Controlled Synthesis of Organic/Inorganic van der Waals Solid for Tunable Light-matter Interactions

Van der Waals (vdW) solids, as a new type of artificial materials that consist of alternating layers bonded by weak interactions, have shed light on fascinating optoelectronic device concepts. As a result, a large variety of vdW devices have been engineered via layer-by-layer stacking of two-dimensional materials, although shadowed by the difficulties of fabrication. Alternatively, direct growth of vdW solids has proven as a scalable and swift way, highlighted by the successful synthesis of graphene/h-BN and transition metal dichalcogenides (TMDs) vertical heterostructures from controlled vapor deposition. Here, we realize high-quality organic and inorganic vdW solids, using methylammonium lead halide (CH3NH3PbI3) as the organic part (organic perovskite) and 2D inorganic monolayers as counterparts. By stacking on various 2D monolayers, the vdW solids behave dramatically different in light emission. Our studies demonstrate that h-BN monolayer is a great complement to organic perovskite for preserving its original optical properties. As a result, organic/h-BN vdW solid arrays are patterned for red light emitting. This work paves the way for designing unprecedented vdW solids with great potential for a wide spectrum of applications in optoelectronics

Optoelectronic properties of atomically thin ReSSe with weak interlayer coupling

Rhenium dichalcogenides, such as ReS2 and ReSe2, have attracted a lot of interests due to the weak interlayered coupling in these materials. Studies of rhenium based dichalcogenide alloys will help us understand the differences between binary rhenium dichalcogenides. They will also extend the applications of two-dimensional (2D) materials through alloying. In this work, we studied the optoelectronic properties of ReSSe with a S and Se ratio of 1 : 1. The band gap of the ReSSe alloy is investigated by optical absorption spectra as well as theoretical calculations. The alloy shows weak interlayered coupling, as evidenced by the Raman spectrum. A field-effect transistor based on ReSSe shows typical n-type behavior with a mobility of about 3 cm2 V-1 s-1 and an on/off ratio of 105, together with the in-plane anisotropic conductivity. The device also shows good photoresponse properties, with a photoresponsivity of 8 A W-1. The results demonstrated here will provide new avenues for the study of 2D materials with weak interlayer interactions and in-plane anisotropy

Multidifferential study of identified charged hadron distributions in ZZ-tagged jets in proton-proton collisions at s=\sqrt{s}=13 TeV

Jet fragmentation functions are measured for the first time in proton-proton collisions for charged pions, kaons, and protons within jets recoiling against a $Z$ boson. The charged-hadron distributions are studied longitudinally and transversely to the jet direction for jets with transverse momentum 20 $< p_{\textrm{T}} < 100$ GeV and in the pseudorapidity range $2.5 < \eta < 4$. The data sample was collected with the LHCb experiment at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 1.64 fb$^{-1}$. Triple differential distributions as a function of the hadron longitudinal momentum fraction, hadron transverse momentum, and jet transverse momentum are also measured for the first time. This helps constrain transverse-momentum-dependent fragmentation functions. Differences in the shapes and magnitudes of the measured distributions for the different hadron species provide insights into the hadronization process for jets predominantly initiated by light quarks.Comment: All figures and tables, along with machine-readable versions and any supplementary material and additional information, are available at https://cern.ch/lhcbproject/Publications/p/LHCb-PAPER-2022-013.html (LHCb public pages

Study of the B−→Λc+Λˉc−K−B^{-} \to \Lambda_{c}^{+} \bar{\Lambda}_{c}^{-} K^{-} decay

