High energy density in artificial heterostructures through relaxation time modulation.

Electrostatic capacitors are foundational components of advanced electronics and high-power electrical systems owing to their ultrafast charging-discharging capability. Ferroelectric materials offer high maximum polarization, but high remnant polarization has hindered their effective deployment in energy storage applications. Previous methodologies have encountered problems because of the deteriorated crystallinity of the ferroelectric materials. We introduce an approach to control the relaxation time using two-dimensional (2D) materials while minimizing energy loss by using 2D/3D/2D heterostructures and preserving the crystallinity of ferroelectric 3D materials. Using this approach, we were able to achieve an energy density of 191.7 joules per cubic centimeter with an efficiency greater than 90%. This precise control over relaxation time holds promise for a wide array of applications and has the potential to accelerate the development of highly efficient energy storage systems.

Remote Epitaxy: Fundamentals, Challenges, and Opportunities.

Advanced heterogeneous integration technologies are pivotal for next-generation electronics. Single-crystalline materials are one of the key building blocks for heterogeneous integration, although it is challenging to produce and integrate these materials. Remote epitaxy is recently introduced as a solution for growing single-crystalline thin films that can be exfoliated from host wafers and then transferred onto foreign platforms. This technology has quickly gained attention, as it can be applied to a wide variety of materials and can realize new functionalities and novel application platforms. Nevertheless, remote epitaxy is a delicate process, and thus, successful execution of remote epitaxy is often challenging. Here, we elucidate the mechanisms of remote epitaxy, summarize recent breakthroughs, and discuss the challenges and solutions in the remote epitaxy of various material systems. We also provide a vision for the future of remote epitaxy for studying fundamental materials science, as well as for functional applications.

Ultrahigh Photoresponsivity of W/Graphene/ß-Ga2O3 Schottky Barrier Deep Ultraviolet Photodiodes.

The integration of graphene with semiconductor materials has been studied for developing advanced electronic and optoelectronic devices. Here, we propose ultrahigh photoresponsivity of ß-Ga2O3 photodiodes with a graphene monolayer inserted in a W Schottky contact. After inserting the graphene monolayer, we found a reduction in the leakage current and ideality factor. The Schottky barrier height was also shown to be about 0.53 eV, which is close to an ideal value. This was attributed to a decrease in the interfacial state density and the strong suppression of metal Fermi-level pinning. Based on a W/graphene/ß-Ga2O3 structure, the responsivity and external quantum efficiency reached 14.49 A/W and 7044%, respectively. These values were over 100 times greater than those of the W contact alone. The rise and delay times of the W/graphene/ß-Ga2O3 Schottky barrier photodiodes significantly decreased to 139 and 200 ms, respectively, compared to those obtained without a graphene interlayer (2000 and 3000 ms). In addition, the W/graphene/ß-Ga2O3 Schottky barrier photodiode was highly stable, even at 150 °C.

Monolithic 3D integration of 2D materials-based electronics towards ultimate edge computing solutions.

Three-dimensional (3D) hetero-integration technology is poised to revolutionize the field of electronics by stacking functional layers vertically, thereby creating novel 3D circuity architectures with high integration density and unparalleled multifunctionality. However, the conventional 3D integration technique involves complex wafer processing and intricate interlayer wiring. Here we demonstrate monolithic 3D integration of two-dimensional, material-based artificial intelligence (AI)-processing hardware with ultimate integrability and multifunctionality. A total of six layers of transistor and memristor arrays were vertically integrated into a 3D nanosystem to perform AI tasks, by peeling and stacking of AI processing layers made from bottom-up synthesized two-dimensional materials. This fully monolithic-3D-integrated AI system substantially reduces processing time, voltage drops, latency and footprint due to its densely packed AI processing layers with dense interlayer connectivity. The successful demonstration of this monolithic-3D-integrated AI system will not only provide a material-level solution for hetero-integration of electronics, but also pave the way for unprecedented multifunctional computing hardware with ultimate parallelism.

