Colloidal Cu2ZnSn(SSe)4 (CZTSSe) Nanocrystals: Shape and CrystalPhase Control to Form Dots, Arrows, Ellipsoids, and RodsShalini Singh,†,‡ Pai Liu,†,‡ Ajay Singh,†,‡,§ Claudia Coughlan,†,‡ Jianjun Wang,†,‡,∥ Matteo Lusi,†,‡

and Kevin M. Ryan*,†,‡

†Materials and Surface Science Institute and ‡Department of Chemical and Environmental Sciences, University of Limerick, Limerick,Ireland

*S Supporting Information

ABSTRACT: Herein, we report shape control in the CZTSSenanocrystal system by tuning the occurrence of polytypismbetween wurtzite and zinc-blende phases. We have isolated thekey control factors in this system and show that the choice ofsolvents/surfactants and precursors and how they areintroduced can allow shape control from dots to ellipsoidsto arrows and rods. The shape evolution is dictated byindependently controlling the respective growth rates of eitherthe zinc-blende or wurtzite regions in the polytypic system. Wefurther show the extension of this synthetic control toeliminate polytypism while retaining anisotropy allowing forsingle-phase wurtzite nanorods of CZTSSe.

■ INTRODUCTION

Colloidal semiconductor nanocrystals comprising earth-abun-dant and low-toxic elements such as Cu2ZnSnS4, Cu2ZnSnSe4,and their alloys Cu2ZnSn(SSe)4 have received much consid-eration due to their significant relevance in solar cells, photo/electrocatalysts and thermoelectrics.1−4 The structure−prop-erty relationships can be tuned for the desired technologicalapplication by affecting composition,5−7 size,8 shape,9 or ligandshell.10,11 These can be altered respectively by tuning the cationor anion ratios, crystal phase, or the nature of the organic−inorganic interface.12

Formation of anisotropic geometries is appealing as thefunctional properties such as electrical and thermal conductiv-ities, total absorption, and photon emission have aspect ratiodependence. When these properties are collectively harnessedin assemblies, they can allow for maximized absorption inphotovoltaics, enhanced conductivity in thermoelectric devices,or directional emission in displays.13−15 In the colloidalsynthesis of compound copper chalcogenide nanocrystals, thewurtzite phase is best suited for switching from isotropicspherical nanocrystal growth to anisotropic growth of 1Dnanorods.16 A suitable balance between the type of ligands,nature of metal precursor, and temperature suppresses thegrowth of selective facets in the crystal allowing the elongationof the nanocrystals along the [001] direction.9,17 To date, 1Dgrowth in copper based multicomponent nanocrystals in theform of nanorods has been widely reported by differentsynthetic routes for the systems having sulfur as the anion suchas CuInxGa1−xS2,

18−20 Cu2ZnSnS4,9,21 CuInS2,

22,23 andAgInS2,.

24 Notably, complete or partial anionic substitutionby Se to form quaternary (I2−II−IV−Se4) or quinary (I2−II−

IV−(SSe)4) nanocrystals in the crystals quenches directionalgrowth only allowing the formation of pseudosphericalnanodots or nanoplates in single phase systems.25−28 However,shape control has been achieved by different research groups inbiphasic nanocrystals, when the nucleation takes place in onephase and the growth in another phase. Branched Cu2CdxSnSeynanocrystals growing from a tetrahedral core with wurtzite armsvia a twinning mechanism has been reported by Zamani et al.29

We have reported the complete colloidal synthesis of Cu2SnSe3nanocrystals occurring either as linear polytypes with a wurtzitecore and cubic tips or branched polytypes with cubic cores andwurtzite tips.30,31 Yu and co-workers have reported theformation of complex quinary systems such as polytypicCu2CdSn(S1−xSex)4

32 and CZTSSe33 nanocrystals with wurtzitenucleation and zinc-blende growth. In some applications, suchpolytypism is desirable, for example, in thermoelectrics where asingle particle having different phases with different electricaland thermal characteristics maximizes the Seebeck coeffi-cient.29,34,35 In other applications, such as photovoltaics, singlephase structures are optimal, as occurrence of phase boundariesin a single particle can act as sites for electron trapping.36−38

The ability to control the occurrence of polytypism or eliminateas needed is therefore important to allow optimization fordesired applications. Herein, we study one of the mostimportant compound semiconductors CZTSSe for boththermoelectrics and photovoltaics and show that polytypismcan be tuned in the heterostructures ranging from ellipsoids

Received: April 16, 2015Revised: June 15, 2015

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(two large zinc-blende (ZB) tips on a wurtzite (WZ) disc) toarrow heads (one dominant ZB with small WZ stem) to singlephase wurtzite rods.

