On-Surface Synthesis II by Dimas G. de Oteyza & Celia Rogero

Author:Dimas G. de Oteyza & Celia Rogero
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

3 Characterization Tools and Associated Insight

3.1 Electronic Properties Determination

The electronic structure of GNRs and in particular their band gap (E g ) size and the values of bands’ effective masses (m VB and m CB ), have been addressed locally with scanning tunneling spectroscopy (STS) [34, 40, 42, 43, 45, 50, 51], and with averaging experimental techniques as angle-resolved-, inverse- and two-photon-photoemission [34, 35, 52–54], high-resolution electron energy loss [52, 55] and optical spectroscopy [56].

The most widespread method used so far is STS, which measures the local density of states (LDOS) as a function of tip position and applied sample bias. It allows determining the electronic band dispersion of occupied and empty energy levels for the vary same GNR. Precise quantification of bands’ energy level alignments and effective masses is given by Fourier-transformed scanning tunneling spectroscopy (FT-STS) [57]. The presence of edges in GNRs gives rise to standing wave patterns that arise due to scattering of electronic Bloch wave functions against such termini. Recording standing waves for different energies along one GNRs edge, allows determining the energy versus momentum dispersion of GNRs’ states via Fourier transform processing of STS data. Precise bandgap values of E g = 2.37 ± 0.06 eV and E g = 1.38 ± 0.03 eV have been extrapolated from valence band maximum and conduction band minimum for 7-aGNR (Fig. 4c–f) [50] and 9-aGNRs [45] grown on Au(111), respectively.

Fig. 4Electronic structure of 7-aGNRs. a dI/dV spectrum (red) of a 7-aGNR grown on Au(111). The onsets of valence and conduction bands are indicated by the shaded areas. Electronic features in the gap are due to the Au surface state (visible in the black spectrum). b ARPES intensity plot recorded for 1 ML of 7-aGNRs grown on Au(788) recorded along the ribbon axis. The VB onset is found at −0.7 eV below the Fermi level. c STM image and plot consisting on equidistant dI/dV spectra of occupied states recorded along the edge of a 7-aGNR. d Line-by-line Fourier transform of (c) including a parabolic fit of the VB. The same is done for the CB on a longer ribbon in (e, f).

a Reprinted with permission from [51]. Copyright (2017) American Chemical Society. b Reprinted with permission from [34]. Copyright (2012) American Chemical Society. c–f Reprinted figure with permission from [52]. Copyright (2015) by the American Physical Society

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