Nitrogen-doped Diamond Research Articles

Growth of polyoxomolybdate with a porous pyramidal structure on carbon xerogel nanodiamond as an efficient electro-catalyst for oxygen reduction reaction.

The slow kinetics of the oxygen reduction reaction (ORR) limits the large-scale usage of the fuel cells. Thus, it is crucial to develop an efficient and stable electrocatalyst for the ORR. Herein, facile synthesis of three-dimensional nitrogen-doped carbon xerogel diamond nanoparticles, CDNPs support is reported. The as-prepared CDNPs support was functionalized with a Keggin-type polyoxomolybdate via the hydrothermal process (POM@CDNPs). As the characterization techniques revealed, this nanocomposite possesses a three-dimensional structure, high density of nitrogen doping, and well-dispersed porous pyramidal morphology of POM, making it a promising catalyst for ORR in alkaline medium. The POM@CDNPs nanocomposite exhibits an outstanding activity for ORR with a limiting current density that reaches -7.30 mA cm-2 at 0.17 V vs. RHE. Moreover, a half-wave potential of 0.773 V is delivered with a stability of about 99.9% after the 100th repetitive cycle as this catalyst forces the ORR to the direct-four-electron pathway. This work spots the advantages of hybridizing the sp3 of the nanodiamond with the sp2 of the carbon xerogels to increase the conductivity of the support material. In addition, the role of the porous pyramidal morphology of the POM on the activity of the nanocomposite was evaluated. This study suggests using advanced carbon-based electro-catalysts with outstanding activity and stability.

Research of a weak negative thermal quenching effect of nitrogen vacancy centers in nitrogen-doped diamond

A weak negative thermal quenching (NTQ) effect of nitrogen vacancy centers in nitrogen-doped diamond is investigated by low-temperature photoluminescence technology. Results show that a weak NTQ phenomenon of the NV− centers was observed in the range of 10–40 K. Aiming to explain the observed NTQ effect, three hypotheses are proposed to classify the underlying physical mechanism, including (a) electron transfer from nitrogen atom to NV centers, (b) electron transfer between NV0 and NV− centers, and (c) the shallow traps of charge carriers in diamond. What is more, with incorporating sulfur, sulfur-nitrogen complex defects are formed with an impurity level of 0.65 eV below the conduction band. The activation energies of nonradiative recombination of NV− centers were obviously decreased with doping sulfur, which maybe related to the substitutional nitrogen (deep donor) replaced by sulfur-nitrogen complex structure defect (relatively shallow donor). The presented work provides a comprehensive understanding on the physical mechanism of NTQ effect of nitrogen vacancy centers in diamond, which would provide guidance for application of NV centers, especially in low temperatures.

Nitrogen vacancy luminescence and their phonon sidebands characteristics in nitrogen-doped diamond

In this work, the nitrogen vacancy (NV) luminescence properties and their phonon sidebands in both high and low nitrogen diamonds after electron irradiation and subsequent annealing were thoroughly investigated. From the extracted results, it was demonstrated that the phonon sidebands were caused by the phonon emission behavior of the electrons located in the NV center during the induced transition process from the laser excitation. On top of that, the phonon energies of the NV0 (neutral) and NV— (negatively charged) centers were estimated at about 45 meV and 63 meV, respectively. As the measurement temperature increased, the excited electrons were more likely to return to the ground state by employing non-radiative transitions without generating phonon sidebands. As a result, the synchronous quenching of the NV luminescence and their phonon sidebands effect takes place. The complex lattice environment of the NV centers in the high nitrogen diamond leads also to a smaller thermal quenching activation energy, while the phonon process at a relatively high measurement temperature results in the manifestation of more obvious phonon sidebands of the NV centers.

