Resolving the Chemically Discrete Structure of Synthetic Borophene Polymorphs

Abstract

Atomically thin two-dimensional (2D) materials exhibit superlative properties dictated by their intralayer atomic structure, which is typically derived from a limited number of thermodynamically stable bulk layered crystals (e.g., graphene from graphite). The growth of entirely synthetic 2D crystals, those with no corresponding bulk allotrope, would circumvent this dependence upon bulk thermodynamics and substantially expand the phase space available for structure-property engineering of 2D materials. However, it remains unclear if synthetic 2D materials can exist as structurally and chemically distinct layers anchored by van der Waals (vdW) forces, as opposed to strongly bound adlayers. Here, we show that atomically thin sheets of boron (i.e., borophene) grown on the Ag(111) surface exhibit a vdW-like structure without a corresponding bulk allotrope. Using X-ray standing wave-excited X-ray photoelectron spectroscopy, the positions of boron in multiple chemical states are resolved with sub-angström spatial resolution, revealing that the borophene forms a single planar layer that is 2.4 Å above the unreconstructed Ag surface. Moreover, our results reveal that multiple borophene phases exhibit these characteristics, denoting a unique form of polymorphism consistent with recent predictions. This observation of synthetic borophene as chemically discrete from the growth substrate suggests that it is possible to engineer a much wider variety of 2D materials than those accessible through bulk layered crystal structures.

2D Boron Sheets: Structure, Growth, and Electronic and Thermal Transport Properties

“…Using particle swarm optimization algorithm as implemented in crystal structure analysis by particle swarm optimization (CALYPSO) code, Wu et al and Yu et al further searched the possible structures of 2D borophene. As shown in Figure , Wu et al successfully disclosed several possible borophene structures with cohesive energies close to the previous α borophene (V 1/9 ) and V 1/8 , V 2/15 ‐borophene . Because the energy difference is quite narrow, standard GGA‐PBE is hard to ensure the order of relative stability.…”

“…The structure of borophene depends on the substrate. On the surface of Ag, Au, Cu, and Al substrates, different phases of borophene were synthesized . The coupling strength between borophene and different substrates is different, implying that the phonon dispersion can be changed by the substrate.…”

“…The growth predictions of borophene on Ag/Au/Cu substrate have been all realized by recent experiments . Gozar, a research scientist in Yale University, has recently grown large‐size borophene with 100 µm 2 in size on the Cu(111) surface.…”

“…As shown in Figure 4, Wu et al [22] successfully disclosed several possible borophene structures with cohesive energies close to the previous α borophene (V 1/9 ) [51] and V 1/8 , V 2/15 -borophene. [17] Because the energy difference is quite narrow, standard GGA-PBE is hard to ensure the order of relative stability. A more accurate PBE0 hybrid functional was used to calculate their cohesive energies.…”

“…[13,14] This breakthrough indicates a new family member of 2D materials is born, and the nanodevices based on boron elements could be possible. Then, borophene sheets with different phases and borophene nanoribbons have been experimentally synthesized on the substrates of Ag (111), [13][14][15][16][17] Cu (111), [18] Al (111), [19] and Au (111). [20] However, the experimental researches of borophene are still lack, and the investigations including growth, characterization and applications are still awaiting, which should be the prerequisite for the application of borophene in the next-generation microelectronics.…”