Single Crystal Films Research Articles

Single Crystal Copper and Gold Alloys for the Plasmonically Enhanced CO2 Reduction Reaction

Chemical fixation of carbon dioxide (CO2) into chemical fuels has raised a lot of interest due to its role in removing greenhouse gasses from the atmosphere to store them as a potential source of energy. CO2 removal techniques have been widely studied and are of great interest because they can be used synergistically as a low-cost source of carbon to supplement and support the existing chemical synthesis of feedstocks and hydrocarbon fuels. One of the most common approaches to the remediation of atmospheric CO2 is the electrochemical reduction of CO2. We have been studying the formation of copper (Cu) films and its alloys with gold (Au). Metal alloys have been shown to exhibit interesting properties to enhance chemical processes beyond those of their individual components, improving the applications in the plasmonic and catalysis fields.Cu and Au are both excellent plasmonic and catalytic materials that show tunable optical properties, strong absorptions in the visible and near infrared, and catalytic activity. However, the applications of Cu materials alone are limited due to Cu’s chemical instability and tendency to oxidation, which inhibits their plasmonic properties and reduces their catalytic performance. By alloying Cu with Au, we have overcome copper’s chemical instability and improved the catalytic properties due to synergistic effects.As a result of our studies, we have also developed a method to deposit Cu2O single crystal film, a material that exhibits semiconductor properties and opens a new line of opportunities for plasmonic applications. In this poster, we will show an overview of our results and discuss the potential applications to plasmonic electrocatalysis and sensors.

Domain epitaxial growth of Ta3N5 film on c-plane sapphire substrate

Tritantalum pentanitride (Ta3N5) semiconductor is a promising material for photoelectrolysis of water with high efficiency. Ta3N5 is a metastable phase in the complex system of TaN binary compounds. Growing stabilized single-crystal Ta3N5 films is correspondingly challenging. Here, we demonstrate the growth of a nearly single-crystal Ta3N5 film with epitaxial domains on c-plane sapphire substrate, Al2O3(0001), by magnetron sputter epitaxy. Introduction of a small amount ~2% of O2 into the reactive sputtering gas mixed with N2 and Ar facilitates the formation of a Ta3N5 phase in the film dominated by metallic TaN. In addition, we indicate that a single-phase polycrystalline Ta3N5 film can be obtained with the assistance of a Ta2O5 seed layer. With controlling thickness of the seed layer smaller than 10 nm and annealing at 1000 °C, a crystalline β phase Ta2O5 was formed, which promotes the domain epitaxial growth of Ta3N5 films on Al2O3(0001). The mechanism behind the stabilization of the orthorhombic Ta3N5 structure resides in its stacking with the ultrathin seed layer of orthorhombic β-Ta2O5, which is energetically beneficial and reduces the lattice mismatch with the substrate.

Accurate Adjusting the Lattice Strain of Triple-Cation and Mixed-Halide Perovskites for High-Performance Photodetector

The instability of perovskite optoelectronic devices remains a big barrier to their commercialization. The instability caused by external stimuli has been addressed by encapsulation, such as humidity, oxygen, heat, and ultraviolet light. However, the intrinsic instability of perovskite materials due to the lattice strain has not been fully addressed, which affects the physical properties and device performance to a great extent. Tuning the lattice strain by controlling the perovskite composition and ratio is an effective way to further develop efficient and stable devices. Herein, we prepare a series of triple-cation and mixed-halide (FAPbI3)x(MAPbBr3)y(CsPbI3)1-x-y perovskite single-crystal thin films and study the effect of lattice strain on the perovskite optoelectronic properties. Especially, the perovskite photodetector with a horizontal structure based on (FAPbI3)0.79(MAPbBr3)0.13(CsPbI3)0.08 single-crystal thin films exhibits excellent performance with an enhanced responsivity of 40 A/W, high detectivity of 1.9 × 1013 Jones, external quantum efficiency of 9100%, and superior stability. This can be explained by the fact that the optimal coordination between each element leads to the release of lattice strain and further produces low defect density and long carrier lifetime in (FAPbI3)0.79(MAPbBr3)0.13(CsPbI3)0.08 single-crystal thin films. This research shows the significance of ion ratios in tuning lattice strain and determining the intrinsic device performance and makes the perovskite (FAPbI3)0.79(MAPbBr3)0.13(CsPbI3)0.08 a promising candidate for the next generation of optoelectronic devices.

