Research

 

 
 
http://www.coolarchive.com/bars/143.gif
Text Box: Back to Home
 

 

 

 


Research interests:

·                 Novel enabling technologies on nano-materials

·                 Strong and lightweight CNT fibers

·                 CNT-fiber-reinforced structural nanocomposites

·                 Functional nanomaterials

·                 Nano hybrid structures for energy applications

·                 Nanotube based bio-sensors

·                 Nanomechanics

·                 Nanoscale characterization

 

 

Research Sponsors:

                            

                                       

 

 

 

 Research Projects:

My research involves two groups of materials: Carbon nanotubes and semiconductor materials

 

1.    Carbon Nanotubes

Carbon nanotube (CNT) is an amazing nano-material that possesses fascinating properties, but putting these fascinations into applications is still facing challenges. Bridging the gap between fascinations and applications is our research interest. We are trying to develop so called “CNT enabling technologies” that can offer solutions for several fundamental issues such as:

·       Nano to Macro --- CNT fiber (Mechanical properties);

·       Controlled CNT structure & architectures (Electronic properties);

·       Direct characterization & defect study of single-walled CNTs (Metrology).

 

 

 

Example projects:

 

·       Synthesis of ultralong individual SWCNTs 

Text Box: a: Photograph of a 4.8-cm-long Si substrate on which a 4-cm SWNT was synthesised. The curve was formed by superimposing on the photograph 230 SEM images along the individual 4-cmlong SWNTs. b-d: SEM image of the beginning, middle and end segments of the SWNT. Since the discovery of carbon nanotubes in 1991, there has been great interest in creating long, continuous nanotubes for applications where their properties coupled with extended lengths will enable new technology developments. For example, ultralong nanotubes can be spun into fibres that are more than an order of magnitude stronger than any current structural material, allowing revolutionary advances in lightweight, high-strength applications. Long metallic nanotubes will enable new types of microelectromechanical systems such as micro-electric motors,and can also act as a nanoconducting cable for wiring micro-electronic devices.

We have acchieved the synthesis of 4-cm-long individual single-wall carbon nanotubes (SWNTs) at a high growth rate of 11 μm s–1 by catalytic chemical vapour deposition. This results suggest the possibility of growing SWNTs continuously without any apparent length limitation.

 

 

 

·       Characterization of intramolecular junctions in CNTs

The remarkable electronic properties of SWNTs have been demonstrated to have significant potential for use in nanoscale electronics and field-effect transistor (FET) based logic devices and for nanophotonics applications. Of particular interest for device applications are SWNT intramolecular junctions (IMJs) formed through so-called 5–7 defects of pentagon-heptagon structures that allow merging of different chiral nanotube structures to form direct metallic-metallic (MM), semiconductor-semiconductor (S-S), or metallic-semiconductor (M-S) junctions. Such IMJs are expected to display single nanotube device behavior as diodes, rectifiers, and S-S potential for electro-optics. Additionally, such single nanotube devices may eventually replace complex source-drain structures in nanotube FETs.

We observed a chiral shift along the length of an individual nanotube and are able to spectrally image the region over which this chiral transition takes place. Spectral evolution over this transition region provides an indication of the structural changes required to change chirality. The observed spectra are consistent with a structural change from semiconductor to metallic character to form an M-S junction. These results represent the first optical and vibrational characterization of a nanotube IMJ.

 

·       Superstrong CNT fibers

From the stone ages to modern history, new materials have often been the enablers of revolutionary technologies. For a wide variety of envisioned applications in space exploration, energy-efficient aircraft, and armor, materials must be significantly stronger, stiffer, and lighter than what is currently available. CNTs have extremely high strength, Text Box: SEM images of CNT fibers. very high stiffness, low density, good chemical stability, and high thermal and electrical conductivities. These superior properties make CNTs very attractive for many structural applications and technologies.

We have obtained CNT fibers that are many times stronger and stiffer per weight than the best existing engineering fibers. The specific strength of our strongest CNT fiber is 5.3 times the specific strength of the strongest commercial fiber (T1000), and the specific stiffness of our stiffest CNT fiber is 4.3 times the specific stiffness of the stiffest commercial fiber (M70J).

 

·       CNT cotton for large production fibers

To fully utilize their extremely high strength, CNTs must be spun into continuous fibers. The most efficient way to produce commercial-scale CNT fibers is by the five-thousand-years-old cotton-based spinning technology. Therefore, it is technologically attractive to produce CNT materials that have spinning properties similar to cotton.

Here we report a new form Text Box: CNT cotton on a 35 mm long quartz support. a) Top view; b) Side view. of CNT material, CNT cotton, that is made of ultralong individual CNTs. This CNT cotton is analogous to conventional cotton in many aspects including the color and fluffiness, and is found favorable for spinning.

The CNT cotton may be the easiest form of CNT materials for fiber spinning, and then has the potential to adapt for commercial cotton spinning technologies for large-scale production of nanotube based fibers.

 

 

 

 

 

 

2. Semiconductor materials and devices

·       GaN based materials and devices

1.     Cubic GaN materials and LEDs by MOCVD: Fabricated the first cubic-GaN LED in the world, paper was published on Applied Physics Letter 74, 2498(1999).

2.     Growth kinetics and dynamics of GaN: First observed and explained a new surface phenomenon in MBE: formation of ‘ghost islands’. Paper was published on Physical Review Letter 85, 2352(2000)

3.     Defects deduction in GaN films: threading screw dislocations are reduced by two orders of magnitude while edge dislocations are reduced by one order. The method was published on Applied Physics Letters, 77, 1105(2000), and won me second-class honor, the 6th national challenge cup of China, 1999.

 

·       InP based materials

1.     MOCVD epitaxy of InP/InGaAsP, InP/AlInGaAs QWs for 10G 1300nm and 1550nm DFB lasers.

2.      Improved the controllability and reproducibility of re-growth processes.

 

·       GaAs based materials and devices

1.     High purity high mobility GaAs: Electron mobility as high as 120,000 cm2/V.s (77K).

2.     GaAlAs/GaAs QW lasers: threshold current density as low as 200A/cm2. A news was released on “China Electronics” in 1992. The project won the second-class prize of scientific and technical progress from Chinese Academy of Sciences in 1994.

3.     GaAs on Si substrate: Room temperature CW laser realized.

4.     Quantum wires laser array: 100mW (highest reported at that time) output was reported on Electronics Letters 31(2), 102(1995).

5.     AlGaAs/GaAs high-power 808nm lasers: 4W CW output at room temperature.

6.     InGaAs/GaAs high-power 980nm lasers: 300mW CW single-mode (spatial) at room temperature.

7.     InGaAs/GaAs VCSELs by MOCVD: Pulse work at room temperature.