Visualization of Electron Orbitals in Scanning Tunneling Microscopy

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1 ISSN , JETP Letters, 2014, Vol. 99, No. 12, pp Pleiades Publishing, Inc., Original Russian Text A.N. Chaika, 2014, published in Pis ma v Zhurnal Eksperimental noi i Teoreticheskoi Fiziki, 2014, Vol. 99, No. 12, pp Visualization of Electron Orbitals in Scanning Tunneling Microscopy A. N. Chaika Institute of Solid State Physics, Russian Academy of Sciences, Chernogolovka, Russia Centre for Research on Adaptive Nanostructures and Nanodevices, School of Physics, Trinity College Dublin, Dublin 2, Ireland chaika@issp.ac.ru Received May 5, 2014 Scanning tunneling microscopy (STM) is one of the main techniques for direct visualization of the surface electronic structure and chemical analysis of multi-component surfaces at the atomic scale. This review is focused on the role of the tip orbital structure and tip-surface interaction in STM imaging with picometer spatial resolution. Fabrication of STM probes with well-defined structure and selective visualization of individual electron orbitals in the STM experiments with controlled tunneling gap and probe structure are demonstrated. DOI: /S INTRODUCTION Scanning tunneling microscopy (STM) [1, 2] is one of the most powerful techniques for atomically resolved studies of the surface electronic and magnetic structure [3 5], chemical analysis of surfaces at the atomic scale [6, 7], and fabrication of nanostructures from individual atoms and molecules [8 12]. Scanning tunneling microscopy is based on the effect of quantum tunneling of electrons through the vacuum gap separating the probe apex and the surface. The probe of the scanning tunneling microscope is mounted on the piezoscanner that allows positioning the tip relative to the surface with picometer precision. At nanometer distances between the tip and sample atoms, electrons can tunnel through the vacuum gap from occupied surface states to unoccupied tip states and vice versa depending on the polarity of the bias voltage applied between the tip and the sample (Figs. 1a and 1b). The tunneling current I can be determined using the equation [15]: I ev = 4πe ρ t ( E F ev + ε)ρ s ( E F + ε) M 2 dε, (1) h 0 where ρ t (E) and ρ s (E) are densities of electron states (DOS) of the tip and the sample, respectively; V is the bias voltage, M is the tunneling matrix element, and E F is the Fermi energy. 1 The article was translated by the author. The probability of the electron tunneling depends exponentially on the tip-sample distance d: M 2 e kd, (2) 2mφ k = , (3) where m is the electron mass, is the Planck s constant, and φ is the electron work function. Estimations at typical values of the work functions in metals and semiconductors (4 5 ev) show that tunneling current drops by approximately one order of magnitude with every 1 Å increase in the tip-sample distance. Because of the exponential dependence, most of the tunneling current flows through only two tip and surface atoms closest to each other. Change of the tunneling current during the tip motion across the sample allows imaging the surface atomic and electronic structure with extremely high lateral and vertical resolution which can be about several picometers [16] and smaller than 1 pm [17], respectively. Nevertheless, the questions related to the limit of the spatial resolution and physical origins of the features in high resolution STM images are still open. If the tunneling current is collected by only one atom at the tip apex, the limit of the spatial resolution is determined by the electronic structure of this particular atom. Usually, the sum of different atomic orbitals produces symmetric charge density distribution around the tip apex atom. The approximation of the spherically symmetric tip [18, 19] is generally used in simulations of atomically resolved STM images. When certain orbitals of either tip [13, 20 22] or surface [23] atoms dominate in the total DOS, their direct visual- 731

