Graphene/Strontium Titanate: Approaching Single Crystal–Like Charge Transport in Polycrystalline Oxide Perovskite Nanocomposites through Grain Boundary Engineering

Grain boundaries critically limit the electronic performance of oxide perovskites. These interfaces lower the carrier mobilities of polycrystalline materials by several orders of magnitude compared to single crystals. Despite extensive effort, improving the mobility of polycrystalline materials (to meet the performance of single crystals) is still a severe challenge. In this work, the grain boundary effect is eliminated in perovskite strontium titanate (STO) by incorporating graphene into the polycrystalline microstructure. An effective mass model provides strong evidence that polycrystalline graphene/strontium titanate (G/STO) nanocomposites approach single crystal‐like charge transport. This phenomenological model reduces the complexity of analyzing charge transport properties so that a quantitative comparison can be made between the nanocomposites and STO single crystals. In other related works, graphene composites also optimize the thermal transport properties of thermoelectric materials. Therefore, decorating grain boundaries with graphene appears to be a robust strategy to achieve “phonon glass–electron crystal” behavior in oxide perovskites.     

Covalently Bonded Graphene Sheets on Carbon Nanotubes: Direct Growth and Outstanding Properties

Integrating 1D carbon nanotubes (CNTs) and 2D graphene with covalent bonds can inherit the outstanding properties of both components and obtain additional advantages. Here, this work reports the preparation of covalently bonded graphene/CNT (G/CNT) structure by a normal chemical vapor deposition method. Specifically, the pre-synthesized defects on the sidewall of CNTs act as nucleation sites for the growth of graphene sheets to form a branch-leaf structure. Graphene leaves restrict the sliding and re-stacking of CNTs, endowing G/CNT hybrid demonstrates excellent anti-agglomeration properties that are not present in either graphene or CNTs. In addition, the covalently bonded structure and high graphitization degree of graphene sheets and CNTs significantly enhance the comprehensive properties of the G/CNT hybrid material, such as large specific surface area, excellent thermal stability, and high electrical conductivity. Consequently, the microwave absorption properties of G/CNT are significantly enhanced compared with CNTs. This work provides a feasible pathway to synthesize high-performance covalently bonded G/CNT hybrids.     

石墨烯/硅基异质集成光电子器件

石墨烯/硅基异质集成的光子器件研究在近年来取得了巨大进展,因石墨烯所具有的诸多独特的物理性质如超高载流子迁移率、超高非线性系数等,石墨烯/硅基异质集成器件展现出了诸如超大带宽、超低功耗等优异性能。文章介绍了近年来报道的典型石墨烯/硅基异质集成器件,包括石墨烯/硅基电光调制器、石墨烯/硅基热光调制器和石墨烯/硅基光电探测器,简要阐述了其原理与性能,并对其未来的应用与发展做出了展望。     

Facile Doping and Work‐Function Modification of Few‐Layer Graphene Using Molecular Oxidants and Reductants

Doping of graphene is a viable route toward enhancing its electrical conductivity and modulating its work function for a wide range of technological applications. In this work, the authors demonstrate facile, solution‐based, noncovalent surface doping of few‐layer graphene (FLG) using a series of molecular metal‐organic and organic species of varying n‐ and p‐type doping strengths. In doing so, the authors tune the electronic, optical, and transport properties of FLG. The authors modulate the work function of graphene over a range of 2.4 eV (from 2.9 to 5.3 eV)—unprecedented for solution‐based doping—via surface electron transfer. A substantial improvement of the conductivity of FLG is attributed to increasing carrier density, slightly offset by a minor reduction of mobility via Coulomb scattering. The mobility of single layer graphene has been reported to decrease significantly more via similar surface doping than FLG, which has the ability to screen buried layers. The dopant dosage influences the properties of FLG and reveals an optimal window of dopant coverage for the best transport properties, wherein dopant molecules aggregate into small and isolated clusters on the surface of FLG. This study shows how soluble molecular dopants can easily and effectively tune the work function and improve the optoelectronic properties of graphene.     

