Atomic metallic ion emission, field surface melting and scanning tunneling microscopy tips

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Atomic metallic ion emission, field surface melting and scanning tunneling microscopy tips

Vu Thien Binh, N. Garcia

To cite this version:

Vu Thien Binh, N. Garcia. Atomic metallic ion emission, field surface melting and scanning tunneling microscopy tips. Journal de Physique I, EDP Sciences, 1991, 1 (5), pp.605-612. �10.1051/jp1:1991155�.

�jpa-00246354�

LPhys. I 1

(1991)

^605-612 MM 1991, PAGE 605

Classification

PhysicsAbsmw~

61.16F-68.45-79.70

ShonCou1u1unication

Atomic metallic ion emission, field surface melting and scanning tunneling microscopy tips

Vu Thien Binh

(I

and N. Garcia _(1>2)

(1)

Ddpartement

de

Physique

des

Matdriaux(* ),

Universitd Claude Bernard

Lyon

1, ⁶⁹⁶²² Vlleur- banne Cedex, France

(2)

Departamento

de Rsica de la Materia Condenwda, Universidad Autonoma de Madrid, 28049 Madrid, Spain

(Received4 March 1991,

accepted13

March 1991)

R4sum6. L'obtention de faisceaux d'ions

(environ

^10~

ions/s) partir

d'dmetteurs ayant une di- mension

atomique

est

possible

et nous

pr6sentons

id leur r6alisation

exp6rimentale.

Ce travail repose

sur le

principe

d'une fusion de surface _sons champ une

temp6rature

d'environ _un tiers de la

tempt-

rature de fusion de volume. Ces 6metteurs

pr6sentent

une structure

pyramidale

de dimension _nano-

mdtrique

et terminde par un atome. Le

voisinage

de

plusieurs

dmetteurs, distants aussi de

quelques

nanomltres, ddfinit alors des joints de

grains

de surface. Nos rdsultats fournissent d'une part

l'explica-

tion la

possibilit6

d'obtenir _avec des pointes initialement

macroscopiques

une r6sclution

atomique

en microsccpie I eflet tunnel, et d'autre part ^ils montrent la

possibilitd

de fabriquer de manilre _con- tr6lde des pcintes avec une double,

triple,

etc... structures tdtines. Cette dtude _a dtd rdalisde _avec des

pointes

de

tungstIne

dont les

caractdristiques

sont

analys£es

_par

microsccpies

dlectronique et ionique

de champ.

Abstract. This work presents ^the

physical

realisation of metallic ion beams from atomic emitters vith _currents of

approximately

^10~ ions per ^{second. It also} puts ^{fonvard the idea of field surface} melt-

ing

at

approximately

one third of the bulk

melting

temperature. Under

cooling

this melted surfbce,

experiments

show

pyramidal

structures of nanometer dimensions

ending

in one atom also

separated

by nanometers, then

shaping

surface grain boundaries. Furthermore, this reveals whyit is

possible

to have atomic resolution in STM

experiments.

The formation of double,

triple,

etc. atomic _teton

tips

is also possible. All this is shown ty field ion and field emission

microscopies

and atomic metallic ion emission

experiments

presented here for tungsten tips.

The

possibility

of

having

sources of metallic ions with atomic dimensions has been

largely

in-

vestigated iii

but their obtention has not been

possible

until _now. We show here the

physical

realhation. When _a

large

electric field

(F)

is

applied

to a metal surface

(few

Volts per

Angstrom)

(*

(UA CNRS).

6t6 JOURNAL DE PHYSIQUE I N°5

field

evaporation

has been observed in

tips ill by using

the field ion

microscope (FIM).

At low tem-

peratures (T), liquid nitrogen (LN),

fields of 5-6

VIA

are needed to desorb the metallic ions from a

tungsten tip.

The activation effect is

negligible

at this

temperature

ⁱⁿ the

desorption

processes and also surface diffusion is small _so that the current of ions obtained is

negligibly

small

(about

one ion per

minute).

At 3t© K ion embsion is uncontrollable with

spots

over the whole surface with very low

intensity [ii,

and cannot be used _as metallic ion _sources of atomic dimensions.

Now let us think in the effect of _an electric field

applied

to _a metal surface. The most

impor-

tant

physical

^{effect is} that the metal _screens very

effectively

the

applied

field. In fact selfconsistent calculations [2] ^{show that} even for

large

fields the second

layer

of atoms from the surface does not notice the field. If the field is

positive,

the

charge

is

compressed

into the bulk

(the image plane

moves into the

solid) denuding

the surface atoms from

charge

^and

creating

a

large

surface

dipole

as well _as

polarbation

_upon which the field acts,

pulling

out the ions from the

equilibrium position

do to a new

equflibrium position

under field

dF;

and it _acts because the field b not constant any

more at the surface.

