Stabilization of Epitaxial Î±âFe 2O3 Thin Films ... - ACS Publications

Subscriber access provided by Kaohsiung Medical University

C: Surfaces, Interfaces, Porous Materials, and Catalysis

Stabilization of Epitaxial #-Fe2O3 Thin Films Grown by Pulsed Laser Deposition on Oxide Substrates Aida Serrano, Juan Rubio-Zuazo, Jesus Lopez Sanchez, Iciar Arnay, Eduardo Salas-Colera, and German R. Castro J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02430 • Publication Date (Web): 21 Jun 2018 Downloaded from http://pubs.acs.org on June 22, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Stabilization of Epitaxial α-Fe2O3 Thin Films Grown by Pulsed Laser Deposition on Oxide Substrates Aida Serrano1,*, Juan Rubio-Zuazo1, Jesús López-Sánchez2, Iciar Arnay1, Eduardo Salas-Colera1, Germán R. Castro1 1 CRG BM25-SpLine, The European Synchrotron (ESRF), 38000 Grenoble, France and Instituto de Ciencia de Materiales de Madrid, ICMM-CSIC, 28049 Madrid, Spain 2 Departamento de Física de Materiales, Universidad Complutense de Madrid, 28040 Madrid, Spain and Unidad Asociada IQFR (CSIC)-UCM, 28040 Madrid, Spain

1/20 ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract We have grown epitaxial incommensurate α-Fe2O3 thin films on α-Al2O3(0001), SrTiO3(111) and LaAlO3(001) substrates, identifying hematite as single iron oxide phase stabilized. We demonstrate that a different lattice coupling behavior as a function of the selected oxide substrate mediates the epitaxial character. Single oriented αFe2O3(0001) layers are obtained on α-Al2O3(0001) and SrTiO3(111) substrates whereas on LaAlO3(001) substrate the hematite layer is found to grow along the r-plane in order to adapt its hexagonal lattice on the cubic lattice of substrate, evidencing a single oriented (1102) layer. In the film plane, crystallographic axes of α-Fe2O3(0001) are collinear with the α-Al2O3(001) ones while a rotation of 30o is found between those of 02) adopts an in-plane α-Fe2O3(0001) and SrTiO3(111). On LaAlO3(001), α-Fe2O3(11 orthorhombic structure rotated 45o respect to the substrate lattice. The crystallographic domain size and crystalline order are dependent on the incommensurate lattice coupling mechanism. Larger values are obtained for layers grown on α-Al2O3(0001) while lower values correspond to the hematite deposited on LaAlO3(001). Moreover, an Fe-O elongation and Fe-Fe contraction of first neighbors distances as well as a dependency on surface flatness as a function of the substrate lattice parameter is also found.


Page 2 of 26

Page 3 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Introduction Hematite (α-Fe2O3) is an iron oxide polymorph that presents an extraordinary potential in diverse applications such as biomedicine, energy storage systems, catalysis or gas sensors1–4. Furthermore, it draws attention due to its abundance, low toxicity, stability and relative easy control of morphology5. Hematite is a canted antiferromagnetic material at room temperature and shows a high Neel temperature (~950 K), being attractive for exchange bias and magnetic data storage technologies6. Besides, this iron oxide phase is a semiconductor with a Eg ∼2.2 eV, in the range of the visible spectrum, which make it one of the most interesting claims for its use in solar water splitting7. Hematite crystallizes in the rhombohedral corundum structure with bulk lattice parameters a=5.035 and c = 13.752 Å, belonging to the space group R3c8. In thin film form its epitaxial growth raises interesting physics and technological possibilities for magnetic, electrical or optical applications. The growth of hematite on several substrates has been performed depending on the scientific interest and the application. However, final structural properties of hematite films, and as a consequence their physical features as the Morin transition9 or the magnetization10, can be altered varying the lattice mismatch or substrate crystallographic orientation. For example, the photocatalytic response can be improved choosing the suitable substrate11, but the process is still not clear. Therefore, in this work we examine the epitaxial stabilization of α-Fe2O3 thin films on three types of oxide substrates with different crystallographic orientation and lattice parameter, evaluating the substrate influence on the morphology and crystalline structure of hematite layer.



