NbTe2 is a typical layered CDW material with two different structural phases. At high temperature (above 550 K), it exhibits a high symmetry 1-T phase where each Nb atom is coordinated octahedrally by Te atoms.32 Below 550 K, NbTe2 undergoes a CDW phase transition which results in a displacement of Nb atoms from the octahedral centers to a monoclinically distorted 1-T phase (1-Tʹ phase).28, 30 This 1-Tʹ phase is very stable at room temperature since the phase transition temperature is much higher. The crystal structure of 1-Tʹ NbTe2 is shown in Fig. 1(a). Each monolayer is composed of an Nb layer sandwiched by two Te layers, where the Nb atoms are displaced within the plane to form “trimers,” whereas the Te atoms present an out-of-plane buckling.28, 32 The Te-Nb-Te sandwiches stack with weak van der Waals interactions to form a layered structure.
We prepared single crystal NbTe2 flakes with different thicknesses via mechanical exfoliation. An optical microscopy image of a representative sample is shown in Fig. 1(b). Different contrasts represent areas with different thicknesses. It can be seen that the exfoliated flake presents a flat surface with uniform thickness in the different areas. Figure 1(c) shows the AFM height profile of the NbTe2 flake, which indicates that the thickness of the flake is ⁓ 28 nm. Due to the strong interlayer coupling of NbTe2, it is very difficult to obtain very thin samples using mechanical exfoliation.25 The thinnest flake obtained in our experiments is ⁓ 15 nm. Further AFM images of NbTe2 flakes with different thickness are shown in Figure S2 (Supporting Information). The Raman spectrum of a NbTe2 flake is shown in Fig. 1(d) with an excitation laser at 532 nm. Characteristic peaks at ∼55 cm− 1, ∼83 cm− 1, ∼121 cm− 1, ∼140 cm− 1, ∼157 cm− 1, ∼168 cm− 1, ∼219 cm− 1, and ∼262 cm− 1 can be observed, which correspond to the phonon modes of Ag1, Ag2, Ag4, Ag5, Ag6, Bg4, Ag7, and Ag8 in NbTe2, respectively.33–34 These results indicate the high crystal quality of the samples. Optical absorption spectra (from 400 nm to 900 nm) of NbTe2 flakes with different thicknesses were measured by using a UV-vis spectrometer, as shown in Fig. 1(e). A broadband absorption response with a smooth absorption band in the wavelength range can be observed for all the thicknesses. The thickness-dependent optical bandgap is estimated from a Tauc plot of (αhν)1/2 versus hν based on the Tauc formula (Figure S3), where α and hν represent the optical absorption coefficient and photon energy, respectively. Figure 1(f) shows the measured optical bandgaps as a function of thicknesses, where the bandgap of the NbTe2 decreases from ⁓ 0.8 eV to 0 eV with increasing the sample thickness from 15 nm to 50 nm.
To characterize the photon-excited carrier dynamics, time-resolved pump-probe transient reflection (ΔR) spectroscopy was used with a pump laser at 520 nm and probe laser at 1040 nm. The pump-induced probe reflection change (ΔR = R-R0) was measured by chopping the pump and monitoring the output of the photodiode with a lock-in amplifier, where R and R0 are the probe reflections with and without pump light, respectively. Figures 2(a) ‒ (e) show the time-resolved ΔR curves for flakes with thicknesses from ⁓15 nm to ⁓50 nm. The insets of these figures present the corresponding normalized ΔR curves for 0 delay times. It can be seen that, for all thicknesses, a fast increase of probe reflection from zero to its maximum value (positive ΔR) is observed at zero-delay. The positive ΔR indicates photoinduced bleaching (PB) of the probe light.35 Since the NbTe2 bandgap is much less than the pump photon energy (~ 2.38 eV), the pump can excite electrons directly from the valance to conduction bands. These excited carriers are commonly known to decrease the absorption of the probe light and enhance its reflection due to the filling of states and the Pauli-blocking effect.36−39
After ΔR reaches its maximum, a decay process can be observed in the ΔR curves, which can be mainly separated into two components: a sharp drop of ΔR followed by a slow relaxation process, as shown in Fig. 2(a) ‒ (e). By fitting the experimental data, relaxation time constants during the decay process can be obtained. In our case, a tri-exponential decay function was used to fit the measured ΔR curves, as follows:40−41
$$\frac{\varDelta R\left(t\right)}{{R}_{0}}=A\text{e}\text{x}\text{p}\left(\frac{-t}{{\tau }_{1}}\right)+B\text{e}\text{x}\text{p}\left(\frac{-t}{{\tau }_{2}}\right)+C\text{e}\text{x}\text{p}\left(\frac{-t}{{\tau }_{3}}\right)$$
1
where A, B, and C denote the corresponding amplitudes. t denotes the delay time between the pump and probe, and τ1, τ2, and τ3 are the time constants of relaxation processes. Here, we combine the semi-log fit with the tri-exponential fit for better evaluation of the time constants.
