Atomic discreteness and the nature of structural equilibrium in conductance histograms of electromigrated Cu-nanocontacts

We investigate the histograms of conductance values obtained during controlled electromigration thinning of Cu thin films. We focus on the question whether the most frequently observed conductance values, apparent as peaks in conductance histograms, can be attributed to the atomic structure of the wire. To this end we calculate the Fourier transform of the conductance histograms. We find all the frequencies matching the highly symmetric crystallographic directions of fcc-Cu. In addition, there are other frequencies explainable by oxidation and possibly formation of hcp-Cu. With these structures we can explain all peaks occurring in the Fourier transform within the relevant range. The results remain the same if only a third of the samples are included. By comparing our results to the ones available in the literature on work-hardened nanowires we find indications that even at low temperatures of the environment, metallic nanocontacts could show enhanced electromigration at low current densities due to defects enhancing electron scattering

Near-equilibrium measurement of quantum size effects using Kelvin probe force microscopy

In nano-structures such as thin films electron confinement results in the quantization of energy levels in the direction perpendicular to the film. The discretization of the energy levels leads to the oscillatory dependence of many properties on the film thickness due to quantum size effects. Pb on Si(111) is a specially interesting system because a particular relationship between the Pb atomic layer thickness and its Fermi wavelength leads to a periodicity of the oscillation of two atomic layers. Here, we demonstrate how the combination of scanning force microscopy (SFM) and Kelvin probe force microscopy (KPFM) provides a reliable method to monitor the quantum oscillations in the work function of Pb ultra-thin film nano-structures on Si(111). Unlike other techniques, with SFM/KPFM we directly address single Pb islands, determine their height while suppressing the influence of electrostatic forces, and, in addition, simultaneously evaluate their local work function by measurements close to equilibrium, without current-dependent and non-equilibrium effects. Our results evidence even-odd oscillations in the work function as a function of the film thickness that decay linearly with the film thickness, proving that this method provides direct and precise information on the quantum states.Comment: This document is the unedited author's version of a Submitted Work that was subsequently accepted for publication in Nanoscale, copyright Royal Society of Chemistry after peer review. To access the final edited and published work see doi belo

Atomically resolved scanning force studies of vicinal Si(111)

Well-ordered stepped semiconductor surfaces attract intense attention owing to the regular arrangements of their atomic steps that makes them perfect templates for the growth of one- dimensional systems, e.g. nanowires. Here, we report on the atomic structure of the vicinal Si(111) surface with 10 degree miscut investigated by a joint frequency-modulation scanning force microscopy (FM-SFM) and ab initio approach. This popular stepped surface contains 7 x 7-reconstructed terraces oriented along the Si(111) direction, separated by a stepped region. Recently, the atomic structure of this triple step based on scanning tunneling microscopy (STM) images has been subject of debate. Unlike STM, SFM atomic resolution capability arises from chemical bonding of the tip apex with the surface atoms. Thus, for surfaces with a corrugated density of states such as semiconductors, SFM provides complementary information to STM and partially removes the dependency of the topography on the electronic structure. Our FM-SFM images with unprecedented spatial resolution on steps confirm the model based on a (7 7 10) orientation of the surface and reveal structural details of this surface. Two different FM-SFM contrasts together with density functional theory calculations explain the presence of defects, buckling and filling asymmetries on the surface. Our results evidence the important role of charge transfers between adatoms, restatoms, and dimers in the stabilisation of the structure of the vicinal surface

Shell effects and free-electrons in electromigrated oxidized Cu-nanocontacts

Electromigration in interconnects continues to be an important field of study in integrated circuits as the interconnects are planned to shrink in size at comparable pace as the semiconductor functional elements. Through shrinking the interconnects approach the regime where quantum size effects become important. The observation of quantum size and shell effects is usually restricted either to low-temperatures or vacuum conditions or to chemically inert materials such as Au. Here, we show that in electromigrated Cu nanocontacts such effects can be observed at room temperature and room pressure even in the presence of oxidation. Our data provide evidence that the nanocontacts are nearly spherical objects with a triangular-cylindrical symmetry of their electronic wave functions with a stronger free-electron-like character compared to previous results. We do not observe a detrimental effect of oxygen. The presence of shell effects has implications for the technological use of Cu nanocontacts as interconnects in integrated circuits and could lead to the use of electronic wave functions of shells in such interconnects