■ EXPERIMENTAL DETAILSMaterials. Copper(I) acetate (Cu(I)Ac >97%), copper(II)

acetylacetonate (Cu(II)(acac)2; >99.99%), copper(I)chloride (Cu(I)Cl >97%), tin(IV) acetate (Sn(Ac)4, >99.99%), tin(II)chloride (SnCl2>97%), zinc acetate (Zn(Ac)2, >99.99%), zinc chloride (ZnCl2 >99.99%), trioctylphosphine oxide (TOPO, 99%), 1-dodecanethiol (1-DDT, 98%), diphenyl diselenide (DPDSe 98%), oleylamine (OLA,technical grade, 70%), and 1-octadecene (ODE, 90% tech) werepurchased from Aldrich. n-Tetradecylphosphonic acid (TDPA) and n-hexylphosphonic acid (HPA) were obtained from PolyCarbonIndustries, Inc. (PCI). All chemicals were used as received withoutany further purification.Synthesis of CZTSSe Ellipsoidal Polytypic Nanocrystals

(Reaction Scheme R1). In a typical synthesis, Cu(I)Cl (0.25mmol), ZnAc (0.125 mmol), SnAc (0.125 mmol), TOPO (1.2 mmol),HPA (0.12 mmol), TDPA (0.26 mmol), and OLA (3 mL) were addedin a three-neck flask and evacuated at 50 °C for 20−30 min. Thesolution was then heated to 270 °C under an argon atmosphere. Whenthe temperature reaches 155 °C, a stock solution was made bydissolving DPDSe (0.25 mmol) into 1-DDT (0.5 mL) and injectedinto the flask. After injection, the reaction was allowed to proceed for15 min with continuous stirring. The reaction was terminated byremoval of the heating mantle and allowed to cool to roomtemperature naturally. The product was washed using toluene andethanol two times at 4000 rpm. For obtaining a high degree ofmonodispersity, size selective precipitation was carried out at 1000rpm for 1 min. The supernatant was collected and washed again at4000 rpm, and the final nanocrystals were dispersed in toluene forfurther characterization. Different Se/S compositions of CZTSSearrow shaped nanocrystals were synthesized by varying the DPDSeamount in the stock solution injections.Synthesis of CZTSSe Arrow Shaped Polytypic Nanocrystals

(Reaction Scheme R2). The synthesis strategy is the same as that forthe CZTSSe arrow shaped nanocrystals with the replacement of Cu(I)Cl with Cu(I)Ac (0.25 mmol).Synthesis of CZTSSe Bullet Shaped Polytypic Nanocrystals

(Reaction Scheme R3). Bullet shaped nanocrystals were synthesizedby the post-treatment of OLA based wurtzite CZTSSe nanocrystals.For synthesizing wurtzite nanocrystals with OLA, Cu(acac)2 (0.5mmol), ZnAc (0.25 mmol), SnAc (0.25 mmol), and OLA (5 mL)were evacuated in a three-neck flask in a Schlenk line at 50 °C for 20−30 min. The temperature of the solution was increased to 270 °Cunder an argon atmosphere in 10 min. When the temperature reaches155 °C, stock solution made by dissolving DPDSe (0.25 mmol) and 1-DDT (1 mL) into 0.5 mL of OLA was injected into the flask. Afterinjection, the reaction was allowed to proceed for 15 min withcontinuous stirring. The reaction was terminated by removal of theheating mantle and allowed to cool to room temperature naturally.The product was collected in a vial and kept for further post-treatmentwithout washing.For post-treatment, TOPO (1.2 mmol), HPA (0.12 mmol), TDPA

(0.26 mmol), and OLA (3 mL) were added in a three-neck flask andevacuated at 50 °C for 20−30 min. The solution was then heated to270 °C under an argon atmosphere. At 90 °C, Se/S stock solution (0.1

mmol of DPDSe into 0.5 mmol of 1-DDT) was injected. When thetemperature reaches 270 °C, a crude solution of wurtzite CZTSSenanocrystals (unwashed) was added into the flask, and the reactionwas allowed to proceed for the next 7 min. The product was washedby toluene and ethanol twice for further characterizations.

