Subhrangsu Mukherjee | North Carolina State University (original) (raw)

Papers by Subhrangsu Mukherjee

Journal of Physics: Condensed Matter, 2006

Polyacrylamide (PAM)-silver nanocomposite materials have been synthesized by the reduction of the... more Polyacrylamide (PAM)-silver nanocomposite materials have been synthesized by the reduction of the silver salt in the polyacrylamide matrix. Silver nanoparticles embedded in the polymer matrix, observed through transmission electron microscopy (TEM), suggests attractive polymer-particle interaction. Detailed spectroscopic investigations have been carried out on the nanocomposite samples using a range of techniques to understand the nature of the interaction. The x-ray photoelectron spectroscopy (XPS) core level spectra and Fourier-transform infrared (FTIR) spectroscopy results indicate that the nanoparticles are attached to the pendant group of the polymer through physical interactions. The XPS valence band study confirms that the interaction between the polymer and the silver nanoparticles occurs through partial charge transfer from the metal particles to the nitrogen sites of the polymer side chains. The modifications associated with the silver nanoparticles are mainly due to confinement effects.

Advanced Energy Materials, 2015

Damage to VUV, EUV, and X-ray Optics IV; and EUV and X-ray Optics: Synergy between Laboratory and Space III, 2013

ABSTRACT Here is presented the spectroscopic study of the evolution of the first buried interface... more ABSTRACT Here is presented the spectroscopic study of the evolution of the first buried interfaces of a B4C capped Co/Mo2C multilayer mirror induced by thermal treatment up to 600°C. This kind of study is typically performed to simulate the response of multilayer optics working in extreme conditions, as for instance when irradiated by new high brilliance sources as Free Electron Lasers. In fact, the efficiency of multilayers is related to the optical contrast between the alternating high and low density layers, and then to the degree of interdiffusion and the creation or evolution of interface compounds. The analysis has been performed at the Co L23 edge with different soft x-ray spectroscopic techniques including diffuse and specular reflectivity, total electron and fluorescent yield at the BEAR beamline at Elettra (Trieste) (http://www.elettra.trieste.it/elettra-beamlines/bear.html). The presentation is focused on the spectroscopic results obtained by soft x-ray standing wave enhanced photoemission (XSW) from the Mo 3d, B 1s, C 1s, O 1s core levels by using a photon energy close to the Co L23 edge and corresponding to the first Bragg peak of the multilayer. The experimental results have been compared with simulations to obtain information both on the chemical state (e.g. oxidation state) and interface morphology in terms of profiles of distribution of elements and interdiffusion of B, oxidized B and C in the interface region. In summary, it is possible to conclude in favour of a good stability of the multilayer in the investigated temperature range, as confirmed by the good performance in terms of reflectivity. These results confirm the usefulness of XSW for this kind analysis of multilayer optics.

Advanced Energy Materials, 2015

Advanced Materials

The addition of small quantities of polystyrene (PS) is a simple and economically viable process ... more The addition of small quantities of polystyrene (PS) is a simple and economically viable process that improves the power conversion efficiency of one of the most efficient small molecule donors. Addition of PS increases the solution viscosity, thereby providing thicker layers, and allows the formation of a desirable bulk heterojunction morphology. Moreover, the PS spontaneously accumulates as phase separated domains, away from the electrodes, so as not to interfere with charge extraction.

Advanced materials (Deerfield Beach, Fla.), 2015

Connections are delineated between solar-cell performance, charge-carrier mobilities, and morphol... more Connections are delineated between solar-cell performance, charge-carrier mobilities, and morphology in a highperformance molecular solar cell. The observations show that maximizing the relative phase purity and structural order while simultaneously limiting the domain size may be essential for achieving optimal solar-cell performances in solution-processed small-molecule solar cells .

Advanced materials (Deerfield Beach, Fla.), Jan 19, 2014

A relationship between solvent additive concentration and active layer thickness in small-molecul... more A relationship between solvent additive concentration and active layer thickness in small-molecule solar cells is investigated. Specifically, the additive concentration must scale with the amount of semiconductor material and not as absolute concentration in solution. Devices with a wide range of active layers with thickness up to 200 nm can readily achieve efficiencies close to 6% when the right concentration of additive is used.

