NP Arrays Research Articles

Optical absorption driven by efficient refraction and light concentration for photovoltaic applications

Broadband absorption of the solar spectrum is a prerequisite to any photovoltaic technology. Subwavelength arrays are known to provide various mechanisms for broadband and omnidirectional absorption of the solar spectrum. In the current work we examine optical absorption in NP arrays decorated with SiO2 quasi-nanolenses (qNL arrays), which numerically exhibit 10–15 % increase in broadband absorption. Specifically, near-field scanning optical microscopy (NSOM) is employed to probe the underlying optical absorption mechanisms. Two NSOM methods are employed for the investigation: 1) far-field imaging using near-field excitation which allows the determination as to the origin of the reflected far-field photons, and 2) far-field excitation and near-field scanning to measure the lateral optical propagations in the arrays. We show that the enhanced optical absorption in qNL arrays is due to combination of three mechanisms: 1) each qNL serves as an effective anti-reflection coating, 2) the presence of qNLs increases the optical interactions between the adjacent dielectrics to conclude elevated levels of light trapping, and 3) the presence of qNL induces strong light concentration of the incoming radiation into the dielectric structures.

Preparation of SiO2@Au Nanoparticle Photonic Crystal Array as Surface-Enhanced Raman Scattering (SERS) Substrate.

Surface-enhanced Raman scattering technology plays a prominent role in spectroscopy. By introducing plasmonic metals and photonic crystals as a substrate, SERS signals can achieve further enhancement. However, the conventional doping preparation methods of these SERS substrates are insufficient in terms of metal-loading capacity and the coupling strength between plasmonic metals and photonic crystals, both of which reduce the SERS activity and reproducibility of SERS substrates. In this work, we report an approach combining spin-coating, surface modification, and in situ reduction methods. Using this approach, a photonic crystal array of SiO2@Au core-shell structure nanoparticles was prepared as a SERS substrate (SiO2@Au NP array). To study the SERS properties of these substrates, Rhodamine 6G was employed as the probe molecule. Compared with a Au-SiO2 NP array prepared using doping methods, the SiO2@Au NP array presented better SERS properties, and it reproduced the SERS spectra after one month. The detection limit of the Rhodamine 6G on SiO2@Au NP array reached 1 × 10-8 mol/L; furthermore, the relative standard deviation (9.82%) of reproducibility and the enhancement factor (1.51 × 106) were evaluated. Our approach provides a new potential option for the preparation of SERS substrates and offers a potential advantage in trace contaminant detection, and nondestructive testing.

Visible light‐enhanced and degradable SERS substrates based on g‐C3N4/Au NP hybrid structures

AbstractIn this study, a visible light‐activated and degradable surface‐enhanced Raman spectroscopy (SERS) substrate was fabricated based on graphite carbon nitride (g‐C3N4) and gold nanoparticle (Au NP) hybrid structures. By using porous structure of g‐C3N4 as the support, Au NP arrays can be formed through sputtering of a thin Au film. Typical surface Raman enhancement effect can be observed by using p‐toluenethiol as the probe molecule. Interestingly, it was found that the Raman signal of probe molecules could be enhanced 2.7 times by pre‐illumination of visible light for a short time within 2 min, while the Raman signal decreased significantly about five times with further long‐term irradiation of more than 60 min. The electromagnetic enhancement effect of Au NP array on semiconductor materials with various extinction coefficients was studied by FDTD simulations. Furthermore, a charge transfer enhancement mechanism was proposed based on band structure of g‐C3N4/Au NP hybrid substrate and molecular orbitals. Our experimental and simulation results demonstrated an approach to construct a visible light‐activated SERS substrate with enhancement and degradation properties based on porous organic semiconductors.

Arrays of Fresnel Nanosystems for Enhanced Photovoltaic Performance.

