Visible transparent mid-infrared broadband absorbers based on gradient refractive indexes and multi-size cavity resonances.

We proposed a multi-layered nanorod structure with the same tilt angle and different diameters, which has high visible transmittance and strong 3-5 µm absorption based on the principles of the gradient of the refractive index and the multi-size cavity resonances. The indium tin oxide (ITO) was selected as the target material to fabricate the structure by using a glancing angle deposition method. The experimental results show that when the deposition angle θ is 80°, swing deposition is successively done with the rotation angle φ of ±8°, ± 5°, ± 3°, and 0° on the surface of the substrate, and the quartz crystal microbalance thicknesses of ITO nanorods are 220 nm for each deposition, the average transmittance is 80.5% in the range of 400-800 nm and the integrated absorption is 86% in the 3-5 µm band. Such a simple, low-cost, and easy-to-fabricate device has potential applications in window stealth materials and other related fields.

3-5 µm mid-infrared broadband absorbers composed of layered ITO nanorod arrays with high visible light transmittance.

A mid-infrared broadband absorber with high visible light transmittance is proposed in this paper. The absorber is composed of layered ITO nanorod arrays with increasing angles fabricated by oblique angle deposition technique. The experimental results show that the average transmittance of the absorber reaches 80% in the 400-800 nm band and the integrated absorption reaches 82.9% in the 3-5 µm band, when the QCM thickness of the first layer of film is 100 nm and the deposition angle θ is 10°, the QCM heights of the second to fifth layers of nanorods are all 330 nm, and their deposition angles are 55°, 68°, 80°, and 87°, respectively. The high transmittance in the visible band is attributed to the gradient of the refractive index. The broadband absorption in the mid-infrared band results from different resonances in the empty cavities with different sizes. Such a simple and large-area absorber has potential applications in window materials and infrared cloaking.

ITO/Si/ITO semi-cone-shell chiral complexes on silicon nanocones with broadband circular dichroism in the mid-infrared wavelength.

This paper proposed ITO/Si/ITO semi-cone-shell chiral complexes on silicon nanocones with broadband CD in the mid-infrared band. The experimental results show that when the deposition angle θ = 45°, the first ITO deposition of ta = 100 nm, the second Si deposition of tb = 200 nm with the azimuth angle unchanged, and the third ITO deposition of tc = 200 nm after rotating the azimuth angle of 60°, the prepared chiral structure has a broadband CD response in the mid-infrared band of 2.5-4 µm. The broadband CD effect is produced by the internal resonance of the three-dimensional open cavity. The cone structure can be regarded as a plurality of planar open resonant rings with different diameters, and these rings resonate at different wavelengths. The experimental results also show that the proposed chiral ITO structure exhibits a better broadband CD response than that of the structure composed of traditional metal Ag. Such a chiral structure provides a new method for the design of CD devices in the mid-infrared band.

Large-area mid-infrared broadband absorbers based on spiral ITO resulting from the combination of two different broadening absorption methods.

A large-area mid-infrared broadband absorber is proposed in this paper. The absorber is a spiral ITO structure grown on a hexagonal lattice arrangement of silicon nanopillars by using a glancing angle deposition method. The experimental results show that when the heights of the silicon nanopillars are 1.7 µm and the number of rotation depositions is n = 5, that is, the rotation angle is 150 degrees, the absorber absorbs more than 81% of electromagnetic waves in the 2.5-6 µm spectral range. In the atmospheric window of 3-5 µm, the integral absorption reaches 96%. The experimental results also show that the absorbing ability of the ITO structure in the mid-infrared atmospheric window is significantly stronger than that of the structure composed of silver under the same preparation conditions. The main reasons for the broadband absorption are that the spiral ITO structure has resonant absorption of electromagnetic waves with different wavelengths in the empty cavity regions with different sizes, and ITO has longer penetration depths than noble metals in the mid-infrared band, which brings about stronger broadband absorption. The combination of the two leads to a broadening of the total absorption spectrum. The higher heights of the silicon nanopillars enhance absorption further. Additionally, the loose spiral ITO distributions indicate lower mean plasma concentration and then increase penetration depths further, resulting in stronger light absorption. Such a large-area mid-infrared absorption structure with a simple preparation method has potential applications in mid-infrared cloaking and sensing.

A disposable paper-based hydrophobic substrate for highly sensitive surface-enhanced Raman scattering detection.

