Au-Assisted Growth of Anisotropic and Epitaxial CdSe Colloidal

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Au-assisted growth of anisotropic and epitaxial CdSe colloidal nanocrystals via in-situ dismantling of quantum dots. Víctor Fernàndez-Altable, Mariona Dalmases, Andrea Falqui, Alberto Casu, Pau Torruella, Sonia Estrade, Francesca Peiró, and Albert Figuerola Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm504433y • Publication Date (Web): 03 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015

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Chemistry of Materials

Au‐assisted growth of anisotropic and epitaxial CdSe colloidal nanocrystals via in‐situ dismantling of quantum dots. Víctor Fernàndez‐Altable,†,‡ Mariona Dalmases,†,‡ Andrea Falqui, &,$ Alberto Casu, $ Pau Torrue‐ lla, #,‡ Sònia Estradé, #,‡ Francesca Peiró, #,‡ Albert Figuerola*,†,‡ †

Departament de Química Inorgànica, Universitat de Barcelona, Martí i Franquès 1‐11, 08028 Barcelona, Spain

‡

Institut de Nanociència i Nanotecnologia (IN2UB), Universitat de Barcelona, Martí i Franquès 1‐11, 08028 Barcelo‐ na, Spain

&

King Abdullah University of Science and Technology (KAUST), Biological and Environmental Sciences and Engi‐ neering Division, Thuwal 23955‐6900, Kingdom of Saudi Arabia

$

Department of Nanochemistry, Istituto Italiano di Tecnologia, I.I.T., Via Morego 30, 16163 Genova, Italy

#

Laboratory of Electron Nanoscopies (LENS)‐MIND/IN2UB, Departament d'Electrònica, Universitat de Barcelona, Martí i Franquès 1‐11, 08028 Barcelona, Spain

ABSTRACT: Metallic nanocrystals have revealed in the last years as valuable materials for the catalytic growth of semi‐ conductor nanowires. Yet, only low melting point metals like Bi have been reported to successfully assist the growth of elongated CdX (X = S, Se, Te) systems in solution, and the possibility to use plasmonic noble metals has become a chal‐ lenging task. In this work we show that the growth of anisotropic CdSe nanostructures in solution can also be efficiently catalyzed by colloidal Au nanoparticles, following a preferential crystallographic alignment between the metallic and semiconductor domains. Noteworthy, we report the heterodox use of semiconductor quantum dots as an homogeneous and tunable source of reactive monomer species to the solution. The mechanistic studies reveal that the in‐situ delivery of these cadmium and chalcogen monomer species and the formation of AuxCdy alloy seeds are both key factors for the epi‐ taxial growth of elongated CdSe domains. The implementation of this method suggests an alternative synthetic approach for the assembly of different semiconductor domains into more complex heterostructures.

INTRODUCTION In the last years, nanochemistry has offered an impres‐ sive plethora of size and shape‐controlled semiconductor nanocrystals (NCs) that helped to identify critical param‐ eters for the optimized performance of NC‐based energy conversion devices.1‐4 Among these parameters, the aniso‐ tropic growth of semiconductors in the shape of nanorods or nanowires5‐7 and their decoration with metallic do‐ mains8‐10 clearly enhance charge carriers separation within the particle, and thus such complex systems hold great promise in these fields. The Vapor‐Liquid‐Solid (VLS) and Solution‐Liquid‐ Solid (SLS) approaches represent smart ways to directly produce 1D semiconductor nanostructures decorated with one metallic tip, fulfilling in this way the two previously mentioned requirements for enhanced charge carriers separation.11‐14 The VLS and SLS approaches are based on the catalytic growth of the semiconductor domain assist‐ ed by the presence of metallic nanoparticles that are able to form liquid alloys with one component of the semicon‐ ductor material, from which the nanorod or nanowire grows. Since the diameter of the wire is dictated by the size of the metallic seed, a high control over the size of

the latter is required in order to obtain homogeneous wire samples. Current synthetic methods have been effi‐ ciently developed for the size‐control of Au nanoparti‐ cles,15 and thus this metal has been generally used in the VLS growth method, where high reaction temperatures are used during synthesis.13‐14 However the scenario is significantly different when the semiconductor growth takes place in solution through the analogous SLS ap‐ proach,11‐12 since the reaction temperatures reached are well below those used in the vapor phase and thus Au (Tm = 1,064 °C) can hardly melt. While for the formation of Si, Ge or InP elongated NCs in solution Au nanoparticles still show an efficient catalytic performance,16‐19 this is not so clear in the case of Cd chalcogenide 1D nanostructures. Bismuth is generally used to catalyze the growth of Cd‐ based semiconductors by SLS mechanisms due to its low‐ er melting point (Tm = 271 °C).20‐24 However, and com‐ pared to Au, just a few methods exist leading to well‐ defined and size‐controlled Bi nanoparticles.25‐26 Moreo‐ ver, Bi shows hindered visible plasmonic properties com‐ pared to noble metals like Au or Ag due to important non‐radiative damping.27‐28 Additionally, the high corro‐ sion resistance of Au and its high electronic conductivity have provided a wide range of technological applications

