Characterization of Two Dimensional Materials with High Resolution Instrumentation

High resolution characterization allows researchers to study important problems at the atomic scale in a variety of fields such as condensed matter physics, synthetic chemistry, and materials science and engineering. This talk will explore the development of scanning probe and electron microscopy instrumentation and technique development for high resolution characterization of two dimensional materials. For example, we have developed a low noise all-fiber interferometer for use as the deflection sensor in liquid environment atomic force microscopy (AFM) to allow for the imaging of muscovite mica in different chemical environments with true atomic resolution. The optimization of the sensor toward achieving the highest possible sensitivity and lowest noise density is accomplished by considering the behavior of an ideal interference cavity. For conductive materials, such as graphene, the atomic structure of synthesized material can be imaged by scanning tunneling microscopy (STM). Our work used STM to discover how graphene grown by chemical vapor deposition (CVD) on copper substrates maintains a continuous pristine atomic structure over atomically flat facets, monatomic steps, edges, and vertices of the underlying catalytic surface. On single crystal substrates, growth proceeds without evidence of a strict epitaxial growth, which produces a polycrystalline overlayer with well stitched grain boundaries (GBs). The strength of these boundaries is further studied through correlative microscopy techniques where membranes of interest are first characterized by low voltage transmission electron microscopy (TEM) and subsequently fracture tested using AFM with a custom large radius diamond indenter. The GBs are found to maintain strengths comparable to that of single crystal graphene. This enhanced strength can be understood by investigating the atomic scale strain fields in the material by aberration corrected HRTEM and performing molecular dynamic simulations of the fracture behavior of experimentally observed boundary regions. Finally, I will discuss a current instrumentation development project to push a different resolution boundary- namely time resolution. Using a novel electron source at LBNL, we have recently begun an effort to understand the atomic scale dynamics of two dimensional materials with ultrafast electron diffraction (UED) pump-probe experimentation and simulation.

For further information please contact james.liddle [at] nist.gov (J. Alex Liddle), 301-975-6050.