Influence of heat treatment temperature on the morphological and structural aspects of reticulated vitreous carbon used in polyaniline electrosynthesis

Reticulated vitreous carbon (RVC) was obtained from different heat treatment temperature (HTT), in the range from 700 up to 2000 °C, and used as a substrate for polyaniline growth from electrosynthesis. The influence of HTT on RVC chemical surface was studied by X-ray photoelectron spectroscopy (XPS) and correlated to electrochemical parameters used in the electrosynthesis. XPS analyses have shown that RVC heteroatoms decrease as HTT increases. The results reveal the migration of chemical bonds from oxidized carbon forms towards carbon atoms as the unique final product. Cyclic voltammetry, electrochemical impedance spectroscopy, and stability test of polyaniline films were performed from oxidized and non-oxidized RVC substrates. Cyclic voltammetry in 0.5 mol L⁻¹ H₂SO₄ revealed higher capacitance for the RVC treated at 1000 °C and oxidized in a hot H₂SO₄ solution. The charge accumulation after RVC chemical treatment has increased around ten times. The lowest electric resistivities and impedances were obtained for the RVC treated at 2000 °C, which also showed the highest polyaniline stability.     

Effects of hydrogen etching process on the structural and optical properties of nano-crystalline diamond films

In this work, hydrogen etching method is applied to improve the quality of nano-crystalline diamond (NCD) films grown from hot-filament assisted chemical vapor deposition (HFCVD) system. From the characteristics of the structure and optical property, the grain size and surface roughness decrease while the optical transmission increase obviously under certain deposition parameters (gas pressure and substrate temperature) and longer etching time. Soft X-ray transmission measurements by synchrotron radiation are also carried out on the NCD films. The result shows that the X-ray transmission has an obvious improvement when the NCD film is fabricated from the hydrogen etching method. And the transmittance reaches 53.3% at X-ray photon energy of 258 eV, which has met the requirement for X-ray mask materials.     

The influence of hydrogen plasma pre-treatment on the structure of BDND electrode surface applied for phenol detection

The surface properties of boron-doped nanocrystalline diamond films treated with H₂ plasma was investigated in regard to their electrochemical response for phenol oxidation. The surface of these films is relatively flat formed by crystallites with sizes of about 40 nm. X-ray photoelectron spectroscopy analyses showed that electrode surface has a high amount of C–H bonds. This behavior is in agreement with Mott-Schottky plot measurements concerning the flat band potential that presented a value as expected for hydrogenated diamond surface. This electrode presented the phenol detection limit of 0.08 mg L⁻¹ for low phenol concentrations from 40 to 250 μmol L⁻¹.     

Uniform Fe3O4–PANi/PS composite spheres with conductive and magnetic properties and their hollow spheres

Core–shell multifunctional composite spheres consisting of Fe₃O₄–polyaniline (PANi) shell and polystyrene (PS) core were fabricated using core–shell-structured sulfonated PS spheres (with uniform diameter of 250 nm) as templates. PANi was doped in situ by sulfonic acid resulting the composite spheres are well conductive. Dissolved with solvent, PS cores were removed from the core–shell composite spheres and hollow Fe₃O₄–PANi spheres were obtained. Removing the PANi and PS components by calcinations produced hollow Fe₃O₄ spheres. The cavity size of the hollow spheres was uniformly approximate to 190 nm and the shell thickness was 30 nm. The cavity size and the shell thickness can be synchronously controlled by varying the sulfonation time of the PS templates. The shell thickness in size range was of 20–86 nm when the sulfonation time was changed from 1 to 4 h. These resulting spheres could be arranged in order by self-assembly of the templates. Both the Fe₃O₄–PANi/PS composite spheres and the hollow Fe₃O₄ spheres exhibit a super-paramagnetic behavior. Scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, and X-ray powder scattering were used to characterize these as-prepared spheres. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

