Nanoscale Thin Films Research Articles

Benefits and Limitations of Encapsulating Platinum Electrocatalysts with Niobium Oxide

The development of energy infrastructure based on renewable sources is vital to reduce our reliance on fossil fuels. Technologies such as hydrogen fuel cells and water electrolyzers are able to bridge the gap between the intermittency issues associated with green energy sources such as wind and solar, and the demand for reliable electricity by being able to convert between electrical and chemical energy. Nanostructured and mesoporous platinum (Pt) prepared by electrodeposition or sputter coating have been shown to have increased performance in hydrogen fuel cells compared to the industry standard of Pt nanoparticles supported on carbon.1,2 Over the lifetime of a fuel cell Pt nanostructures and nanoparticles are, however, known to restructure and lose their surface area by mechanisms such as Ostwald ripening, dissolution, and coalescence. Previously, thin film Pt catalysts have been shown to be stabilized by a nanoscale film of niobium oxide.3 Herein, nanoscale thin films of niobium oxide are investigated as a stabilizing material through use by encapsulating nanostructured Pt catalysts.Nanostructured Pt catalysts were electrodeposited onto Pt-coated Si wafer substrates. Subsequently, niobium oxide films with thicknesses of 0.5 nm, 3 nm, and 4.5 nm were deposited onto the Pt catalyst using atomic layer deposition. X-ray fluorescence and conductive atomic force microscopy techniques were used to confirm the presence and coverage of the niobium oxide layer on the Pt catalyst. The performance of the encapsulated and un-encapsulated Pt catalysts as oxygen reduction reaction catalysts over the course of up to a thousand degradation cycles were evaluated using electrochemical techniques. The morphology of the catalyst before and after the degradation testing was also assessed using scanning electron microscopy techniques. Electrochemical impedance spectroscopy indicated that increasing the thickness of the niobium oxide layer increased the resistance of the catalyst towards the oxygen reduction reaction. The combined results of the analyses performed by electrochemical techniques and electron microscopy measurements revealed a range of film thicknesses for the niobium oxide that successfully stabilized the electrochemically active surface area and surface morphology of the Pt catalyst. Paul, M. T. Y.; Gates, B. D. Mesoporous Platinum Prepared by Electrodeposition for Ultralow Loading Proton Exchange Membrane Fuel Cells. Sci Rep 2019, 9, 4161.Debe, M. K. Tutorial on the Fundamental Characteristics and Practical Properties of Nanostructured Thin Film (NSTF) Catalysts. Electrochem. Soc. 2013, 160, F522.Eastcott, J.; Gates, B. D. Nanoscale Thin Films of Niobium Oxide on Platinum Surfaces: Creating a Platform for Optimizing Material Composition and Electrochemical Stability. Can. J. Chem. 2017, 96, 260–266.

Chemical Bonding and Crystal Structure Schemes in Atomic/Molecular Layer Deposited Fe-Terephthalate Thin Films.

Advanced deposition routes are vital for the growth of functional metal-organic thin films. The gas-phase atomic/molecular layer deposition (ALD/MLD) technique provides solvent-free and uniform nanoscale thin films with unprecedented thickness control and allows straightforward device integration. Most excitingly, the ALD/MLD technique can enable the in situ growth of novel crystalline metal-organic materials. An exquisite example is iron-terephthalate (Fe-BDC), which is one of the most appealing metal-organic framework (MOF) type materials and thus widely studied in bulk form owing to its attractive potential in photocatalysis, biomedicine, and beyond. Resolving the chemistry and structural features of new thin film materials requires an extended selection of characterization and modeling techniques. Here we demonstrate how the unique features of the ALD/MLD grown in situ crystalline Fe-BDC thin films, different from the bulk Fe-BDC MOFs, can be resolved through techniques such as synchrotron grazing-incidence X-ray diffraction (GIXRD), Mössbauer spectroscopy, and resonant inelastic X-ray scattering (RIXS) and crystal structure predictions. The investigations of the Fe-BDC thin films, containing both trivalent and divalent iron, converge toward a novel crystalline Fe(III)-BDC monoclinic phase with space group C2/c and an amorphous Fe(II)-BDC phase. Finally, we demonstrate the excellent thermal stability of our Fe-BDC thin films.

