Field-effect polymer gating of low-dimensionality carbon-based materials

The work focuses on the modification of the transport properties of low-dimensional carbon based materials by tuning their surface charge carrier density (up to values of induced charge exceeding 2·10^14 carriers/cm^2) via electrochemical gating with an innovative polymer electrolyte solution. Main subjects of the study are single- and few-layer graphene systems produced by micro-mechanical exfoliation and by CVD growth respectively. Attempts to modulate the superconducting critical temperature of the graphite intercalated compounds CaC6 are also made. In addition, polymer gating was also studied on highly oriented pyrolytic graphite, for the first time, to study its properties under high charge induction

Field-effect polymer gating of low-dimensionality carbon-based materials

The work focuses on the modification of the transport properties of low-dimensional carbon based materials by tuning their surface charge carrier density (up to values of induced charge exceeding 2·10^14 carriers/cm^2) via electrochemical gating with an innovative polymer electrolyte solution. Main subjects of the study are single- and few-layer graphene systems produced by micro-mechanical exfoliation and by CVD growth respectively. Attempts to modulate the superconducting critical temperature of the graphite intercalated compounds CaC6 are also made. In addition, polymer gating was also studied on highly oriented pyrolytic graphite, for the first time, to study its properties under high charge induction

Temperature Dependence of Electric Transport in Few-layer Graphene under Large Charge Doping Induced by Electrochemical Gating

The temperature dependence of electric transport properties of single-layer and few-layer graphene at large charge doping is of great interest both for the study of the scattering processes dominating the conductivity at different temperatures and in view of the theoretically predicted possibility to reach the superconducting state in such extreme conditions. Here we present the results obtained in 3-, 4- and 5-layer graphene devices down to 3.5 K, where a large surface charge density up to about 6.8x10^14 cm^(-2) has been reached by employing a novel polymer electrolyte solution for the electrochemical gating. In contrast with recent results obtained in single-layer graphene, the temperature dependence of the sheet resistance between 20 K and 280 K shows a low-temperature dominance of a T^2 component - that can be associated with electron-electron scattering - and, at about 100 K, a crossover to the classic electron-phonon regime. Unexpectedly this crossover does not show any dependence on the induced charge density, i.e. on the large tuning of the Fermi energy.Comment: 13 pages, 6 color figure

Huge field-effect surface charge injection and conductance modulation in metallic thin films by electrochemical gating

The field-effect technique, popular thanks to its application in common field-effect transistors, is here applied to metallic thin films by using as a dielectric a novel polymer electrolyte solution. The maximum injected surface charge, determined by a suitable modification of a classic method of electrochemistry called double-step chronocoulometry, reached more than 4 × 10^15 charges/cm^2. At room temperature, relative variations of resistance up to 8%, 1.9% and 1.6% were observed in the case of gold, silver and copper, respectively and, if the films are thick enough (>=25 nm), results can be nicely explained within a free-electron model with parallel resistive channels. The huge charge injections achieved make this particular field-effect technique very promising for a vast variety of materials such as unconventional superconductors, graphene and 2D-like material

Huge field-effect surface charge injection and conductance modulation in metal thin films by electrochemical gating

The field-effect technique, popular thanks to its application in common field-effect transistors, is here applied to metallic thin films by using as a dielectric a novel polymer electrolyte solution. The maximum injected surface charge, determined by a suitable modification of a classic method of electrochemistry called double-step chronocoulometry, reached more than 4 × 10^15 charges/cm^2. At room temperature, relative variations of resistance up to 8%, 1.9% and 1.6% were observed in the case of gold, silver and copper, respectively and, if the films are thick enough (>=25 nm), results can be nicely explained within a free-electron model with parallel resistive channels. The huge charge injections achieved make this particular field-effect technique very promising for a vast variety of materials such as unconventional superconductors, graphene and 2D-like material

MoS₂/WS₂ Heterojunction for Photoelectrochemical Water Oxidation

The solar-assisted oxidation of water is an essential half reaction for achieving a complete cycle of water splitting. The search of efficient photoanodes that can absorb light in the visible range is of paramount importance to enable cost-effective solar energy-conversion systems. Here, we demonstrate that atomically thin layers of MoS₂ and WS₂ can oxidize water to O₂ under incident light. Thin films of solution-processed MoS₂ and WS₂ nanosheets display <i>n</i>-type positive photocurrent densities of 0.45 mA cm^–2 and O₂ evolution under simulated solar irradiation. WS₂ is significantly more efficient than MoS₂; however, bulk heterojunctions (B-HJs) of MoS₂ and WS₂ nanosheets results in a 10-fold increase in incident-photon-to-current-efficiency, compared to the individual constituents. This proves that charge carrier lifetime is tailorable in atomically thin crystals by creating heterojunctions of different compositions and architectures. Our results suggest that the MoS₂ and WS₂ nanosheets and their B-HJ blend are interesting photocatalytic systems for water oxidation, which can be coupled with different reduction processes for solar-fuel production

Thickness-Dependent Characterization of Chemically Exfoliated TiS₂ Nanosheets

Monolayer TiS₂ is the lightest member of the transition metal dichalcogenide family with promising applications in energy storage and conversion systems. The use of TiS₂ has been limited by the lack of rapid characterization of layer numbers via Raman spectroscopy and its easy oxidation in wet environment. Here, we demonstrate the layer-number-dependent Raman modes for TiS₂. 1T TiS₂ presents two characteristics of the Raman active modes, A_1g (out-of-plane) and E_g (in-plane). We identified a characteristic peak frequency shift of the E_g mode with the layer number and an unexplored Raman mode at 372 cm^–1 whose intensity changes relative to the A_1g mode with the thickness of the TiS₂ sheets. These two characteristic features of Raman spectra allow the determination of layer numbers between 1 and 5 in exfoliated TiS₂. Further, we develop a method to produce oxidation-resistant inks of micron-sized mono- and few-layered TiS₂ nanosheets at concentrations up to 1 mg/mL. These TiS₂ inks can be deposited to form thin films with controllable thickness and nanosheet density over square centimeter areas. This opens up pathways for a wider utilization of exfoliated TiS₂ toward a range of applications