Article pubs.acs.org/cm
Continuous Shape Tuning of Nanotetrapods: Toward ShapeMediated Self-Assembly Nimai Mishra,*,†,∥ Wen-Ya Wu,†,‡ Bharathi Madurai Srinivasan,§ Ramanarayan Hariharaputran,§ Yong-Wei Zhang,§ and Yinthai Chan†,‡ †
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore Institute of Materials Research and Engineering (IMRE), A*STAR, 2 Fusionopolis Way, Innovis, Singapore 138634, Singapore § Institute of High Performance Computing, A*STAR, 1 Fusionopolis Way, Singapore 138632, Singapore ‡
S Supporting Information *
ABSTRACT: We describe a surfactant-driven method to synthesize highly monodisperse CdSe-seeded CdS tetrapods with differing arm lengths and diameters in order to examine their effects on self-assembly. We exploited the phenomena of weak- and strong-binding capping groups to tune the arm length and diameter with uniform shape and achieved >95% yield. Afterward, we utilize these particles to overcome some of the key problems in the assembly of anisotropic shaped particles. Intriguingly, we found that tetrapods with certain arm lengths pack like fishbone chains, which was greatly dependent on particle shape and size. These ordered assembly phenomena were understood with the assistance of computer simulations, which strongly support our experimental observations. Importantly, this work presents a synthetic route toward shape tuning in CdSe-seeded CdS tetrapod structures, which has great influence on their self-assembly behavior at the solution/substrate interface.
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INTRODUCTION
because of issues associated with low-quality synthesis (polydispersity) and selective functionalization.11,12 The assemblies of spheres, cubes, rods, and star-shaped nanoparticles are extensively studied due to the well-established synthetic approaches for achieving products with high size monodispersity and shape uniformity.13−16 These assemblies are typically formed either in solution or on a substrate. van der Waals attractions between the particles, steric repulsions between the hydrophobic tails of the surface capping groups, depletion forces, Coulomb or dipole attractions, and magnetic forces13−21 are main factors behind these ordered arrangements of nanocrystals. Obtaining an ordered self-assembly of branched tetrapod particles poses more challenges than their spherical and rod-shaped counterparts.22,23 It is necessary to overcome synthetic challenges to obtain sufficiently monodisperse tetrapods with uniform size and shape, including control over arm length and arm diameter, which is required to
1−3
Colloidal core/shell tetrapod nanocrystals are promising materials for use in optoelectronic applications, such as photovoltaics,4−6 lasing,7 and dual electroluminescence.8 This is because of their solution phase processability, relatively large absorption cross sections, long biexciton lifetime, efficient charge separation at the core/arm interface, and enhanced charge transport through the increased percolation pathways afforded by their arms.6,7,9,10 Close-packing or ordered selfassembly of tetrapod-shaped colloidal semiconductor nanocrystals is an important intermediate step toward practical utilization of these materials in devices or fabrication of artificial solids. Despite their promise as optoelectronic materials, the successful achievement of an ordered self-assembly has been stymied by their branched geometry. There are very few published studies concerning tetrapod assembly. The few include lithographically patterned CdTe tetrapods11 and tip-totip connected three-dimensional CdSe-seeded CdTe tetrapod networks.12 There are several limitations toward such assemblies in terms of quality, scalability, and reproducibility © 2016 American Chemical Society
Received: December 11, 2015 Revised: January 22, 2016 Published: January 22, 2016 1187
DOI: 10.1021/acs.chemmater.5b04803 Chem. Mater. 2016, 28, 1187−1195
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Figure 1. (a) Low-resolution transmission electron microscopy (TEM) image of highly monodispersed zb-CdSe-seeded CdS core/shell tetrapods with 44 ± 4 nm average arm length and 7.4 ± 0.8 nm average arm diameter. (b) Schematic drawing of core/shell tetrapod synthesis and the tuning of their arm length and diameter. Here TOP, ODPA, TOPO, and OL stands for trioctylphosphine, n-octadecylphosphonic acid, trioctylphosphine oxide, and oleic acid, respectively. (OL, 90%) were purchased from Sigma-Aldrich. Trioctylphosphine (TOP, 97%) was purchased from Alfa Aesar. Diisooctylphosphinic acid (DIPA, 90%) was purchased from Fluka. n-Octadecylphosphonic acid (ODPA, 97%), TOPO (99%), and n-hexylphosphonic