Patterned forest-assembly of single-wall carbon nanotubes on gold using a non-thiol...

Patterned forest-assembly of single-wall carbon nanotubes on gold using anon-thiol functionalization technique{

Haoyan Wei,a Sejong Kim,b Sang Nyon Kim,b Bryan D. Huey,a Fotios Papadimitrakopoulos*b andHarris L. Marcus*a

Received 17th July 2007, Accepted 27th August 2007

First published as an Advance Article on the web 7th September 2007

DOI: 10.1039/b710854k

An approach for non-thiol functionalization of single-wall carbon nanotubes (SWNTs) on gold

was demonstrated via an Fe3+-assisted self-assembly technique. Upon immersion of gold into a

pH 2.2 aqueous FeCl3 solution, FeCl3 oxidized the gold surface, due to the aqua regia effect,

resulting in the formation of films of FeO(OH)–FeOCl crystallites. Subsequent immersion into a

SWNT dimethylformamide (DMF) dispersion led to needle-like forest assemblies of SWNTs

based on metal-assisted chelation and electrostatic interactions. Two approaches for surface

patterning of these SWNT forests were investigated based on shadow-mask evaporation and

conventional photolithographic lift-off to localize FeO(OH)–FeOCl/Au composite pads on Si

substrates. The strong adhesion of Fe3+ ions onto silica surfaces can be partially overcome by

repeated washes in aqueous HCl solution (pH , 4), and completely overcome by photoresist-

assisted protection which prevents unwanted Fe3+ ions from complexing with the unexposed silica

surfaces. Such patterned Fe3+-functionalized Au structures provided the basis for the site-specific

forest-assembly of SWNTs as characterized by atomic force microscopy (AFM) and resonance

Raman spectroscopy.

Introduction

Carbon nanotubes (CNTs) have attracted considerable atten-

tion due to their unique structures, remarkable mechanical and

electrical properties.1–3 They have found applicability in a

variety of fields such as nanodevices,4–8 sensors,9–12 high-

aspect-ratio AFM probes13–16 and field emitters.17,18 For a

number of these applications, it is very important to be able to

immobilize CNTs at desired sites with controlled orientation

perpendicular to the underlying substrates, in order to

integrate these remarkable nanostructures into functional

devices. A number of researchers have reported on the

perpendicular growth of single- and multi-wall carbon

nanotubes (SWNTs, MWNTs) using chemical vapor deposi-

tion (CVD) on patterned metal catalyst surfaces achieved

via various methods such as standard lithography,19,20 soft

lithography,21 ink-jet printing22 and nano-channel tem-

plates.23,24 Although the CVD-grown CNTs possess the right

orientation to serve as probes, they are loosely packed (density

around 1011 cm22),24 which renders them extremely difficult to

be handled in the presence of solvents and upon drying they

easily collapse. In addition, the co-existence of various types of

SWNTs (metallic (met-) and semiconducting (sem-)) could also

pose serious problems for electronic devices in case only either

semiconducting or metallic nanoprobes are needed.25

Self-assembled shortened-SWNTs (s-SWNTs) have appeared

to be an alternative viable route to form vertically oriented

CNT arrays, which could also take advantage of the achieve-

ment of post-synthesis separation by length26 and type (sem-

versus met-).27,28 Prior research in our laboratory has shown

that densely packed rope-lattice SWNT forests (density is ca.

1013 cm22 or 1.1–1.2 g cm23,29 which compares favorably with

1.33 g cm23 density of a van der Waals rope-lattice crystal

composed of 1.3–1.4 nm diameter SWNTs30) can be readily

obtained by assembling nanotubes from a DMF dispersion

onto an underlying substrate via the linkage of Fe3+ ions.29

Recently, we have demonstrated the patternability of these

forest-assemblies of SWNTs on Nafion31 and SiOx/Si sub-

strates32 with the aid of electron-beam writing by modifying

substrate surface properties to create preferential nucleation

sites for Fe3+ ions.

In the present contribution, we are reporting our initial

findings in assembling s-SWNT forests on gold substrates

using a non-thiol functionalization technique based on the

direct surface oxidation of Au by acidic FeCl3 treatment.33

This offers the possibility to achieve direct metal function-

alization of Au substrates in a very short period of time

(ca. 15 min). The conventional approaches used to immobi-

lize SWNTs on Au usually require the adsorption of

thiol molecules. The long incubation process of thiol

aMaterials Science and Engineering Program, Department of Chemical,Materials and Biomolecular Engineering, Institute of Materials Science,University of Connecticut, Storrs, CT 06269, USA.E-mail: [email protected]; Fax: (+01) 860 486 4745;Tel: (+01) 860 486 4623bNanomaterials Optoelectronics Laboratory, Polymer Program,Institute of Materials Science, Department of Chemistry, University ofConnecticut, Storrs, CT 06269, USA.E-mail: [email protected]; Fax: (+01) 860 486 4745;Tel: (+01) 860 486 3447{ Electronic supplementary information (ESI) available: AFM micro-graph and Auger spectrum of Au substrate after immersion intoFe2(SO4)3; Auger and XPS spectra of Au substrate after immersioninto pH 2.2 HCl aqueous solution; Auger survey of FeO(OH)–FeOCl/Au strip patterns by photolithographic lift-off process. See DOI:10.1039/b710854k

