Advanced Functional Materials

/Advanced Functional Materials

Van der Waals Heterostructure Devices with Dynamically Controlled Conduction Polarity and Multifunctionality

2018-11-17T22:32:33+00:00November 17th, 2018|Categories: Publications|Tags: , , |

A new application of vdWHs to dynamically control and optimize the electronic and optoelectronic properties of 2D materials is demonstrated. The semivertical MoTe2/MoS2 structure allows for a desirable multifunctional integration of field effect transistorswith on/off ratio >107 and diode with rectification ratio >106. Moreover, the devices exhibit strong capability of suppressing the widely observed trap states–related negative photoresponse effect. Abstract Controlling the conduction behavior of 2D materials is an important prerequisite to achieve their electronic and optoelectronic applications. However, most of the reported approaches are aware of the shortcomings of inflexibility and complexity, which limits the possibility of multifunctional integration. Here, taking advantage of van der Waals heterostructure engineering, a simple method to achieve a dynamically controlled binary channel in a semivertical MoTe2/MoS2 field effect transistor is proposed. It is enabled by the high switchability between tunneling and thermal transports through simply changing the sign of voltage bias. In addition, the proposed system allows for multifunctional integration of transistor with on/off ratio >107 and diode with rectification ratio >106. Moreover, the devices show screen capability to negative photoresponse effect that is widely observed in ambipolar materials, hence improving the photodetection reliability and sensitivity. This study broadens the functionalities of van der Waals heterostructures and opens up more possibilities to realize multifunctional devices.

Published in: "Advanced Functional Materials".

Planar Alignment of Graphene Sheets by a Rotating Magnetic Field for Full Exploitation of Graphene as a 2D Material

2018-11-17T22:32:30+00:00November 17th, 2018|Categories: Publications|Tags: |

Planar alignment of suspended graphene sheets is realized with a near‐perfect order parameter and high optical anisotropy by using a rotating magnetic field produced by a pair of small NdFeB magnets, and can be further patterned and immobilized by photolithography. The arbitrary orientational and spatial control has enabled a wide range of device applications of graphene and related materials. Abstract Planar alignment of disc‐like nanomaterials is required to transfer their superior anisotropic properties from microscopic individual structures to macroscopic collective assemblies. However, such alignment by electrical or magnetic field is challenging due to their additional degrees of orientational freedom compared to that of rod‐like nanostructures. Here, the realization of planar alignment of suspended graphene sheets using a rotating magnetic field produced by a pair of small NdFeB magnets and subsequent demonstration of high optical anisotropy and potential novel device applications is reported. Compared to partially aligned sheets with a static magnetic field, planar aligned graphene suspensions exhibit a near‐perfect order parameter, much higher birefringence and anisotropic absorption/transmission. A unique feature of discotic nanomaterial assemblies is that the observed order parameter and optical property can vary from isotropic to partial and complete alignment depending on the experimental configuration. By immobilizing and patterning aligned graphene in a UV‐curable polymer resin, we further demonstrated an all‐graphene permanent display, which exhibits wide‐angle, high dark‐bright contrast in either transmission or reflection mode without any polarizing optics. The ability to control and pattern graphene orientation in all three dimensions opens up new exploration and broad

Published in: "Advanced Functional Materials".

Saddle‐Point Excitons and Their Extraordinary Light Absorption in 2D β‐Phase Group‐IV Monochalcogenides