Figure

la illustrates

graphically

this effect. Its consequences are that the

binding

_energy of the atoms is weakened and ions _can be

desorbed,

but also that the activation energy of the atoms for diffusion at the surface is also reduced. This _was shown

by early

_exper-

iments [3] ^{and is} also confirmed

by

selfconsistent calculations [2].

Figure

16 also illustrates this process. ^The

binding

_energy

T(F)

and the activation energy

A(F)

under field can be written:

T(F)

=To +

E(F) (I)

A(F)

=Ao +

Ea(F) (2)

where To and Ao _are the

energies

without field and

E(F)

and

Ea(F)

the

respective

corrections for the field. On the other hand surface

melting

occurs when the diffusion coefficient is

large enough

that the atoms can move

easily

at the surface but We bulk atoms cannot. Fbrmer

analyses

on

surface

melting

and surface diffusion [4] have shown that when the surface diffusion coefficient is

m

10~~cm2 Is

the surface has melted. From

equations (I)

and

(2)

_one can see that

by applying

a

field to a surface _we have two

parmeters

to melt it: the field that is localized _on the surface and T tllat acts over the whole

body.

The

question

that we have put ^{to ourselves is: would it be}

possible

to melt a surface

by applying

a field at T much lower than the bulk

melting

on Tm, and at the _same time have a localized ion _source of metallic ions _on We surface? The _answer to this

question

is

affirmative and therefore we

present

^the

experimental

evidence.

The

experiments

_are

performed

in _an

apparatus

^{in which} a field emission

microscope (MM)

is

coupled

to a field ion

microscope.

The base _vacuum b better thant 10~~~ lbrr, and the

pumping speed

^{from the FIM}

imaging

_pres-

sure of _rare gas ^of m 10~^~ lbrr to ultra

high

vacuum pressure

(UHV)

takes less than _a few minutes.

FI

imaging

are

performed

at

LN,

and the

heating

of the

tips

to the desired

temperatures

^{is ob-}

tained

through

a Joule

heating loop.

^The

temperature

at the end of the

tip

b controlled

by

an

optical micropyrometer

with _a

precbion

of about 10 K The tungsten

tips

are

prepared

from _a

single crystal

wire with <ill>

orientation,

and _{we use} the thermal

sharpening technique

near

3000 K and in ultra

high

vacuum

(< ^10~~° lbrr)

to obtain the initial clean

tip

[6,

7j.

The _vacuum

during

^metallic ion emissions and FE h _m 10~~~ lbrr. Metallic ion spots, ^{FI and FE} patterns are

visualized

through

a

channel-plate coupled

with _a fluorescent screen; and the

images

are recorded with _a

high

sensitive video camera,

allowing

then further

image processings.

After

having prepared

^the

tip,

we

proceed by sweeping

the

parameters

^{described above that}

regulate

the process: ^{I-e- F and T. The values of F}

applied

are

positive

as to desorb ions from the surface. In

figure

2a _we

present

^the observations of atomic metallic ion emission

(AMIE)

from atomic dimension structures. The

spots

^have an

opening

of 3°

approximately

and _are well local- ized in _some

points

of the surface.

They

also move in time

diffusing

from one

point

to another. At

N°5 ATOMICMETALLICIONEMISSION 607

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~d

~

~

m#Moaf4pWlkdZWW

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~

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°~~~~~~~~~"

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~ith _an

applied

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- c

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. ©

o .

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, -

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, equi

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j

(

~ i

( _Z

(

ith an ^li.d

', z

,-

,"

E<F)

.ithout

ppli.d

Fig. ^I.

(a)

surface

layers

of atoms without field. the

equilibrium

distance isdo. ^the same

layers

but with

applied

field, where the atoms have been

displaced

outwards to a distance dF. the illustation of the

damped

field is also sketched.

(b)

Activation barrier with and without field and the

binding

_energy of _{atoms at} the equilibrium positions and at the saddle points. The parameters are those of equations

(I)

and

(2).

6lB JOURNAL DE PHYSIQUE I N°5

lower

temperatures

^the

spots

^{remain fixed but have lower} intensities. The three frame

figures

from left to

fight correspond

to different times. The values of F and T _{are m} I-S

VIA

_{and m} 1500 K and the

spot

current

intensity

is _m 10~ ions/s. With these

parameters

and

using

_a crude

theory

for ion

desorption [I]

we found that the activation energy for

desorption

is _m 1.8 eV and the

binding

atomic energy m 2.8

eY

much smaller than that of _a

plane

surface _{at zero} field which h 8.7 eV The reduction of the

desorption

barrier b due _on the _one hand _to the fact that the atom is _on the

top

^of a

protrusion

and has a smaller number of

neighbours

than those in _a

plane surface,

and on

the other hand _to the

decreasing

in the

binding

_energy due _to

F,

that _now its effect is increased

by

the smeared out of the electronic

charge

at the

protrusion

_[5j or

Smouluchowsky

effect. We also found that under the

given

field the surface diffusion activation barrier has decreased from 3 eV to 0.7 eV

by scaling

^with former

experimental

results

by

Bettler and Charbonnier [3] ^{at smaller fields}

(0.4 VIA).

