Direct assembly of nanowires by electron beam-induced dielectrophoresis.

Controllable self-assembly is an important tool to investigate interactions between nanoscale objects. Here we present an assembly strategy based on 3D aligned silicon nanowires. By illuminating the tips of nanowires locally by a focused electron beam, an attractive dielectrophoretic force can be induced, leading to elastic deformations and sticking between adjacent nanowires. The whole process is performed feasibly inside a vacuum environment free from capillary or hydrodynamic forces. Assembly mechanisms are discussed for nanowires in both one and two layers, and various ordered organizations are presented. With the help of moisture treatment, a hierarchical assembly can also be achieved. Notably, an unsynchronized assembly is observed in two layers of nanowires. This study helps with a better understanding of nanoscale sticking phenomena and electrostatic actuations in nanoelectromechanical systems, besides, it also provides possibilities to probe quantum effects like Casimir forces and phonon heat transport in a vacuum gap.

High-quality factor, high-confinement microring resonators in 4H-silicon carbide-on-insulator.

Silicon carbide (SiC) exhibits promising material properties for nonlinear integrated optics. We report on a SiC-on-insulator platform based on crystalline 4H-SiC and demonstrate high-confinement SiC microring resonators with sub-micron waveguide cross-sectional dimensions. The Q factor of SiC microring resonators in such a sub-micron waveguide dimension is improved by a factor of six after surface roughness reduction by applying a wet oxidation process. We achieve a high Q factor (73,000) for such devices and show engineerable dispersion from normal to anomalous dispersion by controlling the waveguide cross-sectional dimension, which paves the way toward nonlinear applications in SiC microring resonators.

Oblique angled plasma etching for 3D silicon structures with wiggling geometries.

Three dimensional (3D) silicon micro- and nanostructures have attracted special research interest, particularly in photonic and electrochemical devices, due to the extra degrees of freedom for manipulation of device performance and properties. However, it is still considered to be difficult to fabricate 3D silicon structures with an arbitrary geometric form in a scalable volume, especially with standard fabrication techniques, which are intrinsically directional and anisotropic. In this work we proposed a unique method of oblique-angled plasma etching from various angles, thus multilayered silicon structures with wiggling geometries can be fabricated in a controllable manner both in micro- and nanoscale. The mechanism is explained as induced modifications of substrate topology and surface charging when a glass pad is attached on the sample surface, thus the incoming ion fluxes can be directed to the substrate surface with an off-normal angle. The process is convenient to perform without additional modifications on the plasma etching systems. At the same time, it provides more possibilities in the toolkit for fabricating 3D silicon structures with conventional fabrication technologies.

MEMS micro-coils for magnetic neurostimulation.

Micro-coil magnetic stimulation of brain tissue presents new challenges for MEMS micro-coil probe fabrication. The main challenges are threefold; (i) low coil resistance for high power efficiency, (ii) low leak current from the probe into the in vitro experimental set-up, (iii) adaptive MEMS process technology because of the dynamic research area, which requires agile design changes. Taking on these challenges, we present a MEMS fabrication process that has three main features; (i) multilayer resist lift-off process to pattern up to 1800-nm-thick metal films, and special care is taken to obtain high conductivity thin-films by physical vapor deposition, and (ii) all micro-coil Al wires are encapsulated in at least 200 nm of ALD alumina and 6-µm-thick parylene C such the leak resistance is high (>210 GΩ), (iii) combining a multi-step DRIE process and maskless photolithography for adaptive design and device fabrication. The entire process requires four lithography steps. Because we avoided SOI wafers and lithography mask fabrication, the design-to-device time is shortened significantly. The resulting probes are 4-mm-long, 60-µm-thick, and down to 150 µm-wide. Selected MEMS coil devices were validated in vivo using mice and compared to previous work.

Plasma-Assisted Microcontact Printing.

