Dual Europium Luminescence Centers in Colloidal Ga2O3

Aug 21, 2015 - Wang , T.; Layek , A.; Hosein , I. D.; Chirmanov , V.; Radovanovic , P. V. Correlation between Native Defects and Dopants in Colloidal ...
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Dual Europium Luminescence Centers in Colloidal Ga2O3 Nanocrystals: Controlled in Situ Reduction of Eu(III) and Stabilization of Eu(II) Arunasish Layek, Baran Yildirim, Vahid Ghodsi, Lisa N. Hutfluss, Manu Hegde, Ting Wang,† and Pavle V. Radovanovic* Department of Chemistry, University of Waterloo, 200 University Avenue West, Waterloo, Ontario N2L 3G1, Canada S Supporting Information *

ABSTRACT: Introducing multiple luminescent centers into colloidal nanocrystals is an attractive way to impart new optical properties into this class of materials. Doping disparate ions into specific nanocrystals is often challenging, due to the preferential incorporation of one type of dopant. Here, we demonstrate the coexistence of europium dopants as divalent and trivalent ions in colloidal Ga2O3 nanocrystals, achieved by controlled in situ reduction of Eu3+ to Eu2+. The two dopant species exhibit distinctly different steady-state and time-resolved photoluminescence, and their ratio can be modified via doping concentration, reaction temperature, or thermal treatment of as-synthesized NCs. The Eu2+ ions are proposed to be stabilized internally owing to the attractive interaction with oxygen vacancies, while Eu3+ dopants partly reside in the nanocrystal surface region. The relationship between the electronic structure of the native defects and the dopant centers is discussed in the context of the overall emission properties. The exposure of these samples to X-ray radiation leads to the reduction of Eu3+ to Eu2+, demonstrating an alternative way of manipulating the oxidation state and suggesting the potential application of this material as an X-ray storage phosphor. The coexistence of Eu2+ and Eu3+ and the ability to control their relative fraction over the full oxidation state range in group III oxide nanocrystals allow for the design and preparation of new photonic and light emitting materials.

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section.9 This small molar absorptivity can be overcome by sensitization of the lanthanide dopants by the NC host lattices upon their excitation above the band edge energy.10−13 Some lanthanide ions can also be stabilized in the divalent oxidation state having distinctly different electronic structure and optical properties. A notable example is Eu2+, which emits a relatively broad band in the visible region associated with a parity-allowed d−f transition.14−18 This emission is sensitive to the local environment and has intense excitation and short lifetime compared to Eu3+.9,16−18 However, Eu2+ is generally sensitive to oxidation, and examples of Eu2+ dopants in NCs are still limited.15,17 The coexistence of europium dopants in divalent and trivalent states in NC host lattices can be an effective alternative to codoping and enrichment of the optical properties of NCs. Moreover, the ability to manipulate the oxidation states of europium ions in solid state materials is useful for different technologies, including the fabrication of photoluminescent X-ray storage phosphors for medical imaging.15 Several specific approaches have been proposed to initiate the formation of Eu2+ dopants in inorganic phosphors from Eu3+ precursors, including the growth of the host lattice in

INTRODUCTION The development of new luminescent nanostructures has been a focus of intense research efforts owing to their promise in photonic, optoelectronic, and biomedical applications.1−4 Colloidal luminescent nanocrystals that are capable of emitting light of different colors can be used for sensing and multiplexed imaging.5 Such systems could rely on the coexistence of multiple emissive centers in NCs, which respond to a particular excitation wavelength. Imparting new luminescence centers into colloidal NCs has often been achieved by substitutional doping during NC growth.6−8 However, an attempt to dope two or more disparate ions with targeted optical properties could lead to selective incorporation of only one type of dopant ion and exclusion of the others. Owing to their unique electronic structure, trivalent lanthanide ions are characterized by a variety of attractive spectroscopic properties, including sharp photoluminescence originating from intra-4f orbital transitions, upconversion photoluminescence enabled by long-lived excited states, and resistance to photobleaching due to the shielded nature of the f electrons.9 These properties make lanthanide-doped NCs suitable phosphors for various light emitting devices, active media for laser and telecommunication technologies, and probes for biomedical imaging. A major challenge to the application of trivalent lanthanides is that f−f transitions are parity forbidden, resulting in a very small absorption cross © XXXX American Chemical Society

