⇦Chinese Physics Letters, 2017, Vol. 34, No. 4, Article code 047801 Photoluminescence Characteristics of ZnCuInS-ZnS Core-Shell Semiconductor Nanocrystals * Qiu-Lin Zhong(钟秋霖), Ming-Rui Tan(谭铭瑞), Qing-Hui Liu(刘庆辉)**, Ning Sui(隋宁), Ke Bi(毕克), Mou-Cui Ni(倪牟翠)**, Ying-Hui Wang(王英惠), Han-Zhuang Zhang(张汉壮) Affiliations Key Laboratory of Physics and Technology for Advanced Batteries (Ministry of Education), College of Physics, Jilin University, Changchun 130012 Received 3 December 2016 *Supported by the National Natural Science Foundation of China under Grant Nos 21573094, 11274142, 11474131 and 51502109, and the China Postdoctoral Science Foundation Funded Project under Grant Nos 2011M500927 and 2013T60319.
**Corresponding author. Email: liuqinghui@jlu.edu.cn; nimc@jlu.edu.cn Citation Text: Zhong Q L, Tan M R, Liu Q H, Sui N and Bi K et al 2017 Chin. Phys. Lett. 34 047801 Abstract The photoluminescence (PL) characteristics of ZnCuInS quantum dots (QDs) with varying ZnS shell thicknesses of 0, 0.5, and 1.5 layers are investigated systemically by time-correlated single-photon counting measurements and temperature-dependent PL measurements. The results show that a ZnS shell thickness of 1.5 layers can effectively improve the PL quantum yield in one order of magnitude by depressing the surface trapping states of the core ZnCuInS QDs at room temperature. However, the PL measurements at the elevated temperature reveal that the core-shell nanocrystals remain temperature-sensitive with respect to their relatively thin shells. The temperature sensitivity of these small-sized single-layered core-shell nanocrystals may find applications as effective thermometers for the in vivo detection of biological reactions within cells. DOI:10.1088/0256-307X/34/4/047801 PACS:78.55.-m, 68.65.Hb © 2017 Chinese Physics Society Article Text Colloidal semiconductor quantum dots (QDs) have attracted worldwide considerable interests due to their excellent optical properties and potential applications in light-emitting diodes,[1,2] optical gain media,[3,4] and solar-harvesting devices.[5,6] However, the toxicity of conventional Cd-based QDs present potential hazards to the environment and restrict their potential bio-medical applications in living cells. During the past ten years, heavy-metal-free I–III–VI semiconductor QDs, such as CuInS$_{2}$, have emerged as promising alternatives.[7,8] In addition to their high absorption efficiency, these QDs simultaneously provide a much wider photoluminescence (PL) tunability across the visible to near-infrared (NIR) region by adjusting their size and composition. Note that I–III–VI QDs like CuInS$_2$ suffer from a relatively low photoluminescence (PL) quantum yield (PL-QY) owing to the existence of surface trapping states. One possible strategy to improve the PL-QY is to passivate the surface trapping states of these QDs by depositing a few layers of a wider bandgap semiconductor material, such as ZnS, on the QD surface, resulting in those denoted as a type-I core-shell structure.[9,10] These nanocrystal (NC) structures can enhance the PL-QY, while the important PL properties of these QDs still remain unclear, which may hinder the further development of these core-shell NCs themselves, and limit their performance in optoelectronic devices. In this work, ZnCuInS QDs with ZnS shell thicknesses of 0, 0.5, and 1.5 layers are synthesized, and their PL properties are measured and discussed in detail. The role of the shell thickness in exciton dissociation is identified based on time- and temperature-dependent PL data. The synthesis of ZnCuInS QDs with different ZnS shell thicknesses was conducted according to a previously reported one-pot method with minor modifications.[11] Firstly, 0.05 mmol of copper acetate, 0.05 mmol of indium acetate, and 0.1 mmol of zinc acetate were dissolved in 1.0 mL of n-dodecanethiol together with 0.5 mL of oleic acid and 5.0 mL of 1-octadecene in a 50 mL three necked flask. The mixture was degassed under vacuum for 30 min and was purged with argon three times at 120$^{\circ}\!$C. The flask was then heated to 180$^{\circ}\!