1. Chemical Vapor Transport (CVT)
Analysis of Transition Metal Chalcogenide Materials Grown by Chemical Vapor Transport Matthew A. Bloodgood, Timothy R. Pope and Tina T. Salguero* University of Georgia, Department of Chemistry, Athens, Georgia
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Introduction
Transition metal di- and trichalcogenides are two significant classes of electronic materials possessing important properties such as superconductivity, charge density waves and, metallic conductivity.1,2,3 Both materials are van der Waals materials making simple exfoliation down to the nanoscale regime possible. The transition metal dichalcogenide, MX2, materials are two dimensional with several exhibiting polymorphism. This characteristic is demonstrated in Figure 1 with TaSe2, where variations in layer stacking results in the 1T, s
Characterization of MXy Materials
The MXy materials were analyzed with x-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and electron probe micro-analysis (EPMA). XRD and SEM images of the MXy materials can be found in Figures 4 and 5, while EDS and EPMA data can be found in Tables 1 and 2.
Oxidation Study of TaSe2
An oxidation study of TaSe2 was carried out for both the 1T and 2H polymorphs. The oxidation study consisted of heating single flakes of TaSe2 at 100, 200, 300, 500, and 700 °C for 1 h, and 100 and 200 °C for 6h.
(A)
(B)
results in the 1T, 2H, or 3R polymorphs. For both TaS2 and TaSe2, the 1T phase is easily distinguishable due to its distinctive gold color when compared to the silver color of 2H and 3R forms.
The transition metal tri-chalcogenides, MX3, possess a chain structure found in various orientations. The three known chain orientations can be seen in Figure 2. Generally, each MX3 material is found to possess only one orientation. Interactions between neighboring chains within the structure form layers resulting in the quasi-1D morphology regularly seen in the bulk MX3.
Figure 6. Raman data for (A) 1T-TaSe2 at varying temperature for 1 h; the formation of Ta2O5 begins at 300 °C for 1 h. (B) 1T- and 2H-TaSe2 at 100 and 200 °C for 6h shows the peak formation of Ta2O5 at 250 cm-1.
Figure 4. XRD patterns of (A) 1T-TiSe2, 1T-TaS2, and 1T-TaSe2 matching to JCPDS , , and , respectively. SEM images of (B) 1T-TiSe2, (C) 1T-TaS2, and (D) 1T-TaSe2.
Element
Atomic %
Spot 1
Spot 2
Spot 3 Ta
24.3
24.8
4.5 Se
55.4
34.4
95.5 O
20.3
40.8
Table 1. Table of EDS and EPMA data for 1T-TiSe2, 1T-TaS2, and 1T-TaSe24.
Experimental Wt. %
Experimental At. %
Theor. Wt. %
EDS
EPMA
Theor. At. % Ti
23.30
22.09 -
33.00
31.85 Se
76.70
77.91
66.00
68.15 Ta
73.80
72.49
75.65
31.86
31.73 S
26.20
27.51
28.68
68.23
53.40
56.77
51.86
36.43
32.24
46.60
43.23
47.60
63.57
67.80
Figure 1. Polymorphism in MX2 materials represented by the (A) trigonal 1T, (B) hexagonal 2H, and (C) rhombohedral 3R of TaSe2.2
Figure 7. SEM images, (A) and (B), of 1T-TaSe2 after heating for 6 h at 200 °C at varied magnification showing growth of Se crystallites. (C) EDS data corresponding to spots indicated in (B). a c
Element
Atomic %
Spot 1
Spot 2 Ta
28.6
32.8 Se
55.1
63.2 O
16.3
4.0
Figure 2. Variations of MX3 chains in (A) TaSe3, (B) TaS3, and (C) TiS3. Associations between neighboring chains are illustrated by the dashed lines.
Figure 8. SEM images, (A) and (B), of 1T-TaSe2 after 4 months of ambient conditions. A scratched surface reveals less oxidation below the surface. (C) EDS data corresponding to the spots indicated in (B).
Chemical Vapor Transport (CVT)
The MXy materials were synthesized and crystallized by chemical vapor transport (CVT). Reactions were carried out directly from the elemental powders over 7 – 14 days in a temperature range of 550 – 975 °C with variation in both the temperature and temperature gradient. Synthesis of TiSe2, TaS2, and TaSe2 was carried out at 975 °C with a 100 °C gradient; TiS3 and TaS3 were synthesized at 550 °C with temperature gradients between 50 and 100 °C; TaSe3 was synthesized at 700 °C with temperature gradients of 20 – 40 °C. The MX2 materials were quenched at the end of the reaction period while the MX3 materials were allowed to cool to room temperature naturally.
References
[1] R. Samnakay et al., Nano Letters 15 (2015), p
[2] J. Renteria et al., Journal of Applied Physics 115 (2014), p
[3] E. Canadell et al., Inorganic Chemistry 29 (1990), p
[4] Z. Yan et al., Journal of Applied Physics 114 (2013), p
Figure 5. XRD patterns (A) of TiS3, TaS3, and TaSe3 matching to JCPDS for TiS3, and for TaS3, and for TaSe3. SEM images of (B) TiS3, (C) TaS3, and (D) TaSe3.
Table 2. Table of EDS data for TiS3, TaS3, and TaSe3 and EPMA data for TaSe3.
Acknowledgements
Emerging Frontiers of Research Initiative (EFRI) 2-DARE project: Novel Switching Phenomena in Atomic MX2 Heterostructures for Multifunctional Applications (NSF ).
Experimental Wt. %
Experimental At. %
Theor. Wt. %
EDS
EPMA
Theor. At. % Ti
33.20
33.60 -
25.00
25.31 S
66.80
66.40
75.00
74.69 Ta
65.30
70.32
29.57
34.70
29.69
70.44
43.30
45.06
42.55
28.41
26.53 Se
56.70
54.94
51.74
73.59
73.47
Iodine transport
High Temperature Low Temperature
Figure 3. CVT reaction setup in an evacuated, sealed quartz tube. Temperature gradient orientation and iodine transport agent are also shown.