Dependence of Gundlach Oscillation on Distinct Surface Structures-51

have observed the surface of the superstructure 9×9Ag/Cu(111) by STS of dZ/dV-V spectra. The Moiŕe pattern of Ag film is divided into two regions: the hollow and the protrusive regions, indicated in Fig. 4.13(a). The average spectra taken at the two regions are shown in Fig. 4.13 (b). In this spectrum, there are five standing wave states and are all taken into our discussion. Local variation of the intensity difference is observed in these five peaks. Although the energy levels of the valley and the apexes locate at the same position, the intensities at the valley of the protrusive regions are

Fig. 4.13 (a) STM topography image of a Ag/Cu(111) surface. The image size is 30nm×32nm (b) The average dZ/dV-V spectra of the hollow and protrusive regions on Ag/Cu(111) surface.

A similar phenomenon has been discussed by W. B. Su et al. who observed the three structures on a reconstructed Au(111), as shown in Fig. 4.14. There exhibit higher apexes and lower valleys in the spectra of FCC and HCP regions. W. B. Su et al.

have demonstrated that the transmission background of the regions with weaker intensity (ridge regions on Au(111) surface) is higher than that with stronger intensity (FCC and HCP regions on Au(111) surface)^[4-12]. This means that tunneling current is tended to reflect in the FCC and HCP regions so that leads to a stronger resonance in the standing wave states. W. B. Su et al. have also shown that on a reconstructed Au(111) surface, the peak with higher transmission background has lower intensity at the apex, and vise versa. The phenomenon has also been observed by McMahon et al.^[4-13] and is really fitted to what we obtained in our system. It is believed that the intensity in the spectra is conserved so that a complementary phenomenon occurs. This result indicates that such complementation of intensity should be a general phenomenon.

protrusive

hollow

Fig. 4.14 (a) STM topography image of a reconstructed Au(111) surface (286Å×286Å) (b) The average dZ/dV-V spectra of FCC, HCP and ridge regions.

With this conclusion, the transmissivity of the protrusive regions on Ag film is supposed to be higher than that of the hollow regions. It implies that the transmissivity changes with different orientation of the reconstructed superstructure. Since at the energy of standing wave states, there are differences in intensity in distinct regions. We can have differentiated STM images to understand the contrast variation during the voltage ramping, i.e. Z-V measurement. Not only the surface topography, but also the comparative contrast can be known.

4.3.2 Surface Mapping with Increasing Gap Width

During the Z-V measurement, the tip-sample gap increases with the applied tip bias. Generally the broader the gap width is, the fuzzier the image and worse the resolution become. Therefore it inspires us to find out how the image will become when the displacement is far away from the surface. The displacement can be expected by the formula dV=FdZ, where V is the applied tip bias, F is the electric field, and Z is

FCC ridge HCP

should be as small as possible to obtain the gap width as broad as we could.

Figure 4.15(a) shows the STM topography image of a reconstructed Au(111) surface. The average dZ/dV-V spectrum in Fig. 4.15(b) is measured from Fig. 4.15(a), and implies the small electric field in the tip-sample gap. There are twelve peaks of Gundlach oscillation, but only the last ten were marked and discussed in our system.

Figure 4.15(c) indicated the corresponding bias at each apex and valley, and in Fig.

4.15(d) the corresponding displacement of the tip. Generally the gap width is within 10 Å during normal scanning. However, as we can see in Fig. 4.15(b) and (d), the farthest distance between the tip and the sample should be nearly 60Å. Since the tip can move from the sample surface with such a long distance, it is interesting to understand the STM image performance during the tip movement.

The STM image in Fig. 4.15(a) was differentiated at the biases where the apexes and valleys locate, and shown in Fig. 4.16 (a) and (b) respectively. It is apparent in Fig.

4.16(a) that the FCC regions are slightly brighter than the HCP regions, and the ridges are the darkest among the three structures. The high transmissivity implies low reflectivity as well as stronger surface resonance and brighter performance in the images. It reveals that the ridges have the highest transmissivity on reconstructed Au(111) surface, and FCC region has slightly higher transmissivity than HCP region.

This result is corresponding to the discussion in Sec. 4.3.1. The mappings at the energy of valleys in Fig. 4.16(b) have the same contrast as normal STM images, and Fig.

4.16(a) vice versa. It proves the assumption of the intensity complementary demonstrated by W. B. Su^[4-12].

