Band-Gap States of AgIn5S8 and ZnSâAgIn5S8 Nanoparticles - The

Subscriber access provided by University of Colorado Boulder

Article 5

8

5

8

Band Gap States of AgInS and ZnS-AgInS Nanoparticles Seonghyun Jeong, Hee Chang Yoon, Noh Soo Han, Ji Hye Oh, Seung Min Park, Byoung Koun Min, Young Rag Do, and Jae Kyu Song J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b00043 • Publication Date (Web): 17 Jan 2017 Downloaded from http://pubs.acs.org on January 18, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 25

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

The Journal of Physical Chemistry

Band Gap States of AgIn5S8 and ZnS-AgIn5S8 Nanoparticles

Seonghyun Jeong,† Hee Chang Yoon,‡ Noh Soo Han,† Ji Hye Oh,‡ Seung Min Park,† Byoung Koun Min,§ 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

Clean Energy Research Center, Korea Institute of Science and Technology, Seoul 136-791, Korea

* Corresponding authors. E-mail addresses: [email protected]; [email protected]

1

ACS Paragon Plus Environment


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

ABSTRACT The size-dependent band gap energies of AgIn5S8 nanoparticles were directly measured for the first time using absorption and photoluminescence spectroscopy, which explained the band gap energy evolution with the quantum confinement effect in the AgIn5S8 nanoparticles. The band gap transition in the steady-state and time-resolved photoluminescence spectra indicated that the stable structure of the AgIn5S8 nanoparticles was the cubic phase. The electronic band structures of the Ag-In-S nanoparticles were mainly related to the crystal structures, although the stoichiometry affected the band energies to some extent. Zn-doping led to the solid-solution of ZnS-AgIn5S8, which was supported by the significant change of the electronic band structures of the AgIn5S8 nanoparticles. Controlling the size and stoichiometry enabled the emission of the Ag-In-S nanoparticles to be tunable in the entire visible regime.

2


Page 2 of 25

Page 3 of 25

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


1. INTRODUCTION The semiconductor nanoparticles of I-III-VI2, such as AgInS2 and CuInS2,1-6 have been investigated over a number of decades as alternatives to replace the toxic elements in the currently available nanoparticles, like CdSe and CdS.7-9 The ternary systems of I-III-VI show direct band gap structures with high absorption coefficients, which increase the potential for light-emitting devices and biomolecular markers.1-6 The other representative semiconductor systems of I-III-VI are III-rich compounds with the general formula of I-III5-VI8, which show a superior color tunability and a higher quantum yield.10-14 The In-rich compound of the Ag-In-S semiconductors is AgIn5S8, the cubic spinel structure of which can be derived from CdIn2S4, i.e., two cadmium cations are replaced by a silver cation and an indium cation.11,12 Therefore the structural vacancies are not negligible in the AgIn5S8 sublattice, implying a high defect density in AgIn5S8. Accordingly, the electronic band structures of AgIn5S8 have not been fully understood, while the band gap energy of 1.80 eV is only known for indirect optical transitions.11,12 Although previous research has prepared thin films of AgIn5S8 to fabricate photovoltaic devices,13,14 due to various types of intrinsic and surface defects,13-16 the synthesis process for AgIn5S8 with high crystallinity was less developed, while the well-defined nanoparticle systems of AgIn5S8 remained challenging.17,18 Recently, a few works demonstrated the enhanced quantum yield and emission-color tunability of nanoparticles using off-stoichiometric Ag-In-S nanoparticles.14-16 In addition, the alloy structures of Ag-In-S with ZnS have improved the optical properties.19-25 But these semiconductors with complicated compositions have led to high defect densities, which have been hurdles in the understanding of the electronic band structures. Indeed, most of the studies for AgIn5S8 nanoparticles have focused on the properties of the defect states, such as vacancies and interstitial atoms.13-16 However, the band gap emission has been

3



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

barely reported in AgIn5S8 nanoparticles, although the direct band gap structures are expected. Therefore, the size-dependent electronic band structures have not been elucidated for AgIn5S8 nanoparticles, even though the tunable band gap is the essential property of photonic devices and biomolecular markers. In this study, the band gap energies of AgIn5S8 nanoparticles were investigated to explain the size-dependent quantum confinement effects and the stoichiometry-dependent electronic band structures. The absorption spectra, as well as the steady-state and time-resolved photoluminescence spectra, indicated the cubic phase of the nanoparticles, which clearly showed the quantum confinement effects. The electronic band structures of the Ag-In-S nanoparticles were mainly determined by the crystal structures, despite the noteworthy effects of stoichiometry. Zn-doping significantly affected the electronic band structures of the AgIn5S8 nanoparticles, indicating that the incorporation of Zn into AgIn5S8 led to the solid-solution of nanoparticles.

