Control of nanomaterial dimensions with atomic precision through synthetic methods is essential to understanding and engineering of nanomaterials. For single-layer inorganic materials, size and shape controls have been achieved by self-assembly and surface-catalyzed reactions of building blocks deposited at a surface. However, the scope of nanostructures accessible by such approach is restricted by the limited choice of building blocks that can be thermally evaporated onto surfaces, such as atoms or thermostable molecules. Herein this limitation is bypassed by using mass-selected molecular ions obtained via electrospray ionization as building blocks to synthesize nanostructures that are inaccessible by conventional evaporation methods. As the first example, micron-scale production of MoS2 and WS2 nanoribbons and their heterostructures on graphene are shown by the self-assembly of asymmetrically shaped building blocks obtained from the electrospray. It is expected that judicious use of electrospray-generated building blocks would unlock access to previously inaccessible inorganic nanostructures.|Bottom-up fabrication of materials inaccessible by conventional thermal sublimation techniques is demonstrated. Mass-selected ionic building blocks generated by electrospray ionization are soft-landed at a substrate, generating nanostructures that are subsequently examined by electron microscopy. As the first example, MoS and WS ions are used to show fabrication of micrometer-long and nanometer-wide MoS2 and WS2 nanoribbons and their heterostructures on graphene. image