Optical Systems Design 2012, 2012

ABSTRACT We study a periodic Co/Mo2C multilayer prepared by magnetron sputtering. The period is 4... more ABSTRACT We study a periodic Co/Mo2C multilayer prepared by magnetron sputtering. The period is 4.1 nm and the sample is designed to work around 778 eV, i.e. close to the Co 2p3/2 threshold, at a glancing angle of 11°. In this condition, strong x-ray standing waves set up within the sample. In order to probe different depths within the stack, particularly the interfaces, the glancing angle is moved along the first Bragg peak, while, the B 1s, C 1s, Mo 3d or O 1s photoelectron spectra, the Co Lα x-ray spectrum as well as the drain current of the sample are measured. Boron is present in the 3.5 nm B4C capping layer and oxygen is from surface contamination. The photoelectrons bring information from the superficial zone, i.e. the 5 first nm, while the characteristic x-rays probe the whole stack. Clear modulations of the intensity of the studied signals as well as core level shifts are observed when going through the Bragg peak. In order to understand what happens in the multilayer calculations of depth distributions of the electric field and the energy loss by the radiation are made with the IMD and OPAL codes, respectively. The combination of experimental results and theoretical simulations will enable us to determine from which place originate the various signals and to know if some interaction exists between the Co and Mo2C layers.

The Journal of Physical Chemistry C, 2014

Small gold nanoparticles capped with 4trimethylsilylethynyl-1-acetylthiobenzene (SEB) were prepar... more Small gold nanoparticles capped with 4trimethylsilylethynyl-1-acetylthiobenzene (SEB) were prepared with spherical shape and different mean sizes (5−8 nm). The functionalized gold nanoparticles (AuNPs-SEB) were deposited onto TiO 2 substrates, and the interaction at the molecule−gold interface, the molecular layer thickness, and the ligand organization on the surface of Au nanospheres were investigated by means of synchrotron radiation induced X-ray photoelectron spectroscopy (SR-XPS) and angular dependent near edge X-ray absorption spectroscopy (NEXAFS) at the C K-edge. In order to obtain better insight into the molecular shell features, the same measurements were also carried out on a self-assembling monolayer (SAM) of SEB anchored on a "flat" gold surface (Au/Si(111) wafer). The comparison between angular dependent NEXAFS spectra collected on the self-assembling monolayer and AuNPs-SEB allowed for successfully probing the molecular arrangement of SEB molecules on the gold nanospheres surface. Furthermore, DFT calculations on the free SEB molecule as well as bonded to a small cluster of gold atoms were developed. The comparison with experimental results allowed better understanding of the spectroscopic signatures in the experimental absorption spectra and rationalization of the molecular organization in the SAM, NPs having a thin molecular shell, and NPs covered by a thick layer of ligands. Figure 4. Angular dependent NEXAFS C K-edge spectra collected with normal (black line) and grazing (red line) incidence of the beam on the sample surface for (a) SAM, (b) NP , and (c) NP(2). The blue lines are obtained by subtracting the normal from the grazing signal, and evidence the observed angular (dichroic) effects. To evidence the polarization dependence of the main π* feature, the aromatic ring π* region is zoomed and reported in (d) SAM, (e) NP , and (f) NP(2).

The Journal of Physical Chemistry C, 2014

The device performance and microstructure of a series of PTB7-based polymers with varied molecula... more The device performance and microstructure of a series of PTB7-based polymers with varied molecular weight and degree of fluorination are investigated. Although the energy level of the highest occupied molecular orbital is found to increase with degree of fluorination, a strong relative molecular weight dependence of device performance dominates any underlying fluorination-dependent trend on overall performance. Microstructural investigation using a combination of X-ray techniques reveals a striking effect of polymer molecular characteristics on film morphology, with the size of PC 71 BM domains systematically decreasing with increasing polymer molecular weight. Furthermore, the relative purity of the mixed PTB7:PC 71 BM domain is found to systematically decrease with increasing molecular weight. When domain sizes with and without the use of the solvent additive diiodooctane (DIO) are compared, the effectiveness of DIO in reducing PC 71 BM domain sizes is also found to be strongly dependent on the molecular weight of the polymer. It is found that molecular weights of at least 150 kg mol −1 are required for DIO to be effective in reducing the PC 71 BM domain size in order to produce high short-circuit current densities. Finally, it is shown that relatively high device efficiencies can be achieved with low degrees of fluorination; an efficiency of 4.6% is achieved for a degree of fluorination of only 5.3%.

