WO2016018988A1

WO2016018988A1 - Two dimensional materials produced by the liquid exfoliation of black phosphorus

Info

Publication number: WO2016018988A1
Application number: PCT/US2015/042607
Authority: WO
Inventors: Scott C. Warren; Adam H. WOOMER; Rebekah WELLS; Tyler FARNSWORTH
Original assignee: University of North Carolina at Chapel Hill
Current assignee: University of North Carolina at Chapel Hill
Priority date: 2014-07-31
Filing date: 2015-07-29
Publication date: 2016-02-04
Anticipated expiration: 2017-01-31

Abstract

A process for producing two-dimensional black phosphorus flakes is carried out by: (a) mixing three-dimensional black phosphorus particles with a polar organic solvent to form a liquid dispersion thereof; and then (b) agitating the liquid dispersion for a time and at an intensity sufficient to produce two-dimensional black phosphorus flakes from at least a portion of the three-dimensional black phosphorus particles; and then (c) optionally separating the two-dimensional black phosphorus flakes from the three-dimensional black phosphorus particles. Compositions produced from the process are also described.

Description

TWO DIMENSIONAL MATERIALS PRODUCED BY

THE LIQUID EXFOLIATION OF BLACK PHOSPHORUS

Scott C. Warren, Adam H. Woomer, Rebekah Wells, and Tyler Farnsworth Related Applications

This application claims the benefit of United States Provisional Patent Application Serial No. 62/031,184, filed July 31, 2014, the disclosure of which is incorporated by reference herein in its entirety. Background of the Invention

Since its experimental realization in 2004, the two-dimensional carbon sheets known as graphene have captivated researchers from many fields of science. See, e.g., US Patent No. 7,071,258. Graphene's extraordinary properties have enabled use in a variety of applications, ranging from transistors to chemical sensors to transparent conductors, but it's lack of a bandgap has limited further device implementation within energy conversion and electronics.

Black phosphorus has been suggested as a possible alternative to graphene {see. e.g., A. Castellanos-Gomez, Leonardo Vicarelli et al., Isolation and characterization of few-layer black phosphorus, 2d Materials (2014)), but efficient methods for the production thereof in useful quantities have not heretofore been described.

Summary of the Invention

Described herein is a process for producing thin black phosphorus flakes (for example, 2 -dimensional black phosphorus flakes), comprising the steps of: (a) mixing three- dimensional black phosphorus particles with a polar organic solvent to form a liquid dispersion thereof; and then (b) agitating said liquid dispersion for a time and at an intensity sufficient to produce two-dimensional black phosphorus flakes from at least a portion of said three-dimensional black phosphorus particles; and then (c) optionally separating said thin black phosphorus flakes from said three-dimensional black phosphorus particles.

Liquid exfoliation has been suggested for graphene and related materials in, for example, J. Coleman et al., Science 331, 568-571 (4 Feb. 2011) and Coleman and Nicolosi, PCT Application WO2012/101457. However, phosphorus-phosphorus bonds are far weaker than those in the materials such as graphene. For example, the P-P bond has a strength of about 50 kcal/mol, while a C-C bond has a strength of about 100 kcal/mol. Still further, there has heretofore been limited evidence that 2-D phosphorus even exists, as the initial reports of mechanical (adhesive tape) exfoliation failed to isolate 2-D phosphorus sheets that are thinner than a few nanometers thick. In fact, collectively, through numerous published papers on 2-D phosphorus, there are only 2 or 3 reported observations of sheets that are thinner than 2 nm; these yields are far lower than for other 2-D materials. Hence, it may have heretofore been expected that these materials are too weak to be produced in any significant yield through a process that places the material under large forces. The present invention is explained in greater detail in the drawings herein and the specification set forth below.

Brief Description of the Drawings

Figure 1 A show the structure of black phosphorus.

Figure IB shows the structure of a black phosphorus monolayer.

Figure 1C is a photograph of a solution with 2-D phosphorus in N-methylpyrrolidone

(NMP) solvent (right cuvette) and a solution with only NMP (left).

Figure ID presents the optical properties of 3-D black phosphorus as well as a typical solution of 2-D phosphorus.

