Controlling microscopic and spectroscopic properties of metallic photonic crystals written by interference ablation

The microscopic and spectroscopic properties of the metallic photonic crystals (MPCs) are controlled by adjusting the annealing temperature after the direct writing process using interference ablation. Strong surface tension and possible nano-scale flowability of the molten gold nanoparticles enable reshaping and aspect-ratio adjustment of the gold nanostructures during the annealing processes. This consequently leads to the tuning of the spectroscopic response of localized surface plasmon resonance by changing the annealing temperature, thus enhancing the flexibility and extending the application of the fabrication technique using interference ablation.     

Preparation and comparison between different thiol-protected Au nanoparticles

We report on the synthesis of monolayer-protected gold cluster (MPCs) in aquatic phase under different conditions. Three thiols, i.e., 2-mercaptoethanesulfonic acid (MPS), 3-mercaptopropionic acid (MPA), and cystamine (CYS) have been used as the protecting monolayer. Furthermore, we studied the effect of varying the ratio between the concentrations of the protecting layer, the gold ions, and the reducing agent in the preparation solution. Comparison was based on measuring the average diameter, stability, and size distribution of the MPCs. Best results were obtained for MPS-based MPCs and that the optimum molar concentration ratio between the protecting layer, the gold ions, and the reducing agent was 1:1:2.     

Tunable Aminooxy‐Functionalized Monolayer‐Protected Gold Clusters for Nonpolar and Aqueous Oximation Reactions

Aminooxy (–ONH₂) groups are well known for their chemoselective reactions with carbonyl compounds, specifically aldehydes and ketones. The versatility of aminooxy chemistry has proven to be an attractive feature that continues to stimulate new applications. This work describes application of aminooxy click chemistry on the surface of gold nanoparticles. A trifunctional amine‐containing aminooxy alkane thiol ligand for use in the functionalization of gold monolayer‐protected clusters (Au MPCs) is presented. Diethanolamine is readily transformed into an organic‐soluble aminooxy thiol ( AOT ) ligand using a short synthetic path. The synthesized AOT ligand is coated on ≤2‐nm‐diameter hexanethiolate‐(C₆S)‐capped Au MPCs using a ligand‐exchange protocol to afford organic‐soluble AOT /C₆S (1:1 ratio) Au mixed monolayer‐protected clusters (MMPCs). The synthesis of these Au(C₆S)( AOT ) MMPCs and representative oximation reactions with various types of aldehyde‐containing molecules is described, highlighting the ease and versatility of the chemistry and how amine protonation can be used to switch solubility characteristics.     

Profile calculation of gold nanostructures by dispersed-nanosphere lithography through oblique etching for LSPR applications

Nanosphere lithography is an inexpensive method used to fabricate gold nanostructures on a substrate. Using dispersed-nanosphere lithography, in which the nanospheres are dispersed on a substrate, 2D or 3D nanostructures can be fabricated by obliquely depositing a gold film on the nanospheres and etching the gold film afterward. These nanostructures are tunable and acute, and are thus good emitting elements for the localized surface plasmon resonance applications. So far, for the fabrication of nanostructures on a substrate with dispersed nanospheres, only 2D nanostructures have been reported through perpendicular etching. We report in this paper that the 3D nanostructures fabricated by dispersed-nanosphere lithography are rigid non-conformal structures, and perpendicular gold etching can be expanded to oblique etching, which provides more possibilities for fabricating the gold nanostructures in various shapes. The profiles of gold nanostructures after several varying angle depositions, and their final profiles after perpendicular or oblique etching, are calculated in this paper. Our profile simulations are applicable for nanospheres (or microspheres) within the range of tens of nanometers to tens of micrometers, and are consistent with our fabricated nanostructures observed using scanning electron and atomic force microscopy. Electronic Supplementary Material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Device applications of self-assembled monolayers and monolayer-protected nanoclusters

