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At least 1,045 records · Page 58

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DOE OSTI2015

Long-term stability assessment of AlGaN/GaN field effect transistors modified with peptides: Device characteristics vs. surface properties

AlGaN/GaN Field Effect Transistors (FETs) are promising biosensing devices. Functionalization of these devices is explored in this study using an in situ approach with phosphoric acid etchant and a phosphonic acid derivative. Devices are terminated on peptides and soaked in water for up to 168 hrs to examine FETs for both device responses and surface chemistry changes. Measurements demonstrated threshold voltage shifting after the functionalization and soaking processes, but demonstrated stable FET behavior throughout. X-ray photoelectron spectroscopy and atomic force microscopy confirmed peptides attachment to device surfaces before and after water soaking. Results of this work point to the stability of peptide coated functionalized AlGaN/GaN devices in solution and support further research of these devices as disposable, long term, in situ biosensors.

36 MATERIALS SCIENCE↗

DOE OSTI2011

Stepwise Assembly of Coordination-Based Metal-Organic Networks

Metal-organic networks (MONs) were created by a stepwise solution deposition approach from vinylpyridine-based building blocks and PdCl{sub 2}. The combined experimental and computational study demonstrates the formation of saturated, structurally organized systems on solid supports. The rigid nature and geometry of the components are well-suited to form honeycomb and parallelogram structures, as predicted by a computational study. Detailed structural information of the new MONs was obtained by optical (UV/vis) spectroscopy, ellipsometry, atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and synchrotron X-ray reflectivity (XRR). Notably, the XPS elemental composition indicates the formation of a palladium coordination-based network.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2016

Characterisation of reduced graphene oxides prepared from natural flaky, lump and amorphous graphites

Highlights: • Natural flaky, lumpy and amorphous graphites were used to synthesis rGO. • Investigation the effect of the crystal morphology on the oxidation process of GrO and characteristics of prepared rGO. • Low graphitisation degree, big specific surface area and small lateral size were beneficial to the oxidation of graphite. - Abstract: The characterisation of reduced graphene oxides (rGOs) prepared from natural flaky, lumpy, and amorphous graphites using Hummers method was investigated. The prepared graphite oxides (GrOs) and rGOs were characterised by X-ray diffraction, Fourier transform infrared spectroscopy, Raman spectroscopy, UV–vis spectroscopy, atomic force microscopy and electrochemical performance. The results showed that amorphous graphite was much easier to oxidise than lumpy and flaky graphites and was preferable for preparing single or double layer graphene because low graphitisation degree, high defect degree, high specific surface area and small crystal size were beneficial for (1) the oxidants to attack the exposed carbon atoms, (2) the intercalation of oxidants, and (3) the diffusion of oxidants between graphitic layers. In addition, rGO synthesised from amorphous graphite had the most defects and the smallest size of the in-plane sp{sup 2} domains compared to those obtained from the other two nature graphites.

36 MATERIALS SCIENCE↗

DOE OSTI2014

A facile one-pot method to Au–SnO{sub 2}-graphene ternary hybrid

In this article, we propose a facile one-pot route for synthesizing Au–SnO{sub 2}-graphene ternary hybrid. In the system, SnCl{sub 2} not only as the precursor of SnO{sub 2}, but also is employed as reducing agent for the effective reduction of both GO and HAuCl{sub 4} to graphene and Au nanoparticles, respectively. The obtained Au–SnO{sub 2}-graphene hybrid materials are characterized by atomic force microscopy, transmission electron microscopy, X-ray diffraction, Raman spectrum, X-ray photo-electron spectroscopy, and thermal gravimetric analysis. It is found that the content of Au nanoparticles decorated on the surface of graphene can be simply adjusted by changing the amount of HAuCl{sub 4} used in the synthesis process.

