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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 24 — Nov. 19, 2012
  • pp: 26725–26735
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Modulation of nanocavity plasmonic emission by local molecular states of C60 on Au(111)

Feng Geng, Yang Zhang, Yunjie Yu, Yanmin Kuang, Yuan Liao, Zhenchao Dong, and Jianguo Hou  »View Author Affiliations


Optics Express, Vol. 20, Issue 24, pp. 26725-26735 (2012)
http://dx.doi.org/10.1364/OE.20.026725


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Abstract

We investigate the modulation of C60 monolayers on the nanocavity plasmonic (NCP) emission on Au(111) by tunneling electron excitation from a scanning tunneling microscope (STM) tip. STM induced luminescence spectra show not only suppressed emission, but also significant redshift of NCP emission bands on the C60 molecules relative to the bare metal surface. The redshift, together with the bias- and coverage-dependent emission feature, indicates that the C60 molecules act beyond a pure dielectric spacer, their electronic states are heavily involved in the inelastic tunneling process for plasmonic emission. A modified quantum cutoff relation is proposed to explain qualitatively the observed emission feature at both bias polarities. We also demonstrate molecularly resolved optical contrast on the C60 monolayer and discuss the contrast mechanism briefly.

© 2012 OSA

1. Introduction

Control of the interaction between molecules and plasmonic nanostructures is important for the development of molecular plasmonics and optoelectronics [1

1. J.-J. Greffet, “Applied physics. Nanoantennas for light emission,” Science 308(5728), 1561–1563 (2005). [CrossRef] [PubMed]

]. While photon-excited methods have been widely used to study the quenching and enhancement of molecular fluorescence by plasmonic nanostructures [2

2. S. Kühn, U. Håkanson, L. Rogobete, and V. Sandoghdar, “Enhancement of single-molecule fluorescence using a gold nanoparticle as an optical nanoantenna,” Phys. Rev. Lett. 97(1), 017402 (2006). [CrossRef] [PubMed]

,3

3. P. Anger, P. Bharadwaj, and L. Novotny, “Enhancement and quenching of single-molecule fluorescence,” Phys. Rev. Lett. 96(11), 113002 (2006). [CrossRef] [PubMed]

], these techniques are not effective in exploring local plasmonic emission at the nanoscale. On the other hand, a scanning tunneling microscope (STM), when combined with photon detectors, is powerful in providing local photon emission information because of the highly localized excitation of tunneling electrons, e.g., from nanoscale plasmonic light [4

4. R. Berndt, J. K. Gimzewski, and P. Johansson, “Inelastic tunneling excitation of tip-induced plasmon modes on noble-metal surfaces,” Phys. Rev. Lett. 67(27), 3796–3799 (1991). [CrossRef] [PubMed]

,5

5. G. Schull, M. Becker, and R. Berndt, “Imaging confined electrons with plasmonic light,” Phys. Rev. Lett. 101(13), 136801 (2008). [CrossRef] [PubMed]

] to molecule-specific fluorescence [6

6. X. H. Qiu, G. V. Nazin, and W. Ho, “Vibrationally resolved fluorescence excited with submolecular precision,” Science 299(5606), 542–546 (2003). [CrossRef] [PubMed]

13

13. A. Kabakchiev, K. Kuhnke, T. Lutz, and K. Kern, “Electroluminescence from individual pentacene nanocrystals,” ChemPhysChem 11(16), 3412–3416 (2010). [CrossRef] [PubMed]

]. Such advantages make the STM induced luminescence (STML) an attractive technique in exploring the nature of nanoscale light emission and energy transfer between molecules and plasmons at the nanoscale.

C60 is a molecule that has attracted great attention in the STM community because of its high symmetry, simplicity, and promising applications in nanoelectronics and optoelectronics, and was also one of the few molecules studied in the early days of STML research [14

14. J. G. Hou, Y. Jinlong, W. Haiqian, L. Qunxiang, Z. Changgan, Y. Lanfeng, W. Bing, D. M. Chen, and Z. Qingshi, “Topology of two-dimensional C60 domains,” Nature 409(6818), 304–305 (2001). [CrossRef] [PubMed]

17

17. K. Sakamoto, K. Meguro, R. Arafune, M. Satoh, Y. Uehara, and S. Ushioda, “Light emission spectra of the monolayer-island of C60 molecules on Au(111) induced by scanning tunneling microscope,” Surf. Sci. 502-503, 149–155 (2002). [CrossRef]

]. In 1993, Berndt et al. presented the first photon map of C60 monolayers on Au(110) with molecular resolution [16

16. R. Berndt, R. Gaisch, J. K. Gimzewski, B. Reihl, R. R. Schlittler, W. D. Schneider, and M. Tschudy, “Photon emission at molecular resolution induced by a scanning tunneling microscope,” Science 262(5138), 1425–1427 (1993). [CrossRef] [PubMed]

]. Later, Sakamoto et al. measured the STM induced light emission spectra of C60 monolayer islands on Au(111) and claimed to have detected the fluorescence from the C60 molecules in the monolayer island [17

17. K. Sakamoto, K. Meguro, R. Arafune, M. Satoh, Y. Uehara, and S. Ushioda, “Light emission spectra of the monolayer-island of C60 molecules on Au(111) induced by scanning tunneling microscope,” Surf. Sci. 502-503, 149–155 (2002). [CrossRef]

]. Nevertheless, subsequent reports suggest that, to generate intrinsic molecular luminescence from C60 molecules, a dielectric spacer layer such as NaCl is indispensable so that the excited molecules can be electronically decoupled from the underlying metal substrate [8

