Patent Application
20130273345
The present invention relates to a non linear optical response material.
The intensity and shape of laser pulses need to be manipulated in a number of advanced optical technologies. To achieve the required response speed, passive devices employing nonlinear optical materials are essential. Saturable absorbers exhibit increased transmittance at high optical intensities or fluences, which is useful for pulse compression, Q-switching and mode locking. In contrast optical limiters exhibit decreased transmittance through a variety of mechanisms such as excited-state absorption, two-photon absorption, free-carrier absorption, nonlinear refraction and scattering. This can also be used for pulse shaping and mode locking in advanced optical technologies, but most importantly for protection of optical and focal-plane array sensors from irreversible damage by intense pulses and thus extension of their survivability dynamic range.
Various materials exhibit OL characteristics including giant π-electron systems such as carbon-black suspensions (CBS), single- and multi-wall carbon nanotube (CNT) suspensions, and small π-electron systems such as fullerenes, porphyrins and phthalocyanines. For over a decade now, the performance benchmark for broadband OL has been held by CBS and CNT suspensions. Their OL property arises primarily through nonlinear scattering of breakdown-induced microplasmas. However this mechanism is not effective against pulses shorter than a few ns due to the induction time, or in solid films because it destroys (vaporizes) the material. The small π-electron systems on the other hand can show OL due to excited-state absorption (also called “reverse saturable absorption”) through build-up of the triplet population at the sub-ns time scale. The ratio of excited-to-ground-state absorption coefficients however strongly depends on the pump wavelength and hence these materials cannot provide broadband coverage.
Both graphene and graphene oxide suspensions have recently been reported to show broadband OL in a variety of solvents with nonlinearity thresholds and clamping (i.e. output-limiting) characteristics that are broadly similar to those of CBS and CNT suspensions The measured off-axis scattering suggests indeed a similar nonlinear scattering mechanism. Therefore CBS and CNT suspensions provide the best performance prior to this invention for broadband optical limiting. Despite their many limitations (chiefly, their ineffectiveness in solid thin films), no other materials can match comparable performance.
Furthermore, state-of-the-art work on graphene and graphene oxide suspensions show that they too do not greatly differ from CBS and CNT suspensions in their optical limiting characteristics or mechanism.
For example, WO2010129196 involves dispersing of heavily-oxidized graphene oxide sheets (GOx) in water and unfunctionalized graphene sheets (GS) in water or organic solvents with addition of stabilizing reagents. Evidently, these dispersions are still not very stable and precipitate over time and thus their optical limiting mechanism arises primarily through nonlinear scattering of breakdown-induced microplasmas. Moreover, their optical limiting effect is inferior.
In general terms the invention relates to surface chemically-functionalized graphenes, such as functionalized sub-stoichiometric graphene oxides (sub-GOx), when dispersed as single sheets in appropriate liquid cells (such as solvents containing heavy-atoms) or film matrices. This may provide extremely efficient nanosecond optical-limiting characteristics for pulse shaping and anti-glare application. These dispersions may display an ability to limit the optical power of intense light sources with non-linearity onset thresholds, half-transmittance thresholds and clamping levels that are vastly superior to previously known materials by 5-10 times, and are effective over a broadband of wavelengths from ultraviolet to near infrared, and for pulses from sub-nanosecond to tens of microseconds.
In a first expression of the invention there is provided a method of fabricating an optical response material comprising:
Preferably, the single sheet graphene is functionalized sub-stoichiometric oxidized graphene.
Forming the film may comprise dissolving the graphene in solution and depositing the solution on a substrate. Further, forming the film may comprise dispersing the graphene in a solid film.
Preferably, the onset of the nonlinear optical limiting effect in the thin film or liquid cell is less than 100 mJ/cm2 or more preferably less than 10 mJ/cm2 when the liner transmittance preferably fall between 50-90%.
