Selective binding and periodic arrangement of magic boron clusters on monolayer borophene.

The synthesis and characterization of small boron clusters with unique size and regular arrangement are crucial for boron chemistry and two-dimensional borophene materials. In this study, together with theoretical calculations, the joint molecular beam epitaxy and scanning tunneling microscopy experiments achieve the formation of unique B5 clusters on monolayer borophene (MLB) on a Cu(111) surface. The B5 clusters tend to selectively bind to specific sites of MLB with covalent boron-boron bonds in the periodic arrangement, which can be ascribed to the charge distribution and electron delocalization character of MLB and also prohibits nearby co-adsorption of B5 clusters. Furthermore, the close-packed adsorption of B5 clusters would facilitate the synthesis of bilayer borophene, exhibiting domino effect-like growth mode. The successful growth and characterization of uniform boron clusters on a surface enrich the boron-based nanomaterials and reveal the essential role of small clusters during the growth of borophene.

Unveiling the Anomalous Hall Response of the Magnetic Structure Changes in the Epitaxial MnBi2Te4 Films.

Recently discovered as an intrinsic antiferromagnetic topological insulator, MnBi2Te4 has attracted tremendous research interest, as it provides an ideal platform to explore the interplay between topological and magnetic orders. MnBi2Te4 displays distinct exotic topological phases that are inextricably linked to the different magnetic structures of the material. In this study, we conducted electrical transport measurements and systematically investigated the anomalous Hall response of epitaxial MnBi2Te4 films when subjected to an external magnetic field sweep, revealing the different magnetic structures stemming from the interplay of applied fields and the material's intrinsic antiferromagnetic (AFM) ordering. Our results demonstrate that the nonsquare anomalous Hall loop is a consequence of the distinct reversal processes within individual septuple layers. These findings shed light on the intricate magnetic structures in MnBi2Te4 and related materials, offering insights into understanding their transport properties and facilitating the implementation of AFM topological electronics.

Engineering Plateau Phase Transition in Quantum Anomalous Hall Multilayers.

The plateau phase transition in quantum anomalous Hall (QAH) insulators corresponds to a quantum state wherein a single magnetic domain gives way to multiple domains and then reconverges back to a single magnetic domain. The layer structure of the sample provides an external knob for adjusting the Chern number C of the QAH insulators. Here, we employ molecular beam epitaxy to grow magnetic topological insulator multilayers and realize the magnetic field-driven plateau phase transition between two QAH states with odd Chern number change ΔC. We find that critical exponents extracted for the plateau phase transitions with ΔC = 1 and ΔC = 3 in QAH insulators are nearly identical. We construct a four-layer Chalker-Coddington network model to understand the consistent critical exponents for the plateau phase transitions with ΔC = 1 and ΔC = 3. This work will motivate further investigations into the critical behaviors of plateau phase transitions with different ΔC in QAH insulators.

Strain Solitons in an Epitaxially Strained van der Waals-like Material.

Strain solitons are quasi-dislocations that form in van der Waals materials to relieve the energy associated with lattice or rotational mismatch. Novel electronic properties of strain solitons were predicted and observed. To date, strain solitons have been observed only in exfoliated crystals or mechanically strained crystals. The lack of a scalable approach toward the generation of strain solitons poses a significant challenge in the study of and use of their properties. Here, we report the formation of strain solitons with epitaxial growth of bismuth on InSb(111)B by molecular beam epitaxy. The morphology of the strain solitons for films of varying thickness is characterized with scanning tunneling microscopy, and the local strain state is determined from atomic resolution images. Bending in the solitons is attributed to interactions with the interface, and large angle bending is associated with edge dislocations. Our results enable the scalable generation of strain solitons.

Spin-Orbit Torques and Magnetization Switching in (Bi,Sb)2Te3/Fe3GeTe2 Heterostructures Grown by Molecular Beam Epitaxy.

