ARTICLE pubs.acs.org/JPCC
Photoluminescence Properties and Its Origin of AgInS2 Quantum Dots with Chalcopyrite Structure Yasushi Hamanaka,* Tetsuya Ogawa, and Masakazu Tsuzuki Department of Materials Science and Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan
Toshihiro Kuzuya College of Design and Manufacturing Technology, Muroran Institute of Technology, 27-1 Mizumoto-cho, Muroran 050-8585, Japan
bS Supporting Information ABSTRACT: We report on the photoluminescence (PL) mechanisms and the nature of the related electronic states of AgInS2 quantum dots (QDs) synthesized via a metathesis reaction of metal complexes. A broad PL band with a large Stokes shift is apparent in the PL spectra of AgInS2 QDs whose average diameter is 2.6 nm. The characteristic decay behavior of the PL spectra and the peak shift of the PL band depending on the excitation intensity indicate that the PL is attributed to the donoracceptor (DA) pair recombination. The binding energies of the donor and acceptor are estimated to be 100 and 220 meV. These values are derived from the temperature dependence of the PL intensity and an analysis of the spectral profile of the PL spectrum considering the DA pair recombination processes. Furthermore, we show that the phonon sidebands constitute the dominant contribution to the PL spectra because of the strong electron-phonon interaction of carriers trapped by these donors or acceptors.
’ INTRODUCTION In recent decades, semiconductor nanocrystals have attracted considerable attention because they offer unique electronic and optical properties, which are attributed to quantum confinement effects on their electronic structures.1,2 Such nanocrystals, often called quantum dots (QDs), have potential applications for future photonic and electronic devices such as laser media, lightemitting diodes (LEDs), solar cell materials, nonlinear optical devices, and building blocks of quantum computing devices.2,3 Chemical synthesis methods are suitable for obtaining large quantities of size-controlled semiconductor QDs and are particularly well developed for II-VI-type semiconductor QDs. In fact, the optical properties of CdS and CdSe QDs have been studied for a long time.4-10 Their considerable fluorescent properties (e.g., high quantum yield, tunable emission wavelength, photostability, and narrow emission spectra) have already been used for fluorescent tags in biological research.11,12 However, most of the conventional binary-compound semiconductors contain toxic elements such as Cd, Pb, Hg, and Se. The use of such toxic heavy metals must be inhibited from the viewpoint of restriction of hazardous substances. Therefore, semiconductor QDs with nontoxic constituents are required, and thus, InP QDs and impurity-doped ZnS QDs have been studied as alternatives.13-21 Apart from these two examples of nontoxic QDs, ternary compound QDs that are type I-III-VI2 chalcopyrite semiconductors such as CuInS2, AgInS2, and AgGaS2 have been proposed as environmentally friendly nanomaterials. Chalcopyrite r 2011 American Chemical Society
semiconductors in both bulk and QD form have attracted much interest as a strong candidate for solar cell materials because they have a direct bandgap with energies well matched to the solar spectrum, high absorption coefficients, and high environmental stability.22-24 On the basis of the recently developed chemical fabrication techniques, type I-III-VI2 QDs have been successfully synthesized via solution-phase routes, and CuInS2, CuInSe2, AgInS2, and AgInSe2 QDs with chalcopyrite or orthorhombic structures have been obtained by various processes.25-36 The physical and chemical properties of bulk chalcopyrite have been extensively studied for a long time; however, there have been few studies for investigating the detailed electronic and optical properties of chalcopyrite QDs, aside from those dealing with fabrication techniques. Optical absorption and photoluminescence (PL) spectra of these QDs show peak shifts that depend on the QD size, which were interpreted as a quantum confinement effect on photoexcited carriers. PL quantum efficiencies of I-III-VI2 QDs do not exceed ∼10%, which is significantly lower than the PL quantum efficiencies of the II-VI QDs (>50%). The PL quantum efficiencies of their derivatives such as CuInS2-ZnS core-shell QDs attain ∼60%.37,38 Type IIII-VI2 QDs typically exhibited broad PL spectra (spectral band widths of about 300 meV) and large Stokes shifts between the PL Received: October 31, 2010 Revised: December 18, 2010 Published: January 18, 2011 1786
