ARTICLE Received 16 Oct 2012 | Accepted 21 Feb 2013 | Published 16 Apr 2013 DOI: 10.1038/ncomms2637 A top–down strategy towards monodisperse colloidal lead sulphide quantum dots Jing Yang1, Tao Ling1, Wen-Tian Wu1, Hui Liu1, Min-Rui Gao1, Chen Ling1, Lan Li2 & Xi-Wen Du1 Monodisperse colloidal quantum dots with size dispersions o10% are of great importance in realizing functionality manipulation, as well as building advanced devices, and have been normally synthesized via ‘bottom–up’ colloidal chemistry. Here we report a facile and environmentally friendly ‘top–down’ strategy towards highly crystalline monodisperse colloidal PbS quantum dots with controllable sizes and narrow dispersions 5.5%oso9.1%, based on laser irradiation of a suspension of polydisperse PbS nanocrystals with larger sizes. The colloidal quantum dots demonstrate size-tunable near-infrared photoluminescence, and self-assemble into well-ordered two-dimensional or three-dimensional superlattices due to the small degree of polydispersity and surface capping of 1-dodecanethiol, not only serving as a surfactant but also a sulphur source. The acquisition of monodisperse colloidal PbS quantum dots is ascribed to both the quantum-confinement effect of quantum dots and the size-selective-vaporization effect of the millisecond pulse laser with monochromaticity and low intensity. 1 Tianjin Key Laboratory of Composite and Functional Materials, School of Materials Science and Engineering, Tianjin University, Tianjin 300072, People’s Republic of China. 2 Key Laboratory of Display Materials and Photoelectric Devices, Ministry of Education, Institute of Material Physics, and Tianjin Key Laboratory for Photoelectric Materials and Devices, Tianjin University of Technology, Tianjin 300384, People’s Republic of China. Correspondence and requests for materials should be addressed to X.-W.D. (email: [email protected]). NATURE COMMUNICATIONS | 4:1695 | DOI: 10.1038/ncomms2637 | www.nature.com/naturecommunications 1 & 2013 Macmillan Publishers Limited. All rights reserved. ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms2637 wing to quantum-confinement effects (QCE), semi- method in terms of not only employing low-temperature conductor nanocrystals (quantum dots, QDs) exhibit conditions (0B30 °C), but also involving a low-toxic lead target Oattractive size-tunable electrical and optical properties, and 1-dodecanethiol (DDT, C12H26S). Our strategy is potentially which make them highly desirable in the applications of universal for the synthesis of monodisperse colloidal QDs with optoelectronics and biotechnology1–7. Because of the striking prominent QCE, such as CdSe, PbSe, PbTe and so on. size-dependence of characteristics, monodisperse QDs, with a size dispersion (or polydispersity) so10% (ref. 8), are of great importance in realizing functional control and manipulation, Results such as achieving very sharp photoluminescence and a precise Synthesis of monodisperse PbS QDs. Monodisperse colloidal tuning of the emission by varying QDs sizes9. Moreover, PbS QDs were prepared by adopting a two-step laser ablation/ monodisperse colloidal QDs have been regarded as promising irradiation route (Supplementary Fig. S1a,b). In this process, raw building blocks for advanced crystal superlattices and devices PbS nanocrystals were first produced by laser ablation of a lead with abnormal electrical and optical behaviours, resulting from plate in DDT, which showed a polydisperse size distribution the quantum mechanical and dipolar interactions between varying from 20–50 nm (Supplementary Fig. S2a), and the rock nanocrystal units10. salt structure of PbS nanocrystals (ICCD-5-592) were confirmed General synthesis approaches towards monodisperse colloidal by the X-ray diffraction pattern (Supplementary Fig. S2b). QDs have primarily employed solution-phase colloidal chemistry Afterwards, the second laser treatment (irradiation duration (the so called ‘bottom–up’ strategy), where an effective separation t ¼ 20 min, water-bath temperature Tb ¼ 30 °C) produced PbS of nucleation and growth steps is believed to be the key in QDs with a significant size-reduction and uniformity in both the obtaining monodisperse nanoparticles11. Specifically, several top (Fig. 1a) and bottom (Supplementary Fig. S3) of solution. ‘bottom–up’ approaches have been developed for preparing A statistical analysis on more than 300 nanoparticles in trans- monodisperse colloidal PbS QDs, including the most popular mission electron microscope (TEM) images demonstrates that the organic-phase hot-injection technique12–14, which induces burst as-prepared nanoparticles are highly monodisperse with dia- nucleation by a rapid injection of excess precursor(s) into a hot meters of 5.5±0.3 nm (s ¼ 5.5%) (inset of Fig. 1a). Besides, the surfactant solution. nanoparticles self-assembled into a two-dimensional hexagonal A ‘top–down’ approach, laser fragmentation in liquid, has been superlattice on the copper grid, also indicative of a high degree normally adopted to realize size-reduction and size-distribution- narrowing of colloidal noble metal (gold and