On the nonuniform electron-density distribution in a layer adsorbed on a metal

The probability of a transition from a uniform charge distribution (UCD) in the adsorbed layer to a nonuniform one (NCD) caused by the dipole-dipole adatom repulsion is analyzed within the Anderson-Newns Hamiltonian and a simple density-of-states model for the substrate. Two limiting cases are considered, namely, an infinitely wide and a narrow band of allowed substrate states. Criteria for the UCD→NCD transition are obtained. Experimental data on reconstruction of clean metal faces are analyzed. Fiz. Tverd. Tela (St. Petersburg) 41, 1543–1547 (September 1999)     

Electrons diffusion study on the nitrogen-doped nanocrystalline diamond film grown by MPECVD method

Nitrogen-doped nanocrystalline diamond (NNCD) films were deposited onto p-type silicon substrates with three different layer structures: (i) directly onto the silicon substrate (NNCD/Si), (ii) silicon with undoped nanocrystalline diamond layer which was deposited in the same way as the above mentioned NNCD by the recipe Ar/CH₄/H₂ with a ratio of 98%/1%/1% (NNCD/NCD/Si), and (iii) silicon wafer with 100 nm thickness SiO₂ layer (NNCD/SiO₂/Si). Atomic force microscopy (AFM), X-ray diffraction (XRD) and Raman spectroscopy were employed to characterize the morphology and microstructure of the as-grown nitrogen-doped diamond films. Silver colloid/silver contacts were made at to measure the current-voltage (I-V) characteristics for the three different structures. Electrons from a CVD reactor hydrogen plasma diffuse toward the p-type silicon substrate during a deposition process under the high temperature (∼800 °C). The study concluded that the SiO₂ layer could effectively prevents the diffusion of electrons.     

Growth and field emission properties of nanotip arrays of amorphous carbon with embedded hexagonal diamond nanoparticles

Nanotip arrays of amorphous carbon with embedded hexagonal diamond nanoparticles were prepared at room temperature for use as excellent field emitters by a unique combination of anodic aluminum oxide (AAO) template and filtered cathodic arc plasma (FCAP) technology. In order to avoid nanopore array formation on the AAO surface, an effective multi-step treatment employing anodization and pore-widening processes alternately was adopted. The nanotips were about 100 nm in width at the bottom and 150 nm in height with density up to 10¹⁰ cm⁻². Transmission electron microscopy investigation indicates that many nanoparticles with diameters of about 10 nm were embedded in the amorphous carbon matrix, which was proved to be hexagonal diamond phase by Raman spectrum and selected-area electron diffraction. There is no previous literature report on the field emission properties of hexagonal diamond and its preparation at room temperature under high-vacuum condition. The nanotip arrays with hexagonal diamond phase exhibit a low turn-on field of 0.5 V/μm and a threshold field of 3.5 V/μm at 10 mA/cm². It is believed that the existence of hexagonal diamond phase has improved the field emission properties.     

Study of nanocrystalline diamond film growth in rf hybrid laser deposition systems in O2, H2 and H2+Ar ambients

Pulsed laser deposition (PLD) and hybrid pulsed laser deposition (HPLD) systems were used for nanocrystalline diamond (NCD) film growth on Si(100) and sapphire(0001) substrates. The PLD system was based on a KrF excimer laser (=248 nm,20 ns), which ablated a graphite target in pure oxygen or hydrogen ambient. The HPLD system was based on a combination of PLD and additional capacitively coupled radio-frequency discharges (13.56 MHz) in argon–hydrogen ambient. Radio-frequency power was applied to two plane parallel electrodes or directly to the substrate holder. The working atmosphere pressure was varied from 1 Pa to 220 Pa and the substrate temperature was varied from 20 °C to 660 °C. X-ray diffraction analysis, Raman spectroscopy and a profilometer were used to study the deposited film properties. A band around 1180 cm^-1, which can be attributed to NCD, occurred in the Raman spectra. A characteristic diamond peak at 1332 cm^-1 was not observed. Films prepared by HPLD showed better compositional homogeneity (from Raman analysis) than films created by PLD. PACS 52.80.Pi; 81.07.Bc; 81.15.Fg     

