Article pubs.acs.org/JPCC
Nonlinear Optical Properties of PbS Colloidal Quantum Dots Fabricated via Solvothermal Method Hui Cheng,† Yuhua Wang,*,† Hongwei Dai,‡ Jun-Bo Han,‡ and Xianchang Li§ †
Hubei Province Key Laboratory of Science in Metallurgical Process, Wuhan University of Science and Technology, Wuhan 430081, China ‡ Wuhan National High Magnetic Field Center, Huazhong University of Science and Technology, Wuhan 430074, China § Anyang Institute of Techonology, Anyang 455000, China
ABSTRACT: Lead sulfide (PbS) quantum dots were prepared by a solvothermal method. The as-synthesized quantum dots were dispersed by toluene, n-hexane, and tetrachloromethane separately and characterized by high-resolution transmission electron microscopy. Femtosecond laser pulse-based single-beam Z-scan analysis was employed to investigate the third-order nonlinear optical response of the quantum dots. Dispersants used in the fabrication process play the essential role not only in the nucleation and crystallization of the nanoparticles but also in modifying the nonlinear optical responses of PbS colloidal quantum dots. Additionally, laser power of the detecting beam has a major influence on the accuracy of single-beam Z-scan analysis.
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INTRODUCTION
nonlinear optical properties of the colloidal metal chalcogenide quantum dots synthesized via a solvothermal method. Structure and property analysis methods used in this work included high-resolution transmission electron microscopy (HRTEM) and Z-scan analysis. HRTEM provides straightforward images of morphologies of the nanoparticles, including lattice structure, shape, size, and size distribution. Z-scan is a very sensitive method to separately determine the nonlinear changes in refraction and absorption coefficients and is often used as a standard technique to investigate the nonlinear optical properties of nanoparticles.15,16
Colloidal metal chalcogenide quantum dots (Qdots) have drawn increasing attention as a class of novel nanosized materials and have been applied to a variety of fields.1−3 Among those, high-quality PbM (M = S, Se, Te, ...) nanoparticles have attracted much more attention because they can easily be synthesized by low-cost wet chemistry techniques,4−6 which produce suspensions of well-crystallized monodisperse Qdots. Applications involving lead chalcogenide Qdots are mostly based on PbS, PbSe, and PbTe; Qdot-based electroluminescent optical switches, solar cells, and photo detectors have been demonstrated.7−9 Tremendous effort has been invested in exploring new applications of the traditional colloidal metal chalcogenide quantum dots such as PbS, PbSe, PbTe, CdSe, etc.10−14 We noticed that the physical and chemical properties of these Qdots are very sensitive to temperature, heating time, chemical environment, and other experiment parameters during fabrication according to some researchers. However, the dispersants used were randomly chosen and scarcely covered.10−14 In our work, we focus on how the dispersant affected the local structure and the nonlinear optical response of the PbS quantum dots and discuss the microstructure and © 2015 American Chemical Society
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SAMPLE PREPARATION A stock solution of 0.016 g (0.5 mmol) of Sulfur power dissolved in 15 mL of oleyl amine (OlAm) is prepared by heating the mixture under argon at 80 °C. For the synthesis, 0.56 g (2 mmol) of PbCl2 and 4 mL of OlAm are mixed in a three-neck flask. The mixture of PbCl2 and OlAm is degassed for 30 min under argon at 150 °C. Then, the PbCl2−OlAm solution is cooled to the required injection temperature, which Received: October 9, 2014 Revised: January 19, 2015 Published: January 22, 2015 3288
DOI: 10.1021/jp510214x J. Phys. Chem. C 2015, 119, 3288−3292
Article
The Journal of Physical Chemistry C is 120 °C in our experiment; we then inject 2.25 mL of the OlAm-S stock solution. After that, the mixture is magnetically stirred under an argon atmosphere for 3 h at 120 °C. After centrifugation of the suspension and decantation of the supernatant, the quantum dots are resuspended in 10 mL of three kinds of dispersants. The Qdots dispersed in toluene are marked as PbS-t samples; those dispersed in n-hexane and tetrachloromethane are termed PbS-n and PbS-c samples, respectively. An experimental sample marked as PbS-x1, 2, 5 (x = t, n, c) means that sample is dispersed by x (x = toluene, n-hexane, tetrachloromethane) and investigated under 1, 2, or 5 mW.
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RESULTS AND DISCUSSION The high-resolution transmission electron microscopy analysis of the quantum dots has been run by a JEM 2100 system. The acceleration voltage was 200 kV, and the camera length was 300 mm. Typical TEM images of PbS-t quantum dots (Figure 1a)
Figure 2. (a) Representative TEM images of PbS-n quantum dots at room temperature; (b, c) high-resolution TEM images of the quantum dots; (d) SAED pattern.
Figure 1. (a) Representative TEM images of PbS-t quantum dots at room temperature; (b, c) high-resolution TEM images of the quantum dots; (d) SAED pattern.
demonstrate monodisperse polyhedral shapes and narrow size distribution. HRTEM images of the Qdots (Figure 1b,c) provide microstructural information on the quantum dots. The clear observation of the lattice fringes indicates the high crystallinity of these Qdots. The indexing of the lattice parameters of the selected area electron diffraction (SAED; shown in Figure 1d) was consistent with the (111), (200), (220), (311), (222), (400), (331), and (420) lattice planes of face-centered cubic PbS. According to Figure 1, the PbS Qdots synthesized in our work were highly crystalline and had perfect lattice structure, avoiding stacking faults and lattice defects. The HRTEM images of PbS-n Qdots in Figure 2 indicate that the PbS quantum dots dispersed in n-hexane have relatively smaller nano groups, and more lattice distortion and dislocation accrue in the edge area. It also suggests that the lattice structure of PbS-n samples is not so periodic compared with that of the PbS-t samples, which is a fact proved by Figure 2d. According to Figure 3, PbS-c and PbS-n samples have extremely similar character. They both show lattice disorder and distortion around the surface area of the quantum dots,
Figure 3. (a) Representative TEM images of PbS-c quantum dots at room temperature; (b, c) high-resolution TEM images of the quantum dots; (d) SAED pattern.
which should be the driving force of nonlocal linear interference during Z-scan analysis. As a conclusion, dispersants strongly affect the nucleation and crystallization condition of the quantum dots, and PbS quantum dots dispersed by toluene have better local structure and less lattice distortion than those dispersed by n-hexane or tetrachloromethane. Z-Scan Analysis. The single-beam Z-scan analysis basically follows the configuration first reported by Sheik-Bahae et al.17 In our case, the duration of the laser pulse was 130 fs and it was focused with a 100 mm focal-length lens; the laser repetition rate was 76 MHz. The wavelength of the detecting beam is 780 nm. 3289
DOI: 10.1021/jp510214x J. Phys. Chem. C 2015, 119, 3288−3292
Article
The Journal of Physical Chemistry C
normalized transmittance values at the valley and peak position of these PbS-n samples represents positive correlation to laser power. We believe that the authentic value of normalized transmittance in closed aperture data is approximately from 0.5 to 1.7, empirically, which is in agreement with the latest literature reports.8,10,13,15 Any results out of that scale have large possibility of suffering nonlocal linear interference (thermal effect, etc); however, the standard error of these fitting results is quite small (
