Transition metal dichalcogenides （TMD），a branch of two-dimensional （2D） layered materials，have attracted continuous attention from both academic and industrial societies due to their intriguing electronic and optoelectronic properties^［1-4］. Generally，the bandgap，electron affinity and Fermi-level of layered TMD are tunable with a decreasing layer-thickness^［5-9］. Moreover，each monolayer （ML） is connected by van der Waals force^［10-12］. It spares TMD materials from surface states/charges and lattice mismatch，thus enabling a freewill stacking of heterostructures^［13-14］. Both of these advantages lead to an extremely rich degree of freedom in designing and fabricating nanoscale electronic and optoelectronic devices. However，everything has two sides，the large-scale freedom also means enormous uncertainty in realistic device applications. In detail，the electronic characteristics of TMD materials，like the layer thickness dependent Fermi-level and band-bending，are still out of common sense^［6，15］. For this reason，the fabricated homo- or hetero- structures usually deviate from the designed configuration. Note that the theoretical calculations can give a series of reference data^{［9，16-21］}. However，as is well-documented，those results might deviate from the realistic situation^{［5，7，22］}，to a certain extent.

Scanning Kelvin Probe Microscopy（SKPM） has become a powerful tool for characterizing electronic properties in TMD，which are crucial for the design and optimization of TMD-based electronic and optoelectronic devices. Li et al. ^［23］ and Feng et al. ^［24］ characterized the surface potential variation of MoS₂ using SKPM under different substrates，light intensities，and humidity conditions. Robinson et al. ^［25］ compared the surface potential of MoS₂ flakes exfoliated on substrates and suspended flakes using SKPM. Additionally，Wang et al. ^［26］ utilized SKPM to reveal the work function change/Fermi level shift of WSe₂ as a function of gate voltage. Furthermore，Zhang et al. ^［27］ confirmed the type-II energy band structure and interlayer charge separation in a MoTe₂/MoS₂ vdW heterostructure using SKPM. Overall，SKPM provides a non-destructive and high-resolution method for characterizing TMD materials，which could facilitate the development of TMD-based electronic and optoelectronic devices.

In this work，SKPM method is utilized to characterize the surface potential of TMD materials，MoS₂ and MoTe₂. As a result，a distinct phenomenon has been observed，in which the Fermi-level （E_F） of MoS₂ and MoTe₂ shifts toward the intrinsic level （E_i） with an increasing layer thickness. It indicates that the background doping concentrations of TMD materials are decreasing continuously. These characteristics should be associated with the band structure reconstruction process，in which the formation energy of the intrinsic dopants changes obviously. Finally，we show the alignment of Fermi-level in a typical MoS₂/MoTe₂ heterostructure_{and the effect of light illumination on it. The findings disclosed here might provide key information for the design and fabrication of van der Waals devices.}

Homogeneous structures of MoTe₂ and MoS₂ flakes were mechanically exfoliated onto a silicon substrate covered with a 280 nm-thick thermal oxide layer. To obtain MoTe₂/MoS₂ heterostructures，MoS₂ monolayers were first deposited on Si/SiO₂ substrate by chemical vapor deposition （CVD） and a single layer MoTe₂ was subsequently transferred onto a selected monolayer of MoS₂. All samples were tested in a nitrogen atmosphere to reduce the influence of water and adsorbates，in order to characterize the intrinsic electrical properties of the materials.

A Bruker icon scanning probe microscope with Nanoscope IV controller is used in the experiment. Probes with n-doped Si were selected for both topography and surface potential measurement. We identified thin layers of MoS₂ and MoTe₂ using an optical microscope，and then used atomic force microscopy （AFM） to characterize the film thickness. The amplitude modulation mode（AM-mode） SKPM was performed to investigate the band alignment of homo- and hetero- structures formed by MoTe₂ and MoS₂. To explore the interlayer photoexciting effect on MoTe₂/MoS₂ type-II heterostructures，we performed SKPM analysis under dark and illuminated conditions，with a laser wavelength of 680 nm.

Figures 1（a） and 1（c） show the AFM image and height profile of MoS₂ flake that was mechanically exfoliated and transferred onto a Si/SiO₂ substrate. The stepped topography helps to identify five distinct layer-thicknesses，namely 1L，2L，3L，4L，and 6L MoS₂. The corresponding SKPM image and potential profile are shown in Figs. 1（b） and （d）. One can find that each increase in layer thickness （monolayer’s level） would lead to a decrease of surface potential，from 2.2 meV @1L to -8.2 meV @2L，-18 meV @4L and finally -20.3 meV @6L.

It was once thought that oxygen and moisture in the environment would change the surface potential^［6］，thus responsible for the variable SKPM-signal with an increasing layer thickness. However，in our experiments，all samples were tested in a nitrogen atmosphere to minimize the influence of adsorbates （such as oxygen and water molecules） on the intrinsic electrical properties of TMD. Furthermore，as shown in Fig. 2（c），we observe an opposite trend of the evolution of surface potential in MoTe₂ flake，which cannot be explained by the above-mentioned reasons （which predicts a monotonic relationship between potential and layer thickness that is independent of the material category）.

