Department of Physics
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Item Open Access Surface enhanced raman spectroscopic study of some functional molecules adsorbed on noble metal nano particles(University of North Bengal, 2019) Hossain, Washim; Haldar, Sripada; Sarkar, Uttam Kr.The objective of the present thesis is focused on the Surface Enhanced Raman Spectroscopic (SERS) study of different arylazo imidazole (AAI) molecules adsorbed on colloidal silver nanoparticles (SNPs). The AAIs have two N heteroatoms and a pendant azo group and show significant chemical interaction (chemisorption) with Ag nano particles through exchange of electronic charge. These five membered N-heterocycles are poor π-acceptors and hence good π-donors. Thus there is a possibility of charge transfer between the AAIs and the Ag-particles which has been probed by SERS. The significant modification of the physical and chemical properties of the complexes due to presence of different substituents in the heterocyclic ring and side arm to azo functions has also been investigated by a comparative SERS study. The present work gives information for better understanding of the mechanism involved in SERS and finds the possibility of some AAIs of being used as organic semiconducting material. The thesis consists of nine chapters and an outline of each of them is given below: In Chapter 1, the mechanisms involved in Raman scattering as well as in the enormous enhancement of Raman signal of the molecules adsorbed on suitable metal surfaces due to SERS effect are discussed. At the beginning the classical and quantum theories of Raman effect are presented. In the following sections, advantages and limitations of normal Raman spectroscopy are illustrated. Although Raman signals can be used as a fingerprint for a given structure of molecule as different structures exhibit characteristic Raman spectra, normal Raman signals are very weak. That SERS effect enhancing the Raman signal by a factor of 1010 to 1011 overcomes the weak scattering cross-section of normal Raman spectroscopy is discussed in the subsequent sections of this chapter in different sub-sections: Discovery of SERS, Key features of SERS, SERS substrates, Mechanisms involved in SERS and Application of SERS. Chapter 2 gives a brief description of the materials and methods of study along with the experimental details. The instrumentation for the related experiments is also discussed in a nutshell. This chapter consists of sections presenting an overview of the functional properties of the arylazo heterocyclic molecules, preparation of colloidal SNPs and recording of UV−Vis spectra, Raman spectra and Transmission Electron Micrograph (TEM). The computational details for the simulation of Raman and SER spectra for comparison with experimental results are presented in the concluding section. In Chapter 3, the Raman spectra and SERS of 1H-2(phenylazo) imidazole (PaiH) adsorbed on SNPs are presented. A trans-to-cis isomerization of PaiH is suggested by the appearance of the cis-signature peak at 570 cm-1 for a monomolecular layer coverage at a concentration of 5×10-6 M. This hypothesis is further substantiated by the shifting of the π−π* band from 358 nm, in methanol solution to 370 nm, in silver sol along with the appearance of the n−π* band, in silver sol at the optimum concentration, as observed by UV-vis spectroscopy. The excellent agreement of the SER spectra obtained experimentally with that simulated by DFT calculations indicates that electronically, biologically as well as optically important PaiH molecule undergoes chemical interaction with SNPs possibly through the imidazole nitrogen atom. A gradual trans-to-cis dark isomerisation of PaiH is suggested with lowering of concentration: the trans-isomers being the majority and hence predominant at higher concentrations whereas almost all the molecules are present as cis-isomer at monomolecular layer coverage. Chapter 4 presents a pH dependent SERS study of 1H-2(phenylazo) imidazole (PaiH) adsorbed on SNPs. A combination of different conformations of PaiH is evident where majority of the molecules prefer a particular conformation at a given pH. It is revealed that most of the PaiH molecules are in cis-form in the concentration range from 10-6M–10-5 M at pH = 7. In alkaline pH, majority of the molecules undergoes azo to hydrozone conformation whereas at acidic pH the azo-Ns participate in the electronic interaction with silver. Significant reduction of HOMO–LUMO gap indicates the possibility of PaiH of being used as an organic semiconductor In Chapter 5, Raman and SER spectra of 1-H-2 (tolylazo) imidazole (TaiH) are reported. It is observed that monomolecular layer of TaiH is formed on SNPs at a concentration of 