Electrochemical Oxidation and ex-situ STM Observations of ...franklin.chm.colostate.edu/bap/pdf of papers copy/SQHOPG.pdf · Electrochemical Oxidation and ex-situ STM Observations

Electrochemical Oxidation and ex-situ STM Observations ofBis(4-dimethylamino-2-dihydroxyphenyl)squaraine Dye Layers on

HOPG Electrodes†

Norihiko Takeda, Michele E. Stawasz, and Bruce A. Parkinson*Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523-1872, USA

Abstract

Electrochemical oxidation of insoluble highly ordered bis(4-dimethylamino-2-dihydroxyphenyl)squaraine (1-1OHSQ) dye layers

adsorbed on highly oriented pyrolytic graphite (HOPG) electrodes was studied in aqueous electrolytes. Staircase cyclic voltammetry

obtained in chloride electrolytes revealed hysteresis characterized by large peak separations (100 – 200 mV) and sharp redox peaks

(fwhm 10 – 60 mV) the shape and potentials of which depended on electrolyte concentrations. Small stochastic reduction peaks

were observed at more negative potentials that are associated with the reduction of small domains of the oxidized 1-1OHSQ layers.

Peak potentials and peak shapes were also dependent on the identity of the electrolyte anion species. The results support a reaction

scheme where electrolyte anions are incorporated into surface confined one-electron oxidized dye molecular layers. From the results

of peak potential shifts, the preference for ion-pairing is estimated to be in the order of: I- > Br- > Cl- > SO4

2– ≈ ClO4- ≈ F-. Ex-situ

STM observations were performed to reveal structural changes upon electrochemical oxidation of 1-1OHSQ layers in LiCl electrolytes.

Two polymorphs with different angles of the long-axis of molecules to the directions of the molecular row were observed for the

oxidized samples. Both polymorphs had smaller packing densities compared to the reduced form of the dye.

Keywords: Surface confined redox processes; Squaraine dye; 2D surface immiscibility; Cyclic voltammetry; Scanning tunneling

microscopy

1. Introduction

Surface confined voltammetry of molecular adsorbates on

highly oriented pyrolytic graphite (HOPG) was first reported

by Brown, Koval, and Anson for the reduction and re-oxidation

of aromatic compounds including phenanthraquinone [1, 2]. The

voltammograms exhibited typical surface confined reversible

behavior. More recent studies of surface confined

electrochemical reactions have reported systems where there is

considerable hysteresis in the voltammograms, with the

oxidation and reduction peaks separated by as much as 250 mV

or even more [3-27]. These reactions usually involve tightly

packed 2D surface phases, such as azobenzene Langmuir-

Blodgett films [3] or methyl viologen cation radicals [4],

although the peak separation in the latter was rather small.

Similar behavior was also observed for conducting 3D phases

such as the organic conducting salts of tetrathiafulvalene (TTF)

[5-8, 13] and 7,7,8,8-tetracyanoquinodimethane (TCNQ) [5-7,

9-13], C60

crystallites [14-23], and liquid crystal perylene

diimides [24], as well as non-conducting solid organometallic

compounds [25-27]. Often the electrochemical reaction of these

phases involves the incorporation of anions or cations into the

oxidized or reduced phase. The large structural changes,

necessary due to the need to accommodate ions in the adsorbed

film or molecular crystal, results in the large hysteresis in the

voltammograms. The barrier to nucleation and growth of the

new phase formed upon oxidation or reduction was used to

explain and model many of the electrochemical results [4, 8,

11-13, 23, 28]. Recently Scholz et al. [29, 30] have pointed out

that the hysteresis in voltammograms may not be a result of

nucleation kinetics but instead is a thermodynamic result due

to a miscibility gap between the two phases. If the oxidized

and reduced surface phases are both insoluble in the electrolyte

* Corresponding author. Tel: +1-970-4910504; fax: +1-970-

4911801.

E-mail address: [email protected] (B.A.

Parkinson).† This paper is a contribution to this special issue honoring Professor

Fred C. Anson, my mentor and friend, who has exhibited the utmost

class and integrity throughout his distinguished career in electrochem-

istry.

Manuscript accepted for publication in Journal of Electroanalytical Chemistry

2 N. Takeda et al. / Electrochemical Oxidation and ex-situ STM Observations of Squaraine Dye

and immiscible then there is no mechanism for the equilibration

of the two phases. In the case where a neutral surface phase is

oxidized or reduced and ions need to be incorporated into the

new phase, it is not surprising that the 2D neutral and 2D ionic

phases are immiscible.

We report the investigation of a new surface confined redox

system, involving squaraine dye molecules (1-1OHSQ), that

exhibits remarkable voltammetry on HOPG electrodes.

In addition, we use scanning tunneling microscopy (STM)

to study the actual molecular arrangements of these molecules

on the surface of HOPG in both their oxidized and reduced states.

2. Experimental

2.1 Chemicals

Bis(4-dimethylamino-2-dihydroxyphenyl)squaraine (1-

1OHSQ) was a generous gift from the Lexmark International

Corporation and was used as received. Dye stock solutions

were prepared by dissolving 1-1OHSQ in dichloromethane

(Fischer, Spectranalyzed grade). Actual concentration of dye

solutions were checked by measuring UV-vis absorption spectra

using a HP8452A spectrophotometer (ε= 3.3 × 105 cm-1 M-1 at

636 nm [31]). Lithium chloride, sodium bromide, sodium

fluoride, sodium iodide, and sodium carbonate were Fischer

chemical reagents and hydrochloric acid, potassium chloride,

sodium chloride, and sodium sulfate were Mallinckrodt

analytical reagents, and they were used as received. A 2.0 M

sodium perchlorate stock solution was prepared from diluted

perchloric acid solution and sodium carbonate. Aqueous

electrolyte solutions were prepared using water purified with

Millipore Milli-Q system.

2.2 Electrode preparation

Highly Oriented Pyrolytic Graphite (HOPG) was cut into

small pieces to prepare working electrodes. Basal planes of the

HOPG pieces were cleaved with adhesive tape (Scotch, 3M) to

expose fresh surfaces. Deposition of 1-1OHSQ dye layers on

HOPG substrates was carried out by similar methods employed

in our previous photoelectrochemical study of 1-1OHSQ on tin

disulfide [32]. In this study, pieces of freshly cleaved HOPG

were dipped in 10–12 µM 1-1OHSQ dichloromethane solution

for 2 min. For comparison, dye coated electrodes were also

prepared by dropping a known amount of 1-1OHSQ dye solution

onto a freshly cleaved HOPG substrate followed by evaporation

of solvents in ambient air. Dye stock solutions were filtered

through a membrane filter (0.02 µm pore diameter, Whatman

anodisc 13) before use to remove any undissolved dye particles

or inert particles that could act as nucleation sites.