This

yields

_a surface diffusion coefficient of _m 2 _x

10~~ cm2 Is

which is of the order of the estimate for surface

melting

[4]. This is consistent with _our observations and the

possibility

to obtain continuous flux of ions

desorbing

from _one atomic site _at low temperatures. ^However these values have to be

compared

with the

T~

_m 3680 K needed to melt the W Therefore _at values much lower than T~ it is

possible

to have _a

liquid

at the surface

layer,

of the _same atoms _as the

bulk, by applying

a convenient field. Thb also has _a critical range over which the

experiment

is

possible

between 1.2 and 1.6

V/fi~

The remarkable fact is that the

liquid

^is

just

at the surface

layer

because the field has no effect _on the second

layer

of _atoms.

By inserting

the

tip

in LN under the

applied

field a very fast

cooling

takes

place

that freezes the

tip

structure and prevents ^diffusion.

By

_means of

MM,

I.e.

reversing

field

polarity,

we observe the

picture

of the freezed _structure

~fig. 2b).

We find that the MM and AMIE

spots correspond

each other very ^{well in} space,

indicating

that both emissions take

place

from

protrusions

at the surface that have been created

by

the field F at

high

T due to

polarization

and

dipole

forces. These little

pyramidal protrusions,

we called them atomic teton

tips

_[6,

7j. They always

emit the ions from the top atom of the

protrusions

because the

binding

_energy is weaker

(as

dbcussed

above)

and the

effective electric field

larger.

lb check that the

positive

field beams are W ions, we have

analyzed

the

spot

^{deviations due} to the

application

^of a

magnetic

field. Our

findings

are that the electron

spots

move with thb field, but the

positive

^{electric field}

spots ~fig. 2a)

do _not. So the emitted

particles

_are

heavy positive

ions. because the current

intensity

is 10~ ions/s with the _vacuum 10~~~ lbrr and T ₌ 1500

K,

it

seems clear that ionization of residual gas ^or

impurities

is not

possible. Furthermore,

after the

obtention of the

pyramidal protrusion

structure we

applied

to the

tip, always

under UHV but at

LN,

a

positive

field of the same value or greater ^{than that needed} to see the AMIE

spots,

^and we see no

spot

at all. These

experiments

have also been

reproduced

for different W

tips.

lb confirnl further the above observations _we have the FIM _{as an} additional

technique

that is well known to have _an

extremely good

atomic resolution. FlM results for the _same freezed

structures than above _are

presented

in

figure

3a where the very

ending

atomic _teton

tips

_[6,

7j

are

observed.

By

^field

desorption experiments

at LN and

by

FIM _{we can see} the different

layers

of atoms that

configure

the atomic teton

tips

^formed at

high

T. Results of different

decaped layers

are

presented

in

figure

3b and

they

show

clearly

the

pyramidal

bash of the different

protrusions meeting

in the surface

grain boundary.

The existence of these boundaries and the

shape

^of the

tip

indicate that the surface tension of the initial

tip

^{has been}

completely changed by

the presence of the field at

high

T.

The observations desc&ed above are of considerable

importance

to understand STM exper- iments. The

question

that remains _to be answered b: how b it

possible

that with

tips

^of100 nm

radius _or

larger

can be revealed atomic resolution swucture on surfaces? After

performing

these

experiments

we believe that the _answer lies in the formation of the

pyramidal protrusion

end in g in

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a

single

atom that we have described.

Experimentalists working

in the STM field kJ1ow very ^well that before

reaching

atomic resolution different treatments have to be

given

to We

tips

[7j. ^These

treatments consbt

basically

in

establhhing

a

large voltage

between

tip

and

sample

and

passing

a

large

current

during

a short

period

^of time.

By giving

these

pulses,

a

tip

that

provides

atomic resolution _can be obtained. In our

opinion, during

these

pulses

the

tip

surface

temperature

^rakes

due to electron bombardment of the surface because of the

large

current

pulse,

but under the

pulse applied voltage

^that

give

rises to fields of _m 1.5

V/fi~

These are

basically

the _same param-

eters that _we need to form _our

pyramidal protrusions.

In

general

we form many

protrusions

over

the

tip surface,

but in a STM

configuration

^We most

probable

_ocurrence will be a

protrusion

at the apex ^{of We}

tip

because in this _case the field is localized

just

at the apex. ^In our

experiments

the field h

practically

constant over the whole surface before

forming

the

protrusions.

