Challenges in Ceramic Science: A Report from the Workshop on Emerging Research Areas in Ceramic Science

In March 2012, a group of researchers met to discuss emerging topics in ceramic science and to identify grand challenges in the field. By the end of the workshop, the group reached a consensus on eight challenges for the future:—understanding rare events in ceramic microstructures, understanding the phase-like behavior of interfaces, predicting and controlling heterogeneous microstructures with unprecedented functionalities, controlling the properties of oxide electronics, understanding defects in the vicinity of interfaces, controlling ceramics far from equilibrium, accelerating the development of new ceramic materials, and harnessing order within disorder in glasses. This paper reports the outcomes of the workshop and provides descriptions of these challenges

Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems

We present the science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems, targeting an evolution in technology, that might lead to impacts and benefits reaching into most areas of society. This roadmap was developed within the framework of the European Graphene Flagship and outlines the main targets and research areas as best understood at the start of this ambitious project. We provide an overview of the key aspects of graphene and related materials (GRMs), ranging from fundamental research challenges to a variety of applications in a large number of sectors, highlighting the steps necessary to take GRMs from a state of raw potential to a point where they might revolutionize multiple industries. We also define an extensive list of acronyms in an effort to standardize the nomenclature in this emerging field.Peer ReviewedPostprint (published version

Atomic structures and properties of oxide interfaces

This thesis uses computational approaches, mainly first-principles methods, to study interfaces in oxide thin films. One of the difficulties in interface studies is the lack of definitive atomistic models, yet they are essential input for any calculations. Here, this problem is tackled by ab initio random structure searching (AIRSS), or more broadly speaking, random structure searching (RSS). The initial work studies the interfaces in vertically aligned nanocomposites (VANs) that consist of CeO₂ pillars embedded in a SrTiO₃ matrix. Enhanced ionic conductivity has been found in these VANs in prior studies, but the role of vertical interfaces is not explained. The initial interface searches are performed with interatomic potentials due to the large size of the interface, followed by refinement first-principles calculations. Based on the obtained structures, it is shown that the majority interfaces are unlikely to directly enhance ionic conductivity. However, a parallel solid-state O¹⁷ NMR study by our collaborators later obtained interface signals that suggest fast ionic conduction. First-principles NMR calculations show the observed signals are not consistent with the majority interface initially studied; instead, they can be assigned to the minority interfaces that are in different orientations. The following work studies the planar interfaces between epitaxial films of CeO₂ and STO substrates. A significant amount of research has been devoted to fluorite-perovskite interfaces since the controversial report of colossal ionic conductivity enhancement in YSZ/STO heterostructures. However, the exact atomic structures of these interfaces are not well understood. AIRSS is used for finding stable CeO₂/STO planar interfaces taking account of different terminations and local stoichiometries. When the STO terminates with a TiO₂ layer, a rock salt structured CeO layer emerges at the interface. On the other hand, with SrO termination, the stable structure contains a partially occupied anion lattice, which gives rise to lateral diffusion of oxygen anions in molecular dynamics simulations. In both cases, the interfaces are found to attract oxygen vacancies, which hinders ionic transport in the perpendicular direction. The subsequent work starts with addressing the perovskite-perovskite interfaces between La₀.₉Ba₀.₁MnO₃ (LBMO) and STO. LBMO is a ferromagnetic insulator with a relatively high ferromagnetic transition temperature, which makes it an ideal material for spintronics applications. However, thin films of LBMO are conductive except when the thickness is less than eight unit cells. This has been attributed to the octahedral proximity effects, as electron microscopy reveals that octahedral tilting in LBMO is suppressed near the interfaces. Whist some experimental observations are successfully accounted for by the first-principles calculations, the predicted tilt angle suppression is much weaker than that observed. By studying the response of octahedral networks to corner perturbations, it is shown that a competing LBMO phase with an alternative tilt configuration is stable as a result of interface coupling.Cambridge Commonwealth, European and International Trust China Scholarship Counci

