title: Controllable assembly of diverse rare-earth nanocrystals via the Langmuir-Blodgett technique and the underlying size- and symmetry-dependent assembly kinetics.
abstract: The Langmuir-Blodgett (LB) technique provides a facile and robust method for the formation of large-area films of various nanoparticles (NPs), including 24.9 nm NaYF(4):Yb,Er nanospheres, 12.0 nm LiYF(4) nanopolyhedra, 14.1 x 1.8 nm triagonal-shaped LaF(3), 12.6 nm square CaF(2), 9.5 x 2.0 nm hexagonal EuF(3), and so forth. The assembly patterns of the deposited films were studied in accordance with the pi-A isotherms. Combined with the TEM observations, several representative stages of assembly process can be distinguished. The scrutiny of the self-assembly process by means of their pi-A isotherms elucidates that the concentration, size, and symmetry of nanoparticles play crucial roles in this process. The concept of "effective concentration", which is defined as the amount of nanoblocks in the "gas phase" rather than the actual number of nanoparticles at the air-water interface, was first proposed as a control parameter to elucidate the possible assembly kinetics. The similarly shaped 12.0 nm LiYF(4) and the 24.9 nm NaYF(4):Yb,Er were selected as the size-dependent examples. The smaller nanoparticles show a strong tendency of congregation to lower the surface energy. Three representative samples, namely, 24.9 nm NaYF(4):Yb,Er nanospheres (O(h)), 14.1 x 1.8 nm oblate triagonal LaF(3) nanosheets (D(3h)), and 41.3 nm x 24.6 nm NaYF(4) rods (D(6h)), were selected as the shape-dependent samples, which showed that the assembly patterns were contributed by the stability arising from the geometry of the nanoparticles, the tendency of aggregation of nanoparticles, and the probable rotation energy during the compression. More importantly, guided by the above assembly kinetics, for the 9.5 x 2.0 nm hexagonal EuF(3), we can effectively acquire the desirable assembly pattern.
authors.affiliation: Beijing National Laboratory for Molecular Sciences, Peking University, Beijing 100871, China.
authors.forename: Huan-Ping
authors.initials: HP
authors.lastname: Zhou
authors.forename: Chao
authors.initials: C
authors.lastname: Zhang
authors.forename: Chun-Hua
authors.initials: CH
authors.lastname: Yan
mesh_terms: D007700:Kinetics; D008674:Metals, Rare Earth / Q000737:chemistry*; D046529:Microscopy, Electron, Transmission; D008954:Models, Biological; D053758:Nanoparticles / Q000737:chemistry* / Q000648:ultrastructure; D036103:Nanotechnology / Q000379:methods*
chemical_list: D008674:Metals, Rare Earth