Browsing by Author "Demirok, U. K."

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Open Access
Charging/discharging of Au (core)/silica (shell) nanoparticles as revealed by XPS
(American Chemical Society, 2005) Tunc, I.; Demirok, U. K.; Süzer, Şefik; Correa-Duatre, M. A.; Liz-Marzan, L. M.
By recording XPS spectra while applying external voltage stress to the sample rod, we can control the extent of charging developed on core-shell-type gold nanoparticles deposited on a copper substrate, in both steady-state and time-resolved fashions. The charging manifests itself as a shift in the measured binding energy of the corresponding XPS peak. Whereas the bare gold nanoparticles exhibit no measurable binding energy shift in the Au 4f peaks, both the Au 4f and the Si 2p peaks exhibit significant and highly correlated (in time and magnitude) shifts in the case of gold (core)/silica (shell) nanoparticles. Using the shift in the Au 4f peaks, the capacitance of the 15-nm gold (core)/6-nm silica (shell) nanoparticle/nanocapacitor is estimated as 60 aF. It is further estimated that, in the fully charged situation, only 1 in 1000 silicon dioxide units in the shell carries a positive charge during our XPS analysis. Our simple method of controlling the charging, by application of an external voltage stress during XPS analysis, enables us to detect, locate, and quantify the charges developed on surface structures in a completely noncontact fashion. © 2005 American Chemical Society.
Open Access
Electrical circuit modeling of surface structures for X-ray photoelectron spectroscopic measurements
(Elsevier BV * North-Holland, 2008) Tasci, T. O.; Atalar, Ergin; Demirok, U. K.; Süzer, Şefik
We model the X-ray photoelectron spectrometer and the sample with lumped electrical circuit elements, and simulate various types of conditions using a widely used computer program (PSpice) and compare the results with experimental measurements. By using the electrical model simulations, the surface voltage and the spectrum can be estimated under various types of external voltage stimuli, and the zero potential condition can be predicted accurately for obtaining a truly uncharged spectrum. Additionally, effects of several charging mechanisms (taking place during XPS measurements) on the surface potential could easily be assessed. Finally, the model enables us to find electrical properties, like resistance and capacitance of surface structures, under X-ray and low-energy electron exposure.
Open Access
Enhanced peak separation in XPS with external biasing
(Elsevier, 2005-08-15) Ertas, G.; Demirok, U. K.; Süzer, Şefik
We have demonstrated that the An 4f peaks of the capped gold nanoparticles deposited on a SiO2 (20 nm)/Si substrate can be separated form the An 4f peaks of a gold metal strip, in contact with the same sample, by application of an external voltage bias to the sample rod while recording the XPS spectra. The external bias controls the flow of low-energy electrons falling on to the sample which in-turn controls the extent of the differential charging of the oxide layer leading to shifts in the binding energy of the gold nanoparticles in contact with the layer. The method is simple and effective for enhancing peak separation and identification of hetero-structures. (c) 2004 Elsevier B.V. All rights reserved.
Open Access
Time-resolved XPS analysis of the SiO2/Si system in the millisecond range
(2004) Demirok, U. K.; Ertas, G.; Süzer, Şefik
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 C12-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.
Open Access
X-ray photoelectron spectroscopic analysis of Si nanoclusters in SiO 2 matrix
(American Chemical Society, 2006) Dane, A.; Demirok, U. K.; Aydınlı, Atilla; Süzer, Şefik
We investigated silicon nanoclusters Si(nc) in a SiO2 matrix prepared by the plasma-enhanced chemical vapor deposition technique, using X-ray photoelectron spectroscopy (XPS) with external voltage stimuli in both static and pulsed modes. This method enables us to induce an additional charging shift of 0.8 eV between the Si2p peaks of the oxide and the underlying silicon, both in static and time-resolved modes, for a silicon sample containing a 6 nm oxide layer. In the case of the sample containing silicon nanoclusters, both Si2p peaks of Si(nc) and host SiO2 undergo a charging shift that is 1 order of magnitude larger (> 15 eV), with no measurable difference between them (i.e., no differential charging between the silicon nanoclusters and the oxide matrix could be detected). By use of a measured Auger parameter, we estimate the relaxation energy of the Si(nc) in the SiO2 matrix as -0.4 eV, which yields a -0.6 eV shift in the binding energy of the Si(nc) with respect to that of bulk Si in the opposite direction of the expected quantum size effect. This must be related to the residual differential charging between the silicon nanoclusters and the oxide host. Therefore, differential charging is still the biggest obstacle for extracting size-dependent binding energy shifts with XPS when one uses the oxide peak as the reference. © 2006 American Chemical Society.
Open Access
X-ray photoelectron spectroscopy for resistance-capacitance measurements of surface structures
(AIP Publishing, 2005-04-29) Ertas, G.; Demirok, U. K.; Atalar, Abdullah; Süzer, Şefik
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.
Open Access
XPS analysis with pulsed voltage stimuli
(2006) Karabudak, E.; Demirok, U. K.; Süzer, Şefik
We record XPS spectra while applying 0 to +10 V or 0 to -10 V square pulses to the sample rod, which normally results in twinning of all peaks at correspondingly increased (for +10 V) or decreased (for -10 V) binding energies. For poorly conducting samples, like silicon oxide layer on a silicon substrate, the twinned peaks appear at different energies due to differential charging, which also vary with respect to the frequency of the applied pulses. Moreover, the frequency dependence varies with the thickness and can be correlated with the capacitance of the oxide layer. The technique is simple and can lead to extract important information related with dielectric properties of surface structures in a totally non-contact fashion. © 2005 Elsevier B.V. All rights reserved.