Single-layered assembly of vanadium pentoxide nanowires on graphene for nanowire-based lithography technique.

Graphene nanoribbon (GNR)-based materials are a promising device material because of their potential high carrier mobility and atomically thin structure. Various approaches have been reported for preparing the GNR-based materials, from bottom-up chemical synthetic procedures to top-down fabrication techniques using lithography of graphene. However, it is still difficult to prepare a large-scale GNR-based material. Here, we develop a procedure to prepare a large-scale GNR network using networked single-layer inorganic nanowires. Vanadium pentoxide (V2O5) nanowires were assembled on graphene with an interfacial layer of a cationic polymer via electrostatic interaction. A large-scale nanowire network can be prepared on graphene and is stable enough for applying an oxygen plasma. Using plasma etching, a networked graphene structure can be generated. Removing the nanowires results in a networked flat structure whose both surface morphology and Raman spectrum indicate a GNR networked structure. The field-effect device indicates the semiconducting character of the GNR networked structure. This work would be useful for fabricating a large-scale GNR-based material as a platform for GNR junctions for physics and electronic circuits.

Photochemical reaction on graphene surfaces controlled by substrate-surface modification with polar self-assembled monolayers.

The unique thinness of two-dimensional materials enables control over chemical phenomena at their surfaces by means of various gating techniques. For example, gating methods based on field-effect-transistor configurations have been achieved. Here, we report a molecular gating approach that employs a local electric field generated by a polar self-assembled monolayer formed on a supporting substrate. By performing Raman scattering spectroscopy analyses with a proper data correction procedure, we found that molecular gating is effective for controlling solid phase photochemical reactions of graphene with benzoyl peroxide. Molecular gating offers a simple method to control chemical reactions on the surfaces of two-dimensional materials because it requires neither the fabrication of a transistor structure nor the application of an external voltage.

Barrier Formation at the Contacts of Vanadium Dioxide and Transition-Metal Dichalcogenides.

Phase-transition field-effect transistors (FETs) are a class of steep-slope devices that show abrupt on/off switching owing to the metal-insulator transition (MIT) induced in the contacting materials. An important avenue to develop phase-transition FETs is to understand the charge injection mechanism at the junction of the contacting MIT materials and semiconductor channels. Here, toward the realization of high-performance phase-transition FETs, we investigate the contact properties of heterojunctions between semiconducting transition-metal dichalcogenides (TMDCs) and vanadium dioxide (VO2) that undergoes a MIT at a critical temperature (Tc) of approximately 340 K. We fabricated transistors based on molybdenum disulfide (MoS2) and tungsten diselenide (WSe2) in contact with the VO2 source/drain electrodes. The VO2-contacted MoS2 transistor exhibited n-type transport both below and above Tc. Across the MIT, the on-current was observed to increase only by a factor of 5, in contrast to the order-of-magnitude change in the resistance of the VO2 electrodes, suggesting the existence of high contact resistance. The Arrhenius analyses of the gate-dependent drain current confirmed the formation of the interfacial barrier at the VO2/MoS2 contacts, irrespective of the phase state of VO2. The VO2-contacted WSe2 transistor showed ambipolar transport, indicating that the Fermi level lies near the mid gap of WSe2. These observations provide insights into the contact properties of phase-transition FETs based on VO2 and TMDCs and suggest the need for contact engineering for high-performance operations.

Growth of vanadium dioxide thin films on hexagonal boron nitride flakes as transferrable substrates.

Vanadium dioxide (VO2) is an archetypal metal-insulator transition (MIT) material, which has been known for decades to show an orders-of-magnitude change in resistivity across the critical temperature of approximately 340 K. In recent years, VO2 has attracted increasing interest for electronic and photonic applications, along with advancement in thin film growth techniques. Previously, thin films of VO2 were commonly grown on rigid substrates such as crystalline oxides and bulk semiconductors, but the use of transferrable materials as the growth substrates can provide versatility in applications, including transparent and flexible devices. Here, we employ single-crystalline hexagonal boron nitride (hBN), which is an insulating layered material, as a substrate for VO2 thin film growth. VO2 thin films in the polycrystalline form are grown onto hBN thin flakes exfoliated onto silicon (Si) with a thermal oxide, with grains reaching up-to a micrometer in size. The VO2 grains on hBN are orientated preferentially with the (110) surface of the rutile structure, which is the most energetically favorable. The VO2 film on hBN shows a MIT at approximately 340 K, across which the resistivity changes by nearly three orders of magnitude, comparable to VO2 films grown on common substrates such as sapphire and titanium dioxide. The VO2/hBN stack can be picked up from the supporting Si and transferred onto arbitrary substrates, onto which VO2 thin films cannot be grown directly. Our results pave the way for new possibilities for practical and versatile applications of VO2 thin films in electronics and photonics.

