86 results found
Xu J, Fu M, Ji C, et al., 2023, Plasmonic‐enhanced NIR‐II downconversion fluorescence beyond 1500 nm from core–shell–shell lanthanide nanoparticles, Advanced Optical Materials, Vol: 11, ISSN: 2195-1071
This paper reports on the light amplification of NaGdF4:Yb,Er,Ce@NaGdF4:Yb,Nd@NaGdF4 core–shell–shell downconversion nanoparticles (CSS-DCNPs) in the near-infrared second biological window (NIR-II: 1000–1700 nm) by plasmonic nanostructures. Through a precisely controlled plasmonic metallic nanostructure, fluorescence from Yb3+ induced 1000 nm emission, Nd3+ induced 1060 nm emission, and Er3+ induced 1527 nm emission are enhanced 1.6-fold, 1.7-fold, and 2.2-fold, respectively, under an 808 nm laser excitation for the CSS-DCNPs coupled with a gold hole-cap nanoarray (Au-HCNA), while the Er3+ induced 1527 nm emission under a 980 nm laser excitation is enhanced up to 6-fold. To gain insight into the enhancement mechanism, the plasmonic modulation of Er3+ induced NIR-II emission at 1550 nm under 980 nm excitation is studied by FDTD simulation and lifetime measurements, showing the observed fluorescence enhancement can be attributed to a combination of enhanced excitation and an increased radiative decay rate.
Daem N, Heutz S, Cloots R, et al., 2023, Improved photovoltaic performances of lead-free Cs2AgBiBr6 double perovskite solar cells incorporating tetracene as co-hole transport layer, Solar RRL, Vol: 7, Pages: 1-9, ISSN: 2367-198X
Cs2AgBiBr6 double perovskite compounds are increasingly studied in recent years as promising candidates able to counter polluting, harmful, and oxygen-/moisture-sensitive issues intrinsic to traditional lead-containing solar cells. Exhibiting high optical absorption coefficient, low toxicity, and important structural stability, Cs2AgBiBr6 solar cells still suffer from limited absorption of low-energy photons, low carrier mobility, and limited carrier lifetimes induced by defect states. Herein, for the first time, a molecular layer of tetracene is introduced within a Cs2AgBiBr6-based photovoltaic architecture: being incorporated at the interface between the double perovskite photoabsorber and spiro-OMeTAD hole transport material, tetracene allows for a suitably graded cascade of energy bands within the solar cell architecture, which ultimately improves interfacial charge transfers and reduces charge recombination. The performances in photovoltaic devices are consequently enhanced versus tetracene-free configurations, with champion values of open-circuit voltages of 1.1 V (vs. 1.0 V), current densities of 2.5 mA cm−2 (vs. 1.9 mA cm−2), and photoconversion efficiencies of 1.7% (vs. 1.3%) with reduced hysteretic behavior.
Mistry EDR, Lubert-Perquel D, Nevjestic I, et al., 2023, Paramagnetic states in oxygen-doped boron nitride extend light harvesting and photochemistry to the deep visible region, Chemistry of Materials, Vol: 35, Pages: 1858-1867, ISSN: 0897-4756
A family of boron nitride (BN)-based photocatalysts for solar fuel syntheses have recently emerged. Studies have shown that oxygen doping, leading to boron oxynitride (BNO), can extend light absorption to the visible range. However, the fundamental question surrounding the origin of enhanced light harvesting and the role of specific chemical states of oxygen in BNO photochemistry remains unanswered. Here, using an integrated experimental and first-principles-based computational approach, we demonstrate that paramagnetic isolated OB3 states are paramount to inducing prominent red-shifted light absorption. Conversely, we highlight the diamagnetic nature of O–B–O states, which are shown to cause undesired larger band gaps and impaired photochemistry. This study elucidates the importance of paramagnetism in BNO semiconductors and provides fundamental insight into its photophysics. The work herein paves the way for tailoring of its optoelectronic and photochemical properties for solar fuel synthesis.
