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Production of phosphorene nanoribbons

Nature volume 568, pages 216–220 (2019)Cite this article

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Abstract

Phosphorene is a mono-elemental, two-dimensional (2D) substance with outstanding, highly directional properties and a bandgap that depends on the number of layers of the material1,2,3,4,5,6,7,8. Nanoribbons, meanwhile, combine the flexibility and unidirectional properties of one-dimensional nanomaterials, the high surface area of 2D nanomaterials and the electron-confinement and edge effects of both. The structures of nanoribbons can thus lead to exceptional control over electronic band structure, the emergence of novel phenomena and unique architectures for applications5,6,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24. Phosphorene’s intrinsically anisotropic structure has motivated numerous theoretical calculations of phosphorene nanoribbons (PNRs), predicting extraordinary properties5,6,12,13,14,15,16,17,18,19,20,21,22,23,24. So far, however, discrete PNRs have not been produced. Here we present a method for creating quantities of high-quality, individual PNRs by ionic scissoring of macroscopic black phosphorus crystals. This top–down process results in stable liquid dispersions of PNRs with typical widths of 4–50 nm, predominantly single-layer thickness, measured lengths of up to 75 μm and aspect ratios of up to 1,000. The nanoribbons are atomically flat single crystals, aligned exclusively in the zigzag crystallographic orientation. The ribbons have remarkably uniform widths along their entire lengths, and are extremely flexible. These properties—together with the ease of downstream manipulation via liquid-phase methods—should enable the search for predicted exotic states6,12,13,14,17,18,19,21, and an array of applications in which PNRs have been predicted to offer transformative advantages. These applications range from thermoelectric devices to high-capacity fast-charging batteries and integrated high-speed electronic circuits6,14,15,16,20,23,24.

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Fig. 1: TEM characterization of PNRs.
Fig. 2: AFM characterization of PNRs.
Fig. 3: PNR formation and Raman spectroscopy of deposited PNRs.

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Data availability

The datasets generated and/or analysed during this study are available from the corresponding author on reasonable request.

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Acknowledgements

This publication was funded, in part, by the Engineering and Physical Sciences Research Council (EPSRC). L.P. and O.D.P. thank the Royal Academy of Engineering for funding their time and for support, and also O. Ward for help with analysing the HS-AFM data. T.S.M. thanks the EPSRC for support via fellowship EP/P023851/1. P.L.C. thanks the EPSRC for support via fellowship EP/S001298/1. The HS-AFM data were collected using the University of Bristol’s Low Noise Labs. We thank M. Wood for early experiments, and A. Sella and A. Pakpour-Tabrizi for enlightening discussions.

Reviewer information

Nature thanks David J. Lewis and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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Authors and Affiliations

  1. Department of Physics & Astronomy, University College London, London, UK

    Mitchell C. Watts, Patrick L. Cullen, Szymon P. Bartuś, Neal T. Skipper & Christopher A. Howard

  2. Interface Analysis Centre, H. H. Wills Physics Laboratory, University of Bristol, Bristol, UK

    Loren Picco, Freddie S. Russell-Pavier & Oliver D. Payton

  3. Department of Physics, Virginia Commonwealth University, Richmond, VA, USA

    Loren Picco

  4. Electrochemical Innovation Laboratory, Department of Chemical Engineering, University College London, London, UK

    Patrick L. Cullen & Thomas S. Miller

  5. Institute of Materials, École Polytechnique Fédérale de Lausanne, Lausanne, Switzerland

    Vasiliki Tileli

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Contributions

M.C.W. discovered the PNR-production method and produced the samples. M.C.W. developed the production method, with input from P.L.C., N.T.S. and C.A.H. The HS-AFM measurements were performed and analysed by L.P., O.D.P., F.S.R-P. and M.C.W. L.P. wrote the automated HS-AFM ribbon-detection and step-height-measurement algorithms. F.S.R-P. wrote the automated ribbon-width-measurement algorithm and produced the HS-AFM movie. M.C.W. and V.T. performed and analysed TEM. T.S.M., M.C.W. and C.A.H. performed the electroplating. T.S.M. collected and analysed the XPS data. C.A.H. collected and analysed the Raman and photoluminescence data, with input from S.P.B. P.L.C measured the ultraviolet/visible absorption spectra. M.C.W. and C.A.H. planned the experiments and wrote the paper with input from all authors. C.A.H. directed the project.

