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Evidence is accumulating for the crucial role of a solid’s free electrons in the dynamics of solid–liquid interfaces. Liquids induce electronic polarization and drive electric currents as they flow; electronic excitations, in turn, participate in hydrodynamic friction. Yet, the underlying solid–liquid interactions have been lacking a direct experimental probe. Here we study the energy transfer across liquid–graphene interfaces using ultrafast spectroscopy. The graphene electrons are heated up quasi-instantaneously by a visible excitation pulse, and the time evolution of the electronic temperature is then monitored with a terahertz pulse. We observe that water accelerates the cooling of the graphene electrons, whereas other polar liquids leave the cooling dynamics largely unaffected. A quantum theory of solid–liquid heat transfer accounts for the water-specific cooling enhancement through a resonance between the graphene surface plasmon mode and the so-called hydrons—water charge fluctuations—particularly the water libration modes, which allows for efficient energy transfer. Our results provide direct experimental evidence of a solid–liquid interaction mediated by collective modes and support the theoretically proposed mechanism for quantum friction. They further reveal a particularly large thermal boundary conductance for the water–graphene interface and suggest strategies for enhancing the thermal conductivity in graphene-based nanostructures.

The dielectric properties of interfacial water on subnanometer length scales govern chemical reactions, carrier transfer, and ion transport at interfaces. Yet, the nature of the interfacial dielectric function has remained under debate as it is challenging to access the interfacial dielectric with subnanometer resolution. Here we use the vibrational response of interfacial water molecules probed using surface-specific sum-frequency generation (SFG) spectra to obtain exquisite depth resolution. Different responses originate from water molecules at different depths and report back on the local interfacial dielectric environment via their spectral amplitudes. From experimental and simulated SFG spectra at the air/water interface, we find that the interfacial dielectric constant changes drastically across an ∼1 Å thin interfacial water region. The strong gradient of the interfacial dielectric constant leads, at charged planar interfaces, to the formation of an electric triple layer that goes beyond the standard double-layer model.

Several metal-organic frameworks (MOFs) excel in harvesting water from the air or as heat pumps as they show a steep increase in water uptake at 10-30 % relative humidity (RH%). A precise understanding of which structural characteristics govern such behavior is lacking. Herein, CAU-10-H and CAU-10-CH3 are studied with -H, -CH3 corresponding to the functions grafted to the organic linker. CAU-10-H shows a steep water uptake approximate to 18 RH% of interest for water harvesting, yet the subtle replacement of -H by -CH3 in the organic linker drastically changes the water adsorption behavior to less steep water uptake at much higher humidity values. The materials' structural deformation and water ordering during adsorption with in situ sum-frequency generation, in situ X-ray diffraction, and molecular simulations are unraveled. In CAU-10-H, an energetically favorable water cluster is formed in the hydrophobic pore, tethered via H-bonds to the framework mu-OH groups, while for CAU-10-CH3, such a favorable cluster cannot form. By relating the findings to the features of water adsorption isotherms of a series of MOFs, it is concluded that favorable water adsorption occurs when sites of intermediate hydrophilicity are present in a hydrophobic structure, and the formation of energetically favorable water clusters is possible.

Optical cavities, resonant with vibrational or electronic transitions of material within the cavity, enable control of light-matter interaction. Previous studies have reported cavity-induced modifications of chemical reactivity, fluorescence, phase behavior, and charge transport. Here, we explore the effect of resonant cavity-phonon coupling on the transient photoconductivity in a hybrid organic-inorganic perovskite. To this end, we measure the ultrafast photoconductivity response of perovskite in a tunable Fabry-Perot terahertz cavity, designed to be transparent for optical excitation. The terahertz-cavity field-phonon interaction causes apparent Rabi splitting between the perovskite phonon mode and the cavity mode. We explore whether the cavity-phonon interaction affects the material electron-phonon interaction by determining the charge carrier mobility through the photoconductivity. Despite the apparent hybridization of cavity and phonon modes, we show that the perovskite properties, in both ground (phonon response) and excited (photoconductive response) states, remain unaffected by the tunable light-matter interaction. Yet the response of the integral perovskite-terahertz optical cavity system depends critically on the interaction strength of the cavity with the phonon: the transient terahertz response to optical excitation can be increased up to 3-fold by tuning the cavity-perovskite interaction strength. These results enable tunable switches and frequency-controlled induced transparency devices., Comment: 14 pages, 4 figures

There is compelling evidence that charge carriers in organic semiconductors (OSs) self-localize in nano-scale space because of dynamic disorder. Yet, some OSs, in particular recently emerged high-mobility organic molecular crystals, feature reduced mobility at increasing temperature, a hallmark for delocalized band transport. Here we present the temperature-dependent mobility in two record-mobility OSs: DNTT (dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]-thiophene), and its alkylated derivative, C8-DNTT-C8. By combining terahertz photoconductivity measurements with fully atomistic non-adiabatic molecular dynamics simulations, we show that while both crystals display a power-law decrease of the mobility (\mu) with temperature (T, following: \mu \propto T^(-n)), the exponent n differs substantially. Modelling provides n values in good agreement with experiments and reveals that the differences in the falloff parameter between the two chemically closely related semiconductors can be traced to the delocalization of the different states thermally accessible by charge carriers, which in turn depends on the specific electronic band structure of the two systems. The emerging picture is that of holes surfing on a dynamic manifold of vibrationally-dressed extended states with a temperature-dependent mobility that provides a sensitive fingerprint for the underlying density of states., Comment: 28 pages, 5 Figures