Omnidirectional broadband absorption of the solar radiation is pivotal to solar energy harvesting and particularly to low-cost non-tracking photovoltaic (PV) technologies. The current work numerically examines the utilization of surface arrays composed of Fresnel nanosystems (Fresnel arrays), which are reminiscent of the known Fresnel lenses, for the realization of ultra-thin silicon PV cells. Specifically, the optical and electrical performances of PV cells integrated with Fresnel arrays are compared with those of a PV cell incorporated with an optimized surface array of nanopillars (NP array). It is shown that the broadband absorption of specifically tailored Fresnel arrays can provide an enhancement of ∼20% over that of an optimized NP array. The performed analysis suggests that broadband absorption in ultra-thin films decorated with Fresnel arrays is driven by two light trapping mechanisms. The first is light trapping governed by light concentration, induced by the arrays, into the underlying substrates, which increases the optical coupling between the impinging illumination and the substrates. The second mechanism is light trapping motivated by refraction, as the Fresnel arrays induce lateral irradiance in the underlying substrates, which increases the optical interaction length and hence the overall probability for optical absorption. Finally, PV cells incorporated with surface Fresnel arrays are numerically calculated, with short-circuit current densities (Jsc) which are ∼50% higher than that of a PV cell incorporated with an optimized NP array. Also, the effect of increased surface area, due to the presence of Fresnel arrays, and its effect on surface recombination and open-circuit voltage (Voc) are discussed.

Carbon Nanocluster-Mediated Nanoblending Assembly for Binder-Free Energy Storage Electrodes with High Capacities and Enhanced Charge Transfer Kinetics.

The effective spatial distribution and arrangement of electrochemically active and conductive components within metal oxide nanoparticle (MO NP)-based electrodes significantly impact their energy storage performance. Unfortunately, conventional electrode preparation processes have much difficulty addressing this issue. Herein, this work demonstrates that a unique nanoblending assembly based on favorable and direct interfacial interactions between high-energy MO NPs and interface-modified carbon nanoclusters (CNs) notably enhances the capacities and charge transfer kinetics of binder-free electrodes in lithium-ion batteries (LIBs). For this study, carboxylic acid (COOH)-functionalized carbon nanoclusters (CCNs) are consecutively assembled with bulky ligand-stabilized MO NPs through ligand-exchange-induced multidentate binding between the COOH groups of CCNs and the surface of NPs. This nanoblending assembly homogeneously distributes conductive CCNs within densely packed MO NP arrays without insulating organics (i.e., polymeric binders and/or ligands) and prevents the aggregation/segregation of electrode components, thus markedly reducing contact resistance between neighboring NPs. Furthermore, when these CCN-mediated MO NP electrodes are formed on highly porous fibril-type current collectors (FCCs) for LIB electrodes, they deliver outstanding areal performance, which can be further improved through simple multistacking. The findings provide a basis for better understanding the relationship between interfacial interaction/structures and charge transfer processes and for developing high-performance energy storage electrodes.

Ultra-fast and sensitive magneto-optical hydrogen sensors using a magnetic nano-cap array

Magnetism in curved nano-geometries has opened a new path to design novel characteristics and phenomena that could greatly impact the future fundamental and applied studies of materials. In this report, a hexagonally-packed array of magnetic nano-caps (or nano-patches, NP) exhibiting unique magnetic anisotropy and optimal nanostructure for fast hydrogen kinetics is explored as a sensing element for the spark-free magnetic-circular-dichroism (MCD) H2 sensor. The MCD H2 sensor outperforms the state-of-the-art optical sensors reported to date and satisfies the most challenging performance targets imposed by US Department of Energy. In particular, a Pd67Co33 NP sensor exhibits response time of t90 < 0.9 s, recovery time of t10 < 9.0 s over the 1–100 mbar H2 partial pressure range, and limit of detection (LOD) of ∼1 ppm. The LOD improves to < 700 ppb, when the signal-to-noise ratio is enhanced by stacking three NP arrays. Using a 30-nm TAF (Teflon AF 2400) polymer coating, the sorption kinetics of the Pd67Co33/TAF sensor are significantly accelerated, with t90 < 0.4 s and t10 < 2.8 s over the same pressure range. When the Pd67Co33/TAF sensor is further coated with a 100-nm poly(methyl methacrylate) (PMMA) layer, a rapid sorption time of t90 < 0.5 s, LOD < 1 ppm and excellent sensor accuracy (<2.5% full scale) are maintained, while the sensor obtains strong selectivity against interference gases and moisture, and a negligible aging effect. The MCD nano-cap sensor platform may have a great impact on the future deployment of H2 fuel, environmental monitoring H2 sensors, and fast proton-based magneto-ionic devices.

