Advanced Functional Materials

/Advanced Functional Materials

Sodium Storage and Electrode Dynamics of Tin–Carbon Composite Electrodes from Bulk Precursors for Sodium‐Ion Batteries

2019-03-13T12:32:57+00:00March 13th, 2019|Categories: Publications|Tags: |

Tin supported by nitrogen‐doped graphene nanoplatelets is found to be an efficient anode for sodium‐ion batteries. Rate‐dependent measurements and in situ diffraction reveal the asymmetric storage behavior of tin within a cycle as well as a memory effect. The in situ electrochemical dilatometry study shows the active role of the support in mitigating the large volume changes of tin during (de)sodiation. Abstract Here, a Sn–C composite material prepared from bulk precursors (tin metal, graphite, and melamine) using ball milling and annealing is reported. The composite (58 wt% Sn and 42 wt% N‐doped carbon) shows a capacity up to 445 mAh gSn+C−1 and an excellent cycle life (1000 cycles). For the graphite, the ball milling leads to graphene nanoplatelets (GnP) for which the storage mechanism changes from solvent co‐intercalation to conventional intercalation. The final composite (Sn at nitrogen‐doped graphite nanoplatelets (SnNGnP)) is obtained by combining the GnPs with Sn and melamine as the nitrogen source. Rate‐dependent measurements and in situ X‐ray diffraction are used to study the asymmetric storage behavior of Sn, which shows a more sloping potential profile during sodiation and more defined steps during desodiation. The disappearance of two redox plateaus during desodiation is linked to the preceding sodiation current density (memory effect). The asymmetric behavior is also found by in situ electrochemical dilatometry. This method also shows that the effective electrode expansion during sodiation is much smaller (about +14%) compared to what is expected from Sn (+420%), which gives a reasonable explanation for the excellent cycle life for

Published in: "Advanced Functional Materials".

Production and Patterning of Liquid Phase–Exfoliated 2D Sheets for Applications in Optoelectronics

2019-03-13T12:32:55+00:00March 13th, 2019|Categories: Publications|

The wide range of techniques that can be used toward the production and deposition of 2D materials’ inks into space‐confined patterns or continuous thin films are discussed in this review. The importance of reaching precise control over the material’s properties through a detailed understanding of their structure and interconnectivity between deposited sheets is discussed. Abstract 2D materials (2DMs), which can be produced by exfoliating bulk crystals of layered materials, display unique optical and electrical properties, making them attractive components for a wide range of technological applications. This review describes the most recent developments in the production of high‐quality 2DMs based inks using liquid‐phase exfoliation (LPE), combined with the patterning approaches, highlighting convenient and effective methods for generating materials and films with controlled thicknesses down to the atomic scale. Different processing strategies that can be employed to deposit the produced inks as patterns and functional thin‐films are introduced, by focusing on those that can be easily translated to the industrial scale such as coating, spraying, and various printing technologies. By providing insight into the multiscale analyses of numerous physical and chemical properties of these functional films and patterns, with a specific focus on their extraordinary electronic characteristics, this review offers the readers crucial information for a profound understanding of the fundamental properties of these patterned surfaces as the millstone toward the generation of novel multifunctional devices. Finally, the challenges and opportunities associated to the 2DMs’ integration into working opto‐electronic (nano)devices is discussed.

Published in: "Advanced Functional Materials".

Silver‐Adapted Diffusive Memristor Based on Organic Nitrogen‐Doped Graphene Oxide Quantum Dots (N‐GOQDs) for Artificial Biosynapse Applications

