Diamond-coated field-effect sensor for DNA recognition — Influence of material and morphology

The influence of nanocrystalline (< 20 nm grains) and microcrystalline (around 100 nm grains) diamond thin film morphology on the capacitance–voltage (C–V) characteristics of diamond-coated field-effect SiN sensors was characterized with respect to DNA recognition. DNA was grafted via –OH surface termination. The sensor materials and surfaces were characterized by scanning electron microscopy, Raman Spectroscopy, fluorescence microscopy, XPS, and contact angle measurements. The C–V characteristics exhibited generally an order of magnitude higher flat band voltage shifts (∆ V_F) after complementary DNA hybridization for both types of diamond-coated sensors (160 ÷ 300 mV) compared to reference SiN sensor without diamond layer (11 mV), even if incomplete DNA denaturation and ∆ V_F fluctuations (60 mV) were accounted for. While microcrystalline diamond provides the highest response, nanocrystalline diamond provides the highest sensitivity. An explanation based on interfacial charges is proposed.     

Fabrication and electrical properties of SrTiO3/diamond junctions

Strontium titanate (STO) films were directly deposited on Ib (100) single crystal diamond by r.f. magnetron sputtering. The as-deposited STO film was in amorphous state. On the other hand, the crystalline STO film was obtained under the optimized condition of a deposition temperature of 250 °C and a post-annealing temperature of 650 °C. STO/diamond junctions were fabricated on boron-doped homoepitaxial layers grown on p⁺-type single crystal diamond substrates. Electrical properties of the STO/diamond junction were investigated by changing the surface terminations of diamond with hydrogen or oxygen and the crystallinity of the STO film. It was found that the amorphous STO acted like a semi-insulator on H-diamond surface and that the amorphous STO/O-diamond junction behaved like a Schottky diode. The crystalline STO/O-diamond showed a complex rectifying behavior. The crystalline STO film possessed a higher dielectric constant as compared to that of the amorphous one.     

Electrochemical synthesis and electronic properties of polypyrrole on intrinsic diamond

Thin film of polypyrrole is electrochemically deposited on hydrogen terminated surface of intrinsic diamond exhibiting surface conductivity (5 × 10^? 5 S/□). Based on a high bonding strength (40 nN) estimated by AFM-scratching experiments and local changes of surface work function detected by Kelvin force microscopy we suggest that the polypyrrole is covalently attached to the diamond surface via carbon–carbon bond while replacing the hydrogen termination. Electronic measurements show loss of surface conductivity of diamond after the polypyrrole deposition. These results are discussed in terms of electronic and electrochemical properties of polypyrrole–diamond system also in the perspective of its potential device applications.     

Chemical composition,thermal stability and hydrogen plasma treatment of laser-cut single-crystal diamond surface studied by X-ray Photoelectron Spectroscopy and Atomic Force Microscopy

X-ray photoelectron spectroscopy examination shows that after laser cutting under ambient condition, the upper surface of diamond consists of a heavy oxidized layer consisting of a variety of carbon–oxygen chemical states comprising –C═O, –C–O–C– and –C–O–H species. The thickness of the oxide layer was estimated to be ～22 ?. Upon vacuum annealing to 700 °C the thickness of the oxide layer decreases to ～10 A and the upper surface layer becomes more diamond-like through desorption of C–O species. Exposure of the laser cut diamond surface to a microwave hydrogen (MW-H) plasma results in removal of the oxide layer and exposure of the diamond phase. This is evidenced by the appearance of characteristic diamond surface and bulk plasmons which accompanied the C (1s) X-ray photoelectron peak. Our studies show that the surface chemical composition and thermal stability of the laser cut and polished surfaces both after MW-H exposure are nearly similar. The morphology of the laser cut surface shows an ill-defined laminar structure without any characteristic features which is not significantly affected by MW-H plasma exposure. This is in contrast to the polished surfaces for which exposure to the MW-H may result in its planarization.     

