Bilayer sliding mechanism for the wurtzite-to-rocksalt transition

Stokes, Harold T.; Gunter, Jesse; Hatch, Dorian M.; Dong, Jianjun; Wang, Hao; Lewis, James P.

doi:10.1103/physrevb.76.012102

Cited by 18 publications

(17 citation statements)

References 20 publications

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“…Mechanical grinding of wurtzite-type CdSe can induce the formation of the zincblende-type arrangement [26]. Under increasing pressure, the wurtzite structure transforms to the rocksalt structure [27]. Wurtzite CdSe crystallizes in space group P 6 3 mc with the lattice parameters a = 4.302Å and c = 7.014Å.…”

Section: Cadmium Selenidementioning

confidence: 99%

Optical phonons in colloidal CdSe nanorods

Lange

Mohr

Artemyev

et al. 2010

Physica Status Solidi (b)

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Colloidal CdSe nanorods are nanoparticles synthesized in solution. The synthesis allows the growth of CdSe nanorods with well defined diameters and aspect ratios. They have many potential applications in the field of optoelectronics and biotechnology. The nanorods can be epitaxially covered with a graded CdS/ZnS shell of a few monolayers in thickness. The shell leads to an increased quantum efficiency and improved photostability of the nanorods. However, the lattice mismatch between the nanorod core material and the shell material introduces strain into the core lattice. In this work Raman spectroscopy, accompanied by ab-initio calculations, is used to determine the amount of this strain, the exciton-phonon coupling strength in the nanorods and to investigate confinement effects. The longitudinal optical phonons in a nanorod are confined to the nanorod volume. The confinement of a phonon wavefunction leads to a neutralization of the q = 0 rule and the phonon frequency is found to depend on the nanorod diameter. The coupling strength between longitudinal optical phonons and excitons is investigated. It also depends on the nanorod diameter. The total coupling strength is much lower than in bulk material due to a decrease of the influence of the Coulomb interaction in nanoparticles. However, the coupling strength is found to rise for decreasing diameters. This is due to the increasing contributions of higher frequency phonons for smaller nanoparticle sizes. A radial breathing mode with a diameter dependent frequency is deduced from ab-initio calculations and its existence in nanorods is verified experimentally. The diameter-dependence of the modes' frequency can be used to estimate the nanorod diameter from a Raman measurement. In core-shell structures, the coverage of a CdSe nanorod with a ZnS shell leads to a compressive strain of the CdSe core due to the smaller lattice parameter of ZnS compared to CdSe. Ab-initio calculations show that all bonds of the CdSe core are shortened. The bonds in the lateral direction are much more strongly compressed than the bonds parallel to the c-axis. The amount of strain is estimated from the Raman spectra. The compressive nature of the shell decreases for thicker nanorods. The exciton wave function changes with the modified boundary from air to ZnS. This is reflected in a altered exciton-phonon coupling strength and can be monitored in the Raman spectra. ZusammenfassungCdSe Nanorods sind Nanopartikel, die in einer kolloidalen Lösung synthetisiert werden. Für diese existieren viele mögliche Anwendungen im Bereich der Optoelektronik und der Biotechnologie. Der Durchmesser und die Länge der Nanorods lässt sich durch die Syntheseparameter festlegen. Die Nanorods können in eine epitaktische Hülle aus einem Halbleitermaterial mit einer größeren Bandlücke, ZnS, eingebettet werden. Diese Hülle verbessert die optischen Eigenschaften und ermöglicht eine weitere Funktionalisierung der Oberfläche. Der Unterschied der Gitterparameter zwischen ZnS und CdSe führt jedoch zu Verspannung...

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Section: Cadmium Selenidementioning

confidence: 99%

Optical phonons in colloidal CdSe nanorods

Lange

Mohr

Artemyev

et al. 2010

Physica Status Solidi (b)

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“…However, no insight into the atomistic origin of such coexistence has been proposed. Rather, model mechanisms of B3 3 B1 (26) or of B4 3 B1 (18,19,27) transformations are often considered. A combination of B4 and B3 regions in a single material represents a means of achieving the most simple domain boundaries without locally discontinuing the lattice or introducing vacancies.…”

mentioning

confidence: 99%

“…We are interested in studying the mechanisms and length scales of solid-solid transformations, their scaling properties compared with single-domain kinetics in nanocrystals, and the transferability of mechanistic models from nanocrystals to bulk materials. The mechanistic issues of CdSe transformation have been an open problem for many decades (18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28)(29). In experiments on CdSe and ZnO nanocrystals, structural transformations are traced back to a single nucleation event and to simplest kinetics (7,20,21).…”

mentioning

confidence: 99%

Nanodomain fragmentation and local rearrangements in CdSe under pressure

Leoni

Ramlau

Meier

et al. 2008

Proc. Natl. Acad. Sci. U.S.A.

