Photonic materials and techniques for SERS and solar cell light trapping

Resume : The supply of clean, renewable energy is the most important step towards preventing climate change. Photovoltaic devices can convert the abundant power of the sun into electrical energy and thus, deliver carbon dioxide emission free energy. Increasing the efficiency of solar cells will decrease the cost of electricity generated by photovoltaic power plants and lead to economically profitable large scale application. Improving state of the art solar cells requires optimal photon management. This talk will give an overview of different strategies for photon managements pursued in the Atwater research group and present the advances achieved in the field of effectively transparent contacts. A significant portion of the incoming photons is lost at the front contacts of solar cell due to parasitic absorption within the transparent conductive oxide or reflection at contact grid fingers. Better transparency usually leads to deteriorate conductivity which in turn leads to electrical losses in the solar cell. We designed a new contact principle that overcomes shadowing losses and parasitic absorption without decrementing the conductivity. By redirecting the light to the active area of the solar cell micro-scale triangular cross-section grid fingers (ETCs) achieve a measured transparency of up to 99.9 %. Due to their close spacing (around 100 µm) ETCs can replace grid fingers as well as transparent conductive oxides (TCO). We show in simulations and experiments that ETCs can increase the short circuit density in solar cells by around 6 % without discriminating the fill factor.

Resume : Antireflective layers fabricated with subwavelength structures (SWSs) dramatically improve the performance of solar cells by gradually changing the refractive index at broad wavelengths. However, nanolithography techniques for fabrication of SWSs such as e-beam or laser interference lithography are very expensive, complex and not suitable for a large area. In this work, we fabricated Si SWSs of nanohole random arrays on Si substrates. To further reduction of the reflectance, micro-scale inverted pyramid pit arrays were first formed before the nanohole texturization. Nanoholes were formed onto these inverted pyramids by agglomeration of Au thin films and metal-assisted chemical etching (MacEtch) in H2O2/HF aqueous solution. MacEtch followed by metal agglomeration is a new cost-effective anisotropic etch processes. We optimized the feature size of SWSs by controlling agglomeration temperature and etching time. A significant reduction in the reflectance was acquired for 1.1mm height of SWSs. The specular reflectance, which is important for high quantum efficiency of solar cells in a wide angle of incidence, exhibited a strong dependency on the angle of incidence. Results showed that extremely low total reflectance of 2.9% and average reflectance of below 3.6% up to angle of incident of 60 degrees in the wavelength range of 200-1100nm. Si SWSs fabricated with metal-assisted chemical etching combined with metal agglomeration have dramatically improved the reflectance characteristics of antireflective layer with simple and low-cost processes.

Resume : Silver nanowires (Ag NWs) technologies are promising conductive transparent electrodes for replacement of indium tin oxide (ITO) and fluorine-doped tin oxide (FTO). Above all, the junction of metallic nanowires has been an important issue in the bottom-up fabrication of organic solar cells s. Welding technologies are currently required to modulate the heating mechanism appropriately. In this study, microwave-assisted welding was introduced in order to improve the optical transmittance and electrical conductivity of Ag NWs in terms of simple, selective, and rapid processing. Besides, Ag NWs enhance the power conversion efficiency (PCE) of organic solar cell. Because, the Ag NWs absorb light strongly at the plasmon resonance condition. This method includes only two steps for fabricating Ag NWs electrodes. First, Ag NWs dispersed in a solvent were coated on polymer substrates using spin coating. Subsequently, the Ag NWs-coated substrates were exposed to microwave radiation for varying exposure times of 1 min, 2 min, and 3 min and the resulting optical transmittances and electrical conductivities are compared. To examine the suitability of thus welded Ag NWs for flexible substrate electrodes, bending tests were performed. Following these tests, we found that both the optical transmittance and electrical conductivity were preserved. Consequently, the sheet resistance was reduced by 40% and it could be shown that welded Ag NWs have good mechanical strength, even after bending test. The highly flexible and transparent electrodes can be mounted on any non-planar surfaces and applied for various for future flexible electronics.

Resume : Flexible solar cells offer many advantages, such as high throughput production by roll-to-roll manufacturing, versatility regarding shapes and size, and also exceptionally light weight. To minimize costs of the device, flexible transparent polymer substrates such as polyethylene terephthalate (PET) is a convenient alternative. For these types of substrates, usually a barrier layer is necessary to prevent the diffusion of moisture and/or oxygen into the photovoltaic device. An important aspect to achieve high efficiencies in thin-film solar cells is the light management concept. Wet-chemical etching of aluminum doped zinc oxide (ZnO:Al) has been shown to result in light trapping textures that significantly enhance the device performance due to improvement in the short circuit current density. In this study, we investigate the wet-chemical etching and resulting light scattering properties of aluminum-doped zinc oxide, deposited at low temperature (< 140 °C) on PET substrates. Special emphasis is put on the influence of a thin (20 nm) barrier layer of zinc tin oxide (ZnSnOx) on subsequent growth and properties of ZnO:Al layers. We evaluate the electrical, optical and light scattering properties of ZnO:Al layers (textured by chemical etching in in 0.5 % HCl solution) grown on PET and PET/ZnSnOx substrates. Our results demonstrate that in the case of the PET/ZnSnOx substrate a homogeneous crater-like structure, suitable for light trapping in solar cells, evolves. In contrast, on bare PET substrates, the ZnO:Al layer etches rather inhomogeneously and partly completely down to the substrate, suggesting presence of cracks or voids in the bulk of ZnO:Al. We show that performance of amorphous silicon solar cells is significantly enhanced, when a ZnSnOx barrier layer is used, leading to an improvement in Jsc by 2.3 mA/cm² compared to a solar cell on bare PET substrate.

