394783 Micro- and nano-engineered advanced materials for renewable energy, thermal, and electronic applications
My multidisciplinary research interests are in thin films and nanoscale processing for advanced materials – from materials to devices to circuits, blurring the boundaries between chemistry, electronics, physics, materials science and mechanics. With the state-of-the-art micro/nano fabrication techniques and surface engineering, I will explore both the fundamental and theoretical nature of new materials and devices. As an engineer and scientist, I view the area of micro- and nano-engineered advanced materials and next generation electronics as a diverse discipline full of formidable challenges that will engage the research community for decades to come.
Previous and Current Research
Synthesis of graphene films and organic compounds for developing electronic devices: Taking on the challenge to develop inexpensive materials with enhanced functionalities for electronics and in general energy applications, I have focused on low-cost direct synthesis of wafer-scale graphene on a variety of substrates for touch screens, displays, solar cells, and next generation transistors. Graphene, a one-atom-thick sheet of sp2-bonded carbon atoms in a honeycomb structure, has attracted significant interest due to its outstanding mechanical, thermal, and electrical properties. I have synthesized graphene films on a variety of substrates including silicon, quartz, and sapphire, through a single-step thermal annealing process of a Cu/Ni bilayer pre-deposited on the substrate in a low pressure environment through chemical vapor deposition (CVD) technique. My method eliminated the need for solution process transfer of graphene to the target substrate, which usually induces defects in its structure. In addition, doping of graphene and organic thin films is important, as it gives rise to p-type and n-type materials, and it adjusts the work function of the film that is necessary to control charge injection and collection in devices such as solar cells, displays, and printed and flexible electronic devices. I developed a low temperature method to fabricate p- and n-type graphene and graphene p-n junctions by modifying the interface between graphene and its dielectric substrate with covalently bonded self-assembled monolayers (SAMs). In a multidisciplinary collaboration work, I showed that surface modifiers based on polymers containing simple aliphatic amine groups and redox-active solution processable metal-organics substantially adjust the work function of conductors, including metals, transparent conductive metal oxides, conducting polymers, and graphene. These methods can pave the way to simplified manufacturing of low-cost and large-area organic electronic technologies.
CVD polymers-surface modification: Conventional polymer deposition (for e.g. ink-jet printing, spin-coating or dip coating) uses solvents that may have adverse effects during the deposition process—such as degradation of the underlying layers or alteration of mechanical/electrical properties—and that cannot be applied for coating of surfaces with high aspect ratio patterns. Chemical vapor deposition (CVD) as a polymer thin film deposition technique offers a versatile platform for fabrication of a wide range of polymer thin films preserving all the functionalities.5 The solvent-free, conformal vapor phase deposition technique enables the integration of polymer thin films or nanostructures into micro- and nanodevices for improved performance. In my work at MIT, I synthesized thin films of polymers via a substrate-independent and all-dry-initiated chemical vapor deposition (iCVD) technique. Surfaces coated with fluoropolymers exhibited high advancing water contact angle (WCA) and very low WCA hysteresis. These coatings reduce the strength of ice and clathrate hydrate adhesion to the substrates up to five times compared to bare substrates and are very desirable for flow assurance strategies aimed at reducing the occurrence of blockages in oil and gas pipelines. In addition, anti-reflective coatings are significantly enhancing the contrast of today’s high resolution displays, but their usage in touch applications suffers from the issue of fingerprint visibility on the surface of such low-reflective substrates. In general, the appearance of fingerprints and oil residues are of major concern for numerous industries involved with maintaining display clarity for applications where handling occurs. To this end, I am working on developing anti-fingerprint and anti-reflective coating using bio-degradable CVD polymers that exhibit high water/oil contact angle. In order to use such coatings in a touch application it must withstand hundreds of thousands of finger touches and rubs. Additional advantages of the CVD technique for polymer deposition is that the mechanical properties of the synthesized polymers against wear and erosion can be improved via in-situ grafting, multilayer composite polymers, and post-synthesis treatment of polymers. Films with mechanical properties comparable to glass were obtained using this technique.
