Since their discovery in 2004¹, two dimensional (2D) layered materials have been recognized as the foundation of the next generation optoelectronics devices² and photonic elements³ due to their strong and unique light-matter interaction, such as ultrafast and broad optical response in graphene⁴, strong excitonic optical properties in single layer transition-metal dichalcogenides (TMDs)^5-7, and tunable direct optical band gap in black phosphorus (BP)^{8, 9}, etc. Recently, a series of newly emerged 2D materials^10-13 have shown interesting nonlinear optical response. In light of remarkable optical properties, 2D layered materials endow new optoelectronics applications that are radically different from their bulk counterparts^14-17.

MXenes are a new family of 2D materials. It can be produced by selective etching methods. Since the first reported Ti₃C₂ MXenes in 2011¹⁸, over 100 kinds of MXenes have been theoretically predicted¹⁹. They shared a universal formula of M_n+1X_nT_x, where M is a transitional metal, X represents C/N, and T is the surface termination. Due to the diversity of compositions, MXenes have exhibited tunable and fascinating optical, chemical, and electronic properties, prompting the proposal of the concept of MXetronics²⁰. The strong plasmonic feature peaks of different MXenes covering the visible to near-infrared (NIR) spectral range^{21, 22}, resulting in the diversity of photoelectronic²³, photothermal²⁴, and photovoltaic applications²⁵. In terms of their nonlinear optical response, Ti₃C₂T_x thin films were recently reported to have thickness-dependent saturable absorption²⁶, which was attributed to the plasmon-induced increased ground-state absorption at high optical intensity. In addition, the nonlinear optical absorption response of MXenes has been demonstrated to be modulated by the excitation wavelength²⁷ and surface group²⁸, promoting the development of MXenes-based nonlinear optical devices. However, most nonlinear optical research about MXenes is currently focused on reporting the performance or intrinsic properties of a specific MXene. The fundamental understanding of MXenes photo-physics and the strategies for regulating the optical performance of MXenes is still very rudimentary²⁹.

The strategy for the construction of 2D heterostructure is an important tool for improving the photoelectronic performance of 2D materials-based devices^30-32. The advantage of each component in 2D heterostructure can be preserved by careful design, and novel properties such as charge transfer or energy transfer between constituents of the 2D heterostructure may appear due to the interfacial effect. Study on the optical response of metal–semiconductor 2D heterostructures is essential for its photodetection and photocatalysis applications³³. Recently, Nb₂CT_x MXene has been shown to have the highest optical extinction coefficient in the near-infrared region²¹. The few-layer MoS₂ generally demonstrated remarkable optical properties in its excitonic resonance region (~1.8 eV)³⁴. Moreover, 2D MXene/MoS₂ heterostructure have shown the great potential for batteries^{35, 36} and supercapacitor³⁷, which was attributed to its long-term stability.

Inspired by these advantages, we demonstrated an enhanced linear and nonlinear optical performance of an Nb₂C/MoS₂ heterostructure by in situ growing MoS₂ on the surface of Nb₂C nanosheets. Interestingly, the species of surface group in Nb₂C can modulate the work function of Nb₂C/MoS₂, which has been confirmed by X-ray photoelectron spectroscopy (XPS) measurements and density functional theory (DFT) calculation. After comparing the experimental and theoretical results, we determined that the surface group of Nb₂C/MoS₂ was dominated by O termination, leading to the decrease in the work function of Nb₂C after the in situ growth of MoS₂. The Nb₂C/MoS₂ inherited the preponderance at a different wavelength of Nb₂C and MoS₂ in absorption and exhibited enhanced broadband optical absorption, which was confirmed by measurement of UV-vis spectrum and DFT calculation. In addition, the saturable absorption of Nb₂C/MoS₂ and pristine Nb₂C was investigated by the Z-scan technique. The nonlinear absorption coefficient and modulation depth of Nb₂C/MoS₂ are greater than those of Nb₂C, but the saturated intensity comparison shows the opposite result. This demonstrates the superior nonlinear optical performance of Nb₂C/MoS₂ to that of Nb₂C. The improved NLO performance can be attributed to the hole transfer from Nb₂C to MoS₂, which caused nonlinear optical response modulation in the heterostructure when combined with the determined energy level alignment. The current findings demonstrated that the Nb₂C/MoS₂ is a promising candidate for high-performance optoelectronic devices and provided an effective method for regulating the nonlinear optical response of MXenes.

