Patent Application
20120261644
1. Field of the Invention
This invention relates to graphene in a narrow ribbon structure and a method for making the same. More specifically, the invention relates to the use of graphene ribbons in electrical devices.
2. Brief Description of the Prior Art
Graphene is defined as a single layer of graphite with the carbon atoms occupying a two-dimensional (2D) hexagonal lattice. It has been used extensively in the past to model the electronic structure of carbon nanotubes (CNTs) [See R. Saito, G. Dresselhaus, M. S. Dresselhaus, Physical Properties of Carbon nanotubes, Imperial College Press, London, 1998; T Ando, Advances in Solid State Physics, Springer, Berlin, 1998, pp 1-18, S. Reich, C. Thomsen, J. Maultzsch “Carbon Nanotubes” Wiley-VCH, 2004 ISBN 3-527-40386-8]. Graphene is a 2D, zero-gap semiconductor that exhibits a linear relationship between the electronic energy E(p) and the 2D momentum p, i.e. E(p)=v0 p, (where v0 is the carrier velocity, p=η√{square root over (kx2+ky2)}, η is Planck's constant divided by 2π, and kx, and ky are reciprocal space vectors in the x and y direction, respectively), instead of the quadratic energy-momentum relationship that describes the energy bands of common semiconductors [See C. Berger et. al., Science 312, 1191, (2006).]. This implies that the electron effective mass is zero and the charge carriers in graphene can be described as relativistic Dirac Fermions. Graphene layers with fairly large lateral dimensions have been produced either by exfoliation of graphite [KS. Novoselov et. al., Science 306, 666, (2004)], epitaxially on SiC by high temperature decomposition of the latter [C. Berger et al. J. Phys. Chem. B, 108, 19912, 2004], or by chemical vapor deposition (CVD) on metals. [Reina, A. et al. Large Area, Few-Layer Graphene Films on Arbitrary Substrates by Chemical Vapor Deposition. Nano Lett. 9, 30-35, (2009); Li X et al. Large-Area Synthesis of High-Quality and Uniform Graphene Films on Copper Foils Science 324, 1312-1314, (2009)]. Reported studies have revealed the remarkable transport properties of graphene [KS. Novoselov et. al., Nature (2005) 438, 197; Y. Zhang et. al., Nature (2005) 438, 201; C. Berger et. al., Science (2006) 312, 1191; MI. Katsnelson, Materials Today (2007) 10, 20] including electron and hole mobilities of the order of 2×10 cm /V.s, i.e. similar to those reported for single CNTs, or higher. However, in suspended graphene devices, the carrier mobility μ=(neρ)−1, where n is the carrier density and ρ is the resistivity, can exceed 200,000 cm [See Bolotin, K. I. et al. Ultrahigh electron mobility in suspended graphene. Solid State Commun. 146, 351-355 (2008).]
Based on these properties, graphene is being considered for application as the active channel for field effect transistor (FET) applications. However, due to the fact that a macroscopic 2D sheet of graphene is a zero-band-gap semiconductor, it cannot be used in FET for digital, logic applications in its 2D form. A minimal conductivity of approximately e2/h (where e is the electron charge and h is Planck's constant) has been observed experimentally in both single and bilayer graphene [See K. S. Novoselov et. al., Nature (2005) 438, 197; MI. Katsnelson, Materials Today (2007) 10, 20]. This would make it impossible to create FETs with reasonable Ion/Ioff ratios (i.e., on-off current ratios), as Ioff would be too high.
Prior art has shown that a small band gap in the range of 100-250 meV can open in 2D graphene in special situations (e.g. by doping part of a graphene bilayer [T. Ohta, A. Bostwick, T. Seyller, K. Horn, E. Rotenberg, Science, 313, 951, (2006)], due to interaction with the substrate [Zhou S. Y. et al. Substrate-induced bandgap opening in epitaxial graphene Nature Mater. 6, 770, (2007)], or by applying an appropriate combination of antisymmetric electrical fields perpendicular to the plane of a graphene bilayer [Y. Zhang, et al. “Direct observation of a widely tunable bandgap in bilayer graphene,” Nature 459, 820 (2009)]). Band gap values in this range are probably too low for digital applications. Additionally, there are serious inherent problems with all these approaches, including that there is no good method to apply controlled doping in graphene; the choice of substrate cannot be made based solely on the need to open a band gap; the effect of substrate surface defects and non-uniformity on the properties of graphene is pronounced; In traditional field effect transistors, where the active layer is a semiconductor with a fixed band gap, the gate is used to modulate the carrier concentration in the channel, not to modulate the band gap of the semiconductor. Requiring a second role for the gate would seriously complicate or impede the performance of a graphene transistor.
