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The requirements for high performance and low energy consumption call for novel light-weight high-temperature structural materials. A possible answer can be intermetallic γ-TiAl-based alloys, which—in terms of weight—clearly outperform the classical Ni based alloys. However, not only their mechanical properties, such as high specific strength and high creep resistance, are important for device design and use, but also their electrical behavior is of significant importance. In order to correctly interpret the results of electrical material testing techniques, such as eddy current testing, a profound knowledge on the electrical properties is essential. In this study, local-probe techniques, such as conductive atomic force microscopy (CAFM) and micro four-point probe (μ4PP) measurements, were used to determine the specific resistivity of the constituent phases of a Ti-43.5Al-4Nb-1Mo-0.1B (at. %) TNM γ-TiAl based alloy. It turned out that the different phases exhibit noticeably different resistivity values, which vary over two orders of magnitude, whereas the βo phase has the smallest resistivity and the α2 phase the highest. CAFM and μ4PP results were in rather good agreement for the α2 and γ phases with resistivity values of ρα2,CAFM = (1.0 ± 0.7) × 10−5 Ω m and ρα2,4PP = (1.5 ± 1.5) × 10−5 Ω m for the α2-phase, and ργ,CAFM = (6.5 ± 2.1) × 10−6 Ω m, and ργ,4PP = (1.4 ± 1.2) × 10−6 Ω m for the γ-phase. For the βo phase, μ4PP measurements resulted in ρβo,4PP = (9.0 ± 5.0) × 10−7 Ω m. In this case, CAFM values are not reliable due to the formation of a contact barrier that deteriorates the measurements. [ABSTRACT FROM AUTHOR]

Visualization of functionally different domains in bulk heterojunction (BHJ) solar cells is of paramount importance to understand the routes of optimization of their structure for best performance. In this work, a concept of detecting n-type and p-type semiconductor domains in BHJ structures by methods based on atomic force microscopy (AFM) is proposed. It assigns an active role to the semiconducting coating of the AFM probe tip which is able to form different junctions, i.e., p-n anisotype or p+-p, n+-n isotype, with the surfaces inspected. Here, we illustrate this concept on the example of BHJ structures composed of the n-type inorganic microcrystalline semiconductor CdS and mechanochemically prepared p-type kesterite nanopowder and two types of AFM probe tip coatings, i.e., p-type boron-doped diamond and n-type nitrogen-doped diamond coating, respectively. Conductive AFM (CAFM) measurements demonstrated unequivocally the different diode behavior when contacting n- or p-type semiconductor domains in the BHJ structures. Simulation of the energy level alignment at the probe-sample interfaces allowed us to explain the formation of anisotype or isotype junctions depending on the sample domain and probe used. Kelvin probe force microscopy measurements were consistent with the CAFM results and indicated the different contact potentials from the diverse types of domains in the BHJ structure. [ABSTRACT FROM AUTHOR]

We have investigated the growth and stability of molecular ultra-thin films, consisting of rod-like semiconducting para-hexaphenyl (6P) molecules vapor deposited on ion beam modified TiO2(110) surfaces. The ion bombarded TiO2(110) surfaces served as growth templates exhibiting nm-scale anisotropic ripple patterns with controllable parameters, like ripple depth and length. In turn, by varying the ripple depth one can tailor the average local slope angle and the local step density/terrace width of the stepped surface. Here, we distinguish three types of substrates: shallow, medium, and deep rippled surfaces. On these substrates, 6P sub-monolayer deposition was carried out in ultra-high vacuum by organic molecular beam evaporation (OMBE) at room temperature leading to the formation of islands consisting of upright standing 6P molecules, which could be imaged by scanning electron microscopy and atomic force microscopy (AFM). It has been found that the local slope and terrace width of the TiO2 template strongly influences the stability of OMBE deposited 6P islands formed on the differently rippled substrates. This effect is demonstrated by means of tapping mode AFM, where an oscillating tip was used as a probe for testing the stability of the organic structures. We conclude that by increasing the local slope of the TiO2(110) surface the bonding strength between the nearest neighbor standing molecules is weakened due to the presence of vertical displacement in the molecular layer in correspondence to the TiO2 atomic step height. [ABSTRACT FROM AUTHOR]

