Switching from reactant to substrate engineering in the selective synthesis of graphene nanoribbons

The challenge of synthesizing graphene nanoribbons (GNRs) with atomic precision is currently being pursued along a one-way road, based on the synthesis of adequate molecular precursors that react in predefined ways through self-assembly processes. The synthetic options for GNR generation would multiply by adding a new direction to this readily successful approach, especially if both of them can be combined. We show here how GNR synthesis can be guided by an adequately nanotemplated substrate instead of by the traditionally designed reactants. The structural atomic precision, unachievable to date through top-down methods, is preserved by the self-assembly process. This new strategy’s proof-of-concept compares experiments using 4,4′′-dibromo-para-terphenyl as a molecular precursor on flat Au(111) and stepped Au(322) substrates. As opposed to the former, the periodic steps of the latter drive the selective synthesis of 6 atom-wide armchair GNRs, whose electronic properties have been further characterized in detail by scanning tunneling spectroscopy, angle resolved photoemission, and density functional theory calculations.The project leading to this publication has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (Grant Agreement No 635919), from the Spanish Ministry of Economy, Industry and Competitiveness (MINECO, Grant No. MAT2016-78293-C6), from the Basque Government (Grant No. IT-621-13), from the regional Government of Aragon (RASMIA Project), and from the University of Padova (Grant CPDA154322, Project AMNES).Peer reviewe

Shifting from reactant to substrate engineering in the selective synthesis of graphene nanoribbons

Resumen del trabajo presentado al Symposium on Surface Science (3S), celebrado en St. Christoph am Arlberg (Austria) del 25 de febrero al 10 de marzo de 2018.The challenge of synthesizing graphene nanoribbons (GNRs) with atomic precision is currently being pursued along a one-way road, based on the synthesis of adequate molecular precursors that react in predefined ways and form GNRs through self-assembly processes. Adding a new direction to this readily successful approach, the synthetic options for GNRs are expected to multiply, especially if both approaches can be combined. We show here how, still based on self-assembly to maintain the atomic precision not yet achievable with top-down methods, selectivity in the GNR synthesis can be guided, instead of by designed reactants, by an adequately nanotemplated substrate. This new strategy´s proof-of-concept compares experiments using 4,4´´-dibromo-para-terphenyl as molecular precursor on flat Au(111) and stepped Au(322) substrates. On Au(111), the reactant first polymerizes into poly-para-phenylene (PPP) at moderate temperatures below 200 ºC. Annealing to higher temperatures drives the lateral fusion of PPP chains through cyclo-dehydrogenation, ending up with graphene nanoribbons of varying width depending on the number of polymers involved. The resulting samples are thus interesting to study width-dependent properties and phenomena in GNRs. We have used such samples to characterize the width-dependent band gap of GNRs and, most importantly, the associated evolution of the energy level alignment of frontier bands. Doing so, a Fermi level pinning scenario of the valence band has been found for GNRs displaying band gaps below ~1.7 eV.[1] However, from the synthetic point of view, selectivity towards particular GNRs is completely missing. A completely different scenario is found on Au(322). The polymerization and cyclodehydrogenation reactions occur in a similar way. However, the periodic steps of Au(322) limits the number of fusing polymers to two, resulting in the selective synthesis of 6 atomwide armchair GNRs (6-aGNRs). In addition, all GNRs being uniaxially aligned along the substrate´s steps, we have been able to characterize the sample´s valence band properties by angle resolved photoemission spectroscopy. Complemented with scanning tunneling spectroscopy measurements and density functional theory calculations, we end up with a fully coherent and complete picture of the electronic properties of 6-aGNRs.Support by the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No 635919), from the Spanish Ministry of Economy, Industry and Competitiveness (MINECO, Grant No. MAT2016-78293-C6), and from the Basque Government (Grant No. IT-621-13) is gratefully acknowledged.Peer reviewe

Transfer reactions at the neutron dripline with triton target

Two-neutron transfer to $^{9}$Li will populate the ground state of $^{11}$Li as well as low-lying resonances in a way that is complementary to studies of these states performed at higher beam energies. We aim at detecting the charged particles from the transfer reactions as well as neutrons coming from the decay of possible $^{11}$Li resonances

In-gas-jet spectroscopy of actinium isotopes

International audienc

Test of FAZIA prototypes at LNS.

The response of a few silicon-silicon-CsI(Tl) and silicon-silicon telescopes with high quality detectors developed within the FAZIA collaboration [1] is tested in this work. The silicon detectors were manufactured from “random cut” wafers to avoid channeling effects and are characterized by a high dopant homogeneity. One siliconsilicon telescope was mounted on a rotating platform to compare its response in case of front and rear injection. Another silicon detector was mounted on a motorized support, sliding to angles very close to the beam (~0.5°), in order to measure the effects of radiation damage on energy resolution and PSA. Beams of 84Kr and 129Xe at 35A MeV, impinging on targets of natNi, 93Nb, 120Sn and Au, produced fragments over a large range of charge, mass and energy. The aim was to explore the capabilities of various solutions exploiting the digital techniques of Pulse Shape Analisys (PSA) for the Z and A identification of stopped ions. It has been found that PSA is able to fully discriminate the charge of stopped ions up to the maximum available Z (that of the beam, Z=54). The ΔE-E correlations of the first two silicon detectors can separate all the nuclides up to Z~25 and no difference in resolution between front and rear injection is observed. The experimental data also provide some preliminary information about the effects of radiation damage on energy resolution and PSA for high fluences of heavy ions