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Abstract
Transition metal doped quantum dots (d-dots) have attracted much attention owing to the high emission efficiency of the dopant ions together with the large Stocks shifts which can overcome the self-absorption issues. To date, most works focus on improving the optical properties by developing new synthetic routes. However, the integration of these luminescent materials on stretchable substrates is rarely involved. Here, we report the synthesis of stretchable luminescent silica gel-ZnSe:Mn/ZnS films. The as-prepared ZnSe:Mn/ZnS quantum dots (QDs) show a Stocks shift as large as 180 nm and a photoluminescence (PL) quantum yield (QY) as high as 61%. The potential application of the stretchable silica gel-QD films is explored. The prominent properties of the proposed silica gel-QD materials, including their impressive flexibility and highly bright emission suggest that they are promising candidates for smart optoelectronic devices due to their ability of being stretched into arbitrary shapes.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Quantum dots have attracted much interest in opto-electronics, owing to their intensive absorption, solution processability, highly bright emission and size dependent band gaps [1–8]. Although II−VI Cd-based quantum dots (QDs) have achieved continuous success in obtaining excellent optical properties, they are potentially toxic to the environment. Besides, most of the Cd-based QDs have strong self-quenching effects due to their small ensemble Stokes shift [9–12]. This leads to a notable decrease of their emission efficiency. These demerits of undoped quantum dots may hinder their wide applications in light-emitting diodes (LEDs), solid-state lighting, beads-based bar-coding or other circumstances that require high power or close packed QDs [13–15].
Introducing dopants into the QDs (d-dots) has proved an effective strategy for modifying inherent properties of QDs. For an undoped QDs, photons are released by the recombination of exciton with its energy near the absorption band edge, i.e., the so called band-edge emission [16]. The spectral overlap between the absorption and emission is often large, thereby causing strong self-absorption and finally quenching the emission. Unlike those undoped QDs, the emission of d-dots originates from the d-shell orbitals of the doped ions whereas their absorptive features depend more on the host materials. The emission energy gap of the doped ions is noticeably lower than the absorption band gap of the host materials. Therefore, d-dots can present large Stocks shifts. This feature offers great potentials for the applications where high packed QDs are required owing to their merits of suppressing the energy transfer and reabsorption. Moreover, the emission of d-dots have low sensitivity to the changing of the environments and are highly thermally stable, because the emission depends on the atomic-like states of the dopant rather than the exciton delocalization. Owing to these merits, d-dots have become important new class of materials which are good complements to those undoped QDs.
Currently, several dopants are used to modify the optical properties of QDs, including Cu, Mn, Ni and Ag ions [12, 17–21]. Among these, Mn-doped zinc chalcogenides (ZnS and ZnSe) are regarded as promising materials, because the electronic and photophysical properties of QDs can be effectively modified by Mn dopants. For instance, the excitonic states of transition metal Mn2+ locate within the band gap of the host ZnS or ZnSe semiconductor. Therefore, the dopant emission is unaffected by the energy level of the host material, maintaining pure orange spectral window from its intrinsic transition 4T1-6A1. It has been observed that the emission wavelength of the Mn2+ doped QDs is unchanged with the growth time, indicating their independence of the emission with respect to the host materials [22]. In the last decades, continuous successes have been achieved in Mn doped QDs. Using a nucleation doping synthetic route, Peng et al achieved a PL quantum yield of about 50% with a good stability [20, 23]. Later, Zhong et al developed a generic starting materials for the nucleation doping strategy, and further improved the PL QY of the ZnS:Mn d-dots to 55-65% [24]. Recently, Peng et al proposed a reactive Se precursor with a high relative concentration strategy to overcome the reactivity difference of Zn2+ stearate and Mn2+ stearate, which proved an effective way to suppress the separate nucleation. As a result, ZnSe:Mn d-dots showed a nearly single-exponential photoluminescence (PL) decays with extremely long PL lifetimes (up to 1000 us) and a notable PL QY of 70% [25]. Nowadays, most of the efforts are paid in developing new synthetic routes for high quality Mn based d-dots or their applications as emitters. However, the integration of the luminescent materials on stretchable substrates are becoming a more widely discussed topic, because they are promising for further stretchable opto-electronic devices owing to their ability of being stretched into arbitrary shapes [26]. Unfortunately, few reports focus on stretchable devices with Mn based d-dots. Partially because incorporating the d-dots into a matrix may cause agglomeration of the QDs due to improper surface modification, thus leading to a transparency decline. Moreover, free radical species from polymer matrix may also oxidize the surfactant of the QDs, producing surface defect states. As a result, the PL intensity of the d-dots is greatly quenched [27, 28]. Recently, Xu et al. have reported ZnSe:Mn/poly(LMA-co-EGDMA) composite films with high transmittance and remarkable PL QY, owing to the function of the surface decorated 1-dodecanethiol (DDT) in reducing the surface defects, resisting oxidation and enhancing the dispersity [29]. Unlike their efforts on modifying the polymer matrix, we focus on a ZnS shell onto the ZnS:Mn core, aiming to achieve surface passivation. Recent works reveal that the ZnS layer can passivate the surface of the QD core by reducing the dangling bonds [30]. It also prevents the QD core from being oxidized due to its screening effect from the wide band gap energy [31, 32].
