This is a collaborative research project between Yale University and the Universidad Autonoma de Madrid in Spain. The goal of the project is to characterize the chemical reactivity of individual surface atoms on transition metal oxide surfaces using non-contact atomic force microscopy with functionalized tips. For this purpose, a new technique capable of mapping the strengths of chemical interactions on the atomic scale will be developed through experimentation (carried out in Udo Schwarz' and Eric Altman's groups at Yale University) guided by theory (performed by Ruben Perez' group at the Universidad Autonoma de Madrid). The work builds upon recent demonstration of the ability to experimentally obtain the entire three-dimensional force field acting between a probe tip and surface in the short-range chemical forces regime with atomic resolution. Using this method with tips functionalized to expose electrophilic and nucleophilic head groups allows for the characterization and quantification of the key properties that govern oxide catalysis for individual cations and anions on the surface, in specific site geometries. The new method is initially applied to a comparison of the local chemical properties of Ti cations and O anions on the rutile (110) and anatase (001) surfaces, which, despite chemical and structural similarities, behave markedly differently chemically. For a fundamental understanding of the measured tip-sample interaction, the experimental program couples with a synergistic multi-scale theoretical effort that includes a quantum mechanical description of the short-range chemical forces, a classical description of the longer range dispersion forces, and molecular dynamics simulations of the influence of the forces on the probing geometry.
The international collaboration of Yale University and the Universidad Autonoma de Madrid that is at the heart of this project provides a unique educational experience for the graduate and undergraduate students involved in the project. Through extended student exchanges during each year of the project, students are immersed in state-of-the-art experimental and theoretical efforts while being exposed to a truly international research environment. Also, further development of the method as a robust technique promises to impact other fields that are governed by local surface interactions including tribology, thin film growth, electronic device design, and the specific chemical interactions at the center of much of biology.
Many technologically important phenomena such as corrosion, adhesion, friction, and heterogeneous catalysis are governed by surface energetics. To obtain a complete picture of the underlying fundamental mechanisms involved, the ability to measure, identify, and quantify chemical interactions with atomic precision is required. The scientific objective of this project was to demonstrate through collaborative experiments at Yale University and theory at the Universidad Autonoma de Madrid that a species-selective mapping of the chemical reactivity of individual surface atoms on transition metal oxide surfaces could be achieved by combining three-dimensional atomic force microscopy, which has previously been developed in our laboratory at Yale, with simultaneously recorded scanning tunneling microscopy data and ab initio density functional theory calculations. To demonstrate this potential, we chose two metal oxide surfaces, TiO2(110) and a copper oxide surface phase on Cu(100), as technologically relevant model materials, for which we (i) identified the chemical nature of surface sites and surface species, (ii) determined the atomic-scale characteristics of defects, and (iii) quantified defect-induced interaction variations, all of which could not have been achieved with conventional methods. Our approach provides a general scheme for identifying chemically active sites on surfaces with atomic precision and will broadly impact other fields that are governed by local surface interactions including tribology, thin film studies, electronic device design, nanomagnetism, and the specific chemical interactions at the center of much of biology.
