Unusual X-ray excited luminescence spectra of NiO suggestive of a self-trapping of the d-d charge transfer exciton

0.41 MB·English

by  V. I. Sokolov

Unusual X-ray excited luminescence spectra of NiO suggestive of a self-trapping of
the d-d charge transfer exciton
V.I. Sokolov,1 V.A. Pustovarov,2 V.N. Churmanov,2 V.Yu. Ivanov,2
N.B. Gruzdev,1 P.S. Sokolov,3 A.N. Baranov,3 and A.S. Moskvin4
1Institute of Metal Physics UD RAS, S. Kovalevskaya Str. 18, 620990, Ekaterinburg, Russia
2Ural Federal University, Mira Str., 19, 620002 Ekaterinburg, Russia
3Lomonosov Moscow State University, 119991, Moscow, Russia
4Department of Theoretical Physics, Institute of Natural Science,
Ural Federal University, Lenin Str., 51, 620083 Ekaterinburg, Russia
2 (Dated: January 23, 2012)
1
Luminescence spectra of NiO have been investigated under vacuum ultraviolet (VUV) and soft
0
X-ray (XUV) excitation. Photoluminescence (PL) spectra show broad emission bands centered at
2
about2.3and3.2eV.ThePLexcitation(PLE)spectralevolutionandlifetimemeasurementsreveal

n that two mechanisms with short and long decay times, attributed to the d(eg)-d(eg) and p(π)-d
a charge transfer (CT) transitions in the range 4-6eV, respectively, are responsible for the observed
J
emissions, while the most intensive p(σ)-d CT transition at 7eV appears to be a weak if any PL

0 excitation mechanism. The PLE spectra recorded in the 4-7eV range agree with the RIXS and
2 reflectance data. Making use of the XUV excitation allows us to avoid the predominant role of
thesurfaceeffectsinluminescenceandrevealbulkluminescencewithpuzzlingwellisolated doublet
]
of very narrow lines with close energies near 3.3eV characteristic for recombination transitions in
l
e self-trappedd-d CTexcitonsformedbycoupledJahn-TellerNi+ andNi3+ centers. Thisconclusion
- is supported both by a comparative analysis of the luminescence spectra for NiO and solid solu-
r
st tions NixZn1−xO, and by a comprehensive cluster model assignement of different p-d and d-d CT
. transitions, theirrelaxation channels. Tothebest ofourknowledge itisthefirst observation ofthe
at self-trapping for d-d CT excitons. Our paper shows the time resolved luminescence measurements
m provide an instructive tool for elucidation of the p-d and d-d CT excitations and their relaxation
in 3d oxides.
-
d
n
I. INTRODUCTION near 4-5eV reveales a discernible q-dispersion of its en-
o
ergy typical for Mott-Hubbard d-d CT transitions while
c
[ intensive CT band at higher energy peaked near 7-8eV
Explainingtheelectronicpropertiesoftransitionmetal
revealesonlyintensitydispersionwithoutanyvisibledis-
1 monoxides is one of the long-standing problems in the
persion of the energy that is typical for intra-center p-d
v condensed matter physics. Nickel monoxide NiO with
8 CTtransitions. Nevertheless,todatewehavenocompre-
its rather simple rocksalt structure, a large insulating
6 hensive assignment of different spectral features in NiO
gap and an antiferromagnetic ordering temperature of
3 to p-d or d-d CT transitions.
T =523Khasbeenattractingmanyphysicistsasapro-
4 N
. totype oxide for this problem. This strongly correlated Although the optical absorption of NiO have been
1
electronmaterialhasplayedandisplayingaveryimpor-
0 studied experimentally with some detail, their opti-
tantroleinclarifyingtheelectronicstructureandunder-
2 cal emission properties have been scarcely investigated.
