Treating retardation effects in valence EELS spectra for Kramers-Kronig analysis

Retardation effects such as Cerenkov losses and waveguide modes alter the valence electron energy-loss spectrum of semiconductors and insulators as soon as the speed of the probing electron exceeds the speed of light inside the probed medium. This leads to the dilemma, that optical properties from these media cannot be determined correctly using electron energy-loss spectrometry (EELS) if no corrections are applied. In this work we present two ways out of this dilemma: a reduction of the beam energy and the application of an off-line correction. We demonstrate the accuracy of these two methods by using two similar layers of Si(x):H having slightly different refractive indices and discuss the impact of the normalization parameter during Kramers-Kronig analysis (KKA) on the obtained dielectric properties. We further demonstrate that KKA can be applied without the use of standard specimens, if thickness determination using transmission electron microscopy and EELS is accurate enough.     

The impact of surface and retardation losses on valence electron energy-loss spectroscopy

The inelastic scattering of fast electrons transmitting thin foils of silicon (Si), silicon nitride (Si(3)N(4)), gallium arsenide (GaAs), gallium nitride (GaN) and cadmium selenide (CdSe) was analyzed using dielectric theory. In particular, the impact of surface and bulk retardation losses on valence electron energy-loss spectroscopy (VEELS) was studied as a function of the foil thickness. It is shown that for the materials analyzed, surface and retardation losses can cause a systematic, thickness-dependent modulation of the dielectric volume losses, which can hamper the determination of the bulk dielectric data as well as the identification of band-gap and interband transition energies by VEELS. For Si and GaAs, where the dielectric function is strongly peaked with high absolute values, retardation losses lead to additional intensity maxima in the spectrum. For thin films of these materials (below approximately 100 nm), the additional intensity maxima are related to retardation effects due to the finite size of the sample leading to the excitation of guided light modes. For thicker films, exceeding about 200 nm, the intensity maxima are caused by bulk retardation losses, i.e., Cerenkov losses. Although thickness-dependent modulations were observed for Si(3)N(4), GaN and CdSe, the form of the dielectric functions and their lower maxima, means that for TEM samples < 100 nm thick, the band-gap energies of these materials can be accurately identified by VEELS. Guidelines are given that allow for forecasting the impact of surface and retardation losses on VEELS.     

Mapping inelastic intensities in diffraction patterns of magnetic samples using the energy spectrum imaging technique

We present the quantitative measurement of inelastic intensity distributions in diffraction patterns with the aim of studying magnetic materials. The relevant theory based on the mixed dynamic form factor (MDFF) is outlined. Experimentally, the challenge is to obtain sufficient signal for core losses of 3d magnetic materials (in the 700-900eV energy-loss range). We compare two experimental settings in diffraction mode, i.e. the parallel diffraction and the large-angle convergent-beam electron diffraction configurations, and demonstrate the interest of using a spherical aberration corrector. We show how the energy spectrum imaging (ESI) technique can be used to map the inelastic signal in a data cube of scattering angle and energy loss. The magnetic chiral dichroic signal is measured for a magnetite sample and compared with theory.     

Determination of chromium valence over the range Cr(0)–Cr(VI) by electron energy loss spectroscopy

Chromium is a redox active 3d transition metal with a wide range of valences (−2 to +6) that control the geochemistry and toxicity of the element. Therefore, techniques that measure Cr valence are important bio/geochemical tools. Until now, all established methods to determine Cr valence were bulk techniques with many specific to a single, or at best, only a few oxidation state(s). We report an electron energy loss spectroscopy (EELS) technique along with an extensive suite of affined reference spectra that together, unlike other methods, can determine Cr valence (or at least constrain the possible valences) at high-spatial resolution (tens-of-nanometer scale) across a wide valence range, Cr(0)–Cr(VI). Fine structure of Cr-L_2,3 edges was parametrized by measurement of the chemical shift of the L₃ edge and the ratio of integrated intensity under the L₃ and L₂ edges. These two parameterizations were correlated to Cr valence and also the dⁿ orbital configuration which has a large influence on L-edge fine structure. We demonstrate that it is not possible to unambiguously determine Cr valence from only one fine-structure parameterization which is the method employed to determine metal valence by nearly all previous EELS studies. Rather, multiple fine-structure parameterizations must be used together if the full range of possible Cr valences is considered. However even with two parameterizations, there are limitations. For example, distinguishing Cr(IV) from Cr(III) is problematic and it may be difficult to distinguish low-spin Cr(II) from Cr(III). Nevertheless, when Cr is known to be divalent, low- and high-spin dⁿ orbital configurations can be readily distinguished.     