8. E. Cavar, M. C. Blüm, M. Pivetta, F. Patthey, M. Chergui, and W. D. Schneider, “Fluorescence and phosphorescence from individual molecules excited by local electron tunneling,” Phys. Rev. Lett. 95(19), 196102 (2005). [CrossRef] [PubMed]

,11

11. F. Rossel, M. Pivetta, F. Patthey, and W. D. Schneider, “Plasmon enhanced luminescence from fullerene molecules excited by local electron tunneling,” Opt. Express 17(4), 2714–2721 (2009). [CrossRef] [PubMed]

,15

15. F. Rossel, M. Pivetta, and W. D. Schneider, “Luminescence experiments on supported molecules with the scanning tunneling microscope,” Surf. Sci. Rep. 65(5), 129–144 (2010). [CrossRef]

]; Otherwise, molecular fluorescence would be quenched due to strong interactions between the directly adsorbed molecules and metal substrates [18

18. G. Hoffmann, L. Libioulle, and R. Berndt, “Tunneling-induced luminescence from adsorbed organic molecules with submolecular lateral resolution,” Phys. Rev. B 65(21), 212107 (2002). [CrossRef]

21

21. N. L. Schneider, F. Matino, G. Schull, S. Gabutti, M. Mayor, and R. Berndt, “Light emission from a double-decker molecule on a metal surface,” Phys. Rev. B 84(15), 153403 (2011). [CrossRef]

]. Consequently, while the plasmonic nature of emission remains to be clarified experimentally, questions also remain as regards the role of C60 molecules in the nanocavity plasmonic (NCP) emission: Do the molecules merely act as a geometrical spacer? How exactly is the NCP emission modulated by the molecules?

In this work, we investigate the modulation of NCP emission by the local electronic states of adsorbed C60 molecules on Au(111) through the evolution of emission features along with the increase of coverage from sub-monolayer to two monolayers (MLs). By combining bias- and coverage-dependent STML spectra with differential conductance data, the emission is found to be plasmonic in nature, but the C60 molecules function beyond the role of a pure geometrical spacer, their electronic states are involved in the NCP emission process excited by the inelastic tunneling (IET) electrons. For a given bias voltage, the plasmonic emission band on top of the molecular overlayer is found not only suppressed in emission intensities, but also redshifted in both peak energy and cutoff energy in comparison with the emission band on the bare Au surface. Such spectral changes are attributed to the change of hybridization between the molecules and metal substrates at different coverage. As a result, these hybridized unoccupied molecular states change their energy levels and thus modify the IET channels for plasmonic emission. The modulation mechanism of the NCP emission by molecules is qualitatively discussed within the framework of quantum cutoff relations for both bias polarities. In addition, we also present photon mapping results to support the suppression effect of plasmonic emission on the C60 monolayers. We demonstrate molecularly resolved optical contrast on the monolayer and discuss the contrast mechanism briefly.

2. Experiment

The experiments were performed with a low-temperature ultrahigh-vacuum (UHV) STM (Unisoku) at a base pressure of ~8 × 10−11 mbar at ~80 K, operated in the constant-current-topographic mode with the sample biased. The Au(111) substrate was prepared by thermal evaporation of gold (~200 nm thick) on freshly cleaved mica and cleaned in UHV by cycles of Ar+ sputtering and annealing. C60 molecules were thermally evaporated onto Au(111) kept at room temperature. Electrochemically etched silver (Ag) tips were used to perform STM imaging and STML measurements, taking advantage of Ag in producing strong plasmonic field [22

22. C. Zhang, B. Gao, L. G. Chen, Q. S. Meng, H. Yang, R. Zhang, X. Tao, H. Y. Gao, Y. Liao, and Z. C. Dong, “Fabrication of silver tips for scanning tunneling microscope induced luminescence,” Rev. Sci. Instrum. 82(8), 083101 (2011). [CrossRef] [PubMed]

].

Photons emitted from the tunnel junction were collected by a lens inside the UHV chamber and then refocused into an optical fiber by another lens placed outside UHV. The collected light was guided into either a liquid N2-cooled charge-coupled-device (CCD) spectrometer (Princeton Instruments) for steady-state spectral measurements or a multichannel plate photomultiplier tube (PMT, Hamamatsu) for isochromatic spectral measurements using a bandpass filter (680−720 nm). Photon maps were simultaneously acquired during STM scanning through a fiber-free coupling to a single photon avalanched photodiode (SPAD, PerkinElmer). Both the STML spectra and photon intensity presented here were raw data, without calibration for the collection efficiency and sensitivity of the photon detection systems. Except for inverted photon maps, the bright feature corresponds to high emission intensity for all other photon maps presented in this work.

3. Results and discussion

The differences in local electronic structures for the 1st-ML and 2nd-ML C60 molecules are reflected in their light emission spectra induced by tunneling electrons, specifically regarding the suppression of emission and the shift of emission bands. As shown in Fig. 1(d), the STML spectra on top of the C60 molecules show suppressed emission in comparison with that on the bare Au surface, and the suppression effect on the 2nd-ML is stronger than on the 1st-ML. The peak maximum is redshifted ~14 nm for the 1st-ML, but as large as ~40 nm for the 2nd-ML C60 molecules relative to the bare Au. The nature of emission for the case of the 1st-ML C60 is clearly plasmonic, given the spectral similarity to the bare metal surface. While for the 2nd-ML C60, despite the occurrence of the emission band around 1.7 eV that somehow coincides with the photoluminescence data of C60 [8

