Chemical Synthesis, Doping, and Transformation of Magic-Sized

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Chemical Synthesis, Doping, and Transformation of Magic-Sized Semiconductor Alloy Nanoclusters Jiwoong Yang,†,‡,# Franziska Muckel,§,# Woonhyuk Baek,†,‡ Rachel Fainblat,†,‡,§ Hogeun Chang,†,‡ Gerd Bacher,*,§ and Taeghwan Hyeon*,†,‡ †

Center for Nanoparticle Research, Institute for Basic Science (IBS), Seoul 08826, Republic of Korea School of Chemical and Biological Engineering, and Institute of Chemical Processes, Seoul National University, Seoul 08826, Republic of Korea § Werkstoffe der Elektrotechnik und CENIDE, University Duisburg-Essen, Bismarckstraße 81, 47057 Duisburg, Germany ‡

S Supporting Information *

ABSTRACT: Nanoclusters are important prenucleation intermediates for colloidal nanocrystal synthesis. In addition, they exhibit many intriguing properties originating from their extremely small size lying between molecules and typical nanocrystals. However, synthetic control of multicomponent semiconductor nanoclusters remains a daunting goal. Here, we report on the synthesis, doping, and transformation of multielement magic-sized clusters, generating the smallest semiconductor alloys. We use Lewis acid−base reactions at room temperature to synthesize alloy clusters containing three or four types of atoms. Mass spectrometry reveals that the alloy clusters exhibit “magic-size” characteristics with chemical formula of ZnxCd13−xSe13 (x = 0−13) whose compositions are tunable between CdSe and ZnSe. Successful doping of these clusters creates a new class of diluted magnetic semiconductors in the extreme quantum confinement regime. Furthermore, the important role of these alloy clusters as prenucleation intermediates is demonstrated by low temperature transformation into quantum alloy nanoribbons and nanorods. Our study will facilitate the understanding of these novel diluted magnetic semiconductor nanoclusters, and offer new possibilities for the controlled synthesis of nanomaterials at the prenucleation stage, consequently producing novel multicomponent nanomaterials that are difficult to synthesize.

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INTRODUCTION In the last several decades, colloidal chemistry has provided effective ways to synthesize inorganic nanomaterials with unique properties originating from their sizes1 and shapes.2 Recently, research interests have been rapidly shifting from single-component to multicomponent nanostructures.3 Integration of multiple elements in nanoscale materials provides enhanced performance and/or multifunctionality for a wide range of applications such as energy,4 electronic,5 and biomedical applications.6 Various methods including doping,7 alloying,8 ion exchange,9 and heterodeposition10 have been employed to obtain multicomponent nanomaterials. Simultaneously, as the complexity of the reaction systems has increased, researches on the formation mechanism of nanomaterials have gained significant importance.11−13 In addition to the classical crystallization achieved by the addition of monomers,12 it has been revealed that multiple intermediate steps can be involved during nanocrystal formation.13 Thus, the studies on prenucleation stages have been emphasized to comprehend the nonclassical behaviors in nucleation and growth. In this aspect, the development of advanced synthetic © 2017 American Chemical Society

methods by controlling intermediates at the prenucleation stage is one of the desired goals in materials synthesis and will help to expand our understanding on the nanoscale crystallization process, which has yet to be achieved. Magic-sized nanoclusters, which consist of a discrete number of atoms, have been studied as important reaction intermediates between molecules and nanocrystals.14 They are often transiently observed in the prenucleation stage and their role is largely unexplored because of the difficulties in their synthesis. Thus, studying these intermediate species will facilitate our understanding of how molecules evolve into nanocrystals. In addition to their importance in crystallization, their unique properties are of fundamental interest. For example, the ground state of semiconductor nanoclusters is split into multiple fine structure states15 by strong quantum confinement, allowing direct observation of sp-d exchange coupling between each state16a and individual magnetic dopants.16b However, the synthesis of well-defined semiReceived: March 24, 2017 Published: May 8, 2017 6761

DOI: 10.1021/jacs.7b02953 J. Am. Chem. Soc. 2017, 139, 6761−6770

Article

Journal of the American Chemical Society

Figure 1. Synthesis and characterization of alloy II−VI semiconductor clusters. (a) A schematic illustration of the synthesis of the alloy II−VI semiconductor clusters. By simply adjusting the initial ratio of the Cd and Zn precursors, the average composition of the cluster samples could be controlled between ZnSe and CdSe. (b) The average cation molar ratio of the alloy clusters with respect to the molar ratio of the starting reagents. (c) Mass spectrum of the alloy clusters with an average composition of Zn0.6Cd0.4Se. The simulated spectra of ZnxCd13−xSe13 (x = 0−13) are shown below the experimental data for comparison. (d) Absorption spectra of the as-synthesized alloy clusters with various compositions. (e) Absorption spectra from the reaction mixture during the synthesis of the ZnSe clusters. (ZnSe)34 clusters are observed before the formation of the (ZnSe)13 clusters. (f) Absorption spectra from the reaction mixture during the synthesis of alloy clusters. (CdSe)34 clusters are observed before the formation of the ZnxCd13−xSe13 (x = 0−13) clusters.

