Stephen W. Howell

Mass-fabricated one-dimensional silicon nanogaps for hybrid organic/nanoparticle arrays

Optical lithography based on microfabrication techniques was employed to fabricate one-dimensional nanogaps with micrometre edge lengths in silicon. These one-dimensional nanogaps served as a platform on which organic/nanoparticle films were assembled. Characterization of the gaps was performed with high-resolution TEM, SEM, and electrical measurements. Novel self-assembling attachment chemistry, based on the interaction of silicon with a diazonium salt, was used to iteratively build a multi-layer nanoparticle film across a 7 nm gap. By using nanoparticles capped with an easily displaced ligand, a variable conductive path was created across the 1D nanogap. Electrical measurements of the gap showed a dramatic change in the I(V) characteristics after assembly of the nanoparticle film.

Vapor Sensing Using Conjugated Molecule-Linked Au Nanoparticles in a Silica Matrix

Cross-linked assemblies of nanoparticles are of great value as chemiresistor-type sensors. Herein, we report a simple method to fabricate a chemiresistor-type sensor that minimizes the swelling transduction mechanism while optimizing the change in dielectric response. Sensors prepared with this methodology showed enhanced chemoselectivity for phosphonates which are useful surrogates for chemical weapons. Chemoselective sensors were fabricated using an aqueous solution of gold nanoparticles that were then cross-linked in the presence of the silica precursor, tetraethyl orthosilicate with the -, -dithiolate (which is derived from the in situ deprotection of 1,4-di(Phenylethynyl-,-diacetylthio)-benzene (1) with wet triethylamine). The cross-linked nanoparticles and silica matrix were drop coated onto interdigitated electrodes having 8 m spacing. Samples were exposed to a series of analytes including dimethyl methylphosphonate (DMMP), octane, and toluene. A limit of detection was obtained for each analyte. Sensors assembled in this fashion were more sensitive to dimethyl methylphosphonate than to octane by a factor of 1000.

Novel one-dimensional nanogap created with standard optical lithography and evaporation procedures

This article details a simple four-step procedure to create a one-dimensional nanogap on a buried oxide substrate that relies on conventional photolithography performed on a stack of silicon/silicon oxide/silicon, metal evaporation, and hydrofluoric acid oxide removal. Once the nanogap was fabricated it was bridged with an assembly of 1,8-octanedithiol and 5 nm Au nanoparticles capped with a sacrificial dodecylamine coating. Before assembly, characterization of the nanogaps was performed through electrical measurements and SEM imaging. Post assembly, the resistance of the nanogaps was evaluated. The current increased from 70 fA to 200 microA at +1 V bias, clearly indicating a modification due to nanoparticle molecule assembly. Control experiments without nanoparticles or octanedithiol did not show an increase in current.

Near-infrared surface plasmon polariton dispersion control with hyperbolic metamaterials

We demonstrate experimentally signatures and dispersion control of surface plasmon polaritons from 1 to 1.8 µm using periodic multilayer metallo-dielectric hyperbolic metamaterials. The fabricated structures are comprised of smooth films with very low metal filling factor. The measured dispersion properties of these hyperbolic metamaterials agree well with calculations using transfer matrix, finite-difference time-domain, and effective medium approximation methods despite using only 2.5 periods. The enhancement factor in the local photonic density of states from the studied samples in the near-infrared wavelength region is determined to be 2.5-3.5. Development of this type of metamaterial is relevant to sub-wavelength imaging, spontaneous emission and thermophotovoltaic applications.

Observation of the integer quantum Hall effect in high quality, uniform wafer-scale epitaxial graphene films

We report the observation of the integer quantum Hall states at Landau level fillings of ν=2, 6, and 10 in a Hall bar device made of a single-layer epitaxial graphene film on the silicon-face of silicon-carbide prepared via argon-assisted graphitization. The two-dimensional electron gas exhibits a low-temperature (at 4 K) carrier mobility of ∼14u2009000u2002cm2/Vu2009s at the electron density of 6.1×1011u2002cm−2. Furthermore, the sheet resistance obtained from four-probe measurements across the whole area (12×6u2002mm2) of another specimen grown under similar condition displays roughly uniform values (∼1600u2002Ω/square), suggesting that the macroscopic steps and accompanying multilayer graphene domains play a minor role in the low-temperature electronic transport.

