Copper Electrodeposition on Alkanethiolate Covered Gold Electrodes*

Zeitschrift fürPhysikalischeChemie, Bd. 208,S. 107-136(1999) © byR. _{Oldenbourg Verlag,} München 1999

Copper Electrodeposition

on

Alkanethiolate

Covered

Gold Electrodes*

By

O.

_Cavalleri,

A. M.

_Bittner**,

H.

_Kind,

K. Kern

Institut de

_Physique

Expérimentale,EcolePolytechniqueFédéraledeLausanne, CH-1015 Lausanne,Switzerland

andT. Greber

Physik-Institut, UniversitätZürich,CH-8057Zürich,Switzerland

(ReceivedDecember 1, 1997;

accepted

December23, 1997)

Cyclic voltammetry

IFilm

_growth

I

Scanning tunnelling

microscopy

(STM)

I

_{Single crystal}

electrode I

Surface

structureIThiolsI

_{X-ray photoelectron}

_spectroscopy

(XPS)

We have investigated the structure and thermal dynamics of alkanethiolate layers on

Au(Hl)with variabletemperaturescanning tunnelling microscopy(STM), X-ray

photo-electronspectroscopy (XPS)and voltammetry.Theresults build the basis fora studyof

electrodeposition of _copper on alkanethiolate-covered _Au(lll).

Electrodeposition

has been studied as a function ofthe thiolate chainlength, thedeposition

potential

and the

temperature. Time-resolved in situ STM and _voltammetry, both at _{temperatures up} to

345K,andXPS of emersedsamplesshowed that_coppercan

—

depending

onthe

deposi-tion potential

—

eitherformnanometer-sizedislands orlayers, both withoutdestruction ofthe thiolate. We proposeamechanism where_copperpenetratesthe thiolatelayer. The

slowdeposition rate isdetermined

_{only by}

kineticfactors since Cu/Cu2+ exchange

pro-cessescannotoperate.

_Finally

wediscuss therole of thiolatesaspreadsorbedsurfactants.

1. Thiolate

_layers

and the

_{electrodeposition}

of metals

Certain chemical elements or

molecules,

especially

soft bases such _as

iodide, bromide,

sulphide,

carbon monoxide and

_cyanide,

areknown tobind with

_exceptional

_strength

to_{noble metal surfaces. In electrochemical terms,}

the substances adsorb

_{specifically.}

Such

_adlayers

are often

densely packed,

hence the atoms or molecules

prefer (quasi)-hexagonal

_arrangements

[1—7].

* _{Presented at the 5. Ulmer Elektrochemische Tage on "Fundamental Aspects of}

Electrolytic

Metal

_Deposition",

June23-24, 1997.

**

The

_adlayer

_geometry is determined

_by

the

_interplay

of

adsorbate-sub-strate forces that tend to direct the

adsorbing

atoms to

places

that offer a

strong

bond

(often

hollow

sites)

and interadsorbate forces thatcan_be

attrac-tive or

repulsive.

More

complex,

but also muchmore

versatile,

are thiolate

adsorbates. Alkanethiols

CH3(CH2)„ iSH

('C„

thiols')

form

_strongly

adsorb-ed,

ordered thiolate

_layers

CH3(CH2)„_,S/substrate [8-10].

Although

the

adsorption

process does not differ

_principally

from the above-mentioned

cases,theterm

_{'self-assembly'}

is

_employed

to

point

outthat thiolate interad-sorbate forces are _very

_strong

—

long adsorption

times are _necessary to assemble an ordered structure where the

_alkyl

chains

_align

_parallel.

_Up

to

now,

mainly

the

Au(lll)

substrate was

used;

its reconstruction is

readily

lifted

_during

the

_adsorption.

Several studies of vacuum metal

deposition

on thiolate-covered metal

substrates wereconducted;

Jung

and Czanderna

compiled

a_{valuable review}

[11].

The

_{electrodeposition}

of

metals,

e.g. copper, onthiolate

layers

is

tech-nically

much

_simpler,

but moredifficultto

interpret

: onehastoconsider the electrode

_{potential. Especially}

the Cu2+/Cu

_equilibrium

(Nernst)

potential

is

important.

Positive of that

_point,

copper can

only deposit

in a

single layer

and

_only

on some

special

substrates

(underpotential deposition,

UPD) [12—

17].

Negative

of

it,

normal copper bulk

deposition

takes

place

on _any

sub-strate

(overpotential

_deposition,

OPD).

Electrodeposition

of Cu2+ in the OPD range often results in 3D

_growth

_{of copper}on_{metal substrates} as_e.g.

found with in situ STM

[12a, 18-23].

A similar situation shows _{up when copper is}

_deposited

on

alkanethiol-ate-covered

Au(lll)

deep

in the OPD range:

large

copper nodules with diameters in the 100nm _range

develop

[24, 25].

In contrast, we worked at moderate

_{overpotential}

which results in a

complex

scenario that can be

investigated

with in situ STM

[26—29].

We found a

pseudo-layer-by-layer

growth

[27],

i.e. one

layer

is almost finished before the next one starts to

grow. Such a behaviour is very rare on bare metalelectrodes

(some

excep-tions of this rule were found with in situ STM

[12a,

19,

30-32]).

On the

other

_hand,

it is well known that a _{metal surface covered}

_{by strongly}

ad-sorbing

substancescanindeed incite such a2D

growth.

Such substances are

called 'surfactants'; in the

simplest

case, one of the above-mentioned

strongly

adsorbing

atomic ions is a

surfactant,

as known for the

elec-trodeposition

of silver on iodide-covered

Pt(lll) [33].

Usually,

the metal

penetrates

the

_{halogenide layer;}

the

_topmost

structure can be addressed as a

metal-halogenide

coadsorbate

[17, 33-36].

In the presence of a

large

surfactant

molecule,

_crystal

violet,

in the

_electrolyte,

an STM

study

indeed

proved

the 2D

_growth

scenario

_[20,

22].

This situation is

_already

close to

the technical

_application

where

_{'brighteners'}

are added _to metal

plating

baths to

yield

smooth

deposits.

In _{the UPD range where island formation} on bare metal substrates is a rare

_{phenomenon (but}

see

[37]

and

[38]),

we were able to

deposit

copper

Copper ElectrodepositiononAlkanethiolate Covered Gold Electrodes 109 islands with diameters in the nm _range on _{alkanethiolate-covered}

Au(lll).

This process is a

relatively simple

method for

building

nanometer-sized

metal structures

[29].

An often

_neglected

parameter in electrochemical STM studies is the

temperature

on which we shall concentrate _{in this paper. As} we will show

in section

3,

heating

induces several

_stages

of disorder in

_thiolates,

pro-gressing

from the

_(methyl)

end group towards the substrate.

First,

the

methyl

rotation is activated atT& 100 Kas revealed

by

helium diffraction

studies

[10].