2.3 Electrochemical measurements

A Teflon-made electrode holder was designed to use HOPG

pieces as working electrodes and to make samples transferable

between electrochemical measurements and scanning probe

microscopy experiments. For electrochemical measurements,

dye deposited HOPG pieces were mounted on a copper plate

with a silver paste (SPI Supplies) inside the holder, through

which electrical contacts were made. Samples were sandwiched

in a two-piece Teflon holder with a flat Latex washer (4 mm

i.d., 0.6 mm thick) with a 4 mm diameter window in one piece.

The holder was held together with four screws to seal all but

the ~0.13 cm2 active area from the electrolyte.

Electrochemical measurements were conducted using a

conventional three-compartment electrochemical cell. A

saturated calomel electrode (SCE) or a sodium-saturated calomel

electrode (SSCE, in the case of perchlorate electrolyte) were

used as reference electrodes and a Pt gauze was used as a counter

electrode. Staircase cyclic voltammograms (SCVs) and staircase

linear sweep voltammograms were taken with an EG&G

Princeton Applied Research 174A polarographic analyzer

controlled by a MacLab interface (AD Instruments) and a

Macintosh computer. The step height was 1 mV and the step

width depended on scan rate (= (step height)/(scan rate)), which

was 100 ms at scan rate of 10 mV s-1. Current signals were

sampled and averaged over the last 25 % of time duration of

each step at 0.1 ms intervals. After immersing the electrodes,

aqueous electrolyte solutions were purged with nitrogen gas

for at least 15 min prior to measurements. All measurements

were conducted at ambient temperature.

2.4 Scanning probe microscopies

Molecular arrangements of both electrochemically oxidized

and reduced (neutral form before electrochemical oxidation)

forms of 1-1OHSQ layers adsorbed on HOPG prepared by the

dipping method were investigated by scanning tunneling

microscopy. STM experiments were performed ex-situ with a

Digital Instruments Nanoscope III scanning tunneling

microscope operating under ambient conditions. Vibration

isolation was provided by a bungee system enclosed in an

environmental chamber. STM tips were cut to a sharp point

mechanically from (80:20) platinum:iridium 0.2 mm diameter

wire (Alpha Aesar). Images of oxidized or reduced 1-1OHSQ

layers were obtained using typical tunneling parameters of –

1.4 V to –1.6 V (sample negative) and 40 pA with the STM

operating in the constant current mode. This range of sample

bias was slightly different than that reported for imaging of 1-

O

O

N NH3C

H3C

CH3

CH3

HO

OH

Bis(4-dimethylamino-2-dihydroxyphenyl)squaraine (1-1OHSQ)

2

N. Takeda et al. / Electrochemical Oxidation and ex-situ STM Observations of Squaraine Dye 3

1OHSQ (not electrochemically treated) under phenyloctane

solvent [33].

STM samples of oxidized 1-1OHSQ on HOPG were

prepared by anodically scanning non-oxidized samples by

staircase linear sweep voltammetry (scan rate 10 mV s-1) after

subjecting to several potential cycles to check their cyclic

voltammetric behavior. Samples were mounted in the STM

within minutes of the electrochemical experiment. Due to the

ex-situ nature of this experimental setup, it was not possible to

investigate the same exact area of the adsorbate surface upon

exchanging the sample between the STM and the

electrochemical cell for CV cycling. Therefore it was necessary

to rely upon statistical sampling of numerous areas of the surface

to ensure that differences observed in its oxidized state (as

compared to the reduced state) were truly due to electrochemical

reorganization, and not just a local orientational difference. To

reduce adsorbate etching presumably caused by capillary force

(described below) the tip was sometimes immersed into a drop

or two of phenyloctane (Aldrich) deposited onto the sample

surface. This procedure improved the resolution and reduced

the noise when imaging the oxidized form of the dye due to its

negligibly small solubility in phenyloctane. Some of the reduced

form of the dye dissolved in the phenyloctane increasing the

image noise due to the higher solubility of the reduced dye in

this solvent. To prevent dissolution of the reduced squaraine

adsorbate into the phenyloctane layer, the phenyloctane was

presaturated with reduced form of 1-1OHSQ. It was then

deposited on the adsorbate surface in the same manner as the

oxidized films and succeeded in improving image quality and

adsorbate layer stability.

Atomic force microscopy was performed with a Digital

Instruments Nanoscope IIIa scanning probe microscope operated

in tapping mode. Conical silicon AFM tips (Ultralevers™, Park

Scientific Instruments) with a spring constant of ~0.26 or ~0.40

N m-1 were used. An HOPG sample with a freshly deposited 1-

1OHSQ layer prepared by the dropping method was fixed on a

steel plate with a silver paste and then mounted on an AFM

scanner (type-J). All measurements were conducted in ambient

air. After investigating several areas by AFM, the sample was

mounted in the Teflon holder in a same manner as described

above to perform electrochemical studies.

3. Results and discussion

3.1 General trends of cyclic voltammograms of 1-1OHSQ

adsorbed on HOPG

Law et al. reported electrochemical properties of squaraine

dyes dissolved in dichloromethane using Pt as a working

electrode and tetrabutylammonium perchlorate (TBAP) as

supporting electrolyte [34]. The cyclic voltammogram of

squaraines showed two reversible waves with ~60 mV peak

separation as expected for a Nernstian diffusion-controlled one-

electron electrochemical reaction [35]. For the 1-1OHSQ dye,

the first and the second formal oxidation potentials were

determined to be +0.41 and +0.91 V vs Ag | AgCl in

dichloromethane, respectively [34].

