Chaoyang Jiang

Electroluminescence from isolated CdSe∕ZnS quantum dots in multilayered light-emitting diodes

Electro- and photoluminescence spectra of the CdSe∕ZnS core-shell quantum dots (QDs) covered by various organic ligands and incorporated into multilayered light-emitting diodes (LEDs) were recorded by a confocal optical microscope. The QDs were dispersed in a hole transporting material, N,N’-Diphenyl-N,N’-bis(3-methylphenyl)-1,1’-biphenyl-4,4’-diamine (TPD), to investigate the LED performance at different QD concentrations and the effect of different surface modifications on the isolated QDs. No wavelength shift was observed in the electroluminescence spectra from the QD LEDs with or without the TPD. The peak energies of the electro- and photoluminescence showed only small spectral shifts (several nanometer) for the diluted QDs and no dependence on the QD-concentration, surface ligands, or conductive polymers that were used. This suggests that the relative peak shifts are related to the different filling processes in the CdSe QDs under photo excitation and electric injection, rather than to the “chemical”...

Reliable Quantitative SERS Analysis Facilitated by Core–Shell Nanoparticles with Embedded Internal Standards

Quantitative analysis is a great challenge in surface-enhanced Raman scattering (SERS). Core-molecule-shell nanoparticles with two components in the molecular layer, a framework molecule to form the shell, and a probe molecule as a Raman internal standard, were rationally designed for quantitative SERS analysis. The signal of the embedded Raman probe provides effective feedback to correct the fluctuation of samples and measuring conditions. Meanwhile, target molecules with different affinities can be adsorbed onto the shell. The quantitative analysis of target molecules over a large concentration range has been demonstrated with a linear response of the relative SERS intensity versus the surface coverage, which has not been achieved by conventional SERS methods.

Individual nanostructured materials: fabrication and surface-enhanced Raman scattering

The progress of surface-enhanced Raman scattering (SERS) microscopy and spectroscopy on individual nanostructured materials has been reviewed in this feature article. After a brief introduction on individual nanomaterial SERS, we provide a systematic overview on the fabrication and SERS studies of individual nanoparticulates, nano-junctions and hierarchical nano-aggregate. These SERS-active nanomaterials have great potential in designing novel highly sensitive SERS substrates for the development of SERS-based sensing devices with a broad range of applications.

Electrospun Nanofibrous Membranes Surface-Decorated with Silver Nanoparticles as Flexible and Active/Sensitive Substrates for Surface-Enhanced Raman Scattering

The development of novel nanomaterials with well-controlled morphologies/structures to achieve excellent activities/sensitivities in surface-enhanced Raman scattering (SERS) is crucial in advancing the high-performance SERS detections of chemical and biological species. In this study, amidoxime surface-functionalized polyacrylonitrile (ASFPAN) nanofibrous membranes surface-decorated with silver nanoparticles (Ag NPs) were prepared via the technique of electrospinning followed by the method of seed-mediated electroless plating. High SERS activities/sensitivities were observed from the ASFPAN-Ag NPs nanofibrous membranes, while the density and size of Ag NPs had an important impact on the SERS activity/sensitivity. The results confirmed that the enhancement of Raman signals is due to the presence of hot spots between/among Ag NPs on the nanofiber surfaces. Electrospun nanofibrous membranes surface-decorated with Ag NPs were mechanical flexible/resilient and could be used as highly active/sensitive SERS substrates for a broad range of applications.

Preparation and optical properties of silver nanowires and silver-nanowire thin films

Silver nanowires and silver-nanowire thin films have attracted much attention due to their extensive applications in Surface-Enhanced Raman Scattering (SERS) and Surface-Enhanced Fluorescence (SEF). Thin films of silver nanowires within polyelectrolyte layers of poly(allylamine hydrochloride) (PAH) and poly(sodium 4-styrenesulfonate) (PSS) were fabricated by the Spin-Assisted Layer-by-Layer (SA-LbL) method. The surface coverage, thickness, and absorbance properties of the silver-nanowire films were controlled by the number of layers deposited. Both transverse and longitudinal surface plasmon (SP) modes of the silver-nanowires were observed in the absorbance spectra, as was evidence for nanowire interaction. Two-dimensional finite difference time-domain (2D FDTD) simulations predict that the maximum field enhancement occurs at the ends and cross-sectional edges of the wires for the longitudinal and transverse modes, respectively. Silver nanowires were synthesized by a facile, high-yield solvothermal approach, which can be easily manipulated to control the aspect ratio of the nanowires. The effects of polyvinylpyrrolidone (PVP) concentration and molecular weight on the growth of the silver nanowires, which are not documented in the original procedure, are discussed. It is shown that the growth mechanism for silver nanowires in the solvothermal synthesis is similar to that reported for the polyol synthesis.

