Preparation, Controllable Doping And Photoluminescence Properties Of Single-crystalline Silicon Nitride Nanowires

Silicon nitride has attracted considerable attention because of its high strength,high modulus, high temperature resistant, oxidation resistance, radiation resistanceand wide bandgap. Silicon nitride nanowires have superior physical and chemicalproperties than the bulk silicon nitride materials due to their special geometricconstruction. Thus, silicon nitride nanowires are one of the typicalstructural-functional integral bandgap semiconductor materials. However, thefurther commercial application is limited because of the problems like lowproduction, low purity, difficult to be doped, week emission light, and opticalproperties hard to be controlled. Therefore, the preparation of single-crystallinesilicon nitride nanowire, controllable dopant and photoluminescence properties areset as the research purpose in this study. The doping elements are selected in theorder of valence electron at different blocks in the periodic table. And the newprocessing technology for silicon nitride nanowires via nitriding the cryomilled highprurity silicon powder is explored. In order to control and optimize the opticalproperty of silicon nitride nanowires, the influence of doping elements and dopingcontent on the photoluminescence property of single-crystalline silicon nitridenanowires are investigated. Then, the first principle is used to calculate the bandstructure of nanowires and the photoluminescence mechanisms of silicon nitridenanowires are revealed.High purity, single-crystalline α-Si3N4nanowires are prepared usingcryomilling technology and nitridation process. Then, the photoluminescenceproperty of the α-Si3N4nanowires is investigated. The obtained results show thatSi-N bonds have been formed during the nitrogen cryomilling process, whichprevent the oxidation of Si powder. The as-prepared nanowires are homogenouswith the diameter of25nm. The forbidden band gap of the nanowires is5.0eV. Theoptical property shows that there is an emission range from350nm to650nm withfive peaks. The peaks at586nm and542nm are caused by the recombinationbetween Si dangling bond and Si-Si bond or between valence band edge and Sidangling bond. The peak at447nm is caused by the recombination between Sidangling bond and conduction band edge. The peaks at405nm and393nm can be attributed to the recombination between N4+level and valence band edge.Al element of the p-block is chosed as the doping element, with Al metal orAl(NO3)3·9H2O as the doping raw material, and the doping content is from5at.%to10at.%. When the doping content is consistant, doping raw material has littleinfluence on the photoluminescence property of the single-crystalline α-Si3N4nanowires. However, the impact of the doping content on the optical property isobvious. The max emission peak of the spectrum shows a red shift with theincreasing of Al doping content. After doped with10at.%Al, the max emissionpeak of the as-synthesisdnanowires shifts from586nm to621nm compared withthe undoped nanowires. The forbidden band gap of the doped nanowires decreasessignificantly, which is coincident with the calculation results. When the dopingcontent is increased from5at.%to10at.%, the forbidden band gap decreases from4.1eV to3.9eV. N dangling bonds of5at.%Al-doped nanowires from energy levelwith the trap depth of1.2eV from the valence band edge. However, N danglingbonds of10at.%Al-doped nanowires from an energy level at1.1eV. The change ofphotoluminescence properties is mainly caused by the decreasing of forbidden bandgap and the shift of N dangling bands.La element and Y element of the d-block are chosed as the doping elements,and the doping content is from1at.%to10at.%. When using the same dopant, thedoping raw material has little influence on the photoluminescence property of thesingle-crystalline α-Si3N4nanowires. However, the impact of the doping contenton the optical property is obvious. When doped with La, α-Si3N4nanowires show astrong emission with two peaks located at395nm and540nm in the violet-bluespectral range. When the doping content is increased from1at.%to10at.%, thephotoluminescence intensity of the peak at395nm increases obviously and is10times higher than the intensity of the undoped nanowires. The nanowires dopedwith Y element have similar photoluminescence property with the nanowiresdoped with La element. When the doping content is5at.%, two peaks at415nmand530nm are shown, and the photoluminescence intensity is15times higherthan the undoped nanowires. The forbidden band gap of the doped nanowiresdecreases significantly, which is coincident with the calculation results. Theforbidden band gap of5at.%La-doped nanowires is4.0eV, and an impurity level with the trap depth of2.3eV from the valence band edge is formed by the Laelement. The change of photoluminescence property is mainly caused by thedecreasing of forbidden band gap, the formation of the impurity level and theincrease of the N4+defect content casued by the substitute doping of La and Si.Ce element and Tb element of the f-block are chosed as the doping elements,and the doping content is5at.%. The results show that when doped with Ce, anintense violet-blue visible photoluminescence of the single-crystalline α-Si3N4nanowires is observed, and two peaks are located at392n and407nm in the spectralrange. When doped with Tb, there is a strong emission with five peaks at412nm,488nm,543nm,589nm and623nm located in the green spectral range. Theseemissions are assigned to the characteristic transitions of5D3â†'7FJ(J=6,5,4) and5D4â†'7FJ(J=6,5,4,3). The photoluminescence properties of the doped α-Si3N4nanowires are mainly associated with the f layer electron transition of the dopingelement. Silicon nitride nanowires as the substrate material have little influence onthe photoluminescence properties.