Advanced Materials

Yale University's Center for Research on Interface Structures and Phenomena (CRISP) is a Materials Research, Science and Engineering Center funded by the National Science Foundation. The Center is a multidisciplinary collaboration among 23 faculty members across 8 departments. CRISP specializes in the synthesis and characterization of thin film materials with atomic precision and impacts high performance electronics, surface chemistry, lasers, biological materials, and science education.

Recent progress in scanning probe technology, which has led to the establishment of threedimensional atomic force microscopy, is reported on p. 2838 by Mehmet Baykara and co-workers. From the quantitative data on surface force and energy fields the method delivers, lateral force maps with picometer resolution can be generated. All cover and frontispiece artwork by Wendolyn Hill.

The ideal scanning probe microscope should be able to measure surface forces, energies, tunneling currents, and local energy losses with atomic resolution in three dimensions. This progress report describes the latest developments in this direction based on the emerging method of three-dimensional atomic force microscopy (3D-AFM). In addition to current results, future challenges and potential application areas are discussed.

The schematic illustrates how tip-sample interactions, such as tunneling due to electronic overlap, forces due to chemical interactions, and non-linear optical interactions due to the sharp asperity of the gold tip, can be used to characterize individual adsorption, diffusion, and reaction events on a reconstructed WO₃ surface.

Many species of birds produce brilliant non-iridescent colors by light scattering from nanostructures with only short-range order, as explored by Noh and co-workers on page 2871. Forster and co-workers have designed materials composed of polymer nanoparticles to produce color via the same mechanism. TEM and SEM images help visualize the character of these materials.

Color generation by short-range ordered nanostructures in bird feather barbs is investigated with angle-resolved optical spectrometry. The color is produced mostly by the interference of singly scattered light, and is noniridescent under omnidirectional lighting due to structure isotropy.

The hetero-epitaxial interface between the insulating oxides LaAlO₃ and SrTiO₃ exhibits a number of intriguing properties which are not present in either bulk constituent. These novel properties include a quasi two-dimensional conducting electron gas, low temperature superconductivity, and magnetism. The state-of-the-art ab initio theoretical work and its relation to the experimental data are reviewed and some key unresolved issues are discussed.

Recent developments in the design of novel complex oxide multiferroic heterostructures—materials systems where magnetic and ferroelectric orders coexist, as the adjacent figure illustrates—are reviewed. By exploring proximity effects at the interface between different complex oxides, new couplings between the order parameters develop. This results in strong magnetoelectric couplings with added functionality, as required for next generation electronic devices.

An interdisciplinary collaboration involving molecular beam epitaxy thin film deposition, advanced real‒space structural characterization, and first‒principles theory has led to an intimate understanding of the process by which the interface between crystalline oxides and silicon forms, the resulting structure of the interface, and the link between its structure and functionality.

The self-assembly of films that mimic color-producing nanostructures in bird feathers is described. These structures are isotropic and have a characteristic length-scale comparable to the wavelength of visible light. Structural colors are produced when wavelength-independent scattering is suppressed by limiting the optical path length through geometry or absorption.

Interfaces between transition metal oxide compounds provide a setting where correlated behavior can emerge. Essential to interface studies are substrates that have a single atomic plane termination. By tuning the vapor pressure of La above the surface of La_0.18Sr_0.82Al_0.59Ta_0.41O3 (LSAT) during annealing, single-unit-cell steps and predominant A-site (SrO) termination can be achieved.

Precise understanding of structure – property relationships at interfaces is critical for electronic devices, particularly at the nanometer scale, and can be achieved by a synergy of high-quality growth, advanced characterization, and first principles theory (on page 2950).

Oxide-oxide interfaces can have electronic states that are distinctly different than those of either bulk oxide. Ultraviolet photoelectron spectroscopy of substrate/overlayer systems, measured as the overlayer is grown one monolayer at a time, can be used to uniquely identify interfacial electronic states.

Ferroelectric memories, where the ferroelectric polarization state encodes the digital information, are being explored as next generation memory architectures. The operation and prospects of single-transistor memory cells, without a storage capacitor, are reviewed in this work.

Inelastic electron tunneling spectroscopy (IETS) is a powerful tool to characterize both the structural and electrical properties of next generation high-k transistor structures. IETS can address materials issues related to reactions and intermixing at interfaces as well as properties related to carrier mobility, such as the low energy phonon modes of HfO₂ shown in the figure, for device structures that are difficult to accurately characterize by other techniques.

The effect of ferroelectric polarization on CO₂ (yellow and red balls) adsorption on PbTiO₃ (Pb – gray, Ti – blue, O – red) is illustrated through charge transfer plots. Blue (red) highlights areas where electrons accumulate (deplete). CO₂ dissociation is favored on the positively poled surface on the left but not on the negatively poled surface on the right.