Unveiling a √2 a×√2 a superstructure in optimally doped bismuth-based cuprates with non-contact atomic force microscopy

Non-contact atomic force microscopy, bolstered by strategic functionalization of the tip apex with inorganic molecules such as carbon monoxide (CO), has emerged as a powerful tool for atomic-resolution imaging. Here, we employ this methodology to elucidate the surface topography of the BiO plane in optimally dopedBi_2 Sr_2 CaCu_2 O_(8+δ). Single crystal was cleaved and imaged under ultra-high vacuum conditions at 4.5K. This approach uncovers the precise positioning of Bi atoms, thereby exposing the intrinsic structure of the BiO surface. We present a detailed representation of the BiO lattice using constant-height non-contact atomic force microscopy. With the CO-functionalized tip, a √2×√2 superlattice structure on the BiO plane was observed upon decreasing ∆z in constant height mode, a phenomenon hitherto unreported, while the W metal tip reveals only a 1x1 structure of the BiO surface.