Desorption process of absorbed water by drying and heating. (a) A series of PL spectra of microtube after drying (top seven spectra, N2 flow at 50°C, 0 to 15 h) and heating (bottom five spectra, at 200°C under 30 Pa, N2 atmosphere, 15 to 23 h) as time goes on (from top to bottom). The measuring time points can be found in (b). (b) The mode (m = 89) shifts as a function of treating time. The bottom-left and top-right insets schematically illustrate the desorption processes of physically and chemically absorbed water molecules, respectively.