Diamond Surface Research Articles

Exploring the effect of surface chemistry and particle size of boron-doped diamond powder as catalyst and catalyst support for the oxygen reduction reaction

The interest for the electrodes based on conductive boron-doped diamond powder (BDDP) is increasing in recent years due to their excellent physical and chemical stability, the wide potential window in both aqueous and organic electrolytes, the relatively large specific surface area, and their versatility in comparison to boron-doped diamond thin film electrodes. These materials have been proposed as alternative cathode catalyst support due to their high corrosion resistance when subjected to the highly positive potentials originated during the start-stop operations in fuel cells, especially in automobiles. In this work, we present three BDDP supports with different particle sizes and different surface oxygen contents as supports of different iron species for oxygen reduction reaction in alkaline solution. BDDP supports were modified with carbon nitride (C3N4) or phthalocyanines (Pc) as anchoring points for iron. The different electrocatalytic performance observed confirmed the strong influence of the surface chemistry of the BDDP supports on the activity of the metallic sites for FePc samples while, for Fe-C3N4 samples, the determining effect was the particle size of the BDDP support. Additionally, DFT calculations were used to obtain some insights about the interaction of the FePc with the diamond surface.

Surface transfer doped diamond diodes with metal oxide passivation and field-plate

Surface transfer-doping, involving hydrogen terminated diamond surfaces, has been an effective method for producing diamond devices for some years but suffered from poor device longevity and reproducibility. The emergence of metal oxides as an encapsulant has begun to change this situation. Here, HfO2 encapsulated surface transfer doped diamond Schottky diodes with stable device characteristics have been demonstrated. Ideality factor and Schottky barrier heights of the devices did not vary considerably across extended periods of use (up to 39 days). The devices showed excellent blocking capabilities, demonstrating no catastrophic breakdown under the maximum field applied and only a slight increase in leakage current at the reverse bias and field strength of 200 V and 0.167 MV cm−1, respectively. Indeed, a large rectification ratio of up to 108 and a very low leakage current of ≈10−9 A cm−1 were maintained at this reverse bias (200 V). Furthermore, multiple devices were compared across a single substrate, something rarely reported previously for surface transfer doped diamond diodes. Leakage currents and rectification ratios were similar for all of the devices.

Effect of Titanium and Molybdenum Cover on the Surface Restructuration of Diamond Single Crystal during Annealing.

Diamond is an important material for electrical and electronic devices. Because the diamond is in contact with the metal in these applications, it becomes necessary to study the metal-diamond interaction and the structure of the interface, in particular, at elevated temperatures. In this work, we study the interaction of the (100) and (111) surfaces of a synthetic diamond single crystal with spattered titanium and molybdenum films. Atomic force microscopy reveals a uniform coating of titanium and the formation of flattened molybdenum nanoparticles. A thin titanium film is completely oxidized upon contact with air and passes from the oxidized state to the carbide state upon annealing in an ultrahigh vacuum at 800 °C. Molybdenum interacts with the (111) diamond surface already at 500 °C, which leads to the carbidization of its nanoparticles and catalytic graphitization of the diamond surface. This process is much slower on the (100) diamond surface; sp2-hybridized carbon is formed on the diamond and the top of molybdenum carbide nanoparticles, only when the annealing temperature is raised to 800 °C. The conductivity of the resulting sample is improved when compared to the Ti-coated diamond substrates and the Mo-coated (111) substrate annealed at 800 °C. The presented results could be useful for the development of graphene-on-diamond electronics.

Iron-Catalyzed Growth of Vertical Graphitic Layers on the (100) Face of Single-Crystal Diamond

The transformation of diamond into graphite occurs at elevated temperatures, and by controlling this process, sp2/sp3 carbon hybrids can be created. In this work, we study the effect of a thin iron film coating on the reconstruction of a (100) diamond face. We use a synthetic single-crystal and anneal it in ultrahigh vacuum at 1150 °C for 15 min. The electronic state of carbon on a clean face and a coated face is immediately examined by X-ray photoelectron spectroscopy and angle-dependent near-edge X-ray absorption fine structure spectroscopy. Raman spectroscopy and scanning electron microscopy measurements complete the structural characterization of the annealed sample. An analysis of the experimental data reveals the formation of thin sp2-carbon fragments on a clean (100) diamond surface and ordered graphite planes on the iron-coated surface. According to our estimates, iron catalyzes the transformation of roughly 10 nm diamond into graphitic layers growing vertically relative to the (100) face of the diamond.

