Visualization of Nano-Plasmonic Coupling to Molecular Orbital in Light

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Visualization of Nano-Plasmonic Coupling to Molecular Orbital in Light Emission Induced by Tunneling Electrons Arthur Yu, Shaowei Li, Hui Wang, Siyu Chen, Ruqian Wu, and Wilson Ho Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b00613 • Publication Date (Web): 16 Apr 2018 Downloaded from http://pubs.acs.org on April 16, 2018

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Visualization of Nano-Plasmonic Coupling to Molecular Orbital in Light Emission Induced by Tunneling Electrons Arthur Yu†,⊥ Shaowei Li†,⊥ Hui Wang†, Siyu Chen†, Ruqian Wu† and W. Ho*, †, ‡ †

Department of Physics and Astronomy, University of California, Irvine, California 92697-4575, USA ‡

Department of Chemistry, University of California, Irvine, California 92697-2025, USA

ABSTRACT: The coupling between localized plasmon and molecular orbital in the light emission from a metallic nanocavity has been directly detected and imaged with sub-0.1 nm resolution. The light emission intensity was enhanced when the energy difference between the tunneling electrons and the lowest unoccupied molecular orbital (LUMO) of an azulene molecule matches the energy of a plasmon mode of the nanocavity defined by the Ag-tip and Ag (110) substrate of a scanning tunneling microscope (STM). The spatially resolved image of the light emission intensity matches the spatial distribution of the LUMO obtained by scanning tunneling spectroscopy (STS) and density functional theory (DFT) calculations. Our results highlight the near-field coupling of a molecular orbital to the radiative decay of a plasmonic excitation in a confined nanoscale junction. KEYWORDS: scanning tunneling microscope, tunneling-electron-induced

light

emission,

plasmons, molecular orbital

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The demonstration of the optical detection of single molecules via fluorescence or absorption has brought many new details that were not accessible in ensemble measurements.1 The application of far-field2 and near-field3 optical microscopies further enhanced the spatial resolution of the optical signal from individual molecules. In near-field spectroscopy, the lateral resolution of the optical imaging can overcome the Rayleigh criterion through the local field enhancement in the tip-sample gap, but still limited by the spatial extent of surface plasmon that is in the range of a few nanometers. The ultimate spatial resolution of the molecular fluorescence comes from the tunneling electron induced light emission process using the scanning tunneling microscope (STM).4-12 The STM tip can act as a localized electron source, allowing for the coupling of tunneling electrons to molecular orbitals, surface plasmons, and inducing light emission. In general, the spatial resolution of STM induced light emission may also be restricted by the radius of the enhanced field since the emitting light often comes from the decay of surface plasmons10 However, in the case of tunneling electron induced light emission from a molecule adsorbed on a metal surfaces, the surface plasmons are excited by the electrons that inelastically tunnel into the molecular orbital.13,14 Therefore, the plasmon-molecule coupling can be probed with sub-Ångström spatial resolution as in the inelastic electron tunneling process. Because light emission from plasmon excitation and molecular fluorescence follow different selection rules, it becomes possible to probe the effects of molecular orbitals on the light emission process. Previous experiments have demonstrated STM’s ability to induce photon emission from metallic nanoclusters15-16 or molecules7-9,13-19 adsorbed on a decoupling layer grown on a metallic substrate. These studies have shown that tunneling electron induced molecular fluorescence is mediated by surface plasmons. Here we provide direct visualization of the plasmon mediated


