Photoemission and inverse photoemission spectroscopy of NiO

Solid State Comunications, Printed in Great Britain.

Vo1.52,No.9,

AND

PHOTOEMISSION

pp.793-796,

INVERSE

0038-1098/84 $3.00 + .OO Pergamon Press Ltd.

PHOTOEMISSION

S. Hiifner, J. Osterwalder, Laboratorium

1984.

SPECTROSCOPY

T. Riesterer

and F. Hulliger

fiir Festkorperphysik,

CH-8093

Zurich,

OF NiO

ETH

Switzerland

(Received 22 August 1984 by P. Wachter) Photoemission

(UPS) and inverse

a thin film on metallic to the electronic

NiO

structure

is one of the prototype

the investigation d electrons is however This

not completely

is perhaps

fact,

Its electronic

observed There

are three

3d8-3d74s

(orbital

(ii) 2(3d8)-3d7 2p53dg

(charge

has recently

level

scheme

structure

of the 4.0 eV energy

by the

spectra

transfer

gap,

to date.

gap.

transition).

of NiO have

in UPS and XPS spectra

to a d"-l final

is now considered by a charge

to be a state

transfer

to the d band,

band state

Samples

The

generally

published

(d7 in NiO)

obtained vacuum.

screened

from the ligand

observed

(L)

energy.

This Fermi

two BIS studies mation

/4,5/.

accumulated

was

However

with

the infor-

in no case

tran-

793

time

those

and with

crystals

those

cleaved

in

have however

a defined

energy

in

and the

Fermi

is at the top it is known

excess

results

oxygen is pres3+ in Ni ; these ions

leading

to p-type

con-

of the NiO film is

in fig. 1. It is a raw spectrum no background

trum consists

suffi-

films

agree with

band because

An UPS spectrum shown

as

as thin

spectra

spectra

of having

act as acceptors

and photo-

on NiO /l/ as well

The Ni2p

that in NiO always

hybridized.

optical

prepared

The thin film samples

ductivity.

are many

in the

transfer

in the literature

two states

experiments

charge

from single

ent /6/, which

There

samples

that the 4.0 eV gap in

band

the main d band is now interpreted n-l as the d final state, where however the

emission

we have

From this study evidence

were

of the valence

7 eV

below

are strongly

/2,3/

XPS, UPS and BIS

the identical

the advantage

in a 3d8L - state for a ligand hole). In satellite

of NiO and to

solid Ni for a short

XPS valence

resulting

(where L stands turn the so-called

band photoemission

a combined

air to N 800°C.

accepted

ascribed

on the valence

by heating

structure

at the top of the valence

of the new information

valence

to be revised view.

In order

understanding

sition.

from the generally observed

the nature

take advantage

NiO is a 02pdNi3d

It

energy

gap in NiO.

to a closer

can be presented,

that the

of the photoemisson

and to determine

same instrument.

transition),

in NiO.

properties

study using

(iii) 2p63d8_

as

respect

of the optical

performed (i)

with

an unambiguous

to come perhaps

for the

promotion

+ 3dg,

be shown /2,3/,

interpretation band

containing

of the energy

the optical

states

to deduce

established.

possibilities

causing

and empty

cient

at 4.0 eV, is not known

transitions

of NiO grown

for

best demonstrated

that the nature

(BIS) spectra

The data are discussed

of the occupied

materials

of insulators

/l/.

photoemission

Ni are reported.

subtracted.

of three

This

features.

spec-

A signal

PHOTOEMISSION AND INVERSE PHOTOEMISSION SPECTROSCOPY OF NiO

794

is not a trivial NiO

in tests with spectrum

following

In a wide observed

..t -10

-15

I

I

I

I

I

I

I

5 -5 0 Energy relative toE,(eV)

I

I

I

I

I

I

I

,

15

IO

1

Combined

UPS

(21.2 eV) and BIS

(9.7 eV) spectra

of NiO grown

Film on Ni metal.

