- Email: [email protected]
Au clusters deposited on Si(111) and graphite surfaces
Surface and Coatings Technology, 67 (1994)173—182
173
Au clusters deposited on Si(1 11) and graphite surfaces A. ~ A. Kasuyaa, R. Czajkab, N. Horiguchia, Y. Nishinaa ajnstitute for Materials Research, Tohoku University, Katahira 2-1-1, 980 Sendai, Japan binstitute of Physics, Pozna6 Technical University, UI. Piotrowo 3, 60-965 PoznaO, Poland
Abstract The shape, size and electronic properties of Au clusters deposited onto Si(1l1) and graphite substrates are analyzed by scanning tunneling microscopy (STM) in ultrahigh vacuum. Deposits are sputtered from our Au STM tip by applying a sequence of voltage pulses between the tip and the substrate. Their shapes are dome type with well-controlled diameter in the nanometer scale. This technique enables us to generate arrays of Au clusters with desired periodicity and size. Such structures are stable and observable many days after deposition. Our results provide a practical example that one can create nanometer-scale structures by STM. The electronic structure of Au deposits is investigated by means of scanning tunneling spectroscopy. The 1—V characteristics and CITS measurements reveal peculiar properties because ofreduced cluster dimensions.
1. Introduction For the last few decades, miniaturization has been developing very intensively. It has found a very wide application in electronics, communication and data processing. However, the development of miniaturization is continuing, due to new technology and methods of investigation. Especially, the scanning tunneling microscope (STM), invented more than 10 years ago, gives insight into the surface structure of many materials with atomic resolution, The nanoclusters and single atoms are the potential ‘bricks’ that may be used to create the various devices to a very high degree of miniaturization. Nanoclusters are small particles composed of a small number of atoms, starting from a few to several hundred. They are expected to display new properties in comparison with the bulk. The atomic structure and bond character are different from those in bulk crystal. In clusters, say, consisting of 1000 atoms, about one-fourth of them lie on the surface. Their properties should therefore differ markedly from the bulk crystal. Moreover, in small clusters, quantization effects [1] and single electron effects [2] should be detectable. Thus, the main efforts are guided towards the electronic structure investigation of such particles [3—5] and the possibilities for manipulation and positioning at the atomic scale [6—8]. We have been successful in locating gold clusters with a well-controlled geometry on a silicon substrate with 1Permanent address: Institute of Physics, Polish Academy of Sciences, Al. Lotników 32/46, 02-668 Warszawa, Poland.
SSDJ 0257-8972(94)02305-A
very high accuracy by means of STM. Such clusters are very stable at room temperature. Here we present the details of this technique. The electronic structure of such particles is reported elsewhere [9].
2. Experimental details All STM measurements were carried out in situ in an ultrahigh vacuum (UHV) from systemthe at 4STM x 10’° Torr. clusters were deposited gold tip The by applying a sequence of voltage bias pulses between the tip and the substrate. As substrates, we used both Si( 111) wafers and highly oriented pyrolytic graphite (HOPG). The silicon wafers were cut to appropriate dimensions and etched chemically to obtain an atomically smooth surface. In the UHV chamber, they were heated a few times up to 1250 °C.The rate of cooling did not exceed 20°C mm ~ above 900°C and 60°C mm 1 below 900 °C.Such treatment resulted in a high quality surface with small amounts of structural defects. The HOPG substrate was cleaved in laboratory air and immediately transferred into the vacuum chamber. For comparison, we also deposited the gold clusters by laser ablation from the gold target. The purity of target material was better than 99.99%. The same gold tip was used for the deposition procedure and STM measurements. It was prepared by electrochemical etching in 30% HC1. Before mounting to UHV STM, each tip was tested on HOPG in air. Our measurements were carried out with the Digital Instruments (DI) Nanoscope III hardware and software with the McAllister UHV STM head. The
Elsevier Science S.A.
174
A. Wawro et al. / Au clusters on Si(111) and graphite
topography of samples was measured in the constant current mode. Current image tunneling spectroscopy (CITS) was performed in the constant tip—substrate distance mode.
