METHOD OF FORMING NANOTUBES
Methods for the formation of nanotube from thin films are provided. 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.
This invention pertains generally to the formation of nanotubes from thin films, such as graphene sheets and silicon films, using atomic adsorption.BACKGROUND OF THE INVENTION
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, has not yet been accomplished.
Further objects, features, and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.SUMMARY OF THE INVENTION
The present invention provides methods for forming nanotubes from thin films using 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 of the present invention 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.
One embodiment of the invention provides a method for forming the nanotubes. 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.
The present invention provides methods for forming nanotubes from thin films. The films are desirably a single atom thick, as in the case of graphene nanoribbons, or only a few (e.g., 2-10) atomic layers thick, as in the case of semiconductor, metal, or other material films. The methods involve the adsorption of atoms onto the surface of the films. The adsorbed atoms introduce a surface stress, inducing a curvature in the films. As a result of this curvature, edges of the thin films are brought into contact with each other, or with edges of other thin films. Covalent bonds form between the edges, resulting in nanotube formation.
In one embodiment of the present invention, the method for forming a nanotube comprises three steps. In a first step, a thin film is patterned onto a substrate. Various substrate materials may be used, including graphite. The film may comprise a variety of different shapes having different dimensions. The width of the thin films will depend on the desired diameter of the nanotubes. In some embodiments, the width is in the range of 1 to 20 nm. In other embodiments, the width is in the range of 1 to 10 nm. In yet other embodiments, the width is in the range of 1 to 2 nm. The length of the films, while not critical to the method, is generally many times greater than the width of the nanoribbon.
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.
In one variation of the method, the nanotubes are formed from a single thin film, whereby the adsorbent-induced curvature causes opposing edges of the film to meet and form covalent or other chemical bonds. Examples 1 and 2, below, illustrate this variation of the method.
In another variation of the method, a nanotube is formed from a plurality (e.g., two) of layered thin films. In such methods, atoms are adsorbed onto the surface of one or more of the films, causing the edges of the films to contact and form covalent and other chemical bonds, resulting in the formation of a closed tube. In some such embodiments, the widths of the two nanoribbons are substantially the same, while in other embodiments the widths are different. Example 3, below, illustrates this variation of the present methods.
Without wishing to be bound to a particular theory, it is hypothesized that the driving force for the formation of nanotubes from films according to the present invention 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.
As noted above, the magnitude of the bending curvature can be controlled by adjusting the coverage of adsorbed atoms to tune the magnitude of the surface-adsorption-induced stress. This is illustrated for the case of a graphene nanoribbon in
For carbon nanotube formation from graphene nanoribbons, the appropriate surface coverage will determine the width of the patterned nanoribbon. To form a perfectly closed tube, the nanoribbon width, W, should approximate 2πR, where R is the radius of bending curvature as defined by the surface coverage of adsorbed atoms. As shown in
Another important consideration for carbon nanotube formation is that the patterned graphene nanoribbon is not a free-standing film, but is weakly bonded to the underlying substrate through van der Waals attractive forces. The surface stress induced by atomic adsorption should be large enough to overcome this attraction in order for the graphene nanoribbon to detach from the underlying substrate during the bending process. For example, a particular surface coverage of H atoms induces a surface stress, σ, resulting in a bending moment of M=σt/2, where t is the “effective” film thickness, including the adsorbent layer (
Because 2πR≦Wm, the existence of a maximum nanoribbon width sets an upper limit on the radius R of the formed nanotube. In principle, because R ∝ 1/σ, as shown in
The formation of carbon nanotubes from graphene nanoribbons using atomic adsorption is further illustrated by the following non-limiting examples.EXAMPLES
Methods: Unless otherwise specified, the following methods were used in the examples below.First-Principles MD Simulations
