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1、精品论文A Top-down Strategy towards Monodisperse Colloidal PbS Quantum Dots5YANG J, GAO Minrui(School of Materials Science and Engineering, Tianjin University, Tianjin 200072) Abstract: Highly crystalline monodisperse colloidal PbS quantum dots (QDs) with controllable sizes and narrow dispersions 5.4% s
2、igma 8.7% were synthesized by a facile and environmentally friendly “top-down” strategy, i.e., laser irradiation of a suspension of polydisperse raw PbS nanocrystals of tens10ofnanometers.Theas-obtainedcolloidalQDsdemonstratedsize-tunablenearinfrared photoluminescence, and self-assembled into well-o
3、rdered 2D or 3D superlattices on flat substrates dueto small degree of polydispersity (sigma 10%) and the surface capping of 1-dodecanethiol, which not only served as a surfactant but also the sulfur source. The acquisition of monodisperse colloidal QDsupon laser irradiation was primarily based on t
4、he selective evaporation of nanocrystals by low-intensity15laser beam, induced by the robust quantum-confinement-effect-induced bandgap wideness in PbS QDs.Key words: quantum dots; monodisperse; laser irradiation; self-assembly0IntroductionOver the last decade, infrared (IR) semiconductor nanocrysta
5、ls (quantum dots, QDs) have20stimulated a rapid growing of research interest due to their potentially significant applications in optoelectronics and biotechnology. Owing to the quantum confinement effects (QCE), IR QDs exhibit size-tunable optical and electronic properties in the infrared spectral
6、region, which make them highly desirable in applications such as fiber-optic communications1, infrared detectors and photoemitters2, solar and thermal photovoltaic devices3-6, in-vivo biological tagging7 etc. As25one of the important near-infrared (NIR) QDs, lead sulde (PbS) has a direct narrow bulk
7、 bandgap Eg = 0.41 eV and relatively large exciton Bohr radius of 18 nm, which make it facile to tune the bandgap energy of PbS QDs in the NIR region. Particularly, due to the third order nonlinear optical response8 and the multiple exciton generation9 of PbS QDs, many attempts have been made toward
8、s high performance PbS QDs-based solar cells3-5, 10-12. Therefore, monodisperse IR30QDs, uniform-sized with a size dispersion (or polydispersity) s 10%13, are of great importancefor many future applications. Moreover, monodisperse colloidal QDs, synthesized in solution phase and stabilized by a laye
9、r of surface surfactants, have been regarded as promising building blocks for advanced crystal superlattices and devices with abnormal electric, magnetic, and optical behavior, resulting from the quantum mechanical and dipolar interactions between the35nanocrystals units14-16.General synthesis appro
10、aches towards monodisperse colloidal IR QDs have primarily employed the solution-phase colloidal chemistry, the so called “bottom-up” strategy1, 17,18, where the effective separation of the nucleation and growth steps has been believed the key in obtaining monodisperse nanoparticles18. Specifically,
11、 several “bottom-up” approaches have been developed40for the preparation of monodisperse colloidal PbS QDs, including the most popular organic-phase hot-injection technique4, 19-24, which induces burst nucleation by the rapid injection of excess precursor(s) into hot surfactant solution18, meanwhile
12、 a water-based synthesis using nontoxic solvents and low reaction temperatures was also reported recently, nevertheless reaching a polydispersity larger than 20%25, 26.45 Herein we report on a “top-down” strategy to produce high-quality crystalline monodisperseFoundations: Specialized Research Fund
13、for the Doctoral Program of Higher Education (No. 20090032120024 ) Brief author introduction:YANG Jing (1980), Female, Accociate professor, Synthesis of functional nanomaterials via laser ablation/irradiation. E-mail: yang_jing- 11 -colloidal PbS QDs with controllable sizes and narrow dispersions 5.
