Enhancement of Donor–Acceptor Pair Emissions in Colloidal AgInS2

Article pubs.acs.org/JPCC

Enhancement of Donor−Acceptor Pair Emissions in Colloidal AgInS2 Quantum Dots with High Concentrations of Defects Yasushi Hamanaka,*,† Kohei Ozawa,† and Toshihiro Kuzuya‡ †

Department of Materials Science and Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan Department of Materials Science and Technology, Muroran Institute of Technology, 27-1 Mizumoto-cho, Muroran 050-8585, Japan

‡

S Supporting Information *

ABSTRACT: We report on mechanisms of highly fluorescent donor− acceptor pair emissions in chalcopyrite AgInS2 quantum dots (QDs). We observed that photoluminescence quantum yields (PL-QYs) and radiative recombination rates strongly depend on the QD size; smaller QDs exhibit higher PL-QYs and higher radiative recombination rates of up to 60% and 106 s −1, respectively. Such characteristics were examined in terms of concentrations of donors and acceptors estimated from PL decay behaviors analyzed by the theory of Thomas and Hopfield. These analyses revealed that concentrations of donors and acceptors are higher for smaller QDs; this significantly enhances radiative recombination rates and PL-QYs in smaller QDs. In turn, higher concentrations of donors and acceptors in smaller QDs can be attributed to the use of lower synthesis temperatures, which increase the population of donor−acceptor-type lattice defects.

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INTRODUCTION Semiconductor nanocrystals have attracted considerable attention as novel functional materials due to their unique electronic and optical properties, which are attributed to the effects of quantum confinement on their electronic structures.1,2 Such nanocrystals are often called quantum dots (QDs) and have been actively studied because of their potential applications in future photonic and electronic devices, including laser media, light-emitting diodes, solar cell materials, singleelectron transistors, and building blocks for quantum computing devices.2,3 Rapid progress in the colloidal chemical method for synthesizing QDs opens the door for obtaining large quantities and wide varieties of size-controlled semiconductor QDs.3−5 The most well-known QDs are formed from CdSe, which has been used for fluorescent tags in biological imaging.6,7 However, use of CdSe and most other binary-compound semiconductor QDs that contain toxic elements, such as Cd, Pb, Hg, and Se, must be restricted because they are hazardous substances, even though they exhibit considerable fluorescent properties (e.g., high quantum yields, tunable wavelengths, narrow spectra, and high photostabilities).8−12 Recently, nontoxic QDs fabricated from ternary chalcopyrites, such as CuInS2 and AgInS2, have been intensively studied as being environmentally friendly alternatives.13,14 Chalcopyrite QDs exhibit broad photoluminescence (PL) spectra (typical spectral widths of ∼300 meV) with large Stokes shifts (200− 800 meV), indicating that PL originates from recombination of carriers trapped by intragap levels such as a localized defect state, and not from exciton recombination.15−19 These characteristics are opposite of those of CdSe QDs. © 2014 American Chemical Society

PL quantum yields (PL-QYs) of chalcopyrite QDs are usually less than ∼10% unless they are modified, such as by manipulation of elemental compositions or surface coatings with other semiconductors having larger band gap energies. Indeed, PL-QY is enhanced when composition ratios of Cu/In in CuInS2 QDs and Ag/In in AgInS2 QDs are reduced below the stoichiometric ratio or when quaternary QDs are formed by Zn-doping.20−24 PL-QYs are also significantly improved by surface coating with ZnS to form core−shell structures.13,25−27 Maximum PL-QY values above 90% have been reported for ZnS-coated chalcopyrite QDs, which were ascribed to reduction of nonradiative recombination paths by eliminating surface defects.28 However, PL enhancement after growth of a ZnS shell is still controversial, because such a procedure is capable of alloying Zn with the body of QDs, which is equivalent to Zn-doping. In previous publications, we reported on the metathesis synthesis method and PL properties of stoichiometric AgInS2 QDs. Although those QDs were not coated with ZnS layers, their PL-QYs reached ∼40%, which is exceptionally high among chalcopyrite QDs in the absence of any of the modifications mentioned above.29 PL mechanisms were ascribed to a donor−acceptor pair (DAP) recombination in which electrons trapped at the donor sites and holes trapped at the acceptor sites recombine, and the binding energies of these donors and acceptors were estimated to be 100 and 220 meV, respectively.30 In this Article, we report on the mechanisms of Received: February 10, 2014 Revised: June 7, 2014 Published: June 7, 2014 14562

