Browsing by Author "Gong, Yongji"
Now showing 1 - 10 of 10
Results Per Page
Sort Options
Item 2D Materials in Lego Style: Synthesis, Characterizations and Applications(2015-12-04) Gong, Yongji; Ajayan, Pulickel M.; Jun, Lou; Marti, AngelRecently, the emergence and development of 2D materials with various optical and electrical properties has opened up new routes for electronic and optoelectronic device fabrication based on atomically thin layers. For example, graphene behaves as a semi-metal with extremely high mobility, hexagonal boron nitride (h-BN) is a good insulator and monolayer TMDs such as MoS2, MoSe2 and WSe2 are semiconductors with direct band gap. This diversity offers the opportunity to construct atomically thin electronics based entirely on 2D materials. One of the most promising applications is to get 2D integrated circuits to replace the traditional silicon based ones, which will be much thinner and faster. 2D materials can be considered to be analogous to Lego blocks. The Lego game is to use different Lego blocks to get a complicated Lego building. Similarly, we can use different 2D materials to get the corresponding integrated circuits or devices for energy related applications. Based on this purpose, we need different 2D blocks, which are the most fundamental parts in the 2D world, 2D materials with tunable properties, and different strategies to combine the 2D materials together. Chapter 1 focuses on synthesis, characterization and applications of pristine 2D materials, which are the fundamental blocks for the 2D world. In this part, we synthesized different 2D materials such as insulator (h-BN), metal (graphene) and semiconductors (MX2, M = metal and X = chalcogen) for different applications. There are two directions in this part: one is to explore new 2D materials and the other one is to improve the growth of 2D materials to push them closer to their real applications. Moreover, semiconductors with different band gap (from 1.1 eV to 2.8 eV) and different type (p type and n type) have been developed. Furthermore, we improved the growth of different 2D materials to get their millimeter-scale single crystals or even continuous film. In the coming Chapter 2, we focused on the 2D alloys. The purpose of alloying 2D materials is to engineer the phase and band gap by changing the composition in the alloys. By this, we can tune the optical and electrical properties in 2D materials very easily. The first project in this part is about h-BNC system, which can open a band gap in graphene system, resulting in both high mobilities and high ON-OFF ratio in their transistors. Then we developed the MoS2-xSex (x, 0-2) alloys, in which the band gap can be continuously tuned from 1.50 eV to 1.84 eV. At last, RexMo1-xS2 (x, 0-1) system is developed to study the phase transition with different x. In Chapter 3, heterostructures based on different 2D materials are developed by different strategies. For example, we can get h-BN/h-BNC/graphene lateral heterostructure by combing a conversion method and lithography. We also developed the heterostructures based on MoS2/WS2 and MoSe2/WSe2 by a one-step growth method and two-step growth method, respectively. In both of them, we can get the in-plane and vertical heterostructures. The interface of the in-plane interface is atomically seamless and sharp and the bilayer heterostructures have fixed stacking orientations, which are more advantageous than other methods. At last, we developed more complicated heterostructures, which can be composed by 3 or 4 different 2D materials. In Chapter 4, we further developed several different 3D structures constructed by 2D materials for energy storage and conversion. Basically, this part is inspired by graphene aerogel with porous 3D structure. The porous structure enables the access of electrolyte very easily and the graphene network has very good electrical conductivity, advantageous to work as electrochemical applications. In this part, we developed several different structures for different applications, including MoS2/GO as the anode for lithium ion battery, VO2/GO as the cathode for lithium ion battery and h-BNC as ORR catalyst. For the lithium ion battery, the structures developed here have better performance than the commercial ones with higher capacity, better stability and much higher charge and discharge rate. H-BNC aerogel can even beat the performance of commercial Pt/C as the ORR catalyst. In summary, the research based on 2D materials is like the Lego game, including exploring the Lego blocks (pristine 2D materials and their alloys) and combining them together to form the functional devices (2D heterostructures and 3D porous structure from 2D materials).Item Designing artificial 2D crystals with site and size controlled quantum dots(Springer Nature, 2017) Xie, Xuejun; Kang, Jiahao; Cao, Wei; Chu, Jae Hwan; Gong, Yongji; Ajayan, Pulickel M.; Banerjee, KaustavOrdered arrays of quantum dots in two-dimensional (2D) materials would make promising optical