Thickness Determination and Control in Protein-Based Biomaterial Thin Films.

Controlling the thickness and uniformity of biomaterial films is crucial for their application in various fields including sensing and bioelectronics. In this work, we investigated film assemblies of an engineered repeat protein─specifically, the consensus tetratricopeptide repeat (CTPR) protein ─a system with unique robustness and tunability. We propose the use of microreflectance spectroscopy and apparent color inspection for the quick assessment of the thickness and uniformity of protein-based biomaterial films deposited on oxidized silicon substrates. Initially, we characterized the thickness of large, uniform, spin-coated protein films and compared the values obtained from microreflectance spectroscopy with those obtained from other typical methods, such as ellipsometry and atomic force microscopy. The excellent agreement between the results obtained from the different techniques validates the effectiveness of microreflectance as a fast, noninvasive, and affordable technique for determining the thickness of biomaterial films. Subsequently, we applied microreflectance spectroscopy to determine the thickness of drop-casted CTPR-based films prepared from small protein solution volumes, which present a smaller surface area and are less uniform compared to spin-coated samples. Additionally, we demonstrate the utility of apparent color inspection as a tool for assessing film uniformity. Finally, based on these results, we provide a calibration of film thickness as a function of the protein length and concentration for both spin-coated and drop-casted films, serving as a guide for the preparation of CTPR films with a specific thickness. Our results demonstrate the remarkable reproducibility of the CTPR film assembly, enabling the simple preparation of biomaterial films with precise thickness.

Modulation of the Superconducting Phase Transition in Multilayer 2H-NbSe2 Induced by Uniform Biaxial Compressive Strain.

Strain is a powerful tool for tuning the properties of two-dimensional materials. Here, we investigated the effects of large, uniform biaxial compressive strain on the superconducting phase transition of multilayered 2H-NbSe2 flakes. We observed a consistent decrease in the critical temperature of NbSe2 flakes induced by the large thermal compression of a polymeric substrate (>1.2%) at cryogenic temperatures. For thin flakes (∼10 nm thick), a strong modulation of the critical temperature up to 1.5 K is observed, which monotonically decreases with increasing flake thickness. The effects of biaxial compressive strain remain significant even for relatively thick samples up to 80 nm thick, indicating efficient transfer of strain not only from the substrate to the flakes but also across several van der Waals layers. This work demonstrates that compressive strain induced from substrate thermal deformation can effectively tune phase transitions at low temperatures in 2D materials.

Tunable, multifunctional opto-electrical response in multilayer FePS3/single-layer MoS2 van der Waals p-n heterojunctions.

The combination of specific van der Waals semiconductors in vertical stacks leads to atomically sharp heterointerfaces with unique properties, offering versatility and additional functionality for thin, flexible, optoelectronic devices. In this work, we demonstrate heterostructures built from single-layer MoS2 (n-type) and multilayer FePS3 (p-type) as multifunctional p-n junctions where robust photoluminescent light emission and broadband electrical photo-response coexist. This is made possible by the inherent properties of the materials involved and the precise energy band alignment at their interface, which preserves the photoluminescent emission provided by the single-layer MoS2 and confers exceptional tunability to the system. Indeed, through small changes in the applied voltage across the junction, the interplay between photoluminescence and photocurrent generation can be tuned, allowing for a precise control of the light emission of single-layer MoS2 - from severely quenched to an order of magnitude enhancement. Additionally, the broadband photo-response of the system presents an enhanced performance under ultraviolet illumination, in contrast to other van der Waals heterostacks containing single-layer semiconductors. Furthermore, this photo-response can be adjusted by the application of an external electric field, enabling photocurrent generation under both reverse and forward bias, thereby contributing to the overall functionality and versatility of the system.

Large Biaxial Compressive Strain Tuning of Neutral and Charged Excitons in Single-Layer Transition Metal Dichalcogenides.

The absorption and emission of light in single-layer transition metal dichalcogenides are governed by the formation of excitonic quasiparticles. Strain provides a powerful technique to tune the optoelectronic properties of two-dimensional materials and thus to adjust their exciton energies. The effects of large compressive strain in the optical spectrum of two-dimensional (2D) semiconductors remain rather unexplored compared to those of tensile strain, mainly due to experimental constraints. Here, we induced large, uniform, biaxial compressive strain (∼1.2%) by cooling, down to 10 K, single-layer WS2, MoS2, WSe2, and MoSe2 deposited on polycarbonate substrates. We observed a significant strain-induced modulation of neutral exciton energies, with blue shifts up to 160 meV, larger than in any previous experiments. Our results indicate a remarkably efficient transfer of compressive strain, demonstrated by gauge factor values exceeding previous results and approaching theoretical expectations. At low temperatures, we investigated the effect of compressive strain on the resonances associated with the formation of charged excitons. In WS2, a notable reduction of gauge factors for charged compared to neutral excitons suggests an increase in their binding energy, which likely results from the effects of strain added to the influence of the polymeric substrate.

