In addition to graphene, 2D transition-metal chalcogenides as, e.g., MoS2 and WS2 nanostructures are promising materials for applications in electronics and mechanical engineering. Though the structure of these materials causes a highly inert surface with a low defect concentration, defects and edge effects can strongly influence the properties of these nanostructured materials. Therefore, a basic understanding of the interplay between electronic and mechanical properties and the influence of defects, edge states and doping is needed. We demonstrate on the basis of atomistic quantum-chemical simulations of a circular MoS2 platelet, how the mechanical deformation can vary the electronic properties and other device characteristics of such a system.

Carbon-based nanostructures have been the center of intense research and development for more than two decades now. Of these materials, graphene, a two-dimensional (2D) layered material system, has had a significant impact on science and technology in recent years after it was experimentally isolated in single layers in 2004. The recent emergence of other classes of 2D layered systems beyond graphene has added yet more exciting and new dimensions for research and exploration given their diverse and rich spectrum of properties. For example, h-BN a layered material closest in structure to graphene, is an insulator, while NbSe, a transition metal dichalcogenide is metallic and monolayers of other transition metal di-chalcogenides such as MoS2 are direct band-gap semiconductors. The rich variety of properties that 2D layered material systems offer can potentially be engineered on-demand, and creates exciting prospects for their device and technological applications ranging from electronics, sensing, photonics, energy harvesting and flexible electronics in the near future.

Density functional theory and statistical calculations are combined to address the chemical stability and structure of epoxy functionalizations of single-layer graphene. Our computations show that at oxidation levels of O:C<0.5, the Gibbs free energy of formation per epoxide amounts to about 0.6 eV, and the structure of the epoxy functionalizations presents local order and long-range disorder. The positive energy value indicates that in air at p=1 bar and room temperature, epoxy functionalizations of graphene are unstable and prone to spontaneous reduction. Our calculations show also that formation and release of O2 is a slow process whose kinetics is controlled by large energy barriers, the formation of very stable intermediate species, and unlikely electronic transitions.

In this paper, a microporous-containing graphene oxide/polystyrene (M-GO/PS) was designed and prepared by surface-initiated atom transfer radical polymerization (SI-ATRP) of PS from GO surface and then crossrlinking by carbon tetrachloride. The structures of the molecular brush of PS and the related crosslinking M-GO/PS were determined by FTIR, TG, SEM and nitrogen adsorption-desorption analysis. The experimental results showed that PS molecular brush were successfully grown on to the surface of GO. After crosslinking, the PS component was crosslinked into many round nanoparticles with a diameter of 20-30 nm, and therefore the specific surface area of GO/PS obviously increased. This kind of porous M-GO/PS composite was promising for the application in adsorption-desorption energy storage areas.

Mechanical strain creates strong gauge fields in graphene, offering the possibility of controlling its electronic properties. We developed a gauge field theory on a honeycomb lattice valid beyond first-order continuum elasticity. Along the way, we resolve a recent controversy on the theory of strain engineering in graphene: there are no K-point dependent gauge fields.

We report the effects of electron irradiation on graphene Field Effect Transistor (FET) devices. We irradiated the graphene devices with 30keV electrons and measured the electrical transport properties in high vacuum in-situ. Upon electron irradiation, a Raman ‘D’ band appears. In addition, we observed that the doping behavior of the graphene devices changed from P to N type as a result of the irradiation. We also observed a shift of the Dirac point while the graphene FET device stays in vacuum and after it interacted with environmental molecules under ambient conditions.

In recent years, there has been much interest in modelling graphene nanoribbons as they have great potential for use in molecular electronics. We have employed the NEGF formalism to determine the conductivity of graphene nanoribbons in various configurations. The electronic structure calculations were performed within the framework of the Extended Huckel Approximation. Both zigzag and armchair nanoribbons have been considered. In addition, we have also computed the transmission and conductance using the non-equilibrium Greens function formalism for these structures. We also investigated the effect of defects by considering a zigzag nanoribbon with six carbon atoms removed. Finally, the effect of embedding boron nitride aromatic molecules in the nanoribbon has been considered. The results of our calculations are compared with that obtained from recent work carried out using tight-binding model Hamiltonians.

