Quasi-One-Dimensional van der Waals Transition Metal Trichalcogenides

The transition metal trichalcogenides (TMTCs) are quasi-one-dimensional (1D) MX3-type van der Waals layered semiconductors, where M is a transition metal element of groups IV and V, and X indicates chalcogen element. Due to the unique quasi-1D crystalline structures, they possess several novel electrical properties such as variable bandgaps, charge density waves, and superconductivity, and highly anisotropic optical, thermoelectric, and magnetic properties. The study of TMTCs plays an essential role in the 1D quantum materials field, enabling new opportunities in the material research dimension. Currently, tremendous progress in both materials and solid-state devices has been made, demonstrating promising applications in the realization of nanoelectronic devices. This review provides a comprehensive overview to survey the state of the art in materials, devices, and applications based on TMTCs. Firstly, the symbolic structure, current primary synthesis methods, and physical properties of TMTCs have been discussed. Secondly, examples of TMTC applications in various fields are presented, such as photodetectors, energy storage devices, catalysts, and sensors. Finally, we give an overview of the opportunities and future perspectives for the research of TMTCs, as well as the challenges in both basic research and practical applications.


Introduction
The transition metal trichalcogenides (TMTCs) are commonly known as MX 3 compounds with M = Ti, Zr, Hf, V, Nb, and Ta, and X = S, Se, and Te, which are typical quasi-one-dimensional (1D) van der Waals (vdW) structures [1][2][3][4][5][6]. The term "quasi" is used to differentiate it from "true-1D" materials [5,6]. These materials are linked by strong covalent bonds in the 1D chain direction, while weak covalent bonds are between adjacent chains. These additional bonds between the chains further arrange the 1D chains into two-dimensional (2D) sheets. Similar to other 2D structures, these sheets are stacked to form three-dimensional (3D) bulk crystals by weak vdW forces. These individual structural characteristics enable TMTCs both the advantages of 2D materials and quasi-1D properties [7][8][9][10][11]. It is deemed as one of the critical materials for developing the next generation of nano-electronics and has broad application prospects.
In this review, we systematically summarize the recent studies on TMTCs and highlight the structure, synthesis methods, extraordinary properties, and emerging applications. We start by introducing the crystal structures and electronic band structures of TMTCs, in particular their CDW and SC phases. The mainstream methods adopted to synthesize TMTCs, which include chemical vapor transport (CVT), chemical vapor deposition (CVD), atomic layer deposition (ALD), and solid-phase sulfurization method, are discussed next. We then examine the physical properties of TMTCs, including electron transport, optical, thermal, mechanically induced properties, and magnetic properties. Subsequently, potential applications such as photodetectors, energy storage devices, catalysis, and sensors are addressed. Finally, we assess the key challenges and perspectives on the future developments of TMTCs.

Structure Crystal structure
TMTCs are layered materials bound by weak vdW forces (an overview structure of TMTCs is presented in Fig. 1A). The prismatic MX 6 chains of MX 3 share atoms to form 1D chains as the basic structural units of each layer, where the orientation of these chains is parallel to the b-axis direction of the monoclinic cell, as shown in Fig. 1B [3,12,30]. Although the basic structural units of TMTCs are similar, there are some differences in the shape and assembly mode of the 1D chains. The TMTCs can be classified into 3 families with representative structures of ZrSe 3 , NbSe 3 , and TaSe 3 [2,3,58].
The ZrSe 3 family is a monoclinic crystal with high symmetry falling into space group P2 1 /m, which involves all the fourth group MX 3 (M = Ti, Zr, or Hf and X = S, Se, or Te). The typical structure of the ZrSe 3 is shown in Fig. 1B. As illustrated, all MX 3 triangular prism chains are equivalent with an a/2 shift between adjacent chains. Each transition metal atom is connected to the X atom between the chain and the neighboring chain. The X atoms are combined to form a quasi-1D layered structure with anisotropic electronic and optical properties [3,12,59].
The TaSe 3 family is another monoclinic structure crystal. Compared with the ZrSe 3 family, each unit cell contains 4 chains, which can be divided into two types according to the length of the X-X bonds (0.258/0.291 nm) at the bottom of the triangular prism chain [3,60,61].
The triclinic structure is most stable in the NbSe 3 family [62,63]. Compared with the ZrSe 3 family, the NbSe 3 family's layer is constructed by the MX 3 chains with highly broken symmetry. There are three nonequivalent chain structures, indicated as I, II, and III, in the NbSe 3 family [59]. Compared with the electrons of elements in group IV, the excess electrons in the transition metal atoms of group V rupture the part of X-X bonds in the MX 3 chains, which results in local strain. The horizontal c-axis is produced between the trigonal prismatic chain and the layer. The conductance and CDW can be affected by the triclinic structure [64].
The metal coordination in TMTCs is uniformly trigonometric prismatic. In addition to the combination of different chain packing arrangements within the layers described above, varying degrees of metal-metal bonding along the chain can also contribute to structural diversity [5,59]. Among the reported TMTCs, NbS 3 has abundant polymorphisms and their crystal structure parameters are summarized in Table 1 [5,59,[65][66][67]. So far, 7 stable experimental and forecast phases have been reported. These structures can also be attributed to the above three types. The difference lies mainly in the existence of niobium pairing in the chain.

Electronic band structure
The diversity of elements and crystal structures of TMTCs leads to their rich electronic properties, as summarized in Table 2. The electronic properties of the ZrSe 3 family have been extensively studied due to the comparatively simple structure. For example, Abdulsalam and Joubert [36] calculated the energy band structure of ZrSe 3 -based TMTCs by density functional theory (DFT). The results indicated that TMTCs are all indirect bandgap semiconductors with the bandgap range of 0.44 to 2.4 eV except for antimonides and TiSe 3 with metallic properties. NbS 3 with polymorphism are prone to different degrees of Nb-Nb pairing along the chain due to the extra electrons provided by niobium element [5,59]. NbS 3 -I and NbS 3 -IV with Nb-Nb pairing are semiconductors. NbS 3 -V without Nb-Nb  pairing has metallic properties [59,67]. The indirect bandgap of TiS 3 will transition into direct bandgap when the thickness decreases from bulk to monolayer, which is similar to 2H-MoS 2 [37,68 ,69]. There are two near bands with different characters and anisotropies at the valence band maximum of TiS 3 , and the order of bands can be reversed by strain engineering [70]. The monolayer TiSe 3 , ZrS 3 , and ZrSe 3 are indirect bandgap semiconductors with a bandgap range of 0.57 to 1.90 eV, while monolayer TiTe 3 and ZrTe 3 are the metallic characters [37]. From the electronic band structures of TiS 3 , TiSe 3 , and TiTe 3 ( Fig. 2A), it can be seen that the bandgap is decreased with the increase of the halogen atom size. Substituting part of the halogen atoms can tailor the bandgap of TiS 3 . The monolayer TiS 3(1-x) Se 3x is an excellent solar absorber material with a bandgap range of 1.2 to 1.5 eV [71]. However, a solubility limit exists in the alloy formed by element substitution, resulting in a discontinuous alloying [72]. The tripleand fewer HfTe 3 chains confined in carbon nanotubes (CNTs) will rock distortion, driving the metal-insulator transition behavior [73]. The quasi-1D structure of TMTCs makes them susceptible to phase transitions, such as CDW and SC [5,[74][75][76][77].

