Metal dimer nanojunction-magnetic material composites for magnetic field sensing.

Noble metal nanocrystals are used as high sensitivity optoelectronic sensors, such as surface-enhanced Raman scattering, SERS. The sensing performance of metal nanocrystals can be further improved by forming dimer nanojunctions with strong "plasmonic coupling". Since the strength of "plasmonic coupling" is highly sensitive to the sub-nanoscale spacing between plasmonic nanocrystals in nanojunctions, nanojunctions can be used to detect external stimuli that can change the spacing of nanocrystals in the nanojunction and thus change the sensitivity of the Raman scattering spectrum. Here, we utilize this principle to detect the direction and strength of an external magnetic field (MF) using dimer nanojunctions surrounded by magnetic materials as a sensing platform. The results reveal that the changes in nanocrystal spacing in the nanojunction are caused by the rearrangement of the magnetic material under an external MF, which strongly depends on the interaction between the magnetic material and the ligands on the nanocrystal surface and the steric repulsion generated by the ligand configuration on the nanocrystal surface. Compared with the Raman spectrum without an external MF, the enhancement factors of the Raman scattering spectrum under an external MF can reach up to ∼900%, which makes dimer nanojunctions with magnetic materials suitable for "magnetic field" sensing applications.

Effect of morphologies and compositions of silver-based multicomponent heterogeneous nanocrystals on the reduction of 4-nitrophenol.

Silver-based nanocrystals have excellent catalytic performance in various reactions, such as the reduction of 4-nitrophenol. The catalytic performance of nanocrystals varies with several parameters, including nanocrystal morphology, composition, and plasmon-induced hot electrons around nanocrystals. Here, highly heterogeneous nanocrystals (Au-Ag and Ag2S-Ag nanocrystals) fabricated on polymer films via a seed-mediated method are used as catalysts for the reduction of 4-nitrophenol, and the effect of the morphology and composition of nanocrystals on the catalytic performance is investigated. These nanocrystals on polymer films exhibit higher reusability (low catalyst loss) in catalytic applications compared to catalysts dispersed freely in the reaction solution. The excellent catalyst performance of these heterogeneous nanocrystals is attributed to their high surface area/volume ratio (flower-like nanocrystals) and strong synergistic effect (cage-like nanocrystals). These nanocrystals with special morphologies and composites showed higher catalytic performance (higher reactivity at lower catalyst contents) than silver-based nanocrystals reported in the literature. Due to the excellent plasmonic properties of Ag nanocrystals, the catalytic performance of these nanocrystals can be further enhanced by generating hot electrons around the nanocrystals under irradiation. These results demonstrated that by carefully controlling the morphology and composition of nanocrystals, it is possible to design and fabricate excellent catalysts for various reactions.

Investigation of the Performance of Heterogeneous MOF-Silver Nanocube Nanocomposites as CO2 Reduction Photocatalysts by In Situ Raman Spectroscopy.

Here, we fabricated two different heterogeneous nanocomposites, core-shell MOF-AgNC and corner MOF-AgNC, as photocatalysts for CO2 conversion by generating metal-organic frameworks (MOFs) on silver nanocube templates. These MOF-AgNC nanocomposites showed good CO2 adsorption features and high CO2 reduction reactivity. The performances of these MOF-AgNC nanocomposites in CO2 adsorption and CO2 reduction reactions can be characterized by in situ Raman spectrum measurement. The corner MOF-AgNC nanocomposite exhibited a faster CO2 adsorption rate than the core-shell MOF-AgNC nanocomposite, which was due to the higher surface area/volume ratio of the MOF in corner MOF-AgNC. The CO2 reaction reactivity and mechanisms (products of the reaction) of CO2 reduction also depended on the morphologies of MOF-AgNC nanocomposites, which were caused by different reaction environments at the interface between the MOF and AgNCs. The CO2 reduction reactivity of MOF-AgNC nanocomposites also exhibited high sensitivity to the irradiation intensity and wavelength, which was caused by the variation of the number of hot electrons and their positions in AgNCs with the irradiation intensity and irradiation wavelength, respectively. This method for the synthesis of heterogeneous nanocomposites should make it possible to design photocatalysts for various reactions by carefully designing the morphology and composition of nanocomposites.

