Nanometric Films Research Articles

Nanometric Ge Films for Ultrafast Modulation of THz Waves with Flexible Metasurface

AbstractHarnessing confined light on a subwavelength scale within Fano metasurfaces enhances light‐matter interaction on a flexible, low‐loss substrate. This approach enables the integration of ultra‐thin semiconducting films for designing low‐power, ultrafast switchable terahertz, and optical meta devices. Here, an ultra‐thin, ∼λ/6000 germanium overlayers of 25nm or 50nm is thermally evaporated onto a flexible Fano metallic metasurface to achieve ultrafast optical modulation of terahertz radiation. A remarkable 85% modulation depth with an ultrafast speed of 400 GHz is obtained in the resonant transmission. These ultrathin, flexible, and ultrafast functional metasurfaces pave the way for developing flexible terahertz active meta devices based on nanometric scale germanium films, offering the additional advantage of compatibility with complementary metal‐oxide‐semiconductor (CMOS) technology in microelectronics.

A design of compact plasmonic lens consisting of high index dielectric gratings and metal nano-film

In this paper, a hybrid ultra-thin planar subwavelength focusing structure consisting of a high refractive index dielectric grating and a nano metal film was designed. The thickness of this structure is only 200 nm. Surface plasmon polaritons (SPPs) were excited from the nano metal film, propagated along the metal-air interface, and were then converted into a radiation field by the dielectric grating, forming a focused spot in free space. By adjusting the grating position and width parameters, the shape of the focused optical field could be controlled. The simulation results showed that, under 532 nm light irradiation, the lens could produce a 270 nm (full width at half-maximum, FWHM) spot size at a focal length of 2.46λ. Moreover, under the illumination of 633 nm and 780 nm light, the designed lenses were found to produce focal spot sizes of 304 nm (0.48λ) and 364 nm (0.47λ), respectively, which were smaller than the diffraction limit. The simplicity of this plasmonic lens design, coupled with its reduced thickness and minimal absorption loss, offered significant advantages in terms of cost-effectiveness and ease of fabrication.

Temporal Separation between Lattice Dynamics and Electronic Spin‐State Switching in Spin‐Crossover Thin Films Evidenced by Time‐Resolved X‐Ray Diffraction

AbstractSpin‐crossover (SCO) complexes have drawn significant attention for the possibility to photoswitch their electronic spin state on a sub‐picosecond timescale at the molecular level. However, the multi‐step mechanism of laser‐pulse‐induced switching in solid state is not yet fully understood. Here, time‐resolved synchrotron X‐ray diffraction is used to follow the dynamics of the crystal lattice in response to a picosecond laser excitation in nanometric thin films of the SCO complex [Fe(HB(1,2,4‐triazol‐1‐yl)3)2]. The observed structural dynamics unambiguously reveal a lattice expansion on the 100 picosecond timescale, which is temporally decoupled both from the ultrafast molecular photoswitching process (occurring within 100 fs) and from the delayed, thermo‐elastic (Arrhenius‐driven) conversion (taking place ≈10 ns). These time‐separated dynamics are also manifested by the observation of damped acoustic oscillations in the time evolution of the lattice volume, whereas no such oscillations are observed in the electronic spin‐state dynamics. Overall, these results suggest the existence of a universal behavior whereby the intramolecular energy barrier between low‐spin and high‐spin states acts as an intrinsic dynamical bottleneck in the out‐of‐equilibrium spin‐state switching dynamics of SCO materials.

