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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.

No MeSH data available.


Variation of index of refraction δn at E = 2 eV as a function of temperature T in epitaxial nanofilms (a)–(c) and bulk crystals (d)–(f) of KTO (a), (d), KNO (b), (e), and NNO (c), (f). Here, the temperature variation δn of the index n(T) is defined as δn = n(T) − n(R), where n(R) is the room temperature value.
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Figure 5: Variation of index of refraction δn at E = 2 eV as a function of temperature T in epitaxial nanofilms (a)–(c) and bulk crystals (d)–(f) of KTO (a), (d), KNO (b), (e), and NNO (c), (f). Here, the temperature variation δn of the index n(T) is defined as δn = n(T) − n(R), where n(R) is the room temperature value.

Mentions: Our results suggest that new structural phases of epitaxial nanofilms are primarily responsible for complex changes in the electronic band structure. Most significantly, these include bandgap narrowing in the presence of robust FE polarization for structural phases of KNO and NNO with higher symmetry than their bulk crystals. Enhancements of the FE phases in the KTO, KNO, and NNO nanofilms are confirmed by the temperature evolution of the index of refraction (n(T)) as shown in figure 5. The index n(T) of the KTO nanofilm increases on cooling at high temperatures, which is typical for the PE state [53] and qualitatively similar to the behavior of the fully PE KTO bulk crystal. The decrease of n below 650 K, however, evidences the presence of FE polarization [34, 53]. The thermo-optical behavior of NNO and KNO nanofilms also differs from that of their bulk crystals. The PE–FE phase transition occurs at 725 K in the KNO crystal as manifested by an anomaly in the n(T) curve. The non-monotonic positive slope of n(T) for the KNO nanofilm indicates a stable FE state up to at least 780 K (the maximum temperature in this measurement). This upward shift of the FE phase transition is in qualitative agreement with the strain–temperature phase diagram of epitaxial perovskite FE films [54]. The structural phase transition, indicated by an anomaly of n(T) around 485 K in the KNO crystal, does not exist in the tetragonal KNO nanofilm. An absence of bulk-like phase transitions has also been found for FE BaTiO3 films [54]. The presence of a stable FE state with high transition temperature (>780 K) and the absence of distinct phase transitions at low temperatures are also found for the NNO nanofilm, which again differs considerably from the characteristics of the NNO bulk crystal. The data of figure 5 thus clearly indicate an enhancement of the FE phase in all three nanofilms.


Concurrent bandgap narrowing and polarization enhancement in epitaxial ferroelectric nanofilms
Variation of index of refraction δn at E = 2 eV as a function of temperature T in epitaxial nanofilms (a)–(c) and bulk crystals (d)–(f) of KTO (a), (d), KNO (b), (e), and NNO (c), (f). Here, the temperature variation δn of the index n(T) is defined as δn = n(T) − n(R), where n(R) is the room temperature value.
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Related In: Results  -  Collection

License 1 - License 2
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Figure 5: Variation of index of refraction δn at E = 2 eV as a function of temperature T in epitaxial nanofilms (a)–(c) and bulk crystals (d)–(f) of KTO (a), (d), KNO (b), (e), and NNO (c), (f). Here, the temperature variation δn of the index n(T) is defined as δn = n(T) − n(R), where n(R) is the room temperature value.
Mentions: Our results suggest that new structural phases of epitaxial nanofilms are primarily responsible for complex changes in the electronic band structure. Most significantly, these include bandgap narrowing in the presence of robust FE polarization for structural phases of KNO and NNO with higher symmetry than their bulk crystals. Enhancements of the FE phases in the KTO, KNO, and NNO nanofilms are confirmed by the temperature evolution of the index of refraction (n(T)) as shown in figure 5. The index n(T) of the KTO nanofilm increases on cooling at high temperatures, which is typical for the PE state [53] and qualitatively similar to the behavior of the fully PE KTO bulk crystal. The decrease of n below 650 K, however, evidences the presence of FE polarization [34, 53]. The thermo-optical behavior of NNO and KNO nanofilms also differs from that of their bulk crystals. The PE–FE phase transition occurs at 725 K in the KNO crystal as manifested by an anomaly in the n(T) curve. The non-monotonic positive slope of n(T) for the KNO nanofilm indicates a stable FE state up to at least 780 K (the maximum temperature in this measurement). This upward shift of the FE phase transition is in qualitative agreement with the strain–temperature phase diagram of epitaxial perovskite FE films [54]. The structural phase transition, indicated by an anomaly of n(T) around 485 K in the KNO crystal, does not exist in the tetragonal KNO nanofilm. An absence of bulk-like phase transitions has also been found for FE BaTiO3 films [54]. The presence of a stable FE state with high transition temperature (>780 K) and the absence of distinct phase transitions at low temperatures are also found for the NNO nanofilm, which again differs considerably from the characteristics of the NNO bulk crystal. The data of figure 5 thus clearly indicate an enhancement of the FE phase in all three nanofilms.

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.