Nanosheet

A nanosheet is a two-dimensional nanostructure with thickness in a scale ranging from 1 to 100 nm.^[1][2] A typical example of nanosheet is graphene, the thinnest two-dimensional material (0.34 nm) in the world.^[3] It consists of a single layer of carbon atoms with hexagonal lattices.

Synthesis

TEM image of PbO nanosheets with highly symmetric edge length. The edges of PbO nanosheets are surrounded with Au NPs seeds.^[4]

3D AFM topography image of multilayered palladium nanosheet on silicon wafer.^[5]

The most commonly used nanosheet synthesis methods use a bottom-up approach, e.g., pre-organization and polymerization at interfaces like Langmuir–Blodgett films,^[6] solution phase synthesis and chemical vapor deposition (CVD).^[7] For example, CdTe (cadmium telluride) nanosheets could be synthesized by precipitating and aging CdTe nanoparticles in deionized water.^[8] The formation of free-floating CdTe nanosheets was due to directional hydrophobic attraction and anisotropic electrostatic interactions caused by dipole moment and small positive charges. Molecular simulations through a coarse-grained model with parameters from semi-empirical quantum mechanics calculations can be used to prove the experimental process.

Ultrathin single-crystal PbS (lead sulfur) sheets with micro scale in x-, y- dimensions can be obtained using a hot colloidal synthesis method.^[9] Compounds with linear chloroalkanes like 1,2-dichloroethane containing chlorine were used during the formation of PbS sheets. PbS ultrathin sheets probably resulted from the oriented attachment of the PbS nanoparticles in a two-dimensional fashion. The highly reactive facets were preferentially consumed in the growth process that led to the sheet-like PbS crystal growth.

Nanosheets can also be prepared at room temperature. For instance, hexagonal PbO (lead oxide)) nanosheets were synthesized using gold nanoparticles as seeds under room temperature.^[4] The size of the PbO nanosheet can be tuned by gold NPs and Pb2+
concentration in the growth solution. No organic surfactants were employed in the synthesis process. Oriented attachment, in which the sheets form by aggregation of small nanoparticles that each has a net dipole moment,^[10] and ostwald ripening^[11] are the two main reasons for the formation of the PbO nanosheets.

Carbon nanosheets have been produced using industrial hemp bast fibres with a technique that involves heating the fibres at over 350F (180C) for 24 hours. The result is then subjected to intense heat causing the fibers to exfoliate into a carbon nanosheet. This has been used to create an electrode for a supercapacitor with electrochemical qualities ‘on a par with’ devices made using graphene.^[12]

Metal nanosheets have also been synthesized from solution-based method by reducing metal precursors, including palladium,^[13] rhodium,^[14] and gold.^[15]

See also

• Graphene
• Two-dimensional polymer
• Colloidal gold
• Nanocrystal solar cell
• Nanoparticle
• Quantum dot
• Langmuir–Blodgett film

References