acid (HPA, 97%) were purchased from Strem. All chemicals were used without further purification. Synthesis of Spherical Zinc Blende (zb) CdSe Seeds. Nearly monodispersed zb-CdSe nanocrystals (NCs) were synthesized via a previously reported method with slight modifications.25 In a 50 mL three-neck round-bottom flask, 0.3 mmol of CdO, 0.6 mmol of myristic acid, and 5 mL of 1-ODE were degassed at 90 °C for 1 h. The solution was then heated to 250 °C for ∼10−15 min to yield a clear solution, followed by the addition of 12 mL of ODE before cooling to 90 °C to degas for another hour. Upon cooling to room temperature, 0.012 g (0.15 mmol) of 100 mesh Se powder (99.999%) was added to the reaction mixture and degassed at 50 °C for 20 min. Upon heating to 240 °C under N2, color changes from colorless to yellow at ∼150 °C and then to orange-red color upon reaching 240 °C were observed, signifying the formation of zb-CdSe nuclei. A degassed mixture of 0.5 mL of oleic acid and 0.5 mL of oleylamine in 2 mL of 1-ODE was subsequently added dropwise to the reaction mixture. As a guideline, the growth time for a ∼3 nm diameter NC was approximately 1.5 h. As-synthesized zb-CdSe NCs were precipitated out from the growth solution by adding acetone and were subsequently allowed to undergo two more cycles of redispersion and precipitation in toluene and methanol, respectively. Figure S1 in Supporting Information shows the absorption, photoluminescence (PL), and powder X-ray diffraction (PXRD) data of typical zb-CdSe NCs. Synthesis of Spherical Wurtzite (wz) CdSe Seeds. Synthesis of monodispersed wz-CdSe NCs proceeded via a previously reported procedure with slight modifications.3 A bath of 9g of TOPO (90%), 6 g of HDA, and 0.25 mL of DIPA was degassed at 100 °C for 1.5 h. A precursor solution comprising 317 mg of Cd(acac)2 and 567 mg of HDDO in 6 mL of ODE was degassed at 120 °C for 1.5 h, followed by addition of 4 mL of 1.5 M trioctylphosphine selenide (TOP-Se was synthesized in a glovebox by dissolving the desired amount of Se powder in TOP (97%) followed by stirring overnight) at room temperature. This precursor solution was then rapidly injected into the bath at 360 °C and allowed to cool to 80 °C. The resulting CdSe NCs were subsequently processed by three cycles of precipitation in a butanol/methanol mixture and redispersed in toluene for further use. Preparation of Stock Solution of CdSe Seeds. Processed CdSe NCs (zb/wz) were dispersed in a minimal amount of toluene, and their concentration was determined by measuring their absorbance at 350 nm, whose molar absorptivity is known.26 The toluene was then
systematically investigate their structure-dependent self-assembly. At very high concentrations, the tetrahedral shape of these particles could restrict the rotational and translational degrees of freedom, thus leading to a disordered assembly.24 Furthermore, different interparticle interactions, such as van der Waals, electrostatic, and dipole interactions, make it difficult to achieve ordered structures out of these tetrapods.24 Many of these problems are unavoidable, as they are inherent properties of branched tetrahedral geometries. It is more plausible to address synthetic problems in order to achieve highly monodisperse and uniformly shaped tetrapods for ordered assembly experiments. In our previous work, we have successfully overcome some of these challenges by developing a synthesis scheme to create highly monodisperse and uniformly shaped CdSe-seeded CdS tetrapods.3 In this work, we report further investigation on various synthetic parameters, including the role of anionic precursor (trioctylphosphine sulfide, TOPS) and the amount of phosphonic acid, that have allowed us to prepare CdSe-seeded CdS tetrapods with unprecedented shape homogeneity and monodispersity (synthetic yield >95%), as seen in Figure 1a. To understand the role of arm length and diameter on the rotational and translational motions and sizeand shape-dependent interparticle interactions during their assemblies at the solution/substrate interface, we explored the effects of surface capping groups to tune the arm diameter and length. The schematic in Figure 1b shows our effort in order to tune the diameter and length of the CdSe-seeded CdS tetrapods. We present a general and simple approach for the assembly of tetrapods on the solution/substrate interface. Monte Carlo simulations were carried out to understand the mechanism of self-assembly. We have identified that arm lengths and evaporation conditions were the two most important factors for the interlocked-chain-structured selfassembly, which corroborates with our experimental findings.