Effect of diamond surface orientation on the thermal boundary conductance between diamond and aluminum

[100] and [111] oriented diamond substrates were treated using Ar:H and Ar:O plasma treatments, and 1:1 HNO₃:H₂SO₄ heated at 200 °C. Subsequent to these treatments, an aluminum layer was either evaporated or sputteredon the substrates. The thermal boundary conductance (TBC) as well as the interfacial acoustical reflection coefficient between this layer and the diamond substrate was then measured using a Time Domain ThermoReflectance (TDTR) experiment. For the Ar:H plasma treated surfaces the [111] oriented faces exhibited conductances 40% lower than the [100] oriented ones with the lowest measured TBC at 32 ± 5 MWm^− 2 K^− 1. The treatments that led to oxygen-terminated diamond surfaces (extiti.e. acid or Ar:O plasma treatments) showed no TBC anisotropy and the highest measured value was 230 ± 25 MWm^− 2 K^− 1 for samples treated with Ar:O plasma with a sputtered Al layer on top. Sputtered layers on oxygen-terminated surfaces showed systematically higher TBC than their evaporated counterparts. The interfacial acoustic reflection coefficient correlated qualitatively with TBC when comparing samples with the same type of surface terminations (O or H) but this correlation failed when comparing H and O terminated interfaces with each other.     

In situ reactivation of low-temperature thermionic electron emission from nitrogen doped diamond films by hydrogen exposure

Nitrogen doped, hydrogen terminated diamond films have shown a work function of less than 1.5 eV and thermionic electron emission (TE) has been detected at temperatures less than 500 °C. However, ambient exposure or extended operation leads to a deterioration of the emission properties. In this study thermionic electron emission has been evaluated for as-received surfaces and for surfaces after 18 months of ambient exposure. The initial TE current density of the freshly deposited diamond film was ~ 5 × 10^− 5 A/cm² at 500 °C. In contrast, the initial TE current density of a film aged for 18 months was ~ 1.8 × 10^− 9 A/cm² at 500 °C. The decreased emission current density is presumed to be a consequence of oxidation, surface adsorption of contaminants and hydrogen depletion from the surface layer. In situ reactivation of the aged film surface was achieved by introducing hydrogen at a pressure of 1.3 × 10^− 4 mbar and using a hot filament of a nearby ionization gauge to generate atomic and/or excited molecular hydrogen. After 2 h of exposure with the sample at 500 °C, the surface exhibited a stable emission current density of ~ 2.3 × 10^− 6 A/cm² (an increase by a factor of ~ 1300). To elucidate the reactivation process thermionic electron energy distribution (TEED) and XPS core level spectra were measured during in situ hydrogen exposure at 5 × 10^− 8 mbar. During the isothermal exposure it was determined that atomic or excited hydrogen resulted in a much greater increase of the TE in comparison to exposure to molecular hydrogen. During exposure at 400 °C the surface oxygen was substantially reduced, the TEED cut-off energy, which indicates the effective work function, decreased by ~ 200 meV, and the TE intensity increased by a factor of ~ 100. The increase in thermionic emission with hydrogen was ascribed to the reactivation of the surface through the formation of a uniform surface dipole layer and a reduction of the surface work function.     

Using fluorine and chlorine in the diamond CVD process

We deposited diamond films at low substrate temperatures T_sub using the halogen containing precursor gases CHF₃ and C₂H₅Cl with an abundance of hydrogen. Diamond film growth was possible down to T_sub=370 °C using a hot-filament chemical vapor deposition process. The possibility of low temperature growth of diamond could be correlated with an increase of radical density in the gas phase caused by the halogen addition. These radicals, especially atomic hydrogen, fluorine and chlorine are responsible for the creation of active surface sites on the diamond surface. Chlorine especially is able to break surface bonds even at low substrate temperatures. Recent secondary ion mass spectroscopy measurements revealed that halogens are involved in surface reactions and that fluorine and chlorine were incorporated in the deposited films especially at low T_sub. Along with the creation of active surface sites the surface diffusion is important for the diamond growth, which is strongly limited by reduction of the substrate temperature. We succeeded in good quality diamond growth on glasses and aluminium substrates at T_sub=490 °C. A further decrease of T_sub leads to a decrease of diamond film quality and a poor adhesion of the diamond films to the substrate.     

