Publications by authors named "Roberto Longo"

21 Publications

  • Page 1 of 1

The Emotional Impact of the Operator in the Care of Patients With Mental Disorders during the Pandemic: Measure of Interventions on Compassion Fatigue and Burn-Out.

Psychiatr Danub 2021 Sep;33(Suppl 9):108-113

Mental Health Department, ASL TA, Via Piemonte, 51, 72022 Latiano, Italy,

Background: The continuation of the health emergency due to the management of COVID-19 is having a profound effect on all aspects of society, including mental health and physical health. This observational study examined practitioners of psychiatric rehabilitation and therapeutic communities, focusing on the emotional aspects of patient care, in particular the fatigue of compassion, empathy and lack of hope, aspects that could be directly linked to the burnout of health professionals, as found in other similar studies.

Method: In this study, self-administered scale data was collected in 87 healthcare professionals recruited from 3 different psychiatric rehabilitation communities. In particular, we assessed the fatigue of compassion, vicarious trauma, burnout and hope (hopeless), empathy and depressive symptoms in the two months of May and June 2021.

Results: The results obtained after the administration of the following rating scales, ProQOL, BHS, SAVE-9, BDI-II and BEES, showed an overall increase in scores in all professional figures, a significant fatigue of compassion, while the percentage burnout is not present in several groups. The presence of high levels of hope, satisfaction of compassion is indicative of a moderate level of empathy in some professional figures; these high levels can protect workers from the risk of developing work-related stress and depressive symptoms.

Conclusions: The data obtained with this study are not similar to those of previous studies, although they may indicate the importance of factors such as hope, empathy in the care of the patient with psychic disorders in rehabilitation communities, underlining the need for interventions aimed at the emotional management of the care relationship as a tool to improve care and prevent burnout even during times of high stress, such as managing a pandemic.
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September 2021

A Data Fusion Method for Non-Destructive Testing by Means of Artificial Neural Networks.

Sensors (Basel) 2021 Apr 7;21(8). Epub 2021 Apr 7.

ESEO-GSII, 10 Blvd Jean Jeanneteau, 49000 Angers, France.

In the aeronautics sector, aircraft parts are inspected during manufacture, assembly and service, to detect defects eventually present. Defects can be of different types, sizes and orientations, appearing in materials presenting a complex structure. Among the different inspection techniques, Non Destructive Testing (NDT) presents several advantages as they are noninvasive and cost effective. Within the NDT methods, Ultrasonic (US) waves are widely used to detect and characterize defects. However, due the so-called blind zone, they cannot be easily employed for defects close to the surface being inspected. On the other hand, another NDT technique such Eddy Current (EC) can be used only for detecting flaws close to the surface, due to the presence of the EC skin effect. The work presented in this article aims to combine the use of these two NDT methods, exploiting their complementary advantages. To reach this goal, a data fusion method is developed, by using Machine Learning techniques such as Artificial Neural Networks (ANNs). A simulated training database involving simulations of US and EC signals propagating in an Aluminum block in the presence of Side Drill Holes (SDHs) has been implemented, to train the ANNs. Measurements have been then performed on an Aluminum block, presenting tree different SDHs at specific depths. The trained ANNs were used to characterize the different real SDHs, providing an experimental validation. Eventually, particular attention has been addressed to the estimation errors corresponding to each flaw. Experimental results will show that depths and radii estimations error were confined on average within a range of 4%, recording a peak of 11% for the second SDHs.
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http://dx.doi.org/10.3390/s21082598DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8068067PMC
April 2021

Estimation of Heart Rate From Tracheal Sounds Recorded for the Sleep Apnea Syndrome Diagnosis.

IEEE Trans Biomed Eng 2021 10 20;68(10):3039-3047. Epub 2021 Sep 20.