In a second aspect of the invention there is provided an optical-limiter comprising:
Preferably, the chemical functionalization comprises both sub-stoichiometric oxidation, and attachment of surface modifier groups, wherein the surface-modifier group may comprise solubilizing groups from the class including alkyl, cycloalkyl, aryl, arylalkyl, fluoroalkyl and fluoroaryl groups; and/or ionic groups from the class including carboxylic acid, sulfonic acid, phosphonic acid and their salts, quarternary ammonium; and/or polar groups from the class including ester, amide, nitro, cyano; sulfone, sulfoxide; and/or heavy atoms from the class including sulfur, chlorine, bromine, iodine, cadmium, mercury, silver, gold platinum, palladium, yttrium, zirconium, lanthanum, cerium, caesium, barium; and/or electron withdrawing groups; and/or electron donating groups.
The liquid cell may be from the class of heavy atoms solvents with its atomic number bigger than 20. Preferably, the liquid cell is from the class of haloaromatics including chlorobenzene, dichlorobenzenes, trichlorobenzenes, bromobenzene, dibromobenzenes and tribrombenzenes, and their higher halogenated or mixed halogenated analogues. More preferably, the liquid cell is from the class of electron-withdrawing and/or electron-donating solvents.
The solid thin film may be from the class of transparent matrices, including polymers such as polycarbonates, polyimides, polyesters, polyacrylates, polycarbazoles, epoxy polymers, novalak, formaldehyde polymers, polymer containing heavy-atoms, polymer containing electron withdrawing groups, polymer containing electron donating group and sol-gel materials, including sol-gel silica, sol-gel titania, silsequioxanes.
The optical-limiting mechanism may occur by excited state absorption.
In a third aspect, there is provided a thin film comprising an optical limiting layer according to any of the above features.
In a fourth aspect, there is provided a device selected from the group consisting of an anti-glare treated device and a sensor protected with pulse shaping, comprising an optical limiter according to any of the above features.
Embodiments may have the benefits of:
One or more example embodiments of the invention will now be described, with reference to the following figures, in which:
According to one embodiment when graphenes, and sub-stoichiometric graphene oxides (sub-GOx) which are representative members of the class of functionalized graphenes, are dispersed as single sheets in heavy-atom solvents, electron-withdrawing solvents, electron-donating solvents or in film matrices, these functionalized (and unfunctionalized) graphenes may exhibit a giant OL response with nonlinearity thresholds that may be ten times lower than (and hence superior to) the previous benchmarks set by CBS and CNTs, and may also exhibit desirable broadband absorption and optical limiting. The functionalized graphenes may exhibit dispersability in a variety of solvents and solid matrices.
Functionalized graphenes may be dispersed substantially as single sheets rather than aggregated multilayer stacks as previously used.
Various surface chemically-functionalized sub-GOx and ultrasonically-exfoliated unfunctionalized graphene nanosheets may show this giant OL enhancement. Graphene nanosheets means graphene and graphene oxide sheets having a basal plane fraction of carbon atoms in the sp2-hybridized state between 0.1 and 0.9, wherein the remainder fraction of carbons atoms comprises sp3-hybridized carbon atoms which are bonded to oxygen groups selected from hydroxyl and/or epoxy and/or carboxylic acid. Therefore the effect is a feature of the extended pi-electron system that is present in the entire class of graphenes, whether functionalized or not. Appropriate surface chemical functionalization may improve dispersion at higher concentrations (between 1 mg/mL and 15 mg/mL; unfunctionalized graphene sheets not be sufficiently dispersed in concentrations above 0.1 mg/mL). Chemical functionalization may achieve “single-sheet” dispersion (SLG), meaning a state in which that the graphene sheets remain permanently dispersed in the solvent without aggregating (i.e., re-stacking) to give bilayer graphene (BLG) few-layer graphene (FLG) or multi-sheet objects.
