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.
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