Modulation of the electronic states of perovskite SrCrO₃ thin films through protonation via low-energy hydrogen plasma implantation approaches

Abstract

Hydrogenation of transition metal oxides offers a powerful platform to tailor physical functionalities as well as for potential applications in modern electronic technologies. An ideal nondestructive and efficient hydrogen incorporation approach is important for the realistic technological applications. We demonstrate the proton injection on SrCrO₃ thin films via an efficient low-energy hydrogen plasma implantation experiments, without destroying the original lattice framework. Hydrogen ions accumulate largely at the interfacial regions with amorphous character which extend about one-third of the total thickness. The H_xSrCrO₃ (HSCO) thin films appear like exfoliated layers which however retain the fully strained state with distorted perovskite structure. Proton doping induces the change of Cr oxidation state from Cr⁴⁺ to Cr³⁺ in HSCO thin films and a transition from metallic to insulating phase. Our investigations suggest an attractive platform in manipulating the electronic phases in proton-based approaches and may offer a potential peeling off strategy for nanoscale devices through low-energy hydrogen plasma implantation approaches.

Graphical abstract

Keywords

transition metal oxide thin film / metal–insulator transition / hydrogenation

1 Introduction

Functional materials have attracted significant interests due to their novel electronic, photonics behaviors over the past decades [1-3]. Strongly correlated 3d transition metal oxides are characterized by the interplay of spin, charge, orbital degrees of freedom of the valence electrons, where a burst of strategies are proposed to manipulate their intimate couplings and uncover new functionalities [4, 5]. Among them doping exhibits as a controlling parameter with notable characters, e.g., the entering to superconducting state in high T_c superconductors [6]; the appearance of versatile ground states including the ferromagnetic metallic phase associated with colossal magnetoresistance in manganites [7]; the development of high concentrated n/p-type carries for electronic and optoelectronic devices [8]; the new emergent properties across the interface between various iso-structural compounds via interfacial structural proximity effect [9], etc..

The doping of hydrogen ion into transition metal oxides is unique due to the bonding flexibility, which can be forms of different types of charge states, i.e., positively charged proton H⁺, or negatively charged hydride ion H^-, or charge neutral H⁰ as reported in metallic hydrides. The introduction of hydride ion H^- in transition metal oxides, namely transition metal oxyhydride, has been reported to create a heavily n-type doped system in BaTiO_3-xH_x [10], to form the special M-H^--M bonds (M denoting the transition metal ions) resulting in an antiferromagntic ground state with high Neel temperatures in SrVO₂H, LaSrCoO₃H_0.7 and SrCrO₂H compounds [11-14]. The protonation of transition metal complexes attracts considerable attentions, accompanying with the tunability of physical properties and the emergent novel electronic phases. For instance, the new HSrCoO_2.5 insulating ferromagnetic phase emerged in electric-field controlled insertion/extraction of H⁺ ions in strontium cobaltite thin film via ionic liquid gating method [15]; the proton ion concentration controlled insulating (pristine)-metal-insulating phase modulation in VO₂ films through hydrogen spillover method [16]. Proton doping thus serves as an attractive strategy showing the capabilities in strong modulations of the crystal structures and the phase diagrams as well as in exploring novel electronic phases. Different protonation methods have been reported in literatures, e.g., the Pt-catalyst-assisted hydrogen spillover method [17], the different electrolyte gating methods [15, 16, 18], and the hydrogen bombardment method with several keV to MeV high energy hydrogen ion irradiation [19]. These protonation processes involve either time-consuming or chemical reaction or high energy ion irradiation environments which restrict the realist technological applications. Therefore, the search for an alternative nondestructive and efficient approach is demanding.