Topological insulators (TIs) hold promise for manipulating the magnetization of a ferromagnet (FM) through the spin-orbit torque (SOT) mechanism. However, integrating TIs with conventional FMs often leads to significant device-to-device variations and a broad distribution of SOT magnitudes. In this work, we present a scalable approach to grow a full van der Waals FM/TI heterostructure by molecular beam epitaxy, combining the charge-compensated TI (Bi,Sb)2Te3 with 2D FM Fe3GeTe2 (FGT). Harmonic magnetotransport measurements reveal that the SOT efficiency exhibits a non-monotonic temperature dependence and experiences a substantial enhancement with a reduction of the FGT thickness to 2 monolayers. Our study further demonstrates that the magnetization of ultrathin FGT films can be switched with a current density of Jc ∼ 1010 A/m2, with minimal device-to-device variations compared to previous investigations involving traditional FMs.

Coexistence of Superconductivity and Antiferromagnetism in Topological Magnet MnBi2Te4 Films.

The interface of two materials can harbor unexpected emergent phenomena. One example is interface-induced superconductivity. In this work, we employ molecular beam epitaxy to grow a series of heterostructures formed by stacking together two nonsuperconducting antiferromagnetic materials, an intrinsic antiferromagnetic topological insulator MnBi2Te4 and an antiferromagnetic iron chalcogenide FeTe. Our electrical transport measurements reveal interface-induced superconductivity in these heterostructures. By performing scanning tunneling microscopy and spectroscopy measurements, we observe a proximity-induced superconducting gap on the top surface of the MnBi2Te4 layer, confirming the coexistence of superconductivity and antiferromagnetism in the MnBi2Te4 layer. Our findings will advance the fundamental inquiries into the topological superconducting phase in hybrid devices and provide a promising platform for the exploration of chiral Majorana physics in MnBi2Te4-based heterostructures.

Offcut Substrate-Induced Defect Trapping at Step Edges.

We report step edge-induced localized defects suppressing subsequent antiphase boundary formation in the bulk structure of a trilayer oxide heterostructure. The heterostructure encompasses a layer of La0.66Sr0.34MnO3 sandwiched between a superconducting La1.84Sr0.16CuO4 bottom layer and an insulating La2CuO4 top layer. The combination of a minor a-axis mismatch (0.11 Å) and a pronounced c-axis mismatch (2.73 Å) at the step edges leads to the emergence of localized defects exclusively forming at the step edge. Employing atomically resolved electron energy-loss spectroscopy maps, we discern the electronic state of those structures in the second La0.66Sr0.34MnO3 unit cell near the step edge. In particular, a reduction in the pre-edge region of the O-K edge indicates the formation of oxygen vacancies induced by the strained step edge. This study underscores our capability to control defects at the nanoscale.

Controlling the Charge Density Wave Transition in Single-Layer TiTe2xSe2(1-x) Alloys by Band Gap Engineering.

Closing the band gap of a semiconductor into a semimetallic state gives a powerful potential route to tune the electronic energy gains that drive collective phases like charge density waves (CDWs) and excitonic insulator states. We explore this approach for the controversial CDW material monolayer (ML) TiSe2 by engineering its narrow band gap to the semimetallic limit of ML-TiTe2. Using molecular beam epitaxy, we demonstrate the growth of ML-TiTe2xSe2(1-x) alloys across the entire compositional range and unveil how the (2 × 2) CDW instability evolves through the normal state semiconductor-semimetal transition via in situ angle-resolved photoemission spectroscopy. Through model electronic structure calculations, we identify how this tunes the relative strength of excitonic and Peierls-like coupling, demonstrating band gap engineering as a powerful method for controlling the microscopic mechanisms underpinning the formation of collective states in two-dimensional materials.

Wafer-Scale Two-Dimensional Semiconductors for Deep UV Sensing.

2D semiconductors (2SEM) can transform many sectors, from information and communication technology to healthcare. To date, top-down approaches to their fabrication, such as exfoliation of bulk crystals by "scotch-tape," are widely used, but have limited prospects for precise engineering of functionalities and scalability. Here, a bottom-up technique based on epitaxy is used to demonstrate high-quality, wafer-scale 2SEM based on the wide band gap gallium selenide (GaSe) compound. GaSe layers of well-defined thickness are developed using a bespoke facility for the epitaxial growth and in situ studies of 2SEM. The dominant centrosymmetry and stacking of the individual van der Waals layers are verified by theory and experiment; their optical anisotropy and resonant absorption in the UV spectrum are exploited for photon sensing in the technological UV-C spectral range, offering a scalable route to deep-UV optoelectronics.