Obstructive sleep apnea is a common sleep disorder with a high prevalence and often accompanied by significant snoring activity. To diagnose this condition, polysomnography is the standard method, where a neck microphone could be added to record tracheal sounds. These can then be used to study the characteristics of breathing, snoring or apnea. In addition cardiac sounds, also present in the acquired data, could be exploited to extract heart rate. The paper presents new algorithms for estimating heart rate from tracheal sounds, especially in very loud snoring environment. The advantage is that it is possible to reduce the number of diagnostic devices, especially for compact home applications. Three algorithms are proposed, based on optimal filtering and cross-correlation. They are tested firstly on one patient presenting significant pathology of apnea syndrome, with a recording of 509 min. Secondly, an extension to a database of 16 patients is proposed (16 hours of recording). When compared to a reference ECG signal, the final results obtained from tracheal sounds reach an accuracy of 81% to 98% and an RMS error from 1.3 to 4.2 bpm, according to the level of snoring and to the considered algorithm.
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http://dx.doi.org/10.1109/TBME.2021.3061734DOI Listing
October 2021

LiS growth on graphene: Impact on the electrochemical performance of Li-S batteries.

J Chem Phys 2020 Jan;152(1):014701

Department of Chemical Engineering, Texas A&M University, College Station, Texas 77843, USA.

Lithium-sulfur batteries show remarkable potential for energy storage applications due to their high-specific capacity and the low cost of active materials, especially sulfur. However, whereas there is a consensus about the use of lithium metal as the negative electrode, there is not a clear and widely accepted architectural design for the positive electrode of sulfur batteries. The difficulties arise when trying to find a balance between high-surface-area architectures and practical utilization of the sulfur content. Intensive understanding of the interfacial mechanisms becomes then crucial to design optimized carbon-hosted sulfur architectures with enhanced electrochemical performance. In this work, we use density functional theory (DFT)-based first principles calculations to describe and characterize the growing mechanisms of LiS active material on graphene, taken as an example of a nonencapsulated carbon host for the positive electrode of Li-S batteries. We first unravel the two growing mechanisms of LiS supported nanostructures, which explain recent experimental findings on real-time monitoring of interfacial deposition of lithium sulfides during discharge, obtained by means of in situ atomic force microscopy. Then, using a combination of mathematical tools and DFT calculations, we obtain the first cycle voltage plot, explaining the three different regions observed that ultimately lead to the formation of high-order polysulfides upon charge. Finally, we show how the different LiS supported nanostructures can be characterized in X-ray photoelectron spectroscopy measurements. Altogether, this work provides useful insights for the rational design of new carbon-hosted sulfur architectures with optimized characteristics for the positive electrode of lithium-sulfur batteries.
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http://dx.doi.org/10.1063/1.5135304DOI Listing
January 2020

Identification and evaluation of cognitive deficits in schizophrenia using "Machine learning".

Psychiatr Danub 2019 Sep;31(Suppl 3):261-264

Città Solidale Società Cooperativa Sociale, Via Anania Lamarina, 75, 72022 Latiano (BR), Italy,

Background: Schizophrenia can be interpreted as a pathology involving the neocortex whose cognitive dysfunctions represent a central and persistent characteristic of the disease, as well as one of the more important symptoms in relation to the impairment of psychosocial functioning and the resulting disabilities. Given the implication of cognitive functions in everyday life, they can better predict the degree of schizophrenia. The study proposes to use Machine Learning techniques to identify the specific cognitive deficits of schizophrenia that mostly characterize the disorder, as well as to develop a predictive system that can diagnose the presence of schizophrenia based on neurocognitive tests.

Background: The study employs a dataset of neurocognitive assessments carried out on 201 people (86 schizophrenic patients and 115 healthy patients) recruited by the Neuroscience Group of the University of Bari "A. Moro". A data analysis process has been carried out, with the aim of selecting the most relevant features as well as to prepare data for training a number of "off-the-shelf" machine learning methods (Decision Tree, Random Forest, Logistic Regression, k-Nearest Neighbor, Neural Network, Support Vector Machine), which have been evaluated in terms of classification accuracy according to stratified 20-fold cross-validation.

