Publications

The following is an overview of our recent publications, including the full text in PDF format where possible. Please note: These articles may be downloaded for personal use only. Any other use requires prior permission of the author and the publisher.



2014

Carbon monoxide-assisted size confinement of bimetallic alloy nanoparticles, C. Cui, L. Gan, M. Neumann, M. Heggen, B. Roldan Cuenya, P. Strasser, JACS Comm. (2014), in print. [download]

Colloid-based chemical synthesis methods of bimetallic alloy nanoparticles (NPs) provide good monodispersity, yet generally show a strong variation of the resulting mean particle size with alloy composition. This severely compromises accurate correlation between composition of alloy particles and their size-dependent properties. To address this issue, a general CO adsorptionassisted capping-ligand-free solvothermal synthesis method is reported which provides homogeneous bimetallic nanoparticles with almost perfectly constant particle size over an unusually wide compositional range. Using Pt-Ni alloy nanoparticles as an example, we show that variation of the reaction temperature between 160 and 240 oC allows for precise control of the resulting alloy particle bulk composition between 15 and 70 atomic % Ni , coupled with a constant mean particle size of about 4 nm. The sizeconfining and Ni content-controlling role of CO during the nucleation and growth processes are investigated and discussed. Data suggest that size-dependent CO surface chemisorption and reversible Ni-carbonyl formation are key factors for the achievement of a constant particle size and temperature controlled Ni content...


Structural and electronic properties of micellar Au nanoparticles: size and ligand effects, F. Behafarid, J. Matos, S. Hong, L. Zhang, T.S. Rahman, B. Roldan Cuenya, ACS Nano (2014), in print. [download]

Gaining experimental insight into the intrinsic properties of nanoparticles (NPs) represents a scientific challenge due to the difficulty of deconvoluting these properties from various environmental effects such as the presence of adsorbates or a support. A synergistic combination of experimental and theoretical tools, including X-ray absorption fine-structure spectroscopy, scanning transmission electron microscopy, atomic force microscopy, and density functional theory was used in this study to investigate the structure and electronic properties of small (1-4 nm) Au NPs synthesized by an inverse micelle encapsulation method...


Shape Dependent catalytic oxidation of 2-butanol over Pt nanoparticles supported on γ-Al2O3, H. Mistry, F. Behafarid, E. Zhou, E. Zhou, E. Zhou, L.K. Ono, L. Zhang, B. Roldan Cuenya,  ACS Catalysis 4, 109 (2014). [download]

This study illustrates the effect of nanoparticle (NP) shape on the reactivity of size-selected Pt/γ-Al2O3 nanocatalysts for 2-butanol oxidation. Nanoparticles similar in size [transmission electron microscopy (TEM) diameter of 1 nm] but with different shapes were prepared via encapsulation in inverse micelles. The NP shape was resolved by combining information extracted from extended X-ray absorption fine structure spectroscopy (EXAFS) data, TEM, and modeling. A correlation was observed between the average first nearest neighbor coordination number of atoms at the NP surface and their catalytic activity. In particular, the NPs with the largest number of weakly coordinated surface atoms (i.e., edges and corners) were found to be the least active for the total oxidation of 2-butanol. This result highlights that not only size but also shape control must be achieved to tailor the catalytic properties of nanoscale materials..


Pressure dependent effect of hydrogen adsorption on structural and electronic properties of Pt/γ-Al2O3 nanoparticles, H. Mistry, F. Behafarid, S.R. Bare, B. Roldan Cuenya, Chem. Cat. Chem. 6, 348 (2014). [download]

Using in-situ X-ray absorption fine structure spectroscopy, a striking 2D to a 3D shape transformation is revealed in size- and shape-controlled Pt/γ-Al2O3 nanoparticles under increasing H2 pressure at room temperature. In addition, the ability of these particles to adsorb up to 2.5 hydrogen per platinum atom is demonstrated.



2013

Nano-Gold Diggers: Au-Assisted SiO2-Decomposition and Desorption in Supported Nanocatalysts, L.K. Ono, F. Behafarid, B. Roldan Cuenya, ACS Nano 7, 10327 (2013). [download]

An investigation of the thermal stability of size-selected Au nanoparticles (NPs) synthesized via inverse micelle encapsulation and deposited on SiO2(4 nm)/Si(100) is presented. The size and mobility of individual Au NPs after annealing at elevated temperatures in ultrahigh vacuum (UHV) was monitored via atomic force microscopy (AFM). An enhanced thermal stability against coarsening and lack of NP mobility was observed up to 1343 K. In addition, a drastic decrease in the average NP height was detected with increasing annealing temperature, which was not accompanied by the sublimation of Au atoms/clusters in UHV. The apparent decrease in the Au NP height observed is assigned to their ability to dig vertical channels in the underlying SiO2 support. More specifically, a progressive reduction in the thickness of the SiO2 support underneath and in the immediate vicinity of the NPs was evidenced, leading to NPs partially sinking into the SiO2 substrate. The complete removal of silicon oxide in small patches was observed to take place around the Au NPs after annealing at 1343 K in UHV. These results reveal a Au-assisted oxygen desorption from the support via reverse oxygen spill-over to the NPs.


Long-Range Segregation Phenomena in Shape-Selected Bimetallic Nanoparticles: Chemical State Effects, M. Ahmadi, F. Behafarid, C. Cui, P. Strasser, B. Roldan Cuenya, ACS Nano 7, 9195 (2013). [download]

A study of the morphological and chemical stability of shape-selected octahedral Pt0.5Ni0.5 nanoparticles (NPs) supported on highly oriented pyrolytic graphite (HOPG) is presented. Ex situ atomic force microscopy (AFM) and in situ X-ray photoelectron spectroscopy (XPS) measurements were used to monitor the mobility of Pt0.5Ni0.5 NPs and to study long-range atomic segregation and alloy formation phenomena under vacuum, H2, and O2 environments. The chemical state of the NPs was found to play a pivotal role in their surface composition after different thermal treatments. In particular, for these ex situ synthesized NPs, Ni segregation to the NP surface was observed in all environments as long as PtOx species were present. In the presence of oxygen, an enhanced Ni surface segregation was observed at all temperatures. In contrast, in hydrogen and vacuum, the Ni outward segregation occurs only at low temperature (<200-270C), while PtOx species are still present. At higher temperatures, the reduction of the Pt oxide species results in Pt diffusion toward the NP surface and the formation of a Ni
Pt alloy. A consistent correlation between the NP surface composition and its electrocatalytic CO oxidation activity was established.


Towards the understanding of sintering phenomena at the nanoscale: geometric and environmental effects, F. Behafarid, B. Roldan Cuenya, Topics in Catalysis 56, 1542 (2013). [download]

One of the technologically most important requirements for the application of supported metal nanoparticles (NPs) to the field of heterogeneous catalysis is the achievement of thermally and chemically stable systems under reaction conditions. For this purpose, a thorough understanding of the different pathways underlying coarsening phenomena is needed. In particular, in depth knowledge must be achieved on the role of the NP synthesis method, geometrical features of the NPs (size and shape), initial NP dispersion on the support (interparticle distance), support pre-treatment (affecting its morphology and chemical state), and reaction environment (gaseous or liquid medium, pressure, temperature). This study provides examples of the stability and sintering behavior of nanoscale systems monitored ex situ, in situ, and under operando conditions via transmission electron microscopy, atomic force microscopy, scanning tunneling microscopy, and x-ray absorption fine-structure spectroscopy. Experimental data corresponding to physical-vapor-deposited and micelle-synthesized metal (Pt, Au) NPs supported on TiO2, SiO2 and Al2O3 will be used to illustrate Ostwald-ripening and diffusion coalescence processes. In addition, the role of the annealing environment (H2, O2, water vapor) on the stability of NPs will be discussed.


Correlating catalytic methanol oxidation with the structure and oxidation state of size-selected Pt nanoparticles, L. R. Merte, M. Ahmadi, F. Behafarid, L. K. Ono, E. Lira, J. Matos, L. Li, J.C. Yang, and B. Roldan Cuenya, ACS Catal. 3, 1460 (2013). [download]

We have investigated the structure and chemical state of size-selected platinum nanoparticles (NPs) prepared by micelle encapsulation and supported on γ-Al2O3 during the oxidation of methanol under oxygen-rich reaction conditions following both oxidative and reductive pretreatments. X-ray absorption near-edge structure (XANES) and extended x-ray absorption fine-structure (EXAFS) spectroscopy measurements reveal that in both cases, the catalyst is substantially oxidized under reaction conditions at room temperature and becomes partially reduced when the reactor temperature is raised to 50°C. Reactivity tests show that at low temperatures the pre-oxidized catalyst, where a larger degree of oxidation was observed, is more active than the pre-reduced catalyst. We conclude that the differences in reactivity can be linked to the formation and stabilization of distinct active oxide species during the pretreatment.


Eu-doped ZnO nanowire arrays grown by electrodeposition, O. Lupan, T. Pauporté, B. Viana, P. Aschehoug, M. Ahmadi, B. Roldan Cuenya, Y. Rudzevich, Y. Lin, L. Chow, Appl. Surf. Sci. 282, 782 (2013). [download]

The preparation of efficient light emitting diodes requires active optical layers working at low voltage for light emission. Trivalent lanthanide doped wide-bandgap semiconducting oxide nanostructures are promising active materials in opto-electronic devices. In this work we report on the electrochemical deposition (ECD) of Eu-doped ZnO (ZnO:Eu) nanowire arrays on glass substrates coated with F-doped polycrystalline SnO2. The structural, chemical and optical properties of ZnO:Eu nanowires have been systematically characterized by X-ray diffraction, transmission electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, secondary ion mass spectrometry, and photoluminescence...


Shape-selective bimetallic nanoparticle electrocatalysts: evolution of their atomic-scale structure, chemical composition, and electrochemical reactivity under various chemical environments, C.-H. Cui, M. Ahmadi, F. Behafarid, L. Gan, M. Neumann, M. Heggen, B. Roldan Cuenya, P. Strasser, Faraday Discuss. 162, 91 (2013). [download]

Solid surfaces generally respond sensitively to their environment. Gas phase or liquid phase species may adsorb and react with individual surface atoms altering the solid-gas and solid-liquid electronic and chemical properties of the interface. A comprehensive understanding of chemical and electrochemical interfaces with respect to their responses to external stimuli is still missing. The evolution of the structure and composition of shape-selected octahedral PtNi nanoparticles (NPs) in response to chemical (gas-phase) and electrochemical (liquid-phase) environments was studied, and contrasted to that of pure Pt and spherical PtNi NPs. The NPs were exposed to thermal annealing in hydrogen, oxygen, and vacuum, and the resulting NP surface composition was analyzed using X-ray photoelectron spectroscopy (XPS)...