Growth of Single Crystalline 2D Materials beyond Graphene on Non-metallic Substrates.

The advent of 2D materials has ushered in the exploration of their synthesis, characterization and application. While plenty of 2D materials have been synthesized on various metallic substrates, interfacial interaction significantly affects their intrinsic electronic properties. Additionally, the complex transfer process presents further challenges. In this context, experimental efforts are devoted to the direct growth on technologically important semiconductor/insulator substrates. This review aims to uncover the effects of substrate on the growth of 2D materials. The focus is on non-metallic substrate used for epitaxial growth and how this highlights the necessity for phase engineering and advanced characterization at atomic scale. Special attention is paid to monoelemental 2D structures with topological properties. The conclusion is drawn through a discussion of the requirements for integrating 2D materials with current semiconductor-based technology and the unique properties of heterostructures based on 2D materials. Overall, this review describes how 2D materials can be fabricated directly on non-metallic substrates and the exploration of growth mechanism at atomic scale.

Novel Polytype of III-VI Metal Chalcogenides Nano Crystals Realized in Epitaxially Grown InTe.

III-VI metal chalcogenides have garnered considerable research attention as a novel group of layered van der Waals materials because of their exceptional physical properties and potential technological applications. Here, the epitaxial growth and stacking sequences of InTe is reported, an essential and intriguing material from III-VI metal chalcogenides. Aberration-corrected scanning transmission electron microscopy (STEM) is utilized to directly reveal the interlayer stacking modes and atomic structure, leading to a discussion of a new polytype. Furthermore, correlations between the stacking sequences and interlayer distances are substantiated by atomic-resolution STEM analysis, which offers evidence for strong interlayer coupling of the new polytype. It is proposed that layer-by-layer deposition is responsible for the formation of the unconventional stacking order, which is supported by ab initio density functional theory calculations. The results thus establish molecular beam epitaxy as a viable approach for synthesizing novel polytypes. The experimental validation of the InTe polytype here expands the family of materials in the III-VI metal chalcogenides while suggesting the possibility of new stacking sequences for known materials in this system.

Catalyst-free MBE growth of PbSnTe nanowires with tunable aspect ratio.

Topological crystalline insulators (TCIs) are interesting for their topological surface states, which hold great promise for scattering-free transport channels and fault-tolerant quantum computing. A promising TCI is SnTe. However, Sn-vacancies form in SnTe, causing a high hole density, hindering topological transport from the surface being measured. This issue could be relieved by using nanowires with a high surface-to-volume ratio. Furthermore, SnTe can be alloyed with Pb reducing the Sn-vacancies while maintaining its topological phase. Here we present the catalyst-free growth of monocrystalline PbSnTe in molecular beam epitaxy. By the addition of a pre-deposition stage before the growth, we have control over the nucleation phase and thereby increase the nanowire yield. This facilitates tuning the nanowire aspect ratio by a factor of four by varying the growth parameters. These results allow us to grow specific morphologies for future transport experiments to probe the topological surface states in a Pb1-xSnxTe-based platform.

Optical properties of two-dimensional tin nanosheets epitaxially grown on graphene.

Heterostacks formed by combining two-dimensional materials show novel properties which are of great interest for new applications in electronics, photonics and even twistronics, the new emerging field born after the outstanding discoveries on twisted graphene. Here, we report the direct growth of tin nanosheets at the two-dimensional limit via molecular beam epitaxy on chemical vapor deposited graphene on Al2O3(0001). The mutual interaction between the tin nanosheets and graphene is evidenced by structural and chemical investigations. On the one hand, Raman spectroscopy indicates that graphene undergoes compressive strain after the tin growth, while no charge transfer is observed. On the other hand, chemical analysis shows that tin nanosheets interaction with sapphire is mediated by graphene avoiding the tin oxidation occurring in the direct growth on this substrate. Remarkably, optical measurements show that the absorption of tin nanosheets exhibits a graphene-like behavior with a strong absorption in the ultraviolet photon energy range, therein resulting in a different optical response compared to tin nanosheets on bare sapphire. The optical properties of ultra-thin tin films therefore represent an open and flexible playground for the absorption of light in a broad range of the electromagnetic spectrum and technologically relevant applications for photon harvesting and sensors.