Synthesis of platinum and platinum–ruthenium-modified diamond nanoparticles

With the aim of developing dimensionally stable-supported catalysts for direct methanol fuel cell application, Pt and Pt–Ru catalyst nanoparticles were deposited onto undoped and boron-doped diamond nanoparticles (BDDNPs) through a chemical reduction route using sodium borohydride as a reducing agent. As-received commercial diamond nanoparticles (DNPs) were purified by refluxing in aqueous nitric acid solution. Prompt gamma neutron activation analysis and transmission electron microscopy (TEM) techniques were employed to characterize the as-received and purified DNPs. The purified diamond nanoparticulates, as well as the supported Pt and Pt–Ru catalyst systems, were subjected to various physicochemical characterizations, such as scanning electron microscopy, energy dispersive analysis, TEM, X-ray diffraction, inductively coupled plasma-mass spectrometry, X-ray photoelectron spectroscopy, and infrared spectroscopy. Physicochemical characterization showed that the sizes of Pt and Pt–Ru particles were only a few nanometers (2–5 nm), and they were homogeneously dispersed on the diamond surface (5–10 nm). The chemical reduction method offers a simple route to prepare the well-dispersed Pt and Pt–Ru catalyst nanoparticulates on undoped and BDDNPs for their possible employment as an advanced electrode material in direct methanol fuel cells.     

Enhanced field emission characteristics of boron doped diamond films grown by microwave plasma assisted chemical vapor deposition

Boron doped diamond films were synthesized on silicon substrates by microwave plasma chemical vapor deposition (MPCVD) technique. The effect of B₂O₃ concentration varied from 1000 to 5000 ppm on the field emission characteristics was examined. The surface morphology and quality of films were characterized by scanning electron microscope (SEM) and Raman spectroscopy. The surface morphology obtained by SEM showed variation from facetted microcrystal covered with nanometric grains to cauliflower of nanocrystalline diamond (NCD) particles with increasing B₂O₃ concentration. The Raman spectra confirm the formation of NCD films. The field emission properties of NCD films were observed to improve upon increasing boron concentration. The values of the onset field and threshold field are observed to be as low as 0.36 and 0.08 V/μm, respectively. The field emission current stability investigated at the preset value of ∼1 μA is observed to be good, in each case. The enhanced field emission properties are attributed to the better electrical conductivity coupled with the nanometric features of the diamond films.     

The structure of diamond nanoclusters

A model describing the structure of diamond nanoclusters produced by explosive shocks is proposed. The model is based on experimental data obtained from x-ray diffraction and small-angle x-ray scattering. This model considers the diamond nanocluster as a crystalline diamond core coated by a carbon shell having a fractal structure. The shell structure depends both on the cooling kinetics of the detonation products and on the method used to extract from them the diamond fraction. Fiz. Tverd. Tela (St. Petersburg) 41, 740–743 (April 1999)     

The protective properties of ultra-thin diamond like carbon films for high density magnetic storage devices

With the increase of magnetic storage density, the thickness of the protective diamond like carbon (DLC) film on the surfaces of head and disk is required as thin as possible. In this paper, the structure, mechanical properties and corrosion and oxidation resistance of ultra-thin DLC films are investigated. The ultra-thin DLC films were deposited by using filtered cathodic vacuum arc (FCVA) technique. The exact thickness of the ultra-thin DLC film was determined by high resolution transmission electron microscope (HRTEM). Raman analysis indicates that the ultra-thin DLC film presents ta-C structure with high sp³ fraction. In the wear test, a diamond tip was used to simulate a single-asperity contact with the film surface and the wear marks were produced on the film surface. The wear depths decrease with film thickness increasing. If the film thickness was 1.4 nm or above, the wear depth was much lower than that of Si substrate. This indicates that the ultra-thin DLC film with thickness of 1.4 nm shows excellent wear resistance. Corrosion tests in water and oxidation tests in air were carried out to investigate the diffusion barrier effect of the ultra-thin DLC films. The results show that the DLC film with thickness of 1.4 nm provides adequate coverage on the substrate and has good corrosion and oxidation resistance.     