2. EXPERIMENTAL SECTION 2.1. Syntheses of Nanoparticles. Silver nitrate (AgNO3, 99%, Aldrich), indium(III) acetylacetonate (In(acac)3, 99.99%, Aldrich), sulfur (S, 99.98%, Aldrich), zinc stearate (10–12% Zn basis, Aldrich), dodecanethiol (DDT, 98%, Aldrich), oleic acid (OA, 90%, Aldrich), oleylamine (OLA, 70%, Aldrich), 1-octadecene (ODE, 90%, Aldrich), and trioctylphosphine (TOP, 90%, Aldrich) were used without further purification. In a typical synthetic procedure of AgIn5S8,26 AgNO3 (0.10 mmol, 0.017 g), In(acac)3 (0.50 mmol, 0.21 g), OA (1.5 mmol, 0.47 mL), and ODE (25 mmol, 8.0 mL) were loaded into a three-neck flask. The reaction mixture was purged with nitrogen gas for 20 min at an ambient temperature. This reaction solution was heated to 90 °C, and DDT (4.0 mmol, 1.0 mL) was added to the reaction flask. The mixture was heated

4


Page 4 of 25

Page 5 of 25

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


to different reaction temperatures between 120 and 180 °C. Then, S (0.80 mmol, 0.026 g) dissolved in OLA (4.0 mmol, 1.3 mL) was quickly injected into the reaction solution, which was continuously reacted for 3 min. Afterward, the solution was centrifuged by adding anhydrous ethanol to remove the by-products, which was repeated three times. The purified nanoparticles were dispersed in toluene. The solid-solution formation of ZnS-AgIn5S8 was completed without intermediate purification after the synthesis of AgIn5S8. Zn stearate (0.4 mmol, 0.25 g) and S (0.4 mmol, 0.013 g) were dissolved in TOP (4 mmol, 2 mL), which was added to AgIn5S8 solution at 120 °C, and maintained for 2 h. The final solution was purified using centrifugation, and stored in toluene. 2.2. Characterization of Nanoparticles. The crystal structures of nanoparticles were investigated by X-ray diffraction (XRD, X’pert system, Phillips) with Cu K radiation.26 The chemical composition was determined by energy dispersive spectroscopy (EDS, JSM7401F). The morphology and size were analyzed by transmission electron microscopy (TEM, JEM-4010). The absorbance spectra were obtained by UV-visible spectrophotometry (S-3100). For photoluminescence analysis, nanoparticles were excited by a second harmonic (355 nm) of a cavity-dumped oscillator. The photoluminescence was collected using an optical lens, spectrally resolved using a monochromator, detected using a photomultiplier, and recorded using a timecorrelated single photon counter (PicoHarp). The instrumental response of the entire system was 0.05 ns.

3. RESULTS AND DISCUSSION 3.1. Characterization of AgIn5S8 Nanoparticles. The chemical composition of the nanoparticles synthesized at 120 °C agreed with the chemical formula of AgIn5S8 in the EDS

5



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

analysis ([Ag]/[In]/[S] = 1:5.5:9.4) within the acceptable experimental-error range (Figure S1).17,26 The chemical composition obtained using inductively coupled plasma-atomic emission spectrometry, atomic absorption spectroscopy, and element analysis also indicated the stoichiometry of AgIn5S8.26 The TEM image showed that the nanoparticles were monodispersely synthesized with an average diameter (D) of ~ 2.8 nm (Figure 1a). Although not all lattice fringes could be clearly assigned in the high resolution TEM image, due to the small size of the nanoparticles (Figure S2), the lattice spacing of 0.320 nm was consistent with the (311) plane in a cubic phase of AgIn5S8 (inset of Figure 1a). Moreover, the XRD patterns indicated the cubic phase of AgIn5S8 with the most intense peak of the (311) plane (Figure 1b).13,14,18 However, the phase was not conclusive due to the broad diffraction peaks, which were unavoidable in the small nanoparticles. According to the Scherrer formula,26,27 the full-width at half-maximum (FWHM) of the diffraction peak would be ~ 3° at D = 2.8 nm. The coexistence of two phases was indeed observed in the AgInS2 nanoparticles, although the broad XRD peaks suggested the single phase.29 Nevertheless, when the XRD pattern was simulated from the cubic phase diffraction peaks with FWHM of 3°, the simulated XRD pattern was similar to the observed one (Figure 1b), which suggested that the synthesized AgIn5S8 nanoparticle was of the cubic phase. 3.2. Band Gap Energy of AgIn5S8 Nanoparticles. To determine the phase of the AgIn5S8 nanoparticles more conclusively, the optical properties of the AgIn5S8 nanoparticles were examined, because the electronic band structures afforded clues to determine the phase.29 In the absorption spectrum of the nanoparticles, a hump was observed (Figure 2a), whereby the peak energy estimated from the derivative of the spectrum was 2.78 eV.4 This energy was in good agreement with the value obtained by an extrapolation of the linear part of the absorbance (inset of Figure 2a), suggesting the band gap energy of the AgIn5S8 nanoparticles. In addition, the

