Orientation Of Monolayers Research Articles

Probing the Structure and Orientation of Carboxylic Acid-Terminated Self-Assembled Monolayers.

Self-assembled monolayers (SAMs), such as alkanethiols (AT), are widely used as functional coatings or interfaces between different materials. There is an assumption that the arrangement and alignment of the hydrocarbon chains in films made from carboxyl-terminated alkanethiols are similar to those made from alkanethiols. Here, the structure of the outermost layer and near-surface region of SAMs formed from carboxyl-terminated alkanethiols of various lengths has been analyzed. The chemical composition of the samples was measured using X-ray photoelectron spectroscopy (XPS) and angle-resolved XPS (AR-XPS), allowing the film thickness. Metastable induced photoelectron spectroscopy (MIES) as a surface analytical tool sensitive only for the outermost layer in conjunction with density functional theory (DFT) calculations provided insights into the composition of the topmost layer, showing that it consists mainly of the backbone of the SAM-forming molecules. Through combining AR-XPS concentration depth profiles and the measurement of the composition of the outermost layer, it can be shown that SAMs tend to favor a gauche orientation, enabling interactions between the functional groups.

Quantum chemical modeling of alkane 2D monolayer formation on graphene

The paper presents a quantum chemical approach for assessment of the thermodynamic parameters of association for alkanes CnH2n+2 (n = 6–14) and polyaromatic hydrocarbons (PAH) of the coronene series as model structures of the graphene surface within the framework of semiempirical methods. The enthalpy, entropy and Gibbs energy of formation and binding for alkanes with PAH were calculated using the PM3 and PM6-DH2 methods. It is shown that an adequate description of the interactions in the regarded complexes requires the use of PM6-DH2 method, since it contains corrections for dispersion interactions and hydrogen bonds. The parallel orientation of the alkane molecule relative to the coronene plane is proved to be more energetically preferable than perpendicular one, which is consistent with experimental data.Intermolecular C–H/π interactions are revealed to be crucial in the 2D film formation of alkanes on graphene/graphite. While interactions between alkane molecules make a destabilizing contribution due to the implementation of energetically unfavorable types of intermolecular CH/HC interactions. This stipulates a threshold chain length of alkanes capable of film formation on the graphene/graphite surface at standard temperature: 14 and 19 carbon atoms for parallel and perpendicular oriented alkanes, respectively. The obtained threshold values of the alkane chain length, as well as the geometric parameters of their orientation in 2D monolayers on the graphene/graphite surface are consistent with available experimental data.

Pinecone biochar for the Adsorption of chromium (VI) from wastewater: Kinetics, thermodynamics, and adsorbent regeneration

High concentration of chromium in aquatic environments is the trigger for researchers to remediate it from wastewater environments. However, conventional water treatment methods have not been satisfactory in removing chromium from water and wastewater over the last decade. Similarly, many adsorption studies have been focused on one aspect of the treatment, but this study dealt with all aspects of adsorption packages to come up with a concrete conclusion. Therefore, this study aimed to prepare pinecone biochar (PBC) via pyrolysis and apply it for Cr(VI) removal from wastewater. The PBC was characterized using FTIR, SEM-EDX, BET surface area, pHpzc, Raman analyses, TGA, and XRD techniques. Chromium adsorption was studied under the influence of PBC dose, solution pH, initial Cr(VI) concentration, and contact time. The characteristics of PBC are illustrated by FTIR spectroscopic functional groups, XRD non-crystallite structure, SEM rough surface morphology, and high BET surface area125 m2/g, pore volume, 0.07 cm3/g, and pore size 1.4 nm. On the other hand, the maximum Cr (VI) adsorption of 69% was found at the experimental condition of pH 2, adsorbent dosage 0.25 mg/50 mL, initial Cr concentration 100 mg/L, and contact time of 120 min. Similarly, the experimental data were well-fitted with the Langmuir adsorption isotherm at R2 0.96 and the pseudo-second-order kinetics model at R2 0.99. This implies the adsorption process is mainly attributed to monolayer orientation between the adsorbent and adsorbate. In the thermodynamics study of adsorption, ΔG was found to be negative implying the adsorption process was feasible and spontaneous whereas the positive values of ΔH and ΔS indicated the adsorption process was endothermic and increasing the degree of randomness, respectively. Finally, adsorbent regeneration and reusability were successful up to three cycles. In conclusion, biochar surface modification and reusability improvements are urgently required before being applied at the pilot scale.

