The halogenation of polycrystalline lead surfaces studied using ultraviolet photoelectron spectroscopy

Joumalofb¶oleculmStructure.79(1982)181-184 ElsevierScientificPublishingCompany,Amsterdam-_tedinTheNetherlands

THE

BALOGENATION

OF POLYCRYSTALLINE

LEAD

ULTRAVIOLET

PHOTOELECTRON

SPECTROSCOPY

A-W.

P.J.

D.S_

POTTS,

Department

BRIGDEN,

of Physics,

King's

LAW

and

College,

SURFACES

J.Ul

STUDIED

E.P.F.

LEE

London

WCZR

2LS

USING

(Gt. Britain)

ABSTRACT

U.V.

Photoemission

halogenation

of continuous

the

study

originally

for

of the

lead

occurs

by

gas

island

used

lead

scrape

carried

phase

growth.

Br2

and

I*_

gas

indicates

is supported

of

The have

al-

designed

of the

spectra

stages

deposition

instruments

Comparison

This

early

continuous

out with

surface

the

by Clz,

and

work.

with

to study

phase that

spectra

halogenation

by measurements

of the

of PbX2 in the surface layers as a function of halogen for the halide indicates that a single PbX2 Sd-' feature

The

species

to be

dihalides

proportion dose.

been

of polycrystalline

techniques lowed

has

is formed

at all

coverages.

EXPERIMENTAL Gas

phase

molecular with

spectra

beam

photons

halides

were

continuously tinuous

is produced cleaned

section

the halogen

given

elsewhere

RESULTS U.V.

AND

frequently electron

the

given

a target

ionization

onto

using

the

is crossed

Spectra which

chamber The

scrape.

of rod

to rotate Full

pressure.

into

of

beam

made using a lead rod

dose

time

beam

these

a con-

metal for

was

is rotated surface

received

taken

the photon

details

halide

a clean

halogen

of the

of amorphous

the

and

beam

a

by

the

the

(so-5

set)

spectrometers

are

(ref.1). (The ligand spectra

a strong

on free

spectra

source.

as the product

DISCUSSION bear

light

where

produced

Adsorption studies were In this instrument

continuous

photoelectron

calculations

were

spectrometer.

within

is then

and

from

deposited.

surface

work

spectrometer

an HeI/II

produced

by

in this

photoelectron

from

scrape

continuously

reported

of gas

similarity

molecules

of solids

have

bands) phase

and

to one been

amorphous

another

used

(ref.21

to interpret

(ref.3).

0022-2860/82/000Ct-O000/$02.76@1982 Elsevier ScientificPubkshing

ionic

Company

solids

and Fl.0. the

photo-

182

In our able

of the

to suppose

fashion and

study

of PbX2

doses.

While

of Pbx2

formed

at the halide

than

ations

can be

if halogenation

clearly

detectable is more

to that

growth.

to the

changing

size

stages

of halogenation

is reflected

in the

islands

and

the

amorphous IONIZATION

8'

of

ENERGY IO’

therefore

in a purely

reasonrandom

in the

doses ligand

valence

it would band

e.g.

appear

structure

to that

molecule

band

of the figure

structure that

even

of the

amorphous Our observ1.

in terms of a picture of halogenation The changing band structure is to be related the

islands

the

or PbX2

greater

greater

large

eV_

surface 12’

revealed

one

more

clusters.

regularity

detail with

for

11’

changes

related

free

becoming

solid

9’

it was

proceeds

the

closely

interfere

eventually

7'

doses

interpreted

island

develops

lead

halogen

of the

through

grow

are

for different

halide

of

correlation between the gas phase spectrum of PbX2 formed on the surface should occur for low halogen

there

lowest

surface

that

the best

that

halogenation

diffuse

halogen

of the halide in the

another

In the

the and

islands

spectra.

band

early As

structure

approaching

that

of

doses.

Fig2

A

Pbl, Sd-1

a

Fig.

1. HeIa photoelectron spectra of PbC12. a) free molecule d) amorphous PbC12 c) Pb + 700-L C$* b) Pb + SL Cl Nm-'S) = l-33 x lo(ZI4 = 10'6Torg_sec

Fig.

spectra of 5d-1 ionization for 2. HeIIa photoelectron a) clean lead b) Pb + 5L 12 c) Pb + 1OL 12 d) Pb + 15L e) Pb + 20L 12(IL = 10B6Torr.sec = 1.33 x 10'4Nm'2S).

