interaction with local probes discussed before, this Review focuses on the optical read-out of electronic excitation as shown in Figure 1c. Here, a current between a positionable tip and a conductive sample can induce luminescence from the junction. There are three distance ranges in which different processes dominate (see Figure 2). For tip−sample separations larger than a few nanometers, a rather high voltage (>10 V) is needed to drive a measurable current by field emission from the tip. This geometry provides only moderate spatial resolution of tens of nanometers but may be employed to observe luminescence by electron bombardment (cathodoluminescence) of a sample without being affected by the optical field-enhancement of the tip. For extremely small tip−sample distances (below ∼0.2 nm), in contrast, the junction is near the contact regime, with conductivities close to the quantum of conductance G0 = 2e2/h ≈ 7.8 × 10−5 Ω−1. Low voltages in the millivolt range can lead already to currents in the μA range, so that multielectron50 and hot electron emission pathways51 will prevail. Intermediate distances define the tunneling regime, with much lower conductance on the order of 10−9 Ω−1. Here, a moderate voltage of 1 V results typically in tunneling currents on the order of 1 nA. Excitation processes are dominated by D


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Figure 3. Three-dimensional plot of the typical spatial, temporal, and energy resolutions for the human eye (dark red) and for selected solidstate spectroscopic techniques (see text). For better readability, each colored sphere is projected as a colored circle onto three sides of the box. The diffraction limit restricting optical spectroscopy (for a wavelength of 500 nm) and the uncertainty principle are indicated by two planes. Abbreviations: SRM, super resolution microscopy; TEM, transmission electron microscopy; TR-TEM, time-resolved transmission electron microscopy; OM, optical microscopy; STM, scanning tunneling microscopy; STML, STM-induced luminescence; EELS, electron energy loss spectroscopy.

Figure 2. Three regimes of tip sample−distances and their predominant mechanisms for luminescence generation. Typical voltages required to obtain luminescence lie above the indicated boundary at which the electric field near the tip apex becomes sufficiently strong to provide an electric current.

consecutive charge tunneling, and electron−photon interactions may be studied on a single quantum basis, meaning that single electrons and single photons interact with individual quantum systems. In the tunneling regime, two major processes lead to electroluminescence. First, light may be emitted during an inelastic electron tunneling process by direct energy transfer to the electromagnetic modes of the optical cavity between tip and sample, so-called tip-induced modes (TIP). Second, charge injection into a semiconductor or molecule can result in the formation of an electron−hole pair, which may recombine and generate light emission. In the tunneling regime, voltages and currents can be kept low enough to avoid disruption of weak intermolecular bonds and damage to the molecules. The transition to the larger distance field-emission regime has been explored in a few publications,11,52,53 and the transition from the tunnel regime to the contact regime has been addressed in a series of studies.50,54,55

traded in for the required spatial resolution and vice versa. With all necessary precaution, we can say that, while in any single parameter STML may not be the superior technique, it provides a unique combination of high resolution in all three parameters at the same time. It reaches the ultimate atomic limit in spatial resolution combined with close proximity to the Heisenberg uncertainty limit in terms of energy and time resolution. In an experimental setup with optical access, the topographic and electronic characterization techniques available to STM (section 1.3) can be supplemented by the analysis of the luminescence emitted from the tunnel junction. Tunneling from one solid to another provides a way of conserving energy and momentum at the same time, which would not be possible between a free electron and photon. Figure 4 summarizes the various mechanisms that have been introduced above for scanning probes and are essential for the operation of STML. The electronic and electromagnetic environment in the interaction region, that is, the tunnel junction determines the way an energy exchange can occur. The energy dissipation provided by the tunneling electrons drives the emission of light from the tunnel junction. Injection of a charge into a solid or molecule in the junction can result in an excited state of a quantum system (symbolized by a molecule in Figure 4), which may decay radiatively. The electron tunneling process itself provides another luminescence channel because it can directly couple to the electromagnetic field, which mediates an energy transfer from the electron to a localized plasmonic mode. This process may equivalently be described as being driven by the shot noise in the electron current.54,61 The excitation transfer from the tunnel current to the tip− sample system may extend over a large range of the electromagnetic spectrum from UV light to millimeter radiation.62 Microwave emission appears as an energy-loss process, resulting in a small but measurable energy broadening

1.5. Properties of STML

STM-induced luminescence in the tunneling regime relies on the energy transfer between electrons and photons on a single molecular and atomic scale (0.1 nm), with typically millielectronvolt resolution in both electron energy and photon energy. In addition, the method provides implicit time resolution when single-photon detectors are employed. Figure 3 gives an overview of major spectro-microscopic techniques and their approximate optima of resolution in space, energy, and time. We note that it is challenging to compare the various methods on an equal footing because optimization of one parameter will bring drawbacks for the others. In the figure, we distinguish, for example, electron microscopy into three categories: transmission electron microscopy (TEM) with 0.1 nm spatial resolution;56 electron loss spectroscopy with 10 meV energy resolution,57 and single-shot TEM providing 10 ps time resolution.58 However, all three limits cannot be reached in a single experiment. Similarly, optical super resolution provides spatial resolutions down to ∼10 nm,59,60 while both the temporal evolution of the signal and the spectral separation of the emission are exploited as part of the imaging process and represent no resolution parameters that are independent. The acquisition time for super-resolution imaging can again be E


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(of the order of μeV) due to quasi-elastic tunneling processes. The features have been measured in tunneling spectra at cryogenic temperature (≪1 K) and could be modeled within the framework of the so-called P(E) theory.63,64 The function P(E) expresses the probability density P of a tunneling electron to exchange the amount of energy E with the electromagnetic environment.65 If no coupling were present, the elastic peak would degenerate to a Dirac δ-function. Emission in the visible range at readily detectable photon energies requires a substantial energy transfer by the tunneling electron on the order of 1−3 eV. The process might be identified directly as an inelastic electron tunneling process resulting in a step in the derivative of the tunnel current with respect to voltage (dI/dV), although we are not aware that it has ever been reported. Reasons for the missing observation of light generation as an inelastic tunneling process may be found in the typically large width of plasmonic spectra, the low cross section of the process, and its energy overlap with features in the electronic density of states (DOS). The missing observation contrasts with the reported observation of electron energy losses in plasmonic light emission spectra,66 which represents the reverse coupling. On the other hand, a feasible and well-established way to observe the energy exchange between tunneling electrons and

Figure 4. Light emission in STM is induced by the spatially confined tunnel current (blue) between tip and substrate. The current passes through a quantum system (e.g., a molecule) insulated from the substrate by a decoupling layer (violet). The optical emission of the localized dynamic dipole of the excited quantum system couples to tipinduced cavity plasmons between metallic tip and substrate. From the cavity, electromagnetic waves couple efficiently to free propagating light (yellow) through the lightning-rod effect at the tip apex. In addition, inelastic electron tunneling channels can directly excite cavity plasmons and lead to the emission of photons even in the absence of a quantum emitter.

Figure 5. Selection of measurements available in STML, illustrated by using the example of thin semiconducting C60 films on a crystalline metal substrate. Bottom: Electronic spectroscopy and spectrally resolved surface mapping belong to measurement modes of STM. Upper left: STML adds the capability of optical analysis, like electroluminescence-yield spectroscopy, the highly resolved mapping of electroluminescence by photon maps, and optical-emission spectroscopy from a selected position on the surface. Upper right: Both photon−photon correlation and the transient electroluminescence response to short voltage pulses exploit the time resolution of optical detectors to study dynamics in the nanosecond and picosecond ranges. F


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filter. Such data can be employed to access even minor details of the emission characteristics. Another parameter in STML is the polarization of the luminescence. The light is predominantly polarized perpendicular to the surface plane because the emission is often governed by the electric field direction of the cavity modes.68 Valuable information can be obtained, moreover, from the observation of circular polarization components, for example, for magnetic samples and inorganic semiconductors (see section 4.4.1).70,71

photons is the detection of the emitted light that can be monitored by light detectors at a macroscopic distance from the tunnel junction. 1.6. Extending STM Functionality by Light Detection

The detection of electroluminescence by light detectors brings new types of measurements to the experimental toolbox of STML (Figure 5). STM operates with four parameters: (i) the lateral position on the two-dimensional surface of the sample, (ii) the vertical tip−sample distance, (iii) the applied voltage, and (iv) the current. In addition, STML allows the measurement of optical spectra, which add two parameters: (v) the wavelength (photon energy) and (vi) the intensity at a given wavelength. In the experiment, light detection can be realized by counting photons within a detector-given broad energy range without wavelength (i.e., energy) filtering. In this case, the two electric parameters, voltage and current, can be related to the spectrally integrated light intensity. Plotting the light intensity versus voltage provides information on the excitation mechanism, because the onset voltage of light emission is an important parameter for characterizing its origin. The experimental quantum efficiency ηexp is the number of detected photons per tunneling electron ηexp = Je/I, where J is the detector’s photon counting rate, I is the electric current, and e is the elementary charge. This ratio is easily accessible and thus typically the one given in publications and throughout this Review. However, it relates to the real quantum efficiency η0 of a studied system by a number of factors, some of which may be estimated for a given setup: ηexp = η0ηemηtransηdet

