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Surface lattice resonances (SLRs) sustained by ordered metal arrays are characterized by their narrow spectral features, remarkable quality factors, and the ability to tune their spectral properties based on the periodicity of the array. However, the majority of these structures are fabricated using classical lithographic processes or require postannealing steps at high temperatures to enhance the quality of the metal. These limitations hinder the widespread utilization of these periodic metal arrays in various applications. In this work, we use the scalable technique of template-assisted assembly of metal colloids to produce plasmonic supercrystals over centimeter areas capable of sustaining SLRs with high Q factors reaching up to 270. Our approach obviates the need for any postprocessing, offering a streamlined and efficient fabrication route. Furthermore, our method enables extensive tunability across the entire visible and near-infrared spectral ranges, empowering the design of tailored plasmonic resonant structures for a wide range of applications.

Most metal halide perovskites (MHPs) which under ambient conditions crystallize in a tetragonal or cubic phase exhibit a fairly linear increase of the fundamental gap with increasing temperature, in frank contrast to other conventional semiconductors. Though valid in general for MHPs, it has been shown for the archetypal perovskite MAPbI3 (MA stands for methylammonium) that such a behavior is due to an almost equal footing of the effects of thermal expansion on the electronic states and the temperature-induced gap renormalization due to the electron–phonon interaction. Here, the influence of the incorporation of small amounts of Cs on the gap renormalization was investigated in two CsxMA1–xPbI3 single crystals (x = 0.05 and 0.1). Strikingly, from ambient to a temperature Ton of 250–280 K, the slope of the linear temperature dependence of the energy gap for both Cs-containing samples is almost half that for pure MAPbI3. Below Ton and within the stability range of the tetragonal phase, the slope recovers the value of MAPbI3. Based on density functional theory calculations, this surprising behavior is explained as being due to the appearance of an additional electron–phonon coupling mechanism ascribed to dynamic tilting of the PbI6 octahedrons. The latter is activated by the translational dynamics of the Cs cations between equivalent potential minima inside the inorganic cage voids, which is unfolded above the onset temperature Ton. These results provide further evidence of the key role that the A-site cation dynamics plays in the optoelectronic properties of this fascinating class of materials.

We present an innovative contactless method suitable to study in-plane thermal transport based on beam-offset frequency-domain thermoreflectance using a one-dimensional heat source with uniform power distribution. Using a one-dimensional heat source provides a number of advantages as compared to point-like heat sources, as typically used in time- and frequency-domain thermoreflectance experiments, just to name a few: (i) it leads to a slower spatial decay of the temperature field in the direction perpendicular to the line-shaped heat source, allowing to probe the temperature field at larger distances from the heater, hence, enhancing the sensitivity to in-plane thermal transport; (ii) the frequency range of interest is typically kHz. This rather low frequency range is convenient regarding the cost of the required excitation laser system but, most importantly, it allows the study of materials without the presence of a metallic transducer with almost no influence of the finite optical penetration depth of the pump and probe beams on the thermal phase lag, which arises from the large thermal penetration depth imposed by the used frequency range. We also show that for the case of a harmonic thermal excitation source, the phase lag between the thermal excitation and thermal response of the sample exhibits a linear dependence with their spatial offset, where the slope is proportional to the inverse of the thermal diffusivity of the material. We demonstrate the applicability of this method to the cases of: (i) suspended thin films of Si and PDPP4T, (ii) Bi bulk samples, and (iii) Si, glass, and highly-oriented pyrollitic graphite (HOPG) bulk samples with a thin metallic transducer. Finally, we also show that it is possible to study in-plane heat transport on substrates with rather low thermal diffusivity, e.g., glass, even using a metallic transducer. We achieve this by an original approach based on patterning the transducer using focused ion beam, with the key purpose of

The exceptional optoelectronic properties of metal halide perovskites (MHPs) are presumed to arise, at least in part, from the peculiar interplay between the inorganic metal-halide sublattice and the atomic or molecular cations enclosed in the cage voids. The latter can exhibit a roto-translative dynamics, which is shown here to be at the origin of the structural behavior of MHPs as a function of temperature, pressure and composition. The application of high hydrostatic pressure allows for unraveling the nature of the interaction between both sublattices, characterized by the simultaneous action of hydrogen bonding and steric hindrance. In particular, we find that under the conditions of unleashed cation dynamics, the key factor that determines the structural stability of MHPs is the repulsive steric interaction rather than hydrogen bonding. Taking as example the results from pressure and temperature-dependent photoluminescence and Raman experiments on MAPbBr[Formula: see text] but also considering the pertinent MHP literature, we provide a general picture about the relationship between the crystal structure and the presence or absence of cationic dynamic disorder. The reason for the structural sequences observed in MHPs with increasing temperature, pressure, A-site cation size or decreasing halide ionic radius is found principally in the strengthening of the dynamic steric interaction with the increase of the dynamic disorder. In this way, we have deepened our fundamental understanding of MHPs; knowledge that could be coined to improve performance in future optoelectronic devices based on this promising class of semiconductors.

