Defect Evolution of Nonstoichiometric ZnO Quantum Dots - The

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Defect Evolution of Nonstoichiometric ZnO Quantum Dots Sergej Repp,† Stefan Weber,†,‡ and Emre Erdem*,† †

Institut für Physikalische Chemie, Albert-Ludwigs-Universität Freiburg, Albertstraße 21, 79104 Freiburg, Germany Freiburg Institute for Advanced Studies (FRIAS), Albert-Ludwigs-Universität Freiburg, Albertstraße 19, 79104 Freiburg, Germany

‡

S Supporting Information *

ABSTRACT: ZnO quantum dots with relatively narrow size distribution were produced by the presented precipitation method. Theoretical results from the literature were correlated to Zn-rich and O-rich preparations of ZnO. The investigation of the defect evolution was performed by photoluminescence (PL) measurements and electron paramagnetic resonance (EPR). By a deconvolution of PL spectra, the often mentioned defect-related green luminescence with line centers between 2.72 and 2.26 eV was observed. Since all defect centers have different defect formation enthalpies, additional information on the formation of these centers via annealing was collected. This data led us to the conclusion that VZn shows more prominent properties than usually reported. The VO defect is assigned to be responsible for the blue-light emission. The investigation of nonstoichiometric ZnO including its annealing revealed that the EPR signal at g = 1.959 can be assigned to core defects with a weak coupling to remote hydrogens from water molecules. This assignment is corroborated by the detection of hydrogen-related resonances by electron double resonance (ENDOR) spectroscopy.

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INTRODUCTION Zinc oxide (ZnO) is a direct semiconductor with a wide energy band gap of 3.37 eV at room temperature and a relatively high exciton binding energy of 60 meV as compared to GaN with 21 meV.1 These characteristics promise an important role of ZnO in industrial applications, such as phosphors, ultraviolet (UV) absorbers, light-emitting diode (LED) materials,2 photocatalysts,3 field-effect transistors (FETs),4 varistors,5 and in spintronics.6 Ordinary materials typically exhibit deviations from a perfect crystalline order and contain extrinsic and intrinsic defects, such as impurity ions and lattice vacancies or interstitials, respectively. Moreover, the properties of the materials for the above-mentioned applications can typically be tailored by impacting the electronic and accordingly the defect structures on different size scales. In this respect, either the compounds’ microstructures or the defect structures of all kinds have to be well controlled. Due to the quantum-confinement effect, the excitons gain in energy. This provides a theoretical basis for increasing the band gap, which results in a blue-shift of the irradiation wavelengths of nanosized materials.7 Accordingly, at nanoscale, the surfaceto-volume ratio increases on size reduction, which causes a significant increase of the surface-defect density. Despite the many advantages of nanoscaling ZnO, the lack of data from advanced spectroscopic methods, such as optical and magnetic resonance techniques, on nonstoichiometric ZnO compounds represents a great obstacle for the development of a reproducible p-type conductive material. Advanced ab initio calculations for many specific defect types8,9 provide a wealth of fundamental information on the © 2016 American Chemical Society

nonstoichiometric nature of so-called Zn-rich and O-rich ZnO. Since their calculated formation energies (Ef) of the defect charge states are a function of the Fermi level, it is difficult to build models on that, because the Fermi level is unknown. However, under Zn-rich conditions, the Fermi levels of Zni and VO defects are positioned in the donor energy levels. This fact allows the assumption that the Zn-rich samples should form at higher and the O-rich at lower Fermi energies. In the case of VO (++/0), the calculated formation energy in Zn-rich compounds is around 0.7 eV as compared to around 4.1 eV in O-rich compounds.9 Considering the Ef of VZn(−/− −), it should be mentioned that, under Zn-rich and O-rich conditions, the defect needs almost 4 eV, which is a relatively high energy value for a defect center to form. Also remarkable is the Ef of interstitial zinc (Zni) of around 1 eV under both nonstoichiometric conditions. Thus, in Zn-rich material, VO and Zni should form rather than VZn, but in O-rich samples VO and Zni should form, again, rather than VZn. However, defect pairs VO−VZn can form to compensate each other’s charge and to represent together the Schottky defect-type. Additionally, it is important to note that the paramagnetic (+) or (−) charge states were calculated and that such states are unstable at any conditions in ZnO.9 The nonstoichiometry in a solid phase is described through its elemental composition, whose proportions cannot be represented by integers. The electrical neutrality of such compounds is provided with adjusting of overall charge Received: September 8, 2016 Revised: October 6, 2016 Published: October 9, 2016 25124

