Y. M. Wang

Nature of interfacial defects and their roles in strain relaxation at highly lattice mismatched 3C-SiC/Si (001) interface

Misfit defects in a 3C-SiC/Si (001) interface were investigated using a 200 kV high-resolution electron microscope with a point resolution of 0.194 nm. The [110] high-resolution electron microscopic images that do not directly reflect the crystal structure were transformed into the structure map through image deconvolution. Based on this analysis, four types of misfit dislocations at the 3C-SiC/Si (001) interface were determined. In turn, the strain relaxation mechanism was clarified through the generation of grow-in perfect misfit dislocations (including 90° Lomer dislocations and 60° shuffle dislocations) and 90° partial dislocations associated with stacking faults.

Effects of Cr, Mn on the cohesion of the γ-iron grain boundary

Abstract The effect of alloying elements Cr and Mn on the cohesion of the γ-iron Σ11[1 1 0]/(11 3 ) grain boundary (GB) is investigated based on the thermodynamic model of Rice–Wang by using the first-principles density functional theory. The electronic properties are studied for Cr/Fe and Mn/Fe systems. In these systems, the chemical effect of Cr and Mn is in favor of enhancing the cohesion of the grain boundary due to the anisotropic bonding which weakens the bonds in the grain boundary plane, but strengthens those in planes perpendicular to the grain boundary. However, the structural relaxation effect is detrimental to the cohesion of the grain boundary. After synthesizing these two effects, Cr can act as a cohesion enhancer and Mn is an embrittler.

Vertically standing Ge nanowires on GaAs(110) substrates

The growth of epitaxial Ge nanowires is investigated on (100), (111) B and (110) GaAs substrates in the growth temperature range from 300 to 380u2009°C. Unlike epitaxial Ge nanowires on Ge or Si substrates, Ge nanowires on GaAs substrates grow predominantly along the [Formula: see text] direction. Using this unique property, vertical [Formula: see text] Ge nanowires epitaxially grown on GaAs(110) surface are realized. In addition, these Ge nanowires exhibit minimal tapering and uniform diameters, regardless of growth temperatures, which is an advantageous property for device applications. Ge nanowires growing along the [Formula: see text] directions are particularly attractive candidates for forming nanobridge devices on conventional (100) surfaces.

The effects of 3d alloying elements on grain boundary cohesion in γ-iron: a first principles study on interface embrittlement due to the segregation

By the use of a first principles density functional theory, two kinds of models, namely the Rice-Wang thermodynamics model and the Seah quasi-chemical model, are employed to evaluate the embrittling tendency of a grain boundary (GB) due to the 3d element segregation. The first principles method based on those two models is appropriate for calculating the chemical and structural relaxation contributions to the changes of GB cohesion with the 3d segregants. The effects of the 3d transition elements, such as Ti, V, Cr and Mn, on a stable fcc Fe Sigma 11 [1 (1) over bar0]/(11 (3) over bar) GB are studied and the difference between these two models is interpreted. When the chemical and the structural relaxation effects are taken into account, the calculated results for these two models are coincident for most of the elements studied, except for chromium. After analysing their chemical bonding in detail, we find that this discrepancy may be attributable to a lower susceptibility of the Seah model to the bonding anisotropy caused by Cr in the GB. It is proposed that the Seah model should be prudently used for some elements, especially those lying in the middle of a transition period.

First-principles study of the segregation effects on the cohesion of F.C.C. grain boundary

A scheme to evaluate the Griffith work of interfacial separation, 2 gamma (int), is proposed based on first principles to investigate the segregation effects on a Sigma 11[1 (1) over bar0]/(11 (3) over bar) gamma -iron grain boundary. The chemical interaction of substitutional segregants Cr and Mn is able to enhance the cohesion of the grain boundary by anisotropic bonding, which weakens the bondings in the grain boundary plane and strengthens those in the vertical plane. However, their structural relaxation contributions are both detrimental to the cohesion of the grain boundary. After combining these two contributions, Cr acts as a cohesion enhancer but Mn as an embrittler. The interstitial segregants carbon and nitrogen can strengthen the cohesion of the grain boundary by forming strong bonding with their neighbour Fe atoms and restraining their surrounding Fe atom relaxation. The ability of carbon and nitrogen to improve the property of the grain boundary is relative to the environment of their segregation sites. The consistency between the present work and the previous reports gives evidence for the correctness of the scheme.

First-principles investigations of the pressure-induced structural transitions in Mg(AlH4)2

A systematic investigation is presented of the high-pressure structural stability of Mg(AlH(4))(2) using a plane-wave pseudo-potential method. The total-energy calculations show that under ambient pressure the structure of α-Mg(AlH(4))(2) found by experiments is more stable than the other proposed structures, and with pressure increasing the α to β (δ-Zr(MoO(4))(2)-type structure) and β to γxa0(Ca(BF(4))(2)-type structure) transitions occur at 0.67 and 10.28xa0GPa respectively, accompanied with volume reductions of 6.6% and 8.7%. A detailed study of the electronic structures reveals the bonding characteristics between Al and H and between Mg and H as well as the nonmetallic features of α, β, and γ phases under pressure of up to 20.0 GPa. Their electronic structures are mainly responsible for the relative high-pressure stability of the three phases. Finally, an analysis of their structural relations indicates that it is possible to produce the [Formula: see text] structural transition by applying pressure.