Theoretical calculations have shown that narrow ribbons of graphene with widths ranging in the nanometer scale, defined as nanoribbons here, exhibit an energy band gap. This is because, electrons in graphene, besides their 2D confinement, are further confined by the narrow width of the nanoribbons. The latter confinement results in splitting of the original 2D energy levels of graphene making the graphene nanoribbons semiconductors with a finite energy gap.
Top down approaches for patterning a 2D graphene sheet into graphene nanoribbons from 2D graphene, with specific placement and orientation, for example using some kind of lithography (optical or electron beam lithography), are limited to minimum ribbon width sizes of about 30 nm for making an array of ribbons and in the easier case of an isolated feature just about 10 nm. At 30 nm graphene nanoribbon width the calculated band gap is less 0.1 eV and at 10 nm width it is about 0.2 eV. Using top-down graphene nanoribbon patterning, prior art has produced band gaps in the range of 30 meV [See Z. Chen et al. “Graphene nano-ribbon electronics” Physica E 40, 228, (2007)] to 200 meV [See M. Y. Han et al. “Energy Band-Gap Engineering of Graphene Nanoribbons” Phys. Rev. Lett. 98, 206805 (2007)]. These values are quite low compared to main stream semiconductors (e.g. the gaps of Si, Ge and GaAs are 1.12, 0.66, and 1.42 eV respectively) but in the range of energy band gaps of some low-band-gap compound semiconductors (e.g. InSb has a gap of 0.17 eV). [See S. M. Sze, Physics of Semiconductor Devices, 2nd edition, 1981, p. 849].
Methods of forming graphene nanoribbons by unzipping carbon nanotubes parallel to their long axis have recently been reported, and graphene nanoribbons narrower than 10 nm have been successfully fabricated. Tour et al. recently reported a solution-based oxidative process for producing oxidized graphene nanoribbon structures by lengthwise cutting and unraveling of multiwall carbon nanotube (MWCNT) side walls. [See D. V. Kosynkin et al. Nature 458 872 (2009)]. Then they reduced them to graphene nanoribbons. Dai et al. [See Li X. et al. Science 319, 1229 (2008)] exfoliated commercial expandable graphite (Grafguard 160-50N, Graftech Incorporated, Cleveland, Ohio) by heating to 1000° C. in forming gas (3% hydrogen in argon) for 60 seconds. The resulting exfoliated graphite was dispersed in a 1,2-dichloroethane (DCE) solution of poly (m-phenylenevinylene-co-2,5-dioctoxy-p-phenylenevinylene) (PmPV) by sonication for 30 min to form a homogeneous suspension. Centrifugation then removed large pieces of materials from the supernatant. Atomic force microscopy (AFM) was used to characterize the materials deposited on substrates from the supernatant and numerous Graphene Nanoribbons (GNRs) with various widths ranging from w˜50 nm down to sub-10 nm were observed. Topographic heights of the GNRs (average length˜1 micron) were mostly between 1 and 1.8 nm, which, according to the authors of that report correspond to a single layer or a few layers (mostly≦3 layers). [See Li X. et al. Science 319, 1229 (2008).]
To complicate things further, the crystallographic orientation of the edge of graphene nanoribbons determines if the latter is semiconducting or metallic. Specifically, if the long edge of a graphene nanoribbon has the “zigzag” structure, the layer does not have a band gap. On the other hand, if the long edge has an “armchair” structure the graphene nanoribbon is semiconducting. [See K. Nakada, M Fujita, G. Dresselhaus and M. S. Dresselhaus, Phys. Rev. B (1996) 54, 17954]. Furthermore, if the number of the repeating units, N, in the generally semiconducting armchair graphene nanoribbons has specific values, i.e. when N=3M−1, where M is an integer, the GNR is metallic. [See K Nakada, M Fujita, G. Dresselhaus and M. S. Dresselhaus, Phys. Rev. B (1996) 54, 17954].
