|Carbon-based Nano electro-mechanical systems (NEMS) Sensors and Devices|
Professor Dr. Burhanuddin Yeop Majlis
The Institute of Microengineering and Nanoelectronics
Communication and semiconductor technology had gone to new phase, where some of the devices and systems can work in terahertz frequency and pico-ampere signal. Most of the current transmission lines are based on gold, copper and aluminum alloy which cannot give a good path for the signal to pass through which it would get lost due to the resistance and noise generated by the lines. Some alternative technologies include superconductor tapes, polymer film growth, polymer encapsulation of nanomaterial, and powder-in-tube superconductors, which can offer no-loss DC power transmission with nearly zero resistance. The problems with these technologies are high cost, brittleness, more difficult to form, subjected to both critical current and magnetic quench and require continuous cryogenic cooling. Therefore a new electrical conductor with much greater conductivity than that of common metallic conductors, better tensile strength than steel, which is cost effective, easily formed and not subjected to current density, magnetic field or temperature quench, is needed.
Carbon based material, graphene, offers a possible means of meeting the aforementioned needs, where they have better tensile strength, thermal conductivity, density and electrical as compared to some metals. However, in order to make practical use of these properties, a number of challenges must be met, including not only make these materials in large quantities, but also aligning, embedding, and for safety reasons, enclosing or encapsulating the carbon based material in a protective layer or membrane.
Recent advances in the growth techniques of graphene are expected to enable commercial viability of large-area graphene films. Graphene possesses a combination of properties that make it extremely well suited for use in nanoelectromechanical systems (NEMS). Its exceptional mechanical properties include high stiffness and low mass, which lead to high resonant frequencies; and ultrahigh strength, which allows for strain tuning of frequency over a wide range. Its optical properties and high electronic mobility enable robust optical and electrical transduction, while its chemical inertness enables atomically thin devices.
Vertical oriented graphene (VG) nanosheets have attracted growing interest for a wide range of applications ranging from energy storage, catalysis and field-emission to gas sensing, due to their unique orientation, exposed sharp edges, non stacking morphology and huge surface-to-volume ratio. However, controllable growth of vertical aligned graphene with uniform spacings between the sheet is extremely difficult due to delicate substrate/catalyst preparation, non-uniformity of substrate orientation and the nature of 2D material that inhibit them to freely standing and grow vertically like their allotrope, carbon nanotubes. Vertically aligned graphene will be directly grown on interdigitated electrodes structure to utilize its high surface-to volume ratio to fabricate graphene-based NEMS supercapacitor. The mechanical and piezoelectric behavior of vertical graphene will be evaluated to study its suitability as ultra-high sensitivity NEMS pressure sensors. Finally, the as-grown graphene will be integrated in cavities of photonic crystal for application in NEMS biomolecule sensors.
Chemical vapor deposition (CVD) method is the most promising way to produce large-area graphene at low-cost. The objective of this research is to grow large area graphene at lower temperature in the range of 300-600 °C by employing plasma-assisted CVD method. We planned to use an etched substrate surface that could substitute the role of catalyst as nucleation sites for the formation of graphene monolayer. The template roughness should be in the order of few nanometer to attract free carbons nucleation. Plasma reactor will assist in cracking the hydrocarbons and transform them into radicals and free carbons.
The interplay between plasma and thermal CVD is expected to significantly reduce the temperature needed for the formation of graphene. This lower growth temperature will enable the use of wide range of substrates such as plastics, glass and Silicon. The outcome of this research will provide fundamental insights on the mechanism and feasibility formation of large area graphene monolayer that can be directly grown from etched substrates which later can be integrated onto devices.
Organic light-emitting diodes (OLEDs) are emerging as leading technologies for both high quality display and lighting. OLEDs consist of active organic luminescent structure sandwiched between two electrodes, one of which must be transparent. Currently, indium tin oxide (ITO) is the most widely used material for the transparent conductors (TCs) electrodes. However, the rising price and limited geographical availability of Indium, asks for alternative transparent conductors (TCs) for the next generation of OLED devices. The next generation of optoelectronic devices requires transparent conductive electrodes to be lightweight, flexible, cheap, environmentally attractive, and compatible with large-scale manufacturing methods. Graphene could be ideal for replacing transparent electrodes made of expensive indium tin ITO in OLEDs. However, even the best graphene anode OLEDs made today are incapable of emitting light very efficiently because graphene has a low work function and high sheet resistance. This research proposal is to overcome these problems through two approaches. The first approach is to use a metal oxide film (for example, molybdenum trioxide, MoO3) as an intermediate layer between the graphene and the OLED layers. The second approach is by modifying the surface of the graphene using water-dispersed conducting polymer and a perfluorinated ionomer.