Novel chalcogen and halogen surface functional groups for tuning the mechanical properties of TMDs/MXenes heterojunctions

Abstract

MXenes exhibit remarkable mechanical properties due to their unique structural properties and strong atomic bonding, making them highly competitive among 2D materials. Forming heterojunctions between TMDs and MXenes offers a promising strategy to enhance material performance for advanced applications. Although extensive studies have explored the electronic and chemical properties of MXenes-based heterojunctions, investigations into their mechanical properties, particularly the effects of surface functional groups, remain limited. This work systematically investigates the structural and mechanical properties of pristine MXenes and their heterojunctions with TMDs, focusing on how novel chalcogen (S and Se) and halogen (Cl and Br) surface functional groups tune mechanical behavior, anisotropy, and microscopic failure mechanisms. Our results indicate that while surface functionalization generally reduces tensile strength, it enhances the ductility of the heterojunctions. Among the functional groups studied, –Se functionalization induces the most significant improvement in flexibility, indicating potential for applications in flexible devices. Compared with conventional –O or –F terminations reported in previous studies, these newly synthesized functional groups induce distinct anisotropic mechanical responses and tunable interfacial bonding. These findings provide a deeper understanding of the mechanical tuning of TMDs/MXenes heterojunctions by surface functional groups and offer valuable insights into their design for next-generation flexible electronics devices, high-performance sensors and thin-film batteries.

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1 Introduction

MAX phase materials represent a versatile family of ternary carbides and nitrides, described by the general formula M_n+1AX_n (n = 1, 2, and 3), where M is an early transition metal, A is a group A element (typical IIIA or IVA), and X is either carbon or nitrogen.^1,2 In 2011, Naguib et al.³ successfully synthesized the first MXenes, two-dimensional (2D) Ti₃C₂, by selectively etching the Al atomic layer from Ti₃AlC₂ (a typical member in the MAX family) using hydrofluoric acid (HF). This process led to the emergence of MXenes with the general formula M_n+1X_nT_x (n = 1, 2, and 3), where M denotes the transition metal, X is carbon or nitrogen, and T denotes the surface functional groups.^4,5 Due to their excellent electrical,⁶ thermal,⁷ and mechanical^8,9 properties, MXenes have gained significant attention across diverse applications, especially in energy storage, catalysis, adsorption, and sensing technologies.^5,10–13 Among these, Ti₃C₂ is notable for its unique structure,¹⁴ which enables a significantly higher metal ion storage capacity than other 2D materials.

Previous studies have shown that experimentally synthesized MXenes inevitably undergo surface functionalization;^15,16 as a result, extensive research has focused on the effect of functional groups in tuning the overall properties of MXenes. Notably, surface functional groups are known to have substantial impacts on the essential properties of MXenes enhancing their Li-ion capacity,¹⁷ Seebeck coefficients at low temperatures,¹⁸ metallic conductivity,¹⁹ light-harvesting efficiency²⁰ and high photothermal-conversion performance.²¹ Moreover, functionalized MXenes have demonstrated great potential in biosensing and nanophotonics, enabling advanced applications such as electrochemical biosensors for SARS-CoV-2 detection²² and CRISPR/Cas12a-based plasmonic platforms for rapid virus identification.²³ Furthermore, the integration of these sensors with DNA origami also enables ultrasensitive detection of circulating tumor DNA,²⁴ highlighting their biomedical potential. The effect of conventional functional groups (–O, –F, and –OH) on the mechanical behavior of MXenes has been widely studied.^25,26 Among these, the presence of –O groups has been found to stabilize the surface atomic layers, preventing collapse and enhancing mechanical resilience. Recently, Kamysbayev et al.²⁷ reported the first successful synthesis of Ti₃C₂ terminated with a class of novel surface functional groups (–S, –Se, –Cl, and –Br) through substitution reactions in molten inorganic salts. These newly developed Ti₃C₂X₂ (X = S, Se, Cl, and Br) exhibit promising potential for catalytic applications; for instance, Ti₃C₂S₂ and Ti₃C₂Se₂ exhibit enhanced surface activity and high catalytic performance in the hydrogen evolution reaction (HER).^28,29 However, despite their recent emergence and potential, the influence of these novel functional groups on the mechanical properties of MXenes has not been systematically explored yet.

Beyond individual MXenes, the formation of heterojunctions between MXenes and other 2D materials, particularly transition metal dichalcogenides (TMDs), provides additional opportunities to enhance material properties through synergistic interactions. These heterojunctions combine the unique properties of both monolayer materials, such as the electrical conductivity and chemical stability of MXenes with the catalytic and optoelectronic properties of TMDs. As a result, MXenes/TMDs heterojunctions have gained significant attention for applications in flexible sensors,^30,31 energy storage,^32,33 and catalytic systems.^34,35 Chandran et al.³⁶ found that the addition of MoS₂ reduced the charge transfer resistance in MoS₂/MXenes heterojunctions, resulting in enhanced cycling stability and reversibility, making them suitable for supercapacitor electrodes. Similarly, Tao et al.³⁷ successfully assembled MoSe₂/Ti₃C₂ heterojunctions, which exhibited improved nitrogen reduction reaction activity, positioning them as effective catalysts. Liu et al.³⁸ reported that vertical strain tunes the interfacial properties of MoS₂/Ti₃C₂ heterojunctions, transitioning from Ohmic to Schottky contacts under tensile strain. The role of functional groups in tuning the properties of MXenes-based heterojunctions has also drawn increasing attention. For instance, altering the –O and –F functional groups allows precise tuning of the Schottky barrier height.³⁹ However, previous studies have primarily focused on conventional –O, –F and –OH functional groups. Although Ti₃C₂ functionalized with novel groups (–S, –Se, –Cl, and –Br) has been successfully synthesized,²⁷ most previous studies have continued to focus on conventional –O, –F and –OH functional groups. Consequently, the influence of these novel surface terminations on the mechanical performance and failure mechanisms of TMDs/MXenes heterojunctions remains largely unexplored and lacks systematic investigation. To fill this important gap, it is imperative to explore how these unconventional terminations tune the mechanical behavior of TMDs/MXenes heterostructures.

