Sign of the Gap Temperature Dependence in CsPb(Br,Cl) Nanocrystals Determined by Cs-Rattler-Mediated Electron-Phonon Coupling.

So far, the striking sign reversal in the near-ambient slope of the gap temperature dependence of colloidal CsPbCl perovskite nanocrystals (NCs) compared to its Br counterpart remains unresolved. Pure bromide NCs exhibit a linear gap increase with increasing temperature, to which thermal expansion and electron-phonon interaction equally contribute. In contrast, the temperature slope for the chlorine compound gap is clearly negative.

Spectrum on demand light source (SOLS) for advanced photovoltaic characterization.

We report a multi-purpose spectrum-on-demand light source (SOLS), conceived primarily but not exclusively for the multiple and advanced characterization of photovoltaic (PV) materials and devices. The apparatus is a spectral shaper illumination device, providing a tunable and spectrally shaped light beam produced by modulating the intensity and/or wavelength range of a primary light source. SOLS stands out from the state of the art because it produces almost any spectrum on demand and delivers two types of output: a spectrally shaped and spatially homogeneous beam over its cross section for areal illumination or a spatially and spectrally split beam into its wavelength components, a unique capability suited to characterize lateral-tandem (Rainbow) solar cells.

Using pressure to unravel the structure-dynamic-disorder relationship in metal halide perovskites.

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.

RAINBOW Organic Solar Cells: Implementing Spectral Splitting in Lateral Multi-Junction Architectures.

While multi-junction geometries have the potential to boost the efficiency of organic solar cells, the experimental gains yet obtained are still very modest. This work proposes an alternative spectral splitting device concept in which various individual semiconducting junctions with cascading bandgaps are laid side by side, thus the name RAINBOW. Each lateral sub-cell receives a fraction of the spectrum that closely matches the main absorption band of the given semiconductor.

The Ferroelectric-Ferroelastic Debate about Metal Halide Perovskites.

Metal halide perovskites (MHPs) are solution-processed materials with exceptional photoconversion efficiencies that have brought a paradigm shift in photovoltaics. The nature of the peculiar optoelectronic properties underlying such astounding performance is still controversial. The existence of ferroelectricity in MHPs and its alleged impact on photovoltaic activity have fueled an intense debate, in which unanimous consensus is still far from being reached.

Anisotropic thermoreflectance thermometry: A contactless frequency-domain thermoreflectance approach to study anisotropic thermal transport.

We developed a novel contactless frequency-domain thermoreflectance approach to study thermal transport, which is particularly convenient when thermally anisotropic materials are considered. The method is based on a line-shaped heater geometry, produced with a holographic diffractive optical element, instead of using a spot heater as in conventional thermoreflectance. The heater geometry is similar to the one used in the 3-omega method, however, keeping all the technical advantages offered by non-contact methodologies.

Multifunctional Switch Based on Spin-Labeled Gold Nanoparticles.

The fabrication of multifunctional switches is a fundamental step in the development of nanometer-scale molecular spintronic devices. The anchoring of active organic radicals on gold nanoparticles (AuNPs) surface is little studied and the realization of AuNPs-based switches remains extremely challenging. We report the first demonstration of a surface molecular switch based on AuNPs decorated with persistent perchlorotriphenylmethyl (PTM) radicals.

Disentangling Electron-Phonon Coupling and Thermal Expansion Effects in the Band Gap Renormalization of Perovskite Nanocrystals.

The complex electron-phonon interaction occurring in bulk lead halide perovskites gives rise to anomalous temperature dependences, like the widening of the electronic band gap as temperature increases. However, possible confinement effects on the electron-phonon coupling in the nanocrystalline version of these materials remain unexplored. Herein, we study the temperature (ranging from 80 K to ambient) and hydrostatic pressure (from atmospheric to 0.

Ferroelectricity-free lead halide perovskites.

Direct piezoelectric force microscopy (DPFM) is employed to examine whether or not lead halide perovskites exhibit ferroelectricity. Compared to conventional piezoelectric force microscopy, DPFM is a novel technique capable of measuring piezoelectricity directly. This fact is fundamental to be able to examine the existence of ferroelectricity in lead halide perovskites, an issue that has been under debate for several years.

Equal Footing of Thermal Expansion and Electron-Phonon Interaction in the Temperature Dependence of Lead Halide Perovskite Band Gaps.

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 MAPbI (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.

Towards chemically neutral carbon cleaning processes: plasma cleaning of Ni, Rh and Al reflective optical coatings and thin Al filters for free-electron lasers and synchrotron beamline applications.

The choice of a reflective optical coating or filter material has to be adapted to the intended field of application. This is mainly determined by the required photon energy range or by the required reflection angle. Among various materials, nickel and rhodium are common materials used as reflective coatings for (soft) X-ray mirrors.

Hydroxypropyl cellulose photonic architectures by soft nanoimprinting lithography.

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.

Thermal transport in epitaxial Si Ge alloy nanowires with varying composition and morphology.

We report on structural, compositional, and thermal characterization of self-assembled in-plane epitaxial Si Ge alloy nanowires grown by molecular beam epitaxy on Si (001) substrates. The thermal properties were studied by means of scanning thermal microscopy (SThM), while the microstructural characteristics, the spatial distribution of the elemental composition of the alloy nanowires and the sample surface were investigated by transmission electron microscopy and energy dispersive x-ray microanalysis. We provide new insights regarding the morphology of the in-plane nanostructures, their size-dependent gradient chemical composition, and the formation of a 5 nm thick wetting layer on the Si substrate surface.

Low-temperature resonant Raman asymmetry in 2H-MoS under high pressure.

