Optical properties of PbS nanocrystals

PbS is an excellent semiconductor material because of its narrow band gap ~ 0.41eV and a large exciton Bohr radius of 18 nm [55]. Strong quantum confinement of both charge carriers (electron and hole) can be easily achieved by fabricating PbS crystals with a radius less than 18 nm. The photoluminescence (PL) emission spectra, as shown in Fig. 5-26 (a), are measured from 600 nm to 800 nm at the room temperature and the excitation wavelength is 514 nm. Figure 5-26 (a) indicates that there is no emission signal in pure Pb nanowires but a peak located around 655 nm appears after PbS nanocrystals produce. The PL properties of the PbS nanocrystals are fundamentally changed with respect to the bulk material. This modification is caused by the confinement of the free carriers into the volume of the nanocrystal.

The emission intensity reaches a maximum value after 1 hour sulfization. When sulfization time is longer than 1 hour, the intensity of emission peak begins to decay and its position changes very little as the time increases. Due to the uniform size distribution, the half band width of the emission peak at 655 nm is much narrower than most others. A slight red shift is observed in the maximum of the PL spectrum as the reaction time increases. The probable explanation is that the energy gap shifts to lower energies with the increasing of the particle size due to a quantum size effect.

The experimental spectrum of PbS nanocrystals with 1 hour sulfization has been deconvoluted, as shown in Fig. 5-26 (b). The orange-red emission is fit well by two Gaussian functions with a major peak centered at 655 nm (1.89 eV) and a shoulder at 670 nm (1.85 eV). To understand the physical mechanisms of the luminescence in PbS, many researches have been discussed. The electron state can determine almost any properties of nanocrystals. The electronic structure of PbS nanocrystals has been studied using the parabolic effective mass model [109], hyperbolic-hand approximation [110], tight binding type calculations [111], and four-band

envelope-function formalism [50]. The possibility of energy transitions in both electron and hole levels of PbS nanocrystals was revealed in four major types, the Se-Sh, Se-Ph, Pe-Sh and Pe-Ph transitions [57, 108]. In Fig. 5-26 (b), the emission peak observed at 1.89 eV corresponds to the Se-Sh transition which is a lowest energy transition. This indicates that the photoluminescence is attributed to the direct recombination of free electron and hole, a kind of band to band transition energy, in the PbS nanocrystal. The transitions at higher energies (Se-Ph, Pe-Sh and Pe-Ph) do not appear in our experiment because of the presence of a blue emission band (400~ 600 nm) of the porous anodic alumina membrane [9, 13].

600 650 700 750 800

Intensity (a.u.)

Wavelength (nm)

Pb wire Sulfization 1 hr Sulfization 3 hrs Sulfization 6 hrs Sulfization 8 hrs

(a)

600 650 700 750

Intensity (a.u.)

Wavelength (nm)

1.65 1.93 Energy (eV)1.79

2.07

Fig. 5-26 (a) A series of PL spectra for PbS nanocrystals of different exposure time to H2S gas. (b) Schematic diagram for PL curve of the PbS nanocrystals with 1 hour sulfization time whose spectra have been deconvoluted by Gaussian functions.

Additionally, PL band at the long wavelength side of PbS nanocrystals may be attributed to the defects. There have been a few researches in the PL properties of PbS nanostructures [33, 59, 68, 91, 112]. The emission related to defects in the PbS nanocrystal has not yet been observed. However, the similar result exists in other materials [113-116]. ZnSe [113] nanocrystals produced photoluminescence in a blue region, which had two components peaking at 387 nm and 475 nm. The former was assigned to the excitonic emission and the other was a defect emission. In addition, Lei and his coworkers [114] reported PL spectra of ordered TiO2 nanowire arrays had three peaks which were attributed to self trapped excitons (425 nm), and two kinds of

(b)

oxygen vacancy (465, 525 nm). Furthermore, the defect emission usually has less energy than the major peak originating from the substrate and owns a wide range shoulder. In this case, the emission band peaking at 670 nm can be ascribed to electron transition by the defect energy levels in the band gap, such as vacancies or dislocations formed during sample sulfization. The concentration of defects can be increased markedly in the PbS nanocrystals as the reaction time increases. The defects will lend to nonradiative transition and stronger luminescence quenching.

