Jonathan Winterstein

Metrology of DSA process using TEM tomography

Directed self-assembly (DSA) of block copolymers (BCPs) is a rising technique for sub-20 nm patterning. To fully harness DSA capabilities for patterning, a detailed understanding of the three dimensional (3D) structure of BCPs is needed. By combining sequential infiltration synthesis (SIS) and scanning transmission electron microscopy (STEM) tomography, we have characterized the 3D structure of self-assembled and DSA BCPs films with high precision and resolution. SIS is an emerging technique for enhancing pattern transfer in BCPs through the selective growth of inorganic material in polar BCP domains. Here, Al2O3 SIS was used to enhance the imaging contrast and enable tomographic characterization of BCPs with high fidelity. Moreover, by utilizing SIS for both 3D characterization and hard mask fabrication, we were able to characterize the BCP morphology as well as the alumina nanostructures that would be used for pattern transfer.

New Insights into Sequential Infiltration Synthesis

Sequential infiltration synthesis (SIS) is a process derived from ALD in which a polymer is infused with inorganic material using sequential, self-limiting exposures to gaseous precursors. SIS can be used in lithography to harden polymer resists rendering them more robust towards subsequent etching, and this permits deeper and higher-resolution patterning of substrates such as silicon. Herein we describe recent investigations of a model system: Al2O3 SIS using trimethyl aluminum (TMA) and H2O within the diblock copolymer, poly(styrene-block-methyl methacrylate) (PS-b-PMMA). Combining in-situ Fourier transform infrared absorption spectroscopy, quartz-crystal microbalance, and synchrotron grazing incidence small angle X-ray scattering with high resolution scanning transmission electron microscope tomography, we elucidate important details of the SIS process: 1) TMA adsorption in PMMA occurs through a weakly-bound intermediate; 2) the SIS kinetics are diffusion-limited, with desorption 10× slower than adsorption; 3) dynamic structural changes occur during the individual precursor exposures. These findings have important implications for applications such as SIS lithography.

In Situ Atomic-Scale Probing of the Reduction Dynamics of Two-Dimensional Fe2O3 Nanostructures

Atomic-scale structural dynamics and phase transformation pathways were probed, in situ, during the hydrogen-induced reduction of Fe2O3 nanostructure bicrystals using an environmental transmission electron microscope. Reduction commenced with the α-Fe2O3 → γ-Fe2O3 phase transformation of one part of the bicrystal, resulting in the formation of a two-phase structure of α-Fe2O3 and γ-Fe2O3. The progression of the phase transformation into the other half of the bicrystalline Fe2O3 across the bicrystalline boundary led to the formation of a single-crystal phase of γ-Fe2O3 with concomitant oxygen-vacancy ordering on every third {422} plane, followed by transformation into Fe3O4. Further reduction resulted in the coexistence of Fe3O4, FeO, and Fe via the transformation pathway Fe3O4 → FeO → Fe. The series of phase transformations was accompanied by the formation of a Swiss-cheese-like structure, induced by the significant volume shrinkage occurring upon reduction. These results elucidated the atomistic mechanism of the reduction of Fe oxides and demonstrated formation of hybrid structures of Fe oxides via tuning the phase transformation pathway.

High-Resolution Imaging and Spectroscopy at High Pressure: A Novel Liquid Cell for the Transmission Electron Microscope.

We demonstrate quantitative core-loss electron energy-loss spectroscopy of iron oxide nanoparticles and imaging resolution of Ag nanoparticles in liquid down to 0.24 nm, in both transmission and scanning transmission modes, in a novel, monolithic liquid cell developed for the transmission electron microscope (TEM). At typical SiN membrane thicknesses of 50 nm the liquid-layer thickness has a maximum change of only 30 nm for the entire TEM viewing area of 200×200 µm.

Oxidation-state sensitive imaging of cerium dioxide by atomic-resolution low-angle annular dark field scanning transmission electron microscopy.

Low-angle annular dark field (LAADF) scanning transmission electron microscopy (STEM) imaging is presented as a method that is sensitive to the oxidation state of cerium ions in CeO2 nanoparticles. This relationship was validated through electron energy loss spectroscopy (EELS), in situ measurements, as well as multislice image simulations. Static displacements caused by the increased ionic radius of Ce(3+) influence the electron channeling process and increase electron scattering to low angles while reducing scatter to high angles. This process manifests itself by reducing the high-angle annular dark field (HAADF) signal intensity while increasing the LAADF signal intensity in close proximity to Ce(3+) ions. This technique can supplement STEM-EELS and in so doing, relax the experimental challenges associated with acquiring oxidation state information at high spatial resolutions.

