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Structural, optical and vibrational properties of self-assembled Pbn+1(Ti1-x Fex)nO(3n+1)-δ Ruddlesden-Popper superstructures.

Doig KI, Peters JJ, Nawaz S, Walker D, Walker M, Lees MR, Beanland R, Sanchez AM, McConville CF, Palkar VR, Lloyd-Hughes J - Sci Rep (2015)

Bottom Line: No evidence of macroscopic ferromagnetism was found in SQUID magnetometry.The ultrafast optical response exhibited coherent magnon oscillations compatible with local magnetic order, and additionally was used to study photocarrier cooling on picosecond timescales.An optical gap smaller than that of BiFeO3 and long photocarrier lifetimes may make this system interesting as a ferroelectric photovoltaic.

View Article: PubMed Central - PubMed

Affiliation: University of Oxford, Department of Physics, Clarendon Laboratory, Parks Road, Oxford, OX1 3PU, United Kingdom.

ABSTRACT
Bulk crystals and thin films of PbTi(1-x)FexO3(-δ) (PTFO) are multiferroic, exhibiting ferroelectricity and ferromagnetism at room temperature. Here we report that the Ruddlesden-Popper phase Pbn+1(Ti(1-x)Fex)nO3(n+1)-δ forms spontaneously during pulsed laser deposition of PTFO on LaAlO3 substrates. High-resolution transmission electron microscopy, x-ray diffraction and x-ray photoemission spectroscopy were utilised to perform a structural and compositional analysis, demonstrating that n ≃ 8 and x ≃ 0.5. The complex dielectric function of the films was determined from far-infrared to ultraviolet energies using a combination of terahertz time-domain spectroscopy, Fourier transform spectroscopy, and spectroscopic ellipsometry. The simultaneous Raman and infrared activity of phonon modes and the observation of second harmonic generation establishes a non-centrosymmetric point group for Pbn+1(Ti0.5Fe0.5)nO3(n+1)-δ, a prerequisite for (but not proof of) ferroelectricity. No evidence of macroscopic ferromagnetism was found in SQUID magnetometry. The ultrafast optical response exhibited coherent magnon oscillations compatible with local magnetic order, and additionally was used to study photocarrier cooling on picosecond timescales. An optical gap smaller than that of BiFeO3 and long photocarrier lifetimes may make this system interesting as a ferroelectric photovoltaic.

No MeSH data available.


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Transmission electron microscopy of 100 nm PTFO film.(a) Conventional TEM showing ‘wave' patterns. (b) ADF-STEM image showing that ‘waves' consist of a perovskite-like areas (labelled 1 and 1′, blue and red squares) and a rock-salt-like areas (labelled 2, yellow squares). Cyan lines mark the boundaries between these regions. The vertical white solid lines run through the perovskite A site in area 1, becoming the B site in area 1′. The horizontal white dashed lines show atomic planes separated by a stacking fault with . (c) ADF-STEM image showing a change in c, present in region 1 and 1′. (d) Schematic of structure in (010) plane (left, corresponding to plane of TEM image) and (100) plane (right). A Ruddlesden-Popper unit cell for n = 8 is shaded in blue, while another, offset by an antiphase boundary (see text), is shaded in red. Projection of the right-hand cartoon along [010] yields the left-hand schematic, creating areas where Pb ions appear to be on both the A and B sites.
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f2: Transmission electron microscopy of 100 nm PTFO film.(a) Conventional TEM showing ‘wave' patterns. (b) ADF-STEM image showing that ‘waves' consist of a perovskite-like areas (labelled 1 and 1′, blue and red squares) and a rock-salt-like areas (labelled 2, yellow squares). Cyan lines mark the boundaries between these regions. The vertical white solid lines run through the perovskite A site in area 1, becoming the B site in area 1′. The horizontal white dashed lines show atomic planes separated by a stacking fault with . (c) ADF-STEM image showing a change in c, present in region 1 and 1′. (d) Schematic of structure in (010) plane (left, corresponding to plane of TEM image) and (100) plane (right). A Ruddlesden-Popper unit cell for n = 8 is shaded in blue, while another, offset by an antiphase boundary (see text), is shaded in red. Projection of the right-hand cartoon along [010] yields the left-hand schematic, creating areas where Pb ions appear to be on both the A and B sites.

Mentions: Conventional TEM and ADF-STEM images (see Methods) of the atomic structure of the PTFO-100 film are reported in Fig. 2. Bright and dark wave-like patterns can be seen in the low-magnification image of Fig. 2a, with a periodicity in the growth direction of about 4 nm. The ADF image, showing the Pb columns most clearly (Ti atomic columns give weaker contrast), demonstrates that the “waves” consist of areas that appear perovskite-like (labelled 1 and 1′) and others that appear rock-salt-like (areas 2). The A- and B- cation site positions can be seen to reverse on opposite sides of the rock-salt-like layers: rather than lying on the solid white vertical lines in Fig. 2b, which run through the perovskite A-sites in area 1, the Pb atoms in areas 1′ are instead found on the dashed lines (B-sites of area 1). EELS spectra taken in areas 1, 2 and 1′ showed no difference in Fe or Ti composition (Supplementary Fig. S.1), implying that the different regions have similar stoichiometry. Further, TEM-EDX maps over regions up to 75 nm wide showed no variation in composition across each film (Supplementary Fig. S.2).


Structural, optical and vibrational properties of self-assembled Pbn+1(Ti1-x Fex)nO(3n+1)-δ Ruddlesden-Popper superstructures.

