Longbing He

Liquid-like pseudoelasticity of sub-10-nm crystalline silver particles

In nanotechnology, small-volume metals with large surface area are used as electrodes, catalysts, interconnects and antennae. Their shape stability at room temperature has, however, been questioned. Using in situ high-resolution transmission electron microscopy, we find that Ag nanoparticles can be deformed like a liquid droplet but remain highly crystalline in the interior, with no sign of dislocation activity during deformation. Surface-diffusion-mediated pseudoelastic deformation is evident at room temperature, which can be driven by either an external force or capillary-energy minimization. Atomistic simulations confirm that such highly unusual Coble pseudoelasticity can indeed happen for sub-10-nm Ag particles at room temperature and at timescales from seconds to months.

Size‐Dependent Evolution of Graphene Nanopores Under Thermal Excitation

Graphene nanopores expand when pore diameter is larger than membrane thickness after heat treatment; otherwise, nanopore size shrinks. Such size-dependent evolutionary mechanism of nanopores is considered as thermal-induced migration of uncombined carbon atoms. The amount of carbon adatoms determines the extent of diameter change. This could provide an applicable strategy for nanopore fabrication.

Controllable Atomic‐Scale Sculpting and Deposition of Carbon Nanostructures on Graphene

major reason for this is that EBID depends not only on the electron beam conditions but also on the surface concentration of contaminating hydrocarbons. [ 1,28 ] More seriously, it is hard to quantify this concentration, although we know that the hydrocarbons mainly come from either impurities in the TEM column or from adsorption on the specimen surface, or from both. Hence, it is very diffi cult to realize controllable nanofabrications by EBIS or EBID to date. Here in an attempt to describe the relationship between EBIS and EBID, nanopores in a graphene membrane or carbon nanodots on its surface were fabricated by focused electron beam (diameter smaller than 10 nm) with various current densities or irradiation time. It is found that EBIS is signifi cant when the current density is larger than 500 A/cm 2 , while EBID begins to dominate when the current density is less than 300 A/cm 2 . A simplifi ed model has been proposed to explain the experiments quantitatively. We believe that our results have signifi cance in the fabrication of carbon nanostructures, and also provide an effective way to fabricate nanofl uidics and nanoelectronics. Figure 1 a–c demonstrates a typical sputtering process induced by a high intensity focused electron beam on a graphene membrane. Figure 1 a shows a region of pristine graphene before electron irradiation with the circle denoting the region which was exposed to the electron beam. The current density was ∼2.0 × 10 3 A/cm 2 . Figure 1 b shows the same region after 20 s of electron irradiation. The exposed area is clearly altered and shows obvious damage to the membrane confi rmed by disordered structures. Another 28 seconds later, as shown in Figure 1 c, a nanopore has been formed at the center of the irradiation region, where the electron beam current density is higher. It is noted that there is a damaged region between the pore and the pristine graphene. These can be understood from the common assumption that the amount of irradiation damage is proportional to the accumulated irradiation dose. To further understand the sputtering process, we recorded the relationship between nanopore diameter and irradiation time by drilling tens of nanopores using the same beam conditions. Figure 1 d shows the size of graphene nanopores as a function of irradiation time. The electron beam used in Figure 1 d is shown in Figure 1 e. It has a typical Gaussian intensity distribution with a full width at half maximum of 5 nm and a maximum density of ∼ 4.3 × 10 3 A/cm 2 at the center (Figure S1). The thickness of graphene, which had been determined by imaging the edge of the nanopores, was ∼1.7 nm (5 layers). As shown in Figure 1 d, longer the Nanostructures

Electron Beam Etching of CaO Crystals Observed Atom by Atom

With the rapid development of nanoscale structuring technology, the precision in the etching reaches the sub-10 nm scale today. However, with the ongoing development of nanofabrication the etching mechanisms with atomic precision still have to be understood in detail and improved. Here we observe, atom by atom, how preferential facets form in CaO crystals that are etched by an electron beam in an in situ high-resolution transmission electron microscope (HRTEM). An etching mechanism under electron beam irradiation is observed that is surprisingly similar to chemical etching and results in the formation of nanofacets. The observations also explain the dynamics of surface roughening. Our findings show how electron beam etching technology can be developed to ultimately realize tailoring of the facets of various crystalline materials with atomic precision.

Surface Energy and Surface Stability of Ag Nanocrystals at Elevated Temperatures and Their Dominance in Sublimation-Induced Shape Evolution

The surface energy and surface stability of Ag nanocrystals (NCs) are under debate because the measurable values of the surface energy are very inconsistent, and the indices of the observed thermally stable surfaces are apparently in conflict. To clarify this issue, a transmission electron microscope is used to investigate these problems in situ with elaborately designed carbon-shell-capsulated Ag NCs. It is demonstrated that the {111} surfaces are still thermally stable at elevated temperatures, and the victory of the formation of {110} surfaces over {111} surfaces on the Ag NCs during sublimation is due to the special crystal geometry. It is found that the Ag NCs behave as quasiliquids during sublimation, and the cubic NCs represent a featured shape evolution, which is codetermined by both the wetting equilibrium at the Ag-C interface and the relaxation of the system surface energy. Small Ag NCs (≈10 nm) no longer maintain the wetting equilibrium observed in larger Ag NCs, and the crystal orientations of ultrafine Ag NCs (≈6 nm) can rotate to achieve further shape relaxation. Using sublimation kinetics, the mean surface energy of Ag NCs at 1073 K is calculated to be 1.1-1.3 J m-2 .