Development of functional polymers:
Low-cost and scalable CVD polymers for high efficiency heat recovery: Thermoelectric (TE) materials have recently attracted significant interest for converting thermal energy to electric energy in the automobile industry, for recapturing energy currently lost from the hot effluent of power plant smokestacks, and for harvesting the heat currently generated but not used by photovoltaic cells. By understanding how heat and energy flow through materials, energy conversion mechanisms and processes can be integrated into functional devices. The performance of the thermoelectric material is evaluated by the dimensionless figure of merit (ZT) that is a function of electrical conductivity, thermal conductivity, absolute temperature, and Seebeck coefficient—the ratio of the induced voltage over the temperature gradient across the thermoelectric device. A general principle for engineering good TE materials is to separate electrical from thermal conductivity—in particular, to make them dependent on structures at different length scales. Today’s thermoelectric materials of choice for low temperature energy conversion are bismuth antimony telluride alloys. The constituting atomic elements have a low natural abundance and are therefore expensive. Additionally, low natural abundance goes hand in hand with some level of toxicity for the environment. Therefore, in today’s scenario it is unthinkable to enable a widespread use of thermoelectric installations for waste heat recovery. In that context, widely available, although less efficient, organic thermoelectric materials have become the subjects of interest to the scientific community. Polymers as TE materials have been developed via electro spinning, inkjet printing, and electrochemical deposition. However, these methods are not conformal, use solvents, and require additional treatments to dope the polymer before integrating it into thermoelectric generators. I plan to develop polymer TE materials through oxidative CVD where the conformal, all-dry, substrate-independent, and low-cost characteristics of conducting polymers can be leveraged to make scalable thermoelectric generators. The extent of doping and electrical conductivity of such polymers can be tuned throughout the synthesis process by changing growth conditions. In addition, a state-of-the-art mechanism for grafting such polymers to surfaces provides the robust adhesion required for industrial application. Understanding thermal and electrical properties of such polymers is inherently interdisciplinary, involving backgrounds in synthetic chemistry, polymer physics, condensed matter physics, and materials characterization. Through better understanding of transport in amorphous and disordered materials, it is possible to tailor the thermal and electrical conductivity in synthesized polymers and integrate them into low-cost, scalable, and environmentally friendly organic thermoelectric generators.
Mechanical properties of CVD polymers: CVD polymers fully retain the rich chemistry of organic monomers enabling technology for surface modification of membranes, microfluidic structures, and biomedical devices. These CVD polymeric layers possess mechanical flexibility and conformal coverage attributes and can be integrated into device fabrication schemes for optoelectronics and sensors. While promises have been demonstrated in terms of photovoltaics, organic displays, and sensors, the reliability of these devices ultimately depends on their ability to retain their functionality during operation. In terms of organic electronics, this will present a challenge when considering their need for enhanced mechanical properties as well as the potential for fracture and loss of adhesion at critical interfaces. In this work, I will continue to build on my expertise in studying mechanical properties of ultrathin polymers, synthesized using CVD technique. I will investigate the use of functionalization layers containing hydroxyl, vinyl, and fluorine groups to graft these polymers to the desired substrate to enhance their adhesion. I will also seek to improve the mechanical properties, for example elastic modulus, hardness, and toughness through synthesis of stacked ultrathin multilayer structures, cross-linking of various monomers, and post-synthesis treatment of polymers. The onset cracking strain will be related to film thickness, film composition as determined by XPS/FTIR, mechanical properties will be examined through nanoindentation/nanoscratching, and film defects determined from SEM and optical microscopy techniques. XPS and spectroscopic FTIR will be critical in understanding the early stages of growth of these films and linking this to their formation and mechanical properties. I will also make multilayer stretchable and conductive CVD polymer films and integrate them in flexible and organic electronic/photonic devices and examine their reliability under enhanced flexing and stretching conditions.