Surfactant-Induced Anion Exchange and Morphological Evolution for Composition-Controlled Caesium Lead Halide Perovskites with Tunable Optical Properties.

Environmentally stable lead halide perovskite nanostructures with engineered composition and morphology are attractive because of their exotic optical properties. Here, we report the synthesis of monodispersed (∼20 nm) CsPbI3 cubic perovskite nanocrystals (NCs) using edible olive oil as a solvent as well as a chelating reagent. Thereafter, bromide anion exchange reaction using the cetyl trimethyl ammonium bromide surfactant in hexane is carried out at relatively lower temperatures to synthesize caesium lead halide perovskites with variable halide compositions and tunable band gaps. Interestingly, because of the formation of micelles, continuous morphology evolution varying from NCs of different sizes to nanowires (NWs) and nanosheets is observed. The anion exchange temperature has a distinct effect on the morphology of the CsPbBr3 nanostructure and the anion exchange reaction rate. Finally, an easy solution-processed photoconductive device is demonstrated using as-grown CsPbBr3 NWs, indicating its potential for optoelectronic applications.

Application of tungsten disulfide quantum dot-conjugated antimicrobial peptides in bio-imaging and antimicrobial therapy.

Two-dimensional (2D) tungsten disulfide (WS2) quantum dots offer numerous promising applications in materials and optoelectronic sciences. Additionally, the catalytic and photoluminescence properties of ultra-small WS2 nanoparticles are of potential interest in biomedical sciences. Addressing the use of WS2 in the context of infection, the present study describes the conjugation of two potent antimicrobial peptides with WS2 quantum dots, as well as the application of the resulting conjugates in antimicrobial therapy and bioimaging. In doing so, we determined the three-dimensional solution structure of the quantum dot-conjugated antimicrobial peptide by a series of high-resolution nuclear magnetic resonance (NMR) techniques, correlating this to the disruption of both model lipid and bacterial membranes, and to several key biological performances, including antimicrobial and anti-biofilm effects, as well as cell toxicity. The results demonstrate that particle conjugation enhances the antimicrobial and anti-biofilm potency of these peptides, effects inferred to be due to multi-dendate interactions for the conjugated peptides. As such, our study provides information on the mode-of-action of such conjugates, laying the foundation for their potential use in treatment and monitoring of infections.

Plasmon mediated enhancement and tuning of optical emission properties of two dimensional graphitic carbon nitride nanosheets.

We demonstrate surface plasmon induced enhancement and tunablilty in optical emission properties of two dimensional graphitic carbon nitride (g-C3N4) nanosheets through the attachment of gold (Au) nanoparticles. Raman spectroscopy has revealed surface enhanced Raman scattering that arises due to the combined effect of the charge transfer process and localized surface plasmon induced enhancement in electromagnetic field, both occurring at the nanoparticle-nanosheet interface. Photoluminescence studies suggest that at an optimal concentration of nanoparticles, the emission intensity can be enhanced, which is maximum within the 500-525 nm region. Further, the fabricated electroluminescent devices reveal that the emission feature can be tuned from bluish-green to red (∼160 nm shift) upon attaching Au nanoparticles. We propose that the π*→π transition in g-C3N4 can trigger surface plasmon oscillation in Au, which subsequently increases the excitation process in the nanosheets and results in enhanced emission in the green region of the photoluminescence spectrum. On the other hand, electroluminescence of g-C3N4 can induce plasmon oscillation more efficiently and thus can lead to red emission from Au nanoparticles through the radiative damping of particle plasmons. The influence of nanoparticle size and coverage on the emission properties of two dimensional g-C3N4, nanosheets has also been studied in detail.

Multivalent gold nanoparticle-peptide conjugates for targeting intracellular bacterial infections.

Although nanoparticle-tagged antimicrobal peptides have gained considerable importance in recent years, their structure-function correlation has not yet been explored. Here, we have studied the mechanism of action of a designed antimicrobial peptide, VG16KRKP (VARGWKRKCPLFGKGG), delivered via gold nanoparticle tagging against Salmonella infection by combining biological experiments with high- and low-resolution spectroscopic techniques. In comparison with the free VG16KRKP peptide or gold nanoparticle alone, the conjugated variant, Au-VG16KRKP, is non-cytotoxic to eukaryotic cells, but exhibits strong bacteriolytic activity in culture. Au-VG16KRKP can penetrate host epithelial and macrophage cells as well as interact with intracellular S. Typhi LPS under both in vitro and in vivo conditions. Treatment of mice with Au-VG16KRKP post-infection with S. Typhi resulted in reduced intracellular bacterial recovery and highly enhanced protection against S. Typhi challenge. The three-dimensional high resolution structure of nanoparticle conjugated VG16KRKP depicted the generation of a well-separated amphipathic structure with slight aggregation, responsible for the increase of the local concentration of the peptide, thus leading to potent activity. This is the first report on the structural and functional characterization of a nanoparticle conjugated synthetic antimicrobial peptide that can kill intracellular pathogens and eventually protect against S. Typhi challenge in vivo.

