Visualizing Atomic Quantum Defects in Ultrathin 1T-PtTe2.

Defects are of significant importance to determine and improve the distinct properties of 2D materials, such as electronic, optical, and catalytic performance. In this report, we observe four types of point defects in atomically thin flakes of 1T-PtTe2 by using low-temperature scanning tunnelling microscopy and spectroscopy (STM/S). Through the combination of STM imaging and simulations, such defects are identified as a single tellurium vacancy from each side of the top PtTe2 layer and a single platinum vacancy from the topmost and next layer. The density functional theory (DFT) calculations reveal that the platinum vacancies from both the monolayer and bilayer exhibit a local magnetic moment. In bilayer PtTe2, the interlayer coulomb screening effect reduces the local magnetic momentum of the single platinum vacancy. Our research provides meaningful guidance for further experiments about the effects of intrinsic defects on potential functions of thin 1T-PtTe2, such as catalysis and spintronic applications.

Gate-Tunable Anomalous Hall Effect in Stacked van der Waals Ferromagnetic Insulator-Topological Insulator Heterostructures.

The search of novel topological states, such as the quantum anomalous Hall insulator and chiral Majorana fermions, has motivated different schemes to introduce magnetism into topological insulators. A promising scheme is using the magnetic proximity effect (MPE), where a ferromagnetic insulator magnetizes the topological insulator. Most of these heterostructures are synthesized by growth techniques which prevent mixing many of the available ferromagnetic and topological insulators due to difference in growth conditions. Here, we demonstrate that MPE can be obtained in heterostructures stacked via the dry transfer of flakes of van der Waals ferromagnetic and topological insulators (Cr2Ge2Te6/BiSbTeSe2), as evidenced in the observation of an anomalous Hall effect (AHE). Furthermore, devices made from these heterostructures allow modulation of the AHE when controlling the carrier density via electrostatic gating. These results show that simple mechanical transfer of magnetic van der Waals materials provides another possible avenue to magnetize topological insulators by MPE.

Separation in the Roles of Carrier Transport and Light Emission in Light-Emitting Organic Transistors with a Bilayer Configuration.

To develop high-performance organic light-emitting organic field-effect transistors (LE-OFETs), a fundamental problem in organic semiconductors is to compromise light luminescent efficiency for high carrier mobility and vice versa. Therefore, LE-OFETs can avoid this problem by separating the light-emission and carrier-transport functions. Here, a bilayer LE-OFET composed of a tetracene crystal as a carrier transporter (bottom crystal) and a 4-(dicyanomethylene)-2-methyl-6-( p-dimethylaminostyryl)-4 H-pyran (DCM1)-doped tetracene crystal as a light emitter (top crystal) was fabricated. Red light-emission color, which is distinct from the green emission color of tetracene, was detected in the top crystal. Light emission from the top layer was prohibited when an insulating thin film was inserted between the two crystals. These observations indicate that excitons are formed in the bottom crystal and transferred to the top crystal, emitting reddish light. Bilayer LE-OFETs have the advantage of providing both high current density and a bright emission for high-performance light-emitting FETs.

Carrier-doped aromatic hydrocarbons: a new platform in condensed matter chemistry and physics.

High-quality bulk samples of the first four polyacenes, which are naphthalene, anthracene, tetracene, and pentacene, doped with alkali metal in 1 : 1 and 1 : 2 stoichiometries were prepared and their fundamental properties were systematically studied. A new systematic understanding on the electronic states of electron-doped polyacenes sensitive to the energetic balance among on-site Coulomb repulsion, bandwidth and the Peierls instability was provided. The carrier-doped typical aromatic hydrocarbons showed a large variety of properties as well as charge transfer complexes and metal-doped fullerides. We open a new avenue for organometallic and inorganic chemistry.

Three-dimensional porous graphene networks expand graphene-based electronic device applications.

