Exciton states in monolayer MoSe2 and MoTe2 probed by upconversion spectroscopy

Transitions metal dichalcogenides (TMDs) are direct semiconductors in the atomic monolayer (ML) limit with fascinating optical and spin-valley properties. The strong optical absorption of up to 20 % for a single ML is governed by excitons, electron-hole pairs bound by Coulomb attraction. Excited exciton states in MoSe$_2$ and MoTe$_2$ monolayers have so far been elusive due to their low oscillator strength and strong inhomogeneous broadening. Here we show that encapsulation in hexagonal boron nitride results in emission line width of the A:1$s$ exciton below 1.5 meV and 3 meV in our MoSe$_2$ and MoTe$_2$ monolayer samples, respectively. This allows us to investigate the excited exciton states by photoluminescence upconversion spectroscopy for both monolayer materials. The excitation laser is tuned into resonance with the A:1$s$ transition and we observe emission of excited exciton states up to 200 meV above the laser energy. We demonstrate bias control of the efficiency of this non-linear optical process. At the origin of upconversion our model calculations suggest an exciton-exciton (Auger) scattering mechanism specific to TMD MLs involving an excited conduction band thus generating high energy excitons with small wave-vectors. The optical transitions are further investigated by white light reflectivity, photoluminescence excitation and resonant Raman scattering confirming their origin as excited excitonic states in monolayer thin semiconductors.Comment: 14 pages, 7 figures, main text and appendi

Revealing exciton masses and dielectric properties of monolayer semiconductors with high magnetic fields

In semiconductor physics, many essential optoelectronic material parameters can be experimentally revealed via optical spectroscopy in sufficiently large magnetic fields. For monolayer transition-metal dichalcogenide semiconductors, this field scale is substantial --tens of teslas or more-- due to heavy carrier masses and huge exciton binding energies. Here we report absorption spectroscopy of monolayer MoS$_2$, MoSe$_2$, MoTe$_2$, and WS$_2$ in very high magnetic fields to 91~T. We follow the diamagnetic shifts and valley Zeeman splittings of not only the exciton's $1s$ ground state but also its excited $2s$, $3s$, ..., $ns$ Rydberg states. This provides a direct experimental measure of the effective (reduced) exciton masses and dielectric properties. Exciton binding energies, exciton radii, and free-particle bandgaps are also determined. The measured exciton masses are heavier than theoretically predicted, especially for Mo-based monolayers. These results provide essential and quantitative parameters for the rational design of opto-electronic van der Waals heterostructures incorporating 2D semiconductors.Comment: updated; now also including data on MoTe2. Accepted & in press, Nature Commu

Impact of COVID-19 pandemic on safe abortion and family planning services at a tertiary care women’s hospital in Nepal

Background: The COVID-19pandemic emerged as a major public health crisis, which has affected all dimensions of the health care system. Sexual and reproductive health services were severely affected, leading to a decrease in access and service utilization, affecting the overall health of women.Methods: A two-year comparative study, before and during the COVID-19 pandemic, on safe abortion services and family planning, was conducted at Paropakar maternity and women's hospital to assess the impact of COVID-19 on service utilization.Results: Safe abortion services were decreased by 34.4%, and family planning services by 39%, in 2020 as compared to the previous year. Uptake of long-acting reversible contraceptives and permanent methods was most affected. Utilization of services was affected markedly during lockdown, and showed a persistent decline, even after the lockdown was lifted.Conclusions: The COVID-19 pandemic has seriously affected safe abortion and family planning services in Nepal due to lockdown, travel restriction, home isolation, resource reallocation, health facilities serving only emergencies and confusing messages about COVID-19 control. The decline in these services will create additional demand and pressure on the health care system, resulting from unplanned pregnancies and unsafe abortions. Health care staffs should be reoriented about the essential nature of safe abortion and family planning services during emergencies, and the implications of service disruption, on society and the country. Pragmatic and gender sensitive changes to national policies should be made, to ensure that women's health is safeguarded, and safe abortion and family planning included as essential health care services during emergencies.

A single dominant gene controlling resistance to soil zinc deficiency in common bean

Cultivated soils often are either deficient or possess toxic concentrations of one or more mineral elements that adversely affect emergence, growth, maturity, production potential, and/or nutritional quality of common bean (Phaseolus vulgaris L.). Our objective was to study the inheritance of resistance to soil Zn deficiency. The resistant `Matterhorn' was crossed with the susceptible 'T-39'. The F1 was backcrossed to Matterhorn (BC1) and T-39 (BC2), and advanced to the F2. The two parents, F1, F2, BC1 , and BC2 were evaluated in a Zn deficient field trial at Kimberly, Idaho in 2001. Plants were classified as tall-healthy or stunted with chlorotic leaves. Leaves were sampled from the two types of plants at flowering and analyzed for Zn concentration. The tall plants had an average leaf Zn concentration of 22.5 mg kg- 1 . In contrast, stunted plants had a Zn concentration of 15.0 mg kg- 1. All F1 plants were tall resembling Matterhorn, except that unlike Matterhorn (white flowers and seeds) they had purple flowers and black shiny seeds. Thus, the resistance to Zn deficiency was dominant. A segregation of 45 resistant (R) to 20 susceptible (S) plants was observed in the F2, giving a good fit to 3 R:1 S (x2 = 1.1538, P = 0.28). All plants in BC1 were resistant. In BC2, 142 It and 139 S plants were observed, giving a ratio of 1 R to 1 S (x2 = 0.032, P = 0.86). This supports a single dominant gene controlling soil Zn deficiency resistance. The symbol Znd is proposed for the dominant allele controlling resistance to soil Zn deficiency, and znd for its susceptible counterpart

Corrosion resistant behaviour of PANI–metal bilayer coatings

The present work discusses on the corrosion resistant behaviour of polymer metal bilayer coatings, viz. polyaniline (PANI), polyaniline–nickel (PANI–Ni), nickel–polyaniline (Ni–PANI), polyaniline–zinc (PANI–Zn) and zinc–polyaniline (Zn–PANI). The coatings were synthesized by means of cyclic voltametric method. The coatings thus obtained were uniform in nature and highly adherent to the mild steel substrate. The effectiveness of the coatings in preventing corrosion was tested by electrochemical impedance studies (EIS) using Nyquist and Bode plots and potentiodynamic polarization studies as well. Among the various coatings synthesized, the PANI–Zn coating was found to offer the maximum protection, followed by PANI–Ni coatings. Metal–PANI coatings were found to offer the least resistance to corrosion. The coatings thus obtained were characterized by scanning electron microscopic (SEM) analysis and the results are discussed