Walter Hu

Nanoimprinted Polymer Solar Cell

Among the various organic photovoltaic devices, the conjugated polymer/fullerene approach has drawn the most research interest. The performance of these types of solar cells is greatly determined by the nanoscale morphology of the two components (donor/acceptor) and the molecular orientation/crystallinity in the photoactive layer. A vertically bicontinuous and interdigitized heterojunction between donor and acceptor has been regarded as one of the ideal structures to enable both efficient charge separation and transport. Synergistic control of polymer orientation in the nanostructured heterojunction is also critical to improve the performance of polymer solar cells. Nanoimprint lithography has emerged as a new approach to simultaneously control both the heterojunction morphology and polymer chains in organic photovoltaics. Currently, in the area of nanoimprinted polymer solar cells, much progress has been achieved in the fabrication of nanostructured morphology, control of molecular orientation/crystallinity, deposition of acceptor materials, patterned electrodes, understanding of structure-property correlations, and device performance. This review article summarizes the recent studies on nanoimprinted polymer solar cells and discusses the outstanding challenges and opportunities for future work.

Modeling particle shape-dependent dynamics in nanomedicine.

One of the major challenges in nanomedicine is to improve nanoparticle cell selectivity and adhesion efficiency through designing functionalized nanoparticles of controlled sizes, shapes, and material compositions. Recent data on cylindrically shaped filomicelles are beginning to show that non-spherical particles remarkably improved the biological properties over spherical counterpart. Despite these exciting advances, non-spherical particles have not been widely used in nanomedicine applications due to the lack of fundamental understanding of shape effect on targeting efficiency. This paper intends to investigate the shape-dependent adhesion kinetics of non-spherical nanoparticles through computational modeling. The ligand-receptor binding kinetics is coupled with Brownian dynamics to study the dynamic delivery process of nanorods under various vascular flow conditions. The influences of nanoparticle shape, ligand density, and shear rate on adhesion probability are studied. Nanorods are observed to contact and adhere to the wall much easier than their spherical counterparts under the same configuration due to their tumbling motion. The binding probability of a nanorod under a shear rate of 8 s(-1) is found to be three times higher than that of a nanosphere with the same volume. The particle binding probability decreases with increased flow shear rate and channel height. The Brownian motion is found to largely enhance nanoparticle binding. Results from this study contribute to the fundamental understanding and knowledge on how particle shape affects the transport and targeting efficiency of nanocarriers, which will provide mechanistic insights on the design of shape-specific nanomedicine for targeted drug delivery applications.

Shape-specific polymeric nanomedicine: emerging opportunities and challenges.

Size and shape are fundamental properties of micro/nanoparticles that are critically important for nanomedicine applications. Extensive studies have revealed the effect of particle size on spherical particles with respect to circulation, extravasation and distribution in vivo. In contrast, the importance of particle shape has only recently begun to emerge. For example, cylindrically-shaped filomicelles (diameter 22–60 nm, length 8–18 μm) have shown persistent blood circulation for up to one week after intravenous injection, much longer than their spherical counterparts. Disc-shaped nanoparticles have demonstrated higher in vivo targeting specificity to endothelial cells expressing intercellular adhesion molecule receptors in mice than spherical particles of similar size. In this Minireview, we will discuss the recent advances in the fabrication of shape-specific nanoparticles and their unique biological and pharmacological properties. Computational models are presented to provide mechanistic understanding of the shape effects on cell targeting under flow conditions. Shape-specific nanoparticles have the potential to significantly improve the performance of nanomedicine in diagnostic imaging and targeted drug delivery applications.

Effects of nanoimprinted patterns in tissue-culture polystyrene on cell behavior.

Tissue engineering seeks to develop functional tissues in a biomimetic environment in vitro. As the extracellular environment in vivo is composed of numerous nanostructures, fabrication of nanostructured substrates will be valuable for tissue engineering applications. In this article, we report a simple nanoimprint lithography (NIL) process to pattern nanostructures directly on tissue-culture polystyrene plates. By repeating this NIL process, three-dimensional scaffolds consisting of multiple-layer nanostructures were also fabricated. Bovine pulmonary artery smooth muscle cells were cultured on imprinted gratings ranging from 350 nm to 10 μm. The smooth muscle cells attached and proliferated well on these imprinted substrates without additional surface treatment. Cell elongation and alignment were observed on the micro- and nanopatterns, with the effect significantly more pronounced on the nanostructures.

