Pressure-induced structural transformation of CdSe nanocrystals studied with molecular dynamics X. Ye, 1 D. Y. Sun, 2 and X. G. Gong 1 1 Key Laboratory of Surface and Department of Physics, Fudan University, Shanghai 200433, People’s Republic of China 2 Key Laboratory of Optical and Magnetic Resonance Spectroscopy and Department of Physics, East China Normal University, Shanghai 200062, People’s Republic of China Received 29 July 2007; published 10 March 2008 We have studied the pressure-induced structural transformation of CdSe nanocrystals using constant pressure molecular dynamics simulations for finite system. We have observed the transformation from wurtzite to rocksalt structure, the process of transformation is strongly dependent on the shape and size of the nanocrys- tals, and transformation pressure decreases with increasing nanocrystal size. The spherical CdSe nanocrystals studied undergo nonuniform deformation, while the faceted ones undergo uniform deformation. The reverse transformation from rocksalt backward to wurtzite structure of the nanocrystals happens below 1 GPa at room temperature, and the width of the transformation decreases as the temperature increases. DOI: 10.1103/PhysRevB.77.094108 PACS numbers: 61.50.Ks I. INTRODUCTION Pressure-induced structural transformation of group IV- IV, III-V, and II-VI compound semiconductors, such as GaN, SiC, ZnO, and CdSe, has been a longstanding topic of ex- perimental and theoretical research. Experiments and theo- retical studies have shown that this kind of compound semi- conductor would undergo a transformation from fourfold coordinated structure zinc blende ZB or wurtzite WZ to sixfold coordinated rocksalt RS structure under hydrostatic pressure. 1–27 For example, the highly covalent ZB-type SiC starts to transform into RS at a pressure of 100 GPa, 8,9 whrereas the less covalent WZ-type CdSe starts to transform at a much lower pressure of 2 GPa. 10,11 While the transi- tion pressure of these phase transformation has been studied extensively, the transformation mechanism is still on debate. One of the reasons is that transformations in bulk materials are dominated by growth mechanisms and have inherently irreproducible transition cycles due to the defects generated during the course of the transformation. 28 As an intermediate system between single atoms and bulk material, semiconductor nanocrystals are also of experimen- tal and theoretical interest. Compared with bulk material, nanocrystal behaves as single structural domains. Study of the nanocrystal would help us to understand the mechanism of the transformation which is hard to learn from bulk mate- rials. Theoretical and experimental researches have shown that optical and electronic properties of nanocrystals are closely related to their mechanical and structural prop- erties. 29 Thus, understanding the mechanisms of the struc- tural transformations at the nanoscale would facilitate devel- oping nanomaterials and devices. 30,31 Nanocrystals have quite different elastic and thermodynamic properties from their bulk counterparts because of the much higher surface to volume ratio. The CdSe nanocrystal system has been used as a model for structural studies. In the last few years, Alivisa- tos and co-workers have done a series of high pressure ex- periments on CdSe nanocrystals. 11,28,32–36 The results show that, similar to bulk material, CdSe nanocrystals would trans- form from fourfold coordinated WZ/ZB structure to a more densely packed sixfold coordinated RS structure with 18% reduction in volume under hydrostatic pressure, and the transformation is dependent on the size and shape of the nanocrystals. Recently, theoretical efforts have concerned with the possible mechanism and metastable structure of the transformation of CdSe nanocrystal changing from WZ to RS structure, a new intermediate structure for faceted CdSe nanocrystal during the transformation has been identified. 37 Although many experimental and theoretical efforts have been made to this area, the microscopic transformation mechanism from WZ to RS structure of CdSe at nanoscale has not been completely understood yet. 38–41 All these in- spire us to investigate the pressure-driven transformation in nanocrystals of CdSe using the molecular dynamics ap- proach. In present paper, we report the results of molecular dy- namics MD simulations of CdSe nanocrystals of various sizes and shapes undergoing forward and reverse structural phase transformations under hydrostatic pressure. We find the structural transformation of CdSe nanocrystal is highly affected by its size and shape. In the simulation, obvious hysteresis between the forward and backward transforma- tions is observed. Upon transformation, grain boundaries are found for all spherical nanocrystals, while for the faceted ones, the general trend is to form single-domain structure. II. COMPUTATIONAL DETAILS The present study uses constant-pressure molecular dy- namics method developed for finite systems. 42 We briefly outline this new method here, the details can be found in the original papers and recent review article. 42,43 In this method, the Lagrangian for a system is extended to include a PV term, L extend =  i N p i 2 2m i - r i  + P ext V , 1 where r i , m i , and p i are the position, mass, momentum of the ith atom, respectively,  is the interaction potential, and P ext PHYSICAL REVIEW B 77, 094108 2008 1098-0121/2008/779/0941086 ©2008 The American Physical Society 094108-1