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The zero-bandgap properties of graphene (Gr) limit its various applications. Fluorination is an alternative strategy to open the bandgap to broaden its applications. However, traditional fluorination methods are detrimental to human health and contribute extensively to environmental pollution. Here, we develop a simple and safe process of weak fluorination that introduces fluorine atoms into Gr, breaking its zero-bandgap structure. This strategy is more environmentally friendly, risk-free, and harmless to humans, making it suitable for large-scale production. Through this highly controllable weak fluorination process, micro-area selective fluorination is achieved and induces excellent photoluminescence characteristics but maintains a high mobility, compared to the pristine Gr with zero bandgap. The degree of fluorination determines the photoelectric and transport properties of Gr. This work provides an experimental foundation for developing materials based on fluorinated graphene and designing high-performance functional devices. [ABSTRACT FROM AUTHOR]

The hybrid integration of oxide two-dimensional electron gases (2DEGs) and 2D layered transition-metal dichalcogenides is expected to exhibit diverse physical phenomena, which cannot appear in either material alone. Here, we demonstrate a gate-free tunable 2D/2D heterojunction composed of MoTe2 and 2DEG at the surface of KTaO3. The combination of metallic MoTe2 and n-type 2DEG together forms a Schottky diode with a large on/off current ratio of 104 at room temperature. Moreover, the MoTe2/2DEG diode exhibits largely tunable electrical transport characteristics without gate voltages. By applying bias voltages, the diode shows tunable transport properties ranging from insulating to excellent rectifying behaviors. The bias-voltage-dependent modulation dominantly originates from the tunable Schottky barrier width controlled by the carrier density of the 2DEG. Our results pave a way for the development of 2D nanoelectronic devices such as multi-bit memories and bias sensors. [ABSTRACT FROM AUTHOR]

Persistent photoconductivity (PPC) describes the light-induced persistent enhancement of conductivity after turning off the light, which is tunable by a gating voltage. Here, we report a bias-tunable PPC in van der Waals heterojunctions of black phosphorus/2D electron gas (BP/2DEG) on SrTiO3. With a light pulse applied, the resistance of a BP/2DEG heterojunction decreases dramatically over five orders of magnitude. Such a low resistive state is maintained over 5 days without any sign of recovery. The application of the negative bias voltage can tune the device into a high resistive state. By combining the light and bias voltages, the device enables it to change into any resistance value, indicating a potential application for multi-bit photoelectric memory. Further study demonstrates that the BP/2DEG heterojunction can achieve both optical writing/electric erasing and electric writing/optical erasing. Our work paves the way to control the PPC through the bias voltage. [ABSTRACT FROM AUTHOR]

The heterostructures combining 2D layered material and two-dimensional electron gas (2DEG) at the surface or interfaces of complex oxides may exhibit abundant and interesting physical properties. In this paper, we report the development and photoelectrical properties of a p-n heterojunction composed of a p-type 2D layered material WSe2 and n-type 2DEG at the LaAlO3/KTaO3 interface. The WSe2 flake was first fabricated by mechanical exfoliation and then integrated with 2DEG at the LaAlO3/KTaO3 interface obtained using pulsed laser deposition. The WSe2/2DEG heterostructure shows good rectifying behavior with a low leakage current at room temperature. Additionally, compared with oxide 2DEG, the WSe2/2DEG heterostructure exhibits greatly enhanced visible-light photoresponse. The results imply that the 2D heterostructures based on disparate material systems can be realized and demonstrate potential applications in nanoscale electronics and optoelectronics, which are of great scientific and technological significances. [ABSTRACT FROM AUTHOR]

The hybrid integration of oxide two-dimensional electron gases (2DEGs) and 2D layered transition-metal dichalcogenides is expected to exhibit diverse physical phenomena, which cannot appear in either material alone. Here, we demonstrate a gate-free tunable 2D/2D heterojunction composed of MoTe2 and 2DEG at the surface of KTaO3. The combination of metallic MoTe2 and n-type 2DEG together forms a Schottky diode with a large on/off current ratio of 104 at room temperature. Moreover, the MoTe2/2DEG diode exhibits largely tunable electrical transport characteristics without gate voltages. By applying bias voltages, the diode shows tunable transport properties ranging from insulating to excellent rectifying behaviors. The bias-voltage-dependent modulation dominantly originates from the tunable Schottky barrier width controlled by the carrier density of the 2DEG. Our results pave a way for the development of 2D nanoelectronic devices such as multi-bit memories and bias sensors. [ABSTRACT FROM AUTHOR]

Persistent photoconductivity (PPC) describes the light-induced persistent enhancement of conductivity after turning off the light, which is tunable by a gating voltage. Here, we report a bias-tunable PPC in van der Waals heterojunctions of black phosphorus/2D electron gas (BP/2DEG) on SrTiO3. With a light pulse applied, the resistance of a BP/2DEG heterojunction decreases dramatically over five orders of magnitude. Such a low resistive state is maintained over 5 days without any sign of recovery. The application of the negative bias voltage can tune the device into a high resistive state. By combining the light and bias voltages, the device enables it to change into any resistance value, indicating a potential application for multi-bit photoelectric memory. Further study demonstrates that the BP/2DEG heterojunction can achieve both optical writing/electric erasing and electric writing/optical erasing. Our work paves the way to control the PPC through the bias voltage. [ABSTRACT FROM AUTHOR]