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Abstract
Quantum dots (QDs) exhibit not only wide tunability of luminescence but also complex optical properties because of the large degree of freedom in their structure and chemical composition. Quaternary ZnxAgInSe QDs with different Zn/Ag ratios were synthesized and examined as temperature sensors. The relationship among the luminescence energy, emission intensity, and full-width at half-maximum (FWHM) of the emission band at different temperatures was investigated. To understand the photoluminescence mechanism, time-resolved photoluminescence spectra were recorded. Moreover, the dependence of the luminescence peak energy and FWHM on temperature was investigated, and a small deviation from the actual temperature was observed, indicative of the use of ZnxAgInSe QDs as high sensitivity temperature sensors.
© 2017 Optical Society of America
1. Introduction
Colloidal quantum dots (QDs) have attracted considerable attention because of their specific photometric characteristics, and have been applied in light-emitting diodes, optical lasers, and photovoltaic cells, as well as in biological imaging [1–5]. Thus, most of the high-quality QDs are binary II-VI or IV-VI QDs, e.g., binary CdTe and CdSe QDs [6,7], comprising potentially toxic elements. Hence, it is imperative to formulate a potential strategy to develop QDs comprising less toxic components [8–11]. Composition dependent alloy semiconductors, such as CdSexS1-x, CuxInS2, CuxIn1-xSe2 represent a new category of QDs exhibiting tunable emissions [12–14]. Recently, several efficient approaches have been developed for tuning the QD composition [15–17]. Compared to these ternary systems, quaternary Zn-I-III-VI nanostructures have been attracting increasing attention, because of large differences in the bandgap and luminescent properties between the Zn-VI and I-III-VI2 semiconductors [18]. To better apply QDs, it is imperative to understand the composition-dependent luminescence mechanism of the QDs. Compared to the QDs obtained by changing the reaction time, those obtained by tuning the precursor ratios are composed of homogeneous particles [19,20].
Temperature is one of the fundamental thermodynamic state variables and the most measured physical quantity. Because the geometrical or electronic structure easily changes with thermal agitation, traditional temperature sensors typically suffer from a limited temperature response range and poor thermal stability. Conventional thermometers have limited applications, liquid thermometers are not suitable for corrosive environments, and contact thermometers are not suitable for measurement along the beam path. Operating in hazardous situation and harsh environments require reliable temperature sensor and the most adequate is to use remote sensing devices. Recently, non-invasive thermometers with high spatial resolution have become very attractive for temperature measurements. Luminescence thermometry is a versatile optical technique for measuring the local temperature, where temperature-dependent changes serve as an indicator or probe. Thus, some rare earth ion doped luminescent compounds have been employed as optical temperature probes, such as Eu3+-activated SrZrO3 and Er-doped CaZnOS [21–23]. Compared to the rare earth ion doped phosphor, biocompatible QDs exhibit highly sensitivity. In addition, these QDs can serve as probes in the nanometer region and detect multiple signals. It is challenging to fabricate with highly stable and sensitive optical QDs-based temperature sensors [24,25]. However, several issues related to the mechanism and application of QDs as optical temperature sensors have still not been resolved.
In this paper, the bandgap of quaternary ZnxAgInSe QDs was adjusted by changing their element stoichiometric ratio. Based on the temperature-dependent response of photoluminescence (PL), the luminescence mechanism of ZnxAgInSe QDs with different Zn/Ag ratios was analyzed. In addition, the relationship among the energy level, full-width at half maximum (FWHM), and luminescence intensity at 10-300K was investigated. From the temperature-dependence PL spectrum, activation energy (ΔE), average phonon energy (<hw>) and Huang−Rhys factor (S) were obtained. Based on the fitting results, ZnxAgInSe QDs with a 1/1 Zn/Ag ratio exhibits high sensitivity to temperature. The parameters and high sensitivity of ZnxAgInSe QDs were also applied for examining their potential for temperature sensing. Correspondingly, the PL lifetime was investigated, and the luminescence mechanism was proposed.
