Orientation-preserving transfer and directional light scattering from individual light-bending nanoparticles

A nanocup, or semishell, is an asymmetric plasmonic "Janus" nanoparticle with electric and magnetic plasmon modes; the latter scatters light in a direction controlled by nanoparticle orientation, making it the nanoscale analog of a parabolic antenna. Here we report a method for transferring nanocups from their growth substrate to oxide-terminated substrates that precisely preserves their three-dimensional orientation, enabling their use as nanophotonic components. This enables us to selectively excite and probe the electric and magnetic plasmon modes of individual nanocups, showing how the scattered light depends on the direction of incoming light and the orientation of this nanoparticle antenna.     

Fluorescence Enhancement on Large Area Self‐Assembled Plasmonic‐3D Photonic Crystals

Discontinuous plasmonic‐3D photonic crystal hybrid structures are fabricated in order to evaluate the coupling effect of surface plasmon resonance and the photonic stop band. The nanostructures are prepared by silver sputtering deposition on top of hydrophobic 3D photonic crystals. The localized surface plasmon resonance of the nanostructure has a symbiotic relationship with the 3D photonic stop band, leading to highly tunable characteristics. Fluorescence enhancements of conjugated polymer and quantum dot based on these hybrid structures are studied. The maximum fluorescence enhancement for the conjugated polymer of poly(5‐methoxy‐2‐(3‐sulfopropoxy)‐1,4‐phenylenevinylene) potassium salt by a factor of 87 is achieved as compared with that on a glass substrate due to the enhanced near‐field from the discontinuous plasmonic structures, strong scattering effects from rough metal surface with photonic stop band, and accelerated decay rates from metal‐coupled excited state of the fluorophore. It is demonstrated that the enhancement induced by the hybrid structures has a larger effective distance (optimum thickness ≈130 nm) than conventional plasmonic systems. It is expected that this approach has tremendous potential in the field of sensors, fluorescence‐imaging, and optoelectronic applications.     

Controllable Fabrication of Au Nanocups by Confined‐Space Thermal Dewetting for OCT Imaging

Here, this study reports a novel confined‐space thermal dewetting strategy for the fabrication of Au nanocups with tunable diameter, height, and size of cup opening. The nanocup morphology is defined by the cup‐shaped void space created by a yolk–shell silica template that spontaneously takes an eccentric configuration during annealing. Thermal dewetting of Au, which is sandwiched between the yolk and shell, leads to the desired nanocup morphology. With strong scattering in near infrared, the Au nanocups exhibit superior efficiency as contrast agents for spectral‐domain optical coherence tomography imaging. This confined‐space thermal dewetting strategy is scalable and general, and can be potentially extended to the synthesis of novel anisotropic nanostructures of various compositions that are difficult to produce by conventional wet chemical or physical methods, thus opening up opportunities for many new applications.     

Ultrasound‐Propelled Nanocups for Drug Delivery

Ultrasound‐induced bubble activity (cavitation) has been recently shown to actively transport and improve the distribution of therapeutic agents in tumors. However, existing cavitation‐promoting agents are micron‐sized and cannot sustain cavitation activity over prolonged time periods because they are rapidly destroyed upon ultrasound exposure. A novel ultrasound‐responsive single‐cavity polymeric nanoparticle (nanocup) capable of trapping and stabilizing gas against dissolution in the bloodstream is reported. Upon ultrasound exposure at frequencies and intensities achievable with existing diagnostic and therapeutic systems, nanocups initiate and sustain readily detectable cavitation activity for at least four times longer than existing microbubble constructs in an in vivo tumor model. As a proof‐of‐concept of their ability to enhance the delivery of unmodified therapeutics, intravenously injected nanocups are also found to improve the distribution of a freely circulating IgG mouse antibody when the tumor is exposed to ultrasound. Quantification of the delivery distance and concentration of both the nanocups and coadministered model therapeutic in an in vitro flow phantom shows that the ultrasound‐propelled nanocups travel further than the model therapeutic, which is itself delivered to hundreds of microns from the vessel wall. Thus nanocups offer considerable potential for enhanced drug delivery and treatment monitoring in oncological and other biomedical applications.     

