Srinivasan, Madapusi P
Email Address
chesmp@nus.edu.sg
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Publication Robust, high-density zinc oxide nanoarrays by nanoimprint lithography-assisted area-selective atomic layer deposition(2012-11-08) Suresh, V.; Huang, M.S.; Srinivasan, M.P.; Guan, C.; Fan, H.J.; Krishnamoorthy, S.; CHEMICAL & BIOMOLECULAR ENGINEERINGPolymer templates realized through a combination of block copolymer lithography (BCL) and nanoimprint lithography (NIL) are used to direct atomic layer deposition (ALD) to obtain high-quality ZnO nanopatterns. These patterns present a uniform array of ZnO nanostructures with sub-100 nm feature and spatial resolutions, exhibiting narrow distributions in size and separation, and enhanced mechanical stability. The process benefits from the high lateral resolutions determined by the copolymer pattern, controlled growth rates, material quality and enhanced mechanical stability from ALD and repeatability and throughput from NIL. The protocol is generic and readily extendible to a range of other materials that can be grown through ALD. By virtue of their high feature density and material quality, the electrical characteristics of the arrays incorporated within MOS capacitors display high hole-storage density of 7.39 × 10 18 cm -3, excellent retention of ∼97% (for 1000 s of discharging), despite low tunneling oxide thickness of 3 nm. These attributes favor potential application of these ZnO arrays as charge-storage centers in nonvolatile flash memory devices. © 2012 American Chemical Society.Publication Covalent molecular assembly in supercritical carbon dioxide: Formation of nanoparticles in immobilized dendrimers within a porous silica gel matrix(2009-05-15) Puniredd, S.R.; Nguan, B.C.C.; Srinivasan, M.P.; CHEMICAL & BIOMOLECULAR ENGINEERINGWe report the formation of Pd, Pt and Pd/Pt nanoparticles in dendrimer-laden assemblies immobilized within a silica gel matrix and constructed by covalent layer-by-layer (LbL) assembly using supercritical carbon dioxide (SCCO2) as the processing medium. We show that the dendrimer plays a critical role in retaining the nanoparticle precursor within the silica gel. Smaller particle size, higher yield, a denser distribution of nanoparticles and greater stability were observed when the structures were assembled in SCCO2. © 2008 Elsevier Inc. All rights reserved.Publication Bimodal distribution of damage morphology generated by ion implantation(2005-12-05) Mok, K.R.C.; Jaraiz, M.; Martin-Bragado, I.; Rubio, J.E.; Castrillo, P.; Pinacho, R.; Srinivasan, M.P.; Benistant, F.; CHEMICAL & BIOMOLECULAR ENGINEERINGA nucleation and evolution model of damage based on amorphous pockets (APs) has recently been developed and implemented in an atomistic kinetic Monte Carlo simulator. In the model, APs are disordered structures (InV m), which are agglomerates of interstitials (I) and vacancies (V). This model has been used to study the composition and size distribution of APs during different ion implantations. Depending strongly on the dose rate, ion mass and implant temperature, the APs can evolve to a defect population where the agglomerates have a similar number of I and V (n ≈ m), or to a defect population with pure I (m ≈ 0) and pure V (n ≈ 0) clusters, or a mixture of APs and clusters. This behaviour corresponds to a bimodal (APs/clusters) distribution of damage. As the AP have different thermal stability compared to the I and V clusters, the same damage concentration obtained through different implant conditions has a different damage morphology and, consequently, exhibit a different resistance to subsequent thermal treatments. © 2005 Elsevier B.V. All rights reserved.Publication Synthesis of short chain thiol capped au nanoparticles and their stabilization inside functionalized polymers for organic memory applications(2008) Gupta, R.K.; Srinivasan, M.P.; CHEMICAL & BIOMOLECULAR ENGINEERINGPublication Application of direct covalent molecular assembly in the fabrication of polyimide ultrathin films(2005-04-12) Zhang, F.; Jia, Z.; Srinivasan, M.P.; PHYSICS; CHEMICAL & BIOMOLECULAR ENGINEERINGUltrathin films were fabricated using synthesized hydroxyl polyimide (HPI) in a layer-by-layer fashion on amine-terminated substrates of silicon, quartz, and gold. The interlayer linkages were established by using terephthaloyl chloride as a bridging agent to form ester groups between HPI layers. Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, UV-vis absorption spectroscopy, atomic force microscopy, ellipsometry, and electrochemical impedance spectroscopy were employed to study the interfacial chemistry, stepwise growth, morphology, thickness, optical property, and insulation behavior of the assembled film. The films show excellent stability and strength, which can be attributed to the covalent interlayer linkage. © 2005 American Chemical Society.Publication Covalent molecular assembly in a supercritical medium: Formation of nanoparticles encapsulated in immobilized dendrimers(2007-01-17) Puniredd, S.R.; Srinivasan, M.P.; CHEMICAL & BIOMOLECULAR ENGINEERINGThis work demonstrates the feasibility of forming metallic nanoparticles within the confines of dendrimerladen ultrathin films that are immobilized on a solid surface. The functional property of the poly amido amine dendrimer (PAMAM) as a sequestering agent for Cu nanoparticles was exploited by the introduction of copper acetylacetonate (Cu(acac)2) as a precursor. The nanoparticle precursors were introduced into the matrix using supercritical carbon dioxide and liquid tetrahydrofuran (THF) as