Long-Range Lattice engineering of MoTe 2 by 2D Electride

Transcription

1 Supporting Information Long-Range Lattice engineering of MoTe 2 by 2D Electride Sera Kim 1, Seunghyun Song 2, Jongho Park 1,2, Ho Sung Yu 1,2, Suyeon Cho 2, Dohyun Kim 1, Jaeyoon Baik 3, Duk-Hyun Choe 4, K. J. Chang 4, Young Hee Lee 1,2, Sung Wng Kim 1* and Heejun Yang 1* 1 Department of Energy Science, Sungkyunkwan University, Suwon 16419, Korea. 2 Center for Integrated Nanostructure Physics, Institute for Basic Science (IBS), Suwon 16419, Korea. 3 Pohang Accelerator Laboratory, Pohang University of Science and Technology, Pohang , Korea. 4 Department of Physics, Korea Advanced Institute of Science and Technology, Daejeon 34141, Korea. kimsungwng@skku.edu, h.yang@skku.edu

2 Construction of the friction force baseline for FFM characterization: Prior to the FFM characterization of the MoTe 2 /[Ca 2 N] + e heterostructure, we investigated the friction values of the two MoTe 2 structural phases (semiconducting hexagonal 2H and metallic monoclinic 1T') and the influence of different measurement parameters such as film thickness, tip materials, scanning speed, and contact force. The friction force was measured by the torsional displacement of the cantilever in units of millivolts from the photo diode in the atomic force microscopy (AFM) system. The FFM condition was set to use the Pt-coated tip with a scanning speed of 20 µm/s and a contact force of 30 nn. Carrier density of monoclinic MoTe 2 by Hall measurement: To get the exact carrier density at the top surface of MoTe 2 on [Ca 2 N] + e, we tried Hall measurement with the heterostructure. However, the thick geometry of the heterostructure (~500 nm thick in total) and unexplored conducting properties of the [Ca 2 N] + e underneath the MoTe 2 produced difficulties in fabricating the Hall bar geometry and interpreting the data. In addition, vertically non-uniform carrier density inside the MoTe 2 and inevitable oxidation issues during the fabrication process also limited reliable Hall measurements. High-resolution scanning photoemission spectroscopy (SPEM): An SPEM photon source with an energy resolution of approximately 100 mev contributed by a U 6.8 undulator was set to 750 ev. The incident X-ray exposed the sample surface in a perpendicular direction to a fixed angle of 54 from the electron analyzer (PHI 3057) with a resolution of 200 nm 200. Density functional theory (DFT) calculations for the Raman shifts and charge exchange between [Ca 2 N] + e and MoTe 2 : The DFT calculations were performed by using the generalized gradient approximation (GGA) 1 for the exchange-correlation potential and the projector augmented wave potentials 2, as implemented in the VASP code 3. The wave functions were expanded in plane waves up to an energy cutoff of 400 ev. To calculate the Raman shifts of hexagonal MoTe 2, we used the density functional perturbation theory (DFPT) as implemented in the phonopy program 4. We considered a repeated supercell geometry with a vacuum region larger than 15 Å. We used Γ-centered k- points generated by the and Monkhorst-Pack meshes for Brillouin zone integration in the primitive cell and the 4 4 supercell, respectively. The criteria of numerical convergence for energy and force were set as 10-7 ev and 10-5 ev/å, respectively.

3 Figure S1. Raman spectra and optical image of oxidized [Ca 2 N] + e. The optical image of a. pristine [Ca 2 N] + e flake oxidized in about 5 min and b. exfoliated on MoTe 2 was not changed in the ambient condition. c. Raman spectrum of oxidation in the air and due to MoTe 2. All Raman measurements in this study used a 532 nm wavelength excitation laser with 2.6 mw power and a spatial resolution of 500 nm. The oxidation of [Ca 2 N] + e under MoTe 2 was significantly slower; the sample showed minimal change even after being exposed to the ambient environment for 34 h, as shown in Fig. S1a and b. This result could simply be the MoTe 2 acting as a passivation layer. However, the Raman signature of [Ca 2 N] + e in contact with MoTe 2 was different from that of the pristine [Ca 2 N] + e and similar to the oxidized [Ca 2 N] + e shown in Fig. S1c. Given that the extreme oxidation susceptibility of [Ca 2 N] + e is linked to the low work function of the material, we partially attribute the improved stability of the sample, along with the change in the Raman signatures, to the increase in the work function as a result of losing electrons to MoTe 2 in addition to the

