Enhanced Luminescence with Fast Nanosecond Lifetime in In2S3

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Enhanced Luminescence with Fast Nanosecond Lifetime in InS:Tb Nanophosphors 2

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Zhifang Li, Pan Wang, Tianye Yang, Hai Yu, Bingxin Xiao, and Mingzhe Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b09328 • Publication Date (Web): 12 Nov 2015 Downloaded from http://pubs.acs.org on November 16, 2015

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The Journal of Physical Chemistry

Enhanced Luminescence with Fast Nanosecond Lifetime in In2S3:Tb3+ Nanophosphors Zhifang Li, Pan Wang, Tianye Yang, Hai Yu, Bingxin Xiao and Mingzhe Zhang* State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, People's Republic of China.

∗ Corresponding author E-mail address: [email protected], Tel: +86-431-85168881; fax: +86-431-85168881.

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ABSTRACT: Luminescent inorganic nanoparticles have attracted large amounts of attention because their enhanced luminescence efficiency and shortening of radiative lifetime. Particularly rare earth ion doped nanophosphors that posses excellent luminescent properties as a result of their inner shell electronic transitions between the 4f-4f energy levels. Herein, we have investigated the effect of Tb3+ content on the absorption, photoluminescence and luminescence decay curves. The PL spectra gives two emission processes, which are assigned to the intrinsic emission and the defect-related emission respectively. Excess terbium content leads to reducing intensity of photoemission as a result of concentration quenching. The fitted lifetime values are approximately one nanosecond and get shortened with increase of Tb3+ content, due to the increased non-radiative transition rate. UV absorption corroborates valence-conduction band transition and strong quantum size effect. The energy band gap value for Tb3+-doped In2S3 is higher than the pure sample, which is in good agreement with the band structure calculation by using VASP. The In2S3:Tb3+ nanoparticles show strong quantum confinement effect, large luminescence enhancement, and fast nanosecond lifetime, thus make them a new type of phosphor with potential applications in flat-panel displays, lighting, and field-emission devices. Keywords: Photoluminescence spectra; Energy band; Luminescence decay curves; Electronic structure calculations


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1. INTRODUCTION Inorganic luminescent materials (phosphors) have a wide range of applications in many fields, such as solar energy converters, projection television screens, fluorescent tubes, X-ray detectors, and biomedical probes.1-3 However, the present phosphors have some shortcomings that need to be overcome. For instance, in some applications involving fast-moving images, longer lifetime can cause an afterglow that blur the display. Nanoscale phosphors can provide significant improvements compared with conventional bulk phosphors. Luminescent nanoparticles may have an increase in electron-hole overlap factor because of their quantum size confinement, thus yield a greater oscillator strength.4 Which in turn shortens the radiative lifetime, because it is an inverse ration between the fluorescent decay lifetime and the oscillator strength of a transition.5 During the past decades, luminescent inorganic nanoparticles have received considerable research interests after the observation of enhanced luminescence efficiency and shortening of radiative lifetime. Rare-earth ion doped nanophosphors have attracted an especially fast-growing interest owing to their promising applications and outstanding PL properties. The electronic structure of the rare earth ion in the most frequently occurring trivalent state has a common xenon core and a partly filled 4f inner shell. When 4f electrons transit among different energy levels, they produce excellent absorption and fluorescence spectra.6,7 The f-f transitions of trivalent rare-earth ions (except Ce3+ ion) are forbidden by the parity selection rule, so emission (or excitation) spectra of trivalent rare earth ions appear as sharp, line-like bands giving high color purity in emitted light, which makes them advantageous for device applications. Host materials play an important role in exploring fine optical materials. As promising luminescent semiconductors, II-VI semiconductor nanoparticles such as CdS, ZnS, and CdSe8-10 have received significant attention.11 Moreover, extensive studies also have focused on I-VI


