Lakshminarayana Polavarapu

Bio: Lakshminarayana Polavarapu is an academic researcher from Ludwig Maximilian University of Munich. The author has contributed to research in topics: Perovskite (structure) & Nanoparticle. The author has an hindex of 50, co-authored 92 publications receiving 8232 citations. Previous affiliations of Lakshminarayana Polavarapu include Nanosystems Initiative Munich & Nanjing University of Science and Technology.

Polyvinylpyrrolidone (PVP) in nanoparticle synthesis

Abstract: Colloidal synthesis offers a route to nanoparticles (NPs) with controlled composition and structural features. This Perspective describes the use of polyvinylpyrrolidone (PVP) to obtain such nanostructures. PVP can serve as a surface stabilizer, growth modifier, nanoparticle dispersant, and reducing agent. As shown with examples, its role depends on the synthetic conditions. This dependence arises from the amphiphilic nature of PVP along with the molecular weight of the selected PVP. These characteristics can affect nanoparticle growth and morphology by providing solubility in diverse solvents, selective surface stabilization, and even access to kinetically controlled growth conditions. This Perspective includes discussions of the properties of PVP-capped NPs for surface enhanced Raman spectroscopy (SERS), assembly, catalysis, and more. The contribution of PVP to these properties as well as its removal is considered. Ultimately, the NPs accessed through the use of PVP in colloidal syntheses are opening new applications, and the concluding guidelines provided herein should enable new nanostructures to be accessed facilely.

Highly Luminescent Cesium Lead Halide Perovskite Nanocrystals with Tunable Composition and Thickness by Ultrasonication

Abstract: We describe the simple, scalable, single-step, and polar-solvent-free synthesis of high-quality colloidal CsPbX3 (X=Cl, Br, and I) perovskite nanocrystals (NCs) with tunable halide ion composition and thickness by direct ultrasonication of the corresponding precursor solutions in the presence of organic capping molecules. High angle annular dark field scanning transmission electron microscopy (HAADF-STEM) revealed the cubic crystal structure and surface termination of the NCs with atomic resolution. The NCs exhibit high photoluminescence quantum yields, narrow emission line widths, and considerable air stability. Furthermore, we investigated the quantum size effects in CsPbBr3 and CsPbI3 nanoplatelets by tuning their thickness down to only three to six monolayers. The high quality of the prepared NCs (CsPbBr3) was confirmed by amplified spontaneous emission with low thresholds. The versatility of this synthesis approach was demonstrated by synthesizing different perovskite NCs.

State of the Art and Prospects for Halide Perovskite Nanocrystals.

Abstract: Metal-halide perovskites have rapidly emerged as one of the most promising materials of the 21st century, with many exciting properties and great potential for a broad range of applications, from photovoltaics to optoelectronics and photocatalysis. The ease with which metal-halide perovskites can be synthesized in the form of brightly luminescent colloidal nanocrystals, as well as their tunable and intriguing optical and electronic properties, has attracted researchers from different disciplines of science and technology. In the last few years, there has been a significant progress in the shape-controlled synthesis of perovskite nanocrystals and understanding of their properties and applications. In this comprehensive review, researchers having expertise in different fields (chemistry, physics, and device engineering) of metal-halide perovskite nanocrystals have joined together to provide a state of the art overview and future prospects of metal-halide perovskite nanocrystal research.

Monolayer Graphene as a Saturable Absorber in a Mode-Locked Laser

Abstract: We demonstrate that the intrinsic properties of monolayer graphene allow it to act as a more effective saturable absorber for mode-locking fiber lasers when compared to multilayer graphene. The absorption of monolayer graphene can be saturated at lower excitation intensity compared to multilayer graphene, graphene with wrinkle-like defects, or functionalized graphene. Monolayer graphene has a remarkably large modulation depth of 65.9%, whereas the modulation depth of multilayer graphene is greatly reduced due to nonsaturable absorption and scattering loss. Picosecond ultrafast laser pulses (1.23 ps) can be generated using monolayer graphene as a saturable absorber. Due to the ultrafast relaxation time, larger modulation depth and lower scattering loss of monolayer graphene, it performs better than multilayer graphene in terms of pulse shaping ability, pulse stability, and output energy.

