Point defects at cleaved ${\mathrm{Sr}}_{n+1}{\mathrm{Ru}}_n{\mathrm{O}}_{3n+1}$(001) surfaces

The (001) surfaces of cleaved ${\mathrm{Sr}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$ and ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ samples were investigated using low-temperature scanning tunneling microscopy and density functional theory calculations. Intrinsic defects are not created during cleaving. This experimental observation is consistent with calculations, where the formation energy for a Sr and O vacancy, 4.19 eV and 3.81 eV, respectively, is significantly larger than that required to cleave the crystal, 1.11 eV/(1 × 1) unit cell. Surface oxygen vacancies can be created through electron bombardment, however, and their appearance is shown to vary strongly with the imaging conditions. Point defects observed on as-cleaved surfaces result from bulk impurities and adsorption from the residual gas.

Figure 1

(a) The crystal structure of ${\mathrm{Sr}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$. The DFT calculations predict a cleaving energy of 1.11 eV/(1 × 1) unit cell. (b) Top view of the ${\mathrm{Sr}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$(001) surface, which contains Sr (blue) and O (red, large) atoms only. Smaller red circles indicate second-layer O atoms. The inner (yellow) square marks the tetragonal unit cell; the black square marks the primitive unit cell [conventionally called $c$(2 × 2)], which is caused by the clockwise and counterclockwise rotation of the ${\mathrm{RuO}}_6$ octahedra. (c) Large-scale STM image (1000 × 900 ${\mathrm{nm}}^2$, $U$_sample = +1.5 V, $I_t$ = 0.15 nA, $T$ = 78 K).

Figure 2

STM images of cleaved ${\mathrm{Sr}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$(001) surfaces (39.5 × 15.5 ${\mathrm{nm}}^2$, $T$ = 78 K). (a) $U$_sample = +0.05V, $I_t$ = 0.15 nA. Following cleaving at 105 K, sample #6 is free from defects other than adsorbed CO (blue box) molecules identified in Ref. [28]. (b) $U$_sample = +0.2 V, $I_t$ = 0.15 nA. Following cleaving at room temperature, sample #13 is similarly defect-free, although some adsorbed water molecules are observed (brown box). After dosing ${\mathrm{H}}_2\mathrm{O}$ on a clean surface, the same features can be observed (see inset). (c) $U$_sample = +0.05 V, $I_t$ = 0.15 nA. Many point defects are observed upon cleaving a different ${\mathrm{Sr}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$ crystal (cleaving temperature 105 K). These defects occur either at the Sr or the apical oxygen/Ru lattice sites and are related to bulk impurities present in this crystal [see Table 1].

Figure 3

STM images of a ${\mathrm{Sr}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$(001) surface following bombardment with 1000 eV electrons at 105 K. (a) In empty-states images ($U$_sample = +0.1 V, $I_t$ = 0.15 nA, $T$ = 78 K), oxygen vacancies appear as small dark crosses. They can be easily distinguished from adsorbed CO molecules (see inset). (b) In filled-states images ($U$_sample = −0.4 V, $I_t$ = 0.15 nA, $T$ = 78 K), oxygen vacancies appear as small bright squares. Both features are centered at apical oxygen/subsurface Ru atoms.

Figure 4

STM images of doped ${\mathrm{Sr}}_{n+1}{\mathrm{Ru}}_n{\mathrm{O}}_{3n+1}$ samples. (a) STM image of (${\mathrm{Sr}}_{0.95}{\mathrm{Ca}}_{0.05}{)}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$ ($U$_sample = +0.3 V, $I_t$ = 0.15 nA, $T$ = 78 K). The Ca atoms replace Sr atoms. Ca atoms in the first and second layers are imaged as dark and gray squares, respectively. One surface and one subsurface Ca dopant is marked with a red and blue arrow, respectively. An STM simulation confirms the dark appearance of a Ca dopant in the first layer (see inset). (b) STM image of ${\mathrm{Sr}}_2{\mathrm{Ru}}_{0.97}{\mathrm{Mn}}_{0.03}{\mathrm{O}}_4$ ($U$_sample = −0.01 V, $I_t$ = 0.25 nA, $T$ = 6 K), where the Mn dopants replace the Ru atoms and appear as crosses with bright spots positioned at the Ru/apical oxygen lattice site. (c) STM topography of ${\mathrm{Sr}}_2{\mathrm{Ru}}_{0.97}{\mathrm{Co}}_{0.03}{\mathrm{O}}_4$ ($U$_sample = +0.05 V, $I_t$ = 0.15 nA, $T$ = 78 K), where the Co dopants replace the Ru atoms and appear as bright protrusions positioned at the subsurface Ru/apical oxygen lattice site.