Liquid exfoliation of solvent-stabilized few-layer black phosphorus for applications beyond electronics

Abstract

Few-layer black phosphorus (BP) is a new two-dimensional material which is of great interest for applications, mainly in electronics. However, its lack of environmental stability severely limits its synthesis and processing. Here we demonstrate that high-quality, few-layer BP nanosheets, with controllable size and observable photoluminescence, can be produced in large quantities by liquid phase exfoliation under ambient conditions in solvents such as N-cyclohexyl-2-pyrrolidone (CHP). Nanosheets are surprisingly stable in CHP, probably due to the solvation shell protecting the nanosheets from reacting with water or oxygen. Experiments, supported by simulations, show reactions to occur only at the nanosheet edge, with the rate and extent of the reaction dependent on the water/oxygen content. We demonstrate that liquid-exfoliated BP nanosheets are potentially useful in a range of applications from ultrafast saturable absorbers to gas sensors to fillers for composite reinforcement.

ARTICLE

Received 9 Jan 2015 | Accepted 4 Sep 2015 | Published 15 Oct 2015

Damien Hanlon1,2,*, Claudia Backes1,2,*, Evie Doherty1,2,3, Clotilde S. Cucinotta1,2, Nina C. Berner1,3, Conor Boland1,2, Kangho Lee1,3, Andrew Harvey1,2, Peter Lynch1,2, Zahra Gholamvand1,2, Saifeng Zhang4, Kangpeng Wang1,2,4, Glenn Moynihan1,2, Anuj Pokle1,2,3, Quentin M. Ramasse5, Niall McEvoy1,3, Werner J. Blau1,2, Jun Wang4, Gonzalo Abellan6, Frank Hauke6, Andreas Hirsch6, Stefano Sanvito1,2, David D. ORegan1,2, Georg S. Duesberg1,3, Valeria Nicolosi1,2,3 & Jonathan N. Coleman1,2

Few-layer black phosphorus (BP) is a new two-dimensional material which is of great interest for applications, mainly in electronics. However, its lack of environmental stability severely limits its synthesis and processing. Here we demonstrate that high-quality, few-layer BP nanosheets, with controllable size and observable photoluminescence, can be produced in large quantities by liquid phase exfoliation under ambient conditions in solvents such as N-cyclohexyl-2-pyrrolidone (CHP). Nanosheets are surprisingly stable in CHP, probably due to the solvation shell protecting the nanosheets from reacting with water or oxygen. Experiments, supported by simulations, show reactions to occur only at the nanosheet edge, with the rate and extent of the reaction dependent on the water/oxygen content. We demonstrate that liquid-exfoliated BP nanosheets are potentially useful in a range of applications from ultrafast saturable absorbers to gas sensors to llers for composite reinforcement.

1 School of Physics, Trinity College Dublin, Dublin 2, Ireland. 2 CRANN and AMBER Research Centres, Trinity College Dublin, Dublin 2, Ireland. 3 School of Chemistry, Trinity College Dublin, Dublin 2, Ireland. 4 Key Laboratory of Materials for High-Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China. 5 SuperSTEM Laboratory, STFC Daresbury Campus, Daresbury WA4 4AD, UK. 6 Chair of Organic Chemistry II and Institute of Advanced Materials and Processes (ZMP), Friedrich-Alexander-Universitat Erlangen-Nrnberg (FAU), Henkestrasse 42, 91054 Erlangen, Germany. * These authors contributed equally to this work. Correspondence and requests for materials should be addressed toJ.N.C. (email: mailto:[email protected]

Web End [email protected] ).

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DOI: 10.1038/ncomms9563 OPEN

Liquid exfoliation of solvent-stabilized few-layer black phosphorus for applications beyond electronics

ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms9563

Over the last few years, the study of two-dimensional (2D) materials16 such as graphene, BN and MoS2 have become one of the most exciting areas of nano-science.

However, in the past year a new 2D material has been generating considerable excitement in the research community7. Phosphorene consists of atomically thin, 2D nanosheets of black phosphorus (BP). In BP, the monolayers stack together via van der Waals interactions to form layered crystals in much the same way as graphene stacks together to form graphite. Recently, it was shown that BP can be exfoliated by mechanical cleavage to form mono- and few-layer phosphorene, which we refer to as FL-BP812. This new material has a direct bandgap in mono-, few-layer and bulk forms, which varies with nanosheet thickness from B1.5 eV for monolayer phosphorene to B0.3 eV for bulk BP8,13,14. This is in contrast to graphene1 which has no bandgap and materials such as MoS2, which display direct bandgaps only in the monolayer form6. As a result, BP is extremely attractive both for electronics and optoelectronics and has therefore been extensively studied in applications such as transistors7,12,15, photodetectors15,16 and solar cells9.

In addition, like other 2D materials, it is probable that BP has the potential to perform in a range of applications beyond (opto)electronics. Indeed, BP has already been fabricated into electrodes in lithium ion batteries17. Furthermore, theory predicts that FL-BP shows potential for use in gas sensors18 and thermoelectrics19. For most applications, it will be necessary to produce FL-BP in much larger quantities than can be achieved by mechanical exfoliation. One way to prepare nanosheets in large quantities is by liquid phase exfoliation (LPE)20,21. This technique involves the sonication22,23 or shearing24,25 of layered crystals in appropriate liquids and has previously been applied to graphene and boron nitride, as well as a range of other 2D materials2123,2628.

While phosphorene nanosheets have very recently been produced by liquid exfoliation2931, this method remains problematic, largely because BP is known to be unstable7,8,32, degrading via reactions with water and oxygen. For this method to be useful, ways must be found to stabilize liquid-exfoliated FLBP nanosheets against oxidation. It is known that BP can be protected from reacting with environmental species by encapsulation, suggesting a possible way forward7,24. We hypothesized that LPE of BP may be practical if the solvent is carefully chosen to minimize oxidation of the exfoliated nanosheets in the liquid phase due to the solvation shell acting as a barrier to prevent oxidative species reaching the nanosheet surface.

If this could be achieved it would yield numerous advantages. LPE is a powerful technique to produce nanosheets in large quantities. In addition, the nanosheets are produced directly in the liquid phase and are thus inherently processable and can be easily formed into composites, coatings or lms21, facilitating their use in a range of applications. Furthermore, size-control can be achieved by centrifugation, enabling processing for both applications and fundamental studies. For example, studying the oxidation of BP nanosheets of varying sizes would give insight into whether the degradation mechanism is associated with nanosheet edges or the basal plane. However, most importantly, protection of the nanosheets by an appropriate solvent would allow large-scale production in ambient conditions, dramatically simplifying future development of this material.

In this work, we demonstrate that BP can be exfoliated to give large quantities of FL-BP nanosheets by sonication in solvents such as N-cyclohexyl-2-pyrrolidone (CHP), even in ambient conditions. We show that these nanosheets can be readily size-selected by controlled centrifugation. Such dispersions show direct gap photoluminescence (PL) with PL energy, width and

relative quantum yield scaling strongly with layer number. The BP nanosheets are relatively stable in CHP, but degrade rapidly once water/O2 is added. The reaction occurs predominantly at nanosheet edges, presumably following a different reaction pathway than previously observed32. Finally we demonstrate that liquid-exfoliated BP nanosheets can be used in applications beyond electronics. We show that dispersions of FL-BP have impressive nonlinear optical properties allowing them to be used in optical switching applications. In addition, the nanosheets can be incorporated into polymer matrices resulting in reinforced composites and are highly potent gas sensors.

