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Contrast stability and ''stripe'' formation in scanning tunnelling microscopy imaging of highly oriented pyrolytic graphite: the role of STM-tip orientations

机译:高取向热解石墨的扫描隧道显微镜成像中的对比度稳定性和“条纹”形成:STM尖端取向的作用

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摘要

Highly oriented pyrolytic graphite (HOPG) is an important substrate in many technological applications and is routinely used as a standard in Scanning Tunnelling Microscopy (STM) calibration, which makes the accurate interpretation of the HOPG STM contrast of great fundamental and applicative importance. We demonstrate by STM simulations based on electronic structure obtained from first principles that the relative local orientation of the STM-tip apex with respect to the HOPG substrate has a considerable effect on the HOPG STM contrast. Importantly for experimental STM analysis of HOPG, the simulations indicate that local tip-rotations maintaining a major contribution of the d_{3z2-r2} tip-apex state to the STM current affect only the secondary features of the HOPG STM contrast resulting in 'stripe' formation and leaving the primary contrast unaltered. Conversely, tip-rotations leading to enhanced contributions from m ≠ 0 tip-apex electronic states can cause a triangular-hexagonal change in the primary contrast. We also report a comparison of two STM simulation models with experiments in terms of bias-voltage-dependent STM topography brightness correlations and discuss our findings for the HOPG(0 0 0 1) surface in combination with tungsten tip models of different sharpnesses and terminations.
机译:高取向热解石墨(HOPG)是许多技术应用中的重要基材,并且通常用作扫描隧道显微镜(STM)校准的标准,这使得对HOPG STM对比度的准确解释具有重要的基础和应用重要性。通过基于从第一原理获得的电子结构的STM仿真,我们证明了STM尖端顶点相对于HOPG基材的相对局部取向对HOPG STM对比度有相当大的影响。重要的是,对于HOPG的实验STM分析,仿真表明,保持d_ {3z2-r2}尖端-顶点状态对STM电流起主要作用的局部尖端旋转仅影响HOPG STM对比度的次要特征,从而导致“条纹”的形成并保持主要对比不变。相反,尖端旋转导致m≠0尖端电子状态的贡献增加,会导致主要对比度出现三角六边形变化。我们还报告了两个STM模拟模型与实验的对比,这些模型与实验相关,取决于偏置电压的STM形貌亮度相关性,并讨论了我们结合不同清晰度和端接钨尖模型对HOPG(0 0 0 1)表面的发现。

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