Structure Of Ultrathin Films Research Articles

Growth and Structure of Ultrathin Iron Silicate and Iron Germanate Films

Growth and Structure of Ultrathin Iron Silicate and Iron Germanate Films

Phase Composition and Structure of Ultrathin Nanocrystalline Cu-Ni Film Alloys

The results of studying the morphology, crystal structure and phase composition of ultrathin Cu-Ni alloy films with thicknesses up to d = 10 nm and concentrations 0 < Cu < 100 at.% are presented.

Large Area Film of Highly Crystalline, Cleavable, and Transferable Semi-Conducting 2D-Imine Covalent Organic Framework on Dielectric Glass Substrate.

Two dimensional (2D) imine-based covalent organic framework (COF), 2D-COF, is a newly emerging molecular 2D polymer with potential applications in thin film electronics, sensing, and catalysis. It is considered an ideal candidate due to its robust 2D nature and precise tunability of the electronic and functional properties. Herein, we report a scalable facile synthesis of 2D imine-COF with control over film thickness (ranging from 100 nm to a few monolayers) and film dimension reaching up to 2 cm on a dielectric (glass) substrate. Highly crystalline 2D imine polymer films are formed by maintaining a quasi-equilibrium (very slow, ∼15 h) in Schiff base condensation reaction between p-phenylenediamine (PDA) and benzene-1,3,5-tricarboxaldehyde (TCA) molecules. Free-standing thin and ultrathin films of imine-COF are obtained using sonication exfoliation of 2D-COF polymer. Insights into the microstructure of thin/ultrathin imine-COF are obtained using scanning and transmission electron microscopy (SEM and TEM) and atomic force microscopy (AFM), which shows high crystallinity and 2D layered structure in both thin and ultrathin films. The chemical nature of the 2D polymer was established using X-ray photoelectron spectroscopy (XPS). Optical band gap measurements also reveal a semiconducting gap. This is further established by electronic structure calculation using density functional theory (DFT), which reveals a semiconductor-like band structure with strong dispersion in bands near conduction and valence band edges. The structural characteristics (layered morphology and microscopic structure) of 2D imine-COF show significant potential for its application in thin film device fabrication. In addition, the electronic structure shows strong dispersion in the frontier bands, making it a potential semiconducting material for charge carrier transportation in electronic devices.

A new method for characterizing the atomic composition distribution and interface structure of ultrathin multilayer films using molecular dynamics simulation assisted ARXPS

Characterizing ultrathin multilayer films' atom distribution and interface structure remains challenging. This paper innovatively characterized the chemical composition and distribution of Pt/Co/W ultrathin multilayer films by ARXPS. Molecular dynamics simulated the growth process of films to correspond to that in experiment. The results show that through ARXPS, the actual structure of Pt (2 nm)/Co (t = 0.5–3 nm)/W (2 nm) film could be obtained. For Pt (2 nm)/Co (2 nm)/W (2 nm) film, the actual structure was Pt/Co (1.73 nm)/W (0.78 nm)/WO3 + WO2 (1.48 nm). W at the top layer significantly diffused to the bottom layers, and oxidized W reduced from the surface to the interior of multilayer films. Molecular dynamics simulation confirmed that the interdiffusion degree of Co and W atoms was improved with the increase of Co atoms. The growth modes of Co and W layers were respectively layer-by-layer and island. As the number of Co atoms increased, the relative surface roughness of Co and W layers and the relative interface roughness of Co/W were increased. The in-plane magnetic anisotropy and coercivity were respectively enhanced and decreased due to the change in the surface/interface structure. This paper provides a new idea for analyzing ultrathin multilayer films' atom distribution and interface structure.

