Understanding Electrical Conduction and Nanopore Formation During Controlled Breakdown.

Controlled breakdown has recently emerged as a highly appealing technique to fabricate solid-state nanopores for a wide range of biosensing applications. This technique relies on applying an electric field of approximately 0.4-1 V nm-1 across the membrane to induce a current, and eventually, breakdown of the dielectric. Although previous studies have performed controlled breakdown under a range of different conditions, the mechanism of conduction and breakdown has not been fully explored. Here, electrical conduction and nanopore formation in SiNx membranes during controlled breakdown is studied. It is demonstrated that for Si-rich SiNx , oxidation reactions that occur at the membrane-electrolyte interface limit conduction across the dielectric. However, for stoichiometric Si3 N4 the effect of oxidation reactions becomes relatively small and conduction is predominately limited by charge transport across the dielectric. Several important implications resulting from understanding this process are provided which will aid in further developing controlled breakdown in the coming years, particularly for extending this technique to integrate nanopores with on-chip nanostructures.

Electrotunable Nanoplasmonics for Amplified Surface Enhanced Raman Spectroscopy.

Tuning the properties of optical metamaterials in real time is one of the grand challenges of photonics. Being able to do so will enable a class of adaptive photonic materials for use in applications such as surface enhanced Raman spectroscopy and reflectors/absorbers. One strategy to achieving this goal is based on the electrovariable self-assembly and disassembly of two-dimensional nanoparticle arrays at a metal | liquid interface. As expected, the structure results in plasmonic coupling between NPs in the array but perhaps as importantly between the array and the metal surface. In such a system, the density of the nanoparticle array can be reversibly controlled by the variation of electrode potential. Theory suggests that due to a collective plasmon-coupling effect  less than 1 V variation of electrode potential can give rise to a dramatic simultaneous change in optical reflectivity from ∼93% to ∼1% and the amplification of the SERS signal by up to 5 orders of magnitude. This is experimentally demonstrated using a platform based on the voltage-controlled assembly of 40 nm Au-nanoparticle arrays at a TiN/Ag electrode in contact with an aqueous electrolyte. We show that all the physics underpinning the behavior of this platform works precisely as suggested by the proposed theory, setting the electrochemical nanoplasmonics as a promising direction in photonics research.

Auxetic Thermoresponsive Nanoplasmonic Optical Switch.

Development and use of metamaterials have been gaining prominence in large part due to the possibility of creating platforms with "disruptive" and unique optical properties. However, to date, the majority of such systems produced using micro or nanotechnology are static and can only perform certain target functions. Next-generation multifunctional smart optical metamaterials are expected to have tunable elements with the possibility of controlling the optical properties in real time via variation in parameters such as pressure, mechanical stress, and voltage or through nonlinear optical effects. Here, we address this challenge by developing a thermally controlled optical switch, based on the self-assembly of poly( N-isopropylacrylamide)-functionalized gold nanoparticles on a planar macroscale gold substrate. We show that such meta-surfaces can be tuned to exhibit substantial changes in the optical properties in terms of both wavelength and intensity, through the temperature-controlled variation of the interparticle distance within the nanoparticle monolayer as well as its separation from the substrate. This change is based on temperature-induced auxetic expansion and contraction of the functional ligands. Such a system has potential for numerous applications, ranging from thermal sensors to regulated light harnessing.

Determination of Concentration of Amphiphilic Polymer Molecules on the Surface of Encapsulated Semiconductor Nanocrystals.

We present a method for the determination of the average number of polymer molecules on the surface of A(II)B(VI) luminescent core-shell nanocrystals (CdSe/ZnS, ZnSe/ZnS quantum dots, and CdS/ZnS nanorods) encapsulated with amphiphilic polymer. Poly(maleic anhydride-alt-1-tetradecene) (PMAT) was quantitatively labeled with amino-derivative of fluorescein and the average amount of PMAT molecules per single nanocrystal was determined using optical absorption of the dye in the visible spectral range. The average amount of PMAT molecules grows linearly with the surface area of all studied nanocrystals. However, the surface density of the monomer units increases nonlinearly with the surface area, because of the increased competition between PMAT molecules for Zn-hexanethiol surface binding sites. The average value of zeta potential (ζ = -35 mV) was found to be independent of the size, shape, and chemical composition of nanocrystals at fixed buffer parameters (carbonate-bicarbonate buffer, pH 9.5 and 5 mM ionic strength). This finding is expected to be useful for the determination of the surface density of remaining carboxyl groups in PMAT-encapsulated nanocrystals.