The field of art relates generally to Carbon nanotubes (CNTs); more specifically, techniques for sorting CNTs of different electronic types.
A Carbon nanotube (CNT) can be viewed as a sheet of Carbon that has been rolled into the shape of a tube. CNTs having certain properties (e.g., a “metallic” CNT having electronic properties akin to a metal) may be appropriate for certain applications while CNTs having certain other properties (e.g., a “semiconducting” CNT having electronic properties akin to a semiconductor) may be appropriate for certain other applications. CNT properties tend to be a function of the CNT's “chirality” and diameter. The chirality of a CNT characterizes its arrangement of carbon atoms (e.g., arm chair, zigzag, helical/chiral). The diameter of a CNT is the span across a cross section of the tube.
Because the properties of a CNT can be a function of the CNT's chirality and diameter, the suitably of a particular CNT for a particular application is apt to depend on the chirality and diameter of the CNT. Unfortunately, current CNT manufacturing processes are only capable of manufacturing batches of CNTs whose tube diameters and chiralities are widely varied. The problem therefore arises of not being able to collect CNTs (e.g., for a particular application) whose diameter and chiralities reside only within a narrow range (or ranges of) those that have been manufactured.
CNTs are also known to have poor solubility. Here, owing to van der Waals forces (it is believed), individual CNTs tend to “bundle together” into groups. Thus, when a batch of manufactured CNTs are made to flow in a fluidic stream (such as an aqueous solution), bundled groups of CNTs are observed drifting/flowing through the liquid together.
Success at improving the solubility of CNTs has been reported. For example, Zheng et al. (“DNA-Assisted Dispersion and Separation of Carbon Nanotubes”, Nature Materials 2, pgs. 338-342, 2003) has published a process by which single stranded DNA (ss-DNA) is used to “break-down” a CNT bundle into individual CNTs wrapped in a helical structure of DNA. Here, a CNT that has bonded in some fashion with DNA so as to form a combined structure of DNA and the CNT is referred to as “DNA/CNT hybrid structure”. A DNA/CNT hybrid structure of DNA and a metallic CNT may be referred to as “DNA/metallic CNT hybrid structure”. A DNA/CNT hybrid structure of DNA and a semi-conducting CNT may be referred to as “DNA/semi-conducting CNT hybrid structure”.
According to the technique taught by Zheng et al., an aqueous solution containing bundles of CNTs is subjected to the presence of ss-DNA. Because the binding energy associated with the coupling of ss-DNA to a CNT is comparable to the binding energy associated with the coupling of CNTs to one other, the application of sonic energy to the solution can create dynamic situations in which an individual CNT that is bundled with one or more other CNTs will reach a lower energy state if the CNT binds with ss-DNA molecules instead of the CNTs associated with its bundle. Because physical systems tend to fall to lower energy states, this prompts the formation of an individual (i.e., non bundled) CNT helically wrapped in ss-DNA. That is, the CNT essentially leaves its bundle in favor of being helically wrapped by ss-DNA.
According to follow-up work reported by Zheng et al. in “Structure-Based Carbon Nanotube Sorting by Sequence Dependent DNA Assembly”, Science 28 Nov. 2003; 302: 1545-1548, a particular sequence of ss-DNA can be made to self assemble into a helical structure that wraps around the surface of an individual CNT. Individual CNTs wrapped by ss-DNA can then be sorted according to their electrical characteristics through anion exchange chromatography. In this manner, individual CNTs having specific “sought-for” electrical characteristics can be collected.
The present invention is illustrated by way of example, and not limitation, in the figures of the accompanying drawings in which like references indicate similar elements and in which:
a (prior art) shows a density of states diagram for a metallic CNT;
b (prior art) shows a density of states diagram for a semi-conducting CNT;
Referring to
According to one implementation, single walled Carbon nanotubes (SWNTs) manufactured according to a High Pressure Carbon Monoxide (HiPCO) process by Carbon Nanotechnolgies Inc. are solubilized in an aqueous solution with the aid of a custom-synthesized oligonucleotide supplied by Operon Biotechnologies, Inc. The custom-synthesized oligonucleotide has a sequence of TTTTTTTTTTTTTTTTTTTTTTTTTTTTTT (PolyT30). In a typical preparation, the HiPco SWNTs (˜1 mg) are combined with a stock solution of PolyT30 (˜3 ml, 200 μM in 18 MΩ water) and ultrasonicated in an ice water bath (VWR 75D bath sonicator) for 2-3 hrs. at a power of 90 W with frequent vortexing interspersed throughout.
The solution is then centrifuged at 16,000×g for 90 min. and the supernatant decanted and collected. This is repeated times, then the resulting supernatant is dialyzed (MWCO 60,000) against pure water for a period of 2-3 days to remove any free DNA not coated directly on the SWNTs. This procedure typically yields a final concentration of 0.1-0.2 mg/ml of solubilized SWNTs.
