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Dielectric catastrophe at the Mott and Wigner transitions in a moir'e superlattice. (arXiv:2201.12510v1 [cond-mat.mtrl-sci])

2022-02-01T04:30:36+00:00February 1st, 2022|Categories: Publications|Tags: , , |

The metal-insulator transition (MIT) driven by electronic correlations is a fundamental and challenging problem in condensed-matter physics. Particularly, whether such a transition can be continuous remains open. The emergence of semiconducting moir’e materials with continuously tunable bandwidth provides an ideal platform to study interaction-driven MITs. Although a bandwidth-tuned MIT at fixed full electron filling of the moir’e superlattice has been reported recently, that at fractional filling, which involves translational symmetry breaking of the underlying superlattice, remains elusive. Here, we demonstrate bandwidth-tuned MITs in a MoSe2/WS2 moir’e superlattice at both integer and fractional fillings using the exciton sensing technique. The bandwidth is controlled by an out-of-plane electric field. The dielectric response is probed optically with the 2s exciton in a remote WSe2 sensor layer. The exciton spectral weight is negligible for the metallic state, consistent with a large negative dielectric constant. It continuously vanishes when the transition is approached from the insulating side, corresponding to a diverging dielectric constant or a “dielectric catastrophe”. Our results support continuous interaction-driven MITs in a two-dimensional triangular lattice and stimulate future explorations of exotic quantum phases, such as quantum spin liquids, in their vicinities.

Published : "arXiv Mesoscale and Nanoscale Physics".

Hybridization and localized flat band in the WSe2/MoSe2 heterobilayer grown by molecular beam epitaxy. (arXiv:2201.03322v1 [cond-mat.mtrl-sci])

2022-01-11T02:29:47+00:00January 11th, 2022|Categories: Publications|Tags: , , |

Nearly localized moire flat bands in momentum space, arising at particular twist angles, are the key to achieve correlated effects in transition-metal dichalcogenides. Here, we use angle-resolved photoemission spectroscopy (ARPES) to visualize the presence of a flat band near the Fermi level of van der Waals (vdW) WSe2/MoSe2 heterobilayer grown by molecular beam epitaxy. This flat band is localized near the K point of the Brillouin zone and has a width of several hundred meVs. By combining ARPES measurements with density functional theory (DFT) calculations, we confirm the coexistence of different domains, namely the reference 2H stacking without layer misorientation and regions with arbitrary twist angles. For the 2H-stacked heterobilayer, our ARPES results show strong interlayer hybridization effects, further confirmed by complementary micro- Raman spectroscopy measurements. The spin-splitting of the valence band at K is determined to be 470 meV. The valence band maximum (VBM) position of the heterobilayer is located at the Gamma point. The energy difference between the VBM at Gamma and the K point is of -60 meV, which is a stark difference compared to individual 1L WSe2 and 1L WSe2, showing both a VBM at K.

Published in: "arXiv Material Science".

Direct STM Measurements of R- and H-type Twisted MoSe2/WSe2 Heterostructures. (arXiv:2201.02166v1 [cond-mat.mtrl-sci])

2022-01-07T02:29:38+00:00January 7th, 2022|Categories: Publications|Tags: , , |

When semiconducting transition metal dichalcogenides heterostructures are stacked the twist angle and lattice mismatch leads to a periodic moir’e potential. As the angle between the layers changes, so do the electronic properties. As the angle approaches 0- or 60-degrees interesting characteristics and properties such as modulations in the band edges, flat bands, and confinement are predicted to occur. Here we report scanning tunneling microscopy and spectroscopy measurements on the band gaps and band modulations in MoSe2/WSe2 heterostructures with near 0 degree rotation (R-type) and near 60 degree rotation (H-type). We find a modulation of the band gap for both stacking configurations with a larger modulation for R type than for H-type as predicted by theory. Furthermore, local density of states images show that electrons are localized differently at the valence band and conduction band edges.

Published in: "arXiv Material Science".

