We proposed and demonstrated the first account of large-area, semi-transparent, tandem photosensitive nanocrystal skins (PNSs) constructed on flexible substrates operating on the principle of photogenerated potential buildup, which avoid the need for applying an external bias and circumvent the current-matching limitation between junctions. We successfully fabricated and operated the tandem PNSs composed of single monolayers of colloidal water-soluble CdTe and CdHgTe nanocrystals (NCs) in adjacent junctions on a Kapton polymer tape. Owing to the usage of a single NC layer in each junction, noise generation was significantly reduced while keeping the resulting PNS films considerably transparent. In each junction, photogenerated excitons are dissociated at the interface of the semi-transparent Al electrode and the NC layer, with holes migrating to the contact electrode and electrons trapped in the NCs. As a result, the tandem PNSs lead to an open-circuit photovoltage buildup equal to the sum of those of the two single junctions, exhibiting a total voltage buildup of 128.4 mV at an excitation intensity of 75.8 μW cm(-2) at 350 nm. Furthermore, we showed that these flexible PNSs could be bent over 3.5 mm radius of curvature and cut out in arbitrary shapes without damaging the operation of individual parts and without introducing any significant loss in the total sensitivity. These findings indicate that the NC skins are promising as building blocks to make low-cost, flexible, large-area UV/visible sensing platforms with highly efficient full-spectrum conversion.

Abstract-We propose and demonstrate nanowire (NW) device platforms on-chip integrated using electric-field-assisted self-assembly. This platform integrates from nanoprobes to microprobes, and conveniently allows for on-chip manipulation, capturing, and electrical characterization of nanoparticles (NPs). Synthesizing segmented (Au-Ag-Au) NWs and aligning them across predefined microelectrode arrays under ac electric field, we controllably form nanogaps between the self-aligned end (Au) segments by selectively removing the middle (Ag) segments. We precisely control and tune the size of this middle section for nanogap formation in the synthesis process. Using electric field across nanogaps between these nanoprobes, we capture NPs to electrically address and probe them at the nanoscale. This approach holds great promise for the construction of single NP devices with electrical nanoprobe contacts.

In this work, we demonstrate a proof-of-concept system for generating highly polarized light from colloidal quantum dots (QDs) coupled with magnetically aligned segmented Au/Ni/Au nanowires (NWs). Optical characterizations reveal that the optimized QD-NW coupled structures emit highly polarized light with an s-to p-polarization (s/p) contrast as high as 15:1 corresponding to a degree of polarization of 0.88. These experimental results are supported by the finite-difference time-domain simulations, which demonstrate the interplay between the inter-NW distance and the degree of polarization. © 2014 AIP Publishing LLC