Received: June 21, 2015 Revised: August 3, 2015

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Chemistry of Materials a strongly reducing atmosphere,19 postsynthesis reduction by high energy radiation,20 or intrinsic reduction as a result of charge compensation due to substitution of Eu3+ for a divalent host lattice cation.21,22 Examples of the formation of Eu2+ in solid state are also evident in the synthesis of divalent europium-based NCs, such as europium chalcogenides (EuO, EuS, EuSe)23−26 or dihalides (EuCl2).27 These nanostructures are usually synthesized by a photochemical reduction23 or thermal decomposition of complex single-source precursors.24,25 More recently, colloidal EuSe and EuCl2 NCs have been prepared from Eu3+ by simple amine reduction.26,27 Herein, we demonstrate for the first time the presence of Eu2+ in colloidal Ga2O3 NCs by in situ reduction of Eu3+ during NC growth. The two dopant species exhibit distinctly different steady-state and time-resolved PL properties, and their ratio can be modulated by the overall doping concentration and reaction temperature. The effective oxidation state can also be manipulated by postsynthesis treatment, such as thermal annealing and X-ray exposure of as-synthesized NCs. The spectroscopic properties are discussed in the context of the interaction between the electronic structure of the dopant ions and the host lattice native defects. Controlling the interactions between the dopant ions and electron traps via NC size, defect density, doping concentration, and radiation exposure enables the control of the relative Eu2+ and Eu3+ emissions, allowing for the design and preparation of new light emitting and X-ray storage phosphors.

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Time-Resolved PL Measurements. Time-resolved PL measurements of Eu2+ dopants were performed by time-correlated single photon counting (TCSPC) method using a Horiba Jobin Yvon IBH Ltd. system. A 300 nm NanoLED (IBH Ltd.) was used as the excitation source, and the emission was collected at the right angle geometry. The signal was monitored at the emission maximum (402 nm). Both excitation and emission slit widths were set to 16 nm. The instrument response function (IRF) was recorded by detecting the scattered excitation using a Ludox solution (Sigma-Aldrich). Time-resolved PL decay measurements of Eu3+ were performed with a Varian Cary Eclipse fluorescence spectrometer at room temperature. In a typical experiment, the delay and gate times were set to 3 and 1 μs, respectively, and the excitation wavelength was 230 nm. The signals were detected at 613 nm. X-ray Absorption Measurements and Analysis. X-ray absorption spectroscopy (XAS) measurements were performed at the Canadian Light Source (CLS) synchrotron facility. Europium M4,5edge spectra were collected at the High Resolution Spherical Grating Monochromator (SGM) beamline (11ID-1) using a total electron yield (TEY) detector. Europium L3-edge spectra were collected at the Hard X-ray MicroAnalysis (HXMA) beamline (06ID-1), as previously described.30 Europium L3-edge calibration was performed using an iron foil. Spectra were recorded in fluorescence mode with a 32 element germanium array detector. In this configuration, samples were positioned at ca. 45° relative to the incident beam, and the fluorescence was detected at 90° with respect to the incident beam. Using the Athena software package, suitable detector channels were selected and averaged, followed by subtraction of the pre- and postedge backgrounds. A Shirley algorithm baseline was applied, and Eu2+/Eu3+ peaks were fit using a 50−50 sum of Gaussian and Lorentzian functions.31 The spectral intensity of the Eu3+ reference (Eu2O3) was ca. 1.25 times greater than that of the Eu2+ reference (Eu2Si5N8), which has also been reported in the literature.32 Percentages of Eu2+ and Eu3+ were estimated using the integrated area of each peak divided by the total integrated area of both peaks.

EXPERIMENTAL SECTION

Synthesis. Eu2+/Eu3+-doped Ga2O3 NCs were synthesized according to the previously reported procedure for the synthesis of γ-Ga2O3 NCs.28 All reactions were performed in an argon atmosphere. In a 100 mL three-neck round-bottom flask, 1.0 g of gallium acetylacetonate (Ga(acac)3), 10 g of oleylamine, and varying amounts of EuCl3 (1−15 mol % relative to Ga) were mixed. The mixture was degassed and heated to 300 °C, and the reaction was allowed to proceed for 1 h under continuous stirring. The reaction mixture was then cooled to room temperature, and Eu-doped γ-Ga2O3 NCs were isolated by precipitation with ethanol and centrifugation for 10 min at 3000 rpm. The NCs were then washed two more times with ethanol to remove unreacted ions and oleylamine. All NC precipitates were heated in melted TOPO at 90 °C for 1 h, followed by precipitation with ethanol. This procedure was performed three times in total to achieve the maximum removal of the surface-bound dopant ions.29 The TOPO-treated NCs were dispersed in hexane, giving completely transparent colloidal suspensions. To investigate the effect of the reaction conditions on the oxidation state and emission properties of the dopant ions, the NC synthesis was also performed at 200 °C, followed by the same postsynthesis treatment. Absorption and PL Measurements. The absorption spectra were collected with a Varian Cary 5000 UV−vis-NIR spectrophotometer, using 1 cm path length quartz cuvettes. Steady-state PL excitation and emission spectra were recorded at room temperature with a Varian Cary Eclipse spectrometer, using standard quartz fluorescence cuvettes. For PL emission measurements, the samples were excited at 230 nm (for Eu3+) or at 280 nm (for Eu2+). For PL excitation measurements, the emission was monitored at the maximum of the Eu2+ 4f−5d transition band (402 nm) or at the maximum of the Eu3+ 5D0 →7F2 peak (613 nm). Emission and excitation spectra for Eu3+ dopants were recorded in the phosphorescence mode, using a Xenon flash lamp as the excitation source. For these measurements, both excitation and emission slits were set to 5 nm and the spectra were acquired with a delay time of 0.1 ms and a gate time of 5 ms. High-resolution PL spectra were measured with a Renishaw 1000 spectrometer using a frequency tripled Millenia-pumped Tsunami laser (SpectraPhysics) as the excitation source.