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

This journal is � The Royal Society of Chemistry 2007 J. Mater. Chem., 2007, 17, 4577–4585 | 4577

functionalization34,35 (at least several hours in order to obtain

good coverage of thiol molecules on gold) as well as low

surface coverage35 of CNTs are among the few drawbacks of

that approach. In contrast, direct Fe3+-functionalization of

Au substrates eliminates the time-consuming thiol adsorption

process. Combined with the previously reported Fe3+-assisted

self-assembly of SWNTs,29,31 dense arrays of SWNT forests

can be readily obtained on Au substrates from their DMF

dispersion. Moreover, desired patterns of SWNT forest

assemblies can be generated with the aid of a variety of

surface patterning techniques, such as shadow-mask evapora-

tion or photolithographic lift-off processes.

Experimental

Iron(III) chloride hexahydrate (FeCl3?6H2O, A.C.S. reagent),

nitric acid (98%), sulfuric acid (96.4%) and hydrochloric acid

(38%) were obtained from Aldrich and used as supplied.

Iron(III) sulfate (Fe2(SO4)3, reagent grade) was acquired from

Alfa Aesar. Hydrogen peroxide (30%) was obtained from

Fisher Scientific. A.C.S. reagent dimethylformamide (DMF)

was purchased from J.T.Baker. Millipore quality deionized

water with resistivity greater than 18 MV was used for all

experiments. Si (100) wafers were obtained from Montco

Silicon and cleaned in piranha solution (concentrated H2SO4

and 30% H2O2, 7 : 3 v/v) at 90 uC for 30 min. HiPco SWNTs

were purchased from Carbon Nanotechnologies, Inc (CNI).

Following the previously established protocol,36–38 pristine

SWNTs were shortened in a 3 : 1 mixture of HNO3 and H2SO4

with sonication for 4 h at 70uC, filtered, washed with copious

deionized water until the pH of the filtrate was neutral

and dried overnight in vacuum. Sonicating these purified

shortened-SWNTs (s-SWNTs) in DMF resulted in a stable

dispersion.

Au substrates were prepared in three forms: (i) atomically

flat Pico Au (111), (ii) thermally evaporated Au, and (iii) a

sputtered Au–Pd mixture. Atomically flat Pico Au (111)

substrates were purchased from Molecular Imaging. They were

made by evaporating approximately 150 nm of Au onto freshly

cleaved mica substrates with subsequent hydrogen flame

annealing, and packaged in nitrogen. Home-made Au films

were deposited on Si substrates by thermal evaporation from a

tungsten filament at a base vacuum pressure of less than 5 61026 Torr. Chromium (10 nm thick) was deposited before Au

evaporation to serve as the adhesion layer on the Si substrates.

The evaporation process was monitored by a Maxtex, Inc.

thickness monitor and was completed when the Au film

thickness reached 50–100 nm. Sputtered Au–Pd substrates

were made in a Polaron Instruments SEM sputtering coating

system (E5100) using a blended Au–Pd (60 : 40) target. An Ar

plasma was produced at 2.2 keV with the sputtering current

maintained at 20 mA by controlling the Ar gas flowing rate.

The sputtering process lasted ca. 4 min with the obtained film

thickness of ca. 50–100 nm.

SWNT forests were fabricated using our previously reported

Fe3+-assisted self-assembly technique.29,31,39 The schematic

process is presented in Scheme 1. First a layer of Au was

deposited on the Si substrates using one of the aforementioned

approaches. SWNT/Fe3+ assemblies were then obtained by

sequential dipping of the Au substrates in aqueous FeCl3solution (pH 2.2, 15 min) and DMF dispersed SWNTs (pH 8.5,

30 min) separated by intermediate DMF wash (pH 12.7, to

remove excess water and facilitate the transformation of ferric

ions into their basic hydroxide form).29,39

Since Fe3+ ions act as linkers to anchor the SWNT forests

onto the underlying Au substrates, the appropriate placement

of FeO(OH)–FeOCl crystallites is pivotal to realize the

SWNT patterns.31 To this end, patterned FeO(OH)–FeOCl/

Au substrates were prepared with the aid of either shadow-

mask evaporation or photolithographic lift-off methods.