Synthesis of CZTSSe Wurtzite Nanocrystals (ReactionSchemes R4−R8). Cu(acac)2 (as given in Table 1), Zn(Ac)2 (1mmol), Sn(Ac)4 (1 mmol), and TOPO (3 mmol) were mixed with 5mL of ODE in a three-neck flask and were degassed at roomtemperature for 30 min. The mixture was then heated to 270 °C underargon flow. When the temperature of the flask increases to 155 °C, Se/S stock solution (DPDSe in 2 mL of 1-DDT) was injected to the flask.After injection the color of solution changes from dark green to lightyellow and then finally black. After 15 min, nanocrystal growth wasterminated by removal of the heating mantle and allowed to cool toroom temperature. The nanocrystals were washed 2−3 times in a 1:1ratio of toluene to isopropanol and centrifuged at 4000 rpm for 5 minto yield a blackish centrifuged product. After cleaning, the nanocrystalswere redispersed in fresh toluene for further characterization. Theconcentrations of Cu(acac)2 and DPDSe have been varied for differentreactions (reaction R4−R8) and are reported in Table 1.

Characterization Methods. The morphology of CZTSSe nano-crystals was characterized by transmission electron microscopy (TEM)and angular dark-field scanning transmission electron microscopy(DF-STEM) using a JEOL JEM-2011F, operating at an acceleratingvoltage of 200 kV. X-ray diffraction (XRD) of drop-cast films ofnanocrystals on a glass substrate was carried out on a PANalyticalX’Pert MPD Pro using Cu Kα radiation with a 1D X’Celerator stripdetector. Rietveld refinement was carried out using X’Pert High Scoreplus software. Raman spectroscopy measurements were performed ona Renishaw 1000 Raman spectrometer equipped with a charge-coupleddevice (CCD) detector and a HeNe 633 nm laser at roomtemperature. An objective lens (×20) was used to focus the laserson the samples with 10% laser output power. The laser polarizationangle was set at 0°. The spectra were collected in the range from 0 to1000 cm−1. The crystal modeling was performed by using theCRYSTALMAKER software.

■ RESULTS AND DISCUSSIONThe colloidal synthesis of quinary copper chalcogenides ofCZTSSe is complex as it requires balancing the chemistries of

Table 1. Precursor Concentrations, Phase, and Dimensions for CZTSSe Nanocrystals Synthesized by Using ODE

reactionCu precursor

(mmol)Zn precursor(mmol)

Sn precursor(mmol)

S precursor(mL)a

Se precursor(mmol) phase

length(nm)

diameter(nm)

R4 2 1 1 2 0.25 WZ 10 ± 1 8 ± 0.5R5 1.5 1 1 2 0.25 WZ 18 ± 1.5 12 ± 0.5R6 1.2 1 1 2 0.25 WZ 20 ± 1 10 ± 1R7 1.0 1 1 2 0.25 WZ 25 ± 2 10 ± 1R8 0.8 1 1 2 0.25 WZ + Sn2S3 25 ± 2 7 ± 0.5

a1-DDT is used in excess for the synthesis of CZTSSe nanocrystals as it has dual function of sulfur source and capping ligand.

