Controlled Growth and Chemical Engineering of FeSe‐Based Superconducting Films

Chemical engineering tailors functional materials to meet the demands of physical and chemical properties, accelerating materials discovery and design in a multidisciplinary field. However, stereotyped synthetic paradigms in physical and chemical approaches obstruct the effective integration of multiple advantages of respective materials exploration methods. Solid state reaction is often considered separately with wet chemistry synthesis route, such as film growth using physical vapor deposition and hydrothermal methods. Here, the controlled growth of FeSe thin films by physical deposition techniques followed by the chemical engineering to create heterogenous interface in FeSe films via a solution‐based environment is demonstrated. The two‐step route allows an expanded crystal structure and enhanced superconducting transition temperature from 3.6 to 38 K, providing a new strategy to design functional hybrid materials.

Here, we demonstrate the tunable growth of FeSe thin films by controlling the physical deposition conditions, as well as the chemical engineering through a hydrothermal method to achieve a T c -enhanced superconductor (Li,Fe)OHFeSe. [26,44,45] A co-deposition of Fe and Se sources shows a large flexibility to tune their deposition power separately and its feasibility in experimental practice, compared to those using one target of FeSe compound mixture. Low-power sputtering of Fe source benefits the homogeneity and compactness of FeSe thin films. The superconducting -FeSe phase can grow on oriented magnesium oxide and silicon (MgO, Si) or disoriented Si substrates, while the oriented FeSe films are shown on the MgO. MBE grown superconducting FeSe films are then chemically converted into (Li,Fe)OHFeSe phase via a modified hydrothermal method. The Figure 1. Schematics of physical vapor deposition method and solution-based chemical engineering route used in this work to grow FeSe-based superconducting films. a) The representative magnetron sputtering system is used to co-deposit FeSe films starting from Fe and Se targets by DC and RF bias modes, respectively. The picture shows the vivid deposition scene of plasma arcs. MBE technique that is not shown here requests ultra-high vacuum and thermal evaporation to deposit element clusters onto substrates, similar to sputtering. The crystal structure of superconducting -FeSe shows the layered feature. b) In the environment of ionic solution for hydrothermal reaction, FeSe layered structure is expanded by intercalating (Li,Fe)OH layer, which was realized in the physical deposited FeSe films. The resulted crystal structure after chemical engineering exhibit a new interface between FeSe and (Li,Fe)OH. two-step process combining both physical and chemical routes presented in this study show the growth tunability and interfacial engineering of FeSe thin films, providing a new strategy for improving its superconducting performance.

Synthesis Route
The controlled growth and chemical engineering of FeSe thin films are conducted in two steps, where both sputtering and MBE physical deposition techniques are studied for the film growth and hydrothermal reaction [3,26,[46][47][48] is applied for intercalating (Li,Fe)OH into FeSe lattice to modify their interfacial environment. As physical vapor deposition methods, [49] sputtering and MBE techniques are utilized to evaporate Fe and Se sources and get element clusters aggregated and grown onto the selected substrates (Figure 1a). Figure 1a presents a representative magnetron sputtering system applied for the growth of FeSe thin films in this work. Here, FeSe films were co-deposited by DC and RF magnetron sputtering using Fe and Se targets, respectively. Differing from a general deposition using one target, [36] DC power of Fe source was controlled variably from 1 to 10 W and RF power of Se source was kept constant as 20 W in the codeposition. We apply magnetron co-deposition to study the effect of the growth conditions including substrate temperature, deposition time, and sputtering power. Such co-deposition process is undergoing with plasma arcs flourishing from two targets onto the rotating substrate holder under an elevated temperature. The Fe and Se element clusters aggregate and assembly into thin films on substrates with specific atomic coordination.
Crystal structure of -FeSe shows the layered structure consisting of FeSe 4 tetrahedra. The simplest structural feature in -FeSe provides an excellent platform to chemically design and engineer FeSe structures, such as an interface between a superconducting FeSe layer and an insulating layer (Li,Fe)OH. Figure 1b illustrates the solution-based synthesis procedure for (Li,Fe)OHFeSe, in which chemical intercalation and assembly of Li + , Fe 2+ , and OH − occur among FeSe layers. The assembled (Li,Fe)OHFeSe films inherit the tetragonal structure from FeSe films, where the expanded layer spacing (from 5.53 Å in FeSe to 7.79 Å) and the interface between FeSe and (Li,Fe)OH layers play crucial roles on its physical properties of such lattice structure.