DOI: 10.1021/acs.jpcc.6b09108 J. Phys. Chem. C 2016, 120, 25124−25130

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are used to characterize various inequivalent paramagnetic defect centers and to observe their electronic transitions. We have previously reported the advantages of exploitting EPR and PL spectroscopic data.17 EPR (yielding information on the defect surroundings) and PL (yielding information on excitation energies) results confirmed the core−shell model for ZnO nanocrystals. Recently, oxygen vacancies of ZnO were successfully investigated not only by static PL and EPR but also dynamically using in situ EPR.27 An interesting correlation has been also reported for the intensities of EPR lines and the photoluminescence due to defects.28 Both EPR and PL have been intensively used to learn more on the role of defects on ferromagnetism of undoped and doped ZnO. These sample systems are worth to investigate due to their applications as diluted magnetic semiconductor (DMS) materials.29,30 Here, the phase purity and the average nanocrystal size have been determined by X-ray diffraction (XRD). Additionally, highresolution microscopic methods such as transmission electron microscopy (TEM), scanning electron microscopy (SEM), and atomic force microscopy (AFM) were applied to observe the morphology and size distribution of nanoparticles.

compensation by the formation of new defects or implanting the required charged ions into the lattice. To reach nonstoichiometric conditions, two distinct processes are typically applied in addition to conventional synthesis routes: (i) electron-beam (EB) irradiation,10−12 and (ii) air treatment with subsequent annealing followed by thermal quenching, which causes frozen-in defects.13,14 However, frozen-in defects are sensitive to any annealing steps, which can heal such defects and provide a thermodynamically more stable system.13 For instance, Ton-That et al.15 reported that Zn-rich and O-rich ZnO can be achieved by annealing the samples at temperatures up to 1300 °C in Zn-vapor and oxygen atmosphere, respectively. This high thermal energy allows the samples to easily reach a thermodynamic equilibrium. In the present study, instead of EB irradiation or atmospheric treatment, kinetically stable nonstoichiometric ZnO samples were synthesized by just varying the precipitants’ stoichiometry. By this approach, an excess of Zn and O is forced to form naturally and instantly without any heating during precipitation. According to density functional theory (DFT) calculations, Zn interstitials (Zni) and VO defects behave like shallow and deep donor states, respectively, under Zn-rich conditions, whereas O interstitials (Oi) and VZn defects act as deep acceptor states under O-rich conditions.9,16 Oxygen vacancies have been observed using optically detected magnetic resonance (ODMR), and it was stated that the V O concentration in as-grown samples would be below the sensitivity of electron paramagnetic resonance (EPR).11 EPR is a sensitive experimental technique to detect the spins of unpaired electrons, which are located at defect centers. Detailed introductions on EPR-active defect centers in ZnO have been given previously.17−19 Photoluminescence (PL) spectroscopy provides insight into fluorescence emission of semiconductors. Especially ZnO, which is known for its phosphorescent behavior with its intense defect-related luminescence in the green region of the visible electromagnetic spectrum, has been widely investigated.7 According to the core−shell model,18 the paramagnetic, shallow donor defect states are located at the nanoparticle’s core and exhibit an EPR signal at g = 1.959. Such defects could be the oxidized form of interstitial zinc, Zni+ (4s1), and the reduced form of interstitial oxygen, Oi − (2p5).7,17,20 Since Oi has a very high formation energy, it can be treated as negligible to the defect system. Hydrogen has also been revealed as a shallow donor in ZnO,21,22 as it was predicted to be responsible for ntype conductivity by first-principles investigation.23 Hydrogen is known for a rich defect chemistry in semiconductors, such as replacing oxygen and bonding in a multicoordinated configuration.24 According to the core−shell model, shallow donors are generated in the core of a particle. However, the main hydrogen concentration is expected to be at the surface of the crystal, specifically in the hydroxide-shell.25 Moreover, the hydrogen ions are essential for stabilization of the polar (0001̅) O surface.26 There is an ongoing discussion about the luminescence properties of ZnO, which is mainly focused on its “typical” UV emission at 3.3 eV arising from near band edge (NBE) exciton recombination and the “green” luminescent (GL) region (3−2 eV) originating from the intrinsic defect transitions in the PL spectra.7,17,19 However, so far, conflicting evidence has been reported regarding GL emission.7 The main reason for this is that GL emission specifically depends on the synthesis conditions. In this contribution, EPR and PL spectroscopies

