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The emerging ultrawide-bandgap AlGaN alloy system holds promise for the development of advanced materials in the next generation of power semiconductor and UV optoelectronic devices. Within this context, heterostructures based on III-nitrides are very popular in view of their applications as electronic and optoelectronic components. AlGaN-based deep UV emitters are gaining visibility due to their disinfection capabilities. Likewise, high electron mobility transistors are attracting increasing attention owing to their superior electron transport which yields high-speed and high-power applications. These devices are conventionally made of AlGaN/GaN heterostructures grown on foreign substrates. However, structural defects, including stress induced by a mismatch in unit cell parameters and the presence of dislocations, can not only decrease the efficiency of the light emitters (by facilitating the non-radiative recombination of electron–hole pairs), but also impede electron mobility within the two-dimensional electron gas at the AlGaN/GaN interface. Therefore, the significance of obtaining high-quality AlGaN layers becomes evident. Including a thin AlN interlayer between the GaN buffer layer and AlGaN is a possible answer to address these drawbacks. Not only do we show that a thin AlN layer, approximately ≤3 nm in thickness, between the GaN buffer and AlGaN layers, is effective in decreasing the dislocation densities in the AlGaN layer by around 30%, but also this is responsible for an increase in the electron mobility (approximately 33%) compared to a classical AlGaN/GaN heterostructure. Additionally, the resulting heterostructure exhibits better optical quality, with a 7-fold increase in intensity as well as a 20% reduction in full-width at half-maximum in the AlGaN emission. [ABSTRACT FROM AUTHOR]

Fully inorganic perovskite nanocrystals (NCs) have been widely investigated due to their potential as very interesting active materials for several types of photonic and optoelectronic devices. Despite several experiments designed to investigate the basic emission properties of these NCs, a clear and complete understanding of their photophysics is still missing. In this work, temperature‐dependent steady state and time‐resolved photoluminescence (PL) measurements are used to investigate the nature of the emitting states, the origin of the excitation relaxation dynamics and the effects of aging upon long exposure to wet air for thin films of CsPbBr3 NCs prepared by coprecipitation. It is demonstrated that both free excitons and localized excitons contribute to the NC emission and that electron traps within the conduction band, ≈14 meV and ≈80 meV above the band edge, determine thermal emission quenching. Moreover, it is shown that the non‐exponential PL relaxation dynamics are due to short‐range energy migration within a disordered distribution of localized states between 10 and 100 K, with activation of a second, long‐range diffusion process at higher temperatures. It is also demonstrated that aging determines the variations in defect levels, exciton‐phonon coupling and exciton relaxation dynamics. The results substantially improve the current understanding of the basic photophysics of CsPbBr3 NC films and of the aging effects and are expected to provide a useful guide for future characterization of other similar materials. [ABSTRACT FROM AUTHOR]

Dense arrays of core–shell nanowires possess great potential as superabsorptive media for the fabrication of efficient solar cells. We report on GaAs near-band-edge absorption properties of free-standing GaAs–AlGaAs core–shell nanowires having different shell thicknesses, by detailed line-shape analyses of room-temperature photoreflectance (PR) spectra, employing first-derivative Gaussian and Lorentzian models of the GaAs complex dielectric function. Line-shape analyses of the nanowire PR spectra returned a doublet of resonance lines at energies between 1.410 and 1.422 eV, ascribed to strain-split heavy- and light-hole exciton absorption transitions in the GaAs nanowire cores. The optical oscillator strengths of exciton resonances evaluated by Lorentzian analyses of PR features showed a significant enhancement (up to 30×) of GaAs band-edge optical absorption in nanowires with respect to the reference planar structure. Additionally, values of integrated Lorentzian moduli were normalized to the total GaAs core volume fill fraction (estimated in the range 0.5–7.0% with respect to a planar layer of the same height) within each nanowire ensemble, achieving a first ever experimental estimate of the GaAs near band-edge absorption enhancement factor for GaAs–AlGaAs core–shell nanowires in the range 22–190, depending on the nanowire inner core–shell structure. Such strong absorption enhancement is ascribed to improved wave-guiding of incident light into the GaAs cores by the surrounding AlGaAs shell (its average thickness being estimated between ∼14 and 100 nm in the present nanostructures). [ABSTRACT FROM AUTHOR]