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to this noble metal, especially in electronics, and thus the exploitation of Au nanocrystals as components of nanostructured semiconductor‐based systems is consid‐ ered as particularly appealing. As an alternative, anisotropic Au‐CdX nanostructures (X = S, Se, Te) can be readily prepared in solution by in‐ ducing the heterogeneous growth of Au domains on the surface of previously synthesized CdX nanorods with relatively high versatility in terms of size and location of the metal tip, as reported by Banin et al. in 2004.29‐30 Nev‐ ertheless, these hybrid nanocrystals present small, defec‐ tive and non‐epitaxial interfaces difficult to control dur‐ ing the reaction. In recent years, some of the authors of the present work have intensively studied the tempera‐ ture‐dependent structural and morphological evolution of these solution‐prepared cadmium chalcogenide nanorods decorated with Au domains.31‐32 These studies allowed identifying the optimal conditions for the formation of large and epitaxial metal‐semiconductor interfaces that should make the heterostructures more suitable for use in nanoscale electronic devices. Although these studies were mainly performed on samples deposited on substrates, some reported preliminary results indicated the possibil‐ ity to directly obtain analogous structures in solution, with the consequent advantage of producing larger amounts of elongated Au‐CdX epitaxial hybrid nanocrystals that could be easily processed further in low boiling point solvents. Interestingly, these preliminary studies suggested that Au nanoparticles might be per‐ forming as catalytic sites for the growth of the elongated semiconductor domain, something that seems controver‐ sial with the general accepted idea that only low melting point metals like Bi are capable of catalyzing such growth in colloidal phase. Indeed, that was the starting point for the investigations reported in the present work. To the best of our knowledge, the growth of colloidal Au‐CdX nanorods or nanowires by this procedure still remains as a challenge and therefore, this work is devoted to the study of the practical possibilities of the method and to shed some light on the growth mechanism involved. Herein, we report a novel and facile approach for the synthesis of Au‐cadmium chalcogenide elongated NCs with a head‐to‐tail morphology using QDs as precursor materials. These nanostructures show tunable lengths up to 500 nm and intrinsic absolute selectivity for the posi‐ tion of the gold domain with respect to the semiconduc‐ tor section, while ensuring their colloidal stability in or‐ ganic low boiling point solvents. We show that, despite their high melting point, plasmonic Au NCs are also suit‐ able catalytic sites for the anisotropic and epitaxial growth of the chalcogenide semiconductor in solution by forming an alloy with cadmium, provided that the semi‐ conductor molecular precursors are homogeneously de‐ livered. One of the novelties of this work deals with the gradual and tunable release of reactive monomer species to the solution, achieved via in‐situ dismantling of quan‐ tum dots (QDs) that assists a confined 1D growth of the semiconductor. Several authors have reported the suc‐ cessful use of NCs as precursors for novel nanostruc‐

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tures:33 chemical or morphological transformations on NCs have been performed through ion exchange reac‐ tions,34‐35 atomic diffusion mechanisms36‐37 or focused electron beam exposure among others.32,38 Nevertheless, colloidal NCs have never been explored as molecular monomer sources for the growth of more complex and better engineered hybrid nanomaterials. It is known that Cd chalcogenide QDs can suffer Ostwald ripening pro‐ cesses in the presence of appropriate surfactants, which means that the smaller QDs dismantle or dissolve in the solution and recrystallize on the surface of larger QDs, following a thermodynamically‐driven stabilization pro‐ cess.39‐40 Thus, the dismantling that occurs during diges‐ tive ripening can be effectively seen as a constant source of active monomer species that are gradually released to the solution. Taking into consideration the previous statements, a question arises on whether the induced dismantling of colloidal QDs in the presence of Au NCs could represent a successful alternative for the straight‐ forward formation of Au‐semiconductor elongated nanohybrids. EXPERIMENTAL SECTION Chemicals. CdO powder (99%), Se powder (99.99%), Te powder (99.999%), tri‐n‐octylphosphine oxide (TOPO, 99%), CuCl (99.999%‐Cu), tri‐n‐octylphosphine (TOP, 97%) and tri‐n‐butylphosphine (TBP, 99%) were pur‐ chased from Strem Chemicals. Octadecylphosphonic acid (ODPA, 99%) was purchased from Polycarbon Industries. Gold (III) chloride trihydrate (HAuCl4•3H2O, ≥99.9%), hexamethyldisila‐thiane ((TMS)2S, synthesis grade), oleylamine (OLAm, 70%), oleic acid (OLAc, ≥99%), dodecylamine (DDAm, 98%), 1‐octadecene (ODE, 90%), chloroform (CHCl3, ≥99.8%), toluene (99.9%), isopropa‐ nol (i‐PrOH, ≥99.9%), and methanol (MeOH, ≥99.9%) were purchased from Sigma‐Aldrich. Dibenzyl ether (Bz2O, ≥98%) was purchased from Fluka. Nitric acid (HNO3, 69.0%) and hydrochloric acid (HCl, ≥37%) for trace analysis were purchased from Fluka to prepare pure aqua regia (HNO3:HCl 1:3 v/v) for the digestion of stock solutions of nanoparticles prior to their chemical anal‐ yses. Hydrogen peroxide solution (H2O2, 33% w/v) was purchased from Panreac. All reagents and solvents were used as received, except for the ODE, Bz2O and OLAm used to inject quantum dots into the reaction flask, which had been previously degassed under vacuum for 3 h at 120 ºC. The syntheses of precursor NCs used are based on previously reported methods and are described in detail in the Supporting Information.41‐43 Synthesis of Au‐CdSe HNCs in benzyl ether (Refer‐ ence System). The strategy for synthesis of HNCs was based on hot injection of QDs at room temperature into a suspension of Au nanoparticles at high temperature. Re‐ action time, temperature, concentration of injected QDs and, to a lesser extent, amount of OLAm, were varied in different experiences, whereas solvent volume and amount of Au nanoparticles were fixed in all cases. The general protocol is described here for the typical case of elongated Au–CdSe HNCs as follows: in a 25‐mL three‐


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Figure 1. TEM images of Au‐CdSe HNCs obtained at 250 ºC in Bz2O after 5 min (a), 1 h (b), and 4 h (c). The respective average dimensions are 30 nm x 10 nm, 69 nm x 13 nm, and 105 nm x 14 nm. Scale bar: 100 nm. neck flask, a mixture of 3 mL Bz2O, 250 L OLAm, and 200 L of a 0.74 M Au NCs stock solution (0.148 nmol) was degassed at 120 ºC under vacuum for 1 h while being stirred. Meanwhile, 200 L of a 57 M CdSe QDs stock solution (11.4 nmol) were precipitated with 200 L MeOH in an auxiliary vial. Inside a glove box, the precipitate was re‐dispersed in 1 mL of degassed Bz2O. Under N2 atmos‐ phere, the temperature of the flask suspension was fixed at 250 ºC and the CdSe QDs suspension was swiftly in‐ jected. The reaction was stopped after 4 h by letting the system cool down naturally to room temperature. The resulting suspensions were washed twice with equivalent volumes of isopropanol and finally re‐dispersed in tolu‐ ene. Synthesis of Au‐CdSe HNCs in 1‐octadecene. The synthesis of Au–CdSe HNCs in ODE was adapted from the protocol established for the Reference System in Bz2O described above. In this case, 750 L OLAm were used and temperature of reaction was fixed at 300 ºC. The final suspensions were washed twice with equivalent volumes of isopropanol and re‐dispersed in toluene. A series of analogous experiments was also carried out using lower amounts of injected QDs, down to 20% (2.28 nmol) of that employed in the Reference System. Synthesis of Au‐CdTe HNCs in 1‐octadecene. The synthesis of Au–CdTe HNCs followed the same protocol as for the Au–CdSe system in ODE described above. In order to find the optimal conditions for the production of monodispersed hybrid elongated NCs, several tests were performed at different temperatures and times of reac‐ tion, ranging from 200 ºC to 300 ºC, and from 1 min to 4 h, respectively. The best conditions found corresponded to 280 ºC and 15 min of reaction. The final suspensions were washed twice with equivalent volumes of isopropanol and re‐dispersed in toluene for storage. Synthesis of Au‐CdS HNCs in 1‐octadecene. The syn‐ thesis of Au–CdS HNCs followed the same protocol as for the Au–CdSe system in ODE described above. A mixture of Au‐CdS elongated HNCs and large CdS QDs was ob‐ tained using a reaction temperature of 300 ºC and after 4 h of reaction. The final suspensions were washed twice with equivalent volumes of isopropanol and re‐dispersed in toluene for storage.

