Non-conventional charge transport in organic semiconductors: magnetoresistance and thermoelectricity

Magnetoresistance and thermoelectricity require additional properties of materials beyond their ability to transport charge, namely a large resistive response to a magnetic field, or in the case of thermoelectrics a large Seebeck coefficient combined with low thermal conductivity.</p

Charge transport and mobility engineering in two-dimensional transition metal chalcogenide semiconductors

Two-dimensional (2D) van der Waals semiconductors represent the thinnest, air stable semiconducting materials known. Their unique optical, electronic and mechanical properties hold great potential for harnessing them as key components in novel applications for electronics and optoelectronics. However, the charge transport behavior in 2D semiconductors is more susceptible to external surroundings (e.g. gaseous adsorbates from air and trapped charges in substrates) and their electronic performance is generally lower than corresponding bulk materials due to the fact that surface and bulk coincide. In this article, we review recent progress on the charge transport properties and carrier mobility engineering of 2D transition metal chalcogenides, with a particular focus on the markedly high dependence of carrier mobility on thickness. We unveil the origin of this unique thickness dependence and elaborate the devised strategies to master it for carrier mobility optimization. Specifically, physical and chemical methods towards the optimization of the major factors influencing the extrinsic transport such as electrode/semiconductor contacts, interfacial Coulomb impurities and atomic defects are discussed. In particular, the use of \textit{ad-hoc} molecules makes it possible to engineer the interface with the dielectric and heal the vacancies in such materials. By casting fresh light onto the theoretical and experimental works, we provide a guide for improving the electronic performance of the 2D semiconductors, with the ultimate goal of achieving technologically viable atomically thin (opto)electronics.Comment: 33 pages, 19 figures and 6 table

Conductivity in organic semiconductors hybridized with the vacuum field

Organic semiconductors have generated considerable interest for their potential for creating inexpensive and flexible devices easily processed on a large scale [1-11]. However technological applications are currently limited by the low mobility of the charge carriers associated with the disorder in these materials [5-8]. Much effort over the past decades has therefore been focused on optimizing the organisation of the material or the devices to improve carrier mobility. Here we take a radically different path to solving this problem, namely by injecting carriers into states that are hybridized to the vacuum electromagnetic field. These are coherent states that can extend over as many as 10^5 molecules and should thereby favour conductivity in such materials. To test this idea, organic semiconductors were strongly coupled to the vacuum electromagnetic field on plasmonic structures to form polaritonic states with large Rabi splittings ca. 0.7 eV. Conductivity experiments show that indeed the current does increase by an order of magnitude at resonance in the coupled state, reflecting mostly a change in field-effect mobility as revealed when the structure is gated in a transistor configuration. A theoretical quantum model is presented that confirms the delocalization of the wave-functions of the hybridized states and the consequences on the conductivity. While this is a proof-of-principle study, in practice conductivity mediated by light-matter hybridized states is easy to implement and we therefore expect that it will be used to improve organic devices. More broadly our findings illustrate the potential of engineering the vacuum electromagnetic environment to modify and to improve properties of materials.Comment: 16 pages, 13 figure

DEVELOPMENT AND APPLICATION OF SEGMENTATION AND ANALYSIS METHODS FOR THE MORPHOLOGICAL AND FUNCTIONAL CHARACTERIZATION OF THE LOWER LIMB FROM MRI IMAGES

