Larger Lattice Constant Research Articles

Superior Photovoltaic Properties of Lead Halide Perovskites: Insights from First-Principles Theory

Organic–inorganic methylammonium lead halide perovskites have recently emerged as promising solar photovoltaic absorbers. In this Feature Article, we review our theoretical understanding of the superior photovoltaic properties, such as the extremely high optical absorption coefficient and very long carrier diffusion length of CH3NH3PbI3 perovskites through first-principles theory. We elucidate that the superior photovoltaic properties are attributed to the combination of direct band gap p–p transitions enabled by the Pb lone-pair s orbitals and perovskite symmetry, high iconicity, large lattice constant, and strong antibonding coupling between Pb lone-pair s and I p orbitals. We show that CH3NH3PbI3 exhibits intrinsic ambipolar self-doping behavior with conductivities tunable from p-type to n-type via controlling the growth conditions. We show that the p-type conductivity can be further improved by incorporating some group IA, IB, or VIA elements at I-rich/Pb-poor growth conditions. However, the n-type co...

Interface-induced warping in hybrid two-dimensional materials

Two-dimensional hybrid materials consisting of heterogeneous domains have been of great interest. Using empirical molecular dynamical simulation, we show that the morphology of such hybrid 2D materials can extend into the third dimension via strong warping intrinsic to the interfaces between the domains. The interface warping stems from the compressive stress in the domain with a larger lattice constant and even penetrates into the stretched domain. Based on classic plate theory, we analytically quantify the amplitude, wave length and penetration depth of the interface warping as functions of the lattice mismatch, achieving good agreement with the simulations. Moreover, we propose that periodically placing pentagon-heptagon dislocations along the interface can eliminate the warping in the 2D material and such defective interface can be more favorable than the warped one over a critical domain size, which is consistent with recent experimental observations. Our results suggest that the interface warping in 2D hybrid materials should be considered in further exploring their promising properties.

Effect of InGaAs interlayer on the properties of GaAs grown on Si (111) substrate by molecular beam epitaxy

High-quality GaAs films have been epitaxially grown on Si (111) substrates by inserting an InxGa1−xAs interlayer with proper In composition by molecular beam epitaxy (MBE). The effect of InxGa1−xAs (0 < x < 0.2) interlayers on the properties of GaAs films grown on Si (111) substrates by MBE has been studied in detailed. Due to the high compressive strain between InGaAs and Si, InGaAs undergoes partial strain relaxation. Unstrained InGaAs has a larger lattice constant than GaAs. Therefore, a thin InGaAs layer with proper In composition may adopt a close lattice constant with that of GaAs, which is beneficial to the growth of high-quality GaAs epilayer on top. It is found that the proper In composition in InxGa1−xAs interlayer of 10% is beneficial to obtaining high-quality GaAs films, which, on the one hand, greatly compensates the misfit stress between GaAs film and Si substrate, and on the other hand, suppresses the formation of multiple twin during the heteroepitaxial growth of GaAs film. However, when the In composition does not reach the proper value (∼10%), the InxGa1−xAs adopts a lower strain relaxation and undergoes a lattice constant smaller than unstrained GaAs, and therefore introduces compressive stress to GaAs grown on top. When In composition exceeds the proper value, the InxGa1−xAs will adopt a higher strain relaxation and undergoes a lattice constant larger than unstrained GaAs, and therefore introduces tensile stress to GaAs grown on top. As a result, InxGa1−xAs interlayers with improper In composition introduces enlarged misfit stress to GaAs epilayers grown on top, and deteriorates the quality of GaAs epilayers. This work demonstrates a simple but effective method to grow high-quality GaAs epilayers and brings up a broad prospect for the application of GaAs-based optoelectronic devices on Si substrates.

Lithium silicide nanocrystals: synthesis, chemical stability, thermal stability, and carbon encapsulation.

