Even before deposition onto solid substrates, Langmuir films (monolayers at the air-liquid interface) can be investigated in a number of ways.

Langmuir film investigations

Surface pressure investigations

Surface pressure-area isotherms can reveal to us the average orientation of the molecules at various stages of compression as well as the surface pressures at which a tightly packed film has been reached. The average cross-sectional area per molecule when tightly packed can also be measured to determine the behaviour of any unusual, custom made molecules.

Monolayer stability investigations can also be performed to test the surface tension lowering properties of materials. Testing can be performed by holding films at a chosen target surface pressure or by holding at a constant area and observing the area or surface pressure versus time profile. Materials such as DPPC have been found to reduce the surface tension of water nearly to zero and to remain stable for very long periods of time.

The rates of adsorption of mixtures of materials to the air-water interface when dispensed from beneath the surface, within the subphase, can be investigated by measuring the surface pressure versus time. Vapour exposure from above the monolayer can also be monitored in the form of constant area or constant surface pressure plots.

Microscopy

Nima microscopy troughs enable the researcher to perform optical microscopy of the monolayer while being illuminated from beneath through a sapphire window. Inverted microscopes can also be used underneath the trough when using Inverted Microscopy Langmuir Troughs. This keeps the upper surface of the monolayer free of obstruction so that experiments can be performed on the monolayer while still being imaged. For example, illumination of a monolayer of photoluminescent material, from above, using very low wavelength light, prevents the absorption of UV light that would take place when illuminating from beneath.

UV-Visible Absorbance Spectroscopy

UV-Visible Absorbance Spectroscopy can be performed using a Nima Microscopy Trough. It is possible to examine the effects of neighbouring molecule interactions on the electronic structure of the film by observing the changes to the light absorption in the absorbance spectrum, even in the UV region.

Brewster Angle Microscopy

Imaging and magnification of transparent molecular material at the water surface is possible using Brewster Angle Microscopy. The evolution of monolayer film domain structure during compression cycles can be visualised. For more information on this see the Imaging monolayers using Brewster Angle Microscopy section. A compact and portable version of Brewster Angle Microscope is available from Nima. Please see the Nima MicroBAM page for more details.

Surface Potential measurements

Using a Nima surface potential sensor it is possible to observe alignment of the dipoles of molecules within a monolayer during compression. As the molecules orientate themselves in an ordered fashion during compression, a large enhancement of the electric field at the surface of the film can be observed.

More sophisticated techniques

There are even research groups that perform highly sophisticated X-ray and neutron reflectometry experiments on Langmuir films on Nima Langmuir and LB Deposition Troughs. These enable detailed investigations into the structral properties of monolayers.

Solid thin film investigations

The number of possible investigations that may be performed once monolayers are transferred on to solid substrates is almost limitless. Perhaps surprisingly, not all nanoscale film research requires a large budget. Some nanoscale film property investigations can even be performed without the need for any additional expensive equipment.

Redispersion tests

Changes to the chemical structure of thin films can easily be made by heating the samples. By heating a batch of identical deposited films to a range of temperatures it is possible to determine the critical temperature at which the hydrophobic, organic component of a nanostructure breaks its bond and desorbs from the film. This was successfully performed on a set of thiol encapsulated gold nanoparticle films that had been deposited on a Nima LB trough using the LS deposition technique. After heating, each sample was rinsed in a small amount of chloroform to redisperse the deposited material back into a solution. These solutions were then respread on a Langmuir trough and the isotherm positions related to the quantity of material dislodged by the solvent. Higher temperatures were found to prevent redispersion of the nanoparticles above 140˚C, the temperature found to correspond with desorption of the hydrophobic thiol coating molecules. Later on, at much greater cost, X-ray Photoelectron Spectroscopy chemical analysis was employed and showed the same result!

Water droplet contact angle tests

A complimentary set of tests could be to observe the contact angles of pure water droplets on film surfaces that have received different heat treatments. There should be a good agreement between the extent to which chloroform can dislodge material from the substrate and the change in contact angle of water droplets on the surface.

Water droplet contact angle tests could also reveal much about the surface properties of solid film samples that may have been finished with different top-layers of deposited material. This can demonstrate just how significant the interfacial monolayer is in influencing the properties of a thick sample.

Electrical conductivity tests

Simple electrical conductivity tests can be made of monolayers deposited on glass or other interdigitated array substrates using basic voltage sweeping apparatus or by simply taking resistance measurements at a fixed voltage. Metallic nanoparticle films can be deposited as relatively thick films (still less than 100nm thick) and heated to remove all the hydrophobic, organic content. If sufficient material was deposited, direct electrical pathways will be created across the sample transforming the film to metallic conductivity behaviour. An interesting starter project can be devised to model the expected initial film thickness of coated metallic nanoparticle films in order for them to create a continuous metallic film after heating.