Once the solution is essentially solubilized 101 (i.e., bundles of CNTs are broken down into individual CNTs), the DNA wrapping is substantially removed 102 only from certain “select” CNTs having a specific type of electronic property (e.g., metallic or semi-conducting). In
With DNA being delaminated only from the select CNTs 204_4, 204_7 and 204_10, the solution will be in a state that promotes the bundling of the select CNTs with one another. That is, bundles will be created whose constituent CNTs are only the select CNTs.
Once bundles of the select CNTs are formed, such bundles can be readily segregated from the individual non select CNTs. For example, because of their larger size and weight, the bundled, select CNTs could be precipitated 103 from the individual, non select CNTs through differential centrifugation.
As will be described in more detail further below, in an ideal environment, the photon energy of the electromagnetic radiation 402 (which hereafter will be referred to as “light” for simplicity) is approximately equal to that of a energy difference between van Hove singularities within the select CNTs so that only electrons within the solution's select CNTs are sufficiently excited (in response to the irradiation by the light) to jump to a higher energy band and then transfer to a nearby molecule. In short, select CNTs are meant to behave as electron donors to the molecules that are near to them; whereas, non-select CNTs are not meant to behave as electron donors to the molecules that are near to them.
Accordingly, part of the behavior that is intended to be induced within the solution is the break down of a molecule caused by the molecule's reception of an electron that was donated by a select CNT 302. For example, in the case of the hydrogen peroxide molecule, and as shown at time T1_2 in
One or more of the products produced by the molecule's breakdown are expected to be able to react with a CNT's DNA wrapping so as to decompose the DNA wrapping. Better said, the products produced by the molecule's breakdown are able to essentially “attack” a CNT's DNA wrapping. In the case of the hydroxylproducts, hydroxylproducts are believed to be able to cause single strand breaks and/or dual strand breaks in DNA chains. As such, it is believed that hydroxylproducts are able to successfully fragment the DNA wrapping around a CNT.
The probability of an electron being successfully transferred from a donating CNT to a receiving molecule greatly reduces as the distance between the donating CNT and the receiving molecule grows. Thus, for the most part, only molecules that are “nearby” a donating CNT are expected to be broken down. In an ideal environment, as discussed above, only select CNTs donate electrons to their nearby molecules and all non-select CNTs do not donate any electrons to their nearby molecules. Therefore, in such an ideal environment, only molecules near select CNTs will be broken down.
As a result, particularly for products having a short lifetime (such as hydroxylproducts in an aqueous solution), the CNT closest to the products of a broken down molecule should be the CNT that donated the electron which caused the molecule's break down. Therefore, in an ideal environment in which only select CNTs donate electrons, for the most part, select CNTs will have their DNA wrappings fragmented from reactions with products that result from molecular breakdown.
Regardless, the substantially achieved effect is that the rate at which the DNA wrappings of select CNTs are damaged will be greater than the rate for non-select CNTs. As such, select CNTs will favor bundling with other CNTs substantially earlier than non-select CNTs will (if at all). Therefore, upon the subjection of specially tuned light and appropriate molecules to a solution of DNA solubilized CNTs, bundles of select CNTs are apt to appear in the solution before bundles of non-select CNTs. By ceasing the irradiation of the solution with the light before the appearance of bundles formed with non select CNTs (if any), the solution can be made to produce bundles of only select CNTs. The bundles of select CNTs can then be segregated 303 from the individual non select CNTs (e.g., by precipitation through differential centrifugation).
a, 5b and 6 can be used to describe the relationship between the energy of the incident light and the transfer of donor electrons from select CNTs as opposed to non select CNTs.
A characteristic of CNTs is the presence of van Hove singularities in their density of states vs. energy diagrams. A van Hove singularity appears as a “spike” within a density of states vs. energy diagram. Each of
The Fermi-level 504 is a kind of “water-line” that indicates which energy states are populated with electrons and which energy states are not populated with electrons. Simplistically, under normal conditions, nearly all of the energy states beneath the Fermi-level 504 (which corresponds to region 505 in
Referring to the metallic CNT's density of states diagram in
For simplicity, valence band van Hove singularities are “counted” in decreasing energy order, and conduction band van Hove singularities are “counted” in increasing energy order. That is, for example, referring to
Another point of distinction with respect to the Eii energies of conducting and semi-conducting CNTs is the size of the energy spans. Typically, the Eii energy of a metallic CNT is greater than the Eii energy of a semi-conducting CNT (at least for lower values of x and CNTs of comparable diameter). A comparison of
Here, it should be evident from the discussion above concerning
Here, because a van Hove singularity essentially represents a relative electron energy level that many electrons are permitted to possess, a strategy for achieving the above effect is to use light whose incident energy is aimed at “populating” van Hove singularities of select CNTs but not those of non select CNTs. The diagrams of
Consider the CNTs of
Because of the metallic CNT's large E11 energy 507, the greatest concentrations of electrons in the metallic CNT that could be launched into the conduction band 509 with only 0.4 units of additional energy are those electrons who energies extend over region 521 in the metallic CNT's density of states (i.e., from the Fermi level 504 down toward the “first” van Hove singularity 508 in the valence band 505). Here, very few total electrons have energies in energy region 521 because the metallic CNT's density of states diagram only reaches a value of approximately 0.1 states per unit cell of graphite in this region 521. As such, only few electrons will be excited into the conduction band through photon absorption.
