With superlative physical, chemical, and electronic properties, graphene shows significant promise in a variety of technologies, including transistors, sensors, and energy conversion/storage devices. However, as an atomically thin material, graphene is highly sensitive to its environment, which implies that surface/interface chemistry must be carefully considered when integrating graphene into functional devices. Towards this end, we have been studying a variety of methods for chemically functionalizing graphene including organic self-assembled monolayers 1 and epoxidation with atomic oxygen 2. Among possible covalent modification schemes, hydrogenation is arguably the simplest and thus well suited for fundamental atomic-scale studies. Here, we present an ultra-high vacuum (UHV) scanning tunneling microscopy (STM) study of adsorption and desorption of atomic hydrogen on monolayer and bilayer epitaxial graphene on SiC(0001). Atomic-resolution imaging reveals no measurable differences between the adsorption of atomic hydrogen on monolayer compared to bilayer graphene. At high coverages, the fully hydrogenated graphene surface exhibits a periodicity that is well correlated with the underlying Moiré pattern. The desorption temperature, at which 90% of the hydrogen has desorbed, is observed to be ~120ºC for both monolayer and bilayer graphene. However, at lower temperatures, the desorption rate from bilayer graphene is found to be slightly higher than monolayer graphene, thus suggesting that the underlying substrate can influence reaction kinetics on graphene. 1 Md. Z. Hossain, et al., “Chemically homogeneous and thermally reversible oxidation of epitaxial graphene,” Nature Chemistry, 4, 305 (2012). 2 Q. H. Wang and M. C. Hersam, “Room-temperature molecular-resolution characterization of selfassembled organic monolayers on epitaxial graphene,” Nature Chemistry, 1, 206 (2009).
TeYu Chien is an associate professor of physics and astronomy at the University of Wyoming. Professor Chien completed his postdoctoral studies at Northwestern University and at Argonne National Laboratory.
About Professor Chien's Research:
In our lab, we devoted our efforts to study the physical properties of materials, especially the properties of electrons, phonons (vibrational modes) and their interactions. Our goal is to provide the microscopic view of the macroscopic physical phenomena. Since the quantum physics was proposed in early 20th century, physicists study materials in a novel way: particles could have wave-like identities; while waves could also have particle-like identities. For examples, electron, one of the fundamental particles, is intuitively considered as particle. However, a electron could "tunnel through" a potential wall, with which the classic physics could not explain. Quantum mechanics successively describe the phenomena of "penetrating through a energy barrier" effect by considering the electron as a particle wave. This effect is the fundamental background of the Nobel Prize (year 1986) work - the invention of scanning tunneling microscope (STM), which is one of the state-of-the-art research tools used intensively nowadays. Another example is the atom/ion vibration modes in condensed materials. The vibration modes are intuitively considered as waves propagating heat through the interior of materials. However, this picture is failed when Albert Einstein tried to explain the heat capacity of a material with Einstein model, which treats the solid as many individual, non-interacting quantum harmonic oscillators. This issue was solved by Peter Debye who treat the vibrations as "phonons" (Debye model), which are, in concept, particles, or quasi-particles.
TeYu Chien, Chung-Hong Sham, Mark C. Hersam
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University of Illinois Urbana-Champaign, Urbana, IL