Synthesis of Au‐CdTe‐CdSe heterostructures. With respect to the Au–CdSe system in ODE, 50% of CdSe QDs (5.74 nmol) were dispersed in 500 L ODE, injected into the flask with Au NCs at 280 ºC, and let react for 16 min. 5.80 nmol CdTe QDs dispersed in 250 L ODE were then injected in the same reacting flask and the reaction was finally stopped 15 min later. The suspensions were washed twice with equivalent volumes of isopropanol and re‐ dispersed in toluene. Characterization. The synthesized samples dissolved in toluene were drop‐casted on monocrystalline Si sub‐ strates or carbon‐coated Cu grids, and then naturally dried in air before characterization by XRD and TEM, respectively. The sample solutions were appropriately diluted for UV‐Visible absorbance and photolumines‐ cence measurements and digested for bulk chemical analysis. Further details on characterization methods are described in the Supporting Information. RESULTS Au‐assisted growth of elongated CdSe NCs in or‐ ganic solvents. The general strategy for the synthesis of hybrid NCs (HNCs) is based on injection of a QDs solu‐ tion into a suspension of colloidally stable Au nanoparti‐ cles in the presence of a single amine surfactant that also acts as a dismantling agent of QDs. Transmission electron microscopy (TEM) images in Figure 1 illustrate the evolu‐ tion of a sample with time in a system with Au nanoparti‐ cles and CdSe QDs reacting at 250 ºC using benzyl ether (Bz2O) as solvent and oleylamine (OLAm) as dismantling agent (see Reference System in the Experimental section). The formation of elongated worm‐like Au–CdSe HNCs occurs from early stages of the reaction and the reaction conditions provide optimal control on their growth with time. Thus, as illustrated in Figure 1, the average lengths evolve progressively from 30 nm to 105 nm between 5 min and 4 h of reaction. Interestingly, the process takes place with very high location selectivity of the materials in‐ volved. In particular, Au appears in all cases as a single domain at one tip of the HNCs and no decoration of Au clusters is detected on the surface of the grown CdSe tails regardless of time of reaction. On the other hand, the growth of only one CdSe domain on each Au NC takes



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Figure 2. XRD spectrum of the final Au‐CdSe HNCs sample (a). Room temperature optical absorbance (b) and PL spec‐ tra (c) of aliquots ranging from 5 min to 4 h of reaction at 250 ºC in Bz2O, initially containing Au NCs and CdSe QDs. place in all cases; i.e. no other types of nanostructures are obtained under these conditions. During the first stages of the reaction, Au–CdSe HNCs coexist with CdSe QDs of sizes smaller than those used as precursors. The size of these QDs becomes progressively smaller with reaction time (Figure 1a‐b) until no more QDs are observ‐ able after 4 h (Figure 1c). These observations suggest a process of dismantling of the injected spherical‐like CdSe QDs and the subsequent CdSe recrystallization on the surface of Au NCs as elongated domains. X‐ray diffraction (XRD) analysis from final samples confirms the cubic structure for Au and the hexagonal wurtzite structure for CdSe domains in the HNCs as shown in Figure 2a. The room temperature optical ab‐ sorbance spectra of aliquots taken at different reaction times show a gradual widening and loss of characteristic peaks in accordance with the expected overlapping of absorption bands from Au tips and CdSe tails of slightly different lengths and widths (Figure 2b). As shown in Figure 2c, the reaction has been simultaneously moni‐ tored by photoluminescence (PL) measurements of ali‐ quots taken at different reaction times: ca. 5 nm‐QDs used as precursors exhibit an intense PL band centered at 607 nm that shifts down to 560 nm after only 5 min of reaction. The PL band follows the same trend with addi‐ tional reaction time, shifting to a maximum at 510 nm after 4 h of reaction, indicating that small 1‐2 nm QDs are still present in the last aliquot. Usual precipitation‐ redissolution washing cycles are sufficient so as to com‐ pletely remove them from the solution, and accordingly the final sample shows no PL and it is composed exclu‐ sively of Au‐CdSe HNCs with no reminiscence of precur‐ sor QDs. These results evidence the QDs dismantling or dissolution‐based mechanism through which HNCs grow, and confirm the expected PL quenching in such type of noble metal‐semiconductor nanoparticles.29 In fact, equivalent experiments in the absence of Au NCs also show an initial blue shift of the PL band in early stages corresponding to a partial dismantling of the precursor QDs. However this is quickly followed by a clear red shift at later stages until the emission wavelength of the pre‐ cursor QDs is recovered after 4h of reaction, in accord‐ ance with an expected self‐Ostwald ripening mechanism (see Supporting Figure S1).

