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The diffusion of Si dimers on the Si(001) surface at temperatures between room temperature and 128 ± C is measured using a novel atom-tracking technique that can resolve every diffusion event. The atom tracker employs lateral-positioning feedback to lock the scanning tunneling microscope (STM) probe tip into position above selected atoms with subangstrom precision. Once locked the STM tracks the position of the atoms as they migrate over the crystal surface. By tracking individual atoms directly, the ability of the instrument to measure dynamic events is increased by a factor of ϳ1000 over conventional STM imaging techniques. PACS numbers: 68.35.Fx, 61.16.Ch, 68.10.Jy The evolution of surface morphology during growth or etching depends on the detailed interplay between a myriad of atomic-scale kinetic processes. In order to control particular surface morphologies (e.g., to suppress or enhance overlayer island formation) it is important to achieve an understanding of the balance between thermodynamics and various kinetic processes. The equilibrium surface morphologies are controlled by thermodynamics, that is, the binding or configurational free energies, while the rates at which dynamic events occur on the surface are controlled by the details of the activation barriers-kinetics.To study the evolution of surface morphology investigators often utilize rate-equation analysis [1], Monte Carlo [2], or molecular dynamics simulations [3] which explicitly require the input of specific atomic-scale energy parameters. The values for the input parameters are derived in a variety of ways ranging from first-principles total energy calculations, through semiempirical calculations, e.g., from the embedded-atom method, to purely empirical values chosen to reproduce particular measured morphological features. Of course, the ideal parameter values are derived from quantitative experimental measurements of particular system-specific atomic-scale processes. The quantitative measurements of individual atomic-scale energy parameters not only serve as inputs to realistic simulations and model calculations, but also enable the validation and refinement of such calculations. Such processes on certain metal systems have been studied for some time using field ion microscopy [4].Because of its inherent atomic-scale resolution and access to a broad range of materials, the scanning tunneling microscope (STM) is ideally suited to studies of individual atomic-scale processes on surfaces, particularly since the advent of variable-temperature versions which enable the imaging of the surface as the rates of specific processes are varied by altering the sample temperature. However, conventional STM image acquisition is limited by the rate at which dynamic events can be resolved. In this work I report on the use of a novel atom-tracking technique that allows direct quantitative measurements of the diffusion of adsorbed silicon dimers on the Si(001) surface from room temperature to 128 ± C over which the diffusion rate changes by 4 orders of mag...

The current-voltage characteristics of thin wires are often observed to be nonlinear, and this behavior has been ascribed to Schottky barriers at the contacts. We present electronic transport measurements on GaN nanorods and demonstrate that the nonlinear behavior originates instead from space-charge-limited current. A theory of space-charge-limited current in thin wires corroborates the experiments, and shows that poor screening in high aspect ratio materials leads to a dramatic enhancement of space-charge limited current, resulting in new scaling in terms of the aspect ratio.Nanorods and nanowires made of different semiconducting materials have been extensively characterized electrically [1,2,3,4,5,6,7,8,9]. For most applications, a linear current-voltage relationship is desirable, usually requiring ohmic contact to doped wires. However, even in situations where these conditions should be met, it is often observed that the current-voltage characteristics are nonlinear. Invariably, this behavior is explained by the presence of Schottky barriers at the contacts, despite the fact that such models sometimes give poor descriptions of the experimental data. Properly identifying the factors that influence electrical transport characteristics is important for device design but also because extraction of material parameters such as the mobility relies on analysis with specific models.It is well-known that in bulk insulating and semiconductor materials, space-charge-limited (SCL) current leads to nonlinear, non-exponential I − V characteristics [10,11,12] with the relationship I ∝ V 2 . This behavior occurs in situations of mobility-dominated transport when the effective carrier concentration is low. This can arise due to low intrinsic doping, charge traps, or depletion widths at the contacts that are larger than the channel length (punchthrough). Thin wires should be particularly sensitive to SCL effects for several reasons: first, because electrostatic screening in high aspect ratio systems is poor [13], the injected carriers cannot be effectively screened; second, carrier depletion due to surface states is expected to be more important in thin wires due to the large surface-to-volume ratio [14]; third, charge traps may readily be incorporated during growth [15].In this letter, we present electrical transport in individual GaN nanorods showing symmetric, nonlinear I − V characteristics. We show that the relationship I ∝ V 2 is satisfied in these thin wires, a signature of SCL current. A theory for SCL transport in thin wires is presented and shows that SCL current is unusually strong due to a new scaling with the wire aspect ratio.The growth and microstructural characterization of the GaN nanorods has been described in detail elsewhere [16]. Briefly, the nanorods were grown in a commercial metal-organic chemical vapor-deposition system on GaN/sapphire substrates using selective epitaxy, whereby a 30 nm thick SiN film with lithographically defined holes serves as a mask for the nanorod growth. For this work, the nanoro...

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We present electronic transport measurements in individual Au-catalyst/Ge-nanowire interfaces demonstrating the presence of a Schottky barrier. Surprisingly, the small-bias conductance density increases with decreasing diameter. Theoretical calculations suggest that this effect arises because electron-hole recombination in the depletion region is the dominant charge transport mechanism, with a diameter dependence of both the depletion width and the electron-hole recombination time. The recombination time is dominated by surface contributions and depends linearly on the nanowire diameter.

We study the structure of electronic states in individual PbS nanocrystal quantum dots by scanning tunneling spectroscopy (STS) using one-to-two monolayer nanocrystal films treated with 1, 2-ethanedithiols (EDT). Up to six individual valence and conduction band states are resolved for a range of quantum dot sizes. The measured states' energies are in good agreement with calculations using the k · p four-band envelope function formalism. A comparison of STS and optical absorption spectra indicates that some of the absorption features can only be explained by asymmetric transitions involving the states of different symmetries (e.g., S and P or P and D), which points towards the relaxation of the parity selection rules in these nanostructures. STS measurements also reveal a midgap feature, which is likely similar to one observed in previous charge transport studies of EDT-treated quantum dot films.

Microscale four-leaf clover-shaped structures are formed by self-assembly of anionic and cationic porphyrins. Depending on the metal complexed in the porphyrin macrocycle (Zn or Sn), the porphyrin cores are either electron donors or electron acceptors. All four combinations of these two metals in cationic tetra(N-ethanol-4-pyridinium)porphyrin and anionic tetra(sulfonatophenyl)porphyrin result in related cloverlike structures with similar crystalline packing indicated by X-ray diffraction patterns. The clover morphology transforms as the ionic strength and temperature of the self-assembly reaction are increased, but the structures maintain 4-fold symmetry. The ability to alter the electronic and photophysical properties of these solids (e.g., by altering the metals in the porphyrins) and to vary cooperative interactions between the porphyrin subunits raises the possibility of producing binary solids with tunable functionality. For example, we show that the clovers derived from anionic Zn porphyrins (electron donors) and cationic Sn porphyrins (electron acceptors) are photoconductors, but when the metals are reversed in the two porphyrins, the resulting clovers are insulators.