Monolayers In Air Research Articles

Utilizing Ultraviolet Photons to Generate Single-Photon Emitters in Semiconductor Monolayers.

The understanding and controlled creation of atomic defects in semiconductor transition metal dichalcogenides (TMDs) are highly relevant to their applications in high-performance quantum optics and nanoelectronic devices. Here, we demonstrate a versatile approach in generating single-photon emitters in MoS2 monolayers using widely attainable UV light. We discover that only defects engendered by UV photons in vacuum exhibit single-photon-emitter characteristics, whereas those created in air lack quantum emission attributes. In combination with theoretical calculations, we assign the defects generated in vacuum to unpassivated sulfur vacancies, whose highly localized midgap states give rise to single-photon emission. In contrast, UV irradiation of the MoS2 monolayers in air results in oxygen-passivated sulfur vacancies, whose optical properties are likely governed by their pristine band-to-defect band optical transitions. These findings suggest that widely available light sources such as UV light can be utilized for creating quantum photon sources in TMDs.

Spatially selective reversible charge carrier density tuning in WS2 monolayers via photochlorination

Chlorine-doped tungsten disulfide monolayer (1L-WS2) with tunable charge carrier concentration has been realized by pulsed laser irradiation of the atomically thin lattice in a precursor gas atmosphere. This process gives rise to a systematic shift of the neutral exciton peak towards lower energies, indicating reduction of the crystal’s electron density. The capability to progressively tune the carrier density upon variation of the exposure time is demonstrated; this indicates that the Fermi level shift is directly correlated to the respective electron density modulation due to the chlorine species. Notably, this electron withdrawing process enabled the determination of the trion binding energy of the intrinsic crystal, found to be as low as 20 meV, in accordance to theoretical predictions. At the same time, it is found that the effect can be reversed upon continuous wave laser scanning of the monolayer in air. Scanning auger microscopy (SAM) and x-ray photoelectron spectroscopy (XPS) are used to link the actual charge carrier doping to the different chlorine configurations in the monolayer lattice. The spectroscopic analyses, complemented by density functional theory calculations, reveal that chlorine physisorption is responsible for the carrier density modulation induced by the pulsed laser photochemical reaction process. Such bidirectional control of the Fermi level, coupled with the capability offered by lasers to process at pre-selected locations, can be advantageously used for spatially resolved doping modulation in 1L-WS2 with micrometric resolution. This method can also be extended for the controllable doping of other TMD monolayers.

Vapor-Phase Carbenylation of Hard and Soft Material Interfaces.

This study describes the formation of functional organic monolayers on hard and soft interfaces via a vapor-phase carbene insertion into Si-H and C-H bonds. We demonstrate that functional diazirine molecules can be used to form monomolecular coatings on silicon, silicon nitride, and urethane-acrylate polymers under mild vacuum conditions and exposure to UV light. We investigate the molecular coverage and the long-term stability of the resulting monolayers in air, isopropanol, and water. Our results suggest that vapor-phase carbenylation can be used as a complementary technology to the traditional self-assembly, permitting functionalization of various passivated substrates with stable and functional molecular coatings under mild and scalable conditions.

Backside Anti-Reflecting Absorbing Layer Microscopy for in Situ Graphene Imaging and Modification