How Effective is Plasmonic Enhancement of Colloidal Quantum Dots for Color‐Conversion Light‐Emitting Devices?

Enhancing the fluorescence intensity of colloidal quantum dots (QDs) in case of color‐conversion type QD light‐emitting devices (LEDs) is very significant due to the large loss of QDs and their quantum yields during fabrication processes, such as patterning and spin‐coating, and can therefore improve cost‐effectiveness. Understanding the enhancement process is crucial for the design of metallic nanostructure substrates for enhancing the fluorescence of colloidal QDs. In this work, improved color conversion of colloidal green and red QDs coupled with aluminum (Al) and silver (Ag) nanodisk (ND) arrays designed by in‐depth systematic finite‐difference time domain simulations of excitation, spontaneous emission, and quantum efficiency enhancement is reported. Calculated results of the overall photoluminescence enhancement factor in the substrate of 500 × 500 µm² size are 2.37‐fold and 2.82‐fold for Al ND‐green QD and Ag ND‐red QD structures, respectively. Experimental results are in good agreement, showing 2.26‐fold and 2.66‐fold enhancements for Al ND and Ag ND structures. Possible uses of plasmonics in cases such as white LED and total color conversion for possible display applications are discussed. The theoretical treatments and experiments shown in this work are a proof of principle for future studies of plasmonic enhancement of various light‐emitting materials.     

Tuning gold nanorod-nanoparticle hybrids into plasmonic Fano resonance for dramatically enhanced light emission and transmission

We investigate the optical response of a gold nanorod array coupled with a semicontinuous nanoparticle film. We find that, as the gold nanoparticle film is adjusted to the percolating regime, the nanorod-film hybrids are tuned into plasmonic Fano resonance, characterized by the coherent coupling of discrete plasmonic modes of the nanorod array with the continuum band of the percolating film. Consequently, optical transmission of the percolating film is substantially enhanced. Even more strikingly, electromagnetic fields around the nanorod array become much stronger, as reflected by 2 orders of magnitude enhancement in the avalanche multiphoton luminescence. These findings may prove instrumental in the design of various plasmonic nanodevices.     

Tuning Plasmonic Enhancement of Single Nanocrystal Upconversion Luminescence by Varying Gold Nanorod Diameter

Plasmonic enhancement induced by metallic nanostructures is an effective strategy to improve the upconversion efficiency of lanthanide‐doped nanocrystals. It is demonstrated that plasmonic enhancement of the upconversion luminescence (UCL) of single NaYF₄:Yb³⁺/Er³⁺/Mn²⁺ nanocrystal can be tuned by tailoring scattering and absorption cross sections of gold nanorods, which is synthesized wet chemically. The assembly of the single gold nanorod and single upconversion nanocrystal is achieved by the atomic force microscope probe manipulation. By selecting two kinds of gold nanorods with similar longitudinal surface plasmon resonance wavelength but different diameters (27.3 and 46.7 nm), which extinction spectra are separately dominant by the absorption and scattering, the maximum UCL enhancement by a factor of 110 is achieved with the 46.7 nm‐diameter gold nanorod, while it is 19 for the nanorod with the diameter of 27.3 nm. Such strong enhancement with the larger gold nanorod is due to stronger scattering ability and greater extent of the near‐field enhancement. The enhanced UCL shows a strong dependence on the excitation polarization relative to the nanorod long axis. Time‐resolved measurements and finite‐difference time‐domain simulations unveil that both excitation and emission processes of UCL are accelerated by the nanorod plasmonic effect.     