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The Journal of Physical Chemistry C peak and the absorption peak (200-800 meV).26,31,39-41 These observations suggest that the PL does not originate from exciton recombination but from the recombination of carriers trapped by intragap levels formed by structural defects. Ternary semiconductors exhibit more flexible atomic packing, which arises from their enhanced configurational degree of freedom compared with binary systems, which leads to a variety of defects.42 However, a thorough and accurate investigation of PL mechanisms and defect states that contribute to PL does not yet exist for IIII-VI2 QDs, despite these being very important for the development of ternary QDs for various optoelectronic materials, in particular, for light-emitting devices and solar cell applications. In a previous publication, we preliminarily reported that the PL from chalcopyrite AgInS2 QDs synthesized by the metathesis method that our group developed exhibits characteristics of donor-acceptor (DA) pair recombination.35 In this study, we perform a systematic investigation of the PL properties of AgInS2 QDs by measuring the time-resolved PL spectra and temperature dependence and excitation intensity dependence of the PL spectra. In steady-state and time-resolved PL measurements, a broad PL band spanning the visible to the near-infrared region is observed. The temporal behavior and the excitation intensity dependence of the PL spectra clearly indicate that the PL originates from DA pair emission and that both the broad spectral feature and the large Stokes shift of the PL band are due to the strong interactions between the carriers trapped at the donor and/or acceptor levels and phonons. We estimate the binding energies of these donor and acceptor states and discuss their origin.
’ EXPERIMENTAL METHODS AgInS2 QDs with an average diameter of 2.6 nm were prepared by the metathesis reaction between Ag-In thiolate and S-dodecanethiol complexes with silver acetate, indium acetate, dodecanethiol, and sulfur powder used as starting reagents. The details of the QD synthesis are described in a previous report.35 X-ray diffraction measurements confirm that the crystal structure of the QDs is of the chalcopyrite type (chalcopyrite or zinc blende). However, the crystal structure could not be identified conclusively as chalcopyrite or zinc blende because the X-ray diffraction patterns of chalcopyrite (tetragonal, 2a ¼ 6 c) and zinc blende (cationdisordered cubic, 2a = c) structures are very similar.42 The average QD diameter was determined by both the transmission electron microscope (TEM) and analysis of X-raydiffraction-peak widths using Sherrer’s formula. Examination by energy dispersive X-ray spectroscopy (EDX) indicates that the chemical composition of the QDs is Ag:In:S = 1.00:0.96:2.67. The deviation from the stoichiometric composition is because of excess S content and is probably due to the S atoms of the dodecanethiol molecules because QDs were capped using surfactant dodecanethiol molecules. To investigate the temperature dependence of the optical response down to liquid helium temperatures, AgInS2 QDs were dispersed in polymer films. A colloidal solution of AgInS2 QDs in toluene was mixed with an appropriate amount of a toluene solution of polystyrene, and the mixed solution was spin-coated onto glass substrates. Absorption spectra of QDs were measured using a standard double-beam spectrophotometer. Steady-state PL spectra were measured using a 0.47 m monochromator equipped with a cooled charge-coupled device (CCD) detector. The 532 nm (2.33 eV) line of a continuous-wave (CW) Nd:YVO4 laser was used to
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Figure 1. Absorption spectrum (ABS), steady-state PL spectrum (PL), and PL excitation (PLE) spectrum of AgInS2 QDs at ambient temperature.
excite the PL. To measure the PL decay behavior, the excitation source consisted of a pulsed dye laser with a pulse duration of 10 ns and a repetition rate of 10 Hz. The detection energies were selected by a monochromator, and PL decay curves were recorded by a photomultiplier tube and a digital oscilloscope. Time-resolved PL spectra were constructed from a set of PL decay curves recorded at a series of detection energies within the energy range of the steady-state PL spectrum. Both the steadystate and time-resolved PL spectra were corrected for the spectral efficiency of the monochromator and the relative spectral sensitivity of the CCD and photomultiplier tube. The sample temperature was controlled using a liquid helium cryostat. PL excitation (PLE) spectra were measured by a PLE-measurement apparatus using a variable-wavelength monochromatic excitation source formed by combining a 100 W tungsten-halogen lamp and a 0.2 m monochromator. The PL intensity for a fixed energy region selected by a bandpass filter was measured as a function of the excitation photon energy by lock-in detection technique.
