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.

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.

Heterogeneous integration of single-crystalline complex-oxide membranes.

Complex-oxide materials exhibit a vast range of functional properties desirable for next-generation electronic, spintronic, magnetoelectric, neuromorphic, and energy conversion storage devices1-4. Their physical functionalities can be coupled by stacking layers of such materials to create heterostructures and can be further boosted by applying strain5-7. The predominant method for heterogeneous integration and application of strain has been through heteroepitaxy, which drastically limits the possible material combinations and the ability to integrate complex oxides with mature semiconductor technologies. Moreover, key physical properties of complex-oxide thin films, such as piezoelectricity and magnetostriction, are severely reduced by the substrate clamping effect. Here we demonstrate a universal mechanical exfoliation method of producing freestanding single-crystalline membranes made from a wide range of complex-oxide materials including perovskite, spinel and garnet crystal structures with varying crystallographic orientations. In addition, we create artificial heterostructures and hybridize their physical properties by directly stacking such freestanding membranes with different crystal structures and orientations, which is not possible using conventional methods. Our results establish a platform for stacking and coupling three-dimensional structures, akin to two-dimensional material-based heterostructures, for enhancing device functionalities8,9.

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.

Suppressing ion migration in metal halide perovskite via interstitial doping with a trace amount of multivalent cations.

Cations with suitable sizes to occupy an interstitial site of perovskite crystals have been widely used to inhibit ion migration and promote the performance and stability of perovskite optoelectronics. However, such interstitial doping inevitably leads to lattice microstrain that impairs the long-range ordering and stability of the crystals, causing a sacrificial trade-off. Here, we unravel the evident influence of the valence states of the interstitial cations on their efficacy to suppress the ion migration. Incorporation of a trivalent neodymium cation (Nd3+) effectively mitigates the ion migration in the perovskite lattice with a reduced dosage (0.08%) compared to a widely used monovalent cation dopant (Na+, 0.45%). The photovoltaic performances and operational stability of the prototypical perovskite solar cells are enhanced with a trace amount of Nd3+ doping while minimizing the sacrificial trade-off.

Ledge-directed epitaxy of continuously self-aligned single-crystalline nanoribbons of transition metal dichalcogenides.

Two-dimensional transition metal dichalcogenide nanoribbons are touted as the future extreme device downscaling for advanced logic and memory devices but remain a formidable synthetic challenge. Here, we demonstrate a ledge-directed epitaxy (LDE) of dense arrays of continuous, self-aligned, monolayer and single-crystalline MoS2 nanoribbons on ß-gallium (III) oxide (ß-Ga2O3) (100) substrates. LDE MoS2 nanoribbons have spatial uniformity over a long range and transport characteristics on par with those seen in exfoliated benchmarks. Prototype MoS2-nanoribbon-based field-effect transistors exhibit high on/off ratios of 108 and an averaged room temperature electron mobility of 65 cm2 V-1 s-1. The MoS2 nanoribbons can be readily transferred to arbitrary substrates while the underlying ß-Ga2O3 can be reused after mechanical exfoliation. We further demonstrate LDE as a versatile epitaxy platform for the growth of p-type WSe2 nanoribbons and lateral heterostructures made of p-WSe2 and n-MoS2 nanoribbons for futuristic electronics applications.

Activation of ß-catenin by TGF-ß1 promotes ligament-fibroblastic differentiation and inhibits cementoblastic differentiation of human periodontal ligament cells.

