Investigation of Photophysical Properties of Ternary Zn-Ga-S Quantum Dots: Band Gap versus Sub-Band-Gap Excitations and Emissions.

Highly luminescent ternary Zn-Ga-S quantum dots (QDs) were synthesized via a noninjection method by varying Zn/Ga ratios. X-ray diffraction and Raman investigations demonstrate composition-dependent changes with multiple phases including ZnGa2S4, ZnS, and Ga2S3 in all samples. Two distinct excitation pathways were identified from absorption and photoluminescence excitation spectra; among them, one is due to the band-gap transition appearing at around 375 and 395 nm, whereas another one observed nearby 505 nm originates from sub-band-gap defect states. Photoluminescence (PL) spectra of these QDs depict multiple emission noticeable at around 410, 435, 461, and 477 nm arising from crystallographic point defects formed within the band gap. The origin of these defects including zinc interstitials (IZn), zinc vacancies (VZn), sulfur interstitials (IS), sulfur vacancies (VS), and gallium vacancies (VGa) has been discussed in detail by proposing an energy-level diagram. Further, the time-dependent PL decay curve strongly suggests that the tail emission (appear around 477 nm) in these ternary QDs arises due to donor-acceptor pair recombination. This study enables us to understand the PL mechanism in new series of Zn-Ga-S ternary QDs and can be useful for the future utilization of these QDs in photovoltaic and display devices.

Introduction

Unique optical and electronic properties of semiconductor quantum dots (QDs) make them a potential material for solar cells,[1,2] light-emitting diodes (LEDs),[3,4] spintronics,[5,6] and bioimaging[7,8] applications. Semiconductor QDs with emission over the entire visible range and high quantum yields (QYs) are suitable for display devices.[9] Cadmium (Cd)-containing semiconductor QDs such as CdSe, CdS, CdTe, etc. have excellent optical properties with emission over the entire visible range.[10−12] Further, these QDs have been proved as a potential material for photovoltaic devices in recent decades.[13,14] Meanwhile, the toxicity and environmental aspects of these semiconductor QDs rendered their practical applications difficult, since they contain heavy metal Cd.[15,16]

Recently, Cd-free ternary I–III–VI chalcogenide QDs such as CuInS2, CuInSe, AgInS2, ZnInS, etc. were explored as an emerging material for applications in solar cells and LEDs.[17,18] These ternary semiconductor QDs have composition- and size-dependent optical properties.[19] Further, they have large absorption cross section, favorable charge-transport characteristics, and long exciton lifetimes.[20,21] Besides, they also have high power conversion efficiency (PCE) similar to Cd-containing II–VI QDs. In the materials using ternary nanocrystals as a solar cell, up to 3.5, 6.6, and 11.6% PCE was achieved for AgInSe2, CuInS2, and CuInSe–ZnSe, respectively.[22−24] Moreover, among Cd-free QDs, carbon-based QDs are a new class of materials with excellent optical and biological properties. They are different from ternary QDs in terms of composition and crystal structure.[25]

Meanwhile, the photophysical properties of these ternary QDs are very challenging and complex from II–VI semiconductor QDs. Photoluminescence (PL) emission in II–VI QDs mainly originated from radiative recombination of electrons and holes from band edge.[12,26,27] On the other hand, recent studies revealed that PL emission occurring in ternary QDs is from radiative donor–acceptor pair (DAP) recombination, which originates from intrinsic defects.[20,21,28,29] The uncertainties related to these internal defect states result in the complication of proposing a general model. Another aspect is the broad absorption and emission feature with no apparent excitonic properties. These characteristic features are still debatable and ascribed to be due to their unique electronic properties, such as size and shape inhomogeneity, irregular distribution of elements and composition, and sub-band-gap states. The Kamat group has reported a study on the photodynamics of Cu-In-S2 QDs by varying the Cu/In ratio.[21] It has observed broad emission with two distinct absorptions. In UV–vis absorption spectra, one peak appears from the band edge with excitonic character and other broad tail absorption originates from the Cu-related state lying just within the band gap. Besides, the broad emission is attributed to the transitions involving the band gap (excitonic), sub-band-gap (DAP), and surface states. Another study was carried out by the same group on AgInS2–ZnS quaternary QDs.[20] In this study, the origin of significant Stokes shift and long PL lifetimes has been explained by the DAP mechanism as a dominating radiative process arising from antisite defects within the band gap of the QDs.

