XAFS在凝聚态物质研究中的应用资源-CSDN文库

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X射线吸收精细结构谱（XAFS）是一种用于研究物质局域结构和化学环境的强有力的技术。自其发明以来，已经迅速发展了近三十年，成为当今物质结构表征的重要工具。XAFS技术具有元素特异性，即它能够针对吸收原子附近的局域结构及化学状态进行敏感探测。基于XAFS理论，可以对特定原子周围的近邻物种、距离、配位数以及键长波动等定量结构信息进行精确测量，距离测定的精度可以达到约0.01埃或者更好。XAFS的这些特性使其能够广泛应用于固体、液体和气体状态下的多种系统，无论是晶体材料还是非晶材料，都能有效地探测其局域结构。 XAFS技术主要分为两个部分：X射线吸收近边结构（XANES）和扩展X射线吸收精细结构（EXAFS）。XANES能够提供关于吸收原子周围化学环境和氧化态的信息；而EXAFS则能揭示原子尺度的局域结构信息，包括原子种类、距离以及配位数等。XAFS数据的分析涉及对吸收边缘的精细结构进行解析，通常需要结合理论模拟来拟合实验数据，从而获得准确的结构参数。在凝聚态物质研究中，XAFS技术被应用于多个领域，如薄膜、量子阱和量子点、稀磁半导体等。XAFS能够提供这些材料中元素的局域配位环境的详细信息，进而帮助科研人员理解材料的电子结构和性质。例如，在薄膜研究中，XAFS能够用于表征膜层中的原子结构和化学成分，帮助人们理解薄膜生长机制和界面特性。在量子阱和量子点的研究中，XAFS技术能够揭示量子尺寸效应下材料的局域结构变化。在稀磁半导体的研究中，XAFS用于探索掺杂元素的局域环境以及与磁性相关的电子结构特性。此外，随着技术的发展，出现了多种与XAFS相关的新技术，例如原位XAFS、微区XAFS和时间分辨XAFS。原位XAFS技术能够在材料的合成、加工或使用过程中实时监测其局域结构的变化；微区XAFS则能够在微米或亚微米尺度上进行局部结构分析，这对于研究材料的微结构及其在异质结构中的变化尤为重要；时间分辨XAFS则能够研究随时间变化的动态过程，例如化学反应或光催化过程中的瞬态中间体。 XAFS实验通常需要使用高亮度的同步辐射光源，这样的光源能够提供高强度、高稳定性的X射线束，从而获得高质量的XAFS数据。根据不同的样品和研究目的，研究者可以采用不同的测量方法，如透射XAFS适用于常规材料的分析，荧光XAFS适用于稀有杂质和薄膜样品，磁性XAFS用于自旋研究等。 XAFS技术的进步不仅限于实验技术的改进，还包括理论模型的发展和数据处理方法的提升。例如，多壳层拟合技术被广泛应用于EXAFS数据的分析，通过多壳层拟合可以获得更准确的局域结构信息。同时，利用计算机模拟和第一性原理计算，研究者能够更好地理解XAFS数据背后的物理化学过程，从而提高研究结果的可靠性。 XAFS是一种不可或缺的材料研究工具，它在凝聚态物质研究中扮演着重要角色。随着科技的不断进步，XAFS技术及其相关技术将在材料科学、化学、物理学等领域的研究中继续发挥重要作用。

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XAFS Applications in Condensed Matters

Shiqiang Wei, Zhihu Sun, Zhiyun Pan, Wensheng Yan,

Wenjie Zhong, Zheng Wei, Zhi Xie and Bo He

National Synchrotron Radiation Laboratory,

University of Science and Technology of China

Hefei, Anhui 230029, P. R. China

sqwei@ustc.edu.cn

Xinyi Zhang

Department of Physics, Surface Physics Laboratory (National Key Laboratory),

and Synchrotron Radiation Research Center,

Fudan University,

Shanghai 200433, P. R. China

Abstract

X-ray absorption fine structure (XAFS) has experienced a rapid development in the

last three decades and has proven to be a powerful structural characterization

technique nowadays. In this review we introduce the XAFS basic principles including

the theory, the data analysis and the experiment in detail. To show its strength as a

local structure probe, the XAFS applications in condensed matters are summarized

comprehensively, i.e. thin films, quantum wells and dots, dilute magnetic

semiconductors, etc. In addition, we show some new XAFS-related techniques, such

as in-situ XAFS, micro-XAFS, time-resolved XAFS.

Keywords: X-ray absorption fine structure (XAFS) and related techniques, local

structures, quantum wells and dots, dilute magnetic semiconductors, liquid matter.