Methods α-Fe2O3(0001) thin films have been grown by Pulsed Laser Deposition (PLD) on αAl2O3(0001) (AO(0001)), SrTiO3(111) (STO(111)) and LaAlO3(001) (LAO(001)) substrates from a polycrystalline α-Fe2O3 target. An ultraviolet (UV) pulsed laser source (355 nm) of high power (1 W) was employed to obtain the plasma. The base pressure of the chamber for the film deposition was of 3·10−9 mbar. Samples were deposited under an O2 atmosphere (PO2 of 10−4 mbar) and a growth rate of 0.5 nm/min. During the hematite evaporation, substrates were kept at 400 oC in order to obtain a high crystalline degree on the layer minimizing the chemical interaction at the interface, i.e., flat surface and interface. The deposition process was monitored by in situ Reflection High-Energy Electron Diffraction (RHEED) in order to follow the crystalline growth of the films (Figure S1 in the Supporting Information). Intense RHEED patterns were obtained from the beginning for all samples. The expected in-plane lattice mismatch of α-Fe2O3 film with respect to AO(0001) (a=4.785 Å), STO(111) (a=5.522 Å) and LAO(001) (a=3.820 Å) substrate is about -4.96, 9.68 and -24.13 % respectively, suggesting that the film grown on AO(0001) and LAO(001) would be subjected to a compressive strain and under tensile strain on STO(111). Film morphology was studied by Atomic Force Microscopy (AFM) in non-contact mode at room temperature (RT) by using a Nanotec instrument. The images were analyzed using the WSxM [12] software package from Nanotec. The structural characterization by Grazing Incidence X-ray Diffraction (GIXRD), X-ray Reflectivity (XRR) and X-ray Absorption Spectroscopy (XAS) was carried out at the Spanish CRG beamline BM25-Spline at The European Synchrotron (ESRF), Grenoble (France).


Page 4 of 26

Page 5 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


GIXRD and XRR measurements were carried out with a high precision six-circle diffractometer in vertical geometry13 at RT with photon wavelength of 0.620 Å (hν = 20 keV). XAS measurements were performed at the Fe K-edge (7112 eV) at RT in fluorescence mode using a 13-elements Si(Li) detector, located 90o respect to the incoming X-ray beam. Reference standards signal was collected from transmitted photons with an ionization chamber. XAS data were analysed using Demeter package14. Raman characterization was performed at RT using a Witec ALPHA 300RA Confocal Raman Microscopy (CRM) with a linearly p-polarized Nd:YAG laser (532 nm). Raman measurements were analyzed by using Witec Control Plus Software.

Results and Discussion α-Fe2O3 films grown on AO(0001), STO(111) and LAO(001) oxide substrates show a homogeneous coverage with a quite similar average grain size about 40-50 nm (Figure 1). A rms roughness (and average grain height) of 0.7 (2.3) nm, 0.9 (3.1) nm and 1.1 (4.2) nm is obtained for α-Fe2O3 deposited on STO, AO and LAO, respectively. It should be noticed that larger values of grain average height and rms roughness are promoted for hematite prepared on substrates with larger compressive lattice misfit, as illustrated in Figure 1(g). Besides, from AFM images an island-type growth is shown for all hematite thin films prepared on different oxide substrates. One of the more important factor than influence in the stability of film and its final structure is the system free energy on the substrate. In our hematite-oxide substrate systems, the interaction between hematite atoms is higher than with those of the substrate surface inducing the formation of hematite clusters/islands in order to minimize the interfacial



free energy. However, it should be noticed that even with an island growth mode, a reasonably low average roughness -of the order of 1nm- is found for all films.

α-Fe2O3/AO(0001)

(b)

(a)

400nm

2 0

1

2

X (µm)

3

6

(e) Z (nm)

4

400nm

6

Z (nm)

Z (nm)

(d)

4 2 0

0

1

2

X (µm)

3

αsubstrate (Å)

(c)

400nm

6

0

α-Fe2O3/LAO(001)

(f)

4 2 0

5.52

RMS roughness (nm)

α-Fe2O3/STO(111)

1.4

2

1

2

X (µm)

3

3 .8 2

5

(g) 4

1.2 3 1.0 2 0.8 STO

0

5 4.78

AO

LAO

Average height (nm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 26

1

Substrate

FIG. 1. (a-c) AFM image, (d-f) height profiles measured along the black line indicated on the AFM image and (g) rms roughness and average height values for a α-Fe2O3 thin film grown on STO(111), AO(0001) and LAO(001) substrates.

High resolution GIXRD and XRR are employed to study the substrate effect on the hematite epitaxial character. For the three samples a thickness of 28 ± 2 nm is obtained from low angle XRR data (Figure S2 in the Supporting Information), obtaining continuous thin films with a homogeneous coverage as we observed by AFM results. Representative high angle XRR measurements, shown in Figure 2(a-c), reveal the presence of single phase hematite with (0001) orientation for all layers grown on AO(0001) and STO(111). However single (1102) orientation is found for the layers grown on LAO(001), which corresponds to the growth of hematite along the r-plane15. Such growth favors the coupling of the hexagonal primitive lattice of the hematite on the cubic lattice of LAO(001). The out-of-plane crystallographic orientation relationships between the epitaxial hematite films and AO(0001), STO(111) and


Page 7 of 26

LAO(001)

oxide

substrates

(0001) (111)

are

and

revealed

to

be

(0001) (0001),

02) (001), (11

respectively.