1 standing the rich physical properties of 3d compounds. However,luminescencespectroscopycangiveaprofound
: Recentdiscoveriesofagiantlowfrequencydielectriccon- insightintotheelectronicstructure,electron-holeexcita-
v
stant,bistable resistanceswitching,andother unconven-
i tions and their relaxation in the lattice. Different mea-
X tional properties1 have led to a major resurgence of in-
surementsperformedwithUVexcitationbelowandnear
r terest in NiO. opticalgap6–8 pointtoabroadluminescencebandinthe
a
However, despite several decades of studies2 there is region2-3.5eV.Thetemperaturebehaviorindicatesthat
still no literature consensus on the detailed electronic the broad PL spectra of NiO consist of several compo-
structure of NiO. Different energy gap (E ) values be- nents with relative intensity dependent on the temper-
g
tween 3.7 and 5eV3,4 are reported for NiO and reliable ature and excitation energy. The low-energy excitation
E still remains to be settled. Conventional band theo- (E ≈2.4eV)belowthe charge-transfergapstimulates
g exc
ries which stress the delocalized nature of electrons can- a photoemission in single-crystal NiO with two maxima,
notexplainthislargegapandpredictNiOtobemetallic. one at 1.5-1.6eV and a larger one at 2.2-2.3eV9. Green
NiO has long been viewed as a prototype ”Mott insula- luminescence band with a maximum around 2.25eV has
tor”,2 with gap formed by intersite d-d charge transfer been observedin nanoclusteredNiO atE ≤2.95eV7.
exc
(CT) transitions, however, this view was later replaced The observed emission bands in the visible and near in-
by that of a ”CT insulator”5 with gap formed by p-d frared spectral ranges are usually attributed to Ni2+ in-
CT transitions. Most recent RIXS and optical reflec- trasite, or crystal field d-d transitions. In particular,
tivity measurements4 showed that the CT band peaked the main green luminescence band peaked near 2.3eV is
2
sionatabout150K.Furthermore,the3.2eVbandreveals
a two-peak structure clearly visible at elevated temper-
atures. Different kinetic properties, seemingly different
temperaturebehavior6pointtodifferentrelaxationchan-
nels governing the green and violet luminescence.
The comparison of the PL spectra with the catodolu-
minescence (CL) spectra of the same samples6 reveals
differences in the relative intensity of the excited emis-
sions. The 2.8 and 3.2eV PL bands appear in the CL
spectra as shoulders of a main broad green 2.4eV emis-
sionband. However,forNiOsamplesannealedinvacuum
both the 2.4 and 3.2eV bands have a comparable spec-
tral weight and clearly visible multipeak structure. The
green 2.4eV band is visible in time-resolved PL spectra
recorded at 10 K for different delay times6. At variance
with Ref.8 the PL and CL emission bands observed in
this work6 were attributed to the crystal field d-d tran-
sitions.
TothebestofourknowledgethephotoexcitationofPL
has been restricted by E = 4.43eV6 with no inspec-
exc
tionofthePLphotoexcitationovertheCTband,though
such a study can be an instructive tool to elucidate the
mechanism of the CT transitions and the spectral selec-
tivity of the PL. It is worth noting that all the studies
of the PL in NiO point to a special role of different de-
fects and the surface induced local non-cubic distortions
in photoemission enhancement and a remarkable inho-
mogeneous broadening of the PL bands. Indeed, a most
effective absorptionof photons with the energy ¯hω ∼E
g
occursinathin(10-20nm)surfacelayerwithmoreorless
FIG.1: (Coloronline)Spectraofthed-d,p-d CTtransitions distorted symmetry and enhanced defect concentration.
and intracenter crystal field d-d transitions in NiO. Strong Inotherwords,theUVphotoexcitationcannotstimulate
dipole-allowed σ−σ d-d and p-d transitions are shown by the bulk luminescence mirroring the fundamental mate-
thick solid arrows; weak dipole-allowed π−σ p-d transitions rial properties. These issues did motivate our studies of
bythinsolidarrows;weakdipole-forbiddenlow-energytransi- thephotoluminescencespectrainNiOunderhigh-energy
tions bythindashed arrows, respectively. Dashed linespoint excitationwith makinguseofthe bothVUV andsoftX-
to different EH relaxation channels, dotted lines point to PL ray time-resolved PL excitation technique.
transitions. Spectrumofthecrystalfieldtransitionsisrepro-
duced from Ref.10. Right hand side reproduces a fragment
of the RIXSspectra for NiO15.