2018-11-17T22:32:26+00:00November 17th, 2018|Categories: Publications|Tags: |

Monolayer β‐phase group‐IV monochalcogenides possess saddle‐points in the joint density of states, which leads to a remarkable absorption peak within the fundamental gap. Accordingly, the power conversion efficiencies for monolayer β‐GeSe and β‐SnSe are significantly higher than reported high‐performance ultrathin solar cells using transition metal dichalcogenides. Abstract In 2D materials, saddle‐points in the electronic structure give rise to diverging density of states, which leads to intriguing physical phenomena useful for applications, including magnetism, superconductivity, charge density wave, as well as enhanced optical absorption. Using first‐principles calculations, monolayer β‐phase group‐IV monochalcogenides (MX, M = Ge or Sn; X = S or Se) are shown to be a new class of 2D materials that possess saddle‐points in both the lowest conduction band and the highest valence band as well as in the joint density of states. Due to the existence of saddle‐points, a remarkable absorption peak within the fundamental gap is found in these materials when the light polarization is along the armchair (y) direction. The properties of saddle‐point excitons can be effectively tuned by both the strain and thickness of these materials. Importantly, the strong optical absorbance induced by saddle‐point exciton absorptions and the appropriate bandgap give ideal power conversion efficiencies as large as 1.11% for monolayer β‐SnSe, significantly higher than reported high‐performance ultrathin solar cells using transition metal dichalcogenides. These results not only open new avenues for exploring novel many‐body physics, but also suggest β‐phase MXs could be promising candidates for future optoelectronic devices.

Published in: "Advanced Functional Materials".

Stable Sulfur‐Intercalated 1T′ MoS2 on Graphitic Nanoribbons as Hydrogen Evolution Electrocatalyst

2018-11-17T22:32:24+00:00November 17th, 2018|Categories: Publications|Tags: |

The metastable 1T′ polymorph of MoS2 is an excellent catalyst toward the hydrogen evolution reaction. However, its production is limited by its lower energetic stability compared to the semiconductor 2H MoS2 phase. Stabilization of the 1T′ polymorph can be achieved through intercalation of sulfur‐based compounds without adversely affecting its catalytic properties. Abstract The metastable 1T′ polymorph of molybdenum disulfide (MoS2) has shown excellent catalytic activity toward the hydrogen evolution reaction (HER) in water‐splitting applications. Its basal plane exhibits high catalytic activity comparable to the edges in 2H MoS2 and noble metal platinum. However, the production and application of this polymorph are limited by its lower energetic stability compared to the semiconducting 2H MoS2 phase. Here, the production of stable intercalated 1T′ MoS2 nanosheets attached on graphitic nanoribbons is reported. The intercalated 1T′ MoS2 exhibits a stoichiometric S:Mo ratio of 2.3 (±0.1):1 with an expanded interlayer distance of 10 Å caused by a sulfur‐rich intercalation agent and is stable at room temperature for several months even after drying. The composition, structure, and catalytic activity toward HER are investigated both experimentally and theoretically. It is concluded that the 1T′ MoS2 phase is stabilized by the intercalated agents, which further improves the basal planes′ catalytic activity toward HER.

Published in: "Advanced Functional Materials".

Dipole Formation at the MoO3/Conjugated Polymer Interface

2018-11-17T22:32:20+00:00November 17th, 2018|Categories: Publications|Tags: |

MoO3 forms a strong dipole at the interface to P3HT/PC61BM. The dipole increases with increasing thickness of the MoO3 layer and saturates at 2.2 eV at a thickness around 3 nm of MoO3. The formation of the strong dipole is of high importance for the charge transport over the MoO3/polymer interface. Abstract MoO3 is known as high work function (WF) transparent metal oxides. It is used as anode buffer layer in organic based solar cells because of its capability to extract electrons and inject holes from the active layer due to its high WF. Here a broad range of techniques is used to determine the energy levels of the bulk heterojunction (BHJ) and MoO3 to determine that the minimum deposition thickness to achieve a closed layer is 1 nm due to penetration of the evaporated MoO3 into the BHJ. The investigation shows that upon evaporation of the MoO3, a strong dipole is formed at the extended interface between the active layer and the MoO3 and that the strength of the dipole increases with increasing thickness of the MoO3 layer and saturates at 2.2 eV at a thickness around 3 nm.

Published in: "Advanced Functional Materials".

