Minimization Of Interfacial Energy Research Articles

Exploring mechanisms for wettability alteration in low-salinity waterfloods in carbonate rocks

Low Salinity waterfloods, even in carbonates, have received a great deal of attention because of their potential to significantly increase waterflood recoveries in addition to being cost effective. Wettability alteration has been found to play a major role in affecting higher recoveries from carbonates in low salinity floods. A large volume of research has accumulated for determining the effect of brine chemistry on wettability alteration and consequent recoveries. Both divalent cations (Ca2+, Mg2+) and anions (SO42−) as well as their interactions have been found to affect wettability. Charges on the mineral surface were also linked to observed wettability change. Despite the efforts, a clear understanding of the mechanisms for such wettability changes seems to be still elusive. Here in our work, first we have attempted to find viable alternatives for sulfates which pose fouling risks for reservoirs. Sulfur oxyanions such as bisulfites and metabisulfites have been found to be potential anions capable of affecting the desired wettability change without the risk associated with sulfates. Secondly, an effort has been made to understand the mechanism (s) behind such wettability alteration in carbonates during low salinity waterfloods based on our work and that by various authors. A potential mechanism has been proposed for wettability alteration, which seems to explain the works of many researchers. The mechanism is based on the isoelectric point of the carbonate rock surface, pH and interfacial energy between the aqueous brine solution and the rock surface. It is proposed that carbonates would be strongly oil-wet at the isoelectric point of the carbonate rock crystal, when the aqueous solution-rock interfacial tension is maximum. On the other hand, it would exhibit more water-wet or less oil-wet behavior, the further the pH of the brine solution is away from the isoelectric point of the carbonate rock. The wettability change is as a result of electrostatic attraction between polar water molecules with the charged surface that helps to bring down the brine-rock interfacial energy. This minimization of interfacial energy is manifested as the wettability change at the 3- phase carbonate mineral-oil-water contact line. The shift in the isoelectric point due to changes imposed in brine composition needs to be accounted for in addition to brine pH and salinity in order to optimize recoveries from low-salinity waterfloods.

Evolution of Ternary AuAgPd Nanoparticles by the Control of Temperature, Thickness, and Tri-Layer

Metallic alloy nanoparticles (NPs) possess great potential to enhance the optical, electronic, chemical, and magnetic properties for various applications by the control of morphology and elemental composition. This work presents the fabrication of ternary AuAgPd alloy nanostructures on sapphire (0001) via the solid-state dewetting of sputter-deposited tri-metallic layers. Based on the systematic control of temperature, thickness, and deposition order of tri-layers, the composite AuAgPd alloy nanoparticles (NPs) with various shape, size, and density are demonstrated. The metallic tri-layers exhibit various stages of dewetting based on the increasing growth temperatures between 400 and 900 °C at 15 nm tri-layer film thickness. Specifically, the nucleation of tiny voids and hillocks, void coalescence, the growth and isolated nanoparticle formation, and the shape transformation with Ag sublimation are observed. With the reduced film thickness (6 nm), tiny alloy NPs with improved structural uniformity and spatial arrangement are obtained due to enhanced dewetting. The growth trend of alloy NPs is drastically altered by changing the deposition order of metallic tri-layers. The overall evolution is governed by the surface diffusion and inter-mixing of metallic atoms, Rayleigh-like instability, surface and interface energy minimization, and equilibrium state of the system. The UV-VIS-NIR reflectance spectra reveal the formation of an absorption band and reflectance maxima at specific wavelengths based on the morphology and composition of AuAgPd alloy NPs. In addition, Raman spectra analysis shows the modulation of intensity and peak position of natural vibration modes of sapphire (0001).

