AUTHORS: Shicheng Xu, Yongmin Kim, Joonsuk Park, Drew Higgins, Shih-Jia Shen, Peter Schindler, Dickson Thian, J. Provine, Jan Torgersen, Tanja Graf, Thomas D. Schladt, Marat Orazov, Bernard Haochih Liu, Thomas F. Jaramillo AND Fritz B. Prinz
Controlling the morphology of noble metal nanoparticles during surface depositions is strongly influenced by precursor–substrate and precursor–deposit interactions. Depositions can be improved through a variety of means, including tailoring the surface energy of a substrate to improve precursor wettability, or by modifying the surface energy of the deposits themselves. Here, we show that carbon monoxide can be used as a passivation gas during atomic layer deposition to modify the surface energy of already deposited Pt nanoparticles to assist direct deposition onto a carbon catalyst support. The passivation process promotes two-dimensional growth leading to Pt nanoparticles with suppressed thicknesses and a more than 40% improvement in Pt surface-to-volume ratio. This approach to synthesizing nanoparticulate Pt/C catalysts achieved high Pt mass activities for the oxygen reduction reaction, along with excellent stability likely facilitated by strong catalyst–support interactions afforded by this synthetic technique.
AUTHORS: Simin Nie, Gang Xu, Fritz B. Prinz, and Shou-cheng Zhang
Recognized as elementary particles in the standard model, Weyl fermions in condensed matter have received growing attention. However, most of the previously reported Weyl semimetals exhibit rather complicated electronic structures that, in turn, may have raised questions regarding the underlying physics. Here, we report promising topological phases that can be realized in specific honeycomb lattices, including ideal Weyl semimetal structures, 3D strong topological insulators, and nodal-line semimetal configurations. In particular, we highlight a semimetal featuring both Weyl nodes and nodal lines. Guided by this model, we showed that GdSI, the long-perceived ideal Weyl semimetal, has two pairs of Weyl nodes residing at the Fermi level and that LuSI (YSI) is a 3D strong topological insulator with the right-handed helical surface states. Our work provides a mechanism to study topological semimetals and proposes a platform for exploring the physics of Weyl semimetals as well as related device designs.
Authors: Ioannis Petousis, Wei Chen, Geoffroy Hautier, Tanja Graf, Thomas D. Schladt, Kristin A. Persson, and Fritz B. Prinz
In this paper, we demonstrate a high-throughput density functional perturbation theory (DFPT) methodology capable of screening compounds for their dielectric properties. The electronic and ionic dielectric tensors are calculated for 88 compounds, where the eigenvalues of the total dielectric tensors are compared with single crystal and polycrystalline experimental values reported in the literature. We find that GGA/PBE has a smaller mean average deviation from experiments (MARD=16.2%) when compared to LDA. The prediction accuracy of DFPT is lowest for compounds that exhibit complex structural relaxation effects (e.g., octahedra rotation in perovskites) and/or strong anharmonicity. Despite some discrepancies between DFPT results and reported experimental values, the high-throughput methodology is found to be useful in identifying interesting compounds by ranking. This is demonstrated by the high Spearman correlation factor (ρ=0.92). Finally, we demonstrate that DFPT provides a good estimate for the refractive index of a compound without calculating the frequency dependence of the dielectric matrix (MARD=5.7%).
AUTHORS: HAOTIAN WANG, SHICHENG XU, CHARLOTTE TSAI, YUZHANG LI, CHONG LIU, JIE ZHAO, YAYUAN LIU, HONGYUAN YUAN, FRANK ABILD-PEDERSON, FRITZ. B. PRINZ, JENS K. NORSKOV, YI CUI
We report a method for using battery electrode materials to directly and continuously control the lattice strain of platinum (Pt) catalyst and thus tune its catalytic activity for the oxygen reduction reaction (ORR). Whereas the common approach of using metal overlayers introduces ligand effects in addition to strain, by electrochemically switching between the charging and discharging status of battery electrodes the change in volume can be precisely controlled to induce either compressive or tensile strain on supported catalysts. Lattice compression and tension induced by the lithium cobalt oxide substrate of ~5% were directly observed in individual Pt nanoparticles with aberration-corrected transmission electron microscopy. We observed 90% enhancement or 40% suppression in Pt ORR activity under compression or tension, respectively, which is consistent with theoretical predictions.
AUTHORS: Dickson thian, yonas t. yemane, manca logar, shicheng xu, peter schindler, martin m. winterkorn, j. provine, fritz b. prinz
We demonstrate stable, atomically smooth monolayer oxidation of Si(111) using a remote plasma. Scanning tunneling microscopy (STM) confirms the atomically flat nature of the oxidized surface, while cross-sectional transmission electron microscopy (TEM) proves the monolayer to bilayer oxide thickness. Fourier transform infrared spectroscopy (FTIR) and atomic layer deposition (ALD) indicate oxygen is incorporated onto the silicon surface in the form of Si–O–Si and Si–OH bonds. The incorporation of Si–OH bonds is inferred by using TiCl4, a highly specific ALD precursor, for TiO2 ALD. This plasma technique provides precise control of the surface chemistry and yields abrupt yet stable SiO/Si interfaces. It enables production of atomically flat, ALD-active silicon surfaces that could serve as a well-defined platform for investigation of various surface chemistries via STM. Using this substrate, we present the first ever STM observations of ALD TiO2 on silicon oxide.
AUTHORS: S. Acharya, J. Torgersen, Y. Kim, J. Park, P. Schindler, A. L. Dadlani, M. Winterkorn, S. Xu, S. Walch, T. Usui, C. Schildknecht, F. B. Prinz,
Barium oxide (BaO) is a critical component for a number of materials offering high dielectric constants, high proton conductivity as well as potential applicability in superconductivity. For these properties to keep pace with continuous device miniaturization, it is necessary to study thin film deposition of BaO. Atomic layer deposition (ALD) enables single atomic layer thickness control, conformality on complex shaped substrates, and the ability to precisely tune stoichiometry. Depositing multicomponent BaO containing ALD films in a self-limiting manner at low temperatures may extend the favorable bulk properties of these materials into the ultrathin film regime. Here we report the first temperature and dose independent thermal BaO deposition using a novel pyrrole based Ba precursor (py-Ba) and water (H2O) as the co-reactant. The growth per cycle (GPC) is constant at 0.45 Å with excellent self-terminating behavior. The films are smooth (root mean squared (RMS) roughness 2.1 Å) and contain minimal impurities at the lowest reported deposition temperatures for Ba containing films (180–210 °C). We further show conformal coating of non-planar substrates (aspect ratio ∼ 1:2.5) at step coverages above 90%. Intermixing TiO2 ALD layers, we deposited amorphous barium titanate with a dielectric constant of 35. The presented approach for infusing self-terminating BaO in multicomponent oxide films may facilitate tuning electrical and ionic properties in next-generation ultrathin devices.