Ultralight Conductive and Elastic Aerogel for Skeletal Muscle Atrophy Regeneration

2018-11-17T22:32:16+00:00November 17th, 2018|Categories: Publications|

Methacrylic anhydride–tannic acid (TA–MA), dopamine (DOPA), graphite oxide, and hydrothermal reduction are employed to produce an ultralight conductive and elastic aerogel in multistep base reduction. This aerogel can provide a favorable microenvironment for myogenic differentiation, and its transplantation‐combined electrical stimulation (ES) is proved to be a safe and effective strategy for retarding the disuse muscle atrophy. Abstract Cell‐free materials that can transmit both mechanical and electrical stimulations provide a promising strategy for myoinjury repair. Skeletal muscle is sensitive to electrical stimulation (ES), and accordingly, materials are required to transmit the electrical signal while maintaining their elasticity to build the cellular communication network in denervated muscle for retarding muscle atrophy. Here, tannic acid functionalized with methacrylate group (TA–MA), dopamine, and hydrothermal reduction are employed in multistep base reduction to fabricate a polydopamine (PDA)/reduced graphite oxide (rGO) aerogel. This mussel‐inspired PDA/rGO aerogel possesses good conductivity, electromechanical stability, and appropriate Young’s modulus, which are favorable for the growth and differentiation of C2C12 myoblasts. After the cell‐free PDA/rGO aerogel‐transplanted denervated muscle is loaded with cyclic ES for 3 weeks, the mean muscle fiber size increases by 90% and the maximum contraction force of denervated muscle elevates by 50%, accompanied with a slight inflammation infiltration in muscle. In conclusion, PDA/rGO aerogel is a safe and effective implant for retarding the disuse muscle atrophy.

Published in: "Advanced Functional Materials".

Controlled Synthesis of 2D Palladium Diselenide for Sensitive Photodetector Applications

2018-11-17T22:32:13+00:00November 17th, 2018|Categories: Publications|Tags: |

This study reports on the wafer‐area synthesis of high‐quality 2D PdSe2 layer through a simple selenization method. Both experimental analysis and theoretical simulation reveal that the PdSe2 film exhibits a gradual transition from a semiconductor (monolayer) to semimetal (bulk). Further combination of PdSe2 with Si leads to a fast and sensitive broadband photodiode, with a high responsivity and specific detectivity. Abstract Palladium diselenide (PdSe2), a thus far scarcely studied group‐10 transition metal dichalcogenide has exhibited promising potential in future optoelectronic and electronic devices due to unique structures and electrical properties. Here, the controllable synthesis of wafer‐scale and homogeneous 2D PdSe2 film is reported by a simple selenization approach. By choosing different thickness of precursor Pd layer, 2D PdSe2 with thickness of 1.2–20 nm can be readily synthesized. Interestingly, with the increase in thickness, obvious redshift in wavenumber is revealed by Raman spectroscopy. Moreover, in accordance with density functional theory (DFT) calculation, optical absorption and ultraviolet photoemission spectroscopy (UPS) analyses confirm that the PdSe2 exhibits an evolution from a semiconductor (monolayer) to semimetal (bulk). Further combination of the PdSe2 layer with Si leads to a highly sensitive, fast, and broadband photodetector with a high responsivity (300.2 mA W−1) and specific detectivity (≈1013 Jones). By decorating the device with black phosphorus quantum dots, the device performance can be further optimized. These results suggest the as‐selenized PdSe2 is a promising material for optoelectronic application.

Published in: "Advanced Functional Materials".