Lifetime of Excitons in Janus Monolayer MoSSe Prepared from Exfoliated MoSe_2. (arXiv:2112.11211v1 [cond-mat.mtrl-sci])

2021-12-22T02:29:33+00:00December 22nd, 2021|Categories: Publications|Tags: , |

Janus monolayer transition metal dichalcogenides, where one of the two chalcogen layers is substituted with a different kind of chalcogen atoms, are pushing the properties of two dimensional materials into new territories. Yet only little is known about this new kind of material class, mainly due to the challenging synthesis. In this work we propose a method to prepare high quality Janus MoSSe monolayers from as-exfoliated MoSe2 by thermal sulfurization. With this we aim to pave a way for more exotic Janus monolayers, which have been out of the experimental reach thus far. The synthesized MoSSe is diligently characterized by room- and low-temperature Raman and photoluminescence spectroscopy, atomic force microscopy correlated with Raman mappings, and time-correlated single-photon counting. The latter providing new information on the lifetime of excitons in Janus MoSSe monolayers. In addition, we report an enhanced trion formation at low temperatures and a relatively high excitonic transition energy that is indicative of less defect states and strain, and therefore a high sample quality.

Published in: "arXiv Material Science".

Hole- and electron-injection driven phase transitions in transition metal dichalcogenides and beyond: A unified understanding. (arXiv:2112.02319v1 [cond-mat.mtrl-sci])

2021-12-07T05:29:22+00:00December 7th, 2021|Categories: Publications|Tags: , , |

The phase transitions among polymorphic two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted increasing attention for their potential in enabling distinct functionalities in the same material for making integrated devices. Electron-injection to TMDs has been proved to be a feasible way to drive structural phase transition from the semiconducting H-phase to the semimetal dT-phase. In this contribution, based on density-functional theory (DFT) calculations, firstly we demonstrate that hole-injection drives the transition of the H-phase more efficiently to the metallic T-phase than to the semimetallic dT-phase for group VI-B TMDs (MoS2, WS2, and MoSe2, etc.). The origin can be attributed to the smaller work function of the T-phase than that of the dT-phase. Our work function analysis can distinguish the T and dT phases quantitatively while it is challenging for the commonly used crystal field splitting analysis. In addition, our analysis provides a unified understanding for both hole- and electron-injection induced phase transitions for 2D materials beyond TMDs, such as the newly synthesized MoSi2N4 family. Moreover, the hole-driven T-phase transition mechanism can explain the recent experiment of WS2 phase transition by hole-doping with yttrium (Y) atoms. Using 1/3 Y-doped WS2 and MoSe2 as examples, we show that the Mo and W valency increases to 5+. These above findings open up an avenue to obtain the metallic T-phase, which expands the possible stable phases of 2D materials.

Published in: "arXiv Material Science".

Substitutionally Doped MoSe2 for High‐Performance Electronics and Optoelectronics

2021-11-26T13:23:02+00:00November 26th, 2021|Categories: Publications|Tags: , |

This paper introduces Ta-doped MoSe2 via substitutional doping. The transfer characteristic of the synthesized MoSe2 can be controllably switched from n-type to ambipolar, and to p-type. Based on the Ta-doped MoSe2, outstanding homojunction photodetectors (high external quantum efficiency ≈42% and fast response speed ≈20 µs) and inverters (high voltage gain ≈34) are demonstrated. Abstract 2D materials, of which the carrier type and concentration are easily tuned, show tremendous superiority in electronic and optoelectronic applications. However, the achievements are still quite far away from practical applications. Much more effort should be made to further improve their performance. Here, p-type MoSe2 is successfully achieved via substitutional doping of Ta atoms, which is confirmed experimentally and theoretically, and outstanding homojunction photodetectors and inverters are fabricated. MoSe2 p–n homojunction device with a low reverse current (300 pA) exhibits a high rectification ratio (104). The analysis of dark current reveals the domination of the Shockley–Read–Hall (SRH) and band-to-band tunneling (BTB) current. The homojunction photodetector exhibits a large open-circuit voltage (0.68 V) and short-circuit currents (1 µA), which is suitable for micro-solar cells. Furthermore, it possesses outstanding responsivity (0.28 A W−1), large external quantum efficiency (42%), and a high signal-to-noise ratio (≈107). Benefiting from the continuous energy band of homojunction, the response speed reaches up to 20 µs. Besides, the Ta-doped MoSe2 inverter exhibits a high voltage gain (34) and low power consumption (127 nW). This work lays a foundation for the practical application of 2D material devices.

