Alexander Grüneis

Making graphene nanoribbons photoluminescent

We demonstrate the alignment-preserving transfer of parallel graphene nanoribbons (GNRs) onto insulating substrates. The photophysics of such samples is characterized by polarized Raman and photoluminescence (PL) spectroscopies. The Raman scattered light and the PL are polarized along the GNR axis. The Raman cross section as a function of excitation energy has distinct excitonic peaks associated with transitions between the one-dimensional parabolic subbands. We find that the PL of GNRs is intrinsically low but can be strongly enhanced by blue laser irradiation in ambient conditions or hydrogenation in ultrahigh vacuum. These functionalization routes cause the formation of sp3 defects in GNRs. We demonstrate the laser writing of luminescent patterns in GNR films for maskless lithography by the controlled generation of defects. Our findings set the stage for further exploration of the optical properties of GNRs on insulating substrates and in device geometries.

Environmental control of electron-phonon coupling in barium doped graphene

Two-dimensional superconductivity in alkali- and alkaline-Earth-metal doped monolayer graphene has been explained in the framework of electron–phonon coupling (EPC) and experiments yielded superconducting transition temperatures (T C ) up to 6 K. In contrast to bulk graphite intercalation compounds, the interface of doped graphene with its environment affects its physical properties. Here we present a novel and well-defined BaC8 interface structure in Ba-doped single-layer graphene on Au and Ge substrates. We use angle-resolved photoemission spectroscopy in combination with ab initio modelling to extract the Eliashberg function and EPC for both substrates. This allows us to quantitatively assess the environmental effects for both Au and Ge substrates on superconductivity in graphene. We show that for semiconducting Ge substrates, the doping level and EPC are higher. Our study highlights that both dopant order and the metallicity of the substrate can be used to control EPC and hence superconductivity.

First-principles and angle-resolved photoemission study of lithium doped metallic black phosphorous

First principles calculations demonstrate the metallization of phosphorene by means of Li doping nfilling the unoccupied antibonding pz states. The electron–phonon coupling in the metallic phase is nstrong enough to eventually lead to a superconducting phase at Tc= 17 K for LiP8 stoichiometry. nUsing angle-resolved photoemission spectroscopy we confirm that the surface of black phosphorus ncan be chemically functionalized using Li atoms which donate their 2s electron to the conduction nband. The combined theoretical and experimental study demonstrates the semiconductor-metal ntransition indicating a feasible way to induce a superconducting phase in phosphorene and few-layer nblack phosphorus.

Field-Effect Transistors Based on Networks of Highly Aligned, Chemically Synthesized N = 7 Armchair Graphene Nanoribbons

We report on the experimental demonstration and electrical characterization of N = 7 armchair graphene nanoribbon (7-AGNR) field effect transistors. The back-gated transistors are fabricated from atomically precise and highly aligned 7-AGNRs, synthesized with a bottom-up approach. The large area transfer process holds the promise of scalable device fabrication with atomically precise nanoribbons. The channels of the FETs are approximately 30 times longer than the average nanoribbon length of 30 nm to 40 nm. The density of the GNRs is high, so that transport can be assumed well-above the percolation threshold. The long channel transistors exhibit a maximum ION/IOFF current ratio of 87.5.

Resonance Raman spectrum of doped epitaxial graphene at the Lifshitz transition

We employ ultra-high vacuum (UHV) Raman spectroscopy in tandem with angle-resolved photoemission (ARPES) to investigate the doping-dependent Raman spectrum of epitaxial graphene on Ir(111). The evolution of Raman spectra from pristine to heavily Cs doped graphene up to a carrier concentration of 4.4 × 1014 cm-2 is investigated. At this doping, graphene is at the onset of the Lifshitz transition and renormalization effects reduce the electronic bandwidth. The optical transition at the saddle point in the Brillouin zone then becomes experimentally accessible by ultraviolet (UV) light excitation, which achieves resonance Raman conditions in close vicinity to the van Hove singularity in the joint density of states. The position of the Raman G band of fully doped graphene/Ir(111) shifts down by ∼60 cm-1. The G band asymmetry of Cs doped epitaxial graphene assumes an unusual strong Fano asymmetry opposite to that of the G band of doped graphene on insulators. Our calculations can fully explain these observations by substrate dependent quantum interference effects in the scattering pathways for vibrational and electronic Raman scattering.

