EMERGING AREA www.rsc.org/softmatter | Soft Matter

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On the suitability of carbon nanotube forests as non-stick surfacesfor nanomanipulation

Kjetil Gjerde,a R. T. Rajendra Kumar,a Karin Nordstrøm Andersen,a Jakob Kjelstrup-Hansen,b

Ken B. K. Teo,b William I. Milne,c Christer Persson,d Kristian Mølhave,a Horst-Gunther Rubahnb

and Peter Bøggild*a

Received 2nd July 2007, Accepted 23rd November 2007

First published as an Advance Article on the web 16th January 2008

DOI: 10.1039/b709870g

A carbon nanotube forest provides a unique non-stick surface for nanomanipulation, as the

nanostructuring of the surface allows micro- and nanoscale objects to be easily removed after first

being deposited via a liquid dispersion. A common problem for smooth surfaces is the strong initial

stiction caused by adhesion forces after deposition onto the surface. In this work, carbon nanotube

forests fabricated by plasma-enhanced chemical vapour deposition are compared to structures with

a similar morphology, silicon nanograss, defined by anisotropic reactive ion-etching. While

manipulation experiments with latex microbeads on structured as well as smooth surfaces (gold,

silicon, silicon dioxide, Teflon, diamond-like carbon) showed a very low initial stiction for both carbon

nanotube forests and silicon nanograss, a homogeneous distribution of particles was significantly easier

to achieve on the carbon nanotube forests. Contact-angle measurements during gradual evaporation

revealed that the silicon nanograss was superhydrophic with no contact-line pinning, while carbon

nanotube forests in contrast showed strong contact-line pinning, as confirmed by environmental

scanning electron microscopy of microdroplets. As a consequence, latex microbeads dispersed on the

surface from an aqueous solution distributed evenly on carbon nanotube forests, but formed large

agglomerates after evaporation on silicon nanograss. Lateral manipulation of latex microbeads with

a microcantilever was found to be easier on carbon nanotube forests and silicon nanograss compared

to smooth diamond-like carbon, due to a substantially lower initial stiction force on surfaces with

nanoscale roughness. Nanomanipulation of bismuth nanowires, carbon nanotubes and organic

nanofibres was demonstrated on carbon nanotube forests using a sharp tungsten tip. We find that the

reason for the remarkable suitability of carbon nanotube forests as a non-stick surface for

nanomanipulation is indeed the strong contact-line pinning in combination with the nanostructured

surface, which allows homogeneous dispersion and easy manipulation of individual particles.

1. Introduction

Nanostructures, such as carbon nanotubes (CNTs), III–IV and

silicon semiconductor nanowires, metallic nanowires and

organic nanofibres are candidates for future nanoelectronic

and nanoelectromechanical devices. These are often stored in

liquid solution for convenience, or because they are initially

synthesized in liquid. This solution can then be dispersed on

a surface, leaving the nanostructures randomly scattered after

evaporation of the solvent. This may then be followed by identi-

fication of a suitable specimen and lithographic definition of elec-

trical contacts to the object.1,2 For prototyping, characterisation

aMIC - Dept. of Micro- and Nanotechnology, NanoDTU, TechnicalUniversity of Denmark, Bldg 345 East, DK-2800 Kongens Lyngby,Denmark. E-mail: boggild@mic.dtu.dk; Fax: +45 4588 7762; Tel: +454525 5787bNanoSYD, Mads Clausen Institute, University of Southern Denmark,Alsion 2, DK-6400 Sønderborg, DenmarkcDept. of Engineering, University of Cambridge, Trumpington Street,Cambridge, UK, CB2 1PZdDivision of Materials Engineering, Lund Institute of Technology, LundUniversity, SE-22100 Lund, Sweden

392 | Soft Matter, 2008, 4, 392–399

and metrology, it is often desirable to move the nanostructures

onto a test device such as a substrate for transmission electron

microscopy analysis3 or an electrode pair.4 Picking and placing

of a small structure is, however, highly challenging.5–7 Firstly,

accessing a small structure lying flat on a surface is difficult per

se, and secondly, the ubiquitous adhesion forces8 on the micro-

and nanoscale may even prevent detachment and manipulation

in any reproducible manner.9 Carbon nanotubes and nanowires

tend to stick well to any surface,10 and manipulation is often only

possible if the objects are partly suspended from an edge, or if

they are free-standing on a surface.11 Whereas manipulation of

crystalline, inorganic structures like carbon nanotubes and semi-

conductor nanowires is in some cases feasible due to the stiffness

and high mechanical strength, there is a large class of organic

fibres and macromolecular structures that are interesting from

the point of view of applications as well as basic research, but are

also impossible to manipulate without damaging the structures.

