Patterned ferrocenemethanol modified carbon nanotube electrodes onsilane modified silicon

Benjamin Scott Flavel, Jingxian Yu, Joseph George Shapter* and Jamie Scott Quinton

Received 22nd October 2007, Accepted 23rd October 2007

First published as an Advance Article on the web 29th October 2007

DOI: 10.1039/b716284g

Using atomic force anodisation lithography with a silane

monolayer as a resist, patterned silicon oxide nanostructures

were etched onto a silicon substrate. Condensation reactions

were used to immobilise single-walled carbon nanotubes

(SWCNT) onto the oxide followed by attachment of ferro-

cenemethanol to the nanotubes. These nanostructures are

used as electrochemical electrodes to observe oxidation and

reduction of ferrocene demonstrating that such controlled

nanostructured electrodes are ideal candidates for molecular

electronic devices, particularly molecular memory as informa-

tion may be stored in discrete redox states.

Current device and process scaling in the semiconductor industry

has led to a continuous demand for increased density of silicon-

based electronics and a decrease in the cost per bit. However,

scaling techniques currently employed will soon reach physical

and technical limitations1,2 and alternative methods are now

stimulating great interest. One such method is molecular electro-

nics which has been proposed to circumvent the limitations

associated with current semiconductor devices allowing scaling

down to the molecular level3 whilst dramatically decreasing the

associated cost.1

The field of molecular electronics is very broad. However, two

promising approaches are the use of single-walled carbon nano-

tubes as interconnects4,5 and redox-active molecules as a storage

medium.1,6 Carbon nanotubes have attracted considerable interest

due to their unique electronic properties such as the high current

densities possible and ballistic transport properties making them

ideal candidates for molecular scale wires.5,7 It has been shown by

Gooding et al. that carbon nanotubes attached to gold surfaces

with alkanethiols allow electrical communication between the

underlying electrode surface and redox proteins.8,9 However,

widespread technological application of this approach is

uncertain due to issues related to the long-term stability of

alkanethiols on gold.10

Electroactive molecules have stimulated interest as they afford

discrete states at distinct voltages. It is therefore possible to store

data through the programmed access of the oxidation and

reduction states.1,6 Misra et al. have demonstrated a four state

memory element utilising a mixed self assembled monolayer of

ferrocene and porphyrin on a silicon substrate.11 Furthermore,

Zhou et al. showed that redox molecules interfaced with a nano-

wire can lead to multilevel memory.12 However, for any molecular

electronic approach to be successfully integrated into existing

technologies the ability to precisely control the orientation and

position of molecular interconnects and storage media is of crucial

importance.6,13 Additionally, construction based on silicon sub-

strates will see much more straightforward integration with current

architectures. Attempts to address these issues are the subject of

this communication.

Previously we have demonstrated the assembly of single-walled

carbon nanotubes on lithographically patterned silicon/silane

substrates14 and the electron transfer characteristics of carbon

nanotubes on unpatterned silicon (100) surfaces.10,15,16 The

investigation in this communication combines these two appro-

aches in an attempt to successfully fabricate an electro-active and

site selective array of carbon nanotubes. A scanning probe method

called anodisation lithography is used to create regions of silicon

oxide in a self assembled monolayer of hexadecyltrichlorosilane on

silicon substrates, to which single-walled carbon nanotubes with

high carboxylic acid functionality are directly attached. The

electron transport of this carbon nanotube architecture has then

been demonstrated with cyclic voltammetry of redox-active

ferrocene in solution. Furthermore, ferrocenemethanol has been

attached to the immobilised carbon nanotubes using a condensa-

tion reaction between the carboxylic acid groups on the carbon

nanotube with the alcohol. Cyclic voltammetry is then repeated

in a supporting electrolyte solution demonstrating the potential

application for molecular memory.

The preparation of the single-walled carbon nanotubes

(SWNTs) has been described in detail previously.14 In brief,

SWNTs from Carbon Solutions Inc, California, USA, RFP-

SWNT, were refluxed in dilute nitric acid (HNO3) for 24 hours.

The dilute nitric acid solution was then removed and the

nanotubes were placed into a more aggressive oxidiser, namely

a 3 : 1 v/v solution of concentrated sulfuric acid (98%) and

concentrated nitric acid (70%) and ultrasonicated for 8 hours at

0 uC.8,17 The shortened nanotubes were then diluted in 500 mL

of ultrapure water and then filtered through a 0.45 mm poly-

tetrafluoroethylene (PTFE) filter membrane. The nanotubes were

further washed with MilliQ water to bring the pH to 5–7 and dried

under vacuum.8

Atomic force microscope (AFM) images were taken in air using

commercially available silicon cantilevers (FESP-ESP) with

fundamental resonance frequency between 70 and 85 kHz.

