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This paper describes a simple model for comparing the degree of electronic coupling between molecules and electrodes across different large-area molecular junctions. The resulting coupling parameter can be obtained directly from current–voltage data or extracted from published data without fitting. We demonstrate the generalizability of this model by comparing over 40 different junctions comprising different molecules and measured by different laboratories. The results agree with existing models, reflect differences in mechanisms of charge transport and rectification, and are predictive in cases where experimental limitations preclude more sophisticated modeling. We also synthesized a series of conjugated molecular wires, in which embedded dipoles are varied systematically and at both molecule–electrode interfaces. The resulting current–voltage characteristics vary in nonintuitive ways that are not captured by existing models, but which produce trends using our simple model, providing insights that are otherwise difficult or impossible to explain. The utility of our model is its demonstrative generalizability, which is why simple observables like tunneling decay coefficients remain so widely used in molecular electronics despite the existence of much more sophisticated models. Our model is complementary, giving insights into molecule–electrode coupling across series of molecules that can guide synthetic chemists in the design of new molecular motifs, particularly in the context of devices comprising large-area molecular junctions.

The ever-increasing demand for cleaner sources of energy calls for inexpensive and highly efficient solar cells. Solution- and vacuum-processable organic solar cells have gained significant attention as an attractive source of energy owing to their continuously rising power conversion efficiency, low cost, thin device structure, and mechanical flexibility. The absorption of light in organic solar cells generates a strongly bound electron-hole pair called an exciton. To efficiently dissociate the exciton into charges, a heterojunction composed of electron donating and accepting materials with suitable energy levels is typically used to provide the driving force needed to overcome the Coulomb attraction between the electron-hole pair. The photon-to-charge conversion in organic solar cells is a complex process that is subject to losses; hence, it is critical to carefully monitor every step from photon-to-charge conversion, such as exciton photogeneration, exciton diffusion towards the donor-acceptor interface, and exciton dissociation into charges. This thesis sheds light on the photon-to-charge conversion process in organic photovoltaics using a multi-disciplinary approach that combines results from steady-state and time-resolved spectroscopy, and Monte Carlo simulations.

Photoactuators and photoluminescent dyes utilize light to perform mechanical motion and undergo spontaneous radiation emission, respectively. Combining these two functionalities in a single molecule would benefit the construction of advanced molecular machines. Due to the possible detrimental interaction between the two light-dependent functional parts, the design of hybrid systems featuring both functions in parallel remains highly challenging. Here, we develop a light-driven rotary molecular motor with an efficient photoluminescent dye chemically attached to the motor, not compromising its motor function. This molecular system shows efficient rotary motion and bright photoluminescence, and these functions can be addressed by a proper choice of excitation wavelengths and solvents. The moderate interaction between the two parts generates synergistic effects, which are beneficial for lower-energy excitation and chirality transfer from the motor to the photoluminescent dye. Our results provide prospects towards photoactive multifunctional systems capable of carrying out molecular rotary motion and tracking its location in a complex environment.

This paper describes the conductance of single-molecules and self-assembled monolayers comprising an oligophenyleneethynylene core, functionalized with acenes of increasing length that extend conjugation perpendicular to the path of tunneling electrons. In the Mechanically Controlled Break Junction (MCBJ) experiment, multiple conductance plateaus were identified. The high conductance plateau, which we attribute to the single molecule conformation, shows an increase of conductance as a function of acene length, in good agreement with theoretical predictions. The lower plateau is attributed to multiple molecules bridging the junctions with intermolecular interactions playing a role. In junctions comprising a self-assembled monolayer with eutectic Ga–In top-contacts (EGaIn), the pentacene derivative exhibits unusually low conductance, which we ascribe to the inability of these molecules to pack in a monolayer without introducing significant intermolecular contacts. This hypothesis is supported by the MCBJ data and theoretical calculations showing suppressed conductance through thePCfilms. These results highlight the role of intermolecular effects and junction geometries in the observed fluctuations of conductance values between single-molecule and ensemble junctions, and the importance of studying molecules in both platforms.

This study describes the modulation of tunneling probabilities in molecular junctions by switching one of two parallel intramolecular pathways. A linearly conjugated molecular wire provides a rigid framework that allows a second, cross‐conjugated pathway to be effectively switched on and off by protonation, affecting the total conductance of the junction. This approach works because a traversing electron interacts with the entire quantum‐mechanical circuit simultaneously; Kirchhoff's rules do not apply. We confirm this concept by comparing the conductances of a series of compounds with single or parallel pathways in large‐area junctions using EGaIn contacts and single‐molecule break junctions using gold contacts. We affect switching selectively in one of two parallel pathways by converting a cross‐conjugated carbonyl carbon into a trivalent carbocation, which replaces destructive quantum interference with a symmetrical resonance, causing an increase in transmission in the bias window., Manipulating conductance in parallel molecular pathways is a crucial element in molecular electronics. We explore intramolecular parallel pathways with different bond topology, in form of a quantum circuit, in tunneling junctions comprising self‐assembled monolayers. We employ quantum‐interference switching/gating with an acid, without any structural changes, in single‐molecular junctions.

The bottom-up fabrication of functional nanosystems for light-harvesting applications and excitonic devices often relies on molecular self-assembly. Gaining access to the intermediate species involved in self-assembly would provide valuable insights into the pathways via which the final architecture has evolved, yet difficult to achieve due to their intrinsically short-lived nature. Here, we employ a lab-on-a-chip approach as a means to obtain in situ control of the structural complexity of an artificial light-harvesting complex: molecular double-walled nanotubes. Rapid and stable dissolution of the outer wall was realized via microfluidic mixing thereby rendering the thermodynamically unstable inner tubes accessible to spectroscopy. By measurement of the linear dichroism and time-resolved photoluminescence of both double-walled nanotubes and isolated inner tubes we show that the optical (excitonic) properties of the inner tube are remarkably robust to such drastic perturbation of the system's supramolecular structure as removal of the outer wall. The developed platform is readily extendable to a broad range of practical applications such as e.g. self-assembling systems and molecular photonics devices.

