**6. Conclusions**

In conclusion, we have studied the nanotribological behavior of a well-defined hydrogenterminated diamond(111)/tungsten-carbide single asperity contact in UHV as a function of applied load. Local contact conductance measurements showed no significant changes in the shape of I-V curves for loads up to 1.7 N, as expected from the proportionality between the current and the contact area, which provided us with a direct and independent way of measuring the area of contact. The DMT model provided an excellent fit to the current (contact area) versus load data for a variety of bias voltages, which is in agreement with the finding that = 0.02 < 0.1. Using the DMT-relation for contact area versus load, we found that for this ideal single-asperity contact, *i.e.* one of the hardest, stiffest known heterocontacts, involving materials of great tribological importance, friction is directly proportional to the contact area: *Ff* =  *A*, where = 238 MPa for loads up to 12 nN.

By using AFM and STM with the same conductive WC-coated cantilevers, we were able to study the tribological and electronic properties of nanocontacts and to correlate these properties with the degree of passivation of the interface. Contacts could be classified as clean, half-passivated and fully passivated, depending on whether none, one or both of the participating surfaces are covered with chemically inactive layers. While it would be desirable to obtain detailed information on the specific chemistry and structure of these contaminant species, no technique currently exists for obtaining such information at a confined nanoscale interface. Rather, we are restricted to rely on wide-scale AES measurements of the surfaces. Based on these measurements, we propose that the passivating materials for the WC tips consist mostly of strongly bound O and C species. On the tip they could be removed by sliding contact under high load on the Pt substrate. In the case of Pt, the contaminants were C species.

The radii of the tips used for the Au and Al-Ni-Co samples derived from the forcedistance curves was found to be 30 11 nm, and 70 30 nm respectively. Calculations performed for several values of tip radius and for 0.5 nm for D, are shown in figure 30(a) as a function of step height and step dipole moment (Park et al., 2005a). As we can see the data (difference in the force at + and – bias per unit applied volt) fit well the lines corresponding to step dipole values of 1.6 Debye/nm or 0.45 Debye/step atom for Au(111) monoatomic steps (Park et al., 2005a) with the tip radius of 30 nm, and 2.5 Debye/nm or 1.0 Debye/step atom for the smallest (0.2 nm) quasicrystal steps with the tip radius of 70 nm. We can conclude that the dipole moment scales proportionally to step

The dipole moment obtained for Au(111) is ~3 times larger than the value of 0.16 Debye/atom obtained by Jia *et al.* (Jia et al., 1998a; 1998b) from STM barrier height measurements and ~2 times larger than the 0.20 to 0.27 Debye/atom obtained by Besocke *et al.* (Besocke & Wagner, 1973) from work-function measurements on stepped Au(111). Bartels *et al.* (Bartels et al., 2003) obtained 0.33 Debye/atom for Cu(111) steps from STM spectroscopy of localized states at step edges. Apart from systematic and statistical errors in the measurements, the discrepancy could be related to the very different methods used, tunneling barrier in one case and average work function in another as compared to direct

In conclusion, we have studied the nanotribological behavior of a well-defined hydrogenterminated diamond(111)/tungsten-carbide single asperity contact in UHV as a function of applied load. Local contact conductance measurements showed no significant changes in the shape of I-V curves for loads up to 1.7 N, as expected from the proportionality between the current and the contact area, which provided us with a direct and independent way of measuring the area of contact. The DMT model provided an excellent fit to the current (contact area) versus load data for a variety of bias voltages, which is in agreement with the finding that = 0.02 < 0.1. Using the DMT-relation for contact area versus load, we found that for this ideal single-asperity contact, *i.e.* one of the hardest, stiffest known heterocontacts, involving materials of great tribological importance,

By using AFM and STM with the same conductive WC-coated cantilevers, we were able to study the tribological and electronic properties of nanocontacts and to correlate these properties with the degree of passivation of the interface. Contacts could be classified as clean, half-passivated and fully passivated, depending on whether none, one or both of the participating surfaces are covered with chemically inactive layers. While it would be desirable to obtain detailed information on the specific chemistry and structure of these contaminant species, no technique currently exists for obtaining such information at a confined nanoscale interface. Rather, we are restricted to rely on wide-scale AES measurements of the surfaces. Based on these measurements, we propose that the passivating materials for the WC tips consist mostly of strongly bound O and C species. On the tip they could be removed by sliding contact under high load on the Pt substrate. In the

 *A*, where

= 238 MPa for loads

height, at least for steps up to 1.5 nm.

