Large-area nanoengineering of graphene corrugations for visible-frequency graphene plasmons

Tunable hybridized plasmons-phonons in a graphene/mica-nanofilm heterostructure

Graphene plasmons exhibit significant potential across diverse fields, including optoelectronics, metamaterials, and biosensing. However, the exposure of all surface atoms in graphene makes it susceptible to surrounding interference, including losses stemming from charged impurity scattering, the dielectric environment, and the substrate roughness. Thus, designing a dielectric environment with a long lifetime and tunability is essential. In this study, we created a van der Waals (vdW) heterostructure with graphene nanoribbons and mica nano-films. Through Fourier-transform infrared spectroscopy, we identified hybrid modes resulting from the interaction between graphene plasmons and mica phonons. By doping and manipulating the structure of graphene, we achieved control over the phonon-plasmon ratio, thereby influencing the characteristics of these modes. Phonon-dominated modes exhibited stable resonant frequencies, whereas plasmon-dominated modes demonstrated continuous tuning from 1140 to 1360 cm-1 in resonance frequency, accompanied by an increase in extinction intensity from 0.1% to 1.2%. Multiple phonon couplings limited frequency modulation, yielding stable resonances unaffected by the gate voltage. Mica substrates offer atomic level flatness, long phonon lifetimes, and dielectric functionality, enabling hybrid modes with high confinement, extended lifetimes (up to 1.9 picoseconds), and a broad frequency range (from 750 cm-1 to 1450 cm-1). These properties make our graphene and mica heterostructure promising for applications in chemical sensing and integrated photonic devices.

Resolving heterogeneous particle mobility in deeply quenched liquid iron: an ultra-fast assembly-free two-step nucleation mechanism

Despite intensive research, little is known about the intermediate state of phase transforming materials, which may form the missing link between e.g. liquids and solids on the nanoscale. The unraveling of the nanoscale interplay between the structure and dynamics of the intermediate state of phase transformations (through which e.g. crystal nucleation proceeds) is one of the biggest challenges and unsolved problems of materials science. Here we show using unbiased molecular dynamics simulations and spatially resolved atomic displacement maps (d-maps) that upon deep quenching the solidification of undercooled liquid iron proceeds through the formation of metastable pre-nucleation clusters (PNCs). We also reveal that the hitherto hidden PNCs are nearly immobile (dynamically arrested) and the related heterogeneity in atomic mobilities becomes clearly visible on atomic displacement-maps (d-maps) when atomic jumps are referenced to the final crystalline positions. However, this is in contrast to PNCs found in molecular solutions, in which PNCs tend to aggregate, move and crystallize via an activated process. Coordination filtered d-maps resolved in real space directly demonstrate that previously unseen highly ramified intermediate atomic clusters with a short lifetime emerge after incubation of undercooled liquid iron. The supercooled liquid iron is neither a spinodal system nor a liquid and undergoes a transition into a specific state called a quasi-liquid state within the temperature regime of 700-1250 K (0.5Tm > 0.7Tm, where the melting point is Tm ≈ 1811 K). Below 700 K the supercooled system is spinodal-like and above 1300 K it behaves like an ordinary liquid with long incubation times. A two-step process is proposed to explain the anomalous drop in the incubation time in the temperature regime of 700-1250 K. The 1st step is activated aggregation of small atomic clusters followed by assembly-free nearly barrierless ultrafast growth of early ramified prenucleation clusters called germs. The display and characterization of the hidden PNCs in computer simulations could provide new perspectives on the deeper understanding of the long-standing problem of precursor development during crystal nucleation following deep quenching.

Metal-Cation-Induced Tiny Ripple on Graphene

Ripples on graphene play a crucial role in manipulating its physical and chemical properties. However, producing ripples, especially at the nanoscale, remains challenging with current experimental methods. In this study, we report that tiny ripples in graphene can be generated by the adsorption of a single metal cation (Na+, K+, Mg2+, Ca2+, Cu2+, Fe3+) onto a graphene sheet, based on the density functional theory calculations. We attribute this to the cation-π interaction between the metal cation and the aromatic rings on the graphene surface, which makes the carbon atoms closer to metal ions, causing deformation of the graphene sheet, especially in the out-of-plane direction, thereby creating ripples. The equivalent pressures applied to graphene sheets in out-of-plane direction, generated by metal cation-π interactions, reach magnitudes on the order of gigapascals (GPa). More importantly, the electronic and mechanical properties of graphene sheets are modified by the adsorption of various metal cations, resulting in opened bandgaps and enhanced rigidity characterized by a higher elastic modulus. These findings show great potential for applications for producing ripples at the nanoscale in graphene through the regulation of metal cation adsorption.