Published in: "Small".

Edge‐Mediated Annihilation of Vacancy Clusters in Monolayer Molybdenum Diselenide (MoSe2) under Electron Beam Irradiation

2021-11-26T13:21:58+00:00November 26th, 2021|Categories: Publications|Tags: , |

Understanding defect dynamics in 2D materials is critical for tailoring their properties through defect engineering. Herein, annihilation of vacancy clusters in monolayer molybdenum diselenide under electron beam irradiation is reported and found to be energetically favorable, with electron beam and pre-stress facilitating the barrier-hopping. The findings expand the capability of electron beam for manipulating defects in 2D materials. Abstract Annihilation of vacancy clusters in monolayer molybdenum diselenide (MoSe2) under electron beam irradiation is reported. In situ high-resolution transmission electron microscopy observation reveals that the annihilation is achieved by diffusion of vacancies to the free edge near the vacancy clusters. Monte Carlo simulations confirm that it is energetically favorable for the vacancies to locate at the free edge. By computing the minimum energy path for the annihilation of one vacancy cluster as a case study, it is further shown that electron beam irradiation and pre-stress in the suspended MoSe2 monolayer are necessary for the vacancies to overcome the energy barriers for diffusion. The findings suggest a new mechanism of vacancy healing in 2D materials and broaden the capability of electron beam for defect engineering of 2D materials, a promising way of tuning their properties for engineering applications.

Published in: "Small".

Disorder of Excitons and Trions in Monolayer MoSe2. (arXiv:2111.09683v1 [cond-mat.mes-hall])

2021-11-19T04:30:32+00:00November 19th, 2021|Categories: Publications|Tags: , |

The optical spectra of transition metal dichalcogenide (TMDC) monolayers are dominated by excitons and trions. Here we establish the dependences of these optical transitions on disorder from hyperspectral imaging of h-BN encapsulated monolayer MoSe2. While both exciton and trion energies vary spatially, these two quantities are almost perfectly correlated, with spatial variation in the trion binding energy of only ~0.18 meV. In contrast, variation in the energy splitting between the two lowest energy exciton states is one order of magnitude larger at ~1.7 meV. Statistical analysis and theoretical modeling reveal that disorder results from dielectric and bandgap fluctuations, not electrostatic fluctuations. Our results shed light on disorder in high quality TMDC monolayers, its impact on optical transitions, and the many-body nature of excitons and trions.

Published : "arXiv Mesoscale and Nanoscale Physics".

Electron recoil effect in electrically tunable MoSe2 monolayers. (arXiv:2111.07676v1 [cond-mat.mes-hall])

2021-11-16T04:30:43+00:00November 16th, 2021|Categories: Publications|Tags: |

Radiative recombination of excitons dressed by the interactions with free charge carriers often occurs under simultaneous excitation of either electrons or holes to unbound states. This phenomenon, known as the electron recoil effect, manifests itself in pronounced, asymmetric spectral lineshapes of the resulting emission. We study the electron recoil effect experimentally in electrically-tunable monolayer semiconductors and derive it theoretically using both trion and Fermi-polaron pictures. Time-resolved analysis of the recoil lineshapes is employed to access transient, non-equilibrium states of the exciton-carrier complexes. We demonstrate cooling of the initially overheated populations on the picosecond timescales and reveal the impact of lattice temperature and free carrier density. Both thermally activated phonons and the presence of free charges are shown to accelerate equilibration. Finally, we find strong correlations between relaxation times from recoil analysis and luminescence rise times, providing a consistent interpretation for the initial dynamics of trion/Fermi-polaron states.

Published : "arXiv Mesoscale and Nanoscale Physics".