ResultsExfoliation and basic characterization. To produce large quantities of BP nanosheets (see Fig. 1a for structure), we use LPE22,23. This technique is often carried out in amide solvents such as CHP or N-methyl-2-pyrrolidone (NMP), although isopropanol (IPA) has proven useful in some cases22. Sonication of ground BP crystals (see Fig. 1b for scanning electron microscopic (SEM) image) in CHP yields a brown dispersion (Fig. 1c). In the simplest case, we remove unexfoliated material by centrifugation at 1,000 r.p.m. (106 g) for 180 min to yield a stable dispersion which we refer to as the standard sample (std-BP, see Supplementary Fig. 1 for process optimization).

The successful exfoliation of BP in CHP was conrmed by transmission electron microscopy (TEM) of nanosheets on holey carbon grids (TEM, Fig. 1df and Supplementary Fig. 2). These images show electron-transparent nanosheets with lateral dimensions LB1 mm. As shown by scanning TEM (STEM) and high-angle annular dark eld (HAADF) STEM imaging, the lattice appears intact over wide regions (Fig. 1g,h). This suggests that, as with other 2D materials, BP can be exfoliated in liquids without the introduction of defects2124,33. N.B. we used FL-BP exfoliated in IPA for STEM and HAADF, due to difculties in completely removing the CHP. However, the intact lattice was also observed from the std-BP in CHP (Supplementary Fig. 3). To assess the lateral dimensions of the std-BP, we performed statistical TEM analysis, nding a bimodal size distribution with modes associated with LB100 nm and LB3 mm (Fig. 1i). We note that the larger nanosheets are considerably bigger than is typically observed for other 2D materials (for example, Lo1 mm)22,26,33,34.

To further characterize the std-BP dispersion, we measured the extinction coefcient, e, as shown in Fig. 1j. This parameter is dened via the optical transmittance; T 10 eCl, where l is the

cell length and C is the nanosheet concentration (measured by ltration and weighing). The extinction coefcient includes contributions from both absorbance (a) and scattering (s)33,35, which can be isolated using an integrating sphere and are shown in Fig. 1j. Note that coefcients of extinction, absorbance and scattering are related by e(l) a(l) s(l)33,35. Because of

size-dependent scattering contributions, the extinction coefcient cannot be used to accurately measure nanosheet concentration of varying sizes3335. Instead, we can use the measured absorbance coefcient (a(l 465 nm) 15 l g 1 cm 1) to give

the concentration of all subsequent FL-BP dispersions. For example, Fig. 1k shows the BP concentration measured this way to scale with the sonication time as Cpt0.4, similar to previous observations for graphene exfoliation24,36. Concentrations as high as B1 g l 1 can easily be realized.

To further conrm the structural integrity of the FL-BP, the dispersion was ltered onto alumina membranes and subjected to Raman and X-ray photoelectron spectroscopy (XPS). SEM conrmed the homogeneity of the lm (Fig. 1l inset). The Raman spectrum (Fig. 1l) shows the characteristic Ag1, B2g and Ag2 phonons of FL-BP8,12,13,32. The P2p XPS core-level spectra

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NATURE COMMUNICATIONS | DOI: 10.1038/ncomms9563 ARTICLE

40

Count

20

<L> = 1 m

0 0.1 1 10

L (m)

3,000

1

2,500

P2p3/2 P2p1/2

[afii9825], [afii9830], [afii9846] (l g1 m1 )

[afii9825]

Concentration (g l1 )

0.4

[afii9830]

Intensity (a.u.)

Intensity (a.u.)

2,000

1,500

[afii9846]

A1g A2g

1,000

0.1

B2g

500 300 450 600 750

1 10 100

300 400 500

138 135 132 129 126

Wavelength (nm)

Sonic time (h)

Raman shift (cm1)

Binding energy (eV)

Figure 1 | Basic characterization of exfoliated black phosphorous. (a) Structure of black phosphorus (BP). (b) SEM image of a layered BP crystal (scale bar, 100 mm). (c) Photograph of a dispersion of exfoliated FL-BP in CHP. (df) Representative low-resolution transmission electron microscopy (TEM)

images of FL-BP exfoliated in N-cyclohexyl-2-pyrrolidone (CHP) (scale bars in df: 500, 100 and 500 nm). (g) Bright-eld scanning transmission TEM (STEM) image and (h) Butterworth-ltered high-angle annular dark eld (HAADF) STEM image of FL-BP (exfoliated in isopropanol) showing the intact lattice (scale bars in g and h, 2 and 1 nm). (i) Nanosheet length histogram of the exfoliated FL-BP obtained from TEM (sample size 239). (j) Extinction,

absorbance, scattering coefcient spectra of FL-BP in CHP. (k) Concentration of FL-BP as a function of sonication time. The dashed line shows power law behaviour with exponent 0.4. (l) Raman spectrum (mean of 100 spectra, excitation 633 nm) of a ltered dispersion. Inset: scanning electron microscopic image of thin lm (scale bar, 2 mm). (m) X-ray photoelectron spectroscopy P core-level region.

(Fig. 1m) show the expected contributions from P2p1/2 and

P2p3/2 components. Contributions from PxOy species, presumably as a result of partial degradation (discussed below) are minor (o15%).

We can obtain information about the nanosheet thickness using statistical atomic force microscopy (AFM) analysis (images Fig. 2ac and Supplementary Fig. 4). To overcome the problems associated with deposition of nanosheets onto substrates from high-boiling point solvents, the std-BP in CHP was transferred to IPA prior to drop casting the samples (see Methods). Apparent AFM heights from liquid-exfoliated nanomaterials are usually overestimated due to residual solvent24,33,37, as well as contributions from effects such as capillary forces and adhesion38,39. To overcome these problems and to convert the apparent measured AFM thickness to the number of layers, we have applied a similar approach to that reported for graphene and MoS2 (refs 26,27). This involves AFM analysis of incompletely exfoliated nanosheets (Fig. 2d inset), measuring the height of the steps between terraces (Fig. 2d and Supplementary Fig. 5). By plotting the measured step heights in ascending order (Fig. 2e), groups of steps with similar apparent heights appear. It is clear that the apparent step height is always a multiple of B2 nm, a value which we associate with the apparent monolayer height24,33. By plotting the mean apparent height associated with each group in ascending order (Fig. 5e inset), we nd the apparent monolayer thickness to be 2.060.18 nm even though the real thickness is B0.50.7 nm. We can use this information to convert the measured apparent AFM heights into number of layers, N. We then determine the mean number of layers of std-BP to be oN4 9.41.3 (see Supplementary Fig. 6 for the

impact of uncertainty in apparent monolayer height). Shown in Fig. 2f is a plot of N versus nanosheet area for std-BP, demonstrating a rough correlation between ake thickness and area: N /

A

p , as previously observed for exfoliated MoO3 and GaS nanosheets26,40. The nanosheets are reasonably thin: B70% of the observed nanosheets have Nr10 (Fig. 2g).

