Tunable blue-green-emitting wurtzite ZnS:Mg nanosheet-assembled hierarchical spheres for near-UV white LEDs
© Reddy et al.; licensee Springer. 2014
Received: 28 October 2013
Accepted: 27 December 2013
Published: 13 January 2014
Mg-doped ZnS hierarchical spheres have been synthesized via hydrothermal method using mixed solvents of ethylenediamine and DI water without any surface-active agent. The surface morphology and microstructure studies revealed that the hierarchical spheres were consisted of many well-aligned nanosheets with width 10 nm and length about 50 ~ 100 nm. X-Ray diffraction results show that the ZnS:Mg hierarchical spheres have wurtzite structure with high crystallinity. The absorption edge in the diffuse reflection spectra shifts towards lower wavelength with increasing Mg concentration, indicating an expansion in the bandgap energy that is estimated to be in the range of 3.28 to 3.47 eV. Blue-green photoluminescence with tunable intensity and peak position was observed depending on the Mg content. The Mg2+-activated ZnS phosphor can be good candidates for blue-green components in near-UV white light-emitting diodes.
KeywordsZnS:Mg Hierarchical spheres Wurtzite structure Bandgap expansion Tunable blue-green emission
In recent years, there is an explosive development of inorganic semiconductor nanostructures, particularly low-dimensional nanostructures. A variety of low-dimensional nanostructures such as zero-dimensional (0D) nanoparticles; one-dimensional (1D) nanowires, nanotubes, nanorods, and nanobelts; and two-dimensional (2D) nanosheets are investigated extensively due to their novel and fascinating properties compared to their bulk counterparts [1–3]. In addition, as the dimension of a material is reduced to the nanometer scale level, a large percentage of atoms are located at the surface, which significantly affects the structural and optical properties. The surface defects decorating the nanostructures of compound semiconductors often give rise to a rich visible luminescence that is attractive for applications in optical devices [1–4]. However, when the individual semiconductor devices are connected together to form integrated optical or electronic devices, the non-chemical connections between the units limit their cooperative or collective physical responses because of the multi-boundaries of electronic states . Hence, complicated nanostructures such as hierarchical, tetrapod, branched, and dendritic structures with natural junctions between branches or arms are highly desired for interconnection applications in the bottom-up self-assembly approach towards future nanocircuits and nanodevices .
Among all inorganic semiconductors, ZnS is one important electronic and optoelectronic material with prominent applications in visible-blind UV-light sensors [6, 7], gas sensors , field-emitters , piezoelectric energy generation , bioimaging , photocatalyst in environmental contaminant elimination , H2 evolution , CO2 reduction , determination of nucleic acids , solar cells , infrared windows , optical devices , light-emitting diodes , lasers , logic gates, transistors, etc. . ZnS has a bandgap energy of 3.72 eV for its cubic sphalerite phase and 3.77 eV for the hexagonal wurtzite phase . It is well known that at room temperature, only the cubic ZnS is stable, and it can transform to the hexagonal phases at about 1,020°C . For optoelectronics, wurtzite ZnS is more desirable because its luminescent properties are considerably enhanced than sphalerite . Attempts have been reported for preparation of wurtzite ZnS and related materials at lower temperatures through nanoparticle size control or surface-modifying reagents. However, achieving pure-phased wurtzite ZnS with structural stability at ambient conditions remains a challenging issue .
Luminescent properties can be significantly enhanced when suitable activators are added to phosphors. The choice of dopant materials and method of preparation have a crucial effect on the luminescence characteristics. Up to now, various processing routes have been developed for the synthesis and commercial production of ZnS nanophosphors, such as RF thermal plasma , co-precipitation method , sol-gel method , and hydrothermal/solvothermal method . The hydrothermal technique is simple and inexpensive, and it produces samples with high purity, good uniformity in size, and good stoichiometry. To prepare ZnS-based high-efficiency luminescent phosphors, transition metal and rare earth metal ions have been widely used as dopants [27–32]. However, studies on the effect of alkaline metal ions doping on the properties of ZnS are sparingly available except few reports on cubic structured ZnS nanostructures [33–35]. In this work, we report on the lower temperature synthesis of stable Mg-doped ZnS wurtzite nanostructures using hydrothermal technique and their luminescence properties. Mg was chosen as the dopant material because it has comparable ionic radius with Zn and has been used as an environment-friendly phosphor for many applications [36–39]. No report is available on wurtzite Mg-doped ZnS nanostructures despite of the importance of ZnS. In the present work, a systematic investigation was carried out on the effect of Mg doping on the structural, optical, and photoluminescence properties of ZnS:Mg nanostructures.
