Tantalum(IV) sulfide is aninorganic compound with the formulaTaS2. It is alayered compound with three-coordinate sulfide centres and trigonal prismatic or octahedral metal centres.[2] It is structurally similar tomolybdenum disulfide MoS2, and numerous other transitionmetal dichalcogenides. Tantalum disulfide has threepolymorphs 1T-TaS2, 2H-TaS2, and 3R-TaS2, representing trigonal, hexagonal, and rhombohedral respectively.
The properties of the 1T-TaS2 polytype have been described.[3][4][5] In common with many other transition metal dichalcogenide (TMD) compounds, which are metallic at high temperatures, it exhibits a series of charge-density-wave (CDW)phase transitions from 550 K to 50 K. It is unusual amongst them in showing a low-temperature insulating state below 200 K, which is believed to arise from electron correlations, similar to many oxides. The insulating state is commonly attributed to a Mott state.[6] It is also superconducting under pressure or upon doping, with a familiar dome-like phase diagram as a function of dopant, or substituted isovalent element concentration.
Metastability. 1T-TaS2 is unique, not only amongst TMDs but also amongst 'quantum materials' in general, in showing a metastable metallic state at low temperatures.[7] Switching from the insulating to the metallic state can be achieved either optically or by the application of electrical pulses. The metallic state is persistent below ~20K, but its lifetime can be tuned by changing the temperature. The metastable state lifetime can also be tuned by strain. The electrically-induced switching between states is of current interest, because it can be used for ultrafast energy-efficient memory devices.[8]
Because of the frustrated triangular arrangement of localized electrons, the material is suspected of supporting some form of quantum spin liquid state. It has been the subject of numerous studies as a host forintercalation of electron donors.[9]
(a): Schematic of the David star pattern in 1T-TaS2 where green atoms are S and purple are Ta. (b) and (c) are STM images (6.5 K) before and after application of 2.8 V pulses through the STM tip. Insets show ~10 times magnified images.
Atomic resolution image of 1T-TaS2 (298 K). Acquired using HAADF STEM. Scale bar 2nm.[10]
It can be easily cleaved and has a characteristic golden sheen. Upon extended exposure to air, the formation of an oxide layer causes darkening of the surface. Thin films can be prepared by chemical vapour deposition and molecular beam epitaxy.
Three major crystalline phases are known for TaS2:trigonal 1T with one S-Ta-S sheet perunit cell, hexagonal 2H with two S-Ta-S sheets, andrhombohedral 3R with three S-Ta-S sheets per cell; 4H and 6R phases are also observed, but less frequently. Thesepolymorphs mostly differ by the relative arrangement of the S-Ta-S sheet rather than the sheet structure.[13]
2H-TaS2 is a superconductor with the bulk transition temperature TC = 0.5 K, which increases to 2.2 K in flakes with a thickness of a few atomic layers.[11] The bulk TC value increases up to ~8 K at 10 GPa and then saturates with increasing pressure.[14] In contrast, 1T-TaS2 starts superconducting only at ~2 GPa; as a function of pressure its TC quickly rises up to 5 K at ~4 GPa and then saturates.[6]
At ambient pressure and low temperatures 1T-TaS2 is aMott insulator.[6] Upon heating it changes to a Tricliniccharge density wave (TCDW) state at TTCDW ~ 220 K,[15][16][17] to a nearly commensuratecharge density wave (NCCDW) state at TNCCDW ~ 280 K,[2] to an incommensurate CDW (ICCDW) state at TICCDW ~ 350 K,[2] and to a metallic state at TM ~ 600 K.[10]
In the CDW state the TaS2 lattice deforms to create a periodicStar of David pattern. Application of (e.g. 50fs) optical laser pulses[7] or voltage pulses (~2–3 V) through electrodes[18] or in ascanning tunneling microscope (STM) to the CDW state causes it to drop electrical resistance and creates a "mosaic" or domain state consisting of nanometer-sized domains, where both the domains and their walls exhibit metallic conductivity. This mosaic structure is metastable and gradually disappears upon heating.[12][19][18]
Switching of the material to and from the "mosaic", or domain state, by optical or electrical pulses is used for "Charge configuration memory" (CCM) devices. The distinguishing feature of such devices is that they exhibit very efficient and fast non-thermal resistance switching at low temperatures.[8] Room temperature operation of a charge-density-wave oscillator and thermally-driven GHz modulation of the CDW state has been demonstrated.[20][21]
^abcWilson, J.A.; Di Salvo, F.J.; Mahajan, S. (1975). "Charge-density waves and superlattices in the metallic layered transition metal dichalcogenides".Advances in Physics.24 (2):117–201.doi:10.1080/00018737500101391.
^Grant, A J; Griffiths, T M; Yoffe, A D; Pitt, G D (1974-07-21). "Pressure-induced semimetal-metal and metal-metal transitions in 1T and 2H TaS2".Journal of Physics C: Solid State Physics.7 (14):L249 –L253.Bibcode:1974JPhC....7L.249G.doi:10.1088/0022-3719/7/14/001.ISSN0022-3719.
^abMihailovic, D.; Svetin, D.; Vaskivskyi, I.; Venturini, R.; Lipovsek, B.; Mraz, A. (2021). "Ultrafast non-thermal and thermal switching in charge configuration memory devices based on 1T-TaS2".Appl. Phys. Lett.119 (1): 013106.Bibcode:2021ApPhL.119a3106M.doi:10.1063/5.0052311.S2CID237851661.
^Revelli, J. F.; Disalvo, F. J. (1995). "Tantalum Disulfide (TaS2 ) and Its Intercalation Compounds".Tantalum Disulfide (TaS2) and Its Intercalation Compounds. Inorganic Syntheses. Vol. 30. pp. 155–169.doi:10.1002/9780470132616.ch32.ISBN978-0-470-13261-6.
^Freitas, D. C.; Rodière, P.; Osorio, M. R.; Navarro-Moratalla, E.; Nemes, N. M.; Tissen, V. G.; Cario, L.; Coronado, E.; García-Hernández, M.; Vieira, S.; Núñez-Regueiro, M.; Suderow, H. (2016). "Strong enhancement of superconductivity at high pressures within the charge-density-wave states of 2H−TaS2 and 2H−TaSe2".Physical Review B.93 (18) 184512.arXiv:1603.00425.Bibcode:2016PhRvB..93r4512F.doi:10.1103/PhysRevB.93.184512.S2CID54705510.
^Tanda, Satoshi; Sambongi, Takashi; Tani, Toshiro; Tanaka, Shoji (1984). "X-Ray Study of Charge Density Wave Structure in 1T-TaS2".J. Phys. Soc. Jpn.53 (2): 476.Bibcode:1984JPSJ...53..476T.doi:10.1143/JPSJ.53.476.
^Tanda, Satoshi; Sambongi, Takashi (1985). "X-ray study of the new charge-density-wave phase in 1T-TaS2".Synthetic Metals.11 (2):85–100.doi:10.1016/0379-6779(85)90177-8.
^Coleman, R. V.; Giambattista, B.; Hansma, P.K; Johnson, A.; McNairy, W.W.; Slough, C.G. (1988). "Scanning tunnelling microscopy of charge-density waves in transition metal chalcogenides".Advances in Physics.37 (6):559–644.Bibcode:1988AdPhy..37..559C.doi:10.1080/00018738800101439.
![Atomic resolution image of 1T-TaS2 (298 K). Acquired using HAADF STEM. Scale bar 2nm.[10]](/mt/?noimg=&dark=on&url=https%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aAtomic_resolution_image_of_Tantalum_disulfide.png)
