Controllable Single-Molecule Light Emission by Selective Charge

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C: Physical Processes in Nanomaterials and Nanostructures

Controllable Single-Molecule Light Emission by Selective Charge Injection in Scanning Tunneling Microscopy Xiaoyan Wu, Rulin Wang, Yu Zhang, Bowen Song, and ChiYung Yam J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b02198 • Publication Date (Web): 03 Jun 2019 Downloaded from http://pubs.acs.org on June 4, 2019

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The Journal of Physical Chemistry

Controllable Single-Molecule Light Emission by Selective Charge Injection in Scanning Tunneling Microscopy Xiaoyan Wu,† Rulin Wang,‡,† Yu Zhang,¶ Bowen Song,† and ChiYung Yam∗,†,§ †Beijing Computational Science Research Center, Haidian District, Beijing 100193, China ‡College of Physics, Qingdao University, No. 308 Ningxia Road, Qingdao, 266071, China ¶Physics and Chemistry of Materials, Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States §Department of Chemistry, The University of Hong Kong, Hong Kong E-mail: [email protected] Abstract Tunneling electron-induced light emission in scanning tunneling microscopy (STM) has recently been explored as a novel light source with tuneable properties. Here, using first-principles-based atomistic simulations, we study the STM-induced luminescence of a single free-base phthalocyanine (H2 Pc) molecule and propose a way to control the emission frequency of the STM molecular junction by precisely controlling the charge injection at different molecular positions. The luminescence is found to be directly related to the spatial distribution of the molecular states and thus depends sensitively on the tip position. Via careful refinement of the chemical structures, devices with desirable frequencies at different parts of the emitting unit can be designed. Our proposal has profound implications for applications in nanoscale optoelectronics and future hybrid electronic-photonic circuits.

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Introduction Advanced fabrication technologies for the semiconductor electronics industry enable control over the charge transport at the nanoscale. As the downsizing of electronic devices continues however, the operating frequencies of nanoelectronic circuits have a bandwidth limited to the GHz range. 1,2 To overcome this limit, hybrid electronic-photonic devices have been proposed as future ultrafast devices. While signals carried by photons offer the possibilities of extending the working frequencies by several orders of magnitude, due to the diffraction limit, optical components are too large in size to compete with modern nanoelectronics. Therefore, a key component of these hybrid devices is a nanoscale transducer that allows the conversion of electrical stimuli into optical signals. The atomic-scale resolution of scanning tunneling microscopy (STM) provides the possibility of extremely localized excitations. In addition to the imaging of nanostructures at a subnanometer resolution, nanojunctions in STM can also be used to induce light emission via the inelastic scattering of tunneling electrons. In recent years, STM-induced luminescence has attracted much attention. 3,4 The interest is motivated by its potential as a viable technology for obtaining optical spectroscopic information using the subnanometer spatial resolution of STM. Light emission induced by tunneling electrons in STM has been reported in different systems, including metal surfaces, 5 semiconductor heterostructures, 6 nanoparticles, 7 quantum wells 8 and single molecules. 9–14 In the case of single molecules on a metallic surface, STM-induced molecular luminescence has been observed when the molecules are decoupled from STM substrates. In general, light is emitted when an electron-hole pair recombines in the molecules, which can be realized in STM when two molecular states enter the bias voltage window. The injection of charges into these states results in an excited state of the molecules, which may decay radiatively via photon emission. In this framework, single-molecule junctions can operate as light-emitting devices and have recently been explored as novel molecular light sources with tuneable properties. 15–17 Remarkably, luminescence is sensitive to the tip position on the molecule, a phenomenon that can be attributed to the spatial distribution of charges responsible for the photon gen2