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Correspondence to Christopher A. Howard.

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Extended data figures and tables

Extended Data Fig. 1 Schematic of PNR production.

Black phosphorus crystals were outgassed in a fused silica reaction tube, and lithium metal was added. The tube was cooled to −50 °C and high-purity ammonia (NH3) was condensed onto the black phosphorus and lithium, dissolving the alkali metal to form a Li:NH3 solution. The black phosphorus was left submerged in the Li:NH3 solution for about 24 hours, allowing intercalation, before the ammonia was slowly removed. Without air exposure, the intercalated black phosphorus crystal was transferred to a glovebox and placed into a vial, and NMP was added. This vial was then placed in an ultrasonic bath for 1 hour and centrifuged at low acceleration. This process resulted in solutions of phosphorene nanoribbons.

Extended Data Fig. 2 Gallery of PNRs.

TEM micrographs showing PNRs (darker, ribbon-like features) deposited on ‘holey carbon’ TEM grids (lighter, webbed structure). a, Heterogeneous PNRs of different widths. b, Micrographs of high-aspect-ratio (greater than 100) PNRs with widths less than 5 nm. Dashed boxes demarcate composite images; solid boxes correspond to the extracted zoom insets. To enhance visibility, selected PNRs have been outlined by dashed orange lines; otherwise, red arrows indicate the locations of the ribbon. c, Images of PNRs that demonstrate their flexibility. Composite TEM images are demarcated by dashed black outlines.

Extended Data Fig. 3 Electron microscopy, diffraction and spectroscopy of individual PNRs.

a–f, SAED patterns (scale bar, 10 nm−1) taken at the locations corresponding to the boxes marked in the composite TEM image in g (scale bar, 1 μm). These images show the zigzag crystallographic alignment along the length of the PNR, as indicated by the red arrows in a–g. h, Indexed FFT transform of the HRTEM micrograph shown in Fig. 1h. Scale bar, 10 nm−1. i–k, STEM (i; scale bar, 2.5 µm) and TEM (j; scale bar, 2 µm) images of the same PNR, for which EELS spectra (l, m) were recorded. k, Left, closer view (scale bar, 1 µm) of the PNR shown in i; right; spectrum image chosen at a suspended-in-vacuum part of the ribbon, with the red square indicating the region from which the EEL spectra shown in l and m were averaged. l, Core-loss EELS at the phosphorus L2,3 edge (130 eV), showing features in regions I, II and III that correspond to features found in pristine few-layer black phosphorus31. m, Low-loss EELS spectrum, showing the expected plasmonic signature peak at roughly 19 eV and another feature at around 3.3 eV, the latter being similar to that found for few-layer black phosphorus and attributed to surface plasmons31. The broad feature at around 5 eV that is marked with an asterisk is from delocalized signal coming from the amorphous carbon support.

Extended Data Fig. 4 Heights and roughnesses of one- to five-layer PNRs from statistical analysis of HS-AFM images.

See Methods. Normalized histograms (black data points; error bars are proportional to the square root of the number of counts in each bin) and Gaussian fits (solid blue line) of independent, automated, pixel-height measurements from the HOPG substrate (far left) and for PNRs of one to five layers. Extracted (see Methods) average heights (z) and roughnesses (σ) are shown at the top. The numbers of individual pixel-height measurements (taken over multiple locations, PNRs and substrates) are: HOPG substrate, 886,908 measurements; one-layer PNRs, 61,181; two-layer PNRs, 50,637; three-layer PNRs, 18,740; four-layer PNRs, 78,514; and five-layer PNRs, 28,816. Scaled representations (red) of the crystal structures of the PNRs are overlaid.