Atomic wetting of oil droplets into hexagons and stripes

HypothesisPrevious studies showed that liquid droplets can be spherical caps or polygonal shapes on patterned surfaces. We hypothesize that these phenomena can be narrowed down to nanoscale and affecting liquid molecular wetting. We anticipate observing preferential wetting on the underlying substrate with atomic crystalline lattice. ExperimentsThis study explores the wetting behavior of oil nanodroplets deposited on highly ordered pyrolytic graphite (HOPG) with atomic force microscope (AFM). Oil droplets were scanned with hydrophobic and hydrophilic AFM tips in water in peak force mode AFM, carefully controlling the scanning force. Simple modeling was carried out using Surface Evolver to simulate how oil droplets wet on HOPG with molecular lattice. FindingsThe results showed two distinct wetting behaviors: the hexagonal nanodroplets and stripe nanometric films. In both cases, the droplets appear to wet preferentially along preferred orientations which reflect the atomic periodicity of the underlying substrate. This preferential wetting occurs because the atomic roughness is significant at the length scale of the nanodroplets, while the eventual outcome of the wetting (i.e. whether the droplet settles to a finite contact angle, or wets completely into a nanometric film) is dependent on the spreading parameter of the droplet-substrate system, independent of scanning direction.

Formation of submicron conical morphology of the surface of nanometer-thick films of the Al–Fe alloy under various conditions of ion-assisted deposition on glass

The morphology, topography and wetting with distilled water of Al–1.5 at. % Fe alloy films with a thickness 25–90 nm, formed on glass by ion-assisted deposition using a resonance ion source of vacuum electric arc plasma, have been studied. Using scanning probe microscopy, it is shown that, depending on the mode and the time of deposition, the modification of films is accompanied by changes in longitudinal and transverse roughness parameters, as well as parameters – dimensionless complexes, the measurement of which makes it possible to quantitatively describe the processes of coning in the Al–Fe alloy/glass system. Thus, the arithmetic mean roughness of the films increases with deposition time in the range 20–40 nm. Under self-irradiation conditions, a transition from island-like film growth to layer-by-layer film growth was detected. The influence of the substrate topography on the longitudinal step parameters of the film topography has been established. The size and surface density of the microdroplet fraction particles are studied by scanning electron microscopy. The frequency distributions of the microdroplet fraction by size are satisfactorily approximated by the lognormal distribution. It is found that in the mode of irradiation with intrinsic ions, 60–70% of the microparticles have a size of up to 0.8 μm. For the first time, a double Gaussian function was used to approximate the distribution histograms of local maxima and minima of the film relief, which made it possible to increase the accuracy of the description compared to the normal law. The effectiveness of this approach in analyzing the structure formation of nanometer films at various stages of growth has been demonstrated. Using the bi-Gaussian surface model, the role of topographic characteristics in controlling the wetting of modified coatings is revealed. The mechanism of heterogeneous wetting of hydrophilic films in the Cassie mode with contact angles ranging 50°–80° is discussed. It is found that in the potential mode, with an increase in the deposition duration to 10 h, the distribution of the film relief is close to the normal law, and the formation of a developed submicron conical morphology on the surface leads to blended wetting.

(Corrosion Division Morris Cohen Graduate Student Award) Evolution of Passivity and Passivity-Breakdown for Cr and Cr-Containing Alloys