Superior heterojunction properties of solution processed copper-zinc-tin-sulphide quantum dots on Si.

CZTS nanocrystals have been synthesized via a new facile and environmentally friendly route using olive oil at a relatively low temperature. Nanocrystals synthesized using olive oil have a smaller average size in comparison to those synthesized with a conventional solvent-like ethylenediamine. Nanocrystals with an average diameter of 40, 20 and 6 nm have been extracted from the olive oil at different centrifugation speeds of 500, 1000 and 2000 rpm, respectively. The photovoltaic characteristics of p-CZTS/n-Si heterojunctions fabricated using the synthesized colloidal quaternary nanocrystals are demonstrated. The device fabricated with smallest sized CZTS nanocrystals, having an average diameter of ∼6 nm, exhibits an enhancement in power conversion efficiency of 61% in comparison to that of the device fabricated with the nanocrystals of 40 nm in diameter. A lower reflectance and higher minority carrier life time along with a larger surface-to-volume ratio resulted in an enhanced power conversion efficiency for smaller sized CZTS nanocrystals.

Highly Luminescent WS2 Quantum Dots/ZnO Heterojunctions for Light Emitting Devices.

Sonication induced vertical fragmentation of two-dimensional (2D) WS2 nanosheets into highly luminescent, monodispered, zero-dimensional (0D) quantum dots (QDs) is reported. The formation of 0D structures from 2D sheets and their surface/microstructure characterization are revealed from their microscopic and spectroscopic investigations. Size dependent optical properties of WS2 nanostructures have been explored by UV-vis absorption and photoluminescence spectroscopy. Interestingly, it is observed that, below a critical dimension (∼2 nm), comparable to the Bohr exciton radius, the tiny nanocrystals exhibit strong emission. Finally, the electroluminescence characteristics are demonstrated for the first time, by forming a heterojunction of stabilizer free WS2 QDs and ZnO thin films. The signature of white light emission in the light emitting device is attributed to the adequate intermixing of emission characteristics of WS2 QDs and ZnO. The observation of white electroluminescence may pave the way to fabricate prototype futuristic efficient light emitting devices.

Exfoliation of WS2 in the semiconducting phase using a group of lithium halides: a new method of Li intercalation.

Lithium halide assisted high yield synthesis of few layers of 2H phase semiconducting WS2 in organic solvents is reported. A group of lithium halides (LiCl, LiBr and LiI) has been employed for the first time to intercalate WS2 by using Li, followed by mild sonication to exfoliate in dispersive polar solvents. In contrast to the n-butyllithium (n-BuLi) assisted exfoliation method, which yields only the metallic 1T phase on prolonged reaction (3-7 days) at higher temperatures, the proposed exfoliation method produces only semiconducting 2H WS2 in a much shorter time (5 minute sonication). A very high yield of 19 mg ml(-1) has been obtained using LiI as an exfoliating agent due to its lower lattice energy compared to other alkali halides and the smaller size of the cation. Detailed microscopy and spectroscopic characterization reveals exfoliation of few layered WS2 with stoichiometric composition. Absorption and emission characteristics of the 2D WS2 layer exhibit a characteristic band edge and quantum confined transitions. As a proof-of-concept, we have successfully demonstrated photodetector devices comprising solution proccessed p-WS2/n-Si heterojunctions, which behave as diodes with a high rectification ratio (>10(2)) exhibiting a broad band photoresponse over the entire visible region.

Novel silicon compatible p-WS2 2D/3D heterojunction devices exhibiting broadband photoresponse and superior detectivity.

We report for the first time, the fabrication of novel two-dimensional (2D) p-WS2/n-Si vertical heterostructures with superior junction and photoresponse characteristics. Few layer WS2 has been synthesized by a lithium-ion intercalation technique in hexane and coated on Si substrates for realization of CMOS compatible devices. Atomic force microscopy and Raman spectroscopy have been used to confirm the 2D nature of WS2 layers. Sharp band-edge absorption and emission peaks have indicated the formation of mono-to-few-layers thick direct band gap WS2 films. The electrical and optical responses of the heterostructures have exhibited superior properties revealing the formation of an abrupt heterojunction. The fabricated photodetector device depicts a peak responsivity of 1.11 A W(-1) at -2 V with a broadband spectral response of 400-1100 nm and a moderate photo-to-dark current ratio of ∼10(3). The optical switching characteristics have been studied as a function of applied bias and illuminated power density. A comparative study of the reported results on 2D transition metal chalcogenides indicates the superior characteristics of WS2/n-Si heterostructures for future photonic devices.