This study aims to clarify how novel halogen (Cl and Br) and chalcogen (S and Se) surface functional groups tune the anisotropic mechanical behavior and failure mechanisms of TMDs/MXenes heterojunctions. We have systematically calculated the elastic constants, Poisson's ratios and stress–strain behavior of TMDs/Ti₃C₂ and TMDs/Ti₃C₂X₂ (S, Se, Cl, and Br) heterojunctions. Our analysis reveals that functionalization decreases the stiffness and tensile strength of heterojunctions while enhancing their ductility, with –Se functional groups providing the most significant ductility improvement. Furthermore, the mechanical properties of TMDs, Ti₃C₂X₂ and TMDs/Ti₃C₂X₂ heterojunctions in the zig-zag (x-) and armchair (y-) directions are similar under small deformations. According to the microscopic analysis, it can be observed that Ti–C bonds remain dominant in resisting tensile deformation in the y-direction. These findings provide new insights into the effect of functional groups on the mechanical properties of TMDs/MXenes heterojunctions, with significant implications for flexible electronic devices and thin-film batteries.

2 Computational methods

All density functional theory (DFT) calculations were performed using the Cambridge Sequential Total Energy Package (CASTEP) code within the Material Studio (MS).⁴⁰ For lattice parameters and atomic coordinate optimization, we employed the Perdew–Burke–Ernzerhof (PBE) functional within the generalized gradient approximation (GGA) as the exchange–correlation function.⁴¹ To account for the interlayer interactions in the heterojunctions, a semi-empirical dispersion correction based on the Grimme approach (DFT-D2)⁴² was employed. Although the DFT-D2 method was known to have limitations in accurately describing van der Waals (vdW) interactions, it has been widely employed in previous MXenes-based first-principles studies.^43,44 These studies confirmed that the DFT-D2 approach is capable of accurately capturing the structural and mechanical properties of MXenes systems. Furthermore, its adoption in this work ensures methodological consistency with our earlier publications. Additionally, ultrasoft pseudopotentials were used to accurately model the interactions between ionic cores and valence electrons, facilitating the smoothing of wave functions⁴⁵ and enhancing computational performance. We utilized the [Ar]3p3d4s configuration to represent the valence of Ti, the [He]2s2p configuration for C, the [Ne]3s3p configuration for S and Cl, the [Kr]4d5s configuration for Mo, and the [Ar]4p4s configuration for Br and Se. The plane-wave cutoff energy was set to 500 eV, and the Brillouin zone was sampled using a 9 × 5 × 1 Monkhorst–Pack k-point grid.⁴⁶ In geometry optimization sections, the Broyden–Fletcher–Goldfarb–Shanno (BFGS) method⁴⁷ was utilized with convergence criteria set at 10⁻⁷ eV per atom for total energy and 0.005 eV Å⁻¹ for force,⁴⁸ respectively. Spin polarization effects were not considered, as the local magnetic moments originating from transition metal atoms at the edges of pristine MXenes layers are significantly weakened or even entirely diminished due to interfacial interactions after forming the heterojunctions with TMDs, as reported in previous studies.⁴⁹ To avoid periodic interactions between neighboring layers, a vacuum layer of 15 Å was introduced in the z-direction.^50,51 The choice of this vacuum thickness is validated in previous studies on MXenes-based heterojunctions,^52,53 where a 15 Å vacuum layer was shown to effectively prevent the interactions between adjacent layers. Additionally, our previous work⁵⁴ on the electronic properties of TMDs/MXenes heterojunctions further confirms that this vacuum thickness is sufficient.

3 Results and discussion

3.1 Structural properties

The structural features of monolayer MoS₂, MoSe₂, and Ti₃C₂, as well as the atomic configurations of both pristine TMDs/Ti₃C₂ and functionalized TMDs/Ti₃C₂X₂ (X = S, Se, Cl, and Br) heterojunctions, were comprehensively established and validated in our previous study.⁵⁴ To ensure consistency and comparability, we adopt the most stable stacking configurations with the lowest total binding energies as identified in ref. 54, for all subsequent analyses in this study. Specifically, for the pristine TMDs/Ti₃C₂ heterojunctions, the ZM_SA configuration is adopted for MoS₂/Ti₃C₂ and the SA_ZM configuration is adopted for MoSe₂/Ti₃C₂, as shown in Fig. 1(a). For the functionalized heterojunctions, the functional groups are positioned at the hollow sites of the surface Ti atoms on the surface and aligned vertically to the intermediate Ti layer, as shown in Fig. 1(b). To focus on the intrinsic effect of functional group types, we adopted a fully saturated model with a fixed maximum coverage, where each surface Ti atom is terminated by a chalcogen or halogen atom, forming the Ti₃C₂X₂ (X = S, Se, Cl, and Br) structures.

Fig. 1 The side views of (a) TMDs/Ti₃C₂ heterojunctions in ZM_SA and SA_ZM configurations and (b) TMDs/Ti₃C₂X₂ (X = S, Se, Cl, and Br) heterojunctions.⁵⁴ The atomic alignments at the TMDs/MXenes interfaces are explicitly indicated by red dashed lines.

Earlier work⁵⁴ demonstrated that surface functionalization increases the interlayer distance (d) and induces noticeable Ti–C bond elongation in TMDs/Ti₃C₂ heterojunctions, with MoSe₂/Ti₃C₂Se₂ exhibiting a zero bond length difference (d₅₆) between Mo–S(Se)(1) and Mo–S(Se)(2). While our previous study focused on tuning the electronic properties of TMDs/MXenes heterojunctions through biaxial strain and external electric fields, the present study investigates how surface functionalization and uniaxial strains tune the mechanical performance. These structural properties provide qualitative insights into the expected mechanical behavior. Pristine TMDs/MXenes heterojunctions with a smaller interlayer distance generally exhibit stronger vdW coupling and more efficient load transfer, potentially leading to higher tensile strength but reduced ductility. In contrast, surface functionalization increases the interlayer distance, thereby weakening direct vdW interactions and facilitating interlayer sliding, which enhances ductility while reducing the ultimate strength. For the MoSe₂/Ti₃C₂Se₂ heterojunctions, the bond-length difference d₅₆ = 0 implies negligible vertical distortion of the MoSe₂ lattice and overall weak interfacial coupling, preserving the intrinsic in-plane anisotropy of MoSe₂ and thus leading to the highest expected mechanical anisotropy among the systems studied.

3.2 Electronic properties

Interfacial charge coupling in TMDs/MXenes heterojunctions was examined through the work function analysis of each monolayer, as listed in Table 1. The work function quantifies the energy barrier for electron emission, with higher values indicating stronger electron confinement. Pristine MoS₂ and MoSe₂ exhibit work functions of 4.36 and 4.89 eV, respectively, while bare Ti₃C₂ presents a lower value of 3.96 eV, consistent with its metallic nature. Surface functionalization significantly tunes the electronic properties of Ti₃C₂. Chalcogen terminations (–S and –Se) increase the work function to 5.02 and 5.08 eV, respectively, indicating enhanced electron affinity. In contrast, halogen terminations lead to divergent trends, with Ti₃C₂Cl₂ showing a slight increase (4.19 eV) and Ti₃C₂Br₂ a notable decrease (3.72 eV) in work function.

