The discovery of extraordinary new quantum materials with striking properties has caused great excitement, and promises to transform signal processing and computation. We have performed integrated research on three materials (1) Graphene (G) – electrons that move as massless particles at a constant speed; (2) Topological Insulators (TI) – mobile surface electrons with spins fixed to the direction of motion; and (3) Nitrogen-vacancy (NV) Centers in Diamond – a single spin stores a bit of quantum information.  Remarkably, the quantum phenomena displayed by these materials persists at room temperature, changing the rules for signal processing and computation and opening the way for quantum electronics.   

 Defects in materials effect the propagation of electrons and holes in graphene and topological insulators act in ways that are totally unlike carriers in conventional semiconductors – they move like two-dimensional (2D) massless, ultra-relativistic electrons, except their speed is much less than the speed of light. Because there is no bandgap, an electron can pass through a potential barrier by temporarily turning into a hole, dramatically reducing scattering and improving coherence. In addition, for topological insulators the direction of the spin of a surface electron is tied to its direction of motion, providing an ideal means to transport spin information. 

We have imaged and characterized high quality graphene-like materials, such as hexagonal boron nitride (hBN) and hybrid graphene-hBN structures (Fig. 1). Compared with mechanical exfoliation, CVD synthesis [1-2] can provide larger areas, with wafer-scale monolayer or multilayer graphene sheets. Aberration-corrected electron microscopy has been used to characterize MBE-grown films with high resolution at low beam voltages (40 & 80kV) to directly visualize structural defects and relate them to performance.   

We use a Cs corrected Zeiss Libra TEM to investigate chemical vapor deposition (CVD) graphene with added copper and mercury defects. With TEM we address the question, where the Hg and Co atoms are placed on the graphene. At the same time, we observe the effect of the copper and mercury on the pi electrons in graphene with Raman spectroscopy. Furthermore, we are interested in graphene based hybrid structures, such as graphene oxide embedded in a vanadium pentoxide nanofiber matrix (Fig. 2). The graphene sheets and the nanofibers have approximately the same thickness, leading to a material with enhanced mechanical performance in comparison to pure vanadium pentoxide and pure graphene oxide sheets.

Application of Low-Voltage Electron Microscopy and its development and future directions will be presented.

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References:

[1] A. Reina, X. Jia, J. Ho, D. Nezich, H. Son, V. Bulovic, M.S. Dresselhaus, & J. Kong,Nano Lett. 9, 30–35 (2008).

[2] K.S. Kim, Y. Zhao, H. Jang, S.Y. Lee, J.M. Kim, K.S. Kim, J.H. Ahn, P. Kim, J.Y. Choi, and B.H. Hong,  Nature 457, 706-710 (2009).

[3] This work was supported by the STC Center for Integrated Quantum Materials, NSF Grant No. DMR-1231319.

Figures:

Figure 1. (top) h-BN and Graphene single layer stack figure showing the structure correlation (bottom) TEM image at 40 keV of a stack showing a single defect.

Figure 2. The image shows the structure of a free-standing thin-film composed of vanadium pentoxide nanofibers and graphene oxide nanosheets. Fibres and sheets both have an average thickness of 1 nm and oxygen-containing functional groups which promote the interaction between both components, leading to a material with enhanced mechanical performance in comparison to pure vanadium pentoxide and pure graphene oxide sheets.

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