石墨烯纳米条带的电子输运性质研究(英文)

采用紧束缚近似模型,运用Green函数和Landauer-Büttiker公式计算了并联型Armchair型边界的石墨烯纳米条带的电子输运性质。结果表明,随着并联纳米条带数量的增加,石墨烯纳米条带电导峰(电导谷)将有相应数量的增加;条带之间的间距增宽,中心区电导谷的宽度将减小。通过数值计算,揭示该新型石墨烯结构电子输运的物理机制,为基于石墨烯的新型器件的设计和优化提供理论指导,并对石墨烯纳米条带在未来集成电路设计中的应用提供理论参考。     

Metal–Organic Framework‐Derived Graphitic Nanoribbons Anchored on Graphene for Electroionic Artificial Muscles

To achieve large bending displacement and fast response time under ultralow input voltages, as well as improved durability, advanced high‐performance ionic actuators still face crucial design challenges that must be resolved. Here, hierarchically porous and unzipped graphitic nanoribbons anchored on graphene as an efficient electrode material for high‐performance electroionic artificial muscles are reported. Using controlled solvothermal and pyrolysis methods, nanoarchitectured carbon is derived from a self‐templated potassium‐based metal–organic frameworks–graphene hybrid. The newly designed ionic actuator demonstrates excellent actuation performance, including large bending displacement (17.4 mm) and a strain difference of 0.51% at 0.5 V AC input, very fast response time (700 ms) at 0.5 V DC input, wide frequency response (0.1–15 Hz), and excellent cycling stability (92%) after 25 000 cycles without any delamination of electrodes under continuous electrical operation. The breakthrough in actuation performance mainly stems from the unzipping of hollow nanorods to hierarchical porous graphitic nanoribbons anchored on graphene with the enlarged surface area, large pore volume, stronger mechanical integrity, and emerging charge storage and transport ability. Further, the electroionic actuator shows promise when applied in the demonstration of a biomimicking Venus flytrap.     

A Bubble‐Derived Strategy to Prepare Multiple Graphene‐Based Porous Materials

Graphene‐based porous structures have triggered tremendous attention due to their promising application in many fields. Recent progress has yielded structures with stochastic porous networks, which limit their controllability and potential performance. It still remains a big challenge for the scalable production to integrate the 2D building block into engineered porous architectures in multidimensions. Here, a versatile technique based on soft bubble templating and fixation by freezing is described to fabricate 3D bubble‐derived graphene foams (BGFs) and 2D bubble‐derived graphene porous membranes (BGPMs). These light‐weight novel structures are carefully tuned. The BGFs show high adsorption capabilities for organic solvents and good recovery in structural deformation. Furthermore, applications of BGFs and BGPMs in strain sensors for wearable devices are discussed, working as a combined system which can both detect the compressive and tensile deformation. This technique can be extended to assemble other nanomaterials as building blocks into macroscopic configurations.     

Ab Initio Study of Absorption Resonance Correlations between Nanotubes and Nanoribbons of Graphene and Hexagonal Boron Nitride

Semiconductors - Density functional theory calculations are performed for the electronic band structures and optical absorption spectra of the zigzag nanoribbons and armchair nanotubes of graphene...     

石墨烯光阴极带隙设计

为了使石墨烯光阴极实现光电转化功能,以超晶格形式掺杂六角氮化硼到石墨烯中,形成杂化纳米带。通过基于第一性原理的计算,从能带结构可以看出,这种方法可以在一个很大的范围内(0～2.5 eV)调控带隙大小。结合能带结构和电荷密度分布分析了带隙调控的机理,此外,运用K-P模型理论分析也得到了一致的结果。以这种方式调控石墨烯材料的带隙,锯齿型边缘和扶手椅型边缘的六角氮化硼/石墨烯(h-BN/graphene)超晶格纳米带,其带隙大小均随着其中h-BN所占比例的增加而变大,而且其带隙大小几乎不受纳米带宽度的影响,这样一来材料的尺寸可以做到更加微型化。再者,基于此方法可以制成渐变带隙结构,进而实现同一光阴极对不同范围光谱的响应。     

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.     