Topographic (height) images were obtained. Anodisation litho-

graphy was conducted in tapping mode with platinum/iridium

coated silicon cantilevers (SCM-PIT) with fundamental resonance

frequency between 60 and 75 kHz. Precise movement of the

cantilever was controlled using scripts written in C++. All patterns

were written with an applied cantilever voltage of 211 V, a tip

velocity of 1 mm s21 and an atmospheric humidity level between

School of Chemistry, Physics & Earth Sciences, Flinders University,Sturt Road, Bedford Park, Adelaide SA 5001, Australia.E-mail: joe.shapter@flinders.edu.au; Tel: +61 (08) 82012005

COMMUNICATION www.rsc.org/materials | Journal of Materials Chemistry

This journal is � The Royal Society of Chemistry 2007 J. Mater. Chem., 2007, 17, 4757–4761 | 4757

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30 and 45%. The tip–substrate distance was reduced prior to

patterning by reducing the amplitude set point to y70% of its

original value. All images presented represent background

subtracted data (using the flatten feature in the WSxM software).18

Highly boron doped p-type silicon (100) with resistivity

,0.001 V cm (Virginia Semiconductor, Inc. USA) was cut into

1 cm 6 1 cm sized wafers and ultrasonically cleaned in acetone for

30 seconds before being thoroughly rinsed with MilliQ water. The

wafers were then cleaned with a 1 : 3 (v/v) mixture of 30% H2O2

and 98% H2SO4, (piranha solution) at 80 uC for 15 minutes

followed by rinsing with MilliQ water. It has been shown that

treatment of silicon with piranha solution affords a highly

hydroxyl terminated silicon surface with a density of approxi-

mately 1015 –OH groups cm22.2 The hydroxyl terminated silicon

wafers were then immersed in a 0.5% solution of 90%

hexadecyltrichlorosilane in hexadecane for 40 minutes, then rinsed

with chloroform and allowed to dry.

Using a method developed in our group,10 carbon nanotubes

were directly attached to the silicon oxide lines created using

anodisation lithography. This process is shown schematically in

Fig. 1. After surface patterning, the silicon wafer was immersed in

a 1 : 1 : 5 solution of 30% NH4OH, 30% H2O2 and MilliQ water

for 15 minutes at 80 uC, thoroughly rinsed with MilliQ water and

blown dry under a stream of ultra-pure nitrogen prior to

immersion into a 1 : 1 : 5 solution of 36% HCl, 30% H2O2 and

MilliQ water for 15 minutes at 80 uC. The sample was then once

again rinsed and dried prior to exposure to carbon nanotube

solution. The now hydroxyl terminated silicon oxide10 pattern was

incubated at 80 uC in a DMSO solution containing N,N9-dicyclo-

hexylcarbodiimide (DCC) and functionalised carbon nanotubes

for 8 hours. Prior to use the carbon nanotube solution was

ultrasonicated for 5 hours to evenly disperse the carbon nanotubes

in solution. Finally the silicon wafer was rinsed in acetone and

allowed to dry. Ferrocenemethanol was then attached to the

carboxylic acid end groups of the carbon nanotubes using another

condensation reaction as shown in the final step of Fig. 1. The

attached carbon nanotube substrate was immersed in 3 mL of a

DMSO solution containing 0.3 mg of N,N9-dicyclohexylcarbodi-

imide and 1.5 mg of ferrocenemethanol (Sigma-Aldrich) for

48 hours at 25 uC. The substrate was then rinsed in acetone and

dried with nitrogen.

Electrochemical measurements were performed with a

BAS100B Electrochemical Analyser (Bioanalytical Systems Inc.,

USA), operating in cyclic voltammetry mode using a specially

designed electrochemical cell. The electrochemical cell has been

described previously10 and is shown schematically in Fig. 2.

Briefly, the underside of the silicon substrate was roughened using

a silicon carbide crystal and adhered to aluminium foil to form the

working electrode. Two different electrochemical experiments

were carried out, one with unmodified carbon nanotube electrodes

and the second with modification as shown in Fig. 2.10 In

the unmodified situation, an electrolyte solution containing

1 mmol L21 ferrocene and 1 mmol L21 tetrabutylammonium

perchlorate (TBAP) in acetonitrile was used. For the ferrocene-

methanol modified carbon nanotube electrodes an electrolyte

solution containing 1 mmol L21 of tetrabutylammonium per-

chlorate in acetonitrile was used. The potential was swept initially

in the anodic direction from 100 to 900 mV at 100 mV s21. The

background capacitive current was subtracted using the UTLIS,

Utilities for Data Analysis software (Dirk Heering).

