Aggregation of polymer-grafted nanoparticles in good solvents: a hierarchical modeling method

Dumbbells, chains, and ribbons: anisotropic self-assembly of isotropic nanoparticles

Functionalizing the surface of metal nanoparticles can assure their stability in solution or mediate their self-assembly into aggregates with controlled shapes. Here we present a computational study of the colloidal aggregation of gold nanoparticles (Au NPs) isotropically functionalized by a mixture of charged and hydrophobic ligands. We show that, by varying the relative proportion of the two ligands, the NPs form anisotropic aggregates with markedly different topologies: dumbbells, chains, or ribbons. In all cases, two kinds of connections keep the aggregates together: hydrophobic bonds and ion bridges. We show that the anisotropy of the aggregates derives from the NP shell reshaping due to the formation of the hydrophobic links, while ion bridges are accountable for the "secondary structure" of the aggregates. Our findings provide a general physical principle that can also be exploited in different self-assembled systems: anisotropic/directional aggregation can be achieved starting from isotropic objects with a soft, deformable surface.

Functional-Group Effect of Ligand Molecules on the Aggregation of Gold Nanoparticles: A Molecular Dynamics Simulation Study

In this paper, atomistic molecular dynamics simulations are performed for the systems consisting of functionalized gold nanoparticles (NPs) in a toluene medium. Gold NPs are coated with ligand molecules that have different terminal groups, that is, polar carboxyl (COOH), hydroxyl (OH), amine (NH₂), and nonpolar methyl (CH₃). These functional groups are selected to understand the relation between the aggregation behavior of functionalized gold NPs in toluene and the polarity of terminal groups of ligand molecules. The center-of-mass distances between NP pairs, the radial distribution functions, the mean square displacements, the radius of gyration, and the number of hydrogen bonds (H-bond) between ligand molecules are computed. Our simulation results indicate that functionalized gold NPs exhibit different aggregation/dispersion behaviors depending upon the terminal group of ligands.

Molecular Structure Effect of a Self-Assembled Monolayer on Thermal Resistance across an Interface

Understanding heat transfer across an interface is essential to a variety of applications, including thermal energy storage systems. Recent studies have shown that introducing a self-assembled monolayer (SAM) can decrease thermal resistance between solid and fluid. However, the effects of the molecular structure of SAM on interfacial thermal resistance (ITR) are still unclear. Using the gold–SAM/PEG system as a model, we performed nonequilibrium molecular dynamics simulations to calculate the ITR between the PEG and gold. We found that increasing the SAM angle value from 100° to 150° could decrease ITR from 140.85 × 10⁻⁹ to 113.79 × 10⁻⁹ m² K/W owing to penetration of PEG into SAM chains, which promoted thermal transport across the interface. Moreover, a strong dependence of ITR on bond strength was also observed. When the SAM bond strength increased from 2 to 640 kcal⋅mol−1Å−2, ITR first decreased from 106.88 × 10⁻⁹ to 102.69 × 10⁻⁹ m² K/W and then increased to 123.02 × 10⁻⁹ m² K/W until reaching a steady state. The minimum ITR was obtained when the bond strength of SAM was close to that of PEG melt. The matching vibrational spectra facilitated the thermal transport between SAM chains and PEG. This work provides helpful information regarding the optimized design of SAM to enhance interfacial thermal transport.

Examining the self-assembly of patchy alkane-grafted silica nanoparticles using molecular simulation

In this work, molecular dynamics simulations are used to examine the self-assembly of anisotropically coated "patchy" nanoparticles. Specifically, we use a coarse-grained model to examine silica nanoparticles coated with alkane chains, where the poles of the grafted nanoparticle are bare, resulting in strongly attractive patches. Through a systematic screening process, the patchy nanoparticles are found to form dispersed, string-like, and aggregated phases, dependent on the combination of alkane chain length, coating chain density, and the fractional coated surface area. Correlation analysis is used to identify the ability of various particle descriptors to predict bulk phase behavior from more computationally efficient single grafted nanoparticle simulations and demonstrates that the solvent-accessible surface area of the nanoparticle core is a key predictor of bulk phase behavior. The results of this work enhance our knowledge of the phase space of patchy nanoparticles and provide a powerful approach for future screening of these materials.

Self-assembly of hairy disks in two dimensions - insights from molecular simulations

We report the results of large scale molecular dynamics simulations conducted for sparsely grafted disks in two-dimensional systems. The main goal of this work is to show how the ligand mobility influences the self-assembly of particles decorated with short chains. We also analyze the impact of the chain length on the structure of dense phases. A crossover between the systems controlled by the core-core or by the segment-segment interactions is discussed. We prove that the ligand mobility determines the structure of the system. The particles with fixed tethers are found to order into different structures, an amorphous phase, hexagonal or honeycomb lattices, and a "spaghetti"-like phase containing single strings of cores, depending on the length of attached chains. The disks with mobile monomers assemble into a hexagonal structure, while the particles with longer mobile chains attached to them form a lamellar phase consisting of double strings of cores.