’ RESULTS AND DISCUSSION An absorption spectrum of AgInS2 QDs dispersed in hexane and a PL spectrum measured using CW laser excitation are shown in Figure 1. In the absorption spectrum, a broad peak with very little structure is observed around 2.6 eV. From the extrema of the second derivative of the absorption spectrum, we estimate the peak energy of the absorption band to be 2.58 eV, as marked by the arrow in Figure 1. The PL spectrum shows a broad, nearly Gaussian emission band. The peak position and full width at halfmaximum (fwhm) of the PL band are 1.55 and 0.4 eV, respectively. A PLE spectrum detected using emission in the energy range below 1.3 eV is also shown in Figure 1 and shows a hump around 2.6 eV, which agrees well with the 2.58 eV position of the absorption peak. This result supports the estimation of the peak energy of the absorption spectrum. The energy of the absorption peak is 0.71 eV larger than the bandgap energy of bulk chalcopyrite AgInS2, which is 1.87 eV.43 This result indicates that quantum confinement effects influence the carriers in these QDs. As the radius R of the semiconductor QDs decreases and becomes comparable to their bulk exciton Bohr radius aex, the situation of carrier confinement changes drastically. It is known that different carrier confinement regimes occur depending on the ratio between R and aex.4 In the weak confinement regime (R/aex . 1), the motions of the photoexcited electron and hole 1787
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are strongly correlated via the Coulomb interaction, and QD energy spectra are determined by quantization of the motion of the exciton center of mass. In contrast, in the strong confinement regime (R/aex , 1), the confinement energy is much larger than the Coulomb electron-hole interaction, so photoexcited electrons and holes can be treated as independent particles. The Bohr radius of the exciton can be calculated from the expression p2 ε 1 1 þ ð1Þ aex ¼ 2 e me mh where me* and mh* are the effective mass of the electron and hole, respectively; e is the electron charge; and ε is the dielectric constant. The exciton Bohr radius aex of chalcopyrite AgInS2 is estimated to be 5.5 nm from eq 1. In this estimation, me* = 0.12m0 and mh* = 0.39m0 for bulk AgInSe2, where m0 is the free electron mass, were used because the effective masses for bulk AgInS2 have not been reported. This approximation is reasonable because the electronic band structure of chalcopyrite AgInSe2 and, in particular, the curvature of the conduction-band bottom and valence-band top (which are inversely proportional to the effective masses) are equivalent to those of chalcopyrite AgInS2.44,45 In this calculation, we also use a dielectric constant ε = 9.6 in eq 1, which was reported for chalcopyrite AgInS2.46 The average radius of AgInS2 QDs measured in this study is R = 1.3 nm, which is much smaller than the exciton Bohr radius aex = 5.5 nm. Thus, the QD size satisfies the condition R/aex , 1 and, thus, corresponds to the strong confinement regime. Thus, the photoexcited electron and hole are individually confined in a QD. Consequently, the absorption band at 2.58 eV is attributed to the optical transition between the lowest quantized levels of the conduction band and valence band within the AgInS2 QDs and not to the creation of a confined exciton. The PL band exhibits a Stokes shift of ∼1 eV. Such a large Stokes shift and width of the spectrum suggest that the PL is derived from the recombination of electrons and holes trapped in intragap levels. Similar PL spectra were observed in other ternary semiconductor QDs such as CuInS2, CuInSe2, and AgInS2 systems and were attributed to free-to-bound or bound-to-free transitions or DA transitions.26,31,38,39,41 Figure 2 shows the time-resolved PL spectra of AgInS2 QDs measured at 4.2 K by pulsed laser excitation at 2.58 eV. The spectral range is limited to energies above 1.47 eV because of the low sensitivity of the photomultiplier tube in the near-infrared region. With increasing delay time, the PL spectra exhibit both a pronounced red shift and a decrease in the spectral