A mesoscopic approach for nanoscale evaporation heat transfer characteristics

We propose a mesoscopic approach for investigating steady/transient nanoscale evaporation heat transfer characteristics, whose kinetic nature makes it an ideal tool to model the evaporation kinetics at the liquid-vapor interface and the microscopic wall-fluid interactions. Simulation of the evaporation of a flat nanoscale thin liquid film demonstrates its capability of resolving kinetically limited evaporation and liquid film adsorption. For the simulation of an evaporating meniscus in a nanochannel, our proposed mesoscopic approach considers the conservation of mass, momentum and energy of both liquid and vapor phases, thereby naturally incorporating both the multi-dimensional heat transfer effect and vapor transport resistance in nano-confined space. We demonstrate that solid-fluid interaction plays dominant roles on the interfacial transport during nanoscale evaporation, and vapor transport resistance has nonnegligible influences on the evaporation heat/mass transport in nano-confined spaces. By handling statistical behavior of molecule ensembles, this mesoscopic approach not only preserves microscopic physics but also has high computational efficiencies, enabling the simulation of space and time domains on the practical application level. This work paves the way for quantitative investigations of steady/transient nanoscale evaporation heat transfer characteristics using mesoscopic approach.

Atomic Layer Growth of Rutile TiO2 Films with Ultrahigh Dielectric Constants via Crystal Orientation Engineering.

In general, the electronic and optical properties of oxide films can significantly benefit from highly textured crystallinity. However, oxide films grown by atomic layer deposition (ALD), a powerful technique for the synthesis of high-quality, nanoscale thin films, usually exhibit amorphous or randomly oriented polycrystalline phases. Here, we demonstrate the growth of highly textured rutile phase ALD TiO2 films through rational substrate design. Both a- and c-axis preferentially oriented TiO2 films are obtained by varying the lattice parameters of the initial ALD growth surface. Under optimized conditions, we find that it is possible to deposit high-quality, c-axis preferentially aligned TiO2 films with a bulk dielectric constant approaching 185, rivaling the single crystal limit. These films display a remarkably high dielectric constant of 117 despite thin thickness of 5.2 nm. Moreover, the addition of a single doping sequence of Al2O3 successfully suppresses leakage currents to levels compatible with modern dynamic random access memory cells, all the while maintaining the high bulk dielectric constant of 137. These results clearly highlight the prospect of utilizing crystal orientation engineering in ALD thin films for emerging semiconductor devices.

Two-step process for the fabrication of direct FLG\\MoS2 heterostructures

MoS2 has proven its efficacy in flexible electronics, transistor devices, and various biological and chemical applications. However, it is still challenging to achieve large-area MoS2 monolayers with desired material quality and electrical properties to fulfill the requirement for practical applications. Moreover, the main strategy for the preparation of a 2D heterostructure it is based on the sequential stacking of the layered materials using wet or dry transfer methods which introduces many defects. This paper presents an economically viable and straightforward two-step methodology to obtain MoS2 thin films, encompassing magnetron sputtering deposition of Mo and subsequent annealing in a sulfur-rich environment. This approach successfully yielded MoS2 thin films on Si\\SiO2 substrates. Additionally, heterostructures consisting of few layer graphene (FLG) and MoS2 were directly obtained using the same method. The utilization of grazing incidence X-ray diffraction verified the formation of the hexagonal MoS2 phase, a finding further confirmed by Raman spectroscopy. X-ray photoelectron spectroscopy (XPS) investigations revealed the successful sulfurization process, with surface-bound oxides forming only subsequent to air exposure. Comprehensive assessment involving X-ray reflectivity, atomic force microscopy and XPS collectively inferred the fabrication of thin films comprised of a small number of MoS2 layers covering the entire substrate. Electrical assessments exhibited an electrical hysteresis, demonstrating its potential for memristor applications. Overall, this study outlines a cost-effective fabrication method for producing nanoscale MoS2 thin films with excellent properties, avoiding the use of toxic gases such as H2S. These findings contribute to the potential development of cutting-edge applications.

