Reversible Insulator-Metal Transition by Chemical Doping and Dedoping of a Mott Insulator.

The chemical carrier doping of molecular Mott insulators has been poorly investigated to date due to its difficulty. In this study, iodine doping of a molecular Mott insulator, lithium phthalocyanine crystallized in the x-form (x-LiPc), was performed to obtain metallic x-LiPcI. Crystal structure analysis revealed that iodine atoms penetrated channels of x-LiPc and formed one-dimensional chains. The Raman spectroscopy of x-LiPcI indicated the existence of linear I5 - , demonstrating a transition from a half-filled band of the Mott insulating state to a 2/5-filled band of the metallic state. Electrical resistivity measurements confirmed the metallic nature of x-LiPcI, whereas a thermally activated behavior was observed for pristine x-LiPc. Furthermore, the x-LiPc Mott insulator was reproduced by dedoping iodine from x-LiPcI, suggesting that the electronic state can be reversibly tuned between the Mott insulating and metallic states by chemical doping and dedoping.

Unusual Oxygen Partial Pressure Dependence of Electrical Transport of Single-Crystalline Metal Oxide Nanowires Grown by the Vapor-Liquid-Solid Process.

In general, the electrical conductivities of n-type semiconducting metal oxide nanostructures increase with the decrease in the oxygen partial pressure during crystal growth owing to the increased number of crystal imperfections including oxygen vacancies. In this paper, we report an unusual oxygen partial pressure dependence of the electrical conductivity of single-crystalline SnO2 nanowires grown by a vapor-liquid-solid (VLS) process. The electrical conductivity of a single SnO2 nanowire, measured using the four-probe method, substantially decreases by 2 orders of magnitude when the oxygen partial pressure for the crystal growth is reduced from 10-3 to 10-4 Pa. This contradicts the conventional trend of n-type SnO2 semiconductors. Spatially resolved single-nanowire electrical transport measurements, microstructure analysis, plane-view electron energy-loss spectroscopy, and molecular dynamics simulations reveal that the observed unusual oxygen partial pressure dependence of the electrical transport is attributed to the intrinsic differences between the two crystal growth interfaces (LS and VS interfaces) in the critical nucleation of the crystal growth and impurity incorporation probability as a function of the oxygen partial pressure. The impurity incorporation probability at the LS interface is always lower than that at the VS interface, even under reduced oxygen partial pressures.

Controlled Modification of Superconductivity in Epitaxial Atomic Layer-Organic Molecule Heterostructures.

Self-assembled organic molecules can potentially be an excellent source of charge and spin for two-dimensional (2D) atomic-layer superconductors. Here we investigate 2D heterostructures based on In atomic layers epitaxially grown on Si and highly ordered metal-phthalocyanine (MPc, M = Mn, Cu) through a variety of techniques: scanning tunneling microscopy, electron transport measurements, angle-resolved photoemission spectroscopy, X-ray magnetic circular dichroism, and ab initio calculations. We demonstrate that the superconducting transition temperature (Tc) of the heterostructures can be modified in a controllable manner. Particularly, the substitution of the coordinated metal atoms from Mn to Cu is found to reverse the Tc shift from negative to positive directions. This distinctive behavior is attributed to a competition of charge and spin effects, the latter of which is governed by the directionality of the relevant d-orbitals. The present study shows the effectiveness of molecule-induced surface doping and the significance of microscopic understanding of the molecular states in these 2D heterostructures.

True Vapor-Liquid-Solid Process Suppresses Unintentional Carrier Doping of Single Crystalline Metal Oxide Nanowires.

Single crystalline nanowires composed of semiconducting metal oxides formed via a vapor-liquid-solid (VLS) process exhibit an electrical conductivity even without an intentional carrier doping, although these stoichiometric metal oxides are ideally insulators. Suppressing this unintentional doping effect has been a challenging issue not only for metal oxide nanowires but also for various nanostructured metal oxides toward their semiconductor applications. Here we demonstrate that a pure VLS crystal growth, which occurs only at liquid-solid (LS) interface, substantially suppresses an unintentional doping of single crystalline SnO2 nanowires. By strictly tailoring the crystal growth interface of VLS process, we found the gigantic difference of electrical conduction (up to 7 orders of magnitude) between nanowires formed only at LS interface and those formed at both LS and vapor-solid (VS) interfaces. On the basis of investigations with spatially resolved single nanowire electrical measurements, plane-view electron energy-loss spectroscopy, and molecular dynamics simulations, we reveal the gigantic suppression of unintentional carrier doping only for the crystal grown at LS interface due to the higher annealing effect at LS interface compared with that grown at VS interface. These implications will be a foundation to design the semiconducting properties of various nanostructured metal oxides.

