Lauraine Denault

Kinetics and Initial Stages of Bismuth Telluride Electrodeposition

The kinetics and initial stages of Bi 2 Te 3 electrodeposition on gold from an aqueous electrolyte composed of bismuth and telluryl ions and nitric acid were investigated using voltammetry and chronopotentiometry. For tellurium alone, it is found that the reduction of tellurium ions to tellurium occurs with four exchanging electrons at -0.05 V vs Ag/AgCl (3 M KCl). At -0.6 V, tellurium reduces to H 2 Te under kinetic control and the following simultaneous diffusion-controlled reaction occurs with one exchange electron per tellurium atom: HTeO + 2 + H 2 Te + H + + 2e - → 2Te + 2H 2 O. Bismuth telluride deposition starts at more positive potential than bismuth and tellurium deposit individually. It is suggested that the electrodeposition of Bi 2 Te 3 proceeds via a Stransky-Krastanov mechanism. From the Bard correction to the Sand equation, the diffusion coefficients of bismuth and telluryl ions were determined to be 1.90 X 10 -5 and 1.39 X 10 -6 cm 2 /s, respectively, in the investigated solutions. Due to the low diffusivity and solubility of telluryl ions, the limiting current density for Bi 2 Te 3 deposition in the investigated solution is low; it is only 2 mA/cm 2 . Above the limiting current density, a smooth deposit cannot be obtained without agitation. A smooth deposit can be achieved when the depletion of anions is eliminated.

Epitaxial growth of 20 nm InAs and GaAs quantum dots on GaAs through block copolymer templated SiO2 masks

We report on selective area growth of InAs and GaAs quantum dots (QDs) on GaAs through ∼20 nm SiO2 windows prepared by block copolymer lithography. We discuss the mechanisms of growth through these masks, highlighting the variation of the resulting morphology (dot size, spacing, uniformity, and areal density) as a function of growth parameters. We have obtained highly uniform arrays of InAs and GaAs QDs with mean diameters and areal densities of 20.6 nm and 1×1011 cm−2, respectively. We have also investigated the optical characteristics of these QDs as a function of temperature and drawn correlations between the optical response and their crystalline quality.

Mo2C nanowires and nanoribbons on Si by two-step vapor-phase growth

Transition-metal carbides in bulk form have historically been of technological interest primarily due to their excellent mechanical and refractory properties. As electronic materials these ceramic compounds are also particularly intriguing in that their electrical resistivity is relatively low compared to other ceramics and shows metallic temperature-dependent behavior. Some compositions also have superconducting transitions temperatures above 10°K. However, the synthesis of such materials in the form of one-dimensional nanostructures, which may be of interest for various nanoelectronic applications, is relatively difficult due to their refractory nature (Tmelt⩾2000°C). Here we report the synthesis of well-defined Mo2C nanowires and ribbons using a two-step approach in which we catalytically grow metal oxide nanostructures followed by in situ carburization. The growth mechanisms, microstructure, and initial electrical property measurements are discussed.

Infrared p-i-n photodiodes based on InAs quantum dots grown on 20 nm patterned GaAs

We report on selective area growth of InAs quantum dots on GaAs substrates patterned with a hexagonal array of 20 nm pores using block copolymer lithography. We discuss the mechanisms of growth, highlighting the variation in the resulting morphology as a function of nucleation enhancing AlGaAs layers. We also evaluate the optoelectronic performance of p-i-n photodiodes based on single layer nanopatterned grown InAs quantum dot devices. At low to moderate reverse biases, we observe room temperature photoresponse in both near- and mid-IR regimes. At high biases, we observe strong avalanche effects in the mid-IR range with a gain factor of ∼4000.

Mo2C Nanowires and Ribbons on Si via Two-Step Vapor Phase Growth

Transition metal carbides are an interesting class of electronic materials owing to their high electrical conductivity at room temperature, which is only slightly lower than that of their constituent transition metal elements. For example, the room temperature electrical resistivity of bulk Mo2 C is ∼70 μΩ-cm compared to that of Mo (4.85 μΩ-cm), whereas that of NbC is ∼50 μΩ-cm as compared to 15.2 μΩ-cm for Nb. Indeed, the temperature dependent resistivity of many transition metal carbides suggests metallic-like conduction. Furthermore, certain transition metal carbides are known to become superconducting, with transition temperatures ranging from 1.15 °K for TiC1−x to 14 °K for NbC. [1] They are also able to withstand high temperatures and are chemically stable. Initial synthesis of metal carbide nanorods was demonstrated using the carbon nanotube (CNT) confined reaction mechanism by Lieber and co-workers [2] and subsequent superconducting behavior was shown by Fukunaga et al. [3]. Vapor-liquid-solid growth was employed by Johnson et al. [4] to synthesize micron-sized carbide whiskers. Here, we have successfully synthesized Mo2 C nanorods and ribbons on Si substrates using a novel two-step catalytic approach, which allows for synthesis of such high temperature nanostructures at manufacturable temperatures (≤ 1000 °C) and time scales (≤ 60 min). In the first step we utilize a catalytic vapor phase process to grow Mo and/or molybdenum oxide nanostructures, which are subsequently carburized in situ to form the desired Mo2 C nanostructures. Unlike true VLS growth of carbides, in which high temperature (≤ 1100–1200 °C) is required to adequately dissolve carbon into the catalyst particles, our strategy is to react the nanostructures along their entire length with a carbon vapor source after creating the oxide/metal nanostructures, which for Mo2 C can be achieved at relatively low temperatures. (≤ 1000 °C). The nanorods and ribbons are polycrystalline, with a mean grain size of 20–50 nm and 50–150 nm, respectively. We hypothesize that the growth mechanism is a complex mixture of VLS, VSS, and auto-catalytic growth, in which molten catalyst nanoparticles enter a three phase region once the metal precursor is supplied. The growth then presumably continues via a vapor-solid-solid process and is possible assisted by the presence of various molybdenum oxide species on the surface. Initial single nanowire electrical measurements yield a higher resistivity than in the bulk, which is attributed to the fine grain sizes and/or the presence of an oxide layer. A discussion of the growth mechanism will be presented along with issues relating to single nanowire device fabrication and control of nanowire orientation.Copyright

Mineral Classification Using Computer-Controlled Scanning Electron Microscopy

Computer-controlled scanning electron microscopy (CC-SEM) is an automated technique combining electron microscopy, image analysis, and X-ray spectroscopy to rapidly acquire morphological and chemical information for thousands of individual particles. Measurements are typically performed on polished cross-sections of material embedded in epoxy mounts. Individual particles are first identified from the epoxy matrix based on user-specified criteria (e.g., pre-set grayscale threshold from a representative Back-Scattered Electron image). Following identification, morphologic parameters (e.g., area, perimeter, aspect ratio, etc.) and an X-ray spectrum (EDS) are recorded for each particle. Multiple fields of view can be analyzed, resulting in many thousands of individual particle analyses collected during a single CC-SEM session. The resulting data can then be mined for useful information like particle size distributions (PSDs), phase proportions, and even bulk chemistry.