despite their extremely small size, thus representing the smallest alloy semiconductor. By introducing magnetic Mn2+ ions, we can create extremely small-sized diluted magnetic semiconductor alloys that exhibit giant magneto-optical responses in the highest energy regime reported so far owing to the extremely strong quantum confinement. Importantly, 2D quantum nanoribbons and 1D quantum nanorods of the semiconductor alloys could be synthesized at unusually low temperature conditions using these alloy clusters as precursors, suggesting that the synthesis of nanocrystals can be governed at the prenucleation stage.

conductor nanoclusters with multiple atomic species is still very challenging in contrast to metal nanoclusters.17 In nanoscale metal-chalcogenide semiconductors, heteroatoms (e.g., cations) are usually incorporated by the adsorption on the surface of growing nanoparticles by forming bonds to counterions (e.g., chalcogen).7b,18 Thus, there is almost no chance for heteroatom incorporation into extremely small-sized nanoclusters. In addition, alloying of II−VI semiconductor nanocrystals is usually achieved at high temperature above 250 °C,19 where clusters cannot be stabilized. For these reasons, the synthetic control of magic-sized multielement semiconductor clusters has known to be extremely challenging. Herein we present chemical synthesis, doping, and transformation of magic-sized alloy clusters based on II−VI semiconductors. We can successfully obtain ternary and quaternary alloy clusters using room-temperature Lewis acid− base reactions. The composition of the alloy clusters can be fully controlled from pure CdSe to pure ZnSe, enabling wide range control of the bandgap. Mass spectrometry reveals that the chemical formula of the alloy clusters is ZnxCd13−xSe13 (x = 0−13). The alloy clusters have semiconductor band structures

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RESULTS AND DISCUSSION Synthesis and Characterization of Alloy II−VI Clusters. Alloy II−VI clusters were synthesized by a Lewis acid−base reaction between metal halide-amine complexes and octylammonium selenocarbamate in n-octylamine, which is used as both ligand and solvent (see the Experimental Section, Figure 1a). The reaction effectively proceeds at 20 °C because of the high reactivity (basicity) of the selenium precursor. Any further growth of the clusters into nanocrystals is effectively prevented 6762


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Figure 2. Synthesis of Mn2+-doped ZnSe clusters. (a) Mass spectrum of Mn2+-doped ZnSe clusters (xMn = 6%). Minor peaks are caused by fragmentation. (b) High resolution mass spectrum (reflectance mode) of the main peaks within the dashed box in panel (a). The simulated isotopic distributions of Zn13Se13 (blue), Zn12Mn1Se13 (red), and Zn11Mn2Se13 (green) are displayed below the experimental data (black) for comparison. (c) The final average doping concentration of the ZnSe clusters with respect to the initial ratio of Mn in precursors. (d) Absorption (red), PL (blue), and PLE (black) spectra of Mn2+-doped ZnSe clusters (xMn = 6%). (e) Time-resolved PL spectra of Mn2+-doped ZnSe clusters with different doping concentrations.

at this low reaction temperature. Room temperature alloy formation in these prenucleation clusters is interesting, because alloying of II−VI semiconductor usually proceeds at high temperature of >250 °C.19 By adjusting the initial ratio of the Cd- and Zn-halide precursors, the final composition can be continuously controlled between pure CdSe and pure ZnSe (Figure 1b), which differentiates alloying from doping that usually has limited concentration (e.g., xMn ≤ 10% for (CdSe)13 clusters16). The average Zn:Cd ratio in the final products is similar to that of the precursors because of the high reactivity of the reaction. The synthesized alloy clusters were characterized by laser desorption/ionization time-of-flight mass spectrometry (LDI-TOF MS, Figure 1c). The mass spectrum of the synthesized clusters contains more than ten main peaks, whereas the mass spectra of pure CdSe clusters and ZnSe clusters (Figure S1 in the Supporting Information) contain only one main peak corresponding to Cd13Se13 and Zn13Se13, respectively.14g,k,16a These main peaks can be assigned to the ZnxCd13−xSe13 (x = 0−13) species; contributions from clusters with different sizes are not observed, indicating the size singularity of our cluster products. The calculated isotope distributions of each species match well with the experimental data. The results imply that the alloy clusters are generated by incorporation of Zn atoms instead of the same number of Cd atoms (or vice versa). Although fragmentation and/or ligand detachment during the desorption/ionization process might prevent characterization of precise chemical formula,14 the formation of semiconductor alloy nanoclusters, which is the essence of this work, is clearly demonstrated by many methods in this work. The electronic structure of the alloy clusters was investigated by optical spectroscopy. Since the bandgap of bulk ZnSe exceeds that of bulk CdSe by ∼1 eV, the absorption edge is expected to shift to higher energies with increasing Zn contents. Scattering tails are caused by the self-assembled structure of n-