The present status of self-assembled monolayers (SAMs) on different surfaces (2D systems) as well as monolayer formation on metallic and semiconducting cluster surfaces (3D SAM) to form monolayer-protected nanoclusters (MPCs) and their assemblies is reviewed briefly. Attention is focused mainly on the potential electronic and photonic applications of SAMs, MPCs and their 2D and 3D structures fabricated using covalent and hydrophobic interactions in contrast to the usual electrostatic assemblies. These examples illustrate the rational use of organic molecules and nanoclusters using the concept of self-assembly, where subtle systems of double tunnel junctions, hetero junctions and single-electron transition devices could be developed based on the structure and chemistry of multifunctional molecules. The tailoring of cluster size and cluster–cluster spacing to reveal interesting transitions in electronic properties is also demonstrated using the low temperature behavior of the 3D network of nanoclusters as an example. These devices are believed to play an important role in the coming years as the chip functions and clock frequencies reach orders of magnitude beyond those extrapolated from Moore’s law.     

Internal Structure Tailoring in 3D Nanoplasmonic Metasurface for Surface-Enhanced Raman Spectroscopy

Tunable nanoplasmonic metasurfaces have resulted in many versatile platforms for sensing applications including surface-enhanced Raman scattering (SERS)-based detection. However, to date, their fabrication still faces challenges in uniformity, repeatability, and controllability. Here, a novel large-area and hierarchical nanoplasmonic array with controlled internal structure and tunable plasmonic properties is reported, relying on controllably tailoring the single nanosphere on a uniform double-layered array into a well-defined nanoflower structure. The fabrication involves colloidal self-assembly, lithography, and plasmonic metal coating. First, a uniformly distributed double-layered colloidal array is fabricated via an ethanol-assisted self-assembly technique. Next, with the help of inductively coupled plasma dry etching, the lower layer is transformed to the nanoflower array with well-defined petal shape. Subsequently, a gold film with controlled thickness is deposited onto the nanoflower structured array, resulting in a tunable optical and SERS-active enhancement effect. Furthermore, 3D finite-difference time-domain simulation shows multiple enhancement sites inside the nanoflower array. Such a brand-new 3D structured array has the potential for varied applications, ranging from SERS sensors to light regulation.     

Large-scale protein/antibody patterning with limiting unspecific adsorption

A simple synthetic route based on nanosphere lithography has been developed in order to design a large-scale nanoarray for specific control of protein anchoring. This technique based on two-dimensional (2D) colloidal crystals composed of polystyrene spheres allows the easy and inexpensive fabrication of large arrays (up to several centimeters) by reducing the cost. A silicon wafer coated with a thin adhesion layer of chromium (15 nm) and a layer of gold (50 nm) is used as a substrate. PS spheres are deposited on the gold surface using the floating-transferring technique. The PS spheres were then functionalized with PEG-biotin and the defects by self-assembly monolayer (SAM) PEG to prevent unspecific adsorption. Using epifluorescence microscopy, we show that after immersion of sample on target protein (avidin and anti-avidin) solution, the latter are specifically located on polystyrene spheres. Thus, these results are meaningful for exploration of devices based on a large-scale nanoarray of PS spheres and can be used for detection of target proteins or simply to pattern a surface with specific proteins.     

High performance reflection-type 1D magnetophotonic crystals with flat-top responses

In this paper, a theoretical study on the case of reflection-type one-dimensional magnetophotonic crystals (MPCs) has been carried out to establish high performance structures having concurrent high reflectance and large Kerr rotation with flat-top responses. The introduced MPCs are able to maintain their flat-top responses in a wide range of incident angle. For practical purposes, we have also inquired the influence of the error in the thickness of individual layers on the operational parameters of the MPCs. The reflectance flatness and bandwidth of the MPCs are appreciably stable against the imposed thickness errors.     

