Quantum DotsBy Juan D Pulgarin and David Akaptsu

William Paterson University

Whats a Quantum

dot?A quantum Dot are

nano crystals made of Semiconducting material with an

average size ranging from 1 to 5 nm.

So what does this mean? Due to their small size ,

quantum dots exhibit quantum mechanical properties.These properties arise from the quantum confinement of their electrons.What is defined as small enough: When the exciton Bohr radius is larger that the diameter of the molecule.

Quantum confineme

nt Is the reason why quantum dots behave so differently

from their big brothers bulk matter.

As the crystal lattice gets squeezed , the electrons get more trapped giving rise to quantum confinement.

How does the electron react to this?

Other propertiesQuantum dots are semiconductors and are governed by Band gaps. Different gaps have different energetic requirements.Absorb in the visible electromagnetic side of the spectrum and similarly reflects it.The energy required to promote an electron into the conduction band depends on the size of the energy gap between bands.

Lets connect some “dots”

Gap energy Size of CrystalLight emitted

Experimental GrowthControl

QD are Tunable

Energy levels in Quantum confinement can be

described using the Particle in the Box model

The reason being is that as the particle becomes smaller , energy levels become discrete

We mentioned that energy gaps between energy states is inversely proportional to size of dot(box length) therefore decrease in size represents a larger Enegy Gap.

Larger GapLight emitted of higher energy

and shorter wavelength.

Quantum confinement = particle in a box

As a matter of fact it is possible to demonstrate this quantum confinement effect in the particle in the box using Wolfram Alpha.

V(x) = ∞ x < 0,x > a V(x) = 0 0 ≤ x ≤ a ψ (x)= √2/a sin [(nπx)/a] when n=1,2,3,… n = energy level.

The tangent line varies with change in

the frequency of the waveSecond derivative of the the tangent is related tothe energy of the system

Frequency is dependenton the sizeof the box

PIB behaviorAnother demonstration of PIB behavior in quantum dots has been reported by the University of Indiana.

By reducing the size of the box, the energy spacing changes and so do the Spectroscopical properties of these species. a) Change in absorption peak in CdSe with shifts in crystal size

b) Comparison of the experimental wavelength in Energy and the PIB energyc) which depends on Lˆ2.

d) The quadratic dependance of the experiment is clearly seen and therefore e) well described by the PIB.

SynthesisWill focus on CdSe quantum dots.

classic CdSe synthesis are not suitable for undergraduate labs.Dept of Chem at Princeton University has reported two variations to these methods which are considered much safer for undergrad studies and very efficient.

Synthesis Carried under ambient atmosphere at 165˚C

Reduced Concentration of chemical

Employs safer precursors

Reaction time: 3hr lab session

Both methods display the full spectrum of the emission from blue to red .

Procedure 1

Selenide precursor (1mL injected )

Se Powder : 99mg

Trioctyphospine: 5.5ml

Cd precursor( 1mL injected)

Cadmium acetate:53mg

Oleic Acid: 0.6mL Octadecene: 5.5ml

Growth solution Octadecene: 10mL

Temp: 165ºC

Growth time immediate

reaction time 1.5-6.5 min 10 aliquots extracted (1every .7min)

Ambient light colors

clear clear clear sheer orange sheer red

Colors under UV

blue green yellow orange red

average dot diameter

1.62 to 2.09 nm

Type of emission

Surface trapped States

Procedure 2

Selenide precursor Se Powder : 99mg Trioctyphospine: 5.5ml

Cd Precursor Cadmium acetate:53mg Oleic Acid: 0.6mL Octadecene: 5.5ml

Growth solution Octadecene: 10mL 0.67 mL of oleylamine

Temp 165ºC

Growth time immediate

reaction time 1.5-6.5 min 10 aliquots extracted (1every .7min)

Average dot diameter 2.12-2.27nm

Type of emission Exciton Emissions

Synthesis of CdSe quantum dots. Figure courtesy of Department of Chemistry, Princeton University.

The Cd precursor solution contains cadmium acetate (Cd(Ac)2), oleic acid (R-COOH), and octadecene (ODE). The Se precursor solution contains Se and trioctylphosphine (TOP). ODE is the growth solution for procedures 1 and 2, whereas oleylamine (R- NH2*) is added for procedure 2.

Types of emmisionExciton Emission :electron is promoted to the conduction band forming an electron-hole pair.  Upon recombination(energy decay) , a photon is emitted (this is called florescence)

Surface trapped states emissions:

Now lets measure Characterization of quantum dots is usually carried out by measuring its most notorious feature. Emission and absorbance.

By measuring the wavelength of a light emitted by a QD , it is possible to determine the exciton energy necessary to promote an electron and create a hole and hence its size

An experiment similar , has been developed by St Lawrence university in NY.

More Pchem stuff..This calculation arises from the modeling of this structures using the PIB where energy is defined as:

In a QD system other energies must be also considered besides the electron: Etotal= Ehole+Eelectron+Eg

This equation may be solved , if the radii is known, and from the experiment carried out by the Dept of chemistry at the University of Princeton, the reported radii wavelength relationship was obtained experimentally.

Experimental determination of size.

Measure the spec of the light source

Using a spectrophotometer you would have to measure the vial and determine its

lambda max by illuminating the dots in a direction perpendicular to the light collector

From that wavelength you can find the energy of emission of QD by plugging into

the equation.