octylamine passivated clusters, which is discussed in the following parts. Absorption features of all alloy clusters lie in between the energy positions of the (CdSe)13 and (ZnSe)13 clusters (Figure 1d). The excitonic transitions are continuously blue-shifted from the energy position of (CdSe)13 to that of (ZnSe)13 as the average ratio of Zn in the clusters increases from 0% to 100%. Naturally, the optical transitions of II−VI semiconductor clusters cannot be tuned in a continuous manner as their structure consists of a discrete “magic number” of atoms. Alloy formation in these extremely small-sized materials make it possible to finely tune the electronic structure of semiconductors in an energy regime that has not been commonly exploited. In addition, the temperature-dependent absorption spectra exhibit a monotonic redshift with increasing temperature as common for bulk semiconductor compounds (Figure S2 in the Supporting Information). This demonstrates that the alloy clusters exhibit semiconductor band structures despite their extremely small-size, which highlights them as the smallest semiconductor alloy reported so far. To gain a better understanding on alloy cluster formation, we monitored absorption spectra of the reaction mixture during the cluster synthesis. First, we traced the change in the absorption spectra during ZnSe cluster formation. At the initial stage of the reaction, absorption features originating from (ZnSe)34 clusters14k appear, which gradually diminish and finally disappear as the band-edge transition of the (ZnSe)13 clusters becomes clear (i.e., precursors → (ZnSe)34 → (ZnSe)13). This growth process is similar to that for the (CdSe)13 clusters where (CdSe)34 clusters transiently appear before the formation of (CdSe)13 clusters2c (i.e., precursors → (CdSe)34 → (CdSe)13). This suggests that (CdSe)13 and (ZnSe)13 clusters share a similar formation mechanism, which might help the formation of alloy phases in the prenucleation cluster stage. At the early stage of alloy cluster synthesis, distinct absorption features corresponding to (CdSe)34 appear 6763


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Figure 3. Synthesis and optical properties of Mn2+-doped II−VI alloy clusters. (a) A schematic showing the synthesis of the Mn2+-doped II−VI semiconductor alloy clusters. The composition of the clusters could be controlled between ZnSe and CdSe. (b), Absorption (red), PL (blue), and PLE (black) spectra of Mn2+-doped Zn0.6Cd0.4Se clusters (xMn = 6%). (c) Absorption, (d) PLE, and (e) time-resolved PL spectra of the Mn2+-doped alloy clusters with various compositions (xMn = 6%).

at ∼420 nm (Figure 1f), instead of the blue-shifted peaks that correspond to (ZnyCd1−ySe)34. This is also supported by the appearance of (CdSe)34 in the mass spectrum of the reaction intermediate during the synthesis of the alloy clusters (Figure S3 in the Supporting Information). As the reaction proceeds, the absorption features from the (CdSe)34 clusters decrease while the band-edge transition of the alloy clusters dominates the absorption spectrum. This indicates that alloying occurs in the ZnxCd13−xSe13 clusters instead of larger (ZnyCd1−ySe)34 clusters (i.e., precursors → (ZnSe)34+(CdSe)34 → (alloy)13). Interestingly, this is consistent with the fact that (CdSe)13 clusters can be doped by Mn2+ ions, whereas (CdSe)34 clusters cannot be doped, which highlights the extraordinary properties of magic-sized (MSe)13 clusters that can incorporate impurity ions and thus represent a path to bypass self-purification effects in small CdSe nanostructures. Doping of Alloy II−VI Clusters. The difficulties in incorporating four different atoms in the small-sized clusters opens up fundamental question on the feasibility of doping in the alloy clusters and their function as diluted magnetic semiconductors. It has been recently demonstrated that Mn2+doped magic-sized CdSe clusters exhibit unique magnetooptical properties derived from their extremely small-size.16 For example, the magneto-optically active and inactive components of the band-edge fine structures15 are revealed,16a and digital doping with individual magnetic ions20 is demonstrated.16b We

began with synthesis of Mn2+-doped (ZnSe)13 clusters (Experimental Section); note that even doping of pure magic-sized ZnSe clusters has not been reported. The highresolution LDI-mass spectra of the Mn2+-doped ZnSe clusters (Figure 2a and b) show a broader m/z distribution that is slightly expanded toward lower masses compared to undoped (ZnSe)13 clusters (Figure S1 in the Supporting Information). The calculated isotopic distributions suggest that the result can be attributed to superposition of the peaks from the Zn13−xMnxSe13 species (x = 0, 1, and 2), and the presence of clusters with a higher number of Mn ions (x > 2) cannot be excluded. By varying the initial precursor concentration, the average doping concentration (xMn) of the Mn2+-doped (ZnSe)13 clusters can be controlled up to ∼9%, which is comparable to that of the (CdSe)13 clusters (Figure 2c).16a We cannot observe the formation of MnSe clusters. Optical spectroscopy measurements also demonstrate the successful doping of the ZnSe clusters. In contrast to undoped clusters (compare Figure S4 in the Supporting Information), the photoluminescence (PL) spectrum of Mn2+-doped ZnSe clusters shows an internal manganese transition (4T1−6A1), and the photoluminescence excitation (PLE) spectrum of this manganese transition matches the excitonic absorption states of the ZnSe clusters (Figure 2d). This provides direct evidence that Mn2+ ions are in the tetrahedral sites of the host.21a The recombination lifetimes of the Mn2+ transition (Figure 2e) lie 6764


