random Flashcards
SAMS
-ordered molecular assemblies formed by the adsorption of an active surfactant on a solid surface.
-Order in these 2D systems produced by a spontaneous chemical synthesis at the interface, as the system approaches equilibrium.
-Prepared by immersing a substrate in the solution containing a ligand that is reactive toward the surface, or by exposing the substrate to the vapor of the reactive species.
materials for SAMS
-Substrate materials for SAMs: Au, Ag, Cu… (it shouldn’t form an oxide surface film that would interfere with the layering process)
-Layering materials for SAMs: thiols, sulfides and disulfides (must be capable of being adsorbed onto the substrate surface)
Aplicações de SAMS
ultra thin resist or etching mask, modification of surface
properties
Guided self-assembly:
basicamente padroes formados por um processo top-down seguidos de um processo bottom up
Guided self-assembly: surface topography
-Processo chato pq as nanoparticulas não “caem” para as cavidades visto que a força gravitica é muito reduzida a esta escala
-Acontece devido as forças de capilaridade do liquido em suspensão
-Uma vez na trap a força eletroestatica da suspensão ou forças de van de waals vão segurar a particula na cavidade
-Tambem pode ser feito por dip coating controlando a velocidade em que a amostra sai da soluçao e taxa de evaporação
random networks
spray coating, drop casting, spin coating or rod-coating of a solution
containing dispersed nanostructures
Atomic layer deposition:
ALD can be seen as “staking of SAM” – flexibility of materials to deposit, precise control of film thickness, pinhole-free films, conformal coatings, highly repeatable and scalable process
Atomic layer deposition (ALD), is a vapor-phase deposition technique for preparing ultrathin films with precise growth control. ALD is currently rapidly evolving, mostly driven by the continuous trend to miniaturize electronic devices.
The need for ALD
Miniaturization, new device functionalities, on flexible substrates or on top of 3D shapes → Requires deposition techniques offering thickness control, uniformity, conformability, low temperature deposition.
How to understand if we are getting good-quality films?
Ellipsometry – refractive index as an indication of film density
TEM
TEM is an analytical tool that allows detailed investigation of the morphology, structure, and local chemistry of metals, ceramics, polymers, biological materials and minerals. It also enables the investigation of crystal structures, crystallographic orientations through electron diffraction, as well as second phase, precipitates and contaminants distribution by x-ray and electron-energy analysis.
Accelerating voltage TEM
Higher accelerating voltages give higher resolution, but less
contrast. High accelerating voltages can also result in greater
specimen damage.
Electromagnetic lenses TEM
These consist of a coil of copper wires inside iron pole pieces. This field acts as a convex lens,
bringing off axis rays back to focus. Focal
length can be changed by changing the
strength of the current.
Condenser lens TEM
Illuminates the specimen.
Objective lens
–Forms initial image further magnified by
other lenses
– Responsible for focus
–The larger the aperture used the more
phase contrast
–The smaller the aperture the more
aperture contrast
Appertures TEM
–The condenser aperture controls the fraction of the beam which is allowed to hit the specimen. It therefore helps to control the intensity of illumination.
–The objective aperture is used to select which beams in the diffraction pattern contribute to the image, thus producing diffraction contrast.
–The selected area aperture is used to selected a region of the specimen from which a diffraction pattern is obtained.
Bright field TEM
Bright-field imaging is used for
examination of most microstructural
imaging
Dark field TEM
In dark field (DF) images, the direct beam is blocked by the
aperture while one or more diffracted beams are allowed to
pass the objective aperture. Since diffracted beams have strongly interacted with the specimen, very useful information is present in DF images, e.g., about planar defects, stacking faults or particle size.
TEM – Diffraction techniques
Spot Patterns-are created when electrons are diffracted in a single crystal region of a given specimen. Used for unknown phase identification and identification of crystal structure and orientation
Ring Patterns-created when electron diffraction occurs simultaneously from many grains with different orientations relative to the incident electron beam. Used to identify
unknown phases or characterize the crystallography of
a material.
TEM imaging – phase or mass contrast
Diffraction contrast/Phase - some grains diffract more strongly than others; defects may affect diffraction
Mass-thickness contrast - absorption/scattering. Thicker areas or materials with higher Z are darker
Bright field (stained polymers) TEM
In the case of polymer and biological samples, due low atomic number and similar electron densities, staining helps to increase the imaging contrast and improves the radiation damage. Staining agents work by selective absorption in one of the phases and tends to stain unsaturated C-C bonds.
Electropolishing
Sample is immersed in an electrolyte
and then subjected to a direct electrical
current.
* The sample is maintained anodic, with
the cathodic connection being made to
a nearby metal conductor.
* Anodic dissolution of sample creates
polished surface.
TEM can:
▪ Image morphology of samples,
▪ Analyze the composition and some bonding differences
▪ Perform several in-situ measurements.
▪ View frozen material
▪ Acquire electron diffraction patterns
TEM can’t
▪ TEM cannot take colour images.
▪ TEM cannot image through thick samples
than 200nm.
▪ A standard TEM cannot image surface information.
TEM vantagens
▪ Highest spatial resolution
▪ Local crystallographic and chemical analysis at very high resolution
▪ Quantitative identification of structural defects