First-principles MD simulations were performed using the pseudopotential plane-wave method within the local density approximation as implemented in the VASP code. Kresse, G. & Haiher, J., Ab initio molecular dynamics for liquid metals, Phys. Rev. B 47, 558-561 (1993). Single Γ k-point for Brillouin zone sampling and plane wave cutoffs of 21.3 Ryd (H-C system) and 31.2 Ryd (F-C system) were used. The super cell dimension was 27.016×4.2532×23 Å3 with a vacuum layer of ˜15 Å. As shown in
Classical MD simulations were performed using a modified form of the bond-order potential developed by Brenner et al. Brenner, D. W., et al., A second-generation reactive empirical bond order (REBO) potential energy expression for hydrocarbons, J. Phys.-Cond. Matt. 14, 783-802 (2002). Because the original Brenner potential predicted an H adsorption energy of about 2.6 eV, a new bond-order term was introduced for the C—H bond to give an H adsorption energy of about 0.9 eV on graphite, as predicted by the first-principles calculations. In addition, Lennard-Jones potential parameters were developed to give a ˜42 meV/atom interlayer cohesive energy for graphite, in accordance with experiments. Girifalco, L. A. & Lad, R. A., Energy of cohesion, compressibility, and the potential energy functions of the graphite system, J. Chem. Phys. 25, 693-697 (1956). As shown in
Single-walled carbon nanotubes were simulated using first-principles MD simulations. In the simulations, a 1.7-nm-wide graphene nanoribbon was patterned onto a graphite substrate (
Classical MD simulations were used to simulate armchair and zigzag SWNTs.
Single-walled carbon nanotubes were simulated from two layers of graphene nanoribbons using classical MD simulations. As shown in
For the purposes of this disclosure, and unless otherwise specified, “a” or “an” means “one or more.” All patents, applications, references, and publications cited herein are incorporated by reference in their entirety to the same extent as if they were individually incorporated by reference.
As will be understood by one skilled in the art, for any and all purposes, particularly in terms of providing a written description, all ranges disclosed herein also encompass any and all possible subranges and combinations of subranges thereof. Any listed range can be easily recognized as sufficiently describing and enabling the same range being broken down into at least equal halves, thirds, quarters, fifths, tenths, etc. As a non-limiting example, each range discussed herein can be readily broken down into a lower third, middle third and upper third, etc. As will also be understood by one skilled in the art, all language such as “up to,” “at least,” “greater than,” “less than,” and the like includes the number recited and refers to ranges which can be subsequently broken down into subranges as discussed above. Finally, as will be understood by one skilled in the art, a range includes each individual member.
It is understood that the invention is not confined to the particular embodiments set forth herein as illustrative, but embraces all such forms thereof as come within the scope of the following claims.
1. A method for forming a nanotube from one or more thin films, the method comprising adsorbing atoms onto a surface of at least one of the one or more thin films, wherein the absorbed atoms produce a curvature in the at least one thin film sufficient to bring atoms at the edges of the one or more thin films into close proximity and further wherein bonds form between the atoms at the edges of the one or more thin films to provide a nanotube.
2. The method of claim 1, further comprising desorbing the adsorbed atoms.
3. The method of claim 1, wherein the one or more thin films are a single atom thick.
4. The method of claim 3, wherein the one or more thin films are graphene sheets.
5. The method of claim 4, wherein the width of the one or more graphene sheets is about 1 to about 10 nm.
6. The method of claim 1, wherein the one or more thin films are about 5 to about 10 atoms thick.
7. The method of claim 6, wherein the one or more thin films are semiconductor thin films.
8. The method of claim 1, wherein the atoms are selected from the group consisting of H atoms, F atoms, and combinations thereof
9. The method of claim 1, wherein the adsorption provides a surface coverage of adsorbed atoms in the range of about 40% to 60%.
10. The method of claim 1, wherein the nanotube is formed from a single thin film and the covalent bonds form between the atoms at opposite edges of the thin film to provide the nanotube.
11. The method of claim 1, wherein the nanotube is formed from a plurality of thin films and the covalent bonds form between the atoms at the edges of different thin films to provide the nanotube.
12. The method of claim 11, wherein the nanotube is formed from a first thin film and a second thin film and the covalent bonds form between a first edge of the first thin film and the first edge of the second thin film and between a second edge of the first thin film and a second edge of the second thin film.