Electronic surface barrier properties of boron-doped diamond oxidized by plasma treatment

The electronic surface barrier characteristics of single-crystal and nanocrystalline boron-doped diamond in electrolytes are evaluated. Two cases are compared: Oxidation by RF oxygen plasma treatment and oxidation by anodic polarization in alkaline electrolyte. It is shown that the plasma treatment reduces the surface barrier to about 1.0 eV compared to 1.7 eV when subjected to anodic oxidation. For single-crystalline diamond, the oxygen evolution reaction in 0.1 M H₂SO₄ electrolyte is almost insensitive to the oxidation method while the plasma-treated nanocrystalline diamond electrode shows an enhanced activity of grain boundary defects at anodic potentials. X-ray photoemission spectroscopy measurements reveal that the plasma oxidation induces a higher content of carbonyl surface groups than anodic oxidation as well as a small amount of non-sp³ contents.     

Nano-crystalline diamond electrodes with cap layer decorated by gold particles

We describe the electrochemical characteristics of a nano-crystalline diamond (NCD) electrode with a thin (～ 60 nm) low-doped cap layer on a highly boron-doped diamond contact layer, where the grain boundary areas of the surface are decorated by gold particles by electroplating. The presence of the surface cap layer leads to a reduced background current compared to the highly doped NCD electrodes. At the same time, the electrical resistance to the gold particles via grain boundary defects of the cap layer does not limit the activity of the particles in 0.1 M H₂SO₄ electrolyte. The equivalent circuit of the decorated cap-layer electrodes is discussed using the results of electrochemical impedance spectroscopy and data on gold–diamond Schottky diodes fabricated on identical NCD layers. This equivalent circuit can be reduced to a low number of key elements, which could be useful for optimizing such electrodes for high sensitivity/low noise applications.     

Influence of initial surface condition of lithium metal anodes on surface modification with HF

Surface modification of as-received lithium foils was carried out using acid-base reactions of the native surface films on lithium metal with HF. Two types of as-received lithium foils covered with different native films were used as samples for this surface modification. One was a lithium foil having a very thin native surface film and the other one had a thicker native surface film. The surface condition of the lithium metal was analysed by X-ray photoelectron spectroscopy before and after the surface modification using HF, and the coulombic efficiency was measured electrochemically. The thickness of the surface film on the modified lithium foils was related to the Li₂O layer thickness in the native film on the as-received lithium foils. The modified lithium foil which had the thinner native surface film provided more uniform deposition of lithium and a higher coulombic efficiency during charge and discharge cycles when propylene carbonate electrolyte with 1.0 m LiPF₆ was used as the electrolyte. These results show that the initial condition of the native surface film plays an important role in surface modification with HF.     

Plasma etching treatment for surface modification of boron-doped diamond electrodes

Boron-doped diamond (BDD) thin film surfaces were modified by brief plasma treatment using various source gases such as Cl₂, CF₄, Ar and CH₄, and the electrochemical properties of the surfaces were subsequently investigated. From X-ray photoelectron spectroscopy analysis, Cl and F atoms were detected on the BDD surfaces after 3 min of Cl₂ and CF₄ plasma treatments, respectively. From the results of cyclic voltammetry and electrochemical AC impedance measurements, the electron-transfer rate for Fe(CN)₆^3−/4− and Fe^2+/3+ at the BDD electrodes was found to decrease after Cl₂ and CF₄ plasma treatments. However, the electron-transfer rate for Ru(NH₃)₆^2+/3+ showed almost no change after these treatments. This may have been related to the specific interactions of surface halogen (C-Cl and C-F) moieties with the redox species because no electrical passivation was observed after the treatments. In addition, Raman spectroscopy showed that CH₄ plasma treatment of diamond surfaces formed an insulating diamond-like carbon thin layer on the surfaces. Thus, by an appropriate choice of plasma source, short-duration plasma treatments can be an effective way to functionalize diamond surfaces in various ways while maintaining a wide potential window and a low background current.     

Electrolytic in-process dressing grinding performance of a multi-layer brazed coarse-grained diamond wheel using an electrolyte containing carbon microspheres

An electrolyte containing carbon microspheres (CMSs) was prepared for electrolytic dressing of a multi-layer brazed coarse-grained diamond wheel. And the influence of CMSs on the electrolytic in-process dressing grinding performance of Si₃N₄ ceramics with the brazed wheel was investigated. The results indicate that the CMSs can increase the electrolytic capacity of the electrolyte, which increases the thickness of the oxide film formed on the brazed wheel surface, and the worn diamond grit can easily fall off from the bonding matrix. In addition, the CMSs were adsorbed and distributed on the film, which effectively improves its densification and adhesion strength. This thicker and denser oxide film can improve the polishing effect of the brazed wheel. Thus, a better machined surface quality and less subsurface damage of the Si₃N₄ ceramics can be obtained by the multi-layer brazed coarse-grained diamond wheel with electrolyte containing CMSs.     

