Organic and Polymer Synthesis, Organic Solar Cells, Molecular Electronics, Organic Spintronics
B.S. Chemistry, University of Science and Technology of China (1999); Ph.D. Organic/Polymer Chemistry, University of Chicago (2004); Postdoctoral Fellow, Stanford University (2004-2006); "Excellent Student Fellowship (First Prize)" from University of Science and Technology of China, Hefei, Anhui, P. R. China, (1995); "Panasonic Scholarship" from University of Science and Technology of China, Hefei, Anhui, P. R. China, (1996); "P&G Scholarship" from University of Science and Technology of China, Hefei, Anhui, P. R. China, (1997); "Excellent Student Fellowship (Third Prize)" from University of Science and Technology of China, Hefei, Anhui, P. R. China, (1998); "Excellent Thesis of USTC (Year 1999)" from University of Science and Technology of China, Hefei, Anhui, P. R. China, (1999); "Outstanding Leadership and Dedication," recognized by Consulate General of the People's Republic of China in Chicago, (2002); "Excellence in Graduate Polymer Research," American Chemical Society 228th National Meeting, (2004); DuPont Science and Engineering Grant, (2007-2008);DuPont Young Professor Award, (2008-2010); R.J. Reynolds Junior Faculty Development Award, (2008-2009); NSF CAREER Award, (2010-2015); Tanner Award for Excellence in Undergraduate Teaching, (2011); Camille Dreyfus Teacher-Scholar Award, (2011); Camille Dreyfus Teacher-Scholar Award, (2011); CAPA Distinguished Junior Faculty Award, (2012); Two publications were in "the hottest research of 2011" by Thomas Reuters, (2012); One publication was selected as one of the top 9 articles out of 1300+ for "Best of Macromolecular Journals 2012" by Wiley, (2013); Ruth and Phillip Hettleman Prize for Artistic and Scholarly Achievement, (2013)
The You Group focuses on the synthesis and characterization of novel multifunctional materials for a variety of applications, predominately in electronics and photonics. Our approaches are truly interdisciplinary, interfacing chemistry, physics, materials science and engineering. Students and postdoctoral fellows in the group are exposed to and trained in organic and polymer synthesis, surface chemistry, nano-patterning, device fabrication, and physical properties characterization using state-of-the-art instrumentation.
Listed below are the main research directions we are currently pursuing:
Currently, mainstream photovoltaic (PV) cells are made from inorganic semiconductors, mostly crystalline silicon. Commercially available PV cells often reach 15-18% energy conversion efficiency. However, the high cost of production and installation (currently ~$1.50/W + labor) severely restricts their large-scale application and makes them unattractive in most energy production markets. On the other hand, organic semiconductors offer a number of advantages for PV applications: (1) high optical absorption coefficients; (2) adjustable band gap to harvest a large fraction of the solar spectrum; (3) compatibility with flexible substrates; and (4) fabrication via low cost, high throughput printing techniques. What they currently lack, however, is adequate efficiency and lifetime for commercial viability.
A number of approaches to construct and improve organic (or organic/inorganic hybrid) PV (OPV) cells have been proposed and implemented, including dye-sensitized solid-state heterojunction cells, multilayer devices with stacked small molecule organic solar cells, donor (e.g., polymer)-acceptor (e.g., fullerene) organic bulk heterojunction cells, organic-nanocrystal hybrid solar cells, and more recently, perovskite based solar cells. Significant progress has been achieved in the past decade, but at present, a great deal of progress is still to be gained to help OPVs reach their proposed theoretic output.
Our group is currently seeking to further improve conversion efficiency in OPVs and build a deeper understanding on their fundamental operating principles. Using some of the structures listed above, we are applying the following strategies to achieve these goals: (1) engineering the molecular structure of conjugated polymers to harvest more photons; (2) controlling the interfaces between dissimilar materials to improve the charge transport; (3) pursuing fullerene replacement with more light-absorbing molecules while keeping all desirable features of fullerene in OPV; (4) exploring the incorporation of multiple polymers into OPVs (i.e., ternary blend or more), and (5) investigating multiple exciton generation via molecular design of novel materials. All these efforts in our own group are being substantiated by collaboration with peer researchers in the research community.
