Growth Of Ultrathin Films Research Articles

Controlled growth of MoS2 by atomic layer deposition on patterned gold pads

Approaches to synthesize large-area MoS2 thin films have been extensively investigated in recent years towards system-level micro-/nanoelectronics applications. Methods including chemical vapor deposition (CVD) and atomic layer deposition (ALD) can enable uniform and wafer-scale MoS2 films on insulating substrates, but without selective control on the growth location. Herein, we report a controlled growth of ultra-thin MoS2 films on Au pads patterned on insulating substrates using ALD-based techniques. Clear difference in the incubation period duration of the ALD process among different substrate surfaces has been observed. Selective growth of MoS2 can be achieved on patterned Au on SiO2/Si substrate while the incubation period is shorter on sapphire substrate as compared with that on SiO2/Si. In addition, the influence of deposition temperature on the film growth dynamics on difference surfaces has been studied. Such controlled growth of MoS2 by ALD can be very attractive in future optoelectronics applications and the synthesis on patterned Au is promising in fulfilling its application potentials such as electrocatalyst in hydrogen evolution reaction.

Growth and characterization of nickel oxide ultra-thin films

The oxidation of the Ni(111) surface under ultrahigh-vacuum conditions is studied experimentally with low-energy electron diffraction and high-resolution X-ray photoelectron spectroscopy. Exposure of the clean Ni(111) surface to molecular oxygen at room temperature followed by annealing at 400 K leads to the formation of two different structures (2×2) and (33×33)R30∘, prior to the formation of the NiO(111) monolayer. The O 1s core levels indicate that the obtained oxide is terminated by oxygen atoms while the valence band measurements clearly reveal the band gap of NiO. The energy difference between the Fermi level and the maximum of the valance band is extracted and is found to be 0.47 eV.

Large-area ultrathin Te films with substrate-tunable orientation.

Anisotropy in a crystal structure can lead to large orientation-dependent variations of mechanical, optical, and electronic properties. Material orientation control can thus provide a handle to manipulate properties. Here, a novel sputtering approach for 2D materials enables growth of ultrathin (2.5-10 nm) tellurium films with rational control of the crystalline orientation templated by the substrate. The anisotropic Te 〈0001〉 helical chains align in the plane of the substrate on highly oriented pyrolytic graphite (HOPG) and orthogonally to MgO(100) substrates, as shown by polarized Raman spectroscopy and high-resolution electron microscopy. Furthermore, the films are shown to grow in a textured fashion on HOPG, in contrast with previous reports. These ultrathin Te films cover exceptionally large areas (>1 cm2) and are grown at low temperature (25 °C) affording the ability to accommodate a variety of substrates including flexible electronics. They are robust toward oxidation over a period of days and exhibit the non-centrosymmetric P3121 Te structure. Raman signals are acutely dependent on film thickness, suggesting that optical anisotropy persists and is even enhanced at the ultrathin limit. Hall effect measurements indicate orientation-dependent carrier mobility up to 19 cm2 V-1 s-1. These large-area, ultrathin Te films grown by a truly scalable, physical vapor deposition technique with rational control of orientation/thickness open avenues for controlled orientation-dependent properties in semiconducting thin films for applications in electronic and optoelectronic devices.

A van der Waals epitaxial growth of ultrathin two-dimensional Sn film on graphene covered Cu(111) substrate

Two-dimensional (2D) ultrathin gray tin (α-Sn) has attracted intense research efforts owing to its fascinating physical properties such as high-temperature quantum spin Hall effects, high thermoelectricity, topological superconductivity, and so on. Although monolayer α-Sn was achieved on substrates like Bi2Te3(111), Cu(111), Ag(111), and PbTe(111), the above-mentioned physical properties of α-Sn were lost due to strong Sn-substrate coupling. For promising applications of 2D Sn, it is necessary to decouple the Sn film from the substrate in order to retain the intrinsic properties of the 2D Sn film. Here, we grew ultrathin Sn films (about 6 Ǻ) on monolayer-graphene (MLG) covered Cu(111) by using molecular beam epitaxy. It was found that the van der Waals (vdW) epitaxial growth behavior was enhanced due to the template-effect of MLG, leading to the development of the 2D Sn film with a uniform thickness and continuous morphology. In addition, we also observed that the 2D Sn film on the MLG substrate is long-term stable in air. Our study provides a promising approach toward the controllable fabrication of high-quality 2D Sn films.

