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Moiré superlattices formed by stacking two-dimensional crystals have reinvigorated the pursuit for emergent functionalities of engineered superlattices. Unique optical characteristics can be realized from the interplay between the electronic excitations and the atomic rearrangements owing to their intrinsic softness. Although large-scale reconstructions have been identified at small twist angles, they have been treated as being rigid at large twist angles. Here, we report that moiré superlattices made from single layers of MoS

The hole density of individual copper sulfide nanocrystals (Cu2-xS NCs) is determined from the stoichiometric mismatch (x) between copper and sulfide atoms. Consequently, the electronic properties of the material vary over a range of x. To exploit Cu2-xS NCs in devices, assemblies of NCs are typically required. Herein, we investigate the influence of x, referred to as the stoichiometric doping effect, on the structural, optical, electrical, and thermoelectric properties of electronically coupled Cu2-xS NC assemblies. The doping process is done by immersing the solid NC assemblies into a solution containing a Cu(I) complex for different durations (0-10 min). As Cu+ gradually occupied the copper-deficient sites of Cu2-xS NCs, x could be controlled from 0.9 to less than 0.1. Consequently, the near-infrared (NIR) absorbance of Cu2-xS NC assemblies changes systematically with x. With increasing x, electrical conductivity increased and the Seebeck coefficient decreased systematically, leading to the maximal thermoelectric power factor from a film of Cu2-xS NCs at an optimal doping condition yielding x = 0.1. The physical characteristics of the Cu2-xS NC assemblies investigated herein will provide guidelines for exploiting this emerging class of nanocrystal system based on doping.

The sonochemical synthesis of CsPbBr3 perovskite nanocrystals (PNCs) is an attractive method for the large-scale synthesis and low-cost production of PNC at low temperature; however, there are no r...

We report a combined nano-photocurrent and infrared nanoscopy study of twisted bilayer graphene (TBG) enabling access to the local electronic phenomena at length scales as short as 20 nm. We show that the photocurrent changes sign at carrier densities tracking the local superlattice density of states of TBG. We use this property to identify domains of varying local twist angle by local photo-thermoelectric effect. Consistent with the photocurrent study, infrared nano-imaging experiments reveal optical conductivity features dominated by twist-angle dependent interband transitions. Our results provide a fast and robust method for mapping the electronic structure of TBG and suggest that similar methods can be broadly applied to probe electronic inhomogeneities of moir\'e superlattices in other van der Waals heterostructures.

Control of the interlayer twist angle in two-dimensional (2D) van der Waals (vdW) heterostructures enables one to engineer a quasiperiodic moir\'e superlattice of tunable length scale. In twisted bilayer graphene (TBG), the simple moir\'e superlattice band description suggests that the electronic band width can be tuned to be comparable to the vdW interlayer interaction at a 'magic angle', exhibiting strongly correlated behavior. However, the vdW interlayer interaction can also cause significant structural reconstruction at the interface by favoring interlayer commensurability, which competes with the intralayer lattice distortion. Here we report the atomic scale reconstruction in TBG and its effect on the electronic structure. We find a gradual transition from incommensurate moir\'e structure to an array of commensurate domain structures as we decrease the twist angle across the characteristic crossover angle, $\theta_c$ ~1\deg. In the twist regime smaller than $\theta_c$ where the atomic and electronic reconstruction become significant, a simple moir\'e band description breaks down. Upon applying a transverse electric field, we observe electronic transport along the network of one-dimensional (1D) topological channels that surround the alternating triangular gapped domains, providing a new pathway to engineer the system with continuous tunability.

The hole density of individual copper sulfide nanocrystals (Cu

We report the fabrication and characterization of indium gallium zinc oxide (IGZO)-based synaptic thin-film transistors. Radio-frequency magnetron-sputtered AlO x thin films are embedded in the IGZO channel as charge-trapping layers to provide synaptic behavior. The voltage pulse introduced at the gate electrodes traps or de-traps charges in the embedded AlO x layer thus modulates the channel current, which in turn leads to the ability to mimic biological synaptic behaviors such as excitonic postsynaptic current, paired-pulse facilitation, and potentiation and depression. Simulation results suggest that the device can perform properly as a synaptic unit in an artificial neural network.

