Projects Synthesis Materials Fabrication Devices Characterization Theory
Nanobioarchitecture   AAO template  
Langmuir Blodgett   Electron beam lithography  
Focused ion beam lithography   Surface LBL assembly  
Dielectrophoresis   LSPR  
Nanobioarchitecture
The objective of this project is to apply the current nanotechnology to nanoscaled bio-architectures, that are selective and effective, by considering self-assembled bottom-up approaches on nano-patterned surface. Nano-patterns will be prepared by dip-pen lithography, electron beam lithography, focused ion beam (FIB), or block copolymer synthesis. Nano-structured bio-architecture will be fabricated on nano-patterned surfaces and be applied to detect specific compound in solution. Also, It will be used to produce specific compound in solution by bioconversion.
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AAO template
An anodic aluminum oxide (AAO) template is used much like a caste in this fabrication method. In this project, done in collaboration with Professor Sungho Park in the Chemistry Department of Sungkyunkwan University, precursors in solution of a desired material are inserted into the pores of the AAO. Casting of the precursors is followed by an annealing recipe to realize functional nanostructures, such as nanorods and nanowires. The alumina can be retained around the nanostructures or discarded for use various applications. This technique is currently yielding TiOx, NiOx, and InNx nanorods.

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Langmuir Blodgett
Metallic and semi conducting nanoparticles capped with protecting ligand shells have recently been demonstrated to be promising building blocks for molecular electronics and optoelectronics. For molecular electronics, as well as for other applications, the ability to arrange these nanoparticles into wires, films, or three-dimensional assemblies is a crucial point. In most cases, films of nanoparticles are formed either by spin coating, sedimentation, or by simply dipping a substrate into a solution of particles. With those techniques the thickness of the films is difficult to control. Conversely, the Langmuir- Blodgett technique provides a suitable way to control the thickness of the film, which can be tuned layer by layer.

A notable feature of Langmuir-Blodgett film deposition is the instrument’s capability to maintain the orientation of the molecules during the transfer of the film from air/water interface on to a solid substrate. Symmetric compression achieved by two inwardly moving barriers and film deposition right at the centre of film compression ensures that the molecular arrangement of the film will not be altered during deposition process. The troughs are made of cast molded solid piece of purified PTFE (Polytetrafluoroethylene). Dipping well is integral part of the trough and no glue or compression seals with PTFE or silicon o-rings are used to fasten the dipping well into the trough. Barriers are made of hydrophilic material in order to provide leak proof film compression. Because of hydrophilic barriers it is not necessary to overfill the trough but the water level can be kept even below the edges of the trough for greater surface pressures and for eliminating film leakage. The figure shows a Langmuir-Blodgett film deposition system which can be controlled by computer.


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Electron beam lithography
With a focus on resist development, this research has two main components. One area is the development of new ultra sensitive negative tone resists for the direct writing ofnanostructures. By ultra sensitive we mean that these resists are specifically synthesized to be capable of good development and contrast characteristics for low energy electron beam (100-300 eV) nanolithography.

In addition we are optimizing a set of negative tone electron beam resists that will be used for top-down functional nanostructures fabrication. The goal of this component is not only to direct write, and thus position at will, nanostructures at sub 50 nm resolution. The nanostructures should also possess the desired functional material qualities. Materials are chosen due to their functional qualities, such as ferromagnetic properties, index of refraction, high carrier mobility, and/or conductivity, just to name a few; some materials even possess a combination of these properties. Many material properties are discovered when the material is in bulk but it is known that these features can be retained or enhanced when the material is in e.g. nanowire form, which makes such nanowires suitable choices for various nanodevices. Therefore the goal of this component is to directly write and position functional nanostructures using a catalyst free top-down electron beam lithography nanofabrication technique. In comparison to most traditional nanowire bottom-up approaches, this technique requires no catalyst and structures like nanowires of varying length can be written on the same substrate in a controllable and predictable manner.
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Focused ion beam lithography
A Focused Ion Beam (FIB) is a versatile tool that can be used for nano machining in 2 and 3 dimensions, high resolution and high contrast imaging, analysis, device editins, three dimensional reconstruction, STEM imaging and sample preparation for the TEM.

A Focussed Ion Beam Microscope uses a very narrow beam of metal ions, typically gallium, to image the sample like a scanning electron microscope and locally remove material to create complex shapes with dimensions as small as 10 nm (less than 1/1000 diameter of hair). It can also deposit metal or insulator layers by injecting a gas which decomposes in the ion beam.

The unique combination of 10 nm resolution imaging with the ability both to remove and to deposit material in selected areas provides a means of performing materials studies or device fabrication processes which would otherwise be impossible or unreasonably time-consuming. Hence, FIB faciliates rapid prototyping and assessment.

FIB has great potential for nanoscience and for the future development of nanotechnology. Applications in nano engineering include the fabrication of optical routing using photonic component integration, advanced gratings for photonic bandgaps in active media and quantum dot lasers to investigate high speed modulation.

