Dongshan Zhao

Electron beam-assisted healing of nanopores in magnesium alloys

Nanopore-based sensing has emerged as a promising candidate for affordable and powerful DNA sequencing technologies. Herein, we demonstrate that nanopores can be successfully fabricated in Mg alloys via focused electron beam (e-beam) technology. Employing in situ high-resolution transmission electron microscopy techniques, we obtained unambiguous evidence that layer-by-layer growth of atomic planes at the nanopore periphery occurs when the e-beam is spread out, leading to the shrinkage and eventual disappearance of nanopores. The proposed healing process was attributed to the e-beam-induced anisotropic diffusion of Mg atoms in the vicinity of nanopore edges. A plausible diffusion mechanism that describes the observed phenomena is discussed. Our results constitute the first experimental investigation of nanopores in Mg alloys. Direct evidence of the healing process has advanced our fundamental understanding of surface science, which is of great practical importance for many technological applications, including thin film deposition and surface nanopatterning.

Fabrication of faceted nanopores in magnesium

In this paper, using high resolution transmission electron microscopy, we showed the fabrication of faceted nanopores with various shapes in magnesium by focused electron beam (e-beam). The characteristics of nanopore shapes and the crystallographic planes corresponding to the edges of the nanopores were discussed in detail. Interestingly, by manipulating the e-beam (e.g., irradiation direction and duration), the nanopore shape and size could be effectively controlled along different directions. Our results provide important insight into the nanopore patterning in metallic materials and are of fundamental importance concerning the relevant applications, such as nanopore-based sensor, etc.

In situ transmission electron microscopy observations of precipitation and a new orientation relationship between γ-Mg17Al12 and magnesium-based matrix in an Mg–Al–Zn–Sn alloy

An in situ observation of the precipitation of γ-Mg17Al12 phase in a die-cast Mg–Al–Zn–Sn alloy was performed using a transmission electron microscope equipped with a heating stage maintained at 403 K for 100 min. The addition of a small amount of Sn to the AZ91 system accelerates the development of the γ-Mg17Al12 phase formed during continuous precipitation. A new orientation relationship between the γ-Mg17Al12 precipitate and α-Mg matrix was identified as .

Phase-segregation assisted growth of quasi-aligned ZnO nanorods on a Mg0.6Zn0.4O-coated Si substrate by thermal evaporation

Vertically aligned ZnO nanorods were synthesized on a lattice-mismatched cubic-phase Mg0.6Zn0.4O thin-film-coated Si substrate through thermal evaporation of the mixture of carbon and ZnO powders in Ar flow with an atmospheric pressure. X-ray diffraction (XRD) and Raman scattering spectrum results confirmed that the ZnO nanorods have a wurtzite structure. Transmission electron microscopy (TEM), selected-area electron diffraction (SAED) and high-resolution TEM (HRTEM) studies indicated that the nanorods grow along the [0001] orientation. The room-temperature photoluminescence of a strong ultraviolet (UV) emission at 379?nm and a very weak deep-level emission at 510?nm implied the high optical quality of ZnO nanorods. The influence of growth time and substrate temperature on the morphologies of the nanorods was also investigated. It was found that the growth of ZnO nanorods was assisted by thermally induced phase segregation. A possible growth mechanism is proposed.

Anelasticity of twinned CuO nanowires

The mechanical behavior of CuO nanowires (NWs) was investigated by in situ transmission electron microscopy. During compression, the NWs exhibited high bending capabilities associated with high mechanical stress. Interestingly, anelasticity was consistently observed after stress release. Further investigations indicate that the anelasticity is intrinsic to the CuO NWs, although electronbeam irradiation was proved capable of accelerating the shape recovery. A mechanism based on the cooperative motion of twin-associated atoms is proposed to account for this phenomenon. The results provide insight into the mechanical properties of CuO NWs, which are promising materials for nanoscale damping systems.