**6. Conclusions** 

up to 12 nN.

measurement of the dipole force field in the present work.

friction is directly proportional to the contact area: *Ff* =

case of Pt, the contaminants were C species.

The clean Pt(111) surface could be imaged in STM mode with cantilevers stiff enough to avoid the jump-to-contact instability. When such a surface is brought into contact with a clean tip, strong bonds are formed that cause rupture of the contact in the bulk part of the tip and/or substrate upon separation. Sliding is strongly impeded in this case and always leads to severe cantilever deformations and distorted force-displacement curves.

With passivated tips, low adhesion energy contacts (~1 J/m2) are formed. The friction properties of such contacts depend on whether additional adsorbate layers are also present on the Pt surface. Passivated areas of the surface give rise to low friction and sigmoid-type I-V characteristics, typical of poorly conductive or semiconducting materials. Clean Pt areas produce Ohmic contact characteristics.

Clean Pt can be imaged in contact mode with passivated tips and gives rise to atomic lattice stick-slip friction with the Pt(111) lattice periodicity. This is the first time that a chemically active metal surface has been imaged in UHV in AFM contact-mode, revealing stick-slip with atomic lattice periodicity, and indicates that the passivating layer on the WC tip is bound strongly enough to the tip that material is not transferred to the active Pt even in conditions where substantial energy dissipation takes place during friction.

The results indicate that even in ultrahigh vacuum conditions, transfer of low-conductivity, passivating material can easily occur in nano-scale contacts. This demonstrates that detailed studies of third-body processes at the nanoscale are accessible with this AFM-STM multifunctional approach. The presence of these species substantially effects friction and adhesion. These results are relevant to the understanding of transfer film formation and its influence on the structural evolution and tribology of interfaces, whose inelastic properties are only beginning to be probed and understood at the nanometer scale.

We presented the first results of the combination of PCM and AFM techniques, in which current images, obtained on contacts many nanometers in diameter produced by very high loads (up to 5 GPa), reveal the atomic scale periodicity of the substrate. This surprising observation indicates that, even after averaging over many contact points of atomic dimension, the lattice periodicity does not average out.

We also showed that PCM is capable of measuring variations in local conductivity with a lateral resolution that is similar to the corresponding AFM resolution. Moreover, the technique is capable of separating mechanical and electrical contributions to the measured current. We were able to determine that local conductivity variations arise from different sources, namely, moiré superstructure and the conductivity to the underlying substrate.

We favor point-contact current imaging of lattice resolution as an explanation for many of the STM images on graphite presented in the past, especially in the first decade of STM experiments. In these experiments, it is likely that the tip was in contact with the surface, as in PCM, which explains the weak dependence of "tunneling" current as a function of tip distance.

Point contact current imaging, in conjunction with simultaneous friction and topographic imaging, should be an important tool in current efforts to understand the atomic origin of friction. We are currently applying these techniques to study the tribological behavior of surfaces.

Nanoscale Effects of Friction, Adhesion and Electrical Conduction in AFM Experiments 141

Binnig, G., Rohrer, H., Gerber H., & Weibel E. (1983). 7x7 reconstruction on Si(111) resolved

Binnig, G., Quate, C.F. & Gerber, Ch. (1986). Atomic Force Microscope. *Phys. Rev. Lett.* Vol.

Binnig, G., Fuchs, H., Gerber, C., Rohrer, H., Stoll, E., & Tosatti, E. (1986). Energy-dependent

Bennewitz, R., Gyalog, T., Guggisberg, M., Bammerlin, M., Meyer, E., & Güntherodt, H.J.,

Bennewitz, R., Gnecco, E., Gyalog, T., & Meyer, E., (2001). Atomic friction studies on well-

Dai, Q., Vollmer, R., Carpick, R.W., Ogletree, D.F., & Salmeron, M., (1995). Variable-

Daly, C., & Krim, J. (1996). Friction and damping of Xe/Ag(111). *Surf. Sci.*, Vol. 368, pp. (49-

Derjaguin , B. V., Muller, V. M., Toporov, Y. P., (1975). Effect of contact deformations on the

Enachescu, M., van den Oetelaar R.J.A., Carpick R.W., Ogletree D.F., Flipse C.F.J., &

