and Thickness-Dependent Raman Spectroscopy

Transcription

1 Supporting Information Quasi-1D TiS3 Nanoribbons: Mechanical Exfoliation and Thickness-Dependent Raman Spectroscopy Alexey Lipatov, 1 Michael J. Loes, 1 Haidong Lu, 2 Jun Dai, 1 Piotr Patoka, 3 Nataliia S. Vorobeva, 1 Dmitry S. Muratov, 1,4 Georg Ulrich, 3,5 Bernd Kästner, 5 Arne Hoehl, 5 Gerhard Ulm, 5 Xiao Cheng Zeng, 1,6 Eckart Rühl, 3 Alexei Gruverman, 2,6 Peter A. Dowben, 2,6 Alexander Sinitskii 1,6 * 1 Department of Chemistry, University of Nebraska, Lincoln, NE 68588, United States 2 Department of Physics and Astronomy, University of Nebraska, Lincoln, NE 68588, United States 3 Physical Chemistry, Institut für Chemie und Biochemie, Freie Universität Berlin, Berlin, Germany 4 National University of Science and Technology MISIS, Moscow , Russia 5 Physikalisch-Technische Bundesanstalt (PTB), Abbestraße 2-12, Berlin, Germany 6 Nebraska Center for Materials and Nanoscience, University of Nebraska, Lincoln, NE 68588, United States *Corresponding author: sinitskii@unl.edu and Thickness-Dependent Raman Spectroscopy. Supporting Information Page 1 of 8

2 Figure S1. Photographs of vacuum sealed quartz ampules with Ti and S, which were used for the synthesis of TiS3 crystals. (a) The ampule at 550 ºC. (b) The ampule after the synthesis. TiS3 whiskers were grown via the direct reaction between titanium and sulfur at 550 ºC. In a typical synthesis, a ~0.1 g piece of a Ti foil (99.99+%, Alfa Aesar) and ~0.25 g of S powder (analytical grade, Sigma-Aldrich) were sealed in an evacuated (p ~ 200 mtorr) quartz ampule (Figure S1). The ampule was placed in a tube furnace with one end of the ampule being in the center (the hottest area) of the furnace and the other end being closer to the tube opening, thus creating a temperature gradient (Figure S1a). The furnace was heated up to 550 ºC at the rate of 10 ºC/min and the ampule was annealed for 7 days (longer annealing leads to formation of larger TiS3 crystals). TiS3 crystals formed on the surface of Ti foil as well as on the walls of the ampule. When the synthesis was completed, the furnace was turned off, and while the ampule was cooling down, the excessive sulfur accumulated at its colder end, as shown in Figure S1b. According to the S-Ti phase diagram, 1 TiS3 is an equilibrium phase with a restricted homogeneity range. At 550 C, stoichiometric TiS3 is expected to form when grown with an excess of sulfur, while excessive Ti should result in TiS3-x samples with sulfur vacancies. 1 For the synthesis of our samples we used a significant excess of sulfur in a quartz ampule. Furthermore, for further studies and Thickness-Dependent Raman Spectroscopy. Supporting Information Page 2 of 8

3 we only used crystals grown on the surface of a quartz ampule away from the Ti foil, to ensure that these are stoichiometric TiS3 crystals grown in the conditions with an excess of sulfur. The structure and composition of these crystals were confirmed by single-crystal X-ray diffraction, X- ray photoelectron spectroscopy (XPS; Figure S2) and energy-dispersive X-ray spectroscopy (EDX; Figure S3). Figure S2. XPS analysis of freshly exfoliated TiS3 crystals. (a) XPS S2p spectrum of TiS3. (b) XPS Ti2p spectrum of TiS3. The composition analysis of TiS3 was performed at room temperature using a Thermo Scientific K-Alpha X-ray photoelectron spectrometer with a monochromated Al Kα ( ev) X-ray source. For the XPS analysis, the crystals were mechanically exfoliated to minimize the contribution of the surface contamination. The S2p spectrum (Figure S2a) contains three peaks representing overlapping signals from sulfur in two forms: disulfide (S2 2- ) and sulfide (S 2- ). This spectrum was fitted with spin-orbit doublet peaks consisting of S2p3/2 and S2p1/2 components. The S2p3/2 and S2p1/2 peaks for the sulfide group are located at ev and ev, respectively. The S2p3/2 and S2p1/2 peaks with binding energies of ev and ev are characteristic to the disulfide group. For both sulfide and disulfide groups, the spin-orbit splittings corresponding to the binding energy difference between the S2p3/2 and S2p1/2 components are equal to 1.2 ev, which is characteristic for TiS3 according to the literature data. 2,3 The binding energies of Ti 4+ (2p1/2) and Ti 4+ (2p3/2) components are and ev, respectively (Figure S2b). and Thickness-Dependent Raman Spectroscopy. Supporting Information Page 3 of 8