2 732 CHAIKA d z 2 d z 2 Fig. 1. (Color online) (a, b) Schematic view of the electron tunneling (a) from occupied tip states to empty surface states and (b) from filled surface states to unoccupied tip states [13]. (c) Reciprocity principle of STM [14]. Convolution of d xz tip state and surface electron states is equivalent to convolution of tip orbital and d xz surface orbitals. d z 2 d z 2 ization can be achieved in STM experiments with subångström lateral resolution. According to Eq. (1), tunneling current depends symmetrically on the tip and surface DOS in the selected energy region. This is the basis of the reciprocity principle of STM [14] illustrated by Fig. 1c. Atomically resolved STM images bring the information about the orbital structure of the tip and surface atoms simultaneously. As a result, the convolution of the tip and surface electron states is not modified if the tip and surface orbitals are interchanged, as Fig. 1c illustrates. If nonzero m electron states dominate at the tip apex, contrast inversion and asymmetric features with several subatomic maxima can be observed in STM experiments [24, 25]. The subatomic features can correspond to direct visualization of the tip electron orbitals using the surface atomic orbitals. Interpretation of the high resolution STM images can further be complicated because of modification of the tip and surface electronic structure at small ( Å) tunneling gaps [26 30]. According to the theoretical calculations performed for several tip-sample systems [13, 26, 30], the partial DOS is stronger modified for m = 0 electron states which are further protruded into vacuum along the tip axis (p z, d 2 z ). This review is focused on the role of the tip electronic structure and tip sample interaction in STM experiments with sub-ångström spatial resolution. We discuss the advantages of single crystalline probes with well-defined structure for high resolution surface studies and selective visualization of certain electron orbitals in STM experiments. 2. SINGLE CRYSTALLINE TUNGSTEN PROBES FOR HIGH RESOLUTION STM IMAGING Because of exponential distance dependence, most of the tunneling current is usually collected by just one, closest to the surface atom at the tip apex. This allows atomically resolved imaging on low-index metal and semiconductor surfaces even using nonideal probes with large radii of curvature. The shape and atomic structure of the probe are especially important for STM experiments on high-index (stepped) surfaces. According to Eqs. (1) (3), in the case of ideal pyramid at the tip apex up to 90% of the tunneling current can flow through the tip atom closest to the surface. Therefore, precise control of the apex structure can provide detailed information about the electron states responsible for the STM imaging. Electrochemically etched polycrystalline tungsten probes are generally utilized in STM experiments [31]. After chemical etching they can be cleaned and sharpened in ultrahigh vacuum (UHV) using high temperature annealing [32] or ion sputtering [33 36]. The tip apex can also be prepared for high resolution STM imaging using the electric field applied between the tip and the sample [37 43]. This method is based on growth of nanotips on tungsten probes at high (~10 V) negative sample bias voltages [42]. In a number of works polycrystalline tungsten probes were functionalized by molecules and atoms of light elements (carbon, oxygen, hydrogen) [44 55]. These functionalized probes can provide extremely high resolution in STM experiments but they are usually not stable at small tunneling gaps [49] and possess unknown apex structure. Polycrystalline tungsten probes are preferentially oriented along the 011 crystallographic directions [33]. Nevertheless, the precise apex structure cannot be granted in experiments with polycrystalline tips because of formation of several nanotips at the apex [33]. Monotips with well-known orientation can be fabricated using single crystalline refractory metal ingots [34, 35, 37, 56]. Tungsten probes for STM experiments presented in this review were fabricated from [001]- and [111]-oriented single crystalline bars ( mm) cut from high purity crystals using the spark cut. The monotips with small radii of curvature at the apex were obtained using electrochemical