The measured values of τ1, τ2, and τ3 for different film thicknesses are presented in Fig. 2(f). It can be seen that the sample having the fastest relaxation time was 15-nm thick, and had a τ1 ⁓ 7.4 ps. This is in the same order of magnitude of other TMDCs, such as MoS242–43 and PdSe2.35 This picosecond relaxation process can be attributed to carrier–carrier and carrier–phonon scattering during the carrier-cooling process.44–47 The pump-excited hot carriers initially thermalize to quasi-equilibrium states through carrier-carrier scattering. They then transfer their energy to the NbTe2 lattice and are cooled mainly by electron–phonon scattering. A thickness-dependent behavior can be observed in τ1, where it increases from ⁓ 7.4 ps to 38.3 ps as the sample thickness increases from 15 nm to 50 nm. It has been demonstrated that an increase in thickness in TMDCs can lead to an enhancement of dielectric screening of the long-range Coulomb interaction, weakening the electron–phonon coupling,48–49 which in turn increases the relaxation time τ1 for thicker samples.
The time constant τ2 exhibits a similar trend to τ1 with increasing sample thickness, although with an overall slower lifetime, ranging from ⁓ 83.4 ps for 15-nm to ⁓ 465 ps for the 50-nm flakes, as shown in Fig. 2(f). We attribute this relatively longer relaxation process to the anharmonicity-driven phonon-phonon scattering.50 As discussed above, τ1 denotes carrier relaxation to phonons via fast carrier-phonon scattering processes. The subsequent thermalization of these generated phonons with the rest of the phonon subsystem takes a longer time via the anharmonicity-driven phonon-phonon scattering. This phonon dominating process may also explain the thickness-dependent τ2 because of the slower phonon cooling process occurring in thicker flakes.51 The longest lifetime τ3, is on a nanosecond time scale (inset of Fig. 2(f)), which arises from lattice cooling by dissipating the energy to the substrate.37, 52–53
Figures 3(a) shows pump power dependent ΔR measurements for a 32 nm-flake with pump powers from 40 µW to 80 µW, with the probe power fixed at 35 µW. Similar temporal features in the ΔR curves can be observed for different pump powers, indicating that the carrier relaxation dynamics in NbTe2 are pump power independent, similar to other TMDCs.43, 46 In contrast, for the ΔR amplitudes, a clear increase with pump power is observed. Figure 3(b) plots the corresponding peak amplitudes extracted from the ΔR curves in Fig. 3(a), demonstrating a linear relationship between the amplitude and pump power. The observed linear contribution of the pump power indicates a one-photon excitation of carriers in NbTe2 with the pump beam and contribution to Pauling blocking at the probe wavelength.35, 43 The extracted peak amplitudes as a functions of pump power for other thicknesses are presented in Figure S4.
We investigated the anisotropic ultrafast carrier dynamics via polarization-dependent pump-probe measurements. Angle-resolved polarized Raman spectroscopy was used to analyze the crystal axis of NbTe2 flakes under a parallel configuration, with an excitation laser wavelength of 532 nm. In the experiment, we fixed the sample and rotated the polarizers in the incident and scattered light paths to vary the angle between the sample crystallographic orientation and the polarizations of beams. Figure 4(a) shows the Raman spectra of a flake for different excitation laser polarization angles. To better illustrate the polarization trend, the polarization diagram of Ag2 mode of the sample is plot in Fig. 4(b). It can be seen that the peak intensity of the Ag mode oscillates with a periodicity of 180° as the orientation of the polarization is rotated. Therefore, by using this polarization diagram, the crystallographic orientation of the flakes can easily be determined.
After determining the crystal directions, we conducted the polarization-resolved pump-probe measurements. The pump and probe powers were 40 and 35 µW, respectively, with their polarization angles controlled by rotating a half-wave plate. Figure 4(c) shows the normalized ΔR curves of the 40-nm sample for pump polarization angles of 0° and 90° with respect to the sample orientation. Varying the pump polarization did not change their temporal response, indicating that the photon-excited carrier relaxation process is isotropic in NbTe2 flake. We also measured the peak amplitudes of the ΔR curves under different pump polarization angles (Fig. 4(d)) where a sinusoidal dependence on the polarization angles is observed, originating mainly from the anisotropic pump absorption. This is further verified by the polarization-dependent transmission of pump light in the sample, as shown in Figure S5.