Non-contact atomic force microscopy study of atomic

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Non-contact atomic force microscopy study of atomic manipulation on an insulator surface by nanoindentation

This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2006 Nanotechnology 17 S142 (http://iopscience.iop.org/0957-4484/17/7/S07) View the table of contents for this issue, or go to the journal homepage for more

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INSTITUTE OF PHYSICS PUBLISHING

NANOTECHNOLOGY

Nanotechnology 17 (2006) S142–S147

doi:10.1088/0957-4484/17/7/S07

Non-contact atomic force microscopy study of atomic manipulation on an insulator surface by nanoindentation Ryuji Nishi, Daisuke Miyagawa, Yoshihide Seino, Insook Yi1 and Seizo Morita Division of Electrical, Electronic and Information Engineering, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan E-mail: [email protected]

Received 23 August 2005, in final form 21 October 2005 Published 10 March 2006 Online at stacks.iop.org/Nano/17/S142 Abstract Experimental results on vertical manipulation on an insulator surface using non-contact atomic force microscopy are presented. Cleaved ionic KCl(100) single crystal is used as an insulator surface. With the nanoindentation method used, the vertical manipulation of a single atom in an ionic crystal surface is more difficult than in a semiconductor surface. Therefore, in many cases, more than one surface atom is manipulated while, in rare cases, single-atom manipulation is successfully performed. Lateral manipulation of a vacancy has occasionally succeeded on the KCl(100) surface. We have presumed that the lateral manipulation was induced by pulling. (Some figures in this article are in colour only in the electronic version)

1. Introduction To achieve a breakthrough in the miniaturization of future nanodevices, we need new development of technology. Atom manipulation techniques that can manipulate individual atoms or molecules have such potential, that may overcome the limitation of today’s lithography. Atom manipulation expands atom assembly techniques as bottom up processes. Insulating materials are necessary for the measurement of electronic properties of nanoelectronic devices made on a surface. Non-contact atomic force microscopy (NC-AFM) has shown great potential not only for conductive surfaces, but also for insulator surfaces with atomic resolution [1]. During the last few years, atom manipulation techniques rapidly progressed in the NC-AFM field. In 2003 mechanical vertical manipulation was achieved on semiconductor surfaces of Si(111)-(7×7) [2]. It used nanoindentation by an atomic force microscope operating at low temperature and manipulated selected single atoms. The strong repulsive chemical force interaction between a tip and a sample during soft nanoindentation leads to the removal of selected Si adatoms. It can also deposit a Si atom. In 2005 lateral 1 Author to whom any correspondence should be addressed.

0957-4484/06/070142+06$30.00 © 2006 IOP Publishing Ltd

manipulation on Ge(111)-c(2 × 8) was also reported [3]. This manipulation moved adsorbed Ge atoms laterally using low temperature NC-AFM by scanning a tip at a little larger frequency shift. In 2005, by manipulation, an atom inlay was also fabricated at room temperature on a Sn/Ge(111)-c(2 × 8) surface and resulted in construction of the embedded atom letters ‘Sn’ [4]. In spite of this remarkable progress in manipulation using AFM, all the successful manipulations were demonstrated only on semiconductor surfaces. As is well known, such atomic manipulation has not been demonstrated on insulator surfaces. In principle, atomically resolved imaging of insulator surfaces by NC-AFM is more difficult than that of semiconductor surfaces. However, during the last decade, one of the insulator surfaces, cleaved alkali halide surface, has been frequently observed by (NC-AFM) [5–8]. Such surfaces can be easily prepared by cleaving single crystals. For the ionic crystal surface, wear experiments were reported [9, 10], in which debris or mounds were made by scratching the KBr(100) surface using a lateral force microscope. The scratching of the surface was done with a strong force of over 20 nN. Here, we report vertical manipulation on an insulator surface of ionic crystalline KCl(100) using a similar technique

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NC-AFM study of atomic manipulation on an insulator surface by nanoindentation

