Nicholas R. Hendricks

Solvent-Assisted Soft Nanoimprint Lithography for Structured Bilayer Heterojunction Organic Solar Cells

We introduce a novel method to easily fabricate nanopatterns at ambient conditions using solvent-assisted soft nanolithography. For this purpose, a P3HT/PCBM bilayer, one of well-known standard models of solar cell systems, was chosen to optimize bilayer solar cells using the new lithographic technique. The nanopatterns of P3HT made using this method have improved device efficiency compared to planar bilayer heterojunction of the solar cell. The new patterning process creates solar cell devices with a greater than 2-fold increase in power conversion efficiency (PCE) compared to an otherwise equivalent, flat device. This improvement in efficiency is due to the increased interfacial area created by the patterning process. This result demonstrates the feasibility of extensive applications toward nanolithography, relevant to device fabrication, such as electronic devices.

Solution Processable High Dielectric Constant Nanocomposites Based on ZrO2 Nanoparticles for Flexible Organic Transistors

A solution-based strategy for fabrication of high dielectric constant (κ) nanocomposites for flexible organic field effect transistors (OFETs) has been developed. The nanocomposite was composed of a high-κ polymer, cyanoethyl pullulan (CYELP), and a high-κ nanoparticle, zirconium dioxide (ZrO2). Organic field effect transistors (OFETs) based on neat CYELP exhibited anomalous behavior during device operation, such as large hysteresis and variable threshold voltages, which yielded inconsistent devices and poor electrical characteristics. To improve the stability of the OFET, we introduced ZrO2 nanoparticles that bind with residual functional groups on the high-κ polymer, which reduces the number of charge trapping sites. The nanoparticles, which serve as physical cross-links, reduce the hysteresis without decreasing the dielectric constant. The dielectric constant of the nanocomposites was tuned over the range of 15.6-21 by varying the ratio of the two components in the composite dielectrics, resulting in a high areal capacitance between 51 and 74 nF cm(-2) at 100 kHz and good insulating properties of a low leakage current of 1.8 × 10(-6) A cm(-2) at an applied voltage of -3.5 V (0.25 MV cm(-1)). Bottom-gate, top-contact (BGTC) low operating voltage p-channel OFETs using these solution processable high-κ nanocomposites were fabricated by a contact film transfer (CFT) technique with poly(3-hexylthiophene) (P3HT) as the charge transport layer. Field effect mobilities as high as 0.08 cm(2) V(-1) s(-1) and on/off current ratio of 1.2 × 10(3) for P3HT were measured for devices using the high-κ dielectric ZrO2 nanocomposite. These materials are promising for generating solution coatable dielectrics for low cost, large area, low operating voltage flexible transistors.

Flexible Low-Voltage Polymer Thin-Film Transistors Using Supercritical CO2-Deposited ZrO2 Dielectrics

The fabrication of low-voltage flexible organic thin film transistors using zirconia (ZrO(2)) dielectric layers prepared via supercritical fluid deposition was studied. Continuous, single-phase films of approximately 30 nm thick ZrO(2) were grown on polyimide (PI)/aluminum (Al) substrates at 250 °C via hydrolysis of tetrakis(2,2,6,6-tetramethyl-3,5-heptane-dionato) zirconium in supercritical carbon dioxide. This dielectric layer showed a high areal capacitance of 317 nF cm(-2) at 1 kHz and a low leakage current of 1.8 × 10(-6) A cm(-2) at an applied voltage of -3 V. By using poly(3-hexylthiophene) (P3HT) as a semiconductor, we have fabricated flexible thin film transistors operating at V(DS) = -0.5 V and V(G) in a range from 0.5 V to -4 V, with on/off ratios on the order of 1 × 10(3) and mobility values higher than 0.1 cm(2)/(V s).

Facile Colloidal Lithography on Rough and Non-planar Surfaces for Asymmetric Patterning

Free-standing colloidal arrays can be easily transferred to supported fibers. These films conform and provide the template to have consistent submicrometer and nanometer features transferred to the periphery of rough, 7 μm diameter fibers. This technique is adjustable to a number of fiber surfaces and colloidal template sizes.

