Luminescence of nitrogen-vacancy centers in

PAPER

www.rsc.org/pccp | Physical Chemistry Chemical Physics

Luminescence of nitrogen-vacancy centers in nanodiamonds at temperatures between 300 and 700 K: perspectives on nanothermometry Taras Plakhotnik* and Daniel Gruber Received 18th January 2010, Accepted 16th April 2010 DOI: 10.1039/c001132k It is shown that the intensity of photoluminescence of nitrogen-vacancy (NV) centers in nanodiamond decreases 4-fold (with a wide spread among nanocrystals) when the surrounding temperature rises from 300 to 670 K. The effect is accompanied by a 2.7-fold decrease in the luminescence lifetime but negligible changes in the shape of the emission spectra. The heating–cooling circle is reversible. The effect is suggested to be practically useful for thermometry with nanometre spatial resolution but also stimulates deeper insight into the photophysics and photochemistry of NV-centers.

1. Introduction Temperature sensing on nanoscale is a tough scientific and engineering problem. Although quantitative temperature measurements have been developing since Galileo Galilei,1 the distortion of the results caused by heat transfer between the thermometer and the object whose temperature is of interest still presents a methodological problem. Non-contact techniques based on measuring black body radiation2 lose sensitivity at moderate temperatures and/or when measuring temperature of small objects. Thermal radiation from a monolayer of 10 nm particles is detectable only if their surface density is 10 mm2 or higher.3 Because the optically resolved surface area in such measurements is on the order of 100 mm2 (this is fundamentally limited by the wavelength of the detected radiation), the required minimum size of the object assessable by such thermometers is on the order of 1000 nanoparticles and single-particle operation is clearly out of reach. However, temperature sensing with nanoscale resolution is needed, for example, in nanoelectronics and nanolithography4,5 to improve stability, micro-fluidic devices, medical diagnostics and treatment.6 Nanotechnology takes up the challenge.7 Known examples of nanosensors are maximum-temperature thermometers made of silver nanospheres,8 a more traditionally looking carbon-nanotube based thermometer,9 a field-emission based sensor,10 a nanosized thermocouple at the tip of an atomic-force microscope,11,12 organic molecules,13 semiconductor quantum dots (SQD), and other nanocrystals.14–17 In this paper we investigate how luminescence of nitrogenvacany (NV) centers18 in nanodiamonds is affected by the surrounding temperature. Nanodiamonds in general and those with imbedded NV-centers have attracted much interest in recent years and have became a hot topic in many areas of research due to their unique properties such as magnetosensitivity,19 photostability,20 and chemical inertia,21 which make the material a promising candidate for various applications in nanotechnology.22–28 Not much is known about temperature dependence of NV-emission above room temperature.

School of Mathematics and Physics, University of Queensland, QLD 4072, Australia. E-mail: [email protected]

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Narrowing of the zero-phonon line, minor changes to the phonon-band structure,29 and most interestingly increase of the luminescence intensity at higher temperatures30 were reported below 200 K. Sensitivity of NV-embedding diamonds to surrounding temperatures could complement their already known characteristics and would form a basis for using these crystals also for temperature-mapping with nanometre spatial-accuracy.

2. Results and discussion The two images in Fig. 1a measured at 373 and 623 K show spatial distribution of luminescence intensity (represented by color and the vertical scale) within a 40 40 micron field detected by EMCCD. The sharp peaks indicate presence of NV centers at the corresponding locations. The surface density of the crystals is about 0.03 mm2. All peaks are close to diffraction limited spatial size with full-widths at half maxima of 0.9 micron. To determine the total intensity emitted by each of the crystals, the intensity has been integrated over the 4 4 micron area centered at the pixel with the highest signal. The positions of the particles have been determined as a center P P of gravity: [ x, y] = m,nCmn[m, n]/ m,nCmn, where Cmn is the number of counts detected at the pixel with horizontal and vertical coordinates [m, n]. The average integrated intensity of the observed bright spots (excluding the strongest two) is (10 5) 103 photocounts per second (cps). Given the 0.2% detection efficiency, the observed count rate corresponds to an emission rate of about 5 106 photons per second which is 1/6 of the maximum emission rate determined by the luminescence decay rate (see the caption of Fig. 2). The power density of the laser radiation was about 1/6 of the saturation intensity31 (see also below). The detected signal corresponds on average to the brightness of two NV-centers. The two strongest peaks having integrated intensities of 65 103 and 35 103 cps at the lower temperature are likely to be clusters of several crystals while smaller peaks with relatively narrow height distribution correspond to isolated single crystals. The assignment of this emission to NV-defects in diamond is confirmed by the presence of zero-phonon lines at 638 and 575 nm in the Phys. Chem. Chem. Phys., 2010, 12, 9751–9756 | 9751