All of the references cited herein are incorporated by reference in their entirety.
It is immediately apparent that both of these methods of creating sub-10 nm wide graphene nanoribbons have manufacturability issues. The Dai et al. method, produces a mixture of nanoribbon lengths and widths. The variation in width would produce a variety of band gaps in semiconducting nanoribbons. The Tour et al. method, probably produces nanoribbons with tighter width distribution as the width is controlled by the CNT diameter, but because MWCNTs are used, each layer would produce a different nanoribbon width even if a supply of MWCNT with very tightly controlled diameter were available. However, the prior art does not disclose the CNT diameter control necessary to produce nanoribbons with widths tightly controlled enough for industrial applications. Furthermore, the variety of directions that the unzipping of a CNT wall can take, as described in the relevant reference, would result to various graphene nanoribbon chiralities and edge structures. As we described above, changing the edge structure of a GNR with a specific width from “zigzag” to “armchair” can change the GNR electrical character from metallic to semiconducting, respectively. Thus, not having precise control over the GNR chirality and edge structure renders the GNR property unpredictable and thus not broadly applicable technologically.
None of the prior art, including Tour and Dai, disclose structures of graphene or methods of fabricating these structures with demonstrated control over the type, size (width and length), chirality, and edge structure of the nano ribbons. Further, none of the prior art has methods for producing one specific type of nano ribbon without producing other types. In addition, none of the prior art discloses a structure of two or more nano ribbons of the same type placed together. In addition, none of the prior art discloses straight nano ribbons of a specific type that are placed together.
Even though the prior art has randomly produced graphene nanoribbons of dimension below 9 nm in width, it has failed to disclose nanoribbons with a specific band gap. Further, the band gaps disclosed in random samples in the prior art are randomly distributed and thus it is difficult, expensive, impractical, or impossible to produce structures useful in electrical devices from such samples. One reason for this is that the prior art can not produce structures that are uniform in shape, size, straightness, and chirality; have uniform band gaps; or have predictable placement or orientation. The prior art does not disclose two or more nanoribbons placed together with the same band gap within a given tolerance. This fatal shortcoming prevents use of the prior art in large scale production of electric devices using graphene nanoribbons.
Both methods would suffer strongly from uncertainty of placement of GNRs on a substrate, since a solution method is used. This randomness, does not offer any improvement over the problems of predictably placing CNTs on a substrate, and thus there would be no reason to use GNRs instead of CNTs for electronics applications. Furthermore, graphene nanoribbons produced in these prior-art described ways, cannot directly benefit from recent progress of controlled placement of CNTs.
This clearly shows that due to the small width required to reach large enough band gaps to create FETs with reasonable device characteristics, and, equally importantly, the interatomic bond dimension level accuracy and control required for creating the appropriate edge structures of the nanoribbons in order to make them semiconducting, top-down approaches for creating graphene nanoribbons will fail to produce the required structures for electrical devices, e.g. FETs or diodes. Currently this prediction is shown to be true.
“Atomically precise bottom-up fabrication of graphene nanoribbons” by Jinming Cai et al. [Cal J. et al Nature 466, 470, (2010)] discloses a structure and method for fabricating graphene nanoribbons by surface assisted coupling of molecular precursors into polyphenylenes and subsequent cyclodehyrogenation. The reference used a 10,10′-dibromo-9,9′-bianthryl precursor monomer to create ordered rows of this molecule on a metal surface an then polymerize the molecules by heating to form graphene nanoribbons. However, the Cai reference uses a bianthryl molecule with a rotationally flexible covalent bond which creates a non-rigid molecule with many possible conformations on the substrate surface. Therefore, the Cai reference method has difficulty creating a multitude of parallel ribbons with certainty. Inspection of
An aspect of this invention is a structure and a method to produce a structure of one or more graphene ribbons that are uniform in shape, size, straightness, and/or chirality; have uniform band gaps; and/or have predictable placement or orientation.