Above 200 K the infrared

_probing

ofthe

_{méthylènes'}

scissors vibration

_suggests

_{progressive unlocking}

of the

_alkyl

twist

_(a

rotation around either anS—CoraC-C

bond)

and

_development

of

_gauche

defects in the otherwise all-trans

_configured

chains

[39].

This_{behaviour compares}

well with that of self-assembled thiolates on

gold nanoparticles

[40]

and

that of

_melting

bulk alkanes

_[41].

_Heating

above 300K as we used for this

study

can even affect the Au—S bond: boundaries between different ordered

thiolate domains move or

vanish,

mass

_transport

of the substrate occursand

finally

leads to the

_healing

of the

_typical

substrate vacancies. These

vacan-cies are defects

appearing during

the

self-assembly [42—45].

A discussion of the

_{phenomena occurring}

_upon

_heating

thiolate

_layers

(see

section

3.2)

will

_provide

a valuable basis for

understanding

our most recent

_experiments

where we followed the

electrodeposition

of _copper at elevated temperatures with in situ STM

(see

section

_4).

Electrochemical STMs were_uptonow never

operated

above 300Kalbeit

deposition

studies

should be of

_greatest

_importance

for the

_{understanding}

of technical metal

plating

which is

_mostly

carried out well above 300K. We will also show that the

_temperature

can_{be used}as anextra

_parameter

for

_tuning

the copper

as well as the substrate structure

(see

section

4.3).

Encompassing

our

ex-periments

at 300K

_[26,

_27,

_29]

and therecent

results,

we will focus on the

structure ofthe

_deposited

_copperin section 4.2 and

_{additionally analyse}

the

extremely

slow

_deposition

kinetics insection 4.4.

Our

_experimental

methods are in situ electrochemical STM at _{300 up} to 345

K,

STM in

_nitrogen

at _{300 up} to 370

K,

cyclic voltammetry

and

X-ray photoelectron

spectroscopy

(XPS)

of emersed

_{samples. They}

will be

explained

in the

_following

section.

2.

_{Experimental techniques}

The

_{gold samples}

were

prepared by

evaporation

of 120to 150nm

gold

on

570K hot cleaved mica in a 10~5 mbar vacuum and then annealed for

several hours at —600 K

(2

• 10~6

mbar).

Directly

before

_starting

the

self-assembly,

the

_gold

films were flame annealed at _{very dark red}

glow

and cooled in ethanol

_(p.a.,

Fluka).

For the

_{self-assembly,}

the

_samples

were

"beetle" STM

/

Fig.1. Schematicdrawing of the electrochemicaltemperature-variable STMsetup.

CH3(CH2)„_,SH

with n =

6, 8,

10, 12, 14,

16 and 18. Forthe

experiments

carried out under

_nitrogen,

the

_samples

were

kept

at 300 K for at least

one

_day.

For the electrochemical

experiments,

the

following self-assembly

programwas _used: _{twohours 300}

K,

_two

_days

ca. 325

K,

thereafter several more

days

(or

inrare cases

weeks)

300K.All

samples

were

emersed,

rinsed

with ethanol and

_quickly

used. STM measurementswere

performed

as

de-scribed in

[29].

As shown in

_Fig.

1,

a new feature is a Peltier element

(operated

in the

_{heating configuration)}

that we

_coupled

with a thin copper

spacer to the bare side ofthe mica

_sample,

thus

_attaining

a _stable

_sample

and

_electrolyte

_temperature

_(less

than 1 K

_change

_per

hour).

The

gold-covered sidewas in_contactwith the

electrolyte,

50mM

H2S04

_(p.a.,

Fluka)

+ 0 or 1 mM

_CuS04

_(p.a.,

Fluka),

_either in _water

(Millipore®)

_or

—

for the

_{high-temperature experiments}

—

in 2:1

_HOCH2CH2OH

_{(1,2-ethandiol,}

ethylene glycol,

p.a.,

Fluka): H20.

Ethylene glycol

very

effectively

_stops

the otherwise

_quick

_evaporation.

The reference was a _copper

wire,

etched

in

HN03

and rinsed withwater

_shortly

before use.The

sample

was_mounted

in a PCTFE STM cell

equipped

with a Ptcounter electrode. For the XPS

sample

preparation

as well as for the redox

pair

_{measurements,}

exactly

the same

_set-up

wasused. The redox

electrolytes

contained

Ru(NH3)6Cl3 (Alfa)

or

K3Fe(CN)6 (MicroSelect,

Fluka)

with KC1

(p.a.,

Fluka)

and HC1

(p.a.,

Fluka)

in

_ethylene

_{glycol/water,}

andan

Ag/AgCl

referencewas

dipped

into

the STM cell. All

_potentials

are

quoted

with

_respect

to the Cu/Cu2+

_pair

in 1 mM Cu2+

(i.e.

0 mV read +250 mV on the standard

hydrogen

scale and

+50mV on the

Ag/AgCl

(KC1 sat.) scale).

We

always employed

an EG&G

PAR 400

_{potentiostat.}

The

_potential

_{range above 500}mV and below —200 mV was

usually

not accessed in order to avoid oxidation and

im-Copper ElectrodepositiononAlkanethiolate Covered Gold Electrodes 111

ages are shown derivatised: _steps

_descending

from leftto

_right

show up as

black

lines,

ascending

ones as white lines. The

_{X-ray photoelectron}

_spectra

have been

_performed

in a _{VG ESCALAB 220. After} removal from the

liquid

we mounted the

samples

with electrical contactand introduced them via a fast

_entry

lock into the vacuum

_system.

In order to minimize

_X-ray

tube induced

_damage

the

_spectrometer

transmission was maximized with

lowest

_angular

resolution andanoverall _energyresolution of 1.4 eVFWHM onthe Au

4f7/2

peak

at84.0eV

_binding

_{energy. The}

_X-ray

twin anode that

provides

non-monochromatized

_Mg

Ka

(1253.6 eV)

radiationwas retracted

and run with 140W

input

_power. With

_respect

to the maximum flux these

settings

lower the

_resulting

electron flux from an _aluminum

sample by

a

factor of0.3 to 15 nA/cm2. The emission

_angle

was 0°

(normal emission)

and 80°

_(grazing

emission).

Fromthe

_comparison

ofthetwoemission direc-tions the vertical structure of the chemical

_composition

of the

(Cu)/

CH3(CH2)„_,S/Au(lll)

system

canbe inferred.

3.

Alkanethiolate

_layers

on

Au(lll)

Inorderto

develop

abasis for

understanding

the

complex

copper

_deposition

process we shall here

_present

studies of alkanethiolate

layers

on

Au(lll)

under vacuum, in

nitrogen,

air and immersed in

electrolyte,

all in absence

of copper.

3.1 Structure and

_morphology

Let us first focus on the chemical

composition

of alkanethiolate

layers

on

Au(lll)

which can be detected with XPS. In

Fig.

2 the overview _spectrum

of

CH3(CH2),7S/Au(lll)

(sample

A)

is shown for normal emission. The

spectrum is dominated

_by

the strongemission from the

_gold

substrate and the C Is

_peak.

