Monolayer Deposition Research Articles

Simultaneous Effect of Ultraviolet Radiation and Surface Modification on the Work Function and Hole Injection Properties of ZnO Thin Films

The combined effect of ultraviolet (UV) light soaking and self‐assembled monolayer deposition on the work function (WF) of thin ZnO layers and on the efficiency of hole injection into the prototypical conjugated polymer poly(3‐hexylthiophen‐2,5‐diyl) (P3HT) is systematically investigated. It is shown that the WF and injection efficiency depend strongly on the history of UV light exposure. Proper treatment of the ZnO layer enables ohmic hole injection into P3HT, demonstrating ZnO as a potential anode material for organic optoelectronic devices. The results also suggest that valid conclusions on the energy‐level alignment at the ZnO/organic interfaces may only be drawn if the illumination history is precisely known and controlled. This is inherently problematic when comparing electronic data from ultraviolet photoelectron spectroscopy (UPS) measurements carried out under different or ill‐defined illumination conditions.

Engineering of the Electron Transport Layer/Perovskite Interface in Solar Cells Designed on TiO2 Rutile Nanorods

AbstractThe engineering of the electron transport layer (ETL)/light absorber interface is explored in perovskite solar cells. Single‐crystalline TiO2 nanorod (NR) arrays are used as ETL and methylammonium lead iodide (MAPI) as light absorber. A dual ETL surface modification is investigated, namely by a TiCl4 treatment combined with a subsequent PC61BM monolayer deposition, and the effects on the device photovoltaic performance were evaluated with respect to single modifications. Under optimized conditions, for the combined treatment synergistic effects are observed that lead to remarkable enhancements in cell efficiency, from 14.2% to 19.5%, and to suppression of hysteresis. The devices show JSC, VOC, and fill factor as high as 23.2 mA cm−2, 1.1 V, and 77%, respectively. These results are ascribed to a more efficient charge transfer across the ETL/perovskite interface, which originates from the passivation of defects and trap states at the ETL surface. To the best of our knowledge, this is the highest cell performance ever reported for TiO2 NR‐based solar cells fabricated with conventional MAPI light absorber. Perspective wise, this ETL surface functionalization approach combined with more recently developed and better performing light absorbers, such as mixed cation/anion hybrid perovskite materials, is expected to provide further performance enhancements.

Antimony deposition onto Au(111) and insertion of Mg.

Magnesium-based secondary batteries have been regarded as a viable alternative to the immensely popular Li-ion systems owing to their high volumetric capacity. One of the largest challenges is the selection of Mg anode material since the insertion/extraction processes are kinetically slow because of the large ionic radius and high charge density of Mg2+ compared with Li+. In this work, we prepared very thin films of Sb by electrodeposition on a Au(111) substrate. Monolayer and multilayer deposition (up to 20 monolayers) were characterized by cyclic voltammetry (CV) and scanning tunneling microscopy (STM). Monolayer deposition results in a characteristic row structure; the monolayer is commensurate in one dimension, but not in the other. The row structure is to some extent maintained after deposition of further layers. After dissolution of the Sb multilayers the substrate is roughened on the atomic scale due to alloy formation, as demonstrated by CV and STM. Further multilayer deposition correspondingly leads to a rough deposit with protrusions of up to 3 nm. The cyclic voltammogram for Mg insertion/de-insertion from MgCl2/AlCl3/tetraglyme (MACC/TG) electrolyte into/from a Sb-modified electrode shows a positive shift (400 mV) of the onset potential of Mg deposition compared to that of a bare Au electrode. From the charge of the Mg deposition, we find that the ratio of Mg to Sb is 1:1, which is somewhat less than expected for the Mg3Sb2 alloy.