The

calculated out-of-plane lattice parameters are c / = 13.72 Å ~ c, c / = 13.72 Å~ 2c and d / = 3.69 Å ~ c , which are consistent with those obtained from the analysis of non-specular out-of-plane scans (see Figure S3 in the Supporting Information). It should be noted that we can observe Kiessig fringes around film Bragg peaks indicating the occurrence of high quality and smooth surfaces and an abrupt interface for the case of hematite layers deposited on AO(0001) and STO(111). The absence of Kiessig fringes around film Bragg peaks for α-Fe2O3 film deposited on LAO(001) indicates a destructive interference from finite thickness diffraction signal, which can be caused by the presence of a stepped surface together with structural twins. LAO(001) surface is well-known to be characterized by large terraces along one of the crystallographic axis and small terraces along the perpendicular axis (stepped surface). Such a behavior is accompanied by the presence of twins. This finding agrees with AFM measurements showing larger values of the rms roughness for the hematite film prepared on LAO(001).

α−Fe2O3 (0 0 0 6)

14.5 15.0 15.5 16.0 16.5 17.0

2θ (degrees)

(b) Intensity (arb. units)

α−Al2O3 (0 0 0 6)

(a)

α-Fe2O3/STO(111)

15.0

α-Fe2O3/LAO(001)

STO (3 3 3)

(c) Intensity (arb. units)

α-Fe2O3/AO(0001)

Intensity (arb. units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


α−Fe2O3 (0 0 0 6)

15.5

16.0

16.5

2θ (degrees)

LAO (0 0 2) α−Fe2O3 (2 2 0 4)

17.5 18.0 18.5 19.0 19.5 20.0

2θ (degrees)

FIG. 2. High angle XRR scans for a α-Fe2O3 film grown on (a) AO(0001), (b) STO(111) and (c) LAO(001), identifying the reflections in each case.



Figure 3(a-c) shows representative Reciprocal Space Maps (RSM) for the three layers, in which an incommensurate growth is envisaged based on the non-coincidence of the in-plane diffraction maxima from the layers and the substrates. Based on the analysis of the full set of measured diffraction peaks, we obtain different coupling behavior between the hematite lattice and the corresponding substrate lattice. In the case of the hematite layer grown on AO(0001) both in-plane crystallographic axes remain parallel, while for the case of the layers grown on STO(111) a rotation of 30o occurs between both lattices, as depicted on Figure 3(d,e). For the case of α-Fe2O3(1102) grown on LAO(001) two possible coupling scenarios have been proposed in order to reduce the lattice mismatch15, with the hematite adopting an in-plane orthorhombic structure. Either a parallel alignment of the in-plane crystallographic axes or a rotation of 45o between both lattices. Besides, the presence of two domains rotated by 90o is feasible due to the cubic symmetry of the substrate lattice. Even, a co-existence of both scenarios can also happen. However experimentally we only obtain diffraction peaks from the two domains associated with a rotation of 45o between the hematite and substrate lattices, as shown in model of Figure 3(f). So, we discard the presence of the domains related to a parallel alignment of both lattices that is suggested by Wang et al.15, although any in-plane experimental data is shown in such a work. The occurrence of a rotation between hematite and substrate lattices appears in order to match a coincidence lattice with the substrate on which the layer epitaxial growth is conducted by the oxygen sublattice16, minimizing then the lattice mismatch. The in‐plane epitaxial orientation relationships between the hematite films and AO(0001), STO(111)


Page 8 of 26

Page 9 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and LAO(001) oxide substrates are determined to be 100 100

110

and 100

110 ,

100 ,

respectively. From the

positions at several diffraction peaks in reciprocal space we obtain in-plane lattice parameters of ! = b ~5.08(4) Å for hematite layers deposited on AO(0001) and 02) evaporated on LAO(001) substrate STO(111) substrate, whereas for the α-Fe2O3(11 the lattice parameter of the &1120' direction is measured to be 5.08(4) Å and for the &1102' direction is 16.30(5) Å. The large displayed error is associated with the presence of a slight elongation of the diffraction peak, as can be clearly seen for instance in Figure 3(c). This fact can be related to the existence of different crystallites with small deviations of iron stoichiometry or presence of oxygen vacancies to favour the incommensurate coupling. For all layers the obtained lattice parameters match, within the error, those of the bulk structure.

FIG. 3. (a-c) LH Reciprocal Space Maps (RSM) and (d-f) schematic representation in real space for a α-Fe2O3 film grown on AO(0001), STO(111) and LAO(001) respectively. Experimental



in-plane and out-of-plane lattice parameters of α-Fe2O3 layers are depicted. From in-plane diffraction measurements α-Fe2O3 peaks do not coincide with those of substrates, evidencing an incommensurate epitaxial growth.