Graphene‐Based Actuator with Integrated‐Sensing Function

2018-11-17T22:32:10+00:00November 17th, 2018|Categories: Publications|Tags: , |

A graphene‐based actuator with integrated‐sensing function is developed. It realizes real‐time measurements of the shape deformation of the actuator. A smart gripper based on the actuator demonstrates perfect integration of actuating and sensing functions. It can not only grasp and release an object, but also sense every actuation state of the actuator. Abstract Flexible actuators have important applications in artificial muscles, robotics, optical devices, and so on. However, most of the conventional actuators have only actuation function, lacking in real‐time sensing signal feedbacks. Here, to break the limitation and add functionality in conventional actuators, a graphene‐based actuator with integrated‐sensing function is reported, which avoids the dependence on image post‐processing for actuation detection and realizes real‐time measurement of the shape‐deformation amplitudes of the actuator. The actuator is able to show a large bending actuation (curvature of 1.1 cm−1) based on a dual‐mode actuation mechanism when it is driven by near infrared light. Meanwhile, the relative resistance change of the actuator is −17.5%. The sensing function is attributed to piezoresistivity and thermoresistivity of the reduced graphene oxide and paper composite. This actuator can be used as a strain sensor to monitor human motions. A smart gripper based on the actuators demonstrates perfect integration of the actuating and sensing functions, which can not only grasp and release an object, but also sense every actuation state of the actuator. The developed integrated‐sensing actuator is hopeful to open new application fields in soft robotics, artificial muscles, flexible wearable devices, and other integrated‐multifunctional devices.

Published in: "Advanced Functional Materials".

Graphene Sheets: Planar Alignment of Graphene Sheets by a Rotating Magnetic Field for Full Exploitation of Graphene as a 2D Material (Adv. Funct. Mater. 46/2018)

2018-11-17T22:32:06+00:00November 17th, 2018|Categories: Publications|Tags: |

An aligned graphene assembly amplifies the anisotropic properties of individual sheets, creating in a strong dark/bright contrast in the all‐graphene Panda display. Zhiming M. Wang, Jiming Bao, and co‐workers report in article number 1805255, that such alignment can be achieved with a pair of rotating magnets in an arbitrary direction and space, which opens the opportunity of transfering graphene’s superior properties to a macroscopic graphene assembly.

Published in: "Advanced Functional Materials".

Laser‐Induced Graphene Strain Sensors Produced by Ultraviolet Irradiation of Polyimide

2018-11-17T22:32:05+00:00November 17th, 2018|Categories: Publications|Tags: |

Laser‐induced graphene is obtained by irradiation of polyimide by an ultraviolet laser instead of the typical infrared source, with a fourfold decrease in the penetration depth and doubled spatial resolution. Electromechanical strain sensors are patterned in substrates with different thicknesses and their performance is measured. A low‐cost arterial pulse wave monitor is built, exploring the higher sensitivity of thinner substrates. Abstract Laser‐induced graphene (LIG) can be obtained by irradiation of a polymer by a laser source. The present work demonstrates that it is possible to obtain this kind of material using an ultraviolet laser instead of the typical infrared source. Using this approach, a fourfold decrease in the penetration depth (5 µm) is achieved, while the spatial resolution is doubled. Electromechanical strain LIG sensors are patterned in polyimide substrates with different thicknesses and their performance to strain, bending, and force inputs is measured. A low‐cost arterial pulse wave monitor is built, exploring the high force sensitivity of the sensors produced on the thinner substrates.

Published in: "Advanced Functional Materials".

Humidity‐Controlled Ultralow Power Layer‐by‐Layer Thinning, Nanopatterning and Bandgap Engineering of MoTe2