Epitaxial growth of inch-scale single-crystal transition metal dichalcogenides through the patching of unidirectionally orientated ribbons

Two-dimensional (2D) semiconductors, especially transition metal dichalcogenides (TMDs), have been envisioned as promising candidates in extending Moore’s law. To achieve this, the controllable growth of wafer-scale TMDs single crystals or periodic single-crystal patterns are fundamental issues. Herein, we present a universal route for synthesizing arrays of unidirectionally orientated monolayer TMDs ribbons (e.g., MoS2, WS2, MoSe2, WSe2, MoSxSe2-x), by using the step edges of high-miller-index Au facets as templates. Density functional theory calculations regarding the growth kinetics of specific edges have been performed to reveal the morphological transition from triangular domains to patterned ribbons. More intriguingly, we find that, the uniformly aligned TMDs ribbons can merge into single-crystal films through a one-dimensional edge epitaxial growth mode. This work hereby puts forward an alternative pathway for the direct synthesis of inch-scale uniform monolayer TMDs single-crystals or patterned ribbons, which should promote their applications as channel materials in high-performance electronics or other fields.

The Effects of Substrate Temperature on the Growth, Microstructural and Magnetic Properties of Gadolinium-Containing Films on Aluminum Nitride

To facilitate future novel devices incorporating rare earth metal films and III-V semiconductors on Si substrates, this study investigates the mechanisms of growth via molecular beam epitaxy of gadolinium (Gd) on aluminum nitride (AlN) by determining the impact of substrate temperature on microstructure. The Gd films underwent extensive surface analysis via in situ reflective high energy electron diffraction (RHEED) and ex-situ SEM and AFM. Characterization of the surface features of rare earth metal films is important, as surface geometry has been shown to strongly impact magnetic properties. SEM and AFM imaging determined that Gd films grown on AlN (0001) from 80 °C to 400 °C transition from wetting, nodular films to island–trench growth mode to reduce in-plane lattice strain. XRD and Raman spectroscopy of the films revealed that they were primarily comprised of GdN, Gd and Gd2O3. The samples were also analyzed by a vibrating sample magnetometer (VSM) at room temperature. From the room temperature magnetic studies, the thick films showed superparamagnetic behavior, with samples grown between 240 °C and 270 °C showing high magnetic susceptibility. Increasing GdN (111) 2θ peak position and single-crystal growth modes correlated with increasing peak magnetization of the thin films, suggesting that lattice strain in single-crystal films was the primary driver of enhanced magnetic susceptibility.

Single-crystal thin film growth of the Mott insulator EuVO3 under biaxial substrate strain

We report the single-crystal thin film growth of EuVO3 of a Mott insulator with V3+ (d2,S=1). We used the pulsed laser deposition (PLD) and molecular beam epitaxy (MBE) methods and obtained the well-oriented epitaxial thin films with a pseudo-tetragonal crystal structure of EuVO3 which is distinct from the orthorhombic structure in bulk. The high quality of EuVO3 films is verified by the valence state of europium and vanadium ions with X-ray absorption near-edge structure (XANES) measurements showing clear signs from Eu3+ and V3+. The electrical resistivity exhibits the insulating temperature dependence; however, the magnitude of the resistivity is reduced from that of bulk EuVO3, possibly due to biaxial strain from substrates.

Epitaxial single-crystal hexagonal boron nitride multilayers on Ni (111).

Large-area single-crystal monolayers of two-dimensional (2D) materials such as graphene1-3, hexagonal boron nitride (hBN)4-6 and transition metal dichalcogenides7,8 have been grown. hBN is considered to be the 'ideal' dielectric for 2D-materials-based field-effect transistors (FETs), offering the potential for extending Moore's law9,10. Although hBN thicker than a monolayer is more desirable as substrate for 2D semiconductors11,12, highly uniform and single-crystal multilayer hBN growth has yet to be demonstrated. Here we report the epitaxial growth of wafer-scale single-crystal trilayer hBN by a chemical vapour deposition (CVD) method. Uniformly aligned hBN islands are found to grow on single-crystal Ni (111) at early stage and finally to coalesce into a single-crystal film. Cross-sectional transmission electron microscopy (TEM) results show that a Ni23B6 interlayer is formed (during cooling) between the single-crystal hBN film and Ni substrate by boron dissolution in Ni. There are epitaxial relationships between hBN and Ni23B6 and between Ni23B6 and Ni. We also find that the hBN film acts as a protective layer that remains intact during catalytic evolution of hydrogen, suggesting continuous single-crystal hBN. This hBN transferred onto the SiO2 (300 nm)/Si wafer acts as a dielectric layer to reduce electron doping from the SiO2 substrate in MoS2 FETs. Our results demonstrate high-qualitysingle-crystal multilayered hBN over large areas, which should open up new pathways for making it a ubiquitous substrate for 2D semiconductors.