This work is aimed to the development and application of a segmentation and analysis methods for the morphological and functional characterization of the lower limb from MRI images. Regarding the morphological characterization, an automatic algorithm was developed for the segmentation of Skeletal Muscle (SM), Intermuscular Adipose Tissue (IMAT), and Subcutaneus Adipose Tissue (SAT) compartments from cross-sectional T1-W MRI images, in order to assess thigh regional tissue composition in both young and elderly subjects with different degrees of body adiposity, including obese individuals. A fuzzy c-mean algorithm was employed to perform a classification of the different tissues: SM, Adipose Tissue (AT) and bone. Muscle fascia, which is the anatomical structure that separates SAT and IMAT compartments, was segmented using a procedure based on snake active contour model. We validated the segmentation framework on 15 datasets from 5 young normal weight, 5 older normal weight and 5 older obese females using manual segmentations delineated by an expert operator as gold standard. Segmentation errors were assessed for each structure resulting in mean relative area difference of 1.8%, 2.5% and 2.7% for SM, bone and AT, respectively, and a mean sensitivity for each compartment above 96% in each subject typology. Muscle fascia identification performance resulted in a mean distance between manual and automatic contours of 0.81mm and a mean percentage of contour pixels with distance smaller than 2 pixels of 86.2%. Moreover, manual and automatic IMAT and SAT cross-sectional areas in all subject typologies were found significantly correlated (p < 0.001). These results indicate that the proposed automatic segmentation algorithm, adequately performing thigh tissue discrimination, could be an helpful tool in studies of thigh regional composition. To our knowledge, this is the first published approach which identifies muscle fascia in its anatomical position obtaining promising results from a low level based segmentation perspective. Regarding the functional characterization of the lower limb, the properties of a specific region of the SM: the quadriceps femoris was considered representative of the entire compartment. In this region, size and fat content of 6 normal weight and 5 obese well functioning older females were determined at mid-thigh of the dominant leg, by analysing MRI Dixon images. These data as well as peak knee-extension torque, muscle fascicle length and pennation angle were compared in order to assess functional differences between the two groups. The study showed that muscle tissue composition and pennation angle are important determinants of muscle torque per unit muscle section in well-functioning older women. Moreover, as a result of the greater gravitational load, active obese elderly women have more muscle mass but with a higher fat content compared with normal-weight counterparts. The amount and distribution of SM and AT in different body regions, have a relevant clinical impact. In particular, the assessment of changes in both SM and AT amount and distribution are significant as they correlate with processes related to aging. Namely, a loss of SM mass with an increasing of IMAT have been identified as a negative correlate of SM quality and strength in lower limb, leading to functional impairment of different severity. Moreover, the simultaneous presence of such conditions with an abnormal high accumulation of AT in the lower limb has been defined as sarcopenic obesity and correlated with accelerated functional decline with high risk of diseases and mortality. This work represents a step forward not only to the accomplishment of accurate patient-specific thigh tissues segmentation and quantification, but also to the understanding of processes related to aging with the concomitant presence of obesity

Modulating the charge injection in organic field-effect transistors: fluorinated oligophenyl self-assembled monolayers for high work function electrodes

Financial support from the ERC project SUPRAFUNCTION (GA-257305), the EC Marie-Curie projects IEF-MULTITUDES (PIEF-GA-2012-326666) and ITN iSwitch (GA no. 642196), the Agence Nationale de la Recherche through the LabEx project Chemistry of Complex Systems (ANR-10-LABX-0026_CSC), and the International Center for Frontier Research in Chemistry (icFRC). The computational work was supported by the Interuniversity Attraction Pole Programme (P7/05) initiated by the Belgian Science Policy Office, and by the Belgian National Fund for Scientific Research (FNRS). J.C. is an FNRS research director. Colin Van Dyck is a recipient of a Gustave Boël – Sofina Fellowship of the Belgian American Educational Foundation (BAEF). K.M., F.R. and M.M. acknowledge financial support by the Swiss National Science Foundation (SNF) and the Swiss Nanoscience Institute (SNI)

Nano-Subsidence Assisted Precise Integration of Patterned Two-Dimensional Materials for High-Performance Photodetector Arrays

The spatially precise integration of arrays of micro-patterned two-dimensional (2D) crystals onto three-dimensionally structured Si/SiO$_2$ substrates represents an attractive strategy towards the low-cost system-on-chip integration of extended functions in silicon microelectronics. However, the reliable integration of the arrays of 2D materials on non-flat surfaces has thus far proved extremely challenging due to their poor adhesion to underlying substrates as ruled by weak van der Waals interactions. Here we report on a novel fabrication method based on nano-subsidence which enables the precise and reliable integration of the micro-patterned 2D materials/silicon photodiode arrays exhibiting high uniformity. Our devices display peak sensitivity as high as 0.35 A/W and external quantum efficiency (EQE) of ca. 90%, outperforming most commercial photodiodes. The nano-subsidence technique opens a viable path to on-chip integrate 2D crystals onto silicon for beyond-silicon microelectronics.Comment: 41 pages, 5 figures, with S

Coherent Coupling of WS_2 Monolayers with Metallic Photonic Nanostructures at Room Temperature

Room temperature strong coupling of WS_2 monolayer exciton transitions to metallic Fabry-Perot and plasmonic optical cavities is demonstrated. A Rabi splitting of 101 meV is observed for the Fabry-Perot cavity, more than double those reported to date in other 2D materials. The enhanced magnitude and visibility of WS_2 monolayer strong coupling is attributed to the larger absorption coefficient, the narrower linewidth of the A exciton transition, and greater spin-orbit coupling. For WS_2 coupled to plasmonic arrays, the Rabi splitting still reaches 60 meV despite the less favorable coupling conditions, and displays interesting photoluminescence features. The unambiguous signature of WS_2 monolayer strong coupling in easily fabricated metallic resonators at room temperature suggests many possibilities for combining light-matter hybridization with spin and valleytronics.Comment: 26 pages, including Supporting Informatio

