Properties Of Ultrathin Metal Films Research Articles

Optimized optical/electrical/mechanical properties of ultrathin metal films for flexible transparent conductor applications: review [Invited

Transparent conductors are essential for high-performance optoelectronic devices. Recently, ultrathin metal films have received great attention as emerging transparent conductors to replace status quo indium tin oxide (ITO) due to their excellent optoelectrical properties with mechanical flexibility. Understanding an ultrathin metal film's optoelectrical properties with respect to thickness scaling is the prerequisite for the design of high-performance metal film-based transparent conductors. This review paper aims to focus on the evolution of ultrathin metal film’s optical properties as thickness scales. Such evolution of optical properties will be associated with electrical properties by exploring various resistivity scattering models aiming to better understand a film’s intrinsic physical property at an extremely thin scale and provide a guideline for enhancing the film’s intrinsic optoelectrical properties for transparent conductor application. Next, optical design considerations to enhance transparency at visible and near-infrared range are discussed including recent reinforcement learning methods as a potential strategy for transparent conductor design. Then, mechanical flexibility of various ITO-replacement electrodes is discussed as well as the mechanism for the metal film-based transparent conductor's excellent endurance against mechanical stress. Lastly, some of the unique benefits of using a metal film transparent conductor for optoelectronic device application are discussed.

The Influence of the Percolation Transition on the Electric Conductive and Optical Properties of Ultrathin Metallic Films

The Influence of the Percolation Transition on the Electric Conductive and Optical Properties of Ultrathin Metallic Films

An Oxide‐Dispersed Preparation Strategy for Silver Ultrathin Films with Low Percolation Threshold Thickness, Subnanometer Smoothness, and Prominent Durability

AbstractIn this work, an oxide‐dispersed strategy is developed for Ag ultrathin films with low percolation thickness, ultrasmoothness, and excellent stability, which are all critical for their practical applications. Through simply sputtering of CdO‐containing Ag target, nearly amorphous Ag ultrathin films with CdO dispersion are prepared on glass or polyimide substrates under room temperature. The CdO‐dispersed Ag thin films exhibit a low percolation threshold of 5 nm with a transmittance of 89% at 550 nm and a sheet resistance of 32.9 Ω sq−1. The Ag–CdO films also feature an ultrasmooth growth with roughness remaining at 0.2 nm for further deposition. Furthermore, the CdO‐dispersed Ag thin films remain stable under 250 °C annealing in air, and highly accelerated stress test at 121 °C, 97% relative humidity (RH), and 0.1 MPa for 36 h. Flexible heaters with indium‐tin‐oxide (ITO)/CdO‐dispersed Ag/ITO/polyimide structure can reach a working temperature of 450 °C within a few seconds. In addition, the oxide‐dispersed strategy for preparing Ag ultrathin films with low percolation, subnanometer smoothness, and prominent durability is further verified using SnO2 and ZnO. The present study provides new insights for the growth and properties of ultrathin metallic films, and pushes forward the practical applications of ultrathin metallic films in optoelectronic devices.

Dielectric properties of ultrathin metal films around the percolation threshold

We report on optical reflection measurements of thin Au films at and around the percolation threshold (film thickness 3 to 10 nm) in an extremely broad spectral range from 500 to 35000 cm-1 (0.3 - 20 *10-6 m). Combining spectroscopic ellipsometry and Fourier-transform infrared spectroscopy, the dielectric properties of the films can be described over the whole frequency range by Kramers-Kronig consistent effective dielectric functions. The optical conductivity of the films is dominated by two contributions: by a Drude-component starting at the percolation threshold in the low frequency range and a plasmon in the near-infrared region, which shifts down in frequency with increasing film thickness. The interplay of both components leads to a dielectric anomaly in the infrared region with a maximum of the dielectric constant at the insulator-to-metal transition.

Magnetic field induced superconductor-insulator transitions for ultra-thin Bi films on the different underlayers

This work shows the experimental results of the superconductor-insulator (S-I) transition for ultra-thin Bi films in magnetic fields. The quench-condensed (q-c) Bi film onto insulating underlayers have been interpreted to be homogeneous. In contrast, the Bi film without underlayers has been regarded as a granular film. The electrical transport properties of ultra-thin metal films near the S-I transition depend on the structure of the film. In order to confirm the effect of the underlayer to the homogeneity of the superconducting films, we investigate the characteristics of S-I transitions of q-c nominally homogeneous Bi films on underlayers of two insulating materials, SiO, and Sb. Under almost the same deposition condition except for the material of underlayer, we prepared the Bi films by repeating the additional deposition and performed in-situ electrical measurement. It is found that the transport properties near the S-I transitions show the remarkable difference between two films on different underlayers. As for Bi films on SiO, it turned out that the temperature dependence of resistance per square Rsq(T) of the field-tuned transition and the thickness-tuned transition shows similar behavior; it was a thermally activated form. On the other hand, the Rsq(T) of Bi films on Sb for thickness-tuned S-I transition showed logarithmic temperature dependence, but that for field-tuned S-I transition showed a thermally activated form.