Results: Among all variables, 14 were selected as the most influential for the classification problem. The variables with greater influence are related to working memory, executive functions, attention, verbal fluency, memory. The best algorithms turned out to be Support Vector Machine (SVM) and Neural Network, showing an accuracy of 87.8% and 84.8% on a test set.

Conclusions: Machine Learning provides "cheap" and non-invasive methods that potentially enable early intervention with specific rehabilitation interventions. The results suggest the need to integrate a thorough neuropsychological evaluation into the more general diagnostic evaluation of patients with schizophrenia disorder.
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September 2019

Core-Shell Nanocomposites for Improving the Structural Stability of Li-Rich Layered Oxide Cathode Materials for Li-Ion Batteries.

ACS Appl Mater Interfaces 2018 Jun 25;10(22):19226-19234. Epub 2018 May 25.

Department of Materials Science & Engineering , The University of Texas at Dallas , Richardson , Texas 75080 , United States.

The structural stability of Li-rich layered oxide cathode materials is the ultimate frontier to allow the full development of these family of electrode materials. Here, first-principles calculations coupled with cluster expansion are presented to investigate the electrochemical activity of phase-separation, core-shell-structured xLiMnO·(1 - x)LiNiCoMnO nanocomposites. The detrimental surface effects of the core region can be countered by the LiMnO shell, which stabilizes the nanocomposites. The operational voltage windows are accurately determined to avoid the electrochemical activation of the shell and the subsequent structural evolution. In particular, the dependence of the activation voltage with the shell thickness shows that relatively high voltages can still be obtained to meet the energy density needs of Li-ion battery applications. Finally, activation energies of Li migration at the core-shell interface must also be analyzed carefully to avoid the outbreak of a phase transformation, thus making the nanocomposites suitable from a structural viewpoint.
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http://dx.doi.org/10.1021/acsami.8b03898DOI Listing
June 2018

Ab Initio Study on Surface Segregation and Anisotropy of Ni-Rich LiNiCoMnO (NCM) (y ≤ 0.1) Cathodes.

ACS Appl Mater Interfaces 2018 Feb 12;10(7):6673-6680. Epub 2018 Feb 12.

Materials Science & Engineering Department, The University of Texas at Dallas , Richardson, Texas 75080, United States.

Advances in ex situ and in situ (operando) characteristic techniques have unraveled unprecedented atomic details in the electrochemical reaction of Li-ion batteries. To bridge the gap between emerging evidences and practical material development, an elaborate understanding on the electrochemical properties of cathode materials on the atomic scale is urgently needed. In this work, we perform comprehensive first-principle calculations within the density functional theory + U framework on the surface stability, morphology, and elastic anisotropy of Ni-rich LiNiCoMnO (NCM) (y ≤ 0.1) cathode materials, which are strongly related to the emerging evidence in the degradation of Li-ion batteries. On the basis of the surface stability results, the equilibrium particle morphology is obtained, which is mainly determined by the oxygen chemical potential. Ni-rich NCM particles are terminated mostly by the (012) and (001) surfaces for oxygen-poor conditions, whereas the termination corresponds to the (104) and (001) surfaces for oxygen-rich conditions. Besides, Ni surface segregation predominantly occurs on the (100), (110), and (104) nonpolar surfaces, showing a tendency to form a rocksalt NiO domain on the surface because of severe Li-Ni exchange. The observed elastic anisotropy reveals that an uneven deformation is more likely to be formed in the particles synthesized under poor-oxygen conditions, leading to crack generation and propagation. Our findings provide a deep understanding of the surface properties and degradation of Ni-rich NCM particles, thereby proposing possible solution mechanisms to the factors affecting degradation, such as synthesis conditions, coating, or novel nanostructures.
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http://dx.doi.org/10.1021/acsami.7b17424DOI Listing
February 2018

Charge-transfer modified embedded atom method dynamic charge potential for Li-Co-O system.

J Phys Condens Matter 2017 Nov;29(47):475903

Materials Science & Engineering Department, The University of Texas at Dallas, Richardson, TX 75080, United States of America.