Recent advances of spectrally selective absorbers: Materials, nanostructures, and photothermal power generation

With the increasing development of photothermal techniques in various fields, particularly concentrated solar power (CSP) systems and solar thermoelectric generators (STEGs), the demand for high-performance spectrally selective absorbers (SSAs) has grown significantly. These SSAs are essential in achieving high solar absorption and minimal infrared thermal loss, thereby significantly enhancing solar utilization efficiency. This need becomes particularly critical in CSP systems, where high temperatures are pivotal for improved efficiency. However, the necessity for high temperatures imposes stringent requirements on the fabrication of SSAs, given the inherent trade-off between optical performance and thermal stability. SSAs typically require nanoscale thin films, but they are prone to oxidation and diffusion at high temperatures. Recent developments in photothermal materials, including ceramic composites, MXenes, high-entropy materials, and graphene, offer promising solutions to enhance SSAs’ performance. This review article provides a comprehensive evaluation of the latest advancements in these emerging photothermal materials. We summarize the strategies for integrating these advanced materials with already established nanostructures, which is a highly promising approach for the development of advanced SSAs. Additionally, the review explores the application of SSAs in CSP systems and STEGs to boost power generation efficiency. We conclude by summarizing the challenges and opportunities in the field of high-temperature SSAs, offering valuable insights into the development of high-performance SSAs and their role in solar-thermal power generation systems.

Mesoscopic Model for Disjoining Pressure Effects in Nanoscale Thin Liquid Films and Evaporating Extended Meniscuses.

Disjoining pressure effect is the key to describe contact line dynamics, micro/nanoscale liquid-vapor phase change heat transfer, and liquid transport in nanopores. In this paper, by combining a mesoscopic approach for nanoscale liquid-vapor interfacial transport and a mean-field approximation of the long-range solid-fluid molecular interaction, a mesoscopic model for the disjoining pressure effect in nanoscale thin liquid films is proposed. The capability of this model to delineate the disjoining pressure effect is validated. We demonstrate that the Hamaker constant determined from our model agrees very well with molecular dynamics (MD) simulation and that the transient evaporation/condensation mass flux predicted by this mesoscopic model is also consistent with the kinetic theory. Using this model, we investigate the characteristics of the evaporating extended meniscus in a nanochannel. The nonevaporating film region, the evaporating thin-film region, and the intrinsic meniscus region are successfully captured by our model. Our results suggest that the apparent contact angle and thickness of the nonevaporating liquid film are self-tuned according to the evaporation rate, and a higher evaporation rate results a in larger apparent contact angle and thinner nonevaporating liquid film. We also show that disjoining pressure plays a dominant role in the nonevaporating film region and suppresses the evaporation in this region, while capillary pressure dominates the intrinsic meniscus region. Strong evaporation takes place in the thin-film region, and both the disjoining pressure and capillary pressure contribute to the total pressure difference that delivers the liquid from the intrinsic meniscus region to the evaporating thin-film region, compensating for the liquid mass loss due to strong evaporation. Our work provides a new avenue for investigating thin liquid film spreading, liquid transport in nanopores, and microscopic liquid-vapor phase change heat/mass transfer mechanisms near the three-phase contact line region.

Inkjet Printing of PrBa0.5Sr0.5Co1.5Fe0.5O5+ δ Cathode for High-Performance Proton Ceramic Fuel Cells

