Peter M. Wilson

Highly selective gas sensor arrays based on thermally reduced graphene oxide

The electrical properties of reduced graphene oxide (rGO) have been previously shown to be very sensitive to surface adsorbates, thus making rGO a very promising platform for highly sensitive gas sensors. However, poor selectivity of rGO-based gas sensors remains a major problem for their practical use. In this paper, we address the selectivity problem by employing an array of rGO-based integrated sensors instead of focusing on the performance of a single sensing element. Each rGO-based device in such an array has a unique sensor response due to the irregular structure of rGO films at different levels of organization, ranging from nanoscale to macroscale. The resulting rGO-based gas sensing system could reliably recognize analytes of nearly the same chemical nature. In our experiments rGO-based sensor arrays demonstrated a high selectivity that was sufficient to discriminate between different alcohols, such as methanol, ethanol and isopropanol, at a 100% success rate. We also discuss a possible sensing mechanism that provides the basis for analyte differentiation.

Multilayer Graphitic Coatings for Thermal Stabilization of Metallic Nanostructures

We demonstrate that graphitic coatings, which consist of multilayer disordered graphene sheets, can be used for the thermal protection of delicate metal nanostructures. We studied cobalt slanted nanopillars grown by glancing angle deposition that were shown to melt at temperatures much lower than the melting point of bulk cobalt. After graphitic coatings were conformally grown over the surfaces of Co nanopillars by chemical vapor deposition, the resulting carbon-coated Co nanostructures retained their morphology at elevated temperatures, which would damage the uncoated structures. Thermal stabilization is also demonstrated for carbon-coated Ti nanopillars. The results of this study may be extended to other metallic and possibly even nonmetallic nanostructures that need to preserve their morphology at elevated temperatures in a broad range of applications.

Anisotropy, band-to-band transitions, phonon modes, and oxidation properties of cobalt-oxide core-shell slanted columnar thin films

Highly ordered and spatially coherent cobalt slanted columnar thin films (SCTFs) were deposited by glancing angle deposition onto silicon substrates, and subsequently oxidized by annealing at 475 °C. Scanning electron microscopy, Raman scattering, generalized ellipsometry, and density functional theory investigations reveal shape-invariant transformation of the slanted nanocolumns from metallic to transparent metal-oxide core-shell structures with properties characteristic of spinel cobalt oxide. We find passivation of Co-SCTFs yielding Co-Al2O3 core-shell structures produced by conformal deposition of a few nanometers of alumina using atomic layer deposition fully prevents cobalt oxidation in ambient and from annealing up to 475 °C.

Structural and optical properties of cobalt slanted nanopillars conformally coated with few-layer graphene

Optical characterization of anisotropic multicomponent nanostructures is generally not a trivial task, since the relation between a materials structural properties and its permittivity tensor is nonlinear. In this regard, an array of slanted cobalt nanopillars that are conformally coated with few-layer graphene is a particularly challenging object for optical characterization, as it has a complex anisotropic geometry and comprises several materials with different topologies and filling fractions. Normally, a detailed characterization of such complex nanostructures would require a combination of several microscopic and spectroscopic techniques. In this letter, we demonstrate that the important structural parameters of these graphene-coated sculptured thin films can be determined using a fast and simple generalized spectroscopic ellipsometry test combined with an anisotropic Bruggeman effective medium approximation. The graphene coverage as well as structural parameters of nanostructured thin films agree e...X iv :1 31 2. 53 63 v1 [ co nd -m at .m tr lsc i] 1 8 D ec 2 01 3 Structural and optical properties of cobalt slanted columnar thin films conformally coated with graphene by chemical vapor deposition Peter M. Wilson, Daniel Schmidt, Eva Schubert, Mathias Schubert, Alexander Sinitskii, and Tino Hofmann a) Department of Chemistry, and Center for Nanohybrid Functional Materials, University of Nebraska-Lincoln, Lincoln, Nebraska, 68588, USA Department of Electrical Engineering, and Center for Nanohybrid Functional Materials, University of Nebraska-Lincoln, Lincoln, Nebraska, 68588, USA

Chemical vapour deposition and characterization of uniform bilayer and trilayer MoS2 crystals

Molybdenum disulfide (MoS2) is a promising two-dimensional semiconductor for applications in electronics, optoelectronics and catalysis. Chemical vapor deposition (CVD) is a popular approach for the large-scale growth of thin MoS2 crystals. As the properties of MoS2 strongly depend on the number of layers, it is important to reliably grow MoS2 crystals with different thicknesses. In this paper, we present a CVD procedure for MoS2 growth from MoO3 and S, which yields predominantly bilayer and trilayer MoS2 triangular islands as opposed to monolayer MoS2 triangles typically observed in similar CVD experiments. The growth of bilayer and trilayer MoS2 crystals is achieved by increasing the flow rate of sulfur after the original nucleation of MoS2 triangles. Most bilayer MoS2 crystals are uniform in height, such that in a typical crystal the top layer fully extends to the edges of the bottom layer. While trilayer MoS2 crystals grown by this procedure are in general less uniform than bilayers and often form terraced structures, it is still common to observe uniform trilayer MoS2 triangles as well. In addition to standard characterization methods for MoS2, such as Raman spectroscopy, atomic force microscopy and photoluminescence microscopy we demonstrate that scanning electron microscopy can be used to distinguish between monolayer and few-layered MoS2 flakes at low accelerating voltages. The field-effect transistors based on CVD-grown MoS2 triangles have electron mobilities reaching ∼10 cm2 V−1 s−1 and ON/OFF ratios reaching ∼105. The reported CVD procedure can be used for growing large quantities of uniform bilayer and trilayer MoS2 crystals for materials studies.

Low-temperature thermal reduction of graphene oxide: In situ correlative structural, thermal desorption, and electrical transport measurements

Elucidation of the structural transformations in graphene oxide (GO) upon reduction remains an active and important area of research. We report the results of in situ heating experiments, during which electrical, mass spectrometry, X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and transmission electron microscopy (TEM) measurements were carried out correlatively. The simultaneous electrical and temperature programmed desorption measurements allowed us to correlate the onset of the increase in the electrical conductivity of GO by five orders of magnitude at about 150 °C with the maxima of the rates of desorption of H2O, CO, and CO2. Interestingly, this large conductivity change happens at an intermediate level of the reduction of GO, which likely corresponds to the point when the graphitic domains become large enough to enable percolative electronic transport. We demonstrate that the gas desorption is intimately related to (i) the changes in the chemical structure of GO detected by XPS and Ra...

Oxidative peeling of carbon black nanoparticles

We demonstrate that layered carbon black nanoparticles can be oxidatively peeled via the reaction with potassium permanganate in sulfuric acid. As a result of this reaction, outer layers of carbon nanoparticles “peel” off due to high levels of oxidation while the less oxidized inner cores, though they exhibit remarkable solubility in water, remain mostly intact.