Over the last 10 years, diamond as a technological material has seen a renewed and increasing level of interest with genuine potential. A key requirement to enable some of the latest potential applications for diamond will rely heavily on the ability to control and tailor the fabrication, and understand the behaviour of nanoscale diamond (some less than 5 nm in diameter). Diamond is well known for its extreme properties, including hardness, chemical and biological inertness, high Debye temperature, high thermal conductivity and ease of bio-functionalisation to name a few.
To make the task even more challenging, and scientifically interesting, it is not the diamond crystal itself that is of primary interest, but rather the "defects" incorporated in the diamond crystal host. There is a vast body of literature available on the incorporation and manifestation of impurities in diamond1, and of these, a large number are referred to as "colour centres" whereby they absorb or emit light.
By integrating state-of the art microscopy and spectroscopy techniques for simultaneous interrogation of nanoscale objects, we now have the tools at our disposal to enable full scale characterisation of the optical properties of materials with dramatically reduced dimensionality. Coupled with theoretical modelling of nanoparticles, and a range of materials processing capabilities it is possible to predict, modify and measure the behaviour of nanoscale particles, namely nanodiamonds.
A variety of impurities in diamond show promise, including nitrogen, nickel, chromium and silicon related. The focus here is on the nitrogen-vacancy colour centre (NV) shown pictorially in Figure 1. The NV centre consists of a substitutional nitrogen atom adjacent to a carbon vacancy in the diamond lattice. It is photo-stable at room-temperature and has a high optical cross-section and quantum yield thus enabling the detection of photons from a single impurity2.
Figure 1. The nitrogen-vacancy (NV) centre in diamond consisting of a N-atom and lattice vacancy in C3v symmetry. This "optically active" colour centre absorbs UV-Vis light, and emits in the Vis-NIR region of the electromagnetic spectrum. It has near unity quantum efficiency and is the ideal building block for a range of quantum and biological technologies.
There are three broad areas in which colour centres in nanodiamond are playing a new and significant role: Biomedical imaging, Nanometrology and Quantum technologies. Quantum Materials and Applications (QMApp) research group at Macquarie University has projects within each of these sub-headings underpinned by a strong materials fabrication and characterisation backbone which provides built-in feedback to optimise nanodiamonds for specific applications.
Measuring objects on a scale below conventional limits (eg. limited by diffraction of light, the physical dimensions of a probe or the condition of the sample) is of critical interest to understanding the structure and function of all biological processes. As described below, nanodiamond holds a place in this field for two reasons: it is brightly fluorescing and its optical signal is susceptible to magnetic field fluctuations.
Imaging biological processes using fluorescent biolabels, attached for example to a molecule of interest travelling within a network of cells, is a well established technology, however, for a variety of reasons, the technique has not reached its full potential. Conventional fluorophores often blink on and off during the imaging process, or else bleach irreversibly thus placing a limit on the time and frequency at which the fluorophore can be observed.
Furthermore, the toxicity of certain fluorophores renders live-cell imaging impossible. Nanodiamonds have been recognised as a promising alternative for certain applications. Although some existing fluorophores are superior to nanodiamond for certain implementations, it seems clear that nanodiamonds will fill a niche in bio-imaging, where long-term photostability, resistance to blinking or bleaching and non-cytotoxicity are required.
Tremendous progress has been made worldwide in developing nanodiamonds as fluorescent biolabels, and there is genuine promise3 .One key challenge however is to pack enough optically active defects into a diamond that is small enough so as to not interfere with biological processes. It turns out this step is not so straightforward.
Under a range of fabrication conditions, it is essential to determine the "brightness" as a function of particle size and ideally develop a predictive framework for making bright luminescent nanodiamonds of certain sizes4. To get down to the ideal regime below 5 nm, attention has turned to a material called "detonation nanodiamond", which has a very narrow size distribution centred around 4 nm.
Figure 2. A confocal fluorescence map of NV centres in diamond nanocrystals (left) and the corresponding atomic force microscopy map of the diamond crystal profiles. This combined measurement technique coupled with theoretical modelling now enables the prediction of the stability of NV centres in a range of nanodiamond sizes4.
