Pressure-induced Phase Transitions In Nanostructured Silicon

Silicon is the principal material used for microelectronics,photovoltaics and microelectromechanical systems(MEMS)technologies.Nanostructured Si,such as Si nanoparticle(SiNPs),Si nanowire(SiNWs),and porous Si,opens up a new area of Si materials with unique electronic,mechanical,and optical properties and wide applications.Si has a rich phase diagram with more than ten polymorphs under pressure.The pressure-induced phase transitions in various nanostructured Si have been extensively studied in the past decade.However,it is still not clear how the size and morphology of the nanostructured Si affect the phase transitions.In this thesis,using in-situ high-pressure synchrotron x-ray diffraction we systematically studied the pressure-induced phase transitions in a series of nanostructured Si samples including SiNPs(average size～100 nm and～10 nm),SiNWs(diameter up to～100 nm),and porous SiNWs(particle size～10 nm)to clarify the effect of size and morphology of nanostructured Si on their phase transitions.In bulk Si,the original diamond cubic phase(Si-Ⅰ)transforms to β-Sn phase(Si-Ⅱ)at～12 GPa,followed by a transition to a tetragonal phase(Si-Ⅺ)at～13.2 GPa,and a transition to simple hexagonal phase(Si-Ⅴ)when further compressed to above 15.4 GPa.During decompression,the last two phase transitions(Si-Ⅱ→Si-Ⅺ→Si-Ⅴ)are reversible.Then Si-Ⅱ transforms to a rhombohedral phase(Si-Ⅻ),and eventually to a body-centered cubic phase(Si-III).The large SiNPs with average size～100 nm show quite similar high-pressure behavior with their bulk-counterpart.In contrast,the small SiNPs with average size～10 nm show quite different phase transitions from bulk Si.During compression,the phase-transition pressure increases to 16.4 GPa.More interestingly,Si-Ⅰ transforms directly to Si-Ⅴ instead of Si-Ⅱ.During decompression,the Si-Ⅴ transforms to Si-XI,then to Si-Ⅱ,and eventually to a-Si.These results suggest the decreasing size of SiNPs not only raises the phase-transition pressure in compression,but also changes the phase-transition route.The SiNWs with diameters up to～100 nm show slightly different high-pressure behavior with the large SiNPs,i.e.Si-Ⅰ transforms to Si-Ⅺ at 15.2 GPa,and both Si-Ⅲ and a-Si were formed upon decompression.The difference could be attributed to the relative smaller diameter of the SiNWs compared with the large SiNPs.Moreover,the phase transitions in porous SiN Ws with average particle size～10 nm are quite similar to that in the small SiNPs,although their morphology are quite different.These results imply that size is the dominant factor that controls the phase transitions in nanostructured Si,while morphology alone does not show obvious effect.