Temperature and capping dependence of NIR emission from PbS nano–microcrystallites with different morphologies

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Abstract

We have prepared various morphological PbS nano–microcrystallites by three different techniques namely, facile hydrothermal technique, chemical bath deposition and surfactant-assisted solution growth. This is the first report on the photoluminescence emission bands around 0.60 eV and their evolution with temperature in dendrite/rod shaped PbS microstructures. The 0.60 eV band, which is common to all our samples, quenches at low temperatures up to 60–70 K. However, after surface capping with mercaptoethanol (C2H5OSH), a new band around 1.0 eV appears and evolves with temperature, indicating bimodal size distribution in these dendritic nanostructures. On the other hand, the samples grown by chemical bath deposition exhibited this bimodal size distribution even without surface capping. However, after surface is protected with mercaptoethanol, enhancement in 0.60 eV band intensity (4–5 times) with lowering temperature is a specific characteristic of these samples. Anomalous temperature dependence of photoluminescence intensity with corresponding changes in full width at half maxima is an interesting behaviour that indicates thermalization of the carriers in nano-particles of different sizes. We note that the surface capping is an important step in understanding the bimodal nature of hydrothermal grown nanostructure and microstructure. The present results indicate the possibility of white light emission from dendritic structures.

Introduction

The optical properties of nanocrystals are strongly size and shape dependent [1]. Lead sulfide (PbS) is an important direct narrow-bandgap semiconductor with a bulk bandgap of 0.41 eV at 300 K. The exciton Bohr radius of 18 nm is relatively large compared to many semiconductors, which makes this lead chalcogenide an interesting material to study quantum effects. It has wide applications in many fields such as Pb2+ ion selective sensors, photography, IR detectors, and solar absorbers [2], [3], [4], [5]. Dendrite structures have attracted much attention in recent years due to their potential applications [6], [7], [8] in display devices and other optoelectronic applications. Many workers have prepared PbS with different micro-morphologies by hydrothermal [9], [10], [11], [12], [13], ultrasonic [14], surfactant-assisted solution growth [15], and chemical bath deposition (CBD) [16] techniques. Zhou et al. [17], Thongtem et al. [18], and Xiong et al. [19] had reported photoluminescence (PL) from dendritic PbS structures. Cao et al. reported PL from cubic PbS structures [20]. All of them had observed the PL band around 440 nm by exciting with ultra violet wavelengths of 250 nm and 360 nm, except Zhou et al. who observed the emission around 630 nm by exciting with 470 nm. We also have observed the PL band in our dendrite structures around 440 nm by excitation with 360 nm. But we have concentrated on NIR region by using 514.5 nm for excitation. There are several reports on room temperature photoluminescence in other chalcogenide dendritic systems of CdS [21], PbSe [22], CdSe [23], and InS [24]. Though the crystallites are large in size (a micron or bigger), in all these systems the photoluminescence band is blue shifted from the bulk value and the origin of the luminescence is not clearly understood. We have found no reports on temperature dependent PL studies on dendrite structures.

We have prepared PbS nano–microcrystallites with various morphologies by hydrothermal and CBD techniques. To the best of our knowledge, this is the first report on the observation of a blue-shifted PL band at 0.60 eV in these crystallites, which decays at low temperatures. However, after capping the surface with mercaptoethanol (C2H5OSH), a new band appears at 1.0 eV in dendrite structures. Rod shaped structures has also shown similar luminescence behaviour, which is attributed to the bimodal size distribution in these microstructures. Contrary to the samples grown by hydrothermal method, the CBD grown PbS microstructures have exhibited bimodal size distribution even without surface capping. Anomalous temperature dependence of PL intensity and the existence of optimum full width at half maxima (FWHM) are important characteristics of these nano–microstructures.

Section snippets

Experimental

We have prepared PbS flower-shaped dendrite structures (sample 1) by the hydrothermal method used by Wang et al. [10]. In this method, 0.05 M Pb(NO3)2 and 0.1 M thiourea and 3.5 g acrylamide were dissolved in 70 ml 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

Results and discussion

The SEM and TEM pictures of the samples 1–4 are shown in Fig. 1(a–e). Fig. 1(a) shows sample 1, which is made up of 2–3 μm sized flower-shaped structures with nanometer-sized tips. The electron diffraction pattern displayed in Fig. 1(b) is taken from the tip of the flowery structures and indicates excellent crystalline nature of these structures. PbS sheets and dendrite structures are observed in sample 2, Fig. 1(c), where typical thickness of the sheets varies from 100 nm to 500 nm and smaller

Conclusion

In conclusion, we have prepared PbS nano–microstructures by different techniques. There are two prominent bands around 0.6 and 1.0–1.2 eV in dendrite shape structures. The absence of higher energy bands in rod and cluster type structures indicates their bulk nature. The emission shows a strong dependence on shape and size. The anomalous temperature dependence of PL intensity with corresponding changes in FWHM suggests a bimodal size distribution. SEM pictures also shows the multimodal size

Acknowledgements

One of the authors (NBP) expresses his thanks to CSIR, India for providing scholarship during this research programme. We thank S.S. Rao for useful discussions. We thank DST, India for providing facilities for absorption (FTIR) measurements in the Department of Physics and SEM and TEM measurements at the Nano-center, Indian Institute of Science, Bangalore. Thanks are due to Vijay Kumar for absorption measurements, Keshab Barai for SEM measurements, and Amit for TEM measurements. We also thank

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