Boron carbide coatings on diamond particles

Boron carbide (B₄C) coatings on diamond offer potential for obtaining homogeneous B₄C-diamond composites with improved properties. A method was developed for coating diamond particles with B₄C at 1150 °C under argon atmosphere for dwell times of 2–6 hours in a powder mixture of boric acid (H₃BO₃) and amorphous boron. The B₄C coating showed very good adhesion to the diamond substrate, and an unusual five-fold symmetry thought to be due to a twinned growth mechanism. The sudden onset of nucleation at T > 1000 °C is ascribed to the stabilising effect of hydrogen from the decomposition of H₃BO₃ on the diamond surface reactivity.     

Effect of oxygen plasma treatment on non-thrombogenicity of diamond-like carbon films

The non-thrombogenicity of oxygen-plasma-treated DLC films was investigated as surface coatings for medical devices. DLC films were deposited on polycarbonate substrates by a radio frequency plasma enhanced chemical vapor deposition method using acetylene gas. The deposited DLC films were then treated with plasma of oxygen gas at powers of 15 W, 50 W, and 200 W. Wettability was evaluated by water contact angle measurements and the changes in surface chemistry and roughness were examined by X-ray photoelectron spectroscopy and atomic force microscope analysis, respectively. Each oxygen-plasma-treated DLC film exhibited a hydrophilic nature with water contact angles of 11.1°, 17.7° and 36.8°. The non-thrombogenicity of the samples was evaluated through the incubation with platelet-rich plasma isolated from human whole blood. Non-thrombogenic properties dramatically improved for both 15 W- and 50 W-oxygen-plasma-treated DLC films. These results demonstrate that the oxygen plasma treatment at lower powers promotes the non-thrombogenicity of DLC films with highly hydrophilic surfaces.     

A direct-write,resistless hard mask for rapid nanoscale patterning of diamond

We introduce a simple, resist-free dry etch mask for producing patterns in diamond, both bulk and thin deposited films. Direct gallium ion beam exposure of the native diamond surface to doses as low as 10¹⁶ cm^?2 forms a top surface hard mask resistant to both oxygen plasma chemical dry etching and, unexpectedly, argon plasma physical dry etching. Gallium implant hard masks of nominal 50 nm thickness demonstrate oxygen plasma etch resistance to over 450 nm depth, or 9:1 selectivity. The process offers significant advantages over direct ion milling of diamond including increased throughput due to separation of patterning and material removal steps, allowing both nanoscale patterning resolution as well as rapid masking of areas approaching millimeter scales. Retention of diamond properties in nanostructures formed by the technique is demonstrated by fabrication of specially shaped nanoindenter tips that can perform imprint pattern transfer at over 14 GPa pressure into gold and silicon surfaces. This resistless technique can be applied to curved and non-planar surfaces for a variety of potential applications requiring high resolution structuring of diamond coatings.     

Iron-mediated growth of epitaxial graphene on SiC and diamond

Ordered graphene films have been fabricated on Fe-treated SiC and diamond surfaces using the catalytic conversion of sp³ to sp² carbon. In comparison with the bare SiC (0 0 0 1) surface, the graphitization temperature is reduced from over 1000 °C to 600 °C and for diamond (1 1 1), this new approach enables epitaxial graphene to be grown on this surface for the first time. For both substrates, a key development is the in situ monitoring of the entire fabrication process using real-time electron spectroscopy that provides the necessary precision for the production of films of controlled thickness. The quality of the graphene/graphite layers has been verified using angle-resolved photoelectron spectroscopy, scanning tunneling microscopy and low energy electron diffraction. Graphene is only formed on treated regions of the surface and so this offers a method for fabricating and patterning graphene structures on SiC and diamond in the solid-state at industrially realistic temperatures.     

Formation of nanometer-size high-density pits on epitaxial diamond (100) films

We report the formation of pits having widths of approximately 10 nm and a density of 2.5 × 10¹¹/cm² on epitaxial diamond (100) films. The pits are formed by etching the films using atomic hydrogen at a substrate temperature of approximately 500 °C. Exposure to oxygen followed by etching with atomic hydrogen forms additional pits. We propose that the high-density pits are formed due to etching that occurs both perpendicular and parallel to the surface.     

