We elucidate the crystalline nature and the three-dimensional orientation of isolated organic nanocrystals embedded in a sol-gel matrix, using a polarized nonlinear microscopy technique that combines two-photon fluorescence and second harmonic generation. This technique allows the distinction between mono-crystalline structures and nano-scale poly-crystalline aggregates responsible for incoherent second harmonic signals.PACS numbers: 78.67. Bf, 61.82.Rx The optical properties of nanoparticles have recently attracted much attention. In addition to metallic and semiconductor nanoparticles which are now used as biomarkers and as the building blocks of nanostructured materials [1], their organic counterparts constitute an interesting alternative. Advances in molecular engineering have enabled the design of molecular structures of various resonances and symmetries with optimized one-and two-photon absorption cross sections [2], or combining different optical properties such as luminescence and second harmonic generation (SHG) [3]. In addition, macroscopic molecular arrangements have been optimized using the tensorial oriented gas model [4], which predicts that an enhancement of the SHG efficiency is expected from the non-centrosymmetric crystalline arrangement of efficient nonlinear molecules. Molecular nanocrystals can be therefore envisioned as a new class of multi-functional nano-scale materials. In the case of organic nanocrystals however, the traditional crystalline characterization techniques have raised many practical barriers due to their low concentration and fragility. Consequently, the elucidation of their crystalline nature has been so far indirect or averaged over a large number of nanocrystals [5].In this letter, we show that two-photon nonlinear microscopy permits in-situ characterization of isolated organic nanocrystals grown in an amorphous sol-gel matrix. The diagnostic is based on polarization resolved two-photon excited fluorescence (TPF) and SHG. TPF is an incoherent process allowed in centrosymmetric media, which exhibits a specific anisotropy depending on the medium symmetry. On the other hand, SHG is the signature of a crystalline non-centrosymmetric phase in the sample, with a sensitivity down to the nanometric scale [6,7]. We show that the polarization analysis of both TPF and SHG from nanocrystals allows the unambiguous discrimination between isolated mono-crystalline and poly-crystalline systems. Moreover, once a nanocrystal has been identified as mono-crystalline, a detailed model for both TPF and SHG polarization responses accounting for the unit-cell symmetry allows the determination of its three-dimensional orientation within the host matrix.The organic nanocrystals that we investigate are based upon the α-((4'-methoxyphenyl)methylene)-4-nitro-benzene-acetonitrile) molecule (CMONS), which exhibits efficient luminescence and quadratic nonlinearity under two-photon excitation [8,9,10]. The bulk crystalline phases of such crystals have three possible polymorphic forms, two being noncen...
The effects of the structural morphology of the ZnO thin seed layer composed of nanoparticles grown by dip coating have been investigated on the structural properties of ZnO nanowires grown by chemical bath deposition. It is revealed by scanning electron microscopy that the growth of ZnO nanowires is limited by the mass transport of chemical precursors in solution, leading to the inverse relationship of their average diameter and length with their density. It is shown by transmission electron microscopy and X-ray diffraction measurements that ZnO nanowires epitaxially grow on the seed layer and preferentially nucleate on the free surface of ZnO nanoparticles. The vertical alignment of ZnO nanowires as quantitatively deduced by X-ray pole figures is found to be improved by strengthening the texture of the seed layer along the c axis. Similarly, their density increases, showing that the c polar plane is highly reactive chemically and presents preferential surface nucleation sites. The relationship between the average diameters of ZnO nanoparticles and nanowires is completely driven by the nature of the nucleation site that is strongly dependent upon the growth conditions and upon the structural morphology of the seed layer. The texture, roughness, and porosity of the seed layer are three critical parameters.
Controlling the polarity of ZnO nanowires in addition to the uniformity of their structural morphology in terms of position, vertical alignment, length, diameter, and period is still a technological and fundamental challenge for real-world device integration. In order to tackle this issue, we specifically combine the selective area growth on prepatterned polar c-plane ZnO single crystals using electron-beam lithography, with the chemical bath deposition. The formation of ZnO nanowires with a highly controlled structural morphology and a high optical quality is demonstrated over large surface areas on both polar c-plane ZnO single crystals. Importantly, the polarity of ZnO nanowires can be switched from O- to Zn-polar, depending on the polarity of prepatterned ZnO single crystals. This indicates that no fundamental limitations prevent ZnO nanowires from being O- or Zn-polar. In contrast to their catalyst-free growth by vapor-phase deposition techniques, the possibility to control the polarity of ZnO nanowires grown in solution is remarkable, further showing the strong interest in the chemical bath deposition and hydrothermal techniques. The single O- and Zn-polar ZnO nanowires additionally exhibit distinctive cathodoluminescence spectra. To a broader extent, these findings open the way to the ultimate fabrication of well-organized heterostructures made from ZnO nanowires, which can act as building blocks in a large number of electronic, optoelectronic, and photovoltaic devices.
