Browsing by Author "Yuan, Jiangtan"
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Item Excitonic Resonant Emission–Absorption of Surface Plasmons in Transition Metal Dichalcogenides for Chip-Level Electronic–Photonic Integrated Circuits(American Chemical Society, 2016) Zhu, Zhuan; Yuan, Jiangtan; Zhou, Haiqing; Hu, Jonathan; Zhang, Jing; Wei, Chengli; Yu, Fang; Chen, Shuo; Lan, Yucheng; Yang, Yao; Wang, Yanan; Niu, Chao; Ren, Zhifeng; Lou, Jun; Wang, Zhiming; Bao, JimingThe monolithic integration of electronics and photonics has attracted enormous attention due to its potential applications. A major challenge to this integration is the identification of suitable materials that can emit and absorb light at the same wavelength. In this paper we utilize unique excitonic transitions in WS2 monolayers and show that WS2 exhibits a perfect overlap between its absorption and photoluminescence spectra. By coupling WS2 to Ag nanowires, we then show that WS2 monolayers are able to excite and absorb surface plasmons of Ag nanowires at the same wavelength of exciton photoluminescence. This resonant absorption by WS2 is distinguished from that of the ohmic propagation loss of silver nanowires, resulting in a short propagation length of surface plasmons. Our demonstration of resonant optical generation and detection of surface plasmons enables nanoscale optical communication and paves the way for on-chip electronic–photonic integrated circuits.Item Quantum plasmonic control of trions in a picocavity with monolayer WS2(AAAS, 2019) He, Zhe; Han, Zehua; Yuan, Jiangtan; Sinyukov, Alexander M.; Eleuch, Hichem; Niu, Chao; Zhang, Zhenrong; Lou, Jun; Hu, Jonathan; Voronine, Dmitri V.; Scully, Marlan O.Monitoring and controlling the neutral and charged excitons (trions) in two-dimensional (2D) materials are essential for the development of high-performance devices. However, nanoscale control is challenging because of diffraction-limited spatial resolution of conventional far-field techniques. Here, we extend the classical tip-enhanced photoluminescence based on tip-substrate nanocavity to quantum regime and demonstrate controlled nano-optical imaging, namely, tip-enhanced quantum plasmonics. In addition to improving the spatial resolution, we use the scanning probe to control the optoelectronic response of monolayer WS2 by varying the neutral/charged exciton ratio via charge tunneling in Au-Ag picocavity. We observe trion “hot spots” generated by varying the picometer-scale probe-sample distance and show the effects of weak and strong coupling, which depend on the spatial location. Our experimental results are in agreement with simulations and open an unprecedented view of a new range of quantum plasmonic phenomena with 2D materials that will help to design new quantum optoelectronic devices.Item Spatially-Resolved Photoluminescence of Monolayer MoS2 under Controlled Environment for Ambient Optoelectronic Applications(American Chemical Society, 2018) Birmingham, Blake; Yuan, Jiangtan; Filez, Matthias; Fu, Donglong; Hu, Jonathan; Lou, Jun; Scully, Marlan O.; Weckhuysen, Bert M.; Zhang, ZhenrongMonolayer (ML) MoS2 has become a very promising two-dimensional material for photorelated applications, potentially serving as the basis for an ultrathin photodetector, switching device, or transistors because of its strong interaction with light in ambient conditions. Establishing the impact of individual ambient gas components on the optical properties of MoS2 is a necessary step toward application development. By using in situ Raman microspectroscopy with an environment-controlled reaction cell, the photoluminescence (PL) intensity of chemical vapor deposition (CVD)-grown MoS2 MLs is monitored at different intralayer locations under ambient and controlled gas environments, such as N2, O2, and H2O. This new approach enables us to monitor the optical properties of MoS2 at different locations on the flakes and separate the role of photoreaction of various gases during laser irradiation. Upon mild photoirradiation in ambient conditions, the PL intensity in the interior of the ML MoS2 flakes remains unchanged, while the PL intensity at the edge region increases drastically. Photoirradiation in controlled gas environments reveals that O2 is necessary to increase the PL intensity at the MoS2 flake edges, attributed to the charge transfer of chemisorbed O2. N2 or H2O and N2 environments induce decreasing PL intensity upon repetitive laser irradiation. However, the H2O and O2 gas mixture, a combination designed to mimic ambient conditions, is necessary to maintain the PL intensity at the interior of the ML MoS2 flakes. Our study demonstrates that photoreactions with the gaseous environment on the MoS2 ML flakes should be taken into consideration even upon mild photoirradiation because they strongly impact the flakes’ optical properties.