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A series of ZnAgInS (ZAIS) quantum dots were synthesized and their optical properties were tuned by adjusting the reaction times from 5 to 30 min. The emission spectra were observed ranging from 619 to 667 nm. The temperature-dependent photoluminescence properties of ZAIS QDs were investigated from 10 K to 300 K that show a blue shift of spectra line with increasing intensity as well as broadening of spectral line owing to the coupling of the carrier to acoustic phonon. We have also discussed and investigated the internal luminescence mechanism of ZAIS QDs.
© 2016 Optical Society of America
1. Introduction
Colloidal quantum dots have aroused people's great interest in basic research and the application of science and technology. Compared with organic dyes, quantum dots have many excellent optical properties, such as the tunable of PL spectra, high PL quantum yield, symmetry and narrow photoluminescence spectra, broad excitation spectra, and large stokes shift and so on [1–5]. These characteristics provide the possibility of using quantum dots as fluorescent probe, such as high efficient of quantum dot based on Cd and Pb [6,7]. However, II-VI and IV-VI quantum dots contain toxic elements. Considering from the point of view of material chemistry, how the preparation of low toxicity of quantum dots is challenging [8]. Recently, non-toxic semiconductor nanomaterials have been widely investigated. Particularly, I-III-VI semiconductor nanocrystals such as CuInS2 (1.5 eV), CuInSe2 (1.05 eV), and AgInS2 (1.8 eV) have been used to prepare photoelectric device and used in the biological field [9–14]. Vittal et al. reported the preparation of AgInS2 QDs through thermal decomposition of precursor [(Ph3P)2AgIn(SCOPh)4] firstly [15]. After, Torimoto et al. have obtained color tunable ZnS-AgInS2 QDs by using thermal decomposition of the (AgIn)xZn2(1-x)(S2CN(C2H5)2)4 [16]. But these ternary quantum dots have lower emission efficiency, so how to moderate and prepare the quantum dots with high luminous efficiency become our focus [17,18]. Some papers use the methods of zinc doping to solve the problem of low luminous efficiency, that is, they use zinc diffusion into AgInS2 lattice and obtain the quaternary ZnAgInS quantum dots [19–23]. ZnAgInS quantum dots have the appropriate band gap energy, high absorption coefficient, the stability of the photoluminescence, the tunable emission spectra that have aroused more and more attention by all of us [24]. But now on the study of its luminous performance is less, especially the research of temperature-dependent photoluminescence properties. The radiative and the non-radiative relaxation processes as well as the exciton-phonon coupling process were investigated by the temperature-dependent of PL spectroscopy in colloidal QDs. Thus, it is vital significant to investigate the temperature-dependent optical properties of luminescent materials for some luminescent devices. However, there are seldom reports about the temperature-dependent of PL spectroscopy of ZnAgInS QDs. To make ZnAgInS quantum dots better applied into different kinds of devices [25,26], it is necessary to understand its internal luminescence mechanism. In our paper, a series of ZnAgInS quantum dots with different reaction time were prepared by controlling the size of the nanocrystalline to tune their PL spectra. And we investigated the temperature-dependent photoluminescence properties of ZnAgInS quantum dots. Furthermore, in our paper we report a study about the temperature-dependent of the PL intensity, energy levels and the full width at half maximum (FWHM) of ZnAgInS QDs with different reaction times.
2. Experiment section
Reagent: Indium acetate (In(OAc)3, 99.99%), zinc acetate (Zn(OAc)2, 99.99%), silver acetate (AgOAc, 99.99%), sulfur powder (99.5%), dodecanethiol (DDT, 98.0%), 1-octadecene (ODE, 90%), stearic acid (SA, 99.0%) and octadecylamine (OAm, 98.0%) were purchased from Aladdin to synthesize ZnAgInS quantum dots. All chemicals were purchased without further purification.
Zn precursor: Zn(OAc)2 (0.09174 g), OAm (0.3225 g), and ODE (2 mL) were injected into four-necked bottles fixed onto heating equipment. The reactant mixture was heated to 160°C under an argon atmosphere and kept in the temperature about 10 min until forming a colorless transparent solution. The acquired Zn precursor was reserved at 50°C for the future use.
Ag precursor: AgOAc (0.18163 g), SA (0.5690 g), and ODE (4.5 mL) were injected into four-necked bottles fixed onto heating equipment. Then reactant mixture was heated to 160°C under an argon atmosphere and kept in the temperature about 10 min until forming a transparent solution. The acquired Ag precursor was reserved at 50°C for the future use.
In precursor: In(Ac)3 (0.2920 g), SA (1.1370 g), and ODE (4 mL) were injected into four-necked bottles fixed onto heating equipment. The reactant mixture was heated to 200°C under an argon atmosphere and kept in the temperature about 10 min until forming a colorless transparent solution. The acquired In precursor was reserved at 50°C for the future use.