As an example, the self-assembling para-hexaphenylene (p-6P)

nanofibres are very soft and fragile, breaking at a shear stress of

just 20 MPa.9 It has been shown that once the p-6P nanofibres

were dispersed on a Teflon surface, only very short pieces of

these roughly 50 nm tall and 300 nm wide nanofibres could be

This journal is ª The Royal Society of Chemistry 2008

Kjetil Gjerde obtained his MSc in Physics from the Norwegian University of Science and Technology in 1999. From 2000 to 2003 he

worked with 3D tectonic modeling for the International Research Institute of Stavanger. In 2006 he obtained his PhD from the Technical

University of Denmark, department of Micro and Nanotechnology, under the supervision of Peter Bøggild working on integration of

carbon nanotubes into microsystems. He is currently employed by StatoilHydro ASA conducting research on gas-to-liquids technology.

R. T. Rajendra Kumar worked on fabrication of micro-infrared sensors and received his PhD in Physics from Bharathiar University in

2003. From 2003 to 2005 he worked at Stockholm University as a Post-doctoral Fellow on fabrication of SiO2 nano-capillary arrays

for manipulating highly charged ions. From 2005 to 2006 he worked in Dublin City University as a Post-doctoral Fellow on integration

of ZnO nanorods with silicon for nanodevice applications. Since 2007, he has been working at MIC, Technical University of Denmark

as a Post-doctoral Fellow on fabrication of nanotools and nanosurfaces for advanced nanomanipulation and micro/nanofluidics.

Karin Nordstrøm Andersen received her MSc in Physics and Mathematics in 1999 from the University of Arhus, Denmark. In 2005 she

received her PhD from The Technical University of Denmark working with optical components for telecommunication based on high index

materials. From 2005 she has been working as a Postdoc. at the Department of Nano- and Microtechnology at the Technical University of

Denmark on tools for manipulation of nanostructures. The project is part of a European project, Nano-robotic for Assembly and Charac-

terization, which will allow an untrained operator to interact with nano-scale objects for characterization, sorting and assembly tasks.

Peter Bøggild received his PhD at Copenhagen University in 1998 in the field of experimental low temperature physics, and was employed

as an Associate Professor at MIC from 1998 to 2001 where he investigated applications for micro-scale four-point probes. In 2001 he was

appointed Associate Professor and concentrated on development of nanoscale robotic tools for manipulation, assembly and characteriza-

tion of nanostructures and system-level integration of nanostructures. He is currently heading the Nanointegration research group at MIC

– Department for Micro- and Nanotechnology, at the Technical University of Denmark.

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pushed laterally along the surface using the tip of an atomic force

microscope.9 It was not possible to lift the organic nanofibres

away from the surface at all.

Introduction of roughness and structure on surfaces is a well-

known method of reducing the adhesion and stiction effects.8,12

Recently, we demonstrated that p-6P nanofibres dispersed onto

dense forests of vertically aligned carbon nanotubes,13 could

easily be lifted off the surface using a sharp tip. It was also shown

that the stiction (static friction) of latex microbeads was substan-

tially smaller on CNT forests than on Teflon.

A surface for nanomanipulation of dispersed structures should

fulfil two criteria. Firstly, any dispersed structure should be

possible to remove afterwards, implying a small interaction

between the surface and the object. Secondly, the surface should

be easy to wet, so that the liquid dispersion spreads out and evap-

orate evenly, leaving a homogeneous distribution of structures

on the surface. These two criteria at first seem mutually exclu-

sive; the first criterion is fulfilled by a hydrophobic surface, while

the second points to a hydrophilic surface. For instance, the lotus

leaf is micro- and nanostructured to create a hydrophobic

surface on which water forms nearly spherical droplets. As the

droplets roll off they remove surface dust particles in the process.

The surface tension of the water gives rise to collection and

removal of the particles rather than deposition and distribution.

Roughness on the micro- or nanoscale can also cause pinning of

the contact line,14,15 which may introduce contact-angle hyster-

esis, i.e. a difference of the advancing and receding contact angle.

Wier and McCarthy16 pointed out that apparently superhydro-

phobic surfaces can be wetted after condensation of water

vapour, which drastically increases the hysteresis and thus

reduces the water-repellent properties.

The reason for this complex behaviour is that a highly struc-

tured surface can sustain two different wetting states, which

may both lead to superhydrophobicity. In the Wenzel regime17

the surface is fully wetted, and here the apparent contact angle

q0 is determined through a roughness factor r (ratio of actual

This journal is ª The Royal Society of Chemistry 2008

area of rough surface to the apparent area of the corresponding

smooth surface), and the thermodynamic contact angle q,

cosq0 ¼ r(cosq) (1)

Since r is always greater than 1, for q < 90�, the apparent

contact angle q0 will be smaller, whereas q0 will be higher if q >

90�, effectively enhancing the hydrophobicity. In the Cassie

regime18 the droplet is resting on top of the protrusions, and is

therefore in ‘‘contact’’ with both solid and air pockets trapped

between the protrusions. The apparent contact angle is then

cosq0 ¼ f1(cosq) � f2, (2)

where f1 is the fraction of contact area with the solid, and f2 is the

fraction of contact area with the air. As f1 is reduced, the apparent

contact angle will converge to the air–water contact angle of 180�,

corresponding to the superhydrophobic state. While the Wenzel

regime can both enhance hydrophobic and hydrophilic behav-

iour, the Cassie regime always leads to larger contact angles.