TGF-ß and Wnt/ß-catenin signaling pathways are known to be essential for the development of periodontal tissue. In this study, we examined the crosstalk between TGF-ß and Wnt/ß-catenin signaling in ligament-fibroblastic differentiation of human periodontal ligament cells (hPDLCs). TGF-ß1 treatment significantly increased the expression of ligament-fibroblastic markers, but such expression was preventing by treatment with SB431542, a TGF-ß type I receptor inhibitor. As well as phosphorylation of Smad3, TGF-ß1 increased ß-catenin activation. The depletion of ß-catenin reduced the expression of ligament-fibroblastic markers, suggesting that ß-catenin is essential for ligament differentiation. The effect of TGF-ß1 on ß-catenin activation did not seem to be much correlated with Wnt stimuli, but endogenous DKK1 was suppressed by TGF-ß1, indicating that ß-catenin activation could be increased much more by TGF-ß1. In addition to DKK1 suppression, Smad3 phosphorylation by TGF-ß1 facilitated the nuclear translocation of cytoplasmic ß-catenin. In contrast to ligament-fibroblastic differentiation, inhibition of TGF-ß1 signaling was needed for cementoblastic differentiation of hPDLCs. BMP7 treatment accompanied by inhibition of TGF-ß1 signaling had a synergistic effect on cementoblastic differentiation. In conclusion, ß-catenin activation by TGF-ß1 caused ligament-fibroblastic differentiation of hPDLCs, and the presence of TGF-ß1 stimuli basically determined whether hPDLCs are differentiated into ligament progenitor or cementoblasts.

Integration of bulk materials with two-dimensional materials for physical coupling and applications.

Hybrid heterostructures are essential for functional device systems. The advent of 2D materials has broadened the material set beyond conventional 3D material-based heterostructures. It has triggered the fundamental investigation and use in applications of new coupling phenomena between 3D bulk materials and 2D atomic layers that have unique van der Waals features. Here we review the state-of-the-art fabrication of 2D and 3D heterostructures, present a critical survey of unique phenomena arising from forming 3D/2D interfaces, and introduce their applications. We also discuss potential directions for research based on these new coupled architectures.

Unveiling the carrier transport mechanism in epitaxial graphene for forming wafer-scale, single-domain graphene.

Graphene epitaxy on the Si face of a SiC wafer offers monolayer graphene with unique crystal orientation at the wafer-scale. However, due to carrier scattering near vicinal steps and excess bilayer stripes, the size of electrically uniform domains is limited to the width of the terraces extending up to a few microns. Nevertheless, the origin of carrier scattering at the SiC vicinal steps has not been clarified so far. A layer-resolved graphene transfer (LRGT) technique enables exfoliation of the epitaxial graphene formed on SiC wafers and transfer to flat Si wafers, which prepares crystallographically single-crystalline monolayer graphene. Because the LRGT flattens the deformed graphene at the terrace edges and permits an access to the graphene formed at the side wall of vicinal steps, components that affect the mobility of graphene formed near the vicinal steps of SiC could be individually investigated. Here, we reveal that the graphene formed at the side walls of step edges is pristine, and scattering near the steps is mainly attributed by the deformation of graphene at step edges of vicinalized SiC while partially from stripes of bilayer graphene. This study suggests that the two-step LRGT can prepare electrically single-domain graphene at the wafer-scale by removing the major possible sources of electrical degradation.

Polarity governs atomic interaction through two-dimensional materials.

The transparency of two-dimensional (2D) materials to intermolecular interactions of crystalline materials has been an unresolved topic. Here we report that remote atomic interaction through 2D materials is governed by the binding nature, that is, the polarity of atomic bonds, both in the underlying substrates and in 2D material interlayers. Although the potential field from covalent-bonded materials is screened by a monolayer of graphene, that from ionic-bonded materials is strong enough to penetrate through a few layers of graphene. Such field penetration is substantially attenuated by 2D hexagonal boron nitride, which itself has polarization in its atomic bonds. Based on the control of transparency, modulated by the nature of materials as well as interlayer thickness, various types of single-crystalline materials across the periodic table can be epitaxially grown on 2D material-coated substrates. The epitaxial films can subsequently be released as free-standing membranes, which provides unique opportunities for the heterointegration of arbitrary single-crystalline thin films in functional applications.