The versatile hybrid perovskite nanocrystals are one of the most promising materials for optoelectronics in virtue of their tunable bandgaps and high photoluminescence quantum yields. However, their inherent crystalline chemical structure limits the chiroptical properties achievable with the material. The production of chiral perovskites has become an active field of research for its promising applications in optics, chemistry or biology. Typically, chiral halide perovskites are obtained by the incorporation of different chiral moieties in the material. Unfortunately, these chemically modified perovskites have demonstrated moderate values of chiral photoluminescence so far. Here we introduce a general and scalable approach to produce chiral photoluminescence from arbitrary nano-emitters assembled into 2D-chiral metasurfaces. The fabrication via nanoimprinting lithography employs elastomeric molds engraved with chiral motifs covering millimeter areas that are used to pattern two types of unmodified colloidal perovskite nanocrystal inks: green-emissive CsPbBr3 and red-emissive CsPbBr1 I2 . The perovskite 2D-metasurfaces exhibit remarkable photoluminescence dissymmetry factors (glum ) of 0.16 that can be further improved up to glum of 0.3 by adding a high refractive index coating on the metasurfaces. This scalable approach to produce chiral photoluminescent thin films paves the way for the seamless production of bright chiral light sources for upcoming optoelectronic applications. This article is protected by copyright. All rights reserved.

The transparency of silicon in the infrared region enables the design of nano/microstructures for implementation in devices to harvest the infrared (IR) part of the solar spectrum. Herein we report a strategy that uses arrays of inverted silicon pyramids covered with a thin gold film, which exhibit substantial light absorption in the infrared spectral range (below the gap of Si). The absorption stems from the resonant excitation at infrared wavelengths of surface-plasmon polaritons at the metal/dielectric interface mainly by tuning size and separation of the inverted pyramids. The array-parameter optimization proceeded by iteration of the calculation and measurement of the infrared response using finite difference time-domain simulations and Fourier-transform IR spectroscopy, respectively. We analyse the calculated near-field distributions specifically looking for the presence of hot spots, i.e. nano-sized regions of very high concentration of the electronic charge and strong electromagnetic field enhancement, and discuss their potential for hot-electron generation. We show two fabrication routes for this kind of metal/silicon metamaterial either by photolithography or scalable nanoimprint techniques for a seamless integration in optoelectronic fabrication processes.

The high defect tolerance of metal halide perovskites, in terms of their exceptional optoelectronic properties, is assumed to be due to the very fact that most native point defects are shallow, which does not contribute to the non‐radiative recombination of free carriers. Here, a systematic study is presented, which concerns the evolution of shallow‐defect signatures observed at low temperatures in the photoluminescence (PL) spectra of mixed organic‐cation lead iodide perovskite single crystals (FAxMA1−xPbI3, where MA stands for methylammonium and FA for formamidinium). Below ≈100 K, a number of peak‐like features become clearly apparent in the PL spectra at energies lower than the strong free‐exciton emission, which are related to the radiative recombination of bound exciton complexes associated with native shallow defects (donors and/or acceptors). Based on state‐of‐the‐art ab initio calculations, a tentative assignment is provided for all PL features to different shallow‐defects (Pb, I, and MA vacancies as well as I interstitials) typically present in hybrid perovskites. The defect‐related signatures exhibit a clear trend regarding the mixed‐crystal composition, indicating that the material becomes less prone to defect formation with increasing FA content.

Cesium lead halide perovskite nanocrystals have emerged as one of the most promising candidates for manufacturing portable lasers and light sources. In order to harness and exploit their photoluminescence more effectively, the nanocrystals are often accompanied by a photonic scheme that improves light emission. In this work, one introduces a quasi-3D photonic crystal composed of a 2D-grating on top of a distributed Bragg reflector (DBR) that provides a greater photoluminescence enhancement than the isolated architectures alone. The quasi-3D photonic crystals support both Rayleigh-Wood anomalies and guided modes that populate the photonic bandgap of the Bragg mirror, all of them capable of enhancing the outcoupling of light from the emitting layer. In order to demonstrate the benefits of the quasi-3D system, one prepares 2D-gratings, DBRs, and quasi-3D photonic crystals covered with metal halide perovskite nanocrystals and studies the photoluminescence enhancement produced in each case. Interestingly, the quasi-3D structure exhibits a photoluminescence enhancement of 16 times and an increase in spontaneous emission rate, greatly exceeding the values observed for the separate components.