Ceramic fuel cells have been actively studied and are in the commercialization stage because of their economic and efficiency advantages, including the lack of precious metal catalysts, including Pt, fuel flexibility, and high efficiency. Solid oxide fuel cells (SOFCs) are the most typical type of ceramic fuel cell that conduct oxide ions. However, the operating temperature of SOFCs is high (>700 °C) owing to the high activation energy required for ion conduction. Prolonged operation at high temperatures can cause chemical and mechanical degradation in materials and peripheral parts, discouraging their use as long-time stationary generators, which is the main target market for SOFCs. For these reasons, expectations and interest in proton ceramic fuel cells (PCFCs) have recently increased owing to excellent power and durability exhibited at intermediate temperatures (<600 °C). According to recent reports, the durability of PCFC is superior to that of SOFC, even when methane fuel or city gas is used. The cathode reaction accounts for the highest resistance in PCFC operation. Therefore, the development of a high-performance cathode is the biggest factor for the success of PCFCs. To develop a high-performance cathode, the following factors must be optimized: 1) surface and interface shapes and components that can maximize the charge transfer reaction, and 2) porous microstructure that can effectively deliver oxygen gas to the reaction sites, and 3) electrolyte-electrode adhesion at the interface. The potential of inkjet printing (IJP) as a fabrication technology capable of optimizing these structures and components was recently confirmed by our group. IJP has attracted attention as a thin-film deposition technology that can produce complex and sophisticated microstructures, such as various multi-component ceramic materials, while controlling them at the nano-level. Recently, solution processes such as spin coating, tape casting, and screen printing have been frequently adopted in device manufacturing owing to their advantages such as simple accessibility and convenient management. However, fabricating films with nanoscale thickness is very difficult. Moreover, the precision with which a microstructure with the desired shape can be fabricated is limited. Conventional thin-film processes, such as physical vapor deposition and chemical vapor deposition, require complex and difficult deposition conditions, such as high-temperature or high-vacuum maintenance. In addition, equipment management is difficult, a large number of parts and a large system are required, and large-area deposition is highly challenging. Meanwhile, in the case of IJP technology, it is feasible to deposit high-quality nanoscale thin films relatively conveniently without requiring vacuum or temperature maintenance. Furthermore, they can be manufactured over large areas. IJP can be used to conveniently fabricate various patterns in the desired shape without masking or lithography. Moreover, it can precisely control the concentration and combination of printing inks to produce thin films with desired material compositions and microstructures. We successfully fabricated perovskite-type PBSCF cathodes for PCFCs using thermal inkjet printing (T-IJP) on a BaZrxCe0.8-xY0.1Yb0.1O3-δ electrolyte using an inexpensive desktop printer. Ceramic ink solutions were formulated using PBSCF nanopowder as a solid material and dispersed in anhydrous ethanol/propylene glycol methyl ether (PGME)-based solvents. The ink properties were improved by adding various polymers such as surfactants and dispersants. Printability was verified by analyzing the viscosity, surface tension, and particle size of the pigments. The shape and microstructure of the PBSCF cathode layer were successfully adjusted by controlling the number of printing cycles and grayscale value (color level) using commercial printing software. We verified the tendency of these specimens by observing the images using a scanning electron microscope (SEM) equipped with energy dispersive spectra. We also conducted performance evaluations and electrochemical impedance spectroscopy (EIS). Finally, we obtained an optimized PBSCF cathode (G-80) with adequate thickness and a favorable number of pores and cracks. The samples had identical thicknesses of 6–7 μm. This enabled a more reliable and accurate comparison and examination of the tendencies of the specimen microstructures by using grayscale values. We enhanced the performance compared to that of our previous T-IJP work. Among the four specimens measured in this study, G-80 attained a high power output of 728 mWcm-2 in the sub-IT regime (< 600 °C). This performance is higher than that achieved in previous similar studies.

Electrochemistry-Induced Direct Deposition of Nanoscale Thin Zeolitic Imidazolate Framework-8 Films on Insulator Substrates

Electrochemistry-Induced Direct Deposition of Nanoscale Thin Zeolitic Imidazolate Framework-8 Films on Insulator Substrates

Nano-notch modulated fracture behaviors in nanoscale thin films

This study presents an efficient methodology to assess fracture behavior in nanoscale multilayers. After fabricating a nano-cantilever specimen of Cu/Si/SiN, a notch with the tip radius of 3 ∼ 15 nm was introduced into the SiN layer. By changing the position of notch tip, the delamination of Cu/SiN or Cu/Si interfaces, and the fracture of SiN layer were obtained by in situ TEM bending experiments, respectively. The fracture strength of interfaces and SiN layer was determined using an exponential cohesive zone model (CZM) and the crack propagation from the notch tip was analyzed by phase field simulation. Finally, a fracture phase diagram related to the typical position parameters of nano-notch is proposed to predicted the fracture behaviors including the delamination of different interfaces and the fracture of monolayer in nanoscale multilayers. This study provides significant insights into the fracture behavior occurred at nanoscale and a simple but efficient methodology to evaluate the fracture strength of any part in nanoscale multilayers.