Nitrogen-doping effect on single-crystal diamond synthesis by HFCVD

Nitrogen-doped homoepitaxial single-crystal diamond (SCD) films were grown on an SCD substrate by hot filament chemical vapor deposition (HFCVD) to confirm the potential that can yield high-quality single-crystal diamonds. The SCD films doped with 50 sccm nitrogen, grown by HFCVD, showed excellent surface properties based on optical microscopy, with a growth rate of 3.4 [Formula: see text]m/h, and a Raman peak around 1332 cm[Formula: see text] with a full width at maximum (FWHM) of 5.8 cm[Formula: see text], indicating high crystallinity. The FWHM of the (004) peak in the rocking curve was 392 arcsec for the nitrogen-doped SCD film, and that of the SCD substrate was 489 arcsec. Therefore, the crystalline properties and growth rates of the SCD films grown in the HFCVD system were improved, and the effect of nitrogen doping was verified.

Diamond Structures for Tuning of the Finesse Coefficient of Photonic Devices.

Finesse coefficient is one of the most important parameters describing the properties of a resonant cavity. In this research, a mathematical investigation of the application of diamond structures in a fiber-optic Fabry–Perot measurement head to assess their impact on the finesse coefficient is proposed. We present modeled transmission functions of cavities utilizing a nitrogen-doped diamond, a boron-doped diamond, nanocrystalline diamond sheet and a silver mirror. The diamond structures were deposited using a microwave plasma-assisted chemical vapor deposition system. A SEM investigation of surface morphology was conducted. The modeling took into consideration the fiber-optic Fabry–Perot setup working in a reflective mode, with an external cavity and a light source of 1550 nm. A comparison of the mathematical investigation and experimental results is presented.

Towards optical neuromodulation using nitrogen-doped ultrananocrystalline diamond photoelectrodes

Nitrogen-doped ultrananocrystalline diamond (N-UNCD) is a form of diamond electrode with near-infrared photoresponsivity, making it well suited for physiological applications. N-UNCD's photoresponsivity is strongly influenced by its surface. While it is known that oxygen treatment provides a higher photoresponsivity, a better understanding of its surface processes is needed to tailor the material for optical neuromodulation. This work examines the impact of various oxygen treatment methods, with aim of creating oxygen rich surfaces with different chemical and structural properties. Surface characterisation methods along with electrochemical and photoelectrochemical measurements and modelling were used to investigate the films. It was found that oxygen furnace annealing resulted in orders of magnitude improvement in the near-infrared photoresponsivity, to 3.75 ± 0.05 µA/W. This translates to an approximate 200 times increase in the photocurrent compared to the untreated surface. This enhancement in photocurrent is largely due to the changes in the chemical species present at the surface. The photocurrent is estimated to be sufficient for extra-cellular stimulation of brain neurons within the safe optical exposure limit, positioning N-UNCD as an excellent candidate to be used in next-generation photoelectrodes for photobiomodulation.

Zero-biased and visible-blind UV photodetectors based on nitrogen-doped ultrananocrystalline diamond nanowires

Taking advantage of the superflat surface of ultrananocrystalline diamond (UNCD), highly precise UNCD nanowire (NW) arrays were fabricated to develop high-performance UV photodetectors. The large surface-to-volume ratio of a nanowire significantly increases the number of surface trap states, and the reduced dimensionality effectively confines the active area of the charge carrier and shortens the transit time, which results in an enhanced photoconductivity and response speed. In this paper, the zero-biased UV photodetectors based on nitrogen-incorporated ultrananocrystalline diamond nanowire arrays have been demonstrated and characterized. The estimated responsivity was 2.0 A/W for 350 nm incident light when the device operated at room temperature. The UVA and UVB photocurrent signals from this visible blind photodetector were well defined with a rise and decay time of less than 1 s.

Efficiency of Photoconductive Terahertz Generation in Nitrogen-Doped Diamonds

The efficiency of the generation of terahertz radiation from nitrogen-doped (∼0.1–100 ppm) diamonds was investigated. The synthetic polycrystalline and monocrystalline diamond substrates were pumped by a 400 nm femtosecond laser and tested for the photoconductive emitter operation. The dependency of the emitted THz power on the intensity of the optical excitation was measured. The nitrogen concentrations of the diamonds involved were measured from the optical absorbance, which was found to crucially depend on the synthesis technique. The observed correlation between the doping level and the level of the performance of diamond-based antennas demonstrates the prospects of doped diamond as a material for highly efficient large-aperture photoconductive antennas.