Synthesis of La2−xSrxCuO4 films via atomic layer-by-layer molecular beam epitaxy

Atomic layer-by-layer molecular beam epitaxy (ALL-MBE) is a sophisticated technique to synthesize high-temperature superconductor (HTS) materials. ALL-MBE produces single-crystal HTS films with atomically smooth surfaces and interfaces, as well as precise multilayer heterostructures engineered down to a single atomic layer level. This enables the fabrication of tunnel junctions, nanowires, nanorings, and other HTS devices of interest. Our group has focused on ALL-MBE synthesis and materials science of La2−xSrxCuO4 (LSCO), a representative HTS cuprate. In the past two decades, we have synthesized over three thousand LSCO thin films and characterized them by a range of analytical techniques. Here, we present in full detail a systematic process for the synthesis and engineering of atomically perfect LSCO films. The procedure includes the preparation of substrates, calibration of the elemental sources, the recipe for ALL growth of LSCO films without any secondary-phase precipitates, post-growth annealing of the films, and ex situ film characterization. This report should aid replication and dissemination of this technique of synthesizing single-crystal LSCO films for basic research as well as for HTS electronic applications.

Integration and Characterization of LiTaO₃ Single Crystal Film Pyroelectric Sensor Using Mid-Infrared Metamaterial Perfect Absorber

An infrared sensor with Au/SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Au metamaterial perfect absorber (MPA) structure and Crystal ion slicing (CIS) fabricated LiTaO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> single crystal film to realize wavelength-selective function was proposed and fabricated in this paper. The Au/SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Au multilayer MPA exhibited outstanding wavelength selectivity at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$4.04\mu \text{m}$ </tex-math></inline-formula> with a maximum absorbance of 98% and the spectral voltage response of sensor highly agreed with it. Meanwhile, the sensor performed stable angle independency with radiation incident angle from 0° to 50°. After integration, this sensor showed maximum voltage responsivity of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${1.7} \times {10}^{{4}}\text{V}$ </tex-math></inline-formula> /W, maximum detectivity of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${1.7} \times {10}^{{8}}$ </tex-math></inline-formula> cmHz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup> /W, and 5.8ms response time. These results demonstrate the promising application prospect of high specific and fast response pyroelectric infrared sensor achieved by integrating MPA with LiTaO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> thin film.

Single-crystal epitaxial europium iron garnet films with strain-induced perpendicular magnetic anisotropy: Structural, strain, magnetic, and spin transport properties

Single-crystal europium iron garnet (EuIG) thin films epitaxially strain-grown on gadolinium gallium garnet (GGG)(100) substrates using off-axis sputtering have strain-induced perpendicular magnetic anisotropy (PMA). By varying the sputtering conditions, we have tuned the europium/iron (Eu/Fe) composition ratios in the films to tailor the film strains. The films exhibited an extremely smooth, particle-free surface with roughness as low as 0.1 nm as observed using atomic force microscopy. High-resolution x-ray diffraction analysis and reciprocal space maps showed in-plane epitaxial film growth, very smooth film/substrate interface, excellent film crystallinity with a small full width at half maximum of 0.012$^{\circ}$ in the rocking curve scans, and an in-plane compressive strain without relaxation. In addition, spherical aberration-corrected scanning transmission electron microscopy showed an atomically abrupt interface between the EuIG film and GGG. The measured squarish out-of-plane magnetization-field hysteresis loops by vibrating sample magnetometry in conjunction with the measurements from angle-dependent x-ray magnetic dichroism demonstrated the PMA in the films. We have tailored the magnetic properties of the EuIG thin films, including saturation magnetization ranging from 71.91 to 124.51 emu/c.c. (increase with the (Eu/Fe) ratios), coercive field from 27 to 157.64 Oe, and the strength of PMA field ($H_\bot$) increasing from 4.21 to 18.87 kOe with the in-plane compressive strain from -0.774 to -1.044%. We have also investigated spin transport in Pt/EuIG bi-layer structure and evaluated the real part of spin mixing conductance to be $3.48\times10^{14} {\Omega}^{-1}m^{-2}$. We demonstrated the current-induced magnetization switching with a low critical switching current density of $3.5\times10^6 A/cm^2$, showing excellent potential for low-dissipation spintronic devices.