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Figure 1. Topography images of a KCl(100) surface (a) before and (b) after nanoindentation (12 nm × 12 nm). Circles show the position marker. Arrows show nanoindentation points. (Resonance of the free cantilever oscillation at f 0 = 163.960 kHz, spring constant k = 30 N m−1 , oscillation amplitude A = 6 nm, frequency shift  f = −35 Hz, normalized frequency shift γ = −2.9 fN m1/2 ). (c) Before and (d) after nanoindentation (8 nm × 8 nm). ( f 0 = 163.960 kHz, k = 30 N m−1 , A = 6 nm,  f = −34 Hz, γ = −2.9 fN m1/2 .)

of soft nanoindentation [2]. Imaging mechanisms for semiconductor and insulator surfaces are quite different. The difference between the covalent bonding nature of the semiconductor (Si–Si: 2.32 eV/bond) and the ionic bonding nature of the ionic crystal (K–Cl: 7.2 eV/ion pair) leads to difficulty in atomic manipulation of ionic crystal. Most recently, the Reichling group reported lateral manipulation of atomic size defects on CaF2 (111) surfaces [11]. They showed chain-like features with a periodicity of the CaF2 surface lattice indicating stick–slip motion. We report similar results of lateral manipulation on the KCl(100) surface.

2. Experiment All experiments were performed in ultrahigh vacuum (UHV) at room temperature using a non-contact atomic force microscope with the frequency modulation method. The cantilever oscillation was kept at constant amplitude. The tip–sample distance was controlled to keep the frequency shift constant. The system has been described in detail in [12]. We used a silicon cantilever (provided by Nano-world) with a spring constant of about 30 N m−1 and a resonant

frequency of about 160 kHz. The oscillation amplitude of the cantilever is at a constant value between 6 and 12 nm in each measurement. The quality factor of cantilever is typically 10 000. A tip is sputtered with Ar ions in UHV to clean the tip apex, removing the natural oxide layer. During experiments, the voltage of the tip was fixed at ground level and zero bias was applied to the sample holder because long range electrostatic force is compensated and the compensation voltage is equal to zero in most cases, because if a tip accidentally touches a sample surface with non-zero voltage, the surface will be charged up. An atomically flat KCl(100) surface can be easily prepared by cleaving a single crystal in air, followed by quick transfer to the vacuum chamber and heating it in ultrahigh vacuum at about 400 K for 3 h, for neutralizing the surface charge caused by cleavage. Before manipulation, we confirmed the clean surface. After imaging it, nanoindentation was performed. The cantilever was oscillated in the constant amplitude mode. A distance feedback control was turned off before nanoindentation. A tip gradually approached a sample surface with a constant speed of 0.74 Å s−1 . During the approach, S143

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Figure 2. Topography images of a KCl(100) surface (a) before and (b) after nanoindentation (6 nm × 6 nm). Arrows show the nanoindentation point for single-atom manipulation. (c) Cross-sections of lines in (a) (solid line) and (b) (dotted line). ( f 0 = 163.960 kHz, k = 30 N m−1 , A = 6.5 nm,  f = −43 Hz, γ = −4.1 fN m1/2 .)

the frequency shift, dissipation and amplitude were monitored. When the tip excitation voltage or dissipation increased rapidly, the tip was quickly retracted. Then the distance control feedback was turned on. The excitation voltage typically increased about 1.5 times during nanoindentation. Even under such conditions, the oscillation amplitude is stable and does not show any transient behaviour in most cases. After nanoindentation, the NC-AFM image was again obtained at the same frequency shift value. If the image was not changed, another nanoindentation was performed until the tip–sample distance became closer. In the constant amplitude mode, the energy dissipation caused by the tip–sample interaction (Pts (eV/cycle)) can be estimated from the excitation amplitude Aexc (V), the initial excitation amplitude in the no-interaction region Aexc,0 (V) and the oscillation amplitude A (m) as follows: [13]   Aexc πk A2 Pts = −1 (1) e Q 0 Aexec,0 where k (N m−1 ) is the spring constant of the cantilever, e (C) S144

is the elementary electric charge and Q 0 is the quality factor of the freely oscillating cantilever.