Covalent Bonding and Coulomb Repulsion-Guided AuNP Array: A Tunable and Reusable Substrate for Metabolomic Characterization of Lung Cancer Patient Sera.

Surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS) has gained increased attention in the metabolic characterization of human biofluids. However, the stability and reproducibility of nanoparticle-based substrates remain two of the biggest challenges in high-salt environments. Here, by controlling the extent of Coulomb repulsion of 26 nm positively charged AuNPs, a homogeneous layer of covalently bonded AuNPs on a coverslip with tunable interparticle distances down to 16 nm has been successfully fabricated to analyze small biomolecules in human serum. Compared with the self-assembled AuNP array, the covalently bonded AuNP array showed superior performances on stability, reproducibility, and sensitivity in high-salt environments. The stable attachment of AuNPs maintained a detection reproducibility with a RSD less than 12% and enabled the reusability of the array for 10 experiments without significant signal deterioration (<15%) and carryover effects. Moreover, the closely positioned AuNPs allowed the coupling of photoinduced plasmons to generate an enhanced electric field, which promotes the generation of excited electrons to facilitate the desorption/ionization processes instead of the heat dissipation, thus enhancing the detection sensitivity with detection limits down to the femtomole level. Combined with machine learning methods, the AuNP array has been successfully applied to discover seven biomarkers for differentiating early-stage lung cancer patients from healthy controls. It is anticipated that this simple approach of developing robust AuNP arrays can also be extended to other types of NP arrays for wider applications of SALDI-MS technology.

Plasmonic Local Electric Field-Enhanced Interface toward High-Efficiency Cu2ZnSn(S,Se)4 Thin-Film Solar Cells.

The kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cells have shown a continuous rise in power conversion efficiencies in the past years. However, the encountered interfacial problems with respect to charge recombination and extraction losses at the CdS/CZTSSe heterojunction still hinder their further development. In this work, an additional plasmonic local electric field is imposed into the CdS/CZTSSe interface through the electrostatic assembly of a two-dimensional (2D) ordered Au@SiO2 NP array onto an aminosilane-modified surface absorber. The interfacial electric properties are tuned by controlling the coverage particle distance, and the finite-difference time domain (FDTD) simulation demonstrates that the strong near-field enhancement mainly occurs near the p-n junction interface. It is shown that the imposed local electric field leads to interfacial electrostatic potential (Velec) augmentation and improves the charge extraction and recombination processes. These electric benefits enable remarkable improvements in open-circuit voltage (Voc) and short-circuit current (Jsc), leading to the cell efficiency being increased from 10.19 to 11.50%. This work highlights the dramatic role of the plasmonic local electric field and the use of the 2D Au@SiO2 NP array to modify a surface absorber instead of the extensively used ion passivation, providing a new strategy for p-n junction engineering in kesterite photovoltaics.

Conformational control of two-dimensional gold nanoparticle arrays in a confined geometry within a vesicular wall

The two-dimensional (2D) assembly of gold nanoparticles (AuNPs) in a confined geometry is a rare phenomenon that has not been experimentally verified for complex systems. In this study, this process was investigated in detail using two types of block copolymers with hydrophobic and hydrophilic blocks and a series of AuNPs of three different sizes protected by hydrophobic ligands. In aqueous solutions, the selected block copolymers self-assembled into vesicular nanostructures with a hydrophobic domain in the wall, which functions as a confined geometrical space for hydrophobic AuNPs (i.e., it exerts a confinement effect and restricts the movement of AuNPs). Small-angle X-ray scattering studies revealed that AuNPs of different sizes assembled differently in the same confined geometry of the vesicular wall. In addition, optimal conditions for the formation of a regular NP array in the hydrophobic domain were determined. The AuNPs successfully self-assembled into a regular 2D lattice structure, forming a shell around the vesicle, when their size matched the thickness of the hydrophobic domain of the vesicular nanostructure. This study provides guidelines for the fabrication of nanoparticle arrays with controlled structures, which could enhance the functionality of materials and their physical properties.

