Nanofabrication Technique | Nanomaterials for Energy and Biosensing | Wetting and Spreading | Spiral Photonic Crystal

Research

My research interests are in the area of condensed matter physics with emphasis on nanophysics and nanoscience. Specifically, I am interested in the development of nanostructures for energy conversion and storage, photonics, biomedical, and biophysics applications. At the same time, I am studying the acoustic, optical, and thermal properties of nanostructured materials, especially, nanowire arrays, nanorod-graphene composites, and nanosprings. My work will enable the creation of nanostructures with enhanced or completely new properties for diverse applications ranging from solar cells to solid-state batteries, from thermoelectric devices to nanofluidics and biosensors.

 

The facility installed in my lab includes three deposition systems: 1) e-beam evaporator from Cooke Vacuum; 2) home built DC magnetron sputtering; and 3) home built multi-source UHV thermal evaporator. In characterization of nanostructures, we have an ultra-fast laser system, confocal Raman microscope, AFM, SEM, XRD, TEM, etc. A MOKE system is under construction right now for the study of nanomagets.

 

E-beam evaporator

 
 

 


Magnetron sputtering

 

UHV thermal evaporator

 

 


1. Nanofabrication Technique

 

I have developed a novel technique referred to as dynamic oblique angle deposition to fabricate nanomaterials. Figure 1 shows a schematic scratch of the dynamic oblique angle deposition system with the freedom of substrate rotation through a stepper motor. The source materials are evaporated at sufficiently low base pressures by methods such as thermal evaporation, electron-beam evaporation, and sputtering. Either single elements or compounds can be deposited with a suitable evaporation method. As the incident atoms approaching the surface at an incident angle q, as shown in Fig. 1, they will deposit on the extruding part of the surface. The lower parts of the surface are shadowed by the neighboring structures. This is called “global shadowing effect”. After landing on the surface, the atoms may diffuse across the surface to an energetically favorable site. The substrates can be placed in the chamber and locked in a fixed position, or can be rotated in a designed motion pattern. For both cases, straight nanowires are the result of the competition of shadowing effect and surface diffusion. The difference is that tilted nanowires will grow if the substrate is fixed in one position, while vertical nanowires will grow if the substrate is rotated with sufficient speed. In the case of very slow substrate rotation, the nanowires turn into nanosprings.

 

 

 

Fig. 1 Schematic draw of a dynamic oblique angle deposition.

 

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2. Nanomaterials for Energy and Biosensing Applications

My research on this subject focuses on fabricating 3D nanomaterials that can be used as building blocks for energy applications and complex biomedical analysis. The nanomaterials that I am studying are in the forms of nanorods, nanowires, and nanosprings. With the extremely small feature size and large surface area, these nanomaterials can be used as: high-efficiency photovoltaic and thermoelectric materials; ultra-sensitive biosensors based on the surface-enhanced Raman and plasmonic effects; and nanoscale electrodes in batteries. These nanostructures cannot be obtained by advanced lithographic techniques, such as e-beam direct writing, holography, and X-ray lithography.

I employ the dynamic oblique angle deposition technique to fabricate these structures. In this technique, materials are introduced and deposited on a rotating substrate at a highly oblique angle, producing nanostructures with a very large surface area and controllable porosity, shape and symmetry that are not easily achievable by other techniques. The dynamic oblique angle deposition technique is unique in that practically any materials or combination of materials can be grown on any substrates, including glasses and soft polymers. The deposition system can be easily built on a common sputtering or e-beam evaporation system.

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3. Wetting and Spreading on Nanostructured Surface

Generation and manipulation of sub-micron droplets could have impacts on biological assays. I am interested in the study of ultra-small droplets, especially those containing biological molecules. In this research, 3D nanomaterial arrays will be created and investigated for fundamental understanding of the surface wetting and spreading on a nanometer scale. I will design a nanofluidic device containing layered hydrophobic and hydrophilic nanomaterials to achieve rapid mixing of biological flow through diffusion and capillary effects. The flow on this hybrid surface can be focused and confined to an ultrathin layer determined by the thickness of the hydrophilic layer. Therefore, this hybrid surface can provide unique platforms for bio-diagnostics.

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4. Spiral Photonic Crystal

Photonic crystals are periodic nanostructures with regular dielectric constants that control and manipulate photons. Likewise, phononic crystals are regular arrays of elastic materials that control acoustic waves (or phonons). It is very likely that the nanospring arrays, as shown in Fig. 1c, have both photonic bandgaps and phononic bandgaps due to their unique geometries and elastic properties. I have experimentally demonstrated the complete photonic bandgaps of the nanospring arrays, and I will extend my study to the hypersonic frequencies of the same structures. The study of phonons is acknowledged as a major area of condensed matter physics. However, the knowledge of phonons is extremely limited for 3D nanostructures, such as the nanosprings. This research could have an impact in nano-scale condensed matter physics.

 

 

 

 

Fig. 2 Nanospirals grown by the dynamic oblique angle deposition.

 

 

 

 

 

This research could also be used in heat management and thermal energy conversion. High-efficiency thermoelectric devices may require the combination of high electric conductivity and low phononic heat conductivity. I believe that the phononic heat conductivity can be greatly reduced but that the electric conductivity remains the same in phononic crystals. Therefore, it is possible to greatly improve the performance of the thermoelectric devices through the structural engineering of the devices.

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Last updated 07/10/2010