$C until a clear solution was formed, and then a sulfur solution (1.0 mmol) dissolved in 1.0 mL oleylamine was quickly injected into the reaction solution. After waiting for 20 min, 1.0 mL of the solution was extracted as sample S1. Then, 1.0 mL of 0.2 mM zinc acetate dissolved in 0.2 mL oleic acid and 0.8 mL octadecene was injected into the reaction solution. Samples S2 and S3 were extracted after waiting for 10 and 30 min, respectively. All of these samples were purified by precipitation and centrifugation with toluene and methanol three times to remove any byproducts. Steady-state absorption measurements were conducted using an ultraviolet-visible (UV-Vis) spectrophotometer (TU-1810PC, Purkinje General). Steady PL spectra were recorded by a fiber optic spectrometer (USB4000, Ocean Optics) using a continuous laser with an excitation wavelength of 400 nm. Time-correlated single-photon counting (TCSPC) measurements were conducted using a fluorescence spectrometer (mini-$\tau$, Edinburgh Photonics) equipped with an EPL405 laser diode. The NCs were dissolved into toluene for all tests conducted. Transmission electron microscopy (TEM) studies were conducted using an FEI Tecnai G2 F20 operated at 200 kV. Samples for the TEM study were prepared by placing a 4 μL drop of NCs in toluene solution on an ultrathin carbon-film-coated copper grid, and allowed to dry. The average particle sizes of the as-obtained NCs are indicated by the TEM images shown in the insets of Fig. 1, and are estimated to be 3.5, 3.7, and 4.2 nm for samples S1, S2, and S3, respectively. Therefore, the ZnS shell thicknesses of S2 and S3 are 0.2 and 0.7 nm, which are ascribed to 0.5 and 1.5 layers of ZnS, respectively. In addition, the steady absorption spectra shown in Fig. 1 exhibit no obvious excitonic absorption peaks, which is a typical characteristic of I–III–VI core-shell NCs.[12] All absorption edges of the core and core-shell NCs are around 570 nm, regardless of the shell thickness, indicating that the absorption of the NCs is mainly attributable to the ZnCuInS core.
cpl-34-4-047801-fig1.png
Fig. 1. Absorption (in black) and photoluminescence (in red) spectra of ZnCuInS core quantum dots (S1) and ZnCuInS-ZnS core-shell quantum dots (S2 and S3) with ZnS shell thicknesses ascribed as 0.5 and 1.5 layers, respectively. The mean diameters of the samples are 3.5 nm (S1), 3.7 nm (S2), and 4.2 nm (S3), respectively. The scale bars in the transmission electron microscopy images given as insets are all 5 nm.
After the introduction of a shell layer, the PL-QY of core-shell NCs are typically enhanced by about one order of magnitude compared with the PL-QY of the naked cores. We found that the PL intensities of S2 and S3 obviously increased by about 3.0% and 16.4% relative to S1. With further increasing the ZnS shell thickness, the PL-QY remains stable at around 35%. It is known that the bandgaps of bulk ZnS and ZnCuInS are 3.70 eV and 1.50 eV, respectively. The much larger and wider bandgap makes ZnS a very suitable shell material candidate for type-I core-shell QDs. After the introduction of the shell layers, the PL QY of core-shell QDs could usually be enhanced nearly by one order compared with the corresponding naked cores. In our cases, the PL intensity of Zn-Cu-In-S/ZnS QDs (S2 and S3) obviously increases from about 3.0% to 16.4%, more than 5 times, even with half of one shell layer introduction. As the ZnS shell thickness further increases, the PL yield is stable at around 35%. It should be NCs. However, it is noted that, after the deposition of ZnS deposits onto the core surface, it is usually accompanied with an alloying process. This means that part of ZnS will likely incorporate and penetrate, where a portion of the ZnS penetrates into the inner crystal lattice of the core nanocrystals, leading to an increase of their bandgaps, which is clearly indicated in Fig. 1 according to the obvious blue shift in the PL peak positions from 633 nm (S1) to 617 nm (S2) in the absorption spectrum. After the deposition of another layer of ZnS was deposited, a red shift in the PL peak could be evidenced with the sample of S3. According to the spectral data, it is confirmed that the ZnCuInS QD cores are responsible for the observed PL, but the incorporation of ZnS shell also affects the PL intensity of the core (in the core/shell quantum dots), since a part of ZnS incorporates into the crystal lattice of the core nanocrystals. To further evaluate the PL characteristics