Figure IE shows the relation between a solvent's solubility parameter and the resultant concentration of thin sheets or flakes.

Figure IF further shows the relation between a solvent's solubility parameter and the resultant concentration of thin sheets or flakes.

Figure 1G further shows the relation between a solvent's solubilit parameter and the resultant concentration of thin sheets or flakes.

Figure 1H further shows the relation between a solvent's solubility parameter and the resultant concentration of thin sheets or flakes.

Figure 2A shows that variations on the methods herein produce solutions or suspensions containing flakes with thicknesses ranging from monolayers to multilayered phosphorus sheets. Isolation of few-layer solutions of 2-D black phosphorus via centrifugation. Fractions collected near 5k, 9k, and 13k rpm are shown.

Figure 2B shows the brown color of an NMP solution containing bilayers (right, optical gap = 1.48 eV) is compared to pure NMP (left).

Figure 2C shows that, by selecting flakes of varying thickness (Fig. 2A), the optical absorbance changes. Figure 2D shows Tauc plots yield experimental in-plane (XY ) and out-of-plane (Z) optical gaps. These are plotted vs. layer thickness and compared to DFT.

Figure 3 A shows characterization of the 2-D phosphorus flakes that are produced by the methods described herein. A Monolayer structure of 2-D black phosphorus is shown.

Figure 3B shows TEM image of a monolayer (top left) and few-layer sheet. Inset:

TEM allows sheet thickness to be quantified. The intensity difference (ΔΙ) between flakes and background occurs in steps of 25. Monolayers: ΔΙ = 25; bilayers: ΔΙ = 50.

Figure 3C is an HR-TEM which shows monolayers are crystalline.

Figure 3D is an FFT of (Figure 3C) which reveals 101 spots, evidence of a monolayer.

Figure 4A. X-ray photoelectron spectroscopy shows that exposure of the samples to light and oxygen lead to a considerable degree of oxidation within one hour and that the oxidation increases even more over 24 hours.

Figure 4B. A comparison of air-driven oxidation in either the dark or the light shows that oxidation proceeds more rapidly in the light.

Detailed Description of the Preferred Embodiments

"Three-dimensional black phosphorus material" as used herein refers to materials, generally in the form of particles, powders, or slurries thereof having a lateral size of 1 or 2 microns to 10 or 100 millimeters or more in two of its dimensions, and which have a thickness greater than 15 nm (30 atomic layers) in its third dimension.

"Two-dimensional black phosphorus material" as used herein refers to materials, generally in the form of flakes (including slurries of those flakes), that have a thickness of 30 atomic layers or less. Such materials are generally in the form of a distribution of flakes of various thicknesses, ranging from 1 to 30 atomic layers, which may be further fractionated (e.g., by centrifugation).

As noted above, the present invention provides a process for producing two- dimensional black phosphorus flakes, comprising the steps of: (a) mixing three-dimensional black phosphorus particles with a polar organic solvent to form a liquid dispersion thereof; and then (b) agitating said liquid dispersion for a time and at an intensity sufficient to produce two-dimensional black phosphorus flakes from at least a portion of said three- dimensional black phosphorus particles; and then (c) optionally (but in some cases preferably) separating the two-dimensional black phosphorus flakes from said three- dimensional black phosphorus particles. Suitable solvents include, but arc not limited to N-methylpyrrolidone, isopropanol, acetone, benzaldehyde, benzyl benzoate, benzyl ether, bromobenzene, chlorobenzene, cyclohexylpyrrolidinone, chloroform, cyclohexane, cyclohexanone, dimethylacetamide. dimethylformamide, d i m ethyli m idazo 1 i d i none, dimethylsulphoxide, N-dodecylpyrrolidone. formamide, methanol. N-mcthylformamide.. N-octylpyrrolidone. quinoline. N- vinylpyrrolidonc. and combinations thereof. The solvents may be aqueous or non-aqueous. Particularly preferred are benzonitrile. N -methyl pyrrolidone. isopropanol. or a combination thereof. Some solvents, such as methanol and acetone, do not work when used by themselves, but may be mixed with other solvents to render them operable. In general, and as discussed below, solvents with I lan sen solubility parameters close to that of black phosphorus are preferred.