Morphology evolution and mechanism exploration of the diamond surfaces etched by La2O3 powder

In order to systematically study the morphology evolution and mechanism exploration for the diamond surfaces etched by La2O3 powder in N2, the surface morphology, surface roughness, crystal structure, and etched mechanism were investigated. The results reveal that diamond surfaces can be effectively etched by La2O3 from 820 °C to 980 °C, and the etching process is anisotropic. The intensity of the etching increases as the temperature rises. Because of the various carbon atom configurations on the diamond surfaces, the etched pits' morphologies on the {111} surface are an inverted triangular prism and those on the {100} surface are an inverted pyramid. The Ra value of {100} plane increases from 0.40 μm to 2.28 μm, whereas that on {111} one increases from 0.70 μm to 3.32 μm. Furthermore, the mechanism of the diamond surfaces etched by La2O3 is the graphitization process of the diamond.

Surface Tamm States of 2-5 nm Nanodiamond via Raman Spectroscopy.

We observed resonance effects in the Raman scattering of nanodiamonds with an average size of 2-5 nm excited at a wavelength of 1064 nm (1.16 eV). The resonant Raman spectrum of the 2-5 nm nanodiamonds consists of bands at wavelengths of 1325 and 1600 cm-1, a band at 1100-1250 cm-1, and a plateau in the range from 1420 to 1630 cm-1. When excited away from the resonance (at a wavelength of 405 nm, 3.1 eV), the Raman spectrum consists of only three bands at 1325, 1500, and 1600 cm-1. It is important to note that the additional lines (1500 and 1600 cm-1) belong to the sp3-hybridized carbon bonds. The phonon density of states for the nanodiamonds (~1 nm) was calculated using moment tensor potentials (MTP), a class of machine-learning interatomic potentials. The presence of these modes in agreement with the lattice dynamics indicates the existence of bonds with force constants higher than in single-crystal diamonds. The observed resonant phenomena of the Raman scattering and the increase in the bulk modulus are explained by the presence of Tamm states with an energy of electronic transitions of approximately 1 eV, previously observed on the surface of single-crystal diamonds.

Atomic Imaging of Interface Defects in an Insulating Film on Diamond.

The insulator/semiconductor interface structure is the key to electric device performance, and much interest has been focused on understanding the origin of interfacial defects. However, with conventional techniques, it is difficult to analyze the interfacial atomic structure buried in the insulating film. Here, we reveal the atomic structure at the interface between an amorphous aluminum oxide and diamond using a developed electron energy analyzer for photoelectron holography. We find that the three-dimensional atomic structure of a C-O-Al-O-C bridge between two dimer rows of the hydrogen-terminated diamond surface. Our results demonstrate that photoelectron holography can be used to reveal the three-dimensional atomic structure of the interface between a crystal and an amorphous film. We also find that the photoelectron intensity originating from the C-O bonds is strongly related to the interfacial defect density. We anticipate significant progress in the study of amorphous/crystalline interfaces based on their three-dimensional atomic structures analysis.

Effect of Chlorine Salts on TiC Coating Morphology and Bonding Strength of Diamond Surface in Molten Salt Reaction

To obtain a high-quality TiC coating on the diamond (D) surface and improve the interfacial properties of metal matrix composites, the optimized molten salt method is used to prepare TiC coatings on the D surface. The effects of NaCl, KCl, and NaCl–KCl molten salts on the uniformity, surface morphology, composition, and bonding ability of TiC coatings are investigated by scanning electron microscopy, energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction, and cold–hot cycling experiments. The results show that the boiling and pickling processes after the molten salt reaction can effectively remove the residual chloride salts and Ti powders to obtain clean TiC-coated D. The TiC particles of the coating surfaces prepared by NaCl and KCl molten salts are finer and more uniform, which is due to their higher melting points and shorter real reaction time compared with NaCl–KCl mixed salts. The TiC coating prepared by KCl has the best bonding ability and can withstand 50 times as many cold–hot cycles due to its uniform fine deposition particles and appropriate thickness. And, during cold–hot cycles, it can be found that the TiC coating cracking and shedding occur primarily at the edges and corners of the D with internal thermal stress concentration.

