Study Of Structure Design And Related Electronic Properties For Several Novel Two-dimensional Materials

The success of graphene has completely unlocked the secrets and new physics behind the low-dimensional nanomaterials and technology.Due to their own unique structural features and intriguing physical chemical properties,low-dimensional nanomaterials have attracted enormous research enthusiasm in various areas of applied and fundamental physics in the last decade.Note that significant progress has been made in the development of two-dimensional?2D?material system,and tremendous graphene-like layer materials have been predicted or synthesized,including h-BN,silicene,transition metal dichalcogenides?TMDs?,black phosphorus,arsenene and so on.Up to now,these 2D materials have already formed a family of complete low-dimensional nano-materials system,and are able to meet the requirements of most practical application areas.For example,in the studies of rechargeable batteries,low-dimensional materials serving as electrode material have been widely explored and recognized as one of the promising means to enhance the performance and safety of Li-ion or Na-ion batteries.On the other hand,based on the particularity of the 2D structures,we can construct analogous heterojunctions between different 2D layered materials,which would be of great significance for the realization of diverse electronic,optoelectronic and photovoltaic devices.Different from the traditional Van der Waals heterojunctions,lateral heterojunction can only be achieved by direct chemical vapor deposition?CVD?epitaxial growth,marking the ultimate thickness in heterostructure,but related investigations are still in the initial stage.Next,the fabrication of more novel lateral heterojunctions,revealing underlying excellent new physical phenomena,will become the focus of the subsequent research.Meanwhile,one-dimensional?ID?and 2D metal-organic molecular materials have also made considerable progress,especially for spintronics.Because these metal-organic molecular frameworks can present the desired uniform magnetic ordering as well as the half-metallic nature.In this thesis,we systematically studied the electronic,magnetic and carrier mobility behaviors as well as topological properties of a variety of low-dimensional nanomaterials.The manipulation of properties induced by surface adsorption,external strain and substrate effect are also described and discussed in detail.In addition,our results also reveal interesting physical mechanisms and rich functional applications in these low-dimensional nanomaterials.This thesis contains six chapters.In the first chapter,we briefly outline the recent research background and progress of low dimensional nanomaterials and technology.In the second chapter,we review the corresponding theoretical basis,methods and computational codes for the first-principles calculations.In the third chapter,we introduce the results and discussion of some new designed lateral heterojunction with diverse functional integration.In the fourth chapter,we identify the performance of 2D Mo₂C monolayer material as an anode material for both Li-ion and Na-ion batteries.In the fifth chapter,we explore the manipulation of electronic and magnetic properties of a series of 2D and 1D metal-organic molecular frameworks for spintronics.In the last chapter,the main conclusions and innovations of this thesis are summarized and the potential research directions of low-dimensional functionalized materials are pointed out.The main content and results are listed as follows:?1?We perform a systematic theoretical investigation on h-BN/graphene lateral heterostructures.Our results demonstrate that the electronic structures as well as the magnetic properties of the hybridized monolayers can be modified efficiently.Furthermore,the phase transition from insulator to metal can also be realized by the proposed approaches of adjusting the numbers or the ratios of the zigzag h-BN and zigzag graphene.The investigation of the strain effect reveals that the external strain applied along the Y-axis plays a decisive role in the bandgap engineering.Moreover,the calculated effective masses give a reasonable physical representation of the carrier transport properties.Our results show that the mobility direction of the charge carriers is parallel to the interface.These predictions provide new potential strategies for tuning electronic properties and will allow new device functionalities.?2?Inspired by recent experiments,the unprecedented bidirectional heterostructures of y-graphyne@MoSe₂/WSe2 as well as y-graphyne@MoSe₂ and y-graphyne@WSe2 are proposed and examined.Our results reveal that a novel wrinkled?