Showing papers on "Diamond published in 2022"

Coherent interfaces govern direct transformation from graphite to diamond

Abstract: Abstract Understanding the direct transformation from graphite to diamond has been a long-standing challenge with great scientific and practical importance. Previously proposed transformation mechanisms 1–3 , based on traditional experimental observations that lacked atomistic resolution, cannot account for the complex nanostructures occurring at graphite−diamond interfaces during the transformation 4,5 . Here we report the identification of coherent graphite−diamond interfaces, which consist of four basic structural motifs, in partially transformed graphite samples recovered from static compression, using high-angle annular dark-field scanning transmission electron microscopy. These observations provide insight into possible pathways of the transformation. Theoretical calculations confirm that transformation through these coherent interfaces is energetically favoured compared with those through other paths previously proposed 1–3 . The graphite-to-diamond transformation is governed by the formation of nanoscale coherent interfaces (diamond nucleation), which, under static compression, advance to consume the remaining graphite (diamond growth). These results may also shed light on transformation mechanisms of other carbon materials and boron nitride under different synthetic conditions.

Enhancing dielectric strength of thermally conductive epoxy composites by preventing interfacial charge accumulation using micron-sized diamond

Abstract: Epoxy-resin-based dielectric materials play a crucial role in the fabrication of advanced power electronic devices. However, because of a high voltage stress and high heat generation density caused by device miniaturization and high-power density, epoxy composites must possess a high dielectric strength and thermal conductivity. In this study, a general strategy is developed to improve the dielectric strength of thermally conductive epoxy composites using a micron-sized diamond, by preventing an interfacial charge accumulation at the inorganic fillers–epoxy matrix interface that would reduce the interfacial electric field strength. Therefore, the dielectric strength and thermal conductivity of the epoxy composites were significantly and simultaneously improved without any additional physicochemical modification of the micron-sized diamond surface. The reported percentage increase in the dielectric strength among the highly thermal conductive epoxy composites is the highest in the last decade. In addition, the diamond/epoxy composites exhibited lower relative permittivity and excellent mechanical, thermal, and processing properties. These results suggest that the diamond/epoxy composites can be widely used for the fabrication of power electronic devices with a high voltage stress and high heat generation density.

Accurate description of high-order phonon anharmonicity and lattice thermal conductivity from molecular dynamics simulations with machine learning potential

Abstract: Phonon anharmonicity is critical for accurately predicting the material's thermal conductivity ($\ensuremath{\kappa}$). However, its calculation based on the perturbation theory is a difficult and time-consuming task, especially for the high-order phonon scattering process. In this work, using cubic boron arsenide (BAs) and diamond as examples, we combine the machine learning potential (MLP) with molecular dynamics simulations to predict $\phantom{\rule{4pt}{0ex}}\ensuremath{\kappa}$ and assess the effect of anharmonicity on thermal transport properties. A MLP based on the matrix tensor algorithm is developed in this work, which can accurately describe lattice dynamics behaviors in both BAs and diamond. The phonon spectral energy density analysis reveals that MLP can effectively capture both the phonon mode softening and the linewidth broadening induced by the anharmonicity at finite temperatures in both materials. Compared to diamond, BAs exhibits a stronger anharmonicity revealed by the larger deviation from equilibrium position and more pronounced phonon broadening effect, especially at high temperatures. Furthermore, based on the phonon Boltzmann transport equation and three-phonon scattering process, our calculation results demonstrate that the accuracy of the MLP in predicting the $\ensuremath{\kappa}$ is comparable to that of density-functional theory calculations for both diamond and BAs. However, this framework can only predict $\ensuremath{\kappa}$ of diamond in agreement with experimental measurements, but significantly overestimates the $\ensuremath{\kappa}$ of BAs compared to the experimental results, due to the significant impact of high-order phonon scattering process in BAs. In contrast, the $\ensuremath{\kappa}$ values predicted by equilibrium molecular dynamics simulations combined with MLP agree well with experimental values for both BAs and diamond. Our study suggests that molecular dynamics simulation combined with MLP is a reliable and computationally efficient tool to account for full orders of anharmonicity and provide accurate predictions of material's thermal conductivity without any a priori knowledge of the importance of high-order phonon anharmonicity.