Agitation may be carried out by any suitable technique, including but not limited to sonication (e.g., bath or probe sonication) or by mixing (e.g., in a shear mixer). In general, the agitating step is carried out for a time and at an intensity sufficient to convert at least 0.01, 0.02, or 0.05 percent by weight, and optionally up to 2, 5, or 10 percent by weight, of said three-dimensional black phosphorus particles to said two-dimensional black phosphorus flakes.

As discussed below, agitating is preferably carried out under conditions which exclude oxygen, such as by maintaining the liquid dispersion under an inert gas atmosphere during the agitation step, to reduce or minimize oxidation of the black phosphorus.

The separating step, when included, may be carried out by any suitable technique, including eentrifugation, filtration, or a combination thereof. In some embodiments, the separating step further comprises a fractionating step, for example by higher speed eentrifugation, to narrow the size distribution of the flakes in the composition. The separating step, including combinations of the foregoing, may optionally, but in some embodiments preferably, be carried out to the exclusion of oxygen or under an inert gas atmosphere, in like manner as the agitation step, to reduce or minimize the oxidation of black phosphorus.

Flakes may be concentrated in the course of or separately from the separation step(s), or concentrated by other techniques such as evaporation of the solvent. Again this may be carried out to the exclusion of oxygen or under an inert gas atmosphere, in like manner as the agitation step, to reduce or minimize the oxidation of black phosphorus.

The flakes or flake compositions (with or without solvent) produced by the methods described herein typically have (both individually and cumulatively) an average lateral dimension (or width) of not more than 1 , 2, 4 or 8 microns, and/or not less than 20, 40, or 80 nanometers.

The flakes or Hake compositions may comprise, consist of or consist essentially of flakes not more than 30 atomic layers thick, and in some embodiments may comprise, consist of or consist essentially of flakes not more than 10 atomic layers thick (e.g. two-dimensional black phosphorus flakes).

In some embodiments the flakes or flake compositions are characterized by:

at least 5, 8 or 10 percent (and optionally up to 20 or 30 percent) of said two- dimensional black phosphorus flakes consists of flakes having a thickness of four atomic layers or less; and/or

at least 3, 5 or 7 percent (and optionally up to 15 or 20 percent) of said two- dimensional black phosphorus flakes consists of flakes having a thickness of three atomic layers or less; and/or

at least 1, 2 or 3 percent (and optionally up to 10 or 15 percent) of said two- dimensional black phosphorus flakes consists of flakes having a thickness of two atomic layers or less; and/or

at least 0.5, 1 or 2 percent (and optionally up to 3 or 5 percent) of said two- dimensional black phosphorus flakes consists of flakes having a thickness of one atomic layer.

The flakes as described herein are preferably unoxidized or substantially unoxidized by excluding oxygen, as described above and below. If not utilized directly, the flakes and flake compositions (e.g., flakes dispersed in a solvent) may be sealed in a suitable container to the exclusion of oxygen (e.g., by filing the container entirely with the composition and thereby eliminating any gas "head space" in the container; filling any gas head space with an inert gas atmosphere, etc.)

Flakes and compositions of the present invention as described above may be used in the manufacture of devices by further: (d) coating a substrate (e.g., a conductive, semiconductive. or insulating substrate) with said two-dimensional black phosphorus material to produce a device (e.g., an electronic device such as a transistor; an opto-electronic device such as a photocell, etc.); and then (e) optionally sealing or enclosing said device in a housing or container. The coating step may be carried out in accordance with any technique, such as spraying, dip coating, screen printing, doctor blading, inkjet printing, Mayer rod coating, or Langmuir Blodgett deposition. The present invention is explained in greater detail in the following non-limiting Examples.

