Comprehensive tunneling spectroscopy of quasifreestanding ${\mathrm{MoS}}_{2}$ on graphene on Ir(111)

Comprehensive tunneling spectroscopy of quasifreestanding ${MoS}_{2}$ on graphene on Ir(111)

Clifford Murray, Wouter Jolie, Jeison A. Fischer, Joshua Hall, Camiel van Efferen, Niels Ehlen, Alexander Grüneis, Carsten Busse, and Thomas Michely

Phys. Rev. B 99, 115434 – Published 26 March 2019

Abstract

We apply scanning tunneling spectroscopy to determine the band gaps of mono-, bi-, and trilayer ${MoS}_{2}$ grown on a graphene single crystal on Ir(111). Besides the typical scanning tunneling spectroscopy at constant height, we employ two additional spectroscopic methods giving extra sensitivity and qualitative insight into the $k$ vector of the tunneling electrons. Employing this comprehensive set of spectroscopic methods in tandem, we deduce a band gap of $2.53 \pm 0.08$ eV for the monolayer. This is close to the predicted values for freestanding ${MoS}_{2}$ and larger than is measured for ${MoS}_{2}$ on other substrates. Through precise analysis of the “comprehensive” tunneling spectroscopy we also identify critical point energies in the mono- and bilayer ${MoS}_{2}$ band structures. These compare well with their calculated freestanding equivalents, evidencing the graphene/Ir(111) substrate as an excellent environment upon which to study the many celebrated electronic phenomena of monolayer ${MoS}_{2}$ and similar materials. Additionally, this investigation serves to expand the fledgling field of the comprehensive tunneling spectroscopy technique itself.

Received 25 January 2019
Revised 8 March 2019

DOI:https://doi.org/10.1103/PhysRevB.99.115434

Physics Subject Headings (PhySH)

Band gap Density of states

Transition metal dichalcogenides

Scanning tunneling spectroscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Clifford Murray^1,*, Wouter Jolie^1,2, Jeison A. Fischer¹, Joshua Hall¹, Camiel van Efferen¹, Niels Ehlen¹, Alexander Grüneis¹, Carsten Busse^1,2,3, and Thomas Michely¹

¹II. Physikalisches Institut, Universität zu Köln, Zülpicher Straße 77, 50937 Köln, Germany
²Institut für Materialphysik, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Straße 10, 48149 Münster, Germany
³Department Physik, Universität Siegen, 57068 Siegen, Germany

^*murray@ph2.uni-koeln.de

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Vol. 99, Iss. 11 — 15 March 2019

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Figure 1
Constant current STM topographs of ${MoS}_{2}$ on Gr/Ir(111). (a) ${MoS}_{2}$ coverage of 0.6 layers. (b) ${MoS}_{2}$ coverage of 1.4 layers. Small areas of exposed Gr are visible. The TL forms islands of $\approx 20$ nm diameter. Gr wrinkles are visible in the lower section in both topographs. STM parameters: (a) $V = 1.5$ V, $I = 0.01$ nA; (b) $V = 1.0$ V, $I = 0.08$ nA; each image size is $200 \times 190 {nm}^{2}$ .
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Figure 2
(a) Constant height spectra of ML- and BL- ${MoS}_{2}$ (in blue and red respectively). Assignment of the band gaps based only on this STS method is shown. The spectra were taken at the points marked in topographs (b) and (c). The different areas of each sample are indicated for clarity. In (b) a MTB is seen in the top-left corner. STS/M parameters (with stabilization voltage $V_{st}$ , stabilization current $I_{st}$ ): (a) $V_{st} = 1.5$ V; ML $I_{st} = 0.1$ nA, BL $I_{st} = 1.0$ nA; (b) $V = 0.9$ V, $I = 0.10$ nA, image size $10 \times 10 {nm}^{2}$ ; (c) $V = 1.0$ V, $I = 0.08$ nA, image size $28 \times 14 {nm}^{2}$ .
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Figure 3
Sketched band structures of freestanding (a) ML- and (b) BL- ${MoS}_{2}$ . Redrawn after Ref. [13] and adapted to reflect comparison with other DFT calculations [14, 15, 36], the figure should serve only as a generic outline. The first BZ is shown as an inset, with the high-symmetry points and $Q$ point marked. Also indicated is a local maximum between the $K$ and $Q$ points in the ML CB, labeled here $Π_{K Q}$ . For ease of reference the entire band structures have been rigidly shifted to approximately match our energies, rather than fixing 0 eV at the VBM as is typical in DFT.
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Figure 4
Comprehensive STS of (a–e) ML- and (f–j) BL- ${MoS}_{2}$ . The spectra were obtained at the locations shown in Figs. 2 and 2; note that panels (a) and (f) show the same spectra as Fig. 2, here plotted on a logarithmic intensity scale. (a,f) Constant height ${(d I / d V)}_{Z}$ STS spectra. (b,c,g,h) Constant current ${(d I / d V)}_{I}$ STS spectra performed over the VB and CB edges of the respective systems. (d,e,i,j) $κ$ [recording ${(d I / d Z)}_{I}]$ STS spectra performed over the VB and CB edges. (a–j) Assigned critical point energies are marked by dashed black lines; those critical points which constitute a VBM or CBM are dashed red. STS parameters: (a,c,e,f,h,j) $V_{st} = 1.5$ V, (b,d,g,i) $V_{st} = - 2.5$ V; (a-e) $I_{st} = 0.10$ nA, (f) $I_{st} = 1.00$ nA, (g-j) $I_{st} = 0.05$ nA.
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Figure 5
Comprehensive STS of TL- ${MoS}_{2}$ (black), with BL- ${MoS}_{2}$ (red) for comparison. The TL spectra were obtained at the location shown in the inset STM topograph; the BL spectra elsewhere on the same sample. Note that this is a different BL set from that shown in Figs. 2 and 4, to give an idea of deviation within the spectroscopic data. STS parameters: (a) $V_{st} = 1.5$ V, $I_{st} = 1.0$ nA; (b,c) $V_{st} = - 2.5$ V and 1.5 V (for VB and CB respectively), $I_{st} = 0.1$ nA; TL and BL spectra both obtained with the same parameters. STM (inset): $V = 1.0$ V, $I = 0.07$ nA, scale bar 5 nm.
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Physical Review B

covering condensed matter and materials physics

Comprehensive tunneling spectroscopy of quasifreestanding ${MoS}_{2}$ on graphene on Ir(111)

Clifford Murray, Wouter Jolie, Jeison A. Fischer, Joshua Hall, Camiel van Efferen, Niels Ehlen, Alexander Grüneis, Carsten Busse, and Thomas Michely

Phys. Rev. B 99, 115434 – Published 26 March 2019

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Physical Review B

covering condensed matter and materials physics

Comprehensive tunneling spectroscopy of quasifreestanding MoS2 on graphene on Ir(111)

Clifford Murray, Wouter Jolie, Jeison A. Fischer, Joshua Hall, Camiel van Efferen, Niels Ehlen, Alexander Grüneis, Carsten Busse, and Thomas Michely

Phys. Rev. B 99, 115434 – Published 26 March 2019

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Comprehensive tunneling spectroscopy of quasifreestanding ${MoS}_{2}$ on graphene on Ir(111)