The Journal of Physical Chemistry C, 2013

Self-assembly of butanedithiol (C4DT) monolayers (SAMs) on gold and substitutional assembly by oc... more Self-assembly of butanedithiol (C4DT) monolayers (SAMs) on gold and substitutional assembly by octanethiols from a lying-down doubly tethered phase of butane dithiol, were investigated in a vacuum evaporation experiment by high-resolution photoemission. The intent was to address the question of transitions from a lying-down to a standing-up phase in dithiol self-assembly and whether a standing-up phase of short chain dithiol can be formed. The lying-down doubly tethered dithiol phase, with both sulfur atoms attached to gold, was formed by evaporation and then exposed to octanethiol molecules. A mixed SAM composed of standing-up octanethiol and dithiol molecules is found to be formed. The degree of dithiol replacement, as evidenced by the increasing thickness of the SAM, is found to augment slowly with increasing exposure to octanethiol. A standing-up C4DT phase was found to be formed upon rapid very large exposure to C4DT vapors. Differences in valence band photoemission between the standing-up and lying-down phases were noted and accounted for theoretically. In all cases, measurements indicate more than one possible adsorption configuration as evidenced by different sulfur binding energies.

Physical Review B, 2014

We developed a quantitative approach for the determination of molecular arrangement and electroni... more We developed a quantitative approach for the determination of molecular arrangement and electronic structure in anisotropic organic ultrathin films based on the measurement of polarized reflectivity at the carbon K-edge. The reflectivity spectra were fitted to a parameterized model calculation. The method was applied to a self-assembled monolayer of 1,4-benzenedimethanethiol on gold. To simulate reflectivity, the organic anisotropic film was described by a dielectric tensor, obtained by ab initio calculations for the single molecule and suitable rotations to describe the molecular organization in film domains. Film structure was obtained though the best fit of the simulation to the experiment. Results were consistent with a monolayer-thick film composed of domains of molecules with in-plane isotropic distribution of orientations. In each domain, molecules adopted a standing configuration, with a tilt of 28°relative to the substrate normal. Information on the modification of the molecular electronic states due to chemical bonding was derived.

Physica C: Superconductivity, 2005

Core level X-ray Photoelectron Spectroscopy (XPS) studies have been carried out on polycrystallin... more Core level X-ray Photoelectron Spectroscopy (XPS) studies have been carried out on polycrystalline MgB 2 pellets over the whole binding energy range with a view to having an idea of the charge state of Magnesium (Mg). We observe 3 distinct peaks in Mg 2p spectra at 49.3 eV (trace), 51.3 eV (major) and 54.0 eV (trace), corresponding to metallic Mg, MgB 2 and MgCO 3 or, divalent Mg species respectively. Similar trend has been noticed in Mg 2s spectra. The binding energy of Mg in MgB 2 is lower than that corresponding to Mg(2+), indicative of the fact that the charge state of Mg in MgB 2 is less than (2+). Lowering of the formal charge of Mg promotes the σ→π electron transfer in Boron (B) giving rise to holes on the top of the σ-band which are involved in coupling with B E 2g phonons for superconductivity. Through this charge transfer, Mg plays a positive role in hole superconductivity. B 1s spectra consist of 3 peaks corresponding to MgB 2 , boron and B 2 O 3 . There is also evidence of MgO due to surface oxidation as seen from O 1s spectra.

Materials Chemistry and Physics, 2013

h i g h l i g h t s g r a p h i c a l a b s t r a c t Ag nanoparticles were grown in triblock cop... more h i g h l i g h t s g r a p h i c a l a b s t r a c t Ag nanoparticles were grown in triblock copolymer matrix at different temperatures.

Macromolecules, 2009

There are several pathways through which a polymer can degrade such as thermal, photoinduced, bio... more There are several pathways through which a polymer can degrade such as thermal, photoinduced, biological, chemical and mechanical. The thermal degradation process of polymers is widely studied because of both academic and industrial interest. Here we have investigated the kinetics of thermal degradation and structural modification of polyacrylamide ultrathin films as a result of heat treatment at the degradation onset temperature. The chemical analysis of the material was performed using X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The formation of imide functionalities was found to occur on the polymer chains with simultaneous breakdown of amide groups. The kinetics of the degradation products obtained from quantitative analysis of the XPS spectra shows that the thermal degradation of the major part of the polymer occurs within the first 30 min of heating at the onset temperature of 220°C. The rate of degradation was found to saturate after ∼3 h of heating. The structural aspects of the heated films were studied using X-ray reflectivity (XRR). Analysis of the reflectivity data shows that the thickness of the film decreases and the electron density increases after prolonged heating at 220°C. From the amplitude of the reflectivity data, it was found that the interfacial morphology of the film stays almost unmodified and the film retains the polymeric property even after undergoing thermal degradation. The final onset degradation product obtained was ultrathin films of polymer containing 60% and 40% mono-and bicyclic imide functionalities, respectively.