II. EXPERIMENTAL RESULTS
attributed to a Stokes-shifted 1T (D) → 3A (F) tran- The PL measurements were made on the samples of
2g 2g
sitionwhilethelow-energybandpeakednear1.5eVisre- NiO andseveralsolidsolutions Ni1−xZnxO (x=0.2,0.3,
latedto1E (D)→3A (F)transition9(seeFig.1forthe and 0.6) with rock salt crystal structure. As starting
g 2g
spectrumofthecrystalfieldd-d transitions10). However, material we have used the commercially available pow-
thephotoluminescencespectraofbulksinglecrystalsand der of NiO (99%; Prolabo) and ZnO (99.99%; Alfa Ae-
ceramics of NiO under 3.81eV photoexcitation8 sugges- sar)whichhasbeenpressedintopelletsunderpressureof
tive the bandgapexcitationhaverevealedinadditionto about1250barandplacedintogoldcapsules. Quenching
the green band a more intensive broad violet PL band experiments at 7.7GPa and 1000-1100K have been per-
with a maximum around 3eV. The band was related to formedusing a toroid-typehigh-pressureapparatus. De-
a p-d charge transfer. Radiative recombination of carri- tailes of experimental technique and calibration are de-
ersinpowderedpellets ofNiO underUV excitationwith scribedelsewhere11. Electronmicroscopyanalysisshows
E = 4.43eV (280nm) higher than the CT gap con- the samples to be dense poreless oxide ceramics with
exc
sists at 10K of a broad intensive band peaked at 2.8eV rock salt cubic structure and grain size of about 10-
with a shoulder centered at about 3.2eV6. It is clearly 20µm. The NiO and Ni Zn O ceramic samples has
0.3 0.7
appreciatedthatthe2.8eVemissionbanddecreaseswith been stired and pressed into cellulose to enhance the lu-
increasing temperature, while the 3.2eV band follows a minescence intensity. The PL spectra have been excited
strictly opposite trend and becomes the dominant emis- by the synchrotron radiation (SR) with 1ns pulses run-
3
FIG. 3: (Color online) Time-resolved PLE spectra of NiO: 1
-fastwindow;2-slowwindow;3-time-integratedspectrum.
Arrows point to d(eg)-d(eg) and p(π)-d (t2u(π) → eg and
t1u(π) → eg) CT transitions which are most effective in the
luminescence excitation.
curve 3) has a maximum near 2.6eV and an extended
high-energy tail which is more intensive for the 4.13eV
thanforthe5.9eVexcitation. Time-integratedspectrum
at4.13eVexcitationdoescorrelatewiththePLspectrum
ofNiOpressedpowdersobservedinRef.6atT=10Kand
E = 4.43eV. A significant difference in PL spectra is
FIG. 2: (Color online) Time-resolved PL spectra (VUV- ex- exc
revealed for fast and slow windows. High-energy 5.9eV
citation) of NiO: 1 - fast window; 2 - slow window; 3 - time-
integrated spectrum. excitationresultsin a sizeableenhancementof PLin the
high-energy range 3-4eV for fast window, however, the
4.13eV excitation gives rise to a more pronounced effect
withtheintensivemaximumofthebroadPLbandshifted
ning with 96ns intervals. The PL and PLE spectra were
to 3.15eV, while for slow window one observe a broad
recorded in 2-3.5 and 4-7eV range, respectively12. The
PL band peaked near 2.6eV. It is worth noting that the
measurements of PL spectra under soft X-ray (XUV)
PL spectrumfor fastwindow and4.13eV excitationcor-
excitation were made on a SUPERLUMI station (HA-
relates rather well with that in Ref.8 (E = 3.81eV,
exc
SYLAB (DESY), Hamburg) using an ARC Spectra Pro-
T=10K). The PL excitation spectrum shown in Fig.3
308imonochromatorandR6358PHamamatsuphotomul-
reveals a significant effect of different observation condi-
tiplier. The time-resolved PL and PLE spectra, the PL
tions. Time-integrated spectrum and the spectrum for
decay kinetics in 1-80ns interval were measured in two
slow window present a broad maximum near 6eV and a
timewindows: fastwithadelaytimeδt =0.6ns,spanof
1 shouldernear4.5eV.Forfastwindowtheshouldertrans-
windows ∆t = 2.3ns; and slow delay time δt = 58ns,
1 2 forms into a narrow peak near 4eV comparable in mag-
∆t = 14ns. The time-resolved PL spectra as well as
2 nitude with strong maximum near 6eV.
the PL decay kinetics under XUV excitation has been
It should be emphasized that the CT transition
measured on a BW3 beamline by a VUV monochroma-
peaked near 4eV working only in the fast window
tor (Seya-Namioka scheme) equipped with microchannel
is the most effective in excitation of the high-energy
plate-photomultiplier(MCP1645,Hamamatsu). Thepa-
(violet) part of the PL spectrum. Interestingly, the
rameters of time windows: δt=0.1ns, ∆t=5.7ns. The PLE spectrum correlates both with the reflectance13,
temporal resolution of the whole detection system was electroreflectance14, and RIXS15 spectra of NiO in
250ps. The temporary interval between SR excitation
the region 4-6eV. The PL decay kinetics is shown in
pulses is equal 96ns.
