Electromechanical switching in graphene nanoribbons

We investigate the effect of twisting on the electronic, magnetic and transport properties of zigzag graphene nanoribbon (ZGNR) using density functional theory (DFT) calculations. We compare electronic and magnetic properties of ZGNR in both planar and 180°-twisted geometries. Furthermore, combining DFT with the nonequilibrium Green’s function method (NEGF), we examine the quantum conductance of twisted ZGNR in its antiferromagnetic and ferromagnetic states. Consequently, the structure of local magnetic moments in the region between electrodes of opposite magnetization behaves as a Bloch/Neel domain wall. Our calculations show that ZGNR in its ground state (antiferromagnetic) is insensitive to twisting, there is no band gap change, and the conductance of the twisted ZGNR is almost unchanged as well. We demonstrate that the electromechanical switching can be realized by twisting a ferromagnetic ZGNR, which is an ideal spin valve in case of oppositely polarized leads (after twisting).     

人胎盘来源的造血干细胞和间充质干细胞的分离及鉴定

目的建立人胎盘来源的造血干细胞(placenta hematopoietic stem cells,hP-HSCs)和间充质干细胞(placentaderived mesenchymal stem cells,hP-MSCs)的分离方法,并进行鉴定和组分分析。方法选取10份新生健康婴儿胎盘组织,采用机械破碎法联合磁珠分选法分离h P-HSCs,胎盘绒毛膜组织块贴壁法分离hP-MSCs,利用形态学观察、集落培养、流式细胞术等进行鉴定。结果 hP-HSCs:分离后有核细胞数(total nucleated cell number,TNC)为(11. 82±2. 46)×10~8个,TNC回收率≥80%;细胞活性为(99. 7±0. 3)%;细胞表面抗体CD34~+CD45dim表达率为(8. 69±0. 36)%,CD34~+总数为(108. 0±6. 48)×10~6个;集落形成总数为(1. 88±1. 07)×10~6。hP-MSCs:冻存细胞总数为(40. 78±9. 35)×10~7个;细胞活性为(99. 0±1. 5)%;细胞表面抗体CD34~+CD45~+表达率为(0. 1±0. 1)%,CD44~+CD105~+为(99. 6±0. 2)%,CD14~+CD19~+为(0. 1±0. 1)%,CD90~+CD73+为(98. 9±0. 2)%;且具有良好成脂、成骨分化潜能。结论成功建立了hP-HSCs和hP-MSCs体外分离培养方法,为胎盘的临床应用奠定了基础,并提供了细胞种子资源。     

Spectral phonon thermal properties in graphene nanoribbons

This work provides a comprehensive investigation on the spectral phonon properties in graphene nanoribbons (GNRs) by the normal mode decomposition (NMD) method, considering the effects of edge chirality, width, and temperature. We find that the edge chirality has no significant effect on the phonon relaxation time but has a large influence to the phonon group velocity. As a result, the thermal conductivity of around 707 W/(m K) in the 4.26 nm-wide zigzag GNR at room temperature is higher than that of 467 W/(m K) in the armchair GNR with the same width. As the width decreases or the temperature increases, the thermal conductivity reduces significantly due to the decreasing relaxation times. Good agreement is achieved between the thermal conductivities predicted from the Green–Kubo method and the NMD method. We find that optical phonons dominate the thermal transport in the GNRs while the relative contribution of acoustic phonons to the thermal conductivity is only 10.1% and 13% in the zigzag GNR and the armchair GNR, respectively. Interestingly, the ZA mode is found to contribute only 1–5% to the total thermal transport in GNRs, being much lower than that of 30–70% in single layer graphene.     

血小板裂解液大规模扩增人脐带间充质干细胞

目的用血小板裂解液(platelet lysate,PL)大规模扩增人脐带间充质干细胞(umbilical cord mesenchymal stem cell,UCMSC),并检测其生物学特性,为临床应用提供实验依据。方法分别采用PL和胎牛血清(FBS)低密度扩增人UCMSC,比较两组UCMSC的细胞形态、大小、克隆形成率、增殖能力、细胞表型和分化能力。结果 PL扩增的UCMSC形态细长;直径明显小于FBS扩增的UCMSC(P<0.05);克隆形成率与FBS扩增的UCMSC差异无统计学意义(P>0.05);有更高的细胞累积群倍数;与FBS扩增的UCMSC有相似的细胞表型;与FBS扩增的UCMSC均具有成骨、成脂诱导分化能力,但PL扩增的UCMSC成骨分化能力更强。结论 PL可取代FBS用于大规模扩增UCMSC。     

Impurity effects on polaron dynamics in graphene nanoribbons

The impurity effects on the dynamics of polarons in armchair graphene nanoribbons are numerically investigated in the scope of a two-dimensional tight-binding approach with lattice relaxation. The results show that the presence of an impurity changes significantly the net charge distribution associated to the polaron structure. Moreover, the interplay between external electric field and the local impurities plays the role of drastically modifying the polaron dynamics. Interestingly, nanoribbons containing mobile polarons are noted to take place even when considering high impurity levels, which is associated with the highly conductive character of the graphene nanoribbons. This investigation may enlighten the understanding of the charge transport mechanism in carbon-based nanomaterials.     