Ω 2π

1.7. The Origin of Spatial Resolution in STML

The origin of the feature sizes resolved in luminescence maps on the nanometer scale is an interesting aspect of the technique. First, one has to recall that STML records photon maps by scanning the tip apex, which is the source of localized charge injection. Thus, the observation of a feature in STMinduced luminescence shows, strictly speaking, not the source of light emission but the position where charges must be injected to generate luminescence. While for many systems one may argue that charge injection and light emission must occur in close proximity to each other, it is clear that any quantitative evaluation has to take account of this difference. This holds, especially, if diffusion of charges or excitons comes into play. It has been shown that highly resolved orbital features in STML on molecules can be explained entirely by the spatial variations of different electronic pathways4 as discussed in more detail in section 3.2.2. Experiments with mixed C60/C70 films,16 discussed in section 4.4.2, observed that a molecule-bymolecule distinction of the species within a film cannot be obtained: near a C70 molecule, even charge injection into a well-identified neighboring C60 molecule provides a C 70 luminescence spectrum. This can be rationalized by the combination of two mechanisms; first, an injected charge and a successively formed exciton may both diffuse and get trapped at a nearby lower energy level. In the described experiment, the emission line emerging from C70 lies lower in energy than the C60 line so that it is plausible that a diffusing exciton becomes trapped on the C70 species and finally decays radiatively on that molecule. Second, the optical tip enhancement is spatially extended and still works for molecules a few nanometers away from the apex. Therefore, two mechanisms contribute to the experimental resolution in STML: first, the charge injection through a tunnel barrier can provide atomic scale resolution on the 0.1 nm scale due to the exponential decrease of tunnel rate with tunnel distance;39 second, the coupling to the light field is due to the lightning-rod effect and the extension of local plasmon modes. Both mechanisms depend on the precise curvature and shape of the tip. The concentration of the light field is assumed to provide a reproducible resolution on the order of a few nanometers based on the localization of the tipenhanced electromagnetic field.72,73 This near-field optical resolution is subject to limitations similar to those of the contrast achieved by apertureless SNOM when the same tip geometry is used. The optical resolution limit may be improved by employing higher-order optical processes as it has been suggested to explain the extremely high spatial resolution of tip-enhanced Raman (TERS) processes.74 Higher-order optical processes increase the localization of the interaction and are thus able to boost resolution. Recently, also plasmon-driven injection of tunneling electrons has been suggested as a mechanism contributing to the high resolution observed in TERS.75 Thus, it is still important to analyze the contrast mechanism

(1)

where Ω/2π is the geometric collection coefficient for the solid angle Ω covered by the optical system assuming an isotropic emission. ηem and ηtrans are the coupling probabilities from the tunnel junction to the far-field and the transmission probability through the coupling optics to the detector, respectively. ηdet, finally, is the detection efficiency of the employed detector. Literature values for maximum experimental efficiency strongly depend on the system and the luminescence mechanism and range from 10−7 to 10−4. When the product of ηtransηdetΩ/2π can be estimated for an experimental setup, it is often found in the range of a few percent to 20%.67−69 η0ηem values above 10−3 are regarded as exceptionally high. Thus, the true efficiencies η0 of quantum systems generally stay below ∼1%. STML allows one to record the integrated photon count rate pixel-by-pixel, simultaneously with scanning of the surface topography. This provides images known as “photon maps”, which can be obtained on the time scale of a few minutes. As explained in section 1.3, a map can be run in constant current or constant height mode. When photon maps are normalized at each pixel to the respective tunnel current, they become luminescence efficiency maps. In contrast to the single-valued spectrally integrated intensity, measured, for example, with single photon detectors (see section 2.2), the collection of a full luminescence spectrum requires a period of time usually incompatible with typical scanning velocities. Thus, spectra have to be measured position-by-position, in a time-consuming manner, and a spectrally resolved map can be constructed afterward. The obtained three-dimensional data set allows one to make cross sections for defined emission wavelength intervals, which otherwise could be obtained only by scanning an area repeatedly, each time with a different optical band-pass G


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Table 1. Conversion of Energy Values for the Most Widely Used Physical and Chemical Units atomic energy unit (eV)

wavenumbers (cm−1)

wavelength (nm)

light frequency (THz)

SI unit (J)

chemical unit (kcal/mol)

1

8065.5

1240

241.8

1.602 × 10−19

23.3

the surface plane) of 1 μA, V ≈ 1 V). In contrast to single-electron processes with a linear dependence between luminescence intensity P and current I, such multielectron processes follow a power law with exponents α > 1. P = ηexp, α I α

hole pairs. The underlying processes have been interpreted in terms of an Auger process,136 a triplet−triplet exciton annihilation,138 and a plasmonic pumping141 mechanism. In contrast to the emission of metallic tunnel junctions, in these cases overbias emission is observed already at substantially lower currents, down to a few hundred picoamperes.141 3.2. Modification of the Plasmonic Emission of Noble-Metal Surfaces by Dielectric Films

The question of how organic and inorganic dielectric films modify the emission of tip-induced plasmons has been addressed by a multitude of experiments. It has been observed that molecules adsorbed on noble-metal surfaces can reduce13,143−146 or enhance123,147−152 the emission intensity with respect to the intensity on the pristine metal surface and can cause either a redshift145,146,153 or a blueshift123,149,150,152 of the spectrum. In addition to the radiative recombination of electron−hole pairs (see section 4) and an energy transfer from the electronically excited system to a plasmon mode of the STM junction, organic and inorganic dielectric films may modify the emission of tip-induced gap plasmons. Two major effects can be distinguished: (i) the tip-induced modes of the junction can be modified by the dielectric properties of the inserted films and/or the increased separation between the STM tip and the metal surface; and (ii) the plasmon excitation efficiency can be due to a modified electron−plasmon coupling or branching ratio between elastic and inelastic tunneling channels. 3.2.1. Modification of the Plasmon Modes. Insertion of a dielectric film between the STM tip and a noble-metal substrate typically results in an increase of the tip−metal substrate distance. When the electronic states of the film are located outside the energy window spanned by the Fermi levels of the tip (EF,t) and the sample (EF,s), their influence on the electron tunneling through the junction is negligible and the film behaves like a passive dielectric spacer.154 As discussed in section 3.1.1, the larger tip−metal substrate separation weakens the electromagnetic coupling between the tip and the metal substrate; therefore, the emission intensity reduces and the modes shift to higher energy. On the other hand, the polarization of the film may result in a screening of the surface charges at the tip and the metal substrate, which shifts the plasmon modes to lower energy. Because the spectral shifts caused by both effects counteract each other, the experimentally observed shift with respect to the pristine metal surface is usually rather low, typically a few ten millielectronvolts. 3.2.2. Modification of the Plasmon Excitation Efficiency. The aforementioned arguments can neither explain the enhanced emission intensity observed on some molecular films123,147−152 nor the suppressed intensity that Zhang et al.144 observed at the same tip height on the clean metal surface next to the molecules. However, these observations can be rationalized by a changed tunneling matrix element or a modified branching ratio between elastic and inelastic tunnel channels. A modification of the plasmon emission intensity by the local density of electronic states (LDOS) of the surface has been suggested already in the early days of STM-induced emission to explain the up to 1 order of magnitude reduced emission intensity on top of oxygen atoms on Ti.155 Moreover, spatial variations of the plasmonic emission observed in photon maps of reconstructed Au(111) and Au(110) surfaces have been ascribed to local variations in the LDOS of the

(4)

Spectrally, such processes can be identified by the emission of photons whose energy exceeds the energy difference between the Fermi energies of the tip and the sample, which is referred to as forbidden emission,136 overbias63 or above-threshold131 light emission, 2e light,50,54,55,137 or up-conversion emission.138 Its observation indicates tunneling of hot charge carriers, with an energy above their thermal distribution. Overbias light has been first reported by Pechou et al.139 for the STM-induced luminescence from a Au(111) surface at ambient conditions and a current of 10 nA. The only small excess energy of photons of (γ0 + γpl), where γ0 and γpl are the quantum emitter and plasmon line widths, respectively.254,255 Note that the spectral line width is given by the reciprocal lifetime of the

5.2. Exciton Coupling to the Far-Field

Ever since the first observations of intrinsic fluorescence with STML, it has been observed that, while the energy positions of the spectral lines remained fixed, the relative intensity of the lines changed with the properties of the tip (see Figure 20a).2 These variations were attributed to the different spectral enhancement by the plasmon modes of the different tips. To compensate for the influence of the tip enhancement, fluorescence spectra have to be normalized to their respective plasmon spectrum obtained on the metal substrates (Figure 20b). Such “renormalized” spectra show almost identical intensity ratios between the main line and the vibronic progression (Figure 20c).2,231 Next-order corrections to STML spectra and their coupling with vibrations are discussed in section 6. Later studies investigated the influence of the tip on the plasmon modes. The molecular luminescence of thin porphyrin films (5 ML of TPP) was studied using substrates that do not support plasmonic modes, such as highly oriented pyrolytic graphite (HOPG).261 These experiments show that the plasmon field of the tip is already sufficient to detect molecular emission in the far-field (Figure 21a−c). “Dark” tips that do not induce plasmonic emission on metallic surfaces do not exhibit intrinsic electroluminescence emerging from the molecular films. In contrast, “bright” tips showing plasmonic emission on metals yield intrinsic molecular electroluminescence of the molecules deposited on HOPG. The spectral features are enhanced by tip-induced plasmons, which are specific for each tip (see Figure 21d,e).261 In this context, we note that dividing the observed molecular spectral profile by the spectral profile of the same tip on a metal surface2,231,244 does remove the X


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Figure 21. (a) STML spectra obtained on HOPG using a W tip (±2.5 V, 0.4 nA, 600 s). The bottom curve shows the spectrum of a plasmonically “bright” tip on Ag(111) (+2.5, 0.4 nA, 60 s). The inset is an atomically resolved STM topography of HOPG (+0.5 V, 0.3 nA, 2 × 2 nm2). (b) Bias polarity dependence of STML spectra acquired on 5 ML TPP on HOPG (±2.5 V, 0.4 nA, 600 s) together with the PL spectrum measured in situ. (c and d) Dashed curves, STML spectra of the tip plasmon modes on Ag(111) with a W-tip (+2.5 V, 0.4 nA, 60 s); solid curves, corresponding STML spectra from 5 ML TPP on HOPG using the same tip (+2.8 V, 0.4 nA, 600 s). Adapted with permission from ref 261. Copyright 2012 AIP Publishing LLC.