36 MATERIALS SCIENCE↗

DOE OSTI2016

Enhanced organic photovoltaic properties via structural modifications in PEDOT:PSS due to graphene oxide doping

Highlights: • Graphene oxide(GO) blended with PEDOT:PSS is used as HTL for PTB7:PCBM BHJ solar cells. • Increase in conductivity due to structural alterations in PEDOT:PSS by GO addition. • The structural alterations are reaveled under Raman spectroscopy, XPS and AFM. • PEDOT:PSS changed to extended coil due to addition of GO to PEDOT:PSS. • Enhanced conductivity after GO addition to PEDOT:PSS resulted in enhanced PCE. - Abstract: Poly(3,4-thylenedioxythiophene):poly(styrene sulfonate), PEDOT:PSS is a well-known conductive polymer for hole transport in organic devices, the properties of which can be enhanced by doping. Common dopants are metal oxides and nanoparticles. In this study, addition of graphene oxide (GO) to PEDOT:PSS as a dopant is addressed in organic photovoltaics (OPVs). With GO doping, electrical conductivity and transport properties of PEDOT:PSS increases due to structural alterations in the presence of −COOH and −OH functional groups in GO. These structural alterations have been revealed under detailed study of Raman spectra, X-ray photoelectron spectroscopy (XPS) analysis, Topographical and conductive Atom force microscopy (AFM/C-AFM) mapping. OPVs fabricated using PEDOT:PSS: GO (5:1) as a hole transport layer (HTL) exhibited a power conversion efficiency (PCE) of 7.68%, which was higher than the 7.01% that was obtained for the OPVs using pristine PEDOT:PSS.

36 MATERIALS SCIENCE↗

DOE OSTI2014

Spatial dependence of polycrystalline FTO’s conductance analyzed by conductive atomic force microscope (C-AFM)

Fluorine-doped Tin oxide (FTO) is a highly transparent, electrically conductive polycrystalline material frequently used as an electrode in organic solar cells and optical-electronic devices [1–2]. In this work a spatial analysis of the conductive behavior of FTO was carried out by Conductive-mode Atomic Force Microscopy (C-AFM). Rare highly oriented grains sample give us an opportunity to analyze the top portion of polycrystalline FTO and compare with the border one. It is shown that the current flow essentially takes place through the polycrystalline edge at grain boundaries.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2014

Magnetic properties and interlayer coupling of epitaxial Co/Cu films on Si

Thin films of Co and Co/Cu/Co trilayers with wedged Cu interlayers were grown epitaxially on Cu buffer layers on hydrogen passivated Si(001) wafers. We find that single Co layers have a well-defined four-fold anisotropy but with smaller in-plane anisotropies than observed in Co grown on Cu crystals. Ruderman–Kittel–Kasuya–Yosida (RKKY) interlayer coupling is observed in one Co/Cu/Co sample which is the smoothest of the films as measured by atomic force microscopy. Some of the films also form a dot-like structure on the surface. Intermixing at elevated temperatures between the Cu buffer and Si limits the ability to form flat surfaces to promote RKKY coupling.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2015

Enhancement of micro structural properties of PVA doped with MWCNT’s and metal oxide nanocomposites films

WO{sub 3} nanoparticles were prepared by using precipitation method and the multiwall Carbon nanotubes (MWCNT’s) were functionalized to make Carboxylated MWCNTs. Further, prepared WO{sub 3} and carboxylated MWCNT were doped into PVA matrix by coagulation technique and PVA nanocomposites were prepared by simple solvent casting technique. The films were characterized by XRD, FTIR spectroscopy and AFM. FTIR spectroscopy reveals the intensity of absorption of radiation at 3624.55 cm{sup −1} corresponds to the OH group of PVA. It changes in accordance with dopant concentration causing inter/intra molecular hydrogen bonding between the dopants and PVA back bone which leads to the complex formation. XRD data explores the crystalline nature of the film. It is found that for doping concentration x= 7.5 wt% the percentage crystallinity and crystallite size increases whereas micro structural strain and dislocation density decreases. An atomic force microscopy topographic analysis proves that the doped particles have an average size less than 15 nm, as confirmed by XRD data. It was found that roughness of the sample varies with dopant concentration causing variation in crystallinity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2011

Effect of Pentacene-dielectric Affinity on Pentacene Thin Film Growth Morphology in Organic Field-effect Transistors