Mechanical strain modulation of domain wall currents across LiNbO3 nanosensors

Recently, studies on low-dimensional conducting domain walls (DWs) in insulating ferroelectrics have opened up new research areas that allow information to be mechanically written and electrically read on the nanoscale. Large strains in thin films can change the polarization gradient across the DW region and thus increasing the DW current significantly. This phenomenon can enable the development of high sensitivity mechanical vibration sensors. In this study, the effects of variable uniaxial strain on the structures of 180° conducting DWs in LiNbO3 (LNO) single-crystal thin films bonded onto Si/SiO2 substrates were investigated. After the creation of antiparallel domains within each LNO nanosensor integrated at the film surface, strain modulation of DW currents was observed through simple mechanical bending of the film. The DW current increases under application of tensile strain along the axis of polarization, but decreases under application of in-plane compression by a factor of approximately 25. Phase field simulations showed the dramatic change in polarization gradients around the DW regions under the increase in tensile strain, which reduced the band gap. Repetitive band-gap narrowing/broadening with change in local electric field intensity under vibrating mechanical forces can periodically modulate both the carrier density and the DW conduction in the sensors. This finding not only provides the new fundamental physics to enrich the ferroelectric theory, but also paves the way to the near-future development of bending actuators, piezolighters, and micro-/nano-manipulators, etc.

Heterogeneous Nucleation of an Embryo in the Shape of Square Prism: Effect of Surface Roughness.

The solution-based synthesis in a confined space between two parallel plates has been demonstrated to be a potential approach to grow single-crystal perovskite films of large sizes, such as CH3NH3PbX3 (X = Br and I) single-crystal films. In this work, we study the effects of surface roughness on the separation between two parallel, rough plates, and heterogeneous nucleation of an embryo in the shape of square prism for the Wenzel contact between the embryo and the rough surface. Analytical relations are derived for the separation of two parallel, rough plates under mechanical loading and the critical dimensions of a square embryo on a rough surface. The analytical relation reveals that one can control the thickness of perovskite films grown between two parallel plates by changing the surface tension of the precursor solution and mechanical loading. The critical dimensions of a square embryo and the corresponding formation energy are dependent on interface energies and the root mean square of surface roughness. There exists a critical root mean square of surface roughness, above which it is very difficult to form an embryo in the shape of square prism. The results illustrate the important roles of the interface energies and surface roughness of substrates in the growth of single-crystal films, including perovskites and ionic crystals, and the need to include the anisotropic characteristics of surface/interface energies in the nucleation analysis of crystalline materials.

Zinc oxide family semiconductors for ultraviolet radiation emission – A cathodoluminescence study

• Study of ZnO and MgZnO grown using a variety of techniques. • UV cathodoluminescence seen in bulk ZnO and in epitaxial MgZnO. • Sputtered MgZnO shows no UV cathodoluminescence. • Atomic layer deposition appears to be suitable for growing UV-luminescent MgZnO films . Zinc oxide (ZnO) family semiconductors that include ZnO and various ternary and even quaternary semiconductors formed by the inclusion of suitable other elements to zinc and oxygen are potential candidates for making UV-emitting solid-state devices. This work is a study of the UV-emitting potential of ZnO and MgZnO through electron beam irradiation of various samples. In this work, we studied materials grown or deposited through different techniques in order to assess their suitability for use in UV-emitting devices. Our work throws light on which growth or deposition methods are more suitable for obtaining radiation emission-capable materials from this family of II-VI oxide semiconductors. We find that single crystal thin film material, produced through either metal organic chemical vapor deposition (MOCVD) or atomic layer deposition (ALD) appear to be the best, followed by hydrothermally-grown single crystals. In contrast, sputtered material appears unsuitable for making UV-emitting devices.