-graphyne with narrowed energy gap and strong binding strength is achieved on the planar and smooth substrate.The direct-indirect band gap crossover is also found in terms of interlayer coupling.Furthermore,we demonstrate that electron-hole pairs can be spatially separated,and the carrier mobility would be benefited from the absorbed y-graphyne in the BDHs.These results provide not only new insights into the physical and chemical properties of the vertical and bidirectional heterostructures,but also a new strategy for fabricating unprecedented 2D nanomaterials with exciting properties.?3?We demonstrate a novel type of seamless lateral heterostructures with excellent stabilities formed within pristine arsenene and antimonene.These heterostructures possess direct and reduced energy gaps without any modulations.Moreover,the highly coveted type-II alignment and the high carrier mobility are also identified,marking the enhanced quantum efficiency.The tensile strain can result in efficient bandgap engineering.Besides,the proposed critical condition for favored direct energy gaps would have a guiding significance on the subsequent works.Generally,our predictions not only introduce new vitality into lateral heterostructures,enriching available candidate materials in this field,but also highlight the potential of these lateral heterostructures as appealing materials for future devices.?4?We report a breakthrough in lateral heterostructure based on the monolayer square transition-metal dichalcogenides MX₂?M = W,X = S/Se?modules.The MX₂ lateral heterostructure?1S-MX₂ LHS?possess excellent thermal and dynamical stability.Remarkably,the highly desired two-dimensional topological crystalline insulator phase is confirmed by the calculated mirror Chern number nM =-1.A nontrivial band gap of 65 meV is obtained with SOC,indicating the potential for room-temperature observation and applications.The topologically protected edge states emerge at the edges of two different nanoribbons between the bulk band gap,which is consistent with the mirror Chern number.In addition,a strain-induced topological phase transition in 1S-MX₂ LHS is also revealed,endowing the potential utilities in electronics and spintronics.?5?We demonstrate the design of lateral heterostructures based on brand-new building materials,namely 1S-MX₂ LHS,and exhibits excellent stability,marking the high feasibility in the experiment.The desired bandgap opening,that can endure the application at room temperature,is also confirmed in 1S-MX₂ LHS with spin-orbit coupling?SOC?.Strain strategy further results in efficient bandgap engineering and intriguing phase transition.In addition,we find black phosphorus can serve as competent substrate to support 1S-MX₂ LHS with the coveted type-II band alignment,allowing the versatile functionalized bidirectional heterostructures with built-in device function in the further.Besides,it is interesting to remark that the robust electronic features could be maintained in the 1S-MX₂ LHS with larger components.?6?We investigate the feasibility of Mo₂C monolayer serve as anode materials in rechargeable batteries.After ensuring its dynamical and thermal stabilities,various low energy Li and Na adsorption sites are identified,and the electric conductivity of the host material is also maintained.The calculated minor diffusion barriers imply a high mobility and cycling ability of Mo₂C.In addition,the Li-adsorbed Mo₂C monolayer possesses a high theoretical capacity of 526 mAh·g^-1 and a low average electrode potential of 0.14 eV.Besides,we find that the relatively low capability of Na-adsorbed Mo₂C(132 mAh·g^-1)arises from the proposed competition mechanism.These results highlight the promise of Mo₂C monolayer as an appealing anode material for both lithium-ion and sodium-ion batteries.?7?A practical avenue for achieving the long-cherished nanomaterial via novel 2D periodic metalloporphyrin frameworks?referred to as M-PpO and M-Pp45,M = Cr,Mn,Fe,Co,Ni,Cu and Zn?is provided,and the frameworks can maintain regularly and separately distributed transition-metals?TMs?.The electronic and magnetic properties of these novel 2D systems are systematically investigated.Our results reveal that Ni-PpO and Zn-PpO are nonmagnetic,while Cr-PpO,Fe-PpO and Cu-Pp0 are weak antiferromagnetic and Co-PpO is paramagnetic.For M-Pp45 frameworks,however,the spin couplings are all identified to be paramagnetic arising from their long spin coherence length.Remarkably,the introduced TMs have tremendous influence on the band gap of the M-Pp45 frameworks.What is more interesting is that the Mn-PpO framework exhibits long-range ferromagnetic spin coupling as well as half-metallic nature.By performing Monte Carlo simulations based on the Ising model,we further demonstrate that the Mn-PpO framework would possess a Curie temperature?Tc?of 320 K,suggesting a real sense of room temperature is achieved.?8?With the purpose of searching for new intriguing nanomaterial for spintronics,a series of novel metalloporphyrin nanowires?M-PPNW,M = Cr,Mn,Fe,Co,Ni,Cu and Zn?and hybrid nanowires fabricated by metalloporphyrin and metal-phthalocyanine?M-PCNW?are systematically investigated by means of first-principles calculations.The transition metal atoms?TMs?embedded in the frameworks distribute regularly and separately,without any trend to form clusters,thus leading to the ideally ordered spin distribution.Except for the cases embedded with Ni and Zn,the others are spin-polarized.Remarkably,the Mn-PPNW,Mn-PCNW,MnCu-PPNW,MnCr-PCNW,and MnCu-PCNW frameworks all favor the long-ranged ferromagnetic spin ordering and display half-metallic nature,which are of greatest interest and importance for electronics and spintronics.The predicted Curie temperature for the Mn-PCNW is about 150 K.In addition,it is found that the discrepancy in magnetic coupling for these materials is related to the competition mechanisms of through-bond and through-space exchange interactions.