Composition comprising optically and electronically active phosphorene

Abstract

A composition includes a few-layer phosphorene nanomaterial comprising at least one of mono-, bi-, and n-layer phosphorene nanosheets, where n is an integer selected from 3 to 6; deoxygenated water; and at least one amphiphilic surface active component.

Claims

1. A composition, comprising: a few-layer phosphorene nanomaterial comprising at least one of mono-, bi-, and n-layer phosphorene nanosheets, wherein n is an integer selected from 3 to 6; deoxygenated water; and at least one amphiphilic surface active component, wherein said at least one amphiphilic surface active component is selected from alkali metal alkylsulfates and combinations thereof.

2. The composition of claim 1, wherein said at least one amphiphilic surface active component is sodium dodecylsulfate.

3. The composition of claim 1, wherein said few-layer phosphorene nanomaterial is separable into two or more separation fractions comprising a subpopulation of few-layer phosphorene nanosheets comprising a surface area, each said fraction comprising monolayer phosphorene nanosheets, bilayer phosphorene nanosheets and trilayer phosphorene nanosheets, or combinations thereof.

4. The composition of claim 3, wherein said two or more separation fractions are obtainable by centrifuging said few-layer phosphorene nanomaterial in a deoxygenated fluid medium comprising a density gradient.

5. The composition of claim 4, wherein said deoxygenated fluid medium comprises deoxygenated aqueous iodixanol.

6. The composition of claim 5, wherein said deoxygenated fluid medium comprises a plurality of deoxygenated aqueous iodixanol concentrations, said density gradient comprising a range of concentration densities.

7. The composition of claim 6, wherein one of said two or more separation fractions is within said density gradient at a substantially invariant point during centrifugation, said invariant point comprising a density approximating a buoyant density of the one of said two or more separation fractions.

8. A composition, comprising: a few-layer phosphorene nanomaterial comprising at least one of mono-, bi-, and n-layer phosphorene nanosheets, wherein n is an integer selected from 3 to 6; deoxygenated water; and at least one amphiphilic surface active component, wherein said at least one amphiphilic surface active component is selected from bile salts and combinations thereof, wherein said bile salts comprise potassium salts of conjugated or unconjugated cholates and derivatives thereof.

9. The composition of claim 8, wherein said conjugated or unconjugated cholates and cholate derivatives comprise deoxycholates, chenodeoxycholates, taurodeoxycholates, glycochenodeoxycholates, ursodeoxycholates, and/or glycoursodeoxycholates.

10. A composition, comprising: a few-layer phosphorene nanomaterial comprising at least one of mono-, bi-, and n-layer phosphorene nanosheets, wherein n is an integer selected from 3 to 6; deoxygenated water; and at least one amphiphilic surface active component, wherein said at least one amphiphilic surface active component is selected from non-ionic triblock oxyalkylene copolymers and combinations thereof.

11. The composition of claim 10, wherein said at least one amphiphilic surface active component is a copolymer of oxyethylene and oxypropylene.

12. The composition of claim 10, wherein said few-layer phosphorene nanomaterial is separable into two or more separation fractions comprising a subpopulation of few-layer phosphorene nanosheets comprising a surface area, each said fraction comprising monolayer phosphorene nanosheets, bilayer phosphorene nanosheets and trilayer phosphorene nanosheets, or combinations thereof.

13. The composition of claim 12, wherein said two or more separation fractions are obtainable by centrifuging said few-layer phosphorene nanomaterial in a deoxygenated fluid medium comprising a density gradient.

14. The composition of claim 13, wherein said deoxygenated fluid medium comprises deoxygenated aqueous iodixanol.

15. The composition of claim 14, wherein said deoxygenated fluid medium comprises a plurality of deoxygenated aqueous iodixanol concentrations, said density gradient comprising a range of concentration densities.

16. The composition of claim 15, wherein one of said two or more separation fractions is within said density gradient at a substantially invariant point during centrifugation, said invariant point comprising a density approximating a buoyant density of the one of said two or more separation fractions.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIGS. 1A-H. Digital images and spectra illustrating experimental procedure and characterization of FL-P nanosheets. A, Schematic of the preparation method for FL-P aqueous dispersions. Deoxygenated water with 2% w v.sup.1 SDS was prepared by ultrahigh purity Ar purging. The BP crystal was exfoliated in a sealed container using tip ultrasonication, and then centrifuged to remove unexfoliated BP crystals. The FL-P dispersion was subsequently collected and ultracentrifuged to precipitate large flakes. The supernatant was finally redispersed in deoxygenated water. B, Images of the as-exfoliated BP solution (left) and FL-P solution (right). C, Low-magnification transmission electron microscopy (TEM) image of FL-P nanosheets. D, High-resolution TEM image of a FL-P nanosheet. E, Selected area electron diffraction (SAED) pattern of FL-P nanosheets. F, Solution phase Raman spectrum of FL-P nanosheets. G, Visible photoluminescence (PL) spectrum of FL-P nanosheets using an excitation wavelength of 532 nm with a Si CCD. H, X-ray photoelectron spectroscopy (XPS) analysis of the FL-P nanosheets.