4 passivation by MoTe 2 shielding [Ca 2 N] + e from the ambient exposure. Although the electron loss mechanisms are different, we termed the [Ca 2 N] + e in contact with the MoTe 2 as oxidized [Ca 2 N] + e as well because both formation of calcium oxide in the ambient and charge exchange with MoTe 2 result in electron loss, an aspect of oxidation.

5 Figure S2. Optical image of [Ca 2 N] + e on MoTe 2. The optical image of exfoliated [Ca 2 N] + e on MoTe 2 before (left) and after oxidation (right). Fig. S2 shows the optical images of [Ca 2 N] + e on a MoTe 2 flake immediately after exfoliation (left) and after an ambient exposure (right). The white dashed line indicates the position of the exposed [Ca 2 N] + e. The [Ca 2 N] + e underwent oxidation in a similar time scale (few minutes) to the other stand-alone [Ca 2 N] + e flakes even though it has an interface similar to the MoTe 2 /[Ca 2 N] + e structure. In addition, we observed no evidence of a charge transfer between MoTe 2 and [Ca 2 N] + e through Raman spectra. These results demonstrate that the reaction with the ambient environment is more active than the charge transfer that observed in Fig. 2.

6 Figure S3. Optical images of the [Ca 2 N] + e oxidation process. The optical characterization of pristine [Ca 2 N] + e flake on Si substrate during its oxidation. Fig. S3 shows the process of [Ca 2 N] + e oxidation by optical microscopy. The exfoliated [Ca 2 N] + e flake on a 300 nm SiO 2 substrate, as pristine [Ca 2 N] + e, looks green with its own transmission. [Ca 2 N] + e is oxidized extremely rapidly in the air owing to its low work function (2.6 ev) compared with those of other materials. The oxidation of the edge of the flake was more prominent than that at the inner surface due to the anisotropic nature of [Ca 2 N] + e (Fig. S3 below). The color and transmission of the oxidized [Ca 2 N] + e flake were changed as shown in Fig. S3. As an indication of the increased work function and bandgap of the oxidized [Ca 2 N] + e, the appearance of a gold marker under the [Ca 2 N] + e flake is described; this marker was not visible before the oxidation of the [Ca 2 N] + e. The flake was completely oxidized within approximately 5 minute.

7 Figure S4. Raman spectrum of reference 2H and 1T (blue and black, respectively) and MoTe 2 /[Ca 2 N] + e structure (red).

8 Figure S5. Phonon confinement effect on Raman spectra of monoclinic MoTe 2. From bottom : reference 1T (black), slightly, and moderately disordered 1T (orange and yellow, respectively) Here we expand on the brief discussion regarding phonon confinement effect on the Raman spectra of monoclinic MoTe 2. Fig. S4. shows the evolution of Raman spectra of monoclininc MoTe 2 under increasing disorder. In a back scattering confocal Raman spectroscopy, only the phonons with the zero lateral momontum is visible; if the phonon has a lateral momentum, the emitted photon cannot make it back through the optical path to the detector. However, this restriction relaxes in the presence of disorder (defects, grain boundaries etc) as scattering at these defects allows for momentum transfer. Hence, the phonon at the larger k-space is integrated into the spectrum with the increasing disorder 5. In the presence of disorder, mainly two peaks of monoclinic MoTe 2 change most dramatically; A g and B g peaks at 260 and 160 cm -1. We control the disorder by changing the laser power output and illumination duration. For the reference spectra of both phases (blue and black), we used 2.6 mw power; for the slighly disordered we used 15 mw with 10 second exposure. For the moderately disordered 1T, we kept the illumination for ~ 1 min before taking 10

