More Related Content
What's hot (20)
Similar to Pb S Nanostructures.Work At Ii Sc (20)
Pb S Nanostructures.Work At Ii Sc
- 2. Lead(II) sulfide (PbS), most often purified from the mineral galena is a semiconductors of the II-VI family. The structure of PbS is identical to that of halite, NaCl. The two minerals have the same crystal shapes, symmetry and cleavage. Lead sulfide (PbS) is an important direct narrow - bandgap semiconductor with a bulk bandgap of 0.41 eV at 300K. The exciton Bohr radius of 18nm is relatively large compared to many semiconductors, which makes this lead chalcogenide an interesting material to study quantum effects.
- 3. PbS flower-shaped dendritic nanostructures were prepared by the hydrothermal method. In this method, 0.05M Pb(NO3)2 and 0.1M thiourea and 3.5 g acrylamide were dissolved in 70ml of deionized (D.I.) water. The above solution was loaded into a 100-ml Teflon-lined stainless steel autoclave and heated in an oven at 160 ◦C for 12 h. The system is undisturbed till the oven reaches room temperature and the precipitate is collected washed with D.I. water 4–5 times and dried in vacuum at 60 ◦C.
- 4. (a) Flower structures SAED pattern at the tip of the flower structures
- 5. Mixed structures of dendrites and sheets were also prepared by a similar method. Here, 0.12 g of N-cetyl-N,N,N-trimethyl-ammonium bromide (CTAB)was added to 20ml of D.I.water, then 0.08 ml CS2 was added into the above solution and thoroughly mixed at room temperature using magnetic stirrer. After CTAB dissolved completely, 1.2 ml of aqueous ammonia (NH3·H2O, 25 wt.%) was added into the clear solution, which was followed by the addition of 10 ml of 0.1Maqueous Pb(CH3COO)2. A pale yellow precipitate appeared, which was the basic acetate of lead. After stirring slowly for 5mins, the reaction temperature was increased to 40 ◦C and the system incubated at the same temperature for 3 days without disturbing it. The black precipitate was collected, washed with D.I. water, and dried in air at room temperature.
- 6. Prism shaped rod-like structures were also prepared by the hydrothermal method. Equimolar (0.1M) Pb(CH3COO)2 and Na2S2O3 were dissolved in 80ml D.I. water and the above solution was transferred to the 100-ml Teflon-lined stainless steel autoclave and maintained at 100 ◦C for 5 h. The black precipitate was collected, washed in D.I. water 5–6 times and dried in air at 50 ◦C. It was seen that cubic shaped micro-crystals were obtained in this method at elevated temperature of about 150 ◦C.
- 7. (b)Sheet and Dendrite structures (c)Rod-like structures Cubic shaped micro-crystals
- 8. Chemical bath deposition of the flower-like PbS microstructures was also carried out . Here, equimolar solutions of (0.1M) Pb(CH3COO)2 and thiourea in methanol were prepared separately. Equal volumes of both the solutions were taken in a beaker and stirred for 5min. The beaker was covered with parafilm and left undisturbed. The solution turned brown in 15 min and after 20 min particles appeared. A black precipitate appeared at the bottom of the beaker after 6 h. The precipitate was collected, filtered, washed with methanol 5–6 times and dried in vacuum.
- 9. (d)Nano–microclusters obtained by means of CBD
- 10. The absorption spectra for samples (a) – (d) are shown in Fig. (A) and (B). It is typical of these nano–microstructures that the absorption spectra are quite broad and in few cases reveal some structure.
- 11. But for sample (b), which has absorption extending into visible region till 2.5 eV, all the other samples have absorption in the NIR region between 0.5 and 1.5 eV. Whereas the absorption of samples (a) and (d) tends to increase towards lower energies, samples (b) and (c) exhibit the usual behavior of increasing absorption with increasing energy. The broadness of the absorption spectra indicates the size distribution. The broadness of the absorbance peaks was linked with size difference between primary and sub-branches of the dendrites, and scattering effects also need to be taken into account.
- 12. The temperature dependent Photoluminescence spectra(PL) of these samples (514.5nm excitation source) are shown above. Contrary to the absorption spectra, the PL spectra of all the samples are similar and show emission bands around 0.60 eV, which are blue shifted from the bulk value of 0.41 eV.
- 13. Though the samples have different morphologies, the luminescence band around 0.60 eV in all the samples indicate that its origin can be from the bigger sized crystallites. The temperature dependent PL spectra of 0.60 eV band exhibits similar behavior of decreasing intensity with decreasing temperature till 60–70K. Since the surface is relatively large and plays an important role in these structures the surface passivation/deactivation helps in many ways in enhancing both optical and electrical properties of these nano–microstructures. A few organic ligands get attached to the surface of the nanostructures and passivate the dangling bonds at the surface, which is called capping.
- 14. We have used mercaptoethanol, dodecanethiol, and oleic acid as capping agents to passivate the large surface of these structures. After capping, the samples have shown considerable improvement in their PL spectra as shown below.
- 15. It is noted here that the intensity of the low energy transitions is an order of magnitude smaller compared to the higher energy transitions in the first two samples. This kind of interdependent behaviour reveals the carrier transfer mechanism between different energy states of the quantum dots (from smaller particles to bigger particles). From the above results, it is inferred that the surface capping helps in suppressing some of the nonradiative paths and activates the core of the quantum dots. These results also indicate that even after capping there may be nonradiative paths available for the carriers in the bulk of the samples. From the morphology of these flower structures, we expect that the capping will be effective only to a certain extent at the surface of the microstructures, however deep inside the particulates, i.e., in the bulk of the sample, the passivation might not be effective.
- 16. Thick viscous solutions containing the PbS nano-micro structures in a conducting polymer matrix along with an organic solvent were prepared to test certain electrical behaviour. For this 0.25gm of PbS was mixed in a 1ml solution of PEG in DMF (1ml). Thin films of these solutions were deposited on p-type silicon substrate by spin coating. Electrical characterization of these films gave no particular junction characteristics and behaved almost ohmically. However on illuminating the with visible and near IR radiation the current produced increased by about 10 times.
- 17. mA mA V V With illumination Without illumination
- 18. The mapping of the entire sample was done by means Light Beam Induced Current (LBIC) measurements. The image showed intense charge carrier activity in the regions that contained PEG - PbS complex.
- 19. The photoconductive activity of PbS – PEG films promises a large number of possible applications in photonics etc These PbS nanostructures are also being used in sensors, phtodetectors etc.
- 20. Temperature and capping dependence of NIR emission from PbS nano–micro crystallites with different morphologies by Naresh Babu Pendyala ∗, K.S.R. Koteswara Rao. L. Dong, Y. Chu, Y. Liu, M. Li, F. Yang, L. Li, J. Colloid Interf. Sci. 301 (2006) 503. www.wikipedia.org