In recent years, there has been increasing demand for 3D porous graphene structures with excellent 2D characteristics and great potential. As one avenue, several approaches for fabricating 3D porous graphene network structures have been proposed to realize multi-functional graphene materials with 2D graphene structures. Herein, we overview characteristics of 3D porous graphene for applications in future electronic devices along with physical insights into "2D to 3D graphene", in which the characters of 2D graphene such as massless Dirac fermions are well preserved. The present review thus summarizes recent 3D porous graphene studies with a perspective for providing new and board applications of graphene in electronic devices.

A new electrode design for ambipolar injection in organic semiconductors.

Organic semiconductors have attracted much attention for low-cost, flexible and human-friendly optoelectronics. However, achieving high electron-injection efficiency is difficult from air-stable electrodes and cannot be equivalent to that of holes. Here, we present a novel concept of electrode composed of a bilayer of tetratetracontane (TTC) and polycrystalline organic semiconductors (pc-OSC) covered by a metal layer. Field-effect transistors of single-crystal organic semiconductors with the new electrodes of M/pc-OSC/TTC (M: Ca or Au) show both highly efficient electron and hole injection. Contact resistance for electron injection from Au/pc-OSC/TTC and hole injection from Ca/pc-OSC/TTC are comparable to those for electron injection from Ca and hole injection from Au, respectively. Furthermore, the highest field-effect mobilities of holes (22 cm2 V-1 s-1) and electrons (5.0 cm2 V-1 s-1) are observed in rubrene among field-effect transistors with electrodes so far proposed by employing Ca/pc-OSC/TTC and Au/pc-OSC/TTC electrodes for electron and hole injection, respectively.One of technological challenges building organic electronics is efficient injection of electrons at metal-semiconductor interfaces compared to that of holes. The authors show an air-stable electrode design with induced gap states, which support Fermi level pinning and thus ambipolar carrier injection.

Crossover from localized to itinerant states in hydrocarbon Mott insulators.

Crossover from an itinerant state to an isolated electronic state in electron-doped polycyclic aromatic hydrocarbon (PAH) was studied for the two smallest zigzag-type molecules of naphthalene (NN) and anthracene (AN) by focusing on their 1 : 1 stoichiometry, A1(NN) and A1(AN), with alkali metals (A = K and Rb). The competition between on-site Coulombic repulsion energy (U) and bandwidth (W) was argued in terms of their magnetic and electrical properties upon lattice expansion, when A varies from K, with a smaller ionic radius, to Rb, with a larger ionic radius. The temperature-dependence of magnetic susceptibility shows a pronounced hump associated with antiferromagnetic (AFM) interactions for Rb1(NN), being similar to those of K1(NN) and K1(AN) in the earlier report. On the other hand, Rb1(AN) intriguingly exhibits paramagnetic susceptibility, observed in a nearly localized electron system, being apart from an highly correlated Mott insulating state. Crystal structural analyses of the X-ray diffraction profiles show a small difference in lattice parameters of the ab plane among K1(NN), K1(AN), and Rb1(NN), whereas Rb1(AN) exhibits a significantly larger value than those of the others, being indicative of greatly modified interaction energies. The different magnetic properties observed in Rb1(AN) are interpreted from its modified intermolecular distance. The possibility of an emergent metallic state of K1(AN) under high pressure has also been described, referring to electrical transport measured under high pressure.

Large-Area and Transferred High-Quality Three-Dimensional Topological Insulator Bi2-xSbxTe3-ySey Ultrathin Film by Catalyst-Free Physical Vapor Deposition.

Uniform and large-area synthesis of bulk insulating ultrathin films is an important subject toward applications of a surface of three-dimensional topological insulators (3D-TIs) in various electronic devices. Here we report epitaxial growth of bulk insulating three-dimensional topological insulator (3D-TI) Bi2-xSbxTe3-ySey (BSTS) ultrathin films, ranging from a few quintuple to several hundreds of layers, on mica in a large-area (1 cm2) via catalyst-free physical vapor deposition. These films can nondestructively be exfoliated using deionized water and transferred to various kinds of substrates as desired. The transferred BSTS thin films show good ambipolar characteristics as well as well-defined quantum oscillations arising from the topological surface states. The carrier mobility of 2500-5100 cm2/(V s) is comparable to the high-quality bulk BSTS single crystal. Moreover, tunable electronic states from the massless to the massive Dirac fermion were observed with a decrease in the film thickness. Both the feasible large-area synthesis and the reliable film transfer process can promise that BSTS ultrathin films will pave a route to many applications of 3D-TIs.

