The doped melamine crystals are not only colorful, but also fluorescent. In this section a comprehensive study on the fluorescence of each single crystal is carried out by maping the luminescence intensity as functions of both excitation wavelength λ ex and emisson wavelength λ em in the UV-visible range . The left panels in Fig. display the fluorescence excitation-emission wavelength maps for aqueous THF solutions of TCNE and melamine prepared by mixing 0.1 M aqueous solution of melamine and a THF solution of TCNE with molar concentrations of 0.0002 M (0.2 %), 0.002 M (2.0 %), 0.01 M (10 %), 0.02 M (20 %), 0.1 M (100 %), and 0.2 M (200 %), where the percentages in the parentheses are the molar concentrations of TCNE relative to melamine. The map for 0.1 M aqueous THF solution of pure melamine is also shown at the bottom. Their integrated excitation-and emission-wavelength profiles are plotted in Fig. . The pure melamine solution exhibits a strong emission at λ ex = 310 nm (M ex 1 ) and λ em = 368 nm (M em 1 ). As the molar concentration of TCNE relative to melamine is increased from 0 to 20 %, the M 1 emission of melamine becomes weaker, and the emission at λ ex = 368 nm (S ex 1 ) and λ em = 480 nm (S em 1 ) emerges. At the concentration higher than 100 %, both M 1 emission of melamine and S 1 emission disappear, and multiple emission peaks centred at λ ex = 493 nm (S ex 2 ) and λ em = 593 nm (S em 2 ) emerge. Note that the subscript numbers are given so that possible excitation-emission peak pair have the same number.