Self-assembly of a nine-residue amyloid-forming peptide fragment of SARS corona virus E-protein: mechanism of self aggregation and amyloid-inhibition of hIAPP.

Molecular self-assembly, a phenomenon widely observed in nature, has been exploited through organic molecules, proteins, DNA, and peptides to study complex biological systems. These self-assembly systems may also be used in understanding the molecular and structural biology which can inspire the design and synthesis of increasingly complex biomaterials. Specifically, use of these building blocks to investigate protein folding and misfolding has been of particular value since it can provide tremendous insights into peptide aggregation related to a variety of protein misfolding diseases, or amyloid diseases (e.g., Alzheimer's disease, Parkinson's disease, type-II diabetes). Herein, the self-assembly of TK9, a nine-residue peptide of the extra membrane C-terminal tail of the SARS corona virus envelope, and its variants were characterized through biophysical, spectroscopic, and simulated studies, and it was confirmed that the structure of these peptides influences their aggregation propensity, hence, mimicking amyloid proteins. TK9, which forms a beta-sheet rich fibril, contains a key sequence motif that may be critical for beta-sheet formation, thus making it an interesting system to study amyloid fibrillation. TK9 aggregates were further examined through simulations to evaluate the possible intra- and interpeptide interactions at the molecular level. These self-assembly peptides can also serve as amyloid inhibitors through hydrophobic and electrophilic recognition interactions. Our results show that TK9 inhibits the fibrillation of hIAPP, a 37 amino acid peptide implicated in the pathology of type-II diabetes. Thus, biophysical and NMR experimental results have revealed a molecular level understanding of peptide folding events, as well as the inhibition of amyloid-protein aggregation are reported.

Broadband photoresponse and rectification of novel graphene oxide/n-Si heterojunctions.

We report a novel graphene oxide (GO) based p-n heterojunction on n-Si. The fabricated vertical GO/n-Si heterojunction diode shows a very low leakage current density of 0.25 µA/cm(2) and excellent rectification characteristics upto 1 MHz. The device on illumination shows a broadband (300-1100 nm) spectral response with a characteristic peak at ~700 nm, in agreement with the photoluminescence emission from GO. Very high photo-to-dark current ratio (>10(5)) is observed upon illumination of UV light. The transient photocurrent measurements indicate that the GO based heterojunction diodes can be useful for UV and broadband photodetectors, compatible with silicon device technology.

Chemically reduced graphene oxide for ammonia detection at room temperature.

Chemically reduced graphene oxide (RGO) has recently attracted growing interest in the area of chemical sensors because of its high electrical conductivity and chemically active defect sites. This paper reports the synthesis of chemically reduced GO using NaBH4 and its performance for ammonia detection at room temperature. The sensing layer was synthesized on a ceramic substrate containing platinum electrodes. The effect of the reduction time of graphene oxide (GO) was explored to optimize the response, recovery, and response time. The RGO film was characterized electrically and also with atomic force microscopy and X-ray photoelectron spectroscopy. The sensor response was found to lie between 5.5% at 200 ppm (parts per million) and 23% at 2800 ppm of ammonia, and also resistance recovered quickly without any application of heat (for lower concentrations of ammonia). The sensor was exposed to different vapors and found to be selective toward ammonia. We believe such chemically reduced GO could potentially be used to manufacture a new generation of low-power portable ammonia sensors.

Synthesis and superior optical-limiting properties of fluorene-thiophene-benzothiadazole polymer-functionalized graphene sheets.

A polymer based on fluorene, thiophene, and benzothiadazole as the donor-spacer-acceptor triad is covalently coupled to reduced graphene oxide (rGO) sheets via diazonium coupling with phenyl bromide, followed by Suzuki coupling. These polymer-graphene hybrids show good solubility in organic solvents, such as chloroform, tetrahydrofuran (THF), toluene, dichlorobenzene, and N,N-dimethylformamide (DMF), and exhibit an excellent optical-limiting effect with a 532-nm laser beam. The optical-limiting threshold energy values (0.93 J cm(-2) for G-polymer 1 and 1.12 J cm(-2) for G-polymer 2) of these G-polymer hybrids are better than that of carbon nanotubes (3.6 J cm(-2)).

A new class of solid state ionic conductors for application in all solid state dye sensitized solar cells.

Solid state ionic conductors based on a carbazole-imidazolium cation structure were synthesized for application in solid state dye sensitized solar cells. Solid state electrolytes using designed solid state ionic conductors and iodine provide dual channels for hole/triiodide transportation, giving rise to a good conversion efficiency of 2.85%.

Diamond-based molecular platform for photoelectrochemistry.

Boron doped diamond (BDD) thin film was found to exhibit higher photocurrent conversion efficiencies and photostability compared to commonly used transparent conducting oxides (ITO and FTO) owing to the matching energy levels and strong C-C bonding at the organic/diamond interface.