Aggregation may lead to the settling out of the graphene materials and may suppress the giant OL effect, even for incipient aggregation that has not yet caused precipitation. Functionalization may thus prevent close sheet-to-sheet contact which may be detrimental to the giant OL property. The previous use of aggregated graphenes and sub-GOx in other studies in which FLG or multilayer sheets objects are formed in the suspension may be the main reason why the giant OL effect has not been found before. As long as it is not a single sheet dispersion, aggregation or agglomeration is said to have happened. Aggregation and agglomeration can result in FLG or multi-sheets objects. Typically, in this field, FLG is known to be more than two layer and less than 10 layers of stacked graphene sheets and multi-sheets objects are more than 10 layers of stacked graphene. This functionalization may promote an interaction with the solvent, and may suppress the tendency for sheet-to-sheet stacking. This may be encouraged with an atom group of the nanosheets of diameter bigger than 2 Angstroms. Unfunctionalized sub-GOx may be prone to restacking which may destroy the desired OL properties. Furthermore, it may be important to ensure that the graphene does not become fully oxidized, but remains in the sub-stoichiometric oxidized state, because the OL effect is a property of the extended pi-conjugation present in the sheets. “Functionalized graphenes”, may be derived from the functionalization of sub-stoichiometric graphene oxides, or other compounds, such as graphite intercalation compounds, fluorinated graphite, hydrogenated graphite, or other partially reacted graphite compounds. Thus SLG that has a basal plane fraction of carbon atoms in the sp2-hybridized state between 0.1 and 0.9, wherein the remainder fraction of carbons atoms comprises, consists of sp3-hybridized carbon atoms which are bonded to oxygen groups selected from hydroxyl and/or epoxy and/or carboxylic acid and may be obtained either from functionalized sub-oxidized graphene oxide or directly by functionalizing graphite to give single graphene sheets.
Examples of appropriate chemical functionalizations include surface-grafting with alkyl, cycloalkyl, aryl, arylalkyl, fluorocarbon, alkyleneoxy surface-chains. These chains are chosen principally to provide the desired dispersability in the chosen liquid cell or solid film. To achieve the required molecular compatibilization with the liquid cell or solid matrix, the chains could further optionally be functionalized with functional groups such as ionic groups from the class of carboxylic acid, phosphonic acid, sulfonic acid, or quarternized ammonium, or polar groups such as carbonyl, ester, amide, nitro, or hydroxyl group.
Another design principle for the surface-functionalization may be the use of groups to promote the formation of long-lived excited states. A mechanism for this giant OL effect may arises from a new excited-state absorption mechanism from long-lived spin-unpaired excited states. This mechanism is different from the nonlinear scattering mechanism due to breakdown that operates in CBS and CNT suspensions, and the triplet absorption mechanism in fullerenes and other molecules with small pi-electron systems. It may be a unique feature of the dispersed (i.e., molecularly separated) state of graphene including sub-GOx nanosheets, in which its band electronic structure becomes localized by interaction with the medium at high excitation densities to give long-lived and apparently spin-unpaired states.
In liquid cells, a large “heavy-atom” effect from solvents containing chlorine and bromine may exist. The heavy-atom effect refers to the enhancement of a spin-forbidden presence in the presence of a heavy atom that is a part of or external to the molecule. Heavy atoms are atoms with atomic number bigger than about 20, e.g. the following in an appropriate covalent or ionic form: sulphur, chlorine, bromine, iodine, selenium, cadmium, mercury, silver, gold platinum, palladium, yttrium, zirconium, lanthanum, cerium, caesium, barium. They exhibit large spin-orbit coupling of the electrons that allow the singlet state of neighbouring systems to intersystem cross to a lower-energy triplet state faster than the natural rate. This is a characteristic signature of the participation of triplet states. It may be advantageous to also incorporate such heavy atoms into the surface functionalization of the graphene sheets
For example it was reported previously that graphenes have weak optical-limiting effect in solvents like tetrahydrofuran, N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrolidone, γ-butyrolactone, using a nonlinear scattering mechanism. However according to an embodiment, graphenes may exhibit a giant OL effect in solvents such as chlorobenzene, 1,2-dichlorobenzene, 1,2,4-trichlorobenzene, bromobenzene.
Yet another design principle for the surface-functionalization may be the use of strong electron-withdrawing and/or electron-donating groups. Such groups stabilize and localise the excited state and thereby provide for large absorption cross-sections.
In some cases, this giant OL effect can be further enhanced by strong electron-withdrawing and/or electron-donating effect of solvents.