Lithium silicide (LixSi) is the lithiated form of silicon, one of the most promising anode materials for the next generation of lithium-ion batteries (LIBs). In contrast to silicon, LixSi has not been well studied. Herein we report a facile high-energy ball-milling-based synthesis of four phase-pure LixSi (x = 4.4, 3.75, 3.25, and 2.33), using hexane as the lubricant. Surprisingly, the obtained Li3.75Si phase shows significant downward shifts in all X-ray diffraction peak positions, compared with the standard. Our interpretation is that the high-energy ball-mill-synthesized Li3.75Si presents smaller internal pressures and larger lattice constants. The chemical-stability study reveals that only surface reactions occur after Li4.4Si and Li3.75Si are immersed in several battery-assembly-related chemicals. The thermal-stability study shows that Li4.4Si is stable up to 350 °C and Li3.75Si is stable up to 200 °C. This remarkable thermal stability of Li3.75Si is in stark contrast to the long-observed metastability for electrochemically synthesized Li3.75Si. The carbon encapsulation of Li4.4Si has also been studied for its potential applications in LIBs.

Effect of lattice strain on the oxygen vacancy formation and hydrogen adsorption at CeO2(111) surface

Using first-principles calculation, the effect of lattice strain on the oxygen vacancy formation at CeO2(111) surface has been investigated. The tensile strain facilitates the oxygen vacancy formation at the surface and the compressive strain hinders the process. This is in part due to the strengthening or weakening of the surface Ce–O bond under the lattice strain. On the other hand, a more open surface with a larger lattice constant can better accommodate the larger Ce3+ and thus facilitate the structural relaxation of the reduced surface. The studies on the strain effect on the atomic hydrogen adsorption at the defect-free CeO2(111) surface show that the adsorption strength monotonously increases with the increase of the lattice strain, further confirming the tunable surface chemical activity by lattice strain.

Parametrization of Extended Gaussian Disorder Models from Microscopic Charge Transport Simulations.

Simulations of organic semiconducting devices using drift-diffusion equations are vital for the understanding of their functionality as well as for the optimization of their performance. Input parameters for these equations are usually determined from experiments and do not provide a direct link to the chemical structures and material morphology. Here we demonstrate how such a parametrization can be performed by using atomic-scale (microscopic) simulations. To do this, a stochastic network model, parametrized on atomistic simulations, is used to tabulate charge mobility in a wide density range. After accounting for finite-size effects at small charge densities, the data is fitted to the uncorrelated and correlated extended Gaussian disorder models. Surprisingly, the uncorrelated model reproduces the results of microscopic simulations better than the correlated one, compensating for spatial correlations present in a microscopic system by a large lattice constant. The proposed method retains the link to the material morphology and the underlying chemistry and can be used to formulate structure-property relationships or optimize devices prior to compound synthesis.

Novel Electronic States of Heavy Fermion Compound YbCo2Zn20

We studied the heavy fermion compound YbCo2Zn20 with an electronic specific heat coefficient γ \( \simeq \) 8000 mJ/(K2·mol) by measuring the de Haas–van Alphen (dHvA) oscillation, Hall effect, magnetic susceptibility, and magnetization at ambient pressure, as well as the electrical resistivity in magnetic fields of up to 320 kOe and at pressures of up to 5 GPa. The detected Fermi surfaces are small in volume, reflecting the small Brillouin zone based on the large cubic lattice constant a = 14.005 A. The cyclotron effective masses, which were determined from the dHvA experiment, are found to be markedly reduced in magnetic fields. In other words, the detected cyclotron masses of 2.2–8.9 m0 (m0: the rest mass of an electron) at Hav = 117 kOe are enhanced to 100–500 m0 at 0 kOe. By applying pressure, the heavy fermion state disappears at Pc \( \simeq \) 1.8 GPa and orders antiferromagnetically for P > Pc. The field-induced antiferroquadrupolar phase, which is observed only for \(H\parallel \langle 111\rangl...

