Laser induced graphitization of boron carbide in air

Abstract

The localized formation of graphene and diamond like structures on the surface of boron carbide is obtained due to exposure to high intensity laser illumination. The graphitization involves water vapor interacting with the laser illuminated surface of boron carbide and leaving behind excess carbon. The process can be done on the micrometer scale, allowing for a wide range of electronic applications. Raman is a powerful and convenient technique to routinely characterize and distinguish the composition of Boron Carbide (B.sub.4C), particularly since a wide variation in C content is possible in B.sub.4C. Graphitization of 1-3 μm icosahedral B.sub.4C powder is observed at ambient conditions under illumination by a 473 nm (2.62 eV) laser during micro-Raman measurements. The graphitization, with ˜12 nm grain size, is dependent on the illumination intensity. The process is attributed to the oxidation of B.sub.4C to B.sub.2O.sub.3 by water vapor in air, and subsequent evaporation, leaving behind excess carbon. The effectiveness of this process sheds light on amorphization pathways of B.sub.4C, a critical component of resilient mechanical composites, and also enables a means to thermally produce graphitic contacts on single crystal B.sub.4C for nanoelectronics.

Claims

1. A method of producing graphene, comprising: providing Boron Carbide (B.sub.4C) powder; and exposing the powder in the presence of a gas to laser illumination with laser intensity in a range of 0.6 kW/cm.sup.2 to 600 kW/cm.sup.2, whereby localized formation of graphene on the surface of Boron Carbide is obtained.

2. The method as in claim 1, wherein said powder comprises single crystal B.sub.4C, and exposing said powder comprises laser-writing graphitic contacts on said crystal for nanoelectronics.

3. The method as in claim 1, further including relatively increasing the temperature of said laser illumination for the formation of diamond like structures beyond the graphene formation.

4. The method as in claim 1, wherein said gas includes water vapor which interacts with the laser illuminated surface of Boron Carbide and leaves behind excess carbon.

5. The method as in claim 4, further including leaving behind graphitic material with size on a micrometer scale, for use in electronic applications.

6. The method as in claim 1, wherein exposing the powder to laser illumination comprises tuning one or more parameters of a laser source, where the one or more parameters of the laser source are selected from the group consisting of laser wavelength, laser power, laser energy density, laser resulting temperature operation, and combinations thereof.

7. The method as in claim 6, wherein the laser resulting temperature operation is controlled in a range of 250° K to 1800° K.

8. The method as in claim 6, wherein a wavelength of the laser source is tuned to match an absorbance band of the Boron Carbide powder.

9. The method as in claim 1, further comprising a step of incorporating the graphene into an electronic device.

10. The method as in claim 1, wherein the gas comprises one of air or a mixture of gases.

11. A method for the graphitization of Boron Carbide powder, comprising the steps of providing a specimen of Boron Carbide (B.sub.4C) powder and performing Raman measurements on the specimen with a laser in a controlled environment.

12. The method as in claim 11, wherein said controlled environment comprises one of air and ultra-high purity Argon environments, wherein purity of said ultra-high purity Argon is Argon with no more than 1 ppm H.sub.2O, O.sub.2.

13. The method as in claim 12, wherein: said controlled environment comprises air; and said method further includes controlling the temperature and laser intensity of the laser so that temperature of said laser is controlled in a range of 300° K to 850° K, and intensity of said laser is controlled in a range of 0.6 kW/cm.sup.2 to 600 kW/cm.sup.2.

14. The method as in claim 12, wherein: said controlled environment comprises an ultra-high purity Argon environment, wherein purity of said ultra-high purity Argon is Argon with no more than 1 ppm H.sub.2O, O.sub.2; and said method further includes controlling the temperature and laser intensity of the laser so that temperature of said laser is controlled in a range of 300° K to 1800° K, and intensity of said laser is controlled in a range of 0.6 kW/cm.sup.2 to 30 kW/cm.sup.2.

15. The method as in claim 11, further including controlling the temperature and laser intensity of the laser.

16. The method as in claim 15, wherein temperature of said laser is controlled in a range of 250° K to 1800° K.

17. The method as in claim 15, wherein intensity of said laser is controlled in a range of 0.6 kW/cm.sup.2 to 600 kW/cm.sup.2.

18. The method as in claim 11, further including relatively increasing the temperature of the laser resulting in formation of a diamond coating on the Boron Carbide specimen.