选区激光熔化成型316L不锈钢多孔结构的力学性能

为了减轻或消除人工植入体的“应力屏蔽”效应,提高生物相容性,需要对选区激光熔化（SLM）技术成型多孔结构进行力学性能研究。通过制备316L不锈钢体心立方(BCC)、正十二面体（RD)两种多孔结构,分别进行成型件纵向压缩试验,建立了Gibson-Ashby模型,预测了多孔结构弹性模量值。采用分形插值法,分析了孔隙率、平均孔径、比表面积对多孔结构弹性模量和抗压强度的影响程度。分析试验表明,316L不锈钢多孔结构样件在孔隙率为55.13%～94.74%,平均孔径为1.90～4.22 mm,比表面积0.54～4.33时,其弹性模量为0.375 ～1.716 GPa,抗压强度为43.19~160.31 MPa。对比人骨弹性模量0.9 ～1.7 GPa,满足植入体要求。孔隙率、平均孔径、比表面积对正十二面体多孔结构的弹性模量和抗压强度的幅值变化影响较小,对体心立方多孔结构影响较大。正十二面体多孔结构抗压强度为111.75～160.31 MPa,体心立方多孔结构的抗压强度为43.19～158.03 MPa,正十二面体多孔结构的力学性能比体心立方结构性能更好,为选区激光熔化技术制备316L不锈钢多孔结构的人工植入体研究提供依据。     

Stretch‐Driven Increase in Ultrahigh Thermal Conductance of Hydrogenated Borophene and Dimensionality Crossover in Phonon Transmission

Inspirited by the wide range of applications of graphene and the similarity between boron and carbon, 2D boron sheets have gained extensive research interest. In this work, using first‐principles combined with a nonequilibrium Green's function method, thermal conductance of fully hydrogenated borophene, named borophane, is studied. Interestingly and in contrast to widely perceived sense, at 300 K, it is found that the thermal conductance of borophane in the armchair direction is remarkably larger than that of graphene. More interesting, a dimensionality crossover is observed in phonon transmission where low‐frequency phonons exhibit 2D characteristic, while high‐frequency phonons behave like a 1D system, oriented along armchair direction, which results in the ultrahigh thermal conductance. An anomalous increase of thermal conductance with uniaxial tensile strain is observed, which is well explained by the unique puckered structure and chemical bonding in borophane. The excellent in‐plane stiffness and flexibility together with the high thermal conductance suggest that borophane is promising for soft thermal channel. Moreover, this unique dimensionality crossover in phonon transmission offers a perfect platform for studying the effect of phonon population in mode space, which is of primary importance for thermal transport in low‐dimensional systems.     

Nonequilibrium aspects of armchair graphene nanoribbon conduction

Electron quantum transport is theoretically studied for finite-size armchair graphene nanoribbons biased within source and drain metallic electrodes, using an extended-Hückel-based Green's function coupled to a three-dimensional Poisson solver. The analysis evidences dynamic nonequilibrium electron charging phenomena that can affect the conduction mechanism by provoking electronic structure alterations. The origin of such process can be traced in a tracking relationship between the device's local density of states and the electrochemical potentials of the contacts. Such effect has no equivalent in the semiclassical limit.     

Thermoelectric quantum-dot superlattices with high ZT

Following the experimentally observed Seebeck coefficient enhancement in PbTe quantum wells in Pb_1−xEu_xTe/PbTe multiple-quantum-well structures which indicated the potential usefulness of low dimensionality, we have investigated the thermoelectric properties of PbSe_xTe_1−x/PbTe quantum-dot superlattices for possible improved thermoelectric materials. We have again found enhancements in Seebeck coefficient and thermoelectric figure of merit (ZT) relative to bulk values, which occur through the various physics and materials science phenomena associated with the quantum-dot structures. To date, we have obtained estimated ZT values approximately double the best bulk PbTe values, with estimated ZT as high as about 0.9 at 300 K.     