In this article, we demonstrate the protonation in SrCrO₃ (SCO) films using an elaborated low-energy hydrogen plasma implantation method. Hydrogen ions are incorporated into the SCO thin films avoiding the damage of the original lattice structure and accumulate largely at the interfacial region near the substrate which shows amorphous character. The protonated films (H_xSrCrO₃, or HSCO) appear analogous to exfoliated layers from the substrate which interestingly remain strained by the substrates with a strongly distorted perovskite crystal structure and a big lattice expansion of the out-of-plane lattice constant [as illustrated in Fig. 1(c)]. A band-filling controlled metal (SCO) to insulator (HSCO) transition has been proposed to SCO films upon protonation, associated with the variation of Cr electronic configuration from mainly Cr 3d² to Cr 3d³ as determined from soft X-ray absorption measurements at Cr L_{23}-edge. Detailed configuration interaction (CI) cluster calculations have been performed to explore the multiplet features of HSCO films, which reinforce the existence of Cr³⁺ with a 3d³ electronic configuration of octahedral symmetry.

2 Experimental section

Bulk SCO shows a cubic perovskite structure with a lattice constant of 3.819Å as shown in Fig. 1(a), and adopts a Cr 3d² electronic configuration in the ionic limit [20]. The electronic and magnetic properties of SCO are still under debates, which was reported as a paramagnetic metal originally [21], or a paramagnetic insulator owing to bond length fluctuations based on transport measurement [22], or an antiferromagnetic ground state attributed to orbital ordering based on neutron diffraction analysis [23]. Our SCO thin films were deposited on the compressively strained LaAlO₃ (001) (LAO) substrate by molecular beam epitaxial (MBE) technique followed by annealing in air at 250°C for 2h, which show a fully-strained single crystalline phase with high quality from X-ray diffraction and transmission electron microscopy (TEM) analyses (see Ref. [20] for details) as well as a metallic paramagnetic ground state [24].

The hydrogenation experiments of epitaxial SCO thin films were performed with a home-made plasma facility where an adopted low-frequency inductively coupled plasma system was used to generate high density hydrogen plasma [25]. Plasma is a quasineutral medium with identical densities of positive and negative particles. A negative dc bias of -180 V was applied to control the built-in electric field of the plasma, leading to a dominant accumulation of protons, rather than H^- or H⁰, near the film surface as illustrated in Fig. 1(b). The hydrogen plasma implantation were carried out at a vacuum pressure of 5P_a and a base temperature of 300°C. The hydrogen plasma treatment was optimized with a RF power of 1800 W, H₂ fluxes of 20 sccm, and a treated time of 10 mins, as tabulated in Table 1. The plasma environment has been demonstrated as an attractive platform for synthesis of functional nanostructured materials [26, 27] which can be used for electronic, energy and space technological applications [28-30]. We note that our plasma treatment is a low-energy implantation and diffusion process of the ionized hydrogen gas with less destructive to the samples [25, 31], compared to the high-energy, high-flux plasma implantation in regular semiconductor techniques [32].

TOF-SIMS (time-of-flight secondary-ion mass spectrometry setup from GmbH, Germany) measurements had been performed to provide direct information about hydrogen concentrations. Caesium ion beam was used as the sputtering beam with energy of 1 keV at an incident angle of 45° and a sputtering rate of about 0.142 nm/s calibrated for SiO₂, which is capable for collecting the ions with atomic mass between 0–1000 amu. Taking into account the total thickness of SCO thin films as 50 nm, we estimate a sputtering rate of about 0.1 nm/s for SCO thin films. The depth profiles of different ions are shown in the main text. The atomic structures of HSCO thin films were examined using the JEOL2100F equipment for high resolution TEM measurements. The valence state of Cr is investigated by element-specific X-ray absorption measurements, where the positions of the absorption edges reflect the valence states directly. The absorption measurements were performed at the Magnetic Circular Dichroism beamline (BL12B) of National Synchrotron Radiation Laboratory (NSRL), Hefei. Data are collected in total electron yield mode.

