Valley Zeeman splitting in semiconducting two-dimensional group-VI transition metal dichalcogenides

Atomically thin semiconducting group-VI transition metal dichalcogenides (TMDs) have attracted enormous interest because of their as-born bandgaps and other unique properties giving great potential in next-generation electronic devices, valleytronics, photodetectors and flexible optoelectronics applications. Electrons at K and K^' valleys in 2D group-VI TMDs can be selectively excited by the circularly polarized light but energy degenerated due to the time-reversal symmetry, which is known as the valley degree of freedom. In the presence of an external out-of-plane magnetic field, the energy degeneracy is lifted thus there is an energy difference between the two emissions from the two valleys, known as the valley Zeeman splitting energy due to the breaking of time-reversal symmetry. Such unique features originating from the strong spin-orbital and spin-valley couplings make 2D group-VI TMDs highly competitive over the traditional semiconductors and even promising for the emerging valleytronics. In this thesis, circularly-polarized photoluminescence (PL) spectroscopy has been exploited to investigate valley Zeeman splitting behavior of emerging 2D group-VI TMDs under various circumstances. On the way towards the large-scale integration of potential valley Zeeman splitting based devices, one of the critical issues is whether the valley Zeeman splitting behavior changes with the strength of many body interactions induced by the different doping levels across the sample. Here, spatial variations of valley splitting evolution in exfoliated monolayer WSe2 are investigated through magneto-PL mapping measurements. It is found that for the neutral exciton emission, the valley Zeeman splitting behavior almost stays unchanged across the sample though the PL mapping measurements show the nonuniformity of the PL emission energy, which is caused by the unintentional doping during the sample preparation process. While for trion emission, the valley Zeeman splitting behavior changes a lot with the doping level from the sample center to the edge regions. In order to realize two stable binary states in potential valley Zeeman splitting based devices, a large valley Zeeman splitting energy is on demand even under a small magnetic field. Here, exfoliated monolayer WSe2 samples are transferred onto a ferromagnetic substrate of EuS. The net magnetization of EuS substrate results in a short-range magnetic exchange field (MEF) on the interface between the WSe2 and EuS. And this MEF further leads to enhanced valley Zeeman splitting energies for both trion and exciton emissions of WSe2 on the EuS substrate. The short-range MEF originating from proximity effect can be exploited to tune the valley Zeeman splitting behavior in future valleytronics. Hexagonal boron nitride (hBN) with a layered crystal structure has less lattice mismatch with the group-VI TMDs and is often used as a platform to improve the optical quality of the 2D group-VI TMDs by suppressing the unintentional doping from the oxide substrate. Here, a modified method is developed to directly grow WS2 and MoS2 monolayers on hBN with a high yield and high optical quality. Benefiting from the well-resolved and super sharp exciton and trion PL peaks, the intrinsic valley Zeeman splitting behavior in CVD-grown WS2 and MoS2 monolayers on hBN have been clearly revealed through in-situ magnetic-field-dependent PL imaging and spectroscopy at cryogenic temperature for the first time. This thesis manifests that, valley Zeeman splitting behavior in 2D group-VI TMDs can be tuned not only by the different substrates, but also by the doping levels in such 2D group-VI TMDs. These fundamental studies enable us to step further towards the future valleytronics.Doctor of Philosoph

Spatial variations of valley splitting in monolayer transition metal dichalcogenide

In monolayer group‐VI transition metal dichalcogenides (TMDs), valley splitting features have received a lot of attention since it can be potentially utilized for information storing and processing. Among the known two‐dimensional (2D) TMDs, monolayer WSe2 or MoSe2 has been mostly selected for excitonic and valleytronic physics studies because of its sharp and well‐resolved excitonic spectral features. Meanwhile, their high optical quality leads to a tremendous desire for developing promising WSe2‐ and MoSe2‐based valleytronic devices. Toward this goal, exploring the uniformity of valley features crossing an entire piece of monolayer becomes necessary and critical. Here, we performed the systematic magneto‐photoluminescence mapping measurements on mechanically exfoliated monolayer WSe2 and observed unconventional spatial variations of valley splitting. The observed nonuniformity is attributed to the modulated doping, which is probably due to the different distributions of unintentional absorbates across the sample. Such an unexpected doping effect shows the nonnegligible influence on the valley Zeeman splitting of the trion emission (XT) while affecting that of the neutral exciton emission (X0) trivially, evidencing for the large valleytronic sensitivity of the charged exciton. This work not only enriches the understanding of the doping effect on valley splitting but also is meaningful for developing 2D valleytronics.Ministry of Education (MOE)Published versionNational Natural Science Foundation of China, Grant/Award Numbers: 61774040, 11774170; Singapore Ministry of Education (MOE) Tier 1, Grant/Award Number: RG199/17; The Fudan University-CIOMP Joint Fund, Grant/Award Number: FC2018-002; The National Key R&D Program of China, Grant/Award Number: 2018YFA0703700; The National Young 1000 Talent Plan of China; The Shanghai Municipal Natural Science Foundation, Grant/Award Number: 16ZR1402500; The Shanghai Municipal Science and Technology Commission, Grant/Award Number: 18JC1410300