3 VISUALIZATION OF ELECTRON ORBITALS 733 Fig. 2. Transmission electron microscopy images of the W[001] probe after electron beam heating and co axial ion sputtering [22]. (a) Bright field image of the apex. (b) Diffraction pattern measured from the tip apex. (c, d) Dark field images demonstrat ing that apex is grained by the {011} and {001} planes. etching in 2M NaOH solution with controllable cut off adjusted by the current jump. The length of the ingot immersed into the etchant was in the range of mm while the voltage between two electrodes was in the range of V. These parameters allow fabricating tungsten tips with minimal radii of curva ture [31]. The etched tungsten tips were further cleaned from residual oxides and sharpened in situ in the UHV chamber of the scanning tunneling micro scope. Figure 2 shows the results of the transmission elec tron microscopy (TEM) characterization of the W[001] tip apex [22]. The bright field TEM image (Fig. 2a) proves the formation of the nanoscale pyra mid at the tip apex after electron beam heating and coaxial ion sputtering. The electron diffraction pattern taken from the apex (Fig. 2b) exhibits reflexes, typical for single crystalline tungsten. The dark field TEM images (Fig. 2c and 2d) taken from the diffraction spots C and D in Fig. 2b demonstrate that the tip is grained by the {001} planes further from the apex (Fig. 2d) and has an angle of 90 at the apex that cor JETP LETTERS Vol. 99 No responds to the pyramid grained by the {011} planes. The split reflexes in Fig. 2b indicate the presence of slightly misoriented single crystalline blocks at the apex. This misorientation is presumably caused by the stress applied to the apex at the final stage of the chem ical etching and does not exceed several degrees. 3. CHANGE OF THE ELECTRON ORBITAL CONTRIBUTION AT SMALL DISTANCES BETWEEN THE W[001] TIP AND HOPG(0001) According to the TEM studies [22], the W[001] probe apex is stable in tunneling regime even at very small gaps when STM images can reveal subatomic orbital features. The stability of the tip apex allows higher spatial resolution on the surfaces with compli cated atomic structure [57, 58] and simplifies interpre tation of the obtained STM data. Figure 3 demon strates possibility to control the relative contribution of the tip electron orbitals in STM experiments with oriented single crystalline tungsten probes [13, 21, 22]. Scanning tunneling microscopy images obtained with

4 734 CHAIKA d z 2 Fig. 3. (Color online) (a) Schematic model of a W[001] probe interacting with a graphite (0001) surface. (b) Å2 STM images of the tungsten tip atom electron orbitals measured using carbon atomic orbitals of HOPG(0001) at V = 0.1 V, I = 0.7 na (left panel), V = 35 mv, I = 7.2 na (central panel), V = 0.1 V, I = 1.8 na (right panel). (c) Partial DOS associated with d orbitals of the W[001] tip atom at different tip surface distances (indicated on each panel). Reprinted from [21] with permission from Elsevier. (d) Gap resistance dependence of 7 7 Å2 STM images of a graphite surface measured with the W[001] probe at fixed sample bias voltage V = 35 mv (currents are indicated on each frame). Reproduced from [13] with permission from EPL and IOP Publishing. the W[001] probes on highly oriented pyrolytic graph ite (HOPG) surface (Fig. 3b) qualitatively reproduce the shapes of the electron d orbitals with different momentum projections ( d z2, dxz, and dxy). The sub atomic resolution achieved in the experiments [13, 22] corresponds to direct visualization of the tungsten tip atom d orbitals using atomic orbitals of the graphite surface in accordance with the reciprocity principle of STM [14], as Fig. 3a illustrates. The gap resistance dependences of the HOPG(0001) STM images mea sured with several W[001] probes [13] demonstrate that subatomic orbital features can be resolved only at small tunneling gaps (Fig. 3d). At small currents (large tip sample distances) typical HOPG(0001) STM images with hexagonal symmetry are resolved. This may correspond to imaging one of two non equivalent surface atoms [59] or the hollow sites [60]. With increasing current (decreasing distance) symmetric atomic features transform into asymmetric ones with two, three, and four subatomic maxima. The effect was reproducibly observed in a series of experiments with different W[001] probes sharpened in UHV using the same procedure. The electron microscopy studies revealed that the W[001] tip was not substantially modified after STM experiments with subatomic reso lution [22] confirming the pyramidal model of the W[001] tip shown in Fig. 3a. The transformation of the subatomic features was reproducibly observed in very narrow intervals of the gap resistances (tip sample distances). In accordance with Eqs. (1) (3), an increase in the tunneling current from 2.7 to 9.1 na at fixed bias voltage (Fig. 3d) corresponds to change of the tip sample distance in the range of Å. This result emphasizes exceptional importance of pre cise control of the tunneling gap resistance for improv ing spatial resolution and selective imaging of the elec JETP LETTERS Vol. 99 No

