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Register a new userHigh temperature annealing enhanced diamond 13C hyperpolarization at room temperature
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Methods of optical dynamic nuclear polarization (DNP) open the door to the replenishable hyperpolarization of nuclear spins, boosting their NMR/MRI signature by orders of magnitude. Nanodiamond powder rich in negatively charged Nitrogen Vacancy (NV) defect centers has recently emerged as one such promising platform, wherein 13C nuclei can be hyperpolarized through the optically pumped defects completely at room temperature and at low magnetic fields. Given the compelling possibility of relaying this 13C polarization to nuclei in external liquids, there is an urgent need for the engineered production of highly hyperpolarizable diamond particles. In this paper, we report on a systematic study of various material dimensions affecting optical 13C hyperpolarization in diamond particles -- especially electron irradiation and annealing conditions that drive NV center formation. We discover surprisingly that diamond annealing at elevated temperatures close to 1720C have remarkable effects on the hyperpolarization levels, enhancing them by upto 36-fold over materials annealed through conventional means. We unravel the intriguing material origins of these gains, and demonstrate they arise from a simultaneous improvement in NV electron relaxation time and coherence time, as well as the reduction of paramagnetic content, and an increase in 13C relaxation lifetimes. Overall this points to significant recovery of the diamond lattice from radiation damage as a result of the high-temperature annealing. Our work suggests methods for the guided materials production of fluorescent, 13C hyperpolarized, nanodiamonds and pathways for their use as multi-modal (optical and MRI) imaging and hyperpolarization agents.
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Dynamic nuclear polarization via contact with electronic spins has emerged as an attractive route to enhance the sensitivity of nuclear magnetic resonance (NMR) beyond the traditional limits imposed by magnetic field strength and temperature. Among the various alternative implementations, the use of nitrogen vacancy (NV) centers in diamond - a paramagnetic point defect whose spin can be optically polarized at room temperature - has attracted widespread attention, but applications have been hampered by the need to align the NV axis with the external magnetic field. Here we overcome this hurdle through the combined use of continuous optical illumination and a microwave sweep over a broad frequency range. As a proof of principle, we demonstrate our approach using powdered diamond where we attain bulk 13C spin polarization in excess of 0.25 percent under ambient conditions. Remarkably, our technique acts efficiently on diamond crystals of all orientations, and polarizes nuclear spins with a sign that depends exclusively on the direction of the microwave sweep. Our work paves the way towards the use of hyperpolarized diamond particles as imaging contrast agents for biosensing and, ultimately, for the hyperpolarization of nuclear spins in arbitrary liquids brought in contact with their surface.
In this work we investigated the time behavior of the polarization of bulk 13C nuclei in diamond above the thermal equilibrium. This nonthermal nuclear hyperpolarization is achieved by cross relaxation between two nitrogen related paramagnetic defect species in diamond in combination with optical pumping. The decay of the hyperpolarization at four different magnetic fields is measured. Furthermore, we use the comparison with conventional nuclear resonance measurements to identify the involved distances of the nuclear spin with respect to the defects and therefore the coupling strengths. Also, a careful look at the linewidth of the signal give valuable information to piece together the puzzle of the hyperpolarization mechanism.
Uncooled Terahertz (THz) photodetectors (PDs) showing fast (ps) response and high sensitivity (noise equivalent power (NEP) < $nWHz^{-1/2}$) over a broad (0.5THz-10THz) frequency range are needed for applications in high-resolution spectroscopy (relative accuracy ~ $10^{-11}$), metrology, quantum information, security, imaging, optical communications. However, present THz receivers cannot provide the required balance between sensitivity, speed, operation temperature and frequency range. Here, we demonstrate an uncooled THz PD combining the low (~2000 $k_{B}{mu}m^{-2}$) electronic specific heat of high mobility (> 50000 $cm^{2}V^{-1}s^{-1}$) hBN-encapsulated graphene with the asymmetric field-enhancement produced by a bow-tie antenna resonating at 3 THz. This produces a strong photo-thermoelectric conversion, which simultaneously leads to a combination of high sensitivity (NEP $leq$ 160 $pWHz^{-1/2}$), fast response time ($leq 3.3 ns$) and a four orders of magnitude dynamic range, making our devices the fastest, broadband, low noise, room temperature THz PD to date.
The wide bandgap, high-breakdown electric field, and high carrier mobility makes GaN an ideal material for high-power and high-frequency electronics applications such as wireless communication and radar systems. However, the performance and reliability of GaN-based HEMTs are limited by the high channel temperature induced by Joule-heating in the device channel. High thermal conductivity substrates integrated with GaN can improve the extraction of heat from GaN based HEMTs and lower the device operating temperature. However, heterogeneous integration of GaN with diamond substrates is not trivial and presents technical challenges to maximize the heat dissipation potential brought by the diamond substrate. In this work, two modified room temperature surface activated bonding techniques are used to bond GaN and single crystal diamond with different interlayer thicknesses. TDTR is used to measure the thermal properties from room temperature to 480 K. A relatively large TBC of the GaN-diamond interfaces with a 4nm interlayer was observed and material characterization was performed to link the structure of the interface to the TBC. Device modeling shows that the measured GaN-diamond TBC values obtained from bonding can enable high power GaN devices by taking the full advantage of the high thermal conductivity of single crystal diamond and achieve excellent cooling effect. Furthermore, the room-temperature bonding process in this work do not induce stress problem due to different coefficient of thermal expansion in other high temperature integration processes in previous studies. Our work sheds light on the potential for room-temperature heterogeneous integration of semiconductors with diamond for applications of electronics cooling especially for GaN-on-diamond devices.
Spins of negatively charged nitrogen-vacancy (NV$^-$) defects in diamond are among the most promising candidates for solid-state qubits. The fabrication of quantum devices containing these spin-carrying defects requires position-controlled introduction of NV$^-$ defects having excellent properties such as spectral stability, long spin coherence time, and stable negative charge state. Nitrogen ion implantation and annealing enable the positioning of NV$^-$ spin qubits with high precision, but to date, the coherence times of qubits produced this way are short, presumably because of the presence of residual radiation damage. In the present work, we demonstrate that a high temperature annealing at 1000$^circ$C allows 2 millisecond coherence times to be achieved at room temperature. These results were obtained for implantation-produced NV$^-$ defects in a high-purity, 99.99% $^{12}$C enriched single crystal chemical vapor deposited diamond. We discuss these remarkably long coherence times in the context of the thermal behavior of residual defect spins. [Published in Physical Review B {bf{88}}, 075206 (2013)]
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