Structure Of Domain Boundaries Research Articles

Atomic Structure and Electronic Properties of Pristine and O-Doped Basal-Plane Inversion Domain Boundaries in AlN

Atomic Structure and Electronic Properties of Pristine and O-Doped Basal-Plane Inversion Domain Boundaries in AlN

The Structure of Domain and Antiphase Boundaries in κ-Phase of Gallium Oxide

The Structure of Domain and Antiphase Boundaries in κ-Phase of Gallium Oxide

A multiscale generative model to understand disorder in domain boundaries.

A continuing challenge in atomic resolution microscopy is to identify significant structural motifs and their assembly rules in synthesized materials with limited observations. Here, we propose and validate a simple and effective hybrid generative model capable of predicting unseen domain boundaries in a potassium sodium niobate thin film from only a small number of observations, without expensive first-principles calculations or atomistic simulations of domain growth. Our results demonstrate that complicated domain boundary structures spanning 1 to 100 nanometers can arise from simple interpretable local rules played out probabilistically. We also found previously unobserved, significant, tileable boundary motifs that may affect the piezoelectric response of the material system, and evidence that our system creates domain boundaries with the highest configurational entropy. More broadly, our work shows that simple yet interpretable machine learning models could pave the way to describe and understand the nature and origin of disorder in complex materials, therefore improving functional materials design.

Ferroelectric HfO2 and the importance of strain

Ferroelectric oxides based on ${\mathrm{HfO}}_{2}$ show tremendous promise for the next generation of memory and logic devices. The ferroelectric polymorph is one of several that can be derived from the high symmetry cubic fluorite structure of ${\mathrm{HfO}}_{2}$. A single grain of ${\mathrm{HfO}}_{2}$ may consist of a coherent mixture of multiple orientational and translational variants of different polymorphs. Here, we use symmetry-adapted strain-order parameters to elucidate the relationship between the different ${\mathrm{HfO}}_{2}$ polymorphs and their symmetrically equivalent variants. We use first-principles electronic structure methods to identify minimum energy pathways and map them in subspaces of the symmetry-adapted strain order parameters. We next investigate the atomic structure of domain boundaries that separate coexisting variants of ferroelectric ${\mathrm{HfO}}_{2}$. We rely on Gibbsian excess quantities and a precise specification of mechanical boundary conditions to describe the thermodynamic properties of domain boundaries. Our first-principles calculations show that the O and Hf shuffle arrangement within a domain boundary is closely related to the intermediate shuffle patterns of the homogeneous pathways between ferroelectric variants. Furthermore, the preferred structure within a boundary is very sensitive to local strain constraints imposed by the adjacent ferroelectric variants, leading to highly anisotropic domain boundary energies.

CO adsorption on Co(0001) revisited: High-coverage CO superstructures on the close-packed surface of cobalt

The adsorbate overlayer structures formed by CO on highly covered Co(0001) were investigated using temperature programmed desorption, low energy electron diffraction, reflection absorption infrared spectroscopy and synchrotron-based x-ray photoemission spectroscopy, with the purpose to clarify some recent confusion on CO adsorption at high coverages. TPD shows that the coverage of the (2√3 × 2√3)R30o structure (abbreviated as 2√3 structure) is 7/12 (0.583) ML. Its unit cell contains one COtop and six CObridge species that form three ‘bridge dicarbonyls’ in which two CObridge adsorbates share the same cobalt surface atom. Between 0.58 and 0.63 ML the 2√3 structure breaks up into small 2D domains separated by antiphase domain boundaries that consist of ‘double bridge dicarbonyls’ in which three CObridge adsorbates share two cobalt surface atoms. A phase transition that occurs around 0.63 ML creates previously unknown c(8 × 2) and c(12 × 2) domain boundary structures which consist of high density strips with a (2 × 2)–3CO structure and top/hollow site occupation which are separated by antiphase domain boundaries with a lower local coverage. Up to 0.63 ML the structures found at low temperature in UHV are very similar to those observed by others using in-situ STM at 300 K under CO pressure. This changes above 0.63 ML, where STM shows that moiré structures form instead of the c(n × 2) domain boundary structures. We propose that the moiré and c(n × 2) structures have a very similar enthalpy of formation so that small variations of temperature and CO chemical potential can lead to different structures for the same CO coverage. We show that these high-density phases that exist above 0.5 ML do not form at the typical pressures and temperatures used in applied FTS, but they do need to be considered when CO adsorption at room temperature is used as a diagnostic tool to characterize the catalyst surface.

