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Distance-Dependent Energy Transfer between Ga2O3 Nanocrystal Defect States and Conjugated Organic Fluorophores in Hybrid White-Light-Emitting Nanophosphors Vadim Chirmanov, Paul C. Stanish, Arunasish Layek, and Pavle V. Radovanovic* Department of Chemistry, University of Waterloo, 200 University Avenue West, Waterloo, Ontario N2L 3G1, Canada S Supporting Information *

ABSTRACT: We report a quantitative analysis and development of hybrid white-light-emitting nanoconjugates, prepared by functionalizing colloidal γGa2O3 nanocrystals with selected organic fluorophores. Using the Förster resonance energy transfer (FRET) formalism, we studied the coupling of native defect states in Ga2O3 nanocrystals, as energy donors, with different orange-red-emitting fluorophores bound to nanocrystal surfaces, as energy acceptors. Variations in the average nanocrystal size and dye surface coverage were used to characterize the efficiency of the energy transfer process and the corresponding donor−acceptor separations. The results show that for approximately three rhodamine B molecules per nanocrystal the energy transfer efficiency increases from 23% to 49% by decreasing the NC size from 5.3 to 3.6 nm. These FRET efficiencies correspond to the estimated donor−acceptor distances of 3.55 ± 0.02 and 2.99 ± 0.03 nm, respectively. Similar trends were observed for ATTO 590-conjugated Ga2O3 nanocrystals, although ATTO 590 proved to be a more effective energy acceptor owing to a larger molar extinction coefficient in the conjugated form. The size-dependent luminescence of Ga2O3 nanocrystals and the control of FRET parameters through the variations in the bound dye molecules allow for the generation of tunable blue-orange emission, ultimately resulting in white light with targeted chromaticity and high color rendition.

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INTRODUCTION Tungsten filament lamps, widely used for lighting for more than a century, rely on incandescence to generate visible light and therefore have poor luminous efficacy and a short lifetime. The development of white-light-emitting diodes (WLEDs) for general lighting application has received much attention ever since the introduction of the first practically useful red LED in 1962.1−3 Only recently, however, WLEDs have emerged as a viable energy-efficient and durable substitute for incandescent light bulbs.4,5 Generation of white light using LEDs is typically based on the downconversion of a blue or ultraviolet (UV) LED by various phosphors.5−8 In one possible configuration, a part of a blue LED output is converted by a yellow-emitting phosphor to jointly approximate white light. Alternatively, a UV LED with blue and yellow (or blue, green, and red) phosphors can generate more appealing white light, but with lower efficiency. It is also possible to generate white light by carefully mixing red, green, and blue emission from separate LEDs.9 Such devices may suffer from diminished efficiency due to packaging, LED driver losses, losses associated with optical mixing of individual LEDs, and thermal effects, in addition to high manufacturing cost.10−12 Both of these approaches require two or more independent components to produce white light, and it is not uncommon for such devices to have poor color mixing, resulting in a “cool” white light, which is not suitable for general lighting.13,14 Furthermore, most commercially used phosphors contain rare-earth metals which are considered to be © 2015 American Chemical Society

a strategically important resource, significantly increasing the device cost. Development of a single phosphor, in which multiple fluorophore centers emitting complementary spectral colors are electronically coupled, is highly desirable for generating homogeneous white light with reproducible chromaticity and targeted spectral properties. Such phosphors usually require an elaborate synthetic route and/or rely on complex codoped crystal lattices or heavily modified polymers which also involve rare-earth15,16 or scarce metals such as ruthenium and iridium.17,18 The cost and availability of the rare-earth elements and suboptimal properties of illuminated light in typical phosphorconverted WLEDs have led to the emergence of alternative approaches to generating white light.19−21 White light emission has been demonstrated using semiconductor quantum dots (QDs), owing to their size-dependent photoluminescence (PL).21,22 In one such example, a mixture of red-, blue-, and green-emitting QDs was deposited as a monolayer, and white light was generated through electroluminescence.22 An organic route to generating white emission has also yielded some promising results with the development of a two-component, blue-orange-emitting molecule.23 Other examples include Mnand Cu-codoped ZnSe QDs, where a combination of excitonic Received: December 13, 2014 Revised: February 14, 2015 Published: February 26, 2015 5687

DOI: 10.1021/jp512417h J. Phys. Chem. C 2015, 119, 5687−5696

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The Journal of Physical Chemistry C

isolated with the addition of ethanol followed by centrifugation at 3000 rpm for 10 min. Washing with ethanol was repeated three times. The NCs were then capped with TOPO by adding ∼2.5 g of melted TOPO and stirring the solution for 1 h at 70 °C.35 Finally, the resulting colloidal NCs were dispersed in hexane and stored in the refrigerator. Nanocrystals were stable as colloidal suspensions for at least 12 months. Nanocrystal−Dye Conjugate Preparation. To facilitate binding of organic dyes to the surface of Ga2O3 NCs, the water−hexane bilayer approach was used, as previously described.29 The concentrations of all stock suspensions of Ga2O3 NCs were standardized on the basis of the absorbance of 1.0 at 230 nm, and a series of RhB stock solutions having concentrations of 2.13, 5.16, 7.20, and 10.40 μM were prepared separately. For the preparation of a typical conjugate sample, 4 mL of RhB stock solution was deposited into a vial followed by the addition of 6 mL of Ga2O3 stock suspension to form a bilayer. The sample was left undisturbed for 6 h. The hexane portion was then isolated and sonicated for 1−2 min. Conjugate samples with ATTO 590 (A590) were prepared in a similar manner with the binding time extended to 24 h. Estimation of the Number of Dye Molecules per Nanocrystal. The average number of dye molecules per Ga2O3 NC was determined using two parameters: the number of Ga3+ ions in the colloidal NC stock suspension and the molar extinction coefficient of the bound dye. Inductively coupled plasma atomic emission spectrometry (ICP-AES) was used to determine the Ga3+ ion content and the amount of NCs in the 6 mL stock suspensions of Ga2O3 NCs having different average NC sizes. Ga2O3 stock suspensions were completely evaporated, and the remaining solids were digested with aqua regia and diluted with deionized water to the same volume (6 mL). Preparing and handling aqua regia requires special precautions (see the Supporting Information for more details). The resulting solutions were analyzed by ICP-AES. The measured Ga3+ concentrations were then used in conjunction with the crystallographic data (unit cell parameters) and the density of the γ-Ga2O3 phase to estimate the number of Ga2O3 NCs. To determine the molar extinction coefficients of the bound RhB dye, 4 mL of a 1.0 μM stock solution of RhB in water was added to the stock suspensions of γ-Ga2O3 NCs in hexane. RhB was allowed to bind to NCs for just 1 h to ensure a minimal possible amount of nonbound RhB. The molar extinction coefficient was determined using the Beer−Lambert law. Surface coverage was calculated using the number of Ga2O3 NCs together with the absorbance and molar extinction coefficient of the bound dye and reported as the number of dye molecules per NC. Additional details can be found in the Supporting Information. Spectroscopic Measurements. The absorption measurements were performed with a Varian Cary 5000 UV−vis−NIR spectrophotometer at room temperature using a standard quartz cuvette with a 1 cm path length. Photoluminescence measurements were performed with a Varian Cary Eclipse fluorescence spectrometer. The samples were excited at 230 nm (Ga2O3 NC band gap) or at the S0 → S1 absorption peak maxima of the dye molecules. The quantum yield (QY) was determined using quinine bisulfate as the reference. Time-resolved PL decay measurements involving Ga2O3 excitation were made 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, upon