Sb2S3 films were deposited on unheated glass substrates by thermal evaporation and annealed under vacuum atmosphere for 2 hours at different temperatures. Different characterization techniques were used to better understand the behavior of the Sb2S3 material. XRD and Raman spectroscopy confirmed the formation of pure Sb2S3 powder with orthorhombic lattices. The effect of vacuum annealing on the properties of the films was studied. XRD analysis revealed that as-deposited and annealed films at 150ºC were amorphous in nature whereas those annealed at T ≥ 200°C were polycrystalline. The crystallite size of the films showed a decrease from 75.8 to 62.9 nm with the increase of the annealing temperature from 200 to 250 °C. The Raman showed several peaks corresponding to the stibnite Sb2S3 phase. The surface morphology of the films was examined by AFM. The roughness decreases slightly as the transformation from the amorphous to the crystalline phase occurs. The chemical compositions of Sb2S3 films were analyzed by EDS, revealing that all films were Sb-rich. The optical parameters were estimated from the transmittance and reflectance spectra recorded by UV-Vis spectroscopy. A reduction in the band gap energy from 2.12 to 1.70 eV with the increase of annealing temperature was also found.

Second sound is known as the thermal transport regime where heat is carried by temperature waves. Its experimental observation was previously restricted to a small number of materials, usually in rather narrow temperature windows. We show that it is possible to overcome these limitations by driving the system with a rapidly varying temperature field. High-frequency second sound is demonstrated in bulk natural Ge between 7 K and room temperature by studying the phase lag of the thermal response under a harmonic high-frequency external thermal excitation and addressing the relaxation time and the propagation velocity of the heat waves. These results provide a route to investigate the potential of wave-like heat transport in almost any material, opening opportunities to control heat through its oscillatory nature.

Hybrid lead halide perovskites exhibit an atypical temperature dependence of the fundamental gap for the phases stable at ambient conditions: it decreases in energy with decreasing temperature. Reports ascribe such a behavior to a strong electron-phonon renormalization of the gap, neglecting contributions from thermal expansion. However, high pressure experiments performed on the archetypal perovskite MAPbI3 (MA stands for methylammonium) yield a negative pressure coefficient for the gap of the tetragonal room-temperature phase [1], which speaks against the assumption of negligible thermal expansion effects. Here we show that for MAPbI3 the temperature-induced gap renormalization due to electron-phonon interaction can only account for about 40% of the total energy shift, thus implying thermal expansion to be more if not as important as electron-phonon coupling [2]. Furthermore, this result possesses general validity, holding also for the tetragonal or cubic phase, stable at ambient conditions, of most halide perovskite counterparts. As an example, recent results obtained for a series of FAxMA1-xPbI3 solid solutions, where FA stands for formamidinium [3], will be also presented. A striking result concerns the temperature dependence of the gap of a presumably tetragonal but disordered phase which is stable in a wide range of intermediate compositions and temperatures lower than ca. 250 K. This phase is found to exhibit a quadratic dependence of the band gap with temperature, which is again interpreted in terms of the combined effects of thermal expansion and electron-phonon interaction. [1] Francisco-López, A.; Charles, B.; Weber, O. J.; Alonso, M. I.; Garriga, M.; Campoy-Quiles, M.; Weller, M. T.; Goñi, A. R. Pressure-Induced Locking of Methylammonium Cations Versus Amorphization in Hybrid Lead Iodide Perovskites. J. Phys. Chem. C 2018, 122, 22073-2208. [2] Francisco-López, A.; Charles, B.; Weber, O. J.; Alonso, M. I.; Garriga, M.; Campoy-Quiles, M.; Weller, M. T.; G

Superlattices with scattering mechanisms at multiple length scales efficiently scatter phonons at all relevant wavelengths and provide a convenient route to reduce thermal transport. Here, we show, both experimentally and by atomistic simulations, that SiGe superlattices with well-established compositional gradients and a sufficient number of interfaces exhibit extremely low thermal conductivity. Our results reveal that the thermal conductivity of long-period (30–50 nm) superlattices with thicknesses below 200 nm is still thickness-dependent and higher than that of the corresponding alloy thin film. Increasing the number of periods up to 16 has a strong impact on heat propagation, leading to thermal conductivity values below the thin-film alloy limit. Lattice dynamics calculations confirm that the reduced thermal conductivity stems from the simultaneous effects of mass scattering, graded interface scattering, and coherent interference from the lattice periodicity. This study provides a significant step forward in understanding the role of compositional gradients in heat transport across nanostructures. The strategy of employing long-period graded superlattices with extremely low thermal conductivities has great potential for micro- and nano-thermoelectric generation and cooling of Si-based devices.

Metal nanostructures offer exciting ways to manage light at the nanoscale exploited in fields such as imaging, sensing, energy conversion, and information processing. The optical response of the metallic architectures can be engineered to exhibit photonic properties that span from plasmon resonances to more complex phenomena such as negative refractive index, optical chirality, artificial magnetism, and more. However, the latter optical properties are only observed in intricate architectures, which are highly demanding in terms of nanofabrication and hence less scalable and far away from device implementation. Here, a series of metasurfaces covering centimeter areas and operating in the visible spectrum are presented, which are produced from the combination of nanoimprinting lithography and oblique angle metal evaporation. The potential of this scalable approach is illustrated by easily fabricating metasurfaces engineered to exhibit artificial optical magnetism, tunable linear polarization dependent response, chirality with g‐factor of 0.2, and photoluminescence enhancement of 20 times over a 9 mm2 area.

