Effect of interfacial defects on the electronic properties of MoS2 based lateral T–H heterophase junctions

The coexistence of semiconducting (2H) and metallic (1T) phases of MoS₂ monolayers has further pushed their strong potential for applications in the next generation of electronic devices based on two-dimensional lateral heterojunctions. Structural defects have considerable effects on the properties of these 2D devices. In particular, the interfaces of two phases are often imperfect and may contain numerous vacancies created by phase engineering techniques, e.g. under an electron beam. Here, the transport behaviors of the heterojunctions with the existence of point defects are explored by means of first-principles calculations and non-equilibrium Green''s function approach. While vacancies in semiconducting MoS₂ act as scattering centers, their presence at the interface improves the flow of the charge carriers. In the case of V_Mo, the current has been increased by two orders of magnitude in comparison to the perfect device. The enhancement of transmission was explained by changes in the electronic densities at the T–H interface, which open new transport channels for electron conduction.

Our systematic study shows significant improvement in transport properties of MoS₂-based lateral T–H heterophase junctions when interfacial defects are present.

Among the developing family of two-dimensional (2D) materials, transition metal dichalcogenides (TMDs) provide some of the most diverse electronic properties including acting as topological insulators, semiconductors, (semi)metals and superconductors.^1–3 Noticeably, such a difference in the electronic structure of TMDs correlates with their structural configurations, called phases.⁴ Monolayers of MoS₂ in the H-phase, with trigonal prismatic coordination of metal atoms, is a semiconducting material,^5,6 while T-phase with octahedral coordination shows metallic character. The H-phase monolayer is reported to be a promising material for field-effect transistors (FETs) with small-scale channel lengths and negligible current leakage.^5,6Recent experiments have already shown controlled transitions from one phase to another via external stimuli such as electron beam,⁷ ion intercalation,⁸ or laser irradiation.⁹ These phase-engineered 2D materials with minimum variations in atomic structure and uniform stoichiometry not only demonstrate rich physical behavior but also open up new avenues for the design of electronic devices. The fabrication of lateral metallic/semiconducting heterostructures has been suggested as a practical method to minimize the contact resistance at the interface between 2D semiconductors and metal electrodes. In particular, the formation of covalent bonds between the two phases can introduce paths for carriers to travel across the interfaces, thus, the Schottky barrier and contact resistance are reduced.^10–13 It has also been demonstrated that 1T-phase engineered electrodes in MoS₂ based electronic devices would generate ohmic contacts and, as a result, improve electrical characteristics.^12,14Apart from intrinsic defects, the local phase transitions induced by electron beam irradiation may give rise to the formation of point defects, in particular at the interface of the two phases.^15–22 Defects can also be intentionally introduced during the post-growth stage via ion bombardment, plasma treatment, vacuum annealing, or chemical etching.^15–22 Indeed, theoretical and experimental results showed that the presence of sulfur vacancies can decrease the energy difference between the H and T phases and eventually stabilize the 1T phase in MoS₂ monolayer.^23,24 The presence of point defects in semiconducting MoS₂ monolayers leads to the observation of the localized states in their electronic structure, which act as short-ranged scattering centers for charge carriers.^25–28 Hence, defects were found to deteriorate the mobility of the fabricated devices.^29–31 It was also shown that sulfur line vacancies in MoS₂ can behave like pseudo-ballistic wire for electron transport.³²So far, several theoretical studies have reported the transport properties of phase-engineered devices based on TMDs monolayers including MoS₂ based lateral junctions.^{11,12,33–35,35–37} In most of these studies, however, it is assumed that two phases have a perfect crystalline structure and connected via an atomically sharp and defect-free interface.Here, transport properties of devices based on MLs MoS₂, containing various point vacancies and antisites at the interface between metallic and semiconducting phases, are the subject of the present study. Our systematic investigations show significant improvements in the current, as molybdenum vacancy and vacancy complexes are created at the interfaces of two phases. These findings render defect engineering as an efficient route to further improve the performance of the devices based on the lateral heterojunctions formed from TMDs.     