Macromolecules, 2011

Stimuli-responsive polymers are materials that show potential for well-designed surfaces and inte... more Stimuli-responsive polymers are materials that show potential for well-designed surfaces and interfaces for a wide range of applications from medicine 1 to nanotechnology. 2,3 Response to different physical factors in these materials are known, for instance, response to pH, 4 magnetic field 5 or to temperature 6 changes. Poly(N-isopropylacrylamide) (PNIPAM) is a widely studied thermo sensitive polymer. 7À20 This polymer in water exhibits a reversible 21 phase transition at a lower critical solution temperature (LCST). The solution behavior of PNI-PAM has been investigated by a variety of experimental techniques in the dilute and concentrated regimes. The LCST of PNIPAM in water is approximately at 32°C which is close to human body temperature, which makes PNIPAM an attractive temperature-sensitive polymer for the study of biomedical applications. Recently poly(N-isopropylacrylamide)-grafted hyaluronan (PNIPAMÀHA) and PNIPAM-grafted gelatin (PNIPAMÀgelatin), which exhibit sol-to-gel transformation at physiological temperature, were applied as tissue adhesion prevention material and hemostatic aid, respectively. The temperature-dependent change in the conformation of PNIPAM chains reflects changes in polymer/water interactions. At lower temperatures, intermolecular hydrogen bonds between solvent water and polar groups of PNIPAM keep the polymer soluble. At higher temperature above the LCST the hydrogen bonds break, and hydrophobic associations between the collapsed polymer chains take place. Several models have been described to account for the coil-to-globule transition of PNIPAM in water and the complex water/PNIPAM phase diagram. 24À26 It has recently been stressed that PNIPAM is never completely hydrophobic. In fact, as mentioned by the author, PNIPAM is not completely hydrophilic-it has hydrophilic and hydrophobic domains above and below the LCST. Like amphiphilic proteins and surfactants, oxygen and nitrogen rich domains of PNIPAM are hydrophilic above and below the LCST. Similarly the isopropyl groups and the polymethylene backbones are hydrophobic above and below the LCST.

Journal of Electron Spectroscopy and Related Phenomena, 2010

In case of photoelectron spectroscopy of an insulating material the data obtained from the charge... more In case of photoelectron spectroscopy of an insulating material the data obtained from the charged surface are often distorted due to differentially charged surface domains. Recently we have developed a controlled surface neutralization technique to study the kinetics of the surface charging. Here we demonstrate the application of the technique to study the neutralization kinetics of both thick and thin films of charged polymer-metal nanocomposite material using photoemission. Neutralization kinetics of grounded and floated pure polymer thin films was also studied. It was observed that for the thick sample the transition of positively charged domains to overcompensated ones occurs through percolation. In case of grounded thin films the growth of overcompensated domains exhibit a linear behavior followed by saturation. When electrons appear at both surfaces of a floated thin film, the neutralization kinetics show a completely different behavior. Present investigation indicates that for thin films of insulating materials appearing to be neutral in presence of an electron source, controlled neutralization technique may be an important tool to distinguish between presence of multiple chemical species and differential charging.

In case of photoemission spectroscopy of an insulating material the data obtained from the charge... more In case of photoemission spectroscopy of an insulating material the data obtained from the charged surface are normally distorted due to differential charging. Recently, we have developed a controlled surface neutralization technique to study the kinetics of the surface charging. Using this technique and the associated data analysis scheme with an effective charging model, quantitative information from the apparently distorted photoemission data from PTFE surfaces were extracted. The surface charging was controlled by tuning the electron flood current as well as the X-ray intensity. The effective model was found to describe the charging consistently for both the cases. It was shown that the non-linear neutralization response of differential charging around a critical neutralizing electron flux or a critical X-ray emission current was due to percolation of equipotential surface domains. The obtained value of the critical percolation exponent close to unity indicates a percolation similar to that of avalanche breakdown or chain reaction.