Nanostructured Mixed Layers of Organic Materials Obtained by Nanosphere Lithography and Electrochemical Reduction of Aryldiazonium Salts

In this work, we have combined nanosphere lithography with electrochemical reduction of aryldiazonium salts to elaborate nanostructured mixed layers of organic materials. The strategy consists first in the deposition of a close-packed hexagonal monolayer of microbeads used as a mask for the electroreduction of a first aryldiazonium salt. After removing the beads, an ultrathin organic layer with holes remains. Then, a second aryldiazonium salt is electrochemically reduced selectively inside the holes. The relative thickness of the two deposited materials can be changed, leading to mixed layers of different topographies. Moreover, using diazoniums with complementary redox properties, a modified bifunctional electrode acting as a filter for electron transfer with a low potential gap has been obtained. Such layers are similar to low-band-gap organic semiconductors that can be easily n or p doped. Despite this analogy, the oxidation and reduction of redox probes in solution on such nanostructured surfaces occur on completely separated areas of the mixed layer.

The influence of self-assembled monolayers on tribological properties of Si-DLC coatings

The tribological properties of diamond like carbon (DLC) and silicon-incorporated diamond like carbon (Si-DLC) coatings in contact with Si3N4 counterbody were investigated using a ball-on-disc tribometer. The effect of modification by perfluoro and alkylphosphonic self-assembled monolayers was evaluated in terms of lowering the coefficient of friction (CoF) and wear rate. The tribological tests revealed that the presence of silicon decreases the CoF and slightly increases the wear rate of Si-DLC coatings in comparison with DLC. Further decrease of CoF was achieved for Si-DLC coatings after self-assembled monolayers deposition. The effectiveness of modification, stability on ultraviolet (UV) irradiation and characterization of all coatings was monitored by Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), Raman spectroscopy and measurements of the surface free energy (SFE). The obtained study demonstrated that self-assembled monolayers reduce friction and are highly promising candidates as a good lubricant for tribological and mechanical systems.

(Invited) Electroless Deposition of Pb Monolayer - Prelude to Electroless Atomic Layer Deposition As Surface Selective, Surface Controlled, Self-Terminating Process

The presented experimental results including insitu scanning tunnelling microscopy (STM), electrochemical quartz microbalance (ECQCM) and open circuit potential (OCP) transient as well as our modelling and data analysis demonstrate the new phenomenon of the electroless (e-less) deposition of Pb monolayer (ML) on Pt(111), Ru(0001), Au(111) and Cu(111) substrates. The e-less Pb ML morphology and properties resemble its underpotentially deposited counterpart. The e-less Pb ML deposition, in its nature, represents the surface selective, surface, controlled self-terminating process. These attributes are consequence of the synergy between particular properties of the redox processes and substrates involved in the e-less Pb ML deposition. The e-less Pb ML deposition is enabling phenomenon for the two-step e-less atomic layer deposition (ALD) cycle where the e-less Pb ML serves as a reducing agent in the surface limiting redox replacement (SLRR)[1] reaction with noble metal ions, i.e. noble metal deposition. The full e-less ALD cycle results in an overall deposition of the controlled amount of noble metal which is the function of the areal density of deposited Pb ML and the stoichiometry of the SLRR reaction. The e-less ALD is highly selective to the metal substrates at which Pb forms an e-less ML providing an advantage when certain manufacturing and integration requirements are considered.

Tuning Surface Chemoresistivity of Au Ultrathin Films Using Metal Deposition via Surface-Limited Redox Replacement of the Underpotentially Deposited Pb Monolayer.

The presented work investigates the chemoresistivity of Au ultrathin films, whose surface is modified by deposition of few monolayers of Au, Pd, or AuPd alloy. The model adsorbate in this study was the HS- ion from 0.1 M NaCl solution having concentrations ranging from 0 to 40 ppm. The Au surface modification was carried out using deposition via surface-limited redox replacement of the underpotentially deposited Pb monolayer. Modified Au films have shown higher chemoresistivity than the pristine ones. Our results and analysis suggest that these improvements are due to increased concentration of surface defects and enhanced scattering cross-section per adsorbate induced by chemical modification of the surface by Pd. The significance of our findings is discussed for practical applications shining more light on the importance of surface preparation for chemoresistive sensor design and performance.

