Graphene nanoribbons (GNRs) are excellent candidates for next-generation electronic materials. Unlike GNRs produced by "top-down" methods such as lithographical patterning of graphene and unzipping of carbon nanotubes that cannot reach structural perfection, the fabrication of structurally well-defined GNRs has been achieved by a "bottom-up" organic synthesis via solution-mediated or surface-assisted cyclodehydrogenation. Specifically, non-planar polyphenylene precursors were first "build up" from small molecules, and then "graphitized" and "planarized" to yield GNRs. However, fabrication of processable and longitudinally well-extended GNRs has remained a major challenge. Here we report a "bottom-up" solution synthesis of long (>200 nm), liquid-phase processable GNRs with well-defined structure and a large optical bandgap of 1.88 eV. Scanning probe microscopy demonstrates self-assembled monolayers of GNRs, and non-contact, time-resolved Terahertz conductivity measurements reveal excellent charge-carrier mobility within individual GNRs. Such structurally well-defined GNRs offer great opportunities for fundamental studies into graphene nanostructures, as well as development of GNR-based nanoelectronics.DOI: 10.1038/NCHEM.1819 http://www.nature.com/nchem/journal/v6/n2/abs/nchem.1819.html 2 Graphene nanoribbons (GNRs), defined as nanometre-wide strips of graphene, are attracting increasing attention as highly promising candidates for next generation semiconductor materials 1,2,3,4 . Quantum confinement effects impart GNRs with semiconducting properties, i.e. with a finite bandgap, which critically depends on the ribbon width and its edge structure 1,3 . Fabrication of GNRs has been primarily carried out by "top-down" approaches such as lithographical patterning of graphene 5,6 and unzipping of carbon nanotubes 7,8 , revealing their semiconducting nature and excellent transport properties 1 . However, these methods are generally limited by low yields and lack of structural precision, leading to GNRs with uncontrolled edge structures.In contrast, a "bottom-up" chemical synthetic approach based on solution-mediated 9,10,11,12,13 or surface-assisted 14 cyclodehydrogenation, namely "graphitization" and "planarization", of tailor-made three-dimensional polyphenylene precursors offers an appealing strategy for making structurally well-defined and homogeneous GNRs. The polyphenylene precursors are built up from small molecules, and thus their structures can be tailored within the capabilities of modern synthetic chemistry 15 . However, GNRs (>30 nm) produced by solution-mediated methods have been precluded from unambiguous structural characterization, i.e. microscopic visualization, due to their limited processability 9,12 . On the other hand, GNRs produced by the surface-assisted protocol have been characterized to be atomically precise using scanning tunnelling microscopy (STM) 14 . Nevertheless, this method can only provide a limited amount of GNR material, which is further bound to a metal surface, impeding wide...

The conversion of light into free electron-hole pairs constitutes the key process in the fields of photodetection and photovoltaics. The efficiency of this process depends on the competition of different relaxation pathways and can be greatly enhanced when photoexcited carriers do not lose energy as heat, but instead transfer their excess energy into the production of additional electronhole pairs via carrier-carrier scattering processes. Here we use Optical pump -Terahertz probe measurements to show that in graphene carrier-carrier scattering is unprecedentedly efficient and dominates the ultrafast energy relaxation of photoexcited carriers, prevailing over optical phonon emission in a wide range of photon wavelengths. Our results indicate that this leads to the production of secondary hot electrons, originating from the conduction band. Since hot electrons in graphene can drive currents, multiple hot carrier generation makes graphene a promising material for highly efficient broadband extraction of light energy into electronic degrees of freedom, enabling highefficiency optoelectronic applications. I. MAIN TEXTFor many optoelectronic applications, it is highly desirable to identify materials in which an absorbed photon is efficiently converted to electronic excitations. The unique properties of graphene, such as its gapless band structure, flat absorption spectrum [1] and strong electron-electron interactions [2], make it a highly promising material for efficient broadband photon-electron conversion [3]. Indeed, recent theoretical work has anticipated that in graphene multiple electron-hole pairs can be created from a single absorbed photon during energy relaxation of the primary photoexcited e-h pair [4,5]. A photo-excited carrier relaxes initially trough two competing pathways: carrier-carrier scattering and optical phonon emission. In the former process the energy of photoexcited carriers remains in the electron system, being transferred to secondary electrons that gain energy (become hot), whereas in the phonon emission process the energy is lost to the lattice as heat. While recent experiments have shown that photoexcitation of graphene can generate hot carriers [6,7], it remains unknown how efficient this process is with respect to optical phonon emission.Here we study the energy relaxation process of the primary photoexcited e-h pair in doped single-layer graphene. In particular, we quantify the branching ratio between the two competing relaxation pathways. Given the challenging timescale with which these processes occur, we employ an ultrafast Optical pump -Terahertz (THz) probe measurement technique, where we exploit the variation of the photon energy of the pump light. Changing this photon energy is crucial as it allows us to prepare the system with photoexcited carriers having a prescribed initial energy determined by the photon energy, and follow the ensuing energy relaxation dynamics. We show experimentally, in combination with theoretical modeling, that carrier-carrier scattering is the dominant r...

The nature of the photoconductivity in solution-processed films of methylammonium lead iodide perovskite is investigated by determining the variation of the photoconductive response with temperature. Ultrabroadband terahertz (THz) photoconductivity spectra in the 0.3-10 THz range can be reproduced well by a simple Drude-like response at room temperature, where free charge carrier motion is characterized by an average scattering time. The scattering time determined from Drude fits in the 0.3-2THz region increases from ∼4 fs at 300 K (tetragonal phase; mobility of ∼27 cm(2) V(-1) s(-1)) to almost ∼25 fs at 77 K (orthorhombic phase, mobility of ∼150 cm(2) V(-1) s(-1)). For the tetragonal phase (temperature range 150< T < 300 K) the scattering time shows a ∼T(-3/2) dependence, approaching the theoretical limit for pure acoustic phonon (deformation potential) scattering. Hence, electron-phonon, rather than impurity scattering, sets the upper limit on free charge transport for this perovskite.