Optical properties of water-soluble l-cysteine-capped alloyed CdSeS quantum dot passivated with ZnSeTe and ZnSeTe/ZnS shells

Alloyed quantum dots (QDs) passivated with shell materials have valuable optical characteristics suitable for a wide array of applications. In this work, alloyed ternary CdSeS QDs passivated with ZnSeTe and ZnSeTe/ZnS shells have been synthesized via a hot-injection method and a ligand exchange reaction employing l-cysteine as a thiol ligand has been used to obtain these water-soluble nanocrystals for the first time. The photoluminescence (PL) quantum yield (QY) of alloyed l-cysteine-capped CdSeS was 71.2% but decreased significantly to 5.2% upon passivation with a ZnSeTe shell. The red shift in PL emission of the CdSeS/ZnSeTe QDs was attributed to be strain-induced whilst a lattice-induced process likely created defect states in the core/shell interface hence contributing to the decline in the PL QY. Nonetheless, the fluorescence stability of CdSeS/ZnSeTe QDs in aqueous solution was unperturbed. Further passivation with a ZnS shell (CdSeS/ZnSeTe/ZnS) improved the PL QY to a value of 58.7% and thus indicates that the defect state in the QDs core/shell/shell structure was reduced. PL lifetime exciton measurements indicated that the rates of decay of the QDs influenced their photophysical properties.     

Dark-red-emitting CdTe0.5Se0.5/Cd0.5Zn0.5S quantum dots: Effect of chemicals on properties

CdTe_0.5Se_0.5/Cd_0.5Zn_0.5S core/shell quantum dots (QDs) with a tunable photoluminescence (PL) range from yellow to dark red (up to a PL peak wavelength of 683 nm) were fabricated using various reaction systems. The core/shell QDs created in the reaction solution of trioctylamine (TOA) and oleic acid (OA) at 300 °C exhibited narrow PL spectra and a related low PL efficiency (38%). In contrast, the core/shell QDs prepared in the solution of 1-octadecene (ODE) and hexadecylamine (HDA) at 200 °C revealed a high PL efficiency (70%) and broad PL spectra. This phenomenon is ascribed that the precursor of Cd, reaction temperature, solvents, and ligands affected the formation process of the shell. The slow growth rate of the shell in the solution of ODE and HDA made QDs with a high PL efficiency. Metal acetate salts without reaction with HDA led to the core/shell QDs with a broad size distribution.     

Self‐Assembled Large Au Nanoparticle Arrays with Regular Hot Spots for SERS

The cost‐effective self‐assembly of 80 nm Au nanoparticles (NPs) into large‐domain, hexagonally close‐packed arrays for high‐sensitivity and high‐fidelity surface‐enhanced Raman spectroscopy (SERS) is demonstrated. These arrays exhibit specific optical resonances due to strong interparticle coupling, which are well reproduced by finite‐difference time‐domain (FDTD) simulations. The gaps between NPs form a regular lattice of hot spots that enable a large amplification of both photoluminescence and Raman signals. At smaller wavelengths the hot spots are extended away from the minimum‐gap positions, which allows SERS of larger analytes that do not fit into small gaps. Using CdSe quantum dots (QDs) a 3–5 times larger photoluminescence enhancement than previously reported is experimentally demonstrated and an unambiguous estimate of the electromagnetic SERS enhancement factor of ≈10⁴ is obtained by direct scanning electron microscopy imaging of QDs responsible for the Raman signal. Much stronger enhancement of ≈10⁸ is obtained at larger wavelengths for benzenethiol molecules penetrating the NP gaps.     