The 2 g Nb₂AlC powder (11 Technology Co., Ltd., China) was added to a 40 mL of 49% HF solution (Macklin Inc.) and stirred for 60 h at room temperature. Then, the excrescent HF solution was washed with water by centrifugation until the pH was close to 6. Subsequently, the washed dispersion was added to the 5% 25 mL TMAOH solution (Macklin Inc.) with stirring for 12 h. The few-layer Nb₂C nanosheets dispersed in water were obtained after washing the excrescent TMAOH.

The ammonium thiomolybdate (13.3 mg) was added to as prepared Nb₂C nanosheets dispersion (2 mL, 5 mg/mL) and stirred for 12 h until the MoS₄²⁻ was fully inserted into the surface of the Nb₂C nanosheets in dispersion. Then the precipitate was collected with quick freezing and freeze-drying. After annealing (2 h, 500 °C) in an H₂ (10%)/Ar (90%) atmosphere, we obtained the heterostructure with MoS₂ nanocrystals in situ grown on the surface of Nb₂C nanosheets.

The TEM, HRTEM, STEM, and element mapping scans were acquired using a Talos™F200X S/TEM (Thermo Fisher Scientific). A BRUKER D8 ADVANCE XRD system was employed for X-ray diffraction (XRD) phase characterizing. A Renishaw InVia Qontor confocal Raman microscope system was employed for collecting Raman spectra. The XPS measurement was performed with the help of a Thermo Scientific ESCALAB Xi+. The optical absorption spectra were characterized with a UV−vis spectrophotometer (UV-2600, Shimadzu).

The nonlinear optical (NLO) absorption was characterized by an open aperture (OA) Z-scan system. A mode-locked Ti: sapphire system operating at 800 nm with 35 fs pulses at a 2 kHz repetition rate and a TOPAS (Light-Conversion) optical parametric amplifier was employed as a laser source. The Z-scan system measures the transmittance of the sample as a function of optical intensity, where the focal length of lens is 175 mm and beam waist is determined to be 24 μm. The signal was collected with the average power of the optical detectors (OPHIR, PD300 IR).

The first-principles calculations of Nb₂C/MoS₂ heterostructure were performed by density functional theory as implemented in the Vienna ab initio Simulation Package (VASP)³⁸ with the projector augmented wave (PAW) pseudopotentials. The generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof (PBE) functional was used. The plane-wave energy cutoff was set to 500 eV with a precision force of 0.01 eV/Å (1Å=10⁻¹⁰ m). The Brillouin zone was sampled with a 9×9×1 k-mesh point setting, and a denser k-mesh 15×15×1 was used for optical properties computations. The vdW interaction was treated with the semi-empirical D2 method (Grimme method). The absorbance A(ω) was determined by the 2D optical conductivity for low-dimensional systems^{39, 40}. For the calculations of the Nb atom, the Hubbard U method was employed to treat strong correlation effects, and the U value of the Nb d orbital was set to 6.5 eV⁴¹.

At first, the few-layer Nb₂C nanosheets are synthesized with selective HF etching and intercalation methods²⁷. The synthesis route of Nb₂C/MoS₂ heterostructure is shown in Fig. 1(a). Briefly, the MoS₄²⁻ was inserted into the surface of Nb₂C nanosheets during dispersion. MoS₂ nanocrystals were grown in situ on the surface of Nb₂C nanosheets after freeze-drying and annealing in an H₂ atmosphere. Fig. 1(b) and Fig. S1(a) illustrate the transmission electron microscopy (TEM) image of Nb₂C/MoS₂, indicating a uniform, transparent flake structure of Nb₂C/MoS₂. The elemental mapping images were shown in Fig. 1(c). The Nb, Mo, and S elements overlap very well, indicating that the MoS₂ and Nb₂C are integrated very well in a single flake. To further characterize the morphology of MoS₂ on the Nb₂C sheets, high-resolution transmission electron microscopy (HRTEM) was performed (Fig. 1(d) and Fig. S1(b)). The surface of the Nb₂C sheet has a clear lattice fringe. After careful measurement, the interplanar crystal spacing was determined to be 0.62 nm, which agrees well with the d-spacing of the (002) plane in MoS₂. The selected area electron diffraction (SAED) pattern of Nb₂C/MoS₂ (Fig. 1(e)) matched well with (002) and (103) planes for Nb₂C, (100) and (110) planes for MoS₂, indicating a good overlap of the Nb₂C and MoS₂.