2019-03-13T12:32:53+00:00March 13th, 2019|Categories: Publications|Tags: , , |

Organic nitrogen‐doped graphene oxide quantum‐dot thin film is fabricated by a solution‐processed method and utilized as an insulator medium for dynamic silver (Ag) migration in use as a biocompatible artificial biosynapse. Characteristic threshold resistive switching of the devices is used to emulate bioinspired synaptic functioning. Specifically, fundamental memorization characteristics are achieved via transition from short‐term to long‐term potentiation. Abstract Carbon‐based electronic devices are suitable candidates for bioinspired electronics due to their low cost, eco‐friendliness, mechanical flexibility, and compatibility with complementary metal‐oxide‐semiconductor technology. New types of materials such as graphene quantum dots (GQDs) have attracted attention in the search for new applications beyond solar cells and energy harvesting due to their superior properties such as elevated photoluminescence, high chemical inertness, and excellent biocompatibility. In this paper, a biocompatible/organic electronic synapse based on nitrogen‐doped graphene oxide quantum dots (N‐GOQDs) is reported, which exhibits threshold resistive switching via silver cation (Ag+) migration dynamics. In analogy to the calcium (Ca2+) ion dynamics of biological synapses, important biological synapse functions such as short‐term potentiation (STP), paired‐pulse facilitation, and transition from STP to long‐term plasticity behaviors are replicated. Long‐term depression behavior is also evaluated and specific spike‐timing dependent plasticity is assessed. In addition, elaborated switching mechanism of biosimilar Ag+ migration dynamics provides the potential for using N‐GOQD‐based artificial synapse in future biocompatible neuromorphic systems.

Published in: "Advanced Functional Materials".

High Thermoelectric Performance Achieved in GeTe–Bi2Te3 Pseudo‐Binary via Van der Waals Gap‐Induced Hierarchical Ferroelectric Domain Structure

2019-03-13T12:32:51+00:00March 13th, 2019|Categories: Publications|

Nanoscale‐charged domain walls and microdomains, together with Ge van der Waals gaps in Bi2Te3 alloyed GeTe, combine to make a complex hierarchical architecture which realizes an outstanding figure of merit ZT ≈ 2.4 at 773 K via simultaneous modulation of charge carriers and phonons. Abstract GeTe is an interesting material presenting both spontaneous polarization (ferroelectrics) and outstanding electrical conductivity (ideal for thermoelectrics). Pristine GeTe exhibits classic 71° and 109° submicron ferroelectric domains, and near unity thermoelectric figure of merit ZT at 773 K. In this work, it is demonstrated that Bi2Te3 alloying in GeTe lattice can introduce vast Ge vacancies which can further evolve into nanoscale van der Waals gaps upon proper heat treatment, and that these vacancy gaps can induce 180° nanoscale ferroelectric domain boundaries. These microstructures eventually become a hierarchical ferroelectric domain structure, with size varying from submicron to nanoscale and polarization from 71°, 109° to 180°. The establishment of hierarchical ferroelectric domain structure, together with the nanoscale Ge vacancy van der Waals gaps, has profound effects on the electrical and thermal transport properties, resulting in a striking peak thermoelectric ZT ≈ 2.4 at 773 K. These findings might provide an alternative conception for thermoelectric optimization via microstructure modulation.

Published in: "Advanced Functional Materials".

Electrical Conduction at the Interface between Insulating van der Waals Materials

2019-03-13T12:32:48+00:00March 13th, 2019|Categories: Publications|Tags: , , , , , |

Highly conducting interfaces between insulating 2D materials are demonstrated in van der Waals heterostructures fabricated by molecular‐beam epitaxy. In situ growth monitoring by reflection high energy electron diffraction confirms layer‐by‐layer fabrication of the heterostructures and the formation of abrupt interfaces. Hall effect measurements reveal that the conducting carriers are holes, and their densities are as large as 1014 cm−2. Abstract Emergent properties of 2D materials attract considerable interest in condensed matter physics and materials science due to their distinguished features that are missing in their bulk counterparts. A mainstream in this research field is to broaden the scope of material to expand the horizons of the research area, while developing functional interfaces between different 2D materials is another indispensable research direction. Here, the emergence of electrical conduction at the interface between insulating 2D materials is demonstrated. A new class of van der Waals heterostructures consisting of two sets of insulating transition‐metal dichalcogenides, group‐VI WSe2 and group‐IV TMSe2 (TM = Zr, Hf), is developed via molecular‐beam epitaxy, and it is found that those heterostructures are highly conducting although all the constituent materials are highly insulating. The WSe2/ZrSe2 interface exhibits more conducting behavior than the WSe2/HfSe2 interface, which can be understood by considering the band alignments between constituent materials. Moreover, by increasing Se flux during heterostructure fabrication, the WSe2/ZrSe2 interface becomes more conducting, reaching nearly metallic behavior. Further improvement of the crystalline quality as well as exploring different material combinations are expected to lead to metallic conduction, providing a novel functionality emerging

Published in: "Advanced Functional Materials".