Molecular machines have caused one of the greatest paradigm shifts in chemistry, and by powering artificial mechanical molecular systems and enabling autonomous motion, they are expected to be at the heart of exciting new technologies. One of the biggest challenges that still needs to be addressed is designing the involved molecules to combine different orthogonally controllable functions. Here, we present a prototype of artificial molecular motors exhibiting the dual function of rotary motion and photoluminescence. Both properties are controlled by light of different wavelengths or by exploiting motors’ outstanding two-photon absorption properties using low-intensity near-infrared light. This provides a noninvasive way to both locate and operate these motors in situ, essential for the application of molecular machines in complex (bio)environments.

In recent years, the dielectric constant (εr) of organic semiconductors (OSCs) has been of interest in the organic photovoltaic (OPV) community due to its potential influence on the exciton binding energy. Despite progress in the design of high εr OSCs and the accurate measurement of the εr, the effects of the synthetic strategies on specific (opto)electronic properties of the OSCs remain uncertain. In this contribution, the effects of εr on the optical properties of five new C70 derivatives and [70]PCBM are investigated. Together with [70]PCBM, the derivatives have a range of εr values that depend on the polarity and length of the side chains. The properties of the singlet excitons are investigated in detail with steady-state and time-resolved spectroscopy and the exciton diffusion length is measured. All six derivatives show similar photophysical properties in the neat films. However, large differences in the crystallinity of the fullerene films influence the exciton dynamics in blend films. This work shows that design principles for OSCs with a higher εr can have a very different influence on the performance of traditional BHJ devices and in neat films and it is important to consider the neat film properties when investigating the optoelectronic properties of new materials for OPV.

We present dynamics of the first prototype of artificial molecular motors exhibiting the dual function of rotary motion and photoluminescence. This provides a non-invasive way to locate and operate these machines in complex (bio)environments.

Donor-acceptor molecules with thiophene fragments as the π-bridge represent a promising class of materials for organic photovoltaics especially in single-component (SC) organic solar cells (OSCs) and other related applications. However, the effect of the oligothiophene π-bridge length on physicochemical properties, photophysics, charge transport, and photovoltaic performance of these materials has not been thoroughly addressed. Here, we report on the synthesis and comprehensive investigation of the series of star-shaped donor-acceptor molecules ( 0T–4T ) with triphenylamine as a donor core linked through an oligothiophene π-bridge of variable length to the terminal hexyl-dicyanovinyl electron-withdrawing groups. We found that variation of the π-bridge length from 0 to 4 thiophene units strongly impacts their properties such as the solubility, highest occupied molecular orbital energy, optical absorption and photophysics, film morphology, phase behavior, and molecular packing as well as the charge carrier mobility . The performance of the SC and bulk heterojunction OSCs and photodetectors is comprehensively studied and compared. The results obtained provide insight into how to fine-tune and predict properties and photovoltaic performance of small molecules for organic solar cells and photodetectors.

The energy transport in natural light-harvesting complexes can be explored in laboratory conditions via self-assembled supramolecular structures. One such structure arises from the amphiphilic dye C8S3 molecules, which self-assemble in an aqueous medium to a double-wall cylindrical nanotube reminiscent of natural light-harvesting complexes found in green sulphur bacteria. In this paper, we report a way to investigate the structure of inner nanotubes (NTs) alone by dissolving the outer NTs in a microfluidic setting. The resulting thermodynamically unstable system was rapidly frozen, preventing the reassembly of the outer NT from the dissolved molecules, and imaged using cryogenic transmission electron microscopy (cryo-TEM). The experimental cryo-TEM images and the molecular structure were compared by simulating high-resolution TEM images, which were based on the molecular modelling of C8S3 NTs. We found that the inner NT with outer walls removed during the flash-dilution process had a similar size to the parent double-walled NTs. Moreover, no structural inhomogeneity was observed in the inner NT after flash-dilution. This opens up exciting possibilities for functionalisation of inner NTs before the reassembly of the outer NT occurs, which can be broadly extended to modify the intra-architecture of other self-assembled nanostructures.

Organic-inorganic hybrid materials have shown a remarkable and rapid development during the past decade because they can be tailored to obtain new device concepts with controlled physical properties. Here, we report on the electronic and magnetic properties of multilayer organic-inorganic hybrid films. Electrical transport properties arising from the pi electrons in the organic layer are characteristic of a metallic state at high temperature and evolve into a state described by two-dimensional variable range hopping when temperature decreases below 150 K. The intrinsic electronic behavior of the hybrid films was further studied via the optical properties in the IR range. The optical response confirms the metallic character of the hybrid films. In the second part, the magnetic properties are discussed. A long-range ferromagnetic order with an ordering temperature of similar to 1 K is revealed in the Gd-based hybrid film. The Cu-based hybrid film, however, shows more extended ferromagnetic exchange interactions than the Gd-based hybrid LB film.