In the present study staircase cyclic voltammograms (SCVs)

of the 1-1OHSQ dye layers adsorbed on HOPG (1-1OHSQ/

HOPG) electrodes prepared by the dipping method showed

remarkably different behavior from that reported for solution

electrochemistry. General trends of observed voltammogramms

are explained below. Figures 1 (a) and (b) show examples of

cyclic voltammograms of 1-1OHSQ/HOPG electrodes obtained

in 0.1 M LiCl aqueous solution at scan rate of 10 mV s-1. In

each panel voltammograms obtained in the first scan (dotted

lines) and after several cycles (solid lines) are compared for

two different samples prepared by similar procedures. Both

samples revealed sharp oxidation and reduction peaks with a

relatively large peak separation of 140 – 230 mV with no

significant currents between them. Usually the first

voltammetric scans gave considerably different peak potentials

and wave shapes than those from subsequent scans. After the

first scan, main peak potentials were relatively stable but wave

0

0.5µA

0

-0.2 -0.1 0 0.1 0.2 0.3

1µA

(a)

(b)

E / V vs SCE

Fig. 1. Staircase cyclic voltammograms of 1-1 OHSQ/HOPG elec-

trodes (dipping method) in 0.1 M LiCl aqueous solutions. In each panel,

the first scan (dotted line) and the eleventh (a) or tenth (b) scan (solid

line) for two different samples are shown. Scan rate 10 mV s–1. Elec-

trode area 0.13 cm2.


shapes and peak currents occasionally changed with repetitive

scanning. In Fig. 1(a), two overlapping oxidation peaks at

+0.129 and +0.159 V vs SCE can be seen in the first scan (dotted

line). In the eleventh scan, shown as a solid line in the figure,

these peaks had merged to become a single peak at +0.137 ±0.002 V vs SCE. A similar behavior appeared in the reverse

negative scan in which a peak at around –0.01 V vs SCE had

evolved into a larger peak. In Fig. 1(b), the oxidation wave in

the first scan had a prewave at the foot of the peak (+0.185 V vs

SCE) which fused with a ~50 mV more negatively shifted peak

at around +0.135 V vs SCE in the following scans. On the

reduction side, a peak at –0.043 V vs SCE split into two

comparable peaks in the tenth scan as shown. Moreover, small

jagged peaks appeared at potentials more negative than the main

peak. In general, similar current spikes often appeared especially

in cathodic scans. Sometimes the magnitude of these currents

was larger than that of the main peak. These current spikes

behaved rather stochastically, that is the potential and peak

heights changed from scan to scan. This phenomenon made

the determination of reduction potentials more ambiguous than

for the oxidation peaks. The small current spikes tended to

disappear after many potential cycles (as seen in Fig. 1).

However, in some cases multiple peaks remained after extensive

cycling.

The total integrations of the anodic and cathodic currents

were approximately the same and were relatively constant with

scan numbers as in Fig. 1. However, in some cases

voltammogram area as well as peak current decreased with

repetitive scanning. It is possible that the oxidized form of the

dye has a slight solubility in aqueous solutions and desorbed

from HOPG electrodes, or that electrochemically inactive areas

could be formed. The total charge passed by electrochemical

oxidation and reduction of 1-1OHSQ/HOPG electrodes differed

from sample to sample although similar procedures were

employed to prepare the samples. This uncertainty is

presumably because the amounts of 1-1OHSQ molecules

adsorbed on HOPG were different, which could be explained

by such conditions as the quality of the HOPG basal plane

cleavage, the withdrawing process of the HOPG from the dye

solutions, or randomness in the nucleation and growth of the

dye layers in the deposition processes.

Some comments about the magnitude of currents obtained

in our SCVs are needed. If nucleation and growth type kinetics

are involved, as suggested below, current transients for each

potential step would peak. Our SCV currents were averaged

over last 25 % of each step. Due to the signal processing method

employed in this study, integrating the currents to determine

charges and surface coverages will underestimate the actual

coverages. Further coulometric studies are needed to provide

more accurate surface coverages. However, it can be safely

said that multilayers of the dye would be formed, at least in

some cases, since the underestimated charges and the STM

results presented below both confirm this.

3.2 Reaction scheme including electrolyte anion incorporation

In the present study, dye modified electrodes were immersed

in aqueous electrolyte solutions in which 1-1OHSQ is not

soluble. Accordingly, the electroactive 1-1OHSQ layers can

be regarded as immobilized on HOPG surfaces. However,

unlike typical surface confined systems including physically

[1, 2] or covalently [36-44] attached redox molecular species,

the present system revealed a large hysteresis in cyclic

voltammograms even at slow scan rates. This indicates large

activation barriers for redox reactions of adsorbed 1-1OHSQ

layers. The large splitting of redox peaks in the voltammograms

is reminiscent of other reported systems such as TTF and TCNQ

salts [5-13], and solid C60

immobilized on electrodes [14-23].

This behavior was often attributed to large structural changes

during redox reactions. Laviron had simulated the voltammetric

shape of surface confined species, and obtained a hysteresis

effect by using strong “interaction parameters” between

adsorbed molecules [45]. As described in the introduction, the

splitting of anodic and cathodic peaks was also explained by

considering the insolubility of the adsorbed redox species and

the immiscibility of the two adsorbed redox phases [29]. In

this case, the Nernst equation cannot be satisfied in a certain

potential range (“miscibility gap”) where the two phases are

immiscible and the redox reaction is suppressed. If the electrode

is further polarized, exceeding the critical potential, the redox

reaction is initiated and a sharp current peak appears. It should

be noted that in this case the potential region halfway between

the anodic and cathodic peaks is not necessarily the value of

the formal potential of the redox reaction [29].

We hypothesize that the redox reaction of 1-1OHSQ

adsorbed on HOPG proceeds as follows. A neutral 1-1OHSQ

molecule is oxidized by one electron to form a positively charged

1-1OHSQ radical cation (reaction (1)), followed by the

association of an electrolyte anion to maintain charge neutrality

(reaction (2)) as given below.

(1-1OHSQ) – e- ⇔ (1-1OHSQ)+ (1)

(1-1OHSQ)+ + X- ⇔ (1-1OHSQ)+X- (2)

X- is the electrolyte anion species and (1-1OHSQ)+ is a one-

electron oxidized 1-1OHSQ molecule. Since these processes

are reversible, re-reduction of the oxidized form of 1-1OHSQ

is accompanied by release of the electrolyte anion from the film.

We assume a one-electron redox reaction because the redox

peaks of immobilized 1-1OHSQ appeared at a similar potential

as the first one-electron oxidation of dissolved 1-1OHSQ in

dichloromethane solution [34] when the same electrolyte anion

(perchlorate) was used (see section 3.4), and the formal potential

of molecular adsorbed species was usually found to be similar

to that of dissolved species [36, 37, 39, 40].

Many of the observations described above can be interpreted

using the reaction scheme represented by reactions (1) and (2).


