Epitaxial Transition Research Articles

New approaches for achieving more perfect transition metal oxide thin films

This perspective considers the enormous promise of epitaxial functional transition metal oxide thin films for future applications in low power electronic and energy applications since they offer wide-ranging and highly tunable functionalities and multifunctionalities, unrivaled among other classes of materials. It also considers the great challenges that must be overcome for transition metal oxide thin films to meet what is needed in the application domain. These challenges arise from the presence of intrinsic defects and strain effects, which lead to extrinsic defects. Current conventional thin film deposition routes often cannot deliver the required perfection and performance. Since there is a strong link between the physical properties, defects and strain, routes to achieving more perfect materials need to be studied. Several emerging methods and modifications of current methods are presented and discussed. The reasons these methods better address the perfection challenge are considered and evaluated.

Germanium quantum dot infrared photodetectors addressed by self-aligned silicon nanowire electrodes

Germanium quantum dots (GeQDs), addressed by self-aligned and epitaxial silicon nanowires (SiNWs) as electrodes, represent the most fundamental and the smallest units that can be integrated into Si optoelectronics for 1550 nm wavelength detection. In this work, individual GeQD photodetectors have been fabricated based on a low temperature self-condensation of uniform amorphous Si (a-Si)/a-Ge bilayers at 300 °C, led by rolling indium (In) droplets. Remarkably, the diameter of the GeQD nodes can be independently controlled to achieve wider GeQDs for maximizing infrared absorption with narrower SiNW electrodes to ensure a high quality Ge/Si hetero-epitaxial connection. Importantly, these hetero GeQD/SiNW photodetectors can be deployed into predesigned locations for scalable device fabrication. The photodetectors demonstrate a responsivity of 1.5 mA W−1 and a photoconductive gain exceeding 102 to the communication wavelength signals, which are related to the beneficial type-II Ge/Si alignment, gradient Ge/Si epitaxial transition and a larger QD/NW diameter ratio. These results indicate a new approach to batch-fabricate and integrate GeQDs for ultra-compact Si-compatible photodetection and imaging applications.

Directly measuring the structural transition pathways of strain-engineered VO2 thin films.

Epitaxial films of vanadium dioxide (VO2) on rutile TiO2 substrates provide a means of strain-engineering the transition pathways and stabilizing of the intermediate phases between monoclinic (insulating) M1 and rutile (metal) R end phases. In this work, we investigate structural behavior of epitaxial VO2 thin films deposited on isostructural MgF2 (001) and (110) substrates via temperature-dependent Raman microscopy analysis. The choice of MgF2 substrate clearly reveals how elongation of V-V dimers accompanied by the shortening of V-O bonds triggers the intermediate M2 phase in the temperature range between 70-80 °C upon the heating-cooling cycles. Consistent with earlier claims of strain-induced electron correlation enhancement destabilizing the M2 phase our temperature-dependent Raman study supports a small temperature window for this phase. The similarity of the hysteretic behavior of structural and electronic transitions suggests that the structural transitions play key roles in the switching properties of epitaxial VO2 thin films.

Construction of a silicate-based epitaxial transition film on a zirconia ceramic surface to improve the bonding quality of zirconia restorations.

The effect of a silicate-based epitaxial transition film on zirconia produced by a silicate solution during zirconia–resin bonding was investigated. The airborne-particle abraded zirconia was placed in different concentrations of silicate solutions and heated at 50 °C. The silicate transition film was characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), energy-dispersive X-ray spectroscopy (EDX), contact angle measurement and profilometry. The silicate-based epitaxial transition film was successfully constructed on the surface of zirconia, and the surface morphology and composition of zirconia changed. After coupling with KH570 hydrolysate, the shear bond strength (SBS) of zirconia–resin after either 24 h water storage or 5000 thermal cycles can be significantly improved by a silicate-based epitaxial transition film on the surface of zirconia, and all the samples had no cytotoxicity. This may provide a new strategy for improving the bonding quality of zirconia restorations.

Two fast and efficient linear semi-implicit approaches with unconditional energy stability for nonlocal phase field crystal equation

The phase-field crystal equation is a sixth-order nonlinear parabolic equation and have received increasing attention in the study of the microstructural evolution of two-phase systems on atomic length and diffusive time scales. This model can be applied to simulate various phenomena such as epitaxial growth, material hardness and phase transition. Compared with the classical local gradient flow and phase field models, the nonlocal models such as nonlocal phase-field crystal equation equipped with nonlocal diffusion operator can describe more practical phenomena for modeling phase transitions. We propose linear semi-implicit approach and scalar auxiliary variable approach with unconditional energy stability for the nonlocal phase-field crystal equation. The first contribution is that we have proved the unconditional energy stability for nonlocal phase-field crystal model and its semi-discrete schemes carefully and rigorously. Secondly, we found a fast procedure to reduce the computational work and memory requirement which the non-locality of the nonlocal diffusion term generates huge computational work and memory requirement. Finally, several numerical simulations are demonstrated to verify the accuracy and efficiency of our proposed schemes.