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Figure 4. Magneto-optical properties of the Mn2+-doped alloy clusters. (a) MCD spectra of the Mn2+-doped ZnSe clusters (xMn = 6%) at 5 K. (b) Giant Zeeman splittings extracted from panel (a). (c) Normalized experimental (solid lines) and simulated (dashed lines) MCD spectra of the alloyed clusters (xMn = 6%) with various compositions. (d) Comparison of the experimental and simulated MCD spectra showing the MCD amplitudes. (e) Temperature-dependent MCD spectra of Mn2+-doped Zn0.4Cd0.6Se clusters at 1.63 T. A room-temperature spectrum (300 K, red line) is shown in the inset. (f) Temperature-dependent behavior of the MCD amplitude of the alloy clusters with different compositions at 1.63 T. Brillouin fits (dashed line) are also displayed. The inset shows the magnified data on a different y scale.

within the general range for Mn2+-doped ZnSe nanomaterials.21b Similar to Mn2+-doped (CdSe)13,16a a decrease in the PL lifetimes is observed with increasing Mn2+ doping concentrations, which can be attributed either to a partial lifting21c of the selection rules because of the exchange-coupled pairs in bidoped clusters or to increase in the nonradiative pathways because of increased lattice distortions induced by the incorporation of Mn2+ ions. Changes in quantum yield (QY) with respect to the average doping concentration also points to an increasing nonradiative decay at high Mn2+ concentration: QY = ∼4%, ∼6%, and ∼1.3% for xMn = 3%, 6%, and 9%, respectively (Table S1 in the Supporting Information). We can successfully incorporate Mn2+ ions into the alloy clusters by introducing the manganese halide precursor in the reaction mixture for the synthesis of the alloy clusters (Figure 3a). Optical spectra of the Mn2+-doped alloy clusters with different Cd and Zn compositions (xMn = 6%) are shown in Figure 3. The energy position and shape of the absorption spectra for the Mn2+-doped alloy clusters are similar to those of the undoped alloy clusters (Figure 3b and c vs Figure 1d), suggesting that the alloy clusters maintain their structure even after doping with Mn2+ ions. The PL spectrum of the doped alloy clusters clearly shows the Mn2+ internal transition (4T1−6A1) around ∼600 nm. The PLE spectra of this Mn2+ emission show a transition corresponding to the clusters and exhibit a systematic energy shift that is dependent on the composition of the alloy clusters, which is analogous to the composition-dependent shift of the absorption energies. The PL and PLE spectra demonstrate that Mn2+ is located in the tetrahedral sites of the host alloy clusters. In addition, the lifetime of the internal Mn2+ transition of the doped alloy clusters is strongly dependent on the composition of the host alloy clusters (Figure 3d). Although the compositions of the alloy clusters are between that of pure CdSe and pure ZnSe, their lifetimes are not. For example, the Zn0.2Cd0.8Se and Zn0.4Cd0.6Se clusters have definitely shorter lifetimes than the pure CdSe clusters. This nonlinearity implies that the contributions of nonradiative recombination pathways are increased by the lattice distortion in the alloys, which is also

supported by the trend of QY (Table S1 in the Supporting Information). We investigated the magneto-optical characteristics of the Mn2+-doped alloy clusters using magnetic circular dichroism (MCD) spectroscopy.22 Distinctive MCD signals are observed for Mn2+-doped alloy cluster samples of various compositions (Figure 4a−c). The giant magneto-optical responses directly prove that Mn2+ is incorporated into the alloy clusters. All samples exhibit MCD spectra with similar shape but are shifted in energy; this can be directly monitored by the shift of the first zero crossings in MCD from 3.7 to 4.4 eV (Figure S5 in the Supporting Information), demonstrating the gradual replacement of Cd and Zn atoms. Because of the extremely strong quantum confinement in these small-sized clusters, MCD signals are detected at significantly high-energy range (∼4.5 eV). These high-energy giant magneto-optical responses expand the spectral range of future spintronic and spinoptoelectronic applications to the high-ultraviolet, where the alloy nature allows tuning the magneto-optical response over a wide energy range. The Zeeman splitting of the first transition in the Mn2+-doped (ZnSe)13 clusters was extracted considering the fine structure states in the clusters15,16 (see the Experimental Section) by following the relation,

