Materials analysis using photon-in photon-out spectroscopy

Recent interest in nanotechnology and Organic Light Emitting Devices (OLED) has prompted intense research in the electronic structure and optical properties of relevant materials. Here we report recent development and applications of X-ray excited optical luminescence (XEOL). XEOL, using tunable synchrotron light as an excitation source, monitors the optical response following inner shell excitation of an element in a light emitting material in both the energy and time domain. XEOL from several prototype materials will be presented to illustrate its application in material analysis. In XEOL measurements, we first record the x-ray absorption near edge structures (XANES) of an element, e.g. carbon K-edge in OLED materials and Si K-edge in silicon nanostructures. Photon energies from below to above the edge are then selected to excite the system. Photoluminescence is recorded with conventional optical monochromator and photomultiplier (PMT) instrumentation. The total luminescence yield (zero order) or the partial luminescence yield (PLY), is in turn used to obtain the XANES. In time-resolved studies, the short synchrotron light pulse is used as a strobe. Here, the PMT signal (∼2 ns resolution) is used as the start and the synchrotron pulse (bunch clock) as the stop. The time interval between the strobes is used to monitor the intensity decay. A time window (e.g. 0-10 ns after excitation) can be selected between pulses. The light emitted within this window is used to record time-resolved XEOL (TRXEOL) and optical XANES. Timing measurements reported here were conducted at the Advanced Photon Source (APS) in a top-up mode, and the Synchrotron Radiation Center (SRC), University of Wisconsin-Madison and the Canadian Light Source (CLS) using a single bunch. The pulse width was <100 ps in all cases and the repetition rate at APS, SRC and CLS was 153 ns, 300 ns, and 570 ns, respectively. A schematic is shown in Fig.1 where the green arrow represents the SR pulse and the red arrow the emission upon the relaxation of the excited state. nanowire was prepared in our laboratory. As-prepared SiNW is encapsulated with a thin layer of oxide. Other samples were obtained commercially.