Article Cite This: Chem. Mater. 2017, 29, 9075-9083
pubs.acs.org/cm
Hierarchical Multicomponent Nanoheterostructures via Facet-toFacet Attachment of Anisotropic Semiconductor Nanoparticles Shashank Gupta,† Wen-Ya Wu,‡ Sabyasachi Chakrabortty,† Mingjie Li,† Yi Wang,† Xuanwei Ong,† 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
‡
S Supporting Information *
ABSTRACT: As performance and functionality requirements for solution-processed nanomaterials become more stringent and demanding, there is an ever-growing need for hierarchical nanostructures with sophisticated architecture and complex composition. However, the production of structurally complex nanomaterials is often not possible by direct synthesis. In this work, we describe synthetic methodology to covalently link presynthesized anisotropic semiconductor nanoparticles of different composition in a stoichiometrically controlled manner via specific facet sites at room temperature. We demonstrate that CdSe nanorods can be cojoined with CdTe tetrapods via a competitive cation-exchange process with Ag+ that results in linking between the tips of the tetrapod arms with only one end of each nanorod via a Ag2Se−Ag2Te interface. This selective linking was engineered by having a large fraction of CdSe nanorods present in the reaction, which sterically hindered homolinking between Ag2Se-tipped CdSe nanorods and Ag2Te-tipped CdTe tetrapods with themselves. Cation back-exchange with Cd2+ and a size-selective purification to remove unlinked products yields samples enriched in heterolinked CdTe tetrapod−CdSe nanorod structures. High-resolution transmission electron microscopy and energy-dispersive X-ray spectroscopy confirmed the structure and composition of the nanorod-linked tetrapods, while time-resolved and pump-dependent photoluminescence data were consistent with a type II band offset at the CdTe−CdSe interface. The synthetic approach to colloidal nanoheterostructures described here is highly distinct from traditional methods involving a series of nucleation and growth steps at elevated temperature.
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INTRODUCTION Colloidal hybrid nanoparticles (NPs) that comprise multiple domains of distinct composition have the potential to exhibit multifunctional and/or enhanced properties due to the physicochemical attributes of the individual components or synergistic interactions at their interfaces. A heterojunction formed at the interface between two different materials within such hybrid structures can possess very different optoelectronic properties compared to a physical mixture of the same two materials. For example, in heterojunction semiconductors, the type of band alignment at the heterojunction viz. type I, inverse type I, or type II critically determines their optoelectronic properties. Subsequently, type I semiconductor nanostructures have generally been exploited in light-emitting diodes,1−3 fluorescence labels,4−7 and lasers,8−11 while their type II counterparts have been harnessed for solar cells12−15 and photodetectors.16 In addition to band alignment, the shape and size of each component within the nanostructure can also profoundly influence its properties. For example, radiative excitonic recombination in spherical core/shell CdSe/CdS nanoparticles typically takes place in the CdSe core even at large pump intensities, whereas it can occur in both the core and arms of CdSe core-seeded CdS tetrapod, enabling dualwavelength emission.17 As the performance requirements for a © 2017 American Chemical Society
rapidly expanding multitude of applications involving colloidal nanoparticles become more stringent and demanding, it is clear that the development of better synthesis routes to more sophisticated and complex nanocrystal architectures is needed. Heterogeneous nucleation and growth, where one crystal nucleates and grows over another, is to date the most common technique used to synthesize colloidal nanoheterostructures with two or more components.18−21 Growth of the secondary material can be isotropic, which leads to spherical core−shell structures,22,23 or anisotropic, which leads to rodlike24 or branched25,26 morphologies. A sequence of carefully planned heterogeneous nucleation and growth steps can produce highorder hybrid NPs, analogous to the conceptual framework of total synthesis in organic chemistry.27,28 However, the shape and size of the secondary (or higher-order) components depend strongly on a very complex parameter space involving precursor reactivity and concentration, structure of the host crystal, reaction temperature, surfactants present, and so on. Accordingly, dictating the shape and size of the components within a hybrid nanoparticle based on a heterogeneous Received: June 27, 2017 Revised: October 16, 2017 Published: October 17, 2017 9075
