Semiconductor Nanomaterials (CdSe): Introduction and Synthesis methods
| Dip Pen Nanolithography|
1. What is it?
2. How to coat tip?
3. deposition rate of ink?
4. How it can be used for top down and bottom up syntheses
|1. Using AFM probe tip coated with "ink" and depositing that ink in a very specific manner through a water meniscus onto a substrate. |
2. Coat tip via vapor deposition or dipping tip in dilution of ink
3. Depends on molecules in ink
4. Can either use DPN to write an etch-resist and etch everything else away (top down). Or you can use it to deposit your NP of interest in a specific manner (bottom up)
|Compare dip pen nanolithography to microcontact printing. Are they the same thing?||For example, the soft lithography method, micro contact printing (μCP), is the current standard for low cost, bench-top micro and nanoscale patterning, so it is easy to understand why DPN is compared directly to micro contact printing. The problem is that the comparisons are usually based upon applications that are strongly suited to μCP, instead of comparing them to some neutral application. μCP has the ability to pattern one material over a large area in a single stamping step, just as photolithography can pattern over a large area in a single exposure. Of course DPN is slow when it is compared to the strength of another technique. DPN is a maskless direct write technique that can be used to create multiple patterns of varying size, shape, and feature resolution, all on a single substrate. No one would try to apply micro contact printing to such a project because the it would never be worth the time and money required to fabricate each master stamp for each new pattern.|
Basically, microcontact printing is like stamping whereas DPN is like writing. Stamping is easier for mass quantities once you have your template, but DPN can be specific all the time, but it cant do mass quantities
|***A solid semiconductor is a material which when extremely pure has:|
1. A x valence band and an x conduction band at T=0K
2. A band gap energy, Eg, greater than 0 eV but less than x eV
3. An electrical conductivity, σ, at xK typically in the range 10^-8 to 10^3 (Ω*m)^-1
4. An electrical conductivity of x at 0K that increases exponentially with increasing temperature
5. A concentration of electrons in the conduction band at 300 K between x per cm^3 and x per cm^3
|1. A filled valence band and an empty conduction band at T=0K|
2. A band gap energy, Eg, greater than 0 eV but less than 3-4 eV
3. An electrical conductivity, σ, at 300K typically in the range 10^-8 to 10^3 (Ω*m)^-1
4. An electrical conductivity of 0 at 0K that increases exponentially with increasing temperature
5. A concentration of electrons in the conduction band at 300 K between 10^4 per cm^3 and 10^14 per cm^3
|Compare electrical conductivity values at 300K for metals, semiconductors, and insulators|| 1. Metals = greater than 10^6 (Ω*m)^-1|
2. Semiconductors = 10^3 to 10^6 (Ω*m)^-1
3. Insulators = less than 10^-8 (Ω*m)^-1
|Are diamond and NaCl metals, semiconductors, or insulators?||insulators|
|Concentration of electrons in the conduction band for metals, semiconductors, and insulators|| 1. metals = 10^22 to 10^23 per cm cubed|
2. semiconductors = 10^17 to 10^20 per cm cubed
3. insulators = less than 10^4 per cm cubed
|Band Gap eV of CdS, CdSe, and Si at 0K.|| 1. CdS = 2.4 eV|
2. CdSe = 1.74 eV
3. Si = 1.11 eV
| ***Using dual source molecular precursors - explain how Rosenthal's group made CdSe quantum dots (Hot injection method). This is how it should be explained on exam. What ligand is used?|
What's the importance of this method for quantum dots?
|1. Prepare Cd and Se precursors (which are Cd(CH3)2 and Se powder in tributylphosphine/TBP) and keep separated.|
2. Just before the reaction, mix the Cd(CH3)2 with the Se-TBP.
3. Inject this mixture via syringe into TOPO at very high temperatures 360ºC. Nucleation occurs
4. Reduce temperature to 300ºC, allowing nanocrystals to grow to desired size, then remove heating mantle from the reaction vessel.
This method separates CdSe nucleation (reagent injection), from nanoparticle growth with growth termination caused by rapid reduction in reaction solution temperature.
|In Rosenthal's hot injection method - where does TOPO bind to?||TOPO (trioctylphosphine oxide) binds to surface Cd atoms, giving the nanocrystals an organic shell and providing solubility in organic solvents and polymers|
|What's the function of TBP (tributyl phosphine)in the Rosenthal method of quantum dot synthesis?|| TBP is complexed with Se powder prior to mixing with the Cd precursor Cd(CH3)2.|
Tributylphosphine functions as a donor solvent/lewis base to solubilize Se to give Se=P(n-butyl)3
| ******As you make CdSe nanoparticles smaller and smaller - what happens to the max absorption wavelength and emission wavelength?|
Color of the nanoparticles?
|Generally, the smaller the size of the crystal, the larger the band gap, the greater the difference in energy between the highest valence band and the lowest conduction band becomes, therefore more energy is needed to excite the dot, and concurrently, more energy is released when the crystal returns to its resting state. For example, in fluorescent dye applications, this equates to higher frequencies of light emitted after excitation of the dot as the crystal size grows smaller, resulting in a color shift from red to blue in the light emitted.|
So, you will have small particles be blue, larger particles be red.
