Heterostructures of quantum dots (QDs) and two-dimensional (2D) materials show promising potential for photodetection applications owing to their combination of high optical absorption and good in-plane carrier mobility. In this work, the performance of QD-2D photodetectors is tuned by band engineering. Devices are fabricated by coating MoS2 nanosheets with InP QDs, type-I core–shell InP/ZnS QDs, and type-II core–shell InP/CdS QDs. Comparative spectroscopic and photoelectric studies of different hybrids show that the energy band alignment and shell thickness can influence the efficiency of charge transfer (CT), energy transfer (ET), and defect-related processes between QDs and MoS2. Benefiting from efficient CT between the QDs and MoS2, a significant enhancement of responsivity and detectivity is observed in thick-shell InP/CdS QD-MoS2 devices. Our results demonstrate the feasibility of using core–shell QDs for regulating the ET and CT efficiency in heterostructures and highlight the importance of interface band design in QD-2D and other low-dimensional photodetectors.
For semiconductor nanocrystals (NCs), the precise knowledge of phonons in the presence of free carriers is important for understanding their electronic and photonic properties in device applications. With Raman spectroscopy, this study investigates the effects of free charge carriers on optical phonon behaviors of NCs. The adoption of the photocharging method allows us to introduce free charge carriers into NCs without inducing other side effects. In the photocharged ZnO NCs, lower longitudinal optical (LO) phonon frequencies and weaker LO overtones relative to the fundamentals were found, which was explained by the screening and band-filling effects caused by the induced free carriers. The free carrier effects on optical phonon behaviors of NCs, usually neglected in previous studies, should be taken into consideration when discussing the electronic and photonic properties of NC-based devices.
Charge transfer at the interface is important for the optoelectronic and photochemical applications of quantum dot-two-dimensional material (QD-2D) hybrids. In this work, Raman spectroscopy was exploited to characterize the CdS QD–graphene hybrids with varied thicknesses of graphene and QD layer. The selection of Raman excitation energies below the QD band gap rules out the photoexcitation effects, and thus, we can focus on equilibrium charge transfer upon hybrid formation. Correlation analysis of Raman spectra shows evidence of electron transfer with concentration on the order of ∼1012 cm–2, as confirmed by electrical measurements. The method used in this study can be applied to characterize the interfacial interaction of various QD-2D hybrids.
Semiconductor junctions are of great significance for the development of electronic and optoelectronic devices. Here, controllable switching is demonstrated from a Schottky junction to a p-n junction in a partially ionic liquid-gated MoS 2 device with two types of metal contacts. Excellent rectification behavior with a current on-off ratio exceeding 10 6 is achieved in both Schottky and p-n junction modes. The formation of Schottky junction at the Pd electrode/MoS 2 contact and p-n junction at the p-MoS 2 /n-MoS 2 interface is revealed by spatially resolved photocurrent mappings. The switching between the two junctions under ionic gate modulation is correlated with the evolution of the energy band, further validated by the finite element simulation. The device exhibits excellent photodetection properties in the p-n junction mode, including an open circuit voltage up to 0.84 V, a responsivity of 0.24 A W −1 , a specific detectivity of 1.7 × 10 11 Jones, a response time of hundreds of microseconds and a linear dynamic range of up to 91 dB. The electric field control of such high-performance Schottky and p-n junctions opens up fresh perspectives for studying the behavior of junction and the development of 2D electronic devices.
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