High-pressure high-temperature (HPHT) nanodiamonds originate from grinding of diamond microcrystals obtained by HPHT synthesis. Here we report on a simple two-step approach to obtain as small as 1.1 nm HPHT nanodiamonds of excellent purity and crystallinity, which are among the smallest artificially prepared nanodiamonds ever shown and characterized. Moreover we provide experimental evidence of diamond stability down to 1 nm. Controlled annealing at 450 °C in air leads to efficient purification from the nondiamond carbon (shells and dots), as evidenced by X-ray photoelectron spectroscopy, Raman spectroscopy, photoluminescence spectroscopy, and scanning transmission electron microscopy. Annealing at 500 °C promotes, besides of purification, also size reduction of nanodiamonds down to ∼1 nm. Comparably short (1 h) centrifugation of the nanodiamonds aqueous colloidal solution ensures separation of the sub-10 nm fraction. Calculations show that an asymmetry of Raman diamond peak of sub-10 nm HPHT nanodiamonds can be well explained by modified phonon confinement model when the actual particle size distribution is taken into account. In contrast, larger Raman peak asymmetry commonly observed in Raman spectra of detonation nanodiamonds is mainly attributed to defects rather than to the phonon confinement. Thus, the obtained characteristics reflect high material quality including nanoscale effects in sub-10 nm HPHT nanodiamonds prepared by the presented method.

Small oxidized silicon nanocrystals of average sizes below 3.5 nm are prepared using modified electrochemical etching of a silicon wafer. Modifications introduced in the etching procedure together with postetching treatment in H2O2 lead to a decrease in the nanocrystalline core size and also, to some extent, to changes in the surface oxide. The interplay between these two factors allows us to blueshift the photoluminescence (PL) spectrum from 680 down to 590 nm, which is accompanied by changes in PL dynamics. This continual development, however, stops at about 590 nm, below which abrupt switching to fast decaying blue emission band at about 430 nm was observed. Discontinuity of the spectral shift and possible relation between both bands are discussed.

Silicon, a semiconductor underpinning the vast majority of microelectronics, is an indirect‐gap material and consequently is an inefficient light emitter. This hampers the ongoing worldwide effort towards the integration of optoelectronics on silicon wafers. Even though silicon nanocrystals are much better light emitters, they retain the indirect‐gap nature. Here, we propose a solution to this long‐standing problem: silicon nanocrystals can be transformed into a material with fundamental direct bandgap via a concerted action of quantum confinement and tensile strain. We document this transformation by DFT calculations mapping the E(k) band‐structure of Si nanocrystals. The experimental proofs are then given firstly by a 10 000× increase in the photon emission rate of strained silicon nanocrystals together with their altered absorbance spectra, both of which point to direct dipole‐allowed transitions, secondly by single nanocrystal spectroscopy, confirming reduced phonon energies and thus the presence of tensile strain, and lastly by photoluminescence studies under external hydrostatic pressure.

Color centers in diamonds have shown excellent potential for applications in quantum information processing, photonics, and biology. Here we report chemical vapor deposition (CVD) growth of nanocrystalline diamond (NCD) films as thin as 5-6 nm with photoluminescence (PL) from silicon-vacancy (SiV) centers at 739 nm. Instead of conventional 4-6 nm detonation nanodiamonds (DNDs), we prepared and employed hydrogenated 2 nm DNDs (zeta potential = +36 mV) to form extremely dense (∼1.3 × 10 cm), thin (2 ± 1 nm), and smooth (RMS roughness < 0.8 nm) nucleation layers on an Si/SiO substrate, which enabled the CVD growth of such ultrathin NCD films in two different and complementary microwave (MW) CVD systems: (i) focused MW plasma with an ellipsoidal cavity resonator and (ii) pulsed MW plasma with a linear antenna arrangement. Analytical ultracentrifuge, infrared and Raman spectroscopies, atomic force microscopy, and scanning electron microscopy are used for detailed characterization of the 2 nm H-DNDs and the nucleation layer as well as the ultrathin NCD films. We also demonstrate on/off switching of the SiV center PL in the NCD films thinner than 10 nm, which is achieved by changing their surface chemistry.