Modulating Blue Phosphorene by Synergetic Codoping: Indirect to Direct Gap Transition and Strong Bandgap Bowing

2019-03-13T12:32:44+00:00March 13th, 2019|Categories: Publications|Tags: |

The band structure of blue phosphorene (BP) is engineered through synergetic codoping with group IV and VI impurities. The indirect to direct bandgap transition is found. The bandgap of BP can be modulated in a wide range, and strong bandgap bowing is found. Lower formation energies indicate that practical fabrication is a possibility. Abstract The structural and electronic properties of synergistically modified blue phosphorene (BP) is investigated. The inversion and threefold rotational symmetries of BP are broken. The codoping of group IV and VI impurities can turn monolayer BP into direct bandgap semiconductors. The underlying physical mechanism is that group IV and VI impurities tailor the valence band maximum and conduction band minimum, respectively, and move them to Γ. All the bandgaps of monolayer, nanoribbons, and quantum dots of BP can be modulated in a wide range, and the strong bandgap bowing is found. In addition, the Coulomb interactions between the screened impurities are revealed. Lower formation energies indicate the fabricating practicability of synergeticly modified BP. Spin–orbit coupling (SOC) can also be tuned by the introduction of impurities.

Published in: "Advanced Functional Materials".

Sodium Storage and Electrode Dynamics of Tin–Carbon Composite Electrodes from Bulk Precursors for Sodium‐Ion Batteries

2019-03-12T02:32:17+00:00March 12th, 2019|Categories: Publications|Tags: |

Tin supported by nitrogen‐doped graphene nanoplatelets is found to be an efficient anode for sodium‐ion batteries. Rate‐dependent measurements and in situ diffraction reveal the asymmetric storage behavior of tin within a cycle as well as a memory effect. The in situ electrochemical dilatometry study shows the active role of the support in mitigating the large volume changes of tin during (de)sodiation. Abstract Here, a Sn–C composite material prepared from bulk precursors (tin metal, graphite, and melamine) using ball milling and annealing is reported. The composite (58 wt% Sn and 42 wt% N‐doped carbon) shows a capacity up to 445 mAh gSn+C−1 and an excellent cycle life (1000 cycles). For the graphite, the ball milling leads to graphene nanoplatelets (GnP) for which the storage mechanism changes from solvent co‐intercalation to conventional intercalation. The final composite (Sn at nitrogen‐doped graphite nanoplatelets (SnNGnP)) is obtained by combining the GnPs with Sn and melamine as the nitrogen source. Rate‐dependent measurements and in situ X‐ray diffraction are used to study the asymmetric storage behavior of Sn, which shows a more sloping potential profile during sodiation and more defined steps during desodiation. The disappearance of two redox plateaus during desodiation is linked to the preceding sodiation current density (memory effect). The asymmetric behavior is also found by in situ electrochemical dilatometry. This method also shows that the effective electrode expansion during sodiation is much smaller (about +14%) compared to what is expected from Sn (+420%), which gives a reasonable explanation for the excellent cycle life for

Published in: "Advanced Functional Materials".

Production and Patterning of Liquid Phase–Exfoliated 2D Sheets for Applications in Optoelectronics