Electrostatic doping tunable magnetic transition and half-metallicity in the monolayer CrCTe3.

The electrical manipulation of the magnetic transition and spin-polarized states has attracted extensive attention in the field of spintronics. In this work, we perform a detailed study on the electronic and magnetic properties of the carrier-doped monolayer CrCTe3by using first-principles calculation. It is found that, the magnetic transition from Néel-antiferomagnetic (nAFM) to ferromagnetic (FM) is observed in the case of the electron doping, while for hole doping a magnetic transition sequence of nAFM→zigzag-AFM→FM is observed in the monolayer CrCTe3. Interestingly, the carrier doping induced FM ground state always exhibits half-metallicity with full spin polarization. Moreover, the spin polarity of the half-metallic electronic states is opposite for electron and hole doping, meaning that the spin polarization direction can be tuned by manipulating a gate voltage. The Monte Carlo calculations show that the magnetic transition temperature of the doped FM CrCTe3is rapidly increased with the increasing doping concentration and is extremely expected to achieve room temperature at a suitable doping concentration. These findings demonstrate that the monolayer AFM system possesses a potential application in spintronic devices with electrically tunable spin polarization.

Enhanced Organic Thin-Film Transistor Stability by Preventing MoO3 Diffusion with Metal/MoO3/Organic Multilayered Interface Source-Drain Contact.

The source-drain electrode with a MoO3 interfacial modification layer (IML) is considered the most promising method to solve electrical contact issues impeding organic thin-film transistors (OTFTs) from commercialization. However, this method raises many concerns because MoO3 might diffuse into organic materials, which causes device instability. In this work, we observed a significant device stability degradation by damaging on/off switching performance caused by MoO3 diffusion. To prevent the MoO3 diffusion, a source-drain electrode with a multilayered interface contact (MIC) consisting of a top-down stack of metal, MoO3 IML, and organic buffer layer (OBL) is proposed. In the MIC device, the MoO3 IML serves well for its intended functions of reducing contact resistance and suppressing minority carrier injection to the OTFT channel. The inclusion of OBL to the MIC helps block MoO3 diffusion and thereby leads to better device stability and an increased on/off ratio. Through combinations with several organic compounds as a buffer layer, the MoO3 diffusion related electrical behaviors of OTFTs are systematically studied. Key parameters related to MoO3 diffusion such as the Fick coefficient and bias-stress stability such as carrier trapping time are extracted from numerical device analysis. Finally, we summarize a general rule of material selection for making robust source-drain contact.

Ferroelectricity and High Curie Temperature in a 2D Janus Magnet.

The breaking of the out-of-plane symmetry makes a two-dimensional (2D) Janus monolayer a new platform to explore the coupling between ferroelectricity and ferromagnetism. Using density functional theory in combination with Monte Carlo simulations, we report a novel phase-switchable 2D multiferroic material VInSe3 with large intrinsic out-of-plane spontaneous electric polarization and a high Curie temperature (Tc). The structural transition energy barrier between the two phases is determined to be 0.4 eV, indicating the switchability of the electric polarizations and the potential ferroelectricity. Carrier doping can boost the Curie temperature above room temperature, attributing to the enhanced magnetic exchange interaction. A transition from the ferromagnetic (FM) state to the antiferromagnetic (AFM) state can be induced by carrier doping in octahedra-VInSe3, while FM coupling is well-preserved in tetrahedron-VInSe3, which can be regulated to be either an XY or Ising magnet at an appropriate carrier concentration. These findings not only enrich the family of high-Tc low-dimensional monolayers but also offer a new direction for the design and multifunctional application of multiferroic materials.

Hole doping induced ferromagnetism and Dzyaloshinskii-Moriya interaction in the two-dimensional group-IVA oxides.