Progressively Exposing Active Facets of 2D Nanosheets toward Enhanced Pseudocapacitive Response and High‐Rate Sodium Storage

Sodium-ion batteries are gradually regarded as a prospective alternative to lithium-ion batteries due to the cost consideration. Here, three kinds of tin (IV) sulfide nanosheets are controllably designed with progressively exposed active facets, leading to beneficial influences on the Na+ storage kinetics, resulting in gradient improvements of pseudocapacitive response and rate performance. Interestingly, different forms of kinetics results are generated accompanying with the morphology and structure evolution of the three nanosheets. Finally, detailed density functional theory simulations are also applied to analyze the above experimental achievements, proving that different exposed facets of crystalline anodes possess dissimilar Na+ storage kinetics. The investigation experiences and conclusions shown in this work are meaningful to explore many other proper structure design routes toward the high-rate and stable metal-ions storage.Ministry of Education (MOE)Accepted versionThis work was mainly supported by MoE Tier 1 (Grant No. RG19/17). W.H. thanks the supports by the National Basic Research Program of China-Fundamental Studies of Perovskite Solar Cells (Grant No. 2015CB932200), Natural Science Foundation of Jiangsu Province (Grant No. BM2012010), Priority Academic Program Development of Jiangsu Higher Education Institutions (Grant No. YX03001), Ministry of Education of China (Grant No. IRT1148), Synergetic Innovation Center for Organic Electronics and Information Displays, and the National Natural Science Foundation of China (Grant Nos. 61136003 and 51173081). H.Z. acknowledges the financial support from NTU under Start-Up Grant (Grant No. M4081296.070.500000), Joint Research Fund for Overseas Chinese, Hong Kong and Macao Scholars (Grant No. 51528201)

High-rate, long cycle-life Li-ion battery anodes enabled by ultrasmall tin-based nanoparticles encapsulation

Tin (Sn)-based materials are potential alternatives to the commercial graphite anode for next generation Li-ion batteries, but their successful application is always impeded by fast capacity fading upon cycling that stemmed from huge volume variations during lithiation and delithiation. We develop an applicable strategy of encapsulating sub-10-nm-sized Sn-based nanoparticles (i.e., Sn and SnO2) in nitrogen/phosphorus codoped hierarchically porous carbon (NPHPC) or NPHPC-reduced graphene oxide hybrid (NPHPC-G) to effectively solve the issues of Sn-based anodes. Benefiting from the peculiar structure, the composites exhibit unprecedented electrochemical behaviors, for example, NPHPC-G@Sn and NPHPC-G@SnO2 deliver a high reversible capacity of ~1158 and ~1366 mAh g-1 at 200 mA g-1, respectively, and maintain at ~1099 mAh g-1 after 500 cycles and ~1117 mAh g-1 after 300 cycles. In situ transmission electron microscopy and ex situ scanning electron microscopy observations unveil that these composites are able to withstand the volume changes of Sn-based nanoparticles while sustaining the framework of the architectures and hence conferring outstanding electrochemical properties. Our present work provides both in situ and ex situ techniques for understanding the so-called synergistic effect between metals or metal oxides and carbons, which may offer rational guidance to design carbon-based functional materials for energy storage.MOE (Min. of Education, S’pore

Evolution of disposable bamboo chopsticks into uniform carbon fibers : a smart strategy to fabricate sustainable anodes for Li-ion batteries

Future development of mini consumer electronics or large electric vehicles/power grids requires Li-ion batteries (LIBs) with not only an outstanding energy-storage performance but also a minimum cost, and the foremost sustainability. Herein, we put forward a smart strategy to convert used disposable bamboo chopsticks into uniform carbon fibers for anodes of LIBs. Bamboo chopsticks waste is recycled and simply treated by a controllable hydrothermal process performed in alkaline solutions, wherein abundant natural cellulose fibers in bamboo in situ get separated and dispersed spontaneously. After carbonization, the evolved carbon fibers exhibit superior anodic performance to the bulky bamboo carbons counterpart, and competitive electrochemical behavior and cost with commercial graphite. The performance of carbon fibers can be further upgraded by growing nanostructured metal oxides (like MnO2) firmly on each fiber scaffold to form a synergetic core–shell electrode architecture. A high reversible capacity of [similar]710 mA h g−1 is maintained without decay up to 300 cycles. Our strategy presents a scalable route to transform chopsticks waste into carbon fibers, offering a very promising way to make sustainable anodes for LIBs and economical multi-functional carbon-based hybrids available for other practical applications.Published versio

Continuous-Wave Vertical Cavity Surface-Emitting Lasers based on Single Crystalline Lead Halide Perovskites