Atomic-Scale Visualization of Polar Domain Boundaries in Ferroelectric In2Se3 at the Monolayer Limit.

Domain boundaries in ferroelectric materials exhibit rich and diverse physical properties distinct from their parent materials and have been proposed for broad applications in nanoelectronics and quantum information technology. Due to their complexity and diversity, the internal atomic and electronic structure of domain boundaries that governs the electronic properties remains far from being elucidated. By using scanning tunneling microscopy and spectroscopy (STM/S) combined with density functional theory (DFT) calculations, we directly visualize the atomic structure of polar domain boundaries in two-dimensional (2D) ferroelectric β'-In2Se3 down to the monolayer limit. We observe a double-barrier energy potential with a width of about 3 nm across the 60° tail-to-tail domain boundaries in monolayer β'-In2Se3. The results will deepen our understanding of domain boundaries in 2D ferroelectric materials and stimulate innovative applications of these materials.

Inversion domain boundaries in wurtzite GaN

We present two models for the atomic structure of inversion domain boundaries in wurzite GaN, that have not been discussed in existing literature. Using density functional theory, we find that one of these models has a lower formation energy than a previously proposed model known as Holt-$IDB$. Although this newly proposed model has a formation energy higher that the accepted lower energy structure, known as $IDB^*$, we argue that it can be formed under typical growth conditions. We present evidence that it may have been already observed in experiments, albeit misidentified as Holt-$IDB$. Our analysis was facilitated by a convenient notation, that we introduced, to characterize these models; it is based on the mismatch in crystal stacking sequence across the $\{10\overline{1}0\}$ plane. Additionally, we introduce an improved method to calculate energies of certain domain walls that challenge the periodic boundary conditions needed for plane-wave density functional theory methods. This new method provides improved estimations of domain wall energies.

Atomically resolved domain boundary structure in lead zirconate-based antiferroelectrics

Domain boundary (DB) structures are of great importance for understanding the structure-property relationship in many ferroic crystals. Here, we present atomically resolved DB configurations in PbZrO3-based antiferroelectric ceramics. The Pb-cation displacement relative to B-site cations is precisely determined using aberration-corrected scanning transmission electron microscopy. We find that 90° DBs in undoped PbZrO3 can be as thin as one primitive cell of the perovskite structure, often appearing curved or zigzagged due to the complex dipole arrangement. In a chemically modified composition, Pb0.99Nb0.02[(Zr0.57Sn0.43)0.95Ti0.05]0.98O3, in which incommensurate modulations are present, the DB has a typical thickness of at least two primitive cells, with more or less aligned dipole moments. Our findings provide insights into establishing the structure-property relationship in antiferroelectrics, shedding light on the design and fabrication of domain-boundary electronics.

Multi‐scale domain structure observation and piezoelectric responses for [001]‐oriented PMN‐33PT single crystal

AbstractMultiple phase coexistence contributes to the extraordinary piezoelectric behavior of (1‐x)Pb(Mg1/3Nb2/3)O3–xPbTiO3 (PMN–xPT) near the morphotropic phase boundaries (MPBs). By incorporating an optical path of crossed polarized light (PLM) into the commercialized Piezoelectric Force Microscope (PFM) (named as PLM‐PFM system), in situ domain structure observation from micro‐ to nanoscale, as well as measurement of the piezoelectric behavior for individual domains can be realized. For [001]‐oriented single crystal of 67Pb(Mg1/3Nb2/3)O3‐33PbTiO3 (PMN‐33PT), fine domain boundary structures of rhombohedral (R), tetragonal (T), and monoclinic (M) phases are revealed. Measurements of the electric field‐induced displacement as a function of the applied DC electric field (VDC) are performed for domains with different polarization vectors. Values for the electric field‐induced displacement are in descending order for c‐domains of the M, R, and T phases. For an individual phase of T or M, the displacement increases when the angle between the polarization vector and the applied electric field decreases. The multi‐scale perspective of the domain structures and the corresponding piezoelectric response helps in understanding the ultra‐high piezoelectric performance for PMN‐PT single crystals near MPB.

Phase Transitions in a Hard Domain Structure of a Ferrite Garnet Film

Spontaneous and magnetic field-induced phase transitions in a hard domain structure of a uniaxial ferrite–garnet film are studied. The temperature and field ranges of the lattice stability of cylindrical magnetic domains are shown to be dependent on the domain-boundary structure.

