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Dimensionally Reduced 1D Chains of TiSe
Tianyang Li, Yi-Hsin Liu, Spencer Porter, and Joshua E. Goldberger Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm400401z • Publication Date (Web): 24 Mar 2013 Downloaded from http://pubs.acs.org on March 26, 2013
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Chemistry of Materials
Dimensionally Reduced 1D Chains of TiSe2 Tianyang Li, Yi-Hsin Liu, Spencer Porter, Joshua E. Goldberger* Department of Chemistry and Biochemistry, The Ohio State University, Columbus, Ohio 43210, United States Dimensional reduction, molecular chains, metal chalcogenide ABSTRACT: Reducing the dimensionality of inorganic lattices down to the single polyhedral level enables the creation of new materials with fundamentally different optical and electronic properties. Here, we demonstrate for the first time the creation of TiSe2(ethylenediamine) and TiSeO(ethylenediamine) which are 1D dimensionally-reduced lattices of the layered metal dichalcogenide TiSe2. These structures consist of 1D zigzag chains of edge-sharing Ti octahedra. Both materials show a strong absorption above 1.21 eV whereas the TiSe2(ethylenediamine) shows pronounced photoluminescence at energies close to the band edge. This 1.4 eV increase in band gap compared to the -0.2 eV band overlap in semimetallic TiSe2 is similar to the previously reported 1.4 eV increase from TiS2 to TiS2(ethylenediamine). This implies that a specific reduction in polyhedral connectivity will lead to a systematic change in band structure.
Layered metal dichalcogenides (LMDC) are one of the most well studied classes of materials due to their fascinating properties and applications1. TiSe2, for example, is a semimetal2 that exhibits correlated electronic phenomena such as charge density waves3 as well as superconductivity upon Cu intercalation4. Dimensionally-reduced hybrid organic-inorganic crystalline phases, or phases in which the polyhedral connectivity of an inorganic lattice is partially disrupted along one or two dimensions, often exhibit unexpected and unique properties5. Recently we have created TiS2(en) (en=ethylenediamine), the first dimensionally-reduced LMDC having a crystal structure composed of zigzag edge-sharing Ti-S octahedral chains6. Dimensional reduction transforms the 0.3 eV indirect band gap of TiS27 into a 1.7 eV direct band gap. Here we demonstrate that it is possible to apply the same synthetic approaches to create dimensionally-reduced derivatives of TiSe2. We have synthesized TiSe2(en) as well as TiSeO(en), which both have the same crystal structure as TiS2(en). TiSe2(en) also exhibits a conversion from an indirect band overlap to a direct band gap with a 1.4 eV increase in the band gap compared to TiSe2. TiSe2(en) was synthesized using TiCl4 and elemental Se as precursors. Se powder was reduced in excess en by NaBH4 before injecting TiCl4. Reactants were annealed in a Parr reactor at 220 ℃ for 5 days. TiSeO(en) was synthesized similarly except Se powder was dissolved in excess en without NaBH4 at 180℃. Water was added before annealing at 220 ℃. The Ti:Se stoichiometry for both samples was confirmed via X-ray fluorescence (Figure S1) and the 2:8:2 C:H:N ratio was confirmed by elemental analysis.
Figure 1. View of (a) TiSe2(en) and (b) TiSeO(en) projected down the c axis. Single distorted octahedron of (c) TiSe2(en) and (d) TiSeO(en). (e) TiSeO(en) chain along c axis. Atom colors: Ti, blue; Se, grey; O, red; N, green; C, black.
