arXiv subjects

Anh D. Phan

Publications and source records attributed to Anh D. Phan.

At least 55 records · Page 3Linked to original sources

arXiv2018

Multilayered plasmonic nanostructures for solar energy harvesting

Optical properties of core-shell-shell Au@SiO2@Au nanostructures and their solar energy harvesting applications are theoretically investigated using Mie theory and heat transfer equations. The theoretical analysis associated with size-dependent modification of the bulk gold dielectric function agrees well with previous experimental results. We use the appropriate absorption cross-section to determine the solar energy absorption efficiency of the nano-heterostructures, which is strongly structure-dependent, and to predict the time-dependent temperature increase of the nanoshell solution under simulated solar irradiation. Comparisons to prior temperature measurements and theoretical evaluation of the solar power conversion efficiency are discussed to provide new insights into underlying mechanisms. Our approach would accelerate materials and structure testing in solar energy harvesting.

physics.app-ph↗

arXiv2018

Dynamic Gradients, Mobile Layers, Tg Shifts, Role of Vitrification Criterion and Inhomogeneous Decoupling in Free-Standing Polymer Films

The force-level Elastically Collective Nonlinear Langevin Equation theory of activated relaxation in glass-forming free-standing thin films is re-visited to improve its treatment of collective elasticity effects. The naive cut off of the isotropic bulk displacement field approximation is improved to explicitly include spatial anisotropy with a modified boundary condition consistent with a step function liquid-vapor interface. The consequences of this improvement on dynamical predictions are quantitative but of significant magnitude and in the direction of further speeding up dynamics and further suppressing Tg. The theory is applied to thin films and also thick films to address new questions for three different polymers of different dynamic fragility. Variation of the vitrification time scale criterion over many orders of magnitude is found to have a minor effect on changes of the film-averaged Tg relative to its bulk value. The mobile layer length scale grows strongly with cooling and correlates in a nearly linear manner with the effective barrier deduced from the corresponding bulk isotropic liquid alpha relaxation time. The theory predicts a new type of spatially inhomogeneous "dynamic decoupling" corresponding to an effective factorization of the total barrier into its bulk temperature-dependent value multiplied by a function that only depends on location in the film. The effective decoupling exponent grows as the vapor surface is approached. Larger reductions of the absolute value of Tg shifts in thin polymer films are predicted for longer time vitrification criteria and more fragile polymers. Quantitative no-fit-parameter comparisons with experiment and simulation for film-thickness-dependent Tg shifts of PS and PC are in reasonable accord with the theory, including a nearly 100 K suppression of Tg in 4 nm PC films. Predictions are made for polyisobutylene thin films.

cond-mat.soft↗

arXiv2018

Elastically Collective Nonlinear Langevin Equation Theory of Dynamics in Glass-Forming Liquids: Transient Localization, Thermodynamic Mapping and Cooperativity

We analyze multiple new issues concerning activated relaxation in glassy hard sphere fluids and molecular and polymer liquids based on the Elastically Collective Nonlinear Langevin Equation (ECNLE) theory. By invoking a high temperature reference state, a near universality of the apparent dynamic localization length scale is predicted for liquids of widely varying fragility, a result that is relevant to recent simulation studies and quasi-elastic neutron scattering measurements. In contrast, in the same format strongly non-universal behavior is found for the activation barrier that controls long time relaxation. Two measures of cooperativity in ECNLE theory are analyzed. A particle-level total displacement associated with the alpha relaxation event is found to be only of order 1-2 particle diameters and weakly increases with cooling. In contrast, an alternative cooperativity length is defined as the spatial scale required to recover the full barrier and bulk alpha time. This length scale grows strongly with cooling due to the emergence in the deeply supercooled regime of collective long range elastic fluctuations required to allow local hopping. It becomes very large as the laboratory Tg is approached, though is relatively modest at degrees of supercooling accessible with molecular dynamics simulation. The alpha time is found to be exponentially related to this cooperativity length over an enormous number of decades of relaxation time that span the lightly to deeply supercooled regimes. Moreover, the effective barrier height increases almost linearly with the growing cooperativity length scale. An alternative calculation of the collective elastic barrier based on a literal continuum mechanics approach is shown to result in very little change of the theoretical results for bulk properties, but leads to a much smaller and less temperature-sensitive cooperativity length scale.

