Visualizing electrostatic gating effects in two-dimensional heterostructures
Paul Nguyen,Natalie C. Teutsch,Nathan P. Wilson,Joshua Kahn,Xue Xia,Abigail J. Graham,Viktor Kandyba,Alessio Giampietri,Alexei Barinov,Gabriel C. Constantinescu,Nelson Yeung,Nicholas D. M. Hine,Xiaodong Xu,David Cobden,Neil R. Wilson +14 more
TL;DR: Changes in the electronic states of two-dimensional semiconductor devices resulting from electrical gating can be monitored directly using micrometre-scale angle-resolved photoemission spectroscopy, providing a powerful way to study not only fundamental semiconductor physics, but also intriguing phenomena such as topological transitions5 and many-body spectral reconstructions under electrical control.
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Abstract: The ability to directly monitor the states of electrons in modern field-effect devices-for example, imaging local changes in the electrical potential, Fermi level and band structure as a gate voltage is applied-could transform our understanding of the physics and function of a device. Here we show that micrometre-scale, angle-resolved photoemission spectroscopy1-3 (microARPES) applied to two-dimensional van der Waals heterostructures4 affords this ability. In two-terminal graphene devices, we observe a shift of the Fermi level across the Dirac point, with no detectable change in the dispersion, as a gate voltage is applied. In two-dimensional semiconductor devices, we see the conduction-band edge appear as electrons accumulate, thereby firmly establishing the energy and momentum of the edge. In the case of monolayer tungsten diselenide, we observe that the bandgap is renormalized downwards by several hundreds of millielectronvolts-approaching the exciton energy-as the electrostatic doping increases. Both optical spectroscopy and microARPES can be carried out on a single device, allowing definitive studies of the relationship between gate-controlled electronic and optical properties. The technique provides a powerful way to study not only fundamental semiconductor physics, but also intriguing phenomena such as topological transitions5 and many-body spectral reconstructions under electrical control.
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Figures
Figure 1. Visualizing electrostatic gating of monolayer graphene. (a) Schematic of a 2D 19 heterostructure device with a stack comprising graphene encapsulated by BN on a graphite back gate. 20 Photoemission is measured with a focused micron-size X-ray beam spot (see Methods). The graphene 21 is grounded while a gate voltage 𝑉𝐺 is applied to the gate. (b) Optical image of a device mounted in a 22 standard dual in-line package. (c) Optical zoom on the dotted box in (b) showing the stack, and (d) 23 scanning photoemission microscopy (SPEM) image of the same area (scale bar, 50 µm). (e) Energy-24 momentum slices near the graphene K-point, along the red line in the inset Brillouin zone, at the 25 labelled gate voltages. The dashed lines are linear dispersion fits; the Dirac point energy 𝐸𝐷 is deduced 26 from their crossing point (scale bars, 0.2 Å-1). (f) Gate dependence of 𝐸𝐷, with error bars obtained from 27 the fitting procedure. The solid line is a fit based on the dispersion of graphene, with the gate-induced 28 electron density 𝑛𝐺 shown on the top axis calculated from the capacitance (see Methods). 29 30 We first demonstrate gate-doping of monolayer graphene. A graphene sheet is capped by 31 monolayer hexagonal boron nitride (BN), supported on a BN flake over a graphite gate (Fig. 1a), and 32 located in a gap between two platinum electrodes on an SiO2/Si substrate chip (Figs. 1b and 1c; see 33 Methods). A similar structure with two contacts to the graphene would function as a high-mobility 34 transistor26. Scanning photoemission microscopy (SPEM) is used to locate the sample in the ARPES 35 chamber (Fig. 1d; see Methods). Fig. 1e shows energy, 𝐸 − 𝐸𝐹, vs momentum for a slice through the 36 Dirac cone near the graphene zone corner 𝐊, acquired at a series of gate voltages 𝑉𝐺 at 105 K. As 37 expected, the Dirac point energy 𝐸𝐷 shifts from above the Fermi level 𝐸𝐹 at 𝑉𝐺 = -5 V to below 𝐸𝐹 at 38 
Figure 4. Renormalization of the band gap and comparison with optical spectroscopy. (a) Energy-14 momentum slices along 𝚪-𝐊 for monolayer WSe2 in Device 1 at a series of 𝑉𝐺 , with doping 𝑛𝐺 also 15 shown (scale bar, 0.3 Å-1). The intensity in the dashed box is multiplied by 20 at +2.05 V and by 40 at 16 higher 𝑉𝐺. The definition of the band gap, 𝐸𝑔, is indicated. (b) Band gap dependence on 𝑛𝐺 for Device 17 1 (red) and also Device 3 (𝑑𝐵𝑁 = 24.5 ± 0.5 nm, solid black circles) at 100 K. Also plotted (black open 18 circles) are the photoluminescence peak positions for the neutral exciton (𝑋0) and negative trion (𝑋−) 19 in Device 3 at the same temperature. The inset shows the photoluminescence data, with an impurity-20 bound exciton peak XI also labelled. The points plotted at 𝑛𝐺 = 0 are measurements of the band gap 21 from other techniques