| Author: | Mao, Jianfeng |
| Title: | Strain-modulated photoelectric and ferroelectric properties of 2D materials via patterned substrates |
| Advisors: | Hao, Jianhua (AP) |
| Degree: | Ph.D. |
| Year: | 2024 |
| Subject: | Two-dimensional materials Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Applied Physics |
| Pages: | xx, 161 pages : color illustrations |
| Language: | English |
| Abstract: | 2D materials are widely considered as a promising avenue for next-generation electronic and optoelectronic devices, thanks to their exceptional physical and chemical properties. In order to unleash their complete potential and meet the diverse demands of different industries, scientists have examined a multitude of methods to manipulate the properties of these materials. One of the most effective approaches among the numerous methods explored by researchers to adjust the electronic and lattice structure of 2D materials is strain engineering. This method can bring about diverse modifications in the properties of the materials. In this study, we propose a new strategy by constructing patterned nanostructures that are compatible with conventional silicon substrates. Specifically, we demonstrate the potential of our approach by growing CVD monolayer MoS2 and then transferred onto the periodical nanocone arrays, resulting in a high-performance MoS2 photodetector through the manipulation of strain distribution engineered at the nanoscale. Compared to the pristine MoS2 counterpart, the strained MoS2 photodetector exhibits enhanced performance, including a high signal-to-noise ratio exceeding 105 and large responsivity of 3.2 × 104 A W-1. We discuss the physical mechanism responsible for this enhancement by combining KPFM with theoretical simulation. The improved performance can be attributed to the enhanced light absorption, the fast separation of photo-excited carriers, and the suppression of dark currents induced by the designed periodical nanocone arrays. Our work represents an alternative approach to achieving high-performance optoelectronic devices based on 2D materials integrated with semiconductor circuits. Furthermore, we observed significantly high d33eff values for strained 1L-MoS2 and 2L-MoS2 samples that can reach 18.77 pm V-1 and 12.40 pm V-1, respectively, which are among the highest values reported for 2D materials. Our approach is successful due to the robust strain gradients created by the nanocones underneath the 2D materials, leading to a substantial enhancement of the piezoelectric coefficients. Moreover, we observed ferroelectric polarization in the strained 2L-MoS2. The observed ferroelectric properties might stem from the strain-generated moiré patterns and strain gradient-induced lattice deformation in 2L-MoS2. Overall, our results highlight the potential of strain-engineered 2D materials for the development of high-performance piezoelectric and ferroelectric devices. Our method is also applicable to various 2D materials, providing new possibilities for designing and fabricating high-performance 2D electronic and optoelectronic devices. In addition, we have successfully synthesized In2Se3 with different morphologies and phases using the CVD method. Our investigation revealed that the out-of-plane piezoelectricity of CVD-grown α-In2Se3 nanosheets exhibits layer-dependent properties, with piezoelectricity significantly increasing as thickness increases. We also measured the mechanical properties of the α-In2Se3 nanosheets through in-situ SEM measurements and found that the E2D was 36 GPa for the sample with a thickness of approximately 24.2 nm. Our findings pave the way for designing atomic-scale electromechanical coupling system devices. In conclusion, this study investigated the effects of strain engineering on the optoelectronic performance of 2D materials. We explored the strain-induced vertical piezoelectricity and ferroelectricity in 2D materials, as well as their mechanical properties. Our findings demonstrate that strain engineering is an effective approach to control and modify the properties of 2D materials. |
| Rights: | All rights reserved |
| Access: | open access |
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