Electrospun TiO2 Nanofelt Surface-Decorated with Ag Nanoparticles as Sensitive and UV-Cleanable Substrate for Surface Enhanced Raman Scattering

In this study, the free-standing electrospun nanofibrous mat (i.e., nanofelt) consisting of anatase-phase TiO2 nanofibers with diameters of ∼200 nm was prepared, and the nanofelt was subsequently surface-decorated with Ag nanoparticles via an electroless plating method. The sensitivity toward surface enhanced Raman scattering (SERS) and UV-cleanable property of electrospun TiO2/Ag nanofelt were then investigated. In the SERS tests, the target analyte (i.e., 4-mercaptobenzoic acid, Rhodamine 6G, and 4-aminothiophenol) was first adsorbed onto the TiO2/Ag nanofelt as the probe analyte; this was followed by the measurements of Raman intensity and SERS maps. Thereafter, the nanofelt adsorbed with target analyte was cleaned and regenerated/recovered upon UV irradiation in O2-saturated water, and the removal of target analyte was attributed to photodegradation property of anatase-phase TiO2. This study suggested that the electrospun TiO2/Ag nanofelt would be promising as SERS-active substrate with UV-cleanable property for cost-effective and reproducible SERS applications.

Surface-Enhanced Raman Scattering on Hierarchical Porous Cuprous Oxide Nanostructures in Nanoshell and Thin-Film Geometries

Understanding the mechanism of surface-enhanced Raman scattering (SERS) of molecules on semiconductor nanostructures is directly related to our capabilities of designing and optimizing new SERS-active substrates for broad applications in the field of molecular detection and characterization. Here, we present an exploration of using cuprous oxide nanostructures with hierarchical porosity for enhancing Raman signals of adsorbed probe molecules. Distinct SERS signals were detected on both individual polycrystalline nanoshells and porous thin films composed of cuprous oxide nanocrystals. The observed enhancement of SERS signals can be interpreted as synergistic effects of strong chemical interactions between the probe molecules and cuprous oxide surfaces, localized electromagnetic field enhancement, and the unique hierarchical porosity of the nanostructures. Our work introduced a novel type of semiconductor substrates for high-performance SERS and extended the applications of cuprous oxide nanostructures to spectroscopy-based molecular sensing and characterizations.

Layer-by-layer assembly of freestanding thin films with homogeneously distributed upconversion nanocrystals

We report a facile and highly-controlled approach to fabricating freestanding upconversion multilayer thin films containing homogeneously distributed lanthanide-doped nanocrystals. Citrate-coated NaYF4:17%Yb, 3%Er nanocrystals were synthesized using a single-phase high-boiling-point-solvent method, followed by ligand exchange. These hydrophilic upconversion nanocrystals were dispersed in freestanding multilayer polyelectrolyte thin films by layer-by-layer assembly over a sacrificial layer. We found that the nanocomposite multilayer thin films possess outstanding mechanical stability and exhibit NIR-to-visible upconversion luminescence. The effect of the hydrophilic ligand exchange on the upconversion properties of these nanocrystals was explored by characterizing the time evolution of upconversion emission following pulsed NIR excitation. It is found that the ligand-exchange process modestly reduces the intrinsic upconversion efficiency of the nanocrystals relative to the as-synthesized oleic acid coated product. Thin films with NIR-to-visible upconversion properties may be suitable for a variety of optical-device and sensing applications.

SERS-active silver nanoparticles on electrospun nanofibers facilitated via oxygen plasma etching

Manipulating the interaction between inorganic building blocks and polymeric supporting materials is crucial in the fabrication and optimization of hybrid hierarchical nanostructures. Herein, oxygen plasma etching was used to modify electrospun nanofibers of poly(methyl methacrylate) (PMMA) for facilitating the growth of silver nanoparticles (Ag NPs). The PMMA nanofibers in the form of overlaid films, surface-decorated with Ag NPs, were explored as active substrates for surface-enhanced Raman scattering (SERS). Strong SERS enhancement was observed from the Ag NP–PMMA films, as well as individual nanofibers. Our work not only fabricated nanocomposite materials with controlled hierarchical structures and remarkable SERS performances, but also provided a versatile method in tuning interfacial interactions within nanostructured materials.

High-resolution Raman microscopy of curled carbon nanotubes

Patterned carbon nanotube assemblies with bent nanotube bundles were investigated with combined atomic force microscopy and confocal Raman imaging spectroscopy to identify conditions of carbon nanotubes in the bent state. We showed that the tangential G mode on Raman spectra systematically shifts downward upon nanotube bending as was predicted earlier. This lower frequency shift is attributed to the tensile stress, which results in the loosening of C–C bonds in the outer nanotube walls.