Oxygen-terminated Surface Research Articles

Thermally Activated Processes at the Co/ZnO Interface Elucidated Using High Energy X-rays

A detailed picture of the thermally activated processes occurring at the Co/ZnO interface is obtained by a combination of high energy X-ray based techniques: X-ray photoelectron and absorption spectroscopies and the kinematical X-ray standing wave method. At room temperature, the growth of a few monolayers of cobalt proceeds by the nucleation of nanometer-sized clusters on the polar oxygen-terminated (0001) surface of a ZnO single crystal. Progressive annealing from 600 to 970 K allows separating the various interfacial reactions. At the lowest annealing temperature, Co clusters coalesce while keeping their metallic character. Above 700 K Co0 is gradually oxidized to Co2+ and a thin Co rich (Zn,Co)O layer is formed. It is observed that rock salt CoO phases may form at the surface when the initial Co thickness exceeds 1 nm. At the highest annealing temperature (970 K), Co diffuses deeper into ZnO and Zn vacancies are created at subsurface sites that were previously occupied by Co.

Nanocrystalline Diamond Nanoelectrode Arrays and Ensembles

In this report, the fabrication of all-nanocrystalline diamond (NCD) nanoelectrode arrays (NEAs) by e-beam lithography as well as of all-diamond nanoelectrode ensembles (NEEs) using nanosphere lithography is presented. In this way, nanostructuring techniques are combined with the excellent properties of diamond that are desirable for electrochemical sensor devices. Arrays and ensembles of recessed disk electrodes with radii ranging from 150 to 250 nm and a spacing of 10 μm have been fabricated. Electrochemical impedance spectroscopy as well as cyclic voltammetry was conducted to characterize arrays and ensembles with respect to different diffusion regimes. One outstanding advantage of diamond as an electrode material is the stability of specific surface terminations influencing the electron transfer kinetics. On changing the termination from hydrogen- to oxygen-terminated diamond electrode surface, we observe a dependence of the electron transfer rate constant on the charge of the analyte molecule. Ru(NH(3))(6)(+2/+3) shows faster electron transfer on oxygen than on hydrogen-terminated surfaces, while the anion IrCl(6)(-2/-3) exhibits faster electron transfer on hydrogen-terminated surfaces correlating with the surface dipole layer. This effect cannot be observed on macroscopic planar diamond electrodes and emphasizes the sensitivity of the all-diamond NEAs and NEEs. Thus, the NEAs and NEEs in combination with the efficiency and suitability of the selective electrochemical surface termination offer a new versatile system for electrochemical sensing.

Characterization of hydrogen dissociation over aluminium-doped zinc oxide using an efficient massively parallel framework for QM/MM calculations

A task-farm parallelization framework has been implemented in the ChemShell computational chemistry environment to provide a facility for parallelizing common chemical calculations, including finite-difference Hessian evaluation, the nudged elastic band method for reaction path optimization, and population-based methods for global optimization. The optimization methods are provided by a parallel interface to the DL-FIND optimization library. As ChemShell can already exploit parallel external programs for energy and gradient evaluations, the new methods result in a two-level approach to parallelization that gives significantly improved performance for massively parallel calculations. For typical systems, speed-up factors of five to eight times have been observed compared with non-task-farmed calculations. The task-farming version of ChemShell has been used to study the heterolytic dissociation of a hydrogen molecule over a polar oxygen-terminated surface of aluminium-doped zinc oxide using an embedded cluster hybrid QM/MM approach. We calculate a 42 kcal mol −1 heat of reaction and a 30 kcal mol −1 activation energy, which is equivalent to a high backward reaction barrier of 72 kcal mol −1 per H 2 molecule, in close agreement with temperature programmed desorption experiments. The dissociation path includes a stable intermediate comprising a hydride ion in an oxygen vacancy and physisorbed atomic hydrogen.

