Silicon nanocrystals (SiNCs) with bright bandgap photoluminescence (PL) are of current interest for a range of potential applications, from solar windows to biomedical contrast agents. Here, we use the liquid precursor cyclohexasilane (Si6H12) for the plasma synthesis of colloidal SiNCs with exemplary core emission. Through size separation executed in an oxygen-shielded environment, we achieve PL quantum yields (QYs) approaching 70% while exposing intrinsic constraints on efficient core emission from smaller SiNCs. Time-resolved PL spectra of these fractions in response to femtosecond pulsed excitation reveal a zero-phonon radiative channel that anticorrelates with QY, which we model using advanced computational methods applied to a 2 nm SiNC. Our results offer additional insight into the photophysical interplay of the nanocrystal surface, quasi-direct recombination, and efficient SiNC core PL.

In this work, we present an all-gas-phase approach for the synthesis of quantum-confined core/shell nanocrystals (NCs) as a promising alternative to traditional solution-based methods. Spherical quantum dots (QDs) are grown using a single-stage flow-through nonthermal plasma, yielding monodisperse NCs, with a concentric core/shell structure confirmed by electron microscopy. The in-flight negative charging of the NCs by plasma electrons keeps the NC cores separated during shell growth. The success of this gas-phase approach is demonstrated here through the study of Ge/Si core/shell QDs. We find that the epitaxial growth of a Si shell on the Ge QD core compressively strains the Ge lattice and affords the ability to manipulate the Ge band structure by modulation of the core and shell dimensions. This all-gas-phase approach to core/shell QD synthesis offers an effective method to produce high-quality heterostructured NCs with control over the core and shell dimensions.

Doped semiconductor nanocrystals represent an exciting new type of plasmonic material with optical resonances in the infrared. Unlike noble metal nanoparticles, the plasmon resonance can be tuned by altering the doping density. Recently, it has been shown that silicon nanocrystals can be doped using phosphorus and boron resulting in highly tunable infrared plasmon resonances. Due to the band structure of silicon, doping with phosphorus contributes light (transverse) and heavy (longitudinal) electrons, while boron contributes light and heavy holes and one would expect two distinct plasmon branches. Here we develop a classical hybridization theory and a full quantum mechanical TDLDA approach for two-component carrier plasmas and show that the interaction between the two plasmon branches results in strongly hybridized plasmon modes. The antibonding mode where the two components move in phase is bright and depends sensitively on the doping densities. The low energy bonding mode with opposite charge alignment can only be observed in the quantum regime when strong Coulomb screening is present. The theoretical results are in good agreement with the experimental data.

Background: The impact of the 2019 coronavirus pandemic on the mental health of millions worldwide has been well documented, but its impact on prevention and treatment of mental and behavioral health conditions is less clear. The COVID-19 pandemic also created numerous challenges and opportunities to implement health care policies and programs under conditions that are fundamentally different from what has been considered to be usual care. Methods: We conducted a qualitative study to determine the impact of the COVID-19 pandemic on implementation of evidence-based policy and practice by State Mental Health Authorities (SMHA) for prevention and treatment of mental health problems in children and adolescents. Semi-structured interviews were conducted with 29 SMHA representatives of 21 randomly selected states stratified by coronavirus positivity rate and rate of unmet services need. Data analysis with SMHA stakeholders used procedures embedded in the Rapid Assessment Procedure—Informed Community Ethnography methodology. Results: The need for services increased during the pandemic due primarily to family stress and separation from peers. States reporting an increase in demand had high coronavirus positivity and high unmet services need. The greatest impacts were reduced out-of-home services and increased use of telehealth. Barriers to telehealth services included limited access to internet and technology, family preference for face-to-face services, lack of privacy, difficulty using with young children and youth in need of substance use treatment, finding a Health Insurance Portability and Accountability Act (HIPAA)-compliant platform, training providers and clients, and reimbursement challenges. Policy changes to enable reimbursement, internet access, training, and provider licensing resulted in substantially fewer appointment cancellations or no-shows, greater family engagement, reduction in travel time, increased access for people living in remote locations, and increased provider communication and collaboration. States with high rates of coronavirus positivity and high rates of unmet need were most likely to continue use of telehealth post-pandemic. Despite these challenges, states reported successful implementation of policies designed to facilitate virtual services delivery with likely long-term changes in practice. Conclusions: Policy implementation during the pandemic provided important lessons for planning and preparedness for future public health emergencies. Successful policy implementation requires ongoing collaboration among policy makers and with providers.