Efficient modified stabilized invariant energy quadratization approaches for phase-field crystal equation

The phase-field crystal equation is a sixth-order nonlinear parabolic equation and can be applied to simulate various phenomena such as epitaxial growth, material hardness, and phase transition. We propose a series of efficient modified stabilized invariant energy quadratization approaches with unconditional energy stability for the phase-field crystal model. Firstly, we propose a more suitable positive preserving function strictly in square root and consider a modified invariant energy quadratization (MIEQ) approach. Secondly, a series of efficient and suitable functionals H(ϕ) in square root are considered and the modified stabilized invariant energy quadratization (MSIEQ) approaches are developed. We prove the unconditional energy stability and optimal error estimates for the semi-discrete schemes carefully and rigorously. A comparative study of classical IEQ, MIEQ, and MSIEQ approaches is considered to show the accuracy and efficiency. Finally, we present various 2D numerical simulations to demonstrate the stability and accuracy.

Surface-orientation-dependent growth of SrRuO3 epitaxial thin films

The growth of epitaxial transition metal oxide thin films depends on various parameters including the substrate temperature, oxygen partial pressure, kinetics of incoming adatoms, Gibbs free energy, and surface energy. Naturally, the change in the crystallographic surface orientation with a distinctive surface energy also influences the growth rate and growth mode of the epitaxial thin films substantially. Using perovskite SrRuO3 as a model system, we studied the growth characteristics by changing the surface orientation of the SrTiO3 substrate. Employing X-ray diffraction and surface atomic and Kelvin probe force microscopy (KPFM), we observed a systematic decrease in the growth rate and a modification in the growth mode from a two-dimensional growth to a three-dimensional island growth with the change in the surface orientation from (100) to (110) to (111). A spin-polarized density functional theory calculation demonstrated the corresponding difference in the surface energy, which was also confirmed experimentally by the KPFM measurements. The difference in the surface energy could explain the observed change in the growth kinetics, based on the modified classical nucleation theory. A combinatorial method using a polycrystalline epitaxial thin film was employed to generalize our understanding of the crystallographic surface-orientation-dependent thin film growth.

Scalable Epitaxial Growth of WSe2 Thin Films on SiO2/Si via a Self-Assembled PtSe2 Buffer Layer

The growth of large-area epitaxial transition metal dichalgogenides (TMDCs) are of central importance for scalable integrated device applications. Different methods have been developed to achieve large-sized high quality films. However, reliable approaches for centimeter-sized or even wafer-level epitaxial growth of TMDCs are still lacking. Here we demonstrate a new method to grow inch-sized epitaxial WSe2 films on SiO2/Si substrates at a much lower temperature with high repeatability and scalability. High quality crystalline films are achieved through direct selenization of a tungsten film with platinum as the underlayer. The self-assembled PtSe2 buffer layer, formed during selenization, assists epitaxial growth of WSe2. Using fabricated WSe2 films, excellent performance memory devices are demonstrated. As a member of the TMDC family, our findings based on WSe2 may also be applied to other TMDC materials for large-scale production of high quality TMDC films for various applications.

Construction of Rod-Forming Single Network Mesophases in Rod–Coil Diblock Copolymers via Inversely Designed Phase Transition Pathways

The string method is introduced to the self-consistent mean-field theory for rod–coil diblock copolymers to investigate the phase transition kinetics. The equal arc length parametrization used in our improved string method is constrained on both the density and orientation order parameters to obtain accurate and stable minimum energy pathways (MEP). Focusing on different single networks formed by rods as the target structures, we developed an inverse design strategy of kinetic pathways. By designing distinct initial structures (lamellae or cylinders) with specific symmetry matching the target structure as the two ends of the string, we calculated the MEP and analyzed the effects of chain rigidity on the phase transition kinetics. The results indicate that the reordering transition between lamellae belonging to the same crystallographic plane group can induce the formation of metastable single networks. The shape and arrangement of the rigid rods will influence the epitaxial phase transition. The long-term stability of the target structures can be regulated by tailoring the block composition as well as the geometrical asymmetry between rigid and flexible segments, which may guide the experiments to fabricate long-lived single networks.