This paper describes an experimental approach to eliminating the loss of reversibility that surface-bound spiropyrans exhibit when switched with light. Although such fatigue can be controlled in other contexts, on surfaces, the photochromic compounds are held in close proximity to each other and relatively few molecules modulate the properties of a device, leading to a loss of functionality after only a few switching cycles. The switching process was characterized by photoelectron spectroscopy and differences in tunneling currents in the spiropyran and merocyanine forms using eutectic Ga-In. Self-assembled monolayers comprising only the photochromic compounds degraded rapidly, while mixed monolayers with hexanethiol showed different behaviors depending on the relative humidity. Under dry conditions, no chemical degradation was observed and the switching process was reversible over at least 100 cycles. Under humid conditions, no degradation occurred, but the switching process became irreversible. The absence of degradation observed in mixed monolayers is ascribed to the lack of solvation, which increases the barrier to a key bond rotation past the available thermal energy. These results highlight important differences in the contexts in which photochromic compounds are utilized and demonstrate that they can be leveraged to extract device-relevant functionality from surface-bound switches by suppressing fatigue and irreversibility.

This thesis presents an investigation of the electrical properties of about 70 different molecules in molecular tunneling junctions (MTJs). It begins by presenting the field of molecular electronics (ME) -- from the advent of quantum mechanics to the first conception of ideas and experiments in ME. Then it discusses different steps in the fabrication of self-assembled monolayers (SAMs) and their incorporation in large-area MTJ, followed by data acquisition and analysis of current-voltage characteristics. Further, the theoretical methodology, which uses density functional theory in conjunction with non-equilibrium Green's function approach to simulate charge transport probability through MTJs, is reported. After the introduction of several concepts, an in-depth study of quantum interference (QI) effect on a series of molecular wires, comprising backbones with different conjugation patterns, is reported. Continuing with the QI studies, having established the effect of functional groups on QI, another approach to modulate the tunneling probabilities in MTJs by switching QI ON and OFF in one of the two parallel intramolecular pathways is presented. Next, the thesis discusses the non-linear tunneling current decay in sigma-pi molecular structures (oligothiophene-terminated butanethiols) with increasing molecular lengths, explained using a two-barrier model. Finally, the final chapter describes a generalized single-level model that compares the degree of electronic coupling between molecules and electrodes across 40 different large-area EGaIn-based MTJs from different laboratories. Thus, starting from the introduction of several ME concepts, experiments, and simulations, this thesis enables the reader through investigations on a big library of molecules that serve manifold purposes in ME.

International audience; Pi-Conjugated push-pull molecules based on triphenylamine and 1,1,4,4-tetracyanobuta-1,3diene (TCBD) have been functionalized with different terminal arene units. In solution, these highly TCBD-twisted systems showed a strong internal charge transfer band in the visible spectrum and no detectable photoluminescence (PL). Photophysical and theoretical investigations revealed very short singlet excited state deactivation time of ~ 10 ps resulting from significant conformational changes of the TCBD-arene moiety upon photoexcitation, opening a pathway for non-radiative decay. The PL was recovered in vacuum-processed films or when the molecules were dispersed in a PMMA matrix leading to a significant increase of the excited state deactivation time. As shown by cyclic voltammetry, these molecules can act as electron-donors compared to C60. Hence, vacuum-processed planar heterojunction organic solar cells were fabricated leading to a maximum power conversion efficiency of ca. 1.9% which decreases with the increase of the arene size.

Triphenylamine-based small push–pull molecules have recently attracted substantial research attention due to their unique optoelectronic properties. Here, we investigate the excited state de-excitation dynamics and exciton diffusion in TPA-T-DCV-Ph-F small molecule, having simple chemical structure with asymmetrical architecture and end-capped with electron-withdrawing p-fluorodicyanovinyl group. The excited state lifetime in diluted solutions (0.04 ns in toluene and 0.4 ns in chloroform) are found to be surprisingly shorter compared to the solid state (3 ns in PMMA matrix). Time-dependent density functional theory indicates that this behavior originates from non-radiative relaxation of the excited state through a conical intersection between the ground and singlet excited state potential energy surfaces. Exciton diffusion length of ~ 16 nm in solution processed films was retrieved by employing time-resolved photoluminescence volume quenching measurements with Monte Carlo simulations. As means of investigating the device performance of TPA-T-DCV-Ph-F, we manufactured solution and vacuum processed bulk heterojunction solar cells that yielded efficiencies of ~ 1.5% and ~ 3.7%, respectively. Our findings demonstrate that the short lifetime in solutions does not hinder per se long exciton diffusion length in films thereby granting applications of TPA-T-DCV-Ph-F and similar push–pull molecules in vacuum and solution processable devices.

Dye-sensitization of a molecular motor allowed its functioning under two-photon near-infrared excitation. Ultrafast transient absorption spectroscopy was used to verify energy transfer from the sensitizer to the motor and motor’s subsequent rotation.

Ultrafast 2D spectroscopy is combined with single-object spectroscopy to disentangle the structural heterogeneity of an artificial light-harvester. The dynamically (~50 fs timescale) fluctuating environment governs the system’s properties, but not structural variations among different harvesters.

Natural light-harvesting antennae employ a dense array of chromophores to optimize energy transport via the formation of delocalized excited states (excitons), which are critically sensitive to spatio-energetic variations of the molecular structure. Identifying the origin and impact of such variations is highly desirable for understanding and predicting functional properties yet hard to achieve due to averaging of many overlapping responses from individual systems. Here, we overcome this problem by measuring the heterogeneity of synthetic analogues of natural antennae-self-assembled molecular nanotubes-by two complementary approaches: single-nanotube photoluminescence spectroscopy and ultrafast 2D correlation. We demonstrate remarkable homogeneity of the nanotube ensemble and reveal that ultrafast (∼50 fs) modulation of the exciton frequencies governs spectral broadening. Using multiscale exciton modeling, we show that the dominance of homogeneous broadening at the exciton level results from exchange narrowing of strong static disorder found for individual molecules within the nanotube. The detailed characterization of static and dynamic disorder at the exciton as well as the molecular level presented here opens new avenues in analyzing and predicting dynamic exciton properties, such as excitation energy transport.

