Room temperature magnetic phase transition in an electrically-tuned van der Waals ferromagnet

Finding tunable van der Waals (vdW) ferromagnets that operate at above room temperature is an important research focus in physics and materials science. Most vdW magnets are only intrinsically magnetic far below room temperature and magnetism with square-shaped hysteresis at room-temperature has yet to be observed. Here, we report magnetism in a quasi-2D magnet Cr1.2Te2 observed at room temperature (290 K). This magnetism was tuned via a protonic gate with an electron doping concentration up to 3.8 * 10^21 cm^-3. We observed non-monotonic evolutions in both coercivity and anomalous Hall resistivity. Under increased electron doping, the coercivities and anomalous Hall effects (AHEs) vanished, indicating a doping-induced magnetic phase transition. This occurred up to room temperature. DFT calculations showed the formation of an antiferromagnetic (AFM) phase caused by the intercalation of protons which induced significant electron doping in the Cr1.2Te2. The tunability of the magnetic properties and phase in room temperature magnetic vdW Cr1.2Te2 is a significant step towards practical spintronic devices.Comment: 18 pages, 4 figure

Ultrafast charge transfer at GaAs/P3HT as a model system for hybrid organic-inorganic heterointerfaces

Polymer bulk heterojunction solar cells are being widely considered for the development of the future generation of photovoltaic devices, which aim at providing high conversion efficiencies at low cost. These promising devices have been conventionally constructed by blending an organic conjugated polymer donor and a fullerene acceptor. The polymer solar cells have demonstrated a few advantages over their inorganic counterparts: simple and easy processability, high throughput fabrication, and compatibility with low-temperature processes. However, few intrinsic factors are still limiting the overall performance of polymer solar cells, particularly the low carrier mobility, the weak absorbance of thin films and the poor chemical stability. To overcome some of these issues while retaining low production costs and scalability, research in polymer photovoltaics is about to converge towards organic-inorganic hybrid architectures where heterojunction is formed between inorganic acceptors from group IV, III-V, IV-VI and organic compounds (small molecules, oligomers, polymers). Most recently, hybrid photovoltaic based on Group III-V and organic compounds were reported in the literature, which seem rather promising for the achievement of high power conversion efficiencies in a near future. The mainstream III-V compounds have demonstrated several advantages for photovoltaic application. The main advantage in the use of such “traditional” semiconductors lies in their high carrier mobility, optimal staggered band alignment relative to the most common conjugated polymers, and the possibility to finely engineer their band structure and density of states through the conventional methods of alloying, doping, and heterostructure formation. Despite substantial progress in hybrid polymer/III-V photovoltaic devices, the primary processes of exciton dissociation and charge transfer at the heterointerface of these unconventional systems, and the coupling between the excitonic species of the polymer and ii the extended states of the inorganic semiconductor are completely unknown. The understanding of these fundamental issues in polymer/III-V composites will provide a general tool for the engineering of hybrid architectures, with high potential to impact the entire organic photovoltaic research field. In this thesis, we address some of the above issues in a prototype hybrid system based on ntype GaAs and a typical hole transporting conjugated polymer regioregular Poly(3- hexylthiophene-2,5-diyl) (rr-P3HT). Hybrid GaAs/P3HT heterointerface is a very interesting localized/delocalized system where conjugated polymer with discrete and localized orbitals meets inorganic semiconductor with continuous bands of delocalized quantum states. We investigate the interactions of the polymer with GaAs surfaces using model systems, P3HT/GaAs (111B and 110). We approach this problem from both theoretical and experimental points of view by using density functional theory (DFT) calculations and combining various spectroscopy measurements. Our goal is to address some issues of fundamental scientific interest regarding charge transfer processes in organic-inorganic heterointerfaces. In this dissertation, addition to charge photogeneration and charge transfer study of hybrid GaAs/P3HT heterointerfaces, long-lived photoexcitations in pristine P3HT film, transient reflectance spectra of GaAs, the effect of polarity of GaAs surface on charge transfer, and correlation between transient absorption and reflectance spectroscopy are also studied. Our ultrafast spectroscopy provides strong evidence of ambipolar charge transfer of electrons and holes across the GaAs/P3HT heterointerface, where signatures of charge transfer are manifested by presence of long-lived photoinduced absorption and photobleaching by exciting either above or below the polymer optical gap. We demonstrate that ambipolar charge transfer can be regarded as an interesting new concept to optimize photovoltaic power conversion efficiency of hybrid organic-inorganic devices.PHYSICS and APPLIED PHYSIC

Electromagnetic modeling and optimization of photoconductive switches for terahertz generation and photocurrent transient spectroscopy