On the interaction of molecular hydrogen with diamonds: An experimental study using nuclear probes and thermal desorption

Hydrogen behavior in monocrystalline diamonds with different concentrations and types of nitrogen defects was studied using Nuclear Reaction and micro-Elastic Recoil Detection Analyses and Thermal Desorption Spectroscopy. Diamonds were studied in as-received state, after HPHT treatment and after hydrogenation in molecular hydrogen. A considerable amount of hydrogen was found to be bonded to diamond surfaces. Kinetics of surface hydrogen desorption is similar to what was reported for plasma-hydrogenated diamonds. The solubility of hydrogen in type IIa diamonds is very low. The efficiency of hydrogen traps in diamond bulk varies with the dominant type of nitrogen-related defects. The cross-section of traps decreases in row Ib > IaA > IaB diamonds, though binding energy in type IaB crystals may be higher, than in type IaA. Dislocations may promote hydrogen diffusion. A marked dependency of the hydrogen content and diffusivity between diamond growth sectors were observed for some samples. The total hydrogen content is higher in octahedral sectors.     

Wettability between Fe-Al alloy and sintered MgO

In the current study, the wettability between Fe-Al alloy and sintered MgO substrate was investigated. The stable contact angle between the sintered MgO substrate and the liquid iron was approximately 134° at 1550 °C, hardly influenced by Al concentrations of 18 ppm and 370 ppm in the liquid iron. By changing hydrogen partial pressure from 0 vol% to 1 vol%, the oxygen partial pressure decreased. Meanwhile, the contact angle between the MgO substrate and the liquid iron with 370 ppm Al increased with the decrease of oxygen partial pressure. The oxygen partial pressure and contact angle were scarcely affected by increasing hydrogen partial pressure from 1 vol% to 5 vol%. In all cases with 370 ppm Al in the liquid iron, oxide layers were detected on the surface of iron samples. The oxidation of iron could be effectively prevented by increasing the hydrogen partial pressure. The MgO substrate was reduced to Mg vapor in the reducing atmosphere at a high temperature. Then the Mg vapor was dissolved into the iron even before iron melting. Under thermodynamic equilibrium condition, an oxide layer containing two components, i.e. MgO·Al₂O₃ phase and CaO-SiO₂-MgO-Al₂O₃ phase, was generated on the surface of the iron sample. Due to the different wettability between the iron and the two phases, MgO·Al₂O₃ phase was repelled, while the CaO-SiO₂-MgO-Al₂O₃ phase adhered to the inside area.     

Effect of anodic and cathodic treatments on the charge transfer of boron doped diamond electrodes

This paper is concerned with the study of the influence of electrochemical pre-treatments on the behavior of highly boron doped diamond electrodes. Anodic and cathodic preconditioning, performed during 10 s either with 10^− 4 A/cm^− 2 (10^− 3 C cm^− 2) or 10^− 1 A/cm^− 2 (1 C cm^− 2), has been studied. Cyclic voltammetry at as-deposited, anodically and cathodically treated electrodes, in presence of 2 redox couples serving as electrochemical probes is analyzed in the light of the surface characterization given by XPS chemical analysis. Ce^4+/3+ redox couple in 0.5 M H₂SO₄ medium and Fe(CN)₆^3−/4− redox couple in 0.1 M KOH medium, have been studied before and after the different treatments. The results of Mott–Schottky plots and current voltage curves are reported and show that the electrochemical response of BDD electrodes is very dependent on the current density involved in the electrochemical preconditioning. The modification of surface bond termination – either hydrogen or oxygen – studied by XPS analyses is also strongly dependent on electrochemical pre-treatment. In particular, it is evidenced that the most important conversion of surface functionalities from hydrogen to oxygen is obtained when the anodic treatment is performed with the smallest current density. Finally, a correlation between surface terminations and charge transfer is evidenced.     