emission with emissions from multiple dopants gives rise to white light.24 The examples cited above15−18,24 utilize energy transfer from high to lower energy states to facilitate a simultaneous emission. Of particular interest is the phenomenon known as Förster (or fluorescence) resonance energy transfer (FRET), which involves a nonradiative energy transfer from a donor species to an acceptor. The possibility of generating white luminescence through FRET has been previously proposed and demonstrated.25−28 Such an approach could potentially result in cheaper and homogeneous quasi-single phosphors for application in WLEDs and other lighting devices. We have recently reported hybrid white-light-emitting nanostructures constructed by conjugation of blue-emitting γ-Ga2O3 nanocrystals (NCs) with secondary organic fluorophores emitting in the orange-red spectral region.29 These studies have demonstrated that white light generation is enabled by FRET from Ga2O3 NCs to the surface-bound dye molecules, resulting in coupled fluorophore behavior. We further suggested that rhodamine B (RhB) dye, as a model system, can bind to NC surfaces via the free carboxyl group, allowing for FRET mechanism.30 In this paper, we quantitatively describe the FRET mechanism involving defect states in Ga2O3 NCs as energy donors and organic fluorophores as energy acceptors to address the origin of white light generation and its control. On the basis of this analysis, we demonstrate the possibility to tune white light by controlling various synthetically accessible parameters, including NC size and surface coverage of dye molecules. While FRET has usually been studied for conjugated II−VI NCs exhibiting excitonic emission,31,32 we show in this work that unique properties can be achieved by FRET involving NC native defect states as donors. The results of this work reinforce the concept of using functional defects as a way of designing and manipulating new functionalities in complex nanoscale materials.

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EXPERIMENTAL SECTION Materials. All reagents and solvents were used as purchased without further purification. Gallium acetylacetonate (Ga(acac)3; 99.99%) and gallium nitrate hydrate (Ga(NO3)3· xH2O; 99.99%) were purchased from Strem Chemicals. Oleylamine (OAm; 70%), tri-n-octylphosphine oxide (TOPO; 90%), [9-(2-carboxyphenyl)-6-(diethylamino)-3xanthenylidene]diethylammonium chloride (rhodamine B; 90%), and 6-(2,5(6)-dicarboxyphenyl)-1,11-diethyl2,2,4,8,10,10-hexamethyl-2,10-dihydro-1H-pyrano [3,2-g:5,6g′]diquinolin-11-ium perchlorate (ATTO 590; ≥90%) were purchased from Sigma-Aldrich Co. Synthesis of Ga2O3 Nanocrystals. A series of colloidal γGa2O3 NCs of variable sizes have been prepared using a sizecontrolled synthesis previously described.33,34 In a typical synthesis, 0.5 g of Ga(acac)3 and 7.0 g of OAm were placed into a 100 mL three-neck round-bottom flask, and the temperature of the mixture was increased to 80 °C to allow for a complete dissolution of the precursor. The solution was degassed for about 5 min with continued stirring, and the reaction flask was filled with argon. The temperature was subsequently raised to the final reaction temperature (200−310 °C) at a heating rate of ∼3 °C/min. The reaction mixture was kept at the desired temperature for 1 h with constant stirring and then gradually cooled to room temperature. By varying the reaction temperature, NCs with average sizes of 3.3, 3.6, 4.1, 5.0, 5.3, and 5.8 nm were obtained. The final product was 5688


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The results relevant for calculating the FRET parameters are summarized in Table S1 (Supporting Information). We chose RhB dye as a model FRET acceptor that exhibits strong PL in the orange-to-red spectral range (Figure S1a, Supporting Information). The absorption band arising from the π to π* (S0 → S1) transition provides significant overlap with the blue emission of Ga2O3 NCs (Figure S1g), enabling the conditions for FRET mechanism. Another important feature which makes RhB a good model system for this study is its ability to undergo lactone ring formation (RhB-L) upon deprotonation of the carboxyl group in a nonpolar solvent.38,39 Such a transformation causes a change in the electronic structure, which allows for spectroscopic differentiation between NC-bound RhB and its free RhB-L counterpart. The Fourier-transform infrared (FTIR) spectroscopic characterization of this type of nanoconjugates has suggested that binding of dye molecules to the surfaces of transparent metal oxide NCs is facilitated by the carboxyl groups.30 While RhB can bind through the carboxyl group to NCs dispersed in hexane or toluene, any solvated molecules in such nonpolar solvents exist in the lactone form, which is distinctly nonemissive in orange-red and does not undergo resonance energy transfer due to the negligible intensity of the S0 → S1 absorption transition (Figure S1b). Figure 1b shows an increase in the PL decay rate of Ga2O3 NC trap emission as a result of binding of RhB, which is one of the common characteristics of FRET (see also Figure S2 in the Supporting Information). The average lifetimes of the NC trap emission for different concentrations of RhB, estimated from fitting of the corresponding PL decay curves, are given in Table S2 (Supporting Information). The change in the defect trap emission decay rate upon NC conjugation appears to be diminished with decreasing NC size. The DAP mechanism described above depends on the distribution and separation of defect sites which are, in turn, dependent on the NC size.36,37 Sufficient decrease in NC size and the corresponding increase in the near-surface defect concentration could lead to the formation of a donor−acceptor complex involving a holetrapping defect (where the NC DAP recombination occurs) and a conjugated dye molecule. The energy transfer within this complex-like structure could result in prompt and complete quenching of the NC PL, similarly to the static quenching mechanism. Dye molecules also experience profound changes in the emission lifetime upon binding to the NC surface. The inherent lifetime of RhB PL directly excited into the S0 → S1 transition increases upon binding to the NC surface (Figure S3, Supporting Information), indicating a reduction in nonradiative relaxation due to restricted molecular motion. When the same Ga 2 O 3 −RhB nanoconjugates were excited at 230 nm (sensitized emission), the corresponding RhB average PL lifetime was extended from ca. 3.5 ns to over 5 μs, resembling the PL decay dynamics of the Ga2O3 NCs (Figures 1c and S3). In this configuration, the excitation of RhB was governed by the DAP recombination of the Ga2O3 NCs as a result of FRET. Consequently, the afterglow characteristic for Ga2O3 NCs is also observed for bound RhB upon excitation at 230 nm, even after a delay of ca. 1 ms (Figure 1c). A590 displayed a similar behavior, having its inherent emission lifetime of ca. 4 ns extended to ca. 6.2 μs upon conjugation, as a result of energy transfer from Ga2O3 NCs photoexcited at 230 nm (Figure S3 and Table S3, Supporting Information). These results indicate an efficient nonradiative energy transfer from Ga2O3 NCs to the bound dye molecules, leading to a characteristic blueorange emission.

