Ya-Qiong Xu

Half-Metallic Ferromagnetism and Structural Stability of Zincblende Phases of the Transition-Metal Chalcogenides

An accurate density-functional method is used to study systematically half-metallic ferromagnetism and stability of zincblende phases of 3d-transition-metal chalcogenides. The zincblende CrTe, CrSe, and VTe phases are found to be excellent half-metallic ferromagnets with large half-metallic gaps (up to 0.88 eV). They are mechanically stable and approximately 0.31-0.53 eV per formula unit higher in total energy than the corresponding nickel-arsenide ground-state phases, and therefore would be grown epitaxially in the form of films and layers thick enough for spintronic applications.

Dry Contact Transfer Printing of Aligned Carbon Nanotube Patterns and Characterization of Their Optical Properties for Diameter Distribution and Alignment

A scalable and facile approach is demonstrated where as-grown patterns of well-aligned structures composed of single-walled carbon nanotubes (SWNT) synthesized via water-assisted chemical vapor deposition (CVD) can be transferred, or printed, to any host surface in a single dry, room-temperature step using the growth substrate as a stamp. We demonstrate compatibility of this process with multiple transfers for large-scale device and specifically tailored pattern fabrication. Utilizing this transfer approach, anisotropic optical properties of the SWNT films are probed via polarized absorption, Raman, and photoluminescence spectroscopies. Using a simple model to describe optical transitions in the large SWNT species present in the aligned samples, polarized absorption data are demonstrated as an effective tool for accurate assignment of the diameter distribution from broad absorption features located in the infrared. This can be performed on either well-aligned samples or unaligned doped samples, allowing simple and rapid feedback of the SWNT diameter distribution that can be challenging and time-consuming to obtain in other optical methods. Furthermore, we discuss challenges in accurately characterizing alignment in structures of long versus short carbon nanotubes through optical techniques, where SWNT length makes a difference in the information obtained in such measurements. This work provides new insight to the efficient transfer and optical properties of an emerging class of long, large diameter SWNT species typically produced in the CVD process.

Integrated micro‐scanning tunneling microscope

Two versions of micro‐scanning tunneling microscopes (micro‐STMs) have been fabricated. The integrated micro‐STMs are fabricated from single crystal silicon using the high‐aspect‐ratio SCREAM process. Each micro‐STM includes integrated xy comb drive actuators and a torsional z actuator with integrated cantilever and tip. One micro‐STM measures approximately 200 μm on‐a‐side and is an example of a STM element for a STM array architecture. Another, larger micro‐STM/atomic force microscope measures 2 mm on‐a‐side including a 1 mm long cantilever with a 20 nm diam tip. We demonstrate the operation of this larger STM by obtaining a STM image of a 200 nm metal conductor on a silicon chip.

Half-metallic ferromagnetism of MnBi in zincblende phase

The full-potential augmented plane wave plus local orbitals method within density-functional theory is used to predict that MnBi in the zincblende phase is a true half-metallic ferromagnet with a magnetic moment of 4.000mu(B) per formula. This phase of MnBi is found to be robust against volume changes from -12% to +30% and remains qualitatively the same under various exchange-correlation approximations

Plasmonic Hot Electron Induced Photocurrent Response at MoS2-Metal Junctions

We investigate the wavelength- and polarization-dependence of photocurrent signals generated at few-layer MoS2-metal junctions through spatially resolved photocurrent measurements. When incident photon energy is above the direct bandgap of few-layer MoS2, the maximum photocurrent response occurs for the light polarization direction parallel to the metal electrode edge, which can be attributed to photovoltaic effects. In contrast, if incident photon energy is below the direct bandgap of MoS2, the photocurrent response is maximized when the incident light is polarized in the direction perpendicular to the electrode edge, indicating different photocurrent generation mechanisms. Further studies show that this polarized photocurrent response can be interpreted in terms of the polarized absorption of light by the plasmonic metal electrode, its conversion into hot electron-hole pairs, and subsequent injection into MoS2. These fundamental studies shed light on the knowledge of photocurrent generation mechanisms in metal-semiconductor junctions, opening the door for engineering future two-dimensional materials based optoelectronics through surface plasmon resonances.

