Nonlinear Micro/Nano-Mechanical Resonators
Extensive development of Micro/Nano-electromechanical system (M/NEMS) has exhibited superb performance over a wide range of applications, while they benefit from high resonant frequencies and Q-factors owing to their small dimensions and high structure quality of materials. However, many of these M/NEMS design pose some limitations originating from typical linear operation including narrow frequency bandwidth, difficulties in tuning frequency, and limited linear operational range.
Motivated by the need to advance current capabilities of M/NEMS, our previous research has been focused on the implementation of intentional intrinsic nonlinearity in the design of micro/nanomechanical resonators. Such intentional integration of nonlinear designs has been already realized through the use of strongly nonlinear attachments of nanoscale size, such as nanotubes or nanomembranes. Our research has proved that harnessing intentional strong nonlinearity into M/NEMS designs enables exploiting various nonlinear phenomena, which are not attainable in linear settings, such as broadband resonances, dynamic instabilities, nonlinear hysteresis, energy localization phenomena and passive targeted energy transfers.
Eventually, this research aims to develop various nonlinear M/NEMS devices with the practically scalable fabrication available for a wide range of applications including sensing and energy harvesting. Previous development of a mass sensor and a damping sensor will be further expanded in terms of their capability based on the practical fabrication: the microcantilever will be functionalized to sense a chemical or measure biomolecular interactions; and we will further engineer completely integrated sensing systems—from actuation and detection to installation and package—for practical and commercializable applications. We are also particularly interested in developing a broadband nonlinear energy system based on piezoelectric materials to harvest mechanical energy from vibration.
Advanced Atomic Force Microscopy
Since the development in the early 1980s, atomic force microscopy (AFM) has been one of the most useful tools in the field of nano- and bioscience. AFM is capable of imaging and characterizing various materials with nanometer-scale spatial resolution under any environmental conditions including in air and liquid. We are interested in applying our understanding of cantilever dynamics to advance the state of art AFM (atomic force microscopy) by (i) interpreting the signal generated by a cantilever’s motion; (ii) designing a new cantilever system to obtain more information about material properties; and (iii) developing new (or advanced) techniques to measure multiphysical properties such as piezoelectricity, ferroelectricity, pyroelectricity, and IR absorptivity.
Indeed, our previous research enhanced the performance of atomic force microscope infrared microscopy (AFM-IR) based on the proper analysis of the cantilever dynamic response during measurement. By extracting the signal where the vibrational energy of the cantilever is highly localized, the SNR of the AFM-IR signal is significantly increased, such that the throughput is increased by 32-fold compared to state of the art. This enhancement also enabled characterizing the IR spectra of ultra-thin polymer nanostructure of height 15 nm, which was about an order of magnitude improvement over state of the art. A nonlinear cantilever design incorporates an essential geometric nonlinearity was also proposed and their dynamics were studied. It was shown that the new design exhibits broadband resonance over a bandwidth several times its linear resonant frequency and possesses an intrinsic stability that virtually eliminates the instability involved in a linear AFM system. Also, a new technique to measure the simultaneous piezoelectric and ferroelectric responses was invented to study engineered ferroelectric thin films that are inhomogeneous in the thickness direction. This technique was applied to characterize the piezoelectric coefficient of compositionally graded PZT thin films.
Recently, our group developed a new design of microcantilever system enabling multi-harmonic AFM (MH-AFM). Under dynamic mode operation of AFM, an intentionally designed 1:n internal resonance between the two leading bending modes of the microcantilever incorporating an inner paddle, leads to magnification of high-frequency harmonics in the paddle response, which is the basis for AFM of improved sensitivity. Detailed theoretical and experimental studies of the proposed nonlinear AFM design demonstrated the capacity for simultaneous topography imaging and compositional mapping with as much as five-fold enhanced sensitivity.
Further understanding the nonlinear dynamics of AFM cantilever will have a significant payoff, advancing the method of nanoscale characterization. This area of research ultimately aims to uncover important physics involved in various types of materials, which is expected to broadly impact fundamental research in the field of fundamental nano- and bio-science.
Material Research Using Advanced AFM
Our advanced capabilities of material characterization using AFM are actively applied to various areas in material research through collaborations.
Energy Systems Based on Multi-Functional Ferroelectric Material
Simultaneous mechanical-thermal-electrical coupling properties of ferroelectric materials make them attractive for various applications including actuators, sensors, memory, and energy harvesting. Our research has been focused on developing an energy harvesting system based on thermal-electrical coupling of ferroelectric materials. The high power pyroelectric energy conversion of thermal to electrical energy was realized from a ferroelectric BaTiO3 thin film. In bulk pyroelectric materials, the power density is fundamentally limited by the rate of heat transfer through the pyroelectric material, as well as practical limits on the maximum applied electric field. The nanometer-scale pyroelectric film allows us to overcome these limitations, with thermal and electrical cycling as fast as 3 kHz, combined with applied electric field as high as 125 kV/cm. We designed a microfabricated platform with fast control of temperature and electric fields that allows exploration of pyroelectric energy cycles in a previously unexplored operating regime.
Our research will further utilize the mechanical-electrical coupling properties to enable piezoelectric energy conversion of vibrational energy to electrical energy. Our ongoing research is particularly interested in developing a broadband nonlinear energy harvesting M/NEMS. The most desirable operational condition for high efficiency is through motion amplification gained by mechanical resonance. This, however, is technically challenging due to the high resonance frequency and narrow bandwidth of current micro/nanodevices, originating from the intrinsic traits of small mass and damping. Thus, it is difficult to accommodate environmental sources with time-varying low frequencies. Such difficulties can be resolved by properly designing nonlinear systems employing essentially nonlinear components having the capacity for broadband resonance. Ultimately, this research aims to combine mechanical-thermal-electrical coupling to create a new class of energy cycles.
We greatly acknowlege the following agencies for funding our research.