Exploring the potential of branched submicron channels-based structures in ultra-thin Ag films for SERS molecular sensing

Noble metal-based nanostructures are actively studied due to their promising prospects for fabricating efficient substrates for Surface Enhanced Raman Spectroscopy (SERS)-based molecular sensing. In this work, we report the gross potential of SERS substrates based on branched submicron channels in ultra-thin Ag films. These films were fabricated by magnetron sputtering and then annealed in high vacuum conditions to induce branched channels growth by thermal dewetting. Different sets of nominally equal samples were tested during a determined period of time, using confocal Raman spectroscopy and methylene blue (MB) as Raman-active molecule, to assess their performance for SERS in ambient aging conditions. Micro-Raman intensity mapping studies demonstrated the emergence of “hot spots” enhancing the Raman signal in the branched submicron channels. A good mass sensitivity and fast spectral acquisition were achieved using these nanostructures, observing an exceptional spectral resolution and identifying all main vibrational states of MB in a few seconds for samples with a MB surface mass density of about 2 ng/mm2. A good spectral resolution was achieved even using shorter measurement times from 1.00 to 0.05 s, suggesting the potential for fast SERS determinations. Samples showed good reproducibility in the Raman spectral response during the tested period, demonstrating the marginal impact of the aging effects on the Raman signal enhancement and ensuring a time-stable SERS performance in the short term. Our results show that the proposed nanostructures are promising candidates for the development of substrates with competitive-sensitivity, time-stability, and fast SERS response, representing a simple and attractive alternative for efficient SERS-based molecular sensing.

Ellipsometry of ultrathin transparent films. Some aspects of optimum experimental conditions choice

Reflection ellipsometry in visible and infrared range, which enables the analysis of isotropic thin films, is one approach that is extremely sensitive and accurate. There are a few restrictions with this technique when analyzing ultra-thin transparent film (UTTF) structures. In our study, the issue of the independent determination of two parameters of a transparent film that is extremely thin (d < 10 nm) is examined from the standpoint of the optimal conditions choosing for an ellipsometric experiment. An original method to calculation the errors in the main parameters of UTTF depending on the experimental conditions is proposed. One analyzed the influence of the substrate optical parameters and the incidence angle of light on the sensitivity of ellipsometric data. The analysis shows that under certain experimental conditions, the sensitivity of ellipsometric measurements allows to independently determine both the refractive index and the thickness of a transparent film down to 3 nm thick.

Emerging Nontrivial Topology in Ultrathin Films of Rare-Earth Pnictides.

Thin films of rare-earth monopnictide (RE-V) semimetals are expected to turn into semiconductors due to quantum confinement effects (QCE), lifting the overlap between electron pockets at Brillouin zone edges (X) and hole pockets at the zone center (Γ). Instead, using LaSb as an example, we find the emergence of the quantum spin Hall (QSH) insulator phase in (001)-oriented films as the thickness is reduced to 7, 5, or 3 monolayers (MLs). This is attributed to a strong QCE on the in-plane electron pockets and the lack of quantum confinement on the out-of-plane pocket projected onto the zone center, resulting in a band inversion. Spin-orbit coupling (SOC) opens a sizable nontrivial gap in the band structure of ultrathin films. Such effect is anticipated to be general in rare-earth monopnictides and may lead to interesting phenomena when coupled with the 4f magnetic moments present in other members of this family of materials.

Interplay of spin–orbit and exchange interaction in a ferromagnet/heavy-metal hybrid system: Ni on W(110)

In a combined experimental and theoretical study, we investigate the interplay of spin–orbit interaction (SOI) and exchange interaction (XI) in the electronic structure of ultrathin Ni films on W(110). Using spin- and angle-resolved inverse photoemission, we observe that the size of the spin splitting of Ni-related exchange-split states differs for opposite magnetization directions. A quenched spin splitting for one of the magnetization directions reveals a contribution of SOI on an equal footing with XI. Using density-functional theory calculations, we explore the underlying mechanisms responsible for the experimentally observed coupling of SOI and XI. We find that a hybridization between adsorbate and substrate states, along with a high probability density of the respective states at the heavy W nuclei, cause the strong influence of SOI on the Ni-related exchange-split states.