While the Cassie equation, eqn (2), should hold for very rough

and/or very hydrophobic surfaces, the air-pockets may not be

stable for slightly or moderately hydrophobic materials despite

a large surface roughness. This suggests an intermediate phase

where both wetting conditions co-exist. Such complex wetting

behaviour was observed by slow condensation and evaporation

on lotus leaves, where Cheng et al.19 noted that lotus leaves in

certain conditions could be wetted despite having high apparent

contact angles corresponding to a superhydrophobic state.

Consider the artificially nanostructured surfaces, carbon nano-

tube forests (CNTF) and the morphologically similar silicon

nanograss (SiNG); wetting of carbon nanotube forests has been

a topic of interest for some time and Li et al.20 and Lau et al.21

reported pronounced hydrophobicity and even superhydropho-

bicity (contact angle larger than 150�) on such surfaces. Vertically

aligned carbon nanotubes exhibit highly hydrophobic behaviour,

Soft Matter, 2008, 4, 392–399 | 393

Fig. 1 SEM images of SiNG (A,C) and of CNTF (C,D). Note that the

overall dimensions of the SiNG structures are roughly 5 times larger,

both in terms of separation, diameter and length. (E) After dispersion

on SiNG, the drop of water containing microbeads forms a nearly-round

droplet that slowly shrinks by evaporation, pulling nearly all microbeads

into a closed-packed sphere. The inset is a magnified view of the surface

of the microbead agglomerate. (F) Image showing scattered latex

microbeads dispersed on a CNTF surface, near the edge of the round

droplet area.

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which is pronounced further when covered with a hydrophobic

coating such as Teflon.21 The structures appear to amplify either

the hydrophobicity or the hydrophilicity: Fan et al.22 found that

as-grown Si nanorods have a strongly enhanced hydrophilicity

compared to a flat Si surface, while H-passivation (hydrofluoric

acid treatment) likewise enhanced the hydrophobicity, reaching

an equilibrium contact angle of 142.7�.

In the present work, an anisotropically etched Silicon nano-

grass (SiNG) surface showed superhydrophobic behaviour

(154–170�) during slow evaporation and no contact-line pinning.

In contrast, the CNTF substrate showed strong contact-line

pinning, with the contact angle decreasing from 125� to zero

during evaporation. This indicates a transition from the Cassie

to the Wenzel state as observed in ref. 16. Whereas manipulation

and removal of latex microbeads is straightforward for both

surfaces, dispersion of particles is far more effective on CNTF

than on the investigated SiNG surfaces, as a consequence of

the mixed wetting behaviour of the CNTF.

2 Wetting of nanostructured and smooth surfaces

2.1 Sample preparation

The wetting properties of four different substrates were investi-

gated: silicon dioxide (SiO2), Teflon, silicon nanograss (SiNG)

and vertically aligned carbon nanotube forests (CNTF). The

SiO2 surface was prepared by thermal oxidation. The ‘‘Teflon’’

substrate was made by deposition of an 80 nm Teflon-like

coating from a C4F8 plasma onto a silicon wafer.23 The SiNG

was fabricated by anisotropic etching of a silicon wafer in a

SF6–O2 99 : 90 sccm mixture for 5 minutes, followed by isotropic

etching (SF6–O2 260 : 26 sccm) and passivation (C4F8 120 sccm)

cycles for 3 minutes with the wafers kept at �10 �C. The CNTF

were prepared in a plasma-enhanced chemical vapour deposition

system (PECVD).24–26 Transmission electron microscopy images

of individual CNTs grown with the same PECVD process

parameters,33 revealed that the structures are well-graphitized

multi-walled carbon nanotubes, often capped with a thin layer

of amorphous carbon. The presence of such an amorphous

carbon layer depends on the detailed growth process.33

Fig. 3A and 3B show that the aspect ratio and overall

morphology of the SiNG compares to the CNTF except for a

scaling of roughly 5. The SiNG are up to 10 mm in length with

a width ranging from 0.3 to 1 mm at the base, decreasing towards

their apex to 0.1–0.2 mmat the top. The typical separation of indi-

vidual SiNG protrusions is around 1–1.5 mm. The CNTs are 2 mm

tall with a more cylindrical shape, and a diameter around 50–120

nm. The average separation of CNTs is 0.2–0.4 mm. Thus, the

roughness parameter r and the fraction of area to the solid, f1, as

encountered in eqn (1) and eqn (2), are in the same range. Due to

the aspect ratio not exceeding 20 and rather large separations,

neither theCNTFor theSiNGstructureswereobserved to collapse

due to surface tension, as reported for multi-walled carbon nano-

tubes in ref. 27. A surface view of the typical morphology of the

SiNGand theCNTF is shown inFig. 3CandFig. 3D, respectively.

