Which structure is a graphene sheet rolled into a tube or sphere?

A graphene sheet rolled into a tube or sphere forms a structure known as a fullerene. Fullerenes are a class of carbon allotropes that can take the shape of a hollow sphere, ellipsoid, or tube. The most famous example of a fullerene is the buckminsterfullerene, which resembles a soccer ball and consists of 60 carbon atoms arranged in a spherical structure.

The formation of fullerenes from graphene is significant in nanotechnology, as it demonstrates how two-dimensional materials can be transformed into three-dimensional structures with unique properties. These properties include the ability to conduct electricity and heat, as well as potential applications in drug delivery, materials science, and nanomedicine.

Other options provided refer to different carbon structures. Quantum dots are semiconductor particles with quantum mechanical properties. Carbon fibers are long, thin strands of carbon that are known for their strength and lightweight characteristics. Graphite is a form of carbon where carbon atoms are arranged in layers, each layer consisting of graphene sheets stacked together, but it does not form tubes or spheres as fullerenes do.