Utility of the inverse partial fluorescence for electronic structure studies of battery materials

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by  H. Wadati

Utility of the inverse partial fluorescence for electronic structure studies of battery
materials
H. Wadati,1,2,∗ A. J. Achkar,3 D. G. Hawthorn,3 T. Z. Regier,4 M. P. Singh,5
K. D. Truong,5 P. Fournier,5 G. Chen,6 T. Mizokawa,7 and G. A. Sawatzky2
1Department of Applied Physics and Quantum-Phase Electronics Center (QPEC),
University of Tokyo, Hongo, Tokyo 113-8656, Japan
2Department of Physics and Astronomy, University of British Columbia, Vancouver, British Columbia V6T 1Z1, Canada
3Department of Physics and Astronomy, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada
4Canadian Light Source, University of Saskatchewan, Saskatoon, Saskatchewan S7N 0X4, Canada
2
5Regroupement Qu´eb´ecois sur les Mat´eriaux de Pointe and D´epartement de Physique,
1
0 Universit´e de Sherbrooke, Sherbrooke, Qu´ebec J1K 2R1, Canada
2 6College of Physics/State Key Laboratory of Superhard Materials,
Jilin University, Changchun 130012, People’s Republic of China
n 7Department of Complexity Science and Engineering,
a
University of Tokyo, Kashiwa, Chiba 277-8561, Japan
J
(Dated: January 5, 2012)
4
X-ray absorption spectroscopy (XAS) is one of the most widely used experimental techniquesto

] studytheelectronicandspatialstructureofmaterials. Fluorescenceyieldmodeisbulk-sensitive,but
i
c has several serious problems coming from saturation effects. In this study, we show the usefulness
s of partial fluorescence yields in addressing these problems. We discuss the different behaviors of
l- La2NiMnO6 and LiMnO2 at the Mn 2p absorption edges. The total fluorescence yield produces
r misleading spectra for LiMnO2 due to the absence of high-Z (Z: atomic number) elements. We
t
m conclude that the measurement of the inverse partial fluorescence yield is essential in studies of
LiMnO2, which is a hotly debated Li-ion battery material.
.
t
a
m
X-rayabsorptionspectroscopy(XAS)isanexperimen- x-rays. Consequently, the FY mode has a large probing
- tal probe of the electronic and spatial structure of ma- depth(>1000˚Ainthesoftx-rayregion),andisparticu-
d
n terials and is widely used in various fields of the natu- larlysuitableforstudyingbulkelectronicstructures. The
o ral sciences [1–3]. XAS is the measurement of photo- FY measurements can be performed by either the total
c absorption by excitation of a core electron into the un- fluorescenceyield(TFY)orthepartialfluorescenceyield
[
occupied states as a function of photon energy. There (PFY). In the TFY mode photons with any energy are

1 arethree measurementmodes for XAS, the transmission counted,whereasthePFYmodeusesanenergy-sensitive
v mode,theelectronyield(EY)mode,andthefluorescence detectorandselects photons within a particularrangeof
8
yield(FY)mode. Inthetransmissionmode,theintensity energies.
2
ofthex-raysismeasuredinfrontofandbehindthesam-
8
0 pleandtheratioofthetransmittedx-raysisdetermined. TFY = PFYMn+XPFYother
. Transmissionmode experimentsarestandardforhardx- X
1 ωMn(Ei,EMn)µMn(Ei)
0 rays, while for soft x-rays they are difficult to perform = C(Ef)µ (E )+µ (E )sinα (1)
2 duetothe stronginteractionofsoftx-rayswiththesam- T i T Mn sinβ
ω (E ,E )µ (E )
1 ple. Analternativetothetransmissionmodeexperiment +C(E ) X i X X i ,
v: has been provided by measuring the decay products of f PX µT(Ei)+µT(EX)ssiinnαβ
i the core holes which are created in the absorption pro- where
X
cess. Thedecayproductsincludeelectrons,photons,and
r ions escaping from the surface of the sample. The yield µT =µMn+XµX.