Here，we attribute the phenomenon in Figs. 1（c） and 1（d） to the intrinsic variation of the Fermi level. This is because the band structure of van der Waals materials is reconstructed with an increasing layer thickness. It could strongly tune the background doping property，e. g. by altering the defect forming energy. Figure 1（e） compares the bandgap with the surface potential at different layer thicknesses. One can find that both characters follow the same trend of evolution，which might be the direct evidence to support the judgement.

To further test this，we performed the SKPM experiment on another TMD material，MoTe₂. The typical result is shown in Fig. 2. The morphology image （Fig. 2（a）） helps to identify three distinct regions，1L，2L and 3L MoTe₂. Correspondingly，the surface potential exhibits an obvious enhancement in the thick-layer counterpart of MoTe₂ （Fig. 2（b））. It is contrary to that of MoS₂，where the potential increases with an increasing layer thickness. Figures 1（f） and 2（e） schematically show the band structures of MoS₂ and MoTe₂，respectively. The Fermi-level is carefully labelled to show the opposite evolution processes.

It is worth noting that there are space charge regions （SCR） in the MoTe₂ sample，especially at the 1L/2L and 2L/3L interfaces （Fig. 2（b），marked by white triangles and blue dashed lines）. It arises from the band offset between different layer-thickness regions，which can be defined as the depletion region of the layer junction. Such a characteristic is much more obvious in MoTe₂，but almost invisible in MoS₂. The difference in charge carrier concentration might be responsible for the phenomenon. In detail，MoTe₂ is lightly p-doped （a well-known bipolar material） ^［28-30］，MoS₂ is heavily n-doped^［31-32］. Thus，the depletion region of MoTe₂ could be ordered wider than that of MoS₂.

Another interesting phenomenon is that the fluctuations of surface potential are clearly recorded in our SKPM experiments （Fig. 2（b）），even if they are sub-nanometer scale and originate from molecules absorption or contamination. As shown in Figs. 2（b） and 2（d），in the monolayer MoTe₂ region，there is a high density of potential fluctuation （as high as 20 meV），which arises from the sub-nanometer molecules that are incorporated during the mechanical exfoliation process. Figure 2（f） shows a statistical analysis on the diameter and potential of the adsorbed molecules or molecule clusters in the monolayer MoTe₂. One can find that the diameter of molecular clusters spans from sub-nanometer to 10 nanometers； the most frequently found molecules are sub-2 nm ones，which account for 77% of the whole community. The histogram plot shows the relation between the potential fluctuations on the scale of absorbed molecule. It makes sense that the potential fluctuation rises from 11 meV to 27 meV with an increase of the molecule diameter.

Furthermore，we prepared a 1L MoTe₂-1L MoS₂ heterostructure. As depicted in Fig. 3（a），the sample was fabricated by transferring a monolayer MoTe₂ onto the 1L-MoS₂. In SKPM experiments，the MoTe₂ film shows a 104 mV drop in potential. It indicates that the 1L MoTe₂-1L MoS₂ heterostructure is a typical pn junction. At the same time，the SKPM experiment helps to identify the depletion region of such vdW hetero-junction. As shown in Fig. 3（c），the transition region between n-MoS₂ and p-MoTe₂ is a classic space charge region. Its width is as large as 750 nm. A more detailed analysis confirms this conclusion. Figure 3（d） plots the dependence of potential drop on the width of MoTe₂ strip. One can find that the potential drop increases significantly （58 to 109 mV） as the MoTe₂ width increases from 350 to 750 nm. In this specific region，the MoTe₂ layer is fully depleted，thus the potential drop is highly dependent on the layer width. By contrast，when the MoTe₂ width is over 750 nm，the potential drop is constant. It implies that the junction cannot deplete more material. The 750 nm should be the final width of the depletion region. It is also worth noting that such vdW hetero-junction shows a significant response to light illumination^［27］，since the potential drop increases by ~20 mV.

We explore the effect of layer thickness on the band alignment of homo- and hetero-structures formed by few-layer MoS₂ and MoTe₂. As the layer thickness increases，the Fermi level of MoS₂ decreases while the Fermi level of MoTe₂ increases，resulting in opposite evolution trends. The observed space charge region in MoTe₂ is attributed to differences in carrier concentrations，while significant surface potential fluctuations in the monolayer region reveal the effects of impurity adsorbents and non-ideal storage conditions. Our findings offer guidance for the fabrication of 2D material-based devices. We also prepared a vdW heterostructure of monolayers MoS₂ and MoTe₂，and observed a classical space charge region with a final depletion region width of 750 nm. The potential drop of the vdW heterostructure increased by 20 mV under illumination，indicating a good photo-response.

In summary，this work expands our understanding of the electronic structure of layered TMD materials，demonstrating that band alignment can be modified by adjusting the layer thickness. This property likely extends to other TMD materials，offering important guidance for the design of next-generation electronic and optoelectronic devices based on TMD homo- or hetero-structures.