(2) FIGS. 2A-B. Raman spectra of BP precipitants and few-layer phosphorene (FL-P). A, The liquid-phase Raman spectrum from BP precipitants shows three modes at 362 cm.sup.1, 439 cm.sup.1, and 467 cm.sup.1 with full width half maximum (FWHM) values of 1.96 cm.sup.1, 2.12 cm.sup.1, and 1.95 cm.sup.1, respectively. B, Conversely, the spectrum from FL-P shows three modes at 362 cm.sup.1, 439 cm.sup.1, and 466 cm.sup.1 with FWHM values of 2.96 cm.sup.1, 3.31 cm.sup.1, and 5.13 cm.sup.1, respectively.

(3) FIGS. 3A-C. XPS data for FL-P prepared with different surfactants. P 2p core level spectra for sodium dodecylsulfate (violet, 3A), Pluronic F68 (blue, 3B), and sodium cholate (green, 3C).

(4) FIGS. 4A-J. Digital images and graphs illustrating surface properties and exfoliation yield of BP in aqueous solution. A, Image of a BP dispersion in deoxygenated water with and without SDS. B, Image of a water contact angle measurement on an as-exfoliated flat BP crystal surface. C, Optical absorbance spectra of BP dispersions with (red) and without (blue) SDS. D, Zeta potential measurement of BP in water (blue), SDS-water (orange), and BP in SDS-water (red). E,F, Images of BP dispersions in SDS-water and NMP after sonication and centrifugation at 0.5 k, 5 k, 10 k, and 15 k r.p.m. G, Concentration of the BP dispersions from part (e) and (f). H,I, AFM height images of BP nanosheets processed in SDS-water and NMP. J, Thickness distribution of BP nanosheets in SDS-water (red) and NMP (green).

(5) FIG. 5. Extinction measurements of FL-P dispersions in sodium dodecylsulfate (SDS)-water. The extinction coefficient is found to be 2092 Lg.sup.1 m.sup.1, using the absorbance per length (A/1) at 660 nm versus the BP concentration. To calculate the concentration, FL-P dispersions with different volumes were filtered on an AAO membrane with 20 nm pore size, and then the mass difference was compared before and after filtration.

(6) FIGS. 6A-B. AFM derived flake thickness distributions. Thickness histogram (A) and area histogram (B) of FL-P nanosheets prepared in SDS-water. The layer thickness was defined as 1 nm from the thickness histogram with the smaller scale (brown).

(7) FIGS. 7A-C. Optical properties of FL-P aqueous dispersions. A, Optical absorbance spectrum of FL-P and a higher resolution plot in the range between 900 nm and 1250 nm (inset). B, PL spectra of FL-P aqueous dispersions. Three peaks corresponding to monolayer, bilayer, and three-plus layer phosphorene are observed at 907 nm, 1215 nm, and 1428 nm, respectively. C, PL spectra of FL-P dispersions (red) and O.sub.2-exposed FL-P dispersions (blue), and XPS spectra of O.sub.2-exposed FL-P dispersions.

(8) FIGS. 8A-D. Digital images and photoluminescence (PL) spectra of exfoliated, monolayer phosphorene. A, Optical image of a monolayer phosphorene flake (blue outline), passivated from ambient oxidation with 2.6 nm of 50 C. AlOx.2 B, PL spectra of the phosphorene flake in (a), excited with a linearly polarized, 532 nm laser at 15 W (100, 0.9 NA objective, 10 s acquisition in air). The phosphorene monolayer emits at 924 nm (1.34 eV), near the expected wavelength, whereas emissions from the SiO2 substrate (purple) are absent. Additional emissions in the visible are present (761 nm, 823 nm, and 867 nm, respectively), potentially related to oxidized BP regions. C, Optical image of a second phosphorene region (blue outline), passivated like (a). D, PL spectra of the flake in (c) after successive spectral acquisitions with a 532 nm laser at 15 W (100, 0.9 NA objective, 20 s acquisition in air). While the phosphorene monolayer has its excitonic emission at 918 nm (1.35 eV), it possesses a small oxide peak at 854 nm (1.45 eV). With additional measurements, the monolayer phosphorene emission decreases in intensity with the concurrent emergence of oxide-related bands (ca. 783 nm and 827 nm, respectively), indicative of photooxidation.

(9) FIGS. 9A-E. Digital images and PL spectra of bilayer phosphorene with three-layer and four-layer regions. A, Optical image of a bilayer phosphorene flake (dark blue) attached to a thick BP region, passivated by 10.0 nm AlO.sub.x (2.5 nm at 50 C. ALD, 7.5 nm at 150 C. ALD). B, AFM height image of the flake in (a), showing bilayer, three-layer, and four-layer phosphorene areas. C, Height profile along the contour in (b). Bilayer (1.3 nm), three-layer (1.7 nm), four-layer (2.2 nm) steps indicated (0.5 nm from instrumental factors, AlO.sub.x overlayer homogeneity, and adhesion). D, PL spectra of the bilayer phosphorene (blue spot, a), excited with a 532 nm laser at 150 W (50, 0.8 NA objective, 5 s acquisition in air). The laser polarization was adjusted to maximize the bilayer emission relative to the 1143 nm (1.08 eV) Si emission (termed parallel orientation: ). Under parallel orientation, the bilayer phosphorene emits at 1279 nm (0.97 eV). To confirm the anisotropic optical properties of BP, the bilayer emission was dampened five-fold by rotating the laser polarization 90 (20) relative to the parallel orientation (termed perpendicular orientation: ). E, PL spectrum from three-layer (green spot) and four-layer (magenta spot) phosphorene steps of (b), excited like (d). Spectrum taken from a single area on the bilayer phosphorene. While the bilayer phosphorene emission persists, emissions occur at 1488 nm (0.83 eV) and 1535 nm (0.81 eV) for three-layer and four-layer phosphorene, respectively.