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semiconductor nanoparticles (Ag2S, Cu2S, etc.).12-14 Note that compared to all semiconductor nanomaterials mentioned above, little attention has been paid to the optical and electronic properties of metal chalcogenides, such as In2S3, which have 1:1.5 stoichiometric ratio of metal atoms to chalcogenide atoms in their unit cells. Indium sulfide is attracting more and more interest as a chalcogenide semiconductor owing to its special spinel structure with numerous vacancies and corresponding optical, acoustic, and electronic properties.15-17 There exist three different crystalline forms in n-type Indium sulfide (In2S3) semiconductor: α-In2S3 (defect cubic), β-In2S3 (defect spinel), and γ-In2S3 (layered structure).18-22 Due to the bulk band gap (2.02.3 eV) and a large Bohr exciton diameter (33.8 nm), the β-In2S3 has been considered to be a promising candidate for the exploration of quantum confinement effects through modifying shapes and sizes.23-26 Moreover, the peculiar luminescence properties of β-In2S3 could potentially be applied to display devices as a phosphor.27 Nagesha et al.17 reported the luminescence of In2S3 nanoparticles with relatively strong excitonic emission at 360-380 nm and excitonic radiative lifetime of 350 ns. Chen et al.26 prepared In2S3 and europium-doped In2S3 nanoparticles with high quantum efficiencies in the blue, green, and red spectral regions. Herein we report In2S3:Tb3+ nanoparticles of about 4-6 nm in size were fabricated by gasliquid phase chemical deposition method. The effect of the Tb3+ ions concentration on the structural, morphological, chemical, and optical properties of In2S3:Tb3+ nanophosphors was investigated. The structure properties of the samples have been characterized by XRD and HRTEM. The detailed chemical analysis of In2S3:Tb3+ was performed by XPS. In addition, the optical properties of In2S3 nanophosphors in terms of absorption, photoluminescence, and luminescence lifetimes were studied in detail. The theoretical calculation applying to the band


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structure and the spin polarization density of states (DOS) was calculated by using VASP, and the theoretically predicted values were compared with the experimental results. 2. EXPERIMENTAL SECTION 2.1. Experimental Details. The In2S3 and In2S3:Tb3+ nanoparticles used in this experiment were synthesized by an easily reproducible gas–liquid phase chemical deposition by using indium acetate (In(COOCH3)3•3H2O(purity 99.99%)), terbium acetate hydrate (Tb(COOCH3)3 •4H2O(purity 99.99%)), and H2S as source materials. In the first procedure, terbium acetate hydrate and indium acetate were mixed to form Tb3+ molar concentration ratios of 0.02, 0.04, and 0.08. Then the surface-active agent mercaptoethanol (HOCH2CH2SH) was added dropwise into the aboved mixed solutions with magnetic stirring where the concentration of HOCH2CH2SH was 60 mmol L-1. The H2S gas was prepared by HCl reacting with Na2S according to the ratio of 2:1. In the synthesis process, the reactive solution was transferred to an ultrasonic atomizing system to generate the fine droplets, and reacted with the H2S gas taken by the flowing nitrogen in a chamber with circulating water (25℃), which keeps the stability of reaction conditions. The chamber was situated in an ultrasonic bath to accelerate the emergence of a new liquid surface and avoid the nanoparticles agglomeration, furthermore guarantee the uniform granularity and the high performance of the generated nanoparticles. At the beginning, massive nuclei were formed and started to grow on gas–liquid interface. The formed particles entered into the solution under the effect of ultrasound, which caused the new liquid surface to emerge. This process continued until the end of the reaction completely. The reaction equations are as follows: 2HCl + Na2S → H2S↑+ 2NaCl In3+ + Tb3+ + HOCH2CH2SH + H2S↑→ In2S3:Tb3+↓


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The precipitates were collected and were centrifuged at 13,000 rpm for 10 min with both deionized water and anhydrous alcohol thrice and the supernatant was decanted. Yellow products were obtained after being repeatedly washed and then dried in a nitrogen atmosphere. 2.2. Characterization. Transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) images were obtained on a JEOL JEM-2200FS microscope operated at 200 kV. The In2S3 and In2S3:Tb3+ nanoparticles were deposited onto a carbon-coated copper grids film. The crystal phases of the synthesized samples were characterized by X-ray powder diffraction (XRD), which was recorded by a Rigaku D/ Max2550 X-ray diffractometer with Cu Kα (λ = 0.15418 nm). The energy dispersive X-ray spectroscopy (EDS) was used to investigate the doping content of the samples. The Tb3+ content is a weighted average value which derive from two same concentration samples tested five times respectively. The valence state of dopant Tb was characterized by X-ray photoelectron spectroscopy (XPS) (ESCALAB MK II). The excitation and emission spectra were performed using a FluoroMax-4 fluorescence spectrophotometer (Horiba Scientific) equipped with a 450 W xenon arc lamp. The luminescence decay profile measurements were carried out with an Edinburgh FLS980 fluorescence spectrophotometer using a picosecond pulsed LASER Diode EPL-(375 nm) as the source of excitation. The room temperature optical absorption spectra were recorded by an UV-visible near-infrared spectrophotometer. The theoretical calculation applying to the band structure and the spin polarization density of states (DOS) was calculated by using Vienna Ab-initio Simulation Package (VASP) based on density functional theory (DFT) and the projector augmented-wave plane-wave (PAW) pseudopotential theory.28 PBE approximation and generalized gradient approximation (GGA) were chosen for exchange-correlated function. In the calculations, a primitive cell containing 40 atoms and the cutoff energy of 300 eV were


