T. Gyalog

Friction experiments on the nanometre scale

In this review, we present various results obtained by friction force microscopy in the last decade. Starting with material-specific contrast, commonly observed in friction force maps, we discuss how the load dependence of friction and the area of contact have been estimated and compared to elasticity theories. The features observed in a sliding process on the atomic scale can be interpreted within the Tomlinson model. An extension of the model, including thermal effects, predicts a smooth velocity dependence of friction, which recent experiments have confirmed. Other subjects like anisotropy of friction, role of environment, topographical effects, electronic friction and tip modifications are also discussed. The growing importance of molecular dynamics simulations in the study of tribological processes on the atomic scale is outlined.

Friction on the atomic scale: An ultrahigh vacuum atomic force microscopy study on ionic crystals

We performed atomic force microscopy in ultrahigh vacuum on the ionic crystal of KBr(001). The morphology and the tribological properties of this cleavage face are characterized and discussed. The local friction coefficient was extracted by means of the two‐dimensional histogram technique. For loads below 3 nN a linear behavior was found between normal and lateral forces yielding a friction coefficient of less than 0.04. In this load regime, wearless friction is observed. For higher loads, the friction coefficient increases to a value of about 0.7–1.2. The corresponding topography images reveal the typical onset of wear. On the atomic scale a periodicity of 4.7 A was found which corresponds to the distance of equally charged ions on the KBr(001) surface. On this scale, the lateral force map exhibits the typical stick‐slip phenomenon which is discussed in terms of a novel theoretical approach.

An Interdisciplinary Virtual Laboratory on Nanoscience

The Swiss Virtual Campus project ”Virtual Nanoscience Laboratory” realises a virtual laboratory for the booming field of Nanoscience. Nanoscience laboratories are expensive and only major companies and organizations sponsored by research programmes can aord their usage. With a concept of distance education, complex and sensible experimental equipment can be shared through the Internet. Three main topics are realized in the framework of a virtual laboratory: user management, communication and co-operation, and the control of virtual experiments. The basic architecture is based on a multitiered client-server model. Each nanoscience experiment is implemented as a stand-alone web-service.

Atomic friction studies on well-defined surfaces

Atomic friction studies have been performed by means of a friction force microscope (FFM) in ultrahigh vacuum, where well-defined surfaces can be prepared. A home-built FFM allows us to study lateral forces as low as 0.05 nN using rectangular silicon cantilevers. Furthermore, comparison with dissipation measurements performed in non-contact mode are possible. Recent experimental results are presented and discussed in the framework of a one-dimensional Tomlinson model which includes thermal activation. Atomic-scale stick–slip processes on a metallic surface could be repeatedly measured on Cu(111), while the Cu(100) surface was distorted by the tip during the scanning process. A logarithmic velocity dependence of atomic friction has been measured on Cu(111) and NaCl(100) for low scanning velocities. The dissipation found in stick–slip measurements is compared to the power loss detected in dynamic non-contact measurement.

Problem-Based Learning Using Mobile Devices

Small handheld devices such as PDAs and smart phones become more and more popular. Thus, we are seeing a growing number of projects using mobile devices for educational purposes. Due to the limitations of screen size, CPU performance, and memory size, software development for mobile devices is challenging. More critical argument is about the lack of pedagogic and didactic concepts on the usage of mobile devices in education. In this paper, we present a system using mobile devices which supports research experiences for students in a laboratory context. The system allows students to efficiently monitor and control their scientific experiments at anytime, from anywhere. We describe several usage scenarios in the area of nanoscience studies, where the remote control of microscopes is mandatory. We present our system architecture and describe the implementation based on open-source software.

