Silicon Growth at the Two-Dimensional Limit on Ag(111)

Abstract

Having fueled the microelectronics industry for over 50 years, silicon is arguably the most studied and influential semiconductor. With the recent emergence of two-dimensional (2D) materials (e.g., graphene, MoS2, phosphorene, etc.), it is natural to contemplate the behavior of Si in the 2D limit. Guided by atomic-scale studies utilizing ultrahigh vacuum (UHV), scanning tunneling microscopy (STM), and spectroscopy (STS), we have investigated the 2D limits of Si growth on Ag(111). In contrast to previous reports of a distinct sp(2)-bonded silicene allotrope, we observe the evolution of apparent surface alloys (ordered 2D silicon-Ag surface phases), which culminate in the precipitation of crystalline, sp(3)-bonded Si(111) nanosheets. These nanosheets are capped with a √3 honeycomb phase that is isostructural to a √3 honeycomb-chained-trimer (HCT) reconstruction of Ag on Si(111). Further investigations reveal evidence for silicon intermixing with the Ag(111) substrate followed by surface precipitation of crystalline, sp(3)-bonded silicon nanosheets. These conclusions are corroborated by ex situ atomic force microscopy (AFM), transmission electron microscopy (TEM), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). Even at the 2D limit, scanning tunneling spectroscopy shows that the sp(3)-bonded silicon nanosheets exhibit semiconducting electronic properties.

Scanning tunnelling spectroscopy and Raman spectroscopy of monolayer silicene on Ag(111)

“…The peak shows a tail toward lower frequencies, that is similar to the one found in microcrystalline silicon due to amorphous/small grain regions [33]. Such a result is in agreement with a previous Raman analysis of the same phase [11].…”

“…The observation of these features is in clear contrast with the typical spectra obtained for silicon oxide materials, that are characterized by a broad and diffuse band centred at 440 cm −1 and a sharp but not too intense peak at 492 cm −1 [38,39]. Such a peak, that was also observed in the spectra of silicene monolayer and layers obtained for higher Si coverages when the samples were measured in air [11], is clearly absent in Fig. 6, suggesting some resistance to air of monolayer silicene in agreement with a previous work [40].…”

“…This interplay leads to peculiar electronic and vibrational couplings that make the physical properties of the Si atoms different from the ones obtained for the Si adatoms on the Si (111) or (100) surfaces. Hints of this difference have been found when the surface is overgrown with Si and has raised unsettled controversies about the physical nature of this silicon multilayer [11][12][13][14]. These properties that deviate from the one of Si atoms in bulk crystal or thin films were also highlighted through chemical reactions such as hydrogenation or oxidation.…”

A theoretical study of gas adsorption on silicene nanoribbons and its application in a highly sensitive molecule sensor

“…21 Takeda and Shiraishi 14 reported silicene for the first time in 1994 and Guzmán-Verri and Voon 15 coined its name in 2007. Although free-standing silicene is not stable, it was experimentally fabricated on Ag, [21][22][23][24] Ir, 25 ZrB 2 , 26 and ZrC. 27 Using firstprinciples calculations, various substrates such as h-BN, 28 SiC, 29 GaS, 30 graphene 31 and ZnS 32 which have weak van der Waals (vdW) interactions with silicene have been also studied to improve its stability.…”

Determining the atomic structure of the ( 4×4 ) silicene layer on Ag(111) by combined grazing-incidence x-ray diffraction measurements and first-principles calculations

“…Such thick Si films were first addressed to as "multi-layer silicene"; however, further studies revealed their diamond bulklike structure and the surfactant behaviour of silver atoms [33][34][35]. The determination of the atomic structure of the reconstructions is of profound interest, being closely related to the electronic bands simulated by DFT calculation.…”