Phononic, photonic and excitonic properties of ∼5 nm diameter aligned CdSe nanowires

The first work on quantum-sized CdSe nanowires (NWs) was made with NWs encapsulated in chrysotile asbestos nanotubes (asb-CdSe-NWs) in 1997. However, asb-CdSe-NWs remain under-investigated compared to widely-known solution-based CdSe NWs. Here, we study ∼5 nm diameter asb-CdSe-NWs aligned along their c-axis. Polarized optical absorption, Raman and photoluminescence spectra (OAS, RS and PLS) of asb-CdSe-NWs are examined. All spectra display a high anisotropy mainly associated with the anisotropic nearly cylindrical shape of NWs and dielectric contrast between NWs and asbestos, NW-light interaction being strong for the E//c and weak for E⊥c light polarizations. Asb-CdSe-NW E//c OAS shows ∼1.93 eV and ∼2.38 eV bands of excitonic transitions between size-quantized electronic states. RS display strong longitudinal-optical-phonon band with a weak surface-mode shoulder resonantly enhanced at the excitation wavelength corresponding to the ∼2.38 eV absorption band related to the 1Π_1/2→1Π_e transition. Acoustic radial breathing mode is observed at ∼14 cm⁻¹. Low-excitation PLS show polarized exciton emission band at ∼1.883 eV while high-excitation PLS display ∼1.836 eV biexciton emission band. The ∼47 meV biexciton binding energy is enhanced due to the low-permittivity environment. Our asb-CdSe-NW results make an important complementary contribution to the studies of CdSe NWs fabricated by a variety of different techniques.