METALLIC CARBON QUANTUM WIRE FROM SELF-ASSEMBLED ALPHALTENE

Abstract

The present disclosure is related to a method of fabricating a stacked nanographene structure which is assembled into quantum wires or ribbons. While it has been demonstrated that nanowires can be fabricated from various raw carbon materials including PAHs, research and industry has not produced a self-assembled nanowire produced from asphaltene materials that exhibits a metallic character and electronic structure. The following methods and materials can be used to produce new class of materials consisting of a self-assembled quantum wire out of asphaltene.

Claims

1. Method of producing a one-dimensional self-assembled molecular wire comprising depositing a dilute thermally activated asphaltene solution on a target location of a substrate under conditions for molecular wire self-assembly.

2. The method of claim 1, where molecular wire is formed by drop-coating.

3. The method of claim 1, wherein the asphaltene solution comprises an aromatic based solvent.

4. The method of claim 3, wherein the aromatic based solvent is chlorobenzene.

5. The method of claim 1, wherein the dilute asphaltene solution comprises 0.001, 0.005, 0.01, 0.05 to 0.5 mg/ml asphaltene.

6. The method of claim 1, wherein the dilute asphaltene solution comprises 0.005 mg/ml asphaltene.

7. The method of claim 1, the thermally activated asphaltene is prepared by heating asphaltene in the absence of air.

8. The method of claim 7, wherein the asphaltene discotic liquid crystals are heated to 350 to 600 C.

9. The method of claim 7, wherein the asphaltene discotic liquid crystals are heated to about 500 C.

10. The method of claim 7, wherein the asphaltene is heated for 1 to 60 minutes.

11. The method of claim 7, wherein the asphaltene is heated for about 10 minutes.

12. The method of claim 7, wherein the asphaltene are produced from mesophase pitch by (a) extracting crude oil with n-alkane; (b) filtering the n-alkane; (c) dissolving the retentate in toluene forming a toluene solution; (d) filtering the toluene solution; (e) evaporating the toluene; and (f) collecting the residual asphaltene.

13. The method of claim 1, wherein the asphaltene is dissolved in an aromatic solvent.

14. The method of claim 13, wherein the aromatic solvent is toluene or chlorobenzene.

15. A self-assembled molecular wire produced by the method of claim 1, wherein the self-assembled molecular wires have a length of 2 to 100 microns.

16. The self-assembled molecular wire of claim 15, wherein the wire has a height of 10 nm.

17. The self-assembled molecular wire of claim 16, wherein the wire exhibits quantum confinement.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The following drawings form part of the present specification and are included to further demonstrate certain aspects of the present invention. The invention may be better understood by reference to one or more of these drawings in combination with the detailed description of the specification embodiments presented herein.

[0021] FIGS. 1A-D. Top (1A) and side view (1B) of asphaltene DLC molecules modeled using DFT with intermolecular interactions. Extracted asphaltene solids (1C). Thermal processing of Asphaltene into a Multi-layer nanographene (1D).

[0022] FIGS. 2A-F. Fourier Transform Infrared Spectroscopy of asphaltene (2A), self-assembled nanowires from heat treated asphaltene sample (2B), X Ray Diffraction patterns from asphaltene (2C) and self-assembled nanowires from heat treated asphaltene (2D) and Raman spectroscopy of asphaltene (2E) and self-assembled nanowires from heat treated asphaltene (2F).

[0023] FIGS. 3A-D. Density Functional Theory calculation of Density of States (3A) for simulated structure (3B) and showing the Fermi Energy at 0 eV (dotted line). Calculation of DOS for stacked structure in compared with single layer nano-graphene (3C) and calculated DOS for stacked structure compared with single layer graphene (3D).

[0024] FIGS. 4A-D. Scanning Tunneling Spectroscopy I-V spectra (4A) of Highly Ordered Pyrolytic Graphite (HOPG) substrate compared with asphaltene precursor and asphaltene heat treated asphaltene. The blue line is the I-V spectrum of bare graphite. The straight red line is the I-V spectrum of the heat treated sample and black line represents the I-V spectrum of the precursor sample indicating that it is less conductive than graphite. Insert (4B) shows the conductance plots (dI/dV) plots obtained from STS data for MLNG and asphaltene. Atomic Force Microscope 3-D (4C) and 1-D image (4D) of self-assembled micron length nanowires wires from MLNG sample on HOPG with 2-D height topography (inset).