Covalently Bonded Graphene–Carbon Nanotube Hybrid for High‐Performance Thermal Interfaces

The remarkable thermal properties of graphene and carbon nanotubes (CNTs) have been the subject of intensive investigations for the thermal management of integrated circuits. However, the small contact area of CNTs and the large anisotropic heat conduction of graphene have hindered their applications as effective thermal interface materials (TIMs). Here, a covalently bonded graphene–CNT (G‐CNT) hybrid is presented that multiplies the axial heat transfer capability of individual CNTs through their parallel arrangement, while at the same time it provides a large contact area for efficient heat extraction. Through computer simulations, it is demonstrated that the G‐CNT outperforms few‐layer graphene by more than 2 orders of magnitude for the c‐axis heat transfer, while its thermal resistance is 3 orders of magnitude lower than the state‐of‐the‐art TIMs. We show that heat can be removed from the G‐CNT by immersing it in a liquid. The heat transfer characteristics of G‐CNT suggest that it has the potential to revolutionize the design of high‐performance TIMs.     

Metal Oxide/Graphene/Metal Sandwich Structure for Efficient Photoelectrochemical Water Oxidation

Single-layer graphene (SLG) has drawn considerable interest in photoelectrochemical (PEC) cells due to its atomically flat pinhole-free structure and remarkable in-plane carrier mobility. It is challenging, however, to obtain efficient SLG-modified photoelectrodes for PEC water splitting mainly due to the inefficient charge transfer interface. Here, a transition metal oxide/SLG/transition metal sandwich structure modified n-Si-based model photoanode is constructed to regulate the interfacial charge transfer behavior for enhanced PEC water oxidation performance. In this sandwich configuration, SLG tailors the morphology, structure, and work function properties of surface metal electrocatalysts to obtain both higher thermodynamic photovoltage and faster kinetical charge transfer at the semiconductor/electrolyte interface. In addition, SLG promotes the surface catalytic reaction as an effective charge trap and storage layer. This study provides a new structural design to engineer the SLG interfacial properties for high-performance energy conversion devices.     

Improved performance of organic photovoltaic devices by doping F4TCNQ onto solution-processed graphene as a hole transport layer

Interfacial engineering is crucial for the stability and efficiency of organic solar cells. PEDOT:PSS, which has been widely used as a hole transport layer, has stability issues when exposed to air because of its acidic and hygroscopic nature. Herein, we investigated the electrical properties of reduced graphene oxide covered with an F₄TCNQ interfacial layer as an alternative and its effect on the photovoltaic performance. Using an array of charge transport, spectroscopic and imaging techniques we found that the reduced graphene oxide film is efficiently hole-doped through an interfacial charge transfer, which enhances its electrical properties and favorably modifies its work function. Consequently, the open-circuit voltage and fill factor of solar cells incorporating such films are improved. P3HT might also be hole-doped by F₄TCNQ, due to the formation of an intermixed interfacial layer, resulting in an increase of power conversion efficiency.     

Sustainable Synthesis and Assembly of Biomass‐Derived B/N Co‐Doped Carbon Nanosheets with Ultrahigh Aspect Ratio for High‐Performance Supercapacitors

The practical application of graphene has still been hindered by high cost and scarcity in supply. It boosts great interest in seeking for low‐cost substitute of graphene for upcoming usage where extremely physical properties are not absolutely critical. The conversion of renewable biomass offers a great opportunity for sustainable and economic fabrication of 2D carbon nanostructures. However, large‐scale production of carbon nanosheets with ultrahigh aspect ratio, satisfied electronic properties, and the capability of organized assembly like graphene has been rarely reported. In this work, a facile yet efficient approach for mass production of flexible boric/nitrogen co‐doped carbon nanosheets with very thin thickness of 5–8 nm and ultrahigh aspect ratio of over 6000–10 000 is demonstrated by assembling the biomass molecule in long‐range order on 2D hard template and subsequent annealing. The advantage of these doped carbon nanosheets over conventional products lies in that they can be readily assembled to multilevel architectures such as freestanding flexible thin film and ultralight aerogels with better electrical properties, which exhibit exceptional capacitive performance for supercapacitor application. The recyclability of boric acid template further reduces the discharge of the waste and processing cost, rendering high cost‐effectiveness and environmental benignity for scalable production.     