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 Two Dimensional Materials for Renewable Energy Harvesting and Energy Efficient Devices(2018-04-20) Yuan, Jiangtan; Lou, JunTwo-dimensional (2D) materials show promising electronical, optical, magnetic and mechanical properties. In this thesis, we investigate the preparation of two dimensional materials through chemical vapor deposition, and perform electronic, optical and magnetic measurements on these materials and their heterstructures, including Molybdenum Disulfide (MoS2), Vanadium Sulfides (VS2, V5S8) and Iron oxide (Fe2O3). We argue that these materials can be potential candidates for harvesting solar energy to generate electricity and hydrogen, as well as making energy efficient spintronic devices. Moisture content is one of the key parameters in controlling the synthesis of 2D MoS2. We found that moisture present in the MoO3 powder will have significant effect on the synthesis of monolayer MoS2. Having understood the role of moisture in the process, we achieved highly-reproducible, high-yield, and high-quality monolayer MoS2 growth by adopting an unglazed surface crucible to absorb the water during the process. The photoluminescence (PL) performance of individual 2D material can be very different when they are combined together. We study the inter-layer coupling behaviors of hetero-bilayers of MoS2 and WS2 and MoS2-black phosphorous (BP), WS2-BP hetero-stacks. The interactions between the MoS2 and WS2 layers retain a direct band gap for MoS2 while form an indirect gap for the WS2. The MoS2-BP hetero-stack shows a type-II band alignment and WS2-BP hetero-stack has a type-I band alignment. The build-in electric fields in these hetero-stacks are strong enough to split the excitons, leading to a significant reduction in their recombination and subsequently a strongly-quenched PL peak of the WS2 and MoS2. Singe crystalline VS2 nanosheets can be prepared by CVD. We determine the hexagonal 1T crystalline structures of VS2 using XRD and TEM. We investigate the electrocatalytic hydrogen evolution reaction (HER) activities of the 1T-VS2, which shows extremely low overpotential of -68 mV at 10 mA cm-2, and small Tafel slopes of ∼34 mV/decade as well as high stability, demonstrating its potential as a candidate non-noble metal catalyst for HER. Finally, two magnetic 2D materials: V5S8 and Fe2O3 were synthesized and studied. V5S8 maintains its antiferromagnetic properties down to ~ 11 nm thickness, and possible quantum phase transition at accessible fields (~18 T). Due to its minimum surface energy, the unique epsilon phase Fe2O3 was found to be a more stable phase among all other phases in 2D forms. Magnetic Kerr rotation measurements indicated the room-temperature ferromagnetism of epsilon phase Fe2O3. Combing its air-stable nature, ultra-thin 2D epsilon Fe2O3 can be an excellent platform for compact and energy efficient spintronic devices.Item Ultrafast formation of interlayer hot excitons in atomically thin MoS2/WS2ﾠheterostructures(Springer Nature, 2016) Chen, Hailong; Wen, Xiewen; Zhang, Jing; Wu, Tianmin; Gong, Yongji; Zhang, Xiang; Yuan, Jiangtan; Yi, Chongyue; Lou, Jun; Ajayan, Pulickel M.; Zhuang, Wei; Zhang, Guangyu; Zheng, JunrongVan der Waals heterostructures composed of two-dimensional transition-metal dichalcogenides layers have recently emerged as a new family of materials, with great potential for atomically thin opto-electronic and photovoltaic applications. It is puzzling, however, that the photocurrent is yielded so efficiently in these structures, despite the apparent momentum mismatch between the intralayer/interlayer excitons during the charge transfer, as well as the tightly bound nature of the excitons in 2D geometry. Using the energy-state-resolved ultrafast visible/infrared microspectroscopy, we herein obtain unambiguous experimental evidence of the charge transfer intermediate state with excess energy, during the transition from an intralayer exciton to an interlayer exciton at the interface of a WS2/MoS2ﾠheterostructure, and free carriers moving across the interface much faster than recombining into the intralayer excitons. The observations therefore explain how the remarkable charge transfer rate and photocurrent generation are achieved even with the aforementioned momentum mismatch and excitonic localization in 2D heterostructures and devices.Item Ultrafast probes of electron–hole transitions between two atomic layers(Springer Nature, 2018) Wen, Xiewen; Chen, Hailong; Wu, Tianmin; Yu, Zhihao; Yang, Qirong; Deng, Jingwen; Liu, Zhengtang; Guo, Xin; Guan, Jianxin; Zhang, Xiang; Gong, Yongji; Yuan, Jiangtan; Zhang, Zhuhua; Yi, Chongyue; Guo, Xuefeng; Ajayan, Pulickel M.; Zhuang, Wei; Liu, Zhirong; Lou, Jun; Zheng, JunrongPhase transitions of electron-hole pairs on semiconductor/conductor interfaces determine fundamental properties of optoelectronics. To investigate interfacial dynamical transitions of charged quasiparticles, however, remains a grand challenge. By employing ultrafast mid-infrared microspectroscopic probes to detect excitonic internal quantum transitions and two-dimensional atomic device fabrications, we are able to directly monitor the interplay between free carriers and insulating interlayer excitons between two atomic layers. Our observations reveal unexpected ultrafast formation of tightly bound interlayer excitons between conducting graphene and semiconducting MoSe2. The result suggests carriers in the doped graphene are no longer massless, and an effective mass as small as one percent of free electron mass is sufficient to confine carriers within a 2D hetero space with energy 10 times larger than the room-temperature thermal energy. The interlayer excitons arise within 1 ps. Their formation effectively blocks charge recombination and improves charge separation efficiency for more than one order of magnitude.