[0025] FIGS. 5A-B. SEM images of micro-probe taken from points along a quantum wire from self-assembled asphaltene on the surface of copper with Voltage-Current measurements from corresponding points on quantum wire as well as the surface of the copper substrate (5A) PDOS plots for Copper cell (A) and for the simulated Nanographene on Copper (5B) where the dotted blue line represents the Fermi energy.

[0026] FIGS. 6A-B. EDX spectra of asphaltene and thermally treated asphaltene on SiO.sub.2 (6A), with corresponding SEM image (6B) and measurement location from thermally treated asphaltene on SiO.sub.2.

DETAILED DESCRIPTION OF THE INVENTION

[0027] The realization of functioning high performance electronic and optoelectronic devices based on nanomaterials, in general, is impacted significantly by the electrical connections, wires and interconnects that link nanomaterials with external circuitry. One dimensional molecular wires described here in can be produced and used to provide electrical connections and interconnects between nanomaterials and other circuitry components.

[0028] Thermal treatment of asphaltene, precursor discotic liquid crystals (DLC) results in self assembles into a one dimensional columnar multi-layer nanographene due to the alignment of stacked aromatic cores and when deposited from dilute solutions further self-assemble into molecular nanowire. Without being bound of theory, it is believed that due to the loss of insulating hydrogen groups, the increase in sp.sup.2 hybridization and the stacking of aromatic cores, the thermal treatment contributes to a large increase in conductivity. Discotic liquid crystals precursors are mesophases formed from disc-shaped molecules known as discotic mesogens. These phases are also referred to as columnar phases. Discotic mesogens are typically composed of an aromatic core surrounded by flexible alkyl chains. The aromatic cores allow charge transfer in the stacking direction through the conjugate systems. The charge transfer allows the discotic liquid crystals or molecular wires produce from DLCs to be electrically semi-conductive along the stacking direction. Self-assembled nanowires microns in length (1 to 100 m) may be fabricated from a dilute thermally activated asphaltene solution where the solvent is aromatic. These nanowires exhibit quantum confinement and range in height between 3 nm to 20 nm, and in certain aspects on average about 10 nm in height. These quantum confinements result in a conductive electronic state and ohmic properties are observed from these structures. In certain aspects the nanowires have a resistance on the same order or lower than that of copper.

EXAMPLES

[0029] The following examples as well as the figures are included to demonstrate preferred embodiments of the invention. It should be appreciated by those of skill in the art that the techniques disclosed in the examples or figures represent techniques discovered by the inventors to function well in the practice of the invention, and thus can be considered to constitute preferred modes for its practice. However, those of skill in the art should, in light of the present disclosure, appreciate that many changes can be made in the specific embodiments which are disclosed and still obtain a like or similar result without departing from the spirit and scope of the invention.

Example 1

A. Materials and Methods

[0030] Synthesis:

[0031] Asphaltene mesophase pitch was used as the precursor discotic liquid crystal (DLC) and was extracted from Crude oil (Mayan) by n-alkane (1:40 v/v). The solution was mixed for 24 hours and filtered (Whatman 40). The retentate was dissolved in toluene and filtered again and the solution was collected and evaporated. 100 mg of sample was heated to 500 C. in the absence of air for 10 minutes in a flask using Schlenk technique and kept under vacuum.

[0032] Nanowire Fabrication:

[0033] After thermal treatment, dilute solutions of samples in chlorobenzene (5 g/ml) were drop coated onto various substrates (Cu, Highly Ordered Pyrolytic Graphite, SiO.sub.2/Si) and nanowires self-assemble on the surface of substrates. Drop coating is the deposition of a volume of material in solution, after which the solvent is evaporated depositing the material at the location of deposition.