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employed. The Brillouin zone was modeled with a k-point mesh of 2×2×2 by the scheme of Monkhorst–Pack .29 3. RESULTS AND DISCUSSION The dopant content of the samples were 0.54 at%, 1.24 at%, and 1.68 at%. The dopant contents were measured by EDS testing and statistical calculations. The X-ray diffraction patterns of the undoped and In2S3:Tb3+ nanoparticles with different Tb3+ contents are shown in Figure 1c. The corresponding XRD pattern evidences that all the diffraction peaks can be indexed to the cubic phase of β-In2S3 structure (JCPDS No. 84-1385, a =10.774 Å) with the Fd3m space group. In addition, no impurity peaks, such as Tb, In2O3 or InS, are detected, which confirms that the obtained product is of high purity. The broader width of the diffraction peaks in the XRD pattern should be an indication of nanosized particles. The In2S3:Tb3+ nanoparticles are still a cubic structure, suggesting that doping cannot change the original crystal structure of the host semiconductor. The EDS spectrum (Figure 1d) further indicates that the atom content ratio of In and S in the products is 1:1.44, which is consistent with its stoichiometric proportion. The morphology and structure characterization of the In2S3:Tb3+ nanoparticles were further characterized by use of high resolution transition electron microscope (HRTEM). From Figure 1a, we can observe that the particles can be clearly distinguished and that the average size of the In2S3:Tb3+ (1.68 at %) particles are 4-6 nm. No diffraction rings corresponding to any imparities are found in Figure 1b, reaffirming only a single phase in the In2S3:Tb3+ nanoparticles.


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Figure 1. (a) HRTEM image; (b) Electron diffraction ring; (c) XRD pattern and (d) EDS pattern of the In2S3:Tb3+ (1.68 at% content) sample.

Figure 2. XPS spectrum of the In2S3:Tb3+ (1.68 at% content) sample: (a) typical XPS survey spectrum of the In2S3:Tb3+ nanoparticles, (b) core level spectrum for In 3d, (c) core level spectrum for S 2p, (d) core level spectrum for Tb 4d. X-ray photoelectron spectroscopy (XPS) was used as a powerful tool to analyze the surface chemical composition of the In2S3:Tb3+ nanoparticles with a Tb3+ content of 1.68 at% and the valence states of the various elements presented in it. A survey spectrum which is shown in Figure 2a confirms the high chemical purity of the In2S3 nanoparticles consisted solely of Tb, In,


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and S as well as C and O impurity. Carbon and Oxygen in the product may come from the reference and absorbed gaseous molecules, respectively. Figure 2b shows the In 3d core level spectrum, which indicates that the observed values of the binding energies for In 3d5/2 and 3d3/2 are 445.0 eV and 452.5 eV respectively, which are almost consistent with the reported literature values.30, 31 Figure 2c illustrates that two peaks (161.49 eV and 162.48 eV) from the 2p3/2 region of S atoms, and the doublet structures observed are attributed to the spin-orbit splitting.32 Figure 2d presents the Tb 4d XPS spectrum. One relatively weak peak is centered at 151.9 eV, which can be assigned to the binding energy of Tb3+.33,34 The results of XPS, combined with EDS analysis, confirm that the dopants are successfully incorporated into In2S3 nanoparticles.

Figure 3. UV-vis absorption spectra of In2S3 and In2S3:Tb3+ nanoparticles. The band gap in the visible part of the spectrum makes β-In2S3 a promising material in photoelectric conversion fields. Figure 3 displays the absorption spectrum of the undoped and Tb3+-doped In2S3 nanoparticles in the UV-vis range. It could be found that β-In2S3 nanoparticles have the step-like absorption band between 200-400 nm, which is in accordance with the characteristic UV-vis absorption shape of the In2S3 nanocrystals synthesized by many other researchers and are attributed to valence-to-conduction-band transition.26, 35 In addition, a strong


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absorption peak is observed at 270 nm with a weak shoulder at around 300 nm. In bulk In2S3, the band gap (Eg) is reported to be 2.07 eV with the corresponding UV band around 598 nm.36 In our work, the absorption spectra present the ultraviolet absorption edge to be around 299 nm (0 at%), 293 nm (0.54 at%), 294 nm (1.24 at%), and 295 nm (1.68 at%), corresponding to the energy band gaps (Eg) of 4.15 eV, 4.23 eV, 4.21 eV and 4.20 eV respectively. Obviously, large blue shift can be found between the result and the reported data for bulk In2S3, which elucidates the strong quantum confinement of the excitonic transition in the obtained products. It is observed that the Eg value of the Tb3+-doped In2S3 is higher than the pure sample. However, the Eg value decreases as the doping concentration increases. The observed higher Eg value at lower Tb3+ concentration might be due to the smaller crystallite size (Figure S1). This could be explained by the fact that the decrease of crystallite size leads to an increasing band gap on the basis of quantum size effect.37 The reduction in the band gap with higher Tb3+ doping also indicates that the Tb3+ ions substitute for the In sites in In2S3.

