Origin of the metallic properties of heavily boron-doped superconducting diamond

Ultrastable halide perovskite CsPbBr3 photoanodes achieved with electrocatalytic glassy-carbon and boron-doped diamond sheets

Halide perovskites exhibit exceptional optoelectronic properties for photoelectrochemical production of solar fuels and chemicals but their instability in aqueous electrolytes hampers their application. Here we present ultrastable perovskite CsPbBr3-based photoanodes achieved with both multifunctional glassy carbon and boron-doped diamond sheets coated with Ni nanopyramids and NiFeOOH. These perovskite photoanodes achieve record operational stability in aqueous electrolytes, preserving 95% of their initial photocurrent density for 168 h of continuous operation with the glassy carbon sheets and 97% for 210 h with the boron-doped diamond sheets, due to the excellent mechanical and chemical stability of glassy carbon, boron-doped diamond, and nickel metal. Moreover, these photoanodes reach a low water-oxidation onset potential close to +0.4 VRHE and photocurrent densities close to 8 mA cm-2 at 1.23 VRHE, owing to the high conductivity of glassy carbon and boron-doped diamond and the catalytic activity of NiFeOOH. The applied catalytic, protective sheets employ only earth-abundant elements and straightforward fabrication methods, engineering a solution for the success of halide perovskites in stable photoelectrochemical cells.

Step-by-step guide for electrochemical generation of highly oxidizing reactive species on BDD for beginners

Selecting the ideal anodic potential conditions and corresponding limiting current density to generate reactive oxygen species, especially the hydroxyl radical (•OH), becomes a major challenge when venturing into advanced electrochemical oxidation processes. In this work, a step-by-step guide for the electrochemical generation of •OH on boron-doped diamond (BDD) for beginners is shown, in which the following steps are discussed: i) BDD activation (assuming it is new), ii) the electrochemical response of BDD (in electrolyte and ferri/ferro-cyanide), iii) Tafel plots using sampled current voltammetry to evaluate the overpotential region where •OH is mainly generated, iv) a study of radical entrapment in the overpotential region where •OH generation is predominant according to the Tafel plots, and v) finally, the previously found ideal conditions are applied in the electrochemical degradation of amoxicillin, and the instantaneous current efficiency and relative cost of the process are reported.

Exploration of AB3Si3 (A = Na/K/Rb/Cs) compounds under moderate pressure

We discovered the composition of ternary AB3Si3 (A = Na/K/Rb/Cs) compounds in the moderate pressure range of 0-100 GPa using first-principles structural prediction and systematically analyzed their structures, stability, electronic and optical properties within the framework of density functional theory. The AB3Si3 compounds exhibit a diverse phase diagram, including nine structures that are selected based on formation energies, along with a known clathrate RbB3Si3 structure with Pm3̄n symmetry. All predicted phases are thermodynamically and dynamically stable within the studied pressure range. In particular, the KB3Si3 compound with a direct band gap of 1.0 eV is identified as a promising candidate for photovoltaic materials beyond silicon-based materials, among which boron atoms form a unique regular octahedral structure; in contrast, NaB3Si3 and RbB3Si3 compounds are shown to have metallicity. Our findings enrich crystal structures of alkali-metal borosilicides and provide valuable insights into their potential applications.

Heteroatom Functionalization of H-Terminated Diamond Surfaces

Diamond surface functionalization has received significant research interest recently. Specifically, H-termination has been widely adopted because it endows the diamond surface with negative electron affinity and the hole carrier is injected in the presence of surface transfer dopants. Exploring different functional groups' attachment on diamond surfaces and their impact on the electronic structure, using wet and dry chemical approaches, would hence be of interest in engineering diamond as a semiconductor. Here, we report the functionalization of the H-terminated diamond surface with nitrogen and sulfur heteroatoms. Surface characterization of functionalized diamond surfaces shows that these groups are well-distributed and covalently bonded to diamonds. Four chemical functional groups (-SH, -S-S-, -S-O, and -S=O) were found on the sulfurized diamond surface, and two groups (-NH2 and =NH) upon amination. We also report co-functionalization of surface with N and S (N-S), where sulfurization promotes sequential amination efficiency with reduced exposure time. Electrical measurement shows that heteroatom-modified diamond surfaces possess higher conductivity than H-terminated diamonds. Density functional theory (DFT) shows that upon functionalization with various N/S ratios, the conduction band minimum and valence band maximum downshift, which lowers the bandgap in comparison to an H-terminated diamond. These observations suggest the possibility of heteroatom functionalizations with enhanced surface electrical conductivity on the diamond that are useful for various electronic applications.