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3. Results and discussion We show first an example of spectroscopic investigations of gold clusters. Knowledge of the electronic struc-
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Fig. 1. The topography image and current maps of gold clusters on graphite substrate for 1.2 V and 0.5 V bias polarization. The scanned area is 41.2 x 41.2 nm2.
A. Wawro et aL
/
Au clusters on Si(111) and graphite
ture of such small particles is indispensable for designing electronic devices with a very large degree of miniaturization. A wider discussion on these properties will be presented soon [9]. Fig. 1 depicts the topography image and two current maps for polarization 0.5 V (Fig. 1(a)) and 1.2 V (Fig. 1(b)) of gold deposits on the graphite surface. The clusters are bumps, so they are visible as white spots. Current maps display the intensity of tunneling current flowing between the STM tip and surface for a given bias voltage. Because CITS measurements were carried out in the constant tip—substrate distance mode, one can assume that the barrier parameters, i.e. barrier height and width, for each I—V characteristic measurement are the same. As a consequence, the current image strongly reflects the density of electronic states of the analyzed surface. It is easy to notice that strong correlation between topography image and current image occurs. For a bias of 0.5 V, the places in the current map are brighter in the cluster positions, suggesting higher intensity of tunneling current. The opposite situation occurs for a bias of 1.2 V, i.e. the intensity of tunneling current flowing to the clusters is lower in Height
175
comparison with the graphite substrate. Therefore, one can conclude that the density of empty electronic states of analyzed gold clusters is lower at 1.2 eV and higher at 0.5 eV above the Fermi level, relative to that of the graphite substrate. We have observed similar effects on gold clusters deposited on Si(11l) [9]. (Disappearance and re-appearance of clusters in the topography STM image is a consequence of the dependence of specific density of states versus energy above Fermi level.) However, in this paper we focus on the controlled cluster positioning on the substrate. Two parameters are very important: the coordinates of clusters on the surface and the size of clusters that have to be reproducible very well. We decided to apply voltage pulses between the STM gold tip and the substrate to position the clusters. Such technique allows us to deposit clusters of desired size and location. However, the first attempts provided unsatisfactory results. Fig. 2 depicts the group of clusters, close to one of the monoatomic steps, on the Si( 111) surface obtained after applying a single bias pulse. The clusters are distributed randomly on an area of ca. 25 nm diameter and their shapes and size are not uniform.
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176
A. Wawro et al.
/
Au clusters on Si(111) and graphite
Looking for a more reproducible procedure, we used the ‘lithography’ option in the Nanoscope III software. This option allowed us to change voltage pulse parameters, a tip movement and position, etc., by writing and executing a special script program. We noticed that a crucial influence is associated with such parameters as: the value of voltage bias, duration of the pulse, distance between the tip and substrate, speed of approach and withdrawal of the tip to and from the substrate (before and after pulse). The intensity of tunneling current has less importance. Finally, the procedure of cluster deposition was found as follows. First, the tip was approached to the substrate. After a certain time, to stabilize the tip position over the substrate, the voltage pulse (+ 2 V, the sign, as applied to the sample) was applied. In the next step, the tip was withdrawn from the substrate and was moved slightly over it. This cycle was repeated three times, but the applied voltage pulse was systematically increased (up to 5 V). It seems that especially approaching the STM tip towards substrate was of great importance. It is plausible that the close distance between the tip and the substrate establishes bonding between
Height
atoms of the tip and the substrate due to chemical forces developing. The electrostatic forces, arising during voltage pulse duration, gain control over the atom transfer from the tip to the substrate. Examples of periodic structures arranged in F shape and in double rows are presented in Figs. 3 and 4. Each cluster deposited in the way described above is visible as a single bump. Their shapes are regular with well-defined geometry. The section analysis (Fig. 5) provides the details of individual lengths characterizing the row of clusters shown in Fig. 3. The diameter of clusters is very well repeatable and kept about 3.3 nm (horizontal distance between AA markers). The height of each cluster is approximately the same (ca. 0.37 nm, vertical distance between BB markers), being comparable to two atomic diameters. Also, Fourier analysis of the profile shape was performed. The calculated spectral period is 10.08 nm. We emphasize that our intention was to deposit clusters in a row every 10 nm. This means that we have succeeded in this procedure with an accuracy better than 0.1 nm. The procedure presented above was highly reproducible, with only few exceptions. Probably the deposition conditions
