Observing the intricate chemical transformation of an individual molecule as it undergoes a complex reaction is a long-standing challenge in molecular imaging. Advances in scanning probe microscopy now provide the tools to visualize not only the frontier orbitals of chemical reaction partners and products, but their internal covalent bond configurations as well. We used noncontact atomic force microscopy to investigate reaction-induced changes in the detailed internal bond structure of individual oligo-(phenylene-1,2-ethynylenes) on a (100) oriented silver surface as they underwent a series of cyclization processes. Our images reveal the complex surface reaction mechanisms underlying thermally induced cyclization cascades of enediynes. Calculations using ab initio density functional theory provide additional support for the proposed reaction pathways.

Photovoltaic devices based on hybrid perovskite materials have exceeded 22% e ciency due to high charge-carrier mobilities and lifetimes. Properties such as photocurrent generation and open-circuit voltage are influenced by the microscopic structure and orientation of the perovskite crystals, but are di cult to quantify on the intra-grain length scale and are often treated as homogeneous within the active layer. Here, we map the local short-circuit photocurrent, open-circuit photovoltage, and dark drift current in state-of-the-art methylammonium lead iodide solar cells using photoconductive atomic force microscopy. We find, within individual grains, spatially correlated heterogeneity in short-circuit current and open-circuit voltage up to 0.6 V. These variations are related to di erent crystal facets and have a direct impact on the macroscopic power conversion e ciency. We attribute this heterogeneity to a facet-dependent density of trap states. These results imply that controlling crystal grain and facet orientation will enable a systematic optimization of polycrystalline and single-crystal devices for photovoltaic and lighting applications. Photocurrent microscopy to probe local e ciency We used two sets of methylammonium lead iodide chloride (MAPbI 3−x Cl x) thin films, which were processed in parallel. One set was used to fabricate planar solar cells by depositing a hole transport layer (HTL, spiro-OMeTAD) and a gold top contact, resulting in an

We provide direct evidence for the existence of isolated, one-dimensional charge density waves at mirror twin boundaries (MTBs) of single-layer semiconducting MoSe 2 . Such MTBs have been previously observed by transmission electron microscopy and have been predicted to be metallic in MoSe 2 and MoS 2 1-7. Our low-temperature scanning tunnelling microscopy/spectroscopy measurements revealed a substantial bandgap of 100 meV opening at the Fermi energy in the otherwise metallic one-dimensional structures. We found a periodic modulation in the density of states along the MTB, with a wavelength of approximately three lattice constants. In addition to mapping the energy-dependent density of states, we determined the atomic structure and bonding of the MTB through simultaneous high-resolution non-contact atomic force microscopy. Density functional theory calculations based on the observed structure reproduced both the gap opening and the spatially resolved density of states.Properties of two-dimensional (2D) transition metal dichalcogenides (TMDs) are highly sensitive to the presence of defects, and a detailed understanding of their structure may lead to tailoring of material properties through 'defect engineering' . Intrinsic defects have been studied extensively in graphene [8][9][10][11][12] . Defects in 2D semiconductors have been explored to a lesser extent, but are expected to substantially modify material properties. 2D TMD semiconductors are particularly interesting because they exhibit direct bandgaps in the visible range [13][14][15] , high charge-carrier mobility 16,17 , extraordinarily enhanced light-matter interactions [18][19][20][21] and potential applications in novel optoelectronic devices 22,23 . Individual atomic-scale defects in 2D TMDs are expected to modify charge transport 24 or introduce ferromagnetism 25 , whereas one-dimensional defects such as grain boundaries and edges may alter electronic 1 and optical properties 1,26 , and introduce magnetic 27 or catalytic 28,29 functionality. Here we report the direct observation of one-dimensional (1D) charge density waves (CDWs) intrinsic to the conducting MTBs of monolayer MoSe 2 . A 1D CDW is a macroscopic quantum state, where atoms in a 1D metallic system relax and break translational symmetry to reduce total energy by opening a small bandgap at the Fermi energy (E F ) and modulating the charge density at the periodicity of the lattice distortion 30,31 . Although CDW order has been observed in 2D TMD metals such as NbSe 2 and TiSe 2 at low temperature 32,33 , CDWs have not previously been associated with 2D TMD semiconductors.Most studies of 1D CDWs have been performed on ensembles of CDWs in conducting polymers, quasi-one-dimensional metals or self-assembled atomic chains adsorbed on semiconducting surfaces, where inter-CDW coupling can significantly impact CDW properties [34][35][36][37][38] . The CDWs observed here are electronically isolated from one another, and have truly one-dimensional character, forming an atomically precise model system t...