Corrosion prevention of commercial alloys by air-water interface grown, edge on oriented, ultrathin squaraine film

Copper is one of the most demanded commercial metal/alloys in world market. The demand for copper in industries such as electrical, electronics, automobile, telecommunications, defence, etc. as well as in daily life has escalated in the recent years due to its versatile physical and chemical properties. However destruction of copper surface by any means, preferably corrosion, can limit its vast application. For protection from corrosion, various techniques are used to coat metal substrates with passivating materials. These techniques are either complex as well as expensive, or provide incomplete protection in acid media. To address these issues, floating film transfer method (FFTM) is utilized in this work for obtaining ultrathin film of squaraine (passivating molecule) as well as their easy and fast transfer over copper substrate. The squaraine film is deposited on copper substrate in layers, viz., 1 to 4 layers. The corrosion behavior is examined in 0.1 M HCl using electrochemical techniques as well as surface characterization techniques, which portray that copper corrosion is hampered in harmony with the layers deposited. Nearly 40% corrosion protection is reached for copper coated with 1 layer of squaraine. However, the protection is amplified up to 98% with 4 layers of squaraine, which clearly substantiates the supremacy of this coating method over reported methods of protection. This technique and the material (squaraine) are both for the first time being used in the field of corrosion protection. The easy growth of ultrathin film at air-water interface as well as its rapid transfer over substrate promotes use of FFTM for efficient corrosion protection on industrial scale.

Advances in E-ALD By Slrr for the Growth of Ultra Thin Films of Metal and Alloys - in Recognition to John Stickney's Contribution to Electrodeposition

The idea realization progress of electrochemical atomic layer deposition by surface limited redox replacement (E-ALD by SLRR) is among the brightest and most significant contributions of Professor John Stickney. The approach proposed by him in early nineties of last century for epitaxial growth of compound semiconductor films was extended in the beginning of 21st century with his active participation and contribution to the deposition of ultra thin coatings of metals and alloys with a variety of applications thus establishing foundation for the development of one of the most useful and versatile thin film growth methods. In the beginning of this presentation an overview of existing approaches for epitaxial metal thin film growth along with E-ALD by SLRR will first be presented. A comparison of the kinetics manipulation approaches will be made with emphasis on the ability of those methods to produce epitaxial, flat and uniform deposits. In the course of this comparison illustrations will be provided with results, illustrating the advantages and limitations of these deposition approaches in the growth of Ag and Cu thin films on Au (111) substrates. To support the discussion, on the approaches employed, results of electrochemical, in-situ scanning tunneling microscopy (STM), X-ray Photoelectron Spectroscopy characterization experiments will be presented and compared. In the second part, the applicability of SLRR and electroless SLRR in the growth of Pt and Pd on Au (111) substrate will be discussed with special emphasis on the contribution and achievements of John Stickney’s research team. Experimentally demonstrated will be the SLRR growth of Pt and Pd layers with controllable thickness taking place in “shuttling” chamber, flow cell, and “one-pot” configuration. Open circuit potentiometry during the replacement reaction will be shown as a way to control the completion of each deposition event. Anodic stripping of the entire multilayer will be used for determining the overall film thickness. Cyclic Voltammetry and STM will be employed to characterize the growth of Pt and Pd layers with up to 10 nm thickness. High-resolution STM results and modelling concepts underlying the foundation of the proposed approaches will shed light on the mechanistic aspects associated with the nature of growth by redox exchange at a monolayer level. In the third component of this presentation an overview of recent work on the design of potent catalysts with application in energy production and storage will be presented. This will include the use of SLRR approach for coating nanoparticles and continuous nanoporous metal layers with ultra thin films of metal / alloy with specific functionality. Successful outcome of this coating renders the final product catalytically active. In this part, a brief description will be provided also on the direct use of SLRR based protocols for the smart design of bimetallic alloy and/or core shell clusters with application in the most commonly used reactions in the field of fuel-cell catalysis. The discussion on the synthetic component of all relevant activities will be concluded with demonstration of results of testing of accordingly developed catalysts. Finally, a glimpse into the future of this rapidly developing field will be also provided.