Since the seminal work on MoS2 monolayers, photoexcitation in atomically-thin transition metal dichalcogenides (TMDCs) has been assumed to result in excitons with large binding energies (~ 200-600 meV). Because the exciton binding energies are order-of-magnitude larger than thermal energy at room temperature, it is puzzling that photocurrent and photovoltage generation have been observed in TMDC-based devices, even in monolayers with applied electric fields far below the threshold for exciton dissociation. Here, we show that the photoexcitation of TMDC monolayers results in a substantial population of free charges. Performing ultrafast terahertz (THz) spectroscopy on large-area, single crystal WS2, WSe2, and MoSe2 monolayers, we find that ~10% of excitons spontaneously dissociate into charge carriers with lifetimes exceeding 0.2 ns. Scanning tunnelling microscopy reveals that photo-carrier generation is intimately related to mid-gap defect states, likely via trap-mediated Auger scattering. Only in state-of-the-art quality monolayers14, with mid-gap trap densities as low as 10^9 cm^-2, does intrinsic exciton physics start to dominate the THz response. Our findings reveal that excitons or excitonic complexes are only the predominant quasiparticles in photo-excited TMDC monolayers at the limit of sufficiently low defect densities., Comment: 18 pages, 5 figures, SI

Two dimensional van der Waals heterostructures (2D are of significant interest due to their intriguing physical properties that are critically defined by the constituent monolayers and their interlayer coupling . However, typical inorganic 2 D vdWhs fall into the weakly coupled region, limiting efficient interfacial charge flow crucial for developing high performance quantum opto electronics. Here, we demonstrate strong interlayer coupling in Cu3 BHT (BHT = benzenehexathiol) graphene vdWhs an organic inorganic bilayer characterized by prominent interlayer charge transfer Monolayer Cu3 BHT with a Kagome lattice is synthesized on the water surface and then coupled with graphene to produce a cm2 scale 2D vdWh. Spectroscopic and electrical studies, along with theoretical calculation s show significant hole transfer from monolayer Cu3 BHT to graphene upon contact , being characteristic fingerprints for strong interlayer coupling This study unveils the great potential of integrating highly pi-conjugated 2D coordination polymers (2DCPs) into 2D vdWhs to explor e intriguing physical phenomena.

The freezing of water into ice is a key process that is still not fully understood. It generally requires an impurity of some description to initiate the heterogeneous nucleation of the ice crystals. The molecular structure, as well as the extent of structural order within the impurity in question, both play an essential role in determining its effectiveness. However, disentangling these two contributions is a challenge for both experiments and simulations. In this work, we have systematically investigated the ice-nucleating ability of the very same compound, cholesterol, from the crystalline (and thus ordered) form to disordered self-assembled monolayers. Leveraging a combination of experiments and simulations, we identify a "sweet spot" in terms of the surface coverage of the monolayers, whereby cholesterol maximises its ability to nucleate ice (which remains inferior to that of crystalline cholesterol) by enhancing the structural order of the interfacial water molecules. These findings have practical implications for the rational design of synthetic ice-nucleating agents.

The abstract is available here: https://uscholar.univie.ac.at/o:1649476

The charging and dissolution of mineral surfaces in contact with flowing liquids are ubiquitous in nature, as most minerals in water spontaneously acquire charge and dissolve. Mineral dissolution has been studied extensively under equilibrium conditions, even though non-equilibrium phenomena are pervasive and substantially affect the mineral-water interface. Here we demonstrate using interface-specific spectroscopy that liquid flow along a calcium fluoride surface creates a reversible, spatial charge gradient, with decreasing surface charge downstream of the flow. The surface charge gradient can be quantitatively accounted for by a reaction-diffusion-advection model, which reveals that the charge gradient results from a delicate interplay between diffusion, advection, dissolution, and desorption/adsorption. The underlying mechanism is expected to be valid for a wide variety of systems, including groundwater flows in nature and microfluidic systems., Comment: 25 pages, 8 figures, Supplementary Information: 21 pages, 7 figures

The controlled introduction of covalent sp$^{3}$ defects into semiconducting single-walled carbon nanotubes (SWCNTs) gives rise to exciton localization and red-shifted near-infrared luminescence. The single-photon emission characteristics of these functionalized SWCNTs make them interesting candidates for electrically driven quantum light sources. However, the impact of sp$^{3}$ defects on the carrier dynamics and charge transport in carbon nanotubes remains an open question. Here, we use ultrafast, time-resolved optical-pump terahertz-probe spectroscopy as a direct and quantitative technique to investigate the microscopic and temperature-dependent charge transport properties of pristine and functionalized (6,5) SWCNTs in dispersions and thin films. We find that sp$^{3}$ functionalization increases charge carrier scattering, thus reducing the intra-nanotube carrier mobility. In combination with electrical measurements of SWCNT network field-effect transistors, these data enable us to distinguish between contributions of intra-nanotube band transport, sp$^{3}$ defect scattering and inter-nanotube carrier hopping to the overall charge transport properties of nanotube networks., ACS Nano 2022

Molecular-level insight into interfacial water at buried electrode interfaces is essential in elucidating many phenomena of electrochemistry, but spectroscopic probing of the buried interfaces remains challenging. Here, using surface-specific vibrational spectroscopy, we probe and identify the interfacial water orientation and interfacial electric field at the calcium fluoride (CaF2)-supported electrified graphene/water interface under applied potentials. Our data shows that the water orientation changes drastically at negative potentials (

The distribution of ions at the air/water interface plays a decisive role in many natural processes. It is generally understood that polarizable ions with low charge density are surface-active, implying they sit on top of the water surface. Here, we revise this established hypothesis by combining surface-specific heterodyne-detected vibrational sum-frequency generation with neural network-assisted ab initio molecular dynamics simulations. Our results directly demonstrate that ions in typical electrolyte solutions are, in fact, located in a subsurface region leading to a stratification of such interfaces into two distinctive water layers. The outermost surface is ion-depleted, and the sub-surface layer is ion-enriched. As a result, an effective liquid/liquid interface buried a few {\AA} inside the solution emerges, creating a second water/electrolyte interface, in addition to the outermost air/water interface.

