Charge-polarized interfacial superlattices in marginally twisted hexagonal boron nitride.

When two-dimensional crystals are brought into close proximity, their interaction results in reconstruction of electronic spectrum and crystal structure. Such reconstruction strongly depends on the twist angle between the crystals, which has received growing attention due to interesting electronic and optical properties that arise in graphene and transitional metal dichalcogenides. Here we study two insulating crystals of hexagonal boron nitride stacked at small twist angle. Using electrostatic force microscopy, we observe ferroelectric-like domains arranged in triangular superlattices with a large surface potential. The observation is attributed to interfacial elastic deformations that result in out-of-plane dipoles formed by pairs of boron and nitrogen atoms belonging to opposite interfacial surfaces. This creates a bilayer-thick ferroelectric with oppositely polarized (BN and NB) dipoles in neighbouring domains, in agreement with our modeling. These findings open up possibilities for designing van der Waals heterostructures and offer an alternative probe to study moiré-superlattice electrostatic potentials.

Direct Observation of Incommensurate-Commensurate Transition in Graphene-hBN Heterostructures via Optical Second Harmonic Generation.

Commensurability effects play a crucial role in the formation of electronic properties of novel layered heterostructures. The interest in these moiré superstructures has increased tremendously since the recent observation of a superconducting state (Nature 2018, 556, 43-50) and metal-insulator transition (Nature 2018, 556, 80-84) in twisted bilayer graphene. In this regard, a straightforward and efficient experimental technique for detection of the alignment of layered materials is desired. In this work, we use optical second harmonic generation, which is sensitive to the inversion symmetry breaking, to investigate the alignment of graphene/hexagonal boron nitride heterostructures. To achieve that, we activate a commensurate-incommensurate phase transition by a thermal annealing of the sample. We find that this structural change in the system can be directly observed via a strong modification of a nonlinear optical signal. Unambiguous interpretation of obtained results reveals the potential of a second harmonic generation technique for probing of structural changes in layered systems.

Stress transfer at the nanoscale on graphene ribbons of regular geometry.

The knowledge of the mechanism of stress transfer from a polymer matrix to a 2-dimensional nano-inclusion such as a graphene flake is of paramount importance for the design and the production of effective nanocomposites. For efficient reinforcement the shape of the inclusion must be accurately controlled since the axial stress transfer from matrix to the inclusion is affected by the axial-shear coupling observed upon loading of a flake of irregular geometry. Herein, we study true axial phenomena on regular- exfoliated-graphene micro-ribbons which are perfectly aligned to the loading direction. We exploit the strain sensitivity of vibrational wave numbers in order to map point-by-point the strain built up along the length of graphene. By considering the balance of shear-to-axial forces, we identify the shear stress at the interface and develop a universal inverse-length parameter that governs the stress transfer process at the nanoscale. An important parameter that has come out of this approach is the prediction and measurement of the transfer length that is required for efficient stress in these systems.

Composite super-moiré lattices in double-aligned graphene heterostructures.

When two-dimensional (2D) atomic crystals are brought into close proximity to form a van der Waals heterostructure, neighbouring crystals may influence each other's properties. Of particular interest is when the two crystals closely match and a moiré pattern forms, resulting in modified electronic and excitonic spectra, crystal reconstruction, and more. Thus, moiré patterns are a viable tool for controlling the properties of 2D materials. However, the difference in periodicity of the two crystals limits the reconstruction and, thus, is a barrier to the low-energy regime. Here, we present a route to spectrum reconstruction at all energies. By using graphene which is aligned to two hexagonal boron nitride layers, one can make electrons scatter in the differential moiré pattern which results in spectral changes at arbitrarily low energies. Further, we demonstrate that the strength of this potential relies crucially on the atomic reconstruction of graphene within the differential moiré super cell.

Macroscopic self-reorientation of interacting two-dimensional crystals.

Microelectromechanical systems, which can be moved or rotated with nanometre precision, already find applications in such fields as radio-frequency electronics, micro-attenuators, sensors and many others. Especially interesting are those which allow fine control over the motion on the atomic scale because of self-alignment mechanisms and forces acting on the atomic level. Such machines can produce well-controlled movements as a reaction to small changes of the external parameters. Here we demonstrate that, for the system of graphene on hexagonal boron nitride, the interplay between the van der Waals and elastic energies results in graphene mechanically self-rotating towards the hexagonal boron nitride crystallographic directions. Such rotation is macroscopic (for graphene flakes of tens of micrometres the tangential movement can be on hundreds of nanometres) and can be used for reproducible manufacturing of aligned van der Waals heterostructures.

