Heat Transfer Enhancement in Tree-Structured Polymer Linked Gold Nanoparticle Networks.

Human brains use a tree-like neuron network for information processing at high efficiency and low energy consumption. Tree-like structures have also been engineered to enhance mass and heat transfer in various applications. In this work, we reveal the heat transfer mechanism in tree-structured polymer linked gold nanoparticle (AuNP) networks using atomistic simulations. We report both upward and downward heat fluxes between root and leaf nodes in tree-structured polyethylene (PE) and poly(p-phenylene) (PPP) linked AuNP networks at tree levels from 1 to 5. We found that the heat conductance increases with an increasing polymer tree level. The heat transfer enhancement is due to the resulting increase in the low-frequency vibrational modes. This and other thermal properties are affected by the location of the AuNPs in the tree. Moreover, complex tree structures with at least five levels were found to be robust in the sense that disabling half of the leaves did not change the overall heat conductance.

Thermal Transport through Polymer-Linked Gold Nanoparticles.

Polymer-nanoparticle networks have potential applications in molecular electronics and nanophononics. In this work, we use all-atom molecular dynamics to reveal the fundamental mechanisms of thermal transport in polymer-linked gold nanoparticle (AuNP) dimers at the molecular level. Attachment of the polymers to AuNPs of varying sizes allows the determination of effects from the flexibility of the chains when their ends are not held fixed. We report heat conductance (G) values for six polymers-viz. polyethylene, poly(p-phenylene), polyacene, polyacetylene, polythiophene, and poly(3,4-ethylenedioxythiophene)-that represent a broad range of stiffness. We address the multimode effects of polymer type, AuNP size, polymer chain length, polymer conformation, system temperature, and number of linking polymers on G. The combination of the mechanisms for phonon boundary scattering and intrinsic phonon scattering has a strong effect on G. We find that the values of G are larger for conjugated polymers because of the stiffness in their backbones. They are also larger in the low-temperature region for all polymers owing to the quenching of segmental rotations at low temperature. Our simulations also suggest that the total G is additive as the number of linking polymers in the AuNP dimer increases from 1 to 2 to 3.

Nanoscale 3D spatial addressing and valence control of quantum dots using wireframe DNA origami.

Control over the copy number and nanoscale positioning of quantum dots (QDs) is critical to their application to functional nanomaterials design. However, the multiple non-specific binding sites intrinsic to the surface of QDs have prevented their fabrication into multi-QD assemblies with programmed spatial positions. To overcome this challenge, we developed a general synthetic framework to selectively attach spatially addressable QDs on 3D wireframe DNA origami scaffolds using interfacial control of the QD surface. Using optical spectroscopy and molecular dynamics simulation, we investigated the fabrication of monovalent QDs of different sizes using chimeric single-stranded DNA to control QD surface chemistry. By understanding the relationship between chimeric single-stranded DNA length and QD size, we integrated single QDs into wireframe DNA origami objects and visualized the resulting QD-DNA assemblies using electron microscopy. Using these advances, we demonstrated the ability to program arbitrary 3D spatial relationships between QDs and dyes on DNA origami objects by fabricating energy-transfer circuits and colloidal molecules. Our design and fabrication approach enables the geometric control and spatial addressing of QDs together with the integration of other materials including dyes to fabricate hybrid materials for functional nanoscale photonic devices.

Development and Validation of the Haze Risk Perception Scale and Influencing Factor Scale-A Study Based on College Students in Beijing.

Although Beijing's air quality has improved, there is still a long way to go for haze governance. In order to understand haze risk perception and related influencing factors among college students in Beijing, we developed and verified two scales, with college students as the survey object, and analyzed the theoretical framework and realistic level of haze risk perception and influencing factors through empirical research. We showed that the reliability and validity of the two scales are excellent, and they can be used as a powerful tool to measure college students' perception of haze. The haze risk perception scale (HRPS) is divided into four dimensions. The degrees of perception ranked from high to low are: direct consequences perception, indirect consequences perception, risk responsibility perception and risk source perception. The haze risk perception influencing factor scale (HRPIFS) is divided into three dimensions. The degrees of influence ranked from high to low are: personal emotion, media communication and government policy; the three influencing factors all have a significant positive correlation to overall haze risk perception, but personal emotions and media communication are only significantly related to the three dimensions of direct consequence perception, indirect consequence perception and risk source perception. Government policy is only significantly related to the three dimensions of direct consequence perception, indirect consequence perception and risk liability perception. This paper proves the important role of media in haze risk perception and puts forward some policy suggestions to guide the public to form a rational risk perception. These findings can help improve theoretical and practical research related to haze risk.