Vinylene-linked two-dimensional covalent organic frameworks (V-2D-COFs) have shown great promise in electronics and optoelectronics. However, only a few reactions for V-2D-COFs have been developed hitherto. Besides the kinetically low reversibility of C=C bond formation, another underlying issue facing the synthesis of V-2D-COFs is the attainment of high ( E )-alkene selectivity to ensure the appropriate symmetry of 2D frameworks. Here, we tailor the E/Z selectivity of the Wittig reaction by employing a proper catalyst (i.e., Cs 2 CO 3 ) to obtain more stable intermediates and evaluate the temperature across the reaction barrier. Subsequently, the Wittig reaction is innovatively utilized for the synthesis of four crystalline V-2D-COFs by combining aldehydes and ylides. Importantly, the efficient conjugation and decent crystallinity of the resultant V-2D-COFs are demonstrated by their high charge carrier mobilities over 10 cm 2 V -1 s -1 , as revealed by non-contact terahertz (THz) spectroscopy.

Advanced materials 34, 2108682 (2022). doi:10.1002/adma.202108682, D transition metal carbides and/or nitrides, so-called MXenes, are noted as ideal fast-charging cation-intercalation electrode materials, which nevertheless suffer from limited specific capacities. Herein, it is reported that constructing redox-active phosphorus−oxygen terminals can be an attractive strategy for Nb$_4$C$_3$ MXenes to remarkably boost their specific capacities for ultrafast Na$^+$ storage. As revealed, redox-active terminals with a stoichiometric formula of PO$_2$- display a metaphosphate-like configuration with each P atom sustaining three P-O bonds and one P=O dangling bond. Compared with conventional O-terminals, metaphosphate-like terminals empower Nb$_4$C$_3$ (denoted PO$_2$-Nb$_4$C$_3$) with considerably enriched carrier density (fourfold), improved conductivity (12.3-fold at 300 K), additional redox-active sites, boosted Nb redox depth, nondeclined Na$^+$-diffusion capability, and buffered internal stress during Na$^+$ intercalation/de-intercalation. Consequently, compared with O-terminated Nb$_4$C$_3$, PO$_2$-Nb$_4$C$_3$ exhibits a doubled Na$^+$-storage capacity (221.0 mAh g$^{-1}$), well-retained fast-charging capability (4.9 min at 80% capacity retention), significantly promoted cycle life (nondegraded capacity over 2000 cycles), and justified feasibility for assembling energy−power-balanced Na-ion capacitors. This study unveils that the molecular-level design of MXene terminals provides opportunities for developing simultaneously high-capacity and fast-charging electrodes, alleviating the energy−power tradeoff typical for energy-storage devices., Published by Wiley-VCH, Weinheim

Many promising optoelectronic devices, such as broadband photodetectors, nonlinear frequency converters, and building blocks for data communication systems, exploit photoexcited charge carriers in graphene. For these systems, it is essential to understand, and eventually control, the cooling dynamics of the photoinduced hot-carrier distribution. There is, however, still an active debate on the different mechanisms that contribute to hot-carrier cooling. In particular, the intrinsic cooling mechanism that ultimately limits the cooling dynamics remains an open question. Here, we address this question by studying two technologically relevant systems, consisting of high-quality graphene with a mobility >10,000 cm$^2$V$^{-1}$s$^{-1}$ and environments that do not efficiently take up electronic heat from graphene: WSe$_2$-encapsulated graphene and suspended graphene. We study the cooling dynamics of these two high-quality graphene systems using ultrafast pump-probe spectroscopy at room temperature. Cooling via disorder-assisted acoustic phonon scattering and out-of-plane heat transfer to the environment is relatively inefficient in these systems, predicting a cooling time of tens of picoseconds. However, we observe much faster cooling, on a timescale of a few picoseconds. We attribute this to an intrinsic cooling mechanism, where carriers in the hot-carrier distribution with enough kinetic energy emit optical phonons. During phonon emission, the electronic system continuously re-thermalizes, re-creating carriers with enough energy to emit optical phonons. We develop an analytical model that explains the observed dynamics, where cooling is eventually limited by optical-to-acoustic phonon coupling. These fundamental insights into the intrinsic cooling mechanism of hot carriers in graphene will play a key role in guiding the development of graphene-based optoelectronic devices.