Redox-switchable devices based on functionalized graphene nanoribbons

The possibility of tuning the electronic properties of graphene by tailoring the morphology at the nanoscale or by chemical functionalization opens interesting perspectives towards the realization of devices for nanoelectronics. Indeed, the integration of the intrinsic high carrier mobilities of graphene with functionalities that are able to react to external stimuli allows in principle the realization of highly efficient nanostructured switches. In this paper, we report a novel approach to the design of reversible switches based on functionalized graphene nanoribbons, operating upon application of an external redox potential, which exhibit unprecedented ON/OFF ratios. The properties of the proposed systems are investigated by electronic structure and transport calculations based on density functional theory and rationalized in terms of valence-bond theory and Clar's sextet theory.     

Ultra-low-edge-defect graphene nanoribbons patterned by neutral beam

Top-down process, comprising lithography and plasma etching is widely used in very-large-scale integration due to its scalability, has the greatest potential to fabricate graphene nanoribbon based nanoelectronic devices for large-scale intergraded circuits. However, conventional plasma etching inevitably introduces plenty of damage or defects to the etched materials, which drastically degrades the performance of nano materials. In this study, extremely low-damage neutral beam etching (NBE) is applied to fabricate ultra-low-defect graphene nanoribbon array (GNR). The ultra-low-edge-defect GNRs are fabricated by E-beam lithography followed by oxygen NBE from large-scale chemical-vapor-deposition-grown graphene. AFM images clearly shows the GNRs patterned by NBE and E-beam lithography, and Raman spectroscopy exhibits extremely low I_D/I_G of GNRs, which indicate that high-quality GNRs can be successfully fabricated by neutral beam. We also demonstrated bottom-gated field-effect transistor with the high-quality GNR and observed a high carrier mobility (>200 cm² V⁻¹ s⁻¹) at room temperature.     

Excitonic properties of hydrogen saturation-edged armchair graphene nanoribbons

First-principle density functional theory calculations with quasiparticle corrections and many body effects are performed to study the electronic and optical properties of armchair graphene nanoribbons (AGNRs) with variant edges saturated by hydrogen atoms. The "effective width" method associated with the reported AGNR family effect is introduced to understand the electronic structures. The method is further confirmed by analyses of the optical transition spectra and the exciton wavefunctions. The optical properties, including the optical transition spectra, exciton binding energies and the distribution of exciton wavefunctions, can be tuned with the hydrogen saturation edge, thus providing an effective way to control the features of the AGNRs.     

Evolution of graphene nanoribbons under low-voltage electron irradiation

Though the all-semiconducting nature of ultrathin graphene nanoribbons (GNRs) has been demonstrated in field-effect transistors operated at room temperature with ～10(5) on-off current ratios, the borderline for the potential of GNRs is still untouched. There remains a great challenge in fabricating even thinner GNRs with precise width, known edge configurations and specified crystallographic orientations. Unparalleled to other methods, low-voltage electron irradiation leads to a continuous reduction in width to a sub-nanometer range until the occurrence of structural instability. The underlying mechanisms have been investigated by the molecular dynamics method herein, combined with in situ aberration-corrected transmission electron microscopy and density functional theory calculations. The structural evolution reveals that the zigzag edges are dynamically more stable than the chiral ones. Preferential bond breaking induces atomic rings and dangling bonds as the initial defects. The defects grow, combine and reconstruct to complex edge structures. Dynamic recovery is enhanced by thermal activation, especially in cooperation with electron irradiation. Roughness develops under irradiation and reaches a plateau less than 1 nm for all edge configurations after longtime exposure. These features render low-voltage electron irradiation an attractive technique in the fabrication of ultrathin GNRs for exploring the ultimate electronic properties.     

Alignment of graphene nanoribbons by an electric field

In this paper, we develop an analytical approach to predict the field-induced alignment of cantilevered graphene nanoribbons. This approach is validated through molecular simulations using a constitutive atomic electrostatic model. Our results reveal that graphene’s field-oriented bending angle is roughly proportional to the square of field strength or to the graphene length for small deformations, while is roughly independent of graphene width. The effective bending stiffness and the longitudinal polarizability are found to be approximately proportional to the square of graphene length. Compared with carbon nanotubes, graphene nanoribbons are found to be more mechanically sensitive to an external electric field.     

