Molecular bandgap engineering of bottom-up synthesized graphene nanoribbon heterojunctions

Abstract

Bandgap engineering is used to create semiconductor heterostructure devices that perform processes such as resonant tunnelling1,2 and solar energy conversion3,4. However, the performance of such devices degrades as their size is reduced5,6. Graphene-based molecular electronics has emerged as a candidate to enable high performance down to the single-molecule scale7,8,9,10,11,12,13,14,15,16,17. Graphene nanoribbons, for example, can have widths of less than 2 nm and bandgaps that are tunable via their width and symmetry6,18,19. It has been predicted that bandgap engineering within a single graphene nanoribbon may be achieved by varying the width of covalently bonded segments within the nanoribbon20,21,22. Here, we demonstrate the bottom-up synthesis of such width-modulated armchair graphene nanoribbon heterostructures, obtained by fusing segments made from two different molecular building blocks. We study these heterojunctions at subnanometre length scales with scanning tunnelling microscopy and spectroscopy, and identify their spatially modulated electronic structure, demonstrating molecular-scale bandgap engineering, including type I heterojunction behaviour. First-principles calculations support these findings and provide insight into the microscopic electronic structure of bandgap-engineered graphene nanoribbon heterojunctions.

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Figure 1: Bottom-up synthesis of 7–13 GNR heterojunctions.
Figure 2: STM dI/dV spectroscopy of 7–13 GNR heterojunction electronic structure.
Figure 3: Comparison of experimental dI/dV maps and theoretical LDOS for a 7–13 GNR heterojunction.
Figure 4: Theoretical electronic structure of 7–13 GNR heterojunction.

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Acknowledgements

This research was supported by the Office of Naval Research BRC Program (molecular synthesis and characterization), by the Director, Office of Science, Office of Basic Energy Sciences of the US Department of Energy under the Nanomachine Program at the Lawrence Berkeley National Laboratory (contract no. DE-AC02-05CH11231, STM instrumentation development, STM operation and simulations) and by National Science Foundation (NSF) awards (DMR-1206512, image analysis; DMR10-1006184, basic theory and formalism). Computational resources were provided by the NSF through XSEDE resources at the Texas Advanced Computing Center (TACC) at the University of Texas at Austin and Lawrence Berkeley National Laboratory's High Performance Computing Services. S.G.L. acknowledges the support of a Simons Foundation Fellowship in Theoretical Physics. D.H. acknowledges a research fellowship from the German Research Foundation (DFG; grant no. Ha 6946/1-1).

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Y-C.C., C.C., Z.P., D.H., D.G.O. and M.F.C. performed STM measurements and analysed STM data. T.C. and S.G.L. carried out GNR calculations and interpretation of STM data. F.R.F. synthesized the precursor molecules. All authors discussed and wrote the paper.

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Correspondence to Felix R. Fischer or Steven G. Louie or Michael F. Crommie.

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The authors declare no competing financial interests.

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Chen, YC., Cao, T., Chen, C. et al. Molecular bandgap engineering of bottom-up synthesized graphene nanoribbon heterojunctions. Nature Nanotech 10, 156–160 (2015). https://doi.org/10.1038/nnano.2014.307

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