Platinum nanoparticles on gallium nitride surfaces: effect of semiconductor doping on nanoparticle reactivity

Platinum nanoparticles supported on n- and p-type gallium nitride (GaN) are investigated as novel hybrid systems for the electronic control of catalytic activity via electronic interactions with the semiconductor support. In situ oxidation and reduction were studied with high pressure photoemission spectroscopy. The experiments revealed that the underlying wide-band-gap semiconductor has a large influence on the chemical composition and oxygen affinity of supported nanoparticles under X-ray irradiation. For as-deposited Pt cuboctahedra supported on n-type GaN, a higher fraction of oxidized surface atoms was observed compared to cuboctahedral particles supported on p-type GaN. Under an oxygen atmosphere, immediate oxidation was recorded for nanoparticles on n-type GaN, whereas little oxidation was observed for nanoparticles on p-type GaN. Together, these results indicate that changes in the Pt chemical state under X-ray irradiation depend on the type of GaN doping. The strong interaction between the nanoparticles and the support is consistent with charge transfer of X-ray photogenerated free carriers at the semiconductor-nanoparticle interface and suggests that GaN is a promising wide-band-gap support material for photocatalysis and electronic control of catalysis.     

High-sensitivity transmission IR spectroscopy for the chemical identification and structural analysis of conjugated molecules on gallium arsenide surfaces

We demonstrate the use of high-sensitivity, off-normal transmission IR spectroscopy with s-polarized light to probe the chemical identity and orientation of quaterphenyldithiol (QPDT) molecular assemblies on GaAs as a function of ammonium hydroxide (NH4OH) concentration. NH4OH is added to the assembly solution to convert the thioacetyl groups on the QPDT precursor to thiolates. When assembled at high NH4OH concentrations, the acetyl groups are completely removed, and QPDT is disordered on GaAs. Assembly at low NH4OH concentrations, however, results in QPDT assemblies that are preferentially upright. The molecular orientation is further quantified with near-edge X-ray absorption fine structure spectroscopy.     

Photochemical functionalization of gallium nitride thin films with molecular and biomolecular layers

We demonstrate that photochemical functionalization can be used to functionalize and photopattern the surface of gallium nitride crystalline thin films with well-defined molecular and biomolecular layers. GaN(0001) surfaces exposed to a hydrogen plasma will react with organic molecules bearing an alkene (C=C) group when illuminated with 254 nm light. Using a bifunctional molecule with an alkene group at one end and a protected amine group at the other, this process can be used to link the alkene group to the surface, leaving the protected amine exposed. Using a simple contact mask, we demonstrate the ability to directly pattern the spatial distribution of these protected amine groups on the surface with a lateral resolution of <12 mum. After deprotection of the amines, single-stranded DNA oligonucleotides were linked to the surface using a bifunctional cross-linker. Measurements using fluorescently labeled complementary and noncomplementary sequences show that the DNA-modified GaN surfaces exhibit excellent selectivity, while repeated cycles of hybridization and denaturation in urea show good stability. These results demonstrate that photochemical functionalization can be used as an attractive starting point for interfacing molecular and biomolecular systems with GaN and other compound semiconductors.     

Chemical functionalization of diamond surfaces by reaction with diaryl carbenes

A rapid route to the chemical functionalization of hydrogen-terminated diamond surfaces deposited by chemical vapor deposition involving their reaction with substituted diaryl carbenes has been investigated. To avoid difficulties in the handling of highly reactive compounds, the carbene is generated in situ from the thermal decomposition at 400 K of a thin film of the corresponding diaryl diazomethane precursor deposited at the diamond interface. X-ray photoelectron spectroscopy (XPS) has been used to verify that surface functionalization using two starting compounds, bis(4-iodophenyl) diazomethane and bis(4-nitrophenyl) diazomethane, can be achieved using this approach in agreement with recent theoretical studies. The surface grafting density is measured to be around 10(14) cm(-2) in each case. The chemistry observed is found to be insensitive to the detailed properties of the diamond film and to the presence of oxygen contamination at the hydrogen-terminated diamond surface. We further demonstrate the utility of the approach, in the case of the bound nitrophenyl compound, by its reduction to the corresponding primary amine followed by reaction with fluorescein isothiocyanate to achieve fluorescent tagging of the diamond interface.     