Oxidation of ultrathin GaSe

Oxidation of exfoliated gallium selenide (GaSe) is investigated through Raman, photoluminescence, Auger, and X-ray photoelectron spectroscopies. Photoluminescence and Raman intensity reductions associated with spectral features of GaSe are shown to coincide with the emergence of signatures emanating from the by-products of the oxidation reaction, namely, Ga2Se3 and amorphous Se. Photoinduced oxidation is initiated over a portion of a flake highlighting the potential for laser based patterning of two-dimensional heterostructures via selective oxidation.

Electrostatic transfer of patterned epitaxial graphene from SiC(0001) to glass

We report on a scalable electrostatic process to transfer epitaxial graphene onto alkali-containing glass substrates. Multilayer epitaxial graphene (MEG) was grown by heating silicon carbide () to high temperatures (1650?1700??C) in an argon-mediated environment. Optical lithography was used to define patterned graphene regions, typically 20?20??m2, which were then transferred to Pyrex substrates. For the electrostatic transfer, a large electric potential (1.2?kV) was applied between the donor MEG sample (anode) and the heated acceptor glass substrate (cathode). Atomic force microscopy scans of the transferred graphene showed that the morphology of the transferred multilayer graphene resembles that of the donor MEG. Raman spectroscopy analysis confirmed that the graphene can be transferred without inducing defects. The sheet resistance of the transferred graphene was as low as 150??/. The transfer of small (1?2??m wide) and large (~70?70??m2) graphene patterns to Zerodur demonstrates the versatility of this transfer technique.

Application of plasmonic subwavelength structuring to enhance infrared detection

Nanoantennas are an enabling technology for visible to terahertz components and may be used with a variety of detector materials. We have integrated subwavelength patterned metal nanoantennas with various detector materials for infrared detection: midwave infrared indium gallium arsenide antimonide detectors, longwave infrared graphene detectors, and shortwave infrared germanium detectors. Nanoantennas offer a means to make infrared detectors much thinner, thus lowering the dark current and improving performance. The nanoantenna converts incoming plane waves to more tightly bound and concentrated surface waves. The active material only needs to extend as far as these bound fields. In the case of graphene detectors, which are only one or two atomic layers thick, such field concentration is a necessity for usable device performance, as single pass absorption is insufficient. The nanoantenna is thus the enabling component of these thin devices. However nanoantenna integration and fabrication vary considerably across these platforms as do the considerations taken into account during design. Here we discuss the motivation for these devices and show examples for the three material systems. Characterization results are included for the midwave infrared detector.

Electron?electron interaction in high-quality epitaxial graphene

Weak localization is studied in two high-quality epitaxial graphene samples grown on silicon-faced 6H-SiC substrates. Following the methodology of Kozikov et al (2010 Phys. Rev. B 82 075424), we measured the temperature dependence of carrier conductivity at zero and low magnetic (B) fields. In both samples, a logarithmic temperature dependence of the carrier conductivity was observed at Bxa0=xa00 and its amplitude was larger than predicted by a single-particle model, suggesting that electron–electron interaction plays an important role in electron transport in epitaxial graphene films.

Chelant Enhanced Solution Processing for Wafer Scale Synthesis of Transition Metal Dichalcogenide Thin Films

It is of paramount importance to improve the control over large area growth of high quality molybdenum disulfide (MoS2) and other types of 2D dichalcogenides. Such atomically thin materials have great potential for use in electronics, and are thought to make possible the first real applications of spintronics. Here in, a facile and reproducible method of producing wafer scale atomically thin MoS2 layers has been developed using the incorporation of a chelating agent in a common organic solvent, dimethyl sulfoxide (DMSO). Previously, solution processing of a MoS2 precursor, ammonium tetrathiomolybdate ((NH4)2MoS4), and subsequent thermolysis was used to produce large area MoS2 layers. Our work here shows that the use of ethylenediaminetetraacetic acid (EDTA) in DMSO exerts superior control over wafer coverage and film thickness, and the results demonstrate that the chelating action and dispersing effect of EDTA is critical in growing uniform films. Raman spectroscopy, photoluminescence (PL), x-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), atomic force microscopy (AFM) and high-resolution scanning transmission electron microscopy (HR-STEM) indicate the formation of homogenous few layer MoS2 films at the wafer scale, resulting from the novel chelant-in-solution method.