The Shankar Research Group - Excitonics and Nanostructures Laboratory



47. Khajeh ARA, Shankar K and Choi P, Prediction of the Active Layer Nanomorphology in Polymer Solar Cells Using Molecular Dynamics Simulation, ACS Applied Materials & Interfaces, 5, 4617-4624, 2013.

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Abstract: Active layer nanomorphology is a major factor that determines the efficiency of bulk heterojunction polymer solar cells (PSCs). Synthesizing diblock copolymers in which acceptor and donor materials are the constituent blocks is the most recent method to control the structure of the active layer. In the current work, a computational method is proposed to predict the nanomorphology of the active layer consisting of a diblock copolymer. Diblock copolymers have a tendency to self-organize and form well-defined nanostructures. The shape of the structure depends on the Flory–Huggins interaction parameter (i.e., χ), the total degree of polymerization (N) and volume fractions of the constituent blocks (φi). In this work, molecular dynamics (MD) simulations were used to calculate χ parameters for two different block copolymers used in PSCs: P3HT-b-poly(S8A2)-C60and P3HT-b-poly(n-butyl acrylate-stat-acrylate perylene) also known as P3HT-b-PPerAcr. Such calculations indicated strong segregation of blocks into cylindrical structure for P3HT-b-poly(S8A2)-C60 and intermediate segregation into cylindrical structure for P3HT-b-PPerAcr. Experimental results of P3HT-b-poly(S8A2)-C60 and P3HT-b-PTP4AP, a diblock copolymer having very similar structure to P3HT-b-PPerAcr, validate our predictions.

46. Mohammadpour A, Farsinezhad S, Hsieh LH and Shankar K, Multipodal and Multilayer TiO2 Nanotube Arrays: Hierarchical Structures for Energy Harvesting and Sensing, Proceedings of the Materials Research Society, vol. 1552 : Nanostructured Metal Oxides for Advanced Applications, 2013.

Abstract: Our ability to fabricate multipodal and multilayer TiO2 nanotube arrays enables us to increase performance and functionality in light harvesting devices such as excitonic solar cells and photocatalysts. Using a combination of simulations and experiments, we show that multilayer nanotube arrays enable photon management in the active toward enhancing the absorption and utilization of incident light. We show that the simultaneous utilization of TiO2 nanotubes with large (∼450 nm) and small (∼80 nm) diameters in stacked multilayer films increased light absorption and photocurrent in solar cells. Such enhanced light absorption is particularly desirable in the near-infrared region of the solar spectrum in which most excitonic solar cells suffer from poor quantum efficiencies and for blue photons at the TiO2 band-edge where significant room exists for improvement of photocatalytic quantum yields. Under AM 1.5 one sun illumination, multilayer nanotube arrays afforded us an approximately 20% improvement in photocurrent over single layer nanotube array films of the same thickness for N-719 sensitized liquid junction solar cells. Also, the possibility of multipodal TiO2 nanotube growth with different electrolyte recipes is presented.

45. Amirsolaimani B, Zhang X, Han F, Farsinezhad S, Mohammadpour A, Dechaine GP and Shankar K, Effect of the Nature of the Metal Co-Catalyst on CO2 Photoreduction Using Fast-Grown Periodically Modulated Titanium Dioxide Nanotube Arrays (PMTiNTs), Proceedings of the Materials Research Society, vol. 1578 : Titanium Dioxide - Fundamentals and Applications, 2013.

Abstract: Anodically formed TiO2 nanotube arrays composed of the anatase phase with periodically modulated diameters (PMTiNTs) are excellent photocatalysts for the sunlight-driven transformation of carbon dioxide into hydrocarbons. Exploiting the full potential of this nanoarchitecture for CO2 photoreduction requires integration with metal nanoparticles that function as catalytic promoters for multistep electron transfer reactions. We studied the effect of different metallic and bimetallic nanoparticles on the rate of generation of light hydrocarbons by the photoreduction of CO2. All the metal nanoparticles were loaded on to the TiO2nanotubes using the technique of photodeposition, which standardized the coating process and enabled examination purely of the effect of different metals. Photodeposition was used not only due to its simplicity but also because it enabled us to engineer very fine coatings possessing excellent uniformity and depth penetration into the nanotubes. The best performing co-catalysts were found to be CuPt (atomic ratio of 0.33:0.67), Pt and NiPt (1:2), which when loaded onto the PMTiNTs yielded total hydrocarbon generation rates of 3.5, 0.85 and 0.8 mL g-1 hr-1 respectively. The time required to form PMTiNTs was reduced by a factor of 160 by using a recently reported recipe based on fluoride ion bearing electrolyte containing lactic acid. PMTiNTs formed using the ultrafast growth lactic acid-based electrolytes exhibited similar photocatalytic properties to samples obtained more slowly using conventional ethylene glycol-based electrolytes.

