Monolayer Thickness Research Articles

Structure and electronic structure of van der Waals interfaces at a Au(1 1 1) surface covered with a well-ordered molecular layer of n-alkanes

The structure and electronic structure of a system comprising molecular layers of n-alkane adsorbed on Au(111) were investigated to understand the origin of the interfacial electric dipole layer. These alkane molecules are inert and thus van der Waals (vdW) interaction is the primary adsorptive force. Although these vdW interfaces are regarded as ideal physical adsorption systems unlikely to chemically interact with the metal surface, a large interfacial electric dipole layer over 1 eV is formed. It is important to elucidate the mechanism underlying the generation of such large dipole layers to understand the electrode interface in organic semiconductor devices. Thus, this paper elucidates the details of the vdW interface, in particular between tetratetracontane (TTC) and Au(111). First, the mechanism of growth of the TTC layer up to monolayer thickness was revealed using a low-energy electron-diffraction method. Second, the interface states were newly formed, as determined using angle-resolved photoelectron spectroscopy, X-ray photoemission spectroscopy, and near-edge absorption fine-structure spectroscopy. Our findings indicate, unexpectedly, that a relatively strong inter-orbital interaction governs this vdW interface, similar to a weak chemisorption system rather than a physisorption system.

Study of Siloxane Adsorption on Hard Disk Media Surface.

Siloxane adsorption on disk surfaces is investigated as a function of lubricant film thickness and carbon film composition through several analytical technologies including FTIR, XPS, TOF-SIMS, GC-MS, and Candela OSA. Disk parameters that govern the propensity for siloxane adsorption are identified. Lubricant film coverage, taken as the fractional monolayer thickness, is found to be the most significant determinant. Siloxane adsorption decreases with increasing lubricant film coverage. The nitrogen content of the underlying carbon film is also found to be a determinant for submonolayer lubricant films: On the one side, siloxane adsorption on carbon films increases with increasing atom % nitrogen in carbon films. On the other side, the lubricant monolayer film thickness increases with increasing atom % N in carbon films, i.e., lubricant film coverage decreases with increasing atom % N. Thus, increasing atom % N in carbon films leads to increasing siloxane adsorption.

Perfect Absorption by an Atomically Thin Crystal

Optical absorption is one of fundamental light-matter interactions. In most materials, optical absorption is a weak perturbation to the light. In this regime, absorption and emission are irreversible, incoherent processes due to strong damping. Excitons in monolayer transition metal dichalcogenides, however, interact strongly with light, leading to optical absorption in the non-perturbative regime where coherent re-emission of the light has to be considered. Between the incoherent and coherent limits, we show that a robust critical coupling condition exists, leading to perfect optical absorption. Up to 99.6% absorption is measured in a sub-nanometer thick MoSe2 monolayer placed in front of a mirror. The perfect absorption is controlled by tuning the exciton-phonon, exciton-exciton, and exciton-photon interactions by temperature, pulsed laser excitation, and a movable mirror, respectively. Our work suggests unprecedented opportunities for engineering exciton-light interactions using two-dimensional atomically thin crystals, enabling novel photonic applications including ultrafast light modulators and sensitive optical sensing.

Biodiesel flames as a unique pyrolyzing carbon source for the synthesis of hydrophobic carbon films

We report the rapid single-step flame synthesis of hydrophobic carbon layers (C-layers) on the surface of stainless-steel (SS) substrates using vaporized biodiesel as the fuel. A co-flow canola methyl ester/air diffusion flame is used to generate a hydrophobic monolayer on the surface of the metal substrate upon its insertion into the reaction zone. Carbon deposition on the surface of the SS substrates varies by changing the SS disk’s position in the post-flame, and by varying its exposure time. The thickness and mass of the flame-formed monolayer varied depending on the substrate’s insertion point into the flame. However, the variation of mass did not significantly impact the C-layer’s uniformity or hydrophobicity. We hypothesize that a small “inner-cone” of the biodiesel flame along with a high soot propensity can result in an ideal medium to form uniform hydrophobic C-layers of unique hierarchical surface structure. This is supported by introducing SS substrates in methane/air flames formed using the same co-flow burner. The hydrophobic property of the carbon deposits was quantified by measuring the contact angle of water droplets placed on the film’s surface. A water droplet drop test was conducted on the flame-formed hydrophobic layers to study their wettability property.