A facile strategy for rapid preparation of graphene spongy balls

Porous three dimensional (3D) graphene macrostructures have demonstrated the potential in versatile applications in recent years, including energy storage, sensors, and environment protection, etc. However, great research attention has been focused on the optimization of the structure and properties of graphene-based materials. Comparatively, there are less reports on how to shape 3D graphene macrostructures rapidly and effortlessly, which is critical for mass production in industry. Here, we introduce a facile and efficient method, low temperature frying to form graphene-based spongy balls in liquid nitrogen with a yield of ~400 balls min−1. Moreover, the fabrication process can be easily accelerated by using multi pipettes working at the same time. The graphene spongy balls show energy storage with a specific capacitance of 124 F g−1 and oil adsorbing with a capacity of 105.4 times its own weight. This strategy can be a feasible approach to overcome the low efficiency in production and speed up the development of porous 3D graphene-based macrostructures in industrial applications.

Simultaneous atomic-level visualization and high precision photocurrent measurements on photoelectric devices by in situ TEM

Herein, a novel in situ transmission electron microscopy (TEM) method that allows high-resolution imaging and spectroscopy of nanomaterials under simultaneous application of different stimuli, such as light excitation, has been reported to directly explore structure–activity relationships targeted towards device optimization. However, the experimental development of a photoelectric system capable of combining atomic-level visualization with simultaneous electrical current measurement with picoampere-precision still remains a great challenge due to light-induced drift while imaging and noise in the electrical components due to background current. Herein, we report a novel photoelectric TEM holder integrating an LED light source covering the whole visible range, a shielding system to avoid current noise, and a picoammeter, which enables stable TEM imaging at the atomic scale while measuring very small photocurrents (pico ampere range). Using this high-precision photoelectric holder, we measured photocurrents of the order of pico amperes for the first time from a prototype quantum dot solar cell assembled inside a TEM and obtained atomic-level imaging of the photo anode under light exposure. This study paves the way towards obtaining mechanistic insights into the operation of photovoltaic devices by providing direct information on the structure–activity relationships that can be used in device optimization.

A Novel Domain-Confined Growth Strategy for In Situ Controllable Fabrication of Individual Hollow Nanostructures

Abstract The manipulation and tailoring of the structure and properties of semiconductor nanocrystals (NCs) is significant particularly for the design and fabrication of future nanodevices. Here, a novel domain‐confined growth strategy is reported for controllable fabrication of individual monocrystal hollow NCs (h‐NCs) in situ inside a transmission electron microscope, which enables the atomic scale monitoring of the entire reaction. During the process, the preformed carbon shells serve as nanoreaction cells for the formation of CdSeS h‐NCs. Electron beam (e‐beam) irradiation is demonstrated to be the key activation factor for the solid‐to‐hollow shape transformation. The formation of CdSeS hollow NCs is also found to be sensitive to the volume ratio of the CdSe/CdS NCs to the carbon shell and only those CdSe/CdS NCs with a volume ratio in the range 0.2–0.8 are successfully converted into hollow NCs. The method paves the way to potentially use an e‐beam for the in situ tailoring of individual semiconductor NCs targeted toward future nanodevice applications.

In Situ Repair of 2D Chalcogenides under Electron Beam Irradiation

Molybdenum disulfide (MoS2 ) and bismuth telluride (Bi2 Te3 ) are the two most common types of structures adopted by 2D chalcogenides. In view of their unique physical properties and structure, 2D chalcogenides have potential applications in various fields. However, the excellent properties of these 2D crystals depend critically on their crystal structures, where defects, cracks, holes, or even greater damage can be inevitably introduced during the preparation and transferring processes. Such defects adversely impact the performance of devices made from 2D chalcogenides and, hence, it is important to develop ways to intuitively and precisely repair these 2D crystals on the atomic scale, so as to realize high-reliability devices from these structures. Here, an in situ study of the repair of the nanopores in MoS2 and Bi2 Te3 is carried out under electron beam irradiation by transmission electron microscopy. The experimental conditions allow visualization of the structural dynamics of MoS2 and Bi2 Te3 crystals with unprecedented resolution. Real-time observation of the healing of defects at atomic resolution can potentially help to reproducibly fabricate and simultaneously image single-crystalline free-standing 2D chalcogenides. Thus, these findings demonstrate the viability of using an electron beam as an effective tool to precisely engineer materials to suit desired applications in the future.