Electronic properties of bottom-up fabricated graphene nanoribbons
Final Report Abstract
The goal of the project was to utilize a thermally activated on-surface synthesis to generate various atomically precise sub-nanometer wide graphene nanoribbons (GNRs) with different widths, edge shapes and degrees of doping to gain insight into the influence of these parameters on the electronic properties of GNRs. This aims at controlling the size of band gaps and position of valence and conduction bands with respect to the Fermi level of a metal electrode. The bottom-up synthesis of the GNRs using particular precursor molecules has been followed by vibrational high resolution electron energy loss spectroscopy (HREELS). Furthermore we investigated the GNR formation process using temperature-programmed desorption (TPD). Their electronic properties have been analyzed by energy-, time- and angle-resolved two-photon photoemission (2PPE) spectroscopy and electronic HREELS. Additional detailed insights into the adsorption properties of the GNRs on noble metal surfaces have been gained by X-ray standing wave (XSW) measurements. Using Br-substituted molecular precursors, the atomic precision can be achieved in a thermally induced twostep reaction following Br dissociation on a Au(111) surface. With TPD and density functional theory (DFT) we demonstrated that Br atoms are bound to the intermediate polyanthrylene chains and during the final cyclodehydrogenation step of the reaction the associative desorption of HBr and molecular hydrogen occurred. Furthermore, Br atoms can be removed from the polyanthrylene chains by providing molecular hydrogen. The subsequent formation of GNR via a cyclodehydrogenation demonstrates that Br does not influence this part of the overall reaction. In the band gap of N = 7 armchair GNRs (7-AGNR) on Au(111) and Au(788) we have identified exctionic states using energy- and angle-resolved 2PPE. Thereby, an exciton binding energy in the 7-GNR on Au(111) of 160 ± 60 meV has been determined. On the stepped Au(788) surface, the exciton binding energy is in the same range. In nitrogen-doped (singly and doubly N-doped) as well as non-N-doped chevron-shaped CGNRs on Au(111) the unoccupied electronic structure and particularly the image potential states (IPSs) have been investigated. Compared to the pristine surface, reduced effective masses between 0.6 and 0.8 electron masses have been observed and the lifetimes of the IPS are below the experimental detection limit, which is in the femtosecond regime. Independent of the concentration of N dopant atoms introduced in the GNR we observe a constant binding energy with respect to the vacuum level of the system. In addition, the electronic structure changes during the formation of CGNRs (singly and doubly N-doped, non-doped) have been studied using electronic HREELS and DFT. Thereby optical gaps of the precursor molecules, the intermediate non-aromatic polymers and finally the aromatic GNRs have been determined. As expected, no influence of N-doping on the size of the optical gaps has been found. The gap of the precursor molecules is around 4.5 eV. Polymerization leads to a reduction of the gap to a value of 3.2 eV due to elongation and thus enhanc ed delocalization. The CGNRs exhibit a band gap of 2.8 eV, thus the gap is further reduced in the nanoribbons , since they exhibit an extended delocalized π-electron system. As determined by XSW measurements, on Au(111) the 7-AGNR, the doubly N-doped and non-doped CGNRs are not flat, i.e., they are bended. This indicates that the bond to the surface is mainly established by the edge carbon atoms and that the interaction between the extended π-system and the surface is weak. For the 2N- CGNRs an additional contribution from the nitrogen atoms has been found. In comparison to the adsorption height on Au(111), on Cu(111) the height of the carbon backbone is significant lower indicating a stronger interaction between the extended π-system and the copper surface.
Publications
- (2017) Electronic structure changes during the on-surface synthesis of nitrogen-doped chevron-shaped graphene nanoribbons. Phys. Rev. B (Physical Review B) 96 (4) 045434
Maaß, Friedrich; Utecht, Manuel; Stremlau, Stephan; Gille, Marie; Schwarz, Jutta; Hecht, Stefan; Klamroth, Tillmann; Tegeder, Petra
(See online at https://doi.org/10.1103/PhysRevB.96.045434) - Image potential states at chevron-shaped graphene nanoribbons/Au(111) interfaces; Phys. Rev. B, 91 (2015) 045428
C. Bronner, A. Haase, and P. Tegeder
(See online at https://doi.org/10.1103/PhysRevB.91.045428) - Tracking and Removing Br during the on-surface synthesis of a graphene nanoribbons; J. Phys. Chem. C, 119 (2015) 486
C. Bronner, J. Björk, and P. Tegeder
(See online at https://doi.org/10.1021/jp5106218) - Excitonic states in narrow armchair graphene nanoribbons on gold surfaces; J. Phys. Chem. C, 120 (2016) 26168
C. Bronner, D. Gerbert, A. Broska, and P. Tegeder
(See online at https://doi.org/10.1021/acs.jpcc.6b10834)