Manifestation of quantum confinement in transport properties of ultrathin metallic films

The influence of quantum size effect on the transport properties of ultrathin Cu and Fe films in the thickness range between 1 and 32 monolayers is studied by solving the linearized Boltzmann equation in the relaxation-time approximation using ab initio calculations within the framework of spin density functional theory. A strong manifestation of quantum confinement is found in the density of states. However, the plasma frequency shows a smooth or oscillating behavior for different film orientations. The results are in qualitative agreement to infrared-absorption experiments at Cu(111), Fe(001), and Fe(111) films.

Electrical transport properties of ultrathin metallic films

Ultrathin metal films of Al, Au, Cr, Cu, Ru, Ta, Co90Fe10, Ni81Fe19, Ir20Mn80 with or without capping layers have been prepared using ultrahigh vacuum sputtering and their morphological and electrical properties have been studied using atomic force microscopy and four-point probe measurement, respectively. It was found that the capping layer could either enhance or reduce the conductivity, depending on the capping layer thickness and surface roughness of the underlying film. The study of magnetoresistance (MR) dependence on capping and free layer thickness is also carried out on a pseudo spin valve.

Charge-carrier transport properties of ultrathin Pb films

The charge-carrier transport properties of ultrathin metallic films are analysed with ab-initio methods using the density functional theory (DFT) on free-standing single crystalline slabs in the thickness range between 1 and 8 monolayers and compared with experiments for Pb films on Si(111). A strong interplay between bandstructure, quantised in the direction normal to the ultrathin film, charge-carrier scattering mechanisms and magnetoconduction was found. Based on the bandstructure obtained from the DFT, we used standard Boltzmann transport theory in two dimensions to obtain results for the electronic transport properties of 2 to 8 monolayers thick Pb(111) slabs with and without magnetic field. Comparison of calculations and experiment for the thickness dependence of the dc conductivity shows that the dominant scattering mechanism of electrons is diffuse elastic interface scattering for which the assumption of identical scattering times for all subbands and directions, used in this paper, is a good approximation. Within this model we can explain the thickness dependences of the electric conductivity and of the Hall coefficient as well as the anomalous behaviour of the first Pb layer.

Tuning the chemistry of metal surfaces: II. Acetylene cyclotrimerization on ultrathin Pd films on Ta(1 1 0)

Chemisorption of acetylene (C 2H 2) and benzene (C 6H 6) on ultrathin Pd films (monolayer, bilayer, trilayer, etc.) deposited on Ta(1 1 0) is weaker than on bulk-terminated Pd surfaces, but it is not as strongly perturbed as was seen for H 2, CO, NO, and C 2H 4. Thus, the well-known alterations of the chemical properties of ultrathin metal films deposited on refractory metal substrates can be modified by strongly adsorbed species. C 2H 2 is predominately reversibly adsorbed on the Pd monolayer ( θ Pd=1) at 91 K, with thermal desorption peaks at 180 and 265 K. C 6H 6 is formed via cyclotrimerization of C 2H 2 and desorbs in a single peak at 407 K, but the yield is small (1% of an adsorbed benzene monolayer on that surface). Bilayer and trilayer Pd films ( θ Pd=2–3) have similar C 2H 2 chemistry to that from thicker films. On a thick film ( θ Pd=5) at 175 K, C 2H 2 adsorption leads to C 2H 2 desorption in a very broad peak near 330 K, and to benzene desorption at 250 and 500 K, with a yield of twice that on the Pd monolayer. Benzene chemisorption on the Pd monolayer is only slightly weaker than that on a thick Pd film ( θ Pd=5) or on Pd(1 1 1), (a reduction of 6 kcal/mol) but is completely reversibly adsorbed.

The adsorption of CO on [formula omitted]: a joint experimental and theoretical study

We present a detailed experimental study of the geometric, electronic and adsorptive properties of ultrathin Ni films on a W(110) surface. In parallel we have performed calculations on clusters representing the bimetallic system in order to improve the understanding of the relationship between the chemical properties of ultrathin metal films and their geometric as well as electronic structure. Our results indicate that the modification of the electronic structure induced by the intermetallic bond is the most important factor in determining the unique properties of metallic overlayers.

