Observation of localized surface plasmons and hybridized surface plasmon polaritons on self-assembled two-dimensional nanocavities.

Large-area patterning of periodic nanostructures using self-assembled nanospheres is of interest for fabricating low-cost plasmonic substrates, such as two-dimensional (2D) metallic gratings. Surface plasmon polaritons (SPPs) excited on metallic gratings have applications in biosensors, thin-film photovoltaics, photoelectrochemical cells, and photodetectors. Here we fabricated large-area metallic gratings using nanosphere lithography, and the geometry of gratings was controlled by the sphere size and distance between nanospheres. Both forward and backward propagating SPPs were observed using the grating coupling geometry. Furthermore, we reported the first observation of localized surface plasmons (LSPs) on this large-area metallic grating by both simulation and experimental studies. Such an LSP mode was confined in the 2D nanocavities and was not supported by dielectric gratings with the same 2D geometry.

Resonance-induced absorption enhancement in colloidal quantum dot solar cells using nanostructured electrodes.

The application of nanostructured indium-doped tin oxide (ITO) electrodes as diffraction gratings for light absorption enhancement in colloidal quantum dot solar cells is numerically investigated using finite-difference time-domain (FDTD) simulation. Resonant coupling of the incident diffracted light with supported waveguide modes in light absorbing layer at particular wavelengths predicted by grating far-field projection analysis is shown to provide superior near-infrared light trapping for nanostructured devices as compared to the planar structure. Among various technologically feasible nanostructures, the two-dimensional nano-branch array is demonstrated as the most promising polarization-independent structure and proved to be able to maintain its performance despite structural imperfections common in fabrication.

Inverse Opal Photonic Crystals as an Optofluidic Platform for Fast Analysis of Hydrocarbon Mixtures.

Most of the reported optofluidic devices analyze liquid by measuring its refractive index. Recently, the wettability of liquid on various substrates has also been used as a key sensing parameter in optofluidic sensors. However, the above-mentioned techniques face challenges in the analysis of the relative concentration of components in an alkane hydrocarbon mixture, as both refractive indices and wettabilities of alkane hydrocarbons are very close. Here, we propose to apply volatility of liquid as the key sensing parameter, correlate it to the optical property of liquid inside inverse opal photonic crystals, and construct powerful optofluidic sensors for alkane hydrocarbon identification and analysis. We have demonstrated that via evaporation of hydrocarbons inside the periodic structure of inverse opal photonic crystals and observation of their reflection spectra, an inverse opal film could be used as a fast-response optofluidic sensor to accurately differentiate pure hydrocarbon liquids and relative concentrations of their binary and ternary mixtures in tens of seconds. In these 3D photonic crystals, pure chemicals with different volatilities would have different evaporation rates and can be easily identified via the total drying time. For multicomponent mixtures, the same strategy is applied to determine the relative concentration of each component simply by measuring drying time under different temperatures. Using this optofluidic sensing platform, we have determined the relative concentrations of ternary hydrocarbon mixtures with the difference of only one carbon between alkane hydrocarbons, which is a big step toward detailed hydrocarbon analysis for practical use.

Digital microelectromechanical sensor with an engineered polydimethylsiloxane (PDMS) bridge structure.

Functional electronic devices integrated on flexible substrates are of great interest in both academia and industry for their potential applications in wearable technologies. Recently, there have been an increasing number of investigations on developing new materials for flexible strain sensors and pressure sensors, with the aim of achieving better sensitivity and detection ranges. However, the analog signal outputs of these sensors are accompanied with challenges regarding device reproducibility and reliability. Here we designed and fabricated a new class of sensors-digital microelectromechanical (MEM) sensors for wearable technologies. Our digital MEM sensors were implemented with the polydimethysiloxane (PDMS) bridge on flexible substrates, and provided digital signal outputs based on electrical insulating-to-conducting transitions. By engineering the PDMS bridge structure, we could tune the sensitivity of the digital MEM sensor for various applications. These digital MEM sensors were used in bending tests: they were integrated on glove fingers and used to detect gestures. These sensors were also used as force sensors: they were used on human wrists to monitor heart rates. The device was experimentally found to maintain its performance level even after 10 000 cycles of bending or pressing. The digital output of our devices allows a higher tolerance for device fabrication to be set. Furthermore, our devices can be engineered for desired specifications in various potential applications.

Numerical Study of Complementary Nanostructures for Light Trapping in Colloidal Quantum Dot Solar Cells.

We have investigated two complementary nanostructures, nanocavity and nanopillar arrays, for light absorption enhancement in depleted heterojunction colloidal quantum dot (CQD) solar cells. A facile complementary fabrication process is demonstrated for patterning these nanostructures over the large area required for light trapping in photovoltaic devices. The simulation results show that both proposed periodic nanostructures can effectively increase the light absorption in CQD layer of the solar cell throughout the near-infrared region where CQD solar cells typically exhibit weak light absorption. The complementary fabrication process for implementation of these nanostructures can pave the way for large-area, inexpensive light trapping implementation in nanostructured solar cells.