The complete phase diagram of organic-cation solid solutions of lead iodide perovskites [FAxMA1–xPbI3, where MA stands for methylammonium, CH3NH3, and FA for formamidinium, CH(NH2)2] with compositions x ranging from 0 to 1 in steps of 0.1 was constructed in the temperature range from 10 to 365 K by combining Raman scattering and photoluminescence (PL) measurements. The occurrence of phase transitions was inferred from both the temperature-induced changes in the optical emission energies and/or the phonon frequencies and line widths, complementing X-ray and neutron scattering literature data. For MA-rich perovskites (x ≤ 0.2), the same structural behavior as for MAPbI3 was observed with decreasing temperature: cubic Pm3̅m → tetragonal-I I4/mcm → orthorhombic Pnma. As the FA molecule is larger and more symmetric but less polar than MA, a tetragonal crystal structure is favored at low temperatures and FA compositions x > 0.4, to the detriment of the orthorhombic phase. As a consequence, with decreasing temperature, the phase transition sequence for FA-rich compounds is cubic Pm3̅m → tetragonal-II P4/mbm → tetragonal-III. The latter presumably belongs to the P4bm symmetry group, according to neutron scattering data. Strikingly, the isostructural (tetragonal-to-tetragonal) transformation, which occurs between 200 and 150 K, exhibits a kind of critical point for x = 0.7. For intermediate FA contents, the perovskite solid solution transforms close to 250 K directly from the cubic phase to the tetragonal-III phase. The latter is characterized by a nonmonotonic dependence of the band-gap energy on temperature. We ascribe such behavior to a substantial tilting of the PbI6 octahedra in the tetragonal-III phase. In this way, we established important links between crystal-phase stability and the electronic as well as vibrational properties of mixed organic-cation halide perovskites, which might impact the current search for more stable best-performing optoelectronic materials.

Lead halide perovskites, which are causing a paradigm shift in photovoltaics, exhibit an atypical temperature dependence of the fundamental gap: it decreases in energy with decreasing temperature. Reports ascribe such a behavior to a strong electron-phonon renormalization of the gap, neglecting contributions from thermal expansion. However, high pressure experiments performed on the archetypal perovskite MAPbI3 (MA stands for methylammonium) yield a negative pressure coefficient for the gap of the tetragonal room-temperature phase [1], which speaks against the assumption of negligible thermal expansion effects. Here I will show that for MAPbI3 the temperature-induced gap renormalization due to electron-phonon interaction can only account for about 40% of the total energy shift, thus implying thermal expansion to be more if not as important as electron-phonon coupling [2]. Furthermore, this result possesses general validity, holding also for the tetragonal or cubic phase, stable at ambient conditions, of most halide perovskite counterparts. As an example, I will also present recent results obtained for MA rich FAxMA1-xPbI3 solid solutions, where FA stands for formamidinium. [1] A. Francisco-López, B. Charles, O. J. Weber, M. I. Alonso, M. Garriga, M. Campoy-Quiles, M. T. Weller, and A. R. Goñi, J. Phys. Chem. C 122, 22073-22082 (2018). [2] A. Francisco-López, B. Charles, O. J. Weber, M. I. Alonso, M. Garriga, M. Campoy-Quiles, M. T. Weller, and A. R. Goñi, J. Phys. Chem. Lett. 10, 2971-2977 (2019).

Lead halide perovskites, which are causing a paradigm shift in photovoltaics, exhibit an atypical temperature dependence of the fundamental gap: it decreases in energy with decreasing temperature. Reports ascribe such a behavior to a strong electron−phonon renormalization of the gap, neglecting contributions from thermal expansion. However, high-pressure experiments performed on the archetypal perovskite MAPbI3 (MA stands for methylammonium) yield a negative pressure coefficient for the gap of the tetragonal room-temperature phase, which speaks against the assumption of negligible thermal expansion effects. Here we show that for MAPbI3 the temperature-induced gap renormalization due to electron−phonon interaction can only account for about 40% of the total energy shift, thus implying thermal expansion to be more if not as important as electron− phonon coupling. Furthermore, this result possesses general validity, holding also for the tetragonal or cubic phase, stable at ambient conditions, of most halide perovskite counterparts.

Quite high efficiencies exceeding 20% have been realized in solar cells incorporating organic-inorganic hybrid perovskites (APbX3), which have a unique structure with a center cation [A = CH3NH3+, HC(NH2) 2 + ] located within a PbX3− cage (X = I, Br, Cl). Superior characteristics of hybrid perovskite solar cells can be understood from the nature of optical transitions and the efficient carrier collection in the device. From these points of view, this chapter provides details on optical properties of various hybrid perovskite materials and carrier dynamics in the solar cells. In particular, based on the first-principles analyses of different perovskite materials, we present universal rules that allow the unified interpretation of the optical absorption phenomenon in APbX3 perovskites. The external quantum efficiency (EQE) analysis further reveals that high short-circuit current densities (>20 mA/cm2) observed in the perovskite solar cells originate from electric-field-assisted carrier collection and the suppressed optical losses in the devices. Although hybrid perovskites have quite favorable characteristics for solar cells, these materials exhibit rather intense phase change upon exposure to humid air. In this chapter, the degradation process of CH3NH3PbI3 in humid air, characterized by applying ellipsometry technique, is further presented and discussed.