3. Results and discussion 3.1. Vertical manipulation of several atoms We attempted to manipulate surface atoms on the ionic crystal surface by the soft nanoindentation method. First, we searched for a marker to find the thermal drift effect. Soft nanoindentation was performed by the method described above. A tip approached toward a sample surface until the excitation signal became 1.5 times larger than that at the free oscillation distance. Figure 1 shows the situation (a) before and (b) after the nanoindentation. Black circles show the marker. Several atoms are removed around the nanoindentation point shown by the arrow. The depth is about 1.0 Å. The shape is like a square in figure 1(b), shown by a dotted line, because the number of removed anions and cations tends to be almost the same in the square. At the edge of the square, atomic corrugation is slightly emphasized [14].

NC-AFM study of atomic manipulation on an insulator surface by nanoindentation

In some cases, removed atoms are adsorbed at neighbouring sites after nanoindentation like in figure 1(c) before and (d) after the nanoindentation. In this case a tip approached a sample surface until the excitation signal became 1.4 times larger than the excitation value at the free oscillation distance. Several atoms were manipulated near the nanoindentation point shown by the arrow. The protruded area neighbouring the indented point of the dented area (depth about 0.7 Å) became as high as 0.7 Å. This is re-adsorbed atoms that were removed by the nanoindentation.

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3.2. Vertical manipulation of a single atom

3.3. Force curve during nanoindentation In the room temperature AFM, thermal drift is inevitable to some extent. When the tip approaches the sample surface, the position where the nanoindentation actually occurs is different from the originally aimed for position. One of the reasons for the difficulty of single-atom manipulation is such thermal drift. We introduced thermal drift compensation software. It simply works to generate constant increments of voltages to compensate for thermal drift for each x and y direction. The voltages generated are added to scanning x and y signals. Increment parameters for x and y are independently adjusted manually, by looking at the image while scanning. With thermal drift compensation, atom manipulation is performed by soft nanoindentation. Figure 3(a) is the result of vertical manipulation by nanoindentation. The circles are shown in bright spot positions. An arrow shows the nanoindentation point, which is very near where the bright spots disappeared. It shows that one (dashed-line circle) or three (dotted-line circle also) atoms (ion pairs) on bright spot positions disappeared. With removal of an odd number of ions, charge is not balanced. Near the site of the removal, ions affected neighbouring sites. Though one ion is removed, neighbouring ions may move slightly or may look lower. Some of the surrounding removed sites are imaged a little brighter than other sites due to charge imbalance.

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In many nanoindentation experiments, single-atom manipulation rarely took place. Figure 2 shows the result of single-atom removal on an ionic crystal surface. Figures 2(a) and (b) are the topography images before and after manipulation, respectively. A single bright spot vanished upon nanoindentation. Line profiles along the lines in these images are shown in figure 2(c). It shows a depth of about 0.4 Å at the single-site defect. This indicates that vertical manipulation of a single site has been performed successfully. The single site is considered to be a single atom because the topography image shows a symmetrical shape. The position of the nanoindentation was near the centre of the image, but the position of the defect manipulated is not at the centre of the image. This difference in positions is caused mainly by thermal drift. Although single-atom manipulation is an attractive technique for making an atomic scale artificial structure, it has been very difficult so far. At present, we cannot control the single-atom manipulation even in the same dissipation value because the tip-apex condition varies.

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Figure 3. Topography image of a KCl(100) surface after nanoindentation (4 nm × 4 nm). The solid-line circles indicate the lattice points. An arrow shows a position of nanoindentation. The dashed-line circle shows the deepest lattice point. (b) During nanoindentation, the frequency shift (solid squares), dissipation (dots) and calculated force curves (black line) versus z distance. The origin for the z distance is decided by having the imaging distance equal to zero. ( f 0 = 163.429 kHz, k = 29 N m−1 , A = 12 nm,  f = −15 Hz, γ = −3.5 fN m1/2 .)