Surface-enhanced ultrafast two-dimensional vibrational spectroscopy with engineered plasmonic nano-antennas.

Development of noble metal nanostructure substrates that provide strong near-field enhancements enables applications of linear and nonlinear infrared (IR) spectroscopies to study minute sample quantities, such as nanometer thick films and molecular monolayers. Large near-field enhancements of the electric fields used for spectroscopic interrogation of molecules at the nanostructure surface result in enhancement of the spectroscopic signatures. This enhancement scales with the nonlinear order of the method, providing particularly large signal gains for third- and fifth-order IR methods, reaching 106 and 108 raw enhancement factors, not adjusted to the amount of interrogated sample. In this perspective, we overview the advances in the development of nano-arrays of antenna-like nanostructures for mid-IR measurements and illustrate their use in linear and especially nonlinear two-dimensional IR approaches. We discuss how studies of the interaction mechanisms between light, plasmonic antennas, and molecular excitations benefit from the nonlinear two-dimensional time-resolved methods, which involve high-order scaling of the signal with the excitation field, high sensitivity to signal localization, and coherence of the excitation over a broad bandwidth. On the other hand, we demonstrate how studies of molecular structure and ultrafast dynamics by these advanced spectroscopic methods benefit from surface enhancement of signals by plasmonic antennas.

Distinct magneto-Raman signatures of spin-flip phase transitions in CrI3

The discovery of 2-dimensional (2D) materials, such as CrI3, that retain magnetic ordering at monolayer thickness has resulted in a surge of both pure and applied research in 2D magnetism. Here, we report a magneto-Raman spectroscopy study on multilayered CrI3, focusing on two additional features in the spectra that appear below the magnetic ordering temperature and were previously assigned to high frequency magnons. Instead, we conclude these modes are actually zone-folded phonons. We observe a striking evolution of the Raman spectra with increasing magnetic field applied perpendicular to the atomic layers in which clear, sudden changes in intensities of the modes are attributed to the interlayer ordering changing from antiferromagnetic to ferromagnetic at a critical magnetic field. Our work highlights the sensitivity of the Raman modes to weak interlayer spin ordering in CrI3.

Vapor Deposition of Magnetic Van der Waals NiI2 Crystals.

The recent discovery of van der Waals magnetic materials has attracted great attention in materials science and spintronics. The preparation of ultrathin magnetic layers down to atomic thickness is challenging and is mostly by mechanical exfoliation. Here, we report vapor deposition of magnetic van der Waals NiI2 crystals. Two-dimensional (2D) NiI2 flakes are grown on SiO2/Si substrates with a thickness of 5-40 nm and on hexagonal boron nitride (h-BN) down to monolayer thickness. Temperature-dependent Raman spectroscopy reveals robust magnetic phase transitions in the as-grown 2D NiI2 crystals down to trilayer. Electrical measurements show a semiconducting transport behavior with a high on/off ratio of 106 for the NiI2 flakes. Lastly, density functional theory calculation shows an intralayer ferromagnetic and interlayer antiferromagnetic ordering in 2D NiI2. This work provides a feasible approach to epitaxy 2D magnetic transition metal halides and also offers atomically thin materials for spintronic devices.