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highly fluorescent properties of these AgInS2 QDs. A primary factor in such anomalously high PL-QY values is investigated from the viewpoint of recombination processes of photoexcited carriers. We find that PL-QY values and radiative recombination rates vary depending on the concentrations of defects in the QDs.

absorption band is observed. The absorption peaks are indistinct, which may be attributed to the inhomogeneity in the size of QDs (2.5 ± 0.3, 3.4 ± 0.5, and 4.3 ± 0.9 nm). From the extremum of the second derivative of each absorption spectrum, we estimated the peak energies of these absorption bands, E0, to be 2.58, 2.40, and 2.25 eV for Davr = 2.5, 3.4, and 4.3 nm, respectively; these are marked by the arrows in Figure 1. These energies are much larger than the band gap energy of the bulk AgInS2 crystal, 1.87 eV, and increase with decreasing QD size due to a quantum confinement effect on the carriers in QDs.33 From such characteristics, the origin of these absorption bands is assigned to the optical transition between the lowest quantized levels of the conduction band and valence band within the AgInS2 QDs, because the QD radii are much smaller than the exciton Bohr radius of chalcopyrite AgInS2 which is estimated to be 5.5 nm.30 Figure 2 shows the time-resolved PL spectra of AgInS2 QDs measured at 11 K between 0 and 3 μs after excitation. PL peaks

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EXPERIMENTAL METHODS AgInS2 QDs with average diameters, Davr, of 2.5(±0.3), 3.4(±0.5), and 4.3(±0.9) nm were prepared by the metathesis reaction between Ag−In thiolate and S−dodecanethiol complexes using silver acetate, indium acetate, sulfur powder, and dodecanethiol as starting reagents. The QDs were capped with surfactant dodecanethiol molecules to passivate the dangling bonds on the QD surface. Elemental compositions of the QDs were near stoichiometric ratios with Ag/In ratios being 0.9−1.1. Details of the QD synthesis and structural analysis are described in our previous report.31 TEM images and size histograms of QDs are given in the Supporting Information. Absorption spectra of QDs were measured using a standard double-beam spectrophotometer (JASCO V570). To measure time-resolved PL spectra, the excitation source consisted of a mode-locked Ti-sapphire laser (Coherent Mira900) equipped with a cavity dumper (Coherent Pulse Switch). Output pulses with a pulse duration, repetition rate, and wavelength of 100 fs, 10 kHz, and 810 nm (1.53 eV) were frequency-doubled (3.06 eV) and focused onto the sample. Time-resolved PL spectra were recorded by a spectrograph (Hamamatsu Photonics C5094) and a streak camera (Hamamatsu Photonics C4334). Spectra were corrected for the spectral efficiency of the spectrograph and relative spectral sensitivity of the detector. A colloidal solution of AgInS2 QDs in hexane was spin-coated onto quartz substrates, and the sample temperature was controlled using a closed-cycle refrigerator cryostat. The PL-QYs were estimated by comparing the spectrally integrated PL intensities measured under excitation with a continuous wave laser to that of the standard dye, rhodamine 101 (PL-QY of 96%).32 Details of the measurements are given in the Supporting Information.

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RESULTS AND DISCUSSION Absorption spectra of AgInS2 QDs dispersed in hexane are shown in Figure 1. In the absorption spectrum of each QD sample, a broad shoulder peak overlapped with the continuous

Figure 2. Time-resolved PL spectra of AgInS2 QDs with average diameters of (a) 2.5 nm, (b) 3.4 nm, and (c) 4.3 nm measured at 11 K.

exhibit size-dependent blue shifts with decreasing QD diameter similar to blue shifts of absorption peaks due to the quantum confinement effect. With increasing delay time, a pronounced red shift of the PL peak and reduction in the spectral width occur for QDs of all three diameters. Such temporal behaviors of the PL spectra indicate that the high-energy region of the PL

Figure 1. Absorption spectra of AgInS2 QDs. Arrows identify the peak positions of each spectrum. 14563

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AgInS2 and CuInS2 QDs are increased in Ag- and Cu-deficient composition ratios (Ag/In, Cu/In < 1) are reasonable because density of vacancies of group I atoms (Ag and Cu) increases in such composition, which results in enhanced DAP emission.20,22,23 To investigate detailed recombination dynamics of AgInS2 QDs, the time-resolved PL spectra were spectrally integrated over the whole energy region and plotted in Figure 4 on a

band decays faster than the low-energy region; this behavior can be ascribed to a DAP recombination, which is a conventional emission mechanism in various types of I−III− VI2 QDs.13−16,20,23,30,34 In DAP recombination, a photon is emitted when an excited electron trapped by donor and a hole trapped by acceptor recombine as shown schematically in Figure 3. The energy of the emitted photon depends on the

