Stereolithography of Semiconductor Silver and Acrylic-Based Nanocomposites.

Polymer nanocomposites (PNCs) attract the attention of researchers and industry because of their potential properties in widespread fields. Specifically, electrically conductive and semiconductor PNCs are gaining interest as promising materials for biomedical, optoelectronic and sensing applications, among others. Here, metallic nanoparticles (NPs) are extensively used as nanoadditives to increase the electrical conductivity of mere acrylic resin. As the in situ formation of metallic NPs within the acrylic matrix is hindered by the solubility of the NP precursors, we propose a method to increase the density of Ag NPs by using different intermediate solvents, allowing preparation of Ag/acrylic resin nanocomposites with improved electrical behaviour. We fabricated 3D structures using stereolithography (SLA) by dissolving different quantities of metal precursor (AgClO4) in methanol and in N,N-dimethylformamide (DMF) and adding these solutions to the acrylic resin. The high density of Ag NPs obtained notably increases the electrical conductivity of the nanocomposites, reaching the semiconductor regime. We analysed the effect of the auxiliary solvents during the printing process and the implications on the mechanical properties and the degree of cure of the fabricated nanocomposites. The good quality of the materials prepared by this method turn these nanocomposites into promising candidates for electronic applications.

Synthesis of Silver Nanocomposites for Stereolithography: In Situ Formation of Nanoparticles.

Additive Manufacturing (AM) offers remarkable advantages in relation to traditional methods used to obtain solid structures, such as the capability to obtain customized complex geometries adapted to individual requirements. The design of novel nanocomposites suitable for AM is an excellent strategy to widen the application field of these techniques. In this work, we report on the fabrication of metal/polymer nanocomposites with enhanced optical/electrical behaviour for stereolithography (SLA). In particular, we analyse the in situ generation of Ag nanoparticles (NPs) from Ag precursors (AgNO3 and AgClO4) within acrylic resins via SLA. Transmission electron microscopy (TEM) analysis confirmed the formation of Ag NPs smaller than 5 nm in all nanocomposites, providing optical activity to the materials. A high density of Ag NPs with a good distribution through the material for the larger concentration of AgClO4 precursor tested was observed, in contrast to the isolated agglomerations found when the precursor amount was reduced to 0.1%. A significant reduction in the electrical resistivity up to four orders of magnitude was found for this material compared to the unfilled resin. However, consumption of part of the photoinitiator in the formation process of the Ag NPs contributed to a reduction in the polymerization degree of the resin and, consequently, degraded the mechanical properties of the nanocomposites. Experiments with longer curing times showed that, for the higher AgClO4 concentrations tested, post-curing times of 300 min allowed an 80% degree of polymerization to be achieved. These conditions turned these materials into promising candidates to obtain solid structures with multifunctional properties.

Inhibition of light emission from the metastable tetragonal phase at low temperatures in island-like films of lead iodide perovskites.

Photonic applications based on halide perovskites, namely CH3NH3PbI3 (MAPbI3), have recently attracted remarkable attention due to the high efficiencies reported for photovoltaic and light emitting devices. Despite these outstanding results, there are many temperature-, laser excitation power-, and morphology-dependent phenomena that require further research to be completely understood. In this work, we have investigated in detail the nature of exciton optical transitions and recombination dynamics below and above the orthorhombic/tetragonal ('O'-/'T'-) temperature phase transition (∼150 K) depending on the material continuity (continuous-like) or discontinuity (island-like) in MAPbI3 films. At low temperatures, continuous thin films of the perovskite can exhibit strain inhomogeneities associated with the formation of different 'T'-defective domains leading to an energy spread of states over more than 200 meV. On the other hand, a single photoluminescence line peak related to the perovskite 'O'-phase (associated with the distortion of the [PbI3]- anion) is observed in the island-like sample that we attribute to strain relaxation for this morphology. Moreover, the predominantly radiative recombination dynamics of the continuous-like sample mainly originates from nongeminate electron-hole formation of excitons in the 'O'-phase and the internal dynamics with carrier trapping levels. This observation is in strong contrast to the free exciton recombination dominantly found in the island-like sample.