Wade J, Salerno F, Kilbride R, et al., 2022, Controlling anisotropic properties by manipulating the orientation of chiral small molecules, Nature Chemistry, Vol: 14, Pages: 1383-1389, ISSN: 1755-4330
Chiral π-conjugated molecules bring new functionality to technological applications and represent an exciting, rapidly expanding area of research. Their functional properties, such as the absorption and emission of circularly polarised light or the transport of spin-polarised electrons, are highly anisotropic. As a result, the orientation of chiral molecules criticallydetermines the functionality and efficiency of chiral devices. Here we present a strategy to control the orientation of a small chiral molecule (2,2’-dicyanohelicene, CN6H): the use of organic and inorganic templating layers. Such templating layers can either force CN6H molecules to adopt a face-on orientation and self-assemble into upright supramolecular columns oriented with their helical axis perpendicular to the substrate, or an edge-onorientation with parallel-lying supramolecular columns. Through such control, we show that low- and high-energy chiroptical responses can be independently ‘turned on’ or ‘turned off’. The templating methodologies described here provide a simple way to engineer orientational control, and by association, anisotropic functional properties of chiral molecular systems for a range of emerging technologies.
Cowen LM, Gilhooly-Finn PA, Giovannitti A, et al., 2022, Critical analysis of self-doping and water-soluble n-type organic semiconductors: structures and mechanisms, Journal of Materials Chemistry C, Vol: 10, Pages: 8955-8963, ISSN: 2050-7526
Self-doping organic semiconductors provide a promising route to avoid instabilities and morphological issues associated with molecular n-type dopants. Structural characterization of a naphthalenetetracarboxylic diimide (NDI) semiconductor covalently bound to an ammonium hydroxide group is presented. The dopant precursor was found to be the product of an unexpected base catalyzed hydrolysis, which was reversible. The reversible hydrolysis had profound consequences on the chemical composition, morphology, and electronic performance of the doped films. In addition, we investigated the degradation mechanism of the quaternary ammonium group and the subsequent doping of NDI. These findings reveal that the products of more than one chemical reaction during processing of films must be considered when utilizing this promising class of water-soluble semiconductors.
Shankar RB, Mistry EDR, Lubert-Perquel D, et al., 2022, A response surface model to predict and experimentally tune the chemical, magnetic and optoelectronic properties of oxygen-doped boron nitride, ChemPhysChem: a European journal of chemical physics and physical chemistry, Vol: 23, ISSN: 1439-4235
Porous boron nitride (BN), a combination of hexagonal, turbostratic and amorphous BN, has emerged as a new platform photocatalyst. Yet, this material lacks photoactivity under visible light. Theoretical studies predict that tuning the oxygen content in oxygen-doped BN (BNO) could lower the band gap. This is yet to be verified experimentally. We present herein a systematic experimental route to simultaneously tune BNO's chemical, magnetic and optoelectronic properties using a multivariate synthesis parameter space. We report deep visible range band gaps (1.50–2.90 eV) and tuning of the oxygen (2–14 at.%) and specific paramagnetic OB3 contents (7–294 a.u. g−1). Through designing a response surface via a design of experiments (DOE) process, we have identified synthesis parameters influencing BNO's chemical, magnetic and optoelectronic properties. We also present model prediction equations relating these properties to the synthesis parameter space that we have validated experimentally. This methodology can help tailor and optimise BN materials for heterogeneous photocatalysis.