Among the reaction conditions tested, none of them leads to a further significant growth of the CdSe tails beyond 100 nm. The relatively high dipolar momentum of Bz2O (1.390 D) most likely plays a role in the stabilization of the molecular monomers in solution, which tends to shift the reaction equilibrium towards the formation of such species, thus limiting the growth of the desired nanostructures. Instead, 1‐octadecene (ODE) is known to be a solvent with a low dipolar momentum (0.369 D), nearly four times lower than Bz2O,44 which prompted us to test its role in such thermodynamic equilibrium. A series of reactions has been carried out in ODE in order to synthesize higher aspect ratio Au–CdSe HNCs. When performing analogous experiments to the one described as Reference System but substituting Bz2O by ODE, a large amount of precursor QDs with sizes comparable to the initial ones are still observed even after 4 h at 250 °C, while Au‐CdSe HNCs found in the sample show shorter average lengths than those obtained in Bz2O (results not shown). These results indicate the slow and inefficient dismantling process occurring in ODE at this temperature compared to Bz2O; they suggest that non‐coordinating solvents with low dipolar momentum do not promote the release of molecular monomers to the solution. Conse‐ quently, a higher temperature is required to enhance precursor QDs dismantling. In particular, a reaction tem‐ perature of 300 °C is necessary in ODE to induce a quanti‐ tative dismantling and thus, experiments described here‐ after have been conducted at this temperature. The amount of injected CdSe QDs has been varied in a range of 20% to 100% with respect to the Reference System and the time to complete the reaction has been adjusted ac‐ cordingly. As illustrated in Figure 3, the formation of elongated Au–CdSe HNCs occurs in all cases with a worm‐like morphology similar to that obtained in Bz2O but reaching much longer tails even from early stages. As deduced from the results, the combined effect of the higher reaction temperature and poor coordinating char‐ acter of ODE allow CdSe QDs to dismantle faster and, subsequently, Au‐CdSe HNCs to grow longer, compared to experiments performed at 250 °C in Bz2O. TEM micrographs in Figure 3 and Supporting Figure S2 show typical images of the samples obtained in ODE at 300 ºC. As shown in Figure 3a, the injection of only 20%


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ily studied by substituting OLAm by another commonly used primary amine such as DDAm, as reported in the Supporting Information (Figures S4 and S5). The use of an equivalent amount of DDAm as surfactant or dismantling agent instead of OLAm changes dramatically the kinetics of the process in a way that elongated HNCs are not ob‐ tained with any of the materials tested (CdSe and Cu2‐ xSe). Contrary to the cases of OLAm systems, the initial twinned structure of Au in the tips is preserved with all materials and at all reaction temperatures tested when DDAm is employed. This fact evidences the much higher degree of stabilisation that this surfactant exerts on the surface of Au NCs, and alters completely the mechanism by which the semiconductor domain grows. As a conse‐ quence, only low aspect ratio nanoworms are obtained, which are better described as heterodimers.

Figure 3. TEM images of growth stages of Au‐CdSe HNCs at 300 ºC in ODE upon injection of 20% CdSe QDs after 5 min (a) and 1 h (b), and of 100% CdSe QDs after 5 min (c) and 1 h (d). Scale bar: 100 nm. QDs results in the absence of QDs already after 5 min of reaction, although the sample is more polydisperse at these initial stages: a wider range of lengths occurs and even some Au NCs can be spotted with very short or no tail grown on them. Additional time of reaction eventual‐ ly leads to significantly narrower distributions of lengths but always limited below 300 nm (Figure 3b). When 100% QDs are injected to the reaction flask, nanoworms of average length of 200 nm grow just in 5 minutes, while small QDs still remain in solution (Figure 3c). After 1 h, no remaining QDs are observed and the nanoworms grow up to ca. 500 nm (Figure 3d). Beyond the length of the CdSe tails, the combined effect of amount of injected QDs and time of reaction has a significant influence on other morphological, structural, and compositional features of the HNCs. The diameter of the tails also increases with time of reaction and with amount of QDs injected. Thus, after 5 min, the injection of 20% QDs leads to diameters that are smaller than those of the Au seeds and tend to decrease with distance from the Au–CdSe interface. In‐ stead, the diameters of nanoworms obtained with 100% QDs are close to those of the Au seeds and remain con‐ stant along the tails (see Supporting Figure S2). Time of reaction tends to increase the diameter in both cases. Interestingly, this is accompanied by a progressive sur‐ rounding of the Au seeds by grown CdSe, up to the point of their encapsulation in the systems loaded with higher concentrations of QDs as illustrated in Supporting Figure S3. On the other hand, a higher concentration of QDs initially produces structures with straighter sections of the CdSe tails, although they evolve towards crooked tails with time of reaction. The effect of the nature of the amine surfactant in the reaction has also been preliminar‐

It is worth stressing at this point that such experiments have been performed with CdSe QD precursors that were prepared via two different synthetic methods. One of them used phosphonic acids and a coordinating phos‐ phine‐based solvent (results shown in the main text), whereas the other one employed less harmful and much cheaper oleic acid surfactant and a non‐coordinating solvent. Similar samples of Au‐CdSe HNCs have been obtained in both cases, and an example of those synthe‐ sized from CdSe QDs prepared in ODE and stabilized with oleic acid is reported in Supporting Figure S6. High resolution structural and chemical analysis. In order to achieve additional insights on the influence of time of reaction and amount of QDs used as precursors on the structure of the HNCs, high resolution TEM (HRTEM) and spectroscopic analyses have been carried out for samples obtained in ODE at 300 °C. At this tem‐ perature, the polycrystalline twinned structure of the original Au NCs used as seeds evolves to a single crystal, in contrast with what happens when the reactions take place at 250 °C. However, different behaviors are observed depending on the amount of CdSe QDs injected: when low amounts (20%) are injected and after 1h reaction, d‐ spacing values corresponding to fcc Au planes in the head, and to hexagonal wurtzite CdSe planes in the tail are easily identified but a random orientation of the Au and CdSe lattices is observed in the elongated nanohybrids, as depicted in Supporting Figure S7. In the case that high amounts (100%) of QDs are injected, the nanostructures formed evolve with time both in terms of size and chemical composition: Interestingly, a detailed HRTEM structural analysis of an aliquot extracted from the highly QDs loaded system at intermediate reaction times (2h) shows metallic tips with lattices that are clearly compatible with AuxCdy alloys as shown in Figure 4a. Alloying in the tips is confirmed by acquisition of Energy Dispersive X‐ray (EDX) spectra, which render a Au/Cd ratio of about 3 according to quantifications based on the Cliff‐Lorimer method (Figure 4b‐c). Yet, typical d‐spacing values of hexagonal Au2Cd are identified from structural analysis. Moreover, the interfaces of such intermediate nanoworms obtained from 100% QDs exhibit a



