Publications

2022

1. APA

   A dielectric study of Br-doped lead-free methylammonium bismuth chloride (CH3NH3)3Bi2BrxCl9−x

   Chandra, Paramesh, Saha, Saroj, and Mandal, Swapan K.

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   We report here the synthesis and dielectric characteristics of lead-free methylammonium bismuth chloride (MABiCl) and bromine (Br) doped methylammonium bismuth chloride (MABiBrCl) powders. The dielectric characteristics of the samples (pressed powder pellet) measured in the frequency range 1 Hz to 1 MHz and temperature range 333–403 K indicate complex electrical transport in these halide perovskite materials. We have observed that the transport is dominated by longrange hopping of carriers and localized charges above and below a critical frequency respectively. The experimental data are fitted with the theoretical models considering grains, grain boundary, and contacts effects and made a comprehensive analysis. The activation energy obtained from ac electrical conductivity measurements is found to be relatively higher for the Br doped MABiBrCl than undoped MABiCl, which might explain some of the properties reported on solar cells and optoelectronic devices.

2. Matter PHYSICA B

   Spin state bistability in (Mn, Zn) doped Fe(phen)2(NCS)2 molecular thin film nanocrystals on quartz

   Saha, Saroj, and Chandra, Paramesh

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   Spin state bistability within high spin (HS) and low spin (LS) state is investigated in undoped and (Zn, Mn) doped spin crossover (SCO) complex Fe(phen)2(NCS)2 thin films (thickness ∼ 300 nm) deposited by dip-coating technique at room temperature on the quartz substrate. The X-ray diffraction studies clearly show the formation of crystalline structure of SCO complexes. The growth of the thin films was indisputably confirmed by electron microscopy and optical studies. The optical absorption peak between 535-557 nm was clearly observed, and that corresponds to 1A1g → 1T1g ligand field absorption in undoped and metal-doped (Zn, Mn) SCO thin films. The high spin (HS) state of the SCO films at room temperature was confirmed by Raman spectra. The bistability of spin states is clearly revealed by the well pronounced thermal hysteresis loop in magnetization measurements. The spin transition temperature (T1/2) and loop width are found to be critically dependent on metal doping and suggested the possibility of tuning these parameters in spin-crossover thin films to design future spin-based devices.

3. PRB

   Evolution of magnetic and transport properties in the Cu-doped pyrochlore iridate Eu 2 ( Ir 1 − x Cu x ) 2 O 7

   Mondal, Sampad, Modak, Mantu, Maji, Bibekananda,  Mandal, Swapan, Ghosh, B., Saha, Surajit, Sardar, Mahdi, and Banerjee, Sangam

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   We have investigated the effect of Cu substitution in Eu2(Ir1-xCux)2O7 with the help of magnetic and transport property measurements. Cu substitution affects the spin orbit coupling (SOC) and interatomic electron correlation (U) in the transition metal site which in turn modifies the physical properties. XPS measurement reveal that each Cu2+ converts Ir4+ to double amount of Ir5+ ions. We observe metal to insulator transition (T_MI ) around 120 K. At lower temperature in the insulating phase below 50 K the temperature dependence of resistance follows power law dependence and the magnitude of the exponent increases with Cu concentration. The temperature dependent thermopower was observed to follow the electrical resistivity till 50 K, except for a sudden drop in thermopower at temperature less than 50 K was observed. We find negligible Hall voltage in the metallic regime of the samples but a sudden Hall voltage is developed at very low temperatures below 50 K. We observe bifurcation in zero field cooled and field cooled (ZFC-FC) magnetization measurement, exchange bias and negative magnetoresistance and in all these the magnitude increases with increase in Cu doping concentration. We observe linear specific heat and the coefficient of it decreases with Cu doping. All these physical properties observed will be explained in this manuscript by invoking antiferromagnetic all-in-all-out spin (AFM AIAO) structure and onset of spin liquid (SL) phase.

4. Frequency and temperature-dependent dielectric characteristics of lead-free Br doped perovskites (CH3NH3)3Bi2Cl9 and (CH3NH3)3Bi2BrxCl9-x

   Chandra, Paramesh, Saha, Saroj, and Mandal, Swapan

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   We report the synthesis and dielectric characteristics of lead-free methylammonium bismuth chloride (MABiCl) and Br doped methylammonium bismuth chloride (MABiBrCl) in powder form. The morphology of the samples showed growth of large flakes with large surface area. The band gaps of the perovskites are obtained from optical absorption data. The dielectric characteristics of the samples were measured in the frequency range 1 Hz to 1 MHz and temperature range 100–300 K. The data shows a complex electrical transport in these halide perovskite materials. The dielectric permittivity data shows the existence of a critical temperature at which the material undergoes a structural phase transition. The transition temperature is found to be 270 K and 266 K for MABiCl and MABiBrCl respectively. The results can be explained by certain change in orientation of methyl-ammonium (MA) ions as the temperature is increased.

2021

1. Mat. PHYSICA B

   Morphology controlled (CH3NH3)3Bi2Cl9 thin film for lead free perovskite solar cell

   Chandra, Paramesh, and Mandal, Swapan K.

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   Traditional perovskite solar cells have made tremendous progress in terms of efficiency in the past few years but the presence of toxic lead (Pb) has restricted their commercialization. We report here a process to fabricate MABiCl perovskite thin film based solar cell. The process shows some intriguing outcomes like uniform surface morphology of the perovskite layer, high open-circuit voltage and good fill-factor. The film shows improved uniform morphology and surface coverage with flakes (dimension 570 nm × 360 nm, thickness ∼ 100–200 nm). The solar cell device shows an open-circuit voltage of about 496 mV and a fill factor 0.55. The improved morphology of the perovskite layer with reduced number of defects and charge traps possibly decreases the probability of recombination of photoexcited charge carriers. The result indicates that the process can be a good alternative to prepare lead free perovskite solar cells with improved performance.

2020

1. EPJ

   Spin transition properties of metal (Zn, Mn) diluted Fe(phen) _\textrm2 (NCS) _\textrm2 spin-crossover thin films

   Saha, Saroj, and Mandal, Swapan K.

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   We report here the effect of metal (Zn and Mn) dilution on the spin transition of Fe(phen) 2(NCS)2thin film spin-crossover (SCO) complex. The SCO complexes are deposited on glass and indium-tin-oxide (ITO) coated glass by dip-coating technique. The growth of the films is clearly confirmed by the appearance of the sharp optical absorption band at 521–540 nm corresponding to 1A1g to 1T1g ligand field absorption of the SCO complex. Although the microstructure of the films remains unaffected by metal dilution, substitution of Fe(II) by either Zn(II) (diamagnetic) or Mn(II) (paramagnetic) results in subtle changes in the bonding environment of the host metal as inferred form X-ray diffraction and Raman studies. The high spin to low spin (or vice versa) transition can be triggered either by electric field or magnetic field as revealed in the measured current ( I)–voltage (V) profile or magnetization data of the films. The data further shows the effect of metal dilution on the spin transition temperatures(T1/2 ), produced hysteresis loop width and loop area, which are the crucial parameter for fabricating spin-based room temperature switching devices.