Journal of Physics: Condensed Matter, 2006

Polyacrylamide (PAM)-silver nanocomposite materials have been synthesized by the reduction of the... more Polyacrylamide (PAM)-silver nanocomposite materials have been synthesized by the reduction of the silver salt in the polyacrylamide matrix. Silver nanoparticles embedded in the polymer matrix, observed through transmission electron microscopy (TEM), suggests attractive polymer-particle interaction. Detailed spectroscopic investigations have been carried out on the nanocomposite samples using a range of techniques to understand the nature of the interaction. The x-ray photoelectron spectroscopy (XPS) core level spectra and Fourier-transform infrared (FTIR) spectroscopy results indicate that the nanoparticles are attached to the pendant group of the polymer through physical interactions. The XPS valence band study confirms that the interaction between the polymer and the silver nanoparticles occurs through partial charge transfer from the metal particles to the nitrogen sites of the polymer side chains. The modifications associated with the silver nanoparticles are mainly due to confinement effects.

Advanced Energy Materials, 2015

Damage to VUV, EUV, and X-ray Optics IV; and EUV and X-ray Optics: Synergy between Laboratory and Space III, 2013

ABSTRACT Here is presented the spectroscopic study of the evolution of the first buried interface... more ABSTRACT Here is presented the spectroscopic study of the evolution of the first buried interfaces of a B4C capped Co/Mo2C multilayer mirror induced by thermal treatment up to 600°C. This kind of study is typically performed to simulate the response of multilayer optics working in extreme conditions, as for instance when irradiated by new high brilliance sources as Free Electron Lasers. In fact, the efficiency of multilayers is related to the optical contrast between the alternating high and low density layers, and then to the degree of interdiffusion and the creation or evolution of interface compounds. The analysis has been performed at the Co L23 edge with different soft x-ray spectroscopic techniques including diffuse and specular reflectivity, total electron and fluorescent yield at the BEAR beamline at Elettra (Trieste) (http://www.elettra.trieste.it/elettra-beamlines/bear.html). The presentation is focused on the spectroscopic results obtained by soft x-ray standing wave enhanced photoemission (XSW) from the Mo 3d, B 1s, C 1s, O 1s core levels by using a photon energy close to the Co L23 edge and corresponding to the first Bragg peak of the multilayer. The experimental results have been compared with simulations to obtain information both on the chemical state (e.g. oxidation state) and interface morphology in terms of profiles of distribution of elements and interdiffusion of B, oxidized B and C in the interface region. In summary, it is possible to conclude in favour of a good stability of the multilayer in the investigated temperature range, as confirmed by the good performance in terms of reflectivity. These results confirm the usefulness of XSW for this kind analysis of multilayer optics.

Advanced Energy Materials, 2015

Advanced Materials

The addition of small quantities of polystyrene (PS) is a simple and economically viable process ... more The addition of small quantities of polystyrene (PS) is a simple and economically viable process that improves the power conversion efficiency of one of the most efficient small molecule donors. Addition of PS increases the solution viscosity, thereby providing thicker layers, and allows the formation of a desirable bulk heterojunction morphology. Moreover, the PS spontaneously accumulates as phase separated domains, away from the electrodes, so as not to interfere with charge extraction.

Advanced materials (Deerfield Beach, Fla.), 2015

Connections are delineated between solar-cell performance, charge-carrier mobilities, and morphol... more Connections are delineated between solar-cell performance, charge-carrier mobilities, and morphology in a highperformance molecular solar cell. The observations show that maximizing the relative phase purity and structural order while simultaneously limiting the domain size may be essential for achieving optimal solar-cell performances in solution-processed small-molecule solar cells .

Advanced materials (Deerfield Beach, Fla.), Jan 19, 2014

A relationship between solvent additive concentration and active layer thickness in small-molecul... more A relationship between solvent additive concentration and active layer thickness in small-molecule solar cells is investigated. Specifically, the additive concentration must scale with the amount of semiconductor material and not as absolute concentration in solution. Devices with a wide range of active layers with thickness up to 200 nm can readily achieve efficiencies close to 6% when the right concentration of additive is used.

