This Small Business Innovation Research Phase I project focuses on a novel carbide-bonded graphene coating technology to modify the surface of silicon wafer based molds through vacuum-assisted thermal exfoliation of functional graphene nanopaper. The graphene coating exhibits a unique combination of unprecedented properties such as lower surface friction coefficient and superior surface smoothness, higher hardness and wear resistance, better chemical resistance and anti-abrasion, lower thermal expansion coefficient and higher thermal conductivity comparing to silicon wafers and other coating materials. Using this new technology, the graphene coated silicon molds are able to produce high quality and high precision microlens and microlens array in advanced glass molding. Such products are difficult to produce in the current glass industry.
The broader impact/commercial potential of this project is that carbide-bonded graphene coating exhibits a unique combination of desired properties including excellent mechanical and bonding strength, high hardness, good electrical and thermal surface conductivities, low surface friction and excellent surface smoothness, strong chemical corrosion resistance and anti-abrasion, good cytocompatibility, easy micropatterning by cleanroom fabrication techniques, and attractive semiconductive and optoelectronic characteristics, thus opens up a new avenue toward engineering applications of graphenes. Microoptics have enormous applications in numerous fields, such as consumer electronics, sensors, optical communications, medical applications, light shaping, and energy. Currently, most low-cost microoptics products are based on plastic materials, which are commonly used in low-cost consumer electronics. However, plastic microoptics have many drawbacks, such as low reflective index, low light permeability, unstable to environmental changes, low hardness, etc. The replacement of plastic microoptics with low-cost precision glass microoptics is indispensable.
1. Intellectual merit Nanomaterial Innovation Ltd and The Ohio State University carried out functional graphene nanopaper synthesis, vacuum aided graphene coating process and Chemical Vapor Deposition (CVD) based graphene coating process on silicon wafers with micro-patterning, and the characterization of coating and molded glass lens. The research activities combined basic nanotechnology, novel CVD graphene coating technology, and low-cost and mass-producible high precision glass molding. These activities improved our understanding of carbide-bonded graphene (CBG) coating technology which has demonstrated potential applications in high precision glass molding industry. 2. Broader of impacts Because of its unique combination of excellent mechanical, physical, optical transparency and biocompatibility properties together with tunable optoelectronic characteristics, this low-cost CVD-CBG technology and novel coating materials can be applied to a very broad range of applications, including advanced thermal management, next generation electronic components and biosensors. 3. We summarize the major findings as follow: Coating properties A CVD based CBG coating process was developed. The coating thickness could be controlled from 15 nm to 1500 nm, depending on the coating time and C/Si ratio. Comparing to diamond-like carbon coating, CVD based CBG coating has less surface roughness, lower friction coefficient, good Young modulus and Hertzian hardness, higher thermal conductivity and tunable electric resistance. Measurements of adhesion with glass Silicon cannot work directly as mold material due to its severe adhesion to glass at elevated temperatures. This adhesion is usually caused by anodic bonding or other chemical reactions. It is critical to eliminate the adhesion problem such that silicon with fine features can be used as molds for precision glass molding. CBG coating is an ideal solution to solve this problem because it can prevent the direct bonding between glass and silicon (Fig.1). CBG coated silicon molds for micro lens molding A Si mold with microwells was fabricated and coated with a thin film graphene. The surface profile of the coated Si mold is shown in Fig. 2a and the scanning electron microscope (SEM) image of the mold is shown in Fig. 2b. Each microwell has a width of 11 μm and an average depth of 1.5 μm. A glass blank was molded on the graphene coated mold, a highly uniformed micrometer pillars were formed on the glass surface with an average height of 1.5 μm, matching the dimensions of the microwells on the Si mold (Fig.2c). This demonstrates that high-precision microfeatures can be successfully transferred to glass surfaces using this method (Fig. 2d~2f). The geometry of the molded freeform optics (Fig. 3) shows two seamless micro lens arrays, which have the same geometry of individual lenslet but different numbers of lenslet, were successfully molded. Each lenslet has an overall dimension of 360 x 360 μm. The radius of curvature of each lenslet is 7.1119 mm which gives the effective focal length of 12.1157 mm. 4. CBG coated silicon molds for IR lens molding The molded surface of infrared glass was also measured (Fig. 4). The periodic feature has a line width of 2.8 μm and a depth of 0.92 μm, while the dot array shows very good replication.