Figure 4. PL emission spectra of pure and In2S3:Tb3+ nanoparticles excited at 376 nm. The photoluminescence (PL) spectra of In2S3 and In2S3:Tb3+ nanoparticles excited at a wavelength of 376nm in anhydrous alcohol solution are shown in Figure 4. The corresponding


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excitation spectrum by detecting 491 nm emission are provided in Figure S2 of the Supporting Information. Three strong emissions are observed at 467 nm, 481 nm and 491 nm. The emission peak at about 467 nm is assigned to the intrinsic emission or excitonic recombination and the lower energy peaks (at 481 nm and 491 nm) can be ascribed to near-impurity/near-defect excitonic luminescence.38 The doping process produced many defects and dangling bonds, which induced to form numerous defective energy levels in the In2S3 band gap. These new defective energy levels will affect and participate in the emission process, so the intrinsic emission at 467 nm is weaker in the Tb3+ doped In2S3 nanoparticles. In addition, the In2S3:Tb3+ nanoparticles show a higher PL intensity than that are obtained in the In2S3 nanoparticles. Which can be due to luminescence of some impurities (e.g., narrow-line f-f emission of rare earth ions, although the wavelengths of these lines do not well correspond to f-f luminescence of Tb3+). Thus, the doping is conducive to visible light absorption activity. However, it can be observed that the emission intensity first increases with the increase of Tb3+ concentration up to 1.24% and then decreases as Tb3+ concentration increases further. The decrease in PL intensity is probably explained by the phenomenon of concentration quenching. Initially, when the doping Tb3+ concentration is low, a small amount of impurity ions can accommodate into the In2S3 host matrix because the ionic radius of Tb3+ (0.923 Å) is slightly greater than In3+ (0.8 Å), which leads to the PL intensity increasing. Further, when the doping Tb3+ concentration is increased, charge imbalance may occur and then trapping centers may be formed. These trapping centers could induce cross relaxation39 and non-radiative transitions which causes concentration quenching.40


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Figure 5. The luminescence decay profile of In2S3 and In2S3:Tb3+ nanoparticles. The luminescence decay profile from pure In2S3 and In2S3:Tb3+ nanoparticles for different Tb3+ concentrations are shown in Figure 5, obtained by monitoring the peak wavelength of their 467 nm emission following excitation at 376 nm. As can be observed from the figure, the luminescence decay curves for all the concentrations can be fit adequately with the double exponential equation: I = A1 exp (-t/τ1) + A2 exp (-t/τ2)

(1)

Where I denotes the luminescence intensity, A1 and A2 are decay constants, t is the time, and τ1 and τ2 are the lifetimes for the exponential components. The two different lifetimes are responding to two emission peaks from the PL spectra. Based on these parameters, the average lifetime values (τ*) can be calculated by using the below given expression: τ* = ( A1τ12 + A2τ22 ) / (A1τ1 + A2τ2)

(2)

The obtained lifetime values have been determined to be 1.154 ns, 0.972 ns, 0.918 ns and 0.839 ns corresponding to 0 at%, 0.54 at%, 1.24 at% and 1.68 at% respectively. It can be seen that the fitted lifetime values gradually decrease as the Tb3+ ion content increases. The possible reason for the shortening of lifetime is that the increased non-radiative transition rate caused by the efficient energy transfer to luminescent killer sites,41 which could be related to some extent of


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concentration quenching.42, 43 The nanosecond lifetime of our samples is slightly shorter than the excitonic luminescence decay lifetime of In2S3 previously reported.17

Figure 6. Calculated band structure of (a) pure and (b) Tb3+-doped In2S3. In order to confirm our experimental observation concerning the bandgap of cubic β-In2S3, the electronic structures of In2S3 and Tb3+-doped In2S3 (2.5 at% doping concentration) were studied by first principle calculations. The calculated band structure along the high symmetry directions in the first Brillouin zone is plotted in Figure 6. The conduction band minimum (CBM) is found away from the G point, being rather located at the F point, whereas the valence band maximum (VBM) is located at the G point, which means that cubic β-In2S3 is an indirect band gap semiconductor.44 The bandgap energy calculated from the band structure is smaller than the bandgap energy obtained from the UV-vis analysis. This is a common feature of DFT


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calculations, also an artifact of the GGA method used for this calculation.45, 46 The obvious result obtained is that the band gap of Tb3+-doped In2S3 (Figure 6b) is increasing from 0.488 to 0.905 eV in comparison with that of the pure In2S3 (Figure 6a), which is in good agreement with the band gap estimated from absorption peaks of UV-vis spectra.

