Mapping the spatial arrangement of chemical functional groups and their interactions is of significant importance to problems ranging from lubrication and adhesion to recognition in biological systems. A force microscope has been used to measure the adhesive and friction forces between molecularly modified probe tips and organic monolayers terminating in a lithographically defined pattern of distinct functional groups. The adhesive interactions between simple CH(3)/CH(3), CH(3)/COOH, and COOH/COOH functional groups correlate directly with friction images of sample surfaces patterned with these groups. Thus, by monitoring the friction between a specifically functionalized tip and sample, one can produce friction images that display predictable contrast and correspond to the spatial distribution of functional groups on the sample surface. Applications of this chemically sensitive imaging technique are discussed.

https://science.sciencemag.org/content/sci/265/5181/2071.full.pdf

Functional Group Imaging by Chemical Force Microscopy

Twenty years ago in a series of amazing discoveries it was found that a large family of ceramic cuprate materials exhibited superconductivity at temperatures above, and in some cases well above, that of liquid nitrogen. Imaginations were energized by the thought of applications for zero-resistance conductors cooled with an inexpensive and readily available cryogen. Early optimism, however, was soon tempered by the hard realities of these new materials: brittle ceramics are not easily formed into long flexible conductors; high current levels require near-perfect crystallinity; and — the downside of high transition temperature — performance drops rapidly in a magnetic field. Despite these formidable obstacles, thousands of kilometres of high-temperature superconducting wire have now been manufactured for demonstrations of transmission cables, motors and other electrical power components. The question is whether the advantages of superconducting wire, such as efficiency and compactness, can outweigh the disadvantage: cost. The remaining task for materials scientists is to return to the fundamentals and squeeze as much performance as possible from these wonderful and difficult materials.

Materials science challenges for high-temperature superconducting wire

Pulsed laser deposition (PLD) is a conceptually and experimentally simple yet highly versatile tool for thin-film and multilayer research. Its advantages for the film growth of oxides and other chemically complex materials include stoichiometric transfer, growth from an energetic beam, reactive deposition, and inherent simplicity for the growth of multilayered structures. With the use of PLD, artificially layered materials and metastable phases have been created and their properties varied by control of the layer thicknesses. In situ monitoring techniques have provided information about the role of energetic species in the formation of ultrahard phases and in the doping of semiconductors. Cluster-assembled nanocrystalline and composite films offer opportunities to control and produce new combinations of properties with PLD.

Synthesis of Novel Thin-Film Materials by Pulsed Laser Deposition

Transmission infrared spectroscopy has been used to probe the structure of alkanethiolate monolayers adsorbed onto nanometer-sized gold clusters. The alkyl chain lengths vary between propanethiolate and tetracosanethiolate; specifically the C3, C4, C5, C6, C7, C8, C10, C12, C16, C20, and C24 alkanethiolates have been examined as solid suspensions in KBr pellets. It has been found that the smaller chain lengths (C3, C4, and C5) are relatively disordered, with large amounts of gauche defects present, and thus most resemble the free alkanes in the liquid state. The longer length alkanethiolates are predominantly in the all trans zigzag conformation. There are detectable amounts of near surface gauche defects, the amount of which decreases with increasing chain length, and a reasonably high percentage of end-gauche defects, the relative amount of which increases with increasing chain length. Internal gauche defects cannot be detected. A model is proposed to explain these observations, and the data are compare...

Infrared Spectroscopy of Three-Dimensional Self-Assembled Monolayers: N-Alkanethiolate Monolayers on Gold Cluster Compounds

The properties of the Y1−xPrxBa2Cu3O7 (YPrBCO) system are reviewed. These include superconducting, normal state, structural, chemical, optical, magnetic, and thermal properties. The destruction of superconductivity with Pr doping is discussed in view of possible models such as hole filling, localization, magnetic pair-breaking, and the role of hybridization. Applications to electronic devices using YBCO/PrBCO/YBCO multilayers are also reviewed.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0884291400017489

A review of the superconducting and normal state properties of Y1−xPrxBa2Cu3O7

Characterization of thin-film amorphous semiconductors using spectroscopic ellipsometry

Pulsed-laser deposition was used to synthesize artificially layered high-temperature superconductors. Thin-film compounds were formed when the constraint of epitaxy was used to stabilize SrCuO2-BaCuO2 superlattices in the infinite layer structure. Using this approach, two new structural families, Ba2Srn–1,Cun+1 O2n+2+δ and Ba4Srn–1 Cun+3O2n+6+δ have been synthesized; these families superconduct at temperatures as high as 70 kelvin.

