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1.
Nano Lett ; 2020 Feb 26.
Artigo em Inglês | MEDLINE | ID: mdl-32078334

RESUMO

Strong many-body interactions in two-dimensional (2D) semiconductors give rise to efficient exciton-exciton annihilation (EEA). This process is expected to result in the generation of unbound high energy carriers. Here, we report an unconventional photoresponse of van der Waals heterostructure devices resulting from efficient EEA. Our heterostructures, which consist of monolayer transition metal dichalcogenide (TMD), hexagonal boron nitride (hBN), and few-layer graphene, exhibit photocurrent when photoexcited carriers possess sufficient energy to overcome the high energy barrier of hBN. Interestingly, we find that the device exhibits moderate photocurrent quantum efficiency even when the semiconducting TMD layer is excited at its ground exciton resonance despite the high exciton binding energy and large transport barrier. Using ab initio calculations, we show that EEA yields highly energetic electrons and holes with unevenly distributed energies depending on the scattering condition. Our findings highlight the dominant role of EEA in determining the photoresponse of 2D semiconductor optoelectronic devices.

2.
ACS Nano ; 13(9): 10768-10775, 2019 Sep 24.
Artigo em Inglês | MEDLINE | ID: mdl-31491079

RESUMO

Controlled substitutional doping of two-dimensional transition-metal dichalcogenides (TMDs) is of fundamental importance for their applications in electronics and optoelectronics. However, achieving p-type conductivity in MoS2 and WS2 is challenging because of their natural tendency to form n-type vacancy defects. Here, we report versatile growth of p-type monolayer WS2 by liquid-phase mixing of a host tungsten source and niobium dopant. We show that crystallites of WS2 with different concentrations of substitutionally doped Nb up to 1014 cm-2 can be grown by reacting solution-deposited precursor film with sulfur vapor at 850 °C, reflecting the good miscibility of the precursors in the liquid phase. Atomic-resolution characterization with aberration-corrected scanning transmission electron microscopy reveals that the Nb concentration along the outer edge region of the flakes increases consistently with the molar concentration of Nb in the precursor solution. We further demonstrate that ambipolar field-effect transistors can be fabricated based on Nb-doped monolayer WS2.

3.
Nano Lett ; 19(10): 7470-7475, 2019 10 09.
Artigo em Inglês | MEDLINE | ID: mdl-31517494

RESUMO

Monolayer WSe2 exhibits luminescence arising from various types of exciton complexes due to strong many-body effects. Here, we demonstrate selective electrical excitation of positive and negative trions in van der Waals metal-insulator-semiconductor (MIS) heterostructure consisting of few-layer graphene (FLG), hexagonal boron nitride (hBN), and monolayer WSe2. Intentional unbalanced injection of electrons and holes is achieved via field-emission tunneling and electrostatic accumulation. The device exhibits planar electroluminescence from either positive trion X+ or negative trion X- depending on the bias conditions. We show that hBN serves as a tunneling barrier material allowing selective injection of electron or holes into WSe2 from FLG layer. Our observation offers prospects for hot carrier injection, trion manipulation, and on-chip excitonic devices based on two-dimensional semiconductors.

4.
Chem Soc Rev ; 48(17): 4639-4654, 2019 Aug 27.
Artigo em Inglês | MEDLINE | ID: mdl-31410435

RESUMO

Research on 2D materials has recently become one of the hottest topics that has attracted broad interdisciplinary attention. 2D materials offer fascinating platforms for fundamental science and technological explorations at the nanometer scale and molecular level, and exhibit diverse potential applications for future advanced nano-photonics and electronics. The chemical vapor deposition (CVD) technique has shown great promise for producing high-quality 2D materials with superior electro-optical performance. However, it is difficult to synthesize continuous single-crystal 2D materials with large domain sizes and good uniformity due to the low vapor pressure of their precursors. It has been observed that the addition of selected synergistic additives to the CVD process under mild conditions can result in uniformly large-area and highly crystalline monolayer 2D materials with exceptional optical/electrical properties. Moreover, the 2D material-based devices chemically modified by synergistic additives can achieve superior performances compared to those previously reported. In this review, we compare several typical synergistic additive-mediated CVD growth processes of 2D materials, as well as their superior properties, and provide some perspectives and challenges for the future of this emerging research field.