Table 1 Calculated work functions (eV) of pristine TMDs, Ti₃C₂ and Ti₃C₂X₂ (X = S, Se, Cl, and Br) monolayers

Formula	Work function (eV)
MoS2	4.36
MoSe2	4.89
Ti3C2	3.96
Ti3C2S2	5.02
Ti3C2Se2	5.08
Ti3C2Cl2	4.19
Ti3C2Br2	3.72

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Notably, the mismatch in the work function between TMDs and functionalized MXenes is a key factor governing the interfacial electron transfer and potential alignment. When the MXenes exhibits a higher work function (Ti₃C₂S₂ and Ti₃C₂Se₂), electrons flow from the TMDs layer toward the MXenes, enhancing interfacial the charge redistribution and strengthening vdW interactions. Conversely, in systems where the TMDs have a higher work function (Ti₃C₂Cl₂ and Ti₃C₂Br₂), electron transfer proceeds in the opposite direction, resulting in weakened interfacial coupling. These results confirm that surface functional groups can effectively regulate the electronic environment of Ti₃C₂ and thus control the direction and magnitude of charge transfer at TMDs/MXenes interfaces.

To further analyze the interlayer electron transport, the electron localization function (ELF) of MoS₂/Ti₃C₂X₂ and MoSe₂/Ti₃C₂X₂ is shown in Fig. S1(a) and (b), respectively. In the MoS₂ and MoSe₂ layers, high electron localization around the Mo and S (Se) atoms indicates strong covalent interactions, with an ELF value of around 0.5. In the Ti₃C₂ layer, electron localization is observed around C and Ti atoms due to lone-pair electrons of Ti atoms. Although these electrons do not participate directly in bonding, they contribute to localized electron density, which can interact with external substances, making them potentially valuable for catalysis or sensing.⁵⁵ The introduction of functional groups reduces electron density around the metal atoms and even at the interfaces, weakening the interactions between the TMDs and Ti₃C₂ layers.⁵⁶ This reduction is most noticeable in –Cl and –Br functionalization, indicating limited electron transport due to the lack of a continuous electron-rich region.

Given these electronic properties, it is feasible to infer that different functional groups could have a substantial influence on interfacial coupling strength and mechanical performance. As demonstrated in our previous study,⁵⁴ –Cl and –Br terminations preserve the Ti-3d dominated metallicity near the Fermi level, whereas –S and –Se functionalization induces pronounced band flattening and electron localization, thereby reducing electrical conductivity. In particular, –Cl and –Br functionalization is expected to preserve strong interlayer interactions due to the sustained metallic character, which may promote efficient charge delocalization and facilitate stress transfer across the interface, thereby maintaining or potentially enhancing the ductility of the heterojunction. In contrast, the strong electronic localization induced by –S and –Se functionalization may weaken interlayer coupling, reduce stress transfer across the interface, and consequently result in increased anisotropy and reduced ductility.

3.3 Mechanical properties

Both monolayer TMDs and Ti₃C₂ possess a 2D hexagonal lattice structure, which presents challenges for the accurate calculation of their mechanical properties. Therefore, we redefined the lattice vectors and converted the original hexagonal lattice into a 2D orthorhombic crystal structure prior to mechanical property calculations, as illustrated in Fig. S2(a) and (b), respectively. Uniaxial strain is applied along the x- and y-directions, corresponding to the zig-zag and armchair orientations, respectively.

We first calculated the elastic properties of pristine TMDs and Ti₃C₂ monolayers as well as functionalized Ti₃C₂X₂ (X = S, Se, Cl, and Br) systems, with the results listed in Table 2. In the thickness direction, the in-plane stiffness constants are expressed in units of N m⁻¹ instead of GPa, because monolayer TMDs consist of atomic layers, and the effective thickness cannot be clearly defined. Our calculated elastic constants for monolayer MoS₂ and MoSe₂ are in close agreement with reported previous DFT results,⁵⁷ with C₁₁ deviations below 5.5 N m⁻¹ and Poisson's ratio differences within 0.01. Similarly, the computed C₁₁ value for pristine Ti₃C₂ is consistent with the value reported by Tang et al.⁵⁸ These results further support the reliability of our computational approach for accurately describing the mechanical properties of MXenes-based systems.

Table 2 Comparison of in-plane stiffness constants (C₁₁, C₂₂, and C₁₂) and Poisson's ratios (v₁₂, v₂₁) for pristine TMDs, Ti₃C₂, and functionalized Ti₃C₂X₂ (X = S, Se, Cl, and Br)

Formula	C 11	C 22	C 12	ν 12	ν 21
MoS2	129.7 N m^−1	129.7 N m^−1	33.7 N m^−1	0.26	0.26
MoS2^57	124.2 N m^−1	124.2 N m^−1	31.1 N m^−1	0.25	0.25
MoSe2	112.3 N m^−1	112.3 N m^−1	24.7 N m^−1	0.22	0.22
MoSe2^57	108.1 N m^−1	108.1 N m^−1	24.9 N m^−1	0.23	0.23
Ti3C2	506.3 GPa	491.2 GPa	91.1 GPa	0.19	0.18
Ti3C2S2	412.6 GPa	397.3 GPa	107.3 GPa	0.27	0.26
Ti3C2Se2	289.1 GPa	280.2 GPa	89.6 GPa	0.32	0.31
Ti3C2Cl2	376.4 GPa	340.2 GPa	105.4 GPa	0.31	0.28
Ti3C2Br2	387.2 GPa	373.4 GPa	104.5 GPa	0.28	0.27

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Notably, monolayer MoS₂ and MoSe₂ exhibit nearly isotropic mechanical properties, with C₁₁ and C₂₂ values of 129.7 N m⁻¹ and 112.3 N m⁻¹, respectively. A higher Poisson ratio generally indicates greater mechanical ductility, which is consistent with recent findings.⁵⁹ The calculated C₁₁ value for MoS₂ is significantly lower than that of h-BN and graphene, despite its higher Poisson's ratio, indicating that monolayer MoS₂ exhibits better ductility. When converting the C₁₁ of Ti₃C₂ from GPa to N m⁻¹, we found that it corresponds to 241.8 N m⁻¹, which exceeds that of MoS₂ on the same scale; however, MoS₂ shows greater stiffness per unit thickness compared to Ti₃C₂.

