Nitrogen vacancy (NV) color centers in diamond have shown great potential for various applications in quantum technology due to their long coherence times, high sensitivity to magnetic fields and atomic scale resolution. However, one major challenge in utilizing near surface NV centers is the decoherence caused by spins and charges fluctuating on the surface, which affects the spin properties of the sensors. To reduce the induced noise, various oxygen surface treatments such as low power oxygen plasma treatment and annealing under oxygen atmosphere have been explored to terminate the diamond surface and reduce its impact on NV coherence. We showed that the NV center’s coherence time can be enhanced up to a factor of 3 over a large spectral range of noise. Double electron–electron resonance measurements revealed an extra source of decoherence, scaling similarly as the P1 spin bath. The improvement in coherence times is accompanied with an increase in measured ketone/ether content and reduction of sp ^2 signal in x-ray photoelectron spectroscopy measurements. Finally we compared the performance of different NV ensembles and surface treatments for sensing external proton spins. The oxygen annealing is an effective procedure of enhancing the spin coherence times and reducing broad band spin noise experienced by shallow implanted ensemble NV centers in diamond.

The excellent performance of quantum magnetometers based on nitrogen-vacancy (NV) centers in diamond, including their high sensitivity, their wide dynamic range, and the possibility for a calibration-free operation, renders them a very promising alternative to classical magnetic field sensors. However, existing lab prototypes of NV center sensors still suffer from a large volume and non-scalable manufacturing technologies. To mitigate this problem, in this paper, we present a miniaturized and scalable microwave electronics platform for quantum magnetometry based on an S-band SiGe BiCMOS transmitter (TX) chip and a custom-designed resonator manufactured on a microwave printed circuit board. The fabricated TX chip can deliver a high saturated output power of 19dBm at a center frequency of 2.87 GHz over a wide relative bandwidth of 38.3% of the center frequency to a 50 Omega load while occupying a compact die area of 0.593mm(2). To manipulate the spin state of NV centers efficiently, we use two of the presented TX chips to drive a newly proposed differential resonator, which provides microwave magnetic fields of B-1= 179 mu T over a large active area of 18.48 x 10(4) mu m(2). Continuous-wave and pulsed optically detected magnetic resonance (ODMR) measurements are used to verify the excellent performance of the proposed platform compared to the state-of-the-art. In these experiments, the presented platform produced Rabi frequencies up to 5.81 MHz.

Quantum sensors based on solid-state defects, such as the nitrogen-vacancy (NV) center in diamond, offer very good room-temperature sensitivity, long-term stability, and the potential for calibration-free measurements. However, most quantum sensors still suffer from a bulky size and weight, low energy efficiency, and high costs, prohibiting their widespread use. Here, we present custom-designed chip-integrated microwave (MW) electronics for a miniaturized, low-cost, and highly scalable quantum magnetometer based on NV centers in diamond. The presented electronics include a quadrature phase-locked loop (QPLL) chip to generate the required local oscillator signal at around 7 GHz with a wide tuning range of 22% and a low phase noise (PN) of $-$ 122 dBc/Hz at 1-MHz offset from a 7-GHz carrier for broadband low-noise magnetometry. In addition, the magnetometer electronics comprise a 4-channel transmitter chip, which can provide currents up to 412 mA $_\text{pp}$ into a custom-designed inductor over a wide frequency range from 6.4 to 8 GHz. In combination with a custom-designed coil, manufactured on a glass substrate for optical transparency, which features a large active area of ( $\pi\ttimes 180 \ttimes 180~\mu \text{m}^2$ ), this current is sufficient to produce strong MW magnetic fields up to $B_\text{1} = (1/2) \cdot B_\text{ac} = 170~{\mu \mathrm{T}}$ , enabling pulsed optically detected magnetic resonance (ODMR) experiments. In proof-of-concept ODMR experiments, the presented chip-based spin control system produces fast Rabi oscillations of 5.49 MHz. The measured dc and ac magnetic field limits of detection (LOD) of the presented magnetometer are 32 nT/Hz $^\text{1/2}$ and 300 pT/Hz $^\text{1/2}$ , respectively.

Solid-state magnetometers based on color centers in diamond are emerging as one of the leading quantum sensors due to their outstanding room-temperature properties, such as high sensitivity and calibration-free long-term stability. However, their integration into compact systems is still an active area of research. To tackle this challenge, in this paper, we present a quantum magnetometer based on negatively charged nitrogen-vacancy (NV) centers using a custom-designed, chip-integrated 4-way transmitter. In combination with a custom-designed microcoil array, the 4-way transmitter delivers microwave magnetic fields up to 226 µT for carrier frequencies around 7 GHz with a conversion gain of ≥32 dB to NV centers. The local oscillator (LO) signal required to drive the on-chip quadrature upconversion mixer is generated by a custom-designed quadrature PLL, which provides a 22% tuning range between 6.4 and 8 GHz, and a low phase noise of -122 dBc/Hz at 1 MHz offset from a 7 GHz carrier, to enable broadband, low-noise magnetometry. To verify the excellent performance of the integrated electronics, we have embedded them into a widefield diamond magnetometer using off-chip scanning optics, achieving a state-of-the-art AC-magnetic field limit of detection of 300 pT/Hz 1/2 .