The Raman spectroscopy analysis of Nb₂C/MoS₂ heterostructure is illustrated in Fig. 1(f). The prominent peaks appeared at 218 cm⁻¹ and 255 cm⁻¹ are attributed to the vibration mode of E²_2g and A_1g of Nb₂C⁴², respectively, and the peaks located at 378 cm⁻¹ and 402 cm⁻¹ were assigned to the vibration mode of E¹_2g and A_1g in MoS₂⁴³, indicating the successful combination of Nb₂C and MoS₂. X-ray diffraction (XRD) was performed to further characterize the structure of Nb₂C/MoS₂. When compared to the XRD pattern of Nb₂AlC (JCDS PDF#30-0033), the (002) peak of Nb₂C downshifts and broadens to 2θ=7.1∘, demonstrating a successful exfoliation of 2D Nb₂C²⁷. For Nb₂C/MoS₂ heterostructure, the peaks located at 14.2°, 33.4°, and 58.7° were attributed to the (002), (101), and (110) planes of MoS₂ (JCDS PDF#37-1492), and the signal at 8.7° can be ascribed to the (002) plane of Nb₂C. Because the (002) peak of MXene represents the interlayer spacing, the downshifting of the (002) peak in Nb₂C indicates decreased layer spacing after combing with MoS₂.

To analyze the bonding and chemical composition, X-ray photoelectron spectroscopy (XPS) of Nb₂C and Nb₂C/MoS₂ was performed. In the XPS survey pattern spectrum (Fig. S1), the peaks of Nb₂C matched well with Nb, C, O, and F elements, which were presented in the XPS spectrum of Nb₂C/MoS₂. Furthermore, the additional peaks can be found in the Nb₂C/MoS₂ spectrum, which were identified as the feature of Mo and S elements. The C 1s peaks of Nb₂C/MoS₂ (Fig. 2(a) upper plane) consist of three components. The peaks at 284.8 eV, 286.2 eV, and 288.9 eV were designated to sp³ C-C bond⁴⁴, C-O bond⁴⁵, and O-C=O bond⁴⁵, respectively. As compared to the results shown in Fig. 2(a) (lower plane), the C bonding properties are nearly unchanged after combing with MoS₂ because the position and intensity of the three peaks do not change. In the Nb 3d spectrum (Fig. 3(b) upper plane), the peaks at 207.5 eV and 210.2 eV are related to the 3d_5/2 and 3d_3/2 orbits of Nb atom⁴⁶, respectively, which could be assigned to the Nb₂O₅ component^{47, 48}. In comparison, the peaks of the same components in pristine Nb₂C are downshifted to 206.8 eV and 209.5 eV, and the ratio of the two components does not change (Fig. 2(b) lower plane). These results indicate that the chemical environment of Nb atoms has changed due to the growth of MoS₂, but the valence state of Nb atoms has not changed. The upper plane of Fig. 2(c) shows the O 1s high-resolution spectrum, which revealed three peaks in Nb₂C/MoS₂: one resulted from Nb-O contaminations at 531.3 eV⁴⁹, one from C-O contaminations at 532.5 eV⁴⁴, and one from OH surface termination groups at 533.7 eV⁴⁴. These three components shifted to 530.0 eV, 531.4 eV, and 532.5 eV for pristine Nb₂C. It is worth noting that the ratio of the C-O component in Nb₂C/MoS₂ is significantly enhanced as compared to the pristine Nb₂C, demonstrating an increase of oxygen content at the surface of Nb₂C after the in situ growth of MoS₂ (Fig. 2(c) lower plane).

The Mo 3d peaks shown in Fig. S2 consist of four components. The main peaks located at 229.0 eV (3d_5/2) and 232.1 eV (3d_3/2) corresponds to the Mo-S bonding⁵⁰. For the S element (Fig. 2(d)), the peaks located at 226.3 eV, 162.4 eV, and 161.2 eV were assigned to S 2s, S 2p_1/2, and S 2p_3/2, respectively³⁶. Moreover, the peaks located at 168.4 eV were ascribed to the Nb-S bonding⁴⁶, indicating a strong combination of Nb₂C and MoS₂. Figure 2(e) shows the high-resolution XPS spectrum of F 1s, it can be seen that the -F surface termination was removed from the Nb₂C/MoS₂ heterostructure, which could be attributed to the F desorption during the heat treatment⁵¹.