width. The peak energies and the fwhm of the time-resolved PL spectra for various delay times are estimated by fitting the PL spectrum to a Gaussian profile. The time evolution of the PL peak energy is shown in the inset of Figure 2, which indicates that the peak energy is 1.72 eV just after excitation and gradually decreases down to 1.57 eV at 2 μs. Simultaneously, the fwhm decreases from 0.45 to 0.33 eV between 0 and 1 μs (data not shown). This temporal behavior of the PL spectrum strongly suggests that the PL of AgInS2 QDs originates from a DA pair recombination process, in which the Coulomb interaction between the donor and acceptor modifies their binding energies. Therefore, the emission energy depends on the donor-acceptor distance, and the photon energy emitted by a single DA pair with a separation r is expressed by E ¼ Eg - ðEA þ ED Þ þ
e2 εr
ð2Þ
Figure 2. Time-resolved PL spectra of AgInS2 QDs at 4.2 K for various delay times (circles, 0 μs; triangles, 0.5 μs; diamonds, 1.5 μs) after pulsed-laser excitation. The solid, dashed, and dot-dashed curves represent the results of the spectral fitting analysis. The inset shows the time evolution of the PL peak energies.
where Eg is the bandgap energy and ED and EA are the donor and acceptor binding energies, respectively.47 Thus, the emission energy from the spatially confined pairs is higher than that from the spatially distant pairs because of the larger Coulomb interactions for the former. In addition, the electron-hole recombination probability for confined pairs is higher than that for distant pairs because of greater wave function overlap. Therefore, the recombination rate for a high-energy transition is larger than that for a low-energy transition, which results in shorter decay times for higher emission energies in the PL band. The wide distribution of the decay times, which depend on the emission energies, leads to a temporal red shift of the emission band and a temporal reduction of the bandwidth, both of which coincide well with the decay behavior of the PL spectra shown in Figure 2. Another characteristic of the DA pair PL is that its emission band shifts as a function of the excitation intensity.47 The peak energy of the DA pair emission band shifts toward higher energy with increasing excitation intensity because of the increasing contribution of confined pairs to the PL band and decreasing contribution of distant pairs. We measured the PL spectra of AgInS2 QDs by varying the excitation fluence by a factor of 10-4 to unity at 4.2 K, with some of the spectra shown in the inset of Figure 3. We see that the PL band shifts to higher energy as the excitation intensity increases. In Figure 3, the PL peak energies are plotted as a function of the normalized excitation intensity, and we find that the PL peak blue shifts by ∼30 meV upon increasing the excitation intensity by a factor of 104. The excitation intensity dependence of the PL spectra as well as the result of the time-resolved PL measurement show a specific characteristic of the DA pair recombination process. Consequently, we conclude that the broad PL band observed in AgInS2 QDs is due to the DA pair transition. The PL mechanisms of the I-III-VI2 semiconductor QDs have been investigated mainly on CuInS2 systems and not on AgInS2. Several research groups have proposed DA pair recombination to explain the PL of chemically synthesized CuInS2 QDs.26,31,39 However, such a conclusion was hypothetical because it was based on observations of spectra with large band widths, large Stokes shifts, and long PL lifetimes up to several hundreds of nanoseconds. More conclusive evidence is provided by both the temporal red shift and the blue shift of the PL band as a function of excitation intensity, but these factors have not heretofore been investigated. In this study, we find definite and sufficient evidence of DA pair recombination, which indicates 1788
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Figure 3. Excitation intensity dependence of the PL peak energy of AgInS2 QDs at 4.2 K. The inset shows the PL spectra at different excitation intensities.