It can _be seen _{that the}

samples

_remain almost oxygen free

although they

have been

_exposed

to air formore than 100s. In all

details,

such a

_spectrum

_is

typical

_{of the}

thiolate/gold

_system

[46, 47].

In

_Fig.

3 the Au

_4f,

C

Is,

O Is

_(and

for

_comparison

Cu

_2p3/2)

XP _spectra

are _{shown for}

_sample

A. The Au

4f7/2

peak

is set to 84.0eV

_binding

_energy. TheC Is

_peak

is foundat 285.0 eV and

_corresponds

tothat of

polyethylene

chains

[48].

We show aswell the

_spectra

from

grazing

(80°)

emission

(data

for carbon and oxygen are not shown since the

_sample

holder contributes

to _{the oxygen and} some of the carbon

emission).

As the

intensity

from

sulphur

(not shown),

the

_intensity

of the

_{gold signal}

is

_{strongly suppressed}

;

this is observed in presence

(as

in

_Fig.

₃₎

and absence

(not

shown)

of copper. The attenuation

by

more than one order of

magnitude

can

only

be

explained by

a

layer

model in which the thiolate covers the

gold

and binds

-1-1-1-1-Au 4) SampleA Au4d Au4f Au4p Cu2p 01s , CuLMM

-~U,

1000 800 600 400 200 0

ElectronBindingEnergy (eV)

Fig.2. Normal emission XPS ofaC18thiolatelayeron Au(lll); X-ray source Mg Ka.

Sample A_corresponds to an untreatedsample, while on sample _{B copper} was

electro-chemicallydeposited.

In

_Fig.

4 the

_grazing

emission

_intensity

of the C

Is,

the Au 4fand the Cu

_2p3/2

levelare_shownas afunction of exposure timetothe

_X-rays.

While the carbon emission remains constant, the

_gold

and _{the copper emission} increase with the exposure time. For

samples

without copper

deposit

we do

observethesame

aging

kinetics

(not shown).

This variation

clearly

indicates a

change

ofthe

sample.

The

overlayer

_{appears to} become more

_transparent

tothe emission from the substrate. The

_gold

andcopperintensities

_perfectly

fitafirstorder kinetics ansatz I «

₍₁

—

ße~"T)

where

_ß

> 1 isa

sensitivity

factor and where x is the time constant. Within the

_uncertainty

of the

data,

the

_sensitivity

_ß

and the time constant x are

independent

whether copper

has been

_deposited

on the

_sample

or not. The

_sensitivity

ß

is 0.45 ±0.07 for copper and

gold,

x is

inversely proportional

to the

_secondary

electron flux from the

_sample.

From the value of x of 2

•

104s and the

secondary

electron emission flux we find that the

secondary

electron current

_density

from an aluminum

sample

times the time constants is in the order of 2• 1015 e"/cm2. This indiates that the thiolate films are

very

susceptible

to

ionizing

radiation as also

reported by

Jäger

etal.

[49]

(other

_{groups found}

much

_larger

timeconstants

[50]).

Turning

the attention towards the interfacial structure

probed by

STM,

let us first note that the

tunnelling

currents

employed

(some

tenths of

nA)

are

essentially

sensitive _tothe

sulphur

_atom

—

the

_{tip easily}

_penetrates

the thiolate chains which cannot offer local electronic states _{in the proper}

Copper ElectrodepositiononAlkanethiolate Covered Gold Electrodes 113

(Fig.

5),

can _thus be ascribed tothe

_sulphur

atoms,

yielding

a _{coverage of}

0.33. Note that thesamestructurewas found for_a

sulphide

_adlayer

_adsorbed on an

Au(lll)

electrode and for

S/Au(lll)

in vacuum

[7].

However,

we detected

_{'super-superstructures',}

_{e.g. the well-known centered}

_(4X2)

distri-O) 'c 3, c O)

J:

0 0 10 20 30 40

X-ray

exposure-t¡me(103

s)

Fig.

4. Grazing photoelectron emission intensities ofC Is, Au 4f and Cu 2p3/2 (C18 thiolate)as afunctionof_exposuretotheMgKoX-raysource.The solid lines in the data of copper andgoldarefitsto first order kinetics.

a)

b)

Fig.5.Molecular resolution STM

_images

takeninnitrogen

showing

thec(4X₂₎

_pinwheel

(a)and the_c(4X₂₎

_zig-zag

_{(b) superstructures}on a_C,0thiolate

_{sample. Tunneling}

param-eters: (a) I = 1.0nA, V = 1000mV; (b) I = 0.6nA, V = 1000mV Image size:

3.5 nmX3.5nm.

bution

_(Fig.

5)

of the

₍₇₃

X

73)R30°

elements

_(in

_{proper notation}

(3X273))

[10, 52-55].

At lower coverage

(attainable

by

vapour

phase

dosing

of thiols

[56]

or

by

_heating

_{above 370}K

[57, 58])

_{and for short}

i i i

i-C1s

CopperElectrodepositionon_{Alkanethiolate Covered Gold}Electrodes 115

Fig. 6. STMimageofaC10thiolate-covered Au(Hl)surface in air. The dark linesare

thiolate domain boundaries which are pinned by the substrate vacancies (dark spots). Tunnelingparameters: I = 0.3nA,V = 900 mV. Image size: 320nmX320nm.

chain thiolates we detected

striped

structures

[58]

(see

also

[56,

57, 59,

60]).

Allthese structurescanexist in different domains

separated by

domain

walls. These walls show up as dark grooves of

roughly

half a

monolayer

depth

and 1 to 3nm width. In

Fig.

6 the walls often extend between

neigh-bouring

substrate vacancies

_(large

dark

_spots),

but

_they

can also terminate

at other walls or at

_steps.

_{During heating}

in

_nitrogen,

the number of grooves in an STM

image

diminuishes,

i.e. the average domain size in-creases

[61].

Further

_healing

_processes that can be followed involve Ostwald

ripen-ing,

coalescence and annihilation

_(at

_steps)

ofthe vacancies

_{[43, 45].}

The latter are ascribed to

_etching

_processes

_{[62, 63]}

and the

_lifting

of the

Au(lll)

reconstruction and concomitant surface diffusion

_during

the

self-assembly

process

[64, 65];

their surface is covered

by

thiolate as are all

otherterraces

[58,

66,

67].

Our latest

_experiments

show that the

₍₇₃

X

_73)R30°

_lattice,

domain walls and vacancies arefoundalso when the

samples

areimmersed in

elec-trolyte

and

_kept

under

_potential

control

_(in

situ

STM).

When we use the

modified

_electrolyte

basedon an

ethylene

glycol/water

mixture,

we can now

additionally

scanthe surface at elevated

_temperature

[28].

As visualized in the

_image

series of

_Fig.

