Liliya Nikolova

Multiple NaNbO3/Nb2O5 Heterostructure Nanotubes: A New Class of Ferroelectric/Semiconductor Nanomaterials

Adv. Mater. 2010, 22, 1741–1745 2010 WILEY-VCH Verlag G Multiple hetero-nanostructures are a promising new class of materials due to their multifunctional character and the possibility of effectively coupling different properties. Hetero-nanostructure systems, while more challenging to synthesize, often offer significant advantages over singlecomponent systems. For example, Si/SiGe-superlattice nanowires exhibit enhanced thermoelectric performance and high electric conductivity relative to Si nanowires. The coupling between ferroelectric and magnetic order parameters in a nanostructured BaTiO3/CoFe2O4 ferroelectromagnet facilitates the interconversion of energies stored in electric and magnetic fields. However, fabrication of hetero-nanostructures, especially multiple heterostructures, from distinct materials is challenging due to the lattice mismatch and the difficulty in balancing two dissimilar growth processes with different microscopic mechanisms and reaction rates. Here we focus on multiple heterostructures of ferroelectric/ semiconductor nanostructures due to their potential for multifunctional device applications. Interfacing ferroelectric and semiconductor materials can lead to novel functionalities that are independent of the individual components and may be tailored to fit a specific application, although realization attempts were so far unsuccessful. To achieve practical devices, a number of hurdles need to be overcome, including the creation of multiple heterojunctions obtaining different combinations of materials and exploring their potential use in devices. Here, we demonstrate major advances on these fronts: multiple NaNbO3 nanoplates are created inside hollow Nb2O5 nanotubes, forming a novel class of multiple ferroelectric (NaNbO3)/semiconductor (Nb2O5) heterostructures (bandgap 3.4 eV). Specifically, we found that the NaNbO3 nanoplates within heterostructure nanotubes exhibit distinct ferroelectric switching due to the presence of the tensile stress at the interface between NaNbO3 and Nb2O5 nanostructures, which facilitates the ferroelectric phase. The semiconductive properties of the host Nb2O5 nanotubes allow for switching of electrical conductivity by an external electric field, even though ferroelectric NaNbO3 nanoplates are attached to the inner surface of nanotubes, showing great potential for the fabrication of memory and other multifunctional ferroelectric/semiconductor devices. The scientific and technological interest in tubular nanostructures such as silicon, carbon, and other inorganic nanotubes stem from their fascinating electronic, mechanical, and chemical properties and their ability to serve as templates for confined growth within the nanotube’s hollow. Herein, we employed a nanotube-confined growth strategy for the one-step synthesis of multiple NaNbO3/Nb2O5 heterostructure nanotubes, which were prepared by a thermolysis of Nb peroxo complex precursors ([Nb(O2)4] 3 ) combined with (slow) fluoride ion corrosion of Nb foil at 180 8C. The formation process is schematically displayed in Figure 1a. Nb2O5 nanotubes are first formed under hydrothermal conditions mostly at stage 1. In stage 2, the nucleation of the NaNbO3 phase preferentially takes place on the inner surface of the Nb2O5 nanotubes to form NaNbO3 nanocrystallites. Finally, in stage 3 the nanocrystallites of NaNbO3 evolve into nanoplates inside the Nb2O5 nanotubes. As shown in a low-magnification scanning electron microscopy (SEM) image (Fig. 1b), the length of the as-prepared multipleheterostructure nanotube is up to several tens of micrometers with a diameter in the range of 300–600 nm. The structural characteristics of the as-prepared sample are clearly visible in the transmission electron microscopy (TEM) images in Figure 1c, which show a linear arrangement of NaNbO3 nanoplates of about

Approaches for ultrafast imaging of transient materials processes in the transmission electron microscope.