4.93×10−6 M in Ag-sol which is less than the concentration required for monomolecular layer coverage of PaiH which is indicative of the effect of the substituent methyl group on the electronic interaction with the metal nanoparticles. Plasmon induced trans-to-cis dark isomerization of TaiH molecules is evidenced by the appearance of signature cis-peaks in SERS and the UV-Vis absorption spectra. Fragmentation of SNPs with significant reduction in the particle size is revealed by the interaction of TaiH. Large SERS enhancement is observed by these uncoupled plasmons. DFT calculations based on single Ag-atom model are in excellent agreement with experimental observations. Chapter 6 highlights the significant changes observed in the SER spectra of 1-H-2(parachloro phenylazo) imidazole (pClPai-H) molecule with respect to those of PaiH molecules. Monomolecular layer of pClPaiH is formed on SNPs at a concentration of 10-5 M. A moderate dark trans-to-cis isomerization of pClPai-H molecule is indicative with lowering of concentration. The influence of the presence of the chlorine atom at the para position of the phenyl ring is discussed. The pClPaiH molecule undergoes chemical interaction with SNPs possibly through azo N atoms. In Chapter 7, Raman spectra and SERS of two larger arylazo imidazole compounds 1-(CH2)17CH3-2(phenylazo) imidazole (Pai-C18) and 1-(CH2)17CH3-2(tolylazo) imidazole (Tai-C18) adsorbed on SNPs are reported. The Pai-C18 molecules which are known to demonstrate some liquid crystal properties show monomolecular layer coverage at a concentration of 10-7 M in Ag-sols, whereas, the first layer coverage of Tai-C18 is observed at a higher concentration of 10-6 M in Ag-sols. Higher concentration for monomolecular layer formation indicates smaller effective coverage of the metal surface by Tai-C18 molecules as compared with Pai-C18 molecules. Chapter 8 illustrates a comparative study of SERS on different arylazo imidazole molecules. The substituents on the imidazole ring as well as on the phenyl ring are observed to change the electron density in the rings significantly. Consequently, the extent of adsorption of the molecules on the metal surface is modified by the presence of these substituent groups. SERS of 1-methyl-2-(phenylazo) imidazole (PaiMe),1-ethyl-2-(tolylazo) imidazole (TaiEt) and 1-H-2-(napthylazo) imidazole (NaaiH) is reported and the substituent effect on the SER spectra of these molecules is also presented in this chapter. Chapter 9 summarizes the experimental results described in chapters 3-8. It is revealed that AAI molecules undergo chemical interaction with the SNPs mainly through the imidazole N atom and/or azo N atoms. Also the 𝜋-electron system of the aryl group significantly contributes to the charge transfer between the AAI molecules and the SNPs depending on the side group attached to it and the concentration in Ag sol. It is inferred that because of the presence of 𝜋-electron system of the aryl group, imidazole N atoms and azo N atoms the arylazo imidazole molecules may find significant use as prospective smart molecules to show various functionality.Item Open Access Formulation and characterization of room temperature ferroelectric and antiferroelectric liquid crystal mixtures(University of North Bengal, 2018) Debnath, Asim; Mandal, Pradip KumarNow-a-days displays have become a field of tremendous importance as they provide the best means for interface between man and machine. Large numbers of displays are presently available in the market, but among all these displays about 90% are controlled by the liquid crystal display (LCD) technology. Most of the LCD devices starting from simplest wrist watches or calculators to complex laptops or flat TV sets mainly use the nematic liquid crystal phase for their operation. Although a tremendous improvement in the quality of display as well as reduction of manufacturing cost has taken place over the years, there are many issues which the LC industry is trying hard to address. Ferroelectric liquid crystals (FLC) are of current interest in the LCD industry since among various other advantages FLC based displays have micro-second order switching compared to milli-second order switching in nematic based displays. To meet the market demand much effort has been made to optimize the physical parameters of FLCs, such as temperature range, spontaneous polarization (PS), helical pitch (p), switching time (τ), tilt angle (θ) and rotational viscosity (γφ). Multicomponent mixtures are, usually formulated to optimize all the required properties for practical applications since no single FLC compound can satisfy the above requirements. Keeping all these in mind we have prepared room temperature ferroelectric and antiferroelectric liquid