Atomic Layer Deposition By Surface-Limited Redox Replacement Synthesis of Highly Active and Durable CuxAu(1–x)catalysts for Nitrate Electroreduction

The nitrate pollution from fertilizer and animal waste has become a severe environmental problem in recent years. High concentration of NO3-ions in surface water could impair the health of human beings.1The global trend for environmental compatibility has led to increasing demand for removing nitrates using environmentally friendly approaches that meet energy conservation standards.2Due to the low cost and controllable selectivity, the electro-reduction of NO3-ions promoted by metallic or bimetallic catalysts has been considered the among the most promising means for denitrification.3 In order to enhance activity and durability, and minimize the use of expensive catalytic materials, deposition of ultrathin metal/alloy films with controllable thickness and composition has attracted broad attention in last decades. Electrochemical atomic layer deposition (E-ALD) approaches for the growth of single metal films such as Pt and Pd have been developed by our group based on surface-limited redox replacement (SLRR) in one cell configuration.4,5The application of SLRR for deposition of alloy films has been taken as challenge and thoroughly studied in this report. CuxAu(1–x)bimetallic ultrathin-film catalysts have been synthesized using E-ALD by SLRR of Pb underpotentially deposited layer.6 The composition of synthesized alloy ultra-thin film could be controlled by the ratio of [Cu2+] ions and [AuCl4 –] complex in the deposition solution along with the application of specifically designed routine facilitating the administration of SLRR cycles. The alloy film composition has been determined by X-ray photoelectron spectroscopy and indirectly estimated by anodic stripping voltammetry. The nitrate electroreduction activity and durability of CuxAu(1–x)thin films, Cu thin film, and bulk Cu have been studied by one- and multiple-cycle voltammetry. The synthesized CuxAu(1–x)thin films feature up to two times higher nitrate electroreduction activity in acidic solution compared to bulk and thin-film Cu counterparts. Highest activity has been measured with a Cu0.70Au0.30alloy catalyst. Durability tests have demonstrated that Cu thin films undergo rapid passivation losing 65% of its peak activity for up to 100 cycles, whereas Cu0.70Au0.30catalysts developed in this study lose about 40% of their top performance. The significantly better durability of alloy films can be attributed to effective resistance to poisoning and/or minimized passivation of Cu active centers. It has been also found that both CuxAu(1–x)and pure Cu thin films show best electroreduction activity at lowest pH. References Moorcroft, M. J.; Davis, J.; Compton, R. G. Detection and determination of nitrate and nitrite: a review. Talanta 2001, 54, 785− 803Martínez, J.; Ortiz, A.; Ortiz, I. State-of-the-art and perspectives of the catalytic and electrocatalytic reduction of aqueous nitrates. Appl. Catal., B 2017, 207, 42−59Duca, M.; Koper, M. T. M. Powering denitrification: the perspectives of electrocatalytic nitrate reduction. Energy Environ. Sci. 2012,5,9726−9742.Dimitrov, N. Recent Advances in the Growth of Metals, Alloys, and Multilayers by Surface Limited Redox Replacement (SLRR) Based Approaches. Electrochim. Acta 2016, 209, 599−622.Dimitrov, N.; Achari, I.; Ambrozik, S. Palladium Ultrathin Film Growth by Surface-Limited Redox Replacement of Cu and H UPD Monolayers: Approaches, Pros, Cons, and Comparison. Electrochem. Soc. Interface 2018, 27, 65−69.Xie, Y.; Dimitrov, N. Highly Active and Durable Cu x Au (1–x) Ultrathin-Film Catalysts for Nitrate Electroreduction Synthesized by Surface-Limited Redox Replacement. ACS Omega. 2018, 3 (12), 17676–17686.