The specific procedures are illustrated in Scheme 2 a and b

respectively:

(1) Shadow-mask evaporation approach (Scheme 2a):

Au/Pd films were first sputtered onto Si substrates through a

TEM grid. Films of FeO(OH)–FeOCl crystallites were then

obtained by sequential dipping of substrates in (i) FeCl3(pH 2.2, 15 min) solution, (ii) multi-step (¢8) wash in aqueous

HCl solution (pH , 4) to remove excess Fe3+ ions adsorbed

in the uncoated Si regions while maintaining Fe3+ surface

functionality in the Au domains, (iii) brief wash in DMF

(pH 12.7) to remove excess water and facilitate the precipita-

tion of Fe3+ ions as FeO(OH)–FeOCl crystallites.31

(2) Photolithographic lift-off approach (Scheme 2b): Si

substrates were first covered with a layer of positive photo-

resist (Shipley S1813) by spin coating and then exposed to UV

light through a shadow mask with desired patterns. During

the developing process, the exposed photoresist regions were

solubilized away in Shipley MF351 developing medium.

Au film was evaporated onto the developed patterns with

chromium as the adhesion layer. FeO(OH)–FeOCl crystallites

were then deposited by sequential dipping of substrates in (i)

aqueous FeCl3 (pH 2.2, 15 min) solution and (ii) non-aqueous

DMF (pH 12.7) solvent. Subsequently the unexposed resist

regions, along with their Au/FeO(OH)–FeOCl overlayers,

were lifted off in DMF with the aid of gentle sonication to

obtain the FeO(OH)–FeOCl/Au patterns for subsequent self-

assembly of SWNT forest arrays.

AFM characterization was performed with an Asylum

Research MFP-3D in AC mode (Tapping Mode) to investigate

both the FeO(OH)–FeOCl/Au and their SWNT forest

topologies. Standard Si AFM probes from Asylum Research

(Model # AC160, spring constant 42 N m21, resonant

Scheme 1 Schematic representation of metal-assisted self-assembly of

SWNT forests on Fe3+-functionalized Au substrates.

4578 | J. Mater. Chem., 2007, 17, 4577–4585 This journal is � The Royal Society of Chemistry 2007

frequency 300 kHz) were used. An Auger survey was

performed to investigate the precipitation of iron deposits on

Au substrates in a Perkin-Elmer/PHI multi-probe surface

analysis system using a PHI model 15-255GAR precision

electron energy analyzer with a built-in electron gun operated

at 3 keV.

Preferential self-assembly of SWNT forests was charac-

terized by both AFM and focused resonance Raman

spectroscopy. The latter was performed using a Renishaw

Ramanscope 2000, equipped with a 514 nm laser source

focused on a 1 mm spot by a 1006 objective lens.

Results and discussion

Direct Fe3+ functionalization of gold substrates for SWNT

forest assemblies

All three forms of Au substrates behaved the same in terms

of absorbing Fe3+ ions independent of surface morphology

and crystal orientation, and were used without further

differentiation. Fig. 1 illustrates a typical AFM topographical

image of the atomically flat Pico Au (111) substrates

which exhibit flat terraces with termination of (111) crystal

planes after hydrogen flame annealing. The terrace domain

size is typically on the order of 300 to 500 nm. The overall

surface roughness RMS value of the Pico Au substrates is

1.1 ¡ 0.2 nm.

After immersion into pH 2.2 FeCl3 solution for 15 min

followed by a brief DMF wash, iron precipitates were observed

on the Pico Au (111) surface, as shown in Fig. 2a. As indicated

in the high resolution image provided in the inset, these

precipitates appear to have crystalline morphology of size ca.

100 and 20 nm in length and diameter respectively. These

crystallites were further identified as FeO(OH)–FeOCl in

structure by TEM studies.31 These iron deposits exhibited a

semi-continuous coverage of the Au surface with one to two

crystallite thickness on average. The overall AFM measured

RMS surface roughness was on the order of 10 nm. Auger

investigation further confirmed that these crystallites were iron

containing deposits as confirmed by the emergence of three

strong Fe LMM (atomic shells) peaks in the spectrum as

shown in Fig. 2b. The relative concentrations of FeO(OH) and

FeOCl were estimated based on the Fe/Cl ratio calculated

from the Auger spectra. It was found that the majority (ca.

76%) was FeO(OH) with the remainder FeOCl.

