Nonlinear optical probes for organic field effect transistors and halide perovskites

As the field of semiconductors is booming at an exponential rate, surpassing popular industry predictions like Moore's law, small molecule and polymer based semiconductors such as organic and hybrid (organic-inorganic) materials are gaining popularity. Field effect transistors (FETs) form the cornerstone of such electronic technology. Unlike MOSFETs, the organic FETs operate in accumulation regime which is influenced by the metal-semiconductor and semiconductor-dielectric interfaces. Traditional current-voltage characterization is often ineffective in accurately probing these devices due to inherent contact resistance issues and high density of trapped states. Thus, a need for alternate techniques for visualizing and measuring transport in organic FETs is much sought. Transient nonlinear optical techniques have emerged as powerful tools in characterizing such devices, not only for estimating intrinsic transport parameters free from contact issues, but also by aiding in visualization of the interface conditions during formation of accumulation layer while the devices are operating. In this thesis, we setup a microimaging and spectroscopic probing system based on an ultrafast laser source equipped with a prism compressor setup for compensating group velocity dispersion effects from a broadband source for studying an array of optoelectronic systems with a focus on organic thin film devices. We demonstrate contact-resistance free transport parameters from p-type organic FETs showing upto 20 times improvement in mobility estimation utilizing time-resolved electric field induced second harmonic generation (TR-EFISHG) techniques. An order of magnitude improvement in mobility estimation is observed in case of n-type FETs. The majority carriers in these devices are easily differentiable using this technique. Further, this technique enables us to map the electric field distribution in the device channel as well as help visualizing the formation of accumulation layer in each of the cases. Comparison of a small molecule semiconductor with a polymer while using different polymer dielectrics and oxide layers help discern the effect of the semiconductor dielectric interface on the device performance thus providing an effective way of improving such performance. Furthermore, we use this nonlinear optical probing system to explore lead halide perovskites. Lead halide perovskites offer an opportunity to investigate nonlinear optical properties with third and higher-order harmonic generation being possible despite the centrosymmetric crystal structure of 3D lead halide perovskites, which prohibits second harmonic generation. This research delves into the third harmonic generation (THG) from CsPbBr3 nanocrystals (NCs) and compares it to the THG from CsPbBr3 NCs with Ruddlesden-Popper planar faults (RP-CsPbBr3). The THG from CsPbBr3 NCs is negligible in comparison to RP-CsPbBr3 NCs for a wide range of femtosecond excitation wavelengths. Furthermore, the THG of a thin film of RP-CsPbBr3 is compared to that of a single crystal of methylammonium lead bromide (MAPbBr3), revealing that the THG efficiency of RP-CsPbBr3 is three times greater than that of MAPbBr3. An effective third-order susceptibility of approximately 10-18 m2 V-2 is obtained for a RP-CsPbBr3 film, indicating the potential of inorganic halide perovskite NCs with planar defects for various nonlinear optical applications.Includes bibliographical references

Riboswitch Detection Using Profile Hidden Markov Models

<p>Abstract</p> <p>Background</p> <p>Riboswitches are a type of noncoding RNA that regulate gene expression by switching from one structural conformation to another on ligand binding. The various classes of riboswitches discovered so far are differentiated by the ligand, which on binding induces a conformational switch. Every class of riboswitch is characterized by an aptamer domain, which provides the site for ligand binding, and an expression platform that undergoes conformational change on ligand binding. The sequence and structure of the aptamer domain is highly conserved in riboswitches belonging to the same class. We propose a method for fast and accurate identification of riboswitches using profile Hidden Markov Models (pHMM). Our method exploits the high degree of sequence conservation that characterizes the aptamer domain.</p> <p>Results</p> <p>Our method can detect riboswitches in genomic databases rapidly and accurately. Its sensitivity is comparable to the method based on the Covariance Model (CM). For six out of ten riboswitch classes, our method detects more than 99.5% of the candidates identified by the much slower CM method while being several hundred times faster. For three riboswitch classes, our method detects 97-99% of the candidates relative to the CM method. Our method works very well for those classes of riboswitches that are characterized by distinct and conserved sequence motifs.</p> <p>Conclusion</p> <p>Riboswitches play a crucial role in controlling the expression of several prokaryotic genes involved in metabolism and transport processes. As more and more new classes of riboswitches are being discovered, it is important to understand the patterns of their intra and inter genomic distribution. Understanding such patterns will enable us to better understand the evolutionary history of these genetic regulatory elements. However, a complete picture of the distribution pattern of riboswitches will emerge only after accurate identification of riboswitches across genomes. We believe that the riboswitch detection method developed in this paper will aid in that process. The significant advantage in terms of speed, of our pHMM-based approach over the method based on CM allows us to scan entire databases (rather than 5'UTRs only) in a relatively short period of time in order to accurately identify riboswitch candidates.</p

