Single-Photon Emission and Quantum Characterization of Zinc Oxide

Jan 16, 2012 - Room temperature single-photon emission and quantum characterization is reported for isolated defects in zinc oxide. The defects are ...
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Single-Photon Emission and Quantum Characterization of Zinc Oxide Defects Anthony J. Morfa,*,†,‡ Brant C. Gibson,*,‡ Matthias Karg,† Timothy J. Karle,‡ Andrew D. Greentree,‡ Paul Mulvaney,† and Snjezana Tomljenovic-Hanic‡ †

Bio21 Institute and School of Chemistry and ‡School of Physics, University of Melbourne, Victoria 3010, Australia ABSTRACT: Room temperature single-photon emission and quantum characterization is reported for isolated defects in zinc oxide. The defects are observed in thin films of both in-house synthesized and commercial zinc oxide nanoparticles. Emission spectra in the red and infrared, second-order photon correlation functions, lifetime measurements, and photon count rates are presented. Both two- and three-state emitters are identified. Sub-band gap absorption and red emission suggest these defects are the zinc vacancy. These results identify a new source of single photons in a readily available wide band gap semiconductor material which has exceptional electrical, optical, and biocompatibility properties. KEYWORDS: Zinc oxide, single-photon source, defect emission, quantum characterization, zinc vacancy, nanoparticle

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debate.22,23,26 Many point defects have been suggested to explain this emission, including oxygen vacancies, oxygen interstitials, zinc interstitials, antisite oxygen, and zinc vacancies.22,23 Fabrication conditions dramatically alter the properties of the observed defects, with stoichiometric control and thermal conditioning critical.27 The so-called ‘red emission’ from ZnO has been attributed to oxygen vacancies VO and zinc vacancies VZn by various researchers.22 Most studies, however, use excitation photon energies above the band gap of ZnO, which perforce introduce carriers and therefore modify the charge environment of the defect. Recently, it has been shown that a particular defect can be optically excited with sub-band gap photons of energy ∼2.4 eV and emit at ∼1.6 eV; and density functional theory (DFT) was used to attribute this to the zinc vacancy.27 Despite ZnO being a well-studied material, no studies have shown emission from single defects, with all of the earlier reports referring to ensembles of emitters. Hence single-photon emission from individual defects has not been demonstrated nor has direct measurement of single defect properties been reported. Such single defect techniques are extremely useful for accurately determining the nature of the defects including emission properties, their crystallographic origins, and the influence of local environmental effects. In addition, singledefect emission from ZnO could allow such defects to be used as a platform for quantum information processing and fundamental quantum optics experiments.20 Here we report the first demonstration of room-temperature single-photon emission from nanoparticle-derived ZnO thin films. Without consensus in the literature, we use the nomenclature of Wang et al27 to assign the red luminescence

raditional methods for characterizing defects in materials have involved bulk measurements, sampling many instances of the defect. However it is often necessary to monitor the properties of a single system, especially to deconvolve issues that combine the local environment of a particular system with ensemble properties. This is especially important when local environmental fluctuations may greatly affect measured quantities, such as absolute fluorescence and nonradiative recombination rates. Single systems are also fundamental building blocks for quantum devices and constitute the absolute limit of dilution for any sample. Emission from single defects or emitters is also important for a range of quantum protocols.1−3 To date, emission from single molecules,4 nanoparticles,5−8 carbon nanotubes,9 and diamond centers has been observed.10−15 By far, the most advanced source of single-photon emitters are self-organized quantum dots of materials, like GaN16 and InAs,17 that can be electrically driven.18 Such systems emit in the UV and infrared, respectively, but must be operated at low temperatures due to shallow confinement.17,19 At present, diamond is the only known robust source of photostable single photons at room temperature as molecular systems rapidly photodegrade, while most inorganic systems require cooling to low temperatures. Recent reports on the theoretical single defect emission from silicon carbide highlight the benefits of using wide band gap semiconductors as single-photon sources,20 and indeed roomtemperature quantum coherence in spin ensembles of silicon carbide defect centers has been observed.21 Zinc oxide is the subject of considerable investigation and has been utilized for numerous applications in optoelectronics.22,23 It is a wide band gap semiconductor that can be readily fabricated into optoelectronic devices. Photoemission from ZnO has been observed from the UV to the visible.24,25 While the origin of the UV emission is well understood to be direct exciton emission, the visible emission source is still a matter of © 2012 American Chemical Society

Received: November 14, 2011 Revised: January 3, 2012 Published: January 16, 2012 949

