Maddox Willie B, Acharya Danda P, Leong G Jeremy, Sutter Peter, Ciobanu Cristian V
Department of Mechanical Engineering, Colorado School of Mines, 1500 Illinois Street, Golden, Colorado 80401, United States.
Global Foundries, 400 Stone Break Extension, Malta, New York 12020, United States.
ACS Omega. 2018 Jun 18;3(6):6540-6545. doi: 10.1021/acsomega.8b01056. eCollection 2018 Jun 30.
The rutile TiO(110) surface has long-served as a well-characterized, prototypical transition-metal oxide surface used in heterogeneous catalysis and photocatalytic water splitting. Naturally occurring defects on this surface, called bridging-oxygen (BO) vacancies, are important as they determine the overall reactivity of the surface. Herein, we report a bias-dependent, scanning tunneling microscopy (STM) signature of the BO vacancies on TiO(110): for sample bias voltages past a threshold of +3 V, the bright vacancies are flanked on either side (along the oxygen row) by two dark spots approximately shaped like half-moons. The BO vacancies have a bright aspect below the threshold bias also but are not surrounded by half-moon dark depressions. Using generalized gradient approximation calculations with Hubbard correction (GGA + ) for projected density of states (DOS) and simulated STM images, we find that the bias-dependent STM signature originates from (i) local DOS maxima of all BOs (lighter background that occurs above the threshold bias) and (ii) the increased separation between the first and second BO atoms neighboring the vacancy which leads to an apparent dip between these neighboring oxygens. These results offer a new striking example of the STM signature that appears without switching the polarity of the bias. Similar approaches can be employed for seeking distinguishing features on the surfaces of other large band gap semiconductors and insulators.
金红石型TiO(110)表面长期以来一直是一种特征明确的典型过渡金属氧化物表面，用于多相催化和光催化水分解。该表面上自然存在的缺陷，称为桥氧（BO）空位，很重要，因为它们决定了表面的整体反应活性。在此，我们报告了TiO(110)上BO空位的一种与偏压有关的扫描隧道显微镜（STM）特征：对于超过+3 V阈值的样品偏压，明亮的空位在两侧（沿氧行）被两个近似半月形的暗斑包围。BO空位在阈值偏压以下也有明亮的外观，但没有被半月形暗凹陷包围。使用带有哈伯德修正（GGA + ）的广义梯度近似计算来计算投影态密度（DOS）并模拟STM图像，我们发现与偏压有关的STM特征源自（i）所有BO的局部DOS最大值（在阈值偏压以上出现的较浅背景）以及（ii）与空位相邻的第一和第二个BO原子之间间距的增加，这导致这些相邻氧原子之间出现明显的凹陷。这些结果提供了一个无需切换偏压极性就出现的STM特征的新的显著例子。类似的方法可用于寻找其他大带隙半导体和绝缘体表面的独特特征。
