Graphene has attracted much interest in both academia and industry. The challenge of making it semiconducting is crucial for applications in electronic devices. A promising approach is to reduce its physical size down to the nanometer scale. Here, we present the surface-assisted bottom-up fabrication of atomically precise armchair graphene nanoribbons (AGNRs) with predefined widths, namely 7-, 14- and 21-AGNRs, on Ag(111) as well as their spatially resolved width-dependent electronic structures. STM/STS measurements reveal their associated electron scattering patterns and the energy gaps over 1 eV. The mechanism to form such AGNRs is addressed based on the observed intermediate products. Our results provide new insights into the local properties of AGNRs, and have implications for the understanding of their electrical properties and potential applications.

Understanding how molecules interact to form large-scale hierarchical structures on surfaces holds promise for building designer nanoscale constructs with defined chemical and physical properties. Here, we describe early advances in this field and highlight upcoming opportunities and challenges. Both direct intermolecular interactions and those that are mediated by coordinated metal centers or substrates are discussed. These interactions can be additive, but they can also interfere with each other, leading to new assemblies in which electrical potentials vary at distances much larger than those of typical chemical interactions. Earlier spectroscopic and surface measurements have provided partial information on such interfacial effects. In the interim, scanning probe microscopies have assumed defining roles in the field of molecular organization on surfaces, delivering deeper understanding of interactions, structures, and local potentials. Self-assembly is a key strategy to form extended structures on surfaces, advancing nanolithography into the chemical dimension and providing simultaneous control at multiple scales. In parallel, the emergence of graphene and the resulting impetus to explore 2D materials have broadened the field, as surface-confined reactions of molecular building blocks provide access to such materials as 2D polymers and graphene nanoribbons. In this Review, we describe recent advances and point out promising directions that will lead to even greater and more robust capabilities to exploit designer surfaces.

Solution-processed lead halide perovskites have established themselves as one of the most important absorber materials in solar cells with power conversion efficiencies now exceeding 22%. [1] Unfortunately, the over reliance on highly toxic Pb 2+ remains a key issue for widespread commercial applications. The significant concentration of Pb 2+ in high performing halide perovskites and its water solubility make it highly hazardous compound to the environment. [2] Another apparent issue is the inherent instability of lead-based halide perovskites in ambient atmosphere. [3] Although the stability of Pb-based halide perovskites has improved impressively in recent times along with the simultaneous development of passivation techniques, the fabrication of lead-based halide perovskites still requires stringent environmental control. [4] To address these potential issues, there is an increased interest toward leadfree halide perovskites and their analogues in photovoltaics. Replacing Pb 2+ with Sn 2+ or Ge 2+ could minimize the toxicity associated with lead-based halide perovskite; however, the increased environmental instability of these compounds poses significant challenges in solar cell development. [5] Even after incorporating 2D/3D mixtures of perovskites, the efficiency of the highest performing Sn-based system reduced to nearly 50% of its original value within 3 d under 20% humidity. [6] Considering atmospheric stability, trivalent cations such as bismuth and antimony-based ternary halides were also investigated as potential absorber materials due to their inherent atmospheric stability and low toxicity. [7] The incorporation of protonated cations such as MA or Cs with Bi-I octahedra forms Bi-based ternary halides (structural formula A 3 Bi 2 I 9 : A = Cs, MA) exhibits high absorption coefficients and facile solution processability. Nevertheless, the photovoltaic performances of Bi-based ternary halides remained poor mostly due to high optical bandgap and low electronic dimensionality. [8] Replacement of the A-site protonated cations with transition metals such as Ag or Cu is a promising strategy to improve the dimensionality. These transition metals also take part in bonding with Bi-I octahedra, resulting in complex halide bismuthates. In comparison to Bismuth-based ternary halides have recently gained a lot of attention as lead-free perovskite materials. However, photovoltaic performances of these devices remain poor, mostly due to their low-dimensional crystal structure and large bandgap. Here, a dynamic hot casting technique to fabricate silver bismuth iodide-based perovskite solar cells under an ambient atmosphere with power conversion efficiencies above 2.5% is demonstrated. Silver bismuth iodides are 3D analogs of complex ternary bismuth halides with a suitable bandgap for a single junction solar cell. As far as it is known, these results represent the highest efficiency for solution processed air-stable lead-free perovskite solar cells. The enhanced solar cell performance via this dynamic hot casting ...

Single-layer molybdenum disulfide (MoS) has attracted significant attention due to its electronic and physical properties, with much effort invested toward obtaining large-area high-quality monolayer MoS films. In this work, we demonstrate a reactive-barrier-based approach to achieve growth of highly homogeneous single-layer MoS on sapphire by the use of a nickel oxide foam barrier during chemical vapor deposition. Due to the reactivity of the NiO barrier with MoO, the concentration of precursors reaching the substrate and thus nucleation density is effectively reduced, allowing grain sizes of up to 170 μm and continuous monolayers on the centimeter length scale being obtained. The quality of the monolayer is further revealed by angle-resolved photoemission spectroscopy measurement by observation of a very well resolved electronic band structure and spin-orbit splitting of the bands at room temperature with only two major domain orientations, indicating the successful growth of a highly crystalline and well-oriented MoS monolayer.

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have revealed many novel properties of interest to future device applications. In particular, the presence of grain boundaries (GBs) can significantly influence the material properties of 2D TMDs. However, direct characterization of the electronic properties of the GB defects at the atomic scale remains extremely challenging. In this study, we employ scanning tunneling microscopy and spectroscopy to investigate the atomic and electronic structure of low-angle GBs of monolayer tungsten diselenide (WSe2) with misorientation angles of 3-6°. Butterfly features are observed along the GBs, with the periodicity depending on the misorientation angle. Density functional theory calculations show that these butterfly features correspond to gap states that arise in tetragonal dislocation cores and extend to distorted six-membered rings around the dislocation core. Understanding the nature of GB defects and their influence on transport and other device properties highlights the importance of defect engineering in future 2D device fabrication.