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eration. The structural details of individual molecules can thus be revealed through optical spectroscopy upon highly localized charge injection at different molecular positions. 13 This has proven to be an invaluable tool for understanding the fundamental phenomena of single molecules and their potential applications in nanoscale devices. 15,16 Recently, STM-induced luminescence was employed to visualize the coherent coupling of intermolecular dipole-dipole interactions between porphyrin molecules. The spatial distribution of the coupling of the transition dipoles can be directly observed from spectrally resolved photon images. 18 Recently, Imada et al. applied the technique to investigate energy transfer between coupled molecular dimers, 19 and Doppagne et al. used it to obtain vibronic signals at a submolecular resolution. 20 On the other hand, this tip-position dependence of light emission can be exploited to control emission frequency at submolecular levels. Here, we present state-of-the-art atomistic simulations of the STM-induced luminescence of a single free-base phthalocyanine (H2 Pc) molecule on a gold substrate and propose a way to control the emission frequency of molecular light-emitting devices. This is achieved via selective charge injection at different molecular positions. The precise control of the tip position allows adjustable coupling to different molecular orbitals where electron-hole recombinations are involved. Based on the non-equilibrium Green’s function (NEGF) method, interactions with the electromagnetic vacuum environment are included 21 to model the STM-induced luminescence of realistic molecules. Taking into account the atomistic details, our approach also provides the possibility to simulate the spectrally resolved photon images from first-principles-based simulations. This work permits a greater understanding of fundamental phenomena and facilitates the rational design of nanoscale optoelectronic devices.

Methods Quantum Transport and Current-induced Luminescence. The Hamiltonian for the

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single-molecule STM junction is given by

H = Hm + Hmt + Hms + Hep

(1)

where Hm describes the Hamiltonian of an extended system composed of the molecular junction with part of the tip and substrate. Hmt and Hms stand for the coupling of the extended system with the tip and substrate, respectively. Both the tip and substrate are assumed to be in thermal equilibrium with a density matrix that can be described by the Fermi-Dirac distribution. Hep represents the electron-plasmon interaction Hamiltonian, which describes the optical transition processes in the system,

Hep =

X e hµ|Ap · p|νid†µ dν m µν

(2)

where d and d† are the annihilation and creation operators of the electron, respectively. 22 Ap is the time-dependent vector potential for the plasmon mode, p is the electron momentum operator, e is the elementary charge, and m is the electron mass. The interaction effects of the last three terms in equation (1) can be incorporated through self-energies. 21,23,24 The lesser self-energy represents the injection of charge carriers from the tip/substrate, which is given by

Σ< t/s (E) = −if (E)Γt/s (E) Σ> t/s (E) = −i[1 − f (E)]Γt/s (E) Γt/s (E) = −i[Σrt/s (E) − Σat/s (E)]

(3)

where Γt/s are the coupling between the system and the tip and substrate, respectively, which are related to the electron escape rates from the molecule to the tip and substrate. f gives the Fermi-Dirac distribution. The lesser and greater self-energies due to electron-plasmon

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interactions can be expressed as

p Σ (E ∓ h ¯ ωp ) + (N + 1)G (E ± h ¯ ωp )] M p ep (E) = M [N G

(4)

where M p is the electron-plasmon coupling matrix for a plasmon mode with frequency ωp , and N is the corresponding plasmon mode occupation. G (E) are the lesser and greater electron Green’s functions. equation (4) is a non-linear equation since Green’s function depends on the self-energies,

a G (E) = Gr (E) Σ (E) + Σ (E) + Σ t s ep (E) G (E) Gr (E) = [Ga (E)]∗ = Gr0 (E) + Gr0 (E)Σrep (E)Gr (E) Gr0 (E) =

1 E − Hm − Σrt (E) − Σrs (E)

(5)

where Gr (E) and Ga (E) are the retarded and advanced Green’s functions, respectively. Gr0 (E) is the retarded Green’s function of the system without electron-plasmon interactions. In this work, the electron Green’s function is calculated at the lowest order with respect to the electron-plasmon coupling. Previous works showed that the approximation is valid in the weak coupling regime. 25–27 In addition, it has been proved that the lowest order expansion guarantees particle conservation and therefore satisfies current continuity. 28 Within the lowest order approximation, the retarded Green’s function is given by

