Minn-Tsong Lin

Nano Approach Investigation of the Conduction Mechanism in Polyaniline Nanofibers

A nanotechnological approach is applied to measurements of the electric field dependence of resistance under a high electric field while in low voltage. With this technique, the conduction mechanism on a mesoscopic scale is explored in a single, nonagglomerated nanofiber. Polyaniline nanofibers are prepared by vigorous mixing of aniline and oxidation agent ammonium persulfate in acid solution. They exhibit a uniform nanoscale morphology rather than agglomeration as that produced via conventional chemical oxidation. The as-synthesized polyaniline nanofibers are doped (dedoped) with a HCl acid (NH(3) base), and their temperature behaviors of resistances follow an exponential function with an exponent of T(-1/2). To measure the conduction mechanism in a single nanofiber, the dielectrophoresis technique is implemented to position nanofibers on top of two electrodes with a nanogap of 100-600 nm, patterned by electron-beam lithography. After the devices are irradiated by electron beam to reduce contact resistances, their temperature behaviors and electric field dependences are unveiled. The experimental results agree well with the theoretical model of charging energy limited tunneling. Other theoretical models such as Efros-Shklovskii and Motts one-dimensional hopping conduction are excluded after comparisons and arguments. Through fitting, the size of the conductive grain, separation distance between two grains, and charging energy per grain in a single polyaniline nanofiber are estimated to be about 4.9 nm, 2.8 nm, and 78 meV, respectively. The nanotechnological approach, where the nanogap and the dielectrophoresis technique are used for single nanofiber device fabrication, is applied for determination of mesoscopic charge transport in a polyaniline conducting polymer.

Growth, morphology, and crystalline structure of ultrathin Fe films on Cu3Au(100)

The growth mode, morphology, and crystalline structure of Fe films on Cu 3 Au(100) are studied for diVerent growth temperatures (300 and 160 K ), using in situ scanning tunneling microscopy and low energy electron diVraction. Multilayer growth is found to be predominant for both growth temperatures. Only in films of 3‐4 monolayers (ML) grown at 300 K is a mixed mode of layer-bylayer growth and island growth observed. An fcc-to-bcc structural transformation, accompanied by a distinct change in the surface topography, starts at about 3.5 ML and 5.5 ML for the growth temperatures of 300 and 160 K, respectively. For both growth temperatures bcc-like Fe in Fe/Cu 3 Au(100) assumes, most likely through a Bain path, a surface plane with the (100) rather than the (110) orientation found in the Fe/Cu(100) system. Both the surface morphology and the onset thickness of the fcc‐bcc structural transformation are shown to be strongly aVected by the growth temperature.

Self-aligned Co nanoparticle chains supported by single-crystalline Al2O3∕NiAl(100) template

We present Co nanoparticle chains grown by vapor deposition over a single-crystalline Al2O3 layers on NiAl(100) with such features as self-limiting size distribution with the average size of ∼2.7nm, well-ordered alignment, and high thermal stability. We attribute these features to peculiar one-dimensional long stripes with ∼4nm interdistance on the surface of the ultrathin Al2O3 template. This nanostructure may open the door to numerous applications, such as catalysis and nanostorage, where large area well-ordered nanodots are desired.

Digitized Charge Transfer Magnitude Determined by Metal–Organic Coordination Number

Well-ordered metal-organic nanostructures of Fe-PTCDA (perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride) chains and networks are grown on a Au(111) surface. These structures are investigated by high-resolution scanning tunneling microscopy. Digitized frontier orbital shifts are followed in scanning tunneling spectroscopy. By comparing the frontier energies with the molecular coordination environments, we conclude that the specific coordination affects the magnitude of charge transfer onto each PTCDA in the Fe-PTCDA hybridization system. A basic model is derived, which captures the essential underlying physics and correlates the observed energetic shift of the frontier orbital with the charge transfer.

