Carborane Nanomembranes

Abstract

We report on the fabrication of a boron-based two-dimensional (2D) material via electron irradiation-induced cross-linking of carborane self-assembled monolayers (SAMs) on crystalline silver substrates. The SAMs of 1,2-dicarba-closo-dodecarborane-9,12-dithiol (O9,12) were prepared on flat crystalline silver substrates and irradiated with low-energy electrons, resulting in a 2D nanomembrane. The mechanical stability and compact character of the carborane nanomembrane were improved by using 12-(1',12'-dicarba-closo-dodecarboran-1'-yl)-1,12-dicarba-closo-dodecarborane-1-thiol (1-HS-bis-pCB), a longer, rod-like SAM precursor with two para-carborane units linked linearly together. The self-assembly, cross-linking process, and transfer of the resulting membranes onto holey substrates were characterized with different complementary surface-sensitive techniques including X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and low-energy electron diffraction (LEED) as well as scanning tunneling and electron microscopies (STM, SEM) to provide insight on the structural changes within the cross-linked SAMs. The presented methodology has potential for the development of boron-based 2D materials for applications in electronic and optical devices.

Keywords: carboranes; electron irradiation induced chemical synthesis; molecular self-assembly; nanomembranes; two-dimensional materials.

Figure 1

Schematic representation of the cross-linking process based on DFT calculations (see Supporting Information (SI) for details). Initially ordered carborane SAMs are converted into short-range-ordered membranes using low-energy electron irradiation.

Figure 2

S 2p, C 1s, and B 1s XP spectra of a) O9,12 and b) 1-HS-bis-pCB SAMs stepwise cross-linked into a nanomembrane via electron irradiation with an energy of 50 eV. The spectra’s intensities have been multiplied by the indicated factor for better representation.

Figure 3

STM and LEED data of a) O9,12 and b) 1-HS-bis-pCB SAMs on Ag(111). Simulated LEED structures of the O9,12 SAM are highlighted in green (one molecule) and blue (two molecules) and of 1-HS-bis-pCB SAM is highlighted in blue (one molecule). (Conditions: a) 45 × 45 nm², 1.5 nA, 0.1 V; 18 × 18 nm², 0.5 nA, 0.1 V; 34 eV; b) 35 × 35 nm², 0.5 nA, 1.4 V; 10 × 10 nm², 0.5 nA, 1.4 V; 36 eV).

Figure 4

STM images of electron-irradiated O9,12 SAMs on Ag(111) with electron doses of a) 0.5, b) 1.5, c) 3.0, and d) 20.0 mC cm^–2 (Conditions: 27 × 27 nm², 1.0 nA, 0.1 V).

Figure 5

a) Optical microscopy image of a transferred O9,12-based membrane onto a silicon/silicon oxide wafer. b) Scanning electron micrograph of the carborane membrane placed on a TEM grid. c, d) Optical microscopy and SEM images of a membrane fabricated from 1-HS-bis-pCB as the precursor.