Scheme 1. CZTSSe Anisotropic Shape Evolution and TheirCorresponding Reaction Routes

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three different metal cation precursors and two differentchalcogen precursors during the nucleation and growth process.A common feature of the successful protocols employ a hightemperature injection strategy to combine separate cation(flask) and anion (injection) solutions to allow the thresholdtemperature and supersaturation required for nucleation to beachieved simultaneously (Scheme 1). The variables that have adramatic effect on the resulting particles are the wide choice ofprecursors for the cations, anions, surfactants, and solvents. Inour observations, by separating out the contributions of each ofthese species it is possible to attain control both of shape andphase in this semiconductor system. The combination of thecoordinating solvent oleylamine (OLA) with phosphonicligands plays a key role favoring the occurrence of polytypism.Figure 1a,b shows uniform ellipsoids comprising a wurtzite core(width of 8 ± 0.5 nm) and two large ZB derived segments (12± 1 nm) synthesized according to reaction scheme R1. The d-spacing values and the angles between the planes are in goodagreement with the theoretical values.33 From aliquot studies(Figure S1a−d), it was confirmed that the nanocrystals firstnucleate as mixed phase binary Cu−S that are pseudo sphericalin shape with progression of growth to a disc shape with thesubsequent inclusion of Zn, Sn, and Se. As the discs grow, theopposite {002} facets dominate, which have a very similar planespacing to the (111) planes of zinc-blende, creating theoptimum conditions for polymorphism to occur. Two factorsare at play here: (1) as the reaction temperature increases, it

Figure 1. (a) Magnified HRTEM image of single nanocrystal showingthe d spacing of different facets of each phase with crystal model imagein the inset; (b) dark field TEM image of CZTSSe ellipsoids; (c,d)corresponding FFT of zinc-blende (ZB) and wurtzite (WZ) segmentsof single nanocrystal; (e) SAED pattern showing the presence of bothZB and WZ phases.

Figure 2. (a) DF-STEM image of arrow shaped nanocrystals, with inset showing the HRTEM image of a single nanocrystal; (b) SAED pattern; (c,d)FFT patterns for corresponding ZB and WZ segments of single nanocrystal; (e) magnified HRTEM image of the interface marked with theinterplanar spacing of different facets; (f) HR-TEM images and the corresponding 3D crystal models from different angles of the tip of thenanocrystals.

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supports the formation of more thermodynamically favorablezinc-blende phase; (2) as CuCl is a reactive precursor (ascompared to other copper precursors) it facilitates rapid growthon both {002} facets and the occurrence of large ZB-derivedtips. Majority of nanocrystals formed by this reaction schemeare ellipsoids with the occurrence of 5−10% of arrow shapedpolytypic CZTSSe nanocrystals. The polytypism in thesenanocrystals is also verified by fast Fourier transform (FFT)

pattern of the core and tip regions of single nanocrystal (Figure1c,d) marked with plane positions of WZ and ZB phases. Theyare further correlated with the selected area electron diffraction(SAED) pattern shown in Figure 1e confirming the presence oftwo distinct phases. As the (111), (220), and (311) planes ofcubic phase overlap with (002), (110), and (112) planes ofwurtzite phase, these rings appear brighter confirming thepolytypic structure.

Figure 3. (a) DF-STEM image of CZTSSe bullets; (b) SAED pattern showing the presence of both ZB and WZ phases; (c) crystal models of thebullets from different angles; (d) HRTEM image of single nanocrystal; (e) corresponding FFT of single nanocrystal; (f) magnified HRTEM image ofthe interface in single nanocrystal with d spacing of each facet.

Figure 4. HR-TEM images of the CZTSSe nanorods formed by reaction scheme (a) R4, (b) R5, (c) R6, (d) R7 (FFT pattern in inset), and (f) R8;(e) SAED pattern of nanorods synthesized using reaction scheme R7.