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Figure 4. a) HRTEM micrograph showing the epitaxial relationship between Au2Cd and CdSe domains in HNCs ob‐ tained at 300 °C in ODE and 100% injected QDs after 2 h reaction; b) High Angle Annular Dark Field–Scanning TEM (HAADF‐STEM) image of HNCs obtained under the same reaction conditions; c) EDX spectrum of the selected area shown in b); d) and e) HRTEM images of CdSe and AuCd domains, respectively, of HNCs obtained at 300 °C in ODE and 100% injected QDs after 4 h reaction; f) HAADF‐STEM image of an HNC obtained under the same reaction conditions as in d) and its corresponding Cd elemental profile dependence obtained from EELS analysis along the X axis. preferential epitaxial orientation according to the crys‐ tallographic relationship: Au2Cd (100) // CdSe (100) and Au2Cd [001] // CdSe [001] where the left term represents the interface and the right term represents the vector alignment. Lattice mis‐ match m was calculated along the Au2Cd (100) // CdSe (100), where m is defined as the absolute difference be‐ tween two lattice spacings (d1 and d2) along a certain direction, relative to the average of the lattice spacings: m = 2d1‐ d2/(d1+d2)) In our case the lattice mismatches were calculated by considering that three unit cells of Au2Cd along the [100], [110] and [001] directions fit in two unit cells of CdSe along the [100], [110] and [001] directions, with calculated mismatch being 2.00%, 1.84% and 2.88%, respectively. These results confirm Cd diffusion into the bulk of Au seeds at high QDs loading already at intermediate reac‐ tion times, and the low values of lattice mismatch meas‐ ured fully justify the feasibility of epitaxial growth of the semiconductor domain. After 4 h reaction, CdSe tails still appear to be well crystallized as depicted in Figure 4d, despite the significant twists that form along the domain with time. Nonetheless, the wurtzite CdSe lattice is found to be severely distorted in the regions that wrap the me‐ tallic tip (expansions as large as 12% along the (‐221) di‐ rection and 9% in the (2‐1‐1) could be measured in several

images). Moreover, the lattice of the metallic tip differs from both that of bulk Au and Au2Cd. In fact, the meas‐ ured d‐spacings and crystalline directions are compatible with an orthorhombic AuCd alloy phase, indicating an increasing degree of Cd diffusion into the metallic seed with time of reaction (Figure 4e). To further characterize the system, a series of Electron Energy Loss spectra (EELS) has been acquired along the nanoworm as shown in Figure 4f. The spectra show a significant cadmium signal increase in the gold seed with respect to that quan‐ tified in the CdSe tail. This relative increase confirms the alloying of cadmium and gold proposed after HRTEM analysis (see Experimental details in the Supporting In‐ formation for spectra quantification). The latter also indi‐ cates that the CdSe domain grows epitaxially adapted to the AuCd lattice according to the following crystallo‐ graphic relationship: CdSe [102] // AuCd [001] with CdSe (010) // AuCd (010) where the left term represents the interface and the right term represents the vector alignment. Multi‐semiconductor heterostructures. Procedures analogous to those developed for the formation of Au– CdSe HNCs in ODE have also been tested to obtain Au‐ CdS and Au–CdTe elongated HNCs and similar results are obtained as described in the Supporting Information (Figures S8‐S10). From these results it can be concluded that the optimal reaction conditions for the synthesis of


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phological features are identified: a denser tip, an elon‐ gated tail, and a thicker trunk intercalated between them as depicted in Figure 5a and Supporting Figure S11. Figure 5 shows a HRTEM image and EDX analyses of areas of each type of domain in one of these NCs. The composi‐ tional analysis of each area indicates that alloying be‐ tween gold and tellurium occurs in the metallic tip, prob‐ ably enhanced by long time electron beam exposure as observed in previous reports.45 Most interestingly, the sequence of domains in these HNCs from head to tail was AuTex–CdTe–CdSe, i.e. the components of the QDs in‐ jected in the second place appear first in the sequence of the structure. Note also that the absence of Cd in the metallic tip discards that the detected Te is part of a CdTe domain covering the gold NC. Likewise, the absence of Se in the trunk indicates CdTe is not growing coaxially onto the tail of a former Au–CdSe hybrid. All in all, these re‐ sults evidence that the growth of the HNCs takes place via incorporation of the molecular precursors generated from the secondly injected QDs at the very interface between the metallic seeds and primary grown CdSe domain of semiconductor, pointing out the importance of formation of a surface or bulk solid solution between Au and Cd necessary for the growth of the semiconductor tail. DISCUSSION

Figure 5. HRTEM image of a single 3‐domain hybrid Au–CdTe–CdSe heterostructure (a) and EDX analyses (b) of the tip (blue), the trunk (green), and the tail (red) areas of the heterostructure shown in (a). elongated Au–CdTe HNCs are also compatible with analogous Au–CdSe HNCs formation. This allowed for the one‐pot consecutive injections of the two types of QDs, namely CdSe QDs first and later CdTe QDs, thus produc‐ ing 3‐domain hybrid heterostructures with a metallic head and two connected semiconductor domains forming the tail. The latter are expected to be formed of different materials provided the lapse between injections was long enough as to consume most of the CdSe QDs injected in first place. By following the same synthetic scheme de‐ scribed so far at 280 ºC, rather elongated heterostructures are obtained in which three domains with distinct mor‐