Charge density wave phases
The CDW is an exciting phenomenon in condensed matter physics, referring to the periodic modulation of electron charge density. The modulation of electron charge density opens a gap on the Fermi surface, decreasing or even disappearing electrical conductivity. The TaSe 3 and NbSe 3 family are typical CDW materials. The complexity of the Fermi surface can be reflected by the degree of inter-chain coupling and crystal symmetry. There are two stable CDW transitions at 144 K and 59 K in the NbSe 3 crystal [78,79]. However, the transition only opens the energy gap on the partial Fermi surface, and the CDW transformed from NbSe 3 still shows metallic behaviors [80,81]. For thick NbSe 3 with a thickness of 20 to 170 nm, two stable CDW can be modulated by surface acoustic waves [82]. The investigation of the conductivity (σ) of NbS 3 crystal and temperature (T) indicated that the high-density condensed charge shows low mobility under the temperature range of 450 to 475 K (the formation temperature of CDW) [83]. The band structure of TaS 3 exhibits 1D metallic properties with Fermi level along the Γ-to-Y direction crossing 8 dispersive bands [61]. The CDW transformation can produce the metal-semiconductor transition in TaS 3 under 210 K. In addition, the CDW current of these kinds of TMTCs is also affected by the photo-irradiation, magnetic field, and impurities [21,[84][85][86].
The ZrTe 3 is another well-known CDW material with a transition temperature of about 63 K [57,78,87]. As shown in Fig. 2B, ZrTe 3 exhibits an anomaly in resistivity along the a-axis instead of b-axis, which is different from TaS 3 and NbSe 3 [87]. Hu et al. demonstrated that the phonon-electron coupling was essential for formatting CDWs in the quasi-1D ZrTe 3 except for the instability of the Fermi surface [88]. In addition, the CDW transition can be observed at 93 K in a single crystal of HfTe 3 [17]. As a semiconductor material, the metal-semiconductor transition around 220 to 250 K in TiS 3 crystal can be detected, and the anomaly transmission phenomenon is related to the CDW transition [89][90][91].

Superconductivity
The CDW transition and SC are co-existent and competitive in quasi-1D TMTCs. The superconducting transition temperature of ZrTe 3 is about 2 K under atmospheric pressure [92]. There is a highly anisotropic resistance transition in SC when the temperature is lower than the CDW transition temperature in ZrTe 3 [57]. Resistance along the a-axis decreased at 4 K, while the resistance along the b-axis started to fall at 2 K. These differences are attributed to the SC caused by the locally bounded electron pairs, which is different from the traditional SC fluctuations. A similar phenomenon can also be observed in HfTe 3 [17]. When the temperature is around 4 to 5 K, HfTe 3 exhibits quasi-1D SC due to the superconducting pairings occurring along the a-axis. The SC transition is also in NbS 3 around 2 K, which is influenced by the morphology [66]. The SC of the TMTCs can be improved by chemical element doping [33,78]. For example, the SC critical temperature of ZrTe 3 can be increased via Se doped because of the suppressed long-range order of CDW (Fig. 2C) [78]. The novel topological SC has attracted considerable attention due to its unique physical properties, such as the capacity to carry Majorana Fermions. However, few topological SC materials have been reported [25]. Recently, the concomitant of SC and topological electronic structure has been observed in TaSe 3 , as displayed in Fig. 2D, which is considered a novel platform for investigating the topological SC [24,25,[93][94][95]. The topological property-derived TaSe 3 crystals can be attributed to the surface states near the terrace edge.

Synthetic methods
The synthesis of TMTCs can be traced back to the 1960s [14]. Several preparation methods have been developed in order to control the morphology, scale, structure, and properties of TMTCs, such as CVT, CVD, ALD, and the solid-phase sulfurization method [3,28,38,44,96]. In the following section, we will summarize the recent progress of TMTCs synthesis along with their advantages and disadvantages. It should be noted that this review focuses on the comparisons of different synthesis strategies and novel growth methods for TMTCs materials. The detailed discussions on the growth parameters such as the reaction temperature, substrate, flow rate, and type of precursors can be found in previous reviews [3,34,35].

Chemical vapor transport
CVT is the primary preparation method for TMTC bulk crystals due to the advantages of large scale and high controllability. The transition metals and halogens are mixed with stoichiometric ratio and sealed in vacuum ampoules, as shown in Fig. 3A. Typically, I 2 , ICI 3 , S 2 Cl 2 , or TeCl 4 was employed as transport agents in the transport process [3,38,68]. The ampoules were transferred into a certain temperature gradient during the growth process, while the reaction materials can be transported to the cooler side of the ampoules for crystal growth. As an ancient technique, CVT has been widely used for bulk TMTC material growth. Until now, TiS 3 , ZrS 3 , HfS 3 , ZrSe 3 , HfSe 3 , ZrTe 3 , and NbSe 3 crystals have been synthesized via CVT. Different NbS 3 structures can also be obtained by tuning the reaction temperature, time, pressure, and other factors [12,15,65,[97][98][99]. Their reaction mechanisms are slightly different [3,12,34]. Due to the complex relationship between reaction factors and reaction processes, a thorough understanding of reaction mechanisms and details is necessary to accurately control crystal growth. Temperature is the most critical factor for the controllable growth of TMTCs. For example, Talib et al. [100] investigated the temperature-dependent morphology evolution of TiS 3 with the stoichiometric ratio of Ti  range of 400-650 °C. The results indicated that the morphology of TiS 3 changed from nanosheets to nanoribbons (NRs) when the temperature increased from 400 to 550 °C ( Fig. 3B to G). The TiS 3 will be decomposed into TiS 2 at a higher temperature. Similar results were also observed in the ZrS 3 crystal [101]. Typically, the TMTCs synthesized via the CVT method are in bulk crystal. To further explore the potential in miniaturized, integrated, and flexible devices, bulk TMTCs have usually been exfoliated into few-layer or monolayer 2D or 1D crystals because of the low vdW force between layers. The theoretical interlayer cleavage energy of TiS 3 crystal is 0.204 J m −2 , which is lower than that of the graphene in graphite crystal (0.320 J m −2 ). Therefore, the bulk TiS 3 crystals could be exfoliated into 2D layers or 1D chains [46,53].
Mechanical and liquid exfoliation are the primary methods for the bulk crystal. Mechanical exfoliation is an effective method and is widely used to exfoliate 2D layered materials. Since the first demonstration by Geim's group to prepare monolayer graphene in 2004, the mechanical exfoliation technique has been devoted to obtain few-layer materials, such as TMDCs and black phosphorus (BP). Mechanical exfoliation has recently been adopted to exfoliate bulk TMTCs, including TiS 3 , ZrS 3 , ZrSe 3 , ZrTe 3 , and NbSe 3 . The bulk crystals were exfoliated by tapes and transferred to specified substrates via dry or wet transfer method for further study [7,82,[102][103][104][105][106][107][108]. Due to the uncontrollable morphology, thickness, and low yields, the mechanical exfoliated few-layer TMTCs have been limited to integration of electronic devices [28].
Beyond mechanical exfoliation, the liquid exfoliation method has largely improved exfoliation efficiency. Solvent direct exfoliation and ion intercalation exfoliation are two primary liquid exfoliation methods [28]. For the direct exfoliation method, fewlayer or single-layer nanostructures are usually obtained by ultrasonic processing of the layered crystals in organic solvents. The matching between the solvents' surface tension and the layers' cleavage energy is vital to the exfoliation. Generally, organic solvents, including ethanol, isopropanol, N-methyl pyrrolidone (NMP), acetone, N,N-dimethylformamide, cyclohexylpyrrolidone, N-vinylpyrrolidone, and dimethyl sulfoxide, have been used in the liquid exfoliation process [28,45,89,109,110]. Baraghani et al. [89] had ultrasonically treated 100 mg of TiS 3 whiskers in 50 ml of absolute ethanol under nitrogen protection to obtain needle-like TiS 3 NRs. Similarly, Liu et al. added 30 mg of TiS 3 bulk crystal into 30 ml of NMP and sonicated it under a thermostatic water bath at 15 °C for 6 h. After centrifugation and washing off the residual NMP, uniform TiS 3 nanobelts with a thickness of about 30 nm were obtained [45]. The ion intercalation exfoliation method introduces excess ions into the interlayers of bulk TMTC materials, followed by ultrasonic treatment in a solvent to achieve exfoliation. In the 1970s, scientists reported that 3 lithium ions were inserted into the vdW interlayer spacing of TMTCs to form Li 3 MX 3 , while the MX 3 chain structure remained [3]. The Li atoms can also be inserted into the region of the adjacent MX 3 along the a-direction, leading to the exfoliated NRs in the process [4]. Both mechanical and liquid phase exfoliation can damage the morphology and surface structure of the material. More lossless stripping methods are needed to study the intrinsic properties of low-dimensional TMTCs, especially single-or few-chain ones.

