Sleep quality, duration, and consistency are associated with better academic performance in college students

Although numerous survey studies have reported connections between sleep and cognitive function, there remains a lack of quantitative data using objective measures to directly assess the association between sleep and academic performance. In this study, wearable activity trackers were distributed to 100 students in an introductory college chemistry class (88 of whom completed the study), allowing for multiple sleep measures to be correlated with in-class performance on quizzes and midterm examinations. Overall, better quality, longer duration, and greater consistency of sleep correlated with better grades. However, there was no relation between sleep measures on the single night before a test and test performance; instead, sleep duration and quality for the month and the week before a test correlated with better grades. Sleep measures accounted for nearly 25% of the variance in academic performance. These findings provide quantitative, objective evidence that better quality, longer duration, and greater consistency of sleep are strongly associated with better academic performance in college. Gender differences are discussed.

Atomic Structure and Dynamics of Self-Limiting Sub-Nanometer Pores in Monolayer WS2

We reveal a self-limiting mechanism during the formation of a specific type of circular nanopore in monolayer WS₂ that limits its diameter to sub-nm. A single W atom vacancy (triangular nanopore) is transformed into the self-limiting nanopore (SLNP) through the atomic restructuring of S atoms around the area, reducing the number of dangling bonds at the nanopore edge by shifting them further in-plane with W-W bonding instead. Bond rotations in WS₂ help accommodate the electron beam induced atomic loss and ensure the stability of the SLNP. The SLNP shows significant improvement in diameter stability during electron beam irradiation compared to other triangular nanopores in WS₂ that typically continue to expand in diameter during atom loss. The atomic structure of these SLNPs is studied using aberration-corrected scanning transmission electron microscopy with an in situ heating holder, revealing that the SLNPs are mostly formed at a temperature of ∼500 °C, which is a balance between thermally activated S vacancy diffusion and sufficient S vacancy density to initiate local atomic reconstruction. At higher temperatures ( i. e., 1000 °C), S vacancies quickly migrate away into long line vacancies, resulting in low S vacancy density and rapidly expanding holes generated at the edges of the line vacancies. At room temperature, S vacancy migration is low and vacancy density is very high, which limits atomic reconstruction, and instead many small holes open up. These results provide insights into the factors that lead to uniform sized nanopores in the sub-nm range in transition-metal dichalcogenides.

Polarity governs atomic interaction through two-dimensional materials

The transparency of two-dimensional (2D) materials to intermolecular interactions of crystalline materials has been an unresolved topic. Here we report that remote atomic interaction through 2D materials is governed by the binding nature, that is, the polarity of atomic bonds, both in the underlying substrates and in 2D material interlayers. Although the potential field from covalent-bonded materials is screened by a monolayer of graphene, that from ionic-bonded materials is strong enough to penetrate through a few layers of graphene. Such field penetration is substantially attenuated by 2D hexagonal boron nitride, which itself has polarization in its atomic bonds. Based on the control of transparency, modulated by the nature of materials as well as interlayer thickness, various types of single-crystalline materials across the periodic table can be epitaxially grown on 2D material-coated substrates. The epitaxial films can subsequently be released as free-standing membranes, which provides unique opportunities for the heterointegration of arbitrary single-crystalline thin films in functional applications.

Ultralong 1D Vacancy Channels for Rapid Atomic Migration during 2D Void Formation in Monolayer MoS2

High-energy irradiation of materials can lead to void formation due to the aggregation of vacancies, reducing the local stress in the system. Studying void formation and its interplay with vacancy clusters in bulk materials at the atomic level has been challenging due to the thick volume of 3D materials, which generally limits high-resolution transmission electron microscopy. The thin nature of 2D materials is ideal for studying fundamental material defects such as dislocations and crack tips and has potential to reveal void formation by vacancy aggregation in detail. Here, using atomic-resolution in situ transmission electron microscopy of 2D monolayer MoS₂, we capture rapid thermal diffusion of S vacancies into ultralong (∼60 nm) 1D S vacancy channels that initiate void formation at high vacancy densities. Strong interactions are observed between the 1D channels and void growth, whereby Mo and S atoms are funneled back and forth between the void edge and the crystal surface to enable void enlargement. Preferential void growth up to 100 nm is shown to occur by rapid digestion of 1D S vacancy channels as they make contact. These results reveal the atomistic mechanisms behind void enlargement in 2D materials under intense high-energy irradiation at high temperatures and the existence of ultralong 1D vacancy channels. This knowledge may also help improve the understanding of void formation in other systems such as nuclear materials, where direct visualization is challenging due to 3D bulk volume.