In this work, we prepared highly bright ZnSe:Mn/ZnS QDs using the nucleation doping method. By optimizing the experimental conditions, the band edge emission of the ZnSe is greatly suppressed and the as-prepared QDs exhibited pure emission from Mn2+. Besides, these d-dots exhibit a large Stocks shift of about 180 nm, implying that the self-absorption effect is greatly reduced [20, 33]. Furthermore, we explore the integration of QDs in the silica gel to form a flexible film and explored their potential applications for anticounterfeiting. The PL intensity is sensitive to the concentration of the d-dots in the film under fixed excitation power. Such unicolor luminescent feature holds promise in the field of optical data storage or advanced anticounterfeiting.
2. Experimental
2.1 Materials
Zinc stearate (ZnSt2, 10-12% Zn), selenium powder (∼100 mesh, 99.999%), Zinc acetate (10-12%Zn), manganese(Π) distearate (MnSt2,), oleic acid (OA, Sigma, Aldrich, 90%), oleylamine (OAm, Across Organics, 80-90%), 1-octodecene (ODE, 90%), 1-dodecanethiol (DDT, 98%) were purchased from Macklin. Transparent silicone resin (YD65-5A, YD65-5B) was purchased from Dow Corning. All chemicals were used without further purification.
2.2 Synthesis of the ZnSe:Mn core
ODE (10 mL), 0.032 g of MnSt2 and 0.1mmol Se were loaded into a 50 mL three neck flask and degassed at 100 °C for 20 min by bubbling with argon. The temperature was then heated to 300 °C. In a separate vial, Zn(AC)2 (2.5mmol), 7 mL of ODE and 3ml of oleic acid solution were mixed and heated to 100 °C for 30 min and then further heated to 160 °C under bubbling with argon. A oleylamine solution (0.6 ml oleylamine) was swiftly injected into the main reaction mixture at 300 °C and kept for 5 min. The color of the resulting solution slightly turned yellowish and intensified with the progress of the reaction. Then the reaction mixture was swiftly cooled to 200 °C followed by injecting a solution of 0.8 ml Zn(AC)2. After 10 min, increased the temperature to the 240 °C and injected more ZnSt2. ZnSt2 solution was injected in three steps with 10 min intervals.
2.3 Synthesis of the ZnSe:Mn/ZnS QDs
ZnS solution was prepared by mixing Zinc acetate (2mmol, 0.367g), 1 ml of DDT, 2 ml of OA and 3 ml of ODE. ZnS stock solution was heated to 100 °C for 30 min to dissolve. The gradually turning red color of the solution indicated the growth of the ZnSe:Mn cores. Then the shell stock solution was added dropwise into the core crude solution by a 10-ml syringe at 230 °C. Then the mixture was heated to 240 °C and maintained at that temperature for optimum shell overcoating for 20-30 min. After that, the synthesized ZnSe:Mn/ZnS QDs were dissolved into the hexane and purified repeatedly with ethanol by a centrifugation (8000 rpm, 5 min). The supernatant was discarded after centrifugation. In order to achieve pure QDs, the above procedures should be repeated more than three times. Finally, pure QDs were obtained and redispersed in hexane.