Electron-stimulated desorption kinetics of ultra-thin LiCl films on graphene

Ultra-thin alkali halide films are an important substrate for studying single quantum objects at the atomic scale, as they serve as decoupling scaffolds. However, their stability upon exposure to electrons and photons, the most common probes used to study nanomaterials, is not fully understood. Here we present a study of the evolution of the structure of ultra-thin LiCl films, grown on graphene, upon low-energy electron irradiation by means of microspot-low-energy electron diffraction and microscopy. We find that the intensity of the LiCl diffraction spots irradiated at various electron energies follows a bi-exponential decay function, which can be rationalized by two different desorption regimes. In addition, we detect a change in the work function caused by the electron irradiation, confirming desorption of the LiCl film from the graphene layer. We understand the underlying mechanisms for the electron-induced desorption of the salt film in terms of the evolution of the elementary quasiparticles involved in the process: holes, excitons and F and H center pairs. Moreover, a direct comparison of the electron-induced and the soft X-ray photon-induced processes reveal that, in addition to LiCl desorption, the intercalation of lithium into graphene reported for X-ray induced desorption does not take place in electron-stimulated desorption.

Equilibrium configurations and electronic structure of fullerene ultrathin films on SrTiO3(0 0 1) surface

Ultrathin fullerene (C60) films on various metal and semiconductor substrates have been intensively studied but not yet on a perovskite oxide substrate. Using scanning tunneling microscopy and spectroscopy, we investigated the molecular arrangement and electronic structure of C60 ultrathin films grown on a SrTiO3(001) surface. Subject to strong coupling and weak screening from the substrate, the monolayer C60 exhibit short-range ordered molecular arrangement with disordered molecular orientation and a substantially large highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gap (∼4.1 eV). In contrast, C60 in the 2nd and 3rd layers are self-assembled into hexagon lattices with a quasi-(2 × 2) superstructure resulted from the arrangement of molecular orientations showing a chiral pinwheel structure. The HOMO-LUMO gap of the multi-layer C60 further expands to ∼ 4.6 eV, very close to that of gas-phase C60, indicating a negligible inter-layer screening effect.

Electronic structure of Sb ultrathin film on Bi(111) with large lattice mismatch

Topological phase transitions have been actively investigated to understand the fundamental properties and to develop applications for spintronics. In this study, we focused on Sb ultrathin films that undergo a topological phase transition by lattice strain. In order to fabricate samples with a lattice constant theoretically predicted to be topologically nontrivial, Sb ultrathin films are prepared on a Bi substrate. The surface structure and electronic states of the Sb/Bi heterostructure were confirmed by the low-energy electron diffraction measurement and angle-resolved photoemission spectroscopy. For 2 and 3 BL Sb films, we demonstrated the possibility of creating well-ordered immiscible ultrathin films of Sb on a Bi substrate and observed a “V”-shaped electronic band, which is significantly different from the electronic state of Bi thin films and freestanding Sb ultrathin films. This suggests that the electronic state of the Sb ultrathin film is affected by the Bi substrate. We propose that the “V”-shaped band originates from a topological phase transition due to expanded lattice constants and band hybridization between Bi and Sb. This heterostructure will provide a new platform for investigating the topological phase transitions and applications.

Electron states in metalized ultrathin film crystals and their thermodynamic behavior

The dispersion law of electron in ultrathin crystalline metal-like film structures was found and analyzed using the method of two-time and temperature-dependent Green’s functions. Unlike unbounded (bulk) structures of the same crystallographic composition, the energy spectra of electrons in the film is strictly discrete, have two gaps and the narrower range of electron energies. Also, the conditions for the appearance and existence of localized electronic states as a function of boundary parameters were investigated. The probability of finding electrons in localized states is higher at more perturbed film boundaries. The electronic contribution to specific heat and entropy depends linearly on temperature, as in bulk structures, but with a different slope coefficient, so it follows that the electronic gas in films is a less ordered system and is closer to the equilibrium state than the same in bulk structures. These results could provide an explanation for the experimental facts about the stronger superconducting properties of ultrathin thin films.

Near surface structure of ultrathin epitaxial Ru films on graphene/amorphous SiO2 revealed by azimuthal RHEED

Near surface structure of ultrathin epitaxial Ru films on graphene/amorphous SiO2 revealed by azimuthal RHEED

Scanning tunneling microscopy of MnOx ultrathin films on Au(111)