cond-mat.soft↗

arXiv2017

Near-infrared photothermal response of plasmonic gold-coated nanoparticles in tissues

We propose a new approach to understand the time-dependent temperature increasing process of gold-silica core-shell nanoparticles injected into chicken tissues under near-infrared laser irradiation. Gold nanoshells strongly absorb near-infrared radiations and efficiently transform absorbed energy into heat. Temperature rise given by experiments and numerical calculations based on bioheat transfer are in good agreement. Our work improves the analysis of a recent study [Richardson et al., Nano Lett. 9, 1139 (2009)] by including effects of the medium perfusion on temperature increase. The theoretical analysis can also be used to estimate the distribution of nanoparticles in experimental samples and provide a relative accuracy prediction for the temperature profile of new systems. This methodology would provide a novel and reliable tool for speeding up photothermal investigations and designing state-of-the-art photothermal devices.

cond-mat.mes-hall↗

arXiv2017

Theory of Activated Glassy Dynamics in Randomly Pinned Fluids

We generalize the force-level, microscopic, Nonlinear Langevin Equation (NLE) theory and its elastically collective generalization (ECNLE theory) of activated dynamics in bulk spherical particle liquids to address the influence of random particle pinning on structural relaxation. The simplest neutral confinement model is analyzed for hard spheres where there is no change of the equilibrium pair structure upon particle pinning. As the pinned fraction grows, cage scale dynamical constraints are intensified in a manner that increases with density. This results in the mobile particles becoming more transiently localized, with increases of the jump distance, cage scale barrier and NLE theory mean hopping time; subtle changes of the dynamic shear modulus are predicted. The results are contrasted with recent simulations. Similarities in relaxation behavior are identified in the dynamic precursor regime, including a roughly exponential, or weakly supra-exponential, growth of the alpha time with pinning fraction and a reduction of dynamic fragility. However, the increase of the alpha time with pinning predicted by the local NLE theory is too small, and severely so at very high volume fractions. The strong deviations are argued to be due to the longer range collective elasticity aspect of the problem which is expected to be modified by random pinning in a complex manner. A qualitative physical scenario is offered for how the three distinct aspects that quantify the elastic barrier may change with pinning. ECNLE theory calculations of the alpha time are then presented based on the simplest effective-medium-like treatment for how random pinning modifies the elastic barrier. The results appear to be consistent with most, but not all, trends seen in recent simulations. Key open problems are discussed with regards to both theory and simulation.

cond-mat.soft↗

arXiv2017

Theoretical model for plasmonic photothermal response of gold nanostructures solutions

Photothermal effects of gold core-shell nanoparticles and nanorods dispersed in water are theoretically investigated using the transient bioheat equation and the extended Mie theory. Properly calculating the absorption cross section is an extremely crucial milestone to determine the elevation of solution temperature. The nanostructures are assumed to be randomly and uniformly distributed in the solution. Compared to previous experiments, our theoretical temperature increase during laser light illumination provides, in various systems, both reasonable qualitative and quantitative agreement. This approach can be a highly reliable tool to predict photothermal effects in experimentally unexplored structures. We also validate our approach and discuss its limitations.

cond-mat.mtrl-sci↗

arXiv2017

Thermal-induced stress of plasmonic magnetic nanocomposites

We present theoretical calculations to interpret optical and mechanical properties of Ag@Fe3O4 nanoflowers. The microstructures and nature of optical peaks of nanoflowers are determined by means of the Mie theory associated with effective dielectric approximation and the experimental absorption spectrum. Under laser illumination, the thermal strain fields inside and outside the structure due to the absorbed optical energy are studied using continuum mechanics approach. Our findings provide simple but comprehensive description of the elastic behaviors of previous experiments.

