Demonstration of Hexagonal Phase Silicon Carbide Nanowire Arrays with Vertical Alignment

SiC nanowire based electronics hold promise for data collection in harsh environments wherein conventional semiconductor platforms would fail. However, the full adaptation of SiC nanowires as a material platform necessitates strict control of nanowire crystal structure and orientation for reliable performance. Toward such efforts, we report the growth of hexagonal phase SiC nanowire arrays grown with vertical alignment on commercially available single crystalline SiC substrates. The nanowire hexagonality, confirmed with Raman spectroscopy and atomic resolution microscopy, displays a polytypic distribution of predominantly 2H and 4H. Employing a theoretical growth model, the polytypic distribution of hexagonal phase nanowires is accurately predicted in the regime of high supersaturation. Additionally, the reduction of disorder-induced phonon density of states is achieved while maintaining nanowire morphology through a postgrowth anneal. The results of this work expand the repertoire of SiC nanowires by implementing a low-temperature method that promotes polytypes outside the well-studied cubic phase and introduces uniform, vertical alignment on industry standard SiC substrates

Conserved Atomic Bonding Sequences and Strain Organization of Graphene Grain Boundaries

The bulk properties of polycrystalline materials are directly influenced by the atomic structure at the grain boundaries that join neighboring crystallites. In this work, we show that graphene grain boundaries are comprised of structural building blocks of conserved atomic bonding sequences using aberration corrected high-resolution transmission electron microscopy. These sequences appear as stretches of identically arranged periodic or aperiodic regions of dislocations. Atomic scale strain and lattice rotation of these interfaces is derived by mapping the exact positions of every carbon atom at the boundary with ultrahigh precision. Strain fields are organized into local tensile and compressive dipoles in both periodic and aperiodic dislocation regions. Using molecular dynamics tension simulations, we find that experimental grain boundary structures maintain strengths that are comparable to idealized periodic boundaries despite the presence of local aperiodic dislocation sequences

Dynamics of Symmetry-Breaking Stacking Boundaries in Bilayer MoS₂

Crystal symmetry of two-dimensional (2D) materials plays an important role in their electronic and optical properties. Engineering symmetry in 2D materials has recently emerged as a promising way to achieve novel properties and functions. The noncentrosymmetric structure of monolayer transition metal dichalcogenides (TMDCs), such as molybdenum disulfide (MoS₂), has allowed for valley control via circularly polarized optical excitation. In bilayer TMDCs, inversion symmetry can be controlled by varying the stacking sequence, thus providing a pathway to engineer valley selectivity. Here, we report the <i>in situ</i> integration of AA′ and AB stacked bilayer MoS₂ with different inversion symmetries by creating atomically sharp stacking boundaries between the differently stacked domains, via thermal stimulation and electron irradiation, inside an atomic-resolution scanning transmission electron microscopy. The setup enables us to track the formation and atomic motion of the stacking boundaries in real time and with ultrahigh resolution which enables in-depth analysis on the atomic structure at the boundaries. In conjunction with density functional theory calculations, we establish the dynamics of the boundary nucleation and expansion and further identify metallic boundary states. Our approach provides a means to synthesize domain boundaries with intriguing transport properties and opens up a new avenue for controlling valleytronics in nanoscale domains via real-time patterning of domains with different symmetry properties

3D reconstruction of tungsten at atomic resolution using electron tomography

Reconstruction of tiltser_W.tif produced using an equally sloped tomography iterative algorithm

Electron tomography series of tungsten at atomic resolution

Equally sloped tomography series of tip of a tungsten needle. Series covers full 180 degree range. Note: Raw data is not aligned

Dynamics of Symmetry-Breaking Stacking Boundaries in Bilayer MoS₂

Crystal symmetry of two-dimensional (2D) materials plays an important role in their electronic and optical properties. Engineering symmetry in 2D materials has recently emerged as a promising way to achieve novel properties and functions. The noncentrosymmetric structure of monolayer transition metal dichalcogenides (TMDCs), such as molybdenum disulfide (MoS₂), has allowed for valley control via circularly polarized optical excitation. In bilayer TMDCs, inversion symmetry can be controlled by varying the stacking sequence, thus providing a pathway to engineer valley selectivity. Here, we report the <i>in situ</i> integration of AA′ and AB stacked bilayer MoS₂ with different inversion symmetries by creating atomically sharp stacking boundaries between the differently stacked domains, via thermal stimulation and electron irradiation, inside an atomic-resolution scanning transmission electron microscopy. The setup enables us to track the formation and atomic motion of the stacking boundaries in real time and with ultrahigh resolution which enables in-depth analysis on the atomic structure at the boundaries. In conjunction with density functional theory calculations, we establish the dynamics of the boundary nucleation and expansion and further identify metallic boundary states. Our approach provides a means to synthesize domain boundaries with intriguing transport properties and opens up a new avenue for controlling valleytronics in nanoscale domains via real-time patterning of domains with different symmetry properties