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Surface LBL assembly
We aim to develop an innovative method for fabrication of complex 2-D or 3-D nanoarchitectures based on prepatterning surfaces with charges. We use electrostatic interactions to order charged nano-objects on a surface.

The charged state on the surface will serve as a foundation of nanoarchitecture. Any charged nano-objects can be subsequently attached by electrostatic interactions. Combining microcontact printing technique, deliberate and selective deposition control is also possible.

Biomolecules, such as proteins and DNAs, are being demonstrated to be excellent building materials for nanoscience and nanotechnology with unique biological recognition properties.

The rich chemistry and biology associated with the biomolecules have enabled specific modification and tailoring to meet specific needs in various nanofabrications. However, most bottom-up approaches with biomolecules are based on solution phase self-assembly, which lack positional control over the build-up of the nanostructures on surface. To use biological nano-assemblies as potential nanoscale sensors and devices, precise positional control over the fabricated nanostructures on surface is needed. We have established a new approach by combining the top-down and bottom-up strategies. Several techniques, including (a) AFM nanolithography, (b) nanopen delivery, (c) focused-ion beam microscopy (FIB), and (d) micro/nano contact printing, are used as the top-down approach to create nanoscale template for selective positioning of biomolecules on surface, while molecular recognition between specific biomolecules are used as the bottom-up to assemble and grow the nanostructures.
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Dielectrophoresis
Dielectrophoresis(DEP) is a phenomenon in which a force is exerted on a dielectric material when it is subjected to a non-uniform electric field. This force does not require the particle to be charged and strength of the force depends strongly on the particle chemical makeup, shape, and size, as well as on the frequency of the electric field.

Using DEP nanowires can be aligned and low cost large-area devices that incorporate aligned arrays of functional nanowires can easily be fabricated. DEP is also easily scaled up, which leads to more probable commercialization of the device when it is fabricated by this method. Conventional methods to fabricate aligned nanowire arrays are available, but have distinct disadvantages such as high-cost, small area and complex set up in comparison to DEP. Large, portable devices apply broadly to industry (e.g. flexible electronic, sensors and display applications) and the DEP fabrication method will be key in realizing such devices for industry.

Before real experiments are done, simulations are carried out. Please visit the simulation section of our theory page for more infromation. A typical experiment may start with a pre-patterned Si-SiO2 substrate and Ti/Au electrodes deposited by sputtering. Long grown nanowires undergo ultrasonication in solvent (e.g. EtOH, DCE of IPA) to make fragmented short wires which are easier to disperse in solution. After ultrasonication, the wires are loaded to Ti/Au electrodes with a syringe or pipette. When loaded nanowires have a larger dielectric constant than the solvent, they tend to move the maximum electric field region (positive dielectrophoresis) whereas nanowires with a smaller dielectric constant tend to move the minimum electric field region (negative dielectrophoresis). In this way the wires can be aligned parallel according to the induced electric field. The samples are observed as they undergo voltage and frequency changes in order to find the optimized condition that leads to an aligned array of nanowires.

Finally, to expand to a large-scale array and control each contact condition so that this technique applies to industrial usage, experiments using various kinds of nanowires and solvents will be performed. Below, the left figure shows the basic scheme of dielectrophoresis.


An injection pump is used to load wires to the electrode. According to applied AC current, nanowires are moved toward or away from the electrode. Furthermore, here not only the integration is observed, but also other properties (this case, the impedence) can be measured. Above, the figure to the right shows a real SEM image of integrated nanowires. This is somewhat different from the simulation image. In this real image the wires are not quite evenly branched and separated which is evidence of the difficulty involved when integrating the nanowires with good contact.

Current issues and future device issues involve how a more elaborate pattern (not just parallel lines) can be achieved and how each nanowire-electrode coupling can be controlled so that the idea might be expanded to a practical device such as memory or transistors. In addition the focus is on increasing the success rate (current success ratio is about 30%) and making the technique reproducible.

References:
1. Microelectronic Engineering 81 2005 83-89
2. J.Phys. D : Appl. Phys 2003 L109-L114
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LSPR
Noble nanoparticle size, shape, separation and dielectric environment are key factors of the localized surface plasmon resonance. For optimization of such parameters finite difference time domain (FDTD) simulation is used. From the simulation key parameters are determined and the nanoparticle pattern can then be made.

For making a pattern, conventional lithography techniques are not capable of creating an exact pattern at the length scale necessary for LSPR applications. One way to solve such a problem is to use the electron beam resist based on the noble metal colloid. Another solution is to make a nanoparticle by synthesizing a new hybrid material. Requirements of such a material would be the ability to deposit it on a transparent substrate and to control the gold nanoparticle size by initial gold thickness. This work demonstrates the area of LSPR sensing using noble metal nanoparticle. We will fabricate chemical, biological, environmental and optical nanodevices based on LSPR technology.

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