⎛ 2e ⎞ ΔA |ΔE| = ⎜ ⎟·σ · ⎝ 2 ⎠ A0

(1)

where ΔA = MCD/1.15, and σ and A0 are the width and height, respectively, of the corresponding absorption transition. An effective g-factor of 81 is obtained for ZnSe clusters (xMn = 6%; Figure 4b), which is similar in magnitude to that for Mn2+doped (CdSe)13. Since the absorption peaks are broadened by an overlap of absorptions by species with different numbers of Zn atoms in the alloy clusters, this procedure cannot be applied straightforward to the alloy clusters. Thus, we simulated the magneto-optical response of the clusters with different overall compositions to comprehend the influence of the different cluster species to the overall MCD signals (see the Experimental Section). First, the simulated 6765


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Figure 5. Synthesis of alloy nanocrystals from the alloy clusters as starting materials. (a) Schematic illustration showing formation mechanism of alloy nanocrystals from alloy clusters. (b) Low resolution TEM and (c) HRTEM images of Mn2+-doped Cd0.8Zn0.2Se quantum nanoribbons (xMn = 6%). HRTEM image of the edges (top) and top view (bottom) of the lamellar structured nanoribbons. The illustration in panel (b) shows the crystallographic nature of the nanoribbons. (d) EDS images of the alloy nanoribbons for Cd, Se, Zn, and Mn. (e) Synchrotron HRPD patterns of the alloy nanoribbons. The diffraction peaks (black bars) for bulk wurtzite-CdSe (JCPDS# 65−3415) are shown. (f) Absorption spectra of the alloy quantum nanoribbons. (g) Absorption (red), PL (blue), and PLE (black) spectra of the Mn2+-doped Cd0.8Zn0.2Se quantum nanoribbons. (h) Absorption spectra of alloy nanorods obtained from the alloy clusters. The inset shows the TEM image of Cd0.8Zn0.2Se nanorods. (i) In situ SAXS data and (j) their intensity and line width as a function of time during alloy nanoribbon synthesis from the alloy clusters. (k) Comparison of the absorption spectra of nanoribbons synthesized starting from Cd0.6Zn0.4Se alloy clusters (green) and a mixture of CdSe clusters and ZnSe clusters (black; CdSe:ZnSe = 0.6:0.4).

suggests that each alloy species itself exhibit a similar magnitude of magneto-optical response as the pure binary species. Furthermore, the temperature-dependence of the magnetooptical response was examined. Figure 4e depicts the temperature-dependent MCD spectra up to room-temperature for Mn2+-doped (Zn0.4Cd0.6Se)13 clusters (See Figure S7 in the Supporting Information for the data of Mn2+-doped ZnSe clusters). As the intrinsic Zeeman splitting exhibits an opposite sign,22e the absence of a sign reversal in the MCD proves that the sp-d exchange coupling determines the magneto-optical properties up to these elevated temperatures. This suggests that the magneto-optical characteristics of our materials can be used at elevated temperatures in future spintronic applications. The MCD amplitude decrease follows a Brillouin function with increasing temperature for the Mn2+-doped alloy clusters with various compositions (Figure 4f), which also proves that the origin of the magneto-optical activity of our alloy clusters is the sp-d exchange coupling between band states of the clusters and spins of the magnetic dopants.22f Transformation of the Alloy Clusters into Alloy Nanocrystals. Nanoclusters have been highlighted as important prenucleation intermediates for nanoparticle synthesis.13a,14 We studied whether our alloy clusters can evolve into alloy nanocrystals (Figure 5a). When the alloy clusters were heated at different temperature (see the Experimental Section

MCD spectra of each species (ZnxCd13−xSe13 (x = 0−13)) were acquired by linearly shifting the energy of the experimental MCD spectrum of the Mn2+-doped (CdSe)13 clusters between pure Mn2+-doped (CdSe)13 and Mn2+-doped (ZnSe)13. Then, the simulated spectra for each species were weighted by the calculated probabilities of each species based on a binomial distribution (Figure S6 in the Supporting Information) to obtain the final simulated spectra of the sample with a specific average composition, xZn. The simulated data match well the experimental data (Figure 4c); both the energetic shift between different samples and the line width of the spectra in the modeling results agree quite well with experimental observation. Figure 4d shows a comparison of the MCD amplitudes of the experimental and simulated data. The alloy clusters exhibit reduced MCD amplitudes compared to Mn2+-doped (CdSe)13 or Mn2+-doped (ZnSe)13 samples in the experiment. In the simulations, we assumed the same MCD amplitude (which is directly related to the Zeeman splitting) for all species. With this assumption, spectra of the cluster samples containing a mixture of different species are simulated and we found a reduced amplitude with a mean factor of ∼4 for the alloy clusters compared to the doped binary clusters. Therefore, most of the reduction of the magneto-optical response of the alloy clusters can be attributed to the mixture of different species, leading to a spectral broadening of the resonances. This 6766


