Atomic and electronic reconstruction at the van der Waals interface in twisted bilayer graphene
Atomic and electronic reconstruction at the van der Waals interface in twisted bilayer graphene, Published online: 15 April 2019; doi:10.1038/s41563-019-0346-z
An investigation of the structural and transport properties of bilayer graphene as a function of the twist angle between the layers reveals atomic-scale reconstruction for twist angles smaller than a critical value.
By coupling two‐dimensional (2D) titanium oxide and carbide sheets at the molecular level, a self‐standing battery electrode with ideal mechanical durability, excellent high‐current performance, and good cycling stability is achieved. Correspondingly, a full flexible battery showing competitive power density while maintaining a satisfactory energy density, even under mechanical deformations, is devised, which demonstrates great potential for use in wearable powered systems.
High‐performance, flexible, and lightweight powering electrodes are urgently needed to meet the increasing interest in deformable electronic devices, particularly those utilizing solid‐state electrolytes and performing at high charging rates, which unfortunately have remained a formidable challenge. Here, by regularly stacking two‐dimensional (2D) titanium oxide and carbide sheets, in which the two kinds of sheets are coupled at the molecular level, a self‐standing electrode is achieved with ideal mechanical durability and excellent electrochemical performance, including superb rate performance (delivering a capacity of 114 mAh g−1 in 3.4 min) and good cycling stability (remaining >93% after 1000 cycles at 1000 mA g−1). Profiting from these advantages, a flexible and safe full lithium‐ion battery, employing a poly(ethylene glycol) diamine‐based gel polymer as the electrolyte, possesses an excellent power density of 1412 W kg−1 while maintaining a high energy density of 59 Wh kg−1, which outperforms most documented flexible batteries that utilize liquid electrolytes and is even comparable with some cells using coin configurations. Importantly, the performance was well maintained under mechanical deformation and after multiple breaking and self‐healing cycles, demonstrating the feasibility for practical application in wearable powering devices. The results highlight the numerous possibilities for utilizing sheet materials to fabricate wearable electrode materials.
Photoaddressable liquid crystalline (LC) solutions of graphene oxide (GO) stabilized with photocleavable brushes display phase transition upon UV light irradiation. The resulting change in the steric stabilization of GO platelets results in LC phase release and the establishment of a percolated network, enabling the design of solution‐processable dielectric/conductive composites.
The multifunctionality of graphene has the potential to unlock important developments in nanocomposite science. However, the manipulation of graphene without interfering with its unique properties and while controlling its spatial organization remains challenging. Here, the formation of a photoaddressable liquid crystalline (LC) solution through the stabilization of graphene oxide (GO) with photocleavable brushes is described. The LC behavior leads to the thermodynamic entrapment of GO into low aspect ratio domains that fail to display the properties typically predicted for graphene nanocomposites. The morphology and structural and electronic performance of these nanocomposites are regenerated through the brush cleavage, which controls the phase transition of the LC phase. These results show that kinetic control of graphene assembly can be an attractive tool toward the dynamic regulation of processable sol states and structured percolated networks for rational composite manufacturing.
Germanium disulfide (GeS2) with a wide bandgap is introduced as an ideal candidate for polarization‐sensitive photodetection in the UV region. In‐plane anisotropy of GeS2 is demonstrated by theoretical and experimental results. In terms of in‐plane anisotropic absorption and wide bandgap in GeS2, GeS2‐based photodetectors show a strong polarization‐dependent photoresponse in the UV region.
Polarization‐sensitive photodetection in the UV region is highly indispensable in many military and civilian applications. UV‐polarized photodetection usually relies on the use of wide bandgap semiconductors with 1D nanostructures requiring complicated nanofabrication processes. Although the emerging anisotropic 2D semiconductors shed light on the detection of polarization with a simple device architecture, bandgaps of such reported 2D semiconductors are too small to be applied for visible–blind UV‐polarized photodetection. Here, germanium disulfide (GeS2), the widest bandgap (>3 eV) in the family of in‐plane anisotropic 2D semiconductors explored to date, is introduced as an ideal candidate for UV‐polarized photodetection. The structural, vibrational, and optical anisotropies of GeS2 are systematically investigated from theory to experiment. GeS2‐based photodetectors show a strong polarization‐dependent photoresponse in the UV region. GeS2 with a wide bandgap and high in‐plane anisotropy not only enriches the family of anisotropic 2D semiconductors but also expands the polarized photodetection from the current visible and near‐infrared to the brand‐new UV region.
Bioinspired 2D MoSe2 nanosheets with high photothermal conversion efficiency are designed to achieve efficient photothermal‐triggered cancer immunotherapy, by activating cytotoxic T lymphocytes, reprogramming tumor‐associated macrophages to the tumoricidal M1 phenotype, and inactivation of PD‐1/PD‐L1 pathway to avoid immunologic escape.
