Nanometer-thick Films Research Articles

Electroprecipitation of Nanometer-Thick Films of Ln(OH)3 [Ln = La, Ce, and Lu] at Pt Microelectrodes and Their Effect on Electron-Transfer Reactions.

We report investigations of the deposition of nanometer-thick Ln(OH)3 films (Ln = La, Ce, and Lu) and their effect on outer-sphere and inner-sphere electron-transfer reactions. Insoluble Ln(OH)3 films are deposited from aqueous solutions of LaCl3 onto the surface of 12.5 μm radius Pt microdisk electrodes during water or oxygen reduction. Both reactions produce interfacial OH-, which complexes with Ln3+, resulting in the precipitation of Ln(OH)3. Surface analyses by scanning electron microscopy (SEM), SEM-energy-dispersive X-ray spectroscopy, and atomic force microscopy indicate the formation of a 1-2 nm thick uniform film. Outer-sphere electron-transfer reactions (Ru(NH3)63+ reduction, FcMeOH oxidation, and Fe(CN)64-/3- oxidation/reduction) were investigated at Ln(OH)3-modified electrodes of different film thicknesses. The results demonstrate that the steady-state transport-limited current for these reactions decreases with an increase in the film thickness. Moreover, the degree of blockage depends upon the redox species, suggesting that the Ln(OH)3 films are free from pinholes greater than the size of the redox molecules. This suggests that the films are either ionically conducting or that electron tunneling occurs across these thin layers. A similar blocking effect was observed for the inner-sphere reductions of H2O and O2. We further demonstrate that the thickness of La(OH)3 films can be controlled by anodic dissolution. Additionally, we show that La3+ lowers the supersaturation of dissolved H2 required to nucleate a stable nanobubble.

Influence of thermal effects on the breakup of thin films of nanometric thickness

We apply a previously developed asymptotic model (J. Fluid. Mech. 915, A133 (2021)) to study instabilities of free surface films of nanometric thickness on thermally conductive substrates in two and three spatial dimensions. While the specific focus is on metal films exposed to laser heating, the model itself applies to any setup involving films on the nanoscale whose material parameters are temperature-dependent. For the particular case of metal films heated from above, an important aspect is that the considered heating is volumetric, since the absorption length of the applied laser pulse is comparable to the film thickness. In such a setup, absorption of thermal energy and film evolution are closely correlated and must be considered self-consistently. The asymptotic model allows for a significant simplification, which is crucial from both modeling and computational points of view, since it allows for asymptotically correct averaging of the temperature over the film thickness. We find that the properties of the thermally conductive substrate -- in particular its thickness and rate of heat loss -- play a critical role in controlling the film temperature and dynamics. The film evolution is simulated using efficient GPU-based simulations which, when combined with the developed asymptotic model, allow for fully nonlinear time-dependent simulations in large three-dimensional computational domains. In addition to uncovering the role of the substrate and its properties in determining the film evolution, one important finding is that, at least for the considered range of material parameters, strong in-plane thermal diffusion in the film results in negligible spatial variations of temperature, and the film evolution is predominantly influenced by temporal variation of film viscosity and surface tension (dictated by average film temperature), as well as thermal conductivity of the substrate.

Development of nanometer-thick graphite film extreme ultraviolet pellicle with hydrogen-resistant TiN capping layer

TiN has beneficial physicochemical properties, such as high hardness, good chemical inertness, and good corrosion resistance. TiN has been used for optical filters and protective coatings to exploit these properties. We deposited TiN using atomic layer deposition as a capping layer for a pellicle. We investigated the hydrogen plasma resistance using Raman spectroscopy, transmission electron microscopy, atomic force microscopy, and x-ray photoelectron spectroscopy. As the hydrogen plasma exposure time increased, bonds formed between the TiN film and nitrogen compounds. With long-term exposure, the thickness of the TiN film decreased owing to etching.