Resume : In this presentation we give an overview of the current approaches we employ in our laboratory to fabricate plasmonic nanostructures exhibiting both spectroscopic versatility and bio-specificity toward implementation in nanomedicine. Firstly, a large variety of plasmonic nanostructures are fabricated on solid substrates (films) by using self- or template-assisted assembling of nanoparticles or nanoimprint lithography. The second route involves chemical synthesis of gold or silver nanoparticles of controlled size and shape (rods, prisms, stars-shaped) coated by biopolymer (chitosan, poly(ethylene) glycol, pluronic, gelatine) to provide both the right optical response and good biocompatibility. For instance a class of biocompatible “optically hot” nanoparticles are efficiently used for both spectroscopic intracellular investigation via SERS and plasmonic-induced hyperthermia. An interesting result represents the demonstration of gold nanorods acting as dual-modal spectroscopic enhancers in SERS and metal-enhanced fluorescence (MEF). Chitosan-coated triangular silver nanoparticles can operate as dual-modal sensors via surface plasmon resonance (LSPR) and SERS, both in solution and on solid substrate and perform intracellular imaging via SERS or trigger localized hyperthermia in tumors. Recently we demonstrate the multimodal activity of small aggregates of gold nanoparticles as both SERS and FLIM nanoprobes and drug delivery carriers. Currently we focus on development of SERS-based plasmonic nanoprobes conjugated to reporters and drugs to perform imaging, diagnostics and therapy (theranostics).

Resume : Plasmonic properties of gold nanostructures deposited on a dielectric substrate have been widely studied. However when the nanostructures are deposited on a gold film, the optical properties change drastically because of the propagation of the surface plasmon and its coupling with the grating and the localized plasmon resonance of the structure. We thus studied the far-field, with extinction measurements and near-field properties with SERS measurements of such substrate, and we compared them with the same structures deposited directly on an ITO substrate. We characterised them at different excitation wavelengths and by tilting the sample to excite different modes. First we demonstrate that the SERS intensity exhibits strong variation depending on the excitation angle and its maximum for specific angles corresponding to the excitation of Bragg modes. This result was confirmed by Finite elements method simulations. Second, we determine the localisation of the field enhancement by the addition of a silica cap on the top of the nanostructures to prevent the probe molecules to graft on this part of the structure. It was observed that for samples deposited on gold film, there is no difference in the SERS intensity between capped samples and uncapped ones. Indicating all the SERS signals comes from the sides of the nanodisks. However, in the case of ITO substrate, we can observe a decrease of more than half of an order of magnitude when the samples are capped with silica. This indicates that on those samples, most of the signal comes from the top of the nanostructures.

Resume : Surface enhanced Raman scattering (SERS) on plasmonic silver (Ag) surfaces with nanorough morphology is a phenomena providing an extremely sensitive Raman spectroscopic investigation of numerous biological compounds. However Ag nanostructures are known to demonstrate SERS-activity at the blue-green excitation wavelengths while red and near-IR ones are more preferable for the study of bioorganic species. This limitation is caused by the position of the surface plasmon resonance (SPR) band of Ag in the short wavelength range of the visible spectrum. In present work we report on fabrication and characterization of SERS substrates based on silvered macroporous silicon (macroPS) which can be used for the ultrasensitive detection of organic molecules at the excitation wavelengths varied in the visible and near-IR ranges. MacroPS was fabricated by anodization of p-Si wafer in a HF-based solution. The depth and diameter of the macropores altered from 500 to 1000 nm meeting the requirements for the dimensions of so-called plasmonic nanovoids. Thus further covering of macroPS with Ag by electroless deposition led to the formation of Ag nanovoids. The mechanism of the SPR broadening and SERS-signal increase due to surface plasmon mode of strong ring component in the area of pore entrance and contribution from multiple rays’ reflection inside the nanovoid is discussed. A detailed description of the Ag nanovoids and an interpretation of SERS spectra of different analytes are presented.

Resume : Raspberry-like nano-objects made of large plasmonic satellites (>20 nm) covering a central dielectric particles have many potential applications as photonic materials, superlenses as well as (bio-) sensing, but their synthesis remains challenging. Herein, we show, how to build a stable and robust raspberry-like nano-system with close-packed satellites, by combining monodisperse silica particles (80 and 100 nm) and oppositely charged noble metal nanoparticles (Au or Ag) with well-defined sizes (10-50 nm). Their spectral characteristics (wavelength, linewidth, extinction cross-section) have been measured using the spatial modulation spectroscopy technique (SMS) and interpreted with a numerical model. The plasmonic nanostructures exhibit numerous hot spots at satellite junctions contributing to the excellent surface-enhanced Raman scattering (SERS) performance of the composite materials. The spectroscopic investigation indicates that the SERS efficiency is highly dependent on the dimension and nature of plasmonic satellites.

Resume : In order to reduce the material cost for silicon solar cells, imec is investigating the feasibility of making cells on very thin monocrystalline silicon foils. We proposed in the past the so-called i2-module approach, which aims for module-level processing (many substrates in parallel) of thin silicon foils bonded to a large glass superstrate. The substrates used for this concept are high-quality epitaxial foils lifted off from a parent silicon substrate using porous silicon made by electrochemical etching. The porous silicon serves both as seed for epitaxy and as detachment layer. In this paper, we will show that by optimizing the porous silicon, we have managed to obtain 100μm thick silicon foils with effective lifetimes up to 1.3ms, and 40μm thick foils with effective lifetimes up to 700μs, both with high detachment yield. We will also review the current status of the process development for solar cells made from these thin foils. In a first approach, heterojunction solar cells are fabricated on freestanding epitaxial wafers of 40μm thickness. In a second approach, heterojunction back-contacted cells are fabricated on thin foils that are bonded to a glass superstrate. Challenges for device processing and limitations in cell performance will be discussed. Finally, we will show how nanophotonics can be used to improve the current density of the solar cells made from these thin silicon foils by nanotexturing the front surface of the foils.

