Sha Shi

  • Sha Shi
    Sha Shi

Research Assistant, University of Minnesota

Large Magneto-Resistance of Organic Molecular Tunnel Junctions with Non-Magnetic Electrodes

We reported large room temperature magneto-resistance for devices composed of self-assembled mono-layers of different oligophenylene thiols sandwiched between gold contacts [ACS Nano, 10, 8571−8577 (2016)]. The experiment showed that the transport mechanism through the organic molecules was non-resonant tunneling. To explain the observed phenomenon, we developed an analytical model based on the interaction of the tunneling charge carrier with an unpaired charge carrier populating a contact/molecule interface state. The Coulomb interaction between carriers gives rise to the selection of special spin combination, which means different the transmission coefficients for different spin orientation. Singlet and triplet pairing of the tunneling and the interface carriers thus correspond to separate conduction channels with different transmission probabilities. Spin relaxation, enabling transitions between the different channels and therefore tending to maximize the tunneling current for a given applied bias. Applied small magnetic fields suppress the transmission between singlet/triplet channels and lead to the observed positive magneto-resistance. Our model yields insight into the physics of the Coulomb interaction and the exchange between states for determining transmission probabilities that depend on spin.

After completion of secondary school in Tieling, Liaoning Province, China, in 2004 I enrolled at Sichuan University, Chengdu, Sichuan Province, where I received BS and MS degrees in Physics in 2008 and 2011, respectively. My research focused on developing lithography methods based on plasmonics, and my contributions were recognized by Sichuan University through the Excellence Fellowship Award and the Outstanding Senior Award. I became a PhD candidate in the Electrical and Computer Engineering department of the University of Minnesota in 2011. My advisor is Prof. P. Paul Ruden, and I expect to graduate in the fall of 2017. I have been awarded the College of Science and Engineering Fellowship and the Graduate School Doctoral Dissertation Fellowship.

In my MS program, I developed high throughput fabrication techniques and explored applications for the fabrication of photonic and electronic semiconductor devices. Plasmonics-assisted mask/non-mask lithography and imaging were proven to improve dramatically the efficiency of light coupling in near field lithography/imaging systems, and to decrease the critical dimensions of aerial patterns. The research also involved the modeling and fabrication of metamaterials that have special optical characteristics and perform as filters in photonic devices. This work is of immediate relevance to large-scale manufacturing of small feature size devices at low cost.
In the doctoral program, my research focuses on the physics of organic semiconductor materials and devices. The work extends from the analysis of the properties of materials to the development of analytical and numerical models for applications in flexible electronic and spintronic devices including organic molecular tunnel junctions and spin valves.

As a specific example for spintronic devices, the transport mechanism through organic molecules in self-assembled mono-layers is explored, and an analytical model was recently developed that elucidates how the Coulomb interaction gives rise to transmission probabilities that depend on spin. It was then investigated how spin relaxation and its dependence on an external magnetic field causes magneto-resistance. This work successfully explained recent experimental data obtained by the group of Prof. C. Daniel Frisbie in the department of Chemical Engineering and Materials Science at the University of Minnesota.
An example in the area of flexible electronic devices is my theoretical work on organic semiconductor field-effect transistors gated with ionic liquids. Very high charge carrier concentrations are induced with modest gate voltages, and intriguing new phenomena associated with the interaction between the semiconductor charge carriers and the ions in the liquid are critical. This work also involves close interaction with the experimental group of Prof. Frisbie. My current efforts in that area focus on the dynamics of these device structures.

Sha Shi’s Research webpage
Research statement