Solution epitaxy of single-crystal and single-domain KNbO3 film with a great photovoltaic current

As a classic ferroelectric material, orthorhombic perovskite KNbO3 demonstrates a large electro-optical coefficient, high refractive index, and excellent nonlinear optical property. These make single-crystal KNbO3 films being long-pursued for lead-free sensor or optoelectronic device applications. However, it remains a challenge to obtain high-quality single-crystal KNbO3 films because of the volatility of potassium at a high growth temperature. In this work, single-crystal KNbO3 films with a tunable thickness have been successfully fabricated by a low-temperature (∼220 °C) hydrothermal epitaxy on cubic single-crystal Nb:SrTiO3 substrates. Interestingly, the KNbO3 film was investigated to adopt a single-domain structure with a polarization direction pointing to the substrate. In-depth characterization reveals an intriguing disordered region of approximately 0.6–1.2 nm because of partial Nb substitution of Ti near the interface, giving rise to the coexistence of Ti3+ and Ti4+ ions. Such region is proposed to release a large interface strain energy from a large lattice misfit (∼2.3%) between {011}KNbO3 and {100}Nb:SrTiO3 and reduce interfacial electrostatic energy via screening of the positive polar surface of KNbO3. This allows to drive a hydrothermal epitaxy of single-crystal and single-domain KNbO3 without forming a typical dislocation. The films demonstrate a great photovoltaic short-circuit current of 15.65 μA/cm2 under the 375 nm laser irradiation of 500 mW/cm2 light intensity, and the corresponding photoresponsivity is the highest among the emerged KNbO3-based materials. Such improvement in the photoresponsivity has been attributed to a successful low-temperature solution epitaxy of single-crystal and single-domain KNbO3 films, where K deficiency can be significantly suppressed. These findings suggest that a solution epitaxy strategy is effective in producing high-quality ferroelectric films for lead-free optoelectronic or self-powered photodetector device applications.

Characterizations of Single-Crystal Lithium Niobate Thin Films

Single-crystal lithium niobate thin films (lithium niobate on insulator, LNOI) are becoming a new material platform for integrating photonics. Investigation into the physical properties of LNOI is important for the design and fabrication of photonic devices. Herein, LNOIs were prepared by two methods: ion implantation and wafer bonding; and wafer bonding and grinding. High-resolution X-ray diffraction (HRXRD) and confocal Raman spectroscopy were used to study the LNOI lattice properties. The full-width at half-maximum (FWHM) of HRXRD and Raman spectra showed a regular crystal lattice arrangement of the LNOIs. The domain inversion voltage and electro-optical coefficient of the LNOIs were close to those of LN bulk material. This study provides useful information for LNOI fabrication and for photonic devices in LNOI.

A disorder-sensitive emergent vortex phase identified in high-T c superconductor (Li,Fe)OHFeSe

The magneto-transport properties are systematically measured under c-direction fields up to 33 T for a series of single-crystal films of intercalated iron-selenide superconductor (Li,Fe)OHFeSe. The film samples with varying degree of disorder are grown hydrothermally. We observe a magnetic-field-enhanced shoulder-like feature in the mixed state of the high-T c (Li,Fe)OHFeSe films with weak disorder, while the feature fades away in the films with enhanced disorder. The irreversibility field is significantly suppressed to lower temperatures with the appearance of the shoulder feature. Based on the experiment and model analysis, we establish a new vortex-phase diagram for the weakly-disordered high-T c (Li,Fe)OHFeSe, which features an emergent dissipative vortex phase intermediate between the common vortex glass and liquid phases. The reason for the emergence of this intermediate vortex state is further discussed based on related experiments and models.

Two-Photon Absorption and Photoluminescence of Individual CsPbBr3 Nanocrystal Superlattices

Nonlinear optical responses such as two-photon absorption and photoluminescence with halide perovskites are promising for the implications of fluorescence microscopy, high-density data storage, and microfabrication. Since a high degree of structural coherence can help improve the nonlinear optical responses, this study experimentally demonstrates that for individual CsPbBr3 nanocrystal superlattices, the two-photon luminescence is enhanced by about 16 times compared with that of randomly oriented nanocrystal films, and the polarization-dependent luminescence spectra reveal that the nanocrystal superlattices possess a 4-fold symmetry, which is similar to the single-crystal film structures. Due to their broad nonlinear absorption responses, wide-range interband photoluminescence is observed below the band gap. Furthermore, the intensity-dependent absorption results demonstrate that the two-photon absorption coefficient can be as large as 0.644 cm/MW, which is comparable with that of the semiconductor materials such as WS2 and ZnO. These superior performances make the CsPbBr3 nanocrystal superlattices promising for the implications of nonlinear devices such as optical power limiting and optical switches.