Organic field-effect transistors (OFETs) are fabricated by depositing a thin film of semiconductor on the functionalized surface of a SiO{sub 2} dielectric. The chemical and morphological structures of the interface between the semiconductor and the functionalized dielectric are critical for OFET performance. We have characterized the effect of the affinity between semiconductor and functionalized dielectric on the properties of the semiconductor-dielectric interface. The crystalline microstructure/nanostructure of the pentacene semiconductor layers, grown on a dielectric substrate that had been functionalized with either poly(4-vinyl pyridine) or polystyrene (to control hydrophobicity), and grown under a series of substrate temperatures and deposition rates, were characterized by X-ray diffraction, photoemission spectroscopy, and atomic force microscopy. By comparing the morphological features of the semiconductor thin films with the device characteristics (field-effect mobility, threshold voltage, and hysteresis) of the OFET devices, the effect of affinity-driven properties on charge modulation, charge trapping, and charge carrier transport could be described.

36 MATERIALS SCIENCE↗

DOE OSTI2011

Hydrogenation and surface density changes in hydrocarbon films during erosion using Ar/H{sub 2} plasmas

We report interactions of low pressure Ar, H{sub 2}, and Ar/H{sub 2} mixture plasmas with a-C:H films. Surface evolution and erosion of a-C:H films were examined for ion energies up to 200 eV by rf biasing the substrates. Film surfaces were characterized using in situ ellipsometry, x-ray photoelectron spectroscopy, and atomic force microscopy. Multilayer models for steady-state modified surface layers are constructed using ellipsometric data and compared with results of molecular dynamics (MD) simulations and transport of ions in matter (TRIM) calculations. We find that Ar plasma causes a modified layer at the surface that is depleted of H atoms. The depth and degree of this modification is strongly depending on Ar ion energies. This depletion saturates quickly during plasma exposure (<1 s) and persists during steady-state erosion. We find that the thickness and density of the H-depleted layer are in good agreement with MD and TRIM simulations. The degree of surface densification decreases when small amounts of H{sub 2} are added to Ar plasmas. When more than 5% H{sub 2} is added to the plasma, long term loss in surface density is observed, indicating rehydrogenation and saturation of H in the film. As the H{sub 2} fraction increases, the near-surface atomic H increases and the ion composition bombarding the surface changes. This causes incorporation of H deeper into the a-C:H film. For a-C:H films exposed to pure H{sub 2} plasmas, H is introduced into the near-surface region to a depth of up to {approx}8 nm from the surface. As the rf bias is increased the ion energy transitions from solely chemical sputtering to one involving physical sputtering, causing the yield of C atoms from the surface to greatly increase. The increasing yield suppresses H incorporation/saturation and decreases the magnitude of the modified surface layer.

36 MATERIALS SCIENCE↗

DOE OSTI2015

Fabrication of ten-fold photonic quasicrystalline structures

Compared to periodic crystals, quasicrystals have higher point group symmetry and are more favorable in achieving complete band-gaps. In this report, a top-cut prism interferometer is designed to fabricate ten-fold photonic quasicrystalline structures. By optimizing the exposing conditions and material characteristics, appropriate quasicrystals have been obtained in the SU8 photoresist films. Atomic Force Microscopy and laser diffraction are used to characterize the fabricated structures. The measurement results show the consistence between the theoretical design and experiments. This will provide guidance for the large-area and fast production of ten-fold quasicrystalline structures with high quality.

36 MATERIALS SCIENCE↗

DOE OSTI2018

Structural Modification of PECVD As{sub 50}S{sub 50} Chalcogenide-Glass Films by Femtosecond Laser Radiation

A thin film of As{sub 50}S{sub 50} chalcogenide glass modified by femtosecond laser radiation at a wavelength of 800 nm has been studied. The character of the modification has been analyzed by interferometry, atomic-force microscopy, and Raman spectroscopy. The relationship between a change in the refractive index upon the sample modification and structural changes in the glass network is determined.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2018

Surface Modification of Melamine-Formaldehyde (MF-R) Macroparticles in Complex Plasma

The surface modification of melamine-formaldehyde (MF-R) macroparticles (4.12 ± 0.09 μm in diameter) in dc glow discharges in neon, argon, and an argon–oxygen mixture (90% Ar, 10% O{sub 2}) was studied experimentally. The macroparticles were treated in the discharge plasma for 10, 20, 40, and 60 min. The macroparticles were placed in ordered plasma–dust structures and then extracted from them. The results of atomic force microscopy of the surface profile are presented. Quantitative data on destruction of the surface layer and aspects of its modification are discussed. The amount of substance removed from the particle surface for the exposure time was calculated using the fractal analysis method.