Thermoelectric Performance for SnSe Hot-Pressed at Different Temperature

Herein, nanoparticles SnSe are prepared by fusion method together with ball-milling technique and the effect of hot-pressing temperatures on the thermoelectric properties of the dense materials is explored. Due to the optimization of carrier concentration, the peak figure of merit (ZT) value of the compacted material reaches 0.73 for SnSe sample hot-pressed at 400°C and 450°C. The present investigation indicates that the thermoelectric performance of the SnSe compound can be significantly improved by sintering with suitable temperature.     

Tunable Assembly of sp3 Cross‐Linked 3D Graphene Monoliths: A First‐Principles Prediction

One of the biggest challenges in graphene applications is how one can fabricate 3D architectures comprising graphene sheets in which the resulting architectures have inherited graphene's excellent intrinsic properties but have overcome its shortcomings. Two series of 3D graphene monoliths (GMs) using zigzag or armchair graphene nanoribbons as building blocks and sp³ carbon chains as junction nodes are constructued, and calculations based on first principles are performed in order to predict their mechanical and electronic properties. The perfect match between sp² nanoribbons and sp³ linkers results in favorable energy and mechanical/dynamic stability. Owing to their tailored motifs, wine‐rack‐like pores, and rigid sp³ linkers, these GMs possess high surface areas, appreciable mechanical strength, and tunable band gaps. Negative linear compressibilities in a wide range are found for the zigzag GMs. By solving the problems of zero gap and dimensionality of graphene sheets simultaneously, these GMs offer a viable strategy towards many applications, e.g., microelectronic devices, energy storage, molecular sieves, sensitive pressure detectors, and telecommunication line systems.     

Transport properties of graphene nanoribbon heterostructures

We study the electronic and transport properties of heterostructures formed by armchair graphene nanoribbons with intersections of finite length. We describe the system by a tight-binding model and calculate the density of states and the conductance within the Green's function formalism based on real-space renormalization techniques. We show the apparition of interface states and bound states in the continuum which present a strong dependence of the heterostructure geometry. We investigate the effects on the conductance of an external perturbation applied on the edges atoms of the intersection region.     

High thermoelectric figures of merit in PbTe quantum wells

High-quality Pb_1−xEu_xTe/PbTe multiple quantum wells (MQWs) have been grown by molecular beam epitaxy. The measured 300K thermoelectric properties have been compared with that of the best bulk PbTe. This experimental investigation is the first detailed study of MQW structures designed to improve ZT of thermoelectric materials and has resulted in a breakthrough in the decades-long ZT ≅ 1 barrier for a room-temperature thermoelectric material. A value of Z_2DT >1.2 has been achieved for these PbTe quantum wells.     

Insulator band gap in graphane nanoribbons

A theoretical treatment of the insulator band gap E _g of graphane nanoribbons, i.e., graphene monolayer nanoribbons with both sides completely saturated with hydrogen, is presented. It is shown that E _g increases with decreasing nanoribbon width and is practically independent of the specific (zigzag or armchair) atomic structure of the nanoribbon edges.     

Yb和Ni共掺杂对α-MgAgSb室温热电材料的性能调控研究

单一Yb元素掺杂能有效提升α-MgAgSb室温热电材料的热电性能,但Yb元素在α-MgAgSb中较小的固溶度严重限制性能的进一步提升。本文采用Yb和Ni共掺杂来适当增加Yb掺杂元素的固溶度,提高α-MgAgSb的热电性能。结果表明,Yb和Ni原子质量和大小差异均会引起声子散射,极大地降低了α-MgAgSb的热导率,而且Yb和Ni共掺杂能延后双极效应。通过Yb和Ni共掺杂,α-MgAgSb的热电优值峰值达到1.05。     

Effects of Multiple Stacking Faults on the Electronic and Optical Properties of Armchair MoS$$_{}$$ Nanoribbons: First-Principles Calculations

