Surface and Defect Structure of Oxide Nanowires on ${\mathrm{SrTiO}}_3$

Processing the ${\mathrm{SrTiO}}_3(001)$ surface results in the self-assembly of reduced titanate nanowires whose widths are approximately 1 nm. We have imaged these nanowires and their defects at elevated temperatures by atomic resolution scanning tunneling microscopy. The nanowire structure is modeled with density functional theory, and defects observed in the center of the nanowire are determined to be ${\mathrm{Ti}}_4{\mathrm{O}}_3$ vacancy clusters. The activation energy for ${\mathrm{Ti}}_4{\mathrm{O}}_3$ vacancy cluster diffusion is explicitly measured as $4.98\pm 0.17\mathrm{eV}$ with an exponential prefactor of $\mu =6.57\times {10}^{29}{\mathrm{s}}^{-1}$.

Figure 1

An STM image of a triline nanostructured surface is shown in (a) with a field of view of $40\times 40{\mathrm{nm}}^2$. The core level XPS Ti $2p$ spectrum shown in (b) is from a representative triline nanostructured surface showing the presence of a ${\mathrm{Ti}}^{2+}$ peak that arises exclusively from triline nanostructures and is attributed to the triline backbone. The ${\mathrm{Ti}}^{4+}$, ${\mathrm{Ti}}^{3+}$, and ${\mathrm{Ti}}^{2+}$ $2p_{1/2}$ peaks are shown at 465.0, 463.1, and 461.5 eV, respectively. The ${\mathrm{Ti}}^{4+}$, ${\mathrm{Ti}}^{3+}$, and ${\mathrm{Ti}}^{2+}$ $2p_{3/2}$ Ti peaks are shown at 459.2, 457.3, and 455.6 eV, respectively. The error associated with each XPS peak binding energy is $\pm 0.1\mathrm{eV}$. In (c), the ball-and-stick structures are shown of the top three layers of a single triline nanostructure, modeled by DFT, are presented in plan view. The structures are shown with the side lobes in the “zigzag” or out-of-phase configuration. “Layer 3” sits atop a bulk SrO layer. An ($8\times 2$) unit cell is indicated. Simulated ($6.2\times 5.5{\mathrm{nm}}^2$) and experimental ($6.8\times 5.5{\mathrm{nm}}^2$) STM images of a triline nanostructure are shown in (d). A ($1\times 1$) mesh overlays the experimental STM image. The ball-and-stick structure of a single triline is shown in (e), presented in side view. The experimental STM imaging conditions are $V_s=+1.86\mathrm{V}$ and $I_t=0.24\mathrm{nA}$ at $T=791\mathrm{K}$.

Figure 2

(a) Two ${\mathrm{Ti}}_4{\mathrm{O}}_3$ vacancy cluster defects are shown in experimental STM and (b) simulated STM images ($3.12\times 3.12{\mathrm{nm}}^2$ each). The ball-and-stick structure of the topmost layer of the defect is shown in (c). In (d), the four Ti atoms from the top layer are found to reside in the second layer. The STM imaging conditions are $V_s=+1.86\mathrm{V}$ and $I_t=0.24\mathrm{nA}$ at $T=791\mathrm{K}$.

Figure 3

(a)–(d) Sequential STM images ($\Delta t=90\sec $) that are close-ups of the inset area of Fig. 1a, and are also shown in the STM movie [39]. The field of view is $9.5\times 9.5{\mathrm{nm}}^2$. The arrows in (a)–(d) locate the starting positions of the indicated vacancies at $t=0\sec$. The images were obtained at a constant temperature, $T=791\mathrm{K}$, and imaging conditions as $V_s=+1.86\mathrm{V}$ and $I_t=0.24\mathrm{nA}$. (e) Arrhenius diffusion plot of the hopping rates $h$ at seven different temperatures ranging from 753–829 K. Each point corresponds to 50–75 observed hopping events.