Machine Learning Enabled Computational Screening of Inorganic Solid Electrolytes for Suppression of Dendrite Formation in Lithium Metal Anodes

Next generation batteries based on lithium (Li) metal anodes have been plagued by the dendritic electrodeposition of Li metal on the anode during cycling, resulting in short circuit and capacity loss. Suppression of dendritic growth through the use of solid electrolytes has emerged as one of the most promising strategies for enabling the use of Li metal anodes. We perform a computational screening of over 12 000 inorganic solids based on their ability to suppress dendrite initiation in contact with Li metal anode. Properties for mechanically isotropic and anisotropic interfaces that can be used in stability criteria for determining the propensity of dendrite initiation are usually obtained from computationally expensive first-principles methods. In order to obtain a large data set for screening, we use machine-learning models to predict the mechanical properties of several new solid electrolytes. The machine-learning models are trained on purely structural features of the material, which do not require any first-principles calculations. We train a graph convolutional neural network on the shear and bulk moduli because of the availability of a large training data set with low noise due to low uncertainty in their first-principles-calculated values. We use gradient boosting regressor and kernel ridge regression to train the elastic constants, where the choice of the model depends on the size of the training data and the noise that it can handle. The material stiffness is found to increase with an increase in mass density and ratio of Li and sublattice bond ionicity, and decrease with increase in volume per atom and sublattice electronegativity. Cross-validation/test performance suggests our models generalize well. We predict over 20 mechanically anisotropic interfaces between Li metal and four solid electrolytes which can be used to suppress dendrite growth. Our screened candidates are generally soft and highly anisotropic, and present opportunities for simultaneously obtaining dendrite suppression and high ionic conductivity in solid electrolytes.

Freestanding Organic Charge-Transfer Conformal Electronics

Wearable conformal electronics are essential components for next-generation humanlike sensing devices that can accurately respond to external stimuli in nonplanar and dynamic surfaces. However, to explore this potential, it is indispensable to achieve the desired level of deformability and charge-transport mobility in strain-accommodating soft semiconductors. Here, we show pseudo-two-dimensional freestanding conjugated polymer heterojunction nanosheets integrated into substrate-free conformal electronics owing to their exceptional crystalline controlled charge transport and high level of mechanical strength. These freestanding and mechanical robust polymer nanosheets can be adapted into a variety of artificial structured surfaces such as fibers, squares, circles, etc., which produce large-area stretchable conformal charge-transfer sensors for real-time static and dynamic monitoring.

Crystal Graph Convolutional Neural Networks for an Accurate and Interpretable Prediction of Material Properties

The use of machine learning methods for accelerating the design of crystalline materials usually requires manually constructed feature vectors or complex transformation of atom coordinates to input the crystal structure, which either constrains the model to certain crystal types or makes it difficult to provide chemical insights. Here, we develop a crystal graph convolutional neural networks framework to directly learn material properties from the connection of atoms in the crystal, providing a universal and interpretable representation of crystalline materials. Our method provides a highly accurate prediction of density functional theory calculated properties for eight different properties of crystals with various structure types and compositions after being trained with 10^{4} data points. Further, our framework is interpretable because one can extract the contributions from local chemical environments to global properties. Using an example of perovskites, we show how this information can be utilized to discover empirical rules for materials design.

Optically-regulated thermal energy storage in diverse organic phase-change materials

Optical regulation of heat storage in diverse sets of organic phase-change materials is demonstrated and compared.

Atomically Flat Zigzag Edges in Monolayer MoS2 by Thermal Annealing

The edges of 2D materials show novel electronic, magnetic, and optical properties, especially when reduced to nanoribbon widths. Therefore, methods to create atomically flat edges in 2D materials are essential for future exploitation. Atomically flat edges in 2D materials are found after brittle fracture or when electrically biasing, but a simple scalable approach for creating atomically flat periodic edges in monolayer 2D transition metal dichalcogenides has yet to be realized. Here, we show how heating monolayer MoS₂ to 800 °C in vacuum produces atomically flat Mo terminated zigzag edges in nanoribbons. We study this at the atomic level using an ultrastable in situ heating holder in an aberration-corrected transmission electron microscope and discriminating Mo from S at the edge, revealing unique Mo terminations for all zigzag orientations that remain stable and atomically flat when cooling back to room temperature. Highly faceted MoS₂ nanoribbon constrictions are produced with Mo rich edge structures that have theoretically predicted spin separated transport channels, which are promising for spin logic applications.