Lithography-free fabrication of large-area plasmonic nanostructures using colloidal gold nanoparticles

We report simple and efficient fabrication of large-area gold nanostructures using solution-processible gold nanoparticles, where lithography and vacuum evaporation techniques are not involved in the fabrication processes. These gold nanoisland structures exhibit strong particle plasmon resonance that is characterized by optical extinction spectroscopy in the visible spectral range. The tunability of the optical response is realized by controlling the annealing temperature and by changing the concentration of the colloidal solutions of gold nanoparticles. This enables a low-cost route for exploiting new photonic devices, biosensors, and optoelectronic devices with localized field-enhancement.     

Comparison of two-dimensional periodic arrays of convex and concave nanostructures for efficient SERS templates

We describe the optical power enhancement on the surface of the 2D (two-dimensional) periodic arrays of convex and concave gold nanostructures for comparing the characteristics of the nanostructures for surface-enhanced Raman spectroscopy (SERS) templates. The optical power enhancement is due to the surface plasmon polaritons, which is calculated by the Finite-Difference Time-Domain (FDTD) method at commercially-available 532 nm pump light. A periodic array of closely-packed gold particles is defined as convex nanostructure, while a periodic array of hemispherical holes, or voids, on gold substrate is defined as concave nanostructure. The peak power enhancement factor, the average power enhancement factor and the activity rate of each structure were compared. The convex nanostructures show a strong enhancement factor in localized hotspots, while the concave nanostructures show not only the peak power enhancement factor comparable to that of convex nanostructures, but also higher spatially-averaged power enhancement factors and activity rates than those observed on the convex nanostructures, meaning that the highly enhanced near-field zone distributes densely on the substrate. We also revealed the dependence of the void diameter on the inter-void distance for the power enhancement in the concave nanostructures system, providing a guideline for the fabrication of the efficient SERS template, which shows a strong power enhancement factor with a high area density.     

Synthesis of a quantum dot superlattice using molecularly linked metal clusters

We report on a synthesis strategy for fabrication of close-packed planar arrays of nanometer-diameter metal clusters that are covalently linked by organic molecular wires. The clusters are gold single crystals, each encapsulated by a monolayer of dodecanethiol molecules. A colloidal suspension of these clusters in mesitylene is spread onto a substrate. On evaporation of the solvent the clusters self-assemble to form a close-packed monolayer. This cluster network is then crosslinked by immersing the substrate in an acetonitrile solution containing a conjugated di-isonitrile molecule (1,4-di(4-isocyanophenylethynyl)2-ethylbenzene). Transmission electron micrographs of the cluster arrays before and after immersion indicate that the diisonitrile molecules partially substitute for the dodecanethiol molecules to produce a crosslinked network of clusters joined by the di-isonitrile. The interesting feature of this network is that it represents a 2D superlattice of metal quantum dots coupled by well defined tunnel junctions. When the gold clusters used to synthesize the network have diameters less than approximately 2 nm, it is predicted that this superlattice will exhibit Coulomb blockade effects at room temperature.     

Influence of the synthesis conditions of gold nanoparticles on the structure and architectonics of dipeptide composites

A wide variety of peptides and their natural ability to self-assemble makes them very promising candidates for the fabrication of solid-state devices based on nano- and mesocrystals. In this work, we demonstrate an approach to form peptide composite layers with gold nanoparticles through in situ reduction of chloroauric acid trihydrate by dipeptide and/or dipeptide/formaldehyde mixture in the presence of potassium carbonate at different ratios of components. Appropriate composition of components for the synthesis of highly stable gold colloidal dispersion with particle size of 34–36 nm in dipeptide/formaldehyde solution is formulated. Infrared spectroscopy results indicate that dipeptide participates in the reduction process, conjugation with gold nanoparticles and the self-assembly in 2D, which accompanied by changing peptide chain conformations. The structure and morphology of the peptide composite solid layers with gold nanoparticles on gold, mica and silica surfaces are characterized by atomic force microscopy. In these experiments, the flat particles, dendrites, chains, mesocrystals and Janus particles are observed depending on the solution composition and the substrate/interface used. The latter aspect is studied on the molecular level using computer simulations of individual peptide chains on gold, mica and silica surfaces.     

