Investigating the unique occurrence of polytypism and the role of available shell precursors in the growth of giant shell quantum dots

Researchers have epitaxially grown thick inorganic shells on the surface of quantum dots (QDs) cores to improve quantum yields, increase photostability and suppress fluorescence intermittency (blinking) in ‘giant quantum dots’ (gQDs). These unique properties make gQDs excellent candidates for applications in lasers, single molecular probes and solid state LEDs. Although a growing wealth of knowledge exists for the photophysical properties of the gQDs, limited research has been directed towards understanding the synthetic intricacies and crystal growth. In this dissertation work I present a detailed study of the growth of CdSe/CdZnS multishell gQDs and focus on crystallographic and morphological evolution. I studied the effect of core crystal structure and shell growth was performed on crystallographically disparate (W, wurtzite and ZB, zinc blende) CdSe cores under identical synthetic conditions. My work revealed that while shell growth transitioned to W type growth in both cases, occurrence of unique W-ZB mixed crystallinity (polytypism) was significant and might result in the final gQDs as a consequence of the ligands and reaction conditions involved in the traditional synthesis. Next, I investigated the influence of the shell anion precursor concentrations on gQD growth employing identical W cores, by altering the mode of addition and three different sources of sulfur. Experimental results indicated that delicate interplay of crystal structure preference and ligands involved in the synthesis resulted in varied morphologies (rod, tripodal, trigonal and polyhedral) and crystal structures (W, ZB, W-ZB and ZB respectively) of gQDs in each of the syntheses

The role of liquid ink transport in the direct placement of quantum dot emitters onto sub-micrometer antennas by dip-pen nanolithography

Dip‐pen nanolithography (DPN) is used to precisely position core/thick‐shell (“giant”) quantum dots (gQDs; ≥10 nm in diameter) exclusively on top of silicon nanodisk antennas (≈500 nm diameter pillars with a height of ≈200 nm), resulting in periodic arrays of hybrid nanostructures and demonstrating a facile integration strategy toward next‐generation quantum light sources. A three‐step reading‐inking‐writing approach is employed, where atomic force microscopy (AFM) images of the pre‐patterned substrate topography are used as maps to direct accurate placement of nanocrystals. The DPN “ink” comprises gQDs suspended in a non‐aqueous carrier solvent, o‐dichlorobenzene. Systematic analyses of factors influencing deposition rate for this non‐conventional DPN ink are described for flat substrates and used to establish the conditions required to achieve small (sub‐500 nm) feature sizes, namely: dwell time, ink‐substrate contact angle and ink volume. Finally, it is shown that the rate of solvent transport controls the feature size in which gQDs are found on the substrate, but also that the number and consistency of nanocrystals deposited depends on the stability of the gQD suspension. Overall, the results lay the groundwork for expanded use of nanocrystal liquid inks and DPN for fabrication of multi‐component nanostructures that are challenging to create using traditional lithographic techniques.F.D. and J.W. contributed equally to this work. F.D. was supported by postdoctoral funding of the Center for Integrated Nanotechnologies (CINT), an Office of Science (OS) Nanoscale Science Research Center (NSRC) and User Facility operated for the U.S. Department of Energy (DOE) by Los Alamos National Laboratory (LANL; Contract No. DE-AC52-06NA25396) and Sandia National Laboratories (Contract No. DE-NA-0003525), and the work was performed in large part at CINT and contributed to CINT User Project, C2013B0048. J.W., P.A.S., S.M., M.T., and J.A.H. acknowledge LANL Directed Research and Development Funds. C.J.S. is a CINT-funded technical specialist. M.R.B. was funded by an LANL Director's Postdoctoral Fellowship, and A.M.D. by a Single Investigator Small Group Research Grant (2009LANL1096), Division of Materials Science and Engineering (MSE), Office of Basic Energy Sciences (OBES), OS, DOE. Los Alamos National Laboratory, an affirmative action equal opportunity employer, is operated by Los Alamos National Security, LLC, for the National Nuclear Security Administration of the DOE under Contract No. DE-AC52-06NA25396. (Center for Integrated Nanotechnologies (CINT), an Office of Science (OS) Nanoscale Science Research Center (NSRC); DE-AC52-06NA25396 - U.S. Department of Energy (DOE); DE-NA-0003525 - U.S. Department of Energy (DOE); C2013B0048 - CINT User Project; LANL Directed Research and Development Funds; CINT; LANL Director's Postdoctoral Fellowship; 2009LANL1096 - Single Investigator Small Group Research Grant, Division of Materials Science and Engineering (MSE), Office of Basic Energy Sciences (OBES), OS, DOE; DE-AC52-06NA25396 - National Nuclear Security Administration of the DOE)Accepted manuscrip

Patient satisfaction at a primary level health-care facility in a district of West Bengal: Are our patients really satisfied?