5.
ACS Nano ; 13(8): 9218-9226, 2019 Aug 27.
Artigo em Inglês | MEDLINE | ID: mdl-31394038

RESUMO

The ideal quantum confinement structure of monolayer semiconductors offers prominent optical modulation capabilities that are mediated by enhanced many-body interactions. Herein, we establish an electrolyte-gating method for tuning the luminescence properties that are in transition metal dichalcogenide (TMDC) monolayers. We fabricate electric double-layer capacitors on TMDC/graphite heterostructures to investigate electric-field- and carrier-density-dependent photoluminescence. The exciton peak energy initially shows a slight quadratic red shift of ∼1 meV without carrier accumulations, which is caused by the quantum-confined Stark effect. In contrast, the exciton resonance exhibits a larger red shift up to 10 meV with the accumulated carrier density above 1013 cm-2. These results indicate that the optical transitions can be largely modulated by the carrier density control in S- and Se-based TMDCs, as triggered by the doping-induced band gap renormalization effect. To further inspire this modulation capability, we also apply our method to electrolyte-based TMDC light-emitting devices. Biasing solely in electrolyte-induced p-i-n junctions yields pronounced red shifts up to 40 meV for exciton and trion electroluminescence. Consequently, our approach reveals that the doping effects in the high-carrier-density regimes are potentially significant for efficient optical modulation in monolayer semiconductors.

6.
Nat Nanotechnol ; 14(10): 945-949, 2019 Oct.
Artigo em Inglês | MEDLINE | ID: mdl-31427750

RESUMO

All-electric magnetization manipulation at low power is a prerequisite for a wide adoption of spintronic devices. Materials such as heavy metals1-3 or topological insulators4,5 provide good charge-to-spin conversion efficiencies. They enable magnetization switching in heterostructures with either metallic ferromagnets or with magnetic insulators. Recent work suggests a pronounced Edelstein effect in Weyl semimetals due to their non-trivial band structure6,7; the Edelstein effect can be one order of magnitude stronger than it is in topological insulators or Rashba systems. Furthermore, the strong intrinsic spin Hall effect from the bulk states in Weyl semimetals can contribute to the spin current generation8. The Td phase of the Weyl semimetal WTe2 (WTe2 hereafter) possesses strong spin-orbit coupling6,9 and non-trivial band structures10 with a large spin polarization protected by time-reversal symmetry in both the surface and bulk states9-11. Atomically flat surfaces, which can be produced with high quality12, facilitate spintronic device applications. Here, we use WTe2 as a spin current source in WTe2/Ni81Fe19 (Py) heterostructures. We report field-free current-induced magnetization switching at room temperature. A charge current density of ~2.96 × 105 A cm-2 suffices to switch the magnetization of the Py layer. With the charge current along the b axis of the WTe2 layer, the thickness-dependent charge-to-spin conversion efficiency reaches 0.51 at 6-7 GHz. At the WTe2/Py interface, a Dzyaloshinskii-Moriya interaction (DMI) with a DMI constant of -1.78 ± 0.06 mJ m-2 induces chiral domain wall tilting. Our study demonstrates the capability of WTe2 to efficiently manipulate magnetization and sheds light on the role of the interface in Weyl semimetal/magnet heterostructures.

7.
ACS Nano ; 13(8): 9587-9594, 2019 Aug 27.
Artigo em Inglês | MEDLINE | ID: mdl-31322858

RESUMO

Demonstration of van der Waals (vdW) semiconductor/metal heterostructures (SMHs) based on transition metal dichalcogenides has been a central approach in high-speed electronics by introducing ultrafast carrier dynamics. In this regard, a Weyl semimetal WTe2 is of great interest due to its vdW layered nature, low work function, and superior electrical properties. However, little is still known about its heterostructures, and a few picoseconds photocarrier lifetimes hinder its applications in high-speed electronics. Here, we propose a SMH: semimetallic Td phase WTe2 with its sister compound of semiconducting 2H phase MoTe2. Time-resolved terahertz (THz) spectroscopy demonstrated that WTe2 exhibited the significantly shorter carrier lifetimes of sub-picosecond when forming a junction with MoTe2. We provided explicit characteristic signatures, revealing charge transfer across the interface and the subsequent interlayer exciton decay. This work not only offers the extension of the THz detection scope of ultrafast phenomena from atomically thin materials but also provides a building block of vertical SMHs for high-speed electronic devices with sub-picosecond photocarrier lifetimes.