An aspect of this invention is a graphene ribbon structure less than 9 nm wide, more preferably less than 1.6 nm wide.
A further aspect of this invention is a graphene ribbon structure that is five fused aromatic rings or less in width fused together in a ribbon length.
A further aspect of this invention is a graphene ribbon structure that is five aromatic rings or less in width fused in a length direction with a width tolerance of less than 0.5 nm in variation, more preferably less than 0.1 nm in variation.
A further aspect of this invention is a graphene ribbon structure that is five aromatic rings or less in width fused in a length direction and has “arm chair” edges.
A further aspect of this invention is a graphene ribbon structure that is five aromatic rings or less in width fused in a length direction that has “arm chair” edges and with a width tolerance of less than 0.5 nm in variation, more preferably less than 0.1 nm in variation.
An aspect of this invention is a graphene ribbon structure less than 9 nm wide, more preferably less than 1.6 nm wide with a predictable placement on a substrate.
An aspect of this invention is a graphene ribbon structure less than 9 nm wide, more preferably less than 1.6 nm wide with a predictable orientation on a substrate.
An aspect of this invention is a graphene ribbon structure less than 9 nm wide, more preferably less than 1.6 nm wide with a predictable orientation on a substrate where the orientation is related to a crystal orientation of the substrate.
An aspect of this invention is a graphene ribbon structure less than 9 nm wide, more preferably less than 1.6 nm wide, that is connected to one or more conductive electrodes using standard patterning techniques, e.g. lithography.
An aspect of this invention is a graphene ribbon structure less than 9 nm wide, more preferably less than 1.6 nm wide, which is used in an electronic device, e.g. an FET or diode.
An aspect of this invention is a graphene ribbon structure less than 9 nm wide, more preferably less than 1.6 nm wide, which is used in a channel region of an electronic device, e.g. an FET.
An aspect of this invention is multiple graphene ribbon structures less than 9 nm wide each, more preferably less than 1.6 nm wide each, which are used in a channel region of an electronic device, e.g. an FET.
An aspect of this invention is multiple graphene ribbon structures less than 9 nm wide each, more preferably less than 1.6 nm wide each, which are layered and used in a channel region of an electronic device, e.g. an FET.
A further aspect of this invention is a graphene ribbon structure that is five fused aromatic rings in width with a width tolerance of less than 0.5 nm in variation, more preferably less than 0.1 nm in variation that uses pentacene molecules as the molecular building block.
A further aspect of this invention is a graphene ribbon structure that is four fused aromatic rings in width with a width tolerance of less than 0.5 nm in variation, more preferably less than 0.1 nm in variation that uses tetracene molecules as the molecular building block.
The present invention is a ribbon of graphene less than 3 nm wide, more preferably less than 1 nm wide. In a more preferred embodiment, there are multiple ribbons of graphene each with a width of one of the following dimensions: the length of 2 phenyl rings fused together, the length of 3 phenyl rings fused together, the length of 4 phenyl rings fused together, and the length of 5 phenyl rings fused together. In another preferred embodiment the edges of the ribbons are parallel to each other. In another preferred embodiment, the ribbons have at least one arm chair edge and may have wider widths.
The invention further comprises a method of making a ribbon of graphene comprising the steps of:
The invention further comprises an electrical device structure having two or more ribbons of graphene in surface to surface contact with a non conductive substrate. Each of the ribbons has a width less than 3 nm and each of the ribbons has edges that are parallel to one another. In a preferred embodiment the ribbons comprise a channel in a Field Effect Transistor (FET).
Therefore, the present invention is a bottom-up approach, i.e. one where the graphene nanoribbons are constructed by using methods for self-assembly of appropriate molecules to make ordered rows of such molecules on appropriate substrate surfaces. The appropriate molecules (preferably anthracene, tetracene, pentacene) are chemically changed while assembled in flat-lying rows on a substrate into aromatic macromolecules with reduced volatility. In a preferred embodiment the chemical aromatic macromolecules are created by energetic beams (e.g. electromagnetic radiation, for example UV light, X-rays or e-beam or other radiation) or plasma that would cause the chemical change before the appropriate precursor molecules evaporate or sublime. The aromatic macromolecules are further converted to form graphene nanoribbons by a combination of heat and/or radiation. Since the aromatic macromolecule has a higher molecular weight the aromatic macromolecule is not volatile and can absorb the higher heat and/or radiation without subliming/evaporating before it converts into graphene nanoribbons (GNR). These GNRs have a width and edge structure accurately defined by the chemical structure of the original (precursor) molecule and the geometric structure of the ordered rows of the flat-lying “acene” precursor molecules.
