Real-time X-ray Photoelectron Spectroscopy Research Articles

Possibilities and Limitations of Kinetic Studies in pls. insert line break here On-Surface Synthesis by pls. insert 2nd linebreak here Real Time X-ray Photoelectron Spectroscopy.

The kinetics of coupling reactions on surfaces can be quantitatively studied in real time by X-ray Photoelectron Spectroscopy (XPS). From fitting experimental data, kinetic reaction parameters such as the rate constant's pre-exponential and activation energy can be deduced and compared to quantum chemical simulations. To elucidate the possibilities and limitations of this approach, we propose studies in which experimental data are first simulated and subsequently fitted. Knowing the exact kinetic parameters used in the simulation allows one to evaluate the accuracy of the fit result. Here, several experimental influences, such as the data point density and the addition of noise, are explored for a model reaction with first-order kinetics. The proposed procedure sheds light on the accuracy with which kinetic parameters can be derived and may also help in the design of future experiments.

High reactivity of H2O vapor on GaN surfaces

ABSTRACT Understanding the process of oxidation on the surface of GaN is important for improving metal-oxide-semiconductor (MOS) devices. Real-time X-ray photoelectron spectroscopy was used to observe the dynamic adsorption behavior of GaN surfaces upon irradiation of H2O, O2, N2O, and NO gases. It was found that H2O vapor has the highest reactivity on the surface despite its lower oxidation power. The adsorption behavior of H2O was explained by the density functional molecular dynamic calculation including the spin state of the surfaces. Two types of adsorbed H2O molecules were present on the (0001) (+c) surface: non-dissociatively adsorbed H2O (physisorption), and dissociatively adsorbed H2O (chemisorption) molecules that were dissociated with OH and H adsorbed on Ga atoms. H2O molecules attacked the back side of three-fold Ga atoms on the (0001̅) (−c) GaN surface, and the bond length between the Ga and N was broken. The chemisorption on the (101̅0) m-plane of GaN, which is the channel of a trench-type GaN MOS power transistor, was dominant, and a stable Ga-O bond was formed due to the elongated bond length of Ga on the surface. In the atomic layer deposition process of the Al2O3 layer using H2O vapor, the reactions caused at the interface were more remarkable for p-GaN. If unintentional oxidation can be resulted in the generation of the defects at the MOS interface, these results suggest that oxidant gases other than H2O and O2 should be used to avoid uncontrollable oxidation on GaN surfaces.

Mobility of indium on the ZnO(0001) surface

The mobility of indium on the Zn-polar (0001) surface of single crystal ZnO wafers was investigated using real-time x-ray photoelectron spectroscopy. A sudden transition in the wettability of the ZnO(0001) surface was observed at ∼520 °C, with indium migrating from the (0001¯) underside of the wafer, around the non-polar (11¯00) and (112¯0) sidewalls, to form a uniform self-organized (∼20 Å) adlayer. The In adlayer was oxidized, in agreement with the first principles calculations of Northrup and Neugebauer that In2O3 precipitation can only be avoided under a combination of In-rich and Zn-rich conditions. These findings suggest that unintentional In adlayers may form during the epitaxial growth of ZnO on indium-bonded substrates.

Stability of the Surface Electron Accumulation Layers on the Nonpolar (101̅0) and (112̅0) Faces of ZnO

The stability of the hydroxyl termination and downward band bending on the m-plane (101̅0) and a-plane (112̅0) faces of ZnO single crystals was investigated using synchrotron and real-time X-ray photoelectron spectroscopy. On these nonpolar surfaces, a strong correlation was found between the surface band bending and the surface OH coverage, both of which could be modified via heat treatment in ultra high vacuum (UHV). On the m-plane (101̅0) face, in particular, a threshold temperature of ∼400 °C was observed, after which there was a sudden increase in OH desorption and upward movement of the near-surface bands, resulting in a metallic-to-semiconductor transition in the electronic nature of the surface, with a change from surface electron accumulation to depletion. This loss of surface metallicity is associated with the disruption of a stable monolayer of chemisorbed hydroxyl groups that form a closed hydrogen-bonded network, across rows of Zn–O dimers, on the m-plane (101̅0) face. The surface electron accumulation layers on both the m-plane (101̅0) and a-plane (112̅0) faces can be modified and eventually removed by simple UHV heat treatment, with important implications for the electrical properties of ZnO nanostructures and catalytic ZnO nanopowders, which contain a high proportion of these nonpolar surfaces.