a
modes can be classified into the EY mode and the FY X
mode. Thetotalelectronyield(TEY)isthe mostwidely Here,µ (E )isthecontributiontothetotalattenuation
X i
used detection technique. The energy of the outgoing coefficient from the excitation of core electron X, and
electronsis notselectedandsimply allthe escapingelec- ω (E )istheprobabilityoffluorescenceatenergyE re-
X f f
tronsarecounted. Thesignalisdominatedbysecondary sultingfromelectronsdecayingtofillinthecoreholeleft
electrons which are created in the cascade process of the by X. E is an incident photon energy. C =η(E )Ω/4π
i f
Auger decay electrons. The estimated probing depth of is a constant given by the detector efficiency η(E ) and
f
this technique using soft x-rays is ∼ 40 ˚A. Besides the the solid angle Ω of the detector. α and β are the angles
EY mode, the fluorescent decay of the core hole can be ofincidenceandemission,respectively,asmeasuredfrom
also used for XAS measurements. In the FY mode, the the sample surface.
photons created in the fluorescent process have a mean A significant limitation of FY mode XAS arises from
free path of the same order of magnitude as the incident saturationeffects[4],whichdistortpeakintensitiesinthe
2
measured spectra and make FY non-linear with respect
ttrootdhueceadbsaonrpatlitoenrncaoteiffivecimenett.hoRdecteontmlye,aAsucrhekathreetabals.orinp-- gy (eV) 870000 (Ma)n 2p La2NiMnO6/S LirneTPrioOfile3Y 1.0
er 0.8
tion spectra which is bulk-sensitive and does not suffer n En 600 0.6
fromsaturationeffects[5]. Theyusedanenergy-sensitive oto 500 0.4
detector and measured the normal (non-resonant) x-ray Ph 0.2
d 400
eambsiossripotniosnpeedctgreutmha(nNXthEeSo)nferopmrobaeddiffreesroenntanetlelymwenitthainnd- Emitte 300 0.0
cidentphotons. TheinverseofthisNXESPFYistermed V) 800 (b)
as the inverse partial fluorescence yield (IPFY), and is a gy (e 700 Mn 2p LiMnO2 1.0
function of the total x-ray absorption coefficient µ(Ei): Ener 600 00..86
IPFY=A(µ(Ei)+B), (2) oton 500 0.4
h
P 0.2
µw(hEefre)siAnα/=sinβ4π. /Yη(Edfen)ΩotωeYs(tEhfe)µcoYr(eEeil)e,ctraonnd, foBr ex=- mitted 430000 0.0
E
ample, O 1s. In Eqn.(2), the constant B is independent 635 640 645 650 655 660 665
of Ei and A depends only weakly on Ei over a narrow Incident Photon Energy (eV)
energy range and can be treated approximately as con-
stant. Therefore,IPFYisproportionaltotheXASµ(E ) FIG. 1: (Color online): Energy-sensitive fluorescence yield
i
plus an offset proportionalto B. fromLa2NiMnO6andLiMnO2. Inpanel(a)[(b)],normalized
In this paper, we show the different behaviors of PFYof La2NiMnO6 [LiMnO2]astheincidentphotonenergy
is scanned through theMn 2p edges.
La NiMnO andLiMnO attheMn2pabsorptionedges.
2 6 2
One can see that the TFY makes peak structures in
La NiMnO and dip structures in LiMnO . This dif-
2 6 2
ference can be explained by the existence or absence of
TheLainLa NiMnO hasamuchlargerabsorptioncross
high-Z (Z: atomic number) elements such as La in this 2 6
sectionthattheotherelementsinthematerialanddomi-
case. Then we show the usefulness of PFY and IPFY
natesthetotalattenuationcoefficient. Therefore,evenat
forthe measurementsofLiMnO . This technique should
2
the Mn 2p edges, the attenuation length of the incident
becomeanindispensabletoolforthemeasurementofsys-
photonsdoesnotchangesignificantlyandsotheintensity
temswithouthigh-Z elementssuchasLi-ionbatteryma-
of the O K emission is not strongly reduced. When we
terials.
measurethe TFYfromthissample,the absorptionspec-
The thin film of La NiMnO was fabricated on a
2 6
trum shows the expected positive structures, but they
SrTiO substrate by the pulsed laser deposition tech-
3
are slightly distorted by saturation effects, at the Mn 2p
nique, and the details were describedin Refs. [6,7]. The
edges. ThesituationinLiMnO isquitedifferent. Inthis
powder samples ofLiMnO weresynthesizedby the pro- 2
2
material,consisting ofonlylow-Z elements,the penetra-
cedure described in Refs. [8, 9]. The thickness of the
tiondepthdecreasesgreatlywhenexcitingacrosstheMn
thin films used for this study was 50 nm. XAS experi-
2pedge. This leadstoasignificantdecreaseinthe OK
mentswereperformedat11ID-1(SGM) oftheCanadian α
emissionthat is largerin magnitude thanthe increasein
Light Source. The TEY spectra were measured using
Mn L emission. The resulting TFY spectrum exhibits
drain current. The TFY spectra were measured with a α
dipstructuresas showninFig.3. The dipstructuresare
channelplate detector. The PFYspectraweremeasured
not apparent in the PFY. For PFY, a threshold photon
usingasilicondriftdetectorwithanenergy-resolutionof
∼120eV.Theresolutionpower(E/∆E)wassetto5000. energy of 600 eV can be used to separate the O K and
Mn L emissions, as shown in Figs. 1 and 2. The PFY
All the spectra were measured at room temperature.