2.4 Synthesis of the ZnSe:Mn/ZnS quantum dot-silica gel nanocomposites
The transparent silicone resin (YD65-5A, YD65-5B, 1:1 volume ratio) containing a certain concentration of the QDs was dropwised into the mould and then dried at 150 °C to form the rectangle-shaped silica gel-QD nanocomposites. The concentration of ZnSe:Mn/ZnS QDs of each rectangle-shaped pattern was 0%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, respectively. The silica gel-QD was coated onto the surface of the matrix when the rectangle-shaped patterns were solidified. The film can be obtained after drying at 150 °C. Finally, the rectangle-shaped patterns of silica gel-QD compounds were obtained.
2.5 Instruments
The material properties were characterized by X-ray diffractometer (Rigaku MiniFlex II X-ray diffractometer), high-resolution transmission electron microscopy (JEM-2010, JEOL Ltd.), UV-vis spectrophotometer (Cary 300, Varian), steady-state photoluminescence spectrophotometer (Cary Eclipse, Varian) and fiber optic spectrometer (Ava Spec −2048TEC-USB2, 0.5nm, FIB-Y-200-L(2)-UV).
3. Results and discussion
3.1 Structural characterizations
The QDs are prepared using a nucleation-doped synthesis process according to previous report [25].The scheme of the synthesis of the ZnSe:Mn/ZnS QDs is shown in Fig. 1. Briefly, the MnSe nanoclusters are first prepared using precursors of Se powder and Stearic acid manganese to provide Mn source. Then a ZnSe shell is coated onto the MnSe core. Due to the self-purification mechanism of the MnSe, the Mn2+ ions will diffuse into the ZnSe lattice spontaneously. At the high temperature, the Mn2+ ions finally spread outward to the whole ZnSe lattice along with the shelling process. The pure MnSe core cannot emit any light, but with the proceeding of the diffusion process, bright orange emission emerges. It is worth mentioning that during the MnSe core synthesis the temperature is 300 °C which is much higher than ZnSe shelling temperature 200 °C. This notable temperature difference will create the thermal insulation at the interface which prompts Mn2+ ions penetrate into the ZnSe shell, as shown in Fig. 1. In order to enhance emission properties of the ZnSe:Mn QDs, a ZnS shell process is introduced to passivate the surface of the ZnSe:Mn QDs [25, 30]. X-ray diffraction (XRD) were performed to characterize the structures of ZnSe:Mn and ZnSe:Mn/ZnS QDs, as shown in Fig. 2(a). The X-ray powder diffraction pattern of the ZnSe:Mn QDs shows three obvious diffraction peaks located at 27.0°, 45.5°, and 53.2°, which are indexed to the (111), (220), and (311) planes of zinc blende phase of ZnSe (JCPDS no.37-1463). Such appearance in the diffraction patterns are consistent with the previous reports as well [34]. After introducing a ZnS shell, the peaks shift to larger angles. Given the large volume fraction of ZnS, this shift can be reasonably ascribed to the shell-induced lattice compression on the ZnSe host. Besides, the characteristic peaks of the (111), (220), and (311) planes become broader with the growth of ZnS shell. This broadening phenomenon is consistent with overlying of diffraction peaks of ZnSe and ZnS [23, 34, 35]. This implies that the ZnS is well shelled onto the surface of the core, thus forming a core/shell heterostructure rather than an alloy compound, and the shell dominates the contributions to the XRD patterns. In order to further demonstrate the structure of ZnSe:Mn/ZnS QDs, the transmission electron microscopy (TEM), high resolution TEM (HR-TEM) images and electron diffraction pattern were carried out, and shown in Figs. 2(b)-2(d). The QDs have well-resolved lattice fringes, implying their good crystallinity. The diameter distributions of ZnSe:Mn/ZnS QD are estimated from a statistic calculation by using at least 10 TEM images. The diameter of ZnSe:Mn/ZnS falls within a narrow nanometer domain with an average diameter of 5.0 nm. Besides, the lattice constants are calculated from the diffraction patterns match well with parameters of the (111), (220) and (311) planes of ZnSe:Mn/Zn from a selected area electron diffraction (SAED) pattern .
Fig. 2 (a) XRD patterns of as-prepared ZnSe:Mn, ZnSe:Mn/ZnS QDs. TEM (b) and HR-TEM (c) images of ZnSe:Mn/ZnS QDs. (d) SAED pattern of ZnSe:Mn/ZnS QDs.