Ti₃C₂ exhibits a remarkably high stiffness constant, which significantly surpasses those of TMDs monolayers. However, surface functionalization leads to a notable reduction in stiffness. Among the functionalized MXenes, Ti₃C₂Se₂ displays the lowest stiffness constants, with C₁₁ and C₂₂ values of 289.1 and 280.2 GPa, respectively. Additionally, Ti₃C₂Se₂ exhibits the highest Poisson's ratio of 0.32, indicating that it experiences a greater degree of transverse expansion under uniaxial tensile stress compared to its counterparts. Furthermore, the elastic constants and Poisson's ratios reveal that the mechanical properties of Ti₃C₂X₂ are largely isotropic, with similar behavior in both the x- and y-directions.

Furthermore, Table 3 lists the in-plane stiffness constants, Young's modulus and Poisson's ratios of the TMDs/Ti₃C₂ and TMDs/Ti₃C₂X₂ heterojunctions. The in-plane stiffness of the TMDs/Ti₃C₂ heterojunctions is lower than that of pristine Ti₃C₂ but higher than that of the MoS₂ and MoSe₂ layers, suggesting a synergistic strengthening effect. The Poisson ratios of the TMDs/Ti₃C₂X₂ (X = S, Cl, Br) heterojunctions exhibit nearly identical values in both the x- and y-directions, indicating isotropic mechanical behavior.

Table 3 The in-plane stiffness constants (C₁₁, C₂₂, and C₁₂), Young's modulus (Y₁₂ and Y₂₁) and Poisson's ratios (v₁₂, v₂₁) of TMDs/Ti₃C₂ and TMDs/Ti₃C₂X₂ (X = S, Se, Cl, and Br) heterojunctions

Formula	C 11 (GPa)	C 22 (GPa)	C 12 (GPa)	Y 1 (GPa)	Y 2 (GPa)	ν 12	ν 21
MoS2/Ti3C2	387.4	478.1	81.2	373.4	461.1	0.17	0.21
MoS2/Ti3C2S2	349.6	321.2	80.3	329.2	302.7	0.25	0.23
MoS2/Ti3C2Se2	260.2	327.1	101.4	228.5	287.6	0.31	0.39
MoS2/Ti3C2Cl2	321.5	309.8	89.9	295.6	284.3	0.29	0.28
MoS2/Ti3C2Br2	329.3	316.8	85.5	306.7	293.9	0.27	0.26
MoSe2/Ti3C2	444.6	412.3	115.4	411.8	382.3	0.28	0.26
MoSe2/Ti3C2S2	335.5	324.2	100.5	303.9	294.2	0.31	0.30
MoSe2/Ti3C2Se2	337.2	599.1	107.8	318.3	525.3	0.18	0.32
MoSe2/Ti3C2Cl2	326.4	350.1	94.5	301.4	322.8	0.27	0.29
MoSe2/Ti3C2Br2	351.7	321.8	77.2	332.5	304.1	0.24	0.22

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It is worth noting that the MoSe₂/Ti₃C₂Se₂ heterojunctions display pronounced anisotropy, with a Poisson ratio of 0.18 in the x-direction and 0.32 in the y-direction. This anisotropy can be attributed to the larger atomic radius of –Se and the severe distortion of the –Se atomic arrangement within the MoSe₂ and Ti₃C₂Se₂ layers under uniaxial strain. This distortion disrupts the hexagonal symmetry of MoSe₂ and brings the –Se atoms in closer proximity. The attraction between the upper and lower Se– atoms in this direction contributes to the observed anisotropy and increased resistance to shear deformation in the MoSe₂/Ti₃C₂Se₂ heterojunctions.

In the MoS₂/Ti₃C₂ heterojunctions, the introduction of functional groups reduces stiffness in both the x- and y-directions. Among the functionalized heterojunctions, MoS₂/Ti₃C₂Se₂ exhibits the lowest Young's modulus in the x-direction, decreasing by 144.9 to 228.5 GPa compared to pristine Ti₃C₂. This significant reduction is largely attributed to –Se functionalization, as the larger atomic radius and weaker bond strength of Se lead to greater lattice distortion and weaker interlayer interactions. In the y-direction, the Young's modulus for MoS₂/Ti₃C₂ is 461.1 GPa, while the –Cl functional groups have notable effects, resulting in a minimum Young's modulus of 284.3 GPa for MoS₂/Ti₃C₂Cl₂, indicating a considerable difference of 176.8 GPa. These results suggest that the heterojunction is more sensitive to strain in the y-direction relative to the x-direction.

In contrast, for MoSe₂/Ti₃C₂ heterojunctions, functionalization shows a different trend in the y-direction. For MoSe₂/Ti₃C₂Se₂ heterojunctions, the Young's modulus increases to 525.3 GPa, indicating a 65.1% improvement in structural stiffness, suggesting that the –Se functional group enhances the resistance to deformation in the y-direction. This effect is possibly due to increased electron density between the –Se functional group and Ti₃C₂, strengthening the heterojunction's mechanical stability. While surface functionalization reduces the stiffness and Young's modulus of pristine MXenes and TMDs/MXenes heterojunctions, it plays a crucial role in tuning surface stability and electronic properties provided in our previous studies.⁵⁴ Moreover, the reduction in stiffness and Young's modulus provides valuable theoretical insights into the development of flexible devices. Therefore, by further systematically investigating the effect of functional groups on the overall mechanical behavior of heterojunctions, we aim to optimize their performance for specific applications.

Both MoS₂ and MoSe₂ exhibit higher tensile strengths in the y-direction relative to the x-direction, as shown in Fig. 2. Specifically, for MoS₂ in the y-direction, the maximum stress reaches 22.47 GPa at a strain of 23%. In comparison, MoSe₂ demonstrates lower stiffness and tensile strength, as evidenced by reduced stress values for corresponding strain percentages. A distinct step-like jump appears in the stress–strain curves of monolayer MoS₂ and MoSe₂ around 8% tensile strain, resulting from a structural transition caused by the relative displacement of the outer S or Se atomic layers. This behavior correlates with a well-known strain-induced semiconductor-to-metal phase transition in TMDs monolayers.⁶⁰

Fig. 2 The stress–strain curves of pristine (a) MoS₂ and (b) MoSe₂ under uniaxial tensile strains along the x- and y-directions. The hollow symbols indicate the peak points of the corresponding curves, representing the onset strain of mechanical instability.

