Structural and Optical Properties of Single- and Few-Layer Magnetic

Transcription

1 SUPPORTING INFORMATION Structural and Optical Properties of Single- and Few-Layer Magnetic Semiconductor CrPS 4 Jinhwan Lee 1, Taeg Yeoung Ko 2, Jung Hwa Kim 3, Hunyoung Bark 4, Byunggil Kang 4, Soon-Gil Jung 5,6, Tuson Park 5,6, Zonghoon Lee 3, Sunmin Ryu 2,7*, and Changgu Lee 1,4* 1 School of Mechanical Engineering, Sungkyunkwan University, 2066 Seobu-ro, Jangan-gu, Suwon, Gyunggi-do 16419, South Korea 2 Department of Chemistry, Pohang University of Science and Technology (POSTECH), 50 Jigokro 127, Pohang, Gyeongbuk 37673, South Korea 3 School of Materials Science and Engineering, Ulsan National Institute of Science and Technology (UNIST), 50 UNIST-gil, Eonyang-eup, Ulsan 44919, South Korea 4 SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, 2066 Seobu-ro, Jangan-gu, Suwon, Gyunggi-do 16419, South Korea 5 Center for Quantum Materials and Superconductivity (CQMS), Sungkyunkwan University, 2066 Seobu-ro, Jangan-gu, Suwon, Gyunggi-do 16419, South Korea 6 Department of Physics, Sungkyunkwan University, 2066 Seobu-ro, Jangan-gu, Suwon,

2 Gyunggi-do 16419, South Korea 7 Division of Advanced Materials Science, Pohang University of Science and Technology (POSTECH), 50 Jigokro 127, Pohang, Gyeongbuk 37673, South Korea contributed equally to this work * Corresponding Authors (C. Lee: peterlee@skku.edu, S, Ryu: sunryu@postech.ac.kr)

3 Figure S1. Schematic illustration of CVT (chemical vapor transport) growth of CrPS 4 crystals from raw materials in a vacuum-sealed ampoule.

4 (a) FESEM image 20 um (b) Element Cr P S Atomic% Ratio Figure S2. (a) FESEM image of CrPS 4. (b) EDS spectrum and the atomic ratio (Inset table) obtained from the selected dashed area in (a).

5 (a) (b) c 1 nm ~0.63 nm a Figure S3. A cross-sectional view of CrPS 4. (a) TEM image and (b) corresponding atomic model.

6 Magnetic Susceptibility (emu/oe)* to C axis to C axis B = 5000 Oe Temperature (K) Figure S4. Temperature dependence of magnetic susceptibility of CrPS 4. The external magnetic field of 5000 Oe was applied with parallel and perpendicular directions to the c-axis.

7 Drain Current (A) VD 5 V 10 V 15 V 20 V μm I D V D Source CrPS 4 SiO 2 Drain V G Gate Voltage (V) Figure S5. Transfer curve of a FET device based on CrPS 4 as a channel material. Inset illustration shows a cross-sectional view of the FET device structure. A thick CrPS 4, thickness of 474 nm, was deposited on the silicon wafer with 300 nm-thick SiO 2 as a dielectric layer for bottom-gate. The channel length and width were 27.9 and 27.3 μm, respectively. The calculated carrier mobility and current on/off ratio were ~1.0x10-4 cm 2 /Vs and 10 3, respectively. The result also revealed that the channel is an n-type semiconducting material.

8 (001) (531) (24-2) (711) Intensity (a.u.) (40-1) (421) (42-1) (222) (200) (221) (20-2) (311)(31-1) (220) (002) (021) (310) (020) (201) (20-1) (111) (11-1) (420) (31-2) (022) (004) (114) (024) (424) (623) (640) (712) (712) (043) (44-2) Intensity (a.u.) (002) (003) (004) (005) (001) ( o ) o Figure S6. Experimental and theoretical XRD patterns of powdered CrPS 4 from 10 to 80 degrees. Facial indices are marked on each peak. The inset shows XRD pattern of a single crystal flake. (20-3) (402) (132) (3-31) (023) (040) (62-1) (530) (241) (313) (422) (422) 2 ( ) obs. cal.

9 intensity (a.u.) intensity (a.u.) (a) (c) (d) ~0.54 nm 0.54 nm 1 μm (b) nm (e) Distance (nm) ~0.356 nm 2 1/nm 2 nm Distance (nm) Figure S7. TEM images of CrPS 4. (a) Low-magnification image. (b) SAED pattern. (c) Highresolution image with interatomic distances along a and b directions. Line profiles of (c): (d) a and (e) b directions.