Chemical modification of diamond surface with linoleic acid by using benzoyl peroxide

In this study, the introduction of the C=C bond, which is known to be chemically active, on diamond surface was attempted using a reaction with unsaturated fatty acid. The chemical reactivity of a hydrogenated diamond surface with linoleic acid, which was most effective for introducing of the C=C bond, using benzoyl peroxide was investigated in detail. The diamond surface modified with the C=C bond was used as the substrate for the surface-modified diamond. Different radical source such as NCS, NBS, AIBN, and TEMPO were allowed to react with the diamond surface treated with linoleic acid. After the reaction with NCS, NBS, or AIBN, IR peaks at 3011 cm^− 1, which correspond to the C-H bond on the C=C bond, significantly decreased in size, and XPS spectra showed that the elements that derived from the reacted radical source, that is Cl, Br, or N, existed on the diamond surface. On the other hand, after the reaction with TEMPO no XPS peak of the N atoms appeared although the IR peak at 3011 cm^− 1 slightly decreased. These experimental results indicate that the C=C bond introduced on diamond surface can participate in chemical reaction. The method used in this study could become a very useful technique if the coverage factor of the C=C bond is improved by optimization of the reaction condition.     

Effect of surface modification using various acids on electrodeposition of lithium

Sulface modification of lithium was carried out using the chemical reaction of the native film with acids (HF, H₃PO₄, HI, HCl) dissolved in propylene carbonate (PC). The chemical composition change of the lithium surface was detected using X-ray photoelectron spectroscopy. The electrodeposition of lithium on the as-received lithium or the modified lithium was conducted in PC containing 1.0 mol dm^–3 LiClO₄ or LiPF₆ under galvanostatic conditions. The morphology of electrodeposited lithium particles was observed with scanning electron microscopy. The lithium dendrites were observed when lithium was deposited on the as-received lithium in both electrolytes. Moreover the dendrites were also formed on the lithium surface modified with H₃PO₄, HI, or HCl. On the other hand, spherical lithium particles were produced, when lithium was electrodeposited in PC containing 1.0 mol dm^–3 LiPF₆ on the lithium surface modified with HE However spherical lithium particles were not obtained, when PC containing 1.0 mol dm^–3 LiClO₄ was used as the electrolyte. The lithium surface modified by H₃PO₄, HI, or HCl was covered with a thick film consisting of Li₃PO₄, Li₂CO₃, LiOH, or Li₂O. The lithium surface modified with HF was covered with a thin bilayer structure film consisting of LiF and Li₂O. These results clearly show that the surface film having the thin bilayer structure (LiF and Li₂O) and the use of PC containing 1.0 mol dm^–3 LiPF₆ enhance the suppression of dendrite formation of lithium.     

叠氮化合物在无机材料表面改性中的应用

综述了叠氮化合物在无机材料化学表面改性中的应用,以铜催化形成三唑基的点击化学反应为主要改性方法,其改性的基材包括纳米碳管、富勒烯、金、磁性材料、聚合物/粘土纳米粒子以及金刚石表面,通过点击化学改性,可拓展上述无机材料在光电与生物技术以及医药和生化传感器等领域的应用范围。     

Chemical reaction of hydrogenated diamond surface with amino acids by using N-chlorosuccinimide

Various organic compounds can be immobilized on a diamond surface if a chemically active organic functional group, such as an NH₂ group, is introduced on the diamond surface. We therefore attempted to introduce an NH₂ group on a diamond surface by using chemical reaction of a hydrogenated diamond surface with an amino acid and NCS (N-chlorosuccinimide). From our previous experimental results (Diamond Relat. Mater., 15, 668 (2006)), we expected that a functional group could be introduced on the diamond surface. However, there was no peak assigned to the NH₂ group in the IR spectra for diamond powder treated with amino acids. The diamond powder treated with L-serine, L(?)-threonine, L(?)-phenylalanine, L(+)-arginine, L-histidine, or L-asparagine monohydrate had a specific peak at 2275 cm^? 1 in the IR spectra. Moreover, the diamond powder treated with L(?)-threonine had the highest intensity of the peak at 2275 cm^? 1. Therefore, the chemical reaction of a hydrogenated diamond surface with L(?)-threonine was further examined in detail. Unfortunately, the assignment of the peak at 2275 cm^? 1 was unclear. However, we speculate from the compounds in the chemical reaction process that the peak is assigned to the CN group, although the peak position is slightly different from that of the normal CN group.     

