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STM light emission spectroscopy of surface micro-structures on granular Au films
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SurfaceScience324 (1995) 282"288
STM light emission spectroscopy of surface micro-structures on granular Au films K. Ito, S. Ohyama, Y. Uehara, S. Ushioda * Research Institute of Electrical Communication, Tohoku University, Sendai 980-77, Japan.
Received 1 September1994;acceptedfor publication11 October1994
Abstract
Light emission spectra from the gap region of a scanning tunneling microscope (STM) have been measured simultaneously with the cross-section of the surface topography. The spectra consist of two components centered about 1.7 and 2.0 eV, The intensity ratio between these components varies as the tip moves along different surface structures on the sample. We have found a correlation between the light emission spectra and the surface topography. We suggest a mechanism that can qualitatively explain the observed position-dependent spectra.
Coombs et al. [1] first observed visible light emission from the gap region of a scanning tunneling microscope (STM). Since then the STM light emission (STM-LE) has been studied by several groups for the purpose of investigating the mechanisms of the STM-LE processes or of developing a new technique that allows investigations of the optical properties of nanometer scale structures on a sample surface. The intensity map and the spectrum of the STM-LE have been measured on various samples [3-14]. Most of the previous studies focused on obtaining the STM-LE intensity maps of sample surfaces, and the optically obtained maps were compared with the surface topographies determined by STM. In these experiments the spectrally integrated intensity of the emitted light was directly measured by a photomulti-
Corresponding author. Fax: +81 22 223 0696; E-mail: ushioda@jpntuvm0.
plier without spectral analysis [10-12]. According to these results, the integrated intensity variation does not always correspond to the surface topographies [8,12]. It is believed that the observed position dependence of the STM-LE intensity arises in part from the shift of spectral features. Since the spectral sensitivity of the photodetector is wavelength dependent, shifting of spectral features causes variations of the integrated intensity. As described below, shifting of spectral features was observed on a semiconductor sample [14], and similar shifting is expected on a metal sample also. Thus it is interesting to measure the dependence of the STM-LE spectra, rather than just the total intensity, on the surface features under the tip. Then one can correlate the emission spectra and the surface structures directly under the STM tip, and further elucidate the light emission mechanisms. There are several reports on STM-LE spectra [3-9]. These spectra were measured by setting the tunneling current much higher than for the usual measurement of surface topography, and by keeping
0039-6028/95/$09.50 © 1995 ElsevierScienceB.V. All rights reserved SSDI 0039-6028(94)00674-1
K. Ito et aL / Surface Science 324 (1995) 282-288
the position of the probe tip fixed over a point for a long time. In this kind of experiment it is difficult to explore the dependence of the STM-LE spectrum on surface features, because of the modification of the sample surface by high tunneling current and the drift of the tip position during the measurement of a spectrum. Nevertheless, the tip position dependence of the STM-LE spectrum was reported by Samuelson et al. [14]. They found a difference in spectral features at two points separated by about 10 nm on the surface of p-type InP. However, the origin of this spectral difference was not explained, because the relation between the spectral difference and the surface image was not analyzed in detail. To understand the origin of the spectral features that depend on the tip location, it is necessary to compare the STM-LE spectra and the surface topography around the position where the spectra are measured. Such an experiment has not been performed yet on a metal sample with a spatial resolution of nanometer order. On the theoretical side, Uehara et al. [15] calculated the emission spectrum by modeling an STM tip and a sample as a metal sphere and a metal plane, respectively. Their theory was based on the theory of Rendell and Scalapino [2]. Uehara et al. showed that the features of the calculated spectrum change depending on the size of the sphere. From their results one may expect that the spectrum will change depending on the relative position of the tip with respect to surface structures (e.g. bumps and pits). In this paper we present the first clear demonstration of the dependence of the STM-LE spectra on the surface features of evaporated Au films. The STM-LE spectra have been obtained simultaneously with the cross-section of the surface topography. Thus we can directly compare the STM-LE spectra with the surface topography. Au film was evaporated in high v a c u u m ( 1 0 - 7 Torr) on a glass plate. The thickness of the Au film was 50 nm, and the evaporation speed was 0.1 n m / s . This sample was once taken out from the evaporation chamber, and then set on the sample holder of a UHV-STM. The experiments were performed in a high vacuum of 10 - 7 Torr. The experimental configuration is illustrated in Fig. 1. The details of our STM have been described previously [7]. We used Pt lr probe tips with a tip curvature of ~ 25 nm. The STM was operated in a constant-tun-
