1.College of Engineering, Bohai University, Jinzhou 121013, China 2.College of New Energy, Bohai University, Jinzhou 121013, China 3.State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, China
Abstract:In this paper, the grain and grain boundary characteristics and mechanisms of phase transition (from wurtzite to zinc-blende to rock-salt phase structure) of ZnS nanocrystallines are investigated via in situ impedance measurement under pressure up to 29.8 GPa. It should be noted that there are two semiarcs can be found from the modulus plots of ZnS under different pressures. The semiarc in high frequency region represents the grain characteristic, and another one in low frequency region refers to the grain boundary characteristic. The former decreases gradually with pressure increasing and the latter shows an opposite trend. This fact indicates that the effect of grain characteristic becomes weaker and weaker, and the role of grain boundary characteristic is just on the contrary. The grain resistance and grain boundary resistance of ZnS nanocrystalline are also studied. In the low pressure region, both resistances increase with different increment rate with pressure increasing, which can be attributed to the enhanced ability of trap charge carriers due to the small size effect of nanoparticles. In addition, two discontinuous points (about 11 and 15 GPa) can be observed in both resistance curves, corresponding to the points of phase transition from wurtzite to zinc-blende to rock-salt phase structure. With pressure increasing, both resistances decrease gradually until 21 GPa, and this point corresponds to the end of transition from zinc-blende to rock-salt phase structure. Their consequent variations are different, grain boundary resistance gradually decreases with the pressure increasing, while the grain resistance is almost a constant. Additionally, the relaxation frequency, as an intrinsic characteristic, is not affected by the geometrical parameters. According to the linear relation between the grain boundary relaxation frequency and pressure in the pressure range of phase transformation, the mechanism of structure transition from wurtzite to zinc-blende to rock-salt phase structure is also discussed in detail. Based on the investigations, the in situ impedance spectroscopy can not only be used to accurately measure the grain and grain boundary characteristics, but also provide information for studying the phase transformation under pressure. Keywords:high pressure/ ZnS/ impedance spectroscopy/ phase transition
图 2 (a) 金刚石对顶砧薄膜电极示意图: 1, Mo电极; 2, 裸露的金刚石砧面; 3, 沉积在金刚石砧面的Al2O3薄膜; 4, 沉积在Mo薄膜上的Al2O3薄膜; (b)金刚石对顶砧剖面示意图 Figure2. (a) The configuration of a complete microcircuit on a diamond anvil: 1, the Mo electrodes; 2, the exposed diamond anvil; 3, the Al2O3 layer deposited on the diamond anvil; 4, the Al2O3 layer deposited on the Mo film; (b) the cross section of the designed diamond-anvil-cell.
与体材料相比, 对纳米材料体系的相变研究可以在一定程度上得到简化, 因为纳米尺度的颗粒可看作为一个单一结构体系[26,27]. 因此, 研究颗粒表面行为及相关因素对相变的影响是至关重要的. 有研究表明[20,28,29], 涉及晶体原子协同运动的纳米晶体是优先于界面生长的. 为了进一步了解ZnS纳米晶的相变机理, 我们研究了材料晶界弛豫频率随压力的变化关系, 如图5所示. 在确定的压力下, 弛豫频率(f )是材料的固有特性(本征特性), 而与材料的几何参数无关[30]. 由Arrhenius公式可知: 图 5 晶界弛豫频率随压力的变化 Figure5. The change of grain boundary relaxation frequency of ZnS nanocrystalline as a function of pressure.
图 1 纤锌矿ZnS纳米晶XRD谱图
图 2 (a) 金刚石对顶砧薄膜电极示意图: 1, Mo电极; 2, 裸露的金刚石砧面; 3, 沉积在金刚石砧面的Al2O3薄膜; 4, 沉积在Mo薄膜上的Al2O3薄膜; (b)金刚石对顶砧剖面示意图
图 3 不同压力下ZnS纳米晶的模谱图 (a) 8.4 GPa; (b) 12.6 GPa; (c) 18.3 GPa; (d) 29.8 GPa
图 4 晶粒电阻(Rg)和晶界电阻(Rgb)随压力的变化关系
图 5 晶界弛豫频率随压力的变化