TEM images were taken by negative staining technique. The samples were loaded onto carbon-coated grids (230-mesh copper grids that had been coated with continuous thick carbon film) to cover the grid surface. Then, the grid was rinsed three times with ddH2O. Carefully, water was allowed to move away from grid and then touch the edge of grid to filter paper sliver. The grid was then stained by 3% uranyl acetate stain solution for another three times, similar to the rinsing steps. After touching the stain solution, the grid was dried for a few minutes, and then, extra stain solution was absorbed using filter paper sliver. After rinsing and staining three times, copper grids with the sample covered were dried in air.