Trends in binding strength of surface species on nanoparticles: how does the adsorption energy scale with the particle size?, M. Peter, J. M. Flores Camacho, S. Adamovski, L. K. Ono, K.-H. Dostert, C. P. O’Brien, B. Roldan Cuenya, S. Schauermann, H.-J. Freund, Angew. Chem. Int. Ed. 52, 1 (2013). [download]


Synthesis and characterization of Cu-doped ZnO one-dimensional structures for miniaturized sensor applications with faster response, L. Chow, O. Lupan, G. Chai, H. Khallaf, L.K. Ono, B. Roldan Cuenya, I.M. Tiginyanu, V.V. Ursaki, V. Sontea, A. Schulte, Sensors and Actuators A 189, 399 (2013). [download]

Detection of chemicals and biological species is an important issue to human health and safety. In this paper, we report the hydrothermal synthesis at 95 ◦C of Cu-doped ZnO low-dimensional rods for room-temperature (RT) sensing applications and enhanced sensor performances. X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, Raman and photoluminescence are used to characterize the material properties. To demonstrate the suitability of the Cu-doped ZnO rods for gas sensor applications and for comparison with pure ZnO, we fabricated a double rod device using Focused Ion Beam. The responses of pure-ZnO and Cu-doped ZnO rods studied in exactly the same condition are reported. We found that Cu-ZnO sensors have enhanced RT sensitivity, faster response time, and good selectivity. Miniaturized Cu-ZnO rod-based sensors can serve as a good candidate for effective H2 detectors with low power consumption.


Metal Nanoparticle Catalysts beginning to Shape-up, B. Roldan Cuenya, Acc. Chem. Res. 46, 1682 (2013). [download]

This Account provides examples of recent advances in the preparation and characterization of NP catalysts with well-defined shapes. It discusses how to resolve the shape of nanometer-sized catalysts via a combination of microscopy and spectroscopic approaches, and how to follow their evolution in the course of a chemical reaction. Finally, it highlights that for structure-sensitive reactions, controlled synthesis can tune catalytic properties such as the reaction rates, onset reaction temperature, activity, and selectivity.



2012

Stability of Pt Nanoparticles Supported on SiO2/Si(111): A High Pressure X-ray Photoelectron Spectroscopy Study, S. Porsgaard, L.R. Merte, L. K. Ono, F. Behafarid, J. Matos, S. Helveg, M. Salmeron, B. Roldan Cuenya, and F. Besenbacher, ACS Nano 6, 10743 (2012). [download]

The stability of Pt nanoparticles (NPs) supported on ultrathin SiO2 films on Si(111) was investigated in situ under H2 and O2 (0.5 Torr) by high pressure x-ray photoelectron spectroscopy (HP-XPS) and ex situ by atomic force microscopy (AFM). No indication of sintering was observed up to 600 °C in both reducing and oxidizing environments for size-selected Pt NPs synthesized by inverse micelle encapsulation. However, HP-XPS revealed a competing effect of volatile PtOx desorption from the Pt NPs (~2 nm and ~4 nm NP sizes) at temperatures above 450 °C in the presence of 0.5 Torr of O2. Under oxidizing conditions the entire NPs were oxidized, although with no indication of a PtO2 phase, with XPS binding energies better matching PtO. The stability of catalytic NPs in hydrogenation and oxidation reactions is of great importance due to the strong structure sensitivity observed in a number of catalytic processes of industrial relevance. An optimum must be found between the maximization of the surface active sites and metal loading (i.e. minimization of the NP size), combined with the maximization of their stability, which, as it will be shown here, is strongly dependent on the reaction environment.


Electrochemical oxidation of size-selected Pt nanoparticles studied using in situ high-energy-resolution x-ray absorption spectroscopy, L.R. Merte, F. Behafarid, D.J. Miller, D. Friebel, S. Cho, F. Mbuga, D. Sokaras, R. Alonso-Mori, T.-C. Weng, D. Nordlund, A. Nilsson, B. Roldan Cuenya, ACS Catal. 2, 2371(2012). [download]

High-energy-resolution fluorescence-detected x-ray absorption spectroscopy (HERFD-XAS) has been applied to study the chemical state of ~1.2 nm size-selected Pt nanoparticles (NPs) in an electrochemical environment under potential control. Spectral features due to chemisorbed hydrogen, chemisorbed O/OH, and platinum oxides can be distinguished with increasing potential. Pt electro-oxidation follows two competitive pathways involving both oxide formation and Pt dissolution.


Size-dependent evolution of the atomic vibrational density of states and thermodynamic properties of isolated Fe nanoparticles, B. Roldan Cuenya, L.K. Ono, J.R. Croy, K. Paredis, A. Kara, H. Heinrich, J. Zhao, E.E. Alp, A. T. DelaRiva, A. Datye, E. A. Stach, W. Keune, Phys. Rev. B 86, 165406(2012). [download]

We have gained insight into the internal degree of atomic disorder in isolated size-selected Fe nanoparticles (NPs) (~2 to 6 nm in size) supported on SiO2/Si(111) and Al2O3(0001) from precise measurements of the low-energy (low-E) part of the phonon density of states [PDOS, g(E)] via 57Fe nuclear resonant inelastic X-ray scattering (NRIXS), combined with transmission electron microscopy (TEM) measurements. An intriguing size-dependent trend was observed, namely, an increase of the low-E excess density of phonon states (as compared to the PDOS of bulk bcc Fe) with increasing NP size. This is unexpected, since usually the enhancement of the density of low-E phonon modes is attributed to low-coordinated atoms at the NP surface, whose relative content increases with decreasing NP size due to the increase in the surface-to-volume ratio. Our NPs are covered by a Ti coating layer, which essentially restores the local neighbourhood of surface Fe surface atoms towards bulk-like coordination, reducing the surface effect. Our data can be qualitatively explained by the existence of low-coordinated Fe atoms located at grain boundaries or other defects with structural disorder in the interior of the large NPs (~3 - 6 nm), while our small NPs (~2 nm) are single-grain and, therefore, characterized by a higher degree of structural order. This conclusion is corroborated by the observation of Debye behaviour at low energy [g(E) ~ En with n ~ 2] for the small NPs, but non-Debye behaviour (with n ~ 1.4) for the large NPs. The PDOS was used to determine thermodynamic properties of the Fe NPs. Finally, our results demonstrate that, in combination with TEM, NRIXS is a suitable technique to investigate atomic disorder/defects in NPs. We anticipate that our findings are universal for similar NPs with bcc structure.


Highly Sensitive and Selective Hydrogen Single-Nanowire Nanosensor, O. Lupan, L. Chow, Th. Pauporté, L.K. Ono, B. Roldan Cuenya, G. Chai, Sensors and Actuators B 173, 772 (2012). [download]

Metal oxides such as ZnO have been used as hydrogen sensors for a number of years. Through doping, the gas response of zinc oxide to hydrogen has been improved. Cadmium-doped ZnO nanowires (NWs) with high aspect ratio have been grown by electrodeposition. Single doped ZnO NWs have been isolated and contacted to form a nanodevice. Such nanosystem demonstrates an enhanced gas response for the detection of hydrogen at room temperature compared to previously reported H2 nanosensors based on pure single-ZnO NWs or multiple NWs. A dependence of the gas response of single Cd-ZnO nanowire on the NW diameter and Cd content was observed. It is shown that cadmium-doping in single-crystal zinc oxide NWs can be used to optimize their response to gases without the requirement of external heaters. The sensing mechanisms responsible for such improved response to hydrogen are discussed.


Electronic properties and charge transfer phenomena in Pt nanoparticles on γ-Al2O3: size, shape, support, and adsorbate effects, F. Behafarid, L. K. Ono, S. Mostafa, J. R. Croy, G. Shafai, S. Hong, T. S. Rahman, Simon R. Bare and B. Roldan Cuenya, Phys. Chem. Chem. Phys. 14, 11766 (2012). [download]

This study presents a systematic detailed experimental and theoretical investigation of the electronic properties of size-controlled free and g-Al2O3-supported Pt nanoparticles (NPs) and their evolution with decreasing NP size and adsorbate (H2) coverage. A combination of in situ X-ray absorption near-edge structure (XANES) and density functional theory (DFT) calculations revealed changes in the electronic characteristics of the NPs due to size, shape, NP–adsorbate (H2) and NP–support interactions. A correlation between the NP size, number of surface atoms and coordination of such atoms, and the maximum hydrogen coverage stabilized at a given temperature is established, with H/Pt ratios exceeding the 1 : 1 ratio previously reported for bulk Pt surfaces.


In situ coarsening study of inverse micelle prepared Pt nanoparticles supported on γ-Al2O3: pretreatment and environmental effects, J. Matos, L.K. Ono, F. Behafarid, J.R. Croy, S. Mostafa, A. T. DeLaRiva, A.K. Datye, A.I. Frenkel, B. Roldan Cuenya, Phys. Chem. Chem. Phys. 14, 11457 (2012). [download]

The thermal stability of inverse micelle prepared Pt nanoparticles (NPs) supported on nanocrystalline γ-Al2O3 was monitored in situ under different chemical environments (H2, O2, H2O) via extended x-ray absorption fine-structure spectroscopy (EXAFS) and ex situ via scanning transmission electron microscopy (STEM). Drastic differences in the stability of identically synthesized NP samples were observed upon exposure to two different pre-treatments. In particular, exposure to O2 at 400ºC before high temperature annealing in H2 (800ºC) was found to result in the stabilization of the inverse micelle prepared Pt NPs, reaching a maximum overall size after coarsening of ~1 nm. Interestingly, when an analogous sample was pre-treated in H2 at ~400ºC, a final size of ~5 nm was reached at 800ºC. The beneficial role of oxygen for the stabilization of small Pt NPs was also observed in situ during annealing treatments in O2 at 450°C for several hours. In particular, while NPs of 0.5 ± 0.1 nm initial average size did not display any significant sintering (0.6 ± 0.2 nm final size), an analogous thermal treatment in hydrogen lead to NP coarsening (1.2 ± 0.3 nm). The same sample pre-dosed and annealed in an atmosphere containing water only displayed moderate sintering (0.8 ± 0.3 nm). Our data suggest that PtOx species, possibly modifying the NP/support interface, play a role in the stabilization of small Pt NPs. Our study reveals the enhanced thermal stability of inverse micelle prepared Pt NPs and the importance of the sample pre-treatment and annealing environment in the minimization of undesired sintering processes affecting the catalytic performance of nanosized particles.