7,

depicting

a _{vacancy that is}

rapidly

filled with

material from a

nearby

_step, we found

highly

mobile surfaces

already

at 335 K. The surface

_appeared

to be moremobile than in

nitrogen, probably

a result of the electrochemical

polarization.

Some further

_{properties, especially}

the

_ability

to transfer electrons

through

athiolate

layer,

canbe

probed

with

cyclic

voltammetry.

As

electro-lytes

we

routinely

used

_sulphuric

acid either inwater orin

ethylene glycol/

water. In

_passing

we note thatwe

proved

that

ethylene glycol

does neither

affect the

_{electrochemistry}

of

thiolate/Au(l 11)

nor that of the copper

re-ductionat

thiolate/Au(l 11)

and bare

_{Au(lll) [28].}

The

_{voltammograms}

are

typical

for nonreactive surfaces since the double

_layer

current is smaller than forabare metal surface. In

general,

the currentfor short chain thiolates

is

_larger

than for

_long

chain thiolates. At

_higher

_temperatures

the current rises

_{considerably.}

Thus electrons canbe transferredacrossthe thiolate

eas-ier when the chain is short and the temperature

high.

However,

one should

keep

in mind thatthe double

_layer

could have a structure andthus

_capacity

differing

from thaton _bare

_gold.

Hence we also

probed

the surfaces with a redox

pair, Fe(CN)s/4".

The

voltammogram

in

_Fig.

8 reveals the

_typical

current waves that result from

kinetic and diffusional

limitations,

but with less

_peak

current than on bare

Au(lll).

Wefound the samebehaviourasfor the double

layer

current

men-tioned above: the redox waves' current rose at

high

_temperatures

and for short chain

_lengths.

Note that the

_temperature

behaviourcan be more

com-plex

[68].

Experiments

with

_Ru(NH3)i/2+

_normally

showed a much

higher

current ;

however,

thesame

dependence

onthe thiolate chain

length

is found

(Fig.

9).

To assess the

findings

we note that the diffusional limitation must

be the same at bare and thiolated

Au(lll).

The rate of the slowest

_(rate

determining)

process

responsible

for the thiolates'

blocking

behaviour can

be the

_charge

transfer across the thiolate or

possibly

a

penetration

of the

complexes

into the thiolate.

In the

_following,

we shall

interpret

the structure and

_morphology

of alkanethiolate-covered

_Au(lll)

at 300K. We will progress from a

single

molecule

(sub-nm)

to the

_mesoscopic

scale

_(pm).

_First,

ourXPS data

con-firm the

_accepted

structure model of a

gold

surface covered

by

sulphur-bound thiolates. The small oxygen contamination is

likely

due toair

expo-sure. It _{may consist of adsorbed oxygen}atoms

(e.g.

atthiolate-free

_defects)

and/orwater

_(difficult

toremove

completely

invacuum

chambers).

However,

oxygen isaminor

species

on a_thiolate

surface,

evenafter exposuretoair.

Fig.7.In situelectrochemical STM imagesequence(time separationca. 1 min)

showing

the temperature induced

_mobility

ofaC6thiolate/Au(lll) interfaceat335 K.Electrode

potential:

300 mV vs. Cu/Cu2+, i.e.

positive

from the UPD _range, thus the surface is

copper-free. Electrolyte:

50mM H2S04 + 1 mM _CuS04 with the solvent

_being

a 2:1

mixture ofHOCH2CH2OH:H20.Tunnelingparameters:I =0.5nA,V = 120 mV.Image

CopperElectrodepositiononAlkanethiolate Covered Gold Electrodes 117

a) 100 -, 50

"s

< ° 3. '" -50 -100 b) 1 0 CM B -1 U < -2 3. - -3 -4 -5 O 200 400 E(mVvs.Cu/Cu2+)

Fig.8.Cyclic voltammograms foranAu(lH)electrodeat 300K(a),fora_C16

thiolate-covered_{Au(lll)electrode}at_{300 K}(b,dashedline)andat335 K(b,solidline).

Electro-lyte: 1000mMKC1+ 1 mMK,Fe(CN)6with the solventbeinga2:1 mixture of HOCH,

CH2OH:H20. Scanrate: 50 mV/s. Potentials weremeasured vs.the

_Ag/AgCl

electrode

and are reported vs. a hypothetical Cu/Cu2+ (1 mM) reference electrode for an easier

comparison

with the

_previously

shown voltammograms.

Extending

the view from a

single

_moleculeto

neighbouring

molecules,

we note_{that the coverage of}

_sulphur

atoms

_(and

thusof the

alkanethiolates)

is in most cases

1/3;

the

sulphur

atoms _occopy a

(73

X

73)R30°

lattice

(see

above).

Exceptions

are encountered for the short-chain thiolates

(e.g.

n = 6 where this lattice and _a

striped

_structure

coexist)

_or for the low

coverages attainable either for

samples

annealed above 370 K or

by

vacuum

deposition

of thiols. As we

already pointed

out in section

1,

the

(73

X

73)R30°

_geometry

is also found for the

S/Au(lll)

_system.

This and other

_{(quasi)hexagonal}

structures

_(e.g.

found when soft bases are adsorbed

at noble metal surfaces; evenwhen the substrate structureis not

hexagonal

[1, 2])

are

typical

for abalance betweenadsorbate-substrate and

adsorbate-adsorbateinteractions.

Clearly,

this balance has to be

_additionally

affected when

_alkyl

chains interact with each other. It is well known that this results in the all-trans

CopperElectrodepositiononAlkanethiolate Covered Gold Electrodes 119

_I_I_I_L

-300 -200 -100 0

E(mVvs.Cu/Cu2+)

Fig. 9.Cyclic

voltammograms

for a C10thiolate (solid line) and a C,4thiolate-covered

(dashed line) Au(lll) electrode at 300K. Electrolyte: 50 mM H2S04 + 1 mM Ru(NH3)6Cl3. Scanrate: 50 mV/s. Potentials were measured vs. the Ag/AgCl electrode

and are reported vs. a

hypothetical

Cu/Cu2+ (1 mM) reference electrode for an easier

comparisonwith the

_previously

shown voltammograms.

| Au OS • C

Fig. 10. Models ofthe CH3(CH2)9S/Au(lll) interface: (a)

alkyl

chains in the all-trans

conformation,(b) alkyl chainspartially ingaucheconformation.

configured alkyl

chains

_being

_{tilted away from the}surfacenormal

_by

about 30°

_(see

_{Fig. 10(a)).}

The azimuthal tilt direction lies betweenthe directions of the next and nearest next

sulphur neighbours.

The above-mentioned

c(4

X

2)

_{lattice appears}tobe basedon anordereddistributionof

alkyl

chain

twist

_{angles (presumably}

a twist around the S—C

axes);

structures based

onordered distributions of

alkyl

twist

angles

exist also in bulk alkanes

[4].