DOI: 10.1021/acs.chemmater.7b02676 Chem. Mater. 2017, 29, 9075−9083
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Chemistry of Materials
g), HPA (0.1657 g), TDPA (0.1543 g), and TOPO (3 g) were mixed in a 3-neck round-bottom flask (RBF) and degassed at 150 °C for 1.5 h under constant stirring. Separately, 2 mL of 1.5 M tri-noctylphosphine selenide (TOPSe) solution and 10 nmol of wurtzite CdSe (w-CdSe) quantum dots (QDs) (see Supporting Information for synthesis details) were mixed and degassed at 90 °C until all of the toluene was removed from the mixture. After degassing, the temperature of the cadmium precursor mixture was increased to 340 °C. As the temperature increased, the color of the solution changed from dark brown to colorless, indicating the formation of the cadmium−alkyl phosphonic acid complex. TOP (1.8 mL) was then swiftly injected into this mixture, and the temperature was allowed to recover to 340 °C. Upon reaching 340 °C, 1.8 mL of the TOPSe/ CdSe QD mixture was injected swiftly into cadmium precursorcontaining mixture, whereupon the temperature dropped to 320 °C. Upon recovery to 340 °C, the reaction solution was maintained at this temperature for ∼3 min before removing the heating mantle and allowing the solution to cool to room temperature. The resulting CdSe nanorods were processed by adding methanol to the crude reaction solution and centrifuging at 3500 rpm for 8 min. The precipitate was redispersed in toluene. This cycle of precipitation in methanol and redispersion in toluene was then repeated. The concentration of nanorods in toluene was determined using a published molar extinction coefficient35 and made up to ∼5 μM for further use. Synthesis of CdTe Tetrapods. CdTe tetrapods were grown via seeded-growth technique as reported by Manna et al.,36 with slight modifications. As an example, CdO (0.102 g), ODPA (0.5124 g), HPA (0.1414 g), and TOPO (3 g) were mixed in a 3-neck RBF and degassed at 150 °C for 1.5 h under constant stirring. Separately, 0.5 mL of a 0.725 M tri-n-octylphosphine telluride(TOPTe) complex (prepared by heating a mixture of 0.046 g of Te powder in 0.5 mL of TOP under N2 flow at 320 °C for 30 min) was mixed with 2.5 nmol of CdTe QDs (see Supporting Information for synthesis details) and degassed at 90 °C until all the toluene was removed from the mixture. After degassing, the temperature of the cadmium precursor-containing mixture was increased to 320 °C. As the temperature increased, the color of the solution changed from dark brown to colorless, indicating formation of cadmium−alkyl phosphonic acid complex. TOP (1.8 mL) was then swiftly injected into this mixture at 320 °C, and the temperature was allowed to recover. As the temperature reached 320 °C again, 0.5 mL of the TOPTe/CdTe QD mixture was injected swiftly into the cadmium precursor-containing mixture, and the reaction temperature was maintained at 320 °C for ∼3 min. The resulting CdTe tetrapods were processed by adding methanol to the crude reaction solution followed by centrifuging at 3500 rpm for 8 min. The precipitate was then redispersed in toluene and reprocessed by another cycle of precipitation in methanol and redispersion in toluene. Finally, tetrapods were redispersed in a minimum amount of toluene and stored as stock solution for further use. The concentration of the processed tetrapod solution was determined using a published molar extinction coefficient35 and made up to ∼5 μM in toluene. Linking Process. In a typical linking reaction, 0.8 mL of a ∼5 μM solution of CdSe nanorods and 0.2 mL of a ∼5 μM solution of CdTe tetrapods in toluene were mixed in a vial. About 6 mg of ODPA was then added to this mixture and sonicated for 2−3 min for it to dissolve. Separately, 1 mL of an aqueous 0.8 mM Ag+ solution was mixed with 1 mL of an ethanolic DDA solution that was prepared by dissolving 0.7 g of DDA in 25 mL of ethanol. The nanoparticlecontaining solution was subsequently added to the Ag+/DDA mixture and stirred vigorously at room temperature for ∼30 min. Thereafter, methanol was added to the mixture to quench the linking reaction and cause the nanoparticles (NPs) to precipitate. The solution was then centrifuged at 1500 rpm for ∼5 min and decanted to remove the supernatant. The remaining precipitate was redispersed in ∼2 mL of toluene. Cadmium Back-exchange. The solution of size-selectively precipitated NPs redispersed in 1 mL of toluene was mixed with 0.5 mL of a 8 mM solution of cadmium nitrate tetrahydrate in ethanol. To this mixture, 50 μL of tributylphosphine was added under constant stirring inside a glovebox. The mixture was stirred for ∼15 min to