At tensile strains below 5%, both MoS₂ and MoSe₂ display near-isotropic mechanical behavior with consistent stress–strain responses along the x- and y-directions. However, beyond the strain of 6%, the stress–strain curves reveal divergent tensile strength values along these two directions, suggesting that higher strain levels induce distortion of the orthorhombic six-membered ring structure in TMDs. This distortion disrupts the original symmetry of the material, leading to pronounced anisotropic mechanical properties. These findings emphasize the susceptibility of TMDs to structural asymmetry under large tensile strains.

As shown in Fig. 3, pristine Ti₃C₂ exhibits nearly isotropic mechanical behavior, with minimal variation in stress–strain response in the x- and y-directions up to a maximum strain of 24%. However, functionalized Ti₃C₂ demonstrates pronounced anisotropy, characterized by higher tensile strength along the x-direction compared to the y-direction. This anisotropic behavior arises from the disruption of the original Ti–C bonding caused by different surface functional groups. Among the Ti₃C₂X₂ heterojunctions, Ti₃C₂Se₂ shows the highest anisotropy with the largest tensile strength difference between the x- and y-directions, evident at the early stage of strain. This suggests that Ti–Se bonding is notably weaker along the y-direction, leading to a substantial reduction in the original symmetry of Ti₃C₂.

Fig. 3 The stress–strain curves of pristine (a) Ti₃C₂ and (b)–(e) Ti₃C₂X₂ (X = S, Se, Cl, and Br) under uniaxial tensile strains along the x- and y-directions. The hollow symbols indicate the peak points of the corresponding curves, representing the onset strain of mechanical instability.

In contrast, the tensile strength of all Ti₃C₂X₂ is lower compared to that of pristine Ti₃C₂, particularly along the y-direction, suggesting that functional groups generally weaken the mechanical properties of Ti₃C₂. Among the functional groups studied, Ti₃C₂Br₂ shows the greatest reduction in tensile strength along the x-direction. While Ti₃C₂S₂ shows the greatest reduction along the y-direction, decreasing from 49.19 GPa in pristine Ti₃C₂ to 26.59 GPa. Moreover, the slopes of the stress–strain curves for Ti₃C₂S₂ and Ti₃C₂Se₂ along the y-direction are significantly smaller compared to those along the x-direction, indicating that the elastic modulus is lower in the y-direction than in the x-direction.

To assess the influence of different functional groups on the mechanical properties of TMDs/MXenes heterojunctions, we further analyzed the uniaxial tensile strains in x- and y-directions of MoS₂/Ti₃C₂ and MoS₂/Ti₃C₂X₂ (X = S, Se, Cl, and Br). As shown in Fig. 4(a), the heterojunctions functionalized with Se and Cl in the x-direction exhibit enhanced resistance to deformation, as indicated by their higher tensile strengths compared to the bare MoS₂/Ti₃C₂ heterojunctions (34.33 GPa), with MoS₂/Ti₃C₂Se₂ exhibiting the highest tensile strength (36.72 GPa). In contrast, as illustrated in Fig. 4(b), functionalization in the y-direction has a more pronounced weakening effect, where the bare MoS₂/Ti₃C₂ heterojunction shows the highest tensile strength (31.81 GPa). Notably, Se functionalization significantly reduces the tensile strength to 23.87 GPa, which can be attributed to weakened Ti–Se bonding.

Fig. 4 The stress–strain curves of MoS₂/Ti₃C₂ and MoS₂/Ti₃C₂X₂ (X = S, Se, Cl, and Br) heterojunctions under uniaxial tensile strains along the (a) x- and (b) y-directions. The hollow symbols indicate the peak points of the corresponding curves, representing the onset strain of mechanical instability.

The trend in critical strain further reveals distinct anisotropic mechanical responses. In the x-direction, –Br, –Se, and –Cl functionalization enhances the ductility, with the bare MoS₂/Ti₃C₂ heterojunctions exhibiting a critical strain of 21% before nonlinear deformation initiates. Notably, –Se and –Cl functionalization contributes the most significant improvements with MoS₂/Ti₃C₂Se₂ and MoS₂/Ti₃C₂Cl₂ withstanding strains of up to 29%, reflecting their excellent mechanical ductility. This enhancement indicates the redistribution of electron density across the interface, where Se functionalization strengthens Ti–Se interactions. Moreover, –Br functionalization also improves ductility with a critical strain close to 26%, whereas –S functionalization leads to a lower critical strain of 18%.

In contrast, as illustrated in Fig. 4(b), all functionalization in the y-direction decreases the ductility of the MoS₂/Ti₃C₂ heterojunction. The bare MoS₂/Ti₃C₂ heterojunction shows a critical strain of approximately 23%, slightly higher than in the x-direction. –Br and –Cl functionalization reduces the critical strain to 21%, while –S and –Se functionalization further decreases the critical strain to 18% and 17%, respectively. This indicates that the critical strain along the y-direction is generally lower compared to the x-direction, suggesting that the heterojunction is more easily disrupted when subjected to strain along the y-direction. This disparity arises from the structural anisotropy, where the Ti–C bonds oriented along the y-direction exhibit reduced resistance to deformation.

Similar to MoS₂-based heterojunctions, the comparison of black curves in Fig. 5(a) and (b) shows that the MoSe₂/Ti₃C₂ heterojunctions exhibit nearly identical tensile strengths and critical strains in both the x- and y-directions, indicating a high degree of mechanical isotropy. However, MoSe₂/Ti₃C₂X₂ (X = S, Se, and Br) heterojunctions show higher tensile strengths and critical strains in the x-direction than in the y-direction, reflecting anisotropic mechanical behavior.

Fig. 5 The stress–strain curves of MoSe₂/Ti₃C₂ and MoSe₂/Ti₃C₂X₂ (X = S, Se, Cl, and Br) heterojunctions under uniaxial tensile strains along the (a) x- and (b) y-directions. The hollow symbols indicate the peak points of the corresponding curves, representing the onset strain of mechanical instability.

In the x-direction, the MoSe₂/Ti₃C₂ heterojunction exhibits a higher tensile strength (34.01 GPa), with only a slight enhancement in the MoSe₂/Ti₃C₂Se₂ heterojunction (36.14 GPa). While in the y-direction, the bare MoSe₂/Ti₃C₂ heterojunction shows the highest tensile strength (36.02 GPa), but functionalization with S, Se, Cl, and Br significantly lowers the tensile strength. Additionally, the tensile strength of the MoSe₂/Ti₃C₂Se₂ heterojunction in the y-direction is less than half of its value in the x-direction, indicating that the number of Ti–C bonds resisting tensile strain in the y-direction is significantly lower than that in the x-direction. Therefore, MoSe₂/Ti₃C₂Se₂ has the largest difference in ultimate strength between the two directions, displaying the most pronounced mechanical anisotropy. These observations are in agreement with the predictions for the mechanical performance of the TMDs/MXenes heterojunctions discussed in Sections 3.1 and 3.2.