Resume : The structural properties of Cu2ZnSn1-xGexS4 (CZTGS) bulk crystals prepared by solid state reaction have been investigated by means of X-ray diffraction. Depending on the germanium content, it has been found that, whereas Cu2ZnSnS4 (x=0) crystalizes in the tetragonal structure, the Cu2ZnSn1- xGexS4 system constitutes a solid solution and adopts the orthorhombic structure for x > 0.25. Below this value, a two-phase region exists. Quaternary chalcogenides with formula I2–II–IV–VI4 have recently attracted much attention due to their anisotropic structures, nonlinear optical properties and energy clean conversions. Numerous applications fields, such as energy, environment and opto-electronics, make them potential candidates as thin-film solar cells absorber layers. Members of one group of materials, copper-based quaternary semiconductors, namely Cu2ZnSn(S,Se)4 (CZTSSe) has achieved an efficiency of 12.6 %. It remains far below the Cu(In,Ga)Se2 (CIGSe) solar cells efficiency (21.7%). Such enhancement of the efficiency for the later is mainly related to the absorber band-gap which varies with the Ga/In substitutions. In the case of CZTSSe material, partially substituting Ge by Sn should lead to an analog result. CZTS compound crystallizes in the tetragonal structure while CZGeS may crystallizes in the tetragonal stannite-type structure (space group ̅ ) or in the orthorhombic structure (space group Pmn21). In this study, we investigate the effect of the Ge content on the structural, vibrational and morphological properties of Cu2ZnSn1-xGexS4 (CZTGS). The CZTGS compounds were synthesized by solid state reaction with various Ge/Sn substitutions. Based on the X-ray analysis, we found that the Cu2ZnSn1-xGexS4 orthorhombic structure type is present for x > 0.25. In this case, all of the lattice parameters (a, b, c and volume) obey the Vegard’s law, and let us confirm that the Cu2ZnSn1-xGexS4 system constitutes a solid solution with space group Pmn21. However, for x < 0.25, a two-phase region was observed. Nevertheless, additional studies are in progress to exactly determine the miscibility gap region.

Resume : Recent years scientific community attention has been transferred from porous silicon to arrays of Si nanowires (SiNWs) formed by metal-assisted chemical etching (MACE) due to high perfection of its crystal lattice and surface properties. Usually in MACE hydrofluoric acid (HF) was used. But in this work we tried to use green chemistry and HF was changed on NH4F. This procedure made MACE safer and environmentally friendly. Optical properties (total reflectance, interband photoluminescence (PL) and Raman scattering) of SiNWs formed by MACE using green chemistry were investigated. The total reflectance of c-Si is near 30%-40% for the visible spectral range, opposite to SiNWs, which exhibit a strong decrease of the total reflectance. The Raman scattering intensity increases strongly for SiNWs in comparison with the corresponding value of c-Si substrate. Also PL in visible spectral range from SiNW arrays was observed. All these results suggest similar optical properties of SiNWs obtained using green chemistry to the SiNWs obtained by the standard MACE method. It gives an opportunity to obtain high quality SiNWs for various applications in photonics and photovoltaics using green chemistry.

Resume : High refractive-index dielectric nanostructures provide original optical properties thanks to the occurrence of size- and shape-dependent optical resonances. They have thus attracted increasing interest as low-loss alternatives to plasmonic particles. Likewise, nonlinear optical effects play an important role in photonics, providing various functionalities such as harmonic generation or all-optical signal processing. Semiconductor nanostructures are promising for enhancing intrisically weak nonlinear effects due to strong electric fields at resonant optical modes. We focus on Second Harmonic Generation (SHG) from silicon nanowires (SiNWs), which is forbidden in the bulk of the centrosymmetric Si lattice. Hence, SHG can occur only if this symmetry is locally broken, for instance at interfaces or due to strong field gradients. Such limitations can be overcome in SiNWs, thanks to the high surface-to-volume ratio together with field enhancement by resonant modes. We thus show that SHG can be enhanced by two orders of magnitude compared to bulk Si. We also show that surface and bulk-like contributions to SHG can be distinguished by appropriate polarization selection rules. Finally, we observe a size-dependent transition in the contributions to SHG. In small NWs SHG from field gradients in the bulk dominates, while in larger NWs surface contributions are mainly responsible for SHG. In summary, Si nanowires provide a useful tool for nonlinear silicon photonics.

Resume : We report on a novel scheme [1] that exploits the radiation pressure to locally push gold nanorods and induce their aggregation in buffered solutions of biomolecules, achieving biomolecular SERS detection at almost neutral pH. The sensor is applied to detect non-resonant amino acids and proteins, namely Phenylalanine, Bovine Serum Albumin (BSA), Lysozyme, Catalase, Hemoglobin, reaching detection limits in the µg/mL range. Quantitative detection of protein is demonstrated for BSA, down to 50 mM concentrations. Our methodology is chemical free , easy to implement, fast to operate, and has potential for integration in microfluidic circuits for biomarkers detection applications. [1] B. Fazio et al., Sci. Rep. 6, 26952 (2016).