Combination of Plasmon-Mediated Photochemistry and Seed-Mediated Methods for Synthesis of Bicomponent Nanocrystals.

Plasmon resonances of metal nanocrystals resulted from free electrons oscillating around nanocrystals, leading to a strong electromagnetic field around them. Because these oscillating electrons possess higher energy than the original ones, also known as hot electrons, these were widely used as photocatalysts for various reactions. Also, the strength and distribution of the electromagnetic field around the nanocrystals strongly depended on their morphology and excited irradiation, which led to the reaction environment around nanocrystals being controllable. Here, we integrated the seed-mediated and plasmon-mediated photochemistry methods for fabricating bimetallic and semiconductor-metal nanocrystals with controllable morphologies and compositions of the nanocrystals, resulting from the highly anisotropic reaction environment around the nanocrystals. The highly anisotropic reaction environment around the template nanocrystal was caused by the distribution of electromagnetic fields around it and its exposure area in the reaction solution. This new synthesis method should enable the fabrication of various multicomponent nanocrystals with desirable functions for potential applications, such as photocatalysts, chemical sensors, biosensors, biomedicines, etc.

Highly-efficient electrically-driven localized surface plasmon source enabled by resonant inelastic electron tunneling.

On-chip plasmonic circuitry offers a promising route to meet the ever-increasing requirement for device density and data bandwidth in information processing. As the key building block, electrically-driven nanoscale plasmonic sources such as nanoLEDs, nanolasers, and nanojunctions have attracted intense interest in recent years. Among them, surface plasmon (SP) sources based on inelastic electron tunneling (IET) have been demonstrated as an appealing candidate owing to the ultrafast quantum-mechanical tunneling response and great tunability. However, the major barrier to the demonstrated IET-based SP sources is their low SP excitation efficiency due to the fact that elastic tunneling of electrons is much more efficient than inelastic tunneling. Here, we remove this barrier by introducing resonant inelastic electron tunneling (RIET)-follow a recent theoretical proposal-at the visible/near-infrared (NIR) frequencies and demonstrate highly-efficient electrically-driven SP sources. In our system, RIET is supported by a TiN/Al2O3 metallic quantum well (MQW) heterostructure, while monocrystalline silver nanorods (AgNRs) were used for the SP generation (localized surface plasmons (LSPs)). In principle, this RIET approach can push the external quantum efficiency (EQE) close to unity, opening up a new era of SP sources for not only high-performance plasmonic circuitry, but also advanced optical sensing applications.

Tunable Optical Property of Plasmonic-Polymer Nanocomposites Composed of Multilayer Nanocrystal Arrays Stacked in a Homogeneous Polymer Matrix.

Layer-by-layer (LbL) synthetic technique has been used to deposit multilayers composed of a wide range of materials including polymers, colloidal particles, and biomolecules. A more complex organization of nanocomponents-within layers (intralayer) and across layers (interlayer)-beyond simple deposition is required for manufacturing next-generation materials and devices. Recently, LbL was used to fabricate multilayer stacked polymer-nanocrystal nanocomposites composed of a stacking sequence of two immiscible polymer thin films. However, the requirement of two immiscible polymers limits its widespread use for the fabrication of various nanocomposites. Here, we presented a new and simplified synthetic method for the fabrication of multilayer stacked nanocomposites composed of multilayer plasmonic nanocrystal arrays stacked in a homogeneous polymer matrix via iterative sequential LbL deposition of polymer thin films and nanocrystal arrays. This novel fabrication technique requires strong attractive interaction between the "ligand shell" on the nanocrystal surface and the polymer matrix [Flory-Huggins interaction parameter of the ligand shell-polymer matrix (χ) < 0], which can dramatically enhance the stability of nanocomposites during the LbL deposition. The optical properties of plasmonic nanocomposites can be manipulated by the adjustment of the intrinsic property of the nanocrystal and/or coupling effect between adjacent nanocrystals from the same layer (intralayer) and/or the neighboring layer (interlayer). Taking advantage of this novel LbL fabrication technique, the properties of multilayer plasmonic nanocrystal arrays stacked in a homogeneous matrix can be manipulated via tuning the interlayer or intralayer coupling between nanocrystals, which can be achieved by sophisticated control of the packing density of two-dimensional nanocrystal arrays in each individual layer or the thickness of the polymer thin film between two adjacent nanocrystal arrays, respectively. These results provide a facile and effective way of designing a more complex multilayer nanostructure with controllable properties in a homogeneous polymer matrix.

