Supplementary Information. Epitaxial Growth of Single Layer Blue Phosphorus: A New Phase of Two-Dimensional Phosphorus

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1 Supplementary Information Epitaxial Growth of Single Layer Blue Phosphorus: A New Phase of Two-Dimensional Phosphorus Jia Lin Zhang, 1,2# Songtao Zhao, 3# Cheng Han, 1,2,4# Zhunzhun Wang, 3,5 Shu Zhong, 1 Shuo Sun, 2 Rui Guo, 1 Xiong Zhou, 1 Cheng Ding Gu, 1 Kai Di Yuan, 2 Zhenyu Li, 3* Wei Chen 1,2,4,6* 1 Department of Chemistry, National University of Singapore, 3 Science Drive 3, , Singapore 2 Department of Physics, National University of Singapore, 2 Science Drive 3, , Singapore 3 Hefei National Laboratory for Physical Sciences at Microscale, CAS Centre for Excellence and Synergetic Innovation Center of Quantum Information & Quantum Physics, University of Science and Technology of China, Hefei , China 4 Center for Advanced 2D Materials and Graphene Research Center, National University of Singapore, 3 Science Drive 3, , Singapore 5 Guizhou Provincial Key Laboratory of Computational Nano-Material Science, Guizhou Normal College, Guiyang , China 6 National University of Singapore (Suzhou) Research Institute, 377 Lin Quan Street, Suzhou Industrial Park, Jiang Su , China

2 Figure S1. Amorphous phosphorus thin film on Au(111). (a) Large scale STM image of clean Au(111) surface (V tip = V, nm 2 ). The inset shows the atomically resolved STM image of Au(111) (V tip = V, 5 2 nm 2 ). The red arrows in the right indicate [11 0] crystal directions of the gold lattice. (b) The lateral profile corresponding to the orange line in panel (a), reveals a single layer height around 2 Å. (c) Large scale STM image of the phosphorus thin film covered Au(111) (V tip = V, nm 2 ). (d) Close-up STM images of the phosphorus thin film with the directions of the vacancies indicated by yellow, orange and black arrows (V tip = V, nm 2 ). The red arrows in the bottom right corner indicate [11 0] crystal directions of the gold lattice.

3 Figure S2. XPS core level spectra of (a) P 2p 3/2 and (b) Au 4f for as deposited BP precursors on Au(111) and after annealing at different temperatures. Figure S3. Single layer phosphorus on Au(111). (a) STM images of single layer phosphorus on Au(111). The single layer phosphorus was obtained by depositing black phosphorus on Au(111) at room temperature and subsequent annealing at

4 250 C for 60 min (V tip = V, nm 2 ). (b) Lateral profile corresponding to the orange line in panel (a), revealing a 2.3 Å height for a single layer phosphorus. Figure S4. Large scale STM image of postannealing induced single layer phosphorus on Au(111). (a), (b) and (c) (V tip = V, nm 2 ; V tip = 1.26 V, nm 2 ; V tip = V, nm 2 ) Figure S5. Large scale STM image of single layer phosphorus on heated Au(111). (a), (b) and (c) (V tip = 1.0 V, nm 2 ; V tip = 1.0 V, nm 2 ; V tip = 1.0 V, nm 2 )

5 Supplementary Note 1. Amorphous phosphorus thin film on Au(111). Figure S1a shows a large scale STM image of the clean Au(111) surface. An atomically resolved STM image is shown in the inset, with the [11 0] crystal directions indicated by the red arrows. The lateral profile in Figure S1b reveals a thickness of ~2 Å for each Au(111) layer. STM images of the surface after 3 min deposition of P at 260 C with the substrate kept at room temperature are shown in Figure S1c. The surface appears to be uniformly covered with the adsorbate, but the surface display a blurry contrast which is indicative of a rough, amorphous morphology. Careful inspection from the close-up STM image in Figure S1d reveals that the phosphorus thin films are decorated with a lot of line vacancies. These line vacancies are orientated along the [11 0] crystal direction of the underlying Au(111) substrate, as indicated by the yellow, orange and black arrows in panel (d). Supplementary Note 2. XPS core level spectra of (a) P 2p 3/2 and (b) Au 4f for as deposited BP precursors on Au(111) and after annealing at different temperatures. XPS measurements were also performed to further confirm that the surface structure was indeed caused by the adsorbed phosphorus atoms. Figure S2a shows the XPS core level spectra for P 2p 3/2. For the as deposited amorphous BP clusters, the P 2p 3/2 spectra shows a peak located at the binding energy of ev, which is very close to the bulk BP value of ev. Further annealing the sample at gradually increased temperature did not induce any significant change of the peak position. Only a slight shift of about 0.45 ev toward lower binding energy was observed. This

6 shifting started at 225 C, which was the threshold temperature for the nucleation of blue phosphorus. Hence, the XPS spectra further confirms the transformation from P-clusters to blue phosphorus film. The XPS core level spectra for Au 4f have also been measured and no shift of the peak position or broadening of the peak or appearance of new peak is observed. This indicates that the interaction between the blue phosphorus and underlying Au(111) is most likely physical interaction.