Luis A. Jauregui

Rational synthesis of ultrathin n-type Bi2Te3 nanowires with enhanced thermoelectric properties.

A rational yet scalable solution phase method has been established, for the first time, to obtain n-type Bi(2)Te(3) ultrathin nanowires with an average diameter of 8 nm in high yield (up to 93%). Thermoelectric properties of bulk pellets fabricated by compressing the nanowire powder through spark plasma sintering have been investigated. Compared to the current commercial n-type Bi(2)Te(3)-based bulk materials, our nanowire devices exhibit an enhanced ZT of 0.96 peaked at 380 K due to a significant reduction of thermal conductivity derived from phonon scattering at the nanoscale interfaces in the bulk pellets, which corresponds to a 13% enhancement compared to that of the best n-type commercial Bi(2)Te(2.7)Se(0.3) single crystals (~0.85) and is comparable to the best reported result of n-type Bi(2)Te(2.7)Se(0.3) sample (ZT = 1.04) fabricated by the hot pressing of ball-milled powder. The uniformity and high yield of the nanowires provide a promising route to make significant contributions to the manufacture of nanotechnology-based thermoelectric power generation and solid-state cooling devices with superior performance in a reliable and a reproducible way.

Electronic transport in chemical vapor deposited graphene synthesized on Cu: Quantum Hall effect and weak localization

We report on electronic properties of graphene synthesized by chemical vapor deposition (CVD) on copper then transferred to SiO2/Si. Wafer-scale (up to 4 in.) graphene films have been synthesized, consisting dominantly of monolayer graphene as indicated by spectroscopic Raman mapping. Low temperature transport measurements are performed on microdevices fabricated from such CVD graphene, displaying ambipolar field effect (with on/off ratio ∼5 and carrier mobilities up to ∼3000 cm2/V s) and “half-integer” quantum Hall effect, a hall-mark of intrinsic electronic properties of monolayer graphene. We also observe weak localization and extract information about phase coherence and scattering of carriers.

Growth of Single Crystal Graphene Arrays by Locally Controlling Nucleation on Polycrystalline Cu Using Chemical Vapor Deposition

Graphene, a single atomic layer of hexagonally packed carbon atoms, has drawn signifi cant attention with its outstanding electrical, [ 1 ] mechanical, [ 2 , 3 ] and chemical properties. [ 4 , 5 ] Various promising applications based on graphene have been demonstrated, such as in electronics, [ 6 , 7 ] optoelectronics, [ 8 , 9 ] and chemical and biological sensing. [ 10–12 ] To further envision graphene technology, it is critical to synthesize high-quality graphene on a large scale. Since the fi rst mechanical isolation of graphene from graphite crystal in 2004, [ 13 ] intense efforts have been made to develop methods for graphene synthesis, including reduction of graphene oxide, [ 14 ] thermal decomposition of SiC, [ 15 , 16 ]

Nontoxic and Abundant Copper Zinc Tin Sulfide Nanocrystals for Potential High-Temperature Thermoelectric Energy Harvesting

Improving energy/fuel efficiency by converting waste heat into electricity using thermoelectric materials is of great interest due to its simplicity and reliability. However, many thermoelectric materials are composed of either toxic or scarce elements. Here, we report the experimental realization of using nontoxic and abundant copper zinc tin sulfide (CZTS) nanocrystals for potential thermoelectric applications. The CZTS nanocrystals can be synthesized in large quantities from solution phase reaction and compressed into robust bulk pellets through spark plasma sintering and hot press while still maintaining nanoscale grain size inside. Electrical and thermal measurements have been performed from 300 to 700 K to understand the electron and phonon transports. Extra copper doping during the nanocrystal synthesis introduces a significant improvement in the performance.

Effect of electron-beam irradiation on graphene field effect devices

Electron beam exposure is a commonly used tool for fabricating and imaging graphene-based devices. Here, we present a study of the effects of electron-beam irradiation on the electronic transport properties of graphene and the operation of graphene field-effect transistors (GFETs). Exposure to a 30 keV electron-beam caused negative shifts in the charge-neutral point (CNP) of the GFET, interpreted as due to n-doping in the graphene from the interaction of the energetic electron beam with the substrate. The shift in the CNP is substantially reduced for suspended graphene devices. The electron beam is seen to also decrease the carrier mobilities and minimum conductivity, indicating defects created in the graphene. The findings are valuable for understanding the effects of radiation damage on graphene and for the development of radiation-hard graphene-based electronics.

