Journal
ACS NANO
Volume 11, Issue 11, Pages 11661-11668Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acsnano.7b06765
Keywords
graphene nanoribbon; on-surface synthesis; Fermi level pinning; Ullmann coupling; dehydrogenation; scanning; tunneling microscopy and spectroscopy; density functional theory
Categories
Funding
- European Research Council (ERC) under European Union [635919]
- European Union [610446]
- Spanish Ministry of Economy, Industry and Competitiveness (MINECO) [MAT2016-78293-C6]
- Basque Department of Education [PI-2016-1-0027]
- University of Padova [CPDA154322]
- European Research Council (ERC) [635919] Funding Source: European Research Council (ERC)
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We report the energy level alignment evolution of valence and conduction bands of armchair-oriented graphene nanoribbons (aGNR) as their band gap shrinks with increasing width. We use 4,4 ''-dibromo-para-terphenyl as the molecular precursor on Au(111) to form extended poly-para-phenylene nanowires, which can subsequently be fused sideways to form atomically precise aGNRs of varying widths. We measure the frontier bands by means of scanning tunneling spectroscopy, corroborating that the nanoribbon's band gap is inversely proportional to their width. Interestingly, valence bands are found to show Fermi level pinning as the band gap decreases below a threshold value around 1.7 eV. Such behavior is of critical importance to understand the properties of potential contacts in GNR-based devices. Our measurements further reveal a particularly interesting system for studying Fermi level pinning by modifying an adsorbate's band gap while maintaining an almost unchanged interface chemistry defined by substrate and adsorbate.
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