An artificial molecular motor is efficiently powered with near-infrared light compatible with in vivo applications., Light-controlled artificial molecular machines hold tremendous potential to revolutionize molecular sciences as autonomous motion allows the design of smart materials and systems whose properties can respond, adapt, and be modified on command. One long-standing challenge toward future applicability has been the need to develop methods using low-energy, low-intensity, near-infrared light to power these nanomachines. Here, we describe a rotary molecular motor sensitized by a two-photon absorber, which efficiently operates under near-infrared light at intensities and wavelengths compatible with in vivo studies. Time-resolved spectroscopy was used to gain insight into the mechanism of energy transfer to the motor following initial two-photon excitation. Our results offer prospects toward in vitro and in vivo applications of artificial molecular motors.

Despite their ubiquity, self-assembled monolayers (SAMs) of thiols on coinage metals are difficult to study and are still not completely understood, particularly with respect to the nature of thiol-metal bonding. Recent advances in molecular electronics have highlighted this deficiency due to the sensitivity of tunneling charge-transport to the subtle differences in the overall composition of SAMs and the chemistry of their attachment to surfaces. These advances have also challenged assumptions about the spontaneous formation of covalent thiol-metal bonds. This paper describes a series of experiments that correlate changes in the physical properties of SAMs to photoelectron spectroscopy to unambiguously assign binding energies of noncovalent interactions to physisorbed disulfides. These disulfides can be converted to covalent metal-thiolate bonds by exposure to free thiols, leading to the remarkable observation of the total loss and recovery of length-dependent tunneling charge-transport. The identification and assignment of physisorbed disulfides solve a long-standing mystery and reveal new, dynamic properties in SAMs of thiols.

Photosynthetic systems that exist in plants, algae and some bacteria are Earth’s powerhouse by harnessing its most abundant energy source: sunlight. As one of the key elements, they employ a functional nano-machinery that is typically built from many thousands of autonomously assembled molecules: so-called light-harvesting antennae. In billions of years of evolution, nature has engineered these systems to maximize light-absorption and enable photosynthesis under physiological (warm and humid) conditions that would be considered adverse to most lab-based applications. In order to replicate nature’s design principles for light-harvesting antennae for potential applications, the essential functional elements have to be identified and their working principles understood. Inspired by the success of natural light-harvesting antennae, artificial (man-made) systems that closely resemble the structure of their natural counterparts, have recently experienced great attention. These synthetic analogues feature a high degree of internal homogeneity (i.e., different systems are identical), while also being easier to produce, control and modify than the natural systems. In this Thesis, we study how one of such systems, double-walled molecular nanotubes react to light, how the absorbed energy is transported, and how the energy transport is affected by the structure and dimensionality of the system. Working hand in hand with molecular dynamics simulations and theoretical models, our findings pave the way to optimizing and incorporating such systems in (opto)electronic devices and light-harvesting applications.

Multiple optical harmonic generation—the multiplication of photon energy as a result of nonlinear interaction between light and matter—is a key technology in modern electronics and optoelectronics, because it allows the conversion of optical or electronic signals into signals with much higher frequency, and the generation of frequency combs. Owing to the unique electronic band structure of graphene, which features massless Dirac fermions1–3, it has been repeatedly predicted that optical harmonic generation in graphene should be particularly efficient at the technologically important terahertz frequencies4–6. However, these predictions have yet to be confirmed experimentally under technologically relevant operation conditions. Here we report the generation of terahertz harmonics up to the seventh order in single-layer graphene at room temperature and under ambient conditions, driven by terahertz fields of only tens of kilovolts per centimetre, and with field conversion efficiencies in excess of 10−3, 10−4 and 10−5 for the third, fifth and seventh terahertz harmonics, respectively. These conversion efficiencies are remarkably high, given that the electromagnetic interaction occurs in a single atomic layer. The key to such extremely efficient generation of terahertz high harmonics in graphene is the collective thermal response of its background Dirac electrons to the driving terahertz fields. The terahertz harmonics, generated via hot Dirac fermion dynamics, were observed directly in the time domain as electromagnetic field oscillations at these newly synthesized higher frequencies. The effective nonlinear optical coefficients of graphene for the third, fifth and seventh harmonics exceed the respective nonlinear coefficients of typical solids by 7–18 orders of magnitude7–9. Our results provide a direct pathway to highly efficient terahertz frequency synthesis using the present generation of graphene electronics, which operate at much lower fundamental frequencies of only a few hundreds of gigahertz. Efficient terahertz harmonic generation—challenging but important for ultrahigh-speed optoelectronic technologies—is demonstrated in graphene through a nonlinear process that could potentially be generalized to other materials.

Quantum interference effects (QI) are of interest in nano-scale devices based on molecular tunneling junctions because they can affect conductance exponentially through minor structural changes. However, their utilization requires the prediction and deterministic control over the position and magnitude of QI features, which remains a significant challenge. In this context, we designed and synthesized three benzodithiophenes based molecular wires; one linearly-conjugated, one crossconjugated and one cross-conjugated quinone. Using eutectic Ga-In (EGaIn) and CP-AFM, we compared them to a well-known anthraquinone in molecular junctions comprising self-assembled monolayers (SAMs). By combining density functional theory and transition voltage spectroscopy, we show that the presence of an interference feature and its position can be controlled independently by manipulating bond topology and electronegativity. This is the first study to separate these two parameters experimentally, demonstrating that the conductance of a tunneling junction depends on the position and depth of a QI feature, both of which can be controlled synthetically.