Wong PKJ, Zhang W, Wee ATS, et al., 2022, Intelligent Spintronics: From Hybrid Materials to Integrative Devices and Computing Architectures, FRONTIERS IN NANOTECHNOLOGY, Vol: 4
Attwood M, Kim DK, Hadden JHL, et al., 2021, Asymmetric N-heteroacene tetracene analogues as potential n-type semiconductors, Journal of Materials Chemistry C, Vol: 9, Pages: 17073-17083, ISSN: 2050-7526
In the search for high performance n-type organic semiconductors (OSCs) a simple strategy might be substitution of aromatic CH groups for nitrogen heteroatoms. Here, we report the synthesis and characterisation of two novel N-heteroacene compounds, namely, 1,5,12-triazatetracene (TrAT1) and 2,5,12-triazatetracene (TrAT2). Their potential as n-type materials is evaluated against 5,12-diazatetracene (DAT) by UV/vis and EPR spectroscopy, cyclic voltammetry, DFT, single crystal X-ray diffraction and thin film characterisation. Increasing the number of N-heteroatoms was found to stabilise the HOMO and LUMO leading to electron affinities for TrAT1 and TrAT2 of ca. −4 eV. Both compounds were found to exhibit columns of co-facial π-stacked molecules. For TrAT1, molecules are also linked by hydrogen bonding, while the crystal structure of TrAT2 was found to be inherently disordered. Thin films of DAT, TrAT1 and TrAT2 were grown by organic molecular beam deposition (OMBD) and found to form discontinuous films, where TrAT1 exhibited a preferential orientation.
Wade J, Higgins SG, Heutz S, et al., 2021, In memoriam Alasdair James Campbell (11 May 1961-27 February 2021), Journal of Materials Chemistry C, Vol: 9, Pages: 6100-6102, ISSN: 2050-7526
Lubert-Perquel D, Szumska AA, Azzouzi M, et al., 2020, Structure dependence of kinetic and thermodynamic parameters in singlet fission processes., Journal of Physical Chemistry Letters, Vol: 11, Pages: 9557-9565, ISSN: 1948-7185
Singlet fission-whereby one absorbed photon generates two coupled triplet excitons-is a key process for increasing the efficiency of optoelectronic devices by overcoming the Shockley-Queisser limit. A crucial parameter is the rate of dissociation of the coupled triplets, as this limits the number of free triplets subsequently available for harvesting and ultimately the overall efficiency of the device. Here we present an analysis of the thermodynamic and kinetic parameters for this process in parallel and herringbone dimers measured by electron paramagnetic resonance spectroscopy in coevaporated films of pentacene in p-terphenyl. The rate of dissociation is higher for parallel dimers than for their herringbone counterparts, as is the rate of recombination to the ground state. DFT calculations, which provide the magnitude of the electronic coupling as well as the distribution of molecular orbitals for each geometry, suggest that weaker triplet coupling in the parallel dimer is the driving force for faster dissociation. Conversely, localization of the molecular orbitals and a stronger triplet-triplet interaction result in slower dissociation and recombination. The identification and understanding of how the intermolecular geometry promotes efficient triplet dissociation provide the basis for control of triplet coupling and thereby the optimization of one important parameter of device performance.
Shankar R, Lubert-Perquel D, Mistry E, et al., 2020, Boron-Doped Boron Nitride Photocatalyst for Visible Light-Driven H2 Evolution and CO2 Photoreduction
<jats:p>Developing robust, multifunctional photocatalysts that can facilitate both hydrogen evolution <jats:italic>via</jats:italic> photoreforming of water and gas phase CO<jats:sub>2</jats:sub> photoreduction is highly desirable with the long-term vision of integrated photocatalytic setups. Here, we present a step-change in the family of boron oxynitride materials by introducing the first example of a B-doped boron oxynitride (B-BNO). This material resolves an on-going bottleneck associated with BN-based materials, i.e. the lack of photoactivity under visible light. Detailed EPR studies revealed distinct hyperfine interactions between the free oxygen radicals and 3 neighbouring boron nuclei. This confirmed isolated OB<jats:sub>3 </jats:sub>sites, which contribute to band gap narrowing, as the radical species and origin of paramagnetism in BNO materials. We show that B-BNO can facilitate both liquid phase H<jats:sub>2 </jats:sub>evolution and gas phase CO<jats:sub>2</jats:sub> photoreduction, using UV-Vis and deep visible irradiation (λ > 550 nm), without any co-catalysts. The evolution rates, quantum efficiencies, and selectivities observed for both reactions with B-BNO exceed those of its porous BNO counterpart, P25 TiO<jats:sub>2</jats:sub> and bulk g-C<jats:sub>3</jats:sub>N<jats:sub>4</jats:sub>.</jats:p>
