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Photo: CUI, Peter Garten

Spectroscopy of semiconductor nanowires

Currently working on this project: Alexander Littig, Andreas Nielsen
Figure 1
Figure 1: Schematic illustration of the formation for CdSe NWs by SLS methodology.

Semiconductor nanowires (NWs) have been prepared in solution by the Solution-Liquid-Solid (SLS) method where low melting point metal such as bismuth (Bi) nanoparticle is used as catalysts (figure 1). We focused on CdE (E = Se, Te, S) structure which is used as prototype material of the II-VI group structures. The CdE NWs can be synthesized by different types of precursors. For instance, we have generated the NWs by using either the soluble molecular precursors or pre-formed CdE nanoclusters as single-source precursors. Varying the parameters of the approaches, homostructure NWs with different sizes are prepared (figure 2). Moreover, the complicated heterostructure NWs such as core-shell, block NWs are capable to be prepared by using the combination of molecular and cluster precursors (figure 1). These core-shell NWs (e.g. CdSe/CdS NWs) and block NWs (e.g. CdSe/CdTe NWs) can play the very important role for further optical and electrical devices.

Figure 2Figure 3

Figure 2: Left a, b) the TEM images of CdSe NWs; c, d) the TEM images of CdTe NWs. Right a) the TEM image of CdSe/CdTe block NW, b) element mapping of block NW; c,d) TEM images of CdSe/CdS core-shell NWs.

Currently working on this project: Dino Behn, Aina Reich

If a single NWs is illuminated, it shows, similar to nanocrystals, a blinking behavior. This means that the fluorescence intensity is not constant like in the bulk material but changes between different intensities over time (figure 4).

Figure 4
Figure 4: Blinking in a single CdSe nanowire.

The blinking is thought to be correlated to the charge of the NW, hence information about the charge state could contribute to a better understanding of the blinking phenomenon. An EFM (an AFM that probes electrostatic forces) can be employed to measure the charge state of the nanowire under various conditions.

Figure 5
Figure 5: Fluorescence, AFM, and EFM image of the same nanowire.
 Due to the fact of thermal broadening it is at room temperature not possible to resolve in photoluminescence measurements the different transitions which could occur in a single nanowire. For that reason the normal photoluminescence signal of a nanowire is a broad peak as it is shown in figure 6.
Figure 6
Figure 7
Figure 6: Photoluminescence of a single nanowire. Left: Room temperature; Right: Low temperature.

To relate these peaks to the different transitions in the semiconductor nanowire we achieve time dependent photoluminescence measurements (Figure 8). The measured Lifetimes give us information about what is happening to the charge carriers in the nanowire.

Figure 8
Figure 8: Time dependent photoluminescence measurement of a nanowire at low temperatures.