Limits of funneling efficiency in non-uniformly strained 2D semiconductors

Photoexcited electron-hole pairs (excitons) in transition metal dichalcogenides (TMDC) experience an effective force when these materials are non-uniformly strained. In the case of strain produced by a sharp tip pressing at the center of a suspended TMDC membrane, the excitons are transported to the point of the highest strain at the center of the membrane. This effect, exciton funneling, can be used to increase photoconversion efficiency in TMDC, to explore exciton transport and to study correlated states of excitons arising at their high densities. Here, we analyze the limits of funneling efficiency in realistic device geometries. The funneling efficiency in realistic monolayer TMDCs is found to be low, $\lt$5% both at room and low temperatures. This results from dominant diffusion at room temperature and short exciton lifetimes at low temperatures. On the other hand, in TMDC heterostructures with long exciton lifetimes the funneling efficiency reaches ~50% at room temperature, as the exciton density reaches thermal equilibrium in the funnel. Finally, we show that Auger recombination limits funneling efficiency for intense illumination sources

Intrinsic and Extrinsic Defect-Related Excitons in TMDCs

We investigate the excitonic peak associated with defects and disorder in low-temperature photoluminescence of monolayer transition metal dichalcogenides (TMDCs). To uncover the intrinsic origin of defect-related (D) excitons, we study their dependence on gate voltage, excitation power, and temperature in a prototypical TMDC monolayer MoS2. Our results suggest that D excitons are neutral excitons bound to ionized donor levels, likely related to sulfur vacancies, with a density of 7 × 1011 cm–2. To study the extrinsic contribution to D excitons, we controllably deposit oxygen molecules in situ onto the surface of MoS2 kept at cryogenic temperature. We find that, in addition to trivial p-doping of 3 × 1012 cm–2, oxygen affects the D excitons, likely by functionalizing the defect sites. Combined, our results uncover the origin of D excitons, suggest an approach to track the functionalization of TMDCs, to benchmark device quality, and pave the way toward exciton engineering in hybrid organic–inorganic TMDC devices

Non‐uniform strain engineering of 2D materials

2D materials are elastic substances that can sustain high strain. While the response of these materials to spatially uniform strain is well studied, the effects of spatially non-uniform strain are understood much less. In this review, we examine the response of two different 2D materials, transition metal dichalcogenides and graphene, under non-uniform strain. First, we analyze pseudo-magnetic fields formed in graphene subjected to highly localized non-uniform strain. Second, we discuss the effect of non-uniform strain on excitons in non-uniformly strained TMDC. We show that while transport or “funneling” of excitons is relatively inefficient, a different process, a strain-related conversion of excitons to trions is dominant. Finally, we discuss the effects of uniform and non-uniform strain in a graphene-based phononic crystal. We find that uniform strain can be used to broadly tune the frequency of the phononic bandgap by more than 350 % and non-uniform strain smears that bandgap

Cauliflower ear - a minimally invasive treatment method in a wrestling athlete: a case report

Acute auricular hematoma can be caused by direct blunt trauma or other injury to the external ear. It is typically seen in those who practice full contact sports such as boxing, wrestling, and rugby. “Cauliflower ear” deformity, fibrocartilage formation during scarring, is a common complication of auricular hematomas. Therefore, acute drainage of the hematoma and postprocedural techniques for preventing recurrence are necessary for preventing the deformity. There are many techniques although no superior method of treatment has been found. In this case report, we describe a novel method using needle aspiration followed by the application of a magnet and an adapted disc to the affected area of the auricular. This minimally invasive, simple, and accessible method could potentially facilitate the treatment of cauliflower ear among full contact sports athletes

The development and impact of heterotopic ossification in burns: A review of four decades of research

Introduction: Heterotopic ossification (HO) is the formation of lamellar bone within connective and other tissue where bone should not form and is a rare complication after burn injury. However, it leads to severe pain and distress, marked reduction in joint range of motion (ROM), impaired function and increased hospital length of stay. The pathophysiology, incidence and risk factors of HO remain poorly understood in burns and other traumas and the management, controversial. The aim of this comprehensive review, therefore, was to synthesise the available evidence on the development and treatment of HO after acute burn injury. Methods: The review was based on a systematic search of five electronic databases PubMed, EMBASE, CINAHL, LILACS and Scopus. Results: Synthesis and analysis of the data highlighted that, despite the passage of time, little translatable evidence is available to guide any prevention, screening, diagnostic or pharmacological or physical management protocols. Discussion: Causes of HO remain confounded, therefore prevention is difficult. Although spontaneous resolution is possible, surgical resection remains the recommended treatment when ROM and activities of daily living are severely affected. Conclusion: The findings from this review indicate that multicentre data pooling is needed to understand the optimum pathway to prevention, identification and treatment of HO in acute burn patients

