| Due to the spontaneous polarization that can be overturned by the external electric field,ferroelectric materials have attracted much attention in non-volatile storage and other aspects.At present,electronic devices are gradually developing in the direction of miniaturization and integration,while traditional three-dimensional ferroelectric materials have limitations in reducing film thickness reduction due to the influence of the critical size effect.Therefore,exploring two-dimensional ferroelectricity has become an important task.In recent years,with the emergence of two-dimensional van der Waals(vdW)materials with strong anisotropy and weak interlayer interaction,it is possible to explore ferroelectricity in two-dimensional systems.α-In2Se3 with both in-plane and out-of-plane ferroelectric polarization attracts great interest from researchers in In2Se3,and many new structures have been found(β’ and β*phase).In addition,the emergence of two-dimensional ferroelectric(FE)materials enables the integration of nonvolatile FE functions into van der Waals heterostructures(HSs),which will achieve nonvolatile regulation of low-dimensional physical properties.However,the current research is more focused on the application of devices,there have been no reports of the direct experimental probing of the modulations of HS electronic structures caused by integrating FE functions,and very few studies have explored FE HSs at the monolayer(ML)limit.Therefore,it is an attractive work to study the electronic structure of the vdW HS integrated with ferroelectric monolayers.Scanning tunneling microscope/spectroscopy(STM/S)has both ultra-precisions real spatial resolution and electronic energy spatial resolution,which is very suitable for studying the surface atomic structure and electronic structure of new two-dimensional materials.In this paper,the ML In2Se3 and its heterostructures constructed with ML WSe2 have been studied in detail by combining the molecular beam epitaxy(MBE)and STM/S.The main research results of this paper are as follows:(1)Large-area ML In2Se3 thin films were prepared through MBE,combined with variable temperature STM,and it found that the phases with the highest proportion at 298 K,77 K,and 4.5 K are α,β’ and β*,respectively.It was further verified by STS that β’phase had in-plane antiferroelectricity and β*had in-plane ferroelectricity.Finally,it was found that β’ and β*constructed a lateral heterostructure with type II band alignment,and the reversible phase transition of β’/β*could be controlled by needle manipulation technology,thus regulating the heterostructure.(2)ML-In2Se3/ML-WSe2 heterostructures were fabricated by using the ultrahigh vacuum MBE.Combined with STM/S,it was determined that there was a type II band alignment between β’/β*-In2Se3 and WSe2.In addition,significant modulations of the valley structures of WSe2 were observed,and in-situ transformations between the FE and AFE In2Se3 phases demonstrated the dominant role of the polarizations in the top ML In2Se3 layer.The observed phenomena could be attributed to the combination of both the linear and quadratic Stark shifts from the out-of-plane electric field.This work demonstrated a new approach for manipulating the corresponding valley structures.(3)The lateral heterostructures constructed by ML In2Se3(five atomic layers)and WSe2(three atomic layers)were also prepared through MBE.Type-Ⅱ band alignment was found to exist in either the lateral heterostructure composed of anti-FE β’-In2Se3 and WSe2 or the lateral heterostructure composed of FE β*-In2Se3 and WSe2,and the band offsets could be tuned by switching between the β’/β*phases.1D interface states were observed to emerge from both the occupied and unoccupied sides,and it demonstrated that ferroelectricity can be an effective toolset tuning the gap size(0.62 eV and 0.45 eV in theβ’/β*-In2Se3-WSe2 HS,respectively).This work not only assisted in the preparation of twodimensional ferroelectric lateral HSs but also demonstrated a new approach for manipulating the one-dimensional lateral interface states. |
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