Synthesis and photoluminescence properties of ZnS nanobowl arrays via colloidal monolayer template
© Liu et al.; licensee Springer. 2014
Received: 19 May 2014
Accepted: 1 August 2014
Published: 11 August 2014
Two-dimensional Zinc sulfide (ZnS) nanobowl arrays were synthesized via self-assembled monolayer polystyrene sphere template floating on precursor solution surface. A facile approach was proposed to investigate the morphology evolution of nanobowl arrays by post-annealing procedure. Photoluminescence (PL) measurement of as-grown nanoarrays shows that the spectrum mainly includes two parts: a purple emission peak at 382 nm and a broad blue emission band centering at 410 nm with a shoulder around 459 nm, and a blue emission band at 440 nm was obtained after the annealing procedure. ZnS nanoarrays with special morphologies and PL emission are benefits to their promising application in novel photoluminescence nanodevice.
Two-dimensional (2D) micro- and nanostructured arrays have attracted much interest because of their potential applications in optoelectronic device, gas sensors, solar cell, self-cleaning surface, etc. [1–5]. The applications and physical properties of 2D nanoarrays are relevant to their morphology. For example, bowl-like nanostructural arrays can be used in various fields, like nanoscale container and size selection of submicron spheres , light emitting diode surface , and chemical and biological sensors . Therefore, the control of nanoarrays' morphology provides opportunities to tune and improve their properties and further promote their practical applications in various fields.
Zinc sulfide (ZnS), as one of direct band gap semiconductor material, is a luminescent material well-known for its photoluminescence (PL) and electroluminescence [9, 10], which enables wide applications in the fields of flat-panel display , sensor and lasers [12, 13], and photodetectors . Up to date, many efforts have been focused on the morphologies control and PL emissions of ZnS nanomaterials [15–20]. It was demonstrated that their PL emissions were dominated by defect states, such as surface states, stoichiometric vacancies, and interstitial lattice defects [16–19]. However, the defect states in ZnS nanomaterial are active, especially at surface site, which will lead to unstable physical properties as well as poor reliability of ZnS-based nanodevices. It had been reported that the PL intensity of ZnS quantum dots diminished after a few days when left in the condition of normal laboratory lighting . Therefore, such active defects in ZnS nanomaterials should be post-treated to keep the stable properties before practical applications. Post-annealing may be a facile and effective method to solve this problem. Defect states at material surface will be greatly reduced with the increased grain size after annealing, while the interstitial lattice defects tend to stabilize. On the other hand, the morphology of ZnS nanomaterials could be changed with the increased grain size, which may provide a facile approach to control the morphology of nanomaterials by post-treatment. It would be desirable to obtain the ZnS nanomaterials with controlled morphology and stable properties by annealing, which is favorable for their special applications.
In recent years, monolayer colloidal crystal template (MCCT) has proved to be an effective approach for fabricating ordered nanoarrays due to their low cost, flexibility, and various morphologies [22, 23]. Moreover, MCCT exhibits excellent compatibility with traditional fabrication technologies, such as sol-gel [24, 25], pulse laser deposition , vapor deposition , and electrochemical deposition [28–30]. It is worth noting that the nanosphere lithography at the solution surface (NSLSS) had been developed by Qi's group [8, 31, 32], which provides a practical route for fabricating high-quality 2D nanoarrays with special morphologies.
In this work, we devoted to the morphology evolution and the PL emission of 2D ZnS nanobowl arrays through selecting the annealing ambient. The 2D ZnS nanobowl arrays were fabricated by NSLSS method at water bath. The special morphologies of nanoarrays can be obtained after annealing in different ambients, while the surface defect-related PL emissions can be prohibited or suppressed, leaving a blue emission band at around 440 nm. The results indicate that the 2D ZnS nanobowl arrays with special morphology may promote their application in PL-related nanodevice and further broaden their promising application in the field of flexible nanoscale device and bionic optical devices.
Prior to preparation of MCCT, glass substrates were ultrasonically cleaned with acetone, ethanol, 98% H2SO4:H2O2 (3:1), H2O:NH3 · H2O:H2O2 (5:1:1), and distilled water for 60 min, respectively. Polystyrene spheres of 600 nm (10 wt%) employed in the experiment was purchased from Duke Scientific Corporation (Pudong, Shanghai, China). MCCT was formed by a gas-liquid interface self-assembly method, which was discussed in detail in our previous work . In brief, a cleaned glass substrate was fixed at the center of a Petri dish surrounded by distilled water. Then, a drop of 10 μl water-ethanol-diluted PS suspension solution was dropped on the glass surface, and the PS suspension spreads freely and self-assembles into a colorful MCCT film on the water surface. The MCCT was lifted by a silicon substrate and the residual water around the MCCT was absorbed by a filter paper, which was used as a template for synthesizing nanoarrays on the solution surface.