Two structurally distinct phases of manganese oxide ultrathin films were grown on Au(111) substrates and imaged at atomic resolution by scanning tunneling microscopy (STM). The so-called MnOx fishbone phase is only a few monolayers thick and nucleates epitaxially on the bare Au(111) substrate. The surface of this phase exhibits two parallelogram unit cells with sizes and included angles of (14.6 ± 0.2) × (5.6 ± 0.2) Å2, 88 ± 1° and (13.6 ± 0.1) × (5.6 ± 0.2) Å2, 80 ± 1°. The other thicker phase is called the square phase. It is only observed growing on top of the fishbone phase and has a surface unit cell of edge dimension 5.8 ± 0.1 Å. The square phase is thought to be a (001) termination of hausmannite Mn3O4. In addition, less common intermediary surface structures are also observed. This study demonstrates the transition of the crystal structure of an oxide film from a unique ultrathin film structure that is epitaxially constrained by the interaction with the Au(111) substrate to that of a thicker film with the structure of a bulk crystal.

Interplay between magnetic order and electronic band structure in ultrathin GdGe2 metalloxene films.

Dimensionality can strongly influence the magnetic structure of solid systems. Here, we predict theoretically and confirm experimentally that the antiferromagnetic (AFM) ground state of bulk gadolinium germanide metalloxene, which has a quasi-layered defective GdGe2 structure, is preserved in the ultrathin film limit. Ab initio calculations demonstrate that ultrathin GdGe2 films present in-plane intra-layer ferromagnetic coupling and AFM inter-layer coupling in the ground state. Angle-resolved photoemission spectroscopy finds the AFM-induced band splitting expected for the 2 and 3 GdGe2 trilayer (TL) films, which disappear above the Néel temperature. The comparative analysis of isostructural ultrathin DyGe2 and GdSi2 films confirms the magnetic origin of the observed band splitting. These findings are in contrast with the recent report of ferromagnetism in ultrathin metalloxene films, which we ascribe to the presence of uncompensated magnetic moments.

Controlled Nanoporous Structures of Ultrathin π-Conjugated Polymer Films for Highly Sensitive Gas Sensors

Conventional OFET-based chemical sensors have severe limitations in terms of sensitivity owing to poor analyte diffusion and poor interaction between the analyte and charge carriers in the CP film. To overcome this drawback, it is imperative to fabricate CP films that can facilitate analyte diffusion and provide large adsorption sites for interactions between the analytes and charge carriers concentrated in the conducting channel of OFETs. This study is to develop a facile strategy to fabricate ultrathin nanoporous Conducting polymer(CP) films for high-performance OFET gas sensors based on shearing-assisted phase separation (SAPS) of polymer blends, followed by selective solvent etching. In this method, the morphological features of nanoporous CP films, such as pore size and film thickness, can be tuned in the nanoscale regime by simply varying the shear rate. This indicates that the nanoporous P3HT films were ultrathin. Owing to these morphological features, the resulting OFET sensors exhibited excellent sensing performance when exposed to NH3 gas. We demonstrated that our approach for modulating the morphological features of CP thin films is also applicable to other CP/insulating-polymer-blend systems, including P3HT/PMMA and DPP-DTT/PS. The proposed strategy is simple and universal and can be used to fabricate ultrathin nanoporous CP films, which in turn can enable the development of high-performance, low-cost, portable OFET sensors for various commercial applications.

Characterization of the Local Atomic Structure of ALD Coated Interfaces

To accelerate technological innovations using atomic layer deposition (ALD) coatings, we need to establish better understanding of structure-property relationships for ALD films. Historically, the ALD community has had difficulty connecting the atomic structure of ALD films with their performance, largely because of the significant challenge in determining the atomic-scale structure of ultrathin ALD films that are often amorphous, polycrystalline, or defective. In this work, we describe a series of recent efforts that collectively aim to improve understanding of the atomic structure of ALD coatings and inform structure-property understanding. These efforts employ experimental measurements including (a) inert-transfer XPS, (b) in-situ synchrotron high energy X-ray diffraction, and (c) cryogenic electron diffraction. Of particular focus in this talk is the use of X-ray and electron diffraction measurements in combination with pair distribution function (PDF) analysis and reverse Monte Carlo (RMC) structural modeling. We describe a newly developed approach employing alternating RMC and molecular statics steps (RMC-MS) that improves the physical accuracy of model structures derived from experimental diffraction data with minimal additional computational cost over conventional RMC modeling. We also describe efforts using localized cryogenic electron diffraction and PDF analysis (cryo-ePDF) with a ≤ 5 nm spot size to measure atomic structure at ALD interfaces. Together, these advances allow us to quantify differences in the atomic structure as a function of position through the depth of ALD films. A simple two-phase model comprised of bulk and interfacial layers with distinct atomic structure features is consistent with our measurements for both aluminum oxide and zinc oxide ALD coatings. The approaches we report yield atomic structure models that can be used to inform computational studies of the properties if ALD films and will aid in the selection and modification of ALD coating chemistries to address technological needs.