10 (a) (b) 5 nm 2 nm 5 n m (c) (d) 0.5 μm 1 nm Figure S8. HRTEM and STEM images of preferred straight edges. (a) Image obtained from an edge area that is marked by the red ellipse in the low-magnification image (inset). (b) Image zoomed-in from the dotted square in (a). It can be seen that the edge is parallel to a diagonal line of Cr atoms that are represented by the blue spheres in the overlaid crystal structure. (c) Exfoliated CrPS 4 with many straight edges. (d) High-magnification STEM image. Red and yellow dashed lines indicate the preferred edge and the alignment of Cr atoms in (c) and (d), respectively.

11 Figure S9. Polarized optical micrographs obtained as a function of the rotational angle: from 0 to 345 degrees.

12 Intensity (a.u.) (a) (b) Point 1 Red Green Blue Point 2 Point 3 Point 4 Point 5 Point Thickness (nm) Figure S10. (a) Optical micrograph of an exfoliated multi-step sample. Six spots were numbered from 1 to 6 with increasing thickness. (b) RGB channel intensities of the six spots that were represented by their thicknesses.

13 (a) Before laser irradiation (b) After laser irradiation 3L 2L 1L 5 μm 18 μw 50 μw 9 μw 5 μm Figure S11. Photoinduced oxidation of CrPS 4 in air. Optical micrographs of exfoliated CrPS 4 on SiO 2 /Si substrate (a) before and (b) after laser irradiation. The arrows mark where photoirradiation was made (average power is noted; irradiation time was 1000 s for each spot)

14 Intensity (arb. u) Intensity (arb. u) (a) F I Bulk (b) 10 I Si + M Bulk A BC D E H G J K L M N OPQ 5 0 A' A H F B CC' D E G J K L NOPQ 6 5L * * 4L 3L 2L 1L Substrate Raman shift (cm -1 ) L 4L 3L 2L 1L Substrate Raman shift (cm -1 ) Figure S12. Raman spectra of few-layered and bulk CrPS 4 obtained with two different excitation wavelengths. (a) 458 nm. The peaks marked with asterisks near 100 cm -1 originated from the excitation laser. (b) 633 nm. Unlike 458 and 514 nm excitation, two additional peaks (A & C ) were observed at 85 and 178 cm -1. The signal at 520 cm -1 is mostly due to Si rather than M peak for thin layers.

15 Absorbance Energy (ev) Figure S13. Absorption spectra of a bulk sample obtained in a transmission mode using a commercial UV/visible spectrophotometer. A thin and large piece of CrPS 4 crystal (> 2 mm in diameter) was transferred onto transparent adhesive tape that served as an optical blank. Since the detector was partially masked for the limited sample size, absorbance larger than 2.0 (energy > 2.2 ev) could not be measured due to decreased sensitivity. Nevertheless, the spectra clearly revealed two absorption edges located at ~1.3 and ~1.9 ev.

16 (a) bulk (cm -1 ) A B C D E F G H I J K L N O P Q (b) bulk / bulk A B C D E F G H I J K L N O P Q Thickness (L) Thickness (L) (c) Width (cm -1 ) A B C D E F G H I J K L M N O P Q (d) Width (cm -1 ) A B C D E F G H I J K L M N O P Q Thickness (L) Thickness (L) Figure S14. Raman peak frequencies and linewidths of 2L ~ 5L and bulk CrPS 4. (a) Frequency (ω) change of each peak as a function of thickness with respect to that of bulk (ω bulk ). (b) Fractional change in frequency defined as (ω ω bulk )/ω bulk. (c) Linewidth of each peak given as a function of thickness. (d) Close-up of (c).

17 (a) 2L 1L Quartz 3L (b) 5L 4L 5 μm 5 μm (c) 1.0 R-R 0 )/R Thickness (L) Figure S15. 2D CrPS 4 supported on quartz substrates. (a) & (b) Optical micrographs of typical samples. (b) Reflectance contrast, calculated from the blue channel of the micrographs, used in the efficient estimation of thickness. The dotted line is a linear fit to the data, which showed that the contrast increases by 0.18 ± 0.01 /L.

18 Table S1. Comparison of d-spacing values observed from powder XRD and SAED pattern with calculated d-spacing values and Miller indices. d obs (Å) d obs (Å) d d (XRD) (SAED) cal (Å) h k l obs (Å) d obs (Å) (XRD) (SAED) d cal (Å) h k l

19 Table S2. Raman frequencies and linewidths of 17 peaks obtained from 2L ~ 5L and bulk CrPS 4 with 514 nm laser. Raman frequency (cm -1 ) Linewidths (cm -1 ) Peak 2-layer 3-layer 4-layer 5-layer bulk 2-layer 3-layer 4-layer 5-layer bulk A B C D E F G H I J K L M N O P Q