Monodispersed Transition Metals Induced Ordinary-Pressure Phase Transformation from Graphite to Diamond: A First-Principles Calculation

High pressure and high temperature are normally required for the transformation of graphite to diamond; thus, finding a method that allows the transformation to occur under ordinary pressure will be extremely promising for diamond synthesis. Here, it is found that graphite spontaneously transforms into diamond without any pressure by adding monodispersed transition metals, and the universal rules that will help predict the role of certain elements in the phase transition were studied. The results show that the favorable transition metals possess an atomic radius of 0.136-0.160 nm and an unfilled d-orbital of d²s²-d⁷s², which allow more charge transfer and accumulation at the proper position between the metal and dangling C atoms, leading to stronger metal-C bonds and a lower energy barrier for the transition. This provides a universal method to prepare diamond from graphite under ordinary pressure and also provides a way for the synthesis from sp² to sp³ bonded materials.

Spectroscopic signatures of nonpolarons: the case of diamond

Polarons are quasi-particles made from electrons interacting with vibrations in crystal lattices. They derive their name from the strong electron-vibration polar interactions in ionic systems, that induce spectroscopic and optical signatures of such quasi-particles. In this paper, we focus on diamond, a non-polar crystal with inversion symmetry which nevertheless shows interesting signatures stemming from electron-vibration interactions, better denoted "nonpolaron" signatures in this case. The (non)polaronic effects are produced by short-range crystal fields, while long-range quadrupoles only have a small influence. The corresponding many-body spectral function has a characteristic energy dependence, showing a plateau structure that is similar to but distinct from the satellites observed in the polar Fröhlich case. We determine the temperature-dependent spectral function of diamond by two methods: the standard Dyson-Migdal approach, which calculates electron-phonon interactions within the lowest-order expansion of the self-energy, and the cumulant expansion, which includes higher orders of electron-phonon interactions. The latter corrects the nonpolaron energies and broadening, providing a more realistic spectral function, which we examine in detail for both conduction and valence band edges.

High-Pressure Synthesis of Superconducting Sn3S4 Using a Diamond Anvil Cell with a Boron-Doped Diamond Heater

High-pressure techniques open exploration of functional materials in broad research fields. An established diamond anvil cell with a boron-doped diamond heater and transport measurement terminals has performed the high-pressure synthesis of a cubic Sn₃S₄ superconductor. X-ray diffraction and Raman spectroscopy reveal that the Sn₃S₄ phase is stable in the pressure range of P > 5 GPa in a decompression process. Transport measurement terminals in the diamond anvil cell detect a metallic nature and superconductivity in the synthesized Sn₃S₄ with a maximum onset transition temperature (T_c^onset) of 13.3 K at 5.6 GPa. The observed pressure-T_c relationship is consistent with that from the first-principles calculation. The observation of superconductivity in Sn₃S₄ opens further materials exploration under high-temperature and -pressure conditions.

Soft X-ray ARPES for three-dimensional crystals in the micrometre region

An endstation dedicated to angle-resolved photoemission spectroscopy (ARPES) using a soft X-ray microbeam has been developed at the beamline BL25SU of SPring-8. To obtain a high photoemission intensity, this endstation is optimized for measurements under the condition of grazing beam incidence to a sample surface, where the glancing angle is 5° or smaller. A Wolter mirror is used for focusing the soft X-rays. Even at the glancing angle of 5°, the smallest beam spot still having a sufficient photon flux for ARPES is almost round on the sample surface and the FWHM diameter is ∼5 µm. There is no need to change the sample orientation for performing k_x - k_y mapping by virtue of the electron lens with a deflector of the photoelectron analyzer, which makes it possible to keep the irradiation area unchanged. A partially cleaved surface area as small as ∼20 µm was made on an Si(111) wafer and ARPES measurements were performed. The results are presented.

Ultrananocrystalline Diamond Nanowires: Fabrication, Characterization, and Sensor Applications

The aim of this review is to provide a survey of the recent advances and the main remaining challenges related to the ultrananocrystalline diamond (UNCD) nanowires and other nanostructures which exhibit excellent capability as the core components for many diverse novel sensing devices, due to the unique material properties and geometry advantages. The boron or nitrogen doping introduced in the gas phase during deposition promotes p-type or n-type conductivity. With the establishment of the UNCD nanofabrication techniques, more and more nanostructure-based devices are being explored in measuring basic physical and chemical parameters via classic and quantum methods, as exemplified by gas sensors, ultraviolet photodetectors, piezoresistance effect-based devices, biological applications and biosensors, and nitrogen-vacancy color center-based magnetic field quantum sensors. Highlighted finally are some of the remaining challenges and the future outlook in this area.

Yu-Shiba-Rusinov bands in ferromagnetic superconducting diamond

The combination of different exotic properties in materials paves the way for the emergence of their new potential applications. An example is the recently found coexistence of the mutually antagonistic ferromagnetism and superconductivity in hydrogenated boron-doped diamond, which promises to be an attractive system with which to explore unconventional physics. Here, we show the emergence of Yu-Shiba-Rusinov (YSR) bands with a spatial extent of tens of nanometers in ferromagnetic superconducting diamond using scanning tunneling spectroscopy. We demonstrate theoretically how a two-dimensional (2D) spin lattice at the surface of a three-dimensional (3D) superconductor gives rise to the YSR bands and how their density-of-states profile correlates with the spin lattice structure. The established strategy to realize new forms of the coexistence of ferromagnetism and superconductivity opens a way to engineer the unusual electronic states and also to design better-performing superconducting devices.

