More Like this

1. Energy-valley-dependent charge transfer in few-layer transition metal dichalcogenide heterostructures

   https://doi.org/10.1103/PhysRevB.108.085302  

   Valencia-Acuna, Pavel; Amos, Stephanie; Peelaers, Hartwin; Zhao, Hui (August 2023, Physical Review B)

   The effect of the energy valley on interlayer charge transfer in transition metal dichalcogenide (TMD) heterostructures is studied by transient absorption spectroscopy and density functional theory. First-principles calculations confirm that the Λmin valley in the conduction band of few-layer WSe2 evolves from above its K valley in the monolayer (1L) to below it in 4L. Heterostructure samples of 𝑛⁢L−WSe2/1⁢L−MoS2, where 𝑛=1,2,3, and 4, are obtained by mechanical exfoliation and dry transfer. Photoluminescence spectroscopy reveals a thickness-dependent WSe2 band structure and efficient interlayer charge transfer. Transient absorption measurements show that the electron transfer time from the Λmin valley of 4L WSe2 to the K valley of MoS2 is on the order of 30 ps. This process is much slower than the K-K charge transfer in 1L/1L TMD heterostructures. The momentum-indirect interlayer excitons formed after charge transfer have lifetimes >1 ns. 

   more » « less
2. Band alignments, conduction band edges and intralayer bandgap renormalisation in MoSe ₂ /WSe ₂ heterobilayers

   https://doi.org/10.1088/2053-1583/ad7b51  

   Graham, A_J; Nguyen, P_V; Park, H.; Nunn, J.; Kandyba, V.; Cattelan, M.; Giampietri, A.; Barinov, A.; Xu, X.; Cobden, D_H; et al (September 2024, 2D Materials)

   Abstract Stacking two semiconducting transition metal dichalcogenide (MX₂) monolayers to form a heterobilayer creates a new variety of semiconductor junction with unique optoelectronic features, such as hosting long-lived dipolar interlayer excitons. Despite many optical, transport, and theoretical studies, there have been few direct electronic structure measurements of these junctions. Here, we apply angle-resolved photoemission spectroscopy with micron-scale spatial resolution (µARPES) to determine the band alignments in MoSe₂/WSe₂heterobilayers, usingin-situelectrostatic gating to electron-dope and thus probe the conduction band edges. By comparing spectra from heterobilayers with opposite stacking orders, that is, with either MoSe₂or WSe₂on top, we confirm that the band alignment is type II, with the valence band maximum in the WSe₂and the conduction band minimum in the MoSe₂. The overall band gap isE_G= 1.43 ± 0.03 eV, and to within experimental uncertainty it is unaffected by electron doping. However, the offset between the WSe₂and MoSe₂valence bands clearly decreases with increasing electron doping, implying band renormalisation only in the MoSe₂, the layer in which the electrons accumulate. In contrast,µARPES spectra from a WS₂/MoSe₂heterobilayer indicate type I band alignment, with both band edges in the MoSe₂. These insights into the doping-dependent band alignments and gaps of MX₂heterobilayers will be useful for properly understanding and ultimately utilizing their optoelectronic properties. 

   more » « less
3. Tuning the Electronic and Photonic Properties of Monolayer MoS ₂ via In Situ Rhenium Substitutional Doping

   https://doi.org/10.1002/adfm.201706950  

   Zhang, Kehao; Bersch, Brian M.; Joshi, Jaydeep; Addou, Rafik; Cormier, Christopher R.; Zhang, Chenxi; Xu, Ke; Briggs, Natalie C.; Wang, Ke; Subramanian, Shruti; et al (February 2018, Advanced Functional Materials)

