350 research outputs found
High-mobility anisotropic transport in few-layer γ-B28 films
No full textRecent reports of successful synthesis of atomically thin boron films have raised great prospects of discovering novel electronic and transport properties in a new type of 2D materials. Here we show by first-principles calculations that monolayer and bilayer γ-B28 films are intrinsically metallic while the thicker films possess intriguing electronic states that exhibit moderate to large bandgaps in all the interior layers but are nearly gapless at the surface. Remarkably, these surface electronic states are tunable by strain, allowing the outermost layer to transition between a semimetal and a narrow-gap semiconductor. Moreover, these surface states almost exclusively occupy a wide energy range around the Fermi level, thus dominating the electronic transport in γ-B28 films. The dispersions of the surface electronic bands are direction sensitive, and with hole injection producing anisotropic and very high carrier mobility up to 104 cm2 V−1 s−1. Surprisingly, surface passivation can open a sizable bandgap, which offers an additional avenue for effective band engineering and explains the experimental observation of a large bandgap in the synthesized film. These results make few-layer γ-B28 films desirable candidate materials for catalysis and electronics applications
Rational molecular design of efficient yellow-red dendrimer TADF for solution-processed OLEDs : a combined effect of substitution position and strength of the donors
No full textFunding: Changfeng Si thanks the China Scholarship Council (201806890001). Dianming Sun acknowledges support from the Royal Academy of Engineering Enterprise Fellowship (EF2122-13106). We thank EPSRC (EP/W015137/1, EP/W524505/1) for financial support. Open Access funding enabled and organized by Projekt DEAL.The development of high-performance solution-processed red organic light-emitting diodes (OLEDs) remains a challenge, particularly in terms of maintaining efficiency at high luminance. Here, we designed and synthesized four novel orange-red thermally activated delayed fluorescence (TADF) dendrimers that are solution-processable: 2GCzBP , 2DPACzBP , 2FBP2GCz and 2FBP2DPACz . We systematically investigated the effect of substitution position and strength of donors on the optoelectronic properties. The reverse intersystem crossing rate constant (kRISC) of the emitters having donors substituted at positions 11 and 12 of the dibenzo[a,c]phenazine (BP) is more than 10-times faster than that of compounds substituted having donors substituted at positions 3 and 6. Compound 2DPACzBP , containing stronger donors than 2GCzBP , exhibits a red-shifted emission and smaller singlet-triplet energy splitting, ΔEST, of 0.01 eV. The solution-processed OLED with 10 wt% 2DPACzBP doped in mCP emitted at 640 nm and showed a maximum external quantum efficiency (EQEmax) of 7.8%, which was effectively maintained out to a luminance of 1,000 cd m−2. Such a device∙s performance at relevant display luminance is among the highest for solution-processed red TADF OLEDs. The efficiency of the devices was improved significantly by using 4CzIPN as an assistant dopant in a hyperfluorescence (HF) configuration, where the 2DPACzBP HF device shows an EQEmax of 20.0% at λEL of 605 nm and remains high at 11.8% at a luminance of 1,000 cd m−2, which makes this device one of the highest efficiency orange-to-red HF SP-OLEDs to date.Peer reviewe
On the Energy Conserved in a Buckling Fung Hyperelastic Cylindrical Shell Subjected to Torsion, Internal Pressure and Axial Tension
No full textThe author has granted permission for their work to be available to the general public.A theoretical model is proposed for the buckling of a three-dimensional vein subjected to torsion, internal pressure, and axial tension using energy conservation methods. The vein is assumed to be an anisotropic hyperelastic cylindrical shell which obeys the Fung constitutive model. Finite deformation theory for thick-walled blood vessels is used to characterize the vessel dilation in the pre-buckling state.