8. E. Cavar, M. C. Blüm, M. Pivetta, F. Patthey, M. Chergui, and W. D. Schneider, “Fluorescence and phosphorescence from individual molecules excited by local electron tunneling,” Phys. Rev. Lett. 95(19), 196102 (2005). [CrossRef] [PubMed]

,17

17. K. Sakamoto, K. Meguro, R. Arafune, M. Satoh, Y. Uehara, and S. Ushioda, “Light emission spectra of the monolayer-island of C60 molecules on Au(111) induced by scanning tunneling microscope,” Surf. Sci. 502-503, 149–155 (2002). [CrossRef]

], the emission nature is still believed to be plasmonic, judging from the broad feature of the emission band and particularly the shift of emission band with bias voltages that will be discussed in detail later in the paper. For molecule-specific emission, spectral peaks are relatively sharp and more importantly, the peak positions remain essentially constant for different bias due to the association with a common HOMO−LUMO gap [7

7. Z. C. Dong, X. L. Guo, A. S. Trifonov, P. S. Dorozhkin, K. Miki, K. Kimura, S. Yokoyama, and S. Mashiko, “Vibrationally resolved fluorescence from organic molecules near metal surfaces in a scanning tunneling microscope,” Phys. Rev. Lett. 92(8), 086801 (2004). [CrossRef] [PubMed]

,8

8. E. Cavar, M. C. Blüm, M. Pivetta, F. Patthey, M. Chergui, and W. D. Schneider, “Fluorescence and phosphorescence from individual molecules excited by local electron tunneling,” Phys. Rev. Lett. 95(19), 196102 (2005). [CrossRef] [PubMed]

,12

12. Z. C. Dong, X. L. Zhang, H. Y. Gao, Y. Luo, C. Zhang, L. G. Chen, R. Zhang, X. Tao, Y. Zhang, J. L. Yang, and J. G. Hou, “Generation of molecular hot electroluminescence by resonant nanocavity plasmons,” Nat. Photonics 4(1), 50–54 (2010). [CrossRef]

]. The absence of molecule-specific emission from C60 molecules in the present case can be attributed to the strong interaction of the adsorbed molecules with the metal substrates and resultant quenching effects [18

18. G. Hoffmann, L. Libioulle, and R. Berndt, “Tunneling-induced luminescence from adsorbed organic molecules with submolecular lateral resolution,” Phys. Rev. B 65(21), 212107 (2002). [CrossRef]

21

21. N. L. Schneider, F. Matino, G. Schull, S. Gabutti, M. Mayor, and R. Berndt, “Light emission from a double-decker molecule on a metal surface,” Phys. Rev. B 84(15), 153403 (2011). [CrossRef]

].

In order to get a panoramic view of the suppressed emission upon the adsorption of C60 molecules on Au(111), we also carried out photon mapping experiments for sub-ML coverage and above 1 ML, respectively. As shown in Figs. 2(a)
Fig. 2 Photon maps of C60 on Au(111) (2.8 V, 200 pA, 15 × 15 nm2, 2 ms/pixel) acquired at a coverage of ~0.6 ML (a) and ~1.3 ML (b), respectively. (c) and (d) are high-resolution images for STM topograph and simultaneously acquired photon map on a 1st-ML C60 island using a different tip (~0.3 ML, 2.8 V, 800 pA, 5 × 5 nm2, 2 ms/pixel). The representative sites for the on-top and inter-molecular positions are marked as “+” and “×”, respectively. (e) Height profile of the line trace indicated in (c). (f) Photon count profile for the same line trace in (d). The shot-noise of the signals (raw) follows the Poissonian counting statistics. (g) Inverted photon map processed from (d), showing very similar image contrast to the STM topograph in (c). (h) Current profile for the same line trace shown in the inset of the simultaneously recorded current image. (i) and (j) are site-specific differential conductance spectra and STML spectra (2.8 V, 800 pA, 10 s) for the representative sites marked in (c), respectively on the top of molecules (“+”) and in-between molecules (“×”). The emission spectra have been subtracted by the background contribution to highlight the net intensity variation and are normalized to the highest emission intensity in the spectra to show the relative intensity ratio.
and 2(b), the NCP emission intensity on the 2nd-ML C60 is weaker than that on the 1st-ML C60, the latter is in turn weaker than that on the bare Au. These observations are in agreement with the spectral data shown in Fig. 1(d). Furthermore, when imaging over a 1st-ML C60 island with a different Ag tip that is sharper and stronger in emission, STM topographs and photon maps with clear molecular resolution were obtained, as shown in Figs. 2(c) and 2(d). Such high-resolution imaging enables us to correlate the topographic feature in Fig. 2(c) with the photon intensity distribution in Fig. 2(d). The C60 molecules appear dark (i.e., weakly emissive) in the photon map while the gap areas between neighboring C60 molecules appear bright (i.e., stronger in emission), which is in contrast to the situation of C60 on Au(110) [16

16. R. Berndt, R. Gaisch, J. K. Gimzewski, B. Reihl, R. R. Schlittler, W. D. Schneider, and M. Tschudy, “Photon emission at molecular resolution induced by a scanning tunneling microscope,” Science 262(5138), 1425–1427 (1993). [CrossRef] [PubMed]

]. The correlation between the topographic feature and emission intensity is clearly illustrated in the height profile [Fig. 2(e)] and photon count profile [Fig. 2(f)] for a given line trace, which follows a qualitatively inverse relation. Figure 2(g) is an inverted photon map processed from Fig. 2(d) with the “bright” feature standing for instead the weak emission associated with the C60 molecules. The contrast of this inverted image is very similar to that of the STM topograph in Fig. 2(c), offering a clear demonstration of the inverse height-count relation. The response of the STM feedback loop was set to be sufficiently fast in these photon mapping experiments to avoid the complication of photon map features by current fluctuations. Figure 2(h) plots the current profile for the same line trace shown in the inset of a current image that was simultaneously acquired with (c) and (d). The current undulations, from 0.79 to 0.81 nA, are less than 3% of the setpoint current (0.8 nA), which is much smaller than the photon count undulations of about 100% ranging from 3 to 9 kHz with respect to an averaged value of 6 kHz. Consequently, the optical contrast observed is irrelevant to the current fluctuations during scanning.