More sophisticated solid thin film investigation techniques

Many more sophisticated analysis techniques are available at a broad range of prices.

UV-Visible absorbance spectroscopy

UV-Visible Absorbance Spectroscopy is a very popular technique for investigating LB film properties. When deposited on glass or quartz, LB films have high visible transmission as they are only a few nanometres thick. Even films built up from a large number of layers allow a significant amount of light to pass through. Some molecules, such as porphyrins, have very distinctive peaks in their absorbance spectrum that change upon exposure to particular gasses. Because LB films are so thin, the response can be very quick, causing absorbance peaks to disappear and new ones to form in just a few seconds.

Surface plasmons in metallic nanoparticles give rise to a broad absorbance peak. The wavelength position of this peak is highly sensitive to many factors, some controlled in the nanoparticle synthesis stages and some controlled after depositing a thin film. A striking change has been seen during heating of nanoparticle thin films causing a dramatic colour change of the sample as tighter packing of the particles followed by fusion and cluster growth occurred. Different encapsulating molecules, particle sizes, heating times and film thicknesses were found to have noticeable effects on the behaviour of the absorbance spectra with changing temperature.

Atomic Force Microscopy

Deposited film structure and thickness data are often desirable. Atomic Force Microscopy (AFM) gives very high resolution surface topography visualisations. Line profiles across exposed substrate regions can also provide accurate film thickness measurements. AFM recordings taken before and after heat treatments and gas/vapour exposure can reveal much information about the mechanisms that may have been observed during other experiments.

Ellipsometry

Ellipsometry is a relatively simple technique for the determination of the optical properties of a material. If these values are assumed, Ellipsometry is a powerful technique used for the determination of thin film thickness. When applied to a range of sample thicknesses it can reveal the consistency of monolayer deposition quality from the first to the last layer. When used in conjunction with a mass determination technique such as QCM it can even be a necessary technique for modelling the exact dimensions of complex nanostructures.

X-ray Photoelectron Spectroscopy

X-ray Photoelectron Spectroscopy (XPS or sometimes ESCA from Electron Spectroscopy for Chemical Analysis) is an ideal technique for understanding any chemical changes that are believed to have taken place within thin films. Although originally intended for the analysis of surfaces of bulk materials, the probing depth of the instruments is typically several nanometres, often the full thickness of a typical LB sample. LB films are often so thin that they might as well be considered as being entirely composed of surface molecules. XPS can identify chemical elements and bond structures before and after modifications to the sample. XPS can also be very useful to identify the presence of particular molecules used during the synthesis of exotic materials, whether they are intentionally included, unintentionally remaining or newly introduced by contamination. The by-products of reactions during exposure of LB films to vapours can also be examined with XPS.

Gas/vapour exposure

Gas/vapour exposure of samples can also be applied while the electrical conductivity, UV-Visible absorbance spectrum and even the mass (by QCM) is being recorded.

Transmission Electron Microscopy

Optically dense, inorganic materials can be clearly imaged using Transmission Electron Microscopy (TEM) when they are deposited on carbon grid substrates. A monolayer of 4nm diameter gold nanoparticles have been clearly observed to adopt a hexagonal close packing structure using this technique.

Quartz Crystal Microbalance

Quartz Crystal Microbalance (QCM) is a technique employed for measuring the mass of any amount of material deposited on a highly sensitive electrode region of an oscillating quartz crystal. As the sensitivity of QCM has been greatly improved since its introduction, the technique might more properly be described as being a nano-balance as it can be used to measure a small number of nanograms with errors typically below 5 nanograms. This means that the mass build up from LB or LS deposited layers can be recorded and compared to modelled quantities to determine the quality of transfer during deposition. QCM substrates are robust enough to withstand high temperatures so can also be used to measure thin film masses before and after heating, to track molecular desorption.

X-ray Reflectometry

X-ray Reflectometry can be used to investigate the layer structure of multilayer films on silicon wafer substrates. The layer spacing (and hence layer thickness) can be calculated from Bragg peak separation and the transfer quality/layer roughness can be inferred from the level of definition in the interference fringes caused by layer boundaries.

Neutron Reflectometry

Neutron Reflectometry can also probe the thin film structure. Because the beam penetrates further than X-rays, it can provide information on the lateral structure and reveal inhomogeneities with a resolution of a fraction of a nanometre.

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