Nanosphere Arrays Research Articles

Light absorption enhancement in ultrathin perovskite solar cells using light scattering of high-index dielectric nanospheres.

Arrays of high-index dielectric nanoparticles supporting both electrical and magnetic resonances have gained increasing attention for their excellent light-trapping (LT) effects, thus greatly improving the performance of ultrathin solar cells. This work explores front-located, high-index dielectric subwavelength nanosphere arrays as an efficient and broadband LT structure patterned on top of an ultrathin perovskite solar cell (PSC) for a greatly enhanced absorption. Combined strong light scattering and anti-reflection properties achieved by optimized geometrical parameters of the LT structure lead to a broadband absorption enhancement in the ultrathin thickness of a photoactive layer (100 nm) yielding the short-circuit current density (Jsc) of 18.7 mA/cm2, which is 31.7% higher than that of a planar counterpart. Moreover, effects of the LT structure on far-field radiation patterns, scattering cross-sections, multipoles' contributions, and asymmetry parameters along with the incidence angle and polarization dependence are investigated. The present strategy could be applied to diverse applications, such as other ultrathin or semitransparent solar cells, absorbers and photodetectors.

Image analysis algorithm for the verification of hexagonal symmetry in spherical nanostructures

Verification of ordering and symmetry is essential to enhance the nanofabrication process of periodic nanostructures. In this paper, we present the open-source software HEXI, which can detect circles and distinguish between perfect hexagonal ordering and defect configurations. The proposed user-friendly image analysis software (implemented in Python) consists of several stages. First, the algorithm identifies circular structures in microscopy (e.g., scanning electron microscopy, atomic force microscopy) images using the Canny edge detector and the Hough circle transform. Then, the detected circles are categorized as hexagonally ordered or non-hexagonally ordered (defects). This classification can be achieved using three different methods: variance in brightness (global or adaptive) and distance. The software generates visual output (detected coloured circles overlaid over the original image) and quantitative data (total/defective circle count, surface coverage, histogram of size distribution).The circle detection and classification are demonstrated on real-world samples (e.g., spin-coated monolayer of polystyrene spheres, arrays of plasma etched nanospheres, holes). They verify the accuracy of the proposed algorithm, which successfully analysed circular nanostructures of various types (close-packed and non-closed packed arrays, unevenly lit large areas) with hexagonal symmetry, independently of preparation technique (nanosphere lithography, electron beam lithography). Finally, the performance and advantages of the software are discussed.

A numerical study of plasmon-induced enhancement of dipole emission by arrays of silver nanospheres and nanoprisms

Plasmon-induced enhancement of the photoluminescence (PL) intensity of quantum emitters by plasmon nanoparticles (PNPs) is a promising approach for the design and fabrication of PL hybrid materials with improved properties for biosensing and optoelectronics applications. This enhancement can be achieved through careful selection of such parameters as the PNP’s shape, material, size, the type of the medium around them, as well as the excitation and emission wavelengths of the emitter. Here, we simulated the nano-hybrid materials based on arrays of silver nanospheres, nanoprisms, and their combinations, located on polymer films with embedded dipole emitters. For these systems, the Purcell effect, as well as the excitation enhancement, transmittance, and absorption were evaluated. The results show the way to improve the PL properties of thin hybrid films by careful design of their composition using developed models.

Conformal Self-Assembly of Nanospheres for Light-Enhanced Airtightness Monitoring and Room-Temperature Gas Sensing.

The fabrication of conformal nanostructures on microarchitectures is of great significance for diverse applications. Here a facile and universal method was developed for conformal self-assembly of nanospheres on various substrates including convex bumps and concave holes. Hydrophobic microarchitectures could be transferred into superhydrophilic ones using plasma treatment due to the formation of numerous hydroxyl groups. Because of superhydrophilicity, the nanosphere suspension spread on the microarchitectures quickly and conformal self-assembly of nanospheres can be realized. Besides, the feature size of the conformal nanospheres on the substrates could be further regulated by plasma treatment. After transferring two-dimensional tungsten disulfide sheets onto the conformal nanospheres, the periodic nanosphere array was demonstrated to be able to enhance the light harvesting of WS2. Based on this, a light-enhanced room-temperature gas sensor with a fast recovery speed (<35 s) and low detecting limit (500 ppb) was achieved. Moreover, the WS2-covered nanospheres on the microarchitectures were very sensitive to the changes in air pressure due to the formation of suspended sheets on the convex bumps and concave holes. A sensitive photoelectronic pressure sensor that was capable of detecting the airtightness of vacuum devices was developed using the WS2-decorated hierarchical architectures. This work provides a simple method for the fabrication of conformal nanospheres on arbitrary substrates, which is promising for three-dimensional microfabrication of multifunctional hierarchical microarchitectures for diverse applications, such as biomimetic compound eyes, smart wetting surfaces and photonic crystals.