In-plane topological p-n junction in the three-dimensional topological insulator Bi2-xSbxTe3-ySey.

A topological p-n junction (TPNJ) is an important concept to control spin and charge transport on a surface of three-dimensional topological insulators (3D-TIs). Here we report successful fabrication of such TPNJ on a surface of 3D-TI Bi2-xSbxTe3-ySey thin films and experimental observation of the electrical transport. By tuning the chemical potential of n-type topological Dirac surface of Bi2-xSbxTe3-ySey on its top half by using tetrafluoro-7,7,8,8-tetracyanoquinodimethane as an organic acceptor molecule, a half surface can be converted to p-type with leaving the other half side as the opposite n-type, and consequently TPNJ can be created. By sweeping the back-gate voltage in the field effect transistor structure, the TPNJ was controlled both on the bottom and the top surfaces. A dramatic change in electrical transport observed at the TPNJ on 3D-TI thin films promises novel spin and charge transport of 3D-TIs for future spintronics.

Electric Properties of Dirac Fermions Captured into 3D Nanoporous Graphene Networks.

Nanoporous graphene- based electric-double-layer transistors (EDLTs) are successfully fabricated. Transport measurements of the EDLTs demonstrate that the ambipolar electronic states of massless Dirac fermions with a high carrier mobility are well preserved in 3D nanoporous graphene along with anomalous nonlinear Hall resistance and exceptional transistor on/off ratio. This study may open a new avenue for device applications of graphene.

2-Positional pyrene end-capped oligothiophenes for high performance organic field effect transistors.

Three new 2-positional pyrene end-capped oligothiophene co-oligomers, BPynT (n = 1, 2, 3), have been synthesized for application in organic field effect transistors (OFETs). BPy2T showed the highest hole mobility of 3.3 cm(2) V(-1) s(-1) in a single crystal OFET and a good photoluminescence efficiency of 32% in the crystalline state. A green light emission was observed for the OFET based on a BPy2T single crystal.

Biphenyl end-capped bithiazole co-oligomers for high performance organic thin film field effect transistors.

Two new regiospecific biphenyl end-capped bithiazole co-oligomers, BP2Tz(in) and BP2Tz(out), have been synthesized for application in thin film field effect transistors (TFTs). BP2Tz(in) with a 2,2'-bithiazole central unit exhibits a field effect hole mobility as high as 3.5 cm(2) V(-1) s(-1). Green light emission is demonstrated for highly balanced ambipoar TFTs based on both BP2Tz(in) and BP2Tz(out).

Superconductivity in the Graphite Intercalation Compound BaC(6).

Among many two-dimensional (2D) high T(C) superconductors, graphite intercalation compounds (GICs) are the most famous intercalation family, which are classified as typical electron-phonon mediated superconductors. We show unambiguous experimental facts that BaC(6), the superconductivity of which has been missing for many years so far among various alkaline earth metal (Ca, Sr, and Ba) intercalted GICs, exhibits superconductivity at T(C)=65 mK. By adding this finding as the additional experimental point, a complete figure displaying the relationship between T(C) and interlayer distance (d) for GICs is now provided, and their possible superconducting mechanisms raised so far are revisited. The present study settles a long-running debate between theories and experiments on the superconductivity in the first stage GICs.

Multifunctional Porous Graphene for High-Efficiency Steam Generation by Heat Localization.

Multifunctional nanoporous graphene is realized as a heat generator to convert solar illumination into high-energy steam. The novel 3D nanoporous graphene demonstrates a highly energy-effective steam generation with an energy conversation of 80%.

A magnetic anti-cancer compound for magnet-guided delivery and magnetic resonance imaging.

