Stable Sulfuric Vapor Transport and Liquid Sulfur Growth on Transition Metal Dichalcogenides

Transition metal dichalcogenides (TMDs) are an emergent class of low-dimensional materials with growing applications in the field of nanoelectronics. However, efficient methods for synthesizing large monocrystals of these systems are still lacking. Here, we describe an efficient synthetic route for a large number of TMDs that were obtained in quartz glass ampoules by sulfuric vapor transport and liquid sulfur. Unlike the sublimation technique, the metal enters the gas phase in the form of molecules, hence containing a greater amount of sulfur than the growing crystal. We have investigated the physical properties for a selection of these crystals and compared them to state-of-the-art findings reported in the literature. The acquired electronic properties features demonstrate the overall high quality of single crystals grown in this work as exemplified by CoS2, ReS2, NbS2, and TaS2. This new approach to synthesize high-quality TMD single crystals can alleviate many material quality concerns and is suitable for emerging electronic devices.

T wo-dimensional (2D) transition metal dichalcogenides are an emergent class of materials with growing applications in the field of nanoelectronics. 1 Some examples of their use are in heterostructures and monolayers based on MoS 2 and WS 2 as transistors, MoS 2 and MoTe 2 as phototransistors, WS 2 , SnS 2 , and TiS 2 as power sources, and MoS 2 , MoSe 2 , and SnS 2 as catalysts for electrochemical water decomposition. Substances containing more sulfur, for example, trisulfides, can also have a similar layered and reduced dimensional structure. Most transition metal chalcogenides melt incongruently. 2 Therefore, it is difficult to obtain single crystals of these substances by melt techniques such as the Bridgman or Czochralski method. Usually, crystals of these substances are obtained by the chemical vapor transport technique and less frequently by the flux technique, whereas slow cooling of the chalcogenide melts 2−5 is the preferred method. For the vapor transport technique, most often halogens and their compounds are used as transport agents. In this case, there is a possibility of halogen incorporation into the crystal structure of the growing crystal. Hence, for the growth of diselenide crystals free from impurities of other elements, some works use selenium vapors instead as the transport agent. Transport of some dichalcogenides was also studied 2 without the presence of free chalcogens at high temperatures. ReS 2 crystals were grown from the Re 1 S 2.01 charge in 900→800°C gradient by Schafer in several days, as reported in the article by Wehmeier et al. 33 Therein, it was assumed that sulfur fugacity was minimal while it should be determined by Re/ReS 2 equilibrium and be significant at the temperature of synthesis. On the other hand, in other articles, Schafer reported on the possibility of using sulfur vapors as a transport agent. 4 The work described the growth of crystals of several sulfides including TiS 2 , V 1+δ S 2 , NbS 2 , TaS 3 , TaS 2 , MoS 2 , WS 2 , FeS, CoS 2 , NiS 2 , PdS, and PtS, usually in temperature gradient 800 → 700°C . Large thin transition metal dichalcogenide (TMD) crystals were grown in the excess of sulfur in 1050 → 950°C gradient. 5 Disulfide and diselenide crystals were obtained by transport in sulfur vapors with the pressure of 9 atmospheres. 5,6 Previous results have shown that it was possible to obtain disulfide and diselenide crystals but not ditellurides. 7−11 In our previous work, 12 we noted that when evaporating selenium or tellurium from the metal and Se (Te) melt at 850−650°C, only selenium or tellurium evaporated. On the other hand, when sulfur is added to the system, the metal could also evaporate forming mixed dichalcogenide crystals in the cooler part, for example, in Nb(Se,S) 2 . Therefore, the motivation of this work is to study the transport of transition metals and the formation of sulfide crystals in sulfur vapors and determine the optimal temperature profile for metal transport in sulfur vapors. The new insights reported in this study allow the design and synthesis of highquality single crystals of transition metal dichalcogenides, opening the door for high-precision studies of the properties of these systems as well as new types of nanoelectronic applications.