Recently, Zhang et al.[30] have reported a new series on ternary Zn–Ga–S QDs with Ag-doping. These ternary QDs have luminescence properties with emission spanning over violet to aqua, via variation of Zn/Ga ratios. Motivated by these results, here, we have synthesized Zn–Ga–S ternary QDs by varying Zn/Ga ratios and first time explored their photophysical properties. PL spectra of as-synthesized QDs show multiple emission arising from multiple defect states lying within the band gap. Using UV–vis absorption, photoluminescence (PL), photoluminescence excitation (PLE), and time-correlated single-photon counting (TCSPC) techniques, the complex recombination processes have been further resolved by proposing an energy-level diagram. The PL quantum yield (QY) of these QDs was found to be very high, 42%, for the Zn/Ga ratio of 1.0. Moreover, the phase structure and morphology of as-synthesized QDs were investigated by X-ray diffraction (XRD), Raman, and TEM analyses. XRD and Raman studies demonstrate a stoichiometry-dependent structure with multiple phases, which is in good agreement with optical characterizations. Transmission electron microscopy (TEM) images depict the near-spherical shape of the nanocrystals with average diameters of 5.5, 6.0, and 7.5 nm for 0.25, 0.50, and 1.0 Zn/Ga ratios, respectively.

Results and Discussion

A one-spot noninjection method was adopted to prepare 1-dodecanethiol (DDT)-capped Zn–Ga–S ternary QDs. The schematic diagram of the synthesis mechanism is shown in Figure A. Three compositions of Zn–Ga–S QDs were synthesized by varying Zn/Ga ratios, that is, 0.25, 0.50, and 1.0. This method has several features like reproducibility and scale-up capability of the work.[31,32] The phase composition of as-synthesized Zn–Ga–S ternary QDs was identified by the X-ray diffraction (XRD) pattern, which is shown in Figure B with increasing Zn/Ga ratios. As shown in the figure, the XRD pattern of Zn–Ga–S QDs well indexed with three mixed phases, namely, tetragonal ZnGa2S4, zinc blende (cubic) ZnS, and hexagonal Ga2S3.[33−35] It is very exciting to see that the XRD pattern shows a composition-dependent mixed phases. For Zn/Ga = 0.25, the planes (1̅11) and (101) of Ga2S3 become more prominent, representing that most of the contribution is from the hexagonal Ga3S3 phase. Moreover, as Zn/Ga ratios increase, the Ga2S3 phase becomes less noticeable and zinc blende ZnS phase becomes more prominent, which is validated by increasing intensities of (111), (220), and (311) planes. Also, all materials contain a tetragonal ZnGa2S4 crystal structure as a common phase. A recent study by Dong et al. has elucidated similar results in their report in the case of Zn-In-Se ternary QDs.[29] They have observed multiple phases including ZnIn2Se4, In2Se4, and ZnSe. In another work on the In–Zn–Se alloy, Lee and co-workers have illustrated In+3-dominated multiple phases such as ZnIn2Se4, ZnSe, and In2Se3.[36]

Figure 1

(A) Schematic diagram for synthesis of Zn–Ga–S QDs, (B) XRD patterns of Zn–Ga–S QDs with varying Zn/Ga ratios, and (C) Raman spectra of Zn–Ga–S QDs with increasing Zn/Ga ratios (bottom to top).

