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High pressure behavior of AlAsnanocrystals: the first-principle studyAnurag Srivastava a & Neha Tyagi aa Advance Material Research Laboratory, ABV-Indian Instituteof Information Technology and Management, Gwalior, MadhyaPradesh, 474010, India

Available online: 20 Dec 2011

To cite this article: Anurag Srivastava & Neha Tyagi (2012): High pressure behavior of AlAsnanocrystals: the first-principle study, High Pressure Research: An International Journal, 32:1, 43-47

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High Pressure ResearchVol. 32, No. 1, March 2012, 43–47

High pressure behavior of AlAs nanocrystals:the first-principle study

Anurag Srivastava* and Neha Tyagi

Advance Material Research Laboratory, ABV-Indian Institute of Information Technologyand Management, Gwalior, Madhya Pradesh 474010, India

(Received 22 August 2011; final version received 19 November 2011)

In this study, the first-principle density functional approach has been used to analyze the pressure-inducedstructural stability and phase transformation in AlAs nanocrystals. This study includes the stability analy-sis of AlAs nanocrystals in their B4-, B3-, B1- and B2-type phases, and we observed that the B3-typephase is the most stable. We also observed the structural transformations in AlAs nanocrystals fromB3 → B1 at around 8.9 GPa, B3 → B2 at 7.12 GPa and B3 → B4 at 3.88 GPa. The stability of the materi-als has been analyzed using local density approximation with the Perdew–Zunger parameterization and alsowith the Perdew–Burke–Ernzerhof (PBE) and revised PBE parameterizations of the generalized gradientapproximation potential.

Keywords: first principle; AlAs; phase transition; nanocrystal

1. Introduction

High pressure techniques for the investigation of nanomaterials have been developed in par-allel to the growth of nanosciences either to better understand the properties of materialsor to provide attractive methods for nanostructuration. The existence of a characteristiclength scale less than ∼100 nm is the mark of nanomaterials. Semiconductor nanocrystalshave attracted much attention from researchers due to their unique electronic and opti-cal properties. These materials have been used in a variety of applications such as lasers,solar cells and single-electron transistors [1–4]. Nanocrystals are also used in biologicalsensors, which are capable of sensing even at the single-molecule level in living cells [5].Experimentally, Chen and Gao [6] have synthesized wurtzite-type one-dimensional ZnSenanocrystals, which show photoluminescence at room temperature. The aluminum nitride(AlN) as well as AlP nanocrystals of about 5–10 nm diameter have been prepared by theresearchers [7,8].

*Corresponding author. Email: profanurag@gmail.com

ISSN 0895-7959 print/ISSN 1477-2299 online© 2012 Taylor & Francishttp://dx.doi.org/10.1080/08957959.2011.643791http://www.tandfonline.com

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44 Anurag Srivastava and Neha Tyagi

Nayak et al. [9] synthesized GaAs nanocrystals using an electrochemical route from the acidicsolutions of metallic gallium and arsenic oxide and performed the structural analysis using trans-mission electron microscopy, which revealed the presence of an orthorhombic phase in contrast tothe usual cubic phase of bulk GaAs. Specifically, the high pressure experiments on semiconductornanocrystals were carried out first by Haase andAlivisatos [10] and Tolbert et al. [11]. The elevatedtransition pressure of the nanocrystals when compared with that of bulk has been attributed to thesurface energy effects with their coherently changing shape by undergoing uniform deformationduring the transformations. For the last few years, III–V semiconductors have been considered tobe the most promising compounds due to their technological and industrial importance; especiallyAlAs exhibits excellent physical properties, which makes it useful for applications in devices. Mostof the binary compounds of III–V groups adopt tetrahedrally bonded semiconducting phase atambient pressure and, by the application of moderate pressure, transform into a metallic phase.Froyen and Cohen [12] and Martin [13] were the first to report the phase transition in AlAs usingan ab initio pseudopotential approach and suggest that the high pressure structure could either berock salt (B1) or NiAs (B8). Wen and Jiang [14] studied the pressure-induced phase transition inCdSe and ZnO nanocrystals and found that the transition pressure increases as the nanocrystalsbecome smaller. Sapra and Sarma [15] analyzed the band-gap variation with sizes in various II–VIsemiconductor nanocrystals using tight-binding approach.

Recently, our group has studied the structural stability of bulk AlAs in its various possiblephases, such as B3, B1, B2 and B8, and reported the transition from B3 → B8, B3 → B1 andB3 → B2 using an ab initio method [16]. While performing the literature survey, it was foundthat no information is available on phase transition in AlAs nanocrystals under high pressures. Tounderstand the behavior ofAlAs at reduced dimensions and its importance in electronic industries,we thought it pertinent to perform the present study.