Structure-mediated thermal transport of monolayer graphene allotropes nanoribbons

We report the study of the thermal transport management of monolayer graphene allotrope nanoribbons (size ∼20 × 4 nm²) by the modulation of their structures via molecular dynamics simulations. The thermal conductivity of graphyne (GY)-like geometries is observed to decrease monotonously with increasing number of acetylenic linkages between adjacent hexagons. Strikingly, by incorporating those GY or GY-like structures, the thermal performance of graphene can be effectively engineered. The resulting hetero-junctions possess a sharp local temperature jump at the interface, and show a much lower effective thermal conductivity due to the enhanced phonon–phonon scattering. More importantly, by controlling the percentage, type and distribution pattern of the GY or GY-like structures, the hetero-junctions are found to exhibit tunable thermal transport properties (including the effective thermal conductivity, interfacial thermal resistance and rectification). This study provides a heuristic guideline to manipulate the thermal properties of 2D carbon networks, ideal for application in thermoelectric devices with strongly suppressed thermal conductivity.     

Graphene nanogrids for selective and fast osteogenic differentiation of human mesenchymal stem cells

Graphene nanogrids (fabricated by graphene nanoribbons obtained through oxidative unzipping of multi-walled carbon nanotubes) were used as two-dimensional selective templates for accelerated differentiation of human mesenchymal stem cells (hMSCs), isolated from umbilical cord blood, into osteogenic lineage. The biocompatible and hydrophilic graphene nanogrids showed high actin cytoskeleton proliferations coinciding with patterns of the nanogrids. The amounts of proliferations were found slightly better than proliferation on hydrophilic graphene oxide (GO) sheets, and significantly higher than non-uniform proliferations on hydrophobic reduced graphene oxide (rGO) sheets and polydimethylsiloxane substrate. In the presence of chemical inducers, the reduced graphene oxide nanoribbon (rGONR) grid showed a highly accelerated osteogenic differentiation of the hMSCs (a patterned differentiation) in short time of 7 days in which the amount of the osteogenesis was ∼2.2 folds greater than the differentiation (a uniform differentiation) on the rGO sheets. We found that although in the absence of any chemical inducers the graphene nanogrids showed slight patterned osteogenic differentiations, the graphene sheets could not present any differentiation. Therefore, the highly accelerated differentiation on the rGONR grid was assigned to both its excellent capability in adsorption of the chemical inducers and physical stresses induced by the surface topographic features of the nanogrids.     

Cellular behavior of human mesenchymal stem cells cultured on single-walled carbon nanotube film

The influences of carboxylic functionalized single-walled carbon nanotubes (SWCNTs) on cell adhesion, spreading and cell lineage commitment of human mesenchymal stem cells (hMSCs) were evaluated. hMSCs were cultured on a thin mesh like layer of SWCNTs with a vertical height of less than 100 nm. The influence of the SWCNT film was significant on the cell spreading and focal adhesion distribution. Cells spread better on a SWCNT film as compared to cover slip (control), resulting in larger cell area and have higher occurrence of filopodia (microspikes) at the cell boundaries. Cytoskeleton arrangement was observed to be less orientated in the cells cultured on a SWCNT film as compared to control. Neurogenic markers such as nestin, glial fibrillary acidic protein and microtubule associated protein 2 genes were transiently upregulated (a process where cellular components, in this case RNA, is increased in response to external variable) over the first week while genes indicative of osteogenesis remained at its nominal level. These results suggest that nano roughness alone is sufficient to modulate cellular behavior and early stage of stem cell lineage commitment without the aid of an induction medium.     

改良组织块贴壁法制备人脐带间充质干细胞

目的采用改良组织块贴壁法制备人脐带间充质干细胞(human umbilical cord mesenchymal stem cells,hUCMSCs)。方法无菌条件下取足月婴儿脐带,采用改良组织块贴壁法分离间充质干细胞,台盼蓝染色法计数单个核细胞数,倒置相差显微镜下观察细胞形态,流式细胞仪检测细胞周期及免疫表型。结果原代hUCMSCs的数量为(1~5)×105个。培养至8 d,组织块边缘处有单细胞爬出;培养至2周,贴壁生长的细胞散在分布或形成小克隆;培养至20 d,细胞呈梭型、三角形或多角形。培养第20天的原代hUCMSCs的G0/G1期细胞占88.12%,S期细胞占1.23%,G2/M期细胞占10.65%,大多数细胞处于细胞增殖的潜伏期,CD105阳性率为97.65%,CD34阳性率为2.54%。结论采用改良组织块贴壁法分离了hUCMSCs,其生物学特性与骨髓间充质干细胞相似,为其临床应用奠定了基础。     

