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Ultramicroscopy 105 (2005) 12–15

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Direct observation of transition between stick-slip andcontinuous sliding in atomic friction

Wen-Hao Huanga,�, Zheng Weib

aDepartment of Precision Machinery and Precision Instrument, University of Science and Technology of China, Hefei,

Anhui 230027, ChinabDepartment of Modern Physics, University of Science and Technology of China, Hefei, Anhui 230027, China

Received 2 August 2004; received in revised form 22 March 2005

Abstract

The distortion and gradual recovery processes after a continuous tunable laser illuminating the graphite surface in air

have been observed and analyzed in detail with a scanning tunneling microscope (STM). The slow response of the tip to

the laser-induced mechanical elongation is responsible for the observed phenomena, which indicate that the transition

between stick-slip and continuous sliding has occurred due to the atomically modulated friction between the STM tip

and the graphite substrate.

r 2005 Elsevier B.V. All rights reserved.

Keywords: Stick-slip; Scanning tunneling microscopy; Graphite; Laser illumination; Mechanical deformation

1. Introduction

The invention of scanning tunneling microscope(STM) has greatly boosted the studies of surfacesand interfaces, which are crucial in fundamentalresearch and industry applications. By combiningthis novel technique with optical excitation, it ispromising to get more in situ information fromphoto-induced currents and voltages to nanos-tructuring at atomic scale [1].

e front matter r 2005 Elsevier B.V. All rights reserve

tramic.2005.06.011

ng author. Tel.: +86551 3603372;

03372.

ss: whuang@ustc.edu.cn (W.-H. Huang).

Surface nanostructuring has been widely studiedand realized by illuminating the scanning probemicroscope’s tips with ultrafast laser or voltagepulses [2–7] while the original mechanism remainsunclear. There are so many factors which mightaccount for the formation of the nanostructures bylaser-assisted STM (e.g. thermal expansion [2–5],field enhancement [6], strong electric field [7] effects)that it is still challenging to fabricate nanostructureswith definite size in a controllable way.In this paper, we present a series of STM images

that clearly show the distortions and gradualrecovery stages in single images after a continuoustunable laser illuminating the graphite surface in

d.

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W.-H. Huang, Z. Wei / Ultramicroscopy 105 (2005) 12–15 13

air. In order to explain these puzzling images, wepropose a two-step model involving laser induceddeformation of the tip and the stick-slip motionbetween the tip and sample. These images indicatethe transition between the stick-slip and thecontinuous sliding does occur on the graphitesurface. The phenomena are helpful to furtherunderstand the atomic friction at mesoscopic scaleand the mechanical response of the nanosized tipto the laser illumination.

2. Experiment

All of the experiments were carried out with acommercial STM (Nanoscope III) in air. The Pt–Irtips were produced by mechanical-cutting andclean surfaces of graphite which were prepared bycleaving the HOPG with adhesive tape. Thetunable Ti: sapphire laser (pumped by Ar+ laser)was focused onto the tunneling gap with a spot

Fig. 1. Topography image reveals the whole process before and after

lines in (a) indicate three rows of b site carbon atoms, which hascorresponding graphite lattice is shown in (b).

diameter of 6mm and an incidence angle of 601with respect to the surface normal. The laser wasapplied for about 1 s and then was switched offimmediately while the STM was raster scanning inthe tunneling range. The feedback was (withresponse time of �0.05ms) on during the entireexperiments. The laser was adjusted to work withdifferent power and wavelengths to study thedegrees and shapes of various distortions of theimages.

3. Results and discussion

The topography image in Fig. 1 is taken withvoltage bias 20mV and tunneling current 2 nA.The upper arrow indicates the time when a 50mW,800 nm laser illuminates the tunneling gap. Oncethe laser is introduced, the tunneling current willincrease greatly according to the rise time of thelaser and the feedback loop will retract the tip

a 50mW, 800nm laser illuminates the graphite surface. Three

different intensity after the illumination. The direction of the

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W.-H. Huang, Z. Wei / Ultramicroscopy 105 (2005) 12–1514

from the sample. After the pulse was over, the tipwill return gradually to its normal position (let uscall it relaxation period).During the illuminating and relaxation period,

the tip-sample distance isn’t constant due to thereal-time adjust of the feedback loop, which causesthe tunneling current and the electronic circuit ofSTM unstable. The corresponding topographyimage in these two parts are fuzzy and out ofour consideration. What interests us is the follow-ing stable stage called ‘‘stick-slip’’: the topographyimages in this period show periodic stripes (withdifferent intensity and bending) instead of thenormal graphite lattices. These stripes imply thatan unexpected mechanism should play a role inthis situation.The long stripes are reminiscent of the nanoscale

