Periodic modulations of electron density from electron–phonon interactions lead to a novel type of ground state called charge density wave (CDW).^[1] A great deal of materials such as layered transition metal dichalcogenides (TMDCs) and transition metal trichalcogenides (TMTCs) have been found to possess the CDW states.^[1,2] Recently, these interesting states show unexpected and exotic properties when approaching the 2D limit. For example, with the thickness decreasing,^[3] a commensurate CDW state completely disappears in TaS ; and the transition temperature can be drastically enhanced in monolayer NbSe and TiSe .^[4–7] More interestingly, the quantum critical behavior may exist in 2D TaS and other correlated systems^[8,9] as the reduced dimensionality can strengthen Peierls instability, electron–phonon interactions, and the fluctuation effect.^[1,7] Therefore, it is indispensable to investigate the effect of the dimensionality on the CDW states and uncover the intrinsic mechanism.

Like TMDCs, many bulk TMTCs exhibit layered structures with interlayer van der Waals interactions.^[10–12] As a typical TMTC, -type crystals (M = Ti, Zr, Hf, Ta, Nb; X = S, Se, Te) are quasi-1D materials, some of which possess charge density waves in the bulk form. And these states emerge and slide along the direction of metal-atom chains unlike in TMDC materials where the various CDW states are developed by several-atoms-clusters in 2D layers.^[13,14] For instance, two phase transitions at 144 K and 59 K in NbSe are the consequence of three structurally different chains.^[15] Monoclinic TaS has two types of chains which induce CDW transitions at 240 K and 160 K.^[16] Importantly, the low dimensionality may exert strong effects on those chain-like quasi-1D materials due to the different crystal structures. Each CDW transition can be variously affected with the change of device scale along different crystal orientations, thus giving rise to an evident anisotropy. However, limited works have been reported on the CDW states in quasi-1D TMTC materials when the dimensionality is reduced to two dimensions.

TiS , as one of the semiconducting TMTCs, shows extremely excellent optical properties.^[17–21] Strong anisotropy from electric transport measurements represents 30% of increase over the black phosphorus.^[19,22] The bulk crystals have phase transitions at 120, 60, and 17 K through quasi-1D transport.^[23–26] with the first two along the perpendicular chains. We anticipate that the nanoribbon structure can have structural fluctuations in horizontal and perpendicular directions that lead to anisotropic CDW behavior upon external electrical gating. However, to date such studies remain lacking.

Here, we report the properties of exfoliated few-layer TiS nanoribbons and find only two phase transitions at 46 K ( and 85 K ( in stark contrast to the three in thick TiS at 29, 53, and 103 K. can be tuned by the back-gate voltage because of the variable carrier concentration whereas does not show an evident change owing to its robustness to the structural fluctuations. Our transport experiments clearly exhibit a strong anisotropy of CDW states in TiS . We also reveal the light-induced melting of charge density wave under 633-nm laser illumination. When the laser intensity exceeds 10 μW, both two transitions disappear. Moreover, a low illumination intensity of 0.02 μW by a 532-nm laser can readily destroy the CDW states.

Bulk TiS crystals were synthesized via chemical vapor transport (CVT) method (sulphuration of titanium powder).^[17]

Figure 1(a) shows a layered structure of TiS ; parallel chains of triangular prisms which are held by van der Waals forces constitute sheets. The parallel chains are responsible for the quasi-1D nature of the material which leads to charge density waves. Each sheet is formed by two titanium layers and four sulfide layers. The as-grown TiS naturally develops groups of black needle-like bulk crystals. Figure 1(b) shows one of the exfoliated TiS samples transferred onto SiO (285 nm)/Si substrate with a number of nanoribbons. Figure 1(c) shows a scanning electron microscopy (SEM) image, where both bulk crystals and thin nanoribbons can be identified. The purple one in the red rectangle is a thin nanoribbon with a thickness of nm. To identify the composition of sample, energy dispersive x-ray spectroscopy (EDX) has been carried out. Figure 1(d) displays the EDX results of the as-grown samples. More than five positions have been randomly picked and measured, all of which show clear peaks of Ti and S with the total error less than 0.5%. The atomic ratio of Ti and S is nearly Ti:S=1:3, indicating a correct composition of TiS . We further carried out Raman spectroscopy to explore the phonon characteristics of materials and identify the chemical bond. Figure 1(e) displays the Raman spectrum of an isolated nanoribbon thicker than 50 nm; the optical image of the sample is shown in the inset under 532-nm laser illumination. While the peak at is due to Raman mode of silicon substrate, the other four peaks located at , , , and correspond to A -type Raman modes of the TiS which are in good agreement with the bulk crystals. Besides, a previous study reports the observation of the peak which has the highest intensity along b axis in nanoribbons,^[27] which is consistent with our results.

