Researches

Wide Bandgap Semiconductors and Devices

Wide bandgap semiconductors such as III-nitrides, diamond, zinc oxide, and others have intrinsic material properties not available in the conventional Si and GaAs semiconductors, and therefore have been found applications in many critical areas, such as high-power, high-temperature, high-frequency electronics, and solid-state lighting, UV and deep UV photonics (laser/LED emitters and photodetectors).

 

With supporting from industry, we currently focus on the development of deep UV photo-emitters and solar-blind photodetectors, which are finding many applications in different market sectors.

 

Phase Change Materials for Tunable Photonics

Solid-state phase change materials (PCMs) can change their crystal phase between two states rapidly and repeatedly under external thermal, electrical, optical, or mechanical controls. These two states typically have a much larger resistance differences by several orders-of-magnitudes and therefore such a phase change is typically called as metal-insulator transition. Electronics applications could take advantage of such huge change in resistivity. Concurrent with the changes in resistivity are changes in refractive index with significant contrast. Therefore, it is promising to employ these phase change materials for tunable photonics, including optical memory, terahertz-IR-vis light wave switching, spatial light modulators, and many others. Integrating PCMs with large electro-optical coefficients onto Si platform for on-chip or in-board ultrafast data communication could be promising.

We have mainly studied the material physics of vanadium oxide (VO2) and explore its potentials for THz wave modulation. We are also exploring other PCMs for tunable and high-speed photonics.

 

Renewable Energy Harvesting and Conversion: Photovoltaics

The solar energy received by the earth in one hour is more than what the world consumes in one year. Harvesting this inexhaustible green energy is clearly the approach for the sustainable development of our human beings. Solar cells (photovoltaics) is one of the most efficient approach, which directly convert the light (photons) into electrons for electricity generation.

Although silicon based photovoltaics has been a mature technology and has been steadily deployed in the past decade or so, its high cost has always been the barrier for large-scale market penetration. Its efficiency, limited at ~ 20%, is another disadvantage. Another dark side of silicon photovoltaics is its large energy consumption and considerable chemical waste generation during manufacturing.Better technologies are always a dream for efficiently harvesting solar energy in a clean approach. Imaging technologies that have much better efficiencies, or they can be produced in a roll-by-roll spray process for large scale production with considerable low cost.

In our group, we have worked on nanomaterial based sensitized solar cells. We are currently focusing more on inorganic-organic hybrid perovskites, a very promising material system toward high efficiency as well as low production cost.

 

Electrochemical Energy Storage (Power Sources): Batteries and Supercapacitors

Electricity is, with doubt, the most convenient and versatile energy carrier, while electrochemical power sources provide the vital link between the energy generation and its actual use. Rechargeable batteries are one of the key technologies that enable the popularity of portable electronics, wearable sensors, and smart phones, among many others, but insatiable power requirements demands better battery technology. Toward the green electrical vehicle technology with 500 mile range, next-generation batteries with much higher energy density and better safety must be invented. To harness the intermittent renewable energy from solar, wind to tidal power, energy storage is the essential component for power grid stabilization and for future smart grids, while the market barrier due to the high cost of the current energy storages has to be overcome through innovations.

In our group, we devote efforts in studying nanomaterials for developing next-generation battery technologies, particularly lithium-Sulfur batteries, which potentially can provide an energy density 3-5 times higher than the current lithium-ion batteries. If your current electric vehicle only runs for 150 miles per charge, you will easily drive 500 miles without worrying the power.

Supercapacitors are a second technology as power sources. They can be fully charged and discharged in a flash speed of seconds, and hence provide much larger instantons power than batteries. They potentially could find many niche applications. Here in our group, we are studying and developing ultrafast supercapacitors based on vertical graphene networks.