By contrast, the E11 energy 510 of the semi-conducting CNT of
Another approach that can be used alternatively or in combination with the former approach (in which the incident optical energy was set equal to the E11 energy of the semi-conducting CNT) is to tune the incident photon energy to the E22 energy of the semi-conducting CNT. This approach is particularly appropriate if the E22 energy of the semi-conducting CNT is less than the E11 energy of the metallic CNT. Referring again to
An incident light energy of 0.8 units of energy will also, like the former example where Ehv=0.4 units of energy, permit electrons to jump from the first van Hove singularity 505 in the valence band to the first singularity 512 in the conduction band. However, this form of high energy electron generation (i.e., where Ehv is substantially different than an Eii energy) is apt to be less efficient because phonon energy will need to be given to the CNT carbon sheet in order for an optically stimulated electron to reach a van Hove singularity in the conduction band (energy must be given to the CNT lattice to account for the energy difference between the incident light and the E11 energy of the CNT). For this reason it is also theoretically possible to target metallic CNTs as the select CNTs by setting the photon energy substantially equal to the E11 and/or E22 energies (and/or the Eii energies generally) of the select, metallic CNTs. Further discussion in this regard is provided in more detail further below.
With respect to the metallic CNT, with an incident optical energy of 0.8 units of energy, more electrons will be able to reach the conduction band than the case where the photon energy is only 0.4 units of energy (with at least some of these reaching the first van Hove singularity 509 in the conduction band 506). As such, greater electron transfer from metallic CNTs to nearby molecules is possible. However, the number of high energy electrons generated in the semi-conducting CNT's conduction band will far outweigh those in the metallic CNT's—particularly considering the height of the second singularities 518, 519 in the semi-conducting CNTs against the height of the first singularities 508, 509 in the metallic CNT. Thus, the semi-conducting CNTs are expected to donate substantially more electrons to their nearby molecules than the metallic CNTs.
The analysis above only compared a pair of CNTs.
From
It is presently understood in the art that different CNT manufacturing processes exist, and that, sufficient control can be exercised over these manufacturing processes such that the diameters of a batch of manufactured CNTs will only fall within a limited range (e.g., a HiPCO process gives a typical diameter may yield a diameter range of 0.9 nm-1.3 nm and a modified supported catalyst chemical vapor deposition process may give a diameter range of 0.7 nm-0.9 nm)).
As a simple example, establishment of optical photon energies for segregating semi-conducting CNTs from metallic CNTs produced will be described for an exemplary CNT manufacturing process that exhibits a yielded diameter range of 1.0 to 1.3 nm. This corresponds to a batch of CNTs that fall within region 600 of
If the maximum optical energy 602 is set to 1.79 eV, a minimum optical energy 603 can be set to 1.23 eV. By sweeping the optical wavelength such that the incident photon energy sweeps from level 602 to level 603, in theory from
Although this particular example yielded optical energies whose wavelengths substantially correspond to the infra-red realm, it is important to note that different manufactured diameter ranges (i.e., manufactured diameter ranges other than 1.0-1.3 nm) may yield different optical energy ranges as well.
It is also important to point out that the technique described above can also be used to bundle one or more CNTs of specific diameter and chirality. That is, rather than a “select” CNT simply being a semi-conducting CNT, a “select” CNT may be a CNT of specific chirality and diameter. The ability to cause the DNA delamination of one or more CNTs of specific chirality and diameter derives from the fact that the E11 energy for a specific CNT is determined by that CNT's chirality and diameter.
Thus, for example, with knowledge of the particular E11 and/or E22 energy for a CNT of specific chirality and diameter, the optical photon energy can be set substantially equal to the E11 and/or E22 energy of the CNT so as to cause DNA delamination and bundling of only those CNTs having the specific diameter and chirality. Here, the CNT can be either a semi-conducting or metallic CNT. Of course, it is possible to use energies other than the E11 and/or E22 energies (e.g., E33 energy, etc.).
Also, the process of “targeting” CNTs of specific diameter and chirality as the select CNTs can be repeated for additional CNTs that a collection of different diameter and chiralities correspond to the collect CNTs. For example, CNTs of a first diameter and chirality combination, and, CNTs of a second diameter and chirality combination are extracted by setting optical energies substantially equal to the E11 and/or E22 energies that correspond to the first diameter and chirality combination and the second diameter and chirality combination. Note that various selection strategies could be chosen with respect to what type of CNTs are targeted as the select CNTs. For example, the first chirality and diameter combination could correspond to a semi-conducting CNT, and, the second chirality and diameter combination could correspond to metallic CNT. In other instances all targeted CNTs could be metallic CNTs, or, all targeted CNTs could be semi-conducting CNTs.
In the foregoing specification, the invention has been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention as set forth in the appended claims. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.