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Fig. 3. Periodic structure of gold clusters deposited on Si(lil). The scanned area is 50 x 50 nm
A. Wawro et al. / Au clusters on Si(111) and graphite
177
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are strongly influenced by the tip shape. If the apex of tip is smooth enough, the electric field generated by bias pulse is too low and deposition is hampered. The evidence of this is that after a certain time of applying the procedure described above, the deposits did not occur on the substrate. Another feature, which favours such deposits for any applications is their stability in time. In particular, the Au/Si system is promising, because mutual solubility of constituents is negligible (very small interdiffusion) and no compounds exist [10]. Fig. 6 depicts the group of gold dots on silicon substrate arranged in the inscription ‘IMR’ (Institute for Material Research). It is an another example of the possibility of locating clusters in desired ways. The images were taken just after deposition (Fig. 6(a)) and 3 days later (Fig. 6(b)). The same IMR pattern and monoatomic steps are also visible. Evidently such structure is very stable in ultrahigh vacuum at room temperature. The number of dots and the shapes of both patterns are the same. Only the contrast of the image taken 3 days after deposition is worse, which we ascribe to contaminations that have covered the surface during that time.
Another evidence that mobility of gold clusters on silicon substrate is very low is presented in Fig. 7. The clusters deposited from the Au target by laser ablation are located randomly on the whole surface. It seems that there are no agglomerations of clusters either on structural defects or along the edges of monoatomic steps. Such distribution is a result of strong chemical bonding and low surface diffusion at room temperature. Fig. 8 depicts the surface of the sample after a few cycles of deposition and thermal heating up to 1250 °C. Here, contrary to the fresh surface before deposition, contaminations or other than 7 x 7 surface reconstructions are visible. They are located as narrow strips along the edges of monoatomic steps and also form lines perpendicular to the substrate steps. This later location is most probably determined by the existence of linear structural defects of the substrate. Observations of fresh substrate under the same conditions have not revealed the existence of such formations. Consequently, one can eliminate that such contaminations are deposited from vacuum. Most probably, the Si—Au chemical bonding is so strong, that thermal heating above melting point of bulk gold does not
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A. Wawro et al. / Au clusters on Si(111) and graphite
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remove the whole Au material previously deposited on Si substrate. Part migrates probably on the surface to places such as steps and surface defects where they find the minimum of energy. Of course, subsequent annealing cycles lead to a more and more clean surface with the 7 x 7 reconstruction. Similar thermal treatment of the HOPG sample with gold clusters deposited by laser ablation and annealed up to 600 °C, results in two phenomena. First, the clusters exhibit a tendency to coalescence (the cluster dimensions are increasing) and secondly, the clusters diffuse to steps on HOPG sample as shown in Fig. 9. The explanation is as follows: the clusters weakly bonded to the HOPG surface via van der Waals forces only, diffuse easily on the HOPG surface at elevated temperatures and coalesce into bigger structures. Finally, they are adsorbed along steps, where the probability of finding unsaturated bonds is much higher than anywhere else on the HOPG surface. The dependence of gold cluster height versus cluster diameter is shown in the Fig. 10. The circles at the origin of this graph reflect the height and diameter of clusters obtained just after laser ablation
on HOPG substrate. Typical heights are up to 0.5 nm and diameters up to 4 nm. After thermal treatment, the coalesced clusters (or rather gold islands) may have diameter up to 15 nm and height up to 2.5 nm. The reason why some exhibit a dome or ‘flat’ shape is worthy of further studies.