The dispersability at the single-sheet level in a variety of solvents is readily confirmed by any of the following: (i) resilience to centrifuge sedimentation at up to 1000 g (g=earth's gravity) for 10 min, (ii) appearance of extensively single-sheet films with the correct step heights upon spin-casting, and (iii) dynamic light scattering indicates the absence of aggregation. Typically these cells are fabricated by dispersing the functionalized graphenes to a concentration of 30-100 microgram/mL in 1.0-mm-pathlength cells, with light sonication as necessary, to give a linear transmittance of 0.7 at the desired blocking wavelength. The exact concentration required depends on the extent of functionalization of the graphene which affects its absorptivity, and can be determined readily by experiments from a standard Beer's law plot, and the path length selected. The concentration required is inversely proportional to this path length. So for example, if the path length is decreased from 1.0 mm to 100 micrometers, for example, through the use of spacers, the required concentration is increased correspondingly by a factor of ten. The functionalized graphenes typically have dispersability well above 1 mg/mL, so it is not difficult to achieve the correct concentration. The exact linear transmittance required will depend on application and the rest of the optical design, but will usually fall between 0.5 and 0.9.
It is also determined that these functionalized graphenes can be dispersed substantially as single sheets in solid thin films with other materials as compatible matrices. Compatibilization between the functionalized graphenes and the matrix is provided by the presence of the surface modifier group. The use of alkyl side chains (hexyl to octadecyl) in the surface-modifier group provides compatibilization with a variety of polymers including poly(methyl methacrylate), polystyrene, poly(dimethylsiloxane), poly(carbonate) and semiconducting conjugated polymers, chiefly by decreasing the tendency of the graphene sheets to re-stack and improving the van der Waals interaction with the polymers. Other solubilising groups that can be incorporated into the surface-modifier group includes cycloalkyl (e.g., adamantyl, cyclohexyl, cyclopentyl), aryl (e.g, phenyl), arylalkyl (e.g., phenylethyl), fluoroalkyl (e.g., perfluorohexyl, perfluorodecyl), fluoroaryl (e.g., pentafluorophenyl). Their selection depends on the nature of the matrix. In general, fluoroalkyl chains will promote compatibility with fluoropolymers, such as poly(perfluoroalkyl methacrylate), and poly[(tetrafluoroethylene)-co-(2,2-bis-trifluoromethyl-4,5-difluoro-1,3-dioxole)]. Furthermore the use of polar groups from the class including ester, amide, nitro, cyano, sulfone, and sulfoxide, in the surface-modifier group will improve the dispersability of the functionalized graphenes into polar engineering polymers or their precursors, including polyimides such as poly(oxydiphenylenepyromellitimide), polyetherimide, and polysulfones such as poly(bisphenol-A-dimethylsulfone).
The use of ionic groups from the class including carboxylic acid, sulfonic acid, phosphonic acid and their salts, and quarternary ammonium groups will improve the dispersability of the functionalized graphenes into water- and other polar-solvent-soluble polymers such as poly(vinyl alcohol), poly(hydroxystyrene), including polyelectrolytes such as poly(styrenesulfonic acid), poly(acrylic acid) and their salts, and sol-gel materials such as silica from tetraethyl orthosilicate or silsesquioxane, titania from titanium tetrachloride, and zirconia from zirconium tetrachloride.
The design principle for the matrix is similar to that for the solvents. Although giant OL effect is already possible in matrices of simple polymers such as poly(methyl methacrylate) and poly(bisphenol-A carbonate), it may be further enhanced by heavy atom effect and/or strong electron-withdrawing and/or electron-donating effects through the appropriate choice of polymers.
The adequacy of the dispersion is evidenced in some case, e.g., of amorphous polymer matrices, such polystyrene and poly(methyl methacrylate), by the shift of their glass transition temperatures (Tg) which is a well-established signature for fine dispersions at the molecular scale in polymers. If the sheets were aggregated, they will have no effect on the Tg of the polymer at the concentrations at which they were used (2-5 w/w %). The authors in fact found measurable changes of a few degrees Celsius as shown in
These composites can be made by dispersing the functionalized graphenes in a solvent in which the matrix material or its precursor is dispersed, and then forming the film by spin-coating, doctor blading or printing. Optionally, standard lithography can also be applied to pattern the film if desired. Then the composite film is dried or cured. Alternatively the functionalized graphene can also be dispersed into the matrix by compounding at elevated temperatures or by ball-milling, followed by film formation.
