Spray-driven halide exchange in solid-state CsPbX3 nanocrystal films

CsPbI₃ perovskite nanocrystals (NCs) are promising building blocks for photovoltaics and optoelectronics. However, they exhibit an essential drawback in the form of phase stability: α-phase, with a ∼1.80 eV bandgap, can easily experience a phase transition to a non-radiative orthorhombic δ-phase in an ambient environment. This leads to the need to carry out the CsPbI₃-based device fabrication in an inert atmosphere, which is technologically inconvenient and expensive. One of the most successful approaches proposed to overcome this problem is synthesizing mixed halide CsPbBr_3-xI_x NCs to improve the stability of the α-phase perovskite structure. However, the formation of high-quality thin films of CsPbBr_3-xI_x NCs with high PLQY is challenging owing to the degradation of their optical properties after deposition on a substrate. This work presents spray coating to carry out a solid-state anion exchange in CsPbBr₃ NCs thin films at ambient conditions with low-demanding reaction conditions. This constitutes a novel open-air and annealing-free technology to manufacture CsPbBr_3-xI_x NC thin films with high optical quality and record high photoluminescence quantum yields (PLQY) based on spray-driven halide (Br^- to I^-) anion exchange in a solid-state phase. Besides, tunable emission wavelengths between 520 and 670 nm can be obtained from CsPbBr_3-xI_x NC films using accurate tuning volumes of HI solution sprayed over the initial surface of CsPbBr₃ film to provide the halide exchange. The optical quality of the halide-exchanged PNCs films remains practically identical to that of initial Br-containing layers, with a remarkable PLQY enhancement after anion exchange, from ∼61% for CsPbBr₃ thin films emitting at 520 nm to ∼84% for mixed halide CsPbBr_3-xI_x film emitting at 640 nm. The huge potential of the system is confirmed by demonstrating a low-threshold amplified spontaneous emission.

Enhanced Nonlinear Optical Coefficients of MAPbI3 Thin Films by Bismuth Doping

The poor photostability under ambient conditions of hybrid halide perovskites has hindered their recently explored promising nonlinear optical properties. Here, we show how Bi³⁺ can partially substitute Pb²⁺ homogeneously in the commonly studied MAPbI₃, improving both environmental stability and photostability under high laser irradiation. Bi content around 2 atom % produces thin films where the nonlinear refractive (n₂) and absorptive coefficients (β), which modify the refractive index (Δn) of the material with light fluence (I), increase up to factors of 4 and 3.5, respectively, compared to undoped MAPbI₃. Higher doping inhibits the nonlinear parameters; however, the samples show higher fluence damage thresholds. Thus, these results provide a road map on how MAPbI₃ can be engineered for practical cost-effective nonlinear applications by means of Bi doping, including optical limiting devices and multiple-harmonic generation into optoelectronics devices.

Structural characterization of bulk and nanoparticle lead halide perovskite thin films by (S)TEM techniques

Lead halide (APbX₃) perovskites, in polycrystalline thin films but also perovskite nanoparticles (NPs) has demonstrated excellent performance to implement a new generation of photovoltaic and photonic devices. The structural characterization of APbX₃ thin films using (scanning) transmission electron microscopy ((S)TEM) techniques can provide valuable information that can be used to understand and model their optoelectronic performance and device properties. However, since APbX₃ perovskites are soft materials, their characterization using (S)TEM is challenging. Here, we study and compare the structural properties of two different metal halide APbX₃ perovskite thin films: bulk CH₃NH₃PbI₃ prepared by spin-coating of the precursors in solution and CsPbBr₃ colloidal NPs synthetized and deposited by doctor blading. Both specimen preparation methods and working conditions for analysis by (S)TEM are properly optimized. We show that CH₃NH₃PbI₃ thin films grown by a one-step method are composed of independent grains with random orientations. The growth method results in the formation of tetragonal perovskite thin films with good adherence to an underlying TiO₂ layer, which is characterized by a photoluminescence (PL) emission band centered at 775 nm. The perovskite thin films based on CsPbBr₃ colloidal NPs, which are used as the building blocks of the film, are preserved by the deposition process, even if small gaps are observed between adjacent NPs. The crystal structure of CsPbBr₃ NPs is cubic, which is beneficial for optical properties due to its optimal band gap. The absorption and PL spectra measured in both the thin film and the colloidal solution of CsPbBr₃ NPs are very similar, indicating a good homogeneity of the thin films and the absence of aggregation of NPs. However, a particular care was required to avoid long electron irradiation times during our structural studies, even at a low voltage of 80 kV, as the material was observed to decompose through Pb segregation.

Optical contrast of 2D InSe on SiO2/Si and transparent substrates using bandpass filters

The particular optical and electronic properties recently reported for 2D InSe depict this 2D material as being very versatile for future electronic and optoelectronic devices with tunable and optimized functionalities. For its fundamental study and the development of practical applications, rapid and accurate identification methods of atomically thin InSe are essential. Here, we demonstrate an enhancement of the optical contrast between InSe nanosheets and the underlying SiO₂/Si substrate by illuminating with a 40 nm wide bandpass filter centered at 500 nm. Moreover, we study the optical contrast of 2D InSe on transparent substrates. Our results suggest that a good optical contrast is achieved for transparent substrates with low real refractive indices such as LiF or a viscoelastic polydimethylsiloxane stamp. In this case, an optimum optical contrast would be achieved by using a bandpass filter centered at 450 nm. These results can be very useful for speeding up the continuously growing research on 2D InSe and its applications.

Efficient excitation of photoluminescence in a two-dimensional waveguide consisting of a quantum dot-polymer sandwich-type structure

In this Letter, we study a new kind of organic polymer waveguide numerically and experimentally by combining an ultrathin (10-50 nm) layer of compactly packed CdSe/ZnS core/shell colloidal quantum dots (QDs) sandwiched between two cladding poly(methyl methacrylate) (PMMA) layers. When a pumping laser beam is coupled into the waveguide edge, light is mostly confined around the QD layer, improving the efficiency of excitation. Moreover, the absence of losses in the claddings allows the propagation of the pumping laser beam along the entire waveguide length; hence, a high-intensity photoluminescence (PL) is produced. Furthermore, a novel fabrication technology is developed to pattern the PMMA into ridge structures by UV lithography in order to provide additional light confinement. The sandwich-type waveguide is analyzed in comparison to a similar one formed by a PMMA film homogeneously doped by the same QDs. A 100-fold enhancement in the waveguided PL is found for the sandwich-type case due to the higher concentration of QDs inside the waveguide.

High-resolution electron-beam patternable nanocomposite containing metal nanoparticles for plasmonics

Polymer nanocomposites containing noble metal nanoparticles are promising materials for plasmonic applications. In this paper, we report on a high-resolution negative-tone nanocomposite resist based on poly(vinyl alcohol) where silver nanoparticles and nanopatterns are simultaneously generated by electron-beam lithography. Our results indicate nanostructures with a relatively high concentration of nanoparticles and, consequently, an electromagnetic coupling among the nanoparticles. Therefore, the patternable nanocomposite described in this work may be a suitable material for future plasmonic circuitry.