To gain further insights into the spectroscopic properties of our LPE FL-BP, we have selected a sample area with nanosheets of

varying sizes and thicknesses by AFM (Fig. 2h) and relocated the same area under a Raman microscope (Supplementary Fig. 7). A spatial Raman map (Ag1 intensity, excitation wavelength 633 nm)

of the region is shown in Fig. 2i. Single spectra (normalized to the Ag1 mode) extracted at the positions indicated in the Figure are displayed in Fig. 2j. These roughly correspond to the nanosheets circled in Fig. 2i. It has been shown that the intensity ratio of the Ag1/Ag2 phonon sensitively depends on sample degradation as a result of oxidation32. We therefore analysed the Ag1/Ag2 intensity ratio of 120 individual Raman spectra after baseline subtraction. The resultant intensity ratio histogram is plotted in Fig. 2k. None of the spectra showed Ag1/Ag2o0.6, conrming the basal planes to be unoxidized32. Analysis of data collected using 532 nm excitation gives very similar results (Supplementary Fig. 8). Furthermore, oxidation typically gives rise to a broad component under the B2g and Ag2 modes32 which is not observed in our samples, further conrming the structural integrity.

Size selection. A great advantage of liquid exfoliation is the ability to perform size-selection24,33,41. This is important, as some applications (for example, mechanical reinforcement)42 require large nanosheets, while others (for example, catalysis)43 benet from small nanosheets. To demonstrate that established size-selection techniques can be applied to liquid-exfoliated FL-BP, we have performed controlled centrifugation to obtain dispersions with varying size distributions (Methods and Supplementary Figs 9-10). Nanosheet length histograms are displayed in Fig. 3ac (and Supplementary Fig. 11) for three representative size distributions with mean L ranging from 190 to 620 nm. Note that, for much of the remainder of the study, in addition of the std-BP, we have studied dispersions containing small (S-BP, oL4 130 nm) and large (L-BP, oL4 2.3 mm) to gain

insights into size effects (see Supplementary Methods and Supplementary Figs 10 and 12).

We have also analysed the ultravioletvisible optical response of dispersions of nanosheets of varying size. For each sample, we have measured the optical extinction spectra as shown in Fig. 3d.

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24

Apparent height (nm)

Apparent height (nm)

60

16

Mean height

(nm)

8 nm 12 nm

20

12

8

4

40

16

0 4 8 0

2

6

Step group

20

10

12

24 nm

8

4

N1/2

0 0.0

Position (m)

0.2

0.4

0 0 20 40 60 80

1 0.01 0.1 1

Step number

Number of layers,

Area (m2)

30

0 350 400 450 500

0

7

<N> = 9.4

5

H

2

5

6

A B2g A2g

1 g

25

Ox.

No oxidation

Intensity (a.u.)

6

5 4 3

2 1

2

20

5

20

Count

4

15

3

Count

1

10

3

3

10

2

6 4

1

6

4

1

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0 10

5 15 20 25 30

0 0.4

0.8 1.2 1.6

Number of layers, N Raman shift (cm1) Intensity A1g/A2g

Figure 2 | Characterization of individual nanosheets. (ac) Representative atomic force microscopic (AFM) images (scale bars in a, b and c: are 500, 200 and 1000 nm). (d) Height prole of the nanosheet in the inset along the line showing clearly resolvable steps, each consisting of multiple monolayers (scale bar, 500 nm). (e) Heights of 470 steps of deposited FL-BP nanosheets in ascending order. The step height clustered in groups and is always found to be a multiple of B2 nm, which is the apparent height of one monolayer. The mean height for each group (the error is the sum of the mean step height error and the s.d. in step height within a given group) is plotted in ascending order in the inset with the slope giving a mean monolayer step height of 2.060.18 nm.

(f) Plot of number of layers per nanosheet (obtained by dividing the apparent height by the step height) as a function of ake area determined from AFM. The dashed line indicates N /

A

p behaviour. (g) Histogram of number of monolayers per nanosheet (sample size 126). The mean number of layers is

determined as 9.41.3 nm (where the error is due to the uncertainty in the step height analysis and the s.e. of the distribution). (h,i) Large area AFM image (h, scale bar, 1 mm) and Raman A1g intensity map (i, excitation wavelength 633 nm) of the same sample region. (j) Raman spectra (normalized to Ag2)

of the nanosheets indicated in h and i (the numbers labelling the spectra in j correspond to the nanosheets marked by numbers in h and i). (k) Histogram of the intensity ratio of the Ag1/Ag2 modes obtained from the analysis of 120 baseline-corrected spectra acquired over an area of 25 25 mm2

(sample size 120) The absence of spectra with an intensity ratio o0.6 strongly suggests that no basal plane oxidation has occurred32.

The absorbance and scattering spectra of the size-selected FL-BP dispersions are also shown in Fig. 3e,f. Very clear spectral changes as a function of size can be seen in all spectra. Such changes are primarily due to the fact that each sample contains a different distribution of nanosheet thicknesses, each of which contributes a signicantly different component to the measured spectrum10. In addition, the effect of edges may also contribute to the size-dependent changes in spectral shape33,44. The scattering spectra shown in Fig. 3f also vary strongly with nanosheet length. We have characterized this in more detail in Supplementary Fig. 13, using this data to generate a metric to extract nanosheet length from scattering spectra.

Photoluminescence. A number of publications have demonstrated PL from mechanically exfoliated BP nanosheets with one to ve layers13,45,46. To investigate the possibility of PL from liquid-exfoliated FL-BP, we prepared dispersions by performing the sonication under inert gas conditions to avoid any partial oxidation of the BP. However, we note that the sample produced using standard centrifugation was very similar to our std-BP exfoliated under ambient conditions (see Supplementary Figs 14 19 for more information). Prior to the PL measurement, additional centrifugation steps were performed (see Methods) to further remove large and thick nanosheets. Shown in Fig. 4a,b are PL emissionexcitation contour maps measured for an FL-BP dispersion in CHP. Very clear emission lines can be seen at B600 and B900 nm with weaker features appearing at B1,150, B1,260 and B1,325 nm. We associate these features with PL from 1L to 5L BP, respectively. Plotted in Fig. 4c is a PL line spectrum, excited at 510 nm with the measured extinction spectrum for comparison. We have tted this spectrum to ve Gaussian (due to inhomogeneous broadening) lines in Fig. 4d, representing

1-, 2-, 3- 4- and 5-layer nanosheets. In agreement with previous results13, we see strong layer-number-dependence of both position and width of the t components, reecting the changes in the band structure of exfoliated BP8,13,14. The PL intensity also depends strongly on nanosheet thickness. To separate out the effects of thickness-dependent internal PL quantum yield (PLQY) and the population of nanosheets of a given thickness, we performed AFM analysis on this sample (inset of Fig. 4g and Supplementary Fig. 16). From such images, we measured nanosheet layer number and area (estimated as length width)

for B300 nanosheets with the layer-number distribution shown in Fig. 4g. We then calculate the relative PLQY by dividing the integrated PL area for each layer number by the volume fraction of nanosheets of that thickness (approximately equivalent to PL/ absorbance for each layer number, Fig. 4h, see Supplementary Table 1). As observed previously13, we nd the PLQY to be extremely sensitive to nanosheet thickness varying by 4 orders of magnitude from one to ve layers. This effect13 is due to the thickness-dependent band structure of BP, as the internal PLQY (and width) is governed by the density of states distribution for electrons/holes, which depends on the number of band maxima and valleys.