5 VISUALIZATION OF ELECTRON ORBITALS 735 Fig. 4. (Color online) (a c) Isosurface of the change in electron density for the interacting (a) He W[011], (b) He W[111], and (c) He W[001] systems. The helium atom is positioned directly below the apex atom, with the tip sample distance d = 4.0 Å for the W[011] tip and d = 3.5 Å for the W[111] and W[001] tips. (d f) Constant-height slices through the DOS corresponding to the systems shown on panels (a c). The positions of the tungsten tip atoms in the first two layers are drawn as black circles. Reproduced from [28] with permission from IOP Publishing. tron states with different symmetry using STM. Note that transformation of symmetric atomic features into asymmetric ones was not observed in our STM experiments with [111]-oriented tungsten probes. This can be related to different electronic structure of the W[001] and W[111] probes interacting with the graphite surface. Similar subatomic features with two-, three-, and four-fold symmetry were observed in unique non-contact atomic force microscopy (AFM) experiments [61]. The subatomic features were explained by direct visualization of the orbital structures of [011]-, [111]-, and [001]-oriented apexes formed on the polycrystalline tungsten probe during the AFM experiments. The STM images measured with stable single crystalline W[001] probes (Fig. 3d) demonstrate that the observed transformation of the subatomic features is related to modification of the tungsten tip atom electronic structure with decreasing tip-surface distance rather than modification of the crystallographic orientation of the apex as suggested in [61]. For explanation of the fine electronic structure effects in the W[001] tip-hopg(0001) system, the density functional theory (DFT) calculations of the electronic structure of the interacting tip and surface atoms [13, 21] were carried out using the VASP (Vienna Ab-initio Simulation Package) software [62]. The calculations revealed drastic reduction of the partial DOS associated with the most extended along the tip axis d 2 z orbital near E F at tunneling gaps d < 2.5 Å because of its stronger overlap with the electron orbitals of the surface carbon atoms. Figure 3c shows the result of the partial DOS calculations for three different tunneling gaps [21]. At larger tip-sample distances and small bias voltages applied in the STM experiments (Fig. 3d), the spatial resolution is determined by a set of the electron d-states with different symmetry at the apex atom. An improvement of the spatial resolution can be anticipated at distances d = Å since at these distances the d 2 z orbital dominates in the tip DOS near E F. At smaller distances ( Å), the spatial resolution in the experiments can be defined by the nonzero m tip electron states (d xz and d xy ) having larger DOS near E F at these tunneling gaps. These conclusions are in agreement with the results indepen-

6 736 CHAIKA Fig. 5. (Color online) Scanning tunneling microscopy images of graphene synthesized on cubic-sic(001) measured with single crystalline W[111] tip. The images demonstrate random distortions typical for quasi-freestanding graphene. The STM images were measured at (a) V = 22 mv and I = 70 pa and (b, c) V = 22 mv and I = 65 pa. dently obtained in [63, 64] which confirmed the asymmetric electronic structure of the W[001] probe at small distances between the tip and graphite surface atoms. The suppression of further protruded along the tip axis p z and d 2 z orbitals of the probe and surface atoms at small (d < 4.0 Å) tunneling gaps were theoretically predicted for several interacting tip sample systems [26, 28, 30, 65]. The modification of the electronic structure of tungsten probes with different crystallographic orientations of the apex at small tip-surface distances was studied theoretically in [28, 63, 64]. Figure 4 illustrates the asymmetric charge density distribution around the W[011] and W[001] tip atoms interacting with a helium atom at d < 2.5 Å [28]. The theoretical calculations (Fig. 4), predict the subatomic features with two and four maxima at small tunneling gaps for the [011]- and [001]-oriented tungsten probes. At the same time, the charge density distribution around the W[111] tip atom is symmetric even at very small distances between the interacting atoms (Fig. 4e) and does