First Principles Calculations of Nanomaterials for Renewable Energy

Nanostructured materials, including nanoparticles, nanoplatelets and Metal Organic Frameworks are promising platforms for numerous applications in the field of renewable energy. Inorganic, semiconducting nanoparticles and nanoplatelets, for example, are ideal materials for the design of novel photovoltaic devices due to optoelectronic properties that are tunable through modification of their shape, size and composition. Much attention has been drawn to improving efficiency and device performance through altering the character of the nanomaterials, but the discovery of design rules for optimal device performance is still an open question. Due to challenges in controlling experimental techniques on an atomic scale, as well as numerous combinations of size, shape, composition and surface termination, experimental material design in this field is best complemented by accurate atomistic calculations. The latter can help interpret experiments, predict new materials and offer physical understanding. In this dissertation, we take a tour of three classes of nanomaterials that span different dimensionalities and offer different opportunities for renewable energy material design. First, we seek to understand the collective properties of a thin film of cubic lead sulfide nanoparticles that display quantum confinement in all three directions and are used in solar cell devices. We look at the combined effects of temperature and interactions between nanoparticles, and show that at finite temperature, interacting nanoparticles are dynamical dipolar systems with average values of dipole moments and polarizabilities substantially increased with respect to those of the isolated building blocks. We also present a critical discussion of various results reported in the literature for the dipole moments of nanoparticles. This work has important implications for understanding the nature of charge transport through nanoparticle thin films within a solar cell, as it is the interactions and spacing between nanoparticles that govern the charge transfer behavior. Next, we transition from the collective properties of a nanoparticle film to the optoelectronic properties of individual quasi-two dimensional cadmium selenide nanoplatelets. We use this quasi-2D material, quantum confined in only one direction, to develop a general, predictive computational protocol for calculations of 2D materials. Through building up this framework, we provide an understanding of the optical gap of CdSe nanoplatelets, a main experimental observable essential for photovoltaic devices. Our investigation of the optical gap is completed through disentangling the interplay between three main effects: biaxial strain, quantum confinement, and dielectric contrast between the material and its environment. We present the first calculations for these materials based on many-body perturbation theory, of both the fundamental gap and exciton binding energies, validating models that enable further investigation of larger and more complex systems. We discuss a series of models of quasiparticle energies that allow for comparison with previous theoretical predictions and provide the ability to directly probe the three key effects. These models provide a simple method to estimate the gap of complex nanoplatelets, with potential implications for the search of optimal nanomaterials for photovoltaic devices. Finally, we end with a short investigation of a recently synthesized 3D Metal Organic Framework that, while macroscopic in nature, exhibits a nanoporous structure ideal for gas storage, separation and catalysis. In conjunction with experiment, we show that these MOFs exist in an energetically favorable anti-ferromagnetic state, with the ferromagnetic and non-magnetic spin configurations inaccessible due to structural rigidity. Together, our first principles predictions of the optoelectronic properties of nanoparticles, nanoplatelets and Metal Organic Frameworks, along with experimental characterization and synthesis of similar materials, is expected to help guide the search for optimal nanomaterials for renewable energy devices

Determining the Structure–Property Relationships of Quasi-Two-Dimensional Semiconductor Nanoplatelets

We report a theoretical study of CdSe nanoplatelets aimed at identifying the main factors determining their photophysical properties. Using atomic configurations optimized with density functional theory calculations, we computed quasiparticle and exciton binding energies of nanoplatelets with two to seven monolayers. We employed many body perturbation theory at the GW level and solved the Bethe-Salpeter equation to obtain absorption spectra and excitonic properties. Our results, which agree well with recent experiments, were then used to design a model that allows us to disentangle the effects of quantum confinement, strain induced by passivating ligands, and dielectric environment on the electronic properties of nanoplatelets. We found that, for the model to accurately reproduce our first principle results, it is critical to account for surface stress and consider a finite potential barrier and energy-dependent effective masses when describing quantum confinement. Our findings call into question previous assumptions on the validity of an infinite barrier to describe carrier confinement in nanoplatelets, suggesting that it may be possible to optimize interfacial charge transfer and extraction by appropriately choosing passivating ligands. The model developed here is generalizable to core–shell platelets and enables the description of system sizes not yet directly treatable by first-principles calculations. © 2021 American Chemical SocietyISSN:1932-7455ISSN:1932-744

Emergent Electronic and Dielectric Properties of Interacting Nanoparticles at Finite Temperature

Lead chalcogenide nanoparticle solids have been successfully integrated into certified solar cells and represent promising platforms for the design of novel photoabsorbers for photoelectrochemical cells. While much attention has been drawn to improving efficiency and device performance through altering the character of the individual nanoparticles, the role of interactions between nanoparticles is not yet well-understood. Using first-principles molecular dynamics and electronic structure calculations, we investigated the combined effect of temperature and interaction on functionalized lead chalcogenide nanoparticles (NPs). Here, we show that at finite temperature, interacting NPs are dynamical dipolar systems, with the average values of dipole moments and polarizabilities substantially increased with respect to those of the isolated building blocks. In addition, we show that the interacting NPs exhibit slightly smaller fundamental gaps that decrease as a function of temperature and that the radiative lifetimes of both the isolated NPs and the solids are greatly reduced at finite temperature compared to <i>T</i> = 0. Finally, we present a critical discussion of various results reported in the literature for the values of dipole moments of nanoparticles

