The complete phase diagram of organic-cation solid solutions of lead iodide perovskites [FA x MA 1−x PbI 3 , where MA stands for methylammonium, CH 3 NH 3 , and FA for formamidinium, CH(NH 2 ) 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 MAPbI 3 was observed with decreasing temperature: cubic Pm3̅ m → 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 FArich compounds is cubic Pm3̅ m → tetragonal-II P4/mbm → tetragonal-III. The latter presumably belongs to the P4bm symmetry group, according to neutron scattering data. Strikingly, the isostructural (tetragonal-totetragonal) 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 PbI 6 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.

Photoelectron spectra from metal overlayers on GaP(llO) show that the photoionization light source may induce a surface photovoltage, causing an energy shift of valence-and core-level peaks. We analyze the dependence of this surface photovoltage on metal coverage, substrate doping, and temperature. The presence of a surface photovoltage seriously affects the determination of surface band bending by photoelectron spectroscopy, a technique which is generally thought to reflect the equilibrium electronic structure of metal-semiconductor interfaces. PACS numbers: 79.60.Eq, 72.40,+w, 73.30.+yDespite a large number of experimental studies, the interpretation of Schottky-barrier (SB) heights at metalsemiconductor interfaces, and the evolution of surface band bending in the low-metal-coverage regime, remains a controversial issue. While most SB-height data for applications-oriented metal-semiconductor interfaces are obtained from C/V or IIV experiments, information about the electronic structure of the interface, as well as the evolution of the SB and its final height, is usually obtained through photoemission experiments. These are carried out on well-defined substrate surfaces and metal overlayers prepared in situ. I2 In these studies, the surface band bending as a function of metal coverage, substrate doping, and temperature is derived from shifts in a substrate core-level line which may be detected even for thick overlayers (up to tens of A). Such shifts are interpreted as reflecting the position of the Fermi level in the band gap, with the implicit assumption that the condition of Fermi-level equilibrium is met. The possibility that the photoemission data could be affected by photon-induced electron-hole-pair creation and transport processes leading to a nonequilibrium charge distribution has been largely overlooked in these studies, in spite of observations that such effects may occur on clean as well as metal-covered semiconductor surfaces. 3,4 Here we show how such processes strongly affect the thicknessdependent determination of surface band bending, and the measurement of SB heights by photoelectron spectroscopy, for metals deposited on GaP(l 10). This observation may have wider implications for the study of other metal-semiconductor interfaces.

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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.

In these geometries, the excitation of antisymmetric plasmonic resonances can result in magnetism at optical frequencies, a key property for magnetic and left-handed components for visible optics. However, achieving a metamaterial composed of split ring resonators is not an easy task and the fabrication of such structures relies heavily on conventional low throughput lithographic processes. Another exciting optical property is chirality, which in plasmonics enables enantiomer detection and separation. [15,16] Despite the great interest to obtain chiral nanostructures, achieving this particular lack of symmetry in a scalable fashion remains a challenge. In general, plasmonic architectures are produced by focused electron or ion beam lithography, techniques that provide high resolution and versatility but that are difficult to scale up. On the other side of the spectrum there are fabrication techniques compatible with mass production but more restrictive in terms of the produced features such as nanosphere lithography (NSL), [17] interference lithography (IL), [18] and nanoimprinting lithography (NIL). [19-22] These lithographies, however, tend to produce symmetric periodic nanostructures which limit the number of optical properties accessible. One way to gain access to polarization-dependent response is by breaking the symmetry of the original array. This can be achieved simply by tilting the sample at the metallization stage, opening up a plethora of possibilities. The combination of NSL with oblique angle deposition (OAD) has led to a variety of asymmetric plasmonic nanostructures [23,24] from split ring resonators [25] to chiral nanostructures [26,27] and complex 3D plasmonic nanostrucures. [28,29] However, in the case of NSL, the microspheres tend to self-assemble almost exclusively into hexagonal arrays whose large-scale homogeneity is severely affected by the presence of cracks and defects of the final assembly, which in turn deteriorates the overall optical response and broadens the plasmonic resonances. Alternatively, NIL is the technique of preference for the upscaling of nanostructures with excellent quality, it is roll-to-roll compatible and it has an excellent resolution without requiring complex optical setups. [30,31] In NIL, prepatterned elastomeric molds are used as printing stamps in which the geometry, feature depth, and lattice parameter can be varied by changing the original master. These nanostructures combined with OAD can result in a rich variety of novel asymmetric plasmonic architectures; however, the versatility and potential 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 i...

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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 CsPbBr1I2. 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

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.