6


Page 6 of 25

Page 7 of 25

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


absorption spectrum was deconvoluted into the monotonous increase of the absorbance and the Gaussian profile centered at 2.78 eV (inset of Figure 2b), more clearly indicating the band gap absorption of the AgIn5S8 nanoparticles. In the photoluminescence spectrum (Figure 2a), two emission bands were observed, while the intense emission at 2.07 eV was the defect emission such as the donor-acceptor pair recombination.12,16 The band gap emission was observed at 2.74 eV with the Stokes shift of 0.04 eV, which was, to our knowledge, the first observation of the band gap emission of AgIn5S8 nanoparticles. In fact, the photoluminescence spectrum was unmistakably decomposed into the band gap and the defect emission (inset of Figure 2b), where the single Gaussian profile of the band gap transition in the absorption and the photoluminescence spectra suggested the single band gap state in the AgIn5S8 nanoparticles (Figure 2b). Notably, the single profile of the band gap transition was different from the overlap of the two transitions (orthorhombic and chalcopyrite) in the AgInS2 nanoparticles,29 which ruled out the possibility of the secondary phase in the AgIn5S8 nanoparticles. The single band gap state of the nanoparticles was supported by the time-resolved photoluminescence spectra. The time profile of the band gap emission (450 nm) in the nanoparticles was fitted by a triple-exponential model (Figure 3a). I ( t )  A1 exp(  t /  1 )  A2 exp(  t /  2 )  A3 exp(  t /  3 )

(1)

where I(t) is the intensity, τ1, τ2, and τ3 are time constants, and A1, A2, and A3 are relative magnitudes. The time constants (τ1 = 4.1 ns, τ2 = 50 ns, and τ3 = 280 ns) indicated the lifetimes of three states. The long time constants (τ2 and τ3) agreed with the lifetimes of the defect states,16,30 which were related to the tail of the defect emission (Figure 2b) with the small magnitude of A2 and A3 (< 0.1). The time profile of the defect emission (600 nm) was fitted by a doubleexponential model with similar time constants with τ2 and τ3 (Figure 3a). In this regard, the short

7



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

time constant (τ1), which was absent in the time profile of the defect emission, indicated the lifetime of the band gap state. Accordingly, the single time constant implied that the single state contributed to the band gap emission of the nanoparticles. In addition, the time-resolved photoluminescence spectra confirmed that the band gap emission of the nanoparticles was from the single state. The time profiles of the nanoparticles were measured from 400 to 510 nm at the intervals of 2 nm, which were employed to reconstruct the time-resolved photoluminescence spectra of the band gap emission. The shape and the peak position of the band gap emission in the time-resolved photoluminescence spectra barely changed with increasing time (inset of Figure 3b), confirming that the single state was predominantly responsible for the band gap emission. Furthermore, the lifetime of the band gap state in the AgIn5S8 nanoparticles (4.1 ns) was similar to that of the band gap state in the AgInS2 nanoparticles (1.1 and 6.0 ns in the orthorhombic and chalcopyrite phases, respectively),29 suggesting a similarity between the band gap states of Ag-In-S systems. The stable crystal structure of AgIn5S8 is the cubic spinel phase,13,14 whose indirect band gap energy was determined in bulk as 1.8 eV by an extrapolation of the linear part of the absorption spectrum.11 From the results of the indirect band gap energy, the direct band gap energy (Eg) of AgIn5S8 was estimated in the region of 1.8−2.1 eV (see Supporting Information and Figure S3 for details).11,14 The Eg of the nanoparticles (2.78 eV) was much larger than that of the bulk, suggesting quantum confinement effects. For the I-III-VI semiconductors, the quantum confinement effects (ΔEg) were in the range of 0.55−0.75 eV at D = 2.8 nm in the finite-depthwell effective mass approximation for the spatial confinement of the carriers,31 where the confining potentials of the surface-covering molecules in the nanoparticles were considered. Moreover, the finite-depth-well approximation estimated a similar range of ΔEg in AgIn5S8,

8


Page 8 of 25

Page 9 of 25

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


because the confinement effect depended on Eg of the bulk and the effective masses of electrons and holes.31,32 Therefore, the observed confinement effect supported the AgIn5S8 nanoparticles being of cubic structure, as indicated by the TEM and XRD results. The AgIn5S8 nanoparticles prepared at other reaction temperatures were also of cubic phase, because the XRD patterns were quite similar (Figure S4). The chemical composition of these nanoparticles also matched the stoichiometry of AgIn5S8 in the EDS analysis (Figure S1). On the other hand, the size of the nanoparticles increased with increasing reaction temperature for D of 3.1, 3.7, and 4.4 nm at the reaction temperatures of 140, 160, and 180 oC, respectively (Figure S2). The humps in the absorption spectra were red-shifted with increasing reaction temperatures due to the increase in nanoparticle size (inset of Figure 4a). In addition, the band gap emission was red-shifted (Figure 4a), while the absorption and the emission spectra indicated that the single phase predominantly contributed to the optical process of the band gap states (Figure S5). The change of band gap transition showed the typical size-dependent quantum confinement effects (Figure 4b), supporting the identical phase of the nanoparticles. Interestingly, the Stokes shift between the absorption and the emission was in the region of the optical phonon modes (26−45 meV),33 which suggested an electron-phonon coupling in the optical process of the band gap transition. The largest shift (35 meV) was observed at the smallest nanoparticles (2.8 nm), indicating that the average number of phonons involved in the optical process was ~ 1 (Figure 4b). On the other hand, the Stokes shift decreased with increasing nanoparticle size, which indicated that the number of the involved phonons became less than one in the large nanoparticles. Such change of the Stokes shift with respect to size was observed in other nanoparticles, such as CdSe and CdS nanoparticles.9,34