Graphene is conceivably the most nonlinear optoelectronic material we know. Its nonlinear optical coefficients in the terahertz frequency range surpass those of other materials by many orders of magnitude. Here, we show that the terahertz nonlinearity of graphene, both for ultrashort single-cycle and quasi-monochromatic multicycle input terahertz signals, can be efficiently controlled using electrical gating, with gating voltages as low as a few volts. For example, optimal electrical gating enhances the power conversion efficiency in terahertz third-harmonic generation in graphene by about two orders of magnitude. Our experimental results are in quantitative agreement with a physical model of the graphene nonlinearity, describing the time-dependent thermodynamic balance maintained within the electronic population of graphene during interaction with ultrafast electric fields. Our results can serve as a basis for straightforward and accurate design of devices and applications for efficient electronic signal processing in graphene at ultrahigh frequencies., D.T. and H.A.H. acknowledge funding from the European Union’s Horizon 2020 Framework Programme under grant agreement no. 964735 (EXTREME-IR). M.G. and B.G. acknowledge support from the European Cluster of Advanced Laser Light Sources (EUCALL) project that has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement no. 654220. K.-J.T. acknowledges funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement no. 804349 (ERC StG CUHL) and financial support through the MAINZ Visiting Professorship. ICN2 was supported by the Severo Ochoa program from Spanish MINECO (grant no. SEV-2017-0706). Parts of this research were carried out at ELBE at the Helmholtz-Zentrum Dresden-Rossendorf e.V., a member of the Helmholtz Association. F.H.L.K. acknowledges support from the Government of Spain (FIS2016-81044; Severo Ochoa CEX2019-000910-S), Fundació Cellex, Fundació Mir-Puig, and Generalitat de Catalunya (CERCA, AGAUR, and SGR 1656). Furthermore, the research leading to these results has received funding from the European Union’s Horizon 2020 under grant agreement no. 881603 (Graphene Flagship Core 3).

We explore the charge transport properties of phytic acid (PA) doped polyaniline thin films prepared by the surfactant monolayer-assisted interfacial synthesis (SMAIS). Structural and elemental analysis confirms the inclusion of PA in the thin films and reveals a progressive loss of crystallinity with the increase of PA doping content. Charge transport properties are interrogated by time-resolved terahertz (THz) spectroscopy. Notably, independently of doping content and hence crystallinity, the frequency-resolved complex conductivity spectra in the THz region can be properly described by the Drude model, demonstrating band-like charge transport in the samples and state-of-the-art charge carrier mobilities of ≈1 cm2V−1s−1. A temperature-dependent analysis for the conductivity further supports band-like charge transport and suggest that charge carrier mobility is primarily limited by impurity scattering. This work highlights the potential of PA doped polyaniline for organic electronics.

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The Drude-Smith model successfully describes the frequency and phase-resolved electrical conductivity data for a surprisingly broad range of systems, especially in the terahertz region. Still, its interpretation is unclear since its original derivation is flawed. We use an intuitive physical framework to derive the Drude-Smith formula for systems where microscopically free charges are accumulated on a mesoscopic scale by localized scatterers. Within this framework, the model allows us to quantify the microscopic momentum relaxation time of the charges and the fraction of mesoscopically localized charges in addition to the direct current limit of the conductivity. We show that the Drude-Smith model is unique among different Drude-Lorentz models because the relaxation time of the free carriers also determines the frequency and damping of the resonance of the bound charges., Comment: The assumptions made in eq. (6) / fig 1 b) are self- contradictory. It is no legitimate to assume that the incoming current stops instantaneously but the back current does not. The state of the art (Ku\v{z}el, P. & N\v{e}mec, H. Adv. Opt. Mater. 8, 1--23 (2020).) is that Drude-Smith is an empirical parameterisation and a general microscopic interpretation of the parameters alone should not be done

Correction to: Nature Communications https://doi.org/10.1038/s41467-021-24270-x, published online 2 July 2021.

The authors acknowledge support from the Engineering and Physical Sciences Research Council (EPSRC) UK National Service for Computational Chemistry Software (NSCCS); from GENCI (Grand equipement national de calcul intensif); and from CINES (Centre informatique national de l’enseignement superieur). LFH and TvM gratefully acknowledge support from the HPC-EUROPA2 project with the support of the European Commission - Capacities Area - Research Infrastructures. LFH is grateful to the EPSRC for studentship support through the Doctoral Training Account scheme (grant code EP/K503162/1).

Additional information, methods, and macro code. (DOCX 57 kb)

Type 2 diabetes mellitus is characterized by the pathological deposition of fibrillized protein, known as amyloids. It is thought that oligomers and/ or amyloid fibrils formed from human islet amyloid polypeptide (hIAPP or amylin) cause cell death by membrane damage. The molecular structure of hIAPP amyloid fibrils is dominated by beta-sheet structure, as probed with conventional infrared and Raman vibrational spectroscopy. However, with these techniques it is not possible to distinguish between the core and the surface structure of the fibrils. Since the fibril surface crucially affects amyloid toxicity, it is essential to know its structure. Here the surface molecular structure and amino acid residue composition of hIAPP fibrils are specifically probed with nanoscale resolution using tip-enhanced Raman spectroscopy (TERS). The fibril surface mainly contains unordered or alpha-helical structures, in contrast to the beta-sheet-rich core. This experimentally validates recent models of hIAPP amyloids based on NMR measurements. Spatial mapping of the surface structure reveals a highly heterogeneous surface structure. Finally, TERS can probe fibrils formed on a lipid interface, which is more representative of amyloids in vivo.

N-terminal acetylation is a commonly used modification technique for synthetic peptides, mostly applied for reasons of enhanced stability, and in many cases regarded as inconsequential. In engineered biosilification - the controlled deposition of silica for nanotechnology applications by designed peptides - charged groups often play a deciding role. Here we report that changing the charge by acetylation of a 14-amino acid leucine-lysine (LK) peptide dramatically changes the morphology of precipitated biosilica; acetylated LK peptides produce nano-spheres, whereas nano-wires are precipitated by the same peptide in a non-acetylated form. By using interface-specific vibrational spectroscopy and coarse-grained molecular simulations, we show that this change in morphology is not the result of modified peptide-silica interactions, but rather caused by the stabilization of the hydrophobic core of peptide aggregates created by the removal of a peptide charge upon acetylation. These results should raise awareness of the potential impact of N-terminal modifications in peptide applications.