Fig.4 for E =4.13eV and different emission ener-
exc
Photoluminescence spectra of NiO under VUV- gies. The decay curves were approximated by a sum
excitation at two energies ¯hω > E and different reg- of two exponentials and a pedestal which describes a
g
istrationregimesarepresentedinFig.2. ThePLspectra slow microsecond decay: I(t) = y + y exp(−t/τ ) +
0 1 1
have a complicated form depending on registration time y exp(−t/τ ), where τ =3.4ns, τ =14.6ns, y =0.11
2 2 1 2 0
andexcitationenergy. Time-integratedspectrum(Fig.2, (E =4.13eV, E =2.8eV); τ =2.3ns, τ =7.2ns,
exc em 1 2
4
FIG. 4: (Color online) PL decay kinetics (VUV - excitation)
of NiO.
y =0.003 (E =4.13eV, E =3.44eV); τ =2.3ns,
0 exc em 1
τ =16.7ns, y =0.006 (E =5.9eV, E =2.8eV).
2 0 exc em
Thus, time-resolved PL and PLE spectra, as well as the
PL decay kinetics point to two relaxation processes in
NiO with a characteristic times of nanoseconds and ten
of nanosecond, respectively. Part of energy is emitted in
micro- or millisecond range.
Luminescence spectra of NiO under XUV excitation
with energy E =130eV and fast window opening by
exc
100ps after the excitation impulse startare presented in FIG. 5: (Color online) XUV excited luminescence spectra
Fig.5. The XUV excited luminescence reveals puzzling of NiO and solid solutions NixZn1−xO (fast window). Up-
spectral features with two close and very narrow lines per panel: Luminescence spectra of the cellulose coated NiO
I and I with a short decay-time τ <400ps peaked for and Ni0.3Zn0.7O samples under XUV excitation with energy
1 2
NiOsampleat3.310eV(linewidth17meV)and3.369eV Eexc=130eV at T=7.2K. Bottom panel: Low-temperature
(linewidth 13meV), respectively, mounted on a weak (T=7.5K) luminescence spectra of the Ni0.2Zn0.8O and
Ni0.6Zn0.4OcellulosefreesamplesunderXUVexcitationwith
broad structureless pedestal in the 2.5-4eV range which
energy Eexc=130eV (solid line) and Eexc=450eV (dashed
is actually observed only for slow window. To the best
line). Luminescence spectra of Ni0.6Zn0.4O underXUVexci-
of our knowledge, such an unusual luminescence has not tationwithenergyEexc=130eVatT=7.2K(solidline)and
been observed to date either in NiO or other 3d oxides. room temperature (dash-and-dottedline).
