Karthik Ramasamy

Transition-metal chalcogenide nanostructures provide a unique material platform to engineer next-generation energy storage devices such as lithium-ion, sodium-ion, and potassium-ion batteries and flexible supercapacitors. The transition-metal chalcogenide nanocrystals and thin films have enhanced electroactive sites for redox reactions and hierarchical flexibility of structure and electronic properties in the multinary compositions. They also consist of more earth-abundant elements. These properties make them attractive and more viable new electrode materials for energy storage devices compared to the traditional materials. This review highlights the recent advances in chalcogenide-based electrodes for batteries and flexible supercapacitors. The viability and structure–property relation of these materials are explored. The use of various chalcogenide nanocrystals supported on carbonaceous substrates, two-dimensional transition metal chalcogenides, and novel MXene-based chalcogenide heterostructures as electrode materials to improve the electrochemical performance of lithium-ion batteries is discussed. The sodium-ion and potassium-ion batteries offer a more viable alternative to lithium-ion technology as they consist of readily available source materials. Application of various transition metal chalcogenides such as MoS₂, MoSe₂, VS₂, and SnS_x, composite materials, and heterojunction bimetallic nanosheets composed of multi-metals as electrodes to enhance the long-term cycling stability, rate capability, and structural strength to counteract the large volume expansion during the ion intercalation/deintercalation processes is highlighted. The promising performances of layered chalcogenides and various chalcogenide nanowire compositions as electrodes for flexible supercapacitors are also discussed in detail. The review also details the progress made in new chalcogenide nanostructures and layered mesostructures for energy storage applications.

While luminescent solar concentrators (LSCs) have been researched for several decades, there is still a lack of commercially available systems, mostly due to scalability, performance, aesthetics, or some combination of these challenges. These obstacles can be overcome by the systematic optimization of a laminated glass LSC design, demonstrated herein. In particular, we first show that it is possible to improve optical and electrical efficiencies of an LSC by fine-tuned optimization of the constituent fluorophore-containing interlayer resin. Further still, an increased understanding of commercially available solar cells allows us to establish a direct correlation between the device's optical and electrical efficiency. Next, optical characterization of LSCs of varying sizes allows us to elucidate the main loss mechanisms in our LSCs, as well as ways to mitigate them. Altogether these optimization steps create opportunities for high-performance multi-interlayer LSC devices with demonstrated electrical power conversion efficiency as high as 1.1% to 4.9% at visual light transmission of 74% to 5%. Furthermore, careful examination of different blue-color (red-band absorbing) dyes provides a path for color-tunability of LSC windows toward neutral regimes. Design iterations of multiple device form factors enabled a color-neutral prototype without significant performance losses by separating color-neutralizing and LSC layers into different panes of an insulated glass unit. This work demonstrates the importance of LSC design optimization in achieving high-performance solar window technology with commercially acceptable aesthetics.

Applications of flexible electronics typically depend less on high computational power, instead exploiting the advantages of devices with a high degree of deformation that are lightweight, disposable or degradable, and low‐cost.

Near-infrared (NIR) emitting quantum dots (QDs) with emission in the biological transparency windows (NIR-I: 650–950 nm and NIR-II: 1000–1350 nm) are promising candidates for deep-tissue bioimaging. However, they typically contain toxic heavy metals such as cadmium, mercury, arsenic, or lead. We report on the biocompatibility of high brightness CuInSe_xS_2–x/ZnS (CISeS/ZnS) QDs with a tunable emission covering the visible to NIR (550–1300 nm peak emission) and quantify the transmission of their photoluminescence through multiple biological components to evaluate their use as imaging agents. In general, CISeS/ZnS QDs were less cytotoxic to mouse fibroblast cells when compared with commercial CdSe/ZnS and InP/ZnS QDs. Surprisingly, InP/ZnS QDs significantly upregulated expression of apoptotic genes in mouse fibroblast cells, while cells exposed to CISeS/ZnS QDs did not. These findings provide insight into biocompatibility and cytotoxicity of CISeS/ZnS QDs that could be used for bioimaging.

Bioregenerative life-support systems (BLSS) involving plants will be required to realize self-sustaining human settlements beyond Earth. To improve plant photosynthetic efficiency in BLSS, the quality of the solar spectrum can be modified by lightweight, luminescent films. CuInS ₂ /ZnS quantum dot (QD) films with peak emissions at 600 and 660 nm were used to redshift ultraviolet and blue photons to increase biomass accumulation in red romaine lettuce. Plant growth parameters, except spectral quality, were held constant among three equivalent production environments. Lettuce grown under the 600 and 660 nm-emitting QD films respectively increased edible dry mass (13% and 9%), edible fresh mass (11% each), and total leaf area (8% and 13%) compared to under a control film that contained no QDs. Spectral modifications by the luminescent QD films improved photosynthetic efficiency in lettuce and could enhance crop productivity in greenhouses on Earth or in space, where further improvements are expected from greater availability of ultraviolet photons for conversion.

Multinary Cu-based chalcogenide nanocrystals are unique as they consist of earth-abundant materials and offer composition-controlled optoelectronic properties for various energy applications.

A general synthetic route is reported leading to cubic phase Sn_xGe_1−xalloy nanocrystals with a composition varying from essentially pure Ge to 95% Sn. Overcoating of alloys with a sulfide layer increases their thermal stability to 500 °C and makes the alloy nanocrystals amenable to easy ligand exchange.

Flexible supercapacitors are highly attractive for the large number of emerging portable lightweight consumer devices. The novelty of a flexible supercapacitor is the incorporation of flexible electrode or substrate material to combine structural flexibility with the inherently high power density of supercapacitors. Flexible supercapacitors can use non-Faradaic energy storage process as seen in the electric double layer capacitor type or a Faradaic mechanism as seen in the pseudocapacitors (PCs). In this review, we account the current progress in pseudocapacitive electrode materials, fabrication techniques and new materials for electric double layer capacitor, and different flexible substrates. Future directions in developing new materials toward improved energy density and cost-effectiveness of the flexible supercapacitors and their usage in combination with lithium-ion batteries are highlighted.

Nanocrystals of wurtzite and defect chalcopyrite phases of CuMSnS₄ (M = In or Ga) have been synthesized that exhibit strong visible light absoprtion with a band gap between 1.15 and 1.40 eV.