The Magnetron Sputtering Grown FeSe Thin Films
The surface morphology and crystallized structure of FeSe thin films grown by magnetron sputtering evolves with the evaporation power and substrate conditions (Table S1 and Figures S1 and S2, Supporting Information). An optimum substrate temperature is required for the nucleation and growth of FeSe crystalline thin films, while room-temperature deposition leads to the amorphous phase (film #1 in Figure S1, Supporting Information).   (002)). The scale bars in all SEM images are 500 nm. c) XRD patterns of those FeSe films (#2, #3, #4, #5) demonstrate the phase transformation from Fe 7 Se 8 to -FeSe by decreasing DC power. d) XRD patterns of FeSe films #6 present the dominant -FeSe phase for all kinds of substrates.
the substrate, but also produced irregular bulk clusters with a large size of ≈710 nm. Therefore, a lower DC power improves the homogeneity and smoothness, as shown in the SEM images of FeSe films (#4 and #5) grown at DC power of 7 and 1 W, respectively. The FeSe thin films become more compact under the growth condition of 1 W and 2 h. A lower substrate temperature of 200°C can further decrease the roughness of FeSe thin films as shown in Figure 2b, where the FeSe thin films (#6) show the homogenous particle size of ≈158 nm regardless of substrate categories. Especially, the FeSe/MgO films present the epitaxial growth with a square size of 91 nm × 215 nm. X-ray diffraction (XRD) patterns of those FeSe films reveal the structural modification with DC power of Fe source and substrate conditions. The film synthesized at 10 W with 30 min deposition time shows Fe 7 Se 8 phase and Fe particles with a negligible -FeSe signal, while a longer deposition time of 2 h allows the formation of -FeSe phase (Figure 2c). The decrease of DC power of Fe source can diminish Fe 7 Se 8 phase and presents complete -FeSe phase at the DC power of 1 W. When the substrate temperature is set to 200°C to obtain homogenous and smooth FeSe films, the corresponding XRD patterns show -FeSe peaks with a preferred orientation of (00l) plane (Figure 2d). The -FeSe films deposited on Si(any) substrate show a lattice spacing of 5.477 Å, while the films deposited on Si(111) present a layer spacing of 5.465 Å, 5.458 Å for Si(100) and 5.497 Å for MgO (200). The reduced layer spacing could be ascribed to the deviating overall atomic (Fe : Se) ratio ( Figure S3, Supporting Information) and epitaxy induced stain effect in films. [50] From the perspective of growth kinetics, MgO substrate may allow a feasible growth of FeSe film due to lattice epitaxy.

