RSC Advances View Article Online PAPER View Journal | View Issue Accessing new 2D semiconductors with optical band gap: synthesis of iron-intercalated titanium Cite this: RSC Adv.,2018,8, 22552 diselenide thin films via LPCVD† Clara Sanchez-Perez, ab Caroline E. Knapp,a Ross H. Colman, c Carlos Sotelo- Vazquez,a Raija Oilunkaniemi, b Risto S. Laitinen *b and Claire J. Carmalt *a Fe-doped TiSe2 thin-films were synthesized via low pressure chemical vapor deposition (LPCVD) of a single 5 5 source precursor: [Fe(h -C5H4Se)2Ti(h -C5H5)2]2 (1). Samples were heated at 1000 C for 1–18 h and cooled to room temperature following two different protocols, which promoted the formation of different phases. The resulting films were analyzed by grazing incidence X-ray diffraction (GIXRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM) and UV/vis spectroscopy. An investigation of the Fe doping limit from a parallel pyrolysis study of FexTiSe2 powders produced in situ during LPCVD Received 13th April 2018 depositions has shown an increase in the Fe–TiSe –Fe layer width with Fe at% increase. Powders were Accepted 12th June 2018 2 Creative Commons Attribution 3.0 Unported Licence. analyzed using powder X-ray diffraction (PXRD) involving Rietveld refinement and XPS. UV/vis DOI: 10.1039/c8ra03174f measurements of the semiconducting thin films show a shift in band gap with iron doping from 0.1 eV rsc.li/rsc-advances (TiSe2) to 1.46 eV (Fe0.46TiSe2). Introduction axis via van der Waals forces.9 Many kinds of atoms and organic molecules can be intercalated into the van der Waals gap sites The continuous search for high-speed/low-power electronics of layered TMDs, causing dramatic changes in physical prop- 10,11 beyond the current silicon-based devices has incentivised the erties of the host materials and generating a wide variety of This article is licensed under a research on these layered materials with band gaps, redirecting magnetic orderings in these so-called intercalated transition 10 interest from graphene to 2D group 4 and 6 TMDs materials and metal dichalcogenides (ITMDs). Owing to their ability to exist heterostructures with tuneable band alignments for a variety of in more than one oxidation state, investigation of transition 1–5 Open Access Article. Published on 20 June 2018. Downloaded 9/28/2021 9:15:28 PM. nanoelectronic/optoelectronic applications. The design of metals and their role in electronic and transport properties of 12 high-efficiency materials which can convert solar to electrical the aforementioned complexes is of great interest. Ternary energy is a continuously increasing research topic, in which the chalcogenides of the series FexTiSe2 have a defect NiAs-type 13,14 main path nowadays involves thin-lm technology.6 Early lattice. The anions are arranged in hexagonal closed-pack transition metal chalcogenides have been reported to have layers sandwiching octahedrally coordinated Ti4+ cations. 7 applications as cathode materials for rechargeable batteries. Previous studies of the FexTiSe2 system as a function of x Many of them are semiconductors with band gaps lying within showed the existence of several phases corresponding to the UV-visible region, and are predicted to be strong absorbers superstructures of the TiSe2 reference cell, as a result of an of light, which makes them interesting candidates in the search ordering of iron and vacancies in the van der Waals gap for 13,15 of next generation solar cell energy devices.8 values of x > 0.2. It was found that hybridization of Ti 3d and Group 4 transition metal diselenides (TMDs) crystallize in Fe 3d states, along with the overlap of titanium and iron dz2 the 1T-CdI2 structure P3m1, with a hexagonal layer of transition electron shells along the c axis leads to covalent bonding metal sandwiched between two hexagonal layers of chalcogen between layers; therefore a lattice compression occurs along the atoms (S, Se, Te). In the crystal structure, layers stack along the c c axis, although the unit cell volume increases linearly with iron concentration.16–18 aUniversity College London, Department of Chemistry, 20 Gordon St, London, WC1H The FexTiSe2 system presents retrograde solubility in the 0AJ, UK. E-mail: [email protected] solid state; an increase in the temperature of the system results bLaboratory of Inorganic Chemistry, Environmental and Chemical Engineering, in iron release, and on further heating it re-enters the lattice. University of Oulu, P. O. Box 3000, FI-90014 Oulu, Finland. E-mail: risto.laitinen@ This phenomenon is caused by the thermal expansion of the oulu. impurity band of the Fe 3d/Ti 3d hybrid states. Thermally cCharles University, Faculty of Mathematics and Physics, Department of Condensed induced phase transitions are of rst order, and therefore the Matter Physics, Ke Karlovu 3, Prague 2, Czech Republic † Electronic supplementary information (ESI) available. See DOI: specimen state at the given temperature can be xed by 18 10.1039/c8ra03174f quenching. 