Quasi-freestanding epitaxial silicene on Ag(111) by oxygen intercalation

Abstract

Silicene is a monolayer allotrope of silicon atoms arranged in a honeycomb structure with massless Dirac fermion characteristics similar to graphene. It merits development of silicon-based multifunctional nanoelectronic and spintronic devices operated at room temperature because of strong spin-orbit coupling. Nevertheless, until now, silicene could only be epitaxially grown on conductive substrates. The strong silicene-substrate interaction may depress its superior electronic properties. We report a quasi-freestanding silicene layer that has been successfully obtained through oxidization of bilayer silicene on the Ag(111) surface. The oxygen atoms intercalate into the underlayer of silicene, resulting in isolation of the top layer of silicene from the substrate. In consequence, the top layer of silicene exhibits the signature of a 1 × 1 honeycomb lattice and hosts massless Dirac fermions because of much less interaction with the substrate. Furthermore, the oxidized silicon buffer layer is expected to serve as an ideal dielectric layer for electric gating in electronic devices. These findings are relevant for the future design and application of silicene-based nanoelectronic and spintronic devices.

Keywords: DFT; Silicene; angle-resolved photoemission spectroscopy; intercalation; quasi-free-standing; scanning tunneling microscopy.

Fig. 1. Topographic images of pristine and oxygen-intercalated epitaxial silicene grown on Ag(111).

(A) STM topographic image of pristine √3 × √3 silicene that was formed on a √13 × √13/4 × 4 buffer layer. Inset is a high-resolution image of √3 × √3 silicene, which demonstrates a honeycomb structure with a lattice constant of 0.64 nm (V_bias= −0.8 V, I = 0.2 nA). (B) Oxygen-intercalated √3 × √3 silicene after an oxygen dose of 600 L (V_bias = 0.6 V, I = 2 nA). (C) Line profile for the straight line in (B). (D and E) Enlarged STM images of intercalated region [red frame in (B)] and nonintercalated √3 × √3 silicene [black frame in (B)], respectively. The red rhombus and black rhombus stand for the unit cells of 1 × 1 silicene and √3 × √3 silicene, respectively (V_bias = 3 mV, I = 4 nA). (F) Oxygen-intercalated silicene layers after an oxygen dose of 1200 L, in which the buffer layer is fully oxidized, whereas the √3 × √3 silicene shows robust structural stability against oxygen intercalation (V_bias = −1.2 V, I = 0.1 nA).

Fig. 2. XPS and Raman spectra of pristine and oxygen-intercalated silicene.

(A) Si 2p core-level XPS spectra of pristine (top) and oxygen-intercalated (middle, lower dose; bottom, higher dose) silicene layers grown on Ag(111). The Si₁ and Si₂ peaks are attributed to Si–Si bonds in √3 × √3 silicene, whereas Si₃ and Si₄ are attributed to the √13 × √13/4 × 4 silicene buffer layer. a.u., arbitrary units. (B) Raman spectra of √13 × √13/4 × 4 silicene buffer layer (black), √3 × √3 silicene with 0.3 monolayer coverage on √13 × √13/4 × 4 buffer layer (pristine sample; red), and oxygen-intercalated sample (blue). The oxidized buffer layer features a broad Raman peak at 494 cm⁻¹ in the spectrum of the oxygen-intercalated sample.

Fig. 3. Simulations for atomic structures of oxygen adsorbed on both buffer layer and top layer silicene.

(A to C) Atomic structures of an O₂ molecule adsorbed on 4 × 4 silicene buffer layer (A), top layer silicene (B), and 4 × 4 buffer layer underneath √3 × √3 silicene (C). (D and E) Atomic structure of silicene/SiO_x/Ag(111) from ab initio molecular dynamics (AIMD) simulation: side view (D) and top view (E) of the top layer silicene only. (F) Simulated (top) and experimental (bottom) high-resolution STM images of silicene/SiO_x/Ag(111), showing the 1 × 1 silicene honeycomb lattice.

Fig. 4. ARPES results of pristine and oxygen-intercalated silicene.

(A to D) Energy versus k dispersion measured by ARPES for clean Ag(111) surface (A), as-grown √3 × √3 silicene formed on buffer layer (B), oxygen-intercalated silicene with an oxygen dose of 600 L (C), and intercalated silicene with an oxygen dose of 1200 L (D). The SSS and the sp band in (A) are attributed to the Ag(111) substrate. The DP in (B) to (D) is lifted up with increased oxygen dose from 0 to 1200 L, indicating less electron doping from the Ag(111) substrate due to oxygen intercalation. (E) ARPES energy cuts reveal a Dirac cone structure in pristine silicene. (F) Schematic view of shifting of Dirac cone due to oxygen intercalation in ARPES measurement.