Electrical Measurements of Sputtering-Grown FeSe Thin Films
The electrical transport measurements of the as-grown FeSe thin films deposited at 200°C reveal -FeSe dominant electronic behaviors. The thin films deposited on Si (100) substrate shows a classic semiconducting behavior from 300 to 2 K (Figure 3a). The resistance slowly increases when temperature is decreased, while its resistance increases rapidly when the film is cooled below ≈50 K. A resistance decrease at ≈4 K is observed in the film deposited on substrate Si (111), exhibiting a semiconducting behavior at higher temperature ( Figure 3b and the inset). This transition is responsive to magnetic field, suggesting a potential superconducting transition in the case of filamentary superconducting phase. The thin films deposited on MgO substrates show a smaller resistance and metallic behavior at below 180 K, even though no zero-resistance is detected at the lowest measurement temperature of 2 K (Figure 3c). Instead of zero-resistance, an upturn in resistance arises at below 9 K (the inset in Figure 3c), which might originate from the localization scattering. The resistance of FeSe deposited on MgO substrates under DC power of 1 W is much lower than those of films deposited under 7 and 10 W (Figure 3d), indicating a high conductivity under a lower deposition power. The FeSe thin films grown by magnetron sput-tering can be chemically tuned via the Se source assisted annealing. The film of Fe 7 Se 8 dominant phase is chosen for further selenization in a sealed vacuum tube. After annealing, XRD pattern shows the phase transformation from Fe 7 Se 8 to FeSe 2 (Figure 4a). Meanwhile, the morphology of thin films becomes more continuous after selenization (Figure 4b,c and Figure S4, Supporting Information).  (Table S2, Supporting Information). The FeSe flakes were formed on the surface deposited at 400°C (Figure 5a,b). The flakes on GaAs with a size of ≈648 nm are larger than those on Si (111). When the substrate temperature increased to 450°C, more irregular bulk particles www.advancedsciencenews.com www.advphysicsres.com  appeared (Figure 5c) with some flakes in a size of ≈326 nm. At a substrate temperature of 500°C, the film surface is almost covered by strip-like flat FeSe flakes in a length of ≈1.44 um (Figure 5d). The difference among those FeSe thin films could be ascribed to not only substrate temperature, but also the homogenous GaAs buffer layer. Before the deposition of FeSe film, the GaAs buffer layer was deposited for 0 min (#7), 35 min (#8), 14 min (#9), and 30 min (#10). The less time deposition of GaAs buffer layer could induce the disordered nucleation and growth that resulted in some irregular bulk particle formation ( Figure  S4, Supporting Information). Temperature dependent resistance confirms the quality of FeSe thin films deposited at 500°C, which shows metallic transport behavior for the full temperature range (Figure 5e). The sharp resistance decrease at 3.6 K of FeSe thin films corresponds to the superconducting transition where its zero resistance is achieved at 2.6 K. Energy dispersive spectroscopy (EDS) analysis reveals the element ratio is close to that of -FeSe ( Figures S6-S9, Supporting Information). Such sharp superconducting transition indicates high homogeneity of as-grown FeSe thin films.

The MBE-Grown FeSe Thin Film and Chemical Engineering
The superconducting FeSe films grown by MBE are utilized for chemical engineering to form the interface between FeSe and (Li,Fe)OH via hydrothermal reaction. After solution-based ionic intercalation and assembly, the crystal structure of FeSe has been transformed as shown in Figure 6a. The XRD peaks of (00l) plane of as-grown (Li,Fe)OHFeSe film are equally separated, indicating the preferred orientation along its c-axis. [51] Figure 6b shows the diamagnetic transition at 38 K that originates from (Li,Fe)OHFeSe films, whose surface morphology is shown in the inset SEM image. This strategy of using a film precursor platform to explore material composition tuning opens up a new route to explore functional thin film materials via chemical engineering. The FeSe is the simplest building block of iron-based superconductor, while its film growth involves rigorous vacuum environment leading to a challenge to incorporate certain chemical components, such as hydroxide ions. Therefore, a solutionbased chemical engineering route on FeSe thin films provides a pathway compatible with aqueous or solvent environment. In the aqueous solution containing of lithium cation, hydroxide anion, iron, and so on, the FeSe lattice is intercalated by (Li,Fe)OH layer and expanded into a heterogenous structure. Through such a two-step synthesis route, new materials are synthesized on the original FeSe films with the formation of interfaces among layers. Therefore, (Li,Fe)OHFeSe film is achieved for a higher T c after the solution-based intercalation. Since (Li,Fe)OHFeSe can show different T c s from superconducting to non-superconducting due to the Fe vacancy in FeSe layer, [46,48] this two-step route also provide an opportunity to further study the stoichiometry effect by controlling hydrothermal condition. [52] Compared to previously reported method using K 0.8 Fe 1.6 Se 2 crystals as ionic sources, [51,52] the two-step synthesis route in this work utilizes a robust precursor and intercalation in solution-based environment, which can be extended into the syntheses of organic and inorganic hybrid functional materials.