[0034] Characterization:

[0035] Fourier Transform Infrared (FTIR) was taken of samples in KBr pellets (Nicolet, Thermo Scientific). X-Ray Diffraction was taken from 10-60 using Cu K.sub.=0.154 nm at a 0.01 scan step (Bruker). Scanning Electron Microscopy was taken of solutions on SiO.sub.2/Si (S-4800, Hitachi). Transmission Electron Microscopy (H-7650, Hitachi, Japan) was performed after drop casting samples on lacy carbon grids. Scanning Probe Microscopy/Scanning Tunneling Spectroscopy (NT-MDT) was performed with Pt/Ir tips (DPT-10, Bruker) on Highly Ordered Pyrolytic Graphite substrates by drop coating solutions (0.05 mg/ml) from chlorobenzene (Sigma). Each sample was applied three times independently and measurements were taken of across a line one micron in length with 10 points and cycled for 5 five minutes to ensure a stable scan. Two point probe measurements were taken using an Omnicron instrument with Tungsten STM probes which were annealed prior to experimentation to remove native oxide.

[0036] Molecular modeling: Simulations were carried out using modules available in Materials Studio v. 7.0, distributed by BIOVIA. The proposed structure was built imbedded within an otherwise empty simple cubic cell of lattice parameter 30 (in order to approximate a molecular model when using periodic DFT algorithms). The structure geometry was as an initial approximation optimized using a Forcite force field minimization with a universal force field. The structure's geometry was further optimized via a DFT method (CASTEP), using a Generalized Gradient Approximation (GGA) with a WC functional (Wu and Cohen, 2006). The electronic Hamiltonian used an energy cutoff of 240.0 eV, a Self-Consistent Field (SCF) tolerance of 2.010-6 eV/atom, a 111 Monkhorst-Pack grid for Brillouin zone k-point sampling, and an ultrasoft pseudopotential. After optimizing the geometry of the structure, electronic Density of States (DOS) and band structure were calculated using an energy band tolerance of 1.010-5 eV, as well as the same electronic parameters as were used for the geometry optimization of the structure

B. Results

[0037] During the heat treatment of the asphaltene, precursor DLC, cyclodehydrogenation occurs and as a result sp.sup.3 hybridized carbon atom is converted to sp.sup.2 carbon. As evidenced by Fourier Transform Infrared (FTIR) spectroscopy and X-Ray Diffraction (XRD), that the structure of the asphaltene is significantly changed by the thermal treatment and can be realized as a stacked sp.sup.2 hybridized carbon structure after the removal of alkyl side chain from the precursor and the rearrangement or stacking of polycyclic asphaltene molecules.

[0038] In the asphaltene, DLC precursor (FIG. 2.A), sp.sup.3 carbons are present as there are strong CH vibration at 1400 cm.sup.1 in the FTIR spectra representing alkyl groups. Also there is a CH stretching vibration is observed at 2960 cm.sup.1 from hydrogen or methyl groups attached to aromatics.

[0039] After heat treatment (FIG. 2B), the MLNG containing DLC sample shows IR-vibrational bands originated from hydrogens atoms attached to carbons via double bonds at around 1100 cm.sup.1, more CC bond stretching at 1600 cm.sup.1 and the total absence of hydrogen attached to aromatic groups. The crystallinity of both the asphaltene and the thermally treated asphaltene, i.e. MLNG sample has been studied using X-ray powder diffraction (XRD) technique as seen in FIGS. 1C and 1D respectively. XRD confirms the presence of alkyl side chains presented by FTIR in the asphaltene. Asphaltene shows the diffraction peaks having 2-theta at around 24.5 2 which originates from the graphitic (002) plane and another broad peak at 2-theta of 20 representing the alkyl side chains on the asphaltenes (FIG. 2C). XRD data shows that thermal treatment of the asphaltene led to a crystalline structure with d-spacing 3.35 intrinsic to -orbital overlapping between aromatic cores along with the removal of alkyl side chains in the final product (FIG. 2D).

