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We demonstrated and characterized Schottky barrier diodes (SBDs) fabricated on heteroepitaxial diamond films grown onto 3C-SiC/Si substrates. SBDs showed clear diode properties with rectification ratios above 109 at ±5 V and maintained above 108 even at 500 K. Temperature dependence of the Schottky barrier height (SBH) and the ideality factor was explained by assuming inhomogeneous Schottky barriers following the Gaussian distribution. For leakage current analysis, two types of defects, linear pits (LPs) and nonepitaxial crystals (NCs), were shown to be fatal defects, causing the flow of leakage currents. The leakage current was found to be an exponential function of the length of the LPs and a linear function of the size of the NCs. These defects can be suppressed by highly oriented heteroepitaxial diamond films with reduction of tilt and twist spread for diamond nuclei and optimized growth condition. Moreover, intrinsic layer thickening and inserting a buffer layer stopping propagation of dislocations are also effective. By reducing both crystal defects, we can obtain device properties comparable to SBDs fabricated on homoepitaxial diamond films because the leakage currents can be suppressed.

Thick CVD diamond film with perfectly aligned nitrogen-vacancy (NV) centers leads to highly sensitive quantum sensors. A misorientation angle (θ mis) of the (111) substrate is a key to determine the film thickness. We investigated the dependence of the growth rate and NV center properties on θ mis(0.4°–9.9°). The growth rate increased from 1.8 to 4.4 μm h−1 and the NV yield (=[NV centers]/[Nitrogen]) increased from 0.4% to 1.1% by increasing θ mis. Finally, a diamond film of thickness ≈120 μm with aligned NV centers was obtained with an area of 300 μm × 400 μm using θ mis of 9.9°.

The NV center in a diamond is a quantum sensor with exceptional quality for highly sensitive nanoscale analysis of NMR spectra and thermometry. In this study, we investigate nanoscale phase change detection of lipid bilayers utilizing ensemble-averaged nuclear spin detection from small volume ~ (6 nm)$^{3}$, which was determined by the depth of the NV center. Analysis of nanoscale NMR signal confirm thickness of lipid bilayer to be 6.2 nm $\pm$ 3.4 nm with proton density of 65 proton/nm$^{3}$ verifying formation of lipid bilayer on top of diamond sample. Correlation spectroscopy from nanoscale volume reveals quantum oscillation at 3.06 MHz corresponding to the Larmor frequency of proton at an applied magnetic field of 71.8 mT. The result of the correlation spectroscopy was compared with the 2D molecular diffusion model constructed by Monte Carlo simulation combined with results from molecular dynamics simulation. There is a change in diffusion constant from 1.5 $\pm$ 0.25 nm$^{2}$/${\mu}$s to 3.0 $\pm$ 0.5 nm$^{2}$/${\mu}$s when the temperature changes from 26.5 $^\circ$C to 36.0 $^\circ$C. Our results demonstrate that simultaneous observation of changes in translational diffusion and temperature is possible in label-free measurements using nanoscale diamond magnetometry. Our method paves the way for label-free imaging of cell membranes for understanding its phase composition and dynamics.

Negatively charged nitrogen-vacancy (NV) centers in diamond have emerged as promising candidates for a wide range of quantum applications, especially quantum sensing of magnetic field. Implementation of nanostructure into diamond is powerful for efficient photon collection of NV centers and chip-scale miniaturization of the device, which is crucial for sensitive and practical diamond magnetometers. However, fabrication of the diamond nanostructure involves technical limitations and can degrade the spin coherence of the NV centers. In this study, we demonstrate the hybrid integration of a silicon nitride grating structure on a single-crystal diamond by utilizing transfer printing. This approach allows the implementation of the nanostructure in diamond using a simple pick-and-place assembly, facilitating diamond-based quantum applications without any complicated diamond nanofabrication. We observed the intensity enhancement in the collected NV emissions both theoretically and experimentally using the integrated grating structure. By applying the increased photon intensity, we demonstrate the improved magnetic sensitivity of the fabricated device. The proposed hybrid integration approach will offer a promising route toward a compact and sensitive diamond NV-based magnetometer.

