Abstract
A material’s magnetic state and its dynamics are of great fundamental research interest and are also at the core of a wide plethora of modern technologies. However, reliable access to ...magnetization dynamics in materials and devices on the technologically relevant ultrafast timescale, and under realistic device-operation conditions, remains a challenge. Here, we demonstrate a method of ultrafast terahertz (THz) magnetometry, which gives direct access to the (sub-)picosecond magnetization dynamics even in encapsulated materials or devices in a contact-free fashion, in a fully calibrated manner, and under ambient conditions. As a showcase for this powerful method, we measure the ultrafast magnetization dynamics in a laser-excited encapsulated iron film. Our measurements reveal and disentangle distinct contributions originating from (i) incoherent hot-magnon-driven magnetization quenching and (ii) coherent acoustically-driven modulation of the exchange interaction in iron, paving the way to technologies utilizing ultrafast heat-free control of magnetism. High sensitivity and relative ease of experimental arrangement highlight the promise of ultrafast THz magnetometry for both fundamental studies and the technological applications of magnetism.
We report efficient photoconductivity multiplication in few-layer 2H-MoTe2 as a direct consequence of an efficient steplike carrier multiplication with near unity quantum yield and high carrier ...mobility (∼45 cm2 V–1 s–1) in MoTe2. This photoconductivity multiplication is quantified using ultrafast, excitation-wavelength-dependent photoconductivity measurements employing contact-free terahertz spectroscopy. We discuss the possible origins of efficient carrier multiplication in MoTe2 to guide future theoretical investigations. The combination of photoconductivity multiplication and the advantageous bandgap renders MoTe2 as a promising candidate for efficient optoelectronic devices.
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IJS, KILJ, NUK, PNG, UL, UM
Abstract
Coupling between vibrational modes is essential for energy transfer and dissipation in condensed matter. For water, different O-H stretch modes are known to be very strongly coupled both ...within and between water molecules, leading to ultrafast dissipation and delocalization of vibrational energy. In contrast, the information on the vibrational coupling of the H-O-H bending mode of water is lacking, even though the bending mode is an essential intermediate for the energy relaxation pathway from the stretch mode to the heat bath. By combining static and femtosecond infrared, Raman, and hyper-Raman spectroscopies for isotopically diluted water with ab initio molecular dynamics simulations, we find the vibrational coupling of the bending mode differs significantly from the stretch mode: the intramode intermolecular coupling of the bending mode is very weak, in stark contrast to the stretch mode. Our results elucidate the vibrational energy transfer pathways of water. Specifically, the librational motion is essential for the vibrational energy relaxation and orientational dynamics of H-O-H bending mode.
Proteins at interfaces play important roles in cell biology, immunology, bioengineering, and biomimetic material design. Many biological processes are based on interfacial protein action, ranging ...from cellular communication to immune responses and the protein-driven mineralization of bone. Despite the importance of interfacial proteins, comparatively little is known about their structure. The standard methods for studying crystalline or solution-phase proteins (X-ray diffraction and NMR spectroscopy) are not well-suited for studying proteins at interfaces, and for these proteins we still lack a corresponding technique that can provide the same level of structural resolution. This is not surprising in view of the challenges involved in probing the structure of proteins within monomolecular films assembled at a very thin interface in situ. Vibrational sum-frequency generation (SFG) spectroscopy has the potential to overcome this challenge and investigate the structure and dynamics of proteins at interfaces at the molecular level with subpicosecond time resolution. While SFG studies were initially limited to simple model peptides, the past decade has seen a dramatic advancement of experimental techniques and data analysis methods that has made it possible to also study interfacial proteins and their folding, binding, orientation, hydration, and dynamics. In this review, we first explain the principles of SFG spectroscopy and the experimental and theoretical methods to measure and analyze protein SFG spectra. Then we give an extensive overview of the interfacial proteins studied to date with SFG. We highlight representative examples to demonstrate recent advances in probing the structure of proteins at the interfaces of liquids, membranes, minerals, and synthetic materials.
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IJS, KILJ, NUK, PNG, UL, UM
Among the rising 2D soft materials, conjugated polymer nanosheets are one of the most promising and new classes of polymeric materials, which are rarely developed because of the challenge in ...controlling the dimensionality and lack of synthetic strategies. In this study, one kind of sulfur‐enriched conjugated polymer nanosheet (2DP‐S) with a high aspect ratio of up to ≈400 is successfully synthesized. On the basis of structural characterization, as‐prepared 2DP‐S possesses the chemical identity of cruciform‐fused polymeric backbone consisting of quinoidal polythiophene and poly(p‐phenylenevinylene) along horizontal and vertical directions, respectively, by sharing two alternating single–double carbon–carbon bonds in each repeat unit. The unique structural conformation of 2DP‐S renders carrier mobilities of up to 0.1 ± 0.05 cm2 V−1 s−1, a figure inferred from Terahertz time domain spectroscopy. Moreover, upon thermal treatment, 2DP‐S is readily converted into N/S dual‐doped porous carbon nanosheets (2DPCs) under ammonia atmosphere, whose N/S ratio can be rationally controlled by adjusting the activation time. The catalytic performance of the oxygen reduction reaction of as‐prepared 2DPCs is well tunable by the rationally controlled N/S contents. These results offer a new pathway for exploring heteroatom‐doped porous carbons applicable for energy conversion and storage.
