Department of Chemistry, University of Utah , 315 South 1400 East, Salt Lake City, Utah 84112-0850, United States.
Langmuir. 2013 Sep 24;29(38):11941-9. doi: 10.1021/la402468k. Epub 2013 Sep 12.
Optically transparent semiconductors allow simultaneous control of interfacial electrical potential and spectroscopic observation of chemistry near the electrode surface. Care must be taken, however, to avoid unwanted photoexcitation-induced charging of the semiconductor electrode that could influence the results. In this work, we investigate the in situ surface charging by photoexcitation well below the band gap of an optically transparent semiconductor, indium-tin oxide (ITO) electrode. Using total-internal-reflection fluorescence microscopy, the population of 100-nm negatively charged carboxylate-polystyrene fluorescent nanoparticles at an ITO-aqueous solution interface could be monitored in situ. At positive applied potentials (0.7 V versus Ag/AgCl), nanoparticles accumulate reversibly in the electrical double-layer of the ITO surface, and the interfacial nanoparticle populations increase with 488-nm excitation intensity. The potential sensitivity of nanoparticle population exhibited no dependence on excitation intensity, varied from 0.1 to 10 W cm(-2), while the onset potential for particle accumulation shifted by as much as 0.3 V. This shift in surface potential appears to be due to photoexcitation-induced charging of the ITO, even though the excitation radiation photon energy, ~2.4 eV, is well below the primary band gap of ITO, >3.5 eV. A kinetic model was developed to determine the photon order of electron-hole generation relative to the electron-hole recombination. The photoexcitation process was found to be first-order in photon flux, suggesting one-photon excitation of an indirect band gap or defect sites, rather than two-photon excitation into the direct band gap. A control experiment was conducted with red-fluorescent carboxylate-polystyrene particles that were counted using 647-nm excitation, where the photon energy is below the indirect band gap or defect site energy and where the optical absorption of the film vanishes. Red illumination between 1 and 15 W cm(-2) produced no detectable shifts in the onset accumulation potential, which is consistent with the negligible optical absorption of the ITO film at this longer wavelength.
透明半导体可实现对界面电势的同时控制，并在电极表面附近对化学物质进行光谱观察。然而，必须小心谨慎，以避免半导体电极因不必要的光激发而产生的充电，因为这可能会影响结果。在这项工作中，我们研究了光透明半导体（ITO）电极的带隙以下的光激发引起的表面原位充电。通过全内反射荧光显微镜，可以原位监测在 ITO-水溶液界面处的约 100nm 带负电荷的羧酸盐聚苯乙烯荧光纳米粒子的数量。在正施加的电势（相对于 Ag/AgCl 为 0.7 V）下，纳米颗粒在 ITO 表面的双电层中可被可逆地累积，并且界面纳米颗粒的数量随 488nm 激发强度增加而增加。纳米颗粒数量的电位灵敏度与激发强度无关，范围从 0.1 到 10 W cm(-2)，而粒子累积的起始电位变化高达 0.3 V。ITO 的光激发诱导充电似乎导致了表面电势的这种变化，尽管激发辐射光子能量，~2.4 eV，远低于 ITO 的主要带隙，>3.5 eV。建立了一个动力学模型来确定电子-空穴产生与电子-空穴复合的光子顺序。发现光激发过程在光子通量中是一级的，这表明间接带隙或缺陷部位的单光子激发，而不是直接带隙的双光子激发。进行了一个对照实验，使用红色荧光羧酸盐聚苯乙烯纳米粒子，用 647nm 激发进行计数，其中光子能量低于间接带隙或缺陷部位能量，并且薄膜的光吸收消失。在 1 到 15 W cm(-2)之间的红色照射不会产生可检测的起始累积电位偏移，这与该更长波长下 ITO 薄膜的可忽略不计的光吸收一致。