Degradation. It has previously been shown that exfoliated BP degrades in the presence of water and oxygen, greatly limiting its application potential7,8,12,32. However, this may actually be less of a problem for liquid exfoliation as the solvation shell may protect the nanosheets from reacting with oxygen or water. To test this, we have monitored the temporal stability of liquid-dispersed BP by tracking the absorbance, A(l 465 nm), of std-BP, S-BP and

L-BP as a function of time (A aCl, Fig. 5a, Supplementary

Fig. 20 and Supplementary Note 1). As we expect the reaction

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NATURE COMMUNICATIONS | DOI: 10.1038/ncomms9563 ARTICLE

50

650

2L 3L

4L

5L

500

40

1.0

Size

600

30

450

Count

[afii9846] (a.u.)[afii9825] (a.u.)[afii9830] (a.u.)

550

20

0.5

Excitation wavelength (nm)

400

Excitation wavelength (nm)

Emission wavelength (nm) Emission wavelength (nm)

PL/Ext 510nm(a.u.)

PL width (eV)

500

10

<L> = 190 nm

0 0 500 1,000 1,500

350

450

0.0 400 600 800

1L

2L

3L

300 600 800 1,000 1,200

400 1,000 1,200 1,400

30

1.5

25

5

Size

Size

PL Ext

20

1.0

4

PL position (eV)PL/Ext 510nm(a.u.)

0.1

Count

0 0 500 1,000 1,500

1

15

3

10

0.5

2

1.0 1.2 1.4

1

<

L

5 > = 435 nm

0.0 400 600 800

0

1.0 1.5 2.0

40

Wavelength (nm) Energy (eV)

30

2

Count

1

20

1.6

1.2

10

BP-CHP

0.1

<

L

> = 620 nm

0 0 500 1,000 1,500

0.1

BP-CHP Lit Yang Lit Zhang Lit Liu

0.8

Lit Yang Lit Zhang Lit Liu

400 600 800

L (nm) Wavelength (nm)

1 2 3 4 5 6

1

1 2 3 4 5 6

Number of layers, N

Number of layers, N

Figure 3 | Size dependence of optical properties of exfoliated Black Phosphorous. (ac) TEM length histograms of size-selected FL-BP in CHP including representative TEM images as insets (scale bars in ac: 100, 200 and 200 nm). Sample sizes in ac are 131, 111 and 140, respectively.(d) Extinction (e) spectra normalized to 355 nm of FL-BP dispersions with different mean nanosheet lengths showing systematic changes as a function of size. Extinction spectra can be split into contributions from absorbance (a) and scattering (s). (e) Absorbance spectra of the same dispersions (normalized to 340 nm) and (f) scattering spectra. Scattering spectra were obtained by subtracting the absorbance spectra from the normalized extinction spectra.

0.01 400 800 1,200 1,600

35

10

30

1

25

0.1

Count

Relative PLQY

20

15

0.01

10

5

1E3

1E4

0 0 10 15 20 25 30

1 2 3 4 5 6

Number of layers, N

5

N (AFM)

products to be molecules with wide HOMO-LUMO gaps32, the measured absorbance will be dominated by the unreacted BP. This is supported by the lack of spectral changes with time. All samples were shaken before measurement to avoid sedimentation effects and measured at similar absorbances/concentrations. In this case, degradation of the FL-BP should result in a fall in measured absorbance over time, which we do indeed observe for BP exfoliated in a number of solvents (Fig. 5a). It is clear that exfoliation in CHP and NMP results in low reactivity, which can be reduced even further by preparing the dispersions in dried and deoxygenated CHP under inert conditions (sample CHP GB). In addition, we see an increase in the reactivity with decreasing nanosheet size (Fig. 5b, Supplementary Table 2 and Supplementary Fig. 21), suggesting that the chemical reaction starts from the edge as previously found for other 2D materials such as TiS2 (ref. 47).

Similar to Martel and co-workers32, we t these curves to exponential decays: A AUnRe ARee t/t, where ARe represents

the total amount of FL-BP which reacts over time and AUnRe

represents the component which never reacts. Typically we observe time constants of 10s100s of hours. This is much longer

than the decay times of B1 h observed by Martel and co-workers32 for mechanically cleaved BP in water, implying the degredation kinetics to be considerably different for solvent exfoliated BP. Such ts allow us to parameterize good solvents as those with long t and small ARe/(AUnRe ARe)

Figure 4 | Photoluminescence of dispersed black phosphorous nanosheets. (a,b) Photoluminescence emissionexcitation contour maps measured on a size-selected BP dispersion in CHP exfoliated under inert gas conditions (see Methods) measured with a 550 nm and 830 nm cut-off lter in emission, respectively. (c) Photoluminescence line spectrum (wavelength, lexc 510 nm) for this BP dispersion in CHP. Also shown is

the extinction spectrum for this dispersion as dashed line. (d) PL line spectrum, plotted versus photon energy, tted to ve Gaussian lines, representing the PL contributions from 1-, 2-, 3-, 4- and 5-layer nanosheets. (e,f) Position (e) and width (f) of t lines shown in d, plotted versus layer number. The dashed lines in e,f show power law decays with exponents of

0.48 and 1.4, respectively. Also shown in e,f are data for mechanically

cleaved BP nanosheets taken from Yang et al.46, Zhang et al.13 and Liuet al.45 (g) AFM image (inset, scale bar 500 nm) and statistical analysis of nanosheet thickness (expressed as layer number, sample size 281) for the

samples used to measure PL. (h) Relative PL quantum yield (by integrated PL area) as a function of layer number.

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(see Supplementary Table 3). In Fig. 5c, we plot t versus ARe/(AUnRe ARe), extracted from Fig. 5a,b). This plot clearly

shows FL-BP exfoliated in dry, deoxygenated CHP to be most stable, followed by NMP and CHP dispersions exfoliated in ambient conditions with ambient IPA dispersions being the least stable among the organic solvents. In addition, we have exfoliated BP in an aqueous surfactant solution with sodium cholate (NaC) as stabilizer (Supplementary Figs 23 and 28). The BP in the aqueous environment degrades most rapidly, although still with considerably longer time constants than micromechanically cleaved BP. This suggests the surfactant shell to offer partial protection, as with carbon nanotubes48.

These results cannot solely be explained by residual water content of these solvents (Supplementary Fig. 22) so we suggest that the solvation shell of CHP does indeed protect the nanosheets from water and/or oxygen. This is consistent with recent computational studies which show NMP, CHP and IPA to form tightly packed solvation shells adjacent to BP surfaces. The same study showed the molecular ordering within the solvation shell to vary from solvent to solvent, perhaps explaining the observed solvent-dependent stability49.

We note that to stably exfoliate and suspend BP, a solvent must full two criteria: it must have the correct surface energy50 to facilitate exfoliation and avoid reaggregation, and it must form a solvation shell which acts as a barrier to oxygen/water. Compared with most 2D materials, where only the rst criterion is important, this implies that relatively few solvents will be successful at both exfoliating and protecting BP. Indeed we have only had success using CHP, NMP and to a lesser extent IPA. While others have used DMF, ethanol and other solvents to suspend FL-BP30,31, the in-solvent stability has generally not been studied making it impossible to assess whether they meet the second criterion. It is clear that more work is needed to expand the list of solvents that can both suspend and protect BP.