Alternative misfit dislocations pattern in semi-coherent FCC {100} interfaces

The character of interface misfit dislocations is determined according to interface crystallography and minimization of interface energy, which includes coherent interface energy and dislocation line energy. The core energy of dislocations is generally ignored in such analysis. In this work, we demonstrate that the core energy of misfit dislocations is dependent on the mechanical and thermal loading condition, and ultimately determines the nature of interface misfit dislocation patterns (MDP). Employing atomistic simulations with empirical interatomic potentials, we show the transformation of conventional MDP consisting of a/2<110> dislocation into an alternative MDP consisting of mixed a<100> and a/2<110> dislocations under elevated temperatures and/or normal-to-interface tensile stresses. Although a<100> type dislocations typically have greater line energy in bulk, molecular statics/dynamics calculations show that a<100> type misfit dislocations are preferred over a/2<110> type under elevated temperatures and/or normal-to-interface tensile stresses due to their reduced core energy. In addition, we found that the a<100> dislocations possess significantly reduced vacancy formation energies compared to the a/2<110> dislocations. The potential application of this unique property of the alternative dislocation pattern for nanoscale multilayered composite as a functional material is discussed.

Highly efficient electrically conductive networks in carbon‐black‐filled ternary blends through the formation of thermodynamically induced self‐assembled hierarchical structures

ABSTRACTHighly efficient electrical conductive networks were constructed in carbon‐black (CB)‐filled polyoxymethylene (POM)–thermoplastic polyurethane (TPU)–polyamide 6 (PA6) ternary blends through the formation of a hierarchical structure composed of a minor PA6 phase as droplets inside one major phase (TPU) and CB particles localized at the TPU–PA6 interface by thermodynamically induced self‐assembly. The hierarchical structure was thermodynamically predicted on the basis of the minimization of total interfacial energies and confirmed by electron microscopy. The degrees of the TPU phase continuity before and after the addition of PA6 were determined by solvent‐extraction experiments. The percolation threshold of CB decreased by 50% compared to that in the POM–TPU binary blend because of the more efficient formation of a CB conductive network through CB‐covered PA6 domains inside the TPU phase. The hierarchical structure not only increased the electrical conductivity of the composites but also improved their thermal stability in comparison with the simple structure formed by the homogeneously dispersed CB particles in POM. The method reported in this article can offer possibilities for improving the comprehensive properties of the conductive composites and the widening of their applications. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 45877.

Crystallization of MgAl2O4 and its effect on strengthening-toughening behavior in TiAlN/MgAl2O4 nanomultilayers

TiAlN/MgAl2O4 nanomultilayers with different MgAl2O4 layer thickness (lMgAl2O4) were fabricated. Studies show that under the template effect of TiAlN layers driven by interfacial energy minimization, amorphous MgAl2O4 crystallized and grew epitaxially with TiAlN layers, forming high-quality layered structure and coherent interface at low lMgAl2O4 (≤0.7 nm) and achieving maximum hardness of ∼46.0 GPa at lMgAl2O4 = 0.7 nm. Nevertheless, the hardness enhancement was obviously weakened by the partial amorphization of MgAl2O4 layer at larger lMgAl2O4. Additionally, the formation of layered-structure in the TiAlN/MgAl2O4 nanomultilayers remarkably improved the fracture toughness by deflecting and blunting cracks through layer interface.

Effects of Contact-Line Pinning on the Adsorption of Nonspherical Colloids at Liquid Interfaces.

The effects of contact-line pinning are well known in macroscopic systems but are only just beginning to be explored at the microscale in colloidal suspensions. We use digital holography to capture the fast three-dimensional dynamics of micrometer-sized ellipsoids breaching an oil-water interface. We find that the particle angle varies approximately linearly with the height, in contrast to results from simulations based on the minimization of the interfacial energy. Using a simple model of the motion of the contact line, we show that the observed coupling between translational and rotational degrees of freedom is likely due to contact-line pinning. We conclude that the dynamics of colloidal particles adsorbing to a liquid interface are not determined by the minimization of interfacial energy and viscous dissipation alone; contact-line pinning dictates both the time scale and pathway to equilibrium.