silver) nanoparticles, 15–17 through nanosecond laser vaporization or picosecond-laser- 5.5 ± 0.3 nm induced Coulomb explosion18 of nanoparticles upon laser (220) irradiation. However, the polydispersity after laser treatments (200) usually remains fairly large (s410%). It was not until very (111) recently that Werner et al.19 achieved a polydispersity of 6.1% by 4567 applying high pressures during nanosecond laser irradiation of Diameter (nm) colloidal gold nanoparticles. Hitherto, the ‘top–down’ strategy has (311) never been used in acquiring monodisperse colloidal semiconductor nanocrystals, although some attempts have been (222) made lately in the cases of Se20, ZnO21, and Si22 QDs with (400) polydispersity s415%. Different from fast and ultrafast lasers, long-pulse lasers, that is, millisecond-pulse lasers, posses rather low power densities, 200 23 24 which usually cause linear absorption and Joule heating of the 220 1,377 1,464 target. On the other hand, the band gap of QDs widens as QDs 020 PbS (200) 721 [001] size reduces due to QCE. Therefore, when a long-pulse 0.295 nm 2,848 monochromatical laser is employed to irradiate polydisperse 2,917 colloidal QDs, those with large sizes and band gaps narrower than 2,955 the photon energy can be heated and even evaporated, while 2,853 2,576 those with small sizes and wider band gaps are transparent and 2,922 Transmittance (Arbitrary units) Transmittance 2,800 2,600 2,400 keep intact, giving rise to monodisperse QDs in a top–down way. 3,000 2,900 2,800 1,500 1,000 As an example, in the present work, monodisperse colloidal PbS –1 QDs are prepared with controllable sizes (3.5B5.5 nm) and Wavenumber (cm ) narrow dispersions (5.5%oso9.1%), simply by means of Figure 1 | PbS QDs produced at water-bath temperature 30 °C. (a) TEM millisecond laser irradiation of a suspension of polydisperse raw image by laser irradiation of raw PbS nanocrystals for 20 min. Scale bar, PbS nanocrystals with relatively large sizes (that is, tens of 20 nm. Inset displays size histogram of QDs and corresponding Gaussian fit. nanometres). The size control of QDs is realized by an (b) Selected area electron diffraction pattern of PbS QDs shown in a, where adjustment of the water-bath temperature from 0–30 °C and labelled diffraction rings match well with PbS face-centered cubic phase. the as-prepared PbS QDs exhibit size-tunable near-infrared (NIR) (c) High-resolution TEM image of one of the QDs shown in a. The scale bar photoluminescence. Quantitative experimental studies on QDs represents 2 nm, and the inset is the corresponding fast-Fourier transform size evolution with laser ablation duration and a theoretical pattern. (d) Fourier transform-infrared spectra of PbS quantum dots coated calculation of complete vaporization threshold confirm that the with DDT (top) and neat/pure DDT (bottom). Note that the top curve is acquisition of monodisperse QDs arise from both the robust magnified 12 times for clarification of the absorption peaks. The inset shows QCE-induced band gap wideness and the selective vaporization of enlarged spectra in the regime of 2,825B2,930 cm À 1. The main absorption À 1 nanocrystals by low-intensity laser beam. To the best of our peaks arise from the asymmetric vibration of CH3 (2,955 cm ), the À 1 À 1 knowledge, this is the first report on the top–down synthesis of asymmetric (2,922 cm ) and symmetric stretching of CH2 (2,853 cm ), À 1 monodisperse colloidal QDs with so10%. This facial top–down the CH2 scissoring mode (1,464 cm ), the CH3 symmetric bending À 1 À 1 synthetic route manifests itself as an environmentally friendly vibration (1,377 cm ) and the CH2 rocking band (721 cm ) (ref. 25). 2 NATURE COMMUNICATIONS | 4:1695 | DOI: 10.1038/ncomms2637 | www.nature.com/naturecommunications & 2013 Macmillan Publishers Limited. All rights reserved. NATURE COMMUNICATIONS | DOI: 10.1038/ncomms2637 ARTICLE monodispersity of the nanoparticles. The diffraction rings shown in the selected area electron diffraction pattern of the nano- 4.4± 0.4 nm particles (Fig. 1b) index well to the cubic rock salt structure of 60 bulk PbS. The high-resolution TEM image of one of the nano- particles (Fig. 1c) reveals its high-crystalline nature with a lattice 40 spacing of 0.295 nm, expected for d200 of bulk PbS crystals (ICDD-5-592), and the corresponding fast-Fourier transform pattern (inset of Fig. 1c) implies the face-centered cubic phase 20 oriented along [001] direction. Moreover, energy-dispersive X-ray Normalized particle count (%) 0 spectroscopy spectrum (Supplementary Fig. S3) proves that the 3456 nanoparticles contain Pb and S atoms, whereas the signals of C Nanoparticle diameter (nm) and Cu ought to arise from the carbon coated copper grid. In the case of colloids with relatively high concentrations, the PbS QDs could form a three-dimensional hexagonal-close-packed super- 3.5± 0.3 nm lattice assembly (Supplementary Fig. S4). Fourier transform-infrared (FTIR) measurements reveal that, 80 in contrast to free DDT, the S-H stretching vibration band at 60 2576 cm À 1 is absent in the spectrum of capped QDs (inset of Fig.