2019-03-12T02:32:15+00:00March 12th, 2019|Categories: Publications|

The wide range of techniques that can be used toward the production and deposition of 2D materials’ inks into space‐confined patterns or continuous thin films are discussed in this review. The importance of reaching precise control over the material’s properties through a detailed understanding of their structure and interconnectivity between deposited sheets is discussed. Abstract 2D materials (2DMs), which can be produced by exfoliating bulk crystals of layered materials, display unique optical and electrical properties, making them attractive components for a wide range of technological applications. This review describes the most recent developments in the production of high‐quality 2DMs based inks using liquid‐phase exfoliation (LPE), combined with the patterning approaches, highlighting convenient and effective methods for generating materials and films with controlled thicknesses down to the atomic scale. Different processing strategies that can be employed to deposit the produced inks as patterns and functional thin‐films are introduced, by focusing on those that can be easily translated to the industrial scale such as coating, spraying, and various printing technologies. By providing insight into the multiscale analyses of numerous physical and chemical properties of these functional films and patterns, with a specific focus on their extraordinary electronic characteristics, this review offers the readers crucial information for a profound understanding of the fundamental properties of these patterned surfaces as the millstone toward the generation of novel multifunctional devices. Finally, the challenges and opportunities associated to the 2DMs’ integration into working opto‐electronic (nano)devices is discussed.

Published in: "Advanced Functional Materials".

Sodium Storage and Electrode Dynamics of Tin–Carbon Composite Electrodes from Bulk Precursors for Sodium‐Ion Batteries

2019-03-11T22:32:29+00:00March 11th, 2019|Categories: Publications|Tags: |

Tin supported by nitrogen‐doped graphene nanoplatelets is found to be an efficient anode for sodium‐ion batteries. Rate‐dependent measurements and in situ diffraction reveal the asymmetric storage behavior of tin within a cycle as well as a memory effect. The in situ electrochemical dilatometry study shows the active role of the support in mitigating the large volume changes of tin during (de)sodiation. Abstract Here, a Sn–C composite material prepared from bulk precursors (tin metal, graphite, and melamine) using ball milling and annealing is reported. The composite (58 wt% Sn and 42 wt% N‐doped carbon) shows a capacity up to 445 mAh gSn+C−1 and an excellent cycle life (1000 cycles). For the graphite, the ball milling leads to graphene nanoplatelets (GnP) for which the storage mechanism changes from solvent co‐intercalation to conventional intercalation. The final composite (Sn at nitrogen‐doped graphite nanoplatelets (SnNGnP)) is obtained by combining the GnPs with Sn and melamine as the nitrogen source. Rate‐dependent measurements and in situ X‐ray diffraction are used to study the asymmetric storage behavior of Sn, which shows a more sloping potential profile during sodiation and more defined steps during desodiation. The disappearance of two redox plateaus during desodiation is linked to the preceding sodiation current density (memory effect). The asymmetric behavior is also found by in situ electrochemical dilatometry. This method also shows that the effective electrode expansion during sodiation is much smaller (about +14%) compared to what is expected from Sn (+420%), which gives a reasonable explanation for the excellent cycle life for

Published in: "Advanced Functional Materials".

Production and Patterning of Liquid Phase–Exfoliated 2D Sheets for Applications in Optoelectronics

2019-03-11T22:32:27+00:00March 11th, 2019|Categories: Publications|

The wide range of techniques that can be used toward the production and deposition of 2D materials’ inks into space‐confined patterns or continuous thin films are discussed in this review. The importance of reaching precise control over the material’s properties through a detailed understanding of their structure and interconnectivity between deposited sheets is discussed. Abstract 2D materials (2DMs), which can be produced by exfoliating bulk crystals of layered materials, display unique optical and electrical properties, making them attractive components for a wide range of technological applications. This review describes the most recent developments in the production of high‐quality 2DMs based inks using liquid‐phase exfoliation (LPE), combined with the patterning approaches, highlighting convenient and effective methods for generating materials and films with controlled thicknesses down to the atomic scale. Different processing strategies that can be employed to deposit the produced inks as patterns and functional thin‐films are introduced, by focusing on those that can be easily translated to the industrial scale such as coating, spraying, and various printing technologies. By providing insight into the multiscale analyses of numerous physical and chemical properties of these functional films and patterns, with a specific focus on their extraordinary electronic characteristics, this review offers the readers crucial information for a profound understanding of the fundamental properties of these patterned surfaces as the millstone toward the generation of novel multifunctional devices. Finally, the challenges and opportunities associated to the 2DMs’ integration into working opto‐electronic (nano)devices is discussed.

Published in: "Advanced Functional Materials".

