(5ad) Atom-by-Atom Metrology of Materials

Metallic and semiconductor nanostructures possess an array of unique magnetic, electronic and photonic behavior that often differs from their bulk counterparts. The overarching goal of the research proposed herein is elucidating the synthesis-structure-performance relations of atomic-scaled materials by advanced transmission electron microscopy (TEM) techniques. The availability of various imaging, diffraction and spectroscopic modes within a single instrument makes of TEM a powerful tool for characterizing functional nanostructures. These efforts enable us to understand the surface chemistry and electronic structure of diverse atomic-scale systems such as nanocatalysts, transistors or functional nanostructures as therapeutic devices. TEM metrology analysis is used to determine feature size/functions and physical interface states of atomically-designed materials.

One of the core-strengths of our research program is the production of wide variety of semiconductor and metal nanocrystals (and nanowires) by liquid-phase colloidal synthesis in aqueous or non-hydrolytic medium. The colloidal synthetic approach is an especially powerful tool for convenient and reproducible synthesis of nanostructures. An example of our current work on the synthesis of shape-controlled nanocrystals corresponds to the fabrication of sphere- and cubic-shaped PbTe and PbSe NCs. These NCs were synthesized using a one-pot approach and the noncoordinating solvent, 1-octadecene (ODE) as capping agent. This one-pot technique does not need cooling of the precursors in a glovebox and is advantageously reproducible. Materials displaying rock salt structures such as PbTe and PbSe show a size-dependent sphere to cube transition, which occurs at high-temperature regimes. The growth of {100} facets leads to a cubic morphology and lowers total surface energy. Therefore, the sphere to cube transition tends to occur at smaller sizes for PbTe NCs compared to those of PbSe NCs. The different surface energies that exists among {111} and {100} faces is greater for PbTe than for PbSe, favoring a faster growth in the <111> direction for PbTe and a predominant {100} face for nanocrystals with reduced sizes. The ultimate goal of this research involves engineering the work function of the floating gate of non-volatile memory devices with semiconductor nanocrystals of different shapes and sizes.

Axial heterostructures, namely core-shell semiconductor nanowires offer unique features for diverse applications including field-effect transistors, sensors, detectors, light-emitting diodes, and solar cells. Our research efforts placed on the synthesis of core-shell nanowires involved using a combination of silane (SiH4) and germane (GeH4) gases by a ultrahigh vacuum chemical vapor deposition (UHV-CVD) method for successful fabrication of a Si1-xGex shell around the Ge nanowires at wafer temperatures of approximately 400 °C. Another example of recent work on metrology of nanostructures-based electronic devices is the cross sectional view of dual-gated graphene field effect transistor (FET). Graphene is a single layer of carbon atoms, densely packed in a honeycomb two-dimensional (2D) lattice. The waviness nature of the graphene acting as channel in this device indicates the presence of mechanical deformations, which could have also been introduced during the fabrication process of the device. The corrugation of graphene in FET devices could dramatically affect its transport properties.

The efforts of this research program demonstrate how electron microscopy can be used to answer several critical issues in chemistry, biology and materials science aimed at revolutionizing the current understanding of nanostructures applications.