Micro Chemical Vapor Deposition for the Synthesis of Nanomaterials

MEMS (Microelectromechanical Systems) technologies have enabled the construction of a micro chemical vapor deposition (µCVD) system for the synthesis of nanomaterials. By means of localized resistive heating via micro-heaters, unique capabilities of the µCVD systems have been utilized to synthesize carbon nanotubes and graphene in this work, including fast stabilization of temperature; rapid exchange of gas species; laminar gas flow due to small Reynolds number; small diffusion length; and enhanced mass transport. In the prototype designs, the µCVD system is composed of a suspended microheater and two contact pads constructed from the device layer of a SOI (silicon-on-insulator) substrate. Both heat transfer and fluidic analyses have been conducted to validate and optimize the key features of the system. Experimental results in the synthesis of single-walled carbon nanotubes (SWNTs) have shown that amorphous carbon formation can be deterred and ultra-long SWNTs can be grown using ethylene as the source gas using µCVD system while similar experimental conditions failed to produce SWNTs in a conventional CVD system.Further applications of the µCVD system have been successfully demonstrated, including direct placement of high-quality, well-aligned SWNTs on temperature sensitive substrates such as thin paper or polymer sheets, and the synthesis of two-dimensional crystalline graphene structure with good uniformity. Specifically, the construction of high-performance CNT-based devices requires high-quality CNTs while conventional transfer and assembly processes often alter and degrade their properties. The localized heating of µCVD allows the direct synthesis of self-aligned SWNTs onto flexible substrates without causing thermal damages. Both Raman spectral and transmission electron microscopy have been used to validate the quality of these SWNTs. This methodology of direct synthesis of nanomaterials could be applicable to other one-dimensional nanostructures for various applications including flexible electronics. In the application to graphene growth, large area graphene with consistent number of layers has been realized on top of a nickel-coated micro platform using the µCVD system. The capability of ultra-fast heating and cooling provided by the MEMS platform is crucial for the successful growth of graphene. In the prototype graphene synthesis experiments, methane is flowed at 1.5% volume ratio with hydrogen to the platform heated to 1000oC and the heating power is cut off after a 5-min growth process. It has been demonstrated that 1~2 layers of graphene structures can be consistently grown throughout the whole 300×300 m 2 platform.