A sulfur‐enriched conjugated polymer nanosheet (2DP‐S) with a high aspect ratio of up to ≈400 is synthesized using a conventional solution method. With three types of common conjugated materials, as‐prepared 2DP‐S exhibits a surprisingly high carrier mobility. 2DP‐S is further used as carbon precursor for the preparation of porous carbon nanosheets (2DPCs) with rationally controlled N/S ratio.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Terahertz-range dielectric properties of the common polymers low-density polyethylene (LDPE), cyclic olefin/ethylene copolymer (TOPAS®), polyamide-6 (PA6), and polytetrafluoroethylene (PTFE or ...Teflon®) are characterized in the ultra-broadband frequency window 2-15 THz, using a THz time-domain spectrometer employing air-photonics for the generation and detection of single-cycle sub-50 fs THz transients. The time domain measurements provide direct access to both the absorption and refractive index spectra. The polymers LDPE and TOPAS® demonstrate negligible absorption and spectrally-flat refractive index across the entire spectroscopy window, revealing the high potential of these polymers for applications in THz photonics such as ultra-broadband polymer-based dielectric mirrors, waveguides, and fibers. Resonant high-frequency polar vibrational modes are observed and assigned in polymers PA6 and PTFE, and their dielectric functions in the complete frequency window 2-15 THz are theoretically reproduced. Our results demonstrate the potential of ultra-broadband air-photonics-based THz time domain spectroscopy as a valuable analytic tool for materials science.
Notwithstanding the success of lead‐halide perovskites in emerging solar energy conversion technologies, many of the fundamental photophysical phenomena in this material remain debated. Here, the ...initial steps following photogeneration of free charge carriers in lead‐iodide perovskites are studied, and timescales of charge carrier cooling and polaron formation, as a function of temperature and charge carrier excess energy, are quantified. It is found, using terahertz time‐domain spectroscopy (THz‐TDS), that the observed femtosecond rise in the photoconductivity can be described very well using a simple model of sequential charge carrier cooling and polaron formation. For excitation above the bandgap, the carrier cooling time depends on the charge carrier excess energy and lattice temperature, with cooling rates varying between 1 and 6 meV fs−1, depending on the cation. While carrier cooling depends on the cation, polaron formation occurs within ≈400 fs in CH3NH3PbI3 (MAPbI3), CH(NH2)2PbI3 (FAPbI3), and CsPbI3. Its formation time is independent of temperature between 160 and 295 K. The very similar polaron formation dynamics observed for the three perovskites points to the critical role of the inorganic lattice, rather than the cations, for polaron formation.
The initial dynamics of the photogenerated carriers in lead‐iodide perovskites are investigated utilizing THz time‐domain spectroscopy. A sequential model of carrier cooling and subsequent polaron formation within 400 fs describes the observed dynamics well. This work sheds additional light on the pivotal role of polarons in the carrier transport in these materials.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
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 fermions
, it has been repeatedly predicted that optical harmonic generation in graphene should be particularly efficient at the technologically important terahertz frequencies
. 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
, 10
and 10
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 magnitude
. 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.
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KISLJ, NUK, SBMB, UL, UM, UPUK
π-Conjugated two-dimensional covalent organic frameworks (2D COFs) are emerging as a novel class of electroactive materials for (opto)electronic and chemiresistive sensing applications. However, ...understanding the intricate interplay between chemistry, structure, and conductivity in π-conjugated 2D COFs remains elusive. Here, we report a detailed characterization for the electronic properties of two novel samples consisting of Zn– and Cu–phthalocyanine-based pyrazine-linked 2D COFs. These 2D COFs are synthesized by condensation of metal–phthalocyanine (M = Zn and Cu) and pyrene derivatives. The obtained polycrystalline-layered COFs are p-type semiconductors both with a band gap of ∼1.2 eV. A record device-relevant mobility up to ∼5 cm2/(V s) is resolved in the dc limit, which represents a lower threshold induced by charge carrier localization at crystalline grain boundaries. Hall effect measurements (dc limit) and terahertz (THz) spectroscopy (ac limit) in combination with density functional theory (DFT) calculations demonstrate that varying metal center from Cu to Zn in the phthalocyanine moiety has a negligible effect in the conductivity (∼5 × 10–7 S/cm), charge carrier density (∼1012 cm–3), charge carrier scattering rate (∼3 × 1013 s–1), and effective mass (∼2.3m 0) of majority carriers (holes). Notably, charge carrier transport is found to be anisotropic, with hole mobilities being practically null in-plane and finite out-of-plane for these 2D COFs.
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