Surface charge is one of the surface properties of atmospheric aerosols, which has been linked to heterogeneous ice nucleation and hence cloud formation, microphysics, and optical properties. Despite the importance of surface charge for ice nucleation, many questions remain on the molecular-level mechanisms at work. Here, we combine droplet-freezing assay studies with vibrational sum frequency generation (SFG) spectroscopy to correlate interfacial water structure to surface nucleation strength. We study immersion freezing of aqueous solutions of various pHs on the atmospherically relevant aluminum oxide α-Al2O3 (0001) surface using an isolated droplet on the surface. The high-pH solutions freeze at temperatures higher than that of the low-pH solution, while the neutral pH has the highest freezing temperature. On the molecular level, the SFG spectrum of the interfacial water changes substantially upon freezing. At all pHs, crystallization leads to a reduction of intensity of the 3400 cm−1 water resonance, while the 3200 cm−1 intensity drops for low pH but increases for neutral and high pHs. We find that charge-induced surface templating suppresses nucleation, irrespective of the sign of the surface charge. Heterogeneous nucleation is most efficient for the nominally neutral surface.

Supplementary methods and data

Type 2 diabetes mellitus is characterized by the pathological deposition of fibrillized protein, known as amyloids. It is thought that oligomers and/ or amyloid fibrils formed from human islet amyloid polypeptide (hIAPP or amylin) cause cell death by membrane damage. The molecular structure of hIAPP amyloid fibrils is dominated by beta-sheet structure, as probed with conventional infrared and Raman vibrational spectroscopy. However, with these techniques it is not possible to distinguish between the core and the surface structure of the fibrils. Since the fibril surface crucially affects amyloid toxicity, it is essential to know its structure. Here the surface molecular structure and amino acid residue composition of hIAPP fibrils are specifically probed with nanoscale resolution using tip-enhanced Raman spectroscopy (TERS). The fibril surface mainly contains unordered or alpha-helical structures, in contrast to the beta-sheet-rich core. This experimentally validates recent models of hIAPP amyloids based on NMR measurements. Spatial mapping of the surface structure reveals a highly heterogeneous surface structure. Finally, TERS can probe fibrils formed on a lipid interface, which is more representative of amyloids in vivo.

Insulin is a widely used peptide in protein research and it is utilized as a model peptide to understand the mechanics of fibril formation, which is believed to be the cause of diseases such as Alzheimer and Creutzfeld-Jakob syndrome. Insulin has been used as a model system due to its biomedical relevance, small size and relatively simple tertiary structure. The adsorption of insulin on a variety of surfaces has become the focus of numerous studies lately. These works have helped in elucidating the consequence of surface/protein hydrophilic/hydrophobic interaction in terms of protein refolding and aggregation. Unfortunately, such model surfaces differ significantly from physiological surfaces. Here we spectroscopically investigate the adsorption of insulin at lipid monolayers, to further our understanding of the interaction of insulin with biological surfaces. In particular we study the effect of minor mutations of insulin's primary amino acid sequence on its interaction with 1,2-Dipalmitoyl-sn-glycero-3-phosphoglycerol (DPPG) model lipid layers. We probe the structure of bovine and human insulin at the lipid/water interface using sum frequency generation spectroscopy (SFG). The SFG experiments are complemented with XPS analysis of Langmuir-Schaefer deposited lipid/insulin films. We find that bovine and human insulin, even though very similar in sequence, show a substantially different behavior when interacting with lipid films.

Many proteins of diverse sequence, structure and function self-assemble into morphologically similar fibrillar aggregates known as amyloids. Amyloids are remarkable polymers in several respects. First of all, amyloids can be formed from proteins with very different amino acid sequences; the common denominator is that the individual proteins constituting the amyloid fold predominantly into a β-sheet structure. Secondly, the formation of the fibril occurs through non-covalent interactions between primarily the β-sheets, causing the monomers to stack into fibrils. The fibrils are remarkably robust, considering that the monomers are bound non-covalently. Finally, a common characteristic of fibrils is their unbranched, straight, fiber-like structure arising from the intertwining of the multiple β-sheet filaments. These remarkably ordered and stable nanofibrils can be useful as building blocks for protein-based functional materials, but they are also implicated in severe neurodegenerative diseases. The overall aim of this article is to highlight recent efforts aimed at obtaining insights into amyloid proteins on different length scales. Starting from molecular information on amyloids, single fibril properties and mechanical properties of networks of fibrils are described. Specifically, we focus on the self-assembly of amyloid protein fibrils composed of peptides and denatured model proteins, as well as the influence of inhibitors of fibril formation. Additionally, we will demonstrate how the application of recently developed vibrational spectroscopic techniques has emerged as a powerful approach to gain spatially resolved information on the structure–function relation of amyloids. While spectroscopy provides information on local molecular conformations and protein secondary structure, information on the single fibril level has been developed by diverse microscopic techniques. The approaches to reveal basic mechanical properties of single fibrils like bending rigidity, shear modulus, ultimate tensile strength and fracture behavior are illustrated. Lastly, mechanics of networks of amyloid fibrils, typically forming viscoelastic gels are outlined, with a focus on (micro-) rheological properties. The resulting fundamental insights are essential for the rational design of novel edible and biodegradable protein-based polymers, but also to devise therapeutic strategies to combat amyloid assembly and accumulation during pathogenic disorders.