Nano Pinstripes: TiO2 Nanostripe Formation by Nanoparticle-Mediated Pinning of Step Edges, F. Behafarid, B. Roldan Cuenya, J. Phys. Chem. Lett. 3, 608 (2012). [download]

The present scanning tunneling microscopy study (STM) describes the high-temperature growth of TiO2 nanostripes with tunable width, orientation, and spacing, mediated by thermally stable micellar Pt and Au NPs deposited on TiO2(110). This phenomenon could not be explained by spillover effects, but is based on the preferential stabilization of step edges on TiO2(110) by the metal NPs. Contrary to the behavior of physical-vapor-deposited NPs, which are known to move toward step edges upon annealing, our micellar NPs remain immobile up to 1000 ºC. Instead, the mobility of TiO2 step edges towards the micellar NPs, where they become stabilized, is observed. Our findings are relevant to the technological application of nanostructured materials in the fields of catalysis, molecular electronics, and plasmonics.


Coarsening Phenomena of Metal Nanoparticles and the Influence of the Support Pre-treatment: Pt/TiO2(110), F. Behafarid, B. Roldan Cuenya, Surf. Sci. 606, 908 (2012). [download]

One of the technologically most important requirements for the application of oxide-supported metal nanoparticles (NPs) to the fields of molecular electronics, plasmonics, and catalysis is the achievement of thermally stable systems. For this purpose, a thorough understanding of the different pathways underlying thermally-driven coarsening phenomena, and the effect of the nanoparticle synthesis method, support morphology, and degree of support reduction on NP sintering is needed. In this study, the sintering of supported metal NPs has been monitored via scanning tunneling microscopy combined with simulations following the Ostwald ripening and diffusion-coalescence models. Modifications were introduced to the diffusion-coalescence model to incorporate the correct temperature dependence and energetics. Such methods were applied to describe coarsening phenomena of physical-vapor deposited (PVD) and micellar Pt NPs supported on TiO2(110). The TiO2(110) substrates were exposed to different pre-treatments, leading to reduced, oxidized and polymer-modified TiO2 surfaces. Such pre-treatments were found to affect the coarsening behavior of the NPs. No coarsening was observed for the micellar Pt NPs, maintaining their as-prepared size of ~3 nm after annealing in UHV at 1060ºC. Regardless of the initial substrate pre-treatment, the average size of the PVD-grown NPs was found to increase after identical thermal cycles, namely, from 0.5 ± 0.2 nm to 1.0 ± 0.3 nm for pristine TiO2, and from 0.8 ± 0.3 nm to 1.3 ± 0.6 nm for polymer-coated TiO2 after identical treatments. Although no direct real-time in situ microscopic evidence is available to determine the dominant coarsening mechanism of the PVD NPs unequivocally, our simulations following the diffusion-coalescence coarsening route were in significantly better agreement with the experimental data as compared to those based on the Ostwald-ripening model. The enhanced thermal stability of the micellar NPs as compared to the PVD clusters might be related to their initial larger NP size, narrower size distribution, and larger interparticle distances.



2011

Thermodynamic properties of Pt nanoparticles: size, shape, support, and adsorbate effects, B. Roldan Cuenya, M. Alcantara-Ortigoza, L.K. Ono, F. Behafarid, S. Mostafa, J.R. Croy, K. Paredis, G. Shafai, T.S. Rahman, L. Li, Z. Zhang, and J.C. Yang, Phys. Rev. B 84, 245438 (2011). [download]

This study presents a systematic investigation of the thermodynamic properties of free and γ-Al2O3-supported size-controlled Pt nanoparticles (NPs) and their evolution with decreasing NP size. A combination of in situ extended x-ray absorption fine-structure spectroscopy (EXAFS), ex situ transmission electron microscopy (TEM) measurements, and NP shape modeling revealed (i) a cross over from positive to negative thermal expansion with decreasing particle size, (ii) size- and shape-dependent changes in the mean square bond-projected bond-length fluctuations, and (iii) enhanced Debye temperatures (Θ_D, relative to bulk Pt) with a bimodal size-dependence for NPs in the size range of 0.8-5.4 nm. For large NP sizes (diameter d >1.5 nm) Θ_D was found to decrease toward Θ_D of bulk Pt with increasing NP size. For NPs ≤1 nm, a monotonic decrease of Θ_D was observed with decreasing NP size and increasing number of low-coordinated surface atoms. Our density functional theory calculations confirm the size- and shape-dependence of the vibrational properties of our smallest NPs and show how their behavior may be tuned by H desorption from the NPs. The experimental results can be partly attributed to thermally induced changes in the coverage of the adsorbate (H2) used during the EXAFS measurements, bearing in mind that the interaction of the Pt NPs with the stiff, high-melting temperature γ -Al2O3 support may also play a role. The calculations also provide good qualitative agreement with the trends in the mean square bond-projected bond-length fluctuations measured via EXAFS. Furthermore, they revealed that part of the Θ_D enhancement observed experimentally for the smallest NPs (d ≤1 nm) might be assigned to the specific sensitivity of EXAFS, which is intrinsically limited to bond-projected bond-length fluctuations.


Nanoepitaxy using Micellar Nanoparticles, F. Behafarid, B. Roldan Cuenya, Nano Lett. 11, 5290 (2011). [download]

The shape of platinum and gold nanoparticles (NPs)synthesized via inverse micelle encapsulation and supported on TiO2(110) has been resolved by scanning tunneling microscopy. Annealing these systems at high temperature (1000C) and subsequent cooling to room temperature produced ordered arrays of well-separated three-dimensional faceted NPs in their equilibrium state. The observed shapes differ from the kinetically limited shapes of conventional physical vapor deposited NPs, which normally form two-dimensional flat islands upon annealing at elevated temperatures. The initial NP volume was found to provide a means to control the final NP shape. Despite the liquid-phase ex situ synthesis of the micellar particles, the in situ removal of the encapsulating ligands and subsequent annealing consistently lead to the development of a well-defined epitaxial relationship of the metal NPs with the oxide support. The observed epitaxial relationships could be explained in terms of the best overlap between the interfacial Pt (or Au) and TiO2 lattices. In most cases, the ratio of {100}/{111} facets obtained for the NP shapes resolved clearly deviates from that of conventional bulklike Wulff structures.


Evolution of the Structure and Chemical State of Pd Nanoparticles during the in situ Catalytic Reduction of NO with H2, K. Paredis, L. K. Ono, F. Behafarid, Z. Zhang, J.C. Yang, A.I. Frenkel, B. Roldan Cuenya, J. Am. Chem. Soc. 133, 13455 (2011). [download]

In-depth understanding of the fundamental structure of catalysts during operation is indispensable for tailoring future efficient and selective catalysts. We report the evolution of the structure and oxidation state of ZrO2-supported Pd nanocatalysts during the in situ reduction of NO with H2 using x-ray absorption fine-structure spectroscopy and X-ray photoelectron spectroscopy. Prior to the onset of the reaction (≤ 120°C), a NO-induced redispersion of our initial metallic Pd nanoparticles over the ZrO2 support was observed, and Pdδ+ species detected. This process parallels the high production of N2O observed at the onset of the reaction (>120°C), while at higher temperatures (≥150°C) the selectivity shifts mainly toward N2 (~80 %). Concomitant with the onset of N2 production, the Pd atoms aggregate again into large (6.5 nm) metallic Pd nanoparticles, which were found to constitute the active phase for the H2-reduction of NO. Throughout the entire reaction cycle, the formation and stabilization of PdOx was not detected. Our results highlight the importance of in situ reactivity studies to unravel the microscopic processes governing catalytic reactivity.


Oxygen Chemisorption, Formation, and Thermal Stability of Pt Oxides on Pt Nanoparticles supported on SiO2/Si(001): Size-effects, L. K. Ono, J. R. Croy, H. Heinrich, B. Roldan Cuenya, J. Phys. Chem. C 115, 16856 (2011). [download]

The changes induced in the structure and chemical state of size-selected Pt nanoparticles (NPs) supported on ultrathin SiO2 films upon exposure to oxygen have been investigated by atomic force microscopy (AFM), transmission electron microscopy (TEM), in situ X-ray photoelectron spectroscopy (XPS), and temperature-programmed desorption (TPD). For low atomic oxygen exposures, chemisorbed oxygen species were detected on all samples. Exposure to higher atomic oxygen coverages at room temperature lead to the formation and stabilization of PtOx species (PtO2 and PtO). On all samples, a two-step thermal decomposition process was observed upon annealing in ultrahigh vacuum: PtO2 à PtO à Pt. For NPs in the 2-6 nm range, the NP size was found to affect the strength of the O-binding. Contrary to the case of Pt(111), where no oxides were detected above 700 K, 10-20 % PtO was detected on the NP samples via XPS at the same temperature, suggesting the presence of strongly bound oxygen species. In addition, for identical atomic oxygen exposures, decreasing the NP size was found to favor their ability to form oxides. Interestingly, regardless of whether the desorption of chemisorbed oxygen species or that of oxygen in PtOx species was considered, our TPD data revealed higher O2 desorption temperatures for the Pt NPs as compared to the Pt(111) surface. Furthermore, a clear size-dependent trend was observed, with an increase in the strength of the oxygen bonding with decreasing NP size.


Comparative study of hydrothermal treatment and thermal annealing effects on the properties of electrodeposited micro-columnar ZnO thin films, O. Lupan, T. Pauporté, I.M. Tiginyanu, V.V. Ursaki, V. Şontea, L.K. Ono, B. Roldan Cuenya, L. Chow, Thin Solid Films 519, 7738 (2011). [download]

We report a comparison of the role played by different sample treatments, namely, a low-temperature hydrothermal treatment by hot H2O vapor in an autoclave versus thermal annealing in air on the properties of ZnO films grown by electrochemical deposition (ECD). Scanning electron microscopy studies reveal a homogeneous micro-columnar morphology and changes in the film surface for the two different treatments. It is found that post-growth hydrothermal treatments of ECD ZnO films at 150 °C under an aqueous environment enhance their structural and optical properties (photoluminescence, transmission, Raman spectra, etc.) similar to thermal annealing in air at higher temperatures (200 °C). The modifications of the structural and optical properties of ZnO samples after thermal annealing in air in the temperature range of 150–600 °C are discussed. The removal of chlorine from the films by the hydrothermal treatment was evidenced which could be the main reason for the improvement of the film quality. The observation of the enhanced photoluminescence peak at 380 nm demonstrates the superior properties of the hydrothermally treated ZnO films as compared to the films annealed in air ambient at the same or higher temperature. This post-growth hydrothermal treatment would be useful for the realization of high performance optoelectronic devices on flexible supports which might not withstand at high temperature annealing treatments.