2. Experiment section
Chemicals: Zinc acetate (Zn(OAc)2, 99.99%), silver acetate (AgOAc, 99.99%), Se powder (99.5%), octadecylamine (OAm, 98.0%), dodecanethiol (DDT, 98.0%), 1-octadecene (ODE, 90%), stearic acid (SA, 99.0%), oleylamine (OA, 99.99%), Indium acetate (In(OAc)3, 99.99%) were purchased from Alfa Aesar. Methanol, acetone and chloroform used for the synthesis of ZnxAgInSe QDs with different Zn/Ag ratio were purchased from Aladdin.
Precursor solution of Zn(OAc)2: A four-neck flask clamped in a heating jacket was charged with Zn(OAc)2 (x = 0.01317 g, 0.03512g, 0.0439g) to synthesize the ZnxAgInSe QDs, OAm (0.0645 g), and ODE (2.0 mL) under argon. Next, the mixed solution was heated to 160°C and maintained for 10 min. The Zn precursor solution was collected and stored at 50°C for subsequent reaction.
Precursor solution of AgOAc: A four-necked flask clamped in a heating jacket was charged with AgOAc (0.0363 g), SA (0.1137 g), and ODE (2.0 mL) under argon. Next, the mixed solution was heated to 160°C and maintained for 10 min. The Ag precursor solution was collected and stored at 50°C for subsequent reaction.
Precursor solution of In(OAc)3: A four-necked flask clamped in a heating jacket was charged with In(OAc)3 (0.0584 g), SA (0.2274 g), and ODE (2.0 mL) under argon. Next, the mixed solution was heated to 160°C and maintained for 10 min. The In precursor solution was collected and stored at 50°C for subsequent reaction.
Precursor solution of Se: A four-neck flask clamped in a heating jacket was charged with Se powder (0.0078 g), OLA (2mL), and DDT (0.1 mL). Next, the mixture was stirred under argon at room temperature until dissolution.
Synthesis of ZnxAgInSe QDs with different Zn/Ag ratios: The Zn(OAc)2, In(OAc)3, and AgOAc precursor solutions, DDT (2 mL), and ODE (6 mL) were added into a four-neck flask. Second, the reaction mixture was slowly heated to 180°C under argon. Third, when the reaction mixture became clear, the Se precursor solution was injected into the flask, and the temperature was maintained at 220°C for 30 min, affording ZnxAgInSe QDs. Then the reaction was immediately terminated by injecting the product into toluene. The untreated ZnxAgInSe QDs were purified thrice by centrifugation with methanol and acetone. The purified ZnxAgInSe QDs were stored into a the chloroform solution. The ZnxAgInSe QDs film was prepared and used as a temperature sensor.
Characterizations: Photoluminescence (PL) spectra were recorded on a Jobin Yvon FluoroLog-3 fluorescence spectrometer, and a 450-W xenon lamp was used as the light source. Absorption spectra were recorded on a Hitachi UV-4100 spectrophotometer. The lifetime of the QDs was measured on a Horiba Jobin Yvon FluoroLog-3-time-correlated single-photon counting (TCSPC) fluorescence spectrometer, which was pumped at a wavelength of 367 nm using a pulsed diode light source. Transmission electron microscopy (TEM, HITACHI-HT7700) was employed to characterize the morphology and sizes of ZnxAgInSe QDs with different Zn/Ag ratios. Scanning electron microscopy (SEM, Hitachi S-8010) equipped with EDX analysis was employed to record the EDX spectra of ZnxAgInSe QDs. X-ray diffraction (XRD, Rigaku D/max 2500v/pc) was used to estimate the composition of QDs via the measurement of their phase structure. Thermal imaging was carried out using Fluke thermal imager Ti10.
3. Results and discussion
Figure 1(a)–1(c) shows the TEM images of ZnxAgInSe QDs with different Zn/Ag ratios. By changing the Zn/Ag precursor ratios, spherical, sharp grains were observed for ZnxAgInSe QDs, with a homogeneous size distribution. The three ZnxAgInSe samples exhibited the same size (5 nm). Figure 1(d) shows the XRD patterns of the resulting ZnxAgInSe QDs. Three diffraction peaks were indexed between those of cubic ZnSe and chalcopyrite AgInSe2. Figure 1(e)-1(g) shows the EDX spectrum, which shows the ZnxAgInSe composition. The EDX spectrum was better to characterize the QD samples, which showed a quaternary composition of Zn, Ag, In, and Se, and the corresponding element ratios were listed in Table 1.