Upon immersion of these FeO(OH)–FeOCl decorated

Au substrates into DMF dispersed s-SWNTs, acid–base

neutralization between the carboxylic acid terminated nano-

tube ends and the basic iron hydroxides provided the initial

driving force for SWNT assembly.29 Because nanotube side

walls are hydrophobic while the Fe3+/Au surface is hydro-

philic, direct contact with each other is energetically unfavor-

able. Thus instead of lying down on the iron deposits these

SWNTs oriented vertically to the substrates. Moreover, they

were attracted to each other due to the strong hydrophobic

interaction between adjacent side walls, which further facili-

tated the bundling along the lateral direction leading to rope-

lattice SWNT forest assemblies. Fig. 3a illustrates a typical

AFM image of the resulting SWNT forests onto Fe3+/Au

Scheme 2 Schematic representation of patterning and Fe3+-function-

alization of Au via a) shadow-mask evaporation and b) photolitho-

graphic lift-off processes.

Fig. 1 Topological AFM image of atomically flat Pico Au (111) in

the as-received state.


substrates after 30 min immersion in a SWNT/DMF disper-

sion. Although the length range of the DMF-dispersed

s-SWNTs was 25–250 nm,26 the average height of SWNT

assemblies was on the order of 30–45 nm,29 in agreement with

prior reports.12,40

Resonance Raman spectroscopy has been a very valuable

tool for detecting and characterizing SWNTs. Fig. 3b shows

a typical resonance Raman spectrum of the SWNT forest

assemblies on Fe3+/Au substrates in the region of 1200–

1700 cm21, providing further evidence for the successful self-

assembly of SWNTs on Fe3+-functionalized Au substrates.

The higher frequency doublet peaks are called G-band and

correspond to the C–C tangential stretching mode. For

SWNTs, this G-band typically splits into two characteristic

features, G+ (ca. 1592 cm21) and G2. The line shape of the G2

peak is largely dependent on nanotube metallicity41 while the

position of the G+ peak is dependent on the redox state of

SWNTs.42 In addition, there is also a moderate band observed

around 1350 cm21 which comes from the defects in the

hexagonal framework of the SWNTs, usually called a disorder-

induced band, or D-band.43 Previous polarization Raman

studies showed a 35-fold increase of G-band intensity if the

substrate was rotated by 90u rendering the nanotube forest

axis parallel to the polarization of the incident laser.29 This

indicates a high degree of nanotube alignment normal to the

substrate resulting in a needle-like forest configuration as

shown in AFM measurements (Fig. 3a).

To rule out the possible contributions to the spectrum from

underlying substrates, control experiments were performed on

samples after each processing step. The obtained Raman

spectra (Fig. 3b) clearly showed that the peak around the

G-band only occurred after immersion in the SWNT disper-

sion. This further affirmed that the emergence of the 1500–

1600 cm21 features originated from the assembled SWNTs

rather than from the underlying substrates.

Possible mechanism of direct assembly of FeO(OH)–FeOCl

films on bare Au substrates

Aqua regia ( otherwise called ‘‘royal water’’) is a mixture of

concentrated nitric acid and hydrochloric acid (ratio 1 : 3).

Aqua regia etches gold and other noble metals such as

platinum although each acid alone in aqua regia won’t do so

because each component plays a different role during the

solubilization reaction. Nitric acid is a strong oxidizing agent

which dissolves a tiny amount of gold (see reaction (I) below).

Then the chloride anions coordinate with Au cations to form

Fig. 2 a) Topological AFM image and b) Auger spectrum of

FeO(OH)–FeOCl crystallites deposited on an atomically flat Pico

Au (111) substrate.

Fig. 3 a) Representative AFM topological micrograph of SWNT

forests on Fe3+/Au substrates after 30 min immersion in DMF

dispersion of s-SWNTs. b) Typical resonance Raman spectra (514 nm

laser source) of SWNTs/FeO(OH)–FeOCl/Au, FeO(OH)–FeOCl/Au

and Au substrates.


complexes (reaction (II)), lowering the Au cation concentra-

tion in the solution. This shifts the oxidation reaction towards

the right side according to Le Chatelier’s principle, resulting in

the continuous dissolution of gold.

Au + 3NO32 + 6H+ A Au3+ + 3NO2 q + 3H2O (I)

Au3+ + 4Cl2 A AuCl42 (II)

From the above analysis, it can be concluded that in order to

etch gold it is imperative that both oxidant and complexant

(lixiviant) must be present in solution. This mechanism has

been applied to hydrometallurgical extraction of gold from

ores (gold leaching) by using alkaline cyanides as lixiviant.44

Due to the increasing environmental concerns of the public

over the use of these harmful solutions, many efforts

have been promoted to seek alternative ecologically-safe

lixiviants to overcome the weakness of cyanides. Thiosulfate

and halides44 receive the most attention because they appear to

be more promising. The accompanying oxidants include

Fe(III),45,46 Cu(II),44 hypochlorite/hypochlorous acid44 and

perchloric acid.47

In the absence of a strong oxidation reagent (such as NO32

ions) in the acidic FeCl3 solution, we turned our attention to

the oxidizing effects of Fe3+ ions. Fe(III) ions can serve as an

effective oxidant while chloride ions work as the complexant.