International audienc

Water-Dispersed Conjugated Polyelectrolyte for Visible-Light Hydrogen Production

10.1002/solr.201800255Solar RRL331800255-180025

Water-dispersed conjugated polyelectrolyte for visible-light hydrogen production

Conjugated polymer-based photocatalysts have shown great potential in H2 production via water splitting, but an intrinsic drawback of conventional hydrophobic polymer photocatalysts is their poor wettability and relatively large particle size in aqueous media, which is favorable for charge recombination with limited interfacial reaction efficiency. Herein, a well-dispersed organic water reduction system using cationic conjugated polyelectrolyte as the photocatalyst has been reported for the first time. In comparison to a model polymer (PFBT) bearing the same conjugated backbone, the polyelectrolyte exhibits significantly enhanced photocatalytic efficiency due to the extended light absorption and improved charge separation of the polymer aggregates.Ministry of Education (MOE)National Research Foundation (NRF)Accepted versionThis work was financially supported by the Singapore National Research Foundation (R279-000-444-281 and R279-000-483-281), National Univer- sity of Singapore (R279-000-482-133), Singapore MOE AcRF-Tier1 (RG 12/15), Singapore MOE AcRF-Tier1 (2016-T1-002-087, RG 120/16), and AcRF-Tier2 (MOE2016-T2-2-056)

Enhancing photocurrent transient spectroscopy by electromagnetic modeling

International audienc

Charge redistribution at GaAs/P3HT heterointerfaces with different surface polarity

The nature of charged photoexcitations at the interface of highly delocalized inorganic crystals and more localized conjugated polymer systems is of great fundamental interest for a number of hybrid photovoltaic applications. Here we study the interaction between mainstream compound semiconductor GaAs and conjugated polymer P3HT by means of density functional theory simulations. When considering both nonpolar GaAs(110) and polar GaAs(111)B surfaces, we find that polarity of the GaAs surface strongly affects the electronic orbitals and charge redistribution: electrons are efficiently transferred to GaAs substrates, implying the formation of hybrid delocalized states at the interface. Furthermore, P3HT can act as an "acceptor" for GaAs(111)B via hole transfer from GaAs valence band states. Overall the intrinsic surface dipole moment of GaAs surfaces is enhanced by the charge displacement induced by adsorbed P3HT. These theoretical predictions correlate well with energy alignments derived by ultraviolet photoelectron spectroscopy and provide a robust methodology for the design of polymer/III-V heterointerfaces that optimize photovoltaic performance. \ua9 2013 American Chemical Society

Precise Molecular Engineering of Photosensitizers with Aggregation-Induced Emission over 800 nm for Photodynamic Therapy

10.1002/adfm.201901791ADVANCED FUNCTIONAL MATERIALS294

Charge Redistribution at GaAs

The nature of charged photoexcitations at the interface of highly delocalized inorganic crystals and more localized conjugated polymer systems is of great fundamental interest for a number of hybrid photovoltaic applications. Here we study the interaction between mainstream compound semiconductor GaAs and conjugated polymer P3HT by means of density functional theory simulations. When considering both nonpolar GaAs(110) and polar GaAs(111)B surfaces, we find that polarity of the GaAs surface strongly affects the electronic orbitals and charge redistribution: electrons are efficiently transferred to GaAs substrates, implying the formation of hybrid delocalized states at the interface. Furthermore, P3HT can act as an “acceptor” for GaAs(111)B via hole transfer from GaAs valence band states. Overall the intrinsic surface dipole moment of GaAs surfaces is enhanced by the charge displacement induced by adsorbed P3HT. These theoretical predictions correlate well with energy alignments derived by ultraviolet photoelectron spectroscopy and provide a robust methodology for the design of polymer/III−V heterointerfaces that optimize photovoltaic performance

Dibenzothiophene‐S,S‐dioxide‐based conjugated polymers : highly efficient photocatalyts for hydrogen production from water under visible light

Three dibenzothiophene-S,S-dioxide-based alternating copolymers were synthesized by facile Suzuki polymerization for visible light-responsive hydrogen production from water (> 420 nm). Without addition of any cocatalyst, FluPh2-SO showed a photocatalytic efficiency of 3.48 mmol h-1 g-1 , while a larger hydrogen evolution rate (HER) of 4.74 mmol h-1 g-1 was achieved for Py-SO, which was ascribed to the improved coplanarity of the polymer that facilitated both intermolecular packing and charge transport. To minimize the possible steric hindrance of FluPh2-SO by replacing 9,9'-diphenylfluorene with fluorene, Flu-SO exhibited a more red-shifted absorption than FluPh2-SO and yielded the highest HER of 5.04 mmol h-1 g-1 . This work highlights the potential of dibenzothiophene-S,S-dioxide as a versatile building block and the rational design strategy for achieving high photocatalytic efficiency.NRF (Natl Research Foundation, S’pore)ASTAR (Agency for Sci., Tech. and Research, S’pore)MOE (Min. of Education, S’pore