J. Liu

High resolution Auger electron spectroscopy and microscopy of a supported metal catalyst

High spatial resolution Auger electron spectra and scanning Auger microscope (SAM) images of supported metal catalysts have been obtained in a UHV scanning transmission electron microscope. Ag/α-Al2O3 was used as a model catalyst system, where silver was evaporated, in situ, onto polycrystalline alumina carriers. Silver particles, as small as 2 nm in diameter, were clearly revealed in SAM images with high contrast. On large islands, an edge resolution < 3 nm was achieved. Information about surface and bulk properties of supported catalysts can be extracted from images formed with different signals generated from the same area which are obtained simultaneously.

Lattice Strain Induced Remarkable Enhancement in Piezoelectric Performance of ZnO-Based Flexible Nanogenerators

In this work, by employing halogen elements (fluorine, chlorine, bromine, and iodine) as dopant we demonstrate a unique strategy to enhance the output performance of ZnO-based flexible piezoelectric nanogenerators. For a halogen-doped ZnO nanowire film, dopants and doping concentration dependent lattice strain along the ZnO c-axis are established and confirmed by the EDS, XRD, and HRTEM analysis. Although lattice strain induced charge separation was theoretically proposed, it has not been experimentally investigated for wurtzite structured ZnO nanomaterials. Tuning the lattice strain from compressive to tensile state along the ZnO c-axis can be achieved by a substitution of halogen dopant from fluorine to other halogen elements due to the ionic size difference between dopants and oxygen. With its focus on a group of nonmetal element induced lattice strain in ZnO-based nanomaterials, this work paves the way for enhancing the performance of wurtzite-type piezoelectric semiconductor nanomaterials via lattice strain strategy which can be employed to construct piezoelectric nanodevices with higher efficiency in a cost-effective manner.

High-resolution scanning transmission electron microscopy

Abstract High-resolution imaging in scanning transmission electron microscopy (STEM) instruments is achieved with a variety of detector configurations including bright-field/dark-field (BF/DF) imaging by coherent phase contrast, high-angle annular dark-field (HAADF) imaging by incoherent atomic number contrast and many other imaging modes. With the use of post-specimen lenses to expand/compress the whole diffraction pattern, the collection angles of both the BF and the ADF detector can be varied continuously and different detector configurations may be realized. Image resolution is improved by increasing the collection angle of the BF detector. The phase and diffraction contrast in a large-angle bright-field (LABF) image is reduced and LABF images have less dependence on the change of beam defocus and the variations of sample thickness. Applications of these high-resolution STEM imaging modes to the study of interfaces are discussed.

Electron resonance reflections from perfect crystal surfaces and surfaces with steps

Abstract Dynamical calculations are carried out to investigate the creation processes of reflection waves under the (660) specular reflection case of GaAs (110) in the geometry of reflection high energy electron diffraction (RHEED). It is shown that the resonance waves are localized at the top two to three atomic surface layers. The monolayer resonance characteristic happens only at some specified low angle incidence cases but not in general. Most of the reflection intensity in the RHEED pattern is created while the electrons are propagating along the surface. The probability of immediate reflections of electrons from a surface is small. This is the reason that the resonance excitation of a surface can greatly enhance the total reflectance of a surface. The propagation of a electron beam at a crystal surface can be characterized by a mean travelling distance, which is 500 to 700 A for GaAs (110). It is pointed out that a column approximation may not be a good treatment for RHEED calculations. Dynamical calculations for surfaces with steps show that the one-atom-high down-step can critically interrupt the resonance propagation of the resonance wave along the surface. The interrupted wave goes out of the surface and forms some “extra” spots in the RHEED pattern, which are observed in RHEED experiments in a scanning transmission electron microscope. This confirms the existence of surface resonance waves and their oscillating propagations along crystal surfaces.

p‐Type MoS2 and n‐Type ZnO Diode and Its Performance Enhancement by the Piezophototronic Effect

A plasma-induced p-type MoS2 flake and n-type ZnO film diode, which exhibits an excellent rectification ratio, is demonstrated. Under 365 nm optical irradiation, this p-n diode shows a strong photoresponse with an external quantum efficiency of 52.7% and a response time of 66 ms. By increasing the pressure on the junction to 23 MPa, the photocurrent can be enhanced by a factor of four through the piezophototronic effect.