8.
Sci Adv ; 5(7): eaaw2347, 2019 Jul.
Artigo em Inglês | MEDLINE | ID: mdl-31334350

RESUMO

Understanding the remarkable excitonic effects and controlling the exciton binding energies in two-dimensional (2D) semiconductors are crucial in unlocking their full potential for use in future photonic and optoelectronic devices. Here, we demonstrate large excitonic effects and gate-tunable exciton binding energies in single-layer rhenium diselenide (ReSe2) on a back-gated graphene device. We used scanning tunneling spectroscopy and differential reflectance spectroscopy to measure the quasiparticle electronic and optical bandgap of single-layer ReSe2, respectively, yielding a large exciton binding energy of 520 meV. Further, we achieved continuous tuning of the electronic bandgap and exciton binding energy of monolayer ReSe2 by hundreds of milli-electron volts through electrostatic gating, attributed to tunable Coulomb interactions arising from the gate-controlled free carriers in graphene. Our findings open a new avenue for controlling the bandgap renormalization and exciton binding energies in 2D semiconductors for a wide range of technological applications.

9.
Nano Lett ; 19(8): 5595-5603, 2019 Aug 14.
Artigo em Inglês | MEDLINE | ID: mdl-31241969

RESUMO

Photodetectors usually operate in the wavelength range with photon energy above the bandgap of channel semiconductors so that incident photons can excite electrons from valence band to conduction band to generate photocurrent. Here, however, we show that monolayer WS2 photodetectors can detect photons with energy even lying 219 meV below the bandgap of WS2 at room temperature. With the increase of excitation wavelength from 620 to 680 nm, photoresponsivity varies from 551 to 59 mA/W. This anomalous phenomenon is ascribed to energy upconversion, which is a combination effect of one-photon excitation and multiphonon absorption through an intermediate state created most likely by sulfur divacancy with oxygen adsorption. These findings will arouse research interests on other upconversion optoelectronic devices, photovoltaic devices, for example, of monolayer transition metal dichalcogenides (TMDCs).

10.
J Phys Chem Lett ; 10(11): 2869-2873, 2019 Jun 06.
Artigo em Inglês | MEDLINE | ID: mdl-31088074

RESUMO

Recently, some organic-inorganic hybrid perovskites (OIHPs) have been reported to exhibit strong subgap broadband luminescence. While the origin of such luminescence has been proposed by several groups, a strategy to prepare OIHP with the desired subgap emission properties has remained elusive. Here, we report controlled synthesis of a broadband-emitting single-crystal 2D OIHP with an average quantum yield of >80 %. We demonstrate that the intensity of broadband emission can be tuned by controlling the excess iodine ion concentration during the synthesis in hydroiodic acid. We show that the emitters exhibit characteristics of localized defects such as limited mobility and saturation at high excitation power. Using density functional theory calculations, we show that bond-state iodine interstitials are responsible for the observed long-lived luminescence.

11.
Nano Lett ; 19(5): 2840-2849, 2019 05 08.
Artigo em Inglês | MEDLINE | ID: mdl-30929451

RESUMO

Controllability of collective electron states has been a long-sought scientific and technological goal and promises development of new devices. Herein, we investigate the tuning of charge density wave (CDW) in 1T-TaS2 via a two-dimensional (2D) van der Waals heterostructure of 1T-TaS2/BP. Unusual gate-dependent conductance oscillations were observed in 1T-TaS2 nanoflake supported on BP in transport measurements. Scanning tunneling microscopy study shows that the nearly commensurate (NC) CDW phase survived to 4.5 K in this system, which is substantially lower than the NC to commensurate CDW phase transition temperature of 180 K. A Coulomb blockade model was invoked to explain the conductance oscillations, where the domain walls and domains in NC phase serve as series of quantum dot arrays and tunnelling barriers, respectively. Density functional theory calculations show that a range of interfacial interactions, including strain and charge transfer, influences the CDW stabilities. Our work sheds light on tuning CDW orders via 2D heterostructure stacking and provides new insights on the CDW phase transition and sliding mechanism.

12.
Nat Commun ; 10(1): 1290, 2019 03 20.
Artigo em Inglês | MEDLINE | ID: mdl-30894524

RESUMO

The nature of Fermi surface defines the physical properties of conductors and many physical phenomena can be traced to its shape. Although the recent discovery of a current-dependent nonlinear magnetoresistance in spin-polarized non-magnetic materials has attracted considerable attention in spintronics, correlations between this phenomenon and the underlying fermiology remain unexplored. Here, we report the observation of nonlinear magnetoresistance at room temperature in a semimetal WTe2, with an interesting temperature-driven inversion. Theoretical calculations reproduce the nonlinear transport measurements and allow us to attribute the inversion to temperature-induced changes in Fermi surface convexity. We also report a large anisotropy of nonlinear magnetoresistance in WTe2, due to its low symmetry of Fermi surfaces. The good agreement between experiments and theoretical modeling reveals the critical role of Fermi surface topology and convexity on the nonlinear magneto-response. These results lay a new path to explore ramifications of distinct fermiology for nonlinear transport in condensed-matter.