Graphene Nanoribbon Based Thermoelectrics: Controllable Self- Doping and Long-Range Disorder

Control of both the regularity of a material ensemble and nanoscale architecture provides unique opportunities to develop novel thermoelectric applications based on 2D materials. As an example, the authors explore the electronic and thermal properties of functionalized graphene nanoribbons (GNRs) in the single-sheet and helical architectures using multiscale simulations. The results suggest that appropriate functionalization enables precise tuning of the doping density in a planar donor/acceptor GNR ensemble without the need to introduce an explicit dopant, which is critical to the optimization of power factor. In addition, the self-interaction between turns of a GNR may induce long-range disorder along the helical axis, which suppresses the thermal contribution from phonons with long wavelengths, leading to anomalous length independent phonon thermal transport in the quasi-1D system.

Atomic Structure and Dynamics of Defects in 2D MoS2 Bilayers

We present a detailed atomic-level study of defects in bilayer MoS₂ using aberration-corrected transmission electron microscopy at an 80 kV accelerating voltage. Sulfur vacancies are found in both the top and bottom layers in 2H- and 3R-stacked MoS₂ bilayers. In 3R-stacked bilayers, sulfur vacancies can migrate between layers but more preferably reside in the (Mo-2S) column rather than the (2S) column, indicating more complex vacancy production and migration in the bilayer system. As the point vacancy number increases, aggregation into larger defect structures occurs, and this impacts the interlayer stacking. Competition between compression in one layer from the loss of S atoms and the van der Waals interlayer force causes much less structural deformations than those in the monolayer system. Sulfur vacancy lines neighboring in top and bottom layers introduce less strain compared to those staggered in the same layer. These results show how defect structures in multilayered two-dimensional materials differ from their monolayer form.

Epitaxial Templating of Two-Dimensional Metal Chloride Nanocrystals on Monolayer Molybdenum Disulfide

We demonstrate the formation of ionic metal chloride (CuCl) two-dimensional (2D) nanocrystals epitaxially templated on the surface of monolayer molybdenum disulfide (MoS₂). These 2D CuCl nanocrystals are single atomic planes from a nonlayered bulk CuCl structure. They are stabilized as a 2D monolayer on the surface of the MoS₂ through interactions with the uniform periodic surface of the MoS₂. The heterostructure 2D system is studied at the atomic level using aberration-corrected transmission electron microscopy at 80 kV. Dynamics of discrete rotations of the CuCl nanocrystals are observed, maintaining two types of preferential alignments to the MoS₂ lattice, confirming that the strong interlayer interactions drive the stable CuCl structure. Strain maps are produced from displacement maps and used to track real-time variations of local atomic bonding and defect production. Density functional theory calculations interpret the formation of two types of energetically advantageous commensurate superlattices via strong chemical bonds at interfaces and predict their corresponding electronic structures. These results show how vertical heterostructured 2D nanoscale systems can be formed beyond the simple assembly of preformed layered materials and provide indications about how different 2D components and their interfacial coupling mode could influence the overall property of the heterostructures.

Atomic structure and formation mechanism of sub-nanometer pores in 2D monolayer MoS2

We use electron-beam nanofabrication to create sub-nanometer (sub-nm) pores in 2D monolayer MoS₂ with fine control over the pore size down to 0.6 nm, corresponding to the loss of a single Mo atom and surrounding S atoms. The sub-nm pores are created in situ with 1 nm spatial precision in the MoS₂ lattice by control of the angstrom sized probe in an aberration corrected scanning transmission electron microscope with real time tracking of the pore creation. Dynamics of the sub-nm pore creation are captured at the atomic scale and reveal the mechanism of nanopore formation at accelerating voltages of 60 and 80 kV to be due to displacing a Mo atom from the lattice site onto the surface of the MoS₂. This process is enabled by the destabilization of the Mo bonding from localized electron beam induced S atom loss. DFT calculations confirm the energetic advantage of having the ejected Mo atom attach on the sheet surface rather than being expelled into vacuum, and indicate sensitivity of the nanopore potential as a function of the adsorption position of the ejected Mo atom. These results provide detailed atomic level insights into the initial process of single Mo loss that underpins the nucleation of a nanopore and explains the formation mechanism of sub-nm pores in MoS₂.