We report on the coherent control of terahertz (THz) spin waves in a canted antiferromagnet yttrium orthoferrite, YFeO3, associated with a quasiferromagnetic (quasi-FM) spin resonance at a frequency of 0.3 THz, using a single-incident THz pulse. The spin resonance is excited impulsively by the magnetic field component of the THz pulse. The intrinsic dielectric anisotropy of YFeO3 in the THz range allows for coherent control of both the amplitude and the phase of the excited spin wave. The coherent control is based on simultaneous generation of two interfering phase-shifted spin waves whose amplitudes and relative phase, dictated by the dielectric anisotropy of the YFeO3 crystal, can be controlled by varying the polarization of the incident THz pulse with respect to the crystal axes. The spatially anisotropic decay of the THz-excited FM spin resonance in YFeO3, leading to an increasingly linear polarization of the THz oscillation at the spin resonance frequency, suggests a key role of magnon–phonon coupling in spin-wave energy dissipation.

About 70% of our planet is covered in water. Most of that water exists as water in the bulk – the neighbors of water molecules are other water molecules – and only a small fraction of molecules are at the air-water interface. Despite the small relative abundance of interfacial water, it is of the utmost importance: it governs the chemistry involving the surface of oceans and seawater aerosols, or the small water droplets forming clouds. Reactions at the air-water interface are directly relevant to world-scale phenomena such as the nutrient cycle in oceans or the evolution of the ozone layer. The air-water interface is also a good model for extended hydrophobic interfaces – the type of interface that occurs when water meets a large surface constituted by many apolar functional groups. Hydrophobic interfaces play a key role in the hydrophobic collapse of proteins (the process by which proteins acquire their 3D structure) or the aggregation of surfactants. Understanding many interfacial phenomena thus requires us to first understand the structure and dynamics of interfacial water. Studies of water at interfaces have only become possible within the past decades, with the advent of surface-sensitive experimental techniques such as vibrational sum-frequency (VSF) spectroscopy and the development of better classical models of water for molecular simulations. In this talk I will describe recent work investigating, using VSF and molecular simulation, the dynamics of two subpopulations of hydroxyl groups at the air-water interface: those OH groups that do not establish a hydrogen bond (called free OH), and those that do. The free OH population is very small in the bulk but much larger at the interface, and so is partially responsible for the interface’s properties. We found that free OH groups rotate several times faster than water in the bulk. In contrast, the net reorientation of bonded OH groups occurs at a rate similar to that of bulk water. Our results on the air-water interface have direct implications for the kinetics of protein folding or ligand binding: they suggest that the kinetics of the de-wetting transitions that necessarily accompany these processes might be tunable by protein topologies that predominantly affect the local population of free OH groups, since only free OH groups have rotational dynamics that is distinct from bulk water.

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The understanding of defects is of paramount importance for the development of stable halide perovskite solar cells (PSCs). However, isolating their distinctive effects on the device efficiency and stability is currently a challenge. Here, we report that adding the organic molecule 3- phosphonopropionic acid (H3pp) to the halide perovskite precursor solution results in unchanged overall optoelectronic performance while having a tremendous impact on the stability of the halide perovskite by means of ion immobilization. As a result, we obtained PSCs with ~21 % efficiency and outstanding operational stability, retaining nearly 100 % of the initial efficiency after 1000 h at the maximum power point under simulated AM1.5 illumination. The strong interaction between the perovskite and the H3pp molecule through two types of hydrogen bonds (H… I and O…H) from the phosphonate group allows immobilization of ions that leads to remarkable device stability. This binding mode results in shallow point defect passivation that has a significant impact on the device stability but not on the nonradiative recombination and device efficiency. We expect that decoupling the effect of defects on the efficiency and stability will have important implications for the current understanding and advancement of operational PSCs.

Van der Waals heterostructures have emerged as promising building blocks that offer access to new physics, novel device functionalities and superior electrical and optoelectronic properties 1-7 . Applications such as thermal management, photodetection, light emission, data communication, high-speed electronics and light harvesting 8-16 require a thorough understanding of (nanoscale) heat flow. Here, using time-resolved photocurrent measurements, we identify an efficient out-of-plane energy transfer channel, where charge carriers in graphene couple to hyperbolic phonon polaritons 17-19 in the encapsulating layered material. This hyperbolic cooling is particularly efficient, giving picosecond cooling times for hexagonal BN, where the high-momentum hyperbolic phonon polaritons enable efficient near-field energy transfer. We study this heat transfer mechanism using distinct control knobs to vary carrier density and lattice temperature, and find excellent agreement with theory without any adjustable parameters. These insights may lead to the ability to control heat flow in van der Waals heterostructures.