Dielectric, piezoelectric and dc bias characteristics of Bi-doped PZT multilayer ceramic actuator

In this study, the microstructure, dielectric, piezoelectric and dc bias properties of the Bi-doped PZT (PZTB) multilayer piezoelectric actuators, have been discussed. The thickness of the dielectric monolayer was about 70 μm, and a relatively uniform distribution of the grain size was developed. The application of a large dc bias field resulted in a decrease in the dielectric constant and dielectric loss of the multilayer piezoelectric ceramics. The shift of the Curie temperature (Tc) towards a higher value and stabilized temperature-dependent dielectric properties were observed under the dc bias field. The increase in parameter γ suggested the enhancement of the degree of diffuseness of the phase transition. The large displacement reached 0.11% of the thickness of the multilayer piezoelectric ceramic under a low driving voltage of 150 V, indicating the potential of commercial applications of the studied multilayer piezoelectric actuators. This study provides specific experimental data to evaluate the practical application of the Bi-doped PZT multilayer piezoelectric actuators.

Synthesis and Dual-Mode Electrochromism of Anisotropic Monoclinic Nb12O29 Colloidal Nanoplatelets.

Transition metal oxide nanocrystals with dual-mode electrochromism hold promise for smart windows enabling spectrally selective solar modulation. We have developed the colloidal synthesis of anisotropic monoclinic Nb12O29 nanoplatelets (NPLs) to investigate the dual-mode electrochromism of niobium oxide nanocrystals. The precursor for synthesizing NPLs was prepared by mixing NbCl5 and oleic acid to form a complex that was subsequently heated to form an oxide-like structure capped by oleic acid, denoted as niobium oxo cluster. By initiating the synthesis using niobium oxo clusters, preferred growth of NPLs over other polymorphs was observed. The structure of the synthesized NPLs was examined by X-ray diffraction in conjunction with simulations, revealing that the NPLs are monolayer monoclinic Nb12O29, thin in the [100] direction and extended along the b and c directions. Besides having monolayer thickness, NPLs show decreased intensity of Raman signal from Nb-O bonds with higher bond order when compared to bulk monoclinic Nb12O29, as interpreted by calculations. Progressive electrochemical reduction of NPL films led to absorbance in the near-infrared region (stage 1) followed by absorbance in both the visible and near-infrared regions (stage 2), thus exhibiting dual-mode electrochromism. The mechanisms underlying these two processes were distinguished electrochemically by cyclic voltammetry to determine the extent to which ion intercalation limits the kinetics, and by verifying the presence of localized electrons following ion intercalation using X-ray photoelectron spectroscopy. Both results support that the near-infrared absorption results from capacitive charging, and the onset of visible absorption in the second stage is caused by ion intercalation.

Growth of van der Waals magnetic semiconductor materials

Magnetic order in two-dimensional semiconducting layered materials is extremely rare. This has recently triggered a great interest in a variety of layered magnetic compounds, determining whether their magnetic properties can be retained down to monolayer thickness. The most promising systems are the binary MX3 and ternary MYZ3 compounds, where M is a metal, X is a halogen element, Y = Si, Ge or P, and Z is a chalcogen. The transition metal incorporates the magnetism, by a partially filled d-shell, thus enabling ferroic responses for enhanced functionalities. In most of these systems, the phase diagrams are not known and the crystal growth process is made challenging by the high fugacity of some elements and the high reaction temperature needed. We review the recent progress in crystal growth of various compounds of these families, namely CrI3, CrCl3, CrSiTe3, CrGeTe3, MnPS3 and MnPSe3, for which we used different growth techniques, such as melt growth, physical vapor transport and chemical vapor transport. The growth parameters are summarized and discussed. The characterization of the materials is focusing on the determination of the crystal structure, chemical composition and magnetic properties. In some cases, exfoliated flakes of these crystals have been successfully used for nano-device fabrication.