Effect of side-chain π-π stacking on the thermal conductivity switching in azobenzene polymers: a molecular dynamics simulation study.

The light switchable thermal conductivity displayed by some polymers makes them promising for applications like data storage, temperature regulation and light switchable devices. In this study, the mechanism of thermal conductivity switching in poly[6-(4-phenyldiazenyl phenoxy)hexyl metharylate] is studied using molecular dynamics (MD) simulations. The π-π stacking and amorphous polymer structures are specifically prepared through different simulation procedures, and the thermal conductivity of these structures is calculated. It is found that due to the π-π stacking structure, the thermal conductivity along the side-chain direction can change by 30-50% (from 0.34 to 0.51 W m-1 K-1). Through heat flux decomposition, it is found that the thermal conductivity change is dominated by the contribution from bonding interactions. This is because π-π stacking, which enforces the trans conformation, extends the side-chains of azobenzene polymers, making thermal transport in the side-chain direction more efficient. Along the polymer main-chain direction, the thermal conductivity is not affected by the π-π stacking of the side chains. This mechanism may be generalized to other types of polymers with azobenzene side-chains, which will develop a class of photo-responsive polymers.

Molecular Structure of Single-Stranded DNA on the ZnS Surface of Quantum Dots.

DNA-based nanoparticle assemblies have emerged as leading candidates in the development of bioimaging materials, photonic devices, and computing materials. Here, we combine atomistic simulations and experiments to characterize the wrapping mechanism of chimeric single-stranded DNA (ssDNA) on CdSe-ZnS (core-shell) quantum dots (QDs) at different ratios of the phosphorothioate (PS) modification of the bases. We use an implicit solvent, all-atom ssDNA model to match the experimentally calculated ssDNA conformation at low salt concentrations. Through simulation, we find that 3-mercaptopropionic acid (MPA) induces electrostatic repulsion and O-(2-mercaptoethyl)-Ó-methyl-hexa (ethylene glycol) (mPEG) induces steric exclusion, and both reduce the binding affinity of ssDNA. In both simulation and experiment, we find that ssDNA is closer to the QD surface when the QD size is larger. The effect of the PS-base ratio on the conformation of ssDNA is also elaborated in this work. We found through MD simulations, and confirmed by transmission electron microscopy, that the maximum valence numbers are 1, 2, and 3 on QDs of 6, 9, and 14 nm in diameter, respectively. We conclude that the maximum ssDNA valence number is linearly related to the QD size, n ∝ R, and justify this finding through an electrostatic repulsion mechanism.

Electric Potential of Citrate-Capped Gold Nanoparticles Is Affected by Poly(allylamine hydrochloride) and Salt Concentration.

The structure near polyelectrolyte-coated gold nanoparticles (AuNPs) is of significant interest because of the increased use of AuNPs in technological applications and the possibility that the acquisition of polyelectrolytes can lead to novel chemistry in downstream environments. We use all-atom molecular dynamics (MD) simulations to reveal the electric potential around citrate-capped gold nanoparticles (cit-AuNPs) and poly(allylamine hydrochloride) (PAH)-wrapped cit-AuNP (PAH-AuNP). We focus on the effects of the overall ionic strength and the shape of the electric potential. The ionic number distributions for both cit-AuNP and PAH-AuNP are calculated using MD simulations at varying salt concentrations (0, 0.001, 0.005, 0.01, 0.05, 0.1, and 0.2 M NaCl). The net charge distribution (Z(r)) around the nanoparticle is determined from the ionic number distribution observed in the simulations and allows for the calculation of the electric potential (ϕ(r)). We find that the magnitude of ϕ(r) decreases with increasing salt concentration and upon wrapping by PAH. Using a hydrodynamic radius (RH) estimated from the literature and fits to the Debye-Hü̈ckel expression, we found and report the ζ potential for both cit-AuNP and PAH-AuNP at varying salt concentrations. For example, at 0.001 M NaCl, MD simulations suggest that ζ = -25.5 mV for cit-AuNP. Upon wrapping of cit-AuNP by one PAH chain, the resulting PAH-AuNP exhibits a reduced ζ potential (ζ = -8.6 mV). We also compare our MD simulation results for ϕ(r) to the classic Poisson-Boltzmann equation (PBE) approximation and the well-known Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. We find agreement with the limiting regimes─with respect to surface charge, salt concentration and particle size─in which the assumptions of the PBE and DLVO theory are known to be satisfied.