Graphene-based heterostructures display a variety of phenomena that are strongly tunable by electrostatic local gates. Monolayer graphene (MLG) exhibits tunable surface plasmon polaritons, as revealed by scanning nano-infrared experiments. In bilayer graphene (BLG), an electronic gap is induced by a perpendicular displacement field. Gapped BLG is predicted to display unusual effects such as plasmon amplification and domain wall plasmons with significantly larger lifetime than MLG. Furthermore, a variety of correlated electronic phases highly sensitive to displacement fields have been observed in twisted graphene structures. However, applying perpendicular displacement fields in nano-infrared experiments has only recently become possible [Li, H.; Nano Lett. 2020, 20, 3106-3112]. In this work, we fully characterize two approaches to realizing nano-optics compatible top gates: bilayer MoS2 and MLG. We perform nano-infrared imaging on both types of structures and evaluate their strengths and weaknesses. Our work paves the way for comprehensive near-field experiments of correlated phenomena and plasmonic effects in graphene-based heterostructures.

Van der Waals heterostructures obtained via stacking and twisting have been used to create moiré superlattices

Molecular-scale manipulation of electronic and ionic charge accumulation in materials is the backbone of electrochemical energy storage1–4. Layered van der Waals (vdW) crystals are a diverse family of materials into which mobile ions can electrochemically intercalate into the interlamellar gaps of the host atomic lattice5,6. The structural diversity of such materials enables the interfacial properties of composites to be optimized to improve ion intercalation for energy storage and electronic devices7–12. However, the ability of heterolayers to modify intercalation reactions, and their role at the atomic level, are yet to be elucidated. Here we demonstrate the electrointercalation of lithium at the level of individual atomic interfaces of dissimilar vdW layers. Electrochemical devices based on vdW heterostructures 13 of stacked hexagonal boron nitride, graphene and molybdenum dichalcogenide (MoX2; X = S, Se) layers are constructed. We use transmission electron microscopy, in situ magnetoresistance and optical spectroscopy techniques, as well as low-temperature quantum magneto-oscillation measurements and ab initio calculations, to resolve the intermediate stages of lithium intercalation at heterointerfaces. The formation of vdW heterointerfaces between graphene and MoX2 results in a more than tenfold greater accumulation of charge in MoX2 when compared to MoX2/MoX2 homointerfaces, while enforcing a more negative intercalation potential than that of bulk MoX2 by at least 0.5 V. Beyond energy storage, our combined experimental and computational methodology for manipulating and characterizing the electrochemical behaviour of layered systems opens new pathways to control the charge density in two-dimensional electronic and optoelectronic devices.

We report the existence of latent order during core relaxation in the high-angle grain boundaries (GBs) of GaN films using atomic-resolution scanning transmission electron microscopy and ab initio density functional theory calculations. Core structures in the high-angle GBs are characterized by two pairs of Ga-N bonds located next to each other. The core type correlates strongly with the bond angle differences. We identify an order of core relaxation hidden in the high-angle GBs by further classifying the 5/7 atom cores into a stable 5/7 core (5/7(S)) and a metastable 5/7 core (5/7(M)). This core-type classification indicates that metastable cores can exist at real high-angle GBs under certain circumstances. Interestingly, 5/7(M) exhibits distinct defect states compared to 5/7(S), despite their similar atomic configurations. We investigate the reconstruction of defect states observed in 5/7(M) by analyzing the real-space wave functions. An inversion occurred between two localized states during the transition from 5/7(S) to 5/7(M). We suggest an inversion mechanism to explain the formation of new defect states in 5/7(M).