Atomic Structure and Dynamics of Single Platinum Atom Interactions with Monolayer MoS2

We have studied atomic level interactions between single Pt atoms and the surface of monolayer MoS₂ using aberration-corrected annular dark field scanning transmission electron microscopy at an accelerating voltage of 60 kV. Strong contrast from single Pt atoms on the atomically resolved monolayer MoS₂ lattice enables their exact position to be determined with respect to the MoS₂ lattice, revealing stable binding sites. In regions of MoS₂ free from surface contamination, the Pt atoms are localized in S vacancy sites and exhibit dynamic hopping to nearby vacancy sites driven by the energy supplied by the electron beam. However, in areas of MoS₂ contaminated with carbon surface layers, the Pt atoms appear at various positions with respect to the underlying MoS₂ lattice, including on top of Mo and in off-axis positions. These variations are due to the Pt bonding with the surrounding amorphous carbon layer, which disrupts the intrinsic Pt-MoS₂ interactions, leading to more varied positions. Density functional theory (DFT) calculations reveal that Pt atoms on the surface of MoS₂ have a small barrier for migration and are stabilized when bound to either a single or double sulfur vacancies. DFT calculations have been used to understand how the catalytic activity of the MoS₂ basal plane for hydrogen evolution reaction is influenced by Pt dopants by variation of the hydrogen adsorption free energy. This strong dependence of catalytic effect on interfacial configurations is shown to be common for a series of dopants, which may provide a means to create and optimize reaction centers.

Molecularly Engineered Azobenzene Derivatives for High Energy Density Solid-State Solar Thermal Fuels

Solar thermal fuels (STFs) harvest and store solar energy in a closed cycle system through conformational change of molecules and can release the energy in the form of heat on demand. With the aim of developing tunable and optimized STFs for solid-state applications, we designed three azobenzene derivatives functionalized with bulky aromatic groups (phenyl, biphenyl, and tert-butyl phenyl groups). In contrast to pristine azobenzene, which crystallizes and makes nonuniform films, the bulky azobenzene derivatives formed uniform amorphous films that can be charged and discharged with light and heat for many cycles. Thermal stability of the films, a critical metric for thermally triggerable STFs, was greatly increased by the bulky functionalization (up to 180 °C), and we were able to achieve record high energy density of 135 J/g for solid-state STFs, over a 30% improvement compared to previous solid-state reports. Furthermore, the chargeability in the solid state was improved, up to 80% charged from 40% charged in previous solid-state reports. Our results point toward molecular engineering as an effective method to increase energy storage in STFs, improve chargeability, and improve the thermal stability of the thin film.

Enhanced Cell Capture on Functionalized Graphene Oxide Nanosheets through Oxygen Clustering

With the global rise in incidence of cancer and infectious diseases, there is a need for the development of techniques to diagnose, treat, and monitor these conditions. The ability to efficiently capture and isolate cells and other biomolecules from peripheral whole blood for downstream analyses is a necessary requirement. Graphene oxide (GO) is an attractive template nanomaterial for such biosensing applications. Favorable properties include its two-dimensional architecture and wide range of functionalization chemistries, offering significant potential to tailor affinity toward aromatic functional groups expressed in biomolecules of interest. However, a limitation of current techniques is that as-synthesized GO nanosheets are used directly in sensing applications, and the benefits of their structural modification on the device performance have remained unexplored. Here, we report a microfluidic-free, sensitive, planar device on treated GO substrates to enable quick and efficient capture of Class-II MHC-positive cells from murine whole blood. We achieve this by using a mild thermal annealing treatment on the GO substrates, which drives a phase transformation through oxygen clustering. Using a combination of experimental observations and MD simulations, we demonstrate that this process leads to improved reactivity and density of functionalization of cell capture agents, resulting in an enhanced cell capture efficiency of 92 ± 7% at room temperature, almost double the efficiency afforded by devices made using as-synthesized GO (54 ± 3%). Our work highlights a scalable, cost-effective, general approach to improve the functionalization of GO, which creates diverse opportunities for various next-generation device applications.

Ultra-high aspect ratio functional nanoporous silicon via nucleated catalysts

Large scale, sub-10 nm high aspect ratio nanoporous silicon is fabricatedviascalable sputtering and a solution-based process.