Extended Data Fig. 5 HS-AFM of PNRs.

a, Magnified image of the PNR shown in Fig. 2e. Scale bar, 5 µm. b, Magnified image of the bifurcation seen in a. Scale bar, 500 nm. c, d, Linecuts taken from b, before (c) and after (d) the bifurcation, demonstrating how the ribbon self-cleaves from its original height of roughly 3.5 nm (seven layers) to two branches with approximate heights of 1.5 nm (three layers) and 2 nm (four layers). e, f, HS-AFM images (scale bars, 200 nm and 500 nm, respectively) of areas over which the stability study in Extended Data Fig. 10a–f was undertaken. In e, the black box corresponds to a HOPG trench used to correct the stability data for tip effects (see Methods), and the blue box corresponds to the bilayer PNR in Extended Data Fig. 10d. In f, the blue box corresponds to the monolayer ribbon in Extended Data Fig. 10a.

Extended Data Fig. 6 Dimensions of spontaneously dissolved PNRs.

Length versus width scatterplots of deposited PNRs, extracted from TEM data (Methods). The plots show the PNRs that have spontaneously dissolved from Li-BP crystals into NMP solvent (red circles) as well as those formed via bath sonication (blue circles). The magnified plot on the right corresponds to the dashed rectangle in the left-hand image and reveals that the spontaneously dissolved PNRs are on average wider than those formed by sonication.

Extended Data Fig. 7 XPS and Raman spectroscopy of electrodeposited PNRs.

a, XPS maps of the platinum-coated positive (left) and negative (right) terminal electrodes following PNR electrodeposition. The maps were constructed from the integrated area under those phosphorus 2p peaks assigned to the P–P environment. Scale bars, 500 µm. b, Schematic representations of the platinum pattern (grey) on the positive (left) and negative (right) electrodes. c, An example phosphorus 2p spectrum from a, fitted with mixed Gaussian/Lorentzian peaks assigned to P–P bonding and P–O surface contamination. d, Patterned electrodes, showing the grid over which the Raman maps in Fig. 3e, f were taken. Scale bar, 20 μm. e, Example Raman spectra. The blue and red dots in d show the specific locations where the example Raman were taken.

Extended Data Fig. 8 Collective optical properties of PNRs.

a, Example Raman spectrum (black circles) taken from PNRs deposited on a graphite substrate in an argon atmosphere (Methods). The spectrum is fitted with a sum (red curve) of three Lorentzian functions (blue curves) corresponding to the Ag1, B2g and Ag2 peaks of black phosphorus40,41. b–d, Histograms showing the positions (top) and widths (bottom) of the Raman modes. e, Integrated intensity (I) ratios of the Ag1/Ag2 peak (top) and the B2g/Ag2 peak (bottom), extracted by fitting 351 independent spectra taken from the same sample of solution-deposited PNRs. From the fits to the distributions in b–d, we determine the mean values (and standard deviations) of the Raman peak positions and widths to be: Ag1 position 362.5 (0.2) cm−1, width 3.3 (0.2) cm−1; B2g position 439.3 (0.2) cm−1, width 5.4 (0.3) cm−1; Ag2 position 467.5 (0.2) cm−1, width 3.7 (0.2) cm−1. f, Photoluminescence (PL) spectrum from PNR aggregates deposited on graphite. High-energy peaks are Raman features from the PNRs or the graphite substrate (the latter marked by asterisks). Mechanically exfoliated single-layer phosphorene emits at around 1.3 eV (ref. 65). g, UV/Vis absorption spectra from a PNR solution (shown in the inset). Red circles mark the positions of weak features below 500 nm.