Room-temperature Operation of Low-voltage, Non-volatile, Compound-semiconductor Memory Cells.

Whilst the different forms of conventional (charge-based) memories are well suited to their individual roles in computers and other electronic devices, flaws in their properties mean that intensive research into alternative, or emerging, memories continues. In particular, the goal of simultaneously achieving the contradictory requirements of non-volatility and fast, low-voltage (low-energy) switching has proved challenging. Here, we report an oxide-free, floating-gate memory cell based on III-V semiconductor heterostructures with a junctionless channel and non-destructive read of the stored data. Non-volatile data retention of at least 104 s in combination with switching at ≤2.6 V is achieved by use of the extraordinary 2.1 eV conduction band offsets of InAs/AlSb and a triple-barrier resonant tunnelling structure. The combination of low-voltage operation and small capacitance implies intrinsic switching energy per unit area that is 100 and 1000 times smaller than dynamic random access memory and Flash respectively. The device may thus be considered as a new emerging memory with considerable potential.

Consulta por extracción dental en niños nicaragüenses: una aproximación a las necesidades de atención bucal

Objetivo: Identificar los factores asociados a las visitas al dentista por motivo de extracción dental en niños nicaragüenses. Material y Métodos: Se realizó un estudio transversal en una muestra de escolares de 6 a 12 años de edad. Se incluyeron 1400 niños de escuelas públicas, a cuyos padres/tutores se les aplicó un cuestionario para determinar una serie de variables sociodemográficas, socioeconómicas y dentales. La variable dependiente fue la visita al dentista por extracción dental (VDED), la cual fue categorizada como: 0=Sin visita al dentista, 1= VDED, pero no en el último año, 2= VDED en el último año. En el análisis estadístico se emplearon pruebas estadísticas no paramétricas. Resultados: El promedio de edad fue de 9.00±2.00 años y las mujeres representaron el 49.9%. Del total de la muestra, 21.6% acudieron al dentista por extracción dental, de los cuales 9.8% lo hicieron hace más de un año y 11.8% durante al año pasado previo a la encuesta. Las variables que mostraron diferencias significativas (p<0.05) en el análisis estadístico de las VDED fueron la edad, la edad de la madre, y la posición socioeconómica. Conclusiones: Uno de cada cinco niños tuvo VDED. Las visitas al dentista por extracciones dentales podrían utilizarse como un indicador del estado de salud bucal de la población, ya que son una aproximación a las necesidades de salud que presentan.

Objective: To identify factors associated with visits to the dentist for dental extraction in Nicaraguan children. Materials and Methods: This was a cross-sectional study that was carried out in a sample of schoolchildren from 6 to 12 years of age. We included 1400 children from public schools, whose parents / guardians were given a questionnaire to determine a series of sociodemographic, socioeconomic and dental variables. The dependent variable was defined as a dental visit for dental extraction (VDED), which was categorized as: 0 = No visit to the dentist, 1 = VDED, but not in the last year, 2 = VDED in the last year. For statistical analysis, nonparametric statistical tests were used. Results: The average age was 9.00 ± 2.00 years, 49.9% of the total were females. Of the total sample, 21.6% went to the dentist for a dental extraction, of which 9.8% did so more than a year ago and 11.8% had gone during the year prior to the survey. The variables that showed significant differences (p <0.05) in the statistical analysis of the VDED were age of the patient, the age of the mother, and the socioeconomic status. Conclusions: One in five children had VDED. Visits to the dentist for dental extractions could be used as an indicator of the oral health status of the population, since they are an approximation the other health needs present in the population.

Socioeconomic Inequalities in Visits to the Dentist to Receive Professionally Applied Topical Fluoride in a Developing Country.