Rocking curves were performed to confirm the crystalline quality of the hematite layers prepared on different oxide substrate (Figure S4 in the Supporting Information). The inplane domain size of the grown films is calculated to be 400, 250 and 100 Å on AO(0001), STO(111) and LAO(001) substrate, respectively. Although such values present a large uncertainty due to the elongated character of the diffracted peaks a clear tendency is obtained. Larger domain size corresponds to the α-Fe2O3(0001)/AO(0001) film in which a more favourable growth occurs based on the absence of lattice rotation respect to the substrate. The presence of a lattice rotation implies the dispersion of single crystallites diminishing the crystals domain sizes, finding the lowest values for the α-Fe2O3(1102)/LAO(001) where the coupling from an hexagonal to a cubic symmetry is mediated by the hematite epitaxial growth along the r-plane. Figure 4 shows the CRM spectra obtained on the different layers. For each sample a Raman average spectrum is obtained from single Raman spectra measured in each pixel of a selected representative area (5x5 µm2 on film surface). Raman measurements confirm the nature of the iron oxide thin films as single phase hematite, identifying the seven phonon modes (2A1g + 5Eg) predicted for the corundum structure of the R3c space group17,18. In addition to these well-known Raman modes, we distinguish the IR active Eu(LO) mode around 665 cm-1 that can be associated with defects or strain induced in the α-Fe2O3 structure. The vibrational mode at 830 cm-1 is assigned to a


Page 10 of 26

Page 11 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


magnon excitation and the 2Eu(LO) mode around 1325 cm-1 is originated from a twophonon scattering that is known to be strongly resonantly enhanced18,19. As evidenced in Figure 4, shifts of Raman positions and changes in relative intensity of the bands are found for the different hematite films. Specifically, for α-Fe2O3(0001) film prepared on STO(111) substrate most of Raman bands are shifted towards smaller wavenumbers that may be explained by an stretching of the bondlength in the crystalline structure. Contrary behavior is detected for α-Fe2O3(1102)/LAO(001), which might indicate a larger compression strain shifting the Raman bands towards greater wavelengths20,21. Such a behavior may be related to a local distortion of α-Fe2O3 lattice induced by the lattice mismatch, which is found to be lower for hematite grown on STO(111). Regarding intensity of some Raman bands, this varies depending on the substrate employed. One example would be the intensity of Eu(LO) vibrational mode that can be associated with the crystalline disorder in α-Fe2O3 layers22,23. Comparing this band to other active band (Eg(5) mode) we observe the lowest intensity ratio (IEu(LO)/IEg(5)) for hematite deposited on AO(0001) substrate (see inset in Figure 3(a)). Such a fact indicates a minor structural disorder for sample α-Fe2O3/AO(0001), which can be correlated with the best lattice coupling and the larger domain size obtained. Larger disorder is found in hematite grown on LAO(001), which may be assigned to several explications: the orientation along the r-plane in order to accommodate the hexagonal lattice of hematite on the cubic lattice of substrate or/and the local distortion at the layer-substrate interface induced by the characteristic substrate micro-twins that exist at the LAO surface, that may prompt an additional disorder in the upper grown hematite24.



IEu(LO)/IEg(5)

Raman intensity (arb. units)

1.8 1.6 1.4 1.2 1.0

STO AO LAO

Substrate

α-Fe2O3/STO(111) α-Fe2O3/AO(0001) α-Fe2O3/LAO(001)

Raman shift (cm-1)

(b)

(a)

200 400 600 800 1000 1200 1400

(Å)

α

420 410

substrate 2 2 5 5 5.52 4.78 3.82 5.52 4.78 3.82 Eg(4)

2Eu(LO) Magnon

300

Eg(3)

290

Eg(2)

250

Eg(1)

220

Raman shift (cm-1)

1320 1300 840 810 660

Eg(5)

240 230

1340

Eu(LO)

A1g(2)

A1g(1)

O A OL A O T S

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 26

640 620 510 480

O O O ST A LA

Substrate

FIG. 4. (a) Raman spectra and (b) Raman shift of vibration modes identified for epitaxial αFe2O3 thin films grown on STO(111), AO(0001) and LAO(001). Raman spectra are normalized to the 2Eu(LO) band and modes are denoted on the Figure 3(b). Grey line indicates the Raman spectrum for the bare STO(111) substrate. Inset of Figure 3a presents I)*() /I)+(,) ratio.

