Holes in valence bands of nanodiamonds discovered

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 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.
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Nanoscale mirrored cavities amplify, connect quantum memories

The idea of computing systems based on controlling atomic spins just got a boost from new research performed at the Massachusetts Institute of Technology (MIT) and the U.S. Department of Energy's (DOE) Brookhaven National Laboratory. By constructing tiny "mirrors" to trap light around impurity atoms in diamond crystals, the team dramatically increased the efficiency with which photons transmit information about those atoms' electronic spin states, which can be used to store quantum information. Such spin-photon interfaces are thought to be essential for connecting distant quantum memories, which could open the door to quantum computers and long-distance cryptographic systems. Crucially, the team demonstrated a spin-coherence time (how long the memory encoded in the electron spin state lasts) of more than 200 microseconds—a long time in the context of the rate at which computational operations take place. A long coherence time is essential for quantum computing systems and long-range cryptographic networks. "Our research demonstrates a technique to extend the storage time of quantum memories in solids that are efficiently coupled to photons, which is essential to scaling up such quantum memories for functional quantum computing systems and networks," said MIT's Dirk Englund, who led the research, now published in ("On-chip detection of non-classical light by scalable integration of single-photon detectors"). Scientists at the Center for Functional Nanomaterials (CFN), a DOE Office of Science User Facility at Brookhaven Lab, helped to fabricate and characterize the materials. one-dimensional diamond crystal cavities 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. Diamond photonic crystal cavities 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.
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Building a graphene-based future for Europe

Graphene is the strongest, most impermeable and conductive material known to man. Graphene sheets are just one atom thick, but 200 times stronger than steel. The European Union is investing heavily in the exploitation of graphene's unique properties through a number of research initiatives such as the SEMANTICS project running at Trinity College Dublin. It is no surprise that graphene, a substance with better electrical and thermal conductivity, mechanical strength and optical purity than any other, is being heralded as the ‘wonder material’ of the 21stcentury, as plastics were in the 20th century. Graphene could be used to create ultra-fast electronic transistors, foldable computer displays and light-emitting diodes. It could increase and improve the efficiency of batteries and solar cells, help strengthen aircraft wings and even revolutionise tissue engineering and drug delivery in the health sector. It is this huge potential which has convinced the European Commission to commit €1 billion to the Future and Emerging Technologies (FET) Graphene Flagship project, the largest-ever research initiative funded in the history of the EU. It has a guaranteed €54 million in funding for the first two years with much more expected over the next decade. Sustained funding for the full duration of the Graphene Flagship project comes from the EU's Research Framework Programmes, principally from Horizon 2020 (2014-2020). The aim of the Graphene Flagship project, likened in scale to NASA’s mission to put a man on the moon in the 1960s, or the Human Genome project in the 1990s, is to take graphene and related two-dimensional materials such as silicene (a single layer of silicon atoms) from a state of raw potential to a point where they can revolutionise multiple industries and create economic growth and new jobs in Europe. The research effort will cover the entire value chain, from materials production to components and system integration. It will help to develop the strong position Europe already has in the field and provide an opportunity for European initiatives to lead in global efforts to fully exploit graphene’s miraculous properties. Under the EU plan, 126 academics and industry groups from 17 countries will work on 15 individual but connected projects. Feeding industry’s hunger for graphene But this is not the only support being provided by the EU for research into the phenomenal potential of graphene. The SEMANTICS research project, led by Professor Jonathan Coleman at Trinity College Dublin, is funded by the European Research Council (ERC) and has already achieved some promising results. The ERC does not assign funding to particular challenges or objectives, but selects the best scientists with the best ideas on the sole criterion of excellence. By providing complementary types of funding, both to individual scientists to work on their own ideas, and to large-scale consortia to coordinate top-down programmes, the EU is helping to progress towards a better knowledge and exploitation of graphene. “It is no overestimation to state that graphene is one of the most exciting materials of our lifetime,” Prof. Coleman says. “It has the potential to provide answers to the questions that have so far eluded us. Technology, energy and aviation companies worldwide are racing to discover the full potential of graphene. Our research will be an important element in helping to realise that potential.” With the help of European Research Council (ERC) Starting and Proof of Concept Grants, Prof. Coleman and his team are researching methods for obtaining single-atom layers of graphene and other layered compounds through exfoliation (peeling off) from the multilayers, followed by deposition on a range of surfaces to prepare films displaying specific behaviour. “We’re working towards making graphene and other single-atom layers available on an economically viable industrial scale, and making it cheaply,” Prof. Coleman continues. “At CRANN, we are developing nanosheets of graphene and other single-atom materials which can be made in very large quantities,” he adds. “When you put these sheets in plastic, for example, you make the plastic stronger. Not only that – you can massively increase its electrical properties, you can improve its thermal properties and you can make it less permeable to gases. The applications for industry could be endless.” Prof. Coleman admits that scientists are regularly taken aback by the potential of graphene. “We are continually amazed at what graphene and other single-atom layers can do,” he reveals. “Recently it has been discovered that, when added to glue, graphene can make it more adhesive. Who would have thought that? It’s becoming clear that graphene just makes things a whole lot better,” he concludes. So far, the project has developed a practical method for producing two-dimensional nanosheets in large quantities. Crucially, these nanosheets are already being used for a range of applications, including the production of reinforced plastics and metals, building super-capacitors and batteries which store energy, making cheap light detectors, and enabling ultra-sensitive position and motion sensors. As the number of application grows, increased demand for these materials is anticipated. In response, the SEMANTICS team has scaled up the production process and is now producing 2D nanosheets at a rate more than 1000 times faster than was possible just a year ago.
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