This content has been downloaded from IOPscience. Please scroll down to see the full text.

Download details:

IP Address: 83.34.129.159

This content was downloaded on 20/03/2014 at 19:44

Please note that terms and conditions apply.

Interaction and dynamics of ambient water adlayers on graphite probed using AFM voltage

nanolithography and electrostatic force microscopy

View the table of contents for this issue, or go to the journal homepage for more

2014 Nanotechnology 25 155304

(http://iopscience.iop.org/0957-4484/25/15/155304)

Home Search Collections Journals About Contact us My IOPscience

Nanotechnology

Nanotechnology 25 (2014) 155304 (9pp) doi:10.1088/0957-4484/25/15/155304

Interaction and dynamics of ambient wateradlayers on graphite probed using AFMvoltage nanolithography and electrostaticforce microscopyT Gowthami1, Narendra Kurra2 and Gargi Raina1

1 Center for Nanotechnology Research, VIT University, Vellore 632014, India2 Chemistry and Physics of Materials Unit and DST Unit on Nanoscience, Jawaharlal Nehru Center forAdvanced Scientific Research, Jakkur PO, Bangalore 560 064, India

E-mail: gargiraina@vit.ac.in

Received 12 September 2013, revised 30 January 2014Accepted for publication 20 February 2014Published 20 March 2014

AbstractIn this work, we report the impact of the interaction and dynamics of increasing ambient wateradlayers on etch patterns on a hydrophobic highly oriented pyrolytic graphite (HOPG) surfaceobtained using atomic force microscopy (AFM) voltage nanolithography in contact mode byapplying a positive bias to the sample. The changes in the dimensions of the etch patterns wereinvestigated as a function of the increasing number of water adlayers present on the HOPG,which is varied by changing the time interval since HOPG cleavage. Changes in the width ofthe etch patterns and the surrounding water droplets were monitored with time, usingintermittent-contact-mode AFM. Electrostatic force microscopy (EFM) has been employed tostudy the charged nature of the etch patterns and the neighboring water film with time. Thewidth of the etch patterns made on freshly cleaved HOPG shows an increase of ∼33% over48 h, whereas nine-day-old cleaved HOPG shows a 79% increase over the same period. Nochanges in the dimensions are observed while imaging in a nitrogen atmosphere soon afterlithography. In ambient conditions, the EFM phase shift of the patterns shows a large changeof ∼84–88% over 30 h. This study demonstrates the effect of the stored electrostatic energy ofa polarized ice-like water adlayer, resulting in changes in the dimensions of the etch patternslong after lithography, whereas liquid-like water droplets do not affect the etch patterns.

Keywords: AFM voltage nanolithography, highly oriented pyrolytic graphite (HOPG),electrostatic force microscopy (EFM), water adlayer

S Online supplementary data available from stacks.iop.org/Nano/25/155304/mmedia

(Some figures may appear in colour only in the online journal)

1. Introduction

Carbon is one of the most important elements: it forms thebasis of all known forms of life on earth. The various carbonallotropes in bulk, namely diamond and graphite, as wellas nanostructured-carbon materials such as fullerenes (0D),nanotubes (1D), and graphene (2D), exhibit an extraordinary

large range of important material properties according to thenature of local bonding. Nanocarbon materials have gainedtremendous importance owing to their fascinating electrical,optical, thermal, and mechanical properties. The recent dis-covery of a simple method to isolate monolayer graphene hasopened a gateway for important applications using this novelmaterial [1]. Graphene-based devices have been commonly

0957-4484/14/155304+09$33.00 1 c© 2014 IOP Publishing Ltd Printed in the UK

Nanotechnology 25 (2014) 155304 T Gowthami et al

fabricated using conventional electron beam lithography andplasma etching techniques [2–5]. However, these techniqueshave certain disadvantages of degrading the electrical transportbehavior of graphene-based devices due to the irradiation ofthe electron beam required for nanoscale patterning [6]. Inthis connection, scanning probe-based lithography techniquesoffer a simple and direct route to patterning at the nanoscalewithout the use of photoresists under ambient conditions[7, 8]. Additionally, these techniques provide surface modifi-cation with crystallographic control for graphene nanoribbonswhile simultaneously allowing atomic resolution imaging [9].