Fig. 1.

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Fig. 1. (color online) Synthesis and characterizations of TiS nanoribbons. (a) The TiS crystal structure. Ti atoms are covalently bonded to six S atoms forming triangular prisms which are stacked to make chains. (b) The optical microscopy image of TiS nanoribbons on an SiO /Si substrate. (c) The SEM image of TiS nanoribbons. Inset: single TiS nanoribbon. (d) The EDX spectrum of TiS nanoribbons with the atomic ratio of Ti/S of 1:3. (e) The Raman spectrum of the individual TiS nanoribbon on SiO /Si substrate at room temperature using a 532-nm laser source.

In order to study the CDW properties, four-terminal devices with Cr/Au (5 nm/100 nm) electrodes were fabricated along the nanoribbon chain direction, i.e., the b axis, by standard e-beam lithography, metal evaporation, and lift-off process. Temperature-dependent resistance (R–T) is measured from room temperature down to 10 K as shown in Fig. 2(a). The inset displays the device image by a conventional optical microscope. The length of the sample is around 20 μm and the width is 2 μm. The thickness of the nanoribbon is determined to be nm, which is relatively thick. The resistivity measured at 300 K has a value of ρ=0.4Ω⋅cm−1 in good agreement with the previously-reported bulk results.^[28] The R–T curves in our experiments are all measured in the cooling process to eliminate the hysteresis induced by the system. As the temperature cools down, the resistance shows several broad anomaly features. In order to clearly identify the transition temperatures, we adopted the widely-used derivative method and plotted against the temperature. It is known that near the transition temperature, the conductivity can be extracted by

Fig. 2.

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Fig. 2. (color online) CDW transitions in a nm thick and 15-nm thin TiS nanoribbon. (a) The temperature dependence of the resistance and the logarithmic derivative . The source–drain current nA. Three CDW transitions are evident at 29, 53, and 103 K. Inset: optical microscopy image of the 50 nm–60 nm TiS3 FET. (b) – curves of the thick TiS3 nanoribbon at different temperatures. (c) The temperature dependence of parameter using different fitting region. The red curve is fitted at V, while the blue one is at V. The inset shows the two fitting regions. (d) Temperature dependence of the resistance and the logarithmic derivative in a device whose thickness is lower than 15 nm. The source drain current nA. Unlike the thick nanoribbon, two CDW transitions appear at 46 K and 85 K. Inset: optical image of the FET device.

The phase transitions at 53 K and 103 K can be explained by the formation of two charge density waves in disordered quasi-1D system. It was found that below 110 K, a CDW state develops in the direction across the chains, like ZrTe , while the other CDW forms along the chains below 60 K.^[23] Our narrow TiS nanoribbon increases the fluctuation in the direction across the chains, thus weakening the CDW, which also explains the weak non-linear transport after the first CDW state at 110 K. When the temperature is below 25 K, the noise at low voltage becomes ignorable; correspondingly the fitting parameter has large errors to be analyzed. As the voltage becomes larger, the Schottky barrier turns to be prominent (the red curve). Below 130 K, the I–V curve turns to be nonlinear, and gets larger with the temperature reduced, but three peaks clearly show up with two of which corresponding to the aforementioned two CDW states. The transition at 25 K, however, is still unknown and needs further investigation. The transition temperatures from the quasi-1D transport analysis and derivative curve are quite similar, which also confirms the three CDW phase transitions.