To determine the change of work function, the comparison of the XPS secondary electron cut-off (SEC) between Nb₂C and Nb₂C/MoS₂ was shown in Fig. 2(f). The SEC correspond to the position where the detected electron has the highest binding energy, which is usually combined with the position of the Fermi edge to determine the escape work of the material: ϕm=hv−EK,maxmeas, where ϕm is the work function of sample, hv and EK,maxmeas are the photon energy and the detected highest binding energy in the SEC. The SEC energy of Nb₂C/MoS₂ is larger than that of Nb₂C. According to the basic principle of photoelectron spectroscopy^{52, 53}, the work function of Nb₂C/MoS₂ is smaller than that of Nb₂C. In other words, in situ MoS₂ construction on the surface of Nb₂C can be used to tailor the work function of Nb₂C.

Figure 3 shows crystal models of Nb₂C with surface groups of -F, -O, and - OH²⁷, monolayer MoS₂, and Nb₂C/MoS₂ heterostructure used to investigate interface carrier-transfer and work function variations. The electrostatic potential of MoS₂ and Nb₂C monolayer with surface groups of -F, -O, and -OH, along the z direction (vertical plane), were shown in Fig. S3(a–d), respectively. The work function is defined as the energy required to extract an electron from the Fermi level to the surface of a solid⁵⁴, which is equal to the energy difference between the vacuum level and Fermi level used in DFT calculation. Hence, the work function was determined to be 5.82 eV, 4.58 eV, 6.53 eV, and 2.33 eV for MoS₂, Nb₂CF₂, Nb₂CO₂, Nb₂C(OH)₂, respectively. Interestingly, the work function of Nb₂C MXene is very sensitive to its surface terminations: changing from O termination to OH termination causes the work function to decrease over 4 eV, which could be attributed to the variation of dipole moment density of MXene with different terminations⁵⁵. The surface termination-dependent work function properties of MXene may provide a golden opportunity to modulate carrier behavior at the 2D heterostructure interface.

Figure 3(a–c) illustrated the work function of the interface of Nb₂CF₂/MoS₂, Nb₂CO₂/MoS₂, and Nb₂C(OH)₂/MoS₂, which give the work function of 4.56 eV, 6.43 eV, and 3.95 eV, respectively. The work function of heterostructures is different from its pristine compound due to charge redistribution and interface interaction. Moreover, the variation of surface terminations in Nb₂C also changes the work function of the heterostructure. As schematically shown in Fig. 3(d), the electron could transfer from MoS₂ to Nb₂CO₂ after the contact because the Fermi level of MoS₂ is higher than that of Nb₂CO₂, resulting in a decrease in work function in Nb₂CO₂ after contact. In contrast, the electron could transfer from Nb₂C(OH)₂ to MoS₂, resulting in the increase of work function in Nb₂C(OH)₂ after contact (Fig. 3(e)). When the XPS results in the valence band region are combined, we can conclude that the O termination dominated the surface group after the synthesis of heterostructures. We did not consider the Nb₂CF₂/MoS₂ situation because the XPS experiment results indicated F termination desorption during the synthesis process.

To analyze the effects of in situ growth of MoS₂ and surface group changes on linear optical properties of Nb₂C, the optical absorption properties of Nb₂C and Nb₂C/MoS₂ were investigated with UV-visible spectrometry combined with DFT calculation. As shown in Fig. 4(a), the strong broadband optical extinction was observed in the near-infrared (NIR) region having a peak at about 850 nm, which was recognized as the optical feature of Nb₂C MXenes²¹. Due to its good dispersibility in water, the absorbance of Nb₂C dispersion is proportional to its concentration. The extinction coefficient was linearly fitted to be 16.13 L cm⁻¹g⁻¹ (Fig. S4) using the Beer-Lambert law. The DFT calculated optical absorption spectra with different terminations were shown in Fig. 4(b). Because the incident directions are random in dispersion during the UV-visible spectrometry, the calculated values of absorption are the average of three directions (x, y, and z). The difference between experimental and calculated results can be attributed to the termination and environment deviations. Even though the calculated results are not consistent with the experimental results, the average of three calculated spectrums (Nb₂CF₂, Nb₂C(OH)₂, and Nb₂CO₂) shows a similar tendency to the experimental results, which indicates that the experimentally synthesized Nb₂C naturally terminated with F, O, and OH. The enhancement in the near-infrared region can be observed in the average curve, which is inherited from Nb₂CF₂, indicating that the broadband optical extinction in the near-infrared region of Nb₂C in Fig. 4(a) was due to the F termination.