3 Results and discussion

Figure 2(a) shows the θ–2\theta X-ray diffraction near the LAO (002) reflection to determine the structural variation before and after hydrogen plasma implantation. The HSCO thin film shows a larger out-of-plane lattice constant of 3.973\pm 0.007 Å compared to 3.829\pm 0.005 Å of pristine SCO. The lattice expansion of the out-of-plane lattice constant is similar to the lattice expansion reported in protonated HSCoO_2.5 and HVO₂ systems, whereas opposite to the structural contraction observed in many transition-metal oxyhydrides with the replacement of O^2- by H^- anion, e.g., LaSrCoO₃H_0.7, BaTiO_3-xH_x, SrCoO_xH_y, and SrVO₂H [10-13], etc.. The HSCO thin films preserve the perovskite crystal structure (as also suggested by the diffraction patterns from high resolution TEM shown in Figs. 3(c, d) where the HSCO film shows the same symmetry as the LAO substrate) and the fully strained state with identical in-plane lattice constant of 3.789 Å as LAO substrate from the reciprocal space maps around (103) reflections shown in Fig. 2(b). The lattice constant ratio c/a\sim 1.048 representing the tetragonality of HSCO film is much larger than the c/a\sim 1.01 before treatment. The change of the crystal structure from cubic to tetragonal symmetry is illustrated in Figs. 1(a) and (c). This suggests the constraint of the in-plane biaxial lattice parameters by compressive strain induced by the substrate whereas only a strong expansion of the out-of-plane lattice constant is accommodated by hydrogen incorporation.

The insertion of hydrogen ions is verified by TOF-SIMS measurements. Since the absolute intensity values are meaningless, the intensity ratios [i.e., I(\text{A})/I(\text{C}\text{r}) with A denoting H, Cr, ¹⁸O, La and Al ions] are used for comparison and a pristine SCO film is used as reference. Figures 2(d, e) present the intensity ratios for different ions with depth profiles. Overall, hydrogen occupies both the thin films layer and to some extent in the substrate layers and the thin films there appears a reduction of the total thickness of \sim 12 nm after ion implantation experiment which might be related to the small amount of plasma etching at the surface region. A pronounced hydrogen content is observed for HSCO film compared to pristine SCO, confirming the existence of hydrogen through implantation experiment. The hydrogen ions show accumulations at the interfacial region near the LAO substrate. Moreover, Cr ion exhibits a reduced intensity at the H accumulated region close to the interface, which is further identified as “buried” areas without well-defined crystallinity from high resolution TEM measurements as shown in Fig. 3. Finally, no clear oxygen vacancies is observed before and after hydrogen plasma implantation as suggested by the comparable normalized I{(}^{18}\text{O})/I(\text{C}\text{r}) intensities, implying that proton incorporation rather than the creation of oxygen deficiencies is the main reason for the variation of electronic configurations as will be presented below.

Subsequent structural analysis of HSCO films with high resolution TEM measurements are shown in Fig. 3. The thickness of the thin film is about 50 nm, as suggested from the bright field TEM image in Fig. 3(a). More than two-thirds of the interfacial regions with hydrogen accumulations exhibit amorphous feature without well-defined crystallinity. The “buried” regions expand about 10 nm in thickness as determined from the high resolution TEM image in Fig. 3(b). The low connectivity of the structural blocks at the interfacial region and the easy solubility feature of amorphous phase compared to solid phase with ionic bonds provide the possibility to be peeled off from the substrate which might allow for the capability to be transferred to other substrates flexibly. The HSCO thin films appear like exfoliated layers which however retain the fully strained state with the same in-plane lattice constant as the substrate as suggested by off-specular reciprocal space map and the similar crystal symmetry as suggested by the TEM diffraction patterns along (001) and (111) directions [Figs. 3(c, d)]. Figure 3(e) shows the high resolution TEM image at the interface between the HSCO and LAO substrates corresponding to the small region without amorphous character as marked in panel (a). The rectangular area is enlarged in Fig. 3(f) showing a good epitaxial relationship between the HSCO film and the LAO substrate, which however shows some missing of atoms near the interfacial region. The atomic positions of the heavy ions indicate a strongly distorted perovskite structure which might be related to the change of the radii of transition metal ion due to the change of valence state from Cr⁴⁺ to Cr³⁺ and the interstitial of protons with attractive force to the negatively charge O^2- ions. The averaged out-of-plane lattice constant of the HSCO film is estimated to be \sim 3.93 Å as shown in Fig. 3(f), which is consistent with the out-of-plane lattice constant expansion determined from X-ray diffraction measurements.