Figure 3. Schematic diagram of energy level structures of AgInS2 QDs and luminescence mechanisms via DAP recombination. In this scenario, photons are emitted due to recombination of electrons trapped by the donor-type defects and holes trapped by the acceptortype defects created by interband excitation. The recombination rate for nearby pair, W(r1), is larger than that for distant pair, W(r2). Here, r1 and r2 are distances between the donors and acceptors (r1 < r2).

Figure 4. Logarithmic plot of time evolutions of spectrally integrated PL intensities of AgInS2 QDs. Circles, triangles, and rectangles correspond to data for QDs with average diameters of 2.5, 3.4, and 4.3 nm, respectively. Dashed lines indicate results from theoretical analysis with N = 3.7 × 1020 cm −3 and Wmax = 2.0 × 106 s −1 for 2.5 nm, 3.7 × 1020 cm −3 and 1.9 × 106 s −1 for 3.5 nm, and 3.7 × 1020 cm −3 and 1.5 × 106 s −1 for 4.3 nm, respectively.

distance between the donor and acceptor, because the Coulomb interaction acts between the ionized donor and acceptor formed after recombination of an electron and a hole. Therefore, in case that the donor and acceptor are separated by distance r, the photon energy is given by

logarithmic scale as a function of time after excitation. The figure shows that, in the time region later than ∼10−6 s after excitation, the decay curves obey a power law:

e2 (1) εr where Eg is the bandgap energy, Ea (Ed) is the binding energy of the acceptor (donor), e is the electron charge, and ε is the dielectric constant. The recombination rate for a single DAP with a distance r between the donor and acceptor is given by hν = Eg − (Ea + Ed) +

⎛ 2r ⎞ ⎟⎟ W (r ) = Wmax exp⎜⎜ − ⎝ ad,a ⎠

I ∝ t −n

(3)

Here, I, t, and n are the intensity, time interval after excitation, and exponent, respectively. Such decay behaviors are theoretically expected in the late-time region of the DAP emission; they are observed in some types of bulk and nanocrystalline semiconductors.36−38 By fitting eq 3 to the data for t > 10 −6 s in Figure 4, the values of n were estimated to be 1.8, 1.6, and 1.3 for diameters Davr of 2.5, 3.4, and 4.3 nm, respectively. These values for the exponent suggest that the PL of smaller QDs decays faster. From the temporal behaviors in Figure 4, PL decay times τPL of 0.78 ± 0.07, 0.80 ± 0.08, and 0.98 ± 0.08 μs were estimated as the 1/e times for Davr of 2.5, 3.4, and 4.3 nm, respectively; these clearly indicate that decay is faster for smaller QDs. The PL-QYs of these QDs also depend strongly on their size; the yields were estimated to be 63 ± 3%, 57 ± 3%, and 36 ± 2% for 2.5, 3.4, and 4.3 nm, respectively. Thus, smaller QDs have higher PL-QYs. Similar trends in PL decay times have been observed in AgInS2 QDs with average diameters of 2−3 nm.34 However, in contrast to our QDs, their PL-QYs were only 3% and independent of QD size, which suggest a different origin for the size-dependent decay times. The recombination of excited electrons and holes takes place through two competing

(2)

Here, Wmax is a constant and ad,a is a Bohr radius of donor or acceptor with a comparatively smaller binding energy.35 Thus, the emission energy from nearer pairs is higher than that from distant pairs, and the recombination rate for nearer pairs is larger; this leads to the temporal red shift and spectral narrowing of the PL band. Although donors and acceptors in I−III−VI2 QDs cannot be specified so far, intrinsic lattice defects are the most possible candidates. Previous studies pointed out that vacancies of group VI atoms, interstitials of group I atoms, and group III atoms substituted at group I sites possibly act as donors, and vacancies of group I atoms and group IIII atoms, group I atoms substituted at group III sites, and interstitials of group VI atoms act as acceptors, respectively.14,15,18,22,30 Recent reports that the PL-QYs of 14564