Structure, chemical composition, and reactivity correlations during the in situ oxidation of 2-propanol, K. Paredis, L. K. Ono, S. Mostafa, L. Li, Z. Zhang, J.C. Yang, L. Barrio, A.I. Frenkel, B. Roldan Cuenya, J. Am. Chem. Soc. 133, 6728 (2011). [download]

Unraveling the complex interaction between catalysts and reactants under operando conditions is a key step towards gaining fundamental insight in catalysis. We report the evolution of the structure and chemical composition of size-selected micellar Pt nanoparticles (~ 1 nm) supported on nanocrystalline γ-Al2O3 during the catalytic oxidation of 2-propanol using X-ray absorption fine-structure spectroscopy. Platinum oxides were found to be the active species for the partial oxidation of 2-propanol (<140°C), while the complete oxidation (>140°C) is initially catalyzed by oxygen-covered metallic Pt nanoparticles, which were found to re-grow a thin surface oxide layer above 200°C. The intermediate reaction regime, where the partial and complete oxidation pathways coexist, is characterized by the decomposition of the Pt oxide species due to the production of reducing intermediates and the blocking of O2 adsorption sites on the nanoparticle surface. The high catalytic activity and low onset reaction temperature displayed by our small Pt particles for the oxidation of 2-propanol is attributed to the large amount of edge and corner sites available, which facilitate the formation of reactive surface oxides. Our findings highlight the decisive role of the nanoparticle structure and chemical state in oxidation catalytic reactions.



2010

Formation and Thermal Stability of Platinum Oxides on Size-Selected Platinum Nanoparticles: Support Effects, L. K. Ono, B. Yuan, H. Heinrich, B. Roldan Cuenya, J. Phys. Chem. C 114, 22119 (2010). [download]

This article presents a systematic study of the formation and thermal stability of Pt oxide species on size-selected Pt nanoparticles (NPs) supported on SiO2, ZrO2 and TiO2 thin films. The studies were carried out in ultrahigh vacuum (UHV) by temperature-dependent X-ray photoelectron spectroscopy (XPS) measurements and ex-situ transmission electron microscopy (TEM) and atomic force microscopy (AFM).The NPs were synthesized by inverse micelle encapsulation and oxidized in UHV at room temperature by an oxygen plasma treatment. For a given particle size distribution, the role played by the NP support on the stability of Pt oxides was analyzed. PtO2 species are formed on all supports investigated after O2-plasma exposure. A two-step thermal decomposition (PtO2 to PtO to Pt) is observed from 300 K to 600 K upon annealing in UHV. The stability of oxidized Pt species was found to be enhanced on ZrO2 under annealing treatments in O2. Strong NP/support interactions and the formation of Pt-Ti-O alloys are detected for Pt/TiO2 upon annealing in UHV above 550 K but not under an identical treatment in O2. Furthermore, thermal treatments in both environments above 700 K lead to the encapsulation of Pt by TiOx. The final shape of the micellar Pt NPs is influenced by the type of underlying support as well as by the post-deposition treatment. Spherical Pt NPs are stable on SiO2, ZrO2, and TiO2 after in-situ ligand removal with atomic oxygen at RT. However, annealing in UHV at 1000 K leads to NP flattening on ZrO2 and to the diffusion of Pt NPs into TiO2. The stronger the nature of the NP/support interaction, the more dramatic is the change in the NP shape (TiO2 > ZrO2 > SiO2).


Shape-dependent Catalytic Properties of Pt Nanoparticles, S. Mostafa, F. Behafarid, J. R. Croy, L. K. Ono, L. Li, J. C. Yang, A. I. Frenkel, B. Roldan Cuenya, J. Am. Chem. Soc. 132, 15714 (2010). [download]

Tailoring the chemical reactivity of nanomaterials at the atomic level is one of the most important challenges in catalysis research. In order to achieve this elusive goal, fundamental understanding of the geometric and electronic structure of these complex systems at the atomic level must be obtained. This article reports the influence of the nanoparticle shape on the reactivity of Pt nanocatalysts supported on γ-Al2O3. Nanoparticles with analogous average size distributions (~0.8-1 nm), but with different shapes, synthesized by inverse micelle encapsulation, were found to display distinct reactivities for the oxidation of 2-propanol. A correlation between the number of undercoordinated atoms at the nanoparticle surface and the onset temperature for 2-propanol oxidation was observed, demonstrating that catalytic properties can be controlled through shape-selective synthesis.


Anomalous lattice dynamics and thermal properties of supported, size-and shape selected Pt nanoparticles, B. Roldan Cuenya, A.I. Frenkel, S. Mostafa, F. Behafarid, J. R. Croy, L. K. Ono, Q. Wang, Phys. Rev. B  82, 155450 (2010). [download]

Anomalous lattice dynamics and thermal behavior have been observed for ligand-free, size-, and shape-selected Pt nanoparticles (NPs) supported on nanocrystalline γ- Al2O3 via extended X-ray absorption fine-structure spectroscopy. Several major differences were observed for the NPs with respect to bulk Pt: (i) a contraction in the interatomic distances, (ii) a reduction in the dynamic (temperature-dependent) bond length disorder and associated increase in the Debye temperature, and (iii) an overall decrease in the bond length expansion coefficient coupled with NP stiffening. The increase of the Debye temperature is explained in terms of the NP size, shape, support interactions and adsorbate effects. For a similar average size, we observe a striking correlation between the shapes of the NPs and their Debye temperature values.


Synthesis and characterization of Ag- or Sb-doped ZnO Nanorods by a facile hydrothermal route, O. Lupan, L. Chow, L. K. Ono, B. Roldan Cuenya, G. Chai, H. Khallaf, S. Park, A. Schulte, J. Phys. Chem. C 114, 12401 (2010). [download]

ZnO nanorods doped with Ag and Sb have been synthesized by a facile hydrothermal technique. Crystal quality, morphology, chemical/electronic composition, local structure, and vibrational properties are investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and micro-Raman spectroscopy. Evidence of dopant incorporation is demonstrated in the XPS measurements of both Sb-doped and Ag-doped ZnO nanorods. From XRD data, it was found that the doped ZnO nanorods have a lower degree of crystallinity. The lattice constants of doped ZnO nanorods were slightly larger than that of the pure samples.


Solving the structure of size-selected Pt nanocatalysts synthesized by inverse micelle encapsulation, B. Roldan Cuenya, J. R. Croy, S. Mostafa, F. Behafarid, L. Li, Z. Zhang, J. C. Yang, Q. Wang, A. I. Frenkel, J. Am. Chem. Soc. 132, 8747 (2010). [download]

The structure, size, and shape of γ-Al2O3-suported Pt nanoparticles (NPs) synthesized by inverse micelle encapsulation have been resolved via a synergistic combination of imaging and spectroscopic tools. It is shown that this synthesis method leads to 3D NP shapes even for sub-nanometer clusters, in contrast to the raft-like structures obtained for the same systems via traditional deposition-precipitation methods. Furthermore, a high degree of atomic ordering is observed for the micellar NPs in H2 atmosphere at all sizes studied, possibly due to H-induced surface reconstruction in these high surface area clusters. Our findings demonstrate that the influence of NP/support interactions on NP structure can be diminished in favor of NP/adsorbate interactions when NP catalysts are prepared by micelle encapsulation methods.


Synthesis and characterization of ZnO nanowires for nanosensor applications, O. Lupan, G.A. Emelchenko, V.V. Ursaki, G. Chai, A.N. Redkin, A.N. Gruzintsev, I.M. Tiginyanu, L. Chow, L.K. Ono, B. Roldan Cuenya, H. Heinrich, and E.E. Yakimov, Mater. Res. Bull. 45, 1026 (2010). [download]

In this paper we report the synthesis of ZnO nanowires via chemical vapor deposition (CVD) at 650 °C. It will be shown that these nanowires are suitable for sensing applications. ZnO nanowires were grown with diameters ranging from 50 to 200 nm depending on the substrate position in a CVD synthesis reactor and the growth regimes. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL), and Raman spectroscopy have been used to characterize the ZnO nanowires. To investigate the suitability of the CVD synthesized ZnO nanowires for gas sensing applications, a single ZnO nanowire device (50 nm in diameter) was fabricated using a focused ion beam (FIB). The response to H2 of a gas nanosensor based on an individual ZnO nanowire is also reported.


Synthesis and catalytic properties of metal nanoparticles: Size, shape, support, composition, and oxidation state effects, B. Roldan Cuenya, Thin Solid Films 518, 3127 (2010). [download]

Exciting new opportunities are emerging in the field of catalysis based on nanotechnology approaches. A new understanding and mastery of catalysis could have broad societal impacts, since about 80% of the processes in the chemical industry depend on catalysts to work efficiently. Efforts in surface science have led to the discovery of new heterogeneous catalysts, however, until recently the only way to develop new or improved catalysts was by empirical testing in trial-and-error experiments. This time-consuming and costly procedure is now rapidly being replaced by rational design methods that utilize fundamental knowledge of catalysts at the nanoscale. The advent of nanoscience and nanotechnology is providing the ability to create controlled structures and geometries to investigate and optimize a broad range of catalytic processes. As a result, researchers are obtaining fundamental insight into key features that influence the activity, selectivity, and lifetime of nanocatalysts. This review article examines several new findings as well as current challenges in the field of nanoparticle based catalysis, including the role played by the particle structure and morphology (size and shape), its chemical composition and oxidation state, and the effect of the cluster support.