The results obtained so far evidence a multistep mech‐ anism by which such elongated Au‐CdSe HNCs grow, which is depicted in Scheme 1: first, alkylamines induce CdSe QDs dismantling, providing monomer species to the solution that are gradually released without leading to an oversaturated medium that would favor homogeneous nucleation. Second, active monomer species in the solu‐ tion start interacting with Au NCs at high temperatures. Since Au is characterized by a high atomic mobility that can be easily enhanced at high temperatures,46‐47 the structure of Au seeds can eventually rearrange into a single crystal at 300 °C in the presence of OLAm. During such process, the incorporation of other metallic atoms present in the solution like Cd is likely to happen. This leads to either surface or bulk diffusion, depending on reaction temperature and amount of injected QDs, to form Au2Cd metallic alloys. Third, the Au2Cd metallic seed is able to promote the nucleation and longitudinal growth of a CdSe domain by consuming most of the avail‐ able monomers in solution and following a specific epitaxy with a low lattice mismatch. In fact, the values of Au2Cd‐CdSe lattice mismatch observed in this work are lower than any reported value of lattice mismatch for Au‐ CdSe nanostructures,31 what explains the need for the formation of the alloy. Interestingly, the supply of mono‐ mers during longitudinal growth takes place preferential‐ ly at the interface between the Au seed and the CdSe tail, as evidenced from our experiments on the formation of multi‐semiconductor heterostructures. All together re‐ veals the indispensable role of Au nanoparticles in the reaction. Noteworthy, the gradual release of Cd and Se monomer species during QDs dismantling seems to be a key aspect for the anisotropic growth of the CdSe domain: analogous experiments performed by injecting similar



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Scheme 1. Reaction and growth mechanism of Au‐CdSe HNCs in solution.

amounts of usual molecular Cd and Se precursors, as done in traditional SLS synthesis with Bi catalysts, leads to an homogeneous nucleation and growth of spherical CdSe QDs or to Au‐CdSe dimer‐like nanostructures in the best of the cases, probably due to uncontrolled growth in oversaturated solutions (see Supporting Figure S12). The in‐situ delivery of monomers presented here, assures a complete homogeneous distribution of reactive species at all stages of the reaction, compared to standard hot injec‐ tion or dropwise addition methods. On the other hand, it is worth considering that longitudinal growth is accom‐ panied by random bendings along the CdSe tail, which definitely indicate the presence of defects in the crystal‐ line lattice. Nonetheless, the presence of defects and grain boundaries in semiconductors can be greatly beneficial for some of their potential applications.48‐49 At last, after prolonged times of reaction, Cd and Se monomers re‐ maining in solution preferentially induce a transversal growth of the CdSe domain at the expense of the initial longitudinal growth, enlarging in this way the CdSe tail diameter. Simultaneously, Cd atoms continue to diffuse into the Au lattice until a Au/Cd atomic ratio of 1 is achieved and they eventually recrystallize as CdSe over the metallic tip, contributing to the AuCd encapsulation at late stages of the reaction. (Supporting Figure S3) AuxCdy binary alloys have been previously observed to be responsible for the growth of Au2Cd@CdSe and Au@CdSe core@shell nanoparticles,50‐51 but an anisotropic and epi‐ taxial growth of the cadmium chalcogenide domain as the one reported here has never been achieved up to now. This longitudinal growth mechanism resembles the very well‐known SLS mechanism reported by Buhro in 1995.12 However, the SLS mechanism relies on the possi‐ bility of the catalytic metallic seeds to melt at the reaction temperature and form liquid alloys with the metallic pre‐ cursor of the semiconductor material. Thus, it has been generally accepted that low melting point metals are re‐ quired for this strategy to be feasible, or alternatively the eutectic alloy formed during synthesis should present a melting temperature attainable in solution.11‐12 Au has a significantly high bulk melting temperature (1,064 °C),

and consequently it has been used mainly in the growth of semiconductors at high temperature and in the vapor phase using Chemical Vapor Deposition (CVD) tech‐ niques and following the VLS mechanism.13‐14 Nonethe‐ less, Au has been shown to efficiently catalyze the growth of Si and Ge nanowires in solution as well.16‐17 This fact is most likely due to the low melting temperatures of the eutectic AuSi0.2 and AuGe0.4 alloys (atomic ratio), which are 363 °C and 361 °C, respectively.52 Note also that Si and Ge amounts required for the formation of the correspond‐ ing eutectic alloys are relatively low, which means that a small amount of Si or Ge diffusion into Au is sufficient as to dramatically drop the melting temperature of the par‐ ticle from 1,064 °C to temperatures below 400 °C, which are relatively easy to reach with common solvents. On the other hand, for the synthesis of CdX nanorods and nan‐ owires in solution phase, Bi has traditionally been used due to its lower melting temperatures. In principle, Au nanoparticles could also be used for these syntheses since Au also forms an eutectic alloy with Cd with a AuCd2.3 composition and a melting temperature of 505 °C.52 How‐ ever, both the high amount of Cd to be diffused within Au to reach the eutectic composition and its high melting temperature have severely limited the Au‐assisted growth of Cd‐based nanowires in solution phase so far. Interestingly, our results confirm that Au NCs can cata‐ lyze the elongated growth of Cd‐based semiconductor materials in solution as well. However, and considering that the eutectic AuCd2.3 composition has not been ob‐ served at any stage of the reaction, it is not clear what mechanism is actually operating for such growth and two possibilities should be considered: first, if only surface diffusion of Cd atoms is assumed, a very thin surface layer of Au nanoparticles could eventually reach the eutectic alloy composition. In that case, and as a result of the very confined dimensions of this alloy shell, the melting tem‐ perature of the eutectic could drop even below its own bulk melting temperature of 505 °C, due to the well known size‐dependent melting‐point supression,53‐54 and it could shift closer to the present reaction temperatures of 250 or 300 °C. The surface liquid alloy would then cata‐