[0040] Raman spectroscopy was performed shows that both asphaltene (FIG. 2E) and as-synthesized MLNG (FIG. 2F) show D and G bands at around 1350 cm.sup.1 and 1584 cm.sup.1, which signifies the presence of graphitized carbon like structures. The G band corresponds to the hexagonal crystal symmetry of graphitic crystal (J. Chem. Phys., 53, 1126-1130, 1970) structure and originates from sp.sup.2 hybridized carbon atoms whereas the D band is due to sp.sup.3 CC stretching vibration. Interestingly, the MLNG shows increase in G band Raman intensity compared to asphaltene and this clearly indicates the increase in sp.sup.2 carbon atoms due to increased - stacking structures of graphitic layers. The ratio of the intensity of G and D bands can be used to estimate the aromatic dimension (La) of both precursor and heat-treated sample according to the Tuinstra and Koenig equation La=4.4I.sub.G/I.sub.D (J. Chem. Phys., 53, 1126-1130, 1970). So, the MLNG reveals higher La vale (9.16 nm) compared to that (6.16 nm) of precursor asphaltene. It is evident from the Raman and the XRD that heat-treatment has the ability to produce organized stacked multi-layer nanographene structures. In addition, after heat-treatment the D band is sufficiently broadened and distorted which is most likely due to the self-assembly of individual graphene layers stacking, resulting in the increased -orbital overlapping in the multi-layer nanographene.

[0041] A nanographene tri-layer single stack was used as the simulated structure (FIG. 3B). The optimized minimum energy state of the structure reveals 3.35 as inter-layer spacing that is fundamentally characteristic of stacked aromatic cores. The total electronic Density of States (DOS) as calculated from the simulation has a large number of states available at or near the Fermi level that remains continuous with no energy gaps near conduction states (FIG. 3A) and would be expected to exhibit metallic behavior as a band gap is not evident in the DOS. Further analysis was performed to compare this simulation to a single ring structure which represents a quantum dot (FIG. 3C) and also single layer graphene (FIG. 3D). The model indicates that this structure has potential to outperform the quantum nano-graphene carbon dot and single layer graphene as the DOS remains higher in overall volume and peak intensity. In stacked ABA multilayers (Bernal Stacked), the n low-energy conduction bands are bilayer-like for an even number of layers (N=2n), but with different effective masses. In an odd number of layers (N=2n+1) a linear band with the same slope as the energy band in monolayer graphene exists next to the n parabolic ones (Koshino and McCann, Phys. Rev. B 81, 115315, 2010). Staking layers of graphene does not lead to graphite immediately and when the number of layers is sufficiently small enough, it can preserve the 2-D nature of the system (Physical Review B, 88, 115414, 2013). The valence bands can be related to the conduction bands by particle-hole symmetry.

[0042] FIG. 4 and FIG. 5 show images of the self-assembled quantum wire on graphite and copper. These images from Transmission Electron Microscope, Scanning Electron Microscope and Atomic Force Microscopy illustrate the consistent self-assembly of the thermally treated asphaltenes. Voltage Current spectroscopy performed using Scanning Tunneling Spectroscopy is done without introducing contacts to the nanomaterial as it uses tunneling effect to probe the materials electronic states.

[0043] Images taken using Transmission Electron Microscope (TEM) indicate the asphaltene comprise of molecules that are spherical with sizes ranging from 10-100 nanometers which lack any formal organization. After thermal processing, the dilute solutions of MLNG remarkably self-assemble into highly ordered nanowires that appear similar to nanoribbons or nanotubes can be seen in TEM images. Scanning Electron Microscope (SEM) images of the precursor and heat treated asphaltene prepared on SiO.sub.2/Si wafers reveal the structure of the asphaltene to appear spherical with large dendritic structures which exemplifies the discotic PAH properties associated with having aromatic cores attached to alkyl chains (J. Chem. Phys. 53, 1126-1130, 1970; Phys. Rev. B. 81, 115315 2010). However, images after thermal treatment show that the MLNG sample formed nanowires consistent with TEM analysis (Fuel, 87, 3481-3482, 2008). They show a remarkable linear organization microns in length and demonstrate an ability to form nanowires from self-assembled 1-D stacks fabricated from asphaltene.