Mapping of valence energy losses via energy-filtered annular dark-field scanning transmission electron microscopy

The advent of electron monochromators has opened new perspectives on electron energy-loss spectroscopy at low energy losses, including phenomena such as surface plasmon resonances or electron transitions from the valence to the conduction band. In this paper, we report first results making use of the combination of an energy filter and a post-filter annular dark-field detector. This instrumental design allows us to obtain energy-filtered (i.e. inelastic) annular dark-field images in scanning transmission electron microscopy of the 2-dimensional semiconductor band-gap distribution of a GaN/Al₄₅Ga₅₅N structure and of surface plasmon resonances of silver nanoprisms. In comparison to other approaches, the technique is less prone to inelastic delocalization and relativistic artefacts. The mixed contribution of elastic and inelastic contrast is discussed.     

Model-based quantification of EELS spectra: treating the effect of correlated noise

Correlated noise is generally present in experimentally recorded electron energy loss spectra due to a non-ideal electron detector. In this contribution we describe a method to experimentally measure the noise properties of the detector as well as the consequences it has for model-based quantification using maximum likelihood. The effect of the correlated noise on the maximum likelihood fitting results can be shown to be negligible for the estimated (co)variance of the parameters while an experimentally obtained scaling factor is required to correct the likelihood ratio test for the reduction of noise power with frequency. Both effects are derived theoretically under a set of approximations and tested for a range of signal-to-noise values using numerical experiments. Finally, an experimental example shows that the correction for correlated noise is essential and should always be included in the fitting procedure.     

Bandgap measurement of thin dielectric films using monochromated STEM-EELS

High-resolution electron energy-loss spectroscopy (HR-EELS), achieved by attaching electron monochromators to transmission electron microscopes (TEM), has proved to be a powerful tool for measuring bandgaps. However, the method itself is still uncertain, due to Cerenkov loss and surface effects that can potentially influence the quality of EELS data. In the present study, we achieved an energy resolution of about 0.13 eV at 0.1 s, with a spatial resolution of a few nanometers, using a monochromated STEM-EELS technique. We also assessed various methods of bandgap measurement for a-SiNx and SiO₂ thin dielectric films. It was found that the linear fit method was more reliable than the onset reading method in avoiding the effects of Cerenkov loss and specimen thickness. The bandgap of the SiO₂ was estimated to be 8.95 eV, and those of a-SiNx with N/Si ratios of 1.46, 1.20 and 0.92 were measured as 5.3, 4.1 and 2.9 eV, respectively. These bandgap-measurement results using monochromated STEM-EELS were compared with those using Auger electron spectroscopy (AES)-reflective EELS (REELS).     

Experimental and theoretical improvements on understanding of the O K-edge of TeO2

Using an electron monochromator attached to an electron microscope, high energy-resolution electron energy-loss spectra collected from TeO2 have revealed new features in the Oxygen K-edge. Using density-functional theory in the local density and the generalized gradient approximation, we find that core-hole strength of 1.3 gives an excellent fit to our high-resolution experimental data. This indicates that screening is not weak in this oxide, as normally assumed, and that neither the ground state nor a full core-hole model is adequate in quantitative reproduction of the O K-edge in the TeO2 system.     

Limits to the spatial, energy and momentum resolution of electron energy-loss spectroscopy

We discuss various factors that determine the performance of electron energy-loss spectroscopy (EELS) and energy-filtered (EFTEM) imaging in a transmission electron microscope. Some of these factors are instrumental and have undergone substantial improvement in recent years, including the development of electron monochromators and aberration correctors. Others, such as radiation damage, delocalization of inelastic scattering and beam broadening in the specimen, derive from basic physics and are likely to remain as limitations. To aid the experimentalist, analytical expressions are given for beam broadening, delocalization length, energy broadening due to core-hole and excited-electron lifetimes, and for the momentum resolution in angle-resolved EELS.     

Image simulation of high resolution energy filtered TEM images

Inelastic image simulation software is presented, implementing the double channeling approximation which takes into account the combination of multiple elastic and single inelastic scattering in a crystal. The approach is described with a density matrix formalism. Two applications in high resolution energy filtered (EFTEM) transmission electron microscopy (TEM) images are presented: thickness-defocus maps for _SrTiO3${\mathrm{SrTiO}}_3$ and exit plane intensities for an (_LaAlO3)3(_SrTiO3)3$({\mathrm{LaAlO}}_3{)}_3({\mathrm{SrTiO}}_3{)}_3$ multilayer system. Both systems show a severe breakdown in direct interpretability which becomes worse for higher acceleration voltages, thicker samples and lower excitation edge energies. Since this effect already occurs in the exit plane intensity, it is a fundamental limit and image simulations in EFTEM are indispensable just as they are indispensable for elastic high resolution TEM images.     

The strong localization of surface plasmon polariton on a metal-coated tip of optical fiber

It is shown that during the propagation of surface plasmon polariton (SPP) on a metal-coated tip of an optical fiber its wavelength essentially decreases and wave fields anomalously increase. Dependence of a degree of localization of SPP on sharpness of the structure and width of the metal layer is defined. The received results can be used to increase the resolution of scanning optical microscope.     