Due to its outstanding electrical properties and chemical stability, graphene finds widespread use in various electrochemical applications. Although the presence of electrolytes strongly affects its electrical conductivity, the underlying mechanism has remained elusive. Here, we employ terahertz spectroscopy as a contact-free means to investigate the impact of ubiquitous cations (Li+, Na+, K+, and Ca2+) in aqueous solution on the electronic properties of SiO2-supported graphene. We find that, without applying any external potential, cations can shift the Fermi energy of initially hole-doped graphene by ∼200 meV up to the Dirac point, thus counteracting the initial substrate-induced hole doping. Remarkably, the cation concentration and cation hydration complex size determine the kinetics and magnitude of this shift in the Fermi level. Combined with theoretical calculations, we show that the ion-induced Fermi level shift of graphene involves cationic permeation through graphene. The interfacial cations located between graphene and SiO2 electrostatically counteract the substrate-induced hole doping effect in graphene. These insights are crucial for graphene device processing and further developing graphene as an ion-sensing material., X.J., Z.L., Z.C., A.N., K.M., M.B., and H.I.W. acknowledge the financial support by the Max Planck Society. X.J. acknowledges support by a Deutsche Forschungsgemeinschaft-funded position through the Excellence Initiative by the Graduate School of Excellence Materials Science in Mainz (MAINZ) (GSC 266) and support from the Max Planck Graduate Center mit der Johannes Gutenberg-Universität Mainz (MPGC). K.-J.T. acknowledges financial support through the MAINZ Visiting Professorship. X.Y. acknowledges the fellowship support from Chinese Scholarship Council (CSC). This project has received funding from the European Union’s Horizon 2020 research and innovation program under Grant Agreement No. 804349. ICN2 is supported by the Severo Ochoa program from Spanish MINECO (Grant No. SEV-2017-0706). F.H.L.K. acknowledges financial support by Fundacio Cellex Barcelona, support from the Spanish Ministry of Economy and Competitiveness, through the “Severo Ochoa” Programme for Centres of Excellence in R&D (SEV-2015-0522), and support by the Generalitat de Catalunya through the CERCA program.

The rise of two-dimensional (2D) materials and their heterostructures has revolutionized modern materials science. Understanding and manipulating charge transport in 2D materials and charge transfer (CT) across their interfaces play a paramount role in determining application performance. Employing time-resolved terahertz spectroscopy, this dissertation investigates interfacial CT in heterostructures and charge transport in 2D covalent organic frameworks (COFs). Following the pump-wavelength dependent CT mechanism at graphene-WS2 interfaces, we find that defects can rapidly trap injected or photogenerated electrons in WS2 within ~1 ps and store them beyond ~1 ns. This leads to long-lived charge separation and efficient photogating in graphene. Further electrochemical modulation of defect occupation in WS2 leads to a flip of the local photogating field, from electron-photogating in p-doped heterostructures to hole-photogating in n-doped heterostructures. In graphene-nanographene heterostructures, strong interlayer π-π interactions enable the ultrafast and efficient transfer of holes from nanographene to graphene, contributing to the excellent photodetection performance. Furthermore, the size of nanographene acts as a structural knob that regulates interfacial vdW interactions, thereby modifying the electronic coupling strength and CT efficiency. Finally, we unveil that crystallinity plays a critical role in determining the charge transport properties of 2D COFs. In highly crystalline 2D COF thin films, band-like transport prevails with record-high local charge mobility up to hundreds of cm2V–1s–1. Our findings elucidate charge transport and transfer effects in emerging 2D materials and their hybrids, providing new insights into the design and optimization of advanced electronic and optoelectronic devices.

Interfacial water can be found at the surface of an aerosol, a glass of water, and on the surface of vast oceans. At the molecular level, the structure of water at an interface is typically very different from the bulk structure. In this thesis, we try to reveal the mechanisms behind several physicochemical processes at the interfaces: (i) evaporation of aqueous binary mixtures, (ii) interfacial chemical reactions using the surfactant monolayer-assisted interfacial synthesis (SMAIS) method, and (iii) energy dissipation at the water-graphene interface. To investigate these systems, we employ surface-selective sum-frequency generation (SFG) spectroscopy and charge carrier-sensitive optical-pump terahertz-probe (OPTP) spectroscopy. Applying SFG spectroscopy – a surface-specific vibrational spectroscopy – to investigate the surface of liquid mixtures, we reveal that water's interfacial alignment becomes randomized upon adding ethanol and methanol to water. In contrast, it becomes ordered upon adding formic acid. This is concluded from an investigation of the H-O-H/H-O-D bending mode of water at the surface. We also provide molecular-level information, such as intermediates and the impact of the surfactant charge on the interfacial chemical reaction of polyaniline and imidine, through in situ SFG measurement. With OPTP spectroscopy, we investigate how the size of the nanographene affects the charge transfer rate between the nanographene and graphene. Such a charge transfer rate is important for photodetector and photocatalysis-related applications. Finally, we track the photoexcited carrier relaxation of graphene in contact with different liquids, clarifying the role of energy and momentum transfer between the graphene electrons and the collective water libration modes.

Understanding photo-physics in low-dimensional materials, including charge generation, cooling, transport and recombination, is essential for both fundamental science and optoelectronic applications. Many-body effects, e.g., how photogenerated electrons interact with other charge carriers (holes or electrons themselves), phonons, and defects, are known to play a critical role in dictating materials’ optical and electrical properties; reducing the material dimensionality can substantially enhance such interactions due to combined effects of spatial confinement, weakened electronic screening and reduced (electronic or phononic) density of states. The thesis is dedicated to understanding the ultrafast charge carrier dynamics in one- and two-dimensional (1D/2D) quantum materials by terahertz (THz) spectroscopy. First, we present an overview of the basic many-body interactions in quantum materials, including electron-electron scattering (e.g., for carrier heating in metals and impact ionization in semiconductors), carrier-phonon interaction (“polaron” effect), and electron-hole interaction (“exciton” effect). Then we introduce the main experimental technique (THz spectroscopy), and details of the data analysis for inferring the optical and charge transport properties in materials of interest. Finally we summarize key results of the thesis with a primary focus on ultrafast carrier-carrier scattering and electron-phonon in 2D materials (e.g., layered MoTe2 and MXenes), and electron-defect scattering in 1D carbon nanotubes. Overall, our findings elucidate the ultrafast dynamics and transport properties of photogenerated charge carriers subjected to many-body effects in quantum materials, providing new insights into the fundamental photophysical processes relevant to (opto-) electronic applications.