Resume : We report that using Ag nanorods as effective surface-enhanced Raman scattering substrates, one can recognize chemicals at trace levels based on their Raman spectra fingerprints, where the degradation of the SERS sensitivity is a major concern. We found that by covering Ag nanorods with ultra thin Al2O3 layers as SERS substartes, its sensitivity, thermal stability, chemical stability, reusability, etc, can be greatly improved. This make Ag nanorods work as SERS substrates even in very harsh environments and corrosive chemicals. Based on the principle components analysis methods, we proposed a triangle rule and a balance rule to calculate the composition of chemical mixtures at trace levels, with which one can easily recognize chemicals at trace levels, and calculate the relative compositions of the chemical mixtures. Based on the partial least square regression method, one can calculate quantitatively the amount and composition of chemicals at trace levels. This make a step forward to the real application of SERS in the chemical detections/sensing devices.

Resume : This work reports on formation of plasmonic metal nanostructures by porous silicon (PS) templating, their properties and application as substrates for surface enhanced Raman scattering (SERS) spectroscopy. We present technological features, which allow to manage surface plasmon band position from blue to near-IR range and to meet principal requirements to SERS substrates such as ultralow limit of detection, spot-to-spot and sample-to-sample reproducibility, long shelf life. Special attention is focused on composition and morphology of the metal nanostructures (dendrites, nanoparticles, nanorods, nanovoids), which can be grown by variation of metal (Ag, Ni, Cu) and PS types (micro, meso, macro). It is shown that PS acts not only as a template for definition of dimensions, shape and mutual arrangement of the metal nanostructures, but also highly improves their stability to oxidation. SERS measurements were performed with 3D scanning laser confocal Raman microscope Confotec NR500. It provided accurate definition of potential “hot spots” location in the substrates and study of kinetics of the Raman signal degradation upon laser excitation. The PS-based SERS substrates demonstrated the limit of detection ranging from nano- to femtomolar concentration depending on the type of analyte. In conclusion, advantages of SERS substrates based on PS such as simplicity, cost-effectiveness and compatibility with silicon technology in comparison to other nanoengineered ones are considered.

Resume : In this paper, we successfully fabricated periodic silver Nano-arrays (SNAs) fabricated on porous anodic titania template by a simple physical exfoliation method as the surface enhanced Raman and fluorescence substrate. SERS was investigated for low concentration R6G which was deposited on the SNAs. SEF was proved by PL spectrum of R6G and the signal of cytoskeletal structure which was labeled by fluorochrome. The experiment showed that the Raman signal and fluorescence strength achieved greatly enhanced and the fluorescence signal of cytoskeleton increased obviously, stronger photo-stability compared with the smooth silver film. Moreover, the template could be repeatedly used to ensure the reproducibility and stability. This method provides a sensitivity and inexpensive technique to create period silver Nano-arrays for various applications such as Medical diagnosis and biosensor.

Resume : Due to the growing need in detection of molecules of organic and non-organic nature at low concentrations, the active design of universal substrates for Surface-Enhanced Raman Scattering (SERS) is carried out. Besides this, researchers move toward optimizing the parameters of the substrates – particles size, their aggregation and arrangement on the surface. One of the most prevalent direction is a development of affordable substrates with colloidal gold and silver nanoparticles for express diagnostics of certain molecular species, that have to consist with the optical parameters of analyte and experimental conditions. SERS-substrates prepared by means of the Tollens' reaction (the mirror reaction) were used for the detection of low concentrations of biological species (Adenine, amino acids, Hemoglobin). Thin island-like films consisting of silver nanoparticles were formed on glass substrates with the Tollens' reaction. The efficiency of SERS-enhancement is known to be very sensitive to the morphology of the silver film. Also, the typical size of the distinct nanoparticles and it`s aggregates made an impact on plasmonic properties of the SERS-substrate. The surface morphology was varied by reaction conditions (reagent concentration and reductant character) and was controlled by Atomic Force Microscopy (AFM) after preparation. Different ways of interface formation between substrate surface and silver film in order to improve SERS substrate reproducibility were studied.

Resume : Directional plasmon excitation and SERS emission were demonstrated for 1D and 2D gold nanostructure arrays deposited on flat gold layer. Extinction spectrum of both arrays exhibits intense resonance bands that are red shifted when the incident angle is increased. Systematic extinction analysis of different grating periods revealed that this band can be assigned to a propagated surface plasmon of the flat gold surface that fulfills the Bragg condition of the arrays (Bragg mode). Directional SERS measurements demonstrated that the SERS intensity can be improved by one order of magnitude, when the Bragg mode positions matched with either the excitation or the Raman wavelengths. Hybridized numerical calculations of Finite Element Method and Fourier Modal Method also proved the presence of Bragg mode plasmon and illustrated that the enhanced electric field of the Bragg mode is particularly localized on the nanostructures regardless of their size.

Resume : Plasmonic properties of arrays of gold nanodisks were investigated in order to study the localization of the near-field enhancement at the surface of the nanodisks. To measure the near field, Surface enhanced Raman spectroscopy (SERS) was performed on different kind of substrates with and without an over layer of silica that disabled the possibility for our SERS probe to adsorb on the top of our structures. Thanks to this we were able to determine, depending on the substrate, which fraction of the SERS enhancement was coming from the top of our structures. We demonstrate that the near-field enhancement is mainly localized at the top of the nano disks in the case of ITO substrate whereas it comes from the nanodisk side for gold substrate.