Effect of Triton X-100 surfactant on the interfacial activity of ionic surfactants SDS, CTAB and SDBS at the air/water interface: A study using molecular dynamic simulations

Molecular dynamics simulations were carried out to investigate the effect of TX-100 surfactant on the interfacial activity of ionic surfactants SDS, CTAB and SDBS at the air/water interface. Here, interfacial properties, molecular orientation of hydrocarbon chains of ionic surfactant and radial distribution functions between hydrophilic headgroups and water molecules at the air/water interface was evaluated by means of MD simulations. Particularly, TX-100 surfactant produces a reduction of the interfacial film thickness obtained using the water layer and increases the hydrophobic film thickness of the systems. In this investigation, we found that TX-100 surfactant present in mixed monolayers and distributed randomly on the water surface can modify the hydrophilic-lipophilic balance of the monolayer increasing the hydrophobic film thickness and reducing the adsorption of ionic surfactants at the air/water interface. Furthermore, the molecular array of these mixed monolayers obtained by the TX-100 surfactant improve the elasticity of the liquid film, which avoid the rupture of foams reducing the liquid drainage velocity of these systems. Finally, the results suggest that hydrophobic film thickness of the mixed monolayers produce by the TX-100 surfactant can reduce the diffusion of gas molecules from air phase toward the water phase, which increase the stability of foams.

Understanding Chemistry of Two-Dimensional Transition Metal Carbides and Carbonitrides (MXenes) with Gas Analysis.

MXenes, a large family of two-dimensional materials that are intensely investigated for a broad range of applications, are unstable in water, spontaneously forming TiO2. Several hypotheses have been proposed recently to explain the transformations of MXenes in aqueous environments based on characterization of solid products and measurements of solution pH. However, no studies of the gaseous products of these reactions have been reported. In this work, we demonstrate the use of Raman spectroscopy and gas chromatography techniques to study the gaseous reaction products of Ti2C, Ti3C2, Ti3CN, and Nb2C MXenes in aqueous environments. Based on the analysis of gases, the reactivities of MXenes with different monolayer thickness and chemical composition have been analyzed. We demonstrate the analysis of gases produced during MXene transformations as a powerful technique that can be used for better understanding of their nontrivial chemistry.

Increasing the Diversity and Understanding of Semiconductor Nanoplatelets by Colloidal Atomic Layer Deposition

Nanoplatelets (NPLs) are a remarkable class of quantum confined materials with size‐dependent optical properties, which are determined by the defined thickness of the crystalline platelets. To increase the variety of species, the colloidal atomic layer deposition method is used for the preparation of increasingly thicker CdSe NPLs. By growing further crystalline layers onto the surfaces of 4 and 5 monolayers (MLs) thick NPLs, species from 6 to 13 MLs are achieved. While increasing the thickness, the heavy‐hole absorption peak shifts from 513 to 652 nm, leading to a variety of NPLs for applications and further investigations. The thickness and number of MLs of the platelet species are determined by high‐resolution transmission electron microscopy (HRTEM) measurements, allowing the interpretation of several contradictions present in the NPL literature. In recent years, different assumptions are published, leading to a lack of clarity in the fundamentals of this field. Regarding the ongoing scientific interest in NPLs, there is a certain need for clarification, which is provided in this study.

Two-Dimensional Functionalized Ultrathin Semi-Insulating CaF2 Layer on the Si(100) Surface at a Low Temperature for Molecular Electronic Decoupling.

The ability to precisely control the electronic coupling/decoupling of adsorbates from surfaces is an essential goal. It is important for fundamental studies not only in surface science but also in several applied domains including, for example, miniaturized molecular electronic or for the development of various devices such as nanoscale biosensors or photovoltaic cells. Here, we provide atomic-scale experimental and theoretical investigations of a semi-insulating layer grown on a silicon surface via its epitaxy with CaF2. We show that, following the formation of a wetting layer, the ensuing organized unit cells are coupled to additional physisorbed CaF2 molecules, periodically located in their surroundings. This configuration shapes the formation of ribbons of stripes that functionalize the semiconductor surface. The obtained assembly, having a monolayer thickness, reveals a surface gap energy of ∼3.2 eV. The adsorption of iron tetraphenylporphyrin molecules on the ribbons of stripes is used to estimate the electronic insulating properties of this structure via differential conductance measurements. Density functional theory (DFT) including several levels of complexity (annealing, DFT + U, and nonlocal van der Waals functionals) is employed to reproduce our experimental observations. Our findings offer a unique and robust template that brings an alternative solution to electronic semi-insulating layers on metal surfaces such as NaCl. Hence, CaF2/Si(100) ribbon of stripe structures, whose lengths can reach more than 100 nm, can be used as a versatile surface platform for various atomic-scale studies of molecular devices.