Spin dependent charge transfer in MoSe2/hBN/Ni hybrid structures. (arXiv:2110.13007v1 [cond-mat.mtrl-sci])

2021-10-26T02:29:32+00:00October 26th, 2021|Categories: Publications|Tags: , |

We present magneto-photoluminescence measurements in a hybrid 2D semiconductor/ferromagnetic structure consisting of MoSe2/hBN/Ni. When the Nickel layer is magnetized, we observe circularly polarized photoluminescence of the trion peak in MoSe2 monolayer under linearly polarized excitation. This build-up of circular polarization can reach a measured value of about 4% when the magnetization of Ni is saturated perpendicularly to the sample plane, and changes its sign when the magnetization is reversed. The circular polarization decreases when the hBN barrier thickness increases. These results are interpreted in terms of a spin-dependent charge transfer between the MoSe2 monolayer and the Nickel film. The build-up of circular polarization is observed up to 120 K, mainly limited by the trion emission that vanishes with temperature.

Published in: "arXiv Material Science".

Dominating Interlayer Resonant Energy Transfer in Type-II 2D Heterostructure. (arXiv:2110.03492v1 [cond-mat.mtrl-sci])

2021-10-08T02:30:12+00:00October 8th, 2021|Categories: Publications|Tags: , , , , |

Type-II heterostructures (HSs) are essential components of modern electronic and optoelectronic devices. Earlier studies have found that in type-II transition metal dichalcogenide (TMD) HSs, the dominating carrier relaxation pathway is the interlayer charge transfer (CT) mechanism. Here, this report shows that, in a type-II HS formed between monolayers of MoSe2 and ReS2, nonradiative energy transfer (ET) from higher to lower work function material (ReS2 to MoSe2) dominates over the traditional CT process with and without a charge-blocking interlayer. Without a charge-blocking interlayer, the HS area shows 3.6 times MoSe2 photoluminescence (PL) enhancement as compared to the MoSe2 area alone. After completely blocking the CT process, more than one order of magnitude higher MoSe2 PL emission was achieved from the HS area. This work reveals that the nature of this ET is truly a resonant effect by showing that in a similar type-II HS formed by ReS2 and WSe2, CT dominates over ET, resulting in a severely quenched WSe2 PL. This study not only provides significant insight into the competing interlayer processes, but also shows an innovative way to increase the PL quantum yield of the desired TMD material using ET process by carefully choosing the right material combination for HS.

Published in: "arXiv Material Science".

Chlorine doping of MoSe2 flakes by ion implantation. (arXiv:2109.11880v1 [cond-mat.mtrl-sci])

2021-09-27T02:29:17+00:00September 27th, 2021|Categories: Publications|Tags: , |

The efficient integration of transition metal dichalcogenides (TMDs) into the current electronic device technology requires mastering the techniques of effective tuning of their optoelectronic properties. Specifically, controllable doping is essential. For conventional bulk semiconductors, ion implantation is the most developed method offering stable and tunable doping. In this work, we demonstrate n-type doping in MoSe2 flakes realized by low-energy ion implantation of Cl+ ions followed by millisecond-range flash lamp annealing (FLA). We further show that FLA for 3 ms with a peak temperature of about 1000 {deg}C is enough to recrystallize implanted MoSe2. The Cl distribution in few-layer-thick MoSe2 is measured by secondary ion mass spectrometry. An increase in the electron concentration with increasing Cl fluence is determined from the softening and red shift of the Raman-active A_1g phonon mode due to the Fano effect. The electrical measurements confirm the n-type doping of Cl-implanted MoSe2. A comparison of the results of our density functional theory calculations and experimental temperature-dependent micro-Raman spectroscopy data indicates that Cl atoms are incorporated into the atomic network of MoSe2 as substitutional donor impurities.

Published in: "arXiv Material Science".