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RESULTS AND DISCUSSION Figure 1a shows a transmission electron microscopy (TEM) image of a typical sample prepared with the starting Eu3+

Figure 1. (a) TEM image of 4.0% Eu-doped Ga2O3 NCs. (b) XRD pattern of the same sample (red) and undoped Ga2O3 NCs prepared under the same conditions (black). (c) Absorption (dashed lines) and PL (solid lines) spectra of Eu-doped Ga2O3 NCs having different doping concentrations, as indicated in the graph; λexc = 230 nm.

concentration of 5.0 mol %. The actual doping concentration for this sample was determined to be 4.0% by both inductively coupled plasma atomic emission spectrometry (ICP-AES) and energy dispersive X-ray spectroscopy (EDX). The obtained NCs have quasi-spherical morphology with an average NC size of 3.8 ± 0.8 nm. Figure 1b shows the X-ray diffraction pattern of the same sample (red trace) together with the XRD pattern B

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Chemistry of Materials of undoped Ga2O3 NCs prepared under identical conditions (black trace). Both patterns match the pattern of cubic Ga2O3 (γ-phase, JCPDS No. 20-0426), although doped NC sample shows larger peak broadening, consistent with smaller average NC size and/or increased crystal lattice disorder. The average size of undoped NCs was estimated to be 6.0 ± 1.0 nm. Intentionally undoped Ga2O3 NCs exhibit an intense broadband PL centered at ca. 460 nm (Figure 1c, solid black line) upon excitation above the Ga2O3 band edge (absorption spectra of the corresponding samples in Figure 1c are shown with dashed lines).28 This blue emission arises from the recombination of the electrons trapped in the shallow donor states formed by oxygen vacancies with holes trapped in the acceptor states formed by gallium vacancies or gallium−oxygen vacancy pairs.33−35 The energy of the donor−acceptor pair recombination emission is determined by the Coulomb interaction between charged donor and acceptor sites and the binding energies of the corresponding states. The synthesis of NCs in the presence of Eu3+ in the reaction mixture leads to the appearance of a set of narrow PL bands in the range of ca. 600− 700 nm. These features are assigned to intra-4f transitions in Eu3+. The peak intensities of these transitions increase with increasing doping concentration for the small-to-intermediate doping levels (ca. 1−5%), concurrently with the quenching and blue shift of the DAP emission. The quenching of the DAP emission suggests the energy transfer from the NC host lattice to the Eu3+ dopant centers, while the blue shift reflects a decrease in the donor and acceptor binding energies, and/or an increase in the Coulomb interactions between donor and acceptor sites.33 As the doping concentration is further increased (green and orange solid lines), a new band at ca. 400 nm becomes a dominant part of the spectrum. We hypothesized that this band originates from the minor excitation of Eu2+ 4f−5d transition, which is somewhat surprising for an oxide host lattice with the +3 oxidation state of the substituted cation, and the lack of high temperature treatment. It should be noted that a broad shoulder with the maximum intensity at ca. 290 nm is also observed in the absorption spectra (dashed green and orange lines; also Figure S1), consistent with a strong parity-allowed transition. To confirm the presence of both europium centers and the origin of the observed emissions, we conducted steady-state and time-resolved PL studies for different excitation and emission energies. Figure 2a (solid lines) shows the emission spectra of Eu-doped Ga2O3 NCs upon excitation at 280 nm, just below the onset of the NC band gap absorption. For doping concentrations below 1%, the PL spectra are dominated by the relatively weak DAP emission, although a much narrower transition is observed as a shoulder at ca. 400 nm (Figure 2a, black line). At higher doping concentrations, the DAP band vanishes, revealing a narrower band characteristic for Eu2+ 4f6 5d1 → 4f7 (8S7/2) transition (blue, green, and red lines). Consequently, colloidal suspensions of highly doped NCs excited via Eu2+ emit a deep blue light (Figure 2a, inset). Furthermore, the excitation spectra (dashed lines) also show a broad band characteristic for this parity-allowed electronic transition. Although the excitation intensity drops off rapidly below 280 nm and reaches the minimum at ca. 230 nm, it does not completely vanish, increasing slightly at higher energies. Consequently, some Eu2+ emission is still observed for high doping concentrations upon excitation into NC band gap, as shown in Figure 1c (top two panels). This emission indicates a small degree of direct Eu2+ excitation and/or energy transfer