Figure C shows Raman spectra of Zn–Ga–S QDs with increasing Zn/Ga ratios (bottom to top). A peak centered at around 263 cm–1 appears in all samples, attributed to transverse optical (TO) mode of ZnS.[37,38] Further, at higher wave number, a broad hump ranging from 300 to 450 cm–1 is observed. This broad hump is deconvoluted into two peaks at around 353 and 390 cm–1 using Gaussian function fitting. The peak centered at around 353 cm–1 appears due to longitudinal optical (LO) phonon mode of ZnS, whereas the peak at 390 cm–1 is assigned to F2 of Ga2S3.[37−39] Moreover, it is also noticed that the intensity of TO and LO peaks increases with increasing Zn/Ga ratios, confirming that the ZnS phase is the dominating phase as the Zn/Ga ratio increases. Similarly, the intensity of the F2 peak also increases with decreasing Zn/Ga ratio, affirming that the Ga2S3 phase is more dominating in the sample when the Zn/Ga ratio decreases. Therefore, similar to the XRD pattern, Raman spectra also demonstrate composition-dependent changes with multiple phases, including ZnS and Ga2S3.

The shape and size, morphology, of Zn–Ga–S QDs were investigated by transmission electron microscopy (TEM). Figure A–C are the TEM images of Zn–Ga–S QDs with varying Zn/Ga ratios. As shown in the picture, Zn–Ga–S QDs possess near-spherical shapes with average diameters 5.5 (±0.9), 6.0 (±0.8), and 7.5 (±0.9) nm for 0.25, 0.50, and 1.0 Zn/Ga ratios, respectively. The particle size histogram is given in Figure D. The inset image of Figure B is its high-resolution TEM (HRTEM) image where interplanar spacing is found to be 0.32 nm, which is ascribed to the (111) plane of zinc blende ZnS. Further, energy-dispersive X-ray spectroscopy (EDX) analysis was carried out to get information about elemental contents of as-synthesized ternary QDs. Table illustrates the normal ratio (as calculated theoretically) and EDX analysis ratio for Zn–Ga–S QDs. The results obtained from the EDX analysis were quite close to the expected value. Details of elemental information, including sulfur contents, are provided in the Supporting Information in Figure S1A–C.

Figure 2

TEM images of Zn–Ga–S QDs with (A) Zn/Ga = 0.25, (B) Zn/Ga = 0.50, the inset image is its HRTEM image, and (C) Zn/Ga = 1.0. (D) Size bar diagram of Zn–Ga–S QDs with different Zn/Ga ratios.

Table 1

Elemental Analysis of Zn–Ga–S QDs Using EDX

s.no.	normal ratio	EDX analysis ratio
1	Zn/Ga = 0.25	Zn/Ga = 0.23
2	Zn/Ga = 0.50	Zn/Ga = 0.55
3	Zn/Ga = 1.0	Zn/Ga = 0.97

Optical Properties

The excitonic feature of multiphase Zn–Ga–S ternary QDs was determined by steady state absorption spectroscopy, which is shown in Figure A. The absorption spectra of all QDs contain multiple peaks: a band edge transition observed at around 390–400 nm and another next excited state transition nearby 375 nm. Moreover, as clearly seen in the absorption spectra, a tail absorption starting from 550 nm having a maximum at around 505 nm is present in all samples. The origin of this tail absorption is possibly a sub-band-gap transition originating from defect states within the band gap. Further, it was found that the absorption peak slightly blue-shifted (4–8 nm) with increasing Zn/Ga ratio. The corresponding optical band gap was calculated by the Tauc relation, which is shown in Figure B. The observed band gap slightly increased with increasing Zn/Ga ratio and was found to be 3.15, 3.18, and 3.21 eV for 0.25, 0.50, and 1.0 of Zn/Ga ratios, respectively. The reason behind the increase in the optical band gap of Zn–Ga–S QDs with an increase in Zn/Ga ratio is due to the higher band gap of ZnS (3.49 eV) in comparison to Ga2S3 (2.5 eV).[30]

Figure 3

(A) UV–vis absorption spectra, (B) Tauc plots, and (C) PL spectra of Zn–Ga–S QDs with different Zn/Ga ratios. The insets show the photographs of QDs under a UV lamp, (D) Gaussian deconvolution of the emission spectrum of Zn–Ga–S with the Zn/Ga ratio of 0.5, (E) PLE spectra of Zn–Ga–S QDs with varying Zn/Ga ratios at an emission wavelength of 600 nm, and (F) schematic energy-level diagram illustrating optical excitation pathways.