Highly oriented pyrolytic graphite (HOPG) is a layeredcarbon material consisting of stacks of several 2D graphenelayers held together by weak van der Waals forces. It hasa well-defined structure with exposed basal {0001} surfaces,and is commonly used as a calibration standard sample forscanning probe microscopy (SPM) techniques [10]. SPMtechniques under ambient conditions are greatly influenced bythe adsorption of water layers. In most of the dynamic-modeatomic force microscopy (AFM) techniques, the AFM probeeither comes very close to the surface or very briefly contactsthe surface, causing perturbation to the water adlayer onthe sample. It is also known that scanning the AFM tip inambient conditions induces condensation of water over thescanned area [11]. The condensation and evaporation of wateradlayers on a mica surface under ambient conditions was firstprobed using scanning polarization force microscopy (SPFM),in which a biased conductive AFM tip is kept several tens ofangstroms away from a grounded sample. It was reported that2D water clusters of size 10–100 nm with apparent heightof ∼0.2 nm (phase I domains) are formed at around 28%humidity. With further increase in humidity to 50%, phase IIdomains develop with similar apparent height, suggesting thethickness of water layer to be of molecular dimensions [12].It was proposed that, at the molecular level, the electric fieldand water are intertwined such that oriented water layers giverise to an electric field and, conversely, charged surfaces helporient the water dipoles in adjacent layers [13].

The presence of molecularly thin water films and waternanodroplets on a hydrophobic HOPG surface in ambientconditions has been reported. It was found that, at relativehumidity ≥90%, flat round water islands (fluid like) of 5 nmin height are formed by scanning the HOPG surface inintermittent-contact-mode AFM (IC-AFM); these transformwith time to 2 nm high flat islands, directed along thecrystallographic orientation of the HOPG. Such 2 nm flatislands consist of six bilayers (puckered bilayer height ofice ∼0.37 nm) of crystalline water, suggesting solid-likebehavior [14]. Yang et al [15] reported a study of interfacialwater on HOPG using 4D ultrafast electron crystallography,and determined that the structure of the vertically stackedbilayers of water on HOPG are highly ordered, pointingto an ice-like structure related to confined water. Latticeperiodicities corresponding to an ice-like structure wereobserved by scanning an AFM tip with high scanning speeds(∼100–200 nm s−1) and low forces (∼4 nN) over the HOPGsurface, kept in a controlled relative humidity (RH) enclosureunder ambient conditions [16]. Recently, the microscopic

structure of adsorbed water on hydrophobic surfaces underambient conditions was visualized using graphene templating.Water nanodroplets of 5–15 nm in height and 100–300 nmin width on HOPG at ∼40% RH were reported [17]. Waterdiffusion between monolayer graphene and SiO2 substratehas been investigated recently using AFM. It was reportedthat a two-layer thick (∼0.8 nm) ice-like structure is formedinitially and further liquid-like water adds over the ice-likelayer, causing wrinkle and fold formation in graphene on anSiO2 substrate [18]. Recently, the structure of confined waterbetween graphene and hydrophilic substrates such as mica andSiO2 has been studied using AFM to probe the intercalationand diffusion characteristics of water [19].

AFM voltage nanolithography is one of the commonlyused SPM-based patterning techniques [20]. AFM voltagenanolithography on HOPG has been reported by severalgroups [21–25]. The scanning probe etching of graphitehas been attributed to the aqueous electrochemical oxidationand removal of surface carbon atoms by the evolution ofvolatile CO and CO2 by-products [22]. The main parametersaffecting the etching reaction are the applied bias betweenthe tip and sample, tip scanning speed, setpoint force, andhumidity [21]. The nanostructure formation on HOPG byAFM bias nanolithography has been explained as a functionof threshold voltage (Vthreshold) of 5–6 V applied to theAFM tip, namely protrusions (below Vthreshold) and trenches(above Vthreshold) [23]. Cuts and pseudo-cuts being formedfor Vtip > Vthreshold and Vtip < Vthreshold have been reported onsingle-layer graphene on SiO2/Si substrates. Cuts created withhigher voltages remain unchanged for several weeks, whereaspseudo-cuts with smaller voltage disappear over hours [8].STM-based lithography of HOPG has been performed undergas environments of methyl alcohol, oxygen, or isopropanolgases to investigate the effect of water in ambient atmosphere.Methyl alcohol was reported to provide the best atmosphere fornarrow line patterning in comparison to water, which degradesthe etch pattern quality due to the large surface tension ofwater [26].

We observed temporal changes in the dimensions ofthe etch patterns formed on HOPG using AFM voltagenanolithography in contact mode by applying a positivesample bias while the tip is grounded [27]. In this study,we investigated the evolution of the etch patterns long afterlithography as a function of increasing number of nanoscalewater adlayers present on the HOPG. The role of the nanoscalewater adlayer on hydrophobic HOPG surface was examinedusing AFM voltage nanolithography and electrostatic forcemicroscopy (EFM). The charged nature of the etch patternsand the surrounding region was investigated using EFM. Thisis the first demonstration of accumulated electrostatic energyof a polarized water adlayer present on HOPG in ambientconditions, contributing to furthering the etching reaction atthe reactive lithography sites, long after the removal of theelectric field.