*Enachescu, M., Schleef, D., Ogletree, D.F. & Salmeron, M.(1999). Integration of Point-Contact* 

Enachescu, M., Carpick, R.W., Ogletree, D.F., & Salmeron, M.(1999). Making, breaking and

Enachescu, M., Carpick, R.W., Ogletree, D.F., & Salmeron, M.(2004). The role of

Carpick, R.W., Agraït, N., Ogletree, D.F., & Salmeron, M. (1996). Measurement of interfacial

Carpick, R. W., Agraït, N., Ogletree, D. F., & Salmeron, M., (1996). Variation of the interfacial

Carpick, R.W. & Salmeron, M., (1997). Scratching the surface: Fundamental investigations of tribology with atomic force microscopy . *Chem. Rev.* Vol. 97, pp. (1163-1194) Carpick R.W., Enachescu M., Ogletree D.F., & Salmeron M., (1998). Making, Breaking and

adhesion of particles. *J. Colloid Interface Sci.*, Vol. 53, pp. (314-326)

area at the nanometer scale. *Trib. Lett.,* Vol. 7, pp. (73-78)

*Graphite/Pt(111). Phys. Rev. B, Vol. 60, pp. (16913-16919)* 

*J. App. Phys.,* Vol. 95, No. 12, pp. (7694-7700)

*Technol.* B, Vol.14, pp. (1289-1295)

(3334-3340)

October 1998

state-density corrugation of a graphite surface as seen by scanning tunneling

(1999). Atomic-scale stick-slip processes on Cu(111). *Phys. Rev. B,* Vol. 60, pp.

temperature ultrahigh-vacuum atomic-force microscope . *Rev. Sci. Instrum.,* Vol. 66,

Salmeron M., (1998). Atomic force microscopy study of an ideally hard contact: The diamond(111)/tungsten carbide interface. *Phys. Rev. Lett.,* Vol. 81, pp. (1877-1880) Enachescu, M., van den Oetelaar, R.J.A., Carpick, R.W., Ogletree, D.F., Flipse C.F.J., &

Salmeron, M., (1999). Observation of proportionality between friction and contact

*Microscopy and Atomic-Force Microscopy: Application to Characterization of* 

sliding of nanometer-scale contacts. *Mat. Res. Soc. Symp. Proc.,* Vol. 539, pp. (93-103)

contaminants in the variation of adhesion, friction, and electrical conduction properties of carbide-coated scanning probe tips and Pt(111) in ultrahigh vacuum.

shear (friction) with an ultrahigh vacuum atomic force microscope, *J. Vac. Sci.* 

shear strength and adhesion of a nanometer-sized contact. *Langmuir,* Vol. 12, pp.

Sliding of Nanometer-Scale Contacts, *Proceedings of the MRS Fall Meeting,* Boston,

in real space. *Phys. Rev. Lett.*, Vol. 50, pp. (120-123)

microscopy. *Europhys. Lett.,* Vol. 1, pp. (31-36)

defined surfaces. *Trib. Lett.*, Vol. 10, pp.( 51-56)

Dowson, D. (1998). *History of Tribology*, Longman, London

56, pp. (930- 933)

(R11301-R11304)

pp. (5266-5271)

54)

Finally, we have shown the existence of localized dipole fields in the vicinity of steps through direct measurements of the forces experienced by a biased STM tip. Together with measurements of the tip radius (from force-distance curves) and tip-sample distance (from current-distance approach curves) in the course of the same experiment, the method provides a direct way to map out and to measure local dipole moments on surfaces that should be of significance in studies of chemical and electronic properties of surfaces.

## **7. Acknowledgments**

The work presented here was done at Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720

Many thoughts, analyses, suggestions, and conclusions in this work were generated from research by and discussions with the following people: Dr. R.W. Carpick, Dr. D. F. Ogletree, Dr. J. Y. Park, Dr. R.J.A. van den Oetelaar, Dr. X. Lei, Mr. D. Schleef and Dr. M. Salmeron.

This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences Division, of the U.S. Department of Energy under Contracts No. DE-AC03-76SF00098, DE-FG02-02ER46016, and DE-AC02-05CH11231. Also, this work was supported by the Ministry of Education, Research, Youth and Sport, Romania, and by the European Union through the European Regional Development Fund, and by Romanian National Authority for Scientific Research, under project POSCCE-O 2.1.2-2009- 2/12689/717.
