AVS2013 Session SS-FrM: Oxides and Semiconductors: Structure and Reactivity

Carbonate, [CO₃]^2- is a stable intermediate in the water-gas shift reaction (CO + H₂O --> H₂ + CO₂) over inverse CeO_X/Au(111) catalysts, relevant to efficient H₂ production [1]. In addition, carbonate has recently been proposed as the high-temperature (600-700 K) intermediate in the dehydrogenation of methanol [2], and possibly in the complex reaction of acetaldehyde, on CeO₂(100).

Here, we probe the formation and reaction of the carbonate species on CeO_X(100) as a function of temperature and Ce oxidation state by adsorbing CO₂ at 180 K. There is a marked variation in the stability of the carbonate species with the crystallographic orientation of the ceria thin film. Temperature-programmed desorption (TPD) identifies CO₂ as the sole product, with desorption states at 230 K, 410 K, 510 K, and 655 K in the case of stoichiometric CeO₂(100). On CeO₂(111), carbonate desorbs entirely as CO₂ by 300 K [1, 3].

On CeO₂(100), soft X-ray photoelectron spectroscopy (sXPS) confirms the persistence of carbonate up to 600 K. Near-edge X-ray absorption fine structure (NEXAFS) at the C k-edge shows a strong angular dependence that becomes more pronounced at higher temperature (lower coverage). The excitation of the pi* resonance at grazing incidence indicates that the carbonate is lying flat on CeO₂(100).

For partially reduced CeO_1.67(100), we observe: (1) A substantial decrease in low-temperature (< 250 K) desorption, (2) The appearance of an intense CO₂ desorption peak at higher temperature (765 K), and (3) The absence of well-defined peaks in the CO₂ desorption between 300 K and 600 K.

The integrated intensity of the [CO₃]^2- peak in the C 1s spectrum following CO₂ adsorption on CeO_1.67(100) at 180 K is a factor of two greater than that for fully oxidized CeO₂(100). On the reduced surface there is also no loss of this initial intensity upon heating to 300 K. At 700 K the carbonate coverage on the reduced surface is one-third of its starting value, whereas on the oxidized surface complete desorption (as CO₂) has occurred. Furthermore, resonant photoemission spectroscopy (RPES) indicates negligible re-oxidation of the CeO_1.67(100) surface. In contrast, an RPES study of CO₂ on reduced CeO_X(111) demonstrated partial re-oxidation at temperatures as low as 300 K [4].

Research sponsored by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, US Department of Energy.

[1] S. D. Senanayake et al., J. Catal. 271, 392 (2010).

[2] P. M. Albrecht and D. R. Mullins, Langmuir 29, 4559 (2013).

[3] S. D. Senanayake and D. R. Mullins, J. Phys. Chem. C 112, 9744 (2008).

[4] T. Staudt et al., J. Catal. 275, 181 (2010).

Semiconductor nanoparticle (NP) surface chemistry has long been recognized as central to the development and optimization of opto-electronic devices (based on NPs photoluminescence properties). Much attention has been directed to group II-VI quantum dots (QDs) for a variety of targeted applications, although their toxicity renders them incompatible with biological systems. Consequently, there is a renewed interest in nanostructured silicon that is more compatible with existing technology and more biologically friendly. Silicon NPs are capable of exhibiting high quantum yield emission and are considered environmentally adventitious compared to current Cd or Pb alternatives, but the origin of Si NP photoluminescence remains elusive. A significant advantage of using Si NPs is the ability to covalently modify their surface rather than employing datively bound ligands that can over time lead to aggregation, i.e. loss of opto-electronic properties. Though covalent surface modification of Si NPs has been realized, the mechanism of hydrosilylation at defect sites (steps) remains unknown, suggesting that work on model step surfaces could bring much needed insight.

Herein we report the functionalization of vicinal silicon (111) surfaces with both monohydride and dihydride step edges using a 9^o off orientation in the (-1-12 &11-2) directions, respectively. Two reactions are studied as a function of immersion time on both model surfaces: i) thermal hydrosilylation with a terminal alkene (known to be concerted on flat surfaces) and ii) nucleophilic addition using small molecule (methanol). It is found that the nucleophilic system reacts preferentially at the step edge forming a bridge complex between the bridge and lower terrace hydrogen atoms (initial transition structure shown in Figure 1), while the alkene reacts with all terrace and step (mono- and di-hydride) sites. Such selectivity opens the door for nanopatterning and provides important insight into the behavior of Si NPs.