Figure 20. ZnEtioI molecule on a partially oxidized NiAl(110) surface. (a) STML spectra for three different experimental runs with three different tips. Spectra 1 (V = 2.35 V, I = 0.5 nA, exposure time = 300 s) and 2 (V = 2.35 V, I = 0.6 nA, exposure time = 200 s) were taken with two different Ag tips. Spectrum 3 was obtained with a W tip (V = 2.3 V, I = 1 nA, exposure time = 600 s; the original data have been multiplied by a factor of 3). (b) NiAl STML spectra measured with the same tips as in (a); the curve sequence is consistent with that of (a). The spectra were acquired at the same voltages as in (a) and scaled, so that the photon yields of different tips could be directly compared. (c) Smoothed molecular spectra from (a), divided by the corresponding smoothed NiAl spectra from (b) and normalized to fit the same scale. The inset shows the photon energy of each peak for the three spectra, as marked in (a). Reproduced with permission from ref 2. Copyright 2003 The American Association for the Advancement of Science.

5.3. Plasmon-Induced Generation of Excitons

Plasmonic fields do not only contribute to coupling near-field emission to the far-field, they can also act as a source for electron−hole pairs. Coupling between photons from external light sources and molecules inside tunnel junctions has been demonstrated to successfully induce charge excitation in individual porphyrins on Al2O3/NiAl(110). For this system, photon absorption and charge excitation has been detected both in the continuous wave mode262 and with femtosecond laser pulses,43 indicating the possibility of generating photoexcited electron−hole pairs in the environment of a tunnel junction. The sole difference in plasmon-induced fluorescent systems is the origin of the electromagnetic field, which originates from plasmons excited by inelastic electron tunneling processes instead of external light sources. The good spectral matching needed for observing intrinsic emission in the far-field induces also resonance enhancement for electron−hole excitation processes on the surface. Plasmon-induced excitation has been suggested to explain the emission from most fluorescent systems showing bipolar electroluminescence.138,141,261 In these systems, the luminescence onset depends only on the absolute value of the voltage and not on its sign. Figure 22 shows energy level diagrams of two major excitation mechanisms of electroluminescence in STML. Mechanism 1 (Figure 22a) involves resonant electron extraction (hole injection) at the lower state, electron injection into the higher state, and subsequent electron−hole pair recombination. Mechanism 2 (Figure 22b) represents the situation where molecular excitons are generated by plasmonic

influence of the cavity’s Purcell factor g(ω), but neglects a possible energy transfer from excited plasmon modes. An important observation that has not only been made on clean metallic substrates (see section 3.1.2) but also for plasmon−exciton coupling is electroluminescence emission at overbias. The intrinsic photon emission above the energy of the tunneling electrons (ℏν > eV) has been observed for thin porphyrin crystals138,141 and was tentatively attributed to a stepwise pumping of the molecular excitation by plasmons inside the STM nanocavity. In contrast to the case of twoelectron light emission from plasmonic contacts,50,51,54 where high currents of several microamperes were employed, the low currents (∼0.5 nA) sufficient for obtaining overbias emission in molecular crystals suggest different mechanisms (see sections 3.1.2, 6.4, and 6.3). Y


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intrinsic molecular absorption and emission features in STML spectra, together with the fact that only plasmons could be electronically excited, provides solid evidence that molecules in the vicinity of the tunnel junction can be excited by means of tip-induced plasmons. (ii) TPP molecules on top of a GaAs surface demonstrate the direct excitation of emission when the STM tip was placed at distances of about 5 nm away from the closest molecule (see section 4.5.2). Also this approach assures that electrons do not pass through the molecules and can not directly excite their luminescence. Thus, the characteristic fluorescence spectra exhibiting vibrationally resolved Q bands must be induced by a plasmon-mediated mechanism.242 (iii) A chromophore integrated in a molecular wire showed a transition of emission from a broad to a narrow line width with increasing distance from the substrate (see section 4.5.4).245 The transition line can be unequivocally attributed to electron−hole pair recombination. The authors demonstrated that the intrinsic emission rate is always proportional to the plasmon intensity at the same bias voltage, which is expected for the optical excitation of an emitter and constitutes a proof of plasmonic excitation. (iv) Recently, the coupling between plasmons and molecular excitons of H2Pc263 has been studied in detail. Sharp peak-dip asymmetric spectral features appeared on the plasmonic band when electrons tunneled near a molecule. These features were attributed to the interference between absorption and re-emission of plasmons by the molecule. Tip-induced polarized plasmonic fields enabled selective excitation of the molecular excitons. The intensity of the spectral interference features was found to depend on the position of the tip with respect to the molecule, indicating that the symmetry of the lowest energy excitations plays an important role in exciton−plasmon coupling. For a theoretical description of plasmon coupling to individual emitters in a tunnel junction, we refer the reader to the following papers and references therein: refs 264−273.

Figure 22. Energy diagrams for the emission process of a quantum system in a double barrier tunnel junction excited by the tunnel current. (a) A hole has to be extracted from the lower state and the system has to remain charged for a sufficiently long time. When the upper state becomes filled by an electron from the sample, an electron−hole pair is formed, which may recombine radiatively. (b) A tunneling electron transfers part of its energy to a plasmon that becomes absorbed by the quantum system.

modes. Here, inelastic electron tunneling induces nanocavity plasmons (i.e., tip-induced plasmons) that act as a near-field light source, creating excitons. This process is observed for the electroluminescence of TPP molecules on HOPG using noblemetal tips (Figure 21),161 where the direct electron injection (mechanism 1) does not play a role. Because the plasmon modes are also essential in efficiently coupling the molecular emission to the far-field, we note that the plasmon field may enter twice in the efficiency of the light generation process. Therefore, one should expect a nonlinear relation between current and luminescence intensity, which would be worth studying. Plasmon-induced generation of excitons in tunnel junctions has been observed through several approaches. (i) In the “tipshaft” approach, molecules were deposited on the tip but removed from its apex (see section 4.4.3), which prevents a direct excitation by the current path.237 The observation of

Figure 23. Plasmonic and excitonic luminescence from C60 multilayers on Ag(111). (a) STML spectra for a C60 film far away from any emission center (EC) for different sample voltages and for the pristine Ag(111) substrate (V = −3 V). (b) STML spectra at the same sample voltages as in panel (a), but on an EC. The spectra in panel (b) and the Ag(111) spectrum in panel (a) are shifted vertically for clarity. The inset in (b) shows a zoom-in on the emission peaks. (c) Energy diagram of the processes leading to the excitation of tip-induced plasmons and (d) to the recombination of electron−hole pairs. (e,f) Constant height photon map and (g,h) simultaneously recorded current map in a region off and at an EC, respectively (V = −3 V). Reproduced with permission from ref 233. Copyright 2017 American Chemical Society. Z


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5.4. Distinguishing Emission from Plasmons and Excitons in STML

electron−hole pairs, the excitation of tip-induced plasmons by inelastic tunneling, or an energy transfer between both.

