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Despite extensive research on the tribological properties of MoS2, the frictional characteristics of other members of the transition-metal dichalcogenide (TMD) family have remained relatively unexplored. To understand the effect of the chalcogen on the tribological behavior of these materials and gain broader general insights into the factors controlling friction at the nanoscale, we compared the friction force behavior for a nanoscale single asperity sliding on MoS2, MoSe2, and MoTe2 in both bulk and monolayer forms through a combination of atomic force microscopy experiments and molecular dynamics simulations. Experiments and simulations showed that, under otherwise identical conditions, MoS2 has the highest friction among these materials and MoTe2 has the lowest. Simulations complemented by theoretical analysis based on the Prandtl-Tomlinson model revealed that the observed friction contrast between the TMDs was attributable to their lattice constants, which differed depending on the chalcogen. While the corrugation amplitudes of the energy landscapes are similar for all three materials, larger lattice constants permit the tip to slide more easily across correspondingly wider saddle points in the potential energy landscape. These results emphasize the critical role of the lattice constant, which can be the determining factor for frictional behavior at the nanoscale.

Ultralow friction can be achieved with 2D materials, particularly graphene and MoS2. The nanotribological properties of these different 2D materials have been measured in previous atomic force microscope (AFM) experiments sequentially, precluding immediate and direct comparison of their frictional behavior. Here, friction is characterized at the nanoscale using AFM experiments with the same tip sliding over graphene, MoS2, and a graphene/MoS2 heterostructure in a single measurement, repeated hundreds of times, and also measured with a slowly varying normal force. The same material systems are simulated using molecular dynamics (MD) and analyzed using density functional theory (DFT) calculations. In both experiments and MD simulations, graphene consistently exhibits lower friction than the MoS2 monolayer and the heterostructure. In some cases, friction on the heterostructure is lower than that on the MoS2 monolayer. Quasi-static MD simulations and DFT calculations show that the origin of the friction contrast is the difference in energy barriers for a tip sliding across each of the three surfaces.

A simple technique of doping graphene by manipulating adsorbed impurity charges is presented. Using a field effect transistor configuration, controlled polarization of a ferroelectric polymer gate is used to compensate and neutralize charges of one type. The uncompensated charges of the opposite type then dope graphene. Both n- and p-type doping are possible by this method, which is non-destructive and reversible. We observe a change in n-type dopant concentration of 8 × 1012 cm−2 and a change in electron mobility of 650%. The electron and hole mobilities are inversely proportional to the impurity concentration, as predicted by theory. Selective doping of graphene can be achieved using this method by patterning gate electrodes at strategic locations and programming them independently. Such charge control without introducing hard junctions, therefore, permits seamless integration of multiple devices on a continuous graphene film. [ABSTRACT FROM AUTHOR]

Nanomechanical resonators are built into phones, as filters or accelerometers, but they lack a knob to effectively tune the frequency at the nanoscale when it's easy to tune on an octave the tone of a classical musical instrument like a guitar string. Moreover, the control of deformation in nanomaterials, as two-dimensional (2D) materials, to tailor their electronic properties, i.e., straintronic, opens up avenues for applications in force detection, bolometry or quantum emitters. An accurate control of the deformation within these materials is thus necessary to fully exploit their potential. The precise study of deformations in 2D materials involves measurements of vibration modes and nanomechanics. By using a suspended MoS2 membrane heated by the Joule effect, we induce a strong softening of the mechanical resonance frequency as a function of the electrothermal heating, over one octave. A simple electrical tension is used to modulate the thermal mechanical tuning. Its amplitude is very large, greater than 100% modulation for one volt, compared to other approaches on 2D or 1D materials and, moreover, a very wide frequency range is accessible. Finally, we have related a photo-induced softening of the membrane over very long times with the current measurements and a photothermal effect. [ABSTRACT FROM AUTHOR]

Safe and powerful energy storage devices are becoming increasingly important. Charging times of seconds to minutes, with power densities exceeding those of batteries, can in principle be provided by electrochemical capacitors--in particular, pseudocapacitors (1,2). Recent research has focused mainly on improving the gravimetric performance of the electrodes of such systems, but for portable electronics and vehicles volume is at a premium (3). The best volumetric capacitances of carbon-based electrodes are around 300 farads per cubic centimetre (4,5); hydrated ruthenium oxide can reach capacitances of 1,000 to 1,500 farads per cubic centimetre with great cyclability, but only in thin films (6). Recently, electrodes made of two-dimensional titanium carbide ([Ti.sub.3][C.sub.2], a member of the 'MXene' family), produced by etching aluminium from titanium aluminium carbide ([Ti.sub.3]Al[C.sub.2],a 'MAX' phase) in concentrated hydrofluoric acid, have been shown to have volumetric capacitances of over 300 farads per cubic centimetre (7,8). Here we report a method of producing this material using a solution of lithium fluoride and hydrochloric acid. The resulting hydrophilic material swells in volume when hydrated, and can be shaped like clay and dried into a highly conductive solid or rolled into films tens of micrometres thick. Additive-free films of this titanium carbide 'clay (7) have volumetric capacitances of up to 900 farads per cubic centimetre, with excellent cyclability and rate performances. This capacitance is almost twice that of our previous report (8), and our synthetic method also offers a much faster route to film production as well as the avoidance of handling hazardous concentrated hydrofluoric acid., In the search for new electrode materials, two-dimensional solids are of particular interest owing to their large electrochemically active surfaces (9). For example, activated graphene electrodes have capacitances of 200-350 [...]