Our overall research interest is centered on the development of nanomaterials and explore their potential applications for photonics, electronics and energy conversion. Scientific research projects carried out in the Li group can divided broadly in to four categories.

I. Low-dimensional Nanostructures for Energy Conversion

Hydrogen generation from photoelectrochemical (PEC) water splitting, H2O -->H2 + 1/2O2, represents a holy grail in chemistry and energy science. Compared to bulk semiconductors, nanostructured photoelectrodes can potentially improve the solar-to-hydrogen conversion efficiency due to their large surface area and short diffusion length for minority carriers. We have recently fabricated PEC devices using dense and vertically aligned metal oxide nanowire arrays, such as TiO2 and ZnO, as photoanodes. We have explored different methods including elemental doping and quantum dot sensitization to improve the visible light absorption of these wide band gap metal oxides. Laser Photonics Rev. 4, 517-528 (2010)

1. Nitrogen-doped ZnO nanowire arrays for photoelectrochemical water splitting

Dense and vertically aligned ZnO nanowires were first prepared from a hydrothermal method, followed by annealing in ammonia to incorporate N as a dopant. Upon illumination at a power density of 100 mW/cm2 (AM 1.5), water splitting is observed in both ZnO and ZnO:N nanowires. N-doped ZnO nanowires showed an order of magnitude increase in photocurrent density compared to undoped ZnO nanowires. Nano Lett. 9, 2331-2336 (2009)

2. Synergistic effect of CdSe quantum dot (QD) sensitization and nitrogen doping of TiO2 nanostructures for photoelectrochemical solar hydrogen generation

TiO2 nanoparticles (anatase) and nanowires (rutile) simultaneously doped with nitrogen and sensitized with CdSe QDs showed significant enhanced photoresponse, compared to N-doped TiO2 or CdSe QD sensitized TiO2 samples. The enhancement is attributed to the synergistic effect of CdSe sensitization and N-doping that facilitate hole transfer/transport from CdSe to TiO2 through oxygen vacancy states mediated by N-doping. Nano Lett. 10, 478-483 (2010)

3. Double-sided CdS and CdSe quantum dot (QD) co-sensitized ZnO nanowire arrays for photoelectrochemical hydrogen generation

Dense ZnO nanowire arrays were grown on an double-sided ITO substrate followed by respective sensitization of CdS and CdSe QDs on each side. The photoanode exhibited strong absorption in nearly the entire visible spectrum up to 650 nm, with a IPCE of 45% at 0 V vs Ag/AgCl. Under a single white light illumination of 100 mW/cm2, the photoanode yielded a significant photocurrent density of 12 mA/cm2 at 0.4 V vs Ag/AgCl. The photocurrent and IPCE were enhanced compared to single QD sensitized structures as a result of the band alignment of CdS and CdSe in electrolyte. Moreover, the double-sided architectures showed improved charge collection efficiency compared to single-sided cosensitized samples, as a result of direct interaction between QDs and nanowires. Nano Lett. 10, 1088-1092 (2010)


II. Complex Nanowire Structures for Functional Devices

Our research objective is to design and synthesize III-nitride semiconductor nanowires and nanowire heterostructures using chemical vapor deposition and metal-organic chemical vapor deposition. Complex nanowire structures with simultaneous composition and doping modulation represents a breakthrough in synthetic strategy that allows engineering of band structure and the 'built-in" of active device elements such as p-n diode or transistor at the nanoscale. The development of III-nitride heterostructures has lead to the demonstration of nanoscale multicolor LEDs, multiple-quantum-well nanolasers and high electron mobility transistors. Materials Today 9 (10), 18-27 (2006)

1. Tunable Nanolasers from Nanowire Structures

We developed a multi-quantum well (MQW) nanowire heterostructure, consisting of a GaN NW core that functions as an optical cavity as well as a number of epitaxial InGaN/GaN MQW shells that serve as the composition tunable gain medium. The whole structure is dislocation-free, single-crystal and has a unique triangular cross section. Optical excitation of individual MQW NW structures yielded lasing with emission engineered from 365 to 494 nm through modulation of QW composition, and threshold dependent on quantum well number. Our work demonstrates a new level of complexity in nanowire structures, which potentially can yield free-standing injection nanolasers. Nature Mater. 7, 701-706 (2008)