Organic solar cells attract both scientific and economic interest due to their potential for clean and cost-effective photovoltaic energy conversion. Continuous evolution of this field relies on materials research, including synthesis of new compounds and fine control over film microstructure, as well as improved device architectures. In this context, spectroscopic ellipsometry is a helpful characterization tool, stretching over material preparation, device structure, and device modelling. This chapter will provide a general perspective of aspects that can be investigated by ellipsometry in these systems. The acquired insights enhance our capability to understand and model the optoelectronic properties of photovoltaic devices.

The development of devices for sensing and monitoring CO2 levels is crucial for many fields such as food packaging and for human safety indoors. Herein the fabrication of an optical CO2 sensor by integration of a metal–organic framework (MOF) onto bimodal optical waveguides is reported. This sensor is constructed via self-assembly of a transparent film of zeolitic imidazolate framework-8 (ZIF-8) nanoparticles (size: 32 ± 5 nm) on the waveguides. The nanoZIF-8-based sensor exhibits a broad linear response, with limits of detection of 3130 ppm at room temperature and 774 ppm at 278 K. Furthermore, it is robust, selective, fast and reusable, and can be stored under humid conditions with no loss in performance.

The structural phase behavior of high-quality single crystals of methylammonium lead iodide (CH3NH3PbI3 or MAPbI3) was revisited by combining Raman scattering and photoluminescence (PL) measurements under high hydrostatic pressure up to ca. 10 GPa. The single crystals were specially grown with the final thickness needed for pressure experiments, retaining their high quality due to a less invasive preparation procedure, which avoids sample thinning. Both PL and Raman spectra show simultaneous changes in their profiles that indicate the occurrence of three phase transitions subsequently at around 0.4, 2.7, and 3.3 GPa. At the second phase transition, the Raman spectra exhibit a pronounced reduction in the line width of the phonon modes of the inorganic cage, similar to the changes observed at the tetragonal-to-orthorhombic phase transition occurring at around 160 K but ambient pressure. This behavior is interpreted as evidence for the locking of the organic cations in the cage voids above 2.7 GPa due to the reduced volume and symmetry of the unit cell. At the third phase transition, reported here for the first time, the PL is greatly affected, whereas the Raman spectrum experiences only subtle changes related to a splitting of some of the peaks. This behavior may indicate a change mostly in the electronic structure with little effect on the crystal structure. Strikingly, the sharp Raman features observed at high pressures do not support amorphization of MAPbI3 with onset at 3 GPa, as claimed by most of the high-pressure (X-ray) literature. We interpret this apparent discrepancy in terms of the degree of disorder introduced at different length scales in the perovskite lattice by the pressure-induced freeze-out of the methylammonium cation motion.

While the performance of some emerging solar cell technologies is rocketing, there are other important parameters that also need to be considered for industrialization, including aesthetics, payback time or long-term stability. More specifically, aesthetical features of the solar cells play a very important role in the path towards the integration of these semi-transparent technologies into the buildings and windows of the future. In this work, we have implemented a theoretical methodology to study in depth colour tuning in polymer-based organic photovoltaic cells that uses experimentally complex refractive indices. We compare quantitatively basic interference effects (through active layer thickness), binaries with different donors and different acceptors, and ternary systems where the third component is either active or a simple dye. For hybrid perovskite solar cells, we compare the colour tuning capability of blending and alloying. Additionally, we show that the same theoretical framework can be employed for the so-called inverse problem, i.e. determining the best solar cell parameters (specific materials, donor:acceptor ratio and photoactive film thickness) that can reproduce a desired set of chromaticity coordinates.

In this Chapter we will start by briefly summarising the basic concepts of the electronic structure of conjugated polymers. This will enable the discussion of the relevant descriptions of the dielectric function. We will relate these descriptions to the model parameterisations which are used in advanced ellipsometric analysis of thin films such as those used in devices for organic photovoltaics (OPVs) and light emitting diodes (OLEDs). Amongst other things, such parametric descriptions are useful to deal with structural changes in conjugated polymer thin films. Once the models are presented, we will provide representative examples of the nexus between morphology and optical constants, and how the latter can be employed to infer aspects of the former. First, we will discuss how chain conformation affects the optical properties. Then, we will explain the anisotropic behaviour of conjugated polymer films due to their intrinsic molecular anisotropy and review different cases (f. i., oriented films or semicrystalline polymers). We will also describe structural changes that occur upon blending polymers with fullerenes and concomitant variations of the optical properties. Here we will focus on state of the art low band gap polymers mixed with fullerenes. Finally, real-time ellipsometric experiments in which these structure-property relationships can be exploited will be presented.