Connection between Atomic Scale Characterization and Electrochemical Behavior during Passivation of Polycrystalline and Single Crystal Ni-Cr and Ni-Cr-Mo Alloys

Ni-Cr and Ni-Cr-Mo alloys owe their superior corrosion resistance to the surface enrichment of passivating Cr-rich oxides or hydroxides and the synergistic effect of Mo in the film1–3. However, the specific role of alloying elements such as Mo are not well-understood, especially with respect to their position relative to the oxide/metal interface and atomistic influence on film growth, localized breakdown, and repassivation processes. The composition, structure, and thickness of the passivating oxide films are additionally challenging to characterize in-situ due to their nanoscale dimensions and the high electric field imposed by aqueous growth. The key processes which take place in oxide films and regulate their behavior in aqueous environments are atomic, ionic, and electronic in nature and currently poorly understood, often requiring study at the resolution and detection limits of characterization methods4. The main goal of this work is to integrate bulk polycrystalline and single crystal electrochemical measurements with nanoscale characterization to advance the fundamental understanding of the structure, molecular, and electronic properties of oxides formed during aqueous passivation and uncover the role of Mo on surface stability in corrosive environments. The alloys under study are Ni-11Cr, Ni-11Cr-6Mo, Ni-22Cr, and Ni-22Cr-6Mo, wt%, polycrystalline solid solutions with additional focus on individual grain orientations in isolation from grain or twin boundaries. DC and AC electrochemical techniques were utilized for exploring the controlled growth of passive oxide films on these materials in aqueous chloride and chloride-free solutions of various acidity. The repassivation kinetics of polycrystalline and single grain crystal surfaces were determined potentiostatically and galvanostatically after previously formed air oxides were reduced. A major challenge to analysis of the oxidation rate from electrochemical data is that film thickening rate must be assessed independently from the total oxidation rate. Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) results indicate that a significant portion of the applied anodic charge during oxidation is associated with direct cation ejection and/or dissolution rather than film thickening. Single frequency electrochemical impedance spectroscopy (SF-EIS) and ICP-MS were thus applied as novel characterization methods to model the electrochemically grown oxides and measure in-situ changes in oxide thickness and elementary dissolution reaction rates during passivation and film dissolution and/or cation emission. The oxide thicknesses calculated using these methods were related to the charge that goes toward oxide film growth and the related oxidation current density. Atomic force microscopy (AFM) topography images and profiles across single grain surfaces at intermediate stages of potentiostatically and galvanostatically controlled oxide growth were obtained to give insight on the nucleation and morphology of oxides formed. The surface roughness was derived from these images in order to quantify the influence of the minor alloying elements, and chloride presence on the morphology and degree of film breakdown during the stages of passivation. 3D atom probe tomography and X-ray photoelectron spectroscopy analysis of the oxides following potentiostatic and galvanostatic growth provided additional information regarding the location and chemical identity of elements across the films. This pairing of operando mesoscale electrochemical techniques with ex-situ microscopy describes the competitive oxidation and dissolution processes in aqueous environments and achieves an atomic-scale understanding of the oxide surface morphology along with the influence of minor alloying elements. The fate of all elements during fast repassivation was tracked operando for analysis of the significant impact of dissolution and cation ejection during passivation using ICP-MS, SF-EIS, and AFM measurements. The role of kinetics on passivation was found to play a crucial role over thermodynamics, resulting in non-conformal and unexpected oxide growth dictated by the minimization of interfacial energy and stresses. Finally, Mo was found to serve a number of roles during passivation and collects at the film/solution interface as well as throughout oxides. ACKNOWLEDGEMENTS This work was funded by the U.S. Office of Naval Research Multi-University Research Initiative under a subcontract from Northwestern University SP0028970-PROJ0007990 with Dr. David A. Shifler. REFERENCES 1. F. Bocher, R. Huang, and J. R. Scully, Corrosion, 66, 1–15 (2010). 2. A. C. Lloyd, J. J. Noël, S. McIntyre, and D. W. Shoesmith, Electrochim. Acta, 49, 3015–3027 (2004). 3. A. C. Lloyd, J. J. Noël, N. S. McIntyre, and D. W. Shoesmith, JOM, 57, 31–35 (2005). 4. P. Jakupi, D. Zagidulin, J. J. Noël, and D. W. Shoesmith, Electrochim. Acta, 56, 6251–6259 (2011).