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Table 1. Photoluminescence Parameters of AgInS2 QDs Davr (nm) 2.5 ± 0.3 3.4 ± 0.5 4.3 ± 0.9

E0 (eV)

PL-QY (%)

τPL (μs)

krad (s−1)

2.58 2.40 2.25

63 ± 3 57 ± 3 36 ± 2

0.78 ± 0.07 0.80 ± 0.08 0.98 ± 0.08

(0.85 ± 0.09) × 10 (0.74 ± 0.08) × 106 (0.38 ± 0.04) × 106

(0.44 ± 0.05) × 106 (0.51 ± 0.06) × 106 (0.65 ± 0.07) × 106

frequent radiative recombinations of photoexcited carriers in comparison with those of larger QDs. To investigate the reason for the QD-size dependence of the radiative recombination rates and PL-QYs observed here, the decay curves of the total PL intensities were analyzed on the basis of the theory of DAP emission proposed by Thomas et al.36 According to their theory, the total PL intensity at time t is given by

processes: radiative recombination and nonradiative recombination.39 The total recombination rate is given by the inverse of the PL decay time, and therefore can be represented by 1 = k rad + k nrad τPL (4) where krad and knrad are the radiative and nonradiative recombination rates, respectively. The PL-QY is also characterized by these two recombination rates: k rad QY = k rad + k nrad

knrad (s−1) 6

I (t ) = −

(5)

d ⟨Q (t )⟩ dt

(6)

where Q(t) is the probability that an electron (hole) is located on a donor (acceptor) at time t, and ⟨Q(t)⟩ represents its ensemble average over the whole crystal. When either the donor or the acceptor is in excess, ⟨Q(t)⟩ is given by

Thus, the values of krad and knrad can be evaluated by substituting the measured values of τPL and PL-QY into eqs 4 and 5. The results are summarized in Table 1, and the values of krad and knrad are plotted in Figure 5b with the corresponding

⟨Q (t )⟩ = exp[4πN

∫0

∞

{exp[−W (r )t ] − 1}r 2 dr ]

(7)

Here, N is the concentration of the major dopant between donor and acceptor. In contrast, if concentrations of donor and acceptor are comparable, the situation is more complicated and the above theory is not applicable. In such cases, the probability ⟨Q(t)⟩ is given by ⟨Q (t )⟩ = exp[4πN r 2 dr ]

∫0

∞

{exp[−W (r )

∫0

t

⟨Q (t )⟩ dt ] − 1} (8)

In each case, we can calculate the time dependence of the total PL intensity using eqs 2, 6, and either 7 or 8. These theories were constructed for bulk semiconductors; thus, the integrations in eqs 7 and 8 are over an infinite interval. However, for a finite volume, such as a QD, the distance between donors and acceptors is restricted by the QD diameter. Therefore, to apply this theory to the PL decay of semiconductor QDs, we should set the QD diameter as the upper limit on the integrals in eqs 7 and 8.40 The PL decay curves for AgInS2 QDs were simulated utilizing the above theory. In this simulation, the parameters Wmax and N were adjustable, and Davr was considered as the upper limit on the integrals in eqs 7 and 8. The binding energies of the donor and acceptor in AgInS2 QDs were estimated to be 100 and 220 meV, as reported in our previous report.30 Thus, ad,a was set to be 0.75 nm, as derived from a binding energy of 100 meV (shallower level) and use of the following relation:35

Figure 5. (a) Values of PL-QY and (b) krad (red ●) and knrad (blue ▲) for AgInS2 QDs as functions of average diameter.

ad,a =

PL-QY values (Figure 5a). As shown in Figure 5, the krad values depend strongly on the QD size, although there exists a large size distribution especially in larger QDs. The radiative recombination rate krad more than doubles from (0.38 ± 0.04) × 106 s −1 for a diameter of 4.3 nm to (0.85 ± 0.09) × 106 s −1 for a diameter of 2.5 nm, while the nonradiative recombination rate knrad decreases by only 30% ((0.65 ± 0.07) × 106 s −1 to (0.44 ± 0.05) × 106 s −1). This suggests that the higher PL-QYs of smaller QDs are mainly because of the more

EH a0 εEd,a

(9)

Here, a0 and EH are the Bohr radius and ionization energy of the hydrogen atom (Rydberg energy), respectively, and ε is the static dielectric constant of AgInS2, 9.6.41 The PL decay profiles shown in Figure 4 were well fitted, assuming that the concentrations of donors and acceptors were comparable (using eq 8). The dashed lines in Figure 4 are examples of the simulated curves. However, the values of Wmax and N could not 14565

