Zhida Xu

Surface-enhanced Raman nanodomes

We demonstrate a surface-enhanced Raman scattering (SERS) substrate consisting of a closely spaced metal nanodome array fabricated on flexible plastic film. We used a low-cost, large-area replica molding process to produce a two-dimensional periodic array of cylinders that is subsequently overcoated with SiO(2) and silver thin films to form dome-shaped structures. Finite element modeling was used to investigate the electromagnetic field distribution of the nanodome array structure and the effect of the nanodome separation distance on the electromagnetic field enhancement. The SERS enhancement from the nanodome array substrates was experimentally verified using rhodamine 6G as the analyte. With a separation distance of 17 nm achieved between adjacent domes using a process that is precisely controlled during thin film deposition, a reproducible SERS enhancement factor of 1.37 × 10(8) was demonstrated. The nanoreplica molding process presented in this work allows for simple, low-cost, high-throughput fabrication of uniform nanoscale SERS substrates over large surface areas without the requirement for high resolution lithography or defect-free deposition of spherical microparticle monolayer templates.

Long‐time present tweedlike precursors and paraelectric clusters in ferroelectrics containing strong quenched randomness

Transmission electron microscopy studies of the (1−x)Pb(Mg1/3Nb2/3)O3–(x)PbTiO3 [PMN‐PT (1‐x)/x] crystalline solution have been performed for x=0.1, 0.2, 0.35, and 0.60. Bright‐field imaging has revealed a common sequence of domainlike states with increasing x. Normal micron‐sized ferroelectric domains were found for x≳0.40. Tweedlike structures were found for x∼0.35. These tweedlike structures are similar to those previously reported in premartensitic states. Paraelectric clusters were found for x<0.30. The paraelectric cluster state was characterized by the lack of self‐assembly amongst embryos and the presence of relaxor behavior in the macroscopic response characteristics. The composition PMN‐PT 65/35 was then modified with La for a detailed study of the transition between the tweedlike precursor and paraelectric cluster states with increasing impurity content.

Rigorous surface enhanced Raman spectral characterization of large-area high-uniformity silver-coated tapered silica nanopillar arrays

Surface enhanced Raman spectroscopy (SERS) has been increasingly utilized as an analytical technique with significant chemical and biological applications (Qian et al 2008 Nat. Biotechnol. 26 83; Fujita et al 2009 J. Biomed. Opt. 14 024038; Chou et al 2008 Nano Lett.8 1729; Culha et al 2003 Anal. Chem. 75 6196; Willets K A 2009 Anal. Bioanal. Chem. 394 85; Han et al 2009 Anal. Bioanal. Chem. 394 1719; Sha et al 2008 J. Am. Chem. Soc. 130 17214). However, production of a robust, homogeneous and large-area SERS substrate with the same ultrahigh sensitivity and reproducibility still remains an important issue. Here, we describe a large-area ultrahigh-uniformity tapered silver nanopillar array made by laser interference lithography on the entire surface of a 6 inch wafer. Also presented is the rigorous optical characterization method of the tapered nanopillar substrate to accurately quantify the Raman enhancement factor, uniformity and repeatability. An average homogeneous enhancement factor of close to 10(8) was obtained for benzenethiol adsorbed on a silver-coated nanopillar substrate.

DNA-Directed Assembly of Asymmetric Nanoclusters Using Janus Nanoparticles

Asymmetric assembly of nanomaterials has attracted broad interests because of their unique anisotropic properties that are different from those based on the more widely reported symmetric assemblies. Despite the potential advantages, programmable fabrication of asymmetric structure in nanoscale remains a challenge. We report here a DNA-directed approach for the assembly of asymmetric nanoclusters using Janus nanoparticles as building blocks. DNA-functionalized spherical gold nanoparticles (AuNSs) can be selectively attached onto two different hemispheres of DNA-functionalized Janus nanoparticle (JNP) through DNA hybridization. Complementary and invasive DNA strands have been used to control the degree and reversibility of the assembly process through programmable base-pairing interactions, resulting in a series of modular and asymmetric nanostructures that allow systematic study of the size-dependent assembly process. We have also shown that the attachment of the AuNSs onto the gold surface of the Janus nanoparticle results in red shifting of the UV-vis and plasmon resonance spectra.

Surface plasmon enhanced broadband spectrophotometry on black silver substrates

We demonstrate surface plasmon-induced enhancements in optical imaging and spectroscopy on silver coated silicon nanocones which we call black silver. The black silver with dense and homogeneous nanocone forest structure is fabricated with a mass-producible nanomanufacturing method. It can efficiently trap and convert incident photons into localized plasmons in broad wavelength range, permitting the enhancement in optical absorption from ultraviolet to near infrared range by 12 times, the visible fluorescence enhancement of ∼30 times and the Raman scattering enhancement factor up to ∼108. We show the potential of the black silver in high sensitivity and broadband optical sensing of molecules.