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molecular florescence of azulene on Ag (110) through STM induced light emission spectroscopy and microscopy. We explore the physical insight into the electronic and optical properties of the electron-plasmon-photon coupling process derived from spatially resolved light emission images of single molecules. The lowest unoccupied molecular orbital (LUMO) of azulene is imaged, based on scanning tunneling spectroscopy (STS) and compared with density functional theory (DFT) calculations.20 Spatially resolved photon images of single azulene molecule reveal a similar pattern as the LUMO, indicating direct coupling of plasmon mode to molecular orbital. The LUMO of azulene is generally optically inaccessible because of its prominent violation of Kasha’s rule.21 This plasmon-orbital coupling enables an alternative experimental approach to study the dependence of the optical properties on the molecular orbital. The experiment was conducted with a home built STM with a background pressure of 3×10-11 Torr.22 The Ag (110) surface was cleaned through repeated cycles of Ne+ sputtering and annealing to 700 K. The tip was electro-chemically etched from high purity Ag wires. Bias was applied to the sample with the tip at virtual ground. Light emission from the tunnel junction was collected with a lens inside the vacuum chamber and guided to a spectrograph equipped with a liquid nitrogen cooled charged couple device (CCD).7-9,16-17 Azulene molecules were dosed in situ onto the surface at 9 K through a variable leak valve, using its vapor pressure at room temperature of 1×10-4 Torr. Carbon monoxide (CO) molecules were also dosed and co-adsorbed on the surface to allow for more detailed structural imaging with a CO-terminated tip. Upon sublimation onto the surface, azulene molecules appear as pear shaped protrusions in constant current topography (Figure 1a-b). Images show there are two equivalent adsorption geometries on the Ag (110) surface, as shown in Figure 1a, both with the axis of the molecule perpendicular to the

direction. To determine the adsorption geometry more precisely,


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topographic images were taken with a CO-terminated tip, showing the atomic structure of the Ag lattice (Figure 1c) as well as the more detailed structure of azulene. At the set point of 30 mV, 2.8 nA, images taken with the CO-tip can resolve the five- and seven-member carbon rings (Figure 1d). This adsorption geometry was confirmed by DFT calculations for azulene on Ag(110). The calculated binding energy of the ground state geometry is 1.33 eV per molecule, about 0.2 eV higher binding than the second-best geometry shown in Figure S1.20 The optimized geometry in Figure 1f shows azulene molecule straddles two Ag rows, with the center of the seven-member carbon ring over the short bridge site. The electronic structure of azulene was obtained from dI/dV spectra recorded over various points of the molecule, as displayed in Figure 2 for adsorption with the five-member carbon ring at the top. A broad peak at 0.6 V is observed in the dI/dV spectrum, indicating the presence of a localized electronic state (Figure 2a). The intensity of the electronic peak in Figure 2a exhibits spatial variations for four points over the molecule as indicated in the inset. The dI/dV peak is more pronounced at the top and sides of the molecule than at the center and bottom. In principle, the intensity of the dI/dV gives the local density of unoccupied electronic states at the corresponding energy. The dI/dV intensity taken over an azulene molecule is higher than Ag(110) when the bias is below 0.7 V, indicating a higher local density of states. With the bias above 0.8 V, the measured density of states is lower over the azulene molecule than over the Ag. The spatial dependence of the electronic state is visualized with dI/dV imaging at different biases (Figure 2c-i). Each image is composed of the dI/dV signals at different pixels obtained with the lock-in technique, and represents an intensity map at the chosen voltage. The dI/dV images taken below 0.6 V show a three-lobe feature that is localized near the five-member ring of azulene. Another lobe is revealed at 0.7 V and localized at the bottom edge of the seven-member ring


