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1.
Artigo em Inglês | MEDLINE | ID: mdl-32498510

RESUMO

A defect dynamic model is proposed for the positive synergistic effect of neutron- and γ-ray-irradiated silicon NPN transistors. The model considers a γ-ray-induced transformation and annihilation of the neutron-induced divacancy defects in the p-type base region of the transistor. The derived model of the base current predicts a growth function of the γ-ray dose that approaches exponentially an asymptotic value, which depends linearly on the neutron-induced initial displacement damage (DD) and a linear decay function of the dose whose slope depends quadratically on the initial DD. Variable fluence and dose neutron-γ-ray irradiation experiments are carried out, and we find all of the novel dose and fluence dependence predicted by the proposed model are confirmed by the measured data. Our work, hence, identifies that the defect evolution under γ-ray irradiation, rather than the widely believed interface Coulomb interaction, is the dominating mechanism of the synergistic effect. Our work also paves the way for the modification of displacement defects in silicon by γ-ray irradiation.

2.
Phys Chem Chem Phys ; 2020 May 13.
Artigo em Inglês | MEDLINE | ID: mdl-32400823

RESUMO

Searching for half-metals in low dimensional materials is not only of scientific importance, but also has important implications for the realization of spintronic devices on a small scale. In this work, we show theoretically that simple bending can induce spin-splitting in bilayer silicene. For bilayer silicene with Bernal stacking, the monolayer has a long range ferromagnetic spin order and between the two monolayers, the spin orders are opposite, giving rise to an antiferromagnetic configuration for the ground state of the bilayer silicene. Under bending, the antiferromagnetic spin order is retained but the energetic degeneracy of opposite spin states is lifted. Due to the unusual deformation potentials of the conduction band minimum (CBM) and valence band maximum (VBM) as revealed by density-functional theory calculations and density-functional tight-binding calculations, this spin-splitting is nearly proportional to the degree of bending deformation. Consequently, the spin-splitting can be significant and the desired half-metallic state may emerge when the bending increases, which has been verified by direct simulation of the bent bilayer silicene using the generalized Bloch theorem. Our results hint that bilayer silicene may be an excellent candidate for half-metallicity.

3.
Phys Chem Chem Phys ; 2020 Mar 25.
Artigo em Inglês | MEDLINE | ID: mdl-32211628

RESUMO

Realization of half-metallicity (HM) in low dimensional materials is a fundamental challenge for nano spintronics and a critical component for developing alternative generations of information technology. Using first-principles calculations, we reveal an unconventional deformation potential for zigzag nanoribbons (NRs) of 2D-Xenes. Both the conduction band minimum (CBM) and valence band maximum (VBM) of the edge states have negative deformation potentials. This unique property, combined with the localization and spin-polarization of the edge states, enables us to induce spin-splitting and HM using an inhomogeneous strain pattern, such as simple in-plane bending. Indeed, our calculation using the generalized Bloch theorem reveals the predicted HM in bent zigzag silicene NRs. Furthermore, the magnetic stability of the long range magnetic order for the spin-polarized edge states is maintained well against the bending deformation. These aspects indicate that it is a promising approach to realize HM in low dimensions with the zigzag 2D-Xene NRs.

4.
Phys Rev Lett ; 124(8): 086401, 2020 Feb 28.
Artigo em Inglês | MEDLINE | ID: mdl-32167367

RESUMO

The existence of Bloch flat bands of electrons provides a facile pathway to obtain exotic quantum phases owing to strong correlation. Despite the established magic angle mechanism for twisted bilayer graphene, understanding of the emergence of flat bands in twisted bilayers of two-dimensional polar crystals remains elusive. Here, we show that due to the polarity between constituent elements in the monolayer, the formation of complete flat bands in twisted bilayers is triggered as long as the twist angle is less than a certain critical value. Using the twisted bilayer of hexagonal boron nitride (hBN) as an example, our simulations using the density-functional tight-binding method reveal that the flat band originates from the stacking-induced decoupling of the highest occupied (lowest unoccupied) states, which predominantly reside in the regions of the moiré superlattice where the anion (cation) atoms in both layers are overlaid. Our findings have important implications for the future search for and study of flat bands in polar materials.