excitation at 230 nm. The signals were detected at the maximum of the Ga2O3 NC or bound dye molecule emission band. For time-resolved measurements involving direct excitation of dye molecules, we used time-correlated singlephoton counting (TCSPC). These measurements were obtained with an IBH Ltd. time-resolved fluorimeter equipped with an IBH 563 nm NanoLED excitation source. All samples were excited at 563 nm in the right angle geometry, and emissions were detected at 575 nm for RhB and 620 nm for A590. Preparation and Characterization of the LEDs. UV LEDs were purchased from Sensor Electronic Technology, Inc. (UVTOP255TO39FW). The LEDs have the emission wavelength maximum centered at ca. 255 nm and a flat top quartz window. The windows of the LEDs were initially cleaned with hexane and ethanol, followed by the sample deposition. A suitable white-light-emitting sample was deposited on the surface of an LED window in 30 μL aliquots, followed by air drying under ambient conditions. The deposition was controlled by periodical checks of the resulting illumination by applying a voltage in the range of 6.0−8.0 V. Analysis of the resulting light was carried out in a dark room using a Konica Minolta CL-500A illuminance spectrophotometer.

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RESULTS AND DISCUSSION A proposed mechanism of white light generation by hybrid RhB-conjugated Ga2O3 NCs is shown in Figure 1a. Ga2O3 NCs

Figure 1. (a) Proposed PL mechanism of Ga2O3 NCs (right) and the resulting FRET-mediated emission of the bound RhB molecules in the Ga2O3−RhB hybrid nanoconjugates (left). (b) Donor Ga2O3 NC PL decay curves before (red) and after (black) binding of RhB to 5.3 nm NCs (λex, 230 nm; λem, 450 nm). (c) Time-gated PL spectra of the 5.3 nm Ga2O3−RhB nanoconjugates in hexane as a function of the delay upon excitation (gate time, 5 ms; λexc, 230 nm).

exhibit a broad blue-green emission arising from the recombination of an electron trapped in a donor with a hole trapped in an acceptor state.34,36,37 This mechanism is known as the donor−acceptor pair (DAP) recombination, and to avoid confusion with the donor and acceptor species associated with FRET, hereinafter we refer to this process simply as a defect trap emission. A series of Ga2O3 NC samples used to prepare Ga2O3 NC−dye conjugates were characterized by absorption and PL spectroscopies, TEM, and ICP-AES elemental analysis. 5689


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the opportunity to further tailor and manipulate the properties of the resulting emission. To quantify the influence of the spectral overlap integral on FRET parameters, we first estimated the extinction coefficients (ε) of dyes bound to Ga2O3 NCs dispersed in hexane. Binding was accomplished using low concentrations of dyes in aqueous solutions to minimize the presence of free molecules in the hexane phase, as described in the Experimental Section and the Supporting Information. Under those conditions, the difference in absorbance of dye stock solutions in water before and after the transfer of molecules to the hexane dispersion of NCs (Figure S5, Supporting Information) mainly corresponds to the amount of dye molecules bound to the NCs. By correlating the amount of dyes transferred to the hexane phase to the corresponding absorbance measurements at the S0 → S1 band maxima, we estimated the ε values of the bound RhB and A590 to be 11500 ± 400 and 31500 ± 100 cm−1 M−1, respectively. These values are significantly different from the ones obtained for free molecules in aqueous solutions (∼98000 and 120000 cm−1 M−1 for RhB and A590, respectively) or in nonpolar media (13000−16000 cm−1 M−1 for RhB-L at 309 nm).39,40 Although the organic dyes bound to NCs are often assumed to have the same extinction coefficients as those in the free form, these results show that they could experience dramatic changes upon binding, which has to be taken into account for the determination of the FRET parameters. The spectral overlap integral J is a key parameter for controlling FRET and is directly determined by the molar extinction coefficient of the acceptor (εA):41

To confirm that the observed FRET is associated with binding of RhB to Ga2O3 NCs rather than a simple proximity effect, a series of Ga2O3 NC suspensions were treated with RhB by both water−hexane transfer and direct addition of free RhBL. Figure 2 shows the absorption (a) and PL (b) spectra,

Figure 2. (a) Absorption spectra of the hexane suspensions of 3.6 nm Ga2O3 NCs (4.92 × 1014 NCs/mL) containing RhB-L (black) and bound RhB (green) in similar concentrations (ca. 2.7 μM). Inset: normalized absorption spectra of 3.6 nm Ga2O3 NCs (red) and Ga2O3−RhB hybrid nanoconjugates prepared using the same NCs in the band gap region (green). (b) PL spectra of the same two samples after excitation at 230 nm. The red line denotes the emission spectrum of the 3.6 nm Ga2O3 NCs without any dye in the suspension.