Simulation Study of Surface Transfer Doping of Hydrogenated Diamond by MoO3 and V2O5 Metal Oxides

In this work, we investigate the surface transfer doping process that is induced between hydrogen-terminated (100) diamond and the metal oxides, MoO₃ and V₂O₅, through simulation using a semi-empirical Density Functional Theory (DFT) method. DFT was used to calculate the band structure and charge transfer process between these oxide materials and hydrogen terminated diamond. Analysis of the band structures, density of states, Mulliken charges, adsorption energies and position of the Valence Band Minima (VBM) and Conduction Band Minima (CBM) energy levels shows that both oxides act as electron acceptors and inject holes into the diamond structure. Hence, those metal oxides can be described as p-type doping materials for the diamond. Additionally, our work suggests that by depositing appropriate metal oxides in an oxygen rich atmosphere or using metal oxides with high stochiometric ration between oxygen and metal atoms could lead to an increase of the charge transfer between the diamond and oxide, leading to enhanced surface transfer doping.

The occupied electronic structure of ultrathin boron doped diamond

Using angle-resolved photoelectron spectroscopy, we compare the electronic band structure of an ultrathin (1.8 nm) δ-layer of boron-doped diamond with a bulk-like boron doped diamond film (3 μm). Surprisingly, the measurements indicate that except for a small change in the effective mass, there is no significant difference between the electronic structure of these samples, irrespective of their physical dimensionality, except for a small modification of the effective mass. While this suggests that, at the current time, it is not possible to fabricate boron-doped diamond structures with quantum properties, it also means that nanoscale boron doped diamond structures can be fabricated which retain the classical electronic properties of bulk-doped diamond, without a need to consider the influence of quantum confinement.

Identifying and Tracking Defects in Dynamic Supramolecular Polymers

A central paradigm of self-assembly is to create ordered structures starting from molecular monomers that spontaneously recognize and interact with each other via noncovalent interactions. In recent years, great efforts have been directed toward perfecting the design of a variety of supramolecular polymers and materials with different architectures. The resulting structures are often thought of as ideally perfect, defect-free supramolecular fibers, micelles, vesicles, etc., having an intrinsic dynamic character, which are typically studied at the level of statistical ensembles to assess their average properties. However, molecular simulations recently demonstrated that local defects that may be present or may form in these assemblies, and which are poorly captured by conventional approaches, are key to controlling their dynamic behavior and properties. The study of these defects poses considerable challenges, as the flexible/dynamic nature of these soft systems makes it difficult to identify what effectively constitutes a defect and to characterize its stability and evolution. Here, we demonstrate the power of unsupervised machine-learning techniques to systematically identify and compare defects in supramolecular polymer variants in different conditions, using as a benchmark 5 Å resolution coarse-grained molecular simulations of a family of supramolecular polymers. We show that this approach allows a complete data-driven characterization of the internal structure and dynamics of these complex assemblies and of the dynamic pathways for defects formation and resorption. This provides a useful, generally applicable approach to unambiguously identify defects in these dynamic self-assembled materials and to classify them based on their structure, stability, and dynamics.

Doping of Carbon Materials for Metal-Free Electrocatalysis

Carbon atoms in the graphitic carbon skeleton can be replaced by heteroatoms with different electronegative from that of the carbon atom (i.e., heteroatom doping) to modulate the charge distribution over the carbon network. The charge modulation can be achieved via direct charge transfer with an electron acceptor/donor (i.e., charge transfer doping) or through introduction of defects (i.e., defective doping). Various doping strategies, including heteroatom doping, charge-transfer doping, and defective doping, have now been devised for modulating the charge distribution of numerous graphite carbon materials to impart new properties to carbon materials. Consequently, carbon nanomaterials with defined doping have recently become prominent members in the carbon family, promising for a variety of applications, including catalysis, energy conversion and storage, environmental remediation, and important chemical production and industrial processes. The purpose of this review is to present an overview on the doping of carbon materials for metal-free electrocatalysis, especially the development of doping strategies and doping-induced structure and property changes for potential catalytic applications. Current challenges and future perspectives in the doped carbon-based metal-free catalyst field are also discussed.

Superconductivity with high hardness in Mo3C2

This work synthesized a high hardness and superconductive polycrystalline Mo₃C₂ material by the HPHT method. Mo₃C₂ exhibits superconductivity below 8.2 K and its hardness is far higher than that of the traditionally used superconductive materials.

