EFFECTS OF DIFFERENT FUNCTIONAL GROUP-CONTAINING ORGANICS ON MORPHOLOGY-CONTROLLED SYNTHESIS OF SILVER NANOPARTICLES AT ROOM TEMPERATURE

Silver nanoparticles with average particles sizes ranging from 2 to 131nm were manipulatively synthesized starting from silver nitrate using different functional group-containing organic modifiers at room temperature. The effects of the organic modifiers on the morphology of the resulting silver nanoparticles were strongly dependent on the intrinsic properties of the functional groups and the reducibility of the reductant. Numerous ether bonds （-0-）present in polyethylene glycol and Tween-80 were beneficial to the formation of silver nanoparticles with particle sizes of several nanometers in a narrow size distribution in both weak and strong reducing environments. Cetyltrimethylammonium bromide induced the formation of nanosized silver triangle plates in a weak reducing environment. The crystal growth of the silver nanoparticles with particle sizes of more than lOnm was postulated through an adhesion process of small-sized particles followed by a subsequent coalescence process under the present reaction conditions.

Direct imaging and determination of the crystal structure of six-layered graphdiyne

Since its discovery, the direct imaging and determination of the crystal structure of few-layer graphdiyne has proven difficult because it is too delicate under irradiation by an electron beam. In this work, the crystal structure of a six-layered graphdiyne nanosheet was directly observed by low-voltage transmission electron microscopy （TEM） using low current density. The combined use of high-resolution TEM （HRTEM） simulation, electron energy-loss spectroscopy, and electron diffraction revealed that the as-synthesized nanosheet was crystalline graphdiyne with a thickness of 2.19 nm （corresponding to a thickness of six layers） and showed ABC stacking. Thus, this work provides direct evidence for the existence and crystal structure of few-layer graphdiyne, which is a new type of two-dimensional carbon material complementary to graphene.

In situ atomic-scale observation of monolayer graphene growth from SiC

Because of its high compatibility with conventional microfabrication processing technology, epitaxial graphene （EG） grown on SiC shows exceptional promise for graphene-based electronics. However, to date, a detailed understanding of the transformation from three-layer SiC to monolayer graphene is still lacking. Here, we demonstrate the direct atomic-scale observation of EG growth on a SiC （11^-00） surface at 1,000℃ by in situ transmission electron microscopy in combination with ab initio molecular dynamics （AIMD） simulations. Our detailed analysis of the growth dynamics of monolayer graphene reveals that three SiC （11^-00） layers decompose successively to form one graphene layer. Sublimation of the first layer causes the formation of carbon clusters containing short chains and hexagonal rings, which can be considered as the nuclei for graphene growth. Decomposition of the second layer results in the appearance of new chains connecting to the as-formed clusters and the formation of a network with large pores. Finally, the carbon atoms released from the third layer lead to the disappearance of the chains and large pores in the network, resulting in a whole graphene layer. Our study presents a clear picture of the epitaxial growth of the monolayer graphene from SiC and provides valuable information for future developments in SiC-derived EG technology.

In situ TEM observation of dissolution and regrowth dynamics of MoO2 nanowires under oxygen

Direct observation of the dissolution behavior of nanomaterials could provide fundamental insight to understanding their anisotropic properties and stability. The dissolution mechanism in solution and vacuum has been well documented. However, the gas-involved dissolution and regrowth have seldom been explored and the mechanisms remain elusive. We report herein, an in situ TEM study of the dissolution and regrowth dynamics of MoO2 nanowires under oxygen using environmental transmission electron microscopy （ETEM）. For the first time, oscillatory dissolution on the nanowire tip is revealed, and, intriguingly, simultaneous layer-by-layer regrowth on the sidewall facets is observed, leading to a shorter and wider nanowire. Combined with first-principles calculations, we found that electron beam irradiation caused oxygen loss in the tip facets, which resulted in changing the preferential growth facets and drove the morphology reshaping.

In-situ liquid-cell TEM study of radial flow-guided motion of octahedral Au nanoparticles and nanoparticle clusters

The dynamic behavior of octahedral gold nanopartides （NPs） and nanoparticle clusters （NPCs） in aqueous solution is studied by in-situ liquid-cell transmission electron microscopy （TEM）. The octahedral Au NPs/NPCs show preferential orientations in the liquid cell, due to the interaction with the SiNx window. The Au NPs show long-range reversible hopping and three-dimensional （3D） rotational motions in the liquid environment. At the same time, the Au NPCs and NPs perform slow stick-sUp and stick-roU motions, respectivel~ with a centripetal trend. The centripetal motions were explained by a liquid evaporation-induced radial flow model in which the NPCs/NPs trajectories are controlled by Stokes forces and surface friction by the silicon nitride window. The calculated radius-dependent force （Fc） on the NPCs/NPs shows a semi-linear correlation with the distance r between the NPCs/NPs and the center of mass, accompanied with stochastic fluctuations, in agreement with the model predictions. This work thus demonstrates the effectiveness of in situ liquid-cell TEM for the in-depth understanding of complicated liquid flow and force interactions in nanomaterials.

In situ atomic-scale analysis of Rayleigh instability in ultrathin gold nanowires

Comprehensive understanding of the structural/morphology stability of ultrathin （diameter 〈 10 nm） gold nanowires under real service conditions （such as under Joule heating） is a prerequisite for the reliable implementation of these emerging building blocks into functional nanoelectronics and mechatronics systems. Here, by using the in situ transmission electron microscopy （TEM） technique, we discovered that the Rayleigh instability phenomenon exists in ultrathin gold nanowires upon moderate heating. Through the controlled electron beam irradiation-induced heating mechanism （with 〈 100 ~C temperature rise）, we further quantified the effect of electron beam intensity and its dependence on Rayleigh instability in altering the geometry and morphology of the ultrathin gold nanowires. Moreover, in situ high-resolution TEM （HRTEM） observations revealed surface atomic diffusion process to be the dominating mechanism for the morphology evolution processes. Our results, with unprecedented details on the atomic-scale picture of Rayleigh instability and its underlying physics, provide critical insights on the thermal/structural stability of gold nanostructures down to a sub-10 nm level which may pave the way for their interconnect applications in future ultra- large-scale integrated ciroaits.