M.Tech U III The effect of chemistry of nanostructures

Unit-III  The effect of chemistry of nanostructures: Modification of nanoparticles, Langmuir Blodgett films, Self assembled surface films, Binding of molecules on solid substrate surfaces, Molecular nanostructures, Strategies of molecular construction, Synthetic supramolecules.

 

Modification of nanoparticles

Nanoparticles and nanocomposites are used in a wide range of applications in various fields, such as medicine, textiles, cosmetics, agriculture, optics, food packaging, optoelectronic devices, semiconductor devices, aerospace, construction and catalysis. Nanoparticles can be incorporated into polymeric nanocomposites. Polymeric nanocomposites consisting of inorganic nanoparticles and organic polymers represent a new class of materials that exhibit improved performance compared to their microparticle counterparts. It is therefore expected that they will advance the field of engineering applications. Incorporation of inorganic nanoparticles into a polymer matrix can significantly affect the properties of the matrix. The resulting composite might exhibit improved thermal, mechanical, rheological, electrical, catalytic, fire retardancy and optical properties. The properties of polymer composites depend on the type of nanoparticles that are incorporated, their size and shape, their concentration and their interactions with the polymer matrix. The main problem with polymer nanocomposites is the prevention of particle aggregation. It is difficult to produce monodispersed nanoparticles in a polymer matrix because nanoparticles agglomerate due to their specific surface area and volume effects. This problem can be overcome by modification of the surface of the inorganic particles. The modification improves the interfacial interactions between the inorganic particles and the polymer matrix. There are two ways to modify the surface of inorganic particles. The first is accomplished through surface absorption or reaction with small molecules, such as silane coupling agents, and the second method is based on grafting polymeric molecules through covalent bonding to the hydroxyl groups existing on the particles. The advantage of the second procedure over the first lies in the fact that the polymer-grafted particles can be designed with the desired properties through a proper selection of the species of the grafting monomers and the choice of grafting conditions.

 

Langmuir, Langmuir-Blodgett, Langmuir-Schaefer Technique

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 film, Langmuir-Blodgett deposition, Langmuir-Schaefer deposition and multilayers obtained after repeated 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

  1. Frame
  2. Barriers
  3. Trough top
  4. Surface pressure sensor
  5. Dipping mechanism (LB option)
  6. Interface unit

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.

Surface pressure—area isotherms of a Langmuir film and molecules in different phases.

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:

Measurement

Information

Isotherms
Structure, area, interactions, phase transitions, compressibility, hysteresis
Isobar/Isochors
Stability
Surface potential*
Dissociation, orientation, interactions
Dilational rheology
Film viscoelastic properties
Kinetics
Polymerization and enzyme kinetics
Conductivity
Lateral conductivity
Environmental monitoring
pH* and temperature

*Optional

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.

Various LB deposition possibilities on hydrophobic and hydrophilic substrates.

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.

Alternate LB deposition with the KSV NIMA LB Trough Alternate

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.

Self-Assembled Monolayers

Self-assembled monolayers (SAMs) are ordered molecular assemblies formed by the adsorption of amphiphilic, surfactant-type molecules on solid surfaces. The substrate is generally immersed into a dilute solution of the film molecules or suitable precursors thereof and a monolayer film forms spontanously in a time span of a few minutes to a few hours.

A typical amphiphilic molecule (octadecyltrichlorosilane), consisting of a long-chain alkyl group (C18H37) and a polar head group (SiCl3), which forms SAMs on various oxidic substrates.

The driving force for the surface aggregation and self-assembly is i) a covalent bond formation of the film molecules with the substrate surface via suitable functional groups and ii) intermolecular, van der Waals-type interactions between the hydrocarbon chains of the film molecules.

Formation of Self-Assembled Monolayers

According to the type of film-substrate bonding, SAMs can be grouped into the following categories:

  • Organosulfur compounds (thiols, thioethers, disulfides etc.) on late transition metals (gold, silver, copper, mercury)
  • Organosilicon compounds (alkylchlorosilanes, alkylalkoxysilanes) on metal and nonmetal oxides (Al2O3, TiO2, SnO2, SiO2, glass)
  • Fatty acids on metal oxides (AgO, CuO, Al2O3)
  • Alkylphosphonic acids on metal and nonmetal surfaces primed with coordinating transition metal ions (Zr4+, Hf4+, Cu2+, Zn2+)

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