As contamination and environmental degradation increase nowadays, there is a huge demand for new eco-friendly materials. Despite its use for thousands of years, cellulose and its derivatives have gained renewed interest as favourable alternatives to conventional plastics, due to their abundance and lower environmental impact. We report the fabrication of photonic and plasmonic structures by moulding hydroxypropyl cellulose into sub-micrometric periodic lattices, using soft lithography. This is an alternative way to achieve structural colour in this material which is usually obtained exploiting its chiral nematic phase. Cellulose based photonic crystals are biocompatible and can be dissolved in water or not depending on the derivative employed. Patterned cellulose membranes exhibit tuneable colours and may be used to boost the photoluminescence of a host organic dye. Furthermore, we show how metal coating these cellulose photonic architectures leads to plasmonic crystals with excellent optical properties acting as disposable surface enhanced Raman spectroscopy substrates.

The design of ultrathin semiconducting materials that achieve broadband absorption is a long-sought-after goal of crucial importance for optoelectronic applications. To date, attempts to tackle this problem consisted either of the use of strong—but narrowband—or broader—but moderate—light-trapping mechanisms. Here, a strategy that achieves broadband optimal absorption in arbitrarily thin semiconductor materials for all energies above their bandgap is presented. This stems from the strong interplay between Brewster modes, sustained by judiciously nanostructured thin semiconductors on metal films, and photonic crystal modes. Broadband near-unity absorption in Ge ultrathin films is demonstrated, which extends from the visible to the Ge bandgap in the near-infrared and is robust against angle of incidence variation. The strategy follows an easy and scalable fabrication route enabled by soft nanoimprinting lithography, a technique that allows seamless integration in many optoelectronic fabrication procedures.

The nucleation of single macroscopic spherulites at desired positions, as well as ordered arrays of multiple spherulites, is demonstrated by combining the use of crystallizable solvents with local control of solvent evaporation during solution deposition. Moreover, the temperature assisted localized frustration of molecular orientation is shown, enabling the fabrication of samples containing both isotropic areas and spherulites. These macroscopic circular polycrystalline structures are characterized using a range of polarized spectroscopic techniques that allow quantifying their large degree of chain orientation. In order to show the potential of these large conjugated polymer spherulites centered at desired locations, graded bilayer organic photovoltaic devices were fabricated to be used as polarimeters, solid state light polarization detectors with no moving parts, and position sensitive photodetectors.

We report on the investigation by spectroscopic ellipsometry of films containing Cd1 − xHgxTe alloy quantum dots (QDs). The alloy QDs were fabricated from colloidal CdTe QDs grown by an aqueous synthesis process followed by an ion-exchange step in which Hg2+ ions progressively replace Cd2+. For ellipsometric studies, several films were prepared on glass substrates using layer-by-layer (LBL) deposition. The contribution of the QDs to the measured ellipsometric spectra is extracted from a multi-sample, transmission and multi- angle-of-incidence ellipsometric data analysis fitted using standard multilayer and effective medium models that include surface roughness effects, modeled by an effective medium approximation. The relationship of the dielectric function of the QDs retrieved from these studies to that of the corresponding II–VI bulk material counterparts is presented and discussed.

We study the complex electronic band structure of low In content InGaAs/GaP quantum dots. A supercell extended-basis tight-binding model is used to simulate the electronic and the optical properties of a pure GaAs/GaP quantum dot modeled at the atomic level. Transitions between hole states confined into the dots and several XZ-like electronic states confined by the strain field in the GaP barrier are found to play the main role on the optical properties. Especially, the calculated radiative lifetime for such indirect transitions is in good agreement with the photoluminescence decay time measured in time-resolved photoluminescence in the µs range. Photoluminescence experiments under hydrostatic pressure are also presented. The redshift of the photoluminescence spectrum with pressure is also in good agreement with the nature of the electronic confined states simulated with the tight-binding model.

The optical constants of methylammonium lead halide single crystals CH3NH3PbX3 (X = I, Br, Cl) are interpreted with high level ab initio calculations using the relativistic quasiparticle self-consistent GW approximation (QSGW). Good agreement between the optical constants derived from QSGW and those obtained from spectroscopic ellipsometry enables the assignment of the spectral features to their respective inter-band transitions. We show that the transition from the highest valence band (VB) to the lowest conduction band (CB) is responsible for almost all the optical response of MAPbI3 between 1.2 and 5.5 eV (with minor contributions from the second highest VB and the second lowest CB). The calculations indicate that the orientation of [CH3NH3]+ cations has a significant influence on the position of the bandgap suggesting that collective orientation of the organic moieties could result in significant local variations of the optical properties. The optical constants and energy band diagram of CH3NH3PbI3 are then used to simulate the contributions from different optical transitions to a typical transient absorption spectrum (TAS).