High Thermoelectric Performance Achieved in GeTe–Bi2Te3 Pseudo‐Binary via Van der Waals Gap‐Induced Hierarchical Ferroelectric Domain Structure

2019-03-10T20:32:53+00:00March 10th, 2019|Categories: Publications|

Nanoscale‐charged domain walls and microdomains, together with Ge van der Waals gaps in Bi2Te3 alloyed GeTe, combine to make a complex hierarchical architecture which realizes an outstanding figure of merit ZT ≈ 2.4 at 773 K via simultaneous modulation of charge carriers and phonons. Abstract GeTe is an interesting material presenting both spontaneous polarization (ferroelectrics) and outstanding electrical conductivity (ideal for thermoelectrics). Pristine GeTe exhibits classic 71° and 109° submicron ferroelectric domains, and near unity thermoelectric figure of merit ZT at 773 K. In this work, it is demonstrated that Bi2Te3 alloying in GeTe lattice can introduce vast Ge vacancies which can further evolve into nanoscale van der Waals gaps upon proper heat treatment, and that these vacancy gaps can induce 180° nanoscale ferroelectric domain boundaries. These microstructures eventually become a hierarchical ferroelectric domain structure, with size varying from submicron to nanoscale and polarization from 71°, 109° to 180°. The establishment of hierarchical ferroelectric domain structure, together with the nanoscale Ge vacancy van der Waals gaps, has profound effects on the electrical and thermal transport properties, resulting in a striking peak thermoelectric ZT ≈ 2.4 at 773 K. These findings might provide an alternative conception for thermoelectric optimization via microstructure modulation.

Published in: "Advanced Functional Materials".

Electrical Conduction at the Interface between Insulating van der Waals Materials

2019-03-10T20:32:47+00:00March 10th, 2019|Categories: Publications|Tags: , , , , , |

Highly conducting interfaces between insulating 2D materials are demonstrated in van der Waals heterostructures fabricated by molecular‐beam epitaxy. In situ growth monitoring by reflection high energy electron diffraction confirms layer‐by‐layer fabrication of the heterostructures and the formation of abrupt interfaces. Hall effect measurements reveal that the conducting carriers are holes, and their densities are as large as 1014 cm−2. Abstract Emergent properties of 2D materials attract considerable interest in condensed matter physics and materials science due to their distinguished features that are missing in their bulk counterparts. A mainstream in this research field is to broaden the scope of material to expand the horizons of the research area, while developing functional interfaces between different 2D materials is another indispensable research direction. Here, the emergence of electrical conduction at the interface between insulating 2D materials is demonstrated. A new class of van der Waals heterostructures consisting of two sets of insulating transition‐metal dichalcogenides, group‐VI WSe2 and group‐IV TMSe2 (TM = Zr, Hf), is developed via molecular‐beam epitaxy, and it is found that those heterostructures are highly conducting although all the constituent materials are highly insulating. The WSe2/ZrSe2 interface exhibits more conducting behavior than the WSe2/HfSe2 interface, which can be understood by considering the band alignments between constituent materials. Moreover, by increasing Se flux during heterostructure fabrication, the WSe2/ZrSe2 interface becomes more conducting, reaching nearly metallic behavior. Further improvement of the crystalline quality as well as exploring different material combinations are expected to lead to metallic conduction, providing a novel functionality emerging

Published in: "Advanced Functional Materials".

Silver‐Adapted Diffusive Memristor Based on Organic Nitrogen‐Doped Graphene Oxide Quantum Dots (N‐GOQDs) for Artificial Biosynapse Applications