Photoluminescence Enhancement by Band Alignment Engineering in MoS2/FePS3 van der Waals Heterostructures.

Single-layer semiconducting transition metal dichalcogenides (2H-TMDs) display robust excitonic photoluminescence emission, which can be improved by controlled changes to the environment and the chemical potential of the material. However, a drastic emission quench has been generally observed when TMDs are stacked in van der Waals heterostructures, which often favor the nonradiative recombination of photocarriers. Herein, we achieve an enhancement of the photoluminescence of single-layer MoS2 on top of van der Waals FePS3. The optimal energy band alignment of this heterostructure preserves light emission of MoS2 against nonradiative interlayer recombination processes and favors the charge transfer from MoS2, an n-type semiconductor, to FePS3, a p-type narrow-gap semiconductor. The strong depletion of carriers in the MoS2 layer is evidenced by a dramatic increase in the spectral weight of neutral excitons, which is strongly modulated by the thickness of the FePS3 underneath, leading to the increase of photoluminescence intensity. The present results demonstrate the potential for the rational design of van der Waals heterostructures with advanced optoelectronic properties.

Large Multidirectional Spin-to-Charge Conversion in Low-Symmetry Semimetal MoTe2 at Room Temperature.

Efficient and versatile spin-to-charge current conversion is crucial for the development of spintronic applications, which strongly rely on the ability to electrically generate and detect spin currents. In this context, the spin Hall effect has been widely studied in heavy metals with strong spin-orbit coupling. While the high crystal symmetry in these materials limits the conversion to the orthogonal configuration, unusual configurations are expected in low-symmetry transition-metal dichalcogenide semimetals, which could add flexibility to the electrical injection and detection of pure spin currents. Here, we report the observation of spin-to-charge conversion in MoTe2 flakes, which are stacked in graphene lateral spin valves. We detect two distinct contributions arising from the conversion of two different spin orientations. In addition to the conventional conversion where the spin polarization is orthogonal to the charge current, we also detect a conversion where the spin polarization and the charge current are parallel. Both contributions, which could arise either from bulk spin Hall effect or surface Edelstein effect, show large efficiencies comparable to the best spin Hall metals and topological insulators. Our finding enables the simultaneous conversion of spin currents with any in-plane spin polarization in one single experimental configuration.

Room-Temperature Spin Hall Effect in Graphene/MoS2 van der Waals Heterostructures.

Graphene is an excellent material for long-distance spin transport but allows little spin manipulation. Transition-metal dichalcogenides imprint their strong spin-orbit coupling into graphene via the proximity effect, and it has been predicted that efficient spin-to-charge conversion due to spin Hall and Rashba-Edelstein effects could be achieved. Here, by combining Hall probes with ferromagnetic electrodes, we unambiguously demonstrate experimentally the spin Hall effect in graphene induced by MoS2 proximity and for varying temperatures up to room temperature. The fact that spin transport and the spin Hall effect occur in different parts of the same material gives rise to a hitherto unreported efficiency for the spin-to-charge voltage output. Additionally, for a single graphene/MoS2 heterostructure-based device, we evidence a superimposed spin-to-charge current conversion that can be indistinguishably associated with either the proximity-induced Rashba-Edelstein effect in graphene or the spin Hall effect in MoS2. By a comparison of our results to theoretical calculations, the latter scenario is found to be the most plausible one. Our findings pave the way toward the combination of spin information transport and spin-to-charge conversion in two-dimensional materials, opening exciting opportunities in a variety of future spintronic applications.

Unexpected edge conduction in mercury telluride quantum wells under broken time-reversal symmetry.

The realization of quantum spin Hall effect in HgTe quantum wells is considered a milestone in the discovery of topological insulators. Quantum spin Hall states are predicted to allow current flow at the edges of an insulating bulk, as demonstrated in various experiments. A key prediction yet to be experimentally verified is the breakdown of the edge conduction under broken time-reversal symmetry. Here we first establish a systematic framework for the magnetic field dependence of electrostatically gated quantum spin Hall devices. We then study edge conduction of an inverted quantum well device under broken time-reversal symmetry using microwave impedance microscopy, and compare our findings to a non-inverted device. At zero magnetic field, only the inverted device shows clear edge conduction in its local conductivity profile, consistent with theory. Surprisingly, the edge conduction persists up to 9 T with little change. This indicates physics beyond simple quantum spin Hall model, including material-specific properties and possibly many-body effects.

Control of single-spin magnetic anisotropy by exchange coupling.

