Visualization of Dislocation Dynamics in ... - Itai Cohen Group

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Visualization of Dislocation Dynamics in ... - Itai Cohen Group

R EPORTSplanes in the strain field of the dislocationgives rise to a local change in the Braggcondition and results in a dark line in theimage of the diffracted beam. A LDM imageof a 0.3 mm by 0.3 mm section of thecolloidal crystal grown on the template withthe ideal lattice constant d 00 1.63 6m isshown in Fig. 1B. Even these crystals, grownon templates with an ideal lattice constant,contain some dislocations (indicated byarrows). Such dislocations are most frequentlyobserved near the template border.We can further exploit the analogy withthe TEM technique and use contrast inversionto verify that the dark lines in Fig. 1B indeedresult from scattering due to the dislocations.When the sample is tilted slightly further, theBragg condition is no longer fulfilled in theperfect lattice and is instead locally satisfiedin the region corresponding to the dislocationstrain field. Thus, the image contrast on thescreen inverts (Fig. 1C). The additional tiltof the sample introduces an excitation error,s 0 q – g (Fig. 1C, upper right inset), whichcauses the intensity in the diffracted beam todecrease (Fig. 1C, upper left inset). Close tothe dislocation, however, the bending oflattice planes locally scatters light into thedirection of the diffracted beam, making thedark lines appear light.To investigate the effect of a lattice mismatch,we grew a crystal on a template withlattice constant d 10 1.65 6m, which is 1.5%larger than d 0. The crystal grown on thestretched template exhibited a similarly lowdensity of dislocations at a crystal thicknessof 22 6m. Strikingly, as the crystal wasgrown to a thickness of 31 6m, a largenumber of dislocations nucleated and grew(Fig. 1, D and E). We determined the averagedislocation line separation in the directionperpendicular to the dislocation lines, 3,from the images. 3 j1 is the number ofdislocations per unit length. Measuring 3 inthree different 0.3 mm by 0.3 mm regions,we obtained an average value of 53 (T10) 6mfor the 31-6m crystal.Remarkably, although the template wasstretched in both spatial directions, dislocationlines were seen in one direction only (Fig. 1, Dand E). The dislocation contrast is visible onlyif the particle displacements in the dislocationstrain field have a component parallel to thediffraction vector used for imaging. The (220)diffraction vector chosen for imaging in Fig.1, B to E, lies along the y direction; therefore,only lattice distortions with a componentalong the y direction showed up in the image.When we instead chose the (220) diffractionvector, which lies along the x direction, weobserved a second set of dislocations (Fig. 1,F and G). Comparing the images in Fig. 1, Dand F, we conclude that the strain field of thedislocations is strictly perpendicular to theimaged dislocation lines.To elucidate the defect structure on themicroscopic scale, we used confocal microscopyto image the individual particles and todetermine their positions (6). The 31-6m-thickfcc colloidal crystal grown on the stretchedtemplate contains characteristic defects. Atthese defects, the nearest neighbor particleconfiguration changes so that particles havethree opposing nearest neighbor pairs, as isthe case in the hexagonal close-packed (hcp)lattice, rather than six as in the fcc lattice. Areconstruction of a 55 6mby556mby176msection of the crystal is shown in Fig. 2A. Thex, y, andz axes correspond to the (110), (110),and (001) directions of the fcc lattice,respectively. Particles with three opposingnearest neighbor pairs are shown in red, andthose with six opposing nearest neighbor pairsin blue. The red particles lie along intersectingplanes embedded in the fcc lattice. By displayingonly the red particles, we show that theplanes are hcp (Fig. 2B). The red planessandwich a stacking fault where the stackingorder of the hcp planes changes from ABC-ABCABC to ABCBCABCA. The stacking faultlies along the (111) plane where the dislocationsmove most easily as a result of theshallow potential wells. This defines the glideplane of the dislocations in the fcc structure.For a closer look at the strain field associatedwith the stacking faults, we display atypical y-z cut through a stacking fault. Thefault ends above the template and is terminatedby a dislocation (Fig. 2C). The firstrow of particles sits in the template holes. Inthe second row of particles, we recognize theemergence of a strain field in the y-z planeassociated with a dislocation line orientedperpendicular to this plane. The dislocationcore (±) lies about two lattice constantsabove the template. The Burgers circuitillustrated by the red line, which would closein the perfect lattice, exhibits a closure failurearound the dislocation core. The Burgersvector b, which connects the starting andending points of the Burgers circuit, is1/6(112). This type of dislocation is knownas a Shockley partial dislocation and is themost prominent dislocation observed in fccFig. 2. (A) Reconstruction of a 556m by556m by176m section ofthe colloidal crystal grown on thestretched template. The red particlesdelineate stacking faults embeddedin an otherwise perfect fcclattice. (B) Crystal reconstructionshowing only the particles adjacentto the stacking faults. (C) Areconstructed y-z section througha stacking fault. The stacking faultis terminated by a Shockley partialdislocation whose core position isindicated by ±. The red loop indicatesa Burgers circuit. The upperright inset is a three-dimensionalillustration of the fcc unit cell. They-z plane is gray; the hcp planeparallel to the stacking fault is red.Az (µm)(001)y (µm) (110)Cx (µm) (110)Bz (µm)(001)y (µm) (110)x (µm) (110)z (µm)y (µm)194624 SEPTEMBER 2004 VOL 305 SCIENCE www.sciencemag.org

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