Figure C shows PL spectra of Zn–Ga–S QDs with variation in the Zn/Ga ratio, at an excitation wavelength of 375 nm. It is clear from the figure that the PL spectra contain multiple emissions for all compositions of Zn–Ga–S QDs. A very slight blue shift (1–3 nm) has been observed in emission spectra with increasing Zn/Ga ratios. The inset images of Figure C are photographs of the QDs under a UV lamp, showing bright violet-to-aqua emission with increasing Zn/Ga ratios.[30] Thus, obtained photos under the UV lamp visibly demonstrate the compositional variation, which was also observed in the XRD pattern. Moreover, we have calculated QY of these ternary QDs using the standard organic dye 9,10-diphenylanthrancene and found maximum up to 42% for a composition with Zn/Ga = 1.0. Details of QY calculations are given in the Supporting Information, Section S3. Figure D is deconvolution of the emission spectrum of Zn–Ga–S QDs with the Zn/Ga ratio of 0.5 using Gaussian fit, and the PL spectrum can be deconvoluted into four peaks.[40] The obtained results from the fitting are summarized in Table . A very sharp peak with a full width at half-maximum (FWHM) of 14 nm is observed at around 410 nm, which is denoted in Figure D by “1”. Further, a high-intensity peak with a FWHM of 20 nm has been deconvoluted at 435 nm, which is indicated by “2”. Moreover, peak “3” has been deconvoluted at 461 nm with a slightly larger FWHM of 25 nm. Finally, at higher wavelength, a vast peak with FWHM of 68 nm appears at around 477 nm, which is denoted as “4”. The origin of these peaks will be discussed in the next section.

Table 2

Fitting Parameter Obtained from Gaussian Deconvolution of Zn–Ga–S QDs with the Zn/Ga Ratio of 0.5

parameters	deconvoluted peaks
	1	2	3	4
emission wavelength (nm)	410	435	461	477
FWHM (nm)	14	20	25	68

The PLE spectra of ternary QDs can provide insight into the origin of sub-band-gap and band-gap transitions and mechanism of the radiative process, as shown earlier by various groups.[20,21,29]Figure E shows PLE spectra of Zn–Ga–S QDs with varying Zn/Ga ratios at an emission wavelength of 600 nm. PLE spectra at other wavelengths are shown in the Supporting Information, Figure S4A–C. As shown in Figure E, three distinct transitions are observed in the PLE spectra of all samples. A broad transition at around 505 nm is observed in all samples. The origin of this transition may be due to the sub-band-gap excitation, including zinc and gallium defect states since energy gaps from these defect states well-matched with this transition.[34,39] The basis of these sub-band-gap defects will be discussed in the next section. Moreover, at a lower wavelength, two more peaks are observed at around 395 and 375 nm and can be assigned to the band edge and next-level excited state transitions, respectively. For a clear understanding of theses transitions, an energy-level diagram is given in Figure F. The Kamat group members observed similar characteristics in their previous studies.[20,21] Thus, PLE spectra clarify that sub-band-gap low-energy optical transitions are responsible for longer-wavelength emission in these ternary QDs.

