J. Phys. Chem. C 2007, 111, 18927-18931
18927
CdTe Nanowire Networks: Fast Self-Assembly in Solution, Internal Structure, and Optical Properties Yury P. Rakovich,† Yuri Volkov,‡ Sameer Sapra,§ Andrei S. Susha,§ Markus Do1 blinger,¶ John F. Donegan,† and Andrey L. Rogach*,§ Semiconductor Photonics Group, School of Physics and CRANN Research Centre, Trinity College Dublin, Dublin 2, Ireland, Dublin Molecular Medicine Centre and Department of Clinical Medicine, Trinity College Dublin, Dublin 8, Ireland, Photonics and Optoelectronics Group, Physics Department and Center for Nanoscience (CeNS), Ludwig-Maximilians-UniVersita¨t Mu¨nchen, Amalienstrasse 54, 80799 Munich, Germany, and Department of Chemistry and Biochemistry and CeNS, Ludwig-Maximilians-UniVersita¨t Mu¨nchen, Butenandtstrasse 5-13, 81377 Munich, Germany ReceiVed: August 17, 2007; In Final Form: October 2, 2007
CdTe nanowire networks were assembled in standard physiological phosphate-buffered solution from individual CdTe nanocrystals on a time scale of minutes. TEM study revealed a chainlike internal structure of nanowires composed of nonfused CdTe nanocrystals. Individual wires within the network preserved the bright luminescence of CdTe nanocrystal building blocks, with remarkable changes in recombination kinetics for the entire nanowire network, along the single wires and at intercrossing and branching points as revealed by microphotoluminescence and fluorescence lifetime imaging microscopy.
1. Introduction The unique properties of colloidal semiconductor and metal nanocrystals (NCs) have attracted great attention from researches during the last 2 decades. Different strategies for the synthesis and surface functionalization of NCs were developed, and sizetunable optical and electrical properties were studied. Applications of NCs as building blocks in photonics, optoelectronics, and biotechnology are currently under exploration,1,2 leading to a strong demand for new materials and functionalities. The need for nanoscale structures in next-generation devices has directed attention toward the use of novel motifs for the construction of interconnected networks. Semiconductor and metal nanomaterials with elongated forms like nanorods and nanowires (NWs) came into the focus of research relatively recently3 and are considered to be of the highest priority for a variety of applications, as both interconnects and active components in nanoscale optical and electronic devices.4 In addition, they represent a model system for studies of quantum confinement effects in one-dimensional (1D) objects.5,6 One possibility to produce NWs is the template-directed assembly. The linear pores and channels in porous materials are among the very attractive templates, and Hornyak et al. showed formation of 1D gold nanostructures by filling the Au NCs into the pores of an alumina or polymer membrane.7 Minelli et al. demonstrated production of gold NW using block copolymers as templates.8 The use of DNA9 and viral10 templates as a scaffold around which NCs are assembled has been proposed. In addition to molecule-based templates, patterned substrates can be explored to direct the assembly of NCs into NWs. Yin and Xia11 and Zhang et al. 12 have demonstrated * Corresponding author. E-mail:
[email protected]. † School of Physics, Trinity College Dublin. ‡ Department of Clinical Medicine, Trinity College Dublin. § Physics Department, Ludwig-Maximilians-Universita ¨ t Mu¨nchen. ¶ Department of Chemistry and Biochemistry, Ludwig-MaximiliansUniversita¨t Mu¨nchen.
Figure 1. (a-c) Series of optical microscopy images demonstrating self-assembly of CdTe NWs in solution in real time: (a) 3 min after mixing PBS solution with CdTe NCs; (b) 5 min after mixing; (c) 8 min after mixing. White circle in (a-c) shows the same feature on all three images, in order to confirm that all images were taken from one and the same sample region. (d and e) NW network formed on a glass support after evaporation of solvent; optical microscopy images were taken from the same sample region in transmission (d) and luminescence (e) modes.
the assembly of spherical NCs into a number of complex 1D structures by templating against structures patterned on the surface of solid substrates. Template-free methods are of a special interest if the inherent (substrate independent) properties of NWs are to be studied or if the large scale synthesis of NWs is anticipated. A variety of
10.1021/jp076622p CCC: $37.00 © 2007 American Chemical Society Published on Web 12/06/2007
18928 J. Phys. Chem. C, Vol. 111, No. 51, 2007
Rakovich et al. NW networks in phosphate-buffered aqueous solutions and report on the structural and optical properties of the CdTe NW networks. 2. Experimental Section
Figure 2. Representative TEM (a and b) and high-resolution TEM (c) images of single CdTe NWs illustrating their continuity and thickness variation. Single CdTe NCs constituting the wire are clearly seen in (c).
