Boron-doped Diamond Samples Research Articles

Raman electronic effect for non‐destructive boron calibration in type IIb semiconducting diamond

AbstractWhereas p type semiconducting diamond is well‐developed with boron doping, it is still challenge to directly determine boron content within type IIb diamonds in material science. In this work, we show a systematic study on analysis of electronic Raman effect in boron‐doped diamond (BDD) containing boron ranging from several to ~104 ppm. Beside a sharp peak near 5.2 cm−1, a shoulder associated with the Raman electronic effect near 12.1 cm−1 presents in the BDD samples and the intensity is observed to increase significantly with the increasing boron content. Our results demonstrate that the electronic line can be employed as an effective indication for simple, non‐destructive, accurate and broadly applicable boron calibration in semiconducting BDD samples.

A cost-efficient approach for simultaneous scanning electrochemical microscopy and scanning ion conductance microscopy

A novel and cost-efficient probe fabrication method yielding probes for performing simultaneous scanning electrochemical microscopy (SECM) and scanning ion conductance microscopy (SICM) is presented. Coupling both techniques allows distinguishing topographical and electrochemical activity information obtained by SECM. Probes were prepared by deposition of photoresist onto platinum-coated, pulled fused silica capillaries, which resulted in a pipette probe with an integrated ring ultramicroelectrode. The fabricated probes were characterized by means of cyclic voltammetry and scanning electron microscopy. The applicability of probes was demonstrated by measuring and distinguishing topography and electrochemical activity of a model substrate. In addition, porous boron-doped diamond samples were investigated via simultaneously performed SECM and SICM.Graphic abstract

Increasing the Electric Double‐Layer Capacitance in Boron‐Doped Diamond Electrodes

AbstractHighly boron‐doped diamond (BDD) electrodes containing sp2 carbon impurities exhibit an increase in the electric double‐layer capacitance (Cdl) after anodic oxidation. Although it is known that the electrochemical properties of BDD electrodes are affected by several factors, including the boron concentration, the amount of sp2 carbon impurity, and the surface termination and orientation, clarification of the mechanism for the increase in Cdl needs to be done. To this end, we prepared several BDD samples under various conditions and the properties of these samples, including Cdl, were systematically investigated. The mechanism for the increase in Cdl in particular, including the effect of the surface termination, is discussed.

Enhancing Electrochemical Efficiency of Hydroxyl Radical Formation on Diamond Electrodes by Functionalization with Hydrophobic Monolayers.

Electrochemical formation of high-energy species such as hydroxyl radicals in aqueous media is inefficient because oxidation of H2O to form O2 is a more thermodynamically favorable reaction. Boron-doped diamond (BDD) is widely used as an electrode material for generating •OH radicals because it has a very large kinetic overpotential for O2 production, thus increasing electrochemical efficiency for •OH production. Yet, the underlying mechanisms of O2 and •OH production at diamond electrodes are not well understood. We demonstrate that boron-doped diamond surfaces functionalized with hydrophobic, polyfluorinated molecular ligands (PF-BDD) have significantly higher electrochemical efficiency for •OH production compared with hydrogen-terminated (H-BDD), oxidized (O-BDD), or poly(ethylene ether)-functionalized (E-BDD) boron-doped diamond samples. Our measurements show that •OH production is nearly independent of surface functionalization and pH (pH = 7.4 vs 9.2), indicating that •OH is produced by oxidation of H2O in an outer-sphere electron-transfer process. In contrast, the total electrochemical current, which primarily produces O2, differs strongly between samples with different surface functionalizations, indicating an inner-sphere electron-transfer process. X-ray photoelectron spectroscopy measurements show that although both H-BDD and PF-BDD electrodes are oxidized over time, PF-BDD showed longer stability (≈24 h of use) than H-BDD. This work demonstrates that increasing surface hydrophobicity using perfluorinated ligands selectively inhibits inner-sphere oxidation to O2 and therefore provides a pathway to increased efficiency for formation of •OH via an outer-sphere process. The use of hydrophobic electrodes may be a general approach to increasing selectivity toward outer-sphere electron-transfer processes in aqueous media.