Introduction: Many recent studies have shown an increased association between patient's satisfaction levels, patient's compliance, and success of treatment. Aim: The aim of this study is to assess the level of satisfaction among patients who have utilized the outpatient department services provided in the primary care level health institution. Materials and Methods: A health center-based observational cross-sectional study was conducted from July 2011 to October 2011 at Guskara Primary health center, Burdwan among 422 patients using a pre-designed pre-tested structured schedule. Results: Overall, mean satisfaction score was 2.97 ± 0.37. Highest satisfaction scores were observed among 18–20 years, males were more satisfied regarding technical quality of care, whereas females reported higher satisfaction regarding interpersonal manner, unmarried/single group reported the highest satisfaction with most of the services, literate group reported higher satisfaction than the illiterate group, affluent patients reported higher satisfaction regarding technical quality of care, financial aspect. Conclusions: Causes of dissatisfaction were long waiting time, the inadequacy of seating arrangement in the waiting area, inadequate cleanliness of surroundings, inadequate toilet facilities, nonavailability of medicines, and behavior of doctor

The Role of Liquid Ink Transport in the Direct Placement of Quantum Dot Emitters onto Sub-Micrometer Antennas by Dip-Pen Nanolithography

Dip-pen nanolithography (DPN) is used to precisely position core/thick-shell (“giant”) quantum dots (gQDs; ≥10 nm in diameter) exclusively on top of silicon nanodisk antennas (≈500 nm diameter pillars with a height of ≈200 nm), resulting in periodic arrays of hybrid nanostructures and demonstrating a facile integration strategy toward next-generation quantum light sources. A three-step reading-inking-writing approach is employed, where atomic force microscopy (AFM) images of the pre-patterned substrate topography are used as maps to direct accurate placement of nanocrystals. The DPN “ink” comprises gQDs suspended in a non-aqueous carrier solvent, o-dichlorobenzene. Systematic analyses of factors influencing deposition rate for this non-conventional DPN ink are described for flat substrates and used to establish the conditions required to achieve small (sub-500 nm) feature sizes, namely: dwell time, ink-substrate contact angle and ink volume. Finally, it is shown that the rate of solvent transport controls the feature size in which gQDs are found on the substrate, but also that the number and consistency of nanocrystals deposited depends on the stability of the gQD suspension. Overall, the results lay the groundwork for expanded use of nanocrystal liquid inks and DPN for fabrication of multi-component nanostructures that are challenging to create using traditional lithographic techniques.F.D. was supported by postdoctoral funding of the Center for Integrated Nanotechnologies (CINT), an Office of Science (OS) Nanoscale Science Research Center (NSRC) and User Facility operated for the U.S. Department of Energy (DOE) by Los Alamos National Laboratory (LANL; Contract No. DE-AC52-06NA25396) and Sandia National Laboratories (Contract No. DE-NA-0003525), and the work was performed in large part at CINT and contributed to CINT User Project, C2013B0048. J.W., P.A.S., S.M., M.T., and J.A.H. acknowledge LANL Directed Research and Development Funds. C.J.S. is a CINT-funded technical specialist. M.R.B. was funded by an LANL Director’s Postdoctoral Fellowship, and A.M.D. by a Single Investigator Small Group Research Grant (2009LANL1096), Division of Materials Science and Engineering (MSE), Office of Basic Energy Sciences (OBES), OS, DOE. Los Alamos National Laboratory, an affirmative action equal opportunity employer, is operated by Los Alamos National Security, LLC, for the National Nuclear Security Administration of the DOE under Contract No. DE-AC52-06NA2539

From Inside Out: How the Buried Interface, Shell Defects, and Surface Chemistry Conspire to Determine Optical Performance in Nonblinking Giant Quantum Dots

