Colloidal Cu 2 ZnSn(SSe) 4 (CZTSSe) Nanocrystals: Shape and Crystal Phase Control to Form Dots, Arrows, Ellipsoids, and Rods Shalini 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 CZTSSe nanocrystal system by tuning the occurrence of polytypism between wurtzite and zinc-blende phases. We have isolated the key control factors in this system and show that the choice of solvents/surfactants and precursors and how they are introduced can allow shape control from dots to ellipsoids to arrows and rods. The shape evolution is dictated by independently controlling the respective growth rates of either the zinc-blende or wurtzite regions in the polytypic system. We further show the extension of this synthetic control to eliminate polytypism while retaining anisotropy allowing for single-phase wurtzite nanorods of CZTSSe. ■ INTRODUCTION Colloidal semiconductor nanocrystals comprising earth-abun- dant and low-toxic elements such as Cu 2 ZnSnS 4 , Cu 2 ZnSnSe 4 , and their alloys Cu 2 ZnSn(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 technological application by affecting composition, 5−7 size, 8 shape, 9 or ligand shell. 10,11 These can be altered respectively by tuning the cation or anion ratios, crystal phase, or the nature of the organic− inorganic interface. 12 Formation of anisotropic geometries is appealing as the functional properties such as electrical and thermal conductiv- ities, total absorption, and photon emission have aspect ratio dependence. When these properties are collectively harnessed in assemblies, they can allow for maximized absorption in photovoltaics, enhanced conductivity in thermoelectric devices, or directional emission in displays. 13−15 In the colloidal synthesis of compound copper chalcogenide nanocrystals, the wurtzite phase is best suited for switching from isotropic spherical nanocrystal growth to anisotropic growth of 1D nanorods. 16 A suitable balance between the type of ligands, nature of metal precursor, and temperature suppresses the growth of selective facets in the crystal allowing the elongation of the nanocrystals along the [001] direction. 9,17 To date, 1D growth in copper based multicomponent nanocrystals in the form of nanorods has been widely reported by different synthetic routes for the systems having sulfur as the anion such as CuIn x Ga 1−x S 2 , 18−20 Cu 2 ZnSnS 4 , 9,21 CuInS 2 , 22,23 and AgInS 2 ,. 24 Notably, complete or partial anionic substitution by Se to form quaternary (I 2 −II−IV−Se 4 ) or quinary (I 2 −II− IV−(SSe) 4 ) nanocrystals in the crystals quenches directional growth only allowing the formation of pseudospherical nanodots or nanoplates in single phase systems. 25−28 However, shape control has been achieved by different research groups in biphasic nanocrystals, when the nucleation takes place in one phase and the growth in another phase. Branched Cu 2 Cd x SnSe y nanocrystals growing from a tetrahedral core with wurtzite arms via a twinning mechanism has been reported by Zamani et al. 29 We have reported the complete colloidal synthesis of Cu 2 SnSe 3 nanocrystals occurring either as linear polytypes with a wurtzite core and cubic tips or branched polytypes with cubic cores and wurtzite tips. 30,31 Yu and co-workers have reported the formation of complex quinary systems such as polytypic Cu 2 CdSn(S 1−x Se x ) 4 32 and CZTSSe 33 nanocrystals with wurtzite nucleation and zinc-blende growth. In some applications, such polytypism is desirable, for example, in thermoelectrics where a single particle having different phases with different electrical and thermal characteristics maximizes the Seebeck coeffi- cient. 29,34,35 In other applications, such as photovoltaics, single phase structures are optimal, as occurrence of phase boundaries in a single particle can act as sites for electron trapping. 36−38 The ability to control the occurrence of polytypism or eliminate as needed is therefore important to allow optimization for desired applications. Herein, we study one of the most important compound semiconductors CZTSSe for both thermoelectrics and photovoltaics and show that polytypism can be tuned in the heterostructures ranging from ellipsoids Received: April 16, 2015 Revised: June 15, 2015 Article pubs.acs.org/cm © XXXX American Chemical Society A DOI: 10.1021/acs.chemmater.5b01399 Chem. Mater. XXXX, XXX, XXX−XXX