Based on the first-principles calculations, we examine the effect of hole doping on the ferromagnetism and Dzyaloshinskii-Moriya interaction (DMI) for PbSnO2, SnO2and GeO2monolayers. The nonmagnetic to ferromagnetic transition and the DMI can emerge simultaneously in the three two-dimensional IVA oxides. By increasing the hole doping concentration, we find the ferromagnetism can be strengthened for the three oxides. Due to different inversion symmetry breaking, isotropic DMI is found in PbSnO2, whereas anisotropic DMI presents in SnO2and GeO2. More appealingly, for PbSnO2with different hole concentrations, DMI can induce a variety of topological spin textures. Interestingly, a peculiar feature of synchronously switch of magnetic easy axis and DMI chirality upon hole doping is found in PbSnO2. Hence, Néel-type skyrmions can be tailored via changing hole density in PbSnO2. Furthermore, we demonstrate that both SnO2and GeO2.with different hole concentrations can host antiskyrmions or antibimerons (in-plane antiskyrmions). Our findings demonstrate the presence and tunability of topological chiral structures in p-type magnets and open up new possibility for spintronics.

A "Click" Reaction to Engineer MoS2 Field-Effect Transistors with Low Contact Resistance.

Two-dimensional (2D) materials with the atomically thin thickness have attracted great interest in the post-Moore's Law era because of their tremendous potential to continue transistor downscaling and offered advances in device performance at the atomic limit. However, the metal-semiconductor contact is the bottleneck in field-effect transistors (FETs) integrating 2D semiconductors as channel materials. A robust and tunable doping method at the source and drain region of 2D transistors to minimize the contact resistance is highly sought after. Here we report a stable carrier doping method via the mild covalent grafting of maleimides on the surface of 2D transition metal dichalcogenides. The chemisorbed interaction contributes to the efficient carrier doping without degrading the high-performance carrier transport. Density functional theory results further illustrate that the molecular functionalization leads to the mild hybridization and the negligible impact on the conduction bands of monolayer MoS2, avoiding the random scattering from the dopants. Differently from reported molecular treatments, our strategy displays high thermal stability (above 300 °C) and it is compatible with micro/nano processing technology. The contact resistance of MoS2 FETs can be greatly reduced by ∼12 times after molecular functionalization. The Schottky barrier of 44 meV is achieved on monolayer MoS2 FETs, demonstrating efficient charge injection between metal and 2D semiconductor. The mild covalent functionalization of molecules on 2D semiconductors represents a powerful strategy to perform the carrier doping and the device optimization.

Hole Concentration Reduction in CuI by Zn Substitution and its Mechanism: Toward Device Applications.

Copper iodide (CuI) is a promising p-type transparent semiconductor with excellent carrier mobility. However, the high hole concentration in conventionally fabricated CuI including the single crystal hinders its applicability to the channel layer of thin-film transistors. We found that Zn substitution into Cu+ sites can effectively reduce the hole concentration. Experimental and computational examinations showed that the dominant mechanism involved the formation of a defect pair, the Zn-substituted Cu site (ZnCu) and Cu vacancy (VCu), and the simultaneous suppression of VCu arising from the stabilization of Cu+ in the Zn-substituted CuI lattice, rather than hole compensation by the electrons generated from Zn2+ substitution into Cu+ sites. Our results show that the hole concentration of Zn-substituted CuI is tunable in the range of 1014-1018 cm-3, making it suitable for thin-film transistors and hole transport layers in OLEDs.

C60Br24/SWCNT: A Highly Sensitive Medium to Detect H2S via Inhomogeneous Carrier Doping.

H2S is a toxic and corrosive gas, whose accurate detection at sub-ppm concentrations is of high practical importance in environmental, industrial, and health safety applications. Herein, we propose a chemiresistive sensor device that applies a composite of single-walled carbon nanotubes (SWCNTs) and brominated fullerene (C60Br24) as a sensing component, which is capable of detecting 50 ppb H2S even at room temperature with an excellent response of 1.75% in a selective manner. In contrast, a poor gas response of pristine C60-based composites was found in control measurements. The experimental results are complemented by density functional theory calculations showing that C60Br24 in contact with SWCNTs induces localized hole doping in the nanotubes, which is increased further when H2S adsorbs on C60Br24 but decreases in the regions, where direct adsorption of H2S on the nanotubes takes place due to electron doping from the analyte. Accordingly, the heterogeneous chemical environment in the composite results in spatial fluctuations of hole density upon gas adsorption, hence influencing carrier transport and thus giving rise to chemiresistive sensing.

Soft x-ray irradiation induced metallization of layered TiNCl.

We have performed soft x-ray spectroscopy in order to study the photoirradiation time dependence of the valence band structure and chemical states of layered transition metal nitride chloride TiNCl. Under the soft x-ray irradiation, the intensities of the states near the Fermi level (EF) and the Ti3+component increased, while the Cl 2pintensity decreased. Ti 2p-3dresonance photoemission spectroscopy confirmed a distinctive Fermi edge with Ti 3dcharacter. These results indicate the photo-induced metallization originates from deintercalation due to Cl desorption, and thus provide a new carrier doping method that controls the conducting properties of TiNCl.