We sketched above that the unequivocal identification of the origin and generation of light in an STML experiment is often difficult. In many cases, the electroluminescence arising from molecular systems does not result from electron−hole pair recombination but is due to the molecular orbital gating of the tunnel current, which leads to spatially and spectrally modified tip-induced plasmons (see section 3.2.2).4,162 A first indication of the luminescence nature (excitonic vs plasmonic) can be obtained from the character of optical spectra. Inelastic tunneling-induced (plasmonic) emission displays typically broad bands of hundreds of meV that exhibit a quantum cutoff shifting upon reduction of the applied bias voltage (at moderate currents 1 (see section 6.1.1). Indeed, density functional theory (DFT) calculations of the neutral ZnEtioI molecule showed that only a few of its 213 vibrational modes are involved in the electroluminescence,267 AC


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Figure 26. Local STML spectra and spectrally resolved photon maps of single metalloporphyrin molecules on an ultrathin film of Al2O3 on Ni(110). (a) STML spectra on a single ZnEtioI molecule acquired on the two positions marked in the inset topography image. (b) Spectrally resolved photon map of the same molecule (V = 2.2 V, I = 0.1 nA) integrated in the region 727−741 nm, (c) 763−784 nm, and (d) 727−784 nm. The unit of integrated intensity is 1000 CCD counts. (e) STML spectra on a single magnesium porphine molecule acquired on the four positions marked in the topography map (V = 2.2 V, I = 0.2 nA) in (h), as well as on the NiAl substrate and the Al2O3 oxide film. (f) Spectrally resolved photon map of the same molecule integrated in the region 750−763 nm and (g) 766−778 nm (V = 2.3 V, I = 0.4 nA). The image size in panels (f)−(h) is 3.5 × 3.5 nm2. Parts (a)−(d) adapted with permission from ref 75. Copyright 2014 American Chemical Society. Parts (e)−(h) adapted with permission from ref 206. Copyright 2010 American Physical Society.

although the authors could not verify the strong electron− vibrational coupling necessary to explain the experimentally observed intensity distribution. The energy of the emission peaks was found to be independent of the applied voltage, which excludes that these peaks arise from inelastic transitions between the electronic states of the tip and those of the substrate or the molecule. Indeed, similar observations have been reported for many other single molecules3,241−243 and molecular films.147,217,226,278,279 In this context, it is not obvious that, at the same time, the emission intensity rises with increasing voltage but constant current,2,147,217,241−243,278,284 which implies a steadily growing photon yield. Commonly, this observation is rationalized by the increasing number of vibrational states or channels available for molecular excitations.2,147,217,241,243,278 However, when the injection current is kept constant and all injected electrons decay to the vibrational ground level of the excited state, as expected from Kasha’s rule, a constant photon yield would rather be expected. Figure 25 shows that the vibrational patterns recorded on different parts of the ZnEtioI molecule vary substantially. Spectrally and submolecularly resolved photon maps of single ZnEtioI molecules in the saddle conformation enabled Lee et al.75 to image three different excited states of the molecule, from which they concluded that the molecular emission originates from the radiative ionization of the excited doubly charged molecule (bianion) to its monoanion. At the topographically lower molecular lobes, the emission is

dominated by a Franck−Condon progression with a weak negative anharmonicity (Figure 26a, red curve), which has been ascribed to the transition from the 1A1g and 3A1g state of the bianion to the 2B1g electronic ground state of the monoanion. At the topographically higher molecular lobes, the emission is governed by a single Fano resonance (Figure 26a, blue curve), attributed to a transition from the Jahn−Teller active 1B1g state of the bianion emerging from the molecular distortion along the b1g coordinate. Photon maps integrated over a larger spectral range show a single nodal plane (Figure 26d), whose reduced intensity was explained by an additional radiationless transition arising from the crossing of the Jahn−Teller states of the bianion and monoanion along the b2g coordinate. The suggested radiative ionization mechanism from the bianion to the monoanion provides an explanation for the surprisingly high emission intensity observed in STML. Indeed, the transition dipole moments of the lowest electronic transitions are all oriented within the molecular plane and thus perpendicular to the tip−sample axis. Therefore, the radiative decay of electron−hole pairs should not couple efficiently to the plasmon modes of the STM cavity. Positioning the STM tip off the molecule center may result in a net perpendicular dipole moment;206 however, this contribution should be rather weak. In the case of the radiative ionization mechanism suggested by Lee et al., in contrast, the departing electron carries a strong dipole moment parallel to the tip−sample axis, which can drive the plasmon modes of the STM cavity. AD


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Figure 27. STML spectra of various free base porphyrins. (a) STML spectrum on different lobes of a single porphyrin molecule on an octanthiol monolayer on Au(111) (colored curves) and the octanthiol monolayer (black curves). (b) STML spectrum on a self-decoupled porphyrin on Au(111) (red curve) and the Au(111) substrate (black curve). (c) STML spectra next to a single porphyrin molecule on GaAs(110) at the denoted sample voltages. (d) STML spectra of different porphyrin-thiophen oligomer strands suspended between a gold tip and a Au(111) substrate. Part (a) adapted with permission from ref 241. Copyright 2016 Chinese Physics Society. Part (b) adapted with permission from ref 243. Copyright 2013 American Chemical Society. Part (c) adapted with permission from ref 242. Copyright 2012 AIP Publishing LLC. Part (d) adapted with permission from ref 246. Copyright 2016 American Chemical Society.

porphyrins in different matrixes303−307 corroborate this assumption and reveal a variety of close-lying vibrational satellites. Polarization measurements on individual photoluminescence lines304,305 and calculated spectra308,309 further show that these lines result almost exclusively from fundamental vibrations, that is, single-phonon transitions of totally symmetric and nontotally symmetric modes. Therefore, the vibrational fine structure of free base porphyrins in STML spectra arises from Franck−Condon and Herzberg−Teller coupling. In two recent studies, Chong et al.245,246 could resolve several very sharp STML emission lines (FWHM ∼2.5 meV245) for on-surface polymerized porphyrin−thiophen copolymer strands (see section 4.5.4 and Figure 27d). The unequal spacing of the observed peaks indicates that they belong to different fundamental vibrations, instead of a common Franck−Condon progression. Their rather weak intensity with respect to the main emission line suggests that most of the peaks arise from Herzberg−Teller or (pseudo-)Jahn−Teller coupling. Additional DFT calculations of the Raman modes of a model compound showed good agreement with the experimental spectrum, in particular for the higher energy modes.245 Indeed, Raman active modes are always of gerade symmetry, like (pseudo-)Jahn− Teller modes, which might explain the partial agreement between the experimental and theoretical spectrum. The asymmetric line shape of the main emission peak was ascribed to the low-energy phonon sideband of the molecular chain or the damping caused by the excitation of electron−hole pairs in the metal electrodes.245 For two of the three investigated porphyrin−thiophen copolymers, Chong et al. observed a series of weak peaks also on the high energy side of the main emission line.246 Because of similar energy shifts to the main emission line than those on the low energy side, the authors suggested that both emission bands are associated with the same vibrational modes of the molecules and the high energy band

The idea of imaging different electronically excited states of molecules by spectrally resolved photon maps has been first demonstrated by Chen et al., who studied the STML of magnesium porphine (MgP) on an ultrathin insulating Al2O3 film.206 Like ZnEtioI, MgP molecules adsorb in various adsorption configurations on the Al2O3 film, with light being emitted only from molecules with a single central protrusion and an elongated shape in topography images. The reduction of the D4h molecule symmetry was explained by the structural deformation of the molecule due to the interaction with the substrate. The structural deformation leads to two energetically slightly different radiative transitions. While the higher-lying state is mostly localized on the orthogonal lobes along the molecular short axis (Figure 26f), the lower-lying state is mostly localized on the lobes along the molecular long axis (Figure 26g). Interestingly, both states result in a different vibrational fine structure in STML spectra (Figure 26e), which was attributed to the different interaction of both states with the underlying substrate. In the case of the lower-lying state, the purely electronic transition is followed by a Franck−Condon progression with equidistant peaks and a spacing of 47 ± 2 meV (Figure 26e, red curves), which has been assigned to the pyrrole (hindered) rotation or tilt modes. For the emission from the higher-lying state, a vibrational analysis is more difficult (Figure 26e, blue curves), albeit the first vibrational satellite occurs with a spacing of ∼89 meV to the purely electronic transition. 6.2.3. Single Free Base Porphyrin Molecules. In contrast to metalloporphyrins, previously reported STML spectra of free base porphyrins lack pronounced Franck− Condon progressions. A summary of the STML spectra reported for different free base porphyrins is depicted in Figure 27. The broad emission peaks in most of the reported STML spectra exhibit noticeable asymmetries and shoulders, which indicate that these peaks contain several emission lines. Site-selective photoluminescence spectra of various free base AE


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Figure 28. Spectral modification of the STML spectrum by the plasmon modes in the STM cavity. (a) STML spectra acquired with different W tips on a Au(111) surface and (b) immediately afterward on a 5 ML TPP film on Au(111). Adapted with permission from ref 141. Copyright 2010 Nature Publishing Group.

arises from a hot electroluminescence process (see sections 3.1.2 and 6.4.

