Published: September 08, 2011 r2011 American Chemical Society 12677 dx.doi.org/10.1021/la202726g | Langmuir 2011, 27, 12677–12683 ARTICLE pubs.acs.org/Langmuir Impact Dynamics of Colloidal Quantum Dot Solids Lejun Qi, †,§ Peter H. McMurry, ‡ David J. Norris,* ,†,|| and Steven L. Girshick* ,‡ † Department of Chemical Engineering and Materials Science and ‡ Department of Mechanical Engineering, University of Minnesota, Minneapolis, Minnesota 55455, United States b S Supporting Information C olloidal semiconductor nanocrystals, or quantum dots, have unique electrical, chemical, and optical properties that can be useful for electronic and optoelectronic devices. 1À6 In addi- tion, nanocrystals can be easily deposited as films of close-packed assemblies from colloidal dispersions, which can potentially sim- plify the fabrication of these devices. However, in most applica- tions the functionality and stability of the nanocrystals strongly depend on the details of the nanocrystal packing. Because nano- crystals are composed of an inorganic semiconductor core sur- rounded by a layer of organic capping ligands, films typically consist of quantum dots separated by organic molecules. Such materials have been termed nanocrystal solids or quantum dot solids. 7À11 A better understanding of their properties is of funda- mental interest and could also lead to improvements in potential devices and their fabrication. Although the optoelectronic characteristics of nanocrystal solids continue to be studied heavily, their mechanical behavior has been much less explored. Understanding their mechanical pro- perties is important not only because they influence the structural integrity of devices but also because they exhibit new phenomena due to the nature of the colloidal assembly. For example, depend- ing on the details of the solid, dramatically different mechanical characteristics have been reported in previous static measure- ments. 12À15 Elastic behavior was observed in monolayer sheets of close-packed gold nanocrystals capped by dodecanethiol. 15 In contrast, nanoindentation measurements showed that films of CdSe nanocrystals capped by trioctylphosphine oxide (TOPO) and trioctylphosphine (TOP) have viscoplastic properties. 13 Furthermore, when the ligands between the nanocrystals were cross-linked, plastic behavior was observed. In particular, ultra- thin films of CdSe nanocrystals crumpled like thin foils, 12 and spherical agglomerates of Au and Pt nanocrystals deformed like putty. 14 In general, these differences can result from changes in the cohesion between the nanocrystals and the granular nature of the films. However, in these early studies the cohesion and granu- larity have not been fully explored. Cohesion was controlled only by the presence or absence of cross-linking. More importantly, only one report has discussed the granular nature of nanocrystal solids. 13 In that case, the granularity of CdSe nanocrystals capped with TOPO and TOP became apparent only after the removal of the capping ligands. When the ligands are present, they dominate the mechanical behavior under the static or quasi-static condi- tions that have been utilized. Here we use aerosol techniques to investigate the cohesive and granular properties of CdSe nanocrystal solids. With this ap- proach, we can tune the cohesion between the nanocrystals while simultaneously accessing conditions where the influence of the granularity can be examined. We produce quantum dot nano- spheres, which are small pieces of a nanocrystal solid, by aerosolizing a colloidal dispersion of semiconductor nanocryst- als. Each nanosphere contains multiple nanocrystals, their or- ganic capping ligands, and possibly other organic residues from the colloidal dispersion. 16,17 We then test the mechanical proper- ties of these quantum dot nanospheres by impacting them on carbon substrates at various velocities. Previously, we used aerodynamic lenses to focus and deposit many quantum dot nanospheres as microscale lines and patterns. 18 Here we direct Received: July 15, 2011 Revised: September 6, 2011 ABSTRACT: We use aerosol techniques to investigate the cohesive and granular properties of solids composed of colloidal semiconductor nanocrystals (quantum dot solids). We form spherical agglomerates of nanocrystals with a nebulizer and direct them toward a carbon substrate at low (∼0.01 m/s) or high (∼100 m/s) velocities. We then study the morphology of the deposit (i.e., the “splat”) after impact. By varying the size of the agglomerate and the spacing between the nanocrystals within it, we observe influences on the mechanical properties of the quantum dot solid. We observe a liquid- to-solid transition as the nanocrystals become more densely packed. Agglomerates with weakly interacting nanocrystals exhibit liquidlike splashing and coalescence of over- lapping splats. More dense agglomerates exhibit arching and thickening effects, which is behavior typical of granular materials.