Imaging of Nanoscale Light Confinement in Plasmonic Nanoantennas by Brownian Optical Microscopy.

The strongly enhanced and confined subwavelength optical fields near plasmonic nanoantennas have been extensively studied not only for the fundamental understanding of light-matter interactions at the nanoscale but also for their emerging practical application in enhanced second harmonic generation, improved inelastic electron tunneling, harvesting solar energy, and photocatalysis. However, owing to the deep subwavelength nature of plasmonic field confinement, conventional optical imaging techniques are incapable of characterizing the optical performance of these plasmonic nanoantennas. Here, we demonstrate super-resolution imaging of ∼20 nm optical field confinement by monitoring randomly moving dye molecules near plasmonic nanoantennas. This Brownian optical microscopy is especially suitable for plasmonic field characterization because of its capabilities for polarization sensitive wide-field super-resolution imaging.

Large optical nonlinearity enabled by coupled metallic quantum wells.

New materials that exhibit strong second-order optical nonlinearities at a desired operational frequency are of paramount importance for nonlinear optics. Giant second-order susceptibility χ (2) has been obtained in semiconductor quantum wells (QWs). Unfortunately, the limited confining potential in semiconductor QWs causes formidable challenges in scaling such a scheme to the visible/near-infrared (NIR) frequencies for more vital nonlinear-optic applications. Here, we introduce a metal/dielectric heterostructured platform, i.e., TiN/Al2O3 epitaxial multilayers, to overcome that limitation. This platform has an extremely high χ (2) of approximately 1500 pm/V at NIR frequencies. By combining the aforementioned heterostructure with the large electric field enhancement afforded by a nanostructured metasurface, the power efficiency of second harmonic generation (SHG) achieved 10-4 at an incident pulse intensity of 10 GW/cm2, which is an improvement of several orders of magnitude compared to that of previous demonstrations from nonlinear surfaces at similar frequencies. The proposed quantum-engineered heterostructures enable efficient wave mixing at visible/NIR frequencies into ultracompact nonlinear optical devices.

Copper Sulfide Nanodisks and Nanoprisms for Photoacoustic Ovarian Tumor Imaging.

Transvaginal ultrasound is widely used for ovarian cancer screening but has a high false positive rate. Photoacoustic imaging provides additional optical contrast to supplement ultrasound and might be able to improve the accuracy of screening. Here, we report two copper sulfide (CuS) nanoparticles types (nanodisks and triangular nanoprisms) as the photoacoustic contrast agents for imaging ovarian cancer. Both CuS nanoprisms and nanodisks were ~6 nm thick and ~26 nm wide and were coated with poly(ethylene glycol) to make them colloidally stable in phosphate buffered saline (PBS) for at least 2 weeks. The CuS nanodisks and nanoprisms revealed strong localized surface plasmon resonances with peak maxima at 1145 nm and 1098 nm, respectively. Both nanoparticles types had strong and stable photoacoustic intensity with detection limits below 120 pM. The circular CuS nanodisk remained in the circulation of nude mice (n=4) and xenograft 2008 ovarian tumors (n=4) 17.9-fold and 1.8-fold more than the triangular nanoprisms, respectively. Finally, the photoacoustic intensity of the tumors from the mice (n=3) treated with CuS nanodisks was 3.0-fold higher than the baseline. The tumors treated with nanodisks had a characteristic peak at 920 nm in the spectrum to potentially differentiate the tumor from adjacent tissues.

Colloidal Plasmonic Nanocomposites: From Fabrication to Optical Function.