19. The method as in claim 11, wherein said Boron Carbide specimen is an icosahedral B.sub.4C powder.

20. The method as in claim 19, wherein said Boron Carbide specimen has a density of about 1.8 g/cm.sup.3 with a specific surface area in a range of about 2-4 m.sup.2/g.

21. The method as in claim 19, wherein said Boron Carbide specimen is crystalline, stoichiometric B.sub.4C powder.

22. The method as in claim 21, wherein said crystalline, stoichiometric B.sub.4C powder has 1-3 μm crystal dimensions.

23. The method as in claim 11, wherein said laser is a 473 nm (2.62 eV) laser.

24. The method as in claim 11, wherein said laser is focused to a spot size of about 2-3 μm.

25. The method as in claim 11, wherein said graphitization provides about 12 nm grain size results.

26. The method as in claim 11, wherein said resulting graphitic material is less than about 17 nm thick.

27. The method as in claim 11, wherein: said controlled environment comprises one of air and ultra-high purity Argon environments, wherein purity of said ultra-high purity Argon is Argon with no more than 1 ppm H.sub.2O, O.sub.2; said Boron Carbide specimen is a crystalline icosahedral B.sub.4C powder having about 1-3 μm crystal dimensions; and said method further includes controlling the temperature of said laser in a range of 250° K to 1800° K, controlling the intensity of said laser in a range of 0.6 kW/cm.sup.2 to 600 kW/cm.sup.2, and controlling the focus of said laser to a spot size of about 2-3 μm.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) A full and enabling disclosure of the presently disclosed subject matter, including the best mode thereof, to one of ordinary skill in the art, is set forth more particularly including the specification, and including reference to the accompanying figures in which:

(2) FIG. 1(a) is a scanning electron microscope (SEM) image of a sample of Boron Carbide (B.sub.4C) powder, collected as a relatively “thick” pile on supporting tape, suitable for use in presently disclosed methodologies;

(3) FIG. 1(b) is a graph representing an effective direct Tauc gap of 1.9 eV for the sample Boron Carbide (B.sub.4C) powder of FIG. 1(a), with UV-Vis diffuse reflectance measurements indicating that such B.sub.4C powder is semiconducting;

(4) FIG. 1(c) is a graph of spectral features of large area, low-intensity Raman; high intensity μ-Raman on reflective substrate; and ATR-FTIR absorbance, respectively in overlay;

(5) FIG. 2(a) is a graph of sequential μ-Raman spectra of a single spot with the relatively “thick” B.sub.4C powder of FIG. 1(a) in Ambient air environment at various estimated laser intensities (and with full scale of the arbitrary vertical line units of FIG. 2(a) correlating with 2400 cm.sup.−1;

(6) FIG. 2(b) is a graph of sequential μ-Raman spectra of another single spot with the relatively “thick” B.sub.4C powder of FIG. 1(a) under ultra-high purity Argon (1 ppm H.sub.2O, O.sub.2 per vendor) flow ˜0.1 slm at various estimated laser intensities; and

(7) FIGS. 3 through 8 illustrate respectively various corresponding scans and graphs of wavenumber and time results relative to exposure intensities for various samples, including G band and D band information for various Boron Carbide B.sub.4C samples.

(8) Repeat use of reference characters in the present specification and drawings is intended to represent the same or analogous features or elements or steps of the presently disclosed subject matter.

DETAILED DESCRIPTION OF THE PRESENTLY DISCLOSED SUBJECT MATTER

(9) Reference will now be made in detail to various embodiments of the presently disclosed subject matter, one or more examples of which are set forth below. Each embodiment is provided by way of explanation of the subject matter, not limitation thereof. In fact, it will be apparent to those skilled in the art that various modifications and variations may be made in the presently disclosed subject matter without departing from the scope or spirit of the subject matter. For instance, features illustrated or described as part of one embodiment, may be used in another embodiment to yield a still further embodiment, and corresponding and/or associated methodologies may be practiced relative to apparatus disclosed and/or suggested herewith, all of which comprise various embodiments of the presently disclosed subject matter.