nucleation and growth approach is extremely difficult. A more radical synthetic route to multicomponent hybrid NPs is to independently synthesize the individual nanocrystal components and then join them together via their facets under mild reaction conditions. A big advantage of this approach is that the individual nanocrystal components can be produced with a high degree of shape control, narrow size distribution, and high yield via their own optimized synthesis protocols. Oriented attachment provides a basis for how NPs can join together into an ordered geometry via their facets. However, arranging the individual particles into an assembly to facilitate their union requires a delicate balance between particle dipole-, ligand-, and solvent-based interactions.29−32 This is extremely challenging to achieve in practice, especially with architecturally complex, shape-anisotropic nanoparticles as building blocks. If the nanoparticles are of different composition, as would be required to form heterostructures, joining them in a stoichiometrically controlled manner without nanoparticles of the same composition forming links with each other raises the difficulty further if carried out via a conventional oriented-attachment approach. Herein, we introduce a facile synthetic route to joining colloidal semiconductor NCs of different shape and composition in a stoichiometrically controlled manner, namely, the facet-to-facet linking of CdTe tetrapods (TPs) with CdSe nanorods (NRs) in solution at room temperature. Unlike previous approaches to the fabrication of architecturally complex semiconductor nanoheterostructures, the CdTe TPs and CdSe NRs were independently synthesized using their most optimal synthetic protocols and then cojoined via a partial cation-exchange process. The partial cation-exchange process converts the tips of the CdTe TPs and CdSe NRs to small domains of Ag2Te and Ag2Se, respectively. By selectively removing the surface ligands that cap the Ag chalcogenide surfaces, fusion between the unpassivated surfaces occurs as they come into contact. By having a large number of CdSe NRs that did not undergo Ag+ exchange, Ag2Te-tipped CdTe TPs are sterically hindered from linking to each other, thereby allowing for heterolinking with Ag2Se-tipped CdSe NRs via an all-inorganic Ag2Te−Ag2Se bridge. Subsequent back-exchange with Cd2+ converts the Ag2Te−Ag2Se bridge into CdTe−CdSe, ensuring electronic coupling between TP and NR.33
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EXPERIMENTAL SECTION
Chemicals. Cadmium oxide (CdO, 99.5%), tri-n-octylphosphine oxide (TOPO, 90% and 99%), cadmium acetylacetonate (Cd(acac)2, 99.9%), hexadecanediol (HDDO, 90%), 1-octadecene (ODE, 90%), dodecylamine (DDA, 98%), cadmium nitrate tetrahydrate (Cd(NO3)2· 4H2O, 98%), myristic acid (MA, 99%), octyl phosphonic acid (OPA, 97%), oleylamine (Oly, 70%), hexadecyltrimethylammonium bromide (CTAB, 95%) and selenium pellets (5 mm) (Se, 99.99%+) were purchased from Sigma-Aldrich. Hexadecylamine (HDA, 90%), ntetradecylphosphonic acid (TDPA, 98%), silver nitrate (AgNO3, 99.9%), and oleic acid (OA, 90%) were purchased from Alfa Aesar. Tri-n-octylphosphine (TOP, 97%), n-hexylphosphonic acid (HPA, 97%), n-octadecylphosphonic acid (ODPA, 97%), tellurium powder (Te, 99.9%), and tri-n-butylphosphine (TBP, 99%) were purchased from Strem Chemicals. Diisooctylphosphonic acid (DIPA, 90%) was purchased from Fluka chemicals. All chemicals were used without further purification. Unless stated otherwise, all reactions were conducted in oven-dried glassware under nitrogen atmosphere via standard Schlenk line techniques. Synthesis of CdSe Nanorods. CdSe nanorods were synthesized via slight modifications of the seeded growth method described by Manna and co-workers.34 In a typical synthesis reaction, CdO (0.1035 9076
DOI: 10.1021/acs.chemmater.7b02676 Chem. Mater. 2017, 29, 9075−9083
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Figure 1. (a) Schematic showing heterolinking of Ag2Se-tipped CdSe NRs (green) to Ag2Te-tipped CdTe TPs (yellow) via their Ag chalcogenide ends. Due to competitive cation exchange, not all CdSe NRs undergo exchange with Ag+ and remain as pristine NRs. (b) HAADF-STEM image of CdSe NR-linked CdTe TPs. The dotted circles denote the linkage regions. (c) HAADF-STEM image of a single CdSe NR-linked CdTe TP with false color to distinguish the CdSe (green) and CdTe (yellow) regions. Points 1−3 are areas where point EDX was performed. (d−f) EDX spectra corresponding to points 1−3 in (c).