Chemical vapor deposition
The CVD method has been widely used in synthesizing various 2D layered materials, such as graphene, TMDCs, and other vdW heterojunctions. The schematic diagram is shown in Fig. 3H. The composition, doping, morphology, and thickness can be precisely controlled. Compared with CVT, the fast synthesis period and low vacuum environment requirement largely expand the universality of the CVD method and further meet the needs for industrialization [28,68,111]. However, there are few reports on the synthesis of 2D TMTCs via the CVD method, because it is still challenging to accurately introduce the stable amount of precursors and avoid the complex intermediate reaction. Yu et al. [112] proposed a modified CVD method to realize the rapid growth of ZrTe 3 NRs. A confined space was formed by inserting wool in a small quartz tube of the CVD tube furnace. Utilizing ZrCl 4 and tellurium powder as the zirconium and tellurium source, the geometric size of ZrTe 3 NRs had been controlled by changing the growth temperature and time. Bartels' group successfully prepared 1D vdW TaSe 3 nanowires (NWs) via the CVD method using metal chloride adduct TaCl 5 [OEt 2 ] for the match between the optimal growth temperature and the vapor pressure of tantalum and selenium precursors ( Fig. 3I and J) [113]. Another example is from Sun et al. [114], in which TiS 3 NRs and rectangular nanosheets were synthesized on mica substrates with thicknesses ranging from several nanometers to tens of nanometers. The aspect ratio of TiS 3 NRs can be tailored by controlling the growth temperature. There were abundant S 2 2− vacancies in the TiS 3 NRs, resulting in high electrical conductivity and ultralow carrier activation barrier.

Other synthetic methods
The ALD, solid-phase sulfurization method, and CNT packaging technology have also been adopted to synthesize TMTCs [44,[115][116][117]. Basuvalingam et al. [115] utilized ALD to synthesize TiS 3 at a lower growth temperature ( Fig. 3K and L). The phase control was achieved by tuning the deposition temperature and copolymer composition. A rapid solid-phase sulfurization method has also been developed using S powder and Ti foil as precursor materials [44]. The advantage of this process is that it is fast and transfer-free. TiS 3 NRs were synthesized by the sulfurization of the intermediate product TiS 2 . Stonemeyer et al. reported the synthesis of stable single-chain and few-chain NbTe 3 , VTe 3 , and TiTe 3 by confining growth with multiwalled CNTs (Fig. 3M) [116,118]. The TMTC chain exhibited behaviors of few-chain quasi-1D structures, such as few-chain helical rotation and triangular antiprism rocking twist. However, CNTs have limited further research on their properties. Currently, these techniques have not been extensively studied and widely applied due to high cost and low efficiency. Exploring synthesis technologies for the efficient and controllable preparation of low-dimensional TMTCs is still challenging. The optimized exfoliation method, direct growth, and removal of CNTs should be studied intensively.