Gate-Tunable Thermal Metal-Insulator Transition in VO2 Monolithically Integrated into a WSe2 Field-Effect Transistor.

Vanadium dioxide (VO2) shows promise as a building block of switching and sensing devices because it undergoes an abrupt metal-insulator transition (MIT) near room temperature, where the electrical resistivity changes by orders of magnitude. A challenge for versatile applications of VO2 is to control the MIT by gating in the field-effect device geometry. Here, we demonstrate a gate-tunable abrupt switching device based on a VO2 microwire that is monolithically integrated with a two-dimensional (2D) tungsten diselenide (WSe2) semiconductor by van der Waals stacking. We fabricated the WSe2 transistor using the VO2 wire as the drain contact, titanium as the source contact, and hexagonal boron nitride as the gate dielectric. The WSe2 transistor was observed to show ambipolar transport, with higher conductivity in the electron branch. The electron current increases continuously with gate voltage below the critical temperature of the MIT of VO2. Near the critical temperature, the current shows an abrupt and discontinuous jump at a given gate voltage, indicating that the MIT in the contacting VO2 is thermally induced by gate-mediated self-heating. Our results have paved the way for the development of VO2-based gate-tunable devices by the van der Waals stacking of 2D semiconductors, with great potential for electronic and photonic applications.

Contact resistance at planar metal contacts on bilayer graphene and effects of molecular insertion layers.

The possible origins of metal-bilayer graphene (BLG) contact resistance are investigated by taking into consideration the bandgap formed by interfacial charge transfer at the metal contacts. Our results show that a charge injection barrier (Schottky barrier) does not contribute to the contact resistance because the BLG under the contacts is always degenerately doped. We also showed that the contact-doping-induced increase in the density of states (DOS) of BLG under the metal contacts decreases the contact resistance owing to enhanced charge carrier tunnelling at the contacts. The contact doping can be enhanced by inserting molecular dopant layers into the metal contacts. However, carrier tunnelling through the insertion layer increases the contact resistance, and thus, alternative device structures should be employed. Finally, we showed that the inter-band transport by variable range hopping via in-gap states is the largest contributor to contact resistance when the carrier type of the gated channel is opposite to the contact doping carrier type. This indicates that the strategy of contact resistance reduction by the contact-doping-induced increase in the DOS is effective only for a single channel transport branch (n- or p-type) depending on the contact doping carrier type.

Substituent-Controlled Reversible Switching of Charge Injection Barrier Heights at Metal/Organic Semiconductor Contacts Modified with Disordered Molecular Monolayers.

Electrically stimulated switching of a charge injection barrier at the interface between an organic semiconductor and an electrode modified with a disordered monolayer (DM) is studied by using various benzenethiol derivatives as DM molecules. The switching behavior is induced by a structural change in the DM molecules and is manifested as a reversible inversion of the polarity of DM-modified Au electrode/rubrene/DM-modified Au electrode diodes. The switching direction is found to be dominantly determined by the push-back effect of the thiol bonding group, while the terminal group modulates the switching strength. A device with 1,2-benzenedithiol DMs exhibited the highest switching ratios of 20, 10(2), and 10(3) for the switching voltages of 3, 5, and 7 V, respectively. A variation in the tilt angle of benzenethiol DMs due to the application of 7 V is estimated to be smaller than 23.6° by model calculations. This study offers an understanding for obtaining highly stable operations of organic electronic devices, especially with molecular modification layers.

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.

Effects of electron-transfer chemical modification on the electrical characteristics of graphene.

Because of the large reactivity of single layer graphene to electron-transfer chemistries, 4-nitrobenzene diazonium tetrafluoroborate is employed to modify the electrical properties of graphene field-effect transistors. After modification, the transfer characteristics of chemically modified graphene show a reduction in the minimum conductivity, electron-hole mobility asymmetry, a decrease in the electron/hole mobility, and a positive shift of the charge neutrality point with broadening of the minimum conductivity region. These phenomena are attributed to a dediazoniation reaction and the adsorbates on the graphene surface.

Ring of C60 polymers formed by electron or hole injection from a scanning tunneling microscope tip.

Carrier (electron or hole) injection from a scanning tunneling microscope tip causes various surface modifications on the molecular scale. We report that injection into C60 close-packed layers forms a ring-shaped distribution of C60 polymers. This can be explained on the basis of the radial propagation and energy dissipation of carriers. Subsequent electron or hole injections enlarge the ring, showing that both carriers can induce both polymerization and depolymerization. Furthermore, we demonstrate visualization of carrier scattering by injecting carriers into C60 layers with grain boundaries.