Professor George W. Flynn of Columbia University is supported by the Macromolecular, Supramolecular, Nanochemistry (MSN) Program in the Division of Chemistry to conduct a series of related studies on graphene to reveal the nanoscale structure and dynamic properties of molecules at interfaces. More specifically, the PI proposes to: (1) investigate the structural and electronic properties of graphene; (2) determine the charge doping characteristics of halogens and metal atoms adsorbed on graphene sheets; (3) study the potential effectiveness of graphene as a membrane filter with controlled porosity; and (4) probe the atomic level structure and properties of hexagonal boron nitride and its potential interactions with graphene and graphite. The studies are primarily conducted using Scanning Tunneling Microscopy (STM).
The overarching objective of these studies is the elucidation of the driving forces that control interfacial properties and self organization of molecules on graphene. The results will impact the next generation nanoscale optical and electronic devices. The project involves direct participation by undergraduate female students. The educational activities also include the development of education material on Nanoscience and Nanotechnology for Columbia University Summer and Saturday science enhancement experiences for high school students.
Intellectual Merit of the Research Activity A series of interconnected Scanning Tunneling Microscopy (STM) studies were carried out to investigate the structural and electronic properties of the novel two-dimensional material graphene (a single, one carbon atom thick sheet of graphite). These studies all take advantage of the remarkable, nanoscale spatial resolution afforded by the Scanning Tunneling Microscope and its ability to reveal the atomic structure and dynamic properties of molecules at interfaces. Temperature tunable STM and Scanning Tunneling Spectroscopy (STS) experiments on graphene sheets mounted on both insulating and conducting substrates were investigated to provide nanoscale characterization of individual atomic sites on this novel, 2-dimensional material. Scientific efforts were focused on determining the distribution of both negative and positive electrical charge on graphene sheets impregnated with a small number of either nitrogen atoms or boron atoms (referred to in the semi-conductor industry as the charge doping characteristics of the material). The impregnated graphene sheets were grown on single crystal copper samples, which provide an atomic scale template for two-dimensional growth of these carbon sheets. STM can resolve both the position and identity (i.e. carbon, nitrogen, or boron) of atoms in the graphene sample. STM can also sense changes in graphene morphology achieved by mounting these atomically thin sheets on different solid materials (e.g. silicon dioxide, mica, copper and cobalt). STS reveals the density and shape of the electron distribution above the graphene sample for electronic quantum states of these materials near the Fermi energy and can sense charge transfer (doping) to or from graphene and the impregnated atoms. Fundamental insights obtained from these studies provide improved understanding of the chemical, structural, and electronic properties of adsorbate decorated and impregnated samples of graphene. Significant Results Chemical doping is a powerful technique for tailoring the electronic properties of materials. In monolayer graphene, substitutional doping during growth can be used to create unique two-dimensional structures. We have obtained the first atomic-scale spectroscopic measurements by scanning tunneling microscopy (STM) of the electronic structure of individual nitrogen atom dopants in monolayer graphene. Individual nitrogen atoms incorporate as graphitic carbon, and a fraction of the extra electron on each nitrogen atom is delocalized into the graphene lattice. The electronic structure of nitrogen-doped graphene is modified strongly only within a few lattice spacings of the site of the nitrogen dopant. We have also shown that the surface of a dielectric supporting substrate (e.g. mica) has substantial effects on the electronic structure of graphene. Rather than random charge impurities or a flat electronic environment as found on other substrates, the relatively large surface charge on mica inherent in a cleaved crystal leads to both positive and negative charge transfer to the graphene (p- and n-type doping) and is the most important factor determining local doping levels in graphene. Interfacial additives offer direct control of the electronic structure by altering electronic charge both immediately above the interfacial compounds and in their vicinity, a result stemming from the mica surface dipole charge distributions interacting with the graphene. Furthermore, the same flat SiO network of mica can lead to graphene being strongly doped with either electrons or holes, depending on the local neighboring environment. The large dipoles and reactivity of the mica surface make it a difficult substrate with which to work due to the large amount of variability on the surface itself. However, a rich array of specific substrate architectures can be envisioned that could be used to tailor the graphene electronic properties to whatever applications are necessary for a given device. Broader Impacts Resulting From This Research Activity This research program has involved direct participation by undergraduate men and women, who are supervised by both graduate students and post-doctoral fellows. Post-doctorals collaborated with the principle investigator to develop education material focused on Nanoscience and Nanotechnology for Columbia University’s Summer and Saturday science enhancement experiences for high school students. 35% of the graduate Ph.D. recipients supported by this research program over the past 30 years have been women. Institutional breadth has been enhanced by collaborations with multiple academic departments (physics, chemistry, applied physics/materials science, and electrical engineering) through Columbia’s Nanocenter ("Center for Electron Transport in Molecular Nanostructures") and through the involvement of multiple institutions (University of Delaware, Tufts University, City University of NY). Benefits to society accrue from this program because of its heavy emphasis on nanoscale processes involving two-dimensional conductors that have multiple potential device applications. The results of this research seem certain to be a significant factor in the development of the next generation of microscopic optical and electrical devices, chemical sensors, solar cells, and bio-medical monitors working on the nanoscale.
