Langmuir, Langmuir-Blodgett and Langmuir-Schaefer Technologies
KSV NIMA products are based on a wide range of technologies. The Langmuir, Langmuir-Blodgett and Langmuir-Schaefer techniques enable fabrication and characterization of single molecule thick films with control over the packing density of molecules. Our range of products for characterization includes KSV NIMA ISR, KSV NIMA PM-IRRAS, KSV NIMA BAM and KSV NIMA SPOT to measure viscoelastic properties, molecular orientation, chemical composition and visualization of thin films.
The Langmuir (L), Langmuir-Blodgett (LB) and Langmuir-Schaefer (LS) techniques enable fabrication and characterization of single molecule thick films with control over the packing density of molecules. They also enable the creation of multilayer structures with varying layer composition.
Langmuir, Langmuir-Blodgett, Langmuir-Schaefer—what is the difference?
When a monolayer is fabricated at the gas-liquid or liquid-liquid interface, the film is named Langmuir film. A Langmuir film can be deposited on a solid surface and is thereafter called Langmuir-Blodgett film (in the case of vertical deposition) or Langmuir-Schaefer film (in the case of horizontal deposition). Langmuir-Schaefer is often seen just as a variant of Langmuir-Blodgett deposition.
Langmuir Troughs (or Langmuir film balance) are used for Langmuir film fabrication and characterization. Langmuir-Blodgett troughs are used for Langmuir-Blodgett or Langmuir-Schaefer deposition. All KSV NIMA Troughs are modular and when equipped with the right modules can be used for Langmuir film fabrication or characterization as well as Langmuir-Blodgett and Langmuir-Schaefer deposition.
The components of L and LB Troughs
Langmuir troughs include a set of barriers (2), a Langmuir trough top (3*) and a surface pressure sensor (4) as standard. The software-controlled barriers are placed at the interface and compress the monolayer. The trough top holds the liquid phase where monolayers are fabricated. The trough top is often made of hydrophobic material that improves sub-phase containment. The surface pressure sensor provides information about monolayer packing density.
Langmuir-Blodgett troughs include a set of barriers (2), a Langmuir-Blodgett trough top (3*), a surface pressure sensor (4) and a dipping mechanism (5) as standard. The Langmuir-Blodgett trough top holds the liquid phase and has a well in the center to allow space for solid substrate dipping through the monolayer. The dipping mechanism holds the solid substrate and enables controlled deposition cycle(s).
Please note that for Langmuir-Schaefer deposition, the Langmuir-Blodgett trough top is not always necessary and can in some cases be replaced by a Langmuir trough top.
KSV NIMA L & LB Trough modules
KSV NIMA troughs are built on a frame (1) that enables outstanding modularity; a Langmuir-Blodgett trough top can be easily switched with a Langmuir trough top. The dipping mechanism can also be added or removed for simple conversion between Langmuir and Langmuir-Blodgett configurations. All KSV NIMA troughs come with an interface unit (6) that controls the instrument and displays key measurements.
Langmuir film fabrication
Prepare the amphiphile molecules that will create a monolayer in a water insoluble solvent. The sub-phase, typically water, is held in the hydrophobic trough top that gives good sub-phase containment. When the amphiphile solution is deposited on the water surface with a microsyringe, the solution spreads rapidly to cover the available area. As the solvent evaporates, a monolayer forms at the air-water interface and a Langmuir film is created. The software-controlled barriers located at the interface then compress the monolayer until the surface pressure sensor indicates maximum packing density.
A compressed, monolayer film can be considered as a two-dimensional solid with a surface area to volume ratio far above that of bulk materials. In these conditions, materials often yield fascinating new properties. Experimentation using Langmuir troughs enables inference and understanding about how particular molecules pack when confined in two dimensions. The surface pressure-area isotherm can also provide a measure of the average area per molecule and the compressibility of the monolayer.
Langmuir film characterization
Langmuir films fabricated in a Langmuir trough can be studied by analyzing surface pressure isotherms, isochors, and other data measured with the trough or with a complementary characterization instrument.
KSV NIMA Langmuir troughs enable measurements of:
|Structure, area, interactions, phase transitions, compressibility, hysteresis|
|Dissociation, orientation, interactions|
|Film viscoelastic properties|
|Polymerization and enzyme kinetics|
|pH* and temperature|
KSV NIMA Microscopy Troughs are special troughs equipped with a sapphire window in the top. The sapphire window allows high optical transmission down to 200 nm, which is suitable for visible light or UV microscopy. Troughs suitable for both upright and inverted microscopes are available.