Metastable surface oxide on CoGa(100): Structure and stability

We investigated the structure and formation of a surface oxide and bulk $\ensuremath{\beta}{\text{-Ga}}_{2}{\text{O}}_{3}$ on CoGa(100) from ultrahigh vacuum to 1 bar oxygen pressure in a temperature range from 300 to 1040 K. We combined in situ surface x-ray diffraction with scanning tunneling microscopy, atomic force microscopy, and density-functional theory calculations. We find that the two-dimensional epitaxial surface oxide layer exhibits a $p2mm$ symmetry with an additional mirror plane as compared to the bulk oxide. The surface oxide layer is found to form under metastable conditions at an oxygen chemical potential $\ensuremath{\sim}1.6\text{ }\text{eV}$ above the stability limit for bulk $\ensuremath{\beta}{\text{-Ga}}_{2}{\text{O}}_{3}$. The formation of the bulk oxide is kinetically hindered by the presence of the oxygen-terminated surface oxide, which most likely hampers dissociative oxygen chemisorption. We observe that below 620 K, the surface oxide is surprisingly stable at 1 bar oxygen pressure. Substrate faceting accompanies the bulk oxide formation at temperatures higher than 1020 K.

Lateral Depletion of Contact to Metallic Nanoparticles on Boron-Doped Diamond Electrodes

A lateral depletion of contact to metallic nanoparticles on semiconducting diamond electrodes was analyzed by a two-dimensional numerical simulation. The theoretical analysis was applied to experimental results on single-crystal diamond electrodes with an oxygen-terminated surface decorated by gold nanoparticles of 15 and size ranges, tested in . According to the proposed model, a surface depletion of diamond in electrolytes may penetrate sidewise under the contact to the nanoparticles by decreasing the potential of p-type electrode. Due to this effect the contact area could eventually be pinched off, which leads to a shift of the electrochemical reactions at the particles to more negative potentials or to their complete suppression. A good correlation was found between the simulation results and the characteristics of the decorated electrodes with different size nanoparticles and different boron doping concentrations in the surface layer.

Photoresponse and morphology of pentacene thin films modified by oxidized and reduced diamond surfaces

Because of its large band gap and variety of stable surface terminations, diamond is a suitable material to study the optical and electronic properties of organic films. Optical absorption and photocurrent experiments with pentacene on hydrogen- and oxygen-terminated diamond surfaces reveal a strong, polarization-dependent photoresponse of pentacene films. The diamond surface reconstruction as well as the molecule-surface interactions influence the morphology and the molecular structure of the films, causing the associated polarization dependence. On oxygen-terminated diamond, the pentacene thin-film phase typical for electronically inert substrates such as ${\text{SiO}}_{2}$ is formed. On hydrogen-terminated diamond, on the other hand, a three-dimensional growth mode of a filamentlike pentacene morphology is observed by atomic force microscopy, with pentacene molecules arranged with their long molecular axis oriented along the hydrogen-terminated diamond surface, as confirmed by x-ray diffraction. Furthermore, on hydrogen-terminated single crystalline diamond, the $\mathbit{b}$ axis of the pentacene unit cell is found to orient preferentially perpendicular to the surface, in agreement with photocurrent and optical-absorption experiments.

Electronic properties of hydrogen- and oxygen-terminated diamond surfaces exposed to the air

The electronic properties of hydrogen- and oxygen-terminated diamond surfaces exposed to the air are investigated by scanning probe microscopy (SPM). The results indicate that for the hydrogen-terminated diamond surface a shallow acceptor above the valence-band-maximum (VBM) appears in the band gap. However, the oxygen-terminated diamond film exhibits a high resistivity with a wide band gap. Based on the density-functional-theory, the densities of states, corresponding to molecular adsorbate in hydrogenated and oxygenated diamond (100) surfaces, are studied. The results show that the shallow acceptor in the band gap for the hydrogen-terminated diamond film can be attributed to the interaction between the surface C–H bonding orbitals and the adsorbate molecules, while for the oxygen-terminated diamond film, the interaction between the surface C–O bonding orbitals and the adsorbate molecules can induce occupied states in the valence-band.

