Laboratory for Surface Modification (LSM)

Seminars Archives

April 2017 | June 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
Research topic 1: Interpretation of X-ray Absorption Spectroscopy
Frank de Groot
Department of Chemistry
Utrecht University
11:00 AM CHEM 260

Research topic 1: Interpretation of X-ray Absorption Spectroscopy

X-ray absorption spectroscopy (XAS) is an important tool to determine the electronic structure of molecules and materials. The power of x-ray absorption experiments is that they can be performed under a wide range of working conditions with high spatial and temporal resolution.

The theoretical understanding of XAS spectra of transition metal systems is divided into

(a) first principle methods to simulate the 1s core excitations and (b) cluster based semi-empirical charge transfer multiplet calculations to simulate the 2p core excitations. For an overview see Core Level Spectroscopy of Solids. CTM4XAS is a user-friendly interface that can be used to simulate core level spectra (XAS, EELS, XPS, RIXS, Auger) of transition metal and rare earth systems. Micron 41, 687 (2010))
Research topic 2: Resonant Inelastic X-ray Scattering (RIXS)

RIXS is a spectroscopic technique that makes use of a monochromator for both the x-rays that excite the system as well as for the emitted x-rays. As a photon-in photon-out experiment, RIXS can also be adapted to various working conditions with a similar large array of possibilities as XAS experiments. Angew. Chem. 52, 1170 (2013)
Research topic 3: In-situ X-ray spectromicroscopy of working catalysts

Transmission X-ray Microscopes can measure soft x-ray absorption spectra with 20 nm spatial resolution. With the use of a nanoreactor we have used STXM spectromicroscopy to image the chemistry of a working catalyst at 1 bar working conditions. more. The 2012 showed that one can now also measure hard x-ray absorption spectra with 20 nm spatial resolution, at 10 bar working conditions. Angew. Chem. 124, 12152 (2012); ChemPhysChem 11, 951 (2010); Nature 456, 222 (2008).

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