Full paperEffect of flexoelectricity on a bilayer molybdenum disulfide Schottky contact
Graphical Abstract
Introduction
Two-dimensional (2D) materials, such as the family of transition metal dichalcogenides (TMDs) composed of a transition metal and two chalcogens with the chemical formula of MX2 (M and X denote Transition metal and Chalcogen, respectively), have unique physical, chemical, electrical, and mechanical properties [1]. Molybdenum disulfide (MoS2) is one of the most representative 2D semiconductor materials with prominent semiconductor properties [2], [3], [4], [5], and has been used in transistors [6], [7], tactile sensors for electronic skin applications [8], [9], resonators [10] and so on. The indirect gap (Eg) of multi-layered MoS2 depends on its number of layers, which increases from the bulk value of 1.29 eV to the monolayer value of about 1.90 eV when the layer number decreases. [11] The monolayer MoS2 has luminous and non-heating characteristics and also tremendous potential for high efficiency optoelectronic facilities. In comparison to the monolayer MoS2, multi-layered MoS2, particularly the bilayer MoS2, exhibits distinct physical and mechanical properties [12], [13], [14], [15]. The bilayer MoS2 can produce exotic physical phenomena such as giant Stark effect [16] through tailoring interlayer interactions in an electrical and chemical way, which is absent in monolayer MoS2. For example, one can realize the manipulation of band gap [17], [18] by applying an electric field perpendicular to the bilayer MoS2, and the tuning of electrical conductance properties [19] of bilayer MoS2 by chemical doping. In addition, the interlayer excitons [20] can appear in the bilayer MoS2 when applying a small bias voltage. Recently, Quan et al. [21] realized diverse correlated electronic phases and optical properties in twist MoS2 bilayers by changing the twist angle, and also found that the local strain leads to the modulation of lattice vibrations. Hence, the bilayer MoS2 has huge potential in novel electronic [22], [23], optoelectronic [24] and superconducting spintronic [25] devices.
Piezoelectricity, describing the interaction between mechanical deformation and polarization charge, is a familiar electromechanical coupling phenomenon in dielectrics. It has been exploited in a variety of applications in practical devices including sensors and actuators. Researchers have theoretically demonstrated the existence of in-plane piezoelectricity in the monolayer MoS2, [26] and experimentally determined the associated piezoelectric coefficient [27].
The other distinctive electromechanical coupling phenomenon is flexoelectricity which describes the interaction between the electric polarization and strain gradient or inhomogeneous deformation [28]. Unlike piezoelectricity, flexoelectricity can exist in all dielectric materials due to the strain-gradient induced symmetry breaking. The flexoelectric polarization is measured by , here is the fourth-order flexoelectric tensor [29], [30], [31] showing the extent of flexoelectric coupling effect, and denotes the third-order strain gradient tensor. Generally, as one kind of intrinsic properties of materials is quite small and unchanged at room temperature. However, the strain gradient in a structure can become large enough and even achieve a specific distribution form as desired through appropriately designing the structural geometry and loading mode. This paves the way to obtain the required flexoelectric field in dielectric structures, and thus has attracted a lot of interest from researchers in exploring the novel applications of flexoelectricity in piezoelectric dielectrics [32], [33], multiferroic materials [34], piezoelectric semiconductors [35], [36], [37], traditional silicon-based semiconductor structures [38], [39], oxide semicondutors [40], and halide perovskites [41].
Recently, researchers have paid particular attention, by experiments, to the flexoelectric coupling effect in 2D materials [23], [42], [43], [44], [45], [46], [47], [48], [49] such as MoS2. For instance, Seo et al. [44] investigated the out-of-plane polarization phenomenon due to flexoelectricity in a monolayer MoS2 structure. Wang et al. [45] observed the flexoelectricity-induced out-of-plane polarization in odd and even layered MoS2 structures with bending deformation, and found that such a flexoelectricity-induced polarization is controllable through changing the curvature and thickness of 2D membranes. Brennan et al. [46], [47] quantitatively demonstrated the out-of-plane electromechanical coupling responses, resulting from flexoelectricity, of monolayer MoS2, MoSe2, WS2, and WSe2 structures by using piezoresponse force microscopy (PFM). More importantly, the flexoelectric mechanism can be utilized to control and enhance the performance of Schottky and p-n junction diode-based devices. For example, the sensitivity of a photodetector made of bilayer MoS2 [23] and the photovoltaic coupling effect in a MoS2 sheet [48] have been greatly enhanced through flexoelectricity.
This paper is to investigate the flexoelectric effect on a Schottky contact fabricated by the bilayer MoS2, and there are relatively small number of studies on this topic at present, especially on the extraction of flexoelectric coefficients of 2D materials [42], [46]. We aim to quantitatively reveal the tuning mechanism of flexoelectricity on the barrier height of the fabricated bilayer MoS2 Schottky contact and evaluate the out-of-plane flexoelectric coefficients of the bilayer MoS2 by using the Conductive Atomic Force Microscope (CAFM) as shown in Fig. 1(a). It should be noted that the bilayer MoS2 is centrosymmetric and thus has no in-plane piezoelectricity [27]. Hence, it is only the flexoelectricity that changes properties of the bilayer MoS2 Schottky contact in this paper.
Section snippets
Results and discussion
The samples are fabricated via two steps of MoS2 continuous monolayer growth followed by transferring with polymethyl methacrylate (PMMA). In the preparation stage of the sample, MoO3 (99.999 % purity) is used as the molybdenum source, solid sulfur (99.999 % purity) as the sulfur source, and argon is used as growth carrier gas. A double-temperature zone tube furnace with an 80 mm tube diameter is used, in which MoO3 and sulfur are heated to 650 ℃ and 180 ℃, respectively, and the growth pressure
Conclusions
In summary, we use CAFM to investigate the effect of flexoelectric polarization on the behavior of bilayer MoS2 Schottky contact. The changes in barrier height of the fabricated bilayer MoS2 Schottky contact under different tip forces mainly result from the induced flexoelectric polarization, which are quantitatively estimated based on the measured data and the physical mechanism of flexoelectricity. The obtained results show that the flexoelectric polarization could be used to effectively
CRediT authorship contribution statement
Chunli Zhang: Conceptualization, Writing-Original draft, Supervision. Xiaoying Zhuang: Supervision, Writing-Review & Editing; Weiqiu Chen: Supervision, Writing-Review & Editing; Liang Sun: Investigation, Formal analysis, Writing-Original draft preparation; B. Javvaji: Investigation, Formal analysis, Software.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the Natural Science Foundation of Zhejiang province, China (No. LR21A020002), the National Natural Science Foundation of China (Nos. 12172326, 11972139, and 12192210), the National Key Research and Development Program of China (Nos. 2020YFA0711700 and 2020YFA0711701).
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