Gr (E) ≈ Gr0 (E) + Gr0 (E)Σrep (E)Gr0 (E)

(6)

The plasmon is assumbed to decay efficiently into far-field photons and the system is therefore coupled to an empty plasmon mode. The interaction self-energy can be simplified as

p Σ G0 (E ± h ¯ ωp ) M p ep (E) = M

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(7)


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Nanocavity Plasmons in STM. The vector potential for the nanocavity plasmon used in this work is given by 29,30

Ap (r, t) = (

1 h ¯ ) 2 ap Up (r)(be−iωp t + b† eiωp t ) 20 ωp Vp

(8)

where 0 is the vacuum permittivity. ωp is the plasmon frequency. ap defines the polarization direction of the plasmon mode. Vp is the effective volume of the nanogap and Up (r) gives the spatial distribution of the plasmon which depends on the tip position. b and b† are the annihilation and creation operators of plasmon mode. In principle, the characteristics of the STM nanogap plasmon can be readily obtained numerically by either classical or quantum mechanical simulations. 31–33 Here, for simplicity, an analytical expression is taken to describe the spatial distribution of the vector potential. 34,35 2 +(r −r 2 z z,D ) ]

Up (r) = e−α[(ry −ry,D )

e−β|rx −rx,D |

(9)

In this work, we chose the parameters based on a model tunnel junction. 34 r D is the position where the maximum field enhancement is located and is set as 3 ˚ A below the tip apex. The parameters specifying the plasmon mode are summarized in Table 1. Table 1: Parameters used for the nanogap plasmon. parameter value ωp 1.3 eV Γp 0.5 eV α 0.31 ˚ A−2 0.15 ˚ A−1 β 0.22 ˚ A−1

explanation plasmon frequency decay rate spatial extent in y-z plane for rx < rx,D for rx > rx,D

Substituting equation(8) into equation (2), we obtain the electron-plasmon coupling matrix,

Mp =

1 e h ¯ ( ) 2 hµ|ap Up (r) · p|νi m 20 ωp Vp

(10)

Previous experiments showed that photon density of states in STM junctions can be enhanced 6


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due to cavity effect, and the enhancement factor is found to be on the order of 104 , 36–39 compared to that in free space. Thus, we define the spectra density of plasmon mode as

Jp (ω) = 104 ∗ Vp ∗

ωp2 Γp 1 (2π)3 c3 2π (ω − ωp )2 + ( Γ2p )2

(11)

where Γp is the decay rate of the plasmon mode. In this work, the plasmon is coupled to free photons and is broadened by a Lorentzian function with width of 0.5 eV to account for the plasmon decay. 40–42 Taking into account of the spectra density in equation (11), the frequency dependent self-energy for electron-plasmon interaction is modified as

p < Σ ¯ ω)M p ep (E, ω) = Jp (ω)M G0 (E ± h

(12)

Finally, the luminescence intensity can be calculated by substituting the interaction selfenergy into the Meir-Wingreen formula 21 2 F (ω) = h ¯

Z

dE > T r Σ< ep (E, ω)G0 (E) 2π

(13)

Photon Image.To construct spatially resolved photon images, we systematically positioned the tip over the H2 Pc molecule and calculated the emission spectrum for each configuration using equation (13). The emission intensity for each peak is then integrated to obtain each pixel in Figures 4d and 4e. Since the molecule has a D2h symmetry, only one-quarter of the image is calculated. Overall, Figures 4d and 4e contain totally 19 × 19 simulation data. To increase the resolution, the figures are interpolated linearly to 37 × 37 pixels.

