Advances in Nanomaterials

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advances in nanomaterials Perovskite oxides: Group shows of pressures, their results showed an tion, the optical properties of the film technique for engineering unexpected phenomenon: Relatively change from transparent to reflective. high oxygen pressure can initially pro- The substrate, sapphire, it turns out, is ‘perfect’ heterointerfaces duce a “shielding layer” of LaAlO , and, opaque to certain infrared wavelengths 3 Oak Ridge National Lab researcher when this was followed by PLD growth and reflects the light back. Thus, the Ho Nyung Lee has studied complex- at a lower pressure, the end result was VO –sapphire interface is, itself, an 2 oxide thin films for more than a decade, a highly ordered, essentially defect-free optical trap. and, during this time, he has been interface. Ramanathan tells the Bulletin that interested in exploiting the potential One apparent advantage of this the optical effect in VO films is very 2 properties of functional complex-oxide development, the group reports, is this sensitive to the quality or purity of the perovskites thin films. In particular, is not an isolated effect, and the atomic film, and, therefore, to the physical Lee leads a research team that has been layering technique appears to be appli- vapor deposition processing parameters. striving to develop techniques to create cable to perovskite oxides in general. “There are many compounds that are perfect or nearly perfect thin films and The group’s work is published possible, and we want to make phase- superlattices by precisely controlling in Advanced Materials in a paper pure samples with as pure a composi- surfaces and interfaces. titled “Atomic Layer Engineering of tion as possible,” he says. “It is only In a newly published paper, Lee and Perovskite Oxides for Chemically recently that we have been able to syn- his fellow researchers report they have Sharp Heterointerfaces” (doi:10.1002/ thesize exceptionally high-quality films. been able to engineer a chemically adma.201202691). n Stabilizing the phase is a very hard stable and atomically sharp lanthanum problem.” The films can be epitaxial, aluminate monolayer (i.e., a perfect New ultrathin VO film device depending on the substrate. The group or nearly perfect interface) between 2 has grown films on several substrates, LaAlO and strontium titanate hetero- perfectly, reproducibly absorbs including sapphire and titania. structures. infrared light The tuning range for a device such3 The core of the group’s findings is A new paper by a research team at as this is approximately 80 percent to that a single unit-cell layer of LaAlO the Harvard University School of Engi- 0.25 percent reflectivity. Ramanathan grown on a SrTiO substrate serves as neering and Applied Sciences reports explains that the transition threshold3 a buffer and is sufficient to dramati- on a new device that is an exceptional- of the film can be controlled by electric3 cally improve the interface quality. In ly efficient, perfect absorber of infrared fields, doping with charge carriers, or an ORNL press release, Lee says, “This light. The team expects it to be useful adjusting the lattice constant with dop- means that we can now create new for a range of applications, such as tem- ants such as W, Cr and Nb, or lattice properties by precisely conditioning the perature measurement, spectroscopy, strain during film deposition. boundary in the process of stacking dif- tunable filters, thermal emitters, radia- In addition, the phase transition is ferent oxides on top of each other.” tion detectors, energy harvesting, and “very reproducible through millions of The group’s general approach is to high-sensitivity thermal cameras. cycles, with the right composition,” says use pulsed laser deposition (PLD) to In a press release, associate profes- Ramanathan. The dielectric constant strates at relatively low oxygen pres- says that near the VO insulator-to- with the VO2 phase transition, so his32sor and coauthor Shriram Ramanathan also undergoes rapid changeof VO- sub on the SrTiO3grow the LaAlO sures. The breakthrough came when metal polymorphic phase transition group has been studying the material2 Lee and the others began to systemati- the film has a “very complex and rich for high-speed switches and other ultra- cally examine how varying the oxygen microstructure in terms of its electronic high-speed electronic applications. pressure would affect the thin-film properties, and it has very unusual opti- This new ability to control the optical structure. After looking at a wide range cal properties.” With the phase transi- characteristics opens up new possibili- ties. “We are starting to think about opto-electronic platforms. What type of . Wiley.) oxide electronic devices can we make?” Adv. Mat The paper is “Ultra-thin Per- fect Absorber Employing a Tun- (Credit: Choi et al.; able Phase Change Material,” M.A. Kats, et al., Appl. Phys. Lett., Schematic of the structure of a typical lanthanum aluminate-strontium titanate interface, (doi:10.1063/1.4767646). n left, and the abrupt, sharp interface obtained through an atomic layer engineering method developed at ORNL. 16 www.ceramics.org | American Ceramic Society Bulletin, Vol. 92, No. 1


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