Two-dimensional Dirac fermions on oxidized black phosphorus.

We explore the oxidation of a single layer of black phosphorus using ab initio density functional theory calculations. We search for the equilibrium structures of phosphorene oxides, POx with various oxygen concentrations x (0 ≤ x ≤ 1). By evaluating the formation energies with diverse configurations and their vibrational properties for each of various x values, we identify a series of stable oxidized structures with x and confirm that the oxidation occurs naturally. We also find that oxidation makes some modes from the P-O bonds and P-P bonds IR-active implying that the infrared spectra can be used to determine the degree of oxidation of phosphorene. Our electronic structure calculations reveal that the fully oxidized phosphorene (PO) has a direct band gap of 0.83 eV similar to the pristine phosphorene. Intriguingly, the PO possesses two nonsymmorphic symmetries with the inversion symmetry broken, guaranteeing symmetry-protected band structures including the band degeneracy and four-fold degenerate Dirac points. Our results provide an important guide in the search for the rare example of a Dirac semimetal with a higher level of degeneracy, giving significant insight into the relations between the symmetry of the lattice and band topology of electrons.

Enhanced carrier transport along edges of graphene devices.

The relation between macroscopic charge transport properties and microscopic carrier distribution is one of the central issues in the physics and future applications of graphene devices (GDs). We find strong conductance enhancement at the edges of GDs using scanning gate microscopy. This result is explained by our theoretical model of the opening of an additional conduction channel localized at the edges by depleting accumulated charge by the tip.

Large-scale assembly of 'type-switchable' field effect transistors based on carbon nanotubes and nanoparticles.

We report the large-scale assembly of type-switchable field effect transistors (FETs) based on carbon nanotubes (CNTs) and nanoparticles (NPs). In this device, the charges stored in NPs adjacent to ambipolar CNT channels were adjusted to control the carrier type and density in the channels. We demonstrated the real-time reconfiguration of individual FET types and logic circuit functionality. Theoretical simulation of a model system was provided to explain this doping effect. This work takes advantage of the ambipolar properties of CNTs and opens up the possibility to build new types of devices with reconfigurable functionalities.

Decelerated Hot Carrier Cooling in Graphene via Nondissipative Carrier Injection from MoS2.

One key to improve the performance of advanced optoelectronic devices and energy harvesting in graphene is to understand the predominant carrier scattering via optical phonons. Nevertheless, low light absorbance in graphene yields a limited photoexcited carrier density, hampering the hot carrier effect, which is strongly correlated to the hot optical phonon bottleneck effect as the energy-loss channel. Here, by integrating graphene with monolayer MoS2 possessing stronger light absorbance, we demonstrate an efficient interfacial hot carrier transfer between graphene and MoS2 in their heterostructure with a prolonged relaxation time using broadband transient differential transmittance spectroscopy. We observe that the carrier relaxation time of graphene in the heterostructure is 4 times slower than that of bare graphene. This is explained by nondissipative interlayer transfer from MoS2 to graphene, which is attributed to the enhanced hot optical phonon bottleneck effect of graphene in the heterostructure by an increased photoexcited carrier population. A significant reduction of both amplitude and relaxation time in A- and B-excitons is another evidence of the interlayer transfer from MoS2 to graphene. The nondissipative interlayer charge transfer from MoS2 to graphene is confirmed by density functional calculations. This provides a different platform to further study the photoinduced hot carrier effect in graphene heterostructures for photothermoelectric detectors or hot carrier solar cells.

Strain-shear coupling in bilayer MoS2.

