Ab Initio Assessment of the Structural and Optoelectronic Properties of Organic–ZnO Nanoclusters

Structural, electronic, and optical properties of a new coumarin dye, zinc oxide (ZnO) nanoclusters of varying sizes, and their complexes have been investigated using density functional theory (DFT). The band gap of oxide nanoclusters varies with size validating quantum confinement effect in small particles. Energy level diagrams of dye, ZnO nanoclusters, and redox electrolyte are in favor of efficient electron injection from dye to nanocluster and regeneration of the ionized dye. The adsorption of the organic dye to nanocluster is tested for anchoring through three different functional groups (cyano, carbonyl, and hydroxyl) of the dye. We have compared simulated absorption spectra of the dye, nanoclusters, and dye functionalized nanoclusters and discussed the matching with the solar irradiance spectrum. A strong new band appeared in the low energy side of the absorption spectra for dye adsorbed nanoclusters. Frontier molecular orbital calculations reveal that the first absorption band of dye–ZnO complexes is charge transfer (CT) in character. Excitation of this band leads to direct electron transfer to the conduction band (CB) of the nanocluster, making dye–ZnO complexes suitable for type II DSSCs as well

Multilayer Approach for Advanced Hybrid Lithium Battery

Conventional intercalated rechargeable batteries have shown their capacity limit, and the development of an alternative battery system with higher capacity is strongly needed for sustainable electrical vehicles and hand-held devices. Herein, we introduce a feasible and scalable multilayer approach to fabricate a promising hybrid lithium battery with superior capacity and multivoltage plateaus. A sulfur-rich electrode (90 wt % S) is covered by a dual layer of graphite/Li₄Ti₅O₁₂, where the active materials S and Li₄Ti₅O₁₂ can both take part in redox reactions and thus deliver a high capacity of 572 mAh g_cathode^–1 (<i>vs</i> the total mass of electrode) or 1866 mAh g_s^–1 (<i>vs</i> the mass of sulfur) at 0.1C (with the definition of 1C = 1675 mA g_s^–1). The battery shows unique voltage platforms at 2.35 and 2.1 V, contributed from S, and 1.55 V from Li₄Ti₅O₁₂. A high rate capability of 566 mAh g_cathode^–1 at 0.25C and 376 mAh g_cathode^–1 at 1C with durable cycle ability over 100 cycles can be achieved. Operando Raman and electron microscope analysis confirm that the graphite/Li₄Ti₅O₁₂ layer slows the dissolution/migration of polysulfides, thereby giving rise to a higher sulfur utilization and a slower capacity decay. This advanced hybrid battery with a multilayer concept for marrying different voltage plateaus from various electrode materials opens a way of providing tunable capacity and multiple voltage platforms for energy device applications

Exploring an Emissive Charge Transfer Process in Zero-Twist Donor–Acceptor Molecular Design as a Dual-State Emitter

The present work reports a new concept on how to diminish dark twist intramolecular charge transfer (TICT) states with the zero-twist D–A systems in order to design frameworks with dual solution and solid-state emission property. The study began with theoretical calculations to understand the structural needs followed by the chemical synthesis of conceptually new two molecular designs, <b>1</b> and <b>2</b>, with zero-twist angle between electron donor and acceptor units linked through a covalent bond and finally their applications in OLED devices. Oxazole was used as an acceptor in combination with the phenothiazene core as the donor, and the effect of enhanced electron donation was studied using methyl and anisole donor groups. DFT studies indicated a partial segregation of HOMO–LUMO levels in molecular designs, and the photophysics of these planar charge transfer molecules have been investigated. Natural transition orbital (NTO) calculations were carried out to understand excited-state transition character in these D–A molecules. Molecular level studies through single-crystal analysis revealed the importance of steric factor in controlling other molecular parameters, particularly short-range molecular forces. The synthesized compounds were eventually utilized in green-emitting OLED devices as a pristine emitting layer. Compound <b>2</b> showed better device efficiency than <b>1</b> in unoptimized devices largely due to the presence of the anisole group which prevented stacking of molecules. Solution-state emission and electroluminescence data of fabricated devices using <b>1</b> and <b>2</b> pointed out that molecular modification helped to enhance emission efficiency of <b>2</b> without shifting the emission wavelength

