The biggest challenge in the field of low-dimensional nanomaterials towards practical applications is scalable production with structural uniformity. As the size of materials is getting smaller, their tendency of the structure-dependent properties which directly affects device reliability of largescale applications is getting stronger due to quantum confinement effects. For example, one-dimensional (1D) carbon nanotubes have various electrical/optical properties based on their structures (e.g., diameter, chirality, etc.). Likewise, two-dimensional (2D) layered materials also exhibit different properties based on their thickness. To overcome such structural heterogeneity, isopycnic density gradient ultracentrifugation (i-DGU) will be introduced to achieve monodispersity of nanomaterials in structure based on their buoyant density differentiations. The i-DGU approach enables to sort 1D carbon nanotubes and 2D layered materials such as graphene, transition metal dichalcogenides, hexagonal boron nitride with high structural purity based on their structure. Various largescale optoelectronic applications, electrically driven light emitters and photodetectors demonstrated based on the monodisperse nanomaterials will be discussed.

Lasing mode regulation is essential for realizing the practical applications such as optical communication and optical sensing/switches. In this work, we etched the slit along the c-axis of a hexagonal ZnO microrod by focusing ion beam etching. This slit can produce very strong local light field under strong incident light. Compared with the bare ZnO sample, whispering gallery mode (WGM) lasing mode is reduced, while the emission intensity decreases and the laser threshold increases. Then Ag nanoparticles filled the slit as nonlinear absorbers. the regulation of WGM laser mode is realized by the coupling effect of non-linear absorption effect of Ag nano-particles and the local light field. Moreover, laser mode intensity is enhanced by strong coupling between ZnO microrod and surface plasmon resonant of Ag NPs of its surface that is proved by PL decay time.

The response of micromachines to the external actions is typically affected by a scattering, which is on its own induced by their microstructure and by stages of the microfabrication process. The progressive reduction in size of the mechanical components, forced by a path towards (further) miniaturization, has recently enhanced the outcomes of the aforementioned scattering, and provided a burst in research activities to address issues linked to its assessment [1,2]. In this work, we discuss the features of an on-chip testing device that we purposely designed to efficiently estimate the two major sources of scattering affecting inertial, polysilicon-based micromachines: the morphology of the silicon film constituting the movable parts of the device, and the etch defect or overetch induced by microfabrication. The coupled electro-mechanical behavior of the statically determinate movable (micro)structure of the on-chip device has been modelled via beam bending theory [3], within which the aforementioned sources of scattering have been accounted for through local fluctuating fields in the compliant part of the structure itself, namely the supporting spring. The proposed stochastic model is shown to outperform former ones available in the literature [4,5], which neglected the simultaneous and interacting effects of the two mentioned sources on the measure response. The model can fully catch the scattering in the C-V plots up to pull-in, hence also in the nonlinear working regime of the device.

References

[1] Zhu, J.; Liu, X.; Shi, Q.; He, T.; Sun, Z.; Guo, X.; Liu, W.; Sulaiman, O.B.; Dong, B.; Lee, C. Development Trends and Perspectives of Future Sensors and MEMS/NEMS. Micromachines 2020, 11.

[2] Molina, J.P.Q.; Rosafalco, L.; Mariani, S. Stochastic Mechanical Characterization of Polysilicon MEMS: A Deep Learning Approach. Proceedings 2020, 42.

[3] Mirzazadeh, R.; Eftekhar Azam, S.; Mariani, S. Micromechanical Characterization of Polysilicon Films through On-Chip Tests. Sensors 2016, 16, 1191.

[4] Mirzazadeh, R.; Ghisi, A.; Mariani, S. Statistical Investigation of the Mechanical and Geometrical Properties of Polysilicon Films through On-Chip Tests. Micromachines 2018, 9, 53.

[5] Ghisi, A.; Mariani, S. Effect of imperfections due to material heterogeneity on the offset of polysilicon MEMS structures. Sensors 2019, 19, 3256.