(10) In general, commercial 99.9% pure B.sub.4C powder nominally 1-3 μm in size, produced by plasma chemical vapor deposition, was used in the measurements. According to the manufacturer, the free B content of the material is <4 wt %, while the free C-content is <3 wt %, <0.001 wt % for Mg, Na, Mo, Fe, Al, N, <0.002 wt % for Ca, and <wt 0.04% for O. The density of B.sub.4C is 1.8 g/cm.sup.3 with a specific surface area of 2-4 m.sup.2/g. The powders were then prepared for characterization in 2 ways: i) directly placed as a pile onto the measurement setup for a “thick” sample, which is strongly optically absorbing and ii) smeared like pencil lead onto a reflective white ceramic plate (R>80%) to produce a “thin” sample, which is weakly absorbing optically.

(11) FIG. 1(a) is a scanning electron microscope (SEM) image per energy dispersive x-ray spectroscopy (EDX) of a sample of Boron Carbide (B.sub.4C) powder, collected as a relatively “thick” pile on supporting tape, suitable for use in presently disclosed methodologies. EDX showed the carbon content to be 18-21 at. % C as expected for stoichiometric B.sub.4C, while the dimensions were indeed in the specified 1-3 μm range.

(12) X-ray diffraction (XRD) was performed on these powders, and confirms that the structure is indeed that of rhombohedral B.sub.4C. A rhombohedral structure can be described by an equivalent hexagonal lattice, with lattice parameters a.sub.0=5.62 Å, and c.sub.0=12.15 Å, which uniquely identifies the carbon content [5, 11], at a value of approximately 16 at. % C in the B.sub.4C crystal. Given that the free carbon content is ˜3 at. % C (from the manufacturer), and that the total C-content from EDX is ˜18-21 at. % C, this is in good agreement with presently conducted EDX measurements.

(13) FIG. 1(b) is a graph representing an effective direct Tauc gap of 1.9 eV for the sample Boron Carbide (B.sub.4C) powder of FIG. 1(a), with UV-Vis diffuse reflectance measurements indicating that such B.sub.4C powder is semiconducting.

(14) FIG. 1(c) is a graph of spectral features of large area, low-intensity Raman; high intensity μ-Raman on reflective substrate; and ATR-FTIR absorbance, respectively in overlay.

(15) More specifically, μ-Raman measurements were performed on these powders using a 473 nm blue laser focused to a spot size of ˜2 μm with a laser power varying from 0.025-25 mW on both “thick” and “thin” samples under both air and 0.1 slm flow of ultra-high purity (UHP) argon (O.sub.2, H.sub.2O content ˜1 ppm) ambient. All laser intensities are estimated from laser power at the sample and spot size, and are given to 1 significant figure. Large area Raman measurements were performed using a 515 nm green laser with ˜3 mm spot size with ˜1 mW power on the “thick” samples in air. “Thin” samples did not give measurable signal with this large area measurement. The μ-Raman measurements on “thin” samples were measured on single particles >2 μm in dimension across the ceramic substrate, giving rise to sharp peaks. The Raman spectrum of boron carbide is characterized by sharp peaks below 600 cm.sup.−1, associated with the sp.sup.3-chains, and 2 large broad features at ˜700 cm.sup.−1 and 1050 cm.sup.−1 associated with the icosahedra [5,6,10]. See FIG. 1(c).

(16) The sharpness of the spa-chain features for single particles at 481 cm.sup.−1 and 534 cm.sup.−1, and their relatively high intensities indicate qualitatively that this is stoichiometric B.sub.4C [10]. The large area 515 nm Raman spectrum represents an average over many different orientations of B.sub.4C in the “thick” powder.

(17) Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy was performed on “thick” samples using a diamond crystal in the 650-4000 cm.sup.−1 range. These reflectance spectra were converted to absorbance spectra without applying an ATR correction. FIG. 1(c) shows the ATR-FTIR overlaid with the “thin” μ-Raman spectrum, and the “thick” large area Raman, showing that the major icosahedral features line up for the 3 techniques. None of these techniques showed any evidence of graphitic inclusions. The major broad and sharp peaks of B.sub.4C are clearly seen below ˜1200 cm.sup.−1, and related overtones above 1500 cm.sup.−1 with no obvious evidence of graphitic inclusions.

(18) UV-Vis diffuse reflectance measurements indicate that the B.sub.4C powder is semiconducting, with an effective direct Tauc gap of 1.9 eV. See FIG. 1(b). The measured value of band gap is in reasonable agreement with other measurements [12,13], with a large scatter due to differing defect topologies produced using varied synthesis techniques [9]. These above characterization techniques clearly show that the starting powder is indeed crystalline, stoichiometric B.sub.4C powder of 1-3 μm dimension, as per the manufacturer specifications.