that the broad PL band of AgInS2 QDs is due to DA pair recombination. As shown in Figure 3, the PL peak energy decreases with decreasing excitation fluence and converges to a constant value of 1.57-1.58 eV. At this weak excitation limit, we consider that the distant-pair recombination contribution comes to dominate the PL spectra with respect to the confined DA pair recombination process. Thus, the PL spectrum measured at the lowest excitation fluence in this study (the relative excitation intensity of 7 10-5) can be regarded as the spectrum due to pair recombination between the most distant pairs in a QD. Such a spectrum is shown in Figure 4(a) as the solid curve and displays a Gaussian spectral profile with a peak position of 1.575 eV and a fwhm of 376 meV. Although the spectrum comes only from the recombination of the most distant DA pairs, the PL spectrum is still broad. These observations imply that the broad spectral feature does not originate from the distribution of the DA pair distance but from other causes, for instance, the strong electron-phonon interaction. Previous reports on II-VI semiconductor QDs such as CdS, CdSe, and ZnS indicated that similar broad DA pair emission band profiles (fwhm of 300-600 meV, Stokes shift of 500-1700 meV) originate in the strong electron-phonon interaction associated with the deep and localized trap states that have donor and acceptor characteristics.20,48,49 Thus, we assume that the broad spectral shape and large Stokes shift observed in this study are also due to the strong electron-phonon interaction. To examine the electron-phonon interaction in AgInS2 QDs and also evaluate the electron-phonon coupling strength, we analyze the PL spectral profile. The electron-phonon coupling can be evaluated in terms of the configuration coordinate model.50 If we assume a single phonon mode with frequency ω coupled to the transition, then the PL spectrum consists of a series of emission lines at energies En ¼ EZPL - npω
ð3Þ
where n is the number of phonons emitted after emission of the photon and EZPL is the energy of the zero-phonon line. In the configuration coordinate model, the relative intensity of an emission line at En is given by In µ
Sn expð - SÞ n!
ð4Þ
Figure 4. (a) PL spectrum of AgInS2 QDs acquired with a relative excitation intensity of 7 10-5 (solid curve) and absorption spectrum (dashed curve). The simulated PL spectrum using the configurationcoordinate model is shown by the series of vertical bars. The arrow marks the position of the transition between the lowest quantized levels Eeh. (b) Same simulated PL spectrum as shown in (a) but plotted on a logarithmic scale. The arrow indicates the position of the zero-phonon line EZPL.
in the low-temperature limit (0 K). The parameter S in eq 4 is a Huang-Rhys factor and represents the average number of emitted phonons and the carrier-phonon coupling strength. The peak energy Epeak of the emission band depends on the parameter S and is given by Epeak ¼ EZPL - Spω
ð5Þ
By taking S, EZPL, and ω as adjustable parameters, we fit the spectral profile calculated from eqs 3-5 to the PL spectrum shown in Figure 4, which was acquired at 4.2 K with the lowest excitation intensity. The line shape of the experimental PL spectrum is well reproduced with the parameter pω = 33 meV, which is within the range of the average values of the longitudinal optical phonon frequencies reported for bulk AgInS2 crystal (27, 36, and 40 meV).51 The well-simulated spectrum with S = 23, EZPL = 2.323 eV, and pω = 33 meV is shown in Figure 4(a) by a series of vertical bars that express the emission intensity for corresponding transition energy. To clarify the position of the zero-phonon line, the same spectrum is also plotted on a logarithmic scale in Figure 4(b). This simulation indicates that the zero-phonon line is positioned on the high-energy tail of the PL spectrum and that its intensity is negligible because of the large electron-phonon interaction represented by the relatively large Huang-Rhys factor of S = 23. Consequently, almost the entire PL spectrum of AgInS2 QDs is identified as phonon sidebands. Such large Huang-Rhys factors have also been reported for other semiconductor QD systems (e.g., S = 18 for CdSe QDs and S = 110 for CdS QDs) and were attributed to the localized electronic states of the deep trapping centers created at the QD surface.48,49 The energy of photons emitted by the DA pair recombination given by eq 2 corresponds to the energy of the zero-phonon line EZPL. We can estimate ED þ EA for DA pair emission by using (i) EZPL for the most distant pair estimated above (∼2.32 eV) and 1789
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Figure 5. Temperature dependence of integrated PL intensities of AgInS2 QDs. The line represents the result of the fitting analysis. The inset shows an expanded view of the same data plotted on a linear scale.