Furthermore, unlike CNTs, where periodic boundary conditions are present, GNRs have edges with localized states [10] that can also affect transport. As very narrow GNRs are needed to achieve a band gap usable for FET applications for logic, the effect of the edges can be critical. Theoretical calculations have shown that when different hetero-atoms occupy are attached, or occupy edge positions in a graphene nanoribbon lattice, the transport properties of the various graphene nanoribbons are substantially affected. Thus, controlling the chemistry of the long edge of a graphene nanoribbon is very desirable. Here too, the prior top-down approaches do not offer the precision and selectivity of placing specific atoms at specific edge sites of a top-down fabricated graphene nanoribbon. On the other hand, the bottom-up approach proposed in this disclosure is very appropriate for doing exactly that. Starting with an appropriately functionalized molecule, i.e. the monomer (preferably acene molecules) from which the nanoribbon is created after polymerization (making the aromatic macromolecule), an appropriate edge functionalization of the graphene nanoribbon can be created, by specifically synthesizing the precursor molecules to comprise the desired atoms at its long ends.
Therefore, by using the preferred acene molecules (shown in Figures as elements 300, 530, and 560) the created graphene has the desired hydrogen termination at both the long edges and ends of the GNR in addition to the desired “arm chair” edge structure. Further, the precursor acene molecules could be modified by adding appropriate atoms other than hydrogen at the ends of the molecule creating acene derivative molecules. Alternatively, these acene derivative molecules could be used to create GNR using this disclosure where the GNR long edge will have the “arm chair” structure but with terminal atoms other than hydrogen. For example, GNR created with acene derivative molecules with Bromine (Chlorine, Nitrogen, etc.) bound to the ends of the acene molecules will produce GNRs with Bromine (etc.,) terminations on the long edges. Doing this could create electrical properties (e.g., electron mobilities) in the GNR that are different and/or superior to those GNR created by top down approaches or using this approach with acene precursors. Potentially, using acene derivative molecules as precursors could create electron mobilities as high as two dimensional graphene.
While
Furthermore, we have added dashed lines, one 240 to indicate intermolecular distance within a row and two others 250 to show the row distance, distance between rows. These distances and restrictions not recognized in the prior art for the purposes of forming aromatic macromolecules. The lines 250 show the clearly existing gap between the van der Waals surfaces of molecules in two nn rows of pentacene (an acene precursor) molecules. We calculate that this gap is about 2 Å. On the other hand, the same image shows that there is no gap 240 between the van der Waals surfaces of such pentacene molecules in the same row. Thus, reaction between molecules in the same row is bound to happen when C—H bond are broken by appropriate energetic radiation, thus initiating the polymerization along a single row of pentacene molecules. Reaction is not expected to happen between molecules in nn rows, due to the gap of 2 Å, row distance 250 that exists between their van der Waals surfaces.
Note that substrates other than gold may be selected to achieve the purposes of this disclosure so long as the substrates can: 1. cause the precursor molecules to lie flat in row, 2. the row distance 250 is large enough to prevent inter-row polymerization, and 3. the intermolecular gap distance 240 between the nn precursor molecules within a row is small enough to create a bond between the precursor molecules. Examples of alternative substrates would be ones with a surface reconstruction that creates a unique orientation on the substrate, for example, silicon (110) and silicon (100) surface reconstruction creating dimer rows. There are many other (110) and (100) surfaces that could be used, e.g., copper (110). Further, these substrates can not form covalent bonds with the precursor molecules.
In addition, insulating substrates are contemplated which have a dimer row surface reconstruction. These surfaces have long range order reconstructions that would yield very long GNRs. An example of an insulator would be silicon carbide.