We developed novel techniques to fabricate atomically thin ${\mathrm{Bi}}_{2.1}{\mathrm{Sr}}_{1.9}{\mathrm{CaCu}}_{2.0}{\mathrm{O}}_{8+\ensuremath{\delta}}$ van der Waals heterostructures down to two unit cells while maintaining a transition temperature ${T}_{c}$ close to the bulk, and carry out magnetotransport measurements on these van der Waals devices. We find a double sign change of the Hall resistance ${R}_{xy}$ as in the bulk system, spanning both below and above ${T}_{c}$. Further, we observe a drastic enlargement of the region of sign reversal in the temperature-magnetic field phase diagram with decreasing thickness of the device. We obtain quantitative agreement between experimental ${R}_{xy}(T,B)$ and the predictions of the vortex dynamics-based description of Hall effect in high-temperature superconductors both above and below ${T}_{c}$.

Reducing the energy bandwidth of electrons in a lattice below the long-range Coulomb interaction energy promotes correlation effects. Moire superlattices—which are created by stacking van der Waals heterostructures with a controlled twist angle1–3—enable the engineering of electron band structure. Exotic quantum phases can emerge in an engineered moire flat band. The recent discovery of correlated insulator states, superconductivity and the quantum anomalous Hall effect in the flat band of magic-angle twisted bilayer graphene4–8 has sparked the exploration of correlated electron states in other moire systems9–11. The electronic properties of van der Waals moire superlattices can further be tuned by adjusting the interlayer coupling6 or the band structure of constituent layers9. Here, using van der Waals heterostructures of twisted double bilayer graphene (TDBG), we demonstrate a flat electron band that is tunable by perpendicular electric fields in a range of twist angles. Similarly to magic-angle twisted bilayer graphene, TDBG shows energy gaps at the half- and quarter-filled flat bands, indicating the emergence of correlated insulator states. We find that the gaps of these insulator states increase with in-plane magnetic field, suggesting a ferromagnetic order. On doping the half-filled insulator, a sudden drop in resistivity is observed with decreasing temperature. This critical behaviour is confined to a small area in the density–electric-field plane, and is attributed to a phase transition from a normal metal to a spin-polarized correlated state. The discovery of spin-polarized correlated states in electric-field-tunable TDBG provides a new route to engineering interaction-driven quantum phases. Twisted double bilayer graphene devices show tunable spin-polarized correlated states that are sensitive to electric and magnetic fields, providing further insights into correlated states in two-dimensional moire materials.

We developed novel techniques to fabricate atomically thin Bi_{2.1}Sr_{1.9}CaCu_{2.0}O_{8+δ} van der Waals heterostructures down to two unit cells while maintaining a transition temperature T_{c} close to the bulk, and carry out magnetotransport measurements on these van der Waals devices. We find a double sign change of the Hall resistance R_{xy} as in the bulk system, spanning both below and above T_{c}. Further, we observe a drastic enlargement of the region of sign reversal in the temperature-magnetic field phase diagram with decreasing thickness of the device. We obtain quantitative agreement between experimental R_{xy}(T,B) and the predictions of the vortex dynamics-based description of Hall effect in high-temperature superconductors both above and below T_{c}.

There have been significant recent developments in the growth of single-crystal gallium nitride (GaN) on unconventional templates for large-area blue or green light-emitting diodes (LEDs) which, together with layer transfer onto foreign substrates, can enable flexible and stretchable lighting applications. Here, the heteroepitaxial growth of GaN on amorphous and single-crystal substrates employing various interlayers and nucleation layers is reviewed, as well as the use of weak interfaces for layer-transfer onto foreign substrates. Recent progress in low-temperature GaN-based red–green–blue (RGB) LEDs on glass substrates, which has exhibited a calculated efficiency of 11% compared with that from commertial LEDs, is discussed. Layer-transfer techniques with various interlayers are also discussed. These heteroepitaxial GaN growth and layer-transfer technologies are expected to lead to new lighting and display devices with high efficiency and full-color tunability, which are suitable for large-area, stretchable display and lighting applications.

The properties of Van der Waals (VdW) heterostructures are determined by the twist angle and the interface between adjacent layers as well as their polytype and stacking. Here, the use of spectroscopic low energy electron microscopy (LEEM) and micro low energy electron diffraction (µLEED) methods to measure these properties locally is described. The authors present results on a MoS2/hBN heterostructure, but the methods are applicable to other materials. Diffraction spot analysis is used to assess the benefits of using hBN as a substrate. In addition, by making use of the broken rotational symmetry of the lattice, the cleaving history of the MoS2 flake is determined, that is, which layer stems from where in the bulk.

