Modulation of Abnormal Poisson's Ratios and Electronic Properties in

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Functional Inorganic Materials and Devices

Modulation of abnormal Poisson’s ratios and electronic properties in mixed-valence perovskite manganite films Shanquan Chen, Changxin Guan, Shanming Ke, Xierong Zeng, Chuanwei Huang, Sixia Hu, Fei Yen, Haoliang Huang, Yalin Lu, and Lang Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19580 • Publication Date (Web): 10 May 2018 Downloaded from http://pubs.acs.org on May 14, 2018

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ACS Applied Materials & Interfaces

Modulation of abnormal Poisson’s ratios and electronic properties in mixedvalence perovskite manganite films

Shanquan Chen,†,♯ Changxin Guan,†,‡,∆,♯ Shanming Ke,† Xierong Zeng,† Chuanwei Huang,†,‡,* Sixia Hu,‡ Fei Yen,‡ Haoliang Huang,§ Yalin Lu,§ and Lang Chen,‡,*

†

Shenzhen Key Laboratory of Special Functional Materials, College of Materials

Science and Engineering, Shenzhen University, Nanshan District, Shenzhen, 518060, Guangdong, China ‡

Department of Physics, Southern University of Science and Technology, Nanshan

District, Shenzhen, 518055, Guangdong, China ∆

Department of Materials Science and Engineering, Hubei University, Wuchang

District, Wuhan, 430062, Hubei, China §

National Synchrotron Radiation Laboratory, University of Science and Technology

of China, Hefei 230026, P. R. China.

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Abstract: Epitaxy and misfit strain imposed by underlying substrates have been intensively used to tailor the microstructure and electronic properties of oxide films, but this approach is largely restricted by commercially limited substrates. In contrast to the conventional epitaxial misfit strains with a positive Poisson’s constant, we show here a tunable Poisson’s ratio with anomalous values from negative, zero, to positive. This permits effective control over the out-of-plane lattice parameters that strongely correlate the magnetic and transport properties in perovskite mixed-valence La1xSrxMnO3

thin films. Our results provide an unconventional approach to better

modulation and understanding of elastic-mediated microstructures and physical properties of oxide films by engineering the Poisson’s ratios.

Keywords: Poisson’s ratio, auxetic materials, mechanical negative index material, mixed-valence materials, thin films

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INTRODUCTION Due to some of its exemplary and unparallel properties and promising multifunctional applications, abnormal mechanical negative index materials have recently attracted considerable interest such as negative Poisson’s ratio, negative compressibility, and negative normal stress.1-3 The Poisson’s ratio describes the proportionality between two mutually orthogonal strains of a material when subjected to deformation making it a fundamental metric for comparing deformations when the sample is strained elastically. Most materials exhibit a positive Poisson’s ratio, meaning lateral expansion (shrinkage) upon compression (stretching). However, interest in some of the more exotic forms of Poisson’s ratios have recently increased significantly,4-6 as not only do they provide better fundamental knowledge but also serve to inspire yet even more functional properties.7 It has been demonstrated that Poisson’s ratios can be changed from positive to negative via tuning of its crystallographic axes,8-10 and chemical composition,11-12 as well as varying the temperature,13-14 electrical field,15 pressure,11 or electronic orbital coupling in functional materials.16 Usually, materials possessing such anomalous Poisson’s ratios possess enormous superior physical properties, with particular applications in various stringent environments such as aerospace and defense areas. For example, materials with a negative Poisson’s ratio (also called auxetic materials) exhibit enhanced

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indentation resistance and fracture toughness, which is desirable for integrated electronic devices.17-18 Materials with zero Poisson’s ratio also possess other unique mechanical properties useful in functional materials.19-20 However, the exploration of how the Poisson’s ratio is coupled to some of its physical properties is still in its infancy

Recently, it was recognized that the Poisson’s ratio plays a crucial role in manipulating the electronic properties of epitaxial films.21-25 Positive Poisson’s ratios with a range of 0.20-0.35 have usually been taken for granted in most functional oxide films.21-24 Based on a positive Poisson effect, strain engineering in epitaxial oxide films has led to the formation of many new structures and extraordinary functionalities using the well-known in-plane lattice misfit strain between the film and the substrate,26-28 which has offered a new platform to tailor various emergent physical properties. However, in sharp contrast to the extensive effort of modulations in in-plane lattice parameters imposed by underlying substrates, the deformation of out-of-plane lattice parameters and accompanying variability of Poisson’s ratios are less universally studied in epitaxial oxide films, which could largely underestimate many related functional properties (i.e., the out-of-plane deformation and associated ferroelectricity) in epitaxial films.7 Another issue raised often is that a continuous

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misfit strain for in-plane strain engineering in epitaxial films is largely restricted by selecting the proper commercial substrates.29 Very recently, abnormal negative Poisson’s ratios have been reported in ultrathin functional oxide films.24-25,