Figure 3(b) shows force, dissipation and frequency shift versus distance curves, during nanoindentation. The force curve is converted from a frequency shift curve according to [15]. The origin of the z distance axis is set to a normal imaging tip–sample distance. This nanoindentation is done with a small dissipation value of 1 eV/cycle and a repulsive frequency shift of 100 Hz. The maximum repulsive force is about 4 nN. These values show typical dissipation and force in this nanoindentation method. 3.4. Lateral manipulation of vacancy When the imaging area has a vacancy and the tip condition is suitable for manipulating atoms, vacancy motion due to scanning of the tip motion was observed. The image is shown in figure 4(a). The slow scanning direction is from the bottom to the top of the image and the fast scanning direction is from S145

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point (shown by a sphere). This suggests that a neighbouring atom is pulled when a tip is on a vacancy of the lattice point. Figure 4(c) shows the explanation model for pulling the atom from a neighbouring site to a vacancy site [16, 17]. Upon one of the scanning strokes, the atom is presumably pulled in a lower right direction in the image into the vacancy position. This result suggests lateral manipulation of a single vacancy on an ionic crystal surface. This result is similar to those for CaF2 (111) surfaces of Hirth [11]. In the case of figure 4(a), we consider that the dark spot is more likely to be a vacancy because its depth is large (about 1.5 Å). The probability that the dark spot is an impurity atom is small. If that is the case, the above-mentioned mechanism is inverted, i.e. the impurity atom was pushed by the tip, not pulled.

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4. Conclusion We have succeeded in carrying out atomic vertical manipulation on an ionic crystal surface by the nanoindentation method. The indentation depth is controlled to some extent by monitoring the excitation voltage. A few atoms are frequently removed by nanoindentation with precise distance control. Single-atom manipulation has also succeeded. We also introduced thermal drift compensation software for precisely approaching a target point. The force during the nanoindentation is calculated from the frequency shift curve at room temperature. This shows that the force for atom removal is about 4 nN. Lateral manipulation of a vacancy is occasionally achieved. This phenomenon could be combined with vacancy fabrication by vertical manipulation; then artificial atomic size structure might be constructed in the future.

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Figure 4. (a) Topography image of a KCl(100) surface (7.6 nm × 7.6 nm). In the right upper part, the vacancy moves along the tip scan (dotted arrow). The slow scanning direction is from bottom to top and the fast scanning direction is from right to left. (b) Line profile of the black solid line in (a) and 12 times average line profile of equivalent lines for the dotted line in (a). (c) Shows a model of a KCl(100) surface which explains the pulling mode manipulation (larger spheres are brighter sites and the smaller spheres are darker sites). ( f 0 = 164.875 kHz, k = 30 N m−1 , A = 7.5 nm,  f = −10.5 Hz, γ = −1.2 fN m1/2 .)

the right to the left of the image. The vacancy moves in the direction to the left and upward, shown by a black dotted line with an arrow. The line profile along black solid line shows the trajectory of the vacancy, like stick–slip motion. The trajectory bends in the middle, which suggests the existence of some defect at the corner of the trajectory. In figure 4(b), a dotted line shows an average line profile (right axis) along 12 lines on the normal lattice that are equivalent to the dotted line in figure 4(a). The black solid line increases steeply at a lattice S146

We would like to thank to Kenichi Morita for provision of the force calculating software. This work was supported by Handai Frontier Research Centre. Financial support was obtained from the Ministry of Education, Culture, Sports, Science and Technology.

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NC-AFM study of atomic manipulation on an insulator surface by nanoindentation

[10] Socoliuc A, Gnecco E, Bennewitz R and Meyer E 2003 Phys. Rev. B 68 115416 [11] Hirth S, Ostendorf F and Reichling M 2006 Nanotechnology 17 S148–54 [12] Yokoyama K, Ochi T, Uchihashi T, Ashino M, Sugawara Y, Suehira N and Morita S 2000 Rev. Sci. Instrum. 71 128

[13] Gotsmann B, Seidel C, Anczykowski B and Fuchs H 1999 Phys. Rev. B 60 11051 [14] Bennewitz R, Pfeiffer O, Sch¨ar S, Barwich V, Meyer E and Kantorovich L N 2002 Appl. Surf. Sci. 188 232 [15] Sader J E and Jarvis S P 2004 Appl. Phys. Lett. 84 1801 [16] Bartels L, Meyer G and Rieder K-H 1997 Phys. Rev. Lett. 79 697 [17] Pizzagalli L and Baratoff A 2003 Phys. Rev. B 68 115427

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