5.
J Phys Chem Lett ; 11(3): 652-658, 2020 Feb 06.
Artigo em Inglês | MEDLINE | ID: mdl-31912739

RESUMO

Recently, the structural stability of all-inorganic halide perovskite nanocrystals has been significantly enhanced. To understand the enhancement, we developed surface-passivation models for cubic CsPbBr3 nanocrystals with anionic (oleate) and cationic (oleylammonium) organic ligands based on first-principles calculations and nuclear magnetic resonance investigations. We propose that the (100) surface is initially terminated with oleate ligand complexes on PbBr2(100) surfaces. Also, the ligand transition to oleylammonium on the Pb-rich surfaces is expected due to the addition of metal halides (ZnBr2) during colloidal synthesis. The significant improvement in the structural stability of the cationic ligand-passivated CsPbBr3 nanocrystals was attributed to the suppressed exposure of the merging-vulnerable (110) surface, caused by the large difference in formation energy between the ligand-passivated (100) and Br-passivated (110) surfaces.

6.
ACS Nano ; 13(11): 13492-13500, 2019 Nov 26.
Artigo em Inglês | MEDLINE | ID: mdl-31689076

RESUMO

Generating single photons at high temperature remains a major challenge, particularly for group III-As and III-P materials widely used in optical communication. Here, we report a high temperature single photon emitter based on a "surface-free" GaAs quantum dot (QD) in a GaAsP nanowire. By using self-catalyzed vapor-liquid-solid growth and simple surface engineering, we can significantly enhance the optical signal from the QDs with a highly polarized photoluminescence at 750 nm. The "surface-free" nanowire quantum dots show photon antibunching up to 160 K and well resolved exciton lines as high as 220 K.

7.
J Phys Chem Lett ; 10(23): 7421-7425, 2019 Dec 05.
Artigo em Inglês | MEDLINE | ID: mdl-31735032

RESUMO

Dislocations are always observed during crystal growth, and it is usually desirable to reduce the dislocation density in high-quality crystals. Here, the annihilation process of the 30° Shockley partial dislocation pairs in CdTe is studied by first-principles calculations. We found that the dislocations can glide relatively easily due to the weak local bonding. Our systematic study of the slipping mechanism of the dislocations suggests that the energy barrier for the annihilation process is low. Band structure calculations reveal that the band bending caused by the charge transfer between the two dislocation cores depends on the core-core distance. A simple linear model is proposed to describe the mechanism of formation of the dislocation pair. More importantly, we demonstrate that hole injection can affect the core structure, increase the mobility, and eventually trigger a spontaneous mutual annihilation, which could be employed as a possible facile way to reduce the dislocation density.

8.
Adv Mater ; 31(17): e1806593, 2019 Apr.
Artigo em Inglês | MEDLINE | ID: mdl-30883950

RESUMO

Tetrahedral coordination structures, e.g. crystalline Si, GaAs, CdTe, and octahedral coordination structures, e.g. perovskites, represent two classes of successful crystal structures hitherto for solar cell absorbers. Here, via first-principles calculations and crystal symmetry analysis, the two classes of semiconductors are shown exhibiting complementary properties in terms of bond covalency/ionicity, optical property, defect tolerance, and stability, which are correlated with their respective coordination number. Therefore, a spinel structure is proposed, which combines tetrahedral and octahedral coordination into a single crystal structure, as an alternative to perovskite and conventional semiconductors for potential photovoltaic applications. The case studies of a class of 105 spinel AB2 X4 systems identify five spinel compounds HgAl2 Se4 , HgIn2 S4 , CdIn2 Se4 , HgSc2 S4 , and HgY2 S4 as promising solar cell absorbers. In particular, HgAl2 Se4 has suitable bandgap (1.36 eV by GW0 calculation), small direct-indirect bandgap difference (24 meV), appropriate carrier effective mass (me = 0.08 m0 , and mh = 0.69 m0 ), strong optical absorption, and high dynamic stability. This study suggests that crystal systems with mixed tetrahedral and octahedral coordination may open a viable route for emerging solar cell absorbers.