We presented the design and experimental results of a new sideband-separating (2SB) mixer for 600-720 GHz (ALMA Band 9). Although designed following the classical quadrature hybrid architecture, the emphasis of the optimization was this time not placed on ultimate phase and amplitude balance in the hybrid, but on the reduction and active suppression of reflections and standing waves. It turns out that, once the phase and the amplitude are sufficiently balanced, the image rejection ratio (IRR) starts being dominated by unbalanced parasitic reflections in the signal path, and further perfection of the hybrid's balance is not the way toward higher IRR. Four operational waveguide blocks were produced. IRRs above 15 dB over the entire band were obtained, depending critically on mixer matching. The noise temperature was not more than 20-30 K above the value expected from the individual mixer devices. With the RF circuits optimized thus, the performance becomes limited by the IF system, where similar interference and standing wave problems seem to occur. These are exacerbated when, for compactness (i.e., in arrays), cryogenic isolators between the mixers and amplifiers are omitted. We show that, because of the high performance of the mixers, an IRR of 10 dB can still be obtained without isolators. When, on the other hand, highly matched amplifiers are used, we expect that the full image rejection performance can be achieved even in array receivers. These 2SB mixers are intended for the future 2SB receivers on the APEX (Chile) and LLAMA (Argentina) observatories and for deployment in any other observatory that would benefit from sideband separation in the 600-720-GHz band.

Intrinsically disordered proteins play an important role in biology, and unraveling their labile structure presents a vital challenge. However, the dynamical structure of such proteins thwarts their study by standard techniques such as X-ray diffraction and NMR spectroscopy. Here, we use a neat liquid composed of N-methylacetamide molecules as a model system to elucidate dynamical and structural properties similar to those one can expect to see in intrinsically disordered proteins. To examine the structural dynamics in the neat liquid, we combine molecular dynamics, response-function-based spectral simulations, and two-dimensional polarization-resolved infrared spectroscopy in the amide I (CO stretch) region. The two-dimensional spectra reveal a delicate interplay between hydrogen bonding and intermolecular vibrational coupling effects, observed through a fast anisotropy decay. The present study constitutes a general platform for understanding the structure and dynamics of highly disordered proteins.

Dmitry Kobak, Sergey Shpilkin and Maxim S. Pshenichnikov find suspicious patterns in data from Russia's nationwide vote on a package of constitutional changes

Unravelling the nature of energy transport in multi-chromophoric photosynthetic complexes is essential to extract valuable design blueprints for light-harvesting applications. Long-range exciton transport in such systems is facilitated by a combination of delocalized excitation wavefunctions (excitons) and remarkable exciton diffusivities. The unambiguous identification of the exciton transport, however, is intrinsically challenging due to the system's sheer complexity. Here we address this challenge by employing a novel spectroscopic lab-on-a-chip approach: A combination of ultrafast coherent two-dimensional spectroscopy and microfluidics working in tandem with theoretical modelling. This allowed us to unveil exciton transport throughout the entire hierarchical supramolecular structure of a double-walled artificial light-harvesting complex. We show that at low exciton densities, the outer layer acts as an antenna that supplies excitons to the inner tube, while under high excitation fluences it protects the inner tube from overburning. Our findings shed light on the excitonic trajectories across different sub-units of a multi-layered supramolecular structure and underpin the great potential of artificial light-harvesting complexes for directional excitation energy transport., Comment: Submitted to Nature Communications; main manuscript 37 pages (incl. references) and 5 figures. SI 59 pages (incl. references) and 25 Figures

In order to translate molecular properties in molecular-electronic devices, it is necessary to create design principles that can be used to achieve better structure–function control oriented toward device fabrication., In order to translate molecular properties in molecular-electronic devices, it is necessary to create design principles that can be used to achieve better structure–function control oriented toward device fabrication. In molecular tunneling junctions, cross-conjugation tends to give rise to destructive quantum interference effects that can be tuned by changing the electronic properties of the molecules. We performed a systematic study of the tunneling charge-transport properties of a series of compounds characterized by an identical cross-conjugated anthraquinoid molecular skeleton but bearing different substituents at the 9 and 10 positions that affect the energies and localization of their frontier orbitals. We compared the experimental results across three different experimental platforms in both single-molecule and large-area junctions and found a general agreement. Combined with theoretical models, these results separate the intrinsic properties of the molecules from platform-specific effects. This work is a step towards explicit synthetic control over tunneling charge transport targeted at specific functionality in (proto-)devices.

Using spatial light interference of ultrafast laser pulses, we generate a lateral modulation in the magnetization profile of an otherwise uniformly magnetized film, whose magnetic excitation spectrum is monitored via the coherent and resonant interaction with elastic waves. We find an unusual dependence of the magnetoelastic coupling as the externally applied magnetic field is angle- and field-tuned relative to the wave vector of the magnetization modulation, which can be explained by the emergence of spatially inhomogeneous spin-wave modes. In this regard, the spatial light interference methodology can be seen as a user-configurable, temporally windowed, on-demand magnonic crystal, potentially of arbitrary two-dimensional shape, which allows control and selectivity of the spatial distribution of spin waves. Calculations of spin waves using a variety of methods, demonstrated here using the plane-wave method and micromagnetic simulation, can identify the spatial distribution and associated energy scales of each excitation, which opens the door to a number of excitation methodologies beyond our chosen elastic wave excitation.

Three-pulse ultrafast spectroscopy reveals interfacial obstacles in methylammonium lead iodide during hot carrier transfer., Halide perovskites are promising materials for development in hot carrier (HC) solar cells, where the excess energy of above-bandgap photons is harvested before being wasted as heat to enhance device efficiency. Presently, HC separation and transfer processes at higher-energy states remain poorly understood. Here, we investigate the excited state dynamics in CH3NH3PbI3 using pump-push-probe spectroscopy. It has its intrinsic advantages for studying these dynamics over conventional transient spectroscopy, albeit complementary to one another. By exploiting the broad excited-state absorption characteristics, our findings reveal the transfer of HCs from these higher-energy states into bathophenanthroline (bphen), an energy selective organic acceptor far above perovskite’s band edges. Complete HC extraction is realized only after overcoming the interfacial barrier formed at the heterojunction, estimated to be between 1.01 and 1.08 eV above bphen’s lowest unoccupied molecular orbital level. The insights gained here are essential for the development of a new class of optoelectronics.