The decay $B^{-} \to \Lambda_{c}^{+} \bar{\Lambda}_{c}^{-} K^{-}$ is studied in proton-proton collisions at a center-of-mass energy of $\sqrt{s}=13$ TeV using data corresponding to an integrated luminosity of 5 $\mathrm{fb}^{-1}$ collected by the LHCb experiment. In the $\Lambda_{c}^+ K^{-}$ system, the $\Xi_{c}(2930)^{0}$ state observed at the BaBar and Belle experiments is resolved into two narrower states, $\Xi_{c}(2923)^{0}$ and $\Xi_{c}(2939)^{0}$, whose masses and widths are measured to be $$m(\Xi_{c}(2923)^{0}) = 2924.5 \pm 0.4 \pm 1.1 \,\mathrm{MeV}, \\ m(\Xi_{c}(2939)^{0}) = 2938.5 \pm 0.9 \pm 2.3 \,\mathrm{MeV}, \\ \Gamma(\Xi_{c}(2923)^{0}) = \phantom{000}4.8 \pm 0.9 \pm 1.5 \,\mathrm{MeV},\\ \Gamma(\Xi_{c}(2939)^{0}) = \phantom{00}11.0 \pm 1.9 \pm 7.5 \,\mathrm{MeV},$$ where the first uncertainties are statistical and the second systematic. The results are consistent with a previous LHCb measurement using a prompt $\Lambda_{c}^{+} K^{-}$ sample. Evidence of a new $\Xi_{c}(2880)^{0}$ state is found with a local significance of $3.8\,\sigma$, whose mass and width are measured to be $2881.8 \pm 3.1 \pm 8.5\,\mathrm{MeV}$ and $12.4 \pm 5.3 \pm 5.8 \,\mathrm{MeV}$, respectively. In addition, evidence of a new decay mode $\Xi_{c}(2790)^{0} \to \Lambda_{c}^{+} K^{-}$ is found with a significance of $3.7\,\sigma$. The relative branching fraction of $B^{-} \to \Lambda_{c}^{+} \bar{\Lambda}_{c}^{-} K^{-}$ with respect to the $B^{-} \to D^{+} D^{-} K^{-}$ decay is measured to be $2.36 \pm 0.11 \pm 0.22 \pm 0.25$, where the first uncertainty is statistical, the second systematic and the third originates from the branching fractions of charm hadron decays.Comment: All figures and tables, along with any supplementary material and additional information, are available at https://cern.ch/lhcbproject/Publications/p/LHCb-PAPER-2022-028.html (LHCb public pages

Measurement of the ratios of branching fractions R(D∗)\mathcal{R}(D^{*}) and R(D0)\mathcal{R}(D^{0})

The ratios of branching fractions $\mathcal{R}(D^{*})\equiv\mathcal{B}(\bar{B}\to D^{*}\tau^{-}\bar{\nu}_{\tau})/\mathcal{B}(\bar{B}\to D^{*}\mu^{-}\bar{\nu}_{\mu})$ and $\mathcal{R}(D^{0})\equiv\mathcal{B}(B^{-}\to D^{0}\tau^{-}\bar{\nu}_{\tau})/\mathcal{B}(B^{-}\to D^{0}\mu^{-}\bar{\nu}_{\mu})$ are measured, assuming isospin symmetry, using a sample of proton-proton collision data corresponding to 3.0 fb${ }^{-1}$ of integrated luminosity recorded by the LHCb experiment during 2011 and 2012. The tau lepton is identified in the decay mode $\tau^{-}\to\mu^{-}\nu_{\tau}\bar{\nu}_{\mu}$. The measured values are $\mathcal{R}(D^{*})=0.281\pm0.018\pm0.024$ and $\mathcal{R}(D^{0})=0.441\pm0.060\pm0.066$, where the first uncertainty is statistical and the second is systematic. The correlation between these measurements is $\rho=-0.43$. Results are consistent with the current average of these quantities and are at a combined 1.9 standard deviations from the predictions based on lepton flavor universality in the Standard Model.Comment: All figures and tables, along with any supplementary material and additional information, are available at https://cern.ch/lhcbproject/Publications/p/LHCb-PAPER-2022-039.html (LHCb public pages