However, such protection is clearly not perfect as we see some reactivity in all samples. To characterize this, we have added water (which also contains dissolved O2) to dispersions of S-BP, std-BP and L-BP in CHP. Addition of water/O2 signicantly increases the reactivity (Fig. 5d, see Supplementary Table 4 and Supplementary Fig. 23) with t falling dramatically with added water content (Fig. 5e). In addition, ARe/(AUnRe ARe) increases

linearly with water contents up to B10 vol% water, above which it saturates (Fig. 5e inset). Plotting t versus ARe/(AUnRe ARe) for

the water addition samples shows a steady deterioration of stability as water is added (Fig. 5f).

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Figure 5 | Stability of exfoliated black phosphorous nanosheets.(a,b) Relative absorbance at 465 nm, measured as a function of time, for (a) the standard FL-BP dispersion (std-BP) exfoliated in CHP, NMP and IPA, as well as BP exfoliated in CHP in a glovebox (CHP GB) and (b) std-BP in CHP compared with size-selected dispersions containing small (S-BP) and large nanosheets (L-BP). The dashed lines represent exponential decays: A AUnRe ARee t/t, where ARe represents the component of BP, which

reacts with water/O2, AUnRe represents the unreacted component and t represents the reaction timescale. (c) Reaction timescale, t, plotted versus the fraction of BP which reacts, ARe/AUnRe ARe, for a number of different

systems. (d) Fitted time-dependent absorbance (465 nm) data for std-BP in CHP and std-BP dispersions with 3 vol% and 12.5 vol% of water added. (e) Plot of t versus volume fraction of added water. The inset shows the fraction of unstable BP plotted versus water content. The dashed line demonstrates this fraction to scale initially linearly with water content.(f) Plot of t versus ARe/AUnRe ARe for BP exfoliated in CHP with and

without the addition of water. (g) Sequence of AFM images of the same sample region of an as-prepared sample, and after 1, 4 and 11 days of exposure to ambient conditions, respectively (scale bar, 600 nm). Errors in c,e,f are statistical errors associated with the ts. (h) Map showing both fractional length, L, and width, w, changes as measured by AFM immediately after exfoliation after day 4. Negative values of Dw/w0 and

DL/L0 indicate that the nanosheets are getting smaller over time. (i) Mean Raman spectra (633 nm excitation) summed over the same sample region of an as-prepared sample, after 4 days and 11 days, respectively. Spectra are normalized to the silicon peak at 521 cm 1. Inset: Raman A1g intensity map of the sample region (freshly prepared and after 11 days). (j) TEM images of the same ake deposited from a freshly prepared dispersion and after 16 days.

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Furthermore, we investigated the degradation of liquid-exfoliated FL-BP after deposition onto substrates using AFM, Raman and TEM. In these experiments, we monitored the same sample region (that is, the same nanosheets) with AFM and Raman immediately after deposition until 11 days later. As shown by the AFM images in Fig. 5g (and Supplementary Figs 2426), we observe broadening and blurring (that is, loss of ne structure) of the nanosheets. This is typically accompanied with shrinking of nanosheet length, L and width, w (Fig. 5h, Supplementary Fig. 27). In some cases, we observe an increased apparent height which is typically accompanied with an increased contrast in the phase images (Supplementary Figs 25,26), attributed to the adsorption of water. However, it is important to note, that even after 11 days of ageing under ambient conditions, the CHP-exfoliated FL-BP is clearly still intact, as the average Raman spectra (normalized to Si) of the same sample region show no spectral changes, especially with respect to Ag1/Ag2 (Fig. 5i and

Supplementary Fig. 28). These data imply that the overall structure of the FL-BP nanosheets does not change with time, consistent with degradation occurring at nanosheet edges rather than the basal plane. This is further conrmed by XPS (Supplementary Fig. 29) and TEM analysis where we tracked the same nanosheets over time (Fig. 3j and Supplementary Fig. 30). While the akes look disrupted and, as with AFM, blurred out at the edges, most of the nanosheets remain intact even after 16 days. It is important to emphasize that we clearly observe degradation when using a less favourable, low boiling point solvent such as IPA (Supplementary Fig. 31). Taken together this suggests that solvents such as CHP protect the nanosheets from degrading at the basal plane with the solvation shell remaining even after the bulk solvent has been removed. Importantly, it also shows that degradation starts at the edges. This implies that the structure of the solvation shell is different at the nanosheet edges in a way that makes water/O2 ingress at the edges much more likely than at the basal plane. This preferential edge degradation has important implications for future applications of BP. Edge degradation leaves the basal plane intact and should not greatly affect the properties of the BP nanosheets other than their size. This makes the environmental instability of BP less problematic for applications than might previously have been thought.

Reaction mechanism. If the basal plane is protected by solvent such that the dominant reaction pathway in liquid-exfoliated nanosheets occurs at the edges, then the reaction mechanism may differ to that proposed by Martel and co-workers32 Here we suggest an alternative pathway involving nanosheet edges, which is supported by density functional theory calculations (see also Supplementary Note 2). Since we experimentally observe a drop in pH with time after water addition and the formation of phosphorous and/or phosphoric acid (XPS), we propose the following acidbase disproportionation reaction involving nanosheet edges (Fig. 6a), which may become dominant when basal plane reactions are suppressed:

BP 3H2O ! BP2VAC PH3 H3PO3 1 This reaction was evaluated for both edge sites and basal plane

P atoms. In both cases, a defective structure BP2VAC, with two P

vacancies in the BP supercell is formed (2.5% vacancy concentration, Supplementary Figs 32 and 33). The reaction energy of process (1) is evaluated as

DE EBP 3E H2O

E BP2VAC

E PH3

E H3PO3

2 where [E(BP) 3E(H2O)] is the energy of the nanosheet and

three isolated water molecules at innite distance from the nanosheet and [E(BP2VAC) E(PH3) E(H3PO3)] is the energy

of the defective nanosheet innitely distant from the other isolated reaction products. This reaction is exothermic (DE 1.2 eV, see Fig. 6) when the process occurs at the edge

but endothermic (DE 0.26 eV) when occurring far from the

edge (see Supplementary Figs 34 and 35). In either case, degradation is a multistep process. Since we are interested in whether a reaction starts at the edge or the basal plane, we have analysed the early steps in the reaction (approach of H2O and

splitting of H2O to hydroxyl group and H atoms chemisorbed to neighbouring P). In both edge and basal plane cases, the water adsorption is slightly exothermic, while the splitting of the water is highly unfavourable on the basal plane (Supplementary Fig. 34). We have furthermore ruled out that the disproportionation reaction could proceed in the middle of BP nanosheet if activated by the formation of the rst hole by studying the degradation process starting from a defective structure BP2VAC

(Supplementary Fig. 36). The reaction is again endothermic by BDE 0.26 eV further supporting that the reaction is unlikely to

occur on the basal plane.

