Jiating He

Thermodynamics versus kinetics in nanosynthesis

One may discover a stone tool by chance but it takes more than luck to make a car or cell phone. With the advance of nanoscience, the synthesis of increasingly sophisticated nanostructures demands a rational design and a systems approach. In this Review, we advocate the distinction between thermodynamically and kinetically controlled scenarios, that is, whether a product forms because it is the most stable state or because the pathway leading to it has the lowest energy barrier. Great endeavours have been made to describe the multiple concurrent processes in typical nanosynthesis phenomena, so that the mechanistic proposals in the literature are brought into a common framework for easy contrast and comparison.

An unconventional role of ligand in continuously tuning of metal–metal interfacial strain

We show that embedding of a surface ligand can dramatically affect the metal-metal interfacial energy, making it possible to create nanostructures in defiance of traditional wisdom. Despite matching Au-Ag lattices, Au-Ag hybrid NPs can be continuously tuned from concentric core-shell, eccentric core-shell, acorn, to dimer structures. This method can be extended to tune even Au-Au and Ag-Ag interfaces.

Reducing the symmetry of bimetallic Au@Ag nanoparticles by exploiting eccentric polymer shells.

We demonstrate a facile colloidal method for synthesizing Janus nanoparticles, whose eccentric polymer shells are exploited to fabricate eccentric bimetallic cores.

Unconventional Chain‐Growth Mode in the Assembly of Colloidal Gold Nanoparticles

The assembly of nano-objects is a fundamental step toward the development of functional nanodevices. For the optimization of properties and functions, it is important to achieve structural precision. Electronic and plasmonic coupling between metallic nanoparticles (NPs) has been known to give novel electronic and optical properties. Such coupling is critically dependent on structural parameters, such as interparticle spacing and the spatial organization of individual NPs. In comparison to higher-order nanostructures, onedimensional (1D) NP chains are more convenient building blocks for circuits in nanoelectronics, optoelectronics, and biosensors. Minimizing structural irregularity is essential: a large gap may break the coupling along a chain, and branching may cause a short circuit. A popular approach for assembling NPs is to utilize templates, such as biomolecules, polymers, and lithographically created patterns. A general problem of these methods is the lack of means to achieve close packing of the NPs in a chain and avoid irregular gaps. On the other hand, simple colloidal aggregation can bring NPs closely together, with short and regular gaps dictated by their ligand layers. However, colloidal-assembly methods based on random aggregation are generally difficult to control. Thus there is great interest to devise controlling methods. Linear assembly of NPs has been achieved by exploiting the anisotropic ligand organization on Au nanorods, or the phase segregation of immiscible ligands on nanospheres. In addition, intrinsic interactions of NPs, such as dipole–dipole interactions or electrostatic charge repulsion, have been proposed to explain the 1D aggregation of NPs. Despite these advances, it remains a great challenge to program the behavior of NPs for a longer-range order, for example, in terms of preventing branching and controlling cross-sectional width. The aggregation of colloidal NPs in a 3D space can be compared to the polymerization of organic monomers. The general characteristic of step-growth polymerization is that all monomers and oligomers are reactive. Their random combination quickly consumes monomers, so that the laterstage reaction mostly involves the coupling of oligomers (i.e., m-to-n coupling; m and n are the numbers of structural units in the chain). If this reaction is inefficient, few long chains will be obtained. In contrast, in chain-growth polymerization the active species (e.g., radicals or initiators) are limited in concentration, thus making the coupling between monomers (1-to-1 coupling) unfavorable. The excess monomers can continuously react with the active species and supply the chain growth through 1-to-n coupling. Because all NPs in a colloid are capable of aggregating, their “polymerization” tends to follow the step-growth mode. Once monomers are depleted, the m-to-n coupling of larger clusters becomes more difficult and less selective, leading to branches and irregularities. Hence, obtaining long chains (> 30 NPs in length) is very difficult. Here, we report the use of polymer shells to assist the 1D assembly of colloidal AuNPs (Figure 1). The system is unique in that the aggregation of

Aggregation emission properties and self-assembly of conjugated oligocarbazoles.