Zn1−xMg x S (x = 0.00, 0.02, 0.03, 0.04, and 0.05) were prepared using hydrothermal method. In a typical synthesis, Zn(CH3COO)2 · 2H2O, CH4N2S, and Mg(CH3COO)2 were dissolved according to stoichiometry into a solution of ethylenediamine (EN) 30 ml and DI water (70 ml). The reaction was carried out at room temperature for 8 h using a magnetic stirrer before hydrothermal treatment at 180°C in a Teflon-lined stainless steel autoclave for 12 h. The obtained precipitates with light yellow color were washed with purified water and dried at 100°C for 2 h.
The morphology and the average particle size were investigated using a HITACHI S-4800 scanning electron microscopy (SEM) equipped with an energy-dispersive spectrometer (EDS, Inca 400, Oxford Instruments, Abingdon, England, UK). The phase determination of the as-prepared powders was performed using an X-ray diffractometer (XRD) with Cu Kα as the X-ray source (Rigaku Miniflex-1, Shibuya-ku, Japan). Fourier-transform infrared spectroscopy (FTIR) spectra were recorded in the spectral range of 4,000 ~ 500 cm−1 with a spectral resolution of 4 cm−1 (JASCO FTIR-4100, Easton, MD, USA). Diffuse reflectance measurements (DRS) on dry powders were performed using a SCINCO S-3100 double beam spectrophotometer (Twin Lakers, WI, USA). Photoluminescence (PL) measurement was performed at room temperature using a 325-nm He-Cd laser as the excitation source.
Results and discussion
Figure 1d shows the typical EDS spectrum of Zn0.97 Mg0.03S with the characteristic peaks corresponding to the binding energy state of Zn, S, and Mg only. No other impurity peaks are detected in the spectrum, which is an indication of the chemical purity of the sample. The inset of Figure 1d gives the quantitative analysis result of the element composition in Zn0.97 Mg0.03S, which confirms that the obtained material has good stoichiometry.
It is interesting to note from Figure 6 that an appreciable blue shift in the PL emission peak position (from 503 to 475 nm) is noticed with increasing Mg content. The emission peak blue shifted with Mg concentration up to 4 at %, then shifted back at higher concentration. This trend is similar with the dependence of bandgap energy on the doping concentration shown in Figure 5. Regarding the PL intensity, the inset of Figure 6 shows the normalized intensity as a function of Mg doping concentration, which also exhibits a maximum at Mg concentration of 4 at %. The blue shift and the enhancement of the PL spectrum could be caused by the generation of new radiation centers or size decrease due to Mg doping . Mg ions could partially fill the tetrahedral interstitial sites or the position of Zn in the lattice of ZnS. Due to the smaller radius of Mg ions, the volume of the unit cell and the crystallite size decreased as discussed in the XRD analysis, which can lead to the blue shift of the absorption and PL spectra. When the Mg concentration is increased beyond 4 at %, the excess dopant ions could cause more deformation of the ZnS lattice that deteriorated the optical properties. Similarly, a small blue shift with Mg doping was reported in cubic structured ZnS:Mg nanoparticles .
Wurtzite Zn1−xMg x S nanosheets assembled hierarchical spheres have been synthesized using a hydrothermal approach with EN. Surface morphology studies show that the hierarchical spheres are composed of nanosheets. XRD studies showed that samples of all compositions crystallized in ZnS wurtzite structure. Widening of the bandgap was observed in Mg-doped ZnS nanostructures compared to undoped ZnS. Enhanced photoluminescence with increase in Mg doping was observed up to 4 at %. The CIE chromaticity diagram indicated that Zn1−xMg x S with various doping concentration of Mg has potential applications for blue-green components in near UV-white LEDs.
This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2012R1A1A3009736, 2012R1A1A2008845, and 2013K2A2A2000644).
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