Among several corrosion prevention strategies for metallic alloys, alloying with so-called ‘passivating’ elements remains the most effective approach. For example, chromium (Cr) remains the key functional alloying element for Fe-based stainless steels, Ni-based Inconel® alloys and Co-based Stellite® alloys, which dominate the family of corrosion resistant alloys (CRAs) used in a wide range of applications. In addition to conventional alloys, recent exploration in the materials community has led to development of an emerging class of alloys, so-called high entropy alloys (known more broadly as multi-principal element alloys (MPEAs)), which contain multiple alloying elements in ‘principal’ fractions – and of which the corrosion performance has not been widely studied to date. It is however challenging to ascertain the key controlling factors responsible for the passivity in MPEAs using conventional electrochemical techniques due to the combination of low sensitivity and the lack of element resolved information. Moreover, the physical and chemical properties of the nano-metric thin passive film upon CRAs are challenging to evaluate using conventional microscopy and spectroscopy. Consequently, several fundamental aspects of passivity of Cr-containing CRAs remain open questions to be elucidated.The work herein sought to investigate the passivity and dissolution of Cr and Cr-containing alloys, including stainless steel and several MPEAs; primarily using an in-situ electrochemical method called atomic-spectroelectrochemistry (ASEC). ASEC uses a modified flow electrochemical cell coupled with an atomic spectrometer, revealing the real-time and element-resolved dissolution rate during electrochemical testing. In addition to ASEC analysis, the project herein employed a range of electrochemical testing, electron microscopy, X-ray photoelectron spectroscopy, and thermodynamic calculations to explore Cr-based passivity.Significance between composition and growth kinetics on the passivity of Cr and Cr-containing alloys was established. It was found that the passive films upon Cr and Cr-containing alloys (including MPEAs) obey a direct-logarithmic growth law, consistent with the assumptions of the point defect model. In addition, the results unveiled a complex interdependence of oxidation rate of material and dissolution rate of the passive film during anodic polarization, found to be an important parameter to evaluate barrier properties of passive films. The influence of microstructural features including grain boundaries and chemical heterogeneities on Cr-based passivity was explored for stainless steel 316L; whereas the role of alloying elements and electrolytic on the evolution of compositional morphology and thickness of the Cr2O3-based passive film was studied for MPEAs exhibiting single-phase microstructures. The origin of passive film breakdown and role of potential and chloride ions on the breakdown mechanism were elucidated for stainless steel 316L; whereas transpassive dissolution and origin of secondary passivation were studied for equiatomic CoCrFeNi and AlTiVCr MPEAs.Such insights advance the knowledge of underlying mechanisms and associated rate controlling factors of passivity in Cr and Cr-containing alloys, aiding towards an ‘oxide-by-design’ paradigm, to guide design of new CRAs.

Structural comparison between polydopamine precipitate and thin coating layers, down to nanometer film thicknesses

Despite the large number of investigations and applications of polydopamine (PDA), an atomic-scale description of the differences between PDA as a coating layer and PDA precipitate (bulk-PDA) based on clear experimental evidence is not reported yet. In this paper, a two-step approach was used to solve this shortcoming: (i) the deuteration of the dopamine’s catechol-ring to obtain deuterated PDA-based structures, i.e., bulk-PDA and PDA coating layers with different thickness deposited onto SiO2 nanospheres, and (ii) application of different solid-state NMR methods (13C/15N cross-polarization and 2H solid-echo) and electronic microscopy for characterization of the prepared samples in both, deuterated and non-deuterated form. It was found that nano-sized PDA coating layers and bulk-PDA are structurally equivalent at all length scales, down to monomeric units composing the final material. Ss-NMR proved sensitive to identify the distinctive contributions of PDA film surfaces in the 13C and 15N ss-NMR spectra for film thicknesses of 5 nm and less. They consist of amplified line intensities for the aliphatic carbons and NH3+, suggesting the larger number of monomeric units carrying these moieties that exist at the film surfaces compared to the inner oligomer layers. The relevance of these findings for better explaining PDA adhesion is discussed.

Probing molecular arrangements of the organic semiconductor 2,7-Dioctyl[1]benzothieno[3,2- b][1]benzothiophene thin film at the interface by UV Resonant Raman scattering

Raman spectroscopy was employed to investigate nanometric thick films of the organic semiconductor 2,7-Dioctyl[1]benzothieno[3,2-b][1]benzothiophene, following a comprehensive vibrational characterization of the compound condensed phases at various excitation wavelengths. UV Raman excitation enabled the characterization of the thin films, revealing that the molecular orientation at the film/air interface is characterized by a different organization and/or a high degree of disorder compared to the bulk phase. The low penetration depth of the UV Raman excitation allows for the retrieval of this information, unlike the XRD data.