RELATION BETWEEN HARDNESS OF (Ti, Al)N BASED MULTILAYERED COATINGS AND PERIODS OF THEIR STACKING

This study aims to model, by using a finite element method, the relationship between the hardness and the period Λ of metal/nitride multilayer coatings (Ti0.54Al0.46/Ti0.54Al0.46N)n in order to understand the increase of the hardness at the low periods [1] and then optimise the multilayer coating architecture to obtain the best mechanical properties. A 2D axisymmetric finite element model of the Berkovich nanoindentation test was developed. The coating was designed as a stacking of Ti0.54Al0.46 and Ti0.54Al0.46N nanolayers with, in the first hypothesis, equal thickness and perfect interface. The elastoplastic behaviours of the metal and nitride layers were identified by Berkovich nanoindentation experiments and inverse analysis on thick monolayer samples. The indentation curves (P-h) obtained by this model depend on the period Λ of the stacking. Simulated (P-h) curves were compared with experimental data on 2 μm thick films with different periods Λ ranging from 10 to 50 nm deposited by RF magnetron sputtering using reactive gas pulsing process (RGPP). The model forecasts are very consistent with the experience for the largest period but the model does not reproduce the hardness increase at the lowest periods. The Λ = 10 nm coating was analysed by electron energy loss spectroscopy (EELS) on a transmission electron microscope. Results show intermixing of the layers with the presence of nitrogen atoms in the metal layer over a few nanometers [1]. It was concluded that the metal/ceramic interface plays an important role at low periods. The addition in the model of a transition layer in the metal/nitride stacking, with an elastoplastic metal/ceramic medium behaviour, allows to reproduce the nanoindentation experimental curves. The thickness of this transition layer deduced from model updating method is in very good agreement with EELS observations.

Photoluminescence properties of epitaxial asymmetric triple CdSe quantum wells

The authors present the results of the photoluminescence characterization of three asymmetric triple quantum well (ATQW) systems. Each one contains three ultrathin CdSe quantum wells of 1, 2, and 3 monolayers (ML) thickness grown by atomic layer epitaxy between ZnSe separating barriers. Two systems have coupled QWs due to thin 5 nm thick ZnSe separating barriers, and the uncoupled system has 100 nm ZnSe separating barriers. Two of them (5 and 100 nm ZnSe barriers) were grown in the sequence 3–2–1, with the 3 ML QW closer to the substrate, while the third ATQW (5 nm ZnSe barrier) was grown in the reverse order. The photoluminescence spectrum of the uncoupled ATQW (3–2–1 sequence) presents three excitonic peaks, one peak for each QW, with the same emission energies of corresponding isolated single QWs. In the case of two coupled ATQW systems (3–2–1 and 1–2–3 growth sequence), the authors observed only one peak, corresponding to the lowest transition energy in the system formed by the three coupled QWs. The calculation of the wavefunctions shows no overlap for 100 nm ZnSe separating barriers, as expected for this thick barrier. For the systems with 5 nm ZnSe barriers, the calculations indicate that the overlap occurs mainly between the wavefunctions of the central 2 ML thick CdSe QW and the neighboring 1 ML and 3 ML QW wavefunctions; the overlap between the 1 and 3 ML QWs is small as a consequence of their 10 nm separation. The authors found that the effect of the coupling between the QWs is more easily identified by the reduction (or absence) of the intensity of the higher energy transitions of the multiple QW system than by the change of the ATQW energy levels with barrier thickness reduction.