Strain Modulation in Graphene/ZnO Nanorod Film Schottky Junction for Enhanced Photosensing Performance

Strain modulation in flexible semiconductor heterojunctions has always been considered as an effective way to modulate the performance of nanodevices. In this work, a graphene/ZnO nanorods film Schottky junction has been constructed. It shows considerable responsivity and fast on‐off switch to the UV illumination. Through utilizing the piezopotential induced by the atoms displacement in ZnO under the compressive strain, 17% enhanced photosensing property is achieved in this hybrid structure when applying ?0.349% strain. This performance improvement can be ascribed to the Schottky barrier height modification by the strain‐induced piezopotential, which results in the facilitation of electron–hole separation in the graphene/ZnO interface. An energy band principle as well as a finite element analysis is proposed to understand this phenomenon. The results here provide a facile approach to boost the optoelectronic performance of graphene/ZnO heterostructure, which may also be applied to other Schottky junction based hybrid devices.     

Large Scale Graphene/Hexagonal Boron Nitride Heterostructure for Tunable Plasmonics

Vertical integration of hexagonal boron nitride (h‐BN) and graphene for the fabrication of vertical field‐effect transistors or tunneling diodes has stimulated intense interest recently due to the enhanced performance offered by combining an ultrathin dielectric with a semi‐metallic system. Wafer scale fabrication and processing of these heterostructures is needed to make large scale integrated circuitry. In this work, by using remote discharged, radio‐frequency plasma chemical vapor deposition, wafer scale, high quality few layer h‐BN films are successfully grown. By using few layer h‐BN films as top gate dielectric material, the plasmon energy of graphene can be tuned by electrostatic doping. An array of graphene/h‐BN vertically stacked micrometer‐sized disks is fabricated by lithography and transfer techniques, and infrared spectroscopy is used to observe the modes of tunable graphene plasmonic absorption as a function of the repeating (G/h‐BN)_n units in the vertical stack. Interestingly, the plasmonic resonances can be tuned to higher frequencies with increasing layer thickness of the disks, showing that such vertical stacking provides a viable strategy to provide wide window tuning of the plasmons beyond the limitation of the monolayer.     

FeCl3‐Based Few‐Layer Graphene Intercalation Compounds: Single Linear Dispersion Electronic Band Structure and Strong Charge Transfer Doping

Graphene has attracted much attention since its first discovery in 2004. Various approaches have been proposed to control its physical and electronic properties. Here, it is reported that graphene‐based intercalation is an efficient method to modify the electronic properties of few‐layer graphene (FLG). FeCl₃ intercalated FLGs are successfully prepared by the two‐zone vapor transport method. This is the first report on full intercalation for graphene samples. The features of the Raman G peak of such FLG intercalation compounds (FLGIC) are in good agreement with their full intercalation structures. The FLGICs present single Lorentzian 2D peaks, similar to that of single‐layer graphene, indicating the loss of electronic coupling between adjacent graphene layers. First principle calculations further reveal that the band structure of FLGIC is similar to single‐layer graphene but with a strong doping effect due to the charge transfer from graphene to FeCl₃. The successful fabrication of FLGIC opens a new way to modify properties of FLG for fundamental studies and future applications.     

Defect‐Rich Ni3FeN Nanocrystals Anchored on N‐Doped Graphene for Enhanced Electrocatalytic Oxygen Evolution

Owing to their unique optical, electronic, and catalytic properties, metal nitrides nanostructures are widely used in optoelectronics, clean energy, and catalysis fields. Despite great progress has been achieved, synthesis of defect‐rich (DR) bimetallic nitride nanocrystals or related nanohybrids remains a challenge, and their electrocatalytic application for oxygen evolution reaction (OER) has not been fully studied. Herein, the DR‐Ni₃FeN nanocrystals and N‐doped graphene (N‐G) nanohybrids (DR‐Ni₃FeN/N‐G) are fabricated through temperature‐programmed annealing and nitridation treatment of NiFe‐layered double hydroxides/graphene oxide precursors by controlling annealing atmosphere. In the nanohybrids, the DR‐Ni₃FeN nanocrystals are anchored on N‐G, and mainly show twin crystal defects besides ≈10% of stacking faults. Such nanohybrids can efficiently catalyze OER in alkaline media with a small overpotential (0.25 V) to attain the current density of 10 mA cm^?2 and a high turnover frequency (0.46 s^?1), superior to their counterparts (the nearly defect‐free Ni₃FeN/N‐G), commercial IrO₂, and the‐state‐of‐art reported OER catalysts. Except for the superior activity, they show better durability than their counterparts yet. As revealed by microstructural, spectroscopic, and electrochemical analyses, the enhanced OER performance of DR‐Ni₃FeN/N‐G nanohybrids originates from the abundant twin crystal defects in Ni₃FeN active phase and the strong interplay between DR‐Ni₃FeN and N‐G.     