2. PRB

   Role of f − d exchange interaction and Kondo scattering in the Nd-doped pyrochlore iridate ( Eu 1 − x Nd x ) 2 Ir 2 O 7

   Mondal, Sampad, Modak, M., Maji, B., Ray, Mayukh K., Mandal, S., Mandal, Swapan K., Sardar, M., and Banerjee, S.

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   We report a study of magnetization, resistivity, magnetoresistance, and specific heat of the pyrochlore iridate (Eu1−xNdx)2Ir2O7 with x=0.0, 0.5 and 1.0, where spin-orbit coupling, electronic correlation, magnetic frustration, and Kondo scattering coexist. The metal insulator transition temperature (TMI) decreases with increase in Nd content, but always coincides with the magnetic irreversibility temperature (field-induced moment). Resistivity below TMI does not fit with either activated (gap) or any power-law (gapless) dependence. The Curie constant shows the surprising result that Nd induces singlet correlation (reduction of paramoment) in the Ir sublattice. Magnetoresistance is negative at low temperatures below 10 K and increases strongly with increase in x, and varies quadratically with field switching over to a linear dependence above 50 kOe. Low-temperature specific heat shows a Schottky peak, coming from Nd moments, showing the existence of a doublet split in the Nd energy level, arising from the f−d exchange interaction. All materials show the presence of a linear specific heat in the insulating region. The coefficient of linear specific heat for x=0.0 does not vary with the external magnetic field, but varies superlinearly for x=1.0 materials. We argue that linear specific heat probably rules out weakly correlated phases such as Weyl fermions. We propose that with the introduction of Nd at the Eu site, the system evolves from a chiral spin liquid with gapless spinon excitations with a very small charge gap to a Kondo-type interaction superposed on a chiral spin liquid coexisting with long-range antiferromagnetic ordering. A huge increase of magnetoresistance with increase in Nd concentrations shows the importance of Kondo scattering in the chiral spin-liquid material by rare-earth moments.

2019

1. Frequency Dependent Charge Transport and Spin State Switching Characteristics of Fe(phen) 2 (NCS) 2 in Polymer

   Mondal, Chaitali, Nanda Goswami, M. L., and Mandal, Swapan K.

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   We report on the bistability in spin states of spin crossover (SCO) compound Fe(phen) 2 (NCS) 2 in polymer (polypyrrole) by frequency (1–100 kHz) and temperature dependent (305–457 K) electrical conductivity measurements. The structure and growth of SCO compounds in conducting polymer are obtained by scanning electron microscopy, X-ray diffraction and optical absorption measurements. The thermal dependence of ac conductivity σ ( \ømega ) shows the clear formation of a hysteresis loop in its cooling and heating cycle due to the difference in conductivity in high spin and low spin state. The size, shape and width of the hysteresis loops are found to be critically dependent on the applied frequency and/or the ratio between SCO and polymer. The ac conductivity is found to exhibit a dispersive behavior following Jonscher’s law: σ ( \ømega ) ∝ \ømega n below a critical frequency \ømega c , above which it is found to monotonically decrease with increasing frequency. The thermal dependence of the exponent n and \ømega c is also explored. The charge transport phenomena are explained in the framework of hopping of charge carriers. The data reveals that addition of polymer can play an important role to tune the conductivity of SCO compounds and its spin state dependence characteristics which may be quite helpful for fabricating future spin-based devices. Temperature dependent magnetic susceptibility measurement also confirms the spin transition behavior of the SCO/ppy composite samples. These SCO/ppy composite samples can be taken as the reliable nanomaterials fabricated with the concept of future spin based nanoarchitectonics.

2018

1. Probing Spin-State Switching in Fe(phen) _\textrm2 (NCS) _\textrm2 Thin Film Nanocrystals on Different Substrates by Electrical Conductivity Measurements

   Mondal, Chaitali, Pal, Suvra, and Mandal, Swapan K

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2017

1. EPJ

   Magnetocaloric as a sensitive tool to study magnetic phase in Ca 4 Mn 3 O 10 - δ

   Mondal, Sampad, Modak, Mantu, Ray, Mayukh,  Mandal, Swapan, Sardar, Mahdi, and Banerjee, Sangam

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   We report here magnetic properties of non-stoichiometric . We have measured magnetisation, electrical resistance, magnetoresistance, specific heat and thermo remanent magnetization (TRM) relaxation measurements. We find that at temperatures above 75 K the electrical transport can be explained in terms of activated hopping of magnetic polarons. These polarons are formed due to oxygen vacancy inducing sites. Below 50K the polarons begins to strongly localize causing loss of moment and the electrical transport is governed by variable range hopping (VRH) conduction. The system shows glassy spin relaxation below 50K, which presumably evolves into cluster spin glass at 12 K due to short range ordering of the canted moments. All these aspect could be well correlated with characteristics signature from magnetocaloric effect indicating magnetocaloric is a sensitive tool to study magnetic phase.

2016

1. On-Surface Synthesis of Single Conjugated Polymer Chains for Single-Molecule Devices

   Okawa, Yuji,  Mandal, Swapan, Makarova, Marina, Verveniotis, Elisseos, and Aono, Masakazu

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   Although single-molecule electronic devices have been of great interest for several decades, the fabrication of practical circuits remains challenging due to the lack of reliable ways to wire individual molecules. On-surface synthesis of single conductive polymer chains will be a key technology to solve this problem. We already found that stimulating a molecular layer of diacetylene compound by the tip of scanning tunneling microscope (STM) could initiate chain polymerization. As a result, we could systematically fabricate a single conjugated polydiacetylene chain at designated positions. Subsequently, we developed a novel method (‘chemical soldering’) for connecting the conjugated polymer chains to single organic molecules. The connection of two polydiacetylene chains to a single phthalocyanine molecule was demonstrated. Nanoscale characteristics of the connection were also experimentally and theoretically investigated. Here, we briefly review tip-induced chain polymerization and the chemical soldering methods. This work will help to advance single-molecule electronics.

2. Multiferroicity in ZnO nanodumbbell/BiFeO 3 nanoparticle heterostructures

   Mahesh, Dabbugalla, and Mandal, Swapan

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   We report here on the multiferroic properties of ZnO–BiFeO3 (BiFeO3 referred hereinafter as BFO) nanocomposite structures obtained by using a facile solution-based synthesis route. ZnO is found to grow in the form of well-crystallized and self-assembled dumbbell-like structures. BFO nanoparticles (NPs) are deposited onto ZnO nanodumbbells (NDs) to obtain ZnO–BFO heterostructures. The nanocomposites show prominent ferroelectric polarization hysteresis loop along with enhanced magnetization in comparison to pure BFO NPs. The ordered alignment of spins along with the suppression of Fe–O–Fe antiferromagnetic super-exchange interactions at the ZnO/BFO interface plausibly gives rise to observed multiferroic properties.

3. Electrically controllable molecular spin crossover switching in Fe(phen)2(NCS)2 thin film

   Mondal, Chaitali, and Mandal, Swapan

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   Spin crossover molecular complex Fe(phen)2(NCS)2 in thin film form (20-300 nm) is obtained by simple dip-coating technique on glass substrates. The growth of the molecular films is confirmed by optical and X-ray diffraction data. The morphology of the samples shows distributed nanocrystals with an average size ca. 12 nm. We measure the current (I)-voltage (V) characteristics of a device with 300 nm film thickness and show that application of electric field can induce spin state switching. The electric field experienced by individual nanocrystals separated by nanometric gap is supposed to be quite high and is plausibly playing the crucial role in instigating switching in molecular nanocrystals. The result is quite significant towards developing room temperature molecular spin cross-over switching devices in the nanoscale limit.