Plasmonic nanostructures are extensively used building blocks for engineering optical materials and device architectures. Plasmonic nanocomposites (pNCs) are an emerging class of materials that integrate these nanostructures into hierarchical and often multifunctional systems. These pNCs can be highly customizable by modifying both the plasmonic and matrix components, as well as by controlling the nano- to macroscale morphology of the composite as a whole. Assembly at the nanoscale plays a particularly important role in the design of pNCs that exhibit complex or responsive optical function. Due to their scalability and tunability, pNCs provide a versatile platform for engineering new plasmonic materials and for facile integration into optoelectronic device architectures. This review provides a comprehensive survey of recent achievements in pNC structure, design, fabrication, and optical function, along with some examples of their application in optoelectronics and sensing.

Supramolecular precursors for the synthesis of anisotropic Cu2S nanocrystals.

Copper alkanethiolates are organometallic precursors that have been used to form Cu2S nanodisks upon thermal decomposition. Here, we demonstrate that molecular assembly of Cu alkanethiolates into an ordered liquid crystalline mesophase plays an essential role in templating the disk morphology of the solid-state product. To examine this templating effect, we synthesize Cu alkanethiolate precursors with alkane tails of varying chain length and sterics. We demonstrate that short chain precursors produce two-dimensional (2D) nanosheets of Cu2S, while longer-chained variants produce Cu2S nanodisks exclusively. This work provides new insights into the use of liquid crystalline phases as templates for nanocrystal synthesis and as a potential route for achieving highly anisotropic inorganic nanostructures.

Tunable and directional plasmonic coupling within semiconductor nanodisk assemblies.

Semiconductor nanocrystals are key materials for achieving localized surface plasmon resonance (LSPR) excitation in the extended spectral ranges beyond visible light, which are critical wavelengths for chemical sensing, infrared detection, and telecommunications. Unlike metal nanoparticles which are already widely exploited in plasmonics, little is known about the near-field behavior of semiconductor nanocrystals. Near-field interactions are expected to vary greatly with nanocrystal carrier density and mobility, in addition to properties such as nanocrystal size, shape, and composition. Here we demonstrate near-field coupling between anisotropic disk-shaped nanocrystals composed of Cu2-xS, a degenerately doped semiconductor whose electronic properties can be modulated by Cu content. Assembling colloidal nanocrystals into mono- and multilayer films generates dipole-dipole LSPR coupling between neighboring nanodisks. We investigate nanodisks of varying crystal phases (Cu1.96S, Cu7.2S4, and CuS) and find that nanodisk orientation produces a dramatic change in the magnitude and polarization direction of the localized field generated by LSPR excitation. This study demonstrates the potential of semiconductor nanocrystals for the realization of low-cost, active, and tunable building blocks for infrared plasmonics and for the investigation of light-matter interactions at the nanoscale.

Localized surface plasmon resonances of anisotropic semiconductor nanocrystals.

We demonstrate that anisotropic semiconductor nanocrystals display localized surface plasmon resonances that are dependent on the nanocrystal shape and cover a broad spectral region in the near-IR wavelengths. In-plane and out-of-plane dipolar resonances were observed for colloidal dispersions of Cu(2-x)S nanodisks, and the wavelengths of these resonances are in good agreement with calculations carried out in the electrostatic limit. The wavelength, line shape, and relative intensities of these plasmon bands can be tuned during the synthetic process by controlling the geometric aspect ratio of the disk or using a postsynthetic thermal-processing step to increase the free carrier densities.

Polyelectrolyte-templated synthesis of bimetallic nanoparticles.

Bimetallic nanoparticles (NPs) are known to exhibit enhanced optical and catalytic properties that can be optimized by tailoring NP composition, size, and morphology. Galvanic deposition of a second metal onto a primary metal NP template is a versatile method for fabricating bimetallic NPs using a scalable, solution-based synthesis. We demonstrate that the galvanic displacement reaction pathway can be controlled through appropriate surface modification of the NP template. To synthesize bimetallic Au-Ag NPs, we used colloidal Ag NPs modified by layer-by-layer (LBL) assembled polyelectrolyte layers to template the reduction of HAuCl(4). NPs terminated with positively and negatively charged polyelectrolytes yield highly contrasting morphologies and Au surface concentrations. We propose that these charged surface layers control galvanic charge transfer by controlling nucleation and diffusion at the deposition front. This surface-directed synthetic strategy can be advantageously used to tailor both overall NP morphology and Au surface concentrations.