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.


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.


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., in press, (2008).

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.


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)


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.


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.


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.


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


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]


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.


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.


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.