Fig. 4.15 (a) STM topography image of a reconstructed Au(111) surface. The image size is 24nm×24nm. (b) dZ/dV-V spectrum acquired on reconstructed Au(111) surface. The first two states are believed as image potential states. (c) Corresponding bias at each apex and valley. (d) Corresponding displacement of the tip at each apex and valley.

nm

Fig. 4.16 (a) Mappings at the apexes of each peak whose biases and displacement are indicated in Fig. 4.12 (c) and (d). The sizes of these mappings are 24nm×24nm. The contrast in these mappings is opposite to that in STM topography image. FCC regions are brighter than the other two structures.

a a ( (1 1. .5 55 5n nm m) ) b b ( (2 2. .2 20 0n nm m) )

f f ( (4 4. .2 27 7n nm m) )

c c ( (2 2. .7 79 9n nm m) ) d d ( (3 3. .2 28 8n nm m) ) e e ( (3 3. .8 82 2n nm m) )

h h ( (5 5. .1 15 5n nm m) ) g g ( (4 4. .7 70 0n nm m) )

j

j ( (5 5. .9 92 2n nm m) ) i

i ( (5 5. .5 57 7n nm m) )

(a)

Fig. 4.16 (b) Mappings at the valleys between all peaks, whose biases and displacement are indicated in Fig. 4.12(b) and (c). The sizes of these mappings are 24nm×24nm. The contrast in these mappings is same as that in STM topography image. The ridge regions are the brightest in the mappings.

f’ f ’ ( (4 4. .4 49 9n nm m) ) h’ h ’ ( (5 5. .3 34 4n nm m) ) e’ e ’ ( (4 4. .1 14 4n nm m) ) a’ a ’ ( (1 1. .8 85 5n nm m) ) b b’ ’ ( (2 2. .4 48 8n nm m) )

d’ d ’ ( (3 3. .5 50 0n nm m) ) c’ c ’ ( (3 3. .0 05 5n nm m) )

i’ i ’ ( (5 5. .7 75 5n nm m) )

g’ g ’ ( (4 4. .9 93 3n nm m) )

(b)

To understand the variation of resolution with increasing tip-sample gap, we measured the Full-Width of Highest Maximum (FWHM) for HCP regions, which are the narrowest among the three local structures in Fig. 4.16. Figure 4.16(a) and (b) show not only the mapping at various biases but also the images obtained with increasing gap width. The dependence of the resolution on the gap width is illustrated in Fig. 4.17. We found that the width of HCP regions varies little with the increasing tip-sample separation, and is averagely 0.1145nm in width. This result stands for a significant meaning that the minimum width we can observe maintains while the tip is moving away.

Fig. 4.17 Dependence of the possible resolution on the increasing tip-sample gap width.

1 2 3 4 5 6

0.06 0.08 0.10 0.12 0.14

0.16 ave. value= 0.1145 nm

width of HCP (nm)

displacement (nm)

CONCLUSION

By STM and STS operation, we can observe the Gundlach Oscillation on metal substrate and ultrathin film by Z-V measurement. Energy separations exhibit in the dZ/dV-V spectra at high energy. Whatever in uniform ultrathin (Ag/Cu(111)) and various (Co/Cu(111)) thin film system, constant energy separation at high voltage can be obviously seen. The constant energy separation in each thin film system is considered as the work function difference between the film and the substrate. This assumption is pretty close to the theoretical calculation and be proved by our model (Fig. 4.3). Even in single-atoms thin film system, constant energy separation occurs.

This means that even isolated atoms devote themselves in work function.

However, when the tip shape is changed in scanning process, tip-sample electronic field varies as well as the distribution of the standing wave states. We executed Z-V measurement on Ag/Cu(111) system with various tip-sample electric field and found that the energy separations at high biases change slightly with various tip-sample field. Hence the energy separation is considered field-independent.

In reconstructed simplex surface, dZ/dV-V spectra of distinct regions are different in intensity. Based on this difference, one can obtain knowledge of the electronic structures with increasing tip-sample gap by differentiating the STM image. Z-V measurement was operated on reconstructed Au(111) surface and the tip was moving away from the surface at most 60Å. At this distance, we can still obtain clear differentiated STM image. The HCP regions on the reconstructed Au(111) surface kept 1.1nm when the tip was moving away from the sample. In other words, one can have a resolution as 1.1nm with a large tip-sample distance of about 60Å. It is such an

it is usually difficult to obtain clear STM images and electronic structures of some minute materials such as carbon nanotubes, deoxyribonucleic acid (DNA). It is not due to the small size, but because the soft materials may be dragged by the tip while scanning since the tip-sample separation is too small (<10Å) and the observed objects may be attracted by the tip while ramping tip bias. This technique enables us to observe some soft material without dragging. However the maximum bias in our Z-V measurement is only 9V which is not large enough. It is possible that the tip-sample distance can be broadened than 60Å if the applied voltage is higher than 9V. If so, the observation of larger soft materials becomes expectable and many problem resulted from tip-dragging will certainly avoided.

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