Effects of Annealing on Properties of ZnO Thin Films Prepared by Electrochemical Deposition in Chloride Medium, O. Lupan, T. Pauporte, L. Chow, B. Viana, F. Pelle, L.K. Ono, B. Roldan Cuenya, H. Heinrich, Appl. Surf. Sci. 256, 1895 (2010). [download]

The development of cost-effective and low-temperature synthesis techniques for the growth of high quality zinc oxide thin films is paramount for fabrication of ZnO-based optoelectronic devices, especially ultraviolet (UV)-light emitting diodes, lasers and detectors. We demonstrate that the properties, especially UV emission observed at room temperature of electrodeposited ZnO thin from chloride medium (at 70C) on fluor-doped tin oxide (FTO) substrates are strongly influenced by the post-growth thermal annealing treatments. X-ray diffraction (XRD) measurements show that the films have preferably grown along (002) direction. Thermal annealing in the temperature range of 150-400C in air has been carried out for these ZnO thin films. The as-grown films contain chlorine which is partially removed after annealing at 400C. Morphological changes upon annealing are discussed in the light of compositional changes observed in the ZnO crystals that constitute the film. The optical quality of ZnO thin films was improved after post-deposition thermal treatment at 150C and 400C in our experiments due to the reducing of defects levels and of chlorine content. The transmission and absorption spectra become steeper and the optical band-gap red shifted to the single-crystal value. These findings demonstrate that electrodeposition has potential for the growth of high-quality ZnO thin films with reduced defects for device applications.



2009

Excitation wavelength-independent sensitized Er³⁺ concentration in as-deposited and low temperature annealed Si-rich SiO₂ films, O. Savchyn, R.M. Todi, K.R. Coffey , L.K. Ono, B. Roldan Cuenya, and P. G. Kik, Appl. Phys. Lett. 95, 231109 (2009). [download]

Erbium sensitization is observed in as-deposited Er3+ doped Si-rich SiO2, ruling out the involvement of Si nanocrystals in the Er3+ excitation in these samples. The Er3+ absorption cross section in this material is similar within a factor 3 to that of samples annealed at 600oC under 355 nm and 532 nm excitation. The density of excitable Er3+ ions is shown to be excitation wavelength independent, while the shape of the Er3+ excitation spectra is governed by a wavelength-dependent Er3+ absorption cross section. These findings enable the use of a broad range of wavelengths for the efficient excitation of this gain medium.


Structure and Phonon Density of States of Supported Size-Selected ⁵⁷FeAu Nanoclusters: a Nuclear Resonant Inelastic X-ray Scattering Study, B. Roldan Cuenya, L.K. Ono, J.R. Croy, A. Naitabdi, H. Heinrich, J. Zhao, E.E. Alp, W. Sturhahn, W. Keune, Appl. Phys. Lett. 95, 143103 (2009). [download]

We have measured the phonon density of states (PDOS) of isolated bcc and fcc FexAu1-x alloy nanoclusters (NCs) by nuclear resonant inelastic X-ray scattering. Drastic deviations were observed with respect to the PDOS of bulk Fe-Au alloys. Important information on the structure and thermodynamic properties of these NCs was obtained.


Phonon density of states of self-assembled isolated Fe-rich Fe-Pt alloy nanoclusters, B. Roldan Cuenya, Jason R. Croy, L.K. Ono, A. Naitabdi, H. Heinrich, W. Keune, J. Zhao, W. Sturhahn, E.E. Alp, M. Hu, Phys. Rev. B 80, 125412 (2009). [download]

The Fe-projected phonon density of states (PDOS) of monolayer-thick films of self-assembled, size-selected, isolated 57Fe1-xPtx alloy nanoclusters (NCs) (height: ~2 nm, diameter: ~8 nm) supported on flat SiO2/Si(111) substrates has been measured by nuclear resonant inelastic X-ray scattering. The samples were characterized by transmission electron microscopy and X-ray photoelectron spectroscopy (XPS). Surface segregation of Pt and PtSi formation at the NC surface due to the sample preparation process is inferred from the XPS data. As compared to the bulk, pronounced modifications of the PDOS beyond the bulk cut-off energy are observed in bcc 57Fe0.8Pt0.2(core)/PtSi(shell) NCs. By contrast, the PDOS of fcc 57Fe0.75Pt0.25(core)/PtSi(shell) NCs retains features of bulk ordered Fe3Pt Invar alloys (presumably due to a thicker PtSi shell), in particular the transverse-acoustic [110] TA1 mode near 9 meV. Apparently, this mode is not affected by size effects. The existence of the [110] TA1 phonon mode is a pre-requisite for the persistence of Invar-related effects in Fe3Pt NCs. Important thermodynamic properties of the NCs are derived, such as the vibrational entropy per atom.


Catalytic Decomposition of Alcohols over Size-Selected Pt Nanoparticles Supported on ZrO2: a Study of Activity, Selectivity and Stability, S. Mostafa, J. R. Croy, H. Heinrich, B. Roldan Cuenya, Appl. Catal. A 366, 353 (2009). [download]

This article discusses the performance of ZrO2-supported size-selected Pt nanoparticles for the decomposition of methanol, ethanol, 2-propanol, and 2-butanol. The potential of each alcohol for H2 production and other relevant products in the presence of a catalyst is studied in a packed-bed mass flow reactor operating at atmospheric pressure. All the alcohols studied show some decomposition activity below 200°C which increased with increasing temperature. In all cases, high selectivity towards H2 formation is observed. With the exception of methanol, all alcohol conversion reactions lead to catalyst deactivation at high temperatures (T >250°C for 2-propanol and 2-butanol, T >325°C for ethanol) due to carbon poisoning. However, long-term catalyst deactivation can be avoided by optimizing reaction conditions such as operating temperature.


Size-selected Pt Nanoparticles Synthesized via Micelle Encapsulation: Effect of Pretreatment and Oxidation State on the Activity for Methanol Decomposition and Oxidation, J.R. Croy, S. Mostafa, H. Heinrich, B. Roldan Cuenya, Catal. Lett. 131, 21 (2009). [download]

The effect of pretreatment conditions on the oxidation state and activity of micelle-synthesized Pt nanoparticles supported on ZrO2 was studied for methanol decomposition and oxidation reactions. An O2-pretreatment is observed to be effective for producing clean, stable, and active nanoparticles. Pt-oxide species formed during such pretreatments were found to have little influence in methanol decomposition reactions due to their tendency to reduce. However, these same species are stable during methanol oxidation and appear to take part in a Mars-van Krevelen-type of process, in which bound-oxygen (nanoparticle shell) may be replenished with oxygen from the gas phase.


Enhanced Thermal Stability and Nanoparticle-mediated Surface Patterning: Pt/TiO₂(110), A. Naitabdi, F. Behafarid, B. Roldan Cuenya, Appl. Phys. Lett. 94, 083102 (2009). [download]

This letter reports (i) the enhanced thermal stability (up to 1060C) against coarsening and/or desorption of self-assembled Pt nanoparticles synthesized by inverse micelle encapsulation and deposited on TiO2(110), and (ii) the possibility of taking advantage of the strong nanoparticle/support interactions present in this system to create patterned surfaces at the nanoscale. Following our approach, TiO2 nanostripes with tunable width, orientation, and uniform arrangement over large surface areas were produced.


Thermal Stability and Segregation Processes in Self-Assembled Size-Selected Au_xFe_1-x Nanoparticles deposited on TiO₂(110): Composition Effects, A. Naitabdi, L. K. Ono, F. Behafarid, B. Roldan Cuenya, J. Phys. Chem. C 113, 1433 (2009). [download]

In-situ scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) measurements have been performed to investigate the formation and thermal stability of mono- and bimetallic AuxFe1-x (x = 1, 0.8, 0.5, 0.2, 0) nanoparticles (NPs) supported on TiO2(110). Nearly hexagonal arrangements of size-selected Au, Fe, and Au-Fe NPs with well-defined interparticle distances have been achieved by diblock-copolymer encapsulation. Upon step-wise annealing from 300°C to 1060°C, a remarkable thermal stability of the Au-Fe NPs was observed, maintaining their original spatial arrangement on the TiO2 surface up to 900°C. A majority phase of a gold-iron alloy (solid solution) was achieved for our Au0.5Fe0.5 NPs in the temperature range of 700°C – 800°C, and for Au0.2Fe0.8 NPs at 800°C, while a phase mixture of bcc Fe and Au-Fe alloy was observed for the Au0.8Fe0.2 system at 800°C-900°C. For all samples the segregation of Au atoms towards the NP surface was detected upon high temperature annealing (800°C) in vacuum. Nearly complete Au desorption was observed by XPS at 900°C for Au0.2Fe0.8 NPs, at 1000°C for Au0.5Fe0.5 NPs, and at 1060°C for Au0.8Fe0.2 NPs. The enhanced thermal stability of Au in the Au0.8Fe0.2 NPs is believed to be related to the formation of core(Fe)/shell(Au) structures. Furthermore, contrary to the case of pure Fe or Fe-rich NPs where nearly complete Fe desorption or Fe diffusion into TiO2 was observed at 1000°C, an Fe signal was detected at this temperature for the Au-rich samples (Au0.8Fe0.2 and Au0.5Fe0.5).


Nanostructured zinc oxide films synthesized by successive chemical solution deposition for gas sensor applications, O. Lupan, L. Chow, S. Shishiyanu, E. Monaico, T. Shishiyanu, V. Sontea, B. Roldan Cuenya, A. Naitabdi, S. Park, A. Schulte , Mater. Research Bull. 44, 63 (2009). [download]

Nanostructured ZnO thin films have been deposited using a successive chemical solution deposition method. The structural, morphological, electrical and sensing properties of the films were studied for different concentrations of Al-dopant and were analyzed as a function of rapid photothermal processing temperatures. The films were investigated by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray photoelectron and micro-Raman spectroscopy. Electrical and gas sensitivity measurements were conducted as well. The average grain size is 240 and 224 Å for undoped ZnO and Al-doped ZnO films, respectively. We demonstrate that rapid photothermal processing is an efficient method for improving the quality of nanostructured ZnO films. Nanostructured ZnO films doped with Al showed a higher sensitivity to carbon dioxide than undoped ZnO films. The correlations between material compositions, microstructures of the films and the properties of the gas sensors are discussed.



2008

Size Effects on the Desorption of O₂ from Au₂O₃/Au⁰ Nanoparticles Supported on SiO₂: a TPD Study, L.K. Ono, B. Roldan Cuenya, J. Phys. Chem. C 112, 18543 (2008). [download]

This article contains an in-depth study of the kinetics of O2-desorption from pre-oxidized size-selected Au nanoparticles (NPs) of average size ~1.5 nm and ~5 nm synthesized by micelle encapsulation and supported on SiO2. Thanks to the narrow NP size distributions achieved using our preparation route, information on the effect of the nanoparticle size on the reaction kinetics could be obtained by temperature programmed desorption (TPD). In addition, the validity of different TPD analysis methods (Redhead, heating rate variation, Arrhenius plots, and the complete analysis) to extract meaningful kinetic parameters on supported NPs is discussed. From the different analysis methods, activation energies for O2 desorption of Ed = 1.3 ± 0.2 eV and 1.7 ± 0.1 eV were obtained for the ~5 nm and ~1.5 nm Au NPs, respectively. These differences in the activation energies are directly related to the nanoparticle size, and not to nanoparticle-support interactions.