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(Figure 2e). All four lobes can be resolved in the images taken between 0.9 V and 1.5 V but the dI/dV intensities over azulene are lower than the Ag(110) background (Figure 2f-i). As displayed in Figure 2b, DFT calculations indicate that the LUMO of azulene is located at 0.5 eV above the Fermi level and weakly hybridizes with the s states of Ag atoms at the surface. The calculated LUMO position and the sizeable broadening (0.4 eV to 1.0 eV) are in good agreement with experiment. The three-dimensional lobes of LUMO as well as the HOMO-LUMO gap of azulene on Ag(110) remain very close to those of isolated azulene molecule (see Figure 2b and Figure S2 in Supplemental Material). The dI/dV images at 0.6 V show features that are contained in the calculated two-dimensional map of electron wave function of LUMO (Figure 2g). In most cases where a molecule is adsorbed on a metal substrate, the molecular orbitals delocalize from intermixing with substrate electronic states, causing them to lose their spatially confined features.23 The fact that the LUMO can be imaged by tunneling spectroscopy indicates a relatively weak hybridization between the azulene and substrate. This is understandable since the density of states of Ag is very low near the Fermi level, as shown in Figure 2b. The electronic state of azulene can strongly couple with the plasmon modes of the STM nano-junction. This coupling can be probed by tunneling electron induced light emission. The light emission spectra taken over the metal substrate and the center of an azulene molecule are shown in Figure 3a, with the tunneling gap set at VB = 2.5 V, IT = 1.0 nA. On the Ag (110) surface, the light emission spectrum features a broad peak centering around 2.2 eV with several lower energy peaks, and the onset of emission is at 1.6 eV. The emission spectrum is attributed to the radiative decay of plasmon modes of the tip-substrate nanocavity.24 A similar spectrum is recorded over the azulene molecule. However, the emission from the plasmon mode around 2.2 eV is enhanced, while the mode below 1.7 eV is suppressed. This enhancement effect is


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observed for all azulene molecules we have examined. To understand the origin of such light emission enhancement, we have studied the bias dependence of the light emission. On both the Ag(110) surface and azulene, the onset of light emission is detected at 1.6 V sample bias. As the bias increases, the enhancement of photon emission over the azulene is observed between 2.1 to 2.4 V (Figure 3b). We attribute the photon intensity increase over azulene to the interaction between azulene and surface plasmon on the Ag(110) substrate. In tunneling electron induced light emission on metal surfaces, the emission mechanism involves the inelastic electron tunneling (IET) process. The tunneling electron gives up energy equal to a surface plasmon resonance (SPR) mode of the substrate, and photon emission occurs as a result of the decay of the plasmon. In the presence of a molecule, the molecular orbital can serve as the final state for the inelastic tunneling process, as depicted in Figure 4a. The light emission intensity and the final state

is proportional to the transition probability

between the initial state

: (eq. 1)19,25

Where

denotes the electron-plasmon coupling operator, and

denotes the local density of

the final state. In our measurement for azulene, the lack of spectroscopic features corresponding to the radiative transitions between molecular orbitals implies that the IET process described in the eq. 1 is the dominant emission mechanism. The LUMO of azulene locates at around 0.6 eV but extends in energy from 0.4 eV to 1.0 eV due to the hybridization with Ag, as seen from the width of the dI/dV peak (Figure 2a), the images (Figure 2c-i) and DFT results (Figure 2b). The bias onset of the SPR starts at around 1.6 V from the light emission spectrum (Figure 3b). When the tip is placed over the azulene with 2.5 V bias, the higher local density of states between 0.2 eV to 0.7 eV associated with the molecular orbital enhances the amplitude of the SPR excitation 6 ACS Paragon Plus Environment

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with energy between 2.3 eV and 1.8 eV and its light emission. The emission below 1.7 eV is suppressed because the density of state above 0.8 eV is lower over the azulene than over the Ag substrate. The higher excitation probability for the plasmon modes between 2.0 eV and 2.4 eV is consistent with the observed threshold at 2.1 V bias for the enhancement of photon emission intensity (Figure 3b). To visualize the electron–plasmon–photon interactions in azulene, photon emission imaging was performed at various bias voltages (Figure 4b–4d). For each image, the total photon intensity at each pixel was recorded. The total light emission intensity

