Joseph A. Stroscio

Atomic and Molecular Manipulation with the Scanning Tunneling Microscope

The prospect of manipulating matter on the atomic scale has fascinated scientists for decades. This fascination may be motivated by scientific and technological opportunities, or from a curiosity about the consequences of being able to place atoms in a particular location. Advances in scanning tunneling microscopy have made this prospect a reality; single atoms can be placed at selected positions and structures can be built to a particular design atom-by-atom. Atoms and molecules may be manipulated in a variety of ways by using the interactions present in the tunnel junction of a scanning tunneling microscope. Some of these recent developments and some of the possible uses of atomic and molecular manipulation as a tool for science are discussed.

Tunneling spectroscopy of the Si(111)2 × 1 surface

Abstract Using a scanning tunneling microscope, the tunneling current versus voltage is measured at fixed values of separation between a tungsten probe-tip and a Si(111)2 × 1 surface. Rectification is observed in the I - V curves and is quantitatively accounted for by an electric-field enhancement due to the finite radius-of-curvature of the probe-tip. The parallel wave-vector of certain states is obtained from the decay length of the tunneling current. A rich spectrum is obtained in the ratio of differential to total conductivity, yielding a direct measure of the Si surface density-of-states. Small shifts are observed in the spectrum as a function of doping, and are attributed to shifts in the position of the surface Fermi level.

Tunneling spectroscopy of the GaAs(110) surface

The scanning tunneling microscope is used to study the spectroscopy of p‐type, n‐type, and oxygen‐covered GaAs(110) surfaces. On the clean surface, three components of the current are identified—tunneling out of valence‐band states, tunneling into conduction‐band states, and tunneling through dopant‐induced states in the semiconductor. The results are compared with a theoretical computation of the tunneling current, including band bending in the semiconductor. Good agreement between theory and experiment is obtained only when tunneling through the space‐charge region of the semiconductor is included. On the oxygen‐covered surface, the spectroscopic results show evidence of band bending due to the oxygen adsorbates.

Observing the quantization of zero mass carriers in graphene.

Resolving Landau Levels in Graphene The charge carriers in a two-dimensional conductor, when placed in a magnetic field, can develop an additional set of quantized energy levels. These Landau levels correspond to the carriers now moving in cyclotron orbits. In graphene, which consists of single-atom-thick sheets of graphite, an unusual set of Landau levels with nonequal energy spacing can develop in graphene layers that have undergone symmetry breaking caused by rotation between adjacent layers. Miller et al. (p. 924) used scanning tunneling microscopy at cryogenic temperatures to map out Landau levels in graphene grown on silicon carbide with high energy and momentum resolution, including the characteristic level in graphene that can occur at zero energy. Scanning tunneling microscopy on graphene reveals non-equally spaced Landau energy levels induced by a magnetic field. Application of a magnetic field to conductors causes the charge carriers to circulate in cyclotron orbits with quantized energies called Landau levels (LLs). These are equally spaced in normal metals and two-dimensional electron gases. In graphene, however, the charge carrier velocity is independent of their energy (like massless photons). Consequently, the LL energies are not equally spaced and include a characteristic zero-energy state (the n = 0 LL). With the use of scanning tunneling spectroscopy of graphene grown on silicon carbide, we directly observed the discrete, non-equally–spaced energy-level spectrum of LLs, including the hallmark zero-energy state of graphene. We also detected characteristic magneto-oscillations in the tunneling conductance and mapped the electrostatic potential of graphene by measuring spatial variations in the energy of the n = 0 LL.

Asymmetries in dislocation densities, surface morphology, and strain of GaInAs/GaAs single heterolayers

The dislocation densities, surface morphology, and strain of Ga1−xInxAs/GaAs epitaxial interfaces as a function of indium composition and layer thickness have been investigated by transmission electron microscopy, medium energy ion blocking, and double‐crystal x‐ray diffractometry. The electron microscopy shows that in the thinnest dislocated films (90 and 160 nm, x=0.07) 60° α dislocations form first in one 〈110〉 direction at the interface. Surprisingly, however, an asymmetry in residual layer strain is not detected in these samples, suggesting that the dislocations have the same Burgers vector or are evenly distributed between two Burgers vectors. Orthogonal arrays of dislocations are observed in films thicker than 300 nm (60° and edge‐type, x=0.07). In this case, dislocation densities in each 〈110〉 direction are equal to within experimental error while an asymmetry in in‐plane strain is measured (18% and 30% for x=0.07, 300, and 580 nm thick, respectively). An unequal distribution of Burgers vectors of...

Manipulation of adsorbed atoms and creation of new structures on room-temperature surfaces with a scanning tunneling microscope

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.

Electromechanical properties of graphene drumheads

Straining Suspended Graphene The electronic properties of graphene are best displayed by suspended sheets free from contact with an underlying substrate. Klimov et al. (p. 1557) probed how deformation of suspended graphene sheets could lead to further tuning of its electronic properties with a scanning tunneling microscope; the graphene sheets could also be deformed via an electric field from an underlying electrode. Spectroscopic studies reveal that the induced strain led to charge-carrier localization into spatially confined quantum dots, an effect consistent with the formation of strain-induced pseudomagnetic fields. Mechanical straining of suspended graphene films leads to confinement of charge carriers into quantum dots. We determined the electromechanical properties of a suspended graphene layer by scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) measurements, as well as computational simulations of the graphene-membrane mechanics and morphology. A graphene membrane was continuously deformed by controlling the competing interactions with a STM probe tip and the electric field from a back-gate electrode. The probe tip–induced deformation created a localized strain field in the graphene lattice. STS measurements on the deformed suspended graphene display an electronic spectrum completely different from that of graphene supported by a substrate. The spectrum indicates the formation of a spatially confined quantum dot, in agreement with recent predictions of confinement by strain-induced pseudomagnetic fields.

Imaging the interface of epitaxial graphene with silicon carbide via scanning tunneling microscopy

Graphene grown epitaxially on SiC has been proposed as a material for carbon-based electronics. Understanding the interface between graphene and the SiC substrate will be important for future applications. We report the ability to image the interface structure beneath single-layer graphene using scanning tunneling microscopy. Such imaging is possible because the graphene appears transparent at energies of

The chemisorption and decomposition of ethylene and acetylene on Ni(110)

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Exposure of Epitaxial Graphene on SiC(0001) to Atomic Hydrogen

above or below the Fermi energy