For more information about Langmuir film microscopy, see:
Popular complementary characterization techniques include: Brewster Angle Microscopy (for film visualization), FTIR spectrometry such as PM-IRRAS (for determination of orientation and chemical composition), Interfacial Shear Rheometry (for viscoelastic properties), Surface Potential Sensing (for determination of changes in packing and orientation), Vibrational spectroscopy, UV-VIS absorbance spectroscopy, and X-ray reflectometry.
For more information, see:
Langmuir-Blodgett film deposition
Langmuir films can be transferred to solid surfaces with preserved density, thickness and homogeneity of the sample. This allows the assembly of organized multilayer structures with varying layer compositions. Compared to other organic thin film deposition techniques, LB is less limited by the molecular structure of the functional molecule and is often the only technique that can be used for bottom-up assembly.
LB deposition is traditionally carried out in the ‘solid’ phase where surface pressure is high enough to ensure sufficient cohesion in the monolayer. This means that attraction between the molecules in the monolayer is sufficient to prevent the monolayer from falling apart during transfer to the solid substrate and ensures the build up of homogeneous multilayers. The surface pressure that gives the best results depends on the nature of the monolayer and is usually established empirically. Generally, amphiphiles can seldom be successfully deposited at surface pressures lower than 10 mN/m, and at surface pressures above 40 mN/m collapse and film rigidity often pose problems. When the solid substrate is hydrophilic (glass, SiO2 etc.) the first layer is deposited by raising the solid substrate from the sub-phase through the monolayer, whereas if the solid substrate is hydrophobic (HOPG, silanized SiO2 etc.) the first layer is deposited by lowering the substrate into the sub-phase through the monolayer.
Monolayers can be held at a constant surface pressure by a computer-controlled feedback between the surface pressure sensor and the compressing barriers. This is useful when producing LB films to guarantee the homogeneity of the film deposited.
In the case of Langmuir-Blodgett (LB) deposition, the solid substrate is dipped through the Langmuir film and extra space is required below the monolayer. It means the Langmuir film has to be fabricated with a LB-trough top with a sufficient well size for the substrate. The dipping mechanism holds the solid substrate and enables controlled deposition cycle(s). The Langmuir-Schaefer (LS) technique can be performed with a Langmuir trough top, as no additional depth is required below the monolayer.
Repeated deposition can be achieved to obtain well-organized multilayers on the solid substrate. LB and LS cycles can also be combined to obtain desired structures and thicknesses. The most common multilayer deposition is the Y-type multilayer, which is produced when the monolayer deposits to the solid substrate in both up and down directions. When the monolayer deposits only in the up or down direction the multilayer structure is called either Z-type or X-type. Intermediate structures are sometimes observed for some LB multilayers and they are often referred to be XY-type multilayers.
Some special LB deposition troughs such as the KSV NIMA Alternate-Layer Langmuir-Blodgett Deposition Trough are designed for fully automatic LB multi-deposition from two different Langmuir films.
There are several parameters that affect the type of LB film produced. These include: the nature of the spread film, the sub-phase composition and temperature, the surface pressure during the deposition and the deposition speed, the type and nature of the solid substrate and the time the solid substrate is stored in air or in the sub-phase between the deposition cycles. The quantity and the quality of the deposited monolayer on a solid support are measured by the transfer ratio (t.r.). This is defined as the ratio between the decrease in monolayer area during a deposition stroke, Al, and the area of the substrate, As. An ideal transfer has a t.r. that is equal to 1.
Langmuir-Blodgett film characterization
Many properties of LB films depend on the properties of the Langmuir film it was created from. LB films can be characterized for additional information and checked for the quality of the deposition. Commonly used techniques are include: PM-IRRAS (FTIR spectrometry), Surface Plasmon Resonance, Quartz Cristal Microbalance, Ellipsometry, Vibrational spectroscopy, UV-VIS absorbance spectroscopy, X-ray reflectometry etc.
