LETTERS 382 nature materials | VOL 2 | JUNE 2003 | www.nature.com/naturematerials N anoscale materials are currently being exploited as active components in a wide range of technological applications in various fields, such as composite materials 1,2 , chemical sensing 3 , biomedicine 4–6 , optoelectronics 7–9 and nanoelectronics 10–12 . Colloidal nanocrystals are promising candidates in these fields, due to their ease of fabrication and processibility. Even more applications and new functional materials might emerge if nanocrystals could be synthesized in shapes of higher complexity than the ones produced by current methods (spheres, rods, discs) 13–19 . Here, we demonstrate that poly- typism, or the existence of two or more crystal structures in different domains of the same crystal, coupled with the manipulation of surface energy at the nanoscale,can be exploited to produce branched inorganic nanostructures controllably. For the case of CdTe, we designed a high yield, reproducible synthesis of soluble, tetrapod-shaped nanocrystals through which we can independently control the width and length of the four arms. Polytypism is generally prevalent in open, tetrahedrally bonded structures, such as those occurring in the group IV, III-V and II-VI semiconductors 20–22 . Crystal structures of these and many other polytypic materials share a common crystal facet, which can be used to achieve branching. The ±{111} facets of the cubic (zinc blende) structure are atomically identical to the ±(0001) facets of the hexagonal (wurtzite) structure (Fig. 1). The most basic branched polytypic crystal that can therefore be produced using these materials is a ‘tetrapod’, consisting of a zinc-blende core with four {111} facets, each projecting a wurtzite rod terminated with the (0001 – ) facet. For such a structure to be formed, there must be a mechanism by which the stability of the two phases reverses during growth. In conventional bulk crystal growth, some examples exist of controlled formation and growth of polytypic structures 20 , and of modulated growth rates of different crystal facets as a function of time 23 . However, the advent of new methods for preparing inorganic nanocrystals with well-controlled sizes and elementary shapes provides a new set of tools that can be adapted to this purpose 13–19 . Tetrapod- shaped crystals with dimensions on the nanometre and micrometre scale have been observed in a variety of II-VI semiconductors 17,24–26 , and a low yield of colloidal semiconductor tetrapods was observed in the syntheses of CdSe nanorods 27 . In a study of several different II-VI semiconductor materials, we have found that it is possible to obtain a high yield of colloidal semiconductor tetrapods with well-controlled nanoscale dimensions for the case of CdTe. A key parameter for achieving tetrapod growth is the energy difference between the wurtzite and the zinc-blende structures, which determines the temperature Controlled growth of tetrapod-branched inorganic nanocrystals LIBERATO MANNA 1,2 , DELIA J. MILLIRON* 1 , ANDREAS MEISEL* 1 , ERIK C. SCHER 1 AND A. PAUL ALIVISATOS †1 1 Department of Chemistry, University of California, Berkeley and Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA 2 Permanent address: National Nanotechnology Lab of INFM, Via Arnesano, 73100 Lecce, Italy *These authors contributed equally to this work † e-mail: alivis@uclink4.berkeley.edu Published online: 25 May 2003; doi:10.1038/nmat902 Figure 1 Proposed model of a CdTe tetrapod. The exploded view of one arm illustrates the identical nature of the (111) zinc blende (ZB) and (0001 – ) wurtzite (WZ) facets of the nucleus and the arms, respectively (Cd atoms are yellow,Te atoms are blue). Phosphonic acid molecules selectively bind to the lateral facets of the arms, as suggested in the figure (for clarity, only two facets are shown covered). High-resolution transmission electron microscope (HRTEM) analysis would further clarify the shape of the cubic nucleus and the relative orientations between the various arms of the tetrapod. ODPA WZ(0001) – ZB(111) © 2003 Nature Publishing Group