2.2 Development of contact angle during slow evaporation

The evolution of the contact angle and baseline with time for

mL-sized water droplets deposited on SiO2, SiNG, Teflon and

394 | Soft Matter, 2008, 4, 392–399

a CNTF samples was studied using an optical microscope and

a Kruss drop shape analysis system DSA10 Mh2. The baseline

is the diameter of the droplet–surface interface. A computer-

controlled syringe was used to deposit small water droplets on

the surface. In the following we compare the wetting properties

of carbon nanotube forests and silicon nanograss, to a hydro-

phobic surface, Teflon, and a hydrophilic surface, SiO2 during

slow evaporation, which is the scenario most relevant for a liquid

dispersion of nanostructures. The optical microscope pictures in

Fig. 1 shows the shape of the water droplets after 0, 5, 10 and 15

minutes after initial deposition on the SiNG, CNTF, Teflon and

SiO2 surfaces.

On the SiNG surface the initial contact angle is 154�, and as

the droplet evaporates, it increases gradually to 170�, which is

a signature of superhydrophobic behaviour. Two droplets,

slightly different in volume, were deposited on the CNTF, as

shown in a series of photographs in Fig. 1. For the left-most

droplet, the contact angle is initially 125�, corresponding to

strong hydrophobicity. In this case, however, the baseline

remains nearly constant while the contact angle is reduced

strongly. This behavior is characteristic of a superhydrophobic

surface with strong contact-line pinning.15,28

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For smooth surfaces where the contact lines move almost

freely, the advancing angle is only slightly larger than the receding

contact angle. On the Teflon surface, it is seen that the receding

contact angle remains close to the initial 105� during the evapora-

tion process. The loss of liquid is accommodated by a reduction

of the baseline, rather than decreasing of the contact angle. On

the SiO2 surface, the initial contact angle is less than 20�,

dropping towards the measurement limit over the 15 minutes.

In Fig. 1B and Fig. 1C the contact angle and baseline as a func-

tion of time are plotted for the four surfaces. The SiNG and

Teflon both exhibit nearly constant contact angles, where the

contact angle of SiNG even increases slightly with evaporation.

Three droplets are plotted for the CNTF: two large size droplets

with an initial baseline around 1 mm, and one small size droplet

with initial baseline around 0.25 mm. All three droplets show

a decreasing contact angle, similar to the SiO2 film. While the

small droplet experience a decline in the baseline after 10 minutes

due to final evaporation of the thin water film, the baseline of the

larger droplets hardly changes during the 15 minutes. Although

water on CNTF initially shows a higher contact angle than on

Teflon, it behaves almost exactly like on SiO2 during evapora-

tion, in that the baseline stays constant and the contact angle

decreases. These results show that the wetting of the CNTF is

much easier than of the SiNG samples investigated here.

Fig. 2 A) Optical microscope image showing the shape of water droplet

upon gradual evaporation after 0, 5 and 10 min of deposition on SiNG,

CNTF, Teflon and SiO2, B) contact angle in degrees, and C) baseline

(diameter of surface–droplet interface) plotted against variation with

evaporation for SiNG, CNTF, Teflon and SiO2.

2.2 Development of individual microdroplets during

condensation

The detailed behaviour of water droplets during condensation

and evaporation can be monitored with environmental scanning

electron microscopy (ESEM), offering a higher resolution than

optical microscopy. The condensation of water on the substrate

inside a FEI XL-30 environmental SEM was controlled by care-

fully regulating the temperature of the substrate with a Peltier

element, as well as the total vapour pressure inside the ESEM

chamber. Condensation of water inside the ESEM was initiated

by lowering the temperature carefully and increasing the water

vapour pressure to around 5 mbar.

In Fig. 2 a series of ESEM pictures shows condensation on the

CNTF. The condensation is initiated at 5.3 mbar and 2.9 �C with

the sudden appearance of spherical microdroplets randomly

distributed over the substrate surface, see Fig. 2A. During

condensation, existing droplets accumulate water vapor and

grow in size, while new droplets are continuously nucleated.