3.2 Optical properties
The wide band gap ZnS shell has already proved an effective material to reduce the dangling bonds and surface traps and enhance the emission efficiency of the QDs [25]. The PL QY of ZnSe:Mn QDs is only 22% due to the surface defects of the nanocrystals [22]. However, after ZnS coating, the PL quantum yield of the ZnSe:Mn/ZnS d-dots reaches 70% at different dopant concentrations. As shown in Fig. 3(a), the PL intensities are significantly enhanced after ZnS shelling. It is worth mentioning that the PL data of the three QDs were recorded at the same excitation conditions with the same absorptive strengths. After ZnS shell coating, the PL intensity herein is enhanced almost by 2 folds as compared to unshelled ZnSe:Mn QDs, implying that radiative recombinations occur in an efficient way. Besides, the outer ZnS shell also helps in improving the stability of the QDs by creating a large physical barrier to prevent the photooxidation. Figure 3(b) shows that the PL intensity of ZnSe:Mn/ZnS decreases less than 5% after 2 days, which is much lower than that of ZnSe:Mn (over 80%), implying a better stability of the ZnSe:Mn/ZnS QDs [36]. To determine the exact quantum yield of quantum dots, absolute PL QYs were tested. The PL QY of ZnSe:Mn/ZnS decreased by less than 10% (from 61% to 55%) in 30 hours. However, the PL QY of ZnSe:Mn decreased from 33% to 5%, over 80% to the initial value. These results agree well with the PL experiments. It is known that the Mn ions will spontaneously diffuse outward in ZnSe. This effect will cause the loss of the emissive Mn centers within the host lattice and the decrease of the PL QY of the d-dots [23, 37]. Several recent reports show that the PL QYs of the d-dots are highly sensitive to the diffusion of Mn ions. However, the after introducing a ZnS shell the diffusion induced defects can be suppressed owing the form of an alloy structure by consuming the escaped Mn ions [30]. It is interesting that the emission wavelength of the ZnSe:Mn/ZnS QDs can also be tuned by ZnS shelling, ranging from 580 to 600 nm. Evidence shows that alloying ZnSe and ZnS will induce a blue shift because of the S element. The red shift from 580 to 600 nm agrees well with those reported values in related systems [30, 38].All the QDs show a single emission peak at ~600 nm and no other peaks are observed. This indicates that the energy transfer from ZnSe to the emitting center Mn2+ is efficient, thus suppressing the exitonic emission of the ZnSe. The Stoke shift of the ZnSe:Mn/ZnS is as large as 180 nm, which is promising to overcome the self-absorption issue [39]. Figure 3(c) shows that the PL intensity of the ZnSe:Mn/ZnS are sensitive to the concentration of the QDs. Besides, the as-prepared QDs also shows a single emission peak and an unchanged emission peak despite different QD concentration because of the great suppress in the emission of the host ZnSe material. These advantages prompt us to explore their potential application for encoding. In order to interpret the inner emission mechanism of the ZnSe:Mn/ZnS QDs, a schematic energy diagram is given to illustrate the energy transfer process and the dopant emission as shown in Fig. 3(d). Excited by ultraviolet photons, the undoped ZnSe quantum dots are promoted to excited state from the ground state. After that, the electrons and holes recombine and emit photons. In contrast, in the Mn2+ doped ZnSe the energy is first absorbed by the ZnSe host and subsequently transferred to the Mn2+ 4T1 level instead of dropping straight to the ground state, because of the matched energy levels of the Mn2+ and ZnSe energy levels. Then, through radiative relaxations, the electrons drop to the Mn2+ 6A1 state, accompanying with red emission at 580 nm [24].The ZnS outer shell deals with redundant surface and the dangling bonds of the ZnSe surface, which helps suppressing the harmful energy transfer channels and guarantees an efficient energy transfer from ZnSe host to Mn2+ ions. Evidence shows that the intrinsic energy levels of Mn2+ ions are almost unaffected by the outer ZnS shell [39, 40]. Therefore, the emission intensity of the ZnSe:Mn/ZnS is strengthened greatly without notable wavelength variations.
Fig. 3 (a) PL and absorption of the ZnSe:Mn and ZnSe:Mn/ZnS QDs. (b) Stability of the QDs. (c) PL and absorption spectra of ZnSe:Mn/ZnS QD solutions at different concentrations. (d) Schematic illustration of energy levels and the energy transfer processes of the QDs.