9



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

Page 10 of 25

The quantum yield of the band gap emission was low (< 1%), due to the high defect densities of the AgIn5S8 nanoparticles.13-16 However, the quantum yield of the defect emission was in the range of 5−20%, which depended on the size of nanoparticles.26 In addition to the quantum yield, the defect emission energy was affected by the size of nanoparticles. With increasing size of nanoparticles, the defect emission was red-shifted (Figure S6), whereby the shift of defect emission was similar to that of band gap emission (Figure 4b). The emission energy of the donor–acceptor pair recombination (EDA) was related to Eg. EDA = Eg – (ED + EA) + e2/ r

(2)

where ED is the donor binding energy, EA is the acceptor binding energy, and r is the distance of the donor–acceptor pair (see Supporting Information for details).4-6 The similar shift between EDA and Eg indicated that ED and EA were not much influenced by size. In other words, the value of ED + EA estimated previously as 0.30 eV was not affected by size to a great extent, although the origins of the donor and acceptor states were not clearly established.12,26 Upon close examination, the difference between EDA and Eg slightly increased with increasing size of the nanoparticles, which was related to the decreasing Coulombic interaction term (e2/ r), rather than the change of ED + EA value (see Supporting Information for details). Therefore, it was suggested that the energy levels of the donor and acceptor states remained nearly unchanged from the conduction and valence bands, respectively, despite the change of band gap energy. 3.3. Stoichiometry Effect on Electronic Band Structures. The band gap energy of the AgIn5S8 nanoparticles at D = 2.8 nm was larger than that of the AgInS2 nanoparticles at the same size, which was 2.46 and 2.58 eV for the chalcopyrite and orthorhombic phases, respectively.29 Indeed, the electronic structures of AgIn5S8 would be different from those of AgInS2, due to the different contributions of the Ag, In, and S orbitals. However, the evolution of the electronic

10


Page 11 of 25

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


band structures in Ag-In-S nanoparticles has not been well understood with respect to stoichiometry, because the accurate band gap energies of Ag-In-S nanoparticles have not been obtained. To understand the stoichiometry effect regarding the electronic band structures of AgIn-S nanoparticles, off-stoichiometric nanoparticles were prepared with [Ag]/[In] = 1:3 at 120 °C, which were compared to AgInS2 ([Ag]/[In] = 1:1) and AgIn5S8 ([Ag]/[In] = 1:5). The chemical composition of the off-stoichiometric nanoparticles agreed with that of AgIn3S5 (AgInxS(3x+1)/2, x = 3), while the sizes of the nanoparticles were similar to those of AgInS2 and AgIn5S8 prepared at the same temperature (Figure S7). With decreasing ratio of Ag/In, the band gap emission of the nanoparticles was blue-shifted (Figure 5a), indicating the stoichiometry effect on the electronic band structures. The conduction band of the Ag-In-S semiconductors was composed of the hybrid orbitals of In(5s5p) and S(3p) orbitals, while the valence band was the hybrid orbitals of Ag(4d) and S(3p).16,35 Therefore, the blue-shift was attributed to the reduced contribution of the Ag orbitals in the electronic bands, which might lower the valence band and consequently enlarge the Eg of the nanoparticles.16,35 However, the shift was not monotonous with the ratio of Ag/In, because the Eg of the offstoichiometric (AgIn3S5) nanoparticles was near that of the AgIn5S8 nanoparticles. In addition, the XRD patterns of the off-stoichiometric nanoparticles agreed with the cubic phase of AgIn5S8 (Figure S7), suggesting that the electronic band structure of the off-stoichiometric nanoparticles would be similar to that of AgIn5S8. The slight decrease in the Eg of the off-stoichiometric nanoparticles, compared to AgIn5S8, was explained by the higher contribution of Ag orbitals, which might elevate the valence band of the off-stoichiometric nanoparticles, compared to that of the AgIn5S8 nanoparticles. The difference between Eg and EDA in the off-stoichiometric nanoparticles was similar to that in the AgIn5S8 nanoparticles (Figure 5b), implying that the