Effects of annealing temperature and duration on the morphological and optical evolution of self-assembled Pt nanostructures on c-plane sapphire.

Metallic nanostructures (NSs) have been widely adapted in various applications and their physical, chemical, optical and catalytic properties are strongly dependent on their surface morphologies. In this work, the morphological and optical evolution of self-assembled Pt nanostructures on c-plane sapphire (0001) is demonstrated by the control of annealing temperature and dwelling duration with the distinct thickness of Pt films. The formation of Pt NSs is led by the surface diffusion, agglomeration and surface and interface energy minimization of Pt thin films, which relies on the growth parameters such as system temperature, film thickness and annealing duration. The Pt layer of 10 nm shows the formation of overlaying NPs below 650°C and isolated Pt nanoparticles above 700°C based on the enhanced surface diffusion and Volmer-Weber growth model whereas larger wiggly nanostructures are formed with 20 nm thick Pt layers based on the coalescence growth model. The morphologies of Pt nanostructures demonstrate a sharp distinction depending on the growth parameters applied. By the control of dwelling duration, the gradual transition from dense Pt nanoparticles to networks-like and large clusters is observed as correlated to the Rayleigh instability and Ostwald ripening. The various Pt NSs show a significant distinction in the reflectance spectra depending on the morphology evolution: i.e. the enhancement in UV-visible and NIR regions and the related optical properties are discussed in conjunction with the Pt NSs morphology and the surface coverage.

Orientations of dendritic growth during solidification

Dendrites are crystalline forms which grow far from the limit of stability of the plane front and adopt an orientation which is as close as possible to the heat flux direction. Dendritic growth orientations for cubic metals, bct Sn, and hcp Zn, can be controlled by thermal conductivity, Young’s modulus, and surface energy. The control factors have been elaborated. Since the dendrite is a single crystal, its properties such as thermal conductivity that influences the heat flux direction, the minimum Young’s modulus direction that influences the strain energy minimization, and the minimum surface energy plane that influences the crystal/liquid interface energy minimization have been proved to control the dendritic growth direction. The dendritic growth directions of cubic metals are determined by the minimum Young’s modulus direction and/or axis direction of symmetry of the minimum crystal surface energy plane. The dendritic growth direction of bct Sn is determined by its maximum thermal conductivity direction and the minimum surface energy plane normal direction. The primary dendritic growth direction of hcp Zn is determined by its maximum thermal conductivity direction and the minimum surface energy plane normal direction and the secondary dendrite arm direction of hcp Zn is normal to the primary dendritic growth direction.

Engineering flame retardant biodegradable polymer nanocomposites and their application in 3D printing

Flame retardant, environmentally sustainable nanocomposites were made by melt blending poly (lactic acid) (PLA) with melamine polyphosphate (MPP) and Cloisite 30B (C-30B). The composition of the nanocomposite was highly specific and guided by interfacial energy minimization principals which balanced enthalpic and mechanical contributions. In this critical range, even small changes in the filler concentration can have a large impact on the performance of the material. We showed that while addition of 17% MPP can increase the flame resistance of PLA, (achieving a UL-94 V2 rating) the mechanical properties were significantly degraded. Addition of only 1% C-30B reversed these effects and yielded a nanocomposite with enhanced mechanical properties and passing the UL-94 V0 flame test. This compound was able to be extruded and fed into a Makerbot Replicator 2X Fused Deposition Modeling (FDM) 3D printer, where the printed samples were indistinguishable mechanically from their molded counterparts and also achieved the UL-94 V0 rating. The enhanced performance occurred only within a very narrow composition window. Cone calorimetry revealed that while a significant decrease in the heat release rate, accompanied by the formation of an intumescent char, was achieved by the addition of 1% C-30B to the MPP/PLA blend, a very poor char, corresponding the increase in the heat release rate, was obtained with addition of 2% C-30B. These results were explained in view of the altered phase morphology observed in Transmission Electron Microscopy (TEM).