Spatiotemporal Evolution of Temperature During Transient Heating of Nanoparticle Arrays.

Nanoparticles (NPs) are promising agents to absorb external energy and generate heat. Clusters of NPs or NP array heating have found an essential role in several biomedical applications, diagnostic techniques, and chemical catalysis. Various studies have shed light on the heat transfer of nanostructures and greatly advanced our understanding of NP array heating. However, there is a lack of analytical tools and dimensionless parameters to describe the transient heating of NP arrays. Here we demonstrate a comprehensive analysis of the transient NP array heating. Firstly, we develop a set of analytical solutions for the NP array heating and provide a useful mathematical description of the spatial-temporal evolution of temperature for 2D, 3D, and spherical NP array heating. Based on this, we introduce the concept of thermal resolution that quantifies the relationship between minimal heating time, NP array size, energy intensity, and target temperature. Lastly, we define a set of dimensionless parameters that characterize the transition from confined heating to delocalized heating. This study advances the understanding of nanomaterials heating and guides the rational design of innovative approaches for NP array heating.

Synthesis and Near-Field Enhancement of Composite Nanoarrays Based on Electrodeposition and in Situ Reaction

Since the morphology and element composition of metal nanostructures strongly affect the surface plasma oscillation characteristics, it has been widely concerned in surface enhanced Raman scattering (SERS). Herein, we proposed a novel route to fabricate composite Au/Ag nanoparticle arrays with synergistic effect for electromagnetic enhancement. Ag nanoparticles were electrodeposited onto a home-made template with highly ordered bowl-like pits. After a novel method of “confined annealing”, we further achieved well-regulated spherical Ag NP arrays, and the composite Au/Ag nanoparticle arrays were finally obtained via in situ replacement. The fabricated composite nanostructures showed stable and sensitive surface enhanced Raman scattering (SERS) performance mainly due to the synergistic effect and abundant “hot spots”, with the enhancement factor (EF) of [Formula: see text] for crystal violet (CV) molecules. In addition, this simple and effective preparation process greatly improved the uniformity of three-dimensional nanostructures, providing a new idea for further improving the stability of SERS signals.

Facile Detection and Quantification of Acetamiprid Using a Portable Raman Spectrometer Combined with Self-Assembled Gold Nanoparticle Array

Rapid and facile determination of pesticides is critically important in food and environmental monitoring. This study developed a self-assembled gold nanoparticle array based SERS method for highly specific and sensitive detection of acetamiprid, a neonicotinoid pesticide that used to be difficult in SERS analysis due to its low affinity with SERS substrates. SERS detection and quantification of acetamiprid was conducted with self-assembled gold nanoparticle arrays at the interface of chloroform and water as the enhancing substrate. Since targets dissolved in chloroform (organic phase) also have access to the hot-spots of Au NP array, the developed method exhibited good sensitivity and specificity for acetamiprid determination. Under the optimal conditions, SERS intensities at Raman shifts of 631 cm−1 and 1109 cm−1 displayed a good linear relationship with the logarithm concentration of acetamiprid in the range of 5.0 × 10−7 to 1.0 × 10−4 mol/L (0.11335 ppm to 22.67 ppm), with correlation coefficients of 0.97972 and 0.97552, respectively. The calculated LOD and LOQ of this method were 1.19 × 10−7 mol/L (0.265 ppb) and 2.63 × 10−7 mol/L (0.586 ppb), respectively, using SERS signal at 631 cm−1, and 2.95 × 10−7 mol/L (0.657 ppb) and 3.86 × 10−7 mol/L (0.860 ppb) using SERS signal at 1109 cm−1, respectively. Furthermore, the developed SERS method was successfully applied in determining acetamiprid on the surface of apple and spinach. This method offers an exciting opportunity for rapid detection of acetamiprid and other organic pesticides considering its advantages of simple preparation process, good specificity and sensitivity, and short detection time (within 1 h).

