To that end, a team of researchers with the U.S. Department ofEnergy (DOE)'s Lawrence Berkeley National Laboratory (Berkeley Lab)have reported the first direct observation of what they have termed"jump-to-contact," the critical step in orientedattachment. "The direct observation of the translational and rotationalaccelerations associated with the jump-to-contact betweennanoparticles enabled us to calculate the forces that driveoriented attachment," said Jim DeYoreo, a scientist with theMolecular Foundry, a DOE nanoscience center at Berkeley Lab wherethis research took place. "This gives us a basis for testingmodels and simulations that could open the door to using orientedattachment in the synthesis of unique new materials." DeYoreo is the corresponding author of a paper in the journal Science that describes this research titled "Direction-specificinteractions control crystal growth by oriented attachment."Co-authoring this paper were Dongsheng Li, Michael Nielsen,Jonathan Lee, Cathrine Frandsen and Jillian Banfield. Ever since a study in 2000 led by co-author Banfield revealed theexistence of nanoparticle oriented attachment, it has become widelyrecognized that the phenomenon is an important mechanism of crystalgrowth in many natural and biomimetic materials, as well as in thesynthesis of nanowires. |
"Such nanocrystal systems often exhibit complex forms rangingfrom quasi-one dimensional chains to three-dimensional hierarchicalsuperstructures, but typically diffract as a single crystal,implying that the primary particles underwent alignment duringgrowth," says Li, first author of the Science paper and memberof DeYoreo's research group. "When particle alignment isaccompanied by coalescence, this growth is characterized asoriented attachment, however, the pathway by which nanoparticlesbecome aligned and attached has been poorly understood." To learn more about the interactions and forces that drive orientedattachment, the Berkeley researchers studied the early crystalgrowth of iron oxide nanoparticles. Iron oxides are abundant inEarth's crust and play an important role in the biogeochemicalprocesses that shape near-surface environments. Using a siliconliquid cell mounted within a high-resolution transmission electronmicroscope at the Molecular Foundry, the research team recordedimages with sufficient resolution to track nanoparticleorientations throughout the growth of the crystals. "We observed the particles undergoing continuous rotation andinteraction until they found a perfect lattice match at which pointa sudden jump-to-contact occurred over a distance of less than onenanometer," DeYoreo says.
"This jump-to-contact isfollowed by lateral atom-by-atom additions initiated at the contactpoint. The measured translational and rotational accelerations showthat strong, highly-direction-specific interactions drive crystalgrowth via oriented attachment." The information gained from this investigation into the orientedattachment of iron oxide nanoparticles should be applicable notonly to the future synthesis of biomimetic materials, but also toenvironmental restoration efforts. Scientists now know thatmineralization in natural environments often proceeds throughparticle-particle attachment events and plays an important part inthe sequestration of contaminants. Understanding the forces behindoriented attachment should also advance the development of branchedor tree-like semiconductor nanowires, structures in which one ormore secondary nanowires grow radially from a primary nanowire. "Branched semiconductor nanowires are being pursued forapplications in photocatalysis, photovoltaics and nanoelectronicsbecause of their large surface areas, small diameters, and abilityto form natural junctions," DeYoreo says.
"Anunderstanding of the underlying mechanisms that control nanowirebranching should help materials scientists develop more effectivestrategies for producing these materials." This research was primarily supported by the DOE Office of Science.
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