Understanding the Raman Spectra of Graphene Nanoribbons for Device Fabrication Wyatt Jebef1,2, Zafer Mutlu2, and Jeffrey Bokor2 1Santa Barbara City College, 2EECS Department, University of California, Berkeley 2020 Transfer-to-Excellence Research Experiences for Undergraduates Program (TTE REU Program) Graphene nanoribbons (GNRs) are quasi-one-dimensional carbon-based semiconductors that possess a tunable bandgap contingent on ribbon width and edge Abstract topology and are therefore a promising alternative for silicon channels in future transistors. The bottom-up synthesis of GNRs provides ultimate control over ribbon design and thus their electronic properties. STM and Raman spectroscopy are the two standard techniques used for characterizing bottom-up synthesized GNRs. Although STM is an effective method to obtain atomic-scale information, its scope is localized and is limited to GNR samples grown on metallic substrates. Contrastingly, Raman spectroscopy is a fast and non-invasive analytical approach that provides complementary information about GNRs on both metallic and insulating substrates at a macroscopic scale. However, most contemporary studies only use Raman spectroscopy to identify GNR type. Herein, we investigate the effect of growth substrates, doping and the transfer process on the Raman features of seven-atom wide armchair GNRs (7-AGNRs) in order to advance their large-scale characterization and implementation in transistors. Motivation Results Bottom-up synthesis enables GNR bandgap engineering via molecular precursor design and Growth on metallic substrates produces uniform GNRs that can serve as key elements for post-silicon CMOS devices.[1] 1617 1.2 Project goal: Increase current understanding of GNR Raman modes to advance the large- ) 1614 -1 Changes in G mode frequency and I /I ratio indicate scale ex-situ characterization of GNRs and improve device fabrication techniques. Ref. 6 1 D G cm 1611 ( Implement GNRs in high-performance transistors. G GNR band structure modification via substrate effects – Long-term goal: 1608 /I 0.8 D I both electronic interactions and strain induced by lattice Armchair shift G 1605 DFT-GW calculations Ref. 5 0.6 mismatch. 10 V 3p + 1 D 1602 9 0.4 GNR 1599 8 3p Au(111) Au(788) Ag(111) Cu(111) 3p+ 2 Metal substrates Chiral 7 1 Doping with HCl and KI2 2 6 3 0.8 10-7 GNRs Dielectric Au(111) ) 22 -8 5 N G HCl -1 10 HCl KI -9 2 cm 0.6 4 Gate ( 10 Band gap (eV)gap Band 20 -10 Zigzag G 10 (A) /I D 3 I 0.4 D -11 - V I G 18 10 2 10-12 FWHMofD 0.2 10-13 1 GNR-based field-effect transistor 16 -14 0 (FET) model. Normalized intensity (a.u.) 10 1570 1580 1590 1600 1610 1620 1630 14 0 4 6 8 10 12 14 16 Raman shift (cm-1) Au(111) HCl KI2 -4 -3 -2 -1 0 1 2 Width (# of C atoms, N) Dopants VG(V) Three GNR edge designs – armchair, Inverse relationship of armchair G shift trend again indicates change in GNR electronic structure. chiral and zigzag. GNR width and bandgap.[2] Increase in the D peak linewidth and ID/IG ratio indicates KI2 is damaging GNRs. Transfer to insulating substrates 1.5 1615 22 ) 1.2 ) -1 -1 Methods 20 cm ( cm 1610 0.9 ( G /I D 18 I 0.6 Bottom-up synthesis [1] 1605 G shift shift G 16 0.3 DBBA Polymer chain 7-AGNR FWHMofD 1600 14 0 Au(111) HCl KI2 Si Cu(111) Au(111) HCl KI2 Si Cu(111) Substrates and dopants Substrates and dopants KI2 is one of the primary causes of defects induced during wet-transfer to SiO2/Si. Deposition on Au(111) Polymerization + Cyclodehydrogenation + heating planarization heating STM image of 7- AGNRs.[3] Conclusion Transfer method and device fabrication Raman spectroscopy has applications beyond GNR type identification. Alternative transfer methods or etchants should be explored to reduce defects induced during transport to insulating substrates. Target With further research, we hope to confirm the trends noted in this work so that the G and D Raman modes may be used to probe V changes in GNR band structure caused by doping and lattice strain. GNRs GNRs D Pd Au(111) Mica W SiO2 HfO2 Si References HCl Gold etchant VG [1] Cai, J. et al., “Atomically precise bottom-up fabrication of graphene nanoribbons.” Nature (2010). [2] Yang, L. et. al., “Quasiparticle Energies and Band Gaps in Graphene Nanoribbons.” Phys. Rev. Lett. (2007). [3] Bennett, P. et al., “Bottom-up graphene nanoribbon field-effect transistors.” Appl. Phys. Lett. (2013). [4] Llinas, J. et al., “Short-channel field-effect transistors with 9-atom and 13-atom wide graphene nanoribbons.” Nature Communications (2017). Wet-transfer [3],[4] Finished device [5] B. V. Senkovskiy et al., “Making Graphene Nanoribbons Photoluminescent.” Nano Lett. (2017). [6] H. Huang et al., “Spatially Resolved Electronic Structures of Atomically Precise Armchair Graphene Nanoribbons.” Scientific Reports, (2012). Raman spectroscopy and STM characterization Microscopic lens 7-AGNRs on Au(111) Acknowledgements 532 nm Laser I would like to thank the following people: Nicole McIntyre and Sam Mountain for their exceptional administrative support, Dr. Zafer Sample G Notch filter Mutlu for his invaluable mentorship and guidance, and Professor Jeffrey Bokor for allowing me to conduct this research as a remote member of the Bokor lab group. I would also like to acknowledge the Center for Energy Efficient Electronics Science as well as the National Science Foundation for funding this project. Raman spectroscopy was performed at the Molecular Foundry at Lawrence D Berkeley National Laboratory (LBNL), supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of C-H(1) Energy (DOE) under contract No. DE-AC02-05CH11231. Device fabrication was performed at the Stanford Nano Shared Facilities 100 nm (SNSF) at Stanford University. Intensity(a.u.) RBLM C-H(2) RBLM 3 STM image of Pd electrodes from GNR-based FET. Contact Information Support Information 500 1000 1500 2000 This work was funded by National CCD Raman Shift (cm-1) Wyatt Jebef Spectroscopic grate Science Foundation Award ECCS- Email: [email protected] 0939514 & ECCS-1461157.