ZnO nanostructures are promising candidates for the development of novel electronic devices due to their unique electrical and optical properties. Here, we present a complementary electrical characterization of individual upright standing and lying ZnO nanorods using conductive atomic force microscopy (C-AFM). Initially, the electrical properties of the arrays of upright standing ZnO NRs were characterized using two-dimensional current maps. The current maps were recorded simultaneously with the topography acquired by contact mode AFM. Further, C-AFM was utilized to determine the local current-voltage (I-V) characteristics of the top and side facets of individual upright standing NRs. Current-voltage characterization revealed a characteristic similar to that of a Schottky diode. Detailed discussion of the electrical properties is based on local I-V curves, as well as on the 2D current maps recorded from specific areas. [ABSTRACT FROM AUTHOR]

A comparative study of steady-state and time-resolved photoluminescence of para-sexiphenyl (PSP) films grown by organic molecular beam epitaxy (OMBE) and hot wall epitaxy (HWE) under comparable conditions is presented. Using different template substrates [mica(001) and KCl(001) surfaces] as well as different OMBE growth conditions has enabled us to vary greatly the morphology of the PSP crystallites while keeping their chemical structure virtually untouched. We prove that the broad redshifted emission band has a structure-related origin rather than being due to monomolecular oxidative defects. We conclude that the growth conditions and type of template substrate impacts substantially on the film morphology (measured by atomic force microscopy) and emission properties of the PSP films. The relative intensity of the defect emission band observed in the delayed spectra was found to correlate with the structural quality of PSP crystallites. In particular, the defect emission has been found to be drastically suppressed when (i) a KCl template substrate was used instead of mica in HWE-grown films, and (ii) in the OMBE-grown films dominated by growth mounds composed of upright standing molecules as opposed to the films consisting of crystallites formed by molecules lying parallel to the substrate. [ABSTRACT FROM AUTHOR]

We report on the morphological and nonlinear optical properties of highly ordered para-sexiphenyl nanofibers grown by hot-wall epitaxy on muscovite mica. For suitable substrate temperatures and deposition times, needle-shaped aggregates are obtained. These nanoaggregates, which are formed by regrouping of small islands of the material, can support optical waveguiding and amplification of the deep-blue emission of para-sexiphenyl. We show evidence of low-threshold coherent random lasing in ensembles of nanofibers and study the dependence of lasing on the nanofibers’ film thickness. We also present spatially resolved lasing measurements on individual nanofiber segments. The achievement of isolated nanofibers with suitable morphology and low optical losses could open the route to unprecedented photonic and optoelectronic devices at the submicrometric scale. [ABSTRACT FROM AUTHOR]

Systematic slow positron implantation spectroscopy (SPIS) and atomic force microscopy studies of various 6H-SiC samples are presented to clarify the role of conductivity type, crystal quality, ion implantation (B+,Al+, and N+), and annealing (1.650 °C) in the formation of continuous long furrows running in one direction across the wafer surface. It is found that the observed changes in surface morphology are primarily the result of step bunching during thermal activation and thus occur regardless of conductivity type, crystal quality, and type of ion implantation. On terraces separating the step bunches, stripelike islands with a discrete height in the nanometer range have been observed which may have some link with the ion implantation chosen. SPIS results clearly indicate the formation of vacancy clusters in n-type material which are connected with the mobility of nitrogen in the samples at elevated temperatures. It is found that defect profiling by SPIS is not influenced by the changes in surface morphology observed due to annealing. [ABSTRACT FROM AUTHOR]