WSe2 Light-Emitting Tunneling Transistors with Enhanced Brightness at Room Temperature.

Monolayers of molybdenum and tungsten dichalcogenides are direct bandgap semiconductors, which makes them promising for optoelectronic applications. In particular, van der Waals heterostructures consisting of monolayers of MoS2 sandwiched between atomically thin hexagonal boron nitride (hBN) and graphene electrodes allows one to obtain light emitting quantum wells (LEQWs) with low-temperature external quantum efficiency (EQE) of 1%. However, the EQE of MoS2- and MoSe2-based LEQWs shows behavior common for many other materials: it decreases fast from cryogenic conditions to room temperature, undermining their practical applications. Here we compare MoSe2 and WSe2 LEQWs. We show that the EQE of WSe2 devices grows with temperature, with room temperature EQE reaching 5%, which is 250× more than the previous best performance of MoS2 and MoSe2 quantum wells in ambient conditions. We attribute such different temperature dependences to the inverted sign of spin-orbit splitting of conduction band states in tungsten and molybdenum dichalcogenides, which makes the lowest-energy exciton in WSe2 dark.

Nonlocal Response and Anamorphosis: The Case of Few-Layer Black Phosphorus.

Few-layer black phosphorus was recently rediscovered as a narrow-bandgap atomically thin semiconductor, attracting unprecedented attention due to its interesting properties. One feature of this material that sets it apart from other atomically thin crystals is its structural in-plane anisotropy which manifests in strongly anisotropic transport characteristics. However, traditional angle-resolved conductance measurements present a challenge for nanoscale systems, calling for new approaches in precision studies of transport anisotropy. Here, we show that the nonlocal response, being exponentially sensitive to the anisotropy value, provides a powerful tool for determining the anisotropy in black phosphorus. This is established by combining measurements of the orientation-dependent nonlocal resistance response with the analysis based on the anamorphosis relations. We demonstrate that the nonlocal response can differ by orders of magnitude for different crystallographic directions even when the anisotropy is at most order-one, allowing us to extract accurate anisotropy values.

Prenatal diagnosis of critical congenital heart disease reduces risk of death from cardiovascular compromise prior to planned neonatal cardiac surgery: a meta-analysis.

OBJECTIVE: To determine if prenatal diagnosis improves the chance that a newborn with critical congenital heart disease will survive to undergo planned cardiac surgery. METHODS: A systematic review of the medical literature identified eight studies which met the following criteria: compared outcomes between newborns with prenatal and those with postnatal diagnosis of critical congenital heart disease; compared groups of patients with the same anatomical diagnosis; provided detailed information on cardiac anatomy; included detailed information on preoperative cause of death. A meta-analysis was performed to assess differences in preoperative mortality rates between newborns with prenatal diagnosis and those with postnatal diagnosis. Patients with established risk factors for increased mortality (high risk) and those whose families chose comfort care rather than cardiac surgery were excluded. RESULTS: In patients with comparable anatomy, standard risk, a parental desire to treat and optimal care, newborns with a prenatal diagnosis of critical congenital heart disease were significantly less likely to die prior to planned cardiac surgery than were those with a comparable postnatal diagnosis (pooled odds ratio, 0.26; 95% CI, 0.08-0.84). CONCLUSIONS: For newborns most likely to benefit from treatment for their critical congenital heart disease, because they did not have additional risk factors and their families pursued treatment, prenatal diagnosis reduced the risk of death prior to planned cardiac surgery relative to patients with a comparable postnatal diagnosis. Further study and efforts to improve prenatal diagnosis of congenital heart disease should therefore be considered.

Twist-controlled resonant tunnelling in graphene/boron nitride/graphene heterostructures.

Recent developments in the technology of van der Waals heterostructures made from two-dimensional atomic crystals have already led to the observation of new physical phenomena, such as the metal-insulator transition and Coulomb drag, and to the realization of functional devices, such as tunnel diodes, tunnel transistors and photovoltaic sensors. An unprecedented degree of control of the electronic properties is available not only by means of the selection of materials in the stack, but also through the additional fine-tuning achievable by adjusting the built-in strain and relative orientation of the component layers. Here we demonstrate how careful alignment of the crystallographic orientation of two graphene electrodes separated by a layer of hexagonal boron nitride in a transistor device can achieve resonant tunnelling with conservation of electron energy, momentum and, potentially, chirality. We show how the resonance peak and negative differential conductance in the device characteristics induce a tunable radiofrequency oscillatory current that has potential for future high-frequency technology.