Two-dimensional nanomaterials and their association into 2-D heterostructures have been extensively studied for the last couple of years. Thus, the development of new, versatile, high-resolution visualization and placement techniques is highly desirable. It is well known that a single layer of graphene can be observed under optical microscopy on Si/SiO2 substrates because of interference effects when the thickness of silicon oxide is optimized.1,2However, differentiating monolayers from bilayers remains a challenge and advanced techniques like Raman mapping, Atomic Force or Scanning Electron Microscopy (AFM, SEM) are more suitable to observe monolayers of graphene. Raman mapping and AFM are relatively slow and SEM is operated in vacuum so that in all cases real-time experiments including chemical modifications are not accessible. Therefore, the development of techniques that combine the large scale and speed of SEM, the topological information of AFM and the simplicity optical microscopy may greatly facilitate the study of nanomaterials. In our group, we are interested in particular forms of graphene derivatives which are graphene oxide (GO) and reduced graphene oxide (r-GO). While a single layer of graphene absorbs 2.3% of the incident light,3 graphene oxide monolayer exhibits a weaker light absorption that makes it almost impossible to distinguish on Si/SiO2 surface. In 2007, Ruoff and co-workers were able to observe directly GO flakes using confocal microscopy; they even obtained higher contrast for GO than for graphene on Si/SiO2. For that they optimized a substrate by depositing silicon nitride layers from 60 to 70 nm on silicon.4 Here we introduce a novel optical microscopy technique based on the use of Anti-Reflecting and Absorbing (ARA) layers yielding to ultra-high contrast reflection imaging of graphene monolayers in air or in water. We name this technique “Backside Absorbing Layer Microscopy” (BALM) and we illustrate its efficiency by in-situ imaging graphene oxide (see Figure) and its chemical modification. Reference: (1) Blake, P.; Hill, E. W.; Castro Neto, A. H.; Novoselov, K. S.; Jiang, D.; Yang, R.; Booth, T. J.; Geim, A. K. Appl. Phys. Lett. 2007, 91, 063124. (2) Roddaro, S.; Pingue, P.; Piazza, V.; Pellegrini, V.; Beltram, F. Nano Lett. 2007, 7, 2707-2710. (3) Nair, R. R.; Blake, P.; Grigorenko, A. N.; Novoselov, K. S.; Booth, T. J.; Stauber, T.; Peres, N. M. R.; Geim, A. K. Science 2008, 320, 1308. (4) Jung, I.; Pelton, M.; Piner, R.; Dikin, D. A.; Stankovich, S.; Watchrotone, S.; Hausner, M.; Ruoff, R. S. Nano Lett. 2007, 7, 3569-3575. Figure 1

First generation newkome-type dendrimer as solubility enhancer of antitumor benzimidazole carbamate

A first generation dendrimer was evaluated as solubility enhancer of the antitumor compound methyl (5-[propylthio]-1H-benzimidazol-2-yl) carbamate. The dendrimer possess carboxylic acid as terminal groups, which provide high water solubility and a low cytotoxic character. The drug-dendrimer association significantly increases active compound solubility in water, avoiding aggregation. The formulation is stable several weeks in a wide temperature range. The formation of Langmuir monolayers in air–water interface of dendrimer-active compound blends evinces the viability of the complex to generate films for surface-mediated drug delivery systems.

Interfacial Fresnel Coefficients and Molecular Structures of Model Cell Membranes: From a Lipid Monolayer to a Lipid Bilayer

During the model membrane formation process from a lipid monolayer of 1,2-dipalmitoyl-sn-glycero-3-phospho-(1′-rac-glycerol) (sodium salt) (DPPG) in air to a lipid bilayer of DPPG/deuterated-DPPG (dDPPG) on water, the intensity of sum frequency generation (SFG) vibrational signals from the DPPG molecules increased by ∼34 times. The increased signal intensity could be caused by inherently different molecular ordering of lipid molecules in the monolayer/bilayer or by optical effects induced by different contacting mediums (prism in air and prism contacting water) (see Figure 2). We resolve the two possibilities by analyzing tilt angles of the methyl groups at DPPG hydrophobic ends for the monolayer/bilayer which reflect the molecular ordering information and by evaluating the Fresnel coefficients which reflect the contacting medium-induced optical effect. DPPG molecules were more ordered after transformation from a lipid monolayer in air to a lipid bilayer on water, which induced a slight signal increase. T...

