pSi introduction and quantum dots demo

Hannah Chen

End video at 6:15

Video questions

1. What are some signs that a chemical reaction is occurring?
2. What is the function of the red and black wires?
3. What do you think happens during the chemical reaction?
4. What does the platinum loop do?

Outcomes

Top-down and cross-section views

Macropores: width > 50 nm

Mesopores: 2 < width < 50 nm

Micropores: width < 2 nm

Nanopores: anything < 100 nm

2-electron reaction (etched)

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4-electron reaction (electropolished)

Quantum dots in solution

• The properties of a material depend on its size
• Fabricated through a straight etch

Quantum dots

• Really fine pores -> when they interact they create quantum confined areas
• Move e- up to a higher energy level when excited, when it falls down it releases energy in the form of light
• Excitation generates e- h+ pairs
• Radiative recombination of e- h+ pairs generates photoluminescence

Quantum confinement

Within these tiny structures, quantum confinement occurs. This means that discrete energy levels are created inside individual atoms through which electrons cannot freely pass. These energy levels “trap” electrons inside and prevent them from interacting with other electrons (see diagram below).

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Excitation

From Wikipedia: Energy may be released from a potential well if sufficient energy is added to the system such that the local maximum is surmounted.

When we add enough energy to the system, we overcome the “local maximum”. The electron thus jumps up through the energy gap, entering its excited higher-energy state.

This happens when we excite the quantum dot with a. The electron jumps up a level, leading a positively charged h+ (hole) and a negatively charged e- (electron) pair. This process continues continuously while the quantum dot is being excited.

Recombination

When the electron falls down to its original energy level, “recombining” with the hole (h+) it left behind, it emits energy in the form of light.

Depending on the size of the band gap (how much energy it takes for the electron to move between the valence and conduction bands), the electron will release more or less energy as it falls back as well.

E = hν = hc/λ

My research last summer:

• Ethanol and other organic solvents have a narrower band gap than silicon
• Exposure to ethanol thus creates surface traps, causing photoluminescence to quench
• Certain treatments can passivate surface traps, essentially creating a protective layer around the material

Blinking and carrier traps

Blinking and carrier traps

Charged quantum dots appear dark because the recombination process does not generate light (non-radiative). Instead, the energy is transferred to the excess particle in the form of kinetic energy.

Core-shell structures

Outer shells protect quantum dots from their environments and allow each electron hole pair to function by itself.

Core-shell structures

Silicon dioxide shells can be formed around silicon quantum dots through thermal oxidation. This helps serve as a protective layer for the actual etched silicon.

Core-shell quantum dots increase PL efficiency.

My research last summer:

• Ethanol and other organic solvents have a narrower band gap than silicon
• Exposure to ethanol thus creates surface traps, causing photoluminescence to quench
• Certain treatments can passivate surface traps, essentially creating a protective layer around the material

My project

• Thermally induced dehydrocoupling: grafting organic molecules onto the porous Si surface
• Another way to preserve the stability of a quantum dot over time

Hydrocarbon grafting

Hydrocarbon-grafted silicon senses ethanol

Applications

• Chemical sensing
• Biological imaging
• Quantum dots fluoresce within the body
• Blood-bound will travel to areas with high activity, most notably tumors
• Silicon is biocompatible and relatively inert, so it has a long life in the human body
• Solar cells
• Movement of electron hole pairs creates current
• Can change band gap through changing size, versus traditional materials
• This can maximize efficiency by using one material with multiple band gaps in one cell