Heterostructures produced from nanosheet-based inks.

The new paradigm of heterostructures based on two-dimensional (2D) atomic crystals has already led to the observation of exciting physical phenomena and creation of novel devices. The possibility of combining layers of different 2D materials in one stack allows unprecedented control over the electronic and optical properties of the resulting material. Still, the current method of mechanical transfer of individual 2D crystals, though allowing exceptional control over the quality of such structures and interfaces, is not scalable. Here we show that such heterostructures can be assembled from chemically exfoliated 2D crystals, allowing for low-cost and scalable methods to be used in device fabrication.

Electronic properties of graphene encapsulated with different two-dimensional atomic crystals.

Hexagonal boron nitride is the only substrate that has so far allowed graphene devices exhibiting micrometer-scale ballistic transport. Can other atomically flat crystals be used as substrates for making quality graphene heterostructures? Here we report on our search for alternative substrates. The devices fabricated by encapsulating graphene with molybdenum or tungsten disulfides and hBN are found to exhibit consistently high carrier mobilities of about 60 000 cm(2) V(-1) s(-1). In contrast, encapsulation with atomically flat layered oxides such as mica, bismuth strontium calcium copper oxide, and vanadium pentoxide results in exceptionally low quality of graphene devices with mobilities of ∼1000 cm(2) V(-1) s(-1). We attribute the difference mainly to self-cleansing that takes place at interfaces between graphene, hBN, and transition metal dichalcogenides. Surface contamination assembles into large pockets allowing the rest of the interface to become atomically clean. The cleansing process does not occur for graphene on atomically flat oxide substrates.

Cloning of Dirac fermions in graphene superlattices.

Superlattices have attracted great interest because their use may make it possible to modify the spectra of two-dimensional electron systems and, ultimately, create materials with tailored electronic properties. In previous studies (see, for example, refs 1-8), it proved difficult to realize superlattices with short periodicities and weak disorder, and most of their observed features could be explained in terms of cyclotron orbits commensurate with the superlattice. Evidence for the formation of superlattice minibands (forming a fractal spectrum known as Hofstadter's butterfly) has been limited to the observation of new low-field oscillations and an internal structure within Landau levels. Here we report transport properties of graphene placed on a boron nitride substrate and accurately aligned along its crystallographic directions. The substrate's moiré potential acts as a superlattice and leads to profound changes in the graphene's electronic spectrum. Second-generation Dirac points appear as pronounced peaks in resistivity, accompanied by reversal of the Hall effect. The latter indicates that the effective sign of the charge carriers changes within graphene's conduction and valence bands. Strong magnetic fields lead to Zak-type cloning of the third generation of Dirac points, which are observed as numerous neutrality points in fields where a unit fraction of the flux quantum pierces the superlattice unit cell. Graphene superlattices such as this one provide a way of studying the rich physics expected in incommensurable quantum systems and illustrate the possibility of controllably modifying the electronic spectra of two-dimensional atomic crystals by varying their crystallographic alignment within van der Waals heterostuctures.

Community-based intervention to optimise falls risk management: a randomised controlled trial.

BACKGROUND: falls are the leading causes of accidental death and fragility fractures in older adults. Interventions that assess and reduce falls risk are underutilised. OBJECTIVE: to evaluate the impact of a multifaceted community-based programme aimed at optimising evidence-based management of patients at risk for fall-related fractures. DESIGN: this was a randomised trial performed from 2003 to 2006. SETTING: community-based intervention in Ontario, Canada. PARTICIPANTS: eligible patients were community-dwelling, aged > or =55 years and identified to be at risk for fall-related fractures. A total of 201 patients were allocated to the intervention group or to usual care. INTERVENTION: components of the intervention included assessment of falls risk, functional status and home environment, and patient education. MEASUREMENTS: primary outcome was the implementation of appropriate falls risk assessment at 6 months. Secondary outcomes included falls and fractures at 6 and 12 months. RESULTS: the mean age of participants was 72 years, and 41% had fallen with injury in the previous year. Compared to usual care, the intervention increased the number of referrals made to physiotherapy [21% (21/101) vs 6.0% (6/100); relative risk (RR) 3.47, 95% confidence interval (CI) 1.46-8.22] and occupational therapy [15% (15/101) vs 0%; RR 30.7, 95% CI 1.86 to >500]. At 12 months, the number of falls in the intervention group was greater than in the usual care group [23% (23/101) vs 11% (11/100); RR 2.07, 95% CI 1.07-4.02]. CONCLUSIONS: compared to usual care, a multi-faceted intervention increased referrals to physiotherapy and occupational therapy but did not reduce risk of falls. Similar falls reduction interventions cannot be recommended based on the results of this study.