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Figure 6 shows that the sets of Wmax and N values are inversely proportional to one another. The values of Wmax are ∼106 s −1, and the donor and acceptor concentrations N are ∼1020 cm −3, respectively. The Wmax values of ∼106 s −1 are comparable with the values reported for PbI2 nanocrystals with Davr of 12 nm and PbS nanocrystals with Davr of 4−6 nm.42,43 Comparing data for Davr of 2.5 and 3.4 nm in Figure 6 with those for 4.3 nm, we can find out that smaller QDs possess higher concentrations of donors and acceptors for the same Wmax values. Such a tendency, which is probably obscured by an inhomogeneity of QD size, suggests that the higher PL-QYs observed in smaller QDs are because of the higher concentrations of donors and acceptors being created in smaller QDs. In ternary chalcopyrites, there are many configurational degrees of freedom that result from the presence of two cation sublattices; this leads to various types of intrinsic point defects with donor and acceptor characteristics, such as vacancies, interstitial atoms, and exchange atoms.44 The DAP emission of chalcopyrite QDs is ascribed to recombinations of electrons and holes trapped by such defects formed in QDs.13−23,30,34 Therefore, relatively high concentrations of donors and acceptors in small QDs correspond to the formation of considerably larger populations of donor−acceptor defects. The dependence of defect concentration on QD size can be attributed to the different growth temperatures used in the synthesis processes (423 K for 2.5 nm, 513 K for 3.4 nm, and 543 K for 4.3 nm). The crystallinity of QDs can be significantly influenced by the growth temperature. High growth temperatures promote atomic motion and reconstruction of the lattice structure, leading to reduction in the population of defects. In contrast, at relatively low growth temperatures, low-crystalline QDs with high concentrations of lattice defects are synthesized. Therefore, smaller QDs synthesized at lower temperatures may contain higher concentrations of donor−acceptor defects and, hence, exhibit larger radiative recombination rates and higher PL-QYs. However, it is well-known that PL-QYs of semiconductor QDs are significantly affected by surface defects.1,6 Surface defects, such as dangling bonds on the surfaces of AgInS2 and CuInS2 QDs, act as nonradiative recombination centers, and the PL intensity exhibits significant enhancement after passivation with ZnS or CdS capping layers.13,25−28 As shown in Figure 5, the nonradiative recombination rate depends weakly on the QD size, in contrast to the radiative recombination rate. This indicates that surface defect concentrations are at approximately the same level for all AgInS2 QD samples; that is, surface defect concentrations are independent of the growth temperature. This is due to sufficient surface passivation of QDs by dodecanethiol molecules, and such passivation is less affected by growth temperature.

be uniquely determined by this analysis. Various sets of Wmax and N values were obtained that could reproduce the observed PL decay curves. These values are summarized in Figure 6. We

Figure 6. Values of Wmax and N estimated for AgInS2 QDs with average diameters of 2.5 nm (red ●), 3.4 nm (blue ▲), and 4.3 nm (■) based on the theory for comparable concentrations of donor and acceptor. Inversely proportional relations between Wmax and N are shown by dashed lines.

have also performed similar analyses based on the theory for excess donors or acceptors (using eq 7). The time dependence of the total PL intensity is simulated using eqs 2, 6, and 7 with the adjustable parameters Wmax and N for Davr of 2.5, 3.4, and 4.3 nm, respectively. In that case, the observed PL decay could be reproduced under the assumption of excessively dilute donors or acceptors for DAP recombination. As an example, sets of Wmax and N values that could reproduce the observed PL decay curves for Davr = 2.5 nm are shown in Figure 7. We can

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CONCLUSIONS We have investigated mechanisms of highly emissive DAP recombination in near-stoichiometric AgInS2 QDs coated with dodecanethiol surfactants. The values for PL-QY and decay curves for PL were measured for different sized QDs synthesized at different growth temperatures. The PL-QYs and radiative recombination rates exhibited strong sizedependence. Higher PL-QYs and larger radiative recombination rates up to 60% and 106 s −1, respectively, were obtained for relatively smaller QDs with Davr of 2.5 nm. Such characteristics were examined in association with donor−acceptor concen-

Figure 7. Values of Wmax and N estimated for AgInS2 QDs with average diameters of 2.5 nm assuming that either the donor or the acceptor is in excess.

