Browsing by Author "Chen, Weibing"
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Item Electron correlation in solids via density embedding theory(AIP Publishing, 2014) Bulik, Ireneusz W.; Chen, Weibing; Scuseria, Gustavo E.Density matrix embeddingᅠtheoryᅠ[G. Knizia and G. K.-L. Chan,ᅠPhys. Rev. Lett.109, 186404 (2012)] and density embeddingᅠtheoryᅠ[I. W. Bulik, G. E. Scuseria, and J. Dukelsky,ᅠPhys. Rev. Bᅠ89, 035140 (2014)] have recently been introduced for model lattice Hamiltonians and molecular systems. In the present work, the formalism is extended to theᅠab initioᅠdescription of infinite systems. An appropriate definition of the impurity Hamiltonian for such systems is presented and demonstrated in cases of 1, 2, and 3 dimensions, usingᅠcoupled clusterᅠtheoryᅠas the impurity solver. Additionally, we discuss the challenges related to disentanglement of fragment and bath states. The current approach yields results comparable toᅠcoupled clusterᅠcalculations of infinite systems even when using a single unit cell as the fragment. Theᅠtheoryᅠis formulated in the basis of Wannier functions but it does not require separate localization of unoccupied bands. The embedding scheme presented here is a promising way of employing highly accurate electronic structure methods for extended systems at a fraction of their original computational cost.Item Interaction of two-dimensional materials with molecules, cells, and substrates(2018-04-26) Chen, Weibing; Lou, JunTwo-dimensional (2D) materials have been widely explored in different fields since 2004. More in-depth understanding of their interaction with the environment becomes more and more vital in designing and implementing novel biological devices. Knowing how to use them, how to evaluate their safety in the human body, and how to prepare them cheaply are three critical questions in the investigations of biomaterials. In this thesis, to cover these three questions, I will examine two-dimensional materials in oxygen sensing, cytotoxicity evaluation, friction tunability, substrate-controlled growth and protective layers in arbitrary substrates. First, I will give a general review of the structure, synthesis, and characterizations of two-dimensional materials used in this thesis. Second, I will switch to the project of studying the interplay of MoS2 and oxygen molecules to unveil the strong correlation between p-type doping and photoluminescence enhancement due to the presence of oxygen. Next, to guarantee the safety of 2D biosensor, I will reveal the tunable friction behaviors of MoS2 via the chemical interface of self-assembled molecules which will be helpful in designing clogging-free biosensors. Furthermore, the toxicity of MoS2 flakes and microparticles will be evaluated via MTT without contaminations of samples during synthesis. The results demonstrate the low toxicity of 2H type MoS2 to 6 six different kinds of human cells. The interaction of 2D materials with substrates will be the focus of the last part of this thesis, where I will demonstrate that patterned substrate controls the growth of monolayer MoSe2 and the grown ultrathin h-BN on iron substrate protects the covered substrate against strong acid very well. The extensive coverage in different fields in this thesis provides us with some essential knowledge of these exciting 2D materials in future biomedical applications.Item Metallic 1T phase source/drain electrodes for field effect transistors from chemical vapor deposited MoS2(AIP, 2014) Kappera, Rajesh; Voiry, Damien; Yalcin, Sibel Ebru; Jen, Wesley; Acerce, Muharrem; Torrel, Sol; Branch, Brittany; Lei, Sidong; Chen, Weibing; Najmaei, Sina; Lou, Jun; Ajayan, Pulickel M.; Gupta, Gautam; Mohite, Aditya D.; Chhowalla, ManishTwo dimensional transitionmetal dichalcogenides (2D TMDs) offer promise as optoelectronic materials due to their direct band gap and reasonably good mobility values. However, most metals form high resistance contacts on semiconducting TMDs such as MoS2. The large contact resistance limits the performance of devices. Unlike bulk materials, low contact resistance cannot be stably achieved in 2D materials by doping. Here we build on our previous work in which we demonstrated that it is possible to achieve low contact resistance electrodes by phase transformation. We show that similar to the previously demonstrated mechanically exfoliated samples, it is possible to decrease the contact resistance and enhance the FET performance by locally inducing and patterning the metallic 1T phase of MoS2 on chemically vapor deposited material. The device properties are substantially improved with 1T phase source/drain electrodes.Item Quantum plasmonic hot-electron injection in lateral WSe2/MoSe2heterostructures(American Physical Society, 2018) Tang, Chenwei; He, Zhe; Chen, Weibing; Jia, Shuai; Lou, Jun; Voronine, Dmitri V.Lateral two-dimensional (2D) transition-metal dichalcogenide (TMD) heterostructures have recently attracted wide attention as promising materials for optoelectronic nanodevices. Due to the nanoscale width of lateral heterojunctions, the study of their optical properties is challenging and requires using subwavelength optical characterization