Previous STM studies showed that the squaraine dyes have

strong intermolecular interactions and form tightly packed well

ordered structures on HOPG substrates [33]. A large structural

change of the ordered 1-1OHSQ molecules occurs in order to

include or exclude the electrolyte anions upon oxidation or

reduction (reaction (2)). The reaction could be initiated at

nucleation sites, possibly at defect sites within the packed

molecular layers or at domain edges, then rapidly expand to

entire domains or layers to give the sharp redox peaks in the

voltammograms.

The fact that the first scan showed a more positive oxidation

peak than successive scans (Fig. 1(b)) indicates that larger

overpotentials are needed to incorporate anions into the 1-

1OHSQ layers prior to reconstruction, as was observed for

another system [19]. Also, the release of solvent which might

have been trapped during the dye deposition processes

(dichloromethane in the present case) could occur after the initial

scan to alter the voltammograms. Furthermore, there could be

time dependent factors, especially for cathodic processes, if

nucleation and growth type kinetics are involved. The multiple

current peaks observed in staircase cyclic voltammograms could

be related to the existence of multiple domains of 1-1OHSQ

layers with different nucleation rates for anion association or

expulsion. The appearance and disappearance of the multiple

current peaks could then be the result of the formation and

annealing of small domains into larger domains with repetitive

cycling. It has been shown that the nucleation rate limits the

formation of ordered liquid crystal layers into small domains

created by controlled-size pits on HOPG [46]. Large areas of

HOPG were quickly covered with ordered layers since the

probability of forming the critical nuclei on some part of the

surface was high [46]. Different polytypes of packed 1-1OHSQ

domains with different formal potentials or overpotentials could

also account for the multiple peaks. The large double peaks

observed in Fig. 1(b) may be the result of the two polymorphs

found in STM studies discussed later.

3.3 Effect of electrolyte concentration

If the above-mentioned reaction scheme is occurring, the

properties of the electrolyte will affect the voltammetric behavior

of 1-1OHSQ layers on HOPG. The effect of electrolyte

concentration was investigated by using three different

concentrations of aqueous LiCl, NaCl, and KCl solutions as

supporting electrolytes (0.01 M, 0.1 M, and 1 M). Dye modified

electrodes were prepared by the same dipping method as in the

preceding section. Changes in the electrolyte cation from lithium

to sodium to potassium had only a small influence on the

voltammetric behavior, as long as the concentrations of the

electrolytes were equal. This is reasonable because reaction

(2) does not involve electrolyte cations but anions. The

electrolyte concentrations did affect some features of

voltammograms. Staircase cyclic voltammograms obtained in

0.01, 0.1, and 1 M of LiCl aqueous solutions after several cycles

are compared in Fig. 2 (scan rate 10 mV s-1). The same 1-

1OHSQ/HOPG sample was used to obtain all of these

voltammograms. It can be seen that both the oxidation peak

potential (Epox) and the reduction peak potential (Ep

red) depend

on the concentration of the supporting electrolyte. Furthermore,

peaks became sharper as the electrolyte concentrations

increased. Values of full width at half maximum (fwhm) of

oxidation and reduction peaks in Fig. 2 are 48 and 42 mV in

0.01 M, 23 and 34 mV in 0.1 M and 9 and 11 mV in 1 M

solutions, respectively.

Oxidation and reduction peak potentials for different

concentrations of LiCl, NaCl, and KCl, determined at scan rate

of 10 mV s-1 after the voltammograms are stabilized by repeated

scanning, are plotted in Fig. 3. In some cases multiple peaks

remained after many potential cycles. The potential of the main

peaks with the largest areas were considered to be the peak

potentials. Regression lines shown in the figure were least-

squares fits to the data obtained for each electrolyte. In each

case, the potentials shifted negatively with increasing

concentrations. Slopes of lines were –70 mV (Epox) and –47

mV (Epred) for LiCl, –70 mV (Ep

ox) and –27 mV (Epred) for NaCl,

and –71 mV (Epox) and –34 mV (Ep

red) for KCl per decade change

of concentration. The magnitude of the potential shift was larger

for Epox than for Ep

red in each case. As a result the peak separation

between the oxidation peak and the reduction peak (Epox – Ep

red)

tended to become larger as the concentration of the electrolyte

was decreased. The different potential shifts could be related

to the differences in the oxidation and reduction processes (for

(a)

(b)

(c)

E / V vs SCE

0

0

2µA

0

-0.2 -0.1 0 0.1 0.2 0.3 0.4


trodes (dipping method) in 0.01 M (a), 0.1 M (b), and 1 M (c) LiCl

aqueous solutions. Scan rate 10 mV s–1. Electrode area 0.13 cm2.


polarization [48]. However, in the system where a large

hysteresis is observed, the occurrence of a sharp peak, much

narrower than the ideal Nernstian system itself, could be

explained by sudden redox reactions after passing the

“miscibility gap” as described in the preceding section [29]. In

such a case, the peak width is also influenced by redox or ion

transfer kinetics. Broadening of the voltammetric peak with

decreasing electrolyte concentration observed in this study might

be due to slower kinetics such as slower diffusion of electrolyte

anions into or out of the 1-1OHSQ layers.

3.4 Effect of electrolyte anion species

Voltammetric behavior of 1-1OHSQ deposited on HOPG

by the dipping method was found to be affected dramatically

by the electrolyte anion. Figure 4 compares cyclic

voltammograms of 1-1OHSQ on HOPG in 0.1 M of NaCl, NaBr,

and NaI aqueous electrolytes (scan rate 10 mV s-1). Again, sharp

redox peaks with a large hysteresis were observed. The peak

potentials were significantly shifted to more negative potentials

in bromide (~150 mV) and iodide (~370 mV) electrolytes when

(a)

(b)

(c)

E / V vs SCE

0

2µA

0

0.4µA

0

-0.4 -0.2 0 0.2

0.2µA


trodes (dipping method) in 0.1 M NaCl (a), NaBr (b), and NaI (c) aque-

ous solutions. Scan rate 10 mV s–1. Electrode area 0.13 cm2.

