Motivation To date, nanosized objects, such as nanospheres, nanowires, nanotubes or nanobelts have been prepared from various materials, such as metals, ceramics, carbon or organic polymers. Two-dimensionally extended nanomaterials have been prepared from Langmuir–Blodgett method, e.g., from polyelectrolyte multilayers, with a thickness between 10 and 15 nm. Extremely flat and two-dimensionally extended objects have been termed “nanosheets” and prepared, for example, by exfoliation of layered inorganic materials, with a thickness down to 1 nm and lateral dimensions up to 100 um. They want a simple controlled preparation of individual, freestanding nanosheets with the thickness of a single biphenyl molecule.(electron-induced crosslinking of self- assembled monolayers and subsequent release of the material from its substrate)

Experimental Fabrication of SAMs: Fabrication of nanosheets: (electron-induced crosslinking) –During electron irradiation, carbon–hydrogen bonds are cleaved initially, followed by formation of carbon–carbon crosslinks between the aromatic units. –SAMs containing only aliphatic chains, such as alkanethiols on gold, are not crosslinked by electron irradiation but undergo C– C- and C–H-bond cleavage and desorption at low electron doses.

By local dissolution of the substrate under a crosslinked biphenyl monolayer, freestanding nanosheets with the thickness of a single molecule have been obtained. For this purpose, SAMs of II have been prepared on window-structured silicon wafers covered on one side with a 30 nm thick silicon nitride (SiNx) layer.

Conclusion They have prepared freestanding nanosheets with the thickness of a single biphenyl molecule and lateral dimensions in the micrometer range from crosslinked, aromatic self-assembled monolayers. Such unimolecular nanosheets could be useful as ultrasensitive sensors that change their vibrational characteristics or optical properties when only very few molecules are adsorbed or chemically attached. The chemical reactivity of the sheets can be selectively tuned by introduction of reactive substituents on either surface. They might also be utilized as ultrathin and highly transparent support materials for special microscopy methods, e.g., as carriers of other materials such as single protein molecules in high resolution transmission electron microscopy. Quite generally, freestanding nanosheets will be useful for the development of methods to handle such ultrathin, two-dimensional objects.