Chemically Specific Dynamic Characterization of Photovoltaic and Photoconductivity Effects of Surface Nanostructures Okan O ¨ ner Ekiz,* Koray Mizrak, and Aykutlu Da ˆ na* UNAM Institute of Materials Science and Nanotechnology, Bilkent University, 06800 Ankara, Turkey X -ray photoelectron spectroscopy (XPS) is a powerful spectroscopic technique for the characterization of surfaces with chemical specificity. 1-5 The photoelectron spectra carries information about the binding states of different atomic species within tens of nanometers of the surface, as well as information on local po- tential variations. If the emitted electrons are not compensated by an external elec- tron gun or directly from the substrate, lo- cal potentials can vary due to local conduc- tivity variations that result in different amounts of accumulated charge. Over- charging by injection of excess electrons was previously used for surface character- ization. 6 Surface potentials can be shifted by direct application of a voltage to the sub- strate. Such shifts resulting from direct volt- age stimulus were previously modeled for static and dynamic voltage excitations, tak- ing into account conductivity and capaci- tances of surface domains. 7-13 Nano- structures are increasingly finding application in photovoltaic technologies. In- vestigation of photovoltaic and photocon- ductivity effects in nanowires, nanocrystals, and nanocomposites are interesting from a fundamental scientific point of view. Con- ventional characterization techniques lack chemical specificity, and most of the time require precision fabrication of contacts on nanostructures using techniques such as electron beam lithography or related lithog- raphies with nanoscale resolution, espe- cially if single nanoparticles are involved. Surface photovoltage spectroscopy (SPV) has been traditionally applied to character- ize photoinduced surface photovoltage changes upon illumination, using the Kelvin probe as a readout method for surface po- tential shifts. 14,15 Also, Kelvin probe micros- copy has been used to observe photovol- taic effects on nanoscale structures. 16 The chemically specific readout of photoin- duced surface potential changes is a highly desirable analytical capability. Such chemi- cally specific measurements are not pos- sible with conventional Kelvin probe mea- surements. Previously, XPS has been used to probe the surface photovoltage of silicon surfaces. 17 Recently, Cohen 18,19 et al. demon- strated that surface potential shifts related to external illumination were observable in XPS spectra of composite semiconductor surfaces. It was demonstrated that static or quasi-static shifts of XPS peaks can be re- lated to photovoltaic and photoconductive effects. The shifts can be quantitatively studied with films on conductive substrates. 18,19 In this article we demonstrate an ap- proach that allows the study of photovol- taic and photoconductivity effects using the XPS, under static or modulated illumina- tion. Surface potentials of domains are in- ternally modulated owing to both photo- voltaic and photoconductivity effects. We show that a circuit model can be used to *Address correspondence to ekiz@bilkent.edu.tr, aykutlu@unam.bilkent.edu.tr. Received for review October 15, 2009 and accepted March 30, 2010. Published online April 9, 2010. 10.1021/nn9014196 © 2010 American Chemical Society ABSTRACT We report characterization of photovoltaic and photoconductivity effects on nanostructured surfaces through light induced changes in the X-ray photoelectron spectra (XPS). The technique combines the chemical specificity of XPS and the power of surface photovoltage spectroscopy (SPV), with the addition of the ability to characterize photoconductivity under both static and dynamic optical excitation. A theoretical model that quantitatively describes the features of the observed spectra is presented. We demonstrate the applicability of the model on a multitude of sample systems, including homo- and heterojunction solar cells, CdS nanoparticles on metallic or semiconducting substrates, and carbon nanotube films on silicon substrates. KEYWORDS: X-ray photoelectron spectroscopy · carbon nanotubes · cadmium sulfide · silicon · dynamic characterization · surface photovoltage · photoconductivity · semiconductor nanoparticles · photovoltaics ARTICLE www.acsnano.org VOL. 4 ▪ NO. 4 ▪ 1851–1860 ▪ 2010 1851