Detection of target analytes with high sensitivity and reproducibility remains a challenge for surface-enhanced Raman scattering (SERS) due to the lack of cost-effective and highly sensitive substrates. In this study, a hydrophobic SERS substrate capable of concentrating nanoparticles and analytes was prepared by spin-coating lubricating liquid onto commercial paper. The condensation effect of the paper-based hydrophobic substrate induced aggregation of gold nanoparticles (Au NPs) to generate ''hot spots'' for SERS and to drive analytes to the hot-spot areas for more sensitive detection. The obtained SERS signal intensity was 5-fold higher than that obtained using common paper, and a detection limit (LOD) of 4.3 × 10-10 M for rhodamine 6G (R6G) was achieved. Randomly selected points on the substrate and different batches of substrates all exhibited high reproducibility, and the relative standard deviation (RSD) at 1362 cm-1 is approximately 11%. A further application of the hydrophobic substrate was demonstrated by the detection of cytochrome C within a linear detection range of 3.90 × 10-8 M-1.25 × 10-6 M. In addition, the prepared substrate can obtain identifiable SERS spectra of cancer cells and non-cancer cells because a large number of AuNP or Au NPs clusters can adhere to cells, resulting in the construction of a 3D hotspot matrix. The disposable hydrophobic paper substrate eliminates the problem of solution diffusion, and also provides an effective platform for biomolecular screening detection.

L-shaped ITO structures fabricated by oblique angle deposition technique for mid-infrared circular dichroism.

This paper proposes a mid-infrared chiral structure, which consists of L-shaped indium tin oxide (ITO) films formed on self-assembled monolayer polystyrene microspheres in two orthogonal directions by oblique angle deposition technique. Experimental results demonstrate that the structure exhibit circular dichroism (CD) responses in the range of 2.5 - 4 µm. As the thickness difference of the ITO films in the two orthogonal directions increases, the CD response enhances. The reason is that the ITO films produce cross dipoles and their bigger differences in thickness bring to bigger phase differences in optical chirality. The experimental results also demonstrate that the CD signals are evidently stronger than those of the structure consisting of silver in the mid-infrared band. This work provides a new idea for the fabrication of mid-infrared chiral structures, which have potential applications in the polarization state control of mid-infrared lasers.

Recyclable three-dimensional Ag nanorod arrays decorated with O-g-C3N4 for highly sensitive SERS sensing of organic pollutants.

A three-dimensional (3D) substrate was developed by assembling a monolayer of graphitic carbon nitride (O-g-C3N4) on Ag nanorod arrays (Ag NRs) for sensitive and recyclable surface enhanced Raman scattering (SERS) detection. The prepared Ag NRs/O-g-C3N4 substrate not only generated a significant Raman enhancement effect as a result of the strong π-π stacking interaction between O-g-C3N4 and the analytes but also possessed excellent self-cleaning property via visible-light irradiation that was attributed to its outstanding catalytic performance. Highly sensitive SERS detection could be achieved with a LOD of 8.2 × 10-10 M for R6 G, and the substrate could be used repeatedly for at least four cycles with tolerable intensity attenuation. In addition, the 3D substrate exhibited long-term stability originating from the electron-donor effect of O-g-C3N4 and high reproducibility due to the uniform decoration of O-g-C3N4 on the Ag NRs through the strong interaction. Furthermore, using Ag NRs/O-g-C3N4, the recyclable detection of antibiotics in a water sample was demonstrated with high sensitivity, which indicates that the 3D Ag NRs/O-g-C3N4 substrate is a promising candidate for eliminating the challenges of single-use SERS substrates and building a portable SERS platform to sense organic molecular species.

Thin layer chromatography combined with surface-enhanced raman spectroscopy for rapid sensing aflatoxins.

A fast and facile method was developed for on-site detection of aflatoxins (AFs) in moldy agricultural products using thin layer chromatography combined with surface-enhanced Raman spectroscopy (TLC-SERS). Four different AFs were successfully separated by TLC and then a small portable Raman spectrometer, with gold colloids as the SERS-active substrate, was applied to identify the separated spots. TLC-SERS application to on-site detection of AFs was systematically investigated. Qualitative and quantitative AF detection was found to be easily accomplished and limits of detection were estimated to be 1.5 × 10-6, 1.1 × 10-5, 1.2 × 10-6, and 6.0 × 10-7 M for AFB1, AFB2, AFG1, and AFG2, respectively. The proposed method was also highly selective, enabling successful AF identifications in complex extracts from moldy peanuts. The study showed that TLC-SERS could be effectively used for separation and detection of these four AFs, demonstrating good prospects for on-site qualitative screening of agricultural products.

Growth and optical properties of Ag-Ti composite nanorods based on oblique angle co-deposition technique.