techniques. We investigated the photoresponse of a lateral 2D WSe2/MoSe2 heterostructure using tip-enhanced photoluminescence (TEPL) with nanoscale spatial resolution and with picoscale tip-sample distance dependence. We demonstrate the observation of quantum plasmonic effects in 2D heterostructures on a nonmetallic substrate, and we report the nano-optical measurements of the lateral 2D TMD heterojunction width of ∼150 nm and the charge tunneling distance of ∼20 pm. Controlling the plasmonic tip location allows for both nano-optical imaging and plasmon-induced hot-electron injection into the heterostructure. By adjusting the tip-sample distance, we demonstrated the controllability of the hot-electron injection via the competition of two quantum plasmonic photoluminescence (PL) enhancement and quenching mechanisms. The directional charge transport in the depletion region leads to the increased hot-electron injection, enhancing the MoSe2 PL signal. The properties of the directional hot-electron injection in the quantum plasmonic regime make the lateral 2D MoSe2/WSe2 heterostructures promising for quantum nanodevices with tunable photoresponse.Item Surface enhanced resonant Raman scattering in hybrid MoSe2@Au nanostructures(Optical Society of America, 2018) Abid, Inès; Chen, Weibing; Yuan, Jiangtan; Najmaei, Sina; Peñafiel, Emil C.; Péchou, Renaud; Large, Nicolas; Lou, Jun; Mlayah, AdnenWe report on the surface enhanced resonant Raman scattering (SERRS) in hybrid MoSe2@Au plasmonic-excitonic nanostructures, focusing on the situation where the localized surface plasmon resonance of Au nanodisks is finely tuned to the exciton absorption of monolayer MoSe2. Using a resonant excitation, we investigate the SERRS in MoSe2@Au and the resonant Raman scattering (RRS) in a MoSe2@SiO2 reference. Both optical responses are compared to the non-resonant Raman scattering signal, thus providing an estimate of the relative contributions from the localized surface plasmons and the confined excitons to the Raman scattering enhancement. We determine a SERRS/RRS enhancement factor exceeding one order of magnitude. Furthermore, using numerical simulations, we explore the optical near-field properties of the hybrid MoSe2@Au nanostructure and investigate the SERRS efficiency dependence on the nanodisk surface morphology and on the excitation wavelength. We demonstrate that a photothermal effect, due to the resonant plasmonic pumping of electron-hole pairs into the MoSe2 layer, and the surface roughness of the metallic nanostructures are the main limiting factors of the SERRS efficiency.Item Synergetic photoluminescence enhancement of monolayer MoS2 via surface plasmon resonance and defect repair(Royal Society of Chemistry, 2018) Zeng, Yi; Chen, Weibing; Tang, Bin; Liao, Jianhui; Lou, Jun; Chen, QingThe weak light-absorption and low quantum yield (QY) in monolayer MoS2 are great challenges for the applications of this material in practical optoelectronic devices. Here, we report on a synergistic strategy to obtain highly enhanced photoluminescence (PL) of monolayer MoS2 by simultaneously improving the intensity of the electromagnetic field around MoS2 and the QY of MoS2. Self-assembled sub-monolayer Au nanoparticles underneath the monolayer MoS2 and bis(trifluoromethane)sulfonimide (TFSI) treatment to the MoS2 surface are used to boost the excitation field and the QY, respectively. An enhancement factor of the PL intensity as high as 280 is achieved. The enhancement mechanisms are analyzed by inspecting the contribution of the PL spectra from A excitons and A− trions under different conditions. Our study takes a further step to developing high-performance optoelectronic devices based on monolayer MoS2.Item Synthesis of High-Quality Graphene and Hexagonal Boron Nitride Monolayer In-Plane Heterostructure on Cu–Ni Alloy(Wiley, 2017) Lu, Guangyuan; Wu, Tianru; Yang, Peng; Yang, Yingchao; Jin, Zehua; Chen, Weibing; Jia, Shuai; Wang, Haomin; Zhang, Guanhua; Sun, Julong; Ajayan, Pulickel M.; Lou, Jun; Xie, Xiaoming; Jiang, MianhengGraphene/hexagonal boron nitride (h-BN) monolayer in-plane heterostructure offers a novel material platform for both fundamental research and device applications. To obtain such a heterostructure in high quality via controllable synthetic approaches is still challenging. In this work, in-plane epitaxy of graphene/h-BN heterostructure is demonstrated on Cu–Ni substrates. The introduction of nickel to copper substrate not only enhances the capability of decomposing polyaminoborane residues but also promotes graphene growth via isothermal segregation. On the alloy surface partially covered by h-BN, graphene is found to nucleate at the corners of the as-formed h-BN grains, and the high growth rate for graphene minimizes the damage of graphene-growth process on h-BN lattice. As a result, high-quality graphene/h-BN in-plane heterostructure with epitaxial relationship can be formed, which is supported by extensive characterizations. Photodetector device applications are demonstrated based on the in-plane heterostructure. The success will have important impact on future research and applications based on this unique material platform.