Engineering optical response through gold bowtie nanoantennas array

We propose the coupled gold bowtie nanoantennas array and investigate its plasmonic properties theoretically. The bowtie antenna consists of a pair of opposing truncated cones. We calculate the transmission spectra and the electric field distributions. The evolution of the transmission spectrum with the gap width of the bowtie, the diameter of the tip of the cone and the distance between adjacent bowties is directly visualized. Furthermore, the sensitivity of the antennas array to dielectric constant changes of the environment is also investigated in detail. We show the electric field distribution of this nanostructure and find that E_x is mainly located at the corners of the cross section, especially at the extremity of the cone. As for the E_y, the electric field enhancement localizes at the external edges and the gap of the bowtie. Our work elucidates further the plasmonic interactions can be useful in the design of optimized, sensitive optical sensors, and the enhancement of the fluorescence of molecules.     

Emission properties of colloidal quantum dots on polyelectrolyte multilayers

We present steady state and time-resolved photoluminescence (PL) characteristics of differently charged CdTe quantum dots (QDs) adsorbed onto a polyelectrolyte (PE) multilayer. The PE multilayer is built up using a layer-by-layer assembly technique. We find that the diffusion of the QDs into the PE multilayer is an important factor in the case of 3-mercapto-1, 2-propanediol stabilized QDs (neutral surface charge), resulting in a ～31-fold enhancement in PL intensity accompanied by a blue shift in the PL spectra and an increase in decay lifetime from 3.74?ns to a maximum of 11.65?ns. These modified emission properties are attributed to the enhanced surface related emission resulting from the interaction of the QD's surface with the PE. We find that diffusion does not occur for thioglycolic acid (TGA) stabilized QDs (negative surface charge) or 2-mercaptoethylamine stabilized QDs (positive surface charge), indicating localization of the QDs on top of the PE multilayer. However, the PL lifetime of the TGA stabilized QDs decreases from 9.58 to 5.78?ns with increasing PE multilayer thickness. This provides evidence for increased intrinsic exciton recombination relative to surface related emission, which results in an overall reduction in the average lifetime. Our studies indicate the importance of the QD surface charge in determining the interaction with the PE multilayers and the subsequent modification of the QD emission properties.     

DNA‐Mediated Self‐Assembly of Plasmonic Antennas with a Single Quantum Dot in the Hot Spot

DNA self‐assembly is a powerful tool to arrange optically active components with high accuracy in a large parallel manner. A facile approach to assemble plasmonic antennas consisting of two metallic nanoparticles (40 nm) with a single colloidal quantum dot positioned at the hot spot is presented here. The design approach is based on DNA complementarity, stoichiometry, and steric hindrance principles. Since no intermediate molecules other than short DNA strands are required, the structures possess a very small gap (≈ 5 nm) which is desired to achieve high Purcell factors and plasmonic enhancement. As a proof‐of‐concept, the fluorescence emission from antennas assembled with both conventional and ultrasmooth spherical gold particles is measured. An increase in fluorescence is obtained, up to ≈30‐fold, compared to quantum dots without antenna.     

Focusing Plasmons in Nanoslits for Surface‐Enhanced Raman Scattering

The focusing of plasmons to obtain a strong and localized electromagnetic‐field enhancement for surface‐enhanced Raman scattering (SERS) is increasing the interest in using plasmonic devices as molecular sensors. In this Full Paper, we report the successful fabrication and demonstration of a solid‐state plasmonic nanoslit–cavity device equipped with nanoantennas on a freestanding thin silicon membrane as a substrate for SERS. Numerical calculations predict a strong and spatially localized enhancement of the optical field in the nanoslit (6 nm in width) upon irradiation. The predicted enhancement factor of SERS was 5.3 × 10⁵, localized in an area of just 6 × 1.5 nm². Raman spectroscopy and imaging confirm an enhancement factor of ≈10⁶ for SERS from molecules chemisorbed at the nanoslit, and demonstrate the electromagnetic‐field‐enhancing function of the plasmonic nanoantennas. The freestanding membrane is open on both sides of the nanoslit, offering the potential for through‐slit molecular translocation studies, and opening bright new perspectives for SERS applications in real‐time (bio)chemical analysis.     