is proportional to the

integral of eq.1 over the photon frequency : (eq. 2) The images give a visual illustration of the interactions. At a set point of VB = 2.0 V, IT = 1.0 nA, the photon image shows almost no spatial variation (Figure 4b), in accordance with the bias dependence curve in Figure 3b. The enhancement effect from azulene does not occur until VB increases to above this value. As we raise the bias to 2.25 V, three faint lobes can be observed in Figure 4c, which become more prominent at VB = 2.5 V (Figure 4d). Comparing the photon emission with dI/dV images (Figure 2c-i), we see the resemblance for the two different absorption types. The similarity is a direct consequence of the interaction between SPR, LUMO of azulene, and the emitted photon. At VB = 2.0 V, the energy difference between the bias voltage and azulene LUMO does not match the SPR, hence no emission enhancement is observed. When enhancement occurs at higher biases, the spatial dependence of the photon emission image resembles the spatial dependence of the electronic state, separated in energy by a plasmon (Figure 4c vs. Figure 2c, and Figure 4d vs. Figure 2e). The photon images demonstrate the prominent role that the azulene LUMO plays in the electron–plasmon–photon interaction. 7 ACS Paragon Plus Environment

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In summary, we have studied the electronic and optical properties of single azulene molecules adsorbed on the Ag(110) surface. The LUMO state of azulene was visualized through dI/dV imaging and DFT wave function, and shown to strongly couple with the tunneling electron induced plasmon excitation processes. The tunneling electron induced light emission spectrum from azulene exhibits selective enhancement of the plasmon mode with an energy matching the difference between the injected electron and the azulene LUMO. Spatially resolved images of light emission contain features that resemble the azulene LUMO, confirming its role as the final state in the plasmon mediated inelastic tunneling process. Our results showcase the unique ability of the STM to resolve and investigate electronic and optical interactions at the single molecule level.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Phone: (949) 824-5234 Author Contributions (A.Y. and S.L.) These authors contributed equally to the work.

⊥

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work is supported by the National Science Foundation Center for Chemical Innovation on Chemistry at the Space-Time Limit (CaSTL) under Grant No. CHE-1414466. In addition, we benefited from valuable discussions with Gregory Czap.


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(17) Yu, A.; Li, S.; Dhital, B.; Lu, H. P.; Ho, W. J. Phys. Chem. C 2016, 120, 21099-21103. (18) Schneider, N. L.; Matino, F.; Schull, G.; Gabutti, S.; Mayor, M.; Berndt, R. Phys. Rev. B 2011, 84, 153403. (19) Geng, F.; Zhang, Y.; Yu, Y.; Kuang, Y.; Liao, Y.; Dong, Z.; Hou, J. Opt. Express 2012, 20, 26725-26735. (20) See supplementary materials for details of DFT calculations. (21) Xin, H.; Gao, X. ChemPlusChem 2017, 82, 945–956. (22) Stipe, B. C.; Rezaei, M. A.; Ho, W. Rev. Sci. Instrum. 1999, 70, 137-143. (23) Qiu, X. H.; Nazin, G. V.; Ho, W. Phys. Rev. Letts. 2004, 92, 206102. (24) Dong, Z. C.; Zhang, X. L.; Gao, H. Y.; Luo, Y.; Zhang, C.; Chen, L.G.; Zhang, R.; Tao, X.; Zhang, Y.; Yang, J. L.; Hou, J. G. Nature Photonics 2010, 4, 50-54. (25) Kuhnke, K.; Große, C.; Merino, P; Kern, K. Chem. Rev. 2017, 117, 5174-5222.