Next, we focus on the thinner nanoribbons. The fabrication method is the same as before, but the sample thickness is reduced to < 15nm as displayed in the inset of Fig. 2(d). Note that experimentally it is challenging to reach monolayer TiS nanoribbon because of the 1D chain structure and the subsequent electric measurements are difficult owing to the extremely large resistance when approaching the atomic thickness. The R–T curve shows anomaly peaks at 46 K and 85 K (the blue curve), which correspond to the two peaks in the derivative curve (in red). The third transition cannot be observed in transport measurements for which the underlying mechanism is not clear yet. We note that the carrier density can strongly affect the CDW phase.^[16,37] Compared with the thick device, in the thin nanoribbons we find a rapid diminishment of the two transition temperatures. We provide two reasons to explain this behavior as follows. First, the scattering mechanism in TiS is dominated by charge impurities at low temperatures (see Fig. S2 in Appendix A). The phase transition is greatly suppressed by impurities in quasi-2D or 1D CDW materials.^[16] Non-isoelectronic impurities can induce potentials which lead to changes of the phase of the condensate, destroy long-range CDW order and lead to a finite phase–phase correlation length .^[38,39] Second, lowering the dimensionality of a system usually tends to reduce the phase space available for the phase parameter to adapt to imperfections, and consequently it suppresses the long-range order and lowers the transition temperature .^[16] Moreover, the Coulombic interaction can modulate transport properties in two dimensions.^[40] With the thickness decreased, the interaction between the SiO /Si substrate and TiS nanoribbon gradually dominates which can also jeopardize the CDW coherence. The size effect of CDW transition temperature can be described by empirical formula^[41]

Fig. 3.

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Fig. 3. (color online) Gate-tuned CDW transitions in thin TiS nanoribbon. (a) Temperature-dependent resistance at various gate voltages from V to 70 V except 40 V in a 15-nm device. Inset: the enlarged view of the R–T curve at low temperature regime. (b) Temperature-dependent logarithmic derivative of R–T under different . Right panel, the curves are multiplied by 10 times to display the evolution of CDW phase transition under external gate voltages. (c) The gate voltage dependence of transition temperature and .

In bulk materials, the doping and high pressure can modulate the carrier density, for instance, to enhance the superconductivity in Cu-doped TiSe .^[43] In quasi-2D materials, the electric-field effect is much more convenient than these two methods: a gate voltage can induce a large number of electrons (holes) into the system so that CDW can be easily tuned.^[3,37] Here, we use a back-gate voltage to tune the phase transitions in 15-nm thick nanoribbon. Figure 3(a) illustrates a set of R–T curves at gate voltage ranging from V to 70 V using a constant source–drain current of 1.5 nA. At 40 K, the resistance changes from 2.6 G to 0.07 G . From V to V, a peak can be clearly identified near the first transition temperature (Fig. 3(a)). The semiconducting phase converting to be metallic looks more evident when we use logarithmic coordinates as demonstrated in the inset. It is possible that the large CDW conduction modulation is due to changes in CDW pinning arising from gate–voltage-induced variations in the CDW gap which was also observed in semiconducting quasi-1D material TaS .^[44] Likewise, figure 3(b) displays the derivative curves to ensure the extraction of the transition temperature. Apparently, two peaks are shifted as a function of the gate voltage. The gate–voltage dependence of conductance at 160 K is displayed in Fig. S2(b). At V, the conductance tends to be zero and the extracted field-effect mobility and on/off ratio are 0.8 cm and , respectively (More information is available in Figs. S2 and S3 in Appendix A). Systematic changes under different gate voltages are summarized in Fig. 3(c). For the first CDW transition, the critical transition temperature does not vary much and it positions at around 40 K. On the contrary, is identified to be 110 K when V and reduced to 93 K at V and in the whole gate region has negative dependence with the value of gate voltage. It can be naturally explained by the fact that the increase of the electron density at positive voltages suppresses , which is also supported by a similar behavior occurring in 1T-TiSe and TaS .^[3,37] Specifically, a large electron density may enhance CDW phase fluctuations, strengthen disorders, and thus ultimately destroy a long-range CDW coherence. More electrons tend to be excited across the gap, the periodic structure of charge density becomes unstable and much lower temperature is needed to reduce the thermal excitation and keep the density wave. This is commonly observed in quasi-1D and quasi-2D systems.^[16] In contrast, decreases a little at larger ; this is presumably caused by its robustness to the fluctuations because of the ribbon structure in which the CDW forms along the chains. However, for the thick nanoribbon sample, the gate cannot tune it well in a sharp contrast to the thin one, as shown in Fig. S4 in the supplementary information (Appendix A).