Giant nonlinear self-phase modulation of large-amplitude spin waves in microscopic YIG waveguides

Nonlinear self-phase modulation is a universal phenomenon responsible, for example, for the formation of propagating dynamic solitons. It has been reported for waves of different physical nature. However its direct experimental observation for spin waves has been challenging. Here we show that exceptionally strong phase modulation can be achieved for spin waves in microscopic waveguides fabricated from nanometer-thick films of magnetic insulator, which support propagation of spin waves with large amplitudes corresponding to angles of magnetization precession exceeding 10°. At these amplitudes, the nonstationary nonlinear dynamic response of the spin system causes an extreme broadening of the spectrum of spin-wave pulses resulting in a strong spatial variation of the spin-wave wavelength and a temporal variation of the spin-wave phase across the pulse. Our findings demonstrate great complexity of nonlinear wave processes in microscopic magnetic structures and importance of their understanding for technical applications of spin waves in integrated devices.

Onset of electronic conductivity in nanometer thick films of yttria stabilized zirconia (YSZ) at high electric fields

Electrochemical cells were prepared on a silicon wafer consisting of a 100 nm thick solid electrolyte of yttria stabilized zirconia (YSZ) prepared by atomic layer deposition (ALD), a bottom Ruthenium and an upper Gold electrode. Constant voltages were applied to the cells and currents were measured at temperatures between 120 °C and 170 °C. After an incubation time stationary currents were measured corresponding to conductivities well above the ionic one of the YSZ. After the polarization an open circuit voltage remained, which was reduced to zero by passing a constant depolarization current through the cell. All results were explained qualitatively and quantitatively assuming reactions with the electrodes and internal reactions within the YSZ. The latter lead to virtual electrodes with high electronic conductivity between which unreacted YSZ exists, i.e. a n/i/p junction forms. A stationary flux of single charged vacancies through the unreacted YSZ is assumed leading to the final stationary current. The calculated characteristic time for reaching this steady state is shown to be the incubation time, which in agreement with the experiment is inverse proportional to both the second power of electric field strength and the diffusion coefficient of oxygen vacancies.

Evaluation of the magnetization dynamics in various thick YIG films using our proposed measurement technique

This paper describes the evaluation of magnetization dynamics in YIG polycrystalline films with various thicknesses using our proposed measurement technique. The saturation magnetostriction (λs) are approximately less than −1.0×10−6 regardless of the film thickness and are lower than that of bulk polycrystalline YIG, which might be attributed to the structural and/or magnetic inhomogeneities. On the other hand, the in-plane effective damping constant (αeff) decreases approximately from 0.00384±0.00029 to 0.00316±0.00013 with the increase of film thickness, and their values are much higher than those of nano-meter thick and micro-meter thick YIG films deposited on GGG substrates. The reason for this difference may be that the extrinsic damping originating from the magnetic inhomogeneities are enhanced in the film plane. In addition, αeff slightly increases as λs decreases. This tendency is opposite to those of Ni-Fe and Fe-Si polycrystalline films, and the correlation between αeff and λs does not appear. Therefore, these results suggest that magnetic inhomogeneities are mainly influenced to both αeff and λs in the film plane.

Nanometer-thick defective graphene films decorated with oriented ruthenium nanoparticles. Higher activity of 101 vs 002 plane for silane-alcohol coupling and hydrogen transfer reduction

Pyrolysis of ammonium alginate films containing Ru(NH3)6Cl3 leads to the formation of Ru nanoparticles (NPs) supported on defective graphene films. The procedure allows controlling the preferential facet orientation of small Ru NPs (5–20 nm), either 002 when the pyrolysis is carried out under Ar atmosphere or the 002 and 101 planes for pyrolysis in the presence of H2. Ru is a metal difficult to prepare in preferential facet orientation compared to noble metals due to its higher reactivity and smaller particle size. Theoretical calculations substantiated the inhibition of Ru(002) growth by H2 adsorption, with restructuration to Ru(002–101) NPs. The defective graphene films of about 15 nm thickness containing one of the two types of Ru NPs (150 ng/cm2) exhibit distinctive catalytic activity for the dehydrogenative coupling of silanes and alcohols and hydrogen transfer reduction of cyclohexanone. Comparison of turnover frequencies indicates that the 101 facet is more efficient than the 002 plane. Overall, this study illustrates that pyrolysis conditions can control the preferential crystallographic orientation of the growing Ru NPs and the relative catalytic activity of their specific crystallographic planes.