The intensity of plasmonic emission in the STM tunnel junction is known to depend on two major factors: one is the local electronic states that determines the IET probability for plasmonic excitation; the other is the junction geometry that determines the electromagnetic coupling and field enhancement [28

28. P. Johansson, R. Monreal, and P. Apell, “Theory for light emission from a scanning tunneling microscope,” Phys. Rev. B Condens. Matter 42(14), 9210–9213 (1990). [CrossRef] [PubMed]

,29

29. J. Aizpurua, S. P. Apell, and R. Berndt, “Role of tip shape in light emission from the scanning tunneling microscope,” Phys. Rev. B 62(3), 2065–2073 (2000). [CrossRef]

]. As shown in the site-specific tunneling spectra of Fig. 2(i), the differential conductance spectrum in the inter-molecular area resembles that on the top of C60 molecules, suggesting the involvement of not only essentially the same molecular states in the tunneling process, but also similar local density of states (LDOS) for the IET excitation of plasmons. Such nearly uniform feature of the local electronic states on the C60 monolayer tends to rule out the possibility of emission undulations by the variations of local electronic states. (More precisely, the LDOS around the LUMO level at the inter-molecular positions is slightly smaller than that on the top of a C60 molecule, which would give smaller IET rates for the inter-molecular positions from the electronic point of view, opposite to the experimental observations.) On the other hand, as shown in Fig. 2(j), although the STML spectra at the on-top and inter-molecular positions are similar in the spectral shape and peak energy, the emission intensity at the on-top position is considerably reduced in comparison with that measured between molecules. Such intensity reduction is qualitatively consistent with the photon map results shown above in Fig. 2(d). (Nevertheless, the STML spectra in (j) were presumably an averaged result over a very small area due to the thermal drift over an exposure time of 10 s, which probably accounts for a reduced intensity ratio of ~1.4, not as large as that observed in the photon map. Note that the emission band is peaked around 1.9 eV for this new Ag tip presumably due to the change of tip shape and resultant change of the NCP modes. However, it should be mentioned that the basic feature of optical contrast remains similar to that given in Fig. 2(d) for all the tips we used.) Having excluded the electronic factor for the emission intensity undulations on the C60 monolayer, the geometric factor based on the undulations of the tip height becomes the predominant factor to produce the observed optical contrast, as strongly supported by the inverse relation between the tip height and photon count addressed above. The insertion of a C60 molecule with a diameter as large as ~0.7 nm into the STM junction would greatly increase the gap distance between the tip and metal substrate, leading to considerably weakened electric field strength and thus suppressed plasmonic emission [28

28. P. Johansson, R. Monreal, and P. Apell, “Theory for light emission from a scanning tunneling microscope,” Phys. Rev. B Condens. Matter 42(14), 9210–9213 (1990). [CrossRef] [PubMed]

]. The larger the gap distance is, the smaller the local field enhancement and thus the weaker the plasmonic emission intensity. This is the primary reason for the observed optical contrast on the C60 monolayer and is also the reason why the light emission on the C60 bilayer is further suppressed. It should be noted that the dielectric property of the tunnel junction can also affect the NCP emission [28

28. P. Johansson, R. Monreal, and P. Apell, “Theory for light emission from a scanning tunneling microscope,” Phys. Rev. B Condens. Matter 42(14), 9210–9213 (1990). [CrossRef] [PubMed]

,29

29. J. Aizpurua, S. P. Apell, and R. Berndt, “Role of tip shape in light emission from the scanning tunneling microscope,” Phys. Rev. B 62(3), 2065–2073 (2000). [CrossRef]

]. The dielectric screening of the C60 molecular layer could reduce the emission energy and intensity [30

30. X. Tao, Z. C. Dong, J. L. Yang, Y. Luo, J. G. Hou, and J. Aizpurua, “Influence of a dielectric layer on photon emission induced by a scanning tunneling microscope,” J. Chem. Phys. 130(8), 084706 (2009). [CrossRef] [PubMed]

]. However, the similar spectral profile with the same peak energy in Fig. 2(j) tends to suggest that the distribution of the dielectric constant is nearly uniform over the molecular layer for both the inter-molecular and on-top positions, which could result in a similar level of emission reduction but cannot account for the large emission undulations on the monolayer shown in Fig. 2(f). Consequently, the molecular resolution in the photon map is believed to be a combined result of the sensitive gap-distance dependent field strength with highly localized excitation by tunneling electrons.

Both the redshift of NCP emission bands and the increase of “turn-on” voltages upon the adsorption of C60 molecules described above are believed to associate with the different electronic states shown in Fig. 1(c). For the 2nd-ML C60 molecules, the change of the onset voltage or the extra energy required to generate similar NCP modes, corresponds nicely to the energy of the LUMO state at ~1.2 V. In other words, the energy of NCP emission in this case follows approximately a modified quantum cutoff relation: hv≤eV-EL, in which EL is the energy position of the C60 LUMO with respect to the Fermi level of the Au substrate. For instance, to generate photon emission with energy up to 1.8 eV, a minimum voltage of 3.0 V would be required, and that is practically what we observed in the experiment [Fig. 3(c) and Fig. 4]. Such correlation also suggests that the LUMO states of C60 serve as a final state for inelastic tunneling that leads to plasmonic emission. For the 1st-ML C60 molecules on Au, since the strong interaction between the molecules and metal substrate makes the LUMO state very broad and extend almost to the Fermi level, the EL value becomes less well-defined. Even in this case, the emission behavior shown in Fig. 3(b) and Fig. 4 still follows qualitatively the above modified quantum cutoff relation, but with a smaller extra-energy for plasmonic excitation and less well-defined onset feature.