Modeling of Radiative Emission from Shallow Color Centers in Single Crystalline Diamond

AbstractOptically active defects in diamond are widely used as bright single‐photon sources for quantum sensing, computing, and communication. For many applications, it is useful to place the emitter close to the diamond surface, where the radiative properties of the emitter are strongly modified by its dielectric environment. It is well‐known that the radiative power from an electric dipole decreases as the emitter approaches an interface with a lower‐index dielectric, leading to an increase in the radiative lifetime. For emitters in crystalline solids, modeling of this effect needs to take into account the crystal orientation and direction of the surface cut, which can greatly impact the emission characteristics. In this paper, a framework for analyzing the emission rates of shallow (<100 nm) defects is provided, in which optical transitions are derived from electric dipoles in a plane perpendicular to their spin axis. The calculations for the depth‐dependent radiative lifetime for color centers in (100)‐, (110)‐, and (111)‐cut diamond are presented, which can be extended to other vacancy defects in diamond.

The Gemstone Cyborg: How Diamond Films Are Creating New Platforms for Cell Regeneration and Biointerfacing

Diamond is a promising material for the biomedical field, mainly due to its set of characteristics such as biocompatibility, strength, and electrical conductivity. Diamond can be synthesised in the laboratory by different methods, is available in the form of plates or films deposited on foreign substrates, and its morphology varies from microcrystalline diamond to ultrananocrystalline diamond. In this review, we summarise some of the most relevant studies regarding the adhesion of cells onto diamond surfaces, the consequent cell growth, and, in some very interesting cases, the differentiation of cells into neurons and oligodendrocytes. We discuss how different morphologies can affect cell adhesion and how surface termination can influence the surface hydrophilicity and consequent attachment of adherent proteins. At the end of the review, we present a brief perspective on how the results from cell adhesion and biocompatibility can make way for the use of diamond as biointerface.

Surface-Dependent Adhesion Properties of Graphene on Diamonds for the Fabrication of Nanodevices: A Molecular Dynamics Investigation

The physical performance of a heterostructure is strongly influenced by the adhesion properties. The study of adhesion properties between graphene and diamond lattice is an inescapable issue in the development of diamond-based graphene multifunctional nanodevices. Herein, a series of adhesion intensities have been theoretically examined, and the optimum facet of diamond and theoretically recommended orientation angle were obtained, respectively. Moreover, the atomistic peeling behavior and the effect of three typical graphene topological defects on adhesion intensity were explored. The study demonstrated that the presence of double-vacancy defects impairs the adhesion strength due to the reduction of the contact area. In contrast, the presence of Stone-Wales defects is conducive to enhancing the adhesion strength. Interestingly, the effect of single-vacancy defects mainly depends on the delicate competition between single-atom removal and single-atom attraction enhancement. Meanwhile, the effects of diamond surface morphology on graphene adhesion were systematically elaborated by the modeling of one-dimensional and two-dimensional surfaces, and randomly rough surfaces. The adhesion details of graphene on regularly tunable diamonds were explored, and the relations of adhesion intensity and graphene morphology with the random roughness of a diamond surface were further revealed in depth. Since the mechanical and electrical performance of a graphene–diamond heterostructure is sensitively influenced by the adhesion intensity, our findings provide insight into the substrate design of graphene–diamond hybrid devices.

Interaction between diamond and alkali metals and its application to machining of diamond

AbstractThe interaction between diamond surfaces and alkali metals was studied experimentally and computationally. Li reacted with diamond above 600°C and significant etching of (100) surface was observed. An attempt was made to apply this etching reaction for electrical discharge machining of diamond tools.

Probing itinerant carrier dynamics at the diamond surface using single nitrogen vacancy centers

Color centers in diamond are widely explored for applications in quantum sensing, computing, and networking. Their optical, spin, and charge properties have extensively been studied, while their interactions with itinerant carriers are relatively unexplored. Here, we show that NV centers situated 10 ± 5 nm of the diamond surface can be converted to the neutral charge state via hole capture. By measuring the hole capture rate, we extract the capture cross section, which is suppressed by proximity to the diamond surface. The distance dependence is consistent with a carrier diffusion model, indicating that the itinerant carrier lifetime can be long, even at the diamond surface. Measuring dynamics of near-surface NV centers offers a tool for characterizing the diamond surface and investigating charge transport in diamond devices.