The time-correlated single-photon measurement was performed to understand carrier dynamics, carrier lifetime, and origin of various emissions. For this measurement, each sample was dispersed in toluene and excited by a pulsed laser with a wavelength of 375 nm. Further, the decay was monitored between 400 and 600 nm emission wavelength over a time window of 65 ns. Figure A–C show normalized PL decay profiles for Zn–Ga–S QDs with increasing Zn/Ga ratios. For all samples, PL decay increases with increasing emission wavelength, which is indicated by an arrow (a) → (e) in the figure. This behavior suggests that at higher wavelength, donor–acceptor pair (DAP) recombination is the dominating radiative process in these new series Zn–Ga–S ternary QDs.[20,21,41] The luminescence decay can be well fitted with a biexponential fitting. Fitting details are given in the Supporting Information, Section S5. When the PL decay curve was monitored near band edge (450 nm), a shorter lifetime (τave) of 22, 11, and 25 ns was observed for 0.25. 0.50, and 1.0 Zn/Ga ratios, respectively. When emission decay was monitored at the longer-wavelength side (600 nm), a longer lifetime of 31, 34, and 64 ns was observed for 0.25. 0.50, and 1.0 Zn/Ga ratios, respectively.

Figure 4

PL decay curves of Zn–Ga–S QDs at different emission wavelengths from 400 to 600 nm: (A) Zn/Ga = 0.25, (B) Zn/Ga = 0.50, and (C) Zn/Ga = 1.0.

The appearance of a less energetic photon with a longer lifetime in the higher-wavelength region indicates DAP recombination. Lower-energy photons generated from donors are far distant to acceptors, leading to lower probability to occur because of their smaller wave function overlap. For higher-energy photon emission, the donor–acceptor pairs are in close proximity to each other and thus have a higher probability of recombinations.[20,42] Therefore, the PL lifetime of DAP recombination increases with increasing emission wavelength.

Origin of Multiple Emission from Zn–Ga–S QDs

In a crystalline material, electronic bands are formed due to translational symmetry. If the material has defects and impurities, then the periodicity of the lattice gets perturbed, leading to the formation of discrete energy levels within the band gap of the material. Depending on impurities and defects, these levels behave as donors or acceptors. Figure is a schematic energy-level diagram for Zn–Ga–S QDs based on the discussion above on XRD, Raman, UV–vis, PL, PLE, and TCSPC results. Five types of point defects are presented, including zinc interstitials (IZn), zinc vacancies (VZn), sulfur interstitials (IS), sulfur vacancies (VS), and gallium vacancies (VGa).[35,43−47] Among them, IZn and VS act as localized donor states, while IS, VZn, and VGa behave as acceptor states. Interstitial states will be formed when an extra atom or ion is positioned at a site that is not occupied in the perfect crystal. As an ion takes such place, the neighboring atoms rearrange themselves. Thus, the ionic radii of the positioned atom decide the extent of deformation. Since zinc ions have smaller ionic radii (0.74 Å) than sulfur ions (1.70 Å), the sulfur interstitials have more lattice strain. This results in the lesser binding energy of electronic levels associated with site IS, whereas IZn sites have more considerable binding energy. Consequently, IS states lie just above the valance band edge than IZn states to the conduction band edge.[43−47] Therefore, peak 1 at 410 nm originates from radiative recombination of conduction band electrons with IS state holes and peak 2 at 435 nm is observed due to the recombination of IZn electrons with valance band holes.[43−47]

Figure 5

Schematic depiction of the emission mechanism in Zn–Ga–S ternary QDs. Since ZnS has a larger band gap than Ga2S3, the Zn–Ga–S band gap lies between both of these band gaps. Process 1 is recombination from the conduction band edge to IS states. Process 2 is recombination from the IZn level to the top of the valance band edge. Process 3 is recombination of VS states to the valance band edge. Finally, process 4 involves recombination from the conduction band edge to VZn, IZn to VZn, and VS to VZn and VGa states.