The oxidation of gold by Fe(III) could involve both 1e2 and

3e2 processes (reactions (III) and (IV)).45 The function of

the dissolved oxygen can further maintain a high ratio of

Fe(III)/Fe(II) through reaction (V).45

Au + Fe3+ A Au+ Fe2+ Eo = 20.65 V (III)

Au+ + 2Fe3+ A Au3++ 2Fe2+ Eo = 20.91 V (IV)

O2 + 4H++ 4Fe2+ A 2H2O + 4Fe3+ (V)

Because the standard reduction potentials (Eo) of reactions

(III) and (IV) are negative, gold dissolution does not take

place spontaneously. A very limited amount of gold can be

oxidized by Fe3+ ions. Previous studies33,45,48 indicated that

the onset of gold dissolution will occur if one or more of the

following conditions are met: elevated temperature (Fe3+ ions

become stronger oxidants), high chloride concentrations,

high Fe(III)/Fe(II) ratio (high O2 concentration), laminar flow

conditions and addition of HCl to FeCl3 (lowered solution

pH value suppresses Fe3+ precipitation to its highly insoluble

hydroxides).

It is well established that both gold(I) and gold(III) can

coordinate with chloride ions to form either compounds

(AuCl, AuCl3) or complexes (AuCl22, AuCl4

2). Electro-

chemical cyclic voltammetry (CV) studies demonstrated that

chloride ions were adsorbed as Au–Cl complexes prior to gold

dissolution.49–51 This chemisorbed chloride layer on the gold

surface49,50,52 has a hexagonal closed packed structure, which

was found to be incommensurate with the gold substrate

bearing interatomic separations on the order of the van der

Waals diameter of chloride.49 Kinetic investigations showed

that the gold dissolution rate was so low under static con-

ditions at room temperature with low chloride concentration

that no apparent Au dissolution was observed.48 Thus this

chemisorbed chloride layer remains on the Au surface, which

in turn attracts large amounts of positively charged Fe3+ ions.

Because of the tendency of Fe3+ ions to precipitate as iron

hydroxides at pH 4.3 and above,39 the formation of iron

hydroxides on the gold surface occurs33 upon exposure to the

basic DMF (pH 12.7). Furthermore, Fe3+-chelated H2O

moieties together with trace amounts of dissolved water in

DMF assist in producing FeO(OH)–FeOCl crystallites on

Au surfaces.

To confirm the combined roles of chloride and Fe3+ ions,

control experiments were performed using either iron(III)

sulfate solution or HCl, which contain either only oxidant

(Fe3+) or only complexant (Cl2), respectively. The Fe2(SO4)3

solution was made to have the same Fe3+ concentration

as FeCl3 solution, and the HCl solution was made to have

the same pH value as FeCl3 solution. Following the same

procedure used with FeCl3, Au substrates were immersed

sequentially in (i) aqueous Fe2(SO4)3 (pH 3.4, 15 min) solution,

(ii) aqueous H2SO4 acid wash (pH , 4), and (iii) non-aqueous

DMF (pH 12.7). Auger investigation and AFM micrography

(Fig. S1 of ESI{) indicated that there was no iron deposit

observed on the Au surface. Similarly, neither Auger nor XPS

measurements (Fig. S2 of ESI{) found any existence of Cl

within the detection limit (0.3%) after dipping Au in HCl

(pH 2.2) for 15 min. This further affirms the necessity of the

co-existence of both oxidant and complexant agents to etch the

gold surface and decorate it with Cl2 ions that further attract

the positive Fe3+ ions from the solution to produce FeO(OH)–

FeOCl crystallites upon subsequent immersion in basic DMF

(pH 12.7).

Localization of FeO(OH)–FeOCl crystallites

The procedure to generate Fe3+/Au patterns using a shadow-

mask evaporation approach is illustrated in Scheme 2a. The

Au/Pd patterns were first obtained by sputtering Au/Pd onto

Si substrates through a TEM grid. Films of FeO(OH)–FeOCl

crystallites were then deposited by spontaneous adsorption

by dipping into FeCl3 solution. The key for the selective

FeO(OH)–FeOCl deposition onto Au/Pd regions is multiple

washing with aqueous HCl solution with pH , 4 to selectively

remove Fe3+ cations chelated on the native SiOx layers (ca.