2019-03-10T20:32:43+00:00March 10th, 2019|Categories: Publications|Tags: , , |

Organic nitrogen‐doped graphene oxide quantum‐dot thin film is fabricated by a solution‐processed method and utilized as an insulator medium for dynamic silver (Ag) migration in use as a biocompatible artificial biosynapse. Characteristic threshold resistive switching of the devices is used to emulate bioinspired synaptic functioning. Specifically, fundamental memorization characteristics are achieved via transition from short‐term to long‐term potentiation. Abstract Carbon‐based electronic devices are suitable candidates for bioinspired electronics due to their low cost, eco‐friendliness, mechanical flexibility, and compatibility with complementary metal‐oxide‐semiconductor technology. New types of materials such as graphene quantum dots (GQDs) have attracted attention in the search for new applications beyond solar cells and energy harvesting due to their superior properties such as elevated photoluminescence, high chemical inertness, and excellent biocompatibility. In this paper, a biocompatible/organic electronic synapse based on nitrogen‐doped graphene oxide quantum dots (N‐GOQDs) is reported, which exhibits threshold resistive switching via silver cation (Ag+) migration dynamics. In analogy to the calcium (Ca2+) ion dynamics of biological synapses, important biological synapse functions such as short‐term potentiation (STP), paired‐pulse facilitation, and transition from STP to long‐term plasticity behaviors are replicated. Long‐term depression behavior is also evaluated and specific spike‐timing dependent plasticity is assessed. In addition, elaborated switching mechanism of biosimilar Ag+ migration dynamics provides the potential for using N‐GOQD‐based artificial synapse in future biocompatible neuromorphic systems.

Published in: "Advanced Functional Materials".

Polarization‐Sensitive Ultraviolet Photodetection of Anisotropic 2D GeS2

2019-03-10T20:32:40+00:00March 10th, 2019|Categories: Publications|

Germanium disulfide (GeS2) with a wide bandgap is introduced as an ideal candidate for polarization‐sensitive photodetection in UV region. In‐plane anisotropy of GeS2 is demonstrated by theoretical and experimental results. In terms of in‐plane anisotropic absorption and wide bandgap in GeS2, GeS2‐based photodetectors show a strong polarization‐dependent photoresponse in UV region. Abstract Polarization‐sensitive photodetection in the UV region is highly indispensable in many military and civilian applications. UV‐polarized photodetection usually relies on the use of wide bandgap semiconductors with 1D nanostructures requiring complicated nanofabrication processes. Although the emerging anisotropic 2D semiconductors shed light on the detection of polarization with a simple device architecture, bandgaps of such reported 2D semiconductors are too small to be applied for visible–blind UV‐polarized photodetection. Here, germanium disulfide (GeS2), the widest bandgap (>3 eV) in the family of in‐plane anisotropic 2D semiconductors explored to date, is introduced as an ideal candidate for UV‐polarized photodetection. The structural, vibrational, and optical anisotropies of GeS2 are systematically investigated from theory to experiment. GeS2‐based photodetectors show a strong polarization‐dependent photoresponse in the UV region. GeS2 with a wide bandgap and high in‐plane anisotropy not only enriches the family of anisotropic 2D semiconductors but also expands the polarized photodetection from the current visible and near‐infrared to the brand‐new UV region.

Published in: "Advanced Functional Materials".

Ultrahigh Electrical Conductivity of Graphene Embedded in Metals

2019-03-10T20:32:35+00:00March 10th, 2019|Categories: Publications|Tags: , |

Ultrahigh electrical conductivity ≈3000 times higher than that of Cu is realized in graphene embedded in metals. As a result, the corresponding graphene/Cu composites show an electrical conductivity significantly higher than that of Ag. Such graphene/metal interactions provide a unique platform to explore electron behaviors in graphene, and the results open up new opportunities for graphene’s applications. Abstract Highly efficient conductors are strongly desired because they can lead to higher working performance and less energy consumption in their wide range applications. However, the improvements on the electrical conductivities of conventional conductors are limited, such as purification and growing single crystal of metals. Here, by embedding graphene in metals (Cu, Al, and Ag), the trade‐off between carrier mobility and carrier density is surmount in graphene, and realize high electron mobility and high electron density simultaneously through elaborate interface design and morphology control. As a result, a maximum electrical conductivity three orders of magnitude higher than the highest on record (more than 3,000 times higher than that of Cu) is obtained in such embedded graphene. As a result, using the graphene as reinforcement, an electrical conductivity as high as ≈117% of the International Annealed Copper Standard and significantly higher than that of Ag is achieved in bulk graphene/Cu composites with an extremely low graphene volume fraction of only 0.008%. The results are of significance when enhancing efficiency and saving energy in electrical and electronic applications of metals, and also of interest for fundamental researches on electron behaviors in graphene.

Published in: "Advanced Functional Materials".

