Bulk Cs 3 Bi 2 I 9 exhibits zero-dimensional (0-D) perovskite crystal structure at the molecular level, providing scopes for novel optical properties compared to three-dimensional perovskite structures. Here, 0-D refers to the crystal structure irrespective of the size of the crystal. We have prepared colloidal Cs 3 Bi 2 I 9 nanocrystals and elucidated the unique optical properties arising from their 0-D crystal structure. Absorption spectrum at 10 K confirms that the electronic band gap of Cs 3 Bi 2 I 9 nanocrystals is at 2.86 eV, along with a sharp excitonic peak at 2.56 eV, resulting in a very high excitonic binding energy, E b X = 300 meV. Interestingly, we observe two peaks in the photoluminescence spectra at room temperature on both sides of the excitonic absorption energy. Because E b X (300 meV) ≫ effective phonon energy (36 meV), the phonon-mediated relaxation of carriers from conduction band minimum to the excitonic state is suppressed to an extent. Consequently, two photoluminescence peaks related to both the bulk band edge and the excitonic transitions are observed. Furthermore, Rb 3 Bi 2 I 9 nanocrystals have also been synthesized, but they exhibit two-dimensional layered structure, unlike the 0-D structure of Cs 3 Bi 2 I 9 .
Herein, carbon nanosphere-decorated vanadium pentoxide (C@V 2 O 5 ) hybrid nanobelts were grown via a single step hydrothermal route with improved electronic conductivity as compared to that of pristine oxide. This hybrid nanomaterial exhibits different complimentary ranges of optimum post-growth annealing temperatures, which are suitable for dual applications either in electro-chromic smart windows or in supercapacitors. C@V 2 O 5 nanobelts annealed at 350 C appear to favor electro-chromic applications. They exhibit maximum dynamic optical transmission modulation as they switch from yellow to dark green, fast switching response, and high visible transmittance. In contrast, C@V 2 O 5 nanobelts annealed at 250 C have been found to be most suitable for supercapacitor applications. They display a high specific capacity and an enhanced diffusion coefficient. Moreover, they exhibit long lifetimes with a capacity retention of $94% even after 5000 cycles of operation. Therefore, the obtained results clearly indicate that optimization of the post-growth annealing temperatures is very important and rather complementary in nature in terms of determining the most favorable device functionalities. It enables us to optimally tune these hybrid nanomaterials for targeted, device-specific, energy applications in either electrochromic or supercapacitor technologies simply based on the annealing temperature alone.
In the era of intelligent automation, smart energy storage devices are highly sought after due to their capability to reveal their state of charge by a visual color change. In principle, this is typically obtained by integrating electrochromic materials which demonstrate a change in optical band gap during redox reactions as electrodes in batteries/supercapacitors. Among various electrochromic materials, transition metal oxides have received significant interests because of their natural abundance, good cycling stability and high electrochemical response. In this work, metal organic framework (MOF) derived carbon embedded NiO (NiO@C) demonstrates very fast switch-ing speeds of ~1.4 s for coloration and ~3.5 s for bleaching with high coloration efficiency of ~135.16 cm 2 /C. These aspects make MOF-derived NiO@C a potential candidate to be integrated as a smart positive electrode in an alkaline zinc (Zn) battery to construct a rechargeable ZnÀ NiO electrochromic (EC) battery. The ZnÀ NiO EC battery delivers an average voltage of ~1.84 V and demonstrates a specific capacity of ~85.3 mAh/m 2 at 0.1 mA/cm 2 current density. It visually displays its energy storage level by changing its color from dark brown (charged state) to colorless (discharged state), making it a potential candidate in smart windows and energy storage displays.
Modern technological trend in smart electronic devices demand more intelligent automation. Simultaneous integration of energy storage and multicolor electrochromism in a single device improve the user-device interfacing based on a...
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