One-Dimensional Confinement and Width-Dependent Bandgap Formation in Epitaxial Graphene Nanoribbons

Result of the Month

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One-dimensional confinement and width-dependent bandgap formation in epitaxial graphene nanoribbons | © CC BY 4.0

Author: Hrag Karakachian Institute: Max Planck Institute for Solid State Research Nature Communications Logo  | © Nature Nature Communications
URL: https://www.nature.com/ncomms/
Date: 2/2021
Instruments: DA30-L, LT STM Lab, LT STM

The ability to define an off state in logic electronics is the key ingredient that is impossible to fulfill using a conventional pristine graphene layer, due to the absence of an electronic bandgap. For years, this property has been the missing element for incorporating graphene into next-generation field effect transistors. In this work, we grow high-quality armchair graphene nanoribbons on the sidewalls of 6H-SiC mesa structures. Angle-resolved photoelectron spectroscopy (ARPES) and scanning tunneling spectroscopy measurements reveal the development of a width-dependent semiconducting gap driven by quantum confinement effects. Furthermore, ARPES demonstrates an ideal one-dimensional electronic behavior that is realized in a graphene-based environment, consisting of well-resolved subbands, dispersing and non-dispersing along and across the ribbons respectively. Our experimental findings, coupled with theoretical tight-binding calculations, set the grounds for a deeper exploration of quantum confinement phenomena and may open intriguing avenues for new low-power electronics.

Image Description 

a, 3D photoelectron intensity distribution I(E,θx,θy) mapping the electronic band structure of armchair graphene nanoribbons (AGNRs). The triangular prism sketch represents the overall shape of the electronic structure in a 2D momentum space. The 1D Brillouin zone of AGNRs is oriented parallel to the [112−0]-direction of SiC (i.e., along the ribbons). b, Second derivative plots along the energy axes of high-resolution ARPES spectra measured along and across the ribbons respectively. c, Perspective AFM view of the mesa structures with a periodicity of 200 nm. The trench depth is 20 nm and the facet inclination is around (26 ± 2). d, First derivative of a STM topography image taken on a single facet displaying the “ladder structure” (V = 0.15 V, I = 1.75 nA). e, Magnified view of the mini-facets highlighting the armchair orientation of the sidewall ribbons (V = 2 V, I = 0.5 nA). The inset shows a high-resolution topography image taken on a single ribbon that has a width of about 2 nm equivalent to N ~ 18 dimers. f, STS spectra measured in the center of the ribbons of different widths displaying both semiconducting and metallic behaviors. The inset shows a zoom-in around the Fermi level.

ARPES Measurements

ARPES measurements were carried out at the Bloch beamline of the MAX IV synchrotron facility in Lund, Sweden. High-resolution energy-momentum cuts were measured using a high performance deflector-based DA30 hemispherical analyzer from Scienta Omicron. The energy and angular resolutions were set to 15 meV and 0.1 respectively. The spot-size of the beam measured 10 × 24 µm2, simultaneously probing around 2000 AGNRs. All ARPES data were acquired in ultra-high vacuum (UHV) at a sample temperature of 80 K.

STM/STS Measurements

The STM/STS measurements were performed in UHV (p < 2 x 10-11 mbar) at 80 K using a commercial Omicron LT-STM. The sample was degassed at an elevated temperature of 800 K for several hours before being transferred to the low-temperature chamber. Several electrochemically etched tungsten tips were utilized during the experiments. A lock-in detection technique was used to accumulate and average over 50 STS spectra taken on each individual ribbon.

 Authors

Hrag Karakachian, T. T. Nhung Nguyen, Johannes Aprojanz, Alexei A. Zakharov, Rositsa Yakimova, Philipp Rosenzweig, Craig M. Polley, Thiagarajan Balasubramanian, Christoph Tegenkamp, Stephen R. Power, and Ulrich Starke.

Institutes

1) Max-Planck-Institut für Festkörperforschung, Heisenbergstraße 1, 70569, Stuttgart, Germany

    Hrag Karakachian, Philipp Rosenzweig & Ulrich Starke

2) Institut für Physik, Technische Universität Chemnitz, Reichenhainer Straße 70, 09126, Chemnitz, Germany

    T. T. Nhung Nguyen, Johannes Aprojanz & Christoph Tegenkamp

3) Institut für Festkörperphysik, Leibniz Universität Hannover, Appelstraße 2, 030167, Hannover, Germany

    Johannes Aprojanz & Christoph Tegenkamp

4) MAX IV Laboratory, Lund University, Fotongatan 2, 22484, Lund, Sweden

    Alexei A. Zakharov, Craig M. Polley & Thiagarajan Balasubramanian

5) IFM, Linköping University, 58183, Linköping, Sweden

    Rositsa Yakimova

6) School of Physics, Trinity College Dublin, Dublin 2, Ireland

    Stephen R. Power

Name and email of corresponding author

Hrag Karakachian

h.karakachian@fkf.mpg.de

 URL of Institute web-pages

https://www.fkf.mpg.de/ga

https://www.tu-chemnitz.de/physik/AFKO/index.html.en

https://www.maxiv.lu.se/accelerators-beamlines/beamlines/bloch/

https://www.maxiv.lu.se/accelerators-beamlines/beamlines/maxpeem/

https://www.ifm.liu.se/materialphysics/semicond/

https://www.tcd.ie/Physics/research/groups/nanoelectronics