The growing field of ultrafast materials science, aimed at exploring short-lived transient processes in materials on the microsecond to femtosecond timescales, has spawned the development of time-resolved, in situ techniques in electron microscopy capable of capturing these events. This article gives a brief overview of two principal approaches that have emerged in the past decade: the stroboscopic ultrafast electron microscope and the nanosecond-time-resolved single-shot instrument. The high time resolution is garnered through the use of advanced pulsed laser systems and a pump-probe experimental platforms using laser-driven photoemission processes to generate time-correlated electron probe pulses synchronized with laser-driven events in the specimen. Each technique has its advantages and limitations and thus is complementary in terms of the materials systems and processes that they can investigate. The stroboscopic approach can achieve atomic resolution and sub-picosecond time resolution for capturing transient events, though it is limited to highly repeatable (>10(6) cycles) materials processes, e.g., optically driven electronic phase transitions that must reset to the materials ground state within the repetition rate of the femtosecond laser. The single-shot approach can explore irreversible events in materials, but the spatial resolution is limited by electron source brightness and electron-electron interactions at nanosecond temporal resolutions and higher. The first part of the article will explain basic operating principles of the stroboscopic approach and briefly review recent applications of this technique. As the authors have pursued the development of the single-shot approach, the latter part of the review discusses its instrumentation design in detail and presents examples of materials science studies and the near-term instrumentation developments of this technique.

Nanocrystallization of amorphous germanium films observed with nanosecond temporal resolution

Using dynamic transmission electron microscopy we measure nucleation and growth rates during laser driven crystallization of amorphous germanium (a-Ge) films supported by silicon monoxide membranes. The films were crystallized using single 532 nm laser pulses at a fluence of ∼128 mJ cm−2. Devitrification processes initiate less than 20 ns after excitation and are complete within ∼55 ns. The nucleation rate was estimated by tracking crystallite density as a function of time and reached a maximum of ∼1.6×1022 nuclei/cm3 s. This study provides information on nanocrystallization phenomena in a-Ge, which is important for the implementation of nanostructured group IV semiconductors in optoelectronics devices.

Single-crystalline BiFeO3 nanowires and their ferroelectric behavior

We report the ferroelectric properties of single-crystalline BiFeO3 nanowires using piezoresponse force microscopy (PFM). The nanowires, synthesized by a hydrothermal approach, have a rhombohedral perovskite structure and a preferential growth of the (211) crystallographic plane perpendicular to the wire axis, as revealed by x-ray and electron diffraction investigations. PFM measurements reveal that the as-synthesized BiFeO3 nanowires, down to 40 nm in diameter, have components of spontaneous polarization along both the axial and radial directions, thereby demonstrating the ferroelectric nature of the wires. The results indicate that such ferroelectric BiFeO3 nanowires should provide promising opportunity for nanoscale nonvolatile memory devices.

In situ laser crystallization of amorphous silicon: Controlled nanosecond studies in the dynamic transmission electron microscope

We describe an in situ method for studying the influence of deposited laser energy on microstructural evolution during nanosecond laser driven crystallization of amorphous Si. By monitoring microstructural evolution as a function of deposited energy in a dynamic transmission electron microscope (DTEM), information on grain size and defect concentration can be correlated directly with processing conditions. This work demonstrates that DTEM studies are a promising approach for obtaining fundamental information on nucleation and growth processes that have technological importance for the development of thin film transistors.

In situ investigation of explosive crystallization in a-Ge: Experimental determination of the interface response function using dynamic transmission electron microscopy

The crystallization of amorphous semiconductors is a strongly exothermic process. Once initiated the release of latent heat can be sufficient to drive a self-sustaining crystallization front through the material in a manner that has been described as explosive. Here, we perform a quantitative in situ study of explosive crystallization in amorphous germanium using dynamic transmission electron microscopy. Direct observations of the speed of the explosive crystallization front as it evolves along a laser-imprinted temperature gradient are used to experimentally determine the complete interface response function (i.e., the temperature-dependent front propagation speed) for this process, which reaches a peak of 16 m/s. Fitting to the Frenkel-Wilson kinetic law demonstrates that the diffusivity of the material locally/immediately in advance of the explosive crystallization front is inconsistent with those of a liquid phase. This result suggests a modification to the liquid-mediated mechanism commonly used to describe this process that replaces the phase change at the leading amorphous-liquid interface with a change in bonding character (from covalent to metallic) occurring in the hot amorphous material.