1 Introduction

X-ray absorption fine structure (XAFS) has been recognized as a powerful structural characterization

method, whose most striking advantages are its element-specificity, sensitivity to the short-range order

(typically several Å) and chemical state. XAFS can provide quantitative structural information at an

atomic scale around the specific atom, i.e., near-neighbor species and distance, coordination number,

fluctuation in bond distance. The distance determination can be accurate to about 0.01 Å or better. The

sensitivity to the short-range order makes XAFS capable of probing the local structure for a diversity of

systems, no matter the studied sample is in solid (crystalline or amorphous), liquid or gas state. Using

high-brilliance synchrotron radiation (SR) light sources, many kinds of measurement methods have

been developed to obtain high quality XAFS data, such as transmission XAFS for usual material,

fluorescence XAFS for dilute impurity and thin film, magnetic XAFS for spin study, in-situ

Support by National Natural Science Foundation of China (Grant No. 10375059, 10404023), the Cooperation

Program between BSRF and NSRL, the Innovation Program of Chinese Academy of Sciences, and Specialized

Research Fund for the Doctoral Program of Higher Education (Grant No.20030358048).

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(high-temperature, high-pressure) XAFS for state or stress transformation investigation,

grazing-incidence XAFS for surface research, space-resolved XAFS and even time-resolved XAFS for

dynamics. Nowadays, XAFS is routinely used in a wide range of scientific fields, including physics,

chemistry, biology, materials sciences, environmental sciences, and so on. In this review, we mainly

focus on the condensed matters.

1.1 Definition of XAFS

XAFS describes the details of how x-rays are absorbed by materials at energies near and above the

absorption edge of a selected atom. It refers to the oscillatory structure in the x-ray absorption

coefficient μ(E) just above the absorption edge. As an example, Fig. 1 shows the Ge K-edge XAFS

spectrum of monocrystalline Ge powder. Generally, XAFS is divided into two regimes: extended X-ray

absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES). EXAFS refers

to the oscillations observed over a wide energy range extending 1000 eV or more above the edge, while

XANES denotes the fine structures around an absorption edge extending typically 30~50 eV. The

fundamental physics of EXAFS is similar to that of XANES. The only distinction of them is the

complexity of their spectra.

10900 11200 11500 11800 12100 12400

0.0

0.5

1.0

1.5

2.0

XANES

EXAFS

μx (Arb. Units)

Energy (eV)

Figure 1: Ge K-edge XAFS spectrum for monocrystalline Ge powder

Due to the nature of the absorption process, XAFS spectra are sensitive to the electronic structure and

local atomic structure of the selected atom. [1] In particular, XANES can distinguish the electronic

state and local coordination geometry (including atomic types, bond angles and distances) surrounding

a specific atom. EXAFS is sensitive to the distance, coordination number and species of the atoms

immediately surrounding the selected element. As a powerful structural probe, XAFS has provided a

relatively simple and practical way to determine the chemical state and local atomic structure in a

variety of systems.

1.2 History of XAFS

The XAFS phenomenon was first observed by Friche and Hertz in 1920. [2] However, the

understanding of this phenomenon has long been controversial. In 1931, Kronig originally interpreted

the XAFS oscillations in crystals as a density-of-states effect in terms of the so-called long-range order

(LRO) theory. [3] But attempts to match EXAFS peaks with predictions of LRO theories proved

unsatisfactory, and afterwards in 1932, Kronig presented an alternative theory for small molecules, [4]

in which the oscillatory structure in EXAFS was attributed to the influence of neighboring atoms on the

transition matrix element in the golden rule, i.e., a short range-order (SRO) effect. The debate between

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the LRO and SRO theories remained for 40 years until in 1971 the pioneering work by Sayers, Stern,

and Lytle performed the k-space Fourier transform to separate peaks corresponding to various atomic

shells. [5] After that it is widely recognized that the SRO theory is more appropriate in interpreting the

origin of XAFS. Another key development in the XAFS theory is that of an accurate treatment of

multiple-scattering (MS) effects. In 1975, Lee and Pendry first proposed the theoretical treatment of

multiple-scattering EXAFS in terms of effective path length. [6] This approach takes the advantage of

the Fourier transform in the EXAFS data analysis. A major advance in the path by path XAFS theory,

which overcomes all the major computational difficulties, is based on a separable representation of the

Green’s-function propagators developed by Rehr and Albers. [7]-[8] With the use of the second

generation machine, XAFS theory was improved gradually [9]-[12] adapted to better resolved fine

structures in the experimental spectra. Nowadays, nearly all parameters (e.g. temperature, [13] angular

[7], [14], [15] and spin [16] dependence) are fixed by ab initio starting conditions, except the treatment

of multi-electron excitations (MEEs) which is phenomenologically described by an amplitude

reduction factor

S (see equation (1) in the next section) weakly depending on energy only and

approximated by a constant in practical calculations.[17] So far, various advancements in the

theoretical models have been obtained such as full multiple-scattering (FMS),[18] self-consistent field

and fully relativistic calculations [19] especially for a quantitative analysis of the near-edge regime.

