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Tunneling Microscopy and Spectroscopy

  1. L. J. Whitman

Published Online: 15 APR 2003

DOI: 10.1002/3527600434.eap542

Encyclopedia of Applied Physics

Encyclopedia of Applied Physics

How to Cite

Whitman, L. J. 2003. Tunneling Microscopy and Spectroscopy. Encyclopedia of Applied Physics. .

Author Information

  1. Naval Research Laboratory, Washington, D.C., U.S.A.

Publication History

  1. Published Online: 15 APR 2003
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Figure 1. The basic elements of scanning tunneling microscopy.

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Figure 2. Three common ways of presenting an STM image: as a set of line scans; in gray scale, with higher points brighter; and as an artificially rendered surface.

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Figure 3. Common elements of an STM, including a tip mounted on a piezoelectric tube scanner, a coarse-approach mechanism (in this case an “inchworm” motor), and a damped vibration-isolation system.

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Figure 4. Photographs of a commercially available STM (AutoProbe® VP, Park Scientific Instruments, Sunnyvale, CA). (a) Complete ultrahigh-vacuum system. (b) STM mounted on vacuum flange. (c) Close-up of STM showing the tip holder being transferred to the scanner.

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Figure 5. Operation of a piezoelectric tube scanner with a four-quadrant outer electrode for horizontal motion and single inner electrode for vertical motion. A horizontal deflection occurs when a voltage is applied across opposing outer electrodes. A vertical deflection occurs when a voltage is applied to the inner electrode with respect to the four outer electrodes.

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Figure 6. Operating principles for two common coarseapproach mechanisms. (a) An “inchworm” motor employs a piezotube and piezoclamps to move an inner shaft by a series of clamping/unclamping and extension/retraction events. (b) Inertial motion of a free mass can be achieved by asymmetrical acceleration.

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Figure 7. (a) Schematic of quantum-mechanical tunneling between a metal tip and metal surface a distance s apart. The tunneling barrier is determined by the work functions of the surface and tip, ϕs and ϕt, respectively, and the bias voltage applied between the two, V. For the case shown, surface positive with respect to the tip, electrons tunnel from bands below the Fermi level of the tip equation image to those above the Fermi level of the surface equation image. (b) Detailed schematic of the role of the density of states near EF on the tunneling process both into and out of empty and filled states, respectively, on a semiconductor surface.

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Figure 8. Filled-state, constant-current STM images of the equation image reconstruction on Au(111) in UHV. (a) 54 × 54 nm view showing the “herringbone” pattern (0.15-V bias, 0.5-nA current, 0.02-nm height range) (Altman and Colton, 1992, reprinted with permission of Elsevier Science—NL). (b) Atomic-resolution close-up, 6.6 × 5.2 nm, of the bulged herringbone corner where the transition between FCC and HCP stacking occurs (0.9 V, 0.5 nA) (Stroscio et al., 1991). The approximate locations of some surface atoms are marked with dots; the change in height from one to the next is about 3 xpm. White lines follow rows of surface atoms across the herringbone corner, highlighting the additional row on the HCP side (indicated by the arrow).

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Figure 9. Atomic-resolution empty- (2.0 V) and filled-state (1.8 V) constant-current images of the same area of a GaAs(110) surface in UHV. The locations of surface Ga atoms (light circles) and As atoms (dark circles) are indicated; the height change from atom-to-atom is about 0.02 nm in the images. The defect observed is a single surface As vacancy (after Lengel et al., 1994).

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Figure 10. The (7 × 7) reconstruction of Si(111) in UHV. Both empty- and filled-state constant-current images are shown (14 × 14 nm, ±2.0 V, 0.2-nm height range); one diamond-shaped (7 × 7) unit cell, 4.66 × 2.69 nm, is indicated (courtesy of A. A. Baski and the author). The structure of the unit cell is shown in top and side view (through the long diagonal) with the topmost atoms largest and darkest. Some of the atoms are labeled to aid correlation between the two views (after Takayanagi et al., 1985).

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Figure 11. Filled-state constant-current image of Mo18O52(100) (1.6 V, 0.25-nm height range) and corresponding structural model. The planar repeat unit is indicated by the black box. The model is a polyhedral representation shaded by height (darker is lower). The sample was prepared in air and then imaged in UHV (Rohrer et al., 1993, reprinted with permission of Elsevier Science—NL).

