Emergence of Two-Dimensional Massless Dirac Fermions, Chiral Pseudospins, and Berry's Phase in Potassium Doped Few-Layer Black Phosphorus

Thin flakes of black phosphorus (BP) are a two-dimensional (2D) semiconductor whose energy gap is predicted being sensitive to the number of layers and external perturbations. Very recently, it was found that a simple method of potassium (K) doping on the surface of BP closes its band gap completely, producing a Dirac semimetal state with a linear band dispersion in the armchair direction and a quadratic one in the zigzag direction. Here, based on first-principles density functional calculations, we predict that, beyond the critical K density of the gap closure, 2D massless Dirac Fermions (i.e., Dirac cones) emerge in K-doped few-layer BP, with linear band dispersions in all momentum directions, and the electronic states around Dirac points have chiral pseudospins and Berry's phase. These features are robust with respect to the spin-orbit interaction and may lead to graphene-like electronic transport properties with greater flexibility for potential device applications

Mechanism and Timescales of Reversible p‐Doping of Methylammonium Lead Triiodide by Oxygen

Understanding and controlling the energy level alignment at interfaces with metal halide perovskites (MHPs) is essential for realizing the full potential of these materials for use in optoelectronic devices. To date, however, the basic electronic properties of MHPs are still under debate. Particularly, reported Fermi level positions in the energy gap vary from indicating strong n- to strong p-type character for nominally identical materials, raising serious questions about intrinsic and extrinsic defects as dopants. ​In this work, photoemission experiments demonstrate that thin films of the prototypical methylammonium lead triiodide (MAPbI3) behave like an intrinsic semiconductor in the absence of oxygen. Oxygen is then shown to be able to reversibly diffuse into and out of the MAPbI3 bulk, requiring rather long saturation timescales of ≈1 h (in: ambient air) and over 10 h (out: ultrahigh vacuum), for few 100 nm thick films. Oxygen in the bulk leads to pronounced p-doping, positioning the Fermi level universally ≈0.55 eV above the valence band maximum. The key doping mechanism is suggested to be molecular oxygen substitution of iodine vacancies, supported by density functional theory calculations. This insight rationalizes previous and future electronic property studies of MHPs and calls for meticulous oxygen exposure protocols.Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659DFG http://dx.doi.org/10.13039/501100001659German Research Foundation http://dx.doi.org/10.13039/501100001659Peer Reviewe

Remodeling Pattern of Spinal Canal after Full Endoscopic Uniportal Lumbar Endoscopic Unilateral Laminotomy for Bilateral Decompression: One Year Repetitive MRI and Clinical Follow-Up Evaluation

Objective: There is limited literature on repetitive postoperative MRI and clinical evaluation after Uniportal Lumbar Endoscopic Unilateral Laminotomy for Bilateral Decompression. Methods: Clinical visual analog scale, Oswestry Disability Index, McNab's criteria evaluation and MRI evaluation of the axial cut spinal canal area of the upper end plate, mid disc and lower end plate were performed for patients who underwent single-level Uniportal Lumbar Endoscopic Unilateral Laminotomy for Bilateral Decompression. From the evaluation of the axial cut MRI, four types of patterns of remodeling were identified: type A: continuous expanded spinal canal, type B: restenosis with delayed expansion, type C: progressive expansion and type D: restenosis. Result: A total of 126 patients with single-level Uniportal Lumbar Endoscopic Unilateral Laminotomy for Bilateral Decompression were recruited with a minimum follow-up of 26 months. Thirty-six type A, fifty type B, thirty type C and ten type D patterns of spinal canal remodeling were observed. All four types of patterns of remodeling had statistically significant improvement in VAS at final follow-up compared to the preoperative state with type A (5.59 +/- 1.58), B (5.58 +/- 1.71), C (5.58 +/- 1.71) and D (5.27 +/- 1.68), p < 0.05. ODI was significantly improved at final follow-up with type A (49.19 +/- 10.51), B (50.00 +/- 11.29), C (45.60 +/- 10.58) and D (45.60 +/- 10.58), p < 0.05. A significant MRI axial cut increment of the spinal canal area was found at the upper endplate at postoperative day one and one year with type A (39.16 +/- 22.73; 28.00 +/- 42.57) mm(2), B (47.42 +/- 18.77; 42.38 +/- 19.29) mm(2), C (51.45 +/- 18.16; 49.49 +/- 18.41) mm(2) and D (49.10 +/- 23.05; 38.18 +/- 18.94) mm(2), respectively, p < 0.05. Similar significant increment was found at the mid-disc at postoperative day one, 6 months and one year with type A (55.16 +/- 27.51; 37.23 +/- 25.88; 44.86 +/- 25.73) mm(2), B (72.83 +/- 23.87; 49.79 +/- 21.93; 62.94 +/- 24.43) mm(2), C (66.85 +/- 34.48; 54.92 +/- 30.70; 64.33 +/- 31.82) mm(2) and D (71.65 +/- 16.87; 41.55 +/- 12.92; 49.83 +/- 13.31) mm(2) and the lower endplate at postoperative day one and one year with type A (49.89 +/- 34.50; 41.04 +/- 28.56) mm(2), B (63.63 +/- 23.70; 54.72 +/- 24.29) mm(2), C (58.50 +/- 24.27; 55.32 +/- 22.49) mm(2) and D (81.43 +/- 16.81; 58.40 +/- 18.05) mm(2) at postoperative day one and one year, respectively, p < 0.05. Conclusions: After full endoscopic lumbar decompression, despite achieving sufficient decompression immediately postoperatively, varying severity of asymptomatic restenosis was found in postoperative six months MRI without clinical significance. Further remodeling with a varying degree of increment of the spinal canal area occurs at postoperative one year with overall good clinical outcomes