from the Mn L emission (> 600 eV) and the IPFY from
Figure1showstheenergyresolvedx-rayemissionspec-
the O K emission (< 600 eV) are shown in Fig. 3.
tra of La NiMnO (a) and LiMnO (b) (normalized to
2 6 2
the incidentphotonintensity)asthe incidentphotonen- InthecaseofLa NiMnO (Fig.3(a)),theTFYshows
2 6
ergy is scanned through the Mn 2p absorption edges. lessdistortionbysaturationeffectsthaninLiMnO . The
2
Emissionspectraforseveralincidentphotonenergiesare Mn L PFY displays stronger saturation effects, and the
showninFig.2,andwecanseex-rayemissionpeakscor- IPFY from O K emission shows increased noise because
responding to the following two transitions Mn L ∼ the intensity of O K emission does not change signifi-
α,β
640 eV (3d → 2p), and O K ∼ 520 eV (2p → 1s). As cantly at this edge, as shown in Fig. 2 (a). The IPFY of
α
expected, the Mn L emission increases as the incident La NiMnO alsoshowssomedistortionbecausethesam-
2 6
photon energy passes through the Mn 2p edges. In con- ple is only 50 nm thick. The IPFY method is based on
trast, the O K emission shows a small dip in the case of the condition of total absorption, which, in this case, is
La NiMnO (a),andabigdipinthecaseofLiMnO (b). notmetatallphotonenergies. ForLiMnO (Fig.3(b)),
2 6 2 2
3
notbereliablyappliedtoeitherLiMnO orLa NiMnO ,
y (arb. units) 1100....2086 L(aa2)NOiM KnaO 6/SrTiO3 666664535655505 eeeeeVVVVV ((MMnn 22pp31//22)) L(biM)nOO K2a 666664535655505 eeeeeVVVVV ((MMnn 22pp31//22)) witnhhgeircLehaiwsaetsoaigmtntsriifibwcuhatniecthltyomtloehsdeseprdraeitsseteontrhcteeioocnfhtaihnne2gtsehtreinonLpgae2l2nyNeatibMrasnotirOob6n6-
sit 0.4 Mn La.b Mn La.b depth when exciting across the Mn L edge. The PFY
n
nte 0.2 in both systems exhibits strong self-absorption effects.
I
0 These effects are overcome by using the inverse of the O
400 500 600 700 800 400 500 600 700 800
Emitted Photon Energy (eV) Emitted Photon Energy (eV) K PFY (IPFY), which is a good representation of the
Mn XAS in LiMnO . As such low-Z systems are highly
2
FIG. 2: (Color online): Emission spectra for several incident relevant battery materials, the IPFY technique will be a
photon energies are shown for La2NiMnO6 (a) and LiMnO2 crucial tool for measuring accurate XAS spectra.
(b). EmissionscorrespondingtotheOKαandMnLα,β tran-
sitions are observed.
This research was made possible with financial sup-
port from the Canadian funding organizations NSERC,
(a) La2NiMnO6/SrTiO3 (b) LiMnO2 CFI, and CIFAR. This research is granted by the Kao
Mn 2p Mn 2p Foundation and also by the Japan Society for the Pro-
TEY
TEY
s) motionofScience(JSPS)throughthe“FundingProgram
unit TFY forWorld-LeadingInnovativeR&DonScienceandTech-
b. TFY
ar nology (FIRST Program),” initiated by the Council for
ensity ( P(MFnY La,b ) P(MFnY La,b ) Science and Technology Policy (CSTP).
nt
I IPFY IPFY
(O Ka ) (O Ka )
640 650 660 640 650 660
Incident Photon Energy (eV) Incident Photon Energy (eV)
∗ Electronic address: [email protected];
URL:http://www.geocities.jp/qxbqd097/index2.htm
FIG. 3: (Color online): TEY, TFY, Mn L edge PFY and O
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K edgeIPFYasafunctionofphotonenergythroughtheMn
Solids (CRC Press, New York,2008).
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2