In the case of vacancies, a similar discussion can be applied. A vacancy can be formed when an atom is absent from its native site. It also puts lattice strain in the crystal by rearranging the neighboring atoms. As we know that sulfur atoms have bigger atomic size than zinc, sulfur vacancy (VS) levels are near to the conduction band edge than those of zinc vacancies (VZn) states.[42−46] Therefore, we attribute peak 3 at 460 nm to radiative recombination of VS state electrons with valance band holes and emission 4 initiates from recombination of conduction band electrons with VZn state holes.[44−46] Furthermore, the peak 4 has larger FWHM than any other peaks, so it could be originated from multiple defect states. As we have observed the Ga2S3 phase in all samples from XRD and Raman measurements, gallium vacancies (VGa) are also formed and could lie just above (at 0.7 eV) the valance band edge of Ga2S3.[35,38] In Figure S2 of the Supporting Information, it has been shown that the width of tail emission increases with increasing Ga concentration in Zn–Ga–S QDs. Accordingly, it also suggests that the tail of peak 4 arises from the contribution of gallium states since it contains broad emission ranging from 450 to 650 nm. Therefore, we can conclude that peak 4 also originates from VS to VGa states.[35,38] In addition, the emission 4 may be contributed from other defect states, including IZn to VZn and VS to VZn as observed in the previous reports.[44,47]

Conclusions

In summary, we examined photophysical properties of highly luminescent Zn–Ga–S ternary QDs by varying the Zn/Ga ratio. The notion of multiphase formation with ZnGa2S4, ZnS, and Ga2S3 in these ternary QDs was supported by XRD patterns as well as Raman spectra. Three optical transitions were recognized, two being from valance to conduction band and the other from sub-band-gap defect states, contributing to the absorption as well as in excitation spectra of these QDs. Moreover, multiple emission was observed in these QDs arising from interstitial and vacancy defects formed within the band gap of the QDs. The dominating radiative mechanism in the higher-wavelength region is believed to be DAP recombination occurring from these defects sites. Therefore, understanding of photophysical properties of these ternary Zn–Ga–S QDs can be a benefit for obtaining highly efficient QDs for LEDs and photovoltaic devices.

Experimental Section

Materials

Gallium acetylacetonate (Ga(acac)3, 99.99%), zinc acetate dihydrate (Zn(OAc)2·2H2O, 99%), sulfur powder (99.99%), 1-dodecanethiol (DDT, 98%), oleic acid (OA, 90%), 1-octadecene (ODE, 90%), toluene (C6H5.CH3), and methanol (CH3.OH) were used. All of the chemicals were bought from Sigma-Aldrich and used as received.

Synthesis

Synthesis of Zn–Ga–S QDs was carried out using a one-pot noninjection approach, a previously reported method.[30] First, 2.0 mL of DDT, 6.0 mL of OA, and 12.0 mL of ODE were mixed in a 100 mL three-neck flask at room temperature. Further, after gassing–degassing (for Zn/Ga = 1 composition), Zn(OAc)2.2H2O (0.2 mmol, 44.0 mg), Ga(acac)3 (0.2 mmol, 73.4 mg), and sulfur powder (0.8 mmol, 25.6 mg) were added to the reaction mixture. Furthermore, the reaction mixture was heated up to 100 °C for half an hour, followed by heating at 240 °C for 30 min under the N2 environment. After cooling up to room temperature, aliquots were purified by toluene and methanol, respectively. Further, the obtained product was dispersed in toluene and stored in vacuum for further characterizations. Similarly, other compositions were synthesized by varying Zn and Ga molar concentrations.

Characterizations

The XRD pattern of the powder samples was obtained using a Rigaku Miniflex-600 diffractometer having a Cu (Kα, λ = 1.5418 Å) source. TEM images were obtained by a JEOL-2100F electron microscope (operating voltage 200 KeV) by drop-casting the dispersed sample in toluene on a carbon-coated copper grid (300 mesh). Raman spectra were obtained by a Wi-Tec alpha300 RA system having an Ar laser source with wavelength 532 nm. For this measurement, QDs were dispersed in toluene solution and drop-cast on a glass substrate. EDX analysis was carried out by SEM, Zeiss EVO40 microscope in which an energy dispersive X-ray analyzer was attached. UV–vis absorption spectra were observed using a Hitachi U-3900 UV–vis spectrophotometer. PL and PLE spectra were obtained by a Hitachi F-4700 fluorescence spectrometer. Fluorescence decay spectra were acquired by time-correlated single-photon counting (TCSPC) FL920, Edinburg Instruments, U.K. setup.