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Structural transformations in extended solids result from local atomic rearrangements and phase growth mechanisms. A broad class of technologically relevant properties critically depends on local structural issues connected with domain sizes, domain boundary geometries, and defects. However, a precise understanding of structural transformation mechanisms and domain formation is still an open question. Here, we demonstrate the feasibility of very detailed mechanistic investigations in real materials as a prerequisite for intelligent property control. We address the problem of domain fragmentation in bulk CdSe under pressure, jointly by molecular dynamics simulations, high-pressure experiments, and HR-TEM imaging. We show that domain fragmentation is taking place in the high-pressure regime, where nucleation events generate both zinc blende (B3) and wurtzite (B4) structural motifs and, in turn, cause the final lamellar appearance observable by highresolution TEM. A changed nucleation pattern and a modified B3/B4 ratio represents the system's response to modified external stress conditions. domains ͉ metastable phases ͉ molecular dynamics ͉ polymorphism ͉ solid-solid phase transition R ecent challenges posed to solid-state sciences in terms of discovery of novel materials and tailoring of properties has made a rational approach to material reactivity and structural polymorphism a task of top priority (1-7). To control properties as diverse as mechanical stability or hardness (8, 9), ferroelectric response (10), and conductivity switching (11), even mass and electron transport properties in general (12, 13), a precise knowledge of atomistic processes in the stadium of phase formation is necessary. Although for a given composition the ground state structures can typically be assigned by theory, the understanding of intermediate, metastable geometries and the prediction of their occurrence implies shedding light on transformation mechanisms between different atomic configurations. Gathering evidence for such mechanisms is an intrinsically difficult task, which in real solids may be further complicated by multidomain scenarios and by high defect concentrations. For this reason, investigations on the kinetics of first-order structural transition have recently turned to nanocrystals as prototypes for single-domain phase transformation (7,14). Therein, the action of a particular mechanism is mapped onto a characteristic, observable change of shape of the nanocrystals. However, a precise atomistic understanding of mechanisms and domain formation and further a reliable extrapolation from nano to bulk material are still open issues.Capturing the complex kinetics of structural transformations from theoretical simulations entails working out an atomistic landscape that allows for a detailed understanding of local structural rearrangements. Recently, we have shown that molecular dynamics (MD) simulations and modeling comprise suitable tools for performing detailed investigations of reconstructive phase transitions at experiment...

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“…Progress in the area of energetically favorable pathways in reconstructive transitions has produced four published papers [42,43,44,45] and one paper submitted [46]. They all obtain a listing of transition pathways (using COMSUBS) and then determine the most favorable pathway using electronic structure methods.…”

mentioning

confidence: 99%

“…Papers in the area of reconstructive transitions analyze the B1 to B2 types of transitions in NaCl and PbS [42], the zinc-blend to rocksalt transition in SiC [45], the α to ω transition in titanium [44], and the wurtzite to rocksalt transition in GaN [46]. As an example let us examine the bilayer sliding mechanism for the zinc-blende to rocksalt transition in SiC.…”

mentioning

confidence: 99%

?Structural Transformations in Ceramics: Perovskite-like Oxides and Group III, IV, and V Nitrides?

Lewis¹,

Co-PI²,

Hatch³

et al. 2006

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Ceramic perovskite-like oxides with the general formula (A A. . .)(B B. . .)O 3 and titanium-based oxides are of great technological interest because of their large piezoelectric and dielectric response characteristics.[1] In doped and nanoengineered forms, titantium dioxide finds increasing application as an organic and hydrolytic photocatalyst. The binary main-group-metal nitride compounds have undergone recent advancements of in-situ heating technology in diamond anvil cells leading to a burst of experimental and theoretical interest. In our DOE proposal, we discussed our unique theoretical approach which applies ab initio electronic calculations in conjunction with systematic group-theoretical analysis of lattice distortions to study two representative phase transitions in ceramic materials: (1) displacive phase transitions in primarily titanium-based perovskite-like oxide ceramics, and (2) reconstructive phase transitions in main-group nitride ceramics. A sub area which we have explored in depth is doped titanium dioxide electrical/optical properties. Work in displacive phase transitions in ceramic-like perovskite alloys has produced six papers in print (or press)[2, 3, 4, 5, 6, 7], one submitted paper[8], and two in preparation[9, 10], which can be grouped into seven main areas: (1) We have used domain average engineering symmetry methods to study heterogeneous domain structures in perovskite ferroelectrics. The domain average symmetry that we consider is applicable to crystals having a large number of domains. For such a case, the domains form a kind of nano-composite. The result[2] was a systematic derivation of all allowed "domain sets", their mesoscopic symmetry, information about the role of domain fractions in determining that symmetry, and the corresponding physical tensor properties for that domain set. The three cases we considered in depth corresponded to polarization (p x , p y , p z) oriented along [100], [110], and [111], resulting in the single domain state symmetries P 4mm, Amm2, and R3m, respectively. For the [111] polarization direction the PZM-PT and PMN-PT crystals are examples of interest. The well known BaTiO 3 is an example of a structural change with polarization along the [100] direction. KNbO 3 is an example of a material which undergoes a transition due to the spontaneous polarization toward the edge diagonals 110. As an example of the results we obtained, the dipole ordering along [100], corresponding to BaTiO 3 , is shown in Table 1. ISOTROPY contains the computer implementation of our algorithm for any space group to subgroup phase transition and is freely available[11]. (2) To validate these theoretical multidomain structures we have developed a fast thermodynamic approach to recreate mesoscopic multidomain ferroelectrics.[3] Using a phase field theory free energy functional we have developed a novel multi-order parameter evolution strategy for ferroelectrics is much faster than previous simulations employing instantaneous mechanical equilibrium[12, 13, 14, 15, 16, 17, 18, 19]....

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Bilayer sliding mechanism for the wurtzite-to-rocksalt transition

Cited by 18 publications

References 20 publications

Optical phonons in colloidal CdSe nanorods

Optical phonons in colloidal CdSe nanorods

Nanodomain fragmentation and local rearrangements in CdSe under pressure

?Structural Transformations in Ceramics: Perovskite-like Oxides and Group III, IV, and V Nitrides?

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