Fig. 1 Thermometry with single diamond crystals based on luminescence intensity. (a) Two micron images of the sample obtained at two temperatures. The vertical scale in the image taken at higher temperature is multiplied by a factor of 2.5. (b) Experimental temperature dependence of integrated intensities for 10 representative nanocrystals (thin color lines with dots indicating the measured points). The thick black line shows the dependence averaged over all diamonds in the field of view (about 100 nanocrystals) and thus represents ensemble averaged intensity dependence.

emission spectra (see Fig. 3b). It is clear that the position and the shape of each peak have striking similarities in the two images. The maximum discrepancy between centers of ‘‘gravity’’ for the images of the same crystal taken at different temperatures was determined to be less than 0.5 mm. The crystals were actually changing their position by up to 20 mm due to thermal deformation of the oven. However, they were brought back to the same spot using an XYZ mechanical stage. A small error in the positioning ensures little dependence of the excitation irradiance on the temperature which could arise since the position of the laser beam was fixed relative to the light focusing/collecting microscope objective and the CCD. However, fluctuations in the intensities due to speckles in the in the illuminated spot were still present and caused irregular fluctuations of the intensities on the level of 10–20%. However, the intensities of the peaks are approximately 2.5 times lower at 623 K than at 373 K. A systematic measurement of temperature dependence for 100 crystals revealed that the integrated intensity decreases on average approximately linearly by about 4-fold (see thick black line in Fig. 1b) when the temperature rises from 320 to 670 K. Linear dependence was observed14 in ensembles of nanoparticles such as CdTe, ZnS:Mn2+ and BaFBr:Eu3+ but as shown here, individual crystals demonstrate broad variation in their temperature sensitivity. Some representative examples are displayed in Fig. 1b as thin lines. Note that for some crystals the intensity starts to change significantly only above 420 K but for others the dependence is strong between 300 and 470 K and then flattens. Sometimes a linear dependence is also observed for a single particle. The microscopic reason for these variations is unclear at present but deserves thorough investigations because of a possibility to have crystals tailored to a specific temperature range. Diversity of mechanisms can be responsible for the temperature effects. Fig. 2a shows the energy-level scheme and the relaxation rates notations. We make a usual assumption that k2 0 2, the relaxation from the vibrationally excited state 2 0 is very fast but that k2 0 3, the direct intersystem crossing from the 9752 | Phys. Chem. Chem. Phys., 2010, 12, 9751–9756

vibrationally excited state is relatively negligible. Next, we neglect spin depolarization and assume that only z-sublevel is populated. In this case, the detected photoluminescence rate in the steady state under continuous excitation reads k31 fkI k31 þ k23 R ; k31 k þ k0 þ k23 Iþ k31 þ k23 s

