Analytical electron microscopy of materials

Analytical electron microscopy enables combined crystallographic and chemical information with a high spatial resolution to be gained from microregions of electron-transparent specimens. This is reached by the combined application of imaging, diffraction and spectroscopic methods, using either a dedicated scanning transmission electron microscope or a conventional high-resolution electron microscope (having a strong objective lens) equipped with suitable X-ray or electron spectrometers. Of the diffraction methods especially the technique of convergent beam diffraction is used, yielding valuable information on crystal structures, lattice parameter changes, symmetry variations and crystal perfection, respectively. For chemical analysis, either energy-dispersive X-ray spectroscopy (EDX) is used or electron energy loss spectroscopy (EELS). Finally, high-resolution electron microscopy in the lateral resolution range of some 0.1 nm allows the reliable geometrical inspection of extreme microregions.     

Spectroscopic imaging techniques for characterization of polymer morphologies: a combination of infrared and electron microscopy

The morphological characterization of polymer blends consisting of polyamide and poly(tetrafluoroethylene) using FT-IR spectroscopy and electron microscopy is described. To enhance the lateral resolution - one of the main limits in infrared spectroscopy - a combination with scanning electron microscopy and analytical electron microscopic methods of a transmission electron microscope was made. The possibilities of electron energy loss spectroscopy and energy filtered transmission electron microscopy (EFTEM) in the area of polymer characterization are outlined.     

Femtosecond Time-Resolved Spectroscopic Photoemission Electron Microscopy for Probing Ultrafast Carrier Dynamics in Heterojunctions

The fast developing semiconductor industry is pushing to shrink and speed up transistors. This trend requires us to understand carrier dynamics in semiconductor heterojunctions with both high spatial and temporal resolutions. Recently, we have successfully set up a timeresolved photoemission electron microscopy (TR-PEEM), which integrates the spectroscopic technique to measure electron densities at specific energy levels in space. This instrument provides us an unprecedented access to the evolution of electrons in terms of spatial location, time resolution, and energy, representing a new type of 4D spectro-microscopy. Here in this work, we present measurements of semiconductor performance with a time resolution of 184 fs, electron kinetic energy resolution of 150 meV, and spatial resolution of about 150 nm or better. We obtained time-resolved micro-area photoelectron spectra and energy-resolved TR-PEEM images on the Pb island on Si(111). These experimental results suggest that this instrument has the potential to be a powerful tool for investigating the carrier dynamics in various heterojunctions, which will deepen our understanding of semiconductor properties in the submicron/nanometer spatial scales and ultrafast time scales.     

Inner-shell excitation spectroscopy and X-ray photoemission electron microscopy of adhesion promoters

The C 1s, Si 2p, Si 2s, and O 1s inner-shell excitation spectra of vinyltriethoxysilane, trimethylethoxysilane, and vinyltriacetoxysilane have been recorded by electron energy loss spectroscopy under scattering conditions dominated by electric dipole transitions. The spectra are converted to absolute optical oscillator strength scales and interpreted with the aid of ab initio calculations of the inner-shell excitation spectra of model compounds. Electron energy loss spectra recorded in a transmission electron microscope on partly cured adhesion promoter, atomic force micrographs, and images and X-ray absorption spectra from X-ray photoemission electron microscopy of as-spun and cured vinyltriacetoxysilane-based adhesion promoter films on silicon are presented. The use of these measurements in assisting chemistry studies of adhesion promoters for electronics applications is discussed.     

Scanning work function microscopy

When a sample is locally excited with a highly focused raster-scanned beam of keV electrons, the variations DeltaPhi of the work function across the surface can be monitored from the shift of the onset energy for secondary electron emission along a fixed energy scale. The performance of that "onset" technique of work function microscopy and its incorporation into scanning Auger microprobes is described. The potentialities of this extremely surface sensitive technique for structural and chemical microanalysis are demonstrated by different experimental examples comprising work function analysis of surface reactions, and sputter depth profiling with in-situ Auger and work function spectroscopy. Scanning work function microscopy for surface microanalysis is shown to supply a lateral resolution down to the 10 nm range with a detection limit below 10(-2) of a monolayer.     

Use of cathode lens in scanning electron microscope for low voltage applications

At a landing energy of 10 eV it is possible to achieve spatial resolution of the same order as at the nominal energy, which is usually 15 keV in the classical scanning electron microscope, by taking advantages of the optical properties of the cathode lens. Two different types of the detection system were designed and tested to learn as much about the optical properties of this system as possible and to start to understand the contrast mechanisms at very low energies. Great changes in the contrast take place when the landing energy is changed from 10 eV to an energy of about 2 keV.     

Imaging modes for scanning confocal electron microscopy in a double aberration-corrected transmission electron microscope.