A statistical analysis of the electrical properties of selective area grown O- and Zn-polar ZnO nanorods by chemical bath deposition is performed by four-point probe resistivity measurements in patterned metal contact and multiprobe scanning tunneling microscopy configurations. We show that ZnO nanorods with either polarity exhibit a bulklike electrical conduction in their core and are highly conductive. O-polar ZnO nanorods with a smaller mean electrical conductivity have a nonmetallic or metallic electrical conduction, depending on the nano-object considered, while the vast majority of Zn-polar ZnO nanorods with a larger mean electrical conductivity present a metallic electrical conduction. We reveal, from Raman scattering and spatially resolved 5 K cathodoluminescence measurements, that the resulting high carrier density of ZnO nanorods with O or Zn polarity is due to the massive incorporation of hydrogen in the form of interstitial hydrogen in bond-centered sites (HBC), substitutional hydrogen on the oxygen lattice site (HO), and multiple O–H bonds in a zinc vacancy (VZn–H n ). While HBC is largely incorporated in ZnO nanorods with either polarity, HO and (VZn–H n ) defect complexes appear as the dominant hydrogen-related species in O- and Zn-polar ZnO nanorods, respectively. These findings reveal that polarity greatly affects the electrical and optical properties of ZnO nanorods. They further cast a light on the dominant role of hydrogen when ZnO nanorods are grown by the widely used chemical bath deposition technique. This work should be considered for any strategy for thoroughly controlling their physical properties as a prerequisite for their efficient integration into nanoscale engineering devices.
The elucidation of the fundamental processes in aqueous solution during the chemical bath deposition of ZnO nanowires (NWs) using zinc nitrate and hexamethylenetetramine is of great significance: however, their extrinsic doping by foreign elements for monitoring their optical and electrical properties is still challenging. By combining thermodynamic simulations yielding theoretical solubility plots and speciation diagrams with in situ pH measurements and structural, chemical, and optical analyses, we report an in-depth understanding of the pH effects on the formation and aluminum doping mechanisms of ZnO NWs. By the addition of aluminum nitrate with a given relative concentration for the doping and of ammonia over a broad range of concentrations, the pH is shown to strongly influence the shape, diameter, length, and doping magnitude of ZnO NWs. Tuning the dimensions of ZnO NWs by inhibition of their radial growth only proceeds over a specific pH range, where negatively charged Al(OH) complexes are predominantly formed and act as capping agents by electrostatically interacting with the positively charged m-plane sidewalls. These complexes further favor the aluminum incorporation and doping of ZnO NWs, which only operate over the same pH range following thermal annealing above 200 °C. These findings reporting a full chemical synthesis diagram reveal the significance of carefully selecting and following the pH to control the morphology of ZnO NWs as well as to achieve their thermally activated extrinsic doping, as required for many nanoscale engineering devices.
ZnO nanowires grown by chemical bath deposition (CBD) are of high interest, but their doping with extrinsic elements including gallium in aqueous solution is still challenging despite its primary importance for transparent electrodes and electronics, as well as mid-infrared plasmonics. We elucidate the formation mechanisms of ZnO nanowires by CBD using zinc nitrate and hexamethylenetetramine as standard chemical precursors, as well as gallium nitrate and ammonia as chemical additives. A complete growth diagram, revealing the effects of both the relative concentration of gallium nitrate and pH, is gained by combining a thorough experimental approach with thermodynamic computations yielding theoretical solubility plots as well as Zn(II) and Ga(III) species. The role of Ga(OH)4complexes is specifically shown as capping agents on the m-plane sidewalls of ZnO nanowires, enhancing their development and hence decreasing their aspect ratio. Additionally, the gallium incorporation into ZnO nanowires is investigated in details by chemical analyses and Raman scattering. They show the predominant formation of gallium substituting for zinc atoms (GaZn) in as-grown ZnO nanowires and their partial conversion into GaZn-VZn complexes after post-deposition annealing under oxygen atmosphere. The conversion is further related to a significant relaxation of the strain level in 2 ZnO nanowires. These findings reporting the physico-chemical processes at work during the formation of ZnO nanowires and the related gallium incorporation mechanisms offer a general strategy for their extrinsic doping and open the way for carefully controlling their physical properties as required for nanoscale engineering devices.
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