Abdelghany TM, Leitch AC, Nevjestic I, et al., 2020, Emerging risk from "environmentally-friendly" solvents: Interaction of methylimidazolium ionic liquids with the mitochondrial electron transport chain is a key initiation event in their mammalian toxicity, FOOD AND CHEMICAL TOXICOLOGY, Vol: 145, ISSN: 0278-6915
Shankar R, Lubert-Perquel D, Mistry E, et al., 2020, Water-Stable Boron-Doped Boron Nitride Photocatalyst for Visible Light-Driven H2 Evolution and CO2 Photoreduction
<jats:p>Developing robust, multifunctional photocatalysts that can facilitate both hydrogen evolution via photoreforming of water and gas phase CO2 photoreduction is highly desirable with the long-term vision of integrated photocatalytic setups. Here, we present a new addition to the boron nitride (BN) photocatalyst material platform, boron-doped boron oxynitride (B-BNO), capable of fulfilling this goal. Detailed EPR studies revealed hyperfine interactions between free charges located on discrete OB3 sites, exhibiting an out-of-plane symmetry, and the nuclei of neighbouring boron atoms. This material resolves two long-standing bottlenecks associated to BN-based materials concomitantly: instability in water and lack of photo activity under visible light. We show that B-BNO maintains prolonged stability in water for at least three straight days and can facilitate both liquid phase H2 evolution and gas phase CO2 photoreduction, using UV-Vis and deep visible irradiation (λ > 550 nm), without any cocatalysts. The evolution rates, apparent quantum yields, and selectivities observed for both reactions with B-BNO exceed those of its porous BNO counterpart, P25 TiO2 and bulk g-C3N4. This work provides scope to expand the BN photocatalyst platform to a wider range of reactions.</jats:p>
Antonio EN, Wicking C, Filip S, et al., 2020, Role of iron speciation in oxidation and deposition at the hexadecane-iron interface, ACS Applied Materials and Interfaces, Vol: 12, Pages: 19140-19152, ISSN: 1944-8244
Interactions between iron surfaces and hydrocarbons are the basis for a wide range of materials synthesis processes and novel applications, including sensing. However, in diesel engines these interactions can lead to deposit formation that reduces performance, lowers efficiency, and increases emissions. Here, we present a global study to understand deposition at iron-hexadecane interfaces. We use a combination of spectroscopy, microscopy, and mass spectrometry to investigate surface reactions, bulk chemistry, and deposition processes. A dynamic equilibrium between the oxidation products, both at the surface and in solution, determines the deposition at the surface. Considering the solution and the surface in parallel, we find that the iron speciation affects the morphology, composition, and quantity of the deposit at the surface, as well as the oxidation of hexadecane. Fe(II) and Fe(III) both promote the decomposition of peroxides-intermediates in the oxidation of hexadecane-but through noncatalytic and catalytic mechanisms, respectively. In contrast, Fe(0) is proposed to initiate hexadecane autoxidation during its oxidation to Fe(III). We find that in all cases, the surfaces exclusively contain Fe(III) following heat treatment with hexadecane. Upon subsequent exposure at room temperature, Fe(III) species are found to promote oxidation; this finding is particularly concerning for hybrid vehicles where longer time periods are expected between engine operation. Our work provides a foundation for the development of strategies that disrupt the role of iron in the degradation of hexadecane to ultimately reduce oxidation and deposition in diesel engines.
Kim DK, Lubert-Perquel D, Heutz S, 2020, Correction: Comparison of organic and inorganic layers for structural templating of pentacene thin films, Materials Horizons, Vol: 7, Pages: 299-299, ISSN: 2051-6355
Correction for ‘Comparison of organic and inorganic layers for structural templating of pentacene thin films’ by Dong Kuk Kim et al., Mater. Horiz., 2019, DOI: 10.1039/c9mh00355j.The authors wish to amend the Acknowledgements section of the originally published manuscript. The correct Acknowledgements section is shown below.