Resume : This work reports on highly efficient surface enhanced Raman spectroscopy (SERS) constructed on low-cost, fully recyclable and highly reproducible cardboard plates, which are commonly used as disposable packaging material. The active optical component is based on plasmonic silver nanoparticle structures separated from the metal surface of the cardboard by a nanoscale dielectric gap. The SERS response of the silver (Ag) nanoparticles of various shapes and sizes were systematically investigated, and a Raman enhancement factor higher than 106 for rhodamine 6G detection was achieved [1]. The spectral matching of the plasmonic resonance for maximum Raman enhancement with the optimal local electric field enhancement produced by 60 nm-sized Ag NPs predicted by the electromagnetic simulations reinforces the outstanding results achieved. Furthermore, the nanoplasmonic SERS substrate exhibited high reproducibility and stability. The SERS signals showed that the intensity variation was less than 5%, and the SERS performance could be maintained for up to at least 6 months. [1] Andreia Araújo, Carlos Caro, Manuel J Mendes, Daniela Nunes, Elvira Fortunato, Ricardo Franco, Hugo Águas and Rodrigo Martins, Highly efficient nanoplasmonic SERS on cardboard packaging substrates, Nanotechnology 25 (2014) 415202.

Resume : Surface enhanced Raman spectroscopy (SERS) is a label-free and non-destructive vibrational spectroscopy technique that allows for ultrasensitive and fast structural detection of trace-level molecules through the enhanced electromagnetic (EM) fields amplified by the excitation of localized surface plasmon Resonances (LSPR). Compared to conventional rigid SERS substrates, e.g. glass and silicon, flexible substrates provide flexibilities of configurable assembly which is critical to the applications such as on-line trace level detection. In addition, large-scale production is another concern for reducing cost. In this work, we demonstrate a new type of double-sided, large-scale, and flexible metasurfaces for ultrasensitive, highly uniform, polarization-independent SERS detection. The SERS substrates are fabricated by simply sputtering densely packed gold nanoparticles on porous polycarbonate membranes which composed of conical pores created by asymmetric etching of ion tracks. Morphological characterizations reveal that the conical pores are uniformly distributed on both sides of membranes and uniform in shape and size. The porous polycarbonate membranes are covered with granular gold films which typically have sub-5 nm gaps among adjacent particles. Such metasurfaces show ultrasensitive SERS detection limit down to 10 pM which is comparable to the reported highest value (1 pM) for flexible substrates. The high uniformity is reflected by a low relative standard deviation (RSD) ~10% over the entire wavenumber range (300-1800 cm-1). Numerical simulations disclose that the enhanced EM fields of granular gold films are further amplified by conical pores. Such hierarchical metasurfaces represent a step forward to low-cost, high performance, and reliable flexible platforms for ultrasensitive SERS detection.

Resume : The development of Mid-IR sensors for monitoring the concentrations of organic pollutants in the aquatic environment is currently a challenge of great importance. Mid-infrared range (4000-400 cm-1) contains the absorption bands related to the vibrations of organic molecules. The mid-infrared sensor based on evanescent wave spectroscopy is a promising analytical tool for simultaneous detection and quantification of a variety of pollutants such as hydrocarbon compounds. Chalcogenide glasses are particularly well adapted for sensing applications due to their high refractive index (between 2 and 3). The aims of this study were to synthetize chalcogenide thin films for developing mid-IR optical integrated platforms and perform their functionalization by polymers in order to increase the sensor sensitivity. Among (GeSe2)100-x(Sb2Se3)x glass compositions, two selenide glass targets were chosen for their mid-IR transparency, stability against crystallization and refractive index contrast suitable for mid-IR optical wave-guiding. Infrared selenide waveguides were fabricated by radiofrequency magnetron sputtering and functionalized by a polyisobutylene polymer. By means of attenuated total reflection spectroscopy, measurements were performed in water to detect aromatic hydrocarbons (benzene, toluene and three xylene isomers) in the concentrations range of 10 ppb to 40 ppm. Consequently to minimize the water absorbance, the ZnSe prism surface was successfully modified by a thin hydrophobic coating. Moreover, hydrocarbons detection was also carried out using selenide thin film deposited on ZnSe prism surface to mimic and assess the efficiency of a chalcogenide optical platform. Detection measurements have been fulfilled using sea and ground water. To increase the sensor sensitivity, the use of metallic nanoparticles is one of the promising solutions based on Surface Enhanced Infrared Absorption. Hetero-structures combining gold nanoparticles/chalcogenide glass and waveguides were fabricated and characterized. To assess Surface Enhanced Infrared Absorption, we used a self-assembled monolayer of 4-nitrothiophenol randomly oriented with strongest bands located at 1336 and 1512 cm-1 attributed to NO2 symmetric and antisymmetric stretching modes, respectively. Using various morphologies, thicknesses or inter-particles coupling, an enhancement factor varying from 25 to 106 was calculated to detect 4-nitrophenol. References : J. Charrier et al "Evanescent wave optical micro-sensor based on chalcogenide glass," Sensors and Actuators B-Chemical 173, 468-476 (2012) F. Verger et al, "RF sputtered amorphous chalcogenide thin films for surface enhanced infrared absorption spectroscopy," Optical Materials Express 3, 2112-2131 (2013). F. Verger et al "Surface enhanced infrared absorption by nanoantenna on chalcogenide glass substrates," Appl. Phys. Lett. 106 (2015) Aknowledgement : Authors are thankful to ANR for LOUISE project funds.

