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-1- Baldelli, Shultz

Quantum Dots

and Modern Electronics

Background: B a n d s t r u c t u r e i n s o l i d s t a t e e l e c t r o n i c s ,electronegativity, size. Bohr model of atomic structure.Other Concepts: Solubility and miscibility.

For the past three decades, transistors used in the

integrated circuit industry have been getting smaller andsmaller. One result is that computers have becomesmaller, faster, and more reliable. Sizes have shrunk sothat we are now entering the age of nano devices:devices where the dimensions are in the nanometer (10-9

meter, abbreviated as nm) range. To put this dimensionin context, remember that atoms range in size from 0.5for hydrogen to 3.3 for the largest atom, cesium. Anangstrom is 10-10meter, so nanoparticles are on theorder of ten times the size of an atom.1 At this size,classical descriptions of solid state properties, e.g.conduction in terms of electron drift, breakdown and wemust begin to think about electron motion in termsreminiscent of the Bohr model in atomic structuretheory.

Nano structured matter is condensed matter of a sizescale larger than atoms, but smaller than bulk solids. Itis too big to behave like atoms or molecules but toosmall to act like the bulk solid. One unique property ofnanoscale materials is that a large percentage of the

atoms are located on the surface or at the interface. Forexample, a 4-nm diameter CdS particle has about 1500

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atoms. Of this number, approximately one-third are onthe surface and interact with adsorbed species or gas-phase molecules. This large surface to volume ratioimparts novel properties to nano particles compared withbulk solids, while the mere existence of surface vs. bulkatoms give properties distinct from atoms or molecules.

Among small scale structures, multilayer structureshave the longest history, growing out of the thin-filmindustry. Recently reported ultra-hard surface films

created from CN and TiN is an example of a sandwichedthin film that imparts uniquely important properties tomaterials.

It is the technological potential of these newmaterials, particularly in the burgeoning electronics andcomputer indus try that has helped fuel their advancement.The low dimensional quantum sizes in these materialsmakes it possible to engineer electronic and photonicproperties that make devices such as high speed

transistors and fast, efficient optical storage devices.Quantum-size materials are classified according to theirdimension as shown in Fig. 1.

Quantum size effects appear when particle sizebecomes comparable to or smaller than the characteristiclength scale responsible for that property. For example:

Ligh t Absorpt ion: Due to spatial restriction ofelectron and hole motion, optical absorption in

semiconducting clusters shifts to higher energy, i.e. theband gap increases as particle size decreases. This is

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known as quantum confinement. Optical properties ofthese clusters are exciting both from a fundamentals cientific and a technological point of view . F r o m afundamental science point of view, nano particles represent abridge betw een atomic and s olid state theories . They can bemodeled using an extension of the Bohr model us ed tounders tand atomic properties or a modification of bandtheory. (S ee Theory s ection below .)

a

b

c

d

Fig. 1: Quantum Device Di m ens ion (a)Zero (b)One (c)Two (d)Three.

En

e

rg

y

Size

Bulk Band Gap

Fig. 2: Variation of Band Gap with Quantum Dot Size.

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Mechanical Properties: Metals Materials that arenormally soft, strengthen when the grain size fallsbelow about 50 nm. At this s ize, sources ofdislocations become difficult to activate withconventional forces. For example, bulk Cu is fairlysoft and bendable due to the large number of easilymoved defects and the large number of close packedplanes in coppers face centered cubic structure.One method for strengthening copper is to limitmobility of defects by terminating them on other

defects as in work hardening it. Another method isto assemble bulk Cu from nanoparticles of 5-7 nmsize range. Solid copper made in this way has ahardness and yield strength up to 500% greater thanconventionally produced Cu.2

Mechanical Properties: InsulatorsCeramics, arenormally quite brittle due to directed, localizedbonding. Fractu res, once st art ed, tend to growcatastrophically due to the thermodynamic energy

released as the bonds break. Plastic deformation ofceramics is extremely limited due to repulsion of likecharged ions forced into close proximity. However,ceramics become ductile when assembled fromclusters of size below about 15 nm.3

Electrical Resistance: Laye r ed s t r uc t u r es o f materials with different magnetic properties can beused to create a material with a dramatically loweredelectrical resistance. Application of a magnetic field

to layered materials causes a cooperative-ordering,regularity to the material. Since regularity is greatly

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increased, scattering of electrons, which is the basiso f r e s i s t a n c e , i s g r e a t l y d e c r e a s e d . T h i smagnetoresistance is the basis for development oflow noise, high signal read heads in the recordingindustry. The issues remaining include maintenanceof the magnetoresistance effect to room temperatureand turning on the low resistance with lowermagnetic fields.