Ag-Ti composite nanorod structures with various Ag compositions were fabricated by the oblique angle co-deposition technique, and their optical transmission spectra are tuned by composition ratios of Ag and Ti, polarization directions, and deposition angles. Such tunable optical properties have potential applications in optoelectronics. Specially, for the Ag80 composite nanorod structures, there exists a wavelength, where it is isotropic. We also show that the transmission spectra of the Ag80 composite nanorod structure for the deposition angle of 87.5° are greater than 90%, while the transmission spectra for the 75° deposition angle are lower than 20%. Utilizing such a property, high or low transmission lenses can be designed.

Large-area broadband optical absorber fabricated by shadowing sphere lithography.

We report a large-area broadband optical absorber consisting of Ag/SiO2 stacked plasmonic layers fabricated on a self-assembly polystyrene sphere monolayer using the glancing angle deposition. Such an absorber can absorb more than 90% of light in the spectral range of 350 - 850 nm when the polystyrene spheres have a diameter of 750 nm. The broadband absorption is due to the overlap of localized plasmonic resonance wavelengths resulting from different patchy sizes and shapes of Ag coating on polystyrene spheres. Such a simple, flexible and large-area absorber has potential applications in light cloaking and energy conversion.

Switchable reflection/transmission utilizing polarization on a plasmonic structure consisting of self-assembly polystyrene spheres with silver patches.

We report a plasmonic structure for switchable reflection and transmission by polarization. The structure is composed of a hexagonal-packed polystyrene sphere array with silver patches on them. Simulations and experiments demonstrated that the conversions between reflected beams and transmitted ones can be performed when the polarization directions of incident beams vary from 0° to 90°. A switchable reflection and transmission at a given wavelength can be obtained, as long as sizes of PS spheres and azimuthal angles are properly chosen. Such a patchy plasmonic structure serving as a switch between reflection and transmission have potential applications in photoelectric control devices.

[Hydration of liquid ethanol probed by Raman spectra].

In order to study the concentration dependence of the molecular interactions in ethanol-water hydrogen bonded system, Raman spectra of ethanol-water mixtures with different water contents were obtained at room temperature. It was found that the positions of the 3 C-H stretching vibration bands of ethanol molecule located in the range of 2 800-3 050 cm(-1) would generally present blue shifts when more water was added into the mixture; at the same time, however, the C-O stretching vibration band located at around 1 048 cm(-1) showed an opposite behavior. The different hydration, which is induced by hydrogen bonding, which happened in different concentration mixtures, was thought to be responsible for this situation, the hydration process of liquid ethanol was thus deduced: when adding a small amount of water into pure ethanol, and clusters containing one water molecule and several ethanol molecules were formed instead of ethanol self-association short chain clusters existing in pure ethanol; the clusters would combine more water molecules to form ringlike clusters through hydrogen bond association when adding more water into the mixture, then a temporary saturation would be seen when the volume percent of water reached 50%, and this saturation state would last until the water content reached 70 vol%; after that, the large number of water molecules would dissociate the ringlike clusters to smaller clusters and then associate to the ends of these dissociated clusters through hydrogen bonding; in addition, the improper hydrogen bonding between oxygen atom of water molecule and C-H bond of ethanol molecule is considered to be formed after the content of water reached a high value.

[Study on the elements of the fluorescence spectra emitted from ethanol-water mixture].

The 236 nm UV-light was adopted to excite the 10 ethanol-water mixture samples in which the volume percent of ethanol ranges from 10% to 95%, and the Gaussian decomposition method was used to decompose the fluorescence spectra obtained above to Gaussian curves, where every Gaussian curve can be attributed to the fluorescence emission of a certain kind of luminescent ethanol-water cluster, which is formed by the association between water molecules and ethanol molecules. It was found that every spectrum of the certain binary mixture contains 8 Gaussian elements, and the emission density and peak wavelength of each element were also obtained. The authors studied the mechanism of the fluorescence emission of the mixtures based on the structural characters of fluorescence substances and concluded that the water molecules play an important role in the fluorescence emission: they enlarge the conjugate system as the electron donors; they bridge the ethanol molecules to form 8 kinds of new clusters at the same time, and the conformation of the clusters is rings and chains composed by several rings. Through the peak wavelengths of the Gaussian elements decomposed from the certain concentration spectrum the authors got the information about the relative size of the new clusters, i.e. the clusters are bigger or show chain structures when the peak wavelength of the Gaussian curves is located at the longer positions. The emission transition energies of the 8 kinds of new clusters were also calculated. In addition, it was found that there was a certain relation between the half-width and the association situation. The research contributes to the study of ethanol-water cluster structures and their physical and chemical characteristics.