Figure 2. PL emission (solid lines) and excitation (dashed lines) spectra of Eu-doped Ga2O3 NCs synthesized at 300 °C having different doping concentrations, as indicated in the graphs. (a) Spectra of Eu2+ (for the emission spectra, λexc = 280 nm; for the excitation spectra, λem = 402 nm). (b) Delayed spectra of Eu3+ measured with a delay time of 0.1 ms and gate time of 5 ms (for emission spectra, λexc = 230 nm; for the excitation spectra, λem = 613 nm).

from the NC host lattice to Eu2+ and can be clearly observed when DAP emission is sufficiently quenched. The full width at half-maximum of the Eu2+ excitation band exhibits some broadening with increasing doping concentration, which could be associated with the host lattice distortion (or decreased crystal order) due to significantly larger ionic radii of Eu2+ (1.31 Å) and Eu3+ (1.09 Å) than that of Ga3+ (0.76 Å). As a control experiment of Eu2+ incorporation into Ga2O3 NCs, we performed the same synthesis but without any Ga3+ precursor in the reaction mixture. The obtained product is amorphous and, unlike Ga2O3 NCs, could not be dispersed in TOPO or hexane. Furthermore, the Eu2+ PL spectrum of this amorphous phase is significantly different from that of Eu-doped Ga2O3 NCs (Figure S2). Figure 2b shows time-gated PL spectra of the same samples upon the excitation above the band gap energy of Ga2O3 NCs. The DAP emission has significantly shorter lifetime relative to Eu3+ f−f transitions, allowing for clear observation and assignment of the characteristic Eu3+ peaks, 0.1 ms upon the excitation. The observed emission peaks are assigned to Eu3+ 5D0 → 7FJ (J = 1−4) transitions, as indicated in Figure 2b. The excitation spectra coincide with the NC band gap absorption (dashed lines), indicating a strong sensitization of Eu3+ by energy transfer from the photoexcited NC host lattice and negligible direct excitation (see also Figure S3). The dominant Eu3+ excitation band overlaps on the low energy side with a much weaker shoulder (Figure S3), which has been associated with Eu3+−O2− charge transfer transition,36 demonstrating an alternative mechanism of Eu3+ PL sensitization. This transition has much weaker absorbance than the overlapping 4f7 → 4f6 5d1 transition in Eu2+, allowing for its observation only in the luminescence center-specific excitation spectrum. We did not observe Eu3+ emission upon excitation of Eu2+ dopants, suggesting insignificant energy transfer between Eu2+ and Eu3+. Figure 3 compares typical time-resolved PL data of europium dopants in Ga2O3 NCs monitored at 402 nm (Figure 3a) and 613 nm (Figure 3b), respectively. Owing to the difference in the electronic structure and the origin of the emission, Eu2+ and Eu3+ dopants have distinctly different PL decay dynamics. The time-resolved PL data in Figure 3 were fit using a multiexponential expression describing the intensity time decay: C

