Borophenes made easy

Abstract

To date, the scalable synthesis of elemental two-dimensional materials beyond graphene still remains elusive. Here, we introduce a versatile chemical vapor deposition (CVD) method to grow borophenes, as well as borophene heterostructures, by selectively using diborane originating from traceable byproducts of borazine. Specifically, metallic borophene polymorphs were successfully synthesized on Ir(111) and Cu(111) single-crystal substrates and conjointly with insulating hexagonal boron nitride (hBN) to form atomically precise lateral borophene-hBN interfaces or vertical van der Waals heterostructures. Thereby, borophene is protected from immediate oxidation by a single hBN overlayer. The ability to synthesize high-quality borophenes with large single-crystalline domains in the micrometer scale by a straight-forward CVD approach opens up opportunities for the study of their fundamental properties and for device incorporation.

Fig. 1.. CVD growth of borophene and borophene-hBN heterostructures on Ir(111).

(A) Schematic of diborane dosage on the preheated Ir(111) surface to obtain borophene. (B) STM image of a single-crystalline borophene domain grown by CVD on Ir(111) (V_bias = 0.1 V). (C) Detailed structure of borophene whose unit cell is depicted in red (V_bias = 2.0 V). (D) Schematic of sequential borazine and diborane dosage to obtain borophene-hBN lateral heterostructures. (E) High-resolution STM image of the lateral heterostructure formed by borophene and hBN (V_bias = 1.2 V). Red lines highlight χ₆ borophene’s wavy appearance, and green solid and dashed rhomboids highlight the unit cell and hexagonal moiré pattern of hBN, respectively. (F) XPS boron and nitrogen 1s core levels measured on borophene. (G) Schematic of the vertical heterostructure, with hBN covering borophene, grown by sequential dosing. (H) Atomically resolved image of the hBN lattice covering the borophene in the vertical heterostructure. (V_bias = 0.10 V; subtle 3D rendering was applied for better visualization). (I) Mass spectra of diborane and borazine gas used to grow borophene and hBN, respectively, measured at partial pressure of 3 × 10⁻⁷ mbar.

Fig. 2.. Borophene-hBN lateral interface on Ir(111).

(A) High-resolution STM image of the atomically sharp heterointerface formed by borophene and hBN (V_bias = − 0.5 V). Subtle 3D rendering was applied for better visualization. The interfacial registry is highlighted by the red and green lines. (B) dI/dV spectra taken on borophene and hBN rim and valley regions, together with (C) simultaneously acquired I(V) curves (stabilization conditions: V_bias = 1.5 V, I_t = 0.25 nA, lock-in modulation voltage V = 50 mV). The borophene spectra represent an average over the unit cell. (D) dI/dV intensity map constructed from the series of dI/dV spectra measured along the blue line marked on the STM image (V_bias = 2.0 V) showing a sharp electronic transition. Spectra stabilized at V_bias = 1.5 V and I_t = 0.4 nA, lock-in modulation voltage V = 50 mV. STM images measured at (E) V_bias = 2.7 and (F) V_bias = − 0.8 V, showing a bias-dependent contrast inversion between borophene and hBN.

Fig. 3.. hBN on borophene: vertical heterostructure on Ir(111).

(A) Atomically resolved STM image of an hBN domain, featuring its honeycomb structure, on χ₆ borophene, showing its stripy appearance on Ir(111) (hBN unit cell in green, V_bias = 1.0 V). Subtle 3D rendering has been applied for better visualization. Inset: LEED pattern acquired at 79 eV (simulated diffraction pattern of hBN in green and borophene in red). (B) Boron and (C) nitrogen 1s XP spectra. The fitted components of hBN and borophene spectral contributions are displayed in green and red, respectively. (D) B 1s peak measured at different photoelectron emission angles θ = 0°, 45°, 55°, 60°, 65°, and 70° (lines from dark to light blue). (E) Angular dependence of the relative intensity of borophene B 1s components and Beer-Lambert law fit in black describing the attenuation effect by the hBN overlayer. (F) Series of B 1s spectra measured on hBN-covered borophene after incremental O₂ exposure intervals reveals no sign of oxidation in contrast to an uncovered borophene sample that shows emergence of oxidized boron (G). Intensity maps at the background are constructed with the presented spectra.

Fig. 4.. CVD growth of borophene on Cu(111).

(A) STM image of a single-crystalline χ₃-like borophene domain (V_bias = 1.3 V). Top right inset shows a fast Fourier transform of the image. The scan area is highlighted in the bottom left inset (tunneling current channel, V_bias = 1.3 V). (B and C) High-resolution STM images of the same borophene domain recorded at V_bias = 0.5 and −3.0 V, respectively. Black vectors indicate the unit cell.