14、4% s 15%. Our strategy is primarily based on a so called55“selective evaporation of nanocrystals” phenomenon in low-intensity-laser irradiation of colloidal nanocrystals, prompted by the robust QCE-induced bandgap wideness in PbS QDs, and thus potentially a universal synthetic strategy towards monod
15、isperse colloidal IR QDs with prominent QCE, such as PbSe, PbTe, etc.1Experimental section601.1General procedure for the preparation of PbS QDsWe synthesized monodisperse colloidal PbS QDs by adopting a two-step laser ablation/irradiation route.First, raw PbS nanocrystals were prepared by laser abla
16、tion. Particularly, a pure lead plate(99.99% purity) supported by a target holder was immersed in DDT (98%, J&K Scientific) with a65thin liquid layer (thickness 5 mm) above the target surface, and ablated by the first harmonic(wavelength 1064 nm) of a millisecond pulsed Nd:YAG laser (pulse width 2 m
17、s, repetition rate 2Hz, pulse energy 4 J/pulse, focal spot diameter 0.2 mm), focused onto the target surface, for a while until the color of solution turned black. During ablation, the solution was kept stirring with a magnetic stirrer and the irradiated spot was moved around occasionally. The whole
18、 setup could70guarantee a considerably high yield of nanoparticles29. Polydisperse raw PbS nanocrystals withsize of 20-50 nm (Fig. 1) were separated from the as-obtained black colloid by centrifugation, rinsed with ethanol, dried, weighed, and redispersed in DDT to make a 4.3 mM of suspension.In the
19、 second step, PbS QDs were produced by laser irradiation. Specifically, 0.25 mL of rawPbS nanocrystals dispersion (0.43 mM) was transferred to a quartz cell with an inner diameter of7510.0 mm and irradiated by the unfocused laser (pulse width 5 ms, repetition rate 2 Hz, spot diameter 8.0 mm, pulse e
20、nergy density 30 J/cm2) for a period of time t (1 20 min). The temperature of the colloid solution was controlled via a water-bath (Tb =0 30 C). A relative uniform irradiation of the colloidal nanocrystals could be guaranteed, for the inner diameter of the cell slightly larger than the laser-spot si
21、ze. During the irradiation, a significant color change of the80colloid from opaque black to transparent light grey was observed, indicative of the formation of small PbS QDs.1.2CharacterizationTEM images were acquired by using an FEI Technai G2 F20 transition electron microscope equipped with a fiel
22、d-emission gun. The compositions of the products were determined by an85Oxford INCA energy-dispersive X-ray spectroscopy (EDS) module attached to the TEM. A Hitachi S-4800 scanning electron microscope (SEM) was used to perform morphology observations of the raw nanocrystals. Powder X-ray diffraction
23、 (XRD) measurements were carried out on a Rigaku D/max 2500v/pc diffractometer. The UV-Vis-NIR absorption spectra were9095100105110115120obtainedwithaHitachiU-4100spectrophotometer.Near-infraredphotoluminescence measurements were performed on a spectroflurometer (FL3-221-TCSPC, HORIBA Jobin Yvon inc
24、) with a 450 W xenon lamp and a photomultiplier tube as detector working in a photon counting mode. Infrared spectra of samples were recorded using a Bruker Tensor 27 FT-IRspectromer in the range of 400 4000 cm-1.Fig. 1 TEM image and XRD pattern of raw PbS nanoparticles.2Results and discussionFigure
25、 2a exhibit the TEM image of as-synthesized PbS QDs corresponding to the irradiation duration t = 20 min in the second step, with water-bath temperature Tb = 30 C. The QDs appeared highly monodisperse with diameters of 5.6 0.3 nm (size dispersion s = 5.4%). The statistical analyses were performed on
26、 100 nanoparticles in TEM images. Besides, the QDs self-assembled into a 2D hexagonal superlattice on the copper grid, also indicative of the high degree monodispersity of the QDs. The high-resolution transmission electron microscope (HRTEM) lattice fringe image of a single QD (Figure 2b) revealed i
27、ts high-crystalline nature with a lattice spacing of 0.295 nm, expected for d200 of bulk PbS crystals (ICCD-5-592), and the corresponding fast-Fourier transform pattern (Figure 2c) implied the face-centered cubic phase oriented along the 001 direction. Moreover, the diffraction rings shown in the se
28、lected area electron diffraction (SAED) patterns of the QDs (Figure 2d) indexed well to the cubic rock salt structure of bulk PbS materials. In the case of colloids with relatively high concentrations, the PbS QDs could form a 3D hexagonal-close-packed supperlattice assembly on the carbon-coated cop
29、per grid (Figure 2e). Figure 2f shows an enlarged view of the absorption spectrum of as-synthesized5.6 nm PbS QDs in the NIR spectral regime. A well-defined excitonic absorption peak located in the NIR regime at 1380 nm was discernible from the broad absorption spectrum, which corresponded to the fi