EXAMPLES

The invention describes a method for producing and isolating black phosphorus flakes with thicknesses from monolayers to multilayers. The method involves three main steps: (1 ) preparing a solution containing black phosphorus and a solvent or solvent mixture, (2) sonicating or shear-mixing the solution to separate the layers of black phosphorus into thin flakes, and (3) fractionating this solution to isolate 2-D phosphorus flakes with narrow thickness distributions. Unlike other 2-D materials, black phosphorus is highly susceptible to oxidation (Figure 4). Throughout the process, important changes in protocol are made with respect to earlier work in order to prevent the oxidation of the starting black phosphorus or the final 2-D material. Most optimally, all steps are performed under an inert (e.g., nitrogen, argon) atmosphere. Less optimally, these steps are performed in an oxidizing atmosphere (e.g., air) but the samples are protected from light. While oxidation is slow in the dark, it is faster in the light (Figure 4b). Least optimally, these steps are performed in an oxidizing atmosphere but at low light intensities. In this case, a significant fraction of the totality of the 2-D phosphorus that is produced will be in an oxidized state and therefore no longer be the desired end product of this process.

EXAMPLE 1

Preparing a solution containing black phosphorus

Our experiments employ black phosphorus that has been ground to have particle sizes that are typically between 1 micron and 1 millimeter. Both smaller and larger pieces can be employed, however, although larger pieces result in a slower exfoliation process while smaller pieces restrict the maximum lateral dimension of the 2-D sheets that are ultimately produced. The black phosphorus starting material is measured into solvent at concentrations of 0.5 - 50 mg/mL. Wider ranges are feasible, although lower concentrations will produce dilute solutions of the 2-D material and more concentrated solutions will not result in larger yields of the 2-D material. Solutions are prepared using a wide range of volumes, with varying concentrations, typically from 5 mL to 1 ,000 mL although we foresee no practical limit to extending the range of volumes further. Solvents used for solution preparation include n-methyl-2-pyrrolidone, cyclopentanone, l-cyclohexyl-2-pyrrolidone, l-dodecyl-2- pyrrolidinone, benzyl benzoate, l-octyl-2-pyrrolidone, l-vinyl-2-pyrrolidone, benzyl ether, l,3-dimethyl-2-imidazolidinone, cyclohexanone, chlorobenzene, dimelhylsulfoxide, benzonitrile, n-methylformamide dimethylformamide, benzaldehyde, and isopropyl alcohol. These solvents were related to their subsequent concentration of thin black phosphorus flakes based on their Hansen and Hildebrand solubility parameters [see Figure 1 (e-h)j. Most optimally, benzonitrile, n-methyl-2-pyrrolidone, cyclopentanone, l-vinyl-2-pyrrolidone, and l ,3-dimethyl-2-imidazolidone give the highest concentration of 2-D phosphorus sheets. Less optimally, l-cyclohexyl-2-pyrrolidone. 1 -dodecyl-2-pyrrolidinone, benzyl benzoate, 1 -octyl- 2-pyrrolidone, dimethylsulfoxide, benzonitrile, n-methylformamide dimethylformamide, benzaldehyde, and isopropyl alcohol are also used to produce thin sheets, though at lower yields. Least optimally, benzyl ether, cyclohexanone, and chlorobenzene are very poor solvents for exfoliating black phosphorus into thin sheets. As mentioned above, Figure 1 (e- h) shows the relation between a solvent's solubility parameter and the resultant concentration of thin sheets. The solvent with Hansen solubility parameters nearest to that of black phosphorus will give the highest concentration. Furthermore, combinations of various solvents to produce solutions with altogether different Hansen solubility parameters that are more similar to black phosphorus enables higher yields of few-layer black phosphorus.

EXAMPLE 2

Agitation of the Mixture

Bath and tip sonication methods may be employed to prepare solutions of thin phosphorus flakes using the above mentioned solvents. Most optimally, bath sonication times range from 10 - 30 hours and give a substantial yield of high quality thin sheets. Bath sonication times > 30 hours give the highest concentration of thin sheets, though with small lateral dimensions. Least optimally, bath sonication times < 10 hours produce a low yield of 2-D phosphorus sheets. Tip sonication is an effective means of producing thin phosphorus flakes at relatively high concentrations. Most optimally, solutions of thin phosphorus sheets are prepared with a duty cycle between 10 - 20 % with an output control of 1 - 3 at a time of 5 - 8 hours. Less optimally, solutions of phosphorus thin sheets also may be prepared with duty cycles between 5 - 30 % and varied output controls ranging between 1 - 10 and times between 1 - 20 hours. We have found that shear mixing is another method for producing thin flakes of black phosphorus. Optimally, the rate of 5,000 - 8,000 rpm is used for 2-6 hours. Less optimally, but still efficient at producing thin flakes, are settings ranging from 3,000 - 10,000 rpm with time varying between 15 minutes and 10 hours. EXAMPLE 3