The nature of metal-metal bonding at bimetallic interfaces

The nature of the electronic and physical properties of ultrathin metal films on several transition metal substrates have been studied using X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD). The results indicate that the admetal monolayers have electron densities that vary significantly from those of the corresponding pure metals. Correlations are found among the electronic perturbations of the monolayer films, the admetal-substrate cohesive bond strength and the chemisorption properties of the monolayer films toward CO. A simple model is described to explain the direction and magnitude of the electronic perturbations.

Chemical and electronic properties of ultrathin metal films: The Pd/Re(0001) and Pd/Ru(0001) systems.

The nature of the electronic and chemical properties of ultrathin Pd films on Re(0001) and Ru(0001) has been studied using x-ray photoelectron spectroscopy (XPS), temperature programmed desorption (TPD), and CO chemisorption. The results indicate that the Pd(3${\mathit{d}}_{5/2}$) binding energy for a monolayer (ML) of Pd on Re(0001) and Ru(0001) is perturbed by +0.60 and +0.30 eV, respectively, from that of the surface atoms of Pd(100). These electronic perturbations induce large changes in the chemical properties of the Pd films. TPD results indicate that the desorption temperature of CO from 1 ML of Pd on Re(0001) and Ru(0001) is \ensuremath{\sim}120 K lower than the corresponding desorption temperature from Pd(100). The XPS and CO-TPD data indicate that Pd transfers charge to the Re and Ru substrates, becoming electron deficient and less efficient at \ensuremath{\pi} backdonation toward CO. By comparison of these results with those reported previously for Pd, Ni, and Cu adlayers, a correlation is observed among the electronic perturbations of the adlayers, the cohesive metal-substrate bond strength, the ability of the film to chemisorb CO, and the CO-induced shift in the metal core-level binding energy. In general, the results indicate that the formation of a metal-metal bond at a surface leads to a gain of electron density by the element initially having the greater fraction of empty states in its valence band. This behavior is completely contrary to that seen in bulk alloys, likely a consequence of the anisotropic character of a surface that changes the relative electronegativities of the metal atoms. On the basis of these results, a qualitative scale of surface electronegativities is developed, showing trends that are very different from those found in three-dimensional bulk alloys.

Ultrathin Metal Films: From One to Three Dimensions Plenary Lecture

AbstractThe growth, structure, thermodynamic and electronic properties of ultrathin metal films is discussed, starting from the submonolayer range up to thicknesses at which bulk properties are reached. The following topics are selected from the wide field of phenomena: one‐dimensional crystals, shape and stability of twodimensional crystals, film‐substrate and lateral interactions, electronic structure and bonding, structural transitions and quantum size effects. The substrates used are mainly W(110), Mo(110) and Si(111) surfaces, the film materials several fcc metals.

Properties of ultrathin metallic films on Si(111) determined by high-resolution electron energy loss spectroscopy

UV photoemission, low energy electron diffraction and high-resolution electron energy loss spectroscopy (EELS) have been used in situ to study the nature of 1–25 Å of Au and Pd deposited on Si(111) at temperatures of 20 and 300 K. New information is obtained regarding the formation, nature and microstructure of these metallic layers. We describe and apply a theoretical analysis of EELS which allows the determination of DC transport properties in these ultrathin layers. All metallic layers exhibit significantly higher resistivities than expected in comparison to bulk metal silicides or metallic glass phases, and can be attributed to diffuse scattering of conduction electrons at the interface. The interaction and behavior of atomic hydrogen with these films as detected by EELS is also described and found to convey additional information about their microstructure.

Properties of ultrathin metallic films on Si(111) determined by high-resolution electron energy loss spectroscopy

Properties of ultrathin metallic films on Si(111) determined by high-resolution electron energy loss spectroscopy

Growth, environmental, and electrical properties of ultrathin metal films

The conduction mechanisms of ultrathin (10−100 Å) metal films are investigated. The electrical conductivity and activation energy of discontinuous Au films grown in UHV are correlated with island size and separation and average film thickness. The relationship between film conductivity and deposition rate and substrate temperature is presented. Film temperature data indicate the validity of the proposed model. Some anomalous conductivity effects are reported for films deposited below the UHV range. The results are explained in terms of substrate and film surface contamination. Data are presented indicating the sensitivity of the conductivity of discontinuous films to ambient gas concentration. Reversible and irreversible situations are discussed.