Physical properties Electrical transport properties
The theoretical calculations of quasi-1D TMTCs indicate that the monolayers of MS 3 and MSe 3 (M = Ti, Zr, Hf, Nb) exhibit semiconducting properties, while MTe 3 exhibits metallic behaviors [63]. Compared with TMDCs, the 1D chain structure of TMTCs is effective in improving or suppressing the edge scattering of carriers, which is crucial for the preparation of high-performance nanoelectronic devices [41]. The carrier mobility of bulk TiS 3 is calculated to be 30 cm 2 V −1 s −1 at room temperature (RT) and around 100 cm 2 V −1 s −1 with a temperature lower than 100 K [119]. Dai and Zeng [39] predicted that the carrier mobility of monolayer TiS 3 was highly anisotropic, where the electrons were the main carrier. Theoretical calculations of TiS 3 monolayers show that the coupling of the electronic bands is strongest along the chain direction, while the coupling of the hole bands is the strongest along the vertical chain direction. Electrons are more likely to propagate along the 1D chain and exhibit high mobility. The electron mobility along the b-axis of the 1D chain direction can reach 10 4 cm 2 V −1 s −1 , which is one magnitude higher than monolayer MoS 2 (1,000 cm 2 V −1 s −1 ). The experimental results demonstrate that the anisotropy ratio of the conductivity of fewlayer TiS 3 nanosheets is 2.1 at RT. The value increased to 4.4 at 25 K, making TiS 3 an ideal material for high-performance fieldeffect transistors (FETs) [7,120,121]. We noted that the carrier mobility of TiS 3 measured using FETs is much lower than that of the theoretical calculation, as shown in Figs. 4A and 3B [7,90,122]. Island et al. [103] investigated the in-plane transport performance of TiS 3 nanosheets with a thickness of 30 nm. The highest carrier mobility was 73 cm 2 V −1 s −1 along the b-axis, which is two orders of magnitude lower than the theoretically calculated value (Fig. 4C and D). They ascribed the low performance of TiS 3 FET to the large number of sulfur vacancies in the nanosheets, which could be improved effectively by reducing them [103]. In addition to the sulfur vacancies, some other factors greatly influenced the device mobility, such as the thickness and surface roughness of TMTC materials, structural defects in bulk crystals, temperature, and geometric parameters (Fig. 4E) [4,90,121,123,124]. For example, monolayer NRs have much lower carrier mobility than nanosheets due to the width-dependent variation [121]. The reduction of external scattering has been proven effective in improving the carrier mobility of 2D graphene and TMDCs [68,122,125,126]. To verify the applicability of this method to TMTCs, Lipatov et al. deposited 30 nm of Al 2 O 3 on mechanically exfoliated single TiS 3 NRs to prepare FET (Fig. 4F). The carrier mobility of the device increased from 20.1 cm 2 V −1 s −1 to 42.6 cm 2 V −1 s −1 , while the on/off ratio increased from 300 to 7,100 ( Fig. 4G) [122]. High current density caused electrical breakdown for miniaturized devices. Therefore, the breakdown current density needs to be considered to evaluate the applications of TMTC materials. It is predicted that the breakdown current density of ZrTe 3 NRs can be 144 MA cm −2 , which has the potential to be a novel interconnect material for the next generation of micro-integrated circuits (Fig. 4H) [127]. TaSe 3 / h-BN NWs heterostructures with a high aspect ratio (widths of 20-70 nm) and Nb 1−x Ta x S 3 nanofibers also have high breakdown current density (10 and 30 MA cm −2 , respectively), which is an order of magnitude higher than copper (Cu) (Fig. 4I) [8,113,128]. The excellent breakdown current density can be attributed to the unique crystal structure of the quasi-1D vdW material [9,129]. The breakdown current density of TiS 3 NRs also reaches 1.7 MA cm −2 , which is higher than most reported nanomaterials [130]. The electrical breakdown of TiS 3 NRs was caused by crystal defects formed by material oxidation and sulfur atom desorption. The TMTCs with high breakdown current density are promising for miniaturized nanoelectronic devices.
The interfacial properties of TMTCs and metal contacts have also been investigated, which are essential for advanced electronic devices [131][132][133]. For example, Gilbert et al. [132] evaluated the electronic properties of Au and Pt metal contacts on (001) planes of TiS 3 via x-ray photoelectron spectroscopy (XPS). The results indicated that an ohmic contact, instead of the Schottky barrier, is formed at the interface of Au and the TiS 3 (001) plane. It was believed element S plays a vital role in this process. The DFT calculations demonstrated that there is no tunneling barrier between the monolayer TiS 3 and 6 metals (Ag, Au, Pt, Pd, Ir, and Ni), which indicated that high carrier injection efficiency was achieved from metal to semiconductor [131]. However, the band structure of the monolayer TiS 3 was affected by the metal of Pd, Pt, Ir, and Ni, which will form covalent bonds at the interface to metalize the semiconductor (Fig. 4J).
Linear dichroism refers to the distinctive absorption of polarized light perpendicular to or parallel to an orientation, which is an essential index for evaluating polarization-dependent optical properties [11,135,146,147]. The dichroic ratio for the (001) plane of TiS 3 and the (001) plane of ZrS 3 can reach 4 and 2.55 in the photocurrent measurement, respectively [136,146 ,147]. The angle-resolved photoemission spectroscopy (ARPES) indicates that the strong polarization sensitivity of TiS 3 and ZrS 3 is caused by the different in-plane symmetries of their electronic energy bands. The optimal excitation energy of TiS 3 photocurrent dichroism is 1.0 to 2.0 eV. The excitons with larger binding energy are the primary photoexcitation in TiS 3 NRs [134]. In the ultrahigh-speed injection state, the exciton can be formed in a subpicosecond time scale with a mobility of 50 cm 2 V −1 s −1 .
Similar to TMDCs, Raman spectroscopy can be used to distinguish the layer number of thin TMTC crystals [46,142]. For example, the TiS 3 has 4 A g Raman activity modes in the range of 100 to 600 cm −1 , including A g rigid (175 cm −1 ), A g internal (300 cm −1 ), A g internal (370 cm −1 ), and A g S-S (560 cm −1 ), as shown in Fig. 5A [46]. With the decrease in layers, the intensity of A g rigid decreases, while the intensity of the A g internal increases. On the contrary, the intensity of A g internal and A g S-S remain constant despite the changed layer number (Fig. 5B). Therefore, the TiS 3 number can be identified using the deviations of peak positions between the A g internal and A g rigid . Similar behavior can also be observed in ZrS 3 and ZrSe 3 [142].
TMTCs have ultrahigh luminescence, which is 5 times higher than BPs, and 10 times higher than ReS 2 at the same thickness [137]. The strong photoluminescence (PL) peaks of high-aspect-ratio HfS 3 NRs were observed at 485, 540, and 600 nm under an excitation wavelength of 400 nm [139]. The angle resolved PL spectroscopy of ZrS 3 flakes indicates that light is efficiently absorbed, and the maximum PL intensity appears when the electron field is polarized along the b-axis chain direction [137]. When the electric field is perpendicular to the b-axis, the absorption decreases because the wavelength of the excitation light is much longer than the chain width ( Fig. 5C and D). The dichroic ratio of PL intensity reached 10.8, similar to 1D materials. The anisotropy of polarization is smaller than 1D materials. The strongly isotropic PL emission of thermally oxidized ZrS 3 NRs has also been investigated, comparable to monolayer direct bandgap semiconductor MoS 2 [141]. Uniform ZrO 2 nanocrystals were formed on the surface of ZrS 3 after thermal oxidation treatment, providing additional degrees of freedom for electro-optical modulation of TMTCs.
The TiS 3 nanosheets had large birefringence, which is larger than the well-known strong birefringence materials, such as TiO 2 and calcite [143]. The exciton effect is believed to play an important role in the large birefringence. The birefringence and linear dichroism of ZrS 3 were investigated by polarizationresolved optical microscopy and azimuth-dependent reflectometry microscopy [138]. It was found that the refractive indices and extinction coefficients have different peak values and change trends along the a-axis and b-axis of ZrS 3 .
The light transmittance measurements show that TiS 3 has a transmittance of 30, which is much larger than other 2D anisotropic materials such as MoS 2 (1) and BP (1.4) (Fig. 5E) [7]. It is because the TiS 3 NRs have a quasi-1D chain-like structure. When the excitation light is polarized along the b-axis chain direction, there will be an absorption similar to wire grid polarizers, resulting in a minimized transmittance (Fig. 5F).

