Lecture 1 Flashcards
What is an interface
A volume in space, often approximated as a surface, which separates a much larger volume of dissimilar substances.
To make life simpler for scientists big substances interfaces are negligible. But when it comes to a nanometer scale the interface between surfaces becomes impossible to ignore
This is demonstrated in Figure 1.1 where the surface-to-volume ratio of a spherical nanocrystal increases with decreasing diameter
What are dangling bonds
Dangling bonds are bonds that occur at a surface when atoms have unfilled outer orbital (valence) shells. These bonds are unsaturated, often bear a partial eclectic charge, and they increase the energy of the surface, and of the whole material by an equal amount of γ=n(db) * Φ/2.
This shows that the surface energy (γ) increases with the increase of the surface density of the dangling bonds n(db), which is determined by the surface’s composition, roughness, and curvature.
In order to best represent surfaces and dangling bonds we can imagine the atoms as boxes as shown in Figure 1.2.
What is the impact of surface energy?
Surface energy determines how a surface interacts with the environment =, a surface with higher energy is more reactive as it is more prone to reduce its energy by causing the dangling bonds to react with the environment
What are the important surface effects that can happen on a nanocrystal?
- surface functionalization: molecules or functional groups are grafted through surface chemistry to the surface dangling bonds. This reduces the surface charge and relives the pressure generated by the surface energy which is indicated by the arrow in Figure 1.3
- Surface functionalization defect: Defects often related to edges and kinks that leaves the surface locally more reactive
- impurities absorption: Absorption of impurities satisfies the dangling bonds and surface charges, thereby decreases the surface energy and thus pressure
- Layer-by-layer assembly: where surface charge can be used in the deposition of monolayer and or multilayers of charged species can
- Surface reconstruction: Where dangling bonds partially recombine and the atoms reposition themselves in order to reduce the surface energy
- convex surfaces: increases solubility compared to bulk material
- concave surfaces: decrease solubility compared to bulk material. concave surfaces can be functionalized with molecules and captivates can be used to entrap guests
- passivation: Where dangling bonds or surface groups react overtime with molecules in the atmosphere to form a passivation layer which makes the surface less reactive
- Melting: where dangling bonds are eliminated via melting
This is all dedicated in Figure 1.3
Why does the solubility of concave and convex materials differ?
First, it is important to mention that all materials behave differently depending on their curvature (whether concave or convex), but due to the definitive size of nanostructures, they are inherently endowed with such high curvatures that such effects are much more pronounced.
Okay, so to begin with imagine that you are removing the outer layer (convex) of the halo nanocrystal in Figure 1.3. Essentially what you are doing is dissolving the crystal. this would lead to a lower surface area making it a favourable process (since less reactive surface now). Now if you remove a layer from the inner part (concave) of the crystal, you will increase the surface causing it to be unfavourable decreasing solubility. Note that when you remove from a flat area no, to small changes happen so no big effect occurs
What is the disadvantage of surface functionalization?
Even though we can get cheap material and coat it with appropriate molecules allowing for surface functionalization, what often happens is that dangling bonds or surface groups react with the atmosphere resulting in a passivation layer lowering teh surface energy
Faceting phenomenon
It is a phenomenon where surface energy is reduced by atomically creating flat surfaces which thus possess fewer dangling bonds.
what is surface pressure reduction expressed by during surface reconstruction (and I think other surface effects)
Essentially atoms at the surface are pushed by an external pressure expressed by Laplace law:
Change in P = 2Y/r (for spherical surface)
note that Y is usually given in J/m^2
Important note
in bulk material surface functionalization, grafting, and absorption only affect the properties of the surface. in nanomaterials, they also affect the properties of the bulk
What is a big advantage of nanomaterials?
Due to their incredibly small size nanomaterials have a large surface area
What are the types of nanovoids?
- Zero-dimensional (halo nanocrystals)
- One-dimensional (nanotubes or mesoporous materials)
- Two-dimensional (layered system)
- Three-dimensional (colloidal crystals)
What is the meso-phenomenon?
a phenomenon that manifests between the bulk (classical) and molecular (quantum) regimes and where the length of a characteristic property is comparable with the object’s size.
What is the range of a nanoscale?
From 1-1000 nm as depicted in Figure 1.5
What does shape affect in nanoscience?
in nanoscience, shape affects the relative size-dependent effects in a directional way and the directionality of self-assembly.
What is an important example of how shape affects size-dependent properties?
The colour of nanoscale metals like gold is determined by the frequency at which the sea of conduction electrons can oscillate. Spherical gold particles (isotropic) are usually red since the conduction electrons oscillate at one frequency, but rod-shaped gold particles (anisotropic) will have two frequencies (longitudinal and transverse) as it has two sizes (the length and diameter).
This results in the colour of nanorods being different as the longitudinal frequency can be tuned to be in the infrared by changing the aspect ratio of the nanorod.
what is a shape-independent effect?
Shape in its size-invariant effect has to mainly do with how the objects interact with one another on a geometrical level. For example the lock and key mechanism
What is the effect of symmetry in nanoscience?
- it influences alot of alot of physical and quantistic properties
- it determines the periodicity of self-assembly
Size-dependent vs Size independent effects
Shown in Figure 1.6 and explained in previous flashcards
Self-assembly of nanoscale building blocks is mainly influenced by?
The blocks surface, shape, and the homogeneity of their size
What is nucleation?
In thermodynamics, nucleation is the first step in the formation of either a new thermodynamic phase or structure via self-assembly or self-organization within a substance or mixture. It first begins when enough stable nuclei form
What is self-assembly?
Apparently spontaneous self-organization of objects; it arrises as a system strives to find minimal free energy
What is the disadvantage of self-assembly and its solution?
When compared with other nanofabrication techniques, self-assembly usually presents defects that are intrinsic and not necessarily desirable. The solution to this is to making the function you look for from a self-assembled structure to be defect-tolerant.
What is top-down and bottom-up?
Bottom-up: it is a procedure where you start from the smallest components and you assemble your desired structure from the ground up. Usually associated with chemistry and synthesis
Top-down: it is a procedure where you start from a chuck of material and you cut it and trim it till you get your nanosized architecture. Usually associated with physical techniques, like vapour deposition or lithography. Mostly planar by nature
What are the different types and sub-types of self-assembly?
2 main types are static self-assembly, and dynamic self-assembly. These two types are processed by either co-assembly, hierarchical assembly, or directed assembly. This show in notes figure 1.7