Microcontact printing, polymer pen lithography, and their variations have attracted interests from a broad spectrum of research fields as a result of the feasibility of defining patterns in micro- and nanoscales. In this work, we have proposed and demonstrated a novel lithography method, named plasma-assisted microcontact printing (PA-µCP). Unlike the previous printing methods, where a direct contact is normally required for the transport of liquid-phase inks, plasma-deposited fluorocarbon (FC) has been employed in PA-µCP as the ink material, which can be transferred from the stamp to substrates through a thermal evaporation process. The geometry of the patterns can be modified by adjusting the design of stamp patterns and the contact time, and transferred FC patterns can be used directly as an etch mask to create microstructures in the substrate materials. We have demonstrated the possibility of performing multi-patterning with PA-µCP, where FC patterns can be generated conformally on structured substrates. Because the height of FC patterns is closely related to the local pattern designs, PA-µCP can be used for grayscale patterning. As a proof of concept, Fabry-Perot planar cavities are fabricated with grayscale PA-µCP for structure color printing. We believe PA-µCP is distinguished from conventional techniques by its printing mechanism, which can pave the way for convenient fabrication of photonic, electronic, and biological devices.

Quasi-Random Gratings Enabled by Wrinkled Photoresist Surfaces on a Rigid Substrate.

Micro- and nanoscale surface wrinkling has been widely studied in artificial systems, mostly in soft substrates like polydimethylsiloxane or polystyrene, where the wrinkling dynamics are triggered by thermal stresses or tensile prestrains. Here, we introduce a new wrinkling regime based on photoresist layers on top of a rigid substrate. By introducing a bending deformation, combined with fluorine-based plasma treatment, wrinkles with a characteristic wavelength less than 1 µm can be created. By adding micropatterns on photoresists with standard UV exposure, ordered wrinkles can also be realized. This technique is demonstrated to be applicable in several commercially available photoresists, and the wrinkled patterns can be employed conveniently to create high-aspect-ratio silicon gratings and large-area silicon dioxide membranes. This unique strategy broadens the spectrum of available materials to create wrinkled surfaces in a controllable manner and provides a platform for the easier fabrication of wrinkle-based devices.

Electron-Beam Patterning of Vapor-Deposited Solid Anisole.

The emerging ice lithography (IL) nanofabrication technology differs from conventional electron-beam lithography by working at cryogenic temperatures and using vapor-deposited organic molecules, such as solid water and alkanes, as e-beam resists. In this paper, we systematically investigate e-beam patterning of frozen anisole and assess its performance as an e-beam resist in IL. Dose curves reveal that anisole has a very low contrast of ∼1, with a very weak dependence on primary beam energy in the investigated range of 5-20 keV. The minimum line width of 60 nm is attainable at 20 keV, limited by stage vibration in our apparatus. Notably, various solid states of anisole have been observed and we can control the deposited anisole from crystalline to amorphous state by decreasing the deposition temperature. The critical temperature for forming an amorphous film is 130 K in the vacuum of a microscope chamber. Smooth patterns with a surface roughness of ∼0.7 nm are achieved in the as-deposited amorphous solid anisole. As a proof of principle of 3D fabrication, we finally fabricate nanoscale patterns on exotic silicon micropillars with a high aspect ratio using this resist.

Oxygen vacancies enhance lithium storage performance in ultralong vanadium pentoxide nanobelt cathodes.

Ultralong V2O5 nanobelts have been successfully synthesized by a facile hydrothermal oxidation route. Oxygen vacancies are generated in the V2O5 nanobelts by annealing under N2 atmosphere at an elevated temperature. The microstructure and chemical composition of the pristine and annealed V2O5 nanobelts are studied by different methods. Compared to the pristine V2O5 nanobelts, the annealed V2O5 nanobelts sample possesses a higher reversible capacity of 177.8 mAhg-1 after 50 cycles at a current density of 0.3 Ag-1, corresponding to ∼0.27% capacity loss per cycle. At a higher current density of 1.2 Ag-1, the reversible capacity of annealed V2O5 electrode can reach 128.5 mAhg-1, which is two times larger than that of pristine V2O5 electrode. Ultralong flexible morphology together with oxygen vacancies in the annealed V2O5 electrode is considered to be responsible for the enhanced lithium storage properties.