Cl– concentration / M

Pea

k po

tent

ial E

p / V

vs

SC

E

-0.1

-0.05

0

0.05

0.1

0.15

0.2

0.25

0.3

0.01 0.1 1

Fig. 3. Plots of peak potentials for the oxidation (Ep

ox, open symbols)

and reduction (Ep

red, closed symbols) of 1-1 OHSQ dye layers on HOPG

electrodes as a function of supporting electrolyte concentration. Sup-

porting elecrolyte: lithium chloride (circles), sodium chloride (triangles),

and potassium chloride (inverse triangles). Scan rate 10 mV s–1. Lines

are least-squares fits to the data.

instance, the asymmetric nature of anion inclusion and

exclusion). On average, there is a 53 mV shift of peak potentials

with a 10-fold increase or decrease of the electrolyte

concentration, close to a ~59 mV shift predicted for Nernstian

systems. A similar dependence of peak potentials on electrolyte

ion concentration was observed for other surface confined

systems such as redox-functionalized self-assembled

monolayers (SAMs) on gold [40-42, 44] and the solid

compounds attached to graphite substrates [8, 12, 20], where

electrolyte ions were involved in redox reactions. The 53 mV

value closer to the 59 mV Nernstein value for a one-electron

process further supporting our assumption that the surface

oxidation/reduction process is a one electron process. A ~30

mV shift would be observed if two anions were involved in

reaction (2).

Narrowing of peak widths with increasing electrolyte

concentrations was also commonly observed in NaCl and KCl

electrolytes as well as in LiCl electrolyte. The fwhm of a one-

electron redox voltammetric peak was predicted to be 90.6 mV

(at 25 ˚C) for an ideal Nernstian surface confined redox systems

[35, 37]. Theoretical treatments including effects of electrolyte

concentration on voltammetric shape and peak potentials have

been developed considering the potential distribution across the

redox-functionalized SAMs [47, 48]. It is interesting to note

that recently Ohtani et al. explained the potential shift of sharply

peaked voltammograms by considering strong ion-pair

formation with the surface confined redox-active molecules,

which increased the potential drop and required a smaller


compared to the chloride electrolytes of the same concentration.

The differences in the magnitude of the observed currents were

probably due to the different surface coverages of 1-1OHSQ

on HOPG that resulted from each sample preparation.

In Fig. 5, cyclic voltammograms obtained in 0.1 M NaF

and 0.1 M Na2SO

4 aqueous electrolytes after several potential

cycles (scan rate 10 mV s-1) are shown. In these electrolytes,

broader redox peaks appeared at much more positive potentials

than those in the halide electrolytes of the same concentration.

In both cases, the anodic peak in the first scan appeared more

positive than that of subsequent scans shown in Fig. 5, whose

behavior was also observed in halide electrolytes. In addition,

the anodic peak observed in the first scan was much larger than

the cathodic peak. There could be some irreversibility for the

incorporation of fluoride or sulfate anions into 1-1OHSQ layers.

Voltammograms obtained with five successive scans in 0.1 M

NaClO4 aqueous electrolyte (scan rate 10 mV s-1) are shown in

Fig. 6. Note that potentials are given versus SSCE reference

electrode in this figure. Redox peaks appeared in a similar

potential range to those observed in fluoride and sulfate

electrolytes. In Fig. 6, significant decreases of the peak current

with repetitive scans can be seen. Many dye molecules are

more soluble as perchlorate salts and we suspect loss of the

oxidized dye due to a small aqueous solubility. Voltammograms

obtained in fluoride, sulfate, and perchlorate electrolytes also

resemble each other since measurable currents appear between

the two peaks. Peaks were also somewhat broader than those

observed in chloride, bromide, and iodide electrolytes. The

miscibility gap concept explains the very low currents between

peaks in the halide electrolytes and the different shapes and

peak separations of the voltammograms of adsorbed 1-1OHSQ

redox reaction. One requirement for the miscibility gap is that

the two phases cannot equilibrate through the solution, thus any

solubility of the oxidized or reduced phase would allow for this

equilibration.

All the results presented above confirm the importance of

the electrolyte anions in the redox reaction of 1-1OHSQ on

HOPG supporting the applicability of the reaction scheme

proposed above (reactions (1) and (2)). The order of oxidation

and reduction peak positions observed in 0.1 M electrolytes was

I- < Br- < Cl- < SO4

2– ≈ ClO4- ≈ F- for the anion species

investigated. As suggested in studies on other surface confined

redox reactions involving electrolyte ions [39-44, 48], shifts of

redox peak potentials could be interpreted by differences in the

degree of association (association constant) of electrolyte anions

with the oxidized form of the 1-1OHSQ molecules. Preferential

ion-pairing of positively charged molecular layers with anions

would shift the formal potential of the redox reaction to more

negative potentials. The dramatic shift of the redox potential

observed in this study (~600 mV difference between I- and F-

electrolytes) suggests that the nature of electrolyte anions plays

an important role in the thermodynamics of the present redox

system. Factors such as hydrodynamic radii of anions and free

E / V vs SCE

0

0.3µA

0

-0.1 0 0.1 0.2 0.3 0.4 0.5

0.4µA

(a)

(b)


trodes (dipping method) obtained in 0.1 M NaF (a), and Na2SO4 (b)

aqueous electrolytes. Scan rate 10mV s–1. Electrode area 0.13 cm2.

E / V vs SSCE

Cur

rent

/ µA

-0.4

-0.2

0

0.2

0.4

0.6

-0.1 0 0.1 0.2 0.3 0.4 0.5


trode (dipping method) obtained in 0.1 M NaClO4 aqueous electrolyte.

Scan rate 10mV s–1. Allows indicate direction of decreasing currents

with repetitive cycling. Electrode area 0.13 cm2.


energy differences between hydrated and dye-associated anions

would also affect the ion-pair formation. Following the observed

redox potentials, the order of affinity of anions would be I- >

Br- > Cl- > SO4

2– ≈ ClO4- ≈ F-. The fact that similar peak

potentials are obtained in sulfate, perchlorate, and fluoride

suggests that interactions of these anions with the oxidized 1-

1OHSQ layers are similar.

3.5 Effect of scan rates

An example of a scan rate dependence for staircase cyclic

voltammograms of 1-1OHSQ on HOPG prepared by the dipping

method and measured in 0.1 M KCl aqueous electrolyte is shown

in Fig. 7. Scan rates were varied from 10 to 100 mV s-1 in panel

(a) and from 1 to 10 mV s-1 in panel (b) of the figure. Both the

oxidation and reduction currents depended on scan rates but

were not linearly proportional as expected for a typical surface

immobilized redox species [35, 37]. Also, peak potentials and

fwhm of peaks often depended on scan rates, as can be seen in

Fig. 7. As the scan rate was increased, redox peaks became

broader and the peak separation also became larger.

Furthermore, tails after current peaks tended to become

significant especially at higher scan rates. It is probable that

diffusion of anions, either from the electrolyte or the solid film

would affect the current and ultimately limit the overall reaction

rate. The current sampling of the staircase current may also be

a factor in interpreting peak height with such sharp peak currents.