High-throughput fabrication of TERS probes using DC bias

Tip-enhanced Raman spectroscopy (TERS) has gained great attentions for sensitive characterization and super-resolution chemical imaging of various materials. The performance of TERS is mainly affected by the probe geometry, such as the diameter of the tip apex, surface roughness, etc. In this regard, the fabrication of a sharp apex from metal wire is the one of the most important factor for TERS experiments. Conventionally, the electrochemical etching is commonly used technique for fabricating those metallic probes. To avoid the surface defects, the pulsed bias was commonly employed in the electrochemical etching process. However, it would make the fabrication system to be complex and sophisticated. Here, we report an simple automated electrochemical etching systems using DC bias for the efficient fabrication of TERS probes. With our optimized condition for DC electrochemical etching, gold probes with a clean surface and a radius of curvature below 200 nm can be obtained with yield of 85%. The fabricated metallic probe is used to detect the brilliant cresyl blue (BCB) molecules on the gold film, and it is confirmed that the enhancement factor is about 3,400.     

Photonic band structure and effect of ε and μ on the reflectivity of one-dimensional magnetic photonic crystal structure

In this paper, we study the photonic band structure and reflection properties in one-dimensional magnetic photonic crystals (MPCs). Investigation of dispersion characteristics shows that in the case of MPCs, photonic band gaps arise due to the contrast in the wave impedance, not due to the contrast in the refractive index, while contrast in the refractive index of the two layers decides the position and number of the band gaps. We also study the effect of permittivity and permeability on reflection bands, which shows that the structure that has larger values of magnetic permeability (μ) than dielectric permittivity (ε) have wider TM-reflection bands, whereas the structure for which ε is greater than μ has wider TE-reflection bands. But the gap to mid-gap frequency ratio for TM-reflection bands is larger than TE-reflection bands. Thus, magnetic permeability has greater impact on the reflectivity of MPCs than dielectric permittivity. Finally, the analysis of the omni-reflectance in MPCs has also been studied.     

Synthesis and characterization of water-soluble gold colloids stabilized with aminoresorcinarene

An aminoresorcinarene (TOMR) with eight amino groups as a novel ligand was synthesized and the fabrication of gold hydrosol stabilized with TOMR was reported in this paper. The TOMR-capped gold nanoparticles were characterized and analyzed by the ultraviolet visible (UV-vis) spectroscopy, Fourier transmission infrared spectra (FT-IR), X-ray diffraction (XRD) and transmission electron microscopy (TEM), respectively. The experiment showed that nearly 10 times of the usual amount of hydrate hydrazine as a reducing agent needed to be used, and the obtained TOMR-stabilized gold hydrosol had a higher level of stability at room temperature, which might be related to the structure of TOMR molecule and the formation of hydrophilic double layer structure of TOMR on the surface of gold core.     

Enhanced dispersion and stability of gold nanoparticles on stoichiometric and reduced TiO2(1 1 0) surface in the presence of molybdenum

Mo, Au and their coadsorbed layers were produced on nearly stoichiometric and oxygen-deficient titania surfaces by physical vapor deposition (PVD) and characterized by low energy ion scattering (LEIS), X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) and scanning tunnelling microscopy (STM). The behavior of Au/Mo bimetallic layers was studied at different relative metal coverages and sample temperatures.

STM data indicated clearly that the deposition of Au on the Mo-covered stoichiometric TiO₂(1 1 0) surface results in an enhanced dispersion of gold at 300 K. The mean size of the Au nanoparticles formed at 300 K on the Mo-covered TiO₂(1 1 0) was significantly less than on the Mo-free titania surface (2 ± 0.5 nm and 4 ± 1 nm, respectively). Interestingly, the deposition of Mo at 300 K onto the stoichiometric TiO₂(1 1 0) surface covered by Au nanoparticles of 3–4 nm (0.5 ML) also resulted in an increased dispersity of gold. The driving force for the enhanced wetting at 300 K is that the Au–Mo bond energy is larger than the Au–Au bond energy in 3D gold particles formed on stoichiometric titania. In contrast, 2D gold nanoparticles produced on ion-sputtered titania were not disrupted in the presence of Mo at 300 K, indicating a considerable kinetic hindrance for breaking of the strong Au-TiO_x bond.