The heights

relative

the assignments

each other.

has been

determined

The Fermi

energy

by scraping

the Ni substrate

It is assumed

that the Fermi energy

pinned

close

is indicated coming

ellite

M7

at the top of

the 02p band and a sat-

eV below

the main

d configuration

d peak.

Since

of NiO is

that diagram similar

fore ascribe

(main line) which

02p+Ni3d

with

hybridized

each other.

transfer they

transition.

sitions

can only with

scribed

in

0.5 eV above

the top of the va-

lcnce band /7/ one is observing ation

shift of almost

consequence

of the strong

the 3d electrons

energy

shows /4/.

a spectrum energy,

not change

of

above

the Fermi

energy

for 9.7 eV

the current

in the measuring the charge

impinging process

equilibrium.

on

does This

assignment

the Ni dihalides charge

any doubt

diagram

in

and

ions and the difenergy

and the

the electron-

energy.

This p--td

p-d

tran-

be de-

the electron

the optical

gap is also supported

to the same Fermi

than

In the case of

in fig. 1 is probably

hole binding

In this case one is observing

referenced

between

transfer

UPS/BIS

the hole are on adjacent ference

a BIS spectrum

provided

the sample

localization

transition

that

This may be an

reservation

the case of an insulator. the optical

is a

in NiO.

In fig. 1 the portion

photons

a relax-

2 eV. This

gap energy.

combined

gap to a

is larger

that even charge

a

added

there-

Note however

separation

known

at most

one would

the 4 eV optical

the measured

are lying

lacking

band). grounds

The 3d8L _ signal has its maximum about 2 eV below the Fermi energy. Since it is levels

to in

lead to very

and one electron

the peak to peak

that the acceptor

easier

transitions

band

On energetic

indication

at all be deduced

(one electron

in the conduction

in

Intraatomic

states

in the valence

3d8 + 3dg& the final d state consists of 8 L state a 3d7 state (satellite) and a 3d are strongly

that differ

It is perhaps

because

final

the

the UPS and BIS specstates

can hardly

charge

be if an

to correlate

by two electrons.

transitions

final

of fig. 1 to optical

because

from that figure. interpret

at this point

or added.

to final

principle

there.

band,

the initial

is

by the

the actual

state energies

transitions,

is

that like

like NiO will

is ejected

tra belong

the d8 configuration

from the d electrons

the valence

spectra.

to the top of the valence

band /7/ and therefore

since

It is not trivial final

off the NiO layer

and measuring

notation

state.

spectra

have been given

in a material

electron

to

and

peak

Note

F' in the case of the photoemission

state

of the spec-

tra have not been normalized

scan a further

it is not clear what

as a thin

state

at 13 eV to the 3d"L

energy

F

the broad

it to the 3d84s

at 17 eV above E

"unrelaxed" Fig.

found

In the BIS

N 3.7 eV above E

to the dg state,

the structure

-20

as has been

insulators.

the structure

is ascribed band

matter

other

Vol. 52, No. 9

the optical

transfer

/8,9/.

for the optical

by other

This

data.

gaps are the

transitions follows

For

beyond

from their

systematic spectra

and from a comparison

gaps range

to 8.5 eV for KNiF3

energies

less between

bital

dihalides assign

transitions

it thus

seems

an energy

3' these

transfer

Also

or-

the 3d8 state. electron

will

but probably observed

tran-

to

energy

of course

reduce

not to more

The situation

again

to the If

state

formaly

states

after

and (d7 + d8L) fig. 1 it is evident

a d electron

promotion

(dg + d"I,) -.

From

are

grounds

relative

only

meaningful1 transition

apparent

to distinguish

because

of the charge

c( transitions

energetic

are facts assignment likely.

that make

charge

tranfer Thus

arise,

in this context,

that NiS, which

seems

to have

commonly

a bandwidth

is metallic

in NiO,

it has a dg state This

energy.

exact

nature

change

shows

is very

Ni3d

that

to the

making

a not

/13/.