Effect of surface roughness and surface modification of indium tin oxide electrode on its potential response to tryptophan

The effect of surface modification of indium tin oxide (ITO) electrode on its potential response to tryptophan was investigated for ITO substrates with different surface roughness. It was found that a small difference in surface roughness, between ∼1 and ∼2 nm of R_a evaluated by atomic force microscopy, affects the rest potential of ITO electrode in the electrolyte. A slight difference in In:Sn ratio at the near surface of the ITO substrates, measured by angle-resolved X-ray photoelectron spectrometry and Auger electron spectroscopy is remarkable, and considered to relate with surface roughness. Interestingly, successive modification of the ITO surface with aminopropylsilane and disuccinimidyl suberate, of which essentiality to the potential response to indole compounds we previously reported, improved the stability of the rest potential and enabled the electrodes to respond to tryptophan in case of specimens with R_a values ranging between ∼2 and ∼3 nm but not for those with R_a of ∼1 nm. It was suggested that there are optimum values of effective work function of ITO for specific potential response to tryptophan, which can be obtained by the successive modification of ITO surface.     

Potential applications of surface channel diamond field-effect transistors

In order to realize high frequency and high power diamond devices, diamond FETs on the hydrogen-terminated diamond surface conductive layer have been fabricated. The fabricated diamond MESFETs show high breakdown voltage and output capability of 1 W mm⁻¹. High transconductance diamond MESFET utilizing a self-aligned gate FET fabrication process has been operated in high frequency for the first time. In the 2 μm gate MESFETs, the obtained cut off frequency f_T and maximum frequency of oscillation f_max are 2.2 and 7 GHz, respectively. It is expected that the diamond MESFET with 0.5 μm gate length fabricated by self-aligned gate process shows 8 GHz of f_T and 30 GHz of f_max.     

Mechanism of glucose electrochemical oxidation on gold surface

The complex oxidation of glucose at the surface of gold electrodes was studied in detail in different conditions of pH, buffer and halide concentration. As observed in previous studies, an oxidative current peak occurs during the cathodic sweep showing a highly linear dependence on glucose concentration, when other electrolyte conditions are unchanged. The effect of the different conditions on the intensity of this peak has stressed the limitations of the previously proposed mechanisms. A mechanism able to explain the presence of this oxidative peak was proposed. The mechanism takes into account ion-sorption and electrochemical adsorption of OH⁻, buffer species (K₂HPO₄/KH₂PO₄) and halides.     

Electrochemical behaviour of the surface treated composite-electrolyte/electrode interface

The effect of surface treatment of the solid electrolyte 50 wt% (4.7 mol% Sc₂O₃+95.3 mol % ZrO₂)+ 50 wt % Al₂O₃ (used in the SIRO₂ low temperature oxygen sensor) on its electrochemical behaviour has been studied by complex impedance spectroscopy. Two techniques, namely, preferential etching of alumina from the surface by a suitable etchant and cosintering of a thin layer of an alumina free electrolyte composition were used. The electrode/electrolyte interfaces prepared by using surface treated electrolyte samples had much lower electrode resistance (by a factor of 3–6) compared with the untreated surfaces. The decrease in the interfacial impedance is attributed mainly to the absence of alumina at the interface (an insulator phase) at which no oxygen exchange reactions could take place.     

Growth of atomically step-free surface on diamond {111} mesas

Diamond homoepitaxy by microwave plasma-enhanced chemical vapor deposition was investigated on {111} substrate. Growth at a low CH₄/H₂ ratio of 0.025% in a gas phase results in the formation of an atomically step-free surface over 10 × 10-µm² mesas of diamond {111} substrate, when there are no screw dislocations in the mesas. This was achieved through ideal lateral growth, in which two-dimensional terrace nucleation was completely suppressed. The application of the selective formation of the step-free surface and the lateral growth of diamond films will open the way for the realization of high-quality electronic devices using diamond.     

pH sensor on O-terminated diamond using boron-doped channel

A concept of an ion sensitive FET (ISFET) on diamond using a thin and highly boron-doped channel is described. The doped channel is in direct contact to the electrolyte solution. The diamond surface is oxygen terminated to provide pH sensitivity and chemical stability. The first set of pH-sensitive ISFET microstructures with the extrinsically doped channel is fabricated on 100-oriented single crystal diamond substrate using a solid doping source technique and a wet chemical oxidation for the O termination. The fabricated structures show pH sensitivity close to the Nernst's limit of 59 mV/pH. The ISFET characteristics have been reproducible and did not degrade after repeated cycling between 0.1 M H₂SO₄ and 0.1 M KOH solutions. It is shown that only a part of the applied gate potential to the diamond–electrolyte interface drops within the delta-doped channel. The possible origin of this effect is discussed by analysing the electrochemical impedance characteristics of the boron-doped diamond electrode.     