crystal mixtures to the best of our knowledge first time by any Indian group, which have properties suitable for FLC based display devices and at par with mixtures used in the industry. Suitability of the formulated mixtures for display applications were characterized by optical polarizing microscopy, differential scanning calorimetry, synchrotron X-ray diffraction, dielectric spectroscopy and electrooptic methods. First of all, six binary mixtures (M1-M6) were formulated by using a biphenylyl benzoate ester with oligomethylene spacer based non-flourinated chiral compound (DP1) as dopant and a three ring pyrimidine compound (H5) as host. Of these, M4 exhibit the wide range (100o) and lowest temperature (24oC) ferroelectric phase at 10wt% concentration of DP1. In addition to I-N*- A*-C* phase sequence all the mixtures found to exhibit weakly temperature dependent optical tilt (~22.5o) and fast response time (~200μs), can be useful for high speed SSFLCD applications where contrast and brightness will be less temperature dependent. Temperature range and response time are also found to improve when the binary mixture M4 is used as chiral dopant in a multicomponent host mixture (HM) to formulate another mixture (M7). Next, six multi-component mixtures (M8-M13) were prepared by doping a nonmesogenic chiral terphenyl compound (DP2) with chiral centers at opposite ends in the multi-component host mixture (HM). This HM was prepared mixing four phenyl pyrimidine compounds. Although DP2 is non-mesogenic but only 2wt% of it induces SmC* phase in the HM at or close to room temperature and with increased concentration it produces SmC* phase below ambient down to at least 12°C with physical properties useful for SSFLCD based applications. An oligomethylene spacer based partially fluorinated chiral liquid crystal (DP3) which has smaller spacer length than DP3 and which has only SmC* phase when doped in the HM, resulting mixture (M14) was found to exhibit very broad temperature range SmC* phase (~93o) from below ambient down to at least 12°C and almost temperature independent high optical tilt, low driving voltage and switching speed around 100μs suitable for display applications. To study the effect of SmC*A- SmC*- SmA* phase sequence and molecular structure, four partially fluorinated chiral compounds DP4, DP5, DP6, DP7, which differ only in the number and position of fluorine atoms in the benzoate ring of the molecular rigid core, were doped in the multi-component host mixture (HM). Number and location of fluorine atoms in the dopant structure shows significant effect on the phase sequences as well as on the different physical properties of the mixtures (M15-M18) critically important for display applications. Mixture M15 and M18 showed extended antiferroelectric phase while M16 and M17 exhibited only ferroelectric phase. A wide range room temperature electroclinic mixture, M19 was obtained when only 10wt% of DP6 was mixed in the HM. The mixture exhibits large field induced optical tilt (~23.5o) with less than 0.1% shrinkage of layers suitable for developing fast electroclinic devices free from defects due to buckling of layers. Most of the results have been published in the peer reviewed journals viz; Journal of Applied Physics, Journal of Molecular Liquids, RSC Advances, Liquid Crystals.Item Open Access Design and development of adaptive optics system in visible and near infra-red-band for IUCAA 2 M Telescope(University of North Bengal, 2019) Paul, Jyotirmay; Bhadra, Arunava; Ramaprakash, A.N.Item Open Access Studies on the mass composition of cosmic rays above 10 14 eV(University of North Bengal, 2019) Dam, Sandip; Dey, Rajat Kumar,Item Open Access Phase transitions in Binary mixtures of calamitic and Bent-Core-Mesogens(University of North Bengal, 2019) Chakraborty, Susanta; Das, Malay KumarItem Open Access Density fluctuation and correlation study of multiperticles production in 28Si-Ag/Br interaction at 14.5A GeV(University of North Bengal, 2016) Mali, Provash; Mukhopadhyay, AmitabhaItem Open Access Pulse Profile studies and hard x-ray properties of neutron stars(University of North Bengal, 2016) Pradhan, Pragati; Paul, Bikash Chandra; Paul, BiswajitItem Open Access X-Ray, Dielectric, and electro-optic studies on fluorinated achiral and chiral liquid crystals(University of North Bengal, 2015) Sinha, Debashis; Mandal, Pradip KumarItem Open Access Probing the sources of cosmic rays with high energy gamma rays and neutrinos(University of North Bengal, 2017) Banik, Prabir; Bhadra, Dr. Arunava, Sarkar, Dr. Samir kumar,Item Open Access Relativistic solutions of massive compact objects and stellar models in different space-time geometries(University of North Bengal, 2018) Deb, Rumi; Paul, Bikash Chandra