Professor Cui’s research addresses the self-assembly of Micro/Nano Electro Mechanical Systems (MEMS/NEMS). His long-term research aims to investigate the fundamental mechanical & electrical principles of new materials for MEMS/ NEMS and advanced manufacturing approaches, utilizing “bottom-up” nanomanufacturing of nanomaterials to effectively enhance the performance of micro- and nano-systems. Present research efforts are the combination of “bottom-up” self-assembly with “top-down” fabrication technologies especially polymer hot embossing and printing. This aims to investigate MEMS/ NEMS including devices and systems using graphene, nanotubes, nanoparticles, nanocomposites, and polymers for new actuators, sensors, microfluidic devices, and photovoltaics for various applications to biomedical devices, renewable energy, and electronics cooling.
Highlights of Professor’s research achievements in his focus area are summarized below. Accomplishments of his research include:
- Exploiting the combination of “bottom-up” nano self-assembly with the “top-down” lithography-based microfabrication techniques for MEMS/NEMS.
- Investigating polymer MEMS based on hot embossing technique for low-cost and bath-fabricated microdevices, and polymer shrink nanolithography.
- Creating graphene bio-sensors and integrated polymer MEMS/NEMS.
As the above contributions address a broad spectrum of research projects, they will be described individually below.
1. Exploiting the combination of “bottom-up” nano self-assembly with the “top-down” lithography-based microfabrication techniques for MEMS/ NEMS
Nano self-assembly is enabling nanotechnology, a solution-based nanomanufacturing approach for spontaneous organization of molecules or objects into stable, well-defined structures by electrostatic forces. Since the process occurs towards the system’s thermodynamic minima, it results in stable and robust structures. Among all types of self-assembly techniques, layer-by-layer (LbL) nano self-assembly is the most promising one. Self-assembly processes have numerous beneficial attributes: very cost-effective, versatile, and facile. LbL nano self-assembly is versatile since it can deposit a variety of polarized nanomaterials such as carbon nanotubes, nanoparticles, or quantum dots on almost any type of substrate. It is very cost-effective since it is a solution-based approach processed in an ambient environment, and it can cover the substrate with optimum thickness with very little waste. It is facile since the process can be well controlled from nanoscale to microscale. The LbL nano self-assembly of alternative layers of oppositely charged polyelectrolyte, nanoparticles, carbon nanotubes can provide the formation of films 5-1000 nm thick with monolayers of various substances growing in a pre-set sequence on any substrates, which make it very suitable for low-cost MEMS/NEMS.
Professor Cui was the first researcher in successfully investigating nano self-assembly with microfabrication for MEMS/NEMS applications. Important achievements include a) development the lithographic approach to pattern the self-assembled multilayer using the modified lift-off technique and the spatial patterning of colloidal nanoparticle-based thin film by a combinative technique of layer-by-layer nano self-assembly and metal mask approach of lithography, b) self-assembly of the ultra-thin cantilevers based on polymer-ceramic nanocomposites, c) fabrication and characterization of MOS-Capacitors and field-effect transistors fabricated by layer-by-layer nano assembly. In addition, my group has generated more than thirty publications and two pending patent applications directly related to this research topic within the last five years. Two examples include self-assembled nanoparticle-based field-effect transistors and self-assembled cantilever beams. Currently, his group is investigating nano self-assembly of carbon nanotubes and graphene for bio-sensing applications, very promising for cancer detections.
2. Investigating polymer MEMS based on hot embossing technique for low-cost and bath-fabricated microdevices, and polymer shrink nanolithography.
Hot Embossing Lithography (HEL), also known as imprint lithography (IL), has gained much interest, in particular as a low-cost and high-volume fabrication approach to define micro- to nano-scale structures. Hot embossing is a type of polymer thermoforming process: a mold insert is pressed into a pre-fabricated polymer under vacuum ambient, the polymer material flows into the mold structures at high temperatures, the polymer material is cooled down to a temperature which provides sufficient strength, and the patterned polymer material can be de-molded. The hot embossing technique ensures highly precise molding of almost any structures in polymers, especially in the fabrication of high-aspect-ratio microstructures. Compared with injection molding, hot embossing is another well-known polymer fabrication technique with several significant advantages such as its flexibility and economy.
Professor Cui was the first one who successfully investigated polymer tunneling sensors and polymer comb drive structures. The PMMA based tunneling sensor shows promising properties with much higher resolution and much higher bandwidth, compared to the silicon tunneling sensor with the same dimension. The vertical tunneling accelerometer was fabricated on PMMA by hot embossing lithography. Silicon mold templates are fabricated with UV lithography, anisotropic bulk wet etching, and ICP dry etching. The PMMA structure is transparent, very suitable for the sensor bonding. A laterally driven comb drive on PMMA was first fabricated with hot embossing lithography by Professor Cui’s group. The electrostatic comb drive is used for lateral dimensional acceleration detection based on the electron tunneling mechanism.
3. Creating graphene biosensors and integrated polymer MEMS/NEMS
Professor Cui’s research on polymer/nanoparticle microelectronics aims to fabricate new polymer microelectronics for biosensing and its integration for MEMS. He combined micro and nanofabrication technologies to realize graphene MEMS devices. This research resulted in a novel and revolutionary microelectronics for bio-sensing and integrated polymer/ nanoparticle-based MEMS with the combination of “top-down” and “bottom-up” technologies, compared to conventional silicon-based devices and systems.
Professor Cui successfully fabricated and characterized graphene-based MEMS sensors for glucose, acetylcholine, immune sensing, and cancer detections. The fabrication is implemented with the very low-cost layer-by-layer nano self-assembly technique. The results presented suggest a new route to inexpensive ion-sensitive field-effect transistors for bio-sensing applications.
Professor Cui has been investigating a wide spectrum of projects on MEMS, with an emphasis on graphene and polymer microsystems. He has pioneered the highly interdisciplinary research of the combination of the “bottom-up” nano self-assembly with the “top-down” micromanufacturing techniques for MEMS/NEMS and polymer shrink nanolithography. He has initially contributed to the achievement of polymer MEMS including highly sensitive polymer-based tunneling sensors and polymer comb drives based on the hot embossing technique. He is also the pioneer on self-assembly of graphene biosensors, with the potential for fully integrated polymer/nanoparticle MEMS/NEMS in the future.