Isolating 2-D phosphorus with narrow thickness distributions Centrifugation is an optional method for separating the thin phosphorus sheets into volume fractions with narrow thickness distributions. Low speed centrifugation rates ( 1 ,000 — 5,000 rpm) sediment the bulk, unexfoliated flakes from the sonicated (or shear mixed) solution while high speed centrifugation rates (5,000— 20,000 rpm) will also sediment thinner sheets. A systematic variation of these rates enables high control over thickness distributions. To achieve a narrow thickness distribution of thin phosphorus sheets, the initial solution is centrifuged at a high speed (e.g., > 10,000 rpm). The supernatant is then collected and re-centrifuged at a slightly higher speed (e.g., 12,000 rpm). The sediment from the second centrifugation can then be re-dispersed into the same or different solvent, as long as the I Iildebrand solubility parameters are suitable. These solutions have 2-D phosphorus sheets with a narrow distribution of sheet thicknesses. The thickness of these sheets in solution is dependent upon the centrifugation rate and time. Variations on the above method effectively produces solutions with thicknesses ranging from monolayers to multilayered phosphorus sheets (Fig. 2A). The optical absorbance of the solutions depends on flake thickness, evidencing optical gaps that are tunable from the near-IR to the visible (Figure 2 b- d). The materials produced in this process are high crystalline despite their atomic thicknesses and typically have dimensions below 2 microns ( Fig. 3 b-d).

EXAMPLE 4

Need for Protection from Oxidizing Conditions

Figure 4 demonstrates the need for one of the distinguishing components of this invention— the protection of the samples from oxygen or other oxidizing conditions, (a) X- ray photoelectron spectroscopy shows that exposure of the samples to light and oxygen lead to a considerable degree of oxidation within one hour and that the oxidation increases even more over 24 hours, (b) A comparison of air-driven oxidation in either the dark or the light shows that oxidation proceeds more rapidly in the light.

The foregoing is illustrative of the present invention, and is not to be construed as limiting thereof. The invention is defined by the following claims, with equivalents of the claims to be included therein.

Claims

THAT WHICH IS CLAIMED IS:

1. A process for producing two-dimensional black phosphorus flakes, comprising the steps of:

(a) mixing three-dimensional black phosphorus particles with a polar organic solvent to form a liquid dispersion thereof; and then

(b) agitating said liquid dispersion for a time and at an intensity sufficient to produce two-dimensional black phosphorus flakes from at least a portion of said three-dimensional black phosphorus particles; and then

(c) optionally separating said two-dimensional black phosphorus flakes from said three-dimensional black phosphorus particles.

2. The method of claim 1 , wherein said agitating step is carried out by sonication (e.g., bath or probe sonication) or by mixing (e.g., in a shear mixer).

3. The method of claim 1 or 2, wherein said agitating step is carried out for a time and at an intensity sufficient to convert at least 0.01 , 0.02, or 0.05 percent by weight, and optionally up to 2, 5, or 10 percent by weight, of said three-dimensional black phosphorus particles to said two-dimensional black phosphorus flakes.

4. The method of claims 1 to 3, wherein said flakes have an (individual and cumulative) average lateral dimension (or width) of not more than 1 , 2, 4 or 8 microns, and/or not less than 20, 40, or 80 nanometers.

5. The method of claim 1 to 4, wherein said two-dimensional black phosphorus flakes comprise, consist of or consist essentially of flakes not more than 30 atomic layers thick.

6. The method of claim 1 to 4, wherein said two-dimensional black phosphorus flakes comprise, consist of or consist essentially of flakes not more than 10 atomic layers.