Friction force microscopy in ultrahigh vacuum: an atomic-scale study on KBr(001)

We present an atomic-scale study on friction performed by a bidirectional atomic force microscope operated in ultrahigh vacuum. Experiments on surfaces of in situ cleaved KBr crystals are presented. On a μm scale the cleavage structure with monoatomic steps of 3.5 ± 0.3 Å is revealed. On the atomically flat terraces, atomic-scale resolution is achieved. The resolved square lattice shows a periodicity of 4.7 Å and corrugations of 0.3–0.7 Å and exhibits the cubic symmetry of KBr(001). The lateral (frictional) force map shows all characteristics of the stick-slip movement of the probing tip. From analysis of the friction loops, the kinetic friction force was determined as a function of load. For a load regime of -4 to 10 nN, lateral force corrugations ranging from 1 to 5 nN were found. A comparison with a novel theoretical model is discussed qualitatively.

Atomic-Scale Stick Slip

Atomic-scale stick-slip is one of the fundamental friction processes. It has been observed on layered materials, such as graphite, or ionic crystals, such as NaCl(001). Recently, wearless friction was also observed on clean metallic surfaces, such as Cu(111).

Collaborative Nanoscience Laboratory with Integrated Learning Modules

Creation and maintenance of virtual laboratories needs an interdisciplinary project team with didactic and technical staff and also experts of the field. NANO-WORLD, a collaborative nanoscience laboratory, developed as part of the Swiss Virtual Campus, is built on top of an e-learning portal. Interactive real-time simulations of nanoscience experiments inspire students to access e-learning modules. Remote experiments with mobile notification services increase the learning motivation of students. They can directly explore phenomena in nanoscience and take over the role of scientists.

Nano-World: A showcase suite for technology-enhanced learning

Over the past couple of years we have defined and implemented a variety of tools and instruments for supporting technology-enhanced teaching within the field of Nanoscience. Among others, the Nano-World showcase suite developed includes the following: a collaborative simulator for learning the basics of atomic force field microscopes; a remote laboratory which offers real-world access to experiments at the nanoscale level; software infrastructure for remote control and steering of ongoing experiments using mobile devices; interactive courseware that teaches the basic laws of physics such as force fields; a web-based platform for 3D visualizations of data collected via nano microscopes; and an interactive game for getting first impressions of atomic manipulations. In the paper we describe the different components and report on lessons learned from using the showcase within the university curriculum as well as an information medium for schools and public audiences. We also report on plans and first steps to interface the showcase suite with LiLa - the forthcoming library of labs.

New Developments in Scanning Probe Microscopy

Four topics will be treated in this article: 1) One of the central questions of contact force microscopy is the determination of contact area. Imaging of well-defined structures is one way to estimate the size of the contact. Recently, continuum elasticity models were used to describe the nanometer-sized contact. The lateral contact stiffness method was found to be particularly interesting, because it is rather independent of the selected formalism. 2) Progress has been made with non-contact force microscopy, where true atomic resolution has been achieved. It is found that the instrument has to be operated at similar tip-sample distances as in STM. On Si(111)7×7, the strongest attraction is found for the adatoms, which is in agreement with theoretical models. The contrast at step sites is found to be influenced by short-range chemical forces and long-range electrostatic or van der Waals forces. Spectroscopic methods were used to investigate the frequency shifts between the upper and lower terrace. Local variations of the contact potential are found to be below the detection limit, whereas variations of electrostatic forces due to changes in the interaction volume are found to be predominant. 3) Some artifacts of scanning probe microscopy are briefly discussed. The tip artifact, where the sample topography is convoluted with the tip geometry is the most common artifact. The second artifact is related to laser beam interference between the sample surface and the rear side of the cantilever, which can cause interference patterns in laser beam deflection force microscopy images. 4) The application of AFM-based technology to the construction of chemical and physical sensors is found to be extremely successful. Micro-machined cantilevers are the central part of these sensors. After the chemieal treatment of the cantilevers (active or functional probes), the conditions of the surface film are monitored with a sensitive deflection sensor. The reaction with environmental gases leads to changes of surface stress (stress mode) or mass changes, which are detected as a resonanee frequency shift (frequency mode). Using the bimetallic effect, small heat changes can be translated into cantilever deflections (calorimeter mode).