2. Computational details

The calculations were performed within the framework of density functional theory (DFT) [17]using an ab initio pseudopotential approach as implemented in the Atomistix ToolKit (ATK) [18].ATK is a further development of TranSIESTA-C [19,20], which, in turn, is based on the technology,models and algorithms developed in the academic code TranSIESTA and, in part, McDCal [21],employing localized basis sets as developed in Spanish Initiative for Electronic Simulations withThousands of Atoms (SIESTA) [22]. This study was performed in a self-consistent manner usingthe steepest descent geometric optimization technique with Pulay algorithm for iteration mixing.Double-ζ eta double polarized basis sets were used as the local combination of the atomic orbitalsto describe the valence electrons. A mesh cut-off of 100 Ryd with 1 × 1 × 50 k-points was taken.The exchange correlation functional was described within the local density approximation (LDA-PZ) proposed by Perdew and Zunger [23], and the generalized gradient approximation (GGA-PBE) proposed by Perdew et al. [24] and the revised PBE (revPBE) approximation proposed byZhang and Yang [25] and Hammer et al. [26] were used to calculate the total energies of the B3-,B4-, B2- and B1-type phases of AlAs nanocrystals. The total energy for the B3-type phase of AlAsNC with LDA-PZ potential was −468.52 eV and with GGA revPBE potential was −318.94 eV,which indicates that the LDA-PZ potential is quite good for the calculation of total energies. Weassumed the roughly spherical nanocrystals whose diameter can be calculated using the followingrelationship:

d = a

[3N

4�

]1/3

,

where a is the lattice parameter and N is the number of atoms present in the nanocrystals.

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High Pressure Research 45

3. Result and discussions

We analyzed the stability in all the four phases of AlAs nanocrystals such as zincblende (B3),wurtzite (B4), CsCl (B2) and NaCl (B1) at 0.9 nm. Through computation, it was found that thelattice parameter for bulk AlAs in the B3-type phase was 5.64Å, which showed a close match withits experimental counterpart (5.66Å) [27]. The lattice parameters for all the four stable phases ofAlAs nanocrystals such as B3, B4, B1 and B2 were 5.761, 4.283, 5.350 and 3.207Å, respectively.However, in the absence of any other reported data on nanocrystals, the present computed values forAlAs nanocrystals cannot be compared. The total energy values for the B3-, B4-, B1- and B2-typephases of AlAs nanocrystals were −468.52, −468.46, −467.84 and −467.77 eV, respectively.Looking at the total energy values of the system, the B3-type phase of AlAs nanocrystals can beconsidered as the most stable one at lower pressures and the B1-type phase can be considered asthe high pressure phase; on comparison with bulk AlAs, we found the B3-type phase to be themost stable at low pressures and the B2-type phase to be the high pressure phase. We calculatedthe total energy of the system at different volumes as shown in Figure 1(a)–(c). This study revealedthat under the application of pressure, the B3-type phase of AlAs nanocrystals first transforms tothe wurtzite (B4)-type phase at 3.88 GPa, then to the CsCl (B2)-type phase at around 7.12 GPa andfinally to the NaCl (B1)-type phase at 8.90 GPa. The calculated values of the transition pressuresfor AlAs nanocrystals were lower than those of their bulk counterparts [16,28,29], and these aresummarized in Table 1. To make a basic comprehension about the hardness of the nanocrystals,we also calculated the amount of volume collapse at the transition pressure. The relative volume(V/V0) as a function of transition pressure is plotted in Figure 2(a)–(c), and the amount of volumecollapse computed for B3 → B4, B3 →B2 and B3→B1 transitions were found to be 5.8%, 3.5%and 1.5%, respectively.

Figure 1. (a)–(c) Energy as a function volume for AlAs nanocrystals.

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46 Anurag Srivastava and Neha Tyagi

Table 1. Phase transition pressure in AlAs nanocrystals.

Phase transition pressure (GPa)

Transition type Nanocrystals (PW) Bulk AlAs (others)

B3 → B4 3.88 –B3 → B1 8.90 8.18 [16], 8.25 [28], 11.88 [28]B3 → B2 7.12 73.43 [16], 77.9 [29]

Note: PW, present work.

Figure 2. (a)–(c) Relative volume as a function transition pressure for AlAs nanocrystals.

4. Conclusion

In this study, we performed a complete theoretical analysis of the structural phase transition inAlAsnanocrystals in their various possible phases using the ab initio pseudopotential approach basedon DFT. The equilibrium lattice constant and total energy were calculated using the LDA-PZexchange–correlation functional. The lattice constant for bulk AlAs was found to be in closeagreement with the experimental data. We observed that the AlAs nanocrystals show the structuraltransformation from B3 → B4, B3 → B2 and B3 → B1 within the pressure range of 3–9 GPa.Probably being the first to report on the phase transition in AlAs nanocrystals, our reported valuesof transition will certainly stimulate the experimental and/or more theoretical work.

Acknowledgements

The authors thank the ABV-Indian Institute of Information Technology and Management, Gwalior, for providing theinfrastructural support. One of the authors (N.T.) thanks IIITM for the award of PhD scholarship.

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High Pressure Research 47

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