Using Surface-Enhanced Raman Spectroscopy to Unravel the Wingtip-Dependent Orientation of N-Heterocyclic Carbenes on Gold Nanoparticles

N-Heterocyclic carbenes (NHCs) are an attractive alternative to thiol ligands when forming self-assembled monolayers on noble-metal surfaces; however, relative to the well-studied thiol monolayers, comparatively little is known about the binding, orientation, and packing of NHC monolayers. Herein, we combine surface-enhanced Raman spectroscopy (SERS) and first-principles theory to investigate how the alkyl "wingtip" groups, i.e., those attached to the nitrogens of N-heterocyclic carbenes, affect the NHC orientation on gold nanoparticles. Consistent with previous literature, smaller wingtip groups lead to stable flat configurations; surprisingly, bulkier wingtips also have stable flat configurations likely due to the presence of an adatom. Comparison of experimental SERS results with the theoretically calculated spectra for flat and vertical configurations shows that we are simultaneously detecting both NHC configurations. In addition to providing information on the adsorbate geometry, this study highlights the extreme SERS enhancement of vibrational modes perpendicular to the surface.

Tripodal Triazatruxene Derivative as a Face-On Oriented Hole-Collecting Monolayer for Efficient and Stable Inverted Perovskite Solar Cells.

Hole-collecting monolayers have drawn attention in perovskite solar cell research due to their ease of processing, high performance, and good durability. Since molecules in the hole-collecting monolayer are typically composed of functionalized π-conjugated structures, hole extraction is expected to be more efficient when the π-cores are oriented face-on with respect to the adjacent surfaces. However, strategies for reliably controlling the molecular orientation in monolayers remain elusive. In this work, multiple phosphonic acid anchoring groups were used to control the molecular orientation of a series of triazatruxene derivatives chemisorbed on a transparent conducting oxide electrode surface. Using infrared reflection absorption spectroscopy and metastable atom electron spectroscopy, we found that multipodal derivatives align face-on to the electrode surface, while the monopodal counterpart adopts a more tilted configuration. The face-on orientation was found to facilitate hole extraction, leading to inverted perovskite solar cells with enhanced stability and high-power conversion efficiencies up to 23.0%.

Effect of cavitation intensity control on self-assembling of alkanethiols on gold in room temperature ionic liquids

This study investigates the effect of cavitation intensity on self-assembling of alkanethiol molecules on gold in room temperature ionic liquids (RTILs) under low frequency ultrasound irradiation (20 kHz). The use of RTILs, with low vapor pressure, enabled cavitation activity to be controlled up to quenching through pressure decrease within an argon-saturated atmosphere. This control possibility was used to acquire deeper insights into the role of cavitation on self-assembling processes. It was shown by electrochemical, contact angles and Polarization Modulation - Infrared Reflection Absorption Spectroscopy (PM-IRRAS) measurements that cavitation activates orientation and organization of self-assembled monolayers (SAM). X-ray Photoelectron Spectroscopy (XPS) revealed that, even if chemical adsorption of molecules is highly activated under ultrasound irradiation, it is not dependent on acoustic cavitation intensity.

A simple magnetic-assisted microfluidic method for rapid detection and phenotypic characterization of ultralow concentrations of bacteria

Isolation and enumeration of bacteria at ultralow concentrations and antibiotic resistance profiling are of great importance for early diagnosis and treatment of bacteremia. In this work, we describe a simple, rapid, and versatile magnetic-assisted microfluidic method for rapid bacterial detection. The developed method enables magnetophoretic loading of bead-captured bacteria into the microfluidic chamber under external static and dynamic magnetic fields in 4 min. A shallow microfluidic chamber design that enables the monolayer orientation and transportation of the beads and a glass substrate with a thickness of 0.17 mm was utilized to allow high-resolution fluorescence imaging for quantitative detection. Escherichia coli (E. coli) with green fluorescent protein (GFP)-expressing gene and streptavidin-modified superparamagnetic microbeads were used as model bacteria and capturing beads, respectively. The specificity of the method was validated using Lactobacillus gasseri as a negative control group. The limit of detection and limit of quantification values were determined as 2 CFU/ml and 10 CFU/ml of E. coli, respectively. The magnetic-assisted microfluidic method is a versatile tool for the detection of ultralow concentrations of viable bacteria with the linear range of 5–5000 CFU/ml E. coli in 1 h, and providing growth curves and phenotypic characterization bead-captured E. coli in the following 5 h of incubation. Our results are promising for future rapid and sensitive antibiotic susceptibility testing of ultralow numbers of viable cells.