Internal marketing and salespeople's out-of-role behaviour: The mediating role of job satisfaction

This study examines the effect of internal marketing (IM) on salespeople's in-role and out-of-role behaviours and how these behaviours affect customer satisfaction and sales performance. It also tests the mediating role of job satisfaction as an emotional response and adopts survey design. Data from 322 companies in Daegu and Gyeongbuk Province in the Republic of Korea are collected and used to test the conceptual model using structural equation modelling. The findings reveal that organisations' IM strategy for motivating employees positively affects salespeople's job satisfaction and out-of-role behaviour. The results also indicate that the salespeople's out-of-role behaviour enhances customer satisfaction and sales performance. This study suggests that organisations should provide guidelines and create an organizational culture that supports various IM activities to encourage employees' spontaneous behaviours. It is among the very few, if not the first, studies that demonstrate how companies' IM strategies could be leveraged to enhance job satisfaction and in-role and out-of-role behaviours in employees and how these behaviours could enhance customer satisfaction and sales performance

Impact of light illumination on the surface structure of two-dimensional Ruddlesden–Popper perovskite in the fabrication process

Two-dimensional Ruddlesden–Popper (2DRP) perovskites are promising owing to their excellent environmental stability and competitive efficiency. During the fabrication process, 2DRP perovskites were often unintentionally exposed to light in the laboratory. However, the influence of light illumination on the surface structure of 2DRP during fabrication is unclear. Herein, the photodegradation of 2DRP perovskite (phenethylammonium lead iodide, PEA _2 PbI _4 ) is comprehensively investigated using x-ray photoelectron spectroscopy, scanning electron microscopy, atomic force microscopy, and photoluminescence spectroscopy. We observed that only high-energy light, including that from a blue laser, air mass 1.5G, and notably, the daily used fluorescent lamp (FL) in the laboratory, significantly degraded PEA _2 PbI _4 . However, the red laser and ultraviolet-filtered FL, which had low energies, did not cause photodegradation. From this systematic study, we can explain the discrepancies in the surface morphologies previously studied. For instance, randomly oriented nanorod or rough surface of PEA _2 PbI _4 mostly stems from photodegradation. We observed that photodegradation occurred more strongly when the films were illuminated during annealing than when they were illuminated after all fabrication processes were completed. We suggest that this difference stems from the completeness of the structure and the photodegraded PbI _2 passivation effect. Our study provides two key guidelines for the fabrication of PEA _2 PbI _4 films. The daily-use FL in the laboratory must be avoided for high-quality samples, and dark conditions are highly recommended, at least during the annealing process

8.9% Single-Stack Inverted Polymer Solar Cells with Electron-Rich Polymer Nanolayer-Modified Inorganic Electron-Collecting Buffer Layers

Enhanced power conversion efficiency (PCE) is reported in inverted polymer solar cells when an electron-rich polymer nanolayer (poly(ethyleneimine) (PEI)) is placed on the surface of an electron-collecting buffer layer (ZnO). The active layer is made with bulk heterojunction films of poly[[4,8-bis[(2- ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl] [3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM). The thickness of the PEI nanolayer is controlled to be 2 nm to minimize its insulating effect, which is confirmed by X-ray photoelectron spectroscopy and optical absorption measurements. The Kelvin probe and ultraviolet photoelectron spectroscopy measurements demonstrate that the enhanced PCE by introducing the PEI nanolayer is attributed to the lowered conduction band energy of the ZnO layer via the formation of an interfacial dipole layer at the interfaces between the ZnO layer and the PEI nanolayer. The PEI nanolayer also improves the surface roughness of the ZnO layer so that the device series resistance can be noticeably decreased. As a result, all solar cell parameters including short circuit current density, open circuit voltage, fill factor, and shunt resistance are improved, leading to the PCE increase up to ≈8.9%, which is close to the best PCE reported using conjugated polymer electrolyte films. 8.9% power conversion efficiency is achieved using inverted-type polymer:fullerene solar cells. 2 nm-thick electron-rich poly(ethyleneimine) nanolayers are placed on the ZnO electron-collecting buffer layers and the increased built-in electric field caused by the lowered conduction band energy of ZnO layers enhances performance. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.1

Impacts of Molecular Orientation on the Hole Injection Barrier Reduction: CuPc/HAT-CN/Graphene

The molecular orientation affected by the interaction between a substrate and deposited molecules plays an important role in device performance. It is known that the molecular orientation influences not only the charge transport property but also its electronic structure. Therefore, the combined study of morphology and electronic structure is of high importance for device application. As a transparent electrode, graphene has many promising advantages. However, graphene itself does not have an adequate work function for either an anode or a cathode, and thus the insertion of a charge injection layer is necessary for it to be used as an electrode. In this study, the hole injection barrier (HIB) reduction was investigated at the interface of copper phthalocyanine (CuPc)/graphene with the insertion of a hexa­aza­triphenylene hexa­carbonitrile (HAT-CN) layer between them. The insertion of the HAT-CN layer roughens the originally flat graphene surface and it weakens the π-interaction between CuPc and of graphene. This induces face-on and edge-on mixed orientations of CuPc, while CuPc on bare graphene shows merely a face-on orientation. As a result, the HIB is reduced by the contribution of edge-on CuPc having lower ionization energy (0.37 eV) along with the high work function of the HAT-CN layer (0.26 eV)