Submitted to Nano Letters 20 Supplementary material: rR Fo Supplementary figure S1: Actual shape of the nanowire. a. Tomographic reconstruction of the nanowire (surface rendered representation). b, c, d. Reconstructed volumes of the different sections of the nanowire, viewed parallel to the growth direction. The images are volume rendered representations of the i ev reconstructed dataset obtained by STEM tomography. The insets give a schematic view of the outline of the cross-section. So (b.) and (c.) as well as (c.) and (d.) show the triangularity before ew and after a single twin, with the section in (d.)having the same cross-sectional shape as the one in (b.). fid on Supplementary figure S2: .C Experimental and simulated CBED (Convergent Beam Electron Diffraction) patterns of a GaP nanowire; a) experimental pattern, taken on a FEI Tecnai TEM at a cameralength of 200 mm. en The inset shows the 000-disc of a defocused diffraction pattern, which is used to correlate the directions of direct and reciprocal space. The disc displays the bottom end of the nanowire. (The tia defocused diffraction pattern was taken at a camera length of 600 mm; there is no rotation l- between both camera lengths); b) simulated CBED pattern (software package mbfit [K. Tsuda and M. Tanaka, Refinement of crystal structural parameters using two-dimensional energy- AC filtered CBED patterns Acta Cryst. A55, 1999, p. 939-95423]); The matching intensity distribution between observed and simulated patterns show that the growth direction of the wire S 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 Page 20 of 36 is [111]B. The difference between simulation and experiment is attributed to the shape of the rod, which is not taken into account in the simulation. ACS Paragon Plus Environment Page 21 of 36 21 Supplementary figure S3: Six twin pairs, highlighting the facet evolution on the broad {11 }A side. Clearly, after the rR Fo formation of the second „paired‟ twin the formed facet is not {111}, since the facet angle is far from 19.5º. However an overall {11 } facet is formed parallel to the growth direction. The bulging part after the second „paired‟ twin is probably due to the correction of the droplet angle, i ev inducing a slight change in the ratio of the {111} and {100} nanofacets creating the {11 } facets. Six different wires are shown to indicate that this process is slightly different for each ew wire. All images were acquired close to the top of the nanowire to exclude effects of lateral growth. Supplementary figure S4: on .C Schematic representation of the formation of paired twins in combination with the droplet shape. a. Growth of {11 } facets, composed of {111} and {100} subfacets. b. Formation of a random fid twin plane indicated by the yellow dotted line. The shape of the droplet does not change and en remains distorted towards what was the broad {11 }A side facet and now becomes the (broad) {111}B side facet. c. After the formation of the random twin, {111} facets are formed. The tia development of the {111} facets reduces the distortion of the droplet, which evolves towards a more hemispherical shape. This changes the contact angles with respect to the various facet l- planes, e.g. {100}A at 35º and {111}B at 19º, whereby the nucleation probabilities change. d. AC Formation of the second, paired, twin plane, indicated by the red dotted line, at deformation 2  0, i.e. before the growth interface has become hexagonal. S 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 Submitted to Nano Letters ACS Paragon Plus Environment Page 33 of 36 Supplementary 1 rR Fo a d c l- tia en fid b on .C ew i ev S AC 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 Submitted to Nano Letters ACS Paragon Plus Environment Submitted to Nano Letters Supplementary 2 l- tia en fid on .C ew i ev rR Fo S AC 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 Page 34 of 36 ACS Paragon Plus Environment Page 35 of 36 Supplementary 3 rR Fo a b c d e f 5nm 5nm .C ew i ev 5nm 5nm 5nm 5nm l- tia en fid on S AC 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 Submitted to Nano Letters ACS Paragon Plus Environment Submitted to Nano Letters rR Fo Supplementary 4 i ev random twin paired twin {100}B {111}B a {112}A narrow {111}B b {112}B {111}A c d growth growth l- tia en fid growth {100} 35° on broad {111} 19° .C {111}A {100}A c ew S AC 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 Page 36 of 36 ACS Paragon Plus Environment