The pre-buckling state is identified by its midpoint and then perturbed by a displacement vector field dependent on the circumferential and axial directions to define the buckled state. The total potential energy functional of the system is extremized by minimizing the first variation with respect to the elements of the set of all continuous bounded functions on . The Euler-Lagrange equations form three coupled linear partial differential equations with Dirichlet boundary conditions characterizing the buckling displacement field under equilibrium.
A second solution method approximates the first variation of the total potential energy functional using a variational Taylor series expansion. The approximation is minimized and combined with equations of equilibrium derived from elasticity theory to yield a polynomial relating buckling eigenmodes, material parameters, geometric parameters, and the critical angle of twist which induces buckling. Various properties of the total potential energy functional specific to the problem are proved. Another solution method is outlined using the first variation approximation and the basis of the kernel of the linear transformation which maps buckling displacement amplitudes during static equilibrium.Mathematic
Climate Response of Tree Radial Growth at Different Timescales in the Qinling Mountains.
No full textThe analysis of the tree radial growth response to climate is crucial for dendroclimatological research. However, the response relationships between tree-ring indices and climatic factors at different timescales are not yet clear. In this study, the tree-ring width of Huashan pine (Pinus armandii) from Huashan in the Qinling Mountains, north-central China, was used to explore the response differences of tree growth to climatic factors at daily, pentad (5 days), dekad (10 days) and monthly timescales. Correlation function and linear regression analysis were applied in this paper. The tree-ring width showed a more sensitive response to daily and pentad climatic factors. With the timescale decreasing, the absolute value of the maximum correlation coefficient between the tree-ring data and precipitation increases as well as temperature (mean, minimum and maximum temperature). Compared to the other three timescales, pentad was more suitable for analysing the response of tree growth to climate. Relative to the monthly climate data, the association between the tree-ring data and the pentad climate data was more remarkable and accurate, and the reconstruction function based on the pentad climate was also more reliable and stable. We found that the major climatic factor limiting Huashan pine growth was the precipitation of pentads 20-35 (from April 6 to June 24) rather than the well-known April-June precipitation. The pentad was also proved to be a better timescale for analysing the climate and tree growth in the western and eastern Qinling Mountains. The formation of the earlywood density of Chinese pine (Pinus tabulaeformis) from Shimenshan in western Qinling was mainly affected by the maximum temperature of pentads 28-32 (from May 16 to June 9). The maximum temperature of pentads 28-33 (from May 16 to June 14) was the major factor affecting the ring width of Chinese pine from Shirenshan in eastern Qinling
Tree-ring-based drought variability in the eastern region of the Silk Road and its linkages to the Pacific Ocean
No full textDrought variability from 1568 to 2014 was presented for the eastern region of the Silk Road by using five tree ring chronologies and an optimal information extraction method (i.e., one of the composite-plus-scale methods). The inspection results of the split calibration-verification procedure for the transfer functions showed that the reconstructed Palmer drought severity index (PDSI) was credible throughout the entire time interval. During the past 447 years, there were seven dry periods (1616-1622, 1629-1645, 1682-1730, 1760-1778, 1805-1884, 1919-1933 and 1990-2009) and seven wet periods (1573-1615, 1623-1628, 1646-1681, 1731-1759, 1779-1804, 1885-1918 and 1934-1989). The dry periods were well-documented historical drought events. Significant interannual periods of 2.1-3.8 years and interdecadal cycles of 17.1, 18.3, 23.8, 42.7, 51.3 and 73.0 years were identified via the multitaper method for spectral analysis. Similar patterns of drought fluctuations were found in the records of the drought/flood index and other drought reconstructions. By comparing the reconstructed PDSI with the El Nifio-Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO) at the multidecadal scale, it was discovered that when ENSO and PDO were in phase (i.e., high-PDO/warm-ENSO phase or low-PDO/cold-ENSO phase), the study region was dry or wet more often, respectively, especially before 1850. The influence of ENSO and PDO on the decadal variability of drought has been affected by global warming
Solutions to the First-order Buckling Equations of a Fung Hyperelastic Cylindrical Shell Subjected to Torsion, Internal Pressure, and Axial Tension
No full textThe author has granted permission for their work to be available to the general public.In this study a theoretical model is proposed for the buckling of a vein subjected to torsion, internal pressure, and axial tension using a formation of elasticity theory for shells. The vein is assumed to be an anisotropic hyperelastic cylindrical shell which obeys the Fung constitutive model.