Efficient Piezoelectric Energy Harvesting from a Discrete Hybrid Bismuth Bromide Ferroelectric Templated by Phosphonium Cation.

Bismuth containing hybrid molecular ferroelectrics are receiving tremendous attention in recent years owing to their stable and non-toxic composition. However, these perovskite-like structures are primarily limited to ammonium cations. Herein, we report a new phosphonium based discrete perovskite-like hybrid ferroelectric with a formula [Me(Ph)3 P]3 [Bi2 Br9 ] (MTPBB) and its mechanical energy harvesting capability. The Polarization-Electric field (P-E) measurements resulted in a well-defined ferroelectric hysteresis loop with a remnant polarization value of 2.1 μC cm-2 . Piezoresponse force microscopy experiments enabled visualization of the ferroelectric domain structure and evaluation of the piezoelectric strain coefficient (d33 ) for an MTPBB single crystal and thin film sample. Furthermore, flexible devices incorporating MTPBB in polydimethylsiloxane (PDMS) matrix at various concentrations were fabricated and explored for their mechanical energy harvesting properties. The champion device with 20 wt % of MTPBB in PDMS rendered a maximum peak-to-peak open-circuit voltage of 22.9 V and a maximum power density of 7 μW cm-2 at an optimal load of 4 MΩ. Moreover, the potential of MTPBB-based devices in low power electronics was demonstrated by storing the harvested energy in various electrolytic capacitors.

Production of Large‐Area Nucleus‐Free Single‐Crystal Graphene‐Mesh Metamaterials with Zigzag Edges

In addition to conventional monolayer or bilayer graphene films, graphene-mesh metamaterials have attracted considerable research attention within the scientific community owing to their unique physical and optical properties. Currently, most graphene-mesh metamaterials are fabricated using common lithography techniques on exfoliated graphene flakes, which require the deposition and removal of chemicals during fabrication. This process may introduce contamination or doping, thereby limiting their production size and application in nanodevices. Herein, the controlled production of wafer-scale high-quality single-crystal nucleus-free graphene-mesh metamaterial films with zigzag edges is demonstrated. The 13 C-isotopic labeling graphene-growth approach, large-area Raman mapping techniques, and a uniquely designed high-voltage localized-space air-ionization etching method are utilized to directly remove the graphene nuclei. Subsequently, a hydrogen-assisted anisotropic etching process is employed for transforming irregular edges into zigzag edges within the hexagonal-shaped holes, producing a large-scale single-crystal high-quality graphene-mesh metamaterial film on a Cu(111) substrate. The carrier mobilities of the fabricated field-effect transistors on the as-produced films are measured. The findings of this study enable the large-scale production of high-quality low-dimensional graphene-mesh metamaterials and provide insights for the application of integrated circuits based on graphene and other 2D metamaterials.

Pressure-Induced Superconductivity in HgTe Single-Crystal Film.

HgTe film is widely used for quantum Hall well studies and devices, as it has unique properties, like band gap inversion, carrier‐type switch, and topological evolution depending on the film thickness modulation near the so‐called critical thickness (63.5 Å), while its counterpart bulk materials do not hold these nontrivial properties at ambient pressure. Here, much richer transport properties emerging in bulk HgTe crystal through pressure‐tuning are reported. Not only the above‐mentioned abnormal properties can be realized in a 400 nm thick bulk HgTe single crystal, but superconductivity is also discovered in a series of high‐pressure phases. Combining crystal structure, electrical transport, and Hall coefficient measurements, a p‐n carrier type switching is observed in the first high‐pressure cinnabar phase. Superconductivity emerges after the semiconductor‐to‐metal transition at 3.9 GPa and persists up to 54 GPa, crossing four high‐pressure phases with an increased upper critical field. Density functional theory calculations confirm that a surface‐dominated topologic band structure contributes these exotic properties under high pressure. This discovery presents broad and efficient tuning effects by pressure on the lattice structure and electronic modulations compared to the thickness‐dependent critical properties in 2D and 3D topologic insulators and semimetals.