Optical and fluorescence images of NWs were acquired with a Nikon Eclipse TE2000-U inverted microscope with a 100× oil immersion lens and epifluorescence attachment and with a Zeiss Axioscope 2 microscope with a 10× lens equipped with a Cannon Power Shot A80 camera. Samples for transmission electron microscopy (TEM) were prepared by growing NWs on a copper grid coated with a thin carbon film and subsequently evaporating the solvent. TEM images were obtained on a JEOL 2011 instrument equipped with a tungsten cathode operating at 200 kV. The images were recorded using a TVIPS CCD camera (F114). Photoluminescence (PL) spectra of NWs were measured using a RENISHAW micro-PL/Raman system equipped with a microscope objective (×100), a notch and plasma filters, and a CCD camera. An Ar+ laser (wavelength λ ) 488 nm) was used in the micro-PL measurements. PL decays were measured using time-correlated single-photon counting (Time-Harp, PicoQuant). The samples were excited by using 480 nm picosecond pulses generated by using a PicoQuant, LDH-480 laser head controlled by a PDL-800B driver. The setup was operated at a 20 MHz repetition rate with an overall time resolution of ca. 350 ps. Fluorescence lifetime imaging microscopy (FLIM) was conducted with the Microtime200 time-resolved confocal microscope system (PicoQuant) equipped with an Olympus IX71 inverted microscope. Lifetime maps were calculated on a pixelby-pixel basis by fitting the lifetime of each pixel to the logarithm of the intensity. The FLIM system response was negligible compared with typical lifetimes of the NWs. 3. Results and Discussion
Figure 3. Normalized PL spectra (a) and PL decays (b) of CdTe NCs and NWs grown thereof. Decay time τ1/e changes from 14.4 ns for NCs to 1.9 ns for NWs.
methods, including the catalyst-mediated (vapor-liquid-solid, VLS) phase separation approach13 and the seed-mediated growth in solution,14 have been demonstrated. An alternative approach to produce NWs in solution is based on the self-assembly of preformed NCs building blocks.3 Pacholski et al. reported the formation of single-crystalline ZnO NWs through the selforientation and alignment of single ZnO NCs.15 Silver NWs were produced from suspension of Ag NCs at the air-water interface16 and upon applying dielectrophoresis.17 Cho et al. synthesized PbSe NWs making use of the oriented attachment of spherical NCs, whose shape could be varied on demand to be linear, zigzag, helical, centipedes, and rings.18 Water-soluble thiol-capped CdTe NCs19,20 were demonstrated to be ideal building blocks for self-assembling into micrometer-long NWs through dipole-dipole interactions between spherical NCs promoted by destabilization of stabilizer ligand shells.21,22 In this paper we demonstrate fast self-assembly of CdTe NCs into
CdTe NCssbuilding blocks for NWsswere synthesized in aqueous solution using thioglycolic acid as a ligand following the previously reported approach.19,20 Postpreparative sizeselective precipitation19 was applied to the crude solution of NCs to remove the nonreacted species and separate the strongly emitting fractions of NCs with narrow size distribution (PL quantum yield of 25-40% at room temperature). Red-emitting CdTe NCs with a mean particle size of 4 nm have been used to grow the NWs reported in this work. CdTe NWs were obtained by mixing the stock solution of CdTe NCs with concentration of approximately 0.01 mM (particle/L) with the standard physiological phosphate-buffered saline (PBS) at pH 7.2. Typically, 1 µL of aqueous CdTe NC solution was put on a glass slide. Depending upon the density of NWs required, 1-20 µL of PBS was put onto the NC drop, and the two solutions were mixed and allowed to dry at the ambient conditions. The entire process of NW formation was fast and typically took only 10-15 min to complete. Figure 1a-c shows a series of images demonstrating the growth of CdTe NWs in real time. The formation of NWs started in solution close to the edge of the droplet (bottom part of the frames a-c in Figure 1). Further growth proceeded fast on the time scale of minutes, leading to formation of a NW network (Figure 1, parts b and c). We stress that the self-assembly of CdTe NCs into NWs takes place in solution, although it is facilitated by evaporation of solvent (water) which leads to an increased concentration of CdTe NCssbuilding blocks of NWssat the edge of the drying droplet. Our approach is different to the so-called evaporation-induced, or drying-induced assembly,23 a method relying on capillary