6.4. Spectral Modification by Plasmon Modes in the STM Junction

6.3. Vibrational Spectroscopy of Single Phthalocyanine Molecules

In the following part, we discuss the effects of plasmon modes in the STM cavity on the vibronic features of molecules that go beyond the enhancement or quenching of the entire spectral profile discussed in section 5.2. Specific vibronic transitions can be dramatically modified by the frequency-dependent Purcell factor of the STM cavity g(ω), or an energy transfer from excited plasmons Fex(ω). Therefore, the observed emission spectrum Fobs(ω) can substantially deviate from the emission spectrum of an isolated molecule in the gas-phase FSM(ω) and arises as

Phthalocyanines are tetrabenzotetraazaporphyrins, that is, tetrapyrrole macrocyles whose pyrrole subunits are fused to benzene rings and linked by azo bridges, instead of methane groups as in porphyrins. The additional nitrogen atoms break the near degeneracy of the highest occupied states of the porphyrins, which removes the pseudo forbidden character of the optical transitions.299 As a result, phthalocyanins typically exhibit shorter radiative lifetimes and larger Q-band oscillator strength than porphyrins.310 Nevertheless, the emission from single phthalocyanine (Pc) molecules has been investigated only very recently for ZnPc,239 MgPc,236,277 and free base Pc.236 In these cases, emission spectra are dominated by the vibrationless 0−0 transitions and exhibit only faint vibrational satellites (see Figure 19e, lower black spectra). First highly resolved spectra of these satellites on single ZnPc molecules and linear aggregates277 show a good agreement with experimental Raman spectra, photoluminescence spectra of isolated molecules in cryogenic matrixes, and DFT calculations. Overall, 11 different A1g, B1g, and B2g vibrational modes of MgPc could be identified. Spectrally resolved photon maps of a molecular trimer reveal marked differences in the local emission intensities of different vibrational satellites as well as the 0−0 line. For free base phthalocyanine, the main emission line at 1.81 eV is additionally accompanied by a weak emission peak at 1.92 eV.236 These two peaks arise from the splitting of the Qband due to the reduced molecular symmetry and the emission from the lower lying Qx state and the higher lying Qy state, respectively.

Fobs(ω) ∝ FSM(ω) × g (ω) × Fex(ω)

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As an example for the spectral modification by the plasmon modes in the STM cavity, Figure 28b shows emission spectra of a H2TPP porphyrin multilayer film that were recorded with five different tips, whose plasmon spectrum on a clean gold surface is depicted in Figure 28a.141 A similar selective spectral enhancement of the photoluminescence (PL) of dye molecules has been demonstrated by varying the shape of plasmonic structures311−313 and the spacing within spherical nanoparticle dimers.314 As compared to these experiments, the coupling to tip-induced plasmons within an STM cavity may result in the appearance of new emission peaks with no or only extremely weak intensities in PL spectra of the same chromophore in solution. In the specific case of H2TPP, the intensity of the high energy peaks at ∼2.05 and ∼2.20 eV (Figure 28b) is 2 orders of magnitude higher than that in PL spectra of H2TPP in solution.315,316 Because these peaks have higher energies than the main emission line, Dong et al. suggested a radiative transition from higher vibrational levels of the lowest excited electronic state of the molecules, thus supposedly violating AF


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Figure 29. Emission spectra of tubular porphyrin J-aggregates. (a) STML spectra of a J-aggregate and a cluster consisting of few porphyrin molecules (top, V = −2.8 V, I = 100 pA, 30 s) and photoluminescence (PL) spectra of the same aggregates on Au(111) and the porphyrin monomers in aqueous solution. (b) STML spectra acquired on a J-aggregate and the Au(111) substrate (V = −2.8 V, I = 200 pA, 120 s) at the sites marked in the topography image (V = −2.8 V, I = 1 pA) in (c). Adapted with permission from ref 329. Copyright 2015 AIP Publishing LLC.

characteristics of the 2.05 and 2.2 eV emission peaks, absorption spectra, and time-resolved fluorescence anisotropy measurements, they concluded that these peaks result from transitions from the higher-lying Qy state, in contrast to the lower energy emission peaks originating from the Qx state of H2TTP (see section 6.2.1). In other words, the STML spectra of H2TTP molecules might be interpreted in a way analogous to those of magnesium porphine (see section 6.2.2), that is, by the emission from two different excited states arising from the D2h symmetry of the molecule.

Kasha’s rule (see section 6.1.1), and denoted this process as hot electroluminescence.141 In addition to the extremely low intensity of these high-energy peaks in solution PL spectra, two further observations indicate that the plasmon modes in the STM cavity play an important role in their enhancement. First, significant intensities of these peaks can be only observed for single porphyrin molecules241 or multilayers141,147,220,226,278−283 on substrates with strong surface plasmon resonances (Ag, Au), but not on substrates such as GaAs,242 highly ordered pyrolytic graphite (HOPG),261 ITO, or graphene on Ru(0001).283 Second, these high-energy peaks are visible in STML spectra even when the tunneling electrons have an energy lower than the lowest excitation energy of the H2TPP molecules of ∼1.9 eV.141 Such a process resulting in overbias light has been observed also for other systems (see section 3.1.2), however, not in combination with hot electroluminescence. Dong et al. suggested that the observed molecular emission arises from the plasmonic pumping of the molecules (see section 5.3). As the origin of this resonant pumping, they postulated that the radiative transitions from the vibrational excited levels of the excited electronic state become comparable to the relaxation rate to the vibrational ground level of the excited state, as proposed by earlier theoretical considerations by Le Ru et al.311 Calculations of the molecular emission inside a plasmonic cavity of two metal spheres support the assumption that the observed hot electroluminescence results from an enhanced radiative decay rate due to the enhanced photon density of states in the cavity.317,318 The radiative decay rate was found to increase by 3−5 orders of magnitude when the plasmon resonance of the cavity is tuned to the vibronic transitions of the molecules. In contrast, calculations based on a density matrix approach by Tian et al. suggest that the enhanced radiative lifetime inside an STM cavity plays only a minor role, because calculations with an enhanced radiative lifetime could not reproduce the experimentally observed hot electroluminescence peak at bias voltages smaller than the lowest electronic excitation of the molecule.265,266 Instead, the authors explained the experimentally observed spectral modifications by the STM cavity and the hot electroluminescence by the resonant excitation of the molecules by plasmons (plasmonassisted emission). Time-resolved fluorescence spectra of H2TTP in solution by Bialkowski et al. suggest an alternative interpretation of the high-energy emission peaks.315 On the basis of the similar decay

6.5. Intermolecular Interactions in Molecular Aggregates

In the previous sections, we have shown that STML is particularly suitable for investigating local variations in emission spectra on the level of individual molecules. In the following sections, we discuss examples of local variations in the vibrational fine structure within molecular aggregate and what kind of information they reveal about the local intermolecular interactions. 6.5.1. Exciton Coherence Length in J-Aggregates. In molecular van der Waals solids, the electronic excited states are localized on individual molecules and are able to resonantly interact with the neighboring molecules. In so-called Haggregates, the sign of the resonant coupling is positive, which induces a shift of the main absorption peak to higher energies and a quenching of fluorescence. On the other hand, in J-aggregates, the coupling is negative, which results in a redshift of the main absorption peak and might lead to superradiant emission at low temperature.319 The distribution and transfer of excitations have been first discussed in the framework of Kasha’s molecular exciton model.320,321 He proposed that, in the case of strong intermolecular electronic interaction, an electronic excitation is spread over all molecules and only vibrationless electronic transitions become allowed. However, in real molecular aggregates, the distribution of electronic excitations is limited by static and dynamic disorder as well as the coupling between excitons and molecular vibrations, which were neglected by Kasha. Theoretical considerations by Spano322,323 showed that, when the exciton−vibrational coupling involves a single intramolecular vibration, the exciton coherence number Ncoh, that is, the number of molecules over which the exciton wave function extends coherently, can be estimated from photoluminescence spectra. More precisely, Ncoh can be estimated from the corresponding Huang−Rhys factor S of the isolated molecule and the intensity ratio of the AG


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pure electronic 0−0 transition peak (I0−0) and the first vibrational satellite peak (I0−1):322,323

different emission centers (see section 4.4.2, Figure 16). Referencing each spectrum to the strongest line reveals a very similar vibrational fine structure for all emission centers (Figure 30a). Furthermore, the spectra strongly resemble photo-

I 0−0 (10) I 0−1 On the basis of this work, Spano and co-workers determined the exciton coherence number from photoluminescence spectra in tetracene324 and anthracene325 films; later this approach was used also by other groups to determine the exciton coherence number in polymer aggregates326,327 and dinaphthothiophene derivatives.328 Recently, Meng et al. have applied this approach to the STMinduced electroluminescence of tubular structured mesotetrakis(4-sulfonanophenyl) porphyrin (TPPS) J-aggregates.329 The typical redshift and suppression of vibrational peaks as compared to single porphyrin clusters and molecules is clearly visible in STML and photoluminescence spectra (see Figure 29a). From the relative peak intensity of the vibrational features on the J-aggregate and small porphyrin clusters, the authors determined an exciton coherence number of 4 ± 2 molecules, which is in good agreement with the values derived from the spectral narrowing of the main absorption line in solution (5 molecules330 and 5.5−12.9 molecules331). This rather low value explains the moderate site-dependent shifts of the main emission line in STML spectra of ∼10 nm between sites on the same tubular structure that are only a few nanometers apart from each other (Figure 29b,c). 6.5.2. C60 Films and X Traps. First experiments by W.-D. Schneider and co-workers on few layers thin C60 nanocrystals grown on ultrathin NaCl films revealed varying spectral profiles and peak positions.16,230,231 In their first study,230 they interpreted the obtained STML spectra analogously to photoluminescence spectra of single C60 molecules in inert gas matrixes.332,333 The varying spectral profiles were ascribed to a different ratio of fluorescence and phosphorescence. Later, the authors attributed the variations to a different spectral enhancement by the plasmon modes in the STM cavity231 and the observed rigid shifts in the emission peak positions to a site dependence arising from orientational disorder and the presence of lattice defects.16 Apart from that, they adopted the interpretation of van den Heuvel et al.334 and Akimoto et al.235,335 derived from photoluminescence spectra of C60 single crystals and ascribed the obtained STML spectra to the emission from two luminescent species: Emission lines with an energy >1.69 eV were attributed to the monomolecular Frenkel exciton, whereas the main spectral features at lower energies were associated with Frenkel excitons at localized states that extend over two adjacent molecules. In fact, photoluminescence spectra of solid C60234,235,334−336 and C60 clusters337−339 differ markedly from those of single C60 molecules,332,333 which indicates a different emission mechanism. Calculations show that the electronic ground state and the four lowest excited singlet states of C60 have gerade inversion symmetry;340 thus, radiative transitions between them are dipole-forbidden and require intensity borrowing from higher-lying states via Herzberg−Teller coupling (see see section 6.1). The different photoluminescence spectra observed for solid C60 and C60 clusters indicate that the environment substantially affects the emission process. Our own work on C60 multilayer films (5−10 ML) directly adsorbed on noble-metal substrates shows that the molecular emission is restricted to individual emission centers at structural defects, with the energy of the strongest line varying among Ncoh = S