In this work, we present a detailed optical investigation of the effects of the environment on the photoluminescence (PL) spectra and the relaxation dynamics of pristine and aged CsPbBr3 nanocrystal (NC) thin films. We demonstrate that, contrary to previous results on similar NCs, the PL intensity of pristine NCs is higher when the sample is in wet air than in vacuum, due to the passivation of defects reducing the free exciton trapping and the bound excitons non-radiative relaxation. The aged NCs show a PL intensity increase in wet air nine times stronger than the pristine ones, due to an interplay between static and dynamic effects, increasing the number of emitting NCs and reducing the non-radiative recombination rate of free excitons. These results improve the understanding of the possible interactions between perovskite NCs and the environment, which could be relevant for the development of optical gas sensors exploiting perovskite NCs. [ABSTRACT FROM AUTHOR]

In the present work, we investigate in detail the complex morphological, structural and chemical profile of a III–V multistep quantum dot (QD) system grown by molecular beam epitaxy and employed for the realization of enlarged bandgap intermediate band solar cells (IBSCs). The peculiar multistep design, used during the QD growth for the band-gap engineering of the IBSCs and involving a gradual compositional change from quaternary (AlInGaAs) to ternary (InGaAs) and binary (InAs) compounds, affects the morphological, chemical and structural properties of the QDs and then the electro-optical behavior of the IBSC device. These properties are assessed by combining and comparing scanning transmission electron microscopy experiments, based on high angle annular dark field imaging, with strain analysis and energy dispersive X-ray spectroscopy. The analysis demonstrates defect-free QDs embedded in the AlGaAs matrix, with a well-defined shape and faceting. The results of the STEM experiments are correlated with the complex carrier dynamics and electro-optical properties of the solar cells, showing the potential of the proposed structure for further improvement in the design of new IBSCs, by virtue of an extremely confined strain field and high energy barrier characterizing our QD system. [ABSTRACT FROM AUTHOR]

The quest for single-stage deposition of CuInGaSe2 (CIGS) is an open race to replace very effective but capital intensive thin film solar cell manufacturing processes like multiple-stage coevaporation or sputtering combined with high pressure selenisation treatments. In this paper the most recent achievements of Low Temperature Pulsed Electron Deposition (LTPED), a novel single stage deposition process by which CIGS can be deposited at 250 °C, are presented and discussed. We show that selenium loss during the film deposition is not a problem with LTPED as good crystalline films are formed very close to the melting temperature of selenium. The mechanism of formation of good ohmic contacts between CIGS and Mo in the absence of any MoSe2 transition layers is also illustrated, followed by a brief summary of the measured characteristics of test solar cells grown by LTPED. The 17% efficiency target achieved by lab-scale CIGS devices without bandgap modulation, antireflection coating or K-doping is considered to be a crucial milestone along the path to the industrial scale-up of LTPED. The paper ends with a brief review of the open scientific and technological issues related to the scale-up and the possible future applications of the new technology. [ABSTRACT FROM AUTHOR]

Arrays of metal nano-holes have proved to be among of the most promising structures for applications in the field of nano-photonics and optoelectronics. Supporting both localized and propagating surface plasmons resonances, they are characterized by very high versatility thanks to the tunability of these modes, by means of the change of their periodicity, the size of the holes and metal composition. The interaction between different optical features can be exploited to modulate electromagnetic field distribution leading various hot-spots excitations on the metal surfaces. In this work, long range ordered arrays of nano-holes in thin gold films, with different geometrical characteristics, were fabricated by a modified nano-sphere lithography protocol, which allows precise control on holes' dimensions together with the preservation of the order and of the pristine periodicity of the array. An in-depth analysis of the correlation between surface plasmon modes interference and its effect on electromagnetic field distribution is proposed, both by numerical simulations and experimentally. Finally, metal nano-holes arrays are exploited for surface enhanced Raman experiments, evaluating and comparing their performances by the estimation of the enhancement factor. Values close to the single molecule detection are obtained for most of the samples, proving their potentialities in surface enhanced spectroscopy applications. [ABSTRACT FROM AUTHOR]