Strain landscapes and self-organization of free surfaces in complex oxide epitaxy

Abstract

Formation of Single Gyroid Nanostructure by Order–Order Phase Transition Path in ABC Triblock Terpolymers

The kinetics for the formation of the single gyroid (SG) nanostructure in ABC triblock terpolymers is investigated using the self‐consistent field theory combined with the string method. Both simple phases (lamellae, cylinders, and spheres) and networked double diamonds (DD) can transform into SG through order–order phase transition (OOT). In particular, a packing frustration‐induced variation in the epitaxial relationship between DD and SG is demonstrated. By regulating the block interaction, an expected epitaxial phase transition between these two networks without any rotation of the crystallographic directions can be achieved. Interestingly, the hexagonally perforated lamellae (HPL) are encountered in all identified transition pathways to SG. Nucleation kinetics investigation shows that the HPL tends to nucleate from SG easily, which confirms the kinetic origins of the instability of SG in experiments. Therefore, several strategies of preventing the SG being bypassed, such as controlling annealing time and rates during the morphology evolution, are proposed to promote the stabilizing of the SG in kinetic pathways. The findings reported here provide a novel route for fabrication of SG structured materials by manipulating both the epitaxial relationship and the nucleation kinetics in OOT pathways.

Three-Dimensional Visualization and Characterization of Polymeric Self-Assemblies by Transmission Electron Microtomography.

Self-assembling structures and their dynamical processes in polymeric systems have been investigated using three-dimensional transmission electron microscopy (3D-TEM). Block copolymers (BCPs) self-assemble into nanoscale periodic structures called microphase-separated structures, a deep understanding of which is important for creating nanomaterials with superior physical properties, such as high-performance membranes with well-defined pore size and high-density data storage media. Because microphase-separated structures have become increasingly complicated with advances in precision polymerization, characterizing these complex morphologies is becoming increasingly difficult. Thus, microscopes capable of obtaining 3D images are required. In this article, we demonstrate that 3D-TEM is an essential tool for studying BCP nanostructures, especially those self-assembled during dynamical processes and under confined conditions. The first example is a dynamical process called order-order transitions (OOTs). Upon changing temperature or pressure or applying an external field, such as a shear flow or electric field, BCP nanostructures transform from one type of structure to another. The OOTs are examined by freezing the specimens in the middle of the OOT and then observing the boundary structures between the preexisting and newly formed nanostructures in three-dimensions. In an OOT between the bicontinuous double gyroid and hexagonally packed cylindrical structures, two different types of epitaxial phase transition paths are found. Interestingly, the paths depend on the direction of the OOT. The second example is BCP self-assemblies under confinement that have been examined by 3D-TEM. A variety of intriguing and very complicated 3D morphologies can be formed even from the BCPs that self-assemble into simple nanostructures, such as lamellar and cylindrical structures in the bulk (in free space). Although 3D-TEM is becoming more frequently used for detailed morphological investigations, it is generally used to study static nanostructures. Although OOTs are dynamical processes, the actual experiment is done in the static state, through a detailed morphological study of a snapshot taken during the OOT. Developing time-dependent nanoscale 3D imaging has become a hot topic. Here, the two main problems preventing the development of in situ electron tomography for polymer materials are addressed. First, the staining protocol often used to enhance contrast for electrons is replaced by a new contrast enhancement based on chemical differences between polymers. In this case, no staining is necessary. Second, a new 3D reconstruction algorithm allows us to obtain a high-contrast, quantitative 3D image from fewer projections than is required for the conventional algorithm to achieve similar contrast, reducing the number of projections and thus the electron beam dose. Combining these two new developments is expected to open new doors to 3D in situ real-time structural observation of polymer materials.

Magnetic and Magnetodielectric Properties of Epitaxial Iron Vanadate Thin Films

The magnetic and magnetodielectric behavior of epitaxial (001) transition metal oxide spinel iron vanadate (Fe1+xV2−xO4, FVO) thin films grown on (001) SrTiO3 substrates by pulsed laser deposition is reported. X‐ray absorption spectroscopy studies confirm the trivalent and nominally divalent nature of V and Fe ions, respectively, while an excess of Fe3+ ions is also found. Aberration‐corrected cross‐sectional scanning transmission electron microscopy analysis suggests that the excess Fe3+ may be accommodated through Fe3O4 stacking faults along the {111} planes. Temperature‐dependent X‐ray magnetic circular dichroism spectra reveal an anti‐parallel alignment of spins between the Fe2+ and V3+ cations. M–H loops acquired at 10 K, well below the noncollinear ferromagnetic phase transition temperature, show magnetic anisotropy and point to the existence of two distinct phases with different coercive fields. Visual evidence for magnetic domains and their switching is given by magnetic force microscopy, which finds a switching field of ≈3 T in agreement with the M–H loops. Magnetodielectric measurements for a Pd/FVO/Nb:STO heterostructure reveal a weak hysteresis in the magneto‐capacitance loops at 10 K, thus confirming magneto‐dielectric coupling. Detailed temperature and frequency‐dependent dielectric studies find that Maxwell‐Wagner‐type relaxation dominates capacitance behavior above ≈40 K due to a FVO/Nb:SrTiO3 Schottky interface.

