Fang Lin

Growth of Large-Area 2D MoS2(l_,)Se2, Semiconductor

Semiconducting MoS₂(₁-x) Se₂x mono-layers where x = 0-0.40 are successfully grown over large areas. A random arrangement of the S and Se atoms and a tunable bandgap photoluminescence are observed. Atomically thin, 2D semiconductor alloys with tunable bandgaps have potential applications in nano- and opto-electronics. Field-effect transistors fabricated with the monolayers exhibit high on/off ratios of >10(5).

In situ Study of Oxidative Etching of Palladium Nanocrystals by Liquid Cell Electron Microscopy

Oxidative etching has widely prevailed in the synthesis of a crystal and played a critical role in determining the final growth behavior. In this Letter, we report an in situ microscopic study on the oxidative etching of palladium cubic nanocrystals by liquid cell scanning transmission electron microscopy. The etching was realized with oxidative radiation reactants from electron-water interaction in the presence of Br(-) ions. Dissolution dynamics of monodispersed and aggregated nanocrystals were both investigated and compared. Analyses on the dissolution kinetics of nanocrystals and the diffusion kinetics of the dissolved agents were carried out based on the scanning transmission electron microscopy characterizations. The results presented here pave a way toward the quantitative understanding of the oxidative etching reaction and its application in the functionally orientated fabrication of nanocrystals with certain sizes, structures, and morphologies.

Atomic resolution liquid-cell transmission electron microscopy investigations of the dynamics of nanoparticles in ultrathin liquids

We proposed a highly reproducible method that enables atomic resolution transmission electron microscopy investigations of the dynamics of nanoparticles in liquids. An ultrathin liquid layer was established as a result of formation of bubbles that was initiated and tuned by beam radiation. The migration, aggregation and rotation of palladium nanoparticles were observed.

An improved Wiener deconvolution filter for high-resolution electron microscopy images

We propose an improved Wiener deconvolution filter for high-resolution transmission electron microscopy (HRTEM) images and apply it to an experimental graphite image. The improved filter can simultaneously eliminate the modulation transfer function effects of CCD cameras and reduce noise, thereby confirming its superiority over the general Wiener deconvolution and Wiener denoising filters. In the resulting image, graphene lattices appear. In particular, because of our on-estimation of the noise power spectrum, the vacuum remains naturally smooth and very clear graphene-vacuum boundaries generates. Quantitatively, the signal-to-noise ratio of the filtered image improves by a factor of 3. The proposed method is applicable to very noisy HRTEM images of weak scattering objects, such as few-layer graphene or boron nitride.

An improved image alignment procedure for high-resolution transmission electron microscopy

Image alignment is essential for image processing methods such as through-focus exit-wavefunction reconstruction and image averaging in high-resolution transmission electron microscopy. Relative image displacements exist in any experimentally recorded image series due to the specimen drifts and image shifts, hence image alignment for correcting the image displacements has to be done prior to any further image processing. The image displacement between two successive images is determined by the correlation function of the two relatively shifted images. Here it is shown that more accurate image alignment can be achieved by using an appropriate aperture to filter the high-frequency components of the images being aligned, especially for a crystalline specimen with little non-periodic information. For the image series of crystalline specimens with little amorphous, the radius of the filter aperture should be as small as possible, so long as it covers the innermost lattice reflections. Testing with an experimental through-focus series of Si[110] images, the accuracies of image alignment with different correlation functions are compared with respect to the error functions in through-focus exit-wavefunction reconstruction based on the maximum-likelihood method. Testing with image averaging over noisy experimental images from graphene and carbon-nanotube samples, clear and sharp crystal lattice fringes are recovered after applying optimal image alignment.

Revealing the elemental-specific growth dynamics of Pt–Cu multipods by scanning transmission electron microscopy and chemical mapping

In this work, we reported our experimental approach to reveal the detailed growth behavior of platinum (Pt)–copper (Cu) bimetallic multipod nanostructures in a one-pot synthesis by analyzing the intermediate products from different stages by using aberration-corrected scanning transmission electron microscopy and associated energy-dispersive X-ray spectroscopy. An element-specific growth trajectory of Pt–Cu multipod nanostructures with compositional variation couples to geometric morphologies was observed: Ptx–Cu1−x multipods start from Pt-rich seeds (x > 0.6), evolve into a Pt–Cu alloy phase (x ≈ 0.5), and then form Pt-rich branches (with x > 0.8). This could be further explained on considering the different redox potentials of two metals and their interactions through underpotential deposition, galvanic replacement, and phase segregation. The observed combination of geometric morphologies and compositional variations may provide new strategies to potentially aid rational synthesis of alloy catalysts.

