To understand the optical and transport properties of graphene nanoribbons, an unambiguous determination of their electronic band structure is needed. In this work we demonstrate that the photoemission intensity of each valence sub-band, formed due to the quantum confinement in quasi-one-dimensional (1D) graphene nanoribbons, is a peaked function of the two-dimensional (2D) momentum. We resolve the long-standing discrepancy regarding the valence band effective mass (m∗VB) of armchair graphene nanoribbons with a width of N = 7 carbon atoms (7-AGNRs). In particular, angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling spectroscopy report m∗VB .0.2 and .0.4 of the free electron mass (me), respectively. ARPES mapping in the full 2D momentum space identifies the experimental conditions for obtaining a large intensity for each of the three highest valence 1D sub-bands. Our detail map reveals that previous ARPES experiments have incorrectly assigned the second sub-band as the frontier one. The correct frontier valence sub-band for 7-AGNRs is only visible in a narrow range of emission angles. For this band we obtain an ARPES derived effective mass of 0.4 me, a charge carrier velocity in the linear part of the band of 0.63 106 m s.1 and an energy separation of only .60 meV to the second sub-band. Our results are of importance not only for the growing research field of graphene nanoribbons but also for the community, which studies quantum confined systems.

The opening of a band gap in graphene nanoribbons induces novel optical and electronic properties, strongly enhancing their application potential in nanoscale devices. Knowledge of the optical excitations and associated relaxation dynamics are essential for developing and optimizing device designs and functionality. Here we report on the optical excitations and associated relaxation dynamics in surface aligned 7-atom wide armchair graphene nanoribbons as seen by time-resolved spontaneous Stokes and anti-Stokes Raman scattering spectroscopy. On the anti-Stokes side we observe an optically induced increase of the scattering intensity of the Raman active optical phonons which we assign to changes in the optical phonon populations. The optical phonon population decays with a lifetime of ∼2 ps, indicating an efficient optical-acoustic phonon cooling mechanism. On the Stokes side we observe a substantial decrease of the phonon peak intensities which we relate to the dynamics of the optically induced exciton population. The exciton population shows a multi-exponential relaxation on the hundreds of ps time scale and is independent of the excitation intensity, indicating that exciton-exciton annihilation processes are not important and the exsistence of dark and trapped exciton states. Our results shed light on the optically induced phonon and exciton dynamics in surface aligned armchair graphene nanoribbons and demonstrate that time-resolved spontaneous Raman scattering spectroscopy is a powerful method for exploring quasi-particle dynamics in low dimensional materials.

We couple photoluminescent semiconducting 7-atom wide armchair edge graphene nanoribbons to plasmonic nanoantenna arrays and demonstrate an enhancement of the photoluminescence and Raman scattering intensity of the nanoribbons by more than an order of magnitude averaged over large areas, and by three orders of magnitude in the hot spots of plasmonic antennas. The increase in signal allows us to study Raman spectra with high signal-to-noise ratio. Using plasmonic enhancement we are able to detect the off-resonant Raman scattering from the modified radial breathing-like mode (RBLM) due to physisorbed molecules, the 3rd order RBLM, and C-H vibrations. We find excellent agreement between experimental data and simulations describing the spectral dependence of the enhancement and modifications of the polarization anisotropy. The strong field gradients in the optical near-field further allow us to probe the subwavelength coherence properties of the phonon modes in the nanoribbons. We theoretically model this considering a finite phonon correlation length along the GNR direction. Our results allow estimating the correlation length in graphene nanoribbons.

A semiconductor-to-metal transition in N = 7 armchair graphene nanoribbons causes drastic changes in its electron and phonon system. By using angle-resolved photoemission spectroscopy of lithium-doped graphene nanoribbons, a quasiparticle band gap renormalization from 2.4 to 2.1 eV is observed. Reaching high doping levels (0.05 electrons per atom), it is found that the effective mass of the conduction band carriers increases to a value equal to the free electron mass. This giant increase in the effective mass by doping is a means to enhance the density of states at the Fermi level which can have palpable impact on the transport and optical properties. Electron doping also reduces the Raman intensity by one order of magnitude, and results in relatively small (4 cm−1) hardening of the G phonon and softening of the D phonon. This suggests the importance of both lattice expansion and dynamic effects. The present work highlights that doping of a semiconducting 1D system is strikingly different from its 2D or 3D counterparts and introduces doped graphene nanoribbons as a new tunable quantum material with high potential for basic research and applications.

Lateral heterojunctions of atomically precise graphene nanoribbons (GNRs) hold promise for applications in nanotechnology, yet their charge transport and most of the spectroscopic properties have not been investigated. Here, we synthesize a monolayer of multiple aligned heterojunctions consisting of quasi-metallic and wide-bandgap GNRs, and report characterization by scanning tunneling microscopy, angle-resolved photoemission, Raman spectroscopy, and charge transport. Comprehensive transport measurements as a function of bias and gate voltages, channel length, and temperature reveal that charge transport is dictated by tunneling through the potential barriers formed by wide-bandgap GNR segments. The current-voltage characteristics are in agreement with calculations of tunneling conductance through asymmetric barriers. We fabricate a GNR heterojunctions based sensor and demonstrate greatly improved sensitivity to adsorbates compared to graphene based sensors. This is achieved via modulation of the GNR heterojunction tunneling barriers by adsorbates.