Theoretical analysis of efficiency for vacuum photoelectric energy converters with plasmon-enhanced electron emitter

Thermionic energy converters (TECs) convert heat or light into electrical energy based on electron emission in vacuum. By using a cathode consisting of metal nanostructures, plasmonic thermionic energy converters (PTECs) can overcome challenges concerning high operation temperature, which hinders the use of TEC for solar–thermal energy conversion. However, there is lack of theoretical analysis to describe the mechanism behind PTEC and to guide the design of device. In this study, we developed a simple model to calculate the power conversion efficiency of PTEC consisting of metal nanostructure cathodes, also named as vacuum photoelectric energy converter (VPEC) with plasmon-enhanced electron emitter, in this work. The distribution of plasmon-induced hot electrons was calculated using Fermi's golden rule. Under the assumption of ballistic transport and photoemission, the performance of VPEC was analyzed under different operating conditions. The results reveal that the size and shape of the nanostructure cathode influence the hot electron emission efficiency. For a cathode consisting of a single silver nanosphere, an optimal nanosphere diameter of ∼15 nm exists with optimal quantum efficiency and energy conversion of 8.71% and 1.88%, respectively, under the illumination of 339 nm light. Besides, the optimal performance for cathode consisting of a silver nanosphere array is ∼33% of that for the single silver nanosphere. This model provides insights into the dynamics of plasmon-induced hot electrons and guidelines for optimizing hot electron devices for photoelectric conversion applications.

SERS devices with “hedgehog-like” nanosphere arrays for detection of trace pesticides

The development of surface-enhanced Raman scattering (SERS) devices for detection of trace pesticides has attracted more and more attention. In this work, a large-area self-assembly approach assisted with reactive ion etching (RIE) is proposed for preparing SERS devices consisting of Ag-covered “hedgehog-like” nanosphere arrays (Ag/HLNAs). Such a SERS device has an enhancement factor of [Formula: see text], a limit of detection (LOD) up to 10[Formula: see text][Formula: see text]M for Rhodamine 6G (R6G) analytes, and a relative standard deviation (RSD) smaller than 10%, demonstrating high uniformity. Besides, for pesticide detections, the device achieves an LOD of 10[Formula: see text][Formula: see text]M for thiram molecules. It indicates that the proposed SERS device has a promising opportunity in detecting toxic organic pesticides.

Control of Kerr nonlinearity in a four-level quantum system near a plasmonic nanostructure

Abstract We investigate the nonlinear optical response of a four-level double-V-type quantum system interacting with a pair of weak probe fields while located near a two-dimensional array of metal-coated dielectric nanospheres. Such a quantum system contains a V-type subsystem interacting with surface plasmons, and another V-type subsystem interacting with the free-space vacuum. A distinctive feature of the proposed setup is its sensitivity to the relative phase of the applied fields when placed near the plasmonic nanostructure. We demonstrate that due to the presence of the plasmonic nanostructure, the third-order (Kerr-type) susceptibility for one of the laser fields can be significantly modified while another probe field is acting. Moreover, the Kerr nonlinearity of the system can be controlled and even enhanced by varying the distance of the quantum system from the plasmonic nanostructure. We also show that the Kerr nonlinearity of such a system can be controlled by adjusting the relative phase of the applied fields. The results obtained may find potential applications in on-chip nanoscale photonic devices.

Activation of buried p-GaN through nanopipes in large-size GaN-based tunnel junction LEDs

In GaN-based light-emitting diodes (LEDs), tunnel junctions offer a way of replacing the highly resistive p-type GaN (p-GaN) ohmic contact with a low-resistance n-GaN ohmic contact. However, the p-GaN would be re-passivated by hydrogen atoms during the subsequent growth of n-GaN in a metal-organic chemical vapor deposition (MOCVD) chamber. The n-GaN layer, acting as a hydrogen diffusion barrier, hinders the thermal activation of the underlying p-GaN. Here, we report a method to thermally activate the buried p-GaN in tunnel junction LED (TJ-LED) through vertically aligned nanopipe arrays across the top n-GaN layer, which provides a hydrogen outgassing passage. The fabrication of nanopipes is realized via inductive coupled plasma etching using a mask prepared by self-assembled nanosphere arrays. As a result, we attain large-size TJ-LED chips, exhibiting nearly equivalent p-GaN activation and superior light extraction compared to conventional LEDs. Specifically, the light extraction efficiency is boosted by 44% relative to conventional LEDs at an injection current density of 100 A cm−2.

