Si3N4 single-crystal nanowires grown from silicon micro and nanoparticles near the threshold of passive oxidation

A simple and most promising oxide-assisted catalyst-free method is used to prepare silicon nitride nanowires that give rise to high yield in a short time. After a brief analysis of the state of the art, we reveal the crucial role played by the oxygen partial pressure: when oxygen partial pressure is slightly below the threshold of passive oxidation, a high yield inhibiting the formation of any silica layer covering the nanowires occurs and thanks to the synthesis temperature one can control nanowire dimensions.

3 Si 3 N 4 is a common material in microelectronics and optoelectronics. Si 3 N 4 nanowires (Si 3 N 4 -NW) are used in composites due to their good resistance to thermal shock and oxidation, high fracture toughness, low density and high module 1,2 . In addition to these features, onedimensional single-crystal structures with nanometric diameter exhibit enhanced mechanical properties 3 and novel electrical and optical properties. Consequently, single-crystal Si 3 N 4 -NWs have a great potential as reinforcing materials as well as in the development of electronic and optic nanodevices 4 . Thus, several approaches have been implemented for the synthesis of Si 3 N 4 -NWs, such as carbothermal reduction and nitridation of a mixture containing silicon oxide [5][6][7] , combustion synthesis 8,9 , carbon-nanotube-confined chemical reaction 10,11 , catalyst and catalystless reactions of silicon with nitrogen [12][13][14][15][16] , reaction of silicon oxide with ammonia 17 , chemical vapor deposition 18 and reaction of liquid silicon with nitrogen 19 .
Sustained reaction of solid silicon with N 2 is not allowed since solid silicon and Si 3 N 4 are barriers for nitrogen and silicon diffusion respectively 20 . Accordingly, synthesis involves the reaction of N 2 with either silicon in the vapor (CVD) or liquid (VLS) phase, resulting in the formation of α or β-Si 3 N 4 phase respectively 20 . Both mechanisms involve volatization and nitridation of silicon. This means that two conditions should be fulfilled: no protective layer can exist and any competing reaction with nitridation must be minimized. The first condition involves removing any native silica layer from silicon particles and that the process must be kept at temperatures reasonably below the melting temperature to avoid the formation of a liquid silicon layer that could be easily transformed into a solid Si 3 N 4 layer. The second condition entails a very low oxygen partial pressure and high temperatures (around 10 -19 atm at 1350ºC 20,21 ) to avoid silicon oxidation. In addition, carbothermal reduction method introduces another competing reaction resulting in the formation of SiC. As a consequence, the majority of the approaches demand setting up a complex control of gas purity, long reaction times (currently between 5 to 24 hours) and give quite low production yield. By adding some catalyst production yield can be improved at the expense of by-product formation and contamination.
In contrast, high production yield without catalyst has been obtained by means of a new technique based on a CVD mechanism 5,16 . Although it is suggested that oxygen plays an important role, the oxygen partial pressure has not been hitherto properly controlled nor 4 measured. Moreover, the mechanism is not fully understood and there is no indication of how the final structure can be controlled. Finally, Si 3 N 4 -NWs are always covered by a silica shell. In this letter, we report the synthesis of single crystal Si 3 N 4 -NWs in a reliable and straightforward way.
The actual mechanism for Si 3 N 4 -NWs growth is based on a CVD reaction involving SiO and nitrogen. The influence of oxygen and temperature is analyzed in detail.
Experiments have been carried out in a Mettler Toledo thermo-balance (model For an oxygen partial pressure of 2 10 -3 atm, we observe the formation of Si 3 N 4 in the temperature range from 1200 to 1400ºC. Since nitridation proceeds faster for larger specific surface area 23,24 , the reaction rate is higher for Si-NPs than for Si-μPs. The time required for complete nitridation decreases steadily with temperature. In the case of NP, at 1200ºC the reaction actually ends after approximately five hours, while at 1400ºC, it takes less than one hour. From XRD analysis the dominant phase is α-Si 3 N 4 . For instance, for Si-NPs and in the interval from 1300 to 1350ºC, the ratio α/β increases steadily from 5.3 to 6.5. Concerning the actual CVD mechanism, the direct reaction between silicon vapor and nitrogen is ruled out because oxygen partial pressure is too high 21 (>10 -3 atm). However, formation of Si 3 N 4 has been reported at elevated oxygen partial pressure provided that it remains below the threshold for passive oxidation 25 . Indeed, active oxidation is a source of SiO gas, which, in contact with nitrogen, results in the formation of α-Si 3 N 4 21 . That is why we propose the following two step CVD mechanism: i) formation of SiO through active oxidation, ii) reaction of SiO with nitrogen. A CVD mechanism involving SiO has been proposed in the synthesis of Si 3 N 4 -NWs 15,16 as well as other compounds [26][27][28] , in the so-called oxide-assisted catalyst-free method. A characteristic feature of this method is the formation of an unwanted amorphous SiO 2 outer layer. In view of our previous analysis, the oxygen partial pressure is a critical parameter, since it should be high enough to promote the formation of SiO, but it should remain below the threshold of passive oxidation in order to prevent the formation of SiO 2 . Consequently, the formation of a SiO 2 outer layer can be prevented simply by reducing the oxygen partial pressure.  1.b. Therefore, the silica sheath is formed when the oxygen partial pressure is above the passive oxidation threshold. Moreover, the lower the temperature is the lower the threshold. Thus, by reducing the oxygen partial pressure one can produce NW at lower temperature and have a more accurate control on the NW dimensions.
When the oxygen partial pressure is below the threshold for passive oxidation 25