60

%

o < 3 -60 -120 -180

We can conclude that the _{thiolate structure is} based on the _strong Au—S

bond;

the maximum coverage is

governed by

S —S and interchain interac-tions

—

each one exhibits an _energy

minimum,

and their

interdependence

yields

an overall minimum with the

optimum

_geometry

described above.

The

_{densely spaced alkyl}

chains form an effective spacer

layer

that blocks

electron transferas evidenced

by

the voltammetric data.

On the

_length

scale of 10nm,we find the groovesthatform the

bound-aries between

_differently

oriented molecular domains.

_{Unfortunately,}

it is

impossible

todeducea structural model for them:

firstly,

_{the mechanism of}

tunnelling

on thiolates is still under

debate,

hence the

depth

is difficult to

interpret; secondly,

molecular resolution of the grooves is very difficult. The

_simplest

idea is an

antiphase boundary

where the S —S interdomain

distance is

_larger

thanthe intradomain distance of

_7?

atomic

_gold

distances,

but also

_missing

rowsof thiolate molecules have been

suggested

[55].

Finally

let us

shortly

focus on the

mesoscopic

scale

(pm):

this is the

typical

length

of

_gold

substrate

_steps.

_Very

_long

and

_straight

_steps

are

pre-sumably

due to the

_healing

of volume defects

_during

the

_gold

flame

an-nealing (gliding along

internal

₍₁₁₁₎

_planes);

hence

_they

can cross each

other. Shorter and more round

_steps

should

develop

via

_transport

_processes

during

gold

flame

_annealing

and

_quenching.

3.2 Thermal

_dynamics

Substantial

_changes

of the surface

_morphology

manifest themselves in

_step

movements whichcanbe ascribedtothe diffusion of

_gold

adatomsor

single

atom vacancies. Indeed we use the

smoothening

effect ofsurface diffusion

in both the vacuum

heating

and flame

annealing processing

of our

gold

samples.

Naturally,

such processes are

thermally

activated. On

thiolate-covered

Au(lll),

theirrate is much

_higher

than on _bare

Au(lll)

_since the

Au—Au interaction is weakened

_by

the

_strong

Au—S bond. Substrate atom diffusion was also recorded for

_similarly

strongly

bound

halogen

_atoms on

Au(100) [69a], Au(lll) [69b], Au(110] [70]

and

Pt(110) [71, 72].

High

temperature

dosing

of

_halogènes

can even

produce virtually

defect-free

sur-faces

_{[2, 73]}

as can

prolonged annealing

of thiolate-covered

Au(lll)

[42].

For this

_{annealing procedure,}

the vacancies and

_step

_edges

move

by

dif-fusion of

_single

substrate atom vacancies

[26, 45].

This leads to Ostwald

ripening

before the vacancies reach each other

_(vacancy

coalescence)

and

finally

other

_steps

_{(vacancy annihilation).}

We can _{group all such} events under the

_{heading 'strongly}

activated': the

thermodynamic driving

forces

have to be so

large

that

they

result in

desorption

of more

weakly

bound

adsorbates. An

_example

for a

driving

force is the

_temperature

that should

be above ca. 800 K for

gold

flame

annealing

(acting

to desorb

impurities

and

_resulting

in

_large

scale

_step

_movements)

oraboveca. 340 K for vacancy

Copper ElectrodepositiononAlkanethiolate Covered Gold Electrodes 121

Another

_example

for

_'strong

activation' is the electrode

_potential;

changing

it to either more

positive

or more

negative

values can result in

desorption

of adsorbed substances and surface diffusion. In _{most cases,} an

electron is transferred from orto the adsórbate

_during

the

_{desorption. E.g.}

only

below -1100 mV thiolates desorb from

Au(lll) [74, 75],

demon-strating

that

_they

bind

_strongly;

below

—

500 mV

_sulphide

desorbs from

Au(lll) [7];

below —300 mV iodide desorbs from

Pt(110),

causing

surface

mass

_transport

_{of the substrate}

[72]

_{; —200}mV _{sufficetodesorb the weaker}

bonded

_sulphate

ionfrom

_platinum

surfaces

_[76]).

In the

_{positive potential}

range, the situation can be muchmore

complex;

often an adsórbate is first

oxidized and the

_product

can

desorb,

_{e.g. alkanethiolates} are oxidized to

alkanesulphonic

acids. _{In any} cases,

changing

the

potential

will

supply

the

CH3(CH2)„_,S/Au(lll)

interface with additional electrons

(or

draw elec-trons from

_it)

and

_thereby

weaken the Au—Au and Au—S interactions. The firstcaseleadstosubstrate

_mobility,

the lattertoadsórbate surface diffusion

or

desorption.

However,

the

_analogy

to

halogenide

or

sulphide

adlayers

reaches its

limits when we consider that in the course of surface diffusion not

_only

Au—Au,

Au—S and S —S

interactions,

but also interchain forces come into

play. Clearly,

for

_longer

_alkyl

chains

_they

are

_larger

_{and present} an

ad-ditional

barrier;

hence the mobilities will be lowered

(as

observed

_[43]).

Whether thiolate molecules remain bound to a

moving

gold

adatom or_not,

during

a diffusion event interchain van-der-Waals bonds are broken and

reestablished. Thus the thiolate has toreorient which canentrain formation ordestruction of the all-trans conformation and

_change

of azimuth and twist

angle.

At

_temperatures

above ca. 330

K,

a

large

part

of the thiolate molecules

may exist in an

energetically (slightly)

disfavoured state such as distorted twist

_angles

or

gauche

conformations; the

sulphur

atoms remain in a fixed

position.

Let us name such a situation

'moderately

activated'.

Locally

the

effective thickness will be

_lowered,

i.e.

_'spots'

appear in the otherwise dense 'forest' of

_alkyl

chains

_(see

_Fig.

_10(b)).

Such

_'spots'

are not to be confused with

_missing

molecules

(which

are rare ifthe

_{self-assembly}

was

given

sufficient

_time).

Here an ion in the

electrolyte

can

approach

the

gold

surface much more

closely.

It can

thereby

at least

partially

_penetrate

the

thiolate,

facilitating

anelectron transfertoorfrom the

gold

surface. For this reason we detect

higher

double

layer charging

or redox currents when we

increase the

_temperature.

The currents also increase when we use shorter

alkyl

chains

—

in thiscase,the molecular spacer

layer

becomes

thinner,

and

anioncan

(even

without

penetration) approach

the

gold

surfacemore

close-ly

than in the case of

long

chains

(see

Fig.

9).

We note that

_'moderately

activated' processes cannot be

_brought

about

_{by potential changes}

—

this

-100 0 100 200 300 400 E(mVvs.

Cu/Cu2+)

Fig.11._{Cyclic voltammograms}forCl()thiolate-coveredAu(l 11)at300K(a) and 335 K

(b)

showing

the temperature

dependent

blocking behaviour towards copper

deposition.

Electrolyte:

50mM H2S04 + 1 mM _CuS04 with the solvent being a 2:1 mixture of

HOCH2CH2OH:H20. Scanrate: 10mV/s.