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reference spectra of Eu2+ (Eu2Si5N8) and Eu3+ (Eu2O3) are shown in Figure 4a (bottom panel). A significant difference in the energy of the lowest-lying transition can serve as a signature of the two oxidation states. The spectrum of 0.8% Eu-doped Ga2O3 NCs synthesized at 300 °C is shown in Figure 4a (middle panel). This spectrum consists of two peaks coinciding with the reference spectra of Eu2+ and Eu3+, definitively confirming the coexistence of both oxidation states. At high doping concentrations, the relative amount of Eu3+ increases at the expense of Eu2+ (Figure 4a, top panel), indicating the possibility of manipulating the PL properties via preparation conditions during NC growth. To examine the possibility of manipulating the relative fractions of the oxidation states by postsynthesis treatment, we annealed as-synthesized NC samples at different temperatures. High-temperature annealing results in a notable reduction in the Eu2+ and an increase in Eu3+ peak intensity (Figure 4b). The relative amounts of Eu2+ for different doping concentrations in as-synthesized NCs and after high-temperature annealing were determined from the deconvoluted peak fitting of the background-corrected spectra using a 50−50 sum of Gaussian and Lorentzian functions. The obtained results are summarized in Figure 4c. The fraction of Eu2+ in as-synthesized NCs (blue symbols) reaches nearly 25% for ca. 1% doping concentration. As the doping concentration is increased, the relative fraction of Eu2+ gradually decreases. The same trend is clearly confirmed using Eu M4,5-edge spectra, although overlap with the Ga L2,3-edge spectral features prevents detailed quantitative analysis using these spectra (Figure S5). This behavior reflects the reducing strength limit of the given reaction environment. Although oleylamine is a reducing agent and could be solely responsible for the reduction of Eu3+, it should be emphasized that the dependence of the Eu2+ fraction on the doping concentration in Figure 4c is exactly opposite from the trend for the fraction of Eu3+ on the NC surfaces reported previously.12 This inverse relationship indicates that Eu2+ is mostly stabilized as an internal dopant and suggests the role of native defects in stabilizing Eu2+ sites. Given its negative charge relative to Eu3+, Eu2+ can attract and promote the formation of oxygen vacancies (V•O) around the dopant site more strongly,37 stabilizing the Eu2+−oxygen vacancy complex and contributing to the blue shift of the DAP emission.38 It should be emphasized that as-synthesized NC samples for all doping concentrations remain stable indefinitely under ambient conditions, without any observable change in the ratio of Eu2+ and Eu3+. When as-synthesized NCs are annealed at high temperature, the fraction of Eu2+ is decreased (Figure 4b). The general trend of the Eu2+ fraction with respect to the doping concentration remains the same after high temperature treatment (Figure 4c, red symbols). Although Eu3+ emission originates from shielded f−f transitions, the sharp, richly structured peaks can be used as a signature of the local environment and provide further insight into the reduction process. Figure 5 shows Eu3+ spectra of 4.0% Eu-doped Ga2O3 NCs upon annealing at different temperatures. The increasing annealing temperature leads to broadening of the 5D0 → 7FJ transitions, as well as a decrease in the peak resolution (Figure 5a). These spectral changes indicate an inhomogeneous local environment of Eu3+ dopants upon the thermal treatment. More specific information about the dopant sites can be inferred from the 5D0 → 7F0 transition (Figure 5b). This transition involves nondegenerate states, and each peak can be attributed to a specific Eu3+ site rather than a symmetry-

Figure 3. Time-resolved PL decay of the characteristic emission of (a) Eu2+ (monitored at 402 nm after excitation at 300 nm) and (b) Eu3+ (monitored at 613 nm after excitation at 230 nm) for 4.0% Eu-doped Ga2O3 NCs. The solid lines are biexponential fits to the data points. i ⎛ t⎞ I(t ) = I(0) ∑ Ai exp⎜ − ⎟ τi ⎠ ⎝ 1

(1)

where I(t) and I(0) are the luminescence intensities at time t and 0, respectively, and Ai values are the relative fractions of the individual time components τi. The average lifetimes of the corresponding transition were then calculated as ⟨τ ⟩ =

∑i Ai τi 2 ∑i Ai τi

(2)

The average lifetime of the emission at 402 nm is determined to be ca. 200 ns, 4 orders of magnitude shorter than that at 613 nm (1.4 ms). The dramatically faster decay rate of the PL at 402 nm reflects the allowedness of the d−f transition in Eu2+ and is consistent with the presence of divalent europium ions in Ga2O3 NCs. Similar results were obtained for all doping concentrations (Figure S4 and Tables S1 and S2). To quantify the relative amounts of the two oxidation states, we performed X-ray absorption near-edge structure (XANES) spectroscopic measurements at the Eu L3 edge (Figure 4). The

Figure 4. (a) Eu L3-edge XANES spectra of Eu-doped Ga2O3 NCs synthesized at 300 °C for Eu doping concentrations of 0.8% (middle panel) and 7.5% (top panel) and the reference spectra of Eu2+ (Eu2Si5N8, blue) and Eu3+ (Eu2O3, red) (bottom panel). The spectra of NCs were quantitatively analyzed using a peak deconvolution analysis (corresponding dashed lines) upon the background correction (black dotted line). (b) Eu L3-edge XANES spectra of 4.0% Eu-doped Ga2O3 NCs as synthesized (blue) and after annealing at 800 °C (red). (c) The fraction of Eu2+ as a function of the doping concentration for as-synthesized NCs (blue squares) and the same NCs annealed at 800 °C (red dots). D

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Figure 5. (a) High-resolution PL spectra of Eu3+ (5D0 → 7FJ) in Eudoped Ga2O3 NCs as synthesized (bottom) and annealed at 400 °C (middle) and 800 °C (top). (b) Magnified 5D0 → 7F0 spectra of the samples in (a).

