Abstract
In transition metal dichalcogenides （TMD） flakes， the geometry， such as layer thickness， significantly tune the electronic properties， including bandgap， electron affinity and Fermi level. Such characteristic offers a high degree of freedom to tune the functionality of semiconductor device， once the volatile electronic properties are precisely determined. However， to date， there are still significant uncertainties in determining the Fermi-level alignment of TMD homo- or hetero- junctions， which might lead to significant deviations of band-bending and thus device performance. Here， we utilize the Scanning Kelvin Probe Microscopy （SKPM） to characterize the surface-potential/Fermi-level alignment of TMD homo- or hetero- junctions. Through this effort， a distinct phenomenon is verified where the Fermi-levels of MoS2 and MoTe2 shift towards the intrinsic level with an increasing layer thickness （in other words， the background doping concentration is continuously lowering）. Moreover， we show the significant impact of surface contamination （molecular scale） on the surface potential of monolayer TMD. Finally， we fabricate a MoTe2/MoS2 heterojunction， in which we observe the wide depletion region and large photoresponse. Together， those findings might offer a reference to precisely stack van der Waals （vdW） layers as designed for both electronic and optoelectronic applications.
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 propertie
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
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 （EF） of MoS2 and MoTe2 shifts toward the intrinsic level （Ei） 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
Fig. 1 The SKPM experimental results of few-layer MoS2 （a） the AFM image of stepped MoS2， including 1L， 2L， 3L ，4L and 6L， （b） the SKPM image of the corresponding topography of MoS2， （c） the MoS2 height profile corresponding to the white dashed line marked in （a）， （d） the surface potential distribution of few-layer MoS2 extracted from （b）， （e） the bandgap and potential of MoS2 as a function of layer thickness， in black and red， respectively （the bandgap data are from the theoretical calculation results in Ref. 5）， the potential data at 6L are regional average， （f） evolution of the Fermi level of MoS2 with different layers
图1 少层MoS2的SKPM实验结果 （a） 台阶状MoS2的AFM图像，包括1层，2层，3层，4层和6层，（b） MoS2对应形貌的SKPM图像，（c） （a）中白色虚线对应的MoS2高度曲线，（d） （b）中提取的少层MoS2的表面电势分布，（e） MoS2的带隙和电势随层厚的依赖关系，分别用黑色和红色表示，（f） MoS2费米能级随层厚的演变
It was once thought that oxygen and moisture in the environment would change the surface potentia
Fig. 2 The SKPM experimental results of few-layer MoTe2 （a） the AFM image of stepped MoTe2， including 1L， 2L and 3L， （b） the SKPM image of the corresponding topography of MoTe2， the light blue dashed lines are extensions of the potential boundary， The white triangles in white dashed ovals mark topography and potentials due to the module absorption or contamination， （c） the surface potential distribution and corresponding topography of the few-layer MoTe2 extracted from （a） and （b）， （d） the height and potential curves corresponding to the white dotted line in the MoTe2 monolayer region in （b）， （e） evolution of the Fermi level of MoTe2 with different layers， （f） the number and average potential of adsorbates with different diameter ranges in the monolayer region of MoTe2
图2 少层MoTe2的SKPM实验结果 （a） 台阶状MoTe2的AFM图像，包含1层，2层和3层，（b） MoTe2对应形貌的SKPM图像，浅蓝色虚线是电势边界的延长线，白色虚线椭圆内部的白色三角标记了由模块吸收和污染导致的形貌和电势，（c） （a）和（b）中提取的少层MoTe2对应形貌的表面电势分布，（d） （b）中MoTe2单层区域白色虚线对应的高度和电势曲线，（e） MoTe2费米能级随层厚的演变，（f） MoTe2单层区域吸附物不同直径范围的数量和平均电势
Here， we attribute the phenomenon in Figs.
To further test this， we performed the SKPM experiment on another TMD material， MoTe2. The typical result is shown in
It is worth noting that there are space charge regions （SCR） in the MoTe2 sample， especially at the 1L/2L and 2L/3L interfaces （
Another interesting phenomenon is that the fluctuations of surface potential are clearly recorded in our SKPM experiments （
Furthermore， we prepared a 1L MoTe2-1L MoS2 heterostructure. As depicted in
Fig. 3 The SKPM experimental results of MoTe2/MoS2 vdW heterostructures （a） the AFM 3D image of MoTe2/MoS2 vdW heterostructure， （b） the SKPM 3D image of corresponding topography， （c） the topography and surface potential distributions corresponding to the white dashed line in （b）， （d） the size dependence of the potential difference between the MoTe2/MoS2 vdW heterojunction and MoS2 under dark and light conditions， the inset shows the changes in surface potential of the MoTe2/MoS2 heterostructure under dark and light conditions
图3 MoTe2/MoS2异质结构的SKPM实验结果 （a） MoTe2/MoS2异质结构的三维AFM图像，（b） 对应形貌的三维SKPM图像，（c） 对应（b）中白色虚线的形貌和电势分布，（d） 黑暗和光照条件下MoTe2/MoS2异质结和MoS2之间电势差的尺寸依赖性，插图展示了MoTe2/MoS2异质结构的表面电势在黑暗和光照条件下的变化
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.
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