physics.app-ph↗

arXiv2014

Casimir interactions in strained graphene systems

We theoretically study the strain effect on the Casimir interactions in graphene based systems. We found that the interactions between two strained graphene sheets are strongly dependent on the direction of stretching. The influence of the strain on the dispersion interactions is still strong in the presence of dielectric substrates but is relatively weak when the substrate is metallic. Our studies would suggest new ways to design next generation devices.

cond-mat.mes-hall↗

arXiv2013

Electric potential profile of a spherical soft particle with a charged core

The electrostatic potential profile of a spherical soft particle is derived by solving the Poisson-Boltzmann equations on a spherical system both numerically and analytically. The soft particle is assumed to consist of an ion-permeable charged outer layer and a non-permeable charged core with constant charged density. The contribution of the core to the potential profile is calculated for different charges and dielectric constants. Our results show that the charged core heavily influences the local potential within the soft particle. In contrast, the potential distribution outside the particle in the salt solution is found to be weakly dependent on the core features. These findings are consistent with previous experiments showing the minor impact of the core of the MS2 virus on its overall electrical properties. Our studies also indicate that while a change in temperature from 290 K to 310 K only slightly varies the potential, the ionic strength in the range of 1-600 mM has a significant effect on the potential profile. Our studies would provide good understanding for experimental research in the field of biophysics and nanomedicine.

cond-mat.soft↗

arXiv2013

Repulsive interactions of a lipid membrane with graphene in composite materials

The van der Waals interaction between a lipid membrane and a substrate covered by a graphene sheet is investigated using the Lifshitz theory. The reflection coefficients are obtained for a layered planar system submerged in water. The dielectric response properties of the involved materials are also specified and discussed. Our calculations show that a graphene covered substrate can repel the biological membrane in water. This is attributed to the significant changes in the response properties of the system due to the monolayer graphene. It is also found that the van der Waals interaction is mostly dominated by the presence of graphene, while the role of the particular substrate is secondary.

cond-mat.mes-hall↗

arXiv2013

Near-field heat transfer between gold nanoparticle arrays

The radiative heat transfer between gold nanoparticle layers is presented using the coupled dipole method. Gold nanoparticles are modelled as effective electric and magnetic dipoles interacting via electromagnetic fluctuations. The effect of higher-order multipoles is implemented in the expression of electric polarizability to calculate the interactions at short distances. Our findings show that the near-field radiation reduces as the radius of the nanoparticles is increased. Also, the magnetic dipole contribution to the heat exchange becomes more important for larger particles. When one layer is displayed in parallel with respect to the other layer, the near-field heat transfer exhibits oscillatory-like features due to the influence of the individual nanostructures. Further details about the effect of the nanoparticles size are also discussed.

cond-mat.mes-hall↗

arXiv2013

Surface plasmon resonances of protein-conjugated gold nanoparticles on graphitic substrates

We present theoretical calculations for the absorption properties of protein-coated gold nanoparticles on graphene and graphite substrates. As the substrate is far away from nanoparticles, numerical results show that the number of protein bovine serum molecules molecules aggregating on gold surfaces can be quantitatively determined for gold nanoparticles with arbitrary size by means of the Mie theory and the absorption spectra. The presence of graphitic substrate near protein-conjugated gold nanoparticles substantially enhances the red shift of the surface plasmon resonances of the nanoparticles. Our findings show that graphene and graphite provide the same absorption band when the distance between the nanoparticles and the substrate is large. However at shorter distances, the resonant wavelength peak of graphene-particle system differs from that of graphite-particle system. Furthermore, the influence of the chemical potential of graphene on the optical spectra is also investigated.

cond-mat.mes-hall↗

arXiv2013

van der Waals interactions between graphitic nanowiggles

The van der Waals interactions between two parallel graphitic nanowiggles (GNWs) are calculated using the coupled dipole method (CDM). The CDM is an efficient and accurate approach to determine such interactions explicitly by taking into account the discrete atomic structure. Our findings show that the van der Waals forces vary from attraction to repulsion as nanoribbons move along their lengths with respect to each other. This feature leads to a number of stable and unstable positions of the system during the movement process. These positions can be tuned by changing the length of GNW. Moreover, the influence of the thermal effect on the van der Waals interactions is also extensively investigated. This work would give good direction for both future theoretical and experimental studies.

