PacSurf2024 Session NM2-TuE: 2D Materials Based on Carbon and Boron

The hydrogen boride (HB) sheet [1] is a newly synthesized two-dimensional material composed of hydrogen and boron atoms with their ratio of 1:1. The HB sheet has been attracted increasing attention, not only because of its high gravimetric hydrogen content as a hydrogen storage material and its ability to release hydrogen through light [2] and electrochemical [3] means, but also due to its catalytic activities [4]. Furthermore, the HB sheet has been shown to be chemically stable against water [5], unlike many boron hydrides that undergo hydrolysis reactions. The stability is a fascinating considering the practical application of the HB sheet. However, the molecular and mechanistic details of the chemical processes occurring on the HB sheet have yet to be fully addressed. In this talk, I will discuss the electronic and structural properties of the HB sheet [5], as well as the adsorption and reaction on the HB sheet based on our density functional theory calculations. Special emphasis is placed on the interaction with water molecules, and the mechanism of the stability of the HB sheet against water will be discussed [6,7].

References:

[1] H. Nishio, et al., J. Am. Chem. Soc. 139, 13761 (2017).

[2] R. Kawamura, et al., Nat. Commun. 10, 4880 (2019).

[3] S. Kawamura, et al., Small 20, 2310239 (2024).

[4] T. Goto, et al., Commun. Chem. 5, 118 (2022).

[5] L. T. Ta, Y. Morikawa, and I. Hamada, J. Phys. Condens. Matter 35, 435002 (2023).

[6] K. I. M. Rojas, et al., Commun. Mater. 2, 81 (2021).

[7] K. I. M. Rojas, Y. Morikawa, and I. Hamada (submitted).

Molecular monolayers on metal surfaces are two-dimensional (2D) systems of great interest for advanced applications in several fields, such as molecular electronics, photovoltaics and catalysis[1]. The physical and chemical properties of these systems critically depend on the choice of the molecular unit that constitutes the basic building block of the resulting 2D assembly[2].

Recently, N-heterocyclic carbenes (NHCs) have been proposed for the fabrication of robust molecular monolayers, based on their ability to form strong covalent bonds with transition metals[3]. Studies conducted on gold surfaces have shown that NHC-based monolayers can have greater thermal and chemical stability than alkanethiolate 2D layers[4], making them promising for applications in biosensing, surface patterning and corrosion protection[3].

The fabrication of NHC-based monolayers, however, requires a detailed understanding of how the NHC structure and the surface properties affect the molecular adsorption and self-assembly behavior on metals.

Therefore, we have conducted a nanoscale investigation of NHCs on single-crystal metal surfaces under ultra-high vacuum conditions (UHV). Our study includes NCHs with different backbones and N-substituents, deposited onto atomically clean surfaces using a vapor phase method (supplemental document, Fig.1). Low-temperature scanning tunneling microscopy (STM) was used to image in situ the resulting monolayers and obtain molecular-scale information on binding modes, adsorption geometries, and self-assembly patterns.

We observed different supramolecular assemblies (Fig. 2), in which the binding mode and lattice geometry vary depending on the N-substituents, the backbone structure, and the molecular density on the surface. Moreover, the metal surface properties play an important role, affecting the ability of NHCs to extract metal adatoms from the surface and form mobile NHC-metal complexes.

By discussing different factors which influence the NHC behavior on metals, our work contributes to a wider understanding of the coordination of these ligands to metal surfaces and the formation of 2D NHC-based layers.

[1] Goronzy D.P., et al. "Supramolecular assemblies on surfaces: nanopatterning, functionality, and reactivity." ACS Nano 12.8 (2018): 7445-7481.

[2] Barth J.V., et al. "Engineering atomic and molecular nanostructures at surfaces." Nature 437 (2005).

[3] Smith C.A., et al. "N-heterocyclic carbenes in materials chemistry." Chem Rev 119 (2019).