Ultralong natural graphene nanoribbons and their electrical conductivity

AFM Manipulation of Gold Nanowires To Build Electrical Circuits

We introduce scanning-probe-assisted nanowire circuitry (SPANC) as a new method to fabricate electrodes for the characterization of electrical transport properties at the nanoscale. SPANC uses an atomic force microscope (AFM) to manipulate nanowires to create complex and highly conductive nanostructures (paths) that work as nanoelectrodes, allowing connectivity and electrical characterization of other nano-objects. The paths are formed by the spontaneous cold welding of gold nanowires upon mechanical contact, leading to an excellent contact resistance of ∼9 Ω/junction. SPANC is an easy to use and cost-effective technique that fabricates clean nanodevices. Hence, this new method can complement and/or be an alternative to other well-established methods to fabricate nanocircuits such as electron beam lithography (EBL). The circuits made by SPANC are easily reconfigurable, and their fabrication does not require the use of polymers and chemicals. In this work, we present a few examples that illustrate the capabilities of this method, allowing robust device fabrication and electrical characterization of several nano-objects with sizes down to ∼10 nm, well below the current smallest size able to be contacted in a device using the standard available technology (∼30 nm). Importantly, we also provide the first experimental determination of the sheet resistance of thin antimonene flakes.

Electronic and Optoelectronic Applications Based on 2D Novel Anisotropic Transition Metal Dichalcogenides

With the continuous exploration of 2D transition metal dichalcogenides (TMDs), novel high-performance devices based on the remarkable electronic and optoelectronic natures of 2D TMDs are increasingly emerging. As fresh blood of 2D TMD family, anisotropic MTe2 and ReX2 (M = Mo, W, and X = S, Se) have drawn increasing attention owing to their low-symmetry structures and charming properties of mechanics, electronics, and optoelectronics, which are suitable for the applications of field-effect transistors (FETs), photodetectors, thermoelectric and piezoelectric applications, especially catering to anisotropic devices. Herein, a comprehensive review is introduced, concentrating on their recent progresses and various applications in recent years. First, the crystalline structure and the origin of the strong anisotropy characterized by various techniques are discussed. Specifically, the preparation of these 2D materials is presented and various growth methods are summarized. Then, high-performance applications of these anisotropic TMDs, including FETs, photodetectors, and thermoelectric and piezoelectric applications are discussed. Finally, the conclusion and outlook of these applications are proposed.

A Fully Transparent Flexible Sensor for Cryogenic Temperatures Based on High Strength Metallurgical Graphene

Low-temperature electronics operating in below zero temperatures or even below the lower limit of the common -65 to 125 °C temperature range are essential in medical diagnostics, in space exploration and aviation, in processing and storage of food and mainly in scientific research, like superconducting materials engineering and their applications-superconducting magnets, superconducting energy storage, and magnetic levitation systems. Such electronic devices demand special approach to the materials used in passive elements and sensors. The main goal of this work was the implementation of a fully transparent, flexible cryogenic temperature sensor with graphene structures as sensing element. Electrodes were made of transparent ITO (Indium Tin Oxide) or ITO/Ag/ITO conductive layers by laser ablation and finally encapsulated in a polymer coating. A helium closed-cycle cryostat has been used in measurements of the electrical properties of these graphene-based temperature sensors under cryogenic conditions. The sensors were repeatedly cooled from room temperature to cryogenic temperature. Graphene structures were characterized using Raman spectroscopy. The observation of the resistance changes as a function of temperature indicates the potential use of graphene layers in the construction of temperature sensors. The temperature characteristics of the analyzed graphene sensors exhibit no clear anomalies or strong non-linearity in the entire studied temperature range (as compared to the typical carbon sensor).

Strain engineering in semiconducting two-dimensional crystals

One of the fascinating properties of the new families of two-dimensional crystals is their high stretchability and the possibility to use external strain to manipulate, in a controlled manner, their optical and electronic properties. Strain engineering, understood as the field that study how the physical properties of materials can be tuned by controlling the elastic strain fields applied to it, has a perfect platform for its implementation in the atomically thin semiconducting materials. The object of this review is to give an overview of the recent progress to control the optical and electronics properties of 2D crystals, by means of strain engineering. We will concentrate on semiconducting layered materials, with especial emphasis in transition metal dichalcogenides (MoS2, WS2, MoSe2 and WSe2). The effect of strain in other atomically thin materials like black phosphorus, silicene, etc, is also considered. The benefits of strain engineering in 2D crystals for applications in nanoelectronics and optoelectronics will be revised, and the open problems in the field will be discussed.

Graphene in light: design, synthesis and applications of photo-active graphene and graphene-like materials

Graphene functionalized with photo-active units has become one of the most exciting topics of research in the last few years, which remarkably sustains and expands the graphene boom. The rise of photo-active graphene in photonics and optoelectronics is evidenced by a spate of recent reports on topics ranging from photodetectors, photovoltaics, and optoelectronics to photocatalysis. For these applications, the fabrication of photo-active graphene through appropriate chemical functionalization strategies is essential as pristine graphene has zero bandgap and only weak absorption of photons. Written from the chemists' point of view, up-to-date chemical functionalization of graphene with various small organic molecules, conjugated polymers, rare-earth components, and inorganic semiconductors is reviewed. Particular attention is paid to the development of graphene functionalized with light-harvesting moieties, including materials synthesis, characterization, energy/charge-transfer processes, and applications in photovoltaics. Challenges currently faced by researchers and future perspectives in this field are also discussed.