https://science.sciencemag.org/content/sci/265/5181/2074.full.pdf

Superconductivity in SrCuO2-BaCuO2 Superlattices: Formation of Artificially Layered Superconducting Materials

{ital Z}-contrast electron microscopy demonstrates that interdiffusion is not affecting the resistive transitions observed in {ital M}{times}{ital N} YBa{sub 2}Cu{sub 3}O{sub 7{minus}2}/PrBa{sub 2}Cu{sub 3}O{sub 7{minus}{ital x}} superlattices. At moderate supersaturations, island growth is observed, the islands comprising one Ba-Y-Ba structural unit in height terminated at the Cu-chain plane, having sides 20--30 nm long along (100) and (010). The associated gradual roughening of the growing surface has no apprent effect on critical currents for {ital M}{ge}2, the temperature and field dependence being comparable to that of a film.

Interdiffusion, growth mechanisms, and critical currents in YBa2Cu3O7-x/PrBa2Cu3O7-x superlattices.

Measurements of the critical current density in YBa2Cu3O7−x (YBCO) thin films grown on deliberately miscut (1.6 °) LaAlO3 substrates that also contain a small mosaic spread of subgrain orientations have shown an anomalous angular dependence of Jc. A strong, magnetic field-dependent peak in Jc is observed when the magnetic field is oriented between the c-axis (the film normal direction) and the a-b-plane; in this orientation flux pinning is normally not strong. Transmission electron microscopy (TEM) has been used to determine the defects which are responsible for the flux pinning related to the anomalous Jc peak. [001] columnar defects have been observed to penetrate right through the YBCO films. The diameters of the columnar defect regions are ∼3–5 nm, which is near ideal for producing the observed anomalous pinning and high Jc. Numerous stacking faults involving excess CuO layers have also been observed; these can also serve as effective flux pinners. Small-angle planar boundaries perpendicular to the substrate are rarely seen. Images of the YBCO-substrate interface reveal that some of the columnar growth defects initiate at substrate steps/dislocations. The YBCO unit cells grown on the upper and lower terraces of a substrate step have a relative shift of c/3[001]; thus, the CuO planes are broken within a region less than 10 nm from the substrate, presumably reducing the superconducting order parameter in this region and leading to pinned vortices situated on these sites. Thus, the columnar defects initiated from the interface and the stacking faults are useful for pinning flux through the entire film thickness.

Growth-induced columnar defects in YBa2Cu3O7−x thin films grown on miscut

Thin films of Ho1Ba2Cu3O7 − x and Y1Ba2Cu3O7 − x were deposited on SrTiO3 and Al2O3, substrates by pulsed laser deposition of high-Tc bulk superconductor pellets in vacuum. Following annealing in O2 at 800–900 °C the films were superconducting with typical Tc (50%) = 89 K and transition widths of 10 K. Rutherford backscattering spectrometry (RBS) and secondary ion mass spectrometry (SIMS) were utilized to study the stoichiometry of the as-deposited films for laser energy, densities between 0.11 and 4.5 J cm−2. The films were deficient in holmium and yttrium for energy densities below 0.6 and 0.4 J cm −2, respectively. The films were stoichiometric for fluences above 0.6 J cm−2. In addition, preliminary time dependence and spectroscopic observations of the laser-produced plasma are presented. The results indicate an ablation mechanism that at high energy densities preserves stoichiometry. TEM and x-ray characterization of annealed, superconducting Ho1Ba2Cu3O7 − x films on (100) SrTiO3 showed mixed regions of epitaxially oriented 1:2:3 material with either the c axis or a axis oriented along the surface normal. The a-axis-oriented material grew preferentially in the films with b, c, twinning.

D. H. Lowndes

Papers

Characterization of thin-film amorphous semiconductors using spectroscopic ellipsometry

Superconductivity in SrCuO2-BaCuO2 Superlattices: Formation of Artificially Layered Superconducting Materials

Interdiffusion, growth mechanisms, and critical currents in YBa2Cu3O7-x/PrBa2Cu3O7-x superlattices.

Growth-induced columnar defects in YBa2Cu3O7−x thin films grown on miscut

Pulsed laser deposition of thin superconducting films of Ho1Ba2Cu3O7 − x and Y1Ba2Cu3O7 − x