From X-ray Absorption Near-Edge Structure (XANES) measurements we corroborate the hematite single phase character for all samples. Indistinctly of oxide substrate selected, the absorption signal of the grown films follows that for the powder hematite reference as Figure 5(a) shows. Moreover, a fit of experimental XANES spectra to a linear combination of different iron oxides references compounds (not shown) confirms a 100 % of hematite for all films obtained in this work. To investigate the short-range ordering of Fe-cations and the neighbor bondlengths in hematite films depending on oxide substrate, a study by Extended X-ray Absorption Fine Structure (EXAFS) technique is performed. Figure 5(b) displays the modulus of the Fourier Transform (FT) of the EXAFS signal. The FT is performed in the k2 χ(k) weighted EXAFS signal between 2.2 and 10.5 Å-1. The first intense peak in the FT corresponds to 1st neighbor shell with three short plus three long oxygen neighbors


Page 13 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


around the Fe (of the distorted FeO6 octahedron), and the second peak is related to 2nd neighbor shell and include the single and multiple scatterings from the nearest Fe-Fe neighbors25. In the present work, experimental EXAFS results are fitted in R-space in the range 0.8-3.6 Å using the FEFFIT code26. For the fitting, the amplitude reduction factor S./ and the shift at the edge energy E. are calculated from hematite reference and fixed for the hematite layers. Fittings are carried out taking into account the data reported by Hill et al27. The interatomic distance R and the Debye-Waller (DW) factors σ/ for Fe-O and Fe-Fe distances are used as free parameters for the fitting. The obtained DW factors are of similar order for all hematite films and we do not observe a tendency varying the substrate. However, with respect to first neighbors distances we find that for α-Fe2O3(0001)/STO(111) both Fe-O1 (short oxygen neighbors) and Fe-O2 (long oxygen neighbors) distances are larger than for the rest of films deposited on AO(0001) and LAO(001) substrates, while the second neighbors Fe-Fe distances show a slight shrinking (see Figure 5(c)). Hence, a trend can be established: the larger the lattice parameter of oxide substrate, the larger are the Fe-O elongation and Fe-Fe contraction on the first and second coordination shell. These EXAFS results agree with Raman measurements presented above, confirming a local distortion at short-range of epitaxial α-Fe2O3 structure grown on oxide substrates and proving the influence of substrate in the morphological and structural features of hematite films.



α

(a) α-Fe2O3/STO(111)

1.6

α-Fe2O3/AO(0001) α-Fe2O3/LAO(001)

1.2

α-Fe2O3 powder

0.8 0.4 0.0 7120

7160

7200

7240

Energy (eV)

7280

α-Fe2O3/STO(111)

(b)

12

α-Fe2O3/AO(0001)

2.31

2

(Å)

substrate 2 5 5 5.52 4.78 3.82 4.78 3.82

(c)

Fe-Fe3

α-Fe2O3/LAO(001)

10 8

Fe-O (Å)

2.0

Fourier transform (arb. units)

Normalized absorbance

5.52

6 4

Fe-O2

2.12

3.37

Fe-Fe2

Fe-Fe1

1.94 0

1

2

3

4

5

3.00 2.96

Fe-O1

1.96

0

3.39 3.38

2.16

1.98

2

3.40

STO AO LAO STO AO LAO

R(Å)

2.92 2.88

Substrate

FIG. 5. (a) XANES spectra at the Fe K-edge energy, (b) modulus of the FT of the EXAFS signal (lines with symbols) and best-fitting simulations (continuous lines) and (c) first neighbors Fe-O and Fe-Fe distances obtained from experimental EXAFS results measured at RT for αFe2O3 thin films grown on different substrates: STO(111), AO(0001) and LAO(001). XANES spectrum of α-Fe2O3 reference is presented in Figure 5(a) for comparison.

Conclusions In summary, we show the growth of incommensurate epitaxial α-Fe2O3 thin films on AO(0001), STO(111) and LAO(001) substrates. Hematite layers present different epitaxial lattice coupling depending on the oxide substrate. Single oriented (0001) hematite is found on AO(0001) with in-plane crystallographic axes parallel to those of the substrate. Also single oriented (0001) hematite is found on STO(111) however a rotation of the in-plane axes of 30o respect to those of the substrate is presented. The epitaxial growth on LAO(001) is driven along the r-plane defining single oriented (1102) hematite layers with in-plane axes rotated by 45o respect to those of the cubic substrate. We demonstrate the absence of the domain related to the hematite growth along the in-plane crystallographic axes of LAO(001), which is also predicted to


Fe-Fe (Å)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 26

Page 15 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


decrease the lattice mismatch in the same order as the observed domain. Despite the great stability of hematite, a short-range distortion modifying the Fe-O and Fe-Fe distances of hematite structure is identified, which is prompted by residual lattice mismatch strain or a better oxygen sub-lattice coupling between the hematite and the substrate. Also, a larger surface flatness is measured on α-Fe2O3/STO(111) in concordance with a lower compressive lattice misfit. Regarding the structural order and crystallographic domain size a clear dependency is found for the different epitaxial mechanisms. Hematite prepared on AO(0001) shows larger values based on the more favorable growth mechanism, whereas the layers grown on LAO(001) present lower values due to the less favorable growth way. We demonstrate that a control of morphological and structural characteristics of epitaxial hematite can be conducted varying the lattice parameter and the crystallographic orientation of the oxide substrate, opening the possibility of designing the growth and properties of thin films for advanced applications.