11



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

Page 12 of 25

energy levels of the defect states in the off-stoichiometric nanoparticles also followed those in AgIn5S8. However, it was noted that the difference between the Eg and EDA in AgInS2 was larger than that in AgIn5S8, which was ascribed to the larger ED + EA value in AgInS2 (0.32 eV) than that in AgIn5S8 (0.30 eV).4,26 Accordingly, it was suggested that the electronic band structures of the Ag-In-S nanoparticles were either the AgInS2-type or the AgIn5S8-type, although the stoichiometry affected the band structures to some extent by the contributions of the Ag, In, and S orbitals. Another factor to influence the electronic band structures was the doping.19-25 However, the doping effect on the band structures of the Ag-In-S nanoparticles was not investigated in detail, although Zn-doping was extensively studied to improve the quantum yield of the defect emission. To understand the evolution of the electronic band structures in the Ag-In-S nanoparticles with respect to the doping, the Zn-doped nanoparticles were prepared from the AgIn5S8 nanoparticles by a subsequent thermal process. Peaks in the XRD patterns of the Zn-doped nanoparticles were observed between the cubic structure of AgIn5S8 and the cubic structure of ZnS (Figure S8).30 In the high resolution TEM image of the Zn-doped nanoparticles, the lattice spacing of the (311) plane in the cubic phase of AgIn5S8 was slightly increased (Figure S8), which led to the increase in the size of the Zn-doped nanoparticles (D = ~3.0 nm). Since the solid-solution (alloy) implied homogeneous crystalline structures, where one or two kinds of atoms were partly substituted without changing the primary structures, the XRD patterns and the TEM images suggested that the nanoparticles were the solid-solution of ZnS-AgIn5S8. Moreover, the band gap emission of the doped nanoparticles was blue-shifted from that of the undoped nanoparticles (Figure 5a). Since the band gap emission of the ZnS-AgIn5S8 was overlapped with the defect emission, the band gap emission of the ZnS-AgIn5S8 nanoparticles (dotted line) was obtained using the

12


Page 13 of 25

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


deconvolution process (Figure S5). The blue shift was also found in the solid-solution of ZnSCuInS2 and ZnS-AgInS2, due to the large Eg of ZnS (3.91 eV),36-39 which supported the solidsolution of the ZnS-AgIn5S8 nanoparticles. However, the possibility of a core/shell-like alloyed structure could not be totally ruled out in the ZnS-AgIn5S8 nanoparticles, which suggested further research. Nevertheless, the increase in the lattice fringes and the shift of the XRD peaks, as well as the blue-shifted emission, mainly originated from the solid-solution of ZnS-AgIn5S8 nanoparticles. The incorporation of Zn into the crystal lattice of AgIn5S8 was facilitated by Ag vacancies,15 which were common defects in Ag-In-S semiconductors.21-23 Since the solidsolution of ZnS-AgIn5S8 was synthesized from the pre-prepared AgIn5S8 nanoparticles by a subsequent thermal process, Zn was diffused into the crystal lattice of AgIn5S8 through Ag vacancies during the thermal process. The defect emission of ZnS-AgIn5S8 was also blue-shifted from that of the AgIn5S8 nanoparticles (Figure 5b). However, the shift of the defect emission (0.12 eV) was dissimilar to that of the band gap emission (0.08 eV), which implied that the solid-solution changed the energy levels of the defect states from the conduction and valence bands (ED and EA), in addition to the influence on the electronic band structures. In fact, a change of the defect levels was also found in ZnS-AgInS2, where the defect levels in ZnS-AgInS2 were shallower than those in AgInS2.37 In other words, the ED + EA value in ZnS-AgInS2 was smaller than that in AgInS2, which was in good agreement with ZnS-AgIn5S8, and supported the solid-solution effect on the energy levels. The change of defect levels from the conduction and valence bands was observed in AgInS2, due to the stoichiometry change (AgIn5S8 vs. AgInS2), indicating that the defect levels, as well as the band gap energies, were strongly correlated to the chemical composition of the AgIn-S semiconductors.

13



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

Page 14 of 25

Lastly, it is noted that the band gap emission of the Ag-In-S nanoparticles was tunable in the region of 2.5−2.9 eV (430−500 nm) by the stoichiometry change, while the defect emission was in the region of 1.8−2.4 eV (500−700 nm). Overall, the entire visible regime was accessible by the emission of the Ag-In-S nanoparticles (Figure 5), because a fine-tuning of the emission wavelength was possible through controlling the nanoparticle size. Such a color range and tunability indicate the high potential for application of the Ag-In-S nanoparticles for lightemitting devices and biomolecular markers.

4. SUMMARY AgIn5S8 nanoparticles synthesized at various temperatures showed the cubic phase, while the reaction temperature affected the size of the nanoparticles. The blue regime of the optical spectra indicated the band gap transition of the nanoparticles, which was supported by the time-resolved photoluminescence spectra. The change of band gap energy was attributed to the size-dependent quantum confinement effects in the AgIn5S8 nanoparticles. Despite the evolution of the electronic band structure according to the nanoparticle size, the energy levels of the defect states remained nearly unchanged from the electronic band structures. However, the stoichiometry changed the energy levels of the defect states from the electronic band structures, in addition to the influence on the band gap energies. The off-stoichiometric nanoparticles showed that the electronic band structures of the Ag-In-S nanoparticles were mainly determined by the crystal structures, although the stoichiometry effect on the band structures was not negligible. The solidsolution of ZnS-AgIn5S8 led to the significant change of the electronic band structures in the AgIn5S8 nanoparticles. The entire visible regime accessible by the emission of the nanoparticles holds promise for the applicability of the Ag-In-S nanoparticles to potential luminophores.