Lead halide perovskites are intriguing semiconductors for lasers due to high quantum yield, tunable bandgaps and facile solution-process ability. However, limited by the weak optical confinement, continuous-wave (CW) pumped lasing, as one prerequisite for the electrically pumped lasing, is still challenging in bare lead halide perovskites without high-quality factor (Q) artificial optical cavity. Herein, we report the lasing emission in methylammonium lead tribromide (MAPbBr3) incorporated with a vertical microcavity under continuous pumping at 80 K. The single-crystalline MAPbBr3 perovskite nanoplates were fabricated by the two-step solution method. The MAPbBr3 based vertical cavity surface emitting laser (VCSEL) presents a low threshold of 55.2 W cm-2 and a high Q-factor of 1140 at low temperature. The low threshold lasing emission can be attributed to strong optical confinement in the high-Q cavity and great PL enhancement at 80 K, which is induced by a transition from tetragonal to orthorhombic phase, demonstrated by in-situ temperature Raman spectroscopy. These findings envisage the prospective applications of single-crystalline metal halide perovskites in practicable laser devices.Ministry of Education (MOE)National Research Foundation (NRF

Revealing electronic nature of broad bound exciton bands in two-dimensional semiconducting WS2 and MoS2

Owing to unique electronic, excitonic, and valleytronic properties, atomically thin transition metal dichalcogenides are becoming a promising two-dimensional (2D) semiconductor system for diverse electronic and optoelectronic applications. In an ideal 2D semiconductor, efficient carrier transport is very difficult because of lacking free charge carriers. Doping is necessary for electrically driven device applications based on such 2D semiconductors, which requires investigation of electronic structure changes induced by dopants. Therefore probing correlations between localized electronic states and doping is important. Here, we address the electronic nature of broad bound exciton bands and their origins in exfoliated monolayer (1L) WS2 and MoS2 through monitoring low-temperature photoluminescence and manipulating electrostatic doping. The dominant bound excitons in 1L WS2 vary from donor to acceptor bound excitons with the switching from n- to p-type doping. In 1L MoS2, two localized emission bands appear which are assigned to neutral and ionized donor bound excitons, respectively. The deep donor and acceptor states play critical roles in the observed bound exciton bands, indicating the presence of strongly localized excitons in such 2D semiconductors.MOE (Min. of Education, S’pore)Published versio

Intrinsic excitonic emission and valley Zeeman splitting in epitaxial MS2 (M = Mo and W) monolayers on hexagonal boron nitride

Two-dimensional (2D) semiconductors, represented by 2D transition metal dichalcogenides (TMDs), exhibit rich valley physics due to strong spin-orbit/spin-valley coupling. The most common way to probe such 2D systems is to utilize optical methods, which can monitor light emissions from various excitonic states and further help in understanding the physics behind such phenomena. Therefore, 2D TMDs with good optical quality are in great demand. Here, we report a method to directly grow epitaxial WS2 and MoS2 monolayers on hexagonal boron nitride (hBN) flakes with a high yield and high optical quality; these monolayers show better intrinsic light emission features than exfoliated monolayers from natural crystals. For the first time, the valley Zeeman splitting of WS2 and MoS2 monolayers on hBN has been visualized and systematically investigated. This study paves a new way to produce high optical quality WS2 and MoS2 monolayers and to exploit their intrinsic properties in a multitude of applications.MOE (Min. of Education, S’pore

Engineering valley polarization of monolayer WS2 : a physical doping approach

The emerging field of valleytronics has boosted intensive interests in investigating and controlling valley polarized light emission of monolayer transition metal dichalcogenides (1L TMDs). However, so far, the effective control of valley polarization degree in monolayer TMDs semiconductors is mostly achieved at liquid helium cryogenic temperature (4.2 K), with the requirements of high magnetic field and on‐resonance laser, which are of high cost and unwelcome for applications. To overcome this obstacle, it is depicted that by electrostatic and optical doping, even at temperatures far above liquid helium cryogenic temperature (80 K) and under off‐resonance laser excitation, a competitive valley polarization degree of monolayer WS2 can be achieved (more than threefold enhancement). The enhanced polarization is understood by a general doping dependent valley relaxation mechanism, which agrees well with the unified theory of carrier screening effects on intervalley scattering process. These results demonstrate that the tunability corresponds to an effective magnet field of ≈10 T at 4.2 K. This work not only serves as a reference to future valleytronic studies based on monolayer TMDs with various external or native carrier densities, but also provides an alternative approach toward enhanced polarization degree, which denotes an essential step toward practical valleytronic applications.Ministry of Education (MOE)Accepted versionThis work was supported by the National Natural Science Foundation of China (Nos. 61774040, 11774170), the National Young 1000 Talent Plan of China, the Shanghai Municipal Natural Science Foundation (No. 16ZR1402500), the Opening project of State Key Laboratory of Functional Materials for Informatics (Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences), Singapore Ministry of Education (MOE) Tier 1 RG199/17. S. F. thanks Prof. Yuhei Miyauchi and Prof. Hanan Dery for fruitful discussion about potential mechanism of doping induced carrier screening effect on intervalley scattering