Optical Systems Design 2012, 2012

ABSTRACT We study a periodic Co/Mo2C multilayer prepared by magnetron sputtering. The period is 4... more ABSTRACT We study a periodic Co/Mo2C multilayer prepared by magnetron sputtering. The period is 4.1 nm and the sample is designed to work around 778 eV, i.e. close to the Co 2p3/2 threshold, at a glancing angle of 11°. In this condition, strong x-ray standing waves set up within the sample. In order to probe different depths within the stack, particularly the interfaces, the glancing angle is moved along the first Bragg peak, while, the B 1s, C 1s, Mo 3d or O 1s photoelectron spectra, the Co Lα x-ray spectrum as well as the drain current of the sample are measured. Boron is present in the 3.5 nm B4C capping layer and oxygen is from surface contamination. The photoelectrons bring information from the superficial zone, i.e. the 5 first nm, while the characteristic x-rays probe the whole stack. Clear modulations of the intensity of the studied signals as well as core level shifts are observed when going through the Bragg peak. In order to understand what happens in the multilayer calculations of depth distributions of the electric field and the energy loss by the radiation are made with the IMD and OPAL codes, respectively. The combination of experimental results and theoretical simulations will enable us to determine from which place originate the various signals and to know if some interaction exists between the Co and Mo2C layers.

The Journal of Physical Chemistry C, 2014

Small gold nanoparticles capped with 4trimethylsilylethynyl-1-acetylthiobenzene (SEB) were prepar... more Small gold nanoparticles capped with 4trimethylsilylethynyl-1-acetylthiobenzene (SEB) were prepared with spherical shape and different mean sizes (5−8 nm). The functionalized gold nanoparticles (AuNPs-SEB) were deposited onto TiO 2 substrates, and the interaction at the molecule−gold interface, the molecular layer thickness, and the ligand organization on the surface of Au nanospheres were investigated by means of synchrotron radiation induced X-ray photoelectron spectroscopy (SR-XPS) and angular dependent near edge X-ray absorption spectroscopy (NEXAFS) at the C K-edge. In order to obtain better insight into the molecular shell features, the same measurements were also carried out on a self-assembling monolayer (SAM) of SEB anchored on a "flat" gold surface (Au/Si(111) wafer). The comparison between angular dependent NEXAFS spectra collected on the self-assembling monolayer and AuNPs-SEB allowed for successfully probing the molecular arrangement of SEB molecules on the gold nanospheres surface. Furthermore, DFT calculations on the free SEB molecule as well as bonded to a small cluster of gold atoms were developed. The comparison with experimental results allowed better understanding of the spectroscopic signatures in the experimental absorption spectra and rationalization of the molecular organization in the SAM, NPs having a thin molecular shell, and NPs covered by a thick layer of ligands. Figure 4. Angular dependent NEXAFS C K-edge spectra collected with normal (black line) and grazing (red line) incidence of the beam on the sample surface for (a) SAM, (b) NP , and (c) NP(2). The blue lines are obtained by subtracting the normal from the grazing signal, and evidence the observed angular (dichroic) effects. To evidence the polarization dependence of the main π* feature, the aromatic ring π* region is zoomed and reported in (d) SAM, (e) NP , and (f) NP(2).

The Journal of Physical Chemistry C, 2014

The device performance and microstructure of a series of PTB7-based polymers with varied molecula... more The device performance and microstructure of a series of PTB7-based polymers with varied molecular weight and degree of fluorination are investigated. Although the energy level of the highest occupied molecular orbital is found to increase with degree of fluorination, a strong relative molecular weight dependence of device performance dominates any underlying fluorination-dependent trend on overall performance. Microstructural investigation using a combination of X-ray techniques reveals a striking effect of polymer molecular characteristics on film morphology, with the size of PC 71 BM domains systematically decreasing with increasing polymer molecular weight. Furthermore, the relative purity of the mixed PTB7:PC 71 BM domain is found to systematically decrease with increasing molecular weight. When domain sizes with and without the use of the solvent additive diiodooctane (DIO) are compared, the effectiveness of DIO in reducing PC 71 BM domain sizes is also found to be strongly dependent on the molecular weight of the polymer. It is found that molecular weights of at least 150 kg mol −1 are required for DIO to be effective in reducing the PC 71 BM domain size in order to produce high short-circuit current densities. Finally, it is shown that relatively high device efficiencies can be achieved with low degrees of fluorination; an efficiency of 4.6% is achieved for a degree of fluorination of only 5.3%.