Ruddlesden-Popper (RP) halide perovskites are the new kids on the block for high-performance perovskite photovoltaics with excellent ambient stability. The layered nature of these perovskites offers an exciting possibility of harnessing their ferroelectric property for photovoltaics. Adjacent polar domains in a ferroelectric material allow the spatial separation of electrons and holes. Presently, the structure-function properties governing the ferroelectric behavior of RP perovskites are an open question. Herein, we realize tunable ferroelectricity in 2-phenylethylammonium (PEA) and methylammonium (MA) RP perovskite (PEA)2(MA) n̅-1Pb n̅I3 n̅+1. Second harmonic generation (SHG) confirms the noncentrosymmetric nature of these polycrystalline thin films, whereas piezoresponse force microscopy and polarization-electric field measurements validate the microscopic and macroscopic ferroelectric properties. Temperature-dependent SHG and dielectric constant measurements uncover a phase transition temperature at around 170 °C in these films. Extensive molecular dynamics simulations support the experimental results and identified the correlated reorientation of MA molecules and ion translations as the source of ferroelectricity. Current-voltage characteristics in the dark reveal the persistence of hysteresis in these devices, which has profound implications for light-harvesting and light-emitting applications. Importantly, our findings disclose a viable approach for engineering the ferroelectric properties of RP perovskites that may unlock new functionalities for perovskite optoelectronics. NRF (Natl Research Foundation, S’pore) MOE (Min. of Education, S’pore) Accepted version

It is well-recognized in condensed matter physics that structural and magnetic properties are intimately connected; thus controlling structure has been a successful route to controlling material properties. Extending the methodologies to high frequency modulation provides a route to controlled, bidirectional modification to material properties, expanding the range of material functionality and opening new possibilities for future developments in fields as disparate as magnetic control, spintronics, and magnonics. Ultrafast optical techniques provide one manner in which elastic and magnetic dynamics can be controlled in a material, since ultrafast pulses of light are known to excite elastic deformations while also modifying the magnetic properties of the material. Optically generated elastic waves can routinely achieve the frequency range of GHz with wavelength ranges of a few micrometers, which interestingly overlaps almost perfectly with similar wavelength spin waves. In our work, we first explored the emergence of a transient magnonic crystal after the impulsive excitation of transient grating and the phase-locked elastic wave capable of preferentially driving precessional motion in different regions of the material. Secondly we have demonstrated the first instance of elastically and parametrically driven ferromagnetic oscillator, which exhibited sum and difference frequency conversion over a wide range of frequencies. Finally, we approached magnetoelastic effects from a different point of view: the study of magnetoelastic dynamics in Ni nanowires. Connecting these experiments provide possible applications in optomagnonics research which currently utilizes artificially textured materials.

This Ph.D. thesis focuses on studying magnetization dynamics of magnetic materials on sub-picosecond timescales using terahertz (THz) radiation either as excitation or as a sensitive probe. At the beginning of the thesis, coherent excitations of spin precession are studied in thin films of ferrimagnetic Mn3-xGa Heusler alloys using THz emission spectroscopy. In this study, precise tunability of the THz emission frequency from these films is shown, which makes them highly promising candidates for future wireless communication technologies. Furthermore, the frequency dependence of THz driven ultra-fast demagnetization in conducting ferromagnetic CoFeB thin films is investigated. The ultra-fast demagnetization shows a peak behavior around 0.5 THz which is explained as a competition between the efficiency of Elliot-Yafet type spin scattering and the electron scattering in a Drude like model. This study will help to understand the microscopic mechanisms responsible for demagnetization taking place on sub-picosecond timescales. At the end of the thesis, magnon modes in insulating NiO have been studied with resonant excitation utilizing the magnetic field of the THz transients. This study reveals two magnon modes which are distinguished by their different characteristic magnetic field dependencies. The field dependence of these magnon modes is explained using an eight sub-lattice model. In general, the results presented in this thesis shed light on the field of ultra-fast magnetization dynamics at THz frequencies and are believed to provide a pathway to deepen our knowledge of magnetization dynamics at speeds which are technologically relevant for efficient spintronics devices.

The first representatives of star-shaped molecules having 3-alkylrhodanine (alkyl-Rh) electron-withdrawing groups, linked through bithiophene pi-spacer with electron-donating either triphenylamine (TPA) or tris(2-methoxyphenyl)amine (m-TPA) core were synthesized. The physical properties and photovoltaic performance of these novel molecules with 3-ethylrhodanine groups were comprehensively studied and compared to their full analogs having dicyanovinyl (DCV) units as the other type of well-known and frequently used acceptor groups. On one hand, the former demonstrate several advantages such as higher solubility and better photovoltaic performance in bulk-heterojunction (BHJ) organics solar cells (OSCs) as compared to the latter. Nevertheless, the former have slightly lower optical/electrochemical bandgaps and higher thermooxidation stability. On the other hand, molecules of both series based on m-TPA core along with higher solubility and higher position of HOMO energy levels have more pronounced tendency to crystalize as compared to the TPA-based molecules. Detailed investigation of the structure-property relationships for these series of molecules revealed that donor and acceptor unit combinations influence both charge generation and charge transport/recombination properties, as demonstrated by the ultrafast photoinduced absorption spectroscopy, space charge limited current measurements and transient photovoltage technique. These results give more insight how to fine-tune and predict physical properties and photovoltaic performance of small molecules having either alkyl-Rh or DCV units in their chemical structures and thus providing a molecular design guideline for the next generation of high-performance photovoltaic materials. (C) 2016 Elsevier B.V. All rights reserved.