Synthesis and device applications of atom-thin semiconducting chalcogenides

The ongoing scaling of silicon-based metal-oxide-semiconductor field-effect transistor is hindered by the serious short channel effect in sub-10 nm channel length. The discovery of graphene raised the booming development of 2D materials which have the merits of high mobility, efficient current regulation and high level of integration. However, the performance of 2D materials in electronic and optoelectronic applications still need to be improved, which is the aim, such as high mobility, high sensitivity and high gain, of this thesis. Firstly, the aim of the first work is to improve the FET performance of MoS2 via contact engineering of metal/semiconductor (M/S) vdW heterostructure with an atomically sharp interface prepared by CVD method. M/S vdW contact has shown great advantages, such as good stability, larger size and clean interface, in the improvement of M/S contact. In the CVD synthesis, M/S TMDC vdW heterostructures were less explored due to the high melting points and low chemical reactivity of metal oxide feedstocks and the crystal lattice mismatch. In this work, for the first time, the M/S TMDC vdW heterostructure NbS2/MoS2 was synthesized via a one-step halide-assisted CVD method. OM, SEM and XPS were used to identify the M/S vdW heterostructure. This method provides high crystal quality and clean interface of the heterostructure, confirmed by STEM characterization. A growth mechanism was proposed that MoS2 finished the growth first and subsequently served as a superior substrate for the growth of NbS2, verified by Raman mapping. The PL and device measurement provided physical supports of the M/S vdW heterostructure. Secondly, the aim of the second work is to achieve high photodetector performance of Bi2O2Se via transferring high-quality Bi2O2Se onto 280 nm SiO2/Si substrate (Si substrate) by a non-corrosive method. In the chasing of high-mobility 2D semiconductors, competitive candidate CVD-grown Bi2O2Se has superior stability and high mobility, which surpass that of MoS2. However, the photodetector performance of Bi2O2Se regarding on/off ratio and specific detectivity was limited by the large dark current. Compared with f-mica substrate, Si substrate possesses inherent back gating, which could be used to reduce the dark current. However, the hazardous and corrosive HF will etch Bi2O2Se during the transfer, which has been verified by OM, Raman and STEM characterizations. Therefore, a non-corrosive PS-assisted method was developed. The as-transferred Bi2O2Se retained a high quality and showed ultrasensitive phototransistors performance, including high responsivity of 3.5×104 AW−1, high photoconductive gain of 8×104, and fast response rate of sub-millisecond. On Si substrate, the dark current could be reduced to several pA via back gating, which yields an ultrahigh specific detectivity of 9.0×1013 Jones. This is one of the highest values among 2D material photodetectors and two orders of magnitude higher than that of other CVD-grown 2D materials. Thirdly, the aim of this work is to improve the performance and stability of the CMOS inverters based on 2D semiconductors. Although many 2D semiconductors have shown high CMOS inverter performance, such as MoS2, WSe2, MoTe2, their stability is not satisfactory. In this work, CMOS inverters based on high-mobility and stable n-type Bi2O2Se were demonstrated with clear signal inversion on both f-mica and silicon substrate. On f-mica substrate, under the gating of ion liquid, a gain of 3.5 can be obtained at the supply voltage of only 1.0 V. Dynamic voltage response at the frequency of 100 Hz shows good response with almost no voltage lose. On silicon substrate, the CMOS inverter showed a high gain of 2.2 at the supply voltage of 5.0 V, a high noise margin of 5.2 VDD and a low power consumption at the order of 10-9 W.Doctor of Philosoph

硅基底上二维硒氧化铋的化学气相沉积法 合成及其光电探测应用 = Synthesis of two-dimensional Bi2O2Se on silicon substrate by chemical vapor deposition and its photoelectric detection application

半导体加工工艺微缩过程中, 硅基材料的短沟道效应带来的低能效促使研究人员寻找新型半导体替代材料. 二维半导体因其原子级别的厚度以及范德瓦耳斯表面而倍受关注, 二维硒氧化铋就是其中迁移率、稳定性以及成本各方面较为均衡的一种. 然而, 其制备受到基底很严格的限制, 导致器件加工难度较大. 本文利用化学气相沉积法直接在硅片基底上合成出规格为25 µm×51.0 nm(厚度)的二维硒氧化铋, 并通过拉曼光谱、原子力显微镜、扫描电子显微镜、X射线能谱对其进行表征. 同时, 通过场效应晶体管输运的测试得出其迁移率为80.0 cm2/(V·s)以及光电探测得出其具有2.45×104 A/W的光响应度和6×104的光增益等比较出色的表现. 但由于厚度较大, 导致其场效应管开关比低(2500)以及不高的光电探测灵敏度(5×1010 Jones). 由此可知, 硅片基底虽然带来器件加工上的便利性, 但有待进一步优化生长, 并集成更多种材料的应用. As the scaling-down of semiconductor processing technology goes on, it is urgent to find the successor of silicon-based materials since the severe short channel effect lowers down their energy efficiency as logic devices. Owing to its atomic thickness and van der Waals surface, two-dimensional semiconductors have received huge attention in this area, among which Bi2O2Se has achieved a good trade-off among the carrier mobility, stability and costing. However, the synthesis of Bi2O2Se need some polarized substrates, which hinders its processing and application. Here, a Bi2O2Se layer with 25 µm in size and 51.0 nm in thickness is directly synthesized on a silicon substrate via chemical vapor deposition. A Field-effect transistor with a carrier mobility of 80.0 cm2/(V·s) and phototransistor with a photoresponsivity of 2.45×104 A/W and a photogain of 6×104 is also demonstrated, which hpossesses quite outstanding photodetection performance. Nevertheless, the high dark current and low on/off ratio brought by the large thickness leads to a fair detectivity (5×1010 Jones). All in all,, although silicon substrate brings convenience in device fabricating, it is still needed to further optimizing the growth and integrating more applications of various two-dimensional materials.Published versio