Dynamics and efficient conversion of excitons to trions in non-uniformly strained monolayer WS2

In recent years, there has been ongoing effort in achieving efficient transport of excitons in monolayer transition metal dichalcogenides subjected to highly non-uniform strain. Here we investigate the transport of excitons and trions in monolayer semiconductor WS2 subjected to controlled non-uniform mechanical strain. An atomic force microscope (AFM)-based setup is applied to actively control and tune the strain profiles by indenting the monolayer with an AFM tip. Optical spectroscopy is used to reveal the dynamics of the excited carriers. The non-uniform strain configuration locally changes the valence and conduction bands of WS2, giving rise to effective forces attracting excitons and trions towards the point of maximum strain underneath the AFM tip. We observe large changes in the photoluminescence spectra of WS2 under strain, which we interpret using a drift–diffusion model. We show that the transport of neutral excitons, a process that was previously thought to be efficient in non-uniformly strained two-dimensional semiconductors and termed as funnelling, is negligible at room temperature, in contrast to previous observations. Conversely, we discover that redistribution of free carriers under non-uniform strain profiles leads to highly efficient conversion of excitons to trions. Conversion efficiency reaches up to about 100% even without electrical gating. Our results explain inconsistencies in previous experiments and pave the way towards new types of optoelectronic devices

Neutral and charged excitons interplay in non-uniformly strain-engineered WS2

We investigate the response of excitons in two-dimensional semiconductors to nonuniformity of mechanical strain. In our approach to non-uniform strain-engineering, a WS2 monolayer is suspended over a triangular hole. Large (>2%), strongly non-uniform (>0.28% µm–1), and in-situ tunable strain is induced in WS2 by pressurizing it with inert gas. We observe a pronounced shift of the spectral weight from neutral to charged excitons at the center of the membrane, in addition to well-known strain-dependent bandgap modification. We show that the former phenomenon is a signature of a new effect unique for non-uniform strain: funneling of free carriers towards the region of high strain followed by neutral to charged exciton conversion. Our result establishes non-uniform strain engineering as a novel and useful experimental 'knob' for tuning optoelectronic properties of 2D semiconductors

Omapatrilat, an Angiotensin-Converting Enzyme and Neutral Endopeptidase Inhibitor, Attenuates Early Atherosclerosis in Diabetic and in Nondiabetic Low-Density Lipoprotein Receptor–Deficient Mice

Omapatrilat inhibits both angiotensin-converting enzyme (ACE) and neutral endopeptidase (NEP). ACE inhibitors have been shown to inhibit atherosclerosis in apoE-deficient mice and in several other animal models but failed in low-density lipoprotein (LDL) receptor– deficient mice despite effective inhibition of the reninangiotensin- aldosterone system. The aim of the present study was to examine the effect of omapatrilat on atherogenesis in diabetic and nondiabetic LDL receptor–deficient mice. LDL receptor–deficient male mice were randomly divided into 4 groups (n = 11 each). Diabetes was induced in 2 groups by low-dose STZ, the other 2 groups served as nondiabetic controls. Omapatrilat (70 mg/kg/day) was administered to one of the diabetic and to one of the nondiabetic groups. The diabetic and the nondiabetic mice were sacrificed after 3 and 5 weeks, respectively. The aortae were examined and the atherosclerotic plaque area was measured. The atherosclerotic plaque area was significantly smaller in the omapatrilat-treated mice, both diabetic and nondiabetic, as compared to nontreated controls. The mean plaque area of omapatrilattreated nondiabetic mice was 9357 ± 7293 μm2, versus 71977 ± 34610 μm2 in the nontreated mice (P = .002). In the diabetic animals, the plaque area was 8887 ± 5386 μm2 and 23220 ± 10400 μm2, respectively for treated and nontreated mice (P = .001). Plasma lipids were increased by omapatrilat: Meanplasma cholesterol in treated mice, diabetic and nondiabetic combined, was 39.31 ± 6.00 mmol/L, versus 33.12 ± 7.64 mmol/L in the nontreated animals (P = .008). The corresponding combined mean values of triglycerides were 4.83 ± 1.93 versus 3.00 ± 1.26 mmol/L (P = .02). Omapatrilat treatment did not affect weight or plasma glucose levels. Treatment with omapatrilat inhibits atherogenesis in diabetic as well as nondiabetic LDL receptor–deficient mice despite an increase in plasma lipids, suggesting a direct effect on the arterial wall