High-resolution electron microscopy observations of continuous precipitates with Pitsch-Schrader orientation relationship in an Mg–Al based alloy and interpretation with the O-lattice theory

High-resolution electron microscopy was applied to analyze the continuous precipitated particles of the gamma-Mg(17)Al(12) phase with Pitsch-Schrader OR in the heat-treated AZ91 alloy at 473 K for 8 h. The existence of a continuous precipitated particle with the Pitsch-Schrader OR including the selection of the habit plane and the growth direction in Mg-Al system is rationalized by the constrained coincidence site lattice/constrained complete pattern shift lattice (CCSL/CDSCL) model and the O-lattice theory.

Direct atomic-scale observation of layer-by-layer oxide growth during magnesium oxidation

The atomic-scale oxide growth dynamics are directly revealed by in situ high resolution transmission electron microscopy during the oxidation of Mg surface. The oxidation process is characterized by the layer-by-layer growth of magnesium oxide (MgO) nanocrystal via the adatom process. Consistently, the nucleated MgO crystals exhibit faceted surface morphology as enclosed by {200} lattice planes. It is believed that the relatively lower surface energies of {200} lattice planes should play important roles, governing the growth mechanism. These results facilitate the understanding of the nanoscale oxide growth mechanism that will have an important impact on the development of magnesium or magnesium alloys with improved resistance to oxidation.

Transmission electron microscopy analysis of the crystallography of precipitates in Mg-Sn alloys aged at high temperatures

The crystallographic orientation relationships (ORs) of precipitated β-Mg2Sn particles in Mg–9.76 wt% Sn alloy aged at 573 K for 5 h, corresponding to its peak hardness, were investigated by advanced transmission electron microscopy (TEM). OR-3 of (110)β//(0001)α and [\overline 111]β//[1\overline 210]α and OR-4 of (110)β//(0001)α and [001]β//[2\overline 1\overline 10]α are the key ORs of β-Mg2Sn particles in the alloy. The proportions of β-Mg2Sn particles exhibiting OR-3 and OR-4 were determined as 75.1 and 24.3%, respectively. Crystallographic factors determined the predominance of OR-3 in the precipitated β-Mg2Sn particles. This mechanism was analyzed by a three-dimensional invariant line model constructed using a transformation matrix in reciprocal space. Models of the interface of precipitated β-Mg2Sn and the α-Mg matrix were constructed via high-resolution TEM and atomic resolution high-angle annular dark-field scanning TEM.

High-resolution electron microscopy observations of the microstructure of a rapidly solidified Mg–9.0 wt% Al–1.0 wt% Zn–4.0 wt% Sn alloy

The present study examines the microstructure of rapidly solidified ribbons of an Mg–9.0 wt% Al–1.0 wt% Zn–4.0 wt% Sn alloy. XRD and transmission electron microscopy (TEM), including high-resolution transmission electron microscopy (HRTEM), confirmed that the rapidly solidified ribbons consist of α-Mg, γ-Mg17Al12 and β-Mg2Sn phases. The predominant fraction of the γ-Mg17Al12 phase reveals intergranular texture morphologies with the Burgers OR. The minor fraction of the γ phase exists in short-lath morphology with a typical grain size of 30 nm, and its orientation relationship with the matrix is also the Burgers orientation relationship. The β-Mg2Sn particle reveals spherical morphology with the typical grain size of 40 nm, and its orientation relationship with the matrix is in the form of , .

Three-dimensional structures of magnesium nanopores

The optimization of nanopore-based devices is closely related to the nanopore three-dimensional (3D) structures. In this paper, faceted nanopores were fabricated in magnesium (Mg) by aligning the electron beam (e-beam) along the [0001] direction. Detailed structural characterization by transmission electron microscopy reveals the existence of two 3D structures: hexagonal prism-shaped and hourglass-shaped 3D morphologies. Moreover, the 3D structures of nanopores are also found to depend on the widest nanopore diameter-to-thickness ratio (D/t). A plausible formation mechanism for different 3D structures is discussed. Our results incorporate a critical piece of information regarding the nanopore 3D structures in Mg and may serve as an important design guidance for the size- and shape-controllable fabrication of solid-state nanopores applying the e-beam sculpting technique.