Layer‐Number‐Dependent Electronic and Optoelectronic Properties of 2D WSe₂‐Organic Hybrid Heterojunction

Abstract

One of the most attractive features of 2D WSe2 is a tunability in its electronic and optoelectronic properties depending on the layer number. To harness such unique characteristics for device applications, high quality and easily processable heterojunctions are required and relevant layer‐number‐dependent properties must be understood. Herein, a study is reported on hybrid heterojunctions between 2D WSe2 and organic molecules from one‐step solution chemistry and their layer‐number‐dependent properties. Eosin Y (EY) dye is selected as a p‐dopant and uniformly stacked on mechanically exfoliated WSe2 flakes via van der Waals interaction, forming a hybrid heterojunction with a type II alignment. The EY‐WSe2 heterojunction shows significantly enhanced currents compared to pristine WSe2 with a lower barrier height and a longer effective screening length. The work function of the heterostructure is also lower than that of pristine WSe2. The efficient exciton dissociation and doping effect by EY are confirmed by photocurrent and photoluminescence measurements, where WSe2 emission is markedly quenched by EY and exciton contribution decreases with layer number. These findings shed critical insights into layer‐number‐dependent electronic and optoelectronic properties of organic‐WSe2 layers and also provide simple yet effective means to construct transition metal dichalcogenide‐based heterostructures, which should be valuable for developing layered 2D devices.

Selective Chemical Modulation of Interlayer Excitons in Atomically Thin Heterostructures

“…Along the black line in Figure a, the surface potential of MoS 2 is roughly uniform at ∼240 mV, while about 50 mV is obtained from WSe 2 (Figure b). This is expected as MoS 2 has a higher work function than WSe 2 . , Notably, there is a sharp decrease in surface potential when transitioning from MoS 2 to WSe 2 /MoS 2 . Another drastic decrease is observed from the heterolayer to WSe 2 .…”

“…The hybrid multi-heterointerfaces are designed to have distinct energy level alignments including consecutive type-II/type-II (termed Type-A), type-II/type-I (Type-B), and discontinuous type-II/type-II (Type-C) alignments. Monolayer MoS 2 and WSe 2 along with two sets of molecules, tetracyanoquinodimethane (TCNQ) , and Eosin Y (EY), are used to achieve various heterojunction alignments. The combinations of TMDC and organic layers determine photoinduced charge transfer and dark-state doping, ultimately modulating X I s. Interlayer exciton behaviors are probed by PL spectroscopy.…”

“…Recombination of electrons in MoS 2 and holes in WSe 2 gives rise to unique PL signatures distinct and different from MoS 2 and WSe 2 emissions (Figure a) . We irradiated samples with a 633 nm HeNe laser which excites both MoS 2 and WSe 2 but is not absorbed by the organic layers; the energy gaps between their highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are large compared to the light energy. , This allows us to observe the emissions solely from the TMDCs. While both TCNQ and EY dyes are p-dopants for TMDCs, their effects on X I are drastically different.…”

“…As a result, interlayer excitons will be depleted, and the associated PL emission will be quenched as observed experimentally. Further, additional interlayer charge transfer may also occur at the hybrid interface regardless of illumination (i.e., dark-state doping); the Fermi levels of TMDCs (dotted lines in Figure c) are approximately 0.3 eV higher than the reduction potential of TCNQ, which may induce a p-doping on MoS 2 . ,− The interplay between the photoinduced process and dark-state doping will change the charge concentrations in the stacked heterolayers, thus modulating interlayer excitons and MoS 2 PL ( vide infra ).…”

Recent Progress of Optoelectronic and All‐Optical Neuromorphic Devices: A Comprehensive Review of Device Structures, Materials, and Applications

“…To improve the stability of the organic materials, hybridization with various materials has been actively researched recently. [60,61] Oxide materials, especially metal oxide semiconductors, have advantages such as high carrier mobility, high stability, good transparency, and decent mechanical flexibility. [62] Moreover, the oxygen vacancies that exist in oxide semiconductors can be ionized under light illumination, providing a pathway to control the conductivity of the semiconductor.…”