not produce three subatomic maxima as assumed in [61]. Theoretical calculations [28] also revealed that atomic structure of the W[001] probe is stable even at very small distances between the tip and helium atoms (~1.5 Å) while the W[011] and W[111] probes are substantially relaxed at small distances and, most probably, cannot provide stable STM imaging at tunneling gaps below 2.25 and 2.5 Å, respectively. These results prove that selective imaging of the electron d-states with different symmetry can take place only at extremely small tip-sample distances (d < 2.5 Å) and demonstrate the advantages of single crystalline tungsten probes with different crystallographic orientations. It can be assumed that W[001] probes can be more suitable for the scanning tunneling spectroscopy (STS) experiments demanding higher apex stability. At the same time, W[111] probes can provide higher spa- tial resolution in STM experiments on complex surfaces and should not produce subatomic features related to the tip apex atom. 4. VISUALIZATION OF THE CARBON BONDS IN QUASI-FREESTANDING GRAPHENE ON SiC(001) The examples of high resolution STM studies of complex surfaces using a single crystalline W[111] probe can be found in [14, 66]. Figure 5 demonstrates the picometer lateral resolution achieved with the W[111] probe during STM studies of the trilayer graphene grown on cubic-sic(001) surface. The top monolayer of graphene on SiC(001) consists of nanodomains connected to each other through the domain boundaries [14, 66]. The STM images measured in the middle of domains demonstrate the atomic scale rippling (Fig. 5a) typical for freestanding single layer graphene [67]. The lateral and vertical dimensions of the observed ripples are 3 5 nm and 1 Å, respectively, that is in good agreement with the theoretical calculations [67]. The spatial resolution achieved during the STM experiments with the W[111] tip on allows direct imaging of the nanoscale rippling of the graphene surface and visualization of the random picoscale distortions of the carbon bond lengths in the honeycomb lattice. This is illustrated on Fig. 5b by the STM image measured on top of a ripple. This small surface region can be considered as flat that is supported by the same contrast of different carboncarbon bonds in the randomly distorted honeycomb lattice (Fig. 5b). One of the distorted hexagons is shown in Fig. 5c for clarity. The lengths of the sides of a hexagon differ from the value known for ideal twodimensional graphene lattice (142 pm) on 1 16 pm. The observed distortions are in a good agreement with the theory [67]. The picometer lateral resolution obtained in STM studies of the graphene/sic(001)

7 VISUALIZATION OF ELECTRON ORBITALS 737 system is comparable with the spatial resolution achieved in recent non-contact AFM experiments [68]. 5. ATOMIC RESOLUTION IN STM EXPERIMENTS WITH SINGLE-CRYSTAL DIAMOND PROBES Taking into account the reciprocity principle (Fig. 1c), the experiments with single crystalline W[001] probes on HOPG(0001) (Fig. 3) demonstrate the possibility to improve the spatial resolution using STM probes with carbon atom at the tip apex. In this case, STM images will be obtained using more localized carbon s and p orbitals at the apex. This situation was realized in the experiments with polycrystalline tungsten probes functionalized by molecules consisting of carbon, oxygen and hydrogen atoms [44 55]. Despite the sub-ångström lateral resolution achieved [49], these probes are usually not stable at small tunneling gaps. Enhanced tip stability and control of the tip electronic structure can be achieved in STM experiments with single-crystal, conductive diamond probes [69]. The boron-doped diamond probes can be utilized both for STM and STS experiments demanding high apex stability. Note that heavily boron-doped diamond crystals can possess superconductive properties [70] that assume possible applications in low-temperature studies of superconducting nanostructures. Figure 6 illustrates the high spatial resolution obtained with the [111]-oriented single-crystal diamond probe and its advantages compared to the W[001] tip utilized in earlier STM experiments [19, 20, 57, 58]. The images measured with the diamond and tungsten probes (Figs. 6a and 6b) reveal true honeycomb lattice of the graphite surface with both α and β atoms resolved. However, the hollow sites are substantially deeper on STM images measured with the