The rise and fall of an oxide: insights into the phase diagram of bismuth oxide on Au(111)

Abstract

Bismuth oxide (Bi$_2$O$_3$) is a polymorphic material of considerable technological interest, with applications spanning from heterogeneous catalysis to next-generation nanoelectronics. Despite its relevance, systematic investigations of Bi$_2$O$_3$ thin films remain scarce. Here, we report a comprehensive, multi-technique study of bismuth oxide grown on Au(111). By combining synchrotron-based x-ray photoelectron spectroscopy and diffraction with low-energy electron diffraction and scanning tunneling microscopy, we elucidate the structural evolution of the surface during controlled oxidation and subsequent annealing. We find that Bi deposition induces well-defined surface reconstructions, whereas oxidation triggers the formation of a complex sequence of Bi$_2$O$_3$ domains. High-resolution spectroscopic and diffraction data enable us to propose a structural model consistent with the $(201)$ surface of $β$-Bi$_2$O$_3$. In addition, work function measurements reveal substantial electronic modifications at the interface. These results provide benchmark structural and electronic insights into the Bi oxide/Au(111) system and establish a framework for integrating Bi$_2$O$_3$ in devices in combination to two-dimensional semiconductors exploiting its low contact resistance.

Figures (9)

• Figure 1: Characterization of the Au(111) surface. (a) Au 4$f_{7/2}$ core level spectrum acquired at photon energies $h\nu = 400$ and $200$ eV. Acquired data are displayed as points while fitted bulk and surface components in solid areas. The atomic model of the surface is presented. (b)-(c) Stereographic projection of the modulation function $\chi$ for the bulk and surface components acquired at $h\nu = 400$ eV and $h\nu = 200$ eV (corresponding to a kinetic energy of $316$ and $116$ eV, respectively), Acquired data (colored section) is compared with XPD simulation (greyscale) and the $[11\bar{2}]$ direction is indicated as reference. (d) STM image displaying the herringbone reconstruction of the Au(111) surface ($I = 0.40$ nA, $V = 1.60$ V), with low coverage of Bi visible as faint bright protrusions on the herringbone elbows. (e) LEED pattern obtained at $E_k = 137$ eV, the herringbone reconstruction generates the additional spots surrounding the first order Au(111) diffraction peaks.
• Figure 2: Phases for Bi deposition on Au(111). (a)-(b) $\left(\sqrt{37}\times\sqrt{37}\right)R25.3^\circ$ phase. (a) STM image displaying honeycomb structure ($I = 0.22$ nA, $V = 0.23$ V); (b) LEED image acquired at $E_k = 38$ eV with highlighted unit cells (red/blue) associated to different rotational domains. (c)-(d) $(P \times \sqrt{3})$ phase, with $P = 11$. (c) STM image showing striped pattern associated to the moiré supercells ($I = 0.12$ nA, $V = 1.95$ V); (d) LEED image acquired at $E_k = 38$ eV with highlighted unit cells (red/green/blue) associated to different rotational domains. (e) High resolution Bi 4$f_{7/2}$ core level spectrum obtained after bismuth deposition in the multilayer regime, acquired at $h\nu = 325$ eV. (f) Stereographic projections of the modulation function $\chi$ of the Bi 4$f_{7/2}$ peak at different kinetic energies; The acquired data (colored section) are compared with XPD simulations (greyscale).
• Figure 3: STM images of the thin layer of bismuth (3-5 ML) on Au(111) after RT oxidation. (a) A large scale view of the surface, displaying ordered oxide nanoscale domains ($I = 0.16$ nA, $V = 0.98$ V). (b)-(c) The domains on the surface are highlighted with different colors based on the electronic contrast produced ($I = 0.22$ nA, $V = 1.32$ V).
• Figure 4: (a) Bi 4$f$ core level spectra after increasing exposure of Bi/Au(111) to O$_2$ at RT. New components emerge at higher BE that are associated with oxidized Bi$^{+3}$ species. (b) High resolution Bi 4$f_{7/2}$ core level spectra obtained after oxidation at different temperatures, with reference before oxidation. Oxidation at 423 K leads to the almost complete disappearance of the component related to metallic Bi.
• Figure 5: Atomic model of the $\beta$-Bi$_2$O$_3(201)$ termination employed for XPD simulation; Purple - Bi, Red - O.. Colored arrows represent inequivalent directions in the top view of the atomic model here as well as in the diffraction pattern shown in Fig.\ref{['fig:xpd']}(a).