2015

1. EPJ

   Size effect on the magnetic properties of oleic acid stabilized substrate free BiFeO3 nanocrystals

   Mahesh, Dabbugalla, Mahato, Dr Bipul, Barman, Anjan, and Mandal, Swapan

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   We report here on the unique synthesis of BiFeO3 (BFO) nanocrystals of different size by using oleic acid as a capping agent and investigate the structural and magnetic properties. Oleic acid is found to be a good stabilizing agent to obtain different crystal size and distributions of BFO nanocrystals. Structural characterizations by X-ray diffraction (XRD) reveal the phase purity of all the samples. The BFO nanocrystals display strong size dependent magnetic properties showing increase in both magnetization (Ms)aswellasincoercivefield(Hc) with decreasing the crystal size due to the size confinement effect as well as change in magnetocrystalline anisotropy. The smallest BFO nanocrystal of size ∼12 nm shows remarkably high (8.17 emu/g) magnetization value compared to previous reports. The results obtained here will be of immense important for advanced applications in electromagnetic devices.

2. EPJ

   UV emission from self-assembled ZnS nanowires on DNA templates

   Mandal, Swapan

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   We report on the self-assembled growth of 8-10 nm ZnS nanoparticles and subsequently nanowires of size ~100 nm onto DNA templates driven by the electrostatic interaction between mobile cations (Zn2+) and the negative charges of the phosphate group in DNA backbone. The nanowires show free excitonic ultra-violet (UV) emission at 310 nm along with some defect related emission bands at 345-364 nm. The observed excitonic emission is slightly red-shifted from the optical absorption edge at 300 nm plausibly due to splitting of valence band into hole states. The growth kinetics and relatively lesser internal strain are plausibly responsible for the improved emission spectra of the ZnS nanowires.

3. Observation of Pronounced Electric Polarization and Strong Magnetization in Mn Doped BiFeO3 Nanocrystals

   Mahesh, Dabbugalla, Goswami, M N, and Mandal, Swapan

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   We report here the multiferroic properties of Mn doped BiFeO3 (BFO) nanocrystals to address the basic controversial issue concerning the co-existence of magnetoelectric coupling especially in BFO nanocrystals. The crystal structure of BFO nanocrystals shows significant reduction in lattice parameters (c/a ratio) with increasing Mn concentrations giving rise to large compressive strain and lattice distortion. This has probably led to a significant displacement of Bi3+ ions and FeO6 octahedra and Fe–O–Fe bond angles and consequently more cooperative interaction between the Bi and oxygen which is responsible for the observed large electric polarization of 12.07 μC/cm2 for 6% Mn doped BFO sample along with high saturated magnetization value of 17.49 emu/gm. The results are therefore quite important towards obtaining strong magnetoelectric coupling in BiFeO3 system.

2014

1. Semiconductor?metal?semiconductor transition in Bi and Bi?Ag nanowires

   Mandal, Swapan, and Kabir, Lutful

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   We report on the experimental observation of temperature dependent (195?475?K) semiconductor?metal?semiconductor transition in the electrical resistance of 50?nm Bi nanowires grown in polymer. The distorted lattice structure of Bi nanowires opens up a semiconducting band gap (162?meV) in contrast to semi-metallic bulk as supported by the electron transport and infrared absorption data. The electron transport plausibly arises out of both surface electronic and bulk-like states, where the former dominates at lower temperatures and the latter at higher temperatures. In the intermediate temperature range, electron transport is dominated by phonon interactions, giving rise to the metal-like transport of Bi nanowires. Attempts to dope Bi nanowires with silver results in a significant modification of Bi?Ag nanowire surface states without changing the crystal structure. The modification of surface states plays an important role in shifting the transition temperatures significantly without any appreciable change in the semiconducting gap of Bi nanowires.

2013

1. Structural and Optical Properties of Single Crystalline Bismuth Nanoparticles in Polymer

   Kabir, Lutful, and Mandal, Swapan

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   We report here the structural and optical properties of Bi nanoparticles in polymer (polypyrrole) matrix. The nanoparticles are synthesized following a wet chemical route. The X-ray diffraction data clearly shows the growth of single crystalline Bi nanoparticles within the host polymer. The microstructure of the Bi nanoparticles obtained by transmission electron microscopy (TEM) reveals clearly the formation of spherical shaped nanoparticles of average size˜27 nm with a narrow size distribution. The optical absorption spectrum exhibits a distinct peak at 278 nm which is attributed to the surface plasmon band of Bi nanoparticles. The absorption spectrum is found to be described well following Mie theory.

2. Ordered Monomolecular Layers as a Template for the Regular Arrangement of Gold Nanoparticles

   Makarova, Marina,  Mandal, Swapan, Okawa, Yuji, and Aono, Masakazu

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   Ordered arrays of metal nanoparticles are important for nanoelectronic and nanophotonic applications. Here, we report the formation of self-assembled arrays of gold nanoparticles on molecular layers of diacetylene compounds on a MoS2(0001) substrate. The arrangement of gold nanoparticles is observed using scanning tunneling microscopy. When gold is deposited on a self-assembled monolayer of 10,12-nonacosadiynoic acid or 10,12-octadecadiynoic acid on a MoS2(0001) substrate, the ordered array of diacetylene moieties in the molecular layer serves as a template for the formation of ordered arrays of gold nanoparticles. In contrast, when gold is deposited on a pristine MoS2(0001) surface or on a molecular layer of stearic acid, the gold nanoparticles are randomly distributed on the surface. It is found that the arrangement of gold nanoparticles is largely determined by the deposition rate; faster deposition results in more ordered arrays of gold nanoparticles. Our observations confirm the role of unsaturated π systems in molecules acting as a template for the regular arrangement of gold nanoparticles; this work will open up new possibilities for interfacial nanoarchitectonics.

3. Interconnects with single conjugated polymers

   Okawa, Yuji,  Mandal, Swapan, Makarova, Marina, and Aono, Masakazu

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   We found before that a stimulation with the probe tip of a scanning tunneling microscope (STM) could initiate a chain polymerization of diacetylene compound. Based on these previous studies, here we report a novel method for single molecular wiring, which we call “chemical soldering.” This method enables us to connect single conjugated polymer chains to single functional molecules, which would be an important step in advancing the development of single-molecule electronic circuitry.

