Tian Li (Ph. D.)

Tian Li received Ph.D. degree in Electrical and Computer Engineering advised by Prof. Mario Dagenais from University of Maryland, College Park.

She is currently a Postdoctoral Research Scholar with Prof. Liangbing Hu in University of Maryland, College Park.

Research Concentration 1: Emerging Cellulose Science and Engineering towards Energy, Water, Electronics and Sustainability

Multiscale cellulose structure for applications across length scale

Molecular engineering for ångström scale ion channels

Our work expands the study of cellulose towards molecular scale engineering. Upon crystalline structure conversion, the previously closed molecular chains within the elementary fibers opens up and allows for ion intercalation where new physics and transport mechanism arises. These sub-nm scale or molecular scale ion channels among molecular chains of cellulose allows for interesting interaction among cellulose, water and ions. As a demonstration, the transport of sodium ions within the molecular chains were demonstrated which was utilized to achieve a high ionic thermoelectric performance (nature materials, 18, 6, 2019).

With the aid of advanced characterization methods including small angle neutron scattering and small angle X-ray scattering as well as fundamental understanding of the process-structure-property-application relationship via molecular engineering, enormous opportunities in a myriad of new directions can be foreseen.

Structural engineering for micro- and nano-fluidics

The dimension of the ion transport channels shows excellent tunability via structural engineering. This opens up an exciting new direction using abundant bio-materials—cellulose for fluidic applications. Taking nanofluidics for example, nanoscale ion channels exhibit among fibrillated cellulose and elementary cellulose fibers with a dimension of 2 nm to 50 nm. Wood with numerously aligned nanoscale channels can thus be used as an ion regulation membrane from which an electrically gated ionic transistor was demonstrated with an exchange of electrical signal and ion signal (Science advances).

Energy efficient building materials with light and thermal management

Buildings represent the largest energy sector. Building energy efficiency must be considered as a part of our sustainable energy strategy. With innovative functionalization, wood can be made as an extremely attractive energy efficient building material. Several wood technologies were developed including transparent wood, thermal insulation nanowood, and radiative cooling wood.

Taking cooling wood for example, it features an effective back-scattering of sunlight and minimum absorption coefficient for the realization of the sub-ambient radiative cooling, providing a perpetual path to dissipate heat into the universe via the atmospheric transparency window without energy consumption. The cooling wood is directly derived from mesoporous natural wood. The material exhibits continuous cooling effect with a mechanical strength > 400 MPa and a specific tensile strength of 334.2 MPa cm3/g. The high strength is attributed to the physical entanglement of microscale wood cells and the maximized interaction among aligned nanofibers via hydrogen bonding.

Research Concentration 2: High temperature synthesis-structure-properties

The high temperature synthesis (up to 3300 K) enabled by joule heating of carbon-based substrate opens a new paradigm of nanomaterials. Many new and exciting scientific discoveries on the correlations between high temperature synthesis-structure-properties await. The development of the ultrahigh-temperature operation of thermoelectrics can open up possible applications in many high-power energy systems. To efficiently convert heat to electricity, a high operating temperature is desirable to ensure a high Carnot efficiency. The thermally-reduced solution-processed graphene oxide at 3300 K is shown to be a highly efficient and reliable thermoelectric material up to 3000 K, which makes it a promising candidate for a broad range of applications including concentrated solar power, radiation energy conversion, thermoelectric topping cycles for power plants, and direct energy generation from hydrocarbon combustion, since our maximum operation temperature exceeds the adiabatic flame temperatures of all common fuels in air.

PhD work on Self-Assembled InAs/GaAs/ Quantum Dot Solar Cells and Non-linear Optics

My PhD work focuses on experimental and theoretical studies aimed at establishing the fundamental understanding of the linear and non-linear electrical and optical processes governing the operation of quantum dot solar cells and their feasibility for the realization of intermediate band solar cell.

Uniform performance QD solar cells with high conversion efficiency have been fabricated using carefully calibrated process recipes as the basis of all reliable experimental characterization. We are able to distinguish the nonlinearity effect by 1PA and 2PA process. The observed 2PA current under off-resonant and on-resonant excitation comes from a two-step transition via the tailing states instead of the QD states.

My PhD ~ 5 years experiences in cleanroom fabrication and setting up optoelectronic characterization systems including external quantum efficiency, z-scan, two-photon absorption, automated optical fiber-waveguide coupling stage, et al.

Contact:

mytian1211@gmail.com; tianlee@umd.edu

1147 Energy Research Center

University of Maryland, College Park, 20742