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neling-current mode. The applied bias voltage and the tunneling current settings were fixed at + 0.1 V and 1.0 nA for the measurement of surface topography alone, and at +2.5 V and 5.0 nA for the simultaneous measurements of the STM-LE spectra and the cross-section of surface topography. The electrical potentials are referred to the tip; hence a positive bias means the sample is at a higher potential with respect to the tip. The thermal drift velocity of the tip position was 0.3 n m / m i n along the surface plane. The emitted light radiated at 75 ° to the surface normal was collected by a lens mounted in vacuum, and it was led into a spectrograph through a viewing port and a second lens. The solid angle of the collection was 0.18 sr. The spectra were detected by an optical multichannel detector which covers the spectral range from 1.5 to 2.6 eV in photon energy. The dark-count rate of our light detection system was 0.002 cps/channel, and the integrated darkcounts during the measurement of a single spectrum were about 100 counts integrated over the 512 channels. Integrated signal counts greater than 500 counts were required to obtain a spectrum with a good signal-to-noise ratio. The spectra that are presented in this paper are not corrected for the energy dependent sensitivity of the detection system. The simultaneous measurements were performed in the following manner: surface topography was measured first and an STM surface image was obtained. Then we decided the line along which the tip is moved for a simultaneous measurement. The STM-LE spectra were measured along this predetermined line at several points with a step size of 2.5
K. lto et al. / Surface Science 324 (1995) 282-288
284
nm. Only the surface topography was measured between the neighboring points. The tip was stopped at every spectral measurement point for 100 s to measure the spectrum. The thermal drift of the tip position was 0.5 nm during each spectral measurement. The spatial resolution limited by the thermal drift was smaller than the size of average features on the evaporated Au film. The surface topography of an evaporated Au film is shown in Fig. 2. The scanned area is 200 × 200 nm 2. We see that Au grains with diameters of 10-50 nm and heights of 2 - 5 nm are randomly distributed. We performed the simultaneous measurement of the STM-LE spectra and the cross-section of surface topography along the line (100 nm long) in the direction of the arrow drawn in Fig. 2. We measured the spectra at 40 points (2.5 nm/step) along this line. The results are shown in Figs. 3 and 4. Fig. 3a shows the cross-section of the surface topography. It is clear from Fig. 3a that the tip moved over two Au grains, a and fl, both of whose diameter is ~ 40 nm. Fig. 3b shows the total photon counts integrated over all channels of the detector (512 channels, 1.5-2.6 eV) at each point. We can see the tendency for emission yield to be enhanced at the edges of the Au grains. This tendency has been experimentally observed before. Berndt et al. [8] showed that no emission occurs from the edge of Ag islands in some cases and in other cases emission preferentially occurs from the periphery of the islands. Sivel et al. 10
(9/
/3
Fig. 2. Surface topography of a 200)<200 nm 2 area of the evaporated Au film taken at a sample voltage of +0.1 V and a tunneling current of 1.0 nA. The gray scale corresponds to the height range of 9 nm.
[10] reported that the edge of Au clusters emitted light when a bias voltage greater than 2 V was applied. Fig. 3b shows that the integrated signal intensity is less than 500 counts at some of the locations. Fig. 4 shows a series of the STM-LE spectra measured at points numbered from 1 to 8 in Fig. 3, where the signal was intense enough to obtain the spectra with good S / N ratios. All the spectra consist of two
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broad components that are indicated by two arrows, A and B, in Fig. 4. The peak positions of the components A and B are about 1.7 and 2.0 eV, respectively. The spectra change continuously with the movement of the tip. The intensity ratio between A and B changes from point to point.
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Fig. 6. (a) Cross-section of the surface topography and (b) integrated intensity of the STM-LE. These data were simultaneously measured with the STM-LE spectra shown in Fig. 7 along the arrow drawn in Fig. 5.