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allow Cd2+ exchange to take place. The Cd2+ exchanged NPs were subsequently precipitated by adding excess methanol and centrifuging at 1500 rpm for ∼5 min. The precipitated NPs were then redispersed in 1 mL of toluene for further analysis. Size-Selective Removal of Nanorods. To remove unlinked CdSe nanorods, the solution of redispersed NPs in toluene was exposed to dropwise addition of a 1:2 v/v mixture of methanol and ethanol until it started to look slightly turbid. The slightly turbid solution was then centrifuged at 1500 rpm for 5 min, and the precipitated NPs were redispersed in 1 mL of toluene for further use. Transmission Electron Microscopy. A JEOL JEM 3010 F (300 kV accelerating voltage) microscope was used to obtain bright-field transmission electron microscopy (TEM) images of the NPs. For TEM sample preparation, a drop of the NP solution was placed onto a 300 mesh copper grid covered with a continuous carbon film. Excess solution was removed with an adsorbent paper, and the sample was dried at room temperature. The dark-field, bright-field, high-resolution transmission electron microscopy and nanoparticle elemental analysis were carried out on a FEI Titan 80-300 electron microscope (200 kV accelerating voltage) equipped with an electron beam monochromator, energy-dispersive X-ray spectroscopy (EDX), and a Gatan electron energy loss spectrometer (EELS). The probing electron beam size for the EDX measurement was ∼0.3 nm. The dwell time for each EDX spectrum was 10 s. Time-Resolved and Fluence-Dependent Photoluminescence. Time-resolved photoluminescence (TRPL) measurements on the broad emission band of linked and back-exchanged nanostructures were carried out at room temperature via a frequency-doubled, 1 kHz mode-locked Ti:sapphire with 150 fs laser pulses and a time-correlated single-photon counting (TCSPC) module (PicoQuant PicoHarp 300) detection system. For the fluence-dependent PL measurements, a 100 fs, 1 kHz repetition rate, 400 nm laser pulse from a frequency-doubled regenerative Ti:sapphire amplifier (Coherent, Libra-F-1K-HE-230) was used to excite the sample in a 2 mm thick quartz cuvette. The PL spectra were recorded by a triple grating imaging spectrometer (Princeton Instruments, Acton SpectraPro SP-2500). The same light source was also used for both measurements.
RESULTS AND DISCUSSION For colloidal anisotropic cadmium chalcogenide nanoparticles such as CdS NRs or CdSe seeded CdS TPs, exposure to intermediate concentrations of Ag+ results in cation-exchange occurring nearly exclusively at the tip facets of the particles due to their high surface energy relative to their side facets.33,37,38 In the present work, the cation-exchange reaction is carried out by allowing a mixture of anisotropically shaped CdX (where X = Se or Te) NPs in toluene to come into contact with a mixture of Ag+ in dodecylamine (DDA) and ethanol. The DDA forms a complex with Ag+, allowing it to cross over into the nonpolar toluene mixture39 and undergo cation exchange with the nanoparticles. Subsequently, the reaction produces Ag2X domains that are capped with DDA at the tips of the anisotropic nanoparticles. Exposure to octadecylphosphonic acid (ODPA) results in a spontaneous acid−base reaction with DDA, resulting in an insoluble salt complex that precipitates out of solution (Figure S2). This process effectively removes DDA from the Ag2X domains, causing them to fuse facet-tofacet upon contact and allowing the nanoparticles to link in a stoichiometrically controlled manner. For example, NRs can form at most two junctions with each other while tetrapods can support a maximum of four linkage sites. In contrast, the CdX regions whose native ligands are strongly binding alkylphosphonic and trioctylphosphine ligands40−42 do not undergo fusion upon removal of DDA ligands (see Supporting Information for details). We recently showed that this approach can be used to fabricate complex assemblies such as long polymer-like chains of CdSe-seeded CdS NRs or oligomeric chains of CdSe seeded-CdS TPs.43 Although this approach can in principle allow for semiconductor nanoparticles of different shape and composition to be linked, such as TPs of one composition with NRs of another composition (which we will describe as heterolinking), it cannot prevent the TPs or NRs 9077
DOI: 10.1021/acs.chemmater.7b02676 Chem. Mater. 2017, 29, 9075−9083
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Figure 2. (a) HRTEM image showing tip of w-CdTe TP arm fused with tips of two w-CdSe NRs via a Ag2Te−Ag2Se interface. The dotted red line outlines the Ag2Se−Ag2Te region. (b) XRD spectrum of a thin film of CdTe−CdSe TP−NRs heterolinked via a Ag2Te−Ag2Se interface.
