Krutarth Trivedi

Nano-confinement induced chain alignment in ordered P3HT nanostructures defined by nanoimprint lithography

Control of polymer morphology and chain orientation is of great importance in organic solar cells and field effect transistors (OFETs). Here we report the use of nanoimprint lithography to fabricate large-area, high-density, and ordered nanostructures in conjugated polymer poly(3-hexylthiophene) or P3HT, and also to simultaneously control 3D chain alignment within these P3HT nanostructures. Out-of-plane and in-plane grazing incident X-ray diffraction were used to determine the chain orientation in the imprinted P3HT nanostructures, which shows a strong dependence on their geometry (gratings or pillars). Vertical chain alignment was observed in both nanogratings and nanopillars, indicating strong potential to improve charge transport and optical properties for solar cells in comparison to bulk heterojunction structure. For P3HT nanogratings, pi-pi stacking along the grating direction with an angular distribution of +/-20 degrees was found, which is favorable for OFETs. We propose the chain alignment is induced by the nanoconfinement during nanoimprinting via pi-pi interaction and hydrophobic interaction between polymer chain and mold surfaces.

Quantum Confinement Induced Performance Enhancement in Sub-5-nm Lithographic Si Nanowire Transistors

We demonstrate lithographically fabricated Si nanowire field effect transistors (FETs) with long Si nanowires of tiny cross sectional size (∼3-5 nm) exhibiting high performance without employing complementarily doped junctions or high channel doping. These nanowire FETs show high peak hole mobility (as high as over 1200 cm(2)/(V s)), current density, and drive current as well as low drain leakage current and high on/off ratio. Comparison of nanowire FETs with nanobelt FETs shows enhanced performance is a result of significant quantum confinement in these 3-5 nm wires. This study suggests simple (no additional doping) FETs using tiny top-down nanowires can deliver high performance for potential impact on both CMOS scaling and emerging applications such as biosensing.

Room-Temperature Quantum Confinement Effects in Transport Properties of Ultrathin Si Nanowire Field-Effect Transistors

Quantum confinement of carriers has a substantial impact on nanoscale device operations. We present electrical transport analysis for lithographically fabricated sub-5 nm thick Si nanowire field-effect transistors and show that confinement-induced quantum oscillations prevail at 300 K. Our results discern the basis of recent observations of performance enhancement in ultrathin Si nanowire field-effect transistors and provide direct experimental evidence for theoretical predictions of enhanced carrier mobility in strongly confined nanowire devices.

Cell encapsulation and oxygenation in nanoporous microcontainers

With strides in stem cell biology, cell engineering and molecular therapy, the transplantation of cells to produce therapeutic molecules endogenously is an attractive and achievable alternative to the use of exogenous drugs. The encapsulation of such cell transplants in semi-permeable, nanoporous constructs is often required to protect them from immune attack and to prevent their proliferation in the host. However, effective graft immunoisolation has been mostly elusive owing to the absence of a high-throughput method to create precisely controlled, high-aspect-ratio nanopores. To address the clinical need for effective cell encapsulation and immunoisolation, we devised a biocompatible cell-encapsulating microcontainer and a method to create highly anisotropic nanopores in the microcontainer’s surface. To evaluate the efficacy of these nanopores in oxygenating the encapsulated cells, we engineered 9L rat glioma cells to bioluminesce under hypoxic conditions. The methods described above should aid in evaluating the long term survival and efficacy of cellular grafts.

SU-8-based immunoisolative microcontainer with nanoslots defined by nanoimprint lithography

Cells can secrete biotherapeutic molecules that can replace or restore host function. The transplantation of such cells is a promising therapeutic modality for the treatment of several diseases including type 1 diabetes mellitus. These cellular grafts are encapsulated in semipermeable and immunoisolative membranes to protect them from the host immune system, while allowing the transport of nutrients and small molecules that are required for cell survival and function. The authors report on SU-8-based biocompatible immunoisolative cuboid microcontainers for cell transplantation. Each microcontainer comprises a 300×300×250 or a 1100×1100×250 μm(3) SU-8 hollowed cuboid base that houses the cells and an optically transparent SU-8-based nanoporous lid that closes the device. The hollowed cuboid base was formed by conventional optical lithography to have 8 nl (200×200×200 μm(3)) encapsulation volume for cellular payload. The lid comprises a thick SU-8 slab with an array of cylindrical wells, whose bottom surface is sealed with a thin nanoporous SU-8 membrane. The nanoporous membrane was created from a 100 nm grating (width and spacing) initial silicon mold subjected to a repeated cycle of oxidation and wet etching to achieve a 20 nm wide and 200 nm pitch nano silicon grating. Nanoimprinting and oblique-angle metal deposition, followed by inductively coupled plasma etching were utilized to create 15 nm wide and 350-450 nm deep nanoslots in the thin SU-8 membrane. Isolated mouse islets were encapsulated in the hollowed cuboid base and the nanoporous lid was assembled on top. The penetration of large and small molecules into the microcontainer was observed with fluorescence.