J=

∫ FD(λ) εA(λ) λ 4 dλ

(1)

In the above equation, FD is the donor (Ga2O3 NC) emission integral normalized to 1 in the spectrum plotted in nanometers and εA is the acceptor (RhB or A590) molar extinction coefficient at wavelength λ. The magnitude of J determines the critical distance R0 (also known as the Förster distance) at which the energy transfer efficiency, E, is 50%:41

respectively, of 3.6 nm Ga2O3 NC suspensions containing similar amounts of RhB in bound (green) and lactone (black) forms. Upon excitation above the Ga2O3 band gap energy, we observed a decrease in the donor emission by ∼40% for Ga2O3−RhB accompanied by the appearance of the acceptor emission (Figure 2b, green). On the other hand, the donor NC trap emission decreases by less than 8% in the presence of RhBL, while the corresponding acceptor emission is virtually absent (Figure 2b, black). This observation suggests that free RhB-L in solution cannot accept excitation energy from Ga2O3 NCs and generate the orange emission observed for Ga2O3−RhB nanoconjugates. A small decrease in the donor emission intensity for the colloidal sample containing RhB-L is due to a fraction of the light source associated with direct excitation of RhB-L or minor collisional quenching. Similar results were obtained for other NC sizes and dye molecule concentrations (Figure S4, Supporting Information). Taken together, these results indicate that energy transfer in Ga2O3−RhB nanoconjugates occurs as a result of binding between NCs and dye molecules. A590 is structurally similar to RhB, but its emission is redshifted relative to that of RhB (Figure S1c, Supporting Information). This difference allowed us to systematically investigate the influence of the spectral overlap on the efficiency of the energy transfer and other FRET parameters, providing

⎡ 9000Q (ln 10)κ 2J ⎤1/6 0 ⎥ R0 = ⎢ ⎢⎣ ⎥⎦ 128π 5n 4NA

(2)

In the above equation Q0 is the quantum yield of a donor in the absence of an acceptor, n is the refractive index of the medium, NA is Avogadro’s number, and κ2 is the dipole orientation factor. Using eqs 1 and 2, we obtained the values of J and R0 for Ga2O3−RhB and Ga2O3−A590 hybrid nanoconjugates (Table 1). In these calculations we approximated the dipole orientation factor to 2/3, which is reasonable considering random distributions of both NC defects and binding sites on the Table 1. FRET Parameters for Ga2O3−RhB and Ga2O3− A590 Hybrid Nanoconjugates Prepared Using Different NC Sizes rhodamine B

5690

ATTO 590

NP size (nm)

J

(1013 cm−1 M−1 nm4)

R0 (nm)

J (1013 cm−1 M−1 nm4)

R0 (nm)

3.6 4.1 5.0 5.3

8.01 8.95 11.00 12.10

2.41 2.43 2.45 2.51

9.19 11.86 15.67 18.00

2.47 2.54 2.60 2.68


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Figure 3. Absorption (a) and PL (b) spectra of 3.6 nm Ga2O3−RhB hybrid nanoconjugates with the variable concentration of bound RhB (0.8−2.7 μM). The concentration of Ga2O3 NCs was set to 4.92 × 1014 NCs/mL for all samples, and the excitation wavelength in all cases was 230 nm. (c) Normalized integrated PL intensities of the donor Ga2O3 NCs and the acceptor RhB molecules as a function of the donor/acceptor ratio n. (d) Variations in the donor−acceptor separations and FRET efficiency as a function of n. Solid lines in (c) and (d) show the fits obtained with the corresponding equations, as described in the text; the dashed line in (c) is used as a guide to the eye.

NC surfaces.42−46 Even though the A590 absorption band is red-shifted compared to that of RhB, and therefore has a smaller spectral overlap area with the Ga2O3 NC PL band, its higher ε results in a longer critical distance, rendering A590 a more effective energy acceptor. Furthermore, the higher experimentally determined quantum yield of NC-bound A590 (81%) relative to RhB (27%) makes the Ga2O3−A590 nanoconjugate a more efficient white-light-emitting nanophosphor (vide inf ra). Both J and R0 increase with increasing NC size owing to the red shift of the NC trap emission, resulting in an enhanced area of spectral overlap with a given dye molecule. In contrast with typical semiconductor quantum dot energy donors that exhibit excitonic emission, the FRET efficiency in dye-conjugated Ga2O3 NCs is determined not only by the NC size but also by the distribution of native defects in NCs. The PL of Ga2O3 NCs occurs when an electron trapped in an oxygen vacancy defect state just below the bottom of the conduction band undergoes tunnel transfer to an electron acceptor site (usually described as a Ga−O vacancy pair), where it recombines with a trapped hole.37 The energy donor− acceptor separation (RDA) therefore reflects an average distance between the hole-trapping sites as FRET donors and dyes conjugated on NC surfaces as FRET acceptors. To analyze the energy transfer efficiency and donor−acceptor distances in Ga2O3−RhB and Ga2O3−A590 nanoconjugates, we prepared a series of samples with variable starting concentrations of the dyes. The energy transfer efficiency η can be expressed as41 F η = 1 − DA FD

η=

nR 0 6 nR 0 6 + RDA 6

(4)

where n is the number of acceptors bound to a single donor. This average approximation generally works well when the stoichiometric ratio of the donor and acceptor can be controlled, or if all binding sites are predominantly occupied.32,43,47 It has also been indicated that, for NC−dye conjugates with RDA > R0, eq 4 provides an accurate estimate of the donor−acceptor distances.47 If the number of bound acceptor species is small compared to the number of possible binding sites (such as 1 acceptor per 20 binding sites), then the distribution of the acceptors per donor NC will obey the Poisson statistics and a modified equation for η may be more appropriate:48−50 ∞

η(ξ , R ) =

∑ n=0

6 ⎤ ξ ne−ξ ⎡ nR 0 ⎢ 6 ⎥ 6 n! ⎣ R + nR 0 ⎦

(5)

Here, ξ is the mean number of acceptor molecules bound to each donor NC, and n is the whole actual number of the acceptor molecules. Similarly, RDA for a system with a fixed donor/acceptor ratio can then be calculated using η and R0, assuming a constant acceptor distribution per donor NC:32,43,47 RDA

⎛ n(1 − η) ⎞1/6 = R 0⎜ ⎟ η ⎝ ⎠

(6)

The values of n were determined from the number of NCs for a given average size (Table S1, Supporting Information) and the number of conjugated dye molecules on the basis of their ε values in the bound form. This n value represents the upper limit given the possibility of detachment and solvation of some NC-bound dye molecules. By changing the starting concentration of RhB from ∼2 to 10 μM, the average surface coverage was varied from 1.0 to 3.3 molecules per 3.6 nm NC (Figure 3a

(3)

where FDA and FD are the integrated donor emission intensities in the presence and absence of acceptors, respectively. Another way of expressing η is through the relationship to R0 and the average donor−acceptor separation RDA:32,43 5691