Resume : Surface-enhanced Raman spectroscopy (SERS) is universal method for analysis of small quantities of organic analytes with high sensitivity [1]. It is a multiple times enhancement of the Raman signal from the analyte molecules located on a substrate, comprising individual nanoparticles or nanostructured films of noble metals. As a result of laser irradiation on the metal surface plasmons generated by increasing the electric field around the metal, which increases the Raman signal intensity up to 1011 times. It is remarkable that different analytes absorb (and fluoresce) at different wavelengths, and so lasers of different energies should be used. Therefore, an urgent task is to form a substrate for SERS spectroscopy which has multiband plasmon resonance in a wide energy range. In this project, gold inverse opals with wholly or partly hexagonally ordered pores are proposed as SERS-active substrates. The aim of this work is the formation of gold inverse opals using different electrolytes, study of the correlation of optical properties of the films with their morphology peculiarities, as well as the use of obtained samples as SERS-active substrates. Synthesis of inverse opals consists of several stages: obtaining polystyrene microspheres, formation of colloidal crystals, electrocrystallization of gold in pores of matrix, and its subsequent removal [2]. The quality of the samples at each step influences the final structure of the porous gold film. Au inverse opals were prepared by electrochemical deposition of gold from chloride, citrate or sulfite electrolytes into ordered matrices consisting of microspheres with diameter D = 400 and 530 nm, as well as into disordered matrices consisting of microspheres with D = 200, 300, 500 and 600 nm. (with standard deviation less than 5%). It should be added that the electrolyte composition may affect on particles size, and hence, on plasmon resonance bands. In current project optimal parameters for ordered colloidal crystals formation are found (temperature, concentration of suspension, electric field intensity). Based on them, Au inverse opals are obtained with various normalized thicknesses in the range 0.05 ÷ 1.1 of microsphere diameter in the matrix. For electrodeposition of Au, optimal conditions are found: chloride electrolyte, deposition in potentiostatic mode at Ed = 0.8 V versus Ag / AgCl reference electrode. Figure 1. SEM data of gold inverse opals. Measuring of reflection spectra was performed by varying incidence angles and azimuthal angles. On reflection spectra of the samples a number of various plasmon resonance features are observed. Resonances are associated with the presence of local plasmons of individual particles, Mie and Bragg plasmons in a mixed state [3]. The condition of Mie and Bragg plasmon excitation is determined only by surface morphology depending on period and thickness of porous gold films. Intense local plasmon resonance is typical for all gold films of different crystallinity and its profile depends on grain size varied in the range of 20-50 nm and through this on reflectivity characteristic in 400-1000 nm range. Using XPS method surface composition of the gold films was analyzed to reveal presence of adatoms which could result from different electrolyte compositions. The Raman data, obtained on substrates, show an increased intensity of the Raman signal from fluorescent dyes rhodamine 6G and methylene blue at concentrations up to 10-10 M (volume of aliquot ~ 1 mkl) by their excitation with green (λ = 514 nm) and red (λ = 633 nm) lasers, respectively. The spectra obtained from different parts of the substrates are identical, suggesting about the reproducibility of samples and surface uniformity of substrates. The enhancement factor in the case of methylene blue is G = 106 for most of the concentrations of aliquote. After washing substrates with distilled water, microstructure of the substrates reveals no changes, and SERS effect is observed, that demonstrates the SERS-active substrate to be renewable. Authors wish to acknowledge their colleagues, namely, Dr. K.S. Napolskii and Dr. N.A. Sapoletova for their assistance with experiments and fruitful discussion of experimental results. Authors gratefully acknowledge the support of the Russian Foundation of Basic Research under grant No 15-33-70050-mol_a_mos. References [1] Semenova A A, Goodilin E A, Brazhe N A, Ivanov V K, Baranchikov A E, Lebedev V A, Goldt A E, Sosnovtseva O V, Savilov S V, Egorov A V, Brazhe A R, Parshina E Y, Luneva O G, Maksimov G V, Tretyakov Y D. 2012 J. Mater. Chem. 22 24530 [2] Sapoletova N A, Martynova N A. Napolskii K S, Eliseev A A, Lukashin A V, Kolesnik I V, Petukhov D I, Kushnir S E, Vassilieva A V, Grigoriev S V, Grigoryeva N A, Mistonov A A, Byelov D V, Tretyakov Y D. 2011 Physics of the solid state 53 (6) 1126-1130 [3] Kelf T A, Sugawara Y, Cole R M, Baumberg J J, Abdelsalam M E, Cintra S, Mahajan S, Russell A E, Bartlett P N. 2006 Phys. Rev. B 74 (24) 245415

Resume : Paper substrates, covered with ZnO nanorods (NRs) coated with Ag nanoparticles (NPs), allowed the production of inexpensive, high-performing and highly reproducible SERS platforms. The ZnO NRs were synthesized by a simple, fast and low-temperature hydrothermal method assisted by microwave radiation and made SERS-active by decorating them with a dense array of silver nanoparticles deposited via a single-step thermal evaporation technique.1 Using Rhodamine 6G as the probe molecules, with a concentration down to 10-9 M, the SERS substrates present up to 107 Raman enhancement with good reproducibility. The increase of the sensing area, provided by the ZnO NRs scaffold, dramatically increases the density of hot spots responsible for SERS and is, therefore, key to the pronounced Raman enhancement. The results demonstrate that plasmonic paper, covered with Ag NPs@ZnO NRs, is an efficient SERS platform with the advantages of being recyclable, flexible, lightweight, portable, biocompatible and extremely low-cost. Araújo, A.; Caro, C.; Mendes, M. J.; Nunes, D.; Fortunato, E.; Franco, R.; Águas, H.; Martins, R. Highly Efficient Nanoplasmonic SERS on Cardboard Packaging Substrates. Nanotechnology 2014, 25 (41), 415202.