OBJECTIVE: To determine the frequency and associated factors of visits to the dentist in the last year by Nicaraguan schoolchildren to receive professionally applied topical fluoride (PATF). MATERIAL AND METHODS: A cross-sectional study was designed in children from public schools in the city of León, Nicaragua, were included. A series of socioeconomic, sociodemographic, and behavioural variables were collected through a questionnaire. The dependent variable was the visit to the dentist to receive professionally applied topical fluoride in the last year, which was dichotomised as (0) Did not receive PATF and (1) Yes received PATF. In the statistical analysis, binary logistic regression was used. RESULTS: The mean age of the students included was 9 years, and 49.9% were girls. The prevalence of visits to the dentist in the last year to receive PATF was 3.1%. In the multivariate model, the associated characteristics (p < 0.05) were: female (OR = 2.73, 95% CI = 1.34-4.50); the positive attitude of the mother to the oral health of her child (OR = 2.15, 95% CI = 1.03-4.50); and the best socioeconomic position (OR = 2.68, 95% CI = 1.36-5.31). CONCLUSIONS: The prevalence of visits to the dentist in the last year to receive professionally applied topical fluoride was very low (3.1%). The results of the socioeconomic position suggest the existence of certain inequalities in oral health. It is necessary to implement policies and programs aimed at improving this scenario.

Mapping the plasmonic response of gold nanoparticles embedded in TiO2 thin films.

We present the mapping of the plasmonic properties of gold nanoparticles that are embedded in a TiO2 thin film deposited over two different substrates, glass and silicon. An improved electron energy-loss spectroscopy (EELS) imaging technique was used to extract plasmon maps with nanometre resolution. Several representative cases of randomly dispersed NPs have been examined to carefully evaluate surrounding effects on the optical response of such nanostructured material. Data were compared to analytical calculations and showed good agreement. These results validate previous structural and far-field optical results and provide a clear description of the optical phenomena that take place at a nanometre scale in these materials. They are of primary importance for enlightening the way to the fabrication of thin film materials including metallic nanostructures for photovoltaic applications.

Analysis of electron beam damage of exfoliated MoS2 sheets and quantitative HAADF-STEM imaging.

In this work we examined MoS2 sheets by aberration-corrected scanning transmission electron microscopy (STEM) at three different energies: 80, 120 and 200 kV. Structural damage of the MoS2 sheets has been controlled at 80 kV according a theoretical calculation based on the inelastic scattering of the electrons involved in the interaction electron-matter. The threshold energy for the MoS2 material has been found and experimentally verified in the microscope. At energies higher than the energy threshold we show surface and edge defects produced by the electron beam irradiation. Quantitative analysis at atomic level in the images obtained at 80 kV has been performed using the experimental images and via STEM simulations using SICSTEM software to determine the exact number of MoS22 layers.

Strain analysis for the prediction of the preferential nucleation sites of stacked quantum dots by combination of FEM and APT.

The finite elements method (FEM) is a useful tool for the analysis of the strain state of semiconductor heterostructures. It has been used for the prediction of the nucleation sites of stacked quantum dots (QDs), but often using either simulated data of the atom positions or two-dimensional experimental data, in such a way that it is difficult to assess the validity of the predictions. In this work, we assess the validity of the FEM method for the prediction of stacked QD nucleation sites using three-dimensional experimental data obtained by atom probe tomography (APT). This also allows us to compare the simulation results with the one obtained experimentally. Our analysis demonstrates that FEM and APT constitute a good combination to resolve strain-stress problems of epitaxial semiconductor structures.

Dental plaque, preventive care, and tooth brushing associated with dental caries in primary teeth in schoolchildren ages 6-9 years of Leon, Nicaragua.