Photoconductive terahertz generation in nitrogen-doped single-crystal diamond.

The generation of terahertz radiation in a photoconductive emitter based on nitrogen-doped single-crystal diamond was realized for the first time. Under 400 nm femtosecond laser pumping, the performance of diamond antennas with different dopant levels was investigated and compared with a reference ZnSe antenna. Terahertz waveforms and corresponding spectra were measured. A low saturation level for high-nitrogen-containing diamond substrate was revealed. The results indicate the prospects of doped diamond as a material for high-efficiency large-aperture photoconductive antennas.

Electroluminescence of NV by impact excitation and Stark shift in a MIM diamond structure

Electrically driven emission of nitrogen-vacancy (NV) color centers in diamond has recently emerged as a viable candidate for quantum photonics. However, it is challenging to efficiently excite NV centers by an electric field because of the low density of free carriers in doped diamond. Here, we report efficient electroluminescence of neutrally charged nitrogen-vacancy (NV0) color centers under a high electric field (105 V/cm) and study its Stark effect in a MIM (Metal–Insulator–Metal) diamond device. The MIM device was fabricated by electron beam evaporating molybdenum electrodes and microwave plasma chemical vapor depositing a nitrogen-doped diamond epilayer successively on a single-crystal diamond substrate. We demonstrate the field dependence of the electrically driven emission and extract the NV0 ground state electric field susceptibility of 15.83 ± 0.86 GHzmMV−1 from the Stark effect at 5.5 K. Our findings provide a perspective toward electrically exciting color centers in diamond and help one to understand the inherent features of NV0.

Fabrication of inversion p-channel MOSFET with a nitrogen-doped diamond body

A normally-off inversion p-channel metal-oxide-semiconductor field-effect transistor (MOSFET) with a nitrogen (N)-doped diamond body deposited using microwave plasma-enhanced chemical vapor deposition (MPECVD) was fabricated. The MOSFET exhibited a drain current density of −1.7 mA/mm. Thus far, this value is similar to the device performance of the inversion p-channel MOSFET fabricated using a phosphorus (P)-doped n-type diamond body. The N2 used for N-doping is safer than the PH3 used for P-doping; moreover, the doping concentration is highly controllable. Because the MOSFET, which is a classical electronic device, is driven by a gate voltage, smooth functioning was possible even at a deep donor level. The observed characteristics of the classic MOSFET operating via an N-doped body are crucial for the development of diamond power devices. In this paper, we discuss the significance of the N-doped diamond body and electrical characteristics of the inversion p-channel MOSFET fabricated using an N-doped diamond body.

Broadband Nonlinear Optical Response of Nitrogen-Doped Diamond

A nitrogen-doped diamond crystal with (111) orientation was synthesized with an NaN3 additive in the FeNi-C system at a pressure of 6.5 GPa and a temperature of 1,310°C, using the temperature gradient growth (TGG) method. Spectroscopic properties such as the absorption spectrum and the Raman spectrum as well as the Fourier transform infrared (FTIR) spectrum were studied. FTIR spectroscopy of the C-N vibrational modes at 1,344 and 1,130 cm−1 suggested a nitrogen content of 310 ppm. Its nonlinear optical (NLO) response was investigated using the Z-scan technique under the femtosecond regime. Due to the presence of nitrogen defects, the synthesized crystal performed large nonlinear absorption under both 800- and 532-nm wavelength excitations. However, intrinsic diamond only experiences nonlinear refraction under these two wavelength excitations. Its broadband NLO properties indicated that nitrogen-doped diamond crystals were suitable for the application of ultrafast optical devices.