Thermoelectric properties
The thermoelectric conversion efficiency of a material is determined by the quality factor value of ZT (ZT = S 2 σT/κ), where S, σ, T, and κ are Seebeck coefficient, electrical conductivity, absolute temperature, and thermal conductivity, respectively [28,49]. These parameters are controlled by phonon scattering and electronic structure. High power factor (S 2 σ) and low κ are favorable for ideal thermoelectric materials. There are many theoretical and experimental reports on the thermoelectric property of monolayer TiS 3 and ZrS 3 [31,49,120,[148][149][150][151][152]. The extreme anisotropy exists in the in-plane thermal conductivity of TiS 3 material. The thermal conductivity along the b-axis is twice higher than the a-axis, higher than the reported other layered materials, as shown in Fig. 6A [150]. The large dispersion of optical phonons in the chain direction is the principal cause for the high anisotropic thermal conductivity of TiS 3 . The thermal conductivity of monolayer TiS 3 is much lower than TMDCs, which results in a high ZT value of 3.1 under moderate carrier concentration at RT [120]. Both high Seebeck coefficient and electrical conductivity can be obtained in monolayer ZrSe 3 due to the grooved band near the conduction band minimum [51]. The n-type ZT value of monolayer ZrSe 3 with moderate carrier concentration is 2.4 at 800 K. The selenium atoms on the surface of ZrSe 3 play a major role in the heat transport process. The ZT value of monolayer ZrS 3 along the b-axis direction is predicted to be 2.44 at 800 K, ascribed to the excellent thermoelectric performance similar to single-layer ZrSe 3 [49]. There have been several reports on the theoretical calculation of the thermoelectric properties of TMTC materials. For example, Wang et al. [48] analyzed the thermoelectric properties of bilayer ZrS 3 and Janus ZrS 2 Se using the first-principle calculation method. The results indicated that ZrS 3 had a higher power factor and lower lattice thermal conductivity. The thermoelectric properties were improved by partially replacing S atoms with Se atoms. Under 300 K, the optimal ZT values of p-type and n-type doped bilayers Janus ZrS 2 Se are 2.21 and 1.43, respectively, which are higher than bilayer ZrS 3 . The thermoelectric properties of TMTCs are correlative with the thickness. The phonon confinement effect will dominate when the thickness of the TMTCs is smaller than the phonon confinement size. Due to the increased phonon scattering, the in-plane thermal conductivity is negatively correlated with thickness [31,153]. In other words, the phonon thermal conductivity can be effectively decreased by reducing the dimensionality of the crystal structure. It is confirmed that the thermoelectric performance of monolayer ZrS 3 is superior to its bulk crystal, as shown in Fig. 6B [49]. A 25-fold enhancement of κ was observed in NbSe 3 NWs when the diameter was diminished from 26 to 6.8 nm, providing strong experimental evidence for 1D phonon transport [153].
Metallic TaSe 3 and ZrTe 3 with high breakdown current density have been used as connecting channels in nanoscaled electronics. The Joule heating effect of NWs will cause the performance degradation of devices. Thus, their thermal transport properties need to be considered. A theoretical study indicates that the thermal conductivity of TaSe 3 and ZrTe 3 from chain direction is higher than cross-chain and cross-plane directions [154]. The phonon lifetime and mean free path of TaSe 3 are shorter than ZrTe 3 in the low-frequency range. The NbSe 3 and TaS 3 materials exhibit metallicity at RT, while the Peierls transition occurs at low temperature, complicating the electrons' thermal conduction in these two NWs [22,47,155].

Mechanically induced properties
Strain engineering has been proven effective in tuning materials' intrinsic properties. The changes in the lattice structure of TiS 3 and ZrS 3 have been observed under a larger pressure [53,54,56,156]. Specifically, distortion occurred in multilayer ZrS 3 crystal when the pressure reached 2.5 GPa [53]. Further, when the pressure was increased to 10.8 GPa, the S-S bonds were rearranged along the a-axis, resulting in a phase transition in the ZrS 3 crystal. During this process, the original S-S bonds connected to the Zr atoms are broken and reconnected to the S atoms on the adjacent chain, leading to a new S-S bond. The results were confirmed by high-pressure Raman spectroscopy [53]. A similar phenomenon has been observed in TiS 3 crystals, in which the TiS 3 crystals changed from monoclinic to cubic phases under large pressure [156]. The cubic phase exhibits superconducting potential with an estimated transition temperature of 9.3 K at 80 GPa.
The effect of stress on atomic displacements in crystals can also be observed in the optoelectronic properties of TMTC materials. The DFT calculations indicate that the electronic structure of the MX 3 monolayer can be affected by tensile strain [63,157]. Specifically, monolayers of HfS 3 and ZrS 3 could be transformed from an indirect bandgap to a direct bandgap under a tensile strain of ε = 2%. Further, when ε = 6% and ε = 4% strains are applied, the monolayer HfTe 3 and ZrTe 3 will transform from intrinsic metallic into indirect bandgap semiconductors. On the contrary, the monolayer TiS 3 and NbS 3 could keep the bandgap feature under monoaxial and biaxial tensile strains ε ranging from 0% to 8%. The calculation results show that the bandgap transition occurred more easily in monolayer MX 3 than MX 2 under external strain. Experimental studies have also observed an increased bandgap (up to 9%) when tensile stress is applied to TiS 3 along the b-axis [40]. The monolayer and bilayer TiS 3 change from direct to indirect bandgap semiconductor when the compressive strain is generated. Silva-Guillén et al. [70] believed that the change of the energy band in monolayer TiS 3 under strain could be attributed to the interaction between Ti 3d xy and S 3p x orbitals. Thus, the strain can be used to manipulate the anisotropy of TiS 3 materials. The strain effect on the bandgap of monolayer TiS 3 has also been used to tune its optical properties to expand the light absorption range [157]. Anisotropic experimental results have been observed when monoaxial strain experiments are performed on ZrSe 3 using a 3-point bending test apparatus [105]. Specifically, when the strain was applied along the b-axis, an apparent blue shift of the exciton peak (≈ 60 to 95 meV % −1 ) was observed (Fig. 6C). In contrast, the phenomenon did not appear along the a-axis (≈ 0 to 15 meV % −1 ) (Fig. 6D). Lin and colleagues utilized ultrahigh-resolution ARPES to study the electronic structure evolution of TaSe 3 under tensile strain [95,158]. They observed metal-insulator transition on the stressed TaSe 3 .
Both monoaxial and biaxial tensile strains have been theoretically proven to improve the intrinsic mobility of the TMTCs [157]. The results showed that an order of magnitude had increased both the electron mobility and the hole mobility of monolayer TiS 3 . TiS 3 nanosheets exhibited anisotropic piezoresistive effects [159]. The SC of ZrTe 3 was also affected by pressure [55,57]. The superconducting transition temperature of ZrTe 3 increased from 4 K to 7.1 K when the pressure increased to 28 GPa. Localized electronic states near the Fermi level in wrinkled HfTe 3 film are anticipated to enhance the SC transition temperature [160]. The CDW transport performance of MX 3 can also be modulated by mechanical force [19,52,161]. The CDW will periodically stop when the piezoelectric actuator's vibration frequency coincides with the resonance of the TaS 3 whiskers [52].

Magnetic properties
It is believed that TMTCs exhibit diamagnetic properties [112,162]. The weak magnetism has been found in ZrTe 3 crystal, and the corresponding Curie temperature is higher than 300 K (Fig. 6E to G) [112]. There is an interaction between ferromagnetism and diamagnetism in the ZrTe 3 crystal. The hysteresis loop indicated that ferromagnetism plays a dominant role under the magnetic field of −5,000 Oe to 5,000 Oe. The diamagnetism will be dominant beyond the magnetic field range. This RT ferromagnetism can be ascribed to the structural defects and size-reduced edge states of ZrTe 3 . The introduction of structural defects, such as vacancies and grain boundaries, has been reported to be an effective method of generating the local magnetic moments [28,163].
The DFT studies indicate that the magnetic properties are highly dependent on the crystallographic orientation of TMTC materials. The first-principles study on TiS 3 NRs has shown that the a-TiS 3 NRs (grown along the a-axis) exhibited ferromagnetic metallicity properties, while the b-TiS 3 NRs (grown along the b-axis) were the nonmagnetic direct bandgap semiconductor [121]. The net magnetic moment of the a-TiS 3 unit cell changed between 0.2 μB and 0.8 μB under different widths of NRs. The spin-polarized state originates mainly from the unpaired electrons of the band-edge atoms. The discovery of TiS 3 ferromagnetism is supposed to be helpful for the fabrication of spintronic devices. The related results provide new ideas for further study of the magnetic properties of other TMTCs.