[0044] To characterize the electronic structure of the MLNG compared to that of the asphaltene at a nanoscale molecular domain, Scanning Tunneling spectroscopy (STS) was performed for both samples on highly ordered pyrolytic graphite (HOPG) in air. Scans were taken using multiple HOPG substrates with up to 5000 scans per sample over areas 1 micron in length. FIG. 4 in particular demonstrates the difference in electronic states between asphaltene, graphite and the novel nanowire. FIG. 4A shows the current-voltage characterization of the samples as obtained by this method and is shown together with conductance (dI/dV) curves (FIG. 4B). Due to the fact that the spectra were obtained in air, surface contamination could account for some scattering between data points however, a correlation is very evident despite electronic noise. Remarkably, the MLNG sample i.e. the thermally activated asphaltene shows linear I-V curve which is indicative of an ohmic electronic state and is generally described as ohmic conduction in metals. This metal-like ohmic conductivity of the MLNG sample is consistent with the predicted behavior as already obtained by DFT analysis. Graphite clearly exhibits the semi-conducting electronic state illustrated by a curved I-V spectra. This curve represents a band-gap. Compared to the bare HOPG substrate, the asphaltene was much less conductive and behaves as an insulator, consistent with previously reported STS analysis of asphaltene (Ind. Eng. Chem. Res., 33, 2358-2363, 199 4). Clearly the insulating nature of asphaltenes is a result of aliphatic chains attached to the asphaltene aromatic cores and by removing those chains using thermal treatments, the utility of the alignment and self-association of these asphaltene molecules is demonstrated.

[0045] Further analysis on the conductive nature of thermally treated asphaltene was done using a nano-scale two-point-probe (FIG. 5). IV spectra were collected from several points along a quantum wire fabricated on copper foil using tungsten probes which were annealed to remove the native oxide. The voltage was swept from 1V to +1V and five repeat spectra were taken at each point and averaged. It can clearly be seen that the IV curves are ohmic in nature from each point collected, this is in agreement with the STS presented earlier. It can also be seen that the resistance of the wires are of the same order as the copper foil. It is usual to expect the resistance to scale with probe separation, this trend is not observed which we attribute to inconsistent contacts caused by surface contamination.

[0046] Surface scattering is an influential occurrence after the miniaturization of devices and interconnects (Solid State Physics, Elsevier Science, 1985; Phys. Rev. B., 74, 085109, 2006) whereby electrons undergo either elastic or inelastic scattering depending on the local surface states. Furthermore, the surface electron density located near the nanowire surface makes a large contribution to the total conduction electron density. In FIG. 5A, it can be seen this phenomenon occurring at the interface of the Cu and the MLNG self-assembled nanowire. While the structure is too large to contribute to tunneling, simulations show that sp.sup.2 carbon does contribute states to the copper at the Fermi energy (FIG. 5B). Therefore, these surface states demonstrate an influential role in the electrical conductivity of Cu on the nanometer scale. Comparing the partial density of states (PDOS) of nanographene on Cu with the PDOS of pristine Cu, new states present around the Fermi Energy in the simulation of Cu with NG are due to the interaction between Cu 3d electrons and the C 2p electrons. Simulations of the surfaces provides further explanation of electronic behavior and thus electron transport at the interface. These findings are supported by previous theoretical investigations performed on a contact configuration simulated by a metallic single wall carbon nanotube and linear copper chain (Phys. Rev. B., 60, 6074-6079, 1999). Cu atoms positioned in front of the CNT's carbon atom significantly influence the electronic properties of the entire hybrid system. Moreover, the hybrid system was shown to induce a sizeable charge transfer from the Cu 4s and 3d states to the C atom of the tube. Therefore, a higher number of electronic states are made available, thus increasing the systems total DOS compared pristine Cu or C alone. Studies performed on Cu nanoclusters on ziz-zag graphene nanoribons have also revealed the same hybridization of p and d orbitals between C and Cu in DOS simulations (J. Comput. Electron., 14, 270-279, 2015). An additional study on the impact these effects have on the transport of the nanoribbons determined that copper can alter the current-voltage characteristics and create a negative differential resistance (J. Appl. Phys., 116, 093701, 2014).

[0047] Electron Dispersion X-ray (EDX) Elemental analysis shows both the starting material and the novel product to be made up of only carbon and no other elements in FIG. 6.