The electronic and optical properties of armchair MoS\(_{2}\) nanoribbons with multiple stacking faults are investigated using first-principles calculations. It’s interesting that the band gaps approach zero for armchair MoS\(_{2}\) nanoribbons with two and four stacking faults. The gaps of armchair MoS\(_{2}\) nanoribbons with one stacking fault and three stacking faults are converged to 0.46 eV and 0.36 eV, respectively, which is smaller than perfect MoS\(_{2}\) nanoribbons. The partial charge density of armchair MoS\(_{2}\) nanoribbons with two stacking faults shows that the defect levels are originated from stacking faults. The frequency-dependent optical response (dielectric function, absorption, reflectance and electron energy loss spectra) is also presented. The optical results of monolayer MoS\(_{2}\) are in agreement with previous study. The peaks in the imaginary part of perfect armchair MoS\(_{2}\) nanoribbons are about 2.8 eV, 4.0 eV and 5.4 eV and the peaks of the imaginary part of armchair MoS\(_{2}\) nanoribbons with stacking faults are mainly 2.8 eV and 5.4 eV. They are independent of ribbon width. The peaks in electron energy loss spectra move toward larger wavelengths (redshift) due to the introduction of stacking faults.     

Electronic structures and transport properties of BN nanodot superlattices of armchair graphene nanoribbons

The electronic and transport properties of embedded boron nitride(BN) nanodot superlattices of armchair graphene nanoribbons are studied by first-principles calculations.The band structure of the graphene superlattice strongly depends on the geometric shape and size of the BN nanodot,as well as the concentration of nanodots.The conduction bands and valence bands near the Fermi level are nearly symmetric,which is induced by electron-hole symmetry.When B and N atoms in the graphene superlattices with a triangular BN nanodot are exchanged,the valance bands and conduction bands are inverted with respect to the Fermi level due to electron-hole symmetry.In addition,the hybridization ofπorbitals from C and redundant B atoms or N atoms leads to a localized band appearing near the Fermi level.Our results also show a series of resonant peaks appearing in the conductance.This strongly depends on the distance of the two BN nanodots and on the shape of the BN nanodot. Controlling these parameters might allow the modulation of the electronic response of the systems.     

BN-C Hybrid Nanoribbons as Gas Sensors

The effects of carbon monoxide (CO) and ammonia (NH₃) molecules adsorption on the various composites of boron nitride and graphene BN-C hybrid nanoribbons are investigated using the non-equilibrium Green’s function (NEGF) technique based on density functional theory (DFT). The effects of adsorption with possible random configurations on the average of the density of states (DOS), transmission coefficient, and the current–voltage (I–V) characteristics are calculated. The results indicate that, by embedding armchair graphene nanoribbon (AGNR) with boron nitride nanoribbon (BNNR), the various electronic properties can be observed after gas molecule adsorption. The electronic structure and gap of hybrids system is modified due to gas adsorption, and the systems act like the n-type semiconductor by NH₃ molecule adsorption. The hybrid structures due to their tunable band gap are better candidates for gas detecting compared to the pristine BNNRs and AGNRs.     

Ultrahigh Thermoelectric Performance Realized in Black Phosphorus System by Favorable Band Engineering through Group VA Doping

Black phosphorus (BP) has emerged as a promising thermoelectric candidate because of its strong electronic and thermal anisotropy, suggesting a large σ/κ ratio can be realized by controlling carrier transport orientation for a potentially high ZT. Nevertheless, to date, low conversion efficiency (ZT ≈0.08, 300 K) and poor stability of BP remain the major issues that have hampered its practical applications. This work reports a material family in simple composition XP₇, XP₃, and XP (X = N, As, Sb, Bi) with high‐performance thermoelectric properties by first‐principles calculations. Strikingly, an ultrahigh ZT up to 1.21 at 300 K is achieved in p‐type BiP₇ with an optimal carrier concentration of 5.48 × 10¹⁹ cm^?3 and ZT in n‐type NP₃ can reach up to ≈0.87 at the electron concentration of 3.67 × 10¹⁹ cm^?3 along the zigzag direction, owing to their enhanced density of states and multivalley band structures around the Fermi level through the resonant effects of VA guest and host atoms. Additionally, the calculations demonstrate further improvement in thermoelectric performance of pristine BP by ≈4.8 and 4.5 times at 800 K in p‐type NP and n‐type NP₃, respectively. Considering the high stability, current results indicate that N–P based systems are highly promising for novel metal‐free, nontoxic, and ultralight thermoelectrics.