Doping-related broadening of the density of states governs integer-charge transfer in P3HT

Molecular p-doping allows for an increase in the conductivity of organic semiconductors, which is regularly exploited in thermoelectric devices. Upon doping, integer and fractional charge transfer have been identified as the two competing mechanisms to occur, where the former is desired to promote the generation of mobile holes in the semiconductor host. In general, high dopant electron affinity is expected to promote integer-charge transfer, while strong coupling between the frontier molecular orbitals of dopant and host promotes fractional charge transfer instead. Here, we investigate the role that the width of the density of states (DOS) plays in the doping process by doping the conjugated polymer poly(3-hexylthiophene) (P3HT) with tetracyanoquinodimethane (TCNQ) derivatives of different electron affinities at a 2% dopant ratio. Cyclic voltammetry confirms that only the electron affinity of F4TCNQ (tetrafluorotetracyanoquinodimethane) exceeds the ionization energy of P3HT, while TCNQ and FTCNQ (2-fluoro-7,7,8,8-tetracyanoquinodimethane) turn out to have significantly lower but essentially identical electron affinities. From infrared spectroscopy, we learn, however, that ca. 88% of FTCNQ is ionized while all of TCNQ is not. This translates into P3HT conductivities that are increased for F4TCNQ and FTCNQ doping, but surprisingly even reduced for TCNQ doping. To understand the remarkable discrepancy between TCNQ and FTCNQ, we calculated the percentage of ionized dopants and the hole densities in the P3HT matrix resulting from varied widths of the P3HT highest occupied molecular orbital (HOMO)-DOS via a semi-classical computational approach. We find that broadening of the DOS can yield the expected ionization percentages only if the dopants have significantly different tendencies to cause energetic disorder in the host matrix. In particular, while for TCNQ the doping behavior is well reproduced if the recently reported width of the P3HT HOMO-DOS is used, it must be broadened by almost one order of magnitude to comply with the ionization ratio determined for FTCNQ. Possible reasons for this discrepancy lie in the presence of a permanent dipole in FTCNQ, which highlights that electron affinities alone are not sufficient to define the strength of molecular dopants and their capability to perform integer-charge transfer with organic semiconductors

Enhanced Terahertz Spectroscopy of a Monolayer Transition Metal Dichalcogenide

Two-dimensional materials, including transition metal dichalcogenides, are attractive for a variety of applications in electronics as well as photonics and have recently been envisioned as an appealing platform for phonon polaritonics. However, their direct characterization in the terahertz spectral region, of interest for retrieving, e.g., their phonon response, represents a major challenge, due to the limited sensitivity of typical terahertz spectroscopic tools and the weak interaction of such long-wavelength radiation with sub-nanometer systems. In this work, by exploiting an ad-hoc engineered metallic surface enabling a ten-thousand-fold local absorption boost, we perform enhanced terahertz spectroscopy of a monolayer transition metal dichalcogenide (tungsten diselenide) and extract its dipole-active phonon resonance features. In addition, we use these data to obtain the monolayer effective permittivity around its phonon resonance. Via the direct terahertz characterization of the phonon response of such two-dimensional systems, this method opens the path to the rational design of phonon polariton devices exploiting monolayer transition metal dichalcogenides

Degradation of methylammonium lead iodide perovskite structures through light and electron beam driven ion migration

[Image: see text] Organometal halide perovskites show promising features for cost-effective application in photovoltaics. The material instability remains a major obstacle to broad application because of the poorly understood degradation pathways. Here, we apply simultaneous luminescence and electron microscopy on perovskites for the first time, allowing us to monitor in situ morphology evolution and optical properties upon perovskite degradation. Interestingly, morphology, photoluminescence (PL), and cathodoluminescence of perovskite samples evolve differently upon degradation driven by electron beam (e-beam) or by light. A transversal electric current generated by a scanning electron beam leads to dramatic changes in PL and tunes the energy band gaps continuously alongside film thinning. In contrast, light-induced degradation results in material decomposition to scattered particles and shows little PL spectral shifts. The differences in degradation can be ascribed to different electric currents that drive ion migration. Moreover, solution-processed perovskite cuboids show heterogeneity in stability which is likely related to crystallinity and morphology. Our results reveal the essential role of ion migration in perovskite degradation and provide potential avenues to rationally enhance the stability of perovskite materials by reducing ion migration while improving morphology and crystallinity. It is worth noting that even moderate e-beam currents (86 pA) and acceleration voltages (10 kV) readily induce significant perovskite degradation and alter their optical properties. Therefore, attention has to be paid while characterizing such materials using scanning electron microscopy or transmission electron microscopy techniques