2. Experimental details

AFM voltage nanolithography was performed using a CP-IIatomic force microscope (Veeco, USA) with Pt/Ir coated

2

Nanotechnology 25 (2014) 155304 T Gowthami et al

Figure 1. Schematic showing the AFM lithography setup used. Theconducting tip is grounded and the HOPG sample is positivelybiased during the lithography. In this configuration, the lithographyprocess is more susceptible to the surrounding nanoscale wateradlayer and its polarization due to the applied electric field betweenthe AFM tip and HOPG surface during lithography.

Si tips with spring constant of 1–5 N m−1 and resonancefrequency of ∼60 kHz (Bruker Model SCM-PIT) on anHOPG surface (ZYB grade, mosaic spread ∼0.8◦), in aclean room environment (Class 1000). The experiments wereperformed in a controlled humidity enclosure around theatomic force microscope by bubbling nitrogen gas into aheated water beaker. The humidity was monitored using asensor probe type humidity meter (HT-305, Lutron, HongKong) with an accuracy of ±3%. Lithography was per-formed in contact-mode AFM (CM-AFM) under ambientconditions. Subsequently, the imaging of lithography patternswas first done in contact-mode AFM, and thereafter inintermittent-contact-mode AFM (IC-AFM) for different setsof experiments.

The configuration for the lithography experiments uses aconductive grounded AFM tip, whereas the HOPG sampleis positively biased, as shown in figure 1. For all theexperiments described herein, the lithography parameters werekept identical, namely sample bias 8 V, tip speed 0.04µm s−1,set-point force 15 nN, and controlled humidity 58%. We variedthe time period (given in days) since fresh cleavage of HOPGto study the etch/trench pattern formation as a function ofincreasing number of adsorbed water layers on the HOPGkept inside the AFM enclosure. Imaging in contact modewas done with an optimized low set-point force of 15 nNcorresponding to bare minimum force required to track thesample surface. For IC-AFM imaging, the free amplitude ofcantilever oscillation was around 100 nm, and we used aset-point amplitude of 43–45 nm with typical drive percentageof 3% for imaging with minimum perturbation, so as tovisualize clearly both the trench features and the surroundingwater droplets.

Electrostatic force microscopy (EFM) investigations weredone using the same Pt/Ir coated Si tip (SCM-PIT) placed inthe unmounted chip carrier with attached counter electrodepin wire, using a two-pass lift mode measurement. For theEFM investigations of trench patterns described here, a DCbias was applied to the sample through the Proscan DataAcquisition software in the CP-II atomic force microscope.The DC cantilever deflection was monitored to track the EFMphase signal in the second pass using an optimized lift heightof 80–100 nm with minimal influence from topography. ForEFM analysis of bare HOPG, a DC bias was applied to the tip

with the sample being grounded on the AFM system (diInnova,Veeco).

3. Results and discussion

AFM voltage nanolithography was performed on differentdays since the cleavage of HOPG surface, to study the effectof the increasing ambient water adlayers on the lithographypatterns. Figure 2 shows the trenches formed on HOPG, rightafter cleaving (Day 0, figure 2(a)) and nine days after cleaving(Day 9, figure 2(b)) using identical Pt/Ir coated Si tip at58% RH, with the above lithography parameters. Topographyimages of the lithography patterns in IC-AFM mode wereobtained at different time intervals since lithography, tomonitor the evolution of trench dimensions as a function oftime and number of water layers present on the HOPG.

It may be noted that the HOPG was continuously keptinside the AFM enclosure in a clean room environment,so there is no dust accumulation from the ambient. Onlythose lithography features were considered for analysis whichshowed ‘trench-like’ features in both contact and IC-AFMimaging modes. It is observed from the AFM topographyimages (figure 2) that the width of the trench patterns increasedsince the increasing lithography time. For the trenches obtainedon Day 0, the normalized change in width of the patternsshows an increase of about 33% over 48 h (see figure 2(c)). Inthe case of trench patterns formed with more water adlayerspresent on the surface, i.e., Day 9, an increase of ∼79%(figure 2(d)) was observed. Measurements for the trenchdimensions were made using several line profiles along thetrenches, and the error bars, calculated using ±σW and ±σD(where σ refers to standard deviation), on the measurementsof width and depth for the trench patterns in figure 2, aregiven in the supplementary information (figure S1 availableat stacks.iop.org/Nano/25/155304/mmedia). The lithographyexperiment was also performed on two-day-old cleaved HOPGsample (Day 2) with another Pt/Ir coated Si tip (see figureS2(a) available at stacks.iop.org/Nano/25/155304/mmedia),which showed an increase in normalized width of ∼58%over 48 h as shown in figure S2(b) (available at stacks.iop.org/Nano/25/155304/mmedia). Normalized changes in width1W/W0 are considered instead of absolute values of width,to be able to compare the amount of change in W acrossdifferent sets of experiments performed as a function of time.There is a significant increase in normalized width changesince the lithography time for all Day 0, Day 2, and Day 9experiments. There is a progressive increase in 1W/W0 withincreasing number of water adlayers for Day 0, Day 2, andDay 9 experiments over 48 h, despite the change of tip. Thisimplies clearly that the etching rate increases for additionalwater adlayers present on different days. The increase inwidth of the trench patterns was repeatedly observed inmany other sets of experiments performed under the aboveAFM lithography conditions. The corresponding changes inthe depth and length of trenches were also monitored (seefigures S2 and S3 available at stacks.iop.org/Nano/25/155304/mmedia). The normalized change in depth 1D/D0 doesnot show a systematic trend for trench patterns made on