BACKGROUND: Our study aimed to evaluate the effect of various risk indicators for dental caries on primary teeth of Nicaraguan children (from Leon, Nicaragua) ages 6 to 9, using the negative binomial regression model. MATERIAL/METHODS: A cross-sectional study was carried out to collect clinical, demographic, socioeconomic, and behavioral data from 794 schoolchildren ages 6 to 9 years, randomly selected from 25 schools in the city of León, Nicaragua. Clinical examinations for dental caries (dmft index) were performed by 2 trained and standardized examiners. Socio-demographic, socioeconomic, and behavioral data were self-reported using questionnaires. Multivariate negative binomial regression (NBR) analysis was used. RESULTS: Mean age was 7.49 ± 1.12 years. Boys accounted for 50.1% of the sample. Mean dmft was 3.54 ± 3.13 and caries prevalence (dmft >0) was 77.6%. In the NBR multivariate model (p<0.05), for each year of age, the expected mean dmft decreased by 7.5%. Brushing teeth at least once a day and having received preventive dental care in the last year before data collection were associated with declines in the expected mean dmft by 19.5% and 69.6%, respectively. Presence of dental plaque increased the expected mean dmft by 395.5%. CONCLUSIONS: The proportion of students with caries in this sample was high. We found associations between dental caries in the primary dentition and dental plaque, brushing teeth at least once a day, and having received preventive dental care. To improve oral health, school programs and/or age-appropriate interventions need to be developed based on the specific profile of caries experience and the associated risk indicators.

Analysis of the 3D distribution of stacked self-assembled quantum dots by electron tomography.

The 3D distribution of self-assembled stacked quantum dots (QDs) is a key parameter to obtain the highest performance in a variety of optoelectronic devices. In this work, we have measured this distribution in 3D using a combined procedure of needle-shaped specimen preparation and electron tomography. We show that conventional 2D measurements of the distribution of QDs are not reliable, and only 3D analysis allows an accurate correlation between the growth design and the structural characteristics.

Compositional analysis with atomic column spatial resolution by 5th-order aberration-corrected scanning transmission electron microscopy.

We show in this article that it is possible to obtain elemental compositional maps and profiles with atomic-column resolution across an InxGa1-xAs multilayer structure from 5th-order aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images. The compositional profiles obtained from the analysis of HAADF-STEM images describe accurately the distribution of In in the studied multilayer in good agreement with Muraki's segregation model [Muraki, K., Fukatsu, S., Shiraki, Y. & Ito, R. (1992). Surface segregation of In atoms during molecular beam epitaxy and its influence on the energy levels in InGaAs/GaAs quantums wells. Appl Phys Lett 61, 557-559].

Effect of InAlGaAs and GaAs combination barrier thickness on the duration of dot formation in different layers of stacked InAs/GaAs quantum dot heterostructure grown by MBE.

Multilayer stacks of quantum dots (QDs) (10 periods) with a combination barrier layer of In0.21Al0.21 Ga0.58As (30 angstroms) and GaAs (70-180 angstroms) are grown by solid source molecular beam epitaxy (MBE) and reflection high-energy electron diffraction (RHEED) has been used for the in situ determination of the duration of dot formation in the QD layers. The increase in the duration of dot formation in the consecutive layers of the QD heterostructure with thinner barrier is attributed to the indium migration towards the defects in the strained QD layers. A thicker GaAs layer at 590 degrees C overgrown on the InAlGaAs is believed to remove the unevenness of the growth front for the subsequent QD layer resulting in good vertical stacking of islands till the final layer of the multilayer heterostructures.

[Socioeconomic inequalities in oral health: factors associated with tooth brushing frequency among Nicaraguan schoolchildren]. / Desigualdades socioeconómicas en salud bucal: factores asociados a la frecuencia de cepillado dental en escolares nicaragüenses.

OBJECTIVE: To identify the association between tooth brushing frequency and variables of socioeconomic position in Nicaraguan schoolchildren. MATERIAL AND METHODS: A cross sectional study was undertaken in 1353 schoolchildren ages 6 to 12 randomly selected from 25 elementary schools in Leon, Nicaragua. Using a questionnaire addressed to mothers, sociodemographic, socioeconomic and behavioral variables were collected. The dependent variable was tooth brushing frequency, which was dichotomized in 0 "at least one 7 times/week" and 1 "7 or more times/week". A multivariate analysis was carried out with logistic regression in STATA 9. RESULTS: The average age of child participants was 8.99 +/- 2.00 years and 49.7% were women. In the final model, older age (OR = 2.04), female sex (OR = 1.39) and having a mother with positive attitudes toward oral health (OR = 2.5) were positively associated with the tooth brushing frequency (p < 0.05). Larger family size (OR = 0.89) and having low socioeconomic status (1st quartile; OR = 0.54, 2nd quartile; OR = 0.62, 3rd quartile; OR = 0.67) showed a negative relationship with the tooth brushing frequency. To have had at least one preventive dental visit in the previous year was positively associated (p < 0.10) with tooth brushing frequency. CONCLUSION: This study suggested that existence of indicators of socioeconomic inequalities exist even within less developed countries, and thus emphasize the need to target health promotion programs to vulnerable socioeconomic groups.