To overcome the limitation of conventional fixed charge potential methods for the study of Li-ion battery cathode materials, a dynamic charge potential method, charge-transfer modified embedded atom method (CT-MEAM), has been developed and applied to the Li-Co-O ternary system. The accuracy of the potential has been tested and validated by reproducing a variety of structural and electrochemical properties of LiCoO. A detailed analysis on the local charge distribution confirmed the capability of this potential for dynamic charge modeling. The transferability of the potential is also demonstrated by its reliability in describing Li-rich LiCoO and Li-deficient LiCoO compounds, including their phase stability, equilibrium volume, charge states and cathode voltages. These results demonstrate that the CT-MEAM dynamic charge potential could help to overcome the challenge of modeling complex ternary transition metal oxides. This work can promote molecular dynamics studies of Li ion cathode materials and other important transition metal oxides systems that involve complex electrochemical and catalytic reactions.
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http://dx.doi.org/10.1088/1361-648X/aa9420DOI Listing
November 2017

Computational Study of MoS/HfO Defective Interfaces for Nanometer-Scale Electronics.

ACS Omega 2017 Jun 19;2(6):2827-2834. Epub 2017 Jun 19.

Department of Materials Science & Engineering, The University of Texas at Dallas, 800 W. Campbell Road, Richardson, Texas 75080, United States.

Atomic structures and electronic properties of MoS/HfO defective interfaces are investigated extensively for future field-effect transistor device applications. To mimic the atomic layer deposition growth under ambient conditions, the impact of interfacial oxygen concentration on the MoS/HfO interface electronic structure is examined. Then, the effect on band offsets (BOs) and the thermodynamic stability of those interfaces is investigated and compared with available relevant experimental data. Our results show that the BOs can be modified up to 2 eV by tuning the oxygen content through, for example, the relative partial pressure. Interfaces with hydrogen impurities as well as various structural disorders were also considered, leading to different behaviors, such as n-type doping, or introducing defect states close to the Fermi level because of the formation of hydroxyl groups. Then, our results indicate that for a well-prepared interface the electronic device performance should be better than that of other interfaces, such as III-V/high-κ, because of the absence of interface defect states. However, any unpassivated defects, if present during oxide growth, strongly affect the subsequent electronic properties of the interface. The unique electronic properties of monolayer-to-few-layered transition-metal dichalcogenides and dielectric interfaces are described in detail for the first time, showing the promising interfacial characteristics for future transistor technology.
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http://dx.doi.org/10.1021/acsomega.7b00636DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6641027PMC
June 2017

Understanding the Models of Community Hospital rehabilitation Activity (MoCHA): a mixed-methods study.

BMJ Open 2017 02 27;7(2):e010483. Epub 2017 Feb 27.

The Community Hospitals Association, Ilminster, UK.

Introduction: To understand the variation in performance between community hospitals, our objectives are: to measure the relative performance (cost efficiency) of rehabilitation services in community hospitals; to identify the characteristics of community hospital rehabilitation that optimise performance; to investigate the current impact of community hospital inpatient rehabilitation for older people on secondary care and the potential impact if community hospital rehabilitation was optimised to best practice nationally; to examine the relationship between the configuration of intermediate care and secondary care bed use; and to develop toolkits for commissioners and community hospital providers to optimise performance.

Methods And Analysis: 4 linked studies will be performed. Study 1: cost efficiency modelling will apply econometric techniques to data sets from the National Health Service (NHS) Benchmarking Network surveys of community hospital and intermediate care. This will identify community hospitals' performance and estimate the gap between high and low performers. Analyses will determine the potential impact if the performance of all community hospitals nationally was optimised to best performance, and examine the association between community hospital configuration and secondary care bed use. Study 2: a national community hospital survey gathering detailed cost data and efficiency variables will be performed. Study 3: in-depth case studies of 3 community hospitals, 2 high and 1 low performing, will be undertaken. Case studies will gather routine hospital and local health economy data. Ward culture will be surveyed. Content and delivery of treatment will be observed. Patients and staff will be interviewed. Study 4: co-designed web-based quality improvement toolkits for commissioners and providers will be developed, including indicators of performance and the gap between local and best community hospitals performance.

