Phase transition in epitaxial bismuth nanofilms
Raman and coherent phonon spectroscopies were used to investigate the thickness-dependent phononic properties of ultrathin single-crystal Bi ﬁlms prepared by molecular beam epitaxy on Si(111) substrates. Both the A1g and Eg Raman peaks disappeared in the Raman spectra of a 4 nm Bi ﬁlm, indicating a complete transition from the low-symmetry A7 structure to the high-symmetry A17 structure. Coherent phonon signals of the A1g mode also showed a strong dependence on the ﬁlm thickness, where thin samples ( 15 nm) exhibited lower phonon fre- quency and shorter phonon lifetimes than the thick samples ( 30 nm). This difference is attributed to a shallower energy potential barrier caused by both a permanent phase transition, which is determined by the ﬁlm thickness, and a temporary structural transition by photo- excited carriers. Our results not only provide evidence of a phase transition from the A7 to the A17 structure with the decreasing Bi ﬁlm thickness but also reveal the inﬂuence of this phase transition on phonon dynamics. Understanding these material performance traits will facilitate modern application of Bi thin ﬁlms in electronic devices.