Features of the Domain Boundaries of a Highly Anisotropic (S = 1) Antiferromagnet near the Transition to the Quantum Paramagnet Phase

It is shown that the structure of antiphase domain boundaries in the antiferromagnetic (AFM) phase of a highly anisotropic magnet with S = 1 on a two-dimensional square lattice depends greatly on single-ion anisotropy parameter D. Computer modeling on large square lattices illustrates the changes in the boundary structure from the quantum paramagnet (QP) to the XY phase, including the intermediate QP-XY phase at fairly small variations in positive D.

Inversion domain boundary structure of laterally overgrown c-GaN domains including the inversion from Ga to N polarity at a mask pattern boundary

During epitaxial lateral overgrowth, the lateral polarity inversion of c-GaN domains from Ga to N polarity, triggered at the boundary of an SiO2 mask pattern, resulted in inversion domain boundaries (IDBs) forming preferentially on the \{11{\overline 2}0\} plane, although the formation energy of IDBs on the \{1{\overline 1}00\} plane is known to be lower than that on the \{11{\overline 2}0\} plane. A model that takes a geometrical factor into consideration can explain this preferential tendency of IDB formation on the \{11{\overline 2}0\} plane, and computational simulations based on the proposed model are consistent with experimental results. In contrast with the vertically upright IDBs observed in N-to-Ga polarity inversion, vertically slanted IDBs were formed in some samples during the inversion from Ga to N polarity. These polarity inversions, which appeared to randomly occur on the mask pattern, turned out to be triggered at the mask pattern boundaries.

Determination of Silica and Germania Film Network Structures on Ru(0001) at the Atomic Scale

The detailed structure of silica and germania films supported on Ru(0001) metal substrates are compared to each other. Surface science techniques together with density functional theory calculations have been used to gain insights into the atomic arrangement of these prominent glass-forming materials. The monolayer films of these materials both show predominantly crystalline hexagonal lattices with characteristic domain boundary structures. For the germania monolayer films a large variety of ring elements within domain boundaries have been observed. Density functional calculations predict stronger interaction with the metal substrate for bilayer germania as compared to bilayer silica films. Scanning tunneling microscopy images with atomically resolved structural features have given access to silica and germania bilayer film structures. Both bilayer films form characteristic amorphous ring structures. However, the germania bilayer films appear to be more corrugated, pointing to a stronger interaction with the metal support thus giving rise to slightly different connectivity rules.

Origin of band bending at domain boundaries of MoS2: First-principles study

Using first-principles calculations based on density functional theory, the energetics and electronic structure of domain boundaries of MoS2, in which the same polar edges face each other, are investigated. We find that the interface model with homoelemental bonds is not energetically preferred in this system. The domain boundaries have defect levels that have wide distributions inside the band gap of MoS2. The upshift (or downshift) of the MoS2 energy band occurs around the domain boundaries when the occupation number of electrons in the defect levels increases (or decreases). The charge transfer of electrons from the graphite substrate plays an important role in band bending, which is observed in the recent experiments by scanning tunneling microscopy/spectroscopy.

Atomic Structure of Domain and Interphase Boundaries in Ferroelectric HfO2

AbstractThough ferroelectric HfO2 thin films are now well characterized, little is currently known about their grain substructure. In particular, the formation of domain and phase boundaries requires investigation to better understand phase stabilization, switching, and phase interconversion. Here, scanning transmission electron microscopy is applied to investigate the atomic structure of boundaries in these materials. It is found that orthorhombic/orthorhombic domain walls and coherent orthorhombic/monoclinic interphase boundaries form throughout individual grains. The results inform how interphase boundaries can impose strain conditions that may be key to phase stabilization. Moreover, the atomic structure near interphase boundary walls suggests potential for their mobility under bias, which has been speculated to occur in perovskite morphotropic phase boundary systems by mechanisms similar to domain boundary motion.

Влияние локальных корреляций на переход "однородный изолятор-сверхпроводник" в доменных границах фазы зарядового порядка 2D-системы со смешанной валентностью

Abstract —It is demonstrated in the (pseudo)spin S = 1 formalism that the structure of antiphase domain boundaries in the phase of charge ordering of a mixed-valence system of the Cu^1+, 2+, 3+ “triplet” type in cuprates on a two-dimensional square lattice depends to a considerable extent on on-site correlation parameter U . The results of computer modeling on large square lattices illustrate the change in the boundary structure (from a homogeneous monovalent nonconducting structure of the Cu^2+ type to a filamentary superconducting one) induced by a relatively small variation of positive U values.