For example techniques such as those described in WO2009085015, which is incorporated herein by reference, can be used to alkyl-functionalize sub-oxidized graphene oxide sheets. Not all methods of fabricating SLG are applicable especially if the result also contains FLG, BLG or multi-layers objects such that they are unstable and agglomeration occurs over time.
Typically the SLG would be batch produced. It can then be deposited using any solution-processing techniques.
The liquid-cell and solid-film dispersions prepared this way with linear transmittance in the 50-90% range may show a giant optical limiting effect 5-10 times larger than what is previously known. For a typical liquid cell or solid-film transmittance of 70% at 532-nm wavelength, the typical fluence for the onset of optical limiting behaviour (Fon) is 10 mJ/cm2 from 500 nm to 1100 nm wavelength, while the typical half-transmittance threshold (F50) where the transmittance falls to half of the initial (linear) value at low fluences, is 80-100 mJ/cm2, for 3.5 ns pulses. The output clamping response, as given by gradient of the Fout vs Fin curve (T′) can be as low as 0.05 at few hundred mJ/cm2 for an initial transmittance of 0.7 as shown in
In addition, repeated Z-scan measurements on the same spot, shown in
Typically, this partially oxidized graphene oxide can be prepared by oxidation of synthetic graphite (for example graphite powder product code 496596 from Sigma Aldrich) using a modified Staudemaier oxidation in concentrated sulfuric-nitric acid with potassium chlorate at room temperature for 7 d, and recovered by filtration and exhaustive washing with Millipore H2O.
Typically, this is prepared by mixing of 10 mg of sub-GOx, 100 mg of octadecylamine (ODA) and 60 μL 1,3-diisopropylcarbodiimide in 5 mL of 1,2-dichlorobenzene and heated with intermittent sonication to 80° C. for 24 h under N2 to give a homogeneous black dispersion. 0.25 mL of this dispersion was mixed with 5 mL of tetrahydrofuran and sonicated briefly, centrifuged at 8000 revolutions per min (8000 rpm, corresponding to 5580 g) for 1 h to extract unpurified functionalized sub-GOx in the supernatant. The purified functionalized sub-GOx was obtained by repeated precipitated with 5 mL of ethanol and centrifuged at 1000 g 1 h.
Typically, this more heavily oxidized graphene oxide can be prepared by oxidation of synthetic graphite (for example graphite powder product code 496596 from Sigma Aldrich) using a modified Staudemaier oxidation in concentrated sulfuric-nitric acid with potassium dichromate at room temperature for 7 d, and recovered by filtration and exhaustive washing with Millipore H2O.
Typically, this is prepared by mixing of 10 mg of sub-GOx, 100 mg of octadecylamine (ODA) and 60 μL 1,3-diisopropylcarbodiimide in 5 mL of 1,2-dichlorobenzene and heated with intermittent sonication to 80° C. for 24 h under N2 to give a homogeneous black dispersion. 0.25 mL of this dispersion was mixed with 5 mL of tetrahydrofuran and sonicated briefly, centrifuged at 8000 revolutions per min (8000 rpm, corresponding to 5580 g) for 1 h to extract unpurified functionalized GOx in the supernatant. The purified functionalized GOx was obtained by repeated precipitated with 5 mL of ethanol and centrifuged at 1000 g 1 h.
In a typical preparation, synthetic graphite (for example graphite powder product code 496596 from Sigma Aldrich) was dispersed in 1,2,4-trichlorobenzene by sonicating 1 mg in 1 mL of solvent for 2 h, and then centrifuged at 1000 g to give a supernatant containing ca. 80 μg mL−1 of dispersed graphene sheets. It can also be dispersed in chlorobenzene and 1,2-dichlorobenzene. Other sources of natural or synthetic graphites can also be dispersed in these solvents.
ODA-functionalized sub-GOx was dispersed in chlorobenzene, 1,2-dichlorobenzene, 1,2,4-trichlorobenzene and bromobenzene separately by brief sonication to form 0.15 mg/mL dispersion. Higher concentration of 0.9 mg/mL dispersion can also be prepared in the same way.