Nonvolatile Ferroelectric Memory Effect in Ultrathin α‐In2Se3

2019-03-10T20:32:30+00:00March 10th, 2019|Categories: Publications|Tags: , |

A prototype of a 2D ferroelectric memory device is demonstrated. The nonvolatile functionality stems from the room‐temperature out‐of‐plane ferroelectricity in ultrathin α‐In2Se3. The hybrid ferroelectric field‐effect transistor allows not only the control and probe of electric dipoles, but also the quantitative measurement of the electric polarization in atomically thin van der Waals ferroelectrics. Abstract High‐density memory is integral in solid‐state electronics. 2D ferroelectrics offer a new platform for developing ultrathin electronic devices with nonvolatile functionality. Recent experiments on layered α‐In2Se3 confirm its room‐temperature out‐of‐plane ferroelectricity under ambient conditions. Here, a nonvolatile memory effect in a hybrid 2D ferroelectric field‐effect transistor (FeFET) made of ultrathin α‐In2Se3 and graphene is demonstrated. The resistance of the graphene channel in the FeFET is effectively controllable and retentive due to the electrostatic doping, which stems from the electric polarization of the ferroelectric α‐In2Se3. The electronic logic bit can be represented and stored with different orientations of electric dipoles in the top‐gate ferroelectric. The 2D FeFET can be randomly rewritten over more than 105 cycles without losing the nonvolatility. The approach demonstrates a prototype of rewritable nonvolatile memory with ferroelectricity in van der Waals 2D materials.

Published in: "Advanced Functional Materials".

Synthesis of Large‐Area Atomically Thin BiOI Crystals with Highly Sensitive and Controllable Photodetection

2019-03-10T20:32:28+00:00March 10th, 2019|Categories: Publications|

Large‐area atomically thin BiOI crystals are obtained with a domain size over 100 µm via chemical vapor deposition. Photodetectors fabricated from ultrathin BiOI crystals show high sensitivity to 473 nm light. Also, oxygen vacancies can be introduced into BiOI crystals by 254 nm ultraviolet light irradiation. Abstract Compared to the most studied 2D elements and binary compounds, ternary layered compounds with more adjustable physical and chemical properties have exhibited potential applications in electronic and optoelectronic devices. Here, 2D ternary layered BiOI crystals are synthesized first with a domain size up to 100 µm via space‐confined chemical vapor deposition. The photodetectors based on the as‐grown BiOI nanosheets demonstrate high sensitivity to 473 nm light. The I on/I off ratio and detectivity of BiOI photodetectors can reach up to 1 × 105 and 8.2 × 1011 Jones at 473 nm, respectively. Particularly, the contact and dark current of the photodetectors can be controlled by 254 nm ultraviolet light irradiation due to the introduction of oxygen vacancies. The facile synthesis of large‐area atomically thin BiOI and its controllable performance by ultraviolet light irradiation suggest that 2D BiOI crystal is a promising material for fundamental investigations and optoelectronic applications.

Published in: "Advanced Functional Materials".

Self‐Healable Multifunctional Electronic Tattoos Based on Silk and Graphene

2019-03-10T20:32:27+00:00March 10th, 2019|Categories: Publications|Tags: |

A self‐healable silk E‐tattoo, which shows high sensitivity to multistimuli including strain/humidity/temperature, is reported. Customer‐designed E‐tattoos can be facilely prepared through screen printing or direct writing of a graphene/silk fibroin/Ca2+ suspension. Remarkably, the E‐tattoo can be healed with an efficiency of 100% even after being fully fractured within 0.3 s simply by a droplet of water. Abstract Electronic tattoos (E‐tattoos), which can be intimately mounted on human skin for noninvasive and high‐fidelity sensing, have attracted the attention of researchers in the field of wearable electronics. However, fabricating E‐tattoos that are capable of self‐healing and sensing multistimuli, similar to the inherent attributes of human skin, is still challenging. Herein, a healable and multifunctional E‐tattoo based on a graphene/silk fibroin/Ca2+ (Gr/SF/Ca2+) combination is reported. The highly flexible E‐tattoos are prepared through printing or writing using Gr/SF/Ca2+ suspension. The graphene flakes distributed in the matrix form an electrically conductive path that is responsive to environmental changes, such as strain, humidity, and temperature variations, endowing the E‐tattoo with high sensitivity to multistimuli. The performance of the E‐tattoo is investigated as a strain, humidity, and temperature sensor that shows high sensitivity, a fast response, and long‐term stability. The E‐tattoo is remarkably healed after damage by water because of the reformation of hydrogen and coordination bonds at the fractured interface. The healing efficiency is 100% in only 0.3 s. Finally, as proof of concept, its applications for monitoring of electrocardiograms, breathing, and temperature are shown. Based on its unique properties and superior performance, the Gr/SF/Ca2+ E‐tattoo