Further investigation is needed to clarify these issues.

3.6 STM imaging of reduced and oxidized forms of 1-1OHSQ

Dialkylamino squaraines have been found to be highly

surface active and readily amenable to investigation by STM

[33]. It would be useful to summarize here several observations

obtained in an earlier study.1 Squaraine molecules appeared in

most STM images as football-shaped areas of bright contrast

measuring 15.0 ± 1.5 Å in length, 2.5 ± 1.0 Å in width, and

from 0.2 to 1.5 Å in relative height. Occasionally two bright

spots within the football-shaped region were resolved, each

measuring ~3 Å in length, with a peak to peak distance of about

8 Å (see Figure 4 in reference 33). The measured lengths of the

football shapes correspond with the size of the chromophore of

the molecule (from the nitrogen at one end of the molecule to

the nitrogen at the other end). The width of the football

corresponds well with the width of a phenyl group, not including

the oxygen atoms and the hydroxyls. An explanation for why

the oxygens and hydroxyls are not imaged has been presented

elsewhere [33]. The methyl groups in 1-1OHSQ (and longer

alkyl tails for other dialkylamino squaraines) have not been

clearly resolved. Nevertheless, the length, width, and relative

height of the bright football-shaped areas correspond well with

a model of the molecule lying with the chromophore flat on the

HOPG surface. STM measured heights of 1–2 Å are typical for

aromatic systems adsorbed lying flat on the substrate surface

[49]. Since the methyl tails are not resolved, packing of the

bright chromophores must be examined quantitatively to deduce

the orientation of the methyl tails.

Different ordered structures, or plymorphs, for the same

squaraine molecules were frequently observed in the previous

study [33]. Longer-tailed squaraines tended to produce more

stable adsorbate layers with row structures while the herring-

bone structures formed by the shorter-tailed squaraines seemed

less stable since they were more easily stripped-off of the

substrate surface by the rastering motion of the tip [33]. This

was often observed in this investigation of the layers of 1-

1OHSQ deposited from dichloromethane solutions. The

dichloromethane solution concentrations used within this study

often produced multiple layers of 1-1OHSQ on HOPG. Layer

E / V vs SCE

0

50µA

0

-0.2 -0.1 0 0.1 0.2 0.3

25µA

(a)

(b)


trode (dipping method) in 0.1 M KCl aqueous electrolyte obtained at

various scan rates of 10, 20, 50, and 100 mV s–1 (a) and 1, 2, 5, and 10

mV s–1 (b). Allows indicate direction of increasing scan rates. Elec-

trode area 0.13 cm2.

1 In an earlier study [33], molecules were primarily adsorbed from

phenyloctane solution directly on the HOPG surface. It should be also

noted that 1-1OHSQ was denoted 1-1 SQ in Ref. [33].


heights were measured to be from 1.5 to 4.5 Å in thickness (~

one to three layers). The initial STM images showed that the

surface layers of 1-1OHSQ were smooth with domain

boundaries barely visible as fine fractures. A few minutes after

engagement the rastering motion of the tip began to remove

material from the domain boundaries. The removal of the

adsorbate layer also caused the STM image to become noisy

and streaky, presumably due to the interference of the desorbed

dye molecules with the tunneling junction. Both oxidized and

reduced forms of the dye were susceptible to tip erosion.

At smaller scan sizes (<100 nm) the adsorbate layers were

more resistant to tip-induced erosion and were less noisy as

long as no missing molecules defects or domain boundaries were

present. The duration of imaging a particular area was still

dictated by the removal of molecules in that area. At the smaller

scan sizes it was apparent that removal of additional molecules

was occurring at rectangular defects composed of a few (2-3)

missing molecules. These missing-molecule sites usually

occurred within a single molecular row. As the defect grew,

molecules from adjacent rows were also removed. The process

of defect nucleation and growth of holes in the layer can be

seen in Fig. 8. Continual scanning caused the defects to grow

until neighboring holes coalesced. Capillary forces and tip

collisions with domain or defect edges are presumed to be

responsible for the removal of the molecules from the surface.

As mentioned above, layers of reduced 1-1OHSQ appeared

smooth except for step edges and missing molecule defects,

which often revealed the presence of multiple layers (Fig. 9A).

At larger scan sizes the domain intersections are barely visible

except where erosion has occurred between the domains. A

few times small islands of molecules adsorbed on top of the

smooth mono or multiple layers were imaged. These islands

were 60 – 70 nm in length, 10 – 20 nm in width, and 2 – 3nm in

height. They tended to orient with their long axis at 30° or 60°

angles to each another and along crystallographic directions, as

can be seen in Fig. 9B, where some oriented islands are parallel

to a graphite step edge. At smaller scan sizes (<300 nm)

molecular lamella were observed. Widths of the bright stripes

were typically measured to be ~14 Å, a distance which

corresponds to the length of the chromophore of the 1-1OHSQ

molecule. The lamellar domains often showed rectangular

missing molecule defects. Some experiments, in which multiple

layers of 1-1OHSQ were adsorbed on the graphite surface,

exhibited entire missing rows (see Fig. 9C). These row defects

were measured to be one or more layers deep.

Figure 10 (right) shows a 15 nm × 15 nm image of the row

structure observed for the reduced 1-1OHSQ on HOPG. The

chromophores of the molecules appear as football-like shapes

measuring ~14 Å in length. The distance between chromophores

within the same row was measured to be ~10 Å, while the

distance between chromophores in adjacent rows was ~ 24 Å.

The angle of the long axis of the molecule to the direction of

the row was measured to be ~47°. Dialkylaminophenyl moieties

in squaraines have an electron-donating character while the

central four-membered ring has an electron-accepting character

[50]. The 47° angle of molecule to the row aligns adjacent

molecules within the same row such that their donor and acceptor

moieties interact (see the molecular model proposed for this

structure in Fig. 10). The donor-acceptor interaction between

adjacent molecules within the same molecular row has been

observed for other dialkylamino hydroxyphenyl squaraines and

we believe is the dominant interaction driving the ordering of

these squaraines on HOPG [33]. This structure is similar to the

row structure observed for 1-1OHSQ deposited onto HOPG

from phenyloctane solutions. A smearing of the electron density

between adjacent molecules (in the same row) was occasionally

observed and produced a bright zig-zag pattern within the row

(see the left image in Fig. 10). A relatively blunt tip that smears

Fig. 8. Series of 50 nm × 50 nm STM images of reduced 1-1 OHSQ adsorbed in rows on HOPG showing the time-lapsed erosion of the adsorbate

layer. Image (A) shows a number of missing-molecule defects which appear as dark rectangles. Image (B) was taken 1.5 minutes after image (A) and

shows the growth and coalescence of several of the defects. Arrows correlate the defects from image (A) which have grown and coalesced in image

