Interaction between photocatalyst surface nanostructures and ultraviolet light by apertureless scanning nearfield optical microscopy

通过扫描隧道显微镜（STM）观察到，紫外线（UV）照射在二氧化钛表面上，使二氧化钛表面发生了明显的结构变化，这是紫外线对二氧化钛表面进行电子激发的结果。本文用STM在原子尺度上观察了金红石TiO_2（110）表面的局部光响应。（110）表面几乎所有区域对325 nm紫外（UV）光的照射都有响应，符合最小带隙（3.2eV）电子跃迁，其中台阶和（1×2）脊区的响应强度更高。当同样的表面被远低于最小带隙跃迁能量的442nm蓝光照射时，大部分表面区域处于休眠状态，但台阶区域和（1×2）脊结构的某些区域响应蓝光。这些位点与表面缺氧有关，这可能为可见光下二氧化钛光催化剂表面工程的可能性提供了线索。我们还对MDMO-PPV:PCBM共混物组成的“经典”有机太阳能电池系统进行了全面的开尔文探针力显微镜（KPFM）研究。KPFM方法可以在纳米尺度上获得地形和局部功函数的详细信息。实验分别在黑暗和442nm、675nm激光照射下进行，提取局部分辨表面光电压（SPV）。我们发现氯苯和甲苯浇铸共混膜表面的能量学有明显的差异。结合高分辨率扫描电子显微镜（SEM）实验，我们能够解释KPFM结果，并得出一些关于太阳能电池结构中电子向阴极传递的结论。结果表明，在有机太阳能电池中，通常在薄膜顶部蒸发的甲苯浇铸膜表面对电子向阴极的传播具有阻断行为。

Ultraviolet (UV) irradiation on titanium dioxide surface gives apparent structural changes as observed by scanning tunneling microscopy (STM), which are the consequence of electronic excitation of the surface by UV light. Here local photoresponse of a rutile TiO_2(110) surface is visualized at atomic scale by STM. Almost all the areas on the (110) surface responded to the illumination of 325-nm ultra-violet (UV) light, in accordance with the minimum band-gap (3.2eV) electron transition, with steps and (1×2) ridge areas exhibiting higher response intensities. When the same surface is illuminated with blue light at 442nm, which is well below the minimum band-gap transition energy, most of the surface areas were dormant, but certain areas at step regions and (1×2) ridge structures responded to the blue light. These sites are associated with oxygen deficiency on the surface, which may give a clue to the possibility of surface engineering of titania photocatalyst to be employed under visible light.We also conducted a comprehensive Kelvin probe force microscopy (KPFM) study on the "classical" organic solar cell system consisting of MDMO-PPV:PCBM blends. The KPFM method yields detailed information of topography and local work function at nanometer scale. Experiments were performed either in dark or under laser illumination at 442nm or 675nm to extract the locally resolved surface photovoltage (SPV). We identified distinct differences in the energetics on the surface of chlorobenzene and toluene cast blend films. Together with high resolution scanning electron microscopy (SEM) experiments we were able to interpret the KPFM results and to draw some conclusions for the electron transport towards the cathode in the solar cell configuration. The results suggest a blocking behavior of the toluene cast film surfaces against electron propagation towards the cathode, usually evaporated on top of the films in organic solar cells.