The annealing of the coadsorbed layer formed on a strongly reduced surface to 740 K did not cause a decrease in the wetting of titania surface by gold. The preserved dispersion of Au at higher temperatures is attributed to the presence of the oxygen-deficient sites of titania, which were retained through the reaction of molybdenum with the substrate. Our results suggest that using a Mo-load to titania, Au nanoparticles can be produced with high dispersion and high thermal stability, which offers the fabrication of an effective Au catalyst.     

Time-resolved microscopic imaging of the laser-induced forward transfer process

Laser-induced forward transfer (LIFT) technique is available for the fabrication of micro-sized thin film. In this paper, the LIFT process of gold film was investigated by the microscopic two-dimensional laser induced fluorescence (2D-LIF). The dynamic behavior of the gold atoms and emissive particles were observed in vacuum and atmospheric air. Characteristic behavior was observed for different species. The atoms flew with the fastest speed of more than 2 km/s. The influence of ablation laser energy, film thickness, and the presence of the the substrate on the dynamics of the species are reported.     

A review of 3D-printed sensors

Nowadays, sensors play an important role in human life. Among the many manufacturing methods used in the fabrication of sensors, three-dimensional (3D) printing has gradually shown its advantages, particularly with commercial products. Physical sensors, biosensors, and chemical sensors can all be fabricated via 3D printing technology, through either directly printing sensing components, printing molds for casting sensors, or printing platforms to be integrated with commercial sensors. In this article, the varieties of features and applications of 3D printing technologies used in the fabrication of sensors are reviewed. Several types of 3D printing technologies are compared for better understanding of the tools. With the development of new or hybrid manufacturing methods and materials used in the 3D printing technology, this technology will show its great advantages and potential in the fabrication of highly sensitive nanosensors or compound sensors with 3D intricate structures.     

Hexagonally poled lithium niobate: A two-dimensional nonlinear photonic crystal

We report on the fabrication of what we believe is the first example of a two-dimensional (2D) nonlinear photonic crystal [Berger, Phys. Rev. Lett. 81, 4136 (1998)], where the refractive index is constant but where the 2nd order nonlinear susceptibility is spatially periodic. Such crystals allow for efficient quasi-phase-matched 2nd harmonic generation using multiple reciprocal lattice vectors. External 2nd harmonic conversion efficiencies >60% were measured with picosecond pulses. The fabrication technique is extremely versatile and should allow for the fabrication of a broad range of 2D crystals including quasicrystals.     

Self-assembly of water-dispersed gold nanoparticles stabilized by a thiolated glycol derivative

Two-dimensional (2D) hexagonally close-packed arrays of water-dispersed gold nanoparticles (NPs) on highly hydrophilic sputter-deposited SiO₂ surfaces were fabricated via an evaporation-induced self-assembly process. Using a non-ionic amphiphilic glycol derivative with the thiol head group, 1-mercapto-3,6,9-trioxodecane, as a stabilization ligand, high-concentration Au NPs were stably dispersed in water, and self-assembled into μm-sized well-ordered 2D arrays on SiO₂ surfaces during the solvent evaporation on SiO₂ surfaces. Due to the non-ionic character of the ligand, the particle–particle interactions may only depend on the capillary and van der Waals forces, and not on the electric double-layer forces that change with ion/electrolyte concentrations during the solvent evaporation. This study provides an approach to fabrication of close-packed 2D arrays of water-dispersed NPs instead of using toxicological organic solvent or complex water-organic phase transfer process. Meanwhile, this approach will make it easier to study their self-assembly mechanisms in a variety of solvents by simplifying ambiguous particle–particle interactions.