Transparent films of platinum nanoparticles on graphene nanohybrids were synthesized in a two-step process. Reduction of homogeneously dispersed Pt precursor and graphene in water and solution coating/annealing afforded thin films with high catalytic performance as counter electrodes in dye-sensitized solar cells (DSSC). The requisite dispersant consisting of poly(oxyethylene)-(POE) segments and cyclic imide functionalities allowed the in-situ reduction of dihydrogen hexachloroplatinate by ethanol and the formation of nanohybrids of graphene-supported Pt nanoparticles at 4.0 nm diameter. Characterizations of polymeric dispersants by Fourier-transform infrared spectroscopy, thermogravimetric analysis, and nanohybrids by transmission electron microscope were performed. After screening various compositions of Pt/graphene, the nanohybrid film at the specific ratio of 5/1 by weight was fabricated into a counter electrode (CE) for DSSC by the solution casting method. The evaluation of cell performance demonstrated the most improved power conversion efficiency of 8.00%. This is significant achievement in comparison with 7.14% for the DSSC with the conventional platinum sputtered CE. Furthermore, the solution casting method allows the preparation of transparent CE films that are suitable for using as rear-illuminated DSSC. The approach was proven to be feasible by measuring the cell efficiency under rear light illumination. The power efficiency up to 7.01%, comparable to 8.00% by a normally front illumination, has been accomplished. In contrast, the rear illumination at merely 2.36% efficiency was obtained for the DSSC with sputtered platinum CE. Analyses of cyclic voltammetry, electrochemical impedance spectra were well correlated to the high efficiency of the performance caused by this nanohybrid film.

Graphyne is a generic name for a family of carbon allotrope two-dimensional structures where sp2 (single and double bonds) and sp (triple bonds) hybridized states coexists. They exhibit very interesting electronic and mechanical properties sharing some of the unique graphene characteristics. Similarly to graphene, the graphyne electronic properties can be modified by chemical functionalization, such as; hydrogenation, fluorination and oxidation. Oxidation is of particular interest since it can produce significant structural damages.

In this work we have investigated, through fully atomistic reactive molecular dynamics simulations, the dynamics and structural changes of the oxidation of single-layer graphyne membranes at room temperature. We have considered α, β, and γ-graphyne structures. Our results showed that the oxidation reactions are strongly site dependent and that the sp-hybridized carbon atoms are the preferential sites to chemical attacks. Our results also showed that the effectiveness of the oxidation (estimated from the number of oxygen atoms covalently bonded to carbon atoms) follows the α, β, γ-graphyne structure ordering. These differences can be explained by the fact that for α-graphyne structures the oxidation reactions occur in two steps: first, the oxygen atoms are trapped at the center of the large polygonal rings and then they react with the carbon atoms composing of the triple bonds. The small rings of γ-graphyne structures prevent these reactions to occur. The effectiveness of β-graphyne oxidation is between the α- and γ-graphynes.

Graphene is a two-dimensional (2D) hexagonal array of carbon atoms in sp2-hybridized states. Graphene presents unique and exceptional electronic, thermal and mechanical properties. However, in its pristine state graphene is a gapless semiconductor, which poses some limitations to its use in some transistor electronics. Because of this there is a renewed interest in other possible two-dimensional carbon-based structures similar to graphene. Examples of this are graphynes and graphdiynes, which are two-dimensional structures, composed of carbon atoms in sp2 and sp-hybridized states. Graphdiynes (benzenoid rings connecting two acetylenic groups) were recently synthesized and they can be intrinsically nonzero gap systems. These systems can be easily hydrogenated and the amount of hydrogenation can be used to tune the band gap value. In this work we have investigated, through fully atomistic molecular dynamics simulations with reactive force field (ReaxFF), the structural and dynamics aspects of the hydrogenation mechanisms of graphdiyne membranes. Our results showed that depending on whether the atoms are in the benzenoid rings or as part of the acetylenic groups, the rates of hydrogenation are quite distinct and change in time in a very complex pattern. Initially, the most probable sites to be hydrogenated are the carbon atoms forming the triple bonds, as expected. But as the amount of hydrogenation increases in time this changes and then the carbon atoms forming single bonds become the preferential sites. The formation of correlated domains observed in hydrogenated graphene is no longer observed in the case of graphdiynes. We have also carried out ab initio DFT calculations for model structures in order to test the reliability of ReaxFF calculations.