4. Summary We have analyzed the possibility of creating regular, periodic formations of small clusters. The deposition from the STM tip enables us to position the gold clusters with desired size and location with very high accuracy. Analyzed clusters are very stable and observable at room temperature in ultrahigh vacuum for several days. The I—V characteristics measured above small gold clusters and CITS show characteristics different from the bulk material. These features of the gold clusters may find application in devices designed with a very high degree of miniaturization.
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179
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A. Wawro ci al. / Au clusters on Si(111) and graphite
Acknowledgements This work was supported by the New Program from the Ministry of Education, Science and Culture in Japan, and in part by Poznafl Technical University within the statutory activity under DS 62-100 Project.
References [1] RE. Cavicchi and R.H. Silsbee, Phys. Rev., B, 37(1988)706. [2] DV. Avierin and K.K. Likhariev, J. Low Temp. Phi’s., 62 (1986) 345. [3]R.M. Feenstra, Phi’s. Rev. Lelt., 63 (1989( 1412.
[4] P.N. First, J.A. Stroscio, R.A. Dragoset, D.T. Pierce an~ R.J. Celotta, Phys. Rev. Lett., 63 (1989) 1416. [5] M.F. Crommie, C.P. Lutz and D.M. Eigler, Nature, 363 (1993’ 524. [6] D.M. Eigler and E.K. Schweizer, Nature, 344 (1990)524. [7] J.A. Stroscio and D.M. Eigler, Science, 254 (1991) 1319. [8] Ph. Avouris and 1W. Lyo, Appl. Surf. Sci., 60/61 (1992) 426. [9] A. Wawro, R. Czajka, A. Kasuya and Y. Nishina, unpublished. [10] H. Okamoto and TB. Massalski, in T.B. Massalski (ed.), Binart’ Alloy Phase Diagrams, American Society for Metals, Metals Park, OH, p. 312.
173
Au clusters deposited on Si(1 11) and graphite surfaces A. ~ A. Kasuyaa, R. Czajkab, N. Horiguchia, Y. Nishinaa ajnstitute for Materials Research, Tohoku University, Katahira 2-1-1, 980 Sendai, Japan binstitute of Physics, Pozna6 Technical University, UI. Piotrowo 3, 60-965 PoznaO, Poland
Abstract The shape, size and electronic properties of Au clusters deposited onto Si(1l1) and graphite substrates are analyzed by scanning tunneling microscopy (STM) in ultrahigh vacuum. Deposits are sputtered from our Au STM tip by applying a sequence of voltage pulses between the tip and the substrate. Their shapes are dome type with well-controlled diameter in the nanometer scale. This technique enables us to generate arrays of Au clusters with desired periodicity and size. Such structures are stable and observable many days after deposition. Our results provide a practical example that one can create nanometer-scale structures by STM. The electronic structure of Au deposits is investigated by means of scanning tunneling spectroscopy. The 1—V characteristics and CITS measurements reveal peculiar properties because ofreduced cluster dimensions.
1. Introduction For the last few decades, miniaturization has been developing very intensively. It has found a very wide application in electronics, communication and data processing. However, the development of miniaturization is continuing, due to new technology and methods of investigation. Especially, the scanning tunneling microscope (STM), invented more than 10 years ago, gives insight into the surface structure of many materials with atomic resolution, The nanoclusters and single atoms are the potential ‘bricks’ that may be used to create the various devices to a very high degree of miniaturization. Nanoclusters are small particles composed of a small number of atoms, starting from a few to several hundred. They are expected to display new properties in comparison with the bulk. The atomic structure and bond character are different from those in bulk crystal. In clusters, say, consisting of 1000 atoms, about one-fourth of them lie on the surface. Their properties should therefore differ markedly from the bulk crystal. Moreover, in small clusters, quantization effects [1] and single electron effects [2] should be detectable. Thus, the main efforts are guided towards the electronic structure investigation of such particles [3—5] and the possibilities for manipulation and positioning at the atomic scale [6—8]. We have been successful in locating gold clusters with a well-controlled geometry on a silicon substrate with 1Permanent address: Institute of Physics, Polish Academy of Sciences, Al. Lotników 32/46, 02-668 Warszawa, Poland.