Hydrogen absorption and hydrogen-induced reverse segregation in palladium–silver surface

Hydrogen absorption from surface to second subsurface layer of ordered and substitutionally disordered Pd3Ag(110) was investigated by performing density functional theory-based calculations. Ag segregation to topmost layer was found to be energetically favored for clean surface structure. The absorption of H in the alloy surface, for both ordered and substitutionally disordered structures, is generally characterized by lower activation barrier as compared to pure Pd surface. When H is present in the surface or subsurface regions, our results strongly suggest the tendency of subsurface Pd atoms to segregate to the surface or near surface region. The reverse segregation of Pd, along with the large lattice constant and the modified electronic structure of Pd atoms in Pd3Ag alloy could help in understanding the experimentally determined high hydrogen permeability of PdAg membranes.

Optical floating zone crystal growth and magnetic properties of MgCr2O4

Large, high-quality single crystals of the magnesium chromate spinel, MgCr2O4, have been grown by the optical floating zone technique. The impact of experimental parameters including the feed rod density, growth atmosphere, temperature gradient, and the growth rate on eventual crystal quality have been studied. The best single crystals were obtained by growing in purified argon, at high growth rates of 8–24mm/h and using a slow cooling procedure. The crystals were characterized using Laue and powder X-ray diffraction, energy dispersive X-ray analysis, and magnetization measurements. Lattice parameter measurements taken from the crystals prepared at various growth conditions combined with the magnetic data show that off-stoichiometric crystals have larger lattice constants and a higher magnetic susceptibility at TN. Our results demonstrate the importance of correlating growth conditions with structural and physical properties to produce high quality single crystals that have physical properties commensurate with stoichiometric powder samples.

The mystery of abnormally large volume of PbCrO3 with a structurally consistent Hubbard U from first-principles

PbCrO3 is found experimentally to be a cubic perovskite with an abnormally large lattice constant of about 4.00 A. To date, no successful first principle calculations have been carried out for cubic PbCrO3 perovskite. Combining the generalized-gradient approximations and a structurally consistent Hubbard U approach, the crystal, electronic and magnetic structures of PbCrO3 are calculated to explore the mystery of abnormally large volume of PbCrO3 in cubic structure. The crystal structure, magnetic order and half-metallic properties of PbCrO3 are determined. For the first time, a structurally consistent Hubbard U eff of about 8.28 eV for PbCrO3 is calculated based on linear-response approach. With the structurally consistent U obtained by first principles, a \(Pm\bar 3m\) cubic structure with a lattice constant of about 3.98 A for PbCrO3 is obtained successfully. Moreover PbCrO3 is found to be half-metallic ferromagnet with an integral magnet moment of 2.00μ B per unit cell. The anomalously large volume of PbCrO3 in cubic structure results from the strongly correlated electron interaction. The results obtained agree well with experimental data.

Changes in the Structure of Palladium Nanograins in the Carbon Film (C-nPd) under the Influence of Hydrogen

Carbonaceous – nanopalladium (C-nPd) films on insulating substrates, obtained by PVD (Physical Vapor Deposition) followed by the annealing method, are used for hydrogen sensors. In this paper we present the results of XRD, SEM and electrical measurements of C-nPd films grown by PVD and then either annealed in an inert atmosphere or modified by CVD (Chemical Vapor Deposition). The structure of palladium grains were measured in a H2/N2gas mixture atmosphere containing H2in the concentration range 0-4%. Our measurements showed that C-nPd films prepared by PVD and annealing method were flat and they consisted of sphere-like palladium nano-particles, uniformly distributed in the low-ordered carbon matrix. Effect of hydrogen partial pressure on the crystal structure was studied in situ in GIXD measurements, using a specially designed measuring holder cell. It was observed that at low partial pressure of hydrogen, palladium fcc metal particles transformed into solid solution Pd(H) (α-phase), which occurs in the fcc structure as well. Lattice constant of that solid solution increases with increasing hydrogen partial pressure in the gas atmosphere. After exceeding the critical value of the partial pressure of hydrogen, palladium transforms into the structure of palladium hydride PdHx(β-phase), which has significantly larger lattice constant. This critical value of hydrogen partial pressure depends on the initial structure of C-nPd film, and consequently on the parameters of PVD process and annealing. This value usually corresponds to a few percent hydrogen content in gas mixture.