■ RESULTS, DISCUSSION, AND METHODS
Quartz glass ampoules containing elementary liquid sulfur that are heated to high temperatures are extremely dangerous and unstable. The danger is represented by hot fragments of quartz glass and fumes of sulfur. All manipulations with reaction vessels were carried out with the protection of hands, face, and respiratory organs. Sulfur (Labtex 99.9%) and metals with a purity of no less than 99.9% were used as the reagents. We put metals, not the corresponding sulfides, since we believed that the process of transition of the metal to sulfide at 850°C took no more than a few hours. Only the transport of those metals whose sulfides did not sublimate at 800−850°C was studied. Therefore, the transport of silicon, germanium, cadmium, zinc, tin, and mercury was not studied because the crystals of their sulfides are often formed during the synthesis from elements. The reaction vessel (ampoule) was made up of quartz glass tubes with a diameter of 12 mm and a wall thickness of 2 mm (Figure 1). The tube had two bends which were previously made by the oxygen torch. This shape of the reaction vessel fixed the position of the sulfide charge and liquid sulfur. The size and shape of these bends are not important; it is important that they ensure the fixation of the load and liquid sulfur.
About 100−200 mg of metal and 1−2 g of elementary sulfur were put into the ampoules. Sulfur was taken in excess, necessary for the formation of an invariant two-phase association "liquid sulfur + gaseous sulfur". Charged ampoules were evacuated and sealed in the flame of the oxygen torch. The total length of the vessels was 180−200 mm. The vessels were placed into the furnace so that the left part containing the sulfide usually had a temperature of 800−850°C and the right part with the liquid sulfur had a temperature of about 550−600°C. The 550°C temperature of the cooler end provided sulfur fugacity sufficient for substance transport but not enough to destroy the quartz glass vessel. The highest relative safe temperature of the sulfur part is ∼640°C at which the pressure reaches 10 atmospheres. 13 This reaction vessel resembles the ones for the three-zone technique described in Schafer's works. The time scale of the crystal growth ranges from 1 to 4 months.
All of the experiments conducted to obtain crystals by the vapor transport technique with gaseous sulfur are summarized in Table 1. The table shows the temperature range of coexistence with sulfur, the temperatures of both the hot and the cold ends of the reaction vessel, the approximate growth temperature, the time of the synthesis, the size of the crystals, and the transported amount of substance. In the case of iron, nickel, niobium, and tantalum, complete or almost complete transport of the substance up to 100 mg of metal in 2−4 months through the cross-section of 50 mm 2 was observed. In this way, NbS 2 ( Figure (Figure 3e) were obtained in a ribbon shape 10 mm long and up to 0.5 mm wide. In the experiment with platinum, 50 mg of PtS 2 crystals were obtained in 2 months. The crystals were isometric, a few tens of micrometers in size with a well-defined layered structure (Figure 3f). Whiskers in similar conditions of PdS were formed (Figure 3g). Growing PdS 2 crystals at a similar temperature is likely impeded due to their low temperature of stability.
Transport of rhenium, molybdenum, and tungsten was also observed in similar temperature conditions but significantly less in volume. MoS 2 ( Figure 3h) and WS 2 crystals a few tens of micrometers in size and ReS 2 crystals without a well-defined habit (Figure 4a) were obtained. Tungsten disulfide crystals located right next to the sulfur source were silver in color. Black and silver areas had well-defined borders, and therefore, it can be assumed that two crystal modifications of WS 2 were obtained. The possibility of transferring gold is shown in the standard temperature regime during the recrystallization of the ZnS + Au charge (Figure 4b). In the case of ruthenium, rhodium, and osmium, no transport was observed. Bi 2 S 3 whiskers (Figure 4c) were obtained under the most low-temperature regime: charge temperature�620°C and sulfur source temperature�540°C. In the experiments to transport palladium, nickel, and titanium, some additional crystals were found in the sulfur melt, that is, in the coolest part of the reaction system. Most likely gaseous metal compounds dissolved in the liquid sulfur, diffused to the coolest part of the system, and formed the crystals there. This way    gradient for 2 months. The absence of transport in this experiment demonstrates that sulfur fumes are necessary for transport, which makes this technique different from the sublimation technique. Electron backscatter diffraction (EBSD) mapping confirmed the single-crystal nature of the TiS 3 and NbS 2 crystals. The analyses were performed on a JEOL JSM 5610-lv electron scanning microscope equipped with an UltimMax-100 EDS detector and a Symmetry EBSD detector. The uniform Euler color, phase color, and inverse pole figures clearly demonstrate that the studied TiS 3 crystal is a single crystal ( Figure 5). The very same results were obtained with the NbS 2 crystal. The analyzed crystal was relatively big, around 2 mm in diameter (Figure 6a). We analyzed a natural crystal face, only slightly polished, which is manifested in a poor band contrast (Figure 6b). However, the  Euler map is uniform, and the inverse pole figures demonstrate that all of the analyzed points inside the crystal are oriented in one and the same direction.