Nanostructuring of a bulk material is used to change its mechanical, optical, and electronic properties and to enable many new applications. We present a scalable fabrication technique that enables the creation of densely packed diamond nanopillars for quantum technology applications. The process yields tunable feature sizes without the employment of lithographic techniques. High-aspect-ratio pillars are created through oxygen-plasma etching of diamond with a dewetted palladium film as an etch mask. We demonstrate an iterative renewal of the palladium etch mask, by which the initial mask thickness is not the limiting factor for the etch depth. Following the process, 300-400 million densely packed 100 nm wide and 1 μm tall diamond pillars were created on a 3 × 3 mm2 diamond sample. The fabrication technique is tailored specifically to enable applications and research involving quantum coherent defect center spins in diamond, such as nitrogen-vacancy (NV) centers, which are widely used in quantum science and engineering. To demonstrate the compatibility of our technique with quantum sensing, NV centers are created in the nanopillar sidewalls and are used to sense 1H nuclei in liquid wetting the nanostructured surface. This nanostructuring process is an important element for enabling the wide-scale implementation of NV-driven magnetic resonance imaging or NV-driven NMR.

The creation of single, negatively charged silicon vacancy (SiV−) centers in well-defined diamond layers close to the host surface is a crucial step for the development of diamond-based quantum optic devices with many applications in nanophotonics, quantum sensing, or quantum information science. Here, we report on the creation of shallow (10 nm below the surface), single SiV− centers in diamond using low energy Si+ ion implantation with subsequent high temperature annealing at 1500 °C. We show transition linewidths down to 99 MHz and narrow inhomogeneous distributions. Furthermore, we achieved a reduction of homogeneous linewidths by a factor of 2 after removing subsurface damage using oxygen plasma processing. These results not only give insights into the formation process of SiV− centers but also indicate a favorable processing method to fabricate shallow single quantum emitters in diamond perfectly suited for coupling to nanostructures on the diamond surface.

The negatively-charged nitrogen-vacancy (NV) center in diamond has been shown recently as an excellent sensor for external spins. Nevertheless, their optimum engineering in the near-surface region still requires quantitative knowledge in regard to their activation by vacancy capture during thermal annealing. To this aim, we report on the depth profiles of near-surface helium-induced NV centers (and related helium defects) by step-etching with nanometer resolution. This provides insights into the efficiency of vacancy diffusion and recombination paths concurrent to the formation of NV centers. It was found that the range of efficient formation of NV centers is limited only to approximately 10 to 15nm (radius) around the initial ion track of irradiating helium atoms. Using this information we demonstrate the fabrication of nanometric-thin (δ) profiles of NV centers for sensing external spins at the diamond surface based on a three-step approach, which comprises (i) nitrogen-doped epitaxial CVD diamond overgrowth, (ii) activation of NV centers by low-energy helium irradiation and thermal annealing, and (iii) controlled layer thinning by low-damage plasma etching. Spin coherence times (Hahn echo) ranging up to 50 μs are demonstrated at depths of less than 5nm in material with 1.1 % of ^13C (depth estimated by spin relaxation (T_1) measurements). At the end, the limits of the helium irradiation technique at high ion fluences are also experimentally investigated.

Here we report the fabrication of stable, shallow (<5 nm) nitrogen-vacancy (NV) centers in diamond by nitrogen delta doping at the last stage of the chemical vapor deposition growth process. The NVs are stabilized after treating the diamond in SF6 plasma, otherwise the color centers are not observed, suggesting a strong influence from the surface. X-ray photoelectron spectroscopy measurements show the presence of only fluorine atoms on the surface, in contrast to previous studies, indicating very good surface coverage. We managed to detect hydrogen nuclear magnetic resonance signal from protons in the immersion oil, revealing a depth of the NVs of about 5 nm.

Single Nitrogen-Vacancy (NV) centers in diamond close to the crystal surface are very promising magnetic field sensors with very high sensitivity. Here, we report the enhanced creation of very shallow (less than 3 nm below the diamond surface) NV centers by using fluorine and oxygen plasma treatment. We observe a four fold increase—from 0.11% to about 0.45% in the production yield when the sample surface is terminated with fluorine or oxygen atoms. This effect is explained by the stabilization of the NV's negative charge state which is influenced by the various defects present on the diamond surface.

The controlled scaling of diamond defect center based quantum registers relies on the ability to position NVs with high spatial resolution. Using ion implantation, shallow (< 10 nm) NVs can be placed with accuracy below 20nm, but generally show reduced spin properties compared to bulk NVs. We demonstrate the augmentation of spin properties for shallow implanted NV centers using an overgrowth technique. An increase of the coherence times up to an order of magnitude (T_2 = 250 \mu s) was achieved. Dynamic decoupling of defects spins achieves ms decoherence times. The study marks a further step towards achieving strong coupling among defects positioned with nm precision.