44. Kar P and Shankar K, Biodiagnostics Using Oriented and Aligned Inorganic Semiconductor Nanotubes and Nanowires, Journal of Nanoscience and Nanotechnology, 13, 4473-4496 2013.

Abstract: The simplicity of synthesis of deterministically positioned inorganic semiconductor nanorods (NRs) and nanotubes (NTs) coupled with their chemical stability, high surface area, controllable optical properties and tunable surface functionality, have sparked worldwide research efforts towards biodiagnostic applications. Biosensors based on oriented and aligned one-dimensional (1-D) inorganic semiconductor nanostructures have demonstrated remarkable detection sensitivity, high throughput and label-free operability. In comparison to suspensions of nanoparticles and discrete randomly oriented nanowires, nanowire (NW) and nanotube arrays offer continuous charge transport pathways, a major advantage for all-electrical detection and in exploiting electrokinetic effects. We review highly sensitive biosensors based on oriented and aligned NTs/NRs/NWs employing conventional detection methods, inclusive of fluorescence, electrochemistry and electromechanical sensing as well as detection methods unique to nanowires such as field-effect transistors. Entirely new types of sensing applications such as the impaling of living cells to monitor cellular events in situ, and substrates with electrically controlled wetting for surface-assisted laser desorption and ionization are emerging to take advantage of the unique properties of nanowire arrays. Concurrently, we explain the semiconductor materials and architectures employed, and the functionalization procedures used to construct the biosensors. Aligned semiconductor array-based approaches are critically examined in relation to prevailing technologies to get a sense of the exclusive niches that nanotube/nanorod array biosensors inhabit. The versatility of the detection principles that nanowire/nanotube arrays are compatible with are enabling hybrid approaches where combinations of detection methods are used. Such advantages offset the complexity associated with changing the status quo with respect to the current state-of-the-art in biodiagnostic platforms and devices.

43. Zhang X, Han F, Shi B, Farsinezhad S, Dechaine GP and Shankar K, Photocatalytic Conversion of Diluted CO2 into Light Hydrocarbons Using Periodically Modulated Multiwalled Nanotube Arrays, Angewandte Chemie, 51(51), 12732-12735 2013.

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Abstract: Cu–Pt bimetallic shells supported on a double-walled TiO2nanotube (PMTiNT) array are efficient photocatalysts for the room-temperature conversion of CO2 into light hydrocarbons, such as CH4, C2H4, and C2H6. When Cu0.33–Pt0.67/PMTiNT was used for the photoreduction of diluted CO2 (1 % in N2), an average hydrocarbon production rate of 3.7 mL g−1 h−1 or 6.1 mmol m−2 h−1 was realized under AM 1.5 one-sun illumination.

41. Mohammadpour A, Utkin I, Bodepudi SC, Kar P, Fedosejevs R, Pramanik S and Shankar K, Photophysics and Energy Transfer Studies of Alq3 Confined in the Voids of Nanoporous Anodic Alumina, Journal of Nanoscience and Nanotechnology 13, 2647-2655 2013.

Abstract: We report on a hierarchical nanoarchitecture wherein distinct chromophores are deterministically placed at two different types of sites in a nanoporous metal oxide framework. One chromophore, namely Tris(8-hydroxyquinoline)aluminium(III) (Alq3), is embedded in the 1–2 nm sized nanovoids of anodic aluminum oxide (AAO) and another chromophore (carboxyfluorescein or pyrenebutyric acid) is anchored in the form of a monolayer to the surface of the walls of the cylindrical nanopores (∼20 nm in diameter) of AAO. We found the luminescence maximum to occur at 492 nm, blueshifted by at least 18 nm from the value in solutions and thin films. The excited state decay of Alq3 molecules in nanovoids was found to be biexponential with a fast component of 338 ps and a slower component of 2.26 ns, different from Alq3 thin films and solutions. Using a combination of steady state and time-resolved luminescence studies, we found that efficient Forster-type resonance energy transfer (FRET) from Alq3 in the nanovoids to the carboxyfluorescein monolayer could be used to pump the emission of surface-bound chromophores. Conversely, the emission of nanovoid-confined Alq3 could be pumped by energy transfer from a pyrenebutyric acid monolayer. Such intra-nanoarchitecture interactions between chromophores deterministically placed in different spatial locations are important in applications such as organic light emitting diodes, chemical sensors, energy transfer fluorescent labels, light harvesting antennas and organic spintronics.