Since TiS is a light-sensitive semiconductor, a 633-nm laser (1.94 eV) is used to illuminate the TiS surface as displayed in the inset of Fig. 4(a). The R–T curve shows two phase transitions at 40 K and 85 K without laser illumination (purple curve). The illuminating region of TiS is about 20 . As the laser intensity gradually increases, the resistance reduces and the phase transition becomes weaker prior to the disappearance under 1.0 μW. Figure 4(c) displays the derivative curves where the threshold intensity is about 1.0 μW and the two peaks come to be completely invisible above this threshold. Though the resistance is monotonically decreased by increasing the laser power, the position of two peaks do not change below 1.0 μW, which can exclude the heating effect. Like the process of light-induced superconductivity suppression,^[45,46] laser can destroy the condensate in the distorted region in the CDW state and cause a prompt loss of charge order and equilibration of the electron–lattice system when the absorbed optical energy is higher than the condensation energy (the energy difference between the free energy of the CDW state and the normal state).^[47,48] After the excitation, the periodic electron density structure is destroyed and TiS converts to the normal semiconducting state. With a low laser excitation, however, the two phase transitions still exist but are softened with the increasing of the laser intensity, which may be due to the insufficient photons absorbed per unit cell.^[49] The low laser excitation can raise the fluctuation but not sufficient to destroy the condensation energy. Figure 4(b) shows the I–V curves under different incident laser intensities at 25 K. The resistance reduces after the laser excitation and the I–V curves become linear under 11-μW laser intensity which indicates that the CDW states and Schottky barrier both disappear. Under 1.0-μW laser intensity, the curve is nonlinear, indicating the Schottky barrier existence while CDW states are destroyed. The quasi-linear I–V curve also shows the suppression of CDW states under laser illumination. Figure 4(d) displays the 532-nm laser condition: under a laser intensity of 0.02 μW, the CDW transitions are substantially weakened and two peaks in the derivative curve disappear (see the inset in Fig. 4(d)). Lower critical intensity of 532-nm laser may attribute to the larger illumination energy. In short, we conclude that the CDW state strongly depends on the excitation energy, reminiscent of superconductivity, where the CDW pseudo gap state is easily broken by external factors. Below a critical energy, the system needs much more photons to excite the redundant electrons across the gap. Otherwise, the CDW states can still survive.

Fig. 4.

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Fig. 4. (color online) Melting of the CDW states by laser. (a) curves under different laser power (0 μW–11 μW). Inset shows a schematic structure of TiS device incident with a 633-nm laser. (b) I –V curves under different incident laser powers. (c) Temperature-dependent R–T logarithmic derivatives under different laser powers. (d) R–T curves with and without 532-nm laser. Inset displays temperature-dependent R–T logarithmic derivatives.

Finally, let us briefly compare TiS with previously widely studied CDW in TMDC such as NbSe . In NbSe , reducing the dimensionality strengthens the CDW phase and weakens the superconducting phase due to the Fermi surface nesting.^[7] In contrast, TiS shows an opposite trend in which thin TiS exhibits a lower transition temperature. The CDW states in TiS are strong anisotropic resulting in the different gate tuning capability because of its robustness to the structural fluctuations. In conclusion, we have performed a systematic study on charge density wave properties in quasi-1D TiS nanoribbons. Two CDW transition temperatures decrease in the thin nanoribbon. The second transition temperature reduces as the increase of the electron doping, while the first transition shows a negligible change. We also observed the light-induced melting of CDW after 633-nm and 532-nm laser illuminations. CDW properties in TMTCs are different from quasi-2D materials such as TMDCs. Our study shows that TiS is a promising CDW material for the study of collective phases in TMTCs and expanding the functionalities for electronic applications like information processing.^[50]