Electronic structure analysis of (In, Ga, Al) N heterostructures on the nanometre scale using EELS

In this work the local electronic structure of MOVPE-grown (In, Ga, Al) N heterostructures has been investigated by electron energy loss spectroscopy (EELS). The cold field-emission scanning transmission electron microscope (VG HB501) used was equipped with a dedicated parallel EELS system which provides high dispersions at an energy resolution of 0.35 eV with the use of subnanometre electron probes the spatial resolution of the measurements depends on the physical localization of the scattering process itself and thus is in the order of nanometres.
The low-loss region of the energy spectra gives information on plasmon excitations and transitions across the bandgap. The main problem on looking at the bandgap region of EELS spectra is to separate the bandgap signal from the fading tail of the zero-loss peak. High energy resolution and application of suitable deconvolution routines for removal of the zero-loss peak extract improved information from this energy region.
Thus the EEL spectra of different group III nitrides reveal the onset of the bandgap itself and the characteristic shape of the joint density of states. From these results the local optical properties can be deduced via a Kramers–Kronig transformation.
The data obtained show detailed structure on the energy scale and are in excellent agreement with optical ellipsometric results. In comparison with these techniques EELS methods yield a superior spatial resolution of better than 10 nm which enables detailed investigation of the effect of local defects and boundaries on the optical properties.     

锅炉节能监督规程对锅炉制造业的影响与对策

介绍节能规程的主要内容,阐述规程对锅炉设计的新要求,分析节能规程对锅炉制造企业生存发展的影响,剖析了贯彻执行中存在的问题,提出相应的对策和建议。     

滤波器对滤波反投影重建图像质量的影响

在工业CT无损检测中,切片图像质量直接影响到后续的检测.而滤波函数的选取对图像重建质量影响较大.采用了一种新滤波函数.当新滤波函数的参数a取不同值时,可以近似代替R_L、S_L等滤波函数.通过比较了几种滤波器的空频特性,分析了它们对重建图像质量的影响.仿真与实测数据重建的实验表明新滤波器在a=2时重建图像Gibbs效应小,并具有较高的空间分辨率与密度分辨率.     

沉积参数对磁控溅射CrN_x膜结构和性能的影响

本文采用非平衡直流磁控溅射方法制备CrN_x薄膜,研究沉积参数对膜结构和性能的影响。结果表明沉积过程中增加N_2的分压,薄膜的组成由Cr_2N相转变成CrN相,膜的表面形貌也由胞状转化为正四面体。增加样品台的负偏压,沉积膜层更加致密,表面更加平坦。     

The influence of surface roughness on material dislocation of microindentation using bonded interface technique

ABSTRACT

This paper presents the investigation of the surface roughness on the material dislocation using bonded interface technique. The influence of the surface roughness on three different materials was focused on this study. By using the bonded interface technique, the half-specimens of stainless steel, aluminium and brass were bonded and microindentations were conducted on the bonded interface surfaces with roughnesses ranged from 0.05 to 0.37?µm. The results revealed that the increase of surface roughness induced an exponential decrease in material dislocation in vertical direction beneath the permanent indent. Using Nix-Gao model, rougher surface produced higher load independent hardness which the materials experience the hardening effect. From the observation on the plastic deformation zone, the shear bands of harder material stainless showed limited material dislocation compared to softer material aluminium and brass.     

The influence of the regularization parameter and the first estimate on the performance of Tikhonov regularized non-linear image restoration algorithms

This paper reports studies on the influence of the regularization parameter and the first estimate on the performance of iterative image restoration algorithms. We discuss regularization parameter estimation methods that have been developed for the linear Tikhonov–Miller filter to restore images distorted by additive Gaussian noise. We have performed experiments on synthetic data to show that these methods can be used to determine the regularization parameter of non-linear iterative image restoration algorithms, which we use to restore images contaminated by Poisson noise. We conclude that the generalized cross-validation method is an efficient method to determine a value of the regularization parameter close to the optimal value. We have also derived a method to estimate the regularization parameter of a Tikhonov regularized version of the Richardson–Lucy algorithm.   These iterative image restoration algorithms need a first estimate to start their iteration. An obvious and frequently used choice for the first estimate is the acquired image. However, the restoration algorithm could be sensitive to the noise present in this image, which may hamper the convergence of the algorithm. We have therefore compared various choices of first estimates and tested the convergence of various iterative restoration algorithms. We found that most algorithms converged for most choices, but that smoothed first estimates resulted in a faster convergence.     

应用宽频谱电磁脉冲开放血脑屏障对血液流变学、生化和常规指标及血液中成分的结构影响及其恢复情况研究

用生化仪器仪器检测大鼠在宽频谱电磁脉冲开放血脑屏障前后的生化指标、血液流变学指标和血常规指标,看到宽频带脉冲电磁场确实改变了大鼠的血液的生化指标、流变学指标和常规指标,不同场强,不同脉冲次数和不同照射时间产生的效果是不相同的,但在照射七天以后基本上都能恢复到原来的状态。后用FT—IR红外光谱仪测量血液中分子的红外吸收谱,但谱图基本一样,表明所制造的脉冲电磁仪没有改变血液中分子的结构和构象。