By applying novel multifunctional materials and conceptually new device designs, we expect to significantly improve the efficiency and viability of OPVs and help make low cost, highly efficient organic solar cells a strong contributor toward a future of clean and renewable energy.
Molecular electronics can be broadly defined as technologies utilizing single molecules or molecular assemblies, typically with organic based materials, to perform electronic functions. Advances in current electronics have enabled the rapid scaling down of the size of electrical components, promoting faster and more complex functions and capabilities in electronic devices. Molecular electronics represents an exciting frontier in this manner - while silicon and other commonly used materials in electronics will break down in function below a certain size, molecular electronic devices can scale down to the molecular level, providing the ability to create junctions from single atoms. Furthermore, the use of organic molecules provides the ability to incorporate a wide variety of molecular effects, such as spin polarization, quantum interference, and spin crossover, into electrical devices. Extensive research in the last decade has highlighted the exciting potential of organics in these applications and created opportunities for new paradigms in electronic applications.
We are working on two aspects of this exciting field. First, we are designing and studying architectures to incorporate organic materials into large scale, modernly applicable circuits and electrical components. While many techniques; such as scanning tunneling microscopy (STM), atomic force microscopy (AFM), and use of buffer layers such as PEDOT:PSS or EGaIn; are well developed for academic studies of molecular electronic systems, a push is still needed for these systems to be truly commercially viable. We believe the use of permanent metal-molecule-metal (MMM) junctions will help fulfill this goal, and as such we have shown the capability of nanotransfer printing (nTP) for electronic applications and are working toward scaling the technique to a macroscopic scale. We are also using recent advances in 2-D conductive materials, such as graphene and dichalcogenides, to help further this goal.
Second, we seek to study and characterize the properties of unique molecular systems, pushing the boundaries of molecular electronics by exploiting the potential of specialized molecules and designing systems with unique functionality. For example, electron spin is a degree of freedom often ignored or unable to be exploited in electronic applications. However, experimental and theoretical studies have shown the ability of highly conjugated organic systems to work in spintronic applications, even alluding to the potential for certain systems to act as inherent spin filters. Present efforts seek to study the capability of organic systems, such as porphyrin wires and hexylthiophene chains, to function in these spintronic applications. Also, recent studies have highlighted the potential for organics in thermoelectrics, where a temperature gradient generates current in an electrical junction. The inherent resistive nature of many organic materials is beneficial to these applications as heat and electricity transport slowly in comparison to metallic systems and, more importantly, may be transported at different rates, providing an opportunity to design molecules ideal to this application. We are currently studying a variety of molecules for this field and seek to understand the underlying principles of heat versus electrical transport in organic molecules.
Adhesives are ubiquitous and critical components to everyday life. To expand on their uses and discover adhesives capable of attaching to any surface, it is useful to take a biomimetic approach, wherein we look to nature for inspiration. To this end, mylitus edulis foot proteins (mefps) exhibit outstanding adhesion to numerous substrates, including many traditionally "non-stick" materials such as PTFE and PDMS. Furthermore, these mefps can be effectively mimicked by utilizing the polydopamine, obtained from the oxidative polymerization of dopamine. This polymerization yields a macromolecule with many similar properties to various previously studied mefps. We are interested in exploring the scope of applications of these polydopamine based molecules in order to make adhesives with specifically tailored properties. One such example is to incorporate polydopamine into hydroxyapatite-gelatin-silane based bioceramics to improve the mechanical properties and biocompatibility.
New Polymerization Methods
Polymers and plastics are found everywhere in daily life, and substantial effort is exerted towards identifying and optimizing new polymerizations to prepare these ubiquitous materials. In collaboration with Professor David Nicewicz’s group, we are exploring a number of oxopyrylium salts as single electron photooxidants to initiate the polymerizations. These oxopyrylium salts are capable of oxidizing a wide range of alkene containing molecules, opening the door for novel polymerizations previously unobtainable via single electron oxidation. We are interested in developing this chemistry to polymerize various monomers and comparing their properties to similar polymers synthesized via other conventional methods. More importantly, we are interested in gaining fundamental understanding on (a) the novel initiation step by these photooxidants and the subsequent chain propagation and termination, and (b) the full scope of the utility of oxopyrylium initiated polymerizations.