Applications of Transmission Electron Microscopy on Free-standing and Embedded Two-dimensional Materials

Abstract

In the last decade, 2D nanosheets, more commonly referred to as 2D, layered, or van der Waals materials, have garnered significant scientific interest because of their novel material properties at the nanoscale regime compared to their bulk. Their rise in popularity is commonly attributed to the isolation and study of graphene by Geim and Novoselov in 2004 for which they were awarded the Nobel prize in physics in 2010. Since then, more than 1000 unique 2D chemical compounds have been at least theorized if not experimentally isolated. Many of these materials exhibit favorable mechanical, optical, or electronic properties that may also be tunable by controlling their number of layers. With novel materials being continuously synthesized and applied at such a feverish pace, there exists a critical need to characterize and understand the structures and properties of these novel materials that may have been nothing but theoretical predictions a mere decade ago. Herein, analytical scanning transmission electron microscopy (STEM) supported by computational methods is used to study the atomic and electronic structure of numerous free standing 2D materials as well as 2D materials embedded in devices with a spatial resolution of < 1 Å and an energy resolution of < 0.5 eV. Two computational applications are first presented to introduce and highlight the complexities of electron-sample interactions which can be used to extract additional information from experimental results. The first uses experimentally observed Moire patterns to correlate and understanding rotational misalignments of Bi2Se3; the second exploits the channeling of the electron beam in addition to sample tilt to determine the thickness of atomically thin MoS2. The thickness determination method is then experimentally proven using annular dark field-STEM imaging (ADF-STEM) and applied to MoS2 layers of various thicknesses to test the limits of measuring layer-dependent properties in the TEM using electron energy loss spectroscopy (EELS). Subsequently, the atomic and electronic structure of black phosphorus is thoroughly examined using STEM. Its crystal structure including its lattice parameters and stacking order is unambiguously determined by ADF-STEM. Its electronic structure including its conduction band density of states and plasmon excitations are measured using EELS and compared to density functional theory (DFT) calculations. Additionally, the effect of oxidation, a well-known phenomenon when using black phosphorus, on its properties is measured using a similar approach. The results, as measured using the aforementioned techniques in addition to energy dispersive x-ray spectroscopy (EDX), show that oxidation amorphizes black phosphorus transforming the semiconductor into an insulating oxide. Finally, STEM-EELS is applied to study 2D material embedded field effect transistors (FET) in cross-section. Using a layer-by-layer approach, the interactions between MoS2 and metal device contacts are measured to show the non-idealities of the contact/channel interface. These results, supplemented by DFT calculations, are used to understand the phenomenon of Fermi level pinning and the interaction of the metal contact with the MoS2 layers when deposited onto its surface. The results suggest that the chemistry of the metal-MoS2 bond is important in determining the efficacy of the FET and point toward the ultimate limits of which metals and alloys can and cannot be used when ultra-thin mono- and bi- layer MoS2 channels are desired.