The experimentally obtained absorption spectra of Nb₂C, MoS₂, Nb₂C/MoS₂ were shown in Fig. 4(c) for comparison. We normalized the absorption coefficients of three spectra at 300 nm in Fig. 4(c) to highlight the significance of the three groups. The absorption peak of MoS₂ located in the range of 600–700 nm was attributed to the general features of MoS₂⁵⁶. Furthermore, the absorption of MoS₂ increases sharply as the wavelength decreases below 800 nm, whereas the absorption of Nb₂C decreases. It appears that MoS₂ replenishes the Nb₂C absorption in the visible region, resulting in the increased absorption of Nb₂C/MoS₂ heterostructure in the visible region (450 nm – 800 nm). While the increased absorption of Nb₂C/MoS₂ in the NIR region (>800 nm) is inherited from Nb₂C. In other words, Nb₂C/MoS₂ inherited the preponderance of Nb₂C and MoS₂ in absorption, exhibiting broadband enhanced optical absorption. The DFT calculated spectra of MoS₂, Nb₂CO₂/MoS₂, and Nb₂C(OH)₂/MoS₂ shown in Fig. 4(d) are used to explore the mechanism of their outstanding absorption. The calculated MoS₂ spectra exhibited a similar absorption trend to the experimental results, but the peaks in the calculation spectrum show a blue shift when compared to the experimental spectrum. The blue shift could be ascribed to an underestimation of the band gap in the generalized gradient approximation (GGA) level during DFT calculation. Due to the F desorption during the heat treatment, we averaged the calculated spectra of Nb₂CO₂/MoS₂ and Nb₂C(OH)₂/MoS₂. The averaged line is more consistent with the measured Nb₂C/MoS₂ spectrum than the pristine Nb₂CO₂/MoS₂ and Nb₂C(OH)₂/MoS₂ absorption curves, manifesting that the O and OH groups are interacting with the surface of Nb₂C.

In order to further study the nonlinear optical properties of Nb₂C/MoS₂, the OA Z-scan technique was utilized to determine the NLO response of Nb₂C/MoS₂ and Nb₂C in the near-infrared region. The details of the measurement setup can be found in the experimental section. The OA Z-scan results of Nb₂C/MoS₂ and Nb₂C with the excitation wavelength of 1300 nm and 1550 nm are shown in Fig. 5(a) and 5(b), respectively. Both Nb₂C/MoS₂ and Nb₂C exhibit typical saturable absorption (SA), that is, the normalized transmittance increases as the sample approaches z=z0 point. For the control of the NLO experiment, the linear transmittance of Nb₂C/MoS₂ and Nb₂C were set to be approximate consistency. Given this premise, the Nb₂C/MoS₂ exhibited a stronger SA response than that of the Nb₂C at the wavelength of 1300 nm and 1550 nm.

The intrinsic NLO absorption coefficient can be calculated by fitting the OA Z-scan results to the NLO theory. The NLO propagation equation can be written as⁵⁶: dIdz"=−(α0+αNLI)I, where z' is the transmission distance in the sample, α₀ and α_NL are the linear part and nonlinear part of the absorption coefficient, respectively. Giving the optical intensity Iz=I0/(1+z2/z02), where z is the position along the laser propagation direction, z₀ is the Rayleigh length and I₀ is the peak intensity on the axis, the NLO propagation can be solved as:

T=1πq0∫−∞∞ln[1+q0exp(−x2)]dx.（1）

To further assess the saturable absorption properties, a simplified saturable absorption model with a two-level system can be expressed as⁵⁷:

T=exp[−(α1+α21+I/Is]),（2）

where T is transmission, α₁ is the non-saturable loss component, α₂ is modulation depth, and I_s is saturated intensity. Here, the Z-scan curves were converted into a function of transmission with optical intensity for fitting using the Eq. (2). The stand OA Z-scan model with Eq. (1) works very well for the curves in Fig. 5(a) and 5(b). The two-level system model with Eq. (2) also fits the converted traces well in Fig. 5(c) and 5(d). All the NLO fitting parameters are listed in Table 1.

