Surface-Enhanced Raman Scattering and Polarized Photoluminescence from Catalytically Grown CdSe Nanobelts and Sheets

Abstract

We have successfully fabricated single-crystalline CdSe nanowires, nanobelts, and sheets by a chemical vapor deposition (CVD) method assisted with laser ablation. The synthesized CdSe nanostructures have hexagonal wurtzite phase as characterized by X-ray diffraction (XRD). CdSe nanobelts can range in length from several tens to a hundred micrometers, in thickness from 40 to 70 nm, and a tapered width which is approximately 3 microm at one end and tapers off to approximately 100 nm at a catalytic gold particle. Both selected area electron diffraction (SAED) and high-resolution transmission electron microscopic (HRTEM) measurements show that the single-crystalline hexagonal belts and sheets grew along the [0.1-1.0] direction with side surface of +/-(0 0 0 1) and top surface of +/-(2 -1 -1 0). While the growth mechanism of nanobelts complies with a combination of vapor-liquid-solid (VLS) and vapor-solid (VS) processes, the formation of sheets is primarily based on the VS mechanism. For comparison, the phonon modes of CdSe nanobelts and bulk powder have been measured by surface-enhanced Raman scattering (SERS) and normal Raman scattering (NRS) spectroscopies with off- and near-resonant excitations. A blue-shift of 2.4 cm(-1) for the longitudinal optical (LO) phonon of CdSe nanobelts, relative to bulk CdSe, is attributed to a lattice contraction in the belt structure, which is confirmed by the XRD measurement. Room-temperature microphotoluminescence (PL) at approximately 1.74 eV from single CdSe nanobelts shows a 3-fold enhancement compared to that from bulk CdSe powder and displays a partial polarization dependence of emission angles.

Evolution of optical phonons in CdS nanowires, nanobelts, and nanosheets

“…11,13 For instance, multiphonon responses are enhanced in CdSe NBs compared to bulk CdSe, suggesting that the NBs have better crystalline quality. 7 Similarly, we suggest that the significant enhancement of the multiphonon responses in the CdS NBs and NSs is indicative of better crystalline quality in these materials than in the NWs. Pan et al have observed that the intensity ratio of the 2LO to 1LO phonon responses is much larger in CdS NWs than in bulk CdS, suggesting that the exciton-LO phonon coupling strength is much larger in the CdS NWs.…”

“…3,4 Recent progress in growth techniques which control growth parameters makes possible the synthesis not only of onedimensional nanowires ͑NWs͒ but also of two-dimensional nanobelts ͑NBs͒ and nanosheets ͑NSs͒, providing even more diverse possibilities for optoelectronic applications. [5][6][7][8] The optical properties of nanostructures are influenced by size confinement, 9 strain, 7,10-12 crystallinity, 7 and excitonlongitudinal optical ͑LO͒ phonon interaction. 11,[13][14][15] For instance, the LO phonon energies of CdS NWs with diameters larger than 50 nm do not exhibit quantum confinement effects and are mostly affected by lattice contraction.…”

“…9,11,12 Lattice contraction can be induced by surface tension during surface reconstruction in the growth of CdS NWs ͑Refs. 11 and 12͒ and CdSe NBs, 7 resulting in a significant increase of the LO phonon energy. Good crystallinity provides for welldefined and long-lived phonons which may enhance multiphonon responses.…”

“…For instance, Venugopal et al have observed that the multiphonon responses of CdSe NBs are much stronger than those of bulk CdSe source powder, suggesting an improvement in crystallinity in NBs. 7 Moreover, an enhancement of the second-order LO ͑2LO͒ phonon response can be indicative of an increased exciton-LO phonon coupling strength. 11,13 By evaluating the intensity ratio of the 2LO to first-order LO ͑1LO͒ phonon responses, Shiang et al have shown that the exciton-LO phonon coupling strength in CdS quantum dots increases with increasing dot size.…”

“…The The strength of the multiphonon responses also provides useful information on crystalline quality 7 and exciton-LO phonon coupling. 11,13 For instance, multiphonon responses are enhanced in CdSe NBs compared to bulk CdSe, suggesting that the NBs have better crystalline quality.…”