induced splitting. The 5D0 → 7F0 spectral region of assynthesized NCs consists of two narrow peaks (blue line in Figure 5b), indicating a dual-site occupancy. The intensity ratio of these peaks varies with doping concentration.12 The peak at lower energy (581.9 nm) has been assigned to internally doped Eu3+, while that at higher energy (580.3 nm) to Eu3+ residing on NC surfaces.12 Upon annealing at high temperature, the lower energy peak vanishes, while the higher energy peak becomes broader, suggesting an expulsion of internally incorporated dopant ions to NC surfaces. This conclusion is consistent with the overall change in the spectral band shape in Figure 5a. The process of dopant expulsion is reinforced by the significant size difference between europium and Ga3+ ions. These findings should be correlated with the decrease in the Eu2+ concentration upon annealing, as shown in Figure 4c. The simultaneous ejection of Eu2+ dopant ions during the hightemperature thermal treatment leads to their oxidation at the NC surfaces, not only changing the dopant speciation but also decreasing the ratio of blue-to-red emission intensity. Expulsion and oxidation of Eu2+ is likely fostered by a decrease in the concentration of oxygen vacancies due to high-temperature annealing. Taken together, the results in Figures 4 and 5 confirm the presence of both Eu2+ and Eu3+ oxidation states and the ability to impart complex and fundamentally different optical properties in Ga2O3 NCs via manipulation of the dopant oxidation states. The mechanistic information inferred from these spectroscopic studies allows us to further adjust the synthesis conditions to manipulate the optical properties of colloidal Eu-doped Ga2O3 NCs and, specifically, the ratio of Eu2+ and Eu3+ emission intensities over a broad range. We have previously demonstrated that lowering reaction temperature leads to a decrease in Ga2O3 NC size and an increase in the number density of native defects responsible for DAP emission.28,33,38 The increase in the concentration of defects with decreasing synthesis temperature is partly associated with reduced NC sizes and enhanced surface-to-volume ratio, due to the preferential formation of native defects in the vicinity of NC surfaces.34 Consequently, the DAP emission increases in intensity and shifts to higher energy with decreasing reaction temperature. Given the above results, we examined the influence of the reaction temperature on the PL properties of Eu-doped Ga2O3 NCs. Figure 6a,b shows the PL spectra of

Figure 6. (a) PL spectra of Eu2+ dopant centers in Ga2O3 NCs synthesized with 5 mol % starting concentration of EuCl3 at 200 °C (green trace) and 300 °C (black trace), upon excitation at 280 nm. (b) PL spectra of Eu3+ dopant centers in the same Ga2O3 NCs upon excitation at 230 nm. (c,d) The dependence of Eu2+ (blue circles) and Eu3+ (red squares) PL intensity, monitored at 402 and 613 nm, respectively, for the samples synthesized at (c) 300 °C and (d) 200 °C.

Eu2+ and Eu3+, respectively, in ca. 5% Eu-doped Ga2O3 NCs synthesized at 200 °C (green trace) and 300 °C (black trace). In the sample synthesized at 300 °C, the emission from Eu2+ centers dominates, with relatively minor Eu3+ emission. On the other hand, this trend is reversed for the NCs prepared at 200 °C; the PL of Eu3+ is ca. 3.5 times stronger relative to the sample synthesized in the same way at 300 °C, while Eu2+ nearly vanishes. The difference in the PL properties of the samples synthesized at these two temperatures is also evident for other doping concentrations. Figure 6c,d shows the dependence of Eu2+ (blue) and Eu3+ (red) PL intensities on the doping concentration for the samples synthesized at 300 and 200 °C, respectively. The NC concentrations were adjusted to exhibit the same band gap absorbance at 230 nm, and we monitored PL intensity at 613 nm for Eu3+ and 402 nm for Eu2+. At the high synthesis temperature (Figure 6c), the PL intensities of both Eu2+ and Eu3+ increase with increasing doping concentration, although the increase in intensity of Eu2+ is much more pronounced. Although the absolute concentration of Eu2+ increases somewhat with doping concentration in spite of its diminishing fraction, this increase cannot by itself account for the steep increase in Eu2+ PL intensity in Figure 6c. This 4f6 5d1 → 4f7 PL transition is sensitive to the local environment, and we associate its dependence on the doping concentration with the distortion of the NC host lattice induced by the size difference between europium ions and Ga3+. Increasing doping concentration results in a higher degree of local distortion around the Eu2+ sites, causing higher oscillator strength of this transition. At low synthesis temperature (Figure 6d), Eu3+ has comparatively high emission intensity, which gradually decreases with doping concentration due to crossrelaxation (concentration quenching) processes, while the Eu2+ emission remains negligible in the entire doping concentration range. E