Sputtered 2D transition metal dichalcogenides: from growth to device applications. (arXiv:2109.07232v1 [cond-mat.mtrl-sci])

2021-09-16T02:29:19+00:00September 16th, 2021|Categories: Publications|Tags: , , , , |

Starting from graphene, 2D layered materials family has been recently set up more than 100 different materials with variety of different class of materials such as semiconductors, metals, semimetals, superconductors. Among these materials, 2D semiconductors have found especial importance in the state of the art device applications compared to that of the current conventional devices such as (which material based for example Si based) field effect transistors (FETs) and photodetectors during the last two decades. This high potential in solid state devices is mostly revealed by the transition metal dichalcogenides (TMDCs) semiconductor materials such as MoS2 , WS2 , MoSe2 and WSe2 . Therefore, many different methods and approaches have been developed to grow or obtain so far in order to make use them in solid state devices, which is a great challenge in large area applications. Although there are intensively studied methods such as chemical vapor deposition (CVD), mechanical exfoliation, atomic layer deposition, it is sputtering getting attention day by day due to the simplicity of the growth method together with its reliability, large area growth possibility and repeatability. In this review article, we provide benefits and disadvantages of all the growth methods when growing TMDC materials, then focusing on the sputtering TMDC growth strategies performed. In addition, TMDCs for the FETs and photodetector devices grown by RFMS have been surveyed.

Published in: "arXiv Material Science".

Heteroepitaxial growth of high optical quality, wafer-scale van der Waals heterostrucutres. (arXiv:2109.01916v1 [cond-mat.mtrl-sci])

2021-09-07T02:29:34+00:00September 7th, 2021|Categories: Publications|Tags: , , , |

Transition metal dichalcogenides (TMDs) are materials that can exhibit intriguing optical properties like a change of the bandgap from indirect to direct when being thinned down to a monolayer. Well-resolved narrow excitonic resonances can be observed for such monolayers, however only for materials of sufficient crystalline quality, so far mostly available in the form of micrometer-sized flakes. A further significant improvement of optical and electrical properties can be achieved by transferring the TMD on hexagonal boron nitride (hBN). To exploit the full potential of TMDs in future applications, epitaxial techniques have to be developed that not only allow to growlarge-scale, high-quality TMD monolayers, but allow to perform the growth directly on large-scale epitaxial hBN. In this work we address this problem and demonstrate that MoSe2 of high optical quality can be directly grown on epitaxial hBN on an entire two-inch wafer. We developed a combined growth theme for which hBN is first synthesized at high temperature by Metal Organic Vapor Phase Epitaxy (MOVPE) and as a second step MoSe2 is deposited on top by Molecular Beam Epitaxy (MBE) at much lower temperatures. We show that this structure exhibits excellent optical properties, manifested by narrow excitonic lines in the photoluminescence spectra. Moreover, the material is homogeneous on the area of the whole two-inch wafer, with only +/-0.14 meV deviation of excitonic energy. Our mixed growth technique may guide the way for future large-scale production of high quality TMD/hBN heterostructures.

Published in: "arXiv Material Science".

Substrate effect on thermal conductivity of monolayer WS2: Experimental measurement and theoretical analysis. (arXiv:2108.13252v1 [cond-mat.mes-hall])

2021-08-31T04:30:20+00:00August 31st, 2021|Categories: Publications|Tags: , , , |

Monolayer WS2 has been a competitive candidate in electrical and optoelectronic devices due to its superior optoelectronic properties. To tackle the challenge of thermal management caused by the decreased size and concentrated heat in modern ICs, it is of great significance to accurately characterize the thermal conductivity of the monolayer WS2, especially with substrate supported. In this work, the dual-wavelength flash Raman method is used to experimentally measure the thermal conductivity of the suspended and the Si/SiO2 substrate supported monolayer WS2 at a temperature range of 200 K – 400 K. The room-temperature thermal conductivity of suspended and supported WS2 are 28.45 W/mK and 15.39 W/mK, respectively, with a ~50% reduction due to substrate effect. To systematically study the underlying mechanism behind the striking reduction, we employed the Raman spatial mapping analysis combined with the molecular dynamics simulation. The analysis of Raman spectra showed the increase of doping level, reduction of phonon lifetime and suppression of out-of-plane vibration mode due to substrate effect. In addition, the phonon transmission coefficient was mutually verified with Raman spectra analysis and further revealed that the substrate effect significantly enhances the phonon scattering at the interface and mainly suppresses the acoustic phonon, thus leading to the reduction of thermal conductivity. The thermal conductivity of other suspended and supported monolayer TMDCs (e.g. MoS2, MoSe2 and WSe2) were also listed for comparison. Our researches can be extended to understand the substrate effect of other 2D TMDCs and provide guidance for future TMDCs-based electrical and optoelectronic devices.