3

Nanotechnology 25 (2014) 155304 T Gowthami et al

Figure 2. Evolution of width of trenches obtained by AFM voltage nanolithography using sample bias 8 V, tip speed 0.04 µm s−1, andset-point force 15 nN at a controlled humidity of 58%, for increasing water adlayer on HOPG: AFM topography images, in IC-AFM mode,of trenches at denoted time since lithography. (a) Day 0—freshly cleaved HOPG. (b) Day 9—nine days after cleaving HOPG. Percentagechange in normalized width (1W/W0× 100) since lithography time 1T = T − T0 (h) (where T0 refers to initial imaging time) for (c) Day0 and (d) Day 9, showing an increase of about 33% and 79%, respectively, over 48 h.

Day 0 and Day 2 HOPG samples (see figures S3(a) andS2(c) available at stacks.iop.org/Nano/25/155304/mmedia);this may be on account of the varying number of subsurfacecarbon layers. However, the Day 9 trench pattern exhibits alarge gradual increase of 646% up to 48 h after lithography(see figure S3(b) available at stacks.iop.org/Nano/25/155304/mmedia). In contrast, the normalized change in length1L/L0shows a minimal change of about 2–8% in the case of all Day 0,Day 2, and Day 9 experiments. To ascertain whether thewidely differing values of 1W/W0 and 1L/L0 arise dueto the different in-plane (x–y plane) etching rates along thewidth and the length of the trenches, we made a rectangulartrench pattern of width ∼55 nm and length ∼144 nm (asshown in figure S4(a) available at stacks.iop.org/Nano/25/155304/mmedia). The normalized changes in width (1W/W0)and length (1L/L0) of the rectangular trench pattern over48 h were ∼60% and ∼18%, respectively (see figures S4(b)and (d) available at stacks.iop.org/Nano/25/155304/mmedia).This shows a proportionate increase in the normalized changein width and length, which implies a similar in-plane etchingrate along the x and y directions.

The size of the water droplets around the trench region(figures 2(a) and (b)) was also measured as a function of time;see figure S5 (available at stacks.iop.org/Nano/25/155304/mmedia). The average height of the water droplets on Day 9 is∼6.6 nm, approximately 3 times larger than that for Day 0.However, the amount of water present in the scanned regionmay also vary on account of the dynamic nature of waterdroplets on HOPG.

It is known that HOPG supports molecularly thin wateradlayers in ambient conditions [14]. The role of the wateradlayer was examined to investigate the above changes inthe dimensions of the trench patterns long after the AFMlithography process. We performed two separate lithographyexperiments on freshly cleaved HOPG and monitored theevolution of the trench patterns (line 1 and line 2) with time,in ambient conditions and by passing nitrogen over a period of∼8 h (see figure 3). The humidity reduced drastically from 58%to 40% within half an hour of flushing nitrogen inside the AFMenclosure and thereafter reduced slowly to 35% over ∼8 h. Itcan be noticed clearly from the AFM topography images thatthe trenches widen in ambient conditions (figure 3(a)) whilethere is no change in width in the N2 atmosphere (figure 3(b)).Accordingly, it was found that, ∼8 h after lithography, thenormalized change in width (1W/W0) in ambient conditionsshowed a maximum change of∼20%, whereas in the presenceof nitrogen a minimal change of ∼0.9% was obtained (seefigure 3(c)). It may be noted that in the case of ambientconditions (figure 3(c)), 1W/W0 for line 1 and line 2 trenchpatterns is different, being∼12% for line 1 and∼20% for line2 over∼8 h since lithography. This variation in1W/W0 mayarise from the change in the surrounding water adlayer duringthe AFM bias lithography of the line 1 pattern. Hence, thenature of the surrounding water adlayer for the neighboringline 2 region is altered. Thus, the corresponding evolution ofthe relative change in width with time may differ for line 1and line 2. Additionally, it can be seen that the water dropletsare accumulated around the trench region in the case of bothambient conditions and N2 atmosphere, as seen in figures 3(a)