500 mV and -200

_mV)

without noticeable

_changes

of surface

_properties.

In other

words,

in this range the

CH3(CH2)„_,S/Au(lll)

system

is

_ideally

polarizable.

4.

_{Copper electrodeposition}

on

alkanethiolate-covered

Au(lll)

4.1 From UPD islands to OPD

_layers

We

_begin

the

_presentation

_{of the copper}

_deposition

studies with the voltam-metric

_experiments.

Here

_voltammetry

is not

simply

an

analytic

tool,

but

also the methodtoachieve

potential

control of the

_system

(for

the STM and XPS

_experiments,

we started at a

potential

of 400 mV where copper

de-posits

neither on bare nor on thiolated

gold,

then lowered the

potential

in

steps

of

20,

50 or 100

mV).

The

_{voltammograms}

of thiolate-covered

Au(lll)

samples

in contact with Cu2+ in

_electrolytes

basedonwateraswell as on

ethylene glycol/water

are

usually

featureless with low double

layer charging

currents

_(see

Fig.

11).

CopperElectrodepositiononAlkanethiolate Covered Gold Electrodes 123

In the UPD

_regime,

i.e. above the Cu/Cu2+

_{equilibrium potential}

of0

_mV,

no features were found

either,

albeit in some cases _very shallow

peaks

as-signed

to_{copperUPD showed up.} This behaviour reflects acertain

sample-to-sample variability

of thiolate

_layers

as also

_{reported by}

other groups

[24, 77].

Below 0mV the current

drops slightly, pointing

towards copper

deposition

at amuch reducedrate

_compared

to a bare

gold

substrate where

currents

_drop

toseveral ten

pA/cm2

_(compare

_{e.g. the}currenttransients for copper

deposition

on

Au(lll) [78, 79]).

Rising

_the

_temperature

_increases

the double

_layer

current and reveals small UPD

_peaks

for all

_samples

_(see

Fig.

11).

A reasonable

_explanation

is the

_high

_temperature

defect structure of the thiolate

(see

section

3.2)

that should accelerate the reduction of Cu2+. Faradaic and

_charging

currents also increase with

_decreasing

thiolate chain

length.

These

_findings

_{compare well with the behaviour in}

_copper-free

elec-trolytes

(see

section

3.1):

higher

temperatures

and shorter

_alkyl

chains

gen-erally

increase the current, whether it is due to

_charging

or Faradaic

cur-rents

—

in both cases electrons have tobe transferred

_through

the thiolate to the

adjacent electrolyte phase.

After

_regulating

the

_temperature

and

_setting

the

_potential

to a fixed

value,

the surface structure can _{be followed time resolved}

by

_insitu _STM.

We now have to consider three

_parameters

_influencing

the

_morphology:

temperature,

potential

and thiolate chain

_length.

In a certain range of the

parameters

(UPD

for all thiolate chain

_lengths

andtemperatures; OPD

_only

forn > 12 at lower

_{temperatures,}

seeTable

1)

the

_system

showed a

surpris-ing

topography:

monolayer

high1

islands with an _{average diameter of}2nm cover10to 15%of the surface

_(Fig.

12).

They

are

randomly

distributedand

also found in the substrate

vacancies;

they

are not

_pinned

at

_steps

or at domain boundaries. The _{initial nucleation of these islands couldnot}be fol-lowed. Further

_{growth proceeded}

eithernot or

extremely slowly

atless than 0.7 nm in two hours.

_Apart

fromthe fact that island formation in the UPD range is a rare

phenomenon

(but

see

[37]

and

[38]),

the observed islands

have the

_unique

property of

_changing

neither size nor

_shape

_for

_hours,

i.e.

they

can

truly

be addressed as stable or at least metastable nanostructures.

At all other conditions

(OPD

forn < _{12and OPD for all n at 345}

_K)

_a

pseudo-layer-by-layer growth

was found

(Fig.

13):

islands _{grow until}

_they

coalesce.

_Only

islands

_already

nucleated in theUPD

_regime

_grow,no_further

islands nucleate. In _{electrochemical terms, the nucleation is}

_{instantaneous,}

not

progressive,

in contrast to _copper

deposition

on bare

Au(lll) [79].

When a

monolayer

is almost

filled,

the next

_layer

nucleates on

_top.

How-ever, the

growth speed depends strongly

on the

_temperature

and the chain

length.

At 300

_K,

even the shortest chain

(n

=

6)

system

exhibits a _very

slow

_growth

(at

least onehour per

layer).

At

high

_{temperatures,}

all thiolate

layers

allow a

rapid growth

in the range of minutes per

layer.

1

The

_{monolayer heights}

of Au(lll) (0.236nm) and Cu(lll) (0.208nm) are too similartobedistinguishedinourSTM _setup.

Table 1._{Copper growth}scenarioas_{function of alkanethiolate chainlengthn,temperature}

and

_deposition

potential range(OPD: <0mV,UPD: >0_mV).

room temperature elevated temperature short chain UPD 2D islands OPD layer-by-layer UPD 2D islands OPD layer-by-layer

long

chain UPD 2D islands OPD 2Dislands UPD 2D islands OPD layer-by-layer

Fig.

12.In situ STM

_{image showing}

_{the 2D copper island formation} ona _C,8

thiolate-coveredAu(lll)surface in theUPD_range. Electrolyte:50 mMH2S04 + 1 mMCuS04.

Electrodepotential:30mVvs.Cu/Cu2+.Tunneling_parameters:I=0.3nA,V= 200 mV.

Image size: 220nm + 220nm.

For a

special

set of the

_parameters

(short

chain

thiolates,

300

K,

OPD)

we observed a fractal

shape

of the

growing

and

coalescing

islands as

visualized in

_Fig.

14: each island

_develops

three 'arms' which can

again

sym-Copper Electrodepositionon Alkanethiolate Covered GoldElectrodes 125

Fig.

13. In situ STM

_{picture exemplifying}

the _{OPD copper}

_{layer-by-layer}

growthon a

C,othiolate-covered Au(lll) electrode at 300 K. Electrode potential: -90mV vs. Cu/

Cu2+. Electrolyte: 50 mM H2S04 + 1 mM CuS04. Tunneling parameters: I = _0.4nA,

V = _110mV. Image _{size: 220nmX220nm.}

metry

of the

_{underlying gold}

surface and/ortothe

_symmetry

of the

_sulphur

adlattice. Such

_{growth shapes}

are

usually

associated with a

kinetically

hin-dered

_growth

_[80].

To obtain information onthe chemical

_composition

of the

interface,

we

emersed

_C18

thiolate

_samples

on which copper had been

_deposited

for 3 h

at50or₂₀mV_at300

_K,

_i.e.

_samples

covered

_by

_{copper islands.}

_They

were

investigated

with XPS. The overview

_spectra

_(see

_Fig.