Twisting a route for surface plasmons Graphene is an atomically thin material that supports highly confined plasmon polaritons, or nano-light, with very low loss. The properties of graphene can be made richer by introducing and then rotating a second layer so that there is a slight angle between the atomic registry. Sunku et al. show that the moiré patterns that result from such twisted bilayer graphene also provide confined conducting channels that can be used for the directed propagation of surface plasmons. Controlling the structure thereby provides a pathway to control and route surface plasmons for a nanophotonic platform. Science , this issue p. 1153

The epitaxial lateral overgrowth (ELOG) of GaN microdisks on graphene microdots and the fabrication of flexible light-emitting diodes (LEDs) using these microdisks is reported. An ELOG technique with only patterned graphene microdots is used, without any growth mask. The discrete micro-LED arrays are transferred onto Cu foil by a simple lift-off technique, which works reliably under various bending conditions.

The electronic states of oxygen vacancies (VO s) in amorphous oxide semiconductors are shallow donors, deep donors or electron traps; these are determined by the local atomic structure. Because the amorphous phase is metastable compared with the crystalline phase, the degree of structural disorder is likely to decrease, which is referred to as structural relaxation (SR). Thus SR can affect the VO electronic state by changing the local atomic conditions. In this study, we demonstrated that electron doping is possible through the SR of amorphous oxides without redox reactions using a novel device structure that prevents extrinsic reactions with electrodes and ambient atmosphere during annealing. The concentration of VO s in the shallow-donor state in amorphous In-Ga-Zn-O (a-IGZO) increases from ~1016 to ~1019 cm−3 with increasing annealing temperatures between 300 and 450 °C. The SR-driven doping effect is strongly dependent on the annealing temperature but not on the annealing time. The Arrhenius activation energy of the SR-driven doping effect is 1.76 eV, which is similar to the bonding energies in a-IGZO. Our findings suggest that the free volume in a-IGZO decreases during SR, and the VO s in either deep-donor or electron-trap states are consequently transformed into shallow-donor states. A chemical-free method for enhancing the conductivity of transparent transistor materials could benefit flat-panel and flexible displays. Amorphous indium–gallium–zinc oxide (a-IGZO) is a semiconductor that can withstand bending better than silicon technology thanks to its disordered internal structure. However, a process known as structural relaxation slowly reduces the randomness of a-IGZO atoms, which alters the electrical characteristics of a-IGZO. To initiate controlled structural relaxations, Young-Chang Joo from Seoul National University and colleagues annealed a stacked novel metal/a-IGZO/metal device at temperatures below the glass transition point. Electrical measurements revealed the relaxations increased the concentration of shallow-donor-state oxygen vacancies, enabling electron dopants to be added to a-IGZO without using redox chemistry. The temperature-dependent doping can be sustained for prolonged annealing times, making it a promising approach for enhancing the thermal stress resistance of flexible electronics. The effects of structural relaxation (SR) on the electronic state of oxygen vacancies (VO s) in amorphous oxide semiconductors is investigated. Without redox reactions, the concentration of VO s in the shallow-donor state (NDS) increases about 103 times with increases in the annealing temperature from 300 to 450 °C. The reduction in the free volume size and transformation of VO s in either deep-donor or electron-trap states into the shallow-donor state during SR is the primary mechanism responsible for the increase in NDS.