Simultaneous achievement of high visible transmission and near-infrared heat shielding in flexible liquid crystal-based smart windows via electrode design

Light and heat management are the two vital requirements for smart window towards ideal solar control. However, as a representative polymer dispersed liquid crystal (PDLC) device, little attention has been paid to heat management. Herein, a TiO2/Ag(Cu)/TiO2 (TCAT) electrode was designed and optimized to achieve the tunability of transmission in visible range and near-infrared heat shielding simultaneously. The continuous growth of ultrathin Ag films with low percolation threshold was achieved on TiO2 underlayer by doping a certain amount of Cu. Then the trade-off of PDLC devices’ optoelectrical properties was realized. The average ON state specular transmittance of TCAT-based PDLC devices reached 81.9%, 84.9%, 81.9% and 79.9% for Ag(Cu) thicknesses of 7 nm, 9 nm, 11 nm and 13 nm, respectively. In addition, TCAT electrodes exhibited satisfactory ratio of light to solar gain by taking visible transmittance and solar heat gain coefficient into comprehensive consideration. Compared with ITO-based PDLC counterpart, a notable temperature-decreases of 4.6 °C was observed at ON state of TCAT-based device, indicating an excellent heat shielding performance. The proposed TCAT electrode design is promising in smart window application especially for building energy conservation in hot areas or the summer season.

Investigation of ultrathin carbon film growth and etching by in-situ X-ray reflectivity

The results of studies of carbon films in the process of their growth and etching in plasma of radio-frequency discharge in real time by in-situ X-ray reflectometry are presented. The film thickness, growth rate, density and surface roughness were calculated from the time dependence of the reflection coefficient of x-rays at a wavelength of 1.54 A. A method for the production of carbon films with a super-smooth surface was proposed.

Influence of different sputtering conditions on superconductivity of ultra-thin epitaxial niobium nitride film

Superconductors are used in military and commercial applications due to the full diamagnetic and zero resistance properties. Hereinto, niobium nitride (NbN) has been extensively studied for its excellent superconducting properties, such as high transition temperature ( T c), large superconducting energy gap, narrow transition width and good stability. Superconducting NbN films are widely used in low temperature superconducting devices, for instance, hot electron bolometer, superconducting quantum interference devices and rapid single flux quantum. Especially, in 2001 Gol’tsman has successfully detected a single photon response of 810 nm near-infrared using an ultra-thin superconducting NbN, which proved that the high-quality growth of ultra-thin NbN film was the key step for high resolution and sensitivity of the superconducting detector. Up to now, atomic layer deposition (ALD), molecular beam epitaxy (MBE), pulsed laser deposition (PLD) and magnetron sputtering have been successfully used to grow NbN thin films. However, T c is decreasing gradually as the reduced of thickness. Therefore, there is still a challenge to grow ultra-thin superconducting NbN film with high performance. In this work, 5 nm thickness of superconducting epitaxial NbN film has been grown on MgO substrate by magnetron sputtering. We systematically study the effect of different sputtering parameters on the superconductivity properties of the NbN film. The experiment results show that the high vacuum, high sputtering temperature and low working pressure are positively correlated with the superconducting transition temperature of NbN film under the premise of suitable argon-nitrogen ratio or magnetron power. The analyses of physical property measurement system (PPMS) indicate that as-grown 5 nm-thick NbN film has high performance with a superconducting transition temperature as high as 12.5 K under the optimum condition. Morphologic characterizations show that the ultra-thin NbN films have high density and smoothness with 0.116 nm of the surface roughness. Microstructural measurements also illustrate the epitaxial growth of NbN on MgO substrate. This work demonstrates the ultra-thin NbN films can be well controllably grown with high quality and offers a platform to fabricate the ultra-thin superconducting devices in the coming future.

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.

Growth of ultrathin Ru oxide films on perovskite and corundum substrates

The ability to control the dispersion of RuO2 monolayers on a range of perovskite- and corundum-structured oxide substrates was investigated using complementary experiment and theory. Controlled deposition under UHV conditions and subsequent in situ characterization by x-ray photoelectron spectroscopy (XPS), ion scattering spectroscopy (ISS), reflection high energy electron diffraction (RHEED) and low energy electron diffraction (LEED) was used to follow the chemical state, dispersion and structure of the RuO2 layers as a function of growth and post-growth processing conditions. The experimental results reveal that monolayer RuO2 on ferroelectric PbxZr1-xTiO3(001) and non-polar SrTiO3(001) α-Al2O3(11¯02) tends to dewet the surface to form three-dimensional clusters, though some kinetic stability of dispersed RuO2 monolayers was achieved on the SrTiO3 surface. Although an RuO2 termination is not favored for SrRuO3(001), deposition onto SrRuO3(001) yields a stable RuO2 surface which ISS suggests is capped by chemisorbed oxygen. The complementary density functional theory modeling and ab initio thermodynamics indicate that Ru oxide monolayers on the perovskite substrates tend to be unstable under the low oxygen pressure experimental conditions; however, either lower temperatures or higher oxygen pressures can lead to a kinetic and even thermodynamic stability. The theoretical findings also provide guidance on how the instability of dispersed RuO2 monolayers can be overcome by inserting buffer layers and increasing the partial pressure of molecules that selectively, strongly adsorb at coordinatively unsaturated Ru sites on the oxide surface.