0.6–2 nm thick)53 that cover the Si substrates. For typical wet-

chemical cleaning processes of Si, the surface is terminated

with silanol groups (Si–OH) with concentration of y5.0 61014 cm22 (5.0 nm22).54 These silanol groups readily react with

Fe3+ ions through surface complexation reactions (VI).

x(Si–OH) + Fe3+ u (Si–O)xFe32x + xH+ (x = 1–3) (VI)

To inhibit the adsorption of Fe3+ cations on SiOx, a

continuous wash with acidic chemistry is favorable. This

process can shift reaction (VI) to the left direction by

increasing the concentration of product protons (H+) and

lowering the concentration of reactant Fe3+ ions. Our

experimental results indicated that the iron deposits could be

minimized on the piranha cleaned Si wafer after multiple

washes (¢8) with aqueous HCl (pH , 4). Under the same

wash conditions, these iron deposits were able to survive on


the Au substrates due to their relatively stronger adhesion

to Au. Fig. 4a illustrates an AFM topological image of

FeO(OH)–FeOCl crystallites on Au–Pd island patterns made

by the shadow-mask evaporation approach. The squares are

Au–Pd regions which were fully covered with multilayers (¢2)

of FeO(OH)–FeOCl crystallites as shown in Fig. 4b. The

channels were native oxide coated Si substrates on which only

sporadic iron precipitates were observed (Fig. 4c). These

residual iron deposits on Si domains were believed to mainly

originate from the spatial inhomogeneous distribution of

silanol groups. If silanol groups were close enough to each

other, Fe3+ ions would bind with more than one silanol group

(see reaction (VI)). This multiple binding (especially three

silanols bind with Fe3+ ions together) hold the Fe3+ ions

more strongly, and thus make their removal by the acid wash

very difficult.

Due to the extreme difficulty of completely eliminating iron

deposits on the Fe3+-exposed Si domains, we resorted to a

lithographic lift-off process. Photolithography is widely used

for many pattern fabrication tasks due to its easy access,

capability to generate versatile structures and high throughput.