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EXPERIMENTAL SECTION Synthesis. For being precise, we would like to mention here, that the samples named “stoichiometric”, “Zn-rich” and “O-rich” given in Table 1 differ only by the molar ratios of the

Table 1. Applied Stoichiometric Proportions in Precursor Solutions stoichiometric proportions (Zn:OH)

[ZnCl2]

[NaOH]

Zn-rich (1.2:2) stoichiometric (1:2) O-rich (0.8:2)

0.12 M 0.1 M 0.08 M

0.2 M 0.2 M 0.2 M

educts used for their synthesis, which are zinc chloride (ZnCl2) and sodium hydroxide (NaOH). Of course it is not possible to charge the crystal by simply changing the ratio of Zn to O. Therefore, by adding positively charged excess ions, any kind of negatively charged ion or defect is required to be incorporated into the lattice for charge compensation. The driving force of the present precipitation reaction is supposed to be additionally strengthened by coprecipitation of two equivalents of sodium chloride (NaCl). Solutions with different stoichiometric proportions (see Table 1) of ZnCl2 (99.99%, Aldrich) and NaOH (99.99%, trace metal basis, Aldrich) have been separately dissolved in methanol (MeOH, 99.9%, AlanaR Normapure) at room temperature (RT) using an ultrasonic bath for 30 min. The precipitation reaction, eq 1, immediately started when NaOH solution was added to the ZnCl2 solution. Thereafter, the mixture was stirred for another 10 min in an ultrasonic bath. The resulting suspension was stirred overnight (15−17 h) and was subsequently centrifuged at 2567 × g. The two colorless precipitants could be separated by their different density, where NaCl has lower density (2.17 g/cm3) than ZnO (5.61 g/cm3), which collects at the bottom of the centrifuged precipitate. The top phase of the precipitate was removed, and the bottom phase was resuspended in an adequate amount of MeOH. Methanol is able to dissolve NaCl 25 times better than EtOH.27 This purification cycle was repeated several times until the control XRD analysis did not show any NaCl pattern. Finally, purified ZnO quantum dots were dried overnight at RT, 25125

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Figure 1. Deconvolution of PL spectra of as received ZnO NP powders, produced under different conditions: (a) Zn-rich, (b) stoichiometric, and (c) O-rich. The blue shift in the case of Zn-rich and the red shift in the case of O-rich conditions is clearly visible.

photodetector power was set to 775 V. A UG5 filter was placed on the excitation site and a 290 nm cutoff filter on the emission site. Powders were measured in a special sample holder with a quartz window. For spectral analysis, the PL spectra were modeled by Gaussian functions as described in the literature.31,32 X-band (9.86 GHz) EPR measurements were conducted on a Bruker EMX spectrometer using a rectangular TE102 (X-band) resonator. The magnetic field was determined using an NMR gaussmeter (ER 035M, Bruker); for magneticfield calibration, polycrystalline DPPH with g = 2.0036 was used. Q-band pulsed ENDOR spectra were recorded on a commercial pulse EPR spectrometer Bruker E580 (X/Q) (Bruker BioSpin GmbH, Rheinstetten, Germany) in conjunction with a dielectric ring ENDOR resonator Bruker Q-FT (EN 5107D2) All ENDOR spectra were recorded at 3.8 K at the center-field position of the EPR spectrum.