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The shape evolution is also affected by the choice of copperprecursor (Scheme 1b). Both Cu(I)Cl and Cu(I)OAc are thecombination of a soft acid (Cu+) and a hard base (Cl−/OAc−).However, as the acetate is a softer base than the chloride (0.16eV vs 2.49 eV, respectively), this makes Cu(I)OAc less reactivethan Cu(I)Cl.39 Here, we observed that the reactivity differencehas a big influence on the morphology of resulting nanocrystals.In the case of the nanocrystals produced by employingCu(I)OAc as the Cu precursor (reaction scheme R2), the ZBderived tip location has the preference for only one of the{002} facets giving rise to arrow shaped CZTSSe nanocrystals.As the {002} facets of wurtzite terminate in either a row ofmetal or chalcogen, respectively, the opposite ends havedifferent polarity and surface energies and are passivateddifferently by the available surfactant species. With a lessreactive copper precursor, this manifests as a preference forgrowth at one end, most likely the chalcogen terminated as theligand species are optimized for interaction with the metal ions.The DF-TEM image (Figure 2a) highlights the goodmonodispersity in particle size and the HRTEM image of asingle nanocrystal (inset of Figure 2a) illustrates the crystalmorphology, which is comprises a dominant pyramidal ZB-derived tip over a minor wurtzite segment having a length of 38nm and maximum width of 30 nm. The SAED pattern (Figure2b) verifies the coexistence of the ZB and WZ phase, which isfurther confirmed by measurement of the interplanar spacingand the respective angles between the planes as marked in theFFT patterns (Figure 2c−e). To illustrate the exact 3Dstructure of the arrow shaped nanocrystals, TEM images ofnanocrystals lying on the TEM grid in different angles weretaken and studied to resolve the 3D crystal structure (Figure2f). In our study of CZTSSe polytypic nanocrystals, we foundthat the choice of solvent/ligand is the necessary contributor to

the occurrence of polytypism, whereas the metal precursordictates the extent of the zinc-blende to wurtzite ratio andtherefore whether the shape occurs as ellipsoids or monopods.Interesting morphological variations are also attainable if the

separate contributions of TOPO and phosphonic acids areconsidered in this reaction. TOPO is widely used in colloidalnanocrystal chemistry as it stabilizes the complex formation andas a steric ligand controls overall particle size.40 The otherphosphonic acid species (TDPA, HPA) are the main ligands,and subtle differences in their binding energies to differentfacets allow for selective growth. Here, in the initial stages ofpolytypic nanocrystal formation, the phosphonic acids (PAs)favor disc formation of the wurtzite structure by initiallypassivating the (002) facets, restricting anisotropic growth inthe wurtzite phase (Figure S1b,f). When we carried out asynthesis just using OLA without the TOPO/PA combination,wurtzite nanocrystals are formed with nonuniform morpholo-gies (Figure S4). However, if nucleation is initiated without theTOPO/PA ligands but they are subsequently introduced in thegrowth phase, structures are obtained with a large wurtzite stemand a small cubic tip (reaction scheme R3). Here in contrast tothe zinc-blende dominated polytypes of Figures 1 and 2,wurtzite now dominates in a 3.5:1 ratio with the order of atomstacking at the interface having the sequence ···ABC/AB···AB/CBA···. Figure 3a,d highlights the uniformity, shape, and size ofthe structures with FFT, SAED, and plane spacing, confirmingboth phases with the 3D crystal models in different orientationsand confirming the morphology (Figure 3b−f). It is thereforepossible to obtain a degree of control of the respectivesegments by controlling the number and type of surfactantspecies present during specific growth stages.The occurrence of polytypism could be excluded in CZTSSe

nanocrystals when OLA and PA are replaced by a non-coordinating solvent, i.e., 1-octadecene (ODE) with TOPO asthe only coordinating species (Scheme 1d−f). Whenstoichiometric amounts of precursor species of CZTSSe isused under these conditions, nanoparticles (diameter 8 ± 1nm) are formed in the wurtzite phase (Figure 4a). However,preference for 1D growth can only be initiated by adjusting theamount of copper ion in the reaction (Table 1). As the ratio ofcopper to other ion species is progressively decreased, there is acorresponding increase in aspect ratio indicated by a shapeevolution from pseudosphere to conventional rod shape(Figures 4 and S5). The composition remains stable up to ahalf loading of copper, and beyond this, other binary andternary phases occur. Copper poor phases of CZTSSe are ofinterest as these have shown the highest photovoltaicefficiencies compared to their stoichiometric analogues.1,41,42

Similar phenomena have been observed in other systems wherea low concentration of copper monomer suppresses thenumber of nucleation events and favors anisotropic growthon existing crystals.43,44 The XRD patterns of the nanocrystalsobtained from reaction schemes R4−R8 (Figure S6) match wellwith the simulated wurtzite crystal structure. Peaks correspond-ing to the orthorhombic Sn2S3 (ICSD no. 98-000-5989) aredetectable when the copper loading is reduced below 1 mmolin the reaction flask (reaction R8).Rietveld refinement analysis of the XRD pattern of ellipsoids,

arrow shaped, and bullets (Figure 5) shows a wurtzitecomposition of 38.0%, 42.5%, and 63.6% respectively, whichis in good agreement with the TEM observations (crystal dataand atomic coordinates used for simulated patterns areprovided in the Supporting Information). XPS spectra

Figure 5. XRD pattern and Rietveld fit for polytypic CZTSSenanocrystals.