Pulsed laser deposition of monolayer and bilayer graphene

The properties of graphene vary with the number of layers, thus determining its usefulness in different applications. Consequently, it is important to develop a method for precisely controlling the number of layers for various application demands. In this study, high quality monolayer graphene was successfully prepared using pulsed laser deposition. Meanwhile, the growth of monolayer and bilayer graphene can be controlled by adjusting the laser energy density. Raman spectra show that high quality monolayer films grew at a laser energy density of 5.66 J/cm2 and bilayer graphene films grew at a laser energy density of 8.49 J/cm2. X-ray photoelectron spectroscopy indicate that the main chemical state of carbon is the sp2 hybrid state, and the maximum atomic ratio of sp2 and sp3 results from the sample with laser energy density of 5.66 J/cm2. Scanning electron microscope images show that monolayer graphene is connected by small sheets with different sizes, and the largest graphene sheet reaches 20 μm. High resolution transmission electron microscope images and selected area electron diffraction patterns show that graphene samples are highly crystalline and yield significant layer information. UV–Vis spectra indicate that the transmittance through monolayer and bilayer graphene at 550 nm are 96.93% and 95.05%, respectively. Ultraviolet photoelectron spectroscopic measurements show that the work function of graphene ranges from 3.72 eV to 4.76 eV; the variation in the work function is related to growth defects. Finally, the relationship between the structure of the graphene films and laser energy density are investigated in detail, and these results indicate that the controllable growth of graphene can be achieved by adjusting the laser energy density. This means that a suitable laser energy density is conducive to defining an equilibrium between diffusion and condensation of carbon atoms, which is crucial to the growth of high-quality monolayer and bilayer graphene. It is expected that this study might be facilitate the preparation of graphene-based composite films and devices.

Quantitative Visualization of Topology and Morphing of Percolation Path in Nanoparticle Network Array Exhibiting Coulomb Blockade at Room Temperature

A network of one-dimensional (1D) necklaces of 10 nm Au nanoparticles was fabricated by a directed self-assembly to synthesize 1D necklaces followed by self-limiting monolayer deposition to form a two-dimensional (2D) network array. Scanning electron microscope (SEM) image analysis revealed a percolation threshold lower than random 2D arrays signifying the local 1D structure. The topology of (shortest) percolation paths (tortuosity) and the fraction of clusters isolated from the percolating array were quantified to relate the network morphology to the observed non-Ohmic (Coulomb blockade effect) behavior. Leveraging charge contrast in SEM, the morphing of the percolation path as a function of the kinetic energy of the conduction electron was visualized and quantified to understand the dynamic nature of the percolation behavior. The morphology can be systematically tailored by tuning the two self-assembly processes to obtain the same coverage of the array with significantly diverse non-Ohmic behavior. It w...

Substrate stiffness affects particle distribution pattern in a drying suspension droplet

The complexities involved in achieving a tailor-made evaporative deposition pattern have remained a challenge. Here, we show that the morphological pattern of drying suspension droplets can be altered by varying substrate elastic modulus E. We find that the particle spot diameter and spacing between the particles scale with substrate stiffness as ds ∼ E−0.15 and s ∼ E−1.23, respectively. We show that the larger spot diameter and spacing between particles on a softer substrate are attributed to a higher energy barrier U associated with stronger pinning of the contact line. The particle deposition pattern is characterized in terms of deposition index, Id, whose value is <0.50 and >0.75 for centralized (multilayer) and uniform (monolayer) deposition patterns observed for stiffer and softer substrates, respectively. The outcome of the present study may find applications in biochemical characterization and analysis of micro-/nanoparticles.

Vapor-Phase Deposition and Electronic Characterization of 3-Aminopropyltriethoxysilane Self-Assembled Monolayers on Silicon Dioxide.