(19) In summary when Raman measurements were performed over large areas, i.e., low laser intensity, with “thick” B.sub.4C or on a reflective, thermally conductive substrate with “thin” B.sub.4C for the μ-Raman measurements, clear signatures of crystalline B.sub.4C are observed. FIG. 2(a) is a graph of sequential μ-Raman spectra of a single spot with the relatively “thick” B.sub.4C powder of FIG. 1(a) in Ambient air environment at various estimated laser intensities, with an overwhelmingly clear signature of nano-crystalline graphite at high illumination intensities [14,15], small signature of the icosahedral B.sub.4C remains, using the same laser power as the μ-Raman measurements in FIG. 1(c) of 1 c ˜60 kW/cm.sup.2. FIG. 2(b) is a graph of sequential μ-Raman spectra of another single spot with the relatively “thick” B.sub.4C powder of FIG. 1(a) under ultra-high purity Argon (1 ppm H.sub.2O, O.sub.2 per vendor) flow ˜0.1 slm at various estimated laser intensities.

(20) The graphite Raman spectrum consist of the intrinsic G-peak at ˜1580 cm.sup.−1, a large defect related D-peak at ˜1350 cm.sup.−1, and the related 2D and D+G overtones. In addition, there is another graphitic peak at ˜1830 cm.sup.−1 that will be addressed further below. This directly indicates that the B.sub.4C was graphitized due to the selective removal of boron during the μ-Raman measurement. Since B.sub.4C is typically an inert material with melting point >2000° C. [5], this was an unexpected result.

(21) By performing the measurements at significantly lower laser intensity, the correct un-graphitized spectrum of B.sub.4C is recovered (FIGS. 2(a) and (b)). As the laser power is increased, the irreversible graphitization is observed, which shows that the graphitization is due to sample heating.

(22) In particular, in FIG. 2(a), the sequential μ-Raman spectra of a single spot with “thick” B.sub.4C powder in ambient air environment were conducted at respective estimated laser intensities: (i) 0.6 kW/cm.sup.2 (300K) (ii) 6 kW/cm.sup.2 (300K) (iii) 60 kW/cm.sup.2 (600-680K) (iv) 300 kW/cm.sup.2 (730-850K) (v) 600 kW/cm.sup.2 (1150-1400K) (vi) 300 kW/cm.sup.2 (730-850K) (vii) 60 kW/cm.sup.2 (600-680K) (viii) 6 kW/cm.sup.2 (300K).

(23) In FIG. 2(b), the sequential μ-Raman spectra of another single spot with “thick” B.sub.4C powder under ultra-high purity Argon (1 ppm H.sub.2O, O.sub.2 per vendor) flow ˜0.1 slm were conducted at respective estimated laser intensities: (i) 0.6 kW/cm.sup.2 (300K) (ii) 6 kW/cm.sup.2 (670-760K) (iii) 30 kW/cm.sup.2 (1200-1500K) (iv) 60 kW/cm.sup.2 (1500-1800K) (v) 30 kW/cm.sup.2 (1200-1500K) (vi) 6 kW/cm.sup.2 (670-760K) (vii) 0.6 kW/cm.sup.2 (300K).

(24) With reference to the intensities for each of FIGS. 2(a) and 2(b), the temperatures in parenthesis were estimated to 2 s.f. from the red-shift of the graphitic G-peak, 0.024-0.03 cm-1/K [19,20] referenced to room temperature, 300K value of G-1600 cm-1 (300K) at 0.6-6 kW/cm.sup.2 at the end of the runs for both ambients. While the temperature is not explicitly calculated before the emergence of a strong G-peak, it is a reasonable estimate that for the same laser power that the temperatures are comparable, since both graphite, and B.sub.4C are black powders that absorb strongly. The highest temperature spectrum in each plot of respective FIGS. 2(a) and 2(b) is shown as a thick line.