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EXPERIMENTAL SECTION
Materials. Cadmium oxide (CdO, 99.5%), cadmium acetate [Cd(ac)2, 99.99%], 1,2-hexadecanediol (HDDO, 90%), 1-hexadecylamine (HDA, 90%), 1-octadecene (ODE, 90%), sulfur (S, reagent grade), selenium (Se, 99.99%), trioctylphosphine oxide (TOPO, 90%), oleylamine (technical grade, 70%), myristic acid (90%), and oleic acid 1188
DOI: 10.1021/acs.chemmater.5b04803 Chem. Mater. 2016, 28, 1187−1195
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the base, i1(x,y), i2(x,y), and i3(x,y), from which Ci(x,y) can be calculated. During evaporation, the tetrapods undergo the following two motions (see Figure 2): (1) rotation, where the tetrapod flips
removed under vacuum and TOP was added to make up a NC concentration of 100 μM. This mixture will subsequently be referred to as the zb- or wz-CdSe stock solution. Synthesis of zb-CdSe-Seeded CdSe Tetrapods. Tetrapod-like CdSe-seeded CdS heterostructures were synthesized with substantial modifications via the seeded growth approach.3 Briefly, 2.65 g of TOPO (99%), 0.05175 g of CdO, and a mixture of ligands (see Table S1 of the Supporting Information for more details) were degassed at 150 °C for about 1.5 h in a 50 mL three-neck round-bottom flask. The reaction mixture was then heated to 350 °C under N2, whereupon the solution turned from reddish brown to colorless. Separately, a mixture of S, TOP, and CdSe seeds were prepared by first dissolving a predetermined amount of S in TOP at 50 °C before adding 25 μL of the appropriate CdSe stock solution. Upon reaching the desired injection temperature of 350 °C, an additional 1.8 mL of TOP was added, and the temperature was allowed to recover to 350 °C before the mixture of S, TOP, and CdSe was swiftly injected. The temperature was again allowed to recover to 350 °C and the anisotropic CdS arm was grown at this temperature for 10 min. The heating mantle was then removed and the solution was allowed to cool to 80 °C. As-synthesized CdSe-seeded CdS tetrapods were then processed by repeated cycles of precipitation in methanol and redispersion in toluene. A crude approximation of the concentration of the processed CdSe-seeded CdS structures was determined using a previously reported procedure.1 Synthesis of zb-CdSe-Seeded CdS Tetrapods and wz-CdSeSeeded CdS Nanorods in the Presence of Different Amounts of Sulfur. zb/wz-CdSe-seeded CdS heterostructures were synthesized by varying the amount of sulfur via the seeded-growth approach. Briefly, 2.65 g of TOPO (99%), 0.05175 g of CdO, and a mixture of ligands ODPA (97 mg) and OL (0.5 mL) were degassed at 150 °C for about 1.5 h in a 50 mL three-neck round-bottom flask. Upon reaching the desired injection temperature of 350 °C, an additional 1.8 mL of TOP was added, and the temperature was allowed to recover to 350 °C before the mixture of S, TOP, and CdSe (wz/zb) was swiftly injected. The temperature was again allowed to recover to 350 °C and the anisotropic CdS arm was grown at this temperature for 10 min. Different mole ratio of TOP-S was derived by first dissolving a different amount of S with respect to Cd (Cd:S 1:0.5, 1:2, 1:4, 1:6) in the same amount of TOP at 50 °C. Characterization. i. TEM Characterization. A JEOL JEM 1220F (100 kV accelerating voltage) microscope was used to obtain brightfield low-resolution TEM images. For TEM sample preparation, a drop of the nanoparticle solution was placed onto a 300 mesh size copper grid covered with a continuous carbon film. Excess solution was removed by an adsorbent paper and the sample was dried at room temperature before inserting into the TEM chamber. ii. Optical Characterization. UV−visible absorption spectra were obtained with an Agilent 8453 UV−visible spectrophotometer. Photoluminescence (PL) spectra in the NIR region were collected with a Horiba JobinYvon NanoLog spectrofluorimeter. Care was taken to ensure that the concentrations of the core and core-seeded nanostructures were sufficiently dilute to avoid contributions from reabsorption or energy transfer. iii. XRD Characterization. X-ray diffraction (XRD) data was obtained with a diffractometer (Bruker AXS, GADDS) using Cu Kα radiation (λ = 1.540 598 Å) in the range from 20° to 80°. Samples were prepared on a clean silicon wafer by