Tseng HH, Serri M, Harrison N, et al., 2020, Properties and degradation of manganese(III) porphyrin thin films formed by high vacuum sublimation, Porphyrin Science By Women (In 3 Volumes), Pages: 924-931, ISBN: 9789811223556
Manganese porphyrins are of interest due to the optical, electronic and magnetic properties of the central metal ion, coupled to the low bandgap of the polyaromatic ring. These attractive characteristics are harnessed in solutions or in ultra-thin films, such as, for example, self-assembled monolayers. However, for devices, thicker films deposited using a controlled and reproducible method are required. Here we present the morphological, structural, chemical and optical properties of manganese(III) tetraphenylporphyrin chloride (MnTPPCl) thin films deposited using organic molecular beam deposition, typically employed to process analogue molecules for applications such as organic photovoltaics. We find, using a combination of UV-vis and X-ray photoelectron spectroscopies, that the sublimation process leads to the scission of the Mn-Cl bond. The resultant film is a Mn(II)TPP:Mn(III)TPPCl blend where approximately half the molecules have been reduced. Following growth, exposure to air oxidizes the Mn(II)TPP molecule. Through quantitative analysis of the time-dependent optical properties, the oxygen diffusion coefficient (D) ~1.9 × 10-17 cm2/s is obtained, corresponding to a slow bulk oxidation following fast oxidation of a 8-nm-thick surface layer. The bulk diffusion D is lower than for analogous polycrystalline films, suggestion that grain boundaries, rather than molecular packing, are the rate-limiting steps in oxidation of molecular films. Our results highlight that the stability of the axial ligands should be considered when depositing metal porphyrins from the vapor phase, and offer a solvent-free route to obtain reproducible and smooth thin films of complex materials for engineering film functionalities.
Kim DK, Lubert-Perquel D, Heutz S, 2020, Comparison of organic and inorganic layers for structural templating of pentacene thin films, Materials Horizons, Vol: 7, Pages: 289-298, ISSN: 2051-6355
Pentacene is a key organic semiconductor, which has achieved prominence in transistor applications and as an archetypal material for singlet fission, the process whereby the absorption of one photon leads to the formation of two triplet states. Functional properties of molecules are highly anisotropic, and control over the molecular orientation in thin films with structural templating is commonly implemented as a route for governing the morphology and structure of organic films. Among the structural templating layers, 3, 4, 9, 10 perylenetetracarboxylic dianhydride (PTCDA) and copper (I) iodide (CuI) have been shown to effectively template aromatic systems such as phthalocyanines. Here, we extend their use to pentacene thin films and find that a successful transition to a flat-lying arrangement is achieved with CuI films grown at high temperatures, but not with PTCDA. As a result, we postulate a model based on quadrupole interactions as the driving force behind the molecular orientation of pentacene. A 0.25 eV increase in work function and a two-fold increase in absorption are recorded for the induced flat-lying orientation. Therefore, our templating methodology provides design opportunities for optoelectronic devices that require a predominantly flat-lying orientation.
Tseng H-H, Serri M, Harrison N, et al., 2019, Properties and degradation of manganese(III) porphyrin thin films formed by high vacuum sublimation, Journal of Porphyrins and Phthalocyanines, Vol: 23, Pages: 1515-1522, ISSN: 1088-4246
Manganese porphyrins are of interest due to the optical, electronic and magnetic properties of the central metal ion, coupled to the low bandgap of the polyaromatic ring. These attractive characteristics are harnessed in solutions or in ultra-thin films, such as, for example, self-assembled monolayers. However, for devices, thicker films deposited using a controlled and reproducible method are required. Here we present the morphological, structural, chemical and optical properties of manganese(III) tetraphenylporphyrin chloride (MnTPPCl) thin films deposited using organic molecular beam deposition, typically employed to process analogue molecules for applications such as organic photovoltaics. We find, using a combination of UV-vis and X-ray photoelectron spectroscopies, that the sublimation process leads to the scission of the Mn–Cl bond. The resultant film is a Mn(II)TPP:Mn(III)TPPCl blend where approximately half the molecules have been reduced. Following growth, exposure to air oxidizes the Mn(II)TPP molecule. Through quantitative analysis of the time-dependent optical properties, the oxygen diffusion coefficient (D) ∼1.9×10−17cm2/s is obtained, corresponding to a slow bulk oxidation following fast oxidation of a 8-nm-thick surface layer. The bulk diffusion D is lower than for analogous polycrystalline films, suggestion that grain boundaries, rather than molecular packing, are the rate-limiting steps in oxidation of molecular films. Our results highlight that the stability of the axial ligands should be considered when depositing metal porphyrins from the vapor phase, and offer a solvent-free route to obtain reproducible and smooth thin films of complex materials for engineering film functionalities.