Ethics And Dissemination: Publications will be in peer-reviewed journals, reports will be distributed through stakeholder organisations. Ethical approval was obtained from the Bradford Research Ethics Committee (reference: 15/YH/0062).
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http://dx.doi.org/10.1136/bmjopen-2015-010483DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5337721PMC
February 2017

Nanopatterning of Group V Elements for Tailoring the Electronic Properties of Semiconductors by Monolayer Doping.

ACS Appl Mater Interfaces 2017 Jan 5;9(2):1922-1928. Epub 2017 Jan 5.

Department of Materials Science & Engineering, The University of Texas at Dallas , Richardson, Texas 75080, United States.

Control of the electronic properties of semiconductors is primarily achieved through doping. While scaling down the device dimensions to the molecular regime presents an increasing number of difficulties, doping control at the nanoscale is still regarded as one of the major challenges of the electronic industry. Within this context, new techniques such as monolayer doping (MLD) represent a substantial improvement toward surface doping with atomic and specific doping dose control at the nanoscale. Our previous work has explained in detail the atomistic mechanism behind MLD by means of density-functional theory calculations (Chem. Mater. 2016, 28, 1975). Here, we address the key questions that will ultimately allow one to optimize the scalability of the MLD process. First, we show that dopant coverage control cannot be achieved by simultaneous reaction of several group V elements, but stepwise reactions make it possible. Second, using ab initio molecular dynamics, we investigate the thermal decomposition of the molecular precursors, together with the stability of the corresponding binary and ternary dopant oxides, prior to the dopant diffusion into the semiconductor surface. Finally, the effect of the coverage and type of dopant on the electronic properties of the semiconductor is also analyzed. Furthermore, the atomistic characterization of the MLD process raises unexpected questions regarding possible crystal damage effects by dopant exchange with the semiconductor ions or the final distribution of the doping impurities within the crystal structure. By combining all our results, optimization recipes to create ultrashallow doped junctions at the nanoscale are finally proposed.
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http://dx.doi.org/10.1021/acsami.6b13276DOI Listing
January 2017

Electronic properties of MoS/MoO interfaces: Implications in Tunnel Field Effect Transistors and Hole Contacts.

Sci Rep 2016 Sep 26;6:33562. Epub 2016 Sep 26.

Department of Materials Science &Engineering, The University of Texas at Dallas, Richardson, TX 75080 USA.

In an electronic device based on two dimensional (2D) transitional metal dichalcogenides (TMDs), finding a low resistance metal contact is critical in order to achieve the desired performance. However, due to the unusual Fermi level pinning in metal/2D TMD interface, the performance is limited. Here, we investigate the electronic properties of TMDs and transition metal oxide (TMO) interfaces (MoS/MoO) using density functional theory (DFT). Our results demonstrate that, due to the large work function of MoO and the relative band alignment with MoS, together with small energy gap, the MoS/MoO interface is a good candidate for a tunnel field effect (TFET)-type device. Moreover, if the interface is not stoichiometric because of the presence of oxygen vacancies in MoO, the heterostructure is more suitable for p-type (hole) contacts, exhibiting an Ohmic electrical behavior as experimentally demonstrated for different TMO/TMD interfaces. Our results reveal that the defect state induced by an oxygen vacancy in the MoO aligns with the valance band of MoS, showing an insignificant impact on the band gap of the TMD. This result highlights the role of oxygen vacancies in oxides on facilitating appropriate contacts at the MoS and MoO (x < 3) interface, which consistently explains the available experimental observations.
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http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5035990PMC
http://dx.doi.org/10.1038/srep33562DOI Listing
September 2016

Charge Mediated Reversible Metal-Insulator Transition in Monolayer MoTe2 and WxMo1-xTe2 Alloy.

ACS Nano 2016 08 22;10(8):7370-5. Epub 2016 Jul 22.