(25) The black color of the B.sub.4C implies that the powder acts as a black body, absorbing high intensities of laser light, locally heating up, when a suitable heat sink is not employed. From the position of the μ-Raman G-peak, the local temperature of the sample can be inferred [19,20]. As the sample heats up, the graphite lattice expands, leading to a redshift of the Raman peak. For the presently disclosed samples, the G-peak shifts from the room temperature value of ˜1595 cm.sup.−1 to 1560 cm.sup.−1 at the highest laser power under UHP argon, a substantial shift of 35 cm.sup.−1. Using the 0.024-0.03 cm.sup.−1/K shift for the G-peak measured for defective graphite [19, 20], one may infer that temperatures >1400K are reached at the highest laser intensities (FIGS. 2(a) and 2(b)), with an observed graphitization threshold around 600K under both air (FIG. 2(a)) and UHP argon (FIG. 2(b)).

(26) At the highest laser intensities under UHP argon (FIG. 2(b)), the μ-Raman signature of the B.sub.4C disappears completely, showing that the graphite thickness is >50 monolayers [22], or ˜17 nm due to the complete absorption of the 473 nm laser. Under air (FIG. 2(a)), the B.sub.4C signature is still seen, indicating a much thinner layer.

(27) The oxidation of B.sub.4C has been observed at moderate temperatures ˜500° C., for powders in the 1-3 μm range [16]. In the presence of water vapor, an unavoidable constituent of ambient air, as well as in ultra-high purity (UHP) Ar, oxidation of B.sub.4C to B.sub.2O.sub.3 occurs at temperatures as low as 250 C [17].
B.sub.4C+6H.sub.2O.fwdarw.2B.sub.2O.sub.3(l)+C+6H.sub.2 and/or  (Equation 1a)
B.sub.4C+8H.sub.2O.fwdarw.2B.sub.2O.sub.3(l)+CO.sub.2+8H.sub.2  (Equation 1b)

(28) For laser illumination at high intensity, graphitization indicates that the reaction in Equation (1a) dominates, although Equation (1b) cannot be excluded. Litz et al. [17] further showed that the B.sub.2O.sub.3 on the surface can be removed faster than the B.sub.2O.sub.3 formation by the following reaction, also due to the presence of water vapor.
B.sub.2O.sub.3(l)+H.sub.2O.fwdarw.2HBO.sub.2(g)ORH.sub.3BO.sub.3(g)  (Equation 2)

(29) There is no obvious Raman signature of B.sub.2O.sub.3 in presently conducted measurements, contrary to what was observed after annealing of hot-pressed B.sub.4C in air at 800° C. by Erdemir et al. [18], indicating that B.sub.2O.sub.3 has evaporated due to the reaction in equation (2) for samples per the present disclosure. It is noted that the observation of graphite in the as-received samples by Erdemir et al. [18] may be due to in-situ graphitization as seen here, where their 25 mW 632 nm laser was focused to a 2-3 μm spot size, leading to a measurement geometry similar to presently disclosed “thick” samples. In fact, follow on measurements on similar samples [27] showed the disappearance of B.sub.2O.sub.3 and the emergence of graphitic Raman signatures, which may have occurred due to graphitization during the Raman measurements, as in the currently disclosed subject matter.

(30) μ-Raman measurements performed under unfiltered UHP argon (FIG. 2(b)) also showed graphitization at similar laser powers, although the D-peak began to emerge at slightly lower laser intensity ˜6 kW/cm.sup.2 (but similar temperature) compared to in air (FIG. 2(a)). This may be attributed to unavoidable 1 ppm O.sub.2 and H.sub.2O impurities present in as received UHP argon. This suggests that the removal of carbon from B.sub.4C by CO.sub.2 formation reaction (Equation 1b) is suppressed under UHP argon compared to under air due to the lower vapor pressure of H.sub.2O, although the graphitization in (Equation 1a) still proceeds. This highlights how readily the reactions in Equations (1) and (2) occur. Oxidation at such low partial pressures of O.sub.2 is often observed in metals (e.g. [21]), especially with the assistance of low partial pressures of H.sub.2O <1 ppm. It is also noted that the temperatures estimated from μ-Raman G-peak redshift under UHP argon are higher than under air, possibly due to the lack of heat removal by CO.sub.2 generated in reaction (Equation 1b).