grinded in a mortar and annealed at various temperatures. Quantum dots can only be obtained before the agglomeration process takes place via drying or annealing. The photograph and the reflectance spectrum of such diluted QD’s are presented in Figure S1. Samples for AFM were prepared from the resuspended precipitates in ethanol (EtOH, 99.9%, AlanaR Normapure) in order to separate larger agglomerated particles from less agglomerated ones. It has to be taken into account that these particles might be smaller than the overall average particle size (see Table S2). ZnCl 2 + 2NaOH → ZnO + 2NaCl + H 2O

(1)

During precipitation, zinc hydroxide (Zn(OH)2) can be easily formed, because it is the intermediate product of ZnO.25 Due to the strong drying capabilities of MeOH and EtOH, which not only suppress the generation of hydroxide species, but also effectively bind water, the formation of ZnO is supported. The reaction success is highly sensitive to the water content in the solvent. Methods. For controlling the phase purity and the crystallattice characterization, a Bruker D8 Advanced Series powder diffractometer with a Cu−Kα (λ = 0.15406 nm) radiation source was used. The mean nanocrystallite size was calculated from the full width at half-maximum (fwhm) of the reflex pattern by using the Debye−Scherrer equation (see Supporting Information). The morphology and further structural parameters of the ZnO particles were obtained using TEM and SEM with a Carl Zeiss LEO 912 Omega instrument in combination with an energy-dispersive X-ray (EDX) unit at an acceleration voltage of 200 kV. Samples for TEM were prepared by evaporating a dilute suspension of ZnO particles onto a carboncoated copper grid. AFM measurements were conducted under ambient conditions in the tapping mode of an MFP-3D SA microscope (Asylum Research, Oxford Instruments) with a silicon nitride cantilever, a MLCT F model from Bruker AFM Probes, in triangular shape with a nominal tip radius of 20 nm and a nominal eigenfrequency of 125 kHz. The PL characterization of powder samples was carried out on a PerkinElmer (LS 55) spectrometer with 260 nm excitation wavelength, 5 and 6 nm excitation and emission slit, respectively, while the

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RESULTS AND DISCUSSION Structural Analysis. Nanopowders were obtained via wet chemical precipitation without any additives, such as surfactants or stabilizers. Moreover, organic ligands were not used to avoid possible influence of them on the spectroscopic results. Nevertheless, without surface stabilization, agglomeration always takes place, but agglomerates consist of spherical particles. The powder XRD analysis revealed typical hexagonal wurtzite-structured Zn-rich, O-rich, and stoichiometric nanosized ZnO. Using the Debye−Scherrer equation, an average crystallite size between 12 and 26 nm was obtained for the agglomerated particles. From the electron-dispersive X-ray (EDX) characterization (for further details, see Figures S2, S3, and S4), the elemental analysis confirmed the correct atomic ratios of the respective samples (see Table S1) as it was adjusted in the synthesis. The agglomeration status and the average particle size of 7.1 to 12.8 nm could be obtained by using TEM and SEM (see Figures S5, S6, S7 for the pictures). Additionally, consistent results regarding the average particle size were obtained from AFM for the stoichiometric sample; these data are presented and discussed in Figure S8. Electronic Analysis. Deconvolution of PL spectra revealed one broad GL emission at 2.46 and 2.26 eV for stoichiometric and O-rich samples, respectively (see Figure 1). Additionally, 25126

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Figure 2. EPR investigations of annealed stoichiometric and nonstoichiometric synthesized ZnO. The spectra were detected using X-Band continuous-wave EPR at RT after annealing at different temperatures. For the phase purity of stoichiometric samples, refer to Figure S12 in the Supporting Information.