Over the past decade, interest about metal halide perovskites has rapidly increased, as they can find wide application in optoelectronic devices. Nevertheless, although thermal evaporation is crucial for the development and engineering of such devices based on multilayer structures, the optical properties of thermally deposited perovskite layers (spontaneous and amplified spontaneous emission) have been poorly investigated. This paper is a study from a nano- to micro- and macro-scale about the role of light-emitting species (namely free carriers and excitons) and trap states in the spontaneous emission of thermally evaporated thin layers of CH3NH3PbBr3 perovskite after wet air UV light trap passivation. The map of light emission from grains, carried out by SNOM at the nanoscale and by micro-PL techniques, clearly indicates that free and localized excitons (EXs) are the dominant light-emitting species, the localized excitons being the dominant ones in the presence of crystallites. These species also have a key role in the amplified spontaneous emission (ASE) process: for higher excitation densities, the relative contribution of localized EXs basically remains constant, while a clear competition between ASE and free EXs spontaneous emission is present, which suggests that ASE is due to stimulated emission from the free EXs. [ABSTRACT FROM AUTHOR]

Lead halide perovskites are emerging as extremely interesting active materials for a wide variety of optoelectronic and photonic devices. A deep understanding of their photophysics is thus fundamental in order to properly understand the origins of the materials active properties and to provide strategies for improving them. In this work, we exploit the local morphological variations in a drop-cast thin CsPbBr 3 nanocrystal film to show that the aggregation of NCs has strong effects on the peak wavelengths of PL spectra, the linewidth, and the intensity of dependence on temperature. An analysis based on models that are frequently used in the literature led to completely different conclusions about the intrinsic NC emission properties extracted from spectra measured in points with different contribution of the PL from the aggregates. Our results demonstrate that extreme care has to be used in order to correctly correlate the spectral PL features with the intrinsic emission properties of lead halide perovskite NC films. [ABSTRACT FROM AUTHOR]

The authors investigated, by femtosecond pump-probe measurements, the effects on Auger and trapping processes of excitation resonant (R) to the 1S absorption in CdSe nanorods. They demonstrate that R pumping allows one to avoid exciton trapping in high energy defect states and that the presence of empty defect states leads to an enhancement of the Auger relaxation, with respect to the pumping at energies high above the band gap. Moreover, they show that, despite the Auger enhancement, R pumping increases the stimulated emission lifetime demonstrating that it is not limited by Auger scattering, as widely believed, but by photoabsorption processes, involving defect states. [ABSTRACT FROM AUTHOR]

Colloidal ZnSe nanocrystals are coated with a ZnS shell and are investigated by means of time-resolved photoluminescence spectroscopy. These nanocrystals are stable against photooxidation and have photoluminescence quantum efficiencies around 15% even months after the synthesis. Recombination processes are investigated as a function of the excitation intensity and the emission energy. Nonlinear processes such as Auger recombination dominate in the early times after the photogeneration of electron-hole pairs. The lifetime of single exciton states is shorter in smaller ZnSe dots than in larger ones, in accordance with theoretical predictions and as previously observed on similar systems. [ABSTRACT FROM AUTHOR]

Pt/SiO2/W devices of controlled size, thickness, and interfaces were fabricated by electron beam induced deposition (EBID) in a focused ion beam (FIB) system, for possible applications as metal-insulator-metal diodes. A systematic study of the deposition parameters used for the fabrication of the devices was carried out by scanning electron microscopy, energy dispersive x-ray spectroscopy, and transmission electron microscopy (TEM). In particular, the morphological and structural quality of the deposited layers, their mutual interfaces, and the thickness of the insulating layer were accurately investigated, being key issues for the application of these multilayer structures as tunneling diodes; to this aim, a novel TEM sample preparation approach was developed, which allows the in situ FIB preparation of ready-to-be-observed multilayer structures; by using this approach, TEM samples can be directly made, by depositing the structures in the same conditions used for the device fabrication, in the correct geometry for the cross sectional investigations and, without any further preparation steps. The capability of the EBID technique to fabricate submicrometer electronic devices with promising electrical properties, such as asymmetry and nonlinearity, was demonstrated. [ABSTRACT FROM AUTHOR]

We investigate the Amplified Spontaneous Emission (ASE) properties of a prototypical host-guest polymer polymer blend, namely poly(9,9-dioctylfluorene) (PF8) and poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) blend, with different concentration ratio. We show that the initial F8BT content increase causes an increase of the F8BT ASE threshold, even leading to ASE suppression for F8BT contents between 25% and 75%. ASE is then recovered upon further increase of the F8BT relative content. We demonstrate that the ASE properties of the PF8:F8BT are dominated by morphology effects, like submicrometric phase segregation, determining the net gain of the active waveguides. [ABSTRACT FROM AUTHOR]