Voltage-dependent conductance of a single graphene nanoribbon

Graphene nanoribbons could potentially be used to create molecular wires with tailored conductance properties. However, understanding charge transport through a single molecule requires length-dependent conductance measurements and a systematic variation of the electrode potentials relative to the electronic states of the molecule. Here, we show that the conductance properties of a single molecule can be correlated with its electronic states. Using a scanning tunnelling microscope, the electronic structure of a long and narrow graphene nanoribbon, which is adsorbed on a Au(111) surface, is spatially mapped and its conductance then measured by lifting the molecule off the surface with the tip of the microscope. The tunnelling decay length is measured over a wide range of bias voltages, from the localized Tamm states over the gap up to the delocalized occupied and unoccupied electronic states of the nanoribbon. We also show how the conductance depends on the precise atomic structure and bending of the molecule in the junction, illustrating the importance of the edge states and a planar geometry.

Mechanical properties of freely suspended semiconducting graphene-like layers based on MoS2

We fabricate freely suspended nanosheets of molybdenum disulphide (MoS2) which are characterized by quantitative optical microscopy and high-resolution friction force microscopy. We study the elastic deformation of freely suspended nanosheets of MoS2 using an atomic force microscope. The Young's modulus and the initial pre-tension of the nanosheets are determined by performing a nanoscopic version of a bending test experiment. MoS2 sheets show high elasticity and an extremely high Young's modulus (0.30 TPa, 50% larger than steel). These results make them a potential alternative to graphene in applications requiring flexible semiconductor materials.PACS, 73.61.Le, other inorganic semiconductors, 68.65.Ac, multilayers, 62.20.de, elastic moduli, 81.40.Jj, elasticity and anelasticity, stress-strain relations.

Atomically thin mica flakes and their application as ultrathin insulating substrates for graphene

By mechanical exfoliation, it is possible to deposit atomically thin mica flakes down to single-monolayer thickness on SiO2/Si wafers. The optical contrast of these mica flakes on top of a SiO2/Si substrate depends on their thickness, the illumination wavelength, and the SiO2 substrate thickness, and can be quantitatively accounted for by a Fresnel-law-based model. The preparation of atomically thin insulating crystalline sheets will enable the fabrication of ultrathin, defect-free insulating substrates, dielectric barriers, or planar electron-tunneling junctions. Additionally, it is shown that few-layer graphene flakes can be deposited on top of a previously transferred mica flake. Our transfer method relies on viscoelastic stamps, as used for soft lithography. A Raman spectroscopy study shows that such an all-dry deposition technique yields cleaner and higher-quality flakes than conventional wet-transfer procedures based on lithographic resists.

Electrical transport properties of graphene nanoribbons produced from sonicating graphite in solution

A simple one-stage solution-based method was developed to produce graphene nanoribbons by sonicating graphite powder in organic solutions with polymer surfactant. The graphene nanoribbons were deposited on a silicon substrate, and characterized by Raman spectroscopy and atomic force microscopy. Single-layer and few-layer graphene nanoribbons with a width ranging from sub-10 nm to tens of nanometers and lengths ranging from hundreds of nanometers to 1 µm were routinely observed. The electrical transport properties of individual graphene nanoribbons were measured in both the back-gate and polymer-electrolyte top-gate configurations. The mobility of the graphene nanoribbons was found to be over an order of magnitude higher when measured in the latter than in the former configuration (without the polymer-electrolyte), which can be attributed to the screening of the charged impurities by the counter ions in the polymer-electrolyte. This finding suggests that the charge transport in these solution produced graphene nanoribbons is largely limited by charge impurity scattering.

Room-temperature high on/off ratio in suspended graphene nanoribbon field-effect transistors

We have fabricated suspended few-layer (1-3 layers) graphene nanoribbon field-effect transistors from unzipped multi-wall carbon nanotubes. Electrical transport measurements show that current annealing effectively removes the impurities on the suspended graphene nanoribbons, uncovering the intrinsic ambipolar transfer characteristic of graphene. Further increasing the annealing current creates a narrow constriction in the ribbon, leading to the formation of a large bandgap and subsequent high on/off ratio (which can exceed 10(4)). Such fabricated devices are thermally and mechanically stable: repeated thermal cycling has little effect on their electrical properties. This work shows for the first time that ambipolar field-effect characteristics and high on/off ratios at room temperature can be achieved in relatively wide graphene nanoribbons (15-50 nm) by controlled current annealing.

Chemical functionalization effects on armchair graphene nanoribbon transport

We report first-principles transport calculations in chemically functionalized graphene nanoribbons. The effect of the joint attachment of hydroxyl and hydrogen groups on the graphene surface is investigated as a function of defect location and coverage density. The chemical bonding of a single defect pair (C-OH and C-H) is shown to considerably alter the conduction capability of ribbon channels, similarly to an sp(3) type of defect. With transport calculations in disordered ribbons with lengths up to the micrometer scale, the elastic mean free paths and conduction regimes are analyzed. Even in the low grafting density limit, transport properties are found to be severely damaged by the functionalization, indicating a strong tendency toward an insulating regime.