Enhanced Second-Harmonic Generation from Sequential Capillarity-Assisted Particle Assembly of Hybrid Nanodimers

We show enhanced second-harmonic generation (SHG) from a hybrid metal-dielectric nanodimer consisting of an inorganic perovskite nanoparticle of barium titanate (BaTiO3) coupled to a metallic gold (Au) nanoparticle. BaTiO3-Au nanodimers of 100 nm/80 nm sizes are fabricated by sequential capillarity-assisted particle assembly. The BaTiO3 nanoparticle has a noncentrosymmetric crystalline structure and generates bulk SHG. We use the localized surface plasmon resonance of the gold nanoparticle to enhance the SHG from the BaTiO3 nanoparticle. We experimentally measure the nonlinear signal from assembled nanodimers and demonstrate an up to 15-fold enhancement compared to a single BaTiO3 nanoparticle. We further perform numerical simulations of the linear and SHG spectra of the BaTiO3-Au nanodimer and show that the gold nanoparticle acts as a nanoantenna at the SHG wavelength.

Hierarchically structured porous cadmium selenide polycrystals using polystyrene bilayer templates.

In this study, a novel approach is demonstrated to fabricate hierarchically structured cadmium selenide (CdSe) layers with size-tunable nano/microporous morphologies achieved using polystyrene (PS) bilayered templates (top layer: colloidal template) via potentiostatic electrochemical deposition. The PS bilayer template is made in two steps. First, various PS patterns (stripes, ellipsoids, and circles) are prepared as the bottom layers through imprint lithography. In a second step, a top template is deposited that consists of a self-assembled layer of colloidal 2D packed PS particles. Electrochemical growth of CdSe crystals in the voids and selective removal of the PS bilayered templates give rise to hierarchically patterned 2D hexagonal porous CdSe structures. This simple and facile technique provides various unconventional porous CdSe films, arising from the effect of the PS bottom templates.

Formation of hierarchical silica nanochannels through nanoimprint lithography

Hierarchically structured silica nanochannels were fabricated through the combination of supercritical carbon dioxide mediated silica deposition and nanoimprint lithography of a sacrificial polymer template. Highly-ordered mesoporous silica was prepared with either spherical or cylindrical domain level features, ∼5–6 nm in diameter, to compliment the device level structure of the embedded nanochannels. The hierarchical structure was used as a test device for low-k dielectric materials with a dielectric constant of 2.0 observed.

Mesoporous silica/nanoparticle composites prepared by 3-D replication of highly filled block copolymer templates

The incorporation of iron platinum (FePt) nanoparticles (NPs) within the walls of mesoporous silica was achieved at high NP loadings using highly filled amphiphilic poly(ethylene oxide-b-propylene oxide-b-ethylene oxide) (Pluronic®) copolymer templates prepared by exploiting selective hydrogen bonding between the pre-synthesized nanoparticles and the hydrophilic portion of the block copolymer. The mesoporous silica/NP composites were then synthesized by means of phase selective condensation of tetraethylorthosilicate (TEOS) within the NP loaded block copolymer templates dilated with supercritical carbon dioxide (scCO2) followed by calcination. The resulting mesoporous silica composites were prepared at nanoparticle loadings as high as 25 wt% relative to the template (15 wt% relative to mesoporous silica) and were characterized by electron microscopy, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The method described in this report is general and may be applied to a variety of NPs for encapsulation within the pore walls of mesoporous silica.

Direct imprint patterning of 2-D and 3-D nanoparticle/polymer hybrid and crystalline metal oxide structures for printed optical, electronic, and energy devices

Nanoimprint lithography (NIL) offers high precision patterning of structures as small as 50 nm using wafer-based or roll-to-roll process platforms, however current resist systems offer little functionality. We developed hybrid UV-NIL resists containing up to 90 wt. % nanoparticles with excellent optical transparency for direct patterning of device structures including a readily scalable print, lift, and stack approach for producing large-area, 3D photonic crystal (PC) structures and optical gratings. We have also extended the NIL approach to directly print dimensionally stable metal oxide nanostructures using inks containing high concentrations of crystalline nanoparticles.