2. III-Nitride Nanowire Heterostructures for Nanoscale Photonics

We demonstrate efficient injection of carriers into active nanophotonic devices by developing radial nanowire heterostructures. The structures include n-GaN/InGaN/p-GaN core/shell/shell (CSS) and n-GaN/InGaN/p-AlGaN/p-GaN core/multishell (CMS) nanowire structures. The shell thickness and composition can be synthetically controlled. Transport measurements confirmed electron and hole conduction from Si-doped GaN cores and Mg-doped GaN shells, respectively. Electrical devices made by the CSS/CMS nanowires yielded strong electroluminescence in forward bias, with tunable emission consistent with the fundamental band gap of InGaN shell and high external quantum efficiencies. These works suggest substantial potential of complex nanowire structures in photonics. Nano. Lett. 5, 2287-2291 (2005); Nano. Lett. 4, 1975-1979 (2004)

3. Dopant-free GaN/AlN/AlGaN Nanowire Structures as High Mobility Electron Transistors

We report the rational synthesis of dopant-free GaN/AlN/AlGaN radial nanowire heterostructures and their implementation as high electron mobility transistors. Transport measurements confirm the existence of electron gas in the undoped GaN/AlN/AlGaN nanowire heterostructures and also yield an intrinsic electron mobility of 3,100 cm2/Vs and 21, 000 cm2/Vs at room temperature and 5 K, respectively, for the heterostructure. Field-effect transistors fabricated with ZrO2 dielectrics and metal top gates showed excellent gate coupling with near ideal subthreshold slopes of 68 mV/dec, an on/off current ratio of 107, and scaled on-current and transconductance values of 500 mA/mm and 420 mS/mm. Nano Lett. 6, 1468 (2006)


III. Microbial-Semiconductor Interface


1. Solar-Driven Microbial Photoelectrochemical Cells with a Nanowire Photoanode.

The solar MPC consists of a p-type cuprous oxide nanowire-arrayed photocathode and an electricigen (Shewanella oneidensis MR-1)-colonizing anode, which can harvest solar energy and bioenergy, respectively. The photocathode and bioanode are interfaced by matching the redox potentials of bacterial cells and the electronic bands of semiconductor nanowires. We successfully demonstrated substantial current generation of 200 A from the MPC device based on the synergistic effect of the bioanode (projected area of 20 cm2) and photocathode (projected area of 4 cm2) at zero bias under white light illumination of 20 mW/cm2. We identified the transition of rate-limiting step from the photocathode to the bioanode with increasing light intensities. The solar MPC showed self-sustained operation for more than 50 h in batch-fed mode under continuous light illumination. Nano Lett. 10, 4686-4691 (2010)

2. Microbial Reduction of Graphene Oxide by Shewanella

Graphene oxide (GO) was reduced to graphene in a normal aerobic setup under ambient conditions as mediated by microbial respiration of Shewanella cells. The microbially-reduced graphene (MRG) exhibits excellent electrochemical properties. Extracellular electron transfer pathways at the cell/GO interface were systematically investigated, suggesting both direct electron transfer and electron mediators are involved in the GO reduction. Nano Res., in press


IV. Controlled Synthesis of Novel Nanomaterials and Nanostructures

Rational synthesis of new materials with control over their morphology and physical properties can enable big advances in nanoscience and technology. We aim to address the potential via a unique bottom-up synthetic strategy, by using metal-nanoclusters as catalysts to mediate one-dimensional nanowire growth. Our interest lies in the synthesis and characterization of a wide range of nanomaterials and structures, where new functions are expected at the nanoscale.

1. Ultrasmall single-crystal indium antimonide (InSb) nanowires

We report the rational synthesis of ultrasmall InSb nanowires down to 4.5 nm diameter, via a vapor-liquid-solid growth mechanism. The growth was carried out in a home-built three-zone chemical vapor deposition system, which allows continuous tunability of respective In and Sb vapor pressure via separate temperature control. Several parameters are important for achieving successful nanowire growth, including the use of catalysts and tuning of the V/III ratio. InSb nanowires had uniform diameters with lengths up to several micrometers, and their sizes were defined by gold nanoparticles. HRTEM showed that InSb nanowires were single crystals elongating along the <111> direction, regardless of wire diameter. Cryst. Growth Des. 10, 2479 (2010)