Applications of TMTCs Photodetectors
The TMTCs are promising candidates for optoelectronic devices due to their novel physical and chemical properties such as tunable bandgap. There have been several reports on photodetectors based on TMTCs, such as TiS 3 , ZrS 3 , ZrSe 3 , and NbS 3 (Table 3) [42,102 ,164 -167]. Few-layer TiS 3 NRs have distinct response signals to wavelengths in the visible spectrum, enabling high and fast (4 ms) photoresponse up to 2,910 A W −1 [102]. The TiS 3 NRs can be horizontally aligned on the interdigital electrodes by the dielectrophoresis method [164]. The TiS 3 NR-based photodetector exhibited a large detection range with extension to the near-infrared region. The maximum responsivity, quantum efficiency, and detection rate of TiS 3 NR-based photodetector were 5.22 × 10 2 A W −1 , 6.08 × 10 2 , and 1.69 × 10 9 Jones, respectively. The comparative experiments suggested that the photoresponse of horizontally aligned NRs was higher than the randomly oriented NRs. The ZrS 3 NR-based phototransistors fabricated on SiO 2 /Si wafers exhibited remarkable photoresponse from ultraviolet to near-infrared light [165]. The optical power of ZrS 3 NRs was 5.57 mW cm −2 at a bias voltage of 1 V under 405 nm. The photoswitch current ratio can reach 210, while the photoresponse time was less than 0.4 s. The photodetector based on single HfS 3 NR exhibited selectivity photoresponse to 405 nm with a low dark current of 0.04 pA and a large photoswitching current ratio of 337.5 [168]. The aforementioned ZrS 3 and HfS 3 are p-type semiconductors, and holes are the prominent carriers. For p-type semiconductors, the adsorbed oxygen captures photogenerated electrons under light exposure and increases the conductivity and photocurrent of the TMTCs. In contrast, the adsorbed oxygen decreases under vacuum or nitrogen protection, leading to a decreased photocurrent. The result is opposite to the n-type TiS 3 photodetector. The flexible UV-vis photodetectors based on ZrS 3 NR, HfS 3 NR, and TiS 3 fake films were fabricated on transparent polypropylene films and printed paper substrates (Fig. 7A to C) [139,169 ,170]. Though the performance of the flexible photodetectors is not comparable to that on SiO 2 /Si substrates, they can withstand mechanical forces. In addition, spectral selectivity and wide spectral detection range can be maintained, as well as excellent environmental stability, which is expected to expand the applications.
The low symmetric structure and high anisotropy performance of TMTCs make them promising in polarization photodetection [171]. The indirect narrow-bandgap semiconductor NbS 3 has a bandgap of 0.26 eV and 0.42 eV for bulk and monolayer, respectively. 2D NbS 3 Schottky detectors were fabricated with asymmetric electrodes of Cr and Au, as displayed in Fig.  7D and E [172]. High linear dichroic ratios (3.95 and 1.84) and high-quality polarization images were obtained at 637 and 830 nm, respectively ( Fig. 7F and G). In addition, the detection rate exceeds 10 7 Jones at a wavelength of 3 μm under RT. Fast photoresponse (11.6 μs) and lower noise current (4.6 × 10 −25 A 2 Hz -1 ) had also been achieved. The photovoltaic effect is the key feature of the photodetector in the ranges from visible to near-infrared, and the built-in electric field separates the photogenerated carriers in the space-charge region, while the PL thermal effect is dominated in the mid-infrared to the longwave infrared band. The temperature of the device increases with the illumination, which leads to the enhancement of the hot carriers and the increase of the electrical conductivity. The measurements show that the dichromatic ratios of TiS 3 and ZrS 3 reach 4 and 2.55, respectively, exhibiting high sensitivity to polarized light [136,147]. Wang et al. [147] found that ZrS 3 NRs had maximum light absorption along the b-axis chain direction, and the dichroic response was affected by the excitation wavelength. Under the light irradiation of 450 nm, the dichroic ratio was 1.73, while the ratio reduced to 1.14 when the wavelength changed to 532 nm. The tunable dichroic response was attributed to the higher exciton absorption peak of ZrS 3 than 450 nm, which could effectively detect exciton absorption. Meanwhile, the polarization sensitivity of ZrS 3 NRs is dependent on the layer number, the intrinsic band structure, and the optical transitions. Self-powered devices without an external power supply have been of great interest in recent years due to their low power consumption, light weight, and small size. Generally, the p-n junction and Schottky junction have been used to realize self-powered devices. The built-in electric field is generated inside the device due to the separation of carriers. Self-powered photodetectors based on p-n junctions have the advantages of fast response, large linear regions, and low noise. The TiS 3 /Si p-n junction monolithic device can operate in the photovoltaic mode without external bias, or photoconductive mode with a positive or negative bias [173]. Yao et al. constructed a selfpower photodetector in a 1D solar cell capacitance simulator (SCAPS-1D) software [174]. The fluorine-doped tin oxide FTO, PbS, and narrow-bandgap TiS 3 were indicated as the transparent conductive layer, donor, and acceptor, respectively. The offset value of the conduction band between PbS and TiS 3 is about 0.6 eV, which helps the photogenerated electrons move into the Ag electrode. The photogenerated holes were transmitted to the FTO layer due to the shift of the valence band. The simulation results indicated that the PbS/TiS 3 photodetector with the optimal parameters exhibited a responsivity and detection limit of 0.36 A W −1 and 3.9 × 10 13 Jones, respectively. The photodetectors based on the photothermoelectric (PTE) effect can also realize self-driven operation under zero bias voltage. The effect derives from the asymmetry of the electrode material or the temperature distribution along the channel direction. Utilizing the thermally localized enhanced PTE effect, Wu et al.
[10] fabricated a quasi-1D flexible NbS 3 photodetector with broadband detection from ultraviolet (375 nm) to terahertz (118.8 μm) band (Fig. 7J to L). When the light was illuminated at one end of the device, the generated heat produced a temperature gradient at both ends. Since the thermal decay length of the NbS 3 material is short, the heat generated by the illumination was confined to a small area, which resulted in a large temperature difference between the two ends of the device. Under various wavelengths, the optical radiation rate of the device was higher than 1 V W −1 and the response time was less than 10 ms. The flexible NbS 3 photodetector on PET substrate also demonstrated great stability, showing no obvious degradation of photo response performance after 150 bending cycles.