   Abstract Doping is a fundamental requirement for tuning and improving the properties of conventional semiconductors. Recent doping studies including niobium (Nb) doping of molybdenum disulfide (MoS₂) and tungsten (W) doping of molybdenum diselenide (MoSe₂) have suggested that substitutional doping may provide an efficient route to tune the doping type and suppress deep trap levels of 2D materials. To date, the impact of the doping on the structural, electronic, and photonic properties of in situ‐doped monolayers remains unanswered due to challenges including strong film substrate charge transfer, and difficulty achieving doping concentrations greater than 0.3 at%. Here, in situ rhenium (Re) doping of synthetic monolayer MoS₂with ≈1 at% Re is demonstrated. To limit substrate film charge transfer,r‐plane sapphire is used. Electronic measurements demonstrate that 1 at% Re doping achieves nearly degenerate n‐type doping, which agrees with density functional theory calculations. Moreover, low‐temperature photoluminescence indicates a significant quench of the defect‐bound emission when Re is introduced, which is attributed to the MoO bond and sulfur vacancies passivation and reduction in gap states due to the presence of Re. The work presented here demonstrates that Re doping of MoS₂is a promising route toward electronic and photonic engineering of 2D materials. 

   more » « less
4. Controlling exciton transport in monolayer MoSe ₂ by dielectric screening

   https://doi.org/10.1039/C9NH00462A  

   Hao, Shengcai; Bellus, Matthew Z.; He, Dawei; Wang, Yongsheng; Zhao, Hui (January 2020, Nanoscale Horizons)

   Due to their atomic thinness with reduced dielectric screening, two-dimensional materials can possess a stable excitonic population at room temperature. This is attractive for future excitonic devices, where excitons are used to carry energy or information. In excitonic devices, controlling transport of the charge-neutral excitons is a key element. Here we show that exciton transport in a MoSe 2 monolayer semiconductor can be effectively controlled by dielectric screening. A MoSe 2 monolayer was partially covered with a hexagonal boron nitride flake. Photoluminescence measurements showed that the exciton energy in the covered region is about 12 meV higher than that in the uncovered region. Spatiotemporally resolved differential reflection measurements performed at the junction between the two regions revealed that this energy offset is sufficient to drive excitons across the junction for about 50 ps over a distance of about 200 nm. These results illustrate the feasibility of using van der Waals dielectric engineering to control exciton transport and contribute to understanding the effects of the dielectric environment on the electronic and optical properties of two-dimensional semiconductors. 

   more » « less
5. Synthesis of Co‐Doped MoS ₂ Monolayers with Enhanced Valley Splitting

   https://doi.org/10.1002/adma.201906536  

   Zhou, Jiadong; Lin, Junhao; Sims, Hunter; Jiang, Chongyun; Cong, Chunxiao; Brehm, John A.; Zhang, Zhaowei; Niu, Lin; Chen, Yu; Zhou, Yao; et al (February 2020, Advanced Materials)

   Abstract Internal magnetic moments induced by magnetic dopants in MoS₂monolayers are shown to serve as a new means to engineer valley Zeeman splitting (VZS). Specifically, successful synthesis of monolayer MoS₂doped with the magnetic element Co is reported, and the magnitude of the valley splitting is engineered by manipulating the dopant concentration. Valley splittings of 3.9, 5.2, and 6.15 meV at 7 T in Co‐doped MoS₂with Co concentrations of 0.8%, 1.7%, and 2.5%, respectively, are achieved as revealed by polarization‐resolved photoluminescence (PL) spectroscopy. Atomic‐resolution electron microscopy studies clearly identify the magnetic sites of Co substitution in the MoS₂lattice, forming two distinct types of configurations, namely isolated single dopants and tridopant clusters. Density functional theory (DFT) and model calculations reveal that the observed enhanced VZS arises from an internal magnetic field induced by the tridopant clusters, which couples to the spin, atomic orbital, and valley magnetic moment of carriers from the conduction and valence bands. The present study demonstrates a new method to control the valley pseudospin via magnetic dopants in layered semiconducting materials, paving the way toward magneto‐optical and spintronic devices. 

   more » « less