Figure 7. The total and partial density of (a) pure and (b) Tb3+-doped In2S3. In order to clearly illustrate the chemical bonding mechanism of the band structure, the corresponding total density of states (TDOS) and the partial densities of states (PDOS) of pure and Tb3+-doped In2S3 are plotted in Figure 7, where the zero-point energy represents the Fermi level. For pure In2S3 (Figure 7a), the top of VB and the bottom of CB consist predominantly of In-s, In-p, and S-p states. Below the Fermi level, the VB is mainly split into two parts. The


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energetically lower part is located at about −10 eV and has a bandwidth of 6 eV that is mainly composed of S-s states. The upper part of S-p state and some of In-s and In-p, is located at -7 eV below the EF and has a band width of about 7 eV. Compared with pure In2S3, we can clearly see that incorporation of the Tb atom substantially modifies the density of the states of In2S3 (Figure 7b). The peaks of Tb3+-doped In2S3 become sharper between the both sides of the Fermi level, indicating that the overlap between the VB and CB decreases. According to further analysis of TDOS and PDOS, it is clearly observed from the screening plots that the introduced Tb 4d orbital mainly contributes to the bottom of the conduction band, shrinking the impurities’ energy level away from the forbidden band and producing a narrower conduction band in comparison to the pure cases. Meanwhile, the VB of Tb3+-doped In2S3 shifts to lower energy while its CB shifts to higher energy, suggesting that Tb3+ doping strengthens the bonding interaction of In2S3, makes the quantum confinement effect stronger. The peaks of Tb3+-doped In2S3 become sharper between the both sides of the Fermi level, indicating that the overlap between the VB and CB decreases. 4. CONCLUSIONS In summary, these results demonstrate that the undoped and terbium-doped indium sulfide (In2S3:Tb3+) nanophosphors (around 4-6 nm) with a cubic phase structure could be successfully synthesized via an easily reproducible gas–liquid phase chemical deposition method. XRD, and the electron diffraction rings confirmed the formation of a single phase in the In2S3:Tb3+ nanoparticles. The XPS and EDS results further revealed that the dopants were successfully incorporated into In2S3 nanoparticles. The emission spectra exhibited three strong emissions that be determined by the intrinsic emission and the defect-related emission. In addition, the Tb3+ions doping improved their photoluminescence intensity. The nanosecond lifetime values


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obtained from the experiment are shorter than the excitonic luminescence decay lifetime values previously observed in other literatures. The UV-vis measurement revealed that the strong quantum size effect of the synthesized nanoparticles and that the energy band gap (Eg) value become smaller with the increase of doping concentration. In order to confirm our experimental observation concerning the bandgap, a band structure calculation have been performed. In addition, the chemical bonding mechanism of the band structure has also been clearly illustrated by total density of states (TDOS) and the partial densities of states (PDOS) of pure and Tb3+doped In2S3. Further analysis show that Tb3+ doping strengthens the bonding interaction of In2S3, and makes the quantum confinement effect stronger. Our results indicate that high-quality doped nanoparticles like In2S3:Tb3+ with effective doping, large luminescence enhancement, fast nanosecond lifetime, and small size represent a new type of luminescent materials. ACKNOWLEDGMENT This work was funded by the National Science Foundation of China, No. 11174103 and 11474124. This work was also supported by High Performance Computing Center of Jilin University, China. We would also like to thank Lingwei “William” Kong from Pembroke Pines Charter High School for helping us with English revisions. Thanks for anonymous reviewers for their helpful suggestions on the quality improvement of our present paper. REFERENCES (1) Jüstel, T.; Nikol, H.; Ronda, C. New Developments in the Field of Luminescent Materials for Lighting and Displays. Angew. Chem. Int. Ed. 1998, 37, 3084-3103. (2) Abhilash Kumar, R. G.; Hata, S.; Ikeda, K.-i.; Gopchandran, K. G. Luminescence Dynamics and Concentration Quenching in Gd2−xEuxO3 Nanophosphor. Ceram. Int. 2015, 41, 6037-6050.