ODA-functionalized GOx was dispersed in chlorobenzene, 1,2-dichlorobenzene, 1,2,4-trichlorobenzene and bromobenzene separately by brief sonication to form 0.15 mg/mL dispersion. Higher concentration of 0.9 mg/mL dispersion can also be prepared in the same way.
In a typical preparation, a polymer solution of 400 mg mL−1 PC in 10:1 v/v chlorobenzene:1,2,4-trichlorobenzene was prepared at 120° C. in the nitrogen glovebox. 6.0 mg of ODA-functionalized sub-GOx was separately prepared in 0.50 mL CB. 0.50 mL of the PC solution was added to this solution and sonicated 30 min to aid complete dispersion to give 2.9 w/w % ODA-functionalized sub-GOx to total solids. A 2.0-μm-thick film was formed by spin-coating at 3000 rpm on 13-mm-dia. fused silica discs (spectrosil), then baked at 90° C. (hotplate, 5 min).
In a typical preparation, 9.3 mg of ODA-functionalized GOx was separately prepared in 0.50 mL CB. 0.50 mL of PC solution was added to this solution and sonicated 30 min to aid complete dispersion to give 4.5 w/w % ODA-functionalized GOx to total solids. A 2.0-μm-thick film was formed by spin-coating at 3000 rpm on 13-mm-dia. fused silica discs (spectrosil), then baked at 90° C. (hotplate, 5 min).
In a typical preparation, a polymer solution of 400 mg mL−1 of poly(methyl methacrylate) (PMMA) in CB was prepared at 65° C. in the nitrogen glovebox. 12 mg of ODA-functionalized sub-GOx was added to 1.0 mL of this solution and sonicated 30 min to aid complete dispersion to give 2.9 w/w % ODA-functionalized sub-GOx to total solids. A 3.0-μm-thick film was formed by spin-coating at 1000 rpm onto 13-mm-dia. fused silica discs (spectrosil), then baked at 90° C. (hotplate, 5 min).
In a typical preparation, a polymer solution of 400 mg mL−1 of poly(9,9-octylfluorene-alt-triarylamine) (TFB) in CB was prepared at 65° C. in the nitrogen glovebox. 12 mg of ODA-functionalized sub-GOx was added to 1.0 mL of this solution and sonicated 30 min to aid complete dispersion to give 2.9 w/w % ODA-functionalized sub-GOx to total solids. A 3.0-μm-thick film was formed by spin-coating at 1000 rpm onto 13-mm-dia. fused silica discs (spectrosil), then baked at 90° C. (hotplate, 5 min).
In a typical preparation, a polymer solution of 400 mg mL−1 of polystyrene in CB was prepared at 65° C. in the nitrogen glovebox. 12 mg of ODA-functionalized sub-GOx was added to 1.0 mL of this solution and sonicated 30 min to aid complete dispersion to give 2.9 w/w % ODA-functionalized sub-GOx to total solids. A 3.0-μm-thick film was formed by spin-coating at 1000 rpm onto 13-mm-dia. fused silica discs (spectrosil), then baked at 90° C. (hotplate, 5 min).
a shows typical result of the output Fout vs input Fin fluence characteristic of the ODA-functionalized sub-GOx in PC measured for 3.5-ns pulses at 1064-nm wavelength in air. The Fout/Fin ratio gives the internal sample transmittance T. At high fluence, the limiting slope dFout/dFin gives the limiting differential transmittance T′. Thus for a linear T=0.85, the nonlinearity onset Fon≈10 mJ cm−2. The half-transmittance threshold F50 where the normalized T falls to 50% of the initial value is 100 mJ cm−2 with T′=0.17. Therefore this film is OL to 1064-nm-wavelength nanosecond pulses.