rich sample shows a distinct red shift of its GL emission from 2.37 to 2.26 eV; this could be a sign for an alteration of the emission-responsible defect types, such as transition of shallow donors to VZn to transition of VO to valence band. Additionally, it would explain the low intensity of blue emission of the O-rich sample. The broad defect-related GL originates from the VO and VZn from their three different oxidation states:12 neutral, singly ionized, and doubly ionized. According to theoretical studies, the defect formation energies (Ef) for the shallow donor Zni and the deep donor VO are high in Zn-rich ZnO, and therefore these are unlikely to be formed.8,9,37 Based on its low migration barrier of 0.57 eV, the fast diffusion of Zni would decrease its thermal stability.8 On the other hand, under O-rich conditions it is assumed to obtain relatively high VZn (deep acceptors) defect concentrations, and the amount of them should be larger than the VO (deep donors). These propositions are supported by Kohan et al.,37 who reported that the VO and VZn concentrations depend on the Zn partial pressure. Depending on the Fermi level, these defect states preferably trap electrons during their relaxation or excitation paths. The red shift (0.11 eV) of GL of stoichiometric relative to O-rich samples depicted in Figure 1 shows possible Zni migration due to their high mobility and the natural formation of VO and VZn at higher annealing temperatures.8 Furthermore, the antisites ZnO and OZn are unlikely to form under all conditions, because of their higher formation energies.33 Since it is expected to obtain more VZn in O-rich than in Zn-rich samples, it is possible to conclude from our PL results that the GL emission at 2.26 eV is closely related to VZn or Oi. At this point, to support the findings from PL spectral analyses, it is useful to seek a correlation with the results from EPR. The presented NBE-related luminescence in Figure 1 varies with intensity and energy values. The reasons for this observation could be of different origin: The intensity of a PL transition can rise if the light absorbing probability increases or alternative relaxation processes are absent. Both paths could be active in the presented three samples: (i) high Zn concentration was reported to be responsible for the notable

GL at 2.37 eV, which is present in the stoichiometric sample and which was also observed by others previously,33,34 is present. The PL signals in Figure 1 show a red shift of GL in the case of O-rich ZnO and a blue shift in case of Zn-rich ZnO. For deconvolution, PL spectra were fitted to three Gaussian lines, for which the full widths at half-maximum (fwhm) were allowed to be unequal. The line width variation depends on many factors such as the particle size, the exciton lifetime, and the temperature. Different types of defects have different relaxation times. This fact led us to conclude that more than one different electronic transition in the GL emission and one in the NBE emission are present. The specific defect-based transition energies have been assigned to VZn and VO, which have at least two different GL transitions.31,35 From the experimental data given in Figure 1a, one can see that Zn-rich conditions lead to a remarkably different PL spectra and relatively high NBE intensity compared to stoichiometric and O-rich conditions. This effect has also been shown by Zeng et al.,14 who proposed the Zni being responsible for the violet emission (2.72 eV), and their fast decrease during annealing due to their mobility. The annealing of this Zn-rich sample revealed a shift in emission from violet (2.72 eV) to blue (2.5 eV) (see Figure S9). Namely, the proposed transitions are from Zni shallow donor states to the VB (violet) and from the extended Zni state (with slightly lower energy than the normal state) to the VB (blue).14,36 After annealing the Zn-rich powder under air for 2 h at 300 °C, its color changed from white to greyish, although it was treated in the same way as the O-rich and stoichiometric samples (see Figure S10). This color change could be assigned to the formation of Zn metal, generated by reduction of Zn2+ ions at the high annealing temperatures.14 Organic impurities could be excluded, due to the pure XRD patterns of the samples (see Figure S11 and S12). The stoichiometric sample reveals a typical PL spectrum containing partly overlapping blue and green luminescence. The violet transition (2.72 eV) is due to Zni-defects and similar to the one of the Zn-rich sample. Nevertheless, deep defect states emit less energy such as the green luminescence. The O25127