Ignition of Fully Dense Nanocomposite Thermite Powders by an Electric Spark

Electrostatic discharge ignition of nanocomposite thermites prepared by Arrested Reactive Milling was investigated. The powders 2Al–3CuO and were placed in a sample holder and subjected to an electrostatic discharge from a discharging capacitor. Their optical emission was monitored. Two ignition modes were detected: immediate ignition of individual particles and delayed ignition of a cloud of aerosolized powder. Experiments addressed the effects of material composition and morphology, aging, energy input, layer thickness, and environment on the ignition mode and ignition delays. Prepared thermites were blended with pure metal powders to study ignition of such blends. Individual particle ignition was observed for samples prepared as monolayers in air and argon and for any samples in vacuum. For thicker samples in air and in argon, a delayed powder ignition mode was observed. The delays varied from to several milliseconds. Powders of 2Al–3CuO had shorter ignition delays compared with . For both materials, delays decreased for the powders prepared using longer milling times. For 2Al–3CuO, aged powders ignited after longer delays compared with the freshly prepared powders; however, shorter delays were observed for the aged . Electrostatic discharge energy and layer thickness (except for a monolayer) did not affect the ignition delay. Blending nanocomposite thermites with Al and Ti powders reduced their electrostatic discharge ignition sensitivity and caused longer ignition delays.

Dynamics of Molecular Monolayers with Different Chain Lengths in Air and Solvents Probed by Ultrafast 2D IR Spectroscopy

The ultrafast dynamics of functionalized alkylsilane monolayers with two different alkyl chain lengths, C11 and C3, are studied by 2D IR vibrational echo spectroscopy. Terminal sites of the monolayers are functionalized with an IR probe, tricarbonyl (1,10-phenanthroline) rhenium chloride (RePhen(CO)3Cl), to report on the structural dynamics of monolayer-air and monolayer-solvent interfaces. Frequency–frequency correlation functions (FFCF) of symmetric CO stretching mode were extracted from 2D IR spectra. The FFCF provides information on the time evolution of surface structures that contribute to the inhomogeneously broadened IR absorption spectrum of the CO stretch by quantifying spectral diffusion. To elucidate the detailed structural dynamics with accurate time constants, FFCF decays were monitored to 60 ps, a major improvement over prior experiments. Without the presence of solvents, C3 monolayers have significantly slower spectral diffusion (66 ps) than C11 monolayer (38 ps). This difference supports the previous postulate that spectral diffusion associated with the molecular monolayer in air involves the structural dynamics of the tethering alkyl chains. With the C11 and C3 monolayers immersed in dimethylformamide (DMF), in which RePhen(CO)3Cl is soluble, the FFCFs of both samples display biexponential decays with the time constants of 5.6 and 43 ps for C11 and 5.9 and 63 ps for C3. The slower time constants are in good agreement with the spectral diffusion time constants observed in the absence of solvent, indicating that these slower components still reflect the monolayers’ dynamics, which are not affected by the presence of the solvent. Because the faster time constants were independent of chain length and similar to that of the RePhen(CO)3Cl headgroup in bulk DMF solution, they are attributed to the dynamics of the interfacial DMF molecules. When monolayers were immersed in hexadecane, in which RePhen(CO)3Cl is not soluble, slower dynamics were again observed for the C3 monolayer than the C11 monolayer. The orientational dynamics of the IR probes were studied using polarization selective heterodyne detected transient grating experiments as well.

Flash sample heating for scanning tunneling microscopy: Desorption of 1-octanethiolate self-assembled monolayers in air

The authors have developed a method of performing scanning tunneling microscopy experiments with a sample that can be controllably heated and quickly cooled. Temperatures in excess of 100 °C are achievable, and the same scanning area can be imaged multiple times before and after repeated heating cycles. This opens up for study any physical process or chemical reaction where the reactants, products, and/or intermediates can be kinetically trapped on a conductive surface at room temperature. As a demonstration of this approach, the authors have investigated desorption from 1-octanethiolate self-assembled monolayers on Au(111).