First-Principles Studies of NOxChemistry on Agn/α-Al2O3

NOx chemistry on Al2O3 and Agn/Al2O3 (n ≤ 4) is investigated by density functional theory calculations. Three different terminations of α-Al2O3(0001) are considered, namely Al-terminated, fully hydroxyl-terminated, and partially oxygen-terminated. Weak adhesion energies are reported for Agn supported on Al- and hydroxyl-terminated surfaces, whereas Agn supported on partly oxygen-terminated surfaces are strongly bonded. This is connected to the charge state of the clusters: Agn clusters supported on Al- and hydroxyl-terminated surfaces remain close to an Ag0 state, whereas Agn clusters on partially oxygen-terminated surfaces are in an oxidized state. Adsorption properties of several reaction intermediates relevant for NOx chemistry, such as O, NO, NO2, and NO3, are investigated. Marked variations in adsorption energy are calculated as a function of cluster size. For Al- and partial O-termination there is a strong preference to adsorb at the edge of the Agn cluster. On the basis of the present results, the ...

Electronic structure and band-gap modulation of graphene via substrate surface chemistry

We have studied the electronic structure of graphene deposited on a SiO2 surface using density functional methods. The band structure of the graphene monolayer strongly depends on surface characteristics of the underlying SiO2 surface; for an oxygen-terminated surface, the monolayer exhibits a finite energy band gap while the band gap is closed when the oxygen atoms on the substrate are passivated with hydrogen atoms. We find that at least a graphene bilayer is required for a near zero energy gap when deposited on a substrate without H-passivation. Our results are discussed in the light of recent experiments.

Aminosilane Multilayer Formed on a Single-Crystalline Diamond Surface with Controlled Nanoscopic Hardness and Bioactivity by a Wet Process

Diamond could be an excellent support for nanodevices utilizing biomolecules if it is covered with a polymer layer immobilizing a variety of biomolecules. We report a wet method to form a 3-aminopropyltriethoxysilane (APTES) multilayer with a controlled hardness, roughness, and capacity for immobilizing protein. The method is feasible in typical biochemical laboratories where biomolecules are prepared. Atomic force microscopy (AFM) revealed that the surface geometries and nanoscopic hardness of the multilayers on an oxygen-terminated single-crystalline diamond surface depended on the dielectric constant of the solvent; the smaller the constant, the harder the layer. The hard multilayers had holes and APTES aggregates on the surfaces, while less hard ones had homogeneous surfaces with rare holes and little aggregates. The secondary deposition of APTES in a solvent with a large dielectric constant on a hard multilayer removed the holes, and further treatment of the multilayer in acidic ethanol solution diminished the aggregates. Such a surface can immobilize streptavidin with enough specificity against nonspecific adsorption using a combination of polyethylene glycol reagents. The results of a scratching test and nanoindentation test with AFM provided consistent results, suggesting some universality of the scratching test independent of the tip structure of the cantilever. The mechanism of formation of multilayers on the diamond surface and their binding to it is discussed.

STM study of the growth of cerium oxide nanoparticles on Au(1 1 1)

The morphology and structure of nanosized ceria particles prepared by several in vacuo deposition methods on a Au(1 1 1) template were investigated by scanning tunneling microscopy. Cerium metal nanoparticles on gold have limited reactivity toward molecular oxygen and NO 2, due to the formation of Ce–Au alloys, and their oxidation leads to formation of non-uniform substoichiometric three-dimensional (3D) oxide particles. Cerium metal deposition onto condensed multilayers of water or NO 2 generates fully oxidized but poorly ordered ceria particles after annealing. Ultra-thin flat and ordered ceria nanoislands were prepared by deposition of cerium metal on the gold surface at elevated temperatures under an oxygen background pressure. Atomically resolved images show an oxygen-terminated surface of CeO 2(1 1 1) with oxygen vacancies.

O-terminated nano-diamond ISFET for applications in harsh environment

The concept of an ion-sensitive FET (ISFET) on diamond with boron delta-doped channel and oxygen-terminated surface for pH sensing has been successfully transferred to large-area nano-crystalline diamond on silicon substrates. The nano-crystalline diamond layers, including the boron delta-doped channels of the FETs, were grown by hot-filament CVD. The fabricated layers were characterised by their peak concentration of boron of 3 × 10 20 1/cm 3 and FWHM of about 1 nm. This allowed approx. 50% modulation of the channel current within the gate potential range corresponding to the potential window of water electrolysis on the diamond surface. The O-termination by combination of oxygen-plasma and wet chemical treatments resulted in a pH sensitivity of the ISFETs close to the Nernst's limit in the range between pH1 and pH13. The ISFET characteristics were stable even after anodic treatment in KOH. This allows using nano-crystalline diamond ISFETs with O-termination also as electrodes and even at anodic overpotentials.