Formation of primary amines on silicon nitride surfaces: a direct, plasma-based pathway to functionalization

Silicon nitride is the most commonly used passivation layer in biosensor applications where electronic components must be interfaced with ionic solutions. Unfortunately, the predominant method for functionalizing silicon nitride surfaces, silane chemistry, suffers from a lack of reproducibility. As an alternative, we have developed a silane-free pathway that allows for the direct functionalization of silicon nitride through the creation of primary amines formed by exposure to a radio frequency glow discharge plasma fed with humidified air. The aminated surfaces can then be further functionalized by a variety of methods; here we demonstrate using glutaraldehyde as a bifunctional linker to attach a robust NeutrAvidin (NA) protein layer. Optimal amine formation, based on plasma exposure time, was determined by labeling treated surfaces with an amine-specific fluorinated probe and characterizing the coverage using X-ray photoelectron spectroscopy (XPS). XPS and radiolabeling studies also reveal that plasma-modified surfaces, as compared with silane-modified surfaces, result in similar NA surface coverage, but notably better reproducibility.     

Fabrication and optical property of silicon oxide layer coated semiconductor gallium nitride nanowires

Quasi one-dimensional GaN-SiO(2) nanostructures, with a silicon oxide layer coated on semiconductor GaN nanowires, were successfully synthesized through as-synthesized SiO(2) nanoparticles-assisted reaction. The experimental results indicate that the nanostructure consists of single-crystalline wurtzite GaN nanowire core, an amorphous SiO(2) outer shell separated in the radial direction. These quasi one-dimensional nanowires have the diameters of a few tens of nanometers and lengths up to several hundreds of micrometers. The photoluminescence spectrum of the GaN-SiO(2) nanostructures consists of one broad blue-light emission peak at 480 nm and another weak UV emission peak at 345 nm. The novel method, which may results in high yield and high reproducibility, is demonstrated to be a unique technique for producing nanostructures with controlled morphology.     

Rate constants for the reaction of atomic and molecular bromine with gallium arsenide

The reaction of the (100) face of a gallium arsenide single crystal with atomic and molecular bromine has been studied in a discharge flow system at temperatures between 100 and 225°C and pressures between 0.1 and 40 torr. The reaction with Br₂ was found to be first order in Br₂ only at pressures below 1 torr. Temperature dependence studies in the linear range gave the activation energy and preexponential factor for the rate controlling reaction in the low pressure regime. The results are summarized in the following Arrhenius equation: Deviations from linearity at high pressures are discussed in terms of two alternative mechanisms. The reaction of GaAs with atomic bromine was also studied as a function of temperature, and found to have a temperature dependence described by the following Arrhenius equation:      

Gas-phase halo alkylation of C60-fullerene by ion-molecule reaction under chemical ionization

Chemical ionization (CI) mass spectra of C₆₀-fullerene were studied using 1,2-dibromoethane and 1,2-dichloroethane as CI reagents. The ion-molecule reaction between C₆₀ and C₂H₄X⁺ (X=Br and Cl) leads to the formation of (C₆₀+C₂H₄X)⁺ adducts. The collision-induced dissociation of the adducts reveal gas phase halo alkylation of C₆₀-fullerence involving the C?C bond formation.     

Vicinal functionalization of propiolate esters via a tandem catalytic carbocupration-Mukaiyama aldol reaction sequence

The vicinal functionalization of propiolate esters via a tandem catalytic carbocupration-Mukaiyama aldol reaction sequence has been investigated. It has been shown that catalyst loadings as low as 8 mol % readily allow for good yields and excellent diastereoselectivities (>20:1) for a series of Grignard reagents and aldehydes.     

Hydrogen storage of metal nitride by a mechanochemical reaction

Metal imides (Li(2)NH, CaNH), a metal amide (LiNH(2)) and metal hydrides (LiH, CaH(2)) were synthesized by ball milling of their respective metal nitrides (Li(3)N, Ca(3)N(2)) in a H(2) atmosphere at 1 MPa and at room temperature.     

Photochemical functionalization of hydrogen-terminated diamond surfaces: a structural and mechanistic study

Hydrogen-terminated diamond surfaces can be covalently modified with molecules bearing a terminal vinyl (C=C) group via a photochemical process using sub-band-gap light at 254 nm. We have investigated the photochemical modification of hydrogen-terminated surfaces of nanocrystalline and single-crystal diamond (111) to help understand the structure of the films and the underlying mechanism of photochemical functionalization. A comparison of the rates of photochemical modification of single-crystal diamond and nanocrystalline diamond films shows no significant difference in reactivity, demonstrating that the modification process is not controlled by grain boundaries or other structures unique to polycrystalline films. We find that both single-crystal and polycrystalline hydrogen-terminated diamond samples exhibit negative electron affinity and are functionalized at comparable rates, while oxidized surfaces with positive electron affinity undergo no detectable reaction. Gas chromatography-mass spectrometry (GC-MS) analysis shows the formation of new chemical products in the liquid phase that are formed only when the alkenes are illuminated in direct contact with H-terminated diamond, while control experiments with other surfaces and in the dark show no reaction. Our results show that the functionalization is a surface-mediated photochemical reaction and suggest that modification is initiated by the photoejection of electrons from the diamond surfaces into the liquid phase.     