Single-molecule colocalization studies shed light on the idea of fully emitting versus dark single quantum dots

In this report the correlation between the solution photoluminescence (PL) quantum yield and the fluorescence emission of individual semiconductor quantum dots (QDs) is investigated. This is done by taking advantage of previously reported enhancement in the macroscopic quantum yield of water-soluble QDs capped with dihydrolipoic acid (DHLA) when self-assembled with polyhistidine-appended proteins, and by using fluorescence coincidence analysis (FCA) to detect the presence of "bright" and "dark" single QDs in solution. This allows for changes in the fraction of the two QD species to be tracked as the PL yield of the solution is progressively altered. The results clearly indicate that in a dispersion of luminescent nanocrystals, "bright" (intermittently emitting) single QDs coexist with "permanently dark" (non-emitting) QDs. Furthermore, the increase in the fraction of emitting QDs accompanies the increase in the PL quantum yield of the solution. These findings support the idea that a dispersion of QDs consists of two optically distinct populations of nanocrystals--one is "bright" while the other is "dark;" and that the relative fraction of these two populations defines the overall PL yield.     

Dynamic Passivation in Perovskite Quantum Dots for Specific Ammonia Detection at Room Temperature

Perovskite structured CsPbX₃ (X = Cl, Br, or I) quantum dots (QDs) have attracted considerable interest in the past few years due to their excellent optoelectronic properties. Surface passivation is one of the main pathways to optimize the optoelectrical performance of perovskite QDs, in which the amino group plays an important role for the corresponding interaction between lead and halide. In this work, it is found that ammonia gas could dramatically increase photoluminescence of purified QDs and effectively passivate surface defects of perovskite QDs introduced during purification, which is a reversible process. This phenomenon makes perovskite QDs a kind of ideal candidate for detection of ammonia gas at room temperature. This QD film sensor displays specific recognition behavior toward ammonia gas due to its significant fluorescence enhancement, while depressed luminescence in case of other gases. The sensor, in turn‐on mode, shows a wide detection range from 25 to 350 ppm with a limit of detection as low as 8.85 ppm. Meanwhile, a fast response time of ≈10 s is achieved, and the recovery time is ≈30 s. The fully reversible, high sensitivity and selectivity characteristics make CsPbBr₃ QDs ideal active materials for room‐temperature ammonia sensing.     

Synthesis of highly luminescent CdTe/ZnO core/shell quantum dots in aqueous solution

The synthesis and photoluminescence (PL) properties of aqueous CdTe/ZnO core/shell quantum dots (QDs) have been investigated by using thiolglycolic acid as a capping reagent. The highlighted contribution of the present study was CdTe QDs coated with a ZnO shell by controlling the hydrolysis process of Zn(OAc)₂. The QDs benefitted from overcoming the high lattice mismatch between CdTe and ZnO. The PL peak wavelength of the CdTe/ZnO QDs with high PL quantum yields up to 88% was located in a range between 547 and 596 nm by adjusting the size of CdTe cores and the thickness of ZnO shells. The results of X-ray diffraction analysis and transmission electron microscopy observation indicate that the dot-shaped CdTe/ZnO QDs (566 nm) with an average size of 2.2 nm in diameter belong to the cubic CdTe crystal structure. Due to the passivation of surface defects, it is found that the luminescence decay curves accord with a biexponential decay model of exciton and trap radiation behavior. The average PL lifetimes of CdTe (571 nm) and CdTe/ZnO (596 nm) QDs at room temperature are 27.3 and 35.1 ns, respectively.     