Conclusion
In summary, the FeSe thin films are grown by physical deposition techniques of magnetron sputtering and MBE by a co-deposition of Fe and Se sources, and then are considered as precursor materials for interfacial engineering to get intercalated by (Li,Fe)OH layer in a solution-based environment, resulting an expanded crystal structure and enhanced superconducting transition temperature. The high tunability of the deposition process of FeSe thin films is demonstrated by controlling the growth conditions, such as the deposition power, substrate categories and temperatures. Based on the superconducting FeSe films, the two-step synthesis route is designed to achieve (Li,Fe)OHFeSe films in aqueous solution, which provides a new strategy to achieve functional hybrid materials.

Experimental Section
Magnetron Sputtering Growth of FeSe Films: The co-deposition of FeSe films (#1-#6) were conducted on a 2'' target DC and RF sputter deposition system-PRO Line PVD75 (Kurt J. Lesker Company). Fe and Se targets were mounted for DC and RF sputtering modes, respectively. The plasma ignition power was 20 W for Fe and 50 W for Se in the presence of 10 mTorr Ar environment. For each co-deposition, RF power was always set as 20 W, while DC power was varied from 10 to 1 W. Several substrates were used including MgO (200), Si (100), Si (111), and Si without preferred orientations. The substrate temperature was set as room temperature, 200 and 350°C. Deposition time was set as 30 min and 2 h. The Si substrates have been treated with HF to remove the oxide layer.
Annealing Treatment of FeSe Films: 0.3 g Selenium power and FeSe film were loaded into a sealed glass tube for annealing in a furnace. The sample was heated from room temperature to 200°C in 230 min and held for 6 h, then cooled down to 140°C in 480 min, finally cooled naturally to room temperature.
MBE Growth of FeSe Films: The MBE system that was designed and constructed by Dr. David B. Eason, has six solid source effusion cells and 2 RF plasma gas sources, enabling the deposition of FeSe films (#7-#10). Growth process was in situ monitored by reflection high energy electron diffraction (RHEED). 1 × 10 −10 Torr system base pressure was sustained by cryopump with an ion pump option. Fe and Se elements were supplied from pure sources through resistive heating. GaAs buffer layer was pre-deposited for 14-35 min. Substrate GaAs (200) was used and deposition temperature was 400, 450, and 500°C.
Solution-Based Chemical Engineering of FeSe Film: The chemical engineering was conducted in the hydrothermal reaction environment. 4 g Lithium hydroxide monohydrate, 0.21 g Fe powder, 0.75 g selenourea were mixed with 5 mL de-ionized water in a 25 mL Teflon-lined autoclave. Several pieces of FeSe films were put into the Teflon liner for one-day heating at 120°C. After the natural cooling of the reactor, those FeSe films were washed and dried for measurements.
Structural and Morphologic Characterizations: XRD was conducted on the Rigaku Ultima IV (40 kV, 44 mA, Cu K ) with a scanning range from 5°t o 80°and a sweeping rate of 4°min −1 . The morphologies of FeSe films were characterized by Field Emission Scanning Electron Microscope (FE-SEM) Carl Zeiss AURIGA (200 kV) equipped with Oxford Energy-dispersive X-ray Spectrometer (EDS) for element analysis.
Low-Temperature Resistance and Diamagnetism Measurements: Temperature dependent resistance of FeSe films were measured with a fourprobe van der Pauw method on Physical Properties Measurement System EverCool II (Quantum Design). Diamagnetic measurements were conducted on a vibrating-sample magnetometry that is mounted on PPMS.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.