Research Article: Kinetically Controlled Fabrication of Single‐Crystalline TiO2 Nanobrush Architectures with High Energy {001} Facets

Date Published: April 05, 2017

Publisher: John Wiley and Sons Inc.

Author(s): Lisha Fan, Xiang Gao, Dongkyu Lee, Er‐Jia Guo, Shinbuhm Lee, Paul C. Snijders, Thomas Z. Ward, Gyula Eres, Matthew F. Chisholm, Ho Nyung Lee.

http://doi.org/10.1002/advs.201700045

Abstract

This study demonstrates that precise control of nonequilibrium growth conditions during pulsed laser deposition (PLD) can be exploited to produce single‐crystalline anatase TiO2 nanobrush architectures with large surface areas terminated with high energy {001} facets. The data indicate that the key to nanobrush formation is controlling the atomic surface transport processes to balance defect aggregation and surface‐smoothing processes. High‐resolution scanning transmission electron microscopy data reveal that defect‐mediated aggregation is the key to TiO2 nanobrush formation. The large concentration of defects present at the intersection of domain boundaries promotes aggregation of PLD growth species, resulting in the growth of the single‐crystalline nanobrush architecture. This study proposes a model for the relationship between defect creation and growth mode in nonequilibrium environments, which enables application of this growth method to novel nanostructure design in a broad range of materials.

Partial Text

1D nanostructures, including tubes, rods, and wires, provide fascinating structure‐dependent properties and a high potential for nanotechnologies and related applications in electronics, information storage, optoelectronics, electrochemistry, and electromechanical devices.1, 2, 3 Due to the structural perfection offered in single crystals, the ability to fabricate single crystalline 1D nanostructures is highly advantageous for developing high performance nanomaterials and devices. Pulsed laser deposition (PLD) is a widely known nonequilibrium growth method for growing epitaxial oxide thin films and superlattices.4, 5 The growth of atomically flat epitaxial thin films by PLD is kinetically governed by a multitude of atomistic processes, including surface diffusion, nucleation, atom attachment/detachment, and interlayer mass transport.6 On the other hand, many growth instabilities, including uncontrolled aggregation, clustering, and step bunching, compete with surface smoothing processes necessary for ideal 2D growth, potentially leading to a rough growth front or inclusion of growth defects.7, 8 Motivated by the quest for atomically precise heteroepitaxy, extensive efforts have been made on suppressing such growth instabilities. These efforts have revealed the important role of the adatom mobility for 2D epitaxial growth, as determined by the substrate temperature and the reactive gas pressure during growth by PLD.6 The incident kinetic energy of the growth species in the laser plume can also be tuned by various means, including laser fluence, substrate‐to‐target distance, and spot size.9, 10

The plan view scanning electron microscopy (SEM) images of films in Figure1 illustrate the growth temperature and oxygen background pressure parameter space covered in this work. The growth temperature governs the surface diffusivity of the arriving growth species before they are incorporated into the lattice, while the background pressure modulates the kinetic energy of the growth species and the formation of clusters in the plume. The most porous nanostructure is observed in a sample grown at 500 °C in 100 mTorr O2, revealing a spatially separated roof‐like surface morphology (≈200 nm in lateral size) consisting of triangular lamellae that are stacked along the <110> directions. Film densification occurs at a higher substrate temperature of 600 °C or a lower p(O2) of 50 mTorr. The breakdown of the compact 2D epitaxy that can be seen from the sample grown at 500 °C in 100 mTorr is attributed to the reduced adatom mobility and resultant aggregation that prevails when surface diffusion is suppressed. However, further reduction of the temperature or an increase in oxygen pressure resulted in an undesired cauliflower‐like surface morphology.

Combining the growth map in Figure 1 and the STEM observations in Figure 4, it appears that the highly defected areas at the crossing points of the stacking faults serve as effective nucleation sites for the formation of 1D nanostructures at growth conditions with a reduced adatom mobility and suppressed surface diffusion that is locally exacerbated by the defected regions.44 Apparently, the defective domain boundaries developed from the (101)‐type stacking faults initially act as surface transport barriers between the root and the surrounding matrix, resulting in dense roots that remain separated from the matrix by the domain boundaries. The root then grows laterally and vertically within mutually inclined (101)‐oriented domain boundaries, leading to the observed conical structure. This is consistent with cross‐sectional SEM images of the nanobrush film (Figure S2, Supporting Information) that shows cleavage of the lower part of the film mostly occurs at the root edges, indicating a relatively weaker bond across the domain boundaries between the root and the film matrix. The density of the roots and the inclined angle of the (101) domain boundaries lead to two adjacent roots eventually meeting at around 280 nm thickness. An anisotropic growth governed by the geometric shadowing effect eventually leads to the formation of columnar structures.

A novel PLD synthesis strategy for 1D nanostructure fabrication of single crystalline anatase TiO2 was developed by controlling the surface mobility of the growth species to generate defects and growth instabilities. The nanobrush film features a solid root emanating from crossing points of domain boundaries where a high concentration of defects is located. The surface of the crystalline bristles consists of a high density of the {001} facets, suggesting promising applications where a high chemical activity is desired. The microstructural evolution suggests that the nanobrush formation results from a competition between nonequilibrium aggregation and equilibrating surface diffusion processes. This competition can be altered by controlling the mobility of the growth species through the substrate temperature and oxygen pressure. Thus, this work provides new insight into controllable synthesis of 1D structures, highlighting the role of defect creation in nonequilibrium synthesis. We believe that the comprehensive understanding of the growth mechanism of anatase TiO2 nanobrush formation makes this growth strategy attractive for a broad range of materials and architectures.

Anatase‐phase TiO2 films were grown on TiO2‐terminated (001) STO substrates by PLD. The STO substrates were chemically etched by buffered HF for 30 s and then thermally treated at 1100 °C in air for 1.5 h. TiO2 nanobrush films with a thickness of 1.5 µm were deposited with a KrF excimer laser using a TiO2 polycrystalline target. In order to check the viability of growing single crystalline nanoarchitectures by PLD, various growth conditions were explored. In order to control the surface diffusion of adatoms, the substrate temperature (T = 400–600 °C) and oxygen background pressure (p(O2) = 50–200 mTorr) were systematically varied. We grew TiO2 films with a fixed number of laser pulses (50 000 pulses), and the typical deposition rate was 0.3 Å per laser pulse. For structural characterization, XRD was carried out using a high‐resolution X‐ray diffractometer with Cu Kα1 radiation. The plan view morphology of samples was characterized by SEM. High‐angle annular dark‐field (HAADF) imaging was carried out in Na ion UltraSTEM200 operated at 200 kV. The microscope is equipped with a cold field‐emission gun and an aberration corrector for sub‐angstrom resolution. Inner/outer angles of 70/240 mrad were used for HAADF imaging. The convergence semi‐angle for electron probe was set to 30 mrad.

 

Source:

http://doi.org/10.1002/advs.201700045

 

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