Exceptional Thermochemical Stability of Graphene on N-Polar GaN for Remote Epitaxy.

In this study, we investigate the thermochemical stability of graphene on the GaN substrate for metal-organic chemical vapor deposition (MOCVD)-based remote epitaxy. Despite excellent physical properties of GaN, making it a compelling choice for high-performance electronic and light-emitting device applications, the challenge of thermochemical decomposition of graphene on a GaN substrate at high temperatures has obstructed the achievement of remote homoepitaxy via MOCVD. Our research uncovers an unexpected stability of graphene on N-polar GaN, thereby enabling the MOCVD-based remote homoepitaxy of N-polar GaN. Our comparative analysis of N- and Ga-polar GaN substrates reveals markedly different outcomes: while a graphene/N-polar GaN substrate produces releasable microcrystals (µCs), a graphene/Ga-polar GaN substrate yields nonreleasable thin films. We attribute this discrepancy to the polarity-dependent thermochemical stability of graphene on the GaN substrate and its subsequent reaction with hydrogen. Evidence obtained from Raman spectroscopy, electron microscopic analyses, and overlayer delamination points to a pronounced thermochemical stability of graphene on N-polar GaN during MOCVD-based remote homoepitaxy. Molecular dynamics simulations, corroborated by experimental data, further substantiate that the thermochemical stability of graphene is reliant on the polarity of GaN, due to different reactions with hydrogen at high temperatures. Based on the N-polar remote homoepitaxy of µCs, the practical application of our findings was demonstrated in fabrication of flexible light-emitting diodes composed of p-n junction µCs with InGaN heterostructures.

Remote epitaxial interaction through graphene.

The concept of remote epitaxy involves a two-dimensional van der Waals layer covering the substrate surface, which still enable adatoms to follow the atomic motif of the underlying substrate. The mode of growth must be carefully defined as defects, e.g., pinholes, in two-dimensional materials can allow direct epitaxy from the substrate, which, in combination with lateral epitaxial overgrowth, could also form an epilayer. Here, we show several unique cases that can only be observed for remote epitaxy, distinguishable from other two-dimensional material-based epitaxy mechanisms. We first grow BaTiO3 on patterned graphene to establish a condition for minimizing epitaxial lateral overgrowth. By observing entire nanometer-scale nuclei grown aligned to the substrate on pinhole-free graphene confirmed by high-resolution scanning transmission electron microscopy, we visually confirm that remote epitaxy is operative at the atomic scale. Macroscopically, we also show variations in the density of GaN microcrystal arrays that depend on the ionicity of substrates and the number of graphene layers.

Stability of Graphene and Influence of AlN Surface Pits on GaN Remote Heteroepitaxy for Exfoliation.

Remote epitaxy is a promising technology that has recently attracted considerable attention, which enables the growth of thin films that copy the crystallographic characteristics of the substrate through two-dimensional material interlayers. The grown films can be exfoliated to form freestanding membranes, although it is often challenging to apply this technique if the substrate materials are prone to damage under harsh epitaxy conditions. For example, remote epitaxy of GaN thin films on graphene/GaN templates has not been achieved by a standard metal-organic chemical vapor deposition (MOCVD) method due to such damages. Here, we report GaN remote heteroepitaxy on graphene/AlN templates by MOCVD and investigate the influence of surface pits in AlN on the growth and exfoliation of GaN thin films. We first show the thermal stability of graphene before GaN growth, based on which two-step growth of GaN on graphene/AlN is developed. The GaN samples are successfully exfoliated after the first step of the growth at 750 °C, whereas the exfoliation failed after the second step at 1050 °C. In-depth analysis confirms that the pits in AlN templates lead to the degradation of graphene near the area and thus the alteration of growth modes and the failure of exfoliation. These results exemplify the importance of chemical and topographic properties of growth templates for successful remote epitaxy. It is one of the key factors for III-nitride-based remote epitaxy, and these results are expected to be of great help in realizing complete remote epitaxy using only MOCVD.