2018-11-17T22:32:03+00:00November 17th, 2018|Categories: Publications|Tags: |

A precision, laser‐assisted, humidity‐controlled, layer‐by‐layer thinning method in 2D MoTe2 films is presented. Field effect transistors fabricated from thinned layers exhibit an order of magnitude increase in on/off current, enhanced field‐effect mobility, and the fastest photoresponse for (visible) MoTe2 photodetectors reported to date. Localized band gap engineering is also performed, with sub‐200 nm spatial resolution, via the creation of lateral homojunctions. Abstract A highly effective laser thinning method is demonstrated to accurately control the thickness of MoTe2 layers. By utilizing the humidity present in the ambient atmosphere, multilayered MoTe2 films can be uniformly thinned all the way down to monolayer with layer‐by‐layer precision using an ultralow laser power density of 0.2 mW µm−2. Localized bandgap engineering is also performed in MoTe2, by creating regions with different bandgaps on the same film, enabling the formation of lateral homojunctions with sub‐200 nm spatial resolution. Field‐effect transistors fabricated from these thinned layers exhibit significantly improved electrical properties with an order of magnitude increase in on/off current ratios, along with enhancements in on‐current and field‐effect mobility values. Thinned devices also exhibit the fastest photoresponse (45 µs) for an MoTe2‐based visible photodetector reported to date, along with a high photoresponsivity. A highly sensitive monolayer MoTe2 photodetector is also reported. These results demonstrate the efficiency of the presented thinning approach in producing high‐quality MoTe2 films for electronic and optoelectronic applications.

Published in: "Advanced Functional Materials".

Exfoliated Layered Manganese Trichalcogenide Phosphite (MnPX3, X = S, Se) as Electrocatalytic van der Waals Materials for Hydrogen Evolution

2018-11-17T22:32:00+00:00November 17th, 2018|Categories: Publications|Tags: |

Metal phosphorus chalcogenides (MPX 3) have reclaim ample interest as 2D layered materials, due to favorable performances in energy storage and conversion. Herein, MnPX 3 (X = S, Se) are synthetized and submitted to shear force exfoliation. Exfoliated MnPSe3 has the lowest onset potential and best stability for hydrogen evolution. Such materials show a great promise for future in a hydrogen‐based economy. Abstract Layered metal trichalcogen phosphites, also entitled as metal phosphorus chalcogenides (MPX 3), have regained abundant interest, not only due to their magnetic properties, but also due to promising performances in energy storage and conversion. Herein, two different layered manganese trichalcogen phosphites, MnPX3 (X = S, Se), are synthetized and submitted to shear force exfoliation. Structural and morphological characterization point to the fact that exfoliated MPX 3 (exf‐MnPX 3) undergo mainly a downsizing process, alongside with delamination. Layered exf‐MnPSe3 has the lowest onset potential for hydrogen evolution reaction (HER) in both media. In acidic media, a comparative improvement of 350 mV is observed for exf‐MnPSe3 relative to the bulk MnPSe3. The materials stability as electrocatalysts is also tested for HER in a wide pH range, in which exf‐MnPSe3 has a good stability after 100 cycles. The improved performance of exf‐MnPSe3 can be correlated with the lower relative abundance of Mn and P oxides detected in the Mn 2p and P 2p core levels. Such materials show a great promise for future in a hydrogen‐based economy.

Published in: "Advanced Functional Materials".

Exfoliated Layered Manganese Trichalcogenide Phosphite (MnPX3, X = S, Se) as Electrocatalytic van der Waals Materials for Hydrogen Evolution

2018-11-17T02:32:30+00:00November 17th, 2018|Categories: Publications|Tags: |

Metal phosphorus chalcogenides (MPX 3) have reclaim ample interest as 2D layered materials, due to favorable performances in energy storage and conversion. Herein, MnPX 3 (X = S, Se) are synthetized and submitted to shear force exfoliation. Exfoliated MnPSe3 has the lowest onset potential and best stability for hydrogen evolution. Such materials show a great promise for future in a hydrogen‐based economy. Abstract Layered metal trichalcogen phosphites, also entitled as metal phosphorus chalcogenides (MPX 3), have regained abundant interest, not only due to their magnetic properties, but also due to promising performances in energy storage and conversion. Herein, two different layered manganese trichalcogen phosphites, MnPX3 (X = S, Se), are synthetized and submitted to shear force exfoliation. Structural and morphological characterization point to the fact that exfoliated MPX 3 (exf‐MnPX 3) undergo mainly a downsizing process, alongside with delamination. Layered exf‐MnPSe3 has the lowest onset potential for hydrogen evolution reaction (HER) in both media. In acidic media, a comparative improvement of 350 mV is observed for exf‐MnPSe3 relative to the bulk MnPSe3. The materials stability as electrocatalysts is also tested for HER in a wide pH range, in which exf‐MnPSe3 has a good stability after 100 cycles. The improved performance of exf‐MnPSe3 can be correlated with the lower relative abundance of Mn and P oxides detected in the Mn 2p and P 2p core levels. Such materials show a great promise for future in a hydrogen‐based economy.