Surface acoustic waves in one-dimensional piezoelectric-metallic phononic crystal: Effect of a cap layer

We study the propagation of transverse acoustic waves associated with the surface of a semi-infinite superlattice (SL) composed of piezoelectric-metallic layers and capped with a piezoelectric layer. We present closed-form expressions for localized surface waves, the so-called Bleustein-Gulyaev (BG) waves depending on whether the cap layer is open-circuited or short-circuited. These expressions are obtained by means of the Green's function method which enables to deduce also the densities of states. These theoretical results are illustrated by a few numerical applications to SLs made of piezoelectric layers of hexagonal symmetry belonging to the 6 mm class such as PZT4 and ZnO in contact with metallic layers such as Fe, Al, Au, Cu and boron-doped-diamond. We demonstrate a rule about the existence of surface modes when considering two complementary semi-infinite SLs obtained by the cleavage of an infinite SL along a plane parallel to the piezoelectric layers. Indeed, when the surface layers are open-circuited, one obtains one surface mode per gap, this mode is associated with one of the two complementary SLs. However, when the surface layers are short-circuited, this rule is not fulfilled and one can obtain zero, one or two modes inside each gap of the two complementary SLs depending on the position of the plane where the cleavage is produced. We show that in addition to the BG surface waves localized at the surface of the cap layer, there may exist true guided waves and pseudo-guided waves (i.e. leaky waves) induced by the cap layer either inside the gaps or inside the bands of the SL respectively. Also, we highlight the possibility of existence of interface modes between the SL and a cap layer as well as an interaction between these modes and the BG surface mode when both modes fall in the same band gaps of the SL. The strength of the interaction depends on the width of the cap layer. Finally, we show that the electromechanical coupling coefficient (ECC) is very sensitive to the cap layer thickness, in particular we calculate and discuss the behavior of the ECC as a function of the adlayer thickness for the low velocity surface modes of the SL which exhibit the highest ECC values. The effect of the nature of the metallic layers inside the SL on the ECC is also investigated. The different surface modes discussed in this work should have applications in sensing applications.

Nanocrystalline Boron-Doped Diamond as a Corrosion-Resistant Anode for Water Oxidation via Si Photoelectrodes

Due to its high sensitivity to corrosion, the use of Si in direct photoelectrochemical (PEC) water-splitting systems that convert solar energy into chemical fuels has been greatly limited. Therefore, the development of low-cost materials resistant to corrosion under oxidizing conditions is an important goal toward a suitable protection of otherwise unstable semiconductors used in PEC cells. Here, we report on the development of a protective coating based on thin and electrically conductive nanocrystalline boron-doped diamond (BDD) layers. We found that  BDD layers protect the underlying Si photoelectrodes over a wide pH range (1-14) in aqueous electrolyte solutions. A BDD layer maintains an efficient charge carrier transfer from the underlying silicon to the electrolyte solution. Si|BDD photoelectrodes show no sign of performance degradation after a continuous PEC treatment in neutral, acidic, and basic electrolytes. The deposition of a cobalt phosphate (CoPi) oxygen evolution catalyst onto the BDD layer significantly reduces the overpotential for water oxidation, demonstrating the ability of  BDD layers to substitute the transparent conductive oxide coatings, such as indium tin oxide (ITO) and fluorine-doped tin oxide (FTO), frequently used as protective layers in Si photoelectrodes.

Vertical-type two-dimensional hole gas diamond metal oxide semiconductor field-effect transistors

Power semiconductor devices require low on-resistivity and high breakdown voltages simultaneously. Vertical-type metal-oxide-semiconductor field-effect transistors (MOSFETs) meet these requirements, but have been incompleteness in diamond. Here we show vertical-type p-channel diamond MOSFETs with trench structures and drain current densities equivalent to those of n-channel wide bandgap devices for complementary inverters. We use two-dimensional hole gases induced by atomic layer deposited Al₂O₃ for the channel and drift layers, irrespective of their crystal orientations. The source and gate are on the planar surface, the drift layer is mainly on the sidewall and the drain is the p⁺ substrate. The maximum drain current density exceeds 200 mA mm⁻¹ at a 12 µm source-drain distance. On/off ratios of over eight orders of magnitude are demonstrated and the drain current reaches the lower measurement limit in the off-state at room temperature using a nitrogen-doped n-type blocking layer formed using ion implantation and epitaxial growth.

Carbon-Based Metal-Free Electrocatalysis for Energy Conversion, Energy Storage, and Environmental Protection

Abstract

Carbon-based metal-free catalysts possess desirable properties such as high earth abundance, low cost, high electrical conductivity, structural tunability, good selectivity, strong stability in acidic/alkaline conditions, and environmental friendliness. Because of these properties, these catalysts have recently received increasing attention in energy and environmental applications. Subsequently, various carbon-based electrocatalysts have been developed to replace noble metal catalysts for low-cost renewable generation and storage of clean energy and environmental protection through metal-free electrocatalysis. This article provides an up-to-date review of this rapidly developing field by critically assessing recent advances in the mechanistic understanding, structure design, and material/device fabrication of metal-free carbon-based electrocatalysts for clean energy conversion/storage and environmental protection, along with discussions on current challenges and perspectives.