parallel with the TP arm, the majority of NRs are linked at an angle. We propose that the facets at the apexes of the TPs and NRs may not both be predominantly (001̅) or (001) prior to the linking process, leading to kinked TP−NR interfaces. Indeed, previous studies on Au nanoparticle growth at the tips of cadmium chalcogenide nanorods showed that the Au typically grows on the sulfur-rich (101̅) and (101) facets, leading to an angled orientation. Subsequently, control over the directionality of the linked rods will be the subject of future work. Given the relatively low proportion of CdSe NRs that underwent Ag+ exchange, the occurrence of homolinked CdSe NR chains was comparatively rare while pristine isolated CdSe NRs were in abundance. Free TPs or TP chains formed by homolinking between TPs were also scarcely observed. The statistical distribution of the Ag2X linked products is provided in a later figure. Point EDX analysis of a typical HtNS at various regions corroborated our supposition that the CdSe NRs are generally linked to the tips of the CdTe TPs, as evidenced in Figure 1c−f (see also Figure S6). Point EDX measurements at the joint regions show a mixture of signals that are attributed to cadmium, silver, tellurium, and selenium. This is consistent with the notion that the linking takes place via fusion of the Ag+-exchanged facets. To further substantiate our hypothesis of linking via the fusion of silver chalcogenide facets, high-resolution transmission electron microscopy (HRTEM) and thin-film X-ray diffraction (XRD) were performed on the linked HtNS’s. Figure 2a shows the HRTEM image of the joint region of a HtNS in which the tips of two CdSe NRs can be seen fused with the tip of a CdTe TP’s arm via their respective silver chalcogenide domains. A dotted red line encircles the region containing the two different Ag chalcogenide domains. Thin film XRD spectra of HtNS’s (Figure 2b) show peaks arising from wurtzite CdSe (w-CdSe), triclinic Ag2Se, wurtzite CdTe (w-CdTe), and monoclinic Ag2Te, which is consistent with the HRTEM data. To rule out the possibility of Ag deposition at the surface of Ag2Se or Ag2Te, HRTEM and XRD analysis of CdSe NRs and CdTe TPs separately exposed to relatively high concentrations of Ag+ were carried out. The presence of Ag atop the Ag2Se and Ag2Te domains formed at the tips of Ag+exchanged CdSe NRs and CdTe TPs was not observed under both HRTEM (Figure S7) and XRD of thin films of CdSe NRs and CdTe TPs (Figure S9). It is known that CdSe and CdTe have band gaps whose alignments with each other form a type II heterojunction.44 Such heterostructures are potentially useful in the context of
from linking with themselves (which we will label as homolinking). To promote heterolinking and circumvent homolinking from taking place, we introduce in this work the concept of competitive cation exchange as a method to achieve the selective cojoining of two distinct species of Cd chalcogenide nanoparticles. To illustrate this principle, we prepared a mixture of presynthesized CdSe NRs (∼4 nmol, ∼5 nm diameter, and ∼18 nm length) and CdTe TPs (∼1 nmol, ∼5 nm diameter, and ∼55 nm arm length) in toluene (1 mL) and introduced to this mixture a solution comprising dodecylamine (0.15 mmol), EtOH (1 mL) and AgNO3 (0.8 mmol). A competition between the two different Cd chalcogenide nanoparticles to undergo exchange with Ag+ develops and favors the material that has a larger negative change of free energy (ΔG) for the reaction. A crude calculation based on bulk lattice formation energies and aqueous redox potentials indicates that, for the exchange between Cd2+ and Ag+ in bulk CdSe and CdTe, ΔG is about −140.9 and −163.4 kJ/mol, respectively (see Supporting Information for more details). In the absence of ligand effects, CdTe NPs should, from the standpoint of thermodynamic considerations, undergo Ag+ exchange more spontaneously than CdSe NPs of the same size. This means that, for a sample mixture of CdSe and CdTe NPs, exposure to a low concentration of Ag+ would result in most of the CdTe NPs and very few of the CdSe NPs undergoing Ag+ exchange. The number of CdSe NRs used in the reaction mixture was therefore deliberately made much larger than that of CdTe TPs for two reasons: (i) to ensure that, in addition to TPs, a sufficient number of NRs would undergo Ag+ exchange; (ii) the