DOI: 10.1021/acs.jpcc.6b09108 J. Phys. Chem. C 2016, 120, 25124−25130

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The Journal of Physical Chemistry C increase in UV and blue PL intensity,14,38 and (ii) nonradiative relaxation processes could decrease the energy of the excitons to deeper levels, thus providing green emission. A quantumconfinement effect could also be responsible for the energylevel shifts, and it is observed corresponding to the particle size. In Figure 2, EPR data of defects in stoichiometric (Figure 2a), Zn-rich (Figure 2b) and O-rich (Figure 2c) ZnO samples are presented as a function of the annealing temperature. The spectra of the nonstoichiometric samples recorded at RT differ significantly. Annealing of the samples leads to further variations that may provide a better understanding of the underlying defect mechanisms. According to the core−shell model, shallow donors are the core defect states with resonance at g = 1.959.17 Here, the surface defects in the stoichiometric sample reveal a relatively complicated evolution of the EPR spectral pattern at around g ∼ 2.01. Similarly complicated patterns were previously observed for samples generated via planetary ball milling.39 At RT up to 100 °C, EPR spectra comprising three resonances resembling a powder pattern are observed. This is mainly due to the low annealing temperature where the crystallinization of ZnO has not yet started. This is also consistent with the absence of the core signal (at g ∼ 1.96) below 100 °C. Various kinds of defects centers that are localized at the surface and that differ in concentration cause EPR lines at different g-factors. Crystallinization starts at around 100 °C, thus, we observe both g ∼ 1.96 and 2.01 signals together. Interestingly, at 100 °C, the intense signal around 2.01 again splits into three lines, which are due to the anisotropic distribution of a certain defect located at the surface. Since EPR lines are very sensitive to crystal symmetry changes, fully rhombic crystal formation causes additional splittings. Finally, at 300 °C, quantum dots crystallize, and they are located uniformly on the surface; thus, the anisotropy due to symmetry and different localization of different kinds of defects disappears and we observe only the EPR signal from the surface defects, which are environmentally symmetric, homogeneously distributed, and uniformly located. Above 300 °C, core defects become dominant, and compared to surface defects, their EPR signal intensity is very strong. Due to the substantial increase in core defects, the EPR signals at 600 and 900 °C were as shown in Figure 2a and are scaled by a factor of 8 × 104. The investigation of the nonstoichiometric samples revealed the increasing shallow donor concentration when air annealing is applied to Zn-rich and stoichiometric samples. However, this effect develops in the opposite way while annealing the O-rich ZnO, where the g = 1.959 signal is the only observed one, which exists up to 100 °C and disappears by annealing above 300 °C. The expected Zni-signal is absent in the Zn-rich sample, while Zn is not in its EPR-active state (Zni+) or alternatively such center is not formed at RT and 50 °C. Nevertheless, the comparison between the stoichiometric and Zn-rich spectra revealed similar spectral features after annealing above 300 °C. Likewise, annealing of the O-rich ZnO reveals a shallow donor signal even at RT, which increases until 300 °C and decreases above this temperature. Since the assumed Zni+ defect state is unlikely to form under O-rich conditions, and would oxidize at higher temperatures, it can be assumed that the signal at g = 1.959 must have a different origin and possibly arises from hydrogen and not from Zni+. This might be the reason why the core signal at around g = 1.959 appears at RT even without annealing (Figure 2c). This assumption is corroborated by the successful detection of hydrogen-related