Nanometer-Thick Hematite Films as Photoanodes for Solar Water Splitting

Nanometer-Thick Hematite Films as Photoanodes for Solar Water Splitting

Nanometer-thick copper films with low resistivity grown on 2D material surfaces

Thin Copper (Cu) films (15 nm) are deposited on different 2D material surfaces through e-beam deposition. With the assist of van der Waals epitaxy growth mode on 2D material surfaces, preferential planar growth is observed for Cu films on both MoS2 and WSe2 surfaces at room temperature, which will induce a polycrystalline and continuous Cu film formation. Relative low resistivity values 6.07 (MoS2) and 6.66 (WSe2) μΩ-cm are observed for the thin Cu films. At higher growth temperature 200 °C, Cu diffusion into the MoS2 layers is observed while the non-sulfur 2D material WSe2 can prevent Cu diffusion at the same growth temperature. By further increasing the deposition rates, a record-low resistivity value 4.62 μΩ-cm for thin Cu films is observed for the sample grown on the WSe2 surface. The low resistivity values and the continuous Cu films suggest a good wettability of Cu films on 2D material surfaces. The thin body nature, the capability to prevent Cu diffusion and the unique van der Waals epitaxy growth mode of 2D materials will make non-sulfur 2D materials such as WSe2 a promising candidate to replace the liner/barrier stack in interconnects with reducing linewidths.

Estimation of the Young's Modulus of Nanometer-Thick Films Using Residual Stress-Driven Bilayer Cantilevers.

Precise prediction of mechanical behavior of thin films at the nanoscale requires techniques that consider size effects and fabrication-related issues. Here, we propose a test methodology to estimate the Young’s modulus of nanometer-thick films using micromachined bilayer cantilevers. The bilayer cantilevers which comprise a well-known reference layer and a tested film deflect due to the relief of the residual stresses generated during the fabrication process. The mechanical relationship between the measured residual stresses and the corresponding deflections was used to characterize the tested film. Residual stresses and deflections were related using analytical and finite element models that consider intrinsic stress gradients and the use of adherence layers. The proposed methodology was applied to low pressure chemical vapor deposited silicon nitride tested films with thicknesses ranging from 46 nm to 288 nm. The estimated Young’s modulus values varying between 213.9 GPa and 288.3 GPa were consistent with nanoindentation and alternative residual stress-driven techniques. In addition, the dependence of the results on the thickness and the intrinsic stress gradient of the materials was confirmed. The proposed methodology is simple and can be used to characterize diverse materials deposited under different fabrication conditions.

Investigation of the excitations of plasmons and surface exciton polaritons in monoclinic gadolinium sesquioxide by electron energy-loss spectroscopy and plasmon spectroscopic imaging.

The monoclinic gadolinium sesquioxide (denoted as m-Gd2O3) with its lower crystal symmetry exhibits larger dielectric permittivity (κ) than the cubic Gd2O3 (denoted as c-Gd2O3). Recently, a few nanometers thick m-Gd2O3 thin film has been successfully epitaxially grown on a GaN substrate as a promising candidate gate oxide in metal-oxide-semiconductor field-effect transistors (MOSFETs). Thus, it is important to understand the electronic excitations in m-Gd2O3 and investigate them by electron energy loss spectroscopy (EELS) performed with aloof electron beams and electron diffraction to gain the spatial and momentum resolutions. In this study, using scanning transmission electron microscopy combined with EELS (STEM-EELS) in the aloof electron beam setup, we observed low-loss spectral features at 13 eV and 14.5 eV at the specimen edge in a grazing incidence and the material interior, which can be interpreted as a surface plasmon (SP) and a volume plasmon (VP), respectively. Surface exciton polaritons (SEPs), which represents surface resonances associated with excitonic onsets above the bandgap, were also observed at about 7, 10.2, and 36 eV energy loss. Their surface excitation character was confirmed by energy-filtered transmission electron microscopy spectrum imaging (EFTEM-SI) and using relativistic energy versus-momentum (E–k) map calculations. The momentum (q)-dependent EELS indicates that the SEP features near the bandgap represented a function of q and revealed a nondispersive behavior for VP and SEP at 36 eV. The oscillator strengths for VP and SEP at 36 eV dropped at different q values along with different q directions, revealing the anisotropic electronic structures of m-Gd2O3.