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lyze the growth of the semiconductor domain with an elongated shape through a classical SLS approach. Never‐ theless, the observed dispersion of lengths and the pres‐ ence of defects all along the semiconductor domain of the HNCs could possibly suggest that the melting of the sur‐ face layer is unlikely to be homogeneous. This possibility may be favored by the effect of relatively low temperature of reaction on Au seeds with small differences in size and with thermodynamically non‐equivalent crystallographic surface facets. Alternatively, a Solution‐Solid‐Solid (SSS) approach could explain these results as well. In that case the melting of the metallic alloy does not take place and thus, the eutectic composition is not strictly required: Cd atoms would diffuse in the solid state through the Au lattice,55 either through the surface or into the bulk de‐ pending on reaction temperature and amount of QDs injected, forming a single crystal alloy structure with a composition up to AuCd, as observed by HRTEM (Figure 4e), which would better accommodate the Cd‐based sem‐ iconductor lattice catalyzing its elongated growth. The SSS mechanism was reported for the first time by Wang et al. in 2013 for the growth of Cd and Zn‐based chalcogenide nanowires catalyzed by solid‐phase Ag2Se superionic conductor NCs in solution,56 but no equivalent examples with metallic seeds have been reported yet. The experimental evidences collected in the present work discard the classical heterogeneous nucleation mecha‐ nism, but they are not sufficient to discern between SLS and SSS mechanisms, and further experiments are un‐ derway to clarify this point. CONCLUSIONS In summary, this work demonstrates that Au nanopar‐ ticles are suitable metallic materials for the catalyzed growth of long 1D CdX nanostructures in solution, avoid‐ ing the need for low melting point metallic NCs such as Bi or In, or hard reaction conditions of VLS or supercritical fluid‐liquid‐solid (SFLS) approaches. The reaction is driv‐ en by the fine physicochemical equilibrium between QD precursors, molecular monomers, AuxCdy catalytic seeds and epitaxially grown elongated AuxCdy‐CdX HNCs. It has been observed that this equilibrium can be displaced by controlling reaction conditions such as temperature, concentration of QDs, polarity of solvent and structure of the alkylamine used as surfactant. Additionally, the ap‐ proach described here opens the doors for the synthesis of more complex heterostructures made of more than one semiconductor material. The method represents a new chance for Au NCs to become catalytic sites for the SLS or SSS‐based elongated growth of semiconductor materials in solution phase. Ag, Cu, Pt, Pd, Ni or Sn, just like Cd, all show a certain degree of miscibility with Au and thus the validation of the current methodology for the growth of their corresponding semiconductor materials deserves further attention and will become the target of future studies. Solar cells, photocatalytic systems and thermoe‐ lectric devices could greatly benefit from these new col‐ loidal nanostructures with large, epitaxial and thus unique metal‐semiconductor interfaces.

ASSOCIATED CONTENT Supporting Information. Experimental details, results on control experiment without Au, Au‐CdSe HNCs growth evo‐ lution with time and with amount of QDs injected, effect of the chain structure of the amine surfactant, results on Au‐ CdSe HNCs obtained from oleic acid‐stabilized CdSe QDs, HRTEM studies on Au‐CdSe HNCs obtained with low amounts of injected CdSe QDs, results obtained from injec‐ tion of chalcogenide materials other than CdSe, HRTEM images of 3‐domain HNCs, results obtained from injection of Cd and Se molecular species as precursors instead of CdSe QDs. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We acknowledge financial support from the Spanish MINECO through CTQ2012‐32247 and from the Generalitat de Catalunya through 2014 SGR 129. A. F. acknowledges the Spanish MINECO for a Ramón y Cajal Fellowship (RYC‐2010‐ 05821). V. F‐A. acknowledges the European Comission for the Career Development Allowance under the Marie Curie Pro‐ gramme (MRTN‐CT‐2005‐019283). Some of the TEM experi‐ ments were carried out in the Scientific and Technological Centers of the University of Barcelona (CCiT‐UB), with fi‐ nancial support of CSD2009‐00013 and MAT2010‐16407 Span‐ ish research projects.