Bimetallic Pt/Metal catalysts for the decomposition of methanol: Effect of secondary metal on the oxidation state, activity, and selectivity of Pt, J. R. Croy, S. Mostafa, L. Hickman, H. Heinrich, B. Roldan Cuenya, Appl. Catal. A 350, 207 (2008). [download]

We present here a study of methanol (MeOH) decomposition over a series of bimetallic Pt-M catalysts, with M = Au, Pd, Ru, Fe. All samples have the same initial size distribution (~ 3 nm nanoparticle height), support (ZrO2), and preparation conditions. Therefore, differences in the electronic and catalytic properties of the samples tested are related directly to the addition of the secondary metals (M). We find that the oxidation state as well as the activity of Pt is heavily influenced by the addition of the secondary metal. PtO is found to be highly stable in these systems and increasing concentrations of metallic Pt are associated with the surface segregation of metal M due to its affinity for the oxygen present during air annealing.


High-Energy Phonon Confinement in Nanoscale Metallic Multilayers, B. Roldan Cuenya, W. Keune, R. Peters, E. Schuster, B. Sahoo, U. von Hörsten, W. Sturhahn, J. Zhao, T.S. Toellner, E.E. Alp, and S.D. Bader, Phys. Rev. B 77, 165410 (2008). [download]

The Fe-projected vibrational density of states g(E) in nanoscale 57Fe/M multilayers, where M = Cr, Co, Cu, Pd or Ag, was measured by nuclear resonant inelastic X-ray scattering. With decreasing Fe thickness the high-energy phonon peak of Fe near 36 meV is suppressed for the “soft” metals Ag, Pd and Cu, but much less so for the “hard” metals Co and Cr. This effect is attributed to Fe phonon confinement and interface localization due to energy mismatch between g(E) of M and of Fe.


Formation and Thermal Stability of Au₂O₃ on Size-Selected Gold Nanoparticles: Size and Support Effects, L. K. Ono, B. Roldan Cuenya, J. Phys. Chem. C 112, 4676 (2008). [download]

Gold nanoparticles with two different size distributions (average sizes of ~1.5 and ~5 nm) have been synthesized by inverse micelle encapsulation and deposited on reducible (TiO2) and non-reducible (SiO2) supports. The thermal and chemical stability of oxidized gold species formed upon cluster exposure to atomic oxygen has been investigated in ultrahigh vacuum using a combination of temperature-, time- and CO dosing-dependent X-ray photoelectron spectroscopy (XPS), as well as temperature programmed desorption (TPD). Our work demonstrates that (a) low temperature (150 K) exposure to atomic oxygen leads to the formation of surface, as well as sub-surface gold oxide on Au nanoparticles, (b) the presence of the reducible TiO2 substrate leads to a lower gold oxide stability compared to that on SiO2, possibly due to a TiO2 oxygen vacancy-mediated decomposition process, (c) heating to 550 K (Au/SiO2) and 300 K (Au/TiO2) leads to a near-complete reduction of small (~1.5 nm) NPs while a partial reduction is observed for larger clusters (~5 nm), and (d) the desorption temperature of O2 from pre-oxidized Au clusters deposited on SiO2 depends on the cluster size, with smaller clusters showing stronger O2 binding.


Nanostructures: Sensor and catalytic properties, B. Roldan Cuenya, A. Kolmakov, in "Functional Nanostructures: Processing, Characterization, and Applications", ed. S. Seal, Chapter 6, p. 305 (Springer Science, New York, 2008)



2007

Atomic Vibrations in Iron Nanoclusters: Nuclear Resonant Inelastic X-ray Scattering and Molecular Dynamics Simulations , B. Roldan Cuenya, A. Naitabdi, J. Croy, W. Sturhahn, J.Y. Zhao, E.E. Alp, R. Meyer, D. Sudfeld, E. Schuster, W. Keune, Phys. Rev. B 76, 195422 (2007) [download]

The lattice vibrational dynamics of supported, self-assembled, isolated 57Fe nanoclusters was studied by nuclear resonant inelastic X-ray scattering and molecular dynamics calculations. The morphological and structural properties and the chemical state of the experimental nanoclusters were investigated by atomic force microscopy, high resolution transmission electron microscopy and X-ray photoelectron spectroscopy. The measured and calculated vibrational density of states (VDOS) reveal an enhancement of the low- and high-energy phonon modes and provide experimental and theoretical proof of non-Debye-like behavior in the low-energy region of the VDOS. Experimentally, this effect was found to depend on the nature of the surface shell (oxide or carbide) of the core/shell nanoclusters. According to the calculations for supported isolated pure Fe nanoclusters, the non-Debye-like behavior appears not only in the surface shell, but also in the bcc-Fe core of the nanocluster due to the hybridization of surface and bulk modes.


Nanofabrication and Characterization of ZnO Nanorod Arrays and Branched Microrods by Aqueous Solution Route and Rapid Thermal Processing, O. Lupan, L. Chow, G. Chai, B. Roldan Cuenya, A. Naitabdi, A. Schulte, H. Heinrich, Materials Science and Engineering B 145, 57 (2007) [download]

This paper presents an inexpensive and fast fabrication method for one-dimensional (1D) ZnO nanorod arrays and branched two-dimensional (2D), three-dimensional (3D) – nanoarchitectures. Our synthesis technique includes the use of an aqueous solution route and post-growth rapid thermal annealing. It permits rapid and controlled growth of ZnO nanorod arrays of 1D – rods, 2D – crosses, and 3D – tetrapods without the use of templates or seeds. The obtained ZnO nanorods are uniformly distributed on the surface of Si substrates and individual or branched nano/microrods can be easily transferred to other substrates. Process parameters such as concentration, temperature and time, type of substrate and the reactor design are critical for the formation of nanorod arrays with thin diameter and transferable nanoarchitectures. X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, transmission electron microscopy and Micro-Raman spectroscopy have been used to characterize the samples.


Formation, Thermal Stability and Surface Composition of Size-Selected AuFe Nanoparticles, A. Naitabdi, B. Roldan Cuenya, Appl. Phys. Lett. 91, 113110 (2007) [download]

The surface composition of isolated Au0.5Fe0.5 nanoparticles (NPs) synthesized by micelle encapsulation and supported on TiO2(110) has been investigated. Our study reveals that phase-segregated structures are present after annealing at 300°C. A subsequent thermal treatment at 700°C resulted in the formation of an AuFe alloy. At this temperature, a state characteristic of Fe was identified at the NPs’ surface. Annealing at 900°C resulted in the disappearance of the Fe surface state, which is attributed to Au segregation to the surface. The initial hexagonal NP arrangement on the TiO2(110) surface was preserved up to 900°C. At 1000°C, Au desorption was observed.


Support Dependence of MeOH Decomposition over Size-Selected Pt Nanoparticles, J. R. Croy, S. Mostafa, J. Liu, Yongho Sohn, H. Heinrich, B. Roldan Cuenya, Catal. Lett. 119, 209 (2007) [download]

We present here the decomposition of methanol over Pt nanoparticles supported on a series of oxide powders. The samples tested may be roughly grouped in two categories consisting of large (~ 15-18 nm) and small (~ 8-9 nm) Pt particles deposited on reducible (CeO2, TiO2) and non-reducible (SiO2, ZrO2, Al2O3) supports. The smallest particles (~ 8 nm), deposited on ZrO2, were found to be cationic and the most active for the decomposition of methanol. Furthermore, the stability of metallic Pt and its oxides was observed to be dependent on the choice of support. In all Pt containing samples the reaction proceeds via the direct decomposition of methanol, as no significant amounts of by-products were detected in the experimental range of 100 - 300°C.


Epitaxial growth, magnetic properties and lattice dynamics of Fe nanoclusters on GaAs(001), B. Roldan Cuenya, A. Naitabdi, E. Schuster, R. Peters, M. Doi, W. Keune, Phys. Rev. B 76, 094403 (2007) [download]

Epitaxial bcc-Fe(001) ultrathin films have been grown at 50 C on reconstructed GaAs(001)-(4x6) surfaces and investigated in-situ in ultrahigh vacuum (UHV) by reflection high-energy electron diffraction (RHEED), scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and ⁵⁷Fe conversion electron Mössbauer spectroscopy (CEMS). For t_Fe = 1 monolayer (ML) Fe coverage, isolated Fe nanoclusters are arranged in rows along the [110] direction. With increasing t_Fe, the Fe clusters first connect along the [-110], but not along the [110] direction at 2.5 ML, then consist of percolated Fe clusters without preferential orientation at 3 ML, and finally form a nearly smooth film at 4 ML coverage. Segregation of Ga atoms within the film and on the Fe surface appears to occur at t_Fe = 4 ML, as evidenced by XPS. For coverages below the magnetic percolation, temperature-dependent in-situ CEMS measurements in zero external field provided superparamagnetic blocking temperatures, T_B, of 62 K, 80 K, and 165 K for t_Fe = 1.9, 2.2, and 2.5 ML, respectively. At T < T_B, freezing of superparamagnetic clusters is inferred from the observed quasi-linear T-dependence of the mean hyperfine magnetic field, . By combining the STM and CEMS results, we have determined a large magnetic anisotropy constant of 5 x 10⁵ J/m³ and 8 x 10⁵ J/m³ at t_Fe = 1.9 - 2.2 ML and 2.5 ML, respectively. For t_Fe = 2.5 ML, our uncoated free Fe clusters exhibit intrinsic magnetic ordering below T_B, contrary to literature reports on metal-coated Fe clusters on GaAs. Our present results demonstrate that the nature of the percolation transition for free Fe nanoclusters on GaAs(001) in UHV is from superparamagnetism to ferromagnetism. From the Mössbauer spectral area a very low Debye temperature, Theta_D, of 196 K is deduced for these uncoated Fe nanoclusters in UHV, indicating strong phonon softening in the clusters.


Size dependent study of MeOH decomposition over size-selected Pt nanoparticles synthesized via micelle encapsulation, J. R. Croy, S. Mostafa, Jing Liu, Yong-ho Sohn, B. Roldan Cuenya, Catal. Lett. 118, 1 (2007) [download]

We present here the size-dependent decomposition of methanol (MeOH) over narrowly distributed Pt nanoparticles supported on nanocrystalline anatase TiO2 powder. Micelle encapsulation has been used to create Pt catalysts with average particle sizes of ~ 4, 6, and 8 nm. A packed bed mass flow reactor and mass spectrometry were employed to quantify the catalyst’s activity and selectivity. Among the catalysts tested the smallest nanoparticles showed the best performance including an onset reaction temperature of ~145°C. No byproducts such as CO2 or CH4 were observed in the test range of 100 to 330°C.