Enhanced Piezoresponse and Nonlinear Optical Properties of Fluorinated Self-Assembled Peptide Nanotubes

Self-assembled L,L-diphenylalanine (FF) nanostructures offer an attractive platform for photonics and nonlinear optics. The nonlinear optical (NLO) coefficients of FF nanotubes depend on the diameter of the tube [S. Khanra et al. Phys. Chem. Chem. Phys. 19(4), 3084-3093 (2017)]. To further enhance the NLO properties of FF, we search for structural modifications. Here, we report on the synthesis of fluorinated FF dipeptides by replacing one ortho-hydrogen atom in each of the phenyl groups of FF by a fluorine atom. Density-functional theoretical calculations yield insights into minimum energy conformers of fluorinated FF (Fl-FF). Fl-FF self-assembles akin to FF into micron-length tubes. The effects of fluorination are evaluated on the piezoelectric response and nonlinear optical properties. The piezoelectric d15 coefficient of Fl-FF is found to be more than 10 times higher than that of FF nanotubes, and the intensity of second harmonic generation (SHG) polarimetry from individual Fl-FF nanotubes is more than 20 times that of individual FF nanotubes. Furthermore, we obtain SHG images to compare the intensities of FF and Fl-FF tubes. This work demonstrates the potential of fluorine substitution in other self-assembled biomimetic peptides for enhancing nonlinear optical response and piezoelectricity

Efficacy of vermitechnology integration with Upflow Anaerobic Sludge Blanket (UASB) and activated sludge for metal stabilization: a compliance study on fractionation and biosorption

Efficacy of vermi-transformation for metal partitioning and transformation from Upflow Anaerobic Sludge Blanket (UASB) and Activated Sludge (AS) was investigated. Sludge samples were mixed with cow dung (CD) in two combinations (1:1 (UASB/AS:CD)) & (2:1(UASB/AS: CD)). Fractionation study revealed that Zn, Cd & Pb were associated with reducible fractions, and Cr, Cu with oxidizable fractions. Higher removal efficiency for 1:1 (UASB/AS: CD) combination over 2:1 (UASB/AS: CD) implies the non-significant contribution of cow dung during the metal stabilization process. After vermi-remediation, maximum metal removal was achieved at 1:1 ratio than 2:1 in AS. In UASB, 1:1 ratio worked better for Cr, Zn & Cd, whereas for Cu & Zn 2:1 ratio resulted in efficient removal. Overall for both AS and UASB, efficiency was found to be higher in 1:1 treatment ratio. The value of Kd (Bio sorption) was highest in Cu followed by Cr, which indicates the closer association with the metal bound organic matter (R2 ≥ 0.99). Based on the compliance study between two estimated sorption coefficients Kd (Biosorption & Fractions), vermi-remediation was found to be effective for AS than UASB. Therefore, the obtained results clearly validate the feasibility of integration of vermi-remediation as a potential promising ecological techniques for removing metal contaminant from the wastewater. Further research is required to study the decontamination of emerging contaminants with such integrated technology, which have physico-chemical properties different than metal ions.by Kaling Taki, Anindita Gogoi, Payal Mazumder, Satya Sunder Bhattacharya and Manish Kuma

Frontier review on the propensity and repercussion of SARS-CoV-2 migration to aquatic environment

Increased concern has recently emerged pertaining to the occurrence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in aquatic environment during the current coronavirus disease 2019 (COVID-19) pandemic. While infectious SARS-CoV-2 has yet to be identified in the aquatic environment, the virus potentially enters the wastewater stream from patient excretions and a precautionary approach dictates evaluating transmission pathways to ensure public health and safety. Although enveloped viruses have presumed low persistence in water and are generally susceptible to inactivation by environmental stressors, previously identified enveloped viruses persist in the aqueous environment from days to several weeks. Our analysis suggests that not only the surface water, but also groundwater, represent SARS-CoV-2 control points through possible leaching and infiltrations of effluents from health care facilities, sewage, and drainage water. Most fecally transmitted viruses are highly persistent in the aquatic environment, and therefore, the persistence of SARS-CoV-2 in water is essential to inform its fate in water, wastewater and groundwater and subsequent human exposure