Isolation and characterization of few-layer black phosphorus

Isolation and characterization of mechanically exfoliated black phosphorus flakes with a thickness down to two single-layers is presented. A modification of the mechanical exfoliation method, which provides higher yield of atomically thin flakes than conventional mechanical exfoliation, has been developed. We present general guidelines to determine the number of layers using optical microscopy, Raman spectroscopy and transmission electron microscopy in a fast and reliable way. Moreover, we demonstrate that the exfoliated flakes are highly crystalline and that they are stable even in free-standing form through Raman spectroscopy and transmission electron microscopy measurements. A strong thickness dependence of the band structure is found by density functional theory calculations. The exciton binding energy, within an effective mass approximation, is also calculated for different number of layers. Our computational results for the optical gap are consistent with preliminary photoluminescence results on thin flakes. Finally, we study the environmental stability of black phosphorus flakes finding that the flakes are very hydrophilic and that long term exposure to air moisture etches black phosphorus away. Nonetheless, we demonstrate that the aging of the flakes is slow enough to allow fabrication of field-effect transistors with strong ambipolar behavior. Density functional theory calculations also give us insight into the water-induced changes of the structural and electronic properties of black phosphorus.Comment: 11 main figures, 7 supporting figure

THz Photodetection in Graphene Field Effect Transistors

This Master thesis (Tesi di Laurea Magistrale) is aimed at the design and test of a graphene-based Terahertz photodetector. We exploit the high-mobility of graphene to fabricate a top-gated Graphene Field Effect Transistor (GFET) that is able to detect THz radiation. The photodetection theory was originally developed by M. Dyakonov and M. S. Shur in 1993 and it is based on the nonlinear transport properties of semiconductor FETs and GFETs, which lead to the rectification of an ac current induced by the incoming radiation. The efficiency of the detector is directly linked to the carrier mobility, which can be quite low in traditional semiconductor FETs at room temperature. On the contrary, graphene shows extremely high-mobilities most independently of temperarute which makes it a promising material for a wide range of applications. In our approach the modulation of the carrier density in the graphene channel is achieved through a metallic top gate, isolated from graphene by a thin layer of Hafnium Oxide that we deposit using Atomic Layer Deposition (ALD). In addition, we exploit integrated log-periodic antennas in order to enhance the coupling between the incident THz wave and the graphene transistor. We successfully detected radiation at 0.3 THz at room temperature, both with monolayer and bilayer graphene devices. We achieved a maximum responsivity of 0.09 V/W and a minimum Noise Equivalent Power (NEP) of 7\times10^{-8}\,\mathrm{W/\sqrt{Hz}} . To our knowledge, this work presents the first THz FET photodetector based on graphene

Micromechanical bolometers for sub-Terahertz detection at room temperature

Fast, room temperature imaging at THz and sub-THz frequencies is an interesting feature which could unleash the full potential of plenty applications in security, healthcare and industrial production. In this Letter we introduce micromechanical bolometers based on silicon nitride trampoline membranes as broad-range detectors, down to the sub-THz frequencies. They show, at the largest wavelengths, room-temperature noise-equivalent-powers comparable to state-of-the-art commercial devices (~100 pW Hz-1/2); adding the good operation speed and the easy, large-scale fabrication process, the trampoline membrane could be the next candidate for cheap, room temperature THz imaging and related applications

Single Electron Precision in the Measurement of Charge Distributions on Electrically Biased Graphene Nanotips Using Electron Holography

We use off-axis electron holography to measure the electrostatic charge density distributions on graphene-based nanogap devices that have thicknesses of between 1 and 10 monolayers and separations of between 8 and 58 nm with a precision of better than a single unit charge. Our experimental measurements, which are compared with finite element simulations, show that wider graphene tips, which have thicknesses of a single monolayer at their ends, exhibit charge accumulation along their edges. The results are relevant for both fundamental research on graphene electrostatics and applications of graphene nanogaps to single nucleotide detection in DNA sequencing, single molecule electronics, plasmonic antennae, and cold field emission sources

Micromechanical Bolometers for Subterahertz Detection at Room Temperature

Fast room-temperature imaging at terahertz (THz) and subterahertz (sub-THz) frequencies is an interesting technique that could unleash the full potential of plenty of applications in security, healthcare, and industrial production. In this Letter, we introduce micromechanical bolometers based on silicon nitride trampoline membranes as broad-range detectors down to sub-THz frequencies. They show, at the longest wavelengths, room-temperature noise-equivalent powers comparable to those of state-of-the-art commercial devices (∼100 pW Hz–1/2), which, along with the good operation speed and the easy, large-scale fabrication process, could make the trampoline membrane the next candidate for cheap room-temperature THz imaging and related applications

Highly dispersive multiplexed micromechanical device array for spatially resolved sensing and actuation

The powerful resource of parallelizing simple devices for realizing and enhancing complex operations comes with the drawback of multiple connections for addressing and controlling the individual elements. Here we report on a technological platform where several mechanical resonators can be individually probed and electrically actuated by using dispersive multiplexing within a single electrical channel. We demonstrate room temperature control of the individual device vibrational motion and spatially-resolved readouts. As the single elements have proven to be excellent bolometers and individual nodes for reservoir computing, our platform can be directly employed for single-channel addressing of multiple devices, with immediate applications for far-infrared cameras, spatial light modulators and recurrent neural networks operating at room temperature. (Figure presented.