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The Journal of Physical Chemistry C and Table S4, Supporting Information). Variation in the dye coverage allows us to control the spectral density distribution by controlling the probability of the sensitized emission of the acceptor via the energy transfer process (Figure 3b). Figure 3c plots the relative integrated PL intensities of Ga2O3 NCs (blue dots, integrated from 285 to 575 nm) and RhB (red dots, integrated from 480 to 660 nm) from Figure 3b, as a function of the number of RhB molecules per NC. With increasing surface coverage (or RhB-to-NC ratio), the NC emission is gradually quenched, which is accompanied by a nearly symmetric increase in the RhB emission intensity, consistent with the FRET mechanism. A decrease in the donor and an increase in the acceptor emission intensities level off at high values of surface coverage in Figure 3c, indicating that maximum surface coverage for 3.6 nm diameter NCs is attained for approximately three RhB molecules per NC. The maximum number of bound molecules appears relatively small considering the surface density of the available cation sites. This limit is largely determined by the bulkiness of RhB and the resulting steric hindrance of the potential binding sites. The data for quenching of the PL intensity of the donor were fit using a combination of eqs 3 and 4 and expressing FDA/FD as a function of n. The plots of FRET efficiency and donor− acceptor separation with respect to the surface coverage are shown in Figure 3d. The data for FRET efficiency were fit with eqs 4 (Figure 3d) and 5 (Figure S6, Supporting Information). Both equations provide a good overall fit to the experimentally obtained data points for all sample series. The applicability of the average approximation represented by eq 4 is likely associated with the relatively small number of actual binding sites due to the small size of NCs and bulkiness of the dye acceptors. The energy transfer efficiency gradually increases from 23% to 49% with increasing RhB/NC ratio, while the calculated RDA values are nearly constant. The constant RDA for different surface coverages reflects a largely random distribution of defect sites and dye molecules on NC surfaces. Additional analysis of Ga2O3−RhB nanoconjugates for 5.3 nm NCs produced similar results (Figure S7 and Table S5, Supporting Information). A series of Ga2O3−RhB nanoconjugate samples prepared with different NC sizes and having a 3:1 RhB-to-NC ratio were prepared to quantify the efficiency of the energy transfer and the corresponding donor−acceptor distances as a function of the NC size. Figure 4a plots the spectral overlap integral (left ordinate) and donor−acceptor separation (right ordinate) as a function of the NC size. Both quantities display a linear dependence as expected for the FRET mechanism. Given the unique nature of the energy donors in Ga2O3 NCs described above, the increase in RDA with increasing NC size is a consequence of two cooperating phenomena: (i) a smaller defect density in larger NCs owing to their lower surface-tovolume ratio and higher synthesis temperature, as demonstrated in our previous studies,33,36,37 and (ii) a reduced surface density of RhB for the same number of dye molecules per NC. In addition to an increase in RDA, increasing NC size also results in a longer R0 due to an enhancement in J. Figure 4b shows the FRET efficiency, calculated using eq 3, as a function of the J integral. Surprisingly, the data in Figure 4b display a decreasing linear trend. This seemingly counterintuitive trend can be associated with the fact that conjugates involving smaller NCs tend to have shorter donor−acceptor distances, as indicated in Figure 4a. Considering a similar acceptor-to-donor ratio for all NC sizes, the likelihood of a dye molecule residing in the

Figure 4. Summary of the FRET analysis for the Ga2O3−RhB hybrid nanoconjugates. (a) Spectral overlap integral (J) and average donor− acceptor separation (RDA) values with respect to the average NC size. (b) FRET efficiency as a function of J for Ga2O3−RhB nanoconjugates prepared using different average NC sizes with a similar RhB-to-Ga2O3 NC ratio (n ≈ 3). (c) Variation in RDA as a function of the FRET efficiency determined by the number of bound RhB molecules in Ga2O3−RhB nanoconjugates prepared using different average NC sizes.

immediate vicinity of the defect site acting as a FRET donor increases with decreasing NC size. A comparison of different sample series revealed a nearly constant value of RDA with respect to the FRET efficiency for all NC sizes (Figure 4c), which is governed by the RhB/NC ratio. The results of this analysis are summarized in Table S6 in the Supporting Information. These results quantitatively describe the control of the FRET parameters involving NC defect states. Controlling the efficiency of energy transfer via various FRET parameters is crucial for generating the desired emission spectrum, chromaticity, and color temperature. Such control can be achieved through variations in the average NC size and manipulation of the configuration and number of acceptor dye molecules with respect to a donor NC. To gain further insights into FRET in this type of hybrid white-light-emitting nanoconjugate, we conducted a similar set of experiments for the Ga2O3−A590 system. Considering the structure and properties of A590 (Figure S1c,f, Supporting Information), trends similar to those observed for the Ga2O3− RhB system were expected for FRET efficiency and donor− acceptor separations. Figure 5 shows the results for Ga2O3− A590 nanoconjugates based on the 3.6 nm NCs having a variable A590 surface coverage. The intensities of the A590 absorption bands in Figure 5a indicate a relatively small amount of A590 molecules bound to NCs, which could be related to the 5692


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Figure 5. Absorption (a) and PL (b) spectra of the 3.6 nm Ga2O3−A590 hybrid nanoconjugates with a variable concentration of the bound A590 (0.12−0.63 μM). The concentration of Ga2O3 NCs was set to 4.92 × 1014 NCs/mL for all samples, and the excitation wavelength in all cases was 230 nm. (c) Normalized integrated PL intensities of the donor Ga2O3 NCs and the acceptor A590 molecules as a function of the donor/acceptor ratio n. (d) Variations in the FRET efficiency and the donor−acceptor separations as a function of n. Solid lines in (c) and (d) show the fits obtained with the corresponding equations, as described in the text; the dashed line in (c) is used as a guide to the eye.