Dynamics of Symmetry-Breaking Stacking Boundaries in Bilayer MoS₂

Crystal symmetry of two-dimensional (2D) materials plays an important role in their electronic and optical properties. Engineering symmetry in 2D materials has recently emerged as a promising way to achieve novel properties and functions. The noncentrosymmetric structure of monolayer transition metal dichalcogenides (TMDCs), such as molybdenum disulfide (MoS₂), has allowed for valley control via circularly polarized optical excitation. In bilayer TMDCs, inversion symmetry can be controlled by varying the stacking sequence, thus providing a pathway to engineer valley selectivity. Here, we report the <i>in situ</i> integration of AA′ and AB stacked bilayer MoS₂ with different inversion symmetries by creating atomically sharp stacking boundaries between the differently stacked domains, via thermal stimulation and electron irradiation, inside an atomic-resolution scanning transmission electron microscopy. The setup enables us to track the formation and atomic motion of the stacking boundaries in real time and with ultrahigh resolution which enables in-depth analysis on the atomic structure at the boundaries. In conjunction with density functional theory calculations, we establish the dynamics of the boundary nucleation and expansion and further identify metallic boundary states. Our approach provides a means to synthesize domain boundaries with intriguing transport properties and opens up a new avenue for controlling valleytronics in nanoscale domains via real-time patterning of domains with different symmetry properties

Enabling Oxidation Protection and Carrier-Type Switching for Bismuth Telluride Nanoribbons via <i>in Situ</i> Organic Molecule Coating

Thermoelectric materials with high electrical conductivity and low thermal conductivity (e.g., Bi2Te3) can efficiently convert waste heat into electricity; however, in spite of favorable theoretical predictions, individual Bi2Te3 nanostructures tend to perform less efficiently than bulk Bi2Te3. We report a greater-than-order-of-magnitude enhancement in the thermoelectric properties of suspended Bi2Te3 nanoribbons, coated in situ to form a Bi2Te3/F4-TCNQ core–shell nanoribbon without oxidizing the core–shell interface. The shell serves as an oxidation barrier but also directly functions as a strong electron acceptor and p-type carrier donor, switching the majority carriers from a dominant n-type carrier concentration (∼1021 cm–3) to a dominant p-type carrier concentration (∼1020 cm–3). Compared to uncoated Bi2Te3 nanoribbons, our Bi2Te3/F4-TCNQ core–shell nanoribbon demonstrates an effective chemical potential dramatically shifted toward the valence band (by 300–640 meV), robustly increased Seebeck coefficient (∼6× at 250 K), and improved thermoelectric performance (10–20× at 250 K)

Direct Observation of a Long-Lived Single-Atom Catalyst Chiseling Atomic Structures in Graphene

Fabricating stable functional devices at the atomic scale is an ultimate goal of nanotechnology. In biological processes, such high-precision operations are accomplished by enzymes. A counterpart molecular catalyst that binds to a solid-state substrate would be highly desirable. Here, we report the direct observation of single Si adatoms catalyzing the dissociation of carbon atoms from graphene in an aberration-corrected high-resolution transmission electron microscope (HRTEM). The single Si atom provides a catalytic wedge for energetic electrons to chisel off the graphene lattice, atom by atom, while the Si atom itself is not consumed. The products of the chiseling process are atomic-scale features including graphene pores and clean edges. Our experimental observations and first-principles calculations demonstrated the dynamics, stability, and selectivity of such a single-atom chisel, which opens up the possibility of fabricating certain stable molecular devices by precise modification of materials at the atomic scale

Characterization of Ordering in A‑Site Deficient Perovskite Ca_1–<i>x</i>La_2<i>x</i>/3TiO₃ Using STEM/EELS

The vacancy ordering behavior of an A-site deficient perovskite system, Ca_1–<i>x</i>La_2<i>x</i>/3TiO₃, was studied using atomic resolution scanning transmission electron microscopy (STEM) in conjunction with electron energy-loss spectroscopy (EELS), with the aim of determining the role of A-site composition changes. At low La content (<i>x</i> = 0.2), adopting <i>P</i><i>b</i><i>n</i><i>m</i> symmetry, there was no indication of long-range ordering. Domains, with clear boundaries, were observed in bright-field (BF) imaging, but were not immediately visible in the corresponding high-angle annular dark-field (HAADF) image. These boundaries, with the aid of displacement maps from A-site cations in the HAADF signal, are shown to be tilt boundaries. At the La-rich end of the composition (<i>x</i> = 0.9), adopting <i>Cmmm</i> symmetry, long-range ordering of vacancies and La³⁺ ions was observed, with alternating La-rich and La-poor layers on (001)_p planes, creating a double perovskite lattice along the <i>c</i> axis. These highly ordered domains can be found isolated within a random distribution of vacancies/La³⁺, or within a large population, encompassing a large volume. In regions with a high number density of double perovskite domains, these highly ordered domains were separated by twin boundaries, with 90° or 180° lattice rotations across boundaries. The occurrence and characteristics of these ordered structures are discussed and compared with similar perovskite systems