Different approaches also address the interaction of the core hole with the photoelectron and thereby

pave the way for the theoretical description of an excited state spectroscopy.[19]-[20] Recently,

theoretical standards eliminate the arbitrariness in the experimental analysis as it was done in the early

days by means of tabulated standards.[21] The EXAFS theory is implemented in various ab initio codes

like CONTINUUM, [22] EXCURV, [23] FEFF, [19] GNXAS, [24] WIEN2k, [25] the Munich

SPRKKR package, [26] NSRL-XAFS[27] and others. [12]

It should be remarked that the controversy between the early XAFS theory, i.e., SRO and LRO, actually

lies in the energy-dependent competition between scattering strength and inelastic losses. The excited

high-energy photoelectron state is not infinitely long lived, but must decay as a function of time and

distance and hence cannot probe long-range effects. This decay is due to the inelastic losses (extrinsic

losses) as it traverses the material, and the intrinsic lifetime of the core-hole state (intrinsic losses).

Therefore, the excited photoelectron outgoing wave dies away as it moves further away from the

absorbing atom and becomes too weak to significantly reflect any waves off of distant atoms. The net

effect is that XAFS can only measure the local atomic structure within a range limited by the effective

mean free path of the excited photoelectron. This range is typically on the order of tens of angstroms.

Furthermore, it turns out that a fully quantitative treatment of the near-edge structures is still

challenging. This is due to various many-body effects, such as the photoelectron-core hole interaction,

multiplet effects, the photoelectron self-energy and inelastic losses. Various theoretical treatments are

based on the final state rule in a one-electron approach. A different approach for the calculation of the

near-edge structures is the atomic multiplet theory.[28] Unfortunately, both approaches have their

speciﬁc drawbacks.[29] Recently, in some of works a GW approach is used.[30] Another approach is

based on a time-dependent density-functional theory (TDDFT), as it has been successfully applied to

optical excitations in atoms.[31] The application of this approach to x-ray absorption spectra at the L2,3

edges for the 3d transition metal series has been demonstrated recently.[32]

As for the experimental development, from 1930’s to 1960, XAFS spectra were taken in transmission

with low-power conventional X-ray tubes. Over the last three decades, the technique of XAFS has

experienced a rapid development. This is primarily due to the application of synchrotron radiation (SR)

which has a number of unique and excellent properties, such as high brilliance, wide and continuous

wavelength, polarizability and low emittance. In the 1970s, with the advent of the first dedicated

synchrotron radiation sources using the ‘white’ synchrotron radiation of bending magnets[33] (i.e.

SSRL), the data quality and the acquisition times for the XAFS spectra were greatly improved, although

those spectra were allowed only for the investigation of the nearest neighbor distance and the local

bonding geometry.[34] The first XAFS spectrum recorded on synchrotron radiation was measured by

Stern et al in 1974,[35] which signaled the beginning of rapid development in measurement techniques.

A transmission technique utilizing a channel-cut monochromator and ion chamber was developed,[36]

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followed shortly by the fluorescence technique based on a solid-state detector for dilute systems,[37] an

energy-dispersive technique for time-resolved experiments[38] and an electron detection technique for

surface-sensitive measurement.[39] In the second generation machine (e. g. BESSY I in 1987), the

improvement in the signal-to-noise ratio and the more detailed fine structure of the oscillatory part

opened the possibility to also investigate peaks in the Fourier transform at distances larger than the

nearest neighbor distances, e.g. in [27]. Furthermore, more attention was paid to the dynamics, as, for

example, to the anisotropy of the surface Debye–Waller factors.[40] In the mid-1990s when the third

generation synchrotron radiation facilities (like ALS, APS, BESSY II, Elettra, ESRF, SPring-8) came

into operation, insertion devices like wigglers and undulators started to be used routinely. These

allowed for the measurement of XAFS with excellent statistics and high energy resolution. It is

nowadays possible to record magnetic EXAFS (normally in the range of 10

−2

–10

−3

of the isotropic

absorption), nearly free of noise with a detailed fine structure. Because of the high spectral purity, even

small oscillatory fine structures in the x-ray absorption coefficient as, for example, the so-called atomic

EXAFS can be identified by polarization-dependent surface EXAFS measurements. Up to now, more

new XAFS techniques have been developed, such as time-resolved XAFS, micro-XAFS, X-ray

magnetic circular dichroism (XMCD), to meet more specific and critical demands in structural

investigations.