Graphical Abstract

Thermally Stable, High Performance Transfer Doping of Diamond using Transition Metal Oxides

We report on optimisation of the environmental stability and high temperature operation of surface transfer doping in hydrogen-terminated diamond using MoO3 and V2O5 surface acceptor layers. In-situ annealing of the hydrogenated diamond surface at 400 °C was found to be crucial to enhance long-term doping stability. High temperature sheet resistance measurements up to 300 °C were performed to examine doping thermal stability. Exposure of MoO3 and V2O5 transfer-doped hydrogen-terminated diamond samples up to a temperature of 300 °C in ambient air showed significant and irreversible loss in surface conductivity. Thermal stability was found to improve dramatically however when similar thermal treatment was performed in vacuum or in ambient air when the oxide layers were encapsulated with a protective layer of hydrogen silsesquioxane (HSQ). Inspection of the films by X-ray diffraction revealed greater crystallisation of the MoO3 layers following thermal treatment in ambient air compared to the V2O5 films which appeared to remain amorphous. These results suggest that proper encapsulation and passivation of these oxide materials as surface acceptor layers on hydrogen-terminated diamond is essential to maximise their environmental and thermal stability.

Unconventional Superconductivity in the BiS_{2}-Based Layered Superconductor NdO_{0.71}F_{0.29}BiS_{2}

We investigate the superconducting-gap anisotropy in one of the recently discovered BiS_{2}-based superconductors, NdO_{0.71}F_{0.29}BiS_{2} (T_{c}∼5  K), using laser-based angle-resolved photoemission spectroscopy. Whereas the previously discovered high-T_{c} superconductors such as copper oxides and iron-based superconductors, which are believed to have unconventional superconducting mechanisms, have 3d electrons in their conduction bands, the conduction band of BiS_{2}-based superconductors mainly consists of Bi 6p electrons, and, hence, the conventional superconducting mechanism might be expected. Contrary to this expectation, we observe a strongly anisotropic superconducting gap. This result strongly suggests that the pairing mechanism for NdO_{0.71}F_{0.29}BiS_{2} is an unconventional one and we attribute the observed anisotropy to competitive or cooperative multiple paring interactions.

Durability-enhanced two-dimensional hole gas of C-H diamond surface for complementary power inverter applications

Complementary power field effect transistors (FETs) based on wide bandgap materials not only provide high-voltage switching capability with the reduction of on-resistance and switching losses, but also enable a smart inverter system by the dramatic simplification of external circuits. However, p-channel power FETs with equivalent performance to those of n-channel FETs are not obtained in any wide bandgap material other than diamond. Here we show that a breakdown voltage of more than 1600 V has been obtained in a diamond metal-oxide-semiconductor (MOS) FET with a p-channel based on a two-dimensional hole gas (2DHG). Atomic layer deposited (ALD) Al₂O₃ induces the 2DHG ubiquitously on a hydrogen-terminated (C-H) diamond surface and also acts as both gate insulator and passivation layer. The high voltage performance is equivalent to that of state-of-the-art SiC planar n-channel FETs and AlGaN/GaN FETs. The drain current density in the on-state is also comparable to that of these two FETs with similar device size and V_B.

Formation of Boron-Carbon Nanosheets and Bilayers in Boron-Doped Diamond: Origin of Metallicity and Superconductivity

The insufficient data on a structure of the boron-doped diamond (BDD) has frustrated efforts to fully understand the fascinating electronic properties of this material and how they evolve with doping. We have employed X-ray diffraction and Raman scattering for detailed study of the large-sized BDD single crystals. We demonstrate a formation of boron-carbon (B-C) nanosheets and bilayers in BDD with increasing boron concentration. An incorporation of two boron atoms in the diamond unit cell plays a key role for the B-C nanosheets and bilayer formation. Evidence for these B-C bilayers which are parallel to {111} planes is provided by the observation of high-order, super-lattice reflections in X-ray diffraction and Laue patterns. B-C nanosheets and bilayers minimize the strain energy and affect the electronic structure of BDD. A new shallow acceptor level associated with B-C nanosheets at ~37 meV and the spin-orbit splitting of the valence band of ~6 meV are observed in electronic Raman scattering. We identified that the superconducting transitions occur in the (111) BDD surfaces only. We believe that the origin of Mott and superconducting transitions is associated with the two-dimensional (2D) misfit layer structure of BDD. A model for the BDD crystal structure, based on X-ray and Raman data, is proposed and confirmed by density functional theoretical calculation.