From the other hand, the well isolated I -I doublet in
1 2
the XUV excited luminescence seems to be a close rela-
tiveofthebroadhigh-energy(violet)bandinPLspectra
peaked near 3.2eV. Dramatic difference in violet lumi- relation of the I -I doublet with an emission produced
1 2
nescencespectraunderXUVandVUVexcitationcanbe by somehow coupled pairs of Ni ions. To exclude con-
explainedif to accountfor different penetrationdepth of ceivable parasitic effect of the cellulose coating we have
VUV and XUV quanta. XUV excitation stimulates the made the measurements of the XUV excited lumines-
bulk luminescence mirroring the fundamental material cence for cellulose free ceramicsamples of solidsolutions
properties while the UV photoexcitation stimulates thin Ni Zn O andNi Zn O (Fig.5, bottom panel). For
0.2 0.8 0.6 0.4
surface layers which irregularities give rise to a strongly the both samples we have observed the same I -I dou-
1 2
enhancedandinhomogeneouslybroadenedluminescence. blet structure of the luminescence spectra with practi-
ToexaminetheoriginoftheunconventionalI -I doublet callythe sameenergyseparationδ ≈60meVandasmall
1 2
and make more reasonable suggestions about its nature 20meVblueshiftascomparedwithNiOandNi Zn O
0.3 0.7
we have made the measurements of the XUV excited lu- samples. Such a shift is believed to arise from small
minescence for solid solution Ni Zn O. As in NiO we strainsinducedbycoatings. Interestingly,thenovellumi-
0.3 0.7
observed the I -I doublet actually with the same ener- nescenceisclearlyvisibleonlyatlowtemperatures: room
1 2
giesandcloselinewidths. However,theintegralintensity temperature measurements do not reveal noticeable ef-
of the I -I doublet in Ni Zn O is appeared to be al- fect(seeRTspectrumforNi Zn OinFig.5typicalfor
1 2 0.3 0.7 0.6 0.4
most ten times weaker than in NiO that points to the othersamples). AsitisseeninFig.5(bottompanel)the
5
XUV excitation with higher energy E =450eV does Ni 3de CT transition then one should expect the low-
exc g
induce nearly the same I -I doublet structure ofthe lu- energy p-d CT counterparts with maximuma around 4,
1 2
minescence spectra. 5, and 6eV respectively, which are related to dipole-
forbidden t (π)→e , weak dipole-allowedt (π)→e ,
1g g 2u g
and relatively strong dipole-allowed t (π) → e CT
1u g
III. DISCUSSION transitions, respectively (see Fig.1). It is worth noting
that the π → σ p-d CT t (π)−e transition borrows
1u g
a portion of intensity from the strongest dipole-allowed
Chargecarriersandexcitonsphotogeneratedinacrys-
σ →σ t (σ)−e CT transition because the t (π) and
tal with strong electron-lattice interaction are known 1u g 1u
t (σ)statesofthesamesymmetryarepartlyhybridized
to relax to self-trapped states causing local lattice de- 1u
due to p-p covalency and overlap. Interestingly that
formation and forming luminescence centers. Despite
thisassignementfindsastrongsupportinthereflectance
the nature of radiative and nonradiative transitions in
(4.9, 6.1, and 7.2eV for allowed p-d CT transitions )13
stronglycorrelated3d oxidesis farfromfullunderstand-
and,particularly,inelectroreflectancespectra13,14 which
ing some reliable semiquantitative predictions can be
detect dipole-forbidden transitions. Indeed, the spectra
made in frames of a simple cluster approach (see, e.g.
clearlypointtoaforbiddentransitionpeakednear3.7eV
Refs.16 and references therein). The method provides
(missed in reflectance spectra) which thus defines a p-d
a clear physical picture of the complex electronic struc-
character of the optical CT gap and can be related with
ture and the energy spectrum, as well as the possibility
theon-settransitionforthewholecomplexp-d CTband.
of a quantitative modelling. In a certain sense the clus-
It should be noted that a peak near 3.8eV has been
tercalculationsmightprovideabetter descriptionofthe
observed in nonlinear absorption spectra of NiO18. A
overall electronic structure of insulating 3d oxides than
ratherstrongp(π)-dCTbandpeakedat6.3eVisclearly
the band structure calculations, mainly due to a better
visible in the absorption spectra of MgO:Ni17.
account for correlation effects. Starting with octahedral
NiO complexwiththepointsymmetrygroupO wedeal Asaresultofthep-d CTtransition,aphoto-generated
6 h
with five Ni3dandeighteenoxygenO2patomic orbitals electronlocalizes on a Ni2+ ionforming Jahn-Teller3d9,
formingbothhybridNi3d-O2pbondingandantibonding or Ni1+ configuration, while a photo-generated hole can
e and t molecular orbitals (MO), and purely oxygen move more or less itinerantly in the O 2p valence band
g 2g
nonbondinga (σ),t (π),t (σ),t (π),t (π)orbitals. determiningthehole-likephotoconductivity3. Itisworth
1g 1g 1u 1u 2u
Groundstateof[NiO ]10− cluster,ornominallyNi2+ ion notingthatanyoxygenπ-holeshavelargereffectivemass
6
corresponds to t6 e2 configuration with the Hund 3A thanσ-holes,thatresultsinthesubstantiallydistinctrole
2g g 2g
ground term. ofp(π)-dandp(σ)-dCTtransitionsbothinphotoconduc-
TypicallyfortheoctahedralMeO clusters,16 thenon- tivity19 and the luminescence stimulation.