Molecular dynamics simulation of the evaporation of thin liquid sodium film on the conical nanostructure surface

Deeply understanding the evaporation of the nanoscale thin liquid film on the nanostructure substrates is significant for further constructing the novel wick structure with nanostructure surfaces, which would be used in the optimal design of the sodium heat pipe. For this purpose, this paper investigates the evaporation of thin liquid film on conical nanostructured surfaces by the molecular dynamics method. A cuboid evaporation system is constructed. It consists of the bottom solid substrate and thin liquid sodium film. A flat solid wall is set as the reference. By changing the nanoscale cone's number and height, five conical nanostructure surfaces are built. The number of gas atoms and the total energy of fluid atoms are observed respectively. The evaporation rate and the average heat flux (q) of six cases are compared. The conical nanostructure suppresses evaporation. With the roughness of the conical nanostructures, the evaporation rate decrement maximum can reach 83% and the q decrement maximum can reach 58%. Based on the thermal resistances in the solid-liquid contact region, the D (Self-diffusion coefficient), and the peak values of RDF (Radial distribution function), the mechanisms of the conical nanostructure surfaces are analyzed. The suppressing effects of the conical nanostructure surfaces are achieved by weakening the heat conduction in the solid-liquid contact region and mitigating the collision heat transfer of liquid atoms.

Determining effective refractive index and elasticity of nanoscale metal - thin, soft polymer bilayers using interference signatures of a glass probe

HypothesisThin metal films like silver (Ag) and gold (Au) on PDMS are perfect candidates for surface-enhanced Raman spectroscopy (SERS) active substrates whose optical and mechanical properties are often subjected to precise tuning. Measurement of such properties often requires multiple sophisticated experimental techniques and numerical analysis. When depositing a nanoscale thin metal film like Ag or Au on the polymer surface, the refractive index and elastic modulus of this bilayer structure could vary with the metal deposition time and properties of soft substrates. ExperimentsIn this work, we demonstrate a single experimental methodology to measure the refractive index and elasticity of thin metal films coated on PDMS. Using Dual-Wavelength Reflection Interference Contrast Microscopy (DW-RICM) measurements of a spherical glass probe of known shape, the refractive indices of Ag/PDMS and Au/PDMS substrates of varying metal deposition time, PDMS elasticity, and PDMS coating thickness are extracted corresponding to two different wavelengths. FindingsThe extracted refractive indices of metal/PDMS substrates decrease with increasing metal deposition time, decreasing PDMS elasticity, and increasing PDMS coating thickness. The extracted refractive indices are found to be in good agreement with corresponding UV/VIS spectroscopy measurements. Lastly, their elastic moduli are also obtained using Hertz's theory on the observed contact radius of the glass sphere on the aforementioned substrates.

Cost-effective measurement of magnetostriction in nanoscale thin films through an optical cantilever displacement method

A cost-effective method for the quantitative characterization of the magnetostrictive effect in thin films is presented. In this method, a sample's magnetostriction is extrapolated from the tip displacement of a thin-film magnetostrictive cantilever. The tip displacement is measured by monitoring the position of a reflected laser beam using two differentially coupled photodiode positioning sensors. In contrast with alternative optical deflection-angle devices designed for educational purposes, the detection limit of our setup resolves submicron-level displacements from nanoscale thin films. The efficacy of the system is demonstrated through measurements using amorphous 200-nm thick Terfenol-D/Si (100) bimorph cantilevers. In these measurements, magnetostriction values of 106 ± 3.5 ppm at ±4300 Oe applied field were attained, where the voltage noise floor was ±0.05 V (a cantilever displacement uncertainty of ±70 nm). In-plane (IP) and out-of-plane (OOP) magnetization curves and crystallographic x-ray diffraction (XRD) were performed to determine the magnetic behavior and confirm the amorphous nature of the films, respectively. The experimental methods and material characterization systems demonstrated here enhance the understanding of complex magnetic phenomena and introduce common measurement techniques to better equip students with the skills for insightful analysis of fundamental magnetic physics.