Figure 30. Emission spectra of C60 films and single C60 molecules. (a) STML spectra on various emission centers of a C60 multilayer film (yellow), STML spectra on few layers thin C60 nanocrystals on an ultrathin NaCl film (upper gray curve, reproduced with permission from ref 231), and photoluminescence spectrum provided by Akimoto et al.235 (lower black curve). All spectra are referenced to the main emission line. (b) Schematic energy diagram and coupling vibrational modes for the emission from C60 X traps. (c) Photoluminescence spectrum of single C60 molecules in argon matrix reported by Sassara et al.333 Part (c) adapted with permission from ref 333. Copyright 1997 AIP Publishing LLC.

luminescence spectra by Akimoto et al. ascribed to structural crystal defects (lower black curve in Figure 30a) as well as STML spectra of few layers thin C60 nanocrystals on an ultrathin NaCl layer231 (gray curve in Figure 30a). Combined photoluminescence and absorption spectra reveal a mirror symmetry at the main emission line,235,335 which identifies this line as a pure electronic 0−0 transition. The emission features AH


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the “slow” time-averaged tunnel current. The signal-to-noise ratio is improved by measuring via a lock-in technique and modulating the pump−probe delay time. One of the shortest voltage pulse lengths that has been realized inside an STM tunnel junction by pure electronic means to date is 120 ps.349 When employing terahertz light pulses coupled into the tunnel junction, the time resolution may even reach the femtosecond range. Terahertz radiation allows one to electronically monitor the dynamics in the junction after the studied system has been triggered, for example, by synchronized ultrashort IR pulses.47 The shaken-pulse-pair-excited-STM (SPPX-STM)44 method uses a pump and a probe light pulse, which are delayed with respect to each other. The delay is slowly varied in the picosecond or nanosecond range, and, similar to the all-electronic method sketched above, the information on the “fast” experiment can be read-out through the low bandwidth (kHz-MHz) tunnel current. An alternative way for accessing molecular dynamics on ultrashort time scales is employing detailed data in the frequency domain as it has been presented in a recent publication on light emission from individual decoupled porphyrin molecules75 (see also Figure 26a in section 6.2.2). In this study, optical STML spectra were modeled by the superposition of a broad emission continuum interfering with sharp vibronic transition lines. Because of the coherent background of self-ionization transitions, the phase of the vibronic lines could be extracted by a Fano fit, and the femtosecond time-evolution of the transition could be reconstructed (Figure 31).

at lower energy can be attributed to the Jahn−Teller active hg vibrational modes and the Franck−Condon active ag modes. Their observation implies that the lowest electronic transition must be locally allowed (see section 6.1.1) at the emission centers, in contrast to single C60 molecules. This fact can be rationalized by the local mixing of higher-lying ungerade electronic states into the excited state and the electronic ground state due to the broken translational symmetry at the structural defects. This example demonstrates that the intermolecular interaction in a molecular film can substantially alter the emission characteristics of the single molecules and can even allow electronic transitions that are dipole-forbidden in isolated molecules. In some cases, the electroluminescence spectrum exhibits an additional weak peak S2 at an energy ∼30 meV above the main emission line S1. The energy separation between the S1 and S2 peaks of 30 meV is significantly smaller than the lowest-energy Herzberg−Teller active gu(1) mode of C60 (43 meV).341 Consequently, we can exclude that the more intense S1 peak results from an intensity borrowing from the S2 transition via Herzberg−Teller coupling. On the basis of the laterally changing intensity ratio of both emission peaks, we can also rule out a hot electroluminescence process as well as an emission from two electronic states localized on the same molecule.233 Instead, we assign the two emission peaks to the pure electronic 0−0 transition of a surface trap and one subsurface trap. Indeed, very similar energy differences for the surface and subsurface excitons in anthracene and tetracene have been reported.342−344 When the weak S2 peak is absent, as for the majority of emission centers, we assume the exciton trap to be fully localized in either the surface layer or a subsurface layer.

7. TIME-RESOLVED STML Scanning tunneling microscopy has been regarded as a rather static imaging method345 when it is discussed in comparison to studies employing ultrafast laser pulses, able to access and even control chemical reactions in real time.346 Time-resolved STM is often taken synonymous for video rate imaging, which requires maximized scanning velocities of the STM tip with microsecond averaging times per pixel.347 However, during the past decade, substantial progress in the observation of even faster processes in STM has been achieved. For a fixed position of the tip, it became possible to monitor spin, charge, and exciton relaxation processes with submolecular resolution and submicrosecond resolution. However, following chemical reactions, conformational changes, or charge separation on a single molecule, processes that occur on the picosecond to femtosecond time frame, remains a vision that may become attainable in the future. The exploration of fast dynamics in adsorbate systems by STM has led to impressive studies such as the locally monitored spin transition in individual atoms.348 The time resolution in that study was achieved by pure electronic means using the combination of a pump-and-probe technique with a lock-in technique. Increasing the time resolution further will require advanced high-frequency optimized STM designs.349 In a typical all-electronic experiment performed at a fixed tip−sample distance, the time delay between a short excitation “pump” and a short read-out “probe” voltage pulse is varied. The shortest pulse duration that can be cleanly realized determines the achievable time resolution. By measuring in a nonlinear section of the I(V) curve of the tunnel junction, the “fast” time-resolved response can be monitored by

Figure 31. Reconstruction of a time-domain snapshot of an electron density that is circulating in a ZnEtio molecule. The dynamics was extracted from the Fano line profiles measured by STML. Adapted with permission from ref 75. Copyright 2014 American Chemical Society.

In the following, we discuss ways of accessing the dynamics by STML directly in the time domain. Time resolutions in the lower picosecond range can be obtained in a straightforward manner by exploiting luminescence detection in STML using commercial optical detectors (see section 2.2) together with time correlation electronics (see section 2.3). The detector time resolution determines the limit for STML experiments. Two experimental methods are sketched in Figure 32. The first one is based on a Hanbury−Brown Twiss (HBT) interferometer, which allows measuring time correlations between photons emitted from the tunnel junction by two time-resolved single photon detectors. The measured histogram of time intervals Δt between two detected photons provides the second-order AI


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luminescence response of the investigated system is measured with respect to the generated voltage pulse. Applied to a metal substrate, which typically exhibits subpicosecond plasmonic luminescence dynamics, the measured light transient serves as a monitor for the tunnel current with gigahertz bandwidth.350 On a semiconductor or molecular system, the luminescence transients will display the slower dynamics of these systems. 7.1. Photon−Photon Time Correlations (Hanbury−Brown Twiss Interferometry)

A pioneering study monitoring the dynamics of molecular systems by STML was reported by Perronet and co-workers97 for molecules in solution using photon−photon correlations in a HBT interferometer351 (Figure 33a). Fully uncorrelated photon sources, for example lasers, exhibit a delay-independent correlation function g(2), whereas sources emitting photon bunches, for example, thermal emitters or blinking light sources, exhibit a maximum correlation at time zero (Δt = 0). Individual emitters, such as quantum dots or single molecules, act as single photon sources for which the occurrences of photons arriving with short intervals Δt are significantly suppressed as compared to the occurrence of long intervals. This behavior, referred to as “anti-bunching”, results in a pronounced minimum at time zero, reaching zero occurrences in the ideal case. Having introduced photon correlation measurements from molecular systems, it seems unexpected that the pioneering study97 observes photon-bunching (Figure 33a) instead of antibunching. However, the observation has been attributed to the diffusion motion of molecules inside the tunnel junction in the liquid. Temporal fluctuations of the photon emission rate are indeed expected to exhibit photon-bunching. They may arise from changes of molecular position and orientation inside the tunnel junction. These positive time correlations appear in the correlation function at the time scale of the respective diffusion mechanism. The behavior is comparable to the

Figure 32. Schematic of the experimental setup for time correlated single photon counting (TCSPC) applied to photon−photon correlation measurements and for light detection correlated with voltage pulses from a pulse generator. For the latter measurement, the start input of the TCSPC card has to be switched to the trigger output of the pulse generator.