majority of CdSe NRs which do not undergo Ag+ exchange would sterically hinder the Ag+-exchanged CdTe TPs from linking with each other. Upon addition of ODPA, linking between the Ag2Te-tipped CdTe TPs and Ag2Se-tipped CdSe NRs occurs spontaneously via fusion of their Ag+-exchanged regions, as illustrated in Figure 1a. This process is energetically driven by the formation of numerous covalent bonds between the bare silver chalcogenide facets upon contact with each other, yielding CdTe TPs with CdSe NRs cojoined at the tips of their arms. Figure 1b is a high-angle annular dark-field (HAADF) image of the NR-linked TPs, where it may be observed that many of the heteronanostructures (HtNS’s) were that of CdTe TPs cojoined with at least one CdSe NR, yielding heterolinked CdTe−Ag2Te/Ag2Se−CdSe structures (see also Figure S5). Although in some cases the CdSe NR is fused in a direction 9078
DOI: 10.1021/acs.chemmater.7b02676 Chem. Mater. 2017, 29, 9075−9083
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Figure 3. (a) HAADF-STEM image of CdSe NR-linked CdTe TP nanostructures after Cd2+ back-exchange and size-selective precipitation. (Inset) Typical Cd2+ back-exchanged CdTe−CdSe (TP−NR) HtNS. Scale bar is 20 nm. (b) HRTEM image of a single Cd2+ back-exchanged HtNS. (Inset) Zoomed-in image of the joint region. Scale bar is 20 nm (c) Thin-film XRD spectrum (black) of Cd2+ back-exchanged CdTe−CdSe (TP−NR) HtNS, alongside standard reference peaks of w-CdTe (blue) and w-CdSe (red). (d) Distribution of products obtained after linking via Ag2X domains (green, sample size = 556 particles) and after Cd2+ back-exchange and size-selective precipitation (red, sample size = 503 particles).
agreement with elemental mapping, point EDX, and elemental analysis data. A histogram depicting the distribution of as-synthesized HtNS’s with Ag2X domains (green) and Cd2+ back-exchanged HtNS’s after size-selective precipitation (red) is given in Figure 3d, where free particles and linked particles bearing at least one or more homo- or heterolinkages are sorted into the categories defined by the x-axis. Thus, a CdTe TP with four CdSe NRs linked to its arms would count as one particle. The percentage yield is therefore the fraction of particles in each category over the total number of particles analyzed. Details of how the statistics were obtained are described in the Supporting Information. Additionally, an analysis of the number of CdSe NRs linked to a CdTe TP shows that most of the TPs have between 1 and 4 NRs linked to their tips (see Figure S13), which reflects the stochastic nature of the oriented-attachmentbased linking process. Comparison between the product distributions in Ag2X-linked nanostructures and Cd2+ backexchanged/size-selectively processed nanostructures shows an obvious decrease in the number of free particles, concomitant with a large dominance in the proportion of heterolinked products. These results indicate the effectiveness of the sizeselective process in purifying as-synthesized samples of linked nanostructures. The Cd2+ back-exchanged CdTe−CdSe HtNS’s were further studied for their optical properties, in particular the type II band offset at the heterojunction between the CdTe TP and the CdSe NR. Photoluminescence (PL) measurements showed a marked difference in spectra between linked nanoparticles and
photodetectors and photovoltaics as a photogenerated exciton is spontaneously separated into its charges at the interface between the two materials. To realize CdSe NR-linked CdTe TPs with a type II heterojunction, a Cd2+ back-exchange process was introduced to convert the Ag2Te/Ag2Se into their Cd-based counterparts. Additionally, a size-selective procedure involving the precipitation of linked TPs using small amounts of antisolvent (a mixture of methanol and ethanol in our case) was used to remove unlinked NRs. As exemplified in Figure 3a, the morphology of the converted CdTe−CdSe TP−NR structure is preserved, as expected from cation-exchange reactions carried out at room temperature. Elemental mapping performed on Cd2+ back-exchanged HtNS’s showed selenium signals arising from rodlike protrusions at the tips of TPs, which themselves displayed strong tellurium signals (Figure S11). The presence of Ag at the joint region was notably absent as suggested by point EDX (Figure S12), suggesting