Enhanced Curie temperatures in Fe and Co magnetic nanoparticle assembly on single-crystalline Al2O3∕NiAl(100) with normal metal capping layer

The ferromagnetism of Fe nanoparticle assembly on Al2O3∕NiAl(100) is observed above 150K with the coverage larger than 5 ML (monolayer). Cu capping layer induces an enhancement of the Curie temperature (TC) in both Fe and Co magnetic nanoparticle assembly. The TC of Fe nanoparticle assembly with 2 and 6 ML Cu capping layer is enhanced by ∼20K and even higher, indicating the critical effects of metallic capping layer in such magnetic nanostructures as nanoparticle assembly. The capping layer effect would be crucial for the ex situ measurements and the nanostorage-related applications.

Detailed studies of a high-density polarized hydrogen gas target for storage rings

A high-density target of polarized atomic hydrogen gas for applications in storage rings was produced by injecting atoms from an atomic beam source into a T-shaped storage cell. The influence of the internal gas target on electron-cooled beams of 27 MeV α-particles and 23 MeV protons in the Heidelberg Test Storage Ring has been studied in detail. Target polarization and target thickness were measured by means of 27 MeV α-particles. For hyperfine states 1 + 2 a target thickness of n = (0.96±0.04) × 1014H→/cm2 was achieved with the cell walls cooled to 100 K. Working with a weak magnetic holding field (≈5 G) the maximum target polarization was PT = 0.84±0.02 when state 1 and PT = 0.46±0.01 when states 1 + 2 were injected. The target polarization was found to be constant over a period of 3 months with a net charge of Q ≈ 100C passing the storage cell.

Long-range ordered nanoaperture array with uniform diameter and interpore spacing

In application of focused-ion-beam lithography and grazing Ar+ milling on the U-shape barrier layer of anodic alumina nanochannels, we fabricated a hexagonally symmetry aperture array with nominal diameter of 12±2nm and interspacing of 100±2nm. Besides long-range spatial ordering, the focused-ion-beam guided-grown process has also significantly improved uniformity of both the interpore spacing and the aperture size. This aperture array membrane can be applied to the fabrication of nanostructures, such as a lithographic contact mask for ordered quantum-dot array.

Spin-Dependent Molecule Symmetry at a Pentacene–Co Spinterface

Incorporating spin-polarized scanning tunneling microscopy (SP-STM) measurements and first-principles calculations, we resolve spin-polarized states and consequent features in a pentacene(PEN)-Co hybrid system. Symmetry reduction of PEN clarifies the PEN adsorption site and the Co stacking methods. Near the Fermi energy, the molecular symmetry is spin-dependently recovered and an inversion of spin-polarization in PEN with respect to Co is observed. The experimental findings and calculation results are interpreted by a pz-d hybridization model, in which spin-dependent bonding-antibonding splitting of molecular orbitals happens at metal-organic spinterfaces.

Controlled growth of Co nanoparticle assembly on nanostructured template Al2O3/NiAl(100)

Based on the systematic studies of the growth temperature, deposition rate, and annealing effects, the control of Co nanoparticle density, size, and alignment is demonstrated to be feasible on a nanostructured template Al2O3∕NiAl(100). At 140–170K, a slow deposition rate (0.027ML∕min) promises both the linear alignment and the high particle density. 1.5 ML Co nanoparticle assembly sustains the density of ∼260∕104nm2 even after 800–1090K annealing. This study also indicates the possibilities of the controlled growth for nanoparticles of different materials.

Large single crystal growth, transport property, and spectroscopic characterizations of three-dimensional Dirac semimetal Cd3As2.

The three dimensional (3D) Dirac semimetal is a new quantum state of matter that has attracted much attention recently in physics and material science. Here, we report on the growth of large plate-like single crystals of Cd3As2 in two major orientations by a self-selecting vapor growth (SSVG) method, and the optimum growth conditions have been experimentally determined. The crystalline imperfections and electrical properties of the crystals were examined with transmission electron microscopy (TEM), scanning tunneling microscopy (STM), and transport property measurements. This SSVG method makes it possible to control the as-grown crystal compositions with excess Cd or As leading to mobilities near 5–105 cm2V−1s−1. Zn-doping can effectively reduce the carrier density to reach the maximum residual resistivity ratio (RRRρ300K/ρ5K) of 7.6. A vacuum-cleaved single crystal has been investigated using angle-resolved photoemission spectroscopy (ARPES) to reveal a single Dirac cone near the center of the surface Brillouin zone with a binding energy of approximately 200 meV.