Emergence of a Metal-Insulator Transition and High Temperature Charge Density Waves in VSe 2 at the Monolayer Limit

Transcription

1 Supporting Information Emergence of a Metal-Insulator Transition and High Temperature Charge Density Waves in VSe 2 at the Monolayer Limit Ganbat Duvjir, Byoung Ki Choi, Iksu Jang, Søren Ulstrup,, Soonmin Kang, #, Trinh Thi Ly, Sanghwa Kim, Young Hwan Choi, Chris Jozwiak, Aaron Bostwick, Eli Rotenberg, Je-Geun Park, #, Raman Sankar,, Ki-Seok Kim, *, Jungdae Kim, *, Young Jun Chang, *, Department of Physics, BRL, and EHSRC, University of Ulsan, Ulsan 44610, Republic of Korea Department of Physics, University of Seoul, Seoul 02504, Republic of Korea Department of Physics, Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of Korea Advanced Light Source (ALS), E. O. Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA Department of Physics and Astronomy, Interdisciplinary Nanoscience Center, Aarhus University, 8000 Aarhus C, Denmark # Center for Correlated Electron Systems, Institute for Basic Science (IBS), Seoul 08826, Republic of Korea Department of Physics and Astronomy, Seoul National University (SNU), Seoul 08826, Republic of Korea Institute of Physics, Academia Sinica, Taipei 10617, Taiwan Center for Condensed Matter Sciences, National Taiwan University, Taipei 10617, Taiwan * tkfkd@postech.ac.kr; kimjd@ulsan.ac.kr; yjchang@uos.ac.kr 1

2 1. The morphology of 0.9 ML VSe 2 grown on BLG/SiC substrate The morphology of 0.9 ML coverage VSe 2 on BLG in Fig. S1 shows that the films mostly consist of ML VSe 2. ML and 2 ML VSe 2 films have roughly 80% and 10% coverage of surfaces, respectively. Figure S1. STM images of 0.9 ML VSe 2 grown on BLG/SiC substrate (200 nm 200 nm). 2

3 2. Structural and chemical characterization The arc distribution of VSe 2 spots in LEED (Fig. S2c) indicates that the rotational misalignment between the ML VSe 2 films and BLG exists in the range of θ ±5. This also agrees with the statistical analysis of STM measurements, shown in Fig. S2e. Figure S2. Structural and chemical analysis of ML VSe 2 grown on BLG. (a) and (b) Reflective high-energy electron diffraction (RHEED) images of bilayer graphene (BLG) and ML VSe 2, indicating epitaxial growth of VSe 2 on BLG. (c) Low-energy electron diffraction (LEED) of ML VSe 2 grown on BLG. LEED pattern at 110 ev electron energy showing the graphene (blue dotted circles) and VSe 2 (red dotted circles) diffraction spots. The red and yellow dashed lines indicate the rotational misalignment of θ ±5 between VSe 2 and graphene lattices. (d) X-ray photoemission spectroscopy (XPS) spectrum of ML VSe 2, showing clear core level signals of V and Se after evaporating the protective Se layer. Both carbon and silicon signals originate from the BLG/SiC substrate. The photon energy was 650 ev for XPS. (e) Statistics of the misalignments between VSe 2 and graphene lattices obtained from the analysis of STM topographies. 3

4 3. STM analysis on the misalignment between ML VSe 2 and BLG Figure S3 shows STM topographic images obtained in different locations of sample. The crystal orientation of VSe 2 is rotationally misaligned with that of graphene in the range of θ ±5 which also agrees with the arc distribution of VSe 2 spot in LEED (Fig. S2c). Figure S3. STM images obtained in different locations of sample. The white and red dashed lines indicate the crystal orientation of graphene and VSe 2, respectively. 4

5 In addition, we investigated the influence of misalignment on the observed superstructure of ( 3 2) and ( 3 7) in STM. The ( 3 2) and ( 3 7) superstructures (the dashed boxes in Fig. S4b-d) are observed in the given rotational disorders. Accordingly, all majority peaks in FFTs appear constantly in the misaligned images (marked by colored circles in the FFT images of Fig. S4f-h). However, it is also noted that subtle alterations exist between the STM images with different alignments. Small misalignment and related interface coupling should also play an important role on those variations of topographic features in Fig. S4. Figure S4. (a) High-resolution STM image of bilayer graphene (BLG). (b-d) STM images obtained with different locations of -4 o, 0 o, and +5 o rotational misalignment between VSe 2 and graphene lattices, respectively. (e-h) show the corresponding FFTs of (a-d), respectively. The orange and pink circles indicate Bragg peaks of graphene and VSe 2, respectively. The white circles represent the CDW modulation. The red and green circles represent lattice distortions. The red and white dashed lines indicate the crystal axis of VSe 2 and graphene lattices. 5

6 4. ARPES data of bulk VSe 2 and temperature-dependence of ML VSe 2 Figure S5. ARPES data of bulk and ML VSe 2. (a) Fermi surface map of bulk VSe 2. Six ellipsoids surround the L points above the bulk CDW temperature (105 K). The shape of these contours disperses strongly with k z. 1 (b) and (c) Band cuts along the L-A-L and the H-L-H directions of bulk VSe 2 (180 K, hv = 115 ev). (d) Fermi surface map of ML VSe 2 at 300 K. (e) Photon energy dependence of momentum dispersion curves (MDCs) of ML VSe 2 at binding energy (E bin ) of ev (top) and ev (bottom). The MDCs exhibit straight vertical lines (indicated with arrows), proving the absence of k z dispersion as expected for the 2D electronic states considered here. (f) Temperaturedependent energy dispersion curve (EDC) map at the -point during warming. (g-i) EDCs of the temperature dependent spectra in Figures 2e, 2f, and S2f, showing the shifts of the leading edges, respectively. 6

