Patterning Oxide Nanopillars at the Atomic Scale by Phase Transformation

Artificially controlled nanoscale chemical reduction in VO2 through electron beam illumination

Chemical reduction in oxides plays a crucial role in engineering the material properties through structural transformation and electron filling. Controlling the reduction at nanoscale forms a promising pathway to harvest functionalities, which however is of great challenge for conventional methods (e.g., thermal treatment and chemical reaction). Here, we demonstrate a convenient pathway to achieve nanoscale chemical reduction for vanadium dioxide through the electron-beam illumination. The electron beam induces both surface oxygen desorption through radiolytic process and positively charged background through secondary electrons, which contribute cooperatively to facilitate the vacancy migration from the surface toward the sample bulk. Consequently, the VO₂ transforms into a reduced V₂O₃ phase, which is associated with a distinct insulator to metal transition at room temperature. Furthermore, this process shows an interesting facet-dependence with the pronounced transformation observed for the c-facet VO₂ as compared with the a-facet, which is attributed to the intrinsically different oxygen vacancy formation energy between these facets. Remarkably, we readily achieve a lateral resolution of tens nanometer for the controlled structural transformation with a commercial scanning electron microscope. This work provides a feasible strategy to manipulate the nanoscale chemical reduction in complex oxides for exploiting functionalities.

In situ generation of sub-10 nm silver nanowires under electron beam irradiation in a TEM

Here, we report a facile irradiation-assisted route to fabricate sub-10 nm Ag nanowires from oxide supports using a TEM. The obtained Ag nanowires show a tunable length/diameter aspect ratio with a minimum diameter of about 9.5 nm. Moreover, the nucleation and growth dynamics of Ag nanowires were uncovered from TEM observations.

Ferroelectric Oxide Thin Film with an Out-of-Plane Electrical Conductivity

Ferroelectricity and electrical conductivity are two fundamentally incompatible properties that are difficult to simultaneously achieve in a material. Here, we combine these two contradictory properties by embedding conducting SrNbO₃ micro/nanopillars into a ferroelectric SrNbO_3.5 (i.e., Sr₂Nb₂O₇) thin film. The high-T_c ferroelectric SrNbO_3.5 thin film is epitaxially grown on a LaAlO₃ substrate by pulsed laser deposition. The conducting SrNbO₃ micro/nanopillars are introduced into the film via an electron-irradiation-induced SrNbO_3.5-to-SrNbO₃ phase transformation triggered by a focused electron beam. The sizes and distribution of the SrNbO₃ micro/nanopillars can be accurately controlled through artificial manipulation of the electron-irradiation-induced SrNbO_3.5-to-SrNbO₃ phase transformation. The ferroelectric SrNbO₃/SrNbO_3.5 thin film with an in-plane polarization exhibits an electrical conductivity in the out-of-plane direction. Such conducting ferroelectric thin films may lead to the discovery of plentiful physical phenomena and have great potential for pyroelectric, photoelectric, and multiferroic applications.

Sculpting Nanoscale Functional Channels in Complex Oxides Using Energetic Ions and Electrons

The formation of metastable phases has attracted significant attention because of their unique properties and potential functionalities. In the present study, we demonstrate the phase conversion of energetic-ion-induced amorphous nanochannels/tracks into a metastable defect fluorite in A₂B₂O₇ structured complex oxides by electron irradiation. Through in situ electron irradiation experiments in a scanning transmission electron microscope, we observe electron-induced epitaxial crystallization of the amorphous nanochannels in Yb₂Ti₂O₇ into the defect fluorite. This energetic-electron-induced phase transformation is attributed to the coupled effect of ionization-induced electronic excitations and local heating, along with subthreshold elastic energy transfers. We also show the role of ionic radii of A-site cations (A = Yb, Gd, and Sm) and B-site cations (Ti and Zr) in facilitating the electron-beam-induced crystallization of the amorphous phase to the defect-fluorite structure. The formation of the defect-fluorite structure is eased by the decrease in the difference between ionic radii of A- and B-site cations in the lattice. Molecular dynamics simulations of thermal annealing of the amorphous phase nanochannels in A₂B₂O₇ draw parallels to the electron-irradiation-induced crystallization and confirm the role of ionic radii in lowering the barrier for crystallization. These results suggest that employing guided electron irradiation with atomic precision is a useful technique for selected area phase formation in nanoscale printed devices.

Atomic-Scale Origin of the Quasi-One-Dimensional Metallic Conductivity in Strontium Niobates with Perovskite-Related Layered Structures

The quasi-one-dimensional (1D) metallic conductivity of the perovskite-related Sr_nNb_nO_3n+2 compounds is of continuing fundamental physical interest as well as being important for developing advanced electronic devices. The Sr_nNb_nO_3n+2 compounds can be derived by introducing additional oxygen into the SrNbO₃ perovskite. However, the physical origin for the transition of electrical properties from the three-dimensional (3D) isotropic conductivity in SrNbO₃ to the quasi-1D metallic conductivity in Sr_nNb_nO_3n+2 requires more in-depth clarification. Here we combine advanced transmission electron microscopy with atomistic first-principles calculations to unambiguously determine the atomic and electronic structures of the Sr_nNb_nO_3n+2 compounds and reveal the underlying mechanism for their quasi-1D metallic conductivity. We demonstrate that the local electrical conductivity in the Sr_nNb_nO_3n+2 compounds directly depends on the configuration of the NbO₆ octahedra in local regions. These findings will shed light on the realization of two-dimensional (2D) electrical conductivity from a bulk material, namely by segmenting a 3D conductor into a stack of 2D conducting thin layers.

Directing Matter: Toward Atomic-Scale 3D Nanofabrication

Enabling memristive, neuromorphic, and quantum-based computing as well as efficient mainstream energy storage and conversion technologies requires the next generation of materials customized at the atomic scale. This requires full control of atomic arrangement and bonding in three dimensions. The last two decades witnessed substantial industrial, academic, and government research efforts directed toward this goal through various lithographies and scanning-probe-based methods. These technologies emphasize 2D surface structures, with some limited 3D capability. Recently, a range of focused electron- and ion-based methods have demonstrated compelling alternative pathways to achieving atomically precise manufacturing of 3D structures in solids, liquids, and at interfaces. Electron and ion microscopies offer a platform that can simultaneously observe dynamic and static structures at the nano- and atomic scales and also induce structural rearrangements and chemical transformation. The addition of predictive modeling or rapid image analytics and feedback enables guiding these in a controlled manner. Here, we review the recent results that used focused electron and ion beams to create free-standing nanoscale 3D structures, radiolysis, and the fabrication potential with liquid precursors, epitaxial crystallization of amorphous oxides with atomic layer precision, as well as visualization and control of individual dopant motion within a 3D crystal lattice. These works lay the foundation for approaches to directing nanoscale level architectures and offer a potential roadmap to full 3D atomic control in materials. In this paper, we lay out the gaps that currently constrain the processing range of these platforms, reflect on indirect requirements, such as the integration of large-scale data analysis with theory, and discuss future prospects of these technologies.