6
bonding t (π) oxygen orbital has the highest energy The most effective channel of the recombinational re-
1g
and forms the first electron removal oxygen state while laxation for the spin-triplet p-d CT states t62ge3gγ(π)
other nonbonding oxygen π-orbitals, t2u(π), t1u(π), and (γ(π)=t1g(π),t2u(π),t1u(π))impliestheπ →π transfer
σ-orbital t1u(σ) have lower energy with the energy sepa- t2g → γ(π) with formation of excited spin-triplet 3T1g
ration ∼1eV in between (see Fig.1). or 3T2g states of the t52ge3g configuration of Ni2+ ion fol-
Thep-d CTtransitioninNiO160− centerisrelatedwith lowedbyfinalrelaxationtolowestsingletterms1T2g and
the transfer of O 2p electron to the partially filled 3de - 1E producing green and red luminescence, respectively.
g g
subshellwithformationonthe Ni-siteofthe (t6 e3)con- Obviously, this relaxation is strongly enhanced by any
2g g
figuration of nominal Ni+ ion isoelectronic to the well- symmetry breakingeffects lifting orweakeningthe selec-
known Jahn-Teller Cu2+ ion. Yet actually instead of tion rules. It means the three π → σ p-d CT transi-
a single p-d CT transition we arrive at a series of O tions t1g(π) → eg, t2u(π) → eg, and t1u(π) → eg are
2pγ →Ni3de CTtransitionsformingacomplexp-d CT expected to effectively stimulate the green luminescence
g
band. It should be noted that each single electron tran- due to large effective mass of π-oxygen holes and a well-
sition gives rise to two many-electron transitions. The localized and long-lived character of the p-d excitation.
band starts with the dipole-forbidden t (π) → e , or On the other hand, the most intensive σ → σ p-d CT
1g g
3A2g → 3T1g,3T2g transitions, then includes two for- transitiont1g(σ)→eg appears to be significantly less ef-
mallydipole-allowedso-calledπ →σ p-d CTtransitions, fective due to small effective mass of the σ-oxygen holes
weak t (π) → e , and relatively strong t (π) → e andrelativelyshortlifetime ofthe respectiveunstable p-
2u g 1u g
CT transitions, respectively, each giving rise to 3A → d CT state. It does explain the lack of the 7eV peak in
2g
3T ,3T transitions. Finally main p-d CT band is the PLE excitation spectra (Fig.3).
1u 2u
endedbythestrongestdipole-allowedσ →σt (σ)→e Thus, the p(π)-d CT transitions are believed to effec-
1u g
(3A → 3T ,3T ) CT transition. Above estimates tivelystimulatethegreenluminescenceofNiO,yetthese
2g 1u 2u
predict the separation between partial p-d bands to be cannot explain the origin of violet luminescence, in par-
∼1eV.Thus,ifthemostintensiveCTbandwithamax- ticular,specific I -I doublet stimulatedby XUV excita-
1 2
imum around 7eV observed in RIXS spectra4,15 to at- tionwhichconcentrationbehaviorpointstoparticipation
tribute to the strongest dipole-allowed O 2pt (σ) → of Ni pairs, or d-d CT transitions rather than isolated
1u
6
can explain all the features observed in the XUV ex-
cited luminescence spectra (Fig.5). Weak line I can be
1
attributed to recombination of the dipole-forbidden S-
exciton, while strong line I with that for dipole-allowed
2
P-exciton. The S-P separation (δ ≈60meV) agrees
tun
with theoretically predicted δ ≈ 2|I | ∼70meV,
tun nnn
if to make use of estimates for I in Ref.2. Finally,
nnn
FIG. 6: Illustration to formation of S- and P-type d-d CT
the d-d, or pair character of this luminescence does ex-
excitons in NiO.
plain its strong nonlinear suppression in solid solution
Ni Zn O. It is worth noting that self-trapped d-d ex-
0.3 0.7
NiO centers. Indeed,strongd-d CT,orMotttransitions citonscanbeformedduetoatrappingoftheoxygenhole
6 bornedbythep-d CTtransitiononthenearestNi2+ ion.