Growth of Ultrathin Al2O3 Films on n-InP Substrates as Insulating Layers by RF Magnetron Sputtering and Study on the Optical and Dielectric Properties

Here, we report an explorative study of an attempt to fabricate ultrathin aluminum oxide films on n-InP substrates by radio-frequency (RF) magnetron sputtering as a candidate for insulating layers in semiconductor lasers for optical communication. Film thickness and morphology were monitored to study the film growth and to explore the minimum thickness of a continuous film that RF magnetron sputtering could achieve. Originating from the weak wettability between the n-InP substrate and the Al2O3 film, Al2O3 films firstly grew in an island pattern which then turned into a layer-by-layer pattern when those islands became connected and continuous. Uniform and compact Al2O3 films were obtained when the film thickness reached 40 nm. The average transmittance, optical band gap, and optical absorption coefficient at a wavelength of 1550 nm of this Al2O3 film were about 80%, 3.72 eV, and 3.0 × 104 cm−1, respectively. At a frequency of 1 MHz, the permittivity, dielectric loss, and electrical resistivity were 8.96, 0.31, and 5 × 1010 Ω·cm, respectively. This work provides valuable references for the application of Al2O3 ultrathin films as insulating layers in micro-and opto-electronics.

The Effect of the Superlattice on the Magnetic and Transport Properties of Epitaxial Magnetite Thin Films

The surface structural, transport, and magnetic properties of superlattice films were examined, focusing on the effects of the superlattice periodicity of (Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> /MgO) layers. The superlattice structures of (Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> /MgO)n ultrathin films were fabricated using molecular-beam epitaxy on MgO(001) substrates, where n is the number of layers. It was observed that these oxide films have a single crystalline quality with the same orientation as the substrates for Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> as thin as approximately three unit cells. The transport and magnetic results showed that the conductivity and magnetization of the superlattice increase as the number of (Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> /MgO) layers is increased. Furthermore, a sharp Verwey transition was found in the (Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> /MgO)n superlattices with thickness layer smaller than seen in an earlier study for a single or superlattice film. These results give the indication that Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> superlattice ultrathin films have better properties than single layers of the same thickness. This artificial growth of superlattice ultrathin films will provide a useful method to realize novel properties for other complex oxide systems.

Effect of Si(1 1 1)√3 × √3-Bi template structure on growth mode and electrical conductance of Au overlayers

Growth of ultra-thin Au films on Si(1 1 1)√3 × √3-Bi surfaces was studied using low energy electron diffraction, scanning tunneling microscopy and in situ measurements of surface conductivity. The Si(1 1 1)√3 × √3-Bi surfaces were represented either by the Si(1 1 1)-α-√3 × √3-Bi phase with 0.33 ML (monolayers) of Bi adatoms or by the Si(1 1 1)-β-√3 × √3-Bi phase with 1.0 ML of Bi atoms arranged into trimers. Structure of the template surface was found to affect greatly the growth mode and conductance of the overgrown Au films in the thickness range from 0.1 to 3.0 ML of Au. Growth of Au films on Si(1 1 1)-β-√3 × √3-Bi follows the Vollmer-Webber mode, while in case of Au films on Si(1 1 1)-α-√3 × √3-Bi the complicated multi-stage growth occurs where formation of Stranski-Krastanov wetting layer is successively followed by island growth and vice versa. Surface conductivity of the samples was found to be dictated by Au film morphology. In the Au/Si(1 1 1)-β-√3 × √3-Bi sample, surface conductivity remains essentially unchanged due to lacking of electrical connections between Au islands. In the Au/Si(1 1 1)-α-√3 × √3-Bi sample, surface conductivity grows with Au thickness starting from overcoming of percolation threshold at ∼0.6 ML of Au.