We present Raman and terahertz absorbance spectra of methylammonium lead halide single crystals (MAPbX3, X = I, Br, Cl) at temperatures between 80 and 370 K. These results show good agreement with density-functional-theory phonon calculations.1 Comparison of experimental spectra and calculated vibrational modes enables confident assignment of most of the vibrational features between 50 and 3500 cm-1. Reorientation of the methylammonium cations, unlocked in their cavities at the orthorhombic-to-tetragonal phase transition, plays a key role in shaping the vibrational spectra of the different compounds. Calculations show that these dynamics effects split Raman peaks and create more structure than predicted from the independent harmonic modes. This explains the presence of extra peaks in the experimental spectra that have been a source of confusion in earlier studies. We discuss singular features, in particular the torsional vibration of the C-N axis, which is the only molecular mode that is strongly influenced by the size of the lattice. From analysis of the spectral linewidths, we find that MAPbI3 shows exceptionally short phonon lifetimes, which can be linked to low lattice thermal conductivity and supressed electron-phonon scattering events.

We report micro-Raman imaging of in-plane SiGe alloy nanowires grown by molecular beam epitaxy on Si(001) substrates. The spatial resolution of the Raman images allows us to study individual nanowires. We observe differences in Raman scattering intensity when the light polarization is parallel or perpendicular to the nanowire axis. These variations are shown to be originated from anisotropic absorption in thin nanowires whereas the intrinsic Raman efficiency is less affected by the nanostructuration. Quantitative analysis of the Raman spectra yields the composition and strain variations in the sample, in particular within each nanowire in which we resolve vertical gradients from the base to the top surfaces. These Raman results provide unique insights into the growth processes.

We study the growth of in-plane SiGe alloy nanowires by molecular beam epitaxy on Si substrates using the vapor-liquid-solid mechanism. These horizontal nanowires grow from liquid AuSi seeds which are obtained by annealing Au layers deposited on clean Si(001) surfaces. By continuously supplying Ge to the liquid droplets, an epitaxial process takes place, where wires crawl along <110> directions of the Si substrate, giving rise to the reproducible achievement of self-assembled in-plane SiGe nanowires. The morphology of the obtained nanostructures is characterized by scanning electronic and atomic force microscopies, and their composition and strain status is evaluated by micro-Raman imaging.

We present a comprehensive experimental and computational study on the electromagnetic field distribution in sphere segment void arrays. Surface plasmon polaritons can be excited in these void arrays, resulting in greatly enhanced electromagnetic fields. With the scanning near-field optical microscope (SNOM) we are able to measure the electromagnetic field distribution at the sample surface. For this purpose, an array of relatively large voids with a sphere diameter of 900 nm was fabricated, allowing for an easy access of the scanning glass-fibre tip and yielding very detailed scans. Complementary, finite-difference time-domain (FDTD) calculations on a complete void array have been performed and compared with the SNOM intensity maps and experimental reflectivity data. We show in a direct way both the existence of extended and localised modes in the Au void array for three different void depths. We also show and discuss the changes that the modes undergo for the different void depths and excitation wavelengths. Moreover, since the simulations were performed for two different void geometries, one containing perfectly spherical void surfaces and another more realistic one, which considers the presence of interstitial wall holes and other imperfections, as observed in scanning electron micrographs, we were able to determine by comparison with the experiment under which conditions an array of idealised sphere segment voids is a meaningful model. This demonstrates that both SNOM and FDTD simulations are powerful tools for understanding the plasmonic response of metallic nanostructures, thus enabling, for instance, a design for applications in ultra-sensitive optical detection. © 2013 Springer Science+Business Media New York.

Epitaxial thin films of ceria-zirconia solid solutions were grown on yttria-stabilized zirconia (YSZ) substrates by chemical solution deposition. Characterization by X-ray diffraction, atomic force microscopy and variable angle spectroscopic ellipsometry show that high-quality, epitaxial, high density films, with a thickness in the range of 20-27 nm were obtained. Compositional dependence of the film optical constants (n and k) was determined from ellipsometry measurements. It was found that the optical properties for the mixed-oxide films smoothly vary between the values for the pure oxides and are best described by means of a Tauc-Lorentz (TL) model. Moreover, we found that the parameters of the TL model have a linear dependence on the cerium concentration. The obtained results can serve as a database for the thickness, composition and porosity characterization of CexZr 1-xO2 thin films. © 2013 Elsevier B.V. All rights reserved.