3.3 Encoding using stretchable silica gel-ZnSe:Mn/ZnS films
Figure 4(a) illustrates the schematic procedures of fabricating the stretchable silica gel grafted ZnSe:Mn/ZnS QD films. After removing the air bubbles and drying, high quality silica gel-QD films are obtained [26]. As shown in Fig. 4(b), the silica gel-QD film shows high transparency under the sunlight. Under the irradiation of a UV lamp, the as-prepared silica gel-QD film keeps bright red emission although it is intensively twisted. It is worth noting that the PL intensities of the silica gel-QD films are quite sensitive to the concentration of the QDs as shown in Fig. 4(c). With increasing concentration of QDs, the emission intensities are dramatically enhanced. Besides, the PL intensities also show linear relationship with the thicknesses of the silica gel-QD films, as shown in Figs. 4(d) and 4(e). Inspired by the direct relationship between the QD concentration, the thickness of the film and the PL intensity, we intend to use the as-prepared silica gel-QD rectangles to fabricate emitting codes.
Fig. 4 (a) Schematic of fabricating silica gel-ZnSe:Mn/ZnS films. (b) Photographs of the twisted silica gel-QD films under the sunlight and an UV lamp. (c) PL spectra of the silica gel-QD films (0.5 mm) with different QD concentrations, excited at 405 nm. (d) PL peak intensity of the silica gel-QD films with different QD concentrations. (e) PL peak intensity of the silica gel-QD films with different thicknesses (QD concentration ratio 5%).
Based on the remarkable two-dimension encryption (one dimension is that intensity tunable, the other dimension is that thickness changeable) properties of the silica gel-QD compound, we next try to enlarge its potential utility to recording and encryption [41]. Figure 5(a) depicts our encoding-reading experiment, including an optical fiber spectrometer, a personal computer, an ultraviolet light source and rectangle silica gel-QD films. Figure 5(b) shows the schematic illustration of the information coding and decoding process. First, we established concentration, thickness-intensity relationship in advance. Because the PL intensity increases linearly with thickness and concentration, we simply express the relationship as I = I0 + kct, where I represents PL intensity, c denotes the concentration of the QDs, t is the thickness of the film, k is the slope. Using the PL data of the silica gel-QD film (0.5 mm) with different QD concentrations, we determine the value of I0 and k. Then a double variable data storage pattern is decorated according to the binary codes of the standard 8 bit ASCII characters. In detail, the concentrations represent 16 different row values while the thicknesses denote 16 different column values. Each couple of concentration and thickness [c, t] is assigned different values from 0 to 255. For the decoding, we tested the PL intensity and the thickness of the film. Then we used the intensity-concentration-thickness relationship to derivate the values of c, and the corresponding assigned value of [c, t]. According to the ASCII binary codes, the capital letter “N” “C” “H” “K” “U” can be translated into “78” “67” “72” “75” “85”,respectively. For instance, through this “PC decoding” step, according to the ASCII principle, the silica gel-QD codes can be easily read and translated into “NCHKU”, the acronym of Nanchang Hangkong University. The thickness and the concentration act as encryptors, because the intensities depend on both of them.
Fig. 5 (a) Digital photograph of our experiment. (b) Schematic illustration of the information coding and decoding process.
4. Conclusion
In summary, we have synthesized the stretchable luminescent silica gel-ZnSe:Mn/ZnS films. The as-prepared ZnSe:Mn/ZnS QDs have shown large Stocks shifts and the self-absorption of the QDs is greatly suppressed. No host emission is observed, indicating that the energy transfer from the host ZnSe material to the Mn fluorescent center proceeds in an efficient way. The as-prepared QDs have been successfully integrated into silica gel to form a stretchable film. The obtained films show excellent flexibility without trade-off their emission properties. Their potential applications in optical recording and encryption have been explored. This result provides a new stretchable material of choice for information technologies.
Funding
Natural Science Foundation of China (11564026, 11774141 and 21563013); China Scholarship Council (201608360030); Natural Science Foundation of Jiangxi Province (20171BAB202036 and 20161BAB212035); Outstanding Youth Funds of Jiangxi Province (20171BCB23051 and 20171BCB23052); Science and Technology Project of the Education Department of Jiangxi Province, China (GJJ160681).
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