Both TiSe2(en) and TiSeO(en) have a similar crystalline structure to TiS2(en) reported previously6 (Figure 1a, b). The crystal structures were determined via Rietveld refinement of X-ray powder diffraction patterns using TOPAS Academic software package (Figure S2-4). Both phases adopt a Rhombohedral unit cell (R-3c space group) with unit cell parameters shown in Table 1. Compared to TiS2(en), the a parameter for TiSe2(en) and TiSeO(en) increase by 0.2 Å and 0.3 Å, respectively whereas the c axis increases by 0.3 Å and decreases by 0.2 Å, respectively. For TiSe2(en) these changes are consistent with replacing S with the larger Se. The refined structures consist of zigzag 1D edge sharing chains of Ti octahedra that
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run parallel to the c axis, due to the incorporation of the cis-binding bidentate ligands into the framework. Neighboring octahedra are connected by two bridging Se atoms for TiSe2(en) or one Se atom and one O atom for TiSeO(en). Within each octahedron, the Ti atom is coordinated by four Se atoms (two Se atoms and two O atoms for TiSeO(en)) and two N atoms from the ligand (Figure 1c, d). Bond angles in the octahedra are distorted away from 90° due to the limitation of the length of the en molecule. The gauche conformation of the en ligand8 was confirmed by Raman spectroscopy (Figure S5). Each 1D chain features a 3-fold periodicity along the c axis (Figure 1e). Table 1. Lattice parameters and band gap energy for TiS2(en),TiSeO(en) and TiSe2(en) a/Å
c/Å
Eg /eV
TiS2(en)
18.596(1)
9.0158(1)
1.70
TiSeO(en)
18.800(2)
8.8154(9)
1.21
TiSe2(en)
18.990(1)
9.3260(3)
1.21
In TiSeO(en), Se and O anions are ordered at distinct sites, based off of Rietveld analysis. Three structures were considered as starting models: 1. O only occupies the trans sites; 2. O only occupies the cis sites; and 3. O/Se atoms are disordered at both sites. The best refinement had the O atoms trans to each other and the Se atoms cis to each other. This gives the Ti-Se and Ti-O bond lengths 2.5058(79) Å and 2.148(17) Å. No significant improvement was observed by introducing O/Se disorder, which gave comparable Rwp (weighed profile R-factor, used to evaluate the goodness of fit) values but unreasonable thermal parameters with only 8 percent disorder. Thus, we are confident that the Se and O atoms are ordered in the structure. The refined positions of oxygen further explain why TiSeO(en) has a smaller c parameter than TiS2(en). The Ti-Se-Ti and Ti-S-Ti bond angles in TiSe2(en) and TiS2(en) are close to 90°. By substituting one Se atom in every edge sharing pair for an O atom, the mismatch in the lengths of the Ti-O and Ti-Se bonds causes the Ti-OTi angle to open up and the Ti-Se-Ti angle to compress (Figure S6). The presence of much shorter Ti-O bonds reduced the distance between Ti atoms in the c-direction thereby reducing the c parameter. The optical properties of TiSe2(en) and TiSeO(en) were investigated using diffuse-reflectance absorption (DRA) and photoluminescence (PL) measurements. Both materials have a broad absorption in visible light region and show an onset at near-infrared (NIR) wavelengths (Figure 2a). The band gap energy for both compounds was determined to be 1.21 eV by linearly fitting the absorption edge on the plot of Kubelka-Munk function (F(R) = (1R)^2/2R, R is the measured reflectance) vs. photon energy. This value is 0.5 eV lower than the band gap of TiS2(en) (1.7 eV). Furthermore, there is a 1.4 eV increase in band
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gap energy with respect to bulk TiSe2, which has a -0.2 eV overlap between conduction band and valence band2. This 1.4 eV increase is almost exactly the same magnitude as the difference between TiS2 and TiS2(en).