periodic frictional force map of the graphitesurface, firstly discovered by Mate [8] with atungsten tip. The phenomena have been studiedintensively by many groups, and the most popularmechanism is ‘‘stick-slip’’ motion of the tip alongthe surfaces because of the relative stiffness of thetip (or cantilever in AFM and SFM) [9] than theoutmost graphite layer. Based on numericalsimulation for a static model of AFM, Sasaki etal. [9] theoretically investigate how the cantileverstiffness, scan direction, anisotropy of the atom tipconnected with the cantilever spring influence theimage of atomic-scale friction of graphite inatomic-force microscopy. Their works show thatas the spring constant of the tip/cantileverdecreases, the total energy curve may change fromnearly parabolic to double barriers correspondingto the two nearest hollow sites in the scandirection, which make the tip prefer staying inthe hollow site corresponding to the local mini-mum total energy until the very other end of thetip (or cantilever) has moved for nearly one unitcell’s length. Then it could jump to the next hollowsite. This is the whole process of ‘‘stick-slip’’.For the case in our experiments, the tip has been

expanded, elongated under the strong electric fieldand laser pulses. The piezoelectric actuator retreatsthe tip from sample until the pulse was over. Thenthe tip will take some time to recover its normalscanning status. Although the controlling andacquisition electronics work well after the illumi-

nation, the Pt/Ir tip might become thinner andsofter due to the laser illumination. This meansthat the resulting decrease of the tip/cantilever’sspring constant make the ‘‘stick-slip’’ processoccurs frequently. In Fig. 1(a) the scan directionhas an angle of 81.51 with the three lines (markedwith A, B, C) where three rows of b site carbonatoms locate. This is in agreement with thetheoretical value of 79.11, as shown in Fig. 1(b).The widths of the three stripes marked with A, B,C respectively imply the staying time at theposition between the hollow sites. After sometimes of the scanning, the tip recovers its normalspring constant and return to its continuoussliding (Figs. 1 and 2).In the following list some of the topography

images recorded by using laser in different powerand wavelengths, which indicate the phenomena arerepeatable and universal when laser illuminate thegraphite surface. It has been reported that thegraphite STM images change from trigonal tohexagonal symmetry when irradiating HOPG witha HeNe laser light with wavelength of 623 nm andthe effect is reversible by simply interrupting thelaser light flux [10]. But the laser power they used(1–2mW) is two orders weaker than ours. Suchweak flux couldn’t provide enough energy to distortthe STM tip by heating effect. This might explainwhy they haven’t observed the laser induced stick-slip motion of the tip on the graphite surface. Theyalso study the time dependence of the relativeintensity at a and b sites after starting or interrupt-ing the HeNe laser. The transition between thetrigonal and hexagonal images also need a fewseconds, which is comparable to our results.We also notice that Socoliuc et al. [11] have

measured the lateral force acting on the tip slidingforward and backward in (1 0 0) direction over theNaCl (0 0 1) surface with a friction force microscope. They observed a transition from stick-slip tocontinuous sliding for atomically modulated frictionwhen different externally applied load was applied.The results further prove atomic friction playimportant role when the tip slides over the substrateand rule out the explanation that the observed‘‘stick-slip’’ stages are due to the environment noisesor electric spikes. The laser induced distortion of thegraphite layers are also ruled out because the

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Fig. 2. Three dynamic topography images with different laser parameters (scan range: 3 nm� 3 nm).

W.-H. Huang, Z. Wei / Ultramicroscopy 105 (2005) 12–15 15

graphite has strong intra-layer covalent bonds,which prevent the carbon atoms from vibratinggreatly in the top layer.

4. Conclusion

In summary, the distortions and gradual recov-ery of STM images were observed by illuminatingthe tunneling gap between HOPG and the STM tipin air with a continuous tunable laser. Detailedanalysis to the dynamic topography images showsthat the obtained image could be divided into fourstages: normal scanning, illuminating, relaxationand ‘‘stick-slip’’ stages. Thanks to the fastresponse of the tip to the laser pulses andmechanical stress, the whole stages—the normalgraphite structure, the laser-induced fuzzy area,the distortion and gradual recovery process—could be recorded with atomic resolution, whichclearly show the transition between the slip–stickand continuous sliding at atomic scale andstrongly support that thermal expansion couldplay a role in the relative weak laser pulses. Theexperiment is helpful for understanding the inter-action between the tip and sample, which is crucialfor the controllable nanostructuring in the future.

Acknowledgement

This work is supported by National ScienceFoundation of China project (50275140, 50335050).

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