Atom Concentration, %

C

Fe

O

Point 1 in Fig.7a

11.10 ± 1.91

88.90 ± 0.74

0

Point 2 in Fig 7a

11.81 ± 1.39

87.35 ± 0.73

0

Point 3 in Fig.7b

13.74 ± 1.89

86.26 ± 0.70

0

Point 4 in Fig.7c

21.39 ± 1.70

73.64 ± 0.63

5.05 ± 0.88