Transition from fractal-dendritic to compact islands for the 2D-ferroelectric SnSe on graphene/Ir(111).

Epitaxial growth is a versatile method to prepare two-dimensional van der Waals ferroelectrics like group IV monochalcogenides which have potential for novel electronic devices and sensors. We systematically study SnSe monolayer islands grown by molecular beam epitaxy, especially the effect of annealing temperature on shape and morphology of the edges. Characterization of the samples by scanning tunneling microscopy reveals that the shape of the islands changes from fractal-dendritic after deposition at room temperature to a compact rhombic shape through annealing, but ripening processes are absent up to the desorption temperature. A two-step growth process leads to large, epitaxially aligned rhombic islands bounded by well-defined110-edges (armchair-like), which we claim to be the equilibrium shape of the stoichiometric SnSe monolayer islands. The relaxation of the energetically favorable edges is detected in atomically resolved STM images. The experimental findings are supported by the results of our first-principles calculations, which provide insights into the energetics of the edges, their reconstructions, and yields the equilibrium shapes of the islands which are in good agreement with the experiment.

Patterning optimization for device realization of patterned GaAsSbN nanowire photodetectors.

Vertically grown nanowires (NWs) are a research interest in optoelectronics and photovoltaic applications due to their high surface to volume ratio and good light trapping capabilities. This study presents the effects of process and design parameters on self-catalyzed GaAsSbN NWs grown by plasma-assisted molecular beam epitaxy on patterned silicon substrates using electron beam lithography. Vertical alignment of the patterned NWs examined via scanning electron microscopy show the sensitivity of patterned NW growth to the parameters of NW diameter, pitch, dose time, etching techniques and growth plan. Diameters range from 90 nm to 250 nm. Pitch lengths of 200 nm, 400 nm, 600 nm, 800 nm, 1000 nm, and 1200 nm were examined. Dry etching of the oxide layer of the silicon substrate and PMMA coating is performed using reactive ion etching (RIE) for 20 s and 120 s respectively. Comparisons of different HF etch durations performed pre and post PMMA removal are presented. Additionally, the report of an observed surfactant effect in dilute nitride GaAsSbN NWs in comparison to non-nitride GaAsSb is presented. Optimizations to patterning, RIE, and HF etching are presented to obtain higher vertical yield of patterned GaAsSbN NWs, achieving ∼80% of the expected NWµm2. Room temperature and 4 K photoluminescence results show the effect of nitride incorporation for further bandgap tuning, and patterned pitch on the optical characteristics of the NWs which gives insights to the compositional homogeneity for NWs grown at each pitch length.

Bending and reverse bending during the fabrication of novel GaAs/(In,Ga)As/GaAs core-shell nanowires monitored byin situx-ray diffraction.

We report on the fabrication of a novel design of GaAs/(In,Ga)As/GaAs radial nanowire heterostructures on a Si 111 substrate, where, for the first time, the growth of inhomogeneous shells on a lattice mismatched core results in straight nanowires instead of bent. Nanowire bending caused by axial tensile strain induced by the (In,Ga)As shell on the GaAs core is reversed by axial compressive strain caused by the GaAs outer shell on the (In,Ga)As shell. Progressive nanowire bending and reverse bending in addition to the axial strain evolution during the two processes are accessed byin situby x-ray diffraction. The diameter of the core, thicknesses of the shells, as well as the indium concentration and distribution within the (In,Ga)As quantum well are revealed by 2D energy dispersive x-ray spectroscopy using a transmission electron microscope. Shell(s) growth on one side of the core without substrate rotation results in planar-like radial heterostructures in the form of free standing straight nanowires.

Novel synthesis approach for "stubborn" metals and metal oxides.