Published in: "Advanced Functional Materials".

Ultralight Conductive and Elastic Aerogel for Skeletal Muscle Atrophy Regeneration

2018-11-16T04:34:08+00:00November 16th, 2018|Categories: Publications|

Methacrylic anhydride–tannic acid (TA–MA), dopamine (DOPA), graphite oxide, and hydrothermal reduction are employed to produce an ultralight conductive and elastic aerogel in multistep base reduction. This aerogel can provide a favorable microenvironment for myogenic differentiation, and its transplantation‐combined electrical stimulation (ES) is proved to be a safe and effective strategy for retarding the disuse muscle atrophy. Abstract Cell‐free materials that can transmit both mechanical and electrical stimulations provide a promising strategy for myoinjury repair. Skeletal muscle is sensitive to electrical stimulation (ES), and accordingly, materials are required to transmit the electrical signal while maintaining their elasticity to build the cellular communication network in denervated muscle for retarding muscle atrophy. Here, tannic acid functionalized with methacrylate group (TA–MA), dopamine, and hydrothermal reduction are employed in multistep base reduction to fabricate a polydopamine (PDA)/reduced graphite oxide (rGO) aerogel. This mussel‐inspired PDA/rGO aerogel possesses good conductivity, electromechanical stability, and appropriate Young’s modulus, which are favorable for the growth and differentiation of C2C12 myoblasts. After the cell‐free PDA/rGO aerogel‐transplanted denervated muscle is loaded with cyclic ES for 3 weeks, the mean muscle fiber size increases by 90% and the maximum contraction force of denervated muscle elevates by 50%, accompanied with a slight inflammation infiltration in muscle. In conclusion, PDA/rGO aerogel is a safe and effective implant for retarding the disuse muscle atrophy.

Published in: "Advanced Functional Materials".

Graphene‐Based Actuator with Integrated‐Sensing Function

2018-11-16T04:34:05+00:00November 16th, 2018|Categories: Publications|Tags: , |

A graphene‐based actuator with integrated‐sensing function is developed. It realizes real‐time measurements of the shape deformation of the actuator. A smart gripper based on the actuator demonstrates perfect integration of actuating and sensing functions. It can not only grasp and release an object, but also sense every actuation state of the actuator. Abstract Flexible actuators have important applications in artificial muscles, robotics, optical devices, and so on. However, most of the conventional actuators have only actuation function, lacking in real‐time sensing signal feedbacks. Here, to break the limitation and add functionality in conventional actuators, a graphene‐based actuator with integrated‐sensing function is reported, which avoids the dependence on image post‐processing for actuation detection and realizes real‐time measurement of the shape‐deformation amplitudes of the actuator. The actuator is able to show a large bending actuation (curvature of 1.1 cm−1) based on a dual‐mode actuation mechanism when it is driven by near infrared light. Meanwhile, the relative resistance change of the actuator is −17.5%. The sensing function is attributed to piezoresistivity and thermoresistivity of the reduced graphene oxide and paper composite. This actuator can be used as a strain sensor to monitor human motions. A smart gripper based on the actuators demonstrates perfect integration of the actuating and sensing functions, which can not only grasp and release an object, but also sense every actuation state of the actuator. The developed integrated‐sensing actuator is hopeful to open new application fields in soft robotics, artificial muscles, flexible wearable devices, and other integrated‐multifunctional devices.

Published in: "Advanced Functional Materials".