These calculations show that BP can react with water, even in the absence of oxygen, to remove P atoms from the nanosheet edge although this reaction may only be important when basal plane reactions32 are suppressed. To test the validity of the proposed reaction, we attempted to experimentally determine the stoichiometric ratio of reacted BP and water. After addition of water to a BP dispersion, the consumption of both water and BP can be tracked by ultravioletvisible spectroscopy (see Supplementary Figs 37 and 38, Supplementary Note 3). Knowing the extinction coefcients of both components, this data can be converted to the concentration of reacted BP and water,

BP + 3H2O BP2Vac+PH3+PO3H3

E= 1.2 eV

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Figure 6 | Reactivity of solvent-stabilized Black Phosphorous with water. (a) Edge selective degradation model for BP exposed to pure neutral water. Top and bottom panel represent reagent (BP edge three water molecules)

and reaction products (BP defective edge phosphine phosphorous

acid), respectively, with the reaction energy also given. Green, red and white balls represent P, O and H atoms, respectively. (b) Experimental data for amount of reacted phosphorus and water, respectively, as a function of time for std-BP GB in CHP after addition of 3 vol% of water. The data was obtained from tracking both water and BP concentrations by ultraviolet visible spectroscopy (see Supplementary Note 3). (c) Molar ratio of water/ BP as a function of time measured for a std-BP GB dispersion in CHP after addition of 3 and 10 vol% of water. The molar ratio is centred at 23, which is reasonably consistent with the proposed edge degradation reaction.

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respectively (Supplementary Fig. 37). The reacted concentration as a function of time for both BP and water is shown in Fig. 6b (using the CHP GB dispersion after addition of 3 vol% of water). It is clear that two to three water molecules are consumed for every P atom. As shown in Fig. 6c, a similar stoichiometry ratio is obtained when analysing a dispersion where initially 10 vol% of water was added. The thus experimentally determined H2O/P ratio of two to three agrees well with the proposed edge reaction.

However, it can unfortunately not distinguish between the proposed edge disproportionation reaction and the reported basal plane oxidation32.

Applications. For liquid-exfoliated FL-BP, the degradation timescale is slow enough to allow processing of nanosheets for applications testing in a number of areas. We believe that if applications potential is demonstrated, practical usage of FL-BP will be enabled by encapsulation7,24. In this work we have chosen three applications, which have not previously been described to demonstrate the broad potential of FL-BP.

Theoretical work has suggested FL-BP nanosheets as gas sensors18. We prepared thin lms of FL-BL nanosheets by vacuum ltration followed by transfer to interdigitated electrode arrays. Two-probe measurements showed relatively high conductivities of B1 S m 1, similar to lms of WTe2 nanosheets51. Shown in Fig. 7a (and Supplementary Fig. 39) is the resistance change of a thin lm of FL-BP nanosheets on exposure to ammonia (NH3) gas. We observe a resistance increase, consistent with NH3 donating electrons to the p-type FL-BP. By extrapolation of the signal-to-noise level (Fig. 7b), and assuming the minimum detectable signal to be three times the root mean square noise level, we estimate a detection threshold of 80 p.p.b. This shows FL-BP networks to be competitive with other nano-sensors5254 and a very promising material for gas detection.

The nonlinear optical response of the FL-BP dispersions was investigated by open-aperture Z-scan using a 340-fs pulsed bre laser55. In Fig. 7c,d, it is clearly seen that normalized transmission of the FL-BP dispersions increases with laser intensity at both 1,030 and 515 nm. Such broadband saturable absorption (SA) suggests that the FL-BP nanosheets could serve as an ultrafast nonlinear saturable absorber, an essential mode-locking element for ultrashort pulsed lasers56,57. Since graphene is a well-known broadband saturable absorber58, we carried out the same nonlinear measurement for graphene dispersions prepared by the similar liquid exfoliation. At the same level of linear transmission, the FL-BP dispersions exhibit much stronger SA response than the graphene dispersions at both wavelengths. The saturable intensity Is is obtained by tting the Z-scan data with the SA model dI/dz aI, where a a0/(1 I/ISat): a0 is the

linear absorption coefcient and I is the excitation intensity. As shown in Fig. 7e, ISat of FL-BP is much lower than that of

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Figure 7 | Applications of liquid-exfoliated FL-BP. (a,b) Sensing of NH3 gas using std-BP lms. (a) Sensor response plot shows percentile resistance change versus time of the FL-BP lms with a bias voltage of 1 V at room temperature, on consequent NH3 exposures at various concentrations from 1 to 10 p.p.m. (b) Plot of signal-to-noise ratio as a function of NH3 concentration from 1 to 3 p.p.m. The error bar represents the s.d. of ve devices and the linear line indicates the tted line. (ce) Saturable absorption of std-BP and graphene in CHP for fematosecond pulses excited at (c) 1,030 nm and (d) 515 nm. Linear transmission T0 is given in the legend. (e) Saturation intensity of FL-BP and graphene as a function of T0.

(f) Representative stressstrain curves for PVC and PVC: FL-BP (0.3 vol%). Inset: low strain regime. (gi) Youngs modulus, including the theoretical constant-strain rule-of-mixtures modulus prediction (blue using the Voigt ReussHill planar-averaged nanosheet modulus, blue-dashed using its Voigt and Reuss bounds) (g), tensile strength (h) and tensile toughness(i) plotted as a function of FL-BP volume fraction. (j) Calculated orientation dependence of the in-plane two-dimensional Youngs modulus (YBP, blue)

and Poissons ratio (n, red), of black phosphorus, together with the VoigtReussHill averaged in-plane modulus (oYBP4, black). Errors in b,e,gi are statistical errors associated with the ts.

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graphene at both 1,030 and 515 nm when the linear transmission is equal. The signicant ultrafast nonlinear property of FL-BP implies a huge potential in the development of nanophotonic devices, such as mode-lockers, Q-switchers, optical switches and so on59.

The impressive mechanical properties of 2D materials in general60 suggest the potential to use FL-BP as a reinforcing ller in composites. Shown in Fig. 7f are the representative stressstrain curves for a lm of polyvinylchloride (PVC) and a PVC:FL-BP (0.3 vol%, see methods and Supplementary Figs 40 and 41). It is clear that the mechanical properties improve considerably both in the high and low strain regimes. Shown in Fig. 7g-i are the composite modulus, strength and tensile toughness, plotted as a function of BP loading content. In each case, mechanical properties increase considerably for loading levels of only 0.3 vol%. The modulus, Y, increases from 500 MPa for PVC to 900 MPa for the 0.3 vol% composite. In addition, at 0.3-vol% loading, the measured strength of the composite doubles while its tensile toughness displays a sixfold increase. These are signicant increases and are competitive with those found using graphene as a ller in both PVC61 and polymers in general62.

We used rst-principles simulations (see Supplementary Note 4) to obtain the bulk BP Youngs modulus as a function of orientation in the plane of the crystal as shown in Fig. 7j, yielding a VoigtReussHill in-plane average modulus of

hYBPi 97 GPa. We also calculated the Poissons ratio nding it

to be highly anisotropic. Assuming the BP nanosheets lie in-plane63, we can use the calculated hYBPi, coupled with the rule of

mixtures62 to predict the composite modulus as a function of BP volume fraction, Vf: Ycomp hYBPiVf Ypoly(1 Vf). We nd this

prediction (blue line) to agree well with the experimental data even though we have not corrected for the nite aspect ratio of the nanosheets or their layered nature.