Published : "arXiv Mesoscale and Nanoscale Physics".

Mixed-Salt Enhanced Chemical Vapor Deposition of Two-Dimensional Transition Metal Dichalcogenides. (arXiv:2108.11599v1 [cond-mat.mtrl-sci])

2021-08-27T02:29:22+00:00August 27th, 2021|Categories: Publications|Tags: , |

The usage of molten salts, e.g., Na2MoO4 and Na2WO4, has shown great success in the growth of two-dimensional (2D) transition metal dichalcogenides (TMDCs) by chemical vapor deposition (CVD). In comparison with the halide salt (i.e., NaCl, NaBr, KI)-assisted growth (Salt 1.0), the molten salt-assisted vapor-liquid-solid (VLS) growth technique (Salt 2.0) has improved the reproducibility, efficiency and scalability of synthesizing 2D TMDCs. However, the growth of large-area MoSe2 and WTe2 is still quite challenging with the use Salt 2.0 technique. In this study, a renewed Salt 2.0 technique using mixed salts (e.g., Na2MoO4-Na2SeO3 and Na2WO4-Na2TeO3) is developed for the enhanced CVD growth of 2D MoSe2 and WTe2 crystals with large grain size and yield. Continuous monolayer MoSe2 film with grain size of 100-250 {mu}m or isolated flakes up to ~ 450 {mu}m is grown on a halved 2-inch SiO2/Si wafer. Our study further confirms the synergistic effect of Na+ and SeO32- in the enhanced CVD growth of wafer-scale monolayer MoSe2 film. And thus, the addition of Na2SeO3 and Na2TeO3 into the transition metal salts could be a general strategy for the enhanced CVD growth of many other 2D selenides and tellurides.

Published in: "arXiv Material Science".

Identifying the Evolution of Se‐Vacancy‐Modulated MoSe2 Pre‐Catalyst in Li–S Chemistry

2021-08-26T13:07:20+00:00August 26th, 2021|Categories: Publications|Tags: , |

Witnessing compositional evolution and identifying actual catalytically-active moiety of electrocatalysts is of paramount importance in Li–S chemistry. Nevertheless, this field remains elusive by far. Herein, we report the scalable salt-templated synthesis of Se-vacancy-incorporated MoSe2 architecture (SeVs-MoSe2 ) and reveal the phase evolution of such defective pre-catalyst in working Li–S batteries. The interaction between lithium polysulfides and SeVs-MoSe2 is probed to induce the transformation from SeVs-MoSe2 to MoSeS. Furthermore, operando Raman spectroscopy/ex-situ X-ray diffraction measurements in combination with theoretical simulations verify that the effectual MoSeS catalyst could help promote the conversion from Li2S2 to Li2S, thereby boosting the capacity performance. Thus-derived Li–S battery accordingly exhibits satisfactory rate and cycling capability even under elevated sulfur loading and lean electrolyte conditions (7.67 mg cm–2 ; 4.0 μL mg–1S ). This work is anticipated to open an avenue for designing efficient electrocatalysts toward boosted sulfur chemistry and deepens the understanding of catalytic mechanisms of defective electrocatalysts.

Published in: "Angewandte Chemie International Edition".