Volume Transition and Adhesion Force of Nanosized Bifunctional Spherical Polyelectrolyte Brushes Observed by Dynamic Light Scattering and Atomic Force Microscopy

Stimuli-responsive nanoparticles have attracted considerable interest due to their enormous potential applications. In this paper, nanosized bifunctional spherical polyelectrolyte brushes (BSPBs) were prepared through grafting random copolymer chains from N-isopropylacrylamide (NIPA) and acrylic acid (AA) onto polystyrene core by photoemulsion polymerization. The pH- and thermo-responses of the BSPB with different ratio of NIPA and AA were investigated by dynamic light scattering. Atomic force microscopy (AFM) was employed to in situ monitor the pH response of the morphology and surface adhesion of BSPB as well as to visualize BSPB individually or as monolayer in air and liquid. The brush-like shell was illegible in electron microscopy images but clearly observed in AFM images. The results demonstrated that AFM was a powerful tool for in situ observing the stimulus-response of brushes on the surface of nanoparticles.

On the Stability of Oligo(ethylene glycol) (C11EG6OMe) SAMs on Gold: Behavior at Elevated Temperature in Contact with Water

In this study the temperature dependent confor-mation of hexa(ethylene glycol) self-assembling monolayers (SAMs) under aqueous conditions (in situ) is investigated. To this end characteristic absorption modes in the fingerprint region (1050-1500 cm(-1)) were monitored with real-time polarization modulation infrared spectroscopy. We found a temperature induced conformational change from predominantly helical to helical/all-trans. The process may be divided into two temperature regimes. Up to 40 °C the process is reversible after drying the monolayers in air and successive reimmersion in water, indicating a strong binding of the water molecules to the SAM. At higher temperatures, the conformational change is irreversible. Additionally, a rapid change to a larger mode width and a shift of the mode position to higher wavenumbers (blue-shift) at about 50 °C indicates structural changes caused by decreasing crystallinity of the SAM. While the conformational changes up to 40 °C are supposed to originate from an increased conformational freedom in combination with a stronger interaction with water molecules, the irreversibility and rapid change of mode characteristics at higher temperatures indicate chemical degradation. Complementary measurements in air show a fast and virtually complete reversibility up to 40 °C underlining the effect of the interaction of the ethylene glycol moiety with water. At temperatures above 50 °C modes indicating ester and formate groups appear, supporting the idea of chemical degeneration. Moreover, the temperature behavior is coverage dependent. At incomplete coverage the structural order of the SAM starts decreasing at lower temperatures. This study shows, that the conformational and structural change of hexa(ethylene glycol) SAMs at elevated temperature is an interplay of conformational changes of the SAM, its interaction with water and at higher temperatures its chemical degradation. Our experiments also underline the importance of the in situ analysis on the film structure.

The Structure of Oleamide Films at the Aluminum/Oil Interface and Aluminum/Air Interface Studied by Sum Frequency Generation (SFG) Vibrational Spectroscopy and Reflection Absorption Infrared Spectroscopy (RAIRS)

The structure of oleamide (cis-9-octadecenamide) films on aluminum has been investigated by sum frequency generation vibrational spectroscopy (SFG) and reflection absorption infrared spectroscopy (RAIRS). Three different film deposition strategies were investigated: (i) films formed by equilibrium adsorption from oleamide solutions in oil, (ii) Langmuir-Blodgett films cast at 1 and 25 mN m(-1), (iii) thick spin-cast films. Both L-B and spin-cast films were examined in air and under oil. The adsorbate formed in the 1 mN m(-1) film in air showed little orientational order. For this film, the spectroscopic results and the ellipsometric thickness point to a relatively conformationally disordered monolayer that is oriented principally in the plane of the interface. Direct adsorption to the metal interface from oil results in SFG spectra of oleamide that are comparable to those observed for the 1 mN m(-1) L-B film in air. In contrast, SFG and RAIRS results for the 25 mN m(-1) film in air and SFG spectra of the spin-cast film in air both show strong conformational ordering and orientational alignment normal to the interface. The 25 mN m(-1) film has an ellipsometric thickness almost twice that of the 1 mN m(-1) L-B film. Taken in combination with the spectroscopic results, this is indicative of a well packed monolayer in air in which the hydrocarbon chain is in an essentially defect-free extended conformation with the methyl terminus oriented away from the surface. A similar structure is also deduced for the surface of the spin-cast film in air. Upon immersion of the 25 mN m(-1) L-B film in oil the SFG spectra show that this film rapidly adopts a relatively disordered structure similar to that seen for the 1 mN m(-1) L-B film in air. Immersion of the spin-cast film in oil results in the gradual disordering of the amide film over a period of several days until the observed spectra become essentially identical to those observed for direct adsorption of oleamide from oil.