Miniaturized Silicon Photodetectors

Silicon (Si) technologies provide an excellent platform for the design of microsystems where photonic and microelectronic functionalities are monolithically integrated on the same substrate. In recent years, a variety of passive and active Si photonic devices have been developed, and among them, photodetectors have attracted particular interest from the scientific community. Si photodiodes are typically designed to operate at visible wavelengths, but, unfortunately, their employment in the infrared (IR) range is limited due to the neglectable Si absorption over 1100 nm, even though the use of germanium (Ge) grown on Si has historically allowed operations to be extended up to 1550 nm. In recent years, significant progress has been achieved both by improving the performance of Si-based photodetectors in the visible range and by extending their operation to infrared wavelengths. Near-infrared (NIR) SiGe photodetectors have been demonstrated to have a “zero change” CMOS process flow, while the investigation of new effects and structures has shown that an all-Si approach could be a viable option to construct devices comparable with Ge technology. In addition, the capability to integrate new emerging 2D and 3D materials with Si, together with the capability of manufacturing devices at the nanometric scale, has led to the development of new device families with unexpected performance. Accordingly, this Special Issue of Micromachines seeks to showcase research papers, short communications, and review articles that show the most recent advances in the field of silicon photodetectors and their respective applications

Synthesis and characterization of functional polymeric materials for use in organic photovoltaics

Norbornene-type monomers with pendant oligothiophene donor and perylene diimide acceptor groups were synthesized and polymerized using ring-opening metathesis polymerization (ROMP) to yield donor and acceptor homopolymers. These semiconducting homopolymers were characterized by UV-Vis and fluorescence spectroscopy to determine absorbance maxima, emission and excitation profiles, optical bandgaps, molar absorbtivities, and quantum yields. The electrochemical behavior of the donor and acceptor materials was characterized by cyclic voltammetry to determine the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the organic semiconductors. Donor-acceptor diblock copolymers were synthesized using ROMP. Fluorescence spectroscopy demonstrated increased donor emission quenching with decreasing block length. Random donor-acceptor copolymers demonstrated almost complete quenching of the donor emission, likely due to increased donor-acceptor interfaces for charge transfer. Electron paramagnetic resonance spectroscopy (EPR) confirmed the formation of persistent donor radical cations and acceptor radical anions in the block copolymers. Small-angle X-ray scattering (SAXS) demonstrated bulk microphase separation with domain sizes between 24-28 nm. Furthermore, the formation of crystalline structure within the ordered microdomains was also observed. All of these studies indicate that the designed materials may be useful as the active layer in organic photovoltaic applications. As a route to functional hybrid materials, block copolymers containing a donor-segment and a Lewis-basic oligoethylene glycol segment, for preferential ZnO nanoparticle growth, were synthesized by ROMP. Photophysical and electrochemical characterization demonstrated that the donor electronic properties were maintained upon incorporation into the copolymer. SAXS was used to demonstrate lamellar morphology in bulk films of the symmetric block copolymers. ZnO nanoparticles were synthesized and incorporated into composite thin films with the block copolymers. These composite films demonstrated high photoluminescence quenching, which increased upon thermal annealing, as a result of favorable charge transfer from the photo-excited donor to the ZnO nanoparticles. These studies demonstrate that improved morphology control and self-assembly can increase charge transfer in hybrid materials through increased interfacial area. As an alternative route to directed ZnO nanoparticle growth, a copolymer containing pendant dipicolylamine moieties was synthesized and characterized by photophysical and electrochemical methods.Chemistr

Thermoelectric property studies on nanostructured N-type Si-Ge Bulk Materials

Thesis advisor: Zhifeng RenSiGe alloys are the only proven thermoelectric materials in power generation devices operating above 600 °C and up to 1000 °C in heat conversion into electricity using a radioisotope as the heat source. In addition to radioisotope applications, SiGe thermoelectric materials have many other potential applications, for example, solar thermal to electricity energy conversion and waste heat recovery. However, traditional SiGe alloy material shows low ZT values of about 0.93 at 900 °C, thus, 8% is the highest device efficiency for commercial SiGe thermoelectric devices. Recently, many efforts have been made to enhance the dimensionless thermoelectric figure-of-merit (ZT) of SiGe alloys. Among them, the nano approach has been recognized as an effective mechanism to obtain thermoelectric materials with good performance. In this approach, dense bulk samples with random nanostructures with high interface densities are synthesized through ball milling and a direct current hot press, leading to an enhancement ZT through reduced phonon thermal conductivity. Such a practical technique produced samples of nanostructured p-type dense bulk bismuth antimony telluride with a peak ZT of 1.4 at 1000 °C from either alloy ingot or elemental chunks. However, the generality of this approach has not been demonstrated. Here, we applied the same technique in SiGe system in order to fabricate a nanostructured n-type SiGe alloy with enhanced thermoelectric properties. In this thesis, numerous nanostructured n-type SiGe alloy samples were successfully pressed. The structure of these nanostructured samples was investigated via XRD, EDS, and TEM. It has been confirmed that many nano grains exist in our nanostructured samples.Thesis (PhD) — Boston College, 2009.Submitted to: Boston College. Graduate School of Arts and Sciences.Discipline: Physics