S precursor: sulfur precursor was prepared by dissolving 1.6 mmol S (0.0513 g) in ODE (4 mL) at 120°C, the acquired S precursor was reserved at 50°C for the future use.
Synthesis of ZAIS QDs: Zn(OAc)2 (0.1mmol), AgOAc (0.1 mmol), In(OAc)3 (0.1 mmol), DDT (1 ml), and ODE (6 ml) were all taken into four-necked bottles. The reactant mixture was then degassed by argon and subsequently the temperature was slowly increased to 180°C. Once a clear solution was obtained, 4 ml S precursor was loaded into the bottles and reacted 60 minutes kept the temperature at 200°C to allow ZnAgInS QDs. Then at regular intervals to extract a reaction liquid and poured into the toluene solution rapidly to end the growth of quantum dots for testing their optical properities. The obtained ZAIS QDs were purified with centrifugation by injecting methanol and acetone respectively into toluene solution and stored in chloroform solution.
Characterizations. The fluorescence spectrometer of Jobin Yvon FluoroLog-3 was used to measure the PL spectra which excitation light source was a 450 w xe lamp. The JEM-2100F transmission electron microscopes (TEM) was used to characterize the morphology and the size of the quantum dots. The absorption spectra were monitored by a Hitachi UV-4100 UV-vis. The X-ray diffraction (XRD) spectra were tested by a Rigaku D/max 2500v/pc X-ray diffractometer equipped with a Cu Kα source. The PL lifetime measurements were carried out through a Jobin Yvon FluoroLog-3-221-TCSPC fluorescence spectroscop. The quantum dots were dissolved into chloroform.
3. Results and discussion
Figures 1(a)-1(c) were the TEM photos of ZAIS quantum dots with different reaction times of 10 min, 20 min and 30 min, respectively. The average size of the sample was closed to (a) 4.5 nm, (b) 4.8 nm, (c) 5.1 nm. They conform to the quantum size effect. The diffraction ring was clearly observed in the selected area electron diffraction (SAED) shown in Fig. 1(d). It reveals the highly crystalline nature of the synthesized ZAIS quantum dots. The energy dispersive X-ray spectroscopy (EDS) spectrum of ZAIS quantum dots synthesised at 30 min. Figure 1(e) shows that the synthesised quantum dots possess a quarternary composition, containing S, Zn, Ag, and In. Table 1 is the composition data (EDS spectra) of ZAIS quantum dots synthesised at 30 min. It further proves the existence of S, Zn, Ag, and In.
Fig. 1 TEM pictures of ZnAgInS QDs prepared at various reaction time (a) 10 min, (b) 20 min, (c) 30 min. (d) Selected area electronic diffraction (SAED). (e) The EDS spectrum of the ZnAgInS QDs synthesised at 30 min.
Table 1. Composition Data (EDS Spectra) of ZnAgInS QDs with 30 min.
We have synthesized a series of ZAIS QDs with the optical properties and UV-vis spectra which adjusted by the reaction time from 5 to 30 min that leading to a tunable emission ranging from 619 to 667 nm in Figs. 2(a) and 2(b). With increasing the reaction time, both emission spectra and absorption spectra show a large redshift which due to quantum size effect. Figure 2(c) (up) shows the photos of ZAIS QDs under the light of room temperature with different reaction times. Figure 2(c) (down) shows the picture of ZAIS QDs under a 365 nm UV lamp irradiation and the emitting light colors change from yellow green to deep red with different reaction times.
Fig. 2 (a) PL, (b) UV-vis spectra of ZnAgInS QDs recorded at room temperature for various reaction times, (c)Digital pictures of the samples with various reaction times under the light of room temperature (from left to right 5, 10, 15, 20, 30 and 60 min) (up) and corresponding photographs of ZnAgInS QDs samples under a 365 nm UV lamp irradiation (down).
Figure 3 shows the XRD spectra of ZAIS QDs with different reaction time. We can see from the XRD spectra, three typical diffraction peaks are situated between two distinct characteristic peaks of AgInS2 (JCPDS 25-1330) and ZnS (JCPDS 65-0309) that shown the formation of ZAIS QDs [26].