Interplay of graphene-DNA interactions: Unveiling sensing potential of graphene materials

Graphene-based materials and DNA probes/nanostructures have emerged as building blocks for constructing powerful biosensors. Graphene-based materials possess exceptional properties, including two-dimensional atomically flat basal planes for biomolecule binding. DNA probes serve as excellent selective probes, exhibiting specific recognition capabilities toward diverse target analytes. Meanwhile, DNA nanostructures function as placement scaffolds, enabling the precise organization of molecular species at nanoscale and the positioning of complex biomolecular assays. The interplay of DNA probes/nanostructures and graphene-based materials has fostered the creation of intricate hybrid materials with user-defined architectures. This advancement has resulted in significant progress in developing novel biosensors for detecting DNA, RNA, small molecules, and proteins, as well as for DNA sequencing. Consequently, a profound understanding of the interactions between DNA and graphene-based materials is key to developing these biological devices. In this review, we systematically discussed the current comprehension of the interaction between DNA probes and graphene-based materials, and elucidated the latest advancements in DNA probe-graphene-based biosensors. Additionally, we concisely summarized recent research endeavors involving the deposition of DNA nanostructures on graphene-based materials and explored imminent biosensing applications by seamlessly integrating DNA nanostructures with graphene-based materials. Finally, we delineated the primary challenges and provided prospective insights into this rapidly developing field. We envision that this review will aid researchers in understanding the interactions between DNA and graphene-based materials, gaining deeper insight into the biosensing mechanisms of DNA-graphene-based biosensors, and designing novel biosensors for desired applications.

Compact plasmon modulator based on the spatial control of carrier density in indium tin oxide

To keep pace with the demands of semiconductor integration technology, a semiconductor device should offer a small footprint. Here, we demonstrate a compact electro-optic modulator by controlling the spatial distribution of carrier density in indium tin oxide (ITO). The proposed structure is mainly composed of a symmetrical metal electrode layer, calcium fluoride dielectric layer, and an ITO propagating layer. The carrier density on the surface of the ITO exhibits a periodical distribution when the voltage is applied on the electrode, which greatly enhances the interaction between the surface plasmon polaritons (SPPs) and the ITO. This structure can not only effectively improve the modulation depth of the modulator, but also can further reduce the device size. The numerical results indicate that when the length, width, and height of the device are 14 µm, 5 µm, and 8 µm, respectively, the modulation depth can reach 37.1 dB at a wavelength of 3.66 µm. The structure can realize a broadband modulation in theory only if we select a different period of the electrode corresponding to the propagating wavelength of SPPs because the modulator is based on the scattering effect principle. This structure could potentially have high applicability for optoelectronic integration, optical communications, and optical sensors in the future.

A strong bimetal-support interaction in ethanol steam reforming

The metal-support interaction (MSI) in heterogeneous catalysts plays a crucial role in reforming reaction to produce renewable hydrogen, but conventional objects are limited to single metal and support. Herein, we report a type of RhNi/TiO₂ catalysts with tunable RhNi-TiO₂ strong bimetal-support interaction (SBMSI) derived from structure topological transformation of RhNiTi-layered double hydroxides (RhNiTi-LDHs) precursors. The resulting 0.5RhNi/TiO₂ catalyst (with 0.5 wt.% Rh) exhibits extraordinary catalytic performance toward ethanol steam reforming (ESR) reaction with a H₂ yield of 61.7%, a H₂ production rate of 12.2 L h^-1 g_cat^-1 and a high operational stability (300 h), which is preponderant to the state-of-the-art catalysts. By virtue of synergistic catalysis of multifunctional interface structure (Rh-Ni^δ--O_v-Ti³⁺; O_v denotes oxygen vacancy), the generation of formate intermediate (the rate-determining step in ESR reaction) from steam reforming of CO and CH_x is significantly promoted on 0.5RhNi/TiO₂ catalyst, accounting for its ultra-high H₂ production.