Resume : The role of dielectric surrounding on Au nanostructure for surface plasmon resonance (SPR) behavior is investigated by scanning near field optical microscopy (SNOM). The near field optical field strengths are correlated with the surface enhanced Raman scattering (SERS) enhancement factors. Discontinuous nanostructured Au thin films are deposited by RF magnatron sputtering at very slow rate on three different dielectric materials, ZnO, TiO2 and SiO2. The discontinuity of Au nanostructure was confirmed through scanning electron microscopy (SEM) and atomic force microscopy (AFM). Transparent, continuous and smooth surface dielectric films of ZnO and TiO2 are grown onto borosilicate glass (BSG) substrate by two different techniques. ZnO films are deposited by thermal evaporation of pure ZnO powder. TiO2 films are obtained by the spin coating of the sol-gel derived solvents. Both films are heat treated to appropriate temperatures, to obtain an optimized situation of relatively good crystallization and low surface roughness. The crystal phase formation and surface roughness are investigated by x-ray diffraction and AFM technique simultaneously. A bare BSG substrate of 150 μm thick is used as the third dielectric surrounding in the present study. The role of dielectric material on the SPR behavior of Au/dielectric nanostructure is demonstrated by recording the SPR absorption in the range of 200-1000nm. These three Au/dielectric nanostructures are investigated using SNOM by illuminating it in near field and collecting in transmission far field configuration. The observed optical near field images of the three different nanostructures are discussed by taking their dielectric constant into the account. These three Au/dielectric nanostructures are investigated for SERS application using Rhodamine 6G dye molecule. The calculated SERS enhancement factors are compared with the optical field strengths derived from the near field optical imaging.

Resume : The interplay between metal enhanced fluorescence/quenching and surface enhanced Raman scattering (SERS) is a factor of paramount importance in determining the applicability of a substrate for SERS of fluorescent molecules. In this contribution we present complimentary studies of fluorescence and SERS detection on graphene oxide (GO) and Ag nanoparticle-graphene oxide (AgGO) composite substrates for three organic dyes; Rhodamine 6G (R6G), Rhodamine B (RhB), and Sulforhodamine 101 (SR101). The three dyes have different quantum yields and overlap different regions of the collective Ag nanoparticle extinction spectrum. While all three dyes exhibit fluorescence quenching on the AgGO composite, R6G and SR101 demonstrate larger SERS peak signal-to-noise ratios (SNRs) and enhancement factors (EF) on the AgGO substrate than observed for RhB. The influence of fluorescence quenching on the SERS detection was explored through characterisation of the dyes’ spectra and time-resolved emission on the GO and AgGO substrates. Strong correlation was observed between the SERS SNR and time-resolved photoluminescence data, and the factors contributing to the overall enhanced SERS detection for each dye could be elucidated. It is seen that emission quenching by the GO component is a more significant factor in the SERS SNR and EF for R6G than for SR101. The time-resolved PL data also shows that SR101 couples most strongly to the Ag NPs, and is thus able to benefit most from direct enhancement of the Raman scattering signal.

Resume : Two different types of nanostructures, Ag nanoislands of ~40 nm average (in-plane) diameter and Ag ultrasmall nanoparticles of ~4 nm diameter, were studied as active plasmonic components to be implemented in SERS platforms. Nanoislands provide an optimized value of plasmon resonance intensity, whereas ultrasmall nanoparticles are envisioned to provide a large hot spot areal density. It has been observed that high surface density silver ultrasmall nanoparticles (>1 monolayer) and percolated silver nanoislands reveal an unambiguous increase of the plasmon resonance intensity with time in the early stages of the process. This can be explained in terms of formation of Ag/AgOx core/shell entities with concomitant suppression of particle connectivity, introducing new hints in the development of silver-based materials for SERS detection. Moreover, unlike what was anticipated, the plasmon resonance in both systems is quite stable and visible throughout a long-term period of time. On the other hand, the graphene layer in graphene/Ag NPs, besides its role as molecule immobilizer and fluorescence quencher, yields delocalization of the plasmon resonance along the visible range. This effect gives additional hints of the SERS performance to a broader excitation wavelength range. Implementation of these two types of nanostructures in SERS platforms (e.g., graphene/Ag NPs/Al2O3/Al/Si heterostructures, with combined surface and interference Raman amplification contributions) are analyzed.

Resume : A good quality SERS substrate should provide a high enhancement factor (EF) combined with surface uniformity, stability and reproducibility of obtained spectra. These features provide highly reliable results also enabling time-lapse measurements which tend to be crucial in biological systems. We introduced novel SERS platforms based on etched gallium nitride coated with a nanometric layer of either gold or gold alloy. This system provided the mechanical stability and high surface roughness of GaN combined with high enhancement factor and chemical stability of gold. This feature enabled time-lapse measurements of various biological systems such as Hepatitis B virus antigen and DNA. In order to increase the EF and the overall quality of the system we introduced new types of coatings (silver-gold and copper-gold alloys) and ways to modify them by various means. We are able to further increase the surface area and chemical composition of existing layer by wet chemical etching. Moreover, by heat treatment we are able to change the shape, size and crystallographic form of gold further increasing the EF of platform. We have also developed methods to tune hydrophobic properties of the sample surface. Above mentioned gold layer and surface modification techniques enable fitting for individual applications making it better tool for new applications. In this presentation we would like to share with You results and highlight new application opportunities and challenges.