Layered materials such as graphite and transition metal dichalcogenides have extremely anisotropic mechanical properties owing to orders of magnitude difference between in-plane and out-of-plane interatomic interaction strengths. Although effects of mechanical perturbations on either intralayer or interlayer interactions have been extensively investigated, mutual correlations between them have rarely been addressed. Here, we show that layered materials have an inevitable coupling between in-plane uniaxial strain and interlayer shear. Because of this, the uniaxial in-plane strain induces an anomalous splitting of the degenerate interlayer shear phonon modes such that the split shear mode along the tensile strain is not softened but hardened contrary to the case of intralayer phonon modes. We confirm the effect by measuring Raman shifts of shear modes of bilayer MoS2 under strain. Moreover, by analyzing the splitting, we obtain an unexplored off-diagonal elastic constant, demonstrating that Raman spectroscopy can determine almost all mechanical constants of layered materials.

Correlation between micrometer-scale ripple alignment and atomic-scale crystallographic orientation of monolayer graphene.

Deformation normal to the surface is intrinsic in two-dimensional materials due to phononic thermal fluctuations at finite temperatures. Graphene's negative thermal expansion coefficient is generally explained by such an intrinsic property. Recently, friction measurements on graphene exfoliated on a silicon oxide surface revealed an anomalous anisotropy whose origin was believed to be the formation of ripple domains. Here, we uncover the atomistic origin of the observed friction domains using a cantilever torsion microscopy in conjunction with angle-resolved photoemission spectroscopy. We experimentally demonstrate that ripples on graphene are formed along the zigzag direction of the hexagonal lattice. The formation of zigzag directional ripple is consistent with our theoretical model that takes account of the atomic-scale bending stiffness of carbon-carbon bonds and the interaction of graphene with the substrate. The correlation between micrometer-scale ripple alignment and atomic-scale arrangement of exfoliated monolayer graphene is first discovered and suggests a practical tool for measuring lattice orientation of graphene.

Second-look arthroscopic assessment of arthroscopic single-bundle posterior cruciate ligament reconstruction: comparison of mixed graft versus achilles tendon allograft.

BACKGROUND: Although controversy continues over the choice of graft tissue, including autografts, allografts, and synthetic ligaments, for posterior cruciate ligament (PCL) reconstruction, the use of a mixed graft consisting of a hamstring (semitendinosus and gracilis) autograft plus tibialis anterior allograft tendon has not been studied in detail. HYPOTHESIS: Outcomes of PCL reconstructions performed with a mixed graft would be superior to those using solely an Achilles tendon allograft in terms of functional knee scores, posterior stability, and the graft appearance. STUDY DESIGN: Cohort study; Level of evidence, 2. METHODS: Fifty-eight patients who underwent isolated single-bundle PCL reconstruction using an arthroscopic trans-septal portal with remnant preservation technique were evaluated. They were divided into group A (mixed tendon; n = 30) and group B (Achilles tendon; n = 28). Knee function was evaluated using the Lysholm knee score, Tegner activity score, and the International Knee Documentation Committee (IKDC) grading scale. Anteroposterior stability was measured using the Telos stress view. Twenty patients (66.7%) from group A and 21 patients (75.0%) from group B underwent hardware removal and a second-look arthroscopic examination. RESULTS: The Lysholm knee scores in groups A and B increased from a respective average of 43 and 50 preoperatively to 90 and 88 at follow-up. The IKDC grade and Tegner activity scores were also significantly improved in both groups. Stability was improved in both groups, with an average posterior laxity of 3.0 mm (group A) and 3.3 mm (group B) at follow-up (P > .05). However, there were 4 intraoperative complications in group B: 2 bone fractures and 2 graft pullouts during precyclic tensioning. Second-look arthroscopy revealed a partial tear in 8 cases (40%) from group A and 15 cases (71.4%) from group B (P = .03). All of the partial tears were located in the femoral aperture area. Complete synovial coverage was demonstrated in 10 patients (50%) from group A and 5 patients (23.8%) from group B (P = .04). However, clinical outcomes and stability were not affected by the arthroscopic graft appearance. CONCLUSION: Satisfactory results were obtained for groups A and B in patients who underwent reconstruction for isolated PCL injury. However, 4 intraoperative complications (14.3%) were encountered with use of the Achilles tendon allograft (group B), with a relative higher rate of partial tear and less synovialization in the femoral aperture area.