Using electric current to surpass the microstructure breakup limit

The elongated droplets and grains can break up into smaller ones. This process is driven by the interfacial free energy minimization, which gives rise to a breakup limit. We demonstrated in this work that the breakup limit can be overpassed drastically by using electric current to interfere. Electric current free energy is dependent on the microstructure configuration. The breakup causes the electric current free energy to reduce in some cases. This compensates the increment of interfacial free energy during breaking up and enables the processing to achieve finer microstructure. With engineering practical electric current parameters, our calculation revealed a significant increment of the obtainable number of particles, showing electric current a powerful microstructure refinement technology. The calculation is validated by our experiments on the breakup of Fe3C-plates in Fe matrix. Furthermore, there is a parameter range that electric current can drive spherical particles to split into smaller ones.

On the origin of the metastable β-Ta phase stabilization in tantalum sputtered thin films

Despite extensive studies since the 1960s, the exact mechanisms underlying phase formation of metastable tetragonal beta-Tantalum (β-Ta) structure commonly found in magnetron sputtered tantalum films have not yet been unraveled. Here, we combine in situ and real-time multiple beam optical stress sensor (MOSS) during sputter-deposition of Ta films together with ex situ X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM) to gain insights into the early growth stages and interface properties. The nucleation conditions were varied by depositing films on different buffer layers, either amorphous (a-Si, a-SiOx, a-SiNx) or crystalline (c-Mo), as well as changing the substrate temperature (Ts = 300–750 K). The influence of the mean deposited energy, in the range of 25–83 eV/atom, as calculated from Monte Carlo simulations, on the resulting stress state and phase formation was also investigated by varying the Ar working pressure (0.12–0.75 Pa) and substrate bias voltage (0 to −190 V). It is shown that the nucleation step is decisive for the stabilization of β-Ta phase on amorphous layers. After the initial formation of an interfacial amorphous Ta(Si) alloyed layer, β-Ta nuclei form due to interface energy minimization and grow columnar with a strong [001] fiber-texture. No transition to the equilibrium bcc alpha-phase (α-Ta) was found with increasing film thickness or variation of the deposited energy. The results are interpreted based on a thermodynamic description of nucleation and growth theory. Such rationale also qualitatively supports experimental findings of α-Ta phase selection with the assistance of substrate heating or by by-passing the nucleation stage upon direct growth in registry on crystalline Mo (110) seed layers.

Morphological Transformation of Surface Femtodroplets upon Dissolution.

Constructing controllable liquid patterns with high resolution and accuracy is of great importance in droplet depositions for a range of applications. Simple surface chemical micropatterns have been popularly used to regulate the shape of liquid droplets and the final structure of deposited materials. In this work, we study the morphological evolution of a dissolving femtoliter droplet pinned on multiple microdomains. On the basis of minimization of interfacial energy, the numerical simulations predict various symmetric droplet profiles in equilibrium at different liquid volumes. However, our experimental results show both symmetric and asymmetric shapes of droplets due to contact line pinning and symmetry breaking during droplet dissolution. Upon slow volume reduction, the deposited microdroplet arrays on one single type of simple surface prepatterns spontaneously morphed into a series of complex regular 3D shapes. The findings in this work offer insights into design and prepararion of the rich and complex morphology of liquid patterns via simple surface premicropatterns.