7 5. Detailed analysis of 2D FFT images of the insulating phase at 79 K Figure S6a shows atomic resolution STM images of ML VSe 2 obtained at 79 K. The FFT image of Figure S6a contains extra peaks due to strong lattice distortions (marked by red, green, blue, and gray circles) beside the Bragg peaks (marked by pink circles). To visualize corresponding modulations of each peak in real space, selectively Fourier-filtered images are provided in Fig. S6c f. These images are obtained by inverse transformation of the FFT image while keeping only the circled area with corresponding colors. Figure S6. (a) High-resolution topography and (b) 2D FFT of ML VSe 2 at 79 K (V b = V, I t = 30 pa). Bragg peaks of VSe 2, are marked by pink circles in (b). (c-f) Selectively Fourier-filtered images obtained by taking the region indicated by (c) red, (d) blue, (e) green, and (f) gray circles in (b). 7

8 6. Atomic structure model of ML VSe 2 on graphene At 79 K, the STM image of ML VSe 2 (Figure S7a) shows stripe modulations described by ( 3 2) and ( 3 7) periodicities. The lattice constants of VSe 2 and graphene are 3.35 Å and 2.46 Å, respectively, showing a large lattice mismatch of 26.5%. The ratio of their lattice constants is minimized between (4 4) graphene and (3 3) VSe 2 where the mismatch is 2.1% (Table S1). The combination of ( 3 2) and ( 3 7) periodicities corresponds to tripling the (4 4) graphene and (3 3) VSe 2, i.e. (12 12) graphene and (9 9) VSe 2 (Figure S7b). Assuming rotational disorder of 3 o and 5 o, one can still match the misaligned VSe 2 lattice to the neighboring graphene lattice points within ~ 2% lattice mismatch (Fig. S7c-d). We admit that this model considers the structural match for two endpoints of lattices along one crystal axis while real surfaces contain two-dimensional lattices. The better structural model requires more sophisticated theoretical investigation. Table S1. Lattice mismatch analysis between VSe 2 and graphene. (n, m) n a VSe2 (Å) m a G (Å) Lattice mismatch (%) (1, 1) (1, 2) (2, 3) (3, 4) (4, 5) (5, 6) (5, 7) (6, 7) (7, 8) (7, 9) (8, 9)

9 Figure S7. Atomic structure model of the ML VSe 2 grown on graphene. (a) Highresolution STM image obtained from ML VSe 2 at 79 K (V b = -1.4 V, I t = 30 pa). (b) The overlapped structure model of ML VSe 2 with ( 3 2) and ( 3 7) periodicities and graphene. The unit cells of ( 3 2) and ( 3 7) are marked by dotted boxes. (c,d) The structural models with rotational misalignments of (c) +3 and (d) +5. The blue dashed circles indicate the border line of 2% lattice mismatch. 9

10 7. Experimental clue on the dimerization of V atoms Previous theoretical report indicates that V 3d orbital states dominate the empty states of bulk VSe 2 2, which could contribute to the tunneling spectroscopy (i.e. di/dv). As shown in Fig. S8d, the di/dv image of empty states presents stripe structures parallel to the Se dimerization direction, marked by the blue dashed lines. Fig. S8e shows line profiles along the green and blue arrows in (c) and (d). Interestingly, the stripes of di/dv image are laterally shifted by ~ 0.6 Å from those of topographic image. As we discussed in Fig. 3g, the modulation along the green dashed line is due to the Se dimerization. Similar modulation exists along the blue dashed lines as shown in Fig. S8g. These observations suggest that the V atoms are accordingly dimerized along the same direction of Se dimerization. Figure S8. (a) Filled and (c) empty state STM images of ML VSe 2 obtained at 79 K. (b) and (d) show the corresponding di/dv maps of (a) and (c), respectively. (e) Line profiles along the green and blue arrows in (c) and (d). (f) Top and side view structure model for the top Se (green balls) and V (blue balls) atoms (The bottom Se atoms are not displayed). The dimerization of Se and V atoms are marked by red ovals. (g) Line profiles along the green and blue dashed lines in (c) and (d). 10

11 8. Magnetic measurements of 1.5 ML VSe 2 Figure S9. Magnetic measurements of 1.5 ML VSe 2. (a) and (b) Temperature (T)- dependence and magnetic field (H)-dependence of magnetization (M) of Se-capped 1.5 ML VSe 2 grown on a BLG/SiC substrate along in-plane direction. In (a), blue and red lines indicate measurements during cooling and warming for the field-cooling (FC) condition, while the black line corresponds to the zero-field-cooling (ZFC) condition. In (b), M(H) curves show hysteresis loops for 10 K, 100 K, and 400 K. The saturation magnetization reaches ~1000 emu/cm 3, equivalent to ~5 B per formula unit, similar to the reported value. 3 The inset shows M(H) curves in a wider field range, showing contribution of diamagnetic signal from the substrate. 11

12 9. di/dv spectrum of ML VSe 2 on BLG at 79 K The gap value at point obtained from ARPES is 9 ± 4 mev (Fig. 2g). Since ARPES can only measure the gap from Fermi level to the edge of valence band, full gap value should be 18 ± 8 mev. We have performed STS measurements at 79 K. In Fig. S10, the STS curve shows opening of a ~ 15 mev gap at 79 K, which is consistent with the ARPES measurements. Figure S10. (a) di/dv spectrum of ML VSe 2 on BLG at 79 K. (b) Zoom-in spectrum shows a gap of ~ 15 mev. 12

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