We report the optical observation of lead-vacancy (PbV) centers in diamond fabricated by Pb ion implantation and subsequent high-temperature annealing (2100 {\deg}C) under high pressure (7.7 GPa). Their optical properties were characterized by photoluminescence at varying temperatures down to 5.7 K. We observed intense emission peaks at 550 and 554 nm with a large splitting of approximately 3900 GHz. The two lines are thought to correspond to the zero phonon line (ZPL) of PbV centers with split ground and excited states. A cubic trend of the ZPL width was observed while varying temperature. We performed polarization measurements of the two lines in a single PbV center, showing nearly orthogonal dipole polarizations. These optical measurements strongly indicate that the PbV center possesses D3d symmetry in the diamond lattice. The observed large ground state splitting significantly suppresses the phonon-mediated transition, which causes decoherence of the electron spin state of the group IV color centers in diamond, expecting a long spin coherence time at a temperature of approximately 8 K.

As the motion accuracy of 5-axis machining centers directly influences the geometrical shape accuracy of the machined workpieces, accuracy enhancement of the 5-axis machining centers is strongly needed. To improve the shape accuracy during the machining by a 5-axis machine tool, a method that modifies the CL-data based on the motion trajectory errors normal to the machined surface at each command point has been proposed. In this study, the proposed method is applied to simultaneous 5-axis controlled machining to improve motion accuracy. A normal vector calculation method for the simultaneous 5-axis controlled motion is newly proposed, and the compensation method is applied to turbine blade machining by 5-axis controlled motion. Measurement tests of the cutting motion for blade shape machining by a ball-end mill were carried out with a different control mode of NC. The CL-data for the machining tool path was also modified based on the calculated trajectory of the tool center point. Experimental results reveal that the feed speed and machining accuracy significantly depend on the control mode of NC, and that the shape accuracy can be improved by applying the proposed compensation method without any decrease in motion speed.

HPHT synthesis of diamonds from hydrocarbons attracts great attention due to the opportunity to obtain luminescent nano- and microcrystals of high structure perfection. Systematic investigation of diamond synthesized from the mixture of hetero-hydrocarbons containing dopant elements Si or Ge (C24H20Si and C24H20Ge) with a pure hydrocarbon – adamantane (C10H16) at 8 GPa was performed. The photoluminescence of SiV− and GeV− centers in produced diamonds was found to be saturated when Si and Ge contents in precursors exceed some threshold values. The presence of SiC or Ge as second phases in diamond samples with saturated luminescence indicates that ultimate concentrations of the dopants were reached in diamond. It is shown that SiC inclusions can be captured by growing crystals and be a source of local stresses up to 2 GPa in diamond matrix. No formation of Ge-related inclusions in diamonds was detected, which makes Ge more promising as a dopant in the synthesis method. Surprisingly, the synthesis of diamonds from the C24H20Sn hetero-hydrocarbon was ineffective for SnV− formation: only fluorescence of N-and Si-related color centers was detected at room temperature. As an example of great potential for the synthesis method, mass synthesis of 50-nm diamonds with GeV− centers was realized at 9.4 GPa. Single GeV− production in individual nanodiamond was demonstrated.

Nitrogen-vacancy (NV) centers in diamond work as a quantum electrometer. Using an ensemble state of NV centers, we propose vector electrometry and demonstrate measurements in a diamond electronic device. A transverse electric field applied to the N-V axis under a high voltage was measured while applying a transverse magnetic field. The response of the energy level shift against the electric field was significantly enhanced compared with that against an axial magnetic field. Repeating the measurement of the transverse electric field for multiple N-V axes, we obtained the components of the electric field generated in the device., Comment: 17 pages, 7 figures

Intracellular thermometry techniques play an important role in elucidating the relationship between the intracellular temperature and stem cell functions. However, there have been few reports on thermometry techniques that can detect the intracellular temperature of cells during several days of incubation. In this study, we developed a novel quantum thermometric sensing and analysis system (QTAS) using fluorescent nanodiamonds (FNDs). FNDs could label adipose tissue-derived stem cells (ASCs) at high efficiency with 24 h of incubation, and no cytotoxicity was observed in ASCs labeled with less than 500 μg mL−1 of FNDs. The peak of FNDs was confirmed at approximately 2.87 GHz with the characteristic fluorescence spectra of NV centers that could be optically detected (optically detected magnetic resonance [ODMR]). The ODMR peak clearly shifted to the high-frequency side as the temperature decreased and gave a mean temperature dependence of −(77.6 ± 11.0) kHz °C−1, thus the intracellular temperature of living ASCs during several days of culturing could be precisely measured using the QTAS. Moreover, the intracellular temperature was found to influence the production of growth factors and the degree of differentiation into adipocytes and osteocytes. These data suggest that the QTAS can be used to investigate the relationship between intracellular temperature and cellular functions.