For more information, see:
Rheology is a branch of science involving the study of material flow. Interfacial rheology concerns the unique two-dimensional systems formed between two immiscible phases, such as liquid/gas, and liquid/liquid. The stability of interface greatly depends on the viscoelastic properties of the interface, which makes it a crucial aspect for many industries where foams, emulsions and dispersions are used such as pharmaceuticals, foods, beverages, cosmetics and coatings.
Interfacial rheology is a challenging field of research because of the magnitude of forces in the interface is exceedingly small. Rotating ring and bicone methods have been developed, but they only work with macromolecular compounds at the interfaces. The KSV NIMA ISR utilizes a small magnetic probe that is moved with an oscillating magnetic field. The method reduces the inertia and enhances the sensitivity of the probe compared to the rotating ring method to enable the measurement of low molecular weight surface-active compounds.
Magnetized probe is moved at air-water or oil-water interface using magnetic field created by Helmholtz coil. The probe movement is recorded optically from above. The complex surface modulus is calculated from the strain and signal phase shift and can be divided into elastic and viscous properties of the thin film.
Interfacial Shear Rheology instruments use a Helmholtz coil to create a uniform magnetic field in the area of measurement. A Helmholtz coil uses two identical circular magnetic coils that are separated from each other with the same distance as the coil radius. This allows the magnetized probe to move in the measuring area by controlling the magnetic field. Since no mechanical connection is used the instrument sensitivity is dramatically increased to allow the measurement of very weak viscoelastic forces.
The measurement probes are designed to float easily on the liquid interface due to their lightweight and water repellent coating. The metal inside the probe is magnetized and the probe is ready to use. Since thin film viscoelasticity can vary between different samples there are different sizes of probes to match the experimental conditions.
The probe moves inside a glass channel that creates a small meniscus on both sides of the surface. This channel guides the probe in a straight line and ensures uniform flow geometry. To ensure the glass channel is centered every time a channel holder is used to guide it in the center of the trough. In this way, the magnetic probe easily set in the middle of the magnetic field. The camera and lens are mounted on a xyz-stage to simplify setup.
Dynamic measurement allows the definition of:
- Elastic (storage) modulus, G’
- Viscous (loss) modulus, G’’
- Dynamic interfacial viscosity, μs*
In dynamic rheological experiments three variables are measured: applied stress (σ), strain (γ) and phase angle (φ) between the stress and strain oscillation.
Dynamic viscoelastic modulus G* is obtained as a function of oscillation frequency (ω) from the measurement, and it can be separated into two components, elastic (or storage) modulus G’ and viscous (or loss) modulus G’’. In the measurement either stress
The dynamic testing includes three different measurement types:
- Frequency sweep allows measurement of the viscoelastic properties at different frequencies. This method is extremely useful since rheology is time dependent. It allows defining thin film rheological behavior in different time scales and provides information about the dominance of viscosity or elasticity.
- Single frequency measurement can be used to define the time dependency of viscoelastic properties. When combined with a KSV NIMA Langmuir trough the viscoelastic changes can be defined as a function of surface pressure.
- Amplitude sweep measurement allows defining the linear region of the viscoelastic film and allows defining suitable oscillation amplitude for different thin films. Shear thinning and thickening can also be detected with this method.
Creep testing allows the study of:
- Surface/interfacial viscosity, η
- Elastic moduli, G
- Relaxation times, τ
In creep compliance test mode, the instrument provides information to obtain whether the system behaves more like an ideal Newtonian liquid (dashpot model) or ideal elastic (spring model). Viscoelastic systems are more complex as they combine both elements. These can be modeled with Maxwell and Kelvin-Voigt models. In the creep compliance measurements constant stress (σ) is applied and related strain (γ) is measured.
In the Maxwell model the Elastic modulus, G is calculated from immediate response after stress is applied and the viscosity derives from the time dependent deformation (slope).
In the Kelvin-Voigt model both the spring and the dashpot are parallel and the strain response to applied stress is time dependent and nonlinear. The response can be separated to elastic, G, and viscous, η, components. From these components the relaxation time, τ, can be determined
In most cases the viscoelastic behavior needs to be modeled using different combinations of the Maxwell and the Kelvin-Voigt models, such as Standard Linear Solid model and Generalized Maxwell model.
Instead of strain compliance (J), which is often used in literature, it takes into account the used force.