The Journal of Physical Chemistry C, 2013

Self-assembly of butanedithiol (C4DT) monolayers (SAMs) on gold and substitutional assembly by oc... more Self-assembly of butanedithiol (C4DT) monolayers (SAMs) on gold and substitutional assembly by octanethiols from a lying-down doubly tethered phase of butane dithiol, were investigated in a vacuum evaporation experiment by high-resolution photoemission. The intent was to address the question of transitions from a lying-down to a standing-up phase in dithiol self-assembly and whether a standing-up phase of short chain dithiol can be formed. The lying-down doubly tethered dithiol phase, with both sulfur atoms attached to gold, was formed by evaporation and then exposed to octanethiol molecules. A mixed SAM composed of standing-up octanethiol and dithiol molecules is found to be formed. The degree of dithiol replacement, as evidenced by the increasing thickness of the SAM, is found to augment slowly with increasing exposure to octanethiol. A standing-up C4DT phase was found to be formed upon rapid very large exposure to C4DT vapors. Differences in valence band photoemission between the standing-up and lying-down phases were noted and accounted for theoretically. In all cases, measurements indicate more than one possible adsorption configuration as evidenced by different sulfur binding energies.

Physical Review B, 2014

We developed a quantitative approach for the determination of molecular arrangement and electroni... more We developed a quantitative approach for the determination of molecular arrangement and electronic structure in anisotropic organic ultrathin films based on the measurement of polarized reflectivity at the carbon K-edge. The reflectivity spectra were fitted to a parameterized model calculation. The method was applied to a self-assembled monolayer of 1,4-benzenedimethanethiol on gold. To simulate reflectivity, the organic anisotropic film was described by a dielectric tensor, obtained by ab initio calculations for the single molecule and suitable rotations to describe the molecular organization in film domains. Film structure was obtained though the best fit of the simulation to the experiment. Results were consistent with a monolayer-thick film composed of domains of molecules with in-plane isotropic distribution of orientations. In each domain, molecules adopted a standing configuration, with a tilt of 28°relative to the substrate normal. Information on the modification of the molecular electronic states due to chemical bonding was derived.

Physica C: Superconductivity, 2005

Core level X-ray Photoelectron Spectroscopy (XPS) studies have been carried out on polycrystallin... more Core level X-ray Photoelectron Spectroscopy (XPS) studies have been carried out on polycrystalline MgB 2 pellets over the whole binding energy range with a view to having an idea of the charge state of Magnesium (Mg). We observe 3 distinct peaks in Mg 2p spectra at 49.3 eV (trace), 51.3 eV (major) and 54.0 eV (trace), corresponding to metallic Mg, MgB 2 and MgCO 3 or, divalent Mg species respectively. Similar trend has been noticed in Mg 2s spectra. The binding energy of Mg in MgB 2 is lower than that corresponding to Mg(2+), indicative of the fact that the charge state of Mg in MgB 2 is less than (2+). Lowering of the formal charge of Mg promotes the σ→π electron transfer in Boron (B) giving rise to holes on the top of the σ-band which are involved in coupling with B E 2g phonons for superconductivity. Through this charge transfer, Mg plays a positive role in hole superconductivity. B 1s spectra consist of 3 peaks corresponding to MgB 2 , boron and B 2 O 3 . There is also evidence of MgO due to surface oxidation as seen from O 1s spectra.

Materials Chemistry and Physics, 2013

h i g h l i g h t s g r a p h i c a l a b s t r a c t Ag nanoparticles were grown in triblock cop... more h i g h l i g h t s g r a p h i c a l a b s t r a c t Ag nanoparticles were grown in triblock copolymer matrix at different temperatures.

Macromolecules, 2009

There are several pathways through which a polymer can degrade such as thermal, photoinduced, bio... more There are several pathways through which a polymer can degrade such as thermal, photoinduced, biological, chemical and mechanical. The thermal degradation process of polymers is widely studied because of both academic and industrial interest. Here we have investigated the kinetics of thermal degradation and structural modification of polyacrylamide ultrathin films as a result of heat treatment at the degradation onset temperature. The chemical analysis of the material was performed using X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The formation of imide functionalities was found to occur on the polymer chains with simultaneous breakdown of amide groups. The kinetics of the degradation products obtained from quantitative analysis of the XPS spectra shows that the thermal degradation of the major part of the polymer occurs within the first 30 min of heating at the onset temperature of 220°C. The rate of degradation was found to saturate after ∼3 h of heating. The structural aspects of the heated films were studied using X-ray reflectivity (XRR). Analysis of the reflectivity data shows that the thickness of the film decreases and the electron density increases after prolonged heating at 220°C. From the amplitude of the reflectivity data, it was found that the interfacial morphology of the film stays almost unmodified and the film retains the polymeric property even after undergoing thermal degradation. The final onset degradation product obtained was ultrathin films of polymer containing 60% and 40% mono-and bicyclic imide functionalities, respectively.