Design of Magnetic‐Plasmonic Nanoparticle Assemblies via Interface Engineering of Plasmonic Shells for Targeted Cancer Cell Imaging and Separation

Magnetic‐plasmonic nanoparticles have received considerable attention for widespread applications. These nanoparticles (NPs) exhibiting surface‐enhanced Raman scattering (SERS) activities are developed due to their potential in bio‐sensing applicable in non‐destructive and sensitive analysis with target‐specific separation. However, it is challenging to synthesize these NPs that simultaneously exhibit low remanence, maximized magnetic content, plasmonic coverage with abundant hotspots, and structural uniformity. Here, a method that involves the conjugation of a magnetic template with gold seeds via chemical binding and seed‐mediated growth is proposed, with the objective of obtaining plasmonic nanostructures with abundant hotspots on a magnetic template. To obtain a clean surface for directly functionalizing ligands and enhancing the Raman intensity, an additional growth step of gold (Au) and/or silver (Ag) atoms is proposed after modifying the Raman molecules on the as‐prepared magnetic‐plasmonic nanoparticles. Importantly, one‐sided silver growth occurred in an environment where gold facets are blocked by Raman molecules; otherwise, the gold growth is layer‐by‐layer. Moreover, simultaneous reduction by gold and silver ions allowed for the formation of a uniform bimetallic layer. The enhancement factor of the nanoparticles with a bimetallic layer is approximately 10⁷. The SERS probes functionalized cyclic peptides are employed for targeted cancer‐cell imaging and separation.     

Sensitivity Tuning through Additive Heterogeneous Plasmon Coupling between 3D Assembled Plasmonic Nanoparticle and Nanocup Arrays

Plasmonic substrates have fixed sensitivity once the geometry of the structure is defined. In order to improve the sensitivity, significant research effort has been focused on designing new plasmonic structures, which involves high fabrication costs; however, a method is reported for improving sensitivity not by redesigning the structure but by simply assembling plasmonic nanoparticles (NPs) near the evanescent field of the underlying 3D plasmonic nanostructure. Here, a nanoscale Lycurgus cup array (nanoLCA) is employed as a base colorimetric plasmonic substrate and an assembly template. Compared to the nanoLCA, the NP assembled nanoLCA (NP‐nanoLCA) exhibits much higher sensitivity for both bulk refractive index sensing and biotin–streptavidin binding detection. The limit of detection of the NP‐nanoLCA is at least ten times smaller when detecting biotin–streptavidin conjugation. The numerical calculations confirm the importance of the additive plasmon coupling between the NPs and the nanoLCA for a denser and stronger electric field in the same 3D volumetric space. Tunable sensitivity is accomplished by controlling the number of NPs in each nanocup, or the number density of the hot spots. This simple yet scalable and cost‐effective method of using additive heterogeneous plasmon coupling effects will benefit various chemical, medical, and environmental plasmon‐based sensors.     

Determining the Optimized Interlayer Separation Distance in Vertical Stacked 2D WS2:hBN:MoS2 Heterostructures for Exciton Energy Transfer

The 2D semiconductor monolayer transition metal dichalcogenides, WS₂ and MoS₂, are grown by chemical vapor deposition (CVD) and assembled by sequential transfer into vertical layered heterostructures (VLHs). Insulating hBN, also produced by CVD, is utilized to control the separation between WS₂ and MoS₂ by adjusting the layer number, leading to fine‐scale tuning of the interlayer interactions within the VLHs. The interlayer interactions are studied by photoluminescence (PL) spectroscopy and are demonstrated to be highly sensitive to the input excitation power. For thin hBN separators (one to two layers), the total PL emission switches from quenching to enhancement by increasing the laser power. Femtosecond broadband transient absorption measurements demonstrate that the increase in PL quantum yield results from Förster energy transfer from MoS₂ to WS₂. The PL signal is further enhanced at cryogenic temperatures due to the suppressed nonradiative decay channels. It is shown that (4 ± 1) layers of hBN are optimum for obtaining PL enhancement in the VLHs. Increasing thickness beyond this causes the enhancement factor to diminish, with the WS₂ and MoS₂ then behaving as isolated noninteracting monolayers. These results indicate how controlling the exciton generation rate influences energy transfer and plays an important role in the properties of VLHs.