Growth and thermoelectric properties of few-layered Bismuth Oxyselenide

International audienceThermoelectric (TE) materials are being explored as a renewable energy which is vital in preserving earth as it can serve as a transducer to transfer the heat absorbed to electricity and vice versa. The performance of a TE material is governed by a dimensionless figure of merit (ZT = S2σ/κ) where S is the Seebeck coefficient, σ is the electrical conductivity and κ is the thermal conductivity. To obtain a high ZT, the TE material must exhibit high S and σ with low κ. Strategies such as external doping to increase carrier concentration to achieve higher σ is ineffective in enhancing ZT as both κ and σ are positively correlated by Wiedemann-Franz law while S is negatively correlated to them[1]. The strong interdependencies of the parameters constituting ZT is one of the main challenges to optimize the TE performance of a material. Two-dimension (2D) nanomaterials have been predicted and demonstrated to possess superior TE properties as compared to their 3D counterpart due to quantum confinement effect. In addition, the use of low-dimensional material enables the modulation of the carrier concentration and bandgap of material through field-effect and strain, respectively. Bismuth Oxyselenide (Bi2O2Se) is an emerging 2D ferroelectric material which has attracted much research interest due to its excellent properties such as low thermal conductivity, ultrahigh mobility, Rashba spin-splitting, as well as good air stability[1-4]. Such characteristics in a nano material are required to be an outstanding candidate for thermoelectricity when it comes to clean energy or wearable devices[5]. In this report, we present the Low-Pressure Chemical Vapor Deposition (LPCVD) growth of high quality few-layered Bi2O2Se as well as its high Seebeck coefficient and electrical conductivity measured through the fan-out electrode test structure

Recent progress in the synthesis of novel two-dimensional van der Waals materials

The last decade has witnessed the significant progress of physical fundamental research and great success of practical application in two-dimensional (2D) van der Waals (vdW) materials since the discovery of graphene in 2004. To date, vdW materials is still a vibrant and fast-expanding field, where tremendous reports have been published covering topics from cutting-edge quantum technology to urgent green energy, and so on. Here, we briefly review the emerging hot physical topics and intriguing materials, such as 2D topological materials, piezoelectric materials, ferroelectric materials, magnetic materials and twistronic heterostructures. Then, various vdW material synthetic strategies are discussed in detail, concerning the growth mechanisms, preparation conditions and typical examples. Finally, prospects and further opportunities in the booming field of 2D materials are addressed.Ministry of Education (MOE)National Research Foundation (NRF)Published versionThis work was supported by the National Research Foundation– Competitive Research Program (NRF-CRP21-2019-0007 and NRF-CRP21-2018-0007); the Singapore Ministry of Education Tier 3 Programme ‘Geometrical Quantum Materials’ (MOE2018-T3-1-002), AcRF Tier 2 (2016-T2-2-153) and AcRF Tier 1 (RG7/18 and RG161/19); the National Key R&D Program of China (2020YFA0309200), the National Natural Science Foundation of China (62074025); the Applied Basic Research Program of Sichuan Province (2020ZYD014 and 2021JDGD0026); the Sichuan Province Key Laboratory of Display Science and Technology and the Postdoctoral Innovative Talent Supporting Program (BX20190060)

Synergistic gating of electro‐iono‐photoactive 2D chalcogenide neuristors : coexistence of Hebbian and homeostatic synaptic metaplasticity

Emulation of brain-like signal processing with thin-film devices could lay the foundation for building artificially intelligent learning circuitry in future. Encompassing higher functionalities into single artificial neural elements will allow the development of robust neuromorphic circuitry emulating biological adaptation mechanisms with drastically lesser neural elements, mitigating strict process challenges and high circuit density requirements necessary to match the computational complexity of the human brain. Here, 2D transition metal di-chalcogenide (TMDC) (MoS2) neuristors are designed to mimic intracellular ion endocytosis-exocytosis dynamics / neurotransmitter-release in chemical synapses using three approaches: (i) electronic-mode: a defect modulation approach where the traps at the semiconductor-dielectric interface are perturbed, (ii) ionotronic-mode: where electronic responses are modulated via ionic gating and (iii) photoactive-mode: harnessing persistent photoconductivity or trap-assisted slow recombination mechanisms. Exploiting a novel multi-gated architecture incorporating electrical and optical biases, this incarnation not only addresses different charge-trapping probabilities to finely modulate the synaptic weights, but also amalgamates neuromodulation schemes to achieve “plasticity of plasticity-metaplasticity” via dynamic control of Hebbian spike-time dependent plasticity and homeostatic regulation. Co-existence of such multiple forms of synaptic plasticity increases the efficacy of memory storage and processing capacity of artificial neuristors, enabling design of highly efficient novel neural architectures.NRF (Natl Research Foundation, S’pore)MOE (Min. of Education, S’pore)Accepted versio