Editors' Choice—Review—Cobalt Thin Films: Trends in Processing Technologies and Emerging Applications

Cobalt metallic films are the subject of an ever-expanding academic and industrial interest for incorporation into a multitude of new technological applications. This report reviews the state-of-the art chemistry and deposition techniques for cobalt thin films, highlighting innovations in cobalt metal-organic chemical vapor deposition (MOCVD), plasma and thermal atomic layer deposition (ALD), as well as pulsed MOCVD technologies, and focusing on cobalt source precursors, thin and ultrathin film growth processes, and the resulting effects on film composition, resistivity and other pertinent properties.

On the Structure of Ultrathin FeO Films on Ag(111).

Ultrathin transition metal oxide films exhibit unique physical and chemical properties not observed for the corresponding bulk oxides. These properties, originating mainly from the limited thickness and the interaction with the support, make those films similar to other supported 2D materials with bulk counterparts, such as transition metal dichalcogenides. Ultrathin iron oxide (FeO) films, for example, were shown to exhibit unique electronic, catalytic and magnetic properties that depend on the type of the used support. Ag(111) has always been considered a promising substrate for FeO growth, as it has the same surface symmetry, only ~5% lattice mismatch, is considered to be weakly-interacting and relatively resistant to oxidation. The reports on the growth and structure of ultrathin FeO films on Ag(111) are scarce and often contradictory to each other. We attempted to shed more light on this system by growing the films using different preparation procedures and studying their structure using scanning tunneling microscopy (STM), low energy electron diffraction (LEED) and X-ray photoelectron spectroscopy (XPS). We observed the formation of a previously unreported Moiré superstructure with 45 Å periodicity, as well as other reconstructed and reconstruction-free surface species. The experimental results obtained by us and other groups indicate that the structure of FeO films on this particular support critically depends on the films’ preparation conditions. We also performed density functional theory (DFT) calculations on the structure and properties of a conceptual reconstruction-free FeO film on Ag(111). The results indicate that such a film, if successfully grown, should exhibit tunable thickness-dependent properties, being substrate-influenced in the monolayer regime and free-standing-FeO-like when in the bilayer form.

Tuning of Topological Dirac States via Modification of van der Waals Gap in Strained Ultrathin Bi2Se3 Films

Robust massless Dirac states with helical spin textures were realized at the boundaries of topological insulators such as van der Waals (vdW) layered Bi2Se3 family compounds. Topological properties of massless Dirac states can be controlled by varying the film thickness, external stimuli, or environmental factors. Here, we report single-crystal-quality growth of ultrathin Bi2Se3 films on flexible polyimide sheets and manipulation of the Dirac states by varying the vdW gap. X-ray diffraction unambiguously demonstrates that under uniaxial bending stress the vdW gap substantially changes with interatomic-layer distances unaltered. Terahertz and photoelectron spectroscopy indicate tuning of the number of quantum conducting channels and of work function, by the stress, respectively. Surprisingly, under compressive strain, transport measurements reveal dimensional crossover and suppressed weak antilocalization. First-principles calculations support the observation. Our findings suggest that variation of vdW gap...

Unraveling the Atomic Structure of Fe Intercalated under Graphene on Ir(111): A Multitechnique Approach

The interaction of Fe deposited on graphene grown on Ir(111) was studied in detail to better understand the growth, intercalation, and oxidation of Fe ultrathin films on and under graphene. The stu...

Evolution of Optical Spectra at the Initial Stages of Fe Growth on Si (001)

Optical reflection spectra in the photon energy range of 1.5-6 eV have been studied after the growth of ultrathin iron films on silicon (001) in the 0-1.2 nm thickness range. It has been noted that the reflection coefficient values vary nonmonotonically in thickness near the limiting energy of 1.5 eV and 6 eV. Moreover, they are abnormally large one at 1.5 eV and small one at 6 eV. It is shown that these phenomena can not be explained by simple models of the formation of a metallic, silicide film or amorphous silicon layer at the interface and correspond to structural-phase transformations in the film and in the substrate interface region.