Full Length ArticleZnSe nanocrystals obtained in pores of SiO2 matrix with temperature stable green luminescence
Introduction
Development of the modern optical techniques requires a range of new materials with high efficiency and tunable optical properties [1], [2], [3]. In the recent decade II–VI semiconductor nanocrystals (such as CdSe, CdS, ZnS and ZnSe) have attracted a lot of attention due to their unique size-dependent luminescent characteristics allowing obtaining of emission in the range from red to blue by size variation only, without any change of the chemical composition of the nanoparticle [4]. Modification of the surfaces of II–VI nanocrystals by organic molecules [5] or their incorporation into semiconductors with larger band gaps [6] allows obtaining bright and stable luminophors for the wide range of practical applications. Recent progress in the development of light emission diodes [7], solar cells [8], and biological luminescent probes [9] based on either capped or uncapped II–VI nanocrystals confirms their high significance in the modern optics and material science.
Two main classes of II–VI semiconductor materials are presented by Cd-containing (CdS, CdSe, CdTe) and Zn-containing ones (ZnO, ZnS, ZnSe, ZnTe) [10]. Despite the high potential biological toxicity of the first ones (due to possible release of cadmium into biological tissues), these materials have found wide application as so-called quantum dots, because their band gap in the bulk form is typically less than 2 eV making possible tuning the emission color from red to blue at size reduction, while for the second ones with band gaps of more than 2.5 eV possibility of color tuning is restricted to the range from blue to UV only. For instance, the band gap of one of the common II–VI Zn-containing material, ZnO is about 3.37 eV, so the band edge emission of this material is almost useless for the visualization tasks. However, as was shown recently, ZnO can be effectively applied for the field emission displays (FED) as green low-voltage phosphor due to intensive green emission determined by recombination of the electrons trapped by oxygen vacancies and photoexcited holes from the valence band [11].
ZnSe nanocrystals as well as ZnO ones are rarely investigated as potential quantum dots because the band gap of the bulk ZnSe is about 2.71 eV, so decrease of the size of the nanocrystal can shift the emission band from blue to violet and UV region only. ZnSe was once considered as a potential material for blue-emitting LEDs, but recent research has shown that its efficiency is sufficiently less than the one for GaN diodes widely used now for this application [12]. In our investigation the band edge emission of ZnSe was not considered at all but instead we paid our attention to the green band present both in bulk and nanosize ZnSe. This band has the similar nature as the green band of ZnO relating to the relaxation of electrons trapped by vacancies or different structural defects with the holes from valence band. As we have shown in our paper, intensity of this band for nanocrystals has remarkable temperature stability as compared to the one for ZnSe bulk crystal.
Many practical applications of quantum dots (such as development of flat display panels) require further assembling of single II–VI nanocrystals into larger structures. Such assembling can be achieved either by incorporation of chemically obtained quantum dots into ordered templates or by obtaining quantum dots on the surfaces of templates in situ with template playing a role of substrate at vapor deposition or molecular epitaxy [13], [14]. Proper choice of substrate allows obtaining of the high-stable luminescent materials due to passivation of the surface states of quantum dot by interaction with the states of the substrate matrix. In our research ZnSe nanocrystals were obtained in the pores of SiO2 (silica) sol-gel matrix. The role of SiO2 template was two-fold: as a substrate for deposition of ZnSe from the vapor phase and for restriction of the ultimate size of resulting nanocrystals by the size of the pore. Using two different methods of obtaining nanocrystals in the pores of silica matrices we confirmed of the generality of the experimental results obtained in the paper.
Section snippets
Experimental methods
The SiO2 matrices were synthesized according to the following method [15]. To 3.15 ml of methanol (CH3OH) 3.75 ml of tetramethoxysilane (TMOS) were added, the resulting mixture was stirred for 5 min. Then 4.5 ml of distilled water were added to the mixture which was again stirred. To increase the rate of hydrolysis, 0.525 ml of hydrochloric acid were added. The mixture obtained was poured to Petri dishes of 35×10 mm and sealed. The process of gel formation took 24 h, followed by drying at 45 °С during
Results and discussion
SiO2 sol-gel matrices are widely used for encapsulation of organic molecules [17], metal nanoparticles [18] and quantum dots [19]. Pure SiO2 matrix is almost transparent in the visible region with host related absorption below 200 nm. Luminescence spectra of sol-gel porous SiO2 matrix at 325 nm (He–Cd laser) and 457 nm excitation are shown in Fig. 3a. The spectrum obtained at 325 nm excitation consists of the single wide band with the maximum at 380 nm (at room temperature). The nature of this band
Conclusions
Optical properties of ZnSe nanocrystals obtained in the pores of SiO2 matrix by chemical vapour deposition (CVD) method and method of impregnation of SiO2 matrices with ZnCl2 solution and subsequent annealing in H2Se atmosphere were investigated. Difference in the mechanisms of luminescence temperature quenching in bulk CVD ZnSe crystal and ZnSe nanocrystals obtained in the pores of SiO2 matrix leads to stable nanosecond-range green luminescence band for ZnSe nanocrystals at room temperature
References (31)
Phosphor Handbook
(2006)Handbook of Optical Materials
(2002)Luminescence: From Theory to Applications
(2007)- et al.
Science
(1996) - et al.
Top. Curr. Chem.
(2016) - et al.
Quantum Wells, Wires And Dots: Theoretical and Computational Physics of Semiconductor Nanostructures
(2016) - et al.
Nat. Photonics
(2008) Physica E
(2002)- et al.
Science
(1998) Handbook of Nanophysics: Nanoparticles and Quantum Dots
(2016)
PSS-Rapid Res. Lett.
The Blue Laser Diode: the Complete Story
Appl. Phys. Lett.
Nat. Commun.
Funct. Mater.
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