(ii) the energy between the lowest quantized levels of the conduction and valence band Eeh (= 2.58 eV) into E and Eg of eq 2, respectively. Here, the Coulomb interaction term e2/εr in eq 2 for the distant pair is required and is calculated to be ∼60 meV by assuming that the separation r of the most distant pair is 2.6 nm, which is limited by the average diameter of the QDs. The sum of ED and EA thus estimated is ED þ EA ∼ 320 meV. The temperature-dependent PL measurement was performed between 5 and 310 K under the CW-laser excitation, to determine the donor and acceptor binding energies, ED and EA, separately. Figure 5 and its inset show the temperature dependence of the normalized integrated PL intensities of the DA pair emission band I/I0, where I0 and I represent integrated intensities measured at T = 5 K and other temperatures, respectively. The inset of Figure 5 indicates that the PL intensity slightly increases in the temperature range between 5 and 30 K with an increase of temperature, and is almost constant between 30 and 190 K. The increase of the PL intensity detected below 30 K may be interpreted in terms of thermal detrapping of carriers and following retrapping by deeper defects that is the dominant mechanism in this temperature range as reported in II-VI QDs.49 The PL intensity decreases with further increase in temperature, indicating that a thermal quenching of the PL becomes apparent above 190 K. The thermal quenching of the DA pair emission can be explained in terms of thermal emptying of the donor and acceptor levels and an enhancement of the nonradiative recombination of photoexcited carriers. In this situation, temperature dependence of the PL intensity is given by the following expression Iµ
1 E 1 þ A exp kB T
ð6Þ
where kB and E are the Boltzmann constant and the thermal activation energy (thermal quenching energy), respectively, and A is a constant.47 In Figure 5, the integrated PL intensities are plotted on a logarithmic scale versus the reciprocal temperature in the 50-310 K range. Experimental data (circles) were well fitted by eq 5 (solid line), and an activation energy of E ∼ 100 meV is derived from this analysis, which corresponds to the shallower one between binding energies of donors and acceptors
(ED and EA). The value of ED þ EA is estimated to be ∼320 meV as stated above; hence, we can conclude that the depth of the deeper one between ED and EA is ∼220 meV. According to the previous studies on defect states in bulk chalcopyrite AgInS2, sulfur vacancies (VS), silver vacancies (VAg), sulfur interstisial atoms (Sint), and silver interstitial atoms (Agint) form as native defects.52,53 Among them, VS and Agint act as donors, whereas VAg and Sint act as acceptors. The chemical composition of our AgInS2 QDs is Ag:In:S = 1.00:0.96:2.67, which indicates that these QDs are rich in Ag and S. Excess Ag and S suggests the possible presence of Agint and Sint, which are expected to act as donors and acceptors, respectively. Excess S may also occur due to dodecanethiol molecules, which contain S atoms. Several research groups have observed DA pair emission in bulk AgInS2, which originates from the recombination of carriers trapped by such vacancies and interstitial atoms, and they estimated the binding energies of the donor and acceptor levels. The binding energies and the origin of the defects defined for bulk chalcopyrite AgInS2 are summarized as follows.52-57 (1) ED = 20 and 100 meV for VS, ED = 90 meV for VS or Agint, and ED = 65-70 meV for various charged states of VS or the complex defect related to VS. (2) EA = 180-190 meV for acceptors not assigned and EA = 70 meV for the various charged states of VS or complex defects related to VS. If defects similar to those formed in bulk semiconductors also form in QDs, then their ED and EA would not necessarily agree with those of the bulk because the defect levels created in QDs may also shift to higher energy because of quantum confinement of carriers trapped on defects. This confinement should be more restrictive for shallower levels because of the delocalized nature of the trapped carriers. Therefore, although this study estimates ED and EA to be 100 and 220 meV for AgInS2 QDs, which are comparable to the values reported for the bulk samples mentioned above, we cannot conclude with certainty that