4. Pronounced Multiferroicity in Oleic Acid Stabilized BiFeO3 Nanocrystals at Room Temperature

   Mahesh, Dabbugalla,  Mandal, Swapan, Mahato, Dr Bipul, Rana, Bivas, and Barman, Anjan

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   We report on the experimental observation of pronounced multiferroicity in BiFeO3 nanocrystals (size approximately 40 nm) at room temperature. Large scale BiFeO3 nanocrystals are synthesized using a low temperature chemical route and further stabilized with oleic acid. The nanocrystals exhibit a significant distortion in lattice parameter c compared to the bulk. Oleic acid plays an important role in reducing oxygen vacancies and Fe2+ ions at the nanocrystal surface giving rise to a high resistivity (approximately 10(10) omega-cm at 300 K) of the sample. The direct band gap of nanocrystals is measured to be approximately 4.2 eV (about 1.5 times the bulk value) suggesting a strong quantum confinement effect. The nanocrystals show a remarkably high spontaneous saturation magnetization approximately 4.39 emu/g along with a prominent ferroelectric hysteresis loop at room temperature. Particle size effect leading to the appearance of large number of uncompensated spins and suppression of modulated spin structure have resulted a strong spontaneous magnetization in such nanoscale multiferroic materials.

2012

1. Controlled chain polymerisation and chemical soldering for single-molecule electronics

   Okawa, Yuji, Akai-Kasaya, Megumi, Kuwahara, Yuji,  Mandal, Swapan, and Aono, Masakazu

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   Single functional molecules offer great potential for the development of novel nanoelectronic devices with capabilities beyond today’s silicon-based devices. To realise single-molecule electronics, the development of a viable method for connecting functional molecules to each other using single conductive polymer chains is required. The method of initiating chain polymerisation using the tip of a scanning tunnelling microscope (STM) is very useful for fabricating single conductive polymer chains at designated positions and thereby wiring single molecules. In this feature article, developments in the controlled chain polymerisation of diacetylene compounds and the properties of polydiacetylene chains are summarised. Recent studies of "chemical soldering", a technique enabling the covalent connection of single polydiacetylene chains to single functional molecules, are also introduced. This represents a key step in advancing the development of single-molecule electronics.

2. Methanol sensing characteristics of conducting polypyrrole-silver nanocomposites

   Kabir, Lutful, and Mandal, Swapan

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   Methanol sensing characteristics of conducting polypyrrole-silver nanocomposites are reported here. The nanocomposites are synthesized by wet chemical technique with different amount of silver loadings (5-15 mol%). The sensitivity of the nanocomposites upon exposure to gas molecules is critically dependent on the silver loadings and the concentration of the exposed gas. This is possibly instigated by the modified metal-polymer interface and the polar nature of the constituent metal and the exposed gas. Interaction of the alcohol gas with the polypyrrole chains in the presence of silver effectively determines the change in resistance and hence the sensitivity of the nanocomposites upon exposure to methanol. The adsorption of methanol molecules within the nanocomposites and the subsequent chemical reactions are studied by Fourier transform infrared (FTIR) spectroscopy.

3. DNA-tagged nano gold: A new tool for the control of the armyworm, Spodoptera litura Fab. (Lepidoptera: Noctuidae)

   Chakravarthy, Akshay, Bhattacharyya, Atanu, R., Shashank, Epidi, Timothy, Byasigideri, Doddabasappa, and Mandal, Swapan

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   The efficacy of the DNA tagged gold nanoparticles on the major polyphagous pest, Spodoptera litura Fab. (Lepidoptera: Noctuidae) was evaluated. To determine the efficacy of the test nanoparticle, dilutions viz. 200, 300, 400 and 500 ppm were prepared and 10 µl of the suspension was dispensed on the chickpea (Cicer aritinum) based semi-synthetic diet filled in 5 ml glass vials. Second instar S. litura larvae of uniform age and size were released onto the diet 20 min after surface treatment. 20 larvae of S. litura were exposed to each concentration of DNA tagged gold nanoparticle for 30 s. A control with untreated larvae was also maintained. Larval mortality data sets subjected to analysis of variance (ANOVA) revealed that the particles were effective and caused 50% larval mortality above 500 ppm. As the particle concentration and days after treatment increased, larval mortality also increased. There were statistically significant differences in Spodoptera larval mortality between concentrations. The LC50 (216.91 to 938.95 conc. at 95% CI) increased as the larval age increased. The study demonstrated that DNA tagged gold nanoparticles are effective against S. litura and would therefore be a useful component of an integrated pest management strategy.

2011

1. Chemical Wiring and Soldering toward All-Molecule Electronic Circuitry

   Okawa, Yuji, Mandal, Swapan K., Hu, Chunping, Tateyama, Yoshitaka, Goedecker, Stefan, Tsukamoto, Shigeru, Hasegawa, Tsuyoshi, Gimzewski, James K., and Aono, Masakazu

   Abs

   Key to single-molecule electronics is connecting functional molecules to each other using conductive nanowires. This involves two issues: how to create conductive nanowires at designated positions, and how to ensure chemical bonding between the nanowires and functional molecules. Here, we present a novel method that solves both issues. Relevant functional molecules are placed on a self-assembled monolayer of diacetylene compound. A probe tip of a scanning tunneling microscope is then positioned on the molecular row of the diacetylene compound to which the functional molecule is adsorbed, and a conductive polydiacetylene nanowire is fabricated by initiating chain polymerization by stimulation with the tip. Since the front edge of chain polymerization necessarily has a reactive chemical species, the created polymer nanowire forms chemical bonding with an encountered molecular element. We name this spontaneous reaction "chemical soldering". First-principles theoretical calculations are used to investigate the structures and electronic properties of the connection. We demonstrate that two conductive polymer nanowires are connected to a single phthalocyanine molecule. A resonant tunneling diode formed by this method is discussed.

2. ACSnano

   Rate-Determining Factors in the Chain Polymerization of Molecules Initiated by Local Single-Molecule Excitation

   Mandal, Swapan, Okawa, Yuji, Hasegawa, Tsuyoshi, and Aono, Masakazu

   Abs

   Spontaneous chain polymerization of molecules initiated by a scanning tunneling microscope tip is studied with a focus on its rate-determining factors. Such chain polymerization that happens in self-assembled monolayers (SAM) of diacetylene compound molecules, which results in a π-conjugated linear polydiacetylene nanowire, varies in its rate P depending on domains in the SAM and substrate materials. While the arrangement of diacetylene molecules is identical in every domain on a graphite substrate, it varies in different domains on a MoS(2) substrate. This structural variation enables us to investigate how P is affected by molecular geometry. An important determining factor of P is the distance between two carbon atoms which are to be bound by polymerization reaction, R; as R decreases by 0.1 nm, P increases ∼2 times. P for a MoS(2) substrate is ∼4 times higher (with the same value of R) than that for a graphite substrate because of higher mobility of molecules. The exciting correlation of the chain polymerization rate to the geometrical structure of the diacetylene molecules brings a deeper understanding of the mechanism of chain polymerization kinetics. In addition, the fabrication of one-dimensional conjugated polymer nanowires on a semiconducting MoS(2) substrate as demonstrated here may be of immense importance in the realization of future molecular devices.

3. Fabrication of Luminescent Silver Doped PbS Nanowires in Polymer

   Mandal, Swapan, Mandal, Arup, Barman, Anjan, and Gautam, Ujjal

   Abs

   We report here fabrication of silver (0 to 1.76 mol%) doped PbS nanowires (radius r approximately 1.75 nm) in polymer by a simple wet chemical process. An X-ray photoelectron spectroscopy study clearly confirms the possibility of silver (Ag) doping in PbS nanowires. Both absorption and photoluminescence spectra reveal very strong quantum confinement effect in PbS nanowires as expected for a r/Bohr radius ratio approximately 0.0972 nm. Visible excitonic emission is observed at room temperature in the photoluminescence spectra of undoped and silver doped PbS nanowires in polymer. The excitonic emission is appreciably blue-shifted when doped by silver (1.76 mol%) indicating strong modification of the electronic states by magnetic silver ions. While Ag2+ centers at the substitutional lattice site show an emission band around 525 nm, Ag0 at the interstitial site act as nonradiative recombination centers. Effect of silver doping on the luminescence intensity is also discussed.