Electrochemical characterization of porous boron-doped diamond prepared using SiO2 fiber template

Porous boron-doped diamond (BDD) is fabricated by consecutive plasma enhanced chemical vapor deposition on a 3D porous SiO2 fiber template deposited by spin coating (SC). The fabricated highly doped and mechanically stable porous BDD layers are characterized by scanning electron microscopy (SEM) and Raman spectroscopy. The roughness factor of the prepared porous BDD, depending on the number of porous layers, was determined from cyclic voltammetry (RFCV) and from krypton adsorption isotherms – BET, (RFBET). Differences in determination of the roughness factor using these two methods are discussed. Electrochemical measurements (cyclic voltammetry and galvanostatic charge/discharge cycling) of porous BDD are performed in aqueous electrolyte solutions with different composition and pH values. The highest electric double-layer capacitance of ca. 2 mF·cm−2, related to the projected geometric (2D) surface area, is obtained for the thickest (26 μm) porous BDD electrode measured in 0.5 M H2SO4 electrolyte solution. The capacitance of the same porous BDD normalized to the total physical surface area (3D) determined by BET is 45 μF·cm−2. The electrocatalytic activity of porous BDD electrodes is studied using a hexaammineruthenium(III/II) redox probe, and the electrochemical cycle stability is determined by galvanostatic charge/discharge. The charge retention of the thickest porous BDD samples after removal of non-diamond impurities by an oxidative treatment is ca. 77% after 3000 cycles.

Investigations on Cs-free alternatives for negative ion formation in a low pressure hydrogen discharge at ion source relevant parameters

Caesium (Cs) is applied in high power negative hydrogen ion sources to reduce a converter surface’s work function and thus enabling an efficient negative ion surface formation. Inherent drawbacks with the usage of this reactive alkali metal motivate the search for Cs-free alternative materials for neutral beam injection systems in fusion research. In view of a future DEMOnstration power plant, a suitable material should provide a high negative ion formation efficiency and comply with the RAMI issues of the system: reliability, availability, maintainability, inspectability. Promising candidates, like low work function materials (molybdenum doped with lanthanum (MoLa) and LaB6), as well as different non-doped and boron-doped diamond samples were investigated in this context at identical and ion source relevant parameters at the laboratory experiment HOMER. Negative ion densities were measured above the samples by means of laser photodetachment and compared with two reference cases: pure negative ion volume formation with negative ion densities of about and the effect of H− surface production using an in situ caesiated stainless steel sample which yields 2.5 times higher densities. Compared to pure volume production, none of the diamond samples did exhibit a measurable increase in H− densities, while showing clear indications of plasma-induced erosion. In contrast, both MoLa and LaB6 produced systematically higher densities (MoLa: ×1.60; LaB6: ×1.43). The difference to caesiation can be attributed to the higher work functions of MoLa and LaB6 which are expected to be about 3 eV for both compared to 2.1 eV of a caesiated surface.

A Drude model analysis of conductivity and free carriers in boron-doped diamond films and investigations of their internal stress and strain

Boron-doped diamond (BDD) has seen a substantial increase in interest for use as electrode coating material for electrochemistry and studies of deep brain stimulation mechanism. In this study, we present an alternative method for determining important characteristics, including conductivity, carrier concentration, and time constant, of such material by the signature of Drude-like metallic behavior in the far-infrared (IR) spectral range. Unlike the direct determination of conductivity from the four-point probe method, using far-IR transmittance provides additional information, such as whether the incorporation of boron results in a large concentration of carriers or in inducing defects in the diamond lattice. The slightly doped to medium-doped BDD samples that were produced using chemical vapor deposition and analyzed in this work show conductivities ranging between 5.5 and 11 (Ω cm)-1. Different growth conditions demonstrate that increasing boron concentration results in an increase in the carrier concentration, with values between 7.2 × 1016 and 2.5 × 1017 carriers/cm3. Addition of boron, besides leading to a decrease in the resistivity, also resulted in a decrease in the time constant, limiting BDD conductivity. Investigations, by confocal Raman mapping, of the induced stress in the material due to interaction with the substrate or to the amount of doping are also presented and discussed. The induced tensile stress, which was distributed closer to the film-substrate interface decreased slightly with doping.

