Department of Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina 27695-8204, USA.
Langmuir. 2011 Oct 4;27(19):11883-8. doi: 10.1021/la201161q. Epub 2011 Sep 9.
Self-assembled monolayers (SAMs) of alkanephosphonic acids with chain lengths between 8 and 18 carbon units were formed on thin films of indium tin oxide (ITO) sputter-deposited on silicon substrates with 400 nm thermally grown SiO(2). The silicon substrates, while not intended for use in near-IR or visible optics applications, do provide smooth surfaces that permit systematic engineering of grain size and surface roughness as a function of the sputter pressure. Argon sputter pressures from 4 to 20 mTorr show systematic changes in surface morphology ranging from smooth, micrometer-sized grain structures to <50 nm grains with 3× higher surface roughness. Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy experiments are conducted for alkanephosphonic acids deposited on these wide range of ITO surfaces to evaluate the effects of these morphological features on monolayer ordering. Results indicate that long-chain SAMs are more highly ordered, and have a smaller tilt angle, than short-chain SAMs. Surprisingly, the 1-octadecyl phosphonic acids maintain their order as the lateral grain dimensions of the ITO surface shrink to ∼50 nm. It is only when the ITO surface roughness becomes greater than the SAM chain length (∼15 Å) that SAMs are observed to become relatively disordered.
在硅衬底上溅射沉积的氧化铟锡 (ITO) 薄膜上形成了链长为 8 至 18 个碳原子的烷基亚磷酸单分子层 (SAM)，该硅衬底虽然不打算用于近红外或可见光光学应用，但确实提供了光滑的表面，允许系统地改变晶粒尺寸和表面粗糙度作为溅射压力的函数。氩溅射压力从 4 到 20 毫托显示出表面形态的系统变化，从光滑的、微米级的晶粒结构到具有 3 倍更高表面粗糙度的 <50nm 晶粒。对沉积在这些宽范围 ITO 表面上的烷基亚磷酸进行近边缘 X 射线吸收精细结构 (NEXAFS) 光谱实验，以评估这些形态特征对单层有序性的影响。结果表明，长链 SAM 比短链 SAM 具有更高的有序性和更小的倾斜角。令人惊讶的是，当 ITO 表面的横向晶粒尺寸收缩到约 50nm 时，1-十八烷基膦酸仍能保持其有序性。只有当 ITO 表面粗糙度大于 SAM 链长（约 15Å）时，SAM 才会变得相对无序。