4. High Ferromagnetic Transition Temperature in PbS and PbS:Mn Nanowires

   Mandal, Swapan, Mandal, Arup, and Banerjee, Sangam

   Abs

   Spontaneous magnetization measured in the temperature range 5-300 K with high ferromagnetic transition temperature (T(c)) has been observed in both undoped and Mn doped (2-8 mol %) PbS nanowires (diameter 30 nm) in polymer. For undoped sample, we find T(c) ~ 290 K while for doped samples T(c) varies between 310-340 K depending on Mn concentrations. Both T(c) and coercive fields are critically dependent on Mn concentrations. Coercive fields show a T(0.5) dependence with temperature for a moderate concentration of Mn (4 mol %) in PbS while it deviates from T(0.5) behavior for higher Mn concentrations. Anionic defects arising out of nonstoichiometric growth is solely responsible for the observed magnetism in undoped PbS nanowires. The role of intrinsic strain along with reduced dimensionality in determining such high T(c) and overall magnetizations has been discussed.

2010

1. Polymer stabilized Ni–Ag and Ni–Fe alloy nanoclusters: Structural and magnetic properties

   Kabir, Lutful, Mandal, Arup, and Mandal, Swapan

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   We report here the structural and magnetic behaviors of nickel–silver (Ni–Ag) and nickel–iron (Ni–Fe) nanoclusters stabilized with polymer (polypyrrole). High resolution transmission electron microscopy (HRTEM) indicates Ni–Ag nanoclusters to stabilize in core-shell configuration while that of Ni–Fe nanoclusters in a mixed type of geometry. Structural characterizations by X-ray diffraction (XRD) reveal the possibility of alloying in such bimetallic nanoclusters to some extent even at temperatures much lower than that of bulk alloying. Electron paramagnetic resonance (EPR) spectra clearly reveal two different absorption behaviors: one is ascribed to non-isolated Ni2+ clusters surrounded by either silver or iron giving rise to a broad signal, other (very narrow signal) being due to the isolated superparamagnetic Ni2+ clusters or bimetallic alloy nanoclusters. Results obtained for Ni–Ag and Ni–Fe nanoclusters have been further compared with the behavior exhibited by pure Ni nanoclusters in polypyrrole host. Temperature dependent studies (at 300 and 77 K) of EPR parameters, e.g. linewidth, g-value, line shape and signal intensity indicating the significant influence of surrounding paramagnetic silver or ferromagnetic iron within polymer host on the EPR spectra have been presented.

2. Electron spin resonance in silver-doped PbS nanorods

   Mandal, Arup, and Mandal, Swapan

   Abs

   The electron spin resonance characteristics of silver (Ag)-doped PbS nanorods have been investigated here. We show that both Ag0 and Ag2+ centres can be simultaneously created in PbS nanorod structures grown in polymers. Structural investigations reveal that strain-induced distortion of PbS lattice can lead to the formation of hole traps in as-grown samples. Large strain-induced binding of the Ag+ ions into the Pb2+ vacancies lowers the activation energy of diffusion significantly. The isomorphic substitution of Ag+ in PbS host is then converted into Ag0 and Ag2+ paramagnetic centres to maintain the charge neutrality and cation–anion ratio, confirmed by electron spin resonance investigations.

3. Connecting single conductive polymers to a single functional molecule

   Okawa, Yuji,  Mandal, Swapan, Hu, Chunping, Tateyama, Yoshitaka, Goedecker, Stefan, Tsukamoto, Shigeru, Hasegawa, Tsuyoshi, and Aono, Masakazu

   Abs

   We have developed a method using a scanning tunneling microscope (STM) probe tip to control the chain polymerization of diacetylene compounds in a self-ordered layer, thereby creating conjugated polydiacetylene nanowires. When a small amount of phthalocyanine was deposited on a molecular layer of diacetylene compound, we found adsorbed and stabilized phthalocyanine single molecules. The initiation of chain polymerization on the diacetylene molecular row to which the single phthalocyanine molecule was adsorbed was also demonstrated.

2008

1. Low frequency divergence of the dielectric constant and signature of the Meyer–Neldel rule in the ac conductivity of PbS and PbS:Mn nanorods in polymer

   Mandal, Arup, Kabir, Lutful, and Mandal, Swapan

   Abs

   We report here the temperature (130–296 K) and frequency (100 Hz–1 MHz) dependent dielectric study for PbS and PbS:Mn nanorods of diameter ∼ 30 nm grown in polymer matrix. The frequency dependent capacitance data show the divergence of capacitance in the low frequency regime below a cutoff frequency ∼ 20 kHz, above which it is fairly constant or slowly varying with frequency. The temperature variation of ac conductivity data for such quasi-one-dimensional nanorods reveals the signature of correlated barrier hopping transport in the framework of the Meyer–Neldel rule. We also extend our discussion for Mn doped PbS nanorods to get insight into the effect of magnetic doping on the dielectric properties.

2. Humidity-sensing properties of conducting polypyrrole-silver nanocomposites

   Kabir, Lutful, Mandal, Arup, and Mandal, Swapan

   Abs

   We report here the humidity-sensing characteristics of conducting polypyrrole (PPY)-silver nanocomposites prepared by wet-chemical technique. Addition of silver into the conducting polymer network shows remarkable change in resistance with relative humidity (RH). The resistance of PPY-silver nanocomposites is found to increase with RH till a threshold value, above which the resistance tends to decrease. The threshold RH value is significantly reduced with the increase in silver concentration into the conducting polymer network. The modified metal–polymer interface and the associated interfacial water molecules play the critical role in determining the electrical transport and hence the humidity-response characteristics of the nanocomposites.

2007

1. Fluorescent magnetic emulsion droplets: Potential material for multiplexed optical coding of biomolecules

   Mandal, Swapan

   Abs

   Fluorescent magnetic microbeads can be potentially used in optical coding of biomolecules. The currently commercially available microbeads so far suffer polydispersity in both size and fluorescence intensity. Large statistical fluctuations in intensity level are quite detrimental for their use in optical coding applications. To optimize them for multiplexed optical coding of biological molecules, besides monodispersity, statistical fluctuations in intensity of beads need to be maintained as low as possible (\textless1% and less) in order to obtain a large number of codes. Here, we show that fluorescent and magnetic droplets synthesized through a simple and elegant oil-in-water microemulsion technique can have statistical fluctuations as low as \textless1% with a number of fluorescent labels.