Efficient charge transfer and utilization of near-infrared solar spectrum by ytterbium and thulium codoped gadolinium molybdate (Gd2(MoO4)3:Yb/Tm) nanophosphor in hybrid solar cells

In this work, thulium and ytterbium codoped gadolinium molybdate (Gd₂(MoO₄)₃:Yb/Tm) nanophosphors (NPs) have been synthesized, followed by being incorporated into a photo-catalytic titania (TiO₂) nanoparticle layer. In detail, morphology and phase identification of the prepared NPs are first characterized and then the up-conversion of the Gd₂(MoO₄)₃:Yb/Tm NPs is studied. Electron transfer dynamics after interfacing with bare or NP-doped electron donor TiO₂ and the corresponding photovoltaic performance of solar cells are explored. The results show that Gd₂(MoO₄)₃:Yb/Tm NPs excited at 976 nm exhibit intense blue (460-498 nm) and weak red (627-669 nm) emissions. The lifetime of electron transfer is shortened from 817 to 316 ps after incorporating NPs and correspondingly the electron transfer rate outstrips by 3 times that of the bare TiO₂. Consequently, a notable power conversion efficiency of 4.15% is achieved as compared to 3.17% of pure TiO₂/PTB7. This work demonstrates that the co-doping of robust rare earth ions with different unique functions can widen the harvesting range of the solar spectrum, boost electron transfer rate and eventually strengthen device performance, without complicated interfacial and structural engineering.

Role of ytterbium-erbium co-doped gadolinium molybdate (Gd2(MoO4)3:Yb/Er) nanophosphors in solar cells

Insufficient harvest of solar light energy is one of the obstacles for current photovoltaic devices to achieve high performance. Especially, conventional organic/inorganic hybrid solar cells (HSCs) based on PTB7 as p-type semiconductor can only utilize 400-800 nm solar spectrum. One effective strategy to overcome this obstacle is the introduction of up-conversion nanophosphors (NPs), in the virtue of utilizing the near infrared region (NIR) of solar radiation. Up-conversion can convert low-energy photons to high-energy ones through multi-photon processes, by which the solar spectrum is tailored to well match the absorptive domain of the absorber. Herein we incorporate erbium-ytterbium co-doped gadolinium molybdate (Gd₂(MoO₄)₃, GMO), denoted as GMO:Yb/Er, into TiO₂ acceptor film in HSCs to enhance the light harvest. Here Er³⁺ acts as activator while Yb-MoO₄ ^2- is the joint sensitizer. Facts proved that the GMO:Yb/Er single crystal NPs are capable of turning NIR photons to visible photons that can be easily captured by PTB7. Studies on time-resolved photoluminescence demonstrate that electron transfer rate at the interface increases sharply from 0.65 to 1.42 × 10⁹ s^-1. As a result, the photoelectric conversion efficiency of the GMO:Yb/Er doped TiO₂/PTB7 HSCs reach 3.67%, which is increased by around 25% compared to their neat PTB7/TiO₂ counterparts (2.94%). This work may open a hopeful way to take the advantage of those conversional rare-earth ion doped oxides that function in tailoring solar light spectrum for optoelectronic applications.

Layer-by-layer nanoencapsulation of camptothecin with improved activity

160 nm nanocapsules containing up to 60% of camptothecin in the core and 7-8 polyelectrolyte bilayers in the shell were produced by washless layer-by-layer assembly of heparin and block-copolymer of poly-l-lysine and polyethylene glycol. The outer surface of the nanocapsules was additionally modified with polyethylene glycol of 5 kDa or 20 kDa molecular weight to attain protein resistant properties, colloidal stability in serum and prolonged release of the drug from the capsules. An advantage of the LbL coated capsules is the preservation of camptothecin lactone form with the shell assembly starting at acidic pH and improved chemical stability of encapsulated drug at neutral and basic pH, especially in the presence of albumin that makes such formulation more active than free camptothecin. LbL nanocapsules preserve the camptothecin lactone form at pH 7.4 resulting in triple activity of the drug toward CRL2303 glioblastoma cell.

Self-assembly of polymer-grafted nanoparticles in thin films

We use large-scale molecular dynamics simulations with a coarse-grained model to investigate the self-assembly of solvent-free grafted nanoparticles into thin free-standing films. Two important findings are highlighted. First, for appropriately chosen values of system parameters the nanoparticles spontaneously assemble into monolayer thick films. Further, the nanoparticles self-assemble into a variety of morphologies ranging from dispersed particles, finite stripes, long strings, to percolating networks. The main driving force for these morphologies is the competition between strong short-range attractions of the particle cores and long-range entropic repulsions of the grafted chains. The grafted nanoparticle systems provide practical means to realize two-length-scale systems that have been previously seen, using a simple two-dimensional model [G. Malescio and G. Pellicane, Nat. Mater., 2003, 2, 97], to generate a variety of morphologies. However, there are only relatively narrow ranges of interaction strengths and chain lengths for which anisotropic self-assembly is possible.