Theory of spin diffusion of dilute,polarized fermions for all temperatures

We describe the solution of the transport equation over the entire range of temperature from the Boltzmann to fully degenerate regimes for dilute, polarized Fermi systems. Since spin-polarized systems can show unusual quantum effects involving spin rotation in both Boltzmann and degenerate regimes, a solution of the kinetic equation over the whole temperature range is expected to be useful. Our results for the longitudinal spin diffusion coefficient reduce to the known limits in the Boltzmann and degenerate regimes and also to the expected form in the peculiar high-polarization regime in which one spin species is degenerate and the other described by classical statistics (the degenerate-classical case). We derive numerical results for the spin-rotation quality parameter over the full temperature range. Unlike experimental results that show diminishing anomalously as the temperature decreases toward the degenerate regime, our value for is monotonically increasing. However, the transition to the degenerate-classical regime is found to occur with a rounded-step jump in as a function of polarization.     

Compositional analysis of electrodeposited bismuth telluride thermoelectric thin films using combined electrochemical quartz crystal microgravimetry--stripping voltammetry

Bismuth telluride (Bi 2Te 3 ) is a benchmark material for thermoelectric power generation and cooling applications. Electrodeposition is a versatile technique for preparing thin films of this material; however, it affords films of variable composition depending on the preparation history. A simple and rapid assay of electrodeposited films, therefore, has both fundamental and practical importance. In this study, a new protocol for the electroanalysis of Bi 2Te 3 thin films is presented by combining the two powerful and complementary techniques of electrochemical quartz crystal microgravimetry (EQCM) and stripping voltammetry. First, any free (and excess) tellurium in the electrodeposited film was reduced to soluble Te ( 2- ) species by scanning to negative potentials in a 0.1 M Na 2SO 4 electrolyte, and the accompanying frequency increase (mass loss) was used to determine the content of free tellurium. The film was again subjected to cathodic stripping in the same medium (to generate Bi (0) and soluble Te (2-) from the Bi 2 Te 3 film component of interest), and the EQCM frequency change was used to determine the content of chemically bound Te in the Bi 2Te 3 thin film and thereby the compound stoichiometry. Finally, the EQCM frequency change during Bi oxidation to Bi (3+) and the difference between total Bi and Bi in Bi 2Te 3 resulted in the assay of free (excess) Bi in the electrodeposited film. Problems associated with the chemical/electrochemical stability of the free Bi species were circumvented by a flow electroanalysis approach. Data are also presented on the sensitivity of electrodeposited Bi 2Te 3 film composition to the electrodeposition potential. This newly developed method can be used for the compositional analysis of other thermoelectric thin-film material candidates in general.     

Laterally graded porous silicon optical filter fabricated by diffusion-limited etch process

We report a method of producing a lateral gradient in the optical properties of anodically etched porous silicon layers. Lateral gradation details of the porous silicon layer are governed by the etch mask pattern involved. Unlike other methods that rely on uneven hole current distribution, we believe that in our method the diffusion of reactive ions in the etchant plays a key role. As an implementation of the proposed method, we demonstrate a linearly graded optical bandpass filter operating at the lambda=1550 nm range by employing a tapered etch window opening. The resultant optical filter exhibited a approximate 60 nm tuning range with a sharp transmission bandwidth of approximately 3 nm. Computer simulations indicate that an uneven hole current distribution cannot be the reason for the observed gradient along the taper axis, supporting the view that the diffusion-limited etch process plays the key role.     

Major aromatic VOC in the ambient air in the proximity of an urban landfill facility

In this study, the environmental behavior of major aromatic VOC (including benzene, toluene, ethylbenzene and xylene, commonly called BTEX) in the ambient air was investigated from a mid-size municipal landfill site located in Dae Gu city, Korea in the winter of 2004. A series of field campaigns were conducted in the course of the study to cover eight different locations within and near this landfill site along with a number of VOC vent systems. The mean concentrations of different VOC species in ambient air fell in a comparable range of at or above a few ppb (e.g., the most abundant toluene 10 ppb). An inspection of the VOC data sets at the studied LF sites also indicated that they are quite analogous to those typically found in other urban areas in terms of their absolute magnitude and relative pattern (e.g., the general dominance of toluene over the other species). In light of the fact that there is active ventilation of landfill gas (LFG: e.g., with their LFG concentrations above a few to a few tens of ppm) in the study area with no other distinct source processes, it can be concluded that the effects of the landfill processes may be as important as other point sources in maintaining VOC concentration levels in certain urban areas.     

The effect of environmental factors on the electrical conductivity of a single oligo-DNA molecule measured using single-walled carbon nanotube nanoelectrodes

We present an electrical conductivity study on a double-stranded DNA molecule bridging a single-walled carbon nanotube (SWNT) gap. The amine terminated DNA molecule was trapped between carboxyl functionalized SWNT electrodes by dielectrophoresis. The conductivity of DNA was measured while under the influence of various environmental factors, including salt concentration, counterion variation, pH and temperature. Typically, a current of tens of picoamperes at 1?V was observed at ambient conditions, with a decrease in conductance of about 33% in high vacuum conditions. The counterion variation was analyzed by changing the buffer from sodium acetate to tris(hydroxymethyl) aminomethane, which resulted in a two orders of magnitude increase in the conductivity of the DNA. A reversible shift in the current signal was observed for pH variation. An increase in conductivity of the DNA was also observed at high salt concentrations.     

Finite size effect on hydrogen gas sensing performance in single Pd nanowires

We present the hydrogen sensing performance of individual Pd nanowires grown by electrodeposition into nanochannels of anodized aluminum oxide (AAO) templates investigated as a function of the nanowire diameter. Four-terminal devices based on individual Pd nanowires were found to successfully detect hydrogen gas (H(2)). Our experimental results show that the H(2) sensing sensitivity increases and the response time decreases with decreasing diameter of Pd nanowires with d = 400, 200, 80 and 20?nm, due to the high surface-to-volume ratio and short diffusion paths, respectively. This is in qualitatively good agreement with simulated results obtained from a theoretical model based on a combination of the rate equation and diffusion equation.     

Nanocluster deposition for oxide thin film growth at near room temperature

Metal-organic chemical vapor deposition (MOCVD) at near room temperature would not only enable integration of oxide films on polymers but would provide the capability of conformal coating of high-aspect ratio features required for fabrication of many micro-and nanoelectronic devices. The concept of near room temperature MOCVD (nanocluster deposition: NCD) consists of the production of a single phase with nanosized crystalline nuclei by a chemical vapor reaction at the showerhead maintained above the decomposition temperature of the precursors and consequently deposition of the nanosized crystalline films on unheated substrates. Deposition of the nanosized crystalline nuclei on unheated substrates was performed by controlling both the showerhead temperature and the working pressure. The Bi(3)NbO(7) (BNO) films deposited without substrate heating (real temperature of substrate surface: 50?°C) exhibit a crystalline single phase with smooth and dense morphologies, a dielectric constant of 30, a leakage current density of ～10(-6)?A?cm(-2) at 0.3?MV?cm(-1) and a step coverage of approximately 93% for films deposited at 100?°C on high-aspect ratio features. An NCD provides a new platform for near room temperature deposition of oxide thin films, opening the way for film deposition on polymer substrates to enable a flexible electronic device technology.