Dynamics and white light emission from CdSe nanocrystals TJ Pennycook 1,2 *, JR Mcbride 3 , SJ Rosenthal 3,1,2 , ST Pantelides 1,2 and SJ Pennycook 2,1 1. Department of Physics and Astronomy, Vanderbilt University, Nashville, USA. 2. Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, USA. 3. Department of Chemistry, Vanderbilt University, Nashville, USA. * Now at SuperSTEM, Daresbury, UK. tpennycook@superstem.org STEM, nanocrystals, dynamics For semiconductor nanocrystals, smallness results in quantum confinement, which widens their energy gaps. Their size-tunable optical properties are of major interest for lighting and solar cell applications and as an alternative to fluorescent dyes in biological imaging. The quest to fully exploit quantum confinement brought with it a push to synthesise progressively smaller nanocrystals. Recently it was found that individual ultrasmall sub-2-nm CdSe nanoparticles emit white light [2,3] rather than the monochromatic emission typical of nanocrystals. The white light emission is of interest for its potential in solid state lighting, but also because it cannot be explained by quantum confinement. Ultrasmall particles appear to behave differently from particles that are merely small. Atomic resolution Z-contrast scanning transmission electron microscope (STEM) images of CdSe nanocrystals with diameters of 7 nm, 5 nm and 3 nm are shown in Figure 1. The 7 nm nanocrystals emit red light, the 5 nm nanocrystals emit orange light and the 3 nm nanocrystals emit green light. These particles are composed of wurtzite crystal surrounded by a disordered dynamic surface layer of roughly a nanometer in thickness. Each subsequent STEM image shows a different configuration of this dynamic surface layer, while the crystal cores remain stable. The motions of the surface layer cause it to appear blurry in the images. Figure 2 displays instances of the defects we observe in the small nanocrystals, twin boundaries and stacking faults. Stacking faults cause the intensities seen in the Z-contrast images to change. When viewed down the c-axis, instead of only equivalent columns, we see different intensities in the columns constituting the hexagonal rings, and extra columns in the middle of the rings. These nanocrystals also possess the dynamic surface layer which can again be seen as a blur surrounding their crystal cores. As the nanoparticles are made smaller, the ratio of the thickness of the dynamic surface layer to the size of the particle increases. The transition to white light emission coincides with the size at which this fluxional surface layer envelops the entire particle - the ultrasmall white light emitting CdSe nanoclusters appear entirely fluxional and devoid of any crystal structure. No change in the rate of the dynamic fluctuations was perceivable between accelerating voltages of 60, 100 and 300 kV as well as beam currents varying up to a factor of five. The white light emission is produced by pumping the nanoclusters with ultraviolet light which, like the electron beam, heats the particles. We have simulated the effect of the absorption of ultraviolet photons using density functional theory molecular dynamics. Upon heating an ultrasmall CdSe wurtzite nanocrystal, it immediately disorders. The atoms appear to move around randomly and continuously in a manner resembling the motions observed in the microscope. Cooling the nanocluster down and relaxing it results in a disordered structure significantly lower in energy than when it was a wurtzite nanocrystal. Furthermore, examining the density of states during the molecular dynamics simulation reveals that the structural fluctuations cause the electronic structure to fluctuate as well. The band gap increases and decreases and states appear and disappear inside the gap as the structure morphs over very short time periods. The evolving electronic structure produces a continuous range of possible emission energies that corresponds to the width of the emission spectrum observed experimentally. Thus we attribute the white light emission to the fluxionality of the ultrasmall nanoclusters.