K. Ito et al. // Surface Science 324 (1995) 282-288
286
Now we attempt to relate the change of the spectrum with surface features under the tip. The spectra from 1 to 4 show that component A is intense at the top of grain or, and component B continuously grows as the tip moves toward the edge of grain a. On the other hand, the spectra from 5 to 8 show that component B is intense at the edge of grain /3, and component A grows as the tip moves toward the top of grain /3. These results suggest that the photon emission of lower energy (component A) is intense at the top of Au grains, and the photon emission of higher energy (component B) becomes intense as the tip moves toward the edge of Au grains. The above results were obtained when the tip moved on the grains that had a diameter of ~ 40 nm. Next, we show the spectra that were obtained when the tip moved on a fiat area between grains
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and when it was on a small grain y which has a diameter of ~ 20 nm, as shown in Fig. 5. Fig. 5 shows the surface topography scanned over an area of 100 × 100 nm 2. Figs. 6 and 7 are the results of the simultaneous measurements of the spectra and the surface cross-section along the line in the direction of the arrow drawn in Fig. 5. It is clear from Fig. 7 that all the spectra consist of two components (A and B) similar to those seen in Fig. 4. The spectra measured in the flat area between grains are shown in frames 1 to 4, and the spectra measured on the small grain y are shown in frames 5 to 8. In the flat area the total intensity of the spectrum changes with the tip position, but we cannot see a clear change of the intensity ratio of the two components A and B. On the grain y, component A is intense at the top, and component B grows continuously as the tip moves toward the edge. These results are similar to those seen in frames 1 through 4 of Fig. 4. From the above observations, we conclude that all the spectra consist of two components centered about 1.7 and 2.0 eV independent of the sample surface features. The change of spectra is caused by the change of relative intensities of the two components. When the tip is at the top of a grain, the low energy part of the spectrum is intense. The high energy part becomes intense when the tip is moved toward the
K. lto et al. / Surface Science 324 (1995)282-288 edge of a grain. In contrast, when the tip moves in a flat area between grains, the spectral shape does not change appreciably, but only the total intensity changes with the tip position. Now let us discuss the mechanism that causes the dependence of the STM-LE spectra on the surface features under the tip. As a first step we can model the granular surface of the evaporated Au film as hemispherical bumps and pits (Fig. 8). The STM-LE of metal samples is believed to arise from the localized surface plasmons (LSPs) excited by tunneling electrons [1,3-8]. We consider LSP modes whose resonant frequencies depend on the surface features. When the tip is at the top of a bump (Fig. 8a), the tunneling current excites the modes with a rotational symmetry. This mode may be approximated by the dipole mode of a metallic sphere. The resonant energy of the dipole mode h tab is given by h ta b = h tOp/v~, where h tap is the resonant energy of the bulk plasmon. The electronic transition from the d-band to the Fermi level occurs at h to > h tad, where h tad is the onset of this transition. In Au, h tad = 2.45 eV. The real part of the dielectric constant rapidly approaches zero (but not exactly zero) with increasing energy just below h tad. Thus the optical properties of Au behaves as if the effective h tap lies close to h tad" If we assume that the plasma frequency h tap of Au is effectively given by h too, then h tab = 1.41 eV. When the tip is in a pit as shown in Fig. 8b, the electric field of the dipole mode is partially cancelled. In this situation multipole modes are excited more effectively by the tunneling current. The resonant energy of the multipole mode approaches h tap~ ~ with increasing order of the multipole. This energy becomes exactly h tap~ v~ when the metal surface is perfectly fiat. If we assume h tap h too, then h top/v~- = 1.73 eV. Since the above estimates are quite rough, we do not expect these energies to agree exactly with the observed center positions of the two components at 1.7 and 2.0 eV. However, we see that the ratio between the two energies is close to Vr3/2. Thus we believe that the observed STM-LE spectra originate from the dipole and multipole modes of surface features on the Au film. The component A corresponds to the dipole mode, which has an energy close to h tap/Vr3. The component B corresponds to
287
the multipole modes which have higher resonant energies than the dipole mode. The change of the intensity ratio of these modes accounts for the change in the spectrum: when the tip is located at the top of a grain, the dipole mode is excited effectively and the low energy spectral component A becomes relatively intense. When the tip is moved from the top of a grain down to a concave area, the multipole modes are excited more effectively than the dipole mode. Hence the high energy spectral component B becomes intense. When the tip moves in a flat area the spectral shape does not change, because the excitation ratio of the two components remains constant. Thus we can qualitatively explain the change of the spectrum. In summary we have simultaneously measured the STM-LE spectra and the cross-section of the surface topography of evaporated Au films. The spectra measured in this experiment consist of two components centered about 1.7 and 2.0 eV independent of the tip position. When the tip is at the top of a grain, the low energy spectral component is intense. The high energy component becomes intense as the tip is moved toward the edge of a grain. The spectra do not change appreciably when the tip moves in a flat area between grains. We have discussed the mechanism for the variation of the spectrum, and have qualitatively explained the observed variation: when the tip is located at the top of a grain, the dipole mode is excited effectively; hence the low energy spectral component is intense. When the tip is in a pit the multipole modes are excited more effectively than the dipole mode; hence the high energy component is intense.
