Organometal halide perovskite materials have been studied extensively for photovoltaic applications due to their high power conversion efficiency and compatibility with simple fabrication processes. Despite the theoretical studies and macroscopic electrical characterizations on the electrical and electro-optical properties of the perovskites, a microscopic picture that correlates the chemical composition with electric dipoles in the perovskite solids is still lacking. Herein, we investigate the compositional dependence of electric dipoles in AMX3 (A: organic; M: metal; X: halogen) perovskite structures using modulation electroabsorption (EA) spectroscopy, which measures the change in the reflection of light through a material upon application of a modulated electric field. By sampling various device structures we show that the second harmonic EA spectra reflect the intrinsic, rather than interfacial, properties of the perovskite films. A quantitative analysis of the EA spectra of CH3NH3PbI3, NH2CHNH2PbI3 and CH3NH3Sn0.4Pb0.6I3 is provided to compare the impact of the organic and metal cations on the photoinduced response of dipole moment. Based on the EA results, we propose that the A and M cations could both largely affect the dielectric and dipolar properties of the perovskite materials, but through different mechanisms, such as ionic polarization, rotation of molecular dipoles and charge migration. These processes occur at different time scales and thus result in a frequency-dependent dipole response. We further correlate the dielectric property with charge transport and charge trapping processes in the perovskites using charge modulation spectroscopy, electrochemical impedance spectroscopy and time-resolved THz experiments. Our fundamental spectroscopic measurement together with theoretical calculations can help to improve the understanding on the remarkable electronic properties of the AMX3 perovskites and provide guidelines on the design of perovskite materials with new functionalities.

Robotics can enhance the therapeutic efficacy in cancer therapy beyond what is possible with the manipulation of molecular structures alone which has been the main focus of the pharmaceutical industry. Such robotic enhancement could rely on the implementation of advanced functionalities in each agent designed to transport a therapeutic payload to specific tumor sites that would yield optimized effects. Such functionalities would include navigation capability towards the sites of treatment in order to avoid systemic circulation, self-actuation providing a propelling force sufficient to penetrate the tumor volume beyond the diffusion limits of larger drug molecules, and sensory capability to target regions such as the hypoxic zones that would lead to the best treatment outcomes. Although the field of biomimetics has given rise to new technologies inspired by biological solutions at the macro- and nano-scales, an artificial solution supporting such an embedded capability level is still far beyond present technological feasibility. But methods relying on biology can also be inspired by far-reaching technological concepts. It is shown how such �reverse-biomimetic� approach can be used to implement medical nanorobotic agents having functionalities only envisioned for future medical nanorobots and which are still far beyond present technological feasibility at such a scale.

Molecular packing and morphology of organic semiconductors in the solid state are some of the most crucial yet the most unpredictable parameters controlling the performance of organic electronics. I will describe a new strategy for controlling the co-assembly of p- and n-type organic semiconductors through complementary hydrogen bonding. The dipyrrolopyridine heterocycle is introduced as a p-type (donor) semiconductor component capable of complementary H-bonding with naphthalenediimides (acceptor).[1,2] I will show that the H bonding self-assembly process (i) modulates the charge transfer interactions between the donor and acceptor, (ii) allows for precise control over the heterojunction structure and (ii) leads to a combination of the charge-transport properties of the individual components. These studies provide a foundation for advanced solid state engineering in organic electronics, capitalizing on the complementary H-bonding interactions. [1] H. T. Black, D. F. Perepichka, Angew. Chem. Int. Ed. 2014, 53, 2138-2141 [2] H.T. Black, H. Lin, F. B�langer-Gari�py, D.F. Perepichka, Faraday Discuss. 2014, DOI: 10.1039/C4FD00133H

A new type of Metal-Insulator-Metal (MIM) gas sensor prepared by commonly used semiconductor processing methods is presented. The capacitive device has an extremely simple structure consisting of PECVD-SiO2 and ALD-Al2O3 as dielectric thin films with a thickness in the order of 10 nm sandwiched between two platinum electrodes. At elevated temperatures above 200�C the dielectric films show a distinct polarization mechanism which is attributed to mobile ions that cause a space charge polarization. The adsorption of gas species at the interface of the Pt top electrode and the metal oxides leads to a change in work function or bias offset and thus, a gas-induced change of the impedance of the MIM-structure, sensitive to less than 3 ppm of hydrogen, for example. Based on electrical characterization of these metal-insulator-metal structures using electrical impedance spectroscopy, capacitance-voltage-, and current-voltage-measurements a model is given to explain this unusual gas sensitive permittivity. In particular, this new type of integrated gas sensors show DC bias depending gas selectivity for reducing and oxidizing gases. Furthermore, this new transducer principle can be adapted easily to other gases by choosing appropriate electrode materials.