Hybrid heterostructures of low-dimensional semiconductor nanostructures with two-dimensional (2-D) graphene layers are emerging as new materials for fabricating transferable and/ or fl exible optoelectronic and electronic devices. [ 1–6 ] The semiconductor nanostructures work as effi cient channels for carrier transport and electrical pumping to radiative recombination, thereby improving the device performances greatly in optoelectronics and electronics. Furthermore, the graphene layers, which have excellent electrical and thermal conductivity, high mechanical strength and elasticity, and/or optical transparency, act as novel substrates offering new functionalities such as transferability or fl exibility. [ 7–13 ] In those previous reports, however, the nanostructures were randomly formed on graphene layers since nucleation sites on the graphene layers could not be controlled, preventing practical manipulation of individual nanostructure. It is well known that control of positions and morphology of nanostructures is a prerequisite for the nanostructure to be exploited as building blocks for fabricating various types of nanodevices. [ 14–17 ] Accordingly, the position and morphology control of the low-dimensional nanostructure on the graphene layers is crucial for practical applications. The well-controlled nanostructure on the graphene layers could serve as templates for achieving heteroepitaxy of ultrathin layer on the surface, yielding various quantum heterostructures for diverse applications including optoelectronics, electronics, and photovoltaics. Furthermore, precise position control of the individual nanostructures is a great advantage for realizing the integration of semiconductor nanodevices with graphene electronics for fl exible and/or transparent display applications. Here, we report the positionand morphologycontrolled growth of ZnO nanostructures using artifi cially formed graphene step-edges. Then, the resulting structural and optical properties of the ZnO nanostructure are also discussed. Furthermore, in order to demonstrate the feasibility of those

This review summarizes recent research on GaN nanostructures for light-emitting diode (LED) applications. GaN nanostructure fabrication methods are first discussed, followed by a brief explanation of the basic components of the LED structure based on nitride nanostructures. Various device architectures of nanostructured GaN LEDs, as the main focus of the review, are then presented, covering research from the early LEDs based on a single GaN nanostructure to the most advanced LEDs based on GaN nanostructure arrays on flexible substrates. The research discussed in this review will promote novel applications of GaN LEDs that exploit the advantages of nanostructures.

We report low-temperature ambient synthesis of high-quality, several micrometer thick polycrystalline silicon arrays on soda lime glass substrates by local pyrolysis, where SiH4 gas is locally decomposed near and condensed on the resistively heated microheater arrays with an average growth rate of 50 nm/s. The silicon arrays had nearly perfect crystallinity and a minimum grain size larger than 0.2 μm, as determined by spatially resolved Raman spectroscopy and transmission electron microscopy. Boron-doped silicon arrays by the local pyrolysis was further fabricated to yield concentric p-i-n heterojunction arrays with rectifying current–voltage or photovoltaic characteristics.

The exact role of Cu in the electrical properties of amorphous oxide semiconductors (AOSs) has been unclear, even though Cu has been the key element for the p-type characteristics of crystalline oxide semiconductors. Here, the dynamic changes, determined by diffusion kinetics, in the effect of Cu on the electrical properties of amorphous InGaZnO (a-IGZO) are revealed. In the early stage of annealing, Cu dominantly diffuses into a-IGZO through the free volume and acts as a mobile electron donor, which generates a resistive switching (RS) behavior related to the conductive filaments (CFs). With further annealing, substitutional Cu becomes predominant via In sites. After annealing, supersaturated Cu forms nonuniform, crystalline CuInO clusters in a-IGZO, which decrease the electrical conductivity of a-IGZO and deteriorate the CF-based RS performance. The findings reveal Cu diffusion mechanisms and the role of Cu in the electrical properties of AOSs dependent on the structural location and provide guidelines for modulating the RS characteristics of AOSs through Cu diffusion control.

Plan-view and cross-sectional transmission electron microscopy images show the microstructural properties of GaN thin films grown on graphene layers, including dislocation types and density, crystalline orientation and grain boundaries. The roles of ZnO nanowalls and GaN intermediate layers in the heteroepitaxial growth of GaN on graphene, revealed by cross-sectional transmission electron microscopy, are also discussed.