Influence of the Molecular Orientation and Ionization of Self-Assembled Monolayers in Biosensors: Application to Genosensors of Prostate Cancer Antigen 3

Electrochemical sensing depends on the immobilization of biorecognition elements and charge transfer to the electrode, which can be tuned and sometimes controlled using self-assembled monolayers (SAMs) as matrices. In this study, we show that an efficient immobilization of an ssDNA probe to detect the prostate cancer antigen 3 biomarker is achieved even for less organized SAMs provided that the terminal groups were ionized. These conclusions were reached by comparing the biosensors made with 11-mercaptoundecanoic acid (11-MUA) prepared in ethanol and thioglycolic acid (mercaptoacetic acid: MAA) prepared in ethanolic and aqueous solvents. The highest sensitivity with electrochemical impedance spectroscopy was observed for sensors assembled over MAA monolayers on gold prepared in ethanol and water (at pH 12.5), with limits of detection of 1.2 × 10–9 and 1.7 × 10–9 mol L–1, respectively. The biosensors made on MAA films prepared in water at pH 7.0 and 2.5 were not efficient because they are mostly terminated by protonated carboxylic acid groups, according to sum-frequency generation vibrational and polarization modulation-infrared reflection-absorption (PM-IRRAS) spectroscopies, which prevent an adequate attachment of the biomarker. Well-organized SAMs of 11-MUA also showed good detection performance, but the limit of detection was not as low, 2.1 × 10–9 mol L–1, probably due to its long chains that decrease the charge transfer. Therefore, the matrix fabrication method can control the molecular assembly of biosensors and hence their performance.

Effect of substrate symmetry on the orientations of MoS2 monolayers

Two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) are promising platforms for developing next-generation electronic and optoelectronic devices due to their unique properties. To achieve this, the growth of large single-crystal TMDs is a critical issue. Unraveling the factors affecting the nucleation and domain orientation should hold fundamental significance. Herein, we design the chemical vapor deposition growth of monolayer MoS2 triangles on Au(111) and Au(100) facets, for exploring the substrate facet effects on the domain orientations. According to multi-scale characterizations, we find that, the obtained triangular MoS2 domains present two preferential orientations on the six-fold symmetric Au(111) facet, whereas four predominant orientations on the four-fold symmetric Au(100) facet. Using on-site scanning tunneling microscopy, we further reveal the preferred alignments of monolayer MoS2 triangles along the close-packed directions of both Au(111) and Au(100) facets. Moreover, bunched substrate steps are also found to form along the close-packed directions of the crystal facets, which guides the preferential nucleation of monolayer MoS2 along the step edges. This work should hereby deepen the understanding of the substrate facet/step effect on the nucleation and orientation of monolayer MoS2 domains, thus providing fundamental insights into the controllable syntheses of large single-crystal TMD monolayers.

Substrate-Dependent Study of Chain Orientation and Order in Alkylphosphonic Acid Self-Assembled Monolayers for ALD Blocking.

For years, many efforts in area selective atomic layer deposition (AS-ALD) have focused on trying to achieve high-quality self-assembled monolayers (SAMs), which have been shown by a number of studies to be effective for blocking deposition. Herein, we show that in some cases where a densely packed SAM is not formed, significant ALD inhibition may still be realized. The formation of octadecylphosphonic acid (ODPA) SAMs was evaluated on four metal substrates: Cu, Co, W, and Ru. The molecular orientation, chain packing, and relative surface coverage were evaluated using near-edge X-ray absorption fine structure (NEXAFS), Fourier transform infrared (FTIR) spectroscopy, and electrochemical impedance spectroscopy (EIS). ODPA SAMs formed on Co, Cu, and W showed strong angular dependence of the NEXAFS signal whereas ODPA on Ru did not, suggesting a disordered layer was formed on Ru. Additionally, EIS and FTIR spectroscopy confirmed that Co and Cu form densely packed, "crystal-like" SAMs whereas Ru and W form less dense monolayers, a surprising result since W-ODPA was previously shown to inhibit the ALD of ZnO and Al2O3 best among all the substrates. This work suggests that multiple factors play a role in SAM-based AS-ALD, not just the SAM quality. Therefore, metrological averaging techniques (e.g., WCA and FTIR spectroscopy) commonly used for evaluating SAMs to predict their suitability for ALD inhibition should be supplemented by more atomically sensitive methods. Finally, it highlights important considerations for describing the mechanism of SAM-based selective ALD.

