J. Phys. Chem. C 2009, 113, 13515–13521
13515
Stimulated Emission and Optical Third-Order Nonlinearity in Li-Doped ZnO Nanorods C. Torres-Torres,† M. Trejo-Valdez,*,‡ H. Sobral,§ P. Santiago-Jacinto,| and J. A. Reyes-Esqueda| Seccio´n de Estudios de Posgrado e InVestigacio´n-ESIME-IPN, Zacatenco, Me´xico, D. F., 07738, ESIQIE-Instituto Polite´cnico Nacional, Zacatenco, Me´xico, D. F. 07738, Centro de Ciencias Aplicadas y Desarrollo Tecnolo´gico, UniVersidad Nacional Auto´noma de Me´xico, Me´xico, D. F. 04510, and Instituto de Fı´sica, UniVersidad Nacional Auto´noma de Me´xico, Me´xico, D. F. 04510 ReceiVed: October 29, 2008; ReVised Manuscript ReceiVed: June 8, 2009
We report the structure and optical properties of semiconductor Li-doped ZnO nanorods. The sample was synthesized by an ethylenediamine-assisted low-temperature hydrothermal technique. The obtained morphology was studied by using high-resolution transmission electron microscopy (HRTEM) and X-ray diffraction techniques. Our sample exhibits a crystalline Wurtzite phase of ZnO grown along the [101] direction. However, the lattice fringes corresponding to a ZnO sample intercalated with Li ions in the growing direction correspond to 2.58 Å instead of 2.46 Å, which is the reflection [101] for the ZnO hexagonal wurzite phase. The morphology of the rods changes dramatically. In the case of ZnO rods intercalated, we obtained a sharp pencil morphology. Large values of two photon absorption were measured as well as an important photoemission response. Near resonance, the stimulated emission of the sample was measured with picosecond pulses at 355 nm, finding a lasing threshold of around 30 MW/cm2 (780 µJ/cm2). At this wavelength, the value of β ) 2.4 × 10-4 m/W was measured. Far from resonance, by measuring a self-diffraction effect at 532 nm, we obtained a value of β ) 1 × 10-10 m/W and an important pure electronic nonlinear refraction value n2 ) -1 × 10-15 m2/W. Introduction Nowadays, there is great interest in developing a new generation of materials exhibiting strong potential for photonic applications. In the last years, nanocomposites and nanostructured materials based on metals, semiconductors, and dielectrics were synthesized by different techniques, studying widely their electrical and optical properties,1-3 which make them especially attractive for nonlinear optical applications due to their enhanced third-order optical nonlinearities and fast responses.4-7 Bandgap semiconductors have received special attention because of their emission properties capable to produce efficient shortwavelength laser diodes.8-10 In particular, ZnO has broad band gap energy of 3.37 eV at room temperature and an exciton binding energy of 60 meV, which makes it suitable for efficient excitonic emission at room temperature under low excitation fluence. As an example, very interesting UV emissions in ZnO thin films grown by laserassisted molecular beam epitaxy (MBE)11 and other ZnO nanostructures12,13 have been observed. In addition, varieties of ZnO nanostructures such as nanorods, nanowires, nanocombs, nanobelts, and nanosheets can be grown by using numerous techniques.12-17 For instance, by using a vapor-solid technique, some of these ZnO nanostructures have been prepared at moderate temperatures.12,14 However, hydrothermal techniques allow us to prepare them at even lower temperatures (
