Efficient cyan-emitting solid carbon dots (CDs) were synthesized via a one-pot hydrothermal method. The obtained solid CDs show a broad absorption from 270-460 nm with a maximum around 400 nm, and emit intense cyan light around 500 nm with an internal photoluminescence quantum efficiency of 34.1 % under 400 nm excitation. The emission maximum of the solid CDs remains unchanged under 320-400 nm excitations. Compared with dilute aqueous of CDs (2.5 mg mL-1 ), the emission of solid CDs shows an obvious red-shift of 50 nm. The red-shift is caused by resonant energy transfer due to larger spectral overlap and smaller interparticle distance, together with a new surface state caused by aggregation in solid CDs. A lamp with white LEDs was fabricated by dropping a mixture of solid CDs, CaAlSiN3 :Eu2+ and silicon resin on the top of a near-ultraviolet LED chip. Under an operating current of 20 mA, the as-fabricated white LED generates a high-quality, warm white light with a color rendering index of 86.1, a color temperature of 4340 K, and a luminescence efficiency of 31.3 lm W-1 .