Ultrafast charge transfer in mixed-dimensional WO3-x nanowire/WSe2 heterostructures for attomolar-level molecular sensing

Developing efficient noble-metal-free surface-enhanced Raman scattering (SERS) substrates and unveiling the underlying mechanism is crucial for ultrasensitive molecular sensing. Herein, we report a facile synthesis of mixed-dimensional heterostructures via oxygen plasma treatments of two-dimensional (2D) materials. As a proof-of-concept, 1D/2D WO3-x/WSe2 heterostructures with good controllability and reproducibility are synthesized, in which 1D WO3-x nanowire patterns are laterally arranged along the three-fold symmetric directions of 2D WSe2. The WO3-x/WSe2 heterostructures exhibited high molecular sensitivity, with a limit of detection of 5 × 10-18 M and an enhancement factor of 5.0 × 1011 for methylene blue molecules, even in mixed solutions. We associate the ultrasensitive performance to the efficient charge transfer induced by the unique structures of 1D WO3-x nanowires and the effective interlayer coupling of the heterostructures. We observed a charge transfer timescale of around 1.0 picosecond via ultrafast transient spectroscopy. Our work provides an alternative strategy for the synthesis of 1D nanostructures from 2D materials and offers insights on the role of ultrafast charge transfer mechanisms in plasmon-free SERS-based molecular sensing.

3D Dirac semimetals supported tunable terahertz BIC metamaterials

Based on the 3D Dirac semimetals (DSM) supported tilted double elliptical resonators, the tunable propagation properties of quasi-bound in continuum (BIC) resonance have been investigated in the THz regime, including the effects of rotation angles, DSM Fermi level, and the configuration of resonators. The results manifest that by altering the rotation angle of elliptical resonator, an obvious sharp BIC transmission dip is observed with the Q-factor of more than 60. The DSM Fermi level affects the BIC resonance significantly, a sharp resonant dip is observed if Fermi level is larger than 0.05 eV, resulting from the contributions of reflection and absorption. If Fermi level changes in the range of 0.01-0.15 eV, the amplitude and frequency modulation depths are 92.75 and 44.99%, respectively. Additionally, with the modified configurations of elliptical resonators, e.g. inserting a dielectric hole into the elliptical resonator, another transmission dip resonance is excited and indicates a red shift with the increase of the permittivity of the dielectric filling material. The results are very helpful to understand the mechanisms of DSM plasmonic structures and develop novel tunable THz devices, such as modulators, filters, and sensors in the future.

Large Distortion of Fused Aromatics on Dielectric Interlayers Quantified by Photoemission Orbital Tomography

Polycyclic aromatic compounds with fused benzene rings offer an extraordinary versatility as next-generation organic semiconducting materials for nanoelectronics and optoelectronics due to their tunable characteristics, including charge-carrier mobility and optical absorption. Nonplanarity can be an additional parameter to customize their electronic and optical properties without changing the aromatic core. In this work, we report a combined experimental and theoretical study in which we directly observe large, geometry-induced modifications in the frontier orbitals of a prototypical dye molecule when adsorbed on an atomically thin dielectric interlayer on a metallic substrate. Experimentally, we employ angle-resolved photoemission experiments, interpreted in the framework of the photoemission orbital tomography technique. We demonstrate its sensitivity to detect geometrical bends in adsorbed molecules and highlight the role of the photon energy used in experiment for detecting such geometrical distortions. Theoretically, we conduct density functional calculations to determine the geometric and electronic structure of the adsorbed molecule and simulate the photoemission angular distribution patterns. While we found an overall good agreement between experimental and theoretical data, our results also unveil limitations in current van der Waals corrected density functional approaches for such organic/dielectric interfaces. Hence, photoemission orbital tomography provides a vital experimental benchmark for such systems. By comparison with the state of the same molecule on a metallic substrate, we also offer an explanation why the adsorption on the dielectric induces such large bends in the molecule.

Direct Visualization of Ultrastrong Coupling between Luttinger-Liquid Plasmons and Phonon Polaritons

Ultrastrong coupling of light and matter creates new opportunities to modify chemical reactions or develop novel nanoscale devices. One-dimensional Luttinger-liquid plasmons in metallic carbon nanotubes are long-lived excitations with extreme electromagnetic field confinement. They are promising candidates to realize strong or even ultrastrong coupling at infrared frequencies. We applied near-field polariton interferometry to examine the interaction between propagating Luttinger-liquid plasmons in individual carbon nanotubes and surface phonon polaritons of silica and hexagonal boron nitride. We extracted the dispersion relation of the hybrid Luttinger-liquid plasmon-phonon polaritons (LPPhPs) and explained the observed phenomena by the coupled harmonic oscillator model. The dispersion shows pronounced mode splitting, and the obtained value for the normalized coupling strength shows we reached the ultrastrong coupling regime with both native silica and hBN phonons. Our findings predict future applications to exploit the extraordinary properties of carbon nanotube plasmons, ranging from nanoscale plasmonic circuits to ultrasensitive molecular sensing.