30-32

However, the critical film thickness for the occurrence of negative Poisson’s ratio is usually below several nanometers, and this largely challenges current film deposition technologies and limits their range of applications. For example, negative Poisson’s ratio can only be observed in La1-xSrxMnO3 (LSMO, x=0.3) films with thicknesses less than 3.0 nm.24-25 Another issue of concern is a significant depression of magnetic properties for those ultrathin functional films with negative Poisson’s ratio.25,

33

However, perovskite manganites have been intensively studied in the past decades due to their intriguing electronic properties, such as ferromagnetism and colossal magnetoresistance. The physical properties of manganites could be dramatically influenced by lattice strain,34-35 oxygen stoichiometry,36 ionic radii,37 or hydrostatic pressure,38 due to the intimate coupling among the lattice, orbital, charge, and spin. La0.7Sr0.3MnO3, as one of promising manganites with a high Curie temperature, has been attracted considerable attention. Particularly, the effect of misfit strain at the interface between films and substrates has been widely employed to modify the microstructure and improve the electronic properties of perovskite manganese oxide

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thin films, which recently leads to numerous emergent physical phenomena that could not find in their bulk counterpart.23, 25, 33-34

To alleviate those problems of LSMO films discussed above, we show here a general treatment to unveil the variability of Poisson’s ratios from negative to positive (as depicted in Fig. 1(a)), via the atomic-level modulation of lattice parameters for epitaxial manganite oxide films. The variable Poisson’s ratios induced electronic properties are demonstrated in epitaxial LSMO films by choosing the appropriate deposition pressures, substrates, and film thicknesses. Our results imply that the variability in Poisson’s ratios for epitaxial oxide films provides an alternative to modulate the structural causal functional properties in epitaxial films. Striking modulations are exhibited in the electronic properties of LSMO films with zero Poisson’s ratio.

RESULTS AND DISCUSSION Fig. 1(b-d) shows the θ-2θ XRD scans of the oxygen pressure-dependent LSMO films deposited on different substrates. According to traditional positive Poisson’s ratio, the out-of-plane crystallographic lattice parameter of the films shrink (expand) when the films are epitaxially grown on tensile (compressive) substrates. For example,

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at high deposition oxygen pressures (i.e., 15 Pa in Fig. 1(b)), the (001) and (002) reflection peaks of films are located at the right side of the STO (i.e., asterisk) when films are deposited on the STO substrate with a tensile IP misfit strain. This indicates that the out-of-plane lattice parameter of the LSMO film is shrunken (cfilm=3.85 Å), relative to bulk LSMO (abulk=bbulk=cbulk=3.876 Å).

In contrast, the reflection peaks of the films are located on the left side of the substrates with expanded out-of-plane lattice parameters (cfilm=3.893 Å and cfilm=3.993 Å, respectively) when films were deposited on compressive LSAT and LAO (see Fig. 1(c-d)). Nonetheless, the film reflection peaks gradually shift toward low angles for lower deposition oxygen pressures (i.e., 1.0×10-2 Pa and 5.0×10-4 Pa). This is equivalent to increasing out-of-plane lattice parameters regardless of compressive or tensile substrates. Strikingly, the film peaks shift to the left side of the substrates despite the deposition on a tensile STO. This implies that the out-of-plane lattice parameter (cfilm) of the LSMO films is expanded with low oxygen pressure compared to the bulk one (i.e., up to 3.911 Å for LSMO films deposited at 5.0×10-4 Pa; black curve in Fig.1(b)).

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The out-of-plane and in-plane lattice parameters should be determined simultaneously to best investigate the epitaxy and Poisson’s ratios of LSMO heterostructure systems grown at different oxygen pressures. The RSM measurements of the (103) reflection (plotted in Fig. 2) show that all LSMO films have high epitaxy quality with identical in-plane lattice parameters afilm of LSMO films as that of the substrates (i.e., afilm=asub in those epitaxial LSMO films). Fig. 2 also shows that the extracted out-of-plane lattice parameter by RSM is consistent with that from XRD θ2θ measurements (Fig. 1).

Based on those XRD measurements, all of the in-plane and out-of-plane lattice parameters were confirmed and summarized in Fig. 3. High quality epitaxy was confirmed by RSM, and the in-plane lattice parameters afilm of all LSMO films are equal to those of the underlying substrates. However, the out-of-plane lattice parameters of the LSMO films gradually increase with lower deposition oxygen pressures. It is worth noting from Fig. 3(a) that both the in-plane and out-of-plane lattices are anomalously expanded in the LSMO/STO hetero-structure systems when deposited at low oxygen pressures. This is in sharp contrast to the shrunken out-ofplane lattice parameter based on the conventional positive Poisson effect.23 More importantly, our results demonstrated that the out-of-plane lattice parameters of

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clamped LSMO films can be independently and continuously controlled using oxygen pressure rather than substrates. As shown in Fig. 3(a), the out-of-plane lattice parameters of LSMO films are elongated from 3.851 Å to 3.911 Å when the deposition pressure is gradually lowered, with a remarkable increase up to 0.9% relative to the bulk LSMO.

