Darawish, Rimah (2023). Graphene Nanoribbon Growth and Substrate Transfer for Device Applications. (Thesis). Universität Bern, Bern

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Abstract

Graphene nanoribbons (GNRs) – quasi-one-dimensional stripes of graphene – have emerged as promising materials for next-generation nano-electronic devices. This interest stems from their tunable physicochemical properties, which can be achieved through precise control of ribbon width and edge structure. Atomic precision is achieved by on-surface synthesis, which relies on the metal surface-assisted covalent coupling of specifically designed molecular precursors in ultra-high vacuum. Metallic single-crystal surfaces serve both as a template and a catalyst. However, GNRs located on a metallic substrate are not suitable for many types of characterization and, more importantly, for most applications and device integration. Therefore, the development of efficient substrate transfer procedures for bringing the synthesized GNR materials onto technologically relevant substrates is a necessity. Additionally, it is crucial to optimize growth conditions to maximize GNR length and alignment for high device yield. Finally, GNR quality needs to be maintained and monitored throughout the entire path from GNR growth to device integration. This thesis focuses on the growth, substrate transfer, and characterization of 9-atom-wide armchair GNRs (9-AGNRs) grown on the surface of the regularly stepped Au(788) single crystal. The (788) surface has 3 nm wide (111) terraces separated by monoatomic steps that are running in parallel across the entire single-crystal surface. The GNRs grow along the step edges, leading to an uniaxial GNR alignment across the single crystal surface. Using scanning tunneling microscopy (STM) and Raman and polarized Raman spectroscopy, the quality and the alignment of the GNRs pre- and post-substrate transfer are characterized and the transfer efficiency is quantified. Chapters 1 and 2 introduce the main topics of this thesis and the methods and materials used. The scientific results obtained are presented in Chapters 3 to 7. Chapter 8 gives a short review, discusses the implications of the main findings, and outlines an outlook on the further development of the research topic. Chapter 3 is motivated by previous works that demonstrated a significant increase in device yield, from 10-15% to 85%, by employing uniaxially aligned GNRs. It describes a new model to quantify GNR alignment based on Polarized Raman spectroscopy measurements, which led to a thorough understanding of GNR alignment before and after substrate transfer. In particular, it is shown that low-coverage samples grown on Au(788) exhibit superior uniaxial alignment compared to high-coverage samples, which is attributed to preferential growth along step edges. However, upon transferring the GNRs to a device substrate, the degree of alignment decreases in the low-coverage samples, while it is maintained for high-coverage ones. The loss of alignment can be attributed to the strictly in-plane lateral diffusion of the GNRs that is sterically prevented at high coverages. Chapter 4 investigates in detail the growth dynamics of uniaxial aligned 9-AGNRs using both STM and Raman measurements. It particularly explores the role of the precursor dose – essentially the density of the molecular precursors sublimated onto the substrate – in determining GNR length and quality of alignment. It is found that the GNR growth location on the Au(788) substrate and their degree of alignment are coverage-dependent. The GNR length evolution clearly correlates with the precursor dose and the GNR growth location on the substrate. Also, it is found that GNR alignment after substrate transfer is coverage-dependent. In Chapter 5, a mathematical model is introduced to determine the coverage of GNRs on various substrates using Raman spectroscopy. This allows the quantification of the GNR transfer efficiency by comparing GNR coverages before and after substrate transfer. It is found that the transfer efficiency strongly correlates with the initial coverage of the GNRs on the Au vicinal substrate (Au(788)): It increases from 35 % for low-coverage samples to 52-70 % for high-coverage samples. It is thus concluded that the adsorption of the GNRs next to step edges significantly hinders their transfer compared to a full monolayer. In addition to the exploration of GNR growth and transfer conditions, this thesis also attempts to develop techniques to improve the quality of the transferred samples, which are reported in Chapter 6. The applied GNR substrate transfer process relies on the use of poly-methyl methacrylate (PMMA) to support the GNRs as they are detached from the Au(788) surface. PMMA is notorious for leaving behind residues, and even at low concentrations, it can prevent electrical contact with the GNRs. An optimized cleaning procedure was thus developed that yielded clean enough samples to obtain the first STM images of transferred 9-AGNRs. Finally, Chapter 7 addresses the strong adsorption of GNRs next to the step edges that adversely impact their transfer efficiency. It shows that the step edges can be passivated using the one-dimensional polymer poly-para-phenyelene (PPP). Given PPP's wide band gap, it is expected to act as an insulator without hindering device performance and instead supports aligned GNR growth on terraces and during substrate transfer. Additionally, the intercalation of GNRs with self-assembled monolayers (SAMs) has been investigated as a strategy for decreasing GNR-Au interactions. The work presented in this thesis is foundational for innovations in nano-electronic devices employing GNRs. Notable progress was achieved, particularly in establishing Raman spectroscopy as a quantitative tool to monitor GNR quality, degree of alignment, and quantity. However, the experiments summarized in this thesis also highlight the intrinsic variability of the GNR substrate transfer process that must be addressed to achieve consistent reproducibility. Overall, this thesis offers a deep understanding of GNR growth, alignment, and transfer efficiency, which are key parameters for the prospective use of GNRs in functional devices.

Item Type:	Thesis
Dissertation Type:	Cumulative
Date of Defense:	3 November 2023
Subjects:	500 Science > 530 Physics 500 Science > 540 Chemistry 500 Science > 570 Life sciences; biology
Institute / Center:	08 Faculty of Science > Department of Chemistry, Biochemistry and Pharmaceutical Sciences (DCBP)
Depositing User:	Hammer Igor
Date Deposited:	18 Jun 2024 10:20
Last Modified:	03 Nov 2024 23:25
URI:	https://boristheses.unibe.ch/id/eprint/5141

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