Synthesis and Characterization of Zn-Nix Advanced Alloys Prepared by Mechanical Milling and Sintering at Solid-State Process

Mechanical ball milling assisted by sintering in the solid state was used in this research to produce the Zn-Nix system alloy. The derivative powder compositions of Zn-Nix (x = 0, 5, 10, 15, and 20 wt.%) were obtained to study the Ni effects on the microstructural and mechanical properties. It is worth remarking that conventional methods are not appropriate for the manufacture of the Zn-Nix system alloy. The morphological structure and phases were examined by optical microscopy, X-ray diffraction, and SEM/EDS elemental mapping, whereas the mechanical behavior was accomplished by means of a diamond indentation print (Hardness Vickers). The results showed that the intermetallic γ-ZnNi phase did not form during milling time (&lt;4 h); it appears after the sintering process, which is associated with atomic diffusion mechanism through grain boundary at the minimum interfacial energy (ΔG256°C = −13.83 kJ·mol−1). The powder Zn-Ni10 was found to have better properties. Semispherical coarser particles were seen into the metal matrix (Zn δ-hcp structure) as segregates; however, each particle contains an intermetallic compound Zn-Ni that encloses the Ni (α-fcc structure) pure phase. The Ni-α phase was then transformed into a γ-ZnNi intermetallic compound which shifts to higher values of mechanical hardness from about 60 HV to 400 HV units.

Monodisperse erythrocyte-like and hollow erythrocyte-like silica nanoparticles prepared by a simple template-free and surfactant-free sol–gel route

This paper introduced a simple two-step aqueous method in which monodisperse erythrocyte-like silica nanoparticles could be prepared without surfactant and template by sol–gel method. KOH was used to form the asymmetrical silica nanoparticles through quickly condensation in rotation system. The erythrocyte-like hollow silica nanoparticles with the same external contour could be obtained by further hydrothermal. In addition, double-layer hollow erythrocyte-like and cage-like structures nanoparticles could also be acquired by hydrothermal reaction. A simple and green method was found to work in a dynamic process and produce asymmetrical structure getting rid of the minimization of interfacial energy.

Modeling morphology evolution and densification during solid-state sintering via kinetic Monte Carlo simulation

Microstructure control is an important subject in solid-state sintering and plays a crucial role in determining post-sintering material properties, such as strength, toughness and density, to name but a few. The preponderance of existing numerical sintering simulations model the morphology evolution and densification process driven by surface energy minimization by either dilating the particles to be sintered or using the vacancy annihilation model. Here, we develop a novel kinetic Monte Carlo model to model morphology evolution and densification during free sintering. Specifically, we derive analytically a heterogeneous densification rate of the sintering system by considering sintering stress induced mass transport. The densification of the system is achieved by modeling the sintering stress induced mass transfer via applying effective particle displacement and grain boundary migration with an efficient two-step iterative interfacial energy minimization procedure. Coarsening is also considered in the later stages of the simulations. We show that our model can accurately capture the diffusion-induced evolution of particle morphology, including neck formation and growth, as well as realistically reproduce the overall densification of the sintered material. The computationally obtained dynamic density evolution curves for both two-particle sintering and many-particle material sintering are found to be in excellent agreement with the corresponding experimental master sintering curves. Our model can be utilized to control a variety of structural and physical properties of the sintered materials, such as the pore size and final material density.

Memristors with diffusive dynamics as synaptic emulators for neuromorphic computing.

The accumulation and extrusion of Ca2+ in the pre- and postsynaptic compartments play a critical role in initiating plastic changes in biological synapses. To emulate this fundamental process in electronic devices, we developed diffusive Ag-in-oxide memristors with a temporal response during and after stimulation similar to that of the synaptic Ca2+ dynamics. In situ high-resolution transmission electron microscopy and nanoparticle dynamics simulations both demonstrate that Ag atoms disperse under electrical bias and regroup spontaneously under zero bias because of interfacial energy minimization, closely resembling synaptic influx and extrusion of Ca2+, respectively. The diffusive memristor and its dynamics enable a direct emulation of both short- and long-term plasticity of biological synapses, representing an advance in hardware implementation of neuromorphicfunctionalities.