(10) FIGS. 10A-D. Digital images and polarized PL spectra of three-layer and four-layer phosphorene. A, Optical image of a three-layer phosphorene flake, passivated by 10.0 nm AlO.sub.x (2.5 nm at 50 C. ALD, 7.5 nm at 150 C. ALD). B, Polarization dependent PL spectra for the three-layer phosphorene in (a), ranging from minimum (perpendicular) to maximum (parallel) phosphorene emission (blue-dotted) at 1468 nm (0.84 eV). Polarization invariant Si emission (red-dotted) at 1137 nm (1.09 eV). The sample was excited with a 532 nm laser at 200 W (50, 0.8 NA objective, 5 s acquisition in air). C, Optical image of a four-layer phosphorene flake, encapsulated like (a). D, Polarization dependent PL spectra for the four-layer phosphorene in (c), with maximum phosphorene emission (blue-dotted, parallel) at 1522 nm (0.81 eV) and Si emission (red-dotted) at 1134 nm (1.09 eV). The sample was excited at 250 nW with all other exposure conditions being the same as (b).

(11) FIGS. 11A-D. PL spectra of passivated, five-layer phosphorene. A, Optical image of a five-layer phosphorene flake, passivated by 12.6 nm AlO.sub.x (2.6 nm at 50 C. ALD, 10 nm at 150 C. ALD). B, AFM height image of the boxed region in (a). C, Height profile along the line in (b). Passivated phosphorene flake is 2.7 nm tall, indicative of five-layer phosphorene (0.5 nm additional, as aforementioned). D, PL spectrum of the phosphorene flake in (b), excited in the parallel orientation with a 532 nm laser at 150 W (50, 0.8 NA objective, 5 s acquisition in air). Passivated, five-layer phosphorene emits (blue) at 1582 nm (0.78 eV), and the Si substrate emits (red) at 1129 nm (1.10 eV), as expected.

(12) FIGS. 12A-E. Digital images and solid-state PL spectra of BP nanosheets cast from solution. A, PL spectrum of FL-P aggregates on a 300 nm SiO.sub.2/Si substrate. B, Optical image of FL-P aggregates on the substrate. C, PL mapping image displaying a peak position of 909 nm (1.36 eV). Overlaying (b) with (c) shows that the PL emission comes from the aggregated FL-P. D, PL spectra with respect to UV ozone exposure time. With increasing exposure, the PL emission intensity at 909 nm decreases from monolayer phosphorene oxidation. Furthermore, an emission peak develops at 780 nm (1.59 eV), likely related to PD.sub.x species. Finally, a SDS Raman band disappears with 60 min UV exposure. E, Emission intensities at 780 nm (green) and 909 nm (red) as a function of UV ozone exposure time. The peak at 909 nm decreases monotonically as the FL-P is oxidized. The peak at 780 nm increases initially but ultimately decays from extended chemical modification.

(13) FIGS. 13A-C. Digital images and infrared, solid-state PL spectra of solution cast BP nanosheets. A, Optical image of unencapsulated FL-P aggregates on native oxide (SiO.sub.x) on Si(100). B, Visible PL spectrum for the aggregates in (a), exposed to a 350 W, 532 nm laser (50, 0.8 NA objective, 15 s acquisition in air). Monolayer phosphorene emission is present at 909 nm (1.36 eV), with an additional emission at 797 nm (1.56 eV). Spectrum agrees well with the FL-P aggregate emissions of FIG. 12. C, Infrared PL spectra for the aggregates in (a). Control SiO.sub.x/Si(100) spectrum (purple) shows a weak Si emission (red) at 1121 nm (1.11 eV). FL-P aggregates are broadband emitters, having bilayer (1261 nm, 0.98 eV), three-layer (1459 nm, 0.85 eV), four-layer (1528 nm, 0.81 eV), and five-layer (1588 nm, 0.78 eV) phosphorene emissions. A band tail from monolayer phosphorene (see (b)) also exists (peak at 1035 nm given for reference). Solution-cast phosphorene emission energies are consistent with the mechanically exfoliated BP spectra of FIGS. 8-11. All peak emission wavelengths determined by Voigt fits to the baseline-corrected spectra.

(14) FIGS. 14A-F. Digital images and graphs illustrating size sorting and electrical properties of FL-P aqueous dispersions. A, Images of FL-P solution before (left) and after (right) size sorting using sedimentation-based density gradient ultracentrifugation (s-DGU). B, Histogram of flake areas for fractions f1 to f6. C, False-colored SEM image of an FL-P nanosheet FET with higher magnification image of the channel (inset). D, Output curves for a FL-P FET. E, Transfer curve for a FL-P FET plotted in linear scale (blue) and semi-log scale (green) for Vps values of 1 V (open circles) and 25 mV (closed circles). F, Plot of I.sub.ON/I.sub.OFF versus drive current that compares this work to previously reported BP FETs.

(15) FIGS. 15A-C. Digital images showing area of size-sorted FL-P nanosheets. AFM images of fractions 1, 3, and 5 (FIGS. 15A, 15B, and 15C, respectively) after sedimentation-based density gradient ultracentrifugation (Scale bars: 1 m, Height: 10, 30, and 30 nm, respectively).