Cathodic and anodic pre-treated boron doped diamond with different sp2 content: Morphological, structural, and impedance spectroscopy characterizations

In this work, the influence of cathodic (Red) and anodic (Ox) pre-treatment on boron doped diamond (BDD) films grown with different sp2/sp3 ratios was systematically studied. The sp2/sp3 ratios were controlled by the addition of CH4 of 1,3,5 and 7sccm in the gas inlet during the growth process. The electrodes were treated in 0.5molL−1 H2SO4 at −3 and 3V vs Ag/AgCl, respectively, for 30min. The electrochemical response of BDD films was investigated using electrochemical impedance spectroscopy (EIS) and Mott–Schottky Plot (MSP) measurements. Four film sample sets were produced in a hot filament chemical vapor deposition reactor. During the growth process, an additional H2 line passing through a bubbler containing the B2O3 dissolved in methanol was used to carry the boron. The scanning electron microscopy morphology showed well faced films with a small decrease in their grain size as the CH4 concentration increased. The Raman spectra depicted a pronounced sp2 band, mainly for films with 5 and 7sccm of CH4. MSP showed a decrease in the acceptor concentration as the CH4 increased indicating the CH4 influence on the doping process for Red–BDD and Ox–BDD samples. Nonetheless, an apparent increase in the acceptor concentrations for both Ox–BDD samples was observed compared to that for Red–BDD samples, mainly attributed to the surface conductive layer (SCL) formation after this strong oxidation process. The EIS Nyquist plots for Red–BDD showed a capacitance increase for the films with higher sp2 content (5 and 7sccm). On the other hand, the Nyquist plots for Ox–BDD can be described as semicircles near the origin, at high frequencies, where their charge transfer resistance strongly varied with the sp2 increase in such films.

Superconductivity in diamond

We survey methods of synthesis of boron doped diamond with high pressure-high temperature techniques. New route is proposed for synthesis of relatively large heavily boron doped diamond single crystals, which exhibit superconductivity. Superconducting boron-doped diamond samples were synthesized with isotopes of 10B, 11B, 13C and 12C. We claim the presence of a carbon isotope effect on the superconducting transition temperature, which supports the “diamond–carbon”-related nature of superconductivity and the importance of the electron–phonon interaction as the mechanism of superconductivity in diamond. Isotope substitution permits us to relate almost all bands in the Raman spectra of heavily boron-doped diamond to the vibrations of carbon atoms. The 500 cm − 1 Raman band shifts with either carbon or boron isotope substitution and may be associated with vibrations of paired or clustered boron.

10B/ 11B 1D/2D solid-state high-resolution NMR studies on boron-doped diamond

In this work, three boron-doped diamond samples prepared from a high-pressure high-temperature synthesis method with the same starting materials but with different initial ratios for boron are studied. Magnetic susceptibility measurement shows that the increment of the initial amount of boron does not straightforwardly bring lower superconducting transition temperature. In accordance with our previous 11B high-resolution magic-angle spinning (MAS) NMR results, we show that there are at least four boron signal components and the one at 28.5 ppm is ascribed to the substitutional boron in the diamond structure providing the carriers responsible for conductivity. From observed relative intensities of the four signal components, it is suggested that the excess boron, which does not contribute to the conductivity appears as a broad signal at around 65.5 ppm. We apply two-dimensional (2D) NMR to examine 1H dipolar broadening and 11B– 11B boron spin diffusion, and candidates purported so far for the excess boron, that is, a boron + hydrogen complex and –B–B– and/or –B–C–B– clusters are negated. From its chemical-shift value, it is suggested that the excess boron exists as trigonally coordinated boron. We further apply 2D nutation NMR to examine the size of its quadrupolar coupling and show that it is not very large. As for a structure that compromises the trigonal coordination invoked from the chemical-shift value and the small quadrupolar coupling, we postulate boron locally in a graphite-like structure but the symmetry of the electric field gradient around it is high. Furthermore, we show that 10B MAS NMR is useful to selectively observe the substitutional boron in the diamond structure appearing at 28.5 ppm, whose quadrupolar coupling is much smaller than that of the excess boron at 65.5 ppm.

