Cheol Woo Ha

Photosensitive functionalized surface-modified quantum dots for polymeric structures via two-photon-initiated polymerization technique.

In this paper, the surface modification of CdSe- and CdZnS-based quantum dots (QDs) with a functional silica shell is reported. Functionalized silica shells are prepared by two routes: either by ligand exchange and a modified Stöber process or by a miniemulsion process with amphiphilic poly(oxyethylene) nonylphenylether also know as Igepal CO-520 (IG) as oligomeric amphiphile and modified silica precursors. The polymerizable groups on the functionalized silica shell allow covalent bonding to a polymer matrix and prevent demixing during polymerization and crosslinking. This allows the homogeneous incorporation of QDs in a crosslinked polymer matrix. This paper furthermore demonstrates that the resulting QDs, which are i) shielded with a proper silica shell and ii) functionalized with crosslinkable groups, can be used in two-photon-initiated polymerization processes in combination with different photoresists to obtain highly luminescent 3D structures. The resulting luminescent structures are attractive candidates for photonics and metamaterials research.

Rotational elastic micro joint based on helix-augmented cross-spring design for large angular movement

A new type of micro-joint based on an elastic design concept is proposed for large rotational movement. The proposed new 3D micro-joint was designed based on a cross-spring that has precise and reliable motion. However, the cross-spring has a limitation in the range of rotational angle. To improve the range of rotational movement, the proposed 3D micro-joint was modified with a helical structure. By adding the helical structure, the modified rotational joint can achieve large rotational movement The micro-joint was fabricated by the two-photon stereolithography process (TPS process). The micro-joint was manipulated by optical trapping force. With the same optical trapping force, the advantage of proposed cross-spring on the large rotational movement was evaluated. And the precise movement of the proposed micro-joint was evaluated by calculating the RMS error. It has been shown that the proposed 3D micro-joint has precise and reliable motion for large rotational angle.

Fabrication of submicron-sized metal patterns on a flexible polymer substrate by femtosecond laser sintering of metal nanoparticles

The femtosecond laser sintering of metal nanoparticles was studied in order to fabricate submicron-sized metal patterns on flexible polymer substrates for various applications in the electronic and photonic industries. In this process, a mode-locked Ti:sapphire laser beam was tightly focused on silver nanoparticles. To achieve a homogeneous dispersion of the silver nanoparticles, the nanoparticles were prepared using a two-phase reduction method wherein the silver nanoparticles were encapsulated by functional surfactants. The key advantage of the femtosecond laser sintering process is that it reduces the heat-affected zone during sintering, as the femtosecond (10-15s) laser pulse is shorter than the heat diffusion time (picosecond: 10-12s). Therefore, sintering of metal nanoparticles is limited to the laser focal spot and the thermal diffusion effect is suppressed, enabling the realization of submicron-sized metal patterns on flexible polymer substrates. Through this process, metal conductors with submicron-sized features and high conductivity were successfully fabricated. As demonstrated by the obtained results, the femtosecond laser sintering of metal nanoparticles is a novel process that offers direct, low-temperature, ultra-high-resolution results, and which will have numerous further applications in electronics and photonics.

Photopatternable cadmium-free quantum dots with ene-functionalization

Photopatternable nanoparticles can be easily dispersed into polymeric matrices and used to fabricate optoelectronic devices for display, sensing and quantum information processing applications. Here we report the first instance of a cadmium-free photopatternable quantum dot. A ligand containing dithiolane group at one end and an ene-functionalization at the other end were synthesized for this purpose. The myristic acid ligands on as synthesized red indium zinc phosphide-zinc sulfide (In(Zn)P/ZnS) quantum dots were easily replaced by the newly developed ligand by a simple sonication procedure. The functionalized quantum dots could be easily incorporated into a commercially available photoresist. The quantum dot doped photoresist was used to fabricate three-dimensional quantum dot doped hierarchical microstructures by two-photon lithography. Confocal imaging microscopy was used to verify the uniform incorporation of the nanoparticles in the hybrid microstructure.