placing drops of a concentrated nanoparticle solution onto the silicon surface and drying at 80 °C in the oven. This step was repeated several times until a continuous thin layer of solid was formed on the silicon substrate. Simulation Method. We start with N tetrapods in a square simulation box of size (LN). We assume that the length of all the four arms of a tetrapod are equal (=l) and in each assembly the tetrapods are of uniform size. Hence, the base of the tetrapod is an equilateral triangle, with the core directly above the centroid and the end of the arms at the vertices. Each tetrapod is labeled with a number i running from 1 to N, the total number of tetrapods in the simulation box, and the centroid of the tetrapod is denoted by Ci. The position of the tetrapod is fully described by the coordinates of the three vertices of
Figure 2. Illustration of the two motions, translation and rotation, in tetrapods. randomly and the new base shifts by a distance of l sin(π/3), and (2) translation, where the tetrapod is dragged randomly by a distance proportional to the temperature of evaporation (Te) The probability of both rotation (Pr) and translation (Pt) are inversely proportional to L, since the longer-arm tetrapods are heavier and hence are less probable to rotate or translate. If the distance between two tetrapod cores lc is less than √3l/2 (the distance of a vertex to the centroid), then we consider them locked and they move together. The probability of rotation (Pr ∝ 1/n) of such interlocked units is inversely proportional to the number of units locked. The translation distance is also inversely proportional to the number of units locked. For a temperature of 60 °C and for a chain of length n, the translation distance is dt = l(1 − 0.01n). The probability that one tetrapod can be disjointed from a chain is proportional to the evaporation temperature and inversely proportional to the arm length (Pt ∝ Te/l). We also assume that only the tetrapods at either of the ends of the chain can detach from the chain. This means that the chains of length less than 4 are not very stable. To find the number of units in a chain, we use both the distance between the adjacent tetrapods and the angle formed by a pair of the tetrapods with the previous pair. That is, if a tetrapod j has two neighboring tetrapods i and k within the distance lc, then we calculate the angle between the line connecting Cj and Ci and the line connecting Cj and Ck. If this angle is less than ±10°, then we conclude that the tetrapods i, j, and k form a chain. We do this to the entire assembly to identify the chains and their length. We start from an initial random assembly and at every step we randomly execute the events mentioned above.
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RESULTS AND DISCUSSION A major obstacle to synthesize monodisperse CdSe-seeded CdS tetrapods is the stringent need for the zb-CdSe NC seeds to maintain their phase purity during arm growth. High temperature (>300 °C) is needed in CdSe-seeded CdS tetrapod synthesis for three reasons: (i) to ensure increments in monomer concentration, (ii) to facilitate cleavage of the strong TOP−chalcogenide bond for generating anionic monomer, and (iii) to allow the activation of certain facets of the zb-CdSe seeds for the heterogeneous nucleation.27 This is challenging, given the fact that at high temperature zb-CdSe seeds can be partially converted to wz phase, as the energy difference between bulk wz- and zb-CdSe polytypes is only 1.4 meV/atom.28 Also, phosphonic acids play an important role for the unidirectional wz-CdS arm growth during tetrapod synthesis, but it can detrimentally affect the yield of tetrapods 1189
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Figure 3. Continuously tuning the arm diameter (a−c) and arm length (d−f) of zb-CdSe/CdS tetrapods by changing the surfactant mixture. The average arm length (L) and diameter (D) are shown in the respective TEM images. (g) Graph showing the variance of arm length and diameters within the same sample while the percentage of ODPA changed. Absorption (solid) and PL (dotted) spectra of (h) tetrapods shown in parts a−c and (i) tetrapods shown in parts d−f. All tetrapod samples for optical measurements are prepared in toluene solution.
