Gallium chalcogenides are promising building blocks for novel van der Waals heterostructures. We report on the low-temperature micro-photoluminescence (PL) of GaTe and GaSe films with thicknesses ranging from 200 nm to a single unit cell. In both materials, PL shows a dramatic decrease by 10 4 -10 5 when film thickness is reduced from 200 to 10 nm. Based on evidence from continuouswave (cw) and time-resolved PL, we propose a model explaining the PL decrease as a result of nonradiative carrier escape via surface states. Our results emphasize the need for special passivation of two-dimensional films for optoelectronic applications.

We have decoupled the intrinsic electrostatic effects arising in monolayer and few-layer MoS 2 from those influenced by the flake-substrate interaction. Using ultrasonic force microscopy nanomechanical mapping, we identify the change from supported to suspended flake regions on a trenched substrate. These regions are correlated with the surface potential as measured by scanning Kelvin probe microscopy. Relative to the supported region, we observe an increase in surface potential contrast due to suppressed charge transfer for the suspended monolayer. Using Raman spectroscopy we observe a red shift of the E 1 2g mode for monolayer MoS 2 deposited on Si, consistent with a more strained MoS 2 on the Si substrate compared to the Au substrate.

While mechanical and frictional properties of graphene in air have been extensively studied, graphene's nanomechanical behavior in liquids, vital for its operation in rechargeable batteries, supercapacitors, and sensors, is still largely unexplored. In this paper, we investigate the nanomechanics of normal (adhesive and elastic) and tangential (friction) forces between a stationary, moving, and ultrasonically excited nanoscale atomic force microscope (AFM) tip and exfoliated few layer graphene (FLG) on SiO2 substrate as a function of surrounding media-air, polar (water), and nonpolar (dodecane) liquids. We find that, while the friction coefficient is significantly reduced in liquids, and is always lower for FLG than SiO2, it is higher for graphene in nonpolar dodecane than highly polar water. We also confirm that in ambient environment the water meniscus dominates high adhesion for both hydrophobic FLG and the more hydrophilic SiO2 surface, with the lowest adhesion observed in liquids, in particular for FLG in dodecane, reflecting the low interface energy of this system. By using nanomechanical probing via ultrasonic force microscopy (UFM), we observed a profound reduction of graphene rippling and increase of graphene-substrate contact area in liquid environment. Friction force dependence on ultrasonic modulation amplitude suggests that dodecane at the graphene interface produces a solid-like "cushion" of approximately 2 nm thickness, whereas, in water immersion, the same dependence shows a remarkable similarity with the ambient environment, confirming the presence of a water meniscus in air, and suggesting negligible thickness of a similar water "cushion" on graphene. Dependence of friction on local environment opens new pathways for friction management in microfluidic and micro- and nanoelectromechanical systems.

Scanning probe Microscopy (SPM) represents a powerful tool that, in the past thirty years, has allowed one to investigate material surfaces in unprecedented ways at the nanoscale level. However, SPM has shown very little power of depth penetration, whereas several nanotechnology applications would require it. Subsurface imaging has been achieved only in a few cases, when subsurface features influence the physical properties of the surface, such as the electronic states or the heat transfer. Ultrasonic Force Microscopy (UFM), an adaption of the Atomic Force Microscopy (AFM) contact mode, can dynamically measure the stiffness of the elastic contact between the probing tip and the sample surface. In particular, UFM has proven highly sensitive to the near-surface elastic field in non-homogeneous samples.In this paper, we present an investigation of two-dimensional (2D) materials, namely flakes of graphite and molybdenum disulphide placed on structured polymeric substrates.We show that UFM can non-destructively distinguish suspended and supported areas and localize defects, such as buckling or delamination of adjacent monolayers, generated by residual stress. Specifically, UFM can probe small variations in the local indentation induced by the mechanical interaction between tip and sample. Therefore, any change in the elastic modulus within the volume perturbed by the applied load or the flexural bending of the suspended areas can be detected and imaged. Such a power of investigation is very promising in order to investigate the buried interfaces of nanostructured 2D materials such as in graphene-based devices.

The functionality of graphene and other two-dimensional materials in electronic devices is highly influenced by the film-substrate charge transfer affecting local carrier density. We demonstrate that charges buried under the few layer graphene on/in the insulating substrate can be detected using electromechanical actuation of the conductive atomically thin layers, allowing measurements of areal density of film-substrate transferred charges under few layer graphene and MoS2 suspended films.