7. The method of claim 1 to 4, wherein:

at least 5, 8 or 10 percent (and optionally up to 20 or 30 percent) of said two- dimensional black phosphorus flakes consists of flakes having a thickness of four atomic layers or less; and/or at least 3, 5 or 7 percent (and optionally up to 15 or 20 percent) of said two- dimensional black phosphorus flakes consists of flakes having a thickness of three atomic layers or less; and/or

8. The method of claim 1 to 7, further comprising the step of at least partially fractionating said two-dimensional black phosphorus flakes by thickness.

9. The method of claim 1 to 8, wherein said separating step is present and is carried out by centrifugation, filtration, or a combination thereof.

10. The method of claim 1 to 9, wherein said organic solvent comprises benzonitrile, N-methylpyrrolidone, isopropanol, acetone, benzaldehyde, benzyl benzoate, benzyl ether, bromobenzene, chlorobenzene, cyclohexylpyrrolidinone, chloroform, cyclohexane, cyclohexanone, dimethylacetamide, dimethylformamide, dimethylimidazolidinone, dimethylsulphoxide, N-dodecylpyrrolidone, formamide, methanol, N-methylformamide„ N- octylpyrrolidone, quinoline, N-vinylpyrrolidone, or a combination thereof.

11 The method of claim 1 to 9, wherein said organic solvent comprises benzonitrile, N-methylpyrrolidone, isopropanol, or a combination thereof.

12. The method of claim 1 to 11, wherein said agitating step, and optionally said separating step, is carried out under conditions which exclude oxygen.

13. The method of claim 12, wherein said' agitating step, and optionally said separating step, is carried out with said liquid dispersion under an inert gas atmosphere.

14. The method of claim 1 to 13, further comprising the step of: (d) coating a substrate (e.g., a conductive, semiconductive, or insulating substrate) with said two-dimensional black phosphorus material to produce a device (e.g., an electronic device such as a transistor; an opto-electronic device such as a photocell, etc.); and then

(e) optionally sealing or enclosing said device in a housing or container.

15. The method of claim 14, wherein said coating step is carried out by spraying, dip coating, screen printing, doctor blading, inkjet printing, Mayer rod coating, or Langmuir Blodgett deposition.

16. A composition comprising, consisting of or consisting essentially of two- dimensional black phosphorus flakes,

wherein said flakes have an (individual and cumulative) average lateral dimension (or width) of not more than about 1 , 2, 4 or 8 microns, and/or at least 20, 40, or 80 nanometers; and wherein said two-dimensional black phosphorus flakes comprise, consist of or consist essentially o flakes not more than 30 atomic layers thick.

17. The composition of claim 16 produced by a method of claim 1 to 13.

18. The composition of claim 16, wherein said two-dimensional black phosphorus flakes comprise, consist of or consist essentially of flakes of not more than 10 atomic layers.

19. The composition of claim 18, wherein:

at least 1 , 2 or 3 percent (and optionally up to 10 or 15 percent) of said two- dimensional black phosphorus flakes consists of flakes having a thickness of two atomic layers or less; and/or

20. The composition of claim 16 to 19, further comprising an organic solvent in which said flakes are dispersed.

21. The composition of claim 20, wherein said organic solvent comprises benzonitrile, N-methylpyrrolidone, isopropanol, acetone, benzaldehyde, benzyl benzoate, benzyl ether, bromobenzene, chlorobenzene, cyclohexylpyrrolidinone, chloroform, cyclohexane, cyclohexanone, dimethylacetamide, dimethylformamide, dimethylimidazolidinone, dimethylsulphoxide, N-dodecylpyrrolidone, formamide, methanol, N-mcthylform amide.. N- octylpyrrolidone, quinoline, N-vinylpyrrolidone, or a combination thereof.

22. The composition of claim 20, wherein said organic solvent comprises benzonitrile, N-methylpyrrolidone, isopropanol, or a combination thereof.

23. The composition of claim 16 to 22, wherein said composition is in the form of a film or a liquid suspension (e.g., a suspension of said flakes in the solvent aggregated or dispersed form).

24. The composition of claim 16 to 23, wherein said black phosphorus flakes are unoxidized.

25. The composition of claim 16 to 24, sealed in a container to the exclusion of oxygen (e.g., without a gas head space, or under an inert atmosphere).