2. Strong excitonic confinement effect in ZnS and ZnS:Mn nanorods embedded in polycarbonate membrane pores

   Mandal, Swapan, Mandal, Arup, Das, S., and Bhattacharjee, Baibaswata

   Abs

   We investigate here the optical properties of excitons in a large array of ordered semiconductor-insulator nanorods. A simple and elegant approach is described to obtain such semiconductor-insulator nanorod structures. Here, we investigate ZnS and ZnS:Mn nanorods of diameters ∼ 15–100 nm crystallized within the pores of polycarbonate membrane. The nanorods confined in the dielectric matrix display strong ultraviolet (UV) excitonic absorption and emission bands in both undoped and Mn2+ doped samples. The data reveal that dielectric confinement rather than dimensional quantization has the significant effect in UV-visible optical absorption and photoluminescence (PL) processes in these semiconductor-insulator nanorods of sizes much wider than the Bohr radius. Interestingly, the emission band associated with Mn2+ transition ( ∼ 587–600 nm) is also significantly affected by the size effect as well as dielectric discontinuity at the interface. A detailed investigation of PL emission from these embedded ZnS and ZnS:Mn nanorods is also reported.

3. Polymer assisted preferential growth of PbS and PbS : Mn nanorods: Structural and optical properties

   Mandal, Arup, and Mandal, Swapan

   Abs

   We report here the structural and optical properties of PbS and PbS:Mn nanorods (diameter 30–80 nm) grown in a polymer (polypyrrole) matrix. X-ray diffraction data of nanorods clearly reveals preferential growth of PbS nanorods with a strong lattice distortion from a bulk cubic to tetragonal one. The strain introduced lattice distortion is inherent to the growth process and strongly depends on polymer concentration. The polymer concentration is found to play an important role in controlling the structural properties. The effect of Mn2+ incorporation into PbS lattice shows no appreciable change in the structural properties. Optical absorption behaviors in such PbS and PbS:Mn nanorods are also reported. The absorption peak energy shows blue-shift with increase in Mn2+ concentration to some critical value beyond which red-shift is observed.

2006

1. Electron spin resonance in DNA doped polypyrrole

   Mandal, Swapan

   Abs

   Electron spin resonance (ESR) of electrochemically doped polypyrrole with various concentrations (c) of calf-thymus DNA is reported here. The concentration and thermal dependence of the ESR linewidth (ΔH) clearly depicted a quasi-metallic to metallic transition at a critical doping concentration (c0) of DNA. The thermal and concentration dependences of various ESR parameters are interpreted by considering the possible spin dynamics of carriers and the conformational change in the polypyrrole structure induced by DNA chains.

2. DNA in Nanopores: Negative Capacitance and δ-Relaxation at High Frequency

   Mandal, Swapan

   Abs

   We measured the high frequency dielectric relaxation behavior of DNA molecules confined in nanopores of polycarbonate membrane. The data revealed the existence of a critical frequency omega(c) approximately GHz at which the ac conductivity showed delta-relaxation. Interestingly, the DNA molecules also exhibited a crossover from positive to negative capacitance corresponding to omega(c). The negative capacitance at the critical frequency suggested a strong inductive behavior of DNA molecules in the high frequency regime. The results are interpreted in terms of the confined geometry of the DNA molecules in the nanopores. The interfacial water H-bonded to DNA played a crucial role in determining the high frequency relaxation of DNA molecules. The results indicated that the DNA in nanopores could be designed for application in high frequency bandpass/notch filters.

3. Direct electrical conduction in DNA molecules confined in nanoporous membrane

   Mandal, Swapan

   Abs

   Temperature dependent (80–300 K) dc electrical conductivity (σdc) of deoxyribonucleic acid (DNA) molecules confined in nanopores of polycarbonate membrane is reported here. The DNA confined in such nanopores exhibited metallic resistivity with temperature up to a critical temperature beyond which it became temperature independent. The observed experimental data are explained considering modified electron-phonon interactions in reduced dimensions for the confined DNA molecules.

2005

1. Cobalt doped γ-Fe2O3 nanoparticles: Synthesis and magnetic properties

   Chakrabarti, Supriya,  Mandal, Swapan, and Chaudhuri, S

   Abs

   We demonstrate here the wet chemical synthesis of cobalt doped γ-Fe2O3 nanoparticles and the subsequent effect on magnetic properties with the variation in dopant concentration. It is observed that cobalt can be homogeneously doped into the γ-Fe2O3 lattice up to 5 mol% without any appreciable change in the particle size ( nm). Further increase in cobalt concentration (10 mol% here) resulted in an increase in particle size ( nm) due to possible adsorption of a cobalt layer on the surface of γ-Fe2O3 nanoparticles rather than complete doping in the iron oxide lattice. The ac susceptibility measurements revealed an increase in blocking temperature (TB) with percentage variation in cobalt doping (2–10%), indicating substitution of Fe3+ ions by Co2+ ions in the γ-Fe2O3 lattice. The dc magnetization measurements showed an increase in saturation magnetization only up to 5%, beyond which it significantly diminished. The reduction in saturation magnetization is attributed to the contribution from surface anisotropy in cobalt coated γ-Fe2O3 nanoparticles.

2. Electrodeposited carbon nanotube thin films

   Pal, A.K., Roy, Ratan,  Mandal, Swapan, Gupta, S., and Deb, Biswapriya

   Abs

   A successful attempt to grow carbon nanotubes (CNTs) by electrodeposition technique for the first time is reported here. Carbon nanotubes were grown on Si (001) substrate using acetonitrile (1% v/v) and water as electrolyte at an applied d.c. potential ∼20 V. The films were characterized by X-ray Diffraction (XRD), Scanning Electron Microscope (SEM), Raman, optical absorbance, Fourier Transform Infra Red spectroscopy (FTIR) and Electron Spin Resonance (ESR) measurements. The effect of magnetic field on the growth of nanotubes was studied critically. It was found that the presence of magnetic field during electro-deposition played a crucial role on the growth of carbon nanotubes and hence the electronic properties. Photoluminescence (PL) studies indicated band edge luminescence ∼0.72–0.83 eV. Field emission studies indicated lower turn-on voltage and higher current density for films deposited with magnetic field.

3. Langmuir

   Encapsulation of Magnetic and Fluorescent Nanoparticles in Emulsion Droplets

   Mandal, Swapan, Lequeux, Nicolas, Rotenberg, Benjamin, Tramier, Marc, Fattaccioli, Jacques, Bibette, Jerome, and Dubertret, Benoit

   Abs

   Oils containing both fluorescent semiconductor and magnetic oxide nanoparticles are used to produce oil in water emulsions. This technique produces oil droplets with homogeneous fluorescence and high magnetic nanoparticle concentrations. The optical properties of the oil droplets are studied as a function of the droplet sizes for various concentrations of fluorescent and magnetic nanoparticles. For all concentrations tested, we find a linear variation of the droplet fluorescent intensity as a function of the droplet volume. For a given size and a given quantum dot (QD) concentration, the droplet fluorescence intensity drops sharply as a function of the magnetic nanoparticle concentration. We show that this decrease is due mainly to the strong absorption cross section of the magnetic nanoparticles and to a lesser extent to the dynamic and static quenching of the QD fluorescence. The role of the iron oxide nanoparticle localization in the droplet (surface versus volume) is also discussed.

