Self-organizing high-density single-walled carbon nanotube arrays from surfactant suspensions

Very thin films of oriented and densely packed single-walled carbon nanotubes (SWNTs) can be self-assembled on substrates from surfactant sodium dodecyl sulfate (SDS-) coated SWNT suspensions at ambient conditions. The evaporation of water causes a concentration of the SDS-coated nanotubes above critical micelle concentrations for SDS, and it is believed that self-organization of the SDS molecules serves as a driving force for the oriented and dense assembly of the nanotubes. The high degree of alignment in the SWNT thin films was characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM) and polarized Raman spectroscopy.link_to_subscribed_fulltex

Conductivity of single-walled carbon nanotubes probed by THz time-domain spectroscopy

THz time-domain spectroscopy is applied to probe the transient conductivity single-walled carbon nanotubes (SWNTs). The ultrafast transport properties and carrier dynamics are examined for different levels of laser excitation. © 2005 Optical Society of America

Relationships between Compositional Heterogeneity and Electronic Spectra of (Ga_1–<i>x</i>Zn_<i>x</i>)(N_1–<i>x</i>O_<i>x</i>) Nanocrystals Revealed by Valence Electron Energy Loss Spectroscopy

Many ternary and quaternary semiconductors have been made in nanocrystalline forms for a variety of applications, but we have little understanding of how well their ensemble properties reflect the properties of individual nanocrystals. We examine electronic structure heterogeneities in nanocrystals of (Ga1–xZnx)(N1–xOx), a semiconductor that splits water under visible illumination. We use valence electron energy loss spectroscopy (VEELS) in a scanning transmission electron microscope to map out electronic spectra of (Ga1–xZnx)(N1–xOx) nanocrystals with a spatial resolution of 8 nm. We examine three samples with varying degrees of intraparticle and interparticle compositional heterogeneity and ensemble optical spectra that range from a single band gap in the visible to two band gaps, one in the visible and one in the UV. The VEELS spectra resemble the ensemble absorption spectra for a sample with a homogeneous elemental distribution and a single band gap and, more interestingly, one with intraparticle compositional heterogeneity and two band gaps. We observe spatial variation in VEELS spectra only with significant interparticle compositional heterogeneity. Hence, we reveal the conditions under which the ensemble spectra reveal the optical properties of individual (Ga1–xZnx)(N1–xOx) particles. More broadly, we illustrate how VEELS can be used to probe electronic heterogeneities in compositionally complex nanoscale semiconductors

Control of Elemental Distribution in the Nanoscale Solid-State Reaction That Produces (Ga_1–<i>x</i>Zn_<i>x</i>)(N_1–<i>x</i>O_<i>x</i>) Nanocrystals

Solid-state chemical transformations at the nanoscale can be a powerful tool for achieving compositional complexity in nanomaterials. It is desirable to understand the mechanisms of such reactions and characterize the local-level composition of the resulting materials. Here, we examine how reaction temperature controls the elemental distribution in (Ga_1–<i>x</i>Zn_<i>x</i>)­(N_1–<i>x</i>O_<i>x</i>) nanocrystals (NCs) synthesized <i>via</i> the solid-state nitridation of a mixture of nanoscale ZnO and ZnGa₂O₄ NCs. (Ga_1–<i>x</i>Zn_<i>x</i>)­(N_1–<i>x</i>O_<i>x</i>) is a visible-light absorbing semiconductor that is of interest for applications in solar photochemistry. We couple elemental mapping using energy-dispersive X-ray spectroscopy in a scanning transmission electron microscope (STEM-EDS) with colocation analysis to study the elemental distribution and the degree of homogeneity in the (Ga_1–<i>x</i>Zn_<i>x</i>)­(N_1–<i>x</i>O_<i>x</i>) samples synthesized at temperatures ranging from 650 to 900 °C with varying ensemble compositions (<i>i.e.</i>, <i>x</i> values). Over this range of temperatures, the elemental distribution ranges from highly heterogeneous at 650 °C, consisting of a mixture of larger particles with Ga and N enrichment near the surface and very small NCs, to uniform particles with evenly distributed constituent elements for most compositions at 800 °C and above. We propose a mechanism for the formation of the (Ga_1–<i>x</i>Zn_<i>x</i>)­(N_1–<i>x</i>O_<i>x</i>) NCs in the solid state that involves phase transformation of cubic spinel ZnGa₂O₄ to wurtzite (Ga_1–<i>x</i>Zn_<i>x</i>)­(N_1–<i>x</i>O_<i>x</i>) and diffusion of the elements along with nitrogen incorporation. The temperature-dependence of nitrogen incorporation, bulk diffusion, and vacancy-assisted diffusion processes determines the elemental distribution at each synthesis temperature. Finally, we discuss how the visible band gap of (Ga_1–<i>x</i>Zn_<i>x</i>)­(N_1–<i>x</i>O_<i>x</i>) NCs varies with composition and elemental distribution