Nanoscale liquid-vapor phase change characteristics of binary mixtures from molecular dynamics perspective

Nanoscale thin liquid film boiling of a binary liquid mixture subjected to extremely rapid heating within molecular dynamics framework is the focus of our study. A three-phase MD simulation domain is considered where liquid argon-krypton (Ar-Kr) binary mixture rests over a solid Platinum (Pt) like substrate. The molecular system starting from freezing state is subjected to non-equilibrium boundary heating following a sufficient equilibration period to induce liquid–vapor phase change process. Depending on the mixture composition and boundary heating rate, four distinct phase change scenarios have been observed. Characteristics of different phase change scenarios have been studied in terms of important system parameters such as transient atomistic distribution, temperature, pressure, evaporation history, onset of liquid cluster splashing from the wall along with atomic kinetics parameters like COM (Center of Mass), and MSD (Mean Square Displacement). Addition of Kr with Ar that is less volatile compared to Ar, has been found to result in a delay in the evaporation onset and decrease in interaction energy. As a consequence, with the increase of Kr fraction in the binary mixture, the mobility of the evaporated atoms is hindered and the onset temperature of liquid splashing from the wall that resembles generation of “Leidenfrost Film” changes. Also, the near wall atomistic energetics is significantly influenced by mixture composition. Finally, a contour of various phase change modes for Ar-Kr binary mixtures has been presented as a function of mixture composition and boundary heating rate. Present work has been found to be aligned with contemporary works in the context of the derived heat transfer properties.

Principles of Logic Design with Nanoscale Thin Film Memristive Systems for High Performance Digital Circuit Applications

The characteristic pinched hysteresis behavior of memristors has been reported by stacks of a variety of materials. This paper aims to examine the principles of logic design using such two terminal memristive systems for high performance digital circuit applications. As against logic design with standard CMOS, the benefits of logic design with memristors have been stated. The realization and operation of memristor based AND and OR hybrid logic gates obtained by integrating memristors with standard CMOS logic have been discussed. The IMPLY and MAGIC logic families have been demonstrated by covering MAGIC NOR and NAND logic gate implementation with MAGIC NOR in detail. A qualitative comparison has been drawn towards the end of the paper to conclude on the suitability and application space for each of the logic families studied in this paper. This work also describes the hybrid CMOS-memristive logic family known as MRL (Memristor Ratioed Logic). With the addition of CMOS inverters, this logic family's OR and AND logic gates, which are based on memristive components, are given a full logic structure and signal restoration. The MRL family, in contrast to earlier memristor-based logic families, is compatible with conventional CMOS logic.

Comprehensive analysis of the contribution from the heat capacity in thermal measurement on nanoscale thin films

Nanofilms (NFs) have been widely used in many emerging applications, such as microelectronic devices, spintronics, and optical instruments. Characterizing the thermal conductivity (k) of NFs, kNFs, is nontrivial for both fundamental science and industrial applications. Time-domain thermoreflectance (TDTR) is a powerful technique for thermal characterizations under nano-to-micro-scales. However, both the kNFs and the interface thermal resistance between layers are generally unknown parameters when analyzing TDTR signals. So that an effective thermal resistance model (ETRM) is often utilized, where the impact of heat capacity of NFs, cNFs, has been always ignored. Previous studies have proposed viewpoints on how to validate this assumption, however, which still needs to be verified further by considering the parameters' sensitivities for TDTR signals. In this work, we have highlighted the significance of sensitivity analysis for the investigation of the impact of cNFs with the example experiments on Ni NFs. The error of ETRM has been quantitatively and systematically studied. We found that the application requirements of ETRM in TDTR data analysis rely on the sensitivity relations between kNFs and interface thermal resistance. Finally, suggestions for future characterization of the kNFs have been discussed for reference. Our results and conclusions deepen the understanding of both the transient thermal transport process of multilayer NFs and the ability of TDTR on characterizing kNFs.

