Abstract

Two-dimensional (2D) semiconductors such as molybdenum disulfide (MoS₂) have recently attracted extensive research attention due to their promising compatibility with silicon-based electronics. However, several key challenges still limit their practical integration. Two of the critical issues are: (1) the intrinsic depletion-mode (normally-on) operation of MoS₂ field-effect transistors (FETs), and (2) the large hysteresis commonly observed in the transfer characteristics of MoS₂ FETs due to the inherent sulfur defects. Addressing them is essential for CMOS-compatible 2D-transistor technologies. In this work, we report for the first time that surface modification of the exfoliated multilayer MoS₂ FETs with PBTTT-C₁₄ (poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene)), a p-type conjugated organic polymer, converts the device from depletion-mode to enhancement-mode operation while simultaneously and strongly suppressing hysteresis. Specifically, the threshold voltage (Vth) shifts from −9.6 V to +5.9 V (total shift ~15.5 V), and the hysteresis window decreases from 8.8 V to 1.3 V (~85% reduction). This originates from interfacial charge transfer at the MoS₂/PBTTT-C₁₄ interface, enabled by favourable band alignment. Additionally, donor-like sulfur vacancy states in MoS₂ are involved in interfacial charge transfer to the p-type PBTTT-C₁₄ layer, resulting in effective deactivation of these donor states associated with the generation of free charge carriers and hysteresis. Importantly, this highlights the role of electronic interaction at the interface. To further validate this charge-transfer-driven mechanism, P₃HT (poly(3-hexylthiophene-2,5-diyl)) with similar energy levels to PBTTT-C₁₄ was employed, and it also showed similar enhancement-mode behaviour and hysteresis suppression. In contrast, treatment with a n-type polymer (N₂₂₀₀) (poly{[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)}) with less favourable band alignment which limits interfacial charge transfer, resulted in a smaller Vth shift from −13.8 V to −8.3 V (total shift of ~5.5 V) without inducing enhancement-mode behaviour and a hysteresis reduction from 11.2 V to 5.6 V (~50%). Furthermore, treatment with the high band gap, insulating polymer polydimethylsiloxane (PDMS) leads to no observable change in Vth and hysteresis, supporting the charge-transfer-dominated origin of the observed effects. Overall, this work establishes a clear correlation between polymer energy-level alignment, interfacial charge transfer, and device performance. The results demonstrate that p-type organic polymers with suitable energy levels can simultaneously enable enhancement-mode operation and strong hysteresis suppression in MoS₂ FETs, providing a scalable and CMOS-compatible strategy for high-performance 2D electronics.

Abstract

Two-dimensional (2D) semiconductors such as molybdenum disulfide (MoS₂) have recently attracted extensive research attention due to their promising compatibility with silicon-based electronics. However, several key challenges still limit their practical integration. Two of the critical issues are: (1) the intrinsic depletion-mode (normally-on) operation of MoS₂ field-effect transistors (FETs), and (2) the large hysteresis commonly observed in the transfer characteristics of MoS₂ FETs due to the inherent sulfur defects. Addressing them is essential for CMOS-compatible 2D-transistor technologies. In this work, we report for the first time that surface modification of the exfoliated multilayer MoS₂ FETs with PBTTT-C₁₄ (poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene)), a p-type conjugated organic polymer, converts the device from depletion-mode to enhancement-mode operation while simultaneously and strongly suppressing hysteresis. Specifically, the threshold voltage (Vth) shifts from −9.6 V to +5.9 V (total shift ~15.5 V), and the hysteresis window decreases from 8.8 V to 1.3 V (~85% reduction). This originates from interfacial charge transfer at the MoS₂/PBTTT-C₁₄ interface, enabled by favourable band alignment. Additionally, donor-like sulfur vacancy states in MoS₂ are involved in interfacial charge transfer to the p-type PBTTT-C₁₄ layer, resulting in effective deactivation of these donor states associated with the generation of free charge carriers and hysteresis. Importantly, this highlights the role of electronic interaction at the interface. To further validate this charge-transfer-driven mechanism, P₃HT (poly(3-hexylthiophene-2,5-diyl)) with similar energy levels to PBTTT-C₁₄ was employed, and it also showed similar enhancement-mode behaviour and hysteresis suppression. In contrast, treatment with a n-type polymer (N₂₂₀₀) (poly{[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)}) with less favourable band alignment which limits interfacial charge transfer, resulted in a smaller Vth shift from −13.8 V to −8.3 V (total shift of ~5.5 V) without inducing enhancement-mode behaviour and a hysteresis reduction from 11.2 V to 5.6 V (~50%). Furthermore, treatment with the high band gap, insulating polymer polydimethylsiloxane (PDMS) leads to no observable change in Vth and hysteresis, supporting the charge-transfer-dominated origin of the observed effects. Overall, this work establishes a clear correlation between polymer energy-level alignment, interfacial charge transfer, and device performance. The results demonstrate that p-type organic polymers with suitable energy levels can simultaneously enable enhancement-mode operation and strong hysteresis suppression in MoS₂ FETs, providing a scalable and CMOS-compatible strategy for high-performance 2D electronics.

