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DOE OSTI2012

Evidence of surface states for AlGaN/GaN/SiC HEMTs passivated Si{sub 3}N{sub 4} by CDLTS

In AlGaN/GaN heterostructure field-effect transistors (HEMTs) structures, the surface defects and dislocations may serve as trapping centers and affect the device performance via leakage current and low frequency noise. This work demonstrates the effect of surface passivation on the current-voltage characteristics and we report results of our investigation of the trapping characteristics of Si{sub 3}N{sub 4}-passivated AlGaN/GaN HEMTs on SiC substrates using the conductance deep levels transient spectroscopy (CDLTS) technique. From the measured of CDLTS we identified one electron trap had an activation energy of 0.31 eV it has been located in the AlGaN layer and two hole-likes traps H{sub 1}, H{sub 2}. It has been pointed out that the two hole-likes traps signals did not originate from changes in hole trap population in the channel, but reflected the changes in the electron population in the surface states of the HEMT access regions.

36 MATERIALS SCIENCE↗

DOE OSTI2013

Frequency selective infrared sensors

A frequency selective infrared (IR) photodetector having a predetermined frequency band. The exemplary frequency selective photodetector includes: a dielectric IR absorber having a first surface and a second surface substantially parallel to the first surface; an electrode electrically coupled to the first surface of the dielectric IR absorber; and a frequency selective surface plasmonic (FSSP) structure formed on the second surface of the dielectric IR absorber. The FSSP structure is designed to selectively transmit radiation in the predetermined frequency band that is incident on the FSSP structure substantially independent of the angle of incidence of the incident radiation on the FSSP structure.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗

DOE OSTI2014

Frequency selective infrared sensors

36 MATERIALS SCIENCE↗

DOE OSTI2020

The role of surface hydroxyls on the radiolysis of gibbsite and boehmite nanoplatelets

Understanding mechanistic pathways to radiolytic hydrogen generation by metal oxyhydroxide nanomaterials is challenging because of the difficulties of distinguishing key locations of OH bond scission, from structural interiors to hydroxylated surfaces to physi-sorbed water molecules. In this work we exploited the interface-selectivity of vibrational sum frequency generation (VSFG) to isolate surface versus bulk hydroxyl groups for gibbsite and boehmite nanoplatelets before and after 60 Co irradiation at dose levels of approximately 7.0 and 29.6 Mrad. While high-resolution microscopy revealed no effect on particle bulk and surface structures, VSFG results clearly indicated up to 83% and 94% radiation-induced surface OH bond scission for gibbsite and boehmite, respectively, a substantially higher proportion than observed for interior OH groups by IR and Raman spectroscopy. Electron paramagnetic spectroscopy revealed that the major radiolysis products bound in the mineral structures are trapped electrons, O , O 2 – and possibly F-centers in gibbsite, and H , O and O 3 – in boehmite, which persist on the time frame of several months. The entrapped radiolysis products appear to be highly stable, enduring re-hydration of particle surfaces, and likely reflect a permanent adjustment in the thermodynamic stabilities of these nanomaterials.

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗

DOE OSTI2021

Radiolysis of thermally dehydrated gibbsite

Aluminum hydroxide (gibbsite, Al (OH) 3 ) powders and their thermally dehydrated forms were rehydrated and irradiated with gamma rays to examine the effects of the collapse of the crystalline gibbsite structure as it is converted to alumina on the transport of precursors of molecular hydrogen to the surface. The amount of hydrogen gas, H 2 , production from irradiated gibbsite and its transition phases with adsorbed water increased substantially with increasing temperature for dehydration of the samples, up to a point. Furthermore, the production of H 2 for samples without water was considerably lower than the hydrated samples signifying the importance of surface water and the transport of precursors to the surface. EPR spectroscopy showed that the major radiolytic products are trapped electrons and related O – centers. Thermal gravimetric analysis (TGA) measurements of gibbsite dehydration established the temperatures of phase transitions from gibbsite to alumina. Changes to the surface and structure of gibbsite and its transition phases following irradiation were analyzed using nitrogen adsorption, powder X-ray diffraction (pXRD), Raman spectroscopy, electron paramagnetic resonance (EPR), and scanning electron microscopy (SEM). pXRD and Raman spectra showed amorphorization at about 300 °C, coupled to a substantial increase in specific surface area due to increasing porosity.

38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEA↗

DOE OSTI2016

Phase transition kinetics and surface binding states of methylammonium lead iodide perovskite

We have presented a detailed analysis of phase transition kinetics and binding energy states of solution processed methylammonium lead iodide (MAPbI3) thin films prepared at ambient conditions and annealed at different elevated temperatures. The processing temperature and environmental conditions predominantly control the crystal structure and surface morphology of MAPbI3 thin films. The structural transformation from tetragonal to cubic occurs at 60C for 30 minutes annealing time while 10 minutes annelaed films posses tetragonal structure. The transformed phase is greatly intact even at higher annealing temperature of 150 C and time of 2 hours. Existence of charge transfer interaction between Pb 4f and I 3d oxidation states is quantified by the XPS spin orbit spilitting.

