Recent progress in boron nanomaterials

Abstract

Various types of zero, one, and two-dimensional boron nanomaterials such as nanoclusters, nanowires, nanotubes, nanobelts, nanoribbons, nanosheets, and monolayer crystalline sheets named borophene have been experimentally synthesized and identified in the last 20 years. Owing to their low dimensionality, boron nanomaterials have different bonding configurations from those of three-dimensional bulk boron crystals composed of icosahedra or icosahedral fragments. The resulting intriguing physical and chemical properties of boron nanomaterials are fascinating from the viewpoint of material science. Moreover, the wide variety of boron nanomaterials themselves could be the building blocks for combining with other existing nanomaterials, molecules, atoms, and/or ions to design and create materials with new functionalities and properties. Here, the progress of the boron nanomaterials is reviewed and perspectives and future directions are described.

Keywords: 105 Low-Dimension (1D/2D) materials; 60 New topics / Others; Boron; boron nanotube; boron nanowires; borophene.

Graphical abstract

Figure 1.

Amorphous boron nanowires synthesized by CVD method. (a) SEM image of the boron nanowires. (b) TEM image of the boron nanowires. The inset shows the tip of one nanowire. (c) EELS spectrum recorded on the boron nanowires. Reproduced from Ref. [99] by permission of John Wiley & Sons Ltd.

Figure 2.

Amorphous boron nanowire arrays synthesized by the radio frequency magnetron sputtering method. (a) Low-magnification SEM image of the aligned boron nanowire arrays grown on Si substrates. (b) Cross-sectional SEM image. (c) High-magnification SEM image. Most of the boron nanowire tips have a platform-shaped morphology with a diameter of 60 ± 80 nm. (d) TEM image of a typical boron nanowire. Inset, SAED pattern taken from the nanowire showing some amorphous halo rings. (e) EELS spectra; a) boron nanowire; b) bulk, pure boron; c) bulk B₂O₃. The insets are magnified features of the O-K edges. Reproduced from Ref. [100] by permission of John Wiley & Sons Ltd.

Figure 3.

Size control of the diameter of amorphous boron nanowires in CVD method. TEM images of the boron nanowires synthesized at (a) 750 °C, (b) 800 °C, (c) 900 °C, and (d) 950 °C, respectively, while the thickness of the Au film catalyst was 5 nm. (e) A typical diameter histogram for the boron nanowires prepared at 800 °C. (f) Mean diameter of the boron nanowires as a function of temperature. Reproduced from Ref. [111] by permission of SpringerVerlag.

Figure 4.

Crystalline boron nanowires. Synthesized by CVD method: (a) SEM image of B nanowires, (b), (c) TEM images, and (d) electron-diffraction pattern of nanowire in (c). Reprinted with permission from Ref [112]. Copyright 2002 American Chemical Society. (e) High-resolution TEM image and its corresponding SAED. Reprinted from Ref. [114]. Copyright 2004 with permission from Elsevier. (f) TEM image of dispersive boron nanowires. The upper right inset is the corresponding SAED pattern of the arrowed boron nanowire. (g) High-resolution TEM image of the arrowed boron nanowire, revealing its single-crystal structure. Reprinted from Ref. [115]. Copyright 2004 with permission from Elsevier.

Figure 5.

Effect of quenching on crystallinity and alignment of boron nanowires. SEM images (scale bar 10 μm) and XRD patterns of boron nanowires film synthesized at 1100 °C for 30 minutes followed by (a)(c) slow cooling of ~5 °C/min and (b)(d) quench, respectively. Reprinted with permission from Ref. [117]. Copyright 2004 American Institute of Physics.

Figure 6.

Consecutive TEM images of an individual boron nanowire during different bending stages. Reprinted with permission from Ref. [122]. Copyright 2013 American Chemical Society.

Figure 7.

(a) SEM image of crystalline boron nanowire patterns. (b) Side view SEM image of uniform boron nanowire patterns. (c), (d) Side view SEM image of boron nanowires at the edge and at the inner portion of the pattern. (e), (f) SEM images of the tip and the end of boron nanowires. The white circles indicate the catalysts’ sites. (g), (h) Field emission images of patterned boron nanowires at a current density of 1.4 mA/cm² and 2.1 mA/cm². Reproduced from Ref. [129] by permission of John Wiley & Sons Ltd.

Figure 8.

(a) Raman spectrum and TEM image (inset) of boron nanotube. Owing to the extreme beam sensitivity and charging of the sample, the image is blurred and out of focus. Reprinted with permission from Ref. [130]. Copyright 2004 American Chemical Society. (b) Low-magnification SEM image of large-area boron nanostructures. (c) Magnified SEM image of the boron nanostructures. (d) SEM image of a boron nanotube tip. (e) High-resolution SEM image of the boron nanowires and boron nanotubes at the growth stage. Black and white arrows indicate a boron nanotube and a boron nanowire, respectively. Reproduced from Ref. [131] by permission of The Royal Society of Chemistry.

Figure 9.

(a) TEM image of a boron nanobelt tip. (b) TEM image of a 55 nm-wide boron nanobelt. The surface of this nanobelt is sheathed with amorphous phase, as indicated by white arrows. (c) TEM image of white rectangular region in (b). Some stacking faults, indicated by black arrows, can be seen along the growth direction. A detailed analysis of the corresponding electron diffraction pattern shown in the inset indicated that the crystal structure is tetragonal and the growth direction of this nanobelt is the [001] direction. Reprinted from Ref. [132]. Copyright 2003 with permission from Elsevier.

