Two-dimensional UV photodetector using hybrid structure of monolayer WS₂ and Ti₂N MXene quantum dots

다국어 초록 (Multilingual Abstract)

In this study, high-quality monolayer WS2-based UV photodetectors were successfully fabricated and systematically investigated. The main objective was to enhance the UV photoresponse of 1L-WS2 by hybridizing it with MQDs to improve charge transfer and broaden the optical absorption range. Monolayer WS2 was exfoliated onto Si/SiO2 substrates and characterized by optical microscopy, AFM, and PL spectroscopy with a sharp PL peak at ~1.95 eV indicated a direct bandgap with high crystalline quality. After MQDs incorporation, partial PL quenching was observed, evidencing interfacial charge transfer between WS2 and MQDs.
Electrical characterization under dark conditions showed linear I–V behavior for both pristine and hybrid devices, suggesting Ohmic-type Au–WS2 contacts. The dark current slightly increased in the hybrid device due to the conductive MQDs layer but remained sufficiently low for high detectivity. Optical absorption spectra revealed an extended absorption edge in the UV region after MQDs addition, confirming enhanced photon harvesting. Under illumination, the pristine WS2 exhibited a weak response to 375 nm UV light, while the hybrid 1L-WS2/MQD device showed a clear and strong photocurrent enhancement. At 514 nm excitation, both devices demonstrated relatively low response, indicating wavelength selectivity toward higher-energy photons. The hybrid device achieved a responsivity of ~0.58A/W and detectivity of ~2 × 1011 Jones under 375 nm excitation— about ~60 times higher than the 1L-WS2. The improvement arises from enhanced UV absorption, efficient interfacial charge transfer, and trap-assisted carrier lifetime extension.
These results demonstrate that MQDs sensitization effectively overcomes the intrinsic UV absorption limitations of 1L-WS2 and provides a promising route for high-performance, wavelength-selective UV photodetectors. The findings also highlight the critical role of interfacial charge dynamics in improving the responsivity and detectivity of 2D optoelectronic devices. Although the present work successfully established a UV-selective WS2/MQDs hybrid photodetector with enhanced responsivity and detectivity, there remain several opportunities for further advancement. Future studies should focus on optimizing interfacial architecture, improving stability, and achieving scalable device integration to realize practical applications.
First, interface engineering can be further refined to maximize charge transfer efficiency. Adjusting the QD size, thickness, and surface termination could tailor the band alignment and improve carrier injection efficiency. Controlling the density and uniformity of QDs through optimized spin-coating or layer-by-layer deposition can minimize trap-state scattering and reduce carrier recombination losses. Additionally, incorporating intermediate dielectric spacers such as hexagonal boron nitride (hBN) may suppress nonradiative recombination while preserving fast tunneling, leading to a b alance between sensitivity and speed.
Second, Time-resolved photoluminescence (TRPL) and transient photocurrent measurements could provide further insight into charge-transfer dynamics and recombination lifetimes.
Third, environmental and thermal stability remain important aspects for real-world applications. WS2 and MQDs are both sensitive to ambient oxygen and moisture, which may degrade their interfacial properties over time. Therefore, encapsulation using thin hBN or Al2O3 layers, or device sealing under inert gas, could significantly improve long-term operational reliability. Systematic aging and stability tests under continuous illumination and bias will help to establish the device's durability.
The present work establishes a foundation for understanding and controlling interfacial physics governing 1L-WS2/MQDs hybrid photodetectors. The demonstrated UV-selective enhancement confirms the effectiveness of MQDs as active sensitizers. With continued progress in interface passivation, and heterostructure design, such 0D/2D hybrid architectures hold enormous promise for next-generation ultraviolet optoelectronics, environmental monitoring, and biological sensing applications .

다국어 초록 (Multilingual Abstract)

We report that the integration of transition metal dichalcogenides (TMDs) with MXene quantum dots offers a promising approach for developing high-performance ultraviolet (UV) photodetectors. This hybrid structure combines the strong light absorption and the flexibility of monolayer TMDs and environmental stability of MXenes. Because of the wide bandgap of MXene QDs, the 0D/2D mixed-dimensional hybrid could enhance the UV absorption of TMDs by interfacial energy transfer. Since the monolayer is atomically thin, the generated carriers are created close to the interface and can be collected quickly with less scattering and lower dark current. This thin geometry also allows quantum dots to affect the whole channel of monolayer TMD and strengthens the charge-transfer process.
UV detection with monolayer WS2 is often limited by weak absorption and low carrier generation, especially in the UV range. To overcome this, we integrated Ti2N MXene quantum dots (MQDs) with monolayer WS2 to form a 0D/2D hybrid photodetector aimed at improving UV sensitivity. The hybrid shows a strong and selective enhancement at 375 nm, where both detectivity and responsivity increase sharply compared with pristine WS2. The detectivity reaches ~2x1011Jones and the responsivity approaches ~0.58 A/W at 375 nm, far exceeding the performance of the monolayer device. At higher wavelengths (405 nm–633 nm), hybrid device shows minimal response, confirming that the hybrid structure primarily boosts the UV regime without affecting visible-light selectivity. These results demonstrate that Ti2N MQDs act as effective UV sensitizers, improving charge transfer and light harvesting while maintaining spectral selectivity, making the hybrid WS2/MQDs platform a promising route toward high-performance UV photodetectors.

목차 (Table of Contents)

• Chapter 1. Introduction 1
• 1.1. Photodetectors 1
• 1.1.1. Mechanism of photodetection 3
• 1.1.2. Photodetectors figure-of-merit 5
• 1.2. Advantages and Challenges of TMDs in Optoelectronics 6
• 1.3 Optical Properties of MXene Quantum Dots 9
• 1.4. 0D/2D Hybrid Structures in Optoelectronics 12
• 1.5. Motivation and scope of this thesis 15
• Chapter 2. Experimental Methods 17
• 2.1. Sample preparation 17
• 2.1.1. Synthesis of MQDs 17
• 2.1.2. Preparation for Monolayer WS2 18
• 2.1.3 Patterning of Metal Electrodes 20
• 2.1.4. Preparation of WS2/Ti2N MQDs Hybrid structure 21
• 2.2. Optical Measurements 22
• 2.2.1. Photoluminescence Spectroscopy 22
• 2.2.2. Absorption spectroscopy 23
• 2.3. Device fabrication and electrical measurements 23
• 2.4. Photocurrent measurement setup 24
• Chapter 3. Results and Discussion 26
• 3.1. Optical Properties of WS2 and Hybrid Device 26
• 3.2. Absorption Enhancement of Hybrid in UV range 29
• 3.3. Wavelength-Dependent Photoresponse 31
• 3.4. Responsivity and Detectivity Analysis 35
• 3.5. Interfacial Charge Transfer Mechanism 40
• 3.6. Summary of Photoresponse Behavior 40
• Chapter 4. Summary and Outlook 43