Heteroepitaxial Formation of Aligned Mesostructured Silica Films with Large Structural Periodicities from Mixed Surfactant Systems

Liquid-crystal phases consisting of cylindrical micelles of amphiphilic block copolymers and silica precursors are epitaxially built up on aligned surface micelles formed by an alkyl-PEO surfactant, Brij56, irrespective of the large difference in the intrinsic structural periodicities resulting in the formation of fully aligned mesostructured silica films with large lattice constants. Brij56 works as an alignment controlling agent on rubbing-treated polyimide through selective adsorption from a precursor solution containing the two surfactants, a block copolymer and Brij56, through strong hydrophobic interactions to form an anisotropic surface micelle structure. Aligned mesostructured silica layers with larger periodicities, which dominantly consist of block copolymers, form on these aligned surface micelles by gradually changing the vertical periodicity keeping the lateral intermicelle distance constant. This can be regarded as a kind of heteroepitaxy because the lattice constant at the surface is different from that of the bulk of the film. On the basis of this new concept, highly aligned mesostructured silica films with structural periodicities as large as 10 nm are successfully formed, which has never been achieved when the block copolymers are used alone as the structure-directing agent. The periodicity of the aligned films can precisely be controlled by an appropriate choice of block copolymers and the mixing ratio of the two surfactants, which increases the opportunity for applications of these films with highly anisotropic mesoscale structure.

Effect of Guest Size and Conformation on Crystal Structure and Stability of Structure H Clathrate Hydrates: Experimental and Molecular Dynamics Simulation Studies

To better understand the effect of size and flexibility of large molecule guest substances (LMGSs) on crystal lattice structure and thermodynamic stability of structure H (sH) clathrate hydrates, we performed powder X-ray diffraction (PXRD) measurements and Parrinello–Rahman molecular dynamics (MD) simulations on six alkane and cycloalkane LMGS in the presence of methane help gas. We first quantitatively analyze the dependence of the experimental lattice constants and formation pressure on the average maximum length of the LMGS guests as determined by MD simulation. The PXRD results show that within a family of LMGSs with similar molecular structures molecules of optimum size give better stability of sH hydrate phase. The most stable hydrate in each class has larger lattice constants along the a-axis and smaller lattice constant along the c-axis. The lattice expansion/shrinkage is well reproduced in the MD simulations. From the MD simulations, we determine the changes in the conformation as a result of en...

Strain effect of multilayer FeN structure on GaAs substrate

Overly doped FeN multilayer structure on GaAs substrate was fabricated. After the post-annealing process, FeN martensite in each Fe/FeN layer formed partially chemically ordered Fe16N2, which was observed by X-ray diffraction. To detect the saturation magnetization (Ms) depth profile, polarized neutron reflectivity was conducted. Fe/FeN layer showed a significant improvement of Ms for each layer compared to Ms of Fe. More importantly, different FeN layers showed different Ms according to the physical distance to the substrate GaAs. The most enhanced Ms (exceeding the limit of Fe65Co35 Ms) observed at the bottom part of the film, consistent with previous reports, should be attributed to the lattice strain by GaAs substrate. In order to detect the lattice constant, In-plane X-ray Diffraction was done and a large in-plane lattice constant was determined.

Antiferromagnetic half-metallicity of transition metal nitrides under volume expansion

The transition metal pnictides ABX2 have been recently proposed as half-metallic fully compensated ferrimagnets, also briefly referred to as half-metallic “antiferromagnets” [N. Long, M.Ogura, H. Akai, J. Phys.: Condens. Matter 21, 064241 (2009)]. In this work we carry out a systematic study of the more promising cases of the transition metal nitrides MnCoN2 and NiCrN2 on the basis of density functional theory in the framework of full-potential linearized augmented plane wave method. The electronic structures and the magnetic properties of the above hypothetical compounds in Zinc-blende-type, NaCl-type, and Wurtzite-type structure are calculated within generalized gradient approximation. The results reveal that, although these compounds are metallic in their bulk equilibrium in all three structures, they exhibit antiferromagnetic half-metallicity under negative stress or volume expansion in a limited range of lattice parameters, which is significantly larger than the equilibrium values. This suggests that a situation in which half-metallicity may arise, is when these compounds are coated on semiconducting layers of larger lattice constant.