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(3) Saboktakin, M.; Ye, X.; Oh, S. J.; Hong, S.-H.; Fafarman, A. T.; Chettiar, U. K.; Engheta, N.; Murray, C. B.; Kagan, C. R. Metal-Enhanced Upconversion Luminescence Tunable through Metal Nanoparticle–Nanophosphor Separation. ACS Nano 2012, 6, 87588766. (4) Chen, W.; Sammynaiken, R.; Huang, Y. Luminescence Enhancement of ZnS:Mn Nanoclusters in Zeolite. J. Appl. Phys. 2000, 88, 5188-5193. (5) Cuthbert, J. D.; Thomas, D. G. Fluorescent Decay Times of Excitons Bound to Isoelectronic Traps in GaP and ZnTe. Phys Rev 1967, 154, 763-771. (6) Fan, W.; Feng, J.; Song, S.; Lei, Y.; Zhou, L.; Zheng, G.; Dang, S.; Wang, S.; Zhang, H. Near-Infrared Luminescent Copolymerized Hybrid Materials Built from Tin Nanoclusters and Pmma. Nanoscale 2010, 2, 2096-2103. (7) Gao, D.; Li, Y.; Lai, X.; Wei, Y.; Bi, J.; Li, Y.; Liu, M. Fabrication and Luminescence Properties of Dy3+ Doped CaMoO4 Powders. Mater. Chem. Phys. 2011, 126, 391-397. (8) Peng, Z. A.; Peng, X. Formation of High-Quality CdTe, CdSe, and CdS Nanocrystals Using CdO as Precursor. J. Am. Chem. Soc. 2001, 123, 183-184. (9) Dabbousi, B. O.; Rodriguez-Viejo, J.; 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. (10) Joo, J.; Na, H. B.; Yu, T.; Yu, J. H.; Kim, Y. W.; Wu, F.; Zhang, J. Z.; Hyeon, T. Generalized and Facile Synthesis of Semiconducting Metal Sulfide Nanocrystals. J. Am. Chem. Soc. 2003, 125, 11100-11105.


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(11) Feng, J.; Zhu, H.; Yang, X. A Controllable Growth-Doping Approach to Synthesize Bright White-Light-Emitting Cd:In2S3 Nanocrystals. Nanoscale 2013, 5, 6318-6322. (12) Brelle, M. C.; Zhang, J. Z.; Nguyen, L.; Mehra, R. K. Synthesis and Ultrafast Study of Cysteine- and Glutathione-Capped Ag2S Semiconductor Colloidal Nanoparticles. J. Phys. Chem. A 1999, 103, 10194-10201. (13) Wen, X.; Zhang, W.; Yang, S.; Dai, Z. R.; Wang, Z. L. Solution Phase Synthesis of Cu(OH)2 Nanoribbons by Coordination Self-Assembly Using Cu2S Nanowires as Precursors. Nano Lett. 2002, 2, 1397-1401. (14) Liu, Z.; Xu, D.; Liang, J.; Shen, J.; Zhang, S.; Qian, Y. Growth of Cu2S Ultrathin Nanowires in a Binary Surfactant Solvent. J. Phys. Chem. B 2005, 109, 10699-10704. (15) Amlouk, M.; Saïd, M. A. B.; Kamoun, N.; Belgacem, S.; Brunet, N.; Barjon, D. Acoustic Properties of β-In2S3 Thin Films Prepared by Spray. Jpn. J. Appl. Phys. 1999, 38, 26-30. (16) Cherian, A. S.; Mathew, M.; Kartha, C. S.; Vijayakumar, K. P. Role of Chlorine on the Opto-Electronic Properties of β-In2S3 Thin Films. Thin Solid Films 2010, 518, 1779-1783. (17) Nagesha, D. K.; Liang, X.; Mamedov, A. A.; Gainer, G.; Eastman, M. A.; Giersig, M.; Song, J.-J.; Ni, T.; Kotov, N. A. In2S3 Nanocolloids with Excitonic Emission: In2S3 Vs CdS Comparative Study of Optical and Structural Characteristics. J. Phys. Chem. B 2001, 105, 7490-7498. (18) Chai, B.; Zeng, P.; Zhang, X.; Mao, J.; Zan, L.; Peng, T. Walnut-Like In2S3 Microspheres: Ionic Liquid-Assisted Solvothermal Synthesis, Characterization and Formation Mechanism. Nanoscale 2012, 4, 2372-2377.