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to the zinc vacancy (VZn). As in the work by Wang et al,27 excitation is sub-band gap, and the red luminescence increases proportionally with incident laser power. Two sources of ZnO materials were explored: ZnO nanoparticles prepared in-house (ZnOMelb)28 and commercially available ZnO nanopowder from Sigma Aldrich (ZnOSigma) with an average diameter less than 50 nm. Zinc oxide films were prepared using spin casting29 (ZnOMelb) or drop casting (ZnOSigma). ZnO thin films were postprocessed by thermal annealing to 500 °C in air to form a dense film with grain sizes larger than 20 nm and with average film thicknesses of 40 nm for ZnOMelb samples and 1 μm for ZnOSigma samples. These annealing conditions are known to preserve zinc vacancies and remove oxygen vacancies.27 When samples of ZnOMelb prepared at 175 °C were illuminated, the defect emission rapidly photodegraded, while samples prepared at 600 °C were found to have a very low observable defect density (less than 1 per 10 000 μm2). Samples prepared from ZnOSigma with particle sizes greater than 100 nm contained no identifiable single emitters. Surface topography measurements were performed using atomic force microscopy (AFM). Single-photon emission characteristics were investigated at room temperature under ambient conditions using scanning confocal microscopy. A schematic of the optical characterization is shown in Figure 1.

the ZnO were found to exhibit a characteristic single-photon dip in the second-order correlation function (g(2)) at the zero delay time (t = 0). Figure 2 shows the results of the optical characterization of localized single-photon emitters. The representative antibunching data in Figure 2(a), for the ZnO Melb sample, and 2(b), for the ZnO Sigma sample, demonstrate that localized single-photon emitters are present, and indicate that single-photon emission is not limited to one individual nanoparticle fabrication technique. For the same single-photon emitters measured for Figure 2a,b, photon counts were collected over 600 s for the ZnOMelb sample, shown in Figure 2c, and for the ZnOSigma sample, shown in Figure 2d. These figures show blinking between a bright and a gray state when compared to the background measured 1 μm from the defect location (average background counts of ∼3 k counts s−1 shown as dashed line). As with the blinking behavior, the single-defect emission spectra also show two clear features or states. The background corrected emission spectra are shown in Figure 2e,f for ZnOMelb and ZnOSigma, respectively. The dashed lines in Figure 2e,f are Gaussian fits to each emission line. For the ZnOMelb sample, one emission line is centered at 1.88 eV and the other at 1.77 eV, whereas for the ZnOSigma sample, the states are observed at 1.71 and 1.56 eV. The fact that we observe antibunching on this transition proves that these two spectral features are associated with the same source and not two separate emitters in close spatial proximity with close emission lines. Although blinking between a bright and gray state has previously been observed for semiconductor nanoparticles, the excitonic emission was found to exhibit only one spectral feature. In the case of CdSe nanoparticles, the gray state has been shown to be due to the formation of the trion state.31,32 We observe two spectral peaks in the optical emission from nanoparticle-derived ZnO thin films. Local charging is likely in the ZnO thin films as they have been annealed above their coalescence temperature, however the films still have grain sizes less than 100 nm.29 Thus, we suggest that charges can be bound near a single defect, most likely at grain boundaries. However, this does not explain the two spectral features but merely the blinking resulting from local charging altering the single-defect quantum yield. As mentioned above, since the g(2) confirms that this is a single-photon source, presumably the two states are from the same defect emitting at two different energies. The observation of two peaks with an average separation of ∼0.08 eV is consistent with observations and optical modeling by Shi et al33 (see Figure 2e,f) that determines a phonon energy of ∼0.07 eV in ZnO. Therefore, it is possible that the bright and gray states are the result of emission from a single defect, emitting at its fundamental energy, and a phonon sideband. Interdefect variability in emitted photon energies was almost 300 meV, with different defects emitting from 1.56 eV (793 nm) for the ZnOSigma samples to as high as 1.88 eV (660 nm) for defects in the ZnOMelb sample. This can readily be explained using the emission mechanism determined by Wang et al.27 (see Figure 1 inset). The proposed mechanism involves emission from the conduction band to the zinc vacancy trap. Any alteration to the local environment that affects the band levels will have an effect on the emission properties. It has repeatedly been shown that ZnO surface chemistry results in the oxidation of molecules, which has the effect of accumulating electrons at the ZnO surface.34 Such accumulation of electrons would alter the conduction band level and would obviously alter the emitted photon energy (see Figure 1). As the ZnOMelb

Figure 1. Schematic of experimental setup showing confocal mapping of a ZnO surface prepared from nanoparticles. Following Wang et al,27 we deduce that the red emission is from the zinc vacancy excited by green light. Upper left, measured characteristic second-order correlation function indicating single-photon emission. Upper right, energy level diagram for zinc vacancy, as determined from density functional theory calculations.27 Bottom, SEM micrograph of ZnO surface (1.5 × 1.5 mm2).