Conformal Electroplating of Azobenzene-Based Solar Thermal Fuels onto Large-Area and Fiber Geometries

There is tremendous growth in fields where small functional molecules and colloidal nanomaterials are integrated into thin films for solid-state device applications. Many of these materials are synthesized in solution and there often exists a significant barrier to transition them into the solid state in an efficient manner. Here, we develop a methodology employing an electrodepositable copolymer consisting of small functional molecules for applications in solar energy harvesting and storage. We employ azobenzene solar thermal fuel polymers and functionalize them to enable deposition from low concentration solutions in methanol, resulting in uniform and large-area thin films. This approach enables conformal deposition on a variety of conducting substrates that can be either flat or structured depending on the application. Our approach further enables control over film growth via electrodepsition conditions and results in highly uniform films of hundreds of nanometers to microns in thickness. We demonstrate that this method enables superior retention of solar thermal fuel properties, with energy densities of ∼90 J/g, chargeability in the solid state, and exceptional materials utilization compared to other solid-state processing approaches. This novel approach is applicable to systems such as photon upconversion, photovoltaics, photosensing, light emission, and beyond, where small functional molecules enable solid-state applications.

Chemistry and Structure of Graphene Oxide via Direct Imaging

Graphene oxide (GO) and reduced GO (rGO) are the only variants of graphene that can be manufactured at the kilogram scale, and yet the widely accepted model for their structure has largely relied on indirect evidence. Notably, existing high-resolution transmission electron microscopy (HRTEM) studies of graphene oxide report long-range order of sp(2) lattice with isolated defect clusters. Here, we present HRTEM evidence of a different structural form of GO, where nanocrystalline regions of sp(2) lattice are surrounded by regions of disorder. The presence of contaminants that adsorb to the surface of the material at room temperature normally prevents direct observation of the intrinsic atomic structure of this defective GO. To overcome this, we use an in situ heating holder within an aberration-corrected TEM (AC-TEM) to study the atomic structure of this nanocrystalline graphene oxide from room temperature to 700 °C. As the temperature increases to above 500 °C, the adsorbates detach from the GO and the underlying atomic structure is imaged to be small 2-4 nm crystalline domains within a polycrystalline GO film. By combining spectroscopic evidence with the AC-TEM data, we support the dynamic interpretation of the structural evolution of graphene oxide.

Optimization of the Thermoelectric Figure of Merit in Crystalline C60 with Intercalation Chemistry

Crystalline C60 is an appealing candidate material for thermoelectric (TE) applications due to its extremely low thermal conductivity and potentially high electrical conductivity with metal atom intercalation. We investigate the TE properties of crystalline C60 intercalated with alkali and alkaline earth metals using both classical and quantum mechanical calculations. For the electronic structure, our results show that variation of intercalated metal atoms has a large impact on energy dispersions, which leads to broad tunability of the power factor. For the thermal transport, we show that dopants introduce strong phonon scattering into crystalline C60, leading to considerably lower thermal conductivity. Taking both into account, our calculations suggest that appropriate choice of metal atom intercalation in crystalline C60 could yield figures of merit near 1 at room temperature.

MoS2 Enhanced T-Phase Stabilization and Tunability Through Alloying

Two-dimensional MoS2 is a promising material for nanoelectronics and catalysis, but its potential is not fully exploited since proper control of its multiple phases (H, T, ZT) and electronic properties is lacking. In this theoretical study, alloying is proposed as a method to stabilize the MoS2 T-phase. In particular, MoS2 is alloyed with another material that is known to exist in a monolayer MX2 T-structure, and we show that the formation energy difference among phases decreases even for low impurity concentrations in MoS2, and a relationship between impurity concentration and alloy band gap is established. This method can be potentially applied to many two-dimensional materials to tune/enhance their electronic properties and stabilities in order to suit the desired application.

Nanohole Structuring for Improved Performance of Hydrogenated Amorphous Silicon Photovoltaics

While low hole mobilities limit the current collection and efficiency of hydrogenated amorphous silicon (a-Si:H) photovoltaic devices, attempts to improve mobility of the material directly have stagnated. Herein, we explore a method of utilizing nanostructuring of a-Si:H devices to allow for improved hole collection in thick absorber layers. This is achieved by etching an array of 150 nm diameter holes into intrinsic a-Si:H and then coating the structured material with p-type a-Si:H and a conformal zinc oxide transparent conducting layer. The inclusion of these nanoholes yields relative power conversion efficiency (PCE) increases of ∼45%, from 7.2 to 10.4% PCE for small area devices. Comparisons of optical properties, time-of-flight mobility measurements, and internal quantum efficiency spectra indicate this efficiency is indeed likely occurring from an improved collection pathway provided by the nanostructuring of the devices. Finally, we estimate that through modest optimizations of the design and fabrication, PCEs of beyond 13% should be obtainable for similar devices.