Color centers in diamond are a promising platform for quantum information processing and quantum network applications. Color centers using group-IV elements with an inversion symmetry possess very sharp zero phonon lines and stable emission, which can solve issues on the optical properties of a nitrogen-vacancy center. Especially, here, we show the properties of heavy group-IV color centers using germanium, tin, and lead. In addition to the optical properties, we discuss the spin properties of the group-IV color centers from the orbital relaxation rate to spin coherence time, and show that the heavier group-IV elements have a possibility to achieve a long spin coherence time at higher temperatures, compared with a silicon-vacancy center. In this chapter, we introduce the basics (structure and fabrication methods), optical properties, spin properties, and nanophotonic integration of the heavy group-IV color centers.

The motion trajectories of machine tools directly influence the geometrical shape of machined workpieces. Hence, improvement in their motion accuracy is required. It is known that machined shape errors occurring in numerical control (NC) machine tools can be compensated for by modifying the CL-data, based on the amount of error calculated by the measurement results of the machined shape of the workpiece. However, by using this method the shape errors cannot be compensated accurately in five-axis machining, because the final machining shape may not reflect the motion trajectory of a tool owing to the motion errors of the translational and rotary axes. In this study, a modification method of the cutter location (CL)-data, based on the amount of motion errors of the tool center-point trajectory during the machining motion, is newly proposed. The simulation and experiment of a wing profile machining motion is performed, to confirm the effectiveness of the proposed method. From the results, we confirm that the motion accuracy can be significantly improved by applying the proposed method.

Heteroepitaxial diamond on Si substrates can offer large-area, low-cost diamond wafers for many practical applications. Bias-enhanced nucleation (BEN) has been shown to be one of the best process methods for diamond heteroepitaxy. However, the precise control of the nucleation is difficult because the optimized nucleation time is sensitive to both plasma and substrate conditions. In this study, we performed in situ monitoring for epitaxial diamond nucleation on 3C-SiC/Si substrates that detected the bias current during the BEN process. The bias current was found to decrease at the beginning of the BEN process and then increase during diamond formation. An increase in the bias current of 10% led to epitaxial diamond nucleation with a nucleation density of 1010 cm−2. We found from an analytical model that the nucleation occurs at a constant rate in this region. Similar characteristics were obtained on both (001)- and (111)-oriented substrates. This technique was based on the difference between the secondary electron emission coefficient of diamond and that of the material underneath. Thus, this method would be applicable to optimizing the epitaxial diamond nucleation on other substrates or buffer layers in addition to the 3C-SiC/Si studies here.

Nitrogen-vacancy (NV) centers in diamond have superior physical properties at room temperature for quantum sensing of magnetic fields, electric field, strain, and temperature enabling scalable applications from atomic to macroscopic range. We review the sensor materials, quantum control technology, and applications. The perfectly-aligned NV ensemble well suited for sensing of biological/ medical systems and battery/ power electronics.

We demonstrate vector magnetometry using ensemble of the nitrogen-vacancy (NV) centers in diamond that are perfectly aligned along the [111] direction. By changing the direction and strength of the reference magnetic field, we perform three-dimensional vector measurement of the Oersted field generated by the current flowing in a nearby wire. We had a formula for evaluating the magnetic field sensitivity in the direction perpendicular to the NV axis. We demonstrate that the expected sensitivity is 1.2 times higher than that of the NV ensemble isotropically oriented on four equivalent crystal axes. Our precise method is suitable for time-varying magnetic signals.

We demonstrate a photoelectrical readout of negatively charged nitrogen-vacancy (NV−) centers by utilizing a lateral diamond p–i–n diode structure. We found that the photocarriers generated at NV centers are efficiently extracted in the depletion layer even at zero voltage. The transport mechanism for photoexcited minority carriers was investigated by varying the nitrogen ion implantation dose. The minority carrier diffusion length decreased when the photocarriers passed through the implantation region, particularly at a high nitrogen ion dose. From photoelectrically detected magnetic resonance (PDMR) measurements, the photocurrent was found to exhibit a minimum at approximately 2.87 GHz upon microwave irradiation with a contrast of 12%, while this dip was observed to split upon the application of a static magnetic field. The PDMR signal was found to depend on the measurement position. This study paves the way for efficient electrical detection for quantum sensors based on device technologies.