Although organosilanes, especially 3-aminopropyltriethoxysilane (APTES), are commonly used to functionalize oxide substrates for a variety of applications ranging from molecular/biosensors and electronics to protective layers, reliable and controlled deposition of these molecules remains a major obstacle. In this study, we use surface potential analyses to record and optimize the gas-phase deposition of APTES self-assembled monolayers (SAMs) and to determine the resulting change of the electrokinetic potential and charge at the solid?liquid interface when the system is exposed to an electrolyte. Using a gas-phase molecular layer deposition setup with an in situ molecule deposition sensor, APTES is deposited at room temperature onto ozone-activated SiO2. The resulting layers are characterized using various techniques ranging from contact angle analysis, ellipsometry, fluorescence microscopy, X-ray photoelectron spectroscopy, and electrokinetic analysis to AFM. It turns out that adequate postdeposition treatment is crucial to the formation of perfect molecular SAMs. We demonstrate how a thick layer of APTES molecules is initially adsorbed at the surface; however, the molecules do not bind to SiO2 and are removed if the film is exposed to an electrolyte. Only if the film is kept in a gaseous environment (preferable at low pressure) for a long enough time do APTES molecules start to bind to the surface and form the SAM layer. During this time, superfluous molecules are removed. The resulting modification of the electrokinetic potential at the surface is analyzed in detail for different states.

Differential outcomes of venous and arterial tissue engineered vascular grafts highlight the importance of coupling long-term implantation studies with computational modeling

Electrospinning is commonly used to generate polymeric scaffolds for tissue engineering. Using this approach, we developed a small-diameter tissue engineered vascular graft (TEVG) composed of poly-ε-caprolactone-co-l-lactic acid (PCLA) fibers and longitudinally assessed its performance within both the venous and arterial circulations of immunodeficient (SCID/bg) mice. Based on in vitro analysis demonstrating complete loss of graft strength by 12 weeks, we evaluated neovessel formation in vivo over 6-, 12- and 24-week periods. Mid-term observations indicated physiologic graft function, characterized by 100% patency and luminal matching with adjoining native vessel in both the venous and arterial circulations. An active and robust remodeling process was characterized by a confluent endothelial cell monolayer, macrophage infiltrate, and extracellular matrix deposition and remodeling. Long-term follow-up of venous TEVGs at 24 weeks revealed viable neovessel formation beyond graft degradation when implanted in this high flow, low-pressure environment. Arterial TEVGs experienced catastrophic graft failure due to aneurysmal dilatation and rupture after 14 weeks. Scaffold parameters such as porosity, fiber diameter, and degradation rate informed a previously described computational model of vascular growth and remodeling, and simulations predicted the gross differential performance of the venous and arterial TEVGs over the 24-week time course. Taken together, these results highlight the requirement for in vivo implantation studies to extend past the critical time period of polymer degradation, the importance of differential neotissue deposition relative to the mechanical (pressure) environment, and further support the utility of predictive modeling in the design, use, and evaluation of TEVGs in vivo. Statement of SignificanceHerein, we apply a biodegradable electrospun vascular graft to the arterial and venous circulations of the mouse and follow recipients beyond the point of polymer degradation. While venous implants formed viable neovessels, arterial grafts experienced catastrophic rupture due to aneurysmal dilation. We then inform a previously developed computational model of tissue engineered vascular graft growth and remodeling with parameters specific to the electrospun scaffolds utilized in this study. Remarkably, model simulations predict the differential performance of the venous and arterial constructs over 24 weeks. We conclude that computational simulations should inform the rational selection of scaffold parameters to fabricate tissue engineered vascular grafts that must be followed in vivo over time courses extending beyond polymer degradation.

Sensitive biological detection based on Langmuir–Blodgett films prepared fromdiluted silica suspensions

Self-assembled three-dimensional (3-D) colloidal crystals have been extensively utilized in nanolithography for fabricating a wide spectrum of functional periodic nanostructures for important technological applications. Here a simple and versatile way is reported using silica colloids to prepare close-packed particle multilayers by the Langmuir– Blodgett technique to create highly ordered 3-D colloidal crystals with unusual nanoarrays on a large variety of substrates. Successive deposition of monolayers using amphiphilic molecules in this approach allows control of the layer thickness, particle size, and especially packing structure of them in each layer. It has been demonstrated that the Langmuir–Blodgett film’s transparency and high antiglare optical properties are preserved even after multiple colloidal particle transfers on both straight and curved surfaces. The surface prepared by the Langmuir–Blodgett technique shows remarkable sensitivity for potential biodetection applications. These colloidal templated films have an optical response time of less than 40 s against essential bioproduct vapors. It is shown that the results are consistent with the Kelvin equation (ln P/Po = 􀀀2 Vl /rRT) when the liquid water layer is getting thicker.