Community-based randomised controlled trial evaluating falls and osteoporosis risk management strategies.

BACKGROUND: Osteoporosis-related fractures are a significant public health concern. Interventions that increase detection and treatment of osteoporosis, as well as prevention of fractures and falls, are substantially underutilized. This paper outlines the protocol for a pragmatic randomised trial of a multifaceted community-based care program aimed at optimizing the evidence-based management of falls and fractures in patients at risk. DESIGN: 6-month randomised controlled study. METHODS: This population-based study was completed in the Algoma District of Ontario, Canada a geographically vast area with Sault Ste Marie (population 78,000) as its main city. Eligible patients were allocated to an immediate intervention protocol (IP) group, or a delayed intervention protocol (DP) group. The DP group received usual care for 6 months and then was crossed over to receive the interventions. Components of the intervention were directed at the physicians and their patients and included patient-specific recommendations for osteoporosis therapy as outlined by the clinical practice guidelines developed by Osteoporosis Canada, and falls risk assessment and treatment. Two primary outcomes were measured including implementation of appropriate osteoporosis and falls risk management. Secondary outcomes included quality of life and the number of falls, fractures, and hospital admissions over a twelve-month period. The patient is the unit of allocation and analysis. Analyses will be performed on an intention to treat basis. DISCUSSION: This paper outlines the protocol for a pragmatic randomised trial of a multi-faceted, community-based intervention to optimize the implementation of evidence based management for patients at risk for falls and osteoporosis. TRIAL REGISTRATION: This trial has been registered with clinicaltrials.gov (ID: NCT00465387).

Obturator internus muscle abscess in children.

The authors describe four cases of obturator internus muscle (OIM) abscess in children, including their clinical presentations and treatment. This was a retrospective chart review. Children and adolescents younger than 18 years discharged between July 1, 1985, and September 30, 1998, from Brenner Children's Hospital with the diagnosis of muscle abscess or pelvic abscess were identified. A total of 56 patients were identified with the diagnosis of muscle abscess or pelvic abscess. OIM abscess was defined by radiologic findings of an inflammatory process with fluid collection in the OIM, along with the clinical findings suggestive of an OIM abscess. Four of the patients met the definition of OIM muscle abscess. The common presenting features were fever, limp, and hip pain. Computed tomography or magnetic resonance imaging was diagnostic in all four patients, and Staphylococcus aureus was the causative agent in each. All the patients recovered, one after surgical drainage and the other three after antimicrobial therapy alone or with needle aspiration. The presentation of OIM pyomyositis is similar to that of psoas muscle pyomyositis and other infectious processes of the pelvis and hip. The S. aureus is the most common etiologic agent but not the only one reported. Most patients can be managed without open surgical drainage, but needle aspirations may be helpful both therapeutically and diagnostically.

Impact of different definitions on estimates of accuracy of the diagnosis data in a clinical database.

Computerized medical databases are increasingly used for research. The influence of different definitions of the accuracy of matching on the estimated accuracy of diagnosis data was assessed in a database of visits to a public pediatric clinic. Differences between definitions involved 1) unit of analysis, 2) number of diagnoses required to match per visit, and/or 3) whether database contents are required to match the medical record or medical record contents are required to be matched in the database. Overall, 90% of diagnoses in the database (391/435) were accurately coded relative to the medical record. Alternatively, 77% of diagnoses listed in the medical record (391/506) were accurately coded in the database. When individual visits were used as the unit of analysis, estimates of accuracy using six definitions ranged from 65% to 92%. The most appropriate definition to use for estimating accuracy of diagnosis data likely depends on the purpose of the study. Use of two or more such definitions may enhance portrayal of the accuracy of diagnosis data.

Vertical transmission of multidrug-resistant human immunodeficiency virus type 1 (HIV-1) and continued evolution of drug resistance in an HIV-1-infected infant.

