Graphene Films and Carbon Nanotubes Manufactured Using Molecular Dynamics Without Expensive Post Processing by Utah Scientists

University of Utah researchers have developed a method to form carbon nanotubes and graphene films without using expensive and time consuming post processing systems. The width is in the range of 1 to 20 nm. The length of the films, while not critical to the method, is generally many times greater than the width of the nanoribbon.

The University Of Utah Research Foundation, in U.S. Patent Application 20100119434, discloses methods for the formation of nanotube from thin films. The methods involve the adsorption of atoms to the surface of the films. The adsorbed atoms introduce surface stress, inducing a curvature in the films. The curvature is sufficient to bring atoms at the edges into sufficiently close proximity to form covalent bonds. Other methods include the step of desorbing the atoms from the surface of the film.

The films may comprise a variety of nanomaterials, including graphene sheets and semiconductor thin films, according to inventors Feng Liu, Professor, Department of Materials Science and Engineering, University of Utah, and Decai Yu. The atomic adsorption techniques may also be used to manufacture silicon films. 

The methods for forming nanotubes from thin films use atomic adsorption to induce bending of the films. When the film comprises a single graphene sheet (i.e., a "graphene nanoribbon"), the methods may be used to form single-walled carbon nanotubes (SWNTs). Because graphene nanoribbons can be patterned onto substrates with predefined widths and directions, the size and chirality of the resulting SWNTs may be controlled. Thus, the methods developed at the University of Utah eliminate the need for any post-synthesis steps, allow mass production of nanotubes of uniform size and chirality and offer easy integration of the nanotubes into nanodevices on a substrate.

In the method, a thin film is patterned onto a substrate. Atoms, such as hydrogen (H) and fluorine (F) atoms, are adsorbed to the surface of the film, inducing surface stress that bends the film downward, away from the adsorbed atoms. The radius of curvature of the nanotubes may be controlled through the selection of appropriate adsorbants and surface coverages. When the free ends of the film meet each other or the free ends of another thin film, chemical bond (such as covalent bond) linkages are formed between the edges and a nanotube is formed. The adsorbed atoms may then be desorbed from the surface of the resulting nanotube. 

FIG. 1 shows the formation of SWNTs using first-principles molecular dynamics ("MD") simulations. A 1.7 nm wide graphene nanoribbon is patterned onto a graphite substrate (a(1)). H atoms are adsorbed to a surface coverage of 50% (a(2)). The adsorbed atoms cause the nanoribbon to bend into a SWNT (a(3)-(5)). FIG. 1b (1)-(5) show the formation of a SWNT from a 2.0 nm wide graphene nanoribbon and adsorption of F atoms to a surface coverage of 45%. 

FIG. 2 shows the formation of SWNTs using classical MD simulations. A 1.6-nm-wide (3.5, 3.5) graphene nanoribbon is patterned onto a graphite substrate and H atoms are adsorbed to a surface coverage of 50% (a(1)). The adsorbed atoms cause the nanoribbon to bend into a (4, 4) armchair SWNT (a(2)-(3)). The adsorbed atoms are desorbed at high temperature (a(4)-(6)). FIG. 2c shows a sideview of the resulting armchair SWNT. FIG. 2b (1)-(6) show the formation of a (14, 0) zigzag SWNT from a 3.3-nm-wide (13, 0) graphene nanoribbon and adsorption of H atoms to a surface coverage of 45%. FIG. 2d is a sideview of the resulting zigzag SWNT.

The films may be defined using semiconductor processing techniques, such as lithography, patterning and etching. This is advantageous because it provides an inexpensive parallel process capable of making many identical, or different, sized films in a single run. In the case of graphite, stacks of many layers of graphene may be defined in the substrate. In this embodiment, each graphene layer in the stack provides a separate thin film.

After the thin film has been formed, atoms are adsorbed to its surface. Atoms or combinations of atoms may be adsorbed onto the films. Many different atoms may be used for this purpose. The only requirement is that the selected atoms introduce a surface stress on the thin film that induces a curvature in the film when they are absorbed onto the film. As a result, film edges are brought together, where they undergo covalent or other chemical bonding to form nanotubes. In some embodiments, the adsorbed atoms are selected from the group consisting of H atoms, F atoms, and combinations thereof. The adsorption step can provide different surface coverages of adsorbed atoms. Because the degree of surface coverage will affect the radius of curvature, the degree of surface coverage will depend on the desired diameter of the nanotubes. In some embodiments, the surface coverage is in the range of about 40% to 60%. The adsorption step may be accomplished at room temperature.

Once the nanotube has formed, the adsorbed atoms are desorbed from the surface of the film. Desorption may be accomplished at a high temperature. For example, the desorption of H atoms from a graphene sheet may be carried out at 600 K or higher.

 Without wishing to be bound to a particular theory, Liu and Yu hypothesized that the driving force for the formation of nanotubes from films is the stress induced by the adsorption of atoms on the surface of the films. For a given atomic surface coverage, the thinner the film, the larger the bending curvature, and the smaller the radius of the bending curvature.  

Recent efforts have been devoted to developing technically and economically viable methods for synthesizing carbon nanotubes, which exhibit a wealth of fascinating electrical, optical, and mechanical properties and have a broad range of applications. Currently, there are three major methods for forming carbon nanotubes, including arc discharge, laser ablation, and chemical vapor deposition. Each of the existing methods of carbon nanotube formation is based on bottom-up, self-assembled growth processes that are stochastic in nature.

These stochastic synthetic processes inherently lack control over carbon nanotube size and chirality. Therefore, post-synthesis separation, purification, and sorting of carbon nanotubes are often required, resulting in additional technical difficulties and higher production costs. Although recent progress with chemical vapor deposition methods has led to the production of well-aligned and ordered carbon nanotubes with better control over tube diameter and chirality, synthesizing mass quantities of carbon nanotubes with uniform size and chirality remains a great challenge. A deterministic synthetic approach to carbon nanotube formation, in which the size and chirality of the carbon nanotubes may be predefined, had not been previously accomplished.