A monolithic, back-gated diamond field-effect transistor for tunable color centers

We present an innovative design for a monolithic field effect transistor, where all components consist of the wide-bandgap material diamond. The back gate-electrode is realized by a buried, degenerately boron-doped diamond (resistivity < 10−2 Ω cm), while the dielectric material is made of lightly nitrogen-doped diamond. The 2DHG on the hydrogen-terminated surface serves as the conductive channel of the transistor. We discuss the band structure of this device, the function of each individual component and show the sample preparation routine. Furthermore, we investigate the electrical tunability of the 2DHG and the optical tunability of NV-centers in a first proof-of-principle sample. Additionally, we use the field effect to manipulate the charge state of color centers in the nitrogen-doped film. This vertical and monolithic device structure opens up a range of applications, not only in the diamond semiconductor and quantum information technology, but also for sensing applications where the back-gating is advantageous or where an all-diamond layer sequence is beneficial.

Size-Dependent Electrical Transport Properties in Conducting Diamond Nanostripes.

With the advances in nanofabrication technology, horizontally aligned and well-defined nitrogen-doped ultrananocrystalline diamond nanostripes can be fabricated with widths in the order of tens of nanometers. The study of the size-dependent electron transport properties of these nanostructures is crucial to novel electronic and electrochemical applications. In this paper, 100 nm thick n-type ultrananocrystalline diamond thin films were synthesized by microwave plasma-enhanced chemical vapor deposition method with 5% N2 gas in the plasma during the growth process. Then the nanostripes were fabricated using standard electron beam lithography and reactive ion etching techniques. The electrical transport properties of the free-standing single nanostripes of different widths from 75 to 150 nm and lengths from 1 to 128 μm were investigated. The study showed that the electrical resistivity of the n-type ultrananocrystalline diamond nanostripes increased dramatically with the decrease in the nanostripe width. The nanostripe resistivity was nearly doubted when the width was reduced from 150 nm to 75 nm. The size-dependent variability in conductivity could originate from the imposed diffusive scattering of the nanostripe surfaces which had a further compounding effect to reinforce the grain boundary scattering.

Characterisation of CVD diamond with high concentrations of nitrogen for magnetic-field sensing applications

Ensembles of nitrogen-vacancy (NV) centres in diamond are a leading platform for practical quantum sensors. Reproducible and scalable fabrication of NV-ensembles with desired properties is crucial, as is an understanding of how those properties influence performance. This work addresses these issues by characterising nitrogen-doped diamond produced by the chemical vapour deposition (CVD) method across a range of synthesis conditions. This is shown to produce material with widely differing absorption characteristics, which is linked to the level of defects other than substitutional nitrogen (NS) and NV. In such material, the achievable concentration of NV− ([NV−]) is found to be influenced by the as-grown properties. At the 10–20 ppm level for [NS], the production of CVD-grown material with strain levels sufficient not to limit achievable device sensitivity is demonstrated and a favourable product of [NV−] and is obtained. Additionally, reproducible properties over a batch of 23 samples from a single synthesis run are achieved, which appears promising for the scalability efforts underway in this area of research.

First principles study of structural, elastic, mechanical and electronic properties of nitrogen-doped cubic diamond

We report the structural, elastic, mechanical and electronic properties of nitrogen (N)-doped cubic diamond up to 25%N doping concentrations in the steps of 5%N dopant. Our calculations were performed with the generalized gradient approximation functional of density functional theory with the Perdew–Burke–Ernzerhof exchange–correlation energy through virtual crystal approximation. Cubic diamond shows a structural stability up to 15%N doping and it becomes instable above this dopant concentration. The changes for the typical cubic elastic constants, bulk, shear and Young’s moduli, Poisson ratio, anisotropy, Pugh ratio, Kleinman parameter and electronic band structures of cubic diamond vs. applied doping percentages were also evaluated. The obtained results for these parameters were found to be strictly dependent on the dopant concentration. Although cubic diamond is a well-known insulator, it displays a metallic character even under the doping of 5%N and keeps this trend for higher doping concentrations.

