Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userVisualizing Encapsulated Graphene, its Defects and its Charge Environment by Sub-Micrometer Resolution Electrical Imaging
89
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
Devices made from two-dimensional (2D) materials such as graphene or transition metal dichalcogenides possess interesting electronic properties that can become accessible to experimental probes when the samples are protected from deleterious environmental effects by encapsulating them between hexagonal boron nitride (hBN) layers. While the encapsulated flakes can be detected through post-processing of optical images or confocal Raman mapping, these techniques lack the sub-micrometer scale resolution to identify tears, structural defects or impurities, which is crucial for the fabrication of high-quality devices. Here we demonstrate a simple method to visualize such buried flakes with sub-micrometer resolution, by combining Kelvin force probe microscopy (KPFM) with electrostatic force microscopy (EFM). KPFM, which measures surface potential fluctuations, is extremely effective in spotting charged contaminants within and on top of the heterostructure, making it possible to distinguish contaminated regions in the buried flake. When applying a tip bias larger than the surface potential fluctuations, EFM becomes extremely efficient in highlighting encapsulated flakes and their sub-micron structural defects. We show that these imaging modes, which are standard extensions of atomic force microscopy (AFM), are perfectly suited for locating encapsulated conductors, for visualizing nanometer scale defects and bubbles, and for characterizing their local charge environment.
rate research
Read More
Graphene with high carrier mobility mu is required both for graphene-based electronic devices and for the investigation of the fundamental properties of graphenes Dirac fermions. It is largely accepted that the mobility-limiting factor in graphene is the Coulomb scattering off of charged impurities that reside either on graphene or in the underlying substrate. This is true both for traditional graphene devices on SiO2 substrates and possibly for the recently reported high-mobility suspended and supported devices. An attractive approach to reduce such scattering is to place graphene in an environment with high static dielectric constant kappa that would effectively screen the electric field due to the impurities. However, experiments so far report only a modest effect of high-kappa environment on mobility. Here, we investigate the effect of the dielectric environment of graphene by studying electrical transport in multi-terminal graphene devices that are suspended in liquids with kappa ranging from 1.9 to 33. For non-polar liquids (kappa <5) we observe a rapid increase of mu with kappa and report a record room-temperature mobility as large as ~60,000 cm2/Vs for graphene devices in anisole (kappa=4.3), while in polar liquids (kappa >18) we observe a drastic drop in mobility. We demonstrate that non-polar liquids enhance mobility by screening charged impurities adsorbed on graphene, while charged ions in polar liquids cause the observed mobility suppression. Furthermore, using molecular dynamics simulation we establish that scattering by out-of-plane flexural phonons, a dominant scattering mechanism in suspended graphene in vacuum at room temperature, is suppressed by the presence of liquids. We expect that our findings may provide avenues to control and reduce carrier scattering in future graphene-based electronic devices.
Devices made from two dimensional materials such as graphene and transition metal dichalcogenides exhibit remarkable electronic properties of interest to many subdisciplines of nanoscience. Owing to their 2D nature, their quality is highly susceptible to contamination and degradation when exposed to the ambient environment. Protecting the 2D layers by encapsulation between hexagonal boron nitride layers significantly improves their quality. Locating these samples within the encapsulant and assessing their integrity prior to further processing then becomes challenging. Here we show that conductive scanning probe techniques such as electrostatic force and Kelvin force microscopy makes it possible to visualize the encapsulated layers, their charge environment and local defects including cracks and bubbles on the sub-micrometer scale. Our techniques are employed without requiring electrical contact to the embedded layer, providing valuable feedback on the local electronic quality prior to any device etching or electrode deposition. We show that these measurement modes, which are simple extensions of atomic force microscopy, are perfectly suited for imaging encapsulated conductors and their local charge environments.
In this work we study the behavior of the optical phonon modes in bilayer graphene devices by applying top gate voltage, using Raman scattering. We observe the splitting of the Raman G band as we tune the Fermi level of the sample, which is explained in terms of mixing of the Raman (Eg) and infrared (Eu) phonon modes, due to different doping in the two layers. We theoretically analyze our data in terms of the bilayer graphene phonon self-energy which includes non-homogeneous charge carrier doping between the graphene layers. We show that the comparison between the experiment and theoretical model not only gives information about the total charge concentration in the bilayer graphene device, but also allows to separately quantify the amount of unintentional charge coming from the top and the bottom of the system, and therefore to characterize the interaction of bilayer graphene with its surrounding environment.
We study the intervalley scattering in defected graphene by low-temperature transport measurements. The scattering rate is strongly suppressed when defects are charged. This finding highlights screening of the short-range part of a potential by the long-range part. Experiments on calcium-adsorbed graphene confirm the role of a long-range Coulomb potential. This effect is applicable to other multivalley systems, provided that the charge state of a defect can be electrically tuned. Our result provides a means to electrically control valley relaxation and has important implications in valley dynamics in valleytronic materials.
The technical breakthrough in synthesizing graphene by chemical vapor deposition methods (CVD) has opened up enormous opportunities for large-scale device applications. In order to improve the electrical properties of CVD graphene grown on copper (Cu-CVD graphene), recent efforts have focussed on increasing the grain size of such polycrystalline graphene films to 100 micrometers and larger. While an increase in grain size and hence, a decrease of grain boundary density is expected to greatly enhance the device performance, here we show that the charge mobility and sheet resistance of Cu-CVD graphene is already limited within a single grain. We find that the current high-temperature growth and wet transfer methods of CVD graphene result in quasi-periodic nanoripple arrays (NRAs). Electron-flexural phonon scattering in such partially suspended graphene devices introduces anisotropic charge transport and sets limits to both the highest possible charge mobility and lowest possible sheet resistance values. Our findings provide guidance for further improving the CVD graphene growth and transfer process.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
