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Concurrent bandgap narrowing and polarization enhancement in epitaxial ferroelectric nanofilms

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ABSTRACT

Perovskite-type ferroelectric (FE) crystals are wide bandgap materials with technologically valuable optical and photoelectric properties. Here, versatile engineering of electronic transitions is demonstrated in FE nanofilms of KTaO3, KNbO3 (KNO), and NaNbO3 (NNO) with a thickness of 10–30 unit cells. Control of the bandgap is achieved using heteroepitaxial growth of new structural phases on SrTiO3 (001) substrates. Compared to bulk crystals, anomalous bandgap narrowing is obtained in the FE state of KNO and NNO films. This effect opposes polarization-induced bandgap widening, which is typically found for FE materials. Transmission electron microscopy and spectroscopic ellipsometry measurements indicate that the formation of higher-symmetry structural phases of KNO and NNO produces the desirable red shift of the absorption spectrum towards visible light, while simultaneously stabilizing robust FE order. Tuning of optical properties in FE films is of interest for nanoscale photonic and optoelectronic devices.

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HRTEM images and lattice strains of epitaxial KTO, KNO, and NNO nanofilms on STO (001). (a)–(c) Cross-sectional images along the [010] zone-axis. SAED patterns are shown in the insets. (d)–(f) (200) Fourier filtered images corresponding to the HRTEM images of (a)–(c). (g)–(i) In-plane strain (∊xx) and (j)–(l) out-of-plane strain (∊yy) maps based on the HRTEM images of (a)–(c). The STO substrate lattice was used as a reference in the strain calculations. The white dashed lines in (g)–(l) indicate the interfaces between the layers.
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Figure 2: HRTEM images and lattice strains of epitaxial KTO, KNO, and NNO nanofilms on STO (001). (a)–(c) Cross-sectional images along the [010] zone-axis. SAED patterns are shown in the insets. (d)–(f) (200) Fourier filtered images corresponding to the HRTEM images of (a)–(c). (g)–(i) In-plane strain (∊xx) and (j)–(l) out-of-plane strain (∊yy) maps based on the HRTEM images of (a)–(c). The STO substrate lattice was used as a reference in the strain calculations. The white dashed lines in (g)–(l) indicate the interfaces between the layers.

Mentions: HRTEM analyses confirm epitaxial growth of KTO, KNO, and NNO films with a thickness of 10–30 unit cells on top of STO (figure 2). Selected area electron diffraction (SAED) patterns evidence (001)[100]film//(001)[100]STO cube-on-cube epitaxy. Fourier filtered images do not show any misfit dislocations in KTO and NNO suggesting fully coherent growth of the KTO and NNO films on STO. Dislocations are not formed in KTO up to a film thickness of at least 10 nm. On the other hand, random misfit dislocations are detected in KNO, indicating local strain relaxation.


Concurrent bandgap narrowing and polarization enhancement in epitaxial ferroelectric nanofilms
HRTEM images and lattice strains of epitaxial KTO, KNO, and NNO nanofilms on STO (001). (a)–(c) Cross-sectional images along the [010] zone-axis. SAED patterns are shown in the insets. (d)–(f) (200) Fourier filtered images corresponding to the HRTEM images of (a)–(c). (g)–(i) In-plane strain (∊xx) and (j)–(l) out-of-plane strain (∊yy) maps based on the HRTEM images of (a)–(c). The STO substrate lattice was used as a reference in the strain calculations. The white dashed lines in (g)–(l) indicate the interfaces between the layers.
© Copyright Policy - open-access
Related In: Results  -  Collection

License 1 - License 2
Show All Figures
getmorefigures.php?uid=PMC5036465&req=5

Figure 2: HRTEM images and lattice strains of epitaxial KTO, KNO, and NNO nanofilms on STO (001). (a)–(c) Cross-sectional images along the [010] zone-axis. SAED patterns are shown in the insets. (d)–(f) (200) Fourier filtered images corresponding to the HRTEM images of (a)–(c). (g)–(i) In-plane strain (∊xx) and (j)–(l) out-of-plane strain (∊yy) maps based on the HRTEM images of (a)–(c). The STO substrate lattice was used as a reference in the strain calculations. The white dashed lines in (g)–(l) indicate the interfaces between the layers.
Mentions: HRTEM analyses confirm epitaxial growth of KTO, KNO, and NNO films with a thickness of 10–30 unit cells on top of STO (figure 2). Selected area electron diffraction (SAED) patterns evidence (001)[100]film//(001)[100]STO cube-on-cube epitaxy. Fourier filtered images do not show any misfit dislocations in KTO and NNO suggesting fully coherent growth of the KTO and NNO films on STO. Dislocations are not formed in KTO up to a film thickness of at least 10 nm. On the other hand, random misfit dislocations are detected in KNO, indicating local strain relaxation.

View Article: PubMed Central - PubMed

ABSTRACT

Perovskite-type ferroelectric (FE) crystals are wide bandgap materials with technologically valuable optical and photoelectric properties. Here, versatile engineering of electronic transitions is demonstrated in FE nanofilms of KTaO3, KNbO3 (KNO), and NaNbO3 (NNO) with a thickness of 10–30 unit cells. Control of the bandgap is achieved using heteroepitaxial growth of new structural phases on SrTiO3 (001) substrates. Compared to bulk crystals, anomalous bandgap narrowing is obtained in the FE state of KNO and NNO films. This effect opposes polarization-induced bandgap widening, which is typically found for FE materials. Transmission electron microscopy and spectroscopic ellipsometry measurements indicate that the formation of higher-symmetry structural phases of KNO and NNO produces the desirable red shift of the absorption spectrum towards visible light, while simultaneously stabilizing robust FE order. Tuning of optical properties in FE films is of interest for nanoscale photonic and optoelectronic devices.

No MeSH data available.


Related in: MedlinePlus