4

Nanotechnology 25 (2014) 155304 T Gowthami et al

Figure 3. Comparison of evolution of trenches created on freshly cleaved HOPG at different time intervals since lithography in ambientconditions and the nitrogen atmosphere. Topography images (IC-AFM mode) in (a) ambient conditions and (b) the nitrogen atmosphereover a period of approximately 8 h. Scale bar= 500 nm. Average line profiles obtained across trench 1 and trench 2 inside the box shown ininsets of (a) and (b). (c) Normalized change in width for line 1 and line 2 (1W/W0× 100) with time 1T (h), showing a maximum increaseof 19.6% in ambient conditions and a small change of 0.9% in the N2 atmosphere. (d) Reduction in width and height of water droplets withtime in the N2 atmosphere.

and (b). The presence of water droplets in the vicinity of thelithography region has been explained before as being dueto the surface modification on graphite and change in pH ofthe water [21]. In the N2 atmosphere, there is a considerablereduction in average height of the water droplets from 1.98 to0.94 nm (∼53%) as well as a decrease in average width from52.5 to 31.7 nm (∼40%) over ∼8 h (see figure 3(d)), thoughthe water droplets do not disappear altogether. In contrast,the water droplets near the trenches at ambient humidityof 58%, see figure 3(a), show a much smaller reduction,i.e. ∼24% (1.7–1.3 nm) in height and ∼16% (33.6–28.2 nm)in width over the same time period (see figure S6 available atstacks.iop.org/Nano/25/155304/mmedia). Slow evaporationof attoliter water droplets formed by the electrocondensationprocess on carbonaceous surfaces by a biased AFM tip underambient conditions has been reported previously [28]. Hence,it seems that such sized water droplets may not have a majorrole in the widening of the trenches. We also measured theheights between the HOPG surface and the depth of trenchpatterns 1 and 2 in the ambient condition case (h1A ∼ 1.7 nmand h2A ∼ 1.57 nm) and in the N2 atmosphere (h1N ∼ 0.92 nmand h2N ∼ 0.77 nm) at 0.5 h since lithography (see the averageline profile obtained inside the box region in the insets of

figures 3(a) and (b)). The surface height difference of HOPGbetween ambient conditions and the N2 atmosphere for pattern1 and pattern 2 are∼0.78 nm and∼0.8 nm, respectively, beingclose to the height of two ice-like water adlayers, as reportedby Xu et al [29].

In order to investigate the nature of the ambient wateradlayer further, we performed EFM on a bare HOPG surface.It is observed that, a day after cleaving the HOPG, domain-likestructures with facets were present over the entire scanned areaof the HOPG, which is seen only in the EFM phase images(tip-biased, sample-grounded) and not in the topographyimages (see figure 4). The domain-like structures werealso observed in EFM phase images of the HOPG fortip-grounded and sample-biased configurations. These domainstructures were not, however, observed soon after cleaving theHOPG, implying that these domains are related to the wateradlayer. Domains with polygonal shapes, having domain edgeangles of 60◦ and 120◦, are seen, implying alignment alongcrystallographic directions of the HOPG, which has also beenreported previously [14]. The faceted nature of the domainsmay imply an ice-like structure of the water adlayer. Thereare some rounded structures seen as well, in figures 4(b) and(c) at a few places, which may be related to the liquid-like

5

Nanotechnology 25 (2014) 155304 T Gowthami et al

Figure 4. Bare HOPG surface imaged a day after cleaving. (a) Topography (IC-AFM mode) and corresponding EFM phase images with DCbias applied to the tip, (b) +3 V, and (c) −3 V, and sample kept grounded. Scale bar= 5 µm. A lift-scan height of 100 nm was chosen. EFMphase images of domain-like structures with polygonal facets are seen over the entire scanned area of the HOPG, showing opposite contrastwith reversal of the bias polarity.

water adlayer. Importantly, the faceted domains are charged,as they show opposite EFM phase contrast upon reversal of biaspolarity during EFM imaging (see figures 4(b) and (c)). Thissuggests that the faceted domains show reverse polarizationby the re-orientation of the dipole-oriented water molecules inan ice-like water adlayer. Hence, the domains show oppositeEFM contrast upon reversal of the bias polarity. These domainsare metastable and change by re-scanning the same area, ashas been reported by Verdaguer et al [13].

Further, we performed EFM analysis of the evolution ofthe etch patterns (in figure 3(a)) created soon after cleaving ofHOPG under ambient conditions over a few days (71 h), asshown in figure 5. The EFM phase images show a high EFMcontrast over a wide region (∼530 nm wide), transformingto a narrow zone with small EFM contrast (figures 5(a)–(d)),implying a reduction in the surrounding zone of the chargedlithography pattern. It is seen that for, an increase in normalizedwidth (1W/W0) of ∼33–40% over 30 h (figure 5(e)), thecorresponding normalized change in EFM phase shift,1φ/φ0,shows a decrease of 84% for −1 V and 88% for +1 V(figure 5(f)), which implies a large reduction in the surroundingcharge from the initial time. The Coulombic nature of thelithography trench pattern is confirmed by the opposite EFMcontrast in the EFM phase images and the linear variation inEFM phase shift with reversal of the bias polarity (figure 5(g)).This reduction in EFM phase shift may occur due to the initiallypolarized water adlayer getting de-polarized eventually overtime. This polarized water adlayer may, further, cause aslow etching reaction at the reactive trench sites, long afterthe AFM lithography process. It has also been proposed byVerdaguer et al [13] that such thin dipole-oriented wateradlayers have some accumulated electrostatic energy. Thisis the first demonstration of the effect of how the storedelectrostatic energy of a thin polarized ice-like water adlayercontributes to the etching reaction at the AFM lithographicallymodified sites on the HOPG surface.