Anodic Electrochemical Pretreatment Time and Potential Affect the Electrochemical Characteristics of Moderately Boron-Doped Diamond Electrode

Boron-doped diamond (BDD) electrodes, both as-prepared and electrochemically oxidized were studied. The relation between the anodic oxidation treatment time, anodic potential and electrochemical characteristics has been discussed. Electron transfer processes in all BDD electrode surface were studied by cyclic voltammetry. The ferric/ferrous sulfate and ferri/ ferrocyanide redox systems were chosen respectively to act as the probe of one-electron trans- fer processes. The relation between ∆Ep and ψ, the dimensionless parameter, was obtained with a correlation coefficient greater than 0.9987 by mathematics fitting function. The rate constants of the electron transfer reaction, k0, were evaluated using the ∆Ep values. The k0 values for all BDD electrodes ranged from 3.33 × 10-5 to 4.72 × 10-5 cm s-1 in 0.1 M FeSO4/ 0.1 M H2SO4 and from 1.01 × 10-4 to 2.49 × 10-4 cm s-1 in 0.1 M K4[Fe(CN)6]/0.1 M H2SO4 system, which were in the standard range for a quasi-reversible system, respectively. The electrochemical properties of the BDD electrodes changed as a function of the surface anodic treatment time and potential. The anodic oxidation at low potential stripped mainly the impurities of BDD surface, sp2-carbon, and had little effect on the modification of the surface. While increasing the anodic potential up to +2.0 V, the anodic oxidation stripped the impurities of the BDD surface at first and carried out the modification of the BDD surface from hydrogen-terminated hydrophobic to oxygen-terminated hydrophilic surface with increasing anodic treatment time.

Control of bonding and epitaxy at copper/sapphire interface

The nature of bonding evolving at a metal/oxide interface depends strongly on the termination of oxide surface stabilized in the bonding environment. Specific surface treatments in ultrahigh vacuum allow control of the termination of sapphire (α-Al2O3) (0001) surface varying from hydroxyl-terminated to aluminum-terminated to oxygen-terminated surfaces. With this capability, the interfacial bonding forming between epitaxial Cu films and sapphire (0001) substrates can be tailored. Transmission electron microscopy studies reveal that the interatomic interaction at the interface plays a decisive role in determining wetting, orientation relationship, and thermal behavior of Cu thin films.

Profile Imaging of Reconstructed Polar and Non-polar Surfaces of ZnO

Abstract The atomic scale surface structures of ZnO ( 0 1 1 ¯ 0 ) non-polar as well as ( 0 1 ¯ 1 1 ) and ±(0 0 0 1) polar surfaces have been directly imaged by high-resolution transmission electron microscopy (HRTEM). The observations were made on clean surfaces created by irradiating a single ZnO nanobelt using 400 keV electron beam in TEM, under which ZnO dots were grown epitaxially and in situ on the surface of the nanobelt. A technique is demonstrated for directly distinguishing the surface polarity of the ±(0 0 0 1) polar surfaces. For the ( 0 1 1 ¯ 0 ) non-polar surface, HRTEM images and simulation results indicate that the Zn ions in the first and second layer suffer from inward and outward relaxation, respectively; the oxygen ions in the first and second layer prefer shifting to vicinal Zn ions to shorten the bonding distance. For the oxygen-terminated ( 0 1 ¯ 1 1 ) polar surface, the oxygen ions at the outmost top layer were directly imaged. a × 2 reconstruction has also been observed at the ( 0 1 ¯ 1 1 ) surface, and its atomic structure has been proposed based on image simulation. Oxygen-terminated ( 0 0 0 1 ¯ ) polar surface is flat and shows no detectable reconstruction. For the Zn-terminated (0 0 0 1) polar surface, HRTEM may indicate the existence of Zn vacancies and a possibly c-axis, random outward displacement of the top Zn ions. Our data tend to support the mechanism of removal of surface atoms for maintaining the stability of (0 0 0 1) polar surfaces.