The equilibrium between condensation and evaporation is very

sensitive to small changes in substrate temperature and water

vapor pressure. Roughly 30 seconds later, more water has

condensed, Fig. 2B, causing smaller water droplets to merge as

their surfaces touch. The three smaller droplets in Fig. 2A have

merged into one large droplet in Fig. 2B, which has a more irreg-

ular shape. In Fig. 2B the droplets appear to be pinned at the

previous droplet centers. After further condensation a large

droplet has formed, still pinned at arrow 1, Fig. 2C. Arrow 2

points to an area between two droplets in Fig. 2A and 2B, where

the surface still appears to be hydrophobic. Fig. 2D–E illustrates

merging of two such droplets, where the new droplet is pinned at

the two previous droplet centers. In Fig. 2F the surface shows

mixed hydrophobic-like and hydrophilic-like regions for all

larger droplets. The distinct change of the droplet morphology

This journal is ª The Royal Society of Chemistry 2008

indicates that the droplets experience a transition from the

Cassie state to the Wenzel state from Fig. 2A–F. The condensa-

tion behavior was similar after subsequent drying and re-conden-

sation, indicating that the surface does not change permanently

due to the wetting, which for instance can occur if capillary

forces induce bundling of the CNT. Also, no sign of such

bundling or collapsing was observed in SEM after condensation.

3. Dispersion and manipulation of latex microbeads

Latex microbeads were dispersed on a number of different

substrates: Au, Si, SiO2, Teflon, diamond-like carbon (DLC),

SiNG and CNTF. The SiO2 and Teflon surfaces were prepared

as described in the previous section. The Au and Si substrates

were prepared as described in ref. 13. The diamond-like carbon

substrate was made by depositing a 70 nm DLC film onto a clean

silicon wafer using an S-bend filtered cathodic vacuum arc

system.29

Soft Matter, 2008, 4, 392–399 | 395

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An aqueous solution of 5 mm or 20 mm diameter latex microbe-

ads (Polysciences Inc.) was deposited on SiNG and CNTF

samples in 5–20 mL droplets using a micropipette. The solvent

was evaporated by a low-power heating for 20 minutes. For

the smooth surfaces (Au, Si, SiO2, Teflon, DLC), the trend was

that the microbeads were more evenly dispersed on the high-

energy surfaces (Au, Si, SiO2), while they clustered in agglomer-

ates of up to hundreds of latex beads on the low-energy surfaces

(Teflon, DLC).

On both CNTF and SiNG surfaces, individual latex beads were

found scattered after evaporation. On SiNG, however, the vast

majority of latex beads remaining in the droplet, which either

rolled off the surface and thereby removed the latex beads, or left

a spherical agglomerate of latex beads after evaporation.Remark-

ably, a dense solution led to mm-sized spherical agglomerates

consisting of millions of latex microbeads, as shown in Fig. 3E.

The inset shows closed-packed domains of the microbeads at

the surface of the spherical agglomerate. Even in this case, very

Fig. 3 Environmental scanning electron microscope images of water

droplets on CNTF. A) Condensation of water starts with the appearance

of small spherical micro-droplets. B) Smaller droplets merge into larger

ones, exhibiting pinning at the previous droplet centres. C) After further

condensation a large droplet has been formed, yet still pinned at location

1 (see arrow). (D–E) Sketch illustrating two droplets merging. After

merging (E) the new droplet is pinned at the two previous droplet centres.

(F) In the mixed wetting state many droplets start to have both hydro-

phobic and hydrophilic-like regions. (G) After further condensation the

surface becomes dominated by hydrophilic-like regions (Wenzel regime).

396 | Soft Matter, 2008, 4, 392–399

few latex beads were deposited elsewhere on the surface. For the

CNTF, individual and small clusters ofmicrobeadswere scattered

more evenly across the surface with a tendency of clustering only

near the edges of the droplet area, see Fig. 3F.

The manipulation setup consists of a Klocke Nanotechnik

xyz-stage mounted inside the LEO 1550 SEM chamber, which is

operated by an external computer, and has a resolution of about

10 nm. Using cantilevers with spring constants in the 0.1–10 N

m�1 regime and following the strategy outlined in ref. 13, it was

attempted to release and manipulate latex beads from their initial

position after dispersion and evaporation onto different surfaces:

Au, Si, SiO2, Teflon, DLC, CNTF and SiNG.

On the smooth Au, Si and SiO2 surfaces it was not possible to

release latex beads from their initial position at all, as observed in

ref. 13. On Teflon, it was possible to tear off latex beads,

however, only with a significant bending of the microcantile-

vers.13 On the DLC, SiNG and CNTF surfaces the latex beads

could be pushed on the surface with ease after initial release.

On CNTF, after the initial release of 5–20 mm latex beads, very

little bending of even the softest cantilevers (spring constant 0.1

N m�1) was observed. Fig. 4 shows a group of four 20 mm latex

beads being pushed on the nanotube forest. Here the beads are

sliding and rotating smoothly on the surface Fig. 4A–B, after

which the cluster jumps to a new position, see Fig. 4C. Subse-

quent re-approach allowed further rotation, Fig. 4D. On SiNG

it was likewise possible to push, pull and remove microbeads,

however typically with a more irregular motion pattern, most

likely due to the larger spacing between the protrusions. In

Fig. 5, three latex beads are dragged across a surface by the adhe-

sion force between the cantilever and one bead. During this

process, one of the beads is pinned by an irregularity on the

Fig. 4 SEM images of a micro-cantilever with spring constant 0.1 N m�1

pushing latex beads along a CNTF surface. (A) A cluster of four 20 mm

beads is approached. (B) By pushing it is possible to rotate the cluster.

