Photodriven dipole reordering of the intercalated organic molecules in halide perovskites has been suggested to be a critical degree of freedom, potentially affecting physical properties, device performance, and stability of hybrid perovskite-based optoelectronic devices. However, thus far a direct atomically resolved dipole mapping under device operation condition, that is, illumination, is lacking. Here, we map simultaneously the molecule dipole orientation pattern and the electrostatic potential with atomic resolution using photoexcited cross-sectional scanning tunneling microscopy and spectroscopy. Our experimental observations demonstrate that a photodriven molecule dipole reordering, initiated by a photoexcited separation of electron–hole pairs in spatially displaced orbitals, leads to a fundamental reshaping of the potential landscape in halide perovskites, creating separate one-dimensional transport channels for holes and electrons. We anticipate that analogous light-induced polarization order transitions occur in bulk and are at the origin of the extraordinary efficiencies of organometal halide perovskite-based solar cells as well as could reconcile apparently contradictory materials’ properties.

Repetitive DNA sequences are ubiquitous in life, and changes in the number of repeats often have various physiological and pathological implications. DNA repeats are capable of interchanging between different noncanonical and canonical conformations in a dynamic fashion, causing configurational slippage that often leads to repeat expansion associated with neurological diseases. In this report, we used single-molecule spectroscopy together with biophysical analyses to demonstrate the parity-dependent hairpin structural polymorphism of TGGAA repeat DNA. We found that the DNA adopted two configurations depending on the repeat number parity (even or odd). Transitions between these two configurations were also observed for longer repeats. In addition, the ability to modulate this transition was found to be enhanced by divalent ions. Based on the atomic structure, we propose a local seeding model where the kinked GGA motifs in the stem region of TGGAA repeat DNA act as hot spots to facilitate the transition between the two configurations, which may give rise to disease-associated repeat expansion.DNA tandem repeats | DNA slippage | single-molecule spectroscopy | X-ray crystallography D NA replication is a crucial process in all living organisms. Mishaps in the replication process generally lead to deleterious consequences but also drive biological evolution (1). Changes in the number of tandem copies of a specific DNA sequence within the genome are associated with devastating neuropathies and various types of cancer (2, 3). On the other hand, these changes also help shape normal genomic features such as microsatellite polymorphism, which are often used as markers for population biology studies (4).The unit sizes of repetitive DNA sequences involved in repeat number changes range from a single base (e.g., microsatellites) to dodecanucleotides (12 bases, e.g., in progressive myoclonic epilepsy type 1) (5, 6). DNA slippage is believed to be a primary mechanism driving the change in repeat number of various unit sizes. Repetitive DNA sequences often form alternative structures such as bulges and hairpin loops in addition to canonical DNA conformations (7,8). A repeat unit may slip between being part of a hairpin loop, a bulge, or a duplex in a dynamic fashion, which may alter the course of normal cellular DNA chemistry and ultimately lead to repeat expansion associated with neurological diseases (9). (TGGAA) n repeats, for example, may form noncanonical structures such as a hairpin arm (10, 11) or an antiparallel duplex (12). Expansion of this pentanucleotide sequence has been associated with spinocerebellar ataxia 31 (SCA31), an adult-onset autosomal-dominant neurodegenerative disorder (13).In this article, we probed the conformational heterogeneity and stability of hairpins composed of repetitive TGGAA sequences using single-molecule fluorescence resonance energy transfer [single-molecule FRET (smFRET)] spectroscopy and X-ray crystallography as primary tools. Remarkably, we were able to detect two distinct hairpin confi...

Defect engineering represents a significant approach for atomically thick 2D semiconductor material development to explore the unique material properties and functions. Doping‐induced conversion of conductive polarity is particularly beneficial for optimizing the integration of layered electronics. Here, controllable doping behavior in palladium diselenide (PdSe2) transistor is demonstrated by manipulating its adatom‐vacancy groups. The underlying mechanisms, which originate from reversible adsorption/desorption of oxygen clusters near selenide vacancy defects, are investigated systematically via their dynamic charge transfer characteristics and scanning tunneling microscope analysis. The modulated doping effect allows the PdSe2 transistor to emulate the essential characteristics of photo nociceptor on a device level, including firing signal threshold and sensitization. Interestingly, electrostatic gating, acting as a neuromodulator, can regulate the adaptive modes in nociceptor to improve its adaptability and perceptibility to handle different danger levels. An integrated artificial nociceptor array is also designed to execute unique image processing functions, which suggests a new perspective for extension of the promise of defect engineered 2D electronics in simplified sensory systems toward use in advanced humanoid robots and artificial visual sensors.

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This work demonstrates the direct visualization of atomically resolved quantum-confined electronic structures at organic−inorganic heterointerfaces of twodimensional (2D) organic−inorganic hybrid Ruddlesden−Popper perovskites (RPPs); this is accomplished with scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) by using solvent engineering to prepare perpendicularly oriented 2D RPPs. Atomically resolved band mapping images across the organic−inorganic interfaces of 2D RPPs yield typical quantum-well-like type-I heterojunction band alignment with band gaps depending on the thicknesses or n values of the inorganic perovskite slabs. The presence of edge states within the band gap due to organic cation vacancies is also observed. In addition, real-space visualization of atomic-scale structural phase transition behavior and changes in local electronic band structures are obtained simultaneously. Our results provide an unequivocal observation and explanation of the quantumconfined electronic structures formed at organic−inorganic interfaces of 2D RPPs.