ð1Þ

where f is the photon detection efficiency. The term: k31 k þ k0 þ k23 Is k31 þ k23 s is called saturation intensity (in units of s1 m2). If the irradiance does not saturate the optical transition, the intensity term in the dominator of eqn (1) can be neglected and an increase in nonradiative rate k 0 + k23 (which is equivalent to reduction of the luminescence quantum yield) becomes an obvious reason for decreased luminescence. A partial spin polarization complicates the matter but the expression catches the main physics. The validity of the approximation given in eqn (1) has been investigated experimentally for several nanocrystals. The results are shown in Fig. 2b and c. The fits to the theoretical curves reveal that the saturation intensity, that is the term (k + k 0 + k23)s1k31/(k31 + k23) increases with the temperature significantly (by a factor of 3 for the data in Fig. 2c) while the maximum emission rate, proportional to kk31/(k31 + k23), changes very little (factor of 1.3 in Fig. 2c). Dependence of the radiative rate on temperature earlier was observed for J-aggregates32–34 and quantum dots.35 However the mechanisms responsible for such dependence are not known to be applicable to NV-centers in diamond. In J-aggregates, for example, the reason is the collective nature of the excited states. Thus we conclude that both k and k31/(k31 + k23) are only weakly temperature dependent while s/(k + k 0 + k23) is the main temperature dependent factor. This journal is

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Fig. 2 Transitions and saturation effects in NV-centers. (a) Energy levels of NV-center in diamond and the transitions between these levels. States 1 and 2 are triplet states (2 0 being a vibrationally excited state), and 3 is a metastable singlet state. The nonradiative transitions are shown by wavy lines. Note that there is only a very slow transition (not shown) from the z-sublevel of state 2 (in comparison to the transition from the x- and y-sublevels) to the singlet state. The rates known from the literature:41 k + k 0 E 77 MHz (this rate was measured in bulk samples where the rate is 4 (x,y) approximately two times higher than in nanocrystals), k23 E 30 MHz, k31 E 3.3 MHz, and k(z) 23 E 5 10 k23 . The radiative transitions from x,y-sublevels of state 2 to z-sublevel of state 1 have rates of E 1.5 MHz in bulk. The dashed wavy lines show nonradiative transitions for which no data are available. Generally, k2 0 2 c k and therefore radiative emission from level 2 0 can be neglected and is not included in eqn (1). The optical pumping irradiance at the location of the crystal is I and s is the absorption cross-section of an NV-center. Panels (b) and (c) show saturation effect in two diamond crystals at different temperatures. The solid lines are theoretical curves R = RNI/(I + Is) and the two parameters of the curves are (473) (573) as follows: (b) I(296) = 0.048 MW cm2, R(296) = 0.074 MW cm2, R(473) = 0.10 MW cm2, R(573) s N = 0.12 MHz, Is N = 0.13 MHz, Is N = 0.11 MHz (296) 2 (296) (473) 2 (473) and for the case (c) Is = 0.065 MW cm , RN = 0.21 MHz, Is = 0.181 MW cm , RN = 0.16 MHz. Superscripts in brackets indicate the corresponding temperatures. The intensities do not take into consideration 4% reflection from the quartz substrate.

A second possible reason for the reduced NV-emission could be a change in the value of s at 532 nm, the wavelength of the excitation laser, due to a shift or distortion of the absorption band with changing temperature. We were not able to measure absorption cross-section directly because of strong scattering of the nanodiamonds. Instead we have measured emission spectrum which indirectly represents also absorption due to approximate mirror symmetry between absorption and emission spectra.18 A large change in the emission spectrum itself could contribute to the variations in the detected signal due to the presence of a band-pass optical filter in front of the

EMCCD camera. Three spectra measured at different temperatures are shown in Fig. 3a. No significant changes were detected. Disappearance of the zero-phonon lines visible as little bumps at 638 and 575 nm only at the lowest temperature has a little effect on the intensity passing through the band of the optical filter (see Fig. 3a). Note that even if the mirror symmetry between the absorption and emission spectra holds only approximately, it is highly unlikely that one of them changes while the other does not. Therefore we conclude that changes in the absorption/emission bands have negligible effects on the photoluminescence intensity.