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combined with EDS analysis and line scan measurements(Figures S8−S10) further confirmed the presence of therespective metal and chalcogen ions in the sample and theirhomogeneous distribution.

■ CONCLUSIONIn summary, a range of synthetic approaches were developed toobtain control over shape and phase of anisotropic CZTSSenanocrystals. We have shown that a combination ofcoordinating solvents oleylamine, trioctylphosphine oxide, andalkylphosphonic acids allows the evolution of atypical polytypicheterostructures whose shape could be controlled by the natureof metal precursors. Moreover, we have shown that polytypismcould be ruled out by replacing oleylamine and alkylphosphonicacids with a noncoordinating solvent (1-octadecene). Singlephase wurtzite nanorods are formed whose aspect ratio couldbe increased by gradual reduction of Cu concentrations in theinitial reaction flask. This study confirms the ability tocontrollably engineer the anisotropic shape and phase ofcomplex quinary nanostructures and gives better understandingof shape evolution in compound copper based semiconductornanocrystals. As CZTSSe is of interest for a range of diverseapplications, the ability to control both shape and polytypism atthe nanoscale in a colloidal process offers a reproducible routeto a wide range of geometries and morphologies that can beoptimized as needed.

■ ASSOCIATED CONTENT*S Supporting InformationAliquot studies, more XRD analysis, synthesis result using zincand tin precursors, Rietveld pattern simulation data, and EDSand XPS spectra. The Supporting Information is available freeof charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.5b01399.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: kevin.m.ryan@ul.ie.

Present Addresses§The Molecular Foundry, Lawrence Berkeley NationalLaboratory, 1 Cyclotron Road, Berkeley, California 94720,United States.∥Empa, Swiss Federal Laboratories for Materials Science andTechnology, Dubendorf, Switzerland.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was supported principally by Science FoundationIreland (SFI) under the Principal Investigator Program(Contract No. 11PI-1148) and was conducted under theframework of the Irish Government’s Programme for Researchin Third Level Institutions Cycle 5, National Development Plan2007−2013 with the assistance of the European RegionalDevelopment Fund.