Van der Waals Heterostructure Devices with Dynamically Controlled Conduction Polarity and Multifunctionality

2018-11-16T04:33:51+00:00November 16th, 2018|Categories: Publications|Tags: , , |

A new application of vdWHs to dynamically control and optimize the electronic and optoelectronic properties of 2D materials is demonstrated. The semivertical MoTe2/MoS2 structure allows for a desirable multifunctional integration of field effect transistorswith on/off ratio >107 and diode with rectification ratio >106. Moreover, the devices exhibit strong capability of suppressing the widely observed trap states–related negative photoresponse effect. Abstract Controlling the conduction behavior of 2D materials is an important prerequisite to achieve their electronic and optoelectronic applications. However, most of the reported approaches are aware of the shortcomings of inflexibility and complexity, which limits the possibility of multifunctional integration. Here, taking advantage of van der Waals heterostructure engineering, a simple method to achieve a dynamically controlled binary channel in a semivertical MoTe2/MoS2 field effect transistor is proposed. It is enabled by the high switchability between tunneling and thermal transports through simply changing the sign of voltage bias. In addition, the proposed system allows for multifunctional integration of transistor with on/off ratio >107 and diode with rectification ratio >106. Moreover, the devices show screen capability to negative photoresponse effect that is widely observed in ambipolar materials, hence improving the photodetection reliability and sensitivity. This study broadens the functionalities of van der Waals heterostructures and opens up more possibilities to realize multifunctional devices.

Published in: "Advanced Functional Materials".

Controlled Synthesis of 2D Palladium Diselenide for Sensitive Photodetector Applications

2018-11-16T04:33:48+00:00November 16th, 2018|Categories: Publications|Tags: |

This study reports on the wafer‐area synthesis of high‐quality 2D PdSe2 layer through a simple selenization method. Both experimental analysis and theoretical simulation reveal that the PdSe2 film exhibits a gradual transition from a semiconductor (monolayer) to semimetal (bulk). Further combination of PdSe2 with Si leads to a fast and sensitive broadband photodiode, with a high responsivity and specific detectivity. Abstract Palladium diselenide (PdSe2), a thus far scarcely studied group‐10 transition metal dichalcogenide has exhibited promising potential in future optoelectronic and electronic devices due to unique structures and electrical properties. Here, the controllable synthesis of wafer‐scale and homogeneous 2D PdSe2 film is reported by a simple selenization approach. By choosing different thickness of precursor Pd layer, 2D PdSe2 with thickness of 1.2–20 nm can be readily synthesized. Interestingly, with the increase in thickness, obvious redshift in wavenumber is revealed by Raman spectroscopy. Moreover, in accordance with density functional theory (DFT) calculation, optical absorption and ultraviolet photoemission spectroscopy (UPS) analyses confirm that the PdSe2 exhibits an evolution from a semiconductor (monolayer) to semimetal (bulk). Further combination of the PdSe2 layer with Si leads to a highly sensitive, fast, and broadband photodetector with a high responsivity (300.2 mA W−1) and specific detectivity (≈1013 Jones). By decorating the device with black phosphorus quantum dots, the device performance can be further optimized. These results suggest the as‐selenized PdSe2 is a promising material for optoelectronic application.

Published in: "Advanced Functional Materials".

Humidity‐Controlled Ultralow Power Layer‐by‐Layer Thinning, Nanopatterning and Bandgap Engineering of MoTe2