Independently tunable double Fano-like resonances arising from the interference coupling of localized surface plasmons with waveguide modes

Research on the classical analogy of asymmetric Fano resonances has gained momentum in recent years, owing to the steep dispersion of the Fano-like profile in classical systems, such as plasmonic nanostructures and metamaterials, which has numerous applications. We studied the double Fano-like resonances arising in a waveguide structure made of a periodic array of gold nanospheres placed on the top surface of an indium tin oxide (ITO) film, which is deposited on a silica substrate. These resonances emanate from the interference coupling between the dipolar surface plasmon mode of the individual gold nanospheres and the zero-order transverse electric (TE) and transverse magnetic (TM) waveguide modes within the ITO film. Increasing the array period of the gold nanospheres or thickness of the ITO film shifted the resonance peaks towards the low-frequency side corresponding to the excitation frequencies of the zero-order TE and TM waveguide modes, which were red-shifted. Additionally, the double Fano-like resonance profiles were approximated using an analytical Fano interference model.

Large range LSPR of KTN@Ag core-shell nanoarray dependent of structure size

The localized surface plasmon resonance (LSPR) of Ag nanosphere array and KTN@Ag (nano KTa0.65Nb0.35O3 with Ag at it’s the surface) core-shell nanoarray are elaborately analyzed based on finite-difference time-domain (FDTD). The effects of particle spacing, core size and shell size on LSPR for the KTN@Ag core-shell nanoarray structure are systematically investigated. The results indicate that the core size and shell size for the KTN@Ag core-shell nanoarray structure have a great influence on LSPR. With the increase of the Ag shell radius, the resonance absorption appears blue-shift from 878 nm to 660 nm. With the increase of KTN core radius, the resonance absorption peak exhibits significantly red-shift from 538 nm to 867 nm. Thus, the designed KTN@Ag core-shell nanoarray structure can realize the effective regulation of LSPR in a wider range, which is obviously better than the Ag nanosphere array. The controllable LSPR wavelength enables the KTN@Ag core-shell nanoarray to serve as plasmonic nanosensors. This study provides a theoretical basis for the future application of KTN@Ag core-shell nanoarray structure in optical and optoelectronic devices.

Manufacture of unitary/binary ordered arrays employing self-assembled nanocolloid lithography

This paper details the fabrication of unitary and binary nanocolloid arrays using the spin coating technique. Polystyrene spheres of 900 and 100 nm in size were overlaid on the surface of a silicon substrate using a spin coater. To prepare the unitary nanosphere arrays, nanosphere latex spheres of 900 or 100 nm were first mixed with surfactant solution and then spin-coated onto the silicon substrate. For the binary spin coating, both 900 and 100 nm colloidal nanospheres were mixed with surfactant solutions and then concurrently spin-coated. Fabricated nanosphere arrays were evaluated via both scanning electron and atomic force microscopy. The influence of processing parameters, including type of surfactant, nanosphere to surfactant ratio, spin speed, and spin time on the ordering of colloidal array have been identified; after this step is complete, steps can be taken to optimize the process. These steps provide advantages in terms of improved nanosphere array quality.

Hollow nanosphere arrays with a high-index contrast for enhanced scintillating light output from β-Ga2O3 crystals

β-Ga2O3 is a new type of fast scintillator with potential applications in medical imaging and nuclear radiation detection with high count-rate situations. Because of the severe total internal reflection with its high refractive index, the light extraction efficiency of β-Ga2O3 crystals is rather low, which would limit the performance of detection systems. In this paper, we use hollow nanosphere arrays with a high-index contrast to enhance the light extraction efficiency of β-Ga2O3 crystals. We can increase the transmission diffraction efficiency and reduce the reflection diffraction efficiency through controlling the refractive index and the thickness of the shell of the hollow nanospheres, which can lead to a significant increase in the light extraction efficiency. The relationships between the light extraction efficiency and the refractive index and thickness of the shell of the hollow nanospheres are investigated by both numerical simulations and experiments. It is found that when the refractive index of the shell of the hollow nanospheres is higher than that of β-Ga2O3, the light extraction efficiency is mainly determined by the diffraction efficiency of light transmitted from the surface with the hollow nanosphere arrays. When the refractive index of the shell is less than that of β-Ga2O3, the light extraction efficiency is determined by the ratio of the diffraction efficiency of the light transmitted from the surface with the hollow nanosphere arrays to the diffraction efficiency of the light that can escape from the lateral surface.