Unoccupied surface and interface states in Pd thin films deposited on Fe/Ir(111) surface

We present a systematic first-principles study of the electronic surface states and resonances occuring in thin films of Pd of various thicknesses deposited on a single ferromagnetic monolayer (ML) of Fe on top of Ir(111) substrate. This system is of interest since one Pd layer deposited on Fe/Ir(111) hosts small magnetic skyrmions. The latter are topological magnetic objects with swirling spin-textures with possible implications in the context of spintronic devices since they have the potential to be used as magnetic bits for information technology. The stabilization, detection and manipulation of such non-collinear magnetic entities require a quantitative investigation and a fundamental understanding of their electronic structure. Here we investigate the nature of the unoccupied electronic states in Pd/Fe/Ir(111), which are essential in the large spin-mixing magnetoresistance signature captured using non spin-polarized scanning tunneling microscopy (Crum et al 2015 Nat. Commun. 6 8541, Hanneken et al 2015 Nat. Nanotechnol. 10 1039). To provide a complete analysis, we investigate bare Fe/Ir(111) and Pdn = 2,7/Fe/Ir(111) surfaces. Our results demonstrate the emergence of surface and interface states after deposition of Pd MLs, which are strongly impacted by the large spin–orbit coupling of Ir surface.

New Perspective of Mitigating the Coffee-Ring Effect: Interfacial Assembly

The evaporation of particle-laden droplets on a substrate usually results in ring-like deposits due to particle migration to the contact lines. This ubiquitous phenomenon, known as the coffee-ring effect (CRE), was initially observed in drying coffee droplets and later in many colloidal systems. The CRE has been intensively investigated during the past two decades to unveil the complexity related to its flow patterns, evaporation physics, and deposition structures after solvent evaporation. However, the contribution of colloidal particle assembly and interactions at the air–liquid interface of sessile droplets to the particle deposition requires more attention. The objective of this Review Article is to highlight the recent advances in mitigating or totally suppressing the CRE by means of interfacial assembly via manipulating the multibody interactions, for example, particle–particle, particle–substrate, particle–flow, and particle–interface interactions. Well-ordered monolayer deposition of the colloidal particles, driven by interfacial assembly, has been demonstrated by several research groups. This unique perspective of suppressing the CRE and creating well-ordered monolayer structures by assembling colloidal particles at the air–liquid interface creates a new paradigm in generating coatings and functional devices through liquid processing. General rules and guidelines are established to provide broader prospects of engineering desirable structures of colloidal particle deposition and assembly.

Generalized On-Demand Production of Nanoparticle Monolayers on Arbitrary Solid Surfaces via Capillarity-Mediated Inverse Transfer.

Century-old Langmuir monolayer deposition still represents the most convenient approach to the production of monolayers of colloidal nanoparticles on solid substrates for practical biological and chemical-sensing applications. However, this approach simply yields arbitrarily shaped large monolayers on a flat surface and is strongly limited by substrate topography and interfacial energy. Here, we describe a generalized and facile method of rapidly producing uniform monolayers of various colloidal nanoparticles on arbitrary solid substrates by using an ordinary capillary tube. Our method is based on an interesting finding of inversion phenomenon of a nanoparticle-laden air-water interface by flowing through a capillary tube in a manner that prevents the particles from adhesion to the capillary sidewall, thereby presenting the nanoparticles face-first at the tube's opposite end for direct and one-step deposition onto a substrate. We show that our method not only allows the placement of a nanoparticle monolayer at target locations of solid substrates regardless of their surface geometry and adhesion but also enables the production of monolayers containing nanoparticles with different size, shape, surface charge, and composition. To explore the potential of our approach, we demonstrate the facile integration of gold nanoparticle monolayers into microfluidic devices for the real-time monitoring of molecular Raman signals under dynamic flow conditions. Moreover, we successfully extend the use of our method to developing on-demand Raman sensors that can be built directly on the surface of consumer products for practical chemical sensing and fingerprinting. Specifically, we achieve both the pinpoint deposition of gold nanoparticle monolayers and sensitive molecular detection from the deposited region on clothing fabric for the detection of illegal drug substances, a single grain of rice and an orange for pesticide monitoring, and a $100 bill as a potential anti-counterfeit measure, respectively. We believe that our method will provide unique opportunities to expand the utility of colloidal nanoparticles and to greatly improve the accessibility of nanoparticle-based sensing technologies.