DiscussionIn conclusion, we have shown that the BP crystals can be efciently exfoliated in appropriate solvents to yield high-quality, few-layered nanosheets with controllable size, which can be used for fundamental measurements such as PL studies. Compared with mechanically cleaved nanosheets, liquid-exfoliated FL-BP is remarkably stable in CHP, probably due to protection by the solvation shell. However, addition of water results in degradation of the nanosheets. Both experimental and computational studies indicate the degradation to occur at the nanosheet edge with no basal plane damage and proceed by reaction of the FL-BP with water. Based on this, we propose an alternative BP degradation reaction, which can occur when basal plane reactions are supressed. Importantly, this means that the net result of degradation is reduction on nanosheet size rather than alteration of ake properties. We demonstrate that liquid-exfoliated FL-BP nanosheets have potential for use in applications as gas sensors, saturable absorbers and reinforcing llers for composites. We believe this work is important as it will facilitate the large-scale production of FL-BP. Such a goal is realizable in practise because, even when produced in ambient conditions, CHP-exfoliated FL-BP is stable for B200 h (cf, B1 h for mechanically cleaved

BP). We believe these advances will facilitate its development in a broad range of applications.

Methods

Sample preparation. BP crystals were purchased from Smart Elements (purity99.998%) with all other materials sourced from Sigma Aldrich (all used as received). BP was lightly ground with pestle and mortar and immersed in CHP (concentration 2 g l 1). The dispersion was sonicated for 5 h at 60% amplitude with a horn-probe sonic tip (VibraCell CVX, 750W) under cooling, yielding a stock

dispersion. Aliquots of the stock dispersion were centrifuged at 1,000 r.p.m. (106g) for time periods varying from 5 to 240 min in a Hettich Mikro 220R centrifuge equipped with a xed-angle rotor 1016. The supernatant was decanted and subjected to absorbance spectroscopy. The supernatant with centrifugation conditions 1,000 r.p.m. for 180 min was denoted std-BP. The std-BP was subsequently separated into small and large stable nanosheets. For this purpose, aliquots of std-BP dispersion were subjected to an additional centrifugation of 5 kr.p.m. (2,660g) for 120 min. The supernatant (containing small akes) was decanted and characterized as S-BP, while the sediment (containing large akes) was redispersed in fresh CHP and characterized as L-BP. Alternatively, the FL-BP was size-selected by controlled centrifugation with subsequently increasing rotation speeds. The sediment after2 kr.p.m. (426g, 2 h) was discarded, while the supernatant was subjected to further centrifugation at 3 kr.p.m. (958g, 2 h). The sediment was collected in fresh solvent, while the supernatant was subjected to further centrifugation at 4 kr.p.m. (1,702g, 2 h). Again, the sediment was collected and the supernatant was centrifuged at high r.p.m. This procedure was repeated for 5 kr.p.m. (2,660g, 2 h), 10 kr.p.m. (10,170g, 2 h) and 16 kr.p.m. (25,000g, 2 h) to yield samples with decreasing sizes in the respective sediments. For part of the degradation studies and PL measurement, BP was exfoliated under inert conditions by sonication in an argon-lled glovebox (O2o0.1 p.p.m.; H2Oo0.1 p.p.m.) in pump-freeze deoxygenated and dry CHP with a water content of 29 p.p.m. (0.5 g l 1 BP, 15 ml CHP, tapered tip Bandelin

Sonoplus 3100, 25% amplitude, 2 h, pulse 2 s on, 2 s off). The resultant dispersions were transferred into centrifugation vials, which were sealed and centrifuged for 3 h at 100g to obtain the std-BP GB sample. To increase the population of few-layered species for the PL measurement, this std-BP GB sample was centrifuged for 16 h at 25g. The sediment was discarded and the supernatant centrifuged again at 710g for 180 min. The PL spectra shown in the main manuscript were acquired from the supernatant of this centrifugation after dilution to an optical density of 0.4 cm 1.

All solvent transfer was carried out in the glovebox.

Characterization. Optical extinction and absorbance were measured on a Perkin Elmer 650 spectrometer in quartz cuvettes. To distinguish between contributions from scattering and absorbance to the extinction spectra, dispersions were measured in an integrating sphere using a home-built sample holder to place the cuvette in the centre of the sphere. The absorbance spectrum is obtained from the measurement inside the sphere. A second measurement on each dispersion was performed outside the sphere to obtain the extinction spectrum. This allows for the calculation of the scattering spectrum (extinction absorbance). The experiments to track both water and BP degradations were performed on a Perkin Elmer Lambda 1050 spectrometer in extinction.

Bright-eld TEM imaging was performed using a JEOL 2100, operated at200 kV, while HRTEM was conducted on a FEI Titan TEM (300 kV). High resolution TEM images (Fig. 5f) were taken using an FEI Titan 60300 Ultimate Microscope operated at 300 kV. The FL-BP was dropped on grids using a drop casting method and excess uid was absorbed by an underlying lter membrane. It was then baked in vacuum at 120 C for several hours. The samples were imaged on the day they were received which is termed Day 1. The same ake was then imaged on Day 3 and 16. No changes in nanosheet structure of morphology were observed between Day 1 and Day 3, but by Day 16 a combination of reaction products and water adsorption results in a liquid layer on the ake. This layer can be removed with the beam, and comparison of the shape of the akes between Day 1 and Day 16 shows that the overall shape and size of the ake has not changed. There is also lattice apparent in the ake on Day 16 when using a high magnication ( 300k).

Aberration-corrected STEM images were taken using a Nion Ultrasteme 100 (cold led emission gun at the SuperSTEM Laboratory in Daresbury, UK. The suspended FL-BP akes were dropped onto lacey carbon-coated copper TEM grids as before. The samples were then prebaked at 120 C in a vacuum overnight. The images were recorded using a 100 kV acceleration voltage with a HAADF eld detector and low-pass bright-eld imaging. Figure 1f shows a representative Butterworth-ltered HAADF STEM image and Figure 1g shows a low-by-pass bright-eld STEM image of the material. The lattice shows high uniformity with the presence of very few defects or dislocations. Depending on the zone axis used, the lattice can exhibit a dumbbell-type conguration.

AFM was carried out on a Veeco Nanoscope-IIIa (Digital Instruments) system equipped with an E-head (13 mm scanner) in tapping mode after depositing a drop of the dispersion transferred to IPA on a pre-heated (150 C) Si/SiO2 wafer with an oxide layer of 300 nm. Raman spectroscopy on individual akes was performed using a Horiba Jobin Yvon LabRAM HR800 with 633 nm excitation laser in air under ambient conditions. XPS was performed under ultra-high vacuum conditions (o5 10 10 mbar), using monochromated Al Ka X-rays (1,486.6 eV)

from an Omicron XM1000 MkII X-ray source and an Omicron EA125 energy analyser. An Omicron CN10 electron ood gun was used for charge compensation and the binding energy scale was referenced to the adventitious carbon 1s core level at 284.8 eV. Core-level regions were recorded at an analyser pass energy of 15 eV and with slit widths of 6 mm (entry) and 3 mm 10 mm (exit), resulting in an

instrumental resolution of 0.48 eV. After subtraction of a Shirley background, the core-level spectra were tted with GaussianLorentzian line shapes using the Marquardts algorithm.

