Evidence of thickness-dependent surface-induced ferroelectricity in few-layer germanium sulfide obtained via scanning tunneling spectroscopy.

The discovery of ferroelectricity in two-dimensional van der Waals materials has sparked enormous interest from the scientific community, due to its possible applications in next-generation nanoelectronic devices, such as random-access memory devices, digital signal processors, and solar cells, among others. In the present study, we used vapor phase deposition to synthesize ultrathin germanium sulfide nano-flakes on a highly oriented pyrolytic graphite substrate. Nanostructures of variable thicknesses were characterized using scanning tunneling microscopy and spectroscopy. Tunneling currents under forward and backward biases were measured as a function of nano-flake thickness. Remarkably, we clearly observed a hysteresis pattern, which we attributed to surface ferroelectric behavior, consistent with the screening conditions of polarization charges. The effect increases as the number of layers is reduced. This experimental result may be directly applicable to miniaturized memory devices, given the two-dimensional nature of this effect.

Scanning Tunneling Spectroscopy Method for the Prediction of Semiconductor Heterojunction Performance as a Prequel for Device Development.

The prediction of semiconductor device performance is a persistent challenge in materials science, and the ability to anticipate useful specifications prior to construction is crucial for enhancing the overall efficiency. In this study, we investigate the constituents of a solar cell by employing scanning tunneling microscopy (STM) and spectroscopy (STS). Through our observations, we identify a spatial distribution of the dopant type in thin films of materials that were designed to present major p-doping for germanium sulfide (GeS) and dominant n-doping for tin disulfide (SnS2). By generating separate STS maps for each semiconductor film and conducting a statistical analysis of the gap and doping distribution, we determine intrinsic limitations for the solar cell efficiency that must be understood prior to processing. Subsequently, we fabricate a solar cell utilizing these materials (GeS and SnS2) via vapor phase deposition and carry out a characterization using standard J-V curves under both dark/illuminated irradiance conditions. Our devices corroborate the expected reduced efficiency due to doping fluctuation but exhibit stable photocurrent responses. As originally planned, quantum efficiency measurements reveal that the peak efficiency of our solar cell coincides with the range where the standard silicon solar cells sharply decline. Our STS method is suggested as a prequel to device development in novel material junctions or deposition processes where fluctuations of doping levels are retrieved due to intrinsic material characteristics such as the occurrence of defects, roughness, local chemical segregation, and faceting or step bunching.

Direct production of carbon nanotubes/metal nanoparticles hybrids from a redox reaction between metal ions and reduced carbon nanotubes.

A method to decorate single-walled and multiwalled carbon nanotubes (CNTs) with metal nanoparticles (NPs) based on the formation of a CNT polyelectrolyte is reported. Such a method does not rely on CNT surface functionalization or the use of surfactants. It has been tested for gold (Au) and palladium (Pd). The resulting hybrids present metal NPs highly dispersed along the tube walls and with small size dispersion. The average diameters of the Au and Pd NPs were approximately 5 and approximately 3 nm, respectively. This method paves the way for large-scale decoration of CNTs with metal NPs.