Structure and Properties of Ultrathin SiOx Films on Cu(111).

The metal-oxide interface plays a crucial role in catalysis, and it has attracted increasing interest in recent years. Cu/SiO2, as a common copper-based catalyst, has been widely used in industrial catalysis. However, it is still a challenge to clarify the structures of the interface of Cu-SiOx and the effect on catalytic performance. Herein, we prepared ultrathin SiOx films by evaporating Si onto a Cu(111) surface followed by annealing in an O2 atmosphere, which were characterized by various surface science techniques. The results showed that a SiOx film could grow nearly layer-by-layer on the Cu(111) surface in the present condition. Both X-ray photoelectron spectroscopy (XPS) and high-resolution electron energy loss spectroscopy (HREELS) results confirmed the presence of Cu-O-Si and Si-O-Si species. Thermal stability experiments illustrated that the well-ordered silica films were stable under annealing in vacuum. The feature of CO adsorption suggested a CO-Cuδ+ species induced from the Cuδ+-O-Si. Low-energy ion scattering spectroscopy (LEIS) and XPS results demonstrated that some Cu2O appeared on the surface when the 1 ML SiOx/Cu(111) was exposed in O2 at 353 K.

Subnanomolar Detection of Mercury Cations in Water by an Interfacial Fluorescent Sensor Achieved by Ultrathin Film Structure Optimization.

The objectives of this work were to develop and extend the previously proposed approaches to control the structure of nanoscale planar systems based on modular fluorophore compounds capable of efficient analyte binding and to optimize the architecture of ultrathin films formed from them to create a thin-film sensor element for mercury cations. The possibility of applying the ratiometric approach to the fluorescence measurements to obtain a quantitative analytical signal was shown. It was found that films with the Langmuir-Schaefer film architecture, in which the receptor crown ether groups of the fluoroionophore are oriented toward the studied solution, allow the quantitative determination of mercury cations in water at concentrations below the threshold limit value and are especially effective in the analyte concentration range of 10-10-10-5 M. High selectivity of the obtained thin-film sensitive elements with respect to mercury cations and the possibility of regeneration of such elements after quantitative determination of mercury cations in aqueous solutions are demonstrated.

Kinetic Monte Carlo modeling of oxide thin film growth.

In spite of the increasing interest in and application of ultrathin film oxides in commercial devices, the understanding of the mechanisms that control the growth of these films at the atomic scale remains limited and scarce. This limited understanding prevents the rational design of novel solutions based on precise control of the structure and properties of ultrathin films. Such a limited understanding stems in no minor part from the fact that most of the available modeling methods are unable to access and robustly sample the nanosecond to second timescales required to simulate both atomic deposition and surface reorganization at ultrathin films. To contribute to this knowledge gap, here we have combined molecular dynamics and adaptive kinetic Monte Carlo simulations to study the deposition and growth of oxide materials over an extended timescale of up to ∼0.5ms. In our pilot studies, we have examined the growth of binary oxide thin films on oxide substrates. We have investigated three scenarios: (i) the lattice parameter of both the substrate and thin film are identical, (ii) the lattice parameter of the thin film is smaller than the substrate, and (iii) the lattice parameter is greater than the substrate. Our calculations allow for the diffusion of ions between deposition events and the identification of growth mechanisms in oxide thin films. We make a detailed comparison with previous calculations. Our results are in good agreement with the available experimental results and demonstrate important limitations in former calculations, which fail to sample phase space correctly at the temperatures of interest (typically 300-1000K) with self-evident limitations for the representative modeling of thin films growth. We believe that the present pilot study and proposed combined methodology open up for extended computational support in the understanding and design of ultrathin film growth conditions tailored to specific applications.