To confirm the vertical transmission of multidrug-resistant (MDR) human immunodeficiency virus type 1 (HIV-1) and to assess its impact on further evolution of drug-resistant virus in an infant, proviral DNA amplified from infected peripheral blood mononuclear cell cultures was sequenced to identify reverse transcriptase (RT) and protease (PR) mutations. The infant had proviral DNA with evidence of RT mutations (M41L, L74V, and T215Y) and 3 PR substitutions (K20R, M36I, and V82A). After delivery, the mother's proviral DNA had the same substitutions. Phylogenetic analyses of these HIV-1 RT and PR sequences indicated epidemiological linkage. Plasma drug susceptibility was determined by using a recombinant virus assay. Plasma HIV-1 obtained after the infant's birth demonstrated reduced susceptibility to zidovudine and ritonavir. Thus, vertical transmission of MDR HIV-1 was demonstrated in the setting of detectable maternal plasma viremia. Further accumulation of broad MDR in the infant's virus to 3 antiretroviral classes occurred, despite postnatal therapy.

Synthesis of oligopyridines and their metal complexes as precursors to topological stereoisomers.

[structure: see text]. Palladium-based carbon-carbon coupling reactions in sequence with halogen-exchange chemistry on a series of heterocycles lead to an efficient synthetic strategy for oligopyridines that bind metal ions such as ruthenium to form coordination racks. The particular structures are designed to form terpyridine subunits for octahedral binding. Reaction of 4,6-diiodopyrimidine produces a "double-bay" terpyridine from which binuclear coordination complexes have been formed.

Gender differences in risk behaviors among adolescents who experience date fighting.

OBJECTIVE: Although dating violence frequently begins during adolescence, few studies have focused on date fighting in middle and high school students. Fewer studies have studied gender differences in date violence. This study examines whether gender-specific patterns of risk behaviors exist among adolescents who report date fighting. DESIGN: The study was conducted on data collected from 21 297 students in grades 8 through 12 participating in the Vermont 1995 Youth Risk Behavior Survey. Data were analyzed on 20 724 students (females = 50.1%) who reported: 1) never having been involved in a physical fight (n = 8737); 2) that their last physical fight was with a girlfriend, boyfriend, or other dating partner (n = 432); and 3) that their last fight was with someone other than a dating partner (n = 11 555). Indicators of violence (weapon carrying, being threatened, and fighting), suicide attempts, substance use, sexual behavior, and pregnancy were analyzed with chi(2) tests. Significant variables were analyzed with stepwise logistic regression. RESULTS: Of the males, 1.8% and of the females, 4.2% reported that their last fight was with a boyfriend, girlfriend, or dating partner. Risk behaviors significantly associated among females who only experienced date fighting included the number of male sexual partners in the past 3 months (adjusted odds ratio: 1. 48; 95% confidence interval: 1.26-1.74), number of suicide attempts in the past 12 months (1.55; 1.30-1.85), riding in a car with a drinking driver (1.23; 1.10-1.37), injection of illegal drugs (2.87; 1.10-7.50), use of alcohol before last sexual encounter (1.53; 1. 27-1.86), number of pregnancies (1.66; 1.26-2.21), forced sex (2.92; 2.18-3.91), and inhalant use (1.19; 1.06-1.34). Risk behaviors significantly associated among males who experienced only date fighting were sexual activity (4.11; 2.24-7.53), number of male partners in the past 3 months (1.40; 1.12-1.75), number of times of getting someone pregnant (1.68; 1.17-2.40), experiencing forced sex (2.38; 1.11-5.13), and the number of times threatened with physical violence in past 12 months (1.82; 1.53-2.17). When compared with adolescents who reported fighting with someone other than a date, risk factors significantly associated with date fighting among females were the number of male sexual partners in the past 3 months (1.21; 1.10-1.34), older age (1.21; 1.10-1.34), carrying a weapon in the past 30 days (.77;.66-.90), experiencing forced sex (1.70; 1. 30-2.22), condom non-use (1.96; 1.60-2.41), and number of times of being threatened with physical violence in past 12 months (1.11; 1. 01-1.22). The risk factors among males were the number of male sexual partners in the past 3 months (1.43; 1.28-1.60), experiencing forced sex (1.91; 1.02-3.60), and older age (1.34; 1.14-1.57). CONCLUSIONS: The patterns of risk behaviors differed among male and female adolescents reporting dating violence. Females who reported date fighting were more likely than were nonfighters to have attempted suicide, to engage in sexual and human immunodeficiency virus risk behaviors (use of injectable drugs), to have been pregnant, experienced forced sex, and to have ridden in a car with a drinking driver. Sexual behaviors, including same-gender sexual partners, forced sex, and having been threatened with physical violence, were associated with date fighting among males. These findings are important in screening adolescents at risk for date violence.date fighting, adolescence, risk behaviors, gender.