Wu Z, Robaschik P, Fleet L, et al., 2019, Controlling ferromagnetic ground states and solitons in thin films and nanowires built from iron phthalocyanine chains, Advanced Functional Materials, Vol: 29, ISSN: 1616-301X
Iron phthalocyanine (FePc) is a molecular semiconductor whose building blocks are one-dimensional ferromagnetic chains. We show that its optical and magnetic properties are controlled by the growth strategy, obtaining extremely high coercivities of over 1 T and modulating the exchange constant between 15 and 29 K through tuning the crystal phase by switching from thin films with controlled orientations, to ultralong nanowires. Magnetisation measurements are analysed using concepts and formulas with broad applicability to all one-dimensional ferromagnetic chains. They show that FePc is best described by a Heisenberg model with moments preferentially lying in the molecular planes. The chain Hamiltonian is very similar to that for the classic inorganic magnet CsNiF3, but with ferromagnetic rather than antiferromagnetic interchain interactions. The data at large magnetic fields are well-described by the soliton picture, where the dominant degrees of freedom are moving one-dimensional magnetic domain walls and at low temperatures and fields by the “super-Curie-Weiss” law characteristic of nearly one-dimensional xy and Heisenberg ferromagnets. The ability to control the molecular orientation and ferromagnetism of FePc systems, and produce them on flexible substrates, together with excellent transistor characteristics reported previously for phthalocyanine analogues, makes them potentially useful for magneto-optical and spintronic devices.
Leber R, Wilson L, Robaschik P, et al., 2019, Vacuum deposition of biferrocene thin films: growth strategies for stability and tuneable magnetism, 257th National Meeting of the American-Chemical-Society (ACS), Publisher: AMER CHEMICAL SOC, ISSN: 0065-7727
Lubert-Perquel D, Kim DK, Kay C, et al., 2019, Growth, morphology and structure of mixed pentacene films, Journal of Materials Chemistry C, Vol: 7, Pages: 289-296, ISSN: 2050-7526
Thin films of pentacene and p-terphenyl were grown via organic molecular beam deposition to enable solid-state dilution of functional molecules (pentacene) in an inert matrix (p-terphenyl) at higher concentrations than permitted by traditional crystal growth methods, such as melts. Growth rates were first optimised for single component films to ensure a precise control over the dopant/host concentrations when the mixed films were deposited. Both thin film and bulk phases can be identified in pentacene growths, with the precise lattice parameters dependent on the deposition rates. The effect on the microstructure, resulting from progressive dilution of pentacene in a p-terphenyl host, was then investigated. Although disorder increases and the crystallite size decreases in the mixture, with a minimum at a 1 : 1 ratio, phase segregation is not observed on the length scale (limit) that can be probed in our measurements. This indicates that the mixed films form homogeneous solid-solutions that may be employed for the investigation of solid-state phenomena. Our methodology can be extended to other compatible host-dopant systems used in optoelectronic and spintronic devices.