Energy storage
The layered crystal structure makes TMTCs a promising candidate for energy storage devices. The NbS 3 has been adopted as the cathode of magnesium-zinc solid-state batteries, while the 3D interconnected TaS 3 NWs networks have been used as anode materials for flexible Li-ion batteries (Fig. 8A) [175,176]. Without any protection, the TaS 3 NWs-base batteries exhibit a decent specific capability of 400 mAh g −1 , which is better than commercially available graphite material (372 mAh g −1 ). In addition, compared with the initial charge capacity, only 0.1% of capacity is degraded by per cycle, and a high capacity of 60 mAh g −1 has been observed after 100 cycles, indicating satisfactory cyclic stability, as shown in Fig. 8B. The excellent properties are derived from the conductive network formed by the 3D interconnected TaS 3 NWs. A continuous electron path has been formed for fast electron transfer and provides a large electrolyte contact area. More importantly, the structures of 3D TaS 3 NWs have been retained after 100 cycles, suggesting that the cyclic strain induced by Li insertion/extraction is relieved by the interconnected NWs network. In addition to TaS 3 NWs, NbSe 3 NRs have also been employed as anode with the covering of reduced graphene oxide (rGO) to improve the performance of Li-ion batteries [177]. The volume expansion and structural instability of the electrode material during lithium intercalation and deintercalation were largely suppressed, which enhanced the cycling stability and specific capability. Moreover, the discharge capacity was maintained (300 mAh g −1 ) after 250 cycles.
Most of the redox activity of battery materials is concentrated on transition metals, while the working mechanism of novel battery electrodes indicated that anionic redox also existed in the working process [178]. The reversible transition between S 2 2− and 2S 2− is considered the reason for the high specific capacity of TiS 3 batteries. The electrons of Li ions are simultaneously transferred to alkali metal atoms and sulfur atoms upon intercalation. This mechanism has also been confirmed by nuclear magnetic resonance (NMR) spectroscopy of natural abundance solid-state 33 S in NbS 3 [179]. The DFT calculations show that the valence band mainly depends on the p-state of the chalcogen, while the conduction band typically relies on the d-state of the transition metal in monolayer MX 3 . Such dependence results in a high-power conversion efficiency of 16% to 18% for MX 3 thin-film solar cells, such as ZrS 3 and HfS 3 , which is promising for solar energy storage [1,180]. The 2D vdW hetero-bilayer formed by monolayer ZrS 3 and monolayer TMDCs exhibits type II band alignment [180]. Specifically, the energy of the conduction band edge of monolayer ZrS 3 is lower than that of monolayer TMDCs, making them the acceptor and donor in solar energy conversion, respectively, to achieve a synergistic effect. In theory, the power conversion efficiency of the heterobilayer solar cells can reach 16%, which paves a new way for efficient solar energy conversion on the nanoscale. The exciton binding energy of ZrS 3 along the [100] direction is only 0.15 eV, which is beneficial for separating electron-hole pairs. As a result, the collection of photovoltaic current is expected to be along the [100] direction. The TiS 3 NRs have been used as photoanodes in photoelectrochemical cells, providing a suitable energy band positions for water reduction; i.e., the conduction band position is more negative than the reduction potential of water, and the valence band position is more positive than the oxidation potential of water (Fig.  8C) [181,182]. TiS 3 has also been widely utilized as electrode material for supercapacitors [183,184]. In the battery-type supercapacitors with asymmetric structure, as displayed in Fig. 8D, the highest capacitance of the TiS 3 -based supercapacitor was 235 F g −1 (105 C g −1 ) at 5 mV s −1 . More importantly, the capacitance was retained (about 91%) after 6,000 galvanostatic charge-discharge cycles (Fig. 8E) [183].

Catalysts
The structure of TMTC material can also be expressed as M 4+ X 2− (X 2 2− ), while all the oxidation state of metal atoms is tetravalent. Unlike TMDCs, TMTCs are more likely to generate X 2− and X 2 2− vacancies to form nonstoichiometric TMCs, which is highly desirable for improving catalytic performance [185][186][187]. Ribbon morphology can be easily formed, and abundant unsaturated X 2− ions will be exposed at the edges as reactive active sites [185]. All these characteristics make TMTCs promising photocatalytic and electrocatalytic materials.
In terms of photocatalysis, defective ZrS 3 nanobelts were used to catalyze the reaction of water and oxygen for synthesis of hydrogen peroxide (H 2 O 2 ), and simultaneously oxidize benzylamine to benzonitrile [185]. Under simulated sunlight illumination, the H 2 O 2 and benzonitrile yields with ZrS 3 NRs as photocatalysts are 78.1 ± 1.5 and 32.0 ± 1.2 μmol h −1 , respectively. This is because the conduction band of ZrS 3 mainly consisted of Zr d-orbitals, resulting in a negative potential larger than the reduction potential of O 2 to H 2 O 2 [188]. Both experimental and theoretical studies have shown that S 2 2− is beneficial for separating photogenerated carriers (Fig. 8F to H). The negative ion vacancies on the surface of ZrS 3 NRs are also favorable for transferring holes. Introducing S 2 2− vacancies into ZrS 3 NRs via high-temperature vacuum annealing treatment further enhances photocatalytic efficiency. The Li-based complex has been utilized for solvothermal treatment to induce more S 2− vacancies, improving the electron conduction and hole extraction during the photocatalytic process, and changing benzylamine oxidation kinetics. In addition to ZrS 3 , TiS 3 has also been adopted as a photocatalyst for pollutant degradation [186,189]. TiS 3 with a small bandgap enables the absorption of light from the visible to near-infrared region, which enhances the efficient conversion of solar energy. In the degradation experiment of methyl orange dye under simulated sunlight, TiS 3 showed high catalytic activity compared with other titanium chalcogenides and even higher than TiO 2 . Similar to ZrS 3 , sulfur vacancies can be introduced in TiS 3 NRs after annealing, which further improves its photocatalytic activity. Moreover, the formation of TiO 2 passivation layer leads to the TiO 2 /TiS 3 heterostructure during the photocatalytic process. The heterojunctions and sulfur vacancies help separate photogenerated charges and achieve high-efficiency catalytic reactions. In addition, some reports have shown that the small bandgap of TiS 3 also provides additional thermocatalytic activity [186].
ZrS 3 ultrathin nanosheets have also been used as electrocatalysts for efficient oxygen evolution reactions [110]. There are abundant disulfide bonds on the surface of ZrS 3 . Excellent water oxidation activity has been achieved, which includes low onset overpotential of 244 mV and Tafel slope of 45 mV decade −1 in an alkaline solution at pH 14. The high oxidation activity can be maintained even under neutral conditions (pH 6.9), which has potential applications for total water splitting. The 2D ultrathin nanosheets also increase the reaction contact area and promote the electrocatalytic process. The TiS 3 NRs that are obtained by doping TiS 2 with Nb could be ascribed to the structural transformation from TiS 2 to TiS 3 [190]. The TiS 3 NRs exhibit improved electrocatalytic hydrogen evolution performance.