It is found that the critical strain in the y-direction is significantly lower compared to the x-direction, indicating fewer Ti–C bonds resisting deformation along the y-direction. The bare MoSe₂/Ti₃C₂ heterojunction exhibits the smallest critical strains in the x-direction. For MoSe₂/Ti₃C₂Se₂, the tensile strain in the x-direction reaches over 26% before yielding, while yielding occurs at only 11% tensile strain in the y-direction. A similar trend is observed for MoSe₂/Ti₃C₂S₂, where the yield limit in the x-direction is approximately double that in the y-direction. The introduction of –Br functionalization allows the heterojunction to withstand up to 28% critical strain in the x-direction, but this reduces to 20% in the y-direction. Overall, –Br, –S, Se, and Cl functional groups generally enhance the ductility of the MoSe₂/Ti₃C₂ heterojunction in the x-direction, while only –Cl and –Br improve ductility in the y-direction.

To further quantify the anisotropic mechanical behavior of the investigated systems, Table 4 summarizes the critical strain and ultimate strength of pristine TMDs, Ti₃C₂X₂ (X = S, Se, Cl, and Br), and their heterojunctions under uniaxial tensile strain. Monolayer MoS₂ and MoSe₂ exhibit higher strength and critical strain along the y-direction, reflecting their intrinsic anisotropy, while pristine Ti₃C₂ shows nearly isotropic behavior. After surface functionalization, all Ti₃C₂X₂ systems exhibit reduced ultimate strength in the y-direction. However, the x-direction consistently shows higher strength and ductility for most functionalized systems.

Table 4 Summary of the critical strain (%) and ultimate strength (GPa) of pristine TMDs, Ti₃C₂, Ti₃C₂X₂ (X = S, Se, Cl, and Br) and their heterojunctions under uniaxial tensile strains along the x- and y-directions

Formula	x-direction		y-direction
	Critical strain (%)	Ultimate strength (GPa)	Critical strain (%)	Ultimate strength (GPa)
MoS2	19	15.66	23	22.47
MoSe2	19	12.76	26	20.42
Ti3C2	24	51.11	20	49.19
Ti3C2S2	23	40.66	17	26.59
Ti3C2Se2	25	43.08	18	26.83
Ti3C2Cl2	25	43.58	13	30.10
Ti3C2Br2	30	39.99	18	29.60
MoS2/Ti3C2	21	34.33	23	31.81
MoS2/Ti3C2S2	18	31.04	18	26.95
MoS2/Ti3C2Se2	29	36.72	17	23.87
MoS2/Ti3C2Cl2	29	36.01	21	31.22
MoS2/Ti3C2Br2	26	33.57	21	28.78
MoSe2/Ti3C2	12	34.01	13	36.02
MoSe2/Ti3C2S2	24	32.34	14	23.99
MoSe2/Ti3C2Se2	26	36.14	11	18.51
MoSe2/Ti3C2Cl2	22	31.76	24	29.13
MoSe2/Ti3C2Br2	28	33.57	20	28.62

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When TMDs are combined with Ti₃C₂ and Ti₃C₂X₂ to form heterojunctions, a synergistic mechanical enhancement is observed. MoS₂/Ti₃C₂Cl₂ and MoS₂/Ti₃C₂Se₂ heterojunctions exhibit superior performance in the x-direction, with critical strains of up to 29% and ultimate strengths above 36 GPa. In the y-direction, MoSe₂/Ti₃C₂ shows the highest ultimate strength (36.02 GPa) but with limited ductility. Among all studied heterojunctions, MoSe₂/Ti₃C₂Se₂ displays the greatest mechanical anisotropy, with significantly lower critical strain in the y-direction due to weakened interlayer bonding from –Se functionalization. Overall, –Br and –Cl terminations enhance strength and ductility along the x-direction, while –Se improves stretchability but at the cost of reduced strength, especially in the y-direction. These findings highlight the importance of surface functionalization and strain orientation in tuning mechanical properties for flexible electronics.

The strain energy (E_s) inflection point marks the transition from the elastic to the plastic region, where irreversible structural changes begin to occur.⁶¹ In the linear elastic region, as strain energy increases, the structure retains its honeycomb configuration, allowing it to return to its original undeformed size upon the release of tension. The DFT-D2 scheme provides a computationally efficient correction for vdW interactions; however, it may introduce systematic errors in absolute energy values. Nevertheless, it remains relatively consistent in predicting energy trends and structural changes.⁶² Therefore, our focus is placed on the overall energy trends and critical inflection points, rather than on the absolute energy values. As shown in Fig. 6(a), the E_s of all heterojunctions increases almost linearly with strains of up to around 30%. Notably, the MoS₂/Ti₃C₂Br₂ and MoS₂/Ti₃C₂Cl₂ heterojunctions exhibit the highest E_s, indicating that these systems require more energy to maintain the given strain, which implies stronger bonding interactions. In contrast, the MoS₂/Ti₃C₂Se₂ heterojunction shows a sharp increase in E_s, but requires the least amount of E_s overall, suggesting better ductility but weaker mechanical strength. A similar trend is observed in Fig. 6(b), while the MoSe₂ system displays slightly lower absolute E_s values, which indicates that MoSe₂-based heterojunctions are generally more flexible than their MoS₂ counterparts. Moreover, –Br and –Cl functional groups terminated heterojunctions exhibit higher E_s, indicating stronger bonds that may enhance Ti₃C₂-based heterojunctions through improved interfacial interactions. In contrast, –S and –Se functional groups weaken the structures, with the TMDs/Ti₃C₂Se₂ heterojunctions exhibiting the highest ductility.

Fig. 6 The strain energy (E_s) of (a) MoS₂/Ti₃C₂X₂ and (b) MoSe₂/Ti₃C₂X₂ (X = S, Se, Cl, and Br) heterojunctions under uniaxial tensile strains. The blue/pink areas mark the plastic phase of the heterojunction with the highest E_s.