A requirement of any system to be used in molecular electronics

applications is the important ability to sense and measure current

depending on the electronic state of the system.19 Electrochemical

analysis, namely cyclic voltammetry, which has been used

extensively in the literature,6,11,16,19 was used to probe the ability

of the fabricated carbon nanostructures to conduct electrons and

exhibit distinct states. An AFM image of a carbon nanotube

electrode is shown in Fig. 3 and it can be seen that there is a

relatively small electro-active surface area covered by carbon

nanotubes, which makes it difficult to measure the current from an

individual pattern using the electrochemical setup described. For a

single patterned area of carbon nanotubes, the active surface area

is 4.8 mm2 which is a reduction of active surface area in the order of

105 compared to previous work10 with unpatterned silicon where

the active surface area is 3.84 6 105 mm2. For this reason, the

silicon surface was repeatedly patterned to create an active surface

region of 300 mm2 to increase the current to a level that is

detectable with our electrochemical system. All voltammograms

presented in this paper are the response from a collection of

patterned regions. However, even with repeated patterning the

resultant active electrode surface is still 103 times smaller thanFig. 1 Mechanism for fabrication of a patterned ferrocenemethanol

modified carbon nanotube electrode on silicon (not to scale).

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unpatterned silicon. This is the reason for the reduction in

electrochemical current compared to the previous work.10

Fig. 4 shows the cyclic voltammograms when the carbon

nanotubes structures were used as the working electrode using a

scan rate of 100 mV s21. For both patterned carbon nanotubes

structures both the raw data and a background subtracted

voltammogram are shown along with the electrochemical response

obtained by placing the cell on a region containing only hexa-

decyltrichlorosilane (i.e. no nanotubes). It can be seen that for

unmodified carbon nanotubes when the electrochemical cell is

placed over a region of silane layer the cyclic voltammogram

displays only capacitive current with no observable redox waves.

This combined with previous work,10,16 where it has been shown

that on unpatterned bare silicon substrates redox behaviour is only

observable upon the attachment of carbon nanotubes, proves

that the observed electrochemical response is attributable to the

patterned carbon nanotubes. For the ferrocenemethanol modified

carbon nanotube structures, other small peaks are observable as

a result of the very sensitive scale used. The origin of these

background peaks is discussed in detail later.

For the directly attached carbon nanotube electrode in an elec-

trolyte solution containing 1 mmol L21 ferrocene and 1 mmol L21

tetrabutylammonium perchlorate in acetonitrile distinct redox

waves can be seen. The anodic and cathodic peak positions were

found from the background subtracted data to be at 619 and

403 mV with peak currents of 8.29 and 29.63 nA respectively.

Compared to previous work on unpatterned silicon, this is an

increased separation of the oxidation and reduction peaks by

84 mV. In the previous work on unpatterned silicon a peak current

of approximately 3 mA was obtained for unmodified carbon

nanotubes10 whereas in this work, in the same cell, a peak current

of 8.29 nA is observed, which is a decrease by a factor of 103 and

corresponds well to the decrease in electrode area. Whilst ideally

the measured electrochemical signal should come from an

individual patterned region much more sensitive electronics would

be required and this work simply provides proof of principle. It

also highlights one of the drawbacks to anodisation lithography in

that it is a relatively slow and serial process,20 which in order to

pattern the surface sufficiently to obtain an electrochemical

response requires a great length of time. The peak separation

observed in the unpatterned case compares well to a gold electrode

in ferrocene solution, where the peaks were found to be at 474

and 596 mV respectively.10 The increased peak separation for the

electrochemistry in the case of the patterned electrode is to be

expected due to the increased oxide layer thickness created from

anodisation lithography. This results in a slower electron transfer

rate and hence greater peak separation.16

The increased separation of redox peaks compared to a gold

working electrode can also be explained in terms of contact area

Fig. 3 AFM images of a patterned carbon nanotube electrode.

Nanotube attachment is directly to the patterned oxide through an

ester linkage.

Fig. 2 Schematic of the specially designed electrochemical cell and the two nanotube surfaces examined.

