Oxidized Diamond Surface Research Articles

Surface-Mediated Charge Transfer of Photogenerated Carriers in Diamond.

Solvated electrons are highly reductive chemical species whose chemical properties remain largely unknown. Diamond materials are proposed as a promising emitter of solvated electrons and visible light excitation would enable solar-driven CO2 or N2 reductions reactions in aqueous medium. But sub-bandgap excitation remains challenging. In this work, the role of surface states on diamond materials for charge separation and emission in both gaseous and aqueous environments from deep UV to visible light excitation is elucidated. Four different X-ray and UV-vis spectroscopy methods are applied to diamond materials with different surface termination, doping and crystallinity. Surface states are found to dominate sub-bandgap charge transfer. However, the surface charge separation is drastically reduced for boron-doped diamond due to a very high density of bulk defects. In a gaseous atmosphere, the oxidized diamond surface maintains a negative electron affinity, allowing charge emission, due to remaining hydrogenated and hydroxylated groups. In an aqueous electrolyte, a photocurrent for illumination down to 3.5eV is observed for boron-doped nanostructured diamond, independent of the surface termination. This study opens new perspectives on photo-induced interfacial charge transfer processes from metal-free semiconductors such as diamonds.

Atom-by-Atom and Sheet-by-Sheet Chemical Mechanical Polishing of Diamond Assisted by OH Radicals: A Tight-Binding Quantum Chemical Molecular Dynamics Simulation Study.

Ultraflat and damage-free single-crystal diamond is a promising material for use in electronic devices such as field-effect transistors. Diamond surfaces are conventionally prepared by the chemical mechanical polishing (CMP) method, although the CMP efficiency remains a critical issue owing to the extremely high hardness of diamond. Recently, OH radicals have been demonstrated to be potentially useful for improving the CMP efficiency for diamond; however, the underlying mechanisms are still elusive. In this work, we applied our previously developed CMP-specialized tight-binding quantum chemical molecular dynamics simulator to comprehensively elucidate the CMP mechanisms of diamond assisted by OH radicals. Our simulation results indicate that the diamond surface is oxidized by reactions with OH radicals and then a concomitant surface reconstruction takes place due to the distorted and unstable nature of the oxidized diamond surface structure. Furthermore, we interestingly reveal that the reconstruction of the diamond surface ultimately leads to two distinct removal mechanisms: (i) gradual atom-by-atom removal through the desorption of gaseous molecules (e.g., CO2 and H2CO3) and (ii) drastic sheet-by-sheet removal through the exfoliation of graphitic ring structures. Hence, we propose that promoting the oxidation-induced graphitization of the diamond surface may provide a route to further improving the CMP efficiency.

Atomistic mechanisms of chemical mechanical polishing of diamond (1 0 0) in aqueous H2O2/pure H2O: Molecular dynamics simulations using reactive force field (ReaxFF)

Diamond CMP mechanisms have been widely investigated using the CMP experimental methods. However, the experiment could not observe the details of the removal process, it could not explain the mechanism of removal. The atom removal mechanism of chemical mechanical polishing (CMP) process on the diamond surface polished with a silica abrasive in aqueous H2O2/pure H2O was elucidated using ReaxFF molecular dynamics (MD) simulations. The research shows that the oxidation of diamond surface plays a dual role in the removal of C atoms. First, the diamond surface absorbs OH, O or H to form CO, COH or CH bonds. Then, the C atoms on the oxidized diamond surface are removed under the mechanical action of abrasive in aqueous H2O2. Three types of C atom removal pathway are detected in the CMP process: it can be removed in the form of CO, CO2 or C chain and C atom removal occurs on the first layer. However, no C atoms are removed in pure H2O. The friction in pure H2O is less than that in aqueous H2O2 due to the lubrication of pure H2O. In addition, comparing the different pressures applied to the abrasive, it is found that the greater the pressure applied, the more OH adsorbed on the diamond surface so that the more C atoms are removed. This work shows that the removal of C atoms is the result of the combination of chemical and mechanical effects, which helps understand the removal process of C atoms at the atomic scale in the CMP process and provide an effective method to choose the CMP slurry.