Thermal Response of Langmuir-Blodgett Films of Dipalmitoylphosphatidylcholine Studied by Atomic Force Microscopy and Force Spectroscopy

The topographic evolution of supported dipalmitoylphosphatidylcholine (DPPC) monolayers with temperature has been followed by atomic force microscopy in liquid environment, revealing the presence of only one phase transition event at ∼46°C. This finding is a direct experimental proof that the two phase transitions observed in the corresponding bilayers correspond to the individual phase transition of the two leaflets composing the bilayer. The transition temperature and its dependency on the measuring medium (liquid saline solution or air) is discussed in terms of changes in van der Waals, hydration, and hydrophobic/hydrophilic interactions, and it is directly compared with the transition temperatures observed in the related bilayers under the same experimental conditions. Force spectroscopy allows us to probe the nanomechanical properties of such monolayers as a function of temperature. These measurements show that the force needed to puncture the monolayers is highly dependent on the temperature and on the phospholipid phase, ranging from 120 ± 4 pN at room temperature (liquid condensed phase) to 49 ± 2 pN at 65°C (liquid expanded phase), which represents a two orders-of-magnitude decrease respective to the forces needed to puncture DPPC bilayers. The topographic study of the monolayers in air around the transition temperature revealed the presence of boundary domains in the monolayer surface forming 120° angles between them, thus suggesting that the cooling process from the liquid-expanded to the liquid-condensed phase follows a nucleation and growth mechanism.

Thermal decomposition of perfluorodecylsiloxane self-assembled monolayers in air

The thermal decomposition of perfluorodecylsiloxane self-assembled monolayers (SAMs) in air has been studied using atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and contact angle analysis. It is found that the monolayers are stable in air up to about 300 °C. Above 350 °C the monolayers primarily decompose through C C bond cleavage, with a gradual reduction in chain length.

Alkanethiols on Platinum: Multicomponent Self-Assembled Monolayers

We have studied the formation of self-assembled monolayers (SAMs) of n-alkanethiols on platinum thin films using X-ray photoelectron spectroscopy (XPS), reflection-absorption infrared spectroscopy (RAIRS), spectroscopic ellipsometry (SE), and contact angle (CA) measurements. Specifically, SAMs of 1-hexanethiol, 1-dodecanethiol, and 1-octadecanethiol were grown on polycrystalline Pt films, and the effects of Pt surface preparation, deposition conditions, and solvent treatments on the initial quality and stability of the monolayer in air were investigated. The SAMs prepared under ambient conditions on piranha-cleaned and UV/ozone-cleaned substrates were compared to monolayers formed on template-stripped Pt in an inert atmosphere. We found that alkanethiols deposited from 1 mM ethanolic solutions on piranha-cleaned Pt formed densely packed monolayers in which alkyl chains were oriented close to the surface normal. Stored in the laboratory ambient, these monolayers were unchanged over about 1 week but were largely oxidized in about 1 month. No evidence was found of molecules being weakly bound within the monolayer or having undergone C-S bond scission; however, three distinct sulfur states were observed for all samples in the XPS of the S 2p region. The lowest- and highest-binding-energy components are assigned to alkylthiolate and partially oxidized alkylthiolate species, respectively. The remaining S 2p component (approximately one-third of the sulfur layer), intermediate in binding energy between the other two components, is attributed to a chemisorbed species with a S binding configuration distinct from the majority alkylthiolate: for example, S bound to Pt bound to O, S with a different Pt coordination number, or S in an adsorbed disulfide.

Structural stability study of protein monolayers in air.

