Photoluminescence of Band Gap States in AgInS2 Nanoparticles - The

Oct 14, 2014 - AgInS2 nanoparticles of various sizes were synthesized over a range of reaction temperature from 120 to 180 °C. The band gap energies,...
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Photoluminescence of Band Gap States in AgInS2 Nanoparticles Yong Jin Park,† Ji Hye Oh,‡ Noh Soo Han,† Hee Chang Yoon,‡ Seung Min Park,† Young Rag Do,*,‡ and Jae Kyu Song*,† †

Department of Chemistry, Kyung Hee University, Seoul 130-701, Korea Department of Chemistry, Kookmin University, Seoul 136-702, Korea

‡

S Supporting Information *

ABSTRACT: AgInS2 nanoparticles of various sizes were synthesized over a range of reaction temperature from 120 to 180 °C. The band gap energies, obtained directly from photoluminescence spectra for the first time, were well correlated to the quantum confinement effects as a function of nanoparticle size, because the band gap shift was explained by the finite-depth-well effective mass approximation. The chalcopyrite and orthorhombic phases were observed to coexist in the AgInS2 nanoparticles, although the relative population of each phase depended on the reaction temperature and time. The band gap shift of each phase was comparable, which revealed that the size was the major determinant of the change in the band gap energy. The photodynamics of the band gap states exhibited emission-wavelength dependence, which further supported the coexistence of the two phases. The contributions of each phase in the time profiles matched the relative population of each phase observed in the steady-state photoluminescence spectra.

1. INTRODUCTION

nanoparticles in relation to their shape and size, when they were prepared by the solution-based approaches.16 Recently, most studies of AgInS2 nanoparticles have been focused on the luminescence properties, in addition to the synthesis of nanoparticles with various shapes and sizes, for applications requiring photonic characteristics.13−18 However, clear excitonic absorption and luminescence have not been reported in AgInS2 nanoparticles, despite their direct band gap with a large absorption coefficient.13−18 Indeed, the failure of aligned orientation between Ag and In led to non-negligible density of intrinsic defects. Accordingly, the luminescence properties were mainly examined in defect states to reveal the origins of the defect states, such as vacancies and interstitial atoms in the internal and surface states.16−18 Thus, despite the tunable band gap being one of the most important properties for the adjustability of applications, the size dependence of the band gap has not been systematically clarified in AgInS2 nanoparticles. In this study, the band gap energies of AgInS2 nanoparticles of various sizes were investigated. The band gap energies were well correlated to the quantum confinement effects as a function of size. The coexistence of chalcopyrite and orthorhombic phases was observed from the steady-state photoluminescence (SSPL) and time-resolved photoluminescence (TRPL) spectra, which also revealed that the relative population of each phase depended on the reaction temperature and time. The photodynamics of the band gap states

Semiconductor nanoparticles with a direct band gap are expected to increase applicability to biomolecular markers, solar cells, and light emitting devices.1−5 Among them, II−VI nanoparticles such as CdS and CdSe have been the most extensively investigated, mainly due to the tunable band gap in the visible region, which leads to the development in high crystallinity of nanoparticles with low defect density as well as the tunability of the band gap.6−9 However, a need for alternatives has arisen recently due to toxic elements in these semiconductors. In this regard, the I−III−VI2 semiconductors such as CuInS2 and AgInS2 have been studied,10−14 due to the absence of toxic elements in these semiconductors. These ternary semiconductors have a more complicated composition and structure than the binary semiconductors, because the twocation sublattice leads to many configurational orientations.15 Accordingly, the syntheses of I−III−VI2 nanoparticles have been less developed than those of II−VI ones, showing various types of intrinsic and surface defects.13−18 AgInS2 is usually comprised of two phases, namely, chalcopyrite and orthorhombic, with direct band gap energies of 1.87 and 1.98 eV in bulk, respectively.19 The chalcopyrite (tetragonal) phase of AgInS2 is a thermodynamically stable phase in the lower temperature regime (620 °C). However, the orthorhombic phase is frequently obtained as the metastable phase by the solution-based synthetic approaches, even at a low temperature (3 ns) were recently observed.29 Since the band gap emission of the AgInS2 nanoparticles was directly observed, the dynamics of the band gap states could be investigated using TRPL spectroscopy. The