Figure 2. (a) DRA absorption spectra of TiSe2(en), TiSeO(en) and TiS2(en). (b) Absorption (black) and NIR photoluminescence (red) of TiSe2(en)
NIR PL was observed from TiSe2(en) at room temperature (Figure 2b) and at 77 K (Figure S7) excited at 785 nm. In both conditions, above band gap emission was observed at 1.38 eV and 1.40 eV while an emission close to the band edge (1.20 eV) was also observed at room temperature. No PL was observed for TiSeO(en). Above band gap PL has been observed to occur for numerous materials including single and few layer MoS2 and Tl4InGa3S89, when there are either defect states or low-lying bands that are relatively higher or lower than the energy of conduction band minimum or valence band maximum respectively, with direct transitions and short radiative lifetime. A detailed investigation of the origin and kinetics of this above band gap PL including the influence of excitation wavelength, power, time and temperature, which is beyond the scope of this paper, is currently underway. The simulated band structure of TiSe2(en) further confirms the presence of numerous bands close in energy to the band gap. The electronic structure of TiSe2(en) was calculated using CASTEP (GGA-WC). The result shows that both conduction band minimum and valence band maximum occur at the Г point (Figure 3a), which indicates a direct band gap and is consistent with the observed PL. The band gap appears to be 0.706 eV, 42% lower than the measured value. This ~40% decrease is expected using this level of theory10. For instance, previously we calculated the band gap of TiS2(en) to be 35% lower than the measured gap6. Attempts to simulate the band structure of TiSeO(en) using different level of theory including GGA and LDA proved unsuccessful as the geometry optimization always gave unrealistic Ti-O bond distance (~1.8 Å). Additionally the refined crystal structures were used to calculate the band structure for all three compounds however TiSeO(en) still gave a significantly higher simulated band gap than the other two. We suspect that the lack of PL in TiSeO(en) is due to a change from direct to indirect band gap and elimination of correlated low-lying bands.
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Chemistry of Materials
The partial density of states plots for specific atoms (Figure S9) reveal that the conduction band is mainly composed of unoccupied Ti 3d orbitals and we see a segregation of two sets of bands. The 18 lower energy bands (0.7-2.5 eV) shown in the band diagram in Figure 3a are comprised of three Ti 3d orbitals (t2g), while higher energy bands (2.5-4 eV) are comprised of two other 3d orbitals (eg). The valence band maximum is predominately composed of Se 4p non-bonding orbitals while the N 2p bonding orbitals lie below at -3 eV.
ASSOCIATED CONTENT Supporting Information Experimental details and characterization data, including Raman, PDOS, and Brillion zone. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author
[email protected] Notes The authors declare no competing financial interest.
ACKNOWLEDGMENT We acknowledge Nick Lead for photoluminescence measurements and Raman Spectroscopy Facility of the OSU Chemistry Department, supported by the National Science Foundation under Grant CHE-0639163. J.G. thanks OSU for financial support. This work was partially supported by the Ohio State University Seed Grant Program.
REFERENCES
Figure 3. (a) Local electronic band structure for TiSe2(en). (b) Partial density of states, where s (blue), p (orange), and d (red) orbital contributions are summed (black). (c) Band structure illustrations of TiSe2, TiSe2(en) and TiSeO(en)
TiSeO(en) has the same band gap as TiSe2(en), suggesting that the position of the top of the valence band is not changed despite the fact half of the Se atoms are replaced with O atoms. Because oxygen is more electronegative than selenium, the O 2p non-bonding bands will appear at lower energy than the Se bands (Figure 3c). However the density of Se 4p states near the band edge should be reduced, which explains why TiSeO(en) has a reduced absorption near the band edge than TiSe2(en) (Figure 2b). In conclusion, we have created for the first time dimensionally reduced hybrid zigzag structures of TiSe2(en) and TiSeO(en) which consist of 1D edge-sharing Ti octahedra chains. Both materials have a 1.21 eV band gap whereas TiSe2(en) has a direct gap with NIR photoluminescence. Interestingly, the 1.4 eV increase in band gap from TiSe2 to TiSe2(en) is of the same magnitude as TiS2 to TiS2(en), which implies that dimensional reduction can be applied to different metal chalcogenide lattices with a predictable and systematic change in band structure.
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