REFERENCES (1) Jun, Y.‐w.; Choi, J.‐s.; Cheon, J. Angew. Chem. Int. Ed. 2006, 45, 3414‐3439. (2) Wang, X.; Zhuang, J.; Peng, Q.; Li, Y. Nature 2005, 437, 121‐ 124. (3) Kumar, S.; Nann, T. Small 2006, 2, 316‐329. (4) Hughes,B. K.; Luther, J. M.; Beard, M. C. ACS Nano 2012, 6, 4573‐4579. (5) Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Science 2002, 295, 2425‐2427. (6) Krahne, R.; Morello, G.; Figuerola, A.; George, C.; Deka, S.; Manna, L. Phys. Rep. 2011, 501, 75‐221. (7) Salant, A.; Shalom, M.; Tachan, Z.; Buhbut, S.; Zaban, A.; Banin, U. Nano Lett. 2012, 12, 2095‐2100. (8) Xiang, X.; Chou, L.; Li, X. Phys.Chem. Chem. Phys. 2013, 15, 19545‐19549. (9) Costi, R.; Cohen, G.; Salant, A.; Rabani, E.; Banin, U. Nano Lett. 2009, 9, 2031‐2039. (10) Costi, R.; Saunders, A. E.; Elmalem, E.; Salant, A.; Banin, U. Nano Lett. 2008, 8, 637‐641. (11) Wang, F.; Dong, A.; Sun, J.; Tang, R.; Yu, H.; Buhro, W. E. Inorg. Chem. 2006, 45, 7511‐7521. (12) Trentler, T. J.; Hickman, K. M.; Goel, S. C.; Viano, A. M.; Gibbons, P. C.; Buhro, W. E. Science 1995, 270, 1791–1794. (13) Wagner, R. S.; Ellis, W. C. Appl. Phys. Lett. 1964, 4, 89‐90. (14) Gudiksen, M. S.; Lieber, C. M. J. Am. Chem. Soc. 2000, 122, 8801‐8802. (15) Zhao, P.; Li, N.; Astruc, D. Coord. Chem. Rev. 2013, 257, 638‐665.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(16) Holmes, J.; Johnston, K. P.; Doty, R. C.; Korgel, B. A. Sci‐ ence 2000, 287, 1471‐1473. (17) Hanrath, T.; Korgel, B. A. Adv. Mater. 2003, 15, 437‐440. (18) Chockla, A. M.; Korgel, B. A. J. Mater. Chem. 2009, 19, 996–1001. (19) Shweky, I.; Aharoni, A.; Mokari, T.; Rothenberg, E.; Nadler, M.; Popov, I.; Banin, U. Mat. Sci. Eng. C 2006, 26, 788‐ 794. (20) Yu, H.; Li, J.; Loomis, R. A.; Gibbons, P. C.; Wang, L.‐W.; Buhro, W. E. J. Am. Chem. Soc. 2003, 125, 16168‐16169. (21) Ouyang, L.; Maher, K. N.; Yu, C. L.; McCarty, J.; Park, H. J. Am. Chem. Soc. 2007, 129, 133‐138. (22) Li, Z.; Kurtulus, Ö.; Fu, N.; Wang, Z.; Kornowski, A.; Pietsch, U.; Mews, A. Adv. Funct. Mater. 2009, 19, 3650‐3661. (23) Wang F.; Buhro, W. E. Small 2010, 6, 573–581. (24) Reim, N.; Littig, A.; Behn, D.; Mews, A. J. Am. Chem. Soc. 2013, 135, 18520‐18527. (25) Yarema, M.; Kovalenko, M. V.; Hesser, G.; Talapin, D. V.; Heiss, W. J. Am. Chem. Soc. 2010, 132, 15158–15159. (26) He, M.; Protesescu, L.; Caputo, R.; Krumeich, F.; Kovalenko, M. V. Chem. Mater. 2015, 27, 635–647. (27) Toudert, J.; Serna, R.; Jiménez de Castro, M. J. Phys. Chem. C 2012, 116, 20530‐20539. (28) McMahon, J. M.; Schatz, G. C.; Gray, S. K. Phys. Chem. Chem. Phys. 2013, 15, 5415‐5423. (29) Mokari, T.; Rothenberg, E.; Popov, I.; Costi, R.; Banin, U. Science 2004, 304, 1787‐1790. (30) Mokari, T.; Sztrum, C. G.; Salant, A.; Rabani, E.; Banin, U. Nature Mater. 2005, 4, 855‐863. (31) Figuerola, A.; van Huis, M.; Zanella, M.; Genovese, A.; Ma‐ rras, S.; Falqui, A.; Zandbergen, H.W.; Cingolani, R.; Manna, L. Nano Lett. 2010, 10, 3028‐3036. (32) van Huis, M. A.; Figuerola, A.; Fang, C.; Béché, A.; Zand‐ bergen, H. W.; Manna, L. Nano Lett. 2011, 11, 4555‐4561. (33) Kim, M.; Phan, V. N.; Lee, K. Cryst. Eng. Comm. 2012, 14, 7535‐7548. (34) Robinson, R. D.; Sadtler, B.; Demchenko, D. O.; Erdonmez, C. K.; Wang, L.‐W.; Alivisatos, A. P. Science 2007, 317, 355‐358. (35) Li, H.; Zanella, M.; Genovese, A.; Povia, M.; Falqui, A.; Giannini, C.; Manna, L. Nano Lett. 2011, 11, 4964‐4970. (36) Yin, Y.; Rioux, R. M.; Erdonmez, C. K.; Hughes, S.; Somorjai, G. A.; Alivisatos, A. P. Science 2004, 304, 711‐714. (37) Seo, K.; Bagkar, N.; Kim, S.‐i.; In, J.; Yoon, H.; Jo, Y.; Kim, B. Nano Lett. 2010, 10, 3643‐3647. (38) Wagner, J. B.; Willinger, M.‐G.; Müller, J.‐O.; Su, D. S.; Schlögl, R. Small 2006, 2, 230‐234. (39) Ostwald, W. Z. Z. Phys. Chem. 1901, 37, 385. (40) Saruyama, M.; Kanehara, M.; Teranishi, T. J. Am. Chem. Soc. 2010, 132, 3280‐3282. (41) Carbone, L.; Nobile, C.; De Giorgi, M.; Della Sala, F.; Mo‐ rello, G.; Pompa, P.; Hytch, M.; Snoeck, E.; Fiore, A.; Franchini, I. R.; Nadasan, M.; Silvestre, A. F.; Chiodo, L.; Kudera, S.; Cingola‐ ni, R.; Krahne, R.; Manna, L. Nano Lett. 2007, 7, 2942‐2950. (42) Yu, H.; Chen, M.; Rice, P.M.; Wang, S.X.; White, R.L.; Sun, S.H. Nano Lett. 2005, 5, 379‐382. (43) Kriegel, I.; Jiang, C.Y.; Rodriguez‐Fernandez, J.; Schaller, R.D.; Talapin, D.V.; da Como, E.; Feldmann, J. J. Am. Chem. Soc. 2012, 134, 1583‐1590. (44) Yaws, C. L. Thermophysical properties of chemicals and hydrocarbons; Norwich (USA), William Andrew, 2008. (45) Franchini, I. R.; Bertoni, G.; Falqui, A.; Giannini, C.; Wang, L. W., Manna, L. J. Mater. Chem. 2010, 20, 1357‐1366. (46) Chen, Y.; Palmer, R. E.; Wilcoxon, J. P. Langmuir 2006, 22, 2851–2855. (47) Baletto, F.; Mottet, C.; Ferrando, R. Surf. Sci. 2000, 446, 31–45.

Page 10 of 11

(48) Feser, J. P.; Chan, E. M.; Majumdar, A.; Segalman, R. A.; Urban, J. J. Nano Lett. 2013, 13, 2122‐2127. (49) Zhao, L.‐D.; Hao, S.; Lo, S.‐H.; Wu, C.‐I.; Zhou, X.; Lee, Y.; Li, H.; Biswas, K.; Hogan, T. P.; Uher, C.; Wolverton, C.; Dravid, V. P.; Kanatzidis, M. G. J. Am. Chem. Soc. 2013, 135, 7364‐7370. (50) Guardia, P.; Korobchevskaya, K.; Casu, A.; Genovese, A.; Manna, L.; Comin, A. ACS Nano 2013, 7, 1045‐1053. (51) Lu, W.; Wang, B.; Zeng, J.; Wang, X.; Zhang, S.; Hou, J. G. Langmuir 2005, 21, 3684‐3687. (52) ASM Handbook, Volume 3: Alloy Phase Diagrams (ed Baker, H.) (ASM International, Ohio, USA, 10th edition). (53) Cleveland, C. L.; Luedtke, W. D.; Landman, U. Phys. Rev. B 1999, 60, 5065‐5077. (54) Dick, K.; Dhanasekaran, T.; Zhang, Z.; Meisel, D. J. Am. Chem. Soc. 2002, 124, 2312‐2317. (55) Persson, A. I.; Larsson, M. W.; Stenstrom, S.; Ohlsson, B. J.; Samuelson, L.; Wallenberg, L. R. Nat. Mater. 2004, 3, 677‐681. (56) Wang, J.; Chen, K.; Gong, M.; Xu, B.; Yang, Q. Nano Lett. 2013, 13, 3996‐4000.


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Au-Assisted Growth of Anisotropic and Epitaxial CdSe Colloidal

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