(16) FIG. 16. FET measurements of FL-P nanosheets. The transfer curve of the FET article/device shown in FIG. 14E with the forward and reverse sweep.

(17) FIGS. 17A-B. Histograms of electrical properties of FL-P FETs. Histogram of A, drive current, and B, Io.sub.N/I.sub.OFF ratio with logarithmic scale.

DETAILED DESCRIPTION OF THE INVENTION

(18) As relates to certain non-limiting embodiments, this invention provides a scalable, high-yield, and environmentally benign method for preparing FL-P via ultrasonication in deoxygenated water. FIG. 1A depicts a procedure for obtaining FL-P enriched dispersions and related compositions. To minimize BP chemical degradation, deoxygenated water is prepared by purging deionized water with ultrahigh purity Ar gas in a sealed container. Bulk BP crystals are then exfoliated with tip ultrasonication in a vessel sealed in an Ar environment that contains deoxygenated water with 2% w v.sup.1 sodium dodecylsulfate (SDS). For FL-P enrichment, as-prepared BP solutions undergo two steps of sedimentation-based ultracentrifugation to isolate thin flakes with relatively large lateral area. Initially, the as-exfoliated solution is dark brown (FIG. 1B, left), softening to light pale yellow after solution dilution and thick flake removal (FIG. 1B, right).

(19) The chemical, structural, and optical properties of the resulting FL-P solutions are ascertained through transmission electron microscopy (TEM), Raman spectroscopy, photoluminescence (PL) spectroscopy, and X-ray photoelectron spectroscopy (XPS). FIG. 1C shows a representative, low-magnification TEM image of a FL-P nanosheet, while the high-resolution TEM (HRTEM) image of FIG. 1D reveals the atomic structure. Selected area electron diffraction (SAED) patterns (FIG. 1E) confirm that the FL-P nanosheets are crystalline and orthorhombic in structure. The HRTEM and SAED data further show no evidence of structural disorder or oxidation. In FIG. 1F, a liquid-phase Raman spectrum from the FL-P dispersion shows the three representative BP Raman modes at 362 cm.sup.1 (A.sub.g.sup.1), 439 cm.sup.1 (B.sub.2g), and 466 cm.sup.1 (A.sub.g.sup.2), with corresponding full-width at half-maximum (FWHM) values of 3.0, 3.3, and 5.1 cm.sup.1, respectively. Monolayer and few-layer phosphorene flakes have higher FWHM values than thicker (>5 nm) BP, making the FWHM broadening compared to precipitated, thicker BP dispersions indicative of FL-P enrichment in this process (FIGS. 2A-B). A visible (Si CCD) PL spectrum of the FL-P dispersion taken with 532 nm excitation reveals an emission peak at 1.37 eV, consistent with previous reports for monolayer phosphorene (FIG. 1G). Finally, the chemical quality of the FL-P flakes is assessed using XPS in FIG. 1H. XPS shows that FL-P exhibits the P 2p.sub.3/2 and P 2p.sub.1/2 doublet, characteristic of crystalline BP. Weak oxidized phosphorus (i.e., PDx) sub-bands are also observed at 136 eV in agreement with previous measurements of electronic-grade BP. XPS taken on FL-P prepared with other conventional surfactants (e.g., SDS, Pluronic F68 and sodium cholate) display similar results (FIGS. 3A-C).

(20) Although BP has been described as highly hydrophilic, the FL-P dispersion is only stabilized with the incorporation of amphiphilic surfactants in aqueous solution. While an aqueous BP dispersion prepared with SDS results in a stable dark brown solution (FIG. 4A, left), BP dispersed in water without surfactants precipitates quickly (FIG. 4A, right). To clarify this apparent contradiction, the hydrophilicity of the BP surface was measured with contact angle measurements on a freshly cleaved flat BP crystal. FIG. 4B shows that, immediately following exfoliation, the BP surface has an average contact angle of 57, indicating that the hydrophilicity of BP is between graphene oxide (27) and other 2D nanomaterials (90) such as graphene and transition metal dichalcogenides. From the higher optical absorbance (FIG. 4C) and lower zeta potential value (FIG. 4D), the relatively hydrophobic, freshly exfoliated BP nanosheets are stabilized in aqueous solution with amphiphilic surfactants.

(21) To investigate BP stability, LPE in both SDS-water and NMP was performed under identical exfoliation and centrifugation conditions. Centrifugation in each case occurred in steps ranging from 500 r.p.m. to 15,000 r.p.m. in order to compare the concentration of the resulting dispersions. FIGS. 4E and 4F show that the BP dispersion in NMP possesses a lighter yellow color, indicative of a lower concentration, compared to BP in SDS-water. The actual concentration of BP in SDS-water was calculated from the optical absorbance at 660 nm, using the measured extinction coefficient shown in FIG. 5 (the extinction coefficient that was used for BP in NMP was reported in the literature). This analysis concluded that the concentration of BP in SDS-water is approximately an order of magnitude higher than that of BP in NMP after centrifuging at 15,000 r.p.m. (FIG. 4G). Atomic force microscopy (AFM) images of BP exfoliated in SDS-water (red) and NMP (green) followed by 7,500 r.p.m. centrifugation are shown in FIGS. 4H-I. The thickness histogram in FIG. 4J reveals that FL-P nanosheets prepared in SDS-water have a tighter thickness distribution and thinner average thickness (4.5 nm compared to 17.6 nm) than BP prepared in NMP. Additional thickness and area histograms for FL-P prepared in SDS-water with smaller bins are provided in FIGS. 6A-B. These results illustrate the effectiveness of aqueous surfactant solutions for producing thin FL-P nanosheets compared to organic solvents.

