Application of X-ray photoelectron spectroscopy to characterization of Au nanoparticles formed by ion implantation into SiO2

In X-ray photoelectron spectroscopy (XPS) of Au nanoparticles, the width of 5d valence band changes with Au particle size. This enables us to estimate the size of Au nanoparticles by using XPS. In this work, the 5d-band width has been measured for Au nanoparticles formed by ion implantation into SiO₂. The 5d-band width is found to be correlated strongly with the Au concentration. As the Au concentration increases, the 5d-band width becomes larger, indicating that the Au nanoparticles with the larger size tend to be formed in the vicinity of the projected range of Au ions. This correlation agrees very well with the results from transmission electron microscopy.     

Electronic properties of (Co, Ag) self-organized nano dots on Au(1 1 1) vicinal surfaces

Electronic properties of (Ag, Co) nanostructures grown on Au(1 1 1) vicinal surfaces have been studied by angle resolved photoemission spectroscopy (ARPES), scanning tunneling microscopy and spectroscopy (STM/STS). The growth and self-assembling of Co and Ag nano dots on Au(7 8 8) surface are described. Co island growth leads to the formation of repulsive energy barriers for the surface state, and subsequently to the appearance of confined states in between each group of four Co dots. On the contrary, when Ag nano dots are grown, the potential barrier for the surface electrons is not enough to suppress their dispersive behavior. Nevertheless, inside Ag islands appear new quantized states whose energies can be tailored by varying the deposition rate of the adsorbate and/or the Miller index of the vicinal surfaces. In both systems, high homogeneity of the electronic properties is achieved over a macroscopic scale.     

Bonds and Fermi surfaces studied by photoelectron spectroscopy

Photoelectron spectroscopy with synchrotron radiation employing high energy and angular resolutions is a very efficient tool for experimental investigations of the electronic structure of solids and their surfaces. In addition to standard band-mapping applications, photoemission intensity and line-shape analyses provide valuable information about wave functions, bonds and interactions of a many-electron system. In this report we choose covalent semiconductor surfaces as well as metallic clean and nanostructured surfaces of layered materials to serve as model systems for assessing the spatial origin of photoelectrons and the three-dimensional shape of Fermi surfaces. Received: 11 July 2001 / Accepted: 23 July 2001 / Published online: 3 April 2002     

Structure and surface characterization of ZrO2-Y2O3-Cr2O3 system

3-mol% Y₂O₃ and 0.3 to 3-mol % Cr₂O₃ co-doped ZrO₂ nanopowders were synthesized using co-precipitation technique and investigated by terms of X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy. Structural analysis shows no significant impact of chromium on powders structure except of presence of small amount of m-phase. Surface analysis reveals segregation of yttrium and chromium atoms to the surface along with surface enrichment by oxygen that can be attributed to residual water. Chromium surface atoms present in three oxidation states with catalytically active Cr²⁺ sites possibly controlling m-phase appearance through lattice distortion.     

Surface plasmon assisted photoemission from Au nanoparticles on graphite

The resonant multiple excitation of collective modes in metallic nanoparticles using ultrashort laser pulses leads to an enhanced multiphoton photoemission from the particles. This effect is here demonstrated for the surface-plasmon resonance of Au nanoparticles on graphite. The shape of the photoemission spectra is explained by multiphoton photo-assisted thermionic emission from the nanoparticles and resonant emission via the image-potential state on graphite. Tuning the photon energy between 1.7 eV and 3.2 eV allows the identification of an enhancement of the photoemission yield at 2.1±0.1-eV photon energy that is attributed to the resonant excitation of the surface plasmon in the Au nanoparticles. This identification of the surface-plasmon excitation in this energy range is also supported by electron energy loss spectroscopy. Received: 8 August 2001 / Revised version: 13 September 2001 / Published online: 10 October 2001     

One-dimensional atomic and electronic structures of submonolayer potassium deposited on stepped Ni(7 5 5) surface

We deposited 0.01-0.16 ML (monolayer) of potassium on stepped (7 5 5) [=6(1 1 1) × (1 0 0)] surface of nickel in order to fabricate and to understand the growth process of one-dimensional (1D) potassium atomic-chain structure by observing the low-energy-electron-diffraction (LEED) patterns. The LEED patterns from potassium adsorbates exhibit a distinct coverage dependence that 1× streaks first appeared at low coverages up to 0.04 ML and later 2× streaks appeared at around 0.09 ML. The streaks become spotty at the higher coverage. We explained these coverage-dependent changes in LEED patterns in a thorough comparison with kinematically calculated LEED patterns constructing a reasonable growth model.     

CMOS-compatible light-emitting devices based on thin aluminum nitride film containing Al nanocrystals

Electroluminescence (EL) from Al-rich AlN thin films grown on p-type Si substrate by radio frequency (RF) magnetron sputtering has been observed at room temperature. The light-emitting structure based on the thin films can be driven by an electrical pulse as short as 10⁻⁵ s. No obvious change in the light emission intensity was observed after 10⁶ pulse cycles. It has been found that the light emission intensity increases with the Al concentration. It is shown that the phenomenon is due to the enhancement of the percolative conduction via the Al nanocrystals distributed in the AlN matrix as a result of the increase in Al concentration.     

