Shahab Ahmad

Blue-Green Color Tunable Solution Processable Organolead Chloride–Bromide Mixed Halide Perovskites for Optoelectronic Applications

Solution-processed organo-lead halide perovskites are produced with sharp, color-pure electroluminescence that can be tuned from blue to green region of visible spectrum (425–570 nm). This was accomplished by controlling the halide composition of CH3NH3Pb(BrxCl1–x)3 [0 ≤ x ≤ 1] perovskites. The bandgap and lattice parameters change monotonically with composition. The films possess remarkably sharp band edges and a clean bandgap, with a single optically active phase. These chloride–bromide perovskites can potentially be used in optoelectronic devices like solar cells and light emitting diodes (LEDs). Here we demonstrate high color-purity, tunable LEDs with narrow emission full width at half maxima (FWHM) and low turn on voltages using thin-films of these perovskite materials, including a blue CH3NH3PbCl3 perovskite LED with a narrow emission FWHM of 5 nm.

In Situ Intercalation Dynamics in Inorganic–Organic Layered Perovskite Thin Films

The properties of layered inorganic semiconductors can be manipulated by the insertion of foreign molecular species via a process known as intercalation. In the present study, we investigate the phenomenon of organic moiety (R-NH3I) intercalation in layered metal-halide (PbI2)-based inorganic semiconductors, leading to the formation of inorganic–organic (IO) perovskites [(R-NH3)2PbI4]. During this intercalation strong resonant exciton optical transitions are created, enabling study of the dynamics of this process. Simultaneous in situ photoluminescence (PL) and transmission measurements are used to track the structural and exciton evolution. On the basis of the experimental observations, a model is proposed which explains the process of IO perovskite formation during intercalation of the organic moiety through the inorganic semiconductor layers. The interplay between precursor film thickness and organic solution concentration/solvent highlights the role of van der Waals interactions between the layers, as well as the need for maintaining stoichiometry during intercalation. Nucleation and growth occurring during intercalation matches a Johnson–Mehl–Avrami–Kolmogorov model, with results fitting both ideal and nonideal cases.

Influence of the annealing temperatures on the photoluminescence of KCaBO3:Eu3+ phosphor

Novel red emitting KCaBO3:Eu phosphors have been synthesized by solid-state reaction at various temperatures. Systematic studies on annealing effects and consequent structural evolution and optical properties were investigated by various structural and photoluminescence studies. With an increase in annealing temperature (from 700 °C to 950 °C), these phosphors show a gradual change from a mixed low crystalline phase to a highly crystalline single phase, with minimized volatile impurities. Photoluminescence studies revealed that the low-temperature annealed phosphors showed distinct mixed emission composed of blue and red emissions upon UV excitation. Such dual emission is due to the coexistence of Eu3+ and Eu2+ ions, wherein the reduction of Eu3+ into Eu2+ was attributed to the presence of volatile impurities. Relatively high-temperature annealed phosphors exhibit strong red color photoluminescence due to homogeneously occupied Eu3+ ions in the host crystal charge-compensated (with K+ ions) sites of Ca2+ ions. The dominant red-to-orange emission intensity ratios and Judd–Ofelt parameters of Eu3+ ions support the strong covalent nature and site-occupation of higher asymmetry sites of K+ and Ca2+ ions. High emission life times and efficient and stable photoluminescence at different excitation wavelengths make these phosphors suitable for white LEDs and other display applications.

Strong Photocurrent from Two-Dimensional Excitons in Solution-Processed Stacked Perovskite Semiconductor Sheets.

Room-temperature photocurrent measurements in two-dimensional (2D) inorganic–organic perovskite devices reveal that excitons strongly contribute to the photocurrents despite possessing binding energies over 10 times larger than the thermal energies. The p-type (C6H9C2H4NH3)2PbI4 liberates photocarriers at metallic Schottky aluminum contacts, but incorporating electron- and hole-transport layers enhances the extracted photocurrents by 100-fold. A further 10-fold gain is found when TiO2 nanoparticles are directly integrated into the perovskite layers, although the 2D exciton semiconducting layers are not significantly disrupted. These results show that strong excitonic materials may be useful as photovoltaic materials despite high exciton binding energies and suggest mechanisms to better understand the photovoltaic properties of the related three-dimensional perovskites.

Hierarchical Assemblies of Carbon Nanotubes for Ultraflexible Li-Ion Batteries.

The flexible batteries that are needed to power flexible circuits and displays remain challenging, despite considerable progress in the fabrication of such devices. Here, it is shown that flexible batteries can be fabricated using arrays of carbon nanotube microstructures, which decouple stress from the energy-storage material. It is found that this battery architecture imparts exceptional flexibility (radius ≈ 300 μm), high rate (20 A g(-1) ), and excellent cycling stability.