CdTe Nanowire Networks flow toward the edge of a drying droplet which is widely used to create nanoparticle patterns on solid substrates. In our case, the formation of wires is determined by the gradient of CdTe NC concentration in solution so that the self-assembly of NCs into wires caused by their dipole-dipole interactions (see below) starts in solution near the edge of the drying droplet and proceeds toward the droplet’s center (Figure 1 a-c). Upon complete evaporation of solvent, the luminescent NW network covered the support (Figure 1, parts d and e). The length of single NWs within the network reached hundreds of micrometers, and the maximum observed thickness of these wires as evaluated with an optical microscope (Figure 1d) reached several micrometers. These wires were strongly luminescent (Figure 1e), with a luminescence color very much resembling the emission color of CdTe NC building blocks. This observation points out that the thicker NWs which are easily observed in optical microscope may consist of bundles of thinner wires or even of chains of nonfused CdTe NCs. Indeed, TEM studies revealed the presence of thinner, 10-50 nm NWs within the entire network in addition to the thick wires easily observed in the optical microscope. Representative TEM images of these NWs are shown in Figure 2. They clearly indicate that NW thickness of even the thinnest NWs (Figure 2a) is not of the range of a single NC diameter. Instead, the NWs consist of several NC chains running parallel, forming a single NW (Figure 2a) or a bundle of NWs (Figure 2b). Remarkably, the single NCs which form wires are not fused with each other, presumably being held together by van der Waals or electrostatic interactions between ligand molecules. A high-resolution TEM (HRTEM) image of the NW (Figure 2c) further demonstrates the assembly of NCs with differently oriented lattice planes within the chain. The buildup of NWs from nonfused NCs results in significant thickness variations of the wires. The internal structure of CdTe NWs obtained at our experimental conditions resembles pearl-necklace agglomerates built at the earlier stages of formation of single-crystalline CdTe NWs, as reported previously by the Kotov group.21 As discussed in our previous publication on CdTe-based wires,22 the reason for NWs formation is the screening of the surface charge of nanoparticles by ions present in phosphate buffer solution, facilitating destabilization of NC ligand shells. The driving forces of the self-assembly of CdTe NCs into chainlike aggregates have been identified as dipole-dipole interactions of CdTe NCs24 as well as van der Waals and hydrogen bonding of ligand molecules.25 Recrystallization of initially cubic (zinc blende phase) CdTe NCs constituting chainlike aggregates into single-crystalline wurtzite-phase NWs takes place in solution on a much slower scale of several weeks,17 which was not observed at the conditions of our experiments where the formation of NW networks immobilized on solid substrates was accomplished within several tens of minutes. The NWs reported here still consist of zinc blende phase CdTe NCs, as the HRTEM image analysis show. As long as single CdTe NCs forming NW networks in our experiments remained essentially intact and were still separated from each other through the organic stabilizing shells of thioglycolic acid ligands, the PL color of NWs was determined by the quantum confinement in CdTe NCs and so far resembled the red PL color of CdTe NCs in solution. On the other hand, coupling of electron/hole wavefunctions of closely packed CdTe NCs can be expected in the NWs, despite their mutual separation through the organic shells. The PL spectrum of a NW network on a glass substrate is presented in Figure 3a in comparison with a spectrum taken from original solution of separated CdTe
J. Phys. Chem. C, Vol. 111, No. 51, 2007 18929
Figure 4. FLIM image of a CdTe NW network on a glass substrate (a) and the corresponding PL lifetime histogram (b).
NCs. The PL maximum of the CdTe NWs is red-shifted by 8 nm. The narrowing of the PL line width observed in parallel with the red shift demonstrates the process of energy transfer from smaller toward larger CdTe NCs constituting the wires. A similar effect has been previously reported for solid films of closely packed CdSe26 and CdTe27 NCs where smaller NCs within inhomogeneous size distribution act as donors for larger acceptor nanoparticles, resulting in a decrease of donors contribution and increase of acceptors contribution in overall emission and for bilayers of CdTe NCs of distinctly different sizes.28,29 The PL decay curve for NWs shows a strong (more then 7 times) decrease in the luminescence lifetime (estimated as τ1/e) in comparison with the solution of NCs (Figure 3b). The latter finding points toward a decrease of the electronic confinement in these quasi-1D objects. PL decay curves for the NWs show multiexponential recombination kinetics (Figure 3b) consistent with a PL model that includes multiple emission pathways and is indicative of a broad lifetime distribution caused by the corresponding distribution of defect or trap states due to partial removal of the ligands on the NCs. Considering NWs as a nanoheterogeneous system, the PL decays can be best modeled by continuous distributions of decay times.30 In this case fitting procedures cannot distinguish sufficiently between, for example, a single Gaussian distribution of lifetimes and the sum of two exponentials, or a bimodal Gaussian distribution and the sum of at least three exponentials. Therefore, to gain a better insight into the spatial distribution of lifetimes along NWs, the PL dynamics were evaluated using
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Figure 5. FLIM image (a), corresponding PL decay (b), and lifetime histogram (c) of a single CdTe nanowire. Panel d shows PL spectra recorded from nanowire parts indicated by arrows in panel a.