14


Page 15 of 25

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


SUPPORTING INFORMATION The EDS results, TEM images, XRD patterns, band gap transitions fitted by the Gaussian function, and defect emissions are presented. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENT 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-2015R1A2A2A01002805, NRF-2016R1A5A1012966). This work was also supported by Energy Technology Development Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant (20143030011530).

15



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

Page 16 of 25

REFERENCES (1) Panthani, M. G.; Akhavan, V.; Goodfellow, B.; Schmidtke, J. P.; Dunn, L.; Dodabalapur, A.; Barbara, P. F.; Korgel, B. A. Synthesis of CuInS2, CuInSe2, and Cu(InxGa1-x)Se2 (CIGS) Nanocrystal “inks” for Printable Photovoltaics. J. Am. Chem. Soc. 2008, 130, 16770-16777. (2) 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. (3) 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. (4) 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, 17861792. (5) 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. (6) Hamanaka, Y.; Ozawa, K.; Kuzuya, T. Enhancement of Donor–Acceptor Pair Emissions in Colloidal AgInS2 Quantum Dots with High Concentrations of Defects. J. Phys. Chem. C 2014, 118, 14562-14568. (7) Talapin, D. V.; Rogach, A. L.; Kornowski, A.; Haase, M.; Weller, H. Highly Luminescent Monodisperse CdSe and CdSe/ZnS Nanocrystals Synthesized in a HexadecylamineTrioctylphosphine Oxide-Trioctylphospine Mixture. Nano Lett. 2001, 1, 207-211. (8) Qu, L.; Peng, X. Control of Photoluminescence Properties of CdSe Nanocrystals in Growth. J. Am. Chem. Soc. 2002, 124, 2049-2055. (9) Efros, A. L.; Rosen, M.; Kuno, M.; Nirmal, M.; Norris, D. J.; Bawendi, M. Band-edge Exciton in Quantum Dots of Semiconductors with a Degenerate Valence Band: Dark and Bright Exciton States. Phys. Rev. B 1996, 54, 4843. (10) Allen, P. M.; Bawendi, M. G. Ternary I−III−VI Quantum Dots Luminescent in the Red to Near-Infrared. J. Am. Chem. Soc. 2008, 130, 9240-9241. (11) Usujima, A.; Takeuchi, S.; Endo, S.; Irie, T. Optical and Electrical Properties of CuIn5S8 and AgIn5S8 Single Crystals. Jpn. J. Appl. Phys. 1981, 20, L505-L507. (12) Gasanly, N.; Serpengüzel, A.; Aydinli, A.; Gürlü, O.; Yilmaz, I. Donor-Acceptor Pair Recombination in Agln5S8 Single Crystals. J. Appl. Phys. 1999, 85, 3198-3201. 16


Page 17 of 25

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


(13) Lin, L.; Wu, C.; Lee, T. Growth of Crystalline AgIn5S8 Thin Films on Glass Substrates from Aqueous Solutions. Crystal Growth & Design 2007, 7, 2725-2732. (14) Lin, L.; Wu, C.; Lai, C.; Lee, T. Controlled Deposition of Silver Indium Sulfide Ternary Semiconductor Thin Films by Chemical Bath Deposition. Chem. Mater. 2008, 20, 4475-4483. (15) Chang, J.; Wang, G.; Cheng, C.; Lin, W.; Hsu, J. Strategies for Photoluminescence Enhancement of AgInS2 Quantum Dots and Their Application as Bioimaging Probes. J. Mater. Chem. 2012, 22, 10609-10618. (16) 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 Non-Stoichiometric AgInS2 Nanoparticles. J. Mater. Chem. 2012, 22, 12851-12858. (17) Zhang, W.; Li, D.; Sun, M.; Shao, Y.; Chen, Z.; Xiao, G.; Fu, X. Microwave Hydrothermal Synthesis and Photocatalytic Activity of AgIn5S8 for the Degradation of Dye. Tunable Photoluminescence from the Visible to Near-Infrared Wavelength Region of Non-Stoichiometric AgInS2 Nanoparticles. J. Solid State Chem. 2010, 183, 2466-2474. (18) Li, K.; Chai, B.; Peng, T.; Mao, J.; Zan, L. Preparation of AgIn5S8/TiO2 Heterojunction Nanocomposite and Its Enhanced Photocatalytic H2 Production Property under Visible Light. ACS Catal. 2013, 3, 170-177. (19) 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. (20) Tang, X.; Ho, W. B. A.; Xue, J. M. Synthesis of Zn-doped AgInS2 Nanocrystals and Their Fluorescence Properties. J. Phys. Chem. C 2012, 116, 9769-9773. (21) Rao, M. J.; Shibata, T.; Chattopadhyay, S.; Nag, A. Origin of Photoluminescence and XAFS Study of (ZnS)1–x(AgInS2)x Nanocrystals. J. Phys. Chem. Lett. 2013, 5, 167-173. (22) Torimoto, T.; Kameyama, T.; Kuwabata, S. Photofunctional Materials Fabricated with Chalcopyrite-type Semiconductor Nanoparticles Composed of AgInS2 and Its Solid Solutions. J. Phys. Chem. Lett. 2014, 5, 336-347. (23) Uematsu, T.; Doko, A.; Torimoto, T.; Oohora, K.; Hayashi, T.; Kuwabata, S. Photoinduced Electron Transfer of ZnS–AgInS2 Solid-Solution Semiconductor Nanoparticles: Emission Quenching and Photocatalytic Reactions Controlled by Electrostatic Forces. J. Phys. Chem. C 2013, 117, 15667-15676. (24) Yang, X.; Tang, Y.; Tan, S. T.; Bosman, M.; Dong, Z.; Leck, K. S.; Ji, Y.; Demir, H. V.; Sun, X. W. Facile Synthesis of Luminescent AgInS2–ZnS Solid Solution Nanorods. Small 2013, 9, 2689-2695. 17