“Giant” or core/thick-shell quantum dots (gQDs) are an important class of solid-state quantum emitter characterized by strongly suppressed blinking and photobleaching under ambient conditions, and reduced nonradiative Auger processes. Together, these qualities provide distinguishing and useful functionality as single- and ensemble-photon sources. For many applications, operation at elevated temperatures and under intense photon flux is desired, but performance is strongly dependent on the synthetic method employed for thick-shell growth. Here, a comprehensive analysis of gQD structural properties “from the inside out” as a function of shell-growth method is reported: successive ionic layer adsorption and reaction (SILAR) and high-temperature continuous injection (HT-CI), or sequential combinations of the two. Key correlations across synthesis methods, structural features (interfacial alloying, stacking-fault density and surface-ligand identity), and performance metrics (quantum yield, single-gQD photoluminescence under thermal/photo stress, charging behavior and quantum-optical properties) are identified. Surprisingly, it is found that interfacial alloying is the strongest indicator of gQD stability under stress, but this parameter is not the determining factor for Auger suppression. Furthermore, quantum yield is strongly influenced by surface chemistry and can approach unity even in the case of high shell-defect density, while introduction of zinc-blende stacking faults increases the likelihood that a gQD exhibits charged-state emission

Precision additive nanofabrication: the role of liquid ink transport in the direct placement of quantum dot emitters onto sub-micrometer antennas by dip-pen nanolithography (Small 31/2018)

Back cover graphic.In article number 1801503 , Jennifer A. Hollingsworth and co‐workers demonstrate an advance in nanofabrication using dip‐pen nanolithography (DPN) to directly place nanocrystal quantum dots onto a three‐dimensional nanostructured optical antenna. The results lay the groundwork for the expanded use of DPN and other scanning probe technologies for the additive preparation of functional multi‐component systems and devices at the nanoscale.Published versio

Twist Angle Dependent Interlayer Exciton Lifetimes in van der Waals Heterostructures

In van der Waals (vdW) heterostructures formed by stacking two monolayers of transition metal dichalcogenides, multiple exciton resonances with highly tunable properties are formed and subject to both vertical and lateral confinement. We investigate how a unique control knob, the twist angle between the two monolayers, can be used to control the exciton dynamics. We observe that the interlayer exciton lifetimes in MoSe2/WSe2 twisted bilayers (TBLs) change by one order of magnitude when the twist angle is varied from 1 ◦ to 3.5◦. Using a low-energy continuum model, we theoretically separate two leading mechanisms that influence interlayer exciton radiative lifetimes. The shift to indirect transitions in the momentum space with an increasing twist angle and the energy modulation from the moir´e potential both have a significant impact on interlayer exciton lifetimes. We further predict distinct temperature dependence of interlayer exciton lifetimes in TBLs with different twist angles, which is partially validated by experiments. While many recent studies have highlighted how the twist angle in a vdW TBL can be used to engineer the ground states and quantum phases due to many-body interaction, our studies explore its role in controlling the dynamics of optically excited states, thus, expanding the conceptual applications of “twistronics”.Center for Dynamics and Control of Material

Photophysics of Thermally-Assisted Photobleaching in “Giant” Quantum Dots Revealed in Single Nanocrystals

Quantum dots (QDs) are steadily being implemented as down-conversion phosphors in market-ready display products to enhance color rendering, brightness, and energy efficiency. However, for adequate longevity, QDs must be encased in a protective barrier that separates them from ambient oxygen and humidity, and device architectures are designed to avoid significant heating of the QDs as well as direct contact between the QDs and the excitation source. In order to increase the utility of QDs in display technologies and to extend their usefulness to more demanding applications as, for example, alternative phosphors for solid-state lighting (SSL), QDs must retain their photoluminescence emission properties over extended periods of time under conditions of high temperature and high light flux. Doing so would simplify the fabrication costs for QD display technologies and enable QDs to be used as down-conversion materials in light-emitting diodes for SSL, where direct-on-chip configurations expose the emitters to temperatures approaching 100 °C and to photon fluxes from 0.1 W/mm² to potentially 10 W/mm². Here, we investigate the photobleaching processes of single QDs exposed to controlled temperature and photon flux. In particular, we investigate two types of room-temperature-stable core/thick-shell QDs, known as “giant” QDs for which shell growth is conducted using either a standard layer-by-layer technique or by a continuous injection method. We determine the mechanistic pathways responsible for thermally-assisted photodegradation, distinguishing effects of hot-carrier trapping and QD charging. The findings presented here will assist in the further development of advanced QD heterostructures for maximum device lifetime stability