Kumar, G. R.↗

DOE OSTI2015

Effect of xenon on the structural phase state of the surface layer of cemented carbide under pulsed electron-beam irradiation

A comparative analysis of the surface-layer microstructure of a tungsten-based cemented carbide modified with pulsed high-energy electron beams generated by gas-discharge plasmas and of the tool life of metal-cutting plates prepared from this alloy is performed. The choice of a plasma-forming gas providing for the emission of electrons out of the plasma-filled cathode is shown to have a profound influence both on the formation process of nano-sized structural-phase states in the surface layer of the cemented carbide and on the tool life of the metal-cutting plates prepared from this alloy.

36 MATERIALS SCIENCE↗

DOE OSTI2016

Surface modification of carbon black nanoparticles enhances photothermal separation and release of CO 2

Here, carbon black nanoparticles (CB) were covalently modified to improve the photothermal regeneration of a CO 2 capture nanofluid through decarboxylation. The photothermal release of CO 2 addresses high energy costs associated with regenerating capture fluids. By incorporating sulfonamides on the surface of CB, we enhance the photothermal separation of CO 2 from MEA by approximately 70% more than the unmodified CB. In contrast, with an anionic sulfonate on the surface, the total CO 2 released fell by approximately 60%. We verified the chemical composition and structure of surface modification using complementary techniques including FT-IR, TGA, XPS, and Raman spectroscopy.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2014

Surface-plasmons lasing in double-graphene-layer structures

We consider the concept of injection terahertz lasers based on double-graphene-layer (double-GL) structures with metal surface-plasmon waveguide and study the conditions of their operation. The laser under consideration exploits the resonant radiative transitions between GLs. This enables the double-GL laser room temperature operation and the possibility of voltage tuning of the emission spectrum. We compare the characteristics of the double-GL lasers with the metal surface-plasmon waveguides with those of such laser with the metal-metal waveguides.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2021

Topological surface states on the nonpolar (110) and (111) surfaces of SmB 6

In order to clarify the controversial issue of the topological nature in a mixed-valent Kondo system, SmB 6 , we have explored the surface states on the nonpolar (110) surface of SmB 6 , employing both angle-resolved photoemission spectroscopy (ARPES) experiment and ab initio density-functional theory (DFT) band calculations. Based on ARPES spectroscopic fingerprints and the DFT surface band structures, we ascribe the observed spectral weights at $\bar{X}$ and $\bar{Y}$ on the (110) surface Brillouin zone to topological surface states (TSSs) of “topological insulator (TI)” nature and of “topological crystalline insulator (TCI)” nature, respectively. With varying the chemical potential, the double Dirac cones of the TCI nature exhibit a Lifshitz transition of Fermi surfaces with intriguing spin textures. Further, we have also examined the TSSs on the nearly nonpolar (111) surface of SmB 6 in connection with a recently reported ARPES result and proposed a way to probe the Dirac points that are buried in the bulk-projected bands.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2015

Benzene derivatives adsorbed to the Ag(111) surface: Binding sites and electronic structure

Dispersion corrected Density Functional Theory calculations were employed to study the adsorption of benzenes derivatized with functional groups encompassing a large region of the activated/deactivated spectrum to the Ag(111) surface. Benzenes substituted with weak activating or deactivating groups, such as methyl and fluoro, do not have a strong preference for adsorbing to a particular site on the substrate, with the corrugations in the potential energy surface being similar to those of benzene. Strong activating (N(CH{sub 3}){sub 2}) and deactivating (NO{sub 2}) groups, on the other hand, possess a distinct site preference. The nitrogen in the former prefers to lie above a silver atom (top site), but in the latter a hollow hexagonal-closed-packed (H{sub hcp}) site of the Ag(111) surface is favored instead. Benzenes derivatized with classic activating groups donate electron density from their highest occupied molecular orbital to the surface, and those functionalized with deactivating groups withdraw electron density from the surface into orbitals that are unoccupied in the gas phase. For benzenes functionalized with two substituents, the groups that are strongly activating or deactivating control the site preference and the other groups assume sites that are, to a large degree, dictated by their positions on the benzene ring. The relative stabilities of the ortho, meta, and para positional isomers of disubstituted benzenes can, in some cases, be modified by adsorption to the surface.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2019

Structure and Properties of Gallium-Oxide Films Produced by High-Frequency Magnetron-Assisted Deposition