Research on controlled drug delivery for cancer chemotherapy has focused mainly on ways to deliver existing anti-cancer drug compounds to specified targets, e.g., by conjugating them with magnetic particles or encapsulating them in micelles. Here, we show that an iron-salen, i.e., µ-oxo N,N'- bis(salicylidene)ethylenediamine iron (Fe(Salen)), but not other metal salen derivatives, intrinsically exhibits both magnetic character and anti-cancer activity. X-Ray crystallographic analysis and first principles calculations based on the measured structure support this. It promoted apoptosis of various cancer cell lines, likely, via production of reactive oxygen species. In mouse leg tumor and tail melanoma models, Fe(Salen) delivery with magnet caused a robust decrease in tumor size, and the accumulation of Fe(Salen) was visualized by magnetic resonance imaging. Fe(Salen) is an anti-cancer compound with magnetic property, which is suitable for drug delivery and imaging. We believe such magnetic anti-cancer drugs have the potential to greatly advance cancer chemotherapy for new theranostics and drug-delivery strategies.

Enhanced sensing response of oxidized graphene formed by UV irradiation in water.

A small amount of defects (less than 0.01%) were introduced into graphene by irradiating it with ultraviolet (UV) light in water. The chemisorbed oxygen species caused a limited amount of degradation in the charge carrier mobility, while the physisorbed water molecules caused both a reduction in the mobility and hole doping. The oxidation was nonuniform, owing to variations in the potential caused by the metal contacts. Raman spectroscopy measurements revealed that UV irradiation in water promoted mild oxidation of graphene's basal plane, which enhanced the electrical sensing response of the adsorption of water molecules. The enhanced electrical response was achieved by the high binding energy of the water molecules at the oxidized sites and the near-zero Dirac point voltage, easily obtained by desorbing the physisorbed water molecules.

Tuning of the ground state in electron doped anthracene.

High quality bulk samples of anthracene (AN) doped with potassium (K) in 1 : 1 and 2 : 1 stoichiometries were successfully prepared by a method involving a room temperature solid-state mechanical diffusion process prior to intercalation reactions during heat treatment, and their physical properties were studied using both magnetic and optical measurements. The transfer of almost one electron from K to AN in K1(AN) was confirmed by SQUID and ESR measurements. A pronounced magnetic hump centered at 150 K associated with antiferromagnetic interactions was observed, which can most likely be interpreted in terms of on-site Coulomb repulsions of the Mott insulating states. Optical spectra of K1(AN) clearly showed the insulating states, as well as the electron occupation of the LUMO-derived band of AN. Our results demonstrated tuning of the ground state of a typical bulk hydrocarbon by alkali metal intercalation.

High-quality three-dimensional nanoporous graphene.

We report three-dimensional (3D) nanoporous graphene with preserved 2D electronic properties, tunable pore sizes, and high electron mobility for electronic applications. The complex 3D network comprised of interconnected graphene retains a 2D coherent electron system of massless Dirac fermions. The transport properties of the nanoporous graphene show a semiconducting behavior and strong pore-size dependence, together with unique angular independence. The free-standing, large-scale nanoporous graphene with 2D electronic properties and high electron mobility holds great promise for practical applications in 3D electronic devices.

Bicontinuous nanoporous N-doped graphene for the oxygen reduction reaction.

Bicontinuous nanoporous N-doped graphene with tunable pore size is synthesized by nanoporous Ni-based chemical vapor deposition. The novel 3D graphene material shows an outstanding catalytic activity towards the oxygen reduction reaction with a low onset potential of -0.08 V and a high kinetic current density of 8.2 mA cm(-2) at -0.4 V.

High-sensitivity photodetectors based on multilayer GaTe flakes.

Optoelectronic devices based on layered materials such as graphene have resulted in significant interest due to their unique properties and potential technological applications. The electric and optoelectronic properties of nano GaTe flakes as layered materials are described in this article. The transistor fabricated from multilayer GaTe shows a p-type action with a hole mobility of about 0.2 cm(2) V(-1) s(-1). The gate transistor exhibits a high photoresponsivity of 10(4) A/W, which is greatly better than that of graphene, MoS2, and other layered compounds. Meanwhile, the response speed of 6 ms is also very fast. Both the high photoresponsivity and the fast response time described in the present study strongly suggest that multilayer GaTe is a promising candidate for future optoelectronic and photosensitive device applications.