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reduction of Eu3+ and a higher concentration of native defects in the vicinity of NC surfaces, owing to an increase in the surface-to-volume ratio.33,34 The lack of internally incorporated Eu2+ and more effective sensitization of Eu3+ by surface-based NC defects lead to exclusive Eu3+ emission. It is plausible that some nonoxidized Eu2+ ions remain on the surfaces of NCs, where they are quenched by nonradiative relaxations. These manipulations of the relative fractions of Eu2+ and Eu3+ centers and native defect concentration by reaction conditions, such as temperature, time, and starting dopant precursor concentration, enable the tuning of the PL emission in colloidal Ga2O3 NCs from blue to red. The coexistence of Eu2+ with Eu3+ has not been reported for bulk (β-phase) Ga2O3 prepared by solid state reaction or other high-temperature preparation methods, such as spray pyrolysis.36,40,41 Another important advantage of colloidal NCs is the ability to manipulate the crystal structure, and therefore the properties, of polymorphic materials using NC size.35,42−45 The ability to stabilize divalent lanthanide centers and control the redox conversion by ionizing radiation is the basis for the design of X-ray storage phosphors. These materials are capable of generating an image created by X-ray absorption and have a major application in digital radiography and medical imaging.46 The most understood and widely used X-ray storage phosphor is BaFX:Eu2+ (X = Cl, Br, I).15,46,47 It is proposed that an exposure to ionizing radiation leads to the formation of electron−hole pairs in this phosphor, where the electrons are trapped on anion vacancies (F centers), while holes are trapped on Eu2+ centers leading to their oxidation to Eu3+.47 By visible light photostimulation, the electrons trapped on F centers are again promoted into the conduction band and recaptured by Eu3+, converting them to excited Eu2+ ions which emit blue light. The opposite process is observed in Sm3+-doped BaFCl, for which an exposure to X-ray radiation leads to the generation of Sm2+ ions, most likely by capture of the electrons from the F centers.48 An image created using this X-ray storage phosphor can be recorded by direct photoexcitation of Sm2+. In the context of these results and a high concentration of oxygen vacancies in Ga2O3 NCs, we primarily examined here the effect of X-rays by keeping the samples exposed to ca. 7 keV radiation (Eu L-edge) and periodically collecting the scans. Figure 8 shows the XANES spectra of 0.8% Eu-doped Ga2O3 NCs synthesized at 300 °C before (red line) and after (green line) an exposure to X-ray radiation for 30 min. With increasing exposure time, the amount of Eu2+ increases significantly relative to the amount of Eu3+; upon 30 min exposure, the fraction of Eu2+ increases from ca. 22% to 40%. The X-ray

This dramatic difference in the behavior of the two emission centers with doping concentration at different temperatures can be traced to the reducing power of the reaction mixture, as well as the location and density of native defects in Ga2O3 NCs. The overall scheme of the charge and energy transfer processes responsible for the PL properties of Eu-doped Ga2O3 NCs is shown in Figure 7. The excitation above the Ga2O3 band edge

Figure 7. Schematic representation of the charge and energy transfer processes involved in the dual emission of Eu2+/Eu3+-doped Ga2O3 NCs. Charge transfer is indicated with solid lines and energy transfer with dashed lines.

energy leads to an efficient trapping of the photoexcited electrons and holes in the donor and acceptor states, respectively, resulting in a broad DAP emission of Ga2O3 NCs (thick blue-green arrow). The nature of the donor states has been extensively discussed for β-Ga2O3 single crystals. Electron paramagnetic resonance together with conductivity studies have suggested that the donor states are predominantly associated with one of the three different oxygen vacancy sites (V•O(2)) and that electron transfer takes place along the b crystallographic axis through a hopping or tunneling mechanism, depending on temperature.39 Much less is known about the nature of the oxygen vacancy-based donor states in γGa2O3. Our recent temperature-dependent PL studies have indicated that the donor activation energy in ca. 4 nm diameter colloidal γ-Ga2O3 NCs is approximately 220 meV.33 This activation energy decreases with increasing NC size or europium doping concentration.12,33 The donor and acceptor defect states also play a key role in the sensitization of the europium emission by nonradiative transfer of the DAP exciton energy to the dopant ions (dashed lines in Figure 7). The energy transfer is much more efficient for Eu3+ (thicker dashed line) as evidenced from the excitation spectra in Figure 2. Given the low absorptivity of f−f transitions, the energy transfer is likely to be the Dexter rather than the Förster type. On the other hand, Eu2+ can be readily excited directly (purple arrow), allowing for a control of the NC emission color by selective excitation. The Eu2+ and Eu3+ centers behave independently, with insignificant energy transfer between them, which could be associated with different spatial distribution of the two species. While Eu2+ appears to be stabilized as an internal dopant, Eu3+ is partitioned between the interior and surface of the NCs, with a larger fraction on the NC surfaces. For high synthesis temperatures, Eu3+ in the reaction mixture is readily reduced to Eu2+ (Figure S2). The obtained NCs are also larger and on average contain fewer native defects, a significant fraction of which could be located in the interior of the NCs. These internal defects (i.e., oxygen vacancies with a single positive charge) play a key role in the stabilization of Eu2+, as described above, leading to a strong emission upon direct excitation of the Eu2+ dopant centers. A decrease in the reaction temperature results in a lower degree of