see that the concentration of the major dopant N never exceeds 2.3 × 1020 cm −3, which corresponds to less than only two major dopants in one QD. Such an unreasonably low concentration is contrary to the pair recombination mechanism. Consequently, we conclude that excess donors or acceptors were not present in our QDs. 14566

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(10) Nirmal, M.; Dabbousi, B. O.; Bawendi, M. G.; Macklin, J. J.; Trautman, J. K.; Harris, T. D.; Brus, L. E. Fluorescence Intermittency in Single Cadmium Selenide Nanocrystals. Nature 1996, 383, 802− 804. (11) Eychmüller, A. Structure and Photophysics of Semiconductor Nanocrystals. J. Phys. Chem. B 2000, 104, 6514−6528. (12) Klimov, V. I. Optical Nonlinearities and Ultrafast Carrier Dynamics in Semiconductor Nanocrystals. J. Phys. Chem. B 2000, 104, 6112−6123. (13) Zhong, H.; Bai, Z.; Zou, B. Tuning the Luminescence Properties of Colloidal I−III−VI Semiconductor Nanocrystals for Optoelectronics and Biotechnology Applications. J. Phys. Chem. Lett. 2012, 3, 3167−3175. (14) Aldakov, D.; Lefrançois, A.; Reiss, P. Ternary and Quaternary Metal Chalcogenide Nanocrystals: Synthesis, Properties and Applications. J. Mater. Chem. C 2013, 1, 3756−3776. (15) Castro, S. L.; Bailey, S. G.; Raffaelle, R. P.; Banger, K. K.; Hepp, A. F. Synthesis and Characterization of Colloidal CuInS2 Nanoparticles from a Molecular Single-Source Precursor. J. Phys. Chem. B 2004, 108, 12429−12435. (16) Zhong, H.; Zhou, Y.; Ye, M.; He, Y.; Ye, J.; He, C.; Yang, C.; Li, Y. Controlled Synthesis and Optical Properties of Colloidal Ternary Chalcogenide CuInS2 Nanocrystals. Chem. Mater. 2008, 20, 6434− 6443. (17) Uehara, M.; Watanabe, K.; Tajiri, Y.; Nakamura, H.; Maeda, H. Synthesis of CuInS2 Fluorescent Nanocrystals and Enhancement of Fluorescence by Controlling Crystal Defect. J. Chem. Phys. 2008, 129, 134709−1−134709−6. (18) Nose, K.; Omata, T.; Otsuka-Yao-Matsuo, S. Colloidal Synthesis of Ternary Copper Indium Diselenide Quantum Dots and Their Optical Properties. J. Phys. Chem. C 2009, 113, 3455−3460. (19) Hamanaka, Y.; Kuzuya, T.; Sofue, T.; Kino, T.; Sumiyama, K. Defect-Induced Photoluminescence and Third-Order Nonlinear Optical Response of Chemically Synthesized Chalcopyrite CuInS2 Nanoparticles. Chem. Phys. Lett. 2008, 466, 176−180. (20) Nakamura, H.; Kato, W.; Uehara, M.; Nose, K.; Omata, T.; Otsuka-Yao-Matsuo, S.; Miyazaki, M.; Maeda, H. Tunable Photoluminescence Wavelength of Chalcopyrite CuInS2-Based Semiconductor Nanocrystals Synthesized in a Colloidal System. Chem. Mater. 2006, 18, 3330−3335. (21) Torimoto, T.; Adachi, T.; Okazaki, K.; Sakuraoka, M.; Shibayama, T.; Ohtani, B.; Kudo, A.; Kuwabata, S. Facile Synthesis of ZnS-AgInS2 Solid Solution Nanoparticles for a Color-Adjustable Luminophore. J. Am. Chem. Soc. 2007, 129, 12388−12389. (22) Dai, M.; Ogawa, S.; Kameyama, T.; Okazaki, K.; Kudo, A.; Kuwabata, S.; Tsuboi, Y.; Torimoto, T. Tunable Photoluminescence from the Visible to Near-Infrared Wavelength Region of NonStoichiometric AgInS2 Nanoparticles. J. Mater. Chem. 2012, 22, 12851−12858. (23) Chen, B.; Zhong, H.; Zhang, W.; Tan, Z.; Li, Y.; Yu, C.; Zhai, T.; Bando, Y.; Yang, S.; Zou, B. Highly Emissive and Color-Tunable CuInS2-Based Colloidal Semiconductor Nanocrystals: Off-Stoichiometry Effects and Improved Electroluminescence Performance. Adv. Funct. Mater. 2012, 22, 2081−2088. (24) Mao, B.; Chuang, C.; Lu, F.; Sang, L.; Zhu, J.; Burda, C. Study of the Partial Ag-to-Zn Cation Exchange in AgInS2/ZnS Nanocrystals. J. Phys. Chem. C 2013, 117, 648−656. (25) Li, L.; Daou, T. J.; Texier, I.; Chi, T. T. K.; Liem, N. Q.; Reiss, P. Highly Luminescent CuInS2/ZnS Core/Shell Nanocrystals: Cadmium-Free Quantum Dots for In Vivo Imaging. Chem. Mater. 2009, 21, 2422−2429. (26) Zhong, H.; Wang, Z.; Bovero, E.; Lu, Z.; van Veggel, F. C. J. M.; Scholes, G. D. Colloidal CuInSe2 Nanocrystals in the Quantum Confinement Regime: Synthesis, Optical Properties and Electroluminescence. J. Phys. Chem. C 2011, 115, 12396−12402. (27) Li, L.; Pandey, A.; Werder, D. J.; Khanal, B. P.; Pietryga, J. M.; Klimov, V. I. Efficient Synthesis of Highly Luminescent Copper Indium Sulfide-Based Core/Shell Nanocrystals with Surprisingly Long-Lived Emission. J. Am. Chem. Soc. 2011, 133, 1176−1179.