Tailoring morphologies of diamond thin films for neural stem cells culturing

A controllable growth of neural cells and/or neural stem cells (NSCs) on artificial substrates is highly attractive scientifically as motivated by neurological and neurodegenerative disease treatment. In this contribution, we report on human fetal NSCs attachment and growth on the diamond films. Diamond films of different morphologies were deposited by linear antenna microwave plasma system from H2/CO2/CH4 gas mixtures. We show that the employed NSC lines attach to the oxidized diamond surface rather than hydrogenated. The influences of nanocrystalline diamond morphology (flat film, porous structure, or nano-rods) and surface treatment (oxidized/hydrogenated or laminin coated films) are systematically studied after 8-day cells seeding.

XPS and UPS in situ study of oxygen thermal desorption from nanocrystalline diamond surface oxidized by different process

The chemistry of oxygen bonding on the diamond surface is a rich area of surface science research. It is well known that different surface terminations lead to strong variation of the material work function. This effect in diamond assumes peculiar consequences. In fact the oxidized diamond surface is hydrophilic, due to the high work function it shows a positive electron affinity and it is non conductive. On the contrary hydrogenation completely changes the orientation of the surface dipoles, the surface becomes hydrophobic, the work function lowers leading to a negative electron affinity. In addition hydrogen induces subsurface carriers which render the diamond surface semiconducting. These distinctive electronic properties make the diamond surface very interesting for the fabrication of surface field effect transistors just playing with the oxygen/hydrogen chemistry. Hydrogenation is generally obtained during the diamond synthesis in plasma reactors. Differently, the diamond surface oxidation may be accomplished with different processes (wet chemistry, plasma, UV irradiation).The realization of electronic devices calls for a complete understanding of the carbon–oxygen interactions, their stability and their influence on the electronic properties of diamond. Aim of this work is to explore the properties of diamond surfaces oxidized with piranha mixture, with O 2 plasma and with UV irradiation in a pure O 2 atmosphere. Each of these oxidized surfaces were annealed in situ at different temperatures and analyzed with photoelectron spectroscopies. Decreases of the oxygen concentration obtained via thermal desorption are then correlated with variations of the electronic properties obtained from UPS analyses.

Sequential Electrodeposition of Platinum‐Ruthenium at Boron‐Doped Diamond Electrodes for Methanol Oxidation

Sequential electrodeposition of Pt and Ru on boron‐doped diamond (BDD) films, in 0.5 M H2SO4 by cyclic voltammetry, has been prepared. The potential cycling, in the aqueous solutions of the respective metals, was between 0.00 and 1.00 V versus Ag/AgCl. The catalyst composites, Pt and PtRu, deposited on BDD film substrates, were tested for methanol oxidation. The modified diamond surfaces were also characterized by scanning electron microscopy‐X‐ray fluorescence‐energy dispersive spectroscopy, X‐ray photoelectron spectroscopy, and Auger electron spectroscopy. The scanning Auger electron spectroscopy mapping showed the ruthenium signal only in areas where platinum was electrodeposited. Ruthenium does not deposit on the oxidized diamond surface of the boron‐doped diamond. Particles with 5–10% of ruthenium with respect to platinum exhibited better performance for methanol oxidation in terms of methanol oxidation peak current and chronoamperometric current stability. The electrogenerated •OH radicals on BDD may interact with Pt surface, participating in the methanol oxidation as shown in oxidation current and the shift in the peak position. The conductive diamond surface is a good candidate as the support for the platinum electrocatalyst, because it ensures catalytic activity, which compares with the used carbon, and higher stability under severe anodic and cathodic conditions.

Trapping mechanism on oxygen-terminated diamond surfaces

Mechanisms of the hole trap and detrap on the oxygen-terminated diamond surfaces measured by diamond in-plane-gated field-effect transistors (FETs) have been investigated. Reproducible hysteresis characteristics are observed in the IDS-VGS characteristics of the diamond in-plane-gated FETs. They are caused by carrier trapping in the oxidized diamond surface and detrapping under a light irradiation, the wavelength of which affects the hysteresis width. Carriers are trapped by continuous surface states deeper than 2.0eV from the valence band maximum in the oxidized diamond surface, where the position of the highest occupied level (Fermi level) is located between 2.0 and 2.4eV.

Characterization of locally modified diamond surface using Kelvin probe force microscope

The surface potential difference between an H-terminated surface and a locally oxidized diamond surface produced by an atomic force microprobe was investigated using a Kelvin probe force microscope. The potential of the H-terminated diamond surface was observed to be ∼0.1 V higher than that of the oxidized diamond surface. The surface potential difference can be interpreted in terms of the positions of the vacuum level, the Fermi level, and the conduction and valence band edges, when negative electron affinity and p-type surface conduction are assumed on the H-terminated diamond surface. The surface dipole induced by the electronegativity differences between the surface atoms of the diamond affects the difference in the surface potential between the two surfaces.