Fig. 1 TEM images of ZnxAgInSe QDs with Zn/Ag ratios of (a) 0.3/1; (b) 0.5/1; (c) 1/1. (d) XRD patterns of ZnxAgInSe QDs with different Zn/Ag ratios. (e)-(g) EDX spectrum of ZnxAgInSe QDs with different Zn/Ag ratios.
Table 1. The element ratio of ZnxAgInSe QDs.
The ZnxAgInSe QDs are closely related to their stoichiometries. Figure 2(a) and 2(b) show the recorded PL emission spectra and absorbance of the as-synthesized ZnxAgInSe QDs with different Zn/Ag ratios, respectively. With the increase in the ratio of the Zn/Ag precursor, the emission peak of ZnxAgInSe QDs blue shifted from 682 nm to 643 nm at room temperature and under vacuum, possibly related to the expansion of the energy gap of ZnxAgInSe QDs with increasing Zn content. By the introduction of Zn ions, quaternary ZnxAgInSe QDs can be formed with a wider band-gap (1.8-2.93 eV) via the formation of an alloy with a narrower-band-gap AgInSe (1.24eV) [15]. Meanwhile, with the increase in the Zn content of ZnxAgInSe QDs from 0.3 to 1, the PL intensity increased indicating that the relative change in the different recombination is related to the change in VZn and VAg, and the increase in the Zn content induces the decrease in VAg. The illustration shows the photographs of the ZnxAgInSe QDs samples with different Zn/Ag ratios dispersed in chloroform, which were excited using an ultraviolet 365 nm LED.
Fig. 2 (a) PL emission spectra at room temperature and under vacuum and (b) absorbance of ZnxAgInSe QDs measured at different Zn/Ag ratios. The illustration is photograph of ZnxAgInSe QDs with different Zn/Ag ratios under an ultraviolet LED comprising a 365-nm blue LED (from left to right: 0.3/1, 0.5/1, and 1/1).
The fluorescence lifetime of the ZnxAgInSe QDs were measured (Fig. 3). The data indicated that non-radiative decay significantly increases at a Zn/Ag ratio of 1/1. The triple exponential model was used to fit the PL lifetime of ZnxAgInSe QDs,
where A1, A2, and A3 are the relative weights of the decay components and τ1, τ2, τ3 represent their corresponding decay time. Table 2 summarizes the fitted results with different Zn/Ag ratios. The as-prepared QDs exhibited a long PL lifetime of several hundreds of nanoseconds according to τav = ∑Aiτi2/∑Aiτi. Using this equation, the corresponding average PL lifetimes for ZnxAgInSe QDs were calculated as 254 ns, 234 ns, and 227 ns. These values decrease with increasing Zn/Ag ratio. Previous studies [26–28] have reported that medium τ1 possibly results from the recombination of the conduction-band level and a localized intra-gap level. The longer lifetime τ2 corresponds to the donor-acceptor pair (DAP) transition. The shorter lifetime τ3 corresponds to surface-related radiative recombination. The combination of these three processes leads to the luminescence of ZnxAgInSe QDs.Table 2. PL lifetime fitting parameters of ZnxAgInSe QDs with different Zn/Ag ratio.
In ZnxAgInSe QDs, indium (InAg), as donors, substitutes silver sites and selenium vacancies (VSe), while silver vacancies (VAg) serve as acceptors [28–35]. DAP recombination is predominated (Fig. 4). We can reasonably speculate that the PL luminescence mechanism of ZnxAgInSe QDs is a result of three processes at room temperature, i.e., conduction band-VAg recombination, surface related radiative recombination, and DAP recombination, respectively.
Figure 5 shows the temperature-dependent photoluminescence spectra of ZnxAgInSe QDs recorded at 10-300 K. Notably, with increasing temperature, the PL intensity decrease, the emission spectra red shifted, with line broadening. These results indicate clear changes in the luminescence intensity, peak position, and FWHM.