Lubert-Perquel D, Salvadori E, Dyson M, et al., 2018, Identifying triplet pathways in dilute pentacene films, NATURE COMMUNICATIONS, Vol: 9, ISSN: 2041-1723
Leber R, Wilson LE, Robaschik P, et al., 2017, High Vacuum Deposition of Biferrocene Thin Films on Room Temperature Substrates, Chemistry of Materials, Vol: 29, Pages: 8663-8669, ISSN: 0897-4756
Metallocenes are a promising candidate for future spintronic devices due to their versatile and tunable magnetic properties. However, single metallocenes, e.g., ferrocene, sublimate below room temperature, and therefore the implementation for future applications is challenging. Here, a method to prepare biferrocene thin films using organic molecular beam deposition (OMBD) is presented, and the effect of substrate and deposition rate on the film structure and morphology as well as its chemical and magnetic properties is investigated. On Kapton and Si substrates, biferrocene interacts only weakly with the substrate, and distinct grains scattered over the surface are observed. By incorporating a 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) seeding layer and depositing biferrocene at high deposition rates of 1.0 Å s–1, it is possible to achieve a well-ordered densely packed film. With spintronic applications in mind, the magnetic properties of the thin films are characterized using superconducting quantum interference device (SQUID) magnetometry. Whereas initial SQUID measurements show weak ferromagnetic behavior up to room temperature due to oxidized molecule fragments, measurements of biferrocene on PTCDA capped with LiF show the diamagnetic behavior expected of biferrocene. Through the successful deposition of biferrocene thin films and the ability to control the spin state, these results demonstrate a first step toward metallocene-based spintronics.
Robaschik P, Ma Y, Din S, et al., 2017, Formation of ferromagnetic molecular thin films from blends by annealing, BEILSTEIN JOURNAL OF NANOTECHNOLOGY, Vol: 8, Pages: 1469-1475, ISSN: 2190-4286
We report on a new approach for the fabrication of ferromagnetic molecular thin films. Co-evaporated films of manganese phthalocyanine (MnPc) and tetracyanoquinodimethane (TCNQ) have been produced by organic molecular beam deposition (OMBD) on rigid (glass, silicon) and flexible (Kapton) substrates kept at room temperature. The MnPc:TCNQ films are found to be entirely amorphous due to the size mismatch of the molecules. However, by annealing while covering the samples highly crystalline MnPc films in the β-polymorph can be obtained at 60 °C lower than when starting with pure MnPc films. The resulting films exhibit substantial coercivity (13 mT) at 2 K and a Curie temperature of 11.5 K.
Alexander JA, Scheltens FJ, Drummy LF, et al., 2017, Measuring Optical Absorption in Organic Photovoltaics Using Monochromated Electron Energy-Loss Spectroscopy, IEEE 44th Photovoltaic Specialist Conference (PVSC), Publisher: IEEE, Pages: 966-969, ISSN: 0160-8371
Fleet LR, Stott J, Villis B, et al., 2017, Self-Assembled Molecular Nanowires for High-Performance Organic Transistors, ACS Applied Materials and Interfaces, Vol: 9, Pages: 20686-20695, ISSN: 1944-8244
While organic semiconductors provide tantalizing possibilities for low-cost, light-weight, flexible electronic devices, their current use in transistors—the fundamental building block—is rather limited as their speed and reliability are not competitive with those of their inorganic counterparts and are simply too poor for many practical applications. Through self-assembly, highly ordered nanostructures can be prepared that have more competitive transport characteristics; however, no simple, scalable method has been discovered that can produce devices on the basis of such nanostructures. Here, we show how transistors of self-assembled molecular nanowires can be fabricated using a scalable, gradient sublimation technique, which have dramatically improved characteristics compared to those of their thin-film counterparts, both in terms of performance and stability. Nanowire devices based on copper phthalocyanine have been fabricated with threshold voltages as low as −2.1 V, high on/off ratios of 105, small subthreshold swings of 0.9 V/decade, and mobilities of 0.6 cm2/V s, and lower trap energies as deduced from temperature-dependent properties, in line with leading organic semiconductors involving more complex fabrication. High-performance transistors manufactured using our scalable deposition technique, compatible with flexible substrates, could enable integrated all-organic chips implementing conventional as well as neuromorphic computation and combining sensors, logic, data storage, drivers, and displays.