Fig. 3 Thermometry with NV-centers in diamond based on time-resoled and spectral measurements. (a) Spectra (normalized to have the same integrated intensity) measured at three temperatures. The transparent band shows the transmission band of the band-pass optical filter. Zero-phonon lines of charged NV at 638 nm and neutral NV0 at 575 nm centers are labeled by the arrows. (b) Decay of luminescence at different temperatures measured with pulsed excitation and ICCD as a photo detector. The solid lines show fits to two-exponential decay functions (a)exp(t/t1) + (1 a)exp(t/t2) with the same value of a = 0.2 for all temperatures. Insert shows temperature dependence of the longer decay time t1 deduced from the curves. The straight dashed line shows the expected effect on the lifetime deduced from the solid black line in Fig. 1b, the averaged intensity vs. temperature.

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A reason for increase in the nonradiative rate is a breakdown of the optical polarization effect. Because the relaxation from the singlet state to the ground state populates mainly the z-sublevel but the intersystem crossing from the excited triplet state depopulates almost exclusively x- and y-sublevels, the population of the z-sublevels quickly builds up under continuous excitation. This leads to increased luminescence intensity because of dramatically reduced nonradiative intersystem crossing rate (effectively the nonradiative decay rate close to k 0 in a z-polarized NV-center). When temperature increases, the spin–lattice relaxation rate increases (this rate equals 2 Hz at 100 K and 1 kHz at 300 K) and as a consequence the population of all triplet sublevels equilibrates more effectively. At the equilibrium, the luminescence decay will become double exponential with (x,y) 0 rates k + k0 + k(z) 23 (T) and k + k + k23 (T). Note that intersystem crossing rate k23 may also increase as the temperature rises. However, the required rate of the spin–lattice relaxation rate should be four orders of magnitude larger than the room temperature rate (that is on the order of k31) to have a significant depolarization effect. Thus we conclude that increase of k0 is a far more likely reason of increase in the nonradiative rate. To emphasize the advantage of a single-particle approach to the problem in comparison to bulk measurements we investigated the temperature effect on an ensemble of nanocrystals. After a short pulse excitation, the luminescence decays as R(t) p exp[t(k + k 0 + k23)].

(2)

We measured luminescence decay at different temperatures in an ensemble of nano-crystals covering the quartz substrate at a high concentration. The results are shown in Fig. 3b. The shortening of the lifetime is obvious from the data. Note that the decay curve at the lowest temperature was measured twice: first when the temperature was raised to 720 K (for 10 min) and then after cooling down back to 310 K. Both curves are shown in the figure but they are practically indistinguishable and thus confirm the reversibility of the temperature effect. This is consistent with reported observations of no significant oxidation of nanodiamonds at temperatures below 650 K and that at 700 K the loss of diamond becomes significant on the scale of hours.36 All the curves are not single exponential but can be fit well with two exponents with relative amplitudes 0.8 for faster and 0.2 for slower decays. The fitted curves are shown as solid lines in the figure. At room temperature the two lifetimes were 27 and 4.7 ns. The longer lifetime is close to the value reported for NV-centers in isolated nanodiamonds.37 However, a range of luminescence lifetimes has been reported for NV-centers in diamond38 and therefore multi-exponential character of the curve was expected. At 670 K, the two lifetimes decrease by a factor of 2.7 and 2, respectively. The inset in Fig. 3b shows the dependence of the longer decay time on the temperature. Note that at 720 K the lifetime is four times shorter than at room temperature. The solid line represents a quadratic fit to the experimental points and is shown only as a guide line. The disagreement between the intensity data (dashed line in the inset) and the lifetime shortening is quite clear in the middle of the temperature range. Given the large inhomogeneity in the properties of individual crystals, 9754 | Phys. Chem. Chem. Phys., 2010, 12, 9751–9756