The properties of gallium-oxide films produced by the radio-frequency magnetron-assisted sputtering of a β-Ga{sub 2}O{sub 3} target with deposition onto sapphire substrates are studied. The as-deposited gallium-oxide films are polycrystalline and contain crystallites of the α and β phases. Exposure to oxygen plasma does not bring about the appearance of new crystallites but makes crystallites several times larger in average dimensions in the substrate plane. After annealing at 900°C, the crystallite size becomes twice as large as that in the unannealed film. The films not subjected to thermal annealing exhibit a high resistance at 20°C. In the range of 50–500°C, the conductivity of the samples (G) only slightly depends on temperature (T) and, as T is elevated further, exponentially increases with the activation energy 0.7–1.0 eV. After annealing of the films in argon at 900°C (30 min), the conductivity G starts to sharply increase at T ≈ 350°C. In the dependence of lnG on T {sup –1}, a maximum in the range of 470–520°C and a portion of decreasing conductivity at higher temperatures are observed. The unusual temperature dependence of the conductivity after annealing is attributed to a change in the structure and phase composition of polycrystalline gallium-oxide films and, possibly, to some effects at the surface. The structures produced on insulating substrates are solar blind in the visible wavelength region and sensitive to radiation in the ultraviolet region (222 nm).

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2013

Phase transformations during the Ag-In plating and bonding of vertical diode elements of multijunction solar cells

The conditions of the bonding of silicon multijunction solar cells with vertical p-n junctions using Ag-In solder are studied. The compositions of electrodeposited indium films on silicon wafers silver plated by screen printing and silver and indium films fabricated by layer-by-layer electrochemical deposition onto the surface of silicon vertical diode cells silver plated in vacuum are studied. Studying the electrochemical-deposition conditions, structure, and surface morphology of the grown layers showed that guaranteed bonding is provided by 8-min heat treatment at 400 Degree-Sign C under the pressure of a stack of metallized silicon wafers; however, the ratio of the indium and silver layer thicknesses should not exceed 1: 3. As this condition is satisfied, the solder after wafer bonding has the InAg{sub 3} structure (or InAg{sub 3} with an Ag phase admixture), due to which the junction melting point exceeds 700 Degree-Sign C, which guarantees the functioning of such solar cells under concentrated illumination.

36 MATERIALS SCIENCE↗

DOE OSTI2020

Evolution of nontrivial Fermi surface features in the band structures of the homologous members Pb 5 Bi 6 Se 14 and Pb 5 Bi 12 Se 23

High-quality single crystals of (PbSe) 5 (Bi 2 Se 3 ) 3m were grown and analyzed by nuclear magnetic resonance (NMR) spectroscopy. We report on 77 Se and 207 Pb NMR shifts and nuclear spin-lattice relaxation measurements in the naturally formed heterostructure homology (PbSe) 5 (Bi 2 Se 3 ) 3m with m = 1 (Pb 5 Bi 6 Se 14 ) and m = 2 (Pb 5 Bi 12 Se 23 ). A distinct site-specific contribution has been detected for both nuclei as a function of temperature, which reveals an electronic changeover from a semiconducting (Pb 5 Bi 6 Se 14 ) to a semimetalliclike (Pb 5 Bi 12 Se 23 ) system with nontrivial band structure features near the Fermi level. The temperature dependences of the relaxation rates are dominated by significant changes in the topology of energy dispersions accompanied with band edges and crossings in the region of the Fermi surface. These results, which interrogate nuclear spin interactions from selected atomic sites, clearly expose the effects of the added Bi 2 Se 3 layer on the crystal and electronic structure of (Pb 5 Bi 12 Se 23 ). These findings provide direct microscopic insight into the unconventional and dual nature of the electronic structure of these homologous thermoelectric and topologically nontrivial compounds.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2017

Characterization of Silicon Nanocrystal Surfaces by Multidimensional Solid-State NMR Spectroscopy