These

structures

ending

ⁱⁿ one atom

provide

the atomic resolution and _are the natural

topographical

configuration

after the

tip

has been

heating

under field and then fast freezed. Also Were have been STM observations that _are believed to be due to

multiple tips

[7j. ^These

multiple tips

_may

be the same as those

reported

and visualized in thin paper, ^but now

they

can be built under well controlled

parameters

and conditions. The

applicability

of _a

tip ending

in two

pyramidal protru-

sions of _m 1.5 _nm

height separated by

_m 3 nm in STM can be of

importance

to measure

dynamical

effects. For

example

coherence

length

in

high

Tc

superconductors

[8]. As _a

proof

that this _can be reached with _our field surface

melting

method _we present ^the

experimental

evidence in

figure

4, in which _a

tip ending

in two

pyramides

is

presented

and _{even one can} choose for the

pyramides ending

in three atoms or one atom.

Fig.

4. FIM of a tip ending in two atomic teton

tips

with _m I-S _nm height and separated by _m 3 nm (this is obtained by

counting

rows of atoms in the

picture).

We can make the

tip

finish in three atoms or one atom in _a controlled way. ^these

tips

_{may be of}

importance

for sTM

dynamical

experiments.

In

conclusion,

it _seems that we have shown that the obtention of atomic metallic ions'emhsion is

possible.

This is

accompanied by

the idea put ^{forward of field surface}

melting

that

gives

rhe

to the formation of

pyramidal protrusions

or atomic _teton

tips

and meet in their basis

forming

surface

grain

boundaries. These structures emit ions from a

single

atom source so that ions, _as well

as coherent electron beams, are obtained. MM reveals the

emitting

structure and the exhtence of 2-D

melting.

The obtained

tips

answer also the

question

of the

possibility

of

obtaining

atomic

612 JOURNAL DE PHYSIQUE I N°5

resolution in WM

experiments starting

with a 100 nm radius

tips

as well _as the exhtence of

double, uiple,

etc nano

tips.

Thin may ^have

important physical

consequences ^{and technical}

applications

in

nanotechnology

if _one has clear ideas of how to use thb effecL We

believe,

for

example,

that _our

source of ions _can be of

importance

for

writing

_nano and atomic

conducting

lines _on surfaces. Also

the coherent emission of _our

tips

can make

possible

electron

holography

with three dimensional atomic resolution [7j. ^{Work in this field b} now in progress.

Finally

we would like _to say ^{that We}

experiments

_are

fully reproducible.

In fact the structures obtained _are We

Wermodynamical equilibrium configuration

of _a W

tip

for We temperatures ^and fields

reported

above. We believe that other metal surfaces should show similar

configurations

and the _same idea of field surface

melting

should be

applicable.

Acknowledgements.

We would like to thank Prof. R. Uzan for his interest in thb work and J.

Doglioni

for technical assbtance to set up ^the

system.

^{This work has been}

supported by

an EEC Science

project,

and

by

the French and

Spanish

authorities. We also thank _our partners ^{of the EEC}

project

for support in the realization of these

experiments.

Dbcussions with J.J.

Saenz,

PA~ Serena, M.

Pitaval,

D.

Atlan and G. Gardet _are

apprechted.

References

[1] ^Field Ion

Microscopy,

E.W Muller and II l§ong

(Elsevier Publishing

Co. NY,

1%9);

Held Ion

Microscopy,

Held Ionization and Field

Evaporation, Pwg Su$

^Sci. 1

(1974)

1.

[2] SERENA PA., GARCtA N. and Vu _{MIEN u1NH}

(to

be

published).

[3] BBnLER PC. and OMRmNNIER EM.,_P%ys. Rev l19

(1%0)

85;

Uwuot H. and COMER R., L Chem PhyK 37

(1962)

1706.

Experiments

show that the activation barrier

A(F)

cannot be _zero

(see

^Ref. [1]). This is consistent with _our observations, because below m 1000 K _we do not _see any AMIE _even at

high voltage

due to the lack of diffusion.

[4] DA£H J.G.,

Contemp.

P%ys. ^3@

(1989)

8%

BIENFArr M. and GAY J.-M., Proc. of NATO ASI _on Phase transitions in surface Films, June 19-30 1990, Erice,

Italy.

[5j smouLucHowsxi R~, P%ys. ^{Rev 60}

(1941)

Ml.

[fl

Vu _THtEN BINH,L Mcms. 152

(1988)

355.

[7j ^{see for} example

scanning Tunneling Microscopy

^{and Related} Methods, NATO ASI Sefies E Vol 184, R.J. Behm, N. Garcia and H. Rohrer Eds.. For teton tips see page ^409.

[8] SCHRIEFFER J.R.

(Private communication) september

1990.