Theoretical investigation of superconductivity in diamond: Effects of doping and pressure

The electronic structure, lattice dynamics, and electron–phonon coupling of pure, boron and nitrogen-doped diamond carbon were investigated using first-principle calculations within the generalized-gradient and virtual crystal approximations. To examine the influence of the impurity content and pressure on the superconductivity of these systems, the electron–phonon coupling constant (λ) and the critical temperature (Tc) were calculated as a function of concentrations from 0 to 15% and pressures from 0 to 90 GPa. Regarding the boron-doped diamond, calculations indicated that its electron–phonon coupling strongly relates to the optical phonon modes, and the estimated critical temperatures matched previous theoretical and experimental results. Regarding the nitrogen-doped case, it was observed that both λ and Tc were larger than those obtained for the hole-doped case. The most distinguishing feature of this system was its rising acoustic contribution to the electron–phonon coupling, which led to significant values for λ and Tc. The majority of the scenarios investigated here presented a decreasing critical temperature with increasing pressure. In contrast to the other cases, C0.85N0.15 exhibited a positive dependence between Tc and pressure leading to a superconducting transition temperature of about 122 K at 20 GPa.

Thermoluminescence properties of high-dose gamma-irradiated diamond films

Thermoluminescence (TL) properties of gamma-irradiated chemical vapor deposition (CVD) diamond films at high doses are presented. Samples of undoped and nitrogen-doped CVD diamonds were exposed to 60Co gamma sources and TL intensity as a function of the dose was analyzed. To evaluate the feasibility of using these materials as TL high-dose dosimeters, reproducibility of the TL signal, and the TL response as a function of absorbed dose from 100 Gy up to 10 kGy were studied. Furthermore, the fading of the TL glow curve was analyzed. Finally, to estimate the kinetics parameter values associated with the trap distribution in the CVD diamond, a computerized deconvolution was carried out by using the general order kinetics (GOK) model.

Towards high-quality nitrogen-doped diamond single crystals for X-ray optics.

In this manuscript, characterization of single-crystalline (111) plates prepared from type-Ib diamonds with a nitrogen content of 100-150 ppm by means of high-resolution rocking-curve imaging (RCI) is reported. Contrary to common opinion regarding the intrinsically poor diffraction quality of type-I diamonds, RCI showed the presence of nearly defect-free areas of several millimetres squared in the central part of the diamond plates. The observed broadening of the rocking curves is a result of the cutting and polishing processes, causing strains around the edges of the plates and rare defects. An improvement of the preparation technique will thus allow single-crystalline diamond plates to be made for Laue and Bragg monochromators and beam splitters from type-Ib material with areas large enough to be used as optical elements at fourth-generation synchrotron facilities.

Enhanced electrochemical capacitance of nitrogen-doped ultrananocrystalline diamond through oxygen treatment

The electrochemical capacitance of nitrogen-doped ultrananocrystalline diamond (N-UNCD) can be dramatically increased by treating the surface with an RF-oxygen plasma. Such treated surfaces display excellent properties for use as electrodes in neural stimulation and recording. In the present work, we elucidate the origins of this phenomenon by investigating the effects of different methods of oxygen termination. We found that the increase in electrochemical capacitance is dependent on the details of the method used for oxygen termination. Whilst N-UNCD subjected to UV/ozone treatment, oxygen plasma treatment, and furnace annealing in oxygen gas all displayed increased surface capacitance, the highest capacitance was exhibited by the oxygen annealed sample, with which we achieved ~ 3 orders of magnitude increase in the electrochemical capacitance as compared to the as-grown sample. The maximum recorded capacitance was 3746 ± 132 µF cm−2, which is substantially greater than previously reported N-UNCD electrodes’ electrochemical capacitance (1070 µF cm−2, W. Tong. et al, 2016). Our findings point to the presence of sub-surface solid state capacitance which contributes significantly to the observed electrochemical capacitance of the oxygen terminated N-UNCD electrodes. When combined with the favourable biocompatibility and inertness of the N-UNCD, our approach may provide a route towards the development of advanced neural sensing and stimulating electrodes.