Resume : Core/shell nanostructures are intensively investigated for broadband light trapping and management, because they can play as optical cavities by taking advantage of light-trapping and morphology dependent resonances. The same properties can be exploited in imaging and vibrational spectroscopy (Raman, IR), in order to enhance their analytical sensitivity. [1-5] Moreover, the synergistic combination of plasmonic nanoantennas and dielectric core/shell colloids are leading to the fabrication of near-field optical light concentrators. These structures are very efficient in stimulating photon-driven processes at metal-semiconductor interfaces and show an impressive decrease in degradation time (minutes instead of hours) of organic pollutants. [6] This presentation will review some applications of core/shell light nanoconcentrators as non-plasmonic Raman enhancers, which have been used to investigate biochemical reactions in aqueous environment [3,5] or to detect environmental CO2, [4] and their coupling to plasmonic nanoantennas for promoting and in-situ monitoring light-assisted chemical reactions. [6] References 1) I. Alessandri, J. Am. Chem. Soc. (2013) 135(15), 5541-5544. 2) I. Alessandri et al., RSC Adv. (2014) 4, 38152-38158 3) I. Alessandri et. al., Small (2014) 10, 1294-1298. 4) N. Bontempi et al., Nanoscale (2016) 8, 3226-3231. 5) I. Alessandri et al., ACS-Appl. Mater. Interf., accepted, doi: doi:10.1021/acsami.5b08190. 6) M. Salmistraro et al., Small (2013) 9, 3301-3307.

Resume : We apply a combination of photocurrent and photothermal spectroscopic techniques to experimentally quantify the trade-off between useful and parasitic absorption of light in thin hydrogenated microcrystalline silicon (?c-Si:H) films incorporating self-assembled silver nanoparticle arrays, located at the rear side, for improved light trapping via resonant plasmonic scattering. The photothermal technique is used to measure the total absorptance while the photocurrent spectroscopy accounts only for the photons absorbed in the ?c-Si:H layer (useful absorptance); therefore, the method allows for independent quantification of the useful and parasitic absorptance of the plasmonic (or any other) light trapping structure [1]. We demonstrate that for 0.9 ?m thick ?c-Si:H film the optical losses resulting from the plasmonic light trapping are insignificant below 730 nm, above which they increase rapidly with increasing illumination wavelength. For the films deposited on nanoparticle arrays coupled with a flat silver mirror (plasmonic back reflector), we achieved a significant broadband enhancement of the useful absorption with an average useful absorption of 43% and an average parasitic absorption of 19% over 400?1100 nm wavelength range, achieving 91% of the theoretical Lambertian limit of absorption. [1] S. Morawiec et al. Experimental Quantification of Useful and Parasitic Absorption of Light in Plasmon-Enhanced Thin Silicon Films for Solar Cells Application. Scientific Reports 6 (2016)

Resume : ZnO, a wide band gap semiconductor with 3.3 eV band gap at room temperature, is intensively studied for photovoltaic (PV) applications - mostly as a transparent conductive oxide (TCO) and/or as a n type partner for p-type materials (e.g. Si, CdTe, CIGS…). In this work, we study PV structures based on n type ZnO layers and ZnO nanorods (ZnONR) on p-type Si wafers (2 Ωcm; (100)) with different thicknesses. We used ~50μm and ~200μm Si wafers as a absorbers of solar radiation. Zinc oxide layers and ZnONR were grown by atomic layer deposition method and hydrothermal method, respectively. We focused on the photovoltaic effect in n-ZnO/p-Si solar cells (SC). We evaluated growth conditions, absorbers thicknesses, metal contact etc. in order to obtain the highest PV efficiency. The PV efficiency for such a “new-generation” structures (ZnO:Al/ZnO/ZnONR/Si/Al) equals 14%. This work was partially supported by the National Science Center (decision No.DEC-2012/06/A/ST7/00398), and (Wroclaw group) by the National Laboratory of Quantum Technologies (POIG.02.02.00-00-003/08-00) and Statutory grant S400291.

Resume : We have developed Ni2+-Er3+ sensitized broadband-sensitive upconverters based on oxide hosts to overcome the shortcoming of conventional Er3+-only doped upconverters used for crystalline-silicon (c-Si) solar cells that utilize only a small fraction of solar spectrum around 1.55 μm. We have designed the combination of sensitizers and host materials to utilize photons that are not absorbed by c-Si itself or Er3+ ions. Six-coordinated Ni2+ ions that absorb near infra-red photons are candidates for the sensitizers. However, Ni3+ and four-coordinated Ni2+ do not function as the sensitizers, because they have longer-wavelength emission bands. La(Ga,Sc)O3 and MZrO3 (M = Ca, Sr, Ba) with ABO3-type perovskite structures are suitable for the host materials, because the ionic radii of the A- and B-site ions are close to those of Er3+ and Ni2+, respectively, and the B sites are six-coordinated at the centers of octahedrons. In addition to (1.4-1.6) μm photons (≈2.01016.cm-2.s-1) upconverted by Er3+ ions alone, we have demonstrated the broadband-sensitive (1.1-1.6 μm: ≈ 8.041016.cm-2.s-1) upconversion to 0.98 μm photons by codoping Ni2+-Er3+ ions, which is about four fold larger. Compared with the current density of the present c-Si solar cells (~40 mA.cm-2), the upconverted photons could increase it by ~7.3 mA.cm-2, which corresponds to ~18% improvement. This architecture for broadband-sensitive upconversion may pave a new direction for the improvements in efficiency of the present c-Si solar cells to surpass the limiting conversion efficiency of single-junction solar cells.