Advances in physical vapor deposition techniques have led to a myriad of quantum materials and technological breakthroughs, affecting all areas of nanoscience and nanotechnology which rely on the innovation in synthesis. Despite this, one area that remains challenging is the synthesis of atomically precise complex metal oxide thin films and heterostructures containing "stubborn" elements that are not only nontrivial to evaporate/sublimate but also hard to oxidize. Here, we report a simple yet atomically controlled synthesis approach that bridges this gap. Using platinum and ruthenium as examples, we show that both the low vapor pressure and the difficulty in oxidizing a "stubborn" element can be addressed by using a solid metal-organic compound with significantly higher vapor pressure and with the added benefits of being in a preoxidized state along with excellent thermal and air stability. We demonstrate the synthesis of high-quality single crystalline, epitaxial Pt, and RuO2 films, resulting in a record high residual resistivity ratio (=27) in Pt films and low residual resistivity, ∼6 µΩ·cm, in RuO2 films. We further demonstrate, using SrRuO3 as an example, the viability of this approach for more complex materials with the same ease and control that has been largely responsible for the success of the molecular beam epitaxy of III-V semiconductors. Our approach is a major step forward in the synthesis science of "stubborn" materials, which have been of significant interest to the materials science and the condensed matter physics community.

An Ultrahigh Efficiency Excitonic Micro-LED.

High efficiency micro-LEDs, with lateral dimensions as small as one micrometer, are desired for next-generation displays, virtual/augmented reality, and ultrahigh-speed optical interconnects. The efficiency of quantum well LEDs, however, is reduced to negligibly small values when scaled to such small dimensions. Here, we show such a fundamental challenge can be overcome by developing nanowire excitonic LEDs. Harnessing the large exciton oscillator strength of quantum-confined nanostructures, we demonstrate a submicron scale green-emitting LED having an external quantum efficiency and wall-plug efficiency of 25.2% and 20.7%, respectively, the highest values reported for any LEDs of this size to our knowledge. We established critical factors for achieving excitonic micro-LEDs, including the epitaxy of nanostructures to achieve strain relaxation, the utilization of semipolar planes to minimize polarization effects, and the formation of nanoscale quantum-confinement to enhance electron-hole wave function overlap. This work provides a viable path to break the efficiency bottleneck of nanoscale optoelectronics.

Environmental Doping-Induced Degradation of the Quantum Anomalous Hall Insulators.

The quantum anomalous Hall (QAH) insulator carries dissipation-free chiral edge current and thus provides a unique opportunity to develop energy-efficient transformative information technology. Despite promising advances, the QAH insulator has thus far eluded any practical applications. In addition to its low working temperature, the QAH state in magnetically doped topological insulators usually deteriorates with time in ambient conditions. In this work, we store three QAH devices with similar initial properties in different environments. The QAH device without a protection layer in air shows clear degradation and becomes hole-doped. The QAH device kept in an argon glovebox without a protection layer shows no measurable degradation after 560 h, and the device protected by a 3 nm AlOx protection layer in air shows minimal degradation with stable QAH properties. Our work shows a route to preserve the dissipation-free chiral edge state in QAH devices for potential applications in quantum information technology.

Evolution of Dopant-Concentration-Induced Magnetic Exchange Interaction in Topological Insulator Thin Films.

To date, the quantum anomalous Hall effect has been realized in chromium (Cr)- and/or vanadium(V)-doped topological insulator (Bi,Sb)2Te3 thin films. In this work, we use molecular beam epitaxy to synthesize both V- and Cr-doped Bi2Te3 thin films with controlled dopant concentration. By performing magneto-transport measurements, we find that both systems show an unusual yet similar ferromagnetic response with respect to magnetic dopant concentration; specifically the Curie temperature does not increase monotonically but shows a local maximum at a critical dopant concentration. We attribute this unusual ferromagnetic response observed in Cr/V-doped Bi2Te3 thin films to the dopant-concentration-induced magnetic exchange interaction, which displays evolution from van Vleck-type ferromagnetism in a nontrivial magnetic topological insulator to Ruderman-Kittel-Kasuya-Yosida (RKKY)-type ferromagnetism in a trivial diluted magnetic semiconductor. Our work provides insights into the ferromagnetic properties of magnetically doped topological insulator thin films and facilitates the pursuit of high-temperature quantum anomalous Hall effect.