Binary Self-Assembly of Nanocolloidal Arrays using Concurrent and Sequential Spin Coating Techniques.

This paper reports the binary colloid assembly of nanospheres using spin coating techniques. Polystyrene spheres with sizes of 900 and 100 nm were assembled on top of silicon substrates utilizing a spin coater. Two different spin coating processes, namely concurrent and sequential coatings, were employed. For the concurrent spin coating, 900 and 100 nm colloidal nanospheres of latex were first mixed and then simultaneously spin coated onto the silicon substrate. On the other hand, the sequential coating process first created a monolayer of a 900 nm nanosphere array on the silicon substrate, followed by the spin coating of another layer of a 100 nm colloidal array on top of the 900 nm array. The influence of the processing parameters, including the type of surfactant, spin speed, and spin time, on the self-assembly of the binary colloidal array were explored. The empirical outcomes show that by employing the optimal processing conditions, binary colloidal arrays can be achieved by both the concurrent and sequential spin coating processes.

Engineered nano-sphere array of gold-DNA core-shells and junctions as opto-plasmonic sensors for biodetection.

In this paper, we design opto-plasmonic sensors by the engineered arrangement of gold-nanospheres. We use DNA-gold nanoparticle (GNP) core–shells and DNA rods as junctions between GNPs with a fishnet ground layer for controlling and improving the absorbance and reflection in the range of 100–300 THz. Based on available data, we check the effects of healthy and cancerous cells on the reflection parameter. Here, we demonstrate how the DNA junctions and distance between the nanospheres can be considered to modify the reflection. These structures can be utilized as opto-plasmonic sensors with high sensitivity to distinguish materials in terms of refractive indices. We can use an array of these sensors for both spectroscopy and optical imaging on a real scale. The proposed structures with different topologies are analyzed and their figure of merits (FOM) and sensitivities are obtained. The structure based on the DNA rods as junctions between GNPs shows the best FOM value of 340 RIU−1 and the core–shell heptamer structure has the best sensitivity of about 1287 nm RIU−1.

Integrated EC-SERS Chip with Uniform Nanostructured EC-SERS Active Working Electrode for Rapid Detection of Uric Acid.

Toxemia of pregnancy is a very dangerous disease for pregnant women. The mortality rate of toxemia of pregnancy is close to 10% to 15%. Early detection of pregnancy toxemia is to monitoring uric acid concentration in urine. The current mainstream method for detecting uric acid requires an enzyme (urate oxidase), which needs to be stored in a low-temperature environment, and the method requires complex chemical steps, which takes a longer time and more samples. In this study, we propose an integrated miniature three-electrode electrochemical surface-enhanced Raman spectroscopy chip (EC-SERS chip) suitable for rapid EC-SERS detection applications. The integrated microfluidic reservoir on the chip makes it easy to use, which is very suitable for rapid detection applications. The SERS active working electrode for the proposed integrated EC-SERS chip is a nanocone array polycarbonate (PC) substrate decorated with an evenly distributed and tightly packed array of gold nanospheres. It showed good uniformity and can be easily reproduced. The integrated EC-SERS chip is very small compared to the traditional electrochemical cell, which reduces the sample volume required for the testing. In addition, the chip is for one-time use only. It eliminates the need to clean electrochemical cells for reuse, thereby reducing the possibility of contamination and inaccurate detection. Various low-concentration Rhodamine 6G (R6G) solutions were tested to verify the performance of the developed EC-SERS chip. Experimental results showed that the proposed EC-SERS chip has a strong enhancement factor of up to 8.5 × 106 and a very good EC-SERS uniformity (the relative standard deviation of EC-SERS intensity is as low as 1.41%). The EC-SERS chip developed has been further tested for the detection of uric acid in synthetic urine. The results showed that the EC-SERS signal intensity has a highly linear relationship with the logarithm of the uric acid concentration in synthetic urine, which indicates that the developed EC-SERS chip is suitable for the quantitative detection of uric acid in synthetic urine. Therefore, the developed EC-SERS chip is very promising to be used in routine and early diagnosis of pregnancy toxemia and may be used in many other medical tests, food safety, and biotechnology applications.

Nanosphere Concentrated Photovoltaics with Shape Control

AbstractDielectric colloidal nanospheres (NSs) are promising candidates for light management in photonic devices such as solar cells (SCs). NS arrays can direct the broad incident solar radiation into a set of tighter foci, at which light intensity becomes considerably concentrated, enabling higher photovoltaic conversion efficiency. Furthermore, the NS arrays acting as an effective medium on the SC surface can reduce reflectance and facilitate improved forward scattering. Therefore, uniform arrays of NSs located on top of the SC can behave as antireflection coatings or as microlenses, which can be regarded as a surface distributed light concentrator within the framework of concentrated photovoltaics. Fabrication of NS‐based light‐trapping structures is low‐cost and less complicated than common alternatives such as vacuum evaporated multilayer antireflection coatings. In this work, experimental demonstration and computational confirmation on the shape adjustment of such NS structures for improved light harvesting and efficiency enhancement in Si SCs are studied. The light conversion efficiency of Si solar cells is shown to improve by more than 27% with shape adjustment of NS arrays.