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Page 19 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(19) Yu, S.-H.; Shu, L.; Wu, Y.-S.; Yang, J.; Xie, Y.; Qian, Y.-T. Organothermal Synthesis and Characterization of Nanocrystalline β-Indium Sulfide. J. Am. Ceram. Soc. 1999, 82, 457-460. (20) Diehl, R.; Nitsche, R. Vapour Growth of Three In2S3 Modifications by Iodine Transport. J. Cryst. Growth 1975, 28, 306-310. (21) Gilles, J. M.; Hatwell, H.; Offergeld, G.; van Cakenberghe, J. Photoconductivity in Indium Sulfide. phys. status solidi B 1962, 2, K73-K77. (22) Lokhande, C. D.; Ennaoui, A.; Patil, P. S.; Giersig, M.; Diesner, K.; Muller, M.; Tributsch, H. Chemical Bath Deposition of Indium Sulphide Thin Films: Preparation and Characterization. Thin Solid Films 1999, 340, 18-23. (23) Liu, L.; Xiang, W.; Zhong, J.; Yang, X.; Liang, X.; Liu, H.; Cai, W., Flowerlike Cubic β-In2S3 Microspheres: Synthesis and Characterization. J. Alloys Compd. 2010, 493, 309313. (24) Xiong, Y.; Xie, Y.; Du, G.; Tian, X.; Qian, Y. A Novel in Situ Oxidization–Sulfidation Growth Route Via Self-Purification Process to β-In2S3 Dendrites. J. Solid State Chem. 2002, 166, 336-340. (25) Liu, Y.; Zhang, M.; Gao, Y.; Zhang, R.; Qian, Y. Synthesis and Optical Properties of Cubic In2S3 Hollow Nanospheres. Mater. Chem. Phys. 2007, 101, 362-366. (26) Chen, W.; Bovin, J.-O.; Joly, A. G.; Wang, S.; Su, F.; Li, G. Full-Color Emission from In2S3 and In2S3:Eu3+ Nanoparticles. J. Phys. Chem. B 2004, 108, 11927-11934. (27) Ai, Z. P. Luminescence of In2S3 Nanocrystallites Embedded in Sol–Gel Silica Xerogel. Opt. Mater. 2003, 24, 589-593.


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Page 20 of 31

(28) Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector AugmentedWave Method. Phys. Rev. B 1999, 59, 1758-1775. (29) Monkhorst, H. J.; Pack, J. D. Special Points for Brillouin-Zone Integrations. Phys. Rev. B 1976, 13, 5188-5192. (30) Cai, W.; Zhao, Y.; Hu, J.; Zhong, J.; Xiang, W. Solvothermal Synthesis and Characterization of Zinc Indium Sulfide Microspheres. J. Mater. Sci. Technol. 2011, 27, 559-562. (31) Ye, F.; Du, G.; Jiang, Z.; Zhong, Y.; Wang, X.; Cao, Q.; Jiang, J. Z. Facile and Rapid Synthesis of RGO-In2S3 Composites with Enhanced Cyclability and High Capacity for Lithium Storage. Nanoscale 2012, 4, 7354-7357. (32) Chassaing, E.; Naghavi, N.; Bouttemy, M.; Bockelee, V.; Vigneron, J.; Etcheberry, A.; Lincot, D. Electrodeposition Mechanism of Indium Sulfide and Indium Oxi(Hydroxi)Sulfide Thin Films from In(III)-Thiosulfate Acidic Aqueous Solutions. J. Electrochem. Soc. 2012, 159, D347-D354. (33) Song, Y.; Liu, G.; Wang, J.; Dong, X.; Yu, W. Synthesis and Luminescence Resonance Energy Transfer Based on Noble Metal Nanoparticles and the NaYF4:Tb3+ Shell. Phys. Chem. Chem. Phys. 2014, 16, 15139-15145. (34) Guodong, F.; Changgen, F.; Zhao, Z. Surface and Texture Properties of Tb-Doped Ceria-Zirconia Solid Solution Prepared by Sol-Gel Method. J. Rare Earth 2007, 25, 42-47. (35) Park, K. H.; Jang, K.; Son, S. U. Synthesis, Optical Properties, and Self-Assembly of Ultrathin Hexagonal In2S3 Nanoplates. Angew. Chem. Int. Ed. 2006, 45, 4608-4612.