correlation function g(2)(Δt), which describes the correlations of intensity fluctuations in the emitted photon train. The accessible time resolution in this setup is determined by the timing jitter of electronic pulse formation in the detectors and can be as low as 50 ps for avalanche and photomultiplier detectors (for more detailed detector characteristics, see section 2.2). The second method uses a pulse generator that sends periodically repeated voltage pulses to the tunnel junction (Figure 32). The synchronization trigger of the generator is used as time zero, and one of the single photon detectors provides the arrival time of a photon. In this setup, the

Figure 33. Photon−photon correlation measurements in STML from molecular systems. (a) Time correlation data on the nanosecond scale for different organic molecules in solution at room temperature.350 Abbreviations: squaraine dye (SQ), azobenzene dye (DRPR), triphenylene derivative (H11T). (b) Nanosecond time correlation measurements on an emission center in a C60 film at 4 K (○) for the indicated tunnel currents219 and correlation functions fitted to the data (solid curves). The lifetimes τ were obtained by assuming perfect single photon emission. The upper three curves have been offset for clarity. Part (a) reproduced with permission from ref 350. Copyright 1998 EDP Sciences. AJ


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applied this approach to directly monitor the transient charging of an Ir(ppy)3 molecule in a double tunnel barrier. The system, introduced in section 3.2.2, exhibits a sharp luminescence onset around 3 V (Figure 34a). By applying 104 ns long square pulses

classical example of photon-bunching observed for a molecule whose fluorescence from a short-lived singlet state is interrupted by intersystem crossing (ISC), which leads to extended periods in which the molecule resides in a long-lived triplet state.352 In two early papers, Silly and Charra353,354 reported photon-bunching in STML also in the emission from a gold surface in air and demonstrated an application as a contrast mechanism in STM images. However, the origin of the observed photon-bunching on the time scale of 10−100 ns could not be fully clarified in these publications and may be related to the dynamics of unidentified adsorbates. From our own experience with photon correlations, we note that the observation of bunching is sensitive to any kind of current fluctuations. A pronounced photon-bunching might already result from short instabilities of the tunnel contact, which may be due to the active STM feed-back control or “obstacles” encountered on the surface during tip motion (e.g., during tip scanning). Recently, we demonstrated antibunching of the photon emission from C60 multilayer films (Figure 33b).219 A Hanbury−Brown Twiss setup was combined with the STML instrument shown in Figure 6, which allows light collection in two orthogonal emission directions. In contrast to photonbunching, the antibunching cannot be induced by fluctuations of external parameters but is directly related to the quantum mechanical properties of the light source. Moreover, it might be expected when the tunnel current itself becomes antibunched, for example, by Coulomb blockade, when a charge is located in the tunnel path for a measurable time and prevents the tunneling of further charges. For X traps in a C60 film, photons are generated by exciton recombination at structural defects, localized on a small number of C60 molecules that create a localized trap for charge carriers and excitons (see section 4.4.2).218 The detailed analysis of the photon antibunching allows one to determine the exciton lifetime of 0.75 ns and shows that the observed lifetime is drastically reduced by consecutive charge injection from the STM tip.219 The latter indicates a local charge-exciton annihilation, which is of technological relevance because it is an unwanted process in organic electronic devices. The dynamics of such processes are highly relevant for processes in modern OLEDs and have not been monitored with single molecule resolution before. Therefore, this experiment demonstrates a way to characterize molecule-based light sources that are too densely packed to be accessed individually by diffraction-limited far-field microscopy.

Figure 34. Luminescence response for tunneling through an electronic state of an Ir(ppy)3 molecule, electronically decoupled from the metal substrate by a C60 bilayer. (a) Luminescence intensity (red) as a function of bias voltage recorded by a photon counting detector in a “slow” bias scan taking several seconds. The green dashed line indicates the detector dark count rate. (b) Fast transient luminescence response (orange ■ with error bars) to the field-controlled charging of the Ir(ppy)3 molecule. The 104 ns long pulse switches between two voltages indicated by the blue lines in panel (a). The instrumental time response is indicated by the dashed black line.

crossing the onset voltage, the charge-transfer from the substrate to the molecule is rapidly switched on and off. Figure 34b shows that the observed luminescence transient follows closely the instrumental response, which indicates that the charge-transfer time from the substrate to the molecular state is faster than 1 ns.162 Figure 35 shows an example for a square voltage pulse in the nanosecond range (red curve) applied to measure the luminescence response of an emission center on a molecular C60 film161 (see also section 4.4.2). When the pulse is “off”, the bias voltage is −2.8 V, and the Fermi level of the tip lies within the C60 band gap. No charges are transferred from the STM tip to the film and no light is emitted. During the pulse, the voltage is −3.3 V and the luminescence is “on”: Electrons are locally extracted from the film by the STM tip and recombine with holes injected from the substrate. For the lowest graph in Figure 35, the equilibrium current under “pulse-on” conditions is 2 pA, which corresponds to three tunneling electrons on average during one pulse. The recorded luminescence transients in Figure 35 are averaged over ∼109 pulse repetitions. They exhibit a pronounced nanosecond dynamics, manifested by the slow rising and falling edges. Fitting these edges by exponential functions yields the time constants indicated in the figure. A decrease of the tip−sample distance (bottom to top in Figure 35), which corresponds to an increase of current, reduces the time constants significantly and leads to qualitative changes of the light transient. The measured time constants are much larger than the exciton lifetime at the emission centers determined in the HBT experiments (0.75 ns). Therefore, the observed dynamics can be ascribed to the electron and hole transfer rates into the emission center that depend on the tunnel current and the electric field within the tunnel junction. This experiment shows again that photon emission transients can be employed to access charge carrier dynamics. It can not only be applied to electron−hole recombination but also as a fast monitor for the tunnel current, even close to the limit of

7.2. Electroluminescence Transients

The photon correlations in the previously discussed studies show that local dynamics are accessible even when they are much too fast to be directly followed by the tunnel current. A second type of time-resolved method is to monitor the transient light response to nanosecond voltage pulses. The pulse shapes arriving inside the tunnel junction can markedly differ from the pulse generator’s original pulses, as a result of dispersion and reflections in the cables to the STM junction. By monitoring the plasmonic light emission, the true temporal voltage pulse shape between the two metal electrodes in tunnel contact can be accessed with both millivolt and nanosecond resolution.350 In a next step, the pulses can be shaped to exhibit precisely the form required by the experiment, for example, a pulse with steep edges.350 The optimized pulse form then enables one to study processes of a molecular system in the tunnel junction by time-resolved transient light detection. We AK


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observed molecular emission, which encodes parameters such as vibrational frequencies, local optical coupling, electron−hole interactions, and ultrafast intramolecular dynamics. Accessing these processes at their relevant time scales will be the key for controlling energy transfer, excitation quenching, and electronic decoupling. Finally, one may ask which developments of STML may lie ahead (Figure 36). Submolecular spatial resolution can be

Figure 35. STML transient light intensity (yellow curves) recorded in response to 222 ns long voltage pulses (red curve) on a C60 emission center. When the tip approaches the sample (yellow curves: bottom to top), the indicated time constants at the rising and falling edges are reduced due to the accelerated charge carrier dynamics.

Figure 36. Outlook toward present (green ★) and possible future topics (yellow ★) of STML studies.

reached in a reproducible manner in luminescence maps. Electronic states or vibrations will most likely not exhibit any smaller spatial structures than those that can be resolved today because the smallest features in electronic orbitals is the distance between nodes of the highest accessible electronic wave function, and the smallest unit in a vibration is the bond between neighboring atoms. Both can typically be resolved at 0.1 nm resolution. Optical emission spectra can certainly be accessed with higher spectral resolution than used in most STML studies, because optical spectrometers with resolutions well below the millielectronvolt range are available and could be employed. However, the Heisenberg uncertainty principle links the spectral resolution to the respective dynamic time scale. We discussed in sections 3 and 4 that lifetimes of excitons and charge carriers range in the micro- to picosecond decades and plasmons decay in the subpicosecond range. In these cases, the Heisenberg uncertainty limit is not far from spectral resolution limits. However, there is a clear asset from the local molecular access with respect to spectroscopic information: addressing individual optical emitters by low-temperature STML can substantially reduce or even rule out inhomogeneous line broadening, spectral wandering, and blinking. Thus, STML may provide energy resolutions that are close to the limit determined by the lifetime of the emitting state. Moreover, the spectroscopic range, currently mostly the visible range, may be extended. New superconducting detectors mentioned in section 2 may allow near-infrared or even far-infrared detection and provide access to low-energy excitations together with a good time resolution. The largest potential for innovative studies is, most likely, the access to ultrafast time resolution, which would allow investigations in the lower picosecond and subpicosecond range. The dynamics of conformational changes of molecules, intermolecular and intramolecular excitations and charge transfers, or chemical reactions may become accessible.

single charge tunneling and for moderate photon generation efficiencies. We illustrate this by a numerical example: A 10 ns long pulse, in which the tunnel current is 100 pA, involves six tunneling charge carriers per pulse on average. For a typical conversion efficiency of 1 detected photon per 105 tunneling electrons and a 10 MHz pulse repetition rate (providing a duty cycle of 10%), we obtain a flux of 600 photons per second, that carry the information on the response of the studied system to the pulse. For low background luminescence, an integration time of a few minutes will already provide satisfactory experimental statistics allowing time-resolved measurements of the tunnel current with GHz bandwidth.

