Rempel Jane Y, Trout Bernhardt L, Bawendi Moungi G, Jensen Klavs F
Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
J Phys Chem B. 2005 Oct 20;109(41):19320-8. doi: 10.1021/jp053560z.
Details of the chemical mechanism underlying the growth of colloidal semiconductor nanocrystals remain poorly understood. To provide insight into the subject, we have preformed a comprehensive study of the polar (0001) and (0001) and nonpolar (1120) wurtzite CdSe surfaces that are exposed during crystal growth using first-principles density functional theory (DFT-GGA) calculations. Stabilization of these surfaces by relaxation and reconstruction was considered. Two particular reconstructions of the polar surfaces were examined: vacancy formation on a 2 x 2 unit cell and addition of Se and Cd atoms on the (0001) and (0001) surfaces, respectively. Calculation results indicate that the (1120) is the most stable surface when compared to the two polar surfaces. Furthermore, reconstructions of the (0001) surface are energetically favored when compared to reconstructions of the (0001) facet. Adsorption of Cd and Se atoms and the CdSe molecule on the three relaxed surfaces and two reconstructed (0001) surfaces were also investigated. Several binding sites were considered to determine the most stable binding geometries and energetics. Atomic species preferentially bind in either 2-fold or 3-fold sites, while the CdSe molecule binds parallel to the surface on all of the considered surfaces. Vibrational frequencies of the adspecies were calculated for the most stable binding configurations and were included in the zero point energy correction. Diffusion barriers for the atomic and molecular species were estimated where possible to be between 0.2 and 0.4 eV on the three relaxed surfaces. Thermochemistry of the CdSe molecule binding and dissociation was also investigated. On all considered surfaces, dissociation is preferred to desorption with dissociation only exothermic on the (0001) surface. Comparison of the three relaxed and two reconstructed surfaces indicates that CdSe molecule binding and dissociation is thermodynamically favored on the (0001) surface. This implies that under a reaction-controlled regime, the rate of homoepitaxy would be faster on the (0001) Se terminated surface than on the (0001) and (1120) surfaces, making the (0001) surface of a nanocrystal the primary direction of growth.
胶体半导体纳米晶体生长背后的化学机制细节仍知之甚少。为深入了解这一主题，我们使用第一性原理密度泛函理论（DFT - GGA）计算，对晶体生长过程中暴露的极性（0001）和（0001）以及非极性（1120）纤锌矿CdSe表面进行了全面研究。考虑了通过弛豫和重构对这些表面进行稳定化处理。研究了极性表面的两种特定重构：在2×2晶胞上形成空位以及分别在（0001）和（0001）表面添加Se和Cd原子。计算结果表明，与两个极性表面相比，（1120）表面最稳定。此外，与（0001）晶面的重构相比，（0001）表面的重构在能量上更有利。还研究了Cd和Se原子以及CdSe分子在三个弛豫表面和两个重构的（0001）表面上的吸附情况。考虑了几个结合位点以确定最稳定的结合几何结构和能量。原子物种优先结合在二重或三重位点，而CdSe分子在所有考虑的表面上都平行于表面结合。针对最稳定的结合构型计算了吸附物种的振动频率，并将其纳入零点能量校正。在可能的情况下，估计了原子和分子物种在三个弛豫表面上的扩散势垒在0.2至0.4 eV之间。还研究了CdSe分子结合和解离的热化学。在所有考虑的表面上，解离比脱附更受青睐，仅在（0001）表面上解离是放热的。对三个弛豫表面和两个重构表面的比较表明，CdSe分子的结合和解离在（0001）表面上在热力学上更有利。这意味着在反应控制的条件下，纳米晶体的（0001）Se终止表面上的同质外延速率将比（0001）和（1120）表面上更快，使得纳米晶体的（0001）表面成为主要生长方向。