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with the electrolyte solution. For a gold electrode the contact is a

relatively homogenous, smooth surface. However for a carbon

nanotube electrode it is possible for electrical contact to be made at

the carbon nanotube terminus and through the sidewalls. It has

been shown that electron transfer rate along the length of the

carbon nanotube is significantly faster than though the sidewalls

which are very similar to the graphite basal plane.16,21,22 Whilst the

measured voltammogram from the carbon nanotube structure will

represent a convolution of the two processes16 the response from

electron transfer through the sidewalls is expected to dominate.

This is due to the one dimensional structure of the carbon

nanotubes23,24 leading to dramatically greater ‘sidewall surface

area’ compared to the relatively small ‘terminus surface area’. It is

possible that this behaviour is not observed for carbon nanotubes

attached to bulk silicon because in that case the electrode

represents a ‘forest’ of carbon nanotubes resulting in increased

‘terminus surface area’ with access to the sidewalls hindered by

neighbouring nanotubes.

For the ferrocenemethanol modified carbon nanotubes redox

waves can still be seen, however the measured current is signifi-

cantly reduced. Due to the very small current being measured the

obtained voltammogram is quite noisy, containing many other

peaks. These peaks have been marked with an asterisk (*) and are

attributable to the oxidation and reduction of the silicon substrate

as they are observed when the electrochemical cell is placed over a

bare silicon substrate. These background peaks are also observed

when the electrochemical cell is placed over a region containing

carbon nanotubes or over an unpatterned silane region. This small

amount of oxidation and reduction is possible when placed over a

patterned area due to regions where carbon nanotubes have not

Fig. 4 Cyclic voltammograms of a) carbon nanotubes and b) ferrocenemethanol modified carbon nanotubes attached directly to patterned silicon

electrodes at a scan rate of 100 mV s21.

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been attached. In the case of the silane monolayer, oxidation and

reduction is possible through the many pin holes and inhomo-

geneities present in a self assembled silane layer.2

The anodic and cathodic peak positions were found from the

background subtracted data to be at 500 and 444 mV with peak

currents of 0.2 and 20.17 nA respectively. The reduction in

observed current going from ferrocenemethanol in solution to the

case where the redox active molecule is attached is simply due to a

reduction in the amount of available redox molecules which is

much higher for the solution case. AFM imaging of the nanotube

substrate before and after attachment of the ferrocenemethanol

shows no changes in the substrate meaning variation in observed

current is not due to substrate modification.16

It can be seen that upon attaching the electro-active ferrocene-

methanol molecule directly to the carbon nanotube peak

separation decreases by 150 mV compared to the separation

observed for ferrocene in solution over the unmodified carbon

nanotube electrode. This, along with the reduction in current, can

be explained once again by considering the location on the carbon

nanotube from which electron transfer is occurring. It has been

shown that the oxidation of carbon nanotubes with nitric and

sulfuric acid mixtures will introduce carboxyl groups at the highly

reactive end caps of the carbon nanotube.8,15,25,26 Whilst some

carboxyl functionality will exist along the sidewalls from defects

such as the Stone–Wales defect,26 carboxyl groups will predomi-

nately exist on the end caps25,26 and hence it is expected that

significantly more ferrocenemethanol will be immobilised in these

regions. As a result of this most molecular attachment will occur

on the end of the nanotubes meaning that most electron transfer

occurs along the length of the carbon nanotube shifting the

oxidation and reduction peaks towards each other due to the ease

of electron transfer down the length of the nanotube. However, as

the electroactive species are now on such a small surface area, a

decreased current is observed. This decrease in peak separation has

also been observed for modified carbon nanotubes on unpatterned

silicon substrates where the peaks were seen to shift together by

42 mV, yielding a peak separation of 80 mV compared to 122 mV

for unmodified carbon nanotubes.10,16 Upon looking at the

background subtracted voltammogram further it can be seen that

the oxidation and reduction peaks are also quite broad and

possibly even contain some structure at higher peak separations.

We speculate that this could be attributed to ferrocenemethanol

immobilised on the sidewalls through the defect sites previously

discussed which will yield highly separated peaks and hence a

broadened signal.

In summary, this work presents a new approach for the

fabrication of silicon-based molecular electronic devices meeting

the basic requirement that a current can be measured from

spatially defined regions. Whilst the measured electrochemical

currents are quite small and the electron transfer rate is reduced as

a result of the increased oxide thickness from anodisation

lithography, this approach looks promising for future application

as it uses low energy processes for fabrication. In particular the

attachment of redox active molecules such as ferrocenemethanol to

defined carbon nanotube structures has significant potential in the

field of molecular memory.

Notes and references

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