(22) Following FL-P composition/solution preparation, optical absorbance spectra were measured (FIG. 7A). A higher resolution optical absorbance spectrum shows two peaks between 900 nm and 1250 nm (FIG. 7A, inset), which is consistent with previous reports for monolayer and bilayer phosphorene. PL spectroscopy was also performed on the FL-P solution using an excitation wavelength of 532 nm and a Si CCD for emission wavelengths up to 1000 nm and a N.sub.2-cooled InGaAs array for emission wavelengths between 1000 nm and 1600 nm. In FIG. 7B, the measured PL spectra were fit with Gaussian functions at peak positions of 907 nm (red area), 1215 nm (orange area), and 1428 nm (green area). To correlate these peak positions with the number of phosphorene layers, micromechanically exfoliated BP flakes were prepared on a 300 nm SiO.sub.2/Si substrate and then passivated with thin Al.sub.2O.sub.3 films. PL spectra and corresponding optical microscopy images for flakes ranging in thickness from one layer (1L) to five layers (5L) and more are shown in FIGS. 8-11. Based on the PL peak positions measured for the micromechanically exfoliated BP flakes, the peaks at 907 nm, 1215 nm, and 1428 nm from the FL-P dispersion (FIG. 7B) are assigned to 1L, 2L, and 3L+ phosphorene, respectively.

(23) Additionally, solid-state PL spectra were measured on articles comprising FL-P aggregates deposited onto a SiO.sub.2/Si substrate using an excitation wavelength of 532 nm. The measured PL spectra have a 1L peak position at 909 nm, as shown in FIG. 12A. In FIG. 13, additional peaks were observed for 2L at 1261 nm and 3L+ at 1459 nm, 1528 nm, And 1588 nm using a thermoelectrically cooled InGaAs array. The positions of the PL peaks from FL-P are indicated in Table 1. The peak position differences between the liquid-phase sample and the solid-state sample can be attributed to the different dielectric screening from the surrounding environment.

(24) TABLE-US-00001 TABLE 1 Optical emissions for monolayer and few-layer phosphorene sheets. All emissions probed by solid-state PL spectroscopy. Errors are propagated from the FWHM values of each phosphorene emission. Layer number Isolation scheme Primary emission (eV) Monolayer Mechanical exfoliation 1.34 0.15 eV Bilayer Mechanical exfoliation 0.97 0.16 eV Three-layer Mechanical exfoliation 0.85 0.09 eV Four-layer Mechanical exfoliation 0.82 0.10 eV Five-layer Mechanical exfoliation 0.78 0.06 eV Monolayer Solution cast 1.38 0.18 eV Bilayer Solution cast 0.98 0.79 eV Three-layer Solution cast 0.85 0.03 eV Four-layer Solution cast 0.81 0.05 eV Five-layer Solution cast 0.78 0.03 eV

(25) To verify that the observed PL is not defect-mediated, FL-P aggregates were exposed to UV ozone to intentionally introduce oxygen defects. As the UV ozone exposure time increases, the PL emission intensity at 909 nm decreases and a PDx defect-mediated emission peak emerges at 780 nm (FIG. 12D). The emission intensities at 909 nm (red curve) and 780 nm (green curve) are plotted as a function of UV ozone exposure time, with the 909 nm peak monotonically decreasing as the FL-P is oxidized, and the 780 nm peak initially increasing in intensity due to increased defect concentration and ultimately decaying due to the complete destruction of the material (FIG. 12E). This observation corresponds closely to the time-evolution of PL with respect to photo-oxidation time for micromechanically exfoliated BP (FIG. 8D). Furthermore, PL spectra were measured under the same excitation conditions on the FL-P dispersion before (red area) and after oxidation introduced by O.sub.2 gas bubbling (blue area). Following oxidation, the FL-P shows significantly decreases in PL intensity (FIG. 7C, left) in addition to substantial increases in the PD.sub.x XPS peak (FIG. 7C, right). Consequently, the PL peaks observed in FIG. 7B can be attributed to 1L, 2L, and 3L+ phosphorene, as opposed to defects introduced by solution processing.

(26) To explore the electrical properties of individual FL-P nanosheets, field-effect transistors (FETs) were fabricated by electron-beam lithography (EBL). Prior to FET device fabrication, lateral size sorting of the FL-P solution was performed to enrich large FL-P nanosheets that can bridge the 200 nm electrode gap. Lateral size sorting of the FL-P solution was achieved by sedimentation-based density gradient ultracentrifugation (s-DGU). By using s-DGU, the average flake area was tuned by over an order of magnitude (FIG. 14A). Each fraction (f1 to f6 from the top to the bottom of the centrifuge tube) was collected and deposited onto Si substrates for AFM measurements. The AFM images of f1, f3, and f5 clearly show the size differences between each fraction (FIG. 15). From the AFM measurements, the average areas of f3 to f6 were found to be appropriate for FET fabrication (FIG. 14B).