Gilchrist JB, Heutz S, McComb DW, 2017, Revealing structure and electronic properties at organic interfaces using TEM, Current Opinion in Solid State and Materials Science, Vol: 21, Pages: 68-76, ISSN: 1359-0286
Molecules and atoms at material interfaces have properties that are distinct from those found in the bulk. Distinguishing the interfacial species from the bulk species is the inherent difficulty of interfacial analysis. For organic photovoltaic devices, the interface between the donor and acceptor materials is the location for exciton dissociation. Dissociation is thought to occur via a complex route effected by microstructure and the electronic energy levels. The scale of these devices and the soft nature of these materials create an additional level of difficulty for identification and analysis at these interfaces. The transmission electron microscope (TEM) and the spectroscopic techniques it incorporates can allow the properties of the donor-acceptor interfaces to be revealed. Cross-sectional sample preparation, using modern focused ion beam instruments, enables these buried interfaces to be uncovered with minimal damage for high resolution analysis. This powerful instrument combination has the ability to draw conclusions about interface morphology, structure and electronic properties of organic donor-acceptor interfaces at the molecular scale. Recent publications have demonstrated these abilities, and this article aims to summarise some of that work and provide scope for the future.
Eguchi K, Heutz S, Awaga K, 2017, Templating effects of tetrakis(thiadiazole) porphyrazine on the structure and optical properties of copper phthalocyanine thin films, Journal of Porphyrins and Phthalocyanines, Vol: 21, Pages: 322-326, ISSN: 1088-4246
Molecular templating is an attractive method to improve the crystallinity and control the molecular orientations of organic thin films. Here, we report on the templating effects of an organic nn-type semiconductor, tetrakis(thiadiazole)porphyrazine (H2TTDPz), on the structure and optical absorption of a pp-type semiconductor, copper phthalocyanine (CuPc). X-ray diffraction measurements for the double layer thin films, CuPc/H2TTDPz, indicate a face-on orientation of CuPc, which is replicating the structure of the H2TTDPz thin films, even though the CuPc thin films usually form edge-on-type thin films. The optical absorption measurements show new low-energy transitions in the templated CuPc films.
Gonzalez Arellano DL, Bhamrah Harley J, Yang J, et al., 2017, Room temperature routes towards the creation of zinc oxide films from molecular precursors, ACS Omega, Vol: 2, Pages: 98-104, ISSN: 2470-1343
The advent of “flexible” electronics on plastic substrates with low melting points requires the development of thin film deposition techniques that operate at low temperatures. This is easily achieved with vacuum or solution-processed molecular or polymeric semiconductors, but oxide materials remain a significant challenge. Here we show that zinc oxide (ZnO) can be prepared using only room-temperature processes, using the molecular thin film precursor zinc phthalocyanine (ZnPc), followed by vacuum ultra-violet light treatment to elicit degradation of the organic components and transformation of the deposited film to oxide material. The degradation mechanism was assessed by studying the influence of the atmosphere during the reaction: it was particularly sensitive to oxygen pressure in the chamber and optimal degradation conditions were established as 3 mbar with 40% oxygen in nitrogen. The morphology of the film was relatively unchanged during the reaction, but detailed analysis of itscomposition using both scanning transmission electron microscopy (STEM) and secondary ion mass spectrometry (SIMS) revealed that a 40 nm thick layer containingZnO results from the 100 nm thick precursor after complete reaction. Our methodology represents a simple route for the fabrication of oxides and multilayer structuresthat can be easily integrated into current molecular thin film growth setups,without the need for a high temperature step.
Alexandra J Ramadan, Fearn S, Tim Jones, et al., 2016, Film Formation of Non-Planar Phthalocyanines on Copper (I) Iodide, RSC Advances, Vol: 6, Pages: 95227-95231, ISSN: 2046-2069
Structural templating is frequently used in organic photovoltaic devices to control the properties of the functional layersand therefore improve efficiencies. Modification of the substrate temperatures has also been shown to impact thestructure and morphology of phthalocyanine thin films. Here we combine templating by copper iodide and high substratetemperature growth and study its effect on the structure and morphology of two different non-planar phthalocyanines,chloroaluminium (ClAlPc) and vanadyl (VOPc) phthalocyanine. X-ray diffraction, atomic force microscopy and low energyion scattering show that both the morphology and the structure of the films are starkly different in every case, highlightingthe versatility of phthalocyanine film growth.
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