Chip-integrated metasurface for versatile and multi-wavelength control of light couplings with independent phase and arbitrary polarization.

While metasurfaces are now widely considered in free-space optics, their potential for coupling and tailoring guided waves is not fully explored. Here we transfer the Jones matrix method to target versatile on-chip coupling using metasurface-patterned photonic waveguides around the telecommunication wavelength of 1.55 µm, which can accommodate both propagation and Pancharatnam-Berry phase metasurfaces for guided waves. One can either encode two arbitrary and independent phase profiles to any pair of orthogonal polarizations or deploy complete control over both the phase and polarization of coupled modes. A set of design scenarios synergizing silicon nanoantennas and low-loss silicon-nitride waveguides are proposed, including directional couplers with mode-selectivity and polarization splitters with directionality ranging from 10 to 20 dB. Furthermore, our optimization method can be further extended to cover multiple working wavelengths. Exemplary on-chip color routers are also numerically demonstrated. This chip-integrated metasurface platform further translates the concept of a metasurface into photonic integrated circuits, serving as a positive paradigm for versatile and complete control over waveguide optical signals and motivating chip-scale applications such as polarization/wavelength demultiplexers, optical switches, and multifunctional mode converters.

Large-Area, Ultrathin Metal-Oxide Semiconductor Nanoribbon Arrays Fabricated by Chemical Lift-Off Lithography.

Nanoribbon- and nanowire-based field-effect transistors (FETs) have attracted significant attention due to their high surface-to-volume ratios, which make them effective as chemical and biological sensors. However, the conventional nanofabrication of these devices is challenging and costly, posing a major barrier to widespread use. We report a high-throughput approach for producing arrays of ultrathin (∼3 nm) In2O3 nanoribbon FETs at the wafer scale. Uniform films of semiconducting In2O3 were prepared on Si/SiO2 surfaces via a sol-gel process prior to depositing Au/Ti metal layers. Commercially available high-definition digital versatile discs were employed as low-cost, large-area templates to prepare polymeric stamps for chemical lift-off lithography, which selectively removed molecules from self-assembled monolayers functionalizing the outermost Au surfaces. Nanoscale chemical patterns, consisting of one-dimensional lines (200 nm wide and 400 nm pitch) extending over centimeter length scales, were etched into the metal layers using the remaining monolayer regions as resists. Subsequent etch processes transferred the patterns into the underlying In2O3 films before the removal of the protective organic and metal coatings, revealing large-area nanoribbon arrays. We employed nanoribbons in semiconducting FET channels, achieving current on-to-off ratios over 107 and carrier mobilities up to 13.7 cm2 V-1 s-1. Nanofabricated structures, such as In2O3 nanoribbons and others, will be useful in nanoelectronics and biosensors. The technique demonstrated here will enable these applications and expand low-cost, large-area patterning strategies to enable a variety of materials and design geometries in nanoelectronics.

The Interplay between Trap Density and Hysteresis in Planar Heterojunction Perovskite Solar Cells.

Anomalous current-voltage (J-V) hysteresis in perovskite (PSK) solar cell is open to dispute, where hysteresis is argued to be due to electrode polarization, dipolar polarization, and/or native defects. However, a correlation between those factors and J-V hysteresis is hard to be directly evaluated because they usually coexist and are significantly varied depending on morphology and crystallinity of the PSK layer, selective contacts, and device architecture. In this study, without changing morphology and crystallinity of PSK layer in a planar heterojunction structure employing FA0.9Cs0.1PbI3, a correlation between J-V hysteresis and trap density is directly evaluated by means of thermally induced PbI2 regulating trap density. Increase in thermal annealing time at a given temperature of 150 °C induces growth of PbI2 on the PSK grain surface, which results in significant reduction of nonradiative recombination. Hysteresis index is reduced from 0.384 to 0.146 as the annealing time is increased from 5 to 100 min due to decrease in the amplitude of trap-mediated recombination. Reduction of hysteresis by minimizing trap density via controlling thermal annealing time leads to the stabilized PCE of 18.84% from the normal planar structured FA0.9Cs0.1PbI3 PSK solar cell.

