Mg掺杂ZnO纳米晶的低温共沉淀法制备及光学性质

采用共沉淀(co-precipitation)法制备了Mg掺杂ZnO纳米晶,分别用X射线衍射(XRD)、傅立叶变换红外光谱(FTIR)、紫外可见吸收(UV-Vis)光谱、光致发光(PL)光谱、透射电镜(TEM)、电子顺磁共振(EPR)等分析手段对样品进行了表征。探究了Mg离子在ZnO纳米晶中的存在状态,ZnO纳米晶颗粒尺寸和发射光谱随Mg掺杂浓度的变化,并对其发光机理进行了分析。结果表明:Mg离子在ZnO晶格中以部分晶格位,部分间隙位的方式存在,没有形成MgO表面壳层结构;随Mg掺杂浓度的增大,ZnO纳米晶的颗粒尺寸变小,发射光的光强增大。发射光的最佳激发波长为342nm,中心波长为500nm,荧光量子产率为22.8%。实验分析表明:Mg离子的掺杂在ZnO纳米晶中引入了锌空位(VZn),间隙位的镁离子(IMg),提供了新的复合中心,从而增强了ZnO纳米晶的光致发光。

Luminescent Property of Mg Doped ZnO Nanocrystals Synthesized by Co-precipitation at Low Temperature

ZnO-based luminescent nanomaterials have attracted comprehensive attention because of their great potential applications in ultraviolet laser devices, LED fabrication and bio-imaging due to their large exciton banding energy (60 meV) and wide bandgap (3.37 eV), low toxicity and high photo-stability. ZnO nanomaterials exhibit two kinds of emission: one is ultraviolet (UV) near-band-edge emission at approximately 380 nm and the other is visible deep-level emission in the range 450~730 nm. Generally, ZnO nanomaterials fabricated at high temperature by PLD, CVD and magnetron sputtering get good crystallization and exhibit near-band-edge emission with negligible visible emission. To get highly luminescent visible light emitting ZnO nanomaterials, mass of visible light emitting defects should be introduced to ZnO crystal. Low temperature fabricating and doping are two kinds of methods to introduce defects into crystals.Because the tetrahedral ionic radius of Mg2+ (0.057 nm) is similar to that of Zn2+ (0.060 nm), wide range solubility of Mg2+ in the zinc blend structure is expected. So, Mg2+ doped ZnO nanomaterials have been studied comprehensively. At high temperature, Mg2+ ions take Zn2+ positions in hexagonal structure and form Zn1-xMgxO alloys, band gap of which can be tuned in 3.3~7.8 eV by verifying Mg2+ content. The alloys have been used widely in ultraviolet lasers and Zn1-xMgxO/ZnO heterojunction optoelectronic devices research. However, at low temperature in liquid, because of low free energy and short ionic radius, Mg2+ ions can facilely enter interstitial positions between lattice planes of wurtzite structured ZnO and form Zn vacancies in the crystal, which should introduce energy levels in band gap of ZnO and make the ZnO nanomaterials emit strong visible light. So, it is of both theoretical and practical meanings to study luminescent property of Mg doped ZnO namomaterials prepared at low temperature.In typical preparation, ZnO colloidal nanocrystals were synthesized at 50 ℃ by dropwise adding 10 mL of tetramethylammonium hydroxide 0.552 mol/L in ethanol(AR) dropwise into 30 mL solution of zinc acetate 0.101 mol/L(C4H6O4Zn·2H2O) (AR) in DMSO(AR) under constant stirring. When doping with Mg2+, we added 10 mL of tetramethylammonium hydroxide 0.552 M into 30 mL solution of zinc acetate and magnesium acetate 0.101 mol/L (C4H6O4Mg·4H2O) (AR) in DMSO.When altering Mg2+ concentration in the colloid, we kept the content of Mg2+ plus Zn2+ to be 0.101 M to guarantee a complete reaction. Solid state nanocrystals were precipitated by adding ethyl acetate into the colloid. The precipitates were then centrifugated at 10 000 r/min for 10 min, washed with ethanol and dried at 80 ℃. The phase structure, morphology and optical properties of the samples were characterized by X-ray diffraction (XRD), Fourier transform infrared spectrometry (FTIR), transmission electron microscope (TEM), absorption spectra (UV-vis), room temperature photoluminescence (PL) and electron paramagnetic resonance (EPR). The results indicated that Mg2+ ions partially take interstitial positions in ZnO NCs. When activated by 342 nm laser, the Mg doped ZnO NCs emit strong greenish white light with emission peak at about 500 nm. With higher Mg doping content, the NCs become smaller and the luminescence get stronger. Quantum yield of 20% Mg doped ZnO NCs is 22.8%. The partially interstitial doping Mode introduced Zinc vacancies (VZn) and interstitial Mg2+ (IMg) into the doped NCs, which were responsible for the enhanced greenish white light emission. Our findings provide a comprehensive understanding of effects of Mg doping on the microstructure and electronic properties of ZnO prepared at low temperature.