Properties Of Nanocrystals

[Mrx Wylie]

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We report for the first time that doped nanocrystals of semiconductor can yield both high luminescent efficiencies and lifetime shortening at the same time. Nanocrystals of Mn-doped ZnS with sizes varying from 3.5 to 7.5 nm were prepared by a room temperature chemical process. These nanosized particles have an external photoluminescent quantum efficiency as high as 18% at room temperature and a luminescent decay at least 5 orders of magnitude faster than the corresponding Mn2+ radiative transition in the bulk crystals. Luminescent measurements show that the efficiency increases with decreasing size of the particles, as expected within the framework of an electron-hole localization theory. These results suggest that doped nanocrystals are indeed a new class of materials heretofore unknown.

Cellulose nanocrystals with rod, sphere, and network morphologies were prepared by acid hydrolysis of cotton cellulose, followed by freeze-drying. Hydrolysis with sulfuric acid introduced sulfate groups to these nanocrystal surfaces permitting their dispersion in aqueous as well as organic media, including ethanol and N,N-dimethylformamide, in a matter of seconds. Freeze-drying, on the other hand, induced mesoporosity (91.99 2.57 average pore width) and significantly improved specific surface (13.362 m2/g) that is about 9 times of the original cellulose (1.547 m2/g). Moreover, the nanocrystals exhibited improved thermal conductivity and considerably higher (nearly 30%) carbonaceous residue, possibly due to direct solid-to-gas decomposition. These results demonstrated that a combination of surface charge introduction and fixation of mesoporosity in cellulose nanocrystals is an efficient route to prepare large quantity of high quality cellulose nanocrystals with quick re-dispersion capability for practical applications.

The major challenge of developing the cellulose nanofibers as advanced materials and for further applications is their tendency to form bundles or aggregates. During drying, the abundant hydrogen bonds of cellulose draw the cellulose nanocrystals together to pose significant problems in their re-dispersion for effective processing (Tingaut, Zimmermann, & Lopez-Suevos, 2010). To enable better utilization, it is crucial to develop methods to isolate the nanofibrils after the solvent evaporation in their preparation.

This study was to investigate the hydrolysis and drying processes with the intent to minimize hydrogen bonding, thus reduce and even eliminate aggregation of the cellulose nanocrystals. Homogenous and stable cellulose nanocrystals suspensions were generated by hydrolyzing native cellulose with sulfuric acid to introduce negative charges to the nanocrystal surfaces. Esterification of surface hydroxyl groups of cellulose nanocrystals has shown to introduce sulfate groups to form stable suspensions (Beck-Candanedo et al., 2005). The focus was then to prevent hydrogen bond formation by sustaining repulsion among the cellulose nanocrystals with fast freezing of water among the well-dispersed cellulose nanocrystals with liquid nitrogen to keep them separated and fixed in the solidified ice. The high vacuum in freeze-drying then sublimates the ice in-between the cellulose nanocrystals to substantially reduce or prevent hydrogen bonding, the driving force for the cellulose nanocrystals to aggregate. The induced morphologies and properties of the cellulose nanocrystals were studied to relate to the hydrolysis and drying processes.

Acid hydrolysis of cellulose in sulfuric acid involves rapid protonation of glucosidic oxygen (path 1) or cyclic oxygen (path 2) by protons from the acid, followed by a slow splitting of glucosidic bonds induced by the addition of water (Fig. 1a). This hydrolysis process yields two fragments with shorter chains while preserving the basic backbone structure. In native cellulose, the amorphous regions are more accessible to acid molecules and susceptible to the hydrolytic actions than the

Cellulose nanocrystals with rod, sphere, and network-structured morphologies were prepared by acid hydrolysis and freeze-drying of cotton cellulose. Hydrolysis with sulfuric acid removed amorphous cellulose to produce isolated cellulose nanocrystals with newly introduced sulfate groups on the nanocrystal surfaces. Repulsion among the negatively charged cellulose nanocrystals and quick freezing with liquid nitrogen were very effective in preventing aggregate formation driven by the strong

This research was made possible by funding from the National Textile Center (project M02-CD05), the Jastro-Shields Graduate Research Award, and Summer Graduate Researcher Award from the University of California, Davis.