The EFM phase variation in the nitrogen atmospherefor trench patterns created soon after cleaving HOPG (infigure 3(b)) was also examined (see figures S7(a)–(c) availableat stacks.iop.org/Nano/25/155304/mmedia). The EFM phasecontrast in figures S7(b) and (c) (available at stacks.iop.org/Nano/25/155304/mmedia) also shows opposite contrast

with reversal of the bias polarity, indicating a Coulombicbehavior, as shown in figure S7(e) (available at stacks.iop.org/Nano/25/155304/mmedia). The width of the charged ‘zone’(lithographically modified region) does not show a reductionwith time, in contrast to the ambient conditions case (seefigures S7(b) and (c) available at stacks.iop.org/Nano/25/155304/mmedia). Accordingly, the EFM contrast of the lithographyregion, namely the phase shift (φ), does not show any variationwith time in the N2 atmosphere (figure S7(d) available atstacks.iop.org/Nano/25/155304/mmedia). This is in agree-ment with the minimal variation observed in the width of thetrench patterns in the topography images. This implies that,though the water droplets do not disappear completely in theN2 atmosphere, there is a removal of the thin polarized ice-likewater adlayer by the flushing of N2. This is supported by ourobservation in figures 3(a) and (b), where the height differenceof the HOPG surface and the depth of trench between ambientconditions and the N2 atmosphere 30 min since N2 flushing is∼0.79 nm, corresponding to the height of two ice-like wateradlayers. The removal of the polarized water adlayers mayin turn lead to the ‘zone’ of the charged lithography regionremaining unchanged with time. This is responsible for therebeing no further chemical reaction at the lithography trenchsites, thus maintaining the initial state of the trench pattern.

To understand why the water droplets do not affect thetrench patterns, we performed EFM of the lithography trencheson the HOPG sample long after cleaving (approximately15 days). This was done in order to distinguish clearly theEFM contrast between the large water droplets and that forlithography trench patterns. Figure 6(a) shows a topographyimage of water droplets surrounding trench patterns and thecorresponding EFM phase images. The water droplets have asize range of 0.4–3.8 nm (see figure 6(b)). In comparison, thewater droplets for Day 0 lithography experiments (figure 3(d))are relatively smaller in size, i.e., 0.9–2 nm, and accordinglytheir EFM contrast merges with that for the trench patterns.

The histogram, in figure 6(b), represents all water dropletsof height above 0.4 nm, for which the EFM contrast doesnot show any change upon bias polarity reversal. This isseen in the EFM phase images, which show the same darkcontrast for the encircled regions for bias polarity reversal (−2to 2 V), suggesting that the water droplets are not charged.

6

Nanotechnology 25 (2014) 155304 T Gowthami et al

Figure 5. Evolution of trench lines created (300 nm apart) soon after HOPG cleaving. Topography (IC-AFM mode) and corresponding EFMcontrast at different intervals of time since lithography: (a) 0.5 h, (b) 26 h, (c) 50 h, and (d) 71 h. The EFM phase images show high dark (for+1 V) and high bright (for −1 V) contrast over a broad lithography region evolving to a narrow zone with small EFM contrast over time,suggesting a reduction in the charged zone area. (e) The normalized change in width (1W/W0× 100) in the topography of the trenches,showing an increase of 33–40%, and (f) the normalized change in EFM phase shift (1φ/φ0× 100) with time 1T (h) for +1 and −1 V,showing a decrease of ∼88% (for +1 V) and ∼84% (for −1 V) over 30 h. (g) Phase shift 8 variation with reversal of bias polarity from −3to +3 V, showing a linear variation between −2 and +1 V, pointing to the Coulombic nature of the trench region. The lift-scan height waskept constant at 80 nm for all EFM phase images.

Accordingly, a capacitive interaction is observed, as seen infigure 6(c). This implies that the water droplets of height above0.4 nm are not charged. Hence, their presence around thelithography trench patterns does not contribute to any furtheretching reaction.