In this work, we investigate the influence of alloy composition inhomogeneities on the vibrational properties of strained Si 1 -xGe x/Si layers with x ranging from 0 to 0.5. We show that the frequencies of the principal alloy vibrational modes (Ge-Ge, Si-Ge, and Si-Si) are strongly influenced by the distribution of Ge atoms within the alloy layers, which becomes gradually random following a series of sequential annealing steps. Our measurements suggest that the composition dependence of the optical phonon frequencies in fully random and unstrained alloys is well described by the results previously published by Alonso and Winer [Phys. Rev. B 39, 10056 (1989)]. In the general case of an alloy layer with unknown degree of compositional inhomogeneity and/or strain relaxation, though the analysis of the Raman spectra is not straightforward. Therefore, we propose an analytical/graphical method to accurately estimate the Ge content and residual strain of SiGe layers exhibiting any level of compositional disorder or strain status, by performing a single Raman measurement. This would be extremely useful in situations where x-ray measurements cannot be conducted. We show that our procedure to treat the Raman data holds for the whole compositional range but with different accuracy depending upon the case: (i) For annealed SiGe layers (mostly strain relaxed) the Ge content x can be directly determined with high accuracy of ± 0.01. (ii) For strained samples (usually as-grown samples) an extra criterion must be adopted seeking for a graphical solution, accounting for the degree of compositional inhomogeneity. In this case, the error in the determination of Ge content depends on alloy composition, being the upper bound ± 0.02 for x 0.3 and ± 0.03 for x > 0.3. © 2012 American Institute of Physics.

Laser irradiation of Ge quantum dots (QDs) grown on Si(100) substrates by solid-source molecular beam epitaxy has been performed using a Nd:YAG laser (532 nm wavelength, 5 ns pulse duration) in a vacuum. The evolution of the Ge QD morphology, strain and composition with the number of laser pulses incident on the same part of the surface, have been studied using atomic force microscopy, scanning electron microscopy and Raman spectroscopy. The observed changes in the topographical and structural properties of the QDs are discussed in terms of Ge–Si diffusion processes. Numerical simulations have been developed for the investigation of the temperature evolution of the QDs during laser irradiation. The obtained results indicate that the thermal behaviour and structural variation of the nanostructures differ from conventional thermal annealing treatments and can be controlled by the laser parameters. Moreover, an unusual island motion has been observed under the action of subsequent laser pulses.

The main challenges toward massive fabrication of silicon nanowires are the way to control the crystal orientation and size. In the present work, we report advances in the pattern transfer process to obtain arrays of holes on a Si substrate, aiming at the fabrication of ordered arrays of quantum wires. The wires obtained from this procedure can be uniform in terms of doping profile, crystal orientation and size, while enormously simplifying metallic contacting for applications where parallel biasing is needed. The samples have been fabricated using a combination of electron-beam lithography and reactive-ion etching. Electron-beam lithography is performed at 10 keV on a 100 nm thick 950 K PMMA. A specific recipe for deep reactive-ion etching was developed in order to minimize any widening or under-etching of the holes, as well as any type of wall roughness. Holes with diameters from 30 nm up to 900 nm, and pitch from 90 nm up to 1000 nm were fabricated, achieving no observable scalloping. The sample has been oxidized for further reduction of the size of interconnects (the site between two holes) and interstitials (the site between three holes). As a result, thin nanowires have been fabricated.

Spectroscopic ellipsometry has been used to characterize the dielectric functions of bulk Cu(In1−xGax)5Se8 crystals. Spectra were measured at room temperature over the energy range 0.74–5.2 eV. The dielectric functions as well as the complex refractive index, the absorption coefficient, and the normal-incidence reflectivity have been modeled using a modification of the Adachi model. The results are in a good agreement with the experimental data over the entire range of photon energies. The model parameters (strength, threshold energy, and broadening) have been determined using the simulated annealing algorithm. The transition energies E0 and E1A are found to shift linearly to higher energies as the gallium content increases.

Direct imaging of the electromagnetic fields generated by plasmon excitation at gold nanoscale voids is achieved with high spatial resolution using scanning near-field optical microscopy (SNOM). The spatial intensity distribution of the electromagnetic field in spherical Au void structures strongly depends on the light excitation wavelength and on the diameter and degree of truncation of the voids. Comparing the SNOM results with reflectivity measurements and boundary element calculations allows for a clear identification of the plasmon modes being activated in each case. The unique mapping of the high-field regions provides key information for the use of void nanostructures as substrates for surface-enhanced Raman scattering. © 2008 The American Physical Society.

By combining Raman scattering from the cleaved edge and under hydrostatic pressure, we have accurately determined the tetragonal phonon deformation potentials of strained Si1−xGex alloys in the entire compositional range for the Ge-like, Si-like, and mixed Si–Ge optical modes. A known biaxial strain is induced on thin alloy layers by pseudomorphic epitaxial growth on silicon and subsequent capping. We also determine the strain shift coefficient of the three modes, which are essentially independent of Ge content between 0.4 and 1. This is key information for an effective use of Raman scattering as strain-characterization tool in SiGe nanostructures.

The built-in strain in self-assembled quantum dots has large impact on their physical properties, but both its average value and degree of anisotropy are often unknown. The authors demonstrate that the pressure coefficient of optical phonons might be used as probe for the strain status of the dots. This method was applied to the case of Ge dots capped with Si layers of different thicknesses. The authors observe a transition from a strictly biaxial stress situation for uncapped dots to a status of quasihydrostatic strain for cap-layer thicknesses larger than a critical value of the order of the dot height.