9.
J Phys Chem Lett ; 10(1): 59-66, 2019 Jan 03.
Artigo em Inglês | MEDLINE | ID: mdl-30554516

RESUMO

The intrinsic poor stability of MAPbI3 hybrid perovskites in the ambient environment remains as the major challenge for photovoltaic applications. In this work, complementary first-principles calculations and experimental characterizations reveal that metal cation alloyed perovskite (MABa xPb1- xI3) can be synthesized and exhibit substantially enhanced stability via forming stronger Ba-I bonds. The Ba-Pb alloyed perovskites remain phase-pure in ambient air for more than 15 days. Furthermore, the bandgap of MABa xPb1- xI3 is tailored in a wide window of 1.56-4.08 eV. Finally, MABa xPb1- xI3 is used as a capping layer on MAPbI3 in solar cells, resulting in significantly improved power conversion efficiency (18.9%) and long-term stability (>30 days). Overall, our results provide a simple but reliable strategy toward stable less-Pb perovskites with tailored physical properties.

10.
Nano Lett ; 18(11): 6665-6671, 2018 11 14.
Artigo em Inglês | MEDLINE | ID: mdl-30350652

RESUMO

Nanocrystals (NCs) with identical components and sizes but different crystal structures could not be distinguished by conventional absorption and emission spectra. Herein, we find that circular dichroism (CD) spectroscopy can easily distinguish the CdSe nanoplatelets (NPLs) with different crystal structures of wurtzite (WZ) and zincblende (ZB) with the help of chiral l- or d-cysteine ligands. In particular, the CD signs of the first excitonic transitions in WZ and ZB NPLs capped by the same chiral cysteine are opposite. Theoretic calculation supports the viewpoint of different crystal structures and surfaces arrangements between WZ and ZB NPLs contributing to this significant phenomenon. The CD peaks appearing at the first excitonic transition band of WZ or ZB CdSe NPLs are clearly assigned to the different transition polarizations along 4p( x,y,z),Se → 5sCd or 4p( x,y),Se → 5sCd. This work not only provides a deep insight into the origin of the optical activity inside chiral semiconductor nanomaterials but also proposes the design principle of chiral semiconductor nanocrystals with high optic activity.

11.
Phys Chem Chem Phys ; 20(27): 18455-18462, 2018 Jul 11.
Artigo em Inglês | MEDLINE | ID: mdl-29947383

RESUMO

A simple and efficient top-down strategy, the chemical vapor etching method, is reported for synthesizing corrugated ZnO nano/micro rods (NRs). The stabilization mechanism of this unique nanostructure has been determined through a combination of aberration-corrected field emission scanning electron microscopy, high-resolution transmission electron microscopy, and first-principles calculations. The experimental data are in good agreement with the theoretical calculations, and a remarkable nonpolar-to-polar surface faceting transition is demonstrated. The corrugated-shaped structure results from the remarkable stability of the defect-induced reconstructions (O vacancy, Zn-Zn dimer), which makes the high-index polar {303[combining macron]1} and {101[combining macron]1[combining macron]} planes lower in energy compared to the nonpolar {101[combining macron]0} plane. Based on the results of first-principles surface calculations, a general formula is established to provide an accurate description of the unusual size effect of the length of the corrugated unit vs. the NR diameter, and it also offers direct explanations for certain experimental observations. The present study deepens our atomic-level understanding of the detailed structure and stability of polar surface decorated corrugated ZnO NRs, and points to a viable path towards designing polar-stable wurtzite structures.

12.
Phys Chem Chem Phys ; 20(18): 12916-12922, 2018 May 09.
Artigo em Inglês | MEDLINE | ID: mdl-29701208

RESUMO

Based on first-principles calculations, we present a systematic investigation of the electronic and magnetic properties of armchair phosphorene nanoribbons (APNRs) functionalized by 3d transition metal (TM) atoms. We found that the central hollow site is the most favorable adsorption site for Mn, Co and Ni, while Fe preferentially occupies the edge hollow site. All of the TM atoms bind to the adjacent P and their adsorption energies are in the range of -4.29 eV to -1.59 eV. Meanwhile, the large ratio of the adsorption energy to the cohesive energy of the metal bulk phase indicates that TM atoms have a preferred 2D growth mode on the edge hydrogenated armchair phosphorene nanoribbons (H-APNRs). The magnetic moments reduce by about 2-4 µB, relative to their free atom states, depending on whether the TM atom is in the high-spin or low-spin state. This reduction is mainly attributed to the electrons transferring from the high-level TM 4s shell to the low-lying 3d shell. Our results demonstrate that TM atom adsorption is a feasible approach to functionalizing the H-APNRs chemically, which results in peculiar electronic and magnetic properties for potential applications in nano-electronics and spintronics.