Aberration correction leads to reduced focal depth of field in the electron microscope. This reduced depth of field can be exploited to probe specific depths within a sample, a process known as optical sectioning. An electron microscope fitted with aberration correctors for both the pre- and postspecimen optics can be used in a confocal mode that provides improved depth resolution and selectivity over optical sectioning in the scanning transmission electron microscope (STEM). In this article we survey the coherent and incoherent imaging modes that are likely to be used in scanning confocal electron microscopy (SCEM) and provide simple expressions to describe the images that result. Calculations compare the depth response of SCEM to optical sectioning in the STEM. The depth resolution in a crystalline matrix is also explored by performing a Bloch wave calculation for the SCEM geometry in which the pre- and postspecimen optics are defocused away from their confocal conditions.     

Electron microscopic measurement of the size of the optical focus in laser scanning microscopy

We describe a method for measuring the lateral focal spot size of a multiphoton laser scanning microscope (LSM) with unprecedented accuracy. A specimen consisting of an aluminum film deposited on a glass coverslip was brought into focus in a LSM and the laser intensity was then increased enough to perform nanoablation of the metal film. This process leaves a permanent trace of the raster path usually taken by the beam during the acquisition of an optical image. A scanning electron microscope (SEM) was then used to determine the nanoablated line width to high accuracy, from which the lateral spot size and hence resolution of the LSM can be determined. To demonstrate our method, we performed analysis of a multiphoton LSM at various infrared wavelengths, and we report measurements of optical lateral spot size with an accuracy of 20 nm, limited only by the resolution of the SEM.     

Thin film analysis in the nanometer scale

A survey is presented on the present state of the art in analytical transmission electron microscopy (ATEM). An essential advantage of this method is the simultaneous use of imaging, analytical and microdiffraction techniques with a lateral resolution in the 1.5 nm range. Two different analytical techniques are frequently used as ATEM attachments, energy dispersive X-ray spectrometry (EDXS) and electron energy loss spectrometry (EELS). Microscopic images with nanometer resolution may be also produced by energy selected imaging (ESI) with characteristic energy loss electrons. Advantages and limitations of all these methods will be discussed using actual material problems in the field of thin film research.     

Thin film analysis in the nanometer scale

A survey is presented on the present state of the art in analytical transmission electron microscopy (ATEM). An essential advantage of this method is the simultaneous use of imaging, analytical and microdiffraction techniques with a lateral resolution in the 1...5 nm range. Two different analytical techniques are frequently used as ATEM attachments, energy dispersive X-ray spectrometry (EDXS) and electron energy loss spectrometry (EELS). Microscopic images with nanometer resolution may be also produced by energy selected imaging (ESI) with characteristic energy loss electrons. Advantages and limitations of all these methods will be discussed using actual material problems in the field of thin film research.     

熔体结晶聚乙烯晶体的高分辨电子显微学研究

聚乙烯在一定的不良溶剂中或在熔融结晶的条件下可以得到具有弧形生长边缘的单晶,对于具有弧形边的（200）晶面的形成机理,已有一些研究报道,但由于高分子链具有成千上万个结构单元,使其结晶过程很复杂,可能会导致弧形边的成因有所不同,因此对于弧形边的形成机理有不同的解释。     

Evidence for GP-zone formation in an Al-3% Zn alloy

The presence of small GP-zones in an Al-3 wt% Zn solid solution aged at temperatures below 80° is inferred from calorimetric reversion studies, although this composition lies outside the miscibility gap of the accepted phase diagram. The size of the zones is about 1 to 1.5 nm, as measured by high resolution electron microscopy.We are greatly indebted to the research staff in the electron microscopy laboratory of C.E.N.G. (Grenoble, France) for the use of a high resolution electron microscope.     

基于散射信号的超分辨光学相减显微镜

现有的光学超分辨显微成像技术主要依赖于特殊的荧光标记物,其对于大多数非荧光样品的超分辨成像就变得无能为力。因此我们提出将光学相减显微技术应用到非荧光样品的成像当中,利用普通共聚焦光斑和面包圈型光斑分别激发样品的散射光成像,从而得到样品同一区域的两幅图像,再通过图像相减的方法提高了图像空间分辨率。不同于一般的超分辨成像方法,这种光学相减显微镜不需要特殊的样品预处理过程,同时两次成像的激发光强度可以保持在一个较低水平,避免了样品损伤的影响。随后金纳米小球和有机聚合物微丝的散射成像实验证明了光学相减显微镜可以将空间分辨率提高到215 nm (0.33λ, 1λ = 650 nm),并且通过探测散射信号得到更多的样品细节信息。     

High-resolution analytical TEM of nanostructured materials

This paper briefly reviews the potential applicability of analytical transmission electron microscopy (TEM) to elucidate both structural and chemical peculiarities of materials at high lateral resolution. Examples of analytical TEM investigations performed by energy-dispersive X-ray spectroscopy (EDXS), electron energy loss spectroscopy (EELS), and energy-filtered TEM (EFTEM) are presented for different materials systems including metals, ceramics, and compound semiconductors. In particular, results are given of imaging the element distribution in the interface region between gamma matrix and gamma' precipitate in the nickel-based superalloy SC16 by energy-filtered TEM. For core-shell structured BaTiO(3) particles the chemical composition and even the chemical bonding were revealed by EELS at a resolution of about 1 nm. A sub-nanometer resolution is demonstrated by energy-selective images of the Ga distribution in the surrounding of (In,Ga)As quantum dots. Moreover, the element distribution in (Al,Ga)As/AlAs multilayers with linear concentration gradients in a range of about 10 nm was investigated by EDXS line-profile analyses and EFTEM.     