Acknowledgements This research was supported financially by grants from the Mitsubishi Foundation and a Grant-in-Aid for Science from the Ministry of Education, Science and Culture. We would like to acknowledge the excellent technical support provided by the Machine Shop of our Institute. One of us (K.I.) is supported by a Fellowship from the Japan Society for the Promotion of Science for Japanese Junior Scientists.
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K. lto et al. / Surface Science 324 (1995) 282-288
References [1] J.H. Coombs, J.K. Gimzewski, B. Reihl, J.K. Sass and R.R. Schlittler, J. Microscopy 152 (1988) 325. [2] R.W. Rendell and D.J. Scalapino, Phys. Rev. B 24 (1981) 3276. [3] R. Berndt, J.K. Gimzewski and P. Johansson, Phys. Rev. Lett. 67 (1991) 3796. [4] R. Berndt and J.K. Gimzewski, Surf. Sci. 269/270 (1992) 556. [5] R. Berndt, J.K. Gimzewski and P. Johansson, Phys. Rev. Lett. 71 (1993) 3493. [6] I.I. Smolyaninov, V.S. Edelman and V.V. Zavyalov, Phys. Lett. A 158 (1991) 337. [7] S. Ushioda, Y. Uehara and M. Kuwahara, Appl. Surf. Sci. 60/61 (1992) 448. [8] R. Berndt, A. Baratoff and J.K. Gimzewski, in: Scanning
[9] [10] [11] [12] [13]
[14] [15]
Tunneling Microscopy and Related Methods, NATO Advanced Studies Institutes Series E, Vol. 184, Eds. R.J. Behm, N. Garcia and H. Rohrer (Kluwer, Dordrecht, 1990) p. 269. S. Ushioda, Solid State Commun. 84 (1992) 173. V. Sivel, R. Coratger, F. Ajustron and J. Beauvillain, Phys. Rev. B 45 (1992) 8634. R. Bemdt and J.K. Gimzewski, Phys. Rev. B 48 (1993) 4746. M.J. Gallagher, A. Howells, L. Yi, T. Chen and D. Sarid, Surf. Sci. 278 (1992) 270. D.L. Abraham, A. Veider, Ch. Sch6nenberger, H.P. Meier, D.L. Arent and S.F. Alvarado, Appl. Phys. Lett. 56 (1990) 1546. L. Samuelson, J. Lindahl, L. Montelius and M.-E. Pistol, Phys. Scr. T42 (1992) 149. Y. Uehara, Y. Kimura, S. Ushioda and K. Takeuchi, Jpn. J. Appl. Phys. 31 (1992) 2465.
SurfaceScience324 (1995) 282"288
STM light emission spectroscopy of surface micro-structures on granular Au films K. Ito, S. Ohyama, Y. Uehara, S. Ushioda * Research Institute of Electrical Communication, Tohoku University, Sendai 980-77, Japan.
Received 1 September1994;acceptedfor publication11 October1994
Abstract
Light emission spectra from the gap region of a scanning tunneling microscope (STM) have been measured simultaneously with the cross-section of the surface topography. The spectra consist of two components centered about 1.7 and 2.0 eV, The intensity ratio between these components varies as the tip moves along different surface structures on the sample. We have found a correlation between the light emission spectra and the surface topography. We suggest a mechanism that can qualitatively explain the observed position-dependent spectra.
