BITS Faculty Publications
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Item Highly selective formaldehyde sensing using ZnO nano-rods(AIP, 2023-03) Choudhary, Sumita; Hazra, Arnab; Gangopadhyay, SubhashisEarly detection of formaldehyde emission from any household materials is technologically very demanding as it can be a serious human health hazard. Even indirect inhaling of formaldehyde may cause significant harm to our eyes, skin, mouth or any other organs. Hence, fabrication of a simple and sensitive formaldehyde sensor would be of high practical importance. Within this work, formation of ZnO nano-rods by controlled thermal oxidation of vacuum deposited thin Zn films in air ambient, followed by fabrication of formaldehyde sensor operating at relatively lower operating temperature are reported. The crystal structure, surface morphology and optical properties of the as grown ZnO nano-rods have been investigated using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and Raman spectroscopy, respectively. The XRD patterns of ZnO suggested the formation of highly crystalline oxide films whereas FESEM images have revealed its nano-rods surface morphology with significantly high (length to diameter) aspect ratio. Raman spectroscopy confirms the thermal oxidation of the Zn thin films. As-grown ZnO nano-rods were then subsequently used to fabricate the chemi-resistive formaldehyde sensors. These sensors showed an extremely high formaldehyde sensing performance at a relatively lower operating temperature of 200°C. In a static measurement mode, the sensor exhibited a gas response of about 53% toward 100 ppm of formaldehyde, with a reasonable fast response and recovery time. Moreover, these ZnO nano-rod based sensors have also been tested with similar type of VOCs such as benzene, xylene, alcohols and acetone and appeared with an excellent selectivity towards formaldehyde over the other VOCs.Item Application of Mono Layered Graphene Field Effect Transistors for Gamma Radiation Detection(IEEE, 2018-10) Rao, V. RamgopalIn this work, we report the application of graphene field effect transistors (GFETs) as a gamma radiation sensor. The GFETs were irradiated at room temperature by 60 Co gamma radiation source for 10 kGy and 20 kGy gamma dose. The Electrical measurements and Raman spectroscopy showed that gamma radiation induced p-doping in graphene. Large positive shifts in Dirac point and significant degradation in electron mobility were observed post-gamma irradiation. Thus modulation in transport properties of GFETs was utilized here to measure the absorbed gamma radiations. We propose, a GFET based radiation detector with high sensitivity of + 113 V for 20 kGy gamma dose operating in ambient condition.Item A Passive Gamma Radiation Dosimeter Using Graphene Field Effect Transistor(IEEE, 2020-03) Rao, V. RamgopalIn this work, Graphene-based field-effect transistors (GFETs) are demonstrated as a highly sensitive dosimeter for gamma radiation. Graphene-based field-effect transistors exhibit p-type doping with the Dirac point shifting in the positive direction upon exposure to gamma radiation. Concurrently, an asymmetric degradation in the electron and hole mobility was observed with the former degrading more rapidly. It is shown that change in the Dirac voltage and carrier mobility is strongly dependent on the dose of gamma radiation. A sensitivity of ~1 V/kGy is reported. Gamma radiation causes partial aerial oxidation of graphene-channel which leads to p-doping as confirmed by the emergence of a higher binding energy peak (286.8 eV) in X-ray photoelectron spectra (XPS). The decrease in contact potential difference estimated through Kelvin probe force microscopy (KPFM) confirms this finding. The radiated devices showed a stable response for ~70 days. Our work demonstrates that gamma irradiation can also be used to induce large and stable hole concentrations in graphene. Such highly sensitive GFET can serve as real-time dosimeter operating in ambient conditions.Item Dopamine Molecules on Aucore−Agshell Bimetallic Nanocolloids: Fourier Transform Infrared, Raman, and Surface-Enhanced Raman Spectroscopy Study Aided by Density Functional Theory(ACS, 2009) Pande, SurojitAdsorption of dopamine (DA) on a Aucore−Agshell bimetallic nanocolloidal surface has been investigated using surface-enhanced Raman spectroscopy (SERS). The normal Raman spectra (NRS) of DA molecules in bulk and in aqueous solution have been investigated