Morphological, electrical, and electromagnetic characterization of nanometric films of Cu and Ni and their combination in bilayers

Modern developments in telecommunication and microelectronics have resulted in spurious radiation, which can promote electromagnetic interference and environmental pollution. To minimize this problem, this work studied nanometric metallic films (65 nm/layer) based on Cu and Ni and their combinations in bilayers deposited on a polyethylene terephthalate substrate by magnetron sputtering. Field emission gun scanning electron microscopy analyses showed that the films have different morphological textures, due to the processing parameters and the free energy of the metals. Electrical measurements by the 4-point method and dielectric spectroscopy showed that the films have low conductivity values, due to oxides formed on the surfaces of the films and defects. X-ray diffraction confirmed the formation of metallic oxides in the films. Electromagnetic characterization (8.2-12.4 GHz) of the monolayer films showed poor performance. Meanwhile, the bilayer films (Ni/Cu and Cu/Ni) showed different behaviors with reflection and absorption contributions. Attenuation values close to 95 % for the Cu/Ni film were obtained.

Catalytic Properties of Nanometric Metal Overlayers with Two‐Dimensional Structures

AbstractAlthough most of the currently used solid catalysts possess a three‐dimensional structure of nanoparticles dispersed on a porous support, the nanoparticle structure should not be considered the optimal structure for all catalytic reactions due to some disadvantages such as thermal instability and catalyst poisoning. Herein, we present the unique catalytic performance of a two‐dimensional metal foil‐supported nanometric Rh thin film, referred to as the “Rh overlayer”, for a catalytic CO−NO reaction and a stoichiometric three‐way catalytic reaction under practical conditions. The extremely high turnover frequency for NO reduction using two‐dimensional Rh is a key to understanding this phenomenon, the detailed mechanism of which can be explained by theoretical calculations.

Molecular dynamics simulations of the melting process of Pb nanofilms

Metallic nanofilms have attracted plenty of research interests because of their important applications in integrated circuits and thermoelectric conversion technology. However, there has not been much research discussing the thermodynamic properties of nanometal films with thickness less than 5 nanometers. To fill this research gap, this article investigates the melting behavior of Pb nanofilms through molecular dynamic simulations. The results show that most nanofilms experience a turning point in their potential energy during melting, followed by a sharp decrease due to the collapse of their previous two-dimensional structure. The melting temperature of nanofilms is found to decrease with decreasing thickness, following the trend predicted by the Safaei model, except for nanofilms with 1-3 atomic layers, which exhibit an abnormally high melting point. It is suggested that this anomaly is due to the reconstruction of nanofilms into a more stable structure, possibly the {111} plane. This study can inspire research on the thermal stability and structural evolution of nanometal films and aid in their future design and preparation.

On‐Demand Subwavelength‐Scale Light Sculpting Using Nanometric Holograms

AbstractSpatially or temporally structured light has attracted considerable attention for its intriguing beam characteristics, which have found extensive applications in classical and quantum optics. Extending structured light from macroscale and microscale to nanometricscale brings more prospects both for fundamental and applied science. However, sculpting light at the subwavelength scale remains a challenge since its transverse structure is fragile in the nanometric scale. Here a novelholography for arbitrary light sculpting at the subwavelength scale is demonstrated. A wave phenomenon of diffractive focusing from an amplitude‐only nanometric (50‐nm‐thick) film is introduced, and use of the induced high‐spatial‐frequency waves as carriers to encode information of an object is considered. Using this technique, nanometric holograms are designed and fabricated for generating the well‐defined eigen modes including the zero‐order Bessel beam, vortex beam, vector beam, Airy beam, as well as an arbitrary light pattern, with feature sizes on the deep‐subwavelength scale. The broadband performance of the hologram is examined, and a white‐light nondiffracting beam at the deep‐subwavelength scale is realized. This demonstration paves a way toward on‐demand light sculpting at the nanometric scale, which may find applications such as optical super‐resolution imaging, nanoparticle manipulation, and precise measurements.

