Images from TEM

NaYF4;Yb/Er synthesized at 250 °c in OA:OM:ODE = 1:1:2

OA = oleic acid, OM = oleylamine, ODE = octadecene

NaYF4-YbEr_1

NaYF4-YbEr_2

NaYF4;Yb/Er synthesized at 270 °c in OA:OM:ODE = 1:1:2

NaYF4-YbEr270_1

NaYF4-YbEr270_3

NaYF4-YbEr270_11

NaYF4;Yb/Tm synthesized at 250 °c in OA:OM:ODE = 1:1:2

NaYF4YbTm_2

NaYF4YbTm_5

NaYF4;Yb/Er synthesized at 250 °c in OA:OM:ODE = 2:1:2

NaYF4-YbEr-6OA_4

NaYF4-YbEr-6OA_10

As can be seen from TEM, the nucleation growth seems to be the best at temperature 270 °C with the ratio OA:OM:ODE = 1:1:2. If the temperature a bit higher, perhaps the size of lanthanide nanocrystal decreases. If the concentration of ligands (OA=oleic acid) increases, we can see the nanocrystal agglomerated.

The best result comes from NaYF4;Yb/Er synthesized at 270 °c in OA:OM:ODE = 1:1:2. It produces fairly monodisperse nanocrystal with same morphology. Also there is an emission peak around 55onm indicating green light. The problem is to put the lanthanide ions (Yb3+,Er3+/Tm3+) in the host material NaYF4. But the lanthanide ions seem to be sitting on the surface of nanocrystal.

This is really disgusting. It depends on luck how to control the nucleation growth of the nanocrystal. If the lanthanide ions don’t like to sit in the host material, increasing temperature won’t help.  The hardest topic in surface chemistry is the control the nucleation growth. This is what I’m doing in the lab.

God made the solids but surfaces were the work of devils.

Schlenk line

Last 2 days Shrenk line broke down. I could not do any synthesis on quantum dots. Quantum dots need to be degas under vacuum atmosphere at 80-100 C then purged 3/4 times before heating. Heating takes place at very high temperature, 250-300 C under Argon or Nitrogen flow. It must be done in oxygen-poor environment. Otherwise the oxygen will steal the electron from my quantum dots.

These couple of days I just went to the lab to do spectroscopic measurement; absorbance and fluorescence. Nothing much I can do without Shrenk line working properly. Only 2 weeks left before the oral presentation and thesis submission due date. I hope at least I can do bilayer coating to get some dopants inside the host material. So far the dopants most like to be sitting on the surface of the host material.

This is really disgusting because if there are no dopants inside the host material, then I get no  upconverter quantum dots.

Schlenk line

In the fume hood

Feymann quote

Once Richard Feymann, the founder of Quantum Electrodynamics was asked about the triumph of  Copenhagen interpretation and his point of view for quantum mechanics.

He said in his response, “Well, just shut up and calculate”.

He also made the famous saying, “One who claims to understand quantum mechanics never understand it at all”.

Can’t fool a smarty

In maths, one is never allowed to multiply by zero or infinity, neither for division as well. At library I found a piece of paper written by some people trying to fool n0n-mathematician audience. Here I provide the false of the proof.

x² – x = 2x² – 2x

x(x – 1) = 2x(x – 1)

x = 2x

1 = 2

(fool.!!)

the correct way to solve this is;

x² – x = 2x² – 2x

0 = x² – x

0 = x(x – 1)

x = 0, 1

it removes the paradox of 2 = 1

Beckmann rearrangement

Beckmann rearrangement convert cyclohexanone oxime to ε-caprolactam.

1. protonation on hydroxy group makes it a better leaving group.
2. cleavage of water and attack of alkyl occur simultaneously.
3. the driving force is to stabilize the charge on N atom once water is cleaved and carbon is  better having positive charge rather than nitrogen.
4. once carbocation is formed, water comes back to stabilize the charge.
5. deprotonation stabilizes the positive charge on oxygen.
6. tautomerism occurs to form amide (ε-caprolactam)

sec x tan x

Indefinite integral of sec x tan x

∫ sec x tan x dx = ∫ (1/cos x)(sin x/cos x) dx

= ∫ sin x/cos² x dx = ∫ sin x/(1 – sin² x) dx

Now use substitution, let u=sin x  =>  du=cos x dx  and  dx= du/cos x = du/√1-u²

= ∫ u/(1 – u²)^3/2 du

Now use substituition again let a=u² =>  da =2u du  and u du = 1/2 da

= 1/2 ∫ (1 – a)^-3/2 da

=  (1-a)^-1/2 + constant(=k)

but a=u² and u=sin x  => a=sin² x

= (1 – sin² x)^-1/2 = 1/√cos² x

= 1/cos x = sec x

Pheewiit..  ∫ sec x tan x dx = sec x + constant(=k)

Chemical Bonding

In chemistry, there are 3 types of chemical bonding,

1. covalent bond
2. ionic bond
3. metallic bond

Before proceeding, what is actually chemical bond? Well, the simple answer is the electrostatic intertaction between nucleus-electron and electron-electron. Another type of bonding is Van der Waals interaction which is for intermolecular interaction.