This fact would diminish obviously the intensity of the emission band. The condition is observed in the PL spectrum of the PbS sample with 8 hours sulfization (Fig. 5-26 (a)). Therefore, that a broad emission peak appears at 670 nm should be assigned to the defects produced from the formation of PbS nanocrystals.

5.8 Summary

In this chapter, we fabricated Pb nanowires in porous alumina membranes with different diameters by the pressure casting process. It can produce large quantity of Pb nanowires with 20, 80, 200 and 300 nm average diameters. The diameter of the nanowires can be controlled by selecting templates with a desired size. The formation of nanowires has a high temperature gradient and a slow growth rate besides the directional solidification. According to theoretical calculation, the nanowires prepared by the pressure casting process can be single crystal structure regardless of the diameter. From the TEM experiments, as the diameter of the nanowires increases, the microstructures of Pb wires are easily formed polycrystals due to the influence of mold properties. Therefore, the nanowires with 20 and 80 nm average diameters are single crystals but ones with 200 and 300 nm average diameters are polycrystals.

or H2S gas. When the PbS nanocubes fabricated from sulfur vapor, the sulfur crystals deposited from the nonreactive sulfur species would be the occasion of the surface roughness and prevent the Pb wire from reacting with the S1(g) gas. Therefore, we decided to use the H2S gas to proceed following experiments. According to the results of the TEM analysis, the PbS nanocube consists of several nanocrystals and as reaction time increases the defects in the nanocrystal increases gradually. The DSC analysis shows that there is no exothermic and endothermic reaction happened in the PbS nanocrystals. That is, the PbS nanocrystals have good thermal stability during temperature range (373 K-623 K). The photoluminescence properties are investigated and the prepared samples display a luminescence around 650-680 nm at room temperature. The emission band could be decomposed into two sub-peaks, one centered at 655 nm is contributed by the recombination of the electron and hole in the PbS nanocrystals; the other located around 670 nm is originated from defects.

The existence of the defects would induce the nonradiative transition and substantially decline the intensity of the emission band. Therefore, the optimal condition fabricated the PbS nanocrystals is 1 hours sulfization. A significant quantum confinement effect makes the energy gap of PbS nanocrystals produce a blue shift from 0.41eV to 1.89 eV. On the basis of our present investigation, it may be promising for applications in the fabrication of photoelectric materials.

Chapter 6 Conclusion

1. The blue PL emission band of porous alumina membrane prepared in oxalic acid solution is consisted of two peaks: one centered at 443 nm is attributed to the oxygen defects of the alumina membranes, and the other located around 470 nm is associated with the oxalic impurities produced in the anodization process.

2. The effect of electrolyte has a large influence on the PL properties of porous alumina membrane. The alumina membranes only anodized in oxalic acid solution has a strong blue emission band.

3. The distribution of the oxalic impurities can be profiled based on the PL spectra of porous alumina membranes with different etching times. The density of the oxalic impurities increases with the depth of the pore wall, and impurities concentrate mostly in the intermediate oxide.

4. Pb nanowires with diameters of 20, 80, 200, and 300 nm have been successfully fabricated by casting process. According to theoretical calculations, the nanowires prepared by pressure casting are single crystal structures regardless of their diameters. From TEM experiments, as the diameter of the nanowires increases, the microstructure of the nanowires is easily formed polycrystals due to the effects of mold properties.

5. The PbS nanocrystals were produced after Pb wires with a diameter 20 nm reacted with H2S gas. In order to obtain the great PL properties, the optimal

condition fabricated the PbS nanocrystals is 1 hours sulfization.

6. As indicated in the PL spectra, an orange-red emission band around 650-680 nm appears at room temperature. The emission band can be decomposed into two sub-peaks, one centered at 655 nm is contributed by the recombination of the electron and hole in the PbS nanocrystals; the other located around 670 nm is originated from defects.

7. A significant quantum confinement effect is observed in this study. It makes the energy gap of PbS nanocrystals produce a blue shift from 0.41eV to 1.89 eV.