two phases in single nanoparticles, because the crystallite boundaries were not observed in TEM images. The phase change of the AgInS2 nanoparticles was reported according to the reaction temperature. 23 Thus, AgInS 2 nanoparticles were prepared at a higher reaction temperature (160 °C). The analyzed composition in the nanoparticles synthesized at 160 °C agreed with the chemical formula of AgInS2 (Figure S1).24 Moreover, ICP-AES, AAS, and EA analysis confirmed the chemical formula of AgInS 2 (1:0.89:1.87). The nanoparticles were enlarged with increasing reaction temperature (inset of Figure 3a), despite the same

Figure 3. (a) Photoluminescence (PL) of AgInS2 nanoparticles synthesized at 160 °C is deconvoluted by three Gaussian functions. A broad shape is observed in the photoluminescent excitation (PLE) spectrum. The inset shows a TEM image of AgInS2 nanoparticles synthesized at 160 °C. (b) Photoluminescence spectra of AgInS2 nanoparticles synthesized at 160 °C are shown as a function of the reaction time. The intensity of the higher-energy band increases with increasing reaction time. The inset shows the deconvoluted photoluminescence spectrum of AgInS2 nanoparticles at a reaction time of 10 min.

reaction time (3 min), from D = 2.8 nm at 120 °C to D = 3.3 nm at 160 °C (Figure S2). In addition, the XRD patterns suggested that the increase in size was not the only change of nanoparticles with increasing reaction temperature. In other words, the XRD pattern of the nanoparticles prepared at 160 °C was distinctively different from that at 120 °C (Figure S5), although the broad diffraction peaks again prevented any assignment of a conclusive phase of the nanoparticles. In this regard, the optical spectra were obtained to elucidate the phase change. Since the photoluminescent excitation spectrum suggested an overlap of the band gap transitions (Figure 3a), the photoluminescence spectrum was deconvoluted by three Gaussian functions. The first peak was assigned as the defect emission, while the other peaks (2.44 and 2.57 eV) were the D

dx.doi.org/10.1021/jp5102253 | J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 4. (a) Photoluminescence spectra of AgInS2 nanoparticles are shown as a function of the reaction temperature. The emission energy decreases with increasing reaction temperature. The intensity of the higher-energy band in AgInS2 nanoparticles at 180 °C is much reduced. The upper inset shows the deconvoluted photoluminescence spectrum of AgInS2 nanoparticles synthesized at 180 °C. The lower inset shows a TEM image of AgInS2 nanoparticles synthesized at 180 °C. (b) Relative intensity of the higher-energy band (orthorhombic phase) is shown as a function of the nanoparticle size. The band gap energy (Eg) of the orthorhombic and chalcopyrite phases is also plotted as a function of the size. Figure 5. (a) Time profiles of the band gap emission of the AgInS2 nanoparticles prepared at 120 °C exhibit emission-wavelength dependence. From bottom to top, the time profiles are shown with increasing emission wavelength. The decay profiles are fitted by the triple-exponential model (yellow lines). The inset shows the timeresolved photoluminescence (TRPL) spectra as a function of detection time, which are reconstructed from time profiles measured at 2 nm intervals. Intensities are normalized for comparison purposes. (b) Time profiles of the band gap emission of the AgInS2 nanoparticles prepared at 160 °C also exhibit emission-wavelength dependence. The inset shows the TRPL spectra as a function of detection time. (c) Time profiles of the band gap emission of the AgInS2 nanoparticles are presented as a function of the reaction temperature.