Fig. 5 Temperature-dependent PL spectra of ZnxAgInSe QDs with different Zn/Ag ratios of (a) 0.3/1; (b) 0.5/1; (c) 1/1.
The temperature-dependent PL intensity was investigated for examining the non-radiative relaxation processes in QDs. Figure 6 shows the PL intensities of the ZnxAgInSe QDs with different compositions versus temperature. With increasing temperature, the PL intensities of all QDs decreased. With the increase in the Zn/Ag ratio, the emission intensity increased. Moreover, for ZnxAgInSe QDs with a 1:1 Zn/Ag ratio, the change in the intensity corresponding to a given change in the temperature was considerably greater than that of other samples with different Zn/Ag ratios, indicative of high-sensitivity temperature sensing. The excited luminescent center is thermally activated via the interactions of phonons interaction, which are released through the cross section between the ground and excited states. The non-radiative transition probability induced by thermal activation leads to the decreased emission intensity [25]. Dotted and solid lines represent the actual measured data and fitted curves obtained by Eq. (2), respectively.
where I(0) is the emission intensity at 0 K, ∆E is the activation energy of the thermal quenching process, A is a constant and κ is the Boltzmann constant. Table 3 summarizes the parameters for ZnxAgInSe QDs. The ΔE of ZnxAgInSe QDs was considerably less than those of ZnCuInS (115meV) and Mn:ZnCuInS (56.4meV) QDs [26,36,37]. With increasing Zn content, ΔE correspondingly increased, indicative of the thermal stability of ZnxAgInSe QDs.Fig. 6 PL intensities of ZnxAgInSe QDS with different Zn/Ag ratios as a function of temperature. The fitted curves (solid lines) are obtained using Eq. (2).
Table 3. Fitted parameters of PL intensities of ZnxAgInSe QDs.
From the temperature-dependent PL spectrum, different peak energies for ZnxAgInSe QDs were observed (Fig. 7). With increasing temperature, the PL peak energy red shifted because of the enhancement of the exciton-phonon coupling and lattice deformation [27]. Experimental data for the energy levels were fitted from Eq. (3). This equation is used to fit to not only several semiconductor materials but also several doped materials, such as Mn:ZnCuInS [37], ZnCuInS [26], Ag2Se QDs [27], EuxSi6−zAlzOzN8−z [38], and ZnAgInS [28], and Table 3 summarizes the results. Those parameters from this equation are related to the internal interactions, e.g., exciton-phonon coupling, in the QDs.
where <hw> is the average phonon energy, S is the Huang−Rhys factor, indicative of the excition transitions to the longitudinal optical (LO) phonon, and kB is the Boltzmann constant. The temperature dependence of the fitting value Eg(0) provided values of 1.816 eV, 1.939 eV, and 1.957 eV, respectively, for different Zn/Ag ratios. These values differ from those of ZnCuInS (2.669 eV) and Mn:ZnCuInS (1.97 eV) [26,36,37]. Compared with the fitting value S for other quaternary QDs, those obtained here are between those reported previously for ZnCuInS (2.1) and Mn:ZnCuInS (0.34) [36,37]. Hence, the relatively lower phonon coupling reflects the stability of ZnxAgInSe QDs compared to other quaternary quantum dots. With increasing Zn/Ag ratio, the Huang-Rhys factor decreased, indicative of the enhancement of the electron-phonon coupling. This enhancement is related to the thermal expansion of the crystalline network resulting from the change in the bandgap energy (Table 4).Fig. 7 Temperature-dependent peak energy of ZnxAgInSe QDs with different Zn/Ag ratios. The fitted curves (full lines) are obtained using Eq. (3).
Table 4. Fitting results from the temperature-dependent peak energy of ZnxAgInSe QDs with different Zn/Ag ratios according to Eq. (3).