Highly flexible micro-joint with large rotational movement based on elastic design concept

As nano/micro devices become smaller, it is important to manipulate them with highly accurate control. The design of micro-joints is important for highly accurate movement. An elastic joint, one of type of micro-joint that is commonly used for rotational movement, however, has a limitation in terms of its small rotational range. This paper discusses a new micro-joint based on an elastic design concept for large rotational movement. The proposed micro-joint was designed with a simple shape of two thin plates. For large rotational movement with small force, a modified micro-joint with a helical structure was proposed. Through numerical simulation, the proposed micro-joint is estimated to have a large rotation that can be obtained with only a small force.

Effective direct writing of hierarchical 3D polymer micromeshes by continuous out-of-plane longitudinal scanning

AbstractNano-stereolithography, also known as two-photon direct writing is widely used for fabrication of the three-dimensional microstructure with submicron resolution. Heirarchical three-dimensional meshed microstructures are crucial for microelectromechanical systems (MEMS), photonics and biotechnological applications. Routine study for various applications using such structures requires fabrication of many sets of structures. Conventionally such meshed microstructures are constructed through layer-by-layer accumulation with discrete point-to-point laser scanning. This technique is time consuming, leading to long fabrication times placing constraints on practical applications as well as optimization of structures. In this work we propose continuous longitudinal laser scanning method as an effective means for direct writing of hierarchical three-dimensional meshed microstructures. The advantages of continuously longitudinal laser scanning method are explored for its time economy and fabrication effectiveness; a fabrication window is suggested to determine the fabrication parameter easily according to laser power and structural design.

Direct fabrication of polymer-metal microstructures for micro coils

A two-photon stereolithgraphy process is an effective method to fabricate 3D polymer microstructures. However, additional functionality, such as electrical conductivity, must be realized to allow them to be used in functional micro-devices. Electrical conductivity is a fundamental requirement for transferring signals and powering microsystems. We report a method for the direct fabrication of complex 3D polymer-metal microstructures. Patterning a hybrid material (a polymer and a metal coatable polymer) in chosen regions is achieved by a two-photon stereolithography process using a dual-stage scanning system. Selective metallization of the metal coatable polymer structure is performed through silver deposition by electroless plating. This process can produce arbitrary 3D polymer-metal hybrid microstructures with high resolution. In this paper, a 3D micro coil structure with an arbitrary shape is demonstrated. The fabricated 3D micro coil is operated at 25.4 GHz, which is useful for compact wireless communication devices, wireless power transmission, and small antennas.

Fabrication of micro open structure using 3D laser scanning method in nano-stereolithography

As interest in fabrication methods for three dimensional (3D) structures has increased, various studies have been conducted to fabricate micro- and nano-structures. The nano-stereolithography process has been recognized as one of the most promising technologies for the fabrication of three-dimensional micro structures with submicron fabrication resolution, and to meet the demand for a process of fabrication of complex 3D micro structures. By considering the surface tension during the fabrication process, it is possible to increase both the fabrication efficiency and the success rate. In this paper, an open structure was fabricated using the 3D laser scanning method. For the fabrication of a perfectly supported open structure, the line width, according to various levels of laser power, was estimated experimentally and the fabrication window was derived with a number of experiments. Compared to the closed structure, the open structure has advantages in terms of fabrication and its functional perspective.

Elastic Translational Joint for Large Translation of Motion Using Spiral Structures

As nano/micro devices become smaller, it is important to manipulate them with highly accurate control. The design of micro joint is important for highly accurate movement. This paper discusses a new micro translational joint based on an elastic design concept for precise and large translational motion. By using the elastic deformation of spiral structures, translational joint can manipulate precisely and move long distance. Through finite element method (FEM) simulation, the proposed translational joint was estimated to have a precise and large translational motion. The proposed translational joint is possible to move without friction, and it can be used for micro/nano-sized manipulators. Therefore, it is expected that many applications for highly accurate devices can be manipulated with the proposed translational joint.

Simulative design in macroscale for prospective application to micro-catheters

In this paper, a motion-transforming element is applied to the development of a new catheter device. The motion-transforming element structure allows a reduction of linear movement and converts linear movement to rotational movement. The simulative design of micro-catheters is based on a proposed structure called the Operating Mini Station (OMS). OMS is operated by movement of a motion-transforming element. A new motion-transforming element is designed using multiple links that are connected by hinged joints based on an elastic design. The design of the links and the hinges are optimized for precise and reliable movement of the motion-transforming element. Because of the elastic design, it is possible to realize a catheter that allows various movements in small spaces like capillaries.