Purpose: The purpose of the study was to examine the recording accuracy of faculty observers and standardized patients (SPs) on a clinical performance examination (CPX). Methods: This was a cross-sectional study of a fourth-year medical students' CPX that was held at a medical school in Seoul, Korea. The CPX consisted of 4 cases and was administered to 118 examinees, with the participation of 52 SP and 45 faculty observers. For the study we chose 15 examinees per case, and analyzed 60 student-SP encounters in total. To determine the recording accuracy level, 2 SP trainers developed an answer key for each encounter. First, we computed agreement rates (P) and kappa coefficient (K) values between the answer key–SPs and the answer key–faculty observers. Secondly, we analyzed variance (ANOVA) with repeated measures to determine whether the mean percentage of the correct checklist score differed as a function of the rater, the case, or the interaction between both factors. Results: Mean P rates ranged from 0.72 to 0.86, while mean K values varied from 0.39 to 0.59. The SP checklist accuracy was higher than that of faculty observersat the level of item comparison. Results from ANOVA showed that there was no significant difference between the percentage of correct scores by the answer key, faculty observers and SPs. There was no significant interaction between rater and case factors. Conclusion: Acceptable levels of recording accuracy were obtained in both rater groups. SP raters can replace faculty raters in a large-scale CPX with thorough preparation.

We report the growth of GaN micro-rods and coaxial quantum-well heterostructures on graphene films, together with structural and optical characterization, for applications in flexible optical devices. Graphene films were grown on Cu foil by means of chemical vapor deposition, and used as the substrates for the growth of the GaN micro-rods, which were subsequently transferred onto SiO2/Si substrates. Highly Si-doped, n-type GaN micro-rods were grown on the graphene films using metal–organic chemical vapor deposition. The growth and vertical alignment of the GaN micro-rods, which is a critical factor for the fabrication of high-performance light-emitting diodes (LEDs), were characterized using electron microscopy and X-ray diffraction. The GaN micro-rods exhibited promising photoluminescence characteristics for optoelectronic device applications, including room-temperature stimulated emission. To fabricate flexible LEDs, InxGa1–xN/GaN multiple quantum wells and a p-type GaN layer were deposited coaxially on the GaN micro-rods, and transferred onto Ag-coated polymer substrates using lift-off. Ti/Au and Ni/Au metal layers were formed to provide electrical contacts to the n-type and p-type GaN regions, respectively. The micro-rod LEDs exhibited intense emission of visible light, even after transfer onto the flexible polymer substrate, and reliable operation was achieved following numerous cycles of mechanical deformation.

Background Tumor necrosis factor-α (TNF-α) plays a crucial role in the pathogenesis of rheumatoid arthritis (RA) and monoclonal antibodies targeting TNF-α are used as a popular therapeutic interventions in RA patients. CXCL10 is a chemokine that potentially plays a role in the immunopathogenesis of RA. The development of bispecific antibody (BiAb) as a therapeutic agent for RA may have great clinical potential. We constructed a single-chain BiAb against both TNF-α and CXCL10 (IgG-based format). Objectives We investigated the inhibitory effects of BiAb targeting TNF-α and CXCL10 on in vitro and in vivo model of RA. Methods We performed TNF-α neutralization assay using WEHI cell and tube formation assay of HUVEC cell to investigate TNF-α antagonistic effect of BiAb. Then we performed T cell migration assay induced by CXCL10 using transwell system and inhibitory assay of osteoclastogenesis. The effects of BiAb were determined by arthritis severity score, histological damage in Tg197 human TNF transgenic mice. Results BiAb suppressed TNF-α comparatively with adalimumab in TNF-α neutralization assay, tube formation assay and osteoclast differentiation. Cell migration induced by CXCL10 was inhibited by BiAb dose dependently. Treatment with BiAb resulted in beneficial effects on clinical and histological parameters of Tg197 human TNF transgenic mice model. Conclusions Our results demonstrate the novel BiAb effectively neutralized TNF-α and CXCL10 in vitro and in vivo model of rheumatoid arthritis. We suggest that BiAb could be a promising biologic agent for the treatment of RA. Disclosure of Interest None declared DOI 10.1136/annrheumdis-2014-eular.3074

Microstructural defects in GaN thin films grown on graphene produced via chemical vapor deposition have been investigated using electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). EBSD analysis reveals the preferred orientations of the GaN films. We further examined the microstructural defects such as grain boundaries and threading dislocations present in the films using TEM. Plan-view TEM analysis showed presence of both high- and low-angle grain boundaries and the threading dislocations mostly bound to those grain boundaries. Moreover, the characteristics and behavior of the threading dislocations were also investigated using cross-section TEM analysis.