Quenching of the Excitonic Emission of ZnO Quantum Dots Due to Auger-Assisted Hole Transfer to CdS Quantum Dots

The charge-transfer mechanism in quantum dot (QD) donor–acceptor systems is still poorly understood. Here, we utilize steady-state and time-resolved emission spectroscopy to study photoinduced hole transfer from ZnO to CdS QDs. The observed quenching of the excitonic emission (both intensity and lifetime of ZnO QDs) in the presence of CdS QDs is attributed to the hole transfer from excited ZnO to CdS QDs. We have demonstrated that the variation of the hole-transfer rate with the driving force does not follow the conventional Marcus model but rather fits with a new Auger-assisted transfer mechanism, where the excess energy is used for electronic excitation. Moreover, we have evidenced the consequences of the hole transfer through the measurement of the enhanced photoconductivity of the film made of the blend of ZnO and CdS QDs

Role of ZnS Segment on Charge Carrier Dynamics and Photoluminescence Property of CdSe@CdS/ZnS Quantum Rods

Growing a wide band gap shell on bare core and/or core@shell materials is a fascinating idea for improving the photoluminescence (PL) efficiency and stability. An epitaxially grown shell adds another degree of complexity to the system and modulates the excited-state relaxation dynamics, which remain poorly understood. Employing time-resolved PL and femtosecond transient absorption (TA) spectroscopy, we present a thorough study on charge carrier dynamics of CdSe@CdS and CdSe@CdS/ZnS quantum rods (QRs). Various excitation wavelengths were used to identify the contribution of individual segment toward the optical properties of the QRs. Our femtosecond TA measurements provide a clear evidence of excitation migration from CdS as well as ZnS to CdSe core within few picoseconds of photoexcitation. The excitons recombine faster in the CdSe moiety of the CdSe@CdS/ZnS than that of the CdSe@CdS QRs via an extra decay path. The interband trap states that are created via the formation of extended defects because of lattice strain relaxation (or ion exchange during the formation of ZnS segment) in CdSe@CdS/ZnS QRs could provide the additional decay channel leading to low PL intensity and quantum yield. We believe that our study will help to develop a strategy for enhancing the PL efficiency through energy funneling across semiconductor heterojunctions and to understand the charge carrier dynamics in nanoheterostructures

Redox Species-Based Electrolytes for Advanced Rechargeable Lithium Ion Batteries

Seeking high-capacity cathodes has become an intensive effort in lithium ion battery research; however, the low energy density still remains a major issue for sustainable handheld devices and vehicles. Herein, we present a new strategy of integrating a redox species-based electrolyte in batteries to boost their performance. Taking the olivine LiFePO₄-based battery as an example, the incorporation of redox species (i.e., polysulfide of Li₂S₈) in the electrolyte results in much lower polarization and superior stability, where the dissociated Li⁺/S_<i>x</i>^2– can significantly speed up the lithium diffusion. More importantly, the presence of the S₈^2–/S^2– redox reaction further contributes extra capacity, making a completely new LiFePO₄/Li₂S_<i>x</i> hybrid battery with a high energy density of 1124 Wh kg_cathode^–1 and a capacity of 442 mAh g_cathode^–1. The marriage of appropriate redox species in an electrolyte for a rechargeable battery is an efficient and scalable approach for obtaining higher energy density storage devices

Quantum Phase Transition from Superparamagnetic to Quantum Superparamagnetic State in Ultrasmall Cd_1–<i>x</i>Cr(II)_<i>x</i>Se Quantum Dots?

Despite a long history of success in formation of transition-metal-doped quantum dots (QDs), the origin of magnetism in diluted magnetic semiconductors (DMSs) is yet a controversial issue. Cr(II)-doped II–VI DMSs are half-metallic, resulting in high-temperature ferromagnetism. The magnetic properties reflect a strong p–d exchange interaction between the spin-up Cr(II) t_2g level and the Se 4p. In this study, ultrasmall (∼3.1 nm) Cr(II)-doped CdSe DMSQDs are shown to exhibit room-temperature ferromagnetism, as expected from theoretical arguments. Surprisingly, a low-temperature phase transition is observed at 20 K that is believed to reflect the onset of long-range ordering of the single-domain DMSQD