processing media. The precursors were subsequently reduced to form the nanoparticles. The resulting film assemblies were characterized via X-ray photoelectron and UV-visible absorption spectroscopies, atomic force, scanning electron, transmission electron microscopies, and ellipsometry. Higher yield, a denser distribution of nanoparticles, and greater stability toward polar solvent attack were observed when the structures were assembled in supercritical CO2 than when THF was employed. TEM images revealed that the embedded nanoparticles are fairly tightly distributed in terms of size with an average diameter of 7 nm. © 2007 American Chemical Society.Publication Pd-Pt and Fe-Ni nanoparticles formed by covalent molecular assembly in supercritical carbon dioxide(2008-04-01) Puniredd, S.R.; Weiyi, S.; Srinivasan, M.P.; CHEMICAL & BIOMOLECULAR ENGINEERINGWe report the formation of Pd-Pt nanoparticles within a dendrimer-laden ultrathin film matrix immobilized on a solid support and constructed by covalent layer-by-layer (LbL) assembly using supercritical carbon dioxide (SCCO2) as the processing medium. Particle size distribution and composition were controlled by precursor composition. The precursor compositions are optimized for Pd-Pt nanoparticles and later extended to the formation of Fe-Ni nanoparticles. As an example of the application of nanoparticles in tribology, Fe-Ni nanoparticle-laden films were observed to exhibit better tribological properties than those containing the monometallic species, thereby suggesting that combination of nanoparticles can be used to derive greater benefits. © 2007.Publication Hierarchically built hetero-superstructure arrays with structurally controlled material compositions(2013-09-24) Suresh, V.; Madapusi, S.; Krishnamoorthy, S.; CHEMICAL & BIOMOLECULAR ENGINEERINGHierarchical assemblies are repeatedly encountered in nature, and when replicated in synthetic patterns and materials, can enhance their functionality or impart multifunctionality. In order to assemble a hierarchical superstructure that consists of components made up of multiple nanostructures, control over placement and stoichiometry is desirable. Macroscopic arrays that present up to three levels of hierarchy are demonstrated here and are achieved using the self-assembly of soft, collapsible block copolymer nanospheres for the first two levels, followed by directed self-assembly of metal nanospheres for the third. The fabrication approach combines advantages of soft sphere self-assembly to yield non-close-packed and variable array pitch values, with the inherent chemical functionality presented by the polymer-based soft spheres; these assemblies can then be transformed into a range of different materials, including metal or semiconductor nanostructures, or further tailored with an additional level of complexity. Structural investigation shows the superstructure formation to be governed by generic design rules that can be extended across different material combinations. © 2013 American Chemical Society.Publication Tailored titanium dioxide photocatalysts for the degradation of organic dyes in wastewater treatment: A review(2009-05-15) Han, F.; Kambala, V.S.R.; Srinivasan, M.; Rajarathnam, D.; Naidu, R.; CHEMICAL & BIOMOLECULAR ENGINEERINGOrganic dyes are one of the largest groups of pollutants released into wastewaters from textile and other industrial processes. Because of potential toxicity of the dyes and their visibility in surface waters, removal and degradation of organic dyes have been a matter of considerable interest. A wide range of methods have been developed, amongst which the heterogeneous photocatalysis involving titanium dioxide (TiO2) appears to be the most promising technology. This paper presents a critical review of novel achievements in the modification of TiO2 photocatalytic systems aimed at: enhanced TiO2 photocatalytic efficiency; complete mineralization of organic dyes; efficient utilization of visible and/or solar light; stability and reproducibility of the modified TiO2; recycle and reuse in real wastewater treatment. © 2009 Elsevier B.V. All rights reserved.Publication Synthesis of short chain thiol capped gold nanoparticles, their stabilization and immobilization on silicon surface(2011-10-20) Gupta, R.K.; Srinivasan, M.P.; Dharmarajan, R.; CHEMICAL & BIOMOLECULAR ENGINEERINGTethering between nanoparticles and a supporting substrate, especially in the form of covalent bonding, is a highly useful way of ensuring development of robust systems for various applications. This requires bestowing specific functionalities on nanoparticle surfaces that can also prevent aggregation between nanoparticles. In this work, short chain thiol (4-aminothiophenol)-capped gold nanoparticles functionalized with amine-termination have been synthesized and changes in their morphology by varying the process conditions as well as the dispersion medium were studied. Subsequently, the nanoparticles (which are prone to agglomerate due to amine-amine hydrogen bonding), were stabilized in solution and by anhydride capping. Finally, these stabilized nanoparticles were immobilized on a SiO2 substrate. X-ray photoelectron spectroscopy (XPS) confirmed the absence of non-reacted thiol groups in the synthesized nanoparticles and presence of nanoparticles on the Si surface after the immobilization process. Transmission electron microscope (TEM) and UV-visible spectroscopy have been used to study particle morphology. Cross-sectional HR-TEM images show direct evidence that the particles are anchored to the silicon surface. © 2011 Elsevier B.V.