Pressure Induced Phase Transition in GaN Nanocrystal Anurag Srivastava* and Neha Tyagi Advanced Material Research Laboratory, ABV-Indian Institute of Information Technology & Management, Gwalior (M.P.) 474010, India Structural phase transition in GaN nanocrystal has been studied within the framework of density-functional theory, using generalized gradient approximation as exchange correlation functionals. The study observes that under the application of pressure GaN nanocrystal transforms from zincblende (B3) type phase to hypothetical rocksalt (B1) type phase at 100.13GPa, which is comparatively larger than its bulk counterpart. The lattice parameter, bulk modulus and pressure derivatives of GaN nanocrystal in both the phases have also been computed. The mechanical strength of the GaN nanocrystal has been analysed in terms of volume collapse at transition pressure and bulk modulus.
Introduction:
Nanocrystals are used in variety of applications such as lasers, solar cells, single electron transistors, biological sensors and in storage of electrons etc [1,2]. The change of transition pressure in nanocrystals is governed by three components: the ratio of volume collapses, the surface energy differences and the internal energy differences.
Table-2 The calculated results of structural phase transition for GaN nanocrystal 3 Volume (Å ) -∆Vt/ Energy (eV) Pt Structure B3-GaN B1-GaN V0 (%) B3-GaN B1-GaN (GPa) Nanocrystal 21.29 17.81 13.63 -335.625 -334.218 100.13 Bulk 21.44 17.24 17.05 -336.422 -335.515 40.05,3 5.5a, 38.15a, a 53.8 a Ref.[6]
Tolbert and Alivisatos [3,4] have carried out first high pressure experiments on semiconductor nanocrystals and found the elevated transition pressure of the nanocrystals compared to that of bulk.
From the obtained energy and volume data at the points of structural transition, the volume collapse (-∆Vt/V0) at transition pressure (Pt) has been calculated.
The reason behind this increase in transition pressure is probably the decrease in the volume collapse at the transition pressure.
Computed bulk modulus and volume collapse of nanocrystal are smaller than the bulk crystal, which suggests the softening of material at reduced dimension.
Computational Brief:
The calculated B3 to B1 transition pressure for GaN nanocrystal is larger than its bulk counterpart.
Simulation has been performed on Atomistix Tool Kit-Virtual Nano Lab, which is based on DFT.
Used GGArev-PBE potential as exchange correla- Figure-1 Energy vs. volume curves for GaN tional functional. nanocrystal and bulk crystal. Employed the Steepest descent geometric optimi- Results & Discussion: zation technique with Pulay algorithm for iteration The zincblende (B3) and rocksalt (B1) type phase mixing. of GaN nanocrystal as well as in bulk counter Selected 100Ry energy cutoff and 1x1x50 k-point parts are found stable. sampling. Figure-2 Relative volume vs. pressure curves for GaN To understand the ground state properties of GaN Table-1 The calculated lattice parameter (a), bulk nanocrystal and bulk crystal. nanocrystal in its B3 and B1 type phases the equimodulus (B0) and pressure derivative (B0′) References: librium lattice parameter, bulk modulus and presfor GaN nanocrystal sure derivative have been computed and summaStructure Lattice Bulk B0′ 1. M. Achermann, M.A. Petruska, S.Kos, D.L.Smith, D.D. Koleske rized in table 1. Parameter (a) Modulus (B0) and V.I. Klimov, Nature 429, 642 (2004). B3-GaN B1-GaN B3-GaN Nanocrystal 4.40 4.141 155.79
B1-GaN B3-GaN B1-GaN 172.99 4.68 5.05
Bulk
241.18, 3.45, a 227 , 4.57a, 223a 4.6a
a
4.397, 4.54a 4.49a
4.077, 4.07a
207.55, 185a,177a
5.17, 5.51a, 4.0a, 3.69a
The calculated ground state parameters, transition pressure and volume collapse for bulk GaN are in good agreement with the other reported data.
The transitions have been calculated through the two pairs of E –V curves, and the E and V values at each tangent point, listed in table 2.
2. P. Alivisatos, Nature Biotechnology 22 (1) 47 (2004). 3. M. Haase, and A. P. Alivisatos, J. Phys. Chem. 96, 6756 (1992). 4. S. H. Tolbert, A. B. Herhold, L. E. Brus and A. P. Alivisatos, Phys. Rev. Lett.76 (23) 4384 (1996). 5. F. Benkabou, P. Becker, M. Certier, and H. Aourag, Phys. Stat. Sol. (b) 209, 223 (1998). 6. Bahmed Daoudi and Aomar Boukraa, Annales des Sciences et Technologie 2 (1) 19 (2010).
Ref.[5]
Acknowledgement: One of the author Neha Tyagi is thankful to ABV-IIITM for providing infrastructural support and award of Ph.D scholarship AMRL, ABV-Indian Institute of Information Technology & Management, Gwalior (M.P.) 474010, India Visit us at: www.iiitm.ac.in | Email: *profanurag@gmail.com