the averaged curves are not described accurately by simple equations which work very well for individual crystals. Now we estimate theoretically the sensitivity of nanodiamonds and the time required to achieve a specified accuracy. The errors in the intensity measurements are fundamentally determined by the detected number of photons. Taking the radiative emission rate to be 10 MHz and the collection efficiency of 1%, the detected count rate can be as high as 105 cps per single NV-center. Because of Poisson statistics of the detected counts, 1% accuracy in the measured intensity requires detection of 104 counts and thus can be achieved with 0.1 s integration time. On the other hand, 1% change in the intensity corresponds roughly (depending on the crystal, see Fig. 1b) to a 1 K change in the temperature. The accuracy with which the location of a nanocrystal can be determined is also limited by the photon statistics. In this example it is approximately 1% of the optical resolution (E1 mm) making it as small as 10 nm. In our experiments, the standard deviation between the positions of the same crystals was smaller than 6 nm when two images were taken at the same temperature. The accuracy of a single measurement is 21/2 times better than the difference between two such measurements and is estimated to be about 4 nm (that is 0.25% of the optical resolution). Note that the position estimates, unlike the intensity estimates, depend very little on the irradiance fluctuations which may arise from technical instabilities in the illumination scheme and/or in the laser power but fundamentally statistical fluctuations affect position and temperature measurements in a similar way. Accurate measurement of luminescence intensity may be problematic if the particle is not stationary and its movement causes extra changes in the irradiance at the location of the particle. However, this difficulty can be overcome if the lifetime (which is irradiance independent) is measured instead. Finally, it is known18 that NV centers are stable at least up to 1100 K, therefore in oxygen-free environments the temperature range can be extended beyond that reported in this paper.

3. Experimental Luminescent diamonds were purchased from Academia Sinica production facility 39 and had a relatively broad size distribution between 20 and 35 nm. They were spin-coated on a quartz substrate which was mounted in a homemade oven. The temperature of the oven could be stabilized anywhere between room temperature and 770 K with estimated 5 K accuracy. The substrate with diamonds was heated to 723 K in air, prior to taking the data, to remove a residue of the stabilizer, sodium azide (added to water at 10 ppm concentration) which thermally decomposes above 600 K. A long focal distance micro objective (Nikon, NA 0.55, 50) was placed outside the oven and the emission from the sample was collected through a small (1.2 mm in diameter) window. The position of the luminescence-exciting laser beam was fixed relative to the microscope objective, however, the sample was subject to large (on the order of 100 microns) displacements caused by thermal expansion of the sample holder. To compensate for these displacements, the oven was mounted on an XYZ-translation This journal is

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stage (Melles Griot) and moved into the same position after every temperature increase/decrease. The image of the sample was taken with electron-multiplying CCD (EMCCD, Andor) working as a conventional CCD (for the below 1 Hz readout rate used in these experiments no gain was a better option) cooled down to 60 1C or a 4Picos CCD (Stanford Computer Optics) with an image intensifier (ICCD). The gain of the image intensifier could be controlled with a time resolution of about 0.2 ns and thus provided temporal resolution40 for measuring decay of the photoluminescence after the pulsed excitation. Photoluminescence of NV-centers was excited with either a pulsed fiber laser (laser pulse 0.6 ns, repetition rate 72 kHz, Standa) or CW laser (5W Verdi, Coherent). The detection system operated either in imaging or in spectral modes. In the spectral mode, emission was spectrally dispersed with a 300-mm grating spectrometer (Acton). A long-pass filter (Semrock, 532 nm) was used in both imaging and spectral modes. In the imaging mode, a band-pass optical filter (672–695 nm, transmission 85% within the band) was used to improve the signal-to-noise ratio by a factor of 2. This filter was most useful at elevated temperatures when thermal radiation from the oven became a problem. The overall collection efficiency of our setup was estimated to be 0.2%. The laser beam of 1 W power was focused on a spot of 80 microns in diameter. The power density of the laser radiation was about 0.02 MW cm2 in the middle of the laser spot which is three times lower than the measured saturation intensity (see Fig. 2).

