Supplementary Material Freestanding Cubic ZrN Single-crystalline Films with Two-dimensional Superconductivity Yuqiao Guo, Jing Peng, Wei Qin, Jiang Zeng, Jiyin Zhao, Jiajing Wu, Wangsheng Chu, Linjun Wang, Changzheng Wu,*, and Yi Xie Hefei National Laboratory for Physical Sciences at the Microscale, CAS Center for Excellence in Nanoscience, and CAS Key Laboratory of Mechanical Behavior and Design of Materials, University of Science & Technology of China, Hefei 230026, PR China. International Center for Quantum Design of Functional Materials (ICQD), and Synergetic Innovation Center of Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China. National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui 230029, China. Experimental Details: Synthesis of vdw-zrn crystal and related exfoliation β-zrncl crystal was prepared by the reaction of Zr and NH 4 Cl according to previous procedure 1, and the as-prepared β-zrncl powder was purified and crystallizing through chemical vapor transport with the agent of NH 4 Cl. Then, the obtained ZrNCl crystal was mixed with CaH 2 (with the molar ratio of 1:2) in a Ar-filled glove box, sealed in evacuated quartz tube, and reacted at the temperature of 650 C for four days. To remove the residual CaH 2 and the CaCl 2 byproduct, NH 4 Cl/methanol solution was performed to wash them out and golden vdw-zrn crystal was obtained. To obtain ZrN nanosheet, mechanical exfoliation was used to laminate vdw-zrn crystal using Scotch tape. Characterization of ZrN nanosheet XRD analysis were carried out on a Philips X Pert Pro Super diffractometer with Cu Kα radiation (λ = 1.54178 Å). AFM was performed by AFM (Bruker, Demension Icon) with contact mode. Field-emission scanning electron microscopy (FE-SEM) images were achieved using a JEOL JSM-6700F SEM. Raman spectra were measured at room temperature with a Renishaw Raman System, of which the excitation wavelength is 532nm. XAFS measurements at the Zr K-edge (18008 ev) were carried out in fluorescence mode at the BL14W1 beamline of the Shanghai Synchrotron Radiation Facility (SSRF), China. The storage ring of SSRF was operated at 3.5 GeV with a maximum current of 210 ma. TEM and HAADF-STEM images were obtained using a JEOL-2010 transmission electron microscope at an acceleration voltage of 200 kv. The ZrN device with standard four-point probe geometry was fabricated by subsequent focused ion beam (FIB) process using FEI Helios NanoLab650 FESEM/FIB dual beam system, with the S1
deposition ion beam current of 24 pa. The magnetic-transporting property was measured through the four-probe technique by Quantum Design physical property measurement system (PPMS-9). Before measurement, the temperature of Chamber is set to 380 K for 30 min to remove the possible impurity via surface adsorption. Calculation method: Our first-principles calculations were carried out in the framework of the recently developed strongly constrained and appropriately normed (SCAN) meta-gga with a nonlocal vdw correction as from the revised Vydrov Van Voorhis 2010 (rvv10) functional using the Vienna ab initio simulation package (VASP) 2-4. All self-consistent calculations were performed with a planewave cutoff of 400 ev on a 17 17 1 Monkhorst-Pack k- point mesh. A supercell with a vacuum layer more than 15 Å thick is used to ensure decoupling between neighboring slabs. For structural relaxation, all the atoms are allowed to relax until atomic forces are smaller than 0.003 ev/ Å. Supplementary Figure S1. The Cl atoms in layered ZrNCl crystals were removed by topo-chemical transformation, resulting in ZrN crystal that remains the lamellar structure with a strong orientation of (00l) facet. Atomically thin ZrN nanosheet endowing with high crystallinity is further achieved by mechanically exfoliating the obtained lamellar ZrN crystal with rocksalt-type centrosymmetric crystallographic structure (S. G. Fm3m). S2
Supplementary Figure S2. XRD pattern showing the topotactic transformation process. (a) ZrNCl, (b) vdw-zrn, (c) c-zrn. From Figure S1, layered structure can be clearly seen in the synthesized ZrN obtained from β-zrncl, with the complete removal of chlorine; and the lamellar ZrN has the XRD pattern paralleling that of standard commercial ZrN with cubic lattice framework (Fig. 1a), while the intensity ratio of I(002)/I(111) in the synthesized ZrN was distinctly enhanced from 0.86 to 7.84 when compared to c-zrn, indicating the high orientation of (002) facets arising from layered structure. Supplementary Figure S3. SEM images of vdw-zrn crystal. S3
Supplementary Figure S4. TEM images of exfoliated vdw-zrn crystal. Supplementary Figure S5. Raman spectra of ZrN with different thicknesses. The peak marked by star is Raman vibration from Silicon. Raman spectra were provided to characterize structural information of ZrN crystal and the exfoliated nanosheets. From Supplementary Figure S5, the vibration modes of all samples with different thicknesses can be assigned to acoustic phonons of ZrN. As thickness decreases, the A 2 peak slightly stiffens (blue shift) 5. S4
Supplementary Figure S6. Device image of ZrN nanosheet. (a) AFM topography, the scale bar is 5 μm. (b) The height profile for white line in (a). Supplementary Figure S7. Temperature dependence of normalized resistance for ZrN nanosheets with different thicknesses. As can be seen in Supplementary Figure S7, the superconducting critical temperature (T c ) in ZrN nanosheets gradually decreases as function of thickness, and the resistance of ZrN nanosheets with thickness less than 6.8 nm cannot reach to zero at 2 K. S5
Supplementary Figure S8. (a) Temperature dependence of resistance of c-zrn under different magnetic fields. (b) Magnetoresistance of c-zrn near the onset of the superconducting phase at various temperatures. Supplementary Table S1. Fitting XAFS data of Zr-N and Zr-Zr pairs in the real space (R-space), where R is bond length, σ 2 is degree of disorder and E 0 is bond energy. samples pair Number R (Å) 2 ( 10-3 Å 2 ) E 0 (ev) c-zrn Zr-N 6.0 0.6 2.31 0.02 4.8 1.0-0.2 Zr-Zr 12.0 1.0 3.24 0.03 4.8 0.4-8.4 vdw-zrn Zr-N 5.6 0.6 2.32 0.02 5.0 1.0 0.5 Zr-Zr 10.5 1.0 3.41 0.02 5.7 0.5-7.0 Reference: (1) Feng, F.; Guo, H.; Li, D.; Wu, C.; Wu, J.; Zhang, W.; Fan, S.; Yang, Y.; Wu, X.; Yang, J. Highly efficient photothermal effect by atomic-thickness confinement in two-dimensional ZrNCl nanosheets. ACS nano 2015, 9, 1683-1691. (2) Sabatini, R.; Gorni, T.; de Gironcoli, S. Nonlocal van der Waals density functional made simple and efficient. Phys. Rev. B 2013, 87:041108. (3) Sun, J.; Ruzsinszky, A.; Perdew, J. P. Strongly constrained and appropriately normed semilocal density functional. Phys. Rev. Lett. 2015, 115:036402. (4) Peng, H.; Yang, Z. H.; Perdew, J. P.; Sun, J. Versatile van der Waals density functional based on a meta-generalized gradient approximation. Phys. Rev. X 2016, 6:041005. (5) Spengler, W.; Kaiser, R. First and second order Raman scattering in transition metal compounds. Solid State Commun. 1976, 18, 881. S6