Epitaxial single-crystalline CsPbBr3 perovskite films on mica, prepared ex-situ, are explored using a low-temperature scanning tunneling microscope (STM) by probing the unoccupied electronic states of their surface in ultra-high vacuum (UHV) at 80 K. Light-assisted STM measurements under a broadband illumination with visible light were employed to enhance and stabilize surface conductivity. STM imaging across the surface of macroscopic bulk CsPbBr3 films reveals large flat terraces characterized by a specific type of surface reconstruction, consisting of parallel rows of U-shaped atomic nanostructures. These structures are spaced by 12 angstroms and exhibit an internal periodicity of 5.1 angstroms. Density functional theory (DFT) calculations reproduce the experimental observations and reveal a competition between different orthorhombic CsPbBr3(110) surface reconstructions: a Cs-rich structure, identified as the most energetically stable, and three alternative PbBr rich reconstructions, which are slightly higher in energy yet remain consistent with the STM data. Additional analyses that explicitly account for the mica substrate exclude the cubic CsPbBr3 phase and other orthorhombic surface orientations, while showing that variations in the mica surface termination do not alter the preferred CsPbBr3(110) reconstruction. This combined approach thereby confirms our assignment and resolves previous STM interpretations of CsPbBr3.
preferred CsPbBr₃(110) reconstruction. This combined approach thereby confirms our assignment and resolves previous STM interpretations of CsPbBr₃.
Metal-halide perovskites (MHPs) have emerged as a transformative class of materials across a wide spectrum of scientific and technological domains 1 . These materials are characterized by an ABX3 crystal structure, where an inorganic sublattice of corner sharing metal-halide octahedra (e.g., PbI6 -or PbBr6 -) and another sublattice of inorganic or organic cations (e.g., Cs + or CH3NH3 + ) together form a perovskite structure that exhibits diverse optical, electrical, and structural properties 2,3,4 . These properties have propelled applications of perovskites in photovoltaics 5,6 , light-emitting diodes (LEDs) 7 , photocatalysis 8 , and sensors 9 .
Particularly, their high absorption coefficients 10 , long carrier diffusion lengths 2,11 , and facile synthesis make them a versatile playground for research on the next-generation optoelectronic devices 12 .
In recent years, the research focus on functional semiconducting materials and devices has significantly shifted towards the role of surfaces and interfaces because of their influence on device performance, especially at the nanoscale 13,14,15,16 . Surface states can govern charge recombination 17 , stability, and interfacial interactions. Engineering surface chemistry has thus become pivotal for achieving higher efficiencies and stability in practical applications 18 . Among the broad family of halide perovskites, cesium lead bromide (CsPbBr3) has drawn particular attention due to its superior thermal and structural stability as compared to hybrid (organicinorganic) counterparts 19,20 .
As a fully inorganic perovskite, CsPbBr3 combines a relatively wide bandgap (~ 2.3 eV) with a high photoluminescence quantum yield and robust environmental tolerance. These characteristics make it especially suitable for blue-green light emission and stable photodetector platforms 21 . Moreover, the nanostructuring of CsPbBr3 into quantum dots, nanowires, or thin films has opened exciting avenues in nanoelectronics 22 , where quantum confinement effects, surface-to-volume ratios, and doping are critical 23,24 .
The surface properties of CsPbBr3 films play a crucial role in governing the charge transport properties, interfacial energy alignment, and defect passivation, the key aspects of highperformance electronic devices. By tailoring surface termination or introducing passivating agents, one can, in principle, improve the charge carrier mobility 25,26,27 and device stability 17 .
Thus, from the perspective of the fundamental surface science and cutting-edge electronic design, CsPbBr3 offers a versatile platform for basic and applied research in novel optoelectronic systems 28,29,30 .
Low-temperature (LT) studies of bulk CsPbBr3 are expected to benefit from reduced thermal and electronic noise, as well as improved charge transport, as cooling suppresses ionic motion, reduces phonon scattering, sharpens optical features, 31 leads to an enhanced charge carrier mobility 28 , thus helping to stabilize scanning probe measurements. At cryogenic temperatures, the orthorhombic crystal structure becomes well-defined, enabling the reliable identification of surface terminations and reconstructions. Indeed, cooling CsPbBr3 to 80 K stabilizes the γ-orthorhombic phase, thermodynamically favoring specific distorted PbBr6 octahedral structures 32 . In this context, LT-STM can provide an enhanced spatial resolution and allows a direct mapping of surface and coordination environments with minimal thermal noise.
Therefore, the insights on the nanoscale surface structure obtained with LT-STM imaging would be crucial for the understanding of the intrinsic (i.e., not dominated by static disorder) surface reconstruction and charge transport properties of bulk CsPbBr3 including low-threshold, single-crystal CsPbBr3 field-effect transistors (FETs) 28 .
In this article, we investigate the atomic-scale surface structure of bulk CsPbBr3 perovskite at low temperature (80 K). We use single-crystalline perovskite samples epitaxially grown on mica substrates, mounted on special sample holders compatible with LT-STM measurements.
Due to the low conductivity of pristine CsPbBr3 in the dark, we generate a population of charge carriers in the conduction band by illuminating the samples with a broad-band white light during STM scanning.
Imaging the bulk CsPbBr3 surface with LT-STM reveals a clear and dominant U-shaped rows surface reconstruction, which is observed at the surface of macroscopically large samples.
The surface exhibits step edges and coalesced step structures with relative height differences of STM imaging of the boundaries between adjacent U-shaped domains further reveals a transitional region that connects neighboring terraces, indicating that the observed reconstructions need to accommodate domain matching at their interfaces. When the
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