In order to reveal more clearly the role of molecular states in the NCP emission process at negative bias, the spectra at high bias (e.g., −4.1, −4.3, and −4.5 V) in Fig. 5(a) were divided by the nearly featureless spectrum at −3.9 V, respectively. Figure 5(b) shows such mathematically processed data, treating the spectrum at −3.9 V as a reference background (the horizontal dash line) so as to highlight the spectral changes of the NCP emission associated with the molecular states and particularly the energy cutoff positions [6

6. X. H. Qiu, G. V. Nazin, and W. Ho, “Vibrationally resolved fluorescence excited with submolecular precision,” Science 299(5606), 542–546 (2003). [CrossRef] [PubMed]

,21

21. N. L. Schneider, F. Matino, G. Schull, S. Gabutti, M. Mayor, and R. Berndt, “Light emission from a double-decker molecule on a metal surface,” Phys. Rev. B 84(15), 153403 (2011). [CrossRef]

]. A small bump is shaping up at the excitation bias of −4.1 V, with a high-energy cutoff at ~1.8 eV. Such correlation implies the open-up of an additional IET channel around −2.3 V for the plasmonic excitation, which coincides roughly with the energy of the HOMO states (EH≈−2.3 V) shown in Fig. 1(c). Similarly, for excitation voltages of −4.3 V and −4.5 V, the energy cutoff positions of the NCP emission move to ~2.0 eV and ~2.2 eV, respectively. (On the 1st-ML C60, the HOMO state is too broad to yield clear quantitative relation.) Such evolution of spectral shapes along with the change of negative bias can be understood by attributing the emission to the inelastic tunneling of electrons from the C60 HOMO state to the STM-tip state, as detailed below.

According to the above analyses for the bias-dependent cutoff energies and onset voltages of the NCP emission on the 2nd-ML C60 molecules, we can picture the basic scenario of C60-modulated plasmonic emission as follows (Fig. 6
Fig. 6 Schematic mechanism of NCP emission on C60 molecules showing the IET excitation channels at (a) positive bias and (b) negative bias. Channel 1 refers to the IET process from the tip to C60 LUMO in (a) or from the HOMO state to the tip in (b), while channel 2 refers to the IET between the tip and Au substrate directly.
). There are mainly two IET channels that may contribute to the NCP emission. Channel 1 is the IET channel between the C60 molecular states and tip (or substrate) states, while Channel 2 is the IET excitation between the tip and metal substrate directly. (Note that the elastic tunneling channel, which is dominant in the tunneling process, is not plotted out in Fig. 6.) Channel 2 is believed negligible due to the exponential dependency of tunneling currents on the gap distance upon the insertion of C60 molecules and resultant increase of the tip−substrate distance [21

21. N. L. Schneider, F. Matino, G. Schull, S. Gabutti, M. Mayor, and R. Berndt, “Light emission from a double-decker molecule on a metal surface,” Phys. Rev. B 84(15), 153403 (2011). [CrossRef]

,30

30. X. Tao, Z. C. Dong, J. L. Yang, Y. Luo, J. G. Hou, and J. Aizpurua, “Influence of a dielectric layer on photon emission induced by a scanning tunneling microscope,” J. Chem. Phys. 130(8), 084706 (2009). [CrossRef] [PubMed]

]. Channel 1 is therefore considered to be dominant in the generation of molecule-modulated NCP emission since electron tunneling has to go through molecules. Specifically, the emission process in the positive bias is attributed to the IET tunneling of electrons into the LUMO states from the tip [Fig. 6(a)], while in the negative bias, to the IET tunneling of electrons from the HOMO states to the tip state [Fig. 6(b)]. Both IET processes follow approximately a modified quantum cutoff relation, hv≤|eV-EM|, in which EM refers to the molecular HOMO or LUMO states.

4. Conclusion

We have investigated the modulation of C60 molecules on the NCP emission on the Au(111) surface. Through combined analyses of bias-dependent STML spectra, photon maps, and differential conductance data, we not only justify the plasmonic nature of emission in this system, but also point out that the observed emission suppression is mainly a result of reduced local field strength due to the increase of gap distance. Moreover, the height undulations on the C60 monolayer are found a dominant factor to yield the molecularly resolved optical contrast in the photon map, with the C60 molecules appearing as dark features. More importantly, the NCP emission is found not only redshifted significantly at a given voltage, but also strongly coverage dependent in terms of the onset voltage and energy cutoff. These observations indicate that the C60 molecules act beyond a pure dielectric spacer, their electronic states are actively involved in the NCP emission process. Furthermore, the energy of the NCP emission follows qualitatively a modified quantum cutoff relation, hv≤|eV-EM|, with the molecular state serving as an initial or final state in the IET process. These findings provide richer understanding on the role of molecules in the tunneling process: not only acting as a dominant elastic tunneling channel, but also offering an inelastic tunneling channel for plasmonic emission. The molecular fluorescence is quenched in the present system due to the strong interaction between the molecules and substrate. The generation of molecule-specific emission would need further electronic decoupling of molecular emitters from the metal substrate.