Low frequency modes (LFMs) of proteins in terahertz (THz) frequency range are suggested to be significant for protein’s activity. However, there is lack of understanding of these motions because of the experimental challenges. One-dimensional Raman and linear absorption spectroscopy in THz range measures them directly, but the spectra are often congested by overlapping resonances of different chemical species. Recently developed two-dimensional terahertz-infrared-visible (2D TIRV) spectroscopy can provide site-specific insight into the THz spectrum by measuring correlations between LFMs and chemically-specific high-frequency vibrational modes (HFMs). Measured 2D TIRV spectra however are affected by instrument response function (IRF) which critically complicates their analysis and interpretation. In this thesis, we develop a method to eliminate IRF and derive complex-valued response function (S(3)). S(3) solely represents the molecular vibrational dynamics and permits for accurate analysis of lineshapes. To verify the approach, we use liquid dimethyl sulfoxide (DMSO) as a model system and measure coupling between CH3 stretch modes (HFMs) and low-frequency intramolecular and intermolecular modes (LFMs). Data analysis using newly developed mixed quantum-classical formalism of response function reveals dominating mechanical coupling and correlated fluctuations of HFM and LFM frequencies. Furthermore, we explore selection rules governing the signal generation in 2D TIRV spectroscopy using DMSO as model sample. Finally, we demonstrate the utilization of 2D TIRV spectroscopy to probe site-specific THz motions of proteins. To this end, we measure THz motions of side chains of lysozyme protein in D2O solution and as a dry film.

This thesis covers two topics; one is an investigation into the nature of the bending mode of water, while the other is the development of a new probe of the interfacial molecules by extending sum-frequency generation (SFG) spectroscopy to heterodyne-detected and polarization-dependent SFG. Although the H-O-H bending mode of water is known to be sensitive to the hydrogen bond strength, the information on the vibrational coupling of the bending mode has been poorly understood. Here, we unveil the vibrational coupling and vibrational dynamics of the bending mode by using infrared, Raman, hyper-Raman, SFG, and time-resolved vibrational techniques. We find that the intermolecular coupling of the bending mode is weak compared to the stretching mode, and address the origin of the bending mode SFG signal at the interfaces. Next, we present the use of polarization-dependent heterodyne-detected SFG (PD-HD-SFG) to study molecular interfacial properties. Surface-specific SFG measurements at different polarization combinations have been used for investigating the molecular orientation at interfaces. We demonstrate that PD-HD-SFG can be used not only to estimate the molecular orientation but also to study the depth profile of molecules, and elucidate details of iodide chemistry at the air-water interface. As such, PD-HD-SFG is a very promising technique for qualitative and quantitative analysis of interfacial molecular properties.

Solution-processed metal-halide perovskites (MHPs) have gained tremendous research attention in recent years owing to their superior photovoltaic performance with a certified solar-to-electricity power conversion efficiency over 25%. Employing ultrafast terahertz time-domain spectroscopy, this thesis investigates the photoexcited charge carrier dynamics and charge transport in newly developed MHPs, focusing on all-inorganic black γ-phase CsPbI₃ and lead-free double perovskite Cs₂AgBiBr₆. We first unveil that large polarons form in black γ-phase CsPbI₃ polycrystalline thin films with a high local mobility up to 270 ± 44 cm2V-1s-1, and polaron-longitudinal optical phonon interaction governs the charge transport. With further increasing the photon injection, we conduct the first experimental quantification of the Mott polaron density (~1018 cm-3) in lead halide perovskites which is in line with theoretical estimations based on the Feynman polaron model. This effect is found to be universal and independent of the constituted cations and anions, representing an intrinsic property of lead halide perovskites. Above the Mott density, excess photoinjected carriers annihilate quickly within tens to hundreds of ps to form a stable, long-lived Mott polaron state. In the lead-free double perovskite Cs₂AgBiBr₆, we reveal excess energy-dependent highly conductive hot carriers, originating primarily from the quasi-ballistic transport of hot holes. By constructing Cs₂AgBiBr₆ double perovskite/graphene heterostructure, we achieve an optical control over hole transfer in both direction and efficiency. These fundamental insights play a vital role in determining and optimizing the efficiency of optoelectronics of the new generation perovskites.

A substantial fraction of biomolecular processes occurs at interfaces. Interactions of proteins with lipid membranes at the membrane/water interface present a critical example. Understanding these interactions enables us to unravel the origin of diseases, develop effective drugs, reveal principles for biomimetic material design, and, in general, understand bases of life. However, the immense complexity of a cell and, all the more, of living organisms overcomplicates interpretation of observed effects. At the same time, to get insight into vital (sub)cellular processes, molecular-scale information on the organization of lipids and proteins and their interactions is necessary. For this reason, simple models are helpful to employ. For instance, (sub)cellular biomembrane has been represented by a lipid bilayer, vesicle, or monolayer. In this thesis, we focus on the characterization of biomolecular organization at bio-interfaces and use lipid monolayer to mimic a biomembrane. To probe interfacial molecules, we employ a combination of surface-specific spectroscopic and microscopic techniques. We complement surface-specific studies by information obtained with bulk techniques. Additionally, we pay particular attention to the organization of water at bio-interfaces inspired by the fact that interactions of water with other biomolecules are crucial for cell life and functions. A combination of bulk and interfacial studies allows us to investigate how properties of interface affect protein interfacial adsorption and folding. The powerful experimental toolbox implemented in this thesis suggests a platform to study various aspects of protein and lipid solvation, lipid packing in biomembranes, lipid-protein interactions, and organization and functional roles of water in mediating these interactions.