22552 | RSC Adv.,2018,8, 22552–22558 This journal is © The Royal Society of Chemistry 2018 View Article Online Paper RSC Advances t Bulk TiSe2 shows metallic conductivity and an optical band BuLi (1.7 M in Hexane, Aldrich) and bis(cyclopentadienyl)tita- 19 2+ gap of 0.1 eV. Upon intercalation of high spin Fe in the TiSe2 nium(IV)dichloride (Aldrich) were used as purchased. Dry THF “host” structure, the electrons donated from iron contribute to (99.9% in Argon, Sigma) and dry toluene were stored over the titanium t2g band suppressing its charge density wave a sodium mirror for 24 h prior to use, and pre-dried dichloro- behaviour20 and increasing the resistivity of the material with methane was dried over Mo sieves 3 A˚ (20% m/v) for 48 h prior increasing iron concentration.11 A transition from spin glass to use. All preparations were undertaken using Schlenk line behaviour to an antiferromagnetic regime occurs at percolation techniques, and all glassware was dried for 12 h at 200 C prior threshold of intercalated iron x > 0.2 due to coupling between to use. Synthesis of the precursor was performed under argon, the iron magnetic moments and superexchange which was passed over a drying column. Aer isolation, the interactions.13,17 polycrystalline powder precursor was stored in a glovebox under Conventional methods for preparation of binary and ternary an Argon atmosphere. Synthesis and purication of the transition metal dichalcogenides involve heating of high purity precursor was conrmed by NMR:22 1H NMR (600 MHz) d/ppm 13 1 elements at high temperatures for long periods of time, and (C6D6): 5.67 (s, 10H); 4.35 (m, 8H). C{ H} NMR (600 MHz) d/ requires several homogenizing steps, making their synthesis ppm (C6D6): 110.99 (m, Fc); 111.26 (m, Cp). long and expensive.16,18,21 The decomposition of metal chalco- genolato complexes to metal chalcogenides takes place at Deposition studies – apparatus and characterization signicantly lower temperatures, and they provide high purity materials required for electronic applications. Recent research LPCVD experiments were carried out in a quartz tube under À has highlighted the importance of careful precursor design, in dynamic vacuum (10 1 torr) embedded inside a furnace to allow order to facilitate cleaner decomposition,22 lowering contami- uniform heating. The temperature was controlled using Pt–Rh nation and improving performance of the resultant functional thermocouples. The polycrystalline precursor was spread evenly material. Chemical vapour deposition (CVD) has drawn atten- in a glazed ceramic boat (0.9  1.4  10.3 cm, VWR® Cat. no. – – Creative Commons Attribution 3.0 Unported Licence. tion as a method to synthesis thin lms of functional mate- 459 0224) and heated up to 1000 C for 1 18 h in order to rials,23 including metal selenides.24,25 The use of single source achieve its sublimation and the formation of the nal product. precursors in CVD has a number of advantages, not least the Powders were collected alongside lms on quartz slides (2.5 cm simplication of the decomposition mechanism since all  1.0 cm  2 mm) supplied by Multi-Lab, which were cleaned required elements are delivered to the desired substrate at the using acetone (99%), isopropanol (99%), and distilled water and same time. Earlier this year compounds of the type: [Fe(h5- dried at 200 C overnight prior to use. The precursor (ca. 0.15 g) 5 C5H4Se)2M(h -C5H5)2]2 [M ¼ Ti (1), Zr (2), Hf (3)] have been sublimed and deposited over the slides in the hot zone of the reported22 for potential utilization as single source precursors. reactor. Aer several attempts two cooling protocols were À In particular, the use of the titanium species 1 in the LPCVD established: 1 – cooled at 13 min 1 to 450 C and then À This article is licensed under a 1 fabrication of iron intercalated titanium diselenide FexTiSe2 quenched; 2 – gradual cooling of 13 min to 355 C, followed À could remove any pre-reaction issues or formation of unwanted by 2 C min 1 to room temperature. The tube was then trans- side products and facilitates a facile, one step route to func- ported inside the glovebox, where the remaining powder and Open Access Article. Published on 20 June 2018. Downloaded 9/28/2021 9:15:28 PM. tional semiconductor materials with tuneable band gap. the quartz slides were stored for characterization. The powder There are only a handful of reports using CVD to produce samples were grinded in a metal mortar inside the glovebox, thin lms of metal chalcogenides.19,26–29 The work presented loaded to 3 mm borosilicate capillaries and sealed for herein explores the capability of our precursors to generate high characterization.