To determine the PL emission mechanisms, we studied the PL lifetime of ZAIS quantum dots with different reaction times displayed in Fig. 4. The PL lifetime of ZAIS quantum dots for 5 min, 10 min, 15min and 20 min were tested with the corresponding emission wavelengths of 620 nm, 636 nm, 646 nm and 667 nm, respectively. The triple exponential model was used to fit the PL lifetime of ZAIS quantum dots, where τ1, τ2, τ3 was the decay time of the PL emission, and A1, A2, A3 was the relative ratios of attenuation components. The average lifetime τav is calculated according to τav = ∑Aiτi2/∑Aiτi [27]. Table 2 was the fitting results. The longer component of τ3 for 5 min from the three-exponential analysis was attributed to donor-acceptor pair (DAP) recombination. The shorter component of τ1 and τ2 were attributed to the quantized conduction band to localized intragap state and the recombination of related to the surface radiative increased. However, with increasing the reaction time, the relative ratios of attenuation components of DAP recombination decreased. The recombination of the quantized conduction band to localized intragap state and the recombination of related to the surface radiative increased. It demonstrated that the photoluminescence of ZAIS QDs probably originates from three types of recombinations containing a recombination of the quantized conduction band to localized intragap state, the recombination of related to the surface radiative and DAP recombination. With increasing the reaction times from 5 min to 20 min, the PL lifetime increases correspondingly. It is well known that increasing the reaction time can accelerate the internal atom aggregation and formation of crystal structure. It can decrease the surface defects and reduce the rapid attenuation components that increasing the lifetime of ZAIS QDs [27].
Table 2. Fit Parameters Rooting in I(t) = A1exp(−t/τ1) + A2exp(−t/τ2) + A3exp(−t/τ3)
Mechanism diagrams of ZAIS QDs are shown in Fig. 5. In ZAIS QDs, sulfur vacancy (VS), and indium replace silver site (InAg) act as donors, whereas indium site substituted by silver (AgIn) and indium vacancy (VIn) act as acceptors. The PL lifetime of ZAIS quantum dots was a typical characteristic that dominated the transition from conduction band to an acceptor level similar to ZnCuInS QDs [28]. Thus, DAP recombination was not main for ZAIS quantum dots, the recombination of the quantized conduction band to localized intragap state and the recombination of related to the surface radiative were increased. Therefore, the PL of ZAIS quantum dots probably comes from three types of recombination: related to the surface of recombination, CB-VIn and/or CB–AgIn recombination, and DAP recombination.
Figures 6(a)-6(d) were the photoluminescence spectra that test at low temperature from 300 K to 10 K. The emission peak was observed with a redshift as well as a broadening with increasing the temperature.
Fig. 6 Temperature dependence spectra of ZnAgInS QDs under various reaction time (a) 5, (b) 10, (c) 15, and (d) 20 min.
The energy levels of ZAIS quantum dots as a function of temperature with different reaction times are shown in Fig. 7. The energy level values were obtained from calculating the peak location of PL spectra. The datas in Fig. 7 were fitted by using the traditional Varshni Eq. (1) for the temperature dependence of the energy level [29],
where ∂ and β are constants, and Eg(0) represents the energy levels of the ZAIS QDs at 0 K. The fitting values of Eg(0) are shown in Table 3. The values of Eg(0) were higher than that of the bulk ZAIS materials. The shift of PL band of the ZAIS QDs with different temperature is similar to that of the temperature dependent band gap narrowing of bulk material which own to quantum size effect. We can see the energy levels decreases with increasing the reaction times which follows the quantum size effect. And we find that the fitted values of Eg(0) are different from both bulk ZnS (3.7 eV) and AgInS2 (1.8 eV), which between the two. It further proves the formation of ZAIS QDs.Fig. 7 Temperature-dependent peaks energy of ZnAgInS QDs with different reaction time. Full lines are the fitting result on the basis of Eq. (1).
Table 3. Fitting Results of Temperature-dependent Peaks Energy of ZnAgInS QDs with Different Reaction Time According to Eq. (1)
We can also use another expression to fit the datas in Fig. 7. This expression improves the Varshni Eq. (1) theoretically [30] because the parameters used in this formula are involved with an inner interaction within semiconductors, that is, the electron-phonon coupling [30],
where kB is the Boltzmann constant, S represents the Huang-Rhys factor, <hw> represents the average phonon energy. The fitted results are shown in Fig. 8 and datas are listed in Table 4. The fitted results of Eg(0) are obtained from Eq. (1) and (2) that having the same values. The datas of S indicated that the electron-phonon interaction decreased with increasing the reaction times and were in good agreement with the experimental datas shown in Fig. 1. The values of S are decreased because of ZnS component incorporating into QDs that change the structure of QDs and the luminous mechanism. The decrease of energy gap with increased temperature was owing to the coupling of exciton and acoustic phonon.Fig. 8 Temperature-dependent peaks energy of ZnAgInS QDs with different reaction time. Full lines are the fitting data according to Eq. (2).