Abstract

Two-dimensional (2D) semiconductors such as molybdenum disulfide (MoS₂) have recently attracted extensive research attention due to their promising compatibility with silicon-based electronics. However, several key challenges still limit their practical integration. Two of the critical issues are: (1) the intrinsic depletion-mode (normally-on) operation of MoS₂ field-effect transistors (FETs), and (2) the large hysteresis commonly observed in the transfer characteristics of MoS₂ FETs due to the inherent sulfur defects. Addressing them is essential for CMOS-compatible 2D-transistor technologies. In this work, we report for the first time that surface modification of the exfoliated multilayer MoS₂ FETs with PBTTT-C₁₄ (poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene)), a p-type conjugated organic polymer, converts the device from depletion-mode to enhancement-mode operation while simultaneously and strongly suppressing hysteresis. Specifically, the threshold voltage (Vth) shifts from −9.6 V to +5.9 V (total shift ~15.5 V), and the hysteresis window decreases from 8.8 V to 1.3 V (~85% reduction). This originates from interfacial charge transfer at the MoS₂/PBTTT-C₁₄ interface, enabled by favourable band alignment. Additionally, donor-like sulfur vacancy states in MoS₂ are involved in interfacial charge transfer to the p-type PBTTT-C₁₄ layer, resulting in effective deactivation of these donor states associated with the generation of free charge carriers and hysteresis. Importantly, this highlights the role of electronic interaction at the interface. To further validate this charge-transfer-driven mechanism, P₃HT (poly(3-hexylthiophene-2,5-diyl)) with similar energy levels to PBTTT-C₁₄ was employed, and it also showed similar enhancement-mode behaviour and hysteresis suppression. In contrast, treatment with a n-type polymer (N₂₂₀₀) (poly{[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)}) with less favourable band alignment which limits interfacial charge transfer, resulted in a smaller Vth shift from −13.8 V to −8.3 V (total shift of ~5.5 V) without inducing enhancement-mode behaviour and a hysteresis reduction from 11.2 V to 5.6 V (~50%). Furthermore, treatment with the high band gap, insulating polymer polydimethylsiloxane (PDMS) leads to no observable change in Vth and hysteresis, supporting the charge-transfer-dominated origin of the observed effects. Overall, this work establishes a clear correlation between polymer energy-level alignment, interfacial charge transfer, and device performance. The results demonstrate that p-type organic polymers with suitable energy levels can simultaneously enable enhancement-mode operation and strong hysteresis suppression in MoS₂ FETs, providing a scalable and CMOS-compatible strategy for high-performance 2D electronics.

Abstract

Two-dimensional (2D) semiconductors such as molybdenum disulfide (MoS₂) have recently attracted extensive research attention due to their promising compatibility with silicon-based electronics. However, several key challenges still limit their practical integration. Two of the critical issues are: (1) the intrinsic depletion-mode (normally-on) operation of MoS₂ field-effect transistors (FETs), and (2) the large hysteresis commonly observed in the transfer characteristics of MoS₂ FETs due to the inherent sulfur defects. Addressing them is essential for CMOS-compatible 2D-transistor technologies. In this work, we report for the first time that surface modification of the exfoliated multilayer MoS₂ FETs with PBTTT-C₁₄ (poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene)), a p-type conjugated organic polymer, converts the device from depletion-mode to enhancement-mode operation while simultaneously and strongly suppressing hysteresis. Specifically, the threshold voltage (Vth) shifts from −9.6 V to +5.9 V (total shift ~15.5 V), and the hysteresis window decreases from 8.8 V to 1.3 V (~85% reduction). This originates from interfacial charge transfer at the MoS₂/PBTTT-C₁₄ interface, enabled by favourable band alignment. Additionally, donor-like sulfur vacancy states in MoS₂ are involved in interfacial charge transfer to the p-type PBTTT-C₁₄ layer, resulting in effective deactivation of these donor states associated with the generation of free charge carriers and hysteresis. Importantly, this highlights the role of electronic interaction at the interface. To further validate this charge-transfer-driven mechanism, P₃HT (poly(3-hexylthiophene-2,5-diyl)) with similar energy levels to PBTTT-C₁₄ was employed, and it also showed similar enhancement-mode behaviour and hysteresis suppression. In contrast, treatment with a n-type polymer (N₂₂₀₀) (poly{[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)}) with less favourable band alignment which limits interfacial charge transfer, resulted in a smaller Vth shift from −13.8 V to −8.3 V (total shift of ~5.5 V) without inducing enhancement-mode behaviour and a hysteresis reduction from 11.2 V to 5.6 V (~50%). Furthermore, treatment with the high band gap, insulating polymer polydimethylsiloxane (PDMS) leads to no observable change in Vth and hysteresis, supporting the charge-transfer-dominated origin of the observed effects. Overall, this work establishes a clear correlation between polymer energy-level alignment, interfacial charge transfer, and device performance. The results demonstrate that p-type organic polymers with suitable energy levels can simultaneously enable enhancement-mode operation and strong hysteresis suppression in MoS₂ FETs, providing a scalable and CMOS-compatible strategy for high-performance 2D electronics.