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Journal of the American Chemical Society for details), we could obtain alloy nanocrystals including 2D alloy quantum nanoribbons (∼80 °C) and 1D alloy nanorods (∼180 °C). These results suggest new reaction pathways to obtain alloy nanostructures without a high temperature alloying process (>250 °C19) and/or post-cation exchange reactions.9 Transmission electron microscopy (TEM) images of the Mn2+doped alloy quantum nanoribbons (xMn = ∼6%) are shown in Figures 5b and c. The chemical composition of the alloy nanoribbons is similar to that of the starting alloy clusters, suggesting the preservation of composition during the transformation. They form 2D lamellar self-assemblies and each nanoribbon exhibits a uniform thickness of ∼1.3 nm with the length of a few micrometers and the width of several tens of nanometers, which is similar to that of CdSe quantum nanoribbons.2c Energy dispersive X-ray spectroscopy (EDS) mapping of the alloy nanoribbons shows that all elements are homogeneously distributed in the nanoribbons (Figure 5d and Figure S8−S10 in the Supporting Information). Highresolution powder diffraction (HRPD) using synchrotron radiation (Figure 5e) confirms that the nanoribbons have a hexagonal wurtzite-structure with a huge lattice contraction. The extracted lattice constants of the alloy nanoribbons follow the Vegard’s law (Table S2 and Figure S11 in the Supporting Information), confirming homogeneous alloying in these nanoribbons. The successful formation of alloy 2D quantumwells is also demonstrated by optical spectroscopy. In addition, alloying leads to continuous variation of the bandgap of the 2D nanoribbons, while fine-tuning of the bandgap of 2D quantum nanostructures of II−VI semiconductors has known to be difficult because of their stepwise growth behavior (Figure 5f).2d,23 Regardless of the composition, the band-edge states are split into heavy hole- and light hole-excitonic transitions, which is representative characteristics of 2D semiconductors.2,23 Doping of alloy nanoribbons can be proved by observation of the internal Mn2+ transition (4T1−6A1) in PL and PLE spectra, while they maintain absorption features that are almost identical to that of the undoped nanoribbons with a similar Cd/Zn ratio (Figure 5g). Alloy 1D nanorods can be obtained by simply increasing the reaction temperature (Figure 5a and h, and Figure S12 in the Supporting Information), suggesting the generality of the nanocrystal synthesis from the clusters. To understand the formation mechanism of alloy nanocrystals with various shapes from alloy clusters, we conducted in situ/ex situ small-angle X-ray scattering (SXAS; See the Experimental Section and Figure S13 in the Supporting Information for details on the procedure). In the low temperature condition, the clusters in n-octylamine solvent are already assembled into 2D lamellar structures (Figure S14 in the Supporting Information). As reaction proceeds, the peak of the lamellar structures becomes intense and sharp in in situ measurement (Figure 5i and j). The decrease of line width can be explained by transformation of the clusters into the nanoribbons with lamellar structures. The increase of the peak intensity suggests that more clusters are assembled into the lamellar structures during the growth. This suggests that the 2D lamellar assemblies of clusters lead to the formation of 2D nanocrystals at low temperature (80 °C). However, this lamellar structure is not well maintained at higher temperature of ∼180 °C, resulting in the formation of 1D nanorods. To highlight the importance of alloying in the cluster states, we also conducted a control experiment: synthesis of nanocrystals from a mixture of pure CdSe and ZnSe clusters (See the Experimental Section). The (CdSe)13 and (ZnSe)13

clusters were separately synthesized and then mixed for nanoribbon synthesis. In contrast to the nanoribbon synthesis from the alloy clusters, mixing the CdSe and ZnSe clusters can lead to formation of inhomogeneous products (Figure 5k). In that case, the band-edge transitions of the major products are red-shifted compared to those of the alloy nanoribbons with the similar overall Zn/Cd ratio, which shows that the incorporation of Zn contents into final nanoribbons is not efficient. The result implies that the formation of alloys at the prenucleation stage can facilitate homogeneous alloying in the final nanocrystals even at the low temperature. Our results demonstrate that the overall synthetic process can be governed by controlling the prenucleation cluster intermediates and gives some clues on how the prenucleation stage affects nanocrystal synthetic chemistry.

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CONCLUSION In this study, we report on the synthesis, doping, and transformation of multielement magic-sized alloy clusters using Lewis acid−base reactions at room-temperature. Our method can easily produce alloy clusters (ZnxCd13−xSe13 (x = 0−13)), and their composition can be readily tuned between pure CdSe and pure ZnSe, allowing a wide variation of the bandgap in the high-energy range. Transition metal doping of these alloy clusters produces diluted magnetic alloy semiconductors whose bandgap is controllable up to the highest energy regime reported so far. These results demonstrate that the doped alloy nanoclusters represent good model systems for the study of dilute magnetic semiconductors in the extremely strong quantum confinement regime, expanding the spectral range into the high ultraviolet for future spintronic applications. Importantly, we have showcased the use of these alloy nanoclusters as the starting materials for the low temperature synthesis of alloy nanocrystals of higher dimensionality (2D and 1D), revealing that alloy clusters can be critical for the formation of alloy nanostructures. Our findings expand the fundamental understanding of diluted magnetic semiconductors down to an extremely small-size regime and will help to develop a new synthetic method by governing the prenucleation stage. To gain better understanding, transformation mechanism studies on the atomic scale should be conducted in the future.