9 second spectrum at 15 mw power output. Their relative intensities decrease while the peaks at 220 and increase 6. This can be understood as a integration of phonon in the larger k-space as the two phonons (260 and 160 cm -1 ) are highly dispersive and has much higher phonon density of states at (220 and cm -1 regions) 6,7. Although we speculate that the dominant type of disorder induced by laser (Te vacancies) would be differnet from the Ca 2 N induced MoTe 2 (polycrystalline), we speculate the same behavior would be observed for both cases.

10 Figure S6. Friction study on different phases of MoTe 2. a. Optical image showing the sample prepared for the friction study. The blue and red circles indicate the hexagonal and monoclinic phases of MoTe 2, respectively, with varying thicknesses. b. AFM topography of the heterophase sample. c. Friction mapping corresponding to the topography image. The hexagonal and monoclinic phases are unambiguously distinguishable from each other independent of their thicknesses. d. Friction force of different phases with respect to SiO 2 as a function of the tip material and loading forces. A hexagonal MoTe 2 flake (Fig. S4a) was prepared by mechanical exfoliation onto a 300 nm silicon oxide substrate. A monoclinic MoTe 2 flake was then transferred to the substrate using the transfer technique with a PMMA/PVA/SiO 2 substrate. The topography (Fig. S4b) and friction mapping (Fig. S4c) showed a slight dependency on the film thickness

11 and scanning speed. The friction force was measured by the torsional displacement of the cantilever in units of millivolts from the photo diode in the atomic force microscopy (AFM) system. The hexagonal MoTe 2 showed a smaller friction force compared with that of the SiO 2 substrate (approximately 40 mv less than the SiO 2 ). However, the friction value of the monoclinic MoTe 2 was approximately 10 mv smaller than that of SiO 2. The friction values showed a small dependency on the tip material and contact force as shown in Fig. S4d.

12 Carrier density (cm -2 ) Temperature (K) Figure S7. Carrier density of monoclinic MoTe 2 by Hall measurement. Since the surface friction force and chemical state of Mo and Te atoms in Fig. 3 indicate structural phase transition from hexagonal to monoclinic MoTe 2 at the top surface, temperature-dependent carrier density was obtained by Hall measurement with a monoclinic MoTe 2 single crystal. The carrier-type was electron in the Hall measurement. Given that the surface friction force and chemical state of Mo and Te atoms in Fig. 3 indicate structural phase transition from hexagonal to monoclinic MoTe 2 at the top surface, we conducted the Hall measurement with a monoclinic MoTe 2 single crystal at various temperatures and the results are shown in Fig. S5. It is clearly shown that the carrier concentration in monoclinic MoTe 2 at room temperature is ~ cm -2. This suggests that the top surface MoTe 2, converted to monoclinic phase, would have an electron density higher than cm -2 by the contact to [Ca 2 N] + e, as claimed in our letter.

13 Figure S8. SPEM spectrum. Based on the binding energy of the single-crystalline MoTe 2 measured previously 2, the difference in MoTe 2 on [Ca 2 N] + e and without [Ca 2 N] + e was analyzed. Fig. S6a shows the binding energy from the Mo 3d and Te 3d bands with a wide range, including C 1s and O 1s peaks. The red and blue lines represent exfoliated MoTe 2 on [Ca 2 N] + e and without [Ca 2 N] + e, respectively. Both spectra are calibrated with C 1s. Because all experiments were performed in a glove box to prevent oxidation, the O 1s peak has a significantly smaller intensity than the reference data 2. Moreover, we cannot observe the Ca 2p and N 1s peaks in both cases due to the surface sensitivity of the SPEM study. The mapping image of the SPEM intensity using a 630 ev set photon source is shown in Fig. S6b. The exfoliated hexagonal

14 MoTe 2, which has an 80 nm thickness, completely changed to the monoclinic phase from the interface to the top of the surface on the [Ca 2 N] + e.