As you get smaller nanoparticles , band gap increases, thus it takes higher energy light to create exciton. So, as particle size gets smaller your absorption max and absorbed wavelengths get smaller. Thus, On a graph of absorption vs. wavelength - smaller particle size will shift absorption to smaller wavelengths and wavelength emitted to smaller wavelengths.
|CdSe, like Na+ Cl-, is highly ionic, but at times there is some x bonding||covalent.|
|When CdSe nanoparticle diameter gets below bulk exciton Bohr diameter of x nm, then you get new electronic states, and absorption bands appear|| CdSe bulk exciton Bohr radius = 11 nm.|
So, radius under 5.5 nm gives new phenomena
|Given the λ, wavelength, of light hitting CdSe. What equations allow you to calculate E?|| E = h v = hc/λ |
|CdSe crystallizes in the wurtzite crystal structure, which is a x unit cell with x packing of the selenide anions and 50% of the tetrahedral holes occupied by the cadmium cations. CdSe are polar, because along the c axis .....x||wurtzite crystal structure - hexagonal unit cell with hcp packing. CdSe nanoparticles are polar along the c axis with the top and bottom facets occupied by opposite ions.|
|The size dependence and the emergence of discrete electronic states from the continuum of levels in the valence and conduction bands of the bulk semiconductor result from||quantum confinement|
|In bulk CdSe, the electron hole pair created upon absorption of a photon maintains a characteristic distance known as the x||bulk Bohr exciton radius or bulk Bohr exciton diameter|
| 1. CdSe has a Bohr exciton radius of x Å, so for nanocrystals smaller than x Å in diameter the electron and hole cannot achieve their desired distance and become particles trapped in a box.|
2. The discrete electronic transitions that emerge from the continuum can be labeled as x transitions
| 1. CdSe Bohr exciton radius = 56 Å or 5.6 nm |
CdSe Bohr exciton diameter = 112 Å, or 11.2 nm
|Longer wavelength, smaller energy emission corresponds with larger or smaller CdSe particle sizes||larger|
|***2 ways to synthesize semiconductor salts via controlled precipitation|| 1. Rapid nucleation/controlled growth (Hot injection method)|
2. Inverse micelle method.
|In the zeolite encapsulation method of CdSe synthesis - to what atom in the zeolite cage does Cd2+ bind?||Oxygen|
|What is a zeolite? What's nano about it?||Aluminosilicate materials that form a cage structure around a central hole/pore. The central hole/pore is nano and can be used to confine the synthesis of CdSe nanoparticles. Faujasite (LZY-52) was the name|
|What's the solvothermal method of making semiconductor nanoparticles? What are the conditions and the two examples given?||A reaction done at high pressure and high temperature with nonpolar solvents like benzene and dimethyl ether. This avoids water sensitivity of the reagents. |
Examples are the reaction of GaCl3 and Li3N to make nano GaN particles that are main blue emitting nanoparticles
Note that benzene is heated to a supercritical fluid to solubilize Li3N here.
and the reaction of Na3P with InCl3 to make InP nanoparticles
|In the solvothermal synthesis of GaN semiconductor nanoparticles, Li3N is polar. How do you get it to dissolve in benzene?||Heat reaction to 280 ºC under high pressure, this causes benzene to turn into a supercritical fluid, which solubilizes things|
|Why is the solvothermal method to synthesize GaN and InP nanoparticles even used?||Because the reagents explode in water, and this provides a way to do the reaction in a nonpolar solvent|
|InN nanofibers can be prepared by the solution-liquid solid method of synthesis. What is SLS?||Analogous to VLS, except that the precursor molecules are coming into the elongated glob on top of the nanofiber from solution, not from vapor.|
|In the SLS method, InN nanofibers grow off of the surface of x, where molecules of precursor 1 ([iPr2InN3]n) reductively decompose||Off the surface of In melt droplets in solution.|
|Why would you use diisopropylbenzene solution instead of regular benzene?||because benzene boils at 60ºC, and if you want a higher temp you do use more steric attachments.|
|ZnSe and CdS nanobelts and nanowires can be made by vapor transport - how does this process occur? Where do belts and wires grow from?||You aren't synthesizing these from scratch, just making nanobelts and nanowires.|
1. Heat your solid nanoparticles to gaseous form (CdS solid head to CdS vapor)
2. Vapor deposits on a catalyst consisting of Au film on Si substrate. Au serves to initiate formation of single crystalline CdS or ZnSe nanoparticles because remember how Au forms alloys with things? It will form an alloy with your semiconductor.
3. The CdS or ZnSe nanoparticles on Au surface aggregate to form polycrystalline film, which serves as future nucleation site for nanowires and nanobelts.
4. Nanowires and nanobelts form from the edges and ridges of the CdS or ZnSe polycrystalline films. Gives them a preferential growth direction.
|*****What methods allow you to make nanowires and nanobelts of semiconductor materials?|| 1. InN nanofibers prepared using the Solution Liquid Solid method|
2. ZnSe nanobelt and nanowires by vapor transport
3. CdS nanobelts via vapor liquid solid growth
|*****What method allows you to grow nanowire forests in any pattern or periodicity you choose from a substrate lattice?||GaN nanowire first by Vapor Liquid Solid Epitaxy growth from substrate lattice.|
For example, use a TEM grid as a shadow mask (like spray painting a cutout) to deposit Au only where the holes in the TEM grid were. Remember that Au serves to nucleate the growth of the nanowires by forming the alloy.
This Au film can be on a metal oxide substrate like silica.
|Used in hydrocarbon cracking and fuel production||zeolites|