In addition to the interfacial shear rheology, dilational rheology can be measured as complementary technique using a Langmuir trough. Please note that these techniques are not directly comparable since the surface packing density changes and absorption/desorption may occur in the dilational measurements.
Polarization Modulation Infrared Reflection Adsorption Spectroscopy (PM-IRRAS) is a powerful surface specific spectroscopic method for thin films and floating monolayers. Compared to more conventional IRRAS, the method utilizes the differences in reflectivity of interfaces for p-polarized (perpendicular to surface) and s-polarized (planar to surface) light. This makes even the characterization single molecule thin layers and molecule orientation possible.
In IR spectroscopy, photons of specific wavenumbers between 102 and 104 cm-1 induce a transition of the molecule under study from a vibrational ground state to an excited state. The molecule in the excited state returns quickly to the ground state by thermal relaxation, which is seen as molecular vibrations. This means that when IR radiation is transmitted or reflected from a sample (solid, liquid or gas) radiation is absorbed and molecular vibration is induced. Examination of the transmitted/reflected light reveals how much energy was absorbed at each wavelength. The amount and frequency that is absorbed can be correlated with the molecular structure of the sample, which gives each material its own spectral ‘fingerprint’. Additionally, functional groups have their characteristic frequencies where they absorb.
The technology of the KSV NIMA PM-IRRAS is based on a reflective IR measurement in combination with a polarization unit.
Fresnel equations help describe the amount of reflected (and transmitted) light when light is reflected from a surface. The incoming light can be split into a polarized component parallel to the plane of reflection and a component perpendicular to this plane (see figure below). By knowing that light with different polarizations interacts differently with the interface it is reflected from, it is possible to use polarization modulation to reduce the noise of reflective FTIR-measurements and to compensate for the water vapor absorption bands.
KSV NIMA PM-IRRAS uses a photo-elastic modulator to modulate the polarization of the light and the intensity modulation of the spectrometer (see figure below). The modulations that allow signals to be separated and collected by the detector have different frequencies.
The measured signal at the detector consists of a high frequency component based on the difference (Δ) in light intensities between the p- and s-polarized light in the photo-elastic modulator, and a low frequency component based on the sum (Σ) of these signals from the FTIR unit. The normalized differential reflectivity spectrum S is calculated from the collected difference (ΔR) and sum spectra (ΣR) of the detected intensities of the p- and s- polarized light.
The PM-IRRAS method allows determination of the molecular orientation of the functional groups and the whole molecule. In floating monolayers, the PM-IRRAS has a strong incident angle dependency and this can be used to determine the orientation. Also, the relative peak ratios of functional groups of known orientation can be used to determine the tilt of the molecules compared to the surface. An effect called ‘surface image selection rule’ that is present in PM-IRRAS performed on good conductors leads to enhancement of perpendicular and cancellation of planar dipoles in the spectra. This can be utilized to establish the molecular orientation by comparing peak intensities, or by calculating from a theoretical spectrum.
Changes in the PM-IRRAS signal intensity and position can be used to infer:
- Molecular absorption/desorption behavior and kinetics
- Molecular packing and orientation
- Phase transitions
- Different surface reactions in a thin film
- Griffits, P and Haseth, J., ‘Chemical Analysis 83: Fourier Transformation Infrared Spectrometry’ (1983), Wiley
- V. Zamlynny, I. Zawisza, J. Lipkowski, ‘Langmuir’ 19 (2003) 133
- Kosters, P., ‘FT-IR Spectroscopy of thin biological layers’ (2000), University of Twente
Brewster Angle Microscopy provides a perfect solution for non-invasive imaging of monolayers at the air-water interface. Real-time observation and recording of film structure enables dynamic activity to be captured.
A Brewster Angle Microscope (BAM) enables the visualization of Langmuir monolayers or adsorbate films at the air-water interface. BAM utilizes the reflection-free condition that occurs when p-polarized light is guided towards an air-water interface at a specific incident angle. This angle is called the Brewster angle, and is determined by Snell’s law and depends on the refractive indices of the materials in the system.
tan α = n2 / n1
- α — Brewster angle in radians
- n1 — Refractive index of air (≈1)
- n2 — Refractive index of water (≈1.33)
The Brewster angle for the air-water interface is 53°, and under this condition the image of a pure water surface appears black. Addition of material to the air-water interface modifies the local refractive index (RI) and this causes a small amount of light to be reflected and displayed within the image. The image displayed contains areas of varying brightness determined by the particular molecules and packing densities across the sampling area.
