Scanning Probe Microscopy of Two Dimensional Covalent Organic Frameworks*

Covalent organic frameworks (COFs) are a new class of materials that can be designed to exhibit interesting physical, chemical and electronic properties. In this work, we study the growth and properties of single layer covalent organic frameworks that we synthesize on noble metal surfaces, typically via an Ullmann-like process accomplished through thermal deposition and annealing. An outstanding challenge in 2D-COF on-surface synthesis is to achieve long range, defect free crystalline order over hundreds of nanometers. Using scanning tunneling microscopy (STM), we probe COF growth and island formation under different preparation conditions, from monomer self-assembly to covalent bond formation. We also study the electronic properties of COF islands obtained in this process, including effects of finite size and substrate interaction, using scanning tunneling spectroscopy and comparisons to theoretical first-principles calculations.