14.
J Phys Chem Lett ; 9(1): 31-35, 2018 Jan 04.
Artigo em Inglês | MEDLINE | ID: mdl-29232944

RESUMO

Cs2AgBiBr6 was proposed as one of the inorganic, stable, and nontoxic replacements of the methylammonium lead halides (CH3NH3PbI3, which is currently considered as one of the most promising light-harvesting material for solar cells). However, the wide indirect band gap of Cs2AgBiBr6 suggests that its application in photovoltaics is limited. Using the first-principle calculation, we show that by controlling the ordering parameter at the mixed sublattice, the band gap of Cs2AgBiBr6 can vary continuously from a wide indirect band gap of 1.93 eV for the fully ordered double-perovskite structure to a small pseudodirect band gap of 0.44 eV for the fully random alloy. Therefore, one can achieve better light absorption simply by controlling the growth temperature and thus the ordering parameters and band gaps. We also show that controlled doping in Cs2AgBiBr6 can change the energy difference between ordered and disordered Cs2AgBiBr6, thus providing further control of the ordering parameters and the band gaps. Our study, therefore, provides a novel approach to carry out band structure engineering in the mixed perovskites for optoelectronic applications.

15.
ACS Appl Mater Interfaces ; 9(48): 42149-42155, 2017 Dec 06.
Artigo em Inglês | MEDLINE | ID: mdl-29134796

RESUMO

van der Waals heterojunctions formed by stacking various two-dimensional (2D) materials have a series of attractive physical properties, thus offering an ideal platform for versatile electronic and optoelectronic applications. Here, we report few-layer SnSe/MoS2 van der Waals heterojunctions and study their electrical and optoelectronic characteristics. The new heterojunctions present excellent electrical transport characteristics with a distinct rectification effect and a high current on/off ratio (∼1 × 105). Such type-II heterostructures also generate a self-powered photocurrent with a fast response time (<10 ms) and exhibit high photoresponsivity of 100 A W-1, together with high external quantum efficiency of 23.3 × 103% under illumination by 532 nm light. Photoswitching characteristics of the heterojunctions can be modulated by bias voltage, light wavelength, and power density. The designed novel type-II van der Waals heterojunctions are formed from a combination of a transition-metal dichalcogenide and a group IV-VI layered 2D material, thereby expanding the library of ultrathin flexible 2D semiconducting devices.

16.
Sci Rep ; 7(1): 4563, 2017 07 04.
Artigo em Inglês | MEDLINE | ID: mdl-28676701

RESUMO

Efficient p-type doping in CdTe has remained a critical challenge for decades, limiting the performance of CdTe-based semiconductor devices. Arsenic is a promising p-type dopant; however, reproducible doping with high concentration is difficult and carrier lifetime is low. We systematically studied defect structures in As-doped CdTe using high-purity single crystal wafers to investigate the mechanisms that limit p-type doping. Two As-doped CdTe with varying acceptor density and two undoped CdTe were grown in Cd-rich and Te-rich environments. The defect structures were investigated by thermoelectric-effect spectroscopy (TEES), and first-principles calculations were used for identifying and assigning the experimentally observed defects. Measurements revealed activation of As is very low in both As-doped samples with very short lifetimes indicating strong compensation and the presence of significant carrier trapping defects. Defect studies suggest two acceptors and one donor level were introduced by As doping with activation energies at ~88 meV, ~293 meV and ~377 meV. In particular, the peak shown at ~162 K in the TEES spectra is very prominent in both As-doped samples, indicating a signature of AX-center donors. The AX-centers are believed to be responsible for most of the compensation because of their low formation energy and very prominent peak intensity in TEES spectra.