Effects of atomic hydrogen and active carbon species in 1mm vertically aligned single-walled carbon nanotube growth

A hot filament chemical vapor deposition (HFCVD) method has been used to investigate the effects of atomic hydrogen and active carbon species on the growth of 1mm vertically aligned single-walled carbon nanotubes (VA-SWNTs). Isotopic shifts of the tangential phonon mode of SWNTs indicate that SWNT growth occurs primarily via reactions of acetylene and ethylene. The authors find that the presence of atomic hydrogen and active carbon species such as ethylene and acetylene appears to be essential for both nucleation and growth of VA-SWNTs in this HFCVD.

Coherent magnetization precession in GaMnAs induced by ultrafast optical excitation

The authors use femtosecond optical pulses to induce, control, and monitor magnetization precession in ferromagnetic Ga0.965Mn0.035As. At temperatures below ∼40K, they observe coherent oscillations of the local Mn spins, triggered by an ultrafast photoinduced reorientation of the in-plane easy axis. The amplitude saturation of the oscillations above a certain pump intensity indicates that the easy axis remains unchanged above ∼TC∕2. The authors find that the observed magnetization precession damping (Gilbert damping) is strongly dependent on pump laser intensity, but independent of ambient temperature. They provide a physical interpretation of the observed light-induced collective Mn-spin precession and relaxation.

Supergrowth of nitrogen-doped single-walled carbon nanotube arrays: active species, dopant characterization, and doped/undoped heterojunctions.

We demonstrate the water-assisted supergrowth of vertically aligned single-walled carbon-nitrogen nanotubes (SWNNTs) using a simple liquid/gas-phase precursor system. In situ characterization of gas-phase nitrogen-containing precursors and their correlation to growth identifies HCN as the most active precursor for SWNNT growth, analogous to C(2)H(2) for single-walled carbon nanotubes (SWNTs). Utilizing Raman spectroscopy, combined with XPS and in situ mass spectrometry during growth, we demonstrate the ability to probe N atoms at low concentrations (10(-5) at. % N) in the SWNNT. Additionally, we demonstrate sensitivity of SWNNT optical transitions to N-doping through absorbance measurements, which appear to be a sensitive fingerprint for SWNNT doping. Finally, we demonstrate the fabrication of SWNT/SWNNT heterojunctions in the self-assembled carpet morphology that can be printed to arbitrary host substrates and facilitate potential emerging applications for this material. This work brings together new aspects regarding the growth, characterization, and materials processing that can yield advanced material architectures involving electronically tuned SWNNT array networks.

Near-bandgap wavelength dependence of long-lived traveling coherent longitudinal acoustic phonons in GaSb-GaAs heterostructures

We report first studies of long-lived oscillations in optical pump-probe measurements on GaSb-GaAs heterostructures. The oscillations arise from a photogenerated coherent longitudinal acoustic phonon wave, which travels from the top surface of GaSb across the interface into the GaAs substrate, providing information on the optical properties of the material as a function of time/depth. Wavelength-dependent studies of the oscillations near the bandgap of GaAs indicate strong correlations to the optical properties of GaAs.

Alignment dependence of one-dimensional electronic hopping transport observed in films of highly aligned, ultralong single-walled carbon nanotubes

The temperature dependent electrical conductivity of highly aligned, as-grown, pristine films of ultralong single-walled carbon nanotubes (SWNTs) is investigated in the framework of conduction based on phonon-assisted electron hopping. A change in transport mechanism occurs between conduction normal to and parallel to the SWNT alignment that results in evolution from bulk three-dimensional (3D) hopping conduction to a one-dimensional (1D) hopping conduction mechanism intrinsic to the electronic structure of SWNT. Vacuum annealing is observed to change the magnitude of the film conductivity but does not alter the observed 1D or 3D hopping mechanism.