Exploring the mechanical properties of nanometer-thick elastic films through micro-drop impinging on large-area suspended graphene.

In this work, the dependence of effective Young's modulus on the thickness of suspended graphene was confirmed through a drop impingement method. Large area suspended graphene (LSG) layers with a diameter of up to 400 μm and a nanometer thickness were prepared through transferring chemical vapor deposition grown graphene from copper substrates. 4, 8, and 12-layer LSG samples were found to be crumpled yet defect-free. The mechanical properties of LSG were first studied by observing its interaction with impinging droplets from an ink-jet nozzle. First, the effective Young's modulus was calculated by fitting the instant deformation captured by high speed photography within microseconds. Next, droplets deposited on LSG caused deformation and generated wrinkles and the effective Young's modulus was calculated from the number of wrinkles. The above methods yielded effective Young's modulus values ranging from 0.3 to 3.4 TPa. The results from these methods all indicated that the effective Young's modulus increases with the decreasing thickness or size of suspended graphene layers. Moreover, the crumpled LSG yields higher effective Young's modulus than ideal flat graphene. These comprehensive results from complementary methodologies with precise LSG thickness control down to the nanometer scale provide good evidence to resolve the debate on the thickness dependence of mechanical strength for LSG.

Thermal and Thermoelectric Properties of SAM-Based Molecular Junctions.

In molecular thermoelectrics, the thermopower of molecular junctions is closely interlinked with their thermal properties; however, the detailed relationship between them remains uncertain. This study systematically investigates the thermal properties of self-assembled monolayer (SAM)-based molecular junctions and relates them to the thermoelectric performance of the junctions. The electrode temperatures for the bare AuTS, AuTS/EGaIn, and AuTS/TPT SAM//Ga2O3/EGaIn samples placed on a hot chuck were measured under different conditions, such as air vs vacuum and the presence and absence of thermal grease, which generates a heat conduction channel from a hot chuck to gold. It was revealed that the SAM was the most efficient thermal resistor, which was responsible for the creation of a temperature differential (ΔT) across the junction; ΔT in an air atmosphere is overestimated to some extent, and air mainly contributes to large dispersions of thermovoltage (ΔV) data. While junction measurements in air were possible at low ΔT (up to 13 K), the new optimal condition, under a vacuum and with thermal grease, allowed us to examine a wide temperature range up to ΔT = 40 K and obtain a more reliable Seebeck coefficient (S, μV/K). The value of S under the new condition was ∼1.4 times higher than that measured in air without thermal grease. Our study shows the potential of liquid-metal-based junctions to reliably investigate heat conduction across nanometer-thick organic films and elaborates on how the thermal properties of molecular junctions affect their thermoelectric performance.

Interfacial self-assembly of ultrathin polydiacetylene/graphene oxide nanocomposites: A new method for synergetic enhancement of surface charge transfer without doping

A new method for interfacial self-assembly of graphene oxide (GO) and 10,12-pentacosadiynoic acid into nanometer-thick light-absorbing films exhibiting synergetic enhancement of conductive properties is developed. The blue form of polydiacetylene (PDA) nanopatches with a thickness of 5–10 nm is stabilized through the interactions between GO and PDA. The hole-transferring polymer decreases contact resistance in the GO sublayer through “healing” the gaps between the GO nanosheets. The separation of electron-hole pairs at the PDA/GO interface under applied electric field leads to a doping-free enhancement of the in-plane surface conductivity by 6 orders of magnitude up to 46.4 S·cm−1 with respect to those of individual components. These PDA/GO nanocomposites were applied as ultrathin diodes/photodiodes in a solar cell by creating a PDA/C60 heterojunction and a light-induced cross-layer charge transfer. The method can be adapted to a variety of polymerizable light-absorbing surfactants for fabricating GO-based nanohybrids for photoelectronics.