Ester-functionalized soluble single-walled carbon nanotubes

We report the preparation of soluble ester- functionalized single-wall carbon nanotubes (sSWNT-COO(CH₂)₁₇CH₃). By use of solution phase IR spectroscopy we are able to compare the ratio of the carbon atoms in the SWNT backbone to the carbon atoms in the ester and amide functionalities of s-SWNTs. Received: 16 July 2001 / Accepted: 3 December 2001 / Published online: 4 March 2002     

Conduction switching in aluminum nitride thin films containing Al nanocrystals

Conduction switching, i.e., a sharp change in the conduction from a lower-conductance state to a higher-conductance state or vice versa in aluminum nitride thin films embedded with Al nanocrystals (nc–Al) has been observed in the ramped-voltage and ramped-current current–voltage (I–V) measurements and the time-domain current measurement as well. Each state is well defined and its I–V characteristic follows a power law. It is observed that the conductance decreases (or increases) with charging (or discharging) in the nc–Al. It is shown that the conduction switching is due to the charging and discharging in the nc–Al at certain strategic sites. With the connecting (or breaking) of some conductive tunneling paths formed by the uncharged nc–Al due to the discharging (or charging) in the nc–Al at the strategic sites, a conduction switching occurs.     

Optoelectronic characteristics of CdTe/HgTe/CdTe quantum-dot quantum-well nanoparticles

Optoelectronic characteristics of CdTe/HgTe/CdTe quantum-dot quantum-well (QDQW) nanoparticles synthesized by the colloidal method are investigated in this study. Strong exciton bands were observed in absorption and photoluminescence (PL) spectra taken for the CdTe/HgTe/CdTe QDQW nanoparticles. The energy difference between the exciton absorption and PL bands is larger than those obtained with CdTe and HgTe nanoparticles. Photocurrent-voltage curves and time-dependent photocurrent curves were obtained for the CdTe/HgTe/CdTe QDQW nanoparticles. With regard to the photocurrent mechanism of these QDQW nanoparticles, those charge carriers participating in the formation of excitons may not contribute to the photocurrent, because of the large binding energy of the excitons. Moreover, it is suggested in this paper that free holes in the HgTe quantum-well in the valance band, rather than free electrons, are the main contributors to the photocurrent.     

Time-dependent photocurrent of a CdTe nanoparticle film under the above-gap illumination

Time-dependent photocurrent of a solid film of 1-thioglycerol-capped CdTe nanoparticles under the illumination of the 325-nm wavelength light is characterized to investigate the transport mechanism of photo-generated charge carriers in this nanoparticle film. Under the illumination of the above-gap light, photocurrent rises rapidly, and subsequently it decays or rises slowly, depending on the magnitude of bias voltage. A careful investigation into the variation in the magnitude of the current measured as a function of time while the light was switched on and off periodically reveals that rapidly and slowly respondent photocurrents overlap in the time-dependent photocurrent. Charge carriers contributing to the rapidly and slowly respondent photocurrents are electrons and holes separated from a fraction of excitons excited by the above-gap light. The transport behaviors of these charge carriers may explain the monotonously decay and slow rising of the photocurrent after its rapidly rising under the illumination for the solid film at unbiased and biased voltage, respectively.     

Dissociation of water molecule on the molybdenum doped fullerene

The dissociation of water molecule on molybdenum (Mo) doped fullerene is studied by using the density-functional theory. It is shown that C₂₀ with Mo dopant are very effective in the dissociation of water molecule. Calculations indicate that the dissociated barriers of 0.207 eV between the transition state (TS) and reactant are much lower than the value for the dissociation of bulk water.     

Time-resolved two-photon photoemission spectroscopy of HOPG and Ag nanoparticles on HOPG

Received: 6 November 1998     

Fabrication of Al nanoparticles and their electrical properties studied by capacitance-voltage measurements

Nanometer-scale Al particles are fabricated and are embedded in a GaAs matrix using molecular beam epitaxial technique. The Al particle is self-assembled on GaAs by supplying an Al molecular beam. The average particle size is found to be 25 nm. The density is 7 × 10¹⁰ cm⁻² when Al of 6.2 × 10¹⁵ atoms/cm² is supplied on (1 0 0)GaAs at a substrate temperature of 300 °C. Clear hysteresis and plateaus in capacitance-voltage (C-V) curves are found in an Al-embedded sample, whereas monotonic increase of capacitance is obtained in a reference sample having an AlAs layer instead of Al. This difference results from trapping of electrons by the Al particles, suggesting that the particles have metallic character.     