SSDJ 0257-8972(94)02305-A
very high accuracy by means of STM. Such clusters are very stable at room temperature. Here we present the details of this technique. The electronic structure of such particles is reported elsewhere [9].
2. Experimental details All STM measurements were carried out in situ in an ultrahigh vacuum (UHV) from systemthe at 4STM x 10’° Torr. clusters were deposited gold tip The by applying a sequence of voltage bias pulses between the tip and the substrate. As substrates, we used both Si( 111) wafers and highly oriented pyrolytic graphite (HOPG). The silicon wafers were cut to appropriate dimensions and etched chemically to obtain an atomically smooth surface. In the UHV chamber, they were heated a few times up to 1250 °C.The rate of cooling did not exceed 20°C mm ~ above 900°C and 60°C mm 1 below 900 °C.Such treatment resulted in a high quality surface with small amounts of structural defects. The HOPG substrate was cleaved in laboratory air and immediately transferred into the vacuum chamber. For comparison, we also deposited the gold clusters by laser ablation from the gold target. The purity of target material was better than 99.99%. The same gold tip was used for the deposition procedure and STM measurements. It was prepared by electrochemical etching in 30% HC1. Before mounting to UHV STM, each tip was tested on HOPG in air. Our measurements were carried out with the Digital Instruments (DI) Nanoscope III hardware and software with the McAllister UHV STM head. The
Elsevier Science S.A.
174
A. Wawro et al. / Au clusters on Si(111) and graphite
topography of samples was measured in the constant current mode. Current image tunneling spectroscopy (CITS) was performed in the constant tip—substrate distance mode.
McSTM
nage
3. Results and discussion We show first an example of spectroscopic investigations of gold clusters. Knowledge of the electronic struc-
CITS
nage
0
-
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A. Wawro et aL
/
Au clusters on Si(111) and graphite
ture of such small particles is indispensable for designing electronic devices with a very large degree of miniaturization. A wider discussion on these properties will be presented soon [9]. Fig. 1 depicts the topography image and two current maps for polarization 0.5 V (Fig. 1(a)) and 1.2 V (Fig. 1(b)) of gold deposits on the graphite surface. The clusters are bumps, so they are visible as white spots. Current maps display the intensity of tunneling current flowing between the STM tip and surface for a given bias voltage. Because CITS measurements were carried out in the constant tip—substrate distance mode, one can assume that the barrier parameters, i.e. barrier height and width, for each I—V characteristic measurement are the same. As a consequence, the current image strongly reflects the density of electronic states of the analyzed surface. It is easy to notice that strong correlation between topography image and current image occurs. For a bias of 0.5 V, the places in the current map are brighter in the cluster positions, suggesting higher intensity of tunneling current. The opposite situation occurs for a bias of 1.2 V, i.e. the intensity of tunneling current flowing to the clusters is lower in Height
175
comparison with the graphite substrate. Therefore, one can conclude that the density of empty electronic states of analyzed gold clusters is lower at 1.2 eV and higher at 0.5 eV above the Fermi level, relative to that of the graphite substrate. We have observed similar effects on gold clusters deposited on Si(11l) [9]. (Disappearance and re-appearance of clusters in the topography STM image is a consequence of the dependence of specific density of states versus energy above Fermi level.) However, in this paper we focus on the controlled cluster positioning on the substrate. Two parameters are very important: the coordinates of clusters on the surface and the size of clusters that have to be reproducible very well. We decided to apply voltage pulses between the STM gold tip and the substrate to position the clusters. Such technique allows us to deposit clusters of desired size and location. However, the first attempts provided unsatisfactory results. Fig. 2 depicts the group of clusters, close to one of the monoatomic steps, on the Si( 111) surface obtained after applying a single bias pulse. The clusters are distributed randomly on an area of ca. 25 nm diameter and their shapes and size are not uniform.
