Fu-Rong Chen

High-Quality Thin Graphene Films from Fast Electrochemical Exfoliation

Flexible and ultratransparent conductors based on graphene sheets have been considered as one promising candidate for replacing currently used indium tin oxide films that are unlikely to satisfy future needs due to their increasing cost and losses in conductivity on bending. Here we demonstrate a simple and fast electrochemical method to exfoliate graphite into thin graphene sheets, mainly AB-stacked bilayered graphene with a large lateral size (several to several tens of micrometers). The electrical properties of these exfoliated sheets are readily superior to commonly used reduced graphene oxide, which preparation typically requires many steps including oxidation of graphite and high temperature reduction. These graphene sheets dissolve in dimethyl formamide (DMF), and they can self-aggregate at air-DMF interfaces after adding water as an antisolvent due to their strong surface hydrophobicity. Interestingly, the continuous films obtained exhibit ultratransparency (∼96% transmittance), and their sheet resistance is <1k Ω/sq after a simple HNO3 treatment, superior to those based on reduced graphene oxide or graphene sheets by other exfoliation methods. Raman and STM characterizations corroborate that the graphene sheets exfoliated by our electrochemical method preserve the intrinsic structure of graphene.

Direct Formation of Wafer Scale Graphene Thin Layers on Insulating Substrates by Chemical Vapor Deposition

Direct formation of high-quality and wafer scale graphene thin layers on insulating gate dielectrics such as SiO(2) is emergent for graphene electronics using Si-wafer compatible fabrication. Here, we report that in a chemical vapor deposition process the carbon species dissociated on Cu surfaces not only result in graphene layers on top of the catalytic Cu thin films but also diffuse through Cu grain boundaries to the interface between Cu and underlying dielectrics. Optimization of the process parameters leads to a continuous and large-area graphene thin layers directly formed on top of the dielectrics. The bottom-gated transistor characteristics for the graphene films have shown quite comparable carrier mobility compared to the top-layer graphene. The proposed method allows us to achieve wafer-sized graphene on versatile insulating substrates without the need of graphene transfer.

‘Big Bang’ tomography as a new route to atomic-resolution electron tomography

Until now it has not been possible to image at atomic resolution using classical electron tomographic methods, except when the target is a perfectly crystalline nano-object imaged along a few zone axes. The main reasons are that mechanical tilting in an electron microscope with sub-ångström precision over a very large angular range is difficult, that many real-life objects such as dielectric layers in microelectronic devices impose geometrical constraints and that many radiation-sensitive objects such as proteins limit the total electron dose. Hence, there is a need for a new tomographic scheme that is able to deduce three-dimensional information from only one or a few projections. Here we present an electron tomographic method that can be used to determine, from only one viewing direction and with sub-ångström precision, both the position of individual atoms in the plane of observation and their vertical position. The concept is based on the fact that an experimentally reconstructed exit wave consists of the superposition of the spherical waves that have been scattered by the individual atoms of the object. Furthermore, the phase of a Fourier component of a spherical wave increases with the distance of propagation at a known ‘phase speed’. If we assume that an atom is a point-like object, the relationship between the phase and the phase speed of each Fourier component is linear, and the distance between the atom and the plane of observation can therefore be determined by linear fitting. This picture has similarities with Big Bang cosmology, in which the Universe expands from a point-like origin such that the distance of any galaxy from the origin is linearly proportional to the speed at which it moves away from the origin (Hubble expansion). The proof of concept of the method has been demonstrated experimentally for graphene with a two-layer structure and it will work optimally for similar layered materials, such as boron nitride and molybdenum disulphide.

High-resolution electron microscopy of Cu/MgO and Pd/MgO interfaces

Abstract Cu/(1 1 1)MgO and Pd/(1 1 1)MgO interfaces with a cube-on-cube relation are produced by the internal oxidation technique. High-resolution transmission electron microscopy has been applied to study the interface structures of Cu/(1 1 1)MgO and Pd/(1 1 1)MgO. In contrast to a previous work in which cases the core structures of interfacial dislocations in Cu/(1 1 1)MgO and Pd/(1 1 1)MgO interfaces could not be directly observed from the high-resolution images, we have observed localized interfacial dislocations in both Cu/(1 1 1)MgO and Pd/(1 1 1)MgO interfaces from high-resolution images. Each interfacial dislocation in the Cu/(1 1 1)MgO and Pd/(1 1 1)MgO interfaces actually contributes to sets of zig-zag dislocations in a hexagonal network. Viewing along [110], the overlapped distances of the zig-zag interfacial dislocations in the network are ∾0.5 and 1 nm for the Cu/MgO and Pd/MgO interfaces, respectively, which is comparable with the observed ‘localized width’ of interfacial dislocations from high-resolution images. The atomic structures of these two metal/oxide interfaces are determined from through-focal series of high-resolution images. The terminating lattice plane in the interface is the oxygen lattice plane for both Cu/(1 1 1)MgO and Pd/(1 1 1)MgO interfaces which are composed of distorted structural units of Cu2O and PdO, respectively. Junction dislocations resembling those in grain boundary facets, related to the difference of the rigid body translations, are observed between two energetically equivalent {1 1 1} facets in the Cu/MgO and Pd/MgO interfaces. The Burgers vector of the junction dislocation is 1 6 〈112〉 and a stacking fault in the soft metal matrix is found accompanying this junction dislocation.