Tracking the amorphous to epitaxial transition in RF-sputtered cubic BFO-STO heterojunctions by means of X-ray photoelectron diffraction

The epitaxial growth of cubic BiFeO3 ultrathin films on SrTiO3 (001) substrates by off-axis RF sputtering is demonstrated, suitable to X-ray spectroscopies interface investigation. X-ray photoelectron diffraction is used as a tool to probe the long-range crystal order and to track the transition from amorphous to epitaxial growth as a function of deposition parameters. Further spectroscopic measurements, in particular, X-ray linear dichroism on the Fe L3,2 edge, confirm the heteroepitaxial growth of BiFeO3 and clearly indicate a 3+ valence state for the iron cation. Finally, XPS is used to reconstruct the band alignment diagram, which results in a staggered configuration with a remarkable energy shift of the SrTiO3 band edges which can ultimately favor the n-type doping of SrTiO3.

Martensite adaption through epitaxial nano transition layers in TiNiCu shape memory alloys

Titanium-rich TiNiCu shape memory thin films with ultralow fatigue have been analysed for their structural features by transmission electron microscopy. The stabilization of austenite (B2) and orthorhombic martensite (B19) variants epitaxially connected to Ti2Cu-type precipitates has been observed and found responsible for the supreme mechanical cycling capability of these compounds. Comprehensiveex situandin situcooling/heating experiments have demonstrated the presence of an austenitic nanoscale region in between B19 and Ti2Cu, in which the structure shows a gradual transition from B19 to B2 which is then coupled to the Ti2Cu precipitate. It is proposed that this residual and epitaxial austenite acts as a template for the temperature-induced B2↔B19 phase transition and is also responsible for the high repeatability of the stress-induced transformation. This scenario poses an antithesis to residual martensite found in common high-fatigue shape memory alloys.

Effects of Oscillatory Shear on the Orientation of the Inverse Bicontinuous Cubic Phase in a Nonionic Surfactant/Water System.

The bicontinuous inverse cubic phase (V2 phase) formed in amphiphilic systems consists of bilayer networks with a long-range order. We have investigated effects of oscillatory shear on the orientation of the V2 phase with space group Ia3d formed in a nonionic surfactant (C12E2)/water system by using simultaneous measurements of rheology/small-angle X-ray scattering. It is shown that grain refining occurs by applying the large amplitude oscillatory shear (LAOS) with a strain amplitude (γ0) of ∼20, which gives the ratio of the loss modulus (G″) to the storage modulus (G') (G″/G' = tan δ) of ∼100. On the other hand, orientation of the cubic lattice occurs when the small amplitude (γ0 ≈ 0.0004) oscillatory shear (SAOS) in the linear regime is applied to the sample just after the LAOS. Interestingly, the orientation is strongly enhanced by the "medium amplitude" (γ0 ≈ 0.05) oscillatory shear ("MAOS") after the SAOS. When the MAOS is applied before applying the LAOS, orientation to a particular direction is not observed, indicating that the grain refining process by the LAOS is necessary for the orientation during the MAOS. The results of additional experiments show that the shear sequence "LAOS-MAOS" is effective for the orientation of the cubic lattice. When the LAOS and MAOS are applied to the sample alternatively, grain refining and orientation occur during the LAOS and MAOS, respectively, indicating reversibility of the orientation. It is shown that (i) the degree of the orientation is dependent on γ0 and the frequency (ω) of the MAOS and (ii) relatively higher orientation can be obtained for the combination of γ0 and ω, which gives tan δ = 2-3. The lattice constant does not change throughout all the shearing processes and is equal to that before shearing within the experimental errors, indicating that the shear melting does not occur. These results suggest a possibility to control the orientation of the cubic lattice only by changing the conditions of oscillatory shear without using the epitaxial transition from other anisotropic phases, such as the hexagonal and lamellar phases.