电泳法制备石墨烯场发射阴极的性能研究

电泳法制备的石墨烯阴极由于其独特的优势,目前已经成为石墨烯阴极的主要制备方法。因此,研究电泳电流以及电泳浓度对石墨烯阴极表面形貌和发射性能的影响具有十分重大的意义。通过扫描电镜、拉曼光谱和超高真空测试平台对其进行形貌和性能表征。结果表明,电泳浓度和电泳电流均对石墨烯阴极的表面形貌和发射性能有较大影响。在电泳电流为4mA,电泳液中石墨烯与异丙醇的比例为110mg：200ml时,其发射性能最佳,开启场强约为4.7V/um。     

碳纳米管/石墨烯复合结构的第一性原理计算

为了研究碳纳米管/石墨烯复合结构的电学性质,采用密度泛函理论(DFT)下的第一性原理,对四种T型复合结构进行了几何结构优化,分析了该复合结构的结合能,能带结构,电子态密度,Mulliken电荷分布及功函数.结果表明复合结构均表现出半导体性质,其稳定性及电子结构取决于碳纳米管类型和复合结构的连接方式,而且复合材料的功函数...     

Microwave Combustion for Rapidly Synthesizing Pore‐Size‐Controllable Porous Graphene

Porous graphene has been widely applied in energy storage, electrocatalysis, photoelectron devices, etc. However, the producing process for porous graphene usually needs long time and is a tedious step. In this work, porous graphene is prepared with controllable pore size by using active metal nanoparticles to catalytically oxidize carbon under microwave combustion process within tens of seconds. The ion exchange membrane based on porous graphene with ≈5 nm pore diameter exhibits a great performance for salinity gradient power generation application with a power density output of ≈1.15 W m^?2. This work highlights a new strategy for the design and synthesis of pore‐size‐controllable porous graphene and provides new opportunities for 2D porous nanomaterials.     

Extraordinary Macroscale Wear Resistance of One Atom Thick Graphene Layer

During the last few years, graphene's unusual friction and wear properties have been demonstrated at nano to micro scales but its industrial tribological potential has not been fully realized. The macroscopic wear resistance of one atom thick graphene coating is reported by subjecting it to pin‐on‐disc type wear testing against most commonly used steel against steel tribo‐pair. It is shown that when tested in hydrogen, a single layer of graphene on steel can last for 6400 sliding cycles, while few‐layer graphene (3–4 layers) lasts for 47 000 cycles. Furthermore, these graphene layers are shown to completely cease wear despite the severe sliding conditions including high contact pressures (≈0.5 GPa) observed typically in macroscale wear tests. The computational simulations show that the extraordinary wear performance originates from hydrogen passivation of the dangling bonds in a ruptured graphene, leading to significant stability and longer lifetime of the graphene protection layer. Also, the electronic properties of these graphene sheets are theoretically evaluated and the improved wear resistance is demonstrated to preserve the electronic properties of graphene and to have significant potential for flexible electronics. The findings demonstrate that tuning the atomistic scale chemical interactions holds the promise of realizing extraordinary tribological properties of monolayer graphene coatings.     