Incorporation of Pyrazine and Bipyridine Linkers with High-Spin Fe(II) and Co(II) in a Metal–Organic Framework

A series of isoreticular metal–organic frameworks (MOFs) of the formula M­(BDC)­(L) (M = Fe­(II) or Co­(II), BDC = 1,4-benzenedicarboxylate, L = pyrazine (pyz) or 4,4′-bipyridine (bipy)) has been synthesized and characterized by N₂ gas uptake measurements, single crystal and powder X-ray diffraction, magnetometry, X-ray absorption spectroscopy, and Mössbauer spectroscopy. These studies indicate the formation of a permanently porous solid with high-spin Fe­(II) and Co­(II) centers that are weakly coupled, consistent with first-principles density functional theory calculations. This family of materials represents unusual examples of paramagnetic metal centers coordinated by linkers capable of mediating magnetic or electronic coupling in a porous framework. While only weak interactions are observed, the rigid 3D framework of the MOF dramatically impacts the properties of these materials when compared with close structural analogues

Incorporation of Pyrazine and Bipyridine Linkers with High-Spin Fe(II) and Co(II) in a Metal–Organic Framework

A series of isoreticular metal–organic frameworks (MOFs) of the formula M­(BDC)­(L) (M = Fe­(II) or Co­(II), BDC = 1,4-benzenedicarboxylate, L = pyrazine (pyz) or 4,4′-bipyridine (bipy)) has been synthesized and characterized by N₂ gas uptake measurements, single crystal and powder X-ray diffraction, magnetometry, X-ray absorption spectroscopy, and Mössbauer spectroscopy. These studies indicate the formation of a permanently porous solid with high-spin Fe­(II) and Co­(II) centers that are weakly coupled, consistent with first-principles density functional theory calculations. This family of materials represents unusual examples of paramagnetic metal centers coordinated by linkers capable of mediating magnetic or electronic coupling in a porous framework. While only weak interactions are observed, the rigid 3D framework of the MOF dramatically impacts the properties of these materials when compared with close structural analogues

Comida e Direitos Humanos no Presídio Central de Porto Alegre Food and Human Rights in the Central Prison in Porto Alegre

Este trabalho busca determinar a realidade da preparação, distribuição e quantidade da comida oferecida no Presídio Central de Porto Alegre, RS, aos detentos. Ele foi elaborado utilizando-se metodologia qualitativa, a partir de observações in loco, e entrevistas realizadas no mês de fevereiro de 2010 com presos, policiais e técnicos que atuam na cozinha dessa prisão. Percebeu-se que os presos gostam da comida recebida e não reclamam de questões relativas à higiene ou qualidade. Assim, ainda que a comida não seja produzida em obediência a padrões mínimos de higiene, as críticas nesse sentido provêm apenas por parte de visitantes (políticos, juízes, promotores, defensores de Direitos Humanos), que pertencem a outra classe social e que, portanto, possuem um gosto diverso (Bourdieu). Logo, ainda que os Direitos Humanos não sejam atendidos nessa unidade prisional (em especial no que tange à Superlotação), pode-se afirmar que, no referente à alimentaçâo, o atendimento é satisfatório.<br>This paper seeks to determine the reality of the preparation, distribution and quantity of food offered in the Central Prison of Porto Alegre, Rio Grande do Sul, to the detainees. It was developed using qualitative methodology, from on-site observations and interviews conducted in february 2010 with prisoners, officers and technicians who work in thekitchen of this prison. It was noted that prisoners like the food received and did not complain of issues relating to hygiene or quality. So even though the food is not produced in compliance to minimum standards of hygiene, the criticism in this sense comes only by guests (politicians, judges, prosecutors, and defenders of human rights) that belong to another class and therefore have a different taste (Bourdieu). So, although human rights are not met in this prison unit (especially with regard to over crowding), one can say that, with regard to food, the service is satisfactory

Diving for science - science for diving: volunteer scuba divers support science and conservation in the Mediterranean Sea

Recreational diving engages 20 million people worldwide. Most of the literature refers to tropical destinations but at least 1 million dives per year take place in Mediterranean marine protected areas (MPAs). Divers may negatively affect underwater habitats. However, if effectively engaged, they can contribute to science, territorial management and more sustainable local economies. During 2006\ue2\u80\u932014, volunteers trained by the not-for-profit organization Reef Check Italia (RCI) completed 24 714 observations and 2417 dives in six Mediterranean countries, contributing to a dataset that supports scientific papers about climate change, rare and non-indigenous species (NIS), and informs MPA management decision-making. The wide range of opportunities offered by this dataset is illustrated with two examples relevant to marine conservation in the context of MPA management. They concern: (i) the spread of the NIS Caulerpa cylindracea along the Ligurian coasts, with a focus on Portofino MPA, and (ii) the distribution and abundance of protected species in the Portofino MPA. A diver-focused survey showed that RCI volunteers are highly committed, and that participation in RCI activities has led to a better understanding of, and a sense of stewardship towards, favoured dive sites and the marine world. Knowing who volunteers are, and why they volunteer in their favourite sector, is crucial to designing citizen-science based projects able to achieve their multiple goals. Copyright \uc2\ua9 2016 John Wiley & Sons, Ltd