taken from the literature: STS120 (purple triangle) and STS221 (pink triangle) are 22 from scanning tunnelling spectroscopy measurements, on graphite at T= 4.5 K and 77 K respectively; 23 2ph (brown square) is from two-photon absorption22, on SiO2 at 300 K; ARIPES (black open square) is 24 from inverse photoemission23, on sapphire at 300 K; and Magex (green solid square) is from magneto-25 optical measurements24, encapsulated in BN at 4 K. 26 27 Figure 4a shows spectra from monolayer WSe2 Device 1 at 𝑉𝐺 = 0 (for reference) and at selected 28 gate voltages well above threshold (about +1.5 V). In this regime we derive the gate doping 𝑛𝐺, also 29 shown, from the gate capacitance and threshold voltage (see Methods). The CBE becomes visible at 30 𝐊 for 𝑛𝐺 > ~10 12 cm-2 and at 𝐐 for 𝑛𝐺 > ~10 
Figure 3. Electrostatic gating of monolayer WSe2. Each vertical strip is an energy-momentum slice, 0.6 20 Å-1 wide, through 𝚪 in WSe2 Device 2 (𝑑ℎ𝐵𝑁 = 6.0 ± 0.5 nm) measured at the gate voltage shown on 21 the bottom axis. Δ𝐸Γ is the photoelectron kinetic energy measured relative to the Γ-point maximum at 22 𝑉𝐺 = 0. The dashed line has slope −1/𝑒. Above left is a device schematic indicating the photoemission 23 current 𝐼𝑃𝐸 from the beam spot, current 𝐼𝐶 from the graphene contact, and current 𝐼𝐺 from the gate 24 through the BN due to photoconductivity. The schematic band diagrams indicate the situations at the 25 gate voltages labelled A-E. The gray rectangle is the graphene Fermi sea, the blue lines are the WSe2 26 conduction and valence band edges, and the smaller arrows indicate when 𝐼𝐺 and 𝐼𝐶 are significant. 27 28 
Figure 2. Layer-number dependent conduction band edge (CBE) in WSe2. (a) Schematic of a device 24 incorporating a WSe2 flake, with overlapping graphene top contact grounded and gate voltage 𝑉𝐺 25 applied to the graphite back gate. (b) Optical and (c) SPEM images of WSe2 Device 1 (𝑑𝐵𝑁 = 7.4 ± 0.5 26 nm), with monolayer, bilayer and trilayer regions identified (scale bars, 5 µm). (d)-(f) Energy-27 momentum slices along 𝚪 − 𝐊 for 1L, 2L, and 3L regions respectively. The upper panels are at 𝑉𝐺 = 0 28 and the lower ones at 𝑉𝐺 = +3.35 𝑉. The intensity in the dashed boxes is multiplied by 20. The fuzzy 29 spots signal population of the CBE. Scale bars, 0.3 Å-1. The data have been reflected about 𝚪 to aid 30
Citations
Angle-Resolved Photoemission Studies of Quantum Materials
TL;DR: Angle-resolved photoemission spectroscopy (ARPES) has emerged as a leading experimental probe for studying the complex phenomena in quantum materials, a subject of increasing importance as mentioned in this paper.
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Directly visualizing the momentum forbidden dark excitons and their dynamics in atomically thin semiconductors
Julien Madéo,Michael K. L. Man,Chakradhar Sahoo,Marshall Campbell,Vivek Pareek,E Laine Wong,Abdullah Al Mahboob,Nicholas S. Chan,Arka Karmakar,Bala Murali Krishna Mariserla,Xiaoqin Li,Tony F. Heinz,Ting Cao,Keshav M. Dani +13 more
TL;DR: The momentum state of excitons in a tungsten diselenide monolayer is probed by photoemitting their constituent electrons and resolving them in time, momentum, and energy.
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Electrically Tunable Valley Dynamics in Twisted WSe 2 / WSe 2 Bilayers
Giovanni Scuri,Trond Andersen,You Zhou,Dominik S. Wild,Ji Ho Sung,Ryan J. Gelly,Damien Bérubé,Hoseok Heo,Linbo Shao,Andrew Y. Joe,Andrés M. Mier Valdivia,Takashi Taniguchi,Kenji Watanabe,Marko Loncar,Philip Kim,Mikhail D. Lukin,Hongkun Park +16 more
TL;DR: In this article, the twist angle was used to control the spin-valley properties of transition metal dichalcogenide bilayers by changing the momentum alignment of the valleys in the two layers.
Design of van der Waals Interfaces for Broad-Spectrum Optoelectronics
Nicolas Ubrig,Evgeniy Ponomarev,Johanna Zultak,Johanna Zultak,Daniil Domaretskiy,Viktor Zólyomi,Daniel J. Terry,Daniel J. Terry,James Howarth,James Howarth,Ignacio Gutiérrez-Lezama,Alexander Zhukov,Alexander Zhukov,Zakhar R. Kudrynskyi,Zakhar D. Kovalyuk,Amalia Patanè,Takashi Taniguchi,Kenji Watanabe,Roman V. Gorbachev,Roman V. Gorbachev,Vladimir I. Fal'ko,Vladimir I. Fal'ko,Alberto F. Morpurgo +22 more
TL;DR: Type-II van der Waals interfaces formed by different two-dimensional materials enable robust interlayer optical transitions, regardless of common issues such as lattice constant mismatch, layer misalignment or whether the constituent compounds are direct or indirect band semiconductors.
Signatures of moiré trions in WSe2/MoSe2 heterobilayers
Erfu Liu,Elyse Barré,Elyse Barré,Jeremiah van Baren,Matthew Wilson,Takashi Taniguchi,Kenji Watanabe,Yong-Tao Cui,Nathaniel M. Gabor,Nathaniel M. Gabor,Tony F. Heinz,Tony F. Heinz,Yia-Chung Chang,Chun Hung Lui +13 more
TL;DR: In this article, the authors reported the optical signatures of trions coupled to the moire potential in tungsten diselenide/molybdenum diselinide heterobilayers.
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