Nitrogen doped graphene with diamond-like bonds achieves unprecedented energy density at high power in a symmetric sustainable supercapacitor

Abstract: Supercapacitors have attracted great interest because of their fast, reversible operation and sustainability. However, their energy densities remain lower than those of batteries. In the last decade, supercapacitors with an energy content of ∼110 W h L−1 at a power of ∼1 kW L−1 were developed by leveraging the open framework structure of graphene-related architectures. Here, we report that the reaction of fluorographene with azide anions enables the preparation of a material combining graphene-type sp2 layers with tetrahedral carbon–carbon bonds and nitrogen (pyridinic and pyrrolic) superdoping (16%). Theoretical investigations showed that the C–C bonds develop between carbon-centered radicals, which emerge in the vicinity of the nitrogen dopants. This material, with diamond-like bonds and an ultra-high mass density of 2.8 g cm−3, is an excellent host for the ions, delivering unprecedented energy densities of 200 W h L−1 at a power of 2.6 kW L−1 and 143 W h L−1 at 52 kW L−1. These findings open a route to materials whose properties may enable a transformative improvement in the performance of supercapacitor components.

Electrochemical oxidation of ciprofloxacin in different aqueous matrices using synthesized boron-doped micro and nano-diamond anodes

Abstract: The present work investigates the electrocatalytic performance of two different morphologies of boron doped-diamond film electrode (microcrystalline diamond - MCD, and nanocrystalline diamond - NCD) used in electrochemical oxidation for the removal of the antibiotic ciprofloxacin (CIP). A thorough study was conducted regarding the formation of the MCD and NCD films through the adjustment of methane in CH4/H2 gas mixture, and the two films were compared in terms of crystalline structure, apparent doping level, and electrochemical properties. The physicochemical results showed that the NCD film had higher sp2 carbon content and greater doping level; this contributed to improvements in its surface roughness, as well as its specific capacitance and charge transfer, which consequently enhanced its electrocatalytic activity in comparison with the MCD. The results obtained from CIP removal and mineralization assays performed in sulfate medium also showed that the NCD was more efficient than the MCD under all the current densities investigated. The effects of CIP concentration and the evolution of the final by-products, including short-chain carboxylic acids and inorganic ions, were also investigated. The electrochemical performance of the NCD was evaluated in different aqueous matrices, including chloride medium, real wastewater and simulated urine. The application of the NCD led to complete or almost complete CIP degradation, regardless of the medium employed. The kinetic constant rates obtained under the different media investigated were as follows: synthetic urine (0.0416 min-1 - R2 = 0.991) < real wastewater (0.0923 min-1 R2 = 0.997) < synthetic matrix containing chloride (0.1992 min-1 - R2 = 0.995); this shows that the pollutant degradation was affected by the type of aqueous matrix and the oxidants that were electrogenerated in situ. The results obtained from the analysis of electrical energy per order (EE/O) showed that the treatment of simulated urine spkiked with required the highest energy consumption, followed by the real effluent and synthetic matrix containing chloride. The present study proves the viability of electrocatalytic nanostructured materials to the treatment of antibiotics in complex matrices.

Theoretical study of novel B–C–O compounds with non-diamond isoelectronic

Abstract: Two novel non-isoelectronic with diamond (non-IED) B–C–O phases (tI16-B 8 C 6 O 2 and mP16-B 8 C 5 O 3 ) have been unmasked. The research of the phonon scattering spectra and the independent elastic constants under ambient pressure (AP) and high pressure (HP) proves the stability of these non-IED B–C–O phases. Respective to the common compounds, the research of the formation enthalpies and the relationship with pressure of all non-IED B–C–O phases suggests that HP technology performed in the diamond anvil cell (DAC) or large volume press (LVP) is an important technology for synthesis. Both tI16-B 8 C 6 O 2 and tI12-B 6 C 4 O 2 possess electrical conductivity. mP16-B 8 C 5 O 3 is a small bandgap semiconductor with a 0.530 eV gap. For aP13-B 6 C 2 O 5 , mC20-B 2 CO 2 and tI18-B 4 CO 4 are all large gap semiconductors with gaps of 5.643 eV, 6.113 eV, and 7.105 eV, respectively. The study on the relationship between band gap values and pressure of these six non-IED B–C–O phases states that tI16-B 8 C 6 O 2 and tI12-B 6 C 4 O 2 maintain electrical conductivity, mC20-B 2 CO 2 and tI18-B 4 CO 4 have good bandgap stability and are less affected by pressure. The stress-strain simulation reveals that the max strain and stress of 0.4 GPa and 141.9 GPa respectively, can be sustained by tI16-B 8 C 6 O 2 . Studies on their mechanical properties shows that they all possess elasticity moduli and hard character. And pressure has an obvious effect on their mechanical properties, therein toughness of tI12-B 6 C 4 O 2 , aP13-B 6 C 2 O 5 , mC20-B 2 CO 2 and tI18-B 4 CO 4 all increases, and hardness of mP16-B 8 C 5 O 3 continue to strengthen during the compression. With abundant hardness characteristics and tunable band gaps, extensive attention will be focused on the scientific research of non-IED B–C–O compounds.