Controlled Growth and Atomic-Scale Mapping of Charged Heterointerfaces in PbTiO3/BiFeO3 Bilayers

Functional oxide interfaces have received a great deal of attention owing to their intriguing physical properties induced by the interplay of lattice, orbital, charge, and spin degrees of freedom. Atomic-scale precision growth of the oxide interface opens new corridors to manipulate the correlated features in nanoelectronics devices. Here, we demonstrate that both head-to-head positively charged and tail-to-tail negatively charged BiFeO3/PbTiO3 (BFO/PTO) heterointerfaces were successfully fabricated by designing the BFO/PTO film system deliberately. Aberration-corrected scanning transmission electron microscopic mapping reveals a head-to-head polarization configuration present at the BFO/PTO interface, when the film was deposited directly on a SrTiO3 (001) substrate. The interfacial atomic structure is reconstructed, and the interfacial width is determined to be 5-6 unit cells. The polarization on both sides of the interface is remarkably enhanced. Atomic-scale structural and chemical element analyses exhibit that the reconstructed interface is rich in oxygen, which effectively compensates for the positive bound charges at the head-to-head polarized BFO/PTO interface. In contrast to the head-to-head polarization configuration, the tail-to-tail BFO/PTO interface exhibits a perfect coherency, when SrRuO3 was introduced as a buffer layer on the substrates prior to the film growth. The width of this tail-to-tail interface is estimated to be 3-4 unit cells, and oxygen vacancies are supposed to screen the negative polarization bound charge. The formation mechanism of these distinct interfaces was discussed from the perspective of charge redistribution.

Polarization Rotation in Ultrathin Ferroelectrics Tailored by Interfacial Oxygen Octahedral Coupling

Multiple polar states and giant piezoelectric responses could be driven by polarization rotation in ferroelectric films, which have potential functionalities in modern material applications. Although theoretical calculations have predicted polarization rotation in pure PbTiO3 films without domain walls and strains, direct experiment has rarely confirmed such polar states under this condition. Here, we observed that interfacial oxygen octahedral coupling (OOC) can introduce an oxygen octahedral rotation, which induces polarization rotation in single domain PbTiO3 films with negligible strains. We have grown ultrathin PbTiO3 films (3.2 nm) on both SrTiO3 and Nb:SrTiO3 substrates and applied aberration-corrected scanning transmission electron microscopy (STEM) to study the interfacial OOC effect. Atomic mappings unit cell by unit cell demonstrate that polarization rotation occurs in PbTiO3 films on both substrates. The distortion of oxygen octahedra in PbTiO3 is proven by annular bright-field STEM. The critical thickness for this polarization rotation is about 4 nm (10 unit cells), above which polarization rotation disappears. First-principles calculations manifest that the interfacial OOC is responsible for the polarization rotation state. These results may shed light on further understanding the polarization behavior in ultrathin ferroelectrics and be helpful to develop relevant devices as polarization rotation is known to be closely related to superior electromechanical responses.

Hydrogen sites occupation in alpha-LaNi4Al hydride

The hydrogen sites occupation in La2Ni8Al2H has been investigated by a first-principle calculation to study the effect of the Al atom on the location of hydrogen and to explore the dominating factors to determine the hydrogen sites. It is found that the hydrogen cannot be stationed at interstitial sites surrounded by atoms including Al. Judged from the solution energy of hydrogen, the 12n site is most stable and the 4h site cannot accommodate H. The hydrogen occupation in LaNi4Al shows a distinct way from that in LaNi5, where five nonequivalent interstices (3f, 4h, 6m, 12o, 12n) have been reported to be available for the hydrogen in LaNi5. This indicates that the Al atom has important influence on the hydrogen occupation. The seems to be a parameter to measure the stability of La2Ni8Al2H. By analyzing the density of states combined with the local environment of sites, the bond order (BO) and average Mulliken charge (AMC), we found that the 12n and 3f sites have similar atomic and electronic structure and the covalence and ionicity. The 3f site is also favorite occupation for hydrogen besides the 12n site.

Rhombohedral–Orthorhombic Ferroelectric Morphotropic Phase Boundary Associated with a Polar Vortex in BiFeO3 Films

Strongly correlated oxides exhibit multiple degrees of freedoms, which can potentially mediate exotic phases with exciting physical properties, such as the polar vortex recently found in ferroelectric oxide films. A polar vortex is stabilized by competition between charge, lattice, and/or orbital degrees of freedom, which displays vortex-ferroelectric phase transitions and emergent chirality, making it a potential candidate for designing information storage and processing devices. Here, by a combination of controlled film growth and aberration-corrected scanning transmission electron microscopy, we obtain nanoscale vortex arrays in [110]-oriented BiFeO3 films. These vortex arrays are stabilized in ultrathin BiFeO3 layers sandwiched by two coherently grown orthorhombic scandate layers, exhibiting a ferroelectric morphotropic phase boundary constituted by a mixed-phase structure of polar orthorhombic BiFeO3 and rhombohedral BiFeO3. Clear polarization switching and piezoelectric signals were observed in these multilayers as revealed by piezoresponse force microscopy. This work presents a feature of a polar vortex in BiFeO3 films showing morphotropic phase boundary character, which offers a potential degree of manipulating phase components and properties of ferroelectric topological structures.