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Figure Captions: Figure 1. STM constant current topography of azulene on Ag (110). (a) Image showing two possible types of adsorbed azulene. (b) Zoom in view of one type. Tunnel junction was set at VB = 1 V, IT = 0.1 nA in (a) and VB = 1 V, IT = 1 nA in (b). A co-adsorbed CO molecule is seen in upper right corner of (a). (c-d) Constant current images with CO-terminated tip. The Ag lattice is resolved in (c). Scaled molecular skeleton of azulene is generated by ChemDraw with calculated bond lengths [20] and overlaid onto CO-tip image in (d). Tunnel junction was set at VB = 30 mV, IT = 3 nA in (c) and VB = 30 mV, IT = 2.8 nA in (d). (e) Schematic diagram of optical experiment. Light emission arises from decay of surface plasmon resonance (SPR) excited by tunneling electrons. (f) The ground state adsorption geometry of azulene on Ag(110) surface calculated by DFT. White, dark grey and light blue balls represent hydrogen, carbon and silver atoms, respectively. Figure 2. Scanning tunneling spectroscopy (STS) of azulene electronic states. (a) dI/dV spectra measured over different points of the azulene molecule, as indicated in the topographic image of the azulene shown in the inset. A bump in dI/dV is observed from 0.3 V to 1.0 V, attributed to LUMO state of azulene. All spectra were taken with tunnel junction set at VB = 1.0 V, IT = 1.0 nA, lock-in modulation at 345 Hz with and 10 mV rms. The dashed lines and arrows denote the regions where azulene density of state is higher (left) or lower (right) than Ag(110). (b) Projected density of states of azulene on Ag (110). The Fermi level is set at 0. Inset demonstrates the three dimensional (3D) wave function corresponding to LUMO of azulene on Ag (110). ( (c-i) dI/dV images at different voltages. For each image, tunnel junction was set at VB = 1.0 V, IT = 1.0 nA at each pixel, then with feedback disabled, bias was ramped to the selected voltage, and dI/dV signal was monitored with lock-in amplifier. The images give spatial distribution of the dI/dV


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signal at the selected voltages. (j) Calculated two dimensional (2D) wave function feature of the LUMO of azulene in a plane of ~2 Å above the molecular plane. Figure 3. Tunneling electron induced light emission from azulene. (a) Light emission spectrum taken over Ag (110) (blue curve) and center of azulene (red curve). Tunnel junction was set at VB = 2.5 V and IT = 1.0 nA, and the nitrogen cooled CCD was exposed for 1000 s in both spectra. The dashed lines and arrows denote the plasmon modes with density of final state over azulene lower (left) or higher (right) compared to over Ag(110). (b) Bias dependent photon emission spectrum from Ag (110) (blue curve) and azulene (red curve). To generate each curve, light emission was recorded as a function of the bias voltage and at the zeroth order of the monochromator, representing the total emission intensity for that voltage. Figure 4. Photon emission imaging with the STM. (a) Schematic diagram of the light emission mechanism. Electrons tunnel inelastically by exciting a surface plasmon resonance (SPR). A molecular orbital, such as LUMO of azulene, can serve as the final state in the inelastic tunneling process. (b-d) Photon emission image at IT = 1.0 nA but different bias voltages as noted. Each image is generated by recording the zeroth order light emission spectrum at each pixel, then integrating to obtain total photon intensity at that pixel. CCD was exposed for 3 s at each pixel. Images in (c) and (d) show resemblance to dI/dV images of the LUMO of azulene.


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Figure 1


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Figure 2


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Figure 4


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HI a)

LO

HI

LO

c)

e)

Ag e-

Azulene 1.5 nm b)

Ag (110)

d)

f)

[001]

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4A

[110]

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hv SPR

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dI/dV (nA/V)

a)

LO

3

g)

c)

1

0.4 V

Azulene > Ag Azulene < Ag

b)

HI

2

0

DOS (states/eV)

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0.0

0.3

4

0.6

0.9

Bias (V)

1.2

1.0 V

d)

h)

0.6 V

1.2 V

e)

i)

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1.5 V

f)

j)

1.5

Azulene Ag(110)

3

5A

HOMO-1 HOMO

2 1 0

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Agd -3

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-2

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a)

Ag(110) Azulene

8

Azulene < 6 Ag

4 2

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0 1.6

In t en s i t y (ar b . u n i t s )

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b)

5

1.8

2.0

2.2

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2.6

Ag(110) Azulene

4 3 2 1 0 1.4

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Vacuum

a)

Fermi Level

eV

c)

Tip

HI b) Initial State

SPR

Final State Fermi Level

Azulene

2.25V

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LO

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Visualization of Nano-Plasmonic Coupling to Molecular Orbital in Light

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