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Raja, Shilpa N.

Publications and source records attributed to Raja, Shilpa N..

10 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Mechanisms of local stress sensing in multifunctional polymer films using fluorescent tetrapod nanocrystals

Nanoscale stress-sensing can be used across fields ranging from detection of incipient cracks in structural mechanics to monitoring forces in biological tissues. We demonstrate how tetrapod quantum dots (tQDs) embedded in block-copolymers act as sensors of tensile/compressive stress. Remarkably, tQDs can detect their own composite dispersion and mechanical properties, with a switch in optomechanical response when tQDs are in direct contact. Using experimental characterizations, atomistic simulations and finite-element analyses, we show that under tensile stress, densely-packed tQDs exhibit a photoluminescence peak shifted to higher energies (“blue-shift”) due to volumetric compressive stress in their core; loosely-packed tQDs exhibit a peak shifted to lower energies (“red-shift”) from tensile stress in the core. The stress-shifts result from the tQD's unique branched morphology in which the CdS arms act as antennas that amplify the stress in the CdSe core. Our nanocomposites exhibit excellent cyclability and scalability with no degraded properties of the host polymer. Colloidal tQDs allow sensing in many materials to potentially enable auto-responsive, smart structural nanocomposites that self-predict impending fracture.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microscopic mechanisms of deformation transfer in high dynamic range branched nanoparticle deformation sensors

Nanoscale stress sensing is of crucial importance to biomechanics and other fields. An ideal stress sensor would have a large dynamic range to function in a variety of materials spanning orders of magnitude of local stresses. In this, we show that tetrapod quantum dots (tQDs) exhibit excellent sensing versatility with stress-correlated signatures in a multitude of polymers. We further show that tQDs exhibit pressure coefficients, which increase with decreasing polymer stiffness, and vary >3 orders of magnitude. This high dynamic range allows tQDs to sense in matrices spanning >4 orders of magnitude in Young's modulus, ranging from compliant biological levels (~100 kPa) to stiffer structural polymers (~5 GPa). We use ligand exchange to tune filler-matrix interfaces, revealing that inverse sensor response scaling is maintained upon significant changes to polymer-tQD interface chemistry. We quantify and explore mechanisms of polymer-tQD strain transfer. An analytical model based on Mori-Tanaka theory presents agreement with observed trends.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Characterizing Photon Reabsorption in Quantum Dot-Polymer Composites for Use as Displacement Sensors

The reabsorption of photoluminescence within a medium, an effect known as the inner filter effect (IFE), has been well studied in solutions, but has garnered less attention in regards to solid-state nanocomposites. Photoluminescence from a quantum dot (QD) can selectively excite larger QDs around it resulting in a net red-shift in the reemitted photon. In CdSe/CdS core/shell QD-polymer nanocomposites, we observe a large spectral red-shift of over a third of the line width of the photoluminescence of the nanocomposites over a distance of 100 μm resulting from the IFE. Unlike fluorescent dyes, which do not show a large IFE red-shift, QDs have a component of inhomogeneous broadening that originates from their size distribution and quantum confinement. By controlling the photoluminescence broadening as well as the sample dispersion and concentration, we show that the magnitude of the IFE within the nanocomposite can be tuned. We further demonstrate that this shift can be exploited in order to spectroscopically monitor the vertical displacement of a nanocomposite in a fluorescence microscope. Large energetic shifts in the measured emission with displacement can be maximized, resulting in a displacement sensor with submicrometer resolution. We further show that the composite can be easily attached to biological samples and is able to measure deformations with high temporal and spatial precision.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2016

Tracking Nanoparticle Diffusion and Interaction during Self-Assembly in a Liquid Cell

Nanoparticle self-assembly has been well studied theoretically, but it remains challenging to directly observe and quantify individual nanoparticle interactions. With our custom image analysis method, we track the trajectories of nanoparticle movement with high precision from a stack of relatively noisy images obtained using liquid cell transmission electron microscopy. In a time frame of minutes, Pt-Fe nanoparticles self-assembled into a loosely packed hcp lattice. The energetics and stability of the dynamic assembly were studied quantitatively. From velocity and diffusion measurements, we experimentally determined the magnitude of forces between single particles and the related physical properties. The results illustrate that long-range anisotropic forces drive the formation of chains, which then clump and fold to maximize close range van der Waals interactions.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2016

Encapsulation of Perovskite Nanocrystals into Macroscale Polymer Matrices: Enhanced Stability and Polarization