Thiophene-phenylene co-oligomers (TPCOs) are among the most promising materials for organic light emitting devices. Here we report on record high among TPCO single crystals photoluminescence quantum yield reaching 60%. The solution-grown crystals are stronger luminescent than the vapor-grown ones, in contrast to a common believe that the vapor-processed organic electronic materials show the highest performance. We also demonstrate that the solution grown TPCO single crystals perform in organic field effect transistors as good as the vapor-grown ones. Altogether, the solution-grown TPCO crystals are demonstrated to hold great potential for organic electronics.

Solution-processed furan/phenylene co-oligomer single crystals combine high photoluminescence quantum yield (>65%) and efficient charge transport (mobility 0.12 cm(2) V-1 s(-1)) making them promising materials for printable organic optoelectronics.

Large‐area molecular tunneling junctions comprising self‐assembled monolayers of redox‐active molecules are described that exhibit two‐terminal bias switching. The as‐prepared monolayers undergo partial charge transfer to the underlying metal substrate (Au, Pt, or Ag), which converts their cores from a quinoid to a hydroquinoid form. The resulting rearomatization converts the bond topology from a cross‐conjugated to a linearly conjugated π system. The cross‐conjugated form correlates to the appearance of an interference feature in the transmission spectrum that vanishes for the linearly conjugated form. Owing to the presence of electron‐withdrawing nitrile groups, the reduction potential and the interference feature lie close to the work function and Fermi level of the metallic substrate. We exploited the relationship between conjugation patterns and quantum interference to create nonvolatile memory in proto‐devices using eutectic Ga–In as the top contact.

We explore the feasibility of exciting localized spin-wave modes in ferromagnetic nanostructures using surface acoustic waves. The time-resolved Faraday effect is used to probe the magnetization dynamics of an array of nickel nanowires. The optical-pump pulse excites both spin-wave modes of the nanowires and acoustic modes of the substrate and we observe that, when the frequencies of these modes coincide, the amplitude of magnetization dynamics is substantially enhanced due to magnetoelastic coupling between the two. Notably, by tuning the magnitude of an externally applied magnetic field, optically excited surface acoustic waves can selectively excite either the upper or lower branches of a splitting in the nanowire's spin-wave spectrum, which micromagnetic simulations indicate is caused by localization of spin waves in different parts of the nanowire. Thus, our results indicate the feasibility of using acoustic waves to selectively excite spatially confined spin waves, an approach that may find utility in future magnonic devices where coherent structural deformations could be used as coherent sources of propagating spin waves.

We report on measurements of magnetic field and temperature dependence of antiferromagnetic resonances in the prototypical antiferromagnet NiO. The frequencies of the magnetic resonances in the vicinity of 1 THz have been determined in the time-domain via time-resolved Faraday measurements after selective excitation by narrow-band superradiant terahertz (THz) pulses at temperatures down to 3K and in magnetic fields up to 10 T. The measurements reveal two antiferromagnetic resonance modes, which can be distinguished by their characteristic magnetic field dependencies. The nature of the two modes is discussed by comparison to an eight-sublattice antiferromagnetic model, which includes superexchange between the next-nearest-neighbor Ni spins, magnetic dipolar interactions, cubic magneto-crystalline anisotropy, and Zeeman interaction with the external magnetic field. Our study indicates that a two-sublattice model is insufficient for the description of spin dynamics in NiO, while the magnetic-dipolar interactions and magneto-crystalline anisotropy play important roles., 5 pages, 5 figures

To succeed in commercial applications, donor materials for organic solar cells should combine high stability and simple synthesis with high performance in devices. Here, we present a new small pi-conjugated molecule, TPA-T-DCV-Ph, which meets those requirements. Simple and efficient three-step synthesis produces a push-pull molecule with triphenylamine donor and phenyldicyanovinyl acceptor groups, which is suitable for both solution processing and vacuum deposition. The unique property of TPA-T-DCV-Ph is an unusually long exciton diffusion length of > 25 nm due to the combined effect of long exciton lifetime and surprisingly low energy disorder. This, together with a device engineering, resulted in > 5% efficiency for TPA-T-DCV-Ph:C-70 vacuum-processed solar cells. The results obtained are envisioned to be further improved by optimizing the absorption of the molecule and light management in the device which can push the efficiency even further.

In this Letter, the proof of principle for a scheme providing intrinsic femtosecond-level synchronization between an external laser system and fourth generation light sources is presented. The scheme is applicable at any accelerator-based light source that is based on the generation of coherent radiation from ultrashort electron bunches such as superradiant terahertz (THz) facilities or X-FELs. It makes use of a superradiant THz pulse generated by the accelerator as an intrinsically synchronized gate signal for electrooptical slicing. We demonstrate that the scheme enables a reduction of the timing instability by more than 2 orders of magnitude. This demonstration experiment thereby proves that intrinsically synchronized time-resolved experiments utilizing laser and accelerator-based radiation pulses on few tens of femtosecond (fs) to few fs timescales are feasible. (C) 2018 Optical Society of America

Recent advancements of accelerator technology enable the generation of carrier-envelope-phase stable THz pulses with high fields at adjustable high repetition rates. The appropriate choice of THz radiator allows generation of narrow-band, spectrally dense, multicycle THz transients of tunable THz frequency which are ideally suited to selectively excite low-energy excitations such as magnons or phonons. They also allow one to study the frequency dependence of nonresonant THz-field interactions with various order parameters with high dynamic range. In this paper, we discuss the future prospects of this new type of THz light source for studying the coherent control of magnetic order based on recent results.