materials, but their assembly could prove challenging. Here we demonstrate a scalable, site and size controlled fabrication of quantum dots in monolayer molybdenum disulfide (MoS2), and quantum dot arrays with nanometer-scale spatial density by focused electron beam irradiation induced local 2H to 1T phase change in MoS2. By designing the quantum dots in a 2D superlattice, we show that new energy bands form where the new band gap can be controlled by the size and pitch of the quantum dots in the superlattice. The band gap can be tuned from 1.81 eV to 1.42 eV without loss of its photoluminescence performance, which provides new directions for fabricating lasers with designed wavelengths. Our work constitutes a photoresist-free, top-down method to create large-area quantum dot arrays with nanometer-scale spatial density that allow the quantum dots to interfere with each other and create artificial crystals. This technique opens up new pathways for fabricating light emitting devices with 2D materials at desired wavelengths. This demonstration can also enable the assembly of large scale quantum information systems and open up new avenues for the design of artificial 2D materials.Item Optoelectronic crystal of artificial atoms in strain-textured molybdenum disulphide(Nature Publishing Group, 2015) Li, Hong; Contryman, Alex W.; Qian, Xiaofeng; Ardakani, Sina Moeini; Gong, Yongji; Wang, Xingli; Weisse, Jeffrey M.; Lee, Chi Hwan; Zhao, Jiheng; Ajayan, Pulickel M.; Li, Ju; Manoharan, Hari C.; Zheng, XiaolinThe isolation of the two-dimensional semiconductor molybdenum disulphide introduced a new optically active material possessing a band gap that can be facilely tuned via elastic strain. As an atomically thin membrane with exceptional strength, monolayer molybdenum disulphide subjected to biaxial strain can embed wide band gap variations overlapping the visible light spectrum, with calculations showing the modified electronic potential emanating from point-induced tensile strain perturbations mimics the Coulomb potential in a mesoscopic atom. Here we realize and confirm this ‘artificial atom’ concept via capillary-pressure-induced nanoindentation of monolayer molybdenum disulphide from a tailored nanopattern, and demonstrate that a synthetic superlattice of these building blocks forms an optoelectronic crystal capable of broadband light absorption and efficient funnelling of photogenerated excitons to points of maximum strain at the artificial-atom nuclei. Such two-dimensional semiconductors with spatially textured band gaps represent a new class of materials, which may find applications in next-generation optoelectronics or photovoltaics.Item Optoelectronic crystal of artificial atoms in strain-textured molybdenum disulphide(Springer Nature, 2015) Li, Hong; Contryman, Alex W.; Qian, Xiaofeng; Ardakani, Sina Moeini; Gong, Yongji; Wang, Xingli; Weisse, Jeffrey M.; Lee, Chi Hwan; Zhao, Jiheng; Ajayan, Pulickel M.; Li, Ju; Manoharan, Hari C.; Zheng, XiaolinThe isolation of the two-dimensional semiconductor molybdenum disulphide introduced a new optically active material possessing a band gap that can be facilely tuned via elastic strain. As an atomically thin membrane with exceptional strength, monolayer molybdenum disulphide subjected to biaxial strain can embed wide band gap variations overlapping the visible light spectrum, with calculations showing the modified electronic potential emanating from point-induced tensile strain perturbations mimics the Coulomb potential in a mesoscopic atom. Here we realize and confirm this ‘artificial atom’ concept via capillary-pressure-induced nanoindentation of monolayer molybdenum disulphide from a tailored nanopattern, and demonstrate that a synthetic superlattice of these building blocks forms an optoelectronic crystal capable of broadband light absorption and efficient funnelling of photogenerated excitons to points of maximum strain at the artificial-atom nuclei. Such two-dimensional semiconductors with spatially textured band gaps represent a new class of materials, which may find applications in next-generation optoelectronics or photovoltaics.Item Single atom catalysts in Van der Waals gaps(Springer Nature, 2022) Jiang, Huaning; Yang, Weiwei; Xu, Mingquan; Wang, Erqing; Wei, Yi; Liu, Wei; Gu, Xiaokang; Liu, Lixuan; Chen, Qian; Zhai, Pengbo; Zou, Xiaolong; Ajayan, Pulickel M.; Zhou, Wu; Gong, YongjiSingle-atom catalysts provide efficiently utilized active sites to improve catalytic activities while improving the stability and enhancing the activities to the level of their bulk metallic counterparts are grand challenges. Herein, we demonstrate a family of single-atom catalysts with different interaction types by confining metal single atoms into the van der Waals gap of two-dimensional SnS2. The relatively weak bonding between the noble metal single atoms and the host endows the single atoms with more intrinsic catalytic activity compared to the ones with strong chemical bonding, while the protection offered by