Purely in-plane ferroelectricity in monolayer SnS at room temperature

2D van der Waals ferroelectrics have emerged as an attractive building block with immense potential to provide multifunctionality in nanoelectronics. Although several accomplishments have been reported in ferroelectric switching for out-of-plane ferroelectrics down to the monolayer, a purely in-plane ferroelectric has not been experimentally validated at the monolayer thickness. Herein, an in-plane ferroelectricity is demonstrated for micrometer-size monolayer SnS at room temperature. SnS has been commonly regarded to exhibit the odd–even effect, where the centrosymmetry breaks only in the odd-number layers to exhibit ferroelectricity. Remarkably, however, a robust room temperature ferroelectricity exists in SnS below a critical thickness of 15 layers with both an odd and even number of layers, suggesting the possibility of controlling the stacking sequence of multilayer SnS beyond the limit of ferroelectricity in the monolayer. This work will pave the way for nanoscale ferroelectric applications based on SnS as a platform for in-plane ferroelectrics.

Atomic layer deposition of Y2O3 thin films with a high growth per cycle by Ar multiple boost injection

Atomic layer deposition of Y2O3 thin films has been examined by multiple injections of tris-isopropylcyclopentadienyl yttrium [Y(iPrCp)3] precursors followed by remote oxygen plasma. By boosting or pressurizing the precursor cylinder higher than the chamber pressure, one can effectively deliver the precursor into the chamber. By increasing the number of shots before oxygen plasma exposure, the growth per cycle increases and shows saturation at 0.22 nm/cycle, close to the unit monolayer (ML) thickness of the Y2O3 film (0.23 nm). By selecting a proper number of shots, one can effectively utilize the precursor over other methods including bubbling delivery. A multiple boost injection method allows the utilization of low vapor pressure precursor to deposit films in a high-pressure environment.

Nonlinear fracture of two-dimensional transition metal carbides (MXenes)

As a newly discovered two-dimensional (2D) material group, MXene has exceptional thermal- electronic properties. However, its mechanical behavior, especially fracture, remains unexplored. In-situ SEM tensile experiments show that the fracture of MXene is quite nonlinear with a much-prolonged softening stage in comparison with other 2D materials, which is suspected to be caused by the anisotropy and relatively thick monolayer of MXenes. In this work, molecular dynamic (MD) modeling is conducted to investigate the anisotropic fracture behavior of two types of MXenes (Ti2C and Ti3C2) with different monolayer thickness. Both pristine and defected MXenes are investigated. Results show that: (1) MXene tends to fracture along zigzag direction, (2) atomic vacancies at the crack tip have limited effects on the overall fracture behavior, (3) a thicker monolayer can produce a larger cohesive zone due to the ‘thinning’ process, (4) complex fracture paths are to be expected for mixed-mode fractures.

Surface Plasmon Resonance Imaging: A Technique to Reveal the Dropwise Condensation Mechanism

Abstract To understand the physics behind dropwise condensation, a microscopy technique must be able to measure a sub-nanometer film at a high temporal resolution (>1,000 FPS). In this work, automated Surface Plasmon Resonance imaging (SPRi) is used as a tool to study the existence of a thin film between the dropwise condensate. SPRi is a label-free imaging technique that works based on the attenuated total internal reflection. SPRi can detect changes in the refractive index (RI) of the test medium in the thin region (<300 nm) above the sensing gold layer. The automated, angle-scanning SPRi instrument was developed by integrating linear and rotating motorized stages. This instrument improves conventional SPRi by enhancing the resolution of angle probing, increasing the speed of angle scanning, and minimizing the angle-dependent image artifacts. As a proof of concept, we visualized the three stages of coalescence at 10,000 FPS, including bridge formation, composite peanut-shape droplet formation, and the relaxation stage. Furthermore, we probed the solid-vapor interface during the dropwise condensation to evaluate the existence of a thin film on the substrate. The results of our visualization show that the area between droplets is covered by an adsorbed film with a thickness of a monolayer (0.275 nm) and a surface coverage of less than one (m2/m2). Moreover, the results reveal a dry region forms on the substrate when part of the substrate is exposed to ambient conditions due to the coalescence. The dry zone on the substrate has higher surface energy, as compared to the surrounding area. Therefore, the exposed area serves as a favorable site for vapor molecules to strike the surface and form new nuclei.