We report the enhanced electroluminescence (EL) of GaN light-emitting diodes (LEDs) on glass substrates by controlling GaN crystal morphology, crystallinity, and device fabrication. Depending on the degree of epitaxy, we studied three different GaN morphologies; randomly oriented GaN polycrystals, nearly single-crystalline pyramid arrays, and fully single-crystalline pyramid arrays, which were fabricated by controlling the epitaxial relationship with the substrates. At proper growth temperature, GaN crystallinity was improved with increasing GaN crystal size irrespective of the GaN crystallographic orientation, as determined by spatially resolved cathodoluminescent spectroscopy. All the different GaN morphologies were further fabricated into LEDs to investigate their EL characteristics. The optimized GaN LEDs on glass composed of the nearly single-crystalline GaN pyramid arrays exhibited excellent microscopic EL uniformity and luminance values of about 2700 and 1150 cd m−2 at peak wavelengths of 537 and 480 nm, respectively.

The structure of tubular GaN coaxially grown on ZnO nanorods with thickness of 6–12nm was investigated using x-ray absorption fine structure (XAFS) at the Ga K edge. The XAFS measurements revealed that the GaN had a distorted-wurtzite structure, and that there were more distortions in the bond length of Ga–Ga pairs than in Ga–N pairs. However, no extra disorders were observed in any of the pairs. These results strongly suggest that Ga atoms first bonded to the ZnO template. Unlike other techniques, the XAFS determines structure around a selected species atom in nano-heterostructures.

The local structure of two samples of vertically well-aligned ZnO nanorods with average diameters of 13 and 37nm were studied using orientation-dependent x-ray absorption fine structure (XAFS) at the Zn K edge. The aligned ZnO nanorod samples were fabricated on sapphire (0001) substrates with a catalyst-free metalorganic vapor-phase epitaxy method. The XAFS measurements showed that both nanorod samples have a well-ordered wurtzite structure and that no vacancy was observed at either site of zinc or oxygen atoms. However, we found that in both samples the lattice constants of a and b were shrunken by ∼0.04A while c was elongated by ∼0.1A, compared with those of their bulk counterparts. Furthermore, there was a substantial amount of disorder in the bond length of the only Zn–O pairs located near the ab plane. This may suggest that the terminating atoms at the boundaries of the nanorods are oxygen atoms.

Structural engineering of van der Waals heterostructures via stacking and twisting has recently been used to create moir\'e superlattices, enabling the realization of new optical and electronic properties in solid-state systems. In particular, moir\'e lattices in twisted bilayers of transition metal dichalcogenides (TMDs) have been shown to lead to exciton trapping, host Mott insulating and superconducting states, and act as unique Hubbard systems whose correlated electronic states can be detected and manipulated optically. Structurally, these twisted heterostructures also feature atomic reconstruction and domain formation. Unfortunately, due to the nanoscale sizes (~10 nm) of typical moir\'e domains, the effects of atomic reconstruction on the electronic and excitonic properties of these heterostructures could not be investigated systematically and have often been ignored. Here, we use near-0$^o$ twist angle MoSe$_2$/MoSe$_2$ bilayers with large rhombohedral AB/BA domains to directly probe excitonic properties of individual domains with far-field optics. We show that this system features broken mirror/inversion symmetry, with the AB and BA domains supporting interlayer excitons with out-of-plane (z) electric dipole moments in opposite directions. The dipole orientation of ground-state $\Gamma$-K interlayer excitons (X$_{I,1}$) can be flipped with electric fields, while higher-energy K-K interlayer excitons (X$_{I,2}$) undergo field-asymmetric hybridization with intralayer K-K excitons (X$_0$). Our study reveals the profound impacts of crystal symmetry on TMD excitons and points to new avenues for realizing topologically nontrivial systems, exotic metasurfaces, collective excitonic phases, and quantum emitter arrays via domain-pattern engineering., Comment: 29 pages, 4 figures in main text, 6 figures in supplementary information