From self-organized masks to nanotips: A new concept for the preparation of densely packed arrays of diamond field emitters

Diblock copolymer based gold nanoparticle arrays are used to pattern diamond and silicon surfaces on the nanoscale. Taking advantage of diblock copolymers forming spherical reverse micelles, which self-assemble into hexagonally ordered arrays when deposited onto a surface, gold nanoparticle patterns are prepared from HAuCl₄ loaded poly(styrene)-block-poly(2-vinylpyridine) (PS-b-P2VP) micelles on top of diamond and silicon. By applying these particles as nanomasks for subsequent reactive ion etching, the hexagonal pattern is transferred into the substrate resulting in a corresponding array of diamond nanotips and silicon nanopillars, respectively. In the case of B-doped diamond, these nanotips exhibit a significantly enhanced electron emissivity as compared to a polished surface proving the new functionality resulting from nanopatterning field emitters of an unprecedented areal density.     

Pulsed laser surface modifications of diamond thin films

In view of practical applications requiring diamond films, plates and membranes with very smooth surfaces, ArF excimer laser polishing treatments were applied to thin (30 μm) diamond films grown by CVD on silicon substrates. The as-prepared diamond surfaces and the laser-treated parts of the samples were characterised by SEM analysis, Raman and micro-Raman spectroscopy. The presence on the laser-treated surface of a thin amorphous carbon layer responsible for the higher surface electrical conductivity and for the different optical reflectivity properties was evidenced. Using confocal micro-Raman spectroscopy a comparative depth profile analysis of the phase quality, below the surface in different regions of the films, was carried out. After short (10 min) treatment by H₂ plasma etching in the CVD chamber the graphitic top layer was completely removed from the samples.     

Low surface temperature synthesis and characterization of diamond thin films

Polycrystalline diamond films are deposited on p-type Si(100) and n-type SiC(6H) substrates at low surface deposition temperatures of 370–530 °C using a microwave plasma enhanced chemical vapor deposition (MPECVD) system. The surface temperature during deposition is monitored by an IR pyrometer capable of measuring temperature between 250 and 600 °C in a microwave environment. The lower deposition temperature is achieved by using an especially designed cooling stage. The influence of the deposition conditions on the growth rate and structure of the diamond film is investigated. A very high growth rate up to 1.3 μm/h on SiC substrate at 530 °C surface temperature is attributed to an optimized Ar-rich Ar/H₂/CH₄ gas composition, deposition pressure, and microwave power. The structure and microstructure of the films are characterized by X-ray diffraction, scanning electron microscopy, and Raman spectroscopy. A detailed stress analysis of the deposited diamond films of grain sizes between 2 and 7 μm showed a net tensile residual stress and predominantly sp³-bonded carbon in the deposited films.     

Effects of surface pretreatments on the deposition of adherent diamond coatings on cemented tungsten carbide substrates

Well-adhered microcrystalline diamond (MCD) coatings have been deposited on WC–Co substrates by the microwave plasma enhanced chemical vapor deposition (MPECVD) method. A multi-interlayer system Cr/CrN/Cr was deposited on the cemented carbide substrate before diamond deposition to act as a diffusion barrier. The interlayer-coated substrate was shortly peened by friable diamond powders with an average size of 150 μm to roughen the surface. Diamond coatings deposited on short peened substrates show higher nucleation density and stronger adhesion properties. The X-ray diffraction (XRD) pattern showed that an additional carbide compound layer (Cr₃C₂ and Cr₇C₃) was formed during the CVD diamond deposition to work as an intermediate bonding layer for better adhesion. Rockwell indentation tests with a load of 1470 N were conducted to investigate the coating's adhesion. No delamination outside of the indentation zone was observed for the diamond coating deposited on the roughened sample. Electron probe microanalysis (EPMA) results showed that the delamination in the indentation zone occurred mainly at the diamond/Cr interface and very little Co (less than 1 wt.%) was detected on the Cr failure surface. This suggests that during the CVD process Co/C inter-diffusion was successfully prevented by the Cr/CrN/Cr buffer layers.     

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 O₂ plasma and with UV irradiation in a pure O₂ 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.