Yu Huang Wang

YuHuang WANG: Nanomaterials Chemistry and Manipulation

We focus on discovering new chemistries and physics based on nanomaterials. Some of our current areas of interest include developing ultra-selective carbon network chemistry, elucidating the fundamental principles that govern the assembly of nanostructures into ordered solids and functional networks, and developing novel devices and methods to address problems in electronics, energy and biomedicine.

1. Ultra-selective Carbon Network Chemistry. Single-walled carbon nanotubes (SWNTs) are a class of nanostructured materials that combine remarkable electrical, mechanical, thermal, and optical properties all in one. Unlike other nanostructures such as quantum dots, SWNTs are, in a sense, molecular in nature. Their fascinating properties are not simply a result of size effects, but originate from an intertwining of the one-dimensional confinement of electronic states with the incredible versatility of how the carbon-carbon bonds are arranged within the tubular network. Within a small diameter range (0.4-2 nm), a SWNT can have over 150 possible chiral structures, each uniquely indexed by a pair of integers (n,m). For two SWNTs that differ in diameter by less than 0.01 nm (e.g., [10,10] vs. [11,9]), one has an electrical conductivity rivaling copper while the other is predicted to be a semiconductor with interesting optical properties. This dramatic change of properties with a subtle difference in structure gives rise to a vast new realm of chemistry and physics.

However, all current synthetic methods for making SWNTs yield a complex mixture of all types of nanotubes and moreover, SWNTs are virtually insoluble in any conventional solvent. These two problems not only have hindered a host of potential applications and the full establishment of this new branch of molecular science, but they also have persistently challenged some of the limits of nanoscience and nanotechnology from the perspective of chemical synthesis, catalysis, complex mixture separation, characterization, and nanostructure manipulation. To meet some of these challenges, we are developing new materials strategies to eliminate the insolubility problem facing SWNTs and new methods to effect separation and manipulation on the single chirality level.

2. Electron Networks for Solar Cells. We are researching novel chemical methods and nanofabrication approaches to integrate CNTs for a wide range of basic and applied research including solar cells. The remarkable electron accepting capabilities and charge transport properties of SWNTs have suggested new possibilities for overcoming the efficiency bottleneck currently facing several next-generation solar cells. CNTs can significantly improve the performance of organic photovoltaic cells and the photoconversion efficiency of TiO2-based Grätzel cells by effectively collecting and shuttling the electrons injected from light harvesting components (e.g. porphyrins) or charge separation centers (e.g. TiO2 nanoparticles). The efficient electron transport also facilitates charge separation and prevents charge recombination, thereby may further improving the photoconversion efficiency.

3. "Seeded Crystalization" of Nanostructures: From Discrete Nanoparticles to Organized Solids. The ability to assemble isolated nanostructures into ordered solids and functional networks will be ultimately required for the realization of many potential applications based on nanomaterials, such as nanoelectronics, quantum wires, photonic crystals, and solar cells. Unlike molecules or atoms, little is known about the fundamental principles that govern the self-assembly and directed assembly of nanostructures. We will approach this problem by making an analogy to crystallization. For example, how does self-assembly nucleate on a surface? Can one plant a "seed" to direct the subsequent assembly? Is it possible to introduce long-range order into a self-assembled system by planting a series of "seeds" that are correlated in space and symmetry? Will this bottom-up approach afford both a packing density far beyond the resolution of lithography and long-range registry that is typically missing in self-assembled systems? This research builds upon powerful nanofabrication methods and tools such as massively parallel dip-pen nanolithography that the PI developed while at Northwestern.