(C) The cluster jumps away. (D) Re-approach allowed further rotation

of the cluster. The scale bar represents 40 mm.

This journal is ª The Royal Society of Chemistry 2008

Fig. 5 The series of SEM images shows three 5 mm latex beads being

pulled across a SiNG surface. The small circle (see arrow) marks

a mechanical pinning point on the SiNG surface, which causes the three

beads to rotate during the translation.

Fig. 6 SEM images of STM tipmanipulating (A–B) Bi nanowires, (C–D)

multi-walled CNT, and (E–F) para-hexaphenylene (p-6P) nanowires.

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surface, causing the cluster to rotate, as shown by the small mark

in Fig. 5A–F. On DLC, the initial stiction was substantially

higher than the dynamical stiction; release of the latex beads

right after dispersion was much more difficult. Once released,

the motion was nearly as smooth as on CNTF.

We have used the CNTF for manipulation of various nano-

structures of different size and material. In Fig. 6, three examples

are shown. The manipulation setup is similar to that described

above, only in this case the microcantilever is replaced by a sharp

tungsten tip.13 In Fig. 6A–B a bismuth nanowire with a diameter

of 400 nm is being lifted up from the surface, for subsequent

transfer to an electronic characterisation device. In Fig. 6C–D

we show that even multi-walled carbon nanotubes of submicron

length can be lifted up and deposited in a new position. In

Fig. 6E–F the STM tip is being pushed under a 200 nm wide,

50 nm high para-hexaphenylene nanowire, which is then gently

let down again onto the surface.

4. Discussion

The contact angle observed during evaporation of water on the

SiNG reached 170�, significantly higher than HF-treated silicon

This journal is ª The Royal Society of Chemistry 2008

posts,22 147�, and close to the values obtained with structured

plasma-polymerised heptafluorobutyl acrylate,28 173�, and with

PTFE-coated CNTF.30 In the work by Chen et al.14 absence of

contact-line pinning was observed only for smooth surfaces,

whereas the SiNG investigated here exhibits microscale rough-

ness, in terms of average distance between SiNG protrusions.

All clean silicon surfaces exposed to ambient conditions

develop a natural oxide with a thickness of 1–2 nm, which should

render the surface hydrophilic. The superhydrophobic behaviour

of the SiNG can be explained by the formation of thin fluoro-

carbon films originating from the side-wall passivation process

in the deep reactive ion etch.31,32 In this picture, the low surface

energy associated with such a fluorocarbon layer effectively

prevents the water from entering between Si protrusions, thus

allowing the liquid to only interact with the apex of the SiNG.

In such a scenario, roughness-induced contact-angle hysteresis

should be small, as is observed experimentally.

The wetting properties of CNTF are quite in contrast to the

results obtained on SiNG. The apparent contact angle for the

CNTF was significantly lower than for the SiNG, most likely

because the CNTFs were not treated chemically. The contact

angle of a flat graphite sheet is around 85�.34 A water contact

angle of 125� on CNTF indicates that the water is initially in

the Cassie regime, since a lower contact angle should be expected

for a rough surface compared to a flat graphite surface as per

Wenzel theory. As for the Cassie model, the fraction of the

area interacting with water from the CNTF morphology is f1z 0.11. With q ¼ 85� for graphite, the apparent contact angle

should be 152� from eqn (2), which is significantly higher than

the observed contact angle, 125�. In essence, the experimentally

observed contact angle is too high to be explained by the Wenzel

model (complete wetting) and too low to be explained by the

Cassie model. An explanation could be that the thin layer of

amorphous carbon on the CNT leads to a significantly different

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contact angle and contact-line pinning, as compared to pyrolithic

graphite.

ESEM was used to monitor slowly condensing microdroplets

on CNT, initially having a spherical shape. After development of

multiple pinning points, the contact lines became highly irreg-

ular; eventually what appeared to be a transition to the Wenzel

regime was observed. Co-existence of different wetting states

on various surfaces has been reported by several groups,35–37

while contact-line pinning by micro- and nanoscale protrusions

have been seen for microposts16 and lotus leaves.19 In the work

by Wier and McCarthy16 the wetting of 40 mm-tall silanised

silicon microposts was studied in dry and wet (condensation)

conditions. During condensation, microdroplets formed between

the posts and pinned the contact line, creating irregular drop

shapes qualitatively similar to the ones we showed in Fig. 2.

Compared to Wier and McCarthy16 and Cheng et al.,19 the

average distance between the protrusions for our CNTF is

much smaller than the typical length scale of the droplets, and

the observed contact-line distortions. Also, in ref. 16 the

contact-line hysteresis and reduction of droplet mobility was

observed during condensation, i.e. near the dew point. For the

CNTF investigated here, a strong contact-line pinning was also

observed at room temperature in ambient conditions.