Effect of interparticle interaction on the low temperature oxidation of CO over size-selected Au nanocatalysts supported on ultrathin TiC films, L. K. Ono, B. Roldan Cuenya, Catal. Lett. 113, 86 (2007) [download]

This work aims to get insight into the influence of interparticle interactions on catalysis. The low temperature CO oxidation is used as a model reaction. A strong dependence of the catalytic activity and stability of gold nanoparticles uniformly dispersed on polycrystalline TiC films was observed as a function of the interparticle distance. Two samples with similar height distributions (~2 nm), but with different average interparticle distances (~30 nm and ~80 nm), were synthesized using diblock copolymer encapsulation. Their chemical reactivity was investigated by temperature programmed desorption (TPD), and reactive coarsening and subsequent deactivation was observed for the sample with the smallest interparticle distance. The system with the largest average interparticle distance showed higher stability towards agglomeration and longer lifetime.



2006

In-situ gas-phase catalytic properties of TiC-supported size-selected gold nanoparticles synthesized by diblock copolymer encapsulation, L. K. Ono, D. Sudfeld, B. Roldan Cuenya, Surf. Sci. 600, 5041 (2006). [download]

TiC-supported size- and shape-selected Au nanoparticles with well defined interparticle distances were synthesized by diblock copolymer encapsulation. Atomic force microscopy (AFM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD) have been used to investigate the correlation between the nanocatalyst morphological/electronic structure and its chemical reactivity. Using the low temperature oxidation of CO as a model reaction, our TPD results showed an enhancement of the catalytic activity with decreasing particle size. Two desorption features were observed and assigned to kinks/steps in the gold surface and the Au-TiC interface. The role of the interparticle distance on the activity is discussed. AFM measurements showed drastic morphological changes (Ostwald ripening) on the nanoparticles after CO oxidation when the initial interparticle distance was small (~30 nm). However, no sintering was observed for Au nanoparticles more widely spaced (~80 nm).


Local investigation of the electronic properties of size-selected Au nanoparticles by scanning tunneling spectroscopy, A. Naitabdi, L. K. Ono, B. Roldan Cuenya, Appl. Phys. Lett. 89, 043101 (2006) [download]

The relationship between the structural/morphological and electronic properties of size-selected gold nanoparticles was investigated using scanning tunneling microscopy and spectroscopy. The nanoparticles were synthesized by inverse micelle encapsulation and were dip-coated on TiO2/Ti(15nm)/Si(111). Annealing in vacuum to 500°C resulted in the removal of the polymer and the formation of an ultrathin TiC support. Significant changes in the electronic local density of states (LDOS) of the nanoparticles, in particular, the onset of non-metallic behavior, were observed with decreasing particle size. The nanoparticle-support interactions were studied and evidence for substrate-induced modifications in the LDOS of interfacial gold atoms is found.


Chalcogenide waveguide structures as substrates and guiding layers for evanescent wave Raman spectroscopy of bacteriorhodopsin, A. Pope, A. Schulte, Y. Guo, L. K. Ono, B. Roldan Cuenya, C. Lopez, K. Richardson, K. Kitanovski, T. Winningham, Vibrational Spectroscopy 42, 249 (2006) [download]

We investigate composite structures where a chalcogenide waveguide provides the substrate and the guiding layer for a bacteriorhodopsin film whose Raman spectrum is desired. Plasma exposure of the chalcogenide substrate allows for a hydrophilic surface to which organic layers can easily adhere. Surface morphology, structure and chemical composition are characterized by X-ray photoelectron spectroscopy. A 785 nm beam from a Ti:sapphire layer was coupled into the As2S3 waveguide and the evanescent field was employed to excite Raman scattering in the bacteriorhodopsin film. The Raman spectrum of the bacteriorhodopsin film in the fingerprint region is indicative of the native conformation and it shows that the protein is in the light-adapted state.


Thermal stability, atomic vibrational dynamics and superheating of confined interfacial Sn layers in Sn/Si multilayers, B. Roldan Cuenya, W. Keune, W.A. Adeagbo and P. Entel, Phys. Rev. B 73, 045311 (2006) [download]

Multilayers composed of materials with low (Sn) and high (Si) bulk melting points were grown at room temperature by ultrahigh vacuum deposition. 119Sn Mossbauer spectroscopy has been used to investigate the temperature dependence of the Debye-Waller factor f, the mean-square displacement and the mean-square velocity of 119Sn nuclei in ultrathin (10-A thick) alpha-like Sn layers embedded between 50-A thick Si layers. The f-factor was found to be non-zero with a value of 0.036 +- 0.009 even at 450 C. This provides unequivocal proof of the solid-state of the confined alpha-like Sn layers at least up to 450 C. Melting can only be achieved by superheating to T > 450 C. This temperature is significantly higher than the melting temperature of bulk beta-Sn (231.9 C) and of a non-confined epitaxial alpha-Sn single layer grown on InSb(111) (170 C) previously reported in the literature [T. Osaka et al., Phys. Rev. B 50, 7567 (1994)]. Our molecular dynamics calculations show that melting of bulk-like alpha-Sn starts at 380 C and is complete at 530 C according to the Lindemann criterion. Since we still observe the solid state at 450 C for the confined alpha-like Sn films, considerable superheating is observed for this system. The stability of the ultrathin confined alpha-like Sn layers arises from electronic interactions with the surrounding Si layers, as evidenced by the Mossbauer chemical shift.



2005

Enhanced hyperfine magnetic fields for face-centered tetragonal Fe(110) ultrathin films on vicinal Pd(110), B. Roldan Cuenya, W. Keune, Dongqi Li, and S. D. Bader, Phys. Rev. B 71, 064409 (2005) [download]

The structure and hyperfine magnetic properties of epitaxial Fe ultrathin films on a vicinal Pds110d surface have been investigated by means of low-energy electron diffraction (LEED), reflection high-energy electron diffraction (RHEED) and 57Fe conversion electron Mössbauer spectroscopy (CEMS). LEED and RHEED provide evidence for initial pseudomorphic film growth. The RHEED determination of the in-plane atomic distance versus Fe film thickness demonstrates the stabilization of the metastable fcc-like Fe structure on Pd(110). This interpretation is supported by in situ 57Fe CEMS measurements which indicate an enhanced saturation hyperfine field of ~39 T for a 3-monolayers-thick Fe film at 25 K. This is the highest value ever measured for Fe on a metallic substrate. Our results suggest that ultrathin fcc-like sface-centered tetragonald Fe films on Pd(110) are in a ferromagnetic high-moment state with an enhanced hyperfine field due to electronic 3d-4d hybridization at the Fe/Pd interface.



2004

Chemically induced charge carrier production and transport in Pd/SiO2 /n-Si(111) metal-oxide-semiconductor Schottky diodes, B. Roldan Cuenya, Hermann Nienhaus, and Eric W. McFarland, Phys. Rev. B 70, 115322 (2004) [download]

The energy transfer associated with reactions at metal surfaces produces energetic electrons and holes. Using ultrathin films of Pd on metal-semiconductor (MS) and metal-oxide-semiconductor (MOS) diode structures, we have investigated reaction-induced electrical phenomena associated with a variety of molecular and atomic interactions with the Pd surfaces. Distinct electronic signals are observable for species as diverse as atomic oxygen, xenon, and molecular hydrocarbons. Both MS and MOS devices allowed the detection of the chemically induced excitation of electron-hole pairs for highly exothermic chemisorption. Electronic signals from gas species with low adsorption energies were only observed in MOS devices with a thin oxide layer between the active metal film and the semiconductor. The density and distribution of interfacial states in the MOS devices have been found to be an important factor in understanding the origin and transport pathways of these “chemicurrents.” A dynamic model is introduced to explain the displacement currents in the MOS devices during low-energy gas-surface interactions.


A MIS device structure for detection of chemically induced charge carriers, X. Liu, B. Roldan Cuenya, and E.W. McFarland, Sensors and Actuators B 99, 556 (2004) [download]

Chemically induced electronic excitations were detected from adsorption of molecular ethylene and atomic oxygen on large-area Au/Al2O3/n-Si metal/insulator/semiconductor (MIS) diodes. The devices were fabricated with oxide thicknesses varying between 0 and 8 nm. For devices with an oxide layer less than ∼3 nm and relatively energetic surface adsorption reactions, electrons are excited by non-adiabatic energy transfer above the Fermi level and travel ballistically into the semiconductor. For low energy reactions and thicker oxides, charge transport in the device is dominated by ballistic “hot” holes created below the Fermi level which travel to the metal-insulator junction to electron donor states resulting in an effective forward-bias charging current. The energy-band structure of silicon as well as the density of interfacial states of the metal-insulator junctions strongly influences the electronic characteristics of the device. These MIS devices are suitable as molecular sensors for the detection of chemical species with low adsorption energies, such as hydrocarbons.


Sensors based on Chemicurrents, B. Roldan Cuenya, and E.W. McFarland, Dekker Encyclopedia of Nanoscience and Nanotechnology (2004) p.3527



2003

Size- and Support-Dependent Electronic and Catalytic Properties of Au0/Au3+ Nanoparticles Synthesized from Block Copolymer Micelles, B. Roldan Cuenya, Sung-Hyeon Baeck, Thomas F. Jaramillo, and Eric W. McFarland, J. Am. Chem. Soc. 125, 12928 (2003) [download]

Supported Au nanoclusters synthesized from diblock copolymer micelles can be reliably prepared with well-controlled sizes and dispersions. For particles with diameters between approximately 1 and 6 nm, the particle size and the support were found to strongly influence the oxygen reactivity, the formation and stabilization of a metal-oxide, and the catalytic activity for electrooxidation of carbon monoxide. The smallest particles studied (1.5 nm) were the most active for electrooxidation of CO and had the largest fraction of oxygen associated with gold at the surface as measured by the Au3+/Au0 X-ray photoemission intensities. Conducting and semiconducting substrates, ITO-coated glass and TiO2, respectively, were associated with greater stabilization of Au3+ oxide as compared to insulating, SiO2, substrates.