High-throughput manufacturing of epitaxial membranes from a single wafer by 2D materials-based layer transfer process.

Layer transfer techniques have been extensively explored for semiconductor device fabrication as a path to reduce costs and to form heterogeneously integrated devices. These techniques entail isolating epitaxial layers from an expensive donor wafer to form freestanding membranes. However, current layer transfer processes are still low-throughput and too expensive to be commercially suitable. Here we report a high-throughput layer transfer technique that can produce multiple compound semiconductor membranes from a single wafer. We directly grow two-dimensional (2D) materials on III-N and III-V substrates using epitaxy tools, which enables a scheme comprised of multiple alternating layers of 2D materials and epilayers that can be formed by a single growth run. Each epilayer in the multistack structure is then harvested by layer-by-layer mechanical exfoliation, producing multiple freestanding membranes from a single wafer without involving time-consuming processes such as sacrificial layer etching or wafer polishing. Moreover, atomic-precision exfoliation at the 2D interface allows for the recycling of the wafers for subsequent membrane production, with the potential for greatly reducing the manufacturing cost.

Vertical full-colour micro-LEDs via 2D materials-based layer transfer.

Micro-LEDs (µLEDs) have been explored for augmented and virtual reality display applications that require extremely high pixels per inch and luminance1,2. However, conventional manufacturing processes based on the lateral assembly of red, green and blue (RGB) µLEDs have limitations in enhancing pixel density3-6. Recent demonstrations of vertical µLED displays have attempted to address this issue by stacking freestanding RGB LED membranes and fabricating top-down7-14, but minimization of the lateral dimensions of stacked µLEDs has been difficult. Here we report full-colour, vertically stacked µLEDs that achieve, to our knowledge, the highest array density (5,100 pixels per inch) and the smallest size (4 µm) reported to date. This is enabled by a two-dimensional materials-based layer transfer technique15-18 that allows the growth of RGB LEDs of near-submicron thickness on two-dimensional material-coated substrates via remote or van der Waals epitaxy, mechanical release and stacking of LEDs, followed by top-down fabrication. The smallest-ever stack height of around 9 µm is the key enabler for record high µLED array density. We also demonstrate vertical integration of blue µLEDs with silicon membrane transistors for active matrix operation. These results establish routes to creating full-colour µLED displays for augmented and virtual reality, while also offering a generalizable platform for broader classes of three-dimensional integrated devices.

Non-epitaxial single-crystal 2D material growth by geometric confinement.

Two-dimensional (2D) materials and their heterostructures show a promising path for next-generation electronics1-3. Nevertheless, 2D-based electronics have not been commercialized, owing mainly to three critical challenges: i) precise kinetic control of layer-by-layer 2D material growth, ii) maintaining a single domain during the growth, and iii) wafer-scale controllability of layer numbers and crystallinity. Here we introduce a deterministic, confined-growth technique that can tackle these three issues simultaneously, thus obtaining wafer-scale single-domain 2D monolayer arrays and their heterostructures on arbitrary substrates. We geometrically confine the growth of the first set of nuclei by defining a selective growth area via patterning SiO2 masks on two-inch substrates. Owing to substantial reduction of the growth duration at the micrometre-scale SiO2 trenches, we obtain wafer-scale single-domain monolayer WSe2 arrays on the arbitrary substrates by filling the trenches via short growth of the first set of nuclei, before the second set of nuclei is introduced, thus without requiring epitaxial seeding. Further growth of transition metal dichalcogenides with the same principle yields the formation of single-domain MoS2/WSe2 heterostructures. Our achievement will lay a strong foundation for 2D materials to fit into industrial settings.

Graphene nanopattern as a universal epitaxy platform for single-crystal membrane production and defect reduction.