Surface NMR using quantum sensors in diamond

Abstract: NMR is a noninvasive, molecular-level spectroscopic technique widely used for chemical characterization. However, it lacks the sensitivity to probe the small number of spins at surfaces and interfaces. Here, we use nitrogen vacancy (NV) centers in diamond as quantum sensors to optically detect NMR signals from chemically modified thin films. To demonstrate the method's capabilities, aluminum oxide layers, common supports in catalysis and materials science, are prepared by atomic layer deposition and are subsequently functionalized by phosphonate chemistry to form self-assembled monolayers. The surface NV-NMR technique detects spatially resolved NMR signals from the monolayer, indicates chemical binding, and quantifies molecular coverage. In addition, it can monitor in real time the formation kinetics at the solid-liquid interface. With our approach, we show that NV quantum sensors are a surface-sensitive NMR tool with femtomole sensitivity for in situ analysis in catalysis, materials, and biological research.

Design, realization, and characterization of a novel diamond detector prototype for FLASH radiotherapy dosimetry

Abstract: Abstract Purpose FLASH radiotherapy (RT) is an emerging technique in which beams with ultra‐high dose rates (UH‐DR) and dose per pulse (UH‐DPP) are used. Commercially available active real‐time dosimeters have been shown to be unsuitable in such conditions, due to severe response nonlinearities. In the present study, a novel diamond‐based Schottky diode detector was specifically designed and realized to match the stringent requirements of FLASH‐RT. Methods A systematic investigation of the main features affecting the diamond response in UH‐DPP conditions was carried out. Several diamond Schottky diode detector prototypes with different layouts were produced at Rome Tor Vergata University in cooperation with PTW‐Freiburg. Such devices were tested under electron UH‐DPP beams. The linearity of the prototypes was investigated up to DPPs of about 26 Gy/pulse and dose rates of approximately 1 kGy/s. In addition, percentage depth dose (PDD) measurements were performed in different irradiation conditions. Radiochromic films were used for reference dosimetry. Results The response linearity of the diamond prototypes was shown to be strongly affected by the size of their active volume as well as by their series resistance. By properly tuning the design layout, the detector response was found to be linear up to at least 20 Gy/pulse, well into the UH‐DPP range conditions. PDD measurements were performed by three different linac applicators, characterized by DPP values at the point of maximum dose of 3.5, 17.2, and 20.6 Gy/pulse, respectively. The very good superimposition of three curves confirmed the diamond response linearity. It is worth mentioning that UH‐DPP irradiation conditions may lead to instantaneous detector currents as high as several mA, thus possibly exceeding the electrometer specifications. This issue was properly addressed in the case of the PTW UNIDOS electrometers. Conclusions The results of the present study clearly demonstrate the feasibility of a diamond detector for FLASH‐RT applications.

Excited-State Optically Detected Magnetic Resonance of Spin Defects in Hexagonal Boron Nitride

Abstract: Negatively charged boron vacancy (V_{B}^{-}) centers in hexagonal boron nitride (h-BN) are promising spin defects in a van der Waals crystal. Understanding the spin properties of the excited state (ES) is critical for realizing dynamic nuclear polarization. Here, we report zero-field splitting in the ES of D_{ES}=2160 MHz and its associated optically detected magnetic resonance (ODMR) contrast of 12% at cryogenic temperature. In contrast to nitrogen vacancy (NV^{-}) centers in diamond, the ODMR contrast of V_{B}^{-} centers is more prominent at cryotemperature than at room temperature. The ES has a g factor similar to the ground state. The ES photodynamics is further elucidated by measuring the level anticrossing of the V_{B}^{-} defects under varying external magnetic fields. Our results provide important information for utilizing the spin defects of h-BN in quantum technology.

