The Si2p binding and the Si KLL kinetic energy difference between the SiO 2 layer and Si substrate is shown to be influenced by application of external voltage bias to the sample holder due to the differential charging as was already reported earlier (Ulgut, B.; Suzer, S. J. Phys. Chem. B 2003, 107, 2939). The cause of this bias induced (physical)-shift is now proven to be mostly due to partial neutralization by the stray electrons within the vacuum system by (i) introducing additional stray electrons via a filament and following their influence on the measured binding energy as a function of the applied voltage, (ii) measuring the Auger parameter. It is also shown that citrate-capped gold nanoclusters deposited on the SiO 2 /Si system experience differential charging similar to that of the oxide layer rather than the silicon substrate.

Prolonged exposure to X-rays of HAuCl4 deposited from an aqueous solution onto a SiO2/Si substrate or into a poly(methyl methacrylate) (PMMA) matrix induces reduction of the Au 3+ ions to Au 0 and subsequent nucleation to gold nanoclusters as recorded by X-ray photoelectron spectroscopy. The corresponding major oxidation product is determined as chlorine {HAuCl 4(ads) + X-rays f Au(ads) + (3/2)Cl2(ads) + HCl(ads)}, which is initially adsorbed onto the surface but eventually diffuses out of the system into the vacuum. The reduced gold atoms aggregate (three-dimensionally) into gold nanoclusters as evidenced by the variation in the binding energy during X-ray exposure, which starts as 1.3 eV but approaches a value that is 0.5 eV higher than that of the bulk gold. The disappearance of the oxidation product (Cl2p signal) and the growth of the nanoclusters (related to the measured binding energy difference between the Si2p of the oxide and Au4f of the reduced gold) exhibit first-order kinetics which is approximately 3 times slower than the reduction of Au 3+ , indicating that both of the former processes are diffusion controlled. Similarly, gold ions incorporated into PMMA can also be reduced and aggregated to gold nanoclusters using 254 nm deep UV irradiation in air evidenced by UV-vis-NIR absorption spectrocopy.

Figure 1. The normal incidence reflection spectra for three different strain values of 7.5%, 15% and 23% for grating A (main) and no strain, 6.4% and 12.8% for grating B (inset). Abstract-In this study, we demonstrate that periods of metallic gratings on elastomeric substrates can be tuned with external strain and hence are found to control the resonance condition of surface plasmon polaritons [1]. The periods of the gratings are increased up to 25% by the use of applied mechanical strain. The tunability of the elastomeric substrate provides the opportunity to use such gratings as efficient surface enhanced Raman spectroscopy substrates. It's been demonstrated that the Raman signal can be maximized by tuning the period of the elastomeric grating. Tunable Surface Plasmon Resonance on an Elastomeric GratingThe surface plasmon resonance (SPR) phenomena observed on metal surfaces or nanoparticles has been a great interest in several fields of research such as nanoscale photonics and biological sensing. Continuous metallic films possessing a periodic perturbation exhibit strong extinction and scattering spectra when excited at the SPR condition. The challenge of designing effective structures to manipulate plasmonic fields and utilize them in functional devices still remains. In particular, the use of SPR in surface enhanced Raman spectroscopy (SERS) and biological sensing require an intelligent design in order to maximize the plasmonic enhancement. In this regard, the tunability of the SPR wavelength provides flexibility in many plasmonic sensing applications. Flexible designs utilizing electronic [2], ferroelectric [3], or thermal [4] tuning mechanisms are also reported in the literature. Those methods are reversible and can be applied after the plasmonic structure is fabricated. Such a repeatable process can find wide applications in the field of Raman spectroscopy and plasmonic sensing. It was reported that by controlling the geometry of the nanoshells films, the SERS enhancements can be optimized [5]. A repeatable thermal tuning mechanism using silver nanoparticles for achieving a tunable SERS substrate was reported by Lu et al. [6].In this study we use an elastomeric grating structure in order to excite surface plasmon polaritons (SPP) on its metallic surface. We report a way of tuning the SPR by applying mechanical strain on the elastomeric grating structure. The elongation of the elastomer effectively changes the period of the metallic grating. It can be seen that the SPR wavelength also shifts as the external strain changes the period of the elastomeric grating coated with a thin metallic layer.We fabricated two silicone elastomers with gratings on top using two different methods. The first elastomeric grating was generated using holographic lithography with 665 nm period. The elastomeric grating is then obtained using the replication procedure. Note that the thickness of the elastomer is kept around 5 mm. To generate SPP, the PDMS grating is coated with a 55 nm of silver using thermal evaporation. For the ...

