The You Group focuses on the synthesis and characterization of novel multifunctional materials for versatile applications, mainly electronics, photonics and catalysis. Our research activities involve organic and polymer synthesis, surface chemistry, nano-patterning, physical properties characterization and device fabrication. Our approaches are truly interdisciplinary, interfacing chemistry, physics, materials science and engineering. Students and postdoctoral fellows in the group will be exposed and trained in organic/polymer synthesis, characterization with state-of-the-art instruments and nanofabrications.

Listed below are the four main research directions we will pursue:

1. Organic/Inorganic Hybrid Solar Cells
As the primary energy source (85% of total energy consumption), the fossil-fuels could be depleted as early as 2030. To reduce the greenhouse effect associated with burning fossil fuels and to sustain healthy growth of the world economy, it is imperative to search for alternative, renewable energy sources. Therefore, there has been a tremendous amount of effort to construct solar cells, i.e., photovoltaic (PV) cells, with high energy conversion efficiency and low production cost.

Currently, mainstream PV cells are built upon inorganic semiconductors, mostly crystalline silicon. The energy conversion efficiency of these PV cells can reach 25%; however, the high cost of generated energy (currently ~$4/W) thwarts their large-scale application, making them unattractive to energy production markets. On the other hand, if organic semiconductors could be employed for PV applications, highly efficient PV cells with low production cost could be obtained because of following advantages associated with organic semiconductors: (1) high optical absorption coefficients; (2) adjustable band gap to harvest a large fraction of the solar spectrum; (3) compatibility with flexible substrates and fabrication via low cost, high throughput printing techniques.

A number of approaches to construct organic (or organic/inorganic hybrid) PV cells have been proposed and implemented, including dye-sensitized solid-state heterojunction cells, multilayer devices with stacked small molecule organic solar cells, donor-acceptor organic bulk heterojunctions, organic-nanocrystal hybrid solar cells, and ordered organic-inorganic bulk heterojunctions. However, until now, the highest energy conversion efficiency of organic PV cells is only ~ 5%.

We will apply the following strategies to improve the conversion efficiency: (1) building well ordered materials/structures to capture most excitons and separate them into mobile charges; (2) synthesizing small bandgap materials to harvest more photons; (3) engineering the interface between dissimilar materials to improve the charge transporting; (4) induced/controlled assembly to enhance charge mobilities.

By applying novel multifunctional materials and conceptually new device designs, we expect to push the efficiency toward 20% and beyond. Low cost, highly efficient, next generation solar cells will greatly contribute to the future renewable energy economy.

2. Molecular Electronics
Molecular electronics can be broadly defined as the technology utilizing single molecules, molecular assemblies, carbon nanotubes or other nanoscale metallic or semiconductor wires to perform electronic functions. The rapid progress in the size shrinking of electronic components in accordance with Moore’s law suggests that the ultimate solution to the growing demand in the miniature of size will be constructing electronic circuit at the molecular or atomic level. The first theoretical model of molecular electronics was demonstrated by Aviram and Ratner, in which they proposed that individual molecules of the type donor-spacer-acceptor (D-s-A) between two electrodes would behave as molecular rectifiers under an electrical voltage bias. There have been tremendous research efforts in the area of molecular electronics in the last decade, and numerous nanostructured materials with versatile electronic functions have been prepared and characterized, such as molecular wires, rectifiers, switches, and transistors.

We will work on three aspects: (1) new molecules, such as transitional metal complexes inserted between conjugated moieties; (2) new surfaces, such as platinum, ruthenium and silicon; (3) fabricating nanogaps by using single walled carbon nanotubes (SWNTs) and DNAs.

3. Organic Spintronics
Conventional electronic devices are charge-based: they ignore the spin properties of the charge carriers (electrons). The emerging field of spintronics (spin transport electronics or spin-based electronics), in which the spin degree of freedom (up or down) of the electrons is used in addition to or instead of the charge degree of freedom, is full of opportunities. Since spin can be manipulated at a faster speed and lower energy cost than charges, spin-based devices should have faster switching times and decreased electric power consumption compared with conventional semiconductor devices. Numerous materials for spin-based multifunctional devices have been proposed for or implemented in memory devices, spin valves, spin-transistors, and spin-light emitting diodes.

Most of the research on spintronics has been conducted using inorganic materials, for example, alloys of nickel, iron and cobalt as ferromagnetic materials and copper as a nonmagnetic conductor; only a few reports have tested organic materials as the conducting layer in spin valves, and none of them have scaled down to the molecular level. To address the demand for miniaturization in semiconductor and microelectronics industries, a molecular-scale spintronics device is in imminent need.

There are a number of advantages in introducing organic molecules for spintronics: (1) Organic molecules are small - usually a few nanometers in length - but fully functional, as demonstrated by the burgeoning field of molecular electronics; (2) Spin diffusion length could be as long as 200 nm in organic molecules such as oligo(thiophene)s; (3) With the powerful tool of “unlimited” organic chemistry, the electronic properties of these organic molecules can be fine-tuned by tweaking the structure and functional groups. Furthermore, these molecules can be easily manipulated through self-assembly, whereas fabricating inorganic-based devices requires expensive vacuum deposition facilities.

We will build molecular spin valve through rational design and synthesis of conjugated molecules and investigate the spin transporting properties using Scanning Tunneling Microscope (STM). Furthermore, we will fabricate spin valves through layer-by-layer approach by sequential assembly of organic molecules and ferromagnetic nanoparticles. There will be other important issues to be addressed, such as spin injection, spin transport and spin detection.

We believe these new concepts in materials and nanofabrication will revolutionize the field of spintronics and aid in the discovery of novel device physics and applications.

4. Novel Catalytic System using Carbon Nanotubes
High-surface-area carbons have been the support for various noble metals and other catalysts to effectively catalyze organic transformations. A special type of carbon that is under intensive investigation is single walled carbon nanotubes (SWNTs). Compared with activated carbons, SWNTs have a few advantages, such as well-defined structure and the feasibility of introducing versatile functionality through reliable synthetic methods.

We will explore various methods to functionalize these SWNTs with catalytic centers and investigate the catalytic properties. Besides, these new catalysts are anticipated to be fully reusable. Novel chemistry will be developed and a whole new family of SWNTs-based catalysts is expected.