Macromolecules, 2011

Stimuli-responsive polymers are materials that show potential for well-designed surfaces and inte... more Stimuli-responsive polymers are materials that show potential for well-designed surfaces and interfaces for a wide range of applications from medicine 1 to nanotechnology. 2,3 Response to different physical factors in these materials are known, for instance, response to pH, 4 magnetic field 5 or to temperature 6 changes. Poly(N-isopropylacrylamide) (PNIPAM) is a widely studied thermo sensitive polymer. 7À20 This polymer in water exhibits a reversible 21 phase transition at a lower critical solution temperature (LCST). The solution behavior of PNI-PAM has been investigated by a variety of experimental techniques in the dilute and concentrated regimes. The LCST of PNIPAM in water is approximately at 32°C which is close to human body temperature, which makes PNIPAM an attractive temperature-sensitive polymer for the study of biomedical applications. Recently poly(N-isopropylacrylamide)-grafted hyaluronan (PNIPAMÀHA) and PNIPAM-grafted gelatin (PNIPAMÀgelatin), which exhibit sol-to-gel transformation at physiological temperature, were applied as tissue adhesion prevention material and hemostatic aid, respectively. The temperature-dependent change in the conformation of PNIPAM chains reflects changes in polymer/water interactions. At lower temperatures, intermolecular hydrogen bonds between solvent water and polar groups of PNIPAM keep the polymer soluble. At higher temperature above the LCST the hydrogen bonds break, and hydrophobic associations between the collapsed polymer chains take place. Several models have been described to account for the coil-to-globule transition of PNIPAM in water and the complex water/PNIPAM phase diagram. 24À26 It has recently been stressed that PNIPAM is never completely hydrophobic. In fact, as mentioned by the author, PNIPAM is not completely hydrophilic-it has hydrophilic and hydrophobic domains above and below the LCST. Like amphiphilic proteins and surfactants, oxygen and nitrogen rich domains of PNIPAM are hydrophilic above and below the LCST. Similarly the isopropyl groups and the polymethylene backbones are hydrophobic above and below the LCST.

Journal of Electron Spectroscopy and Related Phenomena, 2010

In case of photoelectron spectroscopy of an insulating material the data obtained from the charge... more In case of photoelectron spectroscopy of an insulating material the data obtained from the charged surface are often distorted due to differentially charged surface domains. Recently we have developed a controlled surface neutralization technique to study the kinetics of the surface charging. Here we demonstrate the application of the technique to study the neutralization kinetics of both thick and thin films of charged polymer-metal nanocomposite material using photoemission. Neutralization kinetics of grounded and floated pure polymer thin films was also studied. It was observed that for the thick sample the transition of positively charged domains to overcompensated ones occurs through percolation. In case of grounded thin films the growth of overcompensated domains exhibit a linear behavior followed by saturation. When electrons appear at both surfaces of a floated thin film, the neutralization kinetics show a completely different behavior. Present investigation indicates that for thin films of insulating materials appearing to be neutral in presence of an electron source, controlled neutralization technique may be an important tool to distinguish between presence of multiple chemical species and differential charging.

In case of photoemission spectroscopy of an insulating material the data obtained from the charge... more In case of photoemission spectroscopy of an insulating material the data obtained from the charged surface are normally distorted due to differential charging. Recently, we have developed a controlled surface neutralization technique to study the kinetics of the surface charging. Using this technique and the associated data analysis scheme with an effective charging model, quantitative information from the apparently distorted photoemission data from PTFE surfaces were extracted. The surface charging was controlled by tuning the electron flood current as well as the X-ray intensity. The effective model was found to describe the charging consistently for both the cases. It was shown that the non-linear neutralization response of differential charging around a critical neutralizing electron flux or a critical X-ray emission current was due to percolation of equipotential surface domains. The obtained value of the critical percolation exponent close to unity indicates a percolation similar to that of avalanche breakdown or chain reaction.