Lead halide perovskites hold promise for photonic devices, due to their superior optoelectronic properties. However, their use is limited by poor stability and toxicity. We demonstrate enhanced water and light stability of high-surface-area colloidal perovskite nanocrystals by encapsulation of colloidal CsPbBr 3 quantum dots into matched hydrophobic macroscale polymeric matrices. This is achieved by mixing the quantum dots with presynthesized high-molecular-weight polymers. In this work, we monitor the photoluminescence quantum yield of the perovskite-polymer nanocomposite films under water-soaking for the first time, finding no change even after >4 months of continuous immersion in water. Furthermore, photostability is greatly enhanced in the macroscale polymer-encapsulated nanocrystal perovskites, which sustain >10 10 absorption events per quantum dot prior to photodegradation, a significant threshold for potential device use. Control of the quantum dot shape in these thin-film polymer composite enables color tunability via strong quantum-confinement in nanoplates and significant room temperature polarized emission from perovskite nanowires. Not only does the high-molecular-weight polymer protect the perovskites from the environment but also no escaped lead was detected in water that was in contact with the encapsulated perovskites for months. In conclusion, our ligand-passivated perovskite-macroscale polymer composites provide a robust platform for diverse photonic applications.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Mechanisms of Local Stress Sensing in Multifunctional Polymer Films Using Fluorescent Tetrapod Nanocrystals

Nanoscale stress-sensing can be used across fields ranging from detection of incipient cracks in structural mechanics to monitoring forces in biological tissues. We demonstrate how tetrapod quantum dots (tQDs) embedded in block copolymers act as sensors of tensile/compressive stress. Remarkably, tQDs can detect their own composite dispersion and mechanical properties with a switch in optomechanical response when tQDs are in direct contact. Using experimental characterizations, atomistic simulations and finite-element analyses, we show that under tensile stress, densely packed tQDs exhibit a photoluminescence peak shifted to higher energies ("blue-shift") due to volumetric compressive stress in their core; loosely packed tQDs exhibit a peak shifted to lower energies ("red-shift") from tensile stress in the core. The stress shifts result from the tQD's unique branched morphology in which the CdS arms act as antennas that amplify the stress in the CdSe core. Our nanocomposites exhibit excellent cyclability and scalability with no degraded properties of the host polymer. Colloidal tQDs allow sensing in many materials to potentially enable autoresponsive, smart structural nanocomposites that self-predict upcoming fracture.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Cavitation-Induced Stiffness Reductions in Quantum Dot–Polymer Nanocomposites

The elastic stiffness of two polymer nanocomposite systems is investigated. The nanoscale fillers comprise cadmium selenide (CdSe, ~4 nm) and cadmium selenide/cadmium sulfide (CdSe/CdS, ~13 nm) quantum dots (QDs). The QDs are embedded within an electrospun structural block copolymer, poly(styrene-ethylene-butylene-styrene) (SEBS). Tensile testing shows a monotonic decrease in the tensile Young's modulus with increasing partially phase-separated QD concentration; this is to be compared to corresponding nanocomposites reinforced with nanorod (NR) and tetrapod (TP)-SEBS nanocomposites which show a monotonic increase with particle loading. While most studies to date emphasize the increase in Young's modulus in polymer nanocomposites at higher reinforcement loadings, few focus on the tunability of the modulus from reductions in stiffness. The present work reveals up to an ~80% reduction in tensile Young's modulus with the addition of 5 vol % of QDs to electrospun SEBS. In this study, we sought mechanistic insight into this reduction in composite stiffness using a 2D lattice spring model. Simulation results reveal that the stiffness decrease with the addition of QD reinforcements is likely due to cavitation in the polymer in the vicinity of the QD aggregates arising from polymer debonding under tension. We anticipate that this study, performed with a commonly used structural rubber, may find use in designing polymer-matrix nanocomposite fibers with specific Young's moduli for applications requiring a tunable lower stiffness material.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Structural diversity in binary superlattices self-assembled from polymer-grafted nanocrystals

Multicomponent nanocrystal superlattices represent an interesting class of material that derives emergent properties from mesoscale structure, yet their programmability can be limited by the alkyl-chain-based ligands decorating the surfaces of the constituent nanocrystals. Polymeric ligands offer distinct advantages, as they allow for more precise tuning of the effective size and ‘interaction softness’ through changes to the polymer’s molecular weight, chemical nature, architecture, persistence length and surrounding solvent. Here we show the formation of 10 different binary nanocrystal superlattices (BNSLs) with both two- and three-dimensional order through independent adjustment of the core size of spherical nanocrystals and the molecular weight of densely grafted polystyrene ligands. These polymer-brush-based ligands introduce new energetic contributions to the interparticle potential that stabilizes various BNSL phases across a range of length scales and interparticle spacings. In conclusion, our study opens the door for nanocrystals to become modular elements in the design of functional particle brush solids with controlled nanoscale interfaces and mesostructures.

77 NANOSCIENCE AND NANOTECHNOLOGY↗