Structural tunability and switchable exciton emission in inorganic-organic hybrids with mixed halides

Room-temperature tunable excitonic photoluminescence is demonstrated in alloy-tuned layered Inorganic-Organic (IO) hybrids, (C12H25NH3)2PbI4(1−y)Br4y (y = 0 to 1). These perovskite IO hybrids adopt structures with alternating stacks of low-dimensional inorganic and organic layers, considered to be naturally self-assembled multiple quantum wells. These systems resemble stacked monolayer 2D semiconductors since no interlayer coupling exists. Thin films of IO hybrids exhibit sharp and strong photoluminescence (PL) at room-temperature due to stable excitons formed within the low-dimensional inorganic layers. Systematic variation in the observed exciton PL from 510 nm to 350 nm as the alloy composition is changed, is attributed to the structural readjustment of crystal packing upon increase of the Br content in the Pb-I inorganic network. The energy separation between exciton absorption and PL is attributed to the modified exciton density of states and diffusion of excitons from relatively higher energy states...

Strong room-temperature ultraviolet to red excitons from inorganic organic-layered perovskites, (R-NH3)2MX4 (M=Pb2+, Sn2+, Hg2+; X=I−, Br−)

Abstract. Many varieties of layered inorganic-organic (IO) perovskite of type (R-NH3)2MX4 (where R: organic moiety, M: divalent metal, and X: halogen) were successfully fabricated and characterized. X-ray diffraction data suggest that these inorganic and organic structures are alternatively stacked up along c-axis, where inorganic mono layers are of extended corner-shared MX6 octahedra and organic spacers are the bi-layers of organic entities. These layered perovskites show unusual room-temperature exciton absorption and photoluminescence due to the quantum and dielectric confinement-induced enhancement in the exciton binding energies. A wide spectral range of optical exciton tunability (350 to 600 nm) was observed experimentally from systematic compositional variation in (i) divalent metal ions (M=Pb2+, Sn2+, Hg2+), (ii) halides (X=I− and Br−), and (iii) organic moieties (R). Specific photoluminescence features are due to the structure of the extended MX42− network and the eventual electronic band structure. The compositionally dependent photoluminescence of these IO hybrids could be useful in various photonic and optoelectronic devices.

Investigation of structural, dielectric, and magnetic properties of hard and soft mixed ferrite composites

Barium ferrite (hard ferrite) and manganese nickel zinc ferrite (soft ferrite) were successfully synthesized by citrate gel combustion technique. They were used to form the composites by mixing them properly in required compositions (x)BaFe12O19‐(1−x)Mn0.2Ni0.4Zn0.4Fe2O4 (0 ≤ x ≤ 1). X-ray diffraction (XRD) and scanning electron microscopy (SEM) were utilized to investigate the different structural and morphological parameters of pure and mixed ferrite composites. XRD and SEM results confirmed the coexistence of both phases in the composite material. Moreover, it has been observed that the composites were constituted by nanosized particles. Structure of pure soft ferrite was found to be cubic and that of pure hard ferrite was hexagonal. Dielectric constant (e′ and e″) and dielectric loss (tan δ) were analyzed as a function of frequency and composition and the behaviour is explained on the basis of Maxwell-Wagner model. It was observed that the dielectric loss decreases with the increase of hard ferrite co...

Naturally Self-Assembled Nanosystems and Their Templated Structures for Photonic Applications

Self-assembly has the advantage of fabricating structures of complex functionalities, from molecular levels to as big as macroscopic levels. Natural self-assembly involves self-aggregation of one or more materials (organic and/or inorganic) into desired structures while templated self-assembly involves interstitial space filling of diverse nature entities into self-assembled ordered/disordered templates (both from molecular to macro levels). These artificial and engineered new-generation materials offer many advantages over their individual counterparts. This paper reviews and explores the advantages of such naturally self-assembled hybrid molecular level systems and template-assisted macro-/microstructures targeting simple and low-cost device-oriented fabrication techniques, structural flexibility, and a wide range of photonic applications.

Direct deposition strategy for highly ordered inorganic organic perovskite thin films and their optoelectronic applications

A direct deposition methodology has been optimized for highly crystalline inorganic-organic (IO) perovskite thin films. The simplest deposition ensures long-range order with high c-oriented thin films, thicknesses ranging from ultra-thin (~20nm) and upto 1.5 µm. These self-assembled layered perovskites are naturally aligned alternative stacking arrangement of inorganic and organic monolayers, resemble multiple quantum wells (MQWs), which offers superior optoelectronic properties such as room-temperature optical excitons, strong electrically induced photo-carrier mobilities etc. The established fabrication is having device-compatible advantage over other conventional solution–processed thin films wherein the optical features are restricted by thickness limitations (<200nm) and with possible corrugated surface morphologies with multi-phases. The universally acceptable ability has been demonstrated for wide varieties of organic moieties (R) as well as different lead halide networks in type (R-NH3)2PbX4 (X = I, Br,Cl).The potential of the direct deposition methodology for demonstrated in 3D template structure fabrication as well as in photocurrent response capability.