(31) From the ratio of the intensities (i.e. area) of the D and G peaks, i.e. I.sub.D/I.sub.G, the average graphitic grain size. L, is estimated using the well-known [14,15,22]

(32) $\begin{array}{cc}L\left(\mathrm{nm}\right)=\left(2.4\times {10}^{-10}\right){\lambda }_{\mathrm{laser}}^4\left(\mathrm{nm}\right){\left(\frac{I_D}{I_G}\right)}^{-1}& \left(\mathrm{Equation}3\right)\end{array}$
where λ.sub.laser is the Raman excitation wavelength. For the presently disclosed laser graphitized samples, I.sub.D/I.sub.G was ˜0.8-1, indicating L˜12 nm, close to the dimensions of the amorphization bands ˜1-5 nm in impact testing of B.sub.4C. Given that in impact-tested B.sub.4C, very small shards may form, of high surface area, it is possible that these may graphitize during μ-Raman mapping of the damage zone, causing ambiguity in the interpretation of amorphization in B.sub.4C-composites. It is presently suggested that single-crystals [6], and hot pressed samples [18] may be less susceptible to this graphitization owing to the high thermal conductivity of single-crystal B.sub.4C as opposed to micro/nano-crystalline powders that have voids and inter-granular boundaries that suppress thermal conduction under laser illumination. The analogous result is in FIG. 1(c), where the μ-Raman measurements on “thin” B.sub.4C immobilized on a thermally conductive ceramic plate prevented graphitization even at laser powers of ˜60 kW/cm.sup.2.

(33) The graphitic Raman peak at ˜1825 cm.sup.−1 seen in FIG. 2(b), line (iii) (under UHP argon) is always observed in indentation induced amorphization studies of single crystal B.sub.4C [5,6,23], and had previously been considered unknown. However, a similar feature is seen under air at ˜1840 cm.sup.−1 in FIG. 2(a) before the onset of graphitization, as well as in the “thin” measurement in FIG. 1(c), indicating that it could be an overtone combination of the <400 cm.sup.−1 B.sub.4C peak and the ˜1520 cm.sup.−1 B.sub.4C peak. This peak shifts to ˜1830 cm.sup.−1 after graphitization (FIG. 2(a), line (iii)), although this may be due to heating effects. Features in this range have been observed in graphitic structures [24,25,26], particularly in non-planar geometries [26]. From the literature, this peak is likely either a combination of in-plane transverse acoustic/longitudinal optical modes (iTALO) [25], or a combination of a low frequency radial/defect related 1620 cm.sup.−1 modes [26]. Based on the current data, however the exact mode assignment and origin of the 1820 cm.sup.−1 feature cannot be unambiguously concluded.

(34) Presently disclosed results show that ambiguities in interpretation of μ-Raman maps for amorphization studies [5,6,23] may be resolved by performing the measurements at low laser intensities to keep the temperature well below 250° C., under extremely high purity dry inert ambients, such as gettered argon, or by immobilizing samples on high thermal conductivity substrates. This result, based on the detailed study of oxidation kinetics [16,17], also shows that the processing of B.sub.4C composites such as sintering, consolidation etc., should also be performed in dry inert ambients, to prevent the formation of B.sub.2O.sub.3, readily removed in air, slippery graphite (this disclosure), and boric acid [18], potentially weakening the structure of the composite. While this water-vapor induced graphitization is a non-ideality that complicates characterization and processing of B.sub.4C, it may be exploited to thermally produce graphitic layers on semiconducting B.sub.4C single-crystal structures for electronics applications [4], as has been done on SiC for example [22,28].

(35) FIGS. 3 through 8 illustrate respectively various corresponding scans and graphs of wavenumber and time results relative to exposure intensities for various samples, including G band and D band information for various Boron Carbide B.sub.4C samples.

(36) In summary, it was observed laser-induced graphitization at high laser intensities of 1-3 μm B.sub.4C powder at ambient conditions. This was attributed to water-vapor induced oxidation of the B.sub.4C, and subsequent oxide removal also aided by water vapor. These presently disclosed results demonstrate that the characterization of amorphization of B.sub.4C by Raman mapping, and processing of B.sub.4C composites for impact-testing should be performed in dry, inert ambients.

(37) While the presently disclosed subject matter has been described in detail with respect to specific embodiments thereof, it will be appreciated that those skilled in the art, upon attaining an understanding of the foregoing, may readily produce alterations to, variations of, and equivalents to such embodiments. Accordingly, the scope of the presently disclosed subject matter is by way of example rather than by way of limitation, and the subject disclosure does not preclude inclusion of such modifications, variations and/or additions to the presently disclosed subject matter as would be readily apparent to one of ordinary skill in the art.

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