Author information Corresponding author *E-mail: [email protected] (A. S.)

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.



Acknowloedgment This work has been supported by the Ministerio Español de Economia, Industria y Competitividad (MINECO) and the Consejo Superior de Investigaciones Cientificas (CSIC) through the projects PIE-2010-OE-013-200014 and MAT2012-38045-C04-03. J. L-S thanks the FPI fellowship. The ESRF, MINECO and CSIC are acknowledged for the provision of synchrotron radiation facilities.

Supporting Information Available Reflection High-Energy Electron Diffraction patterns before and after the deposition of α-Fe2O3 thin films grown on different substrates. Low Angle X-ray Reflectivity measurement for calculating the thickness of thin films. Details on the X-Ray Diffraction study: Out-of-plane high-resolution Grazing Incident X-Ray Diffraction and Rocking Curve Measurements. This material is available free of charge via the Internet at http://pubs.acs.org.

References (1)

Khatavkar, S. N.; Sartale, S. D. α-Fe2O3 Thin Films by Liquid Phase Deposition : LowCost Option for Supercapacitor. J. Solid State Electrochem. 2017, 21, 2555–2566.

(2)

Yan, S.; Wu, Q. A Novel Structure for Enhancing the Sensitivity of Gas Sensors – αFe2O3 Nanoropes Containing a Large Amount of Grain Boundaries and Their Excellent Ethanol Sensing Performance. J. Mater. Chem. A 2015, 3, 5982–5990.

(3)

Cornell, R. M.; Schwertmann, U. The Iron Oxides: Structure, Properties, Reactions, Occurrence and Uses; 2nd edition WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, 2003.

(4)

Liang, H.; Jiang, X.; Qi, Z.; Chen, W.; Wu, Z.; Xu, B.; Wang, Z.; Mi, J.; Li, Q. Hematite Concave Nanocubes and Their Superior Catalytic Activity for Low Temperature CO


Page 16 of 26

Page 17 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Oxidation. Nanoscale 2014, 6, 7199-7203. (5)

Lian, J.; Duan, X.; Ma, J.; Peng, P.; Kim, T.; Zheng, W. Hematite (α-Fe2O3) with Various Morphologies: Ionic Liquid-Assisted Synthesis, Formation Mechanism, and Properties. ACSnano 2009, 3, 3749–3761.

(6)

Martí, X.; Fina, I.; Jungwirth, T. Prospect for Antiferromagnetic Spintronics. IEEE Trans. Magn. 2015, 51, 1-4.

(7)

Tamirat, A. G.; Rick, J.; Dubale, A.; Su, W. Using Hematite for Photoelectrochemical Water Splitting : A Review of Current Progress and. Nanoscale Horiz. 2016, 1, 243–267.

(8)

Authors and editors of the volumes III/17G-41D. Part of the Landolt-Börnstein - Group III Condensed Matter book series (volume 41D). Hematite (alpha-Fe2O3): general characterization, crystal structure, lattice parameters; Springer Berlin Heidelberg, 1969.

(9)

Park, S.; Jang, H.; Kim, J.-Y.; Park, B.-G.; Koo, T.-Y.; Park, J.-H. Strain Control of Morin Temperature in Epitaxial α-Strain Control of Morin Temperature in Epitaxial αFe2O3 (0001) Film. Europhys. Lett. 2013, 103, 27007.

(10)

Grutter, A.; Wong, F.; Arenholz, E.; Liberati, M.; Suzuki, Y.; Grutter, A.; Wong, F.; Arenholz, E.; Liberati, M.; Suzuki, Y. Enhanced Magnetization in Epitaxial Thin Films via Substrate-Induced Strain Enhanced Magnetization in Epitaxial SrRuO3 Thin Films via Substrate- Induced Strain. J. Appl. Phys. 2010, 107, 09E138.

(11)

Schultz, A. M.; Salvador, P. A.; Rohrer, G. S. Enhanced Photochemical Activity of αFe2O3 Films Supported on SrTiO3 Substrates under Visible Light Illumination. Chem. Commun. 2012, 48, 2012-2014.

(12)

Horcas, I.; Fernandez, R.; Gomez-Rodriguez, J. M.; Colchero, J.; Gomez-Herrero, J.; Baro, A. M. WSXM: A Software for Scanning Probe Microscopy and a Tool for Nanotechnology. Rev. Sci. Instrum. 2007, 78, 013705.

(13)

Rubio-Zuazo, J.; Ferrer, P.; López, A.; Gutiérrez-león, A.; Silva, I.; Castro, G. R. The Multipurpose X-Ray Diffraction End-Station of the BM25B-SpLine Synchrotron Beamline at the ESRF. Nucl. Instrum. Methods Phys. Res., Sect. A 2013, 716, 23–28.