Step-templated CVD growth of aligned graphene nanoribbons supported by a single-layer graphene film

We present chemical vapor deposition (CVD) growth of a hybrid structure of aligned graphene nanoribbons (GNRs) supported by a single-layer graphene sheet. The step structure created on the epitaxial Co film is used to segregate arrays of aligned GNRs. Reflecting the highly ordered step structure of the Co catalyst, straight nanoribbons with high aspect ratio (>100) are formed. Analysis suggests that a large-area, single-layer graphene film also grows over the aligned GNRs, making a GNR-graphene hybrid structure. We also demonstrate the isolation of aligned GNRs by oxygen plasma treatment or partial transfer of the hybrid film. These findings on the formation of highly aligned GNRs give new insights into the formation mechanism of graphene and can be applied for more advanced graphene structure for future electronics.     

Magnetic response of conductance peak structure in junction-confined graphene nanoribbons

We have numerically investigated the magnetic response of the conductance peak structures in the transport gap of graphene nanoribbons. It is shown that the magnetic field induces a number of new conductance peaks within the transport gap of graphene nanoribbons confined by structural junctions. In addition, the magnetic field causes a shift of the conductance peak position and broadening of the peak width. This behaviour is due to the disappearance of zero conductance dips at the junction as a result of breaking time-reversal symmetry. Such behaviour is, however, not observed in the electronic transport of graphene nanoribbons confined by potential barriers, i.e. p-n-junctions. Thus, the magnetic response of conductance peaks may be used to distinguish the origin of the conductance peak structure within the transport gap observed in the experiments.     

Laser-induced unzipping of carbon nanotubes to yield graphene nanoribbons

Irradiation of undoped and doped multi-walled carbon nanotubes by an excimer laser (energy ～200-350 mJ) yields graphene nanoribbons (GNRs). The GNRs so obtained have been characterized by transmission electron microscopy, atomic force microscopy and Raman spectroscopy.     

A theoretical analysis of field emission from graphene nanoribbons

The characteristics of field-emission current of a graphene nanoribbon along the direction without finite size effect in the graphene sheet have been theoretically studied. After the work function of graphene nanoribbon is assumed not to change with its width, numerical calculations show that field-emission current density at a given external field decreases with decreasing width of nanoribbon. Such a decrease is especially large when the width of graphene ribbon is a few nanometers. Thus it will lead to an increase in the turn-on electric field. Numerical calculations also show that the effect of the image potential on the field-emission current density decreases with decreased temperature. This implies that to ensure in a workable field-emission device fabricated by graphene nanoribbon, a wider nanoribbon is needed to keep a large field-emission current density.     

Electromechanical properties of armchair graphene nanoribbons under local torsion

While graphene nanoribbons are prone to twist intrinsically, the effect of local twist on the electromechanical properties remains unexplored. By using the density functional theory in combination with the nonequilibrium Green’s function method, we investigate the responses of structural evolution and electrical transport of armchair graphene nanoribbons to local torsion. We show that local twist can alter their transport properties significantly. The current at a given bias can switch on/off or change many times with twist angle, which is related with twist-induced changes in electronic structures of graphene nanoribbons. Our results can provide a valuable guideline for design and implementation of graphene nanoribbons in nanoelectromechanical systems and devices.     

Electronic thermal conductivity and thermopower of armchair graphene nanoribbons

A theoretical investigation of the diffusion contribution to thermopower, S_d, and the electronic thermal conductivity, κ_e, of semiconducting armchair graphene nanoribbons (GNRs) is made for T ⩽ 300 K. Considering the electrons to be scattered by edge roughness, impurities and deformation-potential coupled acoustic phonons and optical phonons, expressions for S_d and κ_e are obtained. Numerical calculations of S_d and κ_e, as functions of temperature and linear carrier density, bring out the relative importance of the contributing scattering mechanisms. A GNR of width 5 nm, supporting an electron density 2 × 10⁸ m⁻¹, is found to exhibit room temperature values of S_d and κ_e as 42 μV/K and 26.5 W/mK, respectively. A decrease in armchair GNR width, is found to enhance S_d and reduce κ_e. The effect of varying the electron density is to increase their magnitude when Fermi energy moves into the second subband. An analysis of thermopower and thermal conductivity data in clean armchair GNR samples will enable better understanding of the electron–phonon interaction.