Setup of a scanning near field infrared microscope (SNIM): imaging of sub-surface nano-structures in gallium-doped silicon

We have realized a scanning near-field infrared microscope in the 3-4 microm wavelength range. As a light source, a tunable high power continuous wave infrared optical parametric oscillator with an output power of up to 2.9 W in the 3-4 microm range has been set up. Using scanning near field infrared microscopy (SNIM) imaging we have been able to obtain a lateral resolution of < or =30 nm at a wavelength of 3.2 microm, which is far below the far-field resolution limit of lambda/2. Using this "chemical nanoscope" we could image a sub-surface structure of implanted gallium ions in a topographically flat silicon wafer giving evidence for a near-field contrast. The observed contrast is explained in terms of the effective infrared reflection as a function of the sub-surface gallium doping concentration. The future use of the setup for nm imaging in the chemically important OH, N-H and C-H stretching vibration is discussed.     

Bismuth nanowires for potential applications in nanoscale electronics technology.

Nanowires of bismuth with diameters ranging from 10 to 200 nm and lengths of 50 microm have been synthesized by a pressure injection method. Nanostructural and chemical compositional studies using environmental and high resolution transmission electron microscopy with electron stimulated energy dispersive X-ray spectroscopy have revealed essentially single crystal nanowires. The high resolution studies have shown that the nanowires contain amorphous Bi-oxide layers of a few nanometers on the surface. In situ environmental high resolution transmission electron microscopy (environmental-HRTEM) studies at the atomic level, in controlled hydrogen and other reducing gas environments at high temperatures demonstrate that gas reduction can be successfully applied to remove th oxide nanolayers and to maintain the dimensional and structural uniformity of the nanowires, which is key to attaining low electrical contact resistance.     

Photoelectron microscopy and applications in surface and materials science

We review the recent achievements of photoelectron microscopy (PEM), which is a rapidly developing technique that is significantly advancing the frontiers of surface and materials science. The operation principles of scanning photoelectron microscopes (SPEM), using different photon optic systems to obtain a micro-probe of sub-micrometer dimensions, and of the full-field imaging microscope, using electrostatic lenses for magnification of the irradiated sample area, are presented. The contrast mechanisms, based on photon absorption and photon-induced electron emission, are described and the expected development in the photon and electron optics and detection systems are discussed. Particular attention is paid to the present state-of-art performance of the microscopes collecting photoelectrons (PEs), which carry specific information about the lateral variations in the chemical, magnetic and electronic properties of the material under investigation. Selected results, obtained recently with instruments installed at synchrotron light facilities, are used to illustrate the potential of PEM in characterising micro-phases and dynamic processes with a lateral resolution better than 100 nm.     

DEPTH RESOLUTION IN PHOTOELECTRON MICROSCOPY OF ORGANIC SURFACES. THE PHOTOELECTRIC EFFECT OF PHTHALOCYANINE THIN FILMS

Abstract— The application of photoelectron microscopy as a general method of imaging organic and biological surfaces requires a knowledge of the photoelectric effect of very thin organic films. In this study, low magnification images of a 7 Å thick pattern of copper phthalocyanine were obtained, demonstrating that it is possible to visualize a monolayer of organic compound in photoelectron microscopy. Relative photoelectron currents were measured for a series of copper phthalocyanine films ranging in thickness up to 1900 Å. The relative photoelectron currents were independent of temperature (90–298°K), suggesting that electron-electron and not electron-phonon scattering is the dominant mechanism. The photoelectric properties measured are determined primarily by the large organic ring structure and not the central metal atom, as evidenced by the fact that substitution of metal-free phthalocyanine for copper phthalocyanine did not substantially alter the values of observed photoelectron currents. An analysis of the data indicates the depth resolution is 15 ű 5 Å, and equals the electron mean free path. This very good depth resolution is a result of the low kinetic energy associated with electrons released by irradiation near the photoemission threshold.     

Ionic liquid-assisted synthesis of silver nanorods by polyol reduction

<正>Silver nanorods have been successfully synthesized in large scale by the ethylene glycol(EG) reduction in the presence of ionic liquid(IL) 1-butyl-3-methylimidazolium tetrafluoroborate(bmimBF_4) and polyvinyl-pyrrolidone(PVP).The silver nanorods were characterized by scanning electron microscopy(SEM),high-resolution transmission electron microscopy(HRTEM),transmission electron microscopy(TEM),electron energy disperse spectroscopy(EDS) and UV-vis spectroscopy.The results showed that the uniform silver nanorods have an average diameter of about 100 nm and the aspect ratio from 15 to 20.IL,bmimBF_4 may play a role of capping agent together with PVP in the formation of silver nanorods.On the other band,bmimBF_4 may accelerate nucleation and improve the stability of the resulting Ag nanorods due to the low interface tension of IL.