Fabrication of Large-Area Uniform Nanometer-Thick Functional Layers and Their Stacks for Flexible Quantum Dot Light-Emitting Diodes.

Large-area fabrication and stacking of various nanometer-thick functional layers from solutions is essentially important for the construction of flexible thin-film optoelectronic devices, but very challenging. The existing fabrication methods suffer from either non-uniformity caused by the coffee-ring effect or serious solution waste (excess of 90% for spin coating), and are hard to scale up and create stacks. Here, it is shown that centrifugal casting is a universal, scalable, and efficient method to fabricate uniform nanometer-thick films and their stacks of various materials. The coffee-ring effect is effectively suppressed, the solution utilization ratio is higher than ≈61%, and the films/stacks show a smooth surface/high-quality interface. Using this method, flexible quantum dot light-emitting diode displays with uniform luminance in a large lighting area of ≈115 cm2 that have not been achieved even on rigid substrates by the existing methods, are realized. This efficient and low-cost solution processing method paves a way for large-area fabrication of various flexible thin-film optoelectronic devices.

Evidence for spin current driven Bose-Einstein condensation of magnons

The quanta of magnetic excitations – magnons – are known for their unique ability to undergo Bose-Einstein condensation at room temperature. This fascinating phenomenon reveals itself as a spontaneous formation of a coherent state under the influence of incoherent stimuli. Spin currents have been predicted to offer electronic control of Bose-Einstein condensates, but this phenomenon has not been experimentally evidenced up to now. Here we show that current-driven Bose-Einstein condensation can be achieved in nanometer-thick films of magnetic insulators with tailored nonlinearities and minimized magnon interactions. We demonstrate that, above a certain threshold, magnons injected by the spin current overpopulate the lowest-energy level forming a highly coherent spatially extended state. We quantify the chemical potential of the driven magnon gas and show that, at the critical current, it reaches the energy of the lowest magnon level. Our results pave the way for implementation of integrated microscopic quantum magnonic and spintronic devices.

Probing Ionomer Swelling and Water Content in Catalyst Layer during Operation Using Small Angle Neutron and X-Ray Scattering

In the fuel cell catalyst layers (CLs), the ionomer plays roles as a binder and a conductor for protons. It is now known that the ionomer not only forms aggregates between the catalyst agglomerates of the porous structure of the CLs, but it also forms a few nanometer thick thin film covering the surface of this catalyst. In this latter form, it modifies the specific activity of the catalyst by altering the structure and behavior of the electrochemical double layer, and it hinders the oxygen transport toward the catalytic sites. In addition, the nanostructure and functional transport properties of O2 and protons of the ionomer depends on its water content which increases with water activity. Especially, the ionomer must be sufficiently hydrated to enable proton transport from the membrane toward the active sites through the CLs and ensure a proper utilization of the electrochemical surface area. However, excess liquid water accumulation in the pores of the CL hinders oxygen diffusion and should be avoided. Therefore, it is necessary to understand the distribution and the quantity of water, being either within the ionomer or in the pores of the catalyst layers, during fuel cell operation to improve their design and constituents. Very recently, and based on our former experience, we pushed forward the in situ and operando Small Angle Scattering techniques to quantify the water content and ionomer swelling in fuel cell CLs during operation. In this work, we report the effect of the conditioning and operating conditions on the ionomer behavior (Figure - Comparison of SANS Spectra obtained in dry state, at 100%RH and during operation at 80%RH, showing the evolution of the ionomer peak related to its swelling). Figure 1

Reorganization Energy upon Controlled Intermolecular Charge‐Transfer Reactions in Monolithically Integrated Nanodevices