A continuous wave pump laser with a wavelength of 532 nm was used to excite the sample, and emission was collected using a previously described instrumental setup.30 Representative photon statistics from each of these materials were measured at room temperature with an optical fiber-coupled Hanbury Brown−Twiss (HBT) interferometer. Fluorescent emitters in 950

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Figure 2. Room-temperature characterization of single-photon emitting defects in ZnO films, from two sources of nanoparticles, prepared in-house (ZnOMelb: a, c, e) and commercially available (ZnOSigma: b, d, f). (a,b) Background subtracted second-order correlation function (g(2)) showing a dip at time zero for both samples. In each case, pump power was 330 μW, highlighting variations in defect emission. (c,d) Blinking traces showing bright and gray level (average background is shown as a dashed blue line). Note that single-photon emission is derived from both levels. (e,f) Background subtracted, single-defect fluorescence showing two broad emission lines in each case. The dash lines are Gaussian fits to each emission line, while the thick black line is the sum of the two Gaussian peaks.

by fitting the g(2) data to eq 1 over a range of excitation powers. The intrinsic spontaneous emission rate is τ21, and the measured time between photon emission events is τ, which converges to τ21 in the limit of no excitation power. Fitting the decay rate, k = τ−1, versus the incidence power to a linear equation provides the zero-power decay rate (see inset of Figure 4). The intrinsic spontaneous emission lifetime is thus τ(0) = k−1(0). For the case of the single defect shown in Figure 3b, the radiative lifetime is found to be 4.16 ns.

samples are 40 nm thick, the average defect is no more than 20 nm from the surface; thus, it is reasonable to ascribe the blue shifting of the emission from 1.56 to 1.88 eV to a changed conduction band level resulting from charge accumulation at the surface. In addition to spatially resolved spectral information, monitoring of isolated defects through a HBT interferometer also provides qualitatively new information. In particular, it is possible to determine whether the optically active transition is of a two- or three-state (i.e., with a shelving state) nature. On the ZnOMelb sample, we identified two defects physically separated by no more than 50 μm, as marked in Figure 3a, with qualitatively different emission properties. The second-order correlation function is shown for each of these locations in Figures 4 and 5a. Location D1 (shown in Figure 3b) exhibits a characteristic antibunching in the g(2) (Figure 4), indicative of a two-state emitter. The lifetime of the emitter can be determined

g(2)(τ) = 1 − e−τ21/ τ

(1)

Conversely, location D2 (shown in Figure 3c) exhibits a pronounced bunching in the g(2) response (Figure 5a), indicating emission occurs via a three-state mechanism. In this case, g(2) must be fitted to eq 2 which has two exponential components. The first, τ21, represents radiative emission from 951

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Figure 5. (a) Second-order photon correlation function for defect D2 from Figure 3. Prominent bunching features indicate that at least three states are involved in the emission process. Decay rates (b) 1/τ21 and (c) 1/τ23,31 as a function of pump power as determined from fitting a three state model. The zero-power intercept shows the spontaneous emission lifetime is 1.01 ns and the nonemissive time is 278.81 ns.

the excited state to the ground state, and the second, τ23/31, represents a slower, nonradiative pathway. As with the two-state defect emission, the nonperturbed lifetime is determined by linearly fitting the decay rates as a function of laser power. The calculated zero-power decay rates correspond to radiative and nonradiative (Figure 5b,c, respectively) lifetimes of 1.01 and 279 ns, respectively.

Figure 3. (a) Scanning confocal fluorescence map of in-house fabricated ZnO films illuminated with 532 nm CW light. Locations marked ‘D1’ and ‘D2’ identify single-photon emitters that are shown in higher resolution in (b) and (c), respectively. Correlation functions for these centers are shown in Figures 4 and 5.

g(2)(τ) = 1 + c e−τ21/ τ − (1 + c)e−τ23/31/ τ

(2)

