Atomic Force Microscopy Studies of Tribochemical Phenomena

Washington State University, Pullman WA

Investigators

Abstract

We request support for a three-year project to examine the consequences of simultaneous application of chemical agents and mechanical stress to a solid surface. We explore the resulting nanometer-scale changes in surface topography due to application of combined highly localized mechanical stress (due to contact with the tip of an atomic force microscope) and exposure to appropriate solutions and solvents. These basic studies impact several technologies in that they provide underlying understanding for areas such as chemical mechanical planarization (CMP), tribochemical wear, tribo-induced redeposition, tribology in biological systems (where biological fluids serve as the active media), and nanotechnology. Any relative motion of surfaces, no matter how small, will involve dissipative forces that need to be understood and controlled in nanometer-scale devices. As device sizes shrink, the need for clean, atomically flat surfaces will become more and more pressing. Our overall goal is to develop descriptive and predictive models for asperity and substrate modification due to combined chemical, thermal, and mechanical processes. Intellectual Merit - Tribochemical phenomena are complex and poorly understood. Our approach is to simplify the system to a single asperity (the AFM tip), which we can characterize topographically and in some cases chemically, and use simple chemical systems (e.g., water). Our role in the science of stress enhanced processes is to identify the underlying mechanisms. Although we work on a nanometer scale, we are not doing nanotechnology - we support others who are doing nanotechnology. Through careful, quantitative measurements, we clarify the details of both asperity and substrate wear, and the connection between asperity wear and changes in friction with a given substrate. More subtle is a new area we have discovered, namely tip-controlled deposition of materials from saturated solution. Scanning at low contact forces in supersaturated solutions can be exploited to locally deposit crystalline material. For relatively flat surfaces with occasional pits, filling in the pits can be a much more efficient method of producing atomically flat surfaces than removing whole surface layers. We have observed fluctuations in the lateral force during scanning of relatively insoluble, ionic materials that correlate strongly with the degree of supersaturation. We propose to explore this tool as a possible probe of transient surface deposits that are expected in supersaturated solutions and to further our understanding of the fundamental mechanisms for the tip induced deposition and structuring that occurs in supersaturated solutions. An innovative, untested idea, we propose is to combine stress+chemical stimulation with exposure to radiation. The proposed radiation sources are electron beams and UV and femtosecond laser beams. Laser irradiation can be applied in situ. Following radiation exposure we seek synergisms with tribochemical stimulation. We anticipate positive results and will explore the underlying mechanisms in light of what we know about bond breaking, defect formation, and other chemical modifications generated by radiation exposure. We hope to find a new way to promote surface modification. Broader impacts. In addition to a graduate student, this work will involve 4-6 undergraduates a year, and at least one high school student. We expect most of our students, as in the past, to publish their results in peer-reviewed journals. These efforts also fulfill undergraduate department and honors college theses requirements. As undergraduate advisor for physics and honors physics instructor, the PI often influences women and minorities to become science/technology majors, double majors, and/or become involved early in our research projects. The research itself, through fundamental understanding, provides benefits to society in need of reduced wear, longer life, and environmentally friendly processes and systems. Efforts in nanotechnology involving moving interfaces and/or needs for atomically flat, clean surfaces or tip/solution generated structures also benefit from our studies.