A multiferroic on the brink: Uncovering the nuances of strain-induced transitions in BiFeO3

Bismuth ferrite (BiFeO$_3$) is one of the very few known single-phase multiferroic materials. While the bulk compound is rhombohedral (R), the discovery of an epitaxial strain-induced structural transition into a so-called 'super tetragonal-phase' (T-phase) in this material incited a flurry of research activity focused on gaining an understanding of this phase transition and its possible functionalities. This metastable phase of BiFeO$_3$ is also multiferroic, with giant ferroelectric polarization and coexisting antiferromagnetic order, but above all it is the strain relaxation-induced phase mixtures and their outstanding piezoelectric and magnetoelectric responses which continue to intrigue and motivate the physicist and materials scientist communities. Here, we review the research into the T-phase and mixed-phase BiFeO$_3$ system. We begin with a brief summary of the history of the T-phase and an analysis of the structure of the various phases reported in the literature. We then address important questions regarding the symmetry and octahedral rotation patterns and the (as yet underexplored) important role of chemistry in the formation of the metastable T-phase. We follow by describing the phase transitions in this material, and how these may hold promise for large magnetoelectric responses. Finally we point out some experimental challenges inherent to the study of such a system, and potential pathways for how they may be overcome. It is our intention with this work to highlight important issues that, in our opinion, should be carefully considered by the community in order to use this fascinating materials system for a new paradigm of functionality.

Atomically flat reconstructed rutile TiO2(001) surfaces for oxide film growth

The availability of low-index rutile TiO2 single crystal substrates with atomically flat surfaces is essential for enabling epitaxial growth of rutile transition metal oxide films. The high surface energy of the rutile (001) surface often leads to surface faceting, which precludes the sputter and annealing treatment commonly used for the preparation of clean and atomically flat TiO2(110) substrate surfaces. In this work, we reveal that stable and atomically flat rutile TiO2(001) surfaces can be prepared with an atomically ordered reconstructed surface already during a furnace annealing treatment in air. We tentatively ascribe this result to the decrease in surface energy associated with the surface reconstruction, which removes the driving force for faceting. Despite the narrow temperature window where this morphology can initially be formed, we demonstrate that it persists in homoepitaxial growth of TiO2(001) thin films. The stabilization of surface reconstructions that prevent faceting of high-surface-energy crystal faces may offer a promising avenue towards the realization of a wider range of high quality epitaxial transition metal oxide heterostructures.

Emergence of ordered network mesophases in kinetic pathways of order-order transition for linear ABC triblock terpolymers.

Applying the string method to the self-consistent field theory (SCFT) of ABC linear triblock copolymers, we developed a new strategy to design kinetic pathways for the formation of stable or metastable network mesophases in order-order transition (OOT) processes. The design principle regarding the kinetic pathways between distinct mesophases is based on the matching relationships of both domain spacing and dominant Fourier components of the density distributions. The results suggest that complex ordered network mesophases, such as alternating diamond (DA) and alternating plumber's nightmare (PA) could be obtained in kinetic pathways between simple phases covering lamellae, cylinders and spheres. By virtue of the minimal free energy pathway (MEP) obtained, we could acquire the epitaxial relationship and phase transition mechanism. Furthermore, we managed to regulate the MEP by changing the block composition to adjust packing frustration. Two new metastable networks, core-shell five-pronged and six-pronged morphologies, were found in the kinetic pathways, further demonstrating the regulating mechanism. The results will contribute to a better understanding of the kinetic relationship between simple phases and complex networks, thus providing a platform for soft materials design via the OOT route and guiding experimental procedures to fabricate ordered network mesophases.

Unravelling the roles of surface chemical composition and geometry for the graphene–metal interaction through C1s core-level spectroscopy

Here we show that by using a combined experimental and theoretical approach it is possible to separate the contributions to the interaction strength between epitaxial graphene and transition metal surfaces arising from the geometrical and chemical properties of the supporting surfaces. This has been achieved by performing photoelectron measurements and numerical simulations of the C1s core level spectral distribution for a large number of graphene–metal systems, which have been obtained by systematic intercalation of different metals (Co, Rh, Ir and Ru) at the graphene–Ir(111) and graphene–Ru(0001) interfaces. We demonstrate that the chemical species of the substrate’s topmost layer plays a major role in determining the coupling between graphene and its substrate. Moreover, we show that both the experimental and the theoretical C1s spectral centres of mass are in linear relationship with the d-band centre of the transition metal substrate, which is considered a reliable descriptor of the graphene–substrate interaction strength. Our results provide a simple method to determine and tailor the properties of graphene–metal contacts.