Experimental Determination of the Absorption Cross-Section and Molar Extinction Coefficient of CdSe and CdTe Nanowires^†

Absorption cross-sections and corresponding molar extinction coefficients of solution-based CdSe and CdTe nanowires (NWs) are determined. Chemically grown semiconductor NWs are made via a recently developed solution−liquid−solid (SLS) synthesis, employing low melting Au/Bi bimetallic nanoparticle “catalysts” to induce one-dimensional (1D) growth. Resulting wires are highly crystalline and have diameters between 5 and 12 nm as well as lengths exceeding 10 μm. Narrow diameters, below twice the corresponding bulk exciton Bohr radius of each material, place CdSe and CdTe NWs within their respective intermediate to weak confinement regimes. Supporting this are solution linear absorption spectra of NW ensembles showing blue shifts relative to the bulk band gap as well as structure at higher energies. In the case of CdSe, the wires exhibit band edge emission as well as strong absorption/emission polarization anisotropies at the ensemble and single-wire levels. Analogous photocurrent polarization anisotropies have been measured in recently developed CdSe NW photodetectors. To further support fundamental NW optical/electrical studies as well as to promote their use in device applications, experimental absorption cross-sections are determined using correlated transmission electron microscopy, UV/visible extinction spectroscopy, and inductively coupled plasma atomic emission spectroscopy. Measured CdSe NW cross-sections for 1 μm long wires (diameters, 6−42 nm) range from 6.93 × 10^-13 to 3.91 × 10^-11 cm² at the band edge (692−715 nm, 1.73−1.79 eV) and between 3.38 × 10^-12 and 5.50 × 10^-11 cm² at 488 nm (2.54 eV). Similar values are obtained for 1 μm long CdTe NWs (diameters, 7.5−11.5 nm) ranging from 4.32 × 10^-13 to 5.10 × 10^-12 cm² at the band edge (689−752 nm, 1.65−1.80 eV) and between 1.80 × 10^-12 and 1.99 × 10^-11 cm² at 2.54 eV. These numbers compare well with previous theoretical estimates of CdSe/CdTe NW cross-sections far to the blue of the band edge, having order of magnitude values of 1.0 × 10^-11 cm² at 488 nm. In all cases, experimental NW absorption cross-sections are 4−5 orders of magnitude larger than those for corresponding colloidal CdSe and CdTe quantum dots. Even when volume differences are accounted for, band edge NW cross-sections are larger by up to a factor of 8. When considered along with their intrinsic polarization sensitivity, obtained NW cross-sections illustrate fundamental and potentially exploitable differences between 0D and 1D materials.