trations, which can be estimated from PL decay behaviors. Theoretical analyses of the PL decay curves show that the donor−acceptor concentrations are ∼1020 cm −3 and higher for smaller QDs. The enhancement of PL-QYs observed for smaller QDs is attributed to an increase in the population of donor−acceptor-type defects under low-temperature synthesis conditions specific to smaller QDs. Our results should make a considerable contribution to building a strategy for improving the fluorescent properties of ternary semiconductor QDs.

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ASSOCIATED CONTENT

* Supporting Information S

TEM and HRTEM images, histograms, and PL-QY measurements of AgInS2 QDs. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +81-52-735-7197. Fax: +81-52-735-7680. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Time-resolved photoluminescence measurements were performed at the Nara Institute of Science and Technology under the support of Kyoto-Advanced Nanotechnology Network, supported by “Nanotechnology Network” of the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), Japan. We gratefully acknowledge Dr. Y. Okajima and Prof. A. Yamamoto for assistance with the time-resolved measurements. This work was partly supported by the Nanotechnology Platform Program (Molecule and Material Synthesis) and a Grant-in-Aid for Scientific Research (C) of MEXT, and a research grant from the Murata Science Foundation.

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REFERENCES

(1) Weller, H. Colloidal Semiconductor Q-Particles: Chemistry in the Transition Region between Solid State and Molecules. Angew. Chem., Int. Ed. Engl. 1993, 32, 41−53. (2) Masumoto, Y.; Takagahara, T. Semiconductor Quantum Dots: Physics, Spectroscopy, and Applications; Springer: Berlin, 2002. (3) Nalwa, H. S. Nanostructured Materials and Nanotechnology; Academic Press: San Diego, CA, 2002. (4) Kwon, S. G.; Hyeon, T. Colloidal Chemical Synthesis and Formation Kinetics of Uniformly Sized Nanocrystals of Metals, Oxides, and Chalcogenides. Acc. Chem. Res. 2008, 41, 1696−1709. (5) Talapin, D. V.; Lee, J.; Kovalenko, M. V.; Shevchenko, E. Prospects of Nanocrystal Solids as Electronic and Optoelectronic Materials. Chem. Rev. 2010, 110, 389−458. (6) Dabbousi, B. O.; Viejo, J. R.; Mikulec, F. V.; Heine, J. R.; Mattoussi, H.; Ober, R.; Jensen, K. F.; Bawendi, M. G. (CdSe)ZnS Core-Shell Quantum Dots: Synthesis and Characterization of a Size Series of Highly Luminescent Nanocrystallites. J. Phys. Chem. B 1997, 101, 9463−9475. (7) Bruchez, M., Jr.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Semiconductor Nanocrystals as Fluorescent Biological Labels. Science 1998, 281, 2013−2016. (8) Ekimov, A. I.; Efros, A. L.; Onuschenko, A. A. Quantum Size Effect in Semiconductor Microcrystals. Solid State Commun. 1985, 56, 921−924. (9) Norris, D. J.; Sacra, A.; Murray, C. B.; Bawendi, M. G. Measurement of the Size Dependent Hole Spectrum in CdSe Quantum Dots. Phys. Rev. Lett. 1994, 72, 2612−2615. 14567