_2,

_sample

_B)

_show the same featuresas in _{absence of copper,} but

additionally

a Cu

2pV2

emis-sion is

_present

_{(correspondingly}

the

_gold

emission

_decreases).

_Intensity

curves

(Fig.

4)

indicate a

_rapid

destruction with

X-ray

_{exposure. The}

oxy-gen contamination is

slightly higher

than for the

copper-free samples,

but still

_{corresponds only}

to a minor

portion

compared

to that of copper. From the

_intensity

_ratio,

the

_{photoemission}

cross sections and the

binding

_energy

of Cu

_2p3/2

(932.1 eV)

weconclude that the

deposited

_{copper did}notoxidize

although

the

_samples

had to endure more than one minute of contact with

laboratory

air which should suffice for

_complete

oxidation ofa _copper

sur-face. Hence the copper is

protected against

oxidation. In addition to the

Fig. 14. In situ electrochemical STM image of fractal islandsthat nucleated during the

growthof thefirst_copperlayeron a_C6thiolate-coveredAu(lll)electrodeat300 K. The

imagewasobtained after80min of OPDpolarizationat —110mVvs.Cu/Cu2+.

Electro-lyte:

50mMH2S04 + 1 mMCuS04.

Tunneling

parameters: I = _0.5 nA, V = 120mV.

Image size: 175 nmX175nm.

grazing

emission

_(see

_Fig.

_3).

This

_emphasizes

that the copper

_penetrated

the thiolate

_layer

_during

the

_deposition.

4.2 Structure of

_{electrodeposited}

copper

The formal electrode

_potential

_(measured

here with the Cu/Cu2+ reference

electrode)

we_quoteisnottheoneofthe

thiolate/electrolyte

interface. Hence

theoretically

one cannot

_expect

a _{copper bulk}

deposition

_{at 0} mV

_(even

at

highest

Cu2+

_{concentrations).}

_However,

the

_potential

_drop

in the thiolate

layers

should not exceed 100 mV _{since the}

_{overpotential}

_{for copper bulk}

deposition

of about 150 mV is the same as on bare

Au(lll)

and

Au(100)

[18].

The reduction of Cu2+ in acidic media

_produces

_{copper metal. In} our case it grows in

islands,

and the first

question

wehave to_poseconcerns the

copper islands' distance from the

gold

surface. We

already

disproved

the idea of copper

residing

inthiolate-free surface areas

[29]

since such areas

would result in clear

Cu/Au(lll)

UPD

_peaks

for all

_samples.

Fig.

15 illustrates the two further

_{principal possibilities}

that arise:

first,

the copper islands could be located between thiolate and

_gold

(CH3(CH2)„_1S/Cu/Au(lll)),

second,

they

could be on

_top

of the thiolate

layer

(Cu/CH3(CH2)„-iS/Au(lll)).

We here discard other models because

un-Copper ElectrodepositiononAlkanethiolate Covered Gold Electrodes 127

H

Au _B Cu os • c

Fig.15. Schematicrepresentationof the

_possible

structures oftheCu/thiolate/gold inter-face: _copperelectrocrystallized below(a) and above(b)the thiolate layer.

likely (e.g.

alloying

into the

_gold

surface that is

_only

known for the

(110)

face

_[81]).

The second model would

_postulate

isolated copper islands

con-nectedtothe

_methyl

_{end groups}

(see

Fig.

15(b)).

Aswe

already pointed

out

[27],

one

might

think that the necessary electron transfer to the

_top

of the chains should

_{nicely explain}

theslow formation

(but

seesection 4.4

!).

How-ever, several

arguments

speak against

the model: ifone observes

UPD,

the

growing

metal should have a

_stronger

interaction with the substrate than

with itself. Forcopperon_topof the thiolate

layer,

thisis

impossible

because

the Cu-H

_{(or Cu-CH3)}

_{interaction is very weak. The islands} even

with-stand

_rinsing

_{with water, i.e.}

_they

are

strongly

bound.

Also,

heating

involv-ing

the formation of

_{alkyl gauche}

defects should shortensome

(but

_{not all}

!)

of the thiolate chains below an island. Then the island that behaves as a

rigid

copper

layer

'disk'

(compared

to the thiolate

_layer)

would remain attached

_only

to the intact

thiolates,

but the distance to the

_gold

surface would be the same as at low

_temperature

(when

all molecules are

intact).

Hence the electron transfer

_probability

would be the same at low and

_high

temperatures,

and copper should

deposit

at the same rate; this is not ob-served.

Furthermore,

let us assumean island that

additionally

has somecontact

tothe substrate

(i.e.

a _{copper column}

through

the

thiolate).

Such an island

of copper atoms

should,

in absence of

strongly

bound coadsorbates

(S02~

does not

qualify!),

continueto _{grow 3D}

[22]

as

Cu/Au(lll),

in contrastto the observations.

_Clearly,

if the thiolate

_layer

is

_{damaged by large negative}

Copper placed

below the thiolate

_(Fig.

_15(a))

is in accordance with the

following

facts:

_firstly,

the XPS data show an

angular dependence

of the

Cu

_2p

emission whichmustbe due toconsiderable amountsof

material,

i.e. thiolate

molecules,

on

_top

of the copper.

_Secondly,

the

resulting

Cu—S bond is

_strong

andcan

easily explain

the

passivation

of_copper

[29]

:it is

impossi-ble to rinse off the

islands,

they

withstand very

positive potentials,

copper doesnotreactwith air which would

produce higher

Ols XPS

signals.

Note that sucha

thiolate/copper

structurecan be called a

coadsorption

_structure,

and

_examples

like

_{I/Ag/Pt(lll),}

detected after

_{electrodeposition}

of silveron

I/Pt(lll) (not

adsorption

of iodide on

Ag/Pt(lll)!)

are well documented

[33, 34, 73].

The

_question

we haveto _{address is whether copper}atoms can

penetrate

a thiolate

layer;

in section 4.4 we will show that this is indeed

possible.

Here we mention that silver

evaporated

on

thiolate/Au(l 11)

in vacuum _{has indeed been observed to}

_penetrate

_the

layer

[11, 82].

_Let us

also mention that thiolates

_readily

adsorb on

Cu(lOO) [83]

and that even

multilayer

copper thiolates can form

[84].

All these

_arguments

lead us to propose a

CH3(CH2)„_,S/Cu/Au(lll)

structure.

Further structural details are much more difficultto observe. This con-cernsthe lattice of the copperatoms which could be

(1

X

₁₎

asina

complete

UPD

_layer

[15, 85, 86],

a

honeycomb (73

X

73)R30°

lattice as in the

incomplete

Cu +

SOJ"

_coadsorption

UPD

_layer

on

Au(lll) [14, 86-88]

or a normal

(73

X

73)R30°

structure. All three would offer reasonable

adsorption places (e.g.

hollow

sites)

for the

_sulphur

atoms. A more intricate

feature is the structureof the islands' rims. Hereone could

probably

find a

topography

that is

_distinctly

different from that in the island

—

thusone_may

explain

the

_surprising

_stop

of

_growth

and hence the instantaneous instead of

progressive

nucleation. Since this is

_only

the case for

long

thiolate chains and low

_{temperatures,}

we can

speculate

that

especially

the

alkyl

chains

adjust

themselves ina

special

structure.