Coombs et al. [1] first observed visible light emission from the gap region of a scanning tunneling microscope (STM). Since then the STM light emission (STM-LE) has been studied by several groups for the purpose of investigating the mechanisms of the STM-LE processes or of developing a new technique that allows investigations of the optical properties of nanometer scale structures on a sample surface. The intensity map and the spectrum of the STM-LE have been measured on various samples [3-14]. Most of the previous studies focused on obtaining the STM-LE intensity maps of sample surfaces, and the optically obtained maps were compared with the surface topographies determined by STM. In these experiments the spectrally integrated intensity of the emitted light was directly measured by a photomulti-
Corresponding author. Fax: +81 22 223 0696; E-mail: ushioda@jpntuvm0.
plier without spectral analysis [10-12]. According to these results, the integrated intensity variation does not always correspond to the surface topographies [8,12]. It is believed that the observed position dependence of the STM-LE intensity arises in part from the shift of spectral features. Since the spectral sensitivity of the photodetector is wavelength dependent, shifting of spectral features causes variations of the integrated intensity. As described below, shifting of spectral features was observed on a semiconductor sample [14], and similar shifting is expected on a metal sample also. Thus it is interesting to measure the dependence of the STM-LE spectra, rather than just the total intensity, on the surface features under the tip. Then one can correlate the emission spectra and the surface structures directly under the STM tip, and further elucidate the light emission mechanisms. There are several reports on STM-LE spectra [3-9]. These spectra were measured by setting the tunneling current much higher than for the usual measurement of surface topography, and by keeping
0039-6028/95/$09.50 © 1995 ElsevierScienceB.V. All rights reserved SSDI 0039-6028(94)00674-1
K. Ito et aL / Surface Science 324 (1995) 282-288
the position of the probe tip fixed over a point for a long time. In this kind of experiment it is difficult to explore the dependence of the STM-LE spectrum on surface features, because of the modification of the sample surface by high tunneling current and the drift of the tip position during the measurement of a spectrum. Nevertheless, the tip position dependence of the STM-LE spectrum was reported by Samuelson et al. [14]. They found a difference in spectral features at two points separated by about 10 nm on the surface of p-type InP. However, the origin of this spectral difference was not explained, because the relation between the spectral difference and the surface image was not analyzed in detail. To understand the origin of the spectral features that depend on the tip location, it is necessary to compare the STM-LE spectra and the surface topography around the position where the spectra are measured. Such an experiment has not been performed yet on a metal sample with a spatial resolution of nanometer order. On the theoretical side, Uehara et al. [15] calculated the emission spectrum by modeling an STM tip and a sample as a metal sphere and a metal plane, respectively. Their theory was based on the theory of Rendell and Scalapino [2]. Uehara et al. showed that the features of the calculated spectrum change depending on the size of the sphere. From their results one may expect that the spectrum will change depending on the relative position of the tip with respect to surface structures (e.g. bumps and pits). In this paper we present the first clear demonstration of the dependence of the STM-LE spectra on the surface features of evaporated Au films. The STM-LE spectra have been obtained simultaneously with the cross-section of the surface topography. Thus we can directly compare the STM-LE spectra with the surface topography. Au film was evaporated in high v a c u u m ( 1 0 - 7 Torr) on a glass plate. The thickness of the Au film was 50 nm, and the evaporation speed was 0.1 n m / s . This sample was once taken out from the evaporation chamber, and then set on the sample holder of a UHV-STM. The experiments were performed in a high vacuum of 10 - 7 Torr. The experimental configuration is illustrated in Fig. 1. The details of our STM have been described previously [7]. We used Pt lr probe tips with a tip curvature of ~ 25 nm. The STM was operated in a constant-tun-
Lens
283
~
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I
Fig. 1. Experimental setup for the measurement of the STM-LE spectrum.