Results Computational Approach. We model the experimental setup of STM-induced luminescence as shown in Figure 1a. For simplicity, the STM tip is modelled as a linear gold chain,

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while the bulk gold substrate is represented by four atomic layers with a (100) surface. An H2 Pc molecule is placed in the nanogap between the tip apex and the substrate. It is known that intrinsic emissions from molecules are quenched by electronic interactions with metal substrates. The luminescence of single molecules, however, has been detected by introducing a thin insulating layer to electronically decouple the systems from the conducting substrates. 9,10 To examine the intrinsic molecular emission, in our model, the H2 Pc molecule is separated from the substrate by 5 ˚ A to avoid strong molecule-substrate interactions, while the distance is short enough to allow electrons to tunnel through the junction and sustain a steady-state current. The simulations of electron transport and luminescence are based on solving the NEGF equations for the nanojunction in the presence of electron-photon interactions, which has been applied to study various nanoscale optoelectronic devices. 21,23,24,43–45 The rate of carrier exchange between the molecule and contacts, and hence the steady-state current, are determined by self-energies Σt and Σs for the tip and substrate, respectively. Electron-photon interactions are included as perturbations via self-energy Σep . To demonstrate our method, the electronic structure of the model is described at the density functional tight-binding (DFTB) level 46,47 using the augorg-1-1 parameter set, which was developed specifically for describing organic molecules in contact with gold systems. 48 It has recently shown that Coulomb interactions are important for accurate description of conductance and optical transitions in molecular junctions. 49,50 In this work, due to computational cost, single-particle Hamiltonian is employed, however, the current approach can be combined with more sophisticated electronic structure methods to take into account electron-electron interactions. 51,52 The local density of states (LDOS) can be obtained by projecting Green’s P function on real space as D(r, E) = − π1 ij Im[Grij (E)]φ∗i (r)φj (r). Here, the imaginary part of the retarded Green’s function Gr describes the electronic structure of the nanogap, and φ(r) represents the atomic basis function. Electronic Properties of the Single-Molecule STM Junction. We first examine the electronic structure of the H2 Pc molecule within the nanogap of the STM configura-

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Figure 1: (a) Atomistic model for the simulations. A single H2 Pc molecule is placed at the nanogap between the tip and substrate of the STM. Inset: Atomic structure of H2 Pc molecule. Red crosses illustrate three different tip positions for acquiring luminescence spectra. PDOS of the (b) gold substrate, (c) H2 Pc molecule and (d) tip. tion. Figure 1b-d shows respectively the projected density of states (DOS) of the substrate, molecule and tip under zero bias voltage condition. It is observed that both the tip and substrate have a continuum of energy states. The DOS of the tip in Figure 1d is consistent with that of an atomic gold wire calculated using density functional theory. Compared to atomic gold tip employed in other studies, 53 the atomic chain adopted in this study has similar electronic structure where the energy band below Fermi level consists mainly d-states. On the other hand, the H2 Pc molecule shows discrete levels. As the molecule is well separated from the substrate, the molecular states only weakly hybridize with the orbitals of gold atoms at the surface. At equilibrium, it is shown that the HOMO of H2 Pc is approximately 0.35 eV below the Fermi level. Due to the central hydrogen atoms, molecular symmetry is reduced from D4h to D2h , and the two-fold degeneracy of the LOMOs of phthalocyanine is split. Consequently, the LUMO and LUMO+1 are 0.91 eV and 1.00 eV above the Fermi level, respectively. 9