Real-space imaging of several molecular layers of C60_{60} in the rotational glass phase

C$_{60}$ is a model system to study molecule–surface interactions and phase transitions due to its high symmetry and strong covalent π bonding within the molecule versus weak van-der-Waals coupling between neighboring molecules. In the solid, at room temperature, the molecule rotates and behaves as a sphere. However, the pentagonal and hexagonal atomic arrangement imposes deviations from the spherical symmetry that become important at low temperatures. The orientation of the C$_{60}$ can be viewed to represent classic spins. For geometrical reasons the preferred orientation of neighboring C$_{60}$ cannot be satisfied for all of the neighboring molecules, making C$_{60}$ a model for disordered spin systems with frustration. We study several molecular layers of C$_{60}$ islands on highly oriented pyrolytic graphite using scanning tunneling microscopy at liquid nitrogen temperatures. By imaging several layers we obtain a limited access to the three-dimensional rotational structure of the molecules in an island. We find one rotationally disordered layer between two partially rotationally ordered layers with hexagonal patterns. This exotic pattern shows an example of the local distribution of order and disorder in geometrically frustrated systems. Scanning tunneling spectroscopy data confirms the weak interactions of neighboring molecules

Real-space imaging of several molecular layers of C60 in the rotational glass phase

C60 is a model system to study molecule–surface interactions and phase transitions due to its high symmetry and strong covalent π bonding within the molecule versus weak van-der-Waals coupling between neighboring molecules. In the solid, at room temperature, the molecule rotates and behaves as a sphere. However, the pentagonal and hexagonal atomic arrangement imposes deviations from the spherical symmetry that become important at low temperatures. The orientation of the C60 can be viewed to represent classic spins. For geometrical reasons the preferred orientation of neighboring C60 cannot be satisfied for all of the neighboring molecules, making C60 a model for disordered spin systems with frustration. We study several molecular layers of C60 islands on highly oriented pyrolytic graphite using scanning tunneling microscopy at liquid nitrogen temperatures. By imaging several layers we obtain a limited access to the three-dimensional rotational structure of the molecules in an island. We find one rotationally disordered layer between two partially rotationally ordered layers with hexagonal patterns. This exotic pattern shows an example of the local distribution of order and disorder in geometrically frustrated systems. Scanning tunneling spectroscopy data confirms the weak interactions of neighboring molecules

Maternal and Perinatal Outcomes of Twin Pregnancy in 23 Low- and Middle-Income Countries

Background: Twin pregnancies in low- and middle-income countries (LMICs) pose a high risk to mothers and newborns due to inherent biological risks and scarcity of health resources. We conducted a secondary analysis of the WHO Global Survey dataset to analyze maternal and perinatal outcomes in twin pregnancies and factors associated with perinatal morbidity and mortality in twins.Methods: We examined maternal and neonatal characteristics in twin deliveries in 23 LMICs and conducted multi-level logistic regression to determine the association between twins and adverse maternal and perinatal outcomes.Results: 279,425 mothers gave birth to 276,187 (98.8%) singletons and 6,476 (1.2%) twins. Odds of severe adverse maternal outcomes (death, blood transfusion, ICU admission or hysterectomy) (AOR 1.85, 95% CI 1.60-2.14) and perinatal mortality (AOR 2.46, 95% CI 1.40-4.35) in twin pregnancies were higher, however early neonatal death (AOR 2.50, 95% CI 0.95-6.62) and stillbirth (AOR 1.22, 95% CI 0.58-2.57) did not reach significance. Amongst twins alone, maternal age 15%, born second, preterm birth and low birthweight were associated with perinatal mortality. Marriage and caesarean section were protective.Conclusions: Twin pregnancy is a significant risk factor for maternal and perinatal morbidity and mortality in low- resource settings; maternal risk and access to safe caesarean section may determine safest mode of delivery in LMICs. Improving obstetric care in twin pregnancies, particularly timely access to safe caesarean section, is required to reduce risk to mother and baby.Australian Postgraduate AwardA & A Saw ScholarshipUniv Western Australia, Sch Populat Hlth, Fac Med Dent & Hlth Sci, Perth, WA 6009, AustraliaUNDP UNFPA UNICEF WHO World Bank Special Programm, Dept Reprod Hlth & Res, World Hlth Org, Geneva, SwitzerlandUniversidade Federal de São Paulo, Dept Obstet, São Paulo, BrazilBrazilian Cochrane Ctr, São Paulo, BrazilUniversidade Federal de São Paulo, Dept Obstet, São Paulo, BrazilWeb of Scienc