Angle
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04011758. 001 Height Fig. 2. A group of gold clusters deposited after accidental single voltage bias pulse applied between the STM tip and the substrate. The scanned 2. area is 50 x 50 nm
176
A. Wawro et al.
/
Au clusters on Si(111) and graphite
Looking for a more reproducible procedure, we used the ‘lithography’ option in the Nanoscope III software. This option allowed us to change voltage pulse parameters, a tip movement and position, etc., by writing and executing a special script program. We noticed that a crucial influence is associated with such parameters as: the value of voltage bias, duration of the pulse, distance between the tip and substrate, speed of approach and withdrawal of the tip to and from the substrate (before and after pulse). The intensity of tunneling current has less importance. Finally, the procedure of cluster deposition was found as follows. First, the tip was approached to the substrate. After a certain time, to stabilize the tip position over the substrate, the voltage pulse (+ 2 V, the sign, as applied to the sample) was applied. In the next step, the tip was withdrawn from the substrate and was moved slightly over it. This cycle was repeated three times, but the applied voltage pulse was systematically increased (up to 5 V). It seems that especially approaching the STM tip towards substrate was of great importance. It is plausible that the close distance between the tip and the substrate establishes bonding between
Height
atoms of the tip and the substrate due to chemical forces developing. The electrostatic forces, arising during voltage pulse duration, gain control over the atom transfer from the tip to the substrate. Examples of periodic structures arranged in F shape and in double rows are presented in Figs. 3 and 4. Each cluster deposited in the way described above is visible as a single bump. Their shapes are regular with well-defined geometry. The section analysis (Fig. 5) provides the details of individual lengths characterizing the row of clusters shown in Fig. 3. The diameter of clusters is very well repeatable and kept about 3.3 nm (horizontal distance between AA markers). The height of each cluster is approximately the same (ca. 0.37 nm, vertical distance between BB markers), being comparable to two atomic diameters. Also, Fourier analysis of the profile shape was performed. The calculated spectral period is 10.08 nm. We emphasize that our intention was to deposit clusters in a row every 10 nm. This means that we have succeeded in this procedure with an accuracy better than 0.1 nm. The procedure presented above was highly reproducible, with only few exceptions. Probably the deposition conditions
Angle
50.0
___________________________
250
10.5 n~i
I::::
Microscope _____ ____
7’
0
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Fig. 3. Periodic structure of gold clusters deposited on Si(lil). The scanned area is 50 x 50 nm
A. Wawro et al. / Au clusters on Si(111) and graphite
177
Microscope NSlll Mc_STM Scan size 83.1 flM Setpoint 307.5 pA Bias 1.7 U Scan rate 5.3 Hz NuMber of’ saMples 256 Data type Height 2 range 1.0 mi
view angle
—
light angle
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-
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-
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Fig. 4. The gold clusters arranged in two five-dots rows. The scanned area is 80 x 80 nm
are strongly influenced by the tip shape. If the apex of tip is smooth enough, the electric field generated by bias pulse is too low and deposition is hampered. The evidence of this is that after a certain time of applying the procedure described above, the deposits did not occur on the substrate. Another feature, which favours such deposits for any applications is their stability in time. In particular, the Au/Si system is promising, because mutual solubility of constituents is negligible (very small interdiffusion) and no compounds exist [10]. Fig. 6 depicts the group of gold dots on silicon substrate arranged in the inscription ‘IMR’ (Institute for Material Research). It is an another example of the possibility of locating clusters in desired ways. The images were taken just after deposition (Fig. 6(a)) and 3 days later (Fig. 6(b)). The same IMR pattern and monoatomic steps are also visible. Evidently such structure is very stable in ultrahigh vacuum at room temperature. The number of dots and the shapes of both patterns are the same. Only the contrast of the image taken 3 days after deposition is worse, which we ascribe to contaminations that have covered the surface during that time.
