Semiconductor nanorods

The aforementioned frequency of the use of these nanomaterial shapes is best attributed to two factors (1) the ease with which these nanoparticle shapes can be synthesized in the laboratory and (2) the availability of these nanomaterials from commercial sources. It cannot be the aim of this review to cover all of the different nanomaterials used so far, but some of the most commonly investigated will be introduced in more detail. For zero-dimensional nanoparticles, emphasis will be put on metallic nanoparticles (mainly gold), semiconductor quantum dots, as well as magnetic (different iron oxides) and ferroelectric nanoparticles. In the area of onedimensional nanomaterials, metal and semiconductor nanorods and nano wires as well as carbon nanotubes will be briefly discussed, and for two-dimensional nanomaterials only nanoclay. Finally, researchers active in the field are advised to seek further information about these and other nanomaterials in the following, very insightful review articles [16, 36-45]. 

One-dimensional semiconductor nanorods, because of their unique electronic, optical, and mechanical properties, are attractive dopants for liquid crystals. Hence, composites of these two distinctive materials undoubtedly have great potential for applications in electronic and optical devices. 

Chen et al. reported on a general approach by which the polarization of the emission from semiconductor nanorods can be manipulated by an external bias. In their device, the composite of a nematic liquid crystal mixture (E7, Merck) and nanorods (CdS) filled into an ITO-coated cell with an optimized concentration of one CdS nanorod per 1010 LC molecules was used to achieve the highest polarization ratio of the suspended nanorods [447, 448]. The nematic liquid crystal in this system acts as a solvent and media whose direction of alignment can be tuned by an applied electric field. Hence, the orientation of the CdS nanorods can be fine-tuned by an external bias because of the anchoring force between the liquid crystal... 

On a side note, Ouskova and co-workers also reported that the composite of magnetic /i-FejOs nanorods in 5CB showed lower threshold voltages than pure 5CB, and that the sensitivity of the nematic liquid crystal to external magnetic fields was increased in the presence of such magnetic nanorods [451]. Finally, several groups interested in the macroscopic organization and orientation of nanorods also reported on the formation of a lyotropic liquid crystal phase induced by the self-assembly of polymer-coated semiconductor nanorods [453—457], which might be used to improve the device performance, for example, of solar cells. 

ELECTRIC FIELD EFFECTS ON OPTICAL PROPERTIES OF SEMICONDUCTOR NANORODS... 

We studied electric field effects on optical properties of CdSe/ZnS nanorods integrated in thin films sandwiched between transparent electrodes. It was demonstrated that P-polarized component of the photoluminescence of CdSe/ZnS nanorods is quenched stronger by external electric field than the S-polarized component. Quantum dots are more sensitive to external electric field than the nanorods. A mechanism of external electric field influence on the luminescence spectrum of semiconductor nanorods is discussed. 

It was previously demonstrated theoretically [1] and experimentally [2] that semiconductor quantum dots (QDs) show strong dependence of optical properties on an electric field. Chemically synthesized semiconductor nanorods also exhibit the electric field effects. For example, quantum-confined Stark effect and luminescence quenching of single nanorods were previously demonstrated [3-5]. Unlike QDs, the nanorods exhibit quantum confinement only in two dimensions. It is reasonable to assume that the electric field applied along a nanorod may result in the strong polarization dependence of photoluminescence (PL). In the present paper, we investigate the influence of an external electric field onto luminescent properties of chemically synthesized CdSe/ZnS nanorods. 

Electric field effects on optical properties of semiconductor nanorods. 132... 

Recently, synthesis of semiconductor nanorods (nanowires or nanoflbers) and investigation of their properties have aroused much interest. Ge et al. (2002) showed that irradiation technique can be very useful in this regard as well. Cadmium sulfide (CdS) nanorods were successfully prepared by y-irradiation at room temperature and ambient pressure using urea as the template. X-ray diffraction (XRD) pattern showed that the phase of the product was hexagonal. The mean diameter of the nanorods was about 40 nm and the length was up to about 100 nm. The ratio between the length and the width is as high as 5 2. More importantly, they demonstrated that the successful obtainment of nanorods was determined by not only the presence of urea in the system but also the crystallization rate of urea from the solution. 

L. Li, J. Walda, L. Manna, A.P. Alivisatos, Semiconductor nanorod liquid crystals. Nano Lett. 2, 557-560 (2002)... 

C. Nobile, L. Carbone, A. Fiore, R. Cingolani, L. Manna, R. Krahne, Self-assembly of highly fluorescent semiconductor nanorods into large scale smectic liquid crystal structures by coffee stain evaporation dynamics. J. Phys. Condens. Matter 21, 264013 (2009)... 

Semiconductor nanorods are very promising for the future development of ET-based switches controlled by an electric field. In particular, CdSe/CdS nanorods exhibit a large quantum-confined Sfark effect at the single-particle level, which enables direct control of the spectral resonance between donor and acceptor required for nanoscopic Forster-type ET in single nanorod-dye couples.The applied electrical field, in fact, allows to tune... 

Figure 32 Electrically tunable energy transfer from a single semiconductor nanorod to a dye molecule. High-resolution (a) and overview (b) transmission electron micrographs showing the structure of the CdSe/CdS nanocrystals used, (c) For a specific set of a single nanocrystal and a single dye molecule no energy transfer occurs because of the lack of spectral overlap between nanocrystal emission and dye absorption, (d) After application of an electric field, the nanocrystal s PL is red shifted, resulting in the resonance of the nanocrystal and dye transitions. This leads to energy transfer to the dye and subsequent emission, (e) Absorption (dashed lines) and PL (solid lines) spectra of nanocrystals (blue lines) and dye (red lines). Absorption spectra were measured in chloroform solution at room temperature, whereas emission spectra in polystyrene/dye blends at 50 K. Note the considerable spectral overlap of nanocrystal emission with dye absorption. The inset shows the solution absorption and PL of the nanocrystal excitonic feature. (Reprinted by permission from Macmillan Publishers Ltd from Ref. 72.)...

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