Dielectrophoresis (DEP) allows for electric field assisted assembly in spatially non-uniform field distribution, where the induced moment is translated into a net force on polarized particles towards the high field gradient. For example, for a spherical particle of radius r with a permittivity constant of ε p in a host medium with the permittivity of ε m , the dielectrophoretic force is given by (1):where r is the particle radius, ω is the angular frequency and E rms is the root mean square electric field. K(ω) is the Clausius-Mossotti function, which depends on the complex permittivity of the spherical particle and the medium [1]. We present a nanoscale device platform constructed with dielectrophoretic self-assembly of our segmented nanowires. These aligned nanowires automatically provide electrical contacts to the captured nanoparticles to allow for electrical probing at the nanoscale. This enables full integration from nanoparticles to nanowires to microelectrodes to macroprobes on a single chip, spanning a size range of more than six orders of magnitude. For processing, we first pattern the bottom metal layer made of Au (bottom electrodes) that provides an electrical path to upper electrodes through AC coupling. We subsequently deposit a silicon nitride layer, which serves as the dielectric layer for capacitive coupling between the lower electrodes and the upper electrodes, and finally pattern the upper gold microelectrodes. The dielectric layer prevents electrical shortage of these electrodes to metallic nanowires during the assembly process and enables characterization of each nanowire individually. The separation between the tips of the second layer (upper) electrodes fingers are designed to be 5-6 µm long, where the nanowires align at the highest field gradient. Using electrodeposition, we synthesize Au-Ag-Au segmented nanowires in porous membranes made of aluminum oxide (Whatman Anodisc) used as the template. We start the deposition of the gold segment using a current level of -1.6 mA at a deposition rate of 60 nm/min while the reduction occurs for a potential range of -1.70 V and -2.65 V. For subsequent Ag segment that determines the size of our gap, we employ a constant current level of -1.6 mA while varying deposition times to make Ag segments of varying lengths tuned from 300 nm down to 20 nm. Our rate of silver deposition is 85 nm/min for a potential range of -1.40 V and -1.70 V. After aligning Au-Ag-Au nanowires, their middle Ag segments are removed chemically etching Ag with dilute nitric acid (HNO 3 ) solution and/or by post-baking process at 200°C for 75 min. This allows for the two ends of the nanowires to be automatically and precisely aligned. Thermal baking enables us to smoothen the nonuniform morphology of the nanogaps, and strengthens the electrical contacts of the gold segment ends of the nanowires to the microelectrodes [2]. Fig. 1 shows different lengths of Ag segments. The length of these Ag segments is linearly proportional to the total charge driven into the pores of the membrane ...

Abstract:We demonstrate lateral arrays of suspended metal nanodiscs, erected in parallel, partially encased in a dielectric-wrap using in-template synthesis with their polarization-dependent properties controlled as a function of disc-gap/-width tailoring their scattering/absorption spectra.Plasmonic structures enable enhanced light utilization for optoelectronic devices owing to their unique, strong and tunable electric field localization leading to increased scattering and absorption properties. Various nanopatterned plasmonic structures have been proposed to date, which typically use the nanofabrication method of electron beam lithography (EBL). Although these techniques offer high resolution and allow for a more deterministic nanostructure layout for plasmonics, these technologies are quite costly and limited in high throughput. Alternatively, solution processed nanostructures have emerged, for example, including nanodiscs [1], which are synthesized using intemplate electro-deposition [2], followed by simple selective etching of sacrificial metal layers. These nanodiscs are promising for nanoplasmonics because their optical properties may be fine-tuned either via the disc-gap or the discwidth, which can range from 20 nm to several hundred nanometers. In this work, we demonstrate lateral arrays of Au nanodiscs with controlled gaps and widths using the in-template synthesis to allow for the tailoring of the scattering and absorption properties. This has resulted in massive numbers of easily and inexpensively fabricated arrays of nanodisc structures. The achievable resolution of our fabrication approach reaches down to approximately 20 nm. Fabricating such three-dimensional suspended and liquid dispersible architectures is not easily possible by other techniques such as EBL. Here, we report the plasmonic properties of these suspended nanodisc structures including the polarization-dependent scattering spectra computed numerically and measured experimentally. We study the polarization dependent absorption and scattering properties of the nanodisc arrays presented in Fig. 1(a) and 1(b). We model our discs to tailor the light spectra scattered from and absorbed in these Au discs with varying disc-widths (w) for different disc gaps (g). Under unpolarized illumination incident on discs, we observe different spectral contributions from the field polarized in vertical and horizontal directions. Figs. 2(b) and 2(c) present the computed quality scattering factors of the nanodisc array via power fraction analysis of vertically and horizontally polarized fields. Here the disc diameter (D) is 250 nm, which is set by our template membrane used during electro-deposition. When we compute the power component normal to the discs (vertical), we observe strong plasmonic coupling depending on the gap size (g) between the discs. The electric field localized between the discs is found to increase with this coupling. We observe a scattering peak at shorter wavelengths in Fig. 2(b) with a disc width (w) of approximately 50 nm and abov...