in depth. The vibrational signatures, as observed from the Raman and FTIR spectra of the molecule, have been assigned from the potential energy distributions. The pH-dependent NRS of the DA molecule in aqueous solution has been recorded to elucidate the protonation effect and preferential existence of different forms of the molecule. The pH-dependent SERS spectra of the molecule adsorbed on the bimetallic Aucore−Agshell nanocolloidal surface are also reported. The enhancement of bands in the pH-dependent SERS spectra suggests that the molecules are adsorbed onto the bimetallic Aucore−Agshell surface with the molecular plane tilted with respect to the silver surface of Aucore−Agshell bimetallic nanoparticles. The model study authenticates the spectral disposition and orientation of the molecule. Thus, experiment and theory keep abreast of the variety of DA structures envisaged from SERS studies on a new substrate.Item Understanding the Enhancement Mechanisms in the Surface-Enhanced Raman Spectra of the 1,10-Phenanthroline Molecule Adsorbed on a Au@Ag Bimetallic Nanocolloid(ACS, 2011-05-11) Pande, SurojitAdsorption of 1,10-phenanthroline (PHEN) on the Aucore–Agshell (Au@Ag) bimetallic nanocolloid surface has been investigated. Bimetallic nanocolloids have been synthesized using β-cyclodextrin (β-CD) in alkaline solution. Nanocolloids of different Aucore:Agshell ratio were synthesized keeping the same Aucore size. Again, for the preparation, the total metal concentrations were always kept constant. The structure and composition of the bimetallic particles were characterized by UV–vis and high-resolution transmission electron microscopy. Detailed normal Raman (NRS) and surface-enhanced Raman spectra (SERS) of PHEN in aqueous solution have been studied. The relative contributions of the electromagnetic (EM) and charge-transfer (CT) mechanism to the overall enhancement of the SERS bands of the PHEN molecule have been estimated. PHEN molecules are adsorbed on the Agshell surface through both the nitrogen atoms with the molecular plane almost perpendicular to the surface, which has been confirmed from Ag–N stretching vibration. The three-dimensional finite difference time domain (3D-FDTD) method has been applied to simulate the local electric field on the spherical Au@Ag nanocolloid for various core/shell ratios. Comparative spectral information revealed the highest SERS effect from the 1:4 Aucore–Agshell bimetallic nanocolloid of 29 nm size. This observation has also been supported by theory. Thus, experiment and theory keep abreast the attachment of PHEN molecules on a new substrate from SERS studies.Item Exploration of Electrostatic Field Force in Surface-Enhanced Raman Scattering: An Experimental Investigation Aided by Density Functional Calculations(ACS, 2008-10-27) Basu, MrinmoyeeSurface chemical properties of metal nanoparticles must be tunable to create chemical specificity and are a key prerequisite for successful sensing and imaging platforms. To relate surface enhanced Raman scattering (SERS) to electrostatic field force, a simple colloidal chemistry approach has been deliberately exploited for syntheses of gold nanoparticles with negative and positive surface charges to study their interactions with charged analytes. We took up the challenge with sulfur-containing analytes because “Au−S” interaction is well-known. Thiocyanate ion, −SCN−, a well-known SERS analyte, has been proved to be chemically ligated/anchored on positively charged gold nanoparticles surface owing to favorable electrostatic attraction. The Au−S vibrational band at ∼240 cm−1 and blue-shifting of the −C≡N stretching frequency by ∼46 cm−1 in conjunction with its intensity enhancement by an order of ∼103 in the SERS spectrum clearly illustrate a chemisorption phenomenon. In contrast, physisorption of the −SCN− ion becomes evident on negatively charged colloid. Again, methylene blue has been shown to remain engrossed on the negatively charged gold surfaces. However, the electrostatic field force could not be accounted for from fluorescence quenching while methylaminopyrene was introduced because of the distance-dependence effect. The feasibility of such coordinative/chemical attachment also has been examined theoretically by density functional theory (DFT). Moreover, employment of this DFT calculation has been performed on five different metal−molecule interaction models to fruitfully interpret the experimental SERS findings and also