4. Growth mechanism of carbon nanotubes deposited by electrochemical technique

   Mandal, Swapan, Hussain, Shamima, and Pal, A

   Abs

   Understanding the nucleation and growth of carbon nanotubes at room temperature by a novel electrochemical process using acetonitrile as the electrolyte is investigated here. Microscopic insight into the nucleation process clearly reveals coalescence of amorphous carbon clusters in a branched network instigated mainly by dangling-bond induced relaxation and subsequent rearrangement of carbon atoms through a quasi-liquid state leading to the formation of multi-walled carbon nanotubes. Such a direct conversion of amorphous carbon into nanotubes is not abrupt, rather driven by a slow thermodynamical process that is inherent to the electrochemical process.

2004

1. Wet chemical synthesis of iron pyrite and characterization by Mössbauer spectroscopy

   Kar, Soumitra,  Mandal, Swapan, Das, D, and Chaudhuri, S

   Abs

   Iron pyrite in powder form was synthesized by using FeCl3 and either thiourea or Na2S as the starting materials. Attempts were made to achieve pure pyrite phase through a simple wet chemical reaction in the liquid state followed by high temperature annealing in the argon atmosphere. The phase purity and the composition of the sample were determined by X-ray diffraction (XRD) and energy dispersive analysis of X-ray (EDAX), respectively. The phase purity and its intrinsic magnetic behavior were further clarified by Fe57 Mössbauer spectroscopic investigations.

2. Charge transport in chemically synthesized, DNA-doped polypyrrole

   Dutta, Prarthana, and Mandal, Swapan

   Abs

   The charge transport properties of chemically synthesized, DNA-doped polypyrrole (PPY) are described here. The polymer showed a significant change in microstructural morphology and optical absorption spectra with increase in DNA concentrations (0.36–14% w/v). Both the dark and photo-induced I–V characteristics of the polymers showed an increase in the degree of linearity with increase in DNA concentration. The temperature-dependent dc electrical conductivity (σ) exhibited a significant dependence on the nature of carrier hopping dominant in this DNA-doped PPY: σ showed a change from T−1/4 to T−1/2 dependence on temperature as the degree of doping increased from 0.36% to 3%, respectively. Further increase in DNA doping to 14% led to a T−1 dependence of the dc electrical conductivity. The results were interpreted in terms of the localized polaronic and bipolaronic states caused by the disorder in polymeric chains, variation in chain length, interchain interactions, etc.

3. Synthesis of DNA-Polypyrrole Nanocapsule

   Mandal, Swapan, and Dutta, Prarthana

   Abs

   A template synthesis technique of encapsulating DNA molecules inside polypyrrole (PPy) nanotubes, termed as DNA-PPy nanocapsule, has been described here. The insertion of DNA molecules inside polypyrrole nanotubes is spontaneous in nature. The structural characterizations and possible applications of DNA-PPy nanocapsule in bio-sensing devices have been discussed.

2003

1. Effect of interfacial alloying on the surface plasmon resonance of nanocrystalline Au-Ag multilayer thin films

   Roy, Ratan,  Mandal, Swapan, and Pal, A.K.

   Abs

   Nanocrystalline Au and Ag in multilayer thin film form with Au/Ag/Au structure were prepared by high pressure (∼40 Pa) d.c. sputtering techniques. The Ag concentrations in AgxAu1-x films were changed from x = 0 to 1. These multilayer films with varying Ag concentration showed significant changes in microstructures obtained from TEM and XRD analyses. The optical absorption spectra of these multilayer films showed a single plasmon band confirming the formation of Au-Ag alloy. We ascribe this alloying to the interfacial reactions in nanophase limited at the Au-Ag interface. The red-shift and broadening of the plasmon bands with the increase in silver concentration could be associated to the increase in size of the nanoparticles and its distribution. The observed red shift in the plasmon band may be associated with the change in electronic structure at the Au-Ag interface due to configuration mixing of the atomic energy levels of Au and Ag.

2. Effect of particle shape distribution on the surface plasmon resonance of Ag–SiO2 nanocomposite thin films

   Mandal, Swapan, Roy, Ratan, and Pal, AK

   Abs

   Silver–silica nanocomposite thin films were prepared by high pressure d.c. sputtering technique. Films deposited at lower substrate temperature showed a narrow distribution of nanoparticles with nearly spherical shape. Increase in substrate temperature resulted in films with a non-uniform size and shape due to the agglomeration of the nanoparticles. This size and shape distribution has a profound effect on the optical absorbance spectra and results in a broad and asymmetric surface plasmon band. A shape distribution introduced in the Maxwell–Garnett or Bruggeman effective medium theory was found to give a reasonable description of the experimentally observed optical absorption spectra.

3. Synthesis of γ-Fe2 O3 nanoparticles coated on silica spheres: Structural and magnetic properties

   Chakrabarti, Supriya,  Mandal, Swapan, Nath, B., Das, Dasarathi, and Chaudhuri, S.

   Abs

   The structural and magnetic properties of {}gamma -Fe 2 O 3 nanoparticles dispersed on silica spheres prepared by sol-gel method were investigated. The properties of {}gamma -Fe 2 O 3 nanoparticles without silica were compared with those on silica spheres. Both the nanoparticle assemblages were characterized by X-ray diffraction (XRD), transmission electron microscope (TEM), Mössbauer (20, 80 and 300 K) and electron paramagnetic resonance (EPR) (80, 300 K) measurements. The XRD spectra clearly indicated the formation of pure {}gamma -Fe 2 O 3 nanoparticles and the absence of any other form of iron oxide. TEM images showed a uniform distribution of the nanoparticles of size {}sim 6 nm on the surfaces of silica spheres (diameter {}sim 35-60 nm). The size of the individual nanoparticles (without silica) varied within 5-6 nm. The low temperature (20 K) Mössbauer spectra consisted of a partially split sextet superimposed on a doublet. The partial magnetic splitting of the sextet at 20 K revealed the effect of surface magnetization and surface modifications of the {}gamma -Fe 2 O 3 nanoparticles coated on silica spheres. The gradual collapse of the partially split sextet into a doublet with increasing temperature indicated the superparamagnetic relaxation exhibited by the {}gamma -Fe 2 O 3 nanoparticles with/without silica. The surface magnetization arising out of mis-aligned spins at the surface as evidenced by Mössbauer spectra was further confirmed by electron paramagnetic resonance (EPR) studies. Copyright Springer-Verlag Berlin/Heidelberg 2003

4. An ellipsometric investigation of Ag/SiO {2} nanocomposite thin films

   Roy, Ram,  Mandal, Swapan, Bhattacharyya, D., and Pal, A.

   Abs

   Dielectric properties of silver/SiO2 nanocomposite thin films grown by high-pressure d.c. sputtering technique were studied by spectroscopic ellipsometry (300-800 nm). The dielectric behavior of the nanocomposite thin films largely depended on the particle size, its number density and the surrounding environments. The films showed semiconductor-like behavior up to a critical particle size and concentration, beyond which the films exhibited the typical surface plasmon resonance characteristics in their optical properties. The refractive index was also found to have a strong dependence on the particle size and its dispersion in the matrix. The results were found to be consistent with those obtained from UV-VIS optical absorbance data. Bruggeman effective medium theory was used to explain the experimental results.