■ DISCUSSION
The synthetic routes reported here produce two-dimensional transition metal dichalcogenides (TMDCs) that can form charge density waves (CDWs), be superconducting or magnetic, gapped or metallic, and overall with a range of physical properties that reflect the underlying electronic structure 16−19 ( Figures S3 and 7). Bulk superconductivity in NbS 2 is illustrated from low-temperature specific heat measurements, as shown in Figure 7a. Remarkably, we find that superconductivity beyond the Pauli limit still exists in bulk single crystals of NbS 2 for a precisely parallel field alignment, and our upper critical field points to the development of a Fulde−Ferrell−Larkin− Ovchinnikov state above the Pauli limit as the main mechanism. This is also consistent with the observation of a magnetic field- driven phase transition in the thermodynamic quantities within the superconducting state near the Pauli limit. In 2H-TaS 2 , the temperature dependence of specific heat and magnetization measurements are illustrated in Figure 7b. A clear maximum of both data at 76 K is an indication of the charge density wave (CDW) transition. Below the CDW transition, the magnetic susceptibility decreases sharply and continuously. This decrease in the susceptibility can be attributed to the decrease in the density of states on the hole band due to the opening of the CDW gap, and additionally, no long-range magnetic order was detected in this material. Bulk superconductivity in TaS 2 is illustrated from low-temperature resistivity and specific heat measurements, see Figure 7c,d. To make a complete report of the properties of the here synthesized compounds, we discuss in the Supporting Information (SI) the calculated electronic structure of several transition metal dichalcogenides (i.e., CoS 2 , ReS 2 , NbS 2 , and TaS 2 ). These systems were selected to provide insight into the interdependence of the crystal structure, chemical composition, and electronic structure (Figures S1 and  S2).
In summary, we have demonstrated that sulfur vapors with a fugacity of approximately 1−5 atmospheres allow us to transport many transition metals and to obtain crystals of sulfides with the maximum possible sulfur content for these conditions. In the absence of sulfur, the transport of these elements was impeded. Normally, crystals grew closer to the cooler part of the ampoule next to liquid sulfur. In several experiments, transported metals partly dissolved in liquid sulfur and crystallized as sulfides right therein. Our synthetic routes reported here open for the design and growth of high-quality single crystals for TMDC research and applications and will allow an unprecedented characterization of the physical properties, which leads to a better understanding of the underlying mechanisms of TMDC research. ■ METHODS SUMMARY Chemical Composition. The chemical composition of the crystals obtained was measured using a Tescan Vega II XMU scanning electron microscope with an INCA Energy 450 energy-dispersive spectrometer at an accelerating voltage of 20 kV. Crystals glued to the conducting substrate as well as embedded into polished epoxy resin were studied. The crystals were ground and examined by an X-ray powder diffraction technique on DRON-7 (Co K α -radiation) or BRUKER (Cu K α1radiation, graphite monochromator) diffractometers. Crystals with an apparent layered structure were checked for monocrystallinity on the BRUKER diffractometer. Traces of transport were found almost in all ampoules after 2−4 months in the furnace when the temperature in the hot end was 850−820 and 570−540°C in the cooler end and if sulfur excess was sufficient. Most often crystals were found closer to the cooler end of the ampoule, that is, the temperature of crystallization was not greater than 650°C.
Magnetization Measurements. Magnetization measurements were performed by using a Quantum Design SC quantum interference magnetometer. The low-T specific heat down to 0.4 K was measured for TaS 2 in its physical property measurement system with the adiabatic thermal relaxation technique. Specific heat measurements were performed down to 70 mK by using a heat-pulse technique within a dilution refrigerator along H||c.
Theory. We have performed spin-polarized plane-wave pseudopotential calculations with norm-conserved projector-augmented wave (PAW) pseudopotentials, as implemented in the Vienna Ab initio Simulation Package (VASP). 20 The exchange−correlation interactions are treated under generalized gradient approximation (GGA) with van der Waals corrections according to the Grimme DFT-D2 method. 21 The cutoff energy for the plane-wave expansion is set as 500 eV.