Transition-Metal Nitride Halide Dielectrics for Transition-Metal Dichalcogenide Transistors. (arXiv:2108.09347v1 [cond-mat.mtrl-sci])

2021-08-24T02:30:13+00:00August 24th, 2021|Categories: Publications|Tags: , , , , , |

Using first-principles calculations, we investigate six transition-metal nitride halides (TMNHs): HfNBr, HfNCl, TiNBr, TiNCl, ZrNBr, and ZrNCl as potential van der Waals (vdW) dielectrics for transition metal dichalcogenide (TMD) channel transistors. We calculate the exfoliation energies and bulk phonon energies and find that the six TMNHs are exfoliable and thermodynamically stable. We calculate both the optical and static dielectric constants in the in-plane and out-of-plane directions for both monolayer and bulk TMNHs. In monolayers, the out-of-plane static dielectric constant ranges from 5.04 (ZrNCl) to 6.03 (ZrNBr) whereas in-plane dielectric constants range from 13.18 (HfNBr) to 74.52 (TiNCl). We show that the bandgap of TMNHs ranges from 1.53 eV (TiNBr) to 3.36 eV (HfNCl) whereas the affinity ranges from 4.01 eV (HfNBr) to 5.60 eV (TiNCl). Finally, we estimate the dielectric leakage current density of transistors with six TMNH monolayer dielectrics with five monolayer channel TMDs (MoS2, MoSe2, MoTe2, WS2, and WSe2). For p-MOS TMD channel transistors, 19 out of 30 combinations have a smaller leakage current compared to monolayer hexagonal boron nitride (hBN), a well-known vdW dielectric. The smallest monolayer leakage current of 2.14*10-9 A/cm2 is predicted for a p-MOS WS2 transistor with HfNCl as a gate dielectric. HfNBr, HfNCl, ZrNBr, and ZrNCl are also predicted to yield small leakage currents in certain p-MOS TMD transistors.

Published in: "arXiv Material Science".

Interfacial ferroelectricity in rhombohedral-stacked bilayer transition metal dichalcogenides. (arXiv:2108.07659v1 [cond-mat.mes-hall])

2021-08-18T02:29:42+00:00August 18th, 2021|Categories: Publications|Tags: , , , , , |

Van der Waals (vdW) materials have greatly expanded our design space of heterostructures by allowing individual layers to be stacked at non-equilibrium configurations, for example via control of the twist angle. Such heterostructures not only combine characteristics of the individual building blocks, but can also exhibit emergent physical properties absent in the parent compounds through interlayer interactions. Here we report on a new family of emergent, nanometer-thick, semiconductor 2D ferroelectrics, where the individual constituents are well-studied non-ferroelectric monolayer transition metal dichalcogenides (TMDs), namely WSe2, MoSe2, WS2, and MoS2. By stacking two identical monolayer TMDs in parallel, we obtain electrically switchable rhombohedral-stacking configurations, with out-of-plane polarization that is flipped by in-plane sliding motion. Fabricating nearly-parallel stacked bilayers enables the visualization of moir’e ferroelectric domains as well as electric-field-induced domain wall motion with piezoelectric force microscopy (PFM). Furthermore, by using a nearby graphene electronic sensor in a ferroelectric field transistor geometry, we quantify the ferroelectric built-in interlayer potential, in good agreement with first-principles calculations. The novel semiconducting ferroelectric properties of these four new TMDs opens up the possibility of studying the interplay between ferroelectricity and their rich electric and optical properties.

Published in: "arXiv Material Science".

Chiral valley phonons and flat phonon bands in moire materials. (arXiv:2108.03965v1 [cond-mat.mtrl-sci])

2021-08-10T02:30:15+00:00August 10th, 2021|Categories: Publications|Tags: , , |

We investigate the chirality of phonon modes in twisted bilayer WSe2. We demonstrate distinct chiral behavior of the K/K’ valley phonon modes for twist angles close to 0 degrees and close to 60 degrees. Moreover, we discover two sets of well-separated chiral valley modes in moire lattices for angles close to 60 degrees. These emergent moire chiral valley phonons originate from inversion symmetry breaking at the moire scale. We also find similar emergent chiral modes in moire patterns of strain-engineered bilayer WSe2 and MoSe2/WSe2 heterostructure. Furthermore, we observe the flattening of bands near the phononic band-gap edges for a broad range of twist angles in twisted bilayer WSe2. Our findings, which are expected to be generic for moire systems composed of two-dimensional materials that break inversion symmetry, are relevant for understanding electron-phonon and exciton-phonon scattering, and for designing phononic crystals to mimic behaviors of electrons in moire materials.

Published in: "arXiv Material Science".

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