Linear optical properties of 2d assembly of interacting gold nanoparticles: analytical approach in dipole approximation

Using a Drude-like model for the conduction electrons of Metal Nanoparticles (MNPs) assembled in a periodic two-dimensional (2D) array, considering dipole-dipole interactions of adjacent particles, an analytical expression is achieved for the MNP permittivity where the functionality of interaction term on the different geometric and physical parameters is evident. A numerical analysis is carried out for a gold NP array for two different sizes of NPs and two different host media of water and glass. It is shown that, interaction of NPs leads to a substantial increase in the extinction cross section, the red-shift of the Surface Plasmon Resonance (SPR) wavelength and decrease in the full width at half maximum (FWHM) of cross section. The absolute values of the real and imaginary parts of complex permittivity of each MNP decrease in comparison with the single particle case.

Broadband Absorption in Thin Films Motivated by Strong Light Bending

Photon management toward the absorption over a wide spectral range of the solar light is important to various absorption‐based photonic devices. The key for high absorption in thin films is the ability to “bend the light” from the normal direction into the in‐plane direction. Broadband absorption enhancement is reported in thin films decorated with top surface light funnel arrays (LF arrays) due to strong bending of the light by the arrays into the films, in contrast with excitation of thin films by top surface nanopillar arrays (NP arrays) that is governed by forward scattering. The broadband absorption in the film below the LF array (LF film) is 40% higher than that in a film below an optimized NP array (NP film). Also, the strong light bending by the LF arrays into the underlying film induces strong absorptivity peaks substantially exceeding the Yablonovitch limit for both the LF film and the total array–film complex. Finally, the superior LF film broadband absorption is due to strong light bending from the normal direction into the in‐plane direction which is manifested in efficient generation of mode hybridizations.

Symmetry breaking of Au nanospheres confined in 1D nanocylinders: exploring helical assembly by 3D transmission electron microscopy

The hybrid assemblies of polystyrene-tethered gold NPs are controlled by polymer chain stretching in a cylindrical confinement. The zigzag NP arrays observed in 2D TEM can be revealed into double- or triple-stranded helical NP arrays using 3D TEM.

The highly sensitive determination of serotonin by using gold nanoparticles (Au NPs) with a localized surface plasmon resonance (LSPR) absorption wavelength in the visible region.

The development of improved methods for the synthesis of monodisperse gold nanoparticles (Au NPs) is of high priority because they can be used as substrates for surface-enhanced Raman scattering (SERS) applications relating to biological lipids. Herein, Au NPs have been successfully synthesized via a seed-mediated growth method. The LSPR peak is controlled via adjusting the gold nanoseed component, and different fabrication methods were studied to establish the effect of sonication time on NP size. The simple, facile, and room-temperature method is based on a conventional ultrasonic bath, which leads to ultrasonic energy effects on the size and morphology of the Au NPs. This research offers new opportunities for the production of highly monodispersed spherical Au NPs without the use of a magnetic stirrer method, as evidenced by ultraviolet-visible reflectance spectra and scanning electron microscopy (SEM) analysis. SEM images indicate that the spherical Au NP colloidal particles are stable and reliable, which paves the way for their use as a nanostructured biosensor platform that can be exploited for multiple applications, for example, in materials science, sensing, catalysis, medicine, food safety, biomedicine, etc. The highest enhancement factor that could be achieved in terms of the SERS enhancement activity of these Au NP arrays was determined using 10−9 M serotonin (5-hydroxytryptamine, 5-HT) as the Raman probe molecules.

The Fabrication of Rigid Crosslinker-Decorated Gold Nanoparticle Array Film for Catalyzing CO2 Cycloaddition

Abstract In order to precisely design the active sites in two-dimensional (2D) gold-based catalysts, we have developed a convenient and versatile plasma-assisted droplet evaporation-rigid crosslinking method for the fabrication of gold nanoparticle (Au NP) array film. Four kinds of Au NP arrays have been decorated respectively with rigid sulfurated crosslinkers i.e. thieno[3,2-b]thiophene, 2,2′-bithiophene, 1,4-benzenedithiol and 4,4′-thiodibenzenethiol, and the density of crosslinkers can be adjusted under plasma treatment. Particularly, the utilization of 4,4′-thiodibenzenethiol gave uniform particle sizes to form a periodical 2D structure, which provides multiple exposed active sites of gold nanoparticles rather than enwinding by the alkyl chains. Meanwhile, the weaker electron-donating effect and steric hindrance of rigid groups in the crosslinkers could also enhance the catalytic activity. In addition, the Au NP array film can be transferred from the glass substrate and further composited with polymers and metal organic framework (MOF) into self-standing composite membrane. Therefore, this rigid crosslinked array film can serve as an environmentally friendly catalyst for CO2 cycloaddition under atmospheric CO2 pressure, which offers a novel application of Au NPs array film, and opens up a new way for the design and fabrication of 2D hybrid materials.