Current collapse in AlGa(In)N/GaN heterostructure field effect transistors (HFETs) persists as a major reliability constraint in the pathways of widespread realization of III-nitride electronic devices. The phenomenon is marked by the significant decrease in microwave output power in RF PAs and prominent increase in dynamic resistance of power switches. Electrically active states are held responsible for this trap related event, whose capture/emission time governs the corresponding trapping/detrapping mechanisms though the exact origin of the concerned deep levels along with the involved dynamics remains a vastly debated topic. In our earlier studies, transient measurements combined with pulse drive responses of nitride HFETs were analyzed to explore the possibilities of dislocations affecting the current dispersion characteristics. The investigations confirmed that dispersion is strongly correlated with the threading dislocation density (TDD) in the epilayers. However, it was not possible to pinpoint a single mechanism responsible for the trapping during OFF-state quiescent bias though either tunnelling or trap-assisted Frenkel-Poole emission or hopping conduction were the plausible transport mechanisms. Based on previous understandings, in the present study, the analysis was further extended for AlGaN/GaN HFETs fabricated on epistructures with TDDs ~2x108/cm2,~ 7x109/cm2, and ~5x1010/cm2. To replicate practical modes of operations, the quiescent bias point was chosen as VGS in soft-pinch off (~VTh), moderate pinch-off(~VTh-3), or deep pinch-off state(~VTh-6) and VDS varied upto 18 V. Among devices in different wafers, both the current dispersion and the OFF-state gate leakage for identical bias values were found to correspond to the respective TDDs. Also, for devices in the same wafer, the dispersion was dependent upon the reverse bias in a particular pattern. Temperature and field-dependent OFF-state terminal current measurements were assessed for each of these quiescent biases to identify the spatial leakage mechanism at any of the bias states. Next, these devices with TDDs spanning three orders were subjected to step stress bias (without the application of storage thermal stress) upto permanent device degradation featuring large irreversible increase in both RON and current compression as well as OFF-state leakage. Moreover, same bias dependent compression and qualitative leakage current measurements were carried out for the stressed devices. Heteroepitaxial growth being the only commercially feasible option for fabrication of nitride devices, significant TDD generation and propagation throughout the active layers is a given owing to the large epilayer-substrate (SiC,Al2O3,Si) lattice and thermal mismatch. In this regard, this study elucidating the correlation among TDDs, OFF-state leakage and current dispersion in fresh devices, and their evolution in degraded devices provides detailed insights into the reliability barriers of III-N HFETs.

Magnetic Nanoparticles are extensively used in theranostics: hyperthermia, enhancement of contrast in MRI, etc. The main problem with nanoparticles is their delivery to the target cells [1]. To this purpose, the overexpression of transferrin-receptor 1 in cancer cells makes transferrin a potential vehicle for nanoparticles [2]. Transferrin solubilizes Fe3+ in sera and when iron-loaded, it is recognized by receptor-1, which is anchored in the plasma membrane [3]. This Triggers the receptor-mediated endocytosis of the two proteins [4,5]. We develop here a "Trojan horse" system based on this endocytosis of the holotransferrine-receptor adduct to intracellular delivery of maghemite nanoparticles. Different sizes of maghemite (Fe2O3) superparamagnetic nanoparticles (5, 10 and 15 nm) were synthesized, coated with 3-aminopropyltriethoxysilane (APTES) and coupled to holotransferrin (TFe2) by amide bonds. Each nanoparticle (NP) carried a known number of holotransferrins (TFe2-NP). This transferrin construct was tested in vitro and remains active after grafting and interacts with its receptor rapidly (50 �s) with an overall dissociation constant (11 nM) [6]. HeLa cells were incubated for several time intervals with rhodamine-labeled TFe2-NP and NPs. Confocal fluorescence microscopy showed that NPs do not cross the plasma membrane within 1 hour, whereas the constructs holotransferrine-maghemite are internalized in the cytosol in endosomes in less than 15 minutes (below). Furthermore, preliminary magnetophoresis results showed, in Lymphocyte T cells, that the rate of internalization of NPs grafted onto holotransferrine is three times larger than that of raw NPs in a culture media containing: 0, 10 and 55 % FCS. These very promising results seem to exclude the formation of a protein corona and validate our strategy. This hypothesis was also confirmed by molecular modeling. Thus, this nanohybrid system constitutes an interesting model for theranostic devices able to follow the main iron-acquisition pathway. References [1] A. Salvati, A.S. Pitek, M.P. Monopoli et al., Nat. Nanotechnol., 8 (2013) 137. [2] T.R. Daniels, et al., Biochim. Biophys. Acta, 1820 (2012) 291. [3] R.R. Crichton, Iron Metabolism: From Molecular Mechanisms to Clinical Consequences. Third Edition ed. West Sussex: J.Wiley & Sons (2009). [4] A. Dautry-Varsat, A. Ciechanover, H.F. Lodish, Proc. Natl. Acad. Sci. USA, 80 (1983) 2258. [5] M. Hemadi, P.H. Kahn, G. Miquel, et al., Biochemistry, 43 (2004) 1736. [6] H. Piraux, J. Hai, P. Verbeke et al., Biochim. Biophys. Acta., 1830 (2013) 4254.

