The effects of surface chemistry on the morphology and phase stability of titanium dioxide nanoparticles have been investigated using a thermodynamic model based on surface free energies and surface tensions obtained from first principles calculations. It has been found that surfaces representing acidic and alkaline conditions have a significant influence on both the shape of the nanocrystals and the anatase-to-rutile transition size. The latter introduces the possibility of inducing phase transitions by changing the surface chemistry.

Nitrogen-vacancy colour centres in diamond can undergo strong, spin-sensitive optical transitions under ambient conditions, which makes them attractive for applications in quantum optics 1 , nanoscale magnetometry 2,3 and biolabelling 4 . Although nitrogen-vacancy centres have been observed in aggregated detonation nanodiamonds 5 and milled nanodiamonds 6 , they have not been observed in very small isolated nanodiamonds 7 . Here, we report the first direct observation of nitrogen-vacancy centres in discrete 5-nm nanodiamonds at room temperature, including evidence for intermittency in the luminescence (blinking) from the nanodiamonds. We also show that it is possible to control this blinking by modifying the surface of the nanodiamonds.Detonation nanodiamond is routinely produced on an industrial scale, and the raw material can be disintegrated into a stable 5-nm monodisperse colloid 8 . The combination of inert core and chemically reactive surface, which can host a variety of moieties, is appealing for chemists, biologists and material scientists 9,10 . Quantum magnetometry 2,3 is an example of an emerging technology that will directly benefit from the availability of nanocrystals with welldefined sizes in the 5-nm range, because the sensitivity to single spins is inversely proportional to the cube of the distance between the sensor (that is, the nitrogen-vacancy (NV) centre) and the spin being detected.Producing and detecting NV colour centres in isolated 5-nm detonation nanodiamond has been controversial, and there has been some scepticism regarding their stability as a useful emitter in a discrete crystal. For example, theoretical calculations of the crystal energy budget favour the location of nitrogen on the surface rather than in the core, which seems to explain the limited observation of NV centres in chemical vapour deposition and high-pressure high-temperature grains of less than 40 nm in size 11,12 , and favours the prediction that nanodiamonds smaller than 10 nm in size do not contain NV centres 7,13 . Although sub-10-nm nanodiamonds with NV centres have been produced using a top-down approach (milling luminescent high-pressure hightemperature microdiamonds into 7-nm particles 6,14 ), the question of NV stability in isolated detonation nanodiamonds persists.In aggregated detonation nanodiamonds (agglomerates and agglutinates 8 ), high-sensitivity, time-gated luminescence and electronic paramagnetic resonance spectroscopy have been used to extract a weak NV signal from a strong luminescence background 5 . The experiments highlight the eclipsing nature of the graphitic surface layers in nanodiamond aggregates-NV centres were simply not visible through the broadband luminescence from the surface and grain boundary material. To distinguish the NV spectral signature from the large grain boundary luminescence overhead, diamond synthesis yielding discrete sub-10-nm detonation nanodiamonds is vital. Here, we use a robust deaggregation and dispersion method, which diminishes the crystal-crystal interaction to...

The development of the next generation of nanotechnologies requires precise control of the size, shape, and structure of individual components in a variety of chemical and engineering environments. This includes synthesis, storage, operational environments and, since these products will ultimately be discarded, their interaction with natural ecosystems. Much of the important information that determines these properties is contained within nanoscale phase diagrams, but quantitative phase maps that include surface effects and critical diameter (along with temperature and pressure) remain elusive. Here we present the first quantitative equilibrium phase map for gold nanoparticles together with experimental verification, based on relativistic ab initio thermodynamics and in situ high-resolution electron microscopy at elevated temperatures.