complete cation exchange with Cd2+. This is further supported by the HRTEM image in Figure 3b, which shows the lattice fringes of the (002) planes of w-CdTe and w-CdSe domains adjacent to each other. Structural characterization of the CdTe−CdSe TP− NR nanoparticles via thin-film X-ray diffraction (XRD), as illustrated in Figure 3c, featured peaks corresponding to those of w-CdTe and w-CdSe. Finally, the Cd2+ back-exchanged nanoparticles were subjected to elemental analysis via inductively coupled plasma−optical emission spectroscopy (ICP-OES), which showed negligible amounts of Ag (see Table S1). The presence of Ag2X was not detected, in 9079
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Figure 4. (a) PL emission spectra of CdSe NR-linked CdTe TP nanostructures in toluene. (b) Time-resolved PL data (blue open circles) measured at the type II emission peak (∼780 nm for the fluence used) for the linked nanostructures. The decay was fitted with a monoexponential function (solid red line). (c) Pump-fluence-dependent PL spectra of linked nanostructures dispersed in toluene. (Inset) Zoomed-in image of PL spectra, showing the evolution of the type II emission at different pump intensities. (d) Plot of the peak energy of the type II emission versus the cube root of the pump fluence. The solid red line is a linear fit to the experimental data.
Figure 5. (a) Histogram showing the distribution of side-linked (black columns) versus tip-linked (red columns) NRs at different Ag+ concentrations. The columns for each Ag+ concentration were constructed from a sample size of ∼500 linked nanostructures. (b−e) Corresponding TEM images (scale bar = 50 nm) of CdSe NRs and CdTe TPs linked using the different concentrations of Ag+ listed in the histogram in (a). (Inset) Representative illustration of each type of branched structure.
a mixture of CdTe TPs and CdSe NRs. As shown in Figure 4a, a broad emission band from ∼710 to 900 nm was observed from the sample of linked nanoparticles, whereas it was notably absent from the PL spectra of two control mixtures of TPs and NRs (see Figure S14a). The nanoparticles in the control mixtures underwent identical chemical processes as the linked nanoparticles (including the size-selective removal of free nanorods), with the exception that either AgNO3 or ODPA was not added to prevent linking from taking place. Although statistically rare in the sample of linked nanostructures, the free CdSe NRs are relatively fluorescent and account for the narrower emission peak at ∼650 nm. Additionally, it was found that the intensity of the broad peak relative to the peak at ∼650 nm was strongly correlated with the number of CdSe/CdTe
linkages seen from TEM data. In summary, these observations suggest that the broad NIR emission band originated from the interface region of the linked nanostructures as opposed to chemically induced trap states at the surface. The decay plot extracted from TRPL measurements, as exemplified in Figure 4b, revealed a relatively long lifetime of ∼323 ns at the emission peak of ∼780 nm. The lengthened lifetime is consistent with type II emission, which in our case is due to radiative recombination of an electron and hole across a CdTe/CdSe interface. In contrast, the PL lifetimes of the control mixtures at ∼750 nm were on the order of hundreds of picoseconds (see Supporting Information) and are most likely dominated by nonradiative relaxation channels. Further, the PL spectrum shows a blue-shift on increasing the pump fluence, as 9080
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Chemistry of Materials
we demonstrated that partially Ag+-exchanged CdSe NRs and CdTe TPs can be joined at their tip facets via a Ag2Se−Ag2Te intermediate layer. The resulting products can then undergo exchange with Cd2+ to yield CdSe NR-linked CdTe TPs exhibiting type II properties at the CdSe/CdTe heterojunction. The ability to independently synthesize nanocrystals of different shapes and compositions and then stoichiometrically and site-specifically join them together via their facets can in many ways be likened to molecular synthesis, where different molecules can be covalently attached to each other via their functional groups. Achieving such a prospect would certainly inspire the synthesis of nanostructures with higher-order functionality and performance.