Polymer solutions in tetrahydrofuran were prepared from two insulating polymers, poly(methyl methacrylate) (PMMA) (17 mg/mL) and polystyrene (PS) (17 mg/mL), and a semiconducting polymer poly(9,9-octylfluorene-alt-triarylamine) (TFB) 25 mg/mL ODA-functionalized sub-GOx was added to the respective polymer solutions to give ODA-GO-to-polymer weight ratio of ca. 25:1. They were then analysed for phase separation by differential scanning calorimetry (DSC). (see
At a higher fluence of 20 mJ cm−2, the photo-bleaching is still present but attenuated by induced absorptions that emerge within the first 0.2 ns (instrument-limited) with dips at 2.1, 1.9 and 1.75 eV. At 30 mJ cm−2, the photo-absorption dips become more pronounced and dominate the response after ca. 2 ns. At 90 mJ cm−2, the transient response is firmly photoinduced absorption across the entire spectral window beginning at the sub-ns time scale. There is a roll-off of the absorption beyond 700 nm, as found also in the wavelength-dependent OL data. Absorption bands are again found at 2.15, 1.95 and 1.80 eV. The dynamics is complicated by the presence of slow rise components having rise times of 1-3 ns, and multiple decay lifetimes from 6 to 45 ns.
The fast emergence of transient absorption demonstrates that the leading mechanism for the giant OL effect in liquid cells is excited-state absorption. This absorption is characteristic of localized excited states typically associated with molecular π-electron systems. The average band spacing is 0.15-0.20 eV, which coincides with the Raman modes of graphene (G band, 0.195 eV; and D band, 0.165 eV) and we assign to the vibronic spacing of excitonic states. This interpretation receives support from (i) the heavy-atom effect, (ii) the large apparent optical cross-section of these states which we estimate from the T′/T ratio to be ≈10−17 cm2 per basal carbon, and (iii) long excitation life times.
The fluence- and time-dependent crossover from induced transparency to induced absorption is different from the usual behavior of small π-electron systems. These characteristics suggest the excitons here are generated not by direct excitation or the usual singlet→triplet intersystem crossing, but by interactions within the initial electron-hole gas in a nonlinear mechanism. This leads us to speculate that the initial electron-hole gas condenses to triplet-like excitons when promoted by spin-orbit coupling with heavy atoms, as schematically illustrated by path (i) of
The spectra were collected on a two-beam UV-Vis-NIR spectrophotometer (Shimadzu UV-3600) with a wide dynamic range (up to optical density OD 6). The spectrum of the functionalized sub-GOx dispersion (0.34 mL) was collected, and the progress of oxidation followed by sequentially adding aliquots of F4TCNQ (Sigma-Aldrich) in CB (1.12 mM) and measuring the spectrum, until the mole ratio of TCNQ to the C2 units is 0.55. These spectra were then corrected by re-scaling for the dilution effect to display at constant functionalized sub-GOx concentration.
The pristine spectrum of the functionalized sub-GOx shows the usual rising absorption towards shorter wavelengths. The difference spectra obtained by subtraction of the pristine spectrum are shown in FIG. 10 (b). When the first aliquot of F4TCNQ was added, a new band emerges at 390 nm (3.18 eV) due to the presence of neutral F4TCNQ molecules (molar absorptivity ε≈25,000 M−1 cm−1) in the mixture. A new set of absorption also emerges at 880 nm (1.41 eV), 769 nm (1.61 eV) and 685 nm (1.81 eV). This is the characteristic electronic spectrum of the F4TCNQ− anion. The three sub-bands form a vibronic progression, with spacing similar to that of the unsubstituted TCNQ− anion (0.2 eV, ≈1600 cm−1).7,8 The ε of the 880 nm sub-band of F4TCNQ− is estimated to be 16,500 M−1 cm−1 from the known ratio of the corresponding bands in TCNQ.8 Similar to TCNQ−, there is another band in F4TCNQ− at 475 nm (2.61 eV) which can be seen as a shoulder on the neutral F4TCNQ band. Therefore it can be firmly concluded that F4TCNQ acts as a p-dopant for sub-GOx. It is cleanly reduced to the F4TCNQ− state, with no dianion state found.