4 Figure S3. EDX spectrum for the TiS3 crystal showing spectral lines corresponding to sulfur (74.9 at. %) and titanium (25.1 at. %) at ~ 3:1 atomic ratio. EDX was performed using a FEI Tecnai Osiris TEM microscope equipped with a Super-X windowless EDX detector system with a SDD technology. Figure S4. High-resolution TEM image of the ab plane of a TiS3 crystal and the corresponding selected area electron diffraction (SAED) pattern. and Thickness-Dependent Raman Spectroscopy. Supporting Information Page 4 of 8

5 Figure S5. SEM images of TiS3 whiskers. The crystals have visible grooves along the b axis. SEM was performed using a Zeiss Supra 40 field-emission scanning electron microscope at the accelerating voltage of 5 kv. Figure S6. AFM images of the exfoliated TiS3 crystals showing narrow and uniform nanoribbons as well as flakes with different thicknesses. and Thickness-Dependent Raman Spectroscopy. Supporting Information Page 5 of 8

6 Figure S7. Raman spectrum of an exfoliated TiS3 crystal on Si/SiO2 in the cm -1 range. Raman spectrum of a Si/SiO2 substrate is shown for comparison. and Thickness-Dependent Raman Spectroscopy. Supporting Information Page 6 of 8

7 Figure S8. Calculation of the cleavage energies in TiS3; see text for details. Δd denotes the separation between two parts of the TiS3 supercell during the cleavage. To calculate the cleavage energies for different crystallographic planes in TiS3, we considered corresponding supercells to ensure that the planes correctly propagate through the crystal. A representative example of these calculations is shown in Figure S8, which illustrates the calculation of the cleavage energy for the (101) planes (Figure 3). The model shown in Figure S8 is a 10-unit TiS3 supercell with H-passivated edges. For each supercell we used the most convenient basis of lattice vectors, which for this particular supercell consisted of a, b and c vectors shown in Figure S8 (the original a, b and c vectors from Figure 3 are presented as well). This model has a finite size in the a direction and is periodical in both b and c directions. A vacuum of 30 Å was used along the a direction to ensure that the interaction between the neighboring parts was negligible. Figure S8 shows a series of simulation images illustrating the cleavage process. We first optimized the crystal structure at Δd = 0 Å until the force on all atoms was less than 0.02 ev/å. Next, we divided this initial structure into two parts and gradually increased the separation between the selected part (blue parallelogram) and the remaining part. The difference between the total energy of the structure at a separation Δd and the total energy of the initial structure (Δd = 0) was calculated and divided by the area of the cleavage plane (b c plane in the supercell) thus producing the cleavage energy. and Thickness-Dependent Raman Spectroscopy. Supporting Information Page 7 of 8

8 References: 1. Murray, J. L. The S Ti (Sulfur-Titanium) System. Bull. Alloy Phase Diagrams 1986, 7, Fleet, M. E.; Harmer, S. L.; Liu, X.; Nesbitt, H. W. Polarized X-ray Absorption Spectroscopy and XPS of TiS3: S K- and Ti L-edge XANES and S and Ti 2p XPS. Surf. Sci. 2005, 584, Yi, H.; Komesu, T.; Gilbert, S.; Hao, G.; Yost, A. J.; Lipatov, A.; Sinitskii, A.; Avila, J.; Chen, C.; Asensio, M. C.; Dowben, P. A. The Band Structure of the Quasi-One-Dimensional Layered Semiconductor TiS3(001). Appl. Phys. Lett. 2018, 112, and Thickness-Dependent Raman Spectroscopy. Supporting Information Page 8 of 8