Structure and superconductivity of isotope-enriched boron-doped diamond

Superconducting boron-doped diamond samples were synthesized with isotopes of 10B, 11B, 13C and 12C. We claim the presence of a carbon isotope effect on the superconducting transition temperature, which supports the ‘diamond-carbon’-related nature of superconductivity and the importance of the electron–phonon interaction as the mechanism of superconductivity in diamond. Isotope substitution permits us to relate almost all bands in the Raman spectra of heavily boron-doped diamond to the vibrations of carbon atoms. The 500 cm−1 Raman band shifts with either carbon or boron isotope substitution and may be associated with vibrations of paired or clustered boron. The absence of a superconducting transition (down to 1.6 K) in diamonds synthesized in the Co–C–B system at 1900 K correlates with the small boron concentration deduced from lattice parameters.

An insight into what superconducts in polycrystalline boron-doped diamonds based on investigations of microstructure.

The discovery of superconductivity in polycrystalline boron-doped diamond (BDD) synthesized under high pressure and high temperatures [Ekimov, et al. (2004) Nature 428:542-545] has raised a number of questions on the origin of the superconducting state. It was suggested that the heavy boron doping of diamond eventually leads to superconductivity. To justify such statements more detailed information on the microstructure of the composite materials and on the exact boron content in the diamond grains is needed. For that we used high-resolution transmission electron microscopy and electron energy loss spectroscopy. For the studied superconducting BDD samples synthesized at high pressures and high temperatures the diamond grain sizes are approximately 1-2 mum with a boron content between 0.2 (2) and 0.5 (1) at %. The grains are separated by 10- to 20-nm-thick layers and triangular-shaped pockets of predominantly (at least 95 at %) amorphous boron. These results render superconductivity caused by the heavy boron doping in diamond highly unlikely.

Crystal-Face-Selective Adsorption of Au Nanoparticles onto Polycrystalline Diamond Surfaces

Crystal-face-selective adsorption of Au nanoparticles (AuNPs) was achieved on polycrystalline boron-doped diamond (BDD) surface via the self-assembly method combined with a UV/ozone treatment. To the best of our knowledge, this is the first report of crystal-face-selective adsorption on an inorganic solid surface. Hydrogen-plasma-treated BDD samples and those followed by UV/ozone treatment for 2 min or longer showed almost no adsorption of AuNP after immersion in the AuNP solution prepared by the citrate reduction method. However, the samples treated by UV/ozone for 10 s showed AuNP adsorption on their (111) facets selectively after the immersion. Moreover, the sample treated with UV/ozone for 40-60 s showed AuNP adsorption on the whole surface. These results indicate that the AuNP adsorption behavior can be controlled by UV/ozone treatment time. This phenomenon was highly reproducible and was applied to a two-step adsorption method, where AuNPs from different batches were adsorbed on the (111) and (100) surface in this order. Our findings may be of great value for the fabrication of advanced nanoparticle-based functional materials via bottom-up approaches with simple macroscale procedures.

Magnetic properties of ion-implanted diamond

Single crystalline diamond samples of type IIa were implanted with boron and CVD diamond samples with fluorine or iron ions. Defect rich surface layers extending to a depth of about 300 nm for boron and 1 μm for fluorine and iron were produced by multiple energy implantation. The ion concentrations at the implanted regions were between 30 and 600 ppm for iron and fluorine and 0.5 to 1.5 at.% for boron. In all samples the magnetic properties were dominated by the diamagnetism of pure diamond. The main influence of iron and fluorine implantation on the magnetic properties of diamond is the creation of paramagnetic centres induced by disorder. Whereas diamond implanted with boron at a temperature of 900 °C does not show detectable paramagnetism. After subtraction of the linear background all implanted samples show small ferromagnetic-like loops. Although these signals are clearly above the detection limit and appear to be caused by ferromagnetism, we show that the measured loops are mainly caused by a SQUID artefact. We did not find any evidence for the existence of superconductivity in boron-doped diamond samples prepared under the used conditions.

