A general method of manipulating adsorbed atoms and molecules on room-temperature surfaces with the use of a scanning tunneling microscope is described. By applying an appropriate voltage pulse between the sample and probe tip, adsorbed atoms can be induced to diffuse into the region beneath the tip. The field-induced diffusion occurs preferentially toward the tip during the voltage pulse because of the local potential energy gradient arising from the interaction of the adsorbate dipole moment with the electric field gradient at the surface. Depending upon the surface and pulse parameters, cesium (Cs) structures from one nanometer to a few tens of nanometers across have been created in this way on the (110) surfaces of gallium arsenide (GaAs) and indium antimonide (InSb), including structures that do not naturally occur.
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We report the structural and electronic properties of Cs adsorbed on room-temperature GaAs and InSb (110) surfaces as observed with scanning tunneling microscopy. Cs initially forms long onedimensional (ID) zigzag chains on both surfaces. Additional Cs adsorption on GaAsO 10) results in the formation of a 2D overlayer consisting of five-atom Cs polygons arranged in a c(4x4) superlattice. The tunneling gap measured over these insulating structures narrows with the transition from ID to 2D, with metallic characteristics observed following saturation with a second Cs overlayer.PACS numbers: 61.16.Di, 68.35.Bs, 73.20.Dx Considerable interest exists in the study of alkalimetal adsorption on semiconductor surfaces as model systems for understanding metal-semiconductor interfaces. During the past sixty years much debate has focused on the degree of charge transfer and the onset of metallicity during alkali-metal adsorption on both metals and semiconductors. 1 ' 11 In an early description of alkali-metal adsorption put forth by Langmuir, l each alkali adatom transfers its single s valence electron completely to the substrate giving rise to the characteristic work-function decrease. More accurate calculations later found that the alkali-metal s level broadens into a resonance with metal substrate levels upon adsorption, resulting in only a partial charge transfer. 2 The theoretical debate has continued into the present, with some calculations finding nearly total charge transfer, 5 and others very little. 3,4 Experimentally, recent core-level photoemission measurements indicate that some charge transfer always occurs, but the extent may vary dramatically from surface to surface. w,u Previous experimental work with scanning tunneling microscopy (STM) has shown that Cs on GaAs(llO) is a unique system for studying metal-semiconductor interfaces, with Cs atoms forming one-dimensional (ID) chains at low coverages. 9 The adsorption of additional Cs offers the possibility of examining the evolution of a metal-semiconductor system from a ID to 2D state. Valence-band photoemission and inverse photoemission studies have revealed that Cs adsorption introduces a series of surface states within the GaAs band gap. 7 On the basis of photoemission core-level line shapes some investigators have proposed that a surface saturated with Cs at room temperature is metallic. 6 The inverse photoemission results support this view, showing evidence of a metallic Fermi edge. 7 Conflicting results, however, were obtained by Wong et al} who concluded in their valence-band photoemission investigation that the surface does not become metallic with room-temperature adsorption. This conflict highlights the difficulty of detecting the onset of metallicity with photoelectron techniques. As an alternative, STM enables the metallicity of atomic-scale surface structures to be determined directly from the zero-bias tunneling conductivity. 12 In this Letter we report the evolution of the geometric and electronic properties of Cs adsorbed on roomtemperature G...
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