The approach uses finite deformation theory for thick-walled blood vessels to characterize the vessel dilation in the pre-buckling state. The pre-buckling state is identified by its midpoint and then perturbed by a displacement vector field dependent on the circumferential and axial directions to define the buckled state. The buckling equations of static equilibrium are derived using the nominal stress measure and traction boundary conditions are applied. A side result is shown proving the existence of a moment traction although typically taken to be zero for torsional problems. Perturbational displacements raised to the power of two or greater are assumed negligible thereby linearizing the coupled partial differential equations of equilibrium. The coupled equations are solved by supposing first-order and single Fourier term trigonometric forms for the displacement field components.
The model and the assumptions used are validated by experimental data for five human great saphenous vein (GSV) samples taken from a previous study. The theoretical model is unstable but using an eigenvalue compatibility condition as a selection method yields strong quantitative results for three out of five GSVs in the entire tested pressure range (6-100 mmHg). The other two sampless showed excessive stiffening upon loading and may indicate limitations of the model although quantitative predictions were still moderately accurate. The strongest results are in the 6-20 mmHg pressure range where all vessels matched well with predicted values. In general the model showed increased error as pressure increased hinting that effects of vessel stiffening are poorly predicted. The eigenmodes predicted were consistently inaccurate indicating the assumptions used in this solution method are inadequate to characterize the buckling modes of a nonlinear material. It may suggest that nonlinear buckling conformation is determined by nonlinear perturbation terms.Biomedical Engineerin
From Toroidal to Rod-Like Nanostructure, a Mechanism Study for the Reversible Morphological Control on Amphiphilic Triblock Copolymer Micelles
No full textMechanism of the morphological changes between toroidal and rod-like nanostructures of P4VP-b-PS-b-P4VP amphiphilic triblock copolymer micelles has been investigated in aqueous solution. This transition is proved to be highly reversible and tunable upon changing temperature. The toroidal structure, evolving from fibers at 20 degrees C, can transform to rod-like morphology as the temperature either gradually or directly increases to 80 degrees C, and vice versa. However, the transition mechanisms are quite different in different temperature-changing processes. The structure and thickness of the micelles are dependent on the specific temperature, whereas the transition mechanism is related to the method of the temperature change. These morphological changes are considered as a result from the interaction parameter between the solvent and the copolymer blocks, especially the hydrophobic block. Our research complements the external control over the reversible morphological transition of block copolymer micelles without changing the composition of the system or introducing additional influencing factors. (C) 2016 Wiley Periodicals, Inc
Angular Momentum Projected Potential-Energy-Surface Calculation: Application to Hf-178
No full textAn angular momentum projected potential-energy-surface (PES) calculation, which takes both rotational symmetry restoration and multi-quasiparticle excitation into account, is developed by using the macroscopic-microscopic model and the projected shell model (PSM). Within this method, it may become possible to modify the excitation spectra which are influenced by shape-softness of nuclei, including high-K states. As our first example, this method is adopted to study the collective and multi-quasiparticle excitations of Hf-178, and the results are in good agreement with the existing experimental data. In addition, as for the dominant structure of non-collective 6(+) bands, the conflict between experimental result and the previous PSM calculation is clarified.http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcApp=PARTNER_APP&SrcAuth=LinksAMR&KeyUT=WOS:000305474800017&DestLinkType=FullRecord&DestApp=ALL_WOS&UsrCustomerID=8e1609b174ce4e31116a60747a720701Physics, Fluids & PlasmasSCI(E)EI中国科学引文数据库(CSCD)0ARTICLE6514-5171
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