(B). Image (C) was taken 1.5 minutes after image (B) and shows the erosion of the adsorbate layer in the upper right. The direction of thermal drift

is shown in the bottom right corner of the figure.


the electron densities from two adjacent aminophenyl moieties

(represented by the circled area in Fig. 10) might be responsible

for this pattern. Unit cell values for both phenyloctane-deposited

and dichloromethane-deposited 1-1OHSQ are shown in Table

1. Only the most commonly observed polymorph deposited

from phenyloctane from reference 33 (90˚ herringbone) was

included in Table 1. This polymorph was not observed in the

experiments reported herein and the phenyloctane-deposited row

polymorph reported herein was not observed previously. None

of the polymorphs (neither reduced nor oxidized) observed for

the dichloromethane deposition reported herein were observed

in reference 33.

Adsorbed 1-1OHSQ that had been electrochemically

oxidized in 0.1 M LiCl aqueous electrolyte showed multilayers

that were not as smooth as those formed by the reduced form of

the dye. As can be seen in Fig. 11A, the oxidized dye layers are

more broken and defective. Domain boundaries are more

obvious and are often rounded in shape as opposed to the

predominantly faceted domains of the reduced form. Domains

are also smaller, typically measuring 100–300 nm in diameter.

At smaller scan sizes lamella are visible that appear to be similar

to those of the reduced form (Fig. 11B). Rectangular missing

molecule defects are also visible. Repeated scanning eventually

expands these defects and erodes away the adsorbate layer again

in a similar fashion to the erosion of the reduced dye layers.

Missing row defects like those observed in the reduced form of

the dye were found in only a very small percentage of the

experiments.

Upon closer inspection of the lamella it was found that two

different row structure polymorphs exist for oxidized 1-1OHSQ.

Polymorphs are commonly observed in 2D surface structures

of squaraine molecules [33]. The unit cells measured for these

polymorphs can be found in Table 1. Figure 12 shows molecular

resolution images and proposed molecular models for both of

these row structure polymorphs. Both polymorphs exhibit

similar distances for both adjacent molecules within the same

row and molecules in adjacent rows. The angles of the molecules

within the row to the direction of the row, however, are different

for each of the polymorphs. Polymorph 1 exhibits a ~67° angle

of the molecules to the direction of the row. This angle does

not offset adjacent molecules from one another enough to

achieve “optimum” donor-acceptor interaction. In addition, the

partially-negatively charged oxygen atoms in the square core

are still relatively close. Repulsion between these two charged

moieties may be mitigated by possible hydrogen-bonding

between the partially charged oxygen and the hydroxyl group

of the molecule adjacent to it. Polymorph 1 was observed in

only ~25% of the experiments. Polymorph 2 exhibits a ~46°angle much like the reduced dye row structure. This angle offsets

adjacent molecules within the same row such that their donor

and acceptor moieties line up much more effectively, with a

minimum of Oδ- – Oδ- repulsion. The only difference between

the oxidized polymorph 2 structure and the reduced structure is

the distance between molecules in adjacent rows. This distance

Fig. 9. (A) 500 nm × 500 nm STM image showing smooth layers of

reduced 1-1 OHSQ on HOPG (dipping method). Multiple layers are

evident at defect sites. (B) 1.5 µm × 1.5 µm scan STM image of reduced

1-1 OHSQ monolayer adsorbed on HOPG terraces. Small islands

oriented at 30° or 60° to one another and parallel to HOPG step edge

are visible. (C) Several missing rows are evident in this 42 nm × 42 nm

STM image. Missing rows were not observed in every experiment and

were only observed in multiple layer adsorption.


Fig. 10. A 15 nm × 15 nm scan STM image (right) showing the row orientation of reduced 1-1 OHSQ. Individual squaraine chromophores are visible

as bright football-like shapes. The black box represents the unit cell, which has been enlarged and projected onto the proposed molecular model

(bottom) for comparison. Donor-acceptor interactions between adjacent molecules within the same row appear to be the dominant interaction driving

this order. A 5 nm × 5 nm imgae (left) shows a zig-zag contrast which is sometimes observed for the reduced form of 1-1 OHSQ. The black ovals

overlayed on both the left image and the model illustrate the coupling of electron density between adjacent molecules within the same row which

might lead to such a zig-zag structure.

etatS tnevloS c hpromyloP a Å/ b Å/ α ˚/ Å/aeraraluceloM 2 elucelom 1-

DER enatcolynehP enobgnirreh˚09 d 2±52 2±52 2±09 91±651

DER enatcolynehP woR e 1±12 1±01 4±53 61±012

DER HC 2 lC 2 woR 6.0±2.42 5.0±9.9 3.1±5.64 9±042

XO HC 2 lC 2 wor1epyT 8.0±4.02 4.0±3.9 0.4±8.76 8±091

XO HC 2 lC 2 wor2epyT 0.1±6.02 0.1±6.9 0.4±2.64 51±002

Table 1

Unit cell parameters for reduced a (RED) and oxidized b (OX) forms of 1-1OHSQ layers adsorbed on HOPG as determined by STM

a Before electrochemical treatments.b Electrochemically oxidized in 0.1 M LiCl aqueous electrolyte; chloride ions were presumably incorporated as counter ions.c Solvents used for deposition of 1-1OHSQ.d Most commonly observed polymorph in the previous STM study (Ref. [33]), but was not observed again in this study. Note that 1-1OHSQ

was denoted as 1-1 SQ in Ref. [33].e Row polymorph observed for the reduced form of 1-1OHSQ deposited from phenyloctane (same manner as in Ref. [33]) in this study was

not observed in Ref. [33].


is ~1 Å longer in the reduced form, resulting in the reduced

structure having a slightly smaller packing density. This is

counterintuitive since it would be expected that the oxidized

structure would be slightly larger to account for the presence of

the counterions within the adsorbate layer. This contraction of

the adsorbate structure may explain the increased observance

of missing molecule defects in the oxidized adsorbate as

compared to the reduced. Also the double peaks sometimes

observed in the SCV (Fig. 1(b)) may be due to different

polymorphs having slightly different redox properties.

