A few years after the invention of the scanning tunneling microscope (STM), the atomic force microscope (AFM) was developed Instead of measuring tunneling current, a new physical quantity could be investigated with atomic-scale resolution: the force between a small tip and a chosen sample surface This paper reviews progress and recent results obtained with AFM and other closely related techniques in the field of nanotribology, and attempts to point out many of the unresolved questions that remain The goal of this paper is to demonstrate that AFM is capable of producing atomic-scale knowledge As such, the authors will focus upon some of the contributions of the AFM to nanotribology They will almost exclusively discuss results that shed light on the actual atomic and molecular processes taking place, as opposed to the more applied investigations of microscale properties which are also carried out with AFM They will accompany this discussion by mentioning related theoretical efforts and simulations, although their main emphasis will be upon experimental results and the techniques used to obtain them, as well as suggested future directions In many ways, AFM techniques for quantitative, fundamental nanotribology are only in a nascent stage; certain key issues such as force calibration,more » tip characterization, and the effects of the experimental environment, are not fully resolved or standardized The authors thus begin with a critical discussion of the relevant technical aspects with using AFM for nanotribology 289 refs« less

/pdf/scratching-the-surface-fundamental-investigations-of-4kjq5s243s.pdf

Scratching the Surface: Fundamental Investigations of Tribology with Atomic Force Microscopy.

Macroscopic laws of friction do not generally apply to nanoscale contacts. Although continuum mechanics models have been predicted to break down at the nanoscale, they continue to be applied for lack of a better theory. An understanding of how friction force depends on applied load and contact area at these scales is essential for the design of miniaturized devices with optimal mechanical performance. Here we use large-scale molecular dynamics simulations with realistic force fields to establish friction laws in dry nanoscale contacts. We show that friction force depends linearly on the number of atoms that chemically interact across the contact. By defining the contact area as being proportional to this number of interacting atoms, we show that the macroscopically observed linear relationship between friction force and contact area can be extended to the nanoscale. Our model predicts that as the adhesion between the contacting surfaces is reduced, a transition takes place from nonlinear to linear dependence of friction force on load. This transition is consistent with the results of several nanoscale friction experiments. We demonstrate that the breakdown of continuum mechanics can be understood as a result of the rough (multi-asperity) nature of the contact, and show that roughness theories of friction can be applied at the nanoscale.

Friction laws at the nanoscale

Magnetron sputtering of transparent conductive oxides (zinc oxide, indium tin oxide, tin oxide) is a promising technique which allows the deposition of films at low temperatures with good optical and electronic properties. A special advantage is the scalability to large areas. The principles underlying magnetron sputtering are reviewed in this paper. The growth process during magnetron sputtering is characterized by the bombardment of the growing film with species from the sputtering target and from the plasma. In addition to sputtered atoms with energies in the eV range, ions from the plasma (mostly argon) and neutral atoms (also argon) reflected at the target hit the growing film. Depending on the energy of these species and on the ion-to-neutral ratio the properties of the films vary. High energies ( 100 eV), which occur mainly at low sputtering pressures lead to damage of the growing film, connected with mechanical stress, small crystallites and bad electrical parameters. Ion assisted growth with low ion energies (below about 50 eV) is advantageous as is a high ion-to-neutral ratio. A compilation of resistivities of magnetron sputtered zinc oxide films yields a limiting resistivity of 2 × 10-4 cm for polycrystalline films. Based on the correlation between plasma parameters and film properties new research fields are anticipated.

https://iopscience.iop.org/article/10.1088/0022-3727/33/4/201/pdf

Magnetron sputtering of transparent conductive zinc oxide: relation between the sputtering parameters and the electronic properties

Chemical force microscopy

Atomic force microscopy is an imaging tool used widely in fundamental research, although it has, like other scanned probe microscopies, provided only limited information about the chemical nature of systems studied. Modification of force microscope probe tips by covalent linking of organic monolayers that terminate in well-defined functional groups enables direct probing of molecular interactions and imaging with chemical sensitivity. This new chemical force microscopy technique has been used to probe adhesion and frictional forces between distinct chemical groups in organic and aqueous solvents. Contact mechanics provide a framework to model the adhesive forces and to estimate the number of interacting molecular groups. In general, measured adhesive and frictional forces follow trends expected from the strengths of the molecular interactions, although solvation also plays an important role. Knowledge of these forces provides a basis for rationally interpretable mapping of a variety of chemical functionalities and processes such as protonation and ionization.