A baseline CV measurement was made using simple TMA predosing in the ALD system to reduce native oxide (Fig 1a). The ex-situ sulfide solution cleaning prior to cyclic atomic H and TMA predosing resulted in improved C-V characteristics illustrated by decreased dispersion across the frequency spectrum. The greatest improvement in dispersion, however, was achieved when combining the ex-situ wet sulfur treatment with in-situ when atomic hydrogen plasma cleaning (Fig 1b.). To understand the role of the ex-situ wet sulfur treatment and in-situ atomic H cleaning, XPS, LEED, and SIMS were performed. Following wet sulfur treatment, in-situ XPS and LEED studies showed that heating to typical ALD temperatures (285°C) both removed the sulfur and produces a 1x1 LEED pattern consistent with less than a monolayer of disorder at the surface. SIM experiments upon the full oxide/GaN structures confirmed minimal sulfur at the buried oxide-semiconductor interface consistent with the main role of sulfur being to protect the surface against oxidation in air instead of providing a chemical passivation at the oxide/semiconductor interface. Angle resolved XPS studies of the ALD nucleation layers showed the primary role of the in-situ atomic hydrogen was to increase the initial growth rate of the gate oxide thereby increasing the ALD nucleation density consistent with previous reports demonstrating that atomic hydrogen completely removes native oxide. Density Functional Theory Molecular Dynamics (DFTMD) calculations(Fig. 2 and 3) were performed to compare the bonding of the amorphous Al₂O₃ gate oxide to a bulk terminated GaN(0001) versus a GaN(0001) surface with a Ga adlayer. The bonding of the amorphous Al₂O₃ gate oxide directly to the bulk terminated GaN(0001) was most consistent with the experimental results since this interface had bulk-like charge states for the interfacial atoms and a very low density of band gap states compared to even the clean GaN(0001) surface.

Directly grafted organic monolayers on Si and Ge surfaces offer an interesting opportunity to explore aspects of surface passivation and control of electrical properties, namely, molecular gating, of semiconductor surfaces. We report our study of the interfacial electronic structures of n-, and p-type of Si(111), Si(100) and Ge(111) surfaces that have been chemically modified with various organic monolayers. The investigated monolayers include octadecane, attached via hydrosilylation and hydrogermylation of 1-octadecene at Si and Ge surfaces, as well as para-substituted phenyl rings, attached by diazonium activation of hydrogenated Si and Ge surfaces. X-ray photoelectron spectroscopy of the modified Si and Ge surfaces, together with four- probe measurements of ultrathin (20-40 nm) silicon-on-insulator channels, functionalized with the monolayer structures, indicate that there is downward band bending, up to 200 meV, associated with assembling these organic monolayers on the semiconductor substrates. This band bending does not directly correlate with the dipole moment or electron withdrawing or donating character of the molecular moieties, pointing to the critical roles of the nature and quality of the self-assembled monolayer, and the intrinsic electronic structure of the semiconductor material in defining the interfacial electronic structure of the passivated Si and Ge surfaces.

Understanding photochemistry on oxide surfaces, a topic of pronounce importance in the context of solar fuel synthesis, includes tracking the path of the charge carriers photoexcited in the bulk to the surface where both, the electron and the hole, may induce chemical transformations in adsorbed molecular species. Shallow trap states which in the case of TiO₂ are known to store charge over extended periods of time (several hours) have a pronounced influence on this bulk-to-surface processes.[1-2] In this contribution we demonstrate that grazing-incidence IR-spectroscopy can be used to probe electrons localized in such states. Loading was carried out by either irradiation with UV-light or exposure to atomic hydrogen. For both, powders as well as a (110) oriented single crystal substrate of rutile titanium dioxide (r-TiO₂), distinct IR absorption features were observed at around 940 and 970 cm^-1. For the single crystal substrates, the IR absorption bands arising from an excitation of the trapped electrons into higher lying final states, are substantially sharper and, in addition, two new bands are resolved at 1205 cm^-1 and 1375 cm^-1. On the basis of this new high-resolution data we propose that the trap state has to be described as a polaron state and that the final states correspond to hydrogenic states within the polaron potential.

[1] M. Xu, Y. Gao, E. M. Moreno, M. Kunst, M. Muhler, Y. Wang, H. Idriss, C. Wöll, Phys. Rev. Lett.106, 138302 (2011).

[2] D. A. Panayotov and J. T. Yates Jr, Chem. Phys. Lett.436, 204 (2007).