FACULTY SPOTLIGHT: DR. JIAN XU
by Matthew Frank (Engineering Science senior)

Two of the driving forces that brought Dr. Jian Xu to the Penn State Department of Engineering Science and Mechanics were spectacular nanofabrication facilities and the freedom to conduct multidisciplinary research. Almost three years since his arrival in August, 2003 his lab is a reflection of those attractions. Dr. Xu is now working with semiconductor nanocrystal quantum dots to pioneer a new material-set, applicable to the next generation of optoelectronic devices and integrated circuits. Quantum dots are special semiconductor nanocrystals whose electrons occupy well defined discrete quantum states. This characteristic allows Dr. Xu to alter their energy structures.

Xu’s research focuses on modifying the structure of quantum dots to specifically tailor their emission and absorption properties to the needs of a wide array of applications which may include energy conversion, optical communications and biochemical sensing.



With a controlled atmosphere glovebox system, Xu’s group mass-produces quantum dots using an inexpensive organic technique. By varying the size of the quantum dots, he elicits specific light emissions properties and a wide range of brilliant colors. Xu envisions these materials as likely candidates to replace liquid crystal display (LCD) devices, such as those found in computer screens and iPods. The advantage of using quantum dots, along with their easily tunable emissions properties, is that they can exhibit a photoluminescence efficiency of up to 90%.

The quantum dots have been incorporated into electronic and photonic devices with spin-cast and thermal-evaporation processes, allowing Xu and his team to explore different applications. As he states, “Right now we are trying to embed these nanoparticles in a micro cavity structure so eventually we can get a very pure light emission device.” Xu hopes to demonstrate that devices built using quantum dots are less restricted in terms of the methods by which they can be fabricated.

Xu sees an opportunity to use quantum dots to improve the efficiency of plastic solar cells, and he has received a seed grant from the Army Research Office to support his research in this area. The Army’s interest in this is for its potential uses in the field. For example, one application of the technology would be to effectively harness energy from the roof of a tent. Advances in plastic solar cell technology would be valuable to a wide range of applications, like providing power to small electronics in remote locations.

Xu explains that, “Currently plastic solar cells experience a number of limitations, one of them being the inability to harvest infrared light and convert it to useful energy.” In this project, he tailors the absorption properties of PbSe quantum dots to capture infrared light (1-2 microns in wavelength) in addition to wavelengths of light within the visible portion of the electromagnetic spectrum. Existing polymer materials used in current plastic solar cells absorb only visible light, wasting other wavelengths. This research could be a crucial step towards opening new avenues of efficiency. With plastic solar cells, Xu hopes to eventually demonstrate levels of efficiency near 8%, the current standard for rigid solar cells.

Xu will also be exploring other material properties associated with the doping of quantum dots, such as electron mobility, to assess their effectiveness in increasing efficiency. “The final goal of my research in this area,” he states, “is to produce a highly efficient plastic solar cell which is flexible, inexpensive, and useful in large surface area applications.”

The ability of quantum dots to both emit and absorb light also makes them a desirable material to integrate into optical devices for fiber optic communications. Xu spin-coats a thin-film of quantum dot-polymer mix onto a silicon substrate. This creates a hybrid device that can act as a detector, an emitter or a modulator. These optoelectronic components are useful at each end of fiber optic communications where signal multiplexing and de-multiplexing must occur.

Xu is also collaborating with the Penn State Department of Physics to construct nanowires, which are capable of linking nanocrystal structures for the purpose of injecting current and building single photon devices. In addition to his work with quantum dots, Dr. Xu conducts research in semiconductor optoelectronics and bioelectronics. In his other projects he is developing semiconductor lasers, detectors, and modulators that will improve the performance of optoelectronic circuits. Xu is also trying to construct an interface that will improve communication between biological systems and semiconductor electronics. He is very excited for next year’s arrival of a Bionanophotonics Teaching Laboratory which will give students the opportunity to build real nano-sized electronic and photonic devices.

Dr. Jian Xu teaches several classes in the Penn State Department of Engineering Science and Mechanics. As professor for the third year course E SC 314 “Engineering Applications of Materials,” he instructs students in the electrical and optical properties of semiconductors, dielectrics, and magnetic materials. Xu has also taught E SC 400H “Electromagnetic Fields.” Most recently he has developed a new 400-level course (E SC 482) on micro-optoelectromechanical and nanophotonic devices and systems. This course is one of several new courses that have recently been developed to support the growing nanofabrication industry.