A new route to apply nanometric alumina coating on powders by fluidized bed chemical vapor deposition

A new route to deposit alumina nanometric thin films on powders from the single source Aluminum Tri-Isopropoxide (ATI) precursor is developed using the Fluidized Bed Chemical Vapor Deposition (FBCVD) process. For this study, an easy-to-fluidize silica powder is used, as a first step to ultimately coat cohesive oxygen sensitive powders. The FBCVD process has been studied under mild temperature conditions (400–500 °C), without any containing oxygen co-reactant. The influence of the deposition parameters (temperature, total flow rate, precursor inlet molar fraction) on the process behavior (ATI vaporization rate, fluidized bed thermal profile, deposition yield) and on the coating characteristics (composition, thickness, morphology) is analysed in details. Results show that for all conditions tested, the fluidization is maintained and the bed remains almost isothermal all along the deposition experiments, allowing for a uniform coating of the whole particles of the bed. Continuous and conformal nanometric films are obtained, mainly formed of non-carbon contaminated alumina, containing however a small amount of hydroxyl bonds that could be due to air contamination. The proof of concept of using FBCVD from ATI to produce nanometric stoichiometric alumina coatings on powders is then demonstrated, opening the way for new applications.

Mullite photonic glasses with exceptional thermal stability for novel reflective thermal barrier coatings

Thermal barrier coatings are essential materials systems for insulating and protecting substrates exposed to high temperatures. In such systems, the heat transfer has three possible paths: conduction, convection and irradiation. The higher the operating temperature, the more important it is to control or protect against the radiative component, since the radiative heat flux becomes non-negligible. The radiation can be controlled by the use of ceramic-based photonic nanostructures, namely photonic crystals and photonic glasses, creating so-called reflective thermal barrier coatings. In this work, mullite inverse photonic glasses (PhG) have been produced by thermally induced reaction on sol–gel-based silica structures coated with nanometric films of Al2O3 by atomic layer deposition. The conversion to mullite was carried out following a two-stage heat-treatment. The pre-annealing associated with the further mullite formation results in an excellent structural stability of these PhGs up to 1500 °C, being able to retain their high reflectivity in the near infrared range. Therefore, this structure can be considered for next-generation reflective thermal barrier coatings.Graphical abstract

Thin-Film-Mediated Deformation of Droplet during Cryopreservation.

Freezing of dispersions is omnipresent in science and technology. While the passing of a freezing front over a solid particle is reasonably understood, this is not so for soft particles. Here, using an oil-in-water emulsion as a model system, we show that when engulfed into a growing ice front, a soft particle severely deforms. This deformation strongly depends on the engulfment velocity V, even forming pointy-tip shapes for low values of V. We find such singular deformations are mediated by interfacial flows in nanometric thin liquid films separating the nonsolidifying dispersed droplets and the solidifying bulk. We model the fluid flow in these intervening thin films using a lubrication approximation and then relate it to the deformation sustained by the dispersed droplet.

Characterization of Nanometric Thin Films with Far‐Field Light

AbstractThe characterization of ultrathin transparent films is paramount for various optoelectronic materials, coatings, and photonics. However, characterizing such thin layers is difficult and it requires specialized clean‐room equipment and trained personnel. Here, a contact‐less, all‐optical method is introduced and validated for characterizing nanometric transparent films using far‐field optics. A series of nanometric, smooth, and homogeneous layered samples are fabricated first, alternating transparent spacer and fluorescent layers in a controlled manner. Fluorescence radiation pattern originating from the thin fluorophore layers is then recorded and analyzed and quantitative image analysis is used to perform in operando measurements of the refractive index, film homogeneity and to estimate axial fluorophore distances at a sub‐wavelength scale with a precision of a few of nanometers. The results compare favorably to measurements obtained through more complicated and involved techniques. Applications in nanometrology and biological axial super‐resolution imaging are presented. It is demonstrated in live cells the precise axial localization of single organelles in cortical astrocytes, an important type of brain cell. The approach is cheap, versatile and it will have applications in various fields of photonics.

Confined ionic liquids films under shear: The importance of the chemical nature of the solid surface.