|Monolayer of DMPE during first-order phase transition, contrast in domains caused by long-range orientation order.|
|Stearic acid monolayer formation during compression.|
For more information, see:
KSV NIMA offers three different Brewster Angle Microscopy models:
The KSV NIMA Surface Potential Sensor instrument offers complementary data about the packing and orientations of surfactant molecules within a sampling region of a Langmuir film. The Surface Potential Sensor measures the potential difference above and below the film and is sensitive to the sum of all the individual dipole moments rather than measuring surface pressure as a change in the surface tension of the air-water interface.
KSV NIMA SPOT measures surface potential by the vibrating plate capacitor method:
Changes in surface pressure are only detected once a closely packed monolayer begins to form. However, changes in the surface potential can be measured as soon as dipolar molecules appear at the surface. As molecular orientation changes during compression, the alignment of the dipoles causes a large change in the surface potential. This is demonstrated in the plot below, which shows surface pressure-area (violet) and surface potential- area (light blue) isotherms of a monolayer of an antiparasitic drug on an air-buffer solution interface. An unusual surface pressure-area transition was observed at mean molecular area of 140 Å2, but no transition was observed in the surface potential-area isotherm. This suggests that the change was not a phase transition but instead that the drug could undergo aggregation, dimerization or conformational change at this mean molecular area.
For more information, see:
The KSV NIMA Surface Potential Sensor is a compact and highly sensitive characterization instrument.
Dip coating is the precision controlled immersion and withdrawal of any substrate into a reservoir of liquid for the purpose of depositing a layer of material. Many chemical and nanomaterial engineering research projects in academia and industry make use of the dip coating technique.
Many factors contribute to determining the final state of the dip coating of a thin film. A large variety of repeatable dip coated film structures and thicknesses can be fabricated by controlling many factors: functionality of the initial substrate surface, submersion time, withdrawal speed, number of dipping cycles, solution composition, concentration and temperature, number of solutions in each dipping sequence, and environment humidity. The dip coating technique can give uniform, high quality films even on bulky, complex shapes.
The dip coating technique is used to make thin films by self-assembly and with the sol-gel technique. Self-assembly can give film thicknesses of exactly one monolayer. The sol-gel technique creates films of increased, precisely controlled thickness that are mainly determined by the deposition speed and solution viscosity.
Self-assembly is the process where components spontaneously organize or assemble into more complex objects, typically by bouncing around in a solution or gas phase until a stable structure of minimum energy is reached.
Self-assembly is crucial to biomolecular nanotechnology, and is a promising method for assembling atomically precise devices. Components in self-assembled structures find their appropriate location based solely on their structural properties (or chemical properties in the case of atomic or molecular self-assembly). Self-assembly is not limited to the nanoscale and can be done at just about any scale, making it a powerful bottom-up assembly method.
Surfactant molecules can assemble into larger aggregates in solutions varying from round balls to circular rods and lamellar structures, whereas if a solid substrate is immersed in a liquid containing (functionalized) surfactant molecules a monolayer of these components can spontaneously form on the solid substrate either by physisorption, covalent binding or electrostatic interactions. A self-assembled monolayer (SAM) is a two-dimensional film, one molecule thick, covalently organized or assembled at an interface. The classical example of a SAM is the reaction of alkanethiols with a gold surface. Another example is the reaction of silanes with glass, quartz or SiO2 surfaces.
The sol-gel technique is deposition method widely used in material science to create protective coatings, optical coatings and ceramics, to name a few. This technique starts with the hydrolysis of a liquid precursor (sol) that undergoes polycondensation to gradually produce a gel. This gel is a bi-phasic system that contains both a liquid phase (solvent) and a solid phase (integrated network, typically polymer network). Step by step, the proportion of liquid is reduced. The rest of liquid can be removed by drying and can be coupled with a thermal treatment to tailor the material properties of the solid.
The layer-by-layer assembly (LBL) is a simple and relatively cheap method to deposit alternate layers of materials. Thin films are created by depositing alternatively layers of opposite charges, providing a high degree of control of the film thickness: negatively and positively charged layers are deposited successively until the desired thickness is reached.
KSV NIMA Dip Coaters are robust computer controlled instruments for precise thin film deposition.