13.
Adv Sci (Weinh) ; 6(4): 1801626, 2019 Feb 20.
Artigo em Inglês | MEDLINE | ID: mdl-30828533

RESUMO

Multiphoton absorption may find many technological applications, such as enhancing the conversion efficiency of solar cells by the utilization of sub-band-energy photons, below-bandgap photodetection through the simultaneous absorption of several infrared photons for photocurrent generation, or light frequency upconversion for high-resolution, 3D imaging. To enhance multiphoton absorption in semiconducting materials, one of the strategies is to explore low-dimensional excitons. Here, a quantum perturbation theory on a giant enhancement in three-photon absorption (3PA) arising from 2D excitons in multilayered crystals of organic-inorganic hybrid perovskites is presented. The maximal 3PA coefficient is predicted to be in the range of 2-7 cm3 GW-2 at 1100 nm, the largest values reported so far for any 2D and bulk semiconductors at room temperature. Excellent agreement between theory and the experimental findings unambiguously demonstrates a pivotal role in the enhancement of 3PA played by 2D excitons. The theory predicts that the resonant 3PA coefficient should be enhanced further by at least two orders of magnitude with very low temperature. The findings are essential for understanding giant 3PA arising from 2D excitons in layered hybrid perovskites and may open new pathways for highly efficient conversion from infrared light energy to either electrical energy or higher-frequency light emission/lasing.

14.
ACS Nano ; 13(3): 3218-3224, 2019 Mar 26.
Artigo em Inglês | MEDLINE | ID: mdl-30768242

RESUMO

Monolayer semiconductors are atomically thin quantum wells with strong confinement of electrons in a two-dimensional (2D) plane. Here, we experimentally study the out-of-plane polarizability of excitons in hBN-encapsulated monolayer WSe2 in strong electric fields of up to 1.6 V/nm (16 MV/cm). We monitor free exciton photoluminescence peaks with increasing electric fields at a constant carrier density, carefully compensating for unintentional photodoping in our double-gated device at 4 K. We show that the Stark shift is smaller than 0.4 meV despite the large electric fields applied, yielding an upper limit of polarizability α z to be ∼10-11 Dm/V. Such a small polarizability, which is nearly two orders of magnitude smaller than the previously reported value for MoS2, indicates strong atomic confinement of electrons in this 2D system and highlights the unusual robustness of free excitons against surface potential fluctuations.

15.
Nat Nanotechnol ; 14(3): 223-226, 2019 03.
Artigo em Inglês | MEDLINE | ID: mdl-30718834

RESUMO

Two-dimensional transition metal dichalcogenide (TMD) materials, albeit promising candidates for applications in electronics and optoelectronics1-3, are still limited by their low electrical mobility under ambient conditions. Efforts to improve device performance through a variety of routes, such as modification of contact metals4 and gate dielectrics5-9 or encapsulation in hexagonal boron nitride10, have yielded limited success at room temperature. Here, we report a large increase in the performance of TMD field-effect transistors operating under ambient conditions, achieved by engineering the substrate's surface morphology. For MoS2 transistors fabricated on crested substrates, we observed an almost two orders of magnitude increase in carrier mobility compared to standard devices, as well as very high saturation currents. The mechanical strain in TMDs has been predicted to boost carrier mobility11, and has been shown to influence the local bandgap12,13 and quantum emission properties14 of TMDs. With comprehensive investigation of different dielectric environments and morphologies, we demonstrate that the substrate's increased corrugation, with its resulting strain field, is the dominant factor driving performance enhancement. This strategy is universally valid for other semiconducting TMD materials, either p-doped or n-doped, opening them up for applications in heterogeneous integrated electronics.

16.
ACS Appl Mater Interfaces ; 11(13): 12184-12189, 2019 Apr 03.
Artigo em Inglês | MEDLINE | ID: mdl-30811179

RESUMO

Layered transition metal dichalcogenides (TMDCs) intercalated with alkali metals exhibit mixed metallic and semiconducting phases with variable fractions. Thermoelectric properties of such mixed-phase structure are of great interest because of the potential energy filtering effect, wherein interfacial energy barriers strongly scatter cold carriers rather than hot carriers, leading to enhanced Seebeck coefficient ( S). Here, we study the thermoelectric properties of mixed-phase Li xMoS2 as a function of its phase composition tuned by in situ thermally driven deintercalation. We find that the sign of Seebeck coefficient changes from positive to negative during initial reduction of the 1T/1T' phase fraction, indicating crossover from p- to n-type carrier conduction. These anomalous changes in Seebeck coefficient, which cannot be simply explained by the effect of deintercalation-induced reduction in carrier density, can be attributed to the hybrid electronic property of the mixed-phase Li xMoS2. Our work shows that careful phase engineering is a promising route toward achieving thermoelectric performance in TMDCs.