LiFePO<Subscript>4</Subscript>/C with high capacity synthesized by carbothermal reduction method

The olivine-type LiFePO₄/C cathode materials were prepared via carbothermal reduction method using cheap Fe₂O₃ as raw material and different contents of glucose as the reducing agent and carbon source. Their structural and morphological properties were investigated by X-ray diffraction, scanning electron microscope, transmission electron microscope, and particle size distribution analysis. The results demonstrated that when the content of the carbon precursor of glucose was 16 wt.%, the synthesized powder had good crystalline and exhibited homogeneous and narrow particle size distribution. Even and thin coating carbon film was formed on the surface of LiFePO₄ particles during the pyrolysis of glucose, resulting in the enhancement of the electronic conductivity. Electrochemical tests showed that the discharge capacity first increased and then decreased with the increase of glucose content. The optimal sample synthesized using 16 wt.% glucose as carbon source exhibited the highest discharge capacity of 142 mAh g⁻¹ at 0.1C rate with the capacity retention rate of 90.4% and 118 mAh g⁻¹ at 0.5C rate.     

Critical role of laser wavelength on carbon films grown by PLD of graphite

The structure of thin films deposited by pulsed laser ablation (PLD) is strongly dependent on experimental conditions, like laser wavelength and fluence, substrate temperature and pressure. Depending on these parameters we obtained various kinds of carbon materials varying from dense, mainly tetrahedral amorphous carbon (ta-C), to less compact vertically oriented graphene nano-particles. Thin carbon films were grown by PLD on n-Si 〈100〉 substrates, at temperatures ranging from RT to 800°C, from a rotating graphite target operating in vacuum. The laser ablation of the graphite target was performed by a UV pulsed ArF excimer laser (λ=193 nm) and a pulsed Nd:YAG laser, operating in the near IR (λ=1064 nm). The film structure and texturing, characterised by X-ray diffraction analysis, performed at grazing incidence (GI-XRD), and the film density, evaluated by X-ray reflectivity measurements, are strongly affected both by laser wavelength and fluence and by substrate temperature. Micro-Raman and GI-XRD analysis established the progressive formation of aromatic clusters and cluster condensation into vertically oriented nano-sized graphene structures as a direct function of increasing laser wavelength and deposition temperature. The film density, negatively affected by substrate temperature and laser wavelength and fluence, in turn, results in a porous bulk configuration and a high macroscopic surface roughness as shown by SEM characterisation. These structural property modifications induce a relevant variation also on the emission properties of carbon nano-structures, as evidenced by field emission measurements. This work is dedicated to our friend Giorgio who passed away 20th August.     

Highly efficient field emission from nanodiamond films treated by fast reactive ion etching process

A fast reactive ion etching (RIE) treatment method is presented for dramatic enhancement of the field emission performances of nanocrystalline diamond (NCD) films. In this method a moment RIE treatment is able to modify the surface morphologies of NCD films and form a large area of nanoneedle-like arrays on the NCD films, in which the diamond nanoparticles were seeded on the film to serve as an etching mask. These elaborated diamond nanoneedle-like structures showed good uniformity and dense morphology with a controllable aspect ratio and distribution density and thereby significantly increased the electron field emission properties of the NCD films due to the formation of more emitting tips and enhanced field enhancement factor.     

Effect of titanium powder assisted surface pretreatment process on the nucleation enhancement and surface roughness of ultrananocrystalline diamond thin films

A superior, easy and single-step titanium (Ti) powder assisted surface pretreatment process is demonstrated to enhance the diamond nucleation density of ultrananocrystalline diamond (UNCD) films. It is suggested that the Ti fragments attach to silicon (Si) surface form bond with carbon at a faster rate and therefore facilitates the diamond nucleation. The formation of smaller diamond clusters with higher nucleation density on Ti mixed nanodiamond powder pretreated Si substrate is found to be the main reason for smooth UNCD film surface in comparison to the conventional surface pretreatment by only nanodiamond powder ultrasonic process. The X-ray photoelectron spectroscopic study ascertains the absence of SiC on the Si surface, which suggests that the pits, defects and Ti fragments on the Si surface are the nucleation centers to diamond crystal formation. The glancing-incidence X-ray diffraction measurements from 100 nm thick UNCD films evidently show reflections from diamond crystal planes, suggesting it to be an alternative powerful technique to identify diamond phase of UNCD thin films in the absence of ultra-violet Raman spectroscopy, near-edge X-ray absorption fine structure and transmission electron microscopy techniques.     