One‐step and sequential SARSCOV‐2 polymerase chain reaction tests would not work every time

IntroductionRT‐PCR is widely used as a diagnostic test for the detection of SARS‐CoV‐2. In this study, we aim to describe the clinical utility of serial PCR testing in the final detection of COVID‐19.MethodWe collected multiple nasopharyngeal swab samples from patients who had negative RT‐PCR test on the first day after hospitalization. RT‐PCR tests were performed on the second day for all patients with initial negative result. For the patients with secondary negative results on day 2, tertiary RT‐PCR tests were performed on day 3 after hospitalization.ResultAmong 68 patients with initial negative test results, at the end of follow‐up, the mortality number was 20 (29.4%). About 33.8% of patients had subsequent positive PCR test results for the second time and 17.4% of the patients who performed third PCR test had positive result.ConclusionBased on this study, serial RT‐PCR testing is unlikely to yield additional information.     

A Combination of Constitutive Damage Model and Artificial Neural Networks to Characterize the Mechanical Properties of the Healthy and Atherosclerotic Human Coronary Arteries

It has been indicated that the content and structure of the elastin and collagen of the arterial wall can subject to a significant alteration due to the atherosclerosis. Consequently, a high tissue stiffness, stress, and even damage/rupture are triggered in the arterial wall. Although many studies so far have been conducted to quantify the mechanical properties of the coronary arteries, none of them consider the role of collagen damage of the healthy and atherosclerotic human coronary arterial walls. Recently, a fiber family‐based constitutive equation was proposed to capture the anisotropic mechanical response of the healthy and atherosclerotic human coronary arteries via both the histostructural and uniaxial data. In this study, experimental mechanical measurements along with histological data of the healthy and atherosclerotic arterial walls were employed to determine the constitutive damage parameters and remodeling of the collagen fibers. To do this, the preconditioned arterial tissues were excised from human cadavers within 5‐h postmortem, and the mean angle of their collagen fibers was precisely determined. Thereafter, a group of quasistatic axial and circumferential loadings were applied to the arterial walls, and the constrained nonlinear minimization method was employed to identify the arterial parameters according to the axial and circumferential extension data. The remodeling of the collagen fibers during the tensile test was also predicted via Artificial Neural Networks algorithm. Regardless of loading direction, the results presented a noteworthy load‐bearing capability and stiffness of the atherosclerotic arteries compared to the healthy ones (P < 0.005). Theoretical fiber angles were found to be consistent with the experimental histological data with less than 2 and 5° difference for the healthy and atherosclerotic arterial walls, respectively. The pseudoelastic damage model data were also compared with that of the experimental data, and interestingly, the arterial mechanical behavior for both the primary loading (up to the elastic region) and the discontinuous softening (up to the ultimate stress) was well addressed. The proposed model predicted well the mechanical response of the arterial tissue considering the damage of collagen fibers for both the healthy and atherosclerotic arterial walls.     

Post‐COVID‐19 maxillary osteonecrosis and floating maxillary teeth due to mucormycosis in two uncontrolled diabetic patients

Mucormycosis is a rare, invasive, quickly progressing fungal infection that generally affects patients who are immunocompromised. If left untreated, the disease is characterized by progressive necrosis and is often fatal. We present two cases of post‐COVID‐19 mucormycosis with a history of several years of uncontrolled diabetic mellitus.     

Structural characterization of the tobramycin-piperacillin reaction product formed at pH 6.0

Tobramycin is an aminoglycoside antibiotic that loses a significant amount of activity in the presence of Zosyn at pH 6. As part of our investigation into ways to improve the compatibility of tobramycin with Zosyn (which contains piperacillin and tazobactam in an 8:1 ratio buffered at pH 6 by sodium citrate) by lowering the pH, we identified the reaction product of tobramycin and piperacillin at pH 6.0 and the order of the pK(a) values of tobramycin. The structure of the main reaction product of tobramycin and piperacillin at pH 6.0 was determined by 2D NMR to be the product of 3″-NH(2) reacting with the β-lactam of piperacillin. The order of the pK(a) values of the nitrogens of tobramycin was determined by (1)H and (15)N NMR titrations to be 6'-NH(2)>2'-NH(2)>1-NH(2)≈3″-NH(2)>3-NH(2). At pH 4.0, the reaction between tobramycin and Zosyn was almost negligible for a period of up to 2?h. The pH can be lowered by adding an acid such as HCl or citric acid to Zosyn to make a pH 4.0 buffer.