Surface termination of hematite at environmental oxygen pressures: Experimental surface phase diagram

We provide an experimental surface phase diagram (surface termination versus chemical oxygen potential) of $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}(0001)$ using a natural single crystal surface. In a partial reduction reoxidation cycle, we observe a sequence from oxygen-, to ferryl-, and again oxygen-terminated surfaces, in better agreement with recent density functional theory in the generalized gradient approximation calculations than with calculations taking into account on-site $\mathrm{Fe}\phantom{\rule{0.2em}{0ex}}3d$ Coulomb repulsion. The nonreversible change in surface termination is accompanied by the formation of basal twins, which act as a sink for the extra Fe ions during the surface transformation processes.

Functionalizing Zn- and O-terminated ZnO with thiols

We have investigated the adsorption of dodecanethiol on zinc- and oxygen-terminated ZnO surfaces. Strong enthalpic adsorption is demonstrated by the stability of sulfur on both ZnO surfaces for temperatures up to 400°C. The minimal presence of the S 2p3∕2 170eV peak suggests absorption of the sulfur as an unoxidized thiol. The results indicate a higher surface coverage of the thiol on the zinc-terminated surface. Evidence from reflection high energy electron diffraction measurements for the surface ordering after thiol treatment of the oxygen-terminated ZnO surface suggests that the dodecanethiol molecules can adsorb in a highly ordered manner. These results further open the possibility for biofunctionalization of ZnO for biosensing applications.

Experimental study of cavitation damage on hydrogen-terminated and oxygen-terminated diamond film surfaces

To investigate the effect of hydrophobic surface on cavitation damage, hydrogen-terminated diamond film and oxygen-terminated diamond film are prepared for the cavitation damage experiment. Because the film surfaces have the same roughness and hardness, the hydrophobic property effect is focused on. Obvious damage pits appear on the hydrogen-terminated surface after 4 h cavitation experiment, while no such pits are found on oxygen-terminated surface. Surface testing results also show that the surface roughness of hydrogen-terminated surface is higher than that of oxygen-terminated surface after the experiment. Such results indicate that the collapse of bubbles growing from the hydrophobic wall to be damaged plays important role in the generation of cavitation damage.

Boron-doped diamond electrodes: The role of surface termination in the oxidation of dopamine and ascorbic acid

Due to the lack of strong adsorption of reactants, intermediates or products on the boron-doped diamond electrode surface, the electrochemical behavior can be simulated based on a model involving only electron transfer, chemical reactions and solution mass transport (DigiSim). The electrochemical behavior of several different biogenic redox-active species, dopamine, ascorbic acid, uric acid and 3,4-dihydroxyphenylacetic acid, was compared experimentally at as-deposited (hydrogen-terminated) and anodically oxidized (oxygen-terminated) boron-doped diamond surfaces, and molecular orbital theory was used to help explain the results. Using semi-empirical calculations, we find that, for dopamine, which is protonated in acid solution, the interaction of the positively charged quaternary ammonium group with the surface is relatively strong and nearly equal for both hydrogen- and oxygen-terminated surfaces, which explains the lack of sensitivity of the cyclic voltammetry to the surface termination. For ascorbic acid, the interaction of the neutral compound with the hydrogen-terminated surface is weaker, while that with the oxygen-terminated surface is very weak, consistent with the highly inhibited electron transfer.

Nanoscale surface modification of diamond for enhanced electrochemical sensing: Electrochemical characteristics of the surface modification

Nanoscale modification of the surface characteristics of boron-doped diamond has been demonstrated by local electrical discharge in electrolyte solutions. The nano-spots were created on (100)-oriented and oxygen-terminated surface of single crystal diamond using a Pt–Ir tip close to the surface. The created nano-spots caused an increase of the electrochemical activity of the diamond surface in 0.1 M H 2SO 4 electrolyte observed in the anodic potential range. The presence of the nano-spots was identified by selective deposition of gold clusters on the processed surface in a gold galvanic solution.