Void-free filling of spin-on dielectric in 22nm wide ultrahigh aspect ratio Si trenches

The authors demonstrate fabrication of ultrahigh aspect ratio nanotrenches, made by nanoimprint lithography and dimension reduction, as test bed shallow trench isolation structures for the 22nm semiconductor node. Polysilazane based spin-on dielectric (SOD) material is spin coated into the nanotrenches, of 22nm width and aspect ratio over 30, to evaluate gap filling property. Fourier transform infrared spectroscopy analysis is used to characterize the curing properties of the SOD, showing that the material can be cured in oxygen at temperatures of 600°C and higher. Transmission electron microscopy images indicate that the filling is complete and void-free along the entirety of the trench.

Biofriendly bonding processes for nanoporous implantable SU-8 microcapsules for encapsulated cell therapy.

Mechanically robust, cell encapsulating microdevices fabricated using photolithographic methods can lead to more efficient immunoisolation in comparison to cell encapsulating hydrogels. There is a need to develop adhesive bonding methods which can seal such microdevices under physiologically friendly conditions. We report the bonding of SU-8 based substrates through (i) magnetic self assembly, (ii) using medical grade photocured adhesive and (iii) moisture and photochemical cured polymerization. Magnetic self-assembly, carried out in biofriendly aqueous buffers, provides weak bonding not suitable for long term applications. Moisture cured bonding of covalently modified SU-8 substrates, based on silanol condensation, resulted in weak and inconsistent bonding. Photocured bonding using a medical grade adhesive and of acrylate modified substrates provided stable bonding. Of the methods evaluated, photocured adhesion provided the strongest and most stable adhesion.

Simulation of Si nanowire biosensor: Effects of surface and biasing on sensitivity and linearity

We present here a numerically simulated pH sensing performance of Si nanowire (SiNW) FETs. The simulation is formulated based on Fermi-Dirac, Poisson-Boltzman, site-binding and Gouy-Chapman-Stern theories. Device characteristics (Vt, SS, On/Off, etc.) and pH sensing linearity/sensitivity from simulation match well with our sensing experiments using SiNW FETs fabricated with CMOS compatible process. Our study quantitatively shows the biasing under strong inversion yields better linearity, while sub-threshold yields better sensitivity. We also show that high sensitivity and linearity would require oxide surface with high density of reactive groups and good SAMs coverage.

Nanogratings containing sub-10-nm wide trenches by dimension reduction from sloped polymer profile

Large area nanograting patterns are useful in many applications but difficult to fabricate. The authors demonstrate a low temperature dimension reduction method, as a cost-effective alternative to high resolution lithography, to define nanogratings as narrow as 8–10nm. In this process, the slope of prepatterned polymer gratings, with pitch of 200nm or larger and width of 100nm or larger, is contrillably changed from the original straight to curved or sloped. Then, shadow metal evaporation is used to coat the sloped polymer profile to define a much narrower opening. This opening is then transferred to underlying material by plasma etching to form sub-10-nm trenches. The width of trenches can be well controlled by both slope of the profile and angle of metal evaporation. Low processing temperature (as low as 55–85°C—depending on polymer) allows this method to be used with a wide variety of materials.

One-Step Combined-Nanolithography-And-Photolithography for a 2d Photonic Crystal TM Polarizer

Photonic crystals have been widely investigated since they have great potential to manipulate the flow of light in an ultra-compact-scale and enable numerous innovative applications. 2D slab photonic crystals for the telecommunication C band at around 1550 nm have multi-scale structures that are typically micron-scale waveguides and deep sub-micron-scale air hole arrays. Several steps of nanolithography and photolithography are usually used for the fabrication of multi-scale photonic crystals. In this work, we report a one-step lithography process to pattern both micron and deep sub-micron features simultaneously for the 2D slab photonic crystal using combined-nanoimprint-and-photolithography. As a demonstrator, a 2D silicon photonic crystal transverse magnetic (TM) polarizer was fabricated, and the operation was successfully demonstrated.