Therefore, –Br and –Cl functionalization enhances the E_s of the heterojunction, increasing its resistance to deformation and stiffness, which is primarily attributed to the strengthened interfacial interactions induced by their high electronegativity. –Br and –Cl functionalization induces significant charge redistribution when terminating the Ti₃C₂X₂ surface, thereby strengthening the bonding between Ti and X (X = Br and Cl). This enhanced bonding increases the vdW interactions between the Ti₃C₂ and TMDs layers, resulting in improved interfacial stability. As a result, the TMDs layer experiences greater constraints, leading to increased stiffness in the heterojunction. Moreover, the stronger bonding between the Ti₃C₂X₂ and TMDs layers effectively restrains interfacial sliding, which is particularly advantageous for flexible electronic applications, as they reduce the risk of delamination or peeling under sustained mechanical stress.

3.4 Microscopic mechanism

The bond length behavior at the atomic scale offers a clearer understanding of the stress–strain curves. Therefore, we further analyzed the bond lengths along the y-direction for both the bare MoS₂/Ti₃C₂ and representative MoS₂/Ti₃C₂Se₂ heterojunctions. As shown in Fig. 7(a), the bond lengths of bonds (2) and (3), located in the middle of the Ti₃C₂ layer, are longer than those of bonds (1) and (4), which are positioned on the top and bottom surfaces. This variation is due to the higher bond energies of bonds (2) and (3), which make them more resistant to elongation. Additionally, interlayer interaction results in bond (1) on the top surface being slightly longer than bond (4) on the bottom surface.

Fig. 7 The Ti₃C₂ bond length of (a) MoS₂/Ti₃C₂ and (b) MoS₂/Ti₃C₂Se₂ heterojunctions under uniaxial tensile strains along the y-direction.

At early stages of tensile strain, the lengths of the Ti–C bonds increase linearly, reflecting the elastic deformation of the Ti₃C₂ layer. The synchronous elongation of bonds (2) and (3) with bonds (1) and (4) is due to uniform stretching of interatomic interactions under tensile load. However, the extension of these Ti–C bonds is limited by the effective radius of the outer electrons of Ti and C atoms, preventing the bonds from stretching indefinitely. As the tensile strain increases, bond lengths (1), (2), and (3) begin to rapidly decrease around 24% strain, indicating bond fracture, which corresponds directly to the material failure observed in Fig. 4(b). Meanwhile, the Mo–S bond within the MoS₂ layer fractures once the tensile strain exceeds 23%, disrupting its interlayer interaction with the Ti–C bonds in the Ti₃C₂ layer. This results in a substantial decrease in the bond lengths of the already fractured Ti–C bonds (1), (2), and (3).

In contrast, as shown in Fig. 7(b), the bond lengths of (5)(6), (2)(3), and (1)(4) exhibit synchronous changes under strain due to the reduced electrical activation of Ti atoms by Se atoms on the Ti₃C₂Se₂ surface. Both the (2)(3) and (1)(4) bonds increase almost linearly with strain, with the (2)(3) bonds displaying a steadier linear increase corresponding to the stress increase before the critical strain. As the strain approaches 17%, the sharp elongation of the (1)(4) bonds show the structural distortion, marking the material's critical point. Compared to the dramatic shortening of the Ti–C bond length at large strains in Fig. 7(a), the Ti–Se bonds exhibit less contraction, which suggests that the Se-functional group increases ductility of the MoS₂/Ti₃C₂ heterojunction. The Ti–Se bonds located on the upper and lower surfaces exhibit longer bond lengths than the Ti–C bonds. Notably, the (5)(6) bond lengths remain unchanged under increasing tensile strain until stress yielding occurs, while the Ti–C bonds show more significant elongation compared to the Ti–Se bonds. This behavior suggests that Ti–C bonds are more resistant to tensile deformation than Ti–Se bonds.

Fig. 8(a) illustrates the variation in Ti–C bond lengths within the MoSe₂/Ti₃C₂ heterojunctions under uniaxial tensile strain along the y-direction. Notably, the (2) and (3) bonds located in the middle of the Ti₃C₂ layer is approximately 0.15 Å longer than the (1) and (4) bonds located on the surface. Furthermore, the (1) bond near the MoSe₂ layer and the (4) bond at the bottom of Ti₃C₂ exhibit nearly identical lengths, leading to overlapping curves in the bond length distribution. This overlap indicates that the vdW interactions between the MoSe₂ and Ti₃C₂ layers are relatively weaker compared to the interlayer coupling. As the tensile strain increases to 15%, the (1) and (4) bonds elongate synchronously, corresponding to the structural failure of the bare MoSe₂/Ti₃C₂ heterojunction, as shown in Fig. 5(a). As a result, the Ti1 atoms are further attracted to the MoSe₂ layer, causing the elongation of the (1) bond. Meanwhile, the elongation of the (4) bond occurs as the MoSe₂ layer shifts, which weakens the vdW attraction and initiating repulsive forces. This dynamic between bond elongation and vdW interactions is essential in determining the mechanical failure of the heterojunction at critical strain levels.

Fig. 8 The Ti₃C₂ bond length of (a) MoSe₂/Ti₃C₂ and (b) MoSe₂/Ti₃C₂Se₂ heterojunctions under uniaxial tensile strains along the y-direction.

As tensile strain is applied, the decoupling of electronic interactions between the MoSe₂ and Ti₃C₂Se₂ layers results in synchronous changes in the bond lengths of bonds (2)(3) and (1)(4), while the bond lengths of bonds (5) and (6) remain nearly constant before fracture, as shown in Fig. 8(b). As the tensile strain increases to 7%, the bond lengths of (2) and (3) in the middle of the Ti₃C₂Se layer reach 2.35 Å and 2.36 Å, respectively, at marking the beginning of failure. When tensile strain increases to 10%, the bond lengths of bonds (5) and (6) start to decrease, indicating that the Mo–Se bond in the upper MoSe₂ layer has fractured, thereby reducing the tensile resistance of the Ti₃C₂Se₂ layer. Continued tensile strain results in an increase in the bond lengths of bonds (5) and (6), indicating the complete failure of the Ti–Se bonds. The Ti–Se bonds exhibit greater ductility and elongate earlier than the Ti–C bonds, suggesting that the Se atoms are particularly sensitive to deformation. Consequently, the Ti–Se bonds in the MoSe₂/Ti₃C₂Se₂ heterojunction are the weakest in resisting tensile deformation, followed by the Ti–C bonds located in the middle of the Ti₃C₂Se₂ layer, and lastly, the Ti–C bonds at the surface. This finding indicates that the presence of Ti–Se bonds reduces the ultimate strength of the MoSe₂/Ti₃C₂ heterojunction.