Double-Layer 3D Macro-Mesoporous Metal Oxide Modified Boron-Doped Diamond with Enhanced Photoelectrochemical Performance

In this work, a TiO₂/Sb-doped SnO₂ electrode was prepared on the boron-doped diamond (BDD) substrate with double-layer three-dimensional macro-mesoporous (DL3DOM-m) structure, using the polystyrene sphere (PS) vertical deposition method. The as-prepared DL3DOM-m TiO₂/SnO₂/BDD was employed for organic contaminant removal, showing excellent photoelectrocatalytic performance. SEM, XRD and XPS indicated that DL3DOM-m electrode possessed a 3D macroporous layered framework with uniform pore size (about 400 nm), nanosized particles (4.5-5.8 nm), and high electroactive surface area (3-fold more than that of BDD). SA-XRD indicated the backbone of DL3DOM-m electrode had mesoporous structure. It was found that the as-prepared electrode exhibited remarkable electrocatalytic activity, high photocurrent and outstanding absorption capability (91.0 μg cm^-2). Furthermore, bisphenol A (BPA) was completely decomposed after 3 h of reaction applying DL3DOM-m electrode as photoanode, and that on BDD was only 58.9%. It indicated that the modified electrode had great potential to be used in practical water treatment with high photoelectrochemical performance.

High resolution core level spectroscopy of hydrogen-terminated (1 0 0) diamond

Synchrotron-based photoelectron spectroscopy experiments are presented that address a long standing inconsistency in the treatment of the C1s core level of hydrogen terminated (1 0 0) diamond. Through a comparison of surface and bulk sensitive measurements we show that there is a surface related core level component to lower binding energy of the bulk diamond component; this component has a chemical shift of [Formula: see text] eV which has been attributed to carbon atoms which are part of the hydrogen termination. Additionally, our results indicate that the asymmetry of the hydrogen terminated (1 0 0) diamond C1s core level is an intrinsic aspect of the bulk diamond peak which we have attributed to sub-surface carbon layers.

Note: Novel diamond anvil cell for electrical measurements using boron-doped metallic diamond electrodes

A novel diamond anvil cell suitable for electrical transport measurements under high pressure has been developed. A boron-doped metallic diamond film was deposited as an electrode on a nano-polycrystalline diamond anvil using a microwave plasma-assisted chemical vapor deposition technique combined with electron beam lithography. The maximum pressure that can be achieved by this assembly is above 30 GPa. We report electrical transport measurements of Pb up to 8 GPa. The boron-doped metallic diamond electrodes showed no signs of degradation after repeated compression.

Soft X-ray angle-resolved photoemission with micro-positioning techniques for metallic V₂O₃

Soft X-ray angle-resolved photoemission has been performed for metallic V2O3. By combining a microfocus beam (40 µm × 65 µm) and micro-positioning techniques with a long-working-distance microscope, it has been possible to observe band dispersions from tiny cleavage surfaces with a typical size of several tens of µm. The photoemission spectra show a clear position dependence, reflecting the morphology of the cleaved sample surface. By selecting high-quality flat regions on the sample surface, it has been possible to perform band mapping using both photon-energy and polar-angle dependences, opening the door to three-dimensional angle-resolved photoemission spectroscopy for typical three-dimensional correlated materials where large cleavage planes are rarely obtained.

Signature of high Tc above 25 K in high quality superconducting diamond

We have observed zero resistivity above 10 K and an onset of resistivity reduction at 25.2 K in a heavily B-doped diamond film. However, the effective carrier concentration is similar to that of superconducting diamond with a lower T_c. We found that the carrier has a longer mean free path and lifetime than in the previous report, indicating that this highest T_c diamond has better crystallinity compared to that of other superconducting diamond films. In addition, the susceptibility shows a small transition above 20 K in the high quality diamond, suggesting a signature of superconductivity above 20 K. These results strongly suggest that heavier carrier doped defect-free crystalline diamond could give rise to high T_c diamond.

Efficient and durable hydrogen evolution electrocatalyst based on nonmetallic nitrogen doped hexagonal carbon

The feasibility of renewable energy technology, hydrogen production by water electrolysis, depends on the design of efficient and durable electrocatalyst composed of earth-abundant elements. Herein, a highly active and stable nonmetallic electrocatalyst, nitrogen doped hexagonal carbon (NHC), was developed for hydrogen production. It exhibited high activity for hydrogen evolution with a low overpotential of only 65 mV, an apparent exchange current density of 5.7 × 10⁻² mA cm⁻² and a high hydrogen production rate of 20.8 mL cm⁻² h⁻¹ at −0.35 V. The superior hydrogen evolution activity of NHC stemmed from the intrinsic electrocatalytic property of hexagonal nanodiamond, the rapid charge transfer and abundance of electrocatalytic sites after nitrogen doping. Moreover, NHC was stable in a corrosive acidic solution during electrolysis under high current density.

Theoretical Study of the Energetic Stability and Geometry of Terminated and B-Doped Diamond (111) Surfaces

The effect of B doping on the surface (111) reactivity has, in the present study, been investigated for various surface terminations, H, OH, Oon-top, and F. This type of surface modification has experimentally been proven to be extremely important for, for example, applications based on surface electrochemistry. Density functional theory (DFT) has here been used to study both the local and more global effects of substitutionally positioned B atoms in the upper part of the diamond (111) surface. For this purpose, adsorption energies for the various terminating species have been calculated, and the observed results have been carefully analyzed in order to gain a deeper knowledge about the atomic-level cause of the observed effects. As a result, the B dopant shows a clear, but local, effect for all terminating species investigated. In addition, it is only the radical O-terminating species that show a special and high reactivity on the diamond surface. The other terminating species show a much lower reactivity, which in addition are very similar.