provideanimportantcontributiontotheopticalresponse
of strongly correlated 3d oxides16. In NiO one expects
a strong d-d CT transition related with the σ−σ-type
e −e charge transfer t6 e2+t6 e2 →t6 e3+t6 e1 be- IV. CONCLUSION
g g 2g g 2g g 2g g 2g g
tweennnnNisiteswiththecreationofelectron[NiO ]11−
6
andhole[NiO ]9− centers(electron-holedimer),ornom- In summary, the luminescence spectra of NiO under
6
inally Ni+ and Ni3+ ions. This unique anti-Jahn-Teller VUV and XUV excitation have been investigated. PL
transition 3A +3A → 2E +2E creates a d-d CT spectrashowbroademissionbandscenteredatabout2.3
2g 2g g g
exciton self-trapped in the lattice due to electron-hole and3.2eV.ThePLEspectrarecordedinthe4-7eVrange
attraction and strong ”double”Jahn-Teller effect for the agreewith the RIXS andreflectance data. The PL spec-
bothelectronandholecenters. Exchangetunnelreaction tralevolutionandlifetimemeasurementsunderVUVex-
Ni++Ni3+↔Ni3++Ni+ due a two-electrontransfer gives citationrevealthat two mechanisms with long and short
rise to two symmetric (S- and P-) excitons (see Fig.6) decaytimes,respectively,areresponsiblefortheobserved
havings-andp-symmetry,respectively,withenergysep- emissions. These mechanisms are related to p(π)-d and
aration δ = δtun = 2|t|, where t is the two-electron d(eg)-d(eg) CT transitions in the range4-6eV, while the
transferintegralwhichmagnitudeisoftheorderofNi2+- mostintensivep(σ)-dCTtransitionat7evappearstobe
Ni2+ exchangeintegral20. InterestinglythatP-excitonis a weakif any PLexcitationmechanism. The d(eg)-d(eg)
dipole-allowed while S-exciton is dipole-forbidden. CT transition peaked near 4eV and working only in the
Strong dipole-allowed Franck-Condon d(e )-d(e ) CT fast window is the most effective one in the excitation of
g g
transition in NiO manifests itself as a strong spectral the high-energy (violet) part of the PL spectrum. For
feature near 4eV clearly visible by the RIXS spectra the first time we succeded to distinctly separate contri-
(4.5eV)4,15, the reflectance spectra (4.3eV)13, the non- butions of p(π)-d, p(σ)-d, and d-d CT transitions.
linear absorption spectra (4.1-4.3eV)18, and, particu- Making use of the XUV excitation allows us to avoid
larly, in our PLE spectra (4.1eV, see Fig.3). Such a the predominant role of the surface effects in lumines-
strong absorption near 4eV is beyond the predictions of cence and reveal bulk luminescence with puzzling well
the p-d CT model and indeed is lacking in absorption isolated I1-I2 doublet of very narrow lines with close
spectraofMgO:Ni17. Accordingly,theCTgapinNiOis energies near 3.3eV in both NiO and solid solutions
believed to be formed by a superposition of the electro- NixZn1−xO. Comparative analysis of the p-d and d-d
dipole forbidden p-d (t (π) → e ) and allowed d(e )- CT transitions, their relaxation channels, and lumines-
1g g g
d(eg) CT transitions with close energies. The energy cence spectra for NiO and Ni0.3Zn0.7O points to recom-
of the S-P doublet of the self-trapped d-d CT exciton binationtransitioninself-trappednnnNi+-Ni3+ d-d CT
is expected to be ∼1.0-1.5eV lower than the energy of excitons as the only candidate source of unconventional
the d(e )-d(e ) peak, that is near 3eV, if to account for luminescence. Tothe bestofourknowledgeitis thefirst
g g
typical estimates for electron-hole attraction (∼0.5eV) observation of the self-trapping for d-d CT excitons.
and Jahn-Teller stabilization energy (∼0.5eV/one E- The authors are grateful to R.V. Pisarev, V.I. Anisi-
type state). It is worth noting that different optical mov, and A.V. Lukoyanov for discussions and Dr. M.
reflectance and absorption measurements10,13,21 did not Kirm for help in BW3-experiments. This work was par-
reveal the I -I doublet. tially supported by the Russian Federal Agency on Sci-
1 2
Obviously the recombination of the self-trapped d-d ence and Innovation (Grant No. 02.740.11.0217) and
CT exciton, or, strictly speaking, of the S-P doublet RFBR Grant No. 10-02-96032.
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