Table 4. Fitting Results of Temperature-dependent Peaks Energy of ZnAgInS QDs with Different Reaction Time According to Eq. (2)
The FWHM for emission spectra of ZAIS quantum dots as a function of temperature with different reaction times are shown in Fig. 9. The experimental datas were fitted by Eq. (3) [31, 32]. Equation (3) stands for the temperature dependence of excitonic peak broadening in bulk semiconductors and QDs [30]. The total line width can be described with three factors: an inhomogeneous broadening factor, and two other factors are homogeneous broadening own to interactions from acoustic to optical phonon-exiton, separately [30].
where Γinh represents the inhomogeneous line width that is temperature independent and is own to the fluctuations in composition, shape and size [33], σ represents the carriers acoustic phonon coupling coefficient, ELO is the LO phonon energy, and ΓLO stands for the stiffness of carriers-LO-phonon coupling. From Fig. 9, the FWHM of the spectra was increased with the increasing of temperature with different particle sizes of ZnAgInS quantum dots. A variety of light-emitting mechanism involved in the spectral line broadening. The fitted results are tabulated in Table 5. It shows that the dominating contribution to the line broadening owes to the exciton−acoustic phonon coupling. Consequently, at the temperature from 10 K to 300 K, both the line broadening and the line shift were mainly caused by the exciton to acoustic phonon coupling [30]. Compared to quaternary ZnCuInS quantum dots [34], the change trend of these fitting values of the parameters is different, this may be due to the different particle size and nonuniform of particle size of the quantum dot material. The main factors of spectra line broadening may be from the carrier to the acoustic phonon mode coupling or from some types of emission electrons and holes in the center of the composite.Fig. 9 Temperature-dependent FWHM of PL spectra for ZnAgInS quantum dots with various reaction time. Full lines are the fitting result on the basis of Eq. (3).
Table 5. Fitting Results of Temperature-dependent FWHM of ZnAgInS QDs with Different Reaction Time According to Eq. (3)
Temperature-dependent of PL intensity involves very complicated processes, such as thermal activation, thermal escape, and nonradiative relaxation. As shown in Fig. 10, the PL intensity of ZAIS quantum dots as a function of temperature with different reaction times are investigated. When the temperature increasing, the photoluminescence intensity of ZAIS quantum dots decreases fastly. This phenomenon can be explained by the fact that the nonradiative relaxation increases with the temperature increasing. This data can be fitted reasonably well by the following expression [35].
where k stands for the Boltzmann constant, A stands for a constant, I(0) stands for the emission intensity at 0 K, and ∆E stands for activation energy for thermal quenching process. The parameter values of ∆E for the samples of ZAIS nanocrystals are listed in Table 6. The ∆E of ZAIS QDs increases at first then decreases with the increase of the reaction times. It reveals that not the more reaction times, the better stability of the samples. The smaller activation energy for the nonradiative relaxation process causes the decrease of PL intensity. As we all know, many defects such as VS, VAg and InAg act as acceptors or donors, in the luminescence process of ZAIS quantum dots, generating the transition of conduction band-acceptor and the recombination of donor-acceptor pair. Thus, the photoluminescence of ZAIS QDs probably originates from thtree types of recombinations containing a recombination of the quantized conduction band to localized intragap state, the recombination of related to the surface radiative and DAP recombination.Fig. 10 PL intensities of ZAIS quantum dots with various reaction times as a function of temperature. Full lines are the fitting result on the basis of Eq. (4).
Table 6. Fitted Parameters of PL Intensities of ZAIS Quantum Dots with Various Particle Size as a Function of Temperature
4. Conclusion
To sum up, a series of ZnAgInS quantum dots have been synthesized and their optical properties were tuned by adjusting the reaction times. Furthermore, we find that the PL lifetime of ZAIS QDs was increasing with the reaction time prolonging. The energy level of ZAIS quantum dots as a function of temperature with different reaction times were fitted through two Eqs. from 10 K to 300 K . The values of S and the average phonon energy were achieved. The FWHM as a function of temperature for emission spectra of ZAIS quantum dots were fitted with different reaction time. From 10 K to 300 K, the results show that the change of the band energy and the PL spectra broadening for ZAIS quantum dots were major induced by carrier to the acoustic phonon coupling. The three main recombination mechanisms possibly include the recombination of the quantized conduction band to localized intragap state, the recombination of related to the surface radiative and DAP recombination.
Funding
This research was supported by the National Key Foundation for Exploring Scientific Instrument of China (Grant No. 2014YQ120351), the National High Technology Research and Development Program of China (863 Program) (Grant No. 2013AA014201), the Natural Science Foundation of Tianjin (Grant Nos. 15JCYBJC16700, 15JCYBJC16800), and International Cooperation Program from Science and Technology of Tianjin (Grant No. 14RCGHGX00872).
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