Second-harmonic imaging microscopy for time-resolved investigations of transition metal dichalcogenides

Two-dimensional transition metal dichalcogenides (TMD) have shown promise for various applications in optoelectronics and so-called valleytronics. Their operation and performance strongly depend on the stacking of individual layers. Here, optical second-harmonic generation in imaging mode is shown to be a versatile tool for systematic time-resolved investigations of TMD monolayers and heterostructures in consideration of the material’s structure. Large sample areas can be probed without the need of any mapping or scanning. By means of polarization dependent measurements, the crystalline orientation of monolayers or the stacking angles of heterostructures can be evaluated for the whole field of view. Pump-probe experiments then allow to correlate observed transient changes of the second-harmonic response with the underlying structure. The corresponding time-resolution is virtually limited by the pulse duration of the used laser. As an example, polarization dependent and time-resolved measurements on mono- and multilayer MoS2 flakes grown on a SiO2/ Si(001) substrate are presented.

Effect of modulation ratio on structure and mechanical properties of WB2/CrN films deposited by direct-current magnetron sputtering

WB2/CrN multilayer films with thick modulation period about 500 nm but different modulation ratios (tWB2:tCrN = 1.2, 2, 3 and 4) were deposited on steel substrates by magnetron sputtering, and the effect of modulation ratio on the structure, residual stress and tribo-mechanical properties of the WB2/CrN multilayer films was systematically studied. Additionally, WB2 and CrN monolayer films with various thicknesses were also prepared to discuss the growth behavior and the related properties of the WB2/CrN multilayer films. All the sample films present the columnar growth, and the particle size in the WB2 monolayers is smaller than that in the corresponding WB2 sublayers. The preferred orientation of WB2 (260–400nm) and CrN monolayers (132–220 nm) is (101) and (111), respectively. However, the multilayered structure changes the orientation of CrN sublayers to (002), and crystalline Cr2N phase is detected in WB2/CrN multilayers caused by the element diffusion at interface. Moreover, the multilayered structure reduces the compressive stress of the WB2 films greatly by diffusion of point defects to interfaces and an indirect effect of interfaces on stress via film structure with soft CrN, and the residual stress of the WB2, CrN and WB2/CrN films is −2.64∼-4.16, −0.85–0.57 and −1.49∼-2.61 GPa, respectively. Furthermore, film hardness ranging from 27.9 to 33.0 GPa mainly obeys the rule of mixture. The adhesive strength drops greatly from 26 to 17 N with the increasing tWB2:tCrN, and the tensile CrN bottom-layer deteriorates the adhesive strength of the WB2/CrN multilayers with tWB2:tCrN = 2. Overall, WB2/CrN films with tWB2:tCrN = 3 show better wear resistance with lower friction coefficient ∼0.35 and wear rate about 3.6 × 10−7 mm3/mN benefiting from their higher hardness, lower roughness, proper toughness and compressive stress.

Kinetic Control over Self-Assembly of Semiconductor Nanoplatelets.

Semiconductor nanoplatelets exhibit spectrally pure, directional fluorescence. To make polarized light emission accessible and the charge transport effective, nanoplatelets have to be collectively oriented in the solid state. We discovered that the collective nanoplatelets orientation in monolayers can be controlled kinetically by exploiting the solvent evaporation rate in self-assembly at liquid interfaces. Our method avoids insulating additives such as surfactants, making it ideally suited for optoelectronics. The monolayer films with controlled nanoplatelets orientation (edge-up or face-down) exhibit long-range ordering of transition dipole moments and macroscopically polarized light emission. Furthermore, we unveil that the substantial in-plane electronic coupling between nanoplatelets enables charge transport through a single nanoplatelets monolayer, with an efficiency that strongly depends on the orientation of the nanoplatelets. The ability to kinetically control the assembly of nanoplatelets into ordered monolayers with tunable optical and electronic properties paves the way for new applications in optoelectronic devices.

