Ln
3+ doped (Yb
3+,Tm
3+ codoped and Yb
3+,Er
3+,Tm
3+ tridoped) NaYF
4/poly(vinyl pyrrolidone)(PVP) (
Mw 1 300 000) composite fibers with an average diameter of 300-800 nm were prepared by electrospinningand characterized by X-ray diffraction, field emission scanning electron micrography, and Fourier transforminfrared spectra. Their upconversion (UC)
luminescence properties were studied in contrast to the correspondingLn
3+ doped NaYF
4 nanoparticles (15-20 nm) under 980-nm excitation. The results demonstrate that in theYb
3+,Tm
3+ codoped composite fibers the b
lue emission of
1G
4-
3H
6 is dominantly strong, while in thenanoparticles the red emission of
3F
2,3-
3H
6 contributes considerably to the increase of the excitation power.This indicates that the color purity of b
lue is improved greatly by the modification of PVP. In the tridopedYb
3+,Er
3+,Tm
3+ composite fibers, white light with more stable color balance (b
lue
1G
4-
3H
6 of Tm
3+, green
2H
11/2/
4S
3/2-
4I
15/2, and red
4F
9/2-
4I
15/2 of Er
3+) was obtained. The improved UC properties in the compositefibers are attributed to the suppressed local thermal effect. The energy transfer and UC populating processesare discussed.