Dental health services utilization and associated factors in children 6 to 12 years old in a low-income country.

OBJECTIVE: To determine the factors associated with the dental health services utilization among children ages 6 to 12 in León, Nicaragua. MATERIAL AND METHODS: A cross-sectional study was carried out in 1,400 schoolchildren. Using a questionnaire, we determined information related to utilization and independent variables in the previous year. Oral health needs were established by means of a dental examination. To identify the independent variables associated with dental health services utilization, two types of multivariate regression models were used, according to the measurement scale of the outcome variable: a) frequency of utilization as (0) none, (1) one, and (2) two or more, analyzed with the ordered logistic regression and b) the type of service utilized as (0) none, (1) preventive services, (2) curative services, and (3) both services, analyzed with the multinomial logistic regression. RESULTS: The proportion of children who received at least one dental service in the 12 months prior to the study was 27.7 percent. The variables associated with utilization in the two models were older age, female sex, more frequent toothbrushing, positive attitude of the mother toward the child's oral health, higher socioeconomic level, and higher oral health needs. CONCLUSION: Various predisposing, enabling, and oral health needs variables were associated with higher dental health services utilization. As in prior reports elsewhere, these results from Nicaragua confirmed that utilization inequalities exist between socioeconomic groups. The multinomial logistic regression model evidenced the association of different variables depending on the type of service used.

[Prevalence of dental caries in 6-12-year-old schoolchildren in Leon, Nicaragua]. / Prevalencia de caries dental en escolares de 6-12 años de edad de León, Nicaragua.

OBJECTIVE: To determine the prevalence of caries, deft and DMFT indices (sum of decay, messing, and filling tooth in primary and permanent dentition), and the Significant Caries Index (SiC) in scholars from Leon, Nicaragua. PATIENTS AND METHOD: Dental data from a representative sample of 1,400 children were collected and analyzed in a cross-sectional study (year 2002). All subjects were clinically evaluated by one of two calibrated and standardized examiners. RESULTS: 28.6% children were caries free in both dentitions. Caries prevalence in primary teeth in 6-years-old children was 72.6% and 45.0% in permanent teeth in 12-years-old children. Mean deft and DMFT were 2.98 +/- 2.93 (n = 1,125) and 0.65 +/- 1.43 (n = 1,379), respectively. The SiC at 12 years of age was 4.12. Children with caries experience in primary teeth were more likely to have caries in permanent teeth (odds ratio = 2.48; 95% confidence interval, 1.66-3.79) than children without caries in primary teeth. CONCLUSION: Low caries prevalence and experience were observed in the permanent dentition, with a substantial proportion of filled teeth. Such favorable finding was not observed in the primary dentition. Levels of caries failed to meet the oral health goal of FDI/WHO for the year 2000 in 6-years-old. However, the levels found in 12-year-olds had already met the goal for 2000. The present findings confirmed prior reports that the caries experience in primary teeth is associated with the experience of caries in permanent teeth.