Increased Work Function in Few‐Layer Graphene Sheets via Metal Chloride Doping

A chemical approach to controlling the work function of few‐layer graphene is investigated. Graphene films are synthesized on Cu foil by chemical vapor deposition. Six metal chlorides, AuCl₃, IrCl₃, MoCl₃, OsCl₃, PdCl₂, and RhCl₃, are used as dopants. The sheet resistance of the doped graphene decreases from 1100 Ω/sq to ≈500–700 Ω/sq and its transmittance at 550 nm also decreases from 96.7% to 93% for 20 mM AuCl₃ due to the formation of metal particles. The sheet resistance and transmittance are reduced with increasing metal chloride concentration. The G peak in the Raman spectra is shifted to a higher wavenumber after metal chloride doping, which indicates a charge transfer from graphene to metal ions. The intensity ratio of I_C?C/I_C?C increases with doping, indicating an electron transfer from graphene sheets to metal ions. Ultraviolet photoemission spectroscopy data shows that the work function of graphene increases from 4.2 eV to 5.0, 4.9, 4.8, 4.68, 5.0, and 5.14 eV for the graphene with 20 mM AuCl₃, IrCl₃, MoCl₃, OsCl₃, PdCl₂, and RhCl₃, respectively. It is considered that spontaneous charge transfer occurs from the specific energy level of graphene to the metal ions, thus increasing the work function.     

CCD多晶硅层间绝缘介质对器件可靠性的影响

报道了使用石墨烯作为阳极材料的GaN肖特基型紫外探测器。介绍了光敏面为1mm×1mm的新型肖特基紫外探测器的制备过程。并对器件进行了响应光谱、I-V特性测试。器件的响应光谱较为平坦,峰值响应度为0.175A/W;通过对石墨烯进行化学修饰,使峰值响应度增加到0.23A/W。并根据热电子发射理论,计算出了器件掺杂前后的肖特基势垒高度分别为0.477eV和0.882eV,验证了器件性能的提高主要原因是石墨烯功函数的增加。 更多还原     

Amorphous FeOOH Quantum Dots Assembled Mesoporous Film Anchored on Graphene Nanosheets with Superior Electrochemical Performance for Supercapacitors

Previous research on iron oxides/hydroxides has focused on the crystalline rather than the amorphous phase, despite that the latter could have superior electrochemical activity due to the disordered structure. In this work, a simple and scalable synthesis route is developed to prepare amorphous FeOOH quantum dots (QDs) and FeOOH QDs/graphene hybrid nanosheets. The hybrid nanosheets possess a unique heterostructure, comprising a continuous mesoporous FeOOH nanofilm tightly anchored on the graphene surface. The amorphous FeOOH/graphene hybrid nanosheets exhibit superior pseudocapacitive performance, which largely outperforms the crystalline iron oxides/hydroxides‐based materials. In the voltage range between ?0.8 and 0 V versus Ag/AgCl, the amorphous FeOOH/graphene composite electrode exhibits a large specific capacitance of about 365 F g^?1, outstanding cycle performance (89.7% capacitance retention after 20 000 cycles), and excellent rate capability (189 F g^?1 at a current density of 128 A g^?1). When the lower cutoff voltage is extended to ?1.0 and ?1.25 V, the specific capacitance of the amorphous FeOOH/graphene composite electrode can be increased to 403 and 1243 F g^?1, respectively, which, however, compromises the rate capability and cycle performance. This work brings new opportunities to design high‐performance electrode materials for supercapacitors, especially for amorphous oxides/hydroxides‐based materials.     

激光偏振拉曼的CVD多晶石墨烯表征

石墨烯具有优异的物理化学性质,在MEMS器件、光电检测材料、柔性显示屏、新能源电池、复合材料等方面成为研究热点。目前大面积石墨烯制备主要依赖于化学气相沉积技术(chemical vapor deposition, CVD),但其生长的晶体质量直接影响到电化学特性和实际应用,因此需要一种快速而准确的表征方法。实验利用背向散射的偏振激光散射装置测量CVD生长的石墨烯拉曼光谱。通过分析实验获得的300 nm SiO2/Si基底上的单层、五层和十层左右的石墨烯角分辨偏振拉曼光谱,发现单层生长的石墨烯偏振特性与机械剥离单晶石墨烯一致;随着层数的增加,G峰偏振响应差异更加明显,表现出明显的椭圆特性;在不同石墨烯层数上的D峰也呈现出一定的偏振响应差异性。偏振拉曼测试结果表明目前CVD生长的缺陷和多晶特性与石墨烯层数呈现正相关特征。