The assessment of the folding and of the structural stability of a protein in air, upon immobilization in the solid state, represents a critical point from both a fundamental point of view and for the development of solid state nanobioelectronics. The recent demonstrations by Rinaldi et al. [R. Rinaldi et al., Adv. Mater. 14, 1453 (2002); Appl. Phys. Lett. 82, 472 (2003); Ann. (N.Y.) Acad. Sci. 1006, 187 (2003)] of protein-based solid state devices and transistors working in air have raised an intriguing question about the behavior of a biomolecule under nonphysiological conditions. The operation principle of the realized devices is based on the physiological electron transfer function of the metalloprotein azurin. This means that azurin should retain its shape and functionality also in the solid state when utilized in air and at room temperature. In this Brief Report, we prove this claim by analyzing the conformational state of the azurin monolayers developed for such devices by means of intrinsic fluorescence spectroscopy. We show that the immobilization of azurins in the solid state under nonliquid conditions, by means of a specific chemisorption process, does not necessarily lead to protein denaturation. This result is of great importance because it opens up interesting perspectives for the development of solid state hybrid nanodevices for electronic applications requiring nonliquid environments.

Thermal Decomposition of Alkylsiloxane Self-Assembled Monolayers in Air

The thermal decomposition of alkylsiloxane self-assembled monolayers in air has been studied using atomic force microscopy, X-ray photoelectron spectroscopy, and contact angle analysis. It is found that the monolayers are stable in air up to about 200 °C. Above 220 °C the monolayers primarily decompose through C−C bond cleavage, with a gradual reduction in chain length. The siloxane headgroups remain on the surface following the decomposition of hydrocarbon fragments in the monolayers.

Pull-off forces measured between hexadecanethiol self-assembled monolayers in air using an atomic force microscope: analysis of surface free energy

The pull-off forces were measured between hexadecanethiol monolayers, self-assembled on gold-coated silicon nitride cantilever tip and silicon wafer, using atomic force microscopy (AFM). The blind tip reconstruction technique was used for determination of the curvature of the AFM tip. The measured pull-off force value remained practically unaffected by a variation of the maximum applied load in a range of 5–80 nN. This result suggests that the use of continuum elastic contact mechanics in the analysis of AFM pull-off force measurements is not as straightforward as usually assumed in the literature reports on similar systems. The surface free energy of hexadecanethiol monolayer on a gold film of γ = 24–27 mJ/m2 was calculated based on the pull-off forces (F), measured using R = 60–80 nm radius tips, and next applying the Derjaguin approximation: F = 4πRγ. These γ-values were found to match the surface free energy value calculated from contact angle data and using Lewis acid–base interfacial free energy theory.

Preparation of Self-Assembled Monolayer (SAM) Modified Silver Substrates for Surface Enhanced Raman Scattering (SERS)

Abstract Self-assembled thiolate and sulfinate monolayers on smooth silver substrates were successfully characterized with SERS by applying for a selective etchant. The enhancement effect (EF) observed was comparable with those achieved by other techniques. The reusability of the resulted substrate was provided by UV photooxidation of the monolayer in air.

Nanometer-Scale Surface Properties of Mixed Phospholipid Monolayers and Bilayers

Mixed distearoylphosphatidylethanolamine (DSPE) and dioleoylphosphatidylethanolamine (DOPE) monolayers and bilayers have been deposited on mica using the Langmuir-Blodgett (LB) technique, as a model system for biomembranes. Investigation with atomic force microscopy revealed phase-separation for both monolayers in air and bilayers in water in the form of microscopic DSPE domains embedded in a DOPE matrix. For the monolayers in air, the step height measured between the higher DSPE phase and the lower DOPE phase was larger than expected from the molecular lengths, and a significant contrast in adhesion and friction was observed despite identical lipid end groups. This unexpected behavior resulted primarily from a difference in the film mechanical properties, the DOPE phase being inelastically deformed by the probe. For the bilayers in water, similar trends were found in terms of height, adhesion, and friction, but an additional short-range repulsive hydration/steric force over the DSPE phase contributed to the observed differences.