Department of Materials Science and Engineering, The University of Texas at Dallas , Richardson, Texas 75080, United States.

Metal-insulator transitions in low-dimensional materials under ambient conditions are rare and worth pursuing due to their intriguing physics and rich device applications. Monolayer MoTe2 and WTe2 are distinguished from other TMDs by the existence of an exceptional semimetallic distorted octahedral structure (T') with a quite small energy difference from the semiconducting H phase. In the process of transition metal alloying, an equal stability point of the H and the T' phase is observed in the formation energy diagram of monolayer WxMo1-xTe2. This thermodynamically driven phase transition enables a controlled synthesis of the desired phase (H or T') of monolayer WxMo1-xTe2 using a growth method such as chemical vapor deposition (CVD) and molecular beam epitaxy (MBE). Furthermore, charge mediation, as a more feasible method, is found to make the T' phase more stable than the H phase and induce a phase transition from the H phase (semiconducting) to the T' phase (semimetallic) in monolayer WxMo1-xTe2 alloy. This suggests that a dynamic metal-insulator phase transition can be induced, which can be exploited for rich phase transition applications in two-dimensional nanoelectronics.
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http://dx.doi.org/10.1021/acsnano.6b00148DOI Listing
August 2016

Underwater Acoustic Wavefront Visualization by Scanning Laser Doppler Vibrometer for the Characterization of Focused Ultrasonic Transducers.

Sensors (Basel) 2015 Aug 13;15(8):19925-36. Epub 2015 Aug 13.

Vrije Universiteit Brussel, 1050 Brussels, Belgium.

The analysis of acoustic wave fields is important for a large number of engineering designs, communication and health-related reasons. The visualization of wavefronts gives valuable information about the type of transducers and excitation signals more suitable for the test itself. This article is dedicated to the development of a fast procedure for acoustic fields visualization in underwater conditions, by means of laser Doppler vibrometer measurements. The ultrasonic probe is a focused transducer excited by a chirp signal. The scope of this work is to evaluate experimentally the properties of the sound beam in order to get reliable information about the transducer itself to be used in many kinds of engineering tests and transducer design.
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http://dx.doi.org/10.3390/s150819925DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4570403PMC
August 2015

Carbonation of wollastonite(001) competing hydration: microscopic insights from ion spectroscopy and density functional theory.

ACS Appl Mater Interfaces 2015 Mar 18;7(8):4706-12. Epub 2015 Feb 18.

Department of Materials Science and Engineering, University of Texas at Dallas , 800 West Campbell Road, Richardson, Texas 75080, United States.

In this paper, we report about the influence of the chemical potential of water on the carbonation reaction of wollastonite (CaSiO3) as a model surface of cement and concrete. Total energy calculations based on density functional theory combined with kinetic barrier predictions based on nudge elastic band method show that the exposure of the water-free wollastonite surface to CO2 results in a barrier-less carbonation. CO2 reacts with the surface oxygen and forms carbonate (CO3(2-)) complexes together with a major reconstruction of the surface. The reaction comes to a standstill after one carbonate monolayer has been formed. In case one water monolayer is covering the wollastonite surface, the carbonation is no more barrier-less, yet ending in a localized monolayer. Covered with multilayers of water, the thermodynamic ground state of the wollastonite completely changes due to a metal-proton exchange reaction (also called early stage hydration) and Ca(2+) ions are partially removed from solid phase into the H2O/wollastonite interface. Mobile Ca(2+) reacts again with CO2 and forms carbonate complexes, ending in a delocalized layer. By means of high-resolution time-of-flight secondary-ion mass spectrometry images, we confirm that hydration can lead to a partially delocalization of Ca(2+) ions on wollastonite surfaces. Finally, we evaluate the impact of our model surface results by the meaning of low-energy ion-scattering spectroscopy combined with careful discussion about the competing reactions of carbonation vs hydration.
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http://dx.doi.org/10.1021/am508313gDOI Listing
March 2015

Impact of intrinsic atomic defects on the electronic structure of MoS2 monolayers.