Observation of Room-Temperature Photoluminescence Blinking in Armchair-Edge Graphene Nanoribbons

By enhancing the photoluminescence from aligned seven-atom wide armchair-edge graphene nanoribbons using plasmonic nanoantennas, we are able to observe blinking of the emission. The on- and off-times of the blinking follow power law statistics. In time-resolved spectra, we observe spectral diffusion. These findings together are a strong indication of the emission originating from a single quantum emitter. The room temperature photoluminescence displays a narrow spectral width of less than 50 meV, which is significantly smaller than the previously observed ensemble line width of 0.8 eV. From spectral time traces, we identify three optical transitions, which are energetically situated below the lowest bulk excitonic state E11 of the nanoribbons. We attribute the emission to transitions involving Tamm states localized at the end of the nanoribbon. The photoluminescence from a single ribbon is strongly enhanced when its end is in the antenna hot spot resulting in the observed single molecule characteristics of the emission. Our findings illustrate the essential role of the end termination of graphene nanoribbons in light emission and allow us to construct a model for photoluminescence from nanoribbons.

Effect of lithium doping on the optical properties of monolayer MoS2

The effect of lithium atoms evaporation on the surface of monolayer MoS2 grown on SiO2/Si substrate is studied using ultrahigh vacuum (∼10−11 mbar) Raman and circularly polarized photoluminescence spectroscopies, at low lithium coverage (up to ∼0.17 monolayer). With increasing Li doping, the dominant E2g1 and A1g Raman modes of MoS2 shift in energy and broaden. Additionally, non zone-center phonon modes become Raman active. This regards, in particular, to double resonance Raman scattering processes, involving longitudinal acoustic phonon modes at the M and K points of the Brillouin zone of MoS2 and defects. It is also accompanied by a significant decrease in the overall intensity and the degree of circular polarization of the photoluminescence spectrum. The observed changes in the optical spectra are understood as a result of electron doping by lithium atoms and disorder-activated intervalley scattering of electrons and holes in the electronic band structure of monolayer MoS2.

Boron-Doped Graphene Nanoribbons: Electronic Structure and Raman Fingerprint

We investigate the electronic and vibrational properties of bottom-up synthesized aligned armchair graphene nanoribbons of N = 7 carbon atoms width periodically doped by substitutional boron atoms (B-7AGNRs). Using angle-resolved photoemission spectroscopy and density functional theory calculations, we find that the dopant-derived valence and conduction band states are notably hybridized with electronic states of Au substrate and spread in energy. The interaction with the substrate leaves the bands with pure carbon character rather unperturbed. This results in an identical effective mass of ≈0.2 m0 for the next-highest valence band compared with pristine 7AGNRs. We probe the phonons of B-7AGNRs by ultrahigh-vacuum (UHV) Raman spectroscopy and reveal the existence of characteristic splitting and red shifts in Raman modes due to the presence of substitutional boron atoms. Comparing the Raman spectra for three visible lasers (red, green, and blue), we find that interaction with gold suppresses the Raman signal from B-7AGNRs and the energy of the green laser (2.33 eV) is closer to the resonant E22 transition. The hybridized electronic structure of the B-7AGNR-Au interface is expected to improve electrical characteristics of contacts between graphene nanoribbon and Au. The Raman fingerprint allows the easy identification of B-7AGNRs, which is particularly useful for device fabrication.

Photophysical properties of semiconducting armchair-edge grapheme nanoribbons

One-dimensional (1D) graphene nanoribbons (GNRs) are promising materials for future electronics and optoelectronics. Their versatility in electronic properties makes it possible to use them as an active element in devices with a tunable band gap. Different from graphene, armchair-edge GNRs (AGNRs) are semiconducting with a direct bandgap [1-3]. However, until now their photophysical characterization has not been addressed properly due to the metal substrate on which they are grown. The substrate hinders measurements such as transmission and photoluminescence (PL). Here we transfer AGNRs with a width of N = 7 atoms from Au(788) to insulating substrates using an alignment-preserving method and investigate their photoluminescence properties.