36 MATERIALS SCIENCE↗

DOE OSTI2018

Statistics-Based Correction to Measurements of Hardness of Rough Surfaces by Instrumented Indentation

Abstract—A procedure for correcting hardness data measured on rough surfaces using instrumented indentation is presented. The correction procedure relies on the knowledge of surface topography, which can be obtained by atomic force microscopy or other appropriate instrumental technique. The analyzed surface is characterized by the standard deviation of height distribution and an autocorrelation function. The latter is proposed to be modeled with an exponential function with either a quadratic or linear exponent.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2013

Photoluminescence spectra of InAs quantum dots embedded in GaAs heterostructure

Self-assembled InAs/GaAs quantum dot (QD) heterostructures grown by solid state molecular beam epitaxy (MBE) were subjected to growth ripening pause of comparatively shorter durations (0-50s) at the growth temperature (520 Degree-Sign C). Atomic Force Microscopy (AFM) and Transmission Electron Microscopy (TEM) studies indicate that the QD islands increase in size with the growth pause and correspondingly their density decreases. Photoluminescence (PL) studies were carried out on these QDs. The temperature dependence of the PL is studied and models to explain it are explored.

36 MATERIALS SCIENCE↗

DOE OSTI2011

Graphene Formation on Step-free 4H-SiC(0001)

Step-free SiC was thermally decomposed in vacuum to better understand graphene formation in the absence of step fronts. Atomic force microscopy revealed graphene nucleating at surface pits that preferentially form along SiC{l_brace}1{bar 1}00{r_brace} planes. The density of these pits is 1 x 10{sup 8} cm{sup -2}, which is three orders of magnitude greater than the measured density of SiC threading dislocations. Additionally, Raman spectroscopy demonstrated that graphene on step-free regions have a redshifted 2D peak position and a smaller peak width than does graphene grown on stepped regions. This difference is attributed to film thickness, which is confirmed by cross-sectional transmission electron microscopy. Stepped regions have a graphitic film nearly 2 nm thick as compared to less than 0.7 nm for step-free regions.

36 MATERIALS SCIENCE↗

DOE OSTI2011

Zinc Oxide Thin Films Fabricated with Direct Current Magnetron Sputtering Deposition Technique

Zinc oxide (ZnO) is a very promising material for emerging large area electronic applications including thin-film sensors, transistors and solar cells. We fabricated ZnO thin films by employing direct current (DC) magnetron sputtering deposition technique. ZnO films with different thicknesses ranging from 100 nm to 1020 nm were deposited on silicon (Si) substrate. The deposition pressure was varied from 12 mTorr to 25 mTorr. The influences of the film thickness and the deposition pressure on structural properties of the ZnO films were investigated using Mahr surface profilometer and atomic force microscopy (AFM). The experimental results reveal that the film thickness and the deposition pressure play significant role in the structural formation of the deposited ZnO thin films. ZnO films deposited on Si substrates are promising for variety of thin-film sensor applications.

36 MATERIALS SCIENCE↗

DOE OSTI2011

Large-scale, nonsubtractive patterning of transparent conducting oxides by ion bombardment

While significant progress has been achieved in the fabrication and performance of transparent electronic devices, substantially less research effort has been devoted to transparent interconnects, despite their critical importance for transparent integrated circuitry. Here, we exploit the crystal disorder induced by Ar{sup +} ion bombardment to achieve efficient fabrication of electrically conductive patterns on indium oxide surfaces. The resulting ion-induced patterns are characterized by conductive atomic force microscopy, secondary ion mass spectrometry, and four-point charge transport measurements. Massively parallel patterning is demonstrated over square centimeter areas with a patterned electrical conductivity of {approx}10{sup 4} S cm{sup -1}.

36 MATERIALS SCIENCE↗