Acknowledgments

This work was supported in part by NBRP (Grant No. 2006CB922003 and 2011CB921402) and NSFC (Grant No. 91021004, 10574117 and 10974186). The authors thank anonymous referees for their constructive suggestions.

References and links

1.

J.-J. Greffet, “Applied physics. Nanoantennas for light emission,” Science 308(5728), 1561–1563 (2005). [CrossRef] [PubMed]

2.

S. Kühn, U. Håkanson, L. Rogobete, and V. Sandoghdar, “Enhancement of single-molecule fluorescence using a gold nanoparticle as an optical nanoantenna,” Phys. Rev. Lett. 97(1), 017402 (2006). [CrossRef] [PubMed]

3.

P. Anger, P. Bharadwaj, and L. Novotny, “Enhancement and quenching of single-molecule fluorescence,” Phys. Rev. Lett. 96(11), 113002 (2006). [CrossRef] [PubMed]

4.

R. Berndt, J. K. Gimzewski, and P. Johansson, “Inelastic tunneling excitation of tip-induced plasmon modes on noble-metal surfaces,” Phys. Rev. Lett. 67(27), 3796–3799 (1991). [CrossRef] [PubMed]

5.

G. Schull, M. Becker, and R. Berndt, “Imaging confined electrons with plasmonic light,” Phys. Rev. Lett. 101(13), 136801 (2008). [CrossRef] [PubMed]

6.

X. H. Qiu, G. V. Nazin, and W. Ho, “Vibrationally resolved fluorescence excited with submolecular precision,” Science 299(5606), 542–546 (2003). [CrossRef] [PubMed]

7.

Z. C. Dong, X. L. Guo, A. S. Trifonov, P. S. Dorozhkin, K. Miki, K. Kimura, S. Yokoyama, and S. Mashiko, “Vibrationally resolved fluorescence from organic molecules near metal surfaces in a scanning tunneling microscope,” Phys. Rev. Lett. 92(8), 086801 (2004). [CrossRef] [PubMed]

8.

E. Cavar, M. C. Blüm, M. Pivetta, F. Patthey, M. Chergui, and W. D. Schneider, “Fluorescence and phosphorescence from individual molecules excited by local electron tunneling,” Phys. Rev. Lett. 95(19), 196102 (2005). [CrossRef] [PubMed]

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11.

F. Rossel, M. Pivetta, F. Patthey, and W. D. Schneider, “Plasmon enhanced luminescence from fullerene molecules excited by local electron tunneling,” Opt. Express 17(4), 2714–2721 (2009). [CrossRef] [PubMed]

12.

Z. C. Dong, X. L. Zhang, H. Y. Gao, Y. Luo, C. Zhang, L. G. Chen, R. Zhang, X. Tao, Y. Zhang, J. L. Yang, and J. G. Hou, “Generation of molecular hot electroluminescence by resonant nanocavity plasmons,” Nat. Photonics 4(1), 50–54 (2010). [CrossRef]

13.

A. Kabakchiev, K. Kuhnke, T. Lutz, and K. Kern, “Electroluminescence from individual pentacene nanocrystals,” ChemPhysChem 11(16), 3412–3416 (2010). [CrossRef] [PubMed]

14.

J. G. Hou, Y. Jinlong, W. Haiqian, L. Qunxiang, Z. Changgan, Y. Lanfeng, W. Bing, D. M. Chen, and Z. Qingshi, “Topology of two-dimensional C60 domains,” Nature 409(6818), 304–305 (2001). [CrossRef] [PubMed]

15.

F. Rossel, M. Pivetta, and W. D. Schneider, “Luminescence experiments on supported molecules with the scanning tunneling microscope,” Surf. Sci. Rep. 65(5), 129–144 (2010). [CrossRef]

16.

R. Berndt, R. Gaisch, J. K. Gimzewski, B. Reihl, R. R. Schlittler, W. D. Schneider, and M. Tschudy, “Photon emission at molecular resolution induced by a scanning tunneling microscope,” Science 262(5138), 1425–1427 (1993). [CrossRef] [PubMed]

17.

K. Sakamoto, K. Meguro, R. Arafune, M. Satoh, Y. Uehara, and S. Ushioda, “Light emission spectra of the monolayer-island of C60 molecules on Au(111) induced by scanning tunneling microscope,” Surf. Sci. 502-503, 149–155 (2002). [CrossRef]

18.

G. Hoffmann, L. Libioulle, and R. Berndt, “Tunneling-induced luminescence from adsorbed organic molecules with submolecular lateral resolution,” Phys. Rev. B 65(21), 212107 (2002). [CrossRef]

19.

Y. Zhang, X. Tao, H. Y. Gao, Z. C. Dong, J. G. Hou, and T. Okamoto, “Modulation of local plasmon mediated emission through molecular manipulation,” Phys. Rev. B 79(7), 075406 (2009). [CrossRef]

20.

Y. Zhang, F. Geng, H. Y. Gao, Y. Liao, Z. C. Dong, and J. G. Hou, “Enhancement and suppression effect of molecules on nanocavity plasmon emissions excited by tunneling electrons,” Appl. Phys. Lett. 97(24), 243101 (2010). [CrossRef]

21.

N. L. Schneider, F. Matino, G. Schull, S. Gabutti, M. Mayor, and R. Berndt, “Light emission from a double-decker molecule on a metal surface,” Phys. Rev. B 84(15), 153403 (2011). [CrossRef]

22.