The atomic and electronic structures of multilayer graphene on a monolayer boron nitride (MLBN) have been investigated by using the pseudopotential method and the local density approximation (LDA) of the density functional theory (DFT). We show that the LDA energy band gap can be tuned in the range 41-278 meV for a multilayer graphene by using MLBN as a substrate. The dispersion of the π/π* bands slightly away from the K point is linear with the electron speed of 0.9×106 and 0.93×106 for graphene (MLG)/MLBN and ABA trilayer graphene (TLG)/MLBN systems, respectively. This behaviour becomes quadratic with a relative effective mass of 0.0021 for the bilayer graphene (BLG)/MLBN system. The calculated binding energies are in the range of 10-43 meV per C atom.

Making field effect transistors (FETs) on thin flake of single crystal isolated from layered materials was pioneered by the success of graphene. To overcome the difficulties of the zero band gap in graphene electronics, we report the fabrication of an electric double layer (EDL) transistor, a variant of FET, based on another layered material, MoS2. Using strong carrier tunability found in EDL coupled by ion movement, MoS2 transistor displayed an unambiguously ambipolar operation in addition to its commonly observed n-type transport. A high on/off ratio >104, large “ON” state conductivity of ∼mS, and a high reachable n 2D ∼ 1×1014 cm-2 confirmed the high performance transistor operation being important for application. The high-density carriers of both holes and electrons can drive the MoS2 channel to metallic states indicating that new electronic phases could be accessed using the protocol established in making EDL gated transistors on layered materials.

We fabricated MoS2 transistor adopting electric double layer (EDL) as gate dielectric. So far, EDL has realized p-type conducting MoS2 in addition to well-known n-type conduction showing ambipolar operation. In our study, field-effect superconducting transition of MoS2 was realized with maximum T C around 10 K. This T C is the highest not only within MoS2 compounds but also among whole TMDs. The highest T C discovered in this study lies in the carrier density region much smaller than chemically investigated region. Such compounds with small doping level have never been successfully synthesized by chemical method. Furthermore, by combining HfO2 (typical high-k material for FETs) gating with EDL gating, continuous control of carrier density, and thus quantum phase, was demonstrated. As a result, we successfully obtained the phase diagram of MoS2. Interestingly, the T C exhibits strong carrier density dependence, showing dome-shaped superconducting phase. Superconducting dome in other materials than cuprates has been reported only a few times in doped 2D semiconductors. Since FET charge accumulation is basically two dimensional, our result implies the existence of common mechanism for superconducting dome in 2D band insulators.

We report on the direct synthesis of multi-layer boron nitride nanosheets (BNNSs) and their electron microscopic characterization. The synthesis process is carried out by irradiating hexagonal boron nitride (h-BN) target using short laser pulses. Scanning electron microscopy showed large area (≈50×50 μm2) flat layers of BNNSs transparent to the electron beam. Low magnification transmission electron microscope (TEM) is used to characterize different areas of nanosheets. TEM revealed that each individual nanosheet is composed of several layers. High resolution TEM (HRTEM) measurements confirmed the layered structure. HRTEM analysis of the edge of a nanosheet showed 10 layers from which we obtained the thickness (3.3nm) of an individual nanosheet. Selected area electron diffraction pattern indicated polycrystalline structure of nanosheets. Raman spectroscopy clearly identified E2g vibrational mode related to h-BN.