Photosensitization of Boron-Doped Diamond by Surface Grafting of Pyrene Groups

Boron-doped diamond samples were exposed successively to singlet oxygen and pyrene-alkylcarboxylic acid molecules. The resulting samples were studied by photoelectrochemistry. It is shown that the expected esterification reaction indeed allowed the grafting of pyrene groups onto the diamond surface. Cathodic photocurrents have been obtained with quantum yields as high as η=0.29% (λ=351 nm, U=-0.38 V vs. a saturated calomel electrode) in oxygen saturated electrolyte. The pyrene coverage is estimated from both photocurrent and photopotential measurements. The role of dissolved oxygen in the photocurrent generation is discussed.

Effects of hydrogen on electronic properties of doped diamond

We have investigated the effects of hydrogen on the electronic structure of diamond doped boron and sulfur using cluster model method within the frame of ab initio density functional theory (DFT). The results show that the presence of hydrogen results in a deep donor level with no change of conductivity type in sulfur-doped diamond samples and the formation of the multiple hydrogen–boron complexes may cause a conductivity type transition in the hydrogen-rich boron-doped diamond samples.

Superconductivity in boron-doped diamond

Here we report the experimental study of a new superconductor, B-doped diamond. Subjecting a mixture of amorphous boron with graphite to a high-temperature and high-pressure treatment, we synthesized polycrystalline boron-doped diamond samples, whose sizes were suitable for specific heat measurements. For this kind of sample, we detected two specific heat anomalies at temperatures (∼4 K) closely corresponding to temperatures at which the resistance and magnetic susceptibility anomalies take place. The most probable origin of these specific heat anomalies is a bimodal B distribution in polycrystalline diamond. The correlation of physical properties, such as specific heat, electrical resistivity, and magnetic susceptibility, near the superconducting transition temperature is clear evidence of bulk superconductivity in these boron-doped diamond samples. Specific features in the low frequency range of Raman spectra correlate with superconducting properties of diamond. These features help to select heavily boron-doped CVD diamond films (DF) grown on (111) natural diamond, which exhibit superconductivity.

Mechanism of p-type-to-n-type conductivity conversion in boron-doped diamond

We report on the role of two boron–hydrogen complexes in the conductivity conversion from p-type to n-type in boron-doped diamond samples. The calculated electronic structures of the simulated clusters show that the boron–hydrogen complex of hydrogen–boron pairs (BH1) is characteristic of p-type conductivity, and that of two hydrogen–boron centers (BH2) or three-hydrogen-boron centers (BH3) have a character of n-type conductivity. The phenomenon of conversion from p-type-to-n-type conductivity through deuteration may be explained by the transformation from the BH1 into BH2 and/or BH3 complexes.

Current image tunneling spectroscopy of chemical vapor deposited diamond films

The effects of boron doping on the electron field emission properties of diamond films were examined using current image tunneling spectroscopy in atomic force microscopy (AFM). Tunneling current–voltage (It–V) characteristics measured by AFM indicate that incorporation of boron species induces the presence of the impurity state. The films doped with 3 sccm boron (B3) possess the smallest emission barrier (Eb=1.65 eV) and the largest emission ratio (η=38%) among the boron-doped diamond samples. Therefore, B3 diamond films can be turned on at the smallest electric field (E0=8.8 V/μm), exhibiting largest field emission capacity (Je=250 μA/cm2). Restated, the electron field emission behavior of diamond films is closely related to the formation of impurity states due to boron doping.

Aerosol doping of flame-grown diamond films

Diamond films have been deposited using a pre-mixed oxy-acetylene flame onto cooled substrates. Doping of these flame-grown diamond films was achieved by introducing aerosol droplets containing the dopant species into the gas flow. The aerosol was generated by a commercial atomizer and was entrained into the oxygen flow before the flame front leading to a uniform and reproducible flame. Micro-Raman measurements show a decrease in the background fluorescence of the boron-doped films as compared with samples containing other dopants. In addition, low-temperature photoluminescence measurements of both the boron- and silicon-doped films show a significant decrease in the defect bands associated with nitrogen vacancy complexes that are often observed in flame-grown diamond films. The boron-doped diamond samples were also characterized by photoluminescence using the UV 351 nm laser line of an argon ion laser and show both green and/or blue emission.