Diamond applications potential for biosensing devices have been highlighted by several authors, especially concerning the long-term stability of covalent functionalisations on its surface. Additionally, in electrochemistry boron doped diamond electrodes (N(Lambda) similar to 10(20) cm(3)) show high corrosion resistance and a large hydrolysis window. These features, recognised and exploited in industrial applications, have up to now found little resonance in the life-sciences. Here we present diamond microelectrode arrays based on (1) nanocrystalline diamond (NCD) thin films and (2) single crystal diamond (SCD). NCD is necessary for large area applications like arrays, but graphitic grain boundaries may influence its behaviour. The ideal case SCD is covered here for comparison. The array design consists of four electrodes whose sensitive area is delimited by means of a patterned photoresist. Two different patterns were used to realise a layout with four independent openings (15 mm diameter) for simultaneous detection on multiple cells and a layout with one single window (25 mm diameter) intersecting all four electrodes to create a quadrupolar detector suitable for mapping the activity of single cells. Early results validated the suitability of both NCD and SCD devices: (1) cyclic-voltammetry measurements confirmed the adrenaline oxidation potential on the presented microelectrodes around 650 mV; (2) alternating applications of 1 mM adrenaline and saline rinsing solutions showed negligible electrode fouling; and (3) interfaced to single adrenal chromaffin cells, the devices clearly detected sustained sequences of quantal events (10-100pA amplitude, 50-100 ms duration) associated to the vesicular release of adrenaline and noradrenaline during exocytosis induced by cell-depolarisation.

The surface conductivity of hydrogen (H)‐terminated diamond still suffers from instabilities at high temperature especially in O‐containing atmosphere and/or under high power FET device operation. For devices based on this H‐induced surface conductivity concept, stabilization is thus essential. In this investigation AlN, deposited by MOCVD at 800 °C, has been used as passivation and gate dielectric in Surface Channel MESFET and MISFET structures. Despite the high deposition temperature, the H‐induced channel is still present in the case of both structures and can be fully modulated. Surface Channel MESFETs yield a maximum output current density of 200 mA/mm (at pS = 9.5 × 1012 cm−2) in enhancement mode of operation. MISFETs display 20 mA/mm current density (at pS = 2.1 × 1012 cm−2) in semi‐enhancement mode of operation, the maximum current being limited by AlN dielectric breakdown. The film shows strong adhesion. To explain all features consistently, it is proposed that the interfacial bonding is provided by a H‐double bond.

Electrochemistry and biochemistry have always been ideal applications for diamond due to its chemical inertness and stability, sensitivity and biocompatibility. Several diamond ChemFET concepts have been proposed to date, however further improvements are still needed to obtain functional devices that can be operated efficiently beyond the reach of the well established silicon ISFET technology [P. Bergveld, Sensor and Actuators B, Chem. 88 (2003), pp. 1]. In this paper we describe a novel ISFET structure in which a boron doped diamond electrochemical gate electrode is combined and monolithically integrated with an InAlN/GaN HEMT structure. The new device merges the high chemical stability of diamond with the high transconductance and low pinch-off voltage of InAlN/GaN heterostructure FETs, resulting in a highly stable ISFET with high sensitivity. First devices have been fabricated and electrochemically characterized, expressing high current levels, a pH sensitivity of about 50 mV/pH, complete current modulation when operated within the electrochemical window of the electrode in the range of pH 1 to pH 13 and high stability upon pH cycling and the application of high anodic overpotentials.

The production of high quality diamond films by microwave plasma assisted CVD, with rapid growth rates and good uniformity over large surfaces, requires perfectly optimized reactors from the microwave design point of view. Most MW plasma assisted CVD reactors used for diamond film deposition work on the resonant cavity principle. The design of such reactors relies on 3 choices: i) choice of a suitable resonant mode (i.e. with an electric field structure conducive to plasma ignition), ii) choice of a MW coupling system to excite the cavity, and iii) choice of a quartz window to delimit a reduced pressure zone inside the cavity, so as to obtain the plasma in front of the substrate [1].In this paper, we present an analysis method for MW plasma reactors relying on EM modelling, which allows for the identification of a resonant mode responsible for plasma ignition, applied to an existing reactor exhibiting plasma instabilities and requiring constant supervision.This analysis method, which can be generalised to any resonant cavity reactor, can describe the device behaviour (shape and location of the plasma, occurrence of instabilities) as a function of the various cavity geometrical configurations and to get a first estimate of the process performance.On the basis of such an analysis, it was possible to propose modifications to the reactor considered in order to improve process stability, and obtain higher growth rates. First growth tests done on diamond mono- and nano-crystalline films show excellent material quality and an increase in growth rate by more than an order of magnitude. (C) 2009 Elsevier B.V. All rights reserved.