Intermolecular electron-transfer reactions are key processes in physics, chemistry, and biology. The electron-transfer rates depend primarily on the system reorganization energy, that is, the energetic cost to rearrange each reactant and its surrounding environment when a charge is transferred. Despite the evident impact of electron-transfer reactions on charge-carrier hopping, well-controlled electronic transport measurements using monolithically integrated electrochemical devices have not successfully measured the reorganization energies to this date. Here, it is shown that self-rolling nanomembrane devices with strain-engineered mechanical properties, on-a-chip monolithic integration, and multi-environment operation features can overcome this challenge. The ongoing advances in nanomembrane-origami technology allow to manufacture the nCap, a nanocapacitor platform, to perform molecular-level charge transport characterization. Thereby, employing nCap, the copper-phthalocyanine (CuPc) reorganization energy is probed, ≈0.93eV, from temperature-dependent measurements of CuPc nanometer-thick films. Supporting the experimental findings, density functional theory calculations provide the atomistic picture of the measured CuPc charge-transfer reaction. The experimental strategy demonstrated here is a consistent route towards determining the reorganization energy of a system formed by molecules monolithically integrated into electrochemical nanodevices.

Nanometer-Thick SiN Films as Gas Barrier Coatings Densified by Vacuum UV Irradiation

Gas barrier films are widely used in electronic and packaging applications. They are also critical components of flexible organic light-emitting diodes (FOLEDs) that require high gas barrier performance. Among the various film manufacturing techniques, solution-processed thin-film encapsulation (TFE) represents a low-cost FOLED fabrication method. The nanometer-thick SiN films produced following the vacuum ultraviolet (VUV)-induced densification of solution-processed perhydropolysilazane (PHPS) films in a N2 atmosphere can potentially serve as TFE barrier films. However, the nanometer-thick PHPS densification process has not been examined in sufficient detail. We investigated and discussed the effects of the Si–N bond number, PHPS film composition, and free volume (present in the produced Si–N network) on the VUV-induced PHPS densification process. It was found that VUV irradiation caused rapid hydrogen release and film densification through the formation of Si–N bonds. The results obtained using the X-ray photoelectron spectroscopy and dynamic secondary ion mass spectrometry techniques, and the calculated residual hydrogen ratios, revealed that the film composition was strongly related to the number of residual hydrogen atoms and Si–N bonds. Notably, nanometer-thick PHPS film densification was a relatively slow process, in which the free volume in the Si–N network was considerably reduced by the atomic rearrangement induced by the simultaneous cleavage of several Si–N bonds during VUV irradiation. We believe that the results presented herein can potentially serve as a guideline for developing solution-processed nanometer-thick SiN films with relatively high density and excellent gas barrier performance (that is comparable to that exhibited by vacuum-processed barrier films).

The combined use of wrinkling and cracking for the characterization of the mechanical properties of the laser-fabricated nanometer-thick amorphous carbon films

Direct laser writing carbonization has been recently developed to enable facile fabrication of patternable nanometer-thick two-dimensional amorphous carbon films, which may find promising applications in high-performance sensing, energy storage, catalysis, biomaterials etc. Given its nanometer-thick character and brittleness in nature, it is quite challenging to evaluate and characterize the mechanical properties of this novel material. With assistance of a sacrificial-layer transfer approach for sample preparation, herein, we rely on wrinkling metrology as a viable tool for characterizing the elastic modulus of the laser-fabricated nanometer thick amorphous carbon films. Moreover, upon combining the wrinkling metrology with channel cracking behavior investigation, the strength-related mechanical properties, including fracture toughness, fracture strength and fracture strain of the carbon thin films of varied thickness have been evaluated comprehensively. Within the experimental conditions being studied, the elastic modulus, fracture toughness, fracture strength and fracture strain of the laser-fabricated nanometer-thick carbon films respectively fall in a range of 23.1–34.9 GPa, 16.3–151.7 J/m2, 0.5–1.4 GPa, and 2.0–4.5%. The results and methodology presented in this study demonstrate the usefulness of combining wrinkling metrology and cracking technique as a simple but generally applicable approach for comprehensive characterization and evaluation of the mechanical properties of different types of brittle and nanometer-thick thin films.