Published in: "Advanced Functional Materials".

Sandwich, Vertical‐Channeled Thick Electrodes with High Rate and Cycle Performance

2019-03-10T20:32:23+00:00March 10th, 2019|Categories: Publications|Tags: , |

The sandwich, vertical channeled graphene‐based thick electrodes achieve low carbon content (14.4 wt%) and excellent rate/cycle performance under a wide range of high mass loadings (11–72 mg cm−2). The vertical channels provide fast Li+ ion transfer capacity, and the ultrathin sandwich structure buffers volume expansion during long cycle test (1000 cycles). The optimized structure for thick electrodes is investigated in detail. Abstract 3D thick electrode design is a promising strategy to increase the energy density of lithium‐ion batteries but faces challenges such as poor rate and limited cycle life. Herein, a coassembly method is employed to construct low‐tortuosity, mechanically robust 3D thick electrodes. LiFe0.7Mn0.3PO4 nanoplates (LFMP NPs) and graphene are aligned along the growth direction of ice crystals during freezing and assembled into sandwich frameworks with vertical channels, which prompts fast ion transfer within the entire electrode and reveals a 2.5‐fold increase in ion transfer performance as opposed to that of random structured electrodes. In the sandwich framework, LFMP NPs are entrapped in the graphene wall in a “plate‐on‐sheet” contact mode, which avoids the detachment of NPs during cycling and also constitutes electron transfer highways for the thick electrode. Such vertical‐channel sandwich electrodes with mass loading of 21.2 mg cm−2 exhibit a superior rate capability (0.2C–20C) and ultralong cycle life (1000 cycles). Even under an ultrahigh mass loading of 72 mg cm−2, the electrode still delivers an areal capacity up to 9.4 mAh cm−2, ≈2.4 times higher than that of conventional electrodes. This study provides a novel strategy for designing

Published in: "Advanced Functional Materials".

High Thermoelectric Performance Achieved in GeTe–Bi2Te3 Pseudo‐Binary via Van der Waals Gap‐Induced Hierarchical Ferroelectric Domain Structure

2019-03-08T10:32:42+00:00March 8th, 2019|Categories: Publications|

Nanoscale‐charged domain walls and microdomains, together with Ge van der Waals gaps in Bi2Te3 alloyed GeTe, combine to make a complex hierarchical architecture which realizes an outstanding figure of merit ZT ≈ 2.4 at 773 K via simultaneous modulation of charge carriers and phonons. Abstract GeTe is an interesting material presenting both spontaneous polarization (ferroelectrics) and outstanding electrical conductivity (ideal for thermoelectrics). Pristine GeTe exhibits classic 71° and 109° submicron ferroelectric domains, and near unity thermoelectric figure of merit ZT at 773 K. In this work, it is demonstrated that Bi2Te3 alloying in GeTe lattice can introduce vast Ge vacancies which can further evolve into nanoscale van der Waals gaps upon proper heat treatment, and that these vacancy gaps can induce 180° nanoscale ferroelectric domain boundaries. These microstructures eventually become a hierarchical ferroelectric domain structure, with size varying from submicron to nanoscale and polarization from 71°, 109° to 180°. The establishment of hierarchical ferroelectric domain structure, together with the nanoscale Ge vacancy van der Waals gaps, has profound effects on the electrical and thermal transport properties, resulting in a striking peak thermoelectric ZT ≈ 2.4 at 773 K. These findings might provide an alternative conception for thermoelectric optimization via microstructure modulation.

Published in: "Advanced Functional Materials".

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