It is important that at least one bond between nn acene molecules within the same row is formed. This is done by applying radiation as with enough energy to dissociate C—H bonds for dehydrogenation and subsequent formation of C—C bonds between nn molecules in the same molecular row. In a preferred embodiment, the radiation applied has an energy spectrum that includes wavelengths shorter that visual light (e.g. ultraviolet radiation, x-ray radiation, electron beam or gamma rays). In a more preferred embodiment, UV light is used with a wave length between 250 and 350 nanometers. This range is selected based on the dissociation energy of the C—H bonds, which is between 3.8 and 4.6 eV (wavelength of 326-270 nm) according to the Table in
In a more preferred embodiment, UV light from a mercury (Hg) light source is used. A representative energy spectrum produced by such a source is shown in
The exposure of the ordered acene monolayer has to take place at a temperature that is well below the sublimation temperature of the specific acene in vacuum. By creating the intermolecular bonds (at least one for each nn molecule pair) between molecules within the same molecular row, a macromolecule is created whose sublimation temperature increases proportionally to each size. Thus, we subsequently raise the temperature (optionally with simultaneous irradiation with the chosen type of radiation) without the possibility of sublimation of the molecules and destruction of the molecular rows. When the temperature is high enough, dehydrogenation (dissociation of C—H bonds and removal of H atoms that sublime into the vacuum chamber) takes place, leaving behind dangling bonds of C, that eventually lead to formation of covalent C—C bonds between nn acene molecules. This is a consequence of the high energy state of two dangling bonds in nn sites compared to the formation of a new C—C bond. At even higher annealing temperatures, approaching 1000° C. but below the melting point of the substrate (e.g. Au), further dehydrogenation and the formation of the sp2 structure typical of graphene will take place, as this is a favored and very stable state, energetically. Prior art provides numerous examples of graphitization and the stability of the formed graphitic species at temperatures between 700 and 1000° C.
As shown in
It is possible that some C=C and C—C bonds in the sp2 structure of an acene molecule will also be broken by impinging photons during the radiation treatment of the acene layer (either at room temperature or higher temperatures). However, carbon atoms are not volatile even at temperatures much higher that 1000° C. (e.g. 1500° C.). Thus, C atoms, even if their C—C bond to a nn C atom is broken, they remain in place on the substrate, and thus they have all the time necessary to reform such C—C bonds and eventually the energetically favored sp2 structure of graphene. Obviously, this is not the case with the H atoms, which are volatile even at room temperature, immediately after a C—H bond is broken. This ensures that intermolecular C to C bonds (and eventually sp2 structure) will be formed while intramolecular C to C bonds are preserved, despite the fact they may be broken temporarily and reformed. Thus, since the kinetics allow it (long time available), eventually the energetically sp2 structure will form, creating the desired GNRs using a bottom-up fabrication scheme.
After long irradiation at a temperature below the sublimation point of the specific acene molecules used in the specific process (can be room temperature), which ensures formation of at least one bond for each nn pair of molecules within the same molecular row, the temperature is ramped up to promote further dehydrogenation and eventually sp2 structure formation at temperatures approaching 1000° C. The ramp rate may vary between 10° C. per minute and 200° C. per minute, followed by anneal at a specific elevated temperature, between 500 and 1000° C., preferably 1000° C.
The process 600 begins by depositing 610 an acene precursor layer on a substrate that cause the acene precursor molecules to assemble in rows, as shown in
As stated above, the acene precursor will align in rows 300 with intermolecular distances 240 that are near zero and row distances 250 of about 2 Å because of the epitaxial relation of the precursor molecules to the substrate.