Guanine alkylation by ethylene oxide: calculation of chemical reactivity

In this paper we report on calculations of the activation free energy for a chemical reaction between ethylene oxide and guanine. Ethylene oxide is biologically relevant per se and is also a model compound for numerous ultimate carcinogens. Calculations were performed on the medium-high ab initio, DFT, and semiempirical MO levels. Effects of solvation were considered using the Langevine dipole method and solvent reaction field method of Tomasi and co-workers. The calculated activation free energies are in reasonable agreement with the experimental value.     

Determination of boron in silicon-doped gallium arsenide by electrothermal atomic absorption spectrometry and ultraviolet-visible spectrophotometry

Two methods have been developed for the determination of boron impurities in silicon-doped gallium arsenide (GaAs) for electronics. The first method employs the electrothermal atomic absorption spectrometry (ETAAS), the second, the UV-Vis molecular absorption spectrophotomety. In both cases the GaAs sample is decomposed with aqua regia (1+1). To prevent Ga(III) interference on the ETAAS determination of boron, a double extraction of the chlorogallic acid (HGaCl4) in diethyl ether is performed. To improve the overall ETAAS performance, the graphite tubes were pre-treated with iridium(III) and tungsten(IV). A mixed chemical modifier containing Ni(II), Sr(II) and citric acid was also used. The characteristic mass (m0) is 301 +/- 47 pg and the detection limit (3sB) is 2.4 microg g(-1). The classic UV-Vis spectrophotometric procedure using curcumin was also extended to the determination of boron in GaAs. By masking Ga(III) with EDTA and a preliminary extraction of boron with 2-ethyl-hexane 1,3-diol, performed on a semi-micro scale, a detection limit of 0.6 microg g(-1) was achieved. Both methods were applied to the analysis of two Si-doped GaAs samples which were suspected of being boron-contaminated. Results are compared with those obtained by direct analysis of the decomposed sample solution using the inductively coupled plasma atomic emission spectrometry (ICP-AES).     

Pseudo-domino palladium-catalyzed allylic alkylation/Mizoroki-Heck coupling reaction: a key sequence toward (±)-podophyllotoxin

A formal synthesis of podophyllotoxin was carried out in nine steps. The key pseudo-domino step was accomplished through the succession of an intermolecular palladium-catalyzed allylic alkylation and an intramolecular Mizoroki-Heck coupling reaction.     

Structural, elastic constant, and vibrational properties of wurtzite gallium nitride: a first-principles approach

Perdew-Wang proposed generalized gradient approximation (GGA) is used in conjunction with ultrasoft pseudopotential to investigate the structural, elastic constant, and vibrational properties of wurtzite GaN. The equilibrium lattice parameters, axial ratio, internal parameter, bulk modulus, and its pressure derivative are calculated. The effect of pressure on equilibrium lattice parameters, axial ratio, internal parameter (u), relative volume, and bond lengths parallel and perpendicular to the c-axis are discussed. At 52 GPa, the relative volume change is observed to be 17.8%, with an abrupt change in bond length. The calculated elastic constants are used to calculate the shear wave speeds in the [100] and [001] planes. The finite displacement method is employed to calculate phonon frequencies and the phonon density of states. The first- and second-order pressure derivative and volume dependent Gruneisen parameter (γ(j)) of zone-center phonon frequencies are discussed. These phonon calculations calculated at theoretical lattice constants agree well with existing literature.     

Regioselective synthesis of 1,4-di(organo)[60]fullerenes through DMF-assisted monoaddition of silylmethyl Grignard reagents and subsequent alkylation reaction

Monoaddition of Grignard reagents, in particular tri(organo)silylmethylmagnesium chlorides, to [60]fullerene took place smoothly in the presence of dimethylformamide to produce (organo)(hydro)[60]fullerenes, C60R(1)H, in good yield (up to 93% isolated yield). The hydrofullerene was then deprotonated to generate the corresponding anion, C60R(-), which was then alkylated to obtain 58pi-electron di(organo)[60]fullerenes, C60R(1)R(2), in good to high yield (up to 93% overall yield). The two-step methodology provides a wide variety of 1,4-di(organo)[60] fullerenes bearing the same or different organic addends on the [60] fullerene core. By changing the addends, one can control the chemical and physical properties of the compounds at the molecular and bulk levels.     

Grignard reagent-promoted selective ring expansion and alkylation of formyl borneol and isoborneol: a new route to highly substituted cyclopentanes

Formyl borneol, a [2.2.1]-bicyclic carbinol, reacts with various Grignard reagents to produce corresponding alkyl [3.2.1]-bicyclic diols, which can be converted to new highly substituted cyclopentanes, and further to 3-acyl-bornylenes. These ring expansion-alkylation reactions are highly selective. Reaction of formyl isoborneol with methyl magnesium bromide gave ring expansion-only and alkylation-only products.