To analyze in further detail the abnormal expansion of the out-of-plane lattice parameter in LSMO/STO systems, we extend prior works by considering the variability of Poisson’s ratio. The Poisson’s ratio of epitaxial films usually describes the two orthogonal distortions due to the clamping effect of the substrates, which can be expressed as υ =

εt

( ε t − 2ε l )

,23,

39

where ε t and ε l are the out-of-plane and in-

plane strain components, respectively. The most straightforward way to obtain the magnitude of the films’ Poisson’s ratios is by simultaneously determining the substrate-induced strain components along the in-plane and out-of-plane (see Supporting Information, Eq. (1-3)), where ε t =(cfilm-abulk)/abulk, ε l =(afilm-abulk)/abulk. Here, cfilm, afilm, and abulk are the lattice parameters along the out-of-plane, in-plane of the films, and the bulk sample, respectively. The oxygen pressure-dependent Poisson’s ratios for different LSMO hetero-structures are depicted in Fig. 3(d). The value of Poisson’s ratio is positive when grown on compressive substrates (i.e., LSAT

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and LAO) regardless of the value of the oxygen pressures changing from 10-4 Pa to 15 Pa. It is worth noting that the Poisson’s ratios of the LSMO/LAST hetero-structures are unusually larger than the theoretical prediction limit of 0.5 (up to 0.76).40 However, LSMO films grown on tensile STO exhibit tunable Poisson’s ratios with different oxygen pressures. Most notable is the fact that an anomalous zero Poisson’s ratio is seen for LSMO films deposited at low oxygen pressures (i.e. 1.0 Pa - 1.0×10-2 Pa). The Poisson’s ratio becomes negative for lower deposition oxygen pressures, in sharp contrast to the reported positive Poisson’s ratios in most oxides.21-23 These findings indicate that the out-of-plane lattice parameters and Poisson’s ratio of the LSMO films can be effectively tuned using a combination of oxygen pressure and misfit strain.

Next, oxygen vacancies were introduced to understand how the out-of-plane lattice parameters and Poisson’s ratios are modulated in epitaxial mixed-valence LSMO films. In the conventional misfit strain tuning approach, the Poisson’s ratio is a positive constant with a contractive (expanded) out-of-plane lattice parameter if the films are epitaxially deposited on tensile (compressive) substrates. However, in this study, the out-of-plane lattice parameter of the LSMO film is adjustable (Fig. 3(a-c)) with tunable Poisson’s ratios from negative to positive in the LSMO/STO systems

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(Fig. 3(d)). The possible underlying reason for this variable Poisson’s ratio is low oxygen pressure during the deposition that forms oxygen vacancies and gradually changes the valence of Mn ions from Mn4+ to Mn3+ in the mixed-valence LSMO films.25, 41-44 For LSMO films deposited at a lower oxygen pressure, the Mn3+/Mn4+ ratio gradually increases based on the XAS measurements (Fig. S5). The volume of octahedral MnO6 (the out-of-plane lattice parameter) is expected to expand due to the larger radius of Mn3+ (i.e., 0.645 Å for Mn3+ and 0.530 Å for Mn4+).25 For epitaxial films, the in-plane lattice parameter is clamped and possesses identical lattice parameters as that of the substrates. Thus, the out-of-plane lattice parameters shown in Fig. 3(a) are steadily increased for LSMO/STO systems deposited with lower oxygen pressures. Similar trends for oxygen vacancy-driven lattice parameters are observed in LSMO films deposited with compressive substrates (Fig. 3(b-c)). The oxygen vacancy-driven expansion of out-of-plane lattice parameters leads to a stretchable

ε t and abnormal negative Poisson’s ratio in LSMO/STO systems in combination with the induced tensile ε l by STO. In contrast, only positive Poisson’s ratios are observed for films deposited on compressive substrates (LAO and LAST) with contractive ε l .

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The thickness could also affect the lattice parameters and Poisson’s ratios in epitaxial films,25 and thus we further studied the dependence of film thickness on Poisson’s ratios in LSMO/STO systems at given oxygen pressures. The XRD measurements (θ-2θ scan and RSM in Fig. S1 and Fig. S2) indicate that the epitaxy can be remained for LSMO films with thicknesses up to 62 nm. It is clear from Fig. 4 that the film thickness could also lead to a noticeable manipulation of out-of-plane lattice parameters and variable Poisson’s ratios from positive Poisson’s ratio to negative Poisson’s ratio (Fig. 4) in LSMO/STO hetero-structures. This behavior suggests that there is no deformation along out-of-plane for LSMO/STO systems grown at 4.0×10-1 Pa, signifying as a zero Poisson’s ratio (black curve in Fig. 4). Furthermore, LSMO films grown at lower oxygen pressures (i.e., 1.0×10-2 Pa) show a thickness-dependent expansion of out-of-plane parameter lattices that indicate negative Poisson’s ratio behavior. These trends clearly revealed that the film thickness is an alternative way to modulate the out-of-plane lattice parameters and Poisson’s ratios from positive to negative for mixed-valence LSMO films. More importantly, the observed critical thicknesses for the occurrences of abnormal negative Poisson’s ratio could be up to 62.0 nm (Fig. 4). This is much larger than the reported thickness (

Modulation of Abnormal Poisson's Ratios and Electronic Properties in

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