Scalable Approach To Construct Free-Standing and Flexible Carbon Networks for Lithium–Sulfur Battery

Reconstructing carbon nanomaterials (e.g., fullerene, carbon nanotubes (CNTs), and graphene) to multidimensional networks with hierarchical structure is a critical step in exploring their applications. Herein, a sacrificial template method by casting strategy is developed to prepare highly flexible and free-standing carbon film consisting of CNTs, graphene, or both. The scalable size, ultralight and binder-free characteristics, as well as the tunable process/property are promising for their large-scale applications, such as utilizing as interlayers in lithium–sulfur battery. The capability of holding polysulfides (i.e., suppressing the sulfur diffusion) for the networks made from CNTs, graphene, or their mixture is pronounced, among which CNTs are the best. The diffusion process of polysulfides can be visualized in a specially designed glass tube battery. X-ray photoelectron spectroscopy analysis of discharged electrodes was performed to characterize the species in electrodes. A detailed analysis of lithium diffusion constant, electrochemical impedance, and elementary distribution of sulfur in electrodes has been performed to further illustrate the differences of different carbon interlayers for Li–S batteries. The proposed simple and enlargeable production of carbon-based networks may facilitate their applications in battery industry even as a flexible cathode directly. The versatile and reconstructive strategy is extendable to prepare other flexible films and/or membranes for wider applications

Variation in the Photocurrent Response Due to Different Emissive States in Methylammonium Lead Bromide Perovskites

Thin films and crystals of methylammonium lead bromide (MAPbBr₃) perovskites have strong photoluminescence (PL). Previous studies have shown that the emission arises from different states. However, the role of these states in the performance of a solar cell has not been reported. We have used photocurrent and photoluminescence microscopies (PCM and PLM) to investigate the correlation between the photocurrent (PC) and the PL behavior in the different regions of MAPbBr₃ thin film solar cells. Our results show that the PC and the PL responses from the different regions in the thin film show poor correlation compared to the correlation between those of a high efficiency GaAs solar cell. Furthermore, we establish a relationship between the different emissive states and the PC and the PL responses. Out of the two emissive states at 2.34 and 2.28 eV that have been reported, only the state at 2.34 eV has a dominant contribution to the PC. Our results suggest that the emission at 2.28 eV is related to traps, which can lower the performance of the solar cells. Finally, the correlation analysis of the PC and the PL responses we have presented can be used in any solar cell made from direct band gap semiconductor to identify the loss channels in the device

Green Strategy to Single Crystalline Anatase TiO₂ Nanosheets with Dominant (001) Facets and Its Lithiation Study toward Sustainable Cobalt-Free Lithium Ion Full Battery

A green hydrothermal strategy starting from the Ti powders was developed to synthesis a new kind of well dispersed anatase TiO₂ nanosheets (TNSTs) with dominant (001) facets, successfully avoiding using the HF by choosing the safe substitutes of LiF powder. In contrast to traditional approaches targeting TiO₂ with dominant crystal facets, the strategy presented herein is more convenient, environment friendly and available for industrial production. As a unique structured anode applied in lithium ion battery, the TNSTs could exhibit an extremely high capacity around 215 mAh g^–1 at the current density of 100 mA g^–1 and preserved capacity over 140 mAh g^–1 enduring 200 cycles at 400 mA g^–1. As a further step toward commercialization, a model of lithiating TiO₂ was built for the first time and analyzed by the electrochemical characterizations, and full batteries employing lithiated TNSTs as carbon-free anode versus spinel LiNi_<i>x</i>Mn_2–<i>x</i>O₄ (x = 0, 0.5) cathode were configured. The full batteries of TNSTs/LiMn₂O₄ and TNSTs/LiNi_0.5Mn_1.5O₄ have the sustainable advantage of cost-effective and cobalt-free characteristics, and particularly they demonstrated high energy densities of 497 and 580 Wh kg_anode^–1 (i.e., 276 and 341 Wh kg_cathode^–1) with stable capacity retentions of 95% and 99% respectively over 100 cycles. Besides the intriguing performance in batteries, the versatile synthetic strategy and unique characteristics of TNSTs may promise other attracting applications in the fields of photoreaction, electro-catalyst, electrochemistry, interfacial adsorption photovoltaic devices etc