The preparation of high quality oriented diamond thin films via low temperature and hydrogen ion etched nucleation

A three-step process was used to prepare high quality [001]-oriented diamond films. First, diamond crystallites were nucleated for 20 min on Si(001) at a temperature around 740 °C by bias-enhance method, during this step the portion of [001]-oriented diamond nuclei was increased in comparison with the nuclei deposited by a two-step method. Then hydrogen ion etching was performed for 30 min by setting an electric potential of −140 V. After the etching step most of the crystallites were [001]-oriented and twinned crystallites disappeared. Finally, diamond thin films were deposited under conventional conditions for [001]-textured growth. SEM was used to analyse the morphology of diamond crystallites and films. The results indicate that large area, uniform and [001]-oriented diamond thin films can be prepared by three-step growth. The films show good Raman characteristics and higher thermal conductivity than those deposited by a two-step process.     

Fabrication of fluorine-terminated diamond-like carbon thin film using a hyperthermal atomic fluorine beam

Surface modification of diamond-like carbon (DLC) film was performed using a hyperthermal atomic fluorine beam on the purpose of production of hydrophobic surface by maintaining the high hardness of DLC film. By the irradiation of atomic fluorine beam of a 1.0 × 10²⁰ atoms/cm², the contact angle of a water drop against the DLC surface increased from 73° to 111°. The formation of CF₃, CF₂ and CF bonding on the modified DLC surface was confirmed from the measurements of X-ray photoelectron spectra and near-edge X-ray absorption fine structure spectra. Irradiation of hyperthermal atomic fluorine beam was concluded to produce insulator fluorine-terminated DLC film, which has high F content on the surface, by the taking of the use of neutral atomic beam as a fluorine source.     

Nanoindentation study of nickel manganite ceramics obtained by a complex polymerization method

The chemical synthesis of nickel manganite powder was performed by a complex polymerization method (CPM). The obtained fine nanoscaled powders were uniaxially pressed and sintered at different temperatures: 1000–1200 °C for 2 h, and different atmospheres: air and oxygen. The highest density was obtained for the sample sintered at 1200 °C in oxygen atmosphere. The energy for direct band gap transition (Eg) calculated from the Tauc plot decreases from 1.51 to 1.40 eV with the increase of the sintering temperature. Indentation experiments were carried out using a three-sided pyramidal (Berkovich) diamond tip, and Young's modulus of elasticity and hardness of NTC (negative temperature coefficient) ceramics at various indentation depths were calculated. The highest hardness (0.754 GPa) and elastic modulus (16.888 GPa) are exhibited by the ceramics sintered at highest temperature in oxygen atmosphere.     

Diamond wear properties in cold plasma jet

Cold plasma jet is applied to control the contact interface and restrain the diamond wear when cutting ferrous metals. A new generation device was developed to generate stable cold plasma jet under atmospheric pressure. Friction & wear tests and thermal analysis experiments were conducted on a mirror steel NAK80/diamond friction pair in atmospheres of air, nitrogen, and nitrogen cold plasma jet (NPJ), respectively. The friction and diamond graphitization rules were determined through the experiments. Results proved that the friction pair exhibited the best friction and wear properties in NPJ atmosphere. When the rotation speed was ≥ 300 r/min, the nitrogen cold plasma atmosphere was found to reduce the friction coefficient of the friction pair, and the friction coefficient tended to be stable when the load was ≥ 50 N. Moreover, the graphitization temperature of diamond increased from 885 °C to 1185 °C after NPJ treatment.     

Thermionic electron emission from nitrogen-doped homoepitaxial diamond

Nitrogen-doped homoepitaxial diamond films were synthesized for application as low-temperature thermionic electron emitters. Thermionic electron emission measurements were conducted where the emission current was recorded as a function of emitter temperature. At a temperature < 600 °C an emission current was detected which increased with temperature, and the emission current density was about 1.2 mA/cm² at 740 °C. The electron emission was imaged with photoelectron emission microscopy (PEEM) and thermionic field-emission electron microscopy (T-FEEM). The image displayed uniform electron emission over the whole surface area. Thermionic emission and ultraviolet photoemission spectroscopy were employed to determine the temperature dependent electron emission energy distribution from the nitrogen-doped homoepitaxial diamond films. The photoemission spectra indicated an effective work function of 2.4 eV at 550 °C. These values indicate reduced band bending and establish the potential for efficient electron emission devices based on nitrogen-doped homoepitaxial diamond.