By applying voltage pulses to the sample rod while recording the spectrum, we show, for the first time, that it is possible to obtain a time-resolved XPS spectrum in the millisecond range. The Si 2p spectrum of a silicon sample containing a ca. 400-nm oxide layer displays a time-dependent charging shift of ca. 1.7 eV with respect to the Au 4f peaks of a gold metal strip in contact with the sample. When gold is deposited as C 12 -thiol-capped nanoclusters onto the same sample, this time the Au 4f peaks also display time-dependent charging behavior that is slightly different from that of the Si 2p peak. This charging/discharging is related to emptying/filling of the hole traps in the oxide layer by the stray electrons within the vacuum system guided by the external voltage pulses applied to the sample rod, which can be used to extract important parameter(s) related to the dielectric properties of surface structures.Charging/discharging is one of the fundamental processes dictating both the thermodynamics and the kinetics of the various physicochemical changes taking place on surfaces. 1 In addition to the standard electrical measurements, STM, AFM, and Kelvin probe techniques are widely used to elucidate the structural changes accompanying these processes. [2][3][4][5][6] When chemical information is also needed, XPS is usually the preferred spectroscopic technique because all elements, except for hydrogen, and their oxidation state(s) can easily be identified. 7 One of the disadvantages of the XPS technique is that additional positive charges are naturally introduced as a consequence of the photoemission process. These charges do not interfere with measurements when the surfaces are electrically conducting but can cause significant binding-energy shifts for poorly conducting samples. [8][9][10][11] Such charges are usually compensated by flooding the sample with low-energy electrons (or sometimes ions) and trying to bring the surfaces to a steady state in terms of electrons going in and out, but the complete elimination of charging is only an ideal. Surfaces can also be negatively charged if more electrons are introduced. This modification, dubbed controlled surface charging (CSC), has been successfully applied to depth profiling in the 1-10 nm range and/or to the lateral differentiation of mesoscopic layers. [12][13][14][15][16] However, prolonged exposure of the surfaces to intense low-energy electrons can cause physical and chemical damage. 9 It was even reported that the surface potential that developed from the added charging caused deintercalation in layered compounds. 17 Lau and co-workers have also utilized the CSC to extract structural and electrical properties of ultrathin dielectrics on semiconductors. [18][19][20][21][22] In a complementary study, we have recently reported that the positive charging developed on the SiO 2 /Si system can also be controlled simply by the application of an external bias to the sample rod to affect the measured binding-energy difference between the Si 4+ overlayer and the...

Cataloged from PDF version of article.In x-ray photoemission measurements, differential charging causes the measured binding energy difference between the Si 2p of the oxide and the silicon substrate to vary nonlinearly as a function of the applied external do voltage stress, which controls the low-energy electrons going into and out of the sample. This nonlinear variation is similar to the system where a gold metal strip is connected to the same voltage stress through an external 10 Mohm series resistor and determined again by x-ray photoelectron spectroscopy (XPS). We utilize this functional resemblance to determine the resistance of the 4 nm SiO2 layer on a silicon substrate as 8 Mohm. In addition, by performing time-dependent XPS measurements (achieved by pulsing the voltage stress), we determine the time constant for charging/discharging of the same system as 2.0 s. Using an equivalent circuit, consisting of a gold metal strip connected through a 10 Mohm series resistor and a 56 nF parallel capacitor, and performing time-dependent XPS measurements, we also determine the time constant as 0.50 s in agreement with the expected value (0.56 s). Using this time constant and the resistance (8.0 Mohm), we can determined the capacitance of the 4 nm SiO2 layer as 250 nF in excellent agreement with the calculated value. Hence, by application of external do and pulsed voltage stresses, an x-ray photoelectron spectrometer is turned into a tool for extracting electrical parameters of surface structures in a noncontact fashion. (c) 2005 American Institute of Physics