Development of a soft X-ray angle-resolved photoemission system applicable to 100 µm crystals

A system for angle-resolved photoemission spectroscopy (ARPES) of small single crystals with sizes down to 100 µm has been developed. Soft X-ray synchrotron radiation with a spot size of ∼40 µm × 65 µm at the sample position is used for the excitation. Using this system an ARPES measurement has been performed on a Si crystal of size 120 µm × 100 µm × 80 µm. The crystal was properly oriented on a sample stage by measuring the Laue spots. The crystal was cleaved in situ with a microcleaver at 100 K. The cleaved surface was adjusted to the beam spot using an optical microscope. Consequently, clear band dispersions along the Γ-X direction reflecting the bulk electronic states were observed with a photon energy of 879 eV.

Stability and mechanical properties of BC(x) crystals: the role of B-B bonds and boron concentration

Based on a random solid solution model, first-principles calculations were performed to investigate the structural stabilities and mechanical properties of cubic BC(x) (1 < x < 63) crystals. Judging by the formation energy, hardness and ductility, a boron concentration between 2.8 × 10(21) and 8.4 × 10(21) cm(-3) (1.56-4.69 at.%) is a compromise choice to balance the structural stabilities and mechanical properties of BC(x) crystals. The ratio of B-B bonds has an evident effect on the structural stability of the cubic BC(x) crystals. Controlling the ratio of B-B bonds in the precursor materials might be a practicable route for synthesizing BC(x) crystals with higher boron concentrations.

Ultrahigh-vacuum cleaving system for sub-100-microm crystals

An ultrahigh-vacuum cleaver has been developed for cleaving small crystals with sizes of less than 100 microm. The cleaver is fully driven by stepping motors in order to control its position on the micrometer scale. A pair of blades with sharp edges is used to nip and cleave crystals. To position the edges of the blades relative to a small crystal, they are observed using an optical microscope with a long working distance. A silicon crystal with a size of approximately 80 microm has been cleaved by using the developed system, and the cleanliness of the obtained surface has been verified by photoemission spectroscopy.

Diamond-metal contacts: interface barriers and real-time characterization

A review of diamond-metal contacts is presented with reference to reported values of interfacial potential (Schottky) barriers and their dependence on macroscopic and microscopic properties of the diamond surface, the interface and the metal. No simple model can account for the overall spread of p-diamond barriers, although there are, for certain metals, correlations with metal electronegativity, interface chemistry and diamond surface preparation. Detailed studies are presented for a selected contact (Al-p-diamond) using real-time monitoring during metal growth from sub-nanometre to bulk films and subsequent in situ heating to 1000 °C. This contact, prepared in a clean vacuum environment on characterized single-crystal substrates, provides a case study for a combined in situ electrical and spectroscopic investigation using IV measurements for macroscopic diodes and real-time photoelectron spectroscopy for nanoscale metal films. Band bending during growth leads to a rectifying contact with a measured IV barrier height of 1.05 V and an ideality factor of 1.4. A transition from layered to clustered growth of the metal film is revealed in the real-time measurements and this is confirmed by AFM. For the annealed contact, a direct correlation is revealed by real-time photoemission between the onset of interfacial carbide formation and the change from a rectifying to an ohmic contact at 482 °C.

Superconductivity in CVD diamond films

A beautiful jewel of diamond is insulator. However, boron doping can induce semiconductive, metallic and superconducting properties in diamond. When the boron concentration is tuned over 3 × 10(20) cm(-3), diamonds enter the metallic region and show superconductivity at low temperatures. The metal-insulator transition and superconductivity are analyzed using ARPES, XAS, NMR, IXS, transport and magnetic measurements and so on. This review elucidates the physical properties and mechanism of diamond superconductor as a special superconductivity that occurs in semiconductors.

Comment on "temperature-dependent localized excitations of doped carriers in superconducting diamond"

A Comment on the Letter by K. Ishizaka et al., Phys. Rev. Lett. 100, 166402 (2008)10.1103/PhysRevLett.100.166402. The authors of the Letter offer a Reply.

Superconducting group-IV semiconductors

Despite the amount of experimental and theoretical work on doping-induced superconductivity in covalent semiconductors based on group IV elements over the past four years, many open questions and puzzling results remain to be clarified. The nature of the coupling (whether mediated by electronic correlation, phonons or both), the relationship between the doping concentration and the critical temperature (T(c)), which affects the prospects for higher transition temperatures, and the influence of disorder and dopant homogeneity are debated issues that will determine the future of the field. Here, we present recent achievements and predictions, with a focus on boron-doped diamond and silicon. We also suggest that innovative superconducting devices, combining specific properties of diamond or silicon with the maturity of semiconductor-based technologies, will soon be developed.

