Minrui Yu

Semiconductor Nanomembrane Tubes: Three-Dimensional Confinement for Controlled Neurite Outgrowth

In many neural culture studies, neurite migration on a flat, open surface does not reflect the three-dimensional (3D) microenvironment in vivo. With that in mind, we fabricated arrays of semiconductor tubes using strained silicon (Si) and germanium (Ge) nanomembranes and employed them as a cell culture substrate for primary cortical neurons. Our experiments show that the SiGe substrate and the tube fabrication process are biologically viable for neuron cells. We also observe that neurons are attracted by the tube topography, even in the absence of adhesion factors, and can be guided to pass through the tubes during outgrowth. Coupled with selective seeding of individual neurons close to the tube opening, growth within a tube can be limited to a single axon. Furthermore, the tube feature resembles the natural myelin, both physically and electrically, and it is possible to control the tube diameter to be close to that of an axon, providing a confined 3D contact with the axon membrane and potentially insulating it from the extracellular solution.

Laser drilling of nano-pores in sandwiched thin glass membranes

We report on a novel method of using an excimer laser to drill ultra-small pores in borosilicate glass membranes. By introducing a thin layer of liquid between sandwiches of two glass slides, we can shrink the pore size and smoothen the surface on the exit side. We are able to push the minimal exit pore diameter down to 90 nm, well below the laser wavelength of 193 nm. This is achieved with substrates over 150 microm thick. Compared to other methods, this technique is fast, inexpensive, and produces high quality smooth pores.

Single-Ion Channel Recordings on Quartz Substrates

We show that a single-crystal quartz substrate provides a working platform for ion channel research. Single-crystal quartz is piezoelectric, so it can be nanomechanically actuated to perform precise membrane deformations. This, along with its superior noise properties, makes single-crystal quartz ideal for analyzing mechanosensitive ion channels.

Effect of surface bonding on semiconductor nanoribbon wiggling structure

SiGe nanomembranes and nanowires provide one important class of stretchable electronic materials. We have investigated a very interesting wiggling phenomenon of SiGe nanoribbons bonded to Si substrate as experimentally observed in a Hall-bar structure. Based on continuum linear stability analysis, we establish a scaling rule between the wiggling period and surface bonding area, in relation to the ratio of strain energy over the interfacial bonding energy.

Local-Wetting-Induced Deformation of Rolled-Up Si/Si-Ge Nanomembranes: A Potential Route for Remote Chemical Sensing

We fabricate curled 3-D objects from semiconductor nanomembranes consisting of single-crystal silicon, which is epitaxially grown on silicon-germanium-on-insulator substrates. The curling is caused by relaxing the strain induced by lattice mismatch between silicon (Si) and germanium (Ge). Depending on the lithographically patterned geometries and their orientation with respect to the crystallographic direction, different shapes of tubes can be realized. Particularly interesting are tubes that are not completely closed, or partially open, whose mechanical response is ultraelastic. We demonstrate that applying acetone on such tubes generates a surface stress imbalance between the Si and Si-Ge layers, resulting in detectable shape changes. This mechanism has potential applications in chemical sensing, where the deformable curled structures act as dynamic-aperture reflector antennas. Our simulation suggests the curvature changes induced in the presence of certain chemical, such as acetone, will lead to distinctive far-field radiation patterns in the terahertz (THz) range.

Wavenumber-Domain Theory of Terahertz Single-Walled Carbon Nanotube Antenna

A theoretical study is presented on the characteristics of terahertz antennas formed by metallic single-walled carbon nanotube (SWCNT) dipoles. The Boltzmann transport equation and Maxwells equations are combined with boundary conditions of the electron distribution function, in order to formulate a wavenumber-domain integral equation for the current, which considers the spatial dispersion and provides higher level of accuracy and generality than existing approaches. Through proper approximations of that equation, the same spatial integral equations from several other studies can be drawn. The radiation properties of the SWCNT antenna are derived from the wavenumber-domain current. Numerical results are given for short dipole antennas and those with length close to the half wavelength in free space. They are compared to the results calculated by other methods. We also investigate the frequency dependence of conductance under different values of relaxation frequency and find the increase of relaxation frequency leads to strong attenuation of surface-wave resonances.

Local etch control for fabricating nanomechanical devices

We report on the fabrication of suspended nanoelectromechanical systems using an etch enhancement technique. Nanoscale beams are defined by conventional electron beam lithography, followed by locally enhanced etching via electron beam exposure. The structures are successfully suspended within the “etch-booster window” by using an HF vapor etch step. The method is simple, does not require a special setup, and allows the spatial and temporal fine-tuning of the underetching process.

Ultra-stable glass microcraters for on-chip patch clamping

We report on a single-step fabrication procedure of borosilicate glass micropores surrounded by a smooth microcrater. By inserting a thin air-gap between a borosilicate glass substrate and a reflective layer, we achieve dual-sided laser ablation of the device. The resultant crater provides a smoother, curved surface onto which cells settle during planar patch clamping. Gigaohm seals, which are more easily achievable on these devices as compared to conventional micropores, are achieved by patch clamping human embryonic kidney (HEK 293) cells. Further, the microcraters show enhanced mechanical stability of the planar patch clamped cells during perfusion. We integrate polydimethylsiloxane microfluidic devices with the microcraters and use passive pumping to perfuse the cells. We find that passive pumping increases the pressure within the device by 1.85 Pa. However, due to the enhanced stability of the microcrater, fluidic shearing reduces the seal resistance by only 6.8 MΩ on average, which is less than one percent of the gigaohm seal resistance.