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Researchers have combined two or three types of nanoparticles to produce new materials with structures known as superlattices. In some instances, the structures display fundamental new properties such as superfluorescence. The researchers' discovery is reported in the journal Nature and featured on the current issue's cover.

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In addition to the hot-injection technique, room-temperature reprecipitation methods have also been used for the synthesis of CsPbX3 NCs of different morphologies. For instance, Huang et al.49 reported the emulsion-based synthesis of perovskite NCs (both hybrid and all-inorganic) at room temperature. In a recent report Li et al.64 showed that CsPbX3 NCs can be synthesized at room temperature, similar to the LARP method used for the MAPbX3 system. Manna and colleagues65 have demonstrated the synthesis of quantum-confined CsPbBr3 nanoplatelets at room temperature. They have shown that anisotropic growth leads to the formation of nanoplatelets at room temperature through the injection of acetone in a mixture of precursor. The low growth temperature facilitates the control of the plate thickness down to three monolayers. In addition, it was shown by Sun et al.26 that the shape of the NCs could be controlled using different ligand molecules. In this regard, CsPbX3 NCs of different shapes such as spherical dots, nanocubes, nanorods and nanoplatelets were prepared by using different organic acids and amine ligands; however, the growth mechanism was not discussed. The room-temperature solution-phase synthesis of nanoplatelets can be easily scaled up as shown by Yu and colleagues.66 Driven by the wish to replace the toxic lead, Jellicoe et al. synthesized CsSnX3 perovskite NCs, with optical properties tunable both by quantum confinement and halide composition.67 The overview of reported studies clearly suggests that reaction temperature and the choice of ligands play a vital role in determining the dimensionality and size of colloidal metal halide perovskite NCs. In general, low reaction temperatures and high ligand concentrations favor anisotropic growth, mainly producing nanoplatelets with controlled thickness and lateral sizes.

As already mentioned above, the optical properties of perovskite NCs strongly depend not only on their constituent metal and halide ions, but also on their dimensionality and size. Quantum confinement effects have been widely investigated in conventional semiconductor nanomaterials of all dimensionalities.80, 81, 82 Optical properties of thin film (2D) perovskites have been studied since the late 80s,83, 84 whereas the latest developments in the size-controlled synthesis of perovskite NCs have enabled detailed investigations of quantum confinement effects in NCs.25, 29, 46, 50, 85, 86, 87 Sichert et al.25 showed that the perovskite crystal structure can be progressively reduced in a lateral dimension, yielding quasi-2D and ultimately 2D nanoplatelets with a thickness of only a single unit cell (Figure 6a). By varying the ratio of two organic cations, the commonly used MA and the significantly longer octylammonium, the thickness of platelets could be controlled, resulting in pronounced quantum size effects in MAPbBr3 nanoplatelets (Figures 6a and b)25 with an integer amount of layers varying from 1 to 7 (Figure 6b). The authors corroborated their experimental findings with quantum-mechanical calculations, showing good agreement with the measured excitonic transition energy of nanoplatelets. It was found out that for these highly confined 2D systems, the exciton binding energy reaches several hundreds of meV, similar to values for bulk layered perovskites.43, 88, 89 Thus, it is possible to tune the optical properties of perovskite nanoplatelets by controlling the number of layers. Similar effects have been also reported by other groups.32, 46, 51, 59 For instance, Snaith and colleagues90 used the same approach to demonstrate tunable PL from MAPbI3 NCs. The increase of octylamine content in the reaction medium resulted similarly in the formation of nanoplatelets.

Friend and colleagues85 observed size-dependent photon emission from MAPbBr3 NCs embedded in an organic matrix, where the NC size and thus their PL peak could be tuned by varying the concentration of the precursors. The PL peak gradually shifted to higher energies with decreasing particle size. Hassan et al.86 showed tunable PL from lead halide perovskite NCs by varying the number of layers, starting from pre-synthesized NC seeds by a controlled addition of alkyl ammonium halide. As shown in Figure 7a, the resulting MAPbI3 NCs emit green, orange and red colors for 1, 2 and 3 layers, respectively. Sapori et al.87 performed a theoretical study on the quantum confinement in 2D perovskite NCs composed of different cations and halide ions. Their results, showing an increase of the band gap as the number of layers in the nanoplatelets decreases, are in good agreement with the experimental results discussed above (Figure 7b).

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