(27) To prepare arrays of the FL-P nanosheets, fractions f3 to f6 were collected on anodic aluminum oxide (AAO) membranes by vacuum filtration, rinsed with deoxygenated water to remove excess surfactant, and transferred onto degenerately doped Si substrates coated with 20 nm of atomic layer deposition (ALD) Al.sub.2O.sub.3 or HfO.sub.2. Following the FL-P transfer, Au/Ni electrodes of 10 m width and 200 nm length were patterned using EBL. A false-colored scanning electron microscopy (SEM) image (FIG. 14C) shows several FL-P nanosheets (green) connected in parallel between two electrodes (yellow). FET output and transfer curves (FIGS. 14D-E) reveal ambipolar behavior for FL-P with a current modulation (I.sub.ON/I.sub.OFF ratio) of 510.sup.3 and maximum drive current (I.sub.DS) of 130 A um.sup.1 at V.sub.DS=1 V (the transfer curve for the forward and reverse gate voltage sweep is shown in FIG. 16). Histograms of the maximum drive current and current modulation from several FL-P FETs are shown in FIG. 17. For comparison, I.sub.ON/I.sub.OFF ratio and maximum I.sub.DS values for BP FETs from the recent literature (both micromechanically exfoliated and organic solvent exfoliated) are plotted in FIG. 14F (Ref 3: Xia, F., Wang, H. & Jia, Y. Rediscovering black phosphorus as an anisotropic layered material for optoelectronics and electronics. Nature Comm. 5, 4458 (2014); Ref 6: Kang, J. et al. Solvent exfoliation of electronic-grade, two-dimensional black phosphorus. ACS Nano 9, 3596-3604 (2015); Ref. 10: Wood, J. D. et al. Effective passivation of exfoliated black phosphorus transistors against ambient degradation. Nano Lett. 14, 6964-6970 (2014); Ref 30: Du, Y., Liu, H., Deng, Y & Ye, P. D. Device perspective for black phosphorus field-effect transistors: contact resistance, ambipolar behavior, and scaling. ACS Nano 8, 1003510042 (2014); Ref 31: Miao, J., Zhang, S., Cai, L., Scherr, M. & Wang, C. Ultrashort channel length black phosphorus field-effect transistors. ACS Nano 9, 9236-9243 (2015); and Ref 32: Wang, H. et al. Black phosphorus radio-frequency transistors. Nano Lett. 14, 6424-6429 (2014).)

(28) Results from the FL-P device from FIG. 14E are also shown as a yellow star on this plot. Since the upper right hand corner of this plot represents an optimal combined performance from the perspective of maximum drive current and current modulation, it is evident that aqueous-exfoliated FL-P compares favorably with competing BP exfoliation methods from the perspectives of electronic properties and utility for electronic devices.

(29) More generally, relating to various embodiments of this invention, a variety of surface active components are effective at dispersing both the three-dimensional starting materials and the exfoliated two-dimensional planar nanomaterials. Without limitation, one or more surface active components can include one or more bile salts and/or alkali salts of linear alkyl sulfonates. Bile salts can be more broadly described as a group of molecularly rigid and planar amphiphiles with one or more charged groups opposing a hydrophobic face. Examples of bile salts include salts (e.g., sodium or potassium salts) of conjugated or unconjugated cholates and cholate derivatives including deoxycholates, chenodeoxycholates, taurodeoxycholates, glycochenodeoxycholates, ursodeoxycholates, and glycoursodeoxycholates.

(30) Useful alkyl sulfates include butane not limited to sodium hexyl sulfate, sodium octyl sulfate, sodium decyl sulfate, sodium undecyl sulfate, sodium dodecyl sulfate, and lithium dodecyl sulfate. Other useful surface active components include non-ionic triblock copolymers of oxyethylene and oxypropylene (or poly(ethylene oxide) and poly(propylene oxide), respectively), known under the Pluronic trade name), which can be represented by the general formula HO(C.sub.2H.sub.4O).sub.a(C.sub.3H.sub.6O)b(C.sub.2H.sub.4O).sub.aH, wherein a and b are integers in the range of about 10 to about 300; and non ionic tetrafunctional block copolymers of oxyethylene and oxypropylene known under the Tetronic trade name. Various other amphiphilic surface active components useful in conjunction with this invention would be well-known to those skilled in the art, limited only by functional precluding capacity to exfoliate phosphorus starting material and disperse resulting few-layer phosphorene.

(31) Generally, density gradient ultracentrifugation uses a fluid medium with a predefined variation in its density as a function of position within a centrifuge tube or compartment (i.e., a density gradient). Fluid media useful with the present teachings are limited only by nanomaterial aggregation therein to an extent precluding at least partial separation. Accordingly, aqueous and non-aqueous fluids can be used in conjunction with any substance soluble or dispersible therein, over a range of concentrations, so as to provide the medium a density gradient for use in the separation techniques described herein. Such substances can be ionic or non-ionic, non-limiting examples of which include inorganic salts and alcohols, respectively. Such a medium can include a range of aqueous iodixanol concentrations and the corresponding gradient of concentration densities. As understood by those skilled in the art, aqueous iodixanol is a common, widely used non-ionic density gradient medium. However, other media can be used in methods of the present teachings, as would be understood by those skilled in the art.