Growth and thermal stability of TiN/ZrAlN: Effect of internal interfaces

Wear resistant hard films comprised of cubic transition metal nitride (c-TMN) and metastable c-AlN with coherent interfaces have a confined operating envelope governed by the limited thermal stability of metastable phases. However, equilibrium phases (c-TMN and wurtzite(w)-AlN) forming semicoherent interfaces during film growth offer higher thermal stability. We demonstrate this concept for a model multilayer system with TiN and ZrAlN layers where the latter is a nanocomposite of ZrN- and AlN- rich domains. The interfaces between the domains are tuned by changing the AlN crystal structure by varying the multilayer architecture and growth temperature. The interface energy minimization at higher growth temperature leads to formation of semicoherent interfaces between w-AlN and c-TMN during growth of 15 nm thin layers. Ab initio calculations predict higher thermodynamic stability of semicoherent interfaces between c-TMN and w-AlN than isostructural coherent interfaces between c-TMN and c-AlN. The combination of a stable interface structure and confinement of w-AlN to nm-sized domains by its low solubility in c-TMN in a multilayer, results in films with a stable hardness of 34 GPa even after annealing at 1150 °C.

Investigation on the texture evolution of epitaxial La2Zr2O7 buffer layer using EBSD techniques

Electron Backscattered Scattering Detection (EBSD) was used to characterize the texture of La2Zr2O7 (LZO) buffer layers prepared by chemical solution deposition on Ni-5at.% W(Ni5W) biaxially textured substrates. We investigated the texture forming process of LZO films with different cation concentrations of the precursor solution. In this paper, the texture evolution mainly depends on the film thickness and on the grain growth. With low cation concentrations in the precursor solution, the discontinuous LZO grains do not completely cover the Ni5W substrates and exhibit large lattice misorientation with respect to Ni5W (defined as cube orientation). A 45° rotation orientation is found to coexist with the cube orientation and is mainly transformed by the cube orientation with increasing thickness. The interface energy minimization may play a dominant role in texture transformation.

Band offset formation at semiconductor heterojunctions through density-based minimization of interface energy

It is well known that the magnitude of band offset (BO) at any semiconductor heterojunction is directly derivable from the distribution of charge at that interface and that the latter is decided by a minimization of total energy. However, the fact that BO formation is governed by energy minimization has not been explicitly used in theoretical BO models, likely because the equilibrium charge densities at heterojunction interfaces appear difficult to predict, except via explicit calculation. In this paper, electron densities at a large number of (100), (110), and (111) oriented heterojunctions between lattice-matched, isovalent semiconductors with the zinc blende (ZB) structure have been calculated by first-principles methods and analyzed in detail for possible common characteristics among energy-minimized densities. Remarkably, the heterojunction electron density was found to largely depend only on the immediate, local atomic arrangement. In fact, it is so much so that a juxtaposition of local electron-densities generated in oligo-cells (LEGOs) accurately reproduced the charge densities that minimize the energy for the heterojunctions. Furthermore, the charge distribution for each bulk semiconductor was found to display a striking separability of its electrostatic effect into two neutral parts, associated with the cation and the anion, which are approximately transferrable among semiconductors. These discoveries form the basis of a neutral polyhedra theory (NPT) that approximately predicts the equilibrium charge density and BO of relaxed heterojunctions from the energy minimization requirement. Well-known experimentally observed characteristics of heterojunctions, such as the insensitivity of BO to heterojunction orientation and the identity of interface bonds, the transitivity rule, etc., are all in good agreement with the NPT. Therefore, energy minimization, which essentially decides the electronic properties of all other solid and molecular systems, also governs the formation of the charge density at these heterojunction interfaces. In particular, the approach presented here eliminates the need to invoke mechanisms that are specific to semiconductor interfaces.