We investigated an effect of interface defects in diamond lateral p-n-junction diodes on their reverse-biased leakage currents. Defects at the p–n interface were observed by cross-sectional transmission electron microscopy from different electron incident directions. The observations revealed that the diodes with the large leakage current had a number of defects localized at the p–n interface. The amount of the defects is related to the magnitude of the reverse leakage currents of the devices. Leakage analysis with applying high voltages strongly suggests that the leakage currents originate from the observed defects. Possible origin of the defect formation is discussed.

Simultaneous 5-axis control machining is used to machine components in complex, deep grooved shapes. In order to maintain actual feed rates in this kind of machining, it is vitally important to secure continuity in the movement of the rotational axis. When the tool reverses its direction of travel, however, its axis of rotation is liable to make sudden movements. In this study, therefore, we first derive candidate tool postures that can meet the conditions for relative postures to machined surfaces while avoiding tool interferences. We then develop a method for automatically planning continuous changes in the command values of the rotating axial angles in the machine coordinate space.

The electric field inside semiconductor devices is a key physical parameter that determines the properties of the devices. However, techniques based on scanning probe microscopy are limited to sensing at the surface only. Here, we demonstrate the direct sensing of the internal electric field in diamond power devices using single nitrogen-vacancy (NV) centers. The NV center embedded inside the device acts as a nanoscale electric field sensor. We fabricated vertical diamond p-i-n diodes containing the single NV centers. By performing optically detected magnetic resonance measurements under reverse-biased conditions with an applied voltage of up to 150 V, we found a large splitting in the magnetic resonance frequencies. This indicated that the NV center senses the transverse electric field in the space-charge region formed in the i-layer. The experimentally obtained electric field values are in good agreement with those calculated by a device simulator. Furthermore, we demonstrate the sensing of the electric field in different directions by utilizing NV centers with different N-V axes. This direct and quantitative sensing method using an electron spin in a wide-band-gap material provides a way to monitor the electric field in operating semiconductor devices.

High temperature characteristics of bipolar-mode operation of normally-off diamond junction field-effect transistors were investigated up to 573 K. As an important factor, the current gain depending on the gate current was analyzed with a theoretical model. We found that the experimental current gain decreased with the rise in the gate current, in agreement with the theoretical estimation considering the recombination at the end regions. We achieved 4–9 times higher drain currents in the bipolar-mode compared with the unipolar-mode operation at a DC current gain of 10. Furthermore, the bipolar-mode currents at the high temperatures of 473 and 573 K became two orders of magnitude larger than the unipolar-mode current at room temperature with a large DC current gain of $10^{2}$ .

Energy conservation and battery life extension are key challenges for the next-generation hybrid electric vehicles. In particular, the temperature and electric currents in a storage battery need to be monitored simultaneously with ∼1 kHz signal bandwidth for optimum battery usage. Here we introduce a centimeter-scale portable quantum sensor head, consisting of a diamond substrate hosting an ensemble of nitrogen-vacancy (NV) color centers with a density of ∼3 × 1017 cm−3. One diamond surface is attached to a multi-mode fiber for simultaneous optical excitation and readout of the NV centers, while the other diamond surface is attached to a coplanar microwave guide for NV spin ground-state mixing. Signal bandwidth of 1 kHz was realized through time-domain multiplexing of the two-tone microwave frequency modulation at 20 kHz. Two microwave frequencies were locked to the two resonance points that were determined from the optically detected magnetic resonance spectrum. From the mean and the difference of the deviation from the two locked frequencies, the temperature and magnetic field were obtained simultaneously and independently, with sensitivities of 3.5 nT/Hz1/2 and 1.3 mK/Hz1/2, respectively. We also showed that our sensor reached a minimum detectable magnetic field of 5 pT by accumulating signals for over 10 000 s.

Charge state control of the nitrogen vacancy center (NV center) in diamond is important to the realization of applications such as magnetometers or quantum information processing, because only negatively charged NV centers have attractive spin properties. We fabricated p–i–n (p-type–intrinsic–n-type) junctions to investigate the charge state properties of the NV centers in an ultrapure i-layer with a low impurity concentration. The photoluminescence of the ensemble of NV centers demonstrated that the charge state can be modulated by external voltage across the p–i–n junction. The charge state of the NV centers was tuned towards NV− by applying a forward bias below the built-in voltage.