Moreover it has the potential to overcome the limitation of

SiOx contamination with Fe3+ ions due to the fact that lift-off

takes place in organic solvents in which Fe3+ ions and their

hydroxides have limited solubility. Scheme 2b illustrates the

procedure to generate FeO(OH)–FeOCl/Au patterns through

photolithographic lift-off. Si substrates were first spin-coated

with positive photoresist Shipley S1813 which contains a

Novolac (phenol-formaldehyde) resin and a photoactive

compound (PAC, diazonaphthoquinone). Due to the acidic

nature of phenols, Novolac polymers are very soluble in

aqueous bases. However, the PAC is not soluble in aqueous

base serving as an inhibitor to make the photoresist less

soluble in base.55 During UV exposure, the PAC undergoes

photochemistry and subsequent hydrolysis leading to the

formation of carboxylic acids, which allows the selective

dissolution of the exposed regions in the basic developing

medium. After photoresist developing, an Au film was

thermally evaporated followed by immersion in FeCl3 solution

to absorb Fe3+ ions. Subsequent dipping in DMF transforms

the absorbed Fe3+ ions into FeO(OH)–FeOCl crystallites while

the silanol groups remain covered by the unexposed photo-

resists. Finally, the unexposed photoresist with its top

FeO(OH)–FeOCl/Au cover was lifted off in DMF solvent

with the aid of gentle sonication, leaving behind iron-free Si

domains. DMF was utilized instead of a normal lift-off solvent

(such as acetone) based on the following reasons: (i) DMF is

the solvent used to facilitate transformation of Fe3+ ions into

their basic hydroxides and to disperse SWNTs; (ii) DMF is a

polar organic solvent. During the immersion in FeCl3 solution,

the acidic nature of FeCl3 solution (pH 2.2) assisted in making

the photoresist more hydrophilic. This further facilitated the

lift-off of the unexposed photoresists with DMF which is

polar; (iii) experimental results indicated that use of acetone

would leave more carbon residues on the sample surfaces

contaminating FeO(OH)–FeOCl crystallites. After lift-off, the

obtained Fe3+/Au strip patterns were characterized with AFM

as shown in Fig. 5a. It can be observed that FeO(OH)–FeOCl

crystallites were completely confined within Au regions

(Fig. 5b) and absent from Si areas (Fig. 5c). In the lightly

basic DMF liff-off solvent, silanol groups were deprotonated,

resulting in negatively charged surfaces (Si–O2) behaving like

Lewis bases.53 This basic FeO(OH) will not adsorb onto Si–O2

surfaces due to the same charge repulsion, therefore main-

taining the Si domains clean. The surface roughness RMS of

these FeO(OH)–FeOCl crystallites on Au regions, as measured

by AFM, was found to be on the order of 12 nm, slightly

higher than that of the iron deposits on uniform samples

discussed previously (y10 nm). This increased roughness was

believed to arise from the removal of some FeO(OH)–FeOCl

crystallites during the sonication resulting in a change in

the surface topography. Auger measurements also indicated

a higher carbon intensity for Fe3+/Au lines formed by

Fig. 4 a) Topological AFM image of FeO(OH)–FeOCl crystallites on

Au–Pd island patterns produced by the shadow-mask evaporation

approach. b) Top inset shows in high magnification the FeO(OH)–

FeOCl crystallites on Au–Pd regions. c) Bottom inset illustrates that

the native oxide coated Si channels were also sporadically function-

alized with FeO(OH)–FeOCl deposits. The scan areas of b) and c)

insets are 2 6 2 mm2.

Fig. 5 a) Topological AFM image of FeO(OH)–FeOCl/Au strips

patterned with photolithographic lift-off process. b) Iron deposits are

completely localized within Au domains and c) absent in Si domains.

The scan areas of b) and c) insets are 2 6 2 mm2.


photolithography (ESI{ Fig. S3) in comparison with the

uniform Fe3+/Au samples (Fig. 2b). It is believed that such

carbonaceous materials came from the limited re-adsorption of

the dissolved Novolac polymer in the DMF solvent. This is

facilitated by the acidic nature of the phenolic groups that tend

to interact strongly with the basic FeO(OH). The impact of the

adsorbed Novolac on the surface of FeO(OH)–FeOCl crystal-

lites on the subsequent self-assembly of SWNTs will be

discussed in the following section.

Forest-assembly of SWNTs on patterned Fe3+/Au substrates

Upon immersion of these FeO(OH)–FeOCl crystallite covered

Au/Pd or Au patterns into DMF dispersion of s-SWNTs,

spontaneous assembly took place resulting a needle-like forest

configuration.29,31 Fig. 6 illustrates a typical AFM image of

the resulting SWNT forests on island-patterned Fe3+/Au–Pd

substrates made by the shadow-mask evaporation approach.

The majority of these forests were localized within the

Fe3+/Au–Pd regions although some SWNTs spread into the

Si regions. The sporadic SWNT stumps on the Si regions

originated from the residual iron deposits left on the Si

substrates as described previously (see Fig. 4c). The Raman

map provides further evidence for the preferential self-

assembly of SWNT on Au–Pd areas as shown in Fig. 7.

The FeO(OH)–FeOCl crystallites in multilayer configuration

fully covered the Au–Pd domains, assuring a clear positively

charged surface that attracted a significantly large number of

SWNTs (red in the inset of Fig. 7). Most Si regions are free of

SWNTs (indicated in blue), although some individual SWNTs

(red spots) can still be observed in the Si regions (blue). The

Raman mapping result is consistent with the previous AFM

measurement.

In comparison with FeO(OH)–FeOCl/Au–Pd island pat-

terns obtained by the shadow mask approach, extended

immersion time (.90 min) in the SWNT/DMF dispersion

was required for Fe3+/Au line patterns made by the photo-

lithography approach in order to achieve the SWNT forest

assemblies. This additional time is attributed to reduced

interactions between SWNTs and FeO(OH)–FeOCl crystal-

lites. At present, it is believed that this mainly arises from the

adsorbed acidic Novolac polymers on the surface of

FeO(OH)–FeOCl crystallites during lift-off, resulting in the

need for extra time required to displace these carbonaceous

materials with s-SWNTs. In addition, the applied sonication

during the lift-off process may also cause loss of some iron

deposits, which has been confirmed by the increased RMS

value described previously. Fig. 8 illustrates a typical AFM

image of the resulting SWNT forests on line-patterned

Fig. 6 Representative topological AFM image of SWNT forests on

shadow-mask evaporated Au–Pd patterns (yellow squares).

Fig. 7 Superimposed optical and resonance Raman (1592 cm21)

images of SWNTs forests on the Au–Pd pads of Fig. 6.

Fig. 8 Representative topological AFM image of SWNT forests on

line patterns generated by photolithographic lift-off process. Unlike

Fig. 6, this lift-off process ensures no FeO(OH)–FeOCl/SWNT

contamination of the native oxide coated Si substrates.