Ultrahigh Throughput Silicon Nanomanufacturing by Simultaneous Reactive Ion Synthesis and Etching

One-dimensional nanostructures, such as nanowhisker, nanorod, nanowire, nanopillar, nanocone, nanotip, nanoneedle, have attracted significant attentions in the past decades owing to their numerous applications in electronics, photonics, energy conversion and storage, and interfacing with biomolecules and living cells. The manufacturing of nanostructured devices relies on either bottom-up approaches such as synthesis or growth process or top-down approaches such as lithography or etching process. Here we report a unique, synchronized, and simultaneous top-down and bottom-up nanofabrication approach called simultaneous plasma enhanced reactive ion synthesis and etching (SPERISE). For the first time the atomic addition and subtraction of nanomaterials are concurrently observed and precisely controlled in a single-step process permitting ultrahigh-throughput, lithography-less, wafer-scale, and room-temperature nanomanufacturing. Rapid low-cost manufacturing of high-density, high-uniformity, light-trapping nanocone arrays was demonstrated on single crystalline and polycrystalline silicon wafers, as well as amorphous silicon thin films. The proposed nanofabrication mechanisms also provide a general guideline to designing new SPERISE methods for other solid-state materials besides silicon.

Structure-property relationships in strontium barium niobate I. Needle-like nanopolar domains and the metastably-locked incommensurate structure

Abstract Transmission electron microscopy investigations have been performed on SrxBa1-xNb2O6 (SBN x(1 - x)). Bright-field imaging of 〈100〉-oriented SBN 60/40 and 75/25 crystals revealed nanopolar domains with a strong shape anisotropy. The morphology of the nanopolar domains was needle like. Selected area electron diffraction (SAED) patterns revealed the existence of 1/x[110] incommensurate modulations. Dark-field imaging of the incommensurate reflections found the existence of nanometre-sized ferroelastic domains. Diffuse scattering was observed in the SAED patterns along the 〈100〉 direction. This diffuse scattering is believed to arise due to strain gradient interactions between the A-site cation distribution and the incommensuration.

Incommensuration in La‐modified antiferroelectric lead zirconate titanate ceramics

Lanthanum‐modified lead zirconate titanate (PLZT) specimens with a La content of 2 at. % and a Zr/Ti ratio of 95/5 (PLZT 2/95/5) have been investigated by dielectric spectroscopy, Sawyer–Tower polarization (P‐E) measurements, and hot‐stage transmission electron microscopy (TEM). La ‘‘impurities’’ have been found to induce a 1/x〈110〉 incommensurate antiferroelectric structure at about 210 °C from a ferroelectric structure when cooled from above. The incommensurate structure was found to be stable below this temperature of the dielectric maximum, evolving slowly with decreasing temperature towards the commensurate antiferroelectric orthorhombic PZ structure. The incommensuration is believed to arise due to a competition between ‘‘broken’’ dipolar (ferroelectric) and sublattice (antiferroelectric) interactions. The ‘‘dirty’’ antiferroelectric Pb‐based system may be the first example of a proper incommensurately displacively modulated ferroelectric‐type material.

Monolithic integrations of slanted silicon nanostructures on 3D microstructures and their application to surface-enhanced raman spectroscopy

We demonstrated fabrication of black silicon with slanted nanocone array on both planar and 3D micro and meso scale structures produced by a high-throughput lithography-free oblique-angle plasma etching process. Nanocones with gradual change in height were created on the same piece of silicon. The relation between the slanted angle of nanocones and incident angle of directional plasma is experimentally investigated. In order to demonstrate the monolithic integration of nanostructures on micro and meso scale non-planar surfaces, nanocone forest is fabricated on non-planar silicon surfaces in various morphologies such as silicon atomic force microscopy (AFM) tips and pyramidal pits. By integrating nanocones on inverse silicon micro-pyramid array devices, we further improved the surface enhanced Raman scattering (SERS) enhancement property of this optimized commercial SERS substrate by several folds even when using 66% less noble metal coating. We investigated the length gradient dependence and asymmetric properties of SERS effects for slanted nanocone with polarized excitation. This versatile and angle-controllable nanocone fabrication and monolithic 3D nano-micro-meso integration method provides new dimensions for production and optimization of SERS and other nanophotonic sensors.

Black silicon solar thin-film microcells integrating top nanocone structures for broadband and omnidirectional light-trapping

Recently developed classes of monocrystalline silicon solar microcells (μ-cell) can be assembled into modules with characteristics (i.e., mechanically flexible forms, compact concentrator designs, and high-voltage outputs) that would be impossible to achieve using conventional, wafer-based approaches. In this paper, we describe a highly dense, uniform and non-periodic nanocone forest structure of black silicon (bSi) created on optically-thin (30 μm) μ-cells for broadband and omnidirectional light-trapping with a lithography-free and high-throughput plasma texturizing process. With optimized plasma etching conditions and a silicon nitride passivation layer, black silicon μ-cells, when embedded in a polymer waveguiding layer, display dramatic increases of as much as 65.7% in short circuit current, as compared to a bare silicon device. The conversion efficiency increases from 8.1% to 11.5% with a small drop in open circuit voltage and fill factor.