Another evidence that mobility of gold clusters on silicon substrate is very low is presented in Fig. 7. The clusters deposited from the Au target by laser ablation are located randomly on the whole surface. It seems that there are no agglomerations of clusters either on structural defects or along the edges of monoatomic steps. Such distribution is a result of strong chemical bonding and low surface diffusion at room temperature. Fig. 8 depicts the surface of the sample after a few cycles of deposition and thermal heating up to 1250 °C. Here, contrary to the fresh surface before deposition, contaminations or other than 7 x 7 surface reconstructions are visible. They are located as narrow strips along the edges of monoatomic steps and also form lines perpendicular to the substrate steps. This later location is most probably determined by the existence of linear structural defects of the substrate. Observations of fresh substrate under the same conditions have not revealed the existence of such formations. Consequently, one can eliminate that such contaminations are deposited from vacuum. Most probably, the Si—Au chemical bonding is so strong, that thermal heating above melting point of bulk gold does not
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Fig. 5. The cross-section profile of the gold cluster pattern from Fig. 3.
remove the whole Au material previously deposited on Si substrate. Part migrates probably on the surface to places such as steps and surface defects where they find the minimum of energy. Of course, subsequent annealing cycles lead to a more and more clean surface with the 7 x 7 reconstruction. Similar thermal treatment of the HOPG sample with gold clusters deposited by laser ablation and annealed up to 600 °C, results in two phenomena. First, the clusters exhibit a tendency to coalescence (the cluster dimensions are increasing) and secondly, the clusters diffuse to steps on HOPG sample as shown in Fig. 9. The explanation is as follows: the clusters weakly bonded to the HOPG surface via van der Waals forces only, diffuse easily on the HOPG surface at elevated temperatures and coalesce into bigger structures. Finally, they are adsorbed along steps, where the probability of finding unsaturated bonds is much higher than anywhere else on the HOPG surface. The dependence of gold cluster height versus cluster diameter is shown in the Fig. 10. The circles at the origin of this graph reflect the height and diameter of clusters obtained just after laser ablation
on HOPG substrate. Typical heights are up to 0.5 nm and diameters up to 4 nm. After thermal treatment, the coalesced clusters (or rather gold islands) may have diameter up to 15 nm and height up to 2.5 nm. The reason why some exhibit a dome or ‘flat’ shape is worthy of further studies.
4. Summary We have analyzed the possibility of creating regular, periodic formations of small clusters. The deposition from the STM tip enables us to position the gold clusters with desired size and location with very high accuracy. Analyzed clusters are very stable and observable at room temperature in ultrahigh vacuum for several days. The I—V characteristics measured above small gold clusters and CITS show characteristics different from the bulk material. These features of the gold clusters may find application in devices designed with a very high degree of miniaturization.
A. Wawro et al. Height
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(b) Fig. 6. The ‘IMR’ inscription 40 nm in height after deposition (top) and 3 days later (bottom).
179
180
A. Wawro et al.
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09211611.001 Height
Fig. 8. The STM image of Si( Ill) surface with gold clusters (deposited by laser ablation) after a few cycles of thermal heating up to 1250 C. The scanned area is 400 x 400 nm2.
A. Wawro et al.
Height
/
Au clusters on Si(11l) and graphite
181
Angle
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_______
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Fig. 9. Gold cluster distribution along steps on HOPG sample after sample annealing at 600 C for 15 mm. The scanned area is 100 x 100 nm
I
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Acknowledgements This work was supported by the New Program from the Ministry of Education, Science and Culture in Japan, and in part by Poznafl Technical University within the statutory activity under DS 62-100 Project.
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[4] P.N. First, J.A. Stroscio, R.A. Dragoset, D.T. Pierce an~ R.J. Celotta, Phys. Rev. Lett., 63 (1989) 1416. [5] M.F. Crommie, C.P. Lutz and D.M. Eigler, Nature, 363 (1993’ 524. [6] D.M. Eigler and E.K. Schweizer, Nature, 344 (1990)524. [7] J.A. Stroscio and D.M. Eigler, Science, 254 (1991) 1319. [8] Ph. Avouris and 1W. Lyo, Appl. Surf. Sci., 60/61 (1992) 426. [9] A. Wawro, R. Czajka, A. Kasuya and Y. Nishina, unpublished. [10] H. Okamoto and TB. Massalski, in T.B. Massalski (ed.), Binart’ Alloy Phase Diagrams, American Society for Metals, Metals Park, OH, p. 312.