Nanotechnology 2014 Sep 27;25(37):375703. Epub 2014 Aug 27.

Department of Materials Science & Engineering, The University of Texas at Dallas, Richardson, Texas 75080, USA.

Monolayer MoS2 is a direct band gap semiconductor which has been recently investigated for low-power field effect transistors. The initial studies have shown promising performance, including a high on/off current ratio and carrier mobility with a high-κ gate dielectric. However, the performance of these devices strongly depends on the crystalline quality and defect morphology of the monolayers. In order to obtain a detailed understanding of the MoS2 electronic device properties, we examine possible defect structures and their impact on the MoS2 monolayer electronic properties, using density functional theory in combination with scanning tunneling microscopy to identify the nature of the most likely defects. Quantitative understanding based on a detailed knowledge of the atomic and electronic structures will facilitate the search of suitable defect passivation techniques. Our results show that S adatoms are the most energetically favorable type of defect and that S vacancies are energetically more favorable than Mo vacancies. This approach may be extended to other transition-metal dichalcogenides (TMDs), thus providing useful insights to optimize TMD-based electronic devices.
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http://dx.doi.org/10.1088/0957-4484/25/37/375703DOI Listing
September 2014

A density-functional theory study of tip electronic structures in scanning tunneling microscopy.

Nanotechnology 2013 Mar 15;24(10):105201. Epub 2013 Feb 15.

Department of Materials Science and Engineering, The University of Texas at Dallas, 800 West Campbell Road, Richardson, TX 75080, USA.

In this work, we report a detailed analysis of the atomic and electronic structures of transition metal scanning tunneling microscopy tips: Rh, Pd, W, Ir, and Pt pyramidal models, and transition metal (TM) atom tips supported on the W surface, by means of ab initio density-functional theory methods. The d electrons of the apex atoms of the TM tips (Rh, Pd, W, Ir, and Pt tetrahedral structures) show different behaviors near the Fermi level and, especially for the W tip, dz(2) states are shown to be predominant near the Fermi level. The electronic structures of larger pyramidal TM tip structures with a single apex atom are also reported. Their obtained density of states are thoroughly discussed in terms of the different d-electron occupations of the TM tips.
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http://dx.doi.org/10.1088/0957-4484/24/10/105201DOI Listing
March 2013

Probing heterogeneity of cortical bone with ultrasound axial transmission.

IEEE Trans Ultrason Ferroelectr Freq Control 2013 Jan;60(1):187-93

Laboratoire d’Imagerie Parametrique, UMR 7623, Universite Pierre et Marie Curie-Paris VI, Paris, France.

In clinical examination of long cortical bones based on ultrasound axial transmission, the parameter currently used as indicator of bone fragility is the velocity of the first arriving signal (VFAS). VFAS is inherently related to the material properties of the bone site. However, experimental uncertainties may hide the true sensitivity of VFAS to elastic characteristics of bone. Measurements are performed with a multi-element compact array placed in contact with the bone. Therefore, VFAS measurements may be biased by variability imposed by geometrical irregularities of the sample below the probe and/or by probe misalignment. In this paper, we test the assumption that despite experimental errors, VFAS variations resulting from material properties can be measured. The methodology was to compare VFAS and velocities of compression bulk waves (VBWs) on carefully matched sites around the circumference of a test sample (bovine femur). VBW was mapped on bone cross-sections using a through-transmission technique. VBW and VFAS were highly correlated [R² = 0.80, root mean square error = 23 m·s⁻¹, p < 10⁻⁵] and the slope of the linear regression was close to 1 except in a part of the circumference with a pronounced curvature. In measurements performed with the same protocol as for clinical measurements, regions with different material properties (reflected by VBW) could be identified. This work demonstrates that within-specimen variations of material properties can be assessed with a technique available for in vivo measurements.
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http://dx.doi.org/10.1109/tuffc.2013.2549DOI Listing
January 2013

Activation of surface hydroxyl groups by modification of H-terminated Si(111) surfaces.