The chemical and photophysical properties of silicon nanocrystals (Si NCs) are strongly dependent on the chemical composition and structure of their surfaces. Here we use fast magic angle spinning (MAS) and proton detection to enable the rapid acquisition of dipolar and scalar 2D 1 H– 29 Si heteronuclear correlation (HETCOR) solid-state NMR spectra and reveal a molecular picture of hydride-terminated and alkyl-functionalized surfaces of Si NCs produced in a nonthermal plasma. 2D 1 H– 29 Si HETCOR and dipolar 2D 1 H– 1 H multiple-quantum correlation spectra illustrate that resonances from surface mono-, di-, and trihydride groups cannot be resolved, contrary to previous literature assignments. Instead the 2D NMR spectra illustrate that there is large distribution of 1 H and 29 Si chemical shifts for the surface hydride species in both the as-synthesized and functionalized Si NCs. However, proton-detected 1 H– 29 Si refocused INEPT experiments can be used to unambiguously differentiate NMR signals from the different surface hydrides. Varying the 29 Si evolution time in refocused INEPT experiments and fitting the oscillation of the NMR signals allows for the relative populations of the different surface hydrides to be estimated. This analysis confirms that monohydride species are the predominant surface species on the as-synthesized Si NCs. A reduction in the populations of the di- and trihydrides is observed upon functionalization with alkyl groups, consistent with our previous hypothesis that the trihydride, or silyl (*SiH 3 ), group is primarily responsible for initiating surface functionalization reactions. Density functional theory (DFT) calculations were used to obtain quantum chemical structural models of the Si NC surface and reproduce the observed 1 H and 29 Si chemical shifts. Furthermore, the approaches outlined here will be useful to obtain a more detailed picture of surface structures for Si NCs and other hydride-passivated nanomaterials.

29Si NMR↗

DOE OSTI2019

Identification of a Pt 3 Co Surface Intermetallic Alloy in Pt–Co Propane Dehydrogenation Catalysts

Bimetallic Pt-Co nanoparticles (NPs) were prepared and characterized by scanning transmission electron microscopy, in situ X-ray absorption spectroscopy, in situ synchrotron X-ray diffraction, and catalytic conversion for propane dehydrogenation with and without added H-2. In addition, the surface extended X-ray absorption fine structure (EXAFS) obtained by fitting the difference spectrum between the fully reduced and room-temperature-oxidized catalysts suggest that the surface structure remains Pt 3 Co, although the core changes from Pt to Pt 3 Co and to PtCo. At low Co loading, the bimetallic nanoparticles form a Pt3Co intermetallic surface alloy with Pt-rich core. With increasing Co loading, a full alloy forms where both the surface and NP compositions are Pt 3 Co. A further increase in Co loading leads to a Co-rich NP core, likely PtCo, with a surface of Pt 3 Co. Although Pt-Co intermetallic alloys form two different phases and several morphologies, the surface structures are similar in all catalysts. Although both monometallic Pt and Co are active for alkane dehydrogenation, all bimetallic Pt-Co catalysts are significantly more olefin selective than either single metal. The turnover rates of the bimetallic catalysts indicate that Pt is the active atom with little contribution from Co atoms. The high olefin selectivity is suggested to be due to Co acting as a less active structural promoter to break up large Pt ensembles in bimetallic NPs.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2018

Theoretical and experimental investigations on H2 sensing properties of flower-like titanium dioxide

Highlights: • Flower-like rutile TiO{sub 2} has been successfully synthesized. • The unique nanostructure show an great enhanced gas-sensing performance. • A sensing mechanism based on our calculation was proposal. - Abstract: Microstructure and gas sensing properties of partly agglomerating flower-like rutile TiO{sub 2} obtained using a one-pot hydrothermal method was investigated. Test results exhibited an excellent sensitivity to H{sub 2} gas and stable reproducibility. Further, we performed a first principles calculation of an established surface model to delve into the sensing mechanism. Adsorbed oxygen preferred to locate upon the Ti{sub 5c} site, while the electronic structure of surface had rather weak changes after hydrogen adsorption. This simulation may pave a new path to further discover the sensing mechanism of rutile TiO{sub 2}-based gas sensor.

77 NANOSCIENCE AND NANOTECHNOLOGY↗

DOE OSTI2014

Annealing effects on structural, electrical and optical properties of antimony-tinoxide thin films deposited by sol gel dip coating technique

Antimony-doped tin dioxide possess interesting physical and chemical properties. These properties have a wide range of applications such as catalysis and optoelectronic devices. In the present study, antimony-doped tin oxide (SnO2:Sb) thin films were deposited on the quartz substrates by sol-gel dip coating technique. The films were annealed at temperatures 350°C, 550°C and 850°C in air for 2 hours. The structure and surface morphologies were observed by X-ray diffraction (XRD) and Scanning electron microscopy (SEM). XRD patterns shows tetragonal structure for the SnO2:Sb films annealed at different temperatures. Crystallite size increased from 6 to 14 nm as annealing temperature increased from 350°C to 850°C. SEM studies reveals crack free and smooth surface for all the films. The grains are found to be homogenously distributed for films annealed at higher temperature. The electrical conductivity of the films annealed at 350°C and 550°C decreased and increased for the films annealed at 850°C. The optical properties of the films were investigated in the UV-visible-NIR region (200-900 nm) using UV spectra. The transmittance of the films decreased for films annealed at higher temperature. The optical energy band gap values (4.13 eV-4.83 eV) increased with the increase in annealing temperature.

36 MATERIALS SCIENCE↗