Staining Block Copolymers using Sequential Infiltration Synthesis for High Contrast Imaging and STEM tomography

T. Segal-Peretz, J. Winterstein, M. Biswas, J.A. Liddle, Jeffrey W. Elam, N. J. Zaluzec, P.F. Nealey 1 Institute for Molecular Engineering, University of Chicago, Chicago, USA 2 Materials Science Division, Argonne National Laboratory, Argonne, USA 3 Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, USA 4 Energy Systems Division, Argonne National Laboratory, Argonne, USA 5 Electron Microscopy Center, NST Division , Argonne National Laboratory, Argonne, USA

Characterizing Multi-layer Pristine Graphene, Its Contaminants, and Their Origin Using Transmission Electron Microscopy

1. Materials Sci. and Tech. Division, U.S. Naval Research Laboratory, Washington, DC, USA 20375 2. Electronics Sci. and Tech. Division, U.S. Naval Research Laboratory, Washington, DC, USA 20375 3. Chemistry Division, U.S. Naval Research Laboratory, Washington, DC, USA 20375 4. NRC Postdoctoral Associate, U.S. Naval Research Laboratory, Washington, DC, USA 20375 5. Dept. of Physics and Nanoscience Technology Ctr., Univ. of Central Florida, Orlando, FL USA 32816 *current address: Dept. of Mat. Sci. and Eng., McMaster Univ., Hamilton, Ontario, Canada L9H 4L7

The Growth of Catalyst-free NiO Nanowires

NiO is a stable p-type semiconductor with wide band gap (3.74 eV). Nanostructured NiO has drawn much attention as a low-cost material for several applications including electrochromic devices, electrode materials in battery systems, and electrochemical supercapacitors. It is also one of the most promising materials for resistive-switching memory devices. There have been reports of different methods to prepare NiO nanocrystals, including evaporation, sputtering, sol-gel techniques and electrochemical deposition using anodic alumina membranes (AAM). To the best of our knowledge, there have been no reports on thermal oxidation-driven NiO nanowire growth. Thermal oxidation is a proven, low-cost, easy-to-control approach for growing oxide nanowires such as CuO [1], α-Fe2O3 [2, 3] and ZnO [4]. Here we present the in situ study of NiO nanowire growth in an environmental transmission electron microscope (ETEM) and elucidate the atomic structure, morphology, and growth mechanism of NiO nanowires from the oxidation of Ni.

In situ Atomic-Scale Visualization of CuO Nanowire Growth

CuO has received much interest owing to its myriad technologically important applications in solar energy conversion, photocatalysts, lithium ion batteries, and gas sensors. Nanostructured CuO is expected to possess improved or unique properties compared to its bulk form and therefore much effort, the majority using chemical synthesis techniques, has been devoted to the production of CuO nanostructures. Among them, thermal oxidation has recently been employed to generate CuO nanostructures due to its technical simplicity and the ease of applying the method to different metals [13]. However, the mechanism of thermal oxidation-driven oxide nanowire formation has widely been debated and is poorly understood [4]. Here we report dynamic, in situ TEM observations of the growth of CuO nanowires during the oxidation of Cu, which provide key insight into the atomic processes for the growth of CuO nanowires.

Creating nanostructured superconductors on demand by local current annealing

Superconductivity results from a Bose condensate of Cooper-paired electrons with a macroscopic quantum wavefunction. Dramatic effects can occur when the region of the condensate is shaped and confined to the nanometer scale. Recent progress in nanostructured superconductors has revealed a route to topological superconductivity, with possible applications in quantum computing. However, challenges remain in controlling the shape and size of specific superconducting materials. Here, we report a new method to create nanostructured superconductors by partial crystallization of the half-Heusler material, YPtBi. Superconducting islands, with diameters in the range of 100 nm, were reproducibly created by local current annealing of disordered YPtBi in the tunneling junction of a scanning tunneling microscope (STM). We characterize the superconducting island properties by scanning tunneling spectroscopic measurements to determine the gap energy, critical temperature and field, coherence length, and vortex formations. These results show unique properties of a confined superconductor and demonstrate that this new method holds promise to create tailored superconductors for a wide variety of nanometer scale applications.