Unidirectional Alignment of CdSe Nanorods

Cadmium selenide nanorods, unique nanostructures which possess bright and up to 100% polarized photoluminescence, now can be arranged unidirectionally in a single line. The method is based on the chemical attachment of nanorods to the surface of a single cleaved semiconductor nanolayer and exploits a combination of wet-chemical and epitaxial growth techniques to produce templated semiconductor nanostructures. The nanotemplate here is a ZnS layer with a thickness less than the diameter of the nanorods. Binding the nanorods to the template, the limited space provokes an enhanced alignment of the rods along the ZnS layer which can be proved by the high polarization degree of the nanorods emission. A “bottom-up” approach in nanotechnology requires welldefined nanoelements to be placed on well-defined positions. II-VI semiconductor nanocrystals may be suitable nanoelements for both nanoelectronic and nanooptical devices due to discrete energy levels controlled by the size of the nanocrystals.1 Recently, CdSe nanocrystals have been proposed as working elements for nanotransistors,2 electrochromic materials,3 and charge-coupling devices.4 The chemical synthesis of high-quality CdSe, CdTe, ZnSe, etc. nanocrystals is now becoming a routine procedure.5 In contrast, the well-controlled one-dimensional arrangement of nanocrystals into electronic circuits, or nanooptical elements, yet is still a “state-of-the-art” problem. Based on electrostatic or chemical attachment, the nanoarrangement of nanocrystals first requires the preparation of a suitable nanotemplate. Recently, charged polymeric chains (polyelectrolytes)6 or single-walled carbon nanotubes7 have been used, as such templates for attachment of highly luminescent CdSe nanocrystals. An important problem is to control precisely either the position of the nanocrystals or their orientation. The latter, while being not essential for spherical nanocrystals, can be very important for nanorods. Synthesized for the first time only few years ago, CdSe nanorods8 came to be exciting materials due to some important properties: up to 100% polarized luminescence.9 Linearly arranged highly luminescent CdSe nanocrystals could be utilized as nanoemitters or high-resolution detectors of polarized light (recently, this idea was demonstrated on InP rods10). To the author’s knowledge there was only one attempt to control the orientation of CdSe nanorods in the liquid crystals regime;11 instead most applications require solid-state materials. In this paper we propose the utilization of a single semiconductor nanolayer as an intrinsic template for linear arrangement of CdSe nanorods. We prepared a single ZnS nanolayer grown on BaF2 substrate and covered by a SiO2 cap layer. First, a 5 nm thickness ZnS layer was vacuum evaporated onto a singlecrystal BaF2 10 × 10 × 1 mm3 substrate. Then, a 100 nm SiO2 layer was deposited in the same vacuum chamber atop the ZnS layer. Finally, we cleaved the sample orthogonally to the surface plane (single-crystal BaF2 is a highly brittle material) which opened a single 5 nm ZnS stripe at the edge (Figure 1a). This ZnS stripe is suitable for further highly spatially localized attachment of CdSe nanorods (as well as CdSe nanodots, if required). This technique is similar to the cleaved-edge overgrowth in molecular beam epitaxy to produce semiconductor nanowires. Because ZnS is a widegap semiconductor and nonabsorbing material in the region of nanorods emission, we assume the absence of possible energy transfer, or sufficient charge injection from nanorods to ZnS nanotemplate which could complicate the phenom-