The stability of conducting atomic force microscope (C-AFM)-induced surface modifications of thermally grown SiO2 under intense illumination in the extreme ultraviolet (EUV) is investigated with low-energy electron microscopy (LEEM) and x-ray photoemission electron microscopy (XPEEM). With LEEM we find that the protrusions are heavily charged after their formation, but this charge is annihilated after exposure of the sample to short pulses of EUV radiation. The spectra obtained from XPEEM reveal that the stripes formed by C-AFM consist of SiO2. After extended EUV exposure, a radiation-induced desorption of the stripes as well as a desorption of the thermal oxide is observed. [ABSTRACT FROM AUTHOR]

The morphological and electrical evolution of HfO2 and ZrO2 thin films is investigated on the nanoscale using conducting atomic-force microscopy in ultrahigh vacuum. Films of different thicknesses have been grown by atomic layer deposition. With increasing film thickness the film structure changes from amorphous to polycrystalline. By conducting atomic-force microscopy using local current–voltage curve statistics and two-dimensional current imaging it is found that the formation of crystallites has different effects on the electrical properties of the two dielectrics. In the case of HfO2, the crystalline fraction causes weak spots in the oxide, whereas for the ZrO2 films the crystallites exhibit lower leakage currents compared to the amorphous matrix and leakage is mainly determined by thickness fluctuations. [ABSTRACT FROM AUTHOR]

The early evolution of GaSb nanoislands induced by ion sputtering was investigated by high-resolution atomic force microscopy. The growth of maximal nanoisland height varies logarithmically with the root-mean-square (rms) roughness in the short-time regime, beyond which it scales as the roughness, as expected from recent theoretical calculation. This is attributed to the gradual evolution of the nanoislands prior to their full formation. Furthermore, it was found that the rms roughness goes through a maximum of ∼8.6 nm before decreasing slightly with further sputtering to reach a constant value of 5–6 nm. The evolution of the island ordering is slower than the full formation of the nanoislands. These findings cannot be explained by the present Kuramoto-Sivashinsky (KS) equation and/or corresponding numerical simulation. We suggest that the more complicated KS equation, i.e., including the variation of the erosion term and its effect on the nonlinear behavior, should be considered to describe the evolution of the nanoislands induced by ion sputtering. [ABSTRACT FROM AUTHOR]

The adsorption, desorption, and growth kinetics as well as the thin film morphology and crystal structure of p-quaterphenyl (4P) grown under ultrahigh vacuum conditions on single crystalline Au(111) have been investigated. Thermal desorption spectroscopy (TDS) reveals two distinct first-order peaks attributed to monolayer desorption followed by a zero-order multilayer desorption. The saturation coverage of the full 4P monolayer has been quantitatively measured with a quartz microbalance to be 8×1013 molecules/cm2. Using low energy electron diffraction the structures of the 0.5 and 1 ML (monolayer) adsorbates have been studied, showing highly regular arrangements of the 4P molecules, which are affected by the (111) surface structure. At the transition from 0.5 to 1 ML a structural compression of the overlayer has been observed. The behavior of thicker 4P films has been investigated by combined TDS-XPS (XPS—x-ray photoelectron spectroscopy). A temperature-induced recrystallization process at about 270 K has been observed for a 7 nm thick 4P film grown at 93 K, corresponding to a transition from a disordered layerlike growth to a crystalline island growth. Ex situ optical microscopy and atomic-force microscopy investigations have revealed needle-shaped 4P islands. Applying x-ray diffraction the crystalline order and epitaxial relationship of the 4P films with 30 nm and 200 nm mean thicknesses have been determined. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Investigations on the stability of conducting atomic force microscope (C-AFM)-induced surface modifications of thermally grown SiO2 under intense illumination in the extreme ultraviolet (EUV) is carried out. The x-ray photoemission electron microscopy (XPEEM) revealed that the stripes formed by C-AFM consist of SiO2.

The morphological and electrical evolution of HfO2 and ZrO2 thin films is investigated on the nanoscale using conducting atomic-force microscopy in ultrahigh vacuum. The formation of crystallites having different effects on the electrical properties of two dielectrics is observed by conducting atomic-force microscopy using local current-voltage curve statistics and two-dimensional current imaging.