Decoupling of CVD graphene by controlled oxidation of recrystallized Cu

Large-area graphene grown by chemical vapour deposition (CVD) is promising for applications; however, the interaction between graphene and the substrate is still not well understood. In this report, we use a combination of two non-destructive characterization techniques, i.e., electron backscatter diffraction (EBSD) and Raman mapping to locally probe the interface between graphene and copper lattices without removing graphene. We conclude that the crystal structure of the Cu grains under graphene layers is governed by two competing processes: (1) graphene induced Cu surface reconstruction favoring the formation of Cu(100) orientation, and (2) recrystallization from bulk Cu favoring Cu(111) formation. The underlying Cu grains, regardless of reconstruction or recrystallization, induce a large hydrostatic compression to the graphene lattice. Interestingly, the strong interaction could be decoupled by allowing the intercalation of a thin cuprous oxide interfacial-layer. The Cu2O layer is mechanically and chemically weak; hence, graphene films can be detached and transferred to arbitrary substrates and the Cu substrates could be re-used for graphene growth.

Phase TEM for biological imaging utilizing a Boersch electrostatic phase plate: theory and practice

A Boersch electrostatic phase plate (BEPP) used in a transmission electron microscope (TEM) system can provide tuneable phase shifts and overcome the low contrast problem for biological imaging. Theoretically, a pure phase image with a high phase contrast can be obtained using a BEPP. However, a currently available TEM system utilizing a BEPP cannot achieve sufficiently high phase efficiency for biological imaging, owing to the practical conditions. The low phase efficiency is a result of the blocking of partial unscattered electrons by BEPP, and the contribution of absorption contrast. The fraction of blocked unscattered beam is related to BEPP dimensions and to divergence of the illumination system of the TEM. These practical issues are discussed in this paper. Phase images of biological samples (negatively stained ferritin) obtained by utilizing a BEPP are reported, and the phase contrast was found to be enhanced by a factor of approximately 1.5, based on the calculation using the Rose contrast criterion. The low gain in phase contrast is consistent with the expectation from the current TEM/BEPP system. A new generation of phase TEM utilizing BEPP and designed for biological imaging with a high phase efficiency is proposed.

Numerical simulation modeling on the effects of grain boundary misorientation on radiation-induced solute segregation in 304 austenitic stainless steels

The purpose of this study is to develop a model to describe the effects of the grain boundary misorientation on the radiation-induced solute segregation (RIS) in 304 stainless steels. A simple rate equation model with modified boundary conditions, which included the fluxes of defects diffusing along the grain boundaries to the grain boundary dislocations, was developed for RIS at boundaries with different Σ values. The results of the model calculations were compared to the experimental results previously reported by us. It was found that the model could clearly predict the same trends that the Cr depletion levels at special boundaries in irradiated 304 stainless steels were increasing with Σ. The model calculations also showed that the widths of the segregation cusps was decreasing with increasing Σ.

Growth of High-Aspect-Ratio Gold Nanowires on Silicon by Surfactant-Assisted Galvanic Reductions

A simple galvanic reduction for direct growth of Au nanowires on silicon wafers is developed. The nanowires were prepared by reacting HAuCl4aq with Sns in the presence of CTACaq (cetyltrimethylammonium chloride) and NaNO3aq, which were important to the product morphology development. The nanowire diameter was 50-100 nm, and the length was more than 20 microm.

In-line three-dimensional holography of nanocrystalline objects at atomic resolution

Resolution and sensitivity of the latest generation aberration-corrected transmission electron microscopes allow the vast majority of single atoms to be imaged with sub-Ångstrom resolution and their locations determined in an image plane with a precision that exceeds the 1.9-pm wavelength of 300 kV electrons. Such unprecedented performance allows expansion of electron microscopic investigations with atomic resolution into the third dimension. Here we report a general tomographic method to recover the three-dimensional shape of a crystalline particle from high-resolution images of a single projection without the need for sample rotation. The method is compatible with low dose rate electron microscopy, which improves on signal quality, while minimizing electron beam-induced structure modifications even for small particles or surfaces. We apply it to germanium, gold and magnesium oxide particles, and achieve a depth resolution of 1–2 Å, which is smaller than inter-atomic distances.

Dynamics of hydrogen nanobubbles in KLH protein solution studied with in situ wet-TEM

Although the stability of the nanobubble remains a controversial issue that is subject to the classical predictions of high Laplace pressure, we demonstrate that a hydrogen nanobubble can be generated and stabilized in an aqueous solution of Keyhole limpet hemocyanin (KLH) protein via an electron radiolysis process. The hydrogen gas inside the nanobubble is in a “dense gas” phase that is characterized by a Knudsen number and number density of hydrogen molecules. The dynamics of nanobubbles are analyzed using time-series electron microscopy images. The growth of small nanobubbles will be affected by the largest neighboring nanobubble; however, a diffusive shielding effect for small nanobubbles is observed. Locally, anti-Ostwald ripening of nanobubbles can be observed; however, the global growth behavior among the nanobubbles is randomly correlated because the characteristic diffusion length of the hydrogen molecules is considerably greater than the average spacing among the nanobubbles.