We also compared the evolution of interlayer distance d in TMDs/Ti₃C₂ and TMDs/Ti₃C₂Se₂ heterojunctions under uniaxial strain along the y-direction, as shown in Fig. 9. The variation in layer spacing reflects the interfacial interaction strength under strain and provides insight into strain-induced charge transfer. At lower strain levels, all heterostructures exhibit a slight increase in interlayer distance due to atomic rearrangement and weak vdW relaxation. With increasing strain, the Poisson effect leads to a significant reduction in interlayer distance, indicating enhanced interfacial charge coupling.

Fig. 9 The interlayer distance d (Å) of MoS₂/Ti₃C₂, MoS₂/Ti₃C₂Se₂, MoSe₂/Ti₃C₂, and MoSe₂/Ti₃C₂Se₂ heterojunctions under uniaxial tensile strains along the y-direction.

Notably, MoSe₂/Ti₃C₂Se₂ exhibit the largest initial interlayer distance (2.96 Å) and the most pronounced reduction under critical strain, which suggests that Se-induced charge redistribution enhances flexibility but weakens interfacial adhesion. For MoS₂/Ti₃C₂ and MoSe₂/Ti₃C₂ heterojunctions, the relatively stable and moderate reduction in the interlayer distance under strain suggests that strong Ti–C and Mo–S (or Mo–Se) bonds effectively mediate interfacial stress transfer. In contrast, MoS₂/Ti₃C₂Se₂ and MoSe₂/Ti₃C₂Se₂ exhibit a larger initial d and more pronounced contraction, indicating that Se functionalization weakens Ti–X and Mo–Se bonding due to the reduced orbital overlap and enhanced charge delocalization. These results highlight the crucial role of interfacial bond strength in determining the mechanical robustness of TMDs/MXenes heterostructures under tensile deformation.

To clarify the atomic-level fracture mechanisms, Mulliken charge populations of key interfacial bonds were analyzed under both strain-free and critical strain conditions, as summarized in Tables S1–S4. In the MoS₂/Ti₃C₂ heterojunction, Ti atoms exhibit a positive charge at the strain-free stage. Upon reaching the critical strain, a noticeable increase in the net charge on Ti and Mo atoms along with enhanced localization on S atoms indicates a charge redistribution from MoS₂ to the Ti₃C₂. These results indicate a weakening of the Mo–S bond and enhanced interfacial charge coupling, which is consistent with the observed interlayer contraction and mechanical failure. A more pronounced charge redistribution is observed in the MoSe₂/Ti₃C₂ heterojunction. Under the critical strain, the Mo atoms exhibit a significant increase in the positive charge, while the Ti atoms show a minimal variation, suggesting that charge transfer predominantly occurs within the TMDs layer. This internal charge redistribution from Mo to Se atoms, indicative of Mo–Se bond weakening, likely contributes to the pronounced mechanical anisotropy and the reduced critical strain observed along the y-direction.

In the MoSe₂/Ti₃C₂Se₂ heterojunction, the Ti atoms exhibit a notable increase in the positive charge, while the surface –Se functional groups show pronounced positive polarization. In contrast, the Mo atoms maintain the negative charge, indicating that electron transfer is predominantly directed from the MoSe₂ layer toward the Ti₃C₂Se₂ interface. This redistribution weakens the Ti–Se interfacial bonding; however, partial Mo–Se bond integrity is preserved due to the localized electron density on Mo atoms prior to mechanical failure. A comparable trend is observed in the MoS₂/Ti₃C₂Se₂ heterojunction, where charge polarization across Ti–X and Mo–S bonds under strain indicates progressive interfacial debonding. Overall, these findings highlight the critical role of interfacial electron redistribution in tuning the bond strength and fracture behavior.

4 Conclusions

In this study, we systematically investigated the structural and mechanical properties of TMDs/MXenes heterojunctions, focusing on the influence of functionalization and the orientation dependence. Our first-principles calculations demonstrate that the halogen (Cl and Br) and chalcogen (S and Se) surface functionalization leads to a significant reduction in in-plane stiffness and a notable increase in Poisson's ratio. Notably, MoSe₂/Ti₃C₂Br₂ and MoS₂/Ti₃C₂Se₂ exhibit the highest critical strain in the x-direction among all functionalized systems, which makes them promising candidates for flexible devices. Although the overall in-plane stiffness decreases compared to pristine Ti₃C₂, most functionalized heterojunctions retain higher elastic moduli than the constituent TMDs monolayers, indicating a synergistic strengthening effect within the heterojunctions.

While most TMDs/Ti₃C₂X₂ heterojunctions exhibit nearly isotropic mechanical responses, MoSe₂/Ti₃C₂Se₂ displays marked anisotropy under large strain, primarily due to the larger atomic radius of Se that affects bonding energy and induces lattice distortion. Mechanistically, bond-level analysis highlights that Ti–C bonds bear the primary tensile load, while Ti–X bonds contribute little to this resistance. The higher bond energy of Ti–C bonds leads to their preferential breakage, which ultimately causes the strength limit of TMDs/Ti₃C₂X₂ heterojunctions to be lower than that of TMDs/Ti₃C₂. Among the functionalized heterostructures, –Cl termination demonstrates the most balanced performance, combining strong interfacial charge coupling with enhanced mechanical flexibility. It maintains metallic character and supports efficient stress transfer under tensile strain, making it particularly suitable for flexible electronic applications. In contrast, –S and –Se terminations induce charge localization and weaken interfacial bonding, leading to diminished tensile strength and greater mechanical anisotropy. ELF analyses further confirm that functional group-induced charge redistribution influences interfacial bonding strength. These findings highlight the impact of functionalization on the mechanical properties of TMDs/MXenes heterojunctions, offering new insights for next-generation flexible devices, sensors, and thin-film batteries.

Author contributions

Yuqian Zhang: data curation, formal analysis, writing – original draft, and visualization. Zhiwei Liu: data curation, validation, and visualization. Siyu Zheng: data curation, methodology and visualization. Changyang Yu: formal analysis, writing – original draft and visualization. Siliang Yue: data curation, formal analysis and visualization. Chenliang Li: conceptualization, writing – review and editing, funding acquisition, software and supervision. Hui Qi: conceptualization, methodology and supervision.

Conflicts of interest

The authors declare that they have no conflicts of interest.

Data availability

The data supporting this article are described in detail in Section 2, Computational methods.

The supplementary information provides ELF plots, structural transformation diagrams, and detailed Mulliken charge population data for various TMDs/MXenes heterostructures under both strain-free and critical strain conditions. See DOI: https://doi.org/10.1039/d5cp01389e

Acknowledgements

This work was supported by the National Natural Science Foundation of China (12172097) and the Natural Science Foundation of Heilongjiang Province, China (LH2021A006).

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