https://cml.harvard.edu/assets/annuRevMaterSci27_381.pdf

Friction measurements on muscovite mica and glass have been performed with a friction force microscope using Si3N4 tips. The environment was changed from ambient conditions to N2‐ or Ar‐gas conditions. In ambient conditions, the friction‐versus‐load curves showed a nonlinear behavior closely following the Hertzian contact mechanics of a single‐asperity contact: friction force ∼load2/3. For the same tip under gaseous conditions the friction force increased linearly with the load, indicating multi‐asperity contact. A new model is proposed to explain this change in the nature of the friction behavior. In this composite‐tip model, the tip is formed by the actual Si3N4 tip and ‘‘solidlike’’ contaminants present in enclosed cavities between tip and sample surface.

Single‐asperity friction in friction force microscopy: The composite‐tip model

Penetration depth λ behavior in reactively sputtered NbN films has been examined. For NbN films deposited at 300 °C or higher by means of dc magnetron technique, λ values around 300 nm were obtained, very short compared with the previously reported values (500–700 nm). The measured λ dependences on deposition parameters are discussed in connection with film structure on the basis of the Ginzburg–Landau–Abrikosov–Gor’kov theory of type II superconductivity.

Magnetic penetration depths in superconducting NbN films prepared by reactive dc magnetron sputtering

Tin-doped indium oxide (ITO) films were deposited by sputtering of sintered oxide targets, employing an electron cyclotron resonance (ECR) microwave plasma system. The effects of Ar ion bombardment on crystallographic and electrical properties of the films were investigated with control of the ion energy by submagnetic field application. It was found that ion bombardment with energies lower than ~ 40 eV enhances crystallization whereas bombardment with higher energies suppresses crystallization. The surface roughness of the film was below 50 A. The smooth surface formation could be attributed to the microstructural modification arising from ion bombardment.

Effects of Magnetic Field Gradient on Crystallographic Properties in Tin-Doped Indium Oxide Films Deposited by Electron Cyclotron Resonance Plasma Sputtering

Details of the relation between Ar ion bombardment effects on NbN surface and I‐V characteristics of NbN/Pb junctions with plasma oxidized barriers have been studied. NbN base electrodes, which behave as a strong coupling superconductor (Tc>15 K, 2Δ/kTc=4.2–4.3), were prepared onto substrates at 300 °C by reactive dc magnetron sputtering. In the NbN/Pb junction fabrication, cathode self‐bias voltage VCSB during Ar sputter cleaning strongly influences the subgap leakage current value, which increases with VCSB . The VCSB also influences normalized normal tunnel resistance ARNN. It is found, in reflection electron diffraction and x‐ray photoelectron spectroscopy studies, that the Ar ion bombardment at VCSB> 200 V on a NbN surface produces an amorphous structure in the surface layer. It is also found that the thickness of amorphous layer increases with Ar ion energy. Based on the experimental results, low subgap leakage current junctions, where Vm exceeds 100 mV at 4.2 K, were fabricated under low energy pla...

Ar ion bombardment effects of NbN/Pb Josephson junctions with plasma oxidized barriers

A new way of obtaining absolute friction coefficients using the friction force microscope is presented. For microfabricated cantilevers it is very difficult to calculate reliable torsion and normal spring constants; both the shear and the Young's modulus are not well known. Their ratio, however, is constant and close to the bulk value. This fact is used to obtain quantitatively reliable friction coefficients. Furthermore, a new calibration method for the friction-signal detector is introduced. The measured friction coefficients for a silicon-nitride tip on mica, silicon and glass are: 0.071, 0.20 and 0.23. Occasionally non-linear effects, indicating possible single-asperity friction, were observed.

Masaru Igarashi

Papers

Single‐asperity friction in friction force microscopy: The composite‐tip model

Magnetic penetration depths in superconducting NbN films prepared by reactive dc magnetron sputtering

Effects of Magnetic Field Gradient on Crystallographic Properties in Tin-Doped Indium Oxide Films Deposited by Electron Cyclotron Resonance Plasma Sputtering

Ar ion bombardment effects of NbN/Pb Josephson junctions with plasma oxidized barriers

Quantitative Determination of Friction Coefficients by Friction Force Microscopy.