Atomic process of oxidative etching in monolayer molybdenum disulfide

The microscopic process of oxidative etching of two-dimensional molybdenum disulfide (2D MoS2) at an atomic scale is investigated using a correlative transmission electron microscope (TEM)-etching study. MoS2 flakes on graphene TEM grids are precisely tracked and characterized by TEM before and after the oxidative etching. This allows us to determine the structural change with an atomic resolution on the edges of the domains, of well-oriented triangular pits and along the grain boundaries. We observe that the etching mostly starts from the open edges, grain boundaries and pre-existing atomic defects. A zigzag Mo edge is assigned as the dominant termination of the triangular pits, and profound terraces and grooves are observed on the etched edges. Based on the statistical TEM analysis, we reveal possible routes for the kinetics of the oxidative etching in 2D MoS2, which should also be applicable for other 2D transition metal dichalcogenide materials like MoSe2 and WS2.

Effects of non-rotationally symmetric aberrations on the quantitative measurement of lattice positions in a graphene monolayer using high-resolution transmission electron microscopy.

It is crucial to determine the position of lattice atoms in a monolayer specimen with high precision using high-resolution transmission electron microscopy (HRTEM). Image simulations indicate that the intensity centers of periodic lattice atoms in graphene may deviate from their intrinsic positions if non-rotationally symmetric aberrations (except for three-fold and six-fold aberrations) exist in the HRTEM imaging system. In this letter, we quantitatively compared the deviations caused by non-rotationally symmetric aberrations, which are equivalent to individually produce a π/4 phase shift in the wave aberration function at a given frequency of 7.2 nm(-1). A two-fold aberration caused a maximum shift of 0.3 Å, and in the images affected by the axial coma, graphene still maintained its hexagonal structure while all of the measured atomic positions deviated. Furthermore, we discovered that atoms on each sublattice tended to shift by similar distances in the image. Based on this rule, we retrieved the intrinsic bond length between neighboring carbon atoms by shifting the measured atom positions in an experimental HRTEM image affected by residual aberrations.

Probing the oxidative etching induced dissolution of palladium nanocrystals in solution by liquid cell transmission electron microscopy

A microscopic study of dissolution process of nanocrystals, an opposite while functioning cooperatively with growth in many cases, is an essential issue in variety aspects of research on nanocrystals. In this work, an in situ study of the dynamic dissolution process of palladium nanocrystals by liquid cell transmission electron microscope (TEM) is presented. The effective critical size (Rcritical) for monodispersed nanocrystals is determined to be about 5nm in the experimental condition of this article. When the size of nanocrystal is above Rcritical, the dissolution rate (dr/dt) is nearly a constant. For the nanocrystal sizing below Rcritical, the dissolution rate (dr/dt) increases with the decrease of the nanocrystal radius r, indicating that high equilibrium solubility must be taken into account in the dissolution rate of small nanocrystals in solution. It is found that the aggregation kinetics and confinement effect between adjacent nanocrystals have effects on the dissolution rate during the reaction, and it has been analyzed in details and discussed in terms of the underlying physics involved. Lastly, the effects of electron beam-water interaction and the iron (III) agents on the oxidative etching are also compared.

Multiple-ellipse fitting method to precisely measure the positions of atomic columns in a transmission electron microscope image

In this paper, we propose a multiple-ellipse fitting method to accurately determine the atomic column positions in transmission electron microscopy (TEM) images. The column is enclosed by a series of ellipses fitted from contour lines at equidistant intensity levels, and each atomic column is shaped by an averaged elliptical shape to obtain its positions. In particular, the intensity profile of the atomic column can be obtained by an elliptically rotational average based on its shape; therefore, the intensities of the neighbouring atomic column can be subtracted for each atomic column during subsequent position refinement. This method can achieve precision in the picometre range, and we quantitatively measure this precision by analysing an image containing two Gaussian-shaped atoms and some simulated high-resolution transmission electron microscopy (HRTEM) images of SrTiO3.