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.

Ultrasensitive protein detection using lithographically defined Si multi-nanowire field effect transistors.

Low-doped silicon multi-nanowire field effect transistors with high ON/OFF ratio over 10(7) and a low subthreshold swing of 60-120 mV dec(-1) are fabricated using lithographic semiconductor processes. The use of multi-nanowires instead of a single nanowire as sensing elements has shown improved device uniformity and stability in buffer solutions. The device stability is further improved with surface silanization and biasing with a solution gate rather than a backgate. pH sensing with a linear response over a range of 2-9 is achieved using these devices. Selective detection of bovine serum albumin at concentrations as low as 0.1 femtomolar is demonstrated.

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.

Silicon multi-nanochannel FETs to improve device uniformity/stability and femtomolar detection of insulin in serum

Here we demonstrate the use of multiple Si nanochannel (NC) or nanograting (NG) instead of the conventional single nanochannel or nanowire design in biosensors. The NG devices can significantly reduce device-to-device variation, and improve device performance, e.g. higher current, higher ON/OFF ratio, smaller subthreshold slope, lower threshold voltage Vt in buffer solution. NG devices also result in higher sensor stability in buffer and diluted human serum. We believe such improvements are due to reduced discrete dopant fluctuation in the Si nanowires and biochemical noise in the solution because of the multiple-channel design. The improved devices allow us to sense pH linearly with 3-aminopropyltriethoxysilane coated devices, and to selectively detect insulin with limit of detection down to 10 fM in both buffer solution and diluted human serum without pre-purification.

Nanoimprinted Perovskite Nanograting Photodetector with Improved Efficiency

Recently, organolead halide-based perovskites have emerged as promising materials for optoelectronic applications, particularly for photovoltaics, photodetectors, and lasing, with low cost and high performance. Meanwhile, nanoscale photodetectors have attracted tremendous attention toward realizing miniaturized optoelectronic systems, as they offer high sensitivity, ultrafast response, and the capability to detect beyond the diffraction limit. Here we report high-performance nanoscale-patterned perovskite photodetectors implemented by nanoimprint lithography (NIL). The spin-coated lead methylammonium triiodide perovskite shows improved crystallinity and optical properties after NIL. The nanoimprinted metal-semiconductor-metal photodetectors demonstrate significantly improved performance compared to the nonimprinted conventional thin-film devices. The effects of NIL pattern geometries on the optoelectronic characteristics were studied, and the nanograting pattern based photodetectors demonstrated the best performance, showing approximately 35 times improvement on responsivity and 7 times improvement on on/off ratio compared with the nonimprinted devices. The high performance of NIL-nanograting photodetectors likely results from high crystallinity and favored nanostructure morphology, which contribute to higher mobility, longer diffusion length, and better photon absorption. Our results have demonstrated that the NIL is a cost-effective method to fabricate high-performance perovskite nanoscale optoelectronic devices, which may be suitable for manufacturing of high-density perovskite nanophotodetector arrays and to provide integration with state-of-the-art electronic circuits.

Lithographically defined uniform worm-shaped polymeric nanoparticles.

We report a nanoimprint lithography method combined with photolithography on a bi-layer polymer setup to define nano-worms laterally in nano-gratings and produce uniform worm-shaped polymeric nanoparticles in aqueous solution by dissolving water soluble sacrificial layer poly(vinyl alcohol) (PVA). Process control of the thin residue layer, SU-8 curing using broadband UV source, and warm development to remove any residue are necessary to ensure success of this technique. The use of water soluble PVA as a releasing layer and elimination of an invasive plasma etching for the releasing process makes this protocol highly compatible with biomaterials. Direct release and suspension of fluorescent worm-shaped nanoparticles (length to width ratio up to 75) in aqueous solution were demonstrated. Compared to the worm-shaped nanoparticles made by self-assembly, these lithographically defined nano-worms have much better controllability and uniformity on the shape, size, and aspect ratio. The availability of these precisely defined non-spherical particles would be important to develop a comprehensive understanding of the shape effects of nanoparticles on their efficacy in nanomedicine applications.