With increasing temperature, the FWHM of ZnxAgInSe QDs with different Zn/Ag ratios increased (Fig. 8). At high temperature, the electron–phonon interaction and phonon density increase were dominant from the carrier to the acoustic phonon mode coupling or from a number of Zn/Ag emission electrons and holes in the center of the composite, broadening the FWHM of the emission spectrum for ZnxAgInSe QDs. The experimental data were fitted to obtain a better understanding of the line broadening using Eq. (4). Table 5 summarizes the fitted results. The PL peak broadening was explained by multi factors of inhomogeneity and phonon-exciton and acoustic interactions, which represent homogeneous broadening.
where ΓLO is the intension of carrier-LO-phonon coupling, Γinh represents the inhomogeneous line width, which is independent of temperature, and because of the variation of the quantum dots in shape, size, and composition [37]. ELO is the longitudinal optical (LO) phonon energy, and σ represents the acoustic phonon coupling coefficient of carriers. Consequently, from 10 K to 300 K, the line broadening and line shift were predominantly related to the exciton to acoustic phonon coupling [26]. Compared to quaternary ZnCuInS QDs [31], a different change trend in these fitting values for the parameters was observed, possibly related to different and nonuniform particle sizes of the QD material. The main factors related to the spectral line broadening possibly originate from the carrier to the acoustic phonon mode coupling or from some type of emission electrons and holes in the composite center. With increasing Zn precursor content, ELO decrease. In addition, with increasing Zn/Ag ratio from 0.3 to 1, the ΓLO of ZnxAgInSe QDs decreased from 187.7 meV to 104.7 meV.Fig. 8 Temperature-dependent FWHM of the PL spectra for ZnxAgInSe QDs with different Zn/Ag ratios. The fitted curves (solid lines) were obtained using Eq. (4).
Table 5. Fitting parameters of temperature-dependent FWHM for ZnxAgInSe QDs.
To further visualize the temperature-sensing capability of ZnxAgInSe QDs, unpackaged ZnxAgInSe QDs (x = 1) were used to fabricate QD-based films. Meanwhile, the PL spectra were recorded at 230 K and 270 K, and the peak energy and FWHM were calculated [Fig. 9(a)]. Comparing the parameters with the fitting curves shown in Fig. 7 and Fig. 8, the peak energies of the ZnxAgInSe were 1.928 and 1.913 corresponding to the temperature of 230.9 K and 269.5K, respectively. In addition, their FWHMs at 231 K and 272 K were 0.394 and 0.396, respectively. An average temperature of 270.75 K between the peak energy and FWHM was obtained. Figure 9(b) shows the thermal imaging photograph at 270 K, corresponding to an actual temperature of 270.7K, thus, an extremely small deviation is observed. The ratio of these two bands varied, which is typically related to the enhancement of non-radiative relaxation, imparting the characteristic of temperature sensing. The dependence of the peak energy and FWHM on the temperature can be theoretically estimated. These results confirmed that the ZnxAgInSe system exhibits good temperature-sensing ability.
Fig. 9 (a) PL spectra of ZnxAgInSe at different temperatures. (b) Thermal imaging photograph of ZnxAgInSe QDs with a 1/1 Zn/Ag ratio at 270 K.
4. Conclusions
In this study, ZnxAgInSe QDs with different compositions were successfully synthesized. By changing the Zn/Ag ratio, the luminescence properties of ZnxAgInSe QDs were successfully tuned. With increasing Zn/Ag ratio from 0.3 to 1, the photoluminescence (PL) blue shifted from 687 nm to 619 nm. By time-resolved PL measurements, the luminescence mechanism of ZnxAgInSe QDs results from three processes, i.e., conduction-band level to a localized intra-gap level, surface-related recombination, and donor-acceptor recombination, respectively. Furthermore, the application of ZnxAgInSe QDs for temperature sensing was examined. The deviation of the measured temperature from the calculated temperature was as low as 0.05 K, indicative of the good temperature-sensing ability of ZnxAgInSe QDs.
Funding
National High Technology Research and Development Program of China (863 Program) (No. 2013AA014201); the National Key Foundation for Exploring Scientific Instrument of China (No. 2014YQ120351); Natural Science Foundation of Tianjin (No. 11JCYBJC00300, 4JCZDJC31200, 15JCYBJC16700 and 15JCYBJC16800).
Acknowledgments
The author is also heartily grateful to Dr. Yang Li, School of Engineering and Applied Sciences, Harvard University, for his valuable discussions and advice for the data analyses.
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