17.
J Am Chem Soc ; 139(7): 2630-2638, 2017 02 22.
Artigo em Inglês | MEDLINE | ID: mdl-28112933

RESUMO

Hybrid organic-inorganic halide perovskites with the prototype material of CH3NH3PbI3 have recently attracted intense interest as low-cost and high-performance photovoltaic absorbers. Despite the high power conversion efficiency exceeding 20% achieved by their solar cells, two key issues-the poor device stabilities associated with their intrinsic material instability and the toxicity due to water-soluble Pb2+-need to be resolved before large-scale commercialization. Here, we address these issues by exploiting the strategy of cation-transmutation to design stable inorganic Pb-free halide perovskites for solar cells. The idea is to convert two divalent Pb2+ ions into one monovalent M+ and one trivalent M3+ ions, forming a rich class of quaternary halides in double-perovskite structure. We find through first-principles calculations this class of materials have good phase stability against decomposition and wide-range tunable optoelectronic properties. With photovoltaic-functionality-directed materials screening, we identify 11 optimal materials with intrinsic thermodynamic stability, suitable band gaps, small carrier effective masses, and low excitons binding energies as promising candidates to replace Pb-based photovoltaic absorbers in perovskite solar cells. The chemical trends of phase stabilities and electronic properties are also established for this class of materials, offering useful guidance for the development of perovskite solar cells fabricated with them.

18.
ACS Appl Mater Interfaces ; 9(4): 3785-3791, 2017 Feb 01.
Artigo em Inglês | MEDLINE | ID: mdl-28055176

RESUMO

The development of catalytic materials for the hydrogen oxidation, hydrogen evolution, oxygen reduction or oxygen evolution reactions with high reaction rates and low overpotentials are key goals for the development of renewable energy. We report here Ru(0) nanoclusters supported on nitrogen-doped graphene as high-performance multifunctional catalysts for the hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR), showing activities similar to that of commercial Pt/C in alkaline solution. For HER performance in alkaline media, sample Ru/NG-750 reaches 10 mA cm-2 at an overpotential of 8 mV with a Tafel slope of 30 mV dec-1. The high HER performance in alkaline solution is advantageous because most catalysts for ORR and oxygen evolution reaction (OER) also prefer alkaline solution environment whereas degrade in acidic electrolytes. For ORR performance, Ru/NG effectively catalyzes the conversion of O2 into OH- via a 4e process at a current density comparable to that of Pt/C. The unusual catalytic activities of Ru(0) nanoclusters reported here are important discoveries for the advancement of renewable energy conversion reactions.

19.
J Phys Condens Matter ; 29(8): 085501, 2017 Mar 01.
Artigo em Inglês | MEDLINE | ID: mdl-28060770

RESUMO

The optical band gap, extracted from absorption measurements, defines the figure of merit for transparent conducting oxides (TCOs). In many oxides, such as [Formula: see text] or [Formula: see text], inversion symmetry introduces a selection rule that blocks transitions from the valence-band maximum to the conduction-band minimum. This raises the absorption threshold and enlarges the optical gap relative to the fundamental band gap. Here, we present density-functional computations identifying two optical gaps, either of which can be detected, depending on the optical light intensity. Under strong illumination, weak transitions from [Formula: see text]-points near the valence-band maximum contribute significantly to the absorption spectrum and define an optical gap matching the fundamental gap. Low optical intensities by contrast give prominence to the large optical gap determined by the selection rule. While experimental conditions have favored observation of the former optical gap in [Formula: see text], in contrast, absorption measurements in [Formula: see text] have focused on the latter. Our findings explain the disparity between the optical and fundamental gaps in bixbyite [Formula: see text] and predict that, measured under low illumination, the optical gap for rutile [Formula: see text] will increase, from 3.60 eV to 4.34 eV.

20.
Phys Rev Lett ; 119(25): 256402, 2017 Dec 22.
Artigo em Inglês | MEDLINE | ID: mdl-29303319

RESUMO

Triply degenerate points (TDPs) in band structure of a crystal can generate novel TDP fermions without high-energy counterparts. Although identifying ideal TDP semimetals, which host clean TDP fermions around the Fermi level (E_{F}) without coexisting with other quasiparticles, is critical to explore the intrinsic properties of this new fermion, it is still a big challenge and has not been achieved up to now. Here, we disclose an effective approach to search for ideal TDP semimetals via selective band crossing between antibonding s and bonding p orbitals along a line in the momentum space with C_{3v} symmetry. Applying this approach, we have successfully identified the NaCu_{3}Te_{2} family of compounds to be ideal TDP semimetals, where two, and only two, pairs of TDPs are located around the E_{F}. Moreover, we demonstrate a fundamental mechanism to modulate energy splitting between a pair of TDPs, and we illustrate the intrinsic features of TDP Fermi arcs in these ideal TDP semimetals.

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