Building blocks for autonomous computing materials: Dimers, trimers, and tetramers.

Autonomous computing materials for data storage and computing offer an opportunity for next generation of computing devices. Patchy nanoparticle networks, for example, have been suggested as potential candidates for emulating neuronal networks and performing brain-like computing. Here, we use molecular dynamics (MD) simulations to show that stable dimers, trimers, and tetramers can be built from citrate capped gold nanoparticles (cit-AuNPs) linked by poly(allylamine hydrochloride) (PAH) chains. We use different lengths of PAHs to build polymer-networked nanoparticle assemblies that can emulate a complex neuronal network linked by axons of varying lengths. We find that the tetramer structure can accommodate up to 11 different states when the AuNP pairs are connected by either of two polymer linkers, PAH200 and PAH300. We find that the heavy AuNPs contribute to the assembly's structure stability. To further illustrate the stability, the AuNP-AuNP distances in dimer, trimer, and tetramer structures are reduced by steering the cit-AuNPs closer to each other. At different distances, these steered structures are all locally stable in a 10 ns MD simulation time scale because of their connection to the AuNPs. We also find that the global potential energy minimum is at short AuNP-AuNP distances where AuNPs collapse because the -NH3 + and -COO- attraction reduces the potential energy. The stability and application of these fundamental structures remain to be further improved through the use of alternative polymer linkers and nanoparticles.

Engineered nanoparticle network models for autonomous computing.

Materials that exhibit synaptic properties are a key target for our effort to develop computing devices that mimic the brain intrinsically. If successful, they could lead to high performance, low energy consumption, and huge data storage. A 2D square array of engineered nanoparticles (ENPs) interconnected by an emergent polymer network is a possible candidate. Its behavior has been observed and characterized using coarse-grained molecular dynamics (CGMD) simulations and analytical lattice network models. Both models are consistent in predicting network links at varying temperatures, free volumes, and E-field (E⃗) strengths. Hysteretic behavior, synaptic short-term plasticity and long-term plasticity-necessary for brain-like data storage and computing-have been observed in CGMD simulations of the ENP networks in response to E-fields. Non-volatility properties of the ENP networks were also confirmed to be robust to perturbations in the dielectric constant, temperature, and affine geometry.

Chain length effect on thermal transport in amorphous polymers and a structure-thermal conductivity relation.

The physics of thermal transport in polymers is important in many applications, such as in heat exchangers and electronics packaging. Even though thermal conductivity models for amorphous polymers have been reported since the 1970s, none of the published models included the chain conformation and chain stiffness effects. In this study, we use molecular dynamics (MD) simulations to study the chain length effect on thermal conductivity of amorphous polyethylene (PE), and the number of repeating C2H4 units ranges from 5 to 200. The total thermal conductivity is decomposed into its contributions from energy convection (k-convection), and heat transfer through nonbonding (k-nonbonding) and bonding (k-bonding) interactions. Each part of the contributions is fitted empirically by using a scaling relationship: k-convection (Einstein's diffusion coefficient model), k-nonbonding ∝ n (Choy's model) and k-bonding (from this study), where Rg is the radius of gyration, n is the number density, and ξ is the persistence length. Summarizing these three components, we emphasize the chain conformation (Rg) and chain stiffness (ξ) effects on thermal conductivity, and we propose a structure-property relation model for amorphous polymers. Our empirical model is compared with Hansen's experimental data [D. Hansen, R. Kantayya and C. Ho, Polym. Eng. Sci., 1966, 6, 260-262] and with our MD results. Our empirical model relies on realistic structural properties to enable accurate predictions. We believe that our model has captured some key structure-property relations in amorphous polymers.