by partially converting the zb phase of CdSe core to wz phase.29 In our previous work, we have examined the effects of different ligands on the stability of the zb-CdSe and introduced a new ligand system, carboxylic acid, which helped retain the zb phase even at high temperatures in the presence of phosphonic acid.3 In this work we have investigated the role of a sulfur precursor (TOP-S) in the polytypism of CdSe. For zb-CdSe seeds with a fixed amount of ODPA and OL, the increasing amount of sulfur (and therefore TOP-S) beyond 1:1 (Cd:S) ratio resulted in an increased yield of nanorods (see the Supporting Information, Figure S4), suggesting the conversion of zb- to wz-CdSe. Subsequently, wz-CdSe cores with sulfurrich precursor ratios resulted in rods with a reduced occurrence of branching. However, wz-CdSe cores exposed to a Cd:S precursor ratio of 1:1 yielded fairly monodispersed Y-shaped CdSe-seeded CdS rods (see the Supporting Information, Figure
S5), indicative of the coexistence of both the wz- and zb-CdSe phases within the same nanoparticle.30 The wz-CdSe stabilizing effect seen by exposure to large amount of TOP-S may be attributed to its displacement of the weakly binding OL as opposed to ODPA, leaving a larger proportion of the longer alkyl chain ODPA, which favors the wurtzite phase.29 The association of TOP-S as a ligand for CdS-related nanoparticles31 is not surprising, given its analogy to TOP-Se as a ligand for CdSe.32,33 While it cannot be ruled out that TOP-S itself stabilizes the wz-CdSe phase, its role in the polytypism of CdSe should be directly confirmed with specialized techniques, such as in situ wide-angle X-ray scattering (WAXS),29 which was not within our means to access in this present work. Furthermore, it is reported in the literature that the excess unreacted trioctylphosphine (TOP) from TOP-S precursor also favors the wz-CdSe phase.34 However, its effect is likely to be 1190
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diameter of the arms via judicious variation of the surface capping molecules. Figure 3a−c shows that the arm diameter of CdSe-seeded CdS tetrapods can be tuned from 6.7 nm (length = 17.5 ± 2 nm ) to 10.1 nm (length = 18.4 ± 2.5 nm) and finally 14.5 nm (length = 20.2 ± 2.5 nm) while the length of the arms is kept almost the same. This was achieved via a systematic increase of the weak-binding oleic acid over strongbinding ODPA (binding affinities with respect to the CdS surface) as in the case of CdSe quantum dots.40 Since OL is a weak capping molecule for the CdS, it allows the shell precursor to initiate nucleation and growth at both the {100} family of facets and the (002) facets of the CdS arm (Figure 4a). In contrast, due to presence of the strongly binding ODPA,
much less pronounced than that of ODPA, as an excess of TOP-S and a reduction of the amount of ODPA relative to OL with wz-CdSe seed in rod samples result in a much larger occurrence of branching. Unlike their more widely studied spherical counterparts, the physical properties of semiconductor tetrapods cannot be attributed solely to a single parameter, such as nanoparticle radius, but are also dependent on factors like branch yield and arm dimensions, which in turn define their shape monodispersity. It was experimentally verified that tetrapods with longer arms aid in better spatial connectivity, thus decreasing charge hopping events and leading to better charge transport.35 Compared to nanorods with a CdS arm of similar length, longarm tetrapods, by virtue of their larger volume, can accommodate multiexcitons without incurring severe nonradiative Auger processes, thereby allowing for efficient dual emission from the arms and core.36 Furthermore, controlling tetrapods’ arm length and diameter while still preserving the shape uniformity is paramount for in-depth study of shapedependent assemblies of these structures. Consequently, the ability to obtain a high degree of synthetic control over size and shape of such complicated structure requires a detailed understanding of a complex synthetic parameter space,1−3 which presently remains elusive. We overcome this problem of shape control in CdSe-seeded CdS tetrapods by a simple and controllable method that involves the judicious choice of species and amount of surface capping groups. This approach is distinguishable from the previous synthesis in terms of nanocrystal growth, where “continuous precursor injection” during the course of reaction, “high amount of precursor” injected at the beginning or “varying the amount of zb phase seeds” is needed to maintain the kinetic regime for anisotropic growth.1,37,38 In all the above approaches, there is a high risk of free nucleation of the shell material under the thermodynamic growth regime due to the presence of high precursor concentration. In this present study, by contrast, varying the amount of surface capping groups to control the