(14)

Ravel, B.; Newville, M. ATHENA , ARTEMIS , HEPHAESTUS : Data Analysis for X-



Ray Absorption Spectroscopy Using IFEFFIT. J. Synchrotron Radiat. 2005, 12, 537– 541. (15)

Wang, Z.; Viswan, R.; Hu, B.; Harris, V. G.; Li, J.; Viehland, D. Phase-Controlled Epitaxial Growth of Iron Oxide Thin Films on MgO (001). Phys. Status Solidi RRL 2012, 91, 89–91.

(16)

Gota, S.; Henriot, M. Atomic-Oxygen-Assisted MBE Growth of Fe3O4 (111) on AlphaAl2O3(0001). Surf. Sci. 2001, 485, 809–816.

(17)

de Faria, D. L. A.; Venancio Silva, S.; De Oliveira, M. T. Raman Microspectroscopy of Some Iron Oxides and Oxyhydroxides. J. Raman Spectrosc. 1997, 28, 873–878.

(18)

Serrano, A.; Fernandez, J. F.; Rodriguez de la Fuente, O.; García, M. A. A Novel Route to Obtain Metal and Oxide Nanoparticles Co-Existing on a Substrate. Mater. Today Chem. 2017, 4, 64–72.

(19)

Serrano, A. Modified Au-Based Nanomaterials Studied by Surface Plasmon Resonance Spectroscopy; Springer Theses, 2015.

(20)

Massey, M. J.; Baier, U.; Merlin, R.; Weber, W. H. Effects of Pressure and Isotopic Substitution on the Raman Spectrum of alpha-Fe2O3: Identification of Two-Magnon Scattering. Phys. Rev. B 1990, 41, 7822–7827.

(21)

Shim, S.-H.; Duffy, T. S. Raman Spectroscopy of Fe2O3 to 62 GPa. Am. Mineral. 2002, 87, 318–326.

(22)

McCarty, K. F. Inelastic Light Scattering in α-Fe2O3: Phonon vs Magnon Scattering. Solid State Commun. 1988, 68, 799–802.

(23)

Cesar, I.; Sivula, K.; Kay, A.; Zboril, R.; Gratzel, M. Influence of Feature Size, Film Thickness, and Silicon Doping on the Performance of Nanostructured Hematite Photoanodes for Solar Water Splitting. J. Phys. Chem. C 2009, 113, 772–782.

(24)

Li, J.-C., Chen, J.-H., Ding, Y., Ma, C.-R., Chen, C.-L., Fang, J.-Y., Zhang, X.-A. Microstructures and Growth Mechanisms of the Polycrystalline CaBaCo2O5+δ Thin Films Grown on (001) LaAlO3 Substrates. J. Funct. Mater. 2014, 45, 17050–17053.

(25)

Sanson, A.; Mathon, O.; Pascarelli, S. Local Vibrational Dynamics of Hematite (α-


Page 18 of 26

Page 19 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fe2O3) Studied by Extended X-Ray Absorption Fine Structure and Molecular Dynamics. J. Chem. Phys. 2014, 140, 224504. (26)

Newville, M.; Ravel, B.; Haskel, D.; Rehra, J. J.; Stern, E. A.; Yacoby, Y. Analysis of Multiple-Scattering XAFS Data Using Theoretical Standards. Phys. B 1995, 208-209, 154–156.

(27)

Hill, A. H.; Jiao, F.; Bruce, P. G.; Harrison, A.; Kockelmann, W.; Ritter, C. Neutron Diffraction Study of Mesoporous and Bulk Hematite, α-Fe2O3. Chem. Mater. 2008, 20, 4891–4899.

TOC Graphic




Page 20 of 26


α-Fe2O3/STO(111)

(b)

(a)

4 2

400nm

(e)

6 4 2

(f)

6

Z (nm)

(d)

6

αsubstrate (Å)

(c)

400nm

400nm

Z (nm)

Z (nm)

α-Fe2O3/LAO(001)

2 5.52

4

1.4

0

1

2

X (m)

3

0

0

1

2

X (m)

5

3.82

(g)

1.2 3 1.0 2 0.8 1

2 0

ACS Paragon 3 Plus Environment 0

5

4

STO 0

4.78

1

2

X (m)

3

AO

LAO

Substrate

Average height (nm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

α-Fe2O3/AO(0001)

RMS roughness (nm)

Page 21 of 26

α-Fe2O3/STO(111)

F

(b)



(a)

Al2O3 1 2 (0 0 0 6) 3 4 5 6 Fe2O3 7 (0 0 0 6) 8 9 10 11 12 13 14 15 16 17 18 19 20 14.5 15.0 15.5 16.0 21 22

16.5 17.0

2(degrees)

Page 22 of 26 α-Fe2O3/LAO(001)


15.0

(c)

STO (3 3 3)