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EXPERIMENTAL SECTION

Synthesis of the Alloy Clusters. The alloy clusters were synthesized using a modified method from the previous report for the synthesis of CdSe clusters.16a In a typical synthesis, a cationic precursor solution was prepared by heating a mixture of 0.2 mmol of CdCl2 (Aldrich, anhydrous, 99.999%), 0.8 mmol of ZnCl2 (Aldrich, anhydrous, 99.999%), and 7 mL of n-octylamine (Aldrich, 99%) at 120 °C for 2 h. The ratio of CdCl2 to ZnCl2 was adjusted to obtain the alloy clusters with desired Cd/Zn ratios. A selenium precursor, octylammonium selenocarbamate (0.67 M), was prepared by bubbling CO gas (Alpha Gas, 99.999%) into 3 mL of n-octylamine containing 2.0 mmol of elemental selenium (Aldrich) for 1 h.24a The solution of octylammonium selenocarbamate was injected into the cationic precursor solution at 20 °C, and then the resulting reaction mixture was kept at that temperature for 24 h. Subsequently 50 mL of ethanol was added into the solution and centrifugation was performed to get noctylamine passivated alloy clusters. Trioctylphosphine (2 mL, Aldrich, 97%) was used for the first washing process to remove excess selenium. Mn2+-doped alloy clusters were synthesized by modifying the synthetic protocol for the undoped alloy clusters. For example, by adding 0.01 mmol of MnCl2 (Aldrich, anhydrous, 99.999%) into the 6767


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Here ΔA and A0 denote the difference in absorption of the left and right circularly polarized light and the mean absorption, respectively (ΔA = MCD/1.15). As the first minimum of the MCD signal is not affected by the overlap with the higher transition, the value of the Zeeman splitting for this transition can be extracted by fitting a Gaussian peak at the position of the zero crossing to the absorption. Calculation of the Relative Ratios of Various Species Using a Binomial Distribution. We calculated the relative probabilities of the different ZnxCd13−xSe13 cluster species (x = 0−13) in an ensemble with an overall Zn concentration of xZn based on a binomial distribution. The calculation ensures that the summation of probabilities of each species is unity at any given overall Zn concentration xZn between 0 and 1, while the total number of Zn2+ ions in the ZnxCd13−xSe13 species (x = 0−13) yields the overall Zn concentration xZn. Note that this simulation is basically performed for undoped particles, which is a reasonably good approximation also for the doped clusters. The results are shown in Figure S6a in the Supporting Information.

cation precursor solution mixture, Mn2+-doped alloy clusters with xMn = 6% were obtained. Synthesis of Alloy Nanocrystals from the Alloy Clusters. The synthesis of undoped and Mn2+-doped alloy quantum nanoribbons (2D) was performed using the alloy clusters as the starting materials. The as-synthesized alloy clusters (2.0 mmol) were dispersed in 20 mL of n-octylamine. The reaction mixture was slowly heated to 80 °C, and kept for 24 h. The products were purified by centrifugation for the further characterization. For the control experiment, pure CdSe and ZnSe clusters were synthesized separately by following the method described above. CdSe clusters (1.2 mmol) and ZnSe clusters (0.8 mmol) were mixed in 20 mL of n-octylamine (i.e., CdSe:ZnSe = 0.6:0.4). For a fair comparison, the same reaction parameters (e.g., temperature, reaction time, concentration) were used. The reaction mixture was slowly heated to 80 °C, and kept for 24 h. The obtained products were compared with the Cd0.6Zn0.4Se quantum nanoribbons from the alloy clusters (Cd:Zn = 0.6:0.4). Alloy nanorods (1D) were obtained by the modified procedure for the synthesis of alloy nanoribbons using the alloy clusters as starting materials. For a typical synthesis, the reaction mixture containing the alloy clusters (0.3 mmol) was rapidly heated to 180 °C, and kept at this temperature for 30 min. The products were purified by centrifugation for the further characterization. Materials Characterization. The elemental composition of the alloy clusters was characterized by inductively coupled plasma-atomic emission spectrometry (ICP-AES, Shimadzu ICPS-7500). The absorption spectra were measured using a CARY 5000E spectrophotometer (Agilent). PL, PLE, and time-resolved PL spectra were obtained using an FLS 980 spectrometer (Edinburgh Instruments). QY of the alloy clusters were determined using rhodamine 6G as the reference. For the decay study, a pulsed xenon flash lamp was used as an excitation source, and the emitted photons were detected by a microchannel plate photomultiplier tube (MCP-PMT). LDI-TOF MS was performed using a Voyager-DE STR Biospectrometry Workstation (Applied Biosystems) installed at the National Center for Interuniversity Research Facilities (NCIRF) at Seoul National University (negative mode). Desorption/ionization process were achieved using a N2 pulsed laser (3 ns pulses) and the intensity of the laser was less than ∼40% of the full power to avoid the fragmentation. The theoretical isotope distribution was calculated by the mMass 4.5.24b Full width at half-maximum of each peak was set to 0.5 for the calculated data. TEM images of the alloy nanoribbons were recorded using JEM-ARM200F and 2100F (JEOL). Elemental mapping was conducted using high-angle annular dark-field scanning TEM (HAADF-STEM) with an EDS detector. Synchrotron HRPD patterns were measured at PLS-II 9B beamline of Pohang Accelerator Laboratory (PAL) in Korea (λ = 0.15183 nm). For magneto-optical measurements, the alloy cluster samples were prepared as a thin film between two quartz glass substrates and were placed in a cryostat (ST300, Janis) between two poles of an electromagnet (EM4-HVA, Lake Shore) in the Faraday geometry. MCD spectroscopy was performed using a home-built setup containing a 75 W xenon lamp (Lot-Oriel) equipped with a monochromator (Omni-λ 150, Lot-Oriel), a photomultiplier (R928, Hamamatsu), and a photoelastic modulator (PEM-90, Hinds Instruments). Extraction of the Zeeman Splitting Energy. MCD signal of the clusters is composed of several magneto-optically active transitions of the degenerated 1S3/21Se band-edge transition. Following the procedure of our previous study on the Mn2+-doped (CdSe)13 clusters,16 we assign the energetically lowest magneto-optically active transition to ±1L states according to a theoretical study of the fine structure states.15 Assuming an A-type MCD transition,24c the first zero crossing of the MCD signal coincides with the corresponding absorption peak maximum. The Zeeman splitting value is then given by