Tetrapodal Diazatriptycene Enforces Orthogonal Orientation in Self-Assembled Monolayers.

Conformationally rigid multipodal molecules should control the orientation and packing density of functional head groups upon self-assembly on solid supports. Common tripods frequently fail in this regard because of inhomogeneous bonding configuration and stochastic orientation. These issues are circumvented by a suitable tetrapodal diazatriptycene moiety, bearing four thiol-anchoring groups, as demonstrated in the present study. Such molecules form well-defined self-assembled monolayers (SAMs) on Au(111) substrates, whereby the tetrapodal scaffold enforces a nearly upright orientation of the terminal head group with respect to the substrate, with at least three of the four anchoring groups providing thiolate-like covalent attachment to the surface. Functionalization by condensation chemistry allows a large variety of functional head groups to be introduced to the tetrapod, paving the path toward advanced surface engineering and sensor fabrication.

Switching the Electronic Properties of ZnO Surfaces with Negative T‐Type Photochromic Pyridyl‐dihydropyrene Layers and Impact of Fermi Level Pinning

AbstractRemote control of the electronic energy levels by external stimuli such as light will enable optoelectronic devices with improved or additional functionalities. Here, it is demonstrated that the electronic properties of ZnO interfaced with negative T‐type photoswitches, that is, pyridyl‐dihydropyrene (Py‐DHP), can indeed be photomodulated. The process of forward switching of Py‐DHP with green light from an isomer with a low energy gap to an isomer with a wider one is followed by a thermally activated backward transfer. Using photoemission spectroscopy and density functional theory modeling, it is shown that Py‐DHP ring closure/opening reactions result in a reversible shift of frontier occupied molecular levels by 0.7 eV with respect to the Fermi level. Notably, in both molecular configurations, the energy level alignment at ZnO/Py‐DHP interfaces is governed by a Fermi level pinning at the lowest unoccupied molecular level. Moreover, upon switching, an increase in the ionization energy for Py‐DHP multilayers compared to that of a monolayer is observed. This is attributed to a different preferred molecular orientation in monolayer versus multilayers. The results show that a dynamic tuning of the energy level alignment at inorganic/organic interfaces by external stimuli is feasible and will aid the development of photoprogrammable optoelectronic devices.

Potential dependent orientation of sulfanylbenzonitrile monolayers monitored by SERS

The change in orientation of 2-, 3-, and 4-sulfanylbenzonitrile (SBN) self assembled monolayers (SAMs) with respect to electrochemical potential was studied using surface-enhanced Raman spectroscopy (SERS). Being able to understand monolayer orientation under potential control has important applications in both sensors and molecular electronics, where orientation can affect sensitivity and electron transfer respectively. We characterise the vibrations of all three monolayers using a combination of experimental and density function theory (DFT) calculated spectra. The results reveal that the SERS intensities of both in plane (IP) and out of plane (OOP) modes of the aromatic ring and nitrile group are reversibly changed by potential, indicating an orientation change via variations in the tilt and/or twist angles. At more negative potentials the nitrile group of all three SAMs is more perpendicular to the electrode surface and at more positive potentials it is more parallel, governed by an electrostatic interaction between the nitrile group and the electrode surface. Additional effects of the applied potential are observed for the orientation of the ring with respect to the electrode surface through aromatic ring-surface π interactions, confirmed by examination of the effects of the applied potential on the peak width of the CC ring breathing mode.

Numerical investigation of initial crack growth in composite laminates under tension

The simplified automated strength model of a fragment of composite laminate at microlevel based on the finite elements (FE) method is offered. The model allows investigating the growth of microcracks which arise in the resin in monolayers, orthogonal to a direction of external tensile force. On the basis of automated strength FE model of a fragment of composite laminate and the method of reduction of stiffness characteristics of the resin, the scenario of growth of primary destructions at the microlevel is offered and proved. The scenario is confirmed with experimental investigations of composite samples under tension with registration of acoustic emission. In the work, the dependences of decrease of strength characteristics of composite laminates are obtained as functions of characteristics of resin, angles of orientation of monolayers, volume ratio of fibres in a composite laminate.