STM Luminescence of H2 Pc Molecule. The STM junction is then driven into nonequilibrium by external voltage and the electroluminescence spectrum is calculated. In STM experiments, a nanocavity is formed between the tip and the substrate, which supports localized plasmons. To simulate photon emission from STM junctions, the interaction between tunneling electrons and the plasmonic field is considered. Due to its strongly confined nature, the spatial dependence of the plasmonic field is taken into account explicitly. 35,44 This allows us to describe the inelastic tunneling and the associated radiative recombinations of charge carriers in the nanojunction. The tunneling-electron-induced light emission of the STM molecular junction is shown in Figure 2a. The emission spectrum is calculated with the tip positioned on top of one of the isoindole units, and the applied voltage is 2.0 V. A sharp emission peak centered at 1.35 eV is observed. For comparison, the emission spectrum of a bare STM junction is also presented in Figure 2a and clearly, a broad featureless emission is observed. Therefore, the sharp peak at 1.35 eV can be attributed to molecular luminescence. The emission frequency is consistent with the energy of the molecular transition between HOMO and LUMO+1 whereas the emission corresponds to HOMO–LUMO transition is missing. The mechanism of the luminescence can be explained by the process shown in Figure 2b. Here, electrons from the tip tunnel through the molecule inelastically by losing energy to a plasmon in the junction. The plasmon is then assumed to decay efficiently into far-field photons. Compared to the vacuum environment, the rate of spontaneous emission is significantly enhanced when the molecule is coupled to the plasmon due to the increased local density of photonic states. In principle, luminescence can occur when excited carriers are injected into the frontier orbitals of the molecule. To understand why emission corresponding to HOMO–LUMO transition is not observed in the simulations, we varied the tip position and investigated the emission properties of the junction. Figure 3a shows the emission spectra calculated under a bias voltage of 2.0 V for three different tip positions as indicated in Figure 1. Interestingly, the spectra are completely different for the three tip positions. The results show that two different optical transitions occurs when the tip is po-

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Figure 2: (a) Emission spectra of the STM molecular junction (black line) and a bare STM junction (red line). (b) Schematic diagram showing the processes of inelastic tunneling of hot carriers and the luminescence. sitioned at different isoindole unit while no emission is observed when the tip is located over the H2 Pc center. It is understood that charge injection from the tip to the molecule directly depends on the spatial distribution of the molecular orbitals. Thus, the luminescence should be sensitive to the tip position. Examining the frontier molecular orbitals of an isolated H2 Pc obtained with DFTB (see Figure S1), it can be seen that the HOMO delocalizes along both molecular axes, while LUMO and LUMO+1 each extends along one of the molecular axes. These molecular states act as channels for charge carriers to tunnel through the junction, which can be seen in the transmission function (see Figure S2). Hence, when the tip is located at P1 , hot electrons are injected from the tip into LUMO+1 and hot holes are injected from substrate to HOMO. The emission peak at 1.35 eV can therefore be attributed to the HOMO–LUMO+1 transition of the molecule. Whereas at P2 , hot carriers are instead injected into LUMO and the emission peak at 1.26 eV is assigned to the HOMO–LUMO transition. On the other hand, no emission is observed when the tip is positioned at the center of H2 Pc, due to the absence of electronic state. The optical transition mechanism is illustrated in the LDOS plotted in Figure 3b. When the bias voltage is increased to 2.0 V, both LUMO and LUMO+1 enter the external bias window. In addition, HOMO is shifted upwards towards the Fermi level due to the change in electrostatic potential across the junction. As a consequence, holes are injected from the substrate into HOMO while electrons 11



are injected from the tip into the unoccupied molecular states. The latter depends strongly on the tip position and the spatial distribution of the corresponding electronic states. Radiative recombination of these charge carriers then gives rise to light emission. It is shown that light emission of the junction is also affected by the molecular states even when there is no molecular luminescence. As illustrated in Fig.S4, at a low bias voltage of 1.2 V, only LUMO is within the applied bias window and transitions between the molecular state and electronic states in substrate contribute to the light emission of the junction.

Figure 3: (a) Emission spectra calculated at three tip positions over the H2 Pc molecule, as indicated in Figure 1a. (b) LDOS of the system along the tunneling current direction. The red dashed lines mark the Fermi level for the tip and substrate. The black dashed lines separate the system into tip, molecule and substrate regions. All properties are calculated under a 2.0 V bias voltage and constant tip height. The subnanometer resolution of STM allows us to construct photon images based on the emission intensities generated by excitation at different molecular positions. To further