Morphology Evolution of High Efficiency Perovskite Solar Cells via Vapor Induced Intermediate Phases.

Morphology is critical component to achieve high device performance hybrid perovskite solar cells. Here, we develop a vapor induced intermediate phase (VIP) strategy to manipulate the morphology of perovskite films. By exposing the perovskite precursor films to different saturated solvent vapor atmospheres, e.g., dimethylformamide and dimethylsufoxide, dramatic film morphological evolution occurs, associated with the formation of different intermediate phases. We observe that the crystallization kinetics is significantly altered due to the formation of these intermediate phases, yielding highly crystalline perovskite films with less defect states and high carrier lifetimes. The perovskite solar cells with the reconstructed films exhibits the highest power conversion efficiency (PCE) up to 19.2% under 1 sun AM 1.5G irradiance, which is among the highest planar heterojunction perovskite solar cells. Also, the perovskite solar cells with VIP processing shows less hysteresis behavior and a stabilized power output over 18%. Our work opens up a new direction for morphology control through intermediate phase formation, and paves the way toward further enhancing the device performances of perovskite solar cells.

Organic solar cells using CVD-grown graphene electrodes.

We report on the development of flexible organic solar cells (OSCs) incorporating graphene sheets synthesized by chemical vapor deposition (CVD) as transparent conducting electrodes on polyethylene terephthalate (PET) substrates. A key barrier that must be overcome for the successful fabrication of OSCs with graphene electrodes is the poor-film properties of water-based poly(3,4-ethylenedioxythiphene):poly(styrenesulfonate) (PEDOT:PSS) when coated onto hydrophobic graphene surfaces. To form a uniform PEDOT:PSS film on a graphene surface, we added perfluorinated ionomers (PFI) to pristine PEDOT:PSS to create 'GraHEL', which we then successfully spin coated onto the graphene surface. We systematically investigated the effect of number of layers in layer-by-layer stacked graphene anode of an OSC on the performance parameters including the open-circuit voltage (Voc), short-circuit current (Jsc), and fill factor (FF). As the number of graphene layers increased, the FF tended to increase owing to lower sheet resistance, while Jsc tended to decrease owing to the lower light absorption. In light of this trade-off between sheet resistance and transmittance, we determined that three-layer graphene (3LG) represents the best configuration for obtaining the optimal power conversion efficiency (PCE) in OSC anodes, even at suboptimal sheet resistances. We finally developed efficient, flexible OSCs with a PCE of 4.33%, which is the highest efficiency attained so far by an OSC with CVD-grown graphene electrodes to the best of our knowledge.

Nondestructive Single-Atom-Thick Crystallographic Scanner via Sticky-Note-Like van der Waals Assembling-Disassembling.

Crystallographic characteristics, including grain boundaries and crystallographic orientation of each grain, are crucial in defining the properties of two-dimensional materials (2DMs). To date, local microstructure analysis of 2DMs, which requires destructive and complex processes, is primarily used to identify unknown 2DM specimens, hindering the subsequent use of characterized samples. Here, a nondestructive large-area 2D crystallographic analytical method through sticky-note-like van der Waals (vdW) assembling-disassembling is presented. By the vdW assembling of veiled polycrystalline graphene (PCG) with a single-atom-thick single-crystalline graphene filter (SCG-filter), detailed crystallographic information of each grain in PCGs is visualized through a 2D Raman signal scan, which relies on the interlayer twist angle. The scanned PCGs are seamlessly separated from the SCG-filter using vdW disassembling, preserving their original condition. The remaining SCG-filter is then reused for additional crystallographic scans of other PCGs. It is believed that the methods can pave the way for advances in the crystallographic analysis of single-atom-thick materials, offering huge implications for the applications of 2DMs.