Stabilization criteria for cubic AlN in TiN/AlN and CrN/AlN bi-layer systems

We combine the finite element (ABAQUS) and ab initio methods to study and predict the equilibrium critical thickness up to which the metastable cubic (c) AlN phase is energetically favoured to the stable wurtzite (w) variant in TiN/AlN and CrN/AlN bi-layer systems. The results show that the w-AlN phase is preferred for all thicknesses in the free-standing configuration (without a substrate) when grown on TiN, while 4 nm thick c-AlN is predicted for CrN bi-layer material systems. The substrate helps to stabilize c-AlN up to 15.8 nm, and for an incoherent interface between the substrate and the TiN or CrN interlayer, the stabilization mechanically supports the interlayer against relaxation. For a coherent interface to the substrate, a small lattice constant (as, e.g., in the case of Al substrate) helps to stabilize c-AlN, whereas a large lattice constant (as, e.g., in the case of MgO) promotes w-AlN. Finally, we predict that 〈1 1 1〉 orientated specimens allow for thicker c-AlN layers than those grown along the 〈1 0 0〉 direction.

Collective light emission of a finite-size atomic chain

The radiative properties of collective electronic states for a finite one-dimensional atomic chain are investigated. The influence of the chain size on the collective damping rates is examined through the dependence on both the chain length and the lattice constant. The symmetric state damping rate is shown to grow up linearly with the length for short chains, in relative to the atomic transition wavelength, and to saturate for long ones. For large lattice constants the saturation is achieved for small number of chain atoms, and vice versa for small ones. The results can be adopted for any chain of active materials, e.g., an array of quantum dots or organic molecules on a linear matrix.

Periodic overlayers and moiré patterns: theoretical studies of geometric properties

Single crystal surfaces with periodic overlayers, such as graphene on hexagonal metal substrates, are found to exhibit, apart from their intrinsic periodicity, additional long-range order expressed by approximate surface lattices with large lattice constants. This phenomenon can be described as geometrically analogous to lateral interference effects resulting in periodic moiré patterns which are characterized by two-dimensional moiré lattices. Here we discuss in detail the mathematical formalism determining such moiré patterns based on concepts of two-dimensional Fourier transformation including coincidence lattices. The formalism provides simple relations that allow one to calculate possible moiré lattice vectors in their dependence on rotation angles α and scaling factors p1,p2 for periodic (p1 × p2)Rα overlayers on substrate surfaces described by general Bravais lattices. Specific emphasis will be given to hexagonal lattices where experimental data are available.

Effect of Grain Boundary on Spinodal Decomposition Using the Phase Field Crystal Method

Spinodal decomposition (SD) with different grain boundaries (GBs) is investigated on the atomic scale using the novel phase field crystal model. It is demonstrated that the decomposition process is initiated by precipitating one phase with a larger lattice constant in the tension region at the GBs and the other one with a smaller lattice constant in the compression region. As the phase separation proceeds, the dislocations comprising the low-angle GBs migrate toward the compositional domain boundaries to relieve the coherent strain energy, and eventually become randomly distributed in the coarsening regime of SD, which leads to the disappearance of the low-angle GBs. For high-angle GBs, the location of GBs remains unchanged, while the atoms rearrange along the GBs to fit the stress field arising from compositional inhomogeneity.

Hierarchical Freezing in a Lattice Model

A certain two-dimensional lattice model with nearest and next-nearest neighbor interactions is known to have a limit-periodic ground state. We show that during a slow quench from the high temperature, disordered phase, the ground state emerges through an infinite sequence of phase transitions. We define appropriate order parameters and show that the transitions are related by renormalizations of the temperature scale. As the temperature is decreased, sublattices with increasingly large lattice constants become ordered. A rapid quench results in a glasslike state due to kinetic barriers created by simultaneous freezing on sublattices with different lattice constants.