Size-Dependent Thermal Stability and Optical Properties of Ultra-Small Nanodiamonds Synthesized under High Pressure

Abstract: Diamond properties down to the quantum-size region are still poorly understood. High-pressure high-temperature (HPHT) synthesis from chloroadamantane molecules allows precise control of nanodiamond size. Thermal stability and optical properties of nanodiamonds with sizes spanning range from <1 to 8 nm are investigated. It is shown that the existing hypothesis about enhanced thermal stability of nanodiamonds smaller than 2 nm is incorrect. The most striking feature in IR absorption of these samples is the appearance of an enhanced transmission band near the diamond Raman mode (1332 cm−1). Following the previously proposed explanation, we attribute this phenomenon to the Fano effect caused by resonance of the diamond Raman mode with continuum of conductive surface states. We assume that these surface states may be formed by reconstruction of broken bonds on the nanodiamond surfaces. This effect is also responsible for the observed asymmetry of Raman scattering peak. The mechanism of nanodiamond formation in HPHT synthesis is proposed, explaining peculiarities of their structure and properties.

Two-dimensional diamonds from sp2-to-sp3 phase transitions

Abstract: The ability to change material properties through phase engineering has long been sought, with the goal of ad hoc tunability of the physical and chemical properties of the transformed phases. The synthesis and study of graphene have made it possible to explore the mechanisms of 2D phase transformations, opening up paths towards the formation of 2D diamond and diamond thin films. This Review examines the state-of-the-art phase transformations in 2D graphitic systems and beyond. The theoretical models formulated to describe the sp2-to-sp3 transitions from graphene to 2D diamond and the experimental processes developed to induce the transition to 2D diamond are discussed, focusing on the transformations induced by chemical functionalization and pressure. The effects of different structural and environmental factors on the evolution of the phase transformations and on the properties of the transformed diamond phases are explored. Without comprehensively reviewing phase transitions in all 2D materials, we briefly mention hexagonal boron nitride, phosphorene, transition metal dichalcogenides and MXenes systems. Finally, the Review delves into the technologies and applications of phase transformations in 2D materials and the opportunities for this field. Mechanically and chemically induced phase transitions in 2D materials offer access to new properties, opening up paths towards future applications. This Review summarizes the theoretical models and experimental processes for inducing phase transformations in 2D materials, especially graphene to 2D diamond, and examines the associated applications.

Biocompatible surface functionalization architecture for a diamond quantum sensor

Abstract: Quantum metrology enables some of the most precise measurements. In the life sciences, diamond-based quantum sensing has enabled a new class of biophysical sensors and diagnostic devices that are being investigated as a platform for cancer screening and ultra-sensitive immunoassays. However, a broader application in the life sciences based on nanoscale nuclear magnetic resonance spectroscopy has been hampered by the need to interface highly sensitive quantum bit (qubit) sensors with their biological targets. Here, we demonstrate a new approach that combines quantum engineering with single-molecule biophysics to immobilize individual proteins and DNA molecules on the surface of a bulk diamond crystal that hosts coherent nitrogen vacancy qubit sensors. Our thin (sub-5 nm) functionalization architecture provides precise control over protein adsorption density and results in near-surface qubit coherence approaching 100 {\mu}s. The developed architecture remains chemically stable under physiological conditions for over five days, making our technique compatible with most biophysical and biomedical applications.

MOSFETs on (110) C–H Diamond: ALD Al₂O₃/Diamond Interface Analysis and High Performance Normally-OFF Operation Realization

Abstract: Hole concentration of 2-D hole gas (2DHG) on (110) diamond is higher than that on other faces, making it the best choice for power device application. Detailed analysis of atomic layer deposition (ALD) Al₂O₃/(110) C–H diamond interface structure is of vital importance. MOSFETs with thin (10 nm) and thick (100 nm) ALD Al₂O₃ layer were made in this study. The microstructure of Al₂O₃ on (110) C–H diamond was analyzed. Abrupt interface of ALD Al₂O₃/C–H diamond was observed through high resolution transmission electron microscope (HRTEM). Cascode structure using diamond MOSFETs and enhancement mode silicon MOSFET is fabricated and its high performance is confirmed.