RESUMEN
Graphene nanoribbons show exciting electronic properties related to the exotic nature of the charge carriers and to local confinement as well as atomic-scale structural details. The local work function provides evidence for such structural, electronic, and chemical variations at surfaces. Kelvin prove force microscopy can be used to measure the local contact potential difference (LCPD) between a probe tip and a surface, related to the work function. Here we use this technique to map the LCPD of graphene nanoribbons grown on a Au(111) substrate. The LCPD data shows charge transfer between the graphene nanoribbons and the gold substrate. Our results are corroborated with density functional theory calculations, which verify that the maps reflect the doping of the nanoribbons. Our results help to understand the relation between atomic structure and electronic properties both in high-resolution images and in the distance dependence of the LCPD.
RESUMEN
Electromigration in interconnects continues to be an important field of study in integrated circuits as the interconnects are planned to shrink in size at comparable pace as the semiconductor functional elements. Through shrinking the interconnects approach the regime where quantum size effects become important. The observation of quantum size and shell effects is usually restricted either to low-temperatures or vacuum conditions or to chemically inert materials such as Au. Here, we show that in electromigrated Cu nanocontacts such effects can be observed at room temperature and room pressure even in the presence of oxidation. Our data provide evidence that the nanocontacts are nearly spherical objects with a triangular-cylindrical symmetry of their electronic wave functions with a stronger free-electron-like character compared to previous results. We do not observe a detrimental effect of oxygen. The presence of shell effects has implications for the technological use of Cu nanocontacts as interconnects in integrated circuits and could lead to the use of electronic wave functions of shells in such interconnects.
RESUMEN
Perovskite photovoltaic (PV) cells have demonstrated power conversion efficiencies (PCE) that are close to those of monocrystalline silicon cells; however, in contrast to silicon PV, perovskites are not limited by Auger recombination under 1-sun illumination. Nevertheless, compared to GaAs and monocrystalline silicon PV, perovskite cells have significantly lower fill factors due to a combination of resistive and non-radiative recombination losses. This necessitates a deeper understanding of the underlying loss mechanisms and in particular the ideality factor of the cell. By measuring the intensity dependence of the external open-circuit voltage and the internal quasi-Fermi level splitting (QFLS), the transport resistance-free efficiency of the complete cell as well as the efficiency potential of any neat perovskite film with or without attached transport layers are quantified. Moreover, intensity-dependent QFLS measurements on different perovskite compositions allows for disentangling of the impact of the interfaces and the perovskite surface on the non-radiative fill factor and open-circuit voltage loss. It is found that potassium-passivated triple cation perovskite films stand out by their exceptionally high implied PCEs > 28%, which could be achieved with ideal transport layers. Finally, strategies are presented to reduce both the ideality factor and transport losses to push the efficiency to the thermodynamic limit.