15 Figure S9. Local density of states along the vertical direction correlated with an atomic model for the multiple-layered hexagonal MoTe 2 /[Ca 2 N] + e interface. a. We considered 7 layers of Ca 2 N and 6 layers of hexagonal MoTe 2 in the calculation. A vacuum region is not included in the calculation. b. Atomic structure and corresponding local density of states for pristine hexagonal MoTe 2. c. Atomic structure and corresponding local density of states for the region indicated in Fig S7a. In order to model the charge exchange between [Ca 2 N] + e and MoTe 2, we considered the interface structure, in which the (001) surface of [Ca 2 N] + e is in contact with the (001) surface of either hexagonal or monoclinic MoTe 2. (mono-layered case is shown in

16 Fig. 4a and multi-layered case is shown in Fig. S9). We employed a repeated supercell geometry, which provides symmetrical interface bonds at the two interfaces. We fixed the inplane lattice parameters of hexagonal MoTe 2 to its optimized values (a = 3.55 and b = Å) and adapted the lattice constants of [Ca 2 N] + e and monoclinic MoTe 2 accordingly. The distance between MoTe 2 layers was fitted to the experimental parameters (c = 13.97, Å for hexagonal and monoclinic MoTe 2, respectively). We used Γ-centered k-points generated by the and Monkhorst-Pack meshes to sample the Brillouin zone of the interface supercell. The structures were relaxed until the residual forces were less than 0.02 ev/å. The interface geometries were modeled by repeating a slab of seven [Ca 2 N] + e layers and a slab of six MoTe 2 layers. Due to the lattice mismatch, [Ca 2 N] + e is under compressive strain by 1.66% along both the [100] and [010] directions, while the lattice variation along the [001] direction is negligible. During the structural optimization, MoTe 2 layers and two [Ca 2 N] + e layers near the interface were allowed to relax, while the other layers were kept fixed.

17 Figure S10. Local density of states along the vertical direction correlated with an atomic model for the multiple-layered monoclinic MoTe 2 /[Ca 2 N] + e interface. a. We considered 7 layers of Ca 2 N and two sets of 3 layer monoclinic MoTe 2 on top and the bottom of Ca 2 N, respectively. A large vacuum region (> 20 Å) is included between the slabs. b. Atomic structure and corresponding local density of states for pristine monoclinic MoTe 2. c. Atomic structure and corresponding local density of states for the region indicated in Fig S8a. DFT calculations using a supercell geometry, in which the cell length is much smaller than the measured thickness of 1 µm and 100 nm for Ca 2 N and MoTe 2, respectively, do not explain the experimentally observed charge distribution beyond the second MoTe 2 layer from the interface.

18 Figure S11. Electron concentration vs. distance from the MoTe 2 /[Ca 2 N] + e interface. The screening of the electric field in layered MoS 2 along the c-axis (vertical direction) has been experimentally demonstrated to be affected by the interlayer interaction 9,10. Phenomenological parameters, such as interlayer resistance or interlayer outof-plane effective mass, have been introduced to explain the poor screening in MoS 2. Following the formulism used to explain the screening in the vertical direction in graphene 10 and MoS 8 2, we examined the charge distribution in MoTe 2 as a function of the distance from the [Ca 2 N] + e /MoTe 2 interface. We considered two different cases where the in-plane effective masses of hexagonal MoTe 2 are m z = 0.02 m e and m z = 0.5 m e, which can be regarded as low and high limits, respectively. Using different out-of-plane effective masses

19 (m z ), it was found that the screening can only extend over a few nanometers before decaying exponentially.

20 Thickness MoTe 2 Ca 2 N Figure 2a >50 nm ~1 um Figure 3a 120 nm 70 nm Figure 3c-e nm 25 nm Figure S6 80 nm 110 nm Table 1. Summary of MoTe 2 and Ca 2 N thicknesses presented in this study

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