neling-current mode. The applied bias voltage and the tunneling current settings were fixed at + 0.1 V and 1.0 nA for the measurement of surface topography alone, and at +2.5 V and 5.0 nA for the simultaneous measurements of the STM-LE spectra and the cross-section of surface topography. The electrical potentials are referred to the tip; hence a positive bias means the sample is at a higher potential with respect to the tip. The thermal drift velocity of the tip position was 0.3 n m / m i n along the surface plane. The emitted light radiated at 75 ° to the surface normal was collected by a lens mounted in vacuum, and it was led into a spectrograph through a viewing port and a second lens. The solid angle of the collection was 0.18 sr. The spectra were detected by an optical multichannel detector which covers the spectral range from 1.5 to 2.6 eV in photon energy. The dark-count rate of our light detection system was 0.002 cps/channel, and the integrated darkcounts during the measurement of a single spectrum were about 100 counts integrated over the 512 channels. Integrated signal counts greater than 500 counts were required to obtain a spectrum with a good signal-to-noise ratio. The spectra that are presented in this paper are not corrected for the energy dependent sensitivity of the detection system. The simultaneous measurements were performed in the following manner: surface topography was measured first and an STM surface image was obtained. Then we decided the line along which the tip is moved for a simultaneous measurement. The STM-LE spectra were measured along this predetermined line at several points with a step size of 2.5
K. lto et al. / Surface Science 324 (1995) 282-288
284
nm. Only the surface topography was measured between the neighboring points. The tip was stopped at every spectral measurement point for 100 s to measure the spectrum. The thermal drift of the tip position was 0.5 nm during each spectral measurement. The spatial resolution limited by the thermal drift was smaller than the size of average features on the evaporated Au film. The surface topography of an evaporated Au film is shown in Fig. 2. The scanned area is 200 × 200 nm 2. We see that Au grains with diameters of 10-50 nm and heights of 2 - 5 nm are randomly distributed. We performed the simultaneous measurement of the STM-LE spectra and the cross-section of surface topography along the line (100 nm long) in the direction of the arrow drawn in Fig. 2. We measured the spectra at 40 points (2.5 nm/step) along this line. The results are shown in Figs. 3 and 4. Fig. 3a shows the cross-section of the surface topography. It is clear from Fig. 3a that the tip moved over two Au grains, a and fl, both of whose diameter is ~ 40 nm. Fig. 3b shows the total photon counts integrated over all channels of the detector (512 channels, 1.5-2.6 eV) at each point. We can see the tendency for emission yield to be enhanced at the edges of the Au grains. This tendency has been experimentally observed before. Berndt et al. [8] showed that no emission occurs from the edge of Ag islands in some cases and in other cases emission preferentially occurs from the periphery of the islands. Sivel et al. 10
(9/
/3
Fig. 2. Surface topography of a 200)<200 nm 2 area of the evaporated Au film taken at a sample voltage of +0.1 V and a tunneling current of 1.0 nA. The gray scale corresponds to the height range of 9 nm.
[10] reported that the edge of Au clusters emitted light when a bias voltage greater than 2 V was applied. Fig. 3b shows that the integrated signal intensity is less than 500 counts at some of the locations. Fig. 4 shows a series of the STM-LE spectra measured at points numbered from 1 to 8 in Fig. 3, where the signal was intense enough to obtain the spectra with good S / N ratios. All the spectra consist of two
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broad components that are indicated by two arrows, A and B, in Fig. 4. The peak positions of the components A and B are about 1.7 and 2.0 eV, respectively. The spectra change continuously with the movement of the tip. The intensity ratio between A and B changes from point to point.
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K. Ito et al. // Surface Science 324 (1995) 282-288
286
Now we attempt to relate the change of the spectrum with surface features under the tip. The spectra from 1 to 4 show that component A is intense at the top of grain or, and component B continuously grows as the tip moves toward the edge of grain a. On the other hand, the spectra from 5 to 8 show that component B is intense at the edge of grain /3, and component A grows as the tip moves toward the top of grain /3. These results suggest that the photon emission of lower energy (component A) is intense at the top of Au grains, and the photon emission of higher energy (component B) becomes intense as the tip moves toward the edge of Au grains. The above results were obtained when the tip moved on the grains that had a diameter of ~ 40 nm. Next, we show the spectra that were obtained when the tip moved on a fiat area between grains
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o.o8
~X-
0.04
1.5
Fig.
2 2.5 Photon Energy (eV)
7. S T M - L E
3
spectra measured
1.5
2 2.5 Photon Energy (eV)
at points 1 to 8 in Fig. 6.
(a)
Au
(b)
Au Fig. 8. Models of the STM configuration in this experiment: (a) when the tip is located at the top of a grain; (b) when the tip is located in a pit.
