"Chemical functionalization of semiconductor surfaces for microelectronic, energy and sensing applications" - The ability to control the atomic structure and chemical composition of surfaces and interfaces remains the single most important challenge in the development of new devices, whether they involve charges, heat, electromagnetic radiation and even magnetic fields. Interfaces are becoming even more important as the size of devices is reduced. The demands for cheaper, sustainable and more efficient products has also motivated the development of new materials and compounds and introduced new types of interfaces. It is therefore critical to develop a fundamental understanding of surface processing and interface formation to cope with the wealth of systems currently being considered. In such efforts, fundamental progress is often based on the ability to prepare model surfaces with atomically well-defined surface structures, and to perform in-situ measurements that can be directly linked to theoretical modeling. In this talk we illustrate this concept focusing on atomically smooth, chemically nanopattern and oxide-free silicon model surfaces, the formation of which is shown here, for applications as diverse as microelectronics (ultra-shallow doping), energy (Si-based photovoltaic cells), and sensing. These examples underscore that, while hydrogen-terminated silicon surfaces are the gateway to the best and most reliable functionalization chemistries because of their simple synthesis, superior chemical stability and excellent electrical properties, their transformative use critically depends on the ability to chemically modify them without oxidizing the Si substrate. To that end, fundamental understanding and control of the chemistry involved in making and modifying H-terminated Si surfaces is being developed in our laboratory by combining extensive characterization techniques (spectroscopy, imaging, electrical measurements) with first principles calculations.