Generating high-order optical and spin harmonics from ferromagnetic monolayers

High-order harmonic generation (HHG) in solids has entered a new phase of intensive research, with envisioned band-structure mapping on an ultrashort time scale. This partly benefits from a flurry of new HHG materials discovered, but so far has missed an important group. HHG in magnetic materials should have profound impact on future magnetic storage technology advances. Here we introduce and demonstrate HHG in ferromagnetic monolayers. We find that HHG carries spin information and sensitively depends on the relativistic spin–orbit coupling; and if they are dispersed into the crystal momentum k space, harmonics originating from real transitions can be k-resolved and carry the band structure information. Geometrically, the HHG signal is sensitive to spatial orientations of monolayers. Different from the optical counterpart, the spin HHG, though probably weak, only appears at even orders, a consequence of SU(2) symmetry. Our findings open an unexplored frontier—magneto-high-order harmonic generation.

Substrate Lattice-Guided Seed Formation Controls the Orientation of 2D Transition-Metal Dichalcogenides.

Two-dimensional (2D) transition-metal dichalcogenide (TMDC) semiconductors are important for next-generation electronics and optoelectronics. Given the difficulty in growing large single crystals of 2D TMDC materials, understanding the factors affecting the seed formation and orientation becomes an important issue for controlling the growth. Here, we systematically study the growth of molybdenum disulfide (MoS2) monolayer on c-plane sapphire with chemical vapor deposition to discover the factors controlling their orientation. We show that the concentration of precursors, that is, the ratio between sulfur and molybdenum oxide (MoO3), plays a key role in the size and orientation of seeds, subsequently controlling the orientation of MoS2 monolayers. High S/MoO3 ratio is needed in the early stage of growth to form small seeds that can align easily to the substrate lattice structures, while the ratio should be decreased to enlarge the size of the monolayer at the next stage of the lateral growth. Moreover, we show that the seeds are actually crystalline MoS2 layers as revealed by high-resolution transmission electron microscopy. There exist two preferred orientations (0° or 60°) registered on sapphire, confirmed by our density functional theory simulation. This report offers a facile technique to grow highly aligned 2D TMDCs and contributes to knowledge advancement in growth mechanism.

PM-IRRAS Determination of Molecular Orientation of Phosphonic Acid Self-Assembled Monolayers on Indium Zinc Oxide.

Self-assembled monolayers (SAMs) of phosphonic acids (PAs) on transparent conductive oxide (TCO) surfaces can facilitate improvement in TCO/organic semiconductor interface properties. When ordered PA SAMs are formed on oxide substrates, interface dipole and electronic structure are affected by the functional group properties, orientation, and binding modes of the modifiers. Choosing octylphosphonic acid (OPA), F13-octylphosphonic acid (F13OPA), pentafluorophenyl phosphonic acid (F5PPA), benzyl phosphonic acid (BnPA), and pentafluorobenzyl phosphonic acid (F5BnPA) as a representative group of modifiers, we report polarization modulation-infrared reflection-absorption spectroscopy (PM-IRRAS) of binding and molecular orientation on indium-doped zinc oxide (IZO) substrates. Considerable variability in molecular orientation and binding type is observed with changes in PA functional group. OPA exhibits partially disordered alkyl chains but on average the chain axis is tilted ∼57° from the surface normal. F13OPA tilts 26° with mostly tridentate binding. The F5PPA ring is tilted 23° from the surface normal with a mixture of bidentate and tridentate binding; the BnPA ring tilts 31° from normal with a mixture of bidentate and tridentate binding, and the F5BnPA ring tilts 58° from normal with a majority of bidentate with some tridenate binding. These trends are consistent with what has been observed previously for the effects of fluorination on orientation of phosphonic acid modifiers. These results from PM-IRRAS are correlated with recent results on similar systems from near-edge X-ray absorption fine structure (NEXAFS) and density functional theory (DFT) calculations. Overall, these results indicate that both surface binding geometry and intermolecular interactions play important roles in dictating the orientation of PA modifiers on TCO surfaces. This work also establishes PM-IRRAS as a routine method for SAM orientation determination on complex oxide substrates.