Figure 6. Dependence of FRET parameters on the average NC size for both Ga2O3−RhB and Ga2O3−A590 nanoconjugates: (a) critical (Förster) distance, (b) spectral overlap integral, and (c) donor−acceptor separation values. (d) Photographs of the emission of 3.6 nm Ga2O3−RhB nanoconjugates in colloidal form excited by a UV lamp (left) and deposited as a remote phosphor on UV LEDs operating at 7.0 V in lit (top right) and dark (bottom right) environments.

associated with the higher ε value of A590 in the NC-bound form. In contrast to Ga2O3−RhB, a small negative slope is observed for RDA plotted against the surface coverage (Figure 5d). This small deviation from the constant RDA value could potentially be attributed to some aggregation of the Ga2O3 NCs at the water−hexane boundary as a result of the increased binding time needed to achieve the desired surface coverage. Such a phenomenon would effectively reduce the concentration of NCs in the hexane phase, leading to a lower intensity of the corresponding PL peak. Since PL measurements were used to determine the efficiency of FRET, such a decrease in intensity would falsely contribute to the value of η. Nevertheless, the

lower binding affinity of A590 compared to RhB. The highest A590/NC ratio for 3.6 nm Ga2O3 NC was determined to be 0.8, even after a 24 h binding period (Table S7, Supporting Information). As a result, a weaker donor emission quenching and lower intensity of the sensitized acceptor emission were observed relative to those of Ga2O3−RhB (Figure 5b), although they exhibit a similar symmetric behavior (Figure 5c). The maximum FRET efficiency was found to be ca. 26% even for the highest A590/NC ratio (Figure 5d). However, given that the average A590 coverage does not exceed one molecule per NC, it is indeed performing as an efficient energy acceptor relative to the bound RhB. The higher efficiency can readily be 5693


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The Journal of Physical Chemistry C overall RDA values were found to be within the expected range and comparable to those obtained for Ga2O3−RhB (Table S7). The results for A590 bound to NCs having different sizes are summarized in the Supporting Information (Figure S8 and Table S8), generally demonstrating the same trend as for 3.6 nm NCs. These results show that both RhB and A590 are suitable energy acceptors when bound to the surface of Ga2O3 NCs. However, the comparison between the FRET parameters for Ga2O3−RhB and Ga2O3−A590 nanoconjugates reveals some important differences, allowing for a targeted design of broad wavelength range emitters with tunable chromaticity. The critical distances (Figure 6a) and spectral overlap integrals (Figure 6b) for Ga2O3−RhB and Ga2O3−A590 nanoconjugates follow a linear dependence on the NC size, but exhibit significantly different values and slopes. Although Ga2O3−RhB has a larger normalized spectral overlap area owing to the blue shift of the S0 → S1 absorption band of the NC-bound RhB relative to A590, the latter has a nearly 3 times higher ε value. The larger ε of bound A590 results in an increased J integral, in spite of the smaller spectral overlap area on the normalized basis. This finding indicates that a fluorophore having a high ε in the NC-bound form could serve as an efficient energy acceptor, allowing for an increased optical density in the red part of the spectrum to generate a softer white light. The average donor−acceptor distances in RhB- and A590-based nanoconjugates were found to be very similar and in the expected range, which reflects an average distribution of defects in NCs, as well as the dimensions of acceptor molecules and their distribution on NC surfaces (Figure 6c and Table S9, Supporting Information). The conjugated systems described here demonstrate high aptitude to generate a broad blue-orange spectrum, as a result of FRET, by a single excitation wavelength. Careful tailoring of the resulting emission chromaticity could be achieved by variations in the average NC size, number of dye molecules per NC, and type of acceptor species (Figure S9, Supporting Information). Matching blue luminescence of the 3.6 nm Ga2O3 NCs with a complementary orange-red sensitized emission of RhB allows for the generation of the near-ideal white light in a colloidal suspension (Figure 6d, left). Deposition of such samples on the commercially available UV LEDs allows for an efficient downconversion of a narrow UV emission to a broad blue-toorange spectrum, ultimately resulting in the generation of white light under low applied voltage, which could be observed even in a well-lit environment (Figure 6d, right). Given that the purpose of this study is to develop a deeper understanding of the energy transfer process involving NC defects in hybrid white-light-emitting nanoconjugates, the dye molecules and the nanoconjugate configuration were not designed to optimize thermal stability or photostability. Nevertheless, preliminary investigations suggest that these systems remain stable at elevated temperatures (ca. 50 °C) for hours. Optimization of the nanoconjugate properties relevant to light-emitting devices is currently in progress.

acceptors, by transfer across the polar−nonpolar solvent interface. The NC size, spectral overlap integral, molar extinction coefficient of the bound acceptor moiety, and donor-to-acceptor ratio all play an integral role in the efficiency of energy transfer and spectral properties of the nanoconjugate. Binding of organic dye molecules to NC surfaces leads to a significant change in their electronic structure and extinction coefficients compared to the nonbound molecules in solution, which determines the spectral overlap integral, characteristic donor−acceptor distance, and ultimately FRET efficiency. Furthermore, we obtained other key parameters, including the average number of dye molecules bound to NCs and average donor−acceptor distances in Ga2O3−RhB and Ga2O3− A590 hybrid nanoconjugates for varying NC sizes, which are also directly related to FRET efficiency. While the donor− acceptor distance is constant for the same NC size regardless of the surface coverage, it increases with size for the same dye/NC ratio. The results of this work illustrate how carefully selecting the NC size and varying the number of acceptor dye molecules bound to each NC could be used to control the FRET parameters and tune the resulting emission over a large chromaticity range. The analysis reported in this paper illustrates the potential of single-phase white-light-emitting hybrid nanophosphors based on FRET coupling between functional native defects in inorganic NCs and conjugated organic fluorophores.

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ASSOCIATED CONTENT

* Supporting Information S

Additional experimental details, spectroscopic properties of Ga2O3 NCs and dye molecules in conjugated and nonconjugated forms, time-resolved PL data for Ga2O3 NCs and dyes in different forms and for different excitations, determination of the molar extinction coefficient of the NCbound dyes, comparison between the average and Poisson model for fitting the FRET efficiency, FRET parameters for different surface coverages, average NC sizes, and dyes, and chromaticity of Ga2O3−RhB nanoconjugates for different NC sizes. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the donors of the Petroleum Research Fund, administered by the American Chemical Society (New Directions Grant 53856-ND4), Ontario Centres of Excellence (Market Readiness, Grant 20074), and Natural Sciences and Engineering Research Council of Canada (Discovery Grant). P.V.R. thanks the Canada Research Chairs Program for generous support. We thank Prof. Jean Duhamel (University of Waterloo) for stimulating discussions and assistance with some time-resolved measurements.