Chemical state and wettability of ion-irradiated diamond surfaces

It is shown that the wettability of diamond can be tailored in a broad range by the surface modification via ion bombardment. The polished MPCVD diamond films were oxidized in air at 550 °C to make the surface hydrophilic and then were irradiated with Ar + ions at 1 keV energy at doses ranging from 1×10 14 to 5×10 16 cm −2. The surface state evolution was monitored in situ with X-ray photoelectron spectroscopy (XPS) and X-ray excited Auger electron spectroscopy (XAES). A progressive sp 3 to sp 2 bond transformation took place with the dose increase, while the irradiated surface becomes hydrophobic with the water contact angle increasing from 24° to 67° as measured with a sessile drop method. The heating of the ion-treated samples in air at 450 °C restores the hydrophilic properties inherent to oxidized diamond surface.

Diamond nanofabrication and characterization for biosensing application

Surface potential of the H-terminated and locally oxidized diamond surfaces is investigated by Kelvin probe force microscope. Potential of H-terminated diamond surface is observed to be ∼0.1 V higher than that of oxidized diamond surface. Surface potential difference is attributed to the surface charge induced by the electronegativity differences of the terminated atoms. The difference of DNA physisorption on the H- and O-terminated diamond surfaces due to the surface charging effect is also observed. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Control wettability of the hydrogen-terminated diamond surface and the oxidized diamond surface using an atomic force microscope

The force–distance curve indicates the difference in wettability between the hydrogen-terminated (H-terminated) diamond surface and the AFM-field-assisted local oxidized (O-terminated) area. Using a hydrophilic (silicon with native oxide) tip and a hydrophobic (coated with gold) tip as AFM tips, the adhesion force mapping measurement shows that the H-terminated surface is hydrophobic and that the O-terminated surface is hydrophilic. After heating in vacuum, the difference in the adhesion force between two surfaces decreased. This means that water adsorbed on the tip and sample surfaces affects the adhesion force. As an alternative measurement, the contact angle measurement of the H-terminated surface and the chemically oxidized surface was performed. It is proved that the oxidized surface is more hydrophilic than the H-terminated surface and that its surface energy is derived from surface polarity such as that involved in hydrogen bonding and electric dipole which is twofold that the H-terminated surface.

Electron emission from hydrogenated and oxidized heteroepitaxial diamond doped with boron

Electron emission from both the hydrogenated and the oxidized boron-doped heteroepitaxial diamond with smooth surface is studied in order to understand the effect of adsorption on electron emission. B-doped heteroepitaxial diamond is grown on iridium (Ir) by direct current plasma chemical vapor deposition (CVD). A relatively smooth surface is obtained. The Cls peaks in X-ray photoelectron spectroscopy (XPS) for the hydrogenated and oxidized diamond surfaces are 284.1 and 285.6 eV, respectively. The threshold voltage of the hydrogenated diamond surface in the electron emission is lower than that of the oxidized diamond surface. The emission barrier height ratio of the hydrogenated diamond surface to the oxidized diamond surface is estimated to be 2.6 from the slopes of Fowler–Nordheim (F–N) plots. The change in the emission properties might be examined by the surface dipole formed on diamond surface from the XPS results and the F–N plots.

Surface Reforming of the Oxidized Diamond Surface with Silane Coupling Reagents.

Diamond surface reforming was carried out using various silane coupling reagents (n-octyltrimethoxysilane, 3-aminopropyltrimethoxysilane, or 3-mercaptopropyltrimethoxysilane). In order to introduce the O-H group, which reacts with the methoxy group in the silane coupling reagents, on the diamond surface, hydrogenated diamond surfaces were treated with mineral acids (H2SO4, H2SO4+HNO3, or aqua regia). The diamond powders treated with H2SO4 or H2SO4+HNO3 were utilized as the diamond substrate for the reaction with the silane coupling reagents. After the treatment with the silane coupling reagents, peaks assigned to the chemical bonds in the silane coupling reagents appeared in the IR spectrum of the diamond powders. Moreover, these peaks remained even after washing 3 times with organic solvents. The oxidized diamond surface was changed from hydrophilic to hydrophobic upon treatment with n-octyltrimethoxysilane. From the results of this study, it was confirmed that the silane coupling reagent is effective for the surface reforming of diamond treated with mineral acid.