intrinsic surface energies of different facets and high-temperature (350 °C) injection to yield large precursor flux drive the asymmetric growth of nanocrystals under the kinetic growth regime. The kinetic growth is sustained through the high monomer concentration with rational choice of surface capping groups and high reaction temperature; thus, strict control of the growth time and continuous precursor injection are not required for the shape control of tetrapods. Moreover, the simple variations of the amount of surface capping groups, such as ODPA and OL, widen the window of control over the morphology. This excellent controllability is superior to the previous reported synthetic method.1−3,37,38 Core/shell tetrapod growth can be simply understood as a two-step process: (i) heterogeneous nucleation of the shell material, which can only be initiated by a sudden increase of monomer concentration above the supersaturation threshold, and (ii) the subsequent arm growth from the resulting shell nuclei with progressive consumption of monomers in solution.9,39 After nucleation, several other factors determine the dynamics of arm growth, which in turn will affect the final morphology of the tetrapods. These include the intrinsic surface energies of the different crystallographic facets where nucleation and growth occur and the role of surface-selective capping ligands. Given that the surface energies of the different facets that support growth of the arms are intrinsic to the core material, we approached the problem of tuning the length and
Figure 4. Schematic drawings show the growth mechanism in the presence of a weak binder (OL) and a strong binder (ODPA) as morphology control ligand. (a) Weak binder (OL) allows the Cd and S monomer to reach both the {100} family of side facets and front (002) facets, which leads to arms with conical shape, whereas (b) stronger binding ODPA allows the growth only in front facet (002) to form arms with cylindrical shape.
which preferentially binds to the {100} family of sides facets of CdS,1 the main growth axis is along (002) (also called the caxis). Monomers thus add to the end facets, extending their growth mainly along the end facets’ (002) direction (Figure 4b). The diameter of the arm is fairly consistent, since growth at the side {100} family of facets is inhibited by the presence of ODPA. Subsequently, an increase of the diameter of the CdS arm was observed when less ODPA was used. As seen in Figure 3d−f, monodisperse tetrapods with arm lengths ranging from 30 nm (diameter = 7.9 ± 1 nm) to 44 nm (diameter = 7.4 ± 0.8 nm) to 80 nm (diameter = 5.1 ± 1 nm) were successfully synthesized. Notably, we found empirically that longer arm lengths could be achieved at the expense of the arm diameter. Measurement of the arm length and diameter of the tetrapods in Figure 3g revealed that tetrapods with the longest arm lengths were also the thinnest, while those with shorter arms normally tend to grow thicker in arm diameter. 1191
DOI: 10.1021/acs.chemmater.5b04803 Chem. Mater. 2016, 28, 1187−1195
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Chemistry of Materials In addition, the horizontal bars of the points in Figure 3g represent the variability of the length and diameter with respect to the percentage of ODPA in the ODPA/OL ligand system. As discussed earlier, the bars follow the trend where an increasing amount of ODPA with respect to OL will lead to less variability of the diameter due to the strong binding of ODPA to the CdS surface. Depending on the length and diameter of the CdS arms, the electron wave function of the photogenerated exciton in the CdSe core was found to be delocalized in the CdS arms to different extents, as it is in the case of CdSe-seeded CdS nanorods.1 Owing to this leakage of the electron wave function into the CdS arms, radiative recombination of the separated electron and hole is expected to yield a red shift of the emission maximum (relative to the emission of the CdSe core only) as the length and diameter of the shell increases.1,38 Figure 3h,i show the detailed UV−vis absorption and PL emission spectra with respect to the change in diameter and arm length. Prominently, the flowerlike tetrapods emit at a longer wavelength compared to the longest arm length tetrapods, indicating that even when the arm length is very long, further red shift of the PL peak does not occur (some blue shifting may occur). The two reasons explaining this phenomenon might be that (i) strong quantum confinement is still present in the axes perpendicular to the long axis of the arm (i.e., along its length) and (ii) leakage of the electron wave function does not take place beyond a certain arm length for a quasi-type-II alignment between CdSe and CdS. In this work, seeded CdSe/CdS tetrapods with different arm lengths and diameters were obtained using the same zb-CdSe core (∼3 nm). The size of the CdSe core (