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CONCLUSIONS In summary, we investigated the electronic structure and properties of Ga2O3 NCs conjugated with organic dyes to facilitate and control distance-dependent FRET, as a means of generating tunable white light. We prepared a series of hybrid samples using different sizes of Ga2O3 NCs, as energy donors, functionalized with orange-red-emitting organic dyes, as energy

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REFERENCES

(1) Holonyak, N.; Bevacqua, S. F. Coherent (Visible) Light Emission from Ga(As1−xPx) Junctions. Appl. Phys. Lett. 1962, 1, 82−83. (2) Schubert, E. F. Light-Emitting Diodes; Cambridge University Press: Cambridge, U.K., 2006. 5694


Article

The Journal of Physical Chemistry C (3) Azevedo, I. L.; Morgan, M. G.; Morgan, F. The Transition to Solid-State Lighting. Proc. IEEE 2009, 97, 481−510. (4) Krames, M. R.; Shchekin, O. B.; Mueller-Mach, R.; Mueller, G. O.; Zhou, L.; Harbers, G.; Craford, M. G. Status and Future of HighPower Light-Emitting Diodes for Solid-State Lighting. IEEE/OSA J. Disp. Technol. 2007, 3, 160−175. (5) Smet, P. F.; Parmentier, A. B.; Poelman, D. Selecting Conversion Phosphors for White Light-Emitting Diodes. J. Electrochem. Soc. 2011, 158, R37−R54. (6) Ye, S.; Xiao, F.; Pan, Y. X.; Ma, Y. Y.; Zhang, Q. Y. Phosphors in Phosphor-Converted White Light-Emitting Diodes: Recent Advances in Materials, Techniques and Properties. Mater. Sci. Eng., R 2010, 71, 1−34. (7) Tanabe, S.; Fujita, S.; Yoshihara, S.; Sakamoto, A.; Yamamoto, S. YAG Glass-Ceramic Phosphor for White LED (II): Luminescence Characteristics. Proc. SPIE 2005, 5941, 594112−594117. (8) Bachmann, V.; Ronda, C.; Meijerink, A. Temperature Quenching of Yellow Ce3+ Luminescence in YAG:Ce. Chem. Mater. 2009, 21, 2077−2084. (9) Moreno, I.; Contreras, U. Color Distribution from Multicolor LED Arrays. Opt. Express 2007, 15, 3607−3618. (10) Xie, J.; Ni, X.; Fan, Q.; Shimada, R.; Ö zgür, Ü .; Morkoç, H. On the Efficiency Droop in InGaN Multiple Quantum Well Blue Light Emitting Diodes and Its Reduction with p-Doped Quantum Well Barriers. Appl. Phys. Lett. 2008, 93, 121107. (11) Gardner, N. F.; Müller, G. O.; Shen, Y. C.; Chen, G.; Watanabe, S.; Götz, W.; Krames, M. R. Blue-Emitting InGaN−GaN DoubleHeterostructure Light-Emitting Diodes Reaching Maximum Quantum Efficiency above 200 A/cm2. Appl. Phys. Lett. 2007, 91, 243506. (12) Monemar, B.; Sernelius, B. E. Defect Related Issues in the “Current Roll-Off” in InGaN Based Light Emitting Diodes. Appl. Phys. Lett. 2007, 91, 181103. (13) Xie, R.-J.; Hirosaki, N.; Suehiro, T.; Xu, F.-F.; Mitomo, M. A Simple, Efficient Synthetic Route to Sr2Si5N8:Eu2+-Based Red Phosphors for White Light-Emitting Diodes. Chem. Mater. 2006, 18, 5578−5583. (14) Setlur, A. A.; Heward, W. J.; Gao, Y.; Srivastava, A. M.; Chandran, R. G.; Shankar, M. V. Crystal Chemistry and Luminescence of Ce3+-Doped Lu2CaMg2(Si,Ge)3O12 and Its Use in LED Based Lighting. Chem. Mater. 2006, 18, 3314−3322. (15) Huang, C.-H.; Liu, W.-R.; Chen, T.-M. Single-Phased WhiteLight Phosphors Ca9Gd(PO4)7:Eu2+,Mn2+ under Near-Ultraviolet Excitation. J. Phys. Chem. C 2010, 114, 18698−18701. (16) Li, G.; Geng, D.; Shang, M.; Zhang, Y.; Peng, C.; Cheng, Z.; Lin, J. Color Tuning Luminescence of Ce3+/Mn2+/Tb3+-Triactivated Mg2Y8(SiO4)6O2 via Energy Transfer: Potential Single-Phase WhiteLight-Emitting Phosphors. J. Phys. Chem. C 2011, 115, 21882−21892. (17) Shunmugam, R.; Tew, G. N. White-Light Emission from Mixing Blue and Red-Emitting Metal Complexes. Polym. Adv. Technol. 2008, 19, 596−601. (18) Zhen, H.; Xu, W.; Yang, W.; Chen, Q.; Xu, Y.; Jiang, J.; Peng, J.; Cao, Y. White-Light Emission from a Single Polymer with Singlet and Triplet Chromophores on the Backbone. Macromol. Rapid Commun. 2006, 27, 2095−2100. (19) Setlur, A. A.; Radkov, E. V.; Henderson, C. S.; Her, J.-H.; Srivastava, A. M.; Karkada, N.; Kishore, M. S.; Kumar, N. P.; Aesram, D.; Deshpande, A.; et al. Energy-Efficient, High-Color-Rendering LED Lamps Using Oxyfluoride and Fluoride Phosphors. Chem. Mater. 2010, 22, 4076−4082. (20) Kamtekar, K. T.; Monkman, A. P.; Bryce, M. R. Recent Advances in White Organic Light-Emitting Materials and Devices (WOLEDs). Adv. Mater. 2010, 22, 572−582. (21) Bowers, M. J., II; McBride, J. R.; Rosenthal, S. J. White-Light Emission from Magic-Sized Cadmium Selenide Nanocrystals. J. Am. Chem. Soc. 2005, 127, 15378−15379. (22) Anikeeva, P. O.; Halpert, J. E.; Bawendi, M. G.; Bulović, V. Electroluminescence from a Mixed Red-Green-Blue Colloidal Quantum Dot Monolayer. Nano Lett. 2007, 7, 2196−2200.