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

Page 18 of 25

(25) Torimoto, T.; Kamiya, Y.; Kameyama, T.; Nishi, H.; Uematsu, T.; Kuwabata, S.; Shibayama, T. Controlling Shape Anisotropy of ZnS-AgInS2 Solid Solution Nanoparticles for Improving Photocatalytic Activity. ACS Appl. Mater. Interfaces 2016, 8, 27151-27161. (26) Hong, S. P.; Park, H. K.; Oh, J. H.; Yang, H.; Do, Y. R. Comparisons of the Structural and Optical Properties of o-AgInS2, t-AgInS2, and c-AgIn5S8 Nanocrystals and Their Solid-Solution Nanocrystals with ZnS. J. Mater. Chem. 2012, 22, 18939-18949. (27) Cullity, B. D.; Weymouth, J. W. Elements of X-ray Diffraction. Am. J. Phys. 1957, 25, 394395. (28) Lodhi, M. Y.; Mahmood, K.; Mahmood, A.; Malik, H.; Warsi, M. F.; Shakir, I.; Asghar, M.; Khan, M. A. New Mg0.5CoxZn0.5−xFe2O4 Nano-Ferrites: Structural Elucidation and Electromagnetic Behavior Evaluation. Curr. Appl. Phys. 2014, 14, 716-720. (29) Park, Y. J.; Oh, J. H.; Han, N. S.; Yoon, H. C.; Park, S. M.; Do, Y. R.; Song, J. K. Photoluminescence of Band Gap States in AgInS2 Nanoparticles. J. Phys. Chem. C 2014, 118, 25677-25683. (30) Song, J.; Jiang, T.; Guo, T.; Liu, L.; Wang, H.; Xia, T.; Zhang, W.; Ye, X.; Yang, M.; Zhu, L. et al. Facile Synthesis of Water-Soluble Zn-Doped AgIn5S8/ZnS Core/Shell Fluorescent Nanocrystals and Their Biological Application. Inorg. Chem. 2015, 54, 1627-1633. (31) Omata, T.; Nose, K.; Otsuka-Yao-Matsuo, S. Size Dependent Optical Band Gap of Ternary I-III-VI2 Semiconductor Nanocrystals. J. Appl. Phys. 2009, 105, 073106. (32) Brus, L. Electronic Wave Functions in Semiconductor Clusters: Experiment and Theory. J. Phys. Chem. 1986, 90, 2555-2560. (33) Bodnar, I.; Karoza, A.; Kudritskaya, E.; Smirnova, A. Vibrational Spectra of In2S3, CuIn5S8 and AgIn5S8 Compounds with a Spinel Structure. Appl. Spectrosc. 1997, 64, 279-282. (34) Yu, Z.; Li, J.; O'Connor, D. B.; Wang, L.; Barbara, P. F. Large Resonant Stokes Shift in CdS Nanocrystals. J. Phys. Chem. B 2003, 107, 5670-5674. (35) Tsuji, I.; Kato, H.; Kobayashi, H.; Kudo, A. Photocatalytic H2 Evolution Reaction from Aqueous Solutions over Band Structure-Controlled (AgIn)xZn2(1-x)S2 Solid Solution Photocatalysts with Visible-Light Response and Their Surface Nanostructures. J. Am. Chem. Soc. 2004, 126, 13406-13413. (36) Tran, T. K. C.; Le, Q. P.; Nguyen, Q. L.; Li, L.; Reiss, P. Time-Resolved Photoluminescence Study of CuInS2/ZnS Nanocrystals. Adv. Nat. Sci.: Nanosci. Nanotechnol. 2010, 1, 025007.