The possibility that the scanning itself produces ice-likestructures between the tip and the sample would not accountfor the systematic increase in the change in width of the etchpatterns as a function of increasing number of water adlayerson different days since HOPG cleavage. Moreover, it was alsoproposed by Jinesh et al [30] that capillary water condensate(ice-like) does not necessarily require confinement between

the tip and the substrate. Thus, the slow etching reaction longafter the AFM lithography process on the HOPG surface ison account of a thin ice-like water adlayer, which is polarizedand remains so over a long period of time due to the slowdissipation of accumulated electrostatic energy.

4. Conclusions

We report the evolution of trench patterns on HOPG obtainedusing AFM voltage nanolithography in contact mode inambient conditions as a function of the increasing number ofwater layers present on the surface of the HOPG. The variation

7

Nanotechnology 25 (2014) 155304 T Gowthami et al

Figure 6. EFM analysis of water droplets surrounding lithographytrench patterns created 15 days after HOPG cleaving. Water dropletscreated at Day 15 were chosen for this analysis to distinguish theEFM contrast between water droplets and the lithography region:(a) Topography (IC-AFM mode) and corresponding EFM phaseimages at +2 and −2 V obtained at the time of lithography.(b) Histogram of height of water droplets showing unchangedcontrast (example encircled droplets) (dark) for reversal of biaspolarity. This implies that water droplets of height above 0.4 nm donot get charged. (c) EFM phase shift (φ) variation of water dropletswith voltage, showing their capacitive nature. A lift-scan height of80 nm was used for the EFM phase images.

in the width of the trench pattern shows a systematic increasewhereas the change in depth of the trench does not follow asystematic trend for fewer water layers present on the HOPG

surface. The length of the trench patterns shows a minimalchange as L�W , and does not seem to be related to differentin-plane etching rates along the x and y directions. The widthof the etch patterns made on freshly cleaved HOPG increasesby about 33% over 48 h, whereas it more than doubles foretches created on a nine-day-old cleaved HOPG sample overthe same period of time.

The evolution of the EFM phase shift of lithography etchpatterns created soon after HOPG cleaving in ambient condi-tions revealed a large change of∼84–88%, corresponding to achange in width of∼33–40% over a time interval of 30 h sincelithography. In the presence of a N2 atmosphere subsequentto lithography, the change in width of trenches created rightafter HOPG cleaving is minimal (∼0.9%), and accordinglyno change in EFM phase shift (contrast) of trenches is seenover 8 h.

In the presence of a N2 atmosphere, a thin polarizedice-like water adlayer is removed, which does not lead to anyfurther etching reaction at the lithography sites. The heightand width of the water droplets are reduced, and these waterdroplets (liquid-like) do not appear to be charged, from theEFM phase images. Hence, it is inferred that the polarizationof the neighboring thin water adlayer during the lithographyprocess contributes to the furthering of the electrochemicalreaction with the functional groups created at the lithographysites. In contrast, the surrounding water droplets (liquid-like)around the lithography trenches are not charged, and thus theydo not contribute towards the widening of trench sites. Thisis the first demonstration of the effect of a thin ice-like polar-ized water adlayer’s stored electrostatic energy contributingtowards an etching reaction on AFM nanolithography reactivesites on HOPG, albeit slow. These considerations have animportant bearing for patterning of carbon-based materialssuch as graphene in ambient conditions.

Acknowledgments

This work was supported in part by VIT University, Vellore,and Department of Science and Technology, Nanomission,New Delhi. TG acknowledges financial support from VITUniversity. NK acknowledges the Council of Scientific andIndustrial Research (CSIR) for funding. The authors are grate-ful to Professor GU Kulkarni, JNCASR, Bangalore, for manythought-provoking discussions and valuable suggestions. TGand GR are grateful to Professor JP Raina, Director, Centerfor Nanotechnology Research, VIT University, for his constantencouragement.

References

[1] Novoselov K S, Geim A K, Morozov S V, Jiang D, Zhang Y,Dubonos S V, Grigorieva I V and Firsov A A 2004 Electricfield effect in atomically thin carbon films Science306 666–9

[2] Russo S, Oostinga J B, Wehenkel D, Heersche H B,Sobhani S S, Vandersypen L M K and Morpurgo A F 2008Observation of Aharonov–Bohm conductance oscillationsin a graphene ring Phys. Rev. B 77 085413

8

Nanotechnology 25 (2014) 155304 T Gowthami et al

[3] Han M Y, Ozyilmaz B, Zhang Y and Kim P 2007 Energyband-gap engineering of graphene nanoribbons Phys. Rev.Lett. 98 206805

[4] Bunch J S, Yaish Y, Brink M, Bolotin K and McEuen P L2005 Coulomb oscillations and hall effect in quasi-2Dgraphite quantum dots Nano Lett. 5 287–90

[5] Stampfer C, Guttinger J, Molitor F, Graf D, Ihn T andEnsslin K 2008 Tunable coulomb blockade innanostructured graphene Appl. Phys. Lett. 92 012102