C. Zhang, B. Gao, L. G. Chen, Q. S. Meng, H. Yang, R. Zhang, X. Tao, H. Y. Gao, Y. Liao, and Z. C. Dong, “Fabrication of silver tips for scanning tunneling microscope induced luminescence,” Rev. Sci. Instrum. 82(8), 083101 (2011). [CrossRef] [PubMed]

23.

S. R. Forrest, “Ultrathin organic films grown by organic molecular beam deposition and related techniques,” Chem. Rev. 97(6), 1793–1896 (1997). [CrossRef] [PubMed]

24.

Z. C. Dong, D. Fujita, and H. Nejoh, “Adsorption and tunneling of atomic scale lines of indium and lead on Si(100),” Phys. Rev. B 63(11), 115402 (2001). [CrossRef]

25.

X. Lu, M. Grobis, K. H. Khoo, S. G. Louie, and M. F. Crommie, “Charge transfer and screening in individual C60 molecules on metal substrates: A scanning tunneling spectroscopy and theoretical study,” Phys. Rev. B 70(11), 115418 (2004). [CrossRef]

26.

J. Kliewer, R. Berndt, E. V. Chulkov, V. M. Silkin, P. M. Echenique, and S. Crampin, “Dimensionality Effects in the Lifetime of Surface States,” Science 288(5470), 1399–1402 (2000). [CrossRef] [PubMed]

27.

M. Grobis, A. Wachowiak, R. Yamachika, and M. F. Crommie, “Tuning negative differential resistance in a molecular film,” Appl. Phys. Lett. 86(20), 204102 (2005). [CrossRef]

28.

P. Johansson, R. Monreal, and P. Apell, “Theory for light emission from a scanning tunneling microscope,” Phys. Rev. B Condens. Matter 42(14), 9210–9213 (1990). [CrossRef] [PubMed]

29.

J. Aizpurua, S. P. Apell, and R. Berndt, “Role of tip shape in light emission from the scanning tunneling microscope,” Phys. Rev. B 62(3), 2065–2073 (2000). [CrossRef]

30.

X. Tao, Z. C. Dong, J. L. Yang, Y. Luo, J. G. Hou, and J. Aizpurua, “Influence of a dielectric layer on photon emission induced by a scanning tunneling microscope,” J. Chem. Phys. 130(8), 084706 (2009). [CrossRef] [PubMed]

31.

R. Berndt and J. K. Gimzewski, “Isochromat spectroscopy of photons emitted from metal-surfaces in an STM,” Ann. Phys. 505(2), 133–140 (1993). [CrossRef]

32.

G. V. Nazin, X. H. Qiu, and W. Ho, “Atomic engineering of photon emission with a scanning tunneling microscope,” Phys. Rev. Lett. 90(21), 216110 (2003). [CrossRef] [PubMed]

OCIS Codes
(180.5810) Microscopy : Scanning microscopy
(240.6680) Optics at surfaces : Surface plasmons
(240.7040) Optics at surfaces : Tunneling
(260.2160) Physical optics : Energy transfer
(300.6280) Spectroscopy : Spectroscopy, fluorescence and luminescence
(250.5403) Optoelectronics : Plasmonics

ToC Category:
Microscopy

History
Original Manuscript: October 8, 2012
Revised Manuscript: November 5, 2012
Manuscript Accepted: November 5, 2012
Published: November 12, 2012

Citation
Feng Geng, Yang Zhang, Yunjie Yu, Yanmin Kuang, Yuan Liao, Zhenchao Dong, and Jianguo Hou, "Modulation of nanocavity plasmonic emission by local molecular states of C60 on Au(111)," Opt. Express 20, 26725-26735 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-24-26725