PL in dispersion was acquired on a Horiba Scientic Fluorolog-3 system equipped with 450 W Xe halogen lamp and a nitrogen-cooled InGaS diode array

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detector (Symphony iHR 320). Spectra were obtained at 5 C using appropriate cutoff lters (see Supplementary Figs 1719).

Modelling. The QuantumEspresso package was used to evaluate the reaction energies. In this case, we selected an energy cutoff of 50 Ryd, a grid of 2 2 1

MonchorstPack k-points (see Supplementary Note 2) and ultrasoft pseudopotentials. Langreth and Lundqvist van der Waals-corrected exchange and correlation functional64 was used and atomic forces were relaxed until they wereo5 10 3 eV A 1.

We also calculated bulk and in-plane mechanical properties of BP (see Supplementary Note 4 and Supplementary Tables 57) to clarify the microscopic origins of an observed dramatic increase in the Youngs modulus of composites reinforced with BP nanosheets. In this case, the PBE functional, augmented with the empirical dispersion correction of the B97-D functional65 was used to compute the elastic constants of bulk phosphorus by nite-difference, the nanosheets in question being closer to the bulk than monolayer regime. The VoigtReussHill approach,66 was used to estimate both isotropic (found to be comparable to current literature67) and nanosheet in-plane-only estimates of the average nanosheet Youngs modulus, as well as upper (Voigt) and lower (Reuss) expected error bounds.

Applications. For nonlinear optical measurements, an open-aperture Z-scan system was used to study the ultrafast nonlinear optical properties of the FL-BP (std-BP) and graphene dispersions. This measures the total transmittance through a sample as a function of incident laser intensity, while the sample is sequentially moved through the focus of a lens (along the z-axis)55,57. All experiments were performed with 340-fs pulses from a mode-locked bre laser, which was operated at 1,030 nm and its second harmonic, 515 nm, with a pulse repetition rate of 1 kHz. All dispersion samples were tested in quartz cuvettes with 1-mm pathlength.

Gas sensing was conducted on the FL-BP (std-BP) prepared in CHP and then subsequently transferred into 2-proponal by centrifugation as conducted for the AFM measurements to facilitate ltration onto a nitrocellulose membrane. Following ltration, the lm was allowed to dry under vacuum conditions. The FLBP lm was cut into 10 2-mm rectangular pieces. These were then transferred

onto silicon dioxide wafer, while the nitrocellulose membrane was dissolved using the transfer method according to the study by Wu et al.68

For gas sensing, gold electrodes were sputtered on top of an adhesion layer of nickel (Ni/Au 30/70 nm) using a metal shadow mask, which has a 2-mm-wide

and 200-mm-long channel. All devices were loaded in a gas-sensing chamber and annealed at 100 C for 1 h to remove residues and adsorbates on the surface. The gas-sensing chamber was kept at room temperature at a pressure of 10 Torr, with a 100 s.c.c.m. ow of the NH3 and N2 mixtures. The resistance change of ve devices on interval gas exposure was simultaneously measured using a Keithley model 2612A SourceMeter and a Keithley 3706 System Switch at a constant bias voltage of 1 V. The initial resistance and root mean square noise were calculated from the rst 500 data points, B2 min before the rst gas injection. NH3 for 2 min and pure N2 for 5 min were periodically introduced to record sensor response and recover, respectively.

A previously prepared nanoller FL-BP dispersion in CHP, centrifuged between 1 kr.p.m. for 180 min and 3 kr.p.m. for 120 min and redispersed in fresh CHP solvent, was subsequently ltered onto a Polyester Membrane Filter (0.2 mm) of known mass. The membrane was dried in a vacuum oven at 100 C for 2 h and the mass of the membrane was remeasured to attain the mass of the ltered nanoller. The nanoller was redispersed by bath sonication (Branson 1510 Model 45 kHz) in a 65:35 tetrahydrofuran (THF), chloroform solvent mixture. PVC was dissolved in a solvent mixture (65:35 THF/chloroform). A range of FL-BP/PVC/THF/ Chloroform dispersions (from 0 to 0.0074 volume fraction) were made by adding the FL-BP/THF/Chloroform ller solution to the PVC/THF/Chloroform solution with varying increments of loading. These solutions were of constant mass (150 mg of FL-BP and polymer) and constant volume (5 ml of FL-BP, polymer and solvent). These samples were sonicated in the same bath as before for 1 h to homogenize after the blending of the solutions. The homogenized solution mixtures were then dropcast into 5 5 1 cm Teon trays and placed in a vacuum oven for 4 h at

40 C under no vacuum to form composite lms. The lms were then kept overnight (B17 h) at 50 C under full vacuum to ensure that the solvent was completely removed and to protect the ller material from decomposition before testing. For mechanical measurements, the lms were cut into 2.25-mm strips and then tested on a Zwick Roell tensile tester with a 100 N load cell at a strain rate of 10 mm min 1.

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Acknowledgements

The research leading to these results has received funding from the European Union Seventh Framework Programme under grant agreement n604391 Graphene Flagship. We have also received support from the Science Foundation Ireland (SFI) funded centre AMBER (SFI/12/RC/2278). In addition, JNC acknowledges the European Research Council (SEMANTICS) and SFI (11/PI/1087) for nancial support. C.B. acknowledges the German research foundation DFG (BA 4856/1-1). E.D., A.P. and V.N. acknowledge ERC 2DNanoCaps, SFI PIYRA and FP7 MoWSeS. G.S.D., K.L., N.McE and N.C.B. acknowledge SFI for PI_10/IN.1/I3030 while W.B acknowledges an SFI Investigator Award (12/IA/1306). STEM experiments were performed at SuperSTEM, the EPSRC UK national facility for aberration-corrected STEM. J.W. and S.Z. acknowledge NSFC (61178007, 61522510 and 61308034), STCSM (12ZR1451800) and the External Cooperation Program of BIC, CAS (No. 181231KYSB20130007). S.S. and C.S.C. have been supported by the European Research Council (Quest project). All reaction energy calculations were performed on the Parsons cluster (project HPC_12_0722), and all mechanical property calculations on the Lonsdale cluster, (project HPC_14_00800), maintained by the Trinity Centre for High Performance Computing. These clusters were funded through grants from Science Foundation Ireland. G.A. acknowledges the EU for a Marie Curie Fellowship (FP7/2013-IEF-627386).

Author contributions

D.H. and G.A. prepared liquid dispersions; D.H., C.B., A.H., N.McE. and G.S.D. performed the spectroscopy; C.B. performed AFM/Raman and PL; D.H. performed low-res TEM; E.D., A.P., Q.M.R. and V.N. performed hi-res TEM; C.Bo, Z.G. and P.L. prepared and tested composites; K.L. and G.S.D. tested gas sensing; C.S.C. and S.S. performed calculations on FL-BP stability; G.M., and D.D.OR performed simulation of composite mechanical reinforcement by FL-BP loading; S.Z., K.W., W.J.B. and J.W. performed nonlinear optical measurements; G.A., A.Hi and F.H. supported the exfoliation under inert conditions; J.N.C. and C.B. planned the experiments and wrote the paper.

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