the origin of the defects associated with DA pair emission in AgInS2 QDs is identical to those in bulk (i.e., VS, Agint, etc.). However, similar defects with donor and acceptor characteristics probably form in the interior of the AgInS2 QDs because of the high configurational degrees of freedom in ternary chalcopyrites that result from the presence of two cation sublattices, which leads to various types of intrinsic defects.58 Another candidate for the defect in AgInS2 QDs is the surface defect. Previous studies on II-VI semiconductor QDs suggest that several surface defects such as vacancies, dangling bonds, and external adatoms can serve as trap sites associated with deep emission.59,60 In several studies, the PL was ascribed to DA pair recombination of electrons and holes trapped at these surface defects, and the large electron-phonon coupling parameters, comparable to those estimated in this study (S ∼ 20), indicated that the optical transitions involving these defects were strongly coupled to phonons.48,49 The deep emission attributed to radiative recombination of electronhole pairs at surface defect states was also observed in chalcopyrite CuInS2 QDs.61 Presumably, the same types of surface defects also form in AgInS2 QDs and create trap sites for DA pair recombination. Further experiments on local crystal structures and valence states in QDs in conjunction with a precise manipulation of the composition of QDs are required to conclusively determine the origin of the defects formed in chemically synthesized AgInS2 QDs. 1790
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’ CONCLUSIONS We have investigated PL properties of chalcopyrite AgInS2 QDs prepared by chemical synthesis via the metathesis reaction of metal complexes. In the absorption spectrum of AgInS2 QDs with diameters of 2.6 nm, we observe an absorption peak around 2.6 eV, which is due to the transition between the lowest quantized levels of the conduction and valence bands. This energy is quite large compared with the bulk bandgap energy and reveals a strong quantum-confinement effect. A broad PL band is observed and attributed to DA pair recombination because of the observed shift in the time-resolved PL spectrum and the dependence on excitation intensity. We attribute the broad PL spectral band to strong electron-phonon coupling as well as to the distribution of distances between donors and acceptors on the basis of the analysis using the configuration coordinate model. The strong electron-phonon interaction also results in a large Stokes shift, which is attributed to the dominant contribution of the phonon sidebands to the entire PL spectrum. From the results of these analyses and the thermalquenching behavior of the PL, we estimate the binding energies of the donor and acceptor to be 100 and 220 meV. The donor and/or acceptor states may have their origin in surface defects that act as trap sites for electrons and holes or in lattice defects which form in the interior of QDs. ’ ASSOCIATED CONTENT
bS
Supporting Information. Details of synthesis method of AgInS2 QDs; TEM and HRTEM images; and XRD data. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Tel.: þ81-52-735-7197. Fax: þ81-52-735-7680. E-mail:
[email protected].
’ ACKNOWLEDGMENT The authors thank Professor K. Sumiyama for informative suggestions and discussions at the beginning of this study. A part of this research was performed under the Nanotechnology Support Project in Central Japan financially supported by the Nanotechnology Network Project of the Ministry of Education, Science, Culture and Sports (MEXT), Japan. T. K. acknowledges financial support by a Grant-in-Aid for Young Scientists (B) 21760596 from MEXT. ’ REFERENCES (1) Weller, H. 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, 2002. (4) Ekimov, A. I.; Efros, A. L.; Onuschenko, A. A. Solid State Commun. 1985, 56, 921–924. (5) Bawendi, M. G.; Wilson, W. L.; Rothberg, L.; Carrol, P. J.; Jedju, T. M.; Steigerwald, M. L.; Brus, L. E. Phys. Rev. Lett. 1990, 65, 1623– 1626. (6) Nirmal, M; Dabbousi, B. O.; Bawendi, M. G.; Macklin, J. J.; Trautman, J. K.; Harris, T. D.; Brus, L. E. Nature 1996, 383, 802–804. (7) Eychm€uller, A. J. Phys. Chem. B 2000, 104, 6514–6528. (8) Klimov, V. I. J. Phys. Chem. B 2000, 104, 6112–6123.
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