Heterogeneous integration of single-crystal materials offers great opportunities for advanced device platforms and functional systems1. Although substantial efforts have been made to co-integrate active device layers by heteroepitaxy, the mismatch in lattice polarity and lattice constants has been limiting the quality of the grown materials2. Layer transfer methods as an alternative approach, on the other hand, suffer from the limited availability of transferrable materials and transfer-process-related obstacles3. Here, we introduce graphene nanopatterns as an advanced heterointegration platform that allows the creation of a broad spectrum of freestanding single-crystalline membranes with substantially reduced defects, ranging from non-polar materials to polar materials and from low-bandgap to high-bandgap semiconductors. Additionally, we unveil unique mechanisms to substantially reduce crystallographic defects such as misfit dislocations, threading dislocations and antiphase boundaries in lattice- and polarity-mismatched heteroepitaxial systems, owing to the flexibility and chemical inertness of graphene nanopatterns. More importantly, we develop a comprehensive mechanics theory to precisely guide cracks through the graphene layer, and demonstrate the successful exfoliation of any epitaxial overlayers grown on the graphene nanopatterns. Thus, this approach has the potential to revolutionize the heterogeneous integration of dissimilar materials by widening the choice of materials and offering flexibility in designing heterointegrated systems.

Chip-less wireless electronic skins by remote epitaxial freestanding compound semiconductors.

Recent advances in flexible and stretchable electronics have led to a surge of electronic skin (e-skin)-based health monitoring platforms. Conventional wireless e-skins rely on rigid integrated circuit chips that compromise the overall flexibility and consume considerable power. Chip-less wireless e-skins based on inductor-capacitor resonators are limited to mechanical sensors with low sensitivities. We report a chip-less wireless e-skin based on surface acoustic wave sensors made of freestanding ultrathin single-crystalline piezoelectric gallium nitride membranes. Surface acoustic wave-based e-skin offers highly sensitive, low-power, and long-term sensing of strain, ultraviolet light, and ion concentrations in sweat. We demonstrate weeklong monitoring of pulse. These results present routes to inexpensive and versatile low-power, high-sensitivity platforms for wireless health monitoring devices.

Synthesis of Uniformly Sized Bi0.5Sb1.5Te3.0 Nanoparticles via Mechanochemical Process and Wet-Milling for Reduced Thermal Conductivity.

In this study, Bi0.5Sb1.5Te3.0 (BST) nanoparticles (NPs) with high crystallinities were synthesized via a mechanochemical process (MCP). X-ray diffraction (XRD), and Raman and X-ray photoelectron spectroscopy (XPS) spectra of the BST NPs showed that the Bi, Sb, and Te powders successfully formed BiSbTe phase and transmission electron microscopy (TEM) images, verifying the high crystallinity and smaller size, albeit agglomerated. The as-synthesized BST NPs with agglomerated clusters were ground into smaller sizes of approximately 41.8 nm with uniform distribution through a simple wet-milling process during 7 days. The thermal conduction behaviors of bulk alloys fabricated by spark plasma sintering (SPS) of the BST NPs were studied by comparing those of samples fabricated from as-synthesized BST NPs and a BST ingot. The thermal conductivities (κ) of the BST nanocomposites were significantly reduced by introducing BST NPs with smaller grain sizes and finer distributions in the temperature range from 300 to 500 K. The BST nanocomposites fabricated from wet-milled BST NPs offered ultralow κ values of 0.84 W m-1 K-1 at approximately 398 K.

Development of High Dielectric Electrostrictive PVDF Terpolymer Blends for Enhanced Electromechanical Properties.