Conjugated oligocarbazoles with a 9,10-divinylanthracene core have been synthesized, and exhibit the transition from aggregation-induced emission (AIE) to aggregation-induced emission enhancement (AIEE) behaviour with extending conjugation length; self-assembly of the Cz4 molecule affords nanorings with high fluorescent efficiency.

Forest of gold nanowires: a new type of nanocrystal growth.

We report a nanowire growth that is highly unconventional: (1) nanowires can grow from substrate-bound seeds but cannot from colloidal seeds under otherwise the same conditions; (2) the nanowires grow from only one side of the seeds, with their diameter independent of the size of the seeds; and (3) vertically aligned ultrathin nanowires are obtained on substrates, using aqueous solution and ambient conditions. With carefully designed experiments, we propose and test a new mechanism that can explain these unusual phenonmena. It turns out that the strong binding of ligands in this system forces selective deposition of Au at the ligand-deficient interface between Au seeds and oxide substrates. This means of promoting anisotropic growth of nanocrystals into nanowires is previously unknown in the literature. We are able to pinpoint the site of active growth and explain the control of nanowire width. The sustained growth at the active site and the inhibited growth at its parameter push the nanocrystals upward into wires; their diameter is dependent on the dynamic competition of the two processes. The site-specific growth from substrate-anchored seeds provides a rare means to create substrate-nanowire hierarchical structures in aqueous solution under ambient conditions. Rendering a surface conductive, particularly one with complex surface morphology, is now made easy.

Strategy for Nano‐Catalysis in a Fixed‐Bed System

For industry applications of nano-catalysts, the main bottlenecks are the low loading per unit support area and the slow flow rate through the support particles. By growing a dense Au nanowire forest on a loose network of glass fibers, continuous-flow catalysis can be achieved with a processing rate about 100 times that of the best literature rate.

Preservation of Lattice Orientation in Coalescing Imperfectly Aligned Gold Nanowires by a Zipper Mechanism

Parallel-stacked gold nanowires (NWs) in a ring conformation are induced to coalesce, forming solid seamless rings. The axial lattice orientation of the original Au NWs is preserved in the coalesced rings (see picture; scale bars 2 nm, insets 50 nm). A zipper mechanism is proposed to reconcile the three major events in coalescing nanocrystals: ligand loss, lattice alignment, and coalescence.

Synthesis, characterization, two-photon absorption, and optical limiting properties of triphenylamine-based dendrimers

Three π-conjugated dendrimers (Ph-G0, Ph-G1 and Ph-G2) bearing triphenylamine moieties have been synthesized through a convergent synthetic strategy without any protection–deprotection chemistry. The linear photophysical properties, two-photon absorption (TPA), and optical limiting behavior of the dendrimers were investigated in solution at room temperature. Linear absorption and emission spectra revealed a bathochromic shift and decreased fluorescence quantum yields with increasing dendrimer generation. A strong cooperative effect in the TPA absorption of these dendrimers was observed. The TPA cross-sections increase gradually with the proportion of triphenylamine units and the maximum value of the TPA cross-section can reach 5690 GM for Ph-G2. These triphenylamine-based dendrimers exhibited efficient two-photon optical limiting under femtosecond excitation.

One-step synthesis of composite vesicles: Direct polymerization and in situ over-oxidation of thiophene

Composite vesicles with embedded Au nanoparticles are directly prepared from thiophene and Au nanoparticles in aqueous solution in the presence of H2O2 and a catalytic amount of FeCl3. The number, shape, and structure of the vesicles thereof can be readily controlled. We provide evidence that these vesicles contain an aqueous phase in their cavities. The direct synthesis offers an economical route to nanoscale containers and also a rare example for the minimal conditions of vesicle formation.