We report on the combined effect of temperature (6 K-300 K) and high pressure (up to 6 GPa) on the resonant Raman scattering by A phonons in bulk 2H-MoS, as the energy of the A exciton is tuned into resonance with an exciting laser at E  =  1.96 eV. As expected, the pressure to be applied for attaining resonant conditions decreases with decreasing temperature.

Dynamic disorder, phonon lifetimes, and the assignment of modes to the vibrational spectra of methylammonium lead halide perovskites.

We present Raman and terahertz absorbance spectra of methylammonium lead halide single crystals (MAPbX, X = I, Br, Cl) at temperatures between 80 and 370 K. These results show good agreement with density-functional-theory phonon calculations. Comparison of experimental spectra and calculated vibrational modes enables confident assignment of most of the vibrational features between 50 and 3500 cm.

Exploring the origin of high optical absorption in conjugated polymers.

The specific optical absorption of an organic semiconductor is critical to the performance of organic optoelectronic devices. For example, higher light-harvesting efficiency can lead to higher photocurrent in solar cells that are limited by sub-optimal electrical transport. Here, we compare over 40 conjugated polymers, and find that many different chemical structures share an apparent maximum in their extinction coefficients.

Photoinduced p- to n-type Switching in Thermoelectric Polymer-Carbon Nanotube Composites.

UV-induced switching from p- to n-type character is demonstrated during deposition of carbon-nanotube-conjugated polymer composites. This opens the possibility to photopattern n-type regions within an otherwise p-type film, which has a potential for complementary circuitry or, as shown here, thermoelectric generators made from a single solution.

Effects of magnetic field gradients on the aggregation dynamics of colloidal magnetic nanoparticles.

We have used low-field (1)H nuclear-magnetic resonance (NMR) spectroscopy and molecular dynamics (MD) to investigate the aggregation dynamics of magnetic particles in ionic ferrofluids (IFFs) in the presence of magnetic field gradients. At the beginning of the experiments, the measured NMR spectra were broad and asymmetric, exhibiting two features attributed to different dynamical environments of water protons, depending on the local strength of the field gradients. Hence, the spatial redistribution of the magnetic particles in the ferrofluid caused by the presence of an external magnetic field in a time scale of minutes can be monitored in real time, following the changes in the features of the NMR spectra during a period of about an hour.

Optical and mechanical properties of nanofibrillated cellulose: Toward a robust platform for next-generation green technologies.

Nanofibrillated cellulose, a polymer that can be obtained from one of the most abundant biopolymers in nature, is being increasingly explored due to its outstanding properties for packaging and device applications. Still, open challenges in engineering its intrinsic properties remain to address. To elucidate the optical and mechanical stability of nanofibrillated cellulose as a standalone platform, herein we report on three main findings: (i) for the first time an experimental determination of the optical bandgap of nanofibrillated cellulose, important for future modeling purposes, based on the onset of the optical bandgap of the nanofibrillated cellulose film at Eg≈275 nm (4.

Red luminescence and ferromagnetism in europium oxynitridosilicates with a β-K2SO4 structure.

The new compounds LaSrSiO3N and LaBaSiO3N activated with Eu(2+) are orange-red light-emitting luminescent materials under excitation in the UV-blue range. They represent the first examples of stoichiometric alkaline earth oxynitridosilicates with a β-K2SO4 structure. The isostructural compound LaEuSiO3N is ferromagnetic with a Curie temperature of 3 K and also shows red luminescence (λmax = 705 nm) under excitation at 405 nm.

In-plane thermal conductivity of sub-20 nm thick suspended mono-crystalline Si layers.

We measure the thermal conductivity of a 17.5-nm-thick single crystalline Si layer by using a suspended structure developed from a silicon-on-insulator wafer, in which the Si layer bridges the suspended platforms. The obtained value of 19 Wm(-1) K(-1) at room temperature represents a tenfold reduction with respect to bulk Si.

Poly(3-hexylthiophene) nanowires in porous alumina: internal structure under confinement.

We study the structure of poly(3-hexylthiophene) (P3HT) subjected to nanoscale confinement in two dimensions (2D) as imposed by the rigid walls of nanopore anodic aluminum oxide (AAO) templates. P3HT nanowires with aspect ratios (length-to-diameter) above 1000 and diameters ranging between 15 nm and 350 nm are produced in the pores of the AAO templates via two processing routes. These are, namely, drying a solution or cooling from the melt.

Effect of structure and interlayer diffusion in organic position sensitive photodetectors based on complementary wedge donor/acceptor layers.

We have developed organic photodetectors based on two complementary wedge layers made of CuPc and C60 and observed a strong spatial dependence of the spectral response on the position of the incident light spot. Photocurrent measurements are correlated with atomic force microscopy (AFM), micro-Raman and ellipsometry maps in order to provide insights into the local donor/acceptor concentration, layer thickness and nature of the donor-acceptor interface along the direction of the thickness gradient. Deviations in spatial dependence between experimental photocurrent values and those predicted with a model assuming a sharp and well defined organic-organic interface are discussed in terms of inter-diffusion layers.

Probing local strain and composition in Ge nanowires by means of tip-enhanced Raman scattering.

Local strain and Ge content distribution in self-assembled, in-plane Ge/Si nanowires grown by combining molecular beam epitaxy and the metal-catalyst assisted-growth method were investigated by tip-enhanced Raman scattering. We show that this technique is essential to study variations of physical properties of single wires at the nanoscale, a task which cannot be achieved with conventional micro-Raman scattering. As two major findings, we report that (i) the Ge distribution in the (001) crystallographic direction is inhomogeneous, displaying a gradient with a higher Ge content close to the top surface, and (ii) in contrast, the (uncapped) wires exhibit essentially the same small residual compressive strain everywhere along the wire.