The feasibility of distributing particles homogeneously on

a surface from a liquid dispersion was here related to the wetting

properties of the surface. While water droplets on CNTF wets

the surface effectively, leading to evaporation over a relatively

large area, the superhydrophobic behaviour of SiNG leads to

a ‘‘self-cleaning’’ effect as observed on lotus plant leaves.19

Although a few microbeads were observed on the SiNG surface

after dispersion, the large majority were effectively lost by the

formation of large agglomerates.

Dense vertically aligned carbon nanotube forests facilitates

homogeneous dispersion of micro- and nanoparticles, and

exhibits negligible initial and dynamic stiction with all types of

structures so far dispersed on this type of surface, including latex

microbeads, and organic nanofibres,13 carbon nanotubes and Bi

nanowires, as shown in Fig. 6. In comparison, SiNG also showed

good performance as a substrate for nanomanipulation of

supported particles; latex microbeads were easily manipulated

and removed directly after evaporation of the solvent, with negli-

gible static and dynamic friction. However, the possibility of

dispersion of particles on SiNG is in general hindered due to

the superhydrophobicity and lack of contact-line pinning, which

effectively prevents wetting; the liquid dispersion rolls off the

surface, and thereby removes nearly all the objects to be

dispersed. This is particularly problematic if the objects in

dispersion are not available in a high concentration, in which

case the density of objects remaining on the surface after evapo-

ration can become critically low. In comparison, the flat DLC

surface behaved almost as well as the CNT film after initial

release. The initial release is however a critically important

step. The ability to remove nanoparticles with minimal force is

essential for successful manipulation of fragile and small nano-

structures, and absence of initial stiction is greatly preferred.

Still, DLC may be a convenient alternative to carbon nanotube

forests for some applications. In relation to manipulation of

nanostructures, only elongated nanostructures like wires, rods

and tubes can be supported by the SiNG samples presented in

398 | Soft Matter, 2008, 4, 392–399

this work, since the large separation between the protrusions is

likely to cause trapping of nanoparticles, which are small in all

three dimensions. Present efforts concentrate on development

of SiNG-based surfaces with similar structural and wetting prop-

erties as the CNTF, since such SiNG structures are likely to be

easier to fabricate than CNTF. This will make a more direct

comparison of the two types of nanosurfaces possible, in relation

to wetting and nanomanipulation.

Future perspectives include particle systems that are locked to

the 2D plane of the surface but effectively detached from any

lateral forces imposed by the underlying solid substrate. This

could enable self-assembly of objects that have not previously

been possible to work with in dry conditions. The nanorough-

ness of the surfaces also adds a new dimension to controlling

object–surface interactions. It was recently shown that bone-cell

growth is strongly affected by an underlying carbon nanotube

surface.38 Vertically aligned carbon nanotube forests present

some intriguing opportunities for the study of interactions,

motility and growth of living organisms, besides offering a perfect

tool for nanomanipulation of fragile nanostructures.

Acknowledgements

We acknowledge important help and assistance from, and valu-

able discussions with, Andy Horsewell, Kristian Mølhave and

Klaus Bo Mogensen. The Bi nanowires were kindly provided

by Thomas Cornelius. The work was partly supported by EU

grants NANOHAND (IP 034274) and NANORAC (STREP

013680).

References

1 B. Bourlon, D. C. Glattli, B. Placais, J. M. Berroir, C. Miko, L. Forroand A. Bachtold, Phys. Rev. Lett., 2004, 92, 026804/026801–026804.

2 B. Bourlon, C. Miko, L. Forro, D. C. Glattli and A. Bachtold, Phys.Rev. Lett., 2004, 93, 176806–176801–176806–176804.

3 K. Molhave, S. B. Gudnason, A. T. Pedersen, C. H. Clausen,A. Horsewell and P. Boggild, Nano Lett., 2006, 6, 1663–1668.

4 P. Avouris, R. Martel and H. R. Shea, Appl. Surf. Sci., 1999, 141,201–209.

5 K. Molhave, T. M. Hansen, D. N. Madsen and P. Boggild,J. Nanosci. Nanotechnol., 2004, 4, 279–282.

6 K. Molhave, T. Wich, A. Kortschack and P. Boggild,Nanotechnology, 2006, 17, 2434–2441.

7 P. Kim and C. M. Lieber, Science, 1999, 286, 2148–2150.8 Y. X. Zhuang and A. Menon, Tribol. Lett., 2005, 19, 111–117.9 J. Kjelstrup-Hansen, O. Hansen, H.-G. Rubahn and P. Bøggild,

Small, 2006, 2, 660.10 Y. Nakayama and S. Akita, New J. Phys., 2003, 5, –.11 K. Molhave, T. M. Hansen, D. N. Madsen and P. Bøggild,

J. Nanosci. Nanotechnol., 2004, 4, 279–282.12 K. Komvopoulos, Wear, 1996, 200, 305–327.13 K. Gjerde, J. Kjelstrup-Hansen, C. H. Claussen, K. K. B. Teo,

W. I. Milne, H.-G. Rubahn and P. Bøggild, Nanotechnology, 2006,17, 4917–4922.

14 W. Chen, A. Y. Fadeev, M. C. Hsieh, D. Oner, J. Youngblood andT. J. McCarthy, Langmuir, 1999, 15, 3395–3399.

15 V. De Jonghe and D. Chatain, in First European Symposium. Fluids inSpace (ESA SP-353), ESA, Editon edn, 1992, pp. 525–532.