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illustrate the above mechanism, we calculate the transition densities and compare with the photo images of the involved optical transitions. Figure 4a and b shows respectively the corresponding transition densities of HOMO–LUMO and HOMO–LUMO+1 transitions. For the photon images, we systematically calculated the emission spectra by moving the STM tip over the molecule. The emission intensity is then integrated for each tip position to construct the photon image. Figure 4c presents the photon image which shows bright features over the four isoindole units and a dark center. The result agrees well with the experimental observations. 13,18 Each pixel of the photon image represents the excitation probability for the radiative process when the tip is placed at that point. The optical transitions can be further revealed spatially through the spectrally resolved photon image. Energy-resolved photon images are constructed by integrating separately for each emission frequency. Figure 4d and e show the STM-induced luminescence images for the emission peaks at 1.26 eV and 1.35 eV, respectively. Each energy has an emission intensity localized along one of the molecular axes. The emission pattern for the lower energy peak closely resembles the transition density associated with the HOMO–LUMO transition in Figure 4a, and the pattern for the higherenergy peak resembles that of the HOMO–LUMO+1 transition in Figure 4b. It has been reported that the molecular luminescence could be suppressed by strong interactions with the metallic substrate. 9,10 This quenching effect is illustrated in Figure S3 which plots the electroluminescence spectrum when the molecule is placed 3.36 ˚ A (the sum of the van de Waals radii) above the substrate. It can be seen that the emission features associated with molecular transitions are missing, and instead, a broad emission which resemble the spectrum of the bare junction is resulted. The corresponding PDOS of the substrate, molecule and tip is plotted in Figure S3b-d. The simulations reveal that the molecular orbitals strongly hybridize with the electronic states of the substrate. This results in charge and energy transfer between these states and molecular luminescence is therefore quenched. Controllable Light Emission in STM Junctions. The position-dependent lumines-

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Figure 4: Transition densities for the (a) HOMO–LUMO and (b) HOMO–LUMO+1 transitions of the H2 Pc molecule calculated with DFTB. (c) Photon image obtained by integrating the whole emission spectrum. Spectrally resolved photon images obtained by integrating emission intensities for the (d) HOMO–LUMO and (e) HOMO–LUMO+1 transitions. All photon images are calculated under a 2.0 V bias voltage. cence of STM junctions can be exploited in optoelectronics as controllable single-molecule emitters. Due to the D2h symmetry of H2 Pc molecules, the LUMOs lying along the two orthogonal molecular axes are split. This gives rise to two possible optical transitions, which can be excited locally by the tunneling electrons. As shown in Figure 3a, the frequency of emitted photons can be controlled by changing the tip position. When the tip is positioned at P2 , radiative recombination between charge carriers in HOMO and LUMO occurs. On the other hand, when the tip is positioned at P1 , charge carriers in HOMO and LUMO+1 recombine. Via the selective injection of charge carriers into the corresponding electronic state, excitation of the radiative process with different frequencies can be chosen. It is therefore possible to design single-molecule light-emitting devices with controllable wavelengths. By careful refinement of the chemical structures, devices with desirable frequencies at different 14


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parts of the emitting unit can be designed. The proposal is therefore versatile for applications in nanoscale optoelectronics and has potential for use in future hybrid electronic-photonic circuits.

Discussion We have presented atomistic simulations of the STM-induced luminescence of single-molecule junctions based on our recently developed NEGF quantum transport formalism. Previous efforts have been made to successfully explain the spectral features using simple models. 54–56 In these theoretical works, many-body effects like electron correlations, electron-phonon interactions can be readily included. Recently, it has been demonstrated that Fano resonance appears in emission spectrum due to the interaction between molecular excitations and plasmons. In the current approach, the many-body effects can be taken into account via interaction self-energies, which have to be solved self-consistently. 57–60 In addition, the effects of molecular excitations on plasmons can also be included by this self-consistent calculation. Due to the highly localized tunneling electrons originating from the STM tip, the spatial distributions of the molecular states play an important role in the transport and optical properties of nanojunctions. The simulations in this work explicitly take into account the nanoscale nature of the experiments, which is crucial for obtaining the spatial variations of the molecular electronic states and their transition probabilities for the radiative process. Due to the highly localized tunneling electrons originating from the STM tip, the spatial distributions of the molecular states play an important role in the transport and optical properties of nanojunctions. These simulations also allow us to generate photon images, and the results agree well with those of the experiments. This work paves the way for understanding the complex interactions between molecular excitations and plasmons in nanojunctions and utilizing them in diverse applications in nanoscale optoelectronics. STM-induced luminescence is proposed as a new route towards nanoscale tuneable light