Wettability of DPPC Monolayers Deposited from the Titanium Dioxide–Chitosan–Hyaluronic Acid Subphases on Glass

The investigations were carried out to determine wettability of the 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) monolayers transferred from the liquid subphases containing chitosan (Ch), hyaluronic acid (HA), and/or titanium dioxide (TiO2) to a glass support by means of the Langmuir–Blodgett (LB) technique. For comparative purposes, the analysis of the plates surfaces emerged from the analogous subphases without the phospholipid film was also made. Characterization of the DPPC monolayers was based on the contact angle measurements using three test liquids (water, formamide, diiodomethane) and a simulated body fluid (SBF) solution in which the concentration of ions was close to that of human plasma. After deposition of the DPPC monolayers on the glass plates, a significant increase in the contact angles of all the probe liquids was observed compared to the plates pulled out from the given subphase without floating DPPC. The presence of phospholipid monolayer increased the hydrophobic character of the surface due to orientation of its molecules with hydrocarbon chains towards the air. In addition, the components of the subphase attached along with DPPC to the glass support modify the surface polarity. The largest changes were observed in the presence of TiO2.

Manipulating thermal resistance at the solid-fluid interface through monolayer deposition.

Heat transfer across an interface between a monolayer coated solid substrate and fluid has been extensively analyzed through a series of non-equilibrium molecular dynamics simulations. The effect of the monolayer was studied by varying its atomic mass (mM) and interaction energy between monolayer particles (εMM). Even though the fluid adsorption plays a role in heat transfer at the solid–fluid interface, we found that the interfacial thermal resistance (Kapitza resistance) is highly affected by the insertion of monolayer without any further change in the liquid structure near the solid surface. The Kapitza length monotonically increases with the increase of mM irrespective of εMM. The observations were explained by analysis of the overlap of the phonon spectrum at the interface using vibrational density of states. The effect of the monolayer on the Kapitza length was summarized by a fourth polynomial function that demonstrates the contribution of both mM and εMM with respect to each other on the Kapitza resistance within the parametric range studied.

Moiré and honeycomb lattices through self-assembly of hard-core/soft-shell microgels: experiment and simulation

Control over microstructure and interparticle spacing in substrate-supported colloidal arrangements is a key challenge in colloidal self-assembly. We demonstrate here the preparation of Moiré and honeycomb monolayer lattices from core/shell microgels with rigid inorganic cores and soft, deformable hydrogel shells. These structures were realized by the sequential double deposition of freely floating monolayers from the air/water interface onto the same, centimetre-scale substrate. Due to the soft and deformable character of the hydrogel shells, the second applied monolayer fully settles into the same plane as the first monolayer. The resulting structural motif is determined by the drying conditions applied to the second deposition step. We support our experimental findings by Brownian dynamics simulations and provide insights into the structure formation process.

Underpotential and Electroless Pb Monolayer Deposition on Ru(0001)

The work reporting a detailed and comparative study of Pb UPD and Pb e-less ML deposition on Ru(0001) substrate is presented. The electrochemical results are analyzed through the scope of the adsorption isotherm formalism where parameters describing the thermodynamics of deposited Pb monolayer obtained by each process are compared. In addition to these results, the in situ STM and electrochemical quartz micro balance data are presented for each process identifying the mutual mechanistic similarities and differences between electroless Pb and underpotential Pb monolayer. Considerable applications of Ru metal in microelectronics and catalysis rise significance of these results for variety of future developments in these areas.