A nanophosphor sheet is presented. The nanophosphor sheet may include a base layer, a plurality of core-shell phosphors dispersed in the base layer, and a coating layer surrounding at least one core-shell phosphor among the plurality of core-shell phosphors. Also presented is a backlight device that includes a light source emitting light, a light guide plate receiving the light, and a plurality of core-shell phosphors positioned to receive the light and convert the light to white light. The core-shell phosphors may be incorporated into the light guide plate or be positioned on the light guide plate as a separate layer.