4. Conclusion This paper reports proof of principle experiments on NV-centers in nanodiamond demonstrating feasibility of measuring temperature with 10-nm spatial and 100-ms temporal resolutions. The corresponding thermometric accuracy depends on precision of calibration but theoretically can be as high as 1 K. NV-centers in diamond nanocrystals demonstrate sensitivity to temperature in a broad temperature range and the temperature can be assessed optically under normal conditions with a wide-field microscope. The main reason for temperature dependent emission rate is the temperature dependent nonradiative rate which increases at higher temperatures. Understanding of the actual mechanism is important for understanding the electronic structure of the center, which is of great interest for quantum information technology,41 and requires further investigations. Such thermometers are compatible with nanoscale electronic, microfluidic devices, and potentially biological cells and macromolecules, if the sensitivity is tailored to the appropriate temperature range. Diamond based films of nanocrystals are widely used for surface protection. The discovered property will add extra functionality to these coatings.

Acknowledgements The authors acknowledge support of ARC, grant DP0771676. This journal is

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References 1 H. C. Bolton, Evolution of the Thermometer, Kessinger Publishing, Whitefish, MT, 2008, pp. 1592–1743. 2 E. F. J. Ring, J. Med. Eng. Technol., 2006, 30, 192–198. 3 M. G. Cerruti, M. Sauthier, D. Leonard, D. Liu, G. Duscher, D. L. Feldheim and S. Franzen, Anal. Chem., 2006, 78, 3282–3288. 4 R. D. Piner, J. Zhu, F. Xu, S. Hong and C. A. Mirkin, Science, 1999, 283, 661–663. 5 L. Huang, Y.-H. Chang, J. J. Kakkassery and C. A. Mirkin, J. Phys. Chem. B, 2006, 110, 20756–20758. 6 X. Huang, I. H. El-Sayed, W. Qian and M. A. El-Sayed, J. Am. Chem. Soc., 2006, 128, 2115–2120. 7 J. Lee and N. A. Kotov, Nanotoday, 2007, 2, 48–51. 8 Y. Lan, H. Wang, X. Chen, D. W. G. Chen and Z. Ren, Adv. Mater., 2009, 21, 4839–6. 9 Y. Gao and Y. Bando, Nature, 2002, 415, 599. 10 C. M. Tan, J. Jia and W. Yu, Appl. Phys. Lett., 2005, 86, 263104. 11 C. C. Williams and H. K. Wickramasinghe, Appl. Phys. Lett., 1986, 49, 1587–1589. 12 W. Haeberle, M. Pantea and J. K. H. Hoerber, Ultramicroscopy, 2006, 106, 678–686. 13 J. M. Lupton, Appl. Phys. Lett., 2002, 81, 2478–2480. 14 S. Wang, S. Westcott and W. Chen, J. Phys. Chem. B, 2002, 106, 11203–11209. 15 G. W. Walker, V. C. Sundar, C. M. Rudzinski, A. W. Wun, M. G. Bawendi and D. G. Noceraa, Appl. Phys. Lett., 2003, 83, 3555–3557. 16 S. Li, K. Zhang, J.-M. Yang, L. Lin and H. Yang, Nano Lett., 2007, 7, 3102–3105. 17 H. Peng, M. I. J. Stich, J. Yu, L.-n. Sun, L. H. Fischer and O. S. Wolfbeis, Adv. Mater., 2009, 21, 625–4. 18 G. Davies and M. F. Hamer, Proc. R. Soc. London, Ser. A, 1976, 348, 285–298. 19 A. Gruber, A. Drabenstedt, T. Tietz, L. Fleury, J. Wrachtrup and C. v. Borczskowski, Science, 1997, 276, 2012. 20 C.-C. Fu, H.-Y. Lee, K. Chen, T.-S. Lim, H.-Y. Wu, P.-K. Lin, P.-K. Wei, P.-H. Tsao, H.-C. Chang and W. Fann, Proc. Natl. Acad. Sci. U. S. A., 2007, 104, 727–732. 21 A. M. Schrand, H. J. Huang, C. Carlson, J. J. Schlager, E. Osawa, S. M. Hussain and L. M. Dai, J. Phys. Chem. B, 2007, 111, 2–7. 22 H. Huang, E. Pierstoff, E. Osawa and D. Ho, Nano Lett., 2007, 7, 3305–3314. 23 P. Neumann, N. Mizuochi, F. Rempp, P. Hemmer, H. Watanabe, S. Yamasaki, V. Jacques, T. Gaebel, F. Jelezko and J. Wrachtrup, Science, 2008, 320, 1326–1329. 24 L. Childress, M. V. G. Dutt, J. M. Taylor, A. S. Zibrov, F. Jelezko, J. Wrachtrup, P. R. Hemmer and M. D. Lukin, Science, 2006, 314, 281–285. 25 G. Balasubramanian, I. Y. Chan, R. Kolesov, M. Al-Hmoud, J. Tisler, C. Shin, C. Kim, A. Wojcik, P. R. Hemmer, A. Krueger, T. Hanke, A. Leitenstorfer, R. Bratschitsch, F. Jelezko and J. Wrachtrup, Nature, 2008, 455, 648–651. 26 J. R. Maze, P. L. Stanwix, J. S. Hodges, S. Hong, J. M. Taylor, P. Cappellaro, L. Jiang, M. V. G. Dutt, E. Togan, A. S. Zibrov, A. Yacoby, R. L. Walsworth and M. D. Lukin, Nature, 2008, 455, 644–647. 27 E. Rittweger, K. Y. Han, S. E. Irvine, C. Eggeling and S. W. Hell, Nat. Photonics, 2009, 3, 144–147. 28 K. B. Holt, Philos. Trans. R. Soc. London, Ser. A, 2007, 365, 2845–2861. 29 V. Hizhnyakov, V. Boltrushko, H. Kaasik and I. Sildos, J. Chem. Phys., 2003, 119, 6290–6295. 30 A. Dra¨benstedt, L. Fleury, C. Tietz, F. Jelezko, S. Kilin, A. Nizovtzev and J. Wrachtrup, Phys. Rev. B: Condens. Matter Mater. Phys., 1999, 60, 11503–11508. 31 E. Rittweger, D. Wildanger and S. W. Hell, Europhys. Lett., 2009, 86, 14001. 32 H. Fidder and D. A. Wiersma, Phys. Status Solidi B, 1995, 188, 285–295. 33 V. F. Kamalov, I. A. Struganova and K. Yoshihara, J. Phys. Chem., 1996, 100, 8640–8644. 34 I. G. Scheblykin, M. M. Bataiev, M. V. d. Auweraer and A. G. Vitukhnovsky, Chem. Phys. Lett., 2000, 316, 37–44.

