We present a scanning tunneling microscopy/spectroscopy (STM/STS) study of synthetic polycrystalline boron-doped diamond in the temperature range 0.5-4.3 K. At 4.3 K the sample-surface was very non-uniform and tunneling IðV Þ spectra were typical for p-type semiconductors. After cooling below the superconducting transition temperature, we detected and measured the superconducting gap of diamonds. At temperatures around 0.5 K the energy gap was around 0.8 and 1 mV (for two different samples).

Diamond properties down to the quantum-size region are still poorly understood. High-pressure high-temperature (HPHT) synthesis from chloroadamantane molecules allows precise control of nanodiamond size. Thermal stability and optical properties of nanodiamonds with sizes spanning range from <1 to 8 nm are investigated. It is shown that the existing hypothesis about enhanced thermal stability of nanodiamonds smaller than 2 nm is incorrect. The most striking feature in IR absorption of these samples is the appearance of an enhanced transmission band near the diamond Raman mode (1332 cm−1). Following the previously proposed explanation, we attribute this phenomenon to the Fano effect caused by resonance of the diamond Raman mode with continuum of conductive surface states. We assume that these surface states may be formed by reconstruction of broken bonds on the nanodiamond surfaces. This effect is also responsible for the observed asymmetry of Raman scattering peak. The mechanism of nanodiamond formation in HPHT synthesis is proposed, explaining peculiarities of their structure and properties.

Spontaneous light emission is known to be affected by the local density of states and enhanced when coupled to a resonant cavity. Here, we report on an experimental study of siliconvacancy (SiV) color center fluorescence and spontaneous Raman scattering from subwavelength diamond particles supporting loworder Mie resonances in the visible range. For the first time to our knowledge, we have measured the size dependences of the SiV fluorescence emission rate and the Raman scattering intensity from individual diamond particles in the range from 200 to 450 nm. The obtained dependences reveal a sequence of peaks, which we explicitly associate with specific multipole resonances. The results are in agreement with our theoretical analysis and highlight the potential of intrinsic optical resonances for developing nanodiamond-based lasers and single-photon sources.

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Bottom-up high-pressure, high-temperature (BU_HPHT) synthesis of nanodiamonds (NDs) from organic precursors has recently enabled unprecedented control over the ND size down to the phonon confinement region (≤3 nm) of a diamond. This allows us to perform a comprehensive multiwavelength Raman spectroscopy analysis of the Raman spectra and size-related effects in BU_HPHT NDs in the size range from 1.2 to 8 nm. We present and discuss difficulties in ND size determination using the diamond 1332 cm −1 Raman line position. In contrast, we demonstrate the utilization of the low-frequency acoustic Raman modes for this purpose, namely, for quantum-sized sub-3 nm NDs. Using Lamb's model for these low-frequency acoustic modes, we calculate the ND size and find a remarkable agreement with the size values obtained by the XRD and HRTEM.

Impurity-vacancy centers in diamond offer a new class of robust photon sources with versatile quantum properties. While individual color centers commonly act as single-photon sources, their ensembles have been theoretically predicted to have tunable photon-emission statistics. Importantly, the particular type of excitation affects the emission properties of a color center ensemble within a diamond crystal. While optical excitation favors non-synchronized excitation of color centers within an ensemble, electron-beam excitation can synchronize the emitters excitation and thereby provides a control of the second-order correlation function g 2(0). In this letter, we demonstrate experimentally that the photon stream from an ensemble of color centers can exhibit g 2(0) both above and below unity, thereby confirming long standing theoretical predictions by Meuret et al. [S. Meuret, L. H. G. Tizei, T. Cazimajou, et al., “Photon bunching in cathodoluminescence,” Phys. Rev. Lett., vol. 114, no. 19, p. 197401, 2015.]. Such a photon source based on an ensemble of few color centers in a diamond crystal provides a highly tunable platform for informational technologies operating at room temperature.

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