The properties of quantum systems interacting with their environment, commonly called open quantum systems, can be affected strongly by this interaction. Although this can lead to unwanted consequences, such as causing decoherence in qubits used for quantum computation, it can also be exploited as a probe of the environment. For example, magnetic resonance imaging is based on the dependence of the spin relaxation times of protons in water molecules in a host's tissue. Here we show that the excitation energy of a single spin, which is determined by magnetocrystalline anisotropy and controls its stability and suitability for use in magnetic data-storage devices, can be modified by varying the exchange coupling of the spin to a nearby conductive electrode. Using scanning tunnelling microscopy and spectroscopy, we observe variations up to a factor of two of the spin excitation energies of individual atoms as the strength of the spin's coupling to the surrounding electronic bath changes. These observations, combined with calculations, show that exchange coupling can strongly modify the magnetic anisotropy. This system is thus one of the few open quantum systems in which the energy levels, and not just the excited-state lifetimes, can be renormalized controllably. Furthermore, we demonstrate that the magnetocrystalline anisotropy, a property normally determined by the local structure around a spin, can be tuned electronically. These effects may play a significant role in the development of spintronic devices in which an individual magnetic atom or molecule is coupled to conducting leads.

Activation energy paths for graphene nucleation and growth on Cu.

The synthesis of wafer-scale single crystal graphene remains a challenge toward the utilization of its intrinsic properties in electronics. Until now, the large-area chemical vapor deposition of graphene has yielded a polycrystalline material, where grain boundaries are detrimental to its electrical properties. Here, we study the physicochemical mechanisms underlying the nucleation and growth kinetics of graphene on copper, providing new insights necessary for the engineering synthesis of wafer-scale single crystals. Graphene arises from the crystallization of a supersaturated fraction of carbon-adatom species, and its nucleation density is the result of competition between the mobility of the carbon-adatom species and their desorption rate. As the energetics of these phenomena varies with temperature, the nucleation activation energies can span over a wide range (1-3 eV) leading to a rational prediction of the individual nuclei size and density distribution. The growth-limiting step was found to be the attachment of carbon-adatom species to the graphene edges, which was independent of the Cu crystalline orientation.

The Kondo effect in ferromagnetic atomic contacts.

Iron, cobalt and nickel are archetypal ferromagnetic metals. In bulk, electronic conduction in these materials takes place mainly through the s and p electrons, whereas the magnetic moments are mostly in the narrow d-electron bands, where they tend to align. This general picture may change at the nanoscale because electrons at the surfaces of materials experience interactions that differ from those in the bulk. Here we show direct evidence for such changes: electronic transport in atomic-scale contacts of pure ferromagnets (iron, cobalt and nickel), despite their strong bulk ferromagnetism, unexpectedly reveal Kondo physics, that is, the screening of local magnetic moments by the conduction electrons below a characteristic temperature. The Kondo effect creates a sharp resonance at the Fermi energy, affecting the electrical properties of the system; this appears as a Fano-Kondo resonance in the conductance characteristics as observed in other artificial nanostructures. The study of hundreds of contacts shows material-dependent log-normal distributions of the resonance width that arise naturally from Kondo theory. These resonances broaden and disappear with increasing temperature, also as in standard Kondo systems. Our observations, supported by calculations, imply that coordination changes can significantly modify magnetism at the nanoscale. Therefore, in addition to standard micromagnetic physics, strong electronic correlations along with atomic-scale geometry need to be considered when investigating the magnetic properties of magnetic nanostructures.

Photophysics of protoporphyrin ions in vacuo: triplet-state lifetimes and quantum yields.

Lifetimes of triplet-state molecules and triplet quantum yields are important parameters in photobiology as they determine the generation of singlet-oxygen upon irradiation with visible light. Here we report lifetimes of protoporphyrin IX (pp) in vacuo measured in an ion storage ring. We find that after 532 nm photon absorption, pp(-) (free base and negatively charged carboxylate) and pp(+) (single protonation of ring nitrogen) have triplet-state lifetimes of 12 and 6 ms, respectively. After 415 or 390 nm absorption the lifetime of the anion is shorter (1.5 and 0.6 ms) as expected from the increase in temperature. Triplet quantum yields of pp(-) and pp(+) are similar, 0.6-0.7, close to values reported for the free base and monocation in solution. The other channel, direct decay to the electronic ground state and subsequent dissociation of vibrationally excited ions, is much faster than triplet-singlet intersystem crossing. We measured lifetimes of 63 micros, 96 micros, and 0.3 ms after 390, 415, and 532 nm excitation, respectively. A fit of a statistical model to the pp(-) decay results in an Arrhenius activation energy of 0.5+/-0.2 eV for CO(2) loss and a low preexponential factor (10(6)-10(10) s(-1)), indicative of an entropic barrier.