the layered material leads to ultrahigh stability compared to the physically adsorbed single-atom catalysts on the surface. Specifically, the trace Pt-intercalated SnS2 catalyst has superior long-term durability and comparable performance to that of commercial 10 wt% Pt/C catalyst in hydrogen evolution reaction. This work opens an avenue to explore high-performance intercalated single-atom electrocatalysts within various two-dimensional materials.Item Thermodynamics of order and randomness in dopant distributions inferred from atomically resolved imaging(Springer Nature, 2021) Vlcek, Lukas; Yang, Shize; Gong, Yongji; Ajayan, Pulickel; Zhou, Wu; Chisholm, Matthew F.; Ziatdinov, Maxim; Vasudevan, Rama K.; Kalinin, Sergei V.Exploration of structure-property relationships as a function of dopant concentration is commonly based on mean field theories for solid solutions. However, such theories that work well for semiconductors tend to fail in materials with strong correlations, either in electronic behavior or chemical segregation. In these cases, the details of atomic arrangements are generally not explored and analyzed. The knowledge of the generative physics and chemistry of the material can obviate this problem, since defect configuration libraries as stochastic representation of atomic level structures can be generated, or parameters of mesoscopic thermodynamic models can be derived. To obtain such information for improved predictions, we use data from atomically resolved microscopic images that visualize complex structural correlations within the system and translate them into statistical mechanical models of structure formation. Given the significant uncertainties about the microscopic aspects of the material’s processing history along with the limited number of available images, we combine model optimization techniques with the principles of statistical hypothesis testing. We demonstrate the approach on data from a series of atomically-resolved scanning transmission electron microscopy images of MoxRe1-xS2 at varying ratios of Mo/Re stoichiometries, for which we propose an effective interaction model that is then used to generate atomic configurations and make testable predictions at a range of concentrations and formation temperatures.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.Item Valley trion dynamics in monolayerᅠMoSe2(American Physical Society, 2016) Gao, Feng; Gong, Yongji; Titze, Michael; Almeida, Raybel; Ajayan, Pulickel M.; Li, HebinCharged excitons called trions play an important role in the fundamental valley dynamics in newly emerging two-dimensional semiconductor materials. We use ultrafast pump-probe spectroscopy to study the valley trion dynamics in aᅠMoSe2ᅠmonolayer grown by using chemical vapor deposition. The dynamics displays an ultrafast trion formation followed by a nonexponential decay. The measurements at different pump fluences show that the trion decay dynamics becomes slower as the excitation density increases. The observed trion dynamics and the associated density dependence are a result of the trapping by two defect states as being the dominating decay mechanism. The simulation based on a set of rate equations reproduces the experimental data for different pump fluences. Our results reveal the important trion dynamics and identify the trapping by defect states as the primary trion decay mechanism in monolayerᅠMoSe2ᅠunder the excitation densities used in our experiment.Item Vertical and in-plane heterostructures from WS2/MoS2 monolayers(Nature Publishing Group, 2014) Gong, Yongji; Lin, Junhao; Wang, Xingli; Shi, Gang; Lei, Sidong; Lin, Zhong; Zou, Xiaolong; Ye, Gonglan; Vajtai, Robert; Yakobson, Boris I.; Terrones, Humberto; Terrones, Mauricio; Tay, Beng Kang; Lou, Jun; Pantelides, Sokrates T.; Liu, Zheng; Zhou, Wu; Ajayan, Pulickel M.Layer-by-layer stacking or lateral interfacing of atomic monolayers has opened up unprecedented opportunities to engineer two-dimensional heteromaterials. Fabrication of such artificial heterostructures with atomically clean and sharp interfaces, however, is challenging. Here, we report a one-step growth strategy for the creation of high-quality vertically stacked as well as in-plane interconnected heterostructures of WS2/MoS2 via control of the growth temperature. Vertically stacked bilayers with WS2 epitaxially grown on top of the MoS2 monolayer are formed with preferred stacking order at high temperature. A strong interlayer excitonic transition is observed due to the type II band alignment and to the clean interface of these bilayers. Vapour growth at low temperature, on the other hand, leads to lateral epitaxy of WS2 on MoS2 edges, creating seamless and atomically sharp in-plane heterostructures that generate strong localized photoluminescence enhancement and intrinsic p–n junctions. The fabrication of heterostructures from monolayers, using simple and scalable growth, paves the way for the creation of unprecedented two-dimensional materials with exciting properties.