Introduction
Fig 1 High quality graphene
Graphene is an exciting discovery in materials science and its unique properties promise a vast range of practical applications. Strongly dependent on the number of atomic planes, physical characteristics of few-layer graphene (FLG) are different from those of single layer graphene (SLG). The problem is how to establish a reliable high-throughput method of graphene identification and quality control on a wafer scale to make sure that what you get is real graphene, either single or few-layered, as required.
Quality control
The key to quality control of graphene is to establish effective methods distinguish between monolayer and few-layered graphene on a substrate.
Fig 2 Schematic of the process for determining the number of layers in graphene
(ACS Nano, 2011, 5 (2), 914–922)
Raman scattering: Micro-Raman spectroscopy currently is a non-destructive tool for determining the number of atomic planes. Raman scattering is an inherently weak effect and long sampling times are required for a single graphene flake which prohibit high-throughput analysis which will be required for large scale fabrication of graphene.
Optical microscopy: It was previously thought that the contrast of graphene on silicon carbide was too low to be observed directly with an optical microscope. As it turn out, researchers have found that analyzing optical images and comparing them to electrical measurements, is able to identify single graphene layers with 0.3 nanometer thick.
Low-energy electron microscopy (LEEM): LEEM is a projection-type microscopy technique collecting low energy (typically 1–10 eV) electrons back-scattered from the samples for imaging. Nanometer-scale spatial resolution and video-rate temporal resolution are compatible by LEEM. In addition, LEEM is a powerful tool for investigating the growth mechanism of graphene and related 2D materials and optimizing their growth conditions.
Atomic force microscopy (AFM): Atomic-force microscopy is a type of scanning probe microscopy with sub-angstrom resolution. It is able to gather data on the mechanical and electrical properties of materials and surfaces by ‘feeling’ or ‘touching’ the surface with a cantilevered mechanical probe controlled by Piezoelectric components.
Other methods: Other methods include scanning electron microscopy (SEM), transmission electron microscopy (TEM), scanning tunneling microscopy (STM), photoelectron spectroscopy (PES), angle-resolved photoelectron spectroscopy (ARPES), photoemission electron microscopy (PEEM), and reflection high-energy electron diffraction (RHEED). These methods can be used for identification and quality analysis of SLG and FLG samples.