2018-11-13T10:32:52+00:00November 13th, 2018|Categories: Publications|Tags: |

A precision, laser‐assisted, humidity‐controlled, layer‐by‐layer thinning method in 2D MoTe2 films is presented. Field effect transistors fabricated from thinned layers exhibit an order of magnitude increase in on/off current, enhanced field‐effect mobility, and the fastest photoresponse for (visible) MoTe2 photodetectors reported to date. Localized band gap engineering is also performed, with sub‐200 nm spatial resolution, via the creation of lateral homojunctions. Abstract A highly effective laser thinning method is demonstrated to accurately control the thickness of MoTe2 layers. By utilizing the humidity present in the ambient atmosphere, multilayered MoTe2 films can be uniformly thinned all the way down to monolayer with layer‐by‐layer precision using an ultralow laser power density of 0.2 mW µm−2. Localized bandgap engineering is also performed in MoTe2, by creating regions with different bandgaps on the same film, enabling the formation of lateral homojunctions with sub‐200 nm spatial resolution. Field‐effect transistors fabricated from these thinned layers exhibit significantly improved electrical properties with an order of magnitude increase in on/off current ratios, along with enhancements in on‐current and field‐effect mobility values. Thinned devices also exhibit the fastest photoresponse (45 µs) for an MoTe2‐based visible photodetector reported to date, along with a high photoresponsivity. A highly sensitive monolayer MoTe2 photodetector is also reported. These results demonstrate the efficiency of the presented thinning approach in producing high‐quality MoTe2 films for electronic and optoelectronic applications.

Published in: "Advanced Functional Materials".

Laser‐Induced Graphene Strain Sensors Produced by Ultraviolet Irradiation of Polyimide

2018-11-13T10:32:49+00:00November 13th, 2018|Categories: Publications|Tags: |

Laser‐induced graphene is obtained by irradiation of polyimide by an ultraviolet laser instead of the typical infrared source, with a fourfold decrease in the penetration depth and doubled spatial resolution. Electromechanical strain sensors are patterned in substrates with different thicknesses and their performance is measured. A low‐cost arterial pulse wave monitor is built, exploring the higher sensitivity of thinner substrates. Abstract Laser‐induced graphene (LIG) can be obtained by irradiation of a polymer by a laser source. The present work demonstrates that it is possible to obtain this kind of material using an ultraviolet laser instead of the typical infrared source. Using this approach, a fourfold decrease in the penetration depth (5 µm) is achieved, while the spatial resolution is doubled. Electromechanical strain LIG sensors are patterned in polyimide substrates with different thicknesses and their performance to strain, bending, and force inputs is measured. A low‐cost arterial pulse wave monitor is built, exploring the high force sensitivity of the sensors produced on the thinner substrates.

Published in: "Advanced Functional Materials".

Saddle‐Point Excitons and Their Extraordinary Light Absorption in 2D β‐Phase Group‐IV Monochalcogenides

2018-11-13T10:32:45+00:00November 13th, 2018|Categories: Publications|Tags: |

Monolayer β‐phase group‐IV monochalcogenides possess saddle‐points in the joint density of states, which leads to a remarkable absorption peak within the fundamental gap. Accordingly, the power conversion efficiencies for monolayer β‐GeSe and β‐SnSe are significantly higher than reported high‐performance ultrathin solar cells using transition metal dichalcogenides. Abstract In 2D materials, saddle‐points in the electronic structure give rise to diverging density of states, which leads to intriguing physical phenomena useful for applications, including magnetism, superconductivity, charge density wave, as well as enhanced optical absorption. Using first‐principles calculations, monolayer β‐phase group‐IV monochalcogenides (MX, M = Ge or Sn; X = S or Se) are shown to be a new class of 2D materials that possess saddle‐points in both the lowest conduction band and the highest valence band as well as in the joint density of states. Due to the existence of saddle‐points, a remarkable absorption peak within the fundamental gap is found in these materials when the light polarization is along the armchair (y) direction. The properties of saddle‐point excitons can be effectively tuned by both the strain and thickness of these materials. Importantly, the strong optical absorbance induced by saddle‐point exciton absorptions and the appropriate bandgap give ideal power conversion efficiencies as large as 1.11% for monolayer β‐SnSe, significantly higher than reported high‐performance ultrathin solar cells using transition metal dichalcogenides. These results not only open new avenues for exploring novel many‐body physics, but also suggest β‐phase MXs could be promising candidates for future optoelectronic devices.

Published in: "Advanced Functional Materials".

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