Secondary electron emission and vacuum electronics

Secondary electron emission serves as the foundation for a broad range of vacuum electronic devices and instrumentation, from particle detectors and multipliers to high-power amplifiers. While secondary yields of at least 3–4 are required in practical applications, the emitter stability can be compromised by surface dynamics during operation. As a result, the range of practical emitter materials is limited. The development of new emitter materials with high yield and robust operation would advance the state-of-the-art and enable new device concepts and applications. In this Perspective article, I first present an analysis of the secondary emission process, with an emphasis on the influence of material properties. From this analysis, ultra-wide bandgap (UWBG) semiconductors and oxides emerge as superior emitter candidates owing to exceptional surface and transport properties that enable a very high yield of low-energy electrons with narrow energy spread. Importantly, exciting advances are being made in the development of promising UWBG semiconductors such as diamond, cubic boron nitride (c-BN), and aluminum nitride (AlN), as well as UWBG oxides with improved conductivity and crystallinity. These advances are enabled by epitaxial growth techniques that provide control over the electronic properties critical to secondary electron emission, while advanced theoretical tools provide guidance to optimize these properties. Presently, H-terminated diamond offers the greatest opportunity because of its thermally stable negative electron affinity (NEA). In fact, an electron amplifier under development exploits the high yield from this NEA surface, while more robust NEA diamond surfaces are demonstrated with potential for high yields in a range of device applications. Although c-BN and AlN are less mature, they provide opportunities to design novel heterostructures that can enhance the yield further.

Phenomenon of photo-regulation on gold/diamond Schottky barriers and its detector applications

A thickness asymmetric electrode structure on an oxygen-terminated type IIa diamond was designed and prepared (one electrode was semitransparent to ultraviolet light and the other blocked the transmission of ultraviolet light). This structure exhibited an apparent photo-induced rectification property under irradiation by a deuterium lamp. This is attributed to the mechanism by which the light penetrating the electrode reduces the metal–diamond contact barrier. Furthermore, we developed a light-modulated Schottky barrier diamond photodetector based on this mechanism. Solar-blind light can lower the Schottky barrier height in situ in the presence of light, which significantly enhances the photocurrent. However, the Schottky barrier is not reduced by light regulation when there is no light; therefore, the low dark current of the detector is still guaranteed. Compared with the non-photo-regulated Schottky barrier detector, the photo-regulated Schottky barrier detector exhibits a 128% increase in responsivity at 220 nm under a 1.6 V/μm bias. For such an obvious difference in detection performance, this mechanism has rarely been a focus of studies on diamond detectors. In addition to diamond detectors, light-modulated barrier technology can also be applied to other fields related to the diamond surface potential, such as color center control and Schottky diodes; it can also be used to control or evaluate device performance.

Manufacturing of Metal-Diamond Composites with High-Strength CoCrCuxFeNi High-Entropy Alloy Used as a Binder.

This paper focuses on the study of the structure and mechanical properties of CoCrCuxFeNi high-entropy alloys and their adhesion to single diamond crystals. CoCrCuxFeNi alloys were manufactured by the powder metallurgy route, specifically via mechanical alloying of elemental powders, followed by hot pressing. The addition of copper led to the formation of a dual-phase FCC + FCC2 structure. The CoCrCu0.5FeNi alloy exhibited the highest ultimate tensile strength (1080 MPa). Reductions in the ductility of the CoCrCuxFeNi HEAs and the tendency for brittle fracture behavior were observed at high copper concentrations. The equiatomic alloys CoCrFeNi and CoCrCuFeNi demonstrated high adhesion strength to single diamond crystals. The diamond surface at the fracture of the composites having the CoCrFeNi matrix had chromium-rich metal matrix regions, thus indicating that chromium carbide, responsible for adhesion, was formed at the composite-diamond interface. Copper-rich areas were detected on the diamond surface within the composites having the CoCrCuFeNi matrix due to the predominant precipitation of the FCC2 phase at the interfaces or the crack propagation along the FCC/FCC2 interface, resulting in the exposure of the Cu-rich FCC2 phase on the surface.