High-angle ADF and high-resolution SE imaging of supported catalyst clusters

Abstract A high-angle annular detector provides a useful imaging signal for the detection of small metal catalyst clusters in light-element supports. The high-angle annular-dark-field (HAADF) signal is mainly generated by quasi-elastic scattering. The collected signal intensity depends on the atomic number of the scatterer, its amplitude of vibration and the total number of atoms encountered by the probe. This imaging mode can reveal high- Z sensitivity and eliminate most of the diffraction contrast present in low-angle ADF images. In a STEM instrument, secondary electron images with a resolution approaching the probe size ( ∼ 0.5 nm in diameter) can be obtained simultaneously with HAADF images, providing topographic information about the metal catalyst-support system. The combination of a variety of imaging and diffraction techniques available in a STEM instrument can yield important information about the distribution of small metal clusters in the supports, crystallographic relationships between the metal clusters and the support, and morphology as well as chemical compositions of the metal catalyst-support system. Samples of Pt/carbon, Pt/γ-alumina and Pt/NaY zeolite catalysts have been examined. Preliminary results reveal useful information about these catalyst systems.

High-resolution scanning electron microscopy of surface reactions

Abstract High-resolution ( A ) and high-quality secondary electron (SE) images can be obtained from specimens in the high-resolution imaging position of a HB5 dedicated STEM instrument, in parallel with STEM or SREM imaging and with microdiffraction and microanalysis using EELS. The additional information obtained by the SEM arises from the possibility of revealing three-dimensional morphology and sensitivity to the chemical composition of surface layers. Examples include evidence of a reaction between Pd and MgO substrate and the formation of Ni by reduction of NiO under electron beam irradiation.

Contrast and resolution in REM, SEM and SAM

Abstract Reflection electron microscopy (REM) in fixed beam and scanning modes, secondary electron microscopy (SEM) and scanning Auger electron microscopy (SAM) are related in that they provide nanometer-scale resolution of surfaces in moderate or ultra-high vacuum using incident electron beams having energies of the order of 100 keV. They may, in many cases, be used in conjunction or in parallel to provide complementary information on crystal structure, morphology and compositional variations with sensitivity to single surface-layers of atoms. Experiments have shown in each case that the achievable resolution is considerably better than had previously been thought possible. Recent theoretical developments and exploratory investigations have provided at least a partial explanation of why sub-nanometer resolutions can be achieved. In the case of REM, the main limitations to resolution and contrast of the images arise from the inelastic scattering processes. Experiments with a new energy-filtering electron microscope have confirmed that images are considerably improved if formed by using only elastically scattered electrons.

Nanometer-resolution surface analysis with Auger electrons

Abstract High spatial resolution Auger electron spectra and images have been obtained in a UHV scanning transmission electron microscope. An edge resolution of ≈ 1 nm has been achieved in Auger electron images of thin specimens. The attenuation lengths of the collected Auger electrons and the escape depth of the high energy background secondary electrons have been estimated from these nanometer-resolution images. Surface steps are clearly visible in Auger electron images with high contrast and high resolution. These techniques have been applied to study a model catalyst system. A method has been developed to estimate the number of atoms in metal particles which are smaller than the probe size. The most recent results show that Ag clusters containing less than 10 atoms can be readily detected when supported on thin substrates.

Sensitivity of the ELNES in reels to the beam reductions at the TiO2(110) surfaces

The sensitivity of the energy-loss near-edge structure (ELNES) in reflection electron energy-loss spectroscopy (REELS), used in parallel with reflection electron microscopy (REM) and diffraction is shown to provide information on modifications of the TiO2(110) surfaces under strong electron irradiation. The ratio of the sharp peaks in the ELNES is strongly dependent on the oxidation state of the surface. The ELNES obtained from REELS reflects the variation of the surface density of states (DOS) of a few atomic layers. The quantitative analysis of the chemical compositions in the TiO2(110) surface before and after being irradiated by the electron beam indicates a relative decrease of the oxygen component, which corresponds to the reduction of the oxide.