We demonstrate an approach to parallel all-optical writing of magnetic domains using spatial and temporal interference of two ultrashort light pulses. We explore how the fluence and grating periodicity of the optical transient grating influence the size and uniformity of the written bits. Using a total incident optical energy of 3.5 μJ, we demonstrate the capability of simultaneously writing 102 spatially separated bits, each featuring a relevant lateral width of ∼1 μm. We discuss viable routes to extend this technique to write individually addressable, sub-diffraction-limited magnetic domains in a wide range of materials.We demonstrate an approach to parallel all-optical writing of magnetic domains using spatial and temporal interference of two ultrashort light pulses. We explore how the fluence and grating periodicity of the optical transient grating influence the size and uniformity of the written bits. Using a total incident optical energy of 3.5 μJ, we demonstrate the capability of simultaneously writing 102 spatially separated bits, each featuring a relevant lateral width of ∼1 μm. We discuss viable routes to extend this technique to write individually addressable, sub-diffraction-limited magnetic domains in a wide range of materials.

A Digital Sideband Separating (DSS) system has been recently applied to a full 2SB receiver, i.e., one with the analog IF hybrid still in place. This concept allows reaching IRR level around 45 dB and it presents additional advantages in calibration stability compared to the case when no IF hybrid is present. If implemented in multipixel cameras, the DSS system relaxes the requirements for the IRR level of the analog receiver substantially enabling to reach at least an IRR of 30 dB with relatively simple hardware. It would be ideal for spectral line surveys since it practically eliminates the line confusion in addition to rejecting the atmospheric noise in the image band. Therefore, the DSS system is a potential option for a future ALMA upgrade. Here we present our study on an important practical question: how wide should the calibration-channel width in order to reach a desired IRR level? This parameter determines, for a large part, the calibration speed of the DSS system and influences the back-end architecture. We estimate that for currently installed ALMA bands (B3-B8), the channel width of the DSS system can be at least 45 MHz to reach a 30db IRR level in entire band.

Localized laser heating creates temperature gradients in all directions leading to three-dimensional electron flux in metallic materials. Temperature gradients in combination with material magnetization generate thermomagnetic voltages. The interplay between these temperature gradients and the magnetization along with their control enable to manipulate the generated voltages in magnetic nanodevices. We present a highly sensitive method to identify the anomalous Nernst effect generated on the nanometer length scale by micrometer-sized temperature gradients in magnetic tunnel junctions with CoFeB electrodes and a MgO tunnel barrier systematically extracted by analyzing the influence of in-plane temperature gradients on the tunnel magneto-Seebeck effect. This method yields an anomalous Nernst effect coefficient of K-N approximate to 1.6 x 10(-8) VT-1 K-1 for CoFeB. Generally, such investigations are motivated by utilizing otherwise wasted heat in magnetic memory devices for read/write operations. The additionally generated anomalous Nernst effect offers a functionality expansion, opening new application fields such as direction-dependent temperature sensing with downscaling potential.

Ultrafast Dynamics of Intra- and Inter-molecular Interactions in Liquids and Films Evgeniia Salamatova Intra- and inter-molecular interactions determine many dynamical properties in bio- and energy-related materials such as energy transfer, vibrational relaxation and exciton diffusion. An understanding of such processes is very helpful for predicting more complex properties (e.g. hydrogen bonding and structural organization) as well as for designing new materials. In this thesis, the dynamical properties of liquids and solid films are studied by ultrafast spectroscopy. We show that the molecules with a single peptide bond (-CONH-) form atypical for liquids well-organized structures, which promote intermolecular energy transfer. If such molecules are dissolved in water, the energy migration between molecules is not largely affected but the molecules begin to experience a hydrophobic collapse. Similarly, spectroscopy on diluted alcohols demonstrates that the dynamical properties of the bulk alcohols are determined by intermolecular interactions while the intramolecular ones play a minor role. In contrast, for solid films of organic photovoltaic materials, both intra- and intermolecular interactions are crucial for energy transfer and exciton dynamics. The current thesis forms a convenient basis for predicting dynamical and structural properties of (bio)molecular systems with natural extension to e.g. DNA and proteins. The obtained results are also instrumental for designing new molecules for organic electronics applications.

While the susceptibility of CH3NH3PbI3 to water is well-documented, the influence of water on device performance is not well-understood. Herein, we use infrared spectroscopy to show that water infiltration into CH3NH3PbI3 occurs much faster and at a humidity much lower than previously thought. We propose a molecular model in which water molecules have a strong effect on the hydrogen bonding between the methylammonium cations and the Pb-I cage. Furthermore, the exposure of CH3NH3PbI3 to the ambient environment increases the photo current of films in lateral devices by more than 1 order of magnitude. The observed slow component in the photocurrent buildup indicates that the effect is associated with enhanced proton conduction when light is combined with water and oxygen exposure.

We show that extended TBTBT structure (T = thiophene, B = benzothiadiazole) can be used as an electron-deficient building block for designing conjugated polymers with deeply lying HOMO energy levels and narrow band gaps. The first carbazole-TBTBT copolymer P2 demonstrated power conversion efficiencies exceeding 6% in bulk heterojunction solar cells in combination with advanced operational stability, unlike conventional donor polymers such as PTB7, PBDTTT-CF, etc.

Ultrafast hydrogen-bond dynamics in alcohols are studied by 2D IR spectroscopy and combined molecular dynamics—quantum mechanical simulations on the OH stretching mode. Fast memory loss in *100 fs are attributed to intact hydrogen-bond fluctuations. Stable (at the experimental timescale) hydrogen bond structures lead to substantial inhomogeneity at longer times.