Vacuum Technology in the study of Graphene

Graphene, an allotrope of carbon is a two-dimensional sheet of covalently bonded carbon atoms that has been attracting great attention in the field of electronics. In a recent review graphene is defined as a flat monolayer of carbon atoms tightly packed into a 2-D honeycomb lattice. A survey has been made of the production processes and instrumentation for characterization of graphene. In the production of graphene, the methods mainly used are Epitaxial growth, oxide reduction, growth from metal-carbon melts, growth from sugar. In the characterization of graphene, the instruments that are mainly used to study the atomic properties, electronic properties, optical properties, spin properties are Scanning Electron Microscopy, Transmission Electron Microscopy, Raman Spectroscopy. In all these instruments high or ultra-high vacuum is required. This paper attempts to correlate vacuum technology in the production and characterization of graphene. 1.Introduction. Graphene, a carbon allotrope with a two-dimensional honeycomb structure, has become an important player at the forefront of condensed matter physics, drawing the attention of theorists and experimentalists alike due to its challenging nature as a many-body problem, its unusual electronic properties and possible technological applications (see Refs. [1] and references therein). Graphene also belongs to a large class of planar condensed-mattersystems, which includes other graphite-related materials as well as high-Tc superconductors. The mobility of carriers in graphene has been reported as high as 15,000 cm2V-1s-1 with the potential to reach 100,000 cm2V-1s-1 with the reduction of impurities [1]. The increase of charge carriers does not significantly reduce the mobility,unlike 3dimensional semiconductors where dopants and phonons act as scattering centers and reduce mobility. From Lattice Monte Carlo simulations of the phase diagram of graphene as a function of the Coulomb coupling between quasiparticles, it was shown that the graphene in vacuum is likely to be an insulator([1]). An important property of graphene is optical transparency.This somewhat limits observation and detection of the material, however Raman spectroscopy and electron microscopy are viable techniques to identify single-layer graphene [1]. The optical transparency of graphene attracts attention because the conductivity is not adversely impacted by the thinness of the material. The single-atom layer is not depleted of charge carriers (in contrast to conventional semiconductors) and may offer solutions to the opto-electronics industry. Graphene can be fabricated in many forms: few-layer sheets vs. free-standing sheet, nano-scale vs. micro-scale.However, to be industrially viable the method needs to be scalable and relatively low cost. The application or end-use of the product usually dictates the method of fabrication. In particular, a International Symposium on Vacuum Science & Technology and its Application for Accelerators IOP Publishing Journal of Physics: Conference Series 390 (2012) 012067 doi:10.1088/1742-6596/390/1/012067 Published under licence by IOP Publishing Ltd 1 method to fabricate large-area free-standing sheets of single-layer graphene needs to be obtained. This requires an approach that is not limited in size to the starting material. For graphene to be competitive with current or other next-generation products it needs to be affordable. This requires a low-tomoderate fabrication expense. The next three sections will describe some of the patented and patentpending methods to fabricate grapheme The inherent properties of graphene appeal to many industries, including the electronics industry. The high mobility of charge carriers and the high thermal conductivity offer the industry a potential alternative to silicon and diamond. The nature of the carriers has the potential to create next generation solid state devices (ballistic transistors, spin transistors, etc.). The optical transparency of grapheme coupled with the electronic properties may provide the optoelectronics industry with photovoltaic solutions such as transparent electrodes. Other industries also have the opportunity to use graphene’s inherent properties. The carbon base of graphene offers compatibility with organic-based materials.High strength, lightweight materials may be formed with the addition of graphene to conventional plastics. This paper attempts to correlate vacuum technology in the production and characterization of graphene. 2.Production Processes. Three primary synthesis methods have emerged for the production of graphene: (i) micromechanical cleavage (Giem 2007, Park 2009), (ii) chemical vapor deposition (CVD) (O’biren, 2010), and (iii) chemical exfoliation (Srinivason 2007, Park 2009).[1] Micromechanical exfoliation of graphite or the ‘scotch tape’ method is relatively easy and can yield high quality, large area (~ 1 mm) sheets; however, this technique is low yield and only useful for experimental purposes. In the CVD or epitaxial approach, graphene is grown on a metal, generally Cu, substrate. Single layer and bi-layer graphene sheets, measuring up to approximately 1 cm square have been produced successfully by this method. However, neither method is currently capable of producing bulk quantities of graphene. The chemical methods of synthesizing graphene (Srinivason 2007, Park 2009) have emerged as the main approach for large quantity production. This can be done through the chemical exfoliation of graphite oxide (GO). The chemical exfoliation starts with natural graphite and reacts with a mixture of strong acids, such as nitric and sulfuric, and an oxygen donating compound. This reaction induces stress within the graphite layers by intercalating the graphite sheets with oxygen atoms and expanding their separation distance (Srinivason, 2007). The end result is graphitic oxide. The level of oxidation varies based on the method and initial type of graphite used (Park, 2009). Each method leaves different functional groups at the edge, or sometimes also on the surface of the graphene sheet that influence the chemical and physical properties of the GO. Prior experimental results show that often, chemical exfoliation method can create irreversible defects in the graphene sheets, which can decrease the conductance by 3 orders magnitude compared to a defect-free graphene-sheet (Park, 2009). Therefore, our efforts have been focused on optimizing the oxidation and exfoliation processes to gain finer control on surface defects and edge/surface functional groups.[1] 3.Characterization Instruments 3.1Scanning Electron Microscope(SEM.). ll electron beam instruments are built around an electron column, which produces a stable electron beam, controls beam current, beam size and beam shape, and rasters the beam. Electron optics are a very close analog to light optics, and most of the principles of an electron beam column can be understood by thinking of the electrons as rays of light and the electron optical components as simply their optical counterparts. International Symposium on Vacuum Science & Technology and its Application for Accelerators IOP Publishing Journal of Physics: Conference Series 390 (2012) 012067 doi:10.1088/1742-6596/390/1/012067