Interface formation evolution of the hot-forged copper-(Cr)diamond composite and its thermal conductivity

Due to the distinction of (111) and (100) faces on the diamond particle, the preferential formation of interface layers on the diamond particle is discovered when adding metal additives like titanium, chromium, aluminum in copper/diamond composites. The formed interface would affect the thermal conductivity of the copper/diamond composites. In this study, copper-55 vol%(Cr-coated) diamond composites were successfully produced by hot forging under 800 ℃, 900 ℃ and 1050 ℃. Heat treatment of Cr-coated diamond powders was conducted under a series of temperatures for studying the evolution of the carbide interface formation on surfaces of diamond particles. It was found that the average grain size of the Cr7C3 on diamond (111) faces was larger than that on (100) faces when the temperature is below 850 ℃, and the opposite trend was observed when the heat treatment temperature is above 850 ℃. The Cr layer was fully transformed into Cr7C3 when the temperature was above 950 ℃. Unlike the results obtained after heat treatment, the interface layers was directly transformed into Cr3C2 due to the non-equilibrium phase transformation after hot forging. The Cr3C2 interface layers were preferentially kept on (100) diamond faces, and exfoliated on (111) faces after hot forging. The grain size of the formed Cr3C2 on diamond (100) faces was increased with raising the hot forging temperature. The 1050 ℃-hot-forged sample showed the highest thermal conductivity, attributed to large grain sizes of the Cr3C2 interface layer. This study elaborates the mechanisms of the evolution of the interface layer formed under different temperatures on the diamond surfaces in copper-(Cr-coated) diamond composites, and the phenomenon of preferential formation of the interface layers is verified.

Enhancing the thermal boundary conductance of Cu/Diamond interface via diamond surface amorphization by molecular dynamics simulation

Based on the non-equilibrium molecular dynamics simulation, a Cu/amorphous diamond/crystalline diamond sandwich model was established to investigate the effects of the amorphous degree diamond surface and the thickness of the amorphous layer on the thermal boundary conductance of Cu/crystalline diamond. The simulation results show that the thermal boundary conductance can be enhanced by diamond surface amorphization, and increases with the increase of the amorphous degree. For the fully amorphous layer, the thermal boundary conductance increases gradually with the increase of the thickness of the amorphous layer and can be enhanced up to 4 times. The analysis of the vibrational density of states, overlap energy and phonon participation ratio shows that the diamond surface amorphization promotes the vibrational coupling between diamond and Cu atoms at low frequencies, as well as the occurrence of phonon inelastic scattering, and thus improves the thermal transport capacity of interface.

Persistent spin helix on a diamond surface

Using first-principles calculations, we determine the impact of hydroxyl (OH) termination on the spin–orbit coupling (SOC) coefficient and spin texture of diamond surfaces. The persistent spin helix (PSH) state is revealed in the OH-terminated diamond (111) surface. The value of coefficient PSH α PSH was 14.2 meV·Å, which is larger than that of the zinc-blende n-type quantum well of GaAs/AlGaAs. The large α PSH is caused by the asymmetric charge density distribution near the surface rather than the size of the atomic SOC. The large α PSH will ensure the wavelength of PSH (λ PSH ), which will aid in the miniaturization of spintronic devices.

Capillary adhesion governs the friction behavior of electrochemically corroded polycrystalline diamond

The friction behavior of rough polycrystalline diamond (PCD) surfaces is important in many applications and devices that are required to operate in various harsh environments and it can be argued that a thorough understanding of the friction behavior is essential to the application performance. However, the interplay between electrochemical corrosion, capillary adhesion and friction behavior of PCD in multi-asperity contacts is still poorly understood. In this work, we quantify the interfacial capillary adhesion at contact interfaces and its effect on the friction response, between a colloidal microsphere and rough PCD films before and after electrochemical corrosion, for water-immersed, low RH, and humid air conditions. For these multi-asperity contacts, we demonstrate how electrochemical corrosion influences the surface hydrophilicity of the PCD surfaces, and how capillary adhesion due to water condensation contributes to the friction force. We estimate the capillary forces from both the microscopic lateral force experiments and elastoplastic boundary element method (BEM) contact calculations. The combined results indicate strongly that the observed increase in friction force on electrochemically-corroded PCD surfaces is governed by enhanced capillary adhesion at the contact interface, as affected by surface hydrophilicity and environmental humidity.