Nonspecific absorption and clearance of nanomaterials during circulation is the major cause for treatment failure in nanomedicine‐based cancer therapy. Therefore, herein bioinspired red blood cell (RBC) membrane is employed to camouflage 2D MoSe2 nanosheets with high photothermal conversion efficiency to achieve enhanced hemocompatibility and circulation time by preventing macrophage phagocytosis. RBC–MoSe2‐potentiated photothermal therapy (PTT) demonstrates potent in vivo antitumor efficacy, which triggers the release of tumor‐associated antigens to activate cytotoxic T lymphocytes and inactivate the PD‐1/PD‐L1 pathway to avoid immunologic escape. Furthermore, in the ablated tumor microenvironment, the tumor‐associated macrophages are effectively reprogrammed to tumoricidal M1 phenotype to potentiate the antitumor action. Taken together, this biomimetic functionalization thus provides a substantial advance in personalized PTT‐triggered immunotherapy for clinical translation.
Van der Waals bipolar junction transistors based on vertically stacked 2D materials (V2D‐BJT) are proposed, and experimental studies are conducted on the V2D‐BJT using an MoS2/WSe2/MoS2 heterostructure in an n‐p‐n configuration. The V2D‐BJT shows excellent gas sensing performance with a low power dissipation (≈2 nW), a fast response (9 s), and a fast recovery (35 s) time.
The majority of microelectronic devices rely on a p‐n junction. The process of making such a junction is complicated, and it is difficult to make layers that form a junction with an atomic thickness. In this study, bipolar junctions are made by using 2D atomic crystalline layers and even a single layer in which 2D layers adhere together to form a heterostructure via van der Waals forces. A vertical 2D bipolar junction transistor (V2D‐BJT) is studied for the first time. It uses an MoS2/WSe2/MoS2 heterostructure and has an n‐p‐n configuration that exhibits a maximum common‐base current gain of ≈0.97 and a stable common‐emitter current gain (β) of 12 with a nanowatt power consumption. In the first attempt at gas sensing, it shows outstanding performance, exhibiting a very fast response and recovery time (9 and 35 s, respectively) with a power dissipation of only 2 nW. This study demonstrates the potential application of the V2D‐BJT in nanowatt power amplifiers as well as fast‐response and low‐power gas sensors.
Ultrafast transition between exciton phases in van der Waals heterostructures
Ultrafast transition between exciton phases in van der Waals heterostructures, Published online: 08 April 2019; doi:10.1038/s41563-019-0337-0
Femtosecond pump–probe measurements of Coulomb correlations in WS2/WSe2 heterostructures reveal the interlayer exciton binding energy, determined from the 1s–2p resonance, as well as the dynamics of the conversion of intra- to interlayer excitons.
Low‐dimensional high‐quality InSb materials are promising candidates for various next‐generation devices due to the high carrier mobility, low effective mass, and large g‐factor of this heavy element compound. Controlled growth of “free‐standing” 2D InSb nanoflakes is realized by setting growth and substrate design parameters. Electronic assessment of the material marks this system as a viable nanoscale 2D platform for future quantum devices, thermal rectifiers, field emitters, infrared detectors, etc.
Low‐dimensional high‐quality InSb materials are promising candidates for next‐generation quantum devices due to the high carrier mobility, low effective mass, and large g‐factor of the heavy element compound InSb. Various quantum phenomena are demonstrated in InSb 2D electron gases and nanowires. A combination of the best features of these two systems (pristine nanoscale and flexible design) is desirable to realize, e.g., the multiterminal topological Josephson device. Here, controlled growth of 2D nanostructures, nanoflakes, on an InSb platform is demonstrated. An assembly of nanoflakes with various dimensions and morphologies, thinner than the Bohr radius of InSb, are fabricated. Importantly, the growth of either nanowires or nanoflakes can be enforced experimentally by setting growth and substrate design parameters properly. Hall bar measurements on the nanostructures yield mobilities up to ≈20 000 cm2 V−1 s−1 and detect quantum Hall plateaus. This allows to see the system as a viable nanoscale 2D platform for future quantum devices.
Perpendicular magnetic anisotropy (PMA) and spin‐orbit torque (SOT) efficiency are greatly enhanced by MoS2. First‐principles calculation and X‐ray absorption reveal that MoS2 results in the modification of orbital hybridization at the Pt/Co interface. These findings may pave a new way to engineer the PMA and SOT efficiency by 2D materials.