In this work we present a solar cell structure where the concept of intermediate band is exploited by a high energy barrier AlGaAs material with embedded InAs-based quantum dots via a multistep growth approach. In this way the intrinsic issues related to different surface kinetics of involved species (Ga, In and Al adatoms) and affecting crystal quality are successfully overcome. With respect to energy band engineering of the cell, this growth approach introduces a two-dimensional quaternary layer and consequently an additional energy band, between the host junction and the dot energy levels. This band results strongly related to the quantum dot states by thermal transferring and inter-level filling processes. Moreover, low temperature (up to 100 K) photocurrent generation via additional infrared absorption is promoted by the employed band engineering, thus representing an effective method to extend intermediate band solar cell design flexibility. [ABSTRACT FROM AUTHOR]

The combination of lead halide perovskite nanocrystals and conjugated polymer in a blend film opens the way to the realization of hybrid active layers with widely tunable optical and electrical properties. However, the interaction between the polymeric and the perovskite component of the blends is mainly unexplored to date. In this work we perform temperature-dependent photoluminescence and time resolved photoluminescence measurements in order to deeply investigate the photophysics of a poly(9,9-dioctylfluorene- co -benzothiadiazole) (F8BT):CsPbI 1.5 Br 1.5 nanocrystal hybrid film. Our results suggest that the primary interaction channel is charge transfer, both from F8BT to the NCs and from the NCs to F8BT, while Förster resonant energy transfer has no visible effects. Moreover, we show that the charge transfer is assisted by energy migration within the F8BT excited state distribution and that it is dependent on the local micromorphology of the film. Our work improves the current understanding of the polymer:perovskite NC interactions in hybrid films, and it is expected to be relevant for the development of hybrid organic-perovskite optoelectronic devices., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Quasi-2D (two-dimensional) hybrid perovskites are emerging as a new class of materials with high photoluminescence yield and improved stability compared to their three-dimensional (3D) counterparts. Nevertheless, despite their outstanding emission properties, few studies have been reported on amplified spontaneous emission (ASE) and a thorough understanding of the photophysics of these layered materials is still lacking. In this work, we investigate the ASE properties of multilayered quasi-2D BA 3 MA 3 Pb 5 Br 16 films through the dependence of the photoluminescence on temperature and provide a novel insight into the emission processes of quasi-2D lead bromide perovskites. We demonstrate that the PL and ASE properties are strongly affected by the presence, above 190 K, of a minor fraction of the high temperature (HT) phase. This phase dominates the PL spectra at low excitation density and strongly affects the ASE properties. In particular, ASE is only present between 13 K and 230 K, and, at higher temperatures, it is suppressed by absorption of charge transfer states of the HT phase. Our results improve the understanding of the difficulties to obtain ASE at room temperature from these quasi-2D materials and are expected to guide possible materials improvement in order to exploit their excellent emission properties also for the realization of low threshold optically pumped lasers.

This work aims to investigate and characterize the photo-ignition phenomenon of MWCNT/ferrocene mixtures by using a continuous wave (CW) xenon (Xe) light source, in order to find the power ignition threshold by employing a different type of light source as was used in previous research (i.e., pulsed Xe lamp). The experimental photo-ignition tests were carried out by varying the weight ratio of the used mixtures, luminous power, and wavelength range of the incident Xe light by using selective optical filters. For a better explanation of the photo-induced ignition process, the absorption spectra of MWCNT/ferrocene mixtures and ferrocene only were obtained. The experimental results show that the luminous power (related to the entire spectrum of the Xe lamp) needed to trigger the ignition of MWCNT/ferrocene mixtures decreases with increasing metal nanoparticles content according to previously published results when using a different type of light source (i.e., pulsed vs CW Xe light source). Furthermore, less light power is required to trigger photo-ignition when moving towards the ultraviolet (UV) region. This is in agreement with the measured absorption spectra, which present higher absorption values in the UV-vis region for both MWCNT/ferrocene mixtures and ferrocene only diluted in toluene. Finally, a chemo-physical interpretation of the ignition phenomenon is proposed whereby ferrocene photo-excitation, due to photon absorption, produces ferrocene itself in its excited form and is thus capable of promoting electron transfer to MWCNTs. In this way, the resulting radical species, FeCp2 +∙ and MWCNT - , easily react with oxygen giving rise to the ignition of MWCNT/ferrocene samples.