the orientation of the SERS analyte. The observed Raman signals have been assigned from the potential energy distributions in terms of internal coordinates of adsorbate from the output of DFT calculations. The results thus provide a benchmark illustration of the value of DFT for aiding interpretation of adsorbate vibrational spectra attainable by using SERS.Item A Green Chemistry Approach for the Synthesis of Flower-like Ag-Doped MnO2 Nanostructures Probed by Surface-Enhanced Raman Spectroscopy(ACS, 2009) Basu, MrinmoyeeNovel hierarchical flower-like nanostructures of Ag-doped MnO2 have been obtained by facile wet chemical and photochemical routes. UV−visible absorption spectroscopy measurement reveals that doping of Ag nanoparticles in MnO2 nanostructures leads to a red shift of the absorption edge and reduces the optical band gap energy from 2.68 to 2.51 eV while compared with undoped MnO2. Raman study reveals that the band broadens and shifts toward higher wavenumbers as the MnO6 octahedron is contorted by Ag doping and thus the loss of translational symmetry activates otherwise Raman-forbidden oxygen vibrations. Finally, SERS activity upsurges from Ag-doped MnO2 with Rhodamine 6G and 2-aminothiophenol as probe molecules.Item Chelate Effect in Surface Enhanced Raman Scattering with Transition Metal Nanoparticles(ACS, 2009-12-17) Basu, MrinmoyeeOver the years, several protocols have been designed to achieve surface enhanced Raman scattering (SERS) from noble/coinage metal nanoparticles preferably with silver and gold owing to the local electromagnetic field enhancement near their surface. However, the higher value of the imaginary component of the dielectric constant and the coupling between conduction and interband electron transitions result in poor SERS intensity for transition metals. Therefore, a good number of approaches such as the development of various surface roughening procedures have been made to increase the SERS sensitivity involving transition metal nanoparticles. This letter reports that chelating ligands such as 1,10-phenanthroline, ethylenediammine, and so forth have been found to be superior alternatives to bring forth the SERS activity from “3d” block transition metal nanoparticles (nickel and cobalt). Thus, a comparative account of SERS efficiency derived from these materials as well as from coinage metal nanoparticles engaging chelating and nonchelating (e.g., pyridine) ligands becomes intriguing.Item Selective and Sensitive Recognition of Cu2+ in an Aqueous Medium: A Surface-Enhanced Raman Scattering (SERS)-Based Analysis with a Low-Cost Raman Reporter(Wiley, 2012-04-04) Basu, MrinmoyeeIn the present study, surface-enhanced Raman spectra of a bifunctional Raman reporter, 2-mercaptobenzimidazole, has been found to be responsive exclusively towards Cu(2+) ions while the reporter remains anchored on the Au nanoparticle surface. Thus a specific Cu(2+)-ion-detection protocol emerges. The simplicity, sensitivity, and reproducibility of the method allow routine and quantitative detection of Cu(2+) ions. An interference study involving a wide number of other metal ions shows the procedure to be uniquely selective and analytically rigorous. A theoretical study was carried out to corroborate the experimental results. Finally, the method is promising for real-time assessment of Cu(2+) ions in aqueous samples and also has the ability to discriminate Cu(I) and Cu(II) ions in solution.Item Synthesis of TiO2 scaffold by a 2 step bi-layer process using a molten salt synthesis technique(Elsiever, 2011-04-10) Roy, BanasriTiO2 scaffolds of anisotropic rutile particles were grown from rutile seeds by using molten salt synthesis techniques. The rutile seeds were either in the form of a separate layer applied on a substrate or a sintered bulk pellet. Mixtures of amorphous titanium hydroxide and salt applied as coatings on the rutile seeds were heat treated. Depending on the morphology of the seed layer, heat treatment temperature, time and salt medium, rutile was grown with different morphologies and microstructures. For NaCl–KCl eutectic salt mixture and heat treatment at 700 °C for 5 h, nano-whiskers of 20–50 nm diameter and 0.5–1 μm length were obtained. For the NaCl salt sample treated at 850 ºC for 20 h, rutile platelets of 2–5 μm thick, 2–10 μm wide and 5–25 μm in length were produced. X-ray diffraction and Raman scattering were used to identify and characterize the rutile phase of the nanowhiskers.