The actual position of the chloride ion within the adsorbate

unit cell is not known at this time. A speculative position is

shown in the molecular models of Fig. 12. Four factors were

taken into account in the placing the chloride at this position:

(1) the most probable site for the oxidation of the squaraine

molecule is the amino groups; (2) the delocalization of this

partial positive charge is most likely over the amino nitrogen

and the phenyl carbon bonded to it; (3) the close proximity of

positive charged parts of adjacent rows to one another; and (4)

the electrochemical data which show that the ion is most likely

associated 1:1 with a 2D phase of oxidized dye molecules rather

than being diffusely associated with the oxidized layer. It is

possible that the counterion resides slightly above the plane of

the adsorbate between the amino groups of adjacent rows nested

above the methyl groups as is if they were the bottom of a

dimethylamino solvation cage around the chloride ion. There

is also a possibility that water or dichloromethane solvent

molecules are incorporated in these structures and are not

resolved in the STM images. Loss of solvent molecules upon

oxidation may also account for the smaller unit cell areas for

the oxidized molecules.

3.7 Effect of morphologies on electrochemistry of 1-1OHSQ

deposits on HOPG

It has been shown that deposition of 1-1OHSQ molecules

from dichloromethane solution by the dipping method produced

electrochemically active 1-1OHSQ layers on HOPG.

Alternatively, deposition could be accomplished by dropping a

known amount of dye solution onto HOPG as described in the

experimental section (approximately 1 ML amount, ~0.8 × 10-

10 mol cm-2 obtained from STM results). However, electrodes

fabricated in this way revealed little or no well-defined redox

peaks in cyclic voltammograms tested in 0.1 M LiCl aqueous

solution. We attribute this to a morphological effect of 1-1OHSQ

molecules on voltammetric behavior as discussed below.

Figure 13 shows a 5 µm × 5 µm scan AFM image of 1-

1OHSQ on HOPG prepared by the dropping method. Many

bright areas attributable to deposited 1-1OHSQ aggregates or

small crystallites can be seen. These structures have typical

heights of several nanometers but sometimes heights exceed

10 nm. The aggregates appear to align along specific directions.

Similar tall aggregate structures of 1-1OHSQ were also observed

on the basal planes of SnS2 single crystals when a similar

deposition method was used [32]. We speculate that the 3D

aggregates have difficulty in incorporating the electrolyte anions

deep into their bulk structure even though a bias positive enough

to oxidize the free molecule was applied. In other words, the

potential drop developed between the electrode surface and the

dye in the resistive 3D aggregates was insufficient to drive the

reaction. A similar logic is true for electrodes coated with

polymer films containing redox active groups that cannot be

permeated by counterions [37].

However, samples prepared by the dipping method showed

electrochemical activity even when the surface coverage of 1-

1OHSQ was estimated to be several layers (judging from total

area of voltammetric peaks and STM observations of the

samples). In some cases a diffusion tail on the SCV peaks did

indicate that diffusion was operating at high coverages (see Fig.

7(a)). The photoelectrochemical behavior of the 3D aggregates

of 1-1OHSQ was shown to be much different than for

monolayers of this dye [32]. The red shift in the photocurrent

Fig. 11. (A) 500 nm × 500 nm STM image showing broken layers of

oxidized 1-1 OHSQ. Rounded domain boundaries are clearly visible.

(B) 3D representation of a 98 × 98 nm scan STM image of an oxidized

layer showing lamella and rectangular defects.


Fig. 12. (A) 20 nm × 20 nm scan STM image showing polymorph 1 type row structure. Oxidized chromophores are evident as bright football-like

shapes. The unit cell is represented by the black box overlayed on the image and projected onto the proposed molecular model. Donor-acceptor

interactions as well as hydrogen-bonding appear to drive this polytype. Possible positions of the chloride counterions are included within the model.

(B) 28 nm × 28 nm scan showing oxidized polymorph 2 type row structure. In this image oxidized chromophores appear as bright dog bone-like

shapes. The slightly more acute angle of the molecular long axis to the direction of the row for this polymorph allows adjacent donor and acceptor

moieties to come into closer proximity.


action spectrum for the tall aggregates is indicative of different

molecule-molecule interactions than in the monolayer.

Incorporation of the anions into these 3D structures is necessary

for the oxidation of the 1-1OHSQ molecules but the 3D structure

may have no mechanism for ion conduction. Thus, the

electrochemically inactive aggregates could have different

aggregate structures than the active layers.

4. Conclusions

The potential for electrochemical oxidation of 1-1OHSQ

dye layers adsorbed on the basal plane HOPG electrodes in

aqueous electrolytes has been shown to be strongly dependent

on the properties of the electrolyte anions. Staircase cyclic

voltammograms (SCVs) of these electrodes revealed very sharp

peaks and a large hysteresis for oxidation and reduction of the

dye layers. The large rearrangement of the adsorbed dye layers,

due to the incorporation of the electrolyte anions upon oxidation,

is responsible for the large hysteresis in the voltammograms.

Whether the hysteresis is explained by the miscibility gap

model [29] or if a nucleation model can explain the results is

still an open question. Nucleation does appear to be important

especially in the reduction of the oxidized dye layers. We

attribute the many current spikes in the SCVs to the reduction

of different domains of oxidized 1-1OHSQ. STM images

showed the presence of many domains in both the oxidized and

reduced forms. The increased stability of the ionic 2D lattice

perhaps is responsible for the difficulty of reducing it and

expelling the anions. If a nucleation event was limiting the re-

reduction of the layers it would be expected that the smaller

domains would be less likely to have a nucleation event and

would require a larger electrochemical potential to initiate

reduction. In most experiments most of the charge was present

in the initial reduction peak and after repeated scanning the small

current spikes would eventually disappear. We attribute this to

annealing the small domains into larger domains during the

continual oxidation-reduction cycles.

STM images also provided information on the molecular

arrangements of both the oxidized and reduced forms of dye

layers. Differences observed between the two phases could be

the results of structural changes associated with anion

incorporation caused by electrochemical oxidation. However,

definitive proof of anion incorporation and their location was

not obtained from the present STM study. Further investigation

of redox processes of surface confined ordered molecular layers

is needed to work out the detailed mechanism of the surface

transformation. AFM and STM experiments done in-situ in an

electrochemical cell will be able to shed light on these processes

as well as the mechanism of transforming the 2D reduced dye

structures into the 2D oxidized structures.

Acknowledgments

We would like to thank the Lexmark International

Corporation for providing squaraine dyes used in this study.

This work was supported by the Department of Energy Office

of Basic Energy Sciences under contract DE-F603-96ER14625.

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