and when it was on a small grain y which has a diameter of ~ 20 nm, as shown in Fig. 5. Fig. 5 shows the surface topography scanned over an area of 100 × 100 nm 2. Figs. 6 and 7 are the results of the simultaneous measurements of the spectra and the surface cross-section along the line in the direction of the arrow drawn in Fig. 5. It is clear from Fig. 7 that all the spectra consist of two components (A and B) similar to those seen in Fig. 4. The spectra measured in the flat area between grains are shown in frames 1 to 4, and the spectra measured on the small grain y are shown in frames 5 to 8. In the flat area the total intensity of the spectrum changes with the tip position, but we cannot see a clear change of the intensity ratio of the two components A and B. On the grain y, component A is intense at the top, and component B grows continuously as the tip moves toward the edge. These results are similar to those seen in frames 1 through 4 of Fig. 4. From the above observations, we conclude that all the spectra consist of two components centered about 1.7 and 2.0 eV independent of the sample surface features. The change of spectra is caused by the change of relative intensities of the two components. When the tip is at the top of a grain, the low energy part of the spectrum is intense. The high energy part becomes intense when the tip is moved toward the
K. lto et al. / Surface Science 324 (1995)282-288 edge of a grain. In contrast, when the tip moves in a flat area between grains, the spectral shape does not change appreciably, but only the total intensity changes with the tip position. Now let us discuss the mechanism that causes the dependence of the STM-LE spectra on the surface features under the tip. As a first step we can model the granular surface of the evaporated Au film as hemispherical bumps and pits (Fig. 8). The STM-LE of metal samples is believed to arise from the localized surface plasmons (LSPs) excited by tunneling electrons [1,3-8]. We consider LSP modes whose resonant frequencies depend on the surface features. When the tip is at the top of a bump (Fig. 8a), the tunneling current excites the modes with a rotational symmetry. This mode may be approximated by the dipole mode of a metallic sphere. The resonant energy of the dipole mode h tab is given by h ta b = h tOp/v~, where h tap is the resonant energy of the bulk plasmon. The electronic transition from the d-band to the Fermi level occurs at h to > h tad, where h tad is the onset of this transition. In Au, h tad = 2.45 eV. The real part of the dielectric constant rapidly approaches zero (but not exactly zero) with increasing energy just below h tad. Thus the optical properties of Au behaves as if the effective h tap lies close to h tad" If we assume that the plasma frequency h tap of Au is effectively given by h too, then h tab = 1.41 eV. When the tip is in a pit as shown in Fig. 8b, the electric field of the dipole mode is partially cancelled. In this situation multipole modes are excited more effectively by the tunneling current. The resonant energy of the multipole mode approaches h tap~ ~ with increasing order of the multipole. This energy becomes exactly h tap~ v~ when the metal surface is perfectly fiat. If we assume h tap h too, then h top/v~- = 1.73 eV. Since the above estimates are quite rough, we do not expect these energies to agree exactly with the observed center positions of the two components at 1.7 and 2.0 eV. However, we see that the ratio between the two energies is close to Vr3/2. Thus we believe that the observed STM-LE spectra originate from the dipole and multipole modes of surface features on the Au film. The component A corresponds to the dipole mode, which has an energy close to h tap/Vr3. The component B corresponds to
287
the multipole modes which have higher resonant energies than the dipole mode. The change of the intensity ratio of these modes accounts for the change in the spectrum: when the tip is located at the top of a grain, the dipole mode is excited effectively and the low energy spectral component A becomes relatively intense. When the tip is moved from the top of a grain down to a concave area, the multipole modes are excited more effectively than the dipole mode. Hence the high energy spectral component B becomes intense. When the tip moves in a flat area the spectral shape does not change, because the excitation ratio of the two components remains constant. Thus we can qualitatively explain the change of the spectrum. In summary we have simultaneously measured the STM-LE spectra and the cross-section of the surface topography of evaporated Au films. The spectra measured in this experiment consist of two components centered about 1.7 and 2.0 eV independent of the tip position. When the tip is at the top of a grain, the low energy spectral component is intense. The high energy component becomes intense as the tip is moved toward the edge of a grain. The spectra do not change appreciably when the tip moves in a flat area between grains. We have discussed the mechanism for the variation of the spectrum, and have qualitatively explained the observed variation: when the tip is located at the top of a grain, the dipole mode is excited effectively; hence the low energy spectral component is intense. When the tip is in a pit the multipole modes are excited more effectively than the dipole mode; hence the high energy component is intense.
Acknowledgements This research was supported financially by grants from the Mitsubishi Foundation and a Grant-in-Aid for Science from the Ministry of Education, Science and Culture. We would like to acknowledge the excellent technical support provided by the Machine Shop of our Institute. One of us (K.I.) is supported by a Fellowship from the Japan Society for the Promotion of Science for Japanese Junior Scientists.
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