TOPIC 1: INTRO TO NANOSCIENCE Flashcards
Describe Nanowires [1D]
- one dimensional structures
- long string of atoms extending in one direction
- can be metallic, semi-conductive or insulating
perfect nanowire would be a single atom thick
Describe 2D nanomaterials
- Ultra thin sheets
- 5nm or less thick
e.g. Graphene
Describe nanoparticles [3D/0D]
- Assemblies of small numbers of atoms that have coalesced
- sizes and shapes are determined by kinetic and thermodynamic factors
Define the Top-Down approach to producing nanomaterials and name an example
- Start with a block of raw material
- use physical or chemical methods to decrease its size to leave desired structure
e.g. chemical etching. photolithography and electron beam lithography
Chemical Etching [Top-Down]
- Engraving method
- Wet chemical method –> liquid phase chemical reagents + high temp chemical spray
- Chemical reactions between etching species [liquid] and etched material [solid]
Si(s) + 4KOH(aq) –> [SiO4]4- + 4K+(aq) + 2H2(g)
- etching creates topographical surface features through selective removal of material using physical or chemical means
- Etching in one direction is anisotropic
- Etching in all directions is isotropic
Photolithography [Top-Down]
- Based on shining a beam onto material [photoresist] supported by a substrate
- Photoresist is chemically altered by exposure to light
- Next step used to etch away exposed area of material [positive resist] or unexposed area [negative resist] depending on nature of material
- Cheaper and throughput than EBL but low resolution
- Makes integrated circuits
- Mask contains a pattern e.g. for a circuit
- Light projected onto mask and then through the mask onto the surface of the photoresist
+ve photoresist –> breaks down when exposed to light ==> MORE SOLUBLE
-ve photoresist –> becomes crosslinked when exposed to light ==> LESS SOLUBLE
Electron Beam Lithography [EBL]
- Focused beam of electrons to bombard the resist
- Allows patterns to be written directly into the resist [focused nature means no mask is required]
- Used to manufacture masks for photolithography + X-ray lithography
- Used to make integrated circuits
- Resist solubility changes when it comes into contact with electrons
Define the Bottom-Up approach to producing nanomaterials and name an example
- Start with atom-scale building blocks
- Assemble them to create desired structure
e.g. Sol-Gel, Self-Assembly, Chemical Vapour Deposition + Molecular Beam Epitaxy
Sol-Gel Method [Bottom-Up]
- Transformation of a sol [solid in liquid colloid] into a gel [liquid in solid colloid]
THREE STEPS:
- Sol formation –> hydrolysis of a metal oxide
- Polymerisation of the sol via formation of hydroxo- or oxo- bridges
- Heat treatment of the remaining organic or inorganic components
- Used in the formation of nanoscale metal oxides
Self-Assembly [Bottum-Up]
- Family of wet-chemical approaches
- E.g. synthesis of Au NPs –> ionic Au salt reduced to form Au atoms which then aggregate [driven by thermodynamics]
- Requires consideration of thermodynamic and kinetic factors
Chemical Vapour Deposition [Bottom-Up]
- Transformation of gas precursors into solid materials in the form of thin film or powder
- Chemistry is based on high temperature catalytic decomposition of precursors materials e.g. organic molecules when forming polysilicon thin films
- Takes place under low pressure - close to vacuum
- Decompose small molecules + deposit thin layer on substrate
- produces silicon dioxide
- produces CNTs and semiconductor materials
CNTs:
CH4(g) —————————-> SWNT + 2H2(g)
@ 700C, 1atm, Fe/Co/Ni cat.
SILICA WAFER:
Si(OC2H5)4(g) —————————-> SiO2(s) + O(C2H5)2(g)
can also be done with silane:
SiH4(g) +O2(g) —————————-> SiO2(s) + 2H2(g)
Molecular Beam Epitaxy [Bottom-Up]
- Ultra high vacuum process - produces high purity thin films with mono layer control
- Thin layers of materials w/ lattice structures identical to the substrate
- Single + clusters of atoms heated in vacuum and deposited onto a hot substrate surface
- Atoms diffuse across the surface + grow into a pure film
- Process requires extremely low pressure + contamination free environment [layer growth is extremely slow, 1mmh-1]
- Requires better vacuum than comparable tech
- Creates some of the most pure films
- Makes diodes and transistors
Can create higher quality NPs –> maintain vacuum for longer –> higher risk of impurities
Top-Down VS Bottom Up
Surface Area
- Important factor in rationalising unique properties of nanomaterials - high surface area (per unit mass) COMPARED to bulk materials
- As particles decrease in size, a greater proportion of their atoms exist at the surface
- Surface atoms have different energy to interior atoms –> affects reactivity, catalytic activity and physical properties.
In most objects [macro] surface atoms make up negligible proportion so have a negligible effect on the properties of objects.
NOT NEGLIGIBLE IN NANOMATERIAL
Surface Layers
- Surface layers tend to be three or less atoms deep
- Surface layers have a lower number of nearest neighbors than interior atoms which then RAISES their energy and in turn REDUCES the STABILITY of the surface layers
- N(surface atoms)/N(total atoms) = 1/2r
Surface Energy
- It costs higher energy to create a new surface because surface atoms have higher energy than the interior
- Natural tendency to minimise surface area
- Results in surface tension
- Surface Tension is the force perunit length that opposes the expansion of a surface area
dw = ydA
Surface Plasmon Resonance [SPR]
- In metallic NPs collective oscillations of the conduction electron density at the surface are observed when the electrons are excited by incoming light of the appropriate frequency
- Colour of the suspension of NP depends on the wavelength of incident radiation, composition of NP, shape, size and orientation.
Wp^2 = Nee^2/e0m
- Higher electron density results in lambda max shifting to shorter wavelengths