time profiles of the band gap emission of the AgInS2 nanoparticles prepared at 120 °C exhibited emission-wavelength dependence (Figure 5a), indicating that more than one state was involved in the band gap emission.30,31 In addition, the decay profiles were fitted most reasonably by a tripleexponential model. I(t ) = A1 exp( −t /τ1) + A 2 exp(−t /τ2) + A3 exp(−t /τ3) (3)

where I(t) is intensity, τ1, τ2, and τ3 are time constants, and A1, A2, and A3 are relative magnitudes. The obtained time constants (τ1 = 1.1 ns, τ2 = 6.0 ns, and τ3 = 23.0 ns) suggested the lifetimes of three types of states. On the other hand, τ3 agreed with the lifetime of the defect states,16,29 which was supported by the small magnitude of A3 (3 ns),29 considering that the chalcopyrite phase was abundant in our experimental conditions. The dynamics of the band gap states was also investigated in the nanoparticles prepared at 160 °C, where more of the orthorhombic phase was synthesized. The time profiles also exhibited wavelength dependence (Figure 5b), which further supported the involvement of two states in the band gap emission. The decay profiles were fitted with two time constants of the bang gap emission (τ1 = 1.2 ns and τ2 = 6.2 ns), in addition to the time constant of the defect emission.16,29 The emission peak was also red-shifted with increasing time in the TRPL spectra (inset of Figure 5b), which indicated the coexistence of the orthorhombic and chalcopyrite phases in the nanoparticles prepared at 160 °C. The relative contribution of the fast decaying component (ratio of A1 to A2) was larger in the nanoparticles prepared at 160 °C than that at 120 °C, supporting that more of the orthorhombic phase was synthesized at 160 °C. On the other hand, the lifetimes of the nanoparticles prepared at 160 °C (1.2 and 6.2 ns) were a little longer than those at 120 °C (1.1 and 6.0 ns). Since the nanoparticles were enlarged, the reduced surface-to-volume ratio in the larger nanoparticles decreased the surface-related defect states and thus nonradiative decay to the defect states, which slightly increased the lifetimes of the nanoparticles. The contribution of the orthorhombic phase was expected to be minimal in the nanoparticles prepared at 180 °C. Indeed, the contribution of the fast decaying component was the smallest (Figure 5c), when the decay profile was fitted by the tripleexponential model. This result indicated that the contribution of the orthorhombic phase was reduced in the TRPL spectrum, which agreed with the SSPL spectrum (Figure 4a). In this regard, the dynamics of the band gap states consistently supported the temperature dependence of the orthorhombic phase population, which was suggested by the SSPL spectra of the band gap.

Article

ASSOCIATED CONTENT

S Supporting Information *

EDS analysis results, size distributions, simulated XRD patterns, band gap transition fitted by the single Gaussian function, XRD patterns at 120, 160, and 180 °C, and TEM images are presented. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (NRF-2012R1A1A2039882, NRF-2011-0017449). This work was also supported by the National Research Foundation of Korea Grant funded by the Korean Government (NRF-2012M1A2A2671716, NRF-2012-M1A2A2671718).

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4. SUMMARY The band gap energies of AgInS2 nanoparticles of various sizes were directly determined from the photoluminescence spectra. The change of the band gap energies clearly indicated the quantum confinement effects, which matched the finite-depthwell effective mass approximation. The metastable orthorhombic phase was found to coexist with the thermodynamically stable chalcopyrite phase, where the relative population of each phase was affected by the reaction temperature and time. The time profiles of the band gap emission exhibited emissionwavelength dependence, indicating that the orthorhombic and chalcopyrite phases had shorter and longer lifetimes, respectively. The relative contributions of the chalcopyrite phase in the TRPL profiles supported the relative population observed in the SSPL spectra. The crystallite phase systems turned out to be a complex matter in AgInS2 nanoparticles, which required an extended research. The photoluminescence of the band gap states are expected to be useful for understanding on the temperature-dependent phase change in relation to their shape and size. F

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