Junctionless Negative-Differential-Resistance Device Using 2D Van-Der-Waals Layered Materials for Ternary Parallel Computing.

Negative-differential-resistance (NDR) devices offer a promising pathway for developing future computing technologies characterized by exceptionally low energy consumption, especially multivalued logic computing. Nevertheless, conventional approaches aimed at attaining the NDR phenomenon involve intricate junction configurations and/or external doping processes in the channel region, impeding the progress of NDR devices to the circuit and system levels. Here, an NDR device is presented that incorporates a channel without junctions. The NDR phenomenon is achieved by introducing a metal-insulator-semiconductor capacitor to a portion of the channel area. This approach establishes partial potential barrier and well that effectively restrict the movement of hole and electron carriers within specific voltage ranges. Consequently, this facilitates the implementation of both a ternary inverter and a ternary static-random-access-memory, which are essential components in the development of multivalued logic computing technology.

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.

Thrombophlebitis of the internal jugular vein (Lemierre syndrome): clinical and CT findings.

BACKGROUND: Thrombophlebitis of the internal jugular vein (IJV) secondary to neck infection (so-called Lemierre syndrome) is a rare disease. PURPOSE: To evaluate the clinical and CT findings in patients with thrombophlebitis of the IJV. MATERIAL AND METHODS: The clinical and contrast-enhanced neck CT findings were retrospective analyzed in 10 patients (eight men, two women; mean age, 62.9 ± 8.3 years) with thrombophlebitis of the IJV. RESULTS: Five patients (50%) had complications, including pneumonia (n = 3), neck abscess (n = 1), and thrombophlebitis of cerebral venous sinus (n = 1). All patients, except two who were lost to follow-up, had improved after antibiotics and anticoagulation therapy. Nine (90%) patients had underlying infectious processes in the neck. Contrast-enhanced neck CT of 12 IJVs (five right, three left, and two bilateral) affected by thrombophlebitis demonstrated > 5 cm in length (n = 8, 67%), ovoid shape (n = 7, 58%), complete occlusion of the lumen (n = 10, 83%), circumferential (n = 11, 92%), smooth (n = 8, 67%), and thick (≥4 mm) (n = 8, 67%) rim enhancement, and adjacent soft tissue swelling (n = 11, 92%). CONCLUSION: Contrast-enhanced CT is useful in the diagnosis of thrombophlebitis of the IJV; characteristic CT findings of this unusual entity may be the main clue to the correct diagnosis.

Incidental thyroid lesions identified by ultrasound in patients with non-thyroidal head and neck cancer.

BACKGROUND: Thyroid cancer is one of the common head and neck malignancies and may be found incidentally with other head and neck cancers. PURPOSE: To evaluate the prevalence and risk of malignancy in incidental thyroid lesions identified by ultrasound (US) in patients with head and neck cancer. MATERIAL AND METHODS: We retrospectively reviewed medical records of all patients with head and neck cancer other than of thyroid origin between January 2004 and December 2011. A total of 690 patients (537 men and 153 women; mean age, 58.9 ± 12.9 years) underwent US of the neck for the evaluation of cervical lymph node status (including thyroid gland). We evaluated the prevalence of patients with incidental thyroid lesions identified by US and the risk of malignancy in these patients. RESULTS: Of the 690 patients with head and neck cancer, 234 (33.9%) had incidental thyroid lesions on US. Based on US findings, 61 patients underwent fine-needle aspiration, with 39 eventually undergoing thyroidectomy. Among these thyroid lesions, 24 incidental thyroid lesions of 22 patients were histologically proven to be malignant (23 papillary and 1 follicular carcinomas). The risk of malignancy was 9.4% on a patient-by-patient basis. CONCLUSION: Screening of the thyroid gland should be included in the preoperative US examination for cervical lymph node metastases in patients with non-thyroidal head and neck cancer.