Direct interaction of elemental fluorine with diamond surfaces

Direct fluorinations of (i) hydrogenated diamond surfaces and (ii) oxidized diamond surfaces have been performed by thermal reaction in elemental fluorine. Chemisorption of fluorine on diamond powder surfaces has been investigated by diffuse-reflectance infrared Fourier transform spectroscopy. The hydrogenated diamond surface is fluorinated by thermal reaction in fluorine molecules. Hydrogen chemisorbed on the diamond surface is abstracted by the reaction with fluorine molecules even at −10 °C. Peaks due to CF stretching vibrations are observed at 1096, 1251 and 1347 cm −1 after the fluorination of the hydrogenated diamond at 100 °C. These CF stretching peaks increase in intensity with increase in fluorination temperature. CO species on the oxidized diamond surface are affected by fluorine molecules above 100 °C. Peaks due to CO stretching vibrations on the oxidized diamond surface remained and shifted into the higher wavenumber region after reaction with fluorine at low temperatures below 400 °C. The fluorination of the oxidized diamond at 500 °C gives similar chemisorbed states of fluorine to those of the hydrogenated diamond. The diamond surface is fully covered with fluorine by the fluorination at 500 °C.

Selective Metallization of CVD Diamond Films

ABSTRACTA process has been developed for the deposition of patterned adherent metal on diamond substrates using low temperature processing conditions. CVD diamond films on Si wafers were oxidized with an RFO2 plasma and subsequently functionalized by attachment of self-assembled ultrathin films (UTFs) to the oxidized diamond surface. The UTFs were exposed to patterned deep UV radiation, and selectively metallized by electroless (EL) deposition. EL Ni and Co patterns, with feature sizes to 20 μm linewidth have been produced. Oxidized and UTF-modified surfaces were characterized by surface spectroscopie and wettability techniques. The EL metal deposits on the diamond substrate passed the Scotch tape adhesion peel test.

Supported metallic catalysts obtained by anchoring metal complexes on carbon supports

Catalysts containing particles of metallic palladium have been synthesized by anchoring metal complex precursors on carbon supports (active carbon, carbon black, ultra-dispersed graphite and diamond). Anchored precursors were obtained by interaction of solutions of tetrachloride or allylcyclopentadienyl palladium complexes with the supports. Based on data on the peculiarities of HgPdCl 4 adsorption on carbon sorbents of different natures, three types of adsorption sites of Pd II ions have been identified and a quantitative evaluation of their strengths made. The size of the Pd crystallite formed upon reduction of the anchored complexes and its catalytic activity in cyclohexene hydrogenation depend upon the strength of the adsorption site. An increase in the strength of the adsorption site leads to a decrease both in the size and catalytic activity of Pd crystallites. The effect observed is associated with a modifying (ligand) action of atoms involved in the adsorption site. Fourier transform IR spectroscopic data indicate that the interaction of C 3H 5PdC 5H 5 with an oxidized diamond surface occurs with participation of the carboxyl groups of the support. According to transmission electron microscopic data, Pd/C catalysts reduced with hydrogen at 300 °C contain Pd particles 10 – 20 Å in size. On increasing the reduction temperature to 600 °C, the dispersion of Pd/diamond remains almost unchanged while the size of the metallic particles on Pd/graphite increases to 100 Å. This difference is due to peculiarities in the interaction of Pd particles with graphite and diamond surfaces in a hydrogen atmosphere. In the structurally insensitive hydrogenation of benzene, the turnover number (TON) activity of the Pd/C catalyst is commensurable with that of Pd/SiO 2, but in the structurally sensitive hydrogenolysis of ethane and hydrogenation of CO to methanol, the TON of Pd/C is one to two orders of magnitude lower, probably due to dissolution of carbon atoms in the Pd particles. Introduction of Zr ions into Pd/C leads to the appearance of a new absorption band at 1600 cm −1 in the IR spectrum of adsorbed CO, an increase in TON activity by 1 – 2 orders of magnitude in methanol synthesis and a drastic decrease in activity in ethane hydrogenolysis, the inhibiting effect of Zr ions being completely suppressed by high-temperature oxidation of the samples. These specific features are similar to those observed earlier for (Pd+Zr)/SiO 2 and Pd/ZrO 2, and seem to be due to the formation of mixed Pd-Zr active sites in the systems under consideration.