Thermal Transport in Graphene Nanostructures: Experiments and Simulations

a Birck Nanotechnology Center, Purdue University, West Lafayette, IN 47907 b School of Electrical and Computer Engineering, Purdue University, West Lafayette, IN 47907 c Department of Mechanical Engineering, Iowa State University, Ames, IA 50011 d School of Physics, Georgia Institute of Technology, Atlanta, GA 30332 e Department of Electrical and Computer Engineering, University of Houston, Houston, Texas 77204 f Department of Physics, Purdue University, West Lafayette, IN 47907 g School of Mechanical Engineering, Purdue University, West Lafayette, IN 47907

Electrical and thermal conductivities of reduced graphene oxide/polystyrene composites

The author reports an experimental study of electrical and thermal transport in reduced graphene oxide (RGO)/polystyrene (PS) composites. The electrical conductivity (σ) of RGO/PS composites with different RGO concentrations at room temperature shows a percolation behavior with the percolation threshold of ∼0.25 vol. %. Their temperature-dependent electrical conductivity follows Efros-Shklovskii variable range hopping conduction in the temperature range of 30–300 K. The thermal conductivity (κ) of composites is enhanced by ∼90% as the concentration is increased from 0 to 10 vol. %. The thermal conductivity of composites approximately linearly increases with increasing temperature from 150 to 300 K. Composites with a higher concentration show a stronger temperature dependence in the thermal conductivity.

Scanning gate microscopy on graphene: charge inhomogeneity and extrinsic doping

We have performed scanning gate microscopy (SGM) on graphene field effect transistors (GFET) using a biased metallic nanowire coated with a dielectric layer as a contact mode tip and local top gate. Electrical transport through graphene at various back gate voltages is monitored as a function of tip voltage and tip position. Near the Dirac point, the response of graphene resistance to the tip voltage shows significant variation with tip position, and SGM imaging displays mesoscopic domains of electron-doped and hole-doped regions. Our measurements reveal substantial spatial fluctuation in the carrier density in graphene due to extrinsic local doping from sources such as metal contacts, graphene edges, structural defects and resist residues. Our scanning gate measurements also demonstrate graphenes excellent capability to sense the local electric field and charges.

Ambipolar graphene field effect transistors by local metal side gates

We demonstrate ambipolar graphene field effect transistors individually controlled by local metal side gates. The side gated field effect can have on/off ratio comparable with that of the global back gate, and can be tuned in a large range by the back gate and/or a second side gate. We also find that the side gated field effect is significantly stronger by electrically floating the back gate compared to grounding the back gate, consistent with the finding from electrostatic simulation.

Probing dark excitons in atomically thin semiconductors via near-field coupling to surface plasmon polaritons

Transition metal dichalcogenide (TMD) monolayers with a direct bandgap feature tightly bound excitons, strong spin-orbit coupling and spin-valley degrees of freedom. Depending on the spin configuration of the electron-hole pairs, intra-valley excitons of TMD monolayers can be either optically bright or dark. Dark excitons involve nominally spin-forbidden optical transitions with a zero in-plane transition dipole moment, making their detection with conventional far-field optical techniques challenging. Here, we introduce a method for probing the optical properties of two-dimensional materials via near-field coupling to surface plasmon polaritons (SPPs). This coupling selectively enhances optical transitions with dipole moments normal to the two-dimensional plane, enabling direct detection of dark excitons in TMD monolayers. When a WSe2 monolayer is placed on top of a single-crystal silver film, its emission into near-field-coupled SPPs displays new spectral features whose energies and dipole orientations are consistent with dark neutral and charged excitons. The SPP-based near-field spectroscopy significantly improves experimental capabilities for probing and manipulating exciton dynamics of atomically thin materials, thus opening up new avenues for realizing active metasurfaces and robust optoelectronic systems, with potential applications in information processing and communication.