⎛13⎞ Pi(x Zn) = B13, xZn (i) = ⎜ ⎟(x Zn)i (1 − x*)13 − i ⎝i⎠ Simulation of MCD Spectra. MCD spectra of the pure species with varying numbers of Zn atoms (ZnxCd13−xSe13, with x = 0−13) were simulated by shifting the MCD spectrum of Mn2+-doped (CdSe)13 linearly between the zero crossings of Mn2+-doped (CdSe)13 and Mn2+-doped (ZnSe)13, both in n-octylamine (Figure S5 in the Supporting Information). Figure S6 in the Supporting Information depicts simulated MCD spectra for selected species. The extrapolated spectrum for Mn2+-doped (ZnSe)13 resembles the experimental data. Finally, simulated spectra are weighted with the probabilities of each species generating the MCD spectra for as-prepared cluster samples with overall Zn contents of 0%, 20%, 40%, 60%, 80%, and 100%. Note that this is a simplification because the Mn2+ cations are neglected in the binominal calculations. However, as the signal of Mn2+-doped (CdSe) also includes particles with 11 or 12 Cd cations, the approach is sufficiently accurate to simulate the influence of the mixture of the different cluster and therefore the superposition of the individual MCD spectra. In Situ/Ex Situ Small-Angle X-ray Scattering Measurement. In situ/ex situ SAXS measurement were conducted at PLS-II 9A beamline of Pohang Accelerator Laboratory (PAL) in Korea. X-ray was monochromated using Si(111) double crystals and focused at the detector using K−B type mirrors (the beam size: 30 μm × 290 μm (V × H)). CCD detector (Rayonix 2D SX165) was used. The beam exposure time was 5 s for each measurement. The overall setup for in situ measurement is shown in Figure S13 in the Supporting Information, which is modified from the previous report using in situ SAXS.10c Synthesis of alloy quantum nanoribbons from the alloy clusters was conducted in the round-bottom flask by the method described in the above parts. A syringe pump (Hamilton microlab 600) was used for sampling reacting mixture of alloy nanoribbons through the polymer tube. In the middle of the tube, a quartz capillary (diameter: 1.5 mm, wall thickness: 0.01 mm, Hampton research) was used as a window. One mL of the sample aliquot was taken from the reaction solution and moved into the quartz capillary that was placed between X-ray source and the detector. The measured aliquot was pushed back to the reaction mixture for further reaction. The syringe pump and beam exposure time were controlled by the home-built program.

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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/jacs.7b02953. Additional data include MS spectra, temperature-dependent absorption spectra, optical spectra, MCD simulation data, MCD spectra, EDS data, XRD data, TEM images,

⎛ 2e ⎞ ΔA |ΔE| = ⎜ ⎟·σ · ⎝ 2 ⎠ A0 6768


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experimental setup for in situ SAXS measurement, and ex situ SAXS data (PDF)

AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] ORCID

Jiwoong Yang: 0000-0002-2346-8197 Author Contributions #

J. Yang and F. Muckel contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS T.H. acknowledges the financial support of the Research Center Program of Institute for Basic Science (IBS-R006-D1) in Korea. G.B. is grateful to the Deutsche Forschungsgemeinschaft for support under contract Ba1422-13. Experiments at PLS-II were supported in part by MSIP and POSTECH.

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NOTE ADDED AFTER ASAP PUBLICATION This paper was published on May 8, 2017. Figure 5 has been corrected and the paper was re-posted on May 17, 2017.

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