First-principles Study Of Novel Two-dimensional Layered Materials For Infrared Detector Applications

With exceptional physical properties such as layer-dependent electronic structures,high carrier mobility,no dangling bonds on the surface and band gaps covering the entire electromagnetic spectrum,two-dimensional（2D）materials have already brought great values to applications in optoelectronics and energy fields.As the optoelectronic properties of 2D materials are influenced by their band structures to a large extent and the response wavelength of an infrared detector is governed by the forbidden band width of the detector material.To apply 2D materials to the infrared detection,we must look for materials with suitable band gaps and excellent physical properties.This thesis explores the modulation of the physical properties of novel 2D materials at the atomic scale by doping,alloying,strain engineering,et al.,based on the density functional theory.The main research contents are listed here:The continuous tuning of the band gap of Hf(S_1-xTe_x)₂ alloys from 1.732 e V（Hf S₂）to 0 e V（Hf Te₂）is realized by varying the Te component.It shows that gap of 2D materials with large gap corresponding to the visible light can be finely regulated by alloying strategy.The possibility of modulating energy gap types of Pt X₂ and Janus Pt XY（X,Y=S,Se,Te）has been investigated via strain engineering.Finally,we take black phosphorus（BP）,which itself is a direct gap semiconductor with a narrow band gap suitable for infrared detectors,as an example to study the influence of different interlayer interactions on energy band of BP as well as strain on the electronic structure.Furthermore,the influence of absorbed O and Se atoms on bands of BP is further explored.Specific stages of progress are as follows:1)Both bulk phase and few-layer Hf S₂ have a relatively large inherent band gap（>1.7e V）,which makes it difficult to be directly used in the infrared detection.Monolayer Hf Te₂ owns a very small band gap（around 0.1 e V）,and bilayer and thicker ones appear semi-metallic.Both bulk phase and few-layer Hf STe belong to narrow band gap semiconductors,and the band gap varies from 0.056 e V to 0.355 e V when the bulk materials are thinned to monolayer.Due to the fixed and single band gap of these three kinds of bulk phase and few-layer materials,there are some limitations in practical application.Therefore,we have achieved the band gap of the Hf(S_1-xTe_x)₂ alloys being widely tuned from 1.732 e V（Hf S₂）to 0 e V（Hf Te₂）by varying the Te components.The infrared detectors based on Hf(S_1-xTe_x)₂ alloys enable the detection of visible to mid-and far-infrared light to meet different requirements.2)We have considered the structural,stability,and electronic properties of Pt X₂and Janus Pt XY monolayers performed by the first-principles calculations.Pt X₂ and Janus Pt XY monolayers are found to be dynamically and mechanically stable and less rigid than graphene and Mo S₂.All six monolayers show an outstanding stretchability with the Young’s modulus one-fifth that of graphene and half that of Mo S₂.At the ground state,all Pt X₂ and Janus Pt XY monolayers are indirect semiconductors with the gap ranging from 0.760 to 1.810 e V at the PBE functional.When a small range of uniaxial strain along the zigzag direction is applied,Pt X₂ and Janus Pt XY monolayers are still indirect bandgap semiconductors.Pt Te₂ becomes metallic first until the uniaxial compression strain reaches-7%.What is more,the electronic properties of these monolayers can be widely tuned by the biaxial strain.When small compressive strain is applied,all monolayers exhibit an indirect band gap with the decrease of values.Pt Te₂,Pt Se Te and Pt STe monolayers become metallic whenεreaches-6,-8 and-10%,respectively.However,all Pt X₂ and Pt XY can undergo a transition from an indirect band gap to a quasi-direct band gap with the increase of tensile strain,which will undoubtedly facilitate the efficiency of photoelectric coupling.This is also confirmed by the results of transition dipole moment.3)The band gap of black phosphorus（BP）under uniaxial strain along the armchair direction varies monotonically under compressive and tensile strain.The gap is tuned from 0.204 e V to 0.511 e V with the strain ranging from-2%to 2%.The results show that all gaps under strain are direct,allowing the BP-based detector to meet the requirement of mid-infrared wavelength detection.Through the exploration of the effect of O/Se atoms on the electronic structure of BP,we find that the defective level induced by the adsorbed atoms is independent of the thickness of BP and only related to the adsorption site.The most stable adsorption site of O/Se atoms has no effect on the band gap of BP.