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References

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  4. R. Berndt, J. K. Gimzewski, and P. Johansson, “Inelastic tunneling excitation of tip-induced plasmon modes on noble-metal surfaces,” Phys. Rev. Lett.67(27), 3796–3799 (1991). [CrossRef] [PubMed]
  5. G. Schull, M. Becker, and R. Berndt, “Imaging confined electrons with plasmonic light,” Phys. Rev. Lett.101(13), 136801 (2008). [CrossRef] [PubMed]
  6. X. H. Qiu, G. V. Nazin, and W. Ho, “Vibrationally resolved fluorescence excited with submolecular precision,” Science299(5606), 542–546 (2003). [CrossRef] [PubMed]
  7. Z. C. Dong, X. L. Guo, A. S. Trifonov, P. S. Dorozhkin, K. Miki, K. Kimura, S. Yokoyama, and S. Mashiko, “Vibrationally resolved fluorescence from organic molecules near metal surfaces in a scanning tunneling microscope,” Phys. Rev. Lett.92(8), 086801 (2004). [CrossRef] [PubMed]
  8. E. Cavar, M. C. Blüm, M. Pivetta, F. Patthey, M. Chergui, and W. D. Schneider, “Fluorescence and phosphorescence from individual molecules excited by local electron tunneling,” Phys. Rev. Lett.95(19), 196102 (2005). [CrossRef] [PubMed]
  9. S. W. Wu, G. V. Nazin, and W. Ho, “Intramolecular photon emission from a single molecule in a scanning tunneling microscope,” Phys. Rev. B77(20), 205430 (2008). [CrossRef]
  10. C. Chen, P. Chu, C. A. Bobisch, D. L. Mills, and W. Ho, “Viewing the interior of a single molecule: vibronically resolved photon imaging at submolecular resolution,” Phys. Rev. Lett.105(21), 217402 (2010). [CrossRef] [PubMed]
  11. F. Rossel, M. Pivetta, F. Patthey, and W. D. Schneider, “Plasmon enhanced luminescence from fullerene molecules excited by local electron tunneling,” Opt. Express17(4), 2714–2721 (2009). [CrossRef] [PubMed]
  12. Z. C. Dong, X. L. Zhang, H. Y. Gao, Y. Luo, C. Zhang, L. G. Chen, R. Zhang, X. Tao, Y. Zhang, J. L. Yang, and J. G. Hou, “Generation of molecular hot electroluminescence by resonant nanocavity plasmons,” Nat. Photonics4(1), 50–54 (2010). [CrossRef]
  13. A. Kabakchiev, K. Kuhnke, T. Lutz, and K. Kern, “Electroluminescence from individual pentacene nanocrystals,” ChemPhysChem11(16), 3412–3416 (2010). [CrossRef] [PubMed]
  14. J. G. Hou, Y. Jinlong, W. Haiqian, L. Qunxiang, Z. Changgan, Y. Lanfeng, W. Bing, D. M. Chen, and Z. Qingshi, “Topology of two-dimensional C60 domains,” Nature409(6818), 304–305 (2001). [CrossRef] [PubMed]
  15. F. Rossel, M. Pivetta, and W. D. Schneider, “Luminescence experiments on supported molecules with the scanning tunneling microscope,” Surf. Sci. Rep.65(5), 129–144 (2010). [CrossRef]
  16. R. Berndt, R. Gaisch, J. K. Gimzewski, B. Reihl, R. R. Schlittler, W. D. Schneider, and M. Tschudy, “Photon emission at molecular resolution induced by a scanning tunneling microscope,” Science262(5138), 1425–1427 (1993). [CrossRef] [PubMed]
  17. K. Sakamoto, K. Meguro, R. Arafune, M. Satoh, Y. Uehara, and S. Ushioda, “Light emission spectra of the monolayer-island of C60 molecules on Au(111) induced by scanning tunneling microscope,” Surf. Sci.502-503, 149–155 (2002). [CrossRef]
  18. G. Hoffmann, L. Libioulle, and R. Berndt, “Tunneling-induced luminescence from adsorbed organic molecules with submolecular lateral resolution,” Phys. Rev. B65(21), 212107 (2002). [CrossRef]
  19. Y. Zhang, X. Tao, H. Y. Gao, Z. C. Dong, J. G. Hou, and T. Okamoto, “Modulation of local plasmon mediated emission through molecular manipulation,” Phys. Rev. B79(7), 075406 (2009). [CrossRef]
  20. Y. Zhang, F. Geng, H. Y. Gao, Y. Liao, Z. C. Dong, and J. G. Hou, “Enhancement and suppression effect of molecules on nanocavity plasmon emissions excited by tunneling electrons,” Appl. Phys. Lett.97(24), 243101 (2010). [CrossRef]
  21. N. L. Schneider, F. Matino, G. Schull, S. Gabutti, M. Mayor, and R. Berndt, “Light emission from a double-decker molecule on a metal surface,” Phys. Rev. B84(15), 153403 (2011). [CrossRef]
  22. C. Zhang, B. Gao, L. G. Chen, Q. S. Meng, H. Yang, R. Zhang, X. Tao, H. Y. Gao, Y. Liao, and Z. C. Dong, “Fabrication of silver tips for scanning tunneling microscope induced luminescence,” Rev. Sci. Instrum.82(8), 083101 (2011). [CrossRef] [PubMed]
  23. S. R. Forrest, “Ultrathin organic films grown by organic molecular beam deposition and related techniques,” Chem. Rev.97(6), 1793–1896 (1997). [CrossRef] [PubMed]
  24. Z. C. Dong, D. Fujita, and H. Nejoh, “Adsorption and tunneling of atomic scale lines of indium and lead on Si(100),” Phys. Rev. B63(11), 115402 (2001). [CrossRef]
  25. X. Lu, M. Grobis, K. H. Khoo, S. G. Louie, and M. F. Crommie, “Charge transfer and screening in individual C60 molecules on metal substrates: A scanning tunneling spectroscopy and theoretical study,” Phys. Rev. B70(11), 115418 (2004). [CrossRef]
  26. J. Kliewer, R. Berndt, E. V. Chulkov, V. M. Silkin, P. M. Echenique, and S. Crampin, “Dimensionality Effects in the Lifetime of Surface States,” Science288(5470), 1399–1402 (2000). [CrossRef] [PubMed]
  27. M. Grobis, A. Wachowiak, R. Yamachika, and M. F. Crommie, “Tuning negative differential resistance in a molecular film,” Appl. Phys. Lett.86(20), 204102 (2005). [CrossRef]
  28. P. Johansson, R. Monreal, and P. Apell, “Theory for light emission from a scanning tunneling microscope,” Phys. Rev. B Condens. Matter42(14), 9210–9213 (1990). [CrossRef] [PubMed]
  29. J. Aizpurua, S. P. Apell, and R. Berndt, “Role of tip shape in light emission from the scanning tunneling microscope,” Phys. Rev. B62(3), 2065–2073 (2000). [CrossRef]
  30. X. Tao, Z. C. Dong, J. L. Yang, Y. Luo, J. G. Hou, and J. Aizpurua, “Influence of a dielectric layer on photon emission induced by a scanning tunneling microscope,” J. Chem. Phys.130(8), 084706 (2009). [CrossRef] [PubMed]
  31. R. Berndt and J. K. Gimzewski, “Isochromat spectroscopy of photons emitted from metal-surfaces in an STM,” Ann. Phys.505(2), 133–140 (1993). [CrossRef]
  32. G. V. Nazin, X. H. Qiu, and W. Ho, “Atomic engineering of photon emission with a scanning tunneling microscope,” Phys. Rev. Lett.90(21), 216110 (2003). [CrossRef] [PubMed]

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