Figure 4(a) displays the Cr L_{23}-edge absorption spectra of SCO and HSCO thin film. The Cr L_{23}-edge absorption spectra compose of Cr L₃ and L₂ edges owing to spin-orbit coupling of the Cr 2p levels. The pristine SCO film shows identical absorption spectra as the SCO film reported in literature in both Cr L_{23}-edge and O K-edge [as shown in Fig. 5(b)], suggesting a mainly Cr⁴⁺ valence state. The peak positions at Cr L_{23}-edge of HSCO films exhibit a shift of approximately 1.15 eV towards lower absorption energies compared to pristine phase, which is a comparable chemical shift as reported in literatures for one electron change in Cr ions and indicates the primarily Cr³⁺ valence state [20]. We further compare the measured absorption spectra to the reference spectra measured in oxygen-deficient SrCrO_2.8 thin films and the spectra reported in literatures for SCO thin film [20], the La_0.75Sr_0.25CrO₃ and the LaCrO₃ thin films [33, 34]. The HSCO film show comparable absorption edges as the La_0.75Sr_0.25CrO₃ and LaCrO₃ thin films with the dominant Cr³⁺ valence state.

To obtain a deep insight into the electronic information of Cr L_{23}-edge X-ray absorption spectrum, we perform the configuration interaction (CI) cluster calculation for a CrO₆ (Cr: 3d³) cluster with 3d³ electronic configuration. CI cluster calculations take the full multiplet effects into account and show the advantages of interpreting the L_{23}-edge excitation spectrum over the last decades [35-37]. We refer to the literatures as well as our previous reports for the backgrounds and details on performing the CI cluster calculations [35, 37-39]. The simulated spectra presented here are considered only the three lowest energy configurations. The simulated spectra are shown in Fig. 4(b) with a variation of the crystal splitting energy 10 Dq from 0.2 eV to 1.8 eV at an interval of 0.4 eV. The other energy parameters used for present calculations are (in units of eV): U_{dd}=5.5, U_{pd}=7.3, \text{Δ}=5.5, pd\sigma =2, which are the same as the values used in LaCrO₃ and Cr₂O₃ compounds [40, 41]. As 10 Dq increases, the peak splitting inside the L₃ and L₂ absorption edges increases due to the increased energy differences between the t_{2g} and e_g levels. A further comprehensive comparison between the energy splitting values at Cr L₃ edge and the intensity ratios at Cr L₂ excitation peaks suggests a 10 Dq value between 1.0 and 1.4 eV which reproduces the experimental spectrum well. Our CI calculations further reinforce that the Cr ions exist with 3+ valence state sharing an octahedral symmetry surrounded by six O atoms in HSCO thin films, whereas Cr with other valence states are not favorable.

A combined valence band X-ray photoelectron spectroscopy (VB XPS) and O K-edge absorption measurements have been performed to investigate the variation of electronic structures close to Fermi level. The valence band XPS spectra are shown in Fig. 5(a), all of which are referenced to the Fermi level using a Au foil with direct electronic contact to the film surface for energy calibration. The VB XPS spectra are characterized by three regions labelled as A₁, A₂ and A₃. The two peaks showing high binding energies relative to the Fermi level (A₂ and A₃) are dominated by the O 2p-derived with minor Cr contributions due to the O 2p-Cr 3d hybridization. Peak A₁ close to the Fermi level can be assigned to the occupied Cr 3d t_{2g} orbitals with a high-spin electronic configuration. The assignments of these features are the same as been proposed for SCO films, LaCrO₃ powder and thin films as well as the band structure calculations by density functional theory therein [33, 34, 42]. A notable variation of the density of states near the Fermi level has been suggested from valence band XPS spectra. In the pristine SCO film, the Fermi level lies inside the Cr 3d band, which is consistent with the metallic electrical conductivity behavior [33] and is expected due to the partially occupied electronic states on the six-fold degenerated t_{2g} manifold. However, the valence band maximum is located below the Fermi level for HSCO films, indicating the formation of a band gap after hydrogen plasma implantation, as will be discussed below.