(23) Park, S.; Kwon, J. E.; Kim, S. H.; Seo, J.; Chung, K.; Park, S.-Y.; Jang, D.-J.; Medina, B. M.; Gierschner, J.; Park, S. Y. A White-LightEmitting Molecule: Frustrated Energy Transfer between Constituent Emitting Centers. J. Am. Chem. Soc. 2009, 131, 14043−14049. (24) Panda, S. K.; Hickey, S. G.; Demir, H. V.; Eychmüller, A. Bright White-Light Emitting Manganese and Copper Co-Doped ZnSe Quantum Dots. Angew. Chem., Int. Ed. 2011, 50, 4432−4436. (25) Smith, R.; Liu, B.; Bai, J.; Wang, T. Hybrid III-Nitride/Organic Semiconductor Nanostructure with High Efficiency Nonradiative Energy Transfer for White Light Emitters. Nano Lett. 2013, 13, 3042−3047. (26) Sanju, K. S.; Neelakandan, P. P.; Ramaiah, D. DNA-Assisted White Light Emission through FRET. Chem. Commun. 2011, 47, 1288−1290. (27) Park, J. H.; Kim, J. Y.; Chin, B. D.; Kim, Y. C.; Kim, J. K.; Park, O. O. White Emission from Polymer/Quantum Dot Ternary Nanocomposites by Incomplete Energy Transfer. Nanotechnology 2004, 15, 1217−1220. (28) Maiti, D. K.; Bhattacharjee, R.; Datta, A.; Banerjee, A. Modulation of Fluorescence Resonance Energy Transfer Efficiency for White Light Emission from a Series of Stilbene−Perylene Based Donor−Acceptor Pair. J. Phys. Chem. C 2013, 117, 23178−23189. (29) Wang, T.; Chirmanov, V.; Chiu, W. H. M.; Radovanovic, P. V. Generating Tunable White Light by Resonance Energy Transfer in Transparent Dye-Conjugated Metal Oxide Nanocrystals. J. Am. Chem. Soc. 2013, 135, 14520−14523. (30) Layek, A.; Stanish, P. C.; Chirmanov, V.; Radovanovic, P. V. Hybrid ZnO-Based Nanoconjugate for Efficient and Sustainable White Light Generation. Chem. Mater. 2015, 27, 1021−1030. (31) Hoffman, J. B.; Choi, H.; Kamat, P. V. Size-Dependent Energy Transfer Pathways in CdSe Quantum Dot−Squaraine Light-Harvesting Assemblies: Förster versus Dexter. J. Phys. Chem. C 2014, 118, 18453−18461. (32) Clapp, A. R.; Medintz, I. L.; Mattoussi, H. Förster Resonance Energy Transfer Investigations Using Quantum-Dot Fluorophores. ChemPhysChem 2006, 7, 47−57. (33) Wang, T.; Farvid, S. S.; Abulikemu, M.; Radovanovic, P. V. SizeTunable Phosphorescence in Colloidal Metastable γ-Ga2O3 Nanocrystals. J. Am. Chem. Soc. 2010, 132, 9250−9252. (34) Wang, T.; Radovanovic, P. V. In Situ Enhancement of the Blue Photoluminescence of Colloidal Ga2O3 Nanocrystals by Promotion of Defect Formation in Reducing Conditions. Chem. Commun. 2011, 47, 7161−7163. (35) Dave, N.; Pautler, B. G.; Farvid, S. S.; Radovanovic, P. V. Synthesis and Surface Control of Colloidal Cr3+-Doped SnO2 Transparent Magnetic Semiconductor Nanocrystals. Nanotechnology 2010, 21, 134023. (36) Wang, T.; Radovanovic, P. V. Size-Dependent Electron Transfer and Trapping in Strongly Luminescent Colloidal Gallium Oxide Nanocrystals. J. Phys. Chem. C 2011, 115, 18473−18478. (37) Hegde, M.; Wang, T.; Miskovic, Z. L.; Radovanovic, P. V. Origin of Size-Dependent Photoluminescence Decay Dynamics in Colloidal γ-Ga2O3 Nanocrystals. Appl. Phys. Lett. 2012, 100, 141903. (38) El-Rayyes, A. A.; Al-Betar, A.; Htun, T.; Klein, U. K. A. Fluorescence Emission from Rhodamine-B Lactone Adsorbed at Solid Catalysts. Chem. Phys. Lett. 2005, 414, 287−291. (39) Klein, U. K. A.; Hafner, F. W. A New Dual Fluorescence with Rhodamine B Lactone. Chem. Phys. Lett. 1976, 43, 141−145. (40) Nishikiori, H.; Fujii, T. Molecular Forms of Rhodamine B in Dip-Coated Thin Films. J. Phys. Chem. B 1997, 101, 3680−3687. (41) Lakowicz, J. R. Principles of Fluorescence Spectroscopy, 3rd ed.; Springer: New York, 2006. (42) Cantor, C. R.; Pechukas, P. Determination of Distance Distribution Functions by Singlet-Singlet Energy Transfer. Proc. Natl. Acad. Sci. U.S.A. 1971, 68, 2099−2101. (43) Clapp, A. R.; Medintz, I. L.; Mauro, J. M.; Fisher, B. R.; Bawendi, M. G.; Mattoussi, H. Fluorescence Resonance Energy Transfer between Quantum Dot Donors and Dye-Labeled Protein Acceptors. J. Am. Chem. Soc. 2004, 126, 301−310. 5695


Article

The Journal of Physical Chemistry C (44) Loura, L. M. S. Simple Estimation of Förster Resonance Energy Transfer (FRET) Orientation Factor Distribution in Membranes. Int. J. Mol. Sci. 2012, 13, 15252−15270. (45) Dennis, A. M.; Bao, G. Quantum Dot-Fluorescent Protein Pairs as Novel Fluorescence Resonance Energy Transfer Probes. Nano Lett. 2008, 8, 1439−1445. (46) Aldeek, F.; Ji, X.; Mattoussi, H. Quenching of Quantum Dot Emission by Fluorescent Gold Clusters: What It Does and Does Not Share with the Förster Formalism. J. Phys. Chem. C 2013, 117, 15429− 15437. (47) Pons, T.; Medintz, I. L.; Wang, X.; English, D. S.; Mattoussi, H. Solution-Phase Single Quantum Dot Fluorescence Resonance Energy Transfer. J. Am. Chem. Soc. 2006, 128, 15324−15331. (48) Dworak, L.; Matylitsky, V. V.; Ren, T.; Basché, T.; Wachtveitl, J. Acceptor Concentration Dependence of Förster Resonance Energy Transfer Dynamics in Dye-Quantum Dot Complexes. J. Phys. Chem. C 2014, 118, 4396−4402. (49) Beane, G.; Boldt, K.; Kirkwood, N.; Mulvaney, P. Energy Transfer between Quantum Dots and Conjugated Dye Molecules. J. Phys. Chem. C 2014, 118, 18079−18086. (50) Funston, A. M.; Jasieniak, J. J.; Mulvaney, P. Complete Quenching of CdSe Nanocrystal Photoluminescence by Single Dye Molecules. Adv. Mater. 2008, 20, 4274−4280.

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