The path towards miniaturization for micro electro-mechanical systems (MEMS) has recently increased the effects of stochastic variability at the (sub)micron scale on the overall performance of the devices. We recently proposed and designed an on-chip testing device to characterize two sources of variability that majorly affect the scattering in the response to the external actions of inertial (statically determinate) micromachines: the morphology of the polysilicon film constituting the movable parts of the device; and the environment-affected overetch linked to the microfabrication process. A fully stochastic model of the entire device has been set to account for these two sources on the measurable response of the devices, e.g. in terms of the relevant C-V curves up to pull-in. A complexity in the mentioned model is represented by the need to assess the stochastic (local) stiffness of polysilicon, depending on its unknown (local) microstructure. In this work, we discuss a deep learning approach to the micromechanical characterization of polysilicon films, based on densely connected neural networks (NNs). Such NNs extract relevant features of the polysilicon morphology from SEM-like Voronoi tessellation-based digital microstructures. The NN-based model or surrogate is shown to correctly catch size effects at a varying ratio between the characteristic size of the structural components of the device, and the morphology-induced length scale of the aggregate of silicon grains. This property of the model looks indeed necessary, to prove the generalization capability of the learning process, and to next feed Monte Carlo simulations resting on the model of the entire device.

Over the last two decades we have gained more and more insight into how to convert patterned polymer precursors into predicable 3D carbon shapes by pyrolysis/carbonization (carbon origami are a more recent example). Over the last four years we have started gaining control over the internal carbon microstructure and its functionality. The key to the latter is a precise control of the polymer precursor chains and the exact polymer atomic composition of the polymer before and during pyrolysis. Contradicting Rosalind Franklin, we have found that it is possible to graphitize even non-graphitizing carbons simply by applying mechanical stresses to align the polymer precursor chains and stabilizing them in position before pyrolysis. Perhaps the most surprising outcome of this work is the demonstration of the conversion of PAN fibers through pyrolysis into turbostratic graphene suspended wires with diameters as small as 2 nanometers. The suspended graphene bridges have a conductivity similar to that of multiwall carbon nanotubes (MWCNTs) a Young’s modulus of > 400 GPa and electrochemically the material behaves like graphene doped with nitrogen. The latter material represents a very electroactive electrode ideally suited for energy and sensing applications. The current fabrication process for graphene doped with nitrogen is lengthy and complicated ours is a one-step simple process that is easily scalable.

We report a Crookes radiometer that rotates at atmospheric pressure using architected microporous dielectric plates, known as nanocardboard, as vanes [1,2]. Compared to most light mills working at tens of Pascals [3,4], the functionality at pressures three orders-of-magnitude larger results from the metamaterial vanes’ unique features: (1) extremely low areal density (0.1 mg/cm²) that reduces the vane mass and hub friction force by almost 100 times; (2) high thermal resistivity that increases the cross-vane temperature difference; and (3) micro-channels that enable through-vane thermal transpiration gas flows.

Each nanocardboard vane features a basketweave-style five-flow-channel pattern to amplify the thermal transpiration force. We manufactured these vanes using microfabrication techniques in four stages: (1) silicon mold creation using photolithography and reactive ion etching; (2) mold conformal coating using atomic layer deposition; (3) carbon nanotube drop-casting and oxygen plasma etching; and (4) mold cleaving and removing using XeF₂ isotropic etching [1, 5]. We 3D-printed a 26-mm-diameter quad-arm hub and mounted the vanes to it using super glue.

We measured the temperature and rotation speed of the radiometer using thermal and video cameras while illuminating it using an octagonal LED array. We found that our radiometer could operate at atmospheric pressure, and that its rotation rate increased with light intensity. To our knowledge, no other radiometers have achieved such functioning in ambient air. Lastly, we simulated the radiometer’s fluid dynamics, obtaining similar trends between its rotation speed and light intensity and achieving order-of-magnitude agreement with our experiments. Our photophoretically-propelled microstructures reveal new possibilities for light sensing and actuation, aerial microflyers, and photo-generators.

References:

[1] Lin, Chen, et al. “Nanocardboard as a nanoscale analog of hollow sandwich plates.” Nature Communications 9.1 (2018): 1-8.

[2] Cortes, John, et al. “Photophoretic Levitation: Photophoretic Levitation of Macroscopic Nanocardboard Plates” (Adv. Mater. 16/2020). Advanced Materials 32.16 (2020): 2070127.

[3] Han, Li-Hsin, et al. “Light-powered micromotor: design, fabrication, and mathematical modeling.” Journal of Microelectromechanical Systems 20.2 (2011): 487-496.

[4] Wolfe, David, Andres Larraza, and Alejandro Garcia. “A horizontal vane radiometer: Experiment, theory, and simulation.” Physics of Fluids 28.3 (2016): 037103.