18


Page 19 of 25

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


(37) Kameyama, T.; Takahashi, T.; Machida, T.; Kamiya, Y.; Yamamoto, T.; Kuwabata, S.; Torimoto, T. Controlling the Electronic Energy Structure of ZnS–AgInS2 Solid Solution Nanocrystals for Photoluminescence and Photocatalytic Hydrogen Evolution. J. Phys. Chem. C 2015, 119, 24740-24749. (38) Kang, X.; Yang, Y.; Wang, L.; Wei, S.; Pan, D. Warm White Light Emitting Diodes with Gelatin-Coated AgInS2/ZnS Core/Shell Quantum Dots. ACS Appl. Mater. Interfaces 2015, 7, 27713-27719. (39) Yoon, H. C.; Oh, J. H.; Ko, M.; Yoo, H.; Do, Y. R. Synthesis and Characterization of Green Zn−Ag−In−S and Red Zn−Cu−In−S Quantum Dots for Ultrahigh Color Quality of DownConverted White LEDs. ACS Appl. Mater. Interfaces 2015, 7, 7342-7350.

19



a)

5 nm

20

30

50

(642) (731)

(553)

(620)

40

(444)

(440)

(511) (422)

(400)

(222)

(311)

b)

(220)

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

Page 20 of 25

60

70

2 (degree)

Figure 1. (a) The TEM image of the AgIn5S8 nanoparticles synthesized at 120 °C. The inset shows that the lattice spacing is consistent with the (311) plane in a cubic phase of AgIn5S8. (b) An XRD pattern of the AgIn5S8 nanoparticles (black line) is compared to that of the cubic phase (JCPDS 00-026-1477). The simulated XRD pattern (red line) is also shown for comparison purposes.

20


Page 21 of 25

Abs PL 2

a)

(h)

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


2.0

1.5

2.0

2.4 2.8 Energy (eV)

2.5

3.0

Energy (eV)

b)

2.6 2.8 Energy (eV)

2.5 3.0 Energy (eV)

3.0

Abs PL

2.2

2.4

2.6

2.8

3.0

3.2

3.4

Energy (eV)

Figure 2. (a) The photoluminescence (PL) spectrum of the nanoparticles shows the emission at 2.74 eV, in addition to the defect emission at 2.07 eV, while the absorption (Abs) spectrum shows the hump at 2.78 eV (arrow). The inset shows the extrapolation of the linear part of (h)2 in the absorption spectrum. (b) The band gap transition of the photoluminescence is compared to that of the absorption in the AgIn5S8 nanoparticles. The left inset shows that the absorption spectrum is decomposed into the monotonous increase of the absorbance and the single Gaussian profile. The right inset shows that the photoluminescence spectrum is decomposed into the band gap emission and the defect emission.

21



Intensity (a.u.)

a)

1

600 nm

0.1

0.01

450 nm

0

50

100

150

200

250

300

350

Time (ns)

b)

2 ns 5 ns 10 ns

1

Intensity (a.u.)

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

Page 22 of 25

2.5

3.0 Energy (eV)

0.1

0.01 0

5

10

15

20

Time (ns)

Figure 3. (a) The time profile of the band gap emission in the AgIn5S8 nanoparticles (450 nm) is fitted by the triple-exponential model (red line). The time profile of the defect emission (600 nm) is fitted by the double-exponential model (pink line). (b) The enlarged time profile of the band gap emission. The inset shows the time-resolved photoluminescence spectra as a function of the detection time. The intensities are normalized for better comparison.

22


Page 23 of 25

a)

2.5 Energy (eV)

3.0

o

120 C o 140 C o 160 C o 180 C

2.6

2.8

3.0

3.2

Energy (eV)

b)

2.8

Energy (eV)

2.7

50 2.6 2.1

0

Stokes shift (meV)

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


2.0

2.5

3.0

3.5

4.0

4.5

Diameter (nm)

Figure 4. (a) The photoluminescence spectra of the AgIn5S8 nanoparticles are shown as a function of the reaction temperature. The emission energy decreases with increasing reaction temperature. The inset shows the absorption spectra of the AgIn5S8 nanoparticles as a function of the reaction temperature. (b) The band gap (black) and defect emission (red) energies are plotted as a function of the nanoparticle size. The Stokes shift (blue) is also plotted.

23



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

a)

Page 24 of 25

AgInS2 AgIn3S5 AgIn5S8 ZnS-AgIn5S8

2.4

2.6

2.8

3.0

3.2

Energy (eV)

b)

AgInS2 AgIn3S5 AgIn5S8 ZnS-AgIn5S8

1.8

2.0

2.2

2.4

Energy (eV)

Figure 5. (a) The band gap emission region of the photoluminescence spectra in the Ag-In-S nanoparticles is shown with respect to the stoichiometry. The band gap emission of the ZnSAgIn5S8 nanoparticles (dotted line) is obtained using the decomposition process (see text for details). (b) The defect emission region of the photoluminescence spectra in the Ag-In-S nanoparticles is shown.

24


Page 25 of 25

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


TOC GRAPHICS

25


Band-Gap States of AgIn5S8 and ZnSâAgIn5S8 Nanoparticles - The

Recommend Documents

Band-Gap States of AgIn5S8 and ZnSâAgIn5S8 Nanoparticles - The