Surface Oxidation State Variations and Insulator–Metal Transition Modulations in Vanadium Oxides with Pulsed Hydrogen Plasma

AbstractSurface often determines the formation and function of nanoscale thin films and nanomaterials. In correlated vanadium oxides, oxidation states and stoichiometry play important roles in regulating reversible insulator–metal transition (IMT) behaviors. This study presents the surface oxidation state reductions and IMT modulations in vanadium oxides. Oxidation states of vanadium in the surface layer are changed in the nanostructures of vanadium oxides through pulsed hydrogen plasma (PHP). The thickness‐dependent variations are also observed with Raman and X‐ray photoelectron spectroscopy (XPS), and verified in vanadium oxides with different oxidation states. Obvious modulations on IMT behaviors are then demonstrated and related to such variation of surface oxidation states in the VO2 thin film and nanobeam. It is expected that such surface manipulations are beneficial to a better understanding of the IMT in correlated vanadium oxides and the fabrication of advanced functional electronic and photoelectronic devices.

Impact of monolayer engineering on ferroelectricity of sub-5 nm Hf0.5Zr0.5O2 thin films

The sub-5 nm Hf0.5Zr0.5O2 (HZO) thin films have been studied for years; however, they still suffer from challenges including poor ferroelectricity, wake-up effect, and the need for high-temperature annealing. In this paper, the concept and the method of “monolayer engineering” by atomic layer deposition (ALD) are proposed and implemented, which leads to dramatic improvements in the crystallinity and ferroelectricity of the HZO thin film with a thickness of only ∼4 nm at a low annealing temperature of only 370 °C. By substituting one ZrO2 for one HfO2 atomic layer, a high ferroelectric remnant polarization (Pr) ∼15 μC/cm2 is achieved in the HZO thin film. Moreover, the pronounced ferroelectric characteristics are demonstrated in the pristine state, indicating the nearly wake-up-free property of the HZO layer. This study manifests that the monolayer engineering by ALD enables atomic tailoring to enhance the material and physical properties of nanoscale thin films.

Sub-resolution contrast in neutral helium microscopy through facet scattering for quantitative imaging of nanoscale topographies on macroscopic surfaces

Nanoscale thin film coatings and surface treatments are ubiquitous across industry, science, and engineering; imbuing specific functional or mechanical properties (such as corrosion resistance, lubricity, catalytic activity and electronic behaviour). Non-destructive nanoscale imaging of thin film coatings across large (ca. centimetre) lateral length scales, crucial to a wide range of modern industry, remains a significant technical challenge. By harnessing the unique nature of the helium atom–surface interaction, neutral helium microscopy images these surfaces without altering the sample under investigation. Since the helium atom scatters exclusively from the outermost electronic corrugation of the sample, the technique is completely surface sensitive. Furthermore, with a cross-section that is orders of magnitude larger than that of electrons, neutrons and photons, the probe particle routinely interacts with features down to the scale of surface defects and small adsorbates (including hydrogen). Here, we highlight the capacity of neutral helium microscopy for sub-resolution contrast using an advanced facet scattering model based on nanoscale features. By replicating the observed scattered helium intensities, we demonstrate that sub-resolution contrast arises from the unique surface scattering of the incident probe. Consequently, it is now possible to extract quantitative information from the helium atom image, including localised ångström-scale variations in topography.

Thermal capillary waves on bounded nanoscale thin films.

The effect of confining walls on the fluctuation of a nanoscale thin film's free surface is studied using stochastic thin-film equations(STFEs). Two canonical boundary conditions are employed to reveal the influence of the confinement: (1) an imposed contact angle and (2) a pinned contact line. A linear stability analysis provides the wave eigenmodes, after which thermal-capillary-wave theory predicts the wave fluctuation amplitudes. Molecular dynamics (MD) simulations are performed to test the predictions, and a Langevin diffusion model is proposed to capture oscillations of the contact lines observed in MD simulations. Good agreement between the theoretical predictions and the MD simulation results is recovered, and it is discovered that confinement can influence the entire film. Notably, a constraint on the length scale of wave modes is found to affect fluctuation amplitudes from our theoretical model, especially for 3D films. This opens up challenges and future lines of inquiry.