FeO(OH)–FeOCl/Au substrates made by the photolitho-

graphic lift-off approach. All SWNT forests were confined

on the FeO(OH)–FeOCl/Au regions as a result of the tight

localization of FeO(OH)–FeOCl crystallites in previous step as

shown in Fig. 5. Fig. 9 provides the corresponding Raman

spectra taken on Au and Si regions respectively. The charac-

teristic G-band (1592 cm21) observed only in Au domains

while absent in Si regions further confirmed the site-specific

assembly of SWNT forests. In comparison to the uniform

SWNT forests (Fig. 3), the reduced Raman signal intensity was

believed to result from the limited adsorption of photoresist

leading to the decreased density of SWNTs. Efforts are

currently underway to improve the photoresist contamination

of FeO(OH)–FeOCl/Au patterns, and these will be presented

in subsequent publications.

Conclusions

In this study we have demonstrated that s-SWNTs could be

immobilized onto gold substrates via Fe3+-assisted self-

assembly using a non-thiol functionalization technique which

eliminated the time-consuming thiol adsorption process used

in conventional methods. The immersion of gold into FeCl3solution led to the formation of films of FeO(OH)–FeOCl

crystallites on the gold surface as a result of the aqua regia

effect. Upon dipping the Fe3+/Au substrates into the DMF

dispersed s-SWNTs, nanotube forests occurred based on

metal-assisted chelation and electrostatic interactions. In order

to pattern the SWNT assemblies, two approaches were

investigated to assist in localizing Fe3+/Au composite pads

using either sputtering Au–Pd though a TEM grid or

conventional photolithography. Such patterned Fe3+-function-

alized Au arrays established the basis for the subsequent site-

specific forest-assembly of SWNTs as demonstrated by AFM

and Raman spectra. These aligned and patterned SWNT

forests could find a number of applications, such as field

emission electron sources17,18 and biomaterial/nanotube

hybrids for biosensor arrays.11,12

Acknowledgements

The authors gratefully acknowledge the financial support of

the U.S. Army Research Office (grant # ARO-DAAD-19-02-

1-0381). This work was also supported in part by PHS grant

ES013557 from the National Institute of Environmental

Health Sciences/National Institutes of Health (NIEHS/NIH).

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Supplementary material (ESI) for Journal of Materials Chemistry

This journal is © The Royal Society of Chemistry 2007

1

Supporting Information

Patterned Forest-Assembly of Single-Wall Carbon

Nanotubes on Gold Using a Non-Thiol

Functionalization Technique

Haoyan Wei,a Sejong Kim,b Sang Nyon Kim,b Bryan D. Huey,a Fotios Papadimitrakopoulos*b and

Harris L. Marcus*a

a Materials Science and Engineering Program, Department of Chemical, Materials and Biomolecular

Engineering, Institute of Materials Science, University of Connecticut, Storrs, CT 06269, USA

b Nanomaterials Optoelectronics Laboratory, Polymer Program, Institute of Materials Science,

Department of Chemistry, University of Connecticut, Storrs, CT 06269, USA

* To whom correspondence should be addressed.

E-mail: [email protected], [email protected]

Harris L. Marcus: Tel: (+01) 860 486 4623. Fax: (+01) 860 486 4745.

Fotios Papadimitrakopoulos: Phone: (+01) 860 486 3447. Fax: (+01) 860 486 4745.



2

0 100 200 300 400 500 600 700 800

-4000

-3000

-2000

-1000

0

1000

2000

dN(E

)

Kinetic Energy (eV)

Au1 75.8%

Au2C 23.4%

O 0.8%Au minorpeaks

Fig. S1 a) AFM micrograph of surface topology and b) Auger spectrum of Pico Au (111) substrate after

immersion into Fe2(SO4)3 solution for 15 min followed by a brief H2SO4 and DMF wash.

(a)

(b)



3

0 100 200 300 400 500 600 700 800

-1500

-1000

-500

0

500

1000

1500

dN(E

)

Kinetic Energy (eV)

Au1

Au2

CAu minorpeaks

181 eV

210 208 206 204 202 200 198 196 194 192 1904400

4500

4600

4700

4800

4900

5000

5100

5200

5300

5400

5500

5600

5700

CPS

Binding Energy (eV)

198.4 eV

Fig. S2 a) Auger and b) XPS spectra of Pico Au (111) substrate after immersion into pH 2.2 HCl

aqueous solution for 15 min. No evidence of chlorine adsorption on Au surface was found. The arrows

in b) indicate the peak position where Cl is supposed to be.

(a)

(b)



4

0 100 200 300 400 500 600 700 800-600

-400

-200

0

200

400

600

dN(E

)

Kinetic Energy (eV)

Au1

Cl

C O

Fe1 Fe2 Fe3

Fig. S3 Typical Auger survey taken in Au domains on FeO(OH)-FeOCl/Au strip patterns by

photolithographic lift-off process. Higher carbon content was observed in comparison with the uniform

FeO(OH)-FeOCl/Au samples (Fig. 2b).

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