5. Electronic conduction processes in DNA-doped polypyrrole nanocomposite films

   Dutta, Prarthana, and Mandal, Swapan

   Abs

   The electron transport process in DNA-doped polypyrrole (PPY) nanocomposite film deposited on tin oxide coated glass by a simple electrodeposition technique is demonstrated here. Optical absorbance spectra clearly exhibited features corresponding to doped PPY nanocomposite film. The I–V characteristics of the films were non-ohmic and showed a significant change when illuminated with light. Photoinduced I–V profiles suggested carrier hopping to be the dominant transport mechanism in the nanocomposite PPY film. The temperature-dependent dc electrical conductivity data showed a crossover from variable-range hopping to thermally activated hopping of electrons with the increase in temperature. The ac electrical transport properties displayed a frequency-independent region below a characteristic frequency of kHz, above which the conductivity showed a strong frequency-dependent behaviour.

2002

1. Surface plasmon resonance in nanocrystalline silver particles embedded in SiO2 matrix

   Mandal, Swapan, Roy, Ratan, and Pal, AK

   Abs

   Nanocomposite films containing silver particles embedded in SiO2 matrix were prepared by high pressure (~40 Pa) d.c. sputtering technique in an argon plasma on fused silica substrate. Particle size and metal volume fraction were tailored by varying the substrate temperature (Ts~233-300 K) and deposition time (15-240 s). Reduction in size and volume fraction of metal particles culminated in blue-shift of the surface plasmon resonance peak in the optical absorbance spectra of the films. Absence of surface plasmon peak in the absorption spectra below a critical particle size and metal concentration of the nanocomposite films and appearance of sharp absorption edge in the absorbance spectra within the UV-VIS range indicated the semiconducting behaviour of the ultrafine silver particles. Experimental absorbance spectra were theoretically simulated by Mie scattering theory and Maxwell-Garnett effective medium theory.

2. Optical properties of Cd1-1Zn1S nanocrystallites in sol-gel silica matrix

   Bhattacharjee, Baibaswata,  Mandal, Swapan, Chakrabarti, Kuntal, and Chaudhuri, S

   Abs

   Cd1−xZnxS nanoparticles with different compositions (0≤x≤1) embedded in silica matrix were prepared in thin film form by the sol–gel technique. The film texture and structural transformation with increasing x value were studied from transmission electron micrographs and electron diffraction patterns. Optical study indicated increase in optical bandgap and decrease in refractive index with increasing zinc content (x value). Photoluminescence studies showed emission from surface states with emission energy blue shifted with increasing zinc content in the films. Annealing behaviour of the films with increasing annealing temperature and time was studied.

3. Photo-Induced Tunneling Current in Single-Wall Carbon Nanotubes investigated by Scanning Tunneling Spectroscopy

   Kazaoui, Said,  Mandal, Swapan, and Minami, N.

   Abs

   The electronic properties of single‐wall carbon nanotubes (SWCNT) under light excitation were investigated by scanning tunneling microscopy and spectroscopy (STM/STS), since this technique allows both imaging and probing individual nanotubes. A photo‐induced tunneling current (PITC) was observed for semiconducting and metallic SWNT only when the excitation energy exceeds the 1st pairs of van Hove singularity in the density of state. Moreover, the PITC effect occurs at both positive and negative sample bias, increases linearly with the light intensity and is reversible. Further experiments are in progress to elucidate the mechanism of PITC and to ultimately reveal the photoconductivity response of individual SWCNT.

2001

1. Electrical and optical properties of ZnS0.05Se0.95 nanocrystalline thin films

   Ganguly, Abhijit,  Mandal, Swapan, Chaudhuri, S., and Pal, A.

   Abs

   ZnS0.05Se0.95 nanocrystalline films were deposited onto fused silica and GaAs (100) substrates by the magnetron sputtering technique. The electrical conductivities of the films deposited onto fused silica substrates were measured in the temperature range of 180–300 K. It was observed that the low temperature conductivity could be explained by hopping of the charge carriers in the Coulomb gap while at elevated temperature Mott hopping is the predominant mode of conduction in these films. A distinct cross over from Efros–Shklovoskii to Mott hopping is observed for all the films. Optical spectra were found to be dominated by the combined effects of optical losses due to absorption and scattering. The absorption spectra indicated a blueshift, the extent of which depended on the crystallite size. © 2001 American Institute of Physics.

1999

1. Morphology of thin silver film grown by dc sputtering on Si(001)

   Kundu, Shudhangshu, Hazra, Satyajit, Banerjee, Sangam, Sanyal, Milan,  Mandal, Swapan, Chaudhuri, S, and Pal, AK

   Abs

   The morphology and growth mechanism of silver films approximately 150 Å in thickness on Si(001) substrates have been studied by atomic force microscopy and x-ray reflectivity. The thin films prepared by dc sputtering at room temperature are composed of islands of silver. The shape and size distribution of these islands are studied using these two complementary measurement techniques.

2. Electron transport process in discontinuous silver film

   Mandal, Swapan, Gangopadhyay, A., Chaudhuri, S., and Pal, A.K.

   Abs

   Discontinuous silver films were deposited with different grain sizes and inter-grain distances by high-pressure magnetron sputtering technique. Temperature dependence of electrical conductivity indicated that variable range hopping is the predominant mode of electron transport in the films. The presence of a Coulomb gap near the Fermi level was also investigated.

3. Optical properties of nanocrystalline ZnS films prepared by high pressure magnetron sputtering

   Mandal, Swapan, Chaudhuri, S, and Pal, A.K

   Abs

   Nanocrystalline ZnS films with different thickness (10???40 nm) were deposited onto quartz and NaCl substrates by magnetron sputtering of a ZnS target in argon plasma. All the films showed a zinc blende structure and the photoluminescence peak positions depended on the surface to volume ratio of the films. The optical absorption in these films could be explained by the combined effects of phonon and inhomogeneity broadening along with optical loss due to light scattering at the nanocrystallites.

4. Nanocrystalline CdTe films deposited by high-pressure sputtering: Carrier transport at low temperature

   Mandal, Swapan, Chaudhuri, S., and Pal, A.K.

   Abs

   Electrical conductivities of nanocrystalline CdTe films deposited onto quartz substrates by high-pressure (~20 Pa) DC magnetron sputtering were studied as a function of temperature (188–298 K). The grain size varied within 4 to 4.7 nm with the variation of deposition temperature (Ts) within 253 to 273 K. The conductivity (σ) showed (T0/T)p dependence with p~0.5 indicating the presence of a Coulomb gap near the Fermi level. Efros–Shklovskii (ES) hopping was found to be the predominant carrier conduction mechanism in the nanocrystalline CdTe films. The width of the Coulomb gap varied within 27–41 meV depending on the deposition conditions. Cross-over from ES to Mott’s hopping was observed. The existing theoretical models were used for estimating hopping energy (29 to 42 meV) and hopping distance (2.8 to 5.1 nm) in the films.

1998

1. Hopping conduction in nanocrystalline ZnSe films deposited by high pressure d.c. magnetron sputtering

   Mandal, S.K., Chaudhuri, S., and Pal, A.K.

   HTML

1997

1. Au/CdS Schottky Diode Fabricated with Nanocrystalline CdS Layer

   Mandal, S. K., Maity, A. B., Dutta, J., Pal, R., Chaudhuri, S., and Pal, A. K.