Laboratory for Surface Modification (LSM)

Seminars Archives

April 2017 | June 2017 | September 2017

Monday, June 05, 2017
Helium-Ion-Beam Etched Encapsulated Graphene Nanoribbons
Paul Alkemade
Kavli Institute of Nanoscience
Delft University of Technology
1:30pm NPL 201

Graphene nanoribbons (GNRs) have been studied extensively since the realization that narrow graphene ribbons have a band-gap. However, much of the predicted novel physics in GNR devices eluded experiments. The methods of fabrication were the bottleneck, being either too complex or not flexible, or introducing contaminations or lattice imperfections. In the last two years one sees a resurgence of this field, mainly driven by the ability to encapsulate graphene, thus greatly reducing the risk of contamination during or after fabrication.
One of the fabrication methods is etching with a subnanometer helium focused ion beam (He-FIB). So far lattice damage and/or contamination precluded reliable electrical measurements in ion-beam etched graphene devices. In the present study we avoided contamination by encapsulating graphene in boron nitride layers. Subsequently we have conducted extensive electrical measurements.

We used a He-FIB to etch encapsulated graphene into nanoribbons of varying width. We determined first the required He+ dose to make an insulating barrier in graphene. Subsequently, we etched GNR devices.
We characterized the exposed graphene and the GNRs by Raman spectroscopy, atomic force microscopy, and electrical (I-V) measurements. The latter measurements revealed the presence of inactive strips at the edges of the GNRs. Conductance measurements at 4 K showed an energy gap, that was largest for the narrowest ribbons. We observed also that the low-temperature I-V measurements are characterized by power-law scaling, demonstrating that the electrical transport in the encapsulated GNRs is governed by Coulomb blockade and charge hopping between islands and localized states. The Coulomb blockade points to beam-induced disorder in the GNRs.
We attribute the almost complete disappearance of conductance in previous studies to surface contamination that is aggravated by beam-induced disorder.


Wednesday, June 21, 2017
X-ray Spectroscopy of Transition Metal Oxides
Frank de Groot
Department of Chemistry
Utrecht University
11:00 AM CHEM 260

New developments in in-situ x-ray absorption (XAS), transmission x-ray microscopy (TXM) and resonant inelastic x-ray scattering (RIXS) will be discussed. First a brief introduction is given of x-ray absorption spectroscopy, including the multiplet interpretation of XAS spectral shapes [1,2].

Nanoscale chemical imaging of catalysts under working conditions is possible with Transmission X-ray Microscopy [3]. The 20 nanometer resolution combined with powerful chemical speciation by XAS and the ability to image materials under reaction conditions opens up new opportunities to study many chemical processes. A comparison is made with the atomic resolution obtained in STEM-EELS [4,5].

The last part of the talk deals with resonant inelastic x-ray scattering (RIXS), In 2p3d RIXS one scans through the 2p XAS edge and measures the optical excitation range. As an example, the RIXS spectra of CoO will be discussed. First-principle theoretical modelling was performed for the ground state and multiplet analysis for the RIXS experiments. The implications for measurements on coordination compounds (cobalt carboxylates) and cobalt nanoparticles and transition metal oxides is discussed, in particular the comparison with optical spectroscopy [6,7]. Related to the RIXS measurements is the analysis of Fluorescence yield (FY) detected x-ray absorption spectra (XAS), including the intrinsic deviations of FY-XAS spectral shape from the XAS spectrum that is important for measurements with x-ray free electron lasers [7].
[1] Core Level Spectroscopy of Solids
Frank de Groot and Akio Kotani (Taylor & Francis CRC press, 2008)
[2] Download the x-ray spectroscopy simulation software at http://www.anorg.chem.uu.nl/CTM4XAS/
[3] E. de Smit et al. Nature 456, 222 (2008).
[4] F.M.F. de Groot et al. ChemPhysChem 11, 951 (2010);
[5] M. van Schooneveld et al. Nature Nanotechnology 5, 538 (2010)
[6] M. van Schooneveld et al. Angew. Chem. 52, 1170 (2012);
[7] Liu et al. Inorg. Chem 55, 10152 (2016); Huang et al. Nature Comm. (accepted).

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