Roles of strain and domain boundaries on the phase transition stability of VO2 thin films

The fundamental phase transition mechanism and the stability of the semiconductor-to-metal phase transition properties during multiple thermal cycles have been investigated on epitaxial vanadium dioxide (VO2) thin films via both ex situ heating and in situ heating by transmission electron microscopy (TEM). VO2 thin films were deposited on c-cut sapphire substrates by pulsed laser deposition. Ex situ studies show the broadening of transition sharpness (ΔT) and the width of thermal hysteresis (ΔH) after 60 cycles. In situ TEM heating studies reveal that during thermal cycles, large strain was accumulated around the domain boundaries, which was correlated with the phase transition induced lattice constant change and the thermal expansion. It suggests that the degradation of domain boundary structures in the VO2 films not only caused the transition property reduction (e.g., the decrease in ΔT and ΔH) but also played an important role in preventing the film from fracture during thermal cycles.

Electronic structure and switching behavior of the metastable silicene domain boundary

Silicene, a silicon allotrope with a buckled honeycomb lattice, has been extensively studied in the search for materials with graphene-like properties. Here, we study the domain boundaries of a silicene 4 × 4 superstructure on an Ag(111) surface at the atomic resolution using scanning tunneling microscopy (STM) and spectroscopy (STS) along with density functional theory calculations. The silicene domain boundaries (β-phases) are formed at the interface between misaligned domains (α-phases) and show a bias dependence, forming protrusions or depressions as the sample bias changes. In particular, the STM topographs of the silicene–substrate system at a bias of ∼2.0 V show brightly protruding domain boundaries, which can be explained by an energy state originating from the Si 3s and 3pz orbitals. In addition, the topographs depicting the vicinity of the domain boundaries show that the structure does not follow the buckled geometry of the atomic ball-and-stick model. Inside the domain, STS data showed a step-up at ∼0.4 V, which originated from the Si 3p orbitals. We found this step-up to have shifted, which may be attributed to the strain effect at the interface regions between silver and silicene and between the domain and its boundary upon performing spatially resolved STS measurements. The metastable characteristic of the domain boundary (β-phase) causes changes, such as creation or annihilation, in the buckling structures (switching behavior). The observed low activation energy for the buckling change between distinct states may find applications in the electronic control of properties related to domain boundary structures in silicene.

Periodic domain boundary ordering in a dense molecular adlayer: Sub-saturation carbon monoxide on Pd(111)

We describe a previously unreported ordered phase of carbon monoxide adsorbed on the (111) facet of single crystal palladium at near-saturation coverage. The adlayer superstructure is identified from low energy electron diffraction to be c(16×2) with respect to the underlying Pd(111) surface net. The ideal coverage is determined to be 0.6875ML, approximately 92% of the 0.75‐ML saturation coverage. Density functional theory calculations support a model for the molecular packing characterized by strips of locally-saturated (2×2) regions, with the CO bound near high-symmetry surface sites, separated by antiphase domain boundaries. The structure exists in a narrow coverage range and is prepared by heating the saturated adlayer to desorb a small fraction of the CO. Comparison of the c(16×2) domain-boundary structure with structural motifs at lower coverages suggests that between 0.6 and 0.6875ML the adlayer order may be more strongly influenced by interadsorbate repulsion than by adsorption-site-specific interactions. The system is an example of the structural complexity that results from the compromise between adsorbate–substrate and adsorbate–adsorbate interactions.

Assessing the amorphousness and periodicity of common domain boundaries in silica bilayers on Ru(0 0 0 1)

Domain boundaries are hypothesized to play a role in the crystalline to amorphous transition. Here we examine domain boundary structures in comparison to crystalline and amorphous structures in bilayer silica grown on Ru(0 0 0 1). Atomically resolved scanning probe microscopy data of boundaries in crystalline bilayer films are analyzed to determine structural motifs. A rich variety of boundary structures including rotational, closed-loop, antiphase, and complex boundaries are identified. Repeating units with ring sizes of 558 and 57 form the two most common domain boundary types. Quantitative metrics are utilized to assess the structural composition and degree of order for the chemically equivalent crystalline, domain boundary, and amorphous structures. It is found that domain boundaries in the crystalline phase show similarities to the amorphous phase in their ring statistics and, in some cases, in terms of the observed ring neighborhoods. However, by assessing order and periodicity, domain boundaries are shown to be distinct from the glassy state. The role of the Ru(0 0 0 1) substrate in influencing grain boundary structure is also discussed.