Masanori Fujiwara

Detonation nanodiamond (DND) is the smallest class of diamond nanocrystal capable of hosting various color centers with a size akin to molecular pores. Their negatively charged nitrogen-vacancy center (NV−) is a versatile tool for sensing a wide range of physical and even chemical parameters at the nanoscale. The NV− is, therefore, attracting interest as the smallest quantum sensor in biological research. Nonetheless, recent NV− enhancement in DND has yet to yield sufficient fluorescence per particle, leading to efforts to incorporate other group-IV color centers into DND. An example is adding a silicon dopant to the explosive mixture to create negatively charged silicon-vacancy centers (SiV−). In this paper, we report on efficient observation (∼50% of randomly selected spots) of the characteristic optically detected magnetic resonance (ODMR) NV− signal in silicon-doped DND (Si-DND) subjected to boiling acid surface cleaning. The NV− concentration is estimated by continuous-wave electron spin resonance spectroscopy to be 0.35 ppm without the NV− enrichment process. A temperature sensitivity of 0.36K/Hz in an NV− ensemble inside an aggregate of Si-DND is achieved via the ODMR-based technique. Transmission electron microscopy survey reveals that the Si-DNDs core sizes are ∼11.2 nm, the smallest among the nanodiamond’s temperature sensitivity studies. Furthermore, temperature sensing using both SiV− (all-optical technique) and NV− (ODMR-based technique) in the same confocal volume is demonstrated, showing Si-DND’s multimodal temperature sensing capability. The results of the study thereby pave a path for multi-color and multimodal biosensors and for decoupling the detected electrical field and temperature effects on the NV− center.

Nanodiamonds with group-IV color centers, such as silicon-vacancy centers and germanium-vacancy (GeV) centers, exhibit excellent properties, including a sharp and stable zero-phonon line, surface functionalization, and low cytotoxicity. Because the line peak wavelength shifts linearly with the temperature under ambient conditions, the nanodiamonds are promising candidates for all-optical nanoscale thermometry inside a living cell. However, the particles used for temperature measurements have been reported to be larger than a few hundred nanometers. Here, we report temperature sensing using GeV detonation nanodiamonds. The GeV nanodiamonds have a mean particle size of 20 nm. These are the smallest particles among GeV-based particles used in thermometry. The sensitivity of the single GeV centers in the detonation nanodiamond is estimated to be almost consistent with the reported ones of the single GeV centers in bulk diamonds, which can potentially reach sub-kelvin temperature accuracy. The GeV detonation nanodiamonds should function as good photoluminescence probes and intracellular temperature sensors.

Control of the charge state of the nitrogen-vacancy (NV) center is crucial because of its instability and its transitions between the negative (NV–) and neutral (NV0) NV charge states under laser irradiation In this study, we fabricated an n-i-n junction, with an i-layer sandwiched between two phosphorus-doped n-layers; then, we measured the charge state of NV centers under steady state and laser irradiation in a known band structure where the Fermi energy changes gradually. The steady-state charge state measured by a nondestructive single shot exhibited stable NV– and NV0 signals when the Fermi level was even slightly above and below the transition level, respectively. This result indicates that the charge state can be significantly stabilized through band engineering. Both charge-state populations were observed only when the Fermi level was close to the transition level. Under continuous green laser irradiation, the ratio of NV– measured by the photoluminescence spectra changed gradually with the Fermi level in the depletion layer because of the balance between excitation from the laser and the supply of charge from the band. This outcome agrees reasonably with the calculated bands. Furthermore, we measured the PL spectra of the ensemble NV centers and discovered that their charge state can be well-controlled, as in the single NV center. The charge state of the i-layer at the interface can be stabilized by depositing a thin n-layer on the surface. These results would contribute significantly to improve sensor performance.

Recently, ultrashort laser processing has attracted attention for creating nitrogen-vacancy (NV) centers because this method can create single NV centers in spatially-controlled positions, which is an advantage for quantum information devices. On the other hand, creating high-density NV centers in a wide region is also important for quantum sensing because the sensitivity is directly enhanced by increasing the number of NV centers. A recent study demonstrated the creation of high-density NV centers by irradiating femtosecond laser pulses, but the created region was limited to micrometer size, and this technique required many laser pulses to avoid graphitization of diamond. Here, we demonstrate the creation of NV centers in a wide region using only an intense single femtosecond laser pulse irradiation. We irradiated a diamond sample with a femtosecond laser with a focal spot size of 41 µm and a laser fluence of up to 54 J/cm², which is much higher than the typical graphitization threshold in multi-pulse processing. We found that single-pulse irradiation created NV centers without post-annealing for a laser fluence higher than 1.8 J/cm², and the region containing NV centers expanded with increasing laser fluence. The diameter of the area was larger than the focal spot size and reached over 100 µm at a fluence of 54 J/cm². Furthermore, we demonstrated the NV centers’ creation in a millimeter-sized region by a single-shot defocused laser pulse over 1100 µm with a fluence of 33 J/cm². The demonstrated technique will bring interest in the fundamentals and applications of fabricating ultrahigh-sensitivity quantum sensors.

The realization of electroluminescence (EL) of negatively charged nitrogen vacancy (NV⁻) centers is important toward all-electrical control of diamond quantum devices. In this study, we demonstrated electrical excitation and detection of EL of single NV⁻ centers by using lateral diamond p⁺–i(n⁻)–n⁺ diodes. It had been grown by homoepitaxy using the plasma enhanced chemical vapor deposition technique. We introduced a lightly phosphorus doped i(n⁻) layer to stabilize the negative state of NV centers. It was estimated that the efficiency of the electrical excitation rate of the NV center was more than 30 times enhanced by introducing lateral diamond p⁺–i(n⁻)–n⁺ diodes structure compared with the previous vertical diode. Furthermore, the EL of a single NV⁻ center embedded in the i(n⁻) layer region was characterized. The results show that the charge state of the single NV centers can be manipulated by the voltage applied to the p⁺–i(n⁻)–n⁺ diode, where the emission of EL is increasingly dominated by NV⁻ in the range of 30 to 50 V.