Effect of Developmentally Adapted Cognitive Processing Therapy for Youth With Symptoms of Posttraumatic Stress Disorder After Childhood Sexual and Physical Abuse

Importance: Despite the high prevalence, evidence-based treatments for abuse-related posttraumatic stress disorder (PTSD) in adolescents have rarely been studied. Objective: To examine whether developmentally adapted cognitive processing therapy (D-CPT) is more effective than a wait-list condition with treatment advice (WL/TA) among adolescents with PTSD related to childhood abuse. Design, Setting, and Participants: This rater-blinded, multicenter, randomized clinical trial (stratified by center) enrolled treatment-seeking adolescents and young adults (aged 14-21 years) with childhood abuse-related PTSD at 3 university outpatient clinics in Germany from July 2013 to June 2015, with the last follow-up interview conducted by May 2016. Of 194 patients, 88 were eligible for randomization. Interventions: Participants received D-CPT or WL/TA. Cognitive processing therapy was enhanced by a motivational and alliance-building phase, by including emotion regulation and consideration of typical developmental tasks, and by higher session frequency in the trauma-focused core CPT phase. In WL/TA, participants received treatment advice with respective recommendations of clinicians and were offered D-CPT after 7 months. Main Outcomes and Measures: All outcomes were assessed before treatment (baseline), approximately 8 weeks after the start of treatment, after the end of treatment (posttreatment), and at the 3-month follow-up. The primary outcome, PTSD symptom severity, was assessed in clinical interview (Clinician-Administered PTSD Scale for Children and Adolescents for DSM-IV [CAPS-CA]). Secondary outcomes were self-reported PTSD severity, depression, borderline symptoms, behavior problems, and dissociation. Results: The 88 participants (75 [85%] female) had a mean age of 18.1 years (95% CI, 17.6-18.6 years). In the intention-to-treat analysis, the 44 participants receiving D-CPT (39 [89%] female) demonstrated greater improvement than the 44 WL/TA participants (36 [82%] female) in terms of PTSD severity (mean CAPS-CA scores, 24.7 [95% CI, 16.6-32.7] vs 47.5 [95% CI, 37.9-57.1]; Hedges g = 0.90). This difference was maintained through the follow-up (mean CAPS-CA scores, 25.9 [95% CI, 16.2-35.6] vs 47.3 [95% CI, 37.8-56.8]; Hedges g = 0.80). Treatment success was greatest during the trauma-focused core phase. The D-CPT participants also showed greater and stable improvement in all secondary outcomes, with between-groups effect sizes ranging from 0.65 to 1.08 at the posttreatment assessment (eg, for borderline symptoms, 14.1 [95% CI, 8.0-20.2] vs 32.0 [95% CI, 23.8-40.2]; Hedges g = 0.91). Conclusions and Relevance: Adolescents and young adults with abuse-related PTSD benefited more from D-CPT than from WL/TA. Treatment success was stable at the follow-up and generalized to borderline symptoms and other comorbidities

Epitaxial Growth of Pentacene on Alkali Halide Surfaces Studied by Kelvin Probe Force Microscopy

In the field of molecular electronics thin films of molecules adsorbed on insulating surfaces are used as the functional building blocks of electronic devices. A control of the structural and electronic properties of the thin films is required for a reliable operating mode of such devices. Here, noncontact atomic force and Kelvin probe force microscopies have been used to investigate the growth and electronic properties of pentacene on KBr(001) and KCl(001) surfaces. Mainly molecular islands of upright standing pentacene are formed, whereas a new phase of tilted molecules appear near step edges on some KBr samples. Local contact potential differences (LCPD) have been studied with both Kelvin experiments and density-functional theory calculations. Large LCPD are found between the substrate and the differently oriented molecules, which may be explained by a partial charge transfer from the pentacene to the surface. The monitoring of the changes of the pentacene islands during dewetting shows that multilayers build up at the expense of monolayers. Moreover, in the Kelvin images, previously unknown line defects appear, which unveil the epitaxial growth of pentacene crystals.Comment: This document is the unedited author's version of a Submitted Work that was subsequently accepted for publication in ACSNano, copyright American Chemical Society after peer review. To access the final edited and published work see doi belo