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Page 21 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(36) Du, W.; Zhu, J.; Li, S.; Qian, X., Ultrathin β-In2S3 Nanobelts: Shape-Controlled Synthesis and Optical and Photocatalytic Properties. Cryst. Growth Des. 2008, 8, 21302136. (37) Jothibas, M.; Manoharan, C.; Ramalingam, S.; Dhanapandian, S.; Bououdina, M. Spectroscopic Analysis, Structural, Microstructural, Optical and Electrical Properties of ZnDoped In2O3 Thin Films. Spectrochim. Acta, Part A 2014, 122, 171-178. (38) Anuja, D.; Amitava, P., Bright White Light Emission from In2S3 : Eu3+ Nanoparticles. J. Phys. D: Appl. Phys. 2009, 42, 145116-145121. (39) Yan, X.; Fern, G. R.; Withnall, R.; Silver, J. Effects of the Host Lattice and Doping Concentration on the Colour of Tb3+ Cation Emission in Y2O2S:Tb3+ and Gd2O2S:Tb3+ Nanometer Sized Phosphor Particles. Nanoscale 2013, 5, 8640-8646. (40) Yang, S.; Xia, H.; Jiang, Y.; Zhang, J.; Shi, Y.; Gu, X.; Zhang, J.; Zhang, Y.; Jiang, H.; Chen, B. Tm3+ Doped α-NaYF4 Single Crystal for 2µm Laser Application. J. Alloys Compd. 2015, 643, 1-6. (41) Chen, J.; Liu, Y.; Liu, H.; Ding, H.; Fang, M.; Huang, Z. Tunable SrAl2Si2O8: Eu Phosphor Prepared in Air Via Valence State-Controlled Means. Opt Mater 2015, 42, 80-86. (42) Linganna, K.; Rathaiah, M.; Vijaya, N.; Basavapoornima, C.; Jayasankar, C. K.; Ju, S.; Han, W. T.; Venkatramu, V. 1.53 µm Luminescence Properties of Er3+-Doped K–Sr–Al Phosphate Glasses. Ceram Int 2015, 41, 5765-5771. (43) Dousti, M. R.; Poirier, G. Y.; de Camargo, A. S. S. Structural and Spectroscopic Characteristics of Eu3+-Doped Tungsten Phosphate Glasses. Opt Mater 2015, 45, 185-190.


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(44) Zhao, Z.; Cao, Y.; Yi, J.; He, X.; Ma, C.; Qiu, J. Band-Edge Electronic Structure of Beta-In2S3: The Role of S or P Orbitals of Atoms at Different Lattice Positions. Chemphyschem 2012, 13, 1551-1556. (45) Li, Y.; Chen, G.; Zhang, H.; Li, Z. Electronic Structure and Photocatalytic Water Splitting of Lanthanum-Doped Bi2AlNbO7. Mater. Res. Bull. 2009, 44, 741-746. (46) Liu, J. W.; Chen, G.; Li, Z. H.; Zhang, Z. G. Electronic Structure and Visible Light Photocatalysis Water Splitting Property of Chromium-Doped SrTiO3. J. Solid State Chem. 2006, 179, 3704-3708.


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TOC.


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Figure captions Figure 1. (a) HRTEM image; (b) Electron diffraction ring; (c) XRD pattern and (d) EDS pattern of the In2S3:Tb3+ (1.68 at% content) sample. Figure 2. XPS spectrum of the In2S3:Tb3+ (1.68 at% content) sample: (a) typical XPS survey spectrum of the In2S3:Tb3+ nanoparticles, (b) core level spectrum for In 3d, (c) core level spectrum for S 2p, (d) core level spectrum for Tb 4d. Figure 3. UV-vis absorption spectra of In2S3 and In2S3:Tb3+ nanoparticles. Figure 4. PL emission spectra of pure and In2S3:Tb3+ nanoparticles excited at 376 nm. Figure 5. The luminescence decay profile of In2S3 and In2S3:Tb3+ nanoparticles. Figure 6. Calculated band structure of (a) pure and (b) Tb3+-doped In2S3. Figure 7. The total and partial density of (a) pure and (b) Tb3+-doped In2S3.


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Figure 1. (a) HRTEM image; (b) Electron diffraction ring; (c) XRD pattern and (d) EDS pattern of the In2S3:Tb3+ (1.68 at% content) sample. 51x38mm (300 x 300 DPI)



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Figure 2. XPS spectrum of the In2S3:Tb3+ (1.68 at% content) sample: (a) typical XPS survey spectrum of the In2S3:Tb3+ nanoparticles, (b) core level spectrum for In 3d, (c) core level spectrum for S 2p, (d) core level spectrum for Tb 4d. 54x42mm (300 x 300 DPI)


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Figure 3. UV-vis absorption spectra of In2S3 and In2S3:Tb3+ nanoparticles. 51x38mm (300 x 300 DPI)



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Figure 4. PL emission spectra of pure and In2S3:Tb3+ nanoparticles excited at 376 nm 49x35mm (300 x 300 DPI)


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Figure 5. The luminescence decay profile of In2S3 and In2S3:Tb3+ nanoparticles 50x37mm (300 x 300 DPI)



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Figure 6. Calculated band structure of (a) pure and (b) Tb3+-doped In2S3 95x133mm (300 x 300 DPI)


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Figure 7. The total and partial density of (a) pure and (b) Tb3+-doped In2S3 94x131mm (300 x 300 DPI)


Enhanced Luminescence with Fast Nanosecond Lifetime in In2S3

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