resonances by Q-band pulsed Davies and Mims ENDOR spectroscopy due to weak coupling of H atoms to the defect centers or water molecules (see Supporting Information for further discussion regarding ENDOR data). The deep donor and acceptor defects where observed at g = 1.99−2.01,4 and include such paramagnetic defect states as singly ionized vacancies (VO+ and VZn−). According to the core−shell model, these deep defect states are located at the surface of the particle.17 Considering the surface-to-volume ratio, the not annealed samples have the largest surface, as expected, and therefore should contain most surface defects. However, the defects are expected to be paramagnetic, and therefore, should be observable by EPR. The analysis of nonstoichiometric samples reveals that a high concentration (8 × 1017 spins/g; for details of spin counting procedure, refer to the Supporting Information) of paramagnetic defect states is present in the stoichiometric sample and stable up to 300 °C. The line-splitting of the signal at g = 1.99 has also been investigated by others.40 Although VO is a topic of speculations and discussions in the literature,7 the calculated defect density is too small to be detectable via EPR, and therefore, alternative methods, such as irradiation and subsequent measuring at very low temperatures, were applied to increase the singly ionized defect density.12 Nevertheless, EPR spectra of nonirradiated spectra were shown,41 but those have been attributed to VZn−.12 Since the presented spectra are also from nonirradiated samples and remain stable until 300 °C, the strong defect signal of the stoichiometric sample can be also attributed to VZn−. The reason why there is no signal at g = 1.99 in O-rich ZnO, while intense PL emission is observed, can be rationalized by the Ef -diagrams from Oba et al. 9 Specifically, under O-rich conditions, the increased Fermi-level is responsible for reduction of ionized VZn− to diamagnetic VZn2−. With the extra electron provided by VZn2−, its energy is increased in the band gap and this exactly corresponds to the decreased PL energy within the band gap. This is due to the fact that, unlike PL, EPR is only sensitive to paramagnetic defects. Additionally, there is a close dependence between Ef and the defect concentration caused by Zn excess or deficit. Therefore, strong signal-to-noise (S/N) changes were observed in the presented EPR spectra (in Figure 2a−c). Under O-rich synthesis conditions, the lowest S/N ratio was detected due to the lowest defect density, which is expected because of the high Ef.9 Subsequently, the more intense EPR signal is reflecting the defect density to some order of magnitude higher in Zn-rich ZnO, because of the very low Ef of VO, ZnO, and Zni.15

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CONCLUSIONS The investigation of nonstoichiometrically synthesized ZnO could corroborate several theoretical predictions. In particular, the presented results agree well with the calculated Ef and the resulting defect densities of the nonstoichiometric samples.9,16 Additionally, the often controversial reports on the defect assignment to the EPR spectra were resolved, and we conclude that they originate from the Ef. Most of the defects are diamagnetic and do not contribute to the EPR spectra. However, they all can contribute to the luminescence properties of the semiconductor. The GL emission peaks at 2.44 and 2.26 eV are assigned to VZn− and VZn2‑, respectively, and are based on the Fermi energy related defect oxidation state. The often discussed defect center VO is extremely unstable in the paramagnetic state VO+, and therefore, remains 25128

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The Journal of Physical Chemistry C silent in EPR. The diamagnetic states VO0 and VO2+ could contribute to GL, however, they were not assigned in this work. The GL is assumed to be correlated to VZn and its oxidation states, but only VZn‑ contributes to the EPR spectra. Broad GL can be quenched by doping with transition metal ions such as Co, Ni or Mn, or by annealing in a hydrogen environment.15 Applying the Davies and Mims pulse ENDOR sequences at 3.8 K revealed an unresolved 1H-matrix signal at 53 MHz at Qband microwave frequency (see Figure S13 for the ENDOR results and the pulse sequence scheme).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b09108. UV−vis spectrum (Figure S1), SEM/EDX results (Figures S2, S3, S4), TEM results (Figures S5, S6, S7), AFM results (Figure S8), photoluminesence spectra (Figure S9), photograph of O- and Zn-rich samples (Figure S10), XRD (Figure S11, S12), spin counting procedure, and Davies and Mims ENDOR results (Figure S13) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; Tel: +49 761 203 6207. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Deutsche Forschungsgemeinschaft (DFG), Grant ER662/1-2. We also would like to thank Dr. R. Thomann for the TEM, SEM, and EDX analyses, L. Heidinger for ENDOR measurements, J. Urban for AFM measurements, A. Qazzazie for lab-work, and Dr. M. Ade for XRD measurements.

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