α-Fe2O3/AO(0001)

Fe2O3 (0 0 0 6)

15.5 Plus Environment 16.0 ACS Paragon

2(degrees)

16.5

LAO (0 0 2) Fe2O3 (2 2 0 4)

17.5 18.0 18.5 19.0 19.5 20.0

2(degrees)

1.6 9.4 1.5 9.2

1.6 4.435 4.800 4.313 Max. Max. 4.339 4.279 4.245 1.5 4.211 4.172 4.177 4.143 4.109 1.4 4.005 4.076 4.042 4.008 3.837 1.3 3.974 3.940 3.906 3.670 3.872 1.2 3.839 3.805 3.502 3.771 3.737 1.1 3.703 3.335 3.669 3.635 1.0 3.602 3.167 3.568 Min. 3.534 3.500 3.000 0.9

(a)

1.4 9.0 1.3 8.8 1.2

8.6 1.1 8.4 1.0

α-Fe2O3/STO(111)

1.6 2.2

4.800 Max. 4.313 4.279 4.245 1.5 4.211 4.177 4.143 1.4 2.1 4.109 4.076 4.042 4.008 1.3 3.974 3.940 3.906 3.872 1.2 2.0 3.839 3.805 3.771 3.737 1.1 3.703 3.669 3.635 1.0 1.9 3.602 3.568 Min. 3.534 3.500 0.9

(b)

LSTO

LLSTO AO

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41


α-Fe2O3/AO(0001)

HAO=KAO

0.9 8.2 3.5 3.6 3.9 1.00 4.0 1.05 4.1 1.10 4.2 0.80 0.85 3.7 0.903.80.95

KSTO=0 3.5

HHSTO AO

(d)

α-Fe2O3/LAO(001)

4.8 Max. 4.3 4.2 4.2 4.2 4.1 4.1 4.1 4.0 4.0 4.0 3.9 3.9 3.9 3.8 3.8 3.8 3.7 3.7 3.7 3.6 3.6 3.6 3.5 Min. 3.5 3.5

(c)

LLAO STO

Page 23 of 26

3.6

3.7

3.8

3.9

4.0

4.1

4.2

HLAO=KLAO 3.5 2.9 3.6

3.7 4.0 3.0 3.83.13.9 3.2

4.1 3.3 4.2

HSTO LAO

HSTO

(e)

(f)

30 o 45 o

ACS Paragon Plus Environment


(b)

1.8 1.6 1.4 1.2 1.0

αsubstrate (Å) 420

2 2 5 5 5.52 4.78 3.82 5.52 4.78 3.82 Eg(4)

STO AO LAO

Substrate

-Fe2O3/STO(111) -Fe2O3/AO(0001) -Fe2O3/LAO(001)

Raman shift (cm-1)

IEu(LO)/IEg(5)

Raman intensity (arb. units)

(a)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Page 24 of 26

410

2Eu(LO) Magnon

300

Eg(3)

1300 840 810

Eg(2)

250

Eg(1)

660 Eg(5)

640 620

240 A1g(1)

A1g(2)

200 400 600 800 1000 1200Plus 1400 ACS Paragon Environment

510 480

220

Raman shift (cm )

1320

Eu(LO)

290

230

-1

1340

ST

O AO AO TO AO AO L L S

Substrate

Page 25 of 26

αsubstrate (Å)


(a) -Fe2O3/STO(111)

1.6

-Fe2O3/AO(0001) -Fe2O3/LAO(001)

1.2

-Fe2O3 powder

0.8 0.4 0.0 7120

7160

7200

7240

Energy (eV)

7280

(b)

12

-Fe2O3/AO(0001)

2.31

(c)

5 4.78 3.82

Fe-Fe3

3.40 3.39

-Fe2O3/LAO(001)

10

3.38

8

Fe-O (Å)

Normalized absorbance

2.0

-Fe2O3/STO(111)

2

5.52

6 4

2.16

Fe-O2

2.12

3.37

Fe-Fe2

2.96

1.98 2

1.96

0

3.00

Fe-O1

Fe-Fe1

2.92

1.94 2.88 0

1 Paragon 2Plus Environment 3 ACS

R(Å)

4

5

STO AO LAO STO AO LAO

Substrate

Fe-Fe (Å)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Fourier transform (arb. units)

2 85 .82 5.52 4.7 3

α-Fe2O3(0001)/α-A2O3(0001)

The Journal of Physical Chemistry Page 26 of 26 1 2 α-Fe2O3( α-Fe2O3(0001)/SrTiO3(111) 3 4 ACS Paragon Plus Environment 5 6 30 o 7

)/LaAlO3(001)

45 o

Stabilization of Epitaxial Î±âFe 2O3 Thin Films ... - ACS Publications

Recommend Documents

Stabilization of Epitaxial Î±âFe 2O3 Thin Films ... - ACS Publications