cond-mat.mes-hall↗

arXiv2012

Interaction of a graphene sheet with a ferromagnetic metal plate

Nanoscale surface forces such as Casimir and the van der Waals forces can have a significant influence on fabrication, handling and assembly processes as well as the performance of micro and nano devices. In this paper, the investigation and the calculation of the Casimir force between a graphene sheet and a ferromagnetic metal substrate in a vacuum are presented. The reflection coefficients of graphene are graphene-conductivity dependent, and the conductivity of graphene is described by the Kubo formalism. There is an effect of magnetic properties of the metal on the Casimir interaction. The magnetic effect plays a significant role at low temperatures or high value of chemical potential. The numerical results also demonstrate that the thickness of a metal slab has a minor influence on the Casimir force. The investigation and findings about the Casimir force in this study would lead to useful information and effective solutions for design and manufacturing of micro and nano devices, especially in the areas of micro and nano machining, fabrication, manipulation, assembly and metrology.

cond-mat.mes-hall↗

arXiv2012

A model of an optical biosensor detecting environment

Heller et. Al. (Science 311, 508 (2006)) demonstrated the first DNA-CN optical sensor by wrapping a piece of double-stranded DNA around the surface of single-walled carbon nanotubes (CN). This new type of optical device can be placed inside living cells and detect trace amounts of harmful contaminants by means of near infrared light. Using a simple exciton theory in nanostructures and the phenomena of B-Z structural phase transition of DNA, we investigate the working principle of this new class of optical biosensor from DNA by using the nanostructure surface as a sensor to detect the property change of DNA as it responds to the presence of target ions. We also propose some new design models by replacing carbon nanotubes with graphene ribbon semiconductors.

cond-mat.mes-hall↗

arXiv2012

Temperature phase transition model for the DNA-CNTs-based nanotweezers

DNA and Carbon nanotubes (CNTs) have unique physical, mechanical and electronic properties that make them revolutionary materials for advances in technology. In state-of-the-art applications, these physical properties can be exploited to design a type of bio-nanorobot. In this paper, we present the behaviors of DNA-based nanotweezers and show the capabilities of controlling the robotic device. The theoretical calculations are based on the Englander-Peyrard-Bishop model for DNA. Furthermore, the influence of the Casimir force between on the opening and closing of nanotweezers is studied in comparison with the stretching forces of DNA.

cond-mat.soft↗

arXiv2012

Temperature dependent graphene suspension due to thermal Casimir interaction

Thermal effects contributing to the Casimir interaction between objects are usually small at room temperature and they are difficult to separate from quantum mechanical contributions at higher temperatures. We propose that the thermal Casimir force effect can be observed for a graphene flake suspended in a fluid between substrates at the room temperature regime. The properly chosen materials for the substrates and fluid induce a Casimir repulsion. The balance with the other forces, such as gravity and buoyancy, results in a stable temperature dependent equilibrium separation. The suspended graphene is a promising system due to its potential for observing thermal Casimir effects at room temperature.

cond-mat.mes-hall↗

arXiv2012

A new type of optical biosensor from DNA wrapped semiconductor graphene ribbons

Based on a model of the optical biosensors (Science 311, 508 (2006)) by wrapping a piece of double-stranded DNA around the surface of single-walled carbon nanotubes (SWCNT), we propose a new design model of this sensor, in which the SWCNT is replaced by a semiconductor graphene ribbon (SGR). Using a simple theory of exciton in SGRs, we investigated transition of DNA secondary structure from the native, right-handed B form to the alternate, left-handed Z form. This structural phase transition of DNA is the working principle of this optical biosensor at the sub cellular level from DNA and semiconductor graphene ribbons.

cond-mat.mes-hall↗