A considerable amount of work on bio-functionalisation5 has already been done, and now fluorescence measurements are showing that NV can be detected in these diamonds6. The goal at present is to optimise the material for a high concentration of NV.
Sensing weak magnetic fields by exploiting the magnetic-field sensitive optical transition of NV colour centres is the most exciting new avenue for nanodiamond based imaging, and will potentially enable imaging sensitivity at the level of single nuclear spins. Simplistically, the measurement is made by coupling magnetic resonance techniques with fluorescence measurements to detect the change of a local magnetic field near an NV centre.
Practically, the concept involves scanning a nanodiamond over magnetic fields (e.g. magnetic domains or nanomagnets) and monitor the change in optical signal as a function of position. Collecting the appropriate signal would then yield a magnetic field image of the sample surface. The nanodiamond "probe" may consist of a nanodiamond attached to an atomic force microscope tip (see Figure 3).
Figure 3. Fluorescence map of nanodiamonds glued to the tip of an atomic force microscopy cantilever tip. Under the right conditions, it will be possible to scan the tip over a surface with magnetic domains, magnetic nano-labels or even single electron or nuclear spins and map the position and strength of the fields.
The feasibility and potential of this technique to enable sensitivity to the level of single electron and nuclear spins was recently reported7. These pioneering experiments consequently highlighted the practical limitations imposed by the size and quality of diamonds available. Indeed the sensitivity of this technique is largely limited by the separation between the NV centre and the sample, and therefore dictated by the size of the crystal host. These results provide significant motivation to pursue a more deep and comprehensive understanding of the behaviour of NV centres in nanoscale diamonds4.
Diamond Quantum Technologies
In quantum computation or communication, the key building block is termed a quantum bit, or qubit. Qubits consist of a single 2-level quantum system in which the value can be 0, or 1 or a superposition between the two. It is sufficient in the context of this short article to understand this simplified explanation. The electron spin sub-levels in the ground state of the NV centre in diamond behaves like an ideal 2-level system; the spin state (or qubit value) can be 0, 1 or a combination of the two. Furthermore, the state of the spin can be "read out" using magnetic resonance and optical signals as described above. The NV centre in diamond is thus a model solid-state system on which to build a quantum information processing technologies8, and there are many groups worldwide pursuing this.
However, the challenge of implementing full-blown quantum devices in diamond again comes down to material quality and advanced fabrication strategies. It is in principle feasible to implement, for example, a small scale quantum processor in diamond, however the difficulties imposed because of limitations in material quality maintain this goal just out of reach.
Progress in developing techniques to incorporate colour centres in diamond, single photon sources in diamond, and coupled qubits in diamond will determine to what degree diamond-based quantum technologies become practical.
1. Zaitsev, A., Optical properties of diamond: a data handbook. (Springer, Berlin, 2001).
2. Kurtsiefer, C., Mayer, S., Zarda, P., and Weinfurter, H., Physical Review Letters 85 (2), 290 (2000); Brouri, R., Beveratos, A., Poizat, J. P., and Grangier, P., Optics Letters 25 (17), 1294 (2000).
3. Chang, Y.-R. et al., Nat Nano 3 (5), 284 (2008).
4. Bradac, C. et al., Nano Letters (2009).
5. Osawa, E., Pure and Applied Chemistry 80 (7), 1365 (2008); Krüger, A. et al., Langmuir 24 (8), 4200 (2008).
6. Smith, B. R. et al., Small 5 (14), 1649 (2009).
7. Balasubramanian, G. et al., Nature 455 (7213), 648 (2008); Maze, J. R. et al., Nature 455 (7213), 644 (2008).
8. Gaebel, T. et al., Nature Physics 2 (6), 408 (2006); Stoneham, A. M., Harker, A. H., and Morley, G. W., Journal of Physics-Condensed Matter 21 (36) (2009); Wrachtrup, J. and Jelezko, F., Journal of Physics-Condensed Matter 18 (21), S807 (2006).
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