diamond probe, as cross-sections of the STM images demonstrate (Figs. 6c and 6d). This is related to different spatial distribution of the tungsten and carbon atomic orbitals which are responsible for the STM imaging in each particular case. The comparison of the cross-sections in Figs. 6c and 6d shows that conductive diamond probes can provide higher lateral and vertical resolution than traditional d-metal probes. On both cross-sections one can see non-equivalence of the α and β atoms of the graphite surface (with and without nearest neighbors in the second layer). The individual atoms in honeycombs are better resolved in the image measured with the diamond probe. The results of the DFT calculations [69] revealed that nonequivalence of the surface atoms in STM experiments with the diamond probe at tip-sample distances d > 3.0 Å corresponds to the different DOS on the α and β atoms. Figure 6f demonstrates that DOS on the Fermi level is approximately 25% larger for the β atoms. This is in agreement with the observed height difference between non-equivalent surface atoms in the cross-sections shown in Figs. 6c and 6d ( Å). Figures 6g and 6h show the charge density map calculated for the graphite (0001) surface at the tipsample distance of 4.5 Å and STM image measured using the diamond probe, respectively. Good agreement between the theoretical and experimental images proves that high resolution HOPG(0001) STM imaging with the boron-doped diamond probes can be achieved at distances in the range of Å. At these distances the electronic structure of the tip and surface atoms are not substantially modified by the tip-sample interaction (Fig. 7). This allows STM imaging of the surface electronic structure unperturbed by the tip-sample interaction (Fig. 6h). Calculations of the partial DOS associated with the electron orbitals of the diamond probe and graphite surface atoms at different tunneling gaps (Fig. 7) show that electronic structure of the interacting atoms is modified at tip-sample distances d < 3.0 Å. The overlapping of the tip and surface atomic orbitals leads to decrease in the partial DOS associated with the p z orbital of the graphite surface atoms when the diamond tip atom is positioned directly above the surface atom (Fig. 7a, right panel). The suppression of the p z orbital of a surface atom at small tunneling gaps (d < 2.5 Å) is in qualitative agreement with the results of the theoretical calculations performed for the adatom of the Si(111)7 7 surface interacting with the tungsten tip atom [30]. If the diamond tip atom is positioned above the hollow site (left panels in Figs. 7a and 7b), the overlapping of the tip and surface atomic orbitals does not take place even at small distances. Therefore, the change of the partial DOS of the tip and surface atoms is minor even at very small tunneling gaps (d = 1.5 Å in Fig. 7). According to DFT calculations [69], the electronic structure of the diamond probe is defined by the carbon p states with domination of the p x and p y orbitals. Scanning tunneling microscopy images in Figs. 6a and 6h can correspond to imaging of the surface p z orbitals by the p x, y orbitals of the diamond tip atom. The STM experiments and DFT calculations demonstrate the advantages of the probes having p orbitals at the tip atom compared to the traditional metallic probes with d orbitals at the apex. The p orbitals of light elements can provide higher spatial resolution and allow atomically resolved STM imaging at larger tip-sample distances without modification of the surface electronic structure. Note that in experiments with the single-crystal diamond probe [69] d orbitals definitely could not be responsible for the high resolution imaging that cannot be excluded in STM experiments with the tungsten probes functionalized by molecules and light element atoms [44 55].

8 738 CHAIKA Fig. 6. (Color online) (a, b) 18 9 Å STM images of HOPG(0001) measured using (a) single-crystal diamond probe at V = 50 mv, I = 0.1 na and (b) W[001] probe at V = 0.4 V, I = 0.18 na. (c, d) Cross-sections (c) 1 2 and (d) 3 4 of the images in panels (a) and (b), respectively. (e, f) Total DOS associated with the α and β atoms of the graphite (0001) surface. (g) Calculated electron density distribution map in the energy range [E F 0.2 ev; E F ] and (h) STM image measured with the diamond probe at V = 50 mv, I = 0.8 na. Reproduced from [69] with permission from IOP Publishing.