8. CONCLUSION AND OUTLOOK In this Review, we have given a broad overview of the field of charge-light interaction in scanning tunneling microscopy and discussed the coupling between charge carriers and electromagnetic (plasmonic) near-fields inside the STM tunnel junction. Tip properties, such as its material and shape, are essential for controlling the tip’s function as an antenna and thus the coupling between the nanoscale and the electromagnetic far-field. We have provided examples that demonstrate mechanisms and possible applications, and reported on studies suggesting mechanisms for future electro-optic elements on a molecular scale by demonstrating the manipulation of electronic molecular states and the control of the optical output. The combination of spatially selective control of charge injection and luminescence detection enables unprecedented studies of individual nanostructures, defects, and molecules in the tunnel junction. Fundamental studies demonstrated the observation of coherent and incoherent interactions in the emission from individual molecules. We have discussed how the electronic and optical properties of the junction determine the AL


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Biographies

However, experimental methods that involve bias-voltage pulses and single-photon detection are at present not fast enough to reach this time domain. Here, combinations with ultrafast laser pulses may turn out to be a viable solution. We witness a time of rapid progress in this field, a fact evidenced by a very recent publication on the real-time observation of molecular oscillation with submolecular resolution.355 Progress in the near future may be seen in the investigation of charge carrier diffusion. For semiconductor nanostructures, studies have observed exciton diffusion lengths of tens and hundreds of nanometers. Because the carrier mobility in organic semiconductors is lower, similar studies require a higher spatial resolution, which can be well achieved by using STML. Interesting systems for future studies may be grain boundaries of homogeneous or heterogeneous organic films, as discussed in section 4.4.2, intrinsic defects as those in C60 films, or molecular impurities in host matrices. The formation, diffusion, and dissociation of excitons as well as the manipulation of their emission properties with molecular precision are still widely unexplored and may be addressed with unprecedented resolution by STML. A promising class of systems that has attained much attraction are two-dimensional materials such as supported layers of graphene or transition metal dichalcogenides. Exciton studies may even be combined with spin-current injection in a magnetized junction as singlet and triplet states can be distinguished by their emission spectra and lifetime. An external magnetic field may be employed to inject charges spinselectively from ferromagnetic tips and substrates, allowing control of the formation of the two exciton species. Local studies of excitons are relevant in the characterization and further development of photovoltaics, organic emitters, and light sources with specific photon statistical properties, necessary for cryptography or quantum computing. Terahertz light sources can be realized by AC−Josephson junctions in the STM and may be employed for optical pumping in the nearfield. Another challenge may be the realization of the strong coupling regime between excitons and localized plasmons in the tunnel junction of an STM. A proof for its realization would be a Rabi splitting of the emission line of a quantum system. Establishing ways to realize strong coupling in STML will foster fundamental studies on electromagnetic coupling inside electrically pumped nanocavities and will enable an extended luminescence tuning by parameters controlled by the STM.

Klaus Kuhnke is senior scientist and group leader at the Nanoscience Department at the Max Planck Institute for Solid State Research in Stuttgart. He holds a degree in Physics and a Ph.D. from the University of Bonn and received a Feodor-Lynen Fellowship in 1991 for his postdoc studies at the Bell Laboratories in the U.S. His research has focused on the structure of and processes at interfaces employing a variety of devoted methods such as thermal energy atom scattering, Xray surface crystallography, X-ray circular dichroism, and time-resolved nonlinear optical spectroscopy. In the past decade, he developed and set up a low-temperature scanning tunneling microscope for luminescence studies. Currently, he leads the nanooptics research group employing this instrument to investigate electroluminescence from molecular systems. Christoph Große studied nanostructure and molecular sciences at the University of Kassel, Germany, and graduated in 2010 after a diploma thesis in the group of Prof. Rainer Waser at the Forschungszentrum Jülich, Germany. He then joined Klaus Kern’s group and received his Ph.D. in physics in 2015 from the École Polytechnique Fédérale de Lausanne, Switzerland. For his work on the STM-induced luminescence of single molecules and molecular films, he received the Wayne B. Nottingham Prize and the Otto Hahn Medal of the Max Planck Society. Currently, Christoph holds a Feodor Lynen research fellowship and works as a postdoctoral researcher in the group of Prof. Jeremy Baumberg at the NanoPhotonics Centre of the University of Cambridge. Pablo Merino is an Alexander von Humboldt postdoctoral fellow in the Nanoscience Department at the Max Planck Institute for Solid State Research in Stuttgart, Germany. He studied Physics at the Universidad Autónoma de Madrid and Université Paris 7 Denis Diderot. He received his Ph.D. in Condensed Matter Physics and Nanotechnology in 2013 for his work at Centro de Astrobiologia under the supervision of Prof. J. A. Martin-Gago. His studies on structural characterization of epitaxial graphene earned him the extraordinary prize for the best thesis in Physics of the Universidad Autónoma de Madrid. For a postdoctoral stay, he joined Prof. K. Kern’s nanooptics group where he is currently exploring photon dynamics on molecular systems and excitons at the nanoscale. Klaus Kern is Director and Scientific Member at the Max Planck Institute for Solid State Research in Stuttgart, Germany, and Professor of Physics at the École Polytechnique Fédérale de Lausanne (EPFL), Switzerland. He also is Honorary Professor at the University of Konstanz, Germany. His present research interests are in nanoscale science, self-ordering phenomena, and chemistry and physics of surfaces and interfaces. He holds a chemistry degree and Ph.D. from the University of Bonn and a honorary doctoral degree from the University of Aalborg. After his doctoral studies, he was staff scientist at the Research Center Jülich and visiting scientist at Bell Laboratories, Murray Hill, before joining the Faculty of EPFL in 1991 and the MaxPlanck-Society in 1998. Professor Kern has authored and coauthored more than 600 scientific publications. He has served frequently on advisory committees to universities, professional societies, and institutions and has received numerous scientific awards and honors, including the 2008 Gottfried-Wilhelm-Leibniz Prize and the 2016 Van’t Hoff Prize. Prof. Kern has educated a large number of leading academics in surface and nanoscience. During the past 25 years, he has mentored 100 Ph.D. students and 60 postdoctoral fellows. Today, more than 50 of his former students and postdocs hold prominent faculty positions at universities around the world.

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Klaus Kuhnke: 0000-0001-9981-1732 Christoph Große: 0000-0003-2474-0997 Present Addresses §

University of Cambridge, Cavendish Laboratory, Cambridge CB3 0HE, United Kingdom. ∥ Instituto de Ciencias de Materiales de Madrid CSIC, Madrid E-28049, Spain. Notes

The authors declare no competing financial interest. AM


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ACKNOWLEDGMENTS We would like to thank Olle Gunnarsson for constantly helpful and instructive discussions. Furthermore, we would like to thank Anna Roslawska, Makus Etzkorn, Markus Ternes, Talat Rahman, and Lyman Baker for critically reading the manuscript. P.M. would like to thank the AvH-Foundation for financial support.

TEM TERS THz-STM TIP UHV UV VB

ABBREVIATIONS

transmission electron microscope tip enhanced Raman scattering terahertz STM tip-induced plasmon ultra high vacuum ultraviolet valence band

REFERENCES

Substances and Compounds

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CNT CuPc DRPR H11T H2TBPP

carbon nanotube Cu phthalocyanine azobenzene dye triphenylene derivative meso-tetrakis (3,5-ditertiarybutylphenyl)porphyrin H2TPP/TPP tetraphenyl porphyrin HOPG highly oriented pyrolytic graphite Ir(ppy)3 fac-tris(2-phenylpyridine)iridium(III) ITO indium tin oxide MgP magnesium porphine SQ squaraine dye TMDC transition metal dichalcogenides TOPO trioctylphosphine oxide TPPS meso-tetrakis(4-sulfonanophenyl) porphyrin ZnEtio/ZnEtiol [ ( 3 , 8 , 1 3 , 1 8 - t e t r a e t h y l - 2 , 7 , 1 2 , 1 7 tetramethylporphyrin)zinc(II)] ZnPc zinc-phthalocyanines ZnTBPP Zn-meso-tetrakis(3,5-ditertiarybutylphenyl)porphyrin Others

(a)SNOM

(apertureless) scanning near-field optical microscope AC alternating current APD avalanche photo diode CB conduction band CCD charge coupled device DFT density functional theory DOS density of states EC emission center FC Franck−Condon HBT Hanbury−Brown Twiss HOMO highest occupied molecular orbital HT Herzberg−Teller IET inelastic electron tunneling IR infrared ISC intersystem crossing JT Jahn−Teller LDOS local density of states LUMO lowest unoccupied molecular orbital ML monolayer N.A numerical aperture OLED organic light-emitting diode PL photoluminescence PMT photomultiplier tube SPPX-STM shaken-pulse-pair-excited-STM SPP surface plasmon polariton STM scanning tunneling microscope/microscopy STML scanning tunneling microscopy-induced luminescence TCSPC time correlated single photon counting AN


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