The nanoarrays were characterized using field emission scanning electronic microscope (FESEM) and energy dispersive X-ray spectroscopy (EDS) (Hitachi S-4800, Hitachi, Tokyo, Japan), X-ray diffractometer (XRD, Bruker AXS D8 Advance, Bruker Corporation, Karlsruhe, Baden-Württemberg, Germany), and photoluminescence spectrometer (Hitachi F-4600) using 325 nm as the excitation wavelength, respectively.
Results and discussion
As well known, the PL emissions of ZnS nanomaterial are closely dependent on their morphologies and the preparation parameters due to their large surface-to-volume ratio. In general, the purple and blue emissions are ascribed to the defect states in ZnS nanomaterials, such as stoichiometric vacancies, interstitial lattice defects, and surface states. Among these defects, interstitial zinc and sulfur vacancies act as shallow donors (electron traps) and interstitial sulfur and zinc vacancies can behave as deep acceptors (hole traps) . The ascending order of the defect-related emission wavelength is interstitial sulfur, interstitial zinc, sulfur vacancies, and zinc vacancies . In our experiment, zinc vacancies-related luminescence is not observed, which is often located about 480 nm . Therefore, the emission peaks at 382 and 402 nm could be assigned to the radiative recombination of interstitial sulfur, and the peaks at 416 and 434 nm are attributed to interstitial zinc and sulfur vacancies, respectively. The interstitial defects are always present at the surface of ZnS nanomaterials prepared by chemical methods. In addition, the peak at 459 nm is associated with the trapped luminescence arising from the surface state .
For explaining the quenched emission peaks (at 382, 402, 416, and 459 nm) and the decreased emission intensity of 440 nm, the effects of annealing on the PL emission are proposed. Firstly, the interstitial defects at the nanobowl surface site, including interstitial zinc and interstitial sulfur, have diffused out the lattice due to the sufficient drive force originating from the annealing process. Hence, the corresponding PL emissions (at 382, 402, and 416 nm) are quenched. Zeng et al.  reported the similar results in ZnO nanomaterials; they believed that the annealing induces the outward diffusion of interstitial zinc and quenching of blue emissions. Secondly, the increased size and the improved crystallization of ZnS nanoparticles after annealing in air and vacuum, as evidenced by SEM and XRD results, will lead to the decrease of their surface area as well as the disappearance of a large number of surface states. Therefore, the surface states-related PL emissions (at 459 nm) were prohibited or attenuated. In the case of argon, the surface states should be passivated by the presence of hydrogen as mentioned in experimental section, and hence their surface defect-related PL emissions (at 459 nm) were vanished. It had been approved that defects at the surface site of ZnS nanomaterials were easily occupied by other impurity atoms, and the corresponding defect-related emission was quenched . The passivation of defect states was usually employed to investigate the luminescence character of nanomaterials [42, 43]. According to the above results, the lower emission intensity (at 440 nm) of the annealed samples could be easily understood. The PL spectra of as-grown sample are compound emissions of different species defects, and the emissions of interstitial defects and surface states are quenched by annealing, which are mainly responsible for the decreased emission intensity. On the other hand, the improved crystal structure (in air and vacuum) or defect-passivated (in argon) ZnS nanomaterials induced by annealing should also lead to the decrease of the defect-related emission intensity. Comparing with the sulfur vacancy-related emission (at 434 nm) of the as-grown sample, a red shift of 6 nm (from 434 to 440 nm) could be arising from the fit error or the changed surrounding of defects after annealing.
These results indicate that a large number of defect states located at the surface site of as-grown ZnS nanobowl and such defect-related PL emissions are unstable, which could be eliminated by annealing. Moreover, a blue emission band around 440 nm obtained from different annealing ambients is very stable, and its intensity is unchanged when the samples are kept in air for several months.
The 2D nanobowl arrays of ZnS were synthesized via a MCCT floating on the precursor solution surface. A facile post-annealing was proposed to investigate the morphology evolution and PL emissions of ZnS nanobowl arrays. The results indicate that various morphologies of ZnS nanobowl arrays could be obtained by annealing in different ambients. The surface defect-related PL emissions were suppressed, while a stable blue emission was obtained after annealing. ZnS nanobowl arrays with special morphologies and stable PL emission are essential to ensure the reliability of PL-based nanodevice and further accelerate their practical applications in various fields. In addition, the morphologies obtained in different annealing ambients may be a promising candidate in the field of flexible nanodevice and bionic optical devices.
This work is supported by the National Natural Science Foundation of China (Grant Nos. 51001078 and 51202155), the Zhejiang Provincial Natural Science Foundation of China (Grant Nos. Y4110207, Y4110547, LY13E010002, and LQ14E020004), the China Postdoctoral Science Foundation (Grant No. 2012 M521263), the Science and Technology Plan of Taizhou City, Zhejiang Province, China (Grant No. 131KY09).
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