Spectroscopic characterization of N = 9 armchair graphene nanoribbons

Abstract

We investigate the N ¼ 9 atoms wide armchair-type graphene nanoribbons (9-AGNRs) by performing a comprehensive spectroscopic and microscopic characterization of this novel material. In particular, we use X-ray photoelectron, near edge X-ray absorption fine structure, scanning tunneling, polarized Raman and angle-resolved photoemission (ARPES) spectroscopies. The ARPES measurements are aided by calculations of the photoemission matrix elements which yield the position in k space having the strongest photoemission cross section. Comparison with well-studied narrow N ¼ 7 AGNRs shows that the effective electron mass in 9-AGNRs is reduced by two times and the valence band maximum is shifted to lower binding energy by $0.6 eV. In polarized Raman measurements of the aligned 9-AGNR, we reveal anisotropic signal depending upon the phonon symmetry. Our results indicate the 9-AGNRs are a novel 1D semiconductor with a high potential in nanoelectronic applications.

Finding the hidden valence band of N = 7 armchair graphene nanoribbons with angle-resolved photoemission spectroscopy

“…This knowledge allows us to visualize the highest three valence sub-bands and determine important param eters for the transport and optical properties, such as effective masses, energy band offsets and the charge carrier velocities. In previous ARPES data the second-highest sub-band of 7-AGNRs was mistakenly considered as the highest one [25,[29][30][31]. Our results highlight that proper application of the ARPES technique for probing the electronic structure of confined systems requires careful consideration of photoemission matrix elements.…”

“…Due to quantum confinement the amplitude of the wave function in the direction perpendicular to nanoribbon axis is changing as sin(q n ⊥ a/2), where q n ⊥ = 2πn a(N+1) is the quantized value of k ⊥ and a is the lattice constant of graphene [37][38][39]. If the final state in the photoemission process is assumed to be a plane wave, then the photoemision matrix element for the π states of GNR in the dipole approximation can be expressed as [31]…”

“…Table 1. Energies of the band maxima regarding to the Fermi level and the effective masses for the first (VB1) and the second (VB2) valence sub-bands of 7-AGNRs on Au 1 1 1 [29], 0.23 [25], 0.22 [31] ribbons axis (k ⊥ = 0) or slightly off (if there is a small deviation in tilt or azimuthal angle for horizontal orientation) the VB 3 and VB 2 should be seen. Indeed, such a picture was observed in previous data [25,29,30].…”

“…The photoemission intensity was calculated using the dipole approximation for matrix element and the plane wave as a final state [31]. Density-functionaltheory (DFT) calculations were carried out using the FPLO-14.00-48 code (improved version of the original FPLO code by Koepernik and Eschrig [56]) utilizing the generalized gradient approximation (GGA) to the exchange-correlation potential.…”

“…On another hand, Fourier-transformed scanning tunneling spectroscopy (FT-STS), where the E(k) is determined from the interference pattern of the electronic waves reflected form the ribbon edges, found m * VB ≈ 0.4 m e [32]. The recent ARPES results of various groups agree to one another, [25,[29][30][31] making erroneous measurements an unlikely explanation. The discrepancy between ARPES and STS is quite surprising, since both methods actually should lead to identical results.…”