Nanodiamonds are tiny crystals only a few nanometres in size. While they possess the crystalline structure of diamonds, their properties diverge considerably from those of their big brothers, because their surfaces play a dominant role in comparison to their extremely small volumes. Suspended in aqueous solutions, they could function as taxis for active substances in biomedical applications, for example, or be used as catalysts for splitting water. But how are the electronic properties of nanodiamonds deposited on a solid-state substrate different from those displayed by nanodiamonds in aqueous solutions? Dr. Tristan Petit working in the HZB team headed by Prof. Emad F. Aziz has now investigated this with the help of absorption and emission spectroscopy at BESSY II. Their results, just published in ("Valence holes observed in nanodiamonds dispersed in water"), demonstrate that nanodiamonds display valence holes in aqueous solutions, which are not observed when characterized as a thin film. 
Nanodiamonds are tiny crystals only a few nanometres in size. (Image: Mohamed Sennour, MINES ParisTech) “The interaction between the nanodiamonds and the neighbouring molecules and ions is especially strong in water”, say Petit. The adsorption of active pharmaceutical ingredients on nanodiamonds can be influenced, for example, by adding salts or changing the pH value. Petit and his colleagues have now discovered that the electronic signature of surface states of nanodiamonds in aqueous dispersions are considerably different from those of nanodiamonds on a solid-state substrate. With the help of micro-jet technology developed by Emad Aziz at HZB, they examined liquid samples in vacuum using X-ray spectroscopy and developed a detailed picture of the filled and unfilled electron states in valence and conduction bands. Their results show that holes, i.e. missing electrons in the valence band, formed on the surfaces of the nanodiamonds in the aqueous dispersion. “This suggests that electrons at the surface of nanodiamonds are donated to the surrounding water molecules”, Petit suggests. The physicists suspect they might also influence the nanoparticles’ chemical, optical, and catalytic properties through changes to their electronic structure. They would like to determine in future studies whether the catalytic effect of nanodiamonds in aqueous environment can be increased in order to split water molecules into oxygen and hydrogen using light.
Nanodiamonds are tiny crystals only a few nanometres in size. (Image: Mohamed Sennour, MINES ParisTech) “The interaction between the nanodiamonds and the neighbouring molecules and ions is especially strong in water”, say Petit. The adsorption of active pharmaceutical ingredients on nanodiamonds can be influenced, for example, by adding salts or changing the pH value. Petit and his colleagues have now discovered that the electronic signature of surface states of nanodiamonds in aqueous dispersions are considerably different from those of nanodiamonds on a solid-state substrate. With the help of micro-jet technology developed by Emad Aziz at HZB, they examined liquid samples in vacuum using X-ray spectroscopy and developed a detailed picture of the filled and unfilled electron states in valence and conduction bands. Their results show that holes, i.e. missing electrons in the valence band, formed on the surfaces of the nanodiamonds in the aqueous dispersion. “This suggests that electrons at the surface of nanodiamonds are donated to the surrounding water molecules”, Petit suggests. The physicists suspect they might also influence the nanoparticles’ chemical, optical, and catalytic properties through changes to their electronic structure. They would like to determine in future studies whether the catalytic effect of nanodiamonds in aqueous environment can be increased in order to split water molecules into oxygen and hydrogen using light.
A scanning electron micrograph of one of the one-dimensional diamond crystal cavities. (Image: MIT) Impurities trapped in diamond The memory elements described in this research are the spin states of electrons in nitrogen-vacancy (NV) centers in diamond. The NV consists of a nitrogen atom in the place of a carbon atom, adjacent to a crystal vacancy inside the carbon lattice of diamond. The up or down orientation of the electron spins on these NV centers can be used to encode information in a way that is somewhat analogous to how the charge of many electrons is used to encode the "0"s and "1"s in a classical computer. The scientists preferentially orient the NV's spin, whose direction is naturally randomly oriented, along a particular direction. This step prepares a quantum state of "0". From there, scientists can manipulate the electron spins into "1" or back into "0" using microwaves. The "0" state has brighter fluorescence than the "1" state, allowing scientists to measure the state in an optical microscope. The trick is getting the electron spins in the NV centers to hold onto the stable spin states long enough to perform these logic-gate operations—and being able to transfer information among the individual memory elements to create actual computing networks. "It is already possible to transfer information about the electron spin state via photons, but we have to make the interface between the photons and electrons more efficient. The trouble is that photons and electrons normally interact only very weakly. To increase the interaction between photons and the NV, we build an optical cavity—a trap for photons—around the NV," Englund said. 
Building quantum memories on a chip: Diamond photonic crystal cavities (ladder-like structures) are integrated on a silicon substrate. Green laser light (green arrow) excites electrons on impurity atoms trapped within the cavities, picking up information about their spin states, which can then be read out as red light (red arrow) emitted by photoluminescence from the cavity. The inset shows the nitrogen-vacancy (NV)-nanocavity system, where a nitrogen atom (N) is substituted into the diamond crystal lattice in place of a carbon atom (gray balls) adjacent to a vacancy (V). Layers of diamond and air keep light trapped within these cavities long enough to interact with the nitrogen atom's spin state and transfer that information via the emitted light. (Image: MIT) Light and mirrors These cavities, nanofabricated at Brookhaven by MIT graduate student Luozhou Li with the help of staff scientist Ming Lu of the CFN, consist of layers of diamond and air tightly spaced around the impurity atom of an NV center. At each interface between the layers there's a little bit of reflection—like the reflections from a glass surface. With each layer, the reflections add up—like the reflections in a funhouse filled with mirrors. Photons that enter these nanoscale funhouses bounce back and forth up to 10,000 times, greatly enhancing their chance of interacting with the electrons in the NV center. This increases the efficiency of information transfer between photons and the NV center's electron spin state. The devices' performance was characterized in part using optical microscopy in a magnetic field at the CFN, performed by CFN staff scientist Mircea Cotlet, Luozhou Li, and Edward Chen, who is also a graduate student studying under the guidance of Englund at MIT. "Coupling the NV centers with these optical resonator cavities seemed to preserve the NV spin coherence time—the duration of the memory," Cotlet said. Added Englund: "These methods have given us a great starting point for translating information between the spin states of the electrons among multiple NV centers. These results are an important part of validating the scientific promise of NV-cavity systems for quantum networking." In addition, said Li, "The transferred hard mask lithography technique that we have developed in this work would benefit most unconventional substrates that aren't suitable for typical high-resolution patterning by electron beam lithography. In our case, we overcame the problem that hundred-nanometer-thick diamond membranes are too small and too uneven. " The methods may also enable the long-distance transfer of quantum-encoded information over fiber optic cables. Such information could be made completely secure, Englund said, because any attempt to intercept or measure the transferred information would alter the photons' properties, thus alerting the sender and the recipient to the possible presence of an eavesdropper.