The Research Of Pearl-necklace-like PbSe/Pb Ordered Arrays

Nanao-materials have great attraction due to the difference to bulk materials in basic research and practical application. Recently, preparation and characterization of nano-materials have been researched. As an important narrow-band gap IV-VI group semiconductor, PbSe is an attractive material characterized by a large bulk exciton Bohr radius (46 nm), which results in a strong confinement of the electron–hole pair and larger optical nonlinearity. PbSe can be potentially employed in various areas, such as laser materials, solar cells, infrared detectors, near-IR luminescence, and thermoelectric devices.Parallel and ordered PbSe/Pb arrays have been prepared by electrodeposition. We observe morphology by the optics microscope, scanning electron microscope and transmission electron microscope. XRD data are corresponding to the data of electron diffraction. The representative EDS patterns of the knotlike and rodlike areas of samples on the cover glass are shown respectively. The data of knotlike areas show only the presence of Pb elements except for the main composition of the glass (O, Si, Na, Ca), which indicates a high purity of Pb in the knotlike areas. Se and Pb elements are found in the rodlike areas, which demonstrates the presence of PbSe.During the electrodeposition, we can find the existence of"ladder-like"morphology. As shown from SEM, the"knotlike"and"rodlike"areas do not appear in the"ladder-like"morphology. The width of"ladder"become larger and larger along the direction of elecrodeposition. The maximal width can reach 20μm. Besides the existence of"ladder-like"morphology, we also find bifurcation. In this ultrathin film electrodeposition, the ions diffusion is slow, the ions convection is reduced, and the electric migration is dominant. In an ultrathin electrolyte layer where the electric migration overwhelms the diffusion, the electrodeposits may have a very low branching rate and self-organize into regular arrays. The studies of crystal growth suggest that branching in electrochemistry is related to the noise in transport fields. Our special experimental design provides an effective way to suppress noise during electrochemical growth. In this way, the role of the transport fields (here including the concentration field and the electrical field) on morphology formation becomes prominent. It is well-known in the theory of crystal growth that the concentration gradient on the two corners of the filament tip is larger than that in its middle part. This difference increases with the widening of the growing interface. For a sufficiently wide filament tip the nucleation on the corner regions will have much higher probability than in the center part. The local electrical field strongly influences the formation of the sample.A room-temperature NIR absorption spectrum for PbSe / Pb ordered arrays is obtained. We can see that the most prominent absorption peak is 5372 cm-1, which was observed in most samples and came from the PbSe area of the nanoheterostructures. As expected from quantum confinement, the onset of absorption is shifted substantially to the blue of the bulk band gap. Infrared absorption peaks show blue shifts due to quantum size effect and small size effect. Small dimension and large surface tension result in the increase of vibration frequency of bonds, which will lead to blue shifts. On the other hand, nanomaterials are of the quantum size effect, which makes energy level spacing widen to lead to the blue shifts of absorption peak.For the electrical properties, we measure the I-V curves of ordered arrays under three different conditions. First of all, we obtain a I-V curve when T=290K and T=20K, separately. The I-V curve exhibits a nonlinear, symmetric variation, in agreement with tunneling conductance, which means that a periodically built-in potential barrier is poorly formed in the sample structure. The potential barriers formed at the interfaces reduce the flow of charge carriers across the interface. As mentioned previously, there are two possible carrier transport mechanisms, namely, thermionic emission and tunneling. This phenomenon attributes to tunneling, which is also corresponding to the theoretical I-V characteristics of thermionic emission and tunneling. At the same time, we find current value increase under the same voltage while the temperature decreases from T=290K to T=20K. For the second condition(T=290K) and third condition(T=20K), we obtain current value increase under the same voltage while the samples are irradiated by green laser. We explain this phenomenon as follows: carrier concentration of the sample increases under green laser irradiation.