Figure 8. Eu L3-edge XANES spectra of 0.8% Eu-doped Ga2O3 NCs as synthesized at 300 °C (red) and upon in situ exposure to X-ray radiation for 30 min (green). F

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radiation clearly leads to the reduction of Eu3+ in Ga2O3 NCs. This phenomenon likely involves trapping of the electrons, generated by ionizing radiation, at the Eu3+ centers. The reduction of Eu3+ can occur by initial trapping of radiationgenerated electrons at oxygen vacancies or direct trapping of these electrons at Eu3+ centers which are also known to be good electron traps.20 Recent EPR studies on aluminoborosilicate glasses have suggested that exposure to ionizing radiation decreases the overall concentration of native defects as well as the ratio of hole to electron trapping defects.20 Upon sufficient exposure to X-rays, the Eu3+ can effectively compete with oxygen vacancies for trapping of the generated electrons, leading to its reduction to Eu2+. The Eu3+ PL intensity, lowered upon the NC exposure to X-rays, partially recovers within ca. 20 min after irradiation stops. The reoxidized Eu2+ is likely located on NC surfaces, while internally incorporated Eu2+ ions are stable to oxidation and do not change in the course of these experiments. These results demonstrate yet another way to manipulate the oxidation state of europium dopants owing to the electronic and defect structure of the host Ga2O3 NCs. The X-ray-induced reduction of Eu3+ dopants is also a powerful evidence that Eu2+ exists in NCs rather than as a separate nanoscale phase. Furthermore, the response of Eu-doped Ga2O3 NCs to X-ray radiation indicates a potential of this system as an X-ray storage phosphor, which warrants further investigations.

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Article

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.5b02383. Additional absorption, excitation, and emission spectra of Eu2+, excitation spectrum of Eu3+ in Ga2O3 NCs, Eu M4,5-edge XANES spectra, and additional time-resolved data and analysis (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address †

T.W.: National Engineering Research Center for Colloidal Materials, Shandong University, Jinan 250100, P. R. China. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Sciences and Engineering Research Council of Canada and Canada Research Chairs Program (award to P.V.R.). We thank the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research (New Directions Grant 53856-ND4). X-ray absorption spectroscopy experiments were performed at the Canadian Light Source (CLS), which is supported by the NSERC, NRC, CIHR, the Province of Saskatchewan, Western Economic Diversification Canada, and the University of Saskatchewan. We thank Dr. Ning Chen (CLS) for providing the europium reference compounds, assistance at the HXMA (06ID-1) beamline, and stimulating discussions. L.N.H. and M.H. acknowledge CLS for Graduate Travel Awards. B.Y. thanks TUBITAK for an International Postdoctoral Research Fellowship (No. 2219).

CONCLUSIONS

In summary, the results of this work demonstrate the coexistence of substitutionally incorporated europium dopant ions in +2 and +3 oxidation states in colloidal Ga2O3 NCs. This coexistence is enabled by in situ reduction of Eu3+ during the NC synthesis in oleylamine acting as a coordinating solvent and a reducing agent. The Eu2+ sites are stabilized in the interior of the NCs, owing to the attractive interactions with oxygen vacancies with a singular positive charge. On the other hand, a significant fraction of Eu3+ ions are located on the NC surfaces. Both steady-state and time-resolved PL properties of divalent and trivalent europium ions are distinctly different, resulting in the new functional properties of these colloidal NCs through complex photophysical interactions. The Eu2+/Eu3+ concentration ratio decreases with increasing starting Eu3+ precursor concentration and thermal treatment of as-synthesized NCs. The PL intensity of Eu2+ is diminished with decreasing reaction temperature, which we associated with a lower degree of Eu3+ reduction and higher surface-to-volume ratio of the resulting NCs. While Eu3+ substitution for a divalent host cation could lead to “abnormal reduction” owing to the charge compensation,21,22 this work represents an unusual example of the substitutional incorporation of Eu2+ in a trivalent cation host lattice, using Eu3+ precursor in the absence of postsynthesis reduction at high temperature. Furthermore, the ability to convert Eu3+ to Eu2+ in NCs using X-ray radiation suggests the potential applicability of these materials in computed radiography and medical imaging. The results of this work provide a guideline for the design of lanthanide-doped transparent metal oxide NCs having complex PL properties that can be controlled via redox chemistry during the colloidal NC synthesis or their subsequent treatment.

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REFERENCES

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