Surface multiferroics in silicon enabled by hole-carrier doping.

We predict a coexistence of magnetic and electric orders on clean Si(0 0 1) surfaces by first-principles calculations. Upon hole-carrier doping, the Si surfaces can be ferromagnetic, with polarized spins concentrated in an atom-thick space near the surface, due to an exchange splitting of localized s-like surface states on surface Si dimers. The surface magnetization can be controlled by reorienting the electric polarization of Si dimers, manifested as a transition from the magnetic antiferroelectric ground state to ferroelectric p(2 × 1) reconstruction that can be driven by an in-plane external electric field. The coupling between magnetic and electric orders can be further enhanced by strain silicon technology, rendering the Si surfaces as the first metal-free material displaying a multiferroic behavior.

Controllable Synthesis of Monodispersed Fe1- xS2 Nanocrystals for High-Performance Optoelectronic Devices.

The optical properties of stoichiometric iron pyrite (FeS2) nanocrystals (NCs) are characterized by strong UV-Visible (UV-Vis) absorption within the cutoff while negligible absorption beyond the cutoff in near-infrared and longer wavelengths. Herein, we show this bandgap limitation can be broken through controllable synthesis of nonstoichiometric Fe1- xS2 NCs ( x = 0.01-0.107) to induce localized surface plasmonic resonance (LSPR) absorption beyond the cutoff to short-wave infrared spectrum (SWIR, 1-3 µm) with remarkably enhanced broadband absorption across UV-Vis-SWIR spectra. To illustrate the benefit of the broadband absorption, colloidal LSPR Fe1- xS2 NCs were printed on graphene to form LSPR Fe1- xS2 NCs/graphene heterostructure photodetectors. Extraordinary photoresponsivity in exceeding 4.32 × 106 A/W and figure-of-merit detectivity D* > 7.50 × 1012 Jones have been demonstrated in the broadband of UV-Vis-SWIR at room temperature. These Fe1- xS2 NCs/graphene heterostructures are printable and flexible and therefore promising for practical optical and optoelectronic applications.

Red Phosphorus: An Elementary Semiconductor for Room-Temperature NO2 Gas Sensing.

Black and blue phosphorus (both allotropes of elementary phosphorus) have recently been widely explored as an active material for electronic devices, and their potential in gas sensing applications has been demonstrated. On the other hand, amorphous red phosphorus (a-RP), a much cheaper and readily available phosphorus allotrope, has seldom been investigated as an electronic material, and its gas sensing properties have never been studied. In this work we have investigated these properties of a-RP by combining experimental characterizations with theoretical calculations. We found that a-RP exhibited an amphoteric character for detecting both commonly regarded reducing and oxidizing gas molecules, featuring a negative correlation between the electrical resistance of a-RP and the gas concentration. Interestingly, the a-RP based sensors appear to be particularly suitable for room-temperature NO2 detection, exhibiting excellent sensitivity and selectivity, as well as fast temporal response and recovery. A unique sensing feature of a-RP toward NO2 was identified, which is associated with the expansion of P-P bonds upon NO2 chemisorption. Based on density functional theory calculations we proposed a physiochemical model to elaborate the synergistic effects of the P-P bond expansion and Langmuir isotherm adsorption on the electronic properties and gas sensing processes of a-RP.

Modification of graphene/SiO2 interface by UV-irradiation: effect on electrical characteristics.

Graphene is a promising material for next-generation electronic devices. The effect of UV-irradiation on the graphene devices, however, has not been fully explored yet. Here we investigate the UV-induced change of the field effect transistor (FET) characteristics of graphene/SiO2. UV-irradiation in a vacuum gives rise to the decrease in carrier mobility and a hysteresis in the transfer characteristics. Annealing at 160 °C in a vacuum eliminates the hysteresis, recovers the mobility partially, and moves the charge neutrality point to the negative direction. Corresponding Raman spectra indicated that UV-irradiation induced D band relating with defects and the annealing at 160 °C in a vacuum removed the D band. We propose a phenomenological model for the UV-irradiated graphene, in which photochemical reaction produces dangling bonds and the weak sp(3)-like bonds at the graphene/SiO2 interface, and the annealing restores the intrinsic graphene/SiO2 interface by removal of such bonds. Our results shed light to the nature of defect formation by UV-light, which is important for the practical performance of graphene based electronics.