of the fabricated NCs, the wavelength-dependent PL decay of the ZnCuInS QDs was investigated, as presented in Fig. 2(a). Here the rate of PL decay is found to be sensitive to the PL wavelength, where the decay rate gradually decreases with increasing the PL wavelength. This indicates that the PL process of the QDs does not represent simple exciton PL, and other mechanisms in addition to pristine exciton PL are involved. This is attributable to the existence of trapping states. Therefore, it is impossible to fit the PL decay using a simple multi-exponential function. Considering the complexity of the PL relaxation process of ZnCuInS QDs, the PL decay was fitted with a continuous decay rate distribution function[13] as follows: $$\begin{align} I(t)=I(0)\int_{\gamma =0}^\infty {\varphi (\gamma )} \exp (-\gamma t)d\gamma,~~ \tag {1} \end{align} $$ where $I(t)$ is the fluorescence intensity as a function of time $t$, and $\varphi(\gamma)$ describes an emitter concentration distribution for emitters with a given decay rate $\gamma$ (ns$^{-1}$), weighted by the corresponding $\gamma_{\rm rad}$.[14] The distribution $\varphi(\gamma)$ is appropriately given as a log-normal distribution function $$\begin{align} \varphi (\gamma)=A\exp(-\ln(\gamma/\gamma _{_{\rm MF}})/w^2),~~ \tag {2} \end{align} $$ where $\gamma _{_{\rm MF}}$ is the most likely decay rate corresponding to the maximum value of $\varphi(\gamma)$, $w$ is a dimensionless width parameter that determines the distribution width $(\Delta\gamma )$ at $1/e$, and $A$ is the normalization constant, ensuring that $\int_{0}^{\infty}\varphi(\gamma)d\gamma=1$. Meanwhile, $\Delta \gamma$ can be calculated as $$\begin{align} \Delta \gamma=2\gamma _{_{\rm MF}}\sinh w.~~ \tag {3} \end{align} $$ As indicated in the inset of Fig. 2(a), $\gamma _{_{\rm MF}}$ are 0.694 (570 nm), 0.677 (590 nm), 0.572 (620 nm), and 0.411 (650 nm) ns$^{-1}$, respectively. In addition, it is also observed that the value of $\Delta \gamma$ gradually decreases with decreasing the PL wavelength. This indicates that the PL mechanisms operative at shorter wavelengths are closer to the pristine exciton PL in comparison with those mechanisms operative at longer wavelengths. Moreover, the integrated PL decays of S1, S2, and S3 are presented in Fig. 2(b). Considering the complexity of the relaxation process of the NCs, the PL decay behaviors were also fitted with a continuous distribution function. The fitted results show that $\gamma _{_{\rm MF}}$ are 0.572 (S3), 0.454 (S2), and 0.367 (S1) ns$^{-1}$, respectively. Assuming that the relaxation rate owing to the effect of trapping states is less than that originating from the pristine exciton relaxation, the values of $\gamma_{\rm MF}$ decreased due to the passivation of the trapping states by the shell, which reduced the extent of relaxation owing to trapping states.
cpl-34-4-047801-fig2.png
Fig. 2. (a) Wavelength-dependent PL decay of ZnCuInS QDs (S1). (b) The wavelength-integrated PL decays of S1, S2, and S3. The insets show the emitter concentration distributions $\varphi(\gamma)$ for emitters with a given decay rate $\gamma$ (ns$^{-1}$), where the values of $\gamma$ at the maximum $\varphi(\gamma)$ for the NCs are 0.694 (570 nm), 0.677 (590 nm), 0.572 (620 nm) and 0.411 (650 nm) ns$^{-1}$ (a), and 0.572 (S3), 0.454 (S2), and 0.367 (S1) ns$^{-1}$ (b).
To further investigate the impact of the shell thickness on the PL characteristics of ZnCuInS-ZnS NCs, we also conducted the PL measurements as a function of the temperature in the range of 299–393 K for S2 and S3. As shown in Fig. 3(a), the PL intensity of S3 gradually decreases as the temperature increases, and is accompanied by a red-shift in the PL peak position, which is similar to the behavior of CdSe QDs.[15] This declining tendency in the PL intensity can be attributed to the increased proportion of non-radiative relaxation relative to radiative relaxation as the temperature increases. Non-radiative relaxation was assisted by photon–electron and longitudinal optical (LO) phonon coupling.[16]
cpl-34-4-047801-fig3.png
Fig. 3. (a) Temperature-dependent PL spectra of ZnCuInSZnS NCs (S3). Temperature-dependent (b) integrated PL intensity (IPLI) and (c) PL peak (PLP) of ZnCuInSZnS NCs (S2 and S3) in an aggregation state.
