With the features of convenience and eco-friendly, the low-temperature solid-state reaction synthesis was successfully developed as a new approach to prepare quantum-sized ZnS nanocrystals. One major achievement is that the size and shape of ZnS nanocrystals can be tuned by adjusting the surfactant and its feed. The UV-Vis absorption spectra of quasispherical and one-dimensional quantum-sized ZnS nanocrystals all showed a blue-shift from the bulk counterpart, indicating large quantum confinement effects of ZnS nanocrystals. These ZnS nanocrystals all showed well-defined excitonic emission features. Contrastive studies on photoluminescence performances indicated that the bandedge emission experienced only the size-dependent quantum confinement effect, while the trap-state emission experienced the size- and shape-dependences. So we can design a purposeful synthesis route to ZnS nanocrystals with target luminescence emission performances.
Owing to fascinating size-, and shape-dependent electronic and optical properties, quantized semiconductor structures have attracted increasing attention [1-3]. It is well known, for instance, that the band gap of semiconductor nanostructure increases as the particle size decreases when the dimension of nanocrystals approach to exciton Bohr radius, due to quantum-size confinement effect [
The low-temperature solid-state reaction synthetic approach provided a relatively simple and powerful method for controlling the size and shape of nanoparticles [12, 13]. In view of the advantages of low cost, convenience, and lack of pollution, we introduced it to synthesize the Q-ZnS firstly. We fulfilled the size-, and shape-control of Q-ZnS through surfactant-assistanted technique, and the tunable optical behavior.
The manipulation for the synthesis of Q-ZnS through one-step solid-state reaction at room temperature was recorded and illustrated in
with an appreciable proportion of SDBS (Q-ZnS-i) or CTAB (Q-ZnS-ii) (MRSDBS or MRCTAB = 1:5, here, the molar ratio of additive/Zn(OAc)2·2H2O was designated as MR) and ground together for 10 min at room temperature, then sodium sulfide hydrate (Na2S·9H2O) was added with 1:1 molar ratio between Zn(OAc)2·2H2O and Na2S·9H2O. The mixtures were ground for about 30 min, and centrifugation washed, respectively. Finally, the QZnS samples were dispersed in deionized water, respectively, and clear solutions were obtained for following characterization.
The X-ray diffraction (XRD) patterns for as-prepared Q-ZnS-i and -ii clearly indicates that they have cubic phase with lattice constant as cross-referenced to JCPDS 65 - 9585 card (
Additionally, the behaviors of SDBS and CTAB capping on the ZnS nanoparticles were identified by FT-IR spectra. The FTIR spectra of Q-ZnS-i and SDBS are compared in
As shown in
Photoluminescence is a luminescence phenomena that occurs in luminescent materials which can be excited to a higher energy state termed “excited state” after absorbing light at a specific wavelength, and then re-emit light with a lower energy through recombination of electron and hole [14,15]. Due to high sensitivity and non-destructive character, the photoluminescence (PL) technique has been widely used to investigate electronic structure of the surface of semiconductor nanoparticles. Many reports illustrated the luminescence mechanism of ZnS nanoparticles using the well-known energy-level diagram (
defect states. Shallow level traps, which are more spatially delocalized and lie near the conduction band or valance band edge, are more likely to participate in the radiative recombination. The emission occurs when a trapped electron recombines with a hole in the valence band or in some acceptor level such as zinc vacancies [
As far as we known, although many groups have put large efforts in studying the optical properties of ZnS nanocrystals, there have been few reports on the excitonic emission feature of ZnS nanocrystals, as well as the sizeand shape-dependent emission characteristics. In most cases, only trap-state emission at around 450 nm was observed and discussed which is termed “tunneling luminescence” and controlled by defect and impurity, the sizeand shape-dependent quantum confinement effects were neglected.
Photoluminescence spectra of the quasi-spherical nanocrystals (Q-ZnS-i) showed broad violet emission peak centered at 342 nm accompanied by the broad asymmetry shoulder at 378 nm, and smaller hump-like blue emission peaks at 422 nm and 439 nm, and two weak shoulders at 456 and 500 nm, respectively. We attribute the 342 nm peak to a band-edge emission. The shoulder centered at 378 nm is assigned to the overlap-ping peaks involving electronic transitions from conduction band to
interstitial sulfur and interstitial zinc to valance band. In addition, blue emission peaks at 422 nm and 439 nm are assigned to the energy level of sulfur vacancies with the holes from the valance band, and interstitial zinc from interstitial sulfur, respectively, while the last peak at 500 nm which can be attributed to recombination of electrons at sulfur vacancies to the holes at zinc vacancies. It is particularly worth noting that the emission band at 456 nm was attributed to dangling sulfur bonds at the interface of ZnS in this paper. Although the deep-trap emission at around 450 nm is ascribed to surface sulfur vacant sites in some literatures [
The emission spectra of near one-dimensional ZnS (Q-ZnS-ii) showed a band-edge emission feature at 340 nm, and high-intensity trap-state emissions about in range of 345 - 475 nm with well-defined peaks at 349, 365, and 432 nm, and broad shoulder at 455 nm. Compared to PL emission of quasi-spherical nanocrystals, the increase of intensity and shift of shallow-trapped emission implied that the near one-dimensional ZnS possess more defects, and the trap-state PL experimenced shapedependent quantum confinements. And the intensity of emission band at 455 nm, which is ascribed to arise from the dangling sulfur bonds at the interface of ZnS, slightly increased. On the other hand, the band-edge emission of 341 nm showed almost the same wavelength as that of quasi-spherical nanocrystals (342 nm). It is concluded in this work that the band-edge emission may be mostly dependent on size rather than shape. To further demonstrate the size-dependence of quantum confinement effect to band-edge and trap-state emission, PL spectrum of 7 nm-sized quasi-spherical nanocrystals were got (
To estimate the applicability of approach adopted in this work, we introduce it to synthesize the CdS nanocrystals with SDBS. The 3 nm-sized quasi-spherical CdS nanocrystals were successfully obtained (
In summary, quantum-sized ZnS nanocrystals with quasispherical and near one-dimensional shapes were synthesized through room-temperature solid-state reaction methodology. The synthetic procedure has the advantages of convenient operation, and low cost, and lack of pollution, and for mass-production and good applicability. The sizeand shape-controls can be achieved through using different additives and adjusting the MR. The quan tum-
sized ZnS nanocrystals exhibited the quantum confinement effect and unique well-defined excitonic emission features. The band-edge PL emission showed the mostly size-dependent quantum confinement effect, but the trapstate PL experienced the sizeand shape-dependences.
This work was funded by Shanghai University Innovation Foundation, National Nature Science Foundation of China (Grant No. 21101132), and National Nature Science Foundation of China (Grant No. 61006089).
The characterization, the images of Q-ZnS-ii nanoparticles with one-dimensional and wave-like structures and CdS QDs, and Schematic energy level diagram.
The crystalline structures of the products were analyzed by a powder X-ray diffractometer (XRD, MXP18AHF, MAC) with Cu-Kα radiation (λ = 0.154056 nm). The morphologies, microstructures, and crystal lattice of the obtained samples were characterized by transmission electron microscopy (TEM, JEM-2010F at 200 KV). The UV spectra were recorded on a spectrophotometer (HITACHI U-3310) at room temperature. The photoluminescence spectra were obtained by using a HITACHI F-4500 fluorescence spectrophotometer at room temperature.