Observation of an isolated single defect emitting via two different pathways is unlikely. Based on the data presented in Figures 4 and 5, we suggest that the emission pathway of the observed defects is sensitive to charging, grain boundaries, surfaces, or other factors. The other possibility is that two different types of defect are present. However the thermal annealing to 500 °C and the sub-band gap excitation make this possibility less likely.27 The radiative lifetimes measured for both defects are significantly shorter than those measured for the so-called ‘green luminescence’,35 typically ascribed to oxygen vacancies. These results are consistent with UV-excited, ensemble measurements of the ‘red luminescence’ which was shown to be shorter than the ‘green luminescence’.36 Additionally, these values are consistent with UV-excited emission lifetimes seen by Koida et al, which were ascribed to VZn trapping and emission processes.37 This provides further evidence that the observed defect is not related to the commonly ascribed oxygen vacancy. In conclusion, optical emission from single, isolated zinc vacancies has been observed in ZnO for the first time. We have shown that these defects are intrinsic to ZnO produced as nanoparticles and are observable in samples annealed to 500 °C. The VZn can be directly excited with 330 μW of 532 nm laser light and exhibits the well-known ‘red luminescence’

Figure 4. Second-order photon correlation function for defect D1 from Figure 3. The lack of bunching indicates that only a two-state emission is observed. The inset shows the variation of 1/τ12 with pump power, determined via a fit to a two-state model. The zero-power intercept shows that the spontaneous emission lifetime is 4.16 ns. 952

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seen in many samples. This defect has a radiative lifetime between 4 and 1 ns at room temperature, depending on whether the emission follows a two- or three-state mechanism. As local surface states are known to alter defect properties,29 the addition of molecules bound to the surface, such as amines or mercaptans, could lead to controllable defect emission energies. The observation of single defects in ZnO provides a new method for probing semiconductor thin films and will permit future studies on the origin and stability of defects in these films. Such single-defect measurements will be extremely useful for accurately determining the nature of the defects including emission properties, their crystallographic origins, and the influence of local environmental effects. More broadly, the observation of a new room-temperature, single-photon emitter in a wide band gap semiconductor provides opportunities that may lead to the use of such defects in fundamental quantum optics experiments and potentially as a platform for quantum information processing. Experimental Section. Nanoparticle ZnO was prepared as described previously.28 Briefly, particles are prepared in dimethyl sulfoxide at room temperature and are heated for 1 h to temperatures as high as 180 °C. This method reduces the green/yellow luminescence ascribed to oxygen vacancies. The particles were annealed in solution for 1 h at 180 °C, which produces particles with a mean diameter of 20 nm (determined by TEM). Thin films of ZnO were prepared on cleaned silicon wafers with nominal silica thickness of ∼2 nm (determined using ellipsometry). Similar results were observed on glass and silica substrates. Wafers were ultrasonically cleaned first in acetone, then in 1-propanol for 10 min each. Films of ZnO were prepared from two sources. For ZnOMelb samples, the films were prepared by spin casting from ZnO nanoparticle solutions in dimethylformamide spun at 1500 rpm for 180 s.28 The film was then annealed to 500 °C for 30 min in air, resulting in a film thickness of 40 nm (with a root-mean-square roughness of 16.2 nm, as determined by AFM). Samples prepared from Sigma Aldrich purchased ZnO nanoparticles had an average size of less than 50 nm (determined by dynamic light scattering) and were made into an ethanol slurry with the concentration of 40 mg/mL. A small volume of this slurry (200 mL) was spread on to a clean wafer (1 cm2). For each source, the dried film was heated to 500 °C for 30 min in air. Confocal microcopy was performed using a homemade instrument. ZnO samples were illuminated through a 100× 0.95NA objective with a 532 nm frequency doubled Nd:YAG continuous wave laser mounted on a computer controlled stage with three-dimensional positioning control of 100 nm and optical resolution of ∼400 nm. For spectral measurements, the emission was filtered by a 560 nm long pass filter to remove the incident beam before being detected on a CCD-based spectrometer (Acton SpectraPro 2300i with Pixis100 camera). Spectra were background corrected by subtracting the signal accumulated 1 μm from the single defect. Statistics of singlephoton emission were measured after being filtered by both a 560 nm long pass and 650−750 nm band pass filter, by a fiberbased HBT interferometer. Samples with milled recognition markers were prepared by focused ion beam milling (∼ 2 μm depth) and measured with SEM, AFM, and confocal microscopy.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected].

ACKNOWLEDGMENTS The authors wish to acknowledge Dr. David Simpson, Alastair Stacey, and Dr. Stefania Castelletto for helpful discussions about this work. This work was produced with the assistance of the University of Melbourne under the Interdisciplinary Seed Funding Scheme. S.T-H. is supported by the ARC Australian Research Fellowship (project number DP1096288). A.D.G. acknowledges the support of an ARC QEII Fellowship (project number DP0880466).

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