Pérdida dental y patrones de caries en preescolares de una comunidad suburbana de Campeche / Tooth loss and caries patterns in schoolchildren from a suburban community of Campeche

Objetivos: la caries dental es una enfermedad crónica, infecciosa, multifactorial y transmisible. Es muy prevalente y es la causa principal de pérdida dental durante la infancia. El objetivo del presente estudio fue determinar la prevalencia de pérdida de dientes y los patrones de caries en niños preescolares de una comunidad suburbana de Campeche, México. Material y métodos: se realizó un estudio transversal en 109 preescolares de 5-6 años de edad. Los criterios para diagnosticar caries dental fueron el índice ceod (OMS) y el "criterio de magnitud de la lesión cariosa" (CMLC). Se consideró como pérdida dental a la ausencia total del diente o al grado 4 del CMLC (pérdida mayor a 2/3 parte de la corona clínica). Los patrones de ataque de caries registrados fueron: 1) patrón anterior, 2) patrón posterior y 3) patrón anterior-posterior. Todos los sujetos fueron evaluados clínicamente por dos cirujanos dentistas capacitados y estandarizados (kappa <0.85). El análisis consistió en el cálculo de medidas de frecuencia, de tendencia central y de dispersión, según la escala de medición de las variables. El análisis bivariado se realizó con la prueba de chi2. Resultados: trece (11.9 por ciento) de los 109 sujetos tuvieron 34 (1.7 por ciento) de dientes perdidos de un total de 2041 piezas temporales examinadas. Los dientes perdidos con mayor frecuencia fueron los 2dos molares inferiores. De los sujetos con caries (n=82) 47.6 por ciento presentaron el patrón anterior-posterior, 46.3 por ciento tuvieron patrón posterior y 6.1 por ciento exhibieron patrón anterior. No existieron diferencias sustanciales en relación a la edad y sexo. Conclusiones: la pérdida dental en esta población fue alta. El patrón de mayor prevalencia fue el anterior-posterior

Atención genética perinatal primaria para la prevención de los defectos del nacimiento / Attention of primary perinatal genetics for the prevention of the birth defects

Indicar la importancia del asesoramiento genético para la prevención de los defectos del nacimiento y una reproducción sana, utilizando una población blanco altamente motivada, tal como la madre de alto riesgo para defectos del nacimiento. Selección de madres de alto riesgo en la consulta prenatal, la consulta de niños sanos, el servicio de ultrasonido y el registro de malformados. Hospital Chiquinquirá de Maracaibo y consultas públicas y privadas de la región. En once años, 576 pacientes, se atendieron en la consulta de genética. La ultrasonografía realizada por el genetista, demostró ser mas eficiente (38,23 por ciento) que la realizada en el servicio de ultrasonido (11,1 por ciento),(p>0,001; prueba de Z de una cola). La atención prenatal a la madre es incompleta sin el asesoramiento genético ya que a través del mismo, es posible la selección de un 10 por ciento a un 50 por ciento de embarazos de alto riesgo para defectos fetales cuya prevención de incidencia y recurrencia es posible, con beneficios económicos cuantificables para la nación

Programa preventivo de defectos del nacimiento: incidencia de anencefalia en Maracaibo, Venezuela: período 1993-96 / Birth defects preventive program: incidence of anencephaly in Maracaibo, Venezuela: years 1993-96

La incidencia de anencefalia en el Estado Zulia, y especialmente en la Costa Oriental del Lago de Maracaibo, zona de industrialización petrolera, se ha denunciado alta desde comienzos de los años 80, coincidiendo con la generalización del uso del ultrasonido diagnóstico en la evaluación fetal. Nosotros hemos analizado la incidencia de defectos del nacimiento a través del Programa Preventivo de Defectos del nacimiento, dentro del Hospital Chinquinquira de Maracaibo, basándose en cuatro estrategias mancomunadas: i) análisis de 32.331 ecogramas realizados entre 1993 y 1996, ii) registro de malformaciones congénitas durante el año de 1995, un estudio caso-control, iii) análisis de malformaciones motinatos remitidos al Servicio de Anatomia Patológia derivados de 4232 partos, y iv) registro de 638 madres con embarazo de alto riesgo fetal. Como población de referencia se utilizaron 345 historias del Servicio de Genética Médica y Diagnóstico Prenatal del Hospital Coromoto registradas a partir de 1981. Este enfoque nos ha permitido concluir que la incidencia de anencefalia en el Estado Zulia es de 0,75/1000, significativamente igual a la esperada en la mayoría de las poblaciones