the FLIM technique, that is, maps of two-dimensional in-plane variations of the PL decay times. Each pixel in the FLIM image provides the lifetime at a particular position in space (x, y), while monitoring the entire PL spectrum. The lifetime image of a CdTe NW network clearly demonstrates the distribution of emitting species over a wide area (Figure 4a). The corresponding PL lifetime histogram (Figure 4b) shows a lifetime distribution with a first maximum centered at 2.4 ns and a full width at half-maximum (fwhm) equal to 1.3 ns followed by a broad (fwhm ) 4.8 ns) band of lifetimes ranging from 5 to 14 ns. The bimodal distribution of PL decay times shown in Figure 4b implies that at least two different mechanisms are involved in the decay processes. A distribution of decay times causing nonexponential decays in II-VI NCs has been generally discussed in terms of a variation in the nonradiative decay rates caused by trap states.30-33 In the case of CdTe NWs under discussion, PL decay kinetics may also be affected by resonance energy transfer between different parts of a single NW or between different NWs forming networks due to intrinsic size distribution of CdTe NCs constituting the wires. Indeed, the observed structure of lifetime distribution is slightly different for different single NWs. Figure 5a shows an FLIM image of a single isolated CdTe NW. Both components observed in the corresponding lifetime histogram (Figure 5c) are shifted as compared to the data presented in Figure 4b for the NW network, displaying a first peak centered at 1.2 ns and the second one which centers at 4.7 ns. It is noteworthy that both peaks are much narrower in the case of a single NW as compared to the entire network (Figure 5c as compared to Figure 4b), with estimated values of fwhm for a single wire equal to 0.6 ns (first peak) and 3.7 ns (second peak). To monitor the recombination kinetics along a single NW,
micro-PL spectra were recorded for different positions of the exciting laser spot (Figure 5d). It turned out that the position of the PL maximum shifts less than 3 nm over a scanning distance of 14 µm, although the corresponding PL intensity depends on the excitation position (Figure 5, parts a and d). A closer look at the NW network revealed different lifetime kinetics in intercrossed (Figure 6a) and branched (Figure 6b) wires. With the setup used, the images shown in Figure 6 were obtained at the limit of optical resolution, and the criterion used to distinguish between branched and intercrossed points was a stronger reduction of PL intensity from the respective region (Figure 6b) as a result of moving the focus plane toward the substrate as compared with the intensity from the region indicated in Figure 6a. The short-lifetime component is observed in both histograms, but the contribution of the long-lifetime component is clearly different. The most remarkable result is the strong suppression of the long-lifetime component in the branching region (Figure 6, parts b and d) which is accompanied by reduction in the emission intensity. The formation of NWs is facilitated by the partial removal of the surface stabilization layer. For the intercrossed wire region, the lifetime dynamics are very similar to the whole NW showing that although these are crossed there is little or no energy transfer occurring between the wires. On the other hand, the sharp reduction in the PL intensity and the shortening of the lifetime give strong evidence for energy transfer in the branched region. In this region, CdTe NCs forming the wire (Figure 2) are in electronic contact, facilitating energy transfer along the wire and a concomitant reduction in the PL emission intensity and lifetime. A thorough investigation of the observed phenomena aimed at their comprehensive understanding is now underway.
CdTe Nanowire Networks
J. Phys. Chem. C, Vol. 111, No. 51, 2007 18931 References and Notes
Figure 6. Magnified FLIM image of intercrossed (a) and branched (b) NWs and the corresponding lifetime histograms (c and d) measured from indicated regions.
4. Conclusions and Outlook We demonstrated a simple and fast self-assembly technique allowing solution-based growth of luminescent CdTe NW networks from the CdTe NC building blocks on the time scale of minutes. Single CdTe NCs do not fuse with each other within the NWs but, rather, form chainlike aggregates. The bright fluorescence of the NW networks allowed us to study optical properties of NWs with micro-PL spectroscopy and FLIM techniques and observe different recombination kinetics for NW networks, along the single wires and at intercrossing and branching points. Variation in size of the CdTe NCs building blocks is an easy way to influence the properties of the assembled NWs, in particular the emission color. The self-assembly approach to obtain NW networks proposed in this work for thiol-capped CdTe NCs emitting in the visible spectral range should be applicable to other water-soluble, thiol-capped NCs building blocks, like UV-blue-emitting ZnSe NCs34 as well as near-IRemitting CdxHg1-xTe35 or HgTe36 NCs. This will allow the expansion of the applicability of NW-based optical and optoelectronic devices over larger spectral ranges. Further work will also concentrate on fabrication of rigid, ideally single-crystalline NWs to be used as functional elements for devices, which can be achieved by annealing of the chainlike wires reported in this paper. Acknowledgment. This work has been supported by Science Foundation Ireland, in particular via the Walton Award for A.L.R. The financial support of the DFG Excellence Initiative via the “Nanosystems Initiative Munich (NIM)” is gratefully acknowledged. S.S. thanks the Alexander von Humboldt Foundation for the research fellowship.
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