The difference spectra reveal that in addition to these bands, there is an increase of absorption with p-doping over a broad spectral range extending from well below 0.77 eV (i.e., longer than 1600 nm) to at least 2.5 eV (ca. 500 nm), masked at even shorter wavelengths by the intense F4TCNQ band. The absorbance ΔAG of this doping-induced broadband is clearly a feature of the p-doped nano-graphene domains in the functionalized sub-GOx sheets. Its intensity tracks very well with the ratio of F4TCNQ− per unit cell, as shown in
This data is re-plotted in
This absorption cross section is one order of magnitude higher than that contributed by a single carbon atom in the ground-state (ca. 2×10−18 cm2 per basal carbon atom). This demonstrates that the doping-induced holes in the dispersed functionalized sub-GOx graphene single sheets are significantly localized. If they were delocalized, they would reside at the K point which bleaches out absorption in the far infrared, but should cause no significant change the absorption spectrum in visible-NIR, aside from a tiny effect related to bandgap renormalization.
It is important to distinguish between these sub-GOx and the heavily-oxidized GOx which can be obtained by exhaustive oxidation of graphite, although this distinction is often lost in the literature. The fully-oxidized stoichiometric GOx does not have π-electrons, while sub-GOx has a significant fraction of sp2-carbon atoms retained in the basal plane. For sub-GOx that is about one-third- to half-oxidized, the sp2-carbon atoms are organized into nano-graphene domains which are really quite large 2-D π-electron systems in the 10-nm size range (estimated by Raman and infrared spectroscopies in Exemplification 16) separated by boundaries comprising a network of epoxy and/or hydroxyl-bonded sp3-carbon atoms (
The octadecylamine-functionalized sub-GOx nano-sheets can be repeatedly isolated in the dry state and re-dispersed in a variety of organic solvents (up to 15 mg mL−1) and polymer matrices. The alkyl-chains prevent the re-stacking of these sheets, and therefore promote their dispersability in various matrices.
The spectra show the characteristic D band at 1350 cm−1 and G band at 1585 cm−1 with a shoulder at 1620 cm−1. The more heavily-oxidized GOx sample shows greater disorder as expected of its higher oxidation state. Both the shape and position of the D and G bands are remarkably similar to those of nanographites made by high temperature annealing of amorphous carbon thin films. Therefore they result from the same Raman scattering mechanism, with the G band from the graphene, and D band from its perimeter adjacent to the spa defects. Hence we can use the known relationship of the D- to G-band intensity ratio and the size of the perfect graphene domain that has been well-established in nanographites:1
to estimate the average size of the nano-graphene domains in our sub-GOx. Note that this average is closer to the area-weighted average rather than an arithmetic average because the XRD method used to calibrate the size is weighted by the area of the domain. In this way we determined the average domain size to be 6-18 nm. After functionalization, the average domain size becomes smaller, 4-12 nm, but this is still very large compared to the graphene cell length of 0.246 nm.
We can estimate the size of the nano-graphene domain that has an electronic transition at this onset using the known size dependence of the π-π* electronic transition energy of benzenoid polycyclic aromatic hydrocarbons (PAHs).2 This approach is valid because very large PAHs (e.g., C222) do indeed show an absorption band profile very similar to what is observed here, i.e., an absorption edge followed by a broad featureless plateau that extends to higher energies. To extrapolate the experimental relationship to the size range found in our samples, we were guided by the simple theory that the energy gap between the highest occupied and lowest unoccupied levels of a large 2-D quantum well should scale as 1/L where L is a linear dimension of the well.
In
to fit the data, and obtained best fit with A=9.13±0.37 and B=0.75±0.13. Thus the π-π* transition onset of ca. 0.30 eV in our functionalized sub-GOx suggests that a population of the nano-graphene domains has a diameter of 30 aromatic sextets or 60 basal C2 unit cells (in the honeycomb lattice). This corresponds to a diameter of 12 nm, which is in excellent agreement with the estimate obtained from Raman spectroscopy.
Hence we can conclude that nano-graphene domain of 4-12-nm across which corresponds to 20-60 unit cells in diameter are present in our functionalized sub-GOx. The presence of such large graphenites is in fact consistent also with the previous observation of band-like transport in gated conductivity measurements, and with scanning tunneling microscopy of the thermal re-graphenization process.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/SG2011/000452 | 12/28/2011 | WO | 00 | 6/28/2013 |
| Number | Date | Country | |
|---|---|---|---|
| 61427570 | Dec 2010 | US |