As shown in Fig. 5(e), the NLO absorption coefficient α_NL of Nb₂C/MoS₂ was determined to be −0.18 cm/GW, and −0.09 cm/GW at 1300, and 1550 nm. For Nb₂C, these values are −0.08 cm/GW, and −0.05 cm/GW at 1300, and 1550 nm. As a comparison, α_NL of different 2D materials are shown in Fig. S6, indicating a comparable NLO performance of Nb₂C/MoS₂ with classical 2D materials. In addition, the saturated intensity I_s of Nb₂C/MoS₂ was fitted as 61.3 GW/cm², and 107.8 GW/cm² at 1300, and 1550 nm, respectively. While I_s of Nb₂C was estimated to be 136.9 GW/cm², and 203.2 GW/cm² at 1300, and 1550 nm, respectively. As expected, the α_NL of Nb₂C/MoS₂ is larger than that of Nb₂C at both 1300 and 1550 nm wavelengths, that is, Nb₂C/MoS₂ possesses a greater ability to alter absorption coefficient than the Nb₂C at the given optical intensity. In comparison, the fitted I_s shows opposite trend with α_NL: Nb₂C/MoS₂ had a lower saturated intensity than Nb₂C, and a shorter wavelength corresponding to a lower saturated intensity. Because I_s represents half of the optical intensity required for a complete bleaching of materials⁵⁸, implying that the Nb₂C/MoS₂ is more easily saturated than Nb₂C.

Key insight into the enhanced saturable absorption process of Nb₂C/MoS₂ was schematically shown in Fig. 5(f). The saturable absorption in the NIR region of Nb₂C/MoS₂ and Nb₂C can be attributed to the Pauli blocking induced by the intense single-photon transition. That is, the excited electrons gradually occupied possible states in the conduction band until all available states are full. After further increasing the incident light intensity, optical bleaching occurs according to Pauli blocking theory. Hence, the intensity of saturable absorption is naturally linked with the number of excited states^{58, 59}. In addition, the single photon transition mainly originated from Nb₂C instead of MoS₂, because the excitation photon energy (< 1 eV) is smaller than the band gap of MoS₂ (~1.8 eV)³⁴. Moreover, the previous characterization, and calculation indicated that the surface group of Nb₂C/MoS₂ was dominated by O elements, implying that the Fermi level of Nb₂C/MoS₂ is close to the valence band of MoS₂ as presented in Fig. 3(d). Because the electron and hole can spontaneously transfer to energies closer to the Fermi level, the photon-excited holes in Nb₂C could transfer to MoS₂, whereas this process is prohibited for the electrons. On the one hand, saturable absorption is the process by which states gradually exhaust. On the other hand, MoS₂ offers the additional occupying state for carriers through the transfer process. Hence, Nb₂C/MoS₂ can offer more states to be exhausted, leading to a stronger and easier saturable absorption than the pristine Nb₂C.

In summary, we have observed the improved linear and nonlinear optical performance of an Nb₂C/MoS₂ heterostructure designed in this study. The XPS characterizing and work function calculation revealed that the dominated surface group of Nb₂C/MoS₂ was O termination, leading to the decrease in the work function of Nb₂C after the in situ growth of MoS₂. Experiment and theoretical calculations revealed that the Nb₂C/MoS₂ heterostructure has broadband-enhanced optical absorption. Furthermore, the OA Z-scan results showed that Nb₂C/MoS₂ has better NLO performance than Nb₂C. The enhanced NLO performance is attributed to the hole transfer from Nb₂C to MoS₂. The 2D Nb₂C/MoS₂ heterostructure was proved to possess excellent nonlinear optical response, indicating that Nb₂C/MoS₂ can be applicated in the field of optoelectronics and ultrafast photonics, such as saturable absorbers, optical switches, and optical diodes. This facile strategy of in situ construction of the 2D Nb₂C/MoS₂ heterostructure provides guidance for achieving robust light modulation and paves the way for the development of broadband optoelectronic devices and optical modulators.