Ionic liquids have generated interest in applications as lubricants and as additives to conventional lubricants due to their unique physical properties. In these applications, the liquid thin film can be subjected simultaneously to extremely high shear and loads in addition to nanoconfinement effects. Here, we use molecular dynamics simulations with a coarse-grained model to study a nanometric film of an ionic liquid confined between two planar solid surfaces both at equilibrium and at several shear rates. The strength of the interaction between the solid surface and the ions was changed by simulating three different surfaces with enhanced interactions with different ions. The increase in the interaction with either the cation or the anion leads to the formation of a solid-like layer that moves alongside the substrates; however, this layer can exhibit different structures and stability. An increase in the interaction with the high symmetry anion produces a more regular structure that is more resistant to the effects of shear and viscous heating. Two definitions were proposed and used for the calculation of the viscosity: a local definition based on the microscopic characteristics of the liquid and an engineering definition based on the forces measured at the solid surfaces, with the former displaying a correlation with the layered structure induced by the surfaces. Because of the shear thinning behavior of the ionic liquids as well as the temperature rise brought on by viscous heating, both the engineering and the local viscosities decrease as the shear rate increases.

MgO nanocube hydroxylation by nanometric water films.

Hydrophilic nanosized minerals exposed to air moisture host thin water films that are key drivers of reactions of interest in nature and technology. Water films can trigger irreversible mineralogical transformations, and control chemical fluxes across networks of aggregated nanomaterials. Using X-ray diffraction, vibrational spectroscopy, electron microscopy, and (micro)gravimetry, we tracked water film-driven transformations of periclase (MgO) nanocubes to brucite (Mg(OH)2) nanosheets. We show that three monolayer-thick water films first triggered the nucleation-limited growth of brucite, and that water film loadings continuously increased as newly-formed brucite nanosheets captured air moisture. Small (8 nm-wide) nanocubes were completely converted to brucite under this regime while growth on larger (32 nm-wide) nanocubes transitioned to a diffusion-limited regime when (∼0.9 nm-thick) brucite nanocoatings began hampering the flux of reactive species. We also show that intra- and inter-particle microporosity hosted a hydration network that sustained GPa-level crystallization pressures, compressing interlayer brucite spacing during growth. This was prevalent in aggregated 8 nm wide nanocubes, which formed a maze-like network of slit-shaped pores. By resolving the impact of nanocube size and microporosity on reaction yields and crystallization pressures, this work provides new insight into the study of mineralogical transformations induced by nanometric water films. Our findings can be applied to structurally related minerals important to nature and technology, as well as to advance ideas on crystal growth under nanoconfinement.

Highly Reactive Diazenyl Radical Species Evidenced during Aryldiazonium Electroreduction

We report the experimental reassessment of the widely admitted concerted reduction mechanism for diazonium electroreduction. Ultrafast cyclic voltammetry was exploited to demonstrate the existence of a stepwise pathway, and real-time spectroelectrochemistry experiments allowed visualization of the spectral signature of an evolution product of the phenyldiazenyl radical intermediate. Unambiguous identification of the diazenyl species was achieved by radical trapping followed by X-ray structure resolution. The electrochemical generation of this transient under intermediate energetic conditions calls into question our comprehension of the layer structuration when surface modification is achieved via the diazonium electrografting technique as this azo-containing intermediate could be responsible for the systematic presence of azo bridges in nanometric films.

Terahertz Emission Spectroscopy of Ultrafast Coupled Spin and Charge Dynamics in Nanometer Ferromagnetic Heterostructures.

Due to its high sensitivity and because it does not rely on the magneto-optical response, terahertz (THz) emission spectroscopy has been used as a powerful time-resolved tool for investigating ultrafast demagnetization and spin current dynamics in nanometer-thick ferromagnetic (FM)/heavy metal (HM) heterostructures. Here, by changing the order of the conductive HM coating on the FM nanometer film, the dominant electric dipole contribution to the laser-induced THz radiation can be unraveled from the ultrafast magnetic dipole. Furthermore, to take charge equilibration into account, we separate the femtosecond laser-induced spin-to-charge converted current and the instantaneous discharging current within the illuminated area. The THz emission spectroscopy gives us direct information into the coupled spin and charge dynamics during the first moments of the light-matter interaction. Our results also open up new perspectives to manipulate and optimize the ultrafast charge current for promising high-performance and broadband THz radiation.