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(2) Guo, Q.; Hillhouse, H. W.; Agrawal, R. Synthesis of Cu2ZnSnS4Nanocrystal Ink and Its Use for Solar Cells. J. Am. Chem. Soc. 2009,131, 11672.(3) Zhou, H.; Hsu, W.-C.; Duan, H.-S.; Bob, B.; Yang, W.; Song, T.-B.; Hsu, C.-J.; Yang, Y. CZTS nanocrystals: a promising approach fornext generation thin film photovoltaics. Energy Environ. Sci. 2013, 6,2822.(4) Ji, S.; Shi, T.; Qiu, X.; Zhang, J.; Xu, G.; Chen, C.; Jiang, Z.; Ye,C. A Route to Phase Controllable Cu2ZnSn(S1-xSex)4 Nanocrystalswith Tunable Energy Bands. Sci. Rep. 2013, 3, 2733.(5) Shin, S. W.; Han, J. H.; Park, C. Y.; Kim, S.-R.; Park, Y. C.;Agawane, G. L.; Moholkar, A. V.; Yun, J. H.; Jeong, C. H.; Lee, J. Y.;Kim, J. H. A facile and low cost synthesis of earth abundant elementCu2ZnSnS4 (CZTS) nanocrystals: Effect of Cu concentrations. J.Alloys Compd. 2012, 541, 192.(6) Singh, A.; Singh, S.; Levcenko, S.; Unold, T.; Laffir, F.; Ryan, K.M. Compositionally tunable photoluminescence emission in Cu2ZnSn-(S(1‑x)Se(x))4 nanocrystals. Angew. Chem., Int. Ed. 2013, 52, 9120.(7) Wang, J.-J.; Xue, D.-J.; Guo, Y.-G.; Hu, J.-S.; Wan, L.-J. BandgapEngineering of Monodispersed Cu2−xSySe1−y Nanocrystals throughChalcogen Ratio and Crystal Structure. J. Am. Chem. Soc. 2011, 133,18558.(8) Khare, A.; Wills, A. W.; Ammerman, L. M.; Norris, D. J.; Aydil, E.S. Size control and quantum confinement in Cu2ZnSnS4 nanocrystals.Chem. Commun. 2011, 47, 11721.(9) Singh, A.; Geaney, H.; Laffir, F.; Ryan, K. M. Colloidal synthesisof wurtzite Cu2ZnSnS4 nanorods and their perpendicular assembly. J.Am. Chem. Soc. 2012, 134, 2910.(10) Liu, Y.; Yao, D.; Shen, L.; Zhang, H.; Zhang, X.; Yang, B.Alkylthiol-Enabled Se Powder Dissolution in Oleylamine at RoomTemperature for the Phosphine-Free Synthesis of Copper-BasedQuaternary Selenide Nanocrystals. J. Am. Chem. Soc. 2012, 134, 7207.(11) Jiang, C.; Lee, J.-S.; Talapin, D. V. Soluble Precursors forCuInSe2, CuIn1−xGaxSe2, and Cu2ZnSn(S,Se)4 Based on ColloidalNanocrystals and Molecular Metal Chalcogenide Surface Ligands. J.Am. Chem. Soc. 2012, 134, 5010.(12) Aldakov, D.; Lefrancois, A.; Reiss, P. Ternary and quaternarymetal chalcogenide nanocrystals: synthesis, properties and applica-tions. J. Mater. Chem. C 2013, 1, 3756.(13) Singh, S.; Singh, A.; Palaniappan, K.; Ryan, K. M. Colloidalsynthesis of homogeneously alloyed CdSe(x)S(1‑x) nanorods withcompositionally tunable photoluminescence. Chem. Commun. (Cam-bridge, U. K.) 2013, 49, 10293.(14) Ryan, K. M.; Singh, S.; Liu, P.; Singh, A. Assembly of binary,ternary and quaternary compound semiconductor nanorods: Fromlocal to device scale ordering influenced by surface charge.CrystEngComm 2014, 16, 9446.(15) Lee, S. M.; Cho, S. N.; Cheon, J. Anisotropic Shape Control ofColloidal Inorganic Nanocrystals. Adv. Mater. 2003, 15, 441.(16) Wang, Y.-H. A.; Zhang, X.; Bao, N.; Lin, B.; Gupta, A. Synthesisof Shape-Controlled Monodisperse Wurtzite CuInxGa1−xS2 Semi-conductor Nanocrystals with Tunable Band Gap. J. Am. Chem. Soc.2011, 133, 11072.(17) Lu, X.; Zhuang, Z.; Peng, Q.; Li, Y. Controlled synthesis ofwurtzite CuInS2 nanocrystals and their side-by-side nanorodassemblies. CrystEngComm 2011, 13, 4039.(18) Coughlan, C.; Singh, A.; Ryan, K. M. Systematic Study into theSynthesis and Shape Development in Colloidal CuInxGa1−xS2Nanocrystals. Chem. Mater. 2013, 25, 653.(19) Singh, A.; Coughlan, C.; Laffir, F.; Ryan, K. M. Assembly ofCuIn1‑xGaxS2 Nanorods into Highly Ordered 2D and 3D Super-structures. ACS Nano 2012, 6, 6977.(20) Singh, A.; Singh, A.; Ciston, J.; Bustillo, K.; Nordlund, D.;Milliron, D. J. Synergistic Role of Dopants on the Morphology ofAlloyed Copper Chalcogenide Nanocrystals. J. Am. Chem. Soc. 2015,137, 6464.(21) Mainz, R.; Singh, A.; Levcenko, S.; Klaus, M.; Genzel, C.; Ryan,K. M.; Unold, T. Phase-transition-driven growth of compound

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Chemistry of Materials Article

DOI: 10.1021/acs.chemmater.5b01399Chem. Mater. XXXX, XXX, XXX−XXX

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