Initial Products in the Oxidation of Si(111)-7×7 Surface Using O2 at 300 K and Those Formation Processes Observed by Real-time Synchrotron Radiation X-ray Photoelectron Spectroscopy

To understand reaction mechanism of the oxidation at an Si(111)-7×7 surface using O2 at 300 K, the determination of initial products is essentially important. Oxygen bonding configurations and silicon oxidation states were observed using real-time X-ray photoelectron spectroscopy for O 1s together with Si 2p. It is found that ins structure initially forms, where one oxygen atom inserts in the backbond of Si adatom. It is ascertained that the chemisorbed molecular oxygen, so-called paul oxygen, is the adsorbate on top of the ins structure. It is also clarified that ad-ins structure and ins-tri structure, where ad means an oxygen atom on top of Si atom and tri means the interstitial oxygen atom, appear after a short time. Our results imply the presence of a mobile O2 on the surface.

Adsorption Dynamics on Si(111)-7 × 7 Surface Induced by Supersonic O2 Beam Studied Using Real-Time Photoelectron Spectroscopy

The adsorption mechanisms of O2 on a Si(111)-7 × 7 surface were studied using real-time X-ray photoelectron spectroscopy with synchrotron radiation in conjunction with a supersonic molecular-beam technique. To study the dependence of the initial sticking probability, s0, saturation coverage of the O2 incident energy, En, and the oxygen uptake curves, which were evaluated from the time evolution of O 1s peak area, were measured over energy ranges from En = 0.03 eV (thermal gas) to 2.26 eV. It was found that a trapping-mediated adsorption mechanism is dominant at En < 0.06 eV. In contrast, an activated adsorption mechanism becomes prominent at En > 0.06 eV, along with the trapping-mediated adsorption mechanism which becomes less prevalent with increasing En. The saturation coverage at energies over 1.9 eV is approximately 1.6 times larger than that at 0.03 eV. Two different components of the O 1s spectrum and their energy dependence were verified. A marked increase in the tri oxygen component is clearly observed in the O 1s photoelectron spectrum for saturation at En > 0.39 eV, which is the evidence for the enhancement of dissociative adsorption of O2 into the subsurface layers; 0.39 eV was taken as the threshold energy for En dependence of both s0 and saturation coverage. The adsorption states induced via the novel reaction channel correlating with En were confirmed by the in situ spectroscopic surface analysis method.

Oxidant prepulsing of Ge (100) prior to atomic layer deposition of Al2O3: In situ surface characterization

We investigate the effects of H2O oxidant pulsing of Ge (100) substrates prior to Al2O3 atomic layer deposition (ALD) to synthesize metal-oxide-semiconductor devices. The prepulsing reduces the hysteresis in capacitance-voltage measurements from 490 to 30 mV at the flatband voltage and appears to reduce the density of fast interface states while causing a small increase in the midgap interface state density. Real-time x-ray photoelectron spectroscopy during prepulsing revealed the dynamic evolution of Ge–O bonding states and the formation of a surface layer with ∼70% hydroxyl termination which appears to produce a less defective interfacial region after subsequent Al2O3 ALD.

Initial Adsorption Dynamics of O2 on Si(111)-7*7 Surface at Room Temperature

Initial adsorption dynamics of O2 on Si(111)-7×7surface at room temperature was investigated by a supersonic molecular beam technique in conjunction with real-time X-ray photoelectron spectroscopy using synchrotron radiation. The initial sticking probability and the saturation coverage as a function of the translational kinetic energy of impinging O2 were measured in the wide range from 0.03 eV, namely thermal gas generated by back-filling condition, to 2.3eV. We proposed a trapping-mediated process as a dominant adsorption mechanism at low kinetic energy region, while at high kinetic energies approximately over 0.06eV the direct adsorption overlapping the trapping-mediated one whose contribution became small with increasing incident energies were expected to be a dominant adsorption mechanism. A gradual increase of the saturation coverage with increasing kinetic energy was clearly observed in the incident energy of 0.4eV to 1.7eV where the direct adsorption took over. The peak area intensity over 1.7eV regions is approximately 1.8 times larger than that for 0.03eV. These results indicate that the incident energy essentially induces the activated adsorption corresponding to the enhancement of the further O2 dissociative adsorption even at room temperature.

