From Chains to Arrays: Substrate-Mediated Self-Assembly of Diboron Molecules

Abstract

In this study, we explore the substrate-mediated control of self-assembly behavior in diboron molecules (C₁₂H₈B₂O₄, B₂Cat₂) using scanning tunneling microscopy (STM). The structural transformation of B₂Cat₂ molecules from one-dimensional (1D) molecular chains to two-dimensional (2D) molecular arrays was achieved by changing the substrate from Au(111) to bilayer graphene (BLG), highlighting the key role of substrate interactions in determining the assembly structure. Notably, the B-B bond in the molecular arrays on BLG is distinctly pronounced, reflecting a more refined molecular resolution with distinct electronic states than that on Au(111). Density functional theory (DFT) calculations confirm the weak interaction between B₂Cat₂ molecules and the BLG substrate, which facilitates the formation of 2D molecular arrays on BLG. This work demonstrates how controlling substrate properties enables the formation of 1D chains and 2D arrays, providing valuable insights for the design of next-generation molecular electronics and catalysis systems.

Keywords: 2D materials; epitaxial graphene; molecular arrays; organo-boron chemistry; scanning tunneling microscopy; substrate-mediated self-assembly.

Figure 1

(a) (left) Schematic of single B₂Cat₂ molecule. Boron atoms are depicted in pink, oxygen atoms in red, carbon atoms in green, and hydrogen atoms in white. (right) Schematic of B₂Cat₂ on Au(111). (b) STM image of B₂Cat₂ deposited on Au(111) with zoom-in structure presented in (c) (parameters: (b) V_b = −1 V, I_t = 5 pA; (c) V_b = 2.8 V, I_t = 100 pA). The length of B₂Cat₂ is 12.9 Å, and its height on Au(111) is 1.5 Å.

Figure 2

(a) Schematic of B₂Cat₂ on bilayer graphene (BLG). (b) STM image of B₂Cat₂ deposited on BLG (V_b = −1.5 V, I_t = 10 pA). The height of B₂Cat₂ on BLG is 2.1 Å. (c) Close-up STM image of B₂Cat₂ on BLG (V_b = −1 V, I_t = 50 pA). The white rectangle indicates the unit cell. (d) Atomic-resolution STM image of BLG taken from the yellow box in (c). (e) Schematic description of B₂Cat₂ self-assembly structure along zig-zag edge of graphene.

Figure 3

(a) STM image of B₂Cat₂ molecular array at 1.8 V with tunneling current 10 pA. (b) DFT simulation of B₂Cat₂ on BLG at LUMO. (c) STM image of B₂Cat₂ molecular array at −2.2 V with tunneling current 10 pA. (d) DFT simulation of B₂Cat₂ on BLG at HOMO.

Figure 4

(a) Top view (left), side view (middle), and line-profile (right) of differential charge density (DCD) for B₂Cat₂ on Au(111). B₂Cat₂ donates 2.36 electrons to Au(111), as calculated by integrating line-profile from A to B. (b) Top view (left), side view (middle), and line-profile (right) of DCD for B₂Cat₂ on BLG. The red (green) regions represent electron accumulation (depletion) with an isosurface value of 2 × 10⁻⁴ e/ų. B₂Cat₂ gains 0.18 electrons from BLG, as calculated by integrating line-profile from A to B.