Sensors
Similar to other 2D materials, monolayer or few-layer TMTCs have high specific surface areas. The sulfur vacancies exist at the edge of the TMTC material, which is conducive to the adsorption of gas molecules, making it promising for gas sensors. The adsorption energy of gas molecules on surfaces affects the gas-sensing properties. The selectivity of the TiS 3 gas sensor has been investigated via the statistical view of surface adsorption and the adsorption strength value, which has been predicted and verified by the programming gas adsorption spectra [191]. It is indicated that five typical adsorption gas molecules (hydrogen, methane, water, oxygen, and ethanol) on the (001) surface of TiS 3 can be divided into two types. One is the strong  shielding efficiency of all samples tested in the extremely high-frequency band (f = 220-320 GHz). Reproduced with permission [196]. Copyright 2021, Wiley-VCH.
adsorption of polar molecules, including ethanol, water, and oxygen, while another is weak adsorption (nonbinding) to nonpolar molecules of hydrogen and methane. This interesting phenomenon is caused by the bipolar interactions between polar molecules and the (001) surface of TiS 3 . The sensing performance of TiS 3 NRs and graphene-TiS 3 heterojunctions has been evaluated with the target gases of ethanol, methanol, and acetone at RT [192]. The graphene-TiS 3 heterojunction sensor exhibits high selectivity to ethanol at RT. The vdW contact between graphene and TiS 3 effectively reduces the metal-induced defect energy level caused by the interaction between TiS 3 and Au, resulting in negligible Fermi level pinning (FLP) at the contact. The larger Schottky barrier of the Au-Gr-TiS 3 sensor is also beneficial for the absorption and desorption of ethanol molecules. TaS 3 nanofibers have also been utilized as a gas sensor, which exhibits good selectivity against common interfering gases produced during fuel combustion ( Fig. 9A and B) [61]. A minimum detection limit of 0.48 ppb for NO has been achieved, which is far below the environmental value of NO x (50 ppb). Moreover, the detection limit is much lower than other reported materials, such as MoS 2 and TaS 2 . These excellent properties make TMTCs attractive in environmental protection.

Other applications
Furthermore, some other intriguing applications of TMTCs have been investigated such as fiber lasers, photoelectric memristor, light-emitting diode (LED), and fillers in functional composites. For example, TiS 3 and ZrTe 3 are attractive in nonlinear optics, which have been utilized as saturable absorbers to fiber lasers [107,193 ,194]. Ultrashort pulses of 506.5 ns and 1.44 ps can be generated for these optical devices. Liu et al. [45] demonstrated a photoelectric memristor based on TiS 3 ( Fig. 9C and D). Multilevel storage of light with different wavelengths (400 to 808 nm) was realized through the large light absorption range of TiS 3 . The biological synapse function was emulated by the conductance modulation of the TiS 3 -based memristor. Based on this artificial synapse, Pavlov's associative learning behavior was established, proving the potential application of TMTCs in optoelectronic neuromorphic computing systems. The electron injection layer (EIL) of 2D ZrS 3 has the ability to reduce the turn-on voltage (2.8 V) of LED [109]. The interface between the electrode and the photoactive layer was more stable compared to a LED without ZrS 3 EIL. The ZrS 3 thin films in this device are fabricated by the slot-die coating method with optimal ZrS 3 ink, which was prepared via liquid phase exfoliation from crystals, as displayed in Fig. 9E and F. The introduction of NbSe 3 nanofibers into base oil can improve the tribological properties and is expected to be used in the industrial lubrication field [195]. The flexible polymer-TaSe 3 NWs composite films demonstrated superior electromagnetic shielding capability in both X-band (8.2 to 12.4 GHz) and sub-terahertz frequency range (220 to 320 GHz) (Fig. 9G to I) [196,197]. The excellent electromagnetic shielding property has been attributed to the efficient coupling of electromagnetic waves and the TaSe 3 NWs. The free carriers in quasi-1D TaSe 3 interact with the electric field of the electromagnetic wave, which has led to the reflection and absorption of electromagnetic energy. The composite film with a TaSe 3 NW concentration of 4.5 vol% provides a total electromagnetic shielding of about 20 dB in X-band frequency range, which is superior to other conventional composites with metals, CNTs, and graphene. In the sub-terahertz band, the total shielding effect varied from 60 dB to more than 70 dB, which has broad applications in the field of 5G communication and beyond.

Conclusion and Outlooks
TMTCs exhibit remarkable electrical and physical properties owing to their unique quasi-1D chain structure. They have great potential in the applications of novel electronic, optoelectronic devices, and high-performance integrated logic circuits. We summarize the state-of-the-art achievements of TMTCs, which include crystalline structure, electronic structure, synthesis method, physical properties, and related applications. In recent years, some substantial progresses have been made in TMTC materials. Compared to other 2D materials, the research on TMTCs is still in its early stage. Several issues still need to be solved for the systematic understanding of the properties of TMTCs and the realization of their practical applications.
Compared to 2D materials, TMTCs can be reduced to the 1D atomic scale, making them promising for higher levels of integration in integrated optoelectronic platforms. Controllable synthesis of high-quality, large-scale monolayer or few-layer TMTCs is a crucial and necessary starting point for fabricating high-performance TMTC devices. Currently, the synthesis of TMTCs mainly relies on the CVT method, which suffers from a long reaction cycle and complicated process to fabricate low-dimensional devices. Thus, effective strategies need to be developed. The CVD method is an excellent strategy for the controllable growth of monolayer and few-layer TMDCs. Nonetheless, monolayer TMTCs have not been synthesized by the CVD method. Integrating machine learning (ML) into the CVD process is beneficial for finding out the key parameters in the growth of monolayer TMTCs. In the CVD growth of quasi-1D TMTCs, it is equally critical to suppress the growth perpendicular to the layer direction as to control the growth perpendicular to the 1D chain direction. Compared to 2D materials, the growth conditions of TMTCs are considerably more demanding. Auxiliary feedback from ML can help to achieve more accurate control of thermodynamic and kinetic parameters such as temperature and gas flow rate during CVD reactions, as well as optimization of growth parameters. At the same time, the introduction of ML can also speed up the exploration process and reduce costs. As a quasi-1D structure, studies on single-chain or few-chain TMTCs are meaningful. It is vital to develop methods to isolate and transfer materials with single-chain or countable-chain TMTCs. The top-down exfoliation method is uncontrollable, and it is difficult to obtain a high-quality and stable 1D chain structure. The template-dependent method is an effective strategy, while eliminating the effect of template on material properties is still a challenging problem.
Another problem caused by the limitations of the synthesis technique is that the theoretical calculation results of the physical and electrical properties of TMTCs lack experimental verification. Tensile strain greatly affects the bandgap, mobility, etc. of TMTCs, but experimental demonstrations in these areas are far behind. DFT calculations show that the TMTC material has a high solar energy conversion efficiency (16%), which needs to be verified by experimental results in the future. Compared with TMDCs, the innovative applications of quasi-1D TMTCs are still waiting to be explored. For example, ZrTe 3 , NbSe 3 , and TaS 3 have been proven to have remarkable CDW property through theoretical and experimental studies. The anticipated application in quantum computing and information processing needs to be implemented. The high breakdown current density (>100 MA cm −2 ) and quasi-1D structural properties make metallic MTe 3 (i.e., ZrTe 3 ) an alternative material for Cu wires. Such materials can become the basis for future nanoelectronics and neural network interconnection technologies. More functional devices based on TMTCs need to be developed, such as bioelectronics, flexible electronics, and other fields. Based on the experience of the development of 2D materials such as graphene, these problems will be overcome in the near future after attracting the attention of a growing number of researchers. It is expected that TMTCs will be a sort of rising star material in the following post-information age.