Electroactive polymers with high dielectric constants and low moduli can offer fast responses and large electromechanical strain under a relatively low electric field with regard to theoretical driving forces of electrostriction and electrostatic force. However, the conventional electroactive polymers, including silicone rubbers and acrylic polymers, have shown low dielectric constants (ca. < 4) because of their intrinsic limitation, although they have lower moduli (ca. < 1 MPa) than inorganics. To this end, we proposed the high dielectric PVDF terpolymer blends (PVTC-PTM) including poly(vinylidene fluoride-trifluoroethylene-chlorofluoro-ethylene) (P(VDF-TrFE-CFE), PVTC) as a matrix and micelle structured poly(3-hexylthiophene)-b-poly(methyl methacrylate) (P3HT-b-PMMA, PTM) as a conducting filler. The dielectric constant of PVTC-PTM dramatically increased up to 116.8 at 100 Hz despite adding only 2 wt% of the polymer-type filler (PTM). The compatibility and crystalline properties of the PVTC-PTM blends were examined by microscopic, thermal, and X-ray studies. The PVTC-PTM showed more compatible blends than those of the P3HT homopolymer filler (PT) and led to higher crystallinity and smaller crystal grain size relative to those of neat PVTC and PVTC with the PT filler (PVTC-PT). Those by the PVTC-PTM blends can beneficially affect the high-performance electromechanical properties compared to those by the neat PVTC and the PVTC-PT blend. The electromechanical strain of the PVTC-PTM with 2 wt% PTM (PVTC-PTM2) showed ca. 2-fold enhancement (0.44% transverse strain at 30 Vpp µm-1) relative to that of PVTC. We found that the more significant electromechanical performance of the PVTC-PTM blend than the PVTC was predominantly due to the electrostrictive force rather than electrostatic force. We believe that the acquired PVTC-PTM blends are great candidates to achieve the high-performance electromechanical strain and take all benefits derived from the all-organic system, including high electrical breakdown strength, processibility, dielectrics, and large strain, which are largely different from the organic-inorganic hybrid nanocomposite systems.

Solvent-free synthesis of Cu2ZnSnS4 nanocrystals: a facile, green, up-scalable route for low cost photovoltaic cells.

Efficient Cu2ZnSnSe4 (CZTSe) solar cells were fabricated with a simple, environmentally friendly, and scalable synthetic method for Cu2ZnSnS4 (CZTS) nanocrystals. CZTS nanoparticles were mechanochemically synthesized from elemental precursors on a relatively large scale (∼20 g), during which no solvents or additives were used, thus alleviating the complex process of particle synthesis. An analysis of the time evolution of the crystalline phase and morphology of precursor powders revealed that the formation of the CZTS compound was completed in 0.5 h once initiated, suggesting that the mechanochemically induced self-propagating reaction prevails. CZTS ink was prepared by dispersing the as-synthesized nanoparticles in an environmentally benign solvent (160 mg mL(-1) in ethanol) without using any additives, after which it was cast onto Mo-coated glass substrates by a doctor-blade method. Subsequent reactive annealing at 560 °C under a Se-containing atmosphere resulted in substantial grain growth along with the nearly complete substitution of Se. The CZTSe solar cells therefrom exhibited power conversion efficiency levels as high as 6.1% (based on the active area, 0.44 cm(2)) with a relatively high open-circuit voltage (0.42 V) in comparison with the bandgap energy of 1.0 eV.

RTA-treated carbon fiber/copper core/shell hybrid for thermally conductive composites.

In this paper, we demonstrate a facile route to produce epoxy/carbon fiber composites providing continuous heat conduction pathway of Cu with a high degree of crystal perfection via electroplating, followed by rapid thermal annealing (RTA) treatment and compression molding. Copper shells on carbon fibers were coated through electroplating method and post-treated via RTA technique to reduce the degree of imperfection in the Cu crystal. The epoxy/Cu-plated carbon fiber composites with Cu shell of 12.0 vol % prepared via simple compression molding, revealed 18 times larger thermal conductivity (47.2 W m(-1) K(-1)) in parallel direction and 6 times larger thermal conductivity (3.9 W m(-1) K(-1)) in perpendicular direction than epoxy/carbon fiber composite. Our novel composites with RTA-treated carbon fiber/Cu core/shell hybrid showed heat conduction behavior of an excellent polymeric composite thermal conductor with continuous heat conduction pathway, comparable to theoretical values obtained from Hatta and Taya model.