(32) More generally, any material or compound stable, soluble or dispersible in a fluid or solvent of choice can be used as a density gradient medium. A range of densities can be formed by dissolving such a material or compound in the fluid at different concentrations, and a density gradient can be formed, for instance, in a centrifuge tube or compartment. More practically, with regard to choice of medium, the two-dimensional nanomaterials in composition with the surface active components should be soluble, stable or dispersible within the fluids/solvent or resulting density gradient. Likewise, from a practical perspective, the maximum density of the gradient medium, as determined by the solubility limit of such a material or compound in the solvent or fluid of choice, should be at least as large as the buoyant density of the nanomaterial-surface active component complexes for a particular medium. Accordingly, any aqueous density gradient medium can be used provided that the nanomaterials are stable; that is, do not aggregate to an extent precluding useful separation. Alternatives to iodixanol include inorganic salts (such as CsCl, Cs.sub.2SO.sub.4, KBr, etc.), polyhydric alcohols (such as sucrose, glycerol, sorbitol, etc.), polysaccharides (such as polysucrose, dextrans, etc.), other iodinated compounds in addition to iodixanol (such as diatrizoate, nycodenz, etc.), and colloidal materials (such as Percoll).

EXAMPLES

(33) The following non-limiting examples and data illustrate various aspects and features relating to the methods, materials, compositions, articles and/or devices of the present invention, including the preparation and/or separation of few-layer phosphorene as are available through the methodologies described herein. In comparison with the prior art, the present methods, compositions and articles/devices provide results and data which are surprising, unexpected and contrary thereto. While the utility of this invention is illustrated through the use of several phosphorene materials, compositions, separation fractions, media and surface active agents which can be used therewith, it will be understood by those skilled in the art that comparable results are obtainable with various other phosphorene materials, compositions, fractions thereof, media and/or surface active agents, as are commensurate with the scope of this invention.

Example 1

(34) FL-P dispersion preparation: BP crystals were purchased from Smart-Elements and stored in a dark Ar glovebox prior to use. Deionized water with 2% w surfactant was purged with ultrahigh purity grade Ar for at least 1 hr to remove dissolved oxygen. A customized tip ultrasonicator setup was prepared by perforating the plastic lid of a 50 mL conical tube with a 0.125 inch sonicator tip. The interface between the tip and the lid was sealed with PDMS several times to block potential diffusion pathways of ambient O.sub.2 and H.sub.2O. The deoxygenated water and BP were placed in this sealed conical tube with an initial concentration of 1 mg mL.sup.1 under Ar atmosphere with less than 10% relative humidity. Additionally, Parafilm and Teflon tapes were used to further seal the vessel from ambient exposure. The sealed container was connected to the ultrasonicator (Fisher Scientific model 500 sonic dismembrator) in ambient conditions, and then BP crystals were exfoliated by ultrasonication. The resulting solution was centrifuged at 7,500 r.p.m. for 2 hrs at 15 C. to enrich FL-P nanosheets (Avanti J-26 XP, Beckman Coulter). Following centrifugation, the supernatant was collected and then ultracentrifuged at 14,000 r.p.m. for 2 hrs at 22 C. using a SW32Ti rotor (Optima L-80 XP, Beckman Coulter) before redispersing in deoxygenated water.

Example 2

(35) Size selection of FL-P: 3 mL of as-prepared FL-P dispersion was carefully placed on top of a 10 mL linear density gradient (1.05 to 1.10 g cm.sup.3) formed using deoxygenated iodixanol that was loaded with 2% w v.sup.1 SDS. Ultracentrifugation was then performed at 10,000 r.p.m. for 1 hr at 22 C. using a SW41 Ti rotor (Beckman-Coulter). Following ultracentrifugation, fractionation was achieved using a piston gradient fractionator (BioComp Instruments).

Example 3

(36) Contact angle measurements: Water droplet contact angles on BP crystals were measured by a contact angle goniometer. Flat BP crystals were purchased from HQ Graphene. Fresh BP surfaces were prepared by micromechanical exfoliation with water droplets placed on the surface within 30 sec. A high-resolution camera attached on the goniometer captured images from which the contact angle was determined.

Example 4

(37) Zeta potential measurement: Zeta potential measurements were carried out using a Zetasizer Nano ZS (Malvern Instruments) with clear disposable zeta cells. A HeNe laser source with a wavelength of 633 nm and a maximum power of 5 mW was used for the measurements. The reported results are averages from three independent measurements at 25 C.

Example 5

(38) Atomic force microscopy (AFM): AFM images were acquired in tapping mode using an Asylum Cypher AFM with Si cantilevers (290 kHz resonant frequency). As-prepared solutions were deposited onto Si substrates, rinsed with deoxygenated water to remove surfactant, and dried on a hot plate at 80 C. for 10 min in a flowing Ar environment. Prior to deposition, Si substrates were rinsed with acetone, methanol, and deionized water and immersed in diluted (3-aminopropyl)-triethoxysilane (APTES) solution to promote adhesion. After BP deposition on the Si substrate, an environmental cell was assembled in a flowing Ar environment and attached to a Cypher ES scanner under a laminar flow of ultrahigh purity grade N2. Images were taken in the repulsive phase regime at a scanning rate of 0.4 Hz using a minimum of 1024 samples per line. During scanning, N2 was continuously flowed through the environmental cell under optical microscopy light illumination.