Figure 10.

Borophene nanoribbons on Ag(110) surface. (a) A derivative STM image shows boronphene nanoribbons grown on Ag(110). The image size is 100 × 100 nm². The nanoribbons run across the substrate steps without losing continuity. (b) Histogram of nanoribbon width. Gaussian fitting is shown as a black line. (c) High-resolution STM image of two boronphene nanoribbons. Image size: 50 × 30 nm². The bias voltages of STM images are (a) −4.5 V and (b) −1.9 V. Reprinted with permission from Ref. [140]. Copyright 2017 American Physical Society.

Figure 11.

Atomic structures of borophene nanoribbons on Ag(110) optimized by DFT. (a)–(d) Top and (e)–(h) side views of optimized P1–P4 borophene nanoribbons on Ag(110) surface, respectively. Color codes: B, small orange spheres; topmost Ag, large white spheres; lower Ag, large blue spheres. (i)–(l) Simulated STM images for P1–P4 based on the calculated electronic density (0–2 eV above the Fermi level). The red frames correspond to the observed unit cells in STM images. Reprinted with permission from Ref. [140]. Copyright 2017 American Physical Society.

Figure 12.

(a)–(c) SEM images for the ultrathin boron nanosheets at different magnifications. (d) Micro-Raman spectrum of the ultrathin boron nanosheets at room temperature. Reprinted from Ref. [81] which is an open access article published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 13.

(a) Simulated empty states STM image (V _sample=1.0 V), with overlaid atomic structure and unit cell of 0.500 nm by 0.289 nm. (b) Experimental STM image (V _sample = 0.1 V, I _t=1.0 nA) of borophene was dominantly observed when it was grown at a high temperature of 700 °C, with overlaid unit cell of 0.51 nm by 0.29 nm. Top (c) and side (d) views of the low-energy monolayer structure corresponding to the δ₆-type borophene sheet (unit cell indicated by the green box). From [46]. Reprinted with permission from AAAS.

Figure 14.

Atomic geometry of periodically undulated borophene on Ag(111). (a) Front (top) and side (bottom) views of a β₁₂-type borophene sheet on silver. (b) Front (top) and side (bottom) views of an undulated β₁₂-type borophene sheet on reconstructed Ag(111). The topmost Ag atoms are colored blue for clarity. (c), (d) Schematic continuum models for the (c) planar and (b) undulated β₁₂-type borophene sheets on a compliant substrate. Insets illustrate slices of charge redistribution between the B sheet (red) and Ag (blue), where dark and light colors represent charge depletion and accumulation (0.001 e/ų) regions, respectively [152]. Copyright 2016 American Chemical Society.

Figure 15.

(a) STM image of β₁₂-type borophene sheet on Ag(111) (S1 phase) grown on 570 K. Unit cell of 0.50 nm × 0.30 nm is marked by a black rectangle. Top (b) and side (c) views of β₁₂-type borophene sheet on Ag(111). (d) STM image of χ₃-type borophene sheet on Ag(111); this phase (S2 phase) appeared only after annealing at 650 K. Top (e) and side (f) views of χ₃-type borophene sheet on Ag(111). Reprinted with permission from Macmillan Publishers Ltd from Ref. [47]. Copyright 2016.

Figure 16.

STM images of two metastable 2D boron sheets on Ag(111). (a) STM topographic image of boron structures on Ag(111). The boron islands are labelled as ‘S1’ and ‘S3’ phases. (b) The derivative STM image of (a). (c) High-resolution STM image of the S3 phases. The S3 unit cell is marked by a black rectangle. (d) STM topographic image of boron structures on Ag(111). The boron islands are labelled as ‘S1’ and ‘S4’ phases. Most boron islands shown in the image are S1 phase. (e) STM image obtained on the area marked by the red dotted rectangle in (d). (f) High-resolution STM image of the S4 phase. The S4 unit cell is marked by a black rhombus [153]. Copyright 2017 IOP Publishing.

Figure 17.

Structure models of the S3 and S4 phases of boron sheets based on DFT calculations. (a) and (b) Top and side views of the S3 model, which correspond to the β₁₂ sheet of 2D boron on a Ag(1 1 1) surface. (c) Simulated STM topographic image of the β₁₂ sheet. (d) and (e) Top and side views of the S4 model, which correspond to the α sheet of 2D boron on Ag(1 1 1). The α sheet was buckled by 1.1 Å in the z axis when put on Ag(1 1 1). (f) Simulated STM topographic image of the α sheet. The orange and grey balls in (a), (b), (d), and (e) represent boron and silver atoms, respectively. The basic vectors of the super cell, including the Ag(1 1 1) substrate, are marked by blue arrows. Models of the β₁₂ and α sheets are superimposed on their simulated STM images [153]. Copyright 2017 IOP Publishing.

Figure 18.

(a) Schematic representation of the two-zone furnace used to obtain atomically thin γ-B₂₈ films by CVD. The temperatures of the source zone (T ₁) and substrate zone (T ₂) were set as 1100 °C and 1000 °C, respectively for the synthesis. (b) Top and side views of the monolayer. (c) Polyhedral structure of the basic unit cell of the monolayer is shown in the bc projection. Boron atoms forming dumbbells are shown as orange spheres. (d) Optical image of a monolayer on Cu foil. Inset: photograph of the monolayer on Cu foil. Reproduced from Ref. [150] by permission of John Wiley & Sons Ltd.