2D transition metal dichalcogenides have attracted much attention in the field of spintronics due to their rich spin‐dependent properties. The promise of highly compact and low‐energy‐consumption spin‐orbit torque (SOT) devices motivates the search for structures and materials that can satisfy the requirements of giant perpendicular magnetic anisotropy (PMA) and large SOT simultaneously in SOT‐based magnetic memory. Here, it is demonstrated that PMA and SOT in a heavy metal/transition metal ferromagnet structure, Pt/[Co/Ni]2, can be greatly enhanced by introducing a molybdenum disulfide (MoS2) underlayer. According to first‐principles calculation and X‐ray absorption spectroscopy (XAS), the enhancement of the PMA is ascribed to the modification of the orbital hybridization at the interface of Pt/Co due to MoS2. The enhancement of SOT by the role played by MoS2 is explained, which is strongly supported by the identical behavior of SOT and PMA as a function of Pt thickness. This work provides new possibilities to integrate 2D materials into promising spintronics devices.
A highly sensitive infrared light photo‐detector is fabricated by transferring multilayer PdSe2 on a germanium nanocones array with a strong light‐trapping effect. The as‐assembled PdSe2/GeNCs hybrid heterojunction devices can also record simple near‐infrared images.
In this study, a sensitive infrared photodetector (IRPD) composed of a germanium nanocones (GeNCs) array and PdSe2 multilayer is presented, which is obtained by a straightforward selenization approach. The as‐assembled PdSe2/GeNCs hybrid heterojunction exhibits obvious photovoltaic behavior to 1550 nm illumination, which renders the IRPD a self‐driven device without external power supply. Further device analysis reveals that the PdSe2/GeNCs hybrid based IRPD exhibits high sensitivity to 1350, 1550, and 1650 nm illumination with excellent stability and reproducibility. The responsivity and external quantum efficiency is as high as 530.2 mA W−1 and 42.4%, respectively. Such a relatively good device performance is related to the strong light trapping effect of GeNCs array, according to the theoretical simulation based on finite‐difference time‐domain. It is also found that the IRPD shows an abnormal sensitivity to IR illumination with a wavelength of 2200 nm. Finally, the present individual IRPD can also record the simple “F” image produced by 1550 nm, suggesting the promising application of the PdSe2/GeNCs hybrid device in future infrared optoelectronic systems.
Because of the high quality of vapor–solid grown material and its excellent contact resistance, Bi2O2Se phototransistors show an ultrahigh photoresponse (22 100 AW−1), record specific detectivity (3.4 × 1015 Jones), and a high on/off ratio (≈109). All these values are much higher than those of phototransistors based on other 2D materials including MoS2 and commercial photodetectors using Si and III–V materials.
Atomically thin 2D materials have received intense interest both scientifically and technologically. Bismuth oxyselenide (Bi2O2Se) is a semiconducting 2D material with high electron mobility and good stability, making it promising for high‐performance electronics and optoelectronics. Here, an ambient‐pressure vapor–solid (VS) deposition approach for the growth of millimeter‐size 2D Bi2O2Se single crystal domains with thicknesses down to one monolayer is reported. The VS‐grown 2D Bi2O2Se has good crystalline quality, chemical uniformity, and stoichiometry. Field‐effect transistors (FETs) are fabricated using this material and they show a small contact resistivity of 55.2 Ω cm measured by a transfer line method. Upon light irradiation, a phototransistor based on the Bi2O2Se FETs exhibits a maximum responsivity of 22 100 AW−1, which is a record among currently reported 2D semiconductors and approximately two orders of magnitude higher than monolayer MoS2. The Bi2O2Se phototransistor shows a gate tunable photodetectivity up to 3.4 × 1015 Jones and an on/off ratio up to ≈109, both of which are much higher than phototransistors based on other 2D materials reported so far. The results of this study indicate a method to grow large 2D Bi2O2Se single crystals that have great potential for use in optoelectronic applications.
In van der Waals heterostructures comprising hexagonal boron nitride (hBN) and vanadium dioxide (VO2), dynamic and reversible tuning of hyperbolic phonon polaritons is achieved via the insulator‐to‐metal phase transition by controlling the temperature. Using infrared nanospectroscopy, opposite tuning trends for in‐plane and out‐of‐plane phonon resonances are demonstrated during the phase transition.
Unlike conventional plasmonic media, polaritonic van der Waals (vdW) materials hold promise for active control of light–matter interactions. The dispersion relations of elementary excitations such as phonons and plasmons can be tuned in layered vdW systems via stacking using functional substrates. In this work, infrared nanoimaging and nanospectroscopy of hyperbolic phonon polaritons are demonstrated in a novel vdW heterostructure combining hexagonal boron nitride (hBN) and vanadium dioxide (VO2). It is observed that the insulator‐to‐metal transition in VO2 has a profound impact on the polaritons in the proximal hBN layer. In effect, the real‐space propagation of hyperbolic polaritons and their spectroscopic resonances can be actively controlled by temperature. This tunability originates from the effective change in local dielectric properties of the VO2 sublayer in the course of the temperature‐tuned insulator‐to‐metal phase transition. The high susceptibility of polaritons to electronic phase transitions opens new possibilities for applications of vdW materials in combination with strongly correlated quantum materials.