However,

STM observations

(which

for the currents we

employed probe

the

sulphur

_atoms rather than the

chains)

didnot

_yield

_anyclues.

4.3 Thermal behaviour

For an

interpretation

of

_temperature

phenomena

we have to consider the

changes

that

_develop

in the thiolate

_{layer during annealing.}

The

_gradual

development

of thiolate defect structures such as

gauche

conformations,

newtwist

angles

or

possibly

azimuthal disorder

(see

section

3.2)

will

again

allow for

_{increasing penetration}

of

ions2,

this time

_including

Cu2+. As for

Fe(CN)6~,

a reduction will occur much faster.

Doubtlessly,

the

complete

penetration

is now even more facile.

However,

we also have to

_explain

a

qualitative change:

islands grown on thiolates of all chain

lengths

can

co-2

On the fate ofthe

_hydration

shellwecanonly speculate.This iseventrueinabsence of thiolate _[89]!

Copper Electrodeposition onAlkanethiolate Covered Gold Electrodes 129

alesce at

_high

_{temperatures.}

This means that the unknown barrier

against

further

_{growth (presumably}

atthe island

_rim)

is overcome

by

thermal

acti-vation. The

_{pseudo-layer-by-layer growth}

is duetothepresenceofthe thiol-atewhich

_always

_stayson_top; otherwisethe

growth

(on

pure

copper)

would carry on much faster and 3D nodules would result. This is the classical

picture

ofa surfactantthathinders the

_growth

_{in the direction}normal_tothe

surface as also observed whencopper was

deposited

on

gold

in presence of

crystal

violet

_[20].

However,

our surfactant is adsorbed in a

well-charac-terized

_geometry

and will neither desorb nor readsorb

during deposition.

Thusweexcludeoneof the many processes that

complicate

the

understand-ing

of surfactant action in

_{electrodeposition.}

At

_high

_temperatures

substrate

_steps

becomemobile

_('strong

activation',

see section

3.2).

Whenwe

_accept

that copper is boundtothe substrate

(see

section

4.2),

copper island rims — copper

step

edges

—

should be at least

as mobile as the substrate

_steps.

This is

equivalent

to an incrased island

perimeter mobility,

i.e. the fractal island

_shapes

transform to

_compact

shapes.

Obviously,

the barrier

_against

further

_growth

that

_prevails

at low

temperature

is overcome, and further

growth

and coalescence can occur.

This

_arguments

are in line with the

theory

of surface diffusion limited

aggregation

that has

_successfully

been

_applied

to

explain

ramified metal islands grown in vacuum.

However,

the fact that

_temperature

also

enhances _copper

_penetration

and thus

_deposition

rate makes a

quantitative

theoretical

_modelling

_very

difficult,

sincetwo

interdependent

barriers

(thiol-ate

penetration

and the barrier at the island

rim)

are overcome at the same

time.

4.4 Potential

_dynamics

—

copper

deposition

rates

We shall start the discussion with a look at the

_permittable

limits of the

potential.

At the

_positive

end,

we dissolve the copper islands _at

_potentials

several hundred mV

_positive

of the Cu/Cu2+

_equilibrium

value. Note that

we haveto wait several hours at 400mV

₍₃₀₀

_K),

and some islands resist

dissolutioneven_at1000mV

[29]

_which

impressively

_{demonstrates the}

pas-sivation. As one

_expects,

the dissolution is much faster when the

tempera-ture is raised.

Inthe cathodic

_{regime, only}

wellbelow —200 mV the

_{layer growth}

can

change

to 3D

deposition

of nodules with diameters above 100nm. Since

voltammograms

show copper UPD

peaks

after such

_negative

excursions,

some bare substrate

_spots

_appear to exist which would

_clearly

cause the

observed

_growth

of

_large

nodules.

For_{the UPD processes,} nocurrent flow is found since the

_growth

is so

slow that the

_resulting

_peaks

would be far too shallow

_(sub-nA

_range).

In the OPD range, the detectedcurrents

_usually

_predict

a

deposition

ofa

mono-layer

of copper

during

several minutes which is

-at 300 K

than observed

_by

STM. The

_discrepancy

canbe

explained by

the destruction

of the

_sample

close to the viton

_ring

where we

occasionally

found visible

copper

deposits;

this means that copper had here

deposited

at least 100 times thicker than elsewhere. Thus the

_global

current is unreliable at least when its value is small. Therefore we estimate the

deposition

rate from STM measurements:

typical

rates are _{10% of}a

monolayer

in one to two

hours

_(UPD),

one

layer

in twohours for copper

layer growth

on _{short and}

medium chain thiolates

(OPD)

and one

layer

per minute at

_high

tempera-tures

(OPD)3.

Assuming

a Cu

(1

X

₁₎

_geometry

on

Au(lll)

with a

mono-layer charge

of 440

_pC/cm2,

this

_gives

rates of 0.01

_pA/cm2,

0.06

_pA/cm2

and 7

_pA/cm2,

_{respectively.}

We can setlimitstothe

_experimental

rates:

0.005

_pA/cm2

< r< 10

pA/cm2.

(1)

In order toexclude or

verify possible

mechanisms,

it is instructive to

com-pare those rates to rates calculated under

_typical

conditions. Let us first assume that bulk diffusion of Cu2+ is the

rate-limiting

(slowest)

_process.

Thenthe Cotrell

_equation

would

_apply

[12]:

r, = 2

Fc0

JUhti

•

[1

-exp

(2

FAE/RT)]

(2)

where F = 96485

C/mol,

D is the diffusion_constantof Cu2+ in

electrolyte

(5.7

• 10~6 cm2/s

[90]),

c0 the Cu2+ concentration

(1 mM),

t the

time,

R = 8.314

J/(mol

K),

AE the

_potential

_step

_causing

the

_deposition

_(e.g.

-100

_mV)

and T the

_temperature.

Weshould then find

r, « 260

pA/cm2

•

JV(tIsj

(2a)

(comparison:

440

_pA/cm2

accounts fora _Cu

(1

X

₁₎

_monolayer

on

Au(lll)

deposited

in one

second)

which isabout 100 times

larger

than the transient we observed

during

the first minute of

polarisation.

This means that the

diffusion isnot the

_limiting

factor.

This could leave the electron transferasthe slowest process. First let us

concentrate onthecase of copper

deposition

on_{copper, e.g. copper islands.}

Then the slowest

_possible

rateis thatat zero

overpotential

where dissolution

and

_deposition

currents cancel each other. Their

value,

the

_exchange

current

density,

is

[91]

r2« 500

pA/cm2

(3)

3

Theestimatesare rough, _{hence the copper}atom

_density

in theislands (seesection