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Iron-Intercalated Zirconium Diselenide Thin Films from The This is an open access article published under a Creative Commons Attribution (CC-BY) License, which permits unrestricted use, distribution and reproduction in any medium, provided the author and source are cited. http://pubs.acs.org/journal/acsodf Article Iron-Intercalated Zirconium Diselenide Thin Films from the Low- η5‑ η5‑ Pressure Chemical Vapor Deposition of [Fe( C5H4Se)2Zr( C5H5)2]2 Clara Sanchez-Perez, Caroline E. Knapp, Ross H. Colman, Carlos Sotelo-Vazquez, Sanjayan Sathasivam, Raija Oilunkaniemi, Risto S. Laitinen,* and Claire J. Carmalt* Cite This: ACS Omega 2020, 5, 15799−15804 Read Online ACCESS Metrics & More Article Recommendations *sı Supporting Information fi ABSTRACT: Transition metal chalcogenide thin lms of the type FexZrSe2 have applications in electronic devices, but their use is limited by current synthetic techniques. fi Here, we demonstrate the synthesis and characterization of Fe-intercalated ZrSe2 thin lms on quartz substrates using the low-pressure chemical vapor deposition of the single-source η5 η5 ff fi precursor [Fe( -C5H4Se)2Zr( -C5H5)2]2. Powder X-ray di raction of the lm scraping and subsequent Rietveld refinement of the data showed the successful synthesis of the Fe0.14ZrSe2 phase, along with secondary phases of FeSe and ZrO2. Upon intercalation, a opt small optical band gap enhancement (Eg(direct) = 1.72 eV) is detected in comparison with that of the host material. ■ INTRODUCTION post-transition20 metals have been intercalated to modulate Transition metal dichalcogenide (TMD) layered materials electronic states and dramatically change the opto-electronic have attracted a great deal of interest due to their unique and magnetic properties of the host material. In some cases, a relatively low intercalation produces a significant change while physical properties beyond those of graphene as they are 13 generally non-toxic and abundant, have energy band gaps well- virtually retaining the host structure. ZrSe is one of the few metal chalcogenide hosts able to suited for solar energy conversion and high absorption 2 1 accommodate Fe both into tetrahedral and octahedral sites coefficients in the visible range. TMDs of group 4, 5, and 6 between layers,15 withtheFerelativesitedistribution metals are diamagnetic semiconductors with band gap values in − 2,3 dependent on the total amount of Fe and the thermal the range of 0.2 2.0 eV. They have received special interest 16 2,4 treatment of the material. At intercalation levels x =0−0.16, due to their mechanical and electrical anisotropy and charge a semiconducting FexZrSe2 phase exists with a trigonal density wave transitions, related to their structures being ̅ structure P3m1 derived from that of ZrSe2, intermediate composed of layers of TX2 (T = transition metal; X = 2,5 between 1T-CdI2 and NiAs types, and at the critical iron chalcogen) stack along the c axis via van der Waals forces. 13 Downloaded via UNIV OF OULU on September 10, 2020 at 06:05:30 (UTC). concentration of x = 0.41, the material becomes metallic. Zirconium diselenide (ZrSe2) in particular has attracted fi Hybridization of the 3d bands of Zr and Fe, along with the considerable attention in the elds of physics, chemistry, and 2 6,7 overlap of their dz electron shells along the c axis leads to a materials due to their unique structural, optical, and ff See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. covalent interaction between ZrSe layers, ultimately a ecting electronic properties. While the use of ZrSe in secondary 2 2 the unit cell lattice constants.19,21 As the intercalation of lithium batteries confronts electrochemical challenges,8,9 it transition metals in TMD 1T polymorphs donates free carriers shows many potential applications as working materials in to their empty conduction band, increasing their conductiv- photovoltaic solar cells10,11 given that its band gap falls in the ity,4,22 they can be selectively used to fine-tune the electric, visible region of the electromagnetic spectrum (E opt = g(indirect) magnetic, optical, and structural properties of the host 1.20 eV; E opt = 1.61 eV).6,12 Group 4 (d0) TMD such as g(direct) materials in a controllable way to fit application requirements.5 ZrSe stabilize 1T structures under normal conditions, with a 2 ZrSe and Fe ZrSe materials have traditionally been CdI (HCP packing) structure and P3̅m1 symmetry.13 This 2 x 2 2 synthesized from high-purity elements8,13,15,16,23 (long multi- structure type allows the intercalation of small molecules or 5 step processes at high temperatures) or chemical vapor atoms (electron donors) between TX2 layers, which transfer electrons from the intercalate to the transition metal unoccupied d band.4 The intercalation of electron-donating Received: January 29, 2020 atoms in TMD layered materials