Hexagonal boron nitride (h-BN), also known as white graphite, is the inorganic analogue of graphite. Single layers of both structures have been already experimentally realized.

In this work we have investigated, through fully atomistic reactive molecular dynamics simulations, the dynamics of hydrogenation of h-BN single-layers membranes.

Our results show that the rate of hydrogenation atoms bonded to the membrane is highly dependent on the temperature and that only at low temperatures there is a preferential bond to boron atoms. Unlike graphanes (hydrogenated graphene), hydrogenated h-BN membranes do not exhibit the formation of correlated domains. Also, the out-of-plane deformations are more pronounced in comparison with the graphene case. After a critical number of incorporated hydrogen atoms the membrane become increasingly defective, lost its two-dimensional character and collapses. The hydrogen radial pair distribution and second-nearest neighbor correlations were also analyzed.

The advent of graphene created a new era in materials science. Graphene is a two-dimensional planar honeycomb array of carbon atoms in sp2-hybridized states. A natural question is whether other elements of the IV-group of the periodic table (such as silicon and germanium), could also form graphene-like structures. Structurally, the silicon equivalent to graphene is called silicene. Silicene was theoretically predicted in 1994 and recently experimentally realized by different groups. Similarly to graphene, silicene exhibits electronic and mechanical properties that can be exploited to nanoelectronics applications.

In this work we have investigated, through fully atomistic molecular dynamics (MD) simulations, the mechanical properties of single-layer silicene under mechanical strain. These simulations were carried out using a reactive force field (ReaxFF), as implemented in the LAMMPS code. We have calculated the elastic properties and the fracture patterns.

Our results show that the dynamics of the whole fracturing processes of silicene present some similarities with that of graphene as well as some unique features.

Electrodes made of single-walled carbon nanotubes (SWCNTs) chemically modified by a series of anthraquinone derivatives (AQ) have been prepared and characterized by cyclic voltammetry in 0.1M H2SO4, using the standard 3 electrode set-up and by Raman spectroscopy. It has been demonstrated that a AQ modified SWCNT electrode provided between 114 to 220% higher specific capacitance, compared to pristine SWCNT electrode, depending on the length of the spacer between SWCNT and AQ.

Concentration- and layer-dependent percolation thresholds can be determined for carbon nanotube (CNT) films deposited from aqueous dispersions on paper substrates at both the surface of the deposited film (in-plane) and through the thickness of the paper (thru-plane) using impedance spectroscopy. By analyzing the impedance spectra as a function of the number of layers (solution concentration is constant) or the solution concentration (number of layers is constant), the electrical properties and percolation thresholds for CNT-paper composites can be determined. In-plane measurements show that percolation occurs at 4 layers when 1 mg/mL solution concentration is used. In the thru-plane direction, the films are already percolated at 1 mg/mL concentration, which is confirmed by varying the concentration of the solution used to deposit 1 layer films. A second percolation event happens between 8 and 12 layers due to an increased number of interconnections of CNTs within the paper substrate. The lowest sheet resistance achieved was 100 Ω/□.

The electric resistance and the transport properties of a carbon nanotube (5,5) adsorbed with a copper chain connected with two copper end electrodes have been calculated by employing the nonequilibrium Green’s function and the Density Function Theory. The properties of the pure carbon nanotube (5,5) with the Cu electrodes have also been calculated as a reference. Both the equilibrium and the nonequilibrium conditions have been investigated. The results have shown that the electrical resistance of the metallic CNT (5,5) has been reduced by the adsorption of the Cu chain due to the interaction between the Cu and the CNT. The change of the I-V curve slope is also explained in terms of the transmission spectrum.

Novel field emission (FE) devices are introduced employing lateral architecture. Ultrathin multiwalled carbon nanotube (MWCNT) sheet were utilized to fabricate the emitter. Effects of basic configuration of sheets, including the orientation of CNTs and sheet thickness were examined. The novel device achieved the threshold field (the electric field at which current density reach 1 mA/cm2) of 0.67 V/µm and enhancement factor larger than 20,000.