17.
Nano Lett ; 18(11): 6898-6905, 2018 11 14.
Artigo em Inglês | MEDLINE | ID: mdl-30260651

RESUMO

Hexagonal boron nitride (h-BN) was recently reported to display single photon emission from ultraviolet to near-infrared range due to the existence of defects. Single photon emission has potential applications in quantum information processing and optoelectronics. These findings trigger increasing research interests in h-BN defects, such as revealing the nature of the defects. Here, we report another intriguing defect property in h-BN, namely photoluminescence (PL) upconversion (anti-Stokes process). The energy gain by the PL upconversion is about 162 meV. The anomalous PL upconversion is attributed to optical phonon absorption in the one-photon excitation process, based on excitation power, excitation wavelength, and temperature-dependence investigations. Possible constitutions of the defects are discussed from the results of scanning transmission electron microscopy (STEM) studies and theoretical calculations. These findings show that defects in h-BN exhibit strong defect-phonon coupling. The results from STEM and theoretical calculations are beneficial for understanding the constitution of the h-BN defects.

18.
Nat Mater ; 17(10): 908-914, 2018 10.
Artigo em Inglês | MEDLINE | ID: mdl-30202109

RESUMO

Due to their layered structure, two-dimensional Ruddlesden-Popper perovskites (RPPs), composed of multiple organic/inorganic quantum wells, can in principle be exfoliated down to few and single layers. These molecularly thin layers are expected to present unique properties with respect to the bulk counterpart, due to increased lattice deformations caused by interface strain. Here, we have synthesized centimetre-sized, pure-phase single-crystal RPP perovskites (CH3(CH2)3NH3)2(CH3NH3)n-1PbnI3n+1 (n = 1-4) from which single quantum well layers have been exfoliated. We observed a reversible shift in excitonic energies induced by laser annealing on exfoliated layers encapsulated by hexagonal boron nitride. Moreover, a highly efficient photodetector was fabricated using a molecularly thin n = 4 RPP crystal, showing a photogain of 105 and an internal quantum efficiency of ~34%. Our results suggest that, thanks to their dynamic structure, atomically thin perovskites enable an additional degree of control for the bandgap engineering of these materials.

19.
Adv Mater ; 30(39): e1803748, 2018 Sep.
Artigo em Inglês | MEDLINE | ID: mdl-30133006

RESUMO

Optical and electrical properties of 2D transition metal dichalcogenides (TMDCs) grown by chemical vapor deposition (CVD) are strongly determined by their microstructure. Consequently, the visualization of spatial structural variations is of paramount importance for future applications. This study demonstrates how grain boundaries, crystal orientation, and strain fields can unambiguously be identified with combined lateral force microscopy and transverse shear microscopy (TSM) for CVD-grown tungsten disulfide (WS2 ) monolayers, on length scales that are relevant for optoelectronic applications. Further, angle-dependent TSM measurements enable the fourth-order elastic constants of monolayer WS2 to be acquired experimentally. The results facilitate high-throughput and nondestructive microstructure visualization of monolayer TMDCs and insights into their elastic properties, thus providing an accessible tool to support the development of advanced optoelectronic devices based on such 2D semiconductors.

20.
Adv Mater ; 30(47): e1802687, 2018 Nov.
Artigo em Inglês | MEDLINE | ID: mdl-30118543

RESUMO

Ultrathin layers of van der Waals inorganic semiconductors represent a new class of excitonic materials with attractive light-emitting properties. Recent observation of valley polarization, optically pumped lasing, exciton-polaritons, and single-photon emission highlights the exciting prospects for two-dimensional (2D) semiconductors for applications in novel photonic devices. Development of efficient and reliable light sources based on excitonic electroluminescence in 2D semiconductors is of fundamental importance toward the practical implementation of photonic devices. Achieving electroluminescence in these atomically thin layers requires unconventional device designs and in-depth understanding of the carrier injection and transport mechanisms. Herein, various strategies for electrically generating excitons in 2D semiconducting transition metal dichalcogenides such as monolayer MoS2 are reviewed and challenges and opportunities are outlined. Furthermore, novel device concepts such as tunable chiral emission, electrically driven quantum emission, and high-frequency modulation are highlighted.

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