In situ positioning of a few hundred micrometer-sized cleaved surfaces for soft-x-ray angle-resolved photoemission spectroscopy by use of an optical microscope

A method to position samples with small cleaved regions has been developed to be applied to the angle-resolved photoemission spectroscopy (ARPES) which uses soft-x-ray synchrotron radiation focused down to 160 x 180 microm(2). A long-working-distance optical microscope is used for the sample observation. A selected region on a sample can be optimally set at the position of measurements, which is realized by the spatial resolution of the photoelectron analyzer. Using this method, electronic band dispersions of bulk silicon have been measured by ARPES for a partially cleaved region with a size of approximately 200 x 500 microm(2).

Electronic structure of boron-doped diamond with B-H complex and B pair

The electronic structure of boron-hydrogen complex and boron pair in diamond are studied by first-principles density-functional calculations with supercell models. The electronic structure calculated for the B-H complexes with C2v or C3v symmetry and the nearest-neighbor B pair is used to interpret recent experimental results such as B 1s x-ray photoemission spectroscopy, 11B nuclear quadruple resonance and B K-edge x-ray absorption spectroscopy, which cannot be explained solely by the isolated substitutional boron.

Superconductivity in compensated and uncompensated semiconductors

We investigate the localization and superconductivity in heavily doped semiconductors. The crossover from the superconductivity in the host band to that in the impurity band is described on the basis of the disordered three-dimensional attractive Hubbard model for binary alloys. The microscopic inhomogeneity and the thermal superconducting fluctuation are taken into account using the self-consistent 1-loop order theory. The superconductor-insulator transition accompanies the crossover from the host band to the impurity band. We point out an enhancement of the critical temperature Tc around the crossover. Further localization of electron wave functions leads to the localization of Cooper pairs and induces the pseudogap. We find that both the doping compensation by additional donors and the carrier increase by additional acceptors suppress the superconductivity. A theoretical interpretation is proposed for the superconductivity in the boron-doped diamond, SiC, and Si.

Superconductivity in carrier-doped silicon carbide

We report growth and characterization of heavily boron-doped 3C-SiC and 6H-SiC and Al-doped 3C-SiC. Both 3C-SiC:B and 6H-SiC:B reveal type-I superconductivity with a critical temperature Tc=1.5 K. On the other hand, Al-doped 3C-SiC (3C-SiC:Al) shows type-II superconductivity with Tc=1.4 K. Both SiC:Al and SiC:B exhibit zero resistivity and diamagnetic susceptibility below Tc with effective hole-carrier concentration n higher than 1020 cm-3. We interpret the different superconducting behavior in carrier-doped p-type semiconductors SiC:Al, SiC:B, Si:B and C:B in terms of the different ionization energies of their acceptors.

Temperature-dependent localized excitations of doped carriers in superconducting diamond

Laser-excited photoemission spectroscopy is used to show that the doped carriers in metallic or superconducting diamond couple strongly to the lattice via high-energy (approximately 150 meV) optical phonons, with direct observations of localized Franck-Condon multiphonon sidebands appearing as Fermi-edge replicas. It exhibits a temperature-dependent spectral weight transfer from higher to lower energy sidebands and zero-phonon Fermi-edge states. The quantified coupling strength shows a systematic increase on lowering temperature, implicating its relation to the normal state transport and superconductivity.

Investigation of low-resistivity from hydrogenated lightly B-doped diamond by ion implantation

We have implanted boron (B) ions (dosage: 5×1014 cm-2) into diamond and then hydrogenated the sample by implantating hydrogen ions at room temperature. A p-type diamond material with a low resistivity of 7.37 mΩ cm has been obtained in our experiment, which suggests that the hydrogenation of B-doped diamond results in a low-resistivity p-type material. Interestingly, inverse annealing, in which carrier concentration decreased with increasing annealing temperature, was observed at annealing temperatures above 600 °C. In addition, the formation mechanism of a low-resistivity material has been studied by density functional theory calculation using a plane wave method.

Metal-to-insulator transition and superconductivity in boron-doped diamond

The experimental discovery of superconductivity in boron-doped diamond came as a major surprise to both the diamond and the superconducting materials communities. The main experimental results obtained since then on single-crystal diamond epilayers are reviewed and applied to calculations, and some open questions are identified. The critical doping of the metal-to-insulator transition (MIT) was found to coincide with that necessary for superconductivity to occur. Some of the critical exponents of the MIT were determined and superconducting diamond was found to follow a conventional type II behaviour in the dirty limit, with relatively high critical temperature values quite close to the doping-induced insulator-to-metal transition. This could indicate that on the metallic side both the electron-phonon coupling and the screening parameter depend on the boron concentration. In our view, doped diamond is a potential model system for the study of electronic phase transitions and a stimulating example for other semiconductors such as germanium and silicon.