Colloidal lead halide perovskite nanocrystals: synthesis, optical properties and applications

Abstract: Lead halide perovskite nanocrystals (NCs) are receiving a lot of attention nowadays, due to their exceptionally high photoluminescence quantum yields reaching almost 100% and tunability of their optical band gap over the entire visible spectral range by modifying composition or dimensionality/size. We review recent developments in the direct synthesis and ion exchange-based reactions, leading to hybrid organic–inorganic (CH3NH3PbX3) and all-inorganic (CsPbX3) lead halide (X=Cl, Br, I) perovskite NCs, and consider their optical properties related to quantum confinement effects, single emission spectroscopy and lasing. We summarize recent developments on perovskite NCs employed as an active material in several applications such as light-emitting devices, solar cells and photodetectors, and provide a critical outlook into the existing and future challenges. Although research into perovskite nanocrystals is still in its infancy, they are expected to be major players in future nanoscience. Lead halide perovskite nanocrystals are attracting much interest because their quantum yields for photoluminescence are approaching 100% and their optical band gap can be tuned over the entire visible wavelength region – properties that make them promising for use in lasers, light-emitting diodes (LEDs) and solar cells. Andrey Rogach of City University of Hong Kong and co-workers review the latest developments in the synthesis (of both hybrid organic-inorganic and all-inorganic nanocrystals), optical properties (quantum confinement effects, single particle emission and lasing studies) and applications (LEDs, solar cells and photodiodes) of these materials. They outline some of the many remaining challenges, but state their confidence that these should soon be overcome. This review summarizes recent developments in the direct synthesis and ion exchange-based reactions leading to hybrid organic–inorganic and all-inorganic lead halide perovskite nanocrystals. Optical properties related to quantum confinement effects, single emission spectroscopy and lasing are considered. Perovskite nanocrystals have been employed as an active material in several applications such as light-emitting devices, solar cells and photodetectors.

Graphene oxide as a chemically tunable platform for optical applications

Abstract: Chemically derived graphene oxide (GO) is an atomically thin sheet of graphite that has traditionally served as a precursor for graphene, but is increasingly attracting chemists for its own characteristics. It is covalently decorated with oxygen-containing functional groups - either on the basal plane or at the edges - so that it contains a mixture of sp(2)- and sp(3)-hybridized carbon atoms. In particular, manipulation of the size, shape and relative fraction of the sp(2)-hybridized domains of GO by reduction chemistry provides opportunities for tailoring its optoelectronic properties. For example, as-synthesized GO is insulating but controlled deoxidation leads to an electrically and optically active material that is transparent and conducting. Furthermore, in contrast to pure graphene, GO is fluorescent over a broad range of wavelengths, owing to its heterogeneous electronic structure. In this Review, we highlight the recent advances in optical properties of chemically derived GO, as well as new physical and biological applications.

Graphene-Based Materials: Synthesis, Characterization, Properties, and Applications

Abstract: Graphene, a two-dimensional, single-layer sheet of sp(2) hybridized carbon atoms, has attracted tremendous attention and research interest, owing to its exceptional physical properties, such as high electronic conductivity, good thermal stability, and excellent mechanical strength. Other forms of graphene-related materials, including graphene oxide, reduced graphene oxide, and exfoliated graphite, have been reliably produced in large scale. The promising properties together with the ease of processibility and functionalization make graphene-based materials ideal candidates for incorporation into a variety of functional materials. Importantly, graphene and its derivatives have been explored in a wide range of applications, such as electronic and photonic devices, clean energy, and sensors. In this review, after a general introduction to graphene and its derivatives, the synthesis, characterization, properties, and applications of graphene-based materials are discussed.

Atomically Precise Colloidal Metal Nanoclusters and Nanoparticles: Fundamentals and Opportunities

Abstract: Colloidal nanoparticles are being intensely pursued in current nanoscience research. Nanochemists are often frustrated by the well-known fact that no two nanoparticles are the same, which precludes the deep understanding of many fundamental properties of colloidal nanoparticles in which the total structures (core plus surface) must be known. Therefore, controlling nanoparticles with atomic precision and solving their total structures have long been major dreams for nanochemists. Recently, these goals are partially fulfilled in the case of gold nanoparticles, at least in the ultrasmall size regime (1–3 nm in diameter, often called nanoclusters). This review summarizes the major progress in the field, including the principles that permit atomically precise synthesis, new types of atomic structures, and unique physical and chemical properties of atomically precise nanoparticles, as well as exciting opportunities for nanochemists to understand very fundamental science of colloidal nanoparticles (such as the s...

Deep learning with coherent nanophotonic circuits

Abstract: Artificial Neural Networks have dramatically improved performance for many machine learning tasks. We demonstrate a new architecture for a fully optical neural network that enables a computational speed enhancement of at least two orders of magnitude and three orders of magnitude in power efficiency over state-of-the-art electronics.

Graphene photonics, plasmonics, and broadband optoelectronic devices.

Abstract: Graphene has been hailed as a wonderful material in electronics, and recently, it is the rising star in photonics, as well. The wonderful optical properties of graphene afford multiple functions of signal emitting, transmitting, modulating, and detection to be realized in one material. In this paper, the latest progress in graphene photonics, plasmonics, and broadband optoelectronic devices is reviewed. Particular emphasis is placed on the ability to integrate graphene photonics onto the silicon platform to afford broadband operation in light routing and amplification, which involves components like polarizer, modulator, and photodetector. Other functions like saturable absorber and optical limiter are also reviewed.