A method to prepare crystal silicon nitride nanofibers by polysis of polymer with the assistant of FeCl

2

catalyst has been presented in this paper.Scanning electronic microscopy(SEM) showed that the size of the nanofibers mostly are 100~200 nm in diameter and up to several microns in length.The growth mechanism can be explained by vapor-liquid-solid(VLS) model.The XRD pattern of the nanofibers indicated that the fiber belongs to α phase and no other crystalline phases were detected.The optical properties of the single crystalline α-Si

3

N

4

were characterized by absorption and photoluminescence(PL).The optical absorption spectrum showed the relationship between absorption coefficient a verse photo energy hν.In order to investigate the optical band gap of α-Si

3

N

4

nanofibers

we plot the relationship between(α

h

ν)

2

and photon energy

h

ν.A linear relationship was observed.That the nanofibers exhibited direct-gap semiconductor behavior with band gap of ～4.80 eV

which is different from pure Si

3

N

4

(5.0~5.3 eV).This is possible because the catalyst of Fe is dopped and the Fe element affects the silicon-nitrogen molecular vibration and the states within the band gap of the silicon nitride.Intensive luminescence was observed by bare-eye.The broad emission band can be divided into two discrete broad peak by gaussian type peak fitting.The intensive optical emission are at 1.71 eV and 2.18 eV in PLspectrum at room temperature.It is believed that there exists Si dangling bonds between N

3

and Si(called K center) and another point defect identified as the N dangling bonds between Si

2

and N(called N center).These point defects consist of an unpaired electron largely localized on a two-coordinated N and on a three coordinated Si atom

respectively.The two defects give rise to different states

the N center corresponds to a level in the valence band

while the K center is associated to a state in the middle of the gap.The PL emission associates to the inherently imperfect Si and N dangling bonds in the α-Si

3

N

4

structure.The PL mechanism and other related properties need to be further studied.