The effect of the block ratio on the thermal conductivity of amorphous polyethylene-polypropylene (PE-PP) diblock copolymers.

Block copolymers have a wide range of applications, such as battery electrolytes and nanoscale pattern generation. The thermal conductivity is a critical parameter in many of these applications (e.g., batteries), which is strongly related to the molecular conformation. In this work, the thermal transport in a representative diblock copolymer, polyethylene (PE)-polypropylene (PP), at different PE to PP block ratios is studied using molecular dynamics (MD) simulations. Our results show that the thermal conductivity of the PE-PP diblock copolymer can be tuned continuously by the block ratio, and it is strongly related to the molecular conformation, characterized by the radius of gyration (Rg). It is found that increasing the PP portion results in an overall decreasing trend in the thermal conductivity since the PP block has a more flexible backbone, which leads to a smaller spatial extension of the whole PE-PP copolymer molecule. Thermal conductivity decomposition shows that the bonding contribution is dominant in both the PE and PP portions of the block copolymer. The findings from this study can help understand thermal transport in general block copolymers.

Molecular Fin Effect from Heterogeneous Self-Assembled Monolayer Enhances Thermal Conductance across Hard-Soft Interfaces.

Thermal transport across hard-soft interfaces is critical to many modern applications, such as composite materials, thermal management in microelectronics, solar-thermal phase transition, and nanoparticle-assisted hyperthermia therapeutics. In this study, we use equilibrium molecular dynamics (EMD) simulations combined with the Green-Kubo method to study how molecularly heterogeneous structures of the self-assembled monolayer (SAM) affect the thermal transport across the interfaces between the SAM-functionalized gold and organic liquids (hexylamine, propylamine and hexane). We focus on a practically synthesizable heterogeneous SAM featuring alternating short and long molecular chains. Such a structure is found to improve the thermal conductance across the hard-soft interface by 46-68% compared to a homogeneous nonpolar SAM. Through a series of further simulations and analyses, it is found that the root reason for this enhancement is the penetration of the liquid molecules into the spaces between the long SAM molecule chains, which increase the effective contact area. Such an effect is similar to the fins used in macroscopic heat exchanger. This "molecular fin" structure from the heterogeneous SAM studied in this work provides a new general route for enhancing thermal transport across hard-soft material interfaces.

Chain conformation-dependent thermal conductivity of amorphous polymer blends: the impact of inter- and intra-chain interactions.

Polymers with high thermal conductivities are of great interest for both scientific research and industrial applications. In this study, model amorphous polymer blends are studied using molecular dynamics simulations. We have examined the effects of inter- and intra-chain interactions on the molecular-level conformations of the blends, which in turn impact their thermal conductivity. It is found that the thermal conductivity of polymer blends is strongly related to the molecular conformation, especially the spatial extent of the molecular chains indicated by their radius of gyration. Tuning the intra-chain van der Waals (vdW) interaction leads to different molecular structures of the minor component in the binary blend, but the thermal conductivity is not changed. However, increasing the inter-chain vdW interactions between the major and the minor components will increase the thermal conductivity of the blend, which is due to the conformation change in the major component that leads to enhanced thermal transport along the chain backbone through the intra-chain bonding interactions. The fundamental structure-property relationship from this study may provide useful guidance for designing and synthesizing polymer blends with desirable thermal conductivity.

Glucose isomerization to fructose from ab initio molecular dynamics simulations.

Car-Parrinello molecular dynamics simulations (CPMD) coupled with metadynamics (MTD) simulations were conducted to investigate glucose isomerization to fructose in acidic aqueous solution. Glucose to fructose isomerization is initiated by protonation of the C2-OH and the formation of a furanose aldehyde intermediate. Fructose is produced via a hydride transfer from C2 to C1 on the furanose aldehyde followed by the rehydration of the C2 carbocation. Hydride 1,2 shift to form a C2 carbocation is an energetically favorable process but the barrier is relatively high at around 35 kcal/mol. The final step during glucose to fructose isomerization involves the rehydration of the C2 carbocation with an estimated barrier of 25 kcal/mol from our CPMD-MTD simulations.