16 K. A. Wier and T. J. McCarthy, Langmuir, 2006, 22, 2433–2436.17 R. N. Wenzel, Ind. Eng. Chem., 1936, 28, 988.18 A. Cassie and S. Baxter, Trans. Faraday Soc., 1944, 40, 546.19 Y. T. Cheng, D. E. Rodak, A. Angelopoulos and T. Gacek, Appl.

Phys. Lett., 2005, 87.20 H. Li, X. Wang, Y. Song, Y. Liu, Q. Li, L. Jiang and D. Shu, Angew.

Chem., Int. Ed., 2001, 40, 1743–1745.

This journal is ª The Royal Society of Chemistry 2008

Publ

ishe

d on

16

Janu

ary

2008

. Dow

nloa

ded

by U

nive

rsity

of

Cal

ifor

nia

- Ir

vine

on

31/1

0/20

14 0

8:12

:10.

View Article Online

21 K. K. S. Lau, J. Bico, K. B. K. Teo, M. Chhowalla,G. A. J. Amaratunga, W. I. Milne, G. H. McKinley andK. K. Gleason, Nano Lett., 2003, 3, 1701–1705.

22 J. G. Fan, X. J. Tang and Y. P. Zhao, Nanotechnology, 2004, 15,501–504.

23 D. Nilson, ‘‘Polymer Based Minituarized Dye Lasers for lab-on-a-chip-systems, PhD Thesis, Technical University of Denmark, 2005.

24 Z. F. Ren, Z. P. Huang, J. W. Xu, J. H. Wang, P. Bush, M. P. Siegaland P. N. Provencio, Science, 1998, 282, 1105–1107.

25 A. Melechko, V. Merkulov, T. McKnight, M. Guillorn, K. Klein,D. Lowndes and M. Simpson, J. Appl. Phys., 2005, 97, 041301–041339.

26 K. B. K. Teo, C. Singh, M. Chhowalla and W. I. Milne, Encycl.Nanosci. Nanotechnol., 2004, 1, 665–686.

27 N. Chakrapani, B. Q. Wei, A. Carrillo, P. M. Ajayan and R. S. Kane,Proc. Natl. Acad. Sci. U. S. A., 2004, 101, 4009–4012.

28 W. Chen, A. Y. Fadeev, M. C. Hsieh, J. Youngblood andT. J. McCarthy, Langmuir, 1999, 15, 3395–3399.

29 K. J. Clay, S. P. Speakman, N. A. Morrison, N. Tomozeiu,W. I. Milne and A. Kapoor, Diamond Relat. Mater., 1998, 7, 1100.

This journal is ª The Royal Society of Chemistry 2008

30 K. K. S. Lau, J. Bico, K. B. K. Teo, M. Chhowalla,G. A. J. Amaratunga, W. I. Milne, G. H. McKinley andK. K. Gleason, Nano Lett., 2003, 3, 1701–1705.

31 K. S. Chen, A. A. Ayon, X. Zhang and S. M. Spearing,J. Microelectromech. Syst., 2002, 11, 264–275.

32 H. Jansen, M. Deboer, R. Legtenberg and M. Elwenspoek,J. Micromech. Microeng., 1995, 5, 115–120.

33 K. B. K. Teo, M. Chhowalla, G. A. J. Amaratunga, W. I. Milne,D. G. Hasko, G. Pirio, P. Legagneux, F. Wyczisk and D. Pribat,Appl. Phys. Lett., 2001, 79, 1534–1536.

34 I. Morcos, J. Chem. Phys., 1972, 57, 1801–&.35 I. Woodward, W. C. E. Schofield, V. Roucoules and J. P. S. Badyal,

Langmuir, 2003, 19, 3432–3438.36 D. O. H. Teare, C. G. Spanos, P. Ridley, E. J. Kinmond,

V. Roucoules, J. P. S. Badyal, S. A. Brewer, S. Coulson andC. Willis, Chem. Mater., 2002, 14, 4566–4571.

37 M. Morra, E. Occhiello and F. Garbassi, J. Colloid Interface Sci.,1989, 132, 504–508.

38 L. P. Zanello, B. Zhao, H. Hu and R. C. Haddon, Nano Lett., 2006, 6,562–567.

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