Example 6

(39) Transmission electron microscopy (TEM): A BP solution droplet was deposited on a holey carbon TEM grid (Ted-Pella) and dried with N2. The TEM grid was assembled with a TEM sample holder after fewer than 5 min of exposure to ambient air. The TEM images were taken with a JEOL JEM-2100 at an accelerating voltage of 200 keV with a TEM column pressure of 10.sup.7 Torr.

Example 7

(40) Raman spectroscopy: Raman spectra of the solutions were obtained using a Horiba LabRAM HR Evolution with an excitation wavelength of 532 nm. A clear quartz cuvette with 10 mm transmitted path length was used for the measurement. Data were collected for 120 sec at 50 mW using an angled cuvette holder for the solution samples.

Example 8

(41) X-ray photoelectron spectroscopy (XPS): XPS measurements were performed using a high vacuum Thermo Scientific ESCALAB 250 Xi XPS system at a base pressure of 110.sup.9 Ton. The XPS data had a binding energy resolution of 0.1 eV using a monochromated Al K X-ray source at 1486.7 eV (400 m spot size). All core-level spectra were the average of five scans taken at a 100 ms dwell time using a pass energy of 15 eV. When using charge compensation, all core levels were charge-corrected to adventitious carbon at 284.8 eV. Using the software suite Avantage (Thermo Scientific), all subpeaks were determined in a manner detailed in the literature. (Kang, J. et al. Solvent exfoliation of electronic-grade, two-dimensional black phosphorus. ACS Nano 9, 3596-3604 (2015).) The p core level spectra for phosphorus and silicon were fit with doublets.

Example 9

(42) Photoluminescence (PL) spectroscopy: PL spectra for the solution samples were obtained using a Horiba Fluorolog-3 spectrofluorometer. Data were measured in a quartz cuvette for 3 sec. A liquid N.sub.2 cooled InGaAs array was used for the spectra at wavelengths longer than 1000 nm. PL spectra for the solid-state samples on 300 nm SiO.sub.2/Si substrates were collected using a Horiba Xplora Raman/PL system with an excitation wavelength of 532 nm. Data were collected for 120 sec using a 100 objective for point spectra and a 20 objective for mapping. This setup was used for the data in FIGS. 7 and 8. For the infrared PL measurements in FIGS. 9-11, and 13, a spectrometer (Princeton Aston) with a NIRvana InGaAs thermoelectrically cooled array at an excitation wavelength of 532 nm was employed. Data were collected for 5 sec using a 100 near-infrared objective. All PL spectral fits were performed with spline baselines and Voigt (Gaussian-Lorentzian) functions. Spectral fits disregarded any lineshape changes due to differences in grating efficiency at off-blaze wavelengths.

Example 10

(43) Optical absorbance spectroscopy: Optical absorbance spectra were obtained using a Cary 5000 spectrophotometer (Agilent Technologies). A quartz cuvette with 10 mm path length was used for the measurements. The baseline from the aqueous surfactant solution was subtracted from the spectra.

Example 11

(44) FL-P nanosheets transfer by PDMS stamping: The few-layer phosphorene nanosheets were collected on anodic aluminum oxide (AAO) membranes with 100 nm pore size by vacuum filtration. Following vacuum filtration, the nanosheets on the membrane were rinsed with 300 mL of deoxygenated water to remove the surfactants. The nanosheets on the membrane were then transferred onto the target substrates using PDMS stamping.

Example 12

(45) Field-effect transistor fabrication and measurement: Field-effect transistors (FETs) were fabricated using electron beam lithography to define 200 nm long, 10 um wide electrodes (5 nm Ni and 30 nm Au). BP FETs were measured in a Lakeshore CRX 4K under a base pressure of less than 510.sup.4 Torr at room temperature. Two Keithley sourcemeter 2400 units were used to measure device performance. Equation 1 was used to measure carrier mobility:

(46) $\begin{array}{cc}{}_{\mathrm{eff}}=\frac{L{g}_d}{{\mathrm{WC}}_{\mathrm{ox}}{V}_{\mathrm{DS}}}& \left(1\right)\end{array}$
where .sub.eff is the field-effect mobility, L is the channel length (obtained from optical micrographs), g.sub.d is the transconductance, W is the channel width (obtained from optical micrographs), Cox is the oxide capacitance (measured values of 1.0210.sup.2 and 2.0010.sup.3 F.Math.em.sup.2 were used for 20 nm thick ALD HfO.sub.2 and Al.sub.2O.sub.3, respectively), and V.sub.DS is the applied source-drain bias.

(47) As demonstrated, effective exfoliation of FL-P nanosheets has been achieved by ultrasonication in deoxygenated water stabilized with surfactants. This method results in stable, highly concentrated few-layer phosphorene, with distinct advantages compared to exfoliation in organic solvents. Comprehensive microscopic and spectroscopic analysis shows that individual FL-P nanosheets possess properties comparable to micromechanically exfoliated BP flakes without chemical degradation following aqueous processing. For example, PL measurements demonstrate that FL-P aqueous dispersions show strong visible and near-infrared emission that is characteristic of 1L, 2L, and 3L+ phosphorene. FL-P aqueous dispersions are also amenable to further size sorting by s-DGU, which allows for the enrichment of flakes with large lateral areas suitable for FET fabrication. The resulting FETs confirm that aqueous-exfoliated FL-P can be incorporated into electronic devices with performance metrics competitive with the best BP transistors to date.