30、rst exciton transition (1Se-1Sh), consistent with the reported studies on theQCE of PbS QDs23, 30.One of the important factors responsible for size controlling of PbS QDs in our synthetic strategy was the temperature of water-bath Tb, which effectively stabilized the temperature of the colloid solut
31、ion, while being irradiated by laser pulses. Besides Tb = 30 C, the irradiation of raw PbS nanocrystals in the second step was also carried out at Tb = 0, 20, 50 and 70 C, for t = 20 min. The as-obtained products for Tb = 20 and 0 C appeared to be quasi-spherical nanodots with uniform sizes, i.e., 4
32、.6 0.4 nm (s = 8.7 %) and 3.5 0.3 nm (s = 8.6 %) for Tb = 20 and 0 C, respectively (Figure 3b, 3c). Figure 3d demonstrates clearly a drastic increasing of the mean diameter of QDs with Tb. However, as Tb further increased to 50 and 70 C, a ripening of QDs became dominant in the final products, leadi
33、ng to polydisperse non-spherical nanocrystals.125130135140Fig. 2 Monodisperse colloidal 5.6 nm PbS quantum dots (QDs) obtained by laser irradiation of raw PbS nanocrystals. (a) Low-magnification TEM image. (b) HRTEM image of a single QD. The scale bar displays 2 nm. (c) Fast-Fourier transform of TEM
34、 image shown in (b). The zone axis indicates 001. (d) SAED pattern of the QDs shown in (a), where labeled diffraction rings match well with PbS face-centered cubic phase. (e) TEM image of a three-dimensional superlattice of the QDs. Inset shows the respective FFT pattern, indicative of ahexagonal-cl
35、ose-packed assembly. (f) An enlarged view of the absorption spectrum in the NIR window. Inset is an extended view.Fig. 3 TEM images of PbS QDs produced at water-bath temperatures (Tb) of 30C (a), 20C (b), and 0C (c). Scale bars represent 20 nm. (d) Normalized size histograms of PbS QDs and correspon
36、ding Gaussian fits for different Tbs. Namely, the sizes of QDs are 5.6 0.3 nm, 4.6 0.4 nm, and 3.5 0.3 nm for 30C, 20C, and 0C.We performed quantitative investigations on the effect of laser ablation duration (t) on the evolution of average size of PbS QDs prepared at Tb = 0 C. As seen in Figure 4,
37、at t = 1 min, raw145150155160165170175PbS nanocrystals with sizes of tens of nanometers were dramatically converted to fairly small PbS QDs with a mean diameter of 4.9 0.8 nm. Meanwhile, as laser irradiation time extended, the average size of as-prepared PbS QDs reduced monotonically until reaching
38、a minimum limit of3.5 0.3 nm around t = 20 min. At the same time, the degree of polydispersity in the QDs also became smaller, from 16.0 % to 8.6%, with increasing laser irradiation duration (inset of Figure4). Consequently, laser irradiation of raw PbS nanocrystals could effectively reduce the aver
39、age size and size dispersion, and finally led to monodisperse QDs. Similar trends also applied in the cases of elevated water-bath temperatures, e.g., Tb = 30 C, where the QDs following 8 min of laser irradiation achieved the minimum size limit of 5.6 nm, and an extension of t to 20 min lowered the
40、size dispersion from 8.9 % (5.6 0.5 nm) to 5.4 % (5.6 0.3 nm), while keeping the average QDs size unchanged.Fig. 4 The evolution of the mean diameter of PbS QDs and respective size dispersion (inset) with laser ablation duration for Tb =0 C.It is noteworthy that the DDT played an important role in p
41、reparing monodisperse colloidal PbS QDs by laser irradiation, as it not only provided the sulfur source, but also served as a coordinating solvent that prevented QDs growth and agglomeration. The surface covering of DDT molecules allowed QDs to self-assemble into well-ordered superlattices due to va
42、n der Waals force. Fourier transform infrared (FTIR) measurements were adopted to reveal the adsorption mechanism of DDT molecules on the surface of PbS QDs. As displayed in Figure 5, the FTIR spectrum of PbS QDs (top curve) showed similar absorption bands to that of pure DDT (bottomcurve), but with
43、 much lower intensities. The absorption peaks in the range of 3000 2800 cm-1were assigned to the asymmetric vibration of CH3 (2955 cm-1), the asymmetric (2922 cm-1) and symmetric stretching of CH2 (2853 cm-1), respectively. While in lower frequency regimes, the peaks between 1500 and 500 cm-1 were d
44、ue to the CH2 scissoring mode (1464 cm-1), the CH3 symmetric bending vibration (1377 cm-1), and the CH2 rocking band (721 cm-1), respectively31,32. Moreover, comparing with free DDT, the S-H stretching vibration band at 2576 cm-1 was absent inthe spectrum of capped QDs (inset of Figure 5), confirming that the S-H bond was broken and the DDT molecules were chemisorbed as thiolates onto the surface of PbS QDs in a monodentate manner31,33,34. Meanwhi