Doig KI, Peters JJ, Nawaz S, Walker D, Walker M, Lees MR, Beanland R, Sanchez AM, McConville CF, Palkar VR, Lloyd-Hughes J - Sci Rep (2015)

Transmission electron microscopy of 100 nm PTFO film.(a) Conventional TEM showing ‘wave' patterns. (b) ADF-STEM image showing that ‘waves' consist of a perovskite-like areas (labelled 1 and 1′, blue and red squares) and a rock-salt-like areas (labelled 2, yellow squares). Cyan lines mark the boundaries between these regions. The vertical white solid lines run through the perovskite A site in area 1, becoming the B site in area 1′. The horizontal white dashed lines show atomic planes separated by a stacking fault with . (c) ADF-STEM image showing a change in c, present in region 1 and 1′. (d) Schematic of structure in (010) plane (left, corresponding to plane of TEM image) and (100) plane (right). A Ruddlesden-Popper unit cell for n = 8 is shaded in blue, while another, offset by an antiphase boundary (see text), is shaded in red. Projection of the right-hand cartoon along [010] yields the left-hand schematic, creating areas where Pb ions appear to be on both the A and B sites.
© Copyright Policy - open-access
Related In: Results  -  Collection

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Show All Figures
getmorefigures.php?uid=PMC4296293&req=5

f2: Transmission electron microscopy of 100 nm PTFO film.(a) Conventional TEM showing ‘wave' patterns. (b) ADF-STEM image showing that ‘waves' consist of a perovskite-like areas (labelled 1 and 1′, blue and red squares) and a rock-salt-like areas (labelled 2, yellow squares). Cyan lines mark the boundaries between these regions. The vertical white solid lines run through the perovskite A site in area 1, becoming the B site in area 1′. The horizontal white dashed lines show atomic planes separated by a stacking fault with . (c) ADF-STEM image showing a change in c, present in region 1 and 1′. (d) Schematic of structure in (010) plane (left, corresponding to plane of TEM image) and (100) plane (right). A Ruddlesden-Popper unit cell for n = 8 is shaded in blue, while another, offset by an antiphase boundary (see text), is shaded in red. Projection of the right-hand cartoon along [010] yields the left-hand schematic, creating areas where Pb ions appear to be on both the A and B sites.
Mentions: Conventional TEM and ADF-STEM images (see Methods) of the atomic structure of the PTFO-100 film are reported in Fig. 2. Bright and dark wave-like patterns can be seen in the low-magnification image of Fig. 2a, with a periodicity in the growth direction of about 4 nm. The ADF image, showing the Pb columns most clearly (Ti atomic columns give weaker contrast), demonstrates that the “waves” consist of areas that appear perovskite-like (labelled 1 and 1′) and others that appear rock-salt-like (areas 2). The A- and B- cation site positions can be seen to reverse on opposite sides of the rock-salt-like layers: rather than lying on the solid white vertical lines in Fig. 2b, which run through the perovskite A-sites in area 1, the Pb atoms in areas 1′ are instead found on the dashed lines (B-sites of area 1). EELS spectra taken in areas 1, 2 and 1′ showed no difference in Fe or Ti composition (Supplementary Fig. S.1), implying that the different regions have similar stoichiometry. Further, TEM-EDX maps over regions up to 75 nm wide showed no variation in composition across each film (Supplementary Fig. S.2).

Bottom Line: No evidence of macroscopic ferromagnetism was found in SQUID magnetometry.The ultrafast optical response exhibited coherent magnon oscillations compatible with local magnetic order, and additionally was used to study photocarrier cooling on picosecond timescales.An optical gap smaller than that of BiFeO3 and long photocarrier lifetimes may make this system interesting as a ferroelectric photovoltaic.

View Article: PubMed Central - PubMed

Affiliation: University of Oxford, Department of Physics, Clarendon Laboratory, Parks Road, Oxford, OX1 3PU, United Kingdom.

ABSTRACT
Bulk crystals and thin films of PbTi(1-x)FexO3(-δ) (PTFO) are multiferroic, exhibiting ferroelectricity and ferromagnetism at room temperature. Here we report that the Ruddlesden-Popper phase Pbn+1(Ti(1-x)Fex)nO3(n+1)-δ forms spontaneously during pulsed laser deposition of PTFO on LaAlO3 substrates. High-resolution transmission electron microscopy, x-ray diffraction and x-ray photoemission spectroscopy were utilised to perform a structural and compositional analysis, demonstrating that n ≃ 8 and x ≃ 0.5. The complex dielectric function of the films was determined from far-infrared to ultraviolet energies using a combination of terahertz time-domain spectroscopy, Fourier transform spectroscopy, and spectroscopic ellipsometry. The simultaneous Raman and infrared activity of phonon modes and the observation of second harmonic generation establishes a non-centrosymmetric point group for Pbn+1(Ti0.5Fe0.5)nO3(n+1)-δ, a prerequisite for (but not proof of) ferroelectricity. No evidence of macroscopic ferromagnetism was found in SQUID magnetometry. The ultrafast optical response exhibited coherent magnon oscillations compatible with local magnetic order, and additionally was used to study photocarrier cooling on picosecond timescales. An optical gap smaller than that of BiFeO3 and long photocarrier lifetimes may make this system interesting as a ferroelectric photovoltaic.

No MeSH data available.


Related in: MedlinePlus