The periodic shell structure and surface reconstruction of metallic FePt nanoparticles with icosahedral structure has been quantitatively studied by high-resolution transmission electron microscopy with focal series reconstruction with sub-angstrom resolution. The icosahedral FePt nanoparticles fabricated by the gas phase condensation technique in vacuum have been found to be surprisingly oxidation resistant and stable under electron beam irradiation. We find the lattice spacing of (111) planes in the surface region to be size dependent and to expand by as much as 9% with respect to the bulk value of Fe52Pt48. Controlled removal of the (111) surface layers in situ results in a similar outward relaxation of the new surface layer. This unusually large layerwise outward relaxation is discussed in terms of preferential Pt segregation to the surface forming a Pt enriched shell around a Fe-rich Fe/Pt core.
We present experimental results on the characterization of commercially available magnetic force microscopy (MFM) thin film tips as a function of an external magnetic field. Well defined magnetic stray fields are produced using current carrying rings with radii ranging between 603 and 2369 nm fabricated by electron-beam lithography directly imaged by MFM. Treating the MFM tip as a point probe, the analysis of the image contrast as a function of both the magnetic stray field and the lift height allows for a quantitative determination of effective magnetic dipole and monopole moments of the tip as well as their imaginary location within the real physical tip. Our systematic study gives a quantitative relationship on how absolute values of the magnetic dipole and monopole moments and their location within the tip depend on a characteristic decay length of the z component of the magnetic field being detected. From this we can estimate the effective tip volume of the real physical thin film tip relevant in MFM imaging.
In order to investigate the change of gas-sensitive properties of undoped tin oxide nanoparticle films depending on particle size, a thin film synthesis technique has been developed. Well-defined tin oxide nanoparticles have been prepared using a gas-phase condensation method. Pure SnO was used as starting material and was evaporated at T=820 °C. The resulting particles were sintered and crystallized in-flight at T=650 °C. Size-selected nanoparticles ranging from 10 to 35 nm were produced to form a nanoparticle film by means of electrostatic precipitation or low pressure impaction. The effect of in-flight oxidation, sintering, and crystallization on the structure, size, and size distribution of nanoparticles have been studied in detail. The samples show n-type semiconductors’ behavior like bulk SnO2. The influence of particle size on gas sensitivity and response behavior is investigated for C2H5OH at operating temperatures 200–300 °C using silicon substrates having an interdigitated contact pattern and an integrated heating system. In the range of 10–35 nm it has been shown unambiguously that decreasing the particle size of tin oxide particles leads to an increase of the sensitivity and a more rapid response on changing gas conditions. The effect is especially clear for films with a particle size of 20 nm or smaller.
Since the direct observation of magnetization reversal processes in nanostructured systems is hard to achieve, we use magnetoresistance measurements as an indirect sensing tool. To avoid dipolar interactions present in nanowire gratings individual Co nanowires are prepared by electron beam lithography on Si substrates. Using a special two step process nonmagnetic gold contacts are attached to the Co nanowires to exclude an influence of the contact structures on the magnetization reversal process. To support the magnetoresistance analysis also the structure and morphology as well as the magnetic properties of the Co nanowires have been characterized using various microscopy and measurement techniques. Depending on the direction of the applied field three different magnetoresistance measurements can be distinguished. Based on the anisotropic magnetoresistance effect the transversal and the polar magnetoresistance reflect a coherent rotation of the magnetization independent of the wire width. In contrast, the longitudinal magnetoresistance shows a wire widths dependence of the magnetization reversal processes. In small wires nucleation processes occur whereas in wide wires the formation of complex domain structures has been observed. Nanowires with a width in between these groups show "transition"-like reversal processes.
Industrial lithography in microelectronics involves a multistep procedure with formation of micrometer-sized metallic patterns on surfaces. 1 In the quest for writing submicron features for this and other purposes, a variety of different approaches have been developed. 2 The manipulation of individual atoms of xenon or silicon by scanning tunneling microscopy (STM) are spectacular examples 3 but require complicated experimental procedures. STM and AFM (atomic force microscopy) have been utilized in yet other ways to fabricate nanometer-sized surface features such as lines or dots. 2,4 In what appears to be a fairly practical approach, direct writing of metallic features is also possible by irradiating thin films of organometallic materials with electron, ion, or photon beams, the written lines typically being about 100-200 nm wide. 2,5 Similar results are obtained by using organometallic precursors in the gas phase. 6 A new strategy involves the photolysis of thin films of Cu 2 (OH 2 ) 2 -(RCO 2 ), which triggers a radical-chain process with formation of copper-containing features having a line width of about 1000 nm. 7 Here we describe a particularly simple three-step lithographic procedure based on electron beam irradiation of preformed surfactant-stabilized metal or bimetal clusters, the width of the corresponding metallic or bimetallic lines being as small as 30 nm. 8 We have previously shown that nanostructured R 4 N + X -stabilized metal clusters (e.g., Pd, Pt, Rh, Ru, Co, Ni, Fe, Cu) and related bimetallic clusters (e.g., Pd/Pt, Pd/Ni, Pt/Sn) are accessible in a clean and size-selective manner using electrochemical methods. 9 A monomolecular layer of the surfactant surrounds the metal core and thereby prevents agglomeration with undesired formation of metal powders. 10 We speculated that, following spin-coating of tetrahydrofuran (THF) solutions of such clusters onto GaAs substrates, electron beam irradiation would lead to the physical removal and/or destruction of the organic stabilizer, a process which should result in the agglomerization of the clusters with concominant formation of insoluble metallic features. Removal of the nonexposed soluble clustercontaining areas by simple rinsing was expected to complete the lithographic procedure (Scheme 1).In an initial experiment a THF solution of a (C 8 H 17 ) 4 N + Br -stabilized 2.0-nm Pd cluster (69% metal content) at a concentration of 150 mg/mL was spin-coated onto a GaAs substrate (3.9 × 3.9 × 0.5 mm) to form a 132-nm-thick film having a surface roughness of about 1 nm. For the electron beam experiments a layout consisting of five lines and broad contact areas necessary for resistivity measurements was chosen. Using a conventional scanning transmission electron microscope (STEM) (operating at an acceleration voltage of 120 kV) equipped with an external computer-driven control system, 11 the film was irradiated with an electron beam at a dosage of 200 000 µC/ cm 2 . The AFM visualization of the surface shows that the exposed areas are reduced in hei...
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