Arbitrary Gold Nanoparticle Arrays Fabricated through AFM Nanoxerography and Interfacial Seeded Growth.

Based on arrays of Au seeds fabricated withatomicforce microscopy (AFM) nanoxerography, the seeded growth of gold nanoparticles (Au NPs) on surface is achieved. The size evolution of Au NPs in each spot is tracked by in situ AFM and SEM images because each spot can be easily localized in the array system. The extinction microspectra extracted in real time with enhanced signals and red-shift can further monitor the increasing size of Au NPs. As a powerful platform, AFM nanoxerography makes it easy to tune the spot size and the intervals among spots in the Au NP arrays without preparing a template. It also allows for fabricating arbitrary patterns including various symbols and graphs. More interestingly, the in situ growth of Au NPs offers an approach to decreasing the interparticle distance, and thus forming closely interconnected Au nanowire assembly, exhibiting immense potential in the nanoelectronic system.

Stable, Flexible, and High-Performance SERS Chip Enabled by a Ternary Film-Packaged Plasmonic Nanoparticle Array.

The high sensitivity and long-term storage stability of a plasmonic substrate are vital for practical applications of the surface-enhanced Raman scattering (SERS) technique in real-world analysis. In this study, a rationally designed, ternary film-packaged, silver-coated gold-nanoparticle (Au@Ag NP) plasmonic array was fabricated and applied as a stable and high-performance SERS chip for highly sensitive sensing of thiabendazole (TBZ) residues in fruit juices. The ternary films played different roles in the plasmonic chip: a newborn poly(methyl methacrylate) (PMMA) film serving as a template for fixing the self-assembled closely packed monolayer Au@Ag NP array that provided an intensive hot spot, a fluorescent quantitative polymerase chain reaction adhesive film (qPCR film) acting as a carrier to retrieve the Au@Ag/PMMA film that was used to improve the robustness of the plasmonic array, and a polyethylene terephthalate (PET) film covered over the Au@Ag/PMMA/qPCR film performing as a barrier to improve the stability of the chip. The Au@Ag/PMMA/qPCR-PET film chip showed high sensitivity with an enhancement factor of 3.14 × 106, long-term storage stability without changing SERS signals for more than 2 months at room temperatures, and a low limit of detection for sensing TBZ in pear juice (21 ppb), orange juice (43 ppb), and grape juice (69 ppb). In addition, the procedure for fabricating the Au@Ag/PMMA/qPCR-PET film SERS chip was easy to handle, offering a new strategy to develop flexible and wearable sensors for on-site monitoring of chemical contaminants with a portable Raman spectrometer in the future.

Resonance reflection of light by ordered silicon nanopillar arrays with the vertical p-n junction

Silicon nanopillar (Si NP) arrays with the axial p-n junction were formed and investigated. A method to fabricate Si NP ordered arrays by means of electron beam lithography using the negative electron resist and reactive ion etching is presented.The effect of strong resonance light scattering – change of the color of separate Si NPs - was demonstrated. One or several minima were registered in the measured reflection spectra. Thereat, the position of reflection minimum was changed with a change in Si NP diameter. A shift of the minimum position towards the longer wavelength spectral region with an increase in Si NP diameter was observed. A shift of the position of minima to the shorter wavelength spectral region with a decrease in Si NP pitch in microarrays with the same Si NP diameter was observed. The quantitative divergence in the position of reflection minima in Si NPs with calculated dependencies for Mie resonances was found. High photosensitivity of Si NP arrays with axial p-n junction to visible and near IR light was discovered. So, these structures may be used for selective photonic sensors.