[6] Jeon S-C, Kim Y-S and Lee D-K 2010 Fabrication of agraphene nanoribbon with electron beam lithography usinga XR-1541/PMMA lift-off process Trans. Electr. Electron.Mater. 11 190–3

[7] Weng L, Zhang L, Chen Y P and Rokhinson L P 2008 Atomicforce microscope local oxidation nanolithography ofgraphene Appl. Phys. Lett. 93 093107

[8] Puddy R K, Scard P H, Tyndall D, Connolly M R, Smith C G,Jones G A C, Lombardo A, Ferrari A C and Buitelaar M R2011 Atomic force microscope nanolithography ofgraphene: cuts, pseudocuts, and tip current measurementsAppl. Phys. Lett. 98 133120

[9] Tapaszto L, Dobrik G, Lambin P and Biro L P 2008 Tailoringthe atomic structure of graphene nanoribbons by scanningtunneling microscope lithography Nature Nanotechnol.3 397–401

[10] Zhang L, Pejakovic D A, Geng B and Marschall J 2011Surface modification of highly oriented pyrolytic graphiteby reaction with atomic nitrogen at high temperatures Appl.Surf. Sci. 257 5647–56

[11] Gil A, Colchero J, Luna M, Gomez-Herrero J and Baro A M2000 Adsorption of water on solid surfaces studied byscanning force microscopy Langmuir 16 5086–92

[12] Hu J, Xiao X-D, Ogletree D F and Salmeron M 1995 Imagingthe condensation and evaporation of molecularly thin filmsof water with nanometer resolution Science 268 267–9

[13] Verdaguer A, Sacha G M, Bluhm H and Salmeron M 2006Molecular structure of water at interfaces: wetting at thenanometer scale Chem. Rev. 106 1478–510

[14] Luna M, Colchero J and Baro A M 1999 Study of waterdroplets and films on graphite by noncontact scanning forcemicroscopy J. Phys. Chem. B 103 9576–81

[15] Yang D-S and Zewail A H 2009 Ordered water structure athydrophobic graphite interfaces observed by 4D, ultrafastelectron crystallography Proc. Natl Acad. Sci. USA106 4122–6

[16] Teschke O 2010 Imaging Ice-like structures formed on HOPGat room temperature Langmuir 26 16986–90

[17] Cao P, Xu K, Varghese J O and Heath J R 2011 Themicroscopic structure of adsorbed water on hydrophobicsurfaces under ambient conditions Nano Lett. 11 5581–6

[18] Lee M J, Choi J S, Kim J-S, Byun I-S, Lee D H, Ryu S, Lee Cand Park B H 2012 Characteristics and effects of diffusedwater between graphene and a SiO2 substrate Nano Res.5 710–7

[19] Kim J-S et al 2013 Between scylla and charybdis:hydrophobic graphene-guided water diffusion onhydrophilic substrates Sci. Rep. 3 2309

[20] Garcia R, Martinez R V and Martinez J 2006 Nano-chemistryand scanning probe nanolithographies Chem. Soc. Rev.35 29–38

[21] Yoshimizu N, Hicks B, Lal A and Pollock C R 2010 Scanningprobe nanoscale patterning of highly ordered pyrolyticgraphite Nanotechnology 21 095306

[22] Kurra N, Prakash G, Basavaraja S, Fisher T S, Kulkarni G Uand Reifenberger R G 2011 Charge storage in mesoscopicgraphitic islands fabricated using AFM bias lithographyNanotechnology 22 245302

[23] Jiang Y and Guo W 2008 Convex and concave nanodots andlines induced on HOPG surfaces by AFM voltages inambient air Nanotechnology 19 345302

[24] Park J G, Zhang C, Liang R and Wang B 2007Nano-machining of highly oriented pyrolytic graphite usingconductive atomic force microscope tips and carbonnanotubes Nanotechnology 18 405306

[25] Kumar K, Strauf S and Yang E H 2010 A systematic study ofgraphite local oxidation lithography parameters using anatomic force microscope Nanosci. Nanotechnol. Lett.2 185–8

[26] Park J, Kim K B, Park J-Y, Choi T and Seo Y 2011 Graphitepatterning in a controlled gas environment Nanotechnology22 335304

[27] Gowthami T, Gadhewal M and Raina G 2013 Study ofstability of local anodic oxidation on HOPG and few layergraphene using AFM in ambient IEEE Trans. Nanotechnol.12 1002–6

[28] Kurra N, Scott A and Kulkarni G U 2010 Electrocondensationand evaporation of attoliter water droplets: directvisualization using atomic force microscopy Nano Res.3 307–16

[29] Xu K, Cao P and Heath J R 2010 Graphene visualizes the firstwater adlayers on mica at ambient conditions Science329 1188–91

[30] Jinesh K B and Frenken J W M 2006 Capillary condensationin atomic scale friction: how water acts like a glue Phys.Rev. Lett. 96 166103

9