We have developed the magnetic probe with a diamond quantum sensor and electromagnetic coils to detect sentinel lymph nodes (SLNs), as the medical application of diamond quantum sensing. The probe magnetizes magnetic nanoparticles (MNPs) accumulated in SLNs and detect the magnetic fields of magnetized MNPs. In this study, we designed a ferromagnetic core that has a unique shape and optimized the magnetic field generation system for improving the detection performance such as the magnetic sensitivity and detectable distance. The proposed magnetic core enhances an excitation magnetic field strength at a longer distance and suppresses a bias field strength at a location of the diamond quantum sensor. The increment of the excitation field is approximately 370% and the decrement of the bias field is approximately 45%. In addition to the proposed magnetic core, even applying a cancellation coil, the excitation field strength is tenfold compared with the coil only structure, suggesting the larger magnetization of MNPs generates the large magnetic field for longer detectable distance.

We developed a novel magnetometer that employs negatively charged nitrogen-vacancy (NV−) centers in diamond, to detect the magnetic field generated by magnetic nanoparticles (MNPs) for biomedical applications. The compact probe system is integrated into a fiber-optics platform allowing for a compact design. To detect signals from the MNPs effectively, we demonstrated, for the first time, the application of an alternating current (AC) magnetic field generated by the excitation coil of several hundred microteslas for the magnetization of MNPs in diamond quantum sensing. In the lock-in detection system, the minimum detectable AC magnetic field (at a frequency of 1.025 kHz) was approximately 57.6 nT for one second measurement time. We were able to detect the micromolar concentration of MNPs at distances of a few millimeters. These results indicate that the magnetometer with the NV− centers can detect the tiny amounts of MNPs, thereby offering potential for future biomedical applications.

The nitrogen-vacancy (NV) centre in diamond is a promising candidate for quantum computing applications and magnetic sensing applications, because it is an atomic-scale defect with stable coherence time (T2) and reliable accessibility at room temperature. We demonstrated a method for improving the NV spin properties (the full width half maximum (FWHM) value of the magnetic resonance spectrum and T2) through a near-field (NF) etching method under ambient conditions. The NF etching method, based on a He-Cd ultraviolet laser (325 nm), which is longer than the absorption edge of the oxygen molecule, enabled selective removal of defects on the nanodiamond surface. We observed a decrease in the FWHM value close to 15% and an increase in T2 close to 25%. Since our technique can be easily reproduced, a wide range of NV centre applications could be improved, especially magnetic sensing applications. Our results are especially attractive, because they have been obtained under ambient conditions and only require a light source with wavelength slightly above the O2 absorption edge.

Diamond lateral p–n junctions were fabricated on (111) substrates by selective growth of n-type diamond around a p-type layer. We investigated the effects of both the step-flow growth direction of n-type diamond and the crystal face of the p-type sidewalls on the structure and electrical properties of the lateral p–n junctions. The direction of the step-flow growth clearly influenced the morphologies of the p–n junctions. We also found that the crystal faces of the p-type sidewalls had an effect on the quality of the lateral p–n junctions. The two facing p–n junctions obtained in this study becomes an n–p–n structure, which could be a key structure for metal-oxide-semiconductor field-effect transistors with an inversion channel.

Tin-vacancy (SnV) color centers were created in diamond by ion implantation and subsequent high temperature annealing up to 2100 {\deg}C at 7.7 GPa. The first-principles calculation suggests that the large atom of tin can be incorporated into the diamond lattice with a split-vacancy configuration, in which a tin atom sits on an interstitial site with two neighboring vacancies. The SnV center shows a sharp zero phonon line at 619 nm at room temperature. This line splits into four peaks at cryogenic temperatures with a larger ground state splitting of ~850 GHz than those of color centers based on other IV group elements, silicon-vacancy (SiV) and germanium vacancy (GeV) centers. The excited state lifetime was estimated to be ~5 ns by Hanbury Brown-Twiss interferometry measurements on single SnV quantum emitters. The order of the experimentally obtained optical transition energies comparing with the SiV and GeV centers is good agreement with the theoretical calculations., Comment: 10 pages, 4 figures and Supplementary 3 pages, 2 figures