The integrated PL intensity (IPLI) values as a function of the temperature for S2 and S3 are compared in Fig. 3(b). As can be seen from Fig. 3(b), the IPLI of S3 decreases more rapidly with respect to the temperature than that of S2. To quantitatively evaluate the role of shell thickness in the IPLI with respect to the absolute temperature $T$, the data in Fig. 3(b) were fit to the expression $$\begin{align} I(T)=I(0)[1+C\exp(-E_{\rm a}/k_{\rm B}T)]^{-1},~~ \tag {4} \end{align} $$ where $I_{0}$ is the IPLI at 0 K, $C$ is a fitting constant, $E_{\rm a}$ is the activation energy, and $k_{\rm B}$ is Boltzmann's constant. From the fitted parameters, $E_{\rm a}$ is determined to be 331 and 350 meV for S2 and S3, respectively. The small difference of 19 meV between the two samples indicates that the NCs exhibit a similar declining tendency, but with a more sensitive response to the elevated temperature for S3. However, we note that these core-shell structured NCs exhibit a much stronger PL than their corresponding naked QDs. The PL peak (PLP) values as a function of the temperature for S2 and S3 are plotted in Fig. 3(c). The observed phenomenon is expected to be caused by changes in the relative positions of the conduction and valence bands that result from the dilatation of the lattice owing to the incorporation of atoms from the shell into the core, and from the interactions of electrons with the lattice.[17] The difference in shell thickness could influence the role of temperature on variance of the lattice. The temperature dependence of the experimental PLM of ZnCuInS-ZnS NCs with different shell thicknesses can be modeled using the Varshni relation$^{16}$ $$\begin{align} E_{\rm PLM}(T)=E_{\rm PLM}(0)-\alpha T^2(T+\beta)^{-1},~~ \tag {5} \end{align} $$ where $E_{\rm PLM}(T)$ is the emission maximum, $E_{\rm PLM}(0)$ is the PLM at 0 K, $\alpha$ is a temperature coefficient, and $\beta$ is approximately the Debye temperature of the semi-conductor. Equation (5) is based on the temperature-dependent dilatation of the core crystal lattice and the interactions between lattice phonons and excitons.[18] The parameters of Eq. (5) were extracted from S2 and S3, and the results are listed in Table 1. Although the introduction of a ZnS shell affects the PL properties of QDs, the PL of QDs is also sensitive to the temperature. In other words, the ZnS shell improves the PL properties of QDs and maintains their temperature sensitivity.
Table 1. The fitted results for samples S2 and S3 obtained from the Varshni function (Eq. (5)).
Sample EPLM (eV) $\alpha$ (10$^{-4}$eV/K) $\beta$ (K)
S2 2.05 3.12$\pm$0.06 119$\pm$0.8
S3 2.07 5.33$\pm$0.12 148$\pm$1.2
In conclusion, the PL properties of ZnCuInS NCs with ZnS shell thicknesses of 0, 0.5, and 1.5 layers have been investigated by careful comparison. The results indicate that the ZnS shell not only suppresses the surface trapping states, but also restrains electron-lattice interactions and reduces non-radiative transitions at room temperature. These findings are expected to advance the present state of understanding regarding the related applications of I–III–VI core-shell NCs to optoelectronic devices under high-temperature working conditions. Moreover, these small-sized single-layered core-shell NCs (less than 5.0 nm in diameter) could provide more sensitive thermometers for the in vivo detection of biological reactions in living cells. References Light-emitting diodes made from cadmium selenide nanocrystals and a semiconducting polymerEfficient Near-Infrared Polymer Nanocrystal Light-Emitting DiodesOptical Gain and Stimulated Emission in Nanocrystal Quantum DotsLasing from Semiconductor Quantum Rods in a Cylindrical MicrocavityQuantum Dot Solar Cells. Harvesting Light Energy with CdSe Nanocrystals Molecularly Linked to Mesoscopic TiO 2 FilmsTunable Photoluminescence Wavelength of Chalcopyrite CuInS 2 -Based Semiconductor Nanocrystals Synthesized in a Colloidal SystemPhase-Selective Synthesis of CuInS 2 NanocrystalsFacile Synthesis of ZnS−CuInS 2 -Alloyed Nanocrystals for a Color-Tunable Fluorchrome and PhotocatalystNear-Band-Edge Electroluminescence from Heavy-Metal-Free Colloidal Quantum DotsA Simple Route for Highly Luminescent Quaternary Cu-Zn-In-S Nanocrystal EmittersEmploying Heavy Metal-Free Colloidal Quantum Dots in Solution-Processed White Light-Emitting DiodesStrongly nonexponential time-resolved fluorescence of quantum-dot ensembles in three-dimensional photonic crystalsStatistical analysis of time-resolved emission from ensembles of semiconductor quantum dots: Interpretation of exponential decay modelsControlled Synthesis and Optical Properties of Colloidal Ternary Chalcogenide CuInS 2 NanocrystalsTemperature dependence of the photoluminescence properties of colloidal Cd Se ∕ Zn S core/shell quantum dots embedded in a polystyrene matrixTemperature dependence of the energy gap in semiconductorsTemperature Dependence of the Energy Gap in Semiconductors
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