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Figure 12. Filled-state images recorded during the room-temperature reaction of H2S with oxygen-covered Ni(110) in UHV; H2S(gas) + O(surface) [RIGHTWARDS ARROW] H2O(gas) + S(surface). (a) 0, (b) 4.0, (c) 10.6, (d) 26.6, (e) 33.3, and (f) 46.6 mbar · sec. All the images are about 9.5 × 9.5 nm, except (f), which is 7.5 × 7.5 nm (Besenbacher et al., 1994, reprinted with permission of the American Vacuum Society).

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Figure 13. Empty-state images (14 × 21.5 nm) of a Si(111)-(7 × 7) surface during the early stages of oxidation in UHV: after exposure to (a) 0.067 and (b) 0.133 mbar · sec of O2. The arrows point to typical reaction sites: not reacted (N), bright (B), dark (D), and two adjacent dark sites (DD). Structural models for the reacted sites are also shown (after Martel et al., 1996).

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Figure 14. Filled-state variable-temperature STM images of the growth of 0.1 layer of Ag on Pt(111) in UHV. The scale bars are all 50 nm, and the images are gray-scales of the derivative of the topography (which highlights edges) (Röder et al., 1993, reprinted with permission of Elsevier Science—NL).

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Figure 15. The topography of a 4-µm-thick GaSb film grown on a GaAs(001) substrate by molecular-beam epitaxy in UHV. Each step in the constant-current image is about 0.3 nm high. The arrows point out where threading dislocations emerge at the surface and create steps. (Courtesy of P. M. Thibado, B. R. Bennett, B. V. Shanabrook, and the author.)

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Figure 16. A selection of constant-current images of PbS(001) acquired during electrochemical oxidation in NaClO4. All images are 200 × 200 nm, with each successive gray level corresponding to a change in height of 0.3 nm. The images were recorded with a tunneling current of 0.17 nA, tip voltage of 0.94 V, and a sample voltage that changed from (a) 0.20 to 0.24, (b) 0.29 to 0.33, (c) 0.38 to 0.43, and (d) 0.38 to 0.34 V during image acquisition. Some surface impurities (I) and the resulting etch pits (P) are labeled (Higgins and Hamers, 1995, reprinted with permission of Elsevier Science—NL).

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Figure 17. Current-versus-voltage tunneling spectra recorded on clean and Cs-covered GaAs(110) in UHV. With increasing Cs coverage long 1D chains, then 2D planar clusters, and finally a disordered 3D overlayer are observed (images not shown). The apparent “band gap” observed for each structure is indicated (Whitman et al., 1991).

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Figure 18. (a) Filled-state image (1.1 V) of 0.4 layer of Cr deposited on Fe(100) at 290 C in UHV. Each step is 0.14 nm high, and the small bumps in the topography are approximately 0.01 nm high. (b) A higher-resolution view of the Cr-induced bumps. (c) A structural model for the Cr film. The Cr atoms are embedded in the substrate, with displaced Fe atoms forming single-layer islands. (d) Conductance spectra recorded over a Cr or Fe surface atom (after Davies et al., 1996).

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Figure 19. Constant-current filled-state images (15 × 15 nm, 50 mV, 1 nA) of NbxTa1−xS2 crystals recorded in a nitrogen-filled glove box at 380 (x = 0) and 315 K (x = 0.07). The large-period corrugation is due to a charge-density wave that is incommensurate with the crystal lattice (the smaller, shorter-period corrugation) (Liu et al., 1996, reprinted with permission of the American Vacuum Society).

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Figure 20. Filled-state image (37.5 × 37.5 nm) of nanometer-scale lithography of Si(001) − (2 × 1) with an STM in UHV. The surface was first covered with atomic hydrogen, forming (2 × 1)-reconstructed rows of H[BOND]Si = Si[BOND]H (the dimmer rows visible in the image). The H was then locally removed (bright rows) by scanning the desired areas with a high sample bias and current (4.5 V, 2.0 nA) (Lyding et al., 1994, reprinted with permission of the American Institute of Physics). The lines are ≤1 nm wide on a 3-nm pitch.

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Figure 21. (a)–(d) Sequential constant-current empty-state images (0.01 V, 1.0 nA) recorded during the assembly of a circle of Fe atoms on Cu(111) at 4 K in UHV. The average diameter of the circle is 14.25 nm. The structure was assembled one atom at a time using the STM tip. The concentric rings seen in the image are variations in the local density of states (not atomic heights) associated with standing waves created by surface electrons trapped inside the circle of Fe atoms (a “quantum corral”). (Courtesy of M. F. Crommie, C. P. Lutz, and D. M. Eigler.)