Magnetism and interface properties of epitaxial Fe films on high-mobility GaAs/Al0.35Ga0.65As(001) two-dimensional electron gas heterostructures, B. Roldan Cuenya, M. Doi, W. Keune, S. Hoch, D. Reuter, A. Wieck, T. Schmitte, and H. Zabel, Appl. Phys. Lett. 82, 1072 (2003) [download]

An optimized heterostructure design and an optimized surface sputter-cleaning procedure allow the growth of high-quality epitaxial Fe(001) thin films at Ts<~50 °C on selectively doped GaAs/Al0.35Ga0.65As heterostructures, while retaining the high quality transport property of the two-dimensional electron gas. Magneto-optic Kerr effect measurements and model calculations indicate a dominant uniaxial in-plane anisotropy (easy axis along [110], hard axis along [1-10]) and small coercivity (~15 Oe). Interface sensitive 57Fe Mössbauer measurements prove the absence of both magnetic ‘‘dead layers’’ and ‘‘half-magnetization’’ phases (compared to pure Fe), and provide evidence for intermixing within a few monolayers, retaining, however, a metallic Fe state and high Fe magnetic moments at the interface.


Catalytic Activity of Supported Au Nanoparticles Deposited from Block Copolymer Micelles, Thomas F. Jaramillo, Sung-Hyeon Baeck, Beatriz Roldan Cuenya, and Eric W. McFarland, J. Am. Chem. Soc. Comm. 125, 7148 (2003) [download]



2002

Epitaxial growth and interfacial structure of Sn on Si(111)-(7x7), B. Roldan Cuenya, M. Doi, and W. Keune, Surf. Sci. 506, 33 (2002) [download]

Room temperature stabilization of up to 3.5 ML epitaxial metastable a-Sn at the Si(1 1 1)-(7 x 7) surface is reported. The a-Sn layers remain stabilized at the interface even after the deposition of thick Sn layers that undergo the a-Sn to b-Sn transformation. Additionally, a small decrease in the s-electron density at the 119Sn nucleus is found for submonolayer of Sn at the Sn/Si(1 1 1)-(7 x 7) interface. The epitaxial relationship between thick b-Sn layers on Si(1 1 1) is also shown. The results were obtained by low and high energy electron diffraction and 119Sn conversion electron Mössbauer spectroscopy.


Magnetic and structural properties of epitaxial Fe thin films on GaAs(001) and interfaces, M. Doi, B.Roldan Cuenya, W. Keune, T. Schmitte, A. Nefedov, H. Zabel, D. Spoddig, R. Meckenstock, and J. Pelzl, J. Magn. Magn. Mater. 240, 407 (2002) [download]

Fe(0 0 1) thin films (70 Å ) with 57Fe(7.2 Å ) tracer layers at the interface were epitaxially grown on GaAs(4x6) surfaces. Magneto-optic Kerr effect and Ferromagnetic resonance measurements indicate a dominant 2-fold in-plane magnetic anisotropy (easy axis along [1 1 0]) superimposed to a 4-fold anisotropy, and small coercivity (~10 Oe). Mössbauer (CEMS) measurements indicate no magnetic ‘‘dead layer’’ and an average Fe moment of ~1.7–2 µB at the Fe/GaAs interface.



2001

Magnetism, structure and vibrational dynamics of nanoscaled heterostructures: interfaces, ultrathin films and multilayers, B. Roldan Cuenya, Ph.D. Thesis, University of Duisburg, Germany (2001) [download]


Observation of the fcc-to-bcc Bain Transformation in epitaxial Fe ultrathin films on Cu3Au(001), B. Roldan Cuenya, M. Doi, S. Löbus, R. Courths, and W. Keune, Surf. Sci. 493, 338 (2001) [download]

A continuous fcc-to-bcc crystallographic transition via a homogeneous tetragonal lattice deformation (Bain transformation) with increasing Fe coverage was observed in molecular-beam grown epitaxial Fe ultrathin films on Cu3Au(001), contrary to the usual case of a discontinuous martensitic transformation of Fe. With increasing Fe film thickness, a continuous compression of the interlayer distance perpendicular to the film plane and a simultaneous continuous expansion of the in-plane atomic distance was observed. We did not find evidence for the coexistence of fcc and bcc phases. In the 1-12 ML (monolayer) thickness range, the films do exhibit some atomic disorder and do not grow pseudomorphous, but form twisted crystallographic domains that are rotated in the film plane about the film normal direction. The atomic volume of tetragonal states was found to follow closely face-centered tetragonal (fct) or body-centered tetragonal (bct) “epitaxial lines” according to strain-energy calculations [Surf. Rev. Lett. 1 (1994) 15], including a crossover from ferromagnetic high-moment high-volume fct to bct Fe. The results were obtained by in situ X-ray photoelectron diffraction, high-energy electron diffraction, and 57Fe conversion-electron Mossbauer spectroscopy. Correlated with the Bain transformation is a Fe spin reorientation from preferentially perpendicular (for fct) to in-plane (for bct) spin direction at 25 K.


Structure and vibrational dynamics of interfacial Sn layers in Sn/Si multilayers, B. Roldan Cuenya, W. Keune, W. Sturhahn, T. S. Toellner, and M. Y. Hu, Phys. Rev. B 64, 235321 (2001) [download]

The structure and vibrational dynamics of room-temperature-grown nanoscale Sn/amorphous (a-)Si multilayers have been studied by x-ray diffraction, Raman scattering, 119Sn Mössbauer spectroscopy, and 119Sn nuclear-resonant inelastic x-ray scattering (NRIXS) of synchrotron radiation. With increasing Sn-layer thickness, the formation of b-Sn was observed, except at the Sn/Si interfaces, where a 10-Å-thick metastable pure amorphous-a-Sn-like layer remains stabilized. By means of NRIXS we have measured the Sn-projected vibrational density of states (VDOS) in these multilayers (in particular, at the interfaces), and in 500-Å-thick epitaxial a-Sn films on InSb(001) as a reference. Further, the Sn-specific Lamb-Mössbauer factor ( f factor), mean kinetic energy per atom, mean atomic force constant, and vibrational entropy per atom were obtained. The VDOS of the amorphous-a-Sn-like interface layer is observed to be distinctly different from that of (bulk) a-Sn and b-Sn, and its prominent vibrational energies are found to scale with those of amorphous Ge and Si. The observed small difference in vibrational entropy (DS/kB= +10.17±0.05 per atom) between a-Sn and interfacial amorphous-a-like Sn does not account for the stability of the latter phase.


Magnetism of step-decorated Fe on Pd(110), Dongqi Li, B. Roldan Cuenya, J. Pearson, and S. D. Bader, Phys. Rev. B 64, 144410 (2001) [download]

We investigate the growth and magnetic properties of submonolayer Fe wedges on a stepped Pd(110) substrate with reflection high-energy electron diffraction (RHEED) and the surface magneto-optic Kerr effect. RHEED suggests that Fe atoms decorate the steps to form nanostripes. These stripes are ferromagnetic above 0.3 monolayer Fe coverage, or ~6 Å average stripe width, and have a magnetic easy axis along the surface normal. The onset temperatures of the broadened transition exhibit finite-size scaling with a shift exponent l of 1.260.3, consistent with two-dimensional Ising expectations. The coercivity is less temperature dependent than the magnetization.


Growth and magnetic properties of ultrathin Fe on Pd(110), B. Roldan Cuenya, J. Pearson, Chengtao Yu, Dongqi Li, and S. D. Bader, J. Vac. Sci. Technol. A 19, 1182 (2001) [download]

We have investigated the growth and magnetic properties of 0–3 ML (monolayers) Fe on stepped Pd(110) with reflection high-energy electron diffraction (RHEED), low-energy electron diffraction, and the surface magneto-optic Kerr effect in order to relate the morphology, structure, and magnetic properties in a low-dimensional system. The Fe films grown at 340 K are smooth and pseudomorphic up to 1.5 ML, where three-dimensional growth and lateral lattice relaxation ensues. The in-plane row spacing along the [110] direction decreases by ;5%–6% at 3 ML. RHEED oscillations with 1 ML period are observed in the (1,0), (2,0), and the center of the (0,0) streak intensity. The tail of the (0,0) streak at low exit angle, however, has a 0.5 ML period oscillation, which suggests step decoration growth. Submonolayer Fe films remain ferromagnetic above ~0.3 ML. The magnetic easy axis is initially perpendicular to the surface and is in-plane for Fe thickness >1.5 ML. Between 0.9 and 1.2 ML, there appear to be mixed magnetic phases as indicated by an increase in coercivity.



2000

Magnetism and structure of epitaxial face-centered tetragonal Fe thin films, B. Roldan Cuenya, M. Doi, T. Ruckert, W. Keune and T. Steffel, Frontiers in Magnetism, J. Phys. Soc. Japan 79, Suppl. A, (2000) 125 [download]

The structural and magnetic properties of Fe films grown by molecular beam epitaxy on Cu3Au(001) and on Pd(001), and of Fe films in a [Fe(15 Å)/Pd(40 Å)]30 multilayer, have been determined by electron diffraction (RHEED and LEED), X-ray Diffraction, 57Fe Mössbauer spectroscopy (CEMS), or SQUID magnetometry. Very thin Fe films are found to have a tetragonally compressed fcc (fct) structure and enhanced saturation hyperfine field relative to that of bulk bcc Fe. The Fe magnetization in the Fe/Pd multilayer is remarkably enhanced with respect to that of bulk bcc Fe. Our results demonstrate that thin fct Fe films in these systems are in a ferromagnetic high-moment state with an Fe atomic volume (or Wigner-Seitz radius) which is rather close to that of bulk bcc Fe due to lattice relaxation.


Reflection High-Energy Electron Diffraction and 119Sn Mössbauer investigations of epitaxial alpha-Sn Films, B. Roldan Cuenya, M. Doi, O. Marks, W. Keune and K. Mibu, in Structure and Dynamics of Heterogeneous Systems, eds. P. Entel and D.E. Wolf, World Scientific, Singapore, (2000) 251 [download]

Structure and growth of epitaxial Sn films on InSb(001), Cu(001) and fcc-Fe/Cu(001) substrates were investigated by reflection-high-energy-electron-diffraction (RHEED) and 119Sn Mossbauer spectroscopy. The Sn films grow epitaxially in the α-Sn(001) phase up to 1100 Å thickness on InSb, up to 5.5 ML on Cu(001), and up to 2.7 ML on fcc-Fe/Cu(001). Various surface reconstructions as a function of Sn coverage have been observed. The in-plane lattice parameter of the α-Sn overlayer was studied as a function of coverage.