depicted in Figure 4c. This observation is in accordance with the band bending induced by Coulombic interactions between spatially separated holes and electrons.45−48 A plot of the peak wavelength versus the cube root of the pump fluence, as shown in Figure 4d, results in a linear plot that is characteristic of type II emission. These results, along with lifetime measurements and comparison with the PL profile of the control mixtures, collectively corroborate our supposition that the broad band is type II emission arising from the heterojunctions of linked nanostructures. While most of the CdSe NRs were linked to the tips of the CdTe TPs (given that the tips have highest reactivity), it may be observed that some of the TPs have CdSe NRs cojoined at the side of the tetrapod arms. A statistical analysis of a processed sample of TP−NR HtNS’s that underwent Cd2+ back-exchange, illustrated in Figure 5a, shows that the percentage occurrence of CdSe NRs linked to the sides of the CdTe TP increases as the concentration of Ag+ added was varied from 0.4 to 1.6 mM. At low concentrations of Ag+, cation exchange with Cd2+ occurs predominantly at the tip facets of the CdTe TPs, which are most reactive, yielding mostly tiplinked TP−NR nanostructures. At higher concentrations of Ag+, small islands of Ag2Te randomly form on both the tips and the sides of the TP arms, even though they are less reactive (see Supporting Information). These trends may be understood by considering the Ag+-exchange process as the heterogeneous nucleation and growth of Ag2Te domains within the CdTe crystal. A higher Ag+ concentration is needed to induce nucleation at the less-reactive side facets of the CdTe TP arms, akin to partial Ag+-exchange reactions observed in CdS nanorods.49 This ultimately results in a significant number of side-linked rods, even surpassing the number of tip-linked rods at an Ag+ concentration of 1.6 mM. Such structures are difficult to synthesize via the heterogeneous nucleation and growth of CdSe on CdTe TPs, which tends to localize the CdSe domains at the tetrapod tips.50 On the other hand, the same high concentration of Ag+ facilitates cation exchange exclusively at the tips of the CdSe NRs, which are generally less prone to Ag+ exchange than CdTe, as explained before. Consequently, the CdSe NRs are always linked, either to CdTe TPs or to each other, via their end facets rather than their side facets. The corresponding TEM images of the TP−NR heterolinked structures at different Ag+ concentrations are displayed in Figure 5b−e, where it is readily seen that, as Ag+ increases, (i) the number of NRs linked to the tip facets of TPs steadily increases; (ii) the occurrence of NRs linked to the sides of the TP arms increases. If the NRs are likened to branches on the TP host, then the linking procedure described here presents a facile and systematic approach to simultaneously controlling the geometry and stoichiometry of branch extension that is not easily achieved by conventional synthesis.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b02676. CdSe and CdTe quantum dots synthesis, free energy calculation for cation exchange, XRD patterns of pristine and silver-exchanged nanoparticles, additional structural and optical characterization of pristine nanoparticles, linked HtNS’s and Cd2+ back-exchanged HtNS’s (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Yinthai Chan: 0000-0002-8471-9009 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge funding support from A*STAR Science & Engineering Research Council Public Sector Funding (Project no. 142100076).
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REFERENCES
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CONCLUSION In summary, we have developed a different form of oriented attachment in which anisotropic semiconductor nanoparticles of different compositions are joined into a single nanostructure via their facets in a stoichiometrically controlled manner. This was facilitated by (i) the introduction of a reactive layer at distinct facets of the semiconductor nanoparticles via a partial cation-exchange process; (ii) removal of the ligands on the reactive layers, resulting in nanoparticles fusing together; and (iii) a cation back-exchange process to return the reactive layers to their original composition. To exemplify this methodology, 9081
DOI: 10.1021/acs.chemmater.7b02676 Chem. Mater. 2017, 29, 9075−9083
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