can be used to modulate the Accepted: June 10, 2020 intrinsic physical properties of the host material4 that result Published: June 24, 2020 into band gap changes,14 phase transition processes,15,16 and development of magnetic and superconducting behaviors.2,4,5 ff 17 15,16,18,19 In the case of ZrSe2 di erent alkali, transition and © 2020 American Chemical Society https://dx.doi.org/10.1021/acsomega.0c00413 15799 ACS Omega 2020, 5, 15799−15804 ACS Omega http://pubs.acs.org/journal/acsodf Article η5 η5 fi Figure 1. (a) Schematic representation of precursor [Fe( -C5H4Se)2Zr( -C5H5)2]2 used in LPCVD to grow FexZrSe2 thin lms. (b, c) SEM images of the thin film at x10k and x40k magnification, respectively. The inset shows the cross-sectional image to reveal a film thickness of ca. 300 nm. transport reactions,2,6,8,17,24 mainly as single crystals and, in 25,26 few cases, in nanoparticle form. The synthesis of ZrSe2 thin films has been mainly reported via electrodeposition − methods,11,27 29 although the synthesis using MBE30 and CVD31 has been reported as an alternative for the requirement of conductive substrates, which can be problematic for device fi applications. The absence of studies of FexZrSe2 in thin lm form is likely due to the lack of precursors for these ternary systems using established deposition methods. Low-pressure chemical vapor deposition (LPCVD) of organometallic precursors appears to be a suitable alternative due to its versatility toward the use of alternative air/moisture-sensitive precursors. This method allows growth of high-quality thin films from volatile, temperature-sensitive reagents and has been used extensively in the past decade for the synthesis of thin Figure 2. Rietveld refined powder diffraction data (crosses), fit (red fi line), difference curve (shifted blue line), and reflection markers for all lms of functional materials, many of them metal 13,35−38 32−34 phases (ticks). Structural analysis was performed using the selenides. In the present paper, we investigate the ability 39 η5 η5 GSAS package, and reference patterns from ICSD were used. of single-source precursor [Fe( -C5H4Se)2Zr( -C5H5)2]2 (1) fi Embedded in the right corner is an image of the FexZrSe2 structure to successfully generate thin lms of Fe-intercalated ZrSe2 (i.e., drawn from structurally refined PXRD data using VESTA,40 showing FexZrSe2) using a convenient single-step deposition process via Zr and Se atoms (gray and black spheres) making up edge-linked LPCVD and record the optical band gap shift resultant from layers of ZrSe2 octahedra in the ab plane, separated by Fe atoms (gold the intercalation. spheres) randomly occupying the octahedral vacancies within the van der Waals gap with a refined occupancy of ∼14%. ■ RESULTS AND DISCUSSION α fi However, small additional crystalline secondary phases of - Following our previous work on FexTiSe2 thin lms via and γ-Fe, hexagonal FeSe, and ZrO were also observed. The η5 2 LPCVD from the single-source precursor [Fe( -C5H4Se)2Ti- secondary phase formation is likely due to a combination of η5 34 fi η5 ( -C5H5)2]2, FexZrSe2 thin lm synthesis from [Fe( - post-deposition oxidation (semi-amorphous nature of ZrO ) η5 2 C5H4Se)2Zr( -C5H5)2]2 (1)(Figure 1a) was investigated. during post-deposition film handling and due to possible Fe fl fi The highly re ective and black FexZrSe2 lms grown using saturation in between the layers (e.g., FeSe and α- and γ-Fe) LPCVD under a dynamic pressure of 10−1 Torr at 1000 °C (Figure 2). The parent compound ZrSe2 is known to crystallize were air/moisture-sensitive but highly robust with complete ̅ in the CdI2 structure type with P3m1 symmetry, identical to coverage of the quartz substrate. Scanning electron microscopy that of TiSe2, as shown from the Rietveld analysis. The (SEM) analysis of the filmsshowedthatthesurface structure exists as sheets of Zr atoms, octahedrally coordinated morphology consisted of irregular quasi-spherical grains by Se atoms, with each Se bonded to three Zr atoms. The two- uniformly distributed over the entire surface of the substrate dimensional ZrSe2 sheets are AA stacked and weakly bound, (Figure 1b,c), in a similar way as previously reported for ZrSe2 with the space between sheets commonly referred to as the van fi 28 fi fi thin lms. The lm is fairly densely packed both laterally and der Waals gap. Structural modi cations to the parent ZrSe2 vertically, and the cross-sectional SEM imaging of the film lattice upon the intercalation of variable amounts of Fe within showed a thickness of ∼300 nm (Figure 1c, inset). the van der Waals gap have previously shown a change of Glancing-angle X-ray diffraction (XRD) analysis of the symmetry due to intercalant site ordering at x ≥ 0.41 (in fi fl 13 FexZrSe2 lms was not possible due to uorescence of iron in Fe ZrSe ), along with the onset of metallicity.
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