Characterization of the bonding structure of nanocrystalline diamond and amorphous carbon films prepared by plasma assisted techniques

Thin nanocrystalline diamond/amorphous carbon (NCD/a-C) composite films and amorphous diamond-like carbon (DLC) films were prepared by three methods: microwave plasma chemical vapour deposition (MWCVD) from methane/nitrogen mixtures (NCD/a-C), RF magnetron sputtering of a pure graphite target in argon/methane ambients, and pulsed laser deposition (PLD) in vacuum or argon atmosphere (DLC). The films prepared by the three techniques were comprehensively characterized with respect to their bonding structure by Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). PACS 81.05.Uw; 82.80.Pv     

Photoluminescence spectra of ultradisperse diamond

The first study of materials based on ultradisperse diamond produced by detonation is reported. A luminescence band in the visible has been observed, and some of its structural features have been interpreted by analogy with the known luminescence bands of centers in synthetic and natural diamonds. A comparison of the spectra obtained from ultradisperse diamond samples with the surface modified by different chemical treatments suggests that their pattern is governed to a considerable extent by the presence of a graphitic layer on the grain surface. Fiz. Tverd. Tela (St. Petersburg) 39, 2156–2158 (December 1997)     

Photoacoustic spectroscopy of diamond powders and polycrystalline films

Photoacoustic spectroscopy is used to study optical absorption in diamond powders and polycrystalline films. The photoacoustic spectra of diamond powders with crystallite sizes in the range from ∼100 μm to 4 nm and diamond films grown by chemical vapor deposition (CVD) had a number of general characteristic features corresponding to the fundamental absorption edge for light with photon energies exceeding the width of the diamond band gap (∼5.4 eV) and to absorption in the visible and infrared by crystal-structure defects and the presence of non-diamond carbon. For samples of thin (∼10 μm) diamond films on silicon, the photoacoustic spectra revealed peculiarities associated with absorption in the silicon substrate of light transmitted by the diamond film. The shape of the spectral dependence of the amplitude of the photoacoustic signal in the ultraviolet indicates considerable scattering of light specularly reflected from the randomly distributed faces of the diamond crystallites both in the polycrystalline films and in the powders. The dependence of the shape of the photoacoustic spectra on the light modulation frequency allows one to estimate the thermal conductivity of the diamond films, which turns out to be significantly lower than the thermal conductivity of single-crystal diamond. Fiz. Tverd. Tela (St. Petersburg) 39, 1787–1791 (October 1997)     

氧离子注入纳米金刚石薄膜的微结构和电化学性能研究

在纳米金刚石薄膜中注入剂量为1012cm-2的氧离子,并进行700,800,900和1000?C的真空退火处理,系统研究薄膜的微结构和电化学性能结果表明,氧离子注入未退火(O120)和氧离子注入1000?C退火(O121000)电极的电势窗口分别为4.60 V和3.61 V,远大于其他电极的电势窗口,并且这两个样品的电极传质效率较高,说明氧离子注入和高温退火有利于提高电极的传质效率.红外光谱测试表明,样品O120和O121000的表面没有碳氢基团终止层,而其他样品均含有氢终止层,说明氧离子注入和高温退火破坏了薄膜表面含碳氢基团的氢终止层,提高了薄膜的电化学性能Raman光谱测试结果表明,金刚石含量较高、内应力较小和非晶石墨相无序化程度较大的样品具有较好的电化学性能;并且薄膜晶界处的非晶碳的团簇数量或者尺寸减小,样品的电化学性能提高.