9 VISUALIZATION OF ELECTRON ORBITALS 739 p x p y p z p x p y p z p x p y p z p x p y p z Fig. 7. (Color online) Partial DOS of (a) the β atom of a graphite (0001) surface closest to the diamond tip and (b) the tip apex atom at different tunneling gaps and lateral positions of the tip (left panels) above the hollow site and (right panels) above the β atom. Reproduced from [69] with permission from IOP Publishing. Fig. 8. (Color online) (a c) 3 3 nm 2 STM images of the GaTe( 102 ) surface measured with a W[001] tip at (a) V = 1 V and I = 120 pa, (b) V = 1 V and I = 50 pa, and (c) V = 0.9 V and I = 50 pa. (d) Schematic model of the GaTe( 102 ) surface [30]. 6. CHEMICAL CONTRAST IN STM EXPERIMENTS ON THE GaTe ( 102 ) SURFACE Figures 8a 8c show STM images of the GaTe( 102 ) surface (Fig. 8d) measured using a single crystalline W[001] probe [58]. The images presented in Figs. 8a and 8b demonstrate preferential visualization of the electronic features corresponding to the tellurium and gallium surface atoms, respectively. The STM image in Fig. 8c reveals more complicated pattern corresponding to the visualization of the electronic features from both surface sublattices. The images in Figs. 8a 8c were obtained at almost the same sample bias voltage. Theoretical calculations [58] showed that DOS of the GaTe( 102 ) surface in the energy interval relevant to the STM experiments is mostly defined by the Te 5p electron states. Therefore, the images with preferential visualization of either tellurium or gallium sublattices cannot be explained by the surface electronic structure without consideration of the tip sample interaction. It can be suggested that chemical contrast in STM images of the GaTe( 102 ) surface is related to modification of the surface electronic structure at small tunneling gaps. With decreasing tip-sample distances the p orbitals of the GaTe( 102 ) surface atoms can be selectively suppressed as it was observed for the p z orbitals of the Si(111)7 7 [30] and graphite (Fig. 7) surface atoms interacting with the tips and for the orbital of the d z 2

10 740 CHAIKA W[001] tip atom interacting with the graphite surface (Fig. 3). Because of different spatial distribution of the Ga 4p and Te 5p orbitals, their modification can take place at different tunneling gaps. The overlapping of the surface and tip atomic orbitals can decrease the relative contribution of the Te 5p orbitals at small tunneling gaps stipulating the chemical selective STM imaging of the GaTe( 102 ) surface (Figs. 8a and 8b). 7. CONCLUSIONS Visualization of individual atomic orbitals corresponds to the limit of spatial resolution in STM experiments. During the last few years several studies demonstrating the direct visualization of either tip or electron orbitals of surface atoms have been published. An example of the direct orbital imaging is presented in Fig. 3. The subatomic features observed in these experiments are related to modification of the electronic structure of the tungsten tip atom interacting with the carbon atom of the graphite surface. Theoretical calculations demonstrate the suppression of the further protruded tip orbitals at small tunneling gaps because of the overlapping of the tip and sample wavefunctions. Taking into account the reciprocity principle of STM, similar results can be obtained during investigations of the d-metal surfaces using the probes possessing symmetric charge density distribution around the tip atom even at small distances. The results obtained with the [111]-oriented single-crystal diamond and tungsten probes demonstrate the feasibility of STM imaging with picometer lateral resolution without subatomic effects of the tip electronic structure. The picometer spatial resolution can be achieved using such probes even on surfaces with complicated atomic structure. The results presented in this review demonstrate the importance of precise control of the tunneling gap and the probe structure for selective visualization of the electron orbitals in STM experiments. 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