The Langmuir (L), Langmuir-Blodgett (LB) and Langmuir-Schaefer (LS) techniques enable fabrication and characterisation of single molecule thick films with control over the packing density of molecules. They also give the possibility to create multilayer structures with varying layer composition.
This page describes the instrumentation used to create Langmuir films, Langmuir-Blodgett films and Langmuir-Schaefer films. It also explains how the instrumentation provides control over the operations.
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.
Illustration: Langmuir film, Langmuir-Blodgett deposition, Langmuir-Schaefer deposition and multilayers obtained after repeated deposition.
The instrument used for Langmuir film fabrication (and characterisation) is called a Langmuir Trough or Langmuir film balance. The instrument is named Langmuir-Blodgett troughs when used for Langmuir-Blodgett or Langmuir-Schaefer deposition. All KSV NIMA Troughs are modular and can be used for both Langmuir film fabrication/characterization and Langmuir-Blodgett/Langmuir-Schaefer deposition) when equipped with the right modules. For that reason, KSV NIMA often refers to Langmuir and Langmuir-Blodgett troughs or just L and LB Troughs.
L and LB Troughs: how does it look like?
A Langmuir trough always come with a set of barriers (2), a Langmuir trough top (3*) and a surface pressure sensor (4). Software controlled barriers are placed at the interface and are used to compress the monolayer. The liquid phase(s) on which the monolayer is fabricated is contained in the trough top chamber. The trough top is often made of hydrophobic material for better subphase containment. The surface pressure sensor provides information on the monolayer packing density.
A Langmuir-Blodgett trough always come with a set of barriers (2), a Langmuir-Blodgett trough top (3*), a surface pressure sensor (4) and a dipping mechanism (5). The Langmuir-Blodgett trough top is also used to contain the liquid phase but in addition, there is a well in its center to make some 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.
Illustration: KSV NIMA L & LB Trough modules
KSV NIMA troughs are built on a flexible frame (1) enabling outstanding modularity. For instance, a Langmuir trough top can be easily replaced by a Langmuir-Blodgett trough top. The dipping mechanism can also be added/removed for easy switch between Langmuir and Langmuir-Blodgett configuration. In addition, all KSV NIMA troughs come with an interface unit (6) allowing direct control of the instrument as well as display of the key measurements.
Illustration: KSV NIMA Trough equipped with a Langmuir-Blodgett trough top. A Langmuir trough top is in all point similar except that there is no well.
For more information about the uniqueness of the KSV NIMA Trough design, please visit the Trough design page.
Langmuir film fabrication
A solution of an amphiphile is first prepared in a water insoluble solvent. The amphiphile molecules will be used as material to create a monolayer. The suphase (usually water) on which the monolayer will be fabricated is contained in the trough top chamber. The trough top is often made of hydrophobic material for better subphase 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 is formed at the air-water interface: a Langmuir film is created.
Software controlled barriers located at the interface are used to compress the monolayer. The monolayer molecules are perfectly contained between the barriers and the packing density increases as the barriers get nearer from each other. The surface pressure is monitored by the surface pressure sensor providing information on the packing density level.
Once compressed, a monolayer film can be considered to be a two-dimensional solid film with a surface area to volume ratio far above that of bulk materials. At these conditions, materials often yield fascinating new properties. A Langmuir Trough allows you to infer how particular molecules pack together while 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.
Illustration: surface pressure - area isotherms of a Langmuir film and molecules in different phases
Langmuir film characterisation
Langmuir films fabricated in a Langmuir Trough can be studied by analyzing surface pressure isotherms, isochors, as well as 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, hysterisis
||Dissociation, orientation, interactions
||Film viscoelastic properties
||Polymerisation and enzyme kinetics
||pH* and temperature
* available as an option
The KSV NIMA Microscopy Trough is a special kind of Langmuir Trough equipped with trough top which contains a sapphire window. The window allows high optical transmission down to a wavelength of 200nm (suitable for visible light or UV microscopy). Troughs suitable for both upright and inverted microscopes are available. More about Langmuir film microscopy here.
Popular complementary characterization techniques are Brewster Angle Microscopy (for film imaging), FTIR spectrometry such as PM-IRRAS (for determination of e.g. orientation and chemical composition) , Interfacial Shear Rheometry (for viscoelastic properties), Vibrational spectroscopy, UV-VIS absorbance spectroscopy, X-ray reflectometry, to name a few.
Langmuir-Blodgett film deposition
As mentioned above, a Langmuir film can also be transferred to a solid surface. Density, thickness and homogeneity properties are preserved when transferring the Langmuir film onto the sample, giving the possibility to make organized multilayer structures with varying layer composition. Compared to other organic thin film deposition techniques, LB is much less limited by the molecular structure of the functional molecule. This means that it is often the only technique that can be used for bottom-up assembly.
The LB deposition is traditionally carried out in the “solid” phase. The surface pressure is then high enough to ensure sufficient cohesion in the monolayer, e.g. the attraction between the molecules in the monolayer is high enough so that the monolayer does not fall apart during transfer to the solid substrate. This also ensures the build up of homogeneous multilayers. The surface pressure value that gives the best results depends on the nature of the monolayer and is usually established empirically. However, 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 subphase 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 subphase through the monolayer.
The monolayer can be held at a constant surface pressure, which is enabled by a computer controlled feedback system 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 using a Langmuir-Blodgett 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.
Illustration: various LB deposition possibilities on hydrophobic and hydrophilic substrates.
Some special Langmuir-Blodgett Deposition Trough such as the KSV NIMA Alternate-Layer Langmuir-Blodgett Deposition Trough are designed for fully automatic LB multi-deposition from 2 different Langmuir films (read dedicated alternate section for more information).
Illustration: Alternate Lb deposition with the KSV NIMA LB Trough Alternate
There are several parameters that affect on what type of LB film is produced. These are the nature of the spread film, the subphase 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 subphase between the deposition cycles. The quantity and the quality of the deposited monolayer on a solid support is measured by a so called 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. For ideal transfer the t.r. is equal to 1.
Langmuir-Blodgett film characterisation
Many properties of the LB film directly depend on the properties of the Langmuir film it was created from. The LB film can be characterized to obtain additional information and sometime check the quality of the deposition. Commonly used techniques are for instance PM-IRRAS (FTIR spectrometry), Surface Plasmon Resonance, Quartz Cristal Microbalance, Ellipsometry, Vibrational spectroscopy, UV-VIS absorbance spectroscopy, X-ray reflectometry etc.
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