Raman

et-al.

assignment

in agreement

effect

with

require

for the optical

In that diagram

the

of

an 02p-_, gap.

d configurations

as a function

of total

it was assumed

the d8 configuration

at the top of the valence t: .z experimentally

or tne

measurements

/14/ which

assignment

r

to the

that the position

from NiO to Ni:MgO

gap is also

energy.

indicating

sensitive

The chal-ge transfer

Merlin

to that

even closer

of the compound

in going

optical

similar

is positioned band,

observed

thus at

Fermi

energy

d"configurations inNiO

trans-

component the "true"

out on

whether

the charge

of the optical

The most

ma-

it has to be

this type of

can be ruled

does

to the concentrated

noted

the tran-

Position

of the dn configurations

in NiO,

as taken

from fig. 1. It was

assumed

that the Fermi

Fig.

grounds.

The question

in the dilute

Also

observed

the

it is just the

out of the d wavefunction. d-

This makes

it is not very

from the ordinary

fer transitions projection

that

in NiO,

t$:rial understandable.

sitions leading to the final states d8L 9 for the optical gap. and d are appropriate It is however

to Ni:MgO.

in NiO are shown

complicated.

then the final (d8 + dgI,) -

that on energetic

system

there

of the dg state

the position

In fig. 2 the various

respect

the initial

However

in the gap energy

resonance

this value

than the 6.5 eV

is more

lowers

increase

Ni 2+ /ll/. with

d-+ d transitions

by the 4s

a gap of

/12/.

d-d interaction

which

on

gives

is considerable

unreasonable

to

measurement

system

is N 9.5 eV

is close

The screening

Ni in MqO which

of the dg state

gap for these

which

absorption

6.3 eV for this

Fermi

7 eV to the

state

dilute

that

it can be seen from

in atomic

one writes

to of

also in NiO which

1 that the d7 final the Fermi

is

of the Ni

out the 4.0 eV optical

is the optical

in NiO relative

are known

reasonable

of about

transitions

transitions.

below

as-

have

This

because

than the charge

3d8+3d74s

as

2

rep-

w 7 eV changing

and KNiF

From the systematics

sitions.

fig.

in essence

less on the electronegativity

the ligand

rules

NiI

behaviour

promotion

molecules

transitions of

the

2 eV for Ni12

the absorptions

of the order

a reasonable

depend

from

(which

NiF2), while 8 to 3d +3d74s

signed

much

with

of the free Ni dihalide

/lO/'. These

resents

795

PHOTOEMISSION AND INVERSE PHOTOEMISSION SPECTROSCOPY OF NiO

Vol. 52, No. 9

there

transfer

2

top of the valence

gap in NiO un-

determined

cited

higher

experiment

energy

band.

d8L - configuration

in energy,

is at the

The experimentally is about

2 eV

PHOTOEMISSION AND INVERSE PHOTOEMISSION SPECTROSCOPY OF NiO

796

or 2 eV above the 3d8L structure. points

to a problem

of the Coulomb

with

like NiO. Using

nition

namely

form the reaction obtains

energy

required

2da_d7

U = 13 eV a value

inverse

photoemission

thiin films grown

defi-

to per-

+ dg one

experiments

on Ni metal

From these

performed.

in a

the classical

the energy

2

the definition

correlation

system

Fig.

Vol. 52, No. 9

transfer

the most

plausible

have been

data an interpreta-

tion of the 4.0 eV optical a charge

on NiO

p+d

gap in NiO as

transition

seems

one.

that seems quite Acknowledgement

high. would

Note that the same way of reasoning lead to U s 6 eV in Ni metal, which

also certainly

is an overestimation.

the other hand, separation state which

On

if one takes the energy 8 state and the dg

of the d

a value

(S.H.) thanks

for its hospitality;

ly indebted Siegmann

to the group

for providing conditions.

photoemission

Nationaler

and

the ETH

he is especial-

of Prof.

H.C.

the excellent

This work was

by the Schweizerischer

appropriate.

In conclusion,

Zurich

working

of U ti 4 eV is obtained

seems more

One of the authors

supported

Nationalfonds

and

Energieforschungsfonds.

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