X2

183 shell

Sd

For

ionization

clean

leads

metallic

to *DS,2

On halogenation ization simple the

(figure

relative

the

in

on the halide

deduced

work

function

electronegativity be

regarded

layer

and the

layer. this

for

the

the

lead

30.4

effect

trefs.1

of dose.

ably

lower

of

possibly metal The not

eV for

reflect

values

the

should halide

oriented

halide

been

adjusted

ionization

energies

both

that

the

the

3 for

knee,

notable

as PbX2

PbC12.

The

oxidation normally

rapidly

saturate

process

laterally

I- ion has

that

and to be

of

at considerand

I 2 and

completion

indicatinq

to

into

bulk

the

reached

lead

associated

after

is taken

the most

of the

of lead

layer is formed -4 Nm-'S for Cl2 13 x 10

appears

'saturation'

percentage for

number

depth

surface

This

atoms.

the

sampling

in figure

It is also

spectra

the

the

an

indicate

most

disruptive

the

of the

metal.

rapidly effect

ab-

for

I2

on the

lattice. form

conform

in terms

of the

adsorption

to simple linear

form

PbX2 site

is largely

an Island isotherm

state

the

linear

Rather

isotherm.

through of the

physisorbed

over the a vacant

isotherm

Langmuir

of halogenation

a precursor

hopping finding

lead

because

essentially of

we have

to represent

previously

exception

the

within

is done

obtained

for Br2.

of the halide

Surprisingly

de-

surface

1 have

of

energy

0.58

These

absolute

to plot

approximately

layer

free

the

form

variable

eV and

the

The

relative

coverage

to any

2D 5,2 peaks

of a uniform

doses,

6.6 x 15 +Nm-'S sence

the

formation

of a surface

due

in table

atoms

This

to that

& 4) with

the

lead

it is possible

is similar

growth

of the

or halogenated

with

given

therefore

intensities

as a function

than

one

the

be

shift

doses.

only

These

between

this

ionization

will

0.75

elements.

potential

all

ion-

concerned.

nm photon

form

are

true

for

rather

for

continuous eV,

spectra,

that

This

of 0.92 the

the

orbitals

eV respectively.

to higher

is however

respectively.

energies

and

halides

Using of

metal

The

iodide

contact

bulk

in

level

For

'atomic' 24.64

shifted

peaks

function.

shifts

changes

Ionization

for

work

and

as the

vacuum

Sd

indicates

doses.

coverage.

chloride,bromide

metal

peaks

all

to the

shift

observed

clean

for

the

eV and

as expected

As

Sd halide

is formed

pending the

2).

from

at 22.14

are

of the

peaks plus

peaks to the

shape

shift

ionization states

relative

dihalide

of the

*D3,2

these

enerqies

is constant

lead

abd

does

it can be explained

growth

can be

and

mechanism.

explained

'physisorbed'

halogen

island surfaces before becoming at an island edge. (ref.5).

The

in terms moiecules

desorbed

or

184 T.ASLE

1

Absolute ionization energies PbX2 (surface) spectra

in eV for

the

principal

features

Pb 5d

ligand PbC12 PbBr2

8.45, 7.85,

9.54, 8.83,

10.87 10.40

Phi,

7.01,

7.97,

9.27,

10.03

25.18, 25.00,

27.80 27.52

24.30,

26.81

+

c

0

10

5 C’2

DOSE

LANGMUIRE

I

I

I

15

20

25

( 1 1 = l-33 rlc4

Nn$s)

-1

Fig. 3.

% PbC12 Pb/PbC12

as indicated as

a

in

from

function

of

the

HeIIa

5d

spectra

of

Cl 2 dose.

REFERENCES

1 A.W. Potts, P.J. Brigden, D.S. Law and E.P.F. Lee, J.Electron In Press. Spectrosc. 2 A.W. Potts and M.L. Lyus, J.Electron Spectrosc. 13 (1978) 305. 3 R.T. Poole, J-A. Nicholson, J.G. Jenkin, R.C.G. Leckey, J.B. Peel and 5. Liesegand, J-Electron Spectrosc. 15 (1979) 91, Faraday II 25 (1979) 1030. 4 D. Chadwick and A-B. Christie, J.C.S. 5 P. Kisliuk, J.Phys. Chem. Solids 3 (1957) 95, 5 (19581 78. ACKNOWLEDGEMENT We

thank

the

S.R.C.

for

financial

support.