Ab Initio Calculation on Self-Assembled Base-Functionalized Single-Walled Carbon Nanotubes

We perform ab initio calculations on the self-assembled base-functionalized single-walled carbon nanotubes （SWNTs） which exhibit the quasi-1D ‘ladder＇ structure. The optimized configuration in the ab initio calculation is very similar to that obtained from molecular dynamics simulation. We also calculate the electronic structures of the self-assembled base-functionalized SWNTs that exhibit distinct difference from the single-branch base-functionalized SWNT with a localized state lying just below the Fermi level, which may result from the coupling interaction between the bases accompanied by the self-assembly behaviour.     

Coulomb interactions of massless Dirac fermions in graphene; pair-distribution functions and exchange-driven spin-polarized phases

The quasi-2D electrons in graphene behave as massless fermions obeying a Dirac-Weyl equation in the low-energy regime near the two Fermi points. The stability of spin-polarized phases (SPP) in graphene is considered. The exchange energy is evaluated from the analytic pair-distribution functions, and the correlation energies are estimated via a closely similar four-component 2D electron fluid which has been investigated previously. SPPs appear for sufficiently high doping, when the exchange energy alone is considered. However, the inclusion of correlations is found to suppress the spin-phase transition in ideal graphene.     

The conductance of SnO2 small nanowires: A study based on density functional and scattering theories

This study is motivated by the recent advances in the fabrication of oxide nanostructures and its purpose is the assessment of the relationship between their structural properties and the conductance. The structures considered are small SnO₂ nanowires whose size and shape reproduce on a smaller scale the structures produced by current technologies. Their electronic configuration and the conductance are evaluated using the density functional and scattering theories with a simplified modelling of the external leads. The study of the electronic configuration shows that the structure of the allowed energy levels and of the charges responds to the details of the nanowire structure and composition. These effects are important in the context of the conductance. In fact, deep resonances are produced by the alignment of the allowed energy levels in the nanowire with the ones in the external leads. For these conductive channels the relationship between the size and the conductance parallels the one between the size and the binding energy.     

Chemical and microstructural study of the oxygen passivation behaviour of nanocrystalline Mg and MgH2

Nanocrystalline Mg and MgH₂ samples have been prepared by high-energy ball milling and gas phase condensation methods. Starting from these materials in their “as received” state without air exposure, a study of the oxygen and air passivation behaviour was carried out by “in situ” analysis of the samples by X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The binding energy and photoemission Auger parameters have been determined for metallic magnesium as well as for magnesium hydride, oxide and hydroxide species. Values of the MgH₂ material were reported for the first time. The study clearly shows the formation of an oxide passivation layer of ca. 3-4 nm in thickness for all the nanocrystalline magnesium samples handled under controlled inert gas atmospheres. A hydroxide like amorphous layer is formed at the topmost surface layers of the nanocrystalline Mg and MgH₂ samples. The implication of these studies for H₂ storage and transport applications of nanocrystalline magnesium is discussed.     

Angle-resolved photoemission study of Ag nanofilms grown on fcc transition-metal (111) substrates

A comparative study of the electronic structures of Ag nanofilms on the pseudomorphic metastable fcc Fe(111) and bulk-like fcc Co(111) substrates has been carried out to investigate their quantized electronic structures. The photoemission spectra of both Ag nanofilms exhibit the fine structures derived from the quantized sates of Ag sp valence electron. The nanofilm-thickness dependences of the binding energy of the quantized states are reproduced by the calculated results based on the phase accumulation model. From the angle-resolved photoemission measurements, the effective masses of the quantized electronic states along the direction parallel to the nanofilm surface were directly determined. We discuss the electronic hybridization effect between quantized states in Ag nanofilm and 3d-derived electronic states in transition-metal substrates.     

Composition of Ta2O5 stacked films on N2O- and NH3-nitrided Si

The composition and microstructure of rf sputtered 20 nm Ta₂O₅ on N₂O or NH₃ Rapid Thermal Nitrided (RTN) Si substrates have been investigated by X-ray photoelectron spectroscopy. RTN at 800 and 850 °C is effective to suppress active oxidation of Si. There is no evidence for the presence of SiO₂ at Si interface. A lightly nitrided surface is established in both cases without a formation of detectable oxynitride layer at Si. A layered nature of the films is observed, with stoichiometric tantalum pentoxide at and close to the films’ surface. In the depth, the films are mixed ones whose composition depends on the nitridation ambient. N₂O treatment stimulates oxidation processes during the film deposition while NH₃ nitridation results to a less effective oxidation and produces Ta-silicate like film. The correlation between the composition of the interfacial regions and the nitridation gas is also discussed. The results suggest that hydrogen, as a component of nitridation ambient, plays significant role in the reactions controlling the exact composition of the deposited Ta₂O₅, activating reactions with nitrogen. Nitrogen related reactions likely occur with NH₃ processing but do not with N₂O one. The presence of nitrogen feature is not detected in N₂O-samples spectra at all. In the integration perspective, preliminary RTN of Si in N₂O or NH₃ could be a suitable way to produce layered Ta₂O₅-based films with more or less presence of tantalum silicate with a trace of nitrogen, either only at the interface with Si (N₂O-process) or in the whole film (NH₃-process).