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sources. The emission frequency of an STM molecular junction can be controlled by precise positioning of the tip. This allows adjustable coupling and charge injection to selected molecular electronic states, where radiative decay occurs consequently. It is therefore possible to realize controllable light emission by the proper design of emitting units at the molecular level. 16,53,61 Recently, the atomically precise design of graphene nanoribbon edges has been realized experimentally. This allows the rational engineering of the topological electronic structures for potential applications in spintronics and quantum computing. 62–64 Specifically designed graphene nanoribbons achieved via edge functionalization can therefore provide a platform for realizing the proposed nanoscale light-emitting device. Our results demonstrate the possibility of controlling light emission from single-molecule junctions. We believe this finding may contribute to the fundamental understanding and lead to the realization of applications in nanoscale optoelectronics and high-resolution spectroscopy.

Supporting Information Description Molecular orbitals of H2 Pc molecule; Transmission spectra at at different bias voltage and the spatial distribution of transmission within the molecular plane; Emission spectra of the STM junction when the molecular orbitals strongly hybridize with the electronic states of the substrate. Formalism for calculating the tunneling currents.

Acknowledgement The financial support from the National Natural Science Foundation of China (21322306, 21673017 and U1530401), Science Challenge Project (TZ2018004) and the National Basic Research Program of China (2014CB921402) is gratefully acknowledged. The computational support from the Special Program for Applied Research on Super Computation of the NSFCGuangdong Joint Fund (the third phase) (U1501501) and the Beijing Computational Science Research Center (CSRC) is also acknowledged. Y.Z. acknowledges the support from the Los 16


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Alamos National Laboratory (LANL) Directed Research and Development Funds (LDRD, LANL is operated by Triad National Security, LLC, for the National Nuclear Security Administration of the U.S. Department of Energy (Contract No. 89233218NCA000001)

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(a) Atomistic model for the simulations. A single H2Pc molecule is placed at the nanogap between the tip and substrate of the STM. Inset: Atomic structure of H2Pc molecule. Red crosses illustrate three different tip positions for acquiring luminescence spectra. PDOS of the (b) gold substrate, (c) H2Pc molecule and (d) tip. 170x89mm (300 x 300 DPI)



(a) Emission spectra of the STM molecular junction (black line) and a bare STM junction (red line). (b) Schematic diagram showing the processes of inelastic tunneling of hot carriers and the luminescence. 80x37mm (300 x 300 DPI)


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(a) Emission spectra calculated at three tip positions over the H2Pc molecule, as indicated in Figure 1a. (b) LDOS of the system along the tunneling current direction. The red dashed lines mark the Fermi level for the tip and substrate. The black dashed lines separate the system into tip, molecule and substrate regions. All properties are calculated under a 2.0 V bias voltage and constant tip height. 82x112mm (300 x 300 DPI)



Transition densities for the (a) HOMO-LUMO and (b) HOMO-LUMO+1 transitions of the H2Pc molecule calculated with DFTB. (c) Photon image obtained by integrating the whole emission spectrum. Spectrally resolved photon images obtained by integrating emission intensities for the (d) HOMO-LUMO and (e) HOMOLUMO+1 transitions. All photon images are calculated under a 2.0 V bias voltage. 82x103mm (300 x 300 DPI)


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82x21mm (300 x 300 DPI)


Controllable Single-Molecule Light Emission by Selective Charge

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