Nanoscale Patterns on Polar Oxide Surfaces

Abstract

Polar ionic surfaces with bulk termination are inherently unstable because of their diverging electrostatic surface energy. Nevertheless, they are frequently observed in nature, mainly because of charge neutralization by adsorbates, but occur also under atomically clean conditions. Several mechanisms have been invoked to explain the stability of atomically clean polar surfaces, but the frequently observed periodic nanoscale pattern formation has not yet been explained. Here we propose that long-range interactions between alternating electropositive and electronegative regions of different surface terminations minimize the electrostatic energy of the surface and thus stabilize the nanoscale pattern. This is illustrated using the example of polar Fe oxide surfaces by combining scanning tunneling microscopy and spectroscopy results with results from density functional theory-based calculations and dipole–dipole interaction models

Incipient adsorption of water and hydroxyl on hematite (0001) surface

“…At the OH coverage of 1/3 ML, Δγ of Fe1 termination is −99 meV/Å 2 , and is nearly the same independent of whether calculated using PBE+U or PBE+U+D3. Added to the clean Fe1-termination value (67 meV/Å 2 [31,40]) it gives for the surface energy −32 meV/Å 2 , which is in very good agreement with the PBE+U result calculated in [18]. On the O3 termination the variation of Δγ upon OH adsorption is weaker compared to that on the Fe1-terminated surface, which reflects a weaker binding of OH to this surface.…”

Heterostructures of ε-Fe2O3 and α-Fe2O3: insights from density functional theory

“…10 and 11) and also for bulk a-Fe 2 O 3 . 20 The U value chosen for the Featoms are same for the surface and bulk atoms as opposed to the work of Lewandowski et al 49 because the Fe-atoms at the interface has the same environment above and below it. Structural optimizations of each slab of Fe 2 O 3 and of their heterostructure and calculation of the density of states of the heterostructure were carried out using a Monkhorst-Pack (M-P) 50 k-point mesh of 5 Â 3 Â 1 points in the Brillouin zone.…”

Structural Evolution of α-Fe2O3(0001) Surfaces Under Reduction Conditions Monitored by Infrared Spectroscopy

“…The structural transformation to a α-Fe 2 O 3 (111)/Fe 1−x O(111)-biphase occurs only when increasing the temperature to about 1,100 K (Condon et al, 1998). In addition, more recent DFT calculations revealed that the α-Fe 2 O 3 (0001) surface can expose O-, Fe-, and ferryl-terminated regions, whose size varies depending on the oxygen chemical potential (Lewandowski et al, 2016). The long-range order was explained in terms of the dipole-dipole interaction between domains with different work functions.…”

“…The STM results revealed the formation of long-range order of α-Fe 2 O 3 (0001) and Fe 1−x O(111) domains, which are characterized by floret-like spots in the low-energy electron diffraction (LEED) pattern described as (√3 × √3)R30° reconstruction (Kurtz and Henrich, 1983; Lad and Henrich, 1988; Condon et al, 1995). In addition, the surface chemistry and physics of α-Fe 2 O 3 (0001) has been studied by a number of theoretical groups [for details see the review by Parkinson, 2016] (Wang et al, 1998; Bergermayer et al, 2004; Trainor et al, 2004; Nguyen et al, 2014; Lewandowski et al, 2016; Ovcharenko et al, 2016).…”