Saban M. Hus

Differentiation of Surface and Bulk Conductivities in Topological Insulators via Four-Probe Spectroscopy

We show a new method to differentiate conductivities from the surface states and the coexisting bulk states in topological insulators using a four-probe transport spectroscopy in a multiprobe scanning tunneling microscopy system. We derive a scaling relation of measured resistance with respect to varying interprobe spacing for two interconnected conduction channels to allow quantitative determination of conductivities from both channels. Using this method, we demonstrate the separation of 2D and 3D conduction in topological insulators by comparing the conductance scaling of Bi2Se3, Bi2Te2Se, and Sb-doped Bi2Se3 against a pure 2D conductance of graphene on SiC substrate. We also quantitatively show the effect of surface doping carriers on the 2D conductance enhancement in topological insulators. The method offers a means to understanding not just the topological insulators but also the 2D to 3D crossover of conductance in other complex systems.

Contactless Determination of Electrical Conductivity of One-Dimensional Nanomaterials by Solution-Based Electro-orientation Spectroscopy.

Nanowires of the same composition, and even fabricated within the same batch, often exhibit electrical conductivities that can vary by orders of magnitude. Unfortunately, existing electrical characterization methods are time-consuming, making the statistical survey of highly variable samples essentially impractical. Here, we demonstrate a contactless, solution-based method to efficiently measure the electrical conductivity of 1D nanomaterials based on their transient alignment behavior in ac electric fields of different frequencies. Comparison with direct transport measurements by probe-based scanning tunneling microscopy shows that electro-orientation spectroscopy can quantitatively measure nanowire conductivity over a 5-order-of-magnitude range, 10(-5)-1 Ω(-1) m(-1) (corresponding to resistivities in the range 10(2)-10(7) Ω·cm). With this method, we statistically characterize the conductivity of a variety of nanowires and find significant variability in silicon nanowires grown by metal-assisted chemical etching from the same wafer. We also find that the active carrier concentration of n-type silicon nanowires is greatly reduced by surface traps and that surface passivation increases the effective conductivity by an order of magnitude. This simple method makes electrical characterization of insulating and semiconducting 1D nanomaterials far more efficient and accessible to more researchers than current approaches. Electro-orientation spectroscopy also has the potential to be integrated with other solution-based methods for the high-throughput sorting and manipulation of 1D nanomaterials for postgrowth device assembly.

Structure and Growth of Quasi One-Dimensional YSi2 Nanophases on Si(100)

Quasi-one-dimensional YSi(2) nanostructures are formed via self-assembly on the Si(100) surface. These epitaxial nanowires are metastable and their formation strongly depends on the growth parameters. Here, we explore the various stages of yttrium silicide formation over a range of metal coverages and growth temperatures, and establish a rudimentary phase diagram for these novel and often coexisting nanophases. We identify, in addition to previously identified stoichiometric wires, several new nanowire systems. These nanowires exhibit a variety of surface reconstructions, which sometimes coexist on a single wire. From a comparison of scanning tunneling microscopy images, tunneling spectra, and first-principles density functional theory calculations, we determine that these surface reconstructions arise from local orderings of yttrium vacancies. Nanowires often agglomerate into nanowire bundles, the thinnest of which are formed from single wire pairs. The calculations show that such bundles are energetically favored compared to well-separated single wires. Thicker bundles are formed at slightly higher temperature. They extend over several microns, forming a robust network of conducting wires that could possibly be employed in nanodevice applications.

Formation of uni-directional ultrathin metallic YSi2 nanowires on Si(110)

Ultrathin YSi2 nanowires were grown epitaxially on the Si(110) surface. High-aspect-ratio nanowire growth is induced by the strongly anisotropic lattice-match between the silicide crystal lattice and the Si(110) surface, similar to the established formation of rare-earth silicide nanowires on Si(100). In contrast to the Si(100) case, however, YSi2 nanowires on Si(110) grow in a single orientation along the [11¯0] direction and exhibit a clear preference of nucleating at step edges when these edges are aligned along the [11¯0] growth direction. This suggests a promising avenue for the fabrication of regular nanowire arrays with controlled wire separation, by varying the miscut angle of the Si wafer. The nanowires are metallic and are embedded in a reconstructed Si(110)-(23×3)R54.7°-Y semiconducting surface layer.

Differentiation of Surface and Bulk Conductivities via Four-probe Spectroscopy

A large number of materials have localized electronic states present on their surfaces. Understanding and controlling the electronic properties of these states does not only constitute the foundations of surface science but also the modern electronic devices. Being studied with mature surface analytic techniques like STM and ARPES, local electronic properties of surface states can be characterized very well. However, little is known about the electron transport through these 2D surface states which can present completely different characteristics than the transport through the underlying 3D bulk. During the last decade however, the demand for understanding the transport through surface states increased significantly due to constantly decreasing size of microelectronic devices and discovery of new materials like topological insulators (TIs) and other 2D layered materials with surface states hosting new and interesting transport properties.