Argonne National Laboratory

Colloquium Series

The Center for Nanoscale Materials holds a regular biweekly colloquium on alternate Wednesdays at 11:00 a.m. in Bldg. 440, Room A105/106. The goal of the series is to provide a forum for topical multidisciplinary talks in areas of interest to the CNM and also to offer a mechanism for fostering interactions with potential facility users.

Committee Members:

  • Xiao-Min Lin (Chair)
  • Pierre Darancet
  • Ralu Divan
  • Xuedan Ma
  • Elena Rozhkova
  • Jianguo Wen
Date Title
May 16, 2018

First Glimpse of a New Type of Two-Dimensional CrystalsGong Gu, University of Tennessee, Host: Lifen Wang

Traditional and new mainstream semiconductors are all sp3-coordinated crystals, including III-V, IV-IV, and, to a lesser extent, II-VI compounds, which are collectively referred to as octet compounds. Among octet compounds that exist in sp3-coordinated polymorphs, only boron nitride is known to exist also in sp2-coordinated forms, the most common of which is hexagonal BN (h-BN). Given the tremendous interest in two-dimensional (2D) crystals, a natural question is whether h-BN-like polymorphs can exist for at least some octet compounds other than BN. A theoretical study [1], based on an energetic consideration, predicted that each cation-anion bilayer in a wurtzite {0001} film would collapse into a planar,  h-BN-like structure if and only if the film thickness is below a certain threshold. This transformation to the nonpolar structure is deemed a new stabilization mechanism for the otherwise polar crystals to avert the polar field in the ultra-thin limit [1]; a multitude of known mechanisms counter the would-be catastrophic divergence of potential due to the polar field for bulk crystals [2,3]. While the h-BN-like ultra-thin films were hailed as “precursors to wurtzite films,” experimental evidences have been illusive despite efforts to grow ultra-thin films [4,5]. This talk presents the discovery of h-BeO, the h-BN-like form of BeO, made in a serendipitous experiment at CNM. Nanocrystals of BeO formed in graphene-sealed liquid cells were identified by HRTEM and EELS. Since h-BeO and the usual wurtzite BeO (w-BeO) have nearly identical basal plane lattice constants, we resorted to the “fine structure” of EELS, or energy loss near edge structure (ELNES), to show the sp2 electron configuration. Furthermore, we measured h-BeO thicknesses significantly larger than the thermodynamic threshold above which w-BeO is more stable. I will explain why this can be achieved, as well as why previous attempts did not lead to h-BN-like films. Our theoretical work further shows that the h-BN-like thin films of the octet compounds with wurtzite bulks are not so much like h-BN. They constitute a new type of 2D materials, of which we just had a first glimpse.

[1] C. L. Freeman et al, Graphitic nanofilms as precursors to wurtzite films: theory, Phys. Rev. Lett. 96, 066102 (2006).

[2] C. Noguera, Polar oxide surfaces, J. Phys. Condens. Matter 12, R367 (2000).

[3] A. Wander et al, Stability of polar oxide surfaces, Phys. Rev. Lett. 86, 3811 (2001).

[4] Tusche et al, Observation of depolarized ZnO(0001) monolayers: formation of unreconstructed planar sheets, Phys. Rev. Lett. 99, 026102 (2007).

[5] Lee et al, Tunable lattice constant and band gap of single- and few-layer ZnO, J. Phys Chem. Lett. 7, 1335 (2016).

May 30, 2018

"Quantum-Sized" Metal Nanoparticles for Photochemical Energy ConversionYugang Sun, Temple University.  Host:  Gary Wiederrecht

Generation of hot carriers in transition metal catalysts through photoexcitation has been demonstrated to be a promising approach capable of significantly lowering activation temperature of the catalysts, which could have a widespread impact on substantially reducing the current energy demands and improving the selectivity of heterogeneous catalysis.  Plasmonic nanoparticles made of Au, Ag, Cu, and Al are recently focused because they can actively absorb light at the corresponding surface plasmon resonance (SPR) frequencies, which are usually in the visible spectral region.  The high optical absorptions lead to the generation of hot carriers in plasmonic nanoparticles, on which the hot carriers can directly drive chemical transformations.  Despite the promise, plasmonic metal nanoparticles are not useful catalysts for a wide range of important reactions.  In contrast, platinum-group metals (PGMs) such as Pt, Pd, Ru or Rh are excellent catalytic materials but exhibit SPR in the ultraviolet (UV) spectral region, which represents a significant disadvantage for photocatalysis due to the poor overlap with the solar spectrum.  Although increasing size of PGM nanoparticles shifts SPR absorption to the red, it increases cost and reduces surface area and thus catalytic activity.  Moreover, increasing the size of metal nanocrystals significantly reduces the yield of hot electron generation, lowering the efficiency of photochemical energy conversion.  In this presentation, a new light absorption model will be discussed to demonstrate a transformative way to enhance optical absorption in small PGM nanoparticles in the visible spectral region by adjusting their dielectric environment instead of changing their size.  In this model, the “quantum-sized” metal nanocrystals are attached to surfaces of transparent silica spheres, which can support a variety of dielectric scattering resonances (e.g., Fabry-Perot or Whispering Gallery modes depending on the size of silica spheres) capable of creating strong electric fields near the silica surface.  The intensified nearfields can dramatically enhance the absorption cross-section of the metal nanocrystals, which are on the silica surface, thus improving the yield of “hot electrons” in the metal nanocrystals.  This new model provides a unique opportunity to efficiently generate hot carriers in the PMG metal nanoparticles upon excitation of solar energy. 

Jun. 13, 2018 Mike Arnold, University of Wisconsin, Host: Nathan Guisinger
Jun. 27, 2018 James Alexander Liddle, National Institute of  Standards and Technology (NIST), Host: Ralu Divan
Jul. 13, 2018 Humphrey Maris, Brown University, Host:  Dafei Jin
Jul. 25, 2018 Quanxi Jia, State University of New York (SUNY), Host:  Liliana Stan
Aug. 8, 2018  Itai Cohens, Cornell University, Host: Xiao-Min Lin
Sep. 5, 2018 Dongling Ma, Institut National de la Recherche Scientifiue (INRS), Host: Gary Wiederrecht
Sep. 19, 2018  
Oct. 3, 2018 Haiyan Wang, Purdue University, Host: Jie (Joyce) Wang
Oct. 17, 2018 Stephan Lany, National Renewable Energy Laboratory (NREL), Host: Maria Chan
Oct. 31, 2018 Haiyan Wang, Purdue University, Host: Joyce (Jie) Wang
Nov. 14, 2018

Stephen G. Sligar, University of Illinois, Host: Elena Rozhkova

Dec. 12, 2018 P. James Schuck, Columbia University, Host: Pierre Darancet
Jan. 16, 2019 Juejun Hu, Massachusetts Institute of Technology (MIT), Host: Peijun Guo