J Am Chem Soc 2012 May 15;134(21):8869-74. Epub 2012 May 15.

Department of Materials Science and Engineering, University of Texas at Dallas, 800 West Campbell Road, Richardson, Texas 75080, USA.

Chemical functionalization of semiconductor surfaces, particularly silicon oxide, has enabled many technologically important applications (e.g., sensing, photovoltaics, and catalysis). For such processes, hydroxyl groups terminating the oxide surface constitute the primary reaction sites. However, their reactivity is often poor, hindering technologically important processes, such as surface phosphonation requiring a lengthy postprocessing annealing step at 140 °C with poor control of the bonding geometry. Using a novel oxide-free surface featuring a well-defined nanopatterned OH coverage, we demonstrate that hydroxyl groups on oxide-free silicon are more reactive than on silicon oxide. On this model surface, we show that a perfectly ordered layer of monodentate phosphonic acid molecules is chemically grafted at room temperature, and explain why it remains completely stable in aqueous environments, in contrast to phosphonates grafted on silicon oxides. This fundamental understanding of chemical activity and surface stability suggests new directions to functionalize silicon for sensors, photovoltaic devices, and nanoelectronics.
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http://dx.doi.org/10.1021/ja300270wDOI Listing
May 2012

Understanding the preferential adsorption of CO2 over N2 in a flexible metal-organic framework.

J Am Chem Soc 2011 Aug 21;133(32):12849-57. Epub 2011 Jul 21.

Department of Materials Science and Engineering, University of Texas at Dallas, Richardson, Texas 75080, United States.

The unusual uptake behavior and preferential adsorption of CO(2) over N(2) are investigated in a flexible metal-organic framework system, Zn(2)(bdc)(2)(bpee), where bpdc = 4,4'-biphenyl dicarboxylate and bpee = 1,2-bis(4-pyridyl)ethylene, using Raman and IR spectroscopy. The results indicate that the interaction of CO(2) with the framework induces a twisting of one of its ligands, which is possible because of the type of connectivity of the carboxylate end group of the ligand to the metal center and the specific interaction of CO(2) with the framework. The flexibility of the bpee pillars allows the structure to respond to the twisting, fostering the adsorption of more CO(2). DFT calculations support the qualitative picture derived from the experimental analysis. The adsorption sites at higher loading have been identified using a modified van der Waals-Density Functional Theory method, showing that the more energetically favorable positions for the CO(2) molecules are closer to the C═C bond of the bpee and the C-C bond of the bpdc ligands instead of the benzene and pyridine rings of these ligands. These findings are consistent with changes observed using Raman spectroscopy, which is useful for detecting both specific guest-host interactions and structural changes in the framework.
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http://dx.doi.org/10.1021/ja2051149DOI Listing
August 2011

A method for crack sizing using Laser Doppler Vibrometer measurements of Surface Acoustic Waves.

Ultrasonics 2010 Jan 9;50(1):76-80. Epub 2009 Aug 9.

Department of Mechanical Engineering, Vrije Universiteit Brussel (VUB), Pleinlaan 2, 1050 Brussels, Belgium.

The goal of non-destructive testing (NDT) is to determine the position and size of structural defects, in order to measure the quality and evaluate the safety of building materials. Most NDT techniques are rather complex, however, requiring specialized knowledge. In this article, we introduce an experimental method for crack detection that uses Surface Acoustic Waves (SAWs) and optical measurements. The method is tested on a steel beam engraved with slots of known depth. A simple model to determine the cracks size is also proposed. At the end of the article, we describe a possible application: fatigue crack sizing on a damaged slat track. This technique represents a first step toward a better understanding of the crack growth, especially in its early stages (preferably when the cracks can still be repaired) and when it is possible to assume a linear propagation of the crack front. The ultimate goal of this research program is to develop a useful method of monitoring aircraft components during fatigue testing.
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http://dx.doi.org/10.1016/j.ultras.2009.08.001DOI Listing
January 2010
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