Coverage-dependent hydrogen adsorption site determination onRh(100)by high-resolution core-level spectroscopy

We show that high-resolution real-time x-ray photoelectron spectroscopy can be used to determine hydrogen adsorption sites as a function of coverage on $\mathrm{Rh}(100)$. The measurement of the surface core-level shifts does not suffer from the lack of direct sensitivity of other surface probes due to the low scattering cross section and high mobility of atomic hydrogen. At low temperatures $(70--140\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ and coverage (below $0.25\phantom{\rule{0.3em}{0ex}}\mathrm{ML}$), we find that hydrogen adsorbs in fourfold hollow sites on $\mathrm{Rh}(100)$, while at higher coverage the bridge site is preferred. Using Monte Carlo simulations, we unequivocally associate each surface component of the $\mathrm{Rh}\phantom{\rule{0.3em}{0ex}}3{d}_{5∕2}$ core level with a specific adsorption configuration. We obtain a value of $0.74\ifmmode\pm\else\textpm\fi{}0.08$ for the hydrogen initial sticking coefficient, in very good agreement with previous reports.

Non-destructive deposition and diffusion–aggregation of size-selected silver nanoclusters on glassy carbon substrates as probed by real-time X-ray photoelectron spectroscopy

Silver nanoclusters, generated and size-selected in the gas phase and soft landed on low-temperature glassy carbon substrates, were characterized by real-time X-ray photoelectron spectroscopy (XPS). It has been found that the 3d core–electron binding energies of the deposited clusters gradually decrease with time in size-specific manners. The observation clearly showed that the clusters are deposited intact on the surface and diffuse and aggregate, leading to larger clusters. This demonstrates that the real-time XPS can provide key information on states and behaviors of size-selected nanoclusters deposited on surfaces.

Is laser heating advantageous for thermal desorption of large polar molecules?

A systematic study on the desorption and dissociation kinetics of ethylene glycol and poly-(ethylene glycol) oligomers (PEG) with masses ranging from 62 to 35 000 Da from a silica surface was undertaken. The measurements include characterization of the PEG-silica adsorption system using temperature programmed desorption, investigation of the surface coverage, and chemical composition during slow surface heating by real time x-ray photoelectron spectroscopy, and determination of translational energy distributions following nanosecond CO2 laser desorption. We found that even for heating rates exceeding 109 K/s desorption can be described by a thermal model. Intact desorption was limited to PEGs with masses ⩽1000 Da for classical as well as for laser heating.

Oxygen and carbon monoxide interactions on Rh(110) studied by real-time X-ray photoemission spectroscopy

The interaction of CO with a (2 × 1)p2mg oxygen layer on (1 × 1)-Rh(110) and a c(2 × 8) oxygen layer on (1 × 4)-Rh(110) has been studied by synchrotron radiation real-time X-ray photoelectron spectroscopy, low energy electron diffraction and mass spectrometry. The structure and composition of the surface layer were continuously monitored during the reaction performed at 200, 290 and 350 K. The intensity of the components of the O 1s spectra reflected the changes in the oxygen and CO coverages during the reaction. The intensity decrease of the O 1s peak at binding energies < 530.3 eV, corresponding to the adsorbed oxygen, was used as a measure of the CO oxidation rate. The emergence and increase of a second O 1s peak at binding energies > 531.0 eV identified the CO uptake. The changes in the CO 2 partial pressure during titration of the oxygen layers with CO were also monitored by a mass spectrometer. The observed difference in the reactivity of the (2 × 1)p2mg and c(2 × 8) layers was related to the structural differences of the two surfaces: the (1 × 4) reconstruction creates adsorption sites where oxygen is more strongly bonded and less reactive. The effect of the structural changes on the reaction rate was observed during the titration reaction at 350 K.

Real-time X-ray photoelectron spectroscopy study of dissociative oxygen adsorption on Rh(110)

The dissociative oxygen adsorption on a Rh(110) surface has been studied by monitoring the changes in the O 1s spectra during oxygen exposure at 270 and 570 K. The site-sensitive O 1s binding energy was used as a fingerprint of the adatom adsorption geometry and the O 1s intensity as a measure of the oxygen coverage. The short data collection time allowed us to follow the conversion between different oxygen bonding configurations and to correlate it to the observed coverage and structural changes. Comparison between unreconstructive (270 K) and reconstructive (570 K) oxygen adsorption was made and discussed.