dx.doi.org/10.1021/jp501429f | J. Phys. Chem. C 2014, 118, 14562−14568

The Journal of Physical Chemistry C

Article

(28) Jang, E.; Song, W.; Lee, K.; Yang, H. Preparation of a PhotoDegradation-Resistant Quantum Dot−Polymer Composite Plate for Use in the Fabrication of a High-Stability White-Light-Emitting Diode. Nanotechnology 2013, 24, 045607−1−045607−9. (29) Hamanaka, Y.; Ogawa, T.; Tsuzuki, M.; Ozawa, K.; Kuzuya, T. Luminescence Properties of Chalcopyrite AgInS2 Nanocrystals: Their Origin and Related Electronic States. J. Lumin. 2013, 133, 121−124. (30) Hamanaka, Y.; Ogawa, T.; Tsuzuki, M.; Kuzuya, T. Photoluminescence Properties and Its Origin of AgInS2 Quantum Dots with Chalcopyrite Structure. J. Phys. Chem. C 2011, 115, 1786−1792. (31) Ogawa, T.; Kuzuya, T.; Hamanaka, Y.; Sumiyama, K. Synthesis of Ag−In Binary Sulfide NanoparticlesStructural Tuning and Their Photoluminescence Properties. J. Mater. Chem. 2010, 20, 2226−2231. (32) Kubin, R. F.; Fletcher, A. N. Fluorescence Quantum Yields of Some Rhodamine Dyes. J. Lumin. 1982/1983, 27, 455−462. (33) Shay, J. L.; Tell, B.; Schiavone, L. M.; Kasper, H. M.; Thiel, F. Energy Bands of AgInS2 in the Chalcopyrite and Orthorhombic Structures. Phys. Rev. B 1974, 9, 1719−1723. (34) Mao, B.; Chuang, C.; Wang, J.; Burda, C. Synthesis and Photophysical Properties of Ternary I-III-VI AgInS2 Nanocrystals: Intrinsic versus Surface States. J. Phys. Chem. C 2011, 115, 8945−8954. (35) Yu, P. Y.; Cardona, M. Fundamentals of Semiconductors: Physics and Materials Properties; Springer: Heidelberg, 2010. (36) Thomas, D. G.; Hopfield, J. J.; Augustyniak, W. M. Kinetics of Radiative Recombination at Randomly Distributed Donors and Acceptors. Phys. Rev. 1965, 140, A202−A220. (37) Era, K.; Shionoya, S.; Washizawa, Y.; Ohmatsu, H. Mechanism of Broad-Band Luminescences in ZnS PhosphorsII. Characteristics of Pair Emission Type Luminescences. J. Phys. Chem. Solids 1968, 29, 1843−1857. (38) Tanaka, M.; Sawai, S.; Sengoku, M.; Kato, M.; Masumoto, Y. Luminescence Properties of ZnS Phosphor Nanocrystals Prepared by the Laser-Induced Gas-Evaporation Method. J. Appl. Phys. 2000, 87, 8535−8540. (39) Pankove, J. I. Optical Processes in Semiconductors; Dover Publications: New York, 1971. (40) O’Neil, M.; Marohn, J.; McLendon, G. Dynamics of ElectronHole Pair Recombination in Semiconductor Clusters. J. Phys. Chem. 1990, 94, 4356−4363. (41) Márquez, R.; Rincón, C. Phys. Status Solidi B 1995, 191, 115− 119. (42) Dag, I.; Lifshitz, E. Dynamics of Recombination Processes in PbI2 Nanocrystals Embedded in Porous Silica Films. J. Phys. Chem. 1996, 100, 8962−8972. (43) Lifshitz, E.; Sirota, M.; Porteanu, H. Continuous and TimeResolved Photoluminescence Study of Lead Sulfide Nanocrystals, Embedded in Polymer Film. J. Cryst. Growth 1999, 196, 126−134. (44) Feng, Z.; Dai, P.; Ma, X.; Zhan, J.; Lin, Z. Monodispersed Cation-Disordered Cubic AgInS2 Nanocrystals with Enhanced Fluorescence. Appl. Phys. Lett. 2010, 96, 013104-1−013104-3.

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