1.3 Outline

This review deals with the application of XAFS to condensed matters. First, we introduce the basic

principles of XAFS, both single-scattering and multiple-scattering theories. Next the detailed data

analysis procedure is described, followed by a brief introduction of commonly used experimental

techniques for recording a XAFS spectrum. Then we review some examples of XAFS applications to

condensed matters, including thin films, quantum wells and dots, dilute magnetic semiconductors, etc.

Some other XAFS-related techniques, such as in-situ XAFS, micro-XAFS, quick-XAFS, and their

applications in condensed matters, are also included. We have tried our best to include a number of

XAFS works in the research field of condensed matters, but it is difficult for us to reach this purpose

well and to cover all the interesting and important studies.

2 Principles of XAFS

In this section, a simple physical description of the XAFS process and the basic EXAFS formula in the

frame of single-scattering approximation will be given. The recently developed multiple-scattering

theory will also be briefly described. The typical XAFS data analysis procedure will be outlined.

2.1 Single-scattering theory

Figure 2: Pictorial view of the scattering of an

outgoing wave by neighboring atoms.

Figure 3: Schematic plot of multiple-scattering

path in which the photoelectron is scattered by

two neighboring atoms.

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The absorption of X-ray by matter is a photoelectric process, in which an X-ray is absorbed by a

core-level (such as K, L,…) with a certain binding energy, and a photoelectron with wave number k is

excited and outgoes away from the absorbing atom. If there is a neighboring atom surrounding the

absorbing atom, the excited photoelectron wave will be scattered back to the absorber by the electrons

of this neighbor. The interference between the excited and scattered photoelectron waves thus gives rise

to the oscillation in the x-ray absorption coefficient: this is the origin of XAFS. Fig. 2 schematically

describes this procedure.

In the framework of single-scattering where the outgoing photoelectron wave is assumed to be scattered

only once by the neighboring atoms, the oscillation part of the absorption coefficient Δμ(E), normalized

to the atomic background μ

(E), is given by [1]:

∫

∑

∞

−

++=

−

)(/2

.)](2)(2sin[(

)()(

)(

)()(

)(

dRkkkRe

kfSN

φδ

μμ

(1)

Here k is the photoelectron wave vector defined as k = [2m(E-E

)/ħ

]

1/2

and E

is the threshold energy.

For randomly oriented polycrystalline or solution samples an isotropic average over angles must be

performed. In this case the contributions from atoms of the same atomic number and at similar distances

from the absorbing atom may not be resolvable from each other, and the atoms must be conceptually

grouped together into “coordination shells” and N

is the number of atoms in the j-th coordination shell.

In Eq. (1), S

is the amplitude reduction factor, R is the distance between the absorbing and scattering

atoms, g(R

) is the pair distribution function of the scattering atom, λ(k) is the mean free path of the

excited photoelectron, f

(k) and δ(k) are the backscattering amplitude and scattering phase shift of the

scattering atom, respectively, and φ

(k) is phase-shift of the absorbing atom.

Equation (1) is the general EXAFS formula which is applicable to all systems with disorder degree from

low to high. Especially for low disorder systems, the g(R) function can be represented by a Gaussian

distribution,

]

)(

exp[)2()(

πσ

−

−=

−

, (2)

then Eq. (1) is reduced to

)](2)(2sin[

)(

)(/22

kkkRee

kfSN

φδχ

λσ

++=

−−

∑

. (3)

Eq. (3) is also called the standard EXAFS formula, where σ

(Debye-waller factor) is the mean square

variation in the distance R. The

−

term accounts for the effects of thermal vibration and structural

disorder which smear out the EXAFS oscillations.

For a large disorder system such as amorphous materials and liquid, the Gaussian distribution of the g(R)

function fails. Instead the g(R) function can be modeled as a convolution P

⊗P

, where P

is a

normalized Gaussian function centered at R = 0, and P

is a weighted exponential distribution function:

⎪

⎩

⎪

⎨

⎧

≥=−−−

000

),2,1,0(];/)(exp[)(

)(

RRmRRRR

(4)

In this case, the EXAFS function (1) is expressed as:

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