Phys. Chem. Chem. Phys., 2010, 12, 9751–9756 | 9755

35 J. F. Angell and M. D. Struge, Phys. Rev. B: Condens. Matter, 1993, 48, 4650–4658. 36 S. Osswald, G. Yushin, V. Mochalin, S. O. Kucheyev and Y. Gogotsi, J. Am. Chem. Soc., 2006, 128, 11635–11642. 37 A. Beveratos, R. Brouri, T. Gacoin, J. P. Poizat and P. Grangier, Phys. Rev. A: At., Mol., Opt. Phys., 2001, 64, 061802. 38 B. R. Smith, D. Gruber and T. Plakhotnik, Diamond Relat. Mater., 2010, 19, 314–318.

9756 | Phys. Chem. Chem. Phys., 2010, 12, 9751–9756

39 Y. R. Chang, H. Y. Lee, K. Chen, C. C. Chang, D. S. Tsai, C. C. Fu, T. S. Lim, Y. K. Tzeng, C. Y. Fang, C. C. Han, H. C. Chang and W. Fann, Nat. Nanotechnol., 2008, 3, 284–288. 40 B. R. Smith, D. W. Inglis, B. Sandnes, J. R. Rabeau, A. V. Zvyagin, D. Gruber, C. J. Noble, R. Vogel, E. Osawa and T. Plakhotnik, Small, 2009, 5, 1649–1653. 41 N. B. Manson, J. P. Harrison and M. J. Sellars, Phys. Rev. B: Condens. Matter Mater. Phys., 2006, 74, 104303.

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