73 research outputs found
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
Two-Dimensional Dirac Fermions Protected by Space-Time Inversion Symmetry in Black Phosphorus
We report the realization of novel symmetry-protected Dirac fermions in a
surface-doped two-dimensional (2D) semiconductor, black phosphorus. The widely
tunable band gap of black phosphorus by the surface Stark effect is employed to
achieve a surprisingly large band inversion up to ~0.6 eV. High-resolution
angle-resolved photoemission spectra directly reveal the pair creation of Dirac
points and their moving along the axis of the glide-mirror symmetry. Unlike
graphene, the Dirac point of black phosphorus is stable, as protected by
spacetime inversion symmetry, even in the presence of spin-orbit coupling. Our
results establish black phosphorus in the inverted regime as a simple model
system of 2D symmetry-protected (topological) Dirac semimetals, offering an
unprecedented opportunity for the discovery of 2D Weyl semimetals
Polymeric tandem organic light-emitting diodes using a self-organized interfacial layer
The authors have demonstrated efficient polymeric tandem organic light-emitting diodes (OLEDs) with a self-organized interfacial layer, which was formed by differences in chemical surface energy. Hydrophilic poly(styrene sulfonate)-doped poly(3,4-ethylene dioxythiophene) (PEDOT:PSS) was spin coated onto the hydrophobic poly(9,9-dyoctilfluorene) (PFO) surface and a PEDOT:PSS bubble or dome was built as an interfacial layer. The barrier heights of PEDOT:PSS and PFO in the two-unit tandem OLED induced a charge accumulation at the interface in the heterojunction and thereby created exciton recombination at a much higher level than in the one-unit reference. This effect was confirmed in both the hole only and the electron only devices. (c) 2008 American Institute of Physicsopen8
Determination of Malignant and Invasive Predictors in Branch Duct Type Intraductal Papillary Mucinous Neoplasms of the Pancreas: A Suggested Scoring Formula
Prediction of malignancy or invasiveness of branch duct type intraductal papillary mucinous neoplasm (Br-IPMN) is difficult, and proper treatment strategy has not been well established. The authors investigated the characteristics of Br-IPMN and explored its malignancy or invasiveness predicting factors to suggest a scoring formula for predicting pathologic results. From 1994 to 2008, 237 patients who were diagnosed as Br-IPMN at 11 tertiary referral centers in Korea were retrospectively reviewed. The patients' mean age was 63.1 ± 9.2 yr. One hundred ninty-eight (83.5%) patients had nonmalignant IPMN (81 adenoma, 117 borderline atypia), and 39 (16.5%) had malignant IPMN (13 carcinoma in situ, 26 invasive carcinoma). Cyst size and mural nodule were malignancy determining factors by multivariate analysis. Elevated CEA, cyst size and mural nodule were factors determining invasiveness by multivariate analysis. Using the regression coefficient for significant predictors on multivariate analysis, we constructed a malignancy-predicting scoring formula: 22.4 (mural nodule [0 or 1]) + 0.5 (cyst size [mm]). In invasive IPMN, the formula was expressed as invasiveness-predicting score = 36.6 (mural nodule [0 or 1]) + 32.2 (elevated serum CEA [0 or 1]) + 0.6 (cyst size [mm]). Here we present a scoring formula for prediction of malignancy or invasiveness of Br-IPMN which can be used to determine a proper treatment strategy
Photocurrent increase in amorphous Si solar cells by increased reflectivity of LiF/Al electrodes
Thin layers of lithium fluoride (LiF), together with aluminum, can be used as rear electrodes in high-efficiency amorphous silicon (a-Si:H) solar cells on rough substrates, as recently demonstrated by Fang et al. We employ numerical modeling to evaluate the optical losses and charge generation profiles in these thin film solar cells. We find that the increase in rear electrode reflectivity by inserting LiF, leading to increased photocurrent, is the dominant factor in device performance improvement, accounting for 7% gain in photocurrent. The simulations are in good agreement with measurements of pin a-Si:H solar cells with varied rear electrodes
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