If I take 2 hydrogen atoms (1 proton,1 electron each) and bring them close together (several femtometer),  where would you think the electrons will reside. Obviously in the middle of the two nuclei (protons). The electron-electron repulsion is offset by proton-electron attraction of both nuclei. This is the ground state of hydrogen molecule where the electrons are sitting in bonding orbital. If you were to excite the electron, then antibonding orbital is occupied, thus change the electron density. It’s all Coulombic interaction. It has stood every possible test and found to be accurate enough in the scale 10^-20 m.

molecular orbital

Now come the question, what prevent the hydrogen atoms coming very very close together and sitting on top each other? It’s the repulsive force, short-range interaction. It comes into role when atoms are very very close together. Electron clouds of the atoms start seeing each other and forming molecular orbital (MO) from superposition of every atomic orbitals. But Pauli exclusion principle states that no 2 electrons can occupy the same states and at most 2 electrons can reside in an orbital with paired spin.

If there is no repulsive force, the atoms come close together until they collapse and form BLACK HOLES. That would be a disaster. Then there is no matter in the universe. Repulsive force saves us.

dihydrogen

molecular orbitals

Solvent participation

When carrying out chemical reaction, the most important thing is the stoichiometric amount between starting material. Even if one reagent is in excess, it should not interfere with the reaction or solvent. But in many cases it does. Another crucial thing is choosing a suitable solvent. Avoid choosing nucleophilic solvent such as acetone in the reaction.

It’s best to choose volatile solvent so that you can easily vaporize that to get the crystal powder. Some good solvent (not the best) are ethanol, toluene or hexane.

For synthesis of quantum dots, common solvents are TOP/TOPO or Oleic Acid/Octadecene. Some people refer octadecene as non-coordinating solvent. I’m sure what does it mean. Perhaps octadecene does not contribute to ligand passivization.

Can’t figure out too simple thing

Well, it was really ashamed when you are in the lab and not knowing what chemical reaction took places. We are too much dependent on the literature, lab manual or what-so-ever. Here I was making the precursor for rare-earth trifluoroacetate with some deionized water as a solvent.

It is high school chemistry and I completely forgot about it.

metal oxides + acid  ->  salt + water

Here metal oxides is just a base.  Similarly ; metal oxides + trifluoroacetate acid  ->  metal trifluoroacetate + water.

The same holds true for metal oleate. It’s just here trifluoroacetic acid and oleate acid are weak base (partially soluble), hence we need some kind of solvent and cook them at 110 C until they all dissolve. The excess trifluoroacetatic acid (bp 72.4 C) and water can be vaporized using rotavapor.

My brain is a bit rusty. (even rust is just simple redox reaction)

Exciton

Exciton is considered as electron-hole pair, not a pair of electron and hole. Typically it is found in semiconductor, nowadays in quantum dots. Once electron leaves the valence band jump up into conduction band, a holes is generated at valence band. External perturbation such as heat or photon is needed to excite the electron into conduction band.

exciton generation

periodic potential

In semiconductor, electron-holes pair (or exciton) behaves differently than individual particles (electron and holes). Hence it needs to be treated as a newly entire system (quasi particle hence came the name exciton) due to Coulombic  attraction between electron and holes. Nevertheless the existence of periodic potential in semiconductor, electrons interaction from neighboring electrons and surface traps give rise to the exciton energy level slightly below the band gap though classically band gap is known as forbidden region.

exciton energy levels

Recombination of electron and holes creates emission. This is the colour we see from semiconductor. However in quantum dots, this band gap hence the wavelength of emitted photon is tunable by manipulating the size of quantum dots. (more on quantum dots later entry).

Quantum mechanically, holes too have mass called effective mass. And to my suprise it can be negative as well. The fully mathematical approached is not shown here. But trust me for the moment unless you have done some advanced calculus to deal with it. But the effective mass of hole is comparable to that of electron (not always similar).  The effective mass might be due to the motion of electron in dielectric medium of semiconductor (not in vacuum trivially), hole generation and electron-electron interaction.

Once exciton is formed, we can characterize the radius of exciton called exciton Bohr radius and model it using hydrogenic model of atom in crystal. But this moves on to another quantum phenomenon when Bohr exciton radius is confined. Such confinement creates quantum dots (more on quantum dots later). We get a particle in spherical box model for quantum dots.