Magnesium hydride MgH2, which is a well-known compound for hydrogen storage, had been investigated as a novel anode material for lithium�ion batteries reported by Oumellal et al.[1] The theoretical capacity of MgH2 is 2038 mA h g�1 if 2 Li+ incorporate into MgH2, which is almost 6 times to that of graphite. However, the capacity fades rapidly and reduces to less than 200 mA h g�1 after only 5 ~ 10 cycles [1,2], and no improvement had been done for years. In this study, we had successfully retained the reversible capacity of MgH2 electrode by using LiBH4 as a solid�state electrolyte. The result shows a stable reversible capacity of approximately 1230 mA h g�1 can be obtained in the cycling test at a current density of 100 mA g�1 between 0.3 and 1.0 V with nearly 100% capacity retention and 100% coulombic efficiency. The electrode evolution upon discharge�charge process at different stages had also been investigated in this study. This work opens a new way to look for high capacity anode materials for lithium�ion batteries. In addition, the electrochemical performance of numerous metal hydrides will be investigated by using LiBH4 based solid�state electrolytes. References: [1] Y. Oumellal, A. Rougier, G. A. Nazri, J. M. Tarascon, L. Aymard, Nat. Mater. 2008, 7, 916. [2] S. Brutti, G. Mulas, E. Piciollo, S. Panero, P. Reale, J. Mater. Chem. 2012, 22, 14531.

Solar cells based on hybrid metal-organic halide perovskites have been polarizing the attention of the photovoltaics community in the past 3 years due to their ever increasing efficiencies, currently over 19%. This success is largely due to methylammonium lead iodide and mixed halide having band gaps approaching the Shockley-Queisser limit (1.55 eV) and electron and hole diffusion lenghts exceeding 1 micron. Additionally, these electronic properties can be tuned by mixing different halides, metal atoms or cations. In this context computational design can assist the search for novel and even more efficient perovskites by exploring a vast materials library in silico. In this work we elucidate the interplay between the electronic properties and the structure of the lead-iodide perovskite network. We model the perovskite unit cell by four ideal corner sharing octahedra, which can rigidly rotate within the constraints of an orthorhombic unit cell. We show that the structure can be uniquely identified two geometrical parameters, the apical and equatorial Pb-I-Pb bond angles. Furthermore we show a strong correlation between the calculated band gap and these angles. In addition, we show that the magnitude of the Pb-I-Pb angles can be directly controlled by changing the size of the cation component at the center of the cuboctahedral cavity. Based on this finding we propose 17 lead-iodide perovskite absorbers containing different cations at the center of the lead-iodide network, 14 of which have not been reported thus far. The band gaps for these potential novel absorbers are theoretically tunable over 1 eV. Moreover, the band gap trends exhibited are consistent accross different computational approaches (density functional theory and GW - with and without spin-orbit coupling) [1,2] Experiments motivated by this study not only confirmed our predicted trends, but also lead to the synthesis of the novel mixed Rb{1-x}Cs{x}PbI3 perovskite. [1] Filip, M. R., Eperon, G., Snaith, H. J. & Giustino, F., http://arxiv.org/abs/1409.6478 (2014) [2] Filip, M. R. & Giustino, F., http://arxiv.org/abs/1410.2029 (2014) This work was supported by the European Research Council (EU FP7 / ERC grant no. 239578), the UK Engineering and Physical Sciences Research Council (Grant NO. EP/J009857/1) and the Leverhulme Trust (Grant RL-2012-001). G.E.E. is supported by the UK EPSRC and Oxford Photovoltaics Ltd. through a Nanotechnology KTN CASE award. Calculations were performed at the Oxford Supercomputing Centre and the Oxford Materials Modelling Laboratory.

In Marder's presentation the design guidelines for molecules and materials that have large two-photon absorption cross-sections and large “real” third-order susceptibilities for all optical switching were provided. Specifically, molecules with large two-photon absorption cross-sections, d, are in great demand for a variety of applications including two-photon-excited fluorescence microscopy and three-dimensional optical-data storage. These applications utilize the quadratic scaling of the rate of two-photon absorption with input intensity, which allows for the excitation of chromophores with a high degree of spatial selectivity in three dimensions through using a tightly focused laser beam. A strategy for the design of molecules with large two-photon absorption cross-sections, d, has been developed, which is based on the concept that symmetric charge transfer (upon excitation), either from the ends of a conjugated system to the center or vice versa, is correlated to enhanced values of d. In addition, the synthesis and characterization of materials with large third-order nonlinear susceptibility, ?(3), were discussed.

 

Carpenter discussed the discovery of the neutron and the demonstration of neutrons as waves, and described the spallation neutron production process. Cosmic-ray protons produce neutrons that are always around us. He depicted the first nuclear reactor, CP-1, and mentioned the demise of the A2R2 Project as the impetus for accelerator-based neutron sources. Additionally, he will discuss spallation neutrons and early spallation neutron sources: ZING-P, ZING-P’, IPNS; Graham’s moderator data and ING data; and, for fun, allude to the MTA.

An interview with Dr. Carpenter about his work and this talk is available here.