Extended Data Fig. 9 Characterization of lithium-intercalated black phosphorus.

a, b, Example Raman spectra for Li-BP, recorded with the incident laser polarization perpendicular (a) or parallel (b) to the vertical features that are visible in the bottom part of c. The top two spectra of a, b show the results obtained in argon; the bottom two spectra were obtained from the same region in the sample after 30 days in air. These spectra were fitted with a sum (red curve) of Lorentzian functions for black phosphorus (blue curves) and intercalation (green curves) Raman modes. See Methods. c, Top, optical micrograph of an Li-BP crystal (scale bar, 200 µm); bottom, a magnified image (scale bar, 18 µm). The bottom image also shows the area (red box) over which the Raman mapping data in f were taken from. d, Histograms showing the Li/P Ag2 peak position in argon (values taken from 396 individual spectra) and after 30 days in air (taken from 324 spectra). e, Histograms showing the Ag1/Ag2 peak intensity ratio for measurements performed with the incident laser polarization parallel (taken from 180 spectra) or perpendicular (taken from 396 spectra) to the vertical stripes in c. f, Map showing the position of the Ag2 peak for data taken over the red box in c. Scale bar, 5 µm. g, TEM image showing in-plane splitting of PNRs from a larger fragment. Scale bar, 500 nm. h, HS-AFM image (top; scale bar, 100 nm) and associated linescan (bottom) of the surface of an Li-BP crystal, showing a ribbon (line profile 2, black) liberated on the surface and the trench left behind (line profile 1, red).

Extended Data Fig. 10 Stability of PNRs.

a, HS-AFM images of the monolayer PNR shown in Extended Data Fig. 5f, taken immediately (top) and after six days of air exposure (bottom). Scale bars, 50 nm. b, Normalized histograms showing the width (left) and height (right) of the PNR in a, taken immediately (blue) and after six days in air (yellow). Each histogram has been fitted with a Gaussian function (red). c, Width (left) and height (right) of the PNR during six days in air, taken from fits to the histograms as shown in b. Data are means ± s.d. from the fit. For the histograms in b, the numbers of individual measurements are: 2,086 (day 0 width); 16,565 (day 6 width); 45,850 (day 0 height), and 281,980 (day 6 height). d, HS-AFM images of the bilayer PNR shown in in Extended Data Fig. 5e. Scale bars, 50 nm. e, f, As for b, c, but taken for the bilayer ribbon in d. For the histograms in e, numbers of individual measurements are: 8,343 (day 0 width); 4,169 (day 6 width); 65,614 (day 0 height), and 40,520 (day 6 height). g, Evolution of the Raman Ag1/Ag2 integrated intensity ratio, the Ag2/background intensity ratio, Ag2 position and Ag2 width (FWHM, full width at half-maximum), extracted by analysing maps of 90–100 independent spectra taken over the same location from a sample of PNRs drop-cast onto graphite (data points are means ± s.d. taken from fits; see Methods). h, Evolution of XPS spectra (black dots, data; red curves, fits to P peaks; blue curves, fits to P–O peaks) taken from a sample of PNRs drop-cast onto graphite as a function of increasing air exposure. i, Normalized absorbance at 465 nm (black dots) as a function of time for the same solution of PNRs in NMP over 50 days without exposure to air. A photograph of the solution is in the inset. The red curve is an exponential function fit to the data (see Methods).

Supplementary information

Video 1

A flyover visualisation of HS-AFM data taken of a phosphorene nanoribbon. Made by manually tracking a PNR over a 22 × 67 µm2 area using HS-AFM, this 3D flyover highlights the PNR’s compliance to the HOPG substrate and the relative heights changes of both the HOPG substrate and the PNR.

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Watts, M.C., Picco, L., Russell-Pavier, F.S. et al. Production of phosphorene nanoribbons. Nature 568, 216–220 (2019). https://doi.org/10.1038/s41586-019-1074-x

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