Two-dimensional plasmon induced grating in a double V-type quantum system

We theoretically study a two-dimensional electromagnetically induced phase grating in a four-level quantum system located near plasmonic metamaterials consisting of a periodic two-dimensional array of metal-coated dielectric nanospheres. The four-level quantum system interacts with a weak-probe laser field and a strong coherent coupling field with a standing-wave pattern. In the presence of plasmonic metamaterials the quantum system becomes sensitive to the relative phase between applied light. Therefore, the linear absorption and Kerr nonlinearity of the medium become phase dependent. Here, due to the standing-wave pattern of coupling light, an absorptive grating or phase grating can be formed in the quantum system. In our proposed model, we show that the diffraction efficiencies of the grating are related to the relative phase between applied fields, when the quantum system is located at different distances from plasmonic metamaterials.

Metastructure-inspired ultraviolet and blue light filter

Currently, many studies are being conducted to apply optical metamaterials to the transmittance control of visible light and near infrared wavelengths. In this study, a metastructure was fabricated with periodic metal holes that can reduce the transmittance of only ultraviolet (UV) and blue light in the wavelength band from UV to visible light. The connection structure of the metal-hole-patterns, with a hole size of tens to hundreds of μm, and the crystallinity of the metal could be controlled through the three-step process of polystyrene (PS) nanosphere array formation, using the air–water interface method, and size adjustment of the PS nanosphere, through the etching process, and through the adjustment of the metal film thickness. The glass substrate, with an Al-hole-patterned metastructure (an Al-hole-pattern size of ∼245 nm and an Al thickness of ∼60 nm), showed a low transmittance of 33% and 28% in the wavelength bands of 350 nm and 470 nm, respectively, and a high transmittance of 67% and 57% in the ranges of 540 nm and 670 nm, respectively, owing to the metacharacteristics manifested in periodically connected metal patterning structures. This Al-hole-patterned metastructure is expected to be applicable to various optical devices that require a reduction in the transmittance of only the UV and blue light wavelength band.

Multipolar interactions in Si sphere metagratings

A thorough theoretical study of the optical properties of periodic Si nanosphere arrays is undertaken, placing a particular emphasis on the synergy between multipolar, electric and magnetic, Mie resonances, which occur in high-refractive-index nanoparticles and can lead to a rich variety of phenomena ranging from perfect reflection to controlled diffraction. Systematic calculations using the layer-multiple-scattering method, which we properly extended to describe periodic arrays with many different scatterers per unit cell, in conjunction with finite-element simulations are presented. It is shown that rectangular arrays of pairs of Si nanospheres can efficiently diffract light in reflection or transmission mode at large angles as well as split light with minimum backreflection by properly adjusting the geometry of the structure. Our rigorous full-electrodynamic calculations highlight the importance of higher-order multipoles, which are not taken into account in the commonly employed dipole approximation, in the description of these effects.

Highly Selective and Sensitive Detection of Hydrogen Sulfide by the Diffraction Peak of Periodic Au Nanoparticle Array with Silver Coating

The two-dimensional (2D) periodic Au nanosphere array with silver coating was prepared by using a colloidal monolayer template to obtain a Au nanosphere array and subsequently depositing silver thin coating on it, which could be used as an optical sensor to effectively detect H2S. Such periodic Au nanosphere array with silver coating displayed a surface plasmonic resonance (SPR) peak and an optical diffraction peak. Compared with the SPR peak, the diffraction peak, originated from the periodic arrangements of the obtained array, demonstrated a more sensitive optical change to detect H2S with a significant redshift as the H2S concentration increased. It was attributed to the increase of the refractive index of the environment around the Au nanosphere arrays with silver coating due to the partial formation of Ag2S after detecting H2S. Furthermore, the H2S sensor based on the change of the optical diffraction peak, showed an excellent selectivity and it was very sensitive to detect H2S from 2 to 30 μM. This method was investigated by the analysis in H2S-spiked blood samples, which indicates that the method has the potential to detect H2S in blood samples. The presented work provides a new strategy of utilizing the optical diffraction peak of the periodic array to develop promising sensors.