[5] Azadi, Mohsen, et al. “Demonstration of Atmospheric-Pressure Radiometer With Nanocardboard Vanes.” Journal of Microelectromechanical Systems 29.5 (2020): 811-817.

Several micro devices, such as micro-mirrors, are subjected to working conditions featuring alternating loadings that can possibly induce fatigue in the thin metal layers, which represent critical structural parts. The quantification of the degradation of the material properties under fatigue loading is a time consuming task, and the effects of environmental conditions (e.g. humidity) and load characteristics (e.g. frequency, stress ratio) must be properly accounted for. In this work, we propose and assess the efficiency of an on-chip test device based on piezoelectric actuators, able to generate a time-varying (sinusoidal) strain in the mentioned thin metal layers and lead to fatigue. The aim of the research activity is the characterization of the stress/strain-induced degradation process of a thin layer located on the top of a lead zirconate titanate (PZT) actuation system. The characterization has been carried out through measurements of resistivity and roughness, respectively carried out via an ohmmeter and a confocal microscope. The proposed testing device has shown capability to qualitatively highlight the degradation of the metal layers. A re-design of the on-chip device is also discussed, in order to also carry out quantitative evaluations.

Ultrasonic motors are characterized by low speed and high torque operation, without the need for gear trains. They can be compact and lightweight and they can also work in the absence of applied loads, due to the frictional coupling between the rotor and the stator induced by the traveling wave. In this work, we discuss a concept design based on thin piezoelectric films, sol-gel directly deposited onto a silicon substrate to provide high-torque motors compatible with wafer integration technologies. Due to the large dielectric constants and the enhanced breakdown strengths of thin piezoelectric films, such ultrasonic micromotors can lead to meaningful improvements over electrostatic ones in terms of energy density. As far as the fabrication of the micromotor at the mm-scale is concerned, an integrated approach is proposed with significant improvements regarding: the comb-tooth structure, to maximize/optimize the motor torque; a back and front etch lithographic process; the design of the electrodes, which provide the electric signal at the central anchor of the stator, taking advantage of low-temperature soldering. The proposed design has been assessed through multiphysics simulations, carried out to evaluate the resonant behavior of the stator and the motor performance in terms of angular velocity, torque, and output power, and it is shown to lead to promising results.

Two-photon laser scanning microscope (TPLSM) provides outstanding optical three dimension section properties and it has been widely used in fundamental science and biomedical application. However, Current 3D two-photon fluorescence (TPF) imaging techniques usually overlook the spatiotemporal evolution of TPF ellipsoid along the axial direction, which might contain fine dynamical information of imaged targets. Here, we develop a spatiotemporal scanning technique and realize the measurement of spatiotemporal scanning of TPF ellipsoid with a semiconducting CsPbBr₃ nanosheet. Results have shown that axial size of TPF ellipsoid present linear growth as a function of excitation fluence by using spatial scanning. Furthermore, we have observed that axial size of TPF ellipsoid exhibits inhomogeneous linear growth with time delay by introducing spatiotemporal scanning technique. We attribute this phenomenon to the fact that surface and bulk region of CsPbBr₃ nanosheet have inhomogeneous timescale on TPF decay lifetime. Our results not only provide new insights for spatiotemporal resolving of TPF ellipsoid, but also helpful to promote the development of fluorescence lifetime microscopy technology.

The hot-compression bonding process is a new technology used to manufacture heavy forgings which can avoid the size effect caused by the traditional casting process. In this new technology, the surface state of substrates is a key factor to guarantee the quality of bonding joints. At present, the influence of different surface states on the quality of interface bonding is uncertain. Therefore, the effect of surface state on the bonding quality of interface was studied in this paper for the first time. Different methods such as optical observation and elemental analysis were used to composite characterize the surface state. Furthermore, the microscopic morphology of the cross-section samples derived from the bonding joints was used to analyze the quality of interface bonding. The influence of the surface state on interface bonding quality was obtained by analyzing the relationship between the surface state and interface bonding quality. The results show that 90% of the interface bonding area can achieve a seamless interface effect after the bonding of two relatively clean substrates, which means that a clean surface state can greatly improve the bonding quality of bonding joints. This study can help to understand the relationship between the surface state of the substrate and the bonding quality of the interface. It is beneficial to guarantee the interface bonding quality of the substrate and is of great significance to further improve the quality of the joint.