The multiplet structures at O K-edge show dramatic different Cr d-O p hybridization characters for Cr⁴⁺ and Cr³⁺, as shown in Fig. 5(b). The peaks B₁-B₄ in SCO film can be assigned to the excitations to empty Crt_{2g}\uparrow t_{2g}\downarrow e_g\uparrow e_g\downarrow states, associated with a metallic state. The O K-edge absorption spectrum of HSCO thin film shows a higher excitation energies which is comparable to that of oxygen deficient SrCrO_2.8 film, but with less spectral features at the Cr-O hybridization region compared to others. The loss of hybridisation feature is similar to the results observed in HSrCoO_2.5 and Li_xFeO₄ upon lithiation [15, 43].

In the ionic limit, pristine SCO has a t_{2g}^2 electronic configuration where the e_g electron levels are higher in energies due to crystal field effect. The small bond length differences between in-plane and out-of-plane directions as well as the small on-site energy differences among d_{xy}, d_{xz} and d_{yz} orbitals result in a metallic ground state in pristine SCO, where the two electrons are shared by three orbitals in t_{2g} level. An electron itinerant model as illustrated in Fig. 5(c) thus is expected due to the unoccupied electronic states on the six-fold degenerated t_{2g} manifold, which agrees well with the presence of density of states near Fermi level as suggested by VB XPS spectrum. The HSCO thin films are transparent in color and are insulators with room temperature resistance above 5 MΩ. The absence of density of states near Fermi level for HSCO film is similar to that of the oxygen deficient SCO_2.8 phase which shows an insulating behavior. Doping of protons induces a single electron to Cr ion with Cr valence state changing from Cr⁴⁺ to Cr³⁺ as confirmed in the Cr L_{23}-edge absorption measurements. Cr³⁺ in HSCO films adopts the t_{2g}^3 electronic configuration. An energy gap is expected, where an energy barrier should be suppressed for electronic conductivity. Band filling of 3d orbitals is one of the main concepts in strongly correlated systems, which determines various electronic ground phases [7]. The metal (SCO)-insulator (HSCO) transition is analogous to the band-filling controlled Mott transitions through proton doping effect, similar as reported in proton-doped SmNiO₃ compound [44]. Moreover, the strongly distorted structure might also contribute to the insulating behavior due to the reduced bandwidth with strong distorted Cr-O-Cr bonds.

4 Conclusions

In conclusion, an efficient and nondestructive low-energy hydrogen plasma implantation experiment has been elaborated to investigate the H accumulation effect and the modulation of the electronic phases in transition metal SCO thin films. Hydrogen ions accumulate largely at the interfacial regions near the LAO substrate. The atomic structure at the interfacial region is examined by high resolution TEM measurements which shows amorphous character. The low connectivity of the structural blocks at the interfacial region and the easy solubility feature of amorphous phase compared to solid phase with ionic bonds suggest that the HSCO films are convenient to peel off and allow to be transferred to other substrates flexibly, which however retain the fully strained state with distorted perovskite structure. Proton introduces an additional electron to the strontium chromium thin films which induces a mainly Cr³⁺ valence state in HSCO films compared to the primarily Cr⁴⁺ in pristine SCO films. The variation of the electronic configuration from 3d² to 3d³ upon protonation drives a change from metallic (SCO) to insulating (HSCO) phases as suggested by the variation of the density of states near Fermi level from VB XPS characterizations, analogous to the band-filling controlled Mott transition mechanism. The multiplet features at Cr L_{23}-edge absorption spectrum of HSCO film can be reproduced well with crystal field splitting 10 Dq between 1.0–1.4 eV using CI cluster calculations considering the Cr t_{2g}^3 electronic configuration of a CrO₆ cluster under octahedral symmetry. This implies the retain of the original crystal structures which avoids the strong disorder of Cr ions due to hydrogen bombardment or the occupancy of the substitutional sites by hydrogen ions rather than interstitial sites. The facile hydrogenation process offers a new possibility to tailor the crystal structure and the electronic configurations towards fascinating physical properties of perovskite oxides, such as hydrogen storage devices as well as integrated electronic and photonic applications [45, 46]. The special interfacial properties between the thin film and the substrate may pave a new strategy for application of strain-mediated transition metal oxide thin films towards nanoscale engineering through peeling off process, and open a new platform to manipulate the interplay between different collective phenomena through non-destructive low-energy hydrogen plasma implantation approach.

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