The next step 620 is forming at least one bond between nn acene molecules within the same row. This is done by applying radiation as describe above in the description of
In step 630, graphene is formed by changing the macromolecule 400 formed in step 620 by adding heat. Since a macromolecule 400 was formed in step 620, the addition of heat will not volatilize the molecule before it is de-hydrogenated to form the GNRs 500. The amount of heat applies preferably is between 250 degrees ° C. but below the melting point of the substrate (e.g. gold layer). Pure gold has a melting point of 1064° C. The heat could be applied from 10 minutes to 10 hours with optimal times determined by experimentation, in an oxygen free atmosphere. In a preferred embodiment, the radiation applied in step 620 will continue throughout the heat application in step 630. The heat preferably will be applied in the vacuum chamber of
The apparatus 700 comprises a known vacuum chamber 710 for general deposition of materials on substrates. These vacuum chambers 710 are well known and can be purchase as a complete unit or in components for assembly. The vacuum chambers 710 are normally evacuated by vacuum pumps (not shown) through the vacuum pump port 730. The vacuum pumps can be one turbo pump and one mechanical pump in series configuration and optionally can include an ion pump and a titanium sublimation pump.
The chamber apparatus 700 comprises a heated substrate holder 720, e.g. a polymeric boron nitride/pyrolytic graphite heater commonly available for this purpose.
The chamber apparatus 700 further comprises a molecular source 750 that can be an effusion cell 750 that is commonly known.
The chamber apparatus 700 further novelty comprises a radiation source 740 that is used to apply radiation, preferably UV light, through a window that is transparent at the spectrum frequencies necessary for breaking carbon-hydrogen bonds. In a preferred embodiment, the window is made from quartz and the radiation source is a mercury (Hg) lamp that will produce the spectrum shown in
The structure in
The structure in
The structure in
In
Pentacene molecules, which have a length just above 1.5 nm, can grow parallel ribbons of the same width separated by a narrow gap from a nearest neighbor (nn) ribbon on appropriate surfaces. The pentacene molecules lie with their long axis practically parallel to the surface and practically perpendicular to the direction of the strip, as shown in
After growth of such self-assembled pentacene single-molecule rows on an appropriate surface, an ultraviolet (UV) radiation or electron beam (e-beam) treatment should be used to make crosslinks (bonds) between the aligned nn pentacene molecules. Radiative treatments at a judiciously chosen temperature are preferred to a simple heat treatment without radiation, because pentacene will most likely evaporate before crosslinking starts by just heating between ca. 150-300° C. depending on the environment and the interaction with the substrate. After the radiation treatment at a temperature below the sublimation temperature of pentacene, some crosslinks should form randomly between neighboring molecules along the same strip (row of pentacene molecules). At that point, the large dimensions of the resulting supermolecule (made by linking many pentacene molecules together with covalent bonds) do not allow its sublimation from the substrate.
Continuation of the radiative treatment at higher temperature than the initial crosslinking process step is expected to push reaction towards the thermodynamically stable state, which would be the graphene ribbon formation, at a temperature lower that the one required for graphene nanoribbon formation by simply heating the supermolecule formed in the initial low temperature radiative step.
If the substrate surface used for the self-assembly of flat lying pentacene molecular rows is not insulating, but conductive, as is the case with the Au (111) surface used in the embodiment described above, then the graphene nanoribbons have to be transferred to an insulating substrate without disturbing their structure. This can be done by first depositing an insulating material 805 on the GNRs, e.g. depositing 10 nm of polyxydroxystyrene-based NFC that wets the graphene surfaces (spreads on them) followed by deposition of a second, thicker insulating layer of HfO2 by atomic layer deposition (ALD), as described in prior art [see Farmer D. B et al. Nano Lett. 9, 4474, (2009)]. Then depositing a metal layer and patterning this layer to form the metal gates 850 of the prospective GNR transistors (
Following that a thick layer of material is molded on the previous substrate to act as a handle wafer 825 (
Then the original substrate 820, on which a Au (111) surface 815 was grown is removed (this corresponds to step 930 in the flow-chart of
Following that step, Au could be patterned (this corresponds to step 940 in the flow-chart of
The method of embodiment 1 can be used with one difference: The pentacene molecule is replaced by tetracene, an acene molecule with four fused aromatic rings instead of the five fused aromatic rings of pentacene (
The method of embodiment 1 can be used with one difference: The pentacene molecule is replaced by anthracene, an acene molecule with three fused aromatic rings instead of the five fused aromatic rings of pentacene. This will result to even shorter GNRs (thus with even wider band gap) than tetracene.
One skilled in the art given this disclosure could envision alternative embodiments of this invention are within the contemplation of the inventor.
