Ultraviolet-laser atom-probe tomographic three-dimensional atom-by-atom mapping of isotopically modulated Si nanoscopic layers

Using ultraviolet-laser assisted local-electrode atom-probe (UV-LEAP) tomography, we obtain three-dimensional (3D) atom-by-atom images of isotopically modulated S²⁸ i and S ³⁰ i ultrathin layers having thicknesses in the range of 5-30 nm. The 3D images display interfaces between the different monoisotopic layers with an interfacial width of ∼1.7 nm, thus demonstrating a significant improvement over isotope mapping achievable using secondary-ion mass-spectrometry or even visible laser-assisted atom-probe tomography. This sharpness is attributed to reduced thermal effects resulting from using a highly focused UV laser beam. Our findings demonstrate that UV-LEAP tomography provides the high accuracy needed to characterize, at the subnanometer scale, the emerging isotopically programmed nanomaterials.