It is very early in the development of applications

and creat ion of these new materials . Althoughunderstanding of the optical properties appears to bewell on the road, control of the mechanical propertiesincluding the optimal elements to be incorporated in thecluster is still very much in its infancy.

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Cluster Characterization:

It is instructive to ask, how the atomic levelstructure of these clusters have been characterized.After all cluster optical properties vary with cluster sizeand shape, so until the connection be tween size andshape and optical absorption was established, opticalmethods could not be used. Conventional solid statetechniques also could not be used because, by definition,quantum size particles are too small to show regularityof atomic arrangement on a long-range scale. An

elegant solution to this problem is to use chemicalactivity. As atoms are added to the surface of a smallcluster, the ability of the cluster to interact withsurrounding molecules changes depending on the atomicstructure of the surface.

Fig. 3: Representation of a 55-Atom Ni Cluster Showing

Location of Three-Fold Hollow and Two-Fold Bridge Sites aswell as Location of a 56thNi Atom Interacting with Water.

56thNiatom

H atom in three-fold hollow site

H atom in two-fold bridge site

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For example, the structure of 55 Ni or Co atoms isthat of an icosahedral sphere. This structure can bind 44hydrogen atoms in triangular-hole and two-atom-bridgesites. Addition of a 56th Ni or Co atom, puts this atomoutside of the sphere, sticking out. This 56thatomshows a high affinity for binding of water or ammonia ina coordinate covalent bond.4

Comparison of clusters formed with similarelements is also instructive. For example, Co and Ni

clusters of the same size form similar cluster structures,but with important differences. The cluster structurechanges as the size changes with some size rangesforming the more closely packed face centered cubic(Ni) or hexagonal close packed (Co) structure of thebulk solid. However, the propensity for forming theicosahedral structure is greater for Ni. This is consistentw ith the decreas e in d-electron bonding in N i relative to Co.

Recall that the extent of dorbital bonding in the solidaffects the density in all three transition series. In the

first transition series, up to Mn, the number of dorbital electrons involved in bonding is roughly equalto the number of dorbitals. Past Mn, this numberdecreases through Zn. After Zn the dorbital electronsare essentially not involved in bonding.

Goal of the Experiment:

This exercise represents an excursion into cutting-edge technology. Both the theory and experimentalconstruction methodology will undoubtedly change asthe field develops. The purpose of this exercise is to

demonstrate how fundamental principles of solubilityand band-gap energy are involved in these very exciting

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materials. In the process you will gain experience withexcluding oxygen from a synthetic system as well ascareful execution of experimental procedure. As youperform this experiment, be particularly attentive towater. Keep this in mind as you read how to create nanoparticle size materials.

This could be an open ended experiment that youcan follow through for a problem of the semesterexercise. Suggestions for further development will be

given at the end of the exercise.

The Confinement System:

One of the technological challenges of dealing withquantum-size materials , both in synthesis and application,is to limit the size of growing clusters and to preventthem from interacting with each other. If clusters dointeract, they condense and collapse into the bulk solid.This interaction must be prevented for the clusters to

exhibit quantum confinement properties. In thisexercise, you will limit cluster size by two methods;growing them in ultra-small vessels called inversemicelles, and in the small, included volume in a coiledpolymer, polyvinyl alcohol. Once formed, cluster mustbe prevented from interacting when assembled into aliquid or solid media for application. That is, they mustbe stabilized without destroying quantum confinementproperties. In this exercise, we shall not stabilize theclusters. However, as a follow up problem of the

semester, you might experiment with systems forstabilizing the micelles or capping the clusters.

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Micelles:

T o u n d e r s t a n d t h eformation of micelles, it isnecessary to understand thes t r u c t u r e o f s u r f a c t a n t s .Surfactants are also known assoap. Although soaps differ intheir detailed structure, all

consist of a long chain organic portion attached to anionic group known as the head group. In water,

collections of these molecules conglomerate with polarhead-groups pointing toward the water and the longchain organic portion imbedded in any organic basedgrease or dirt. This unit consisting of a cluster ofsurfactant molecules with ionic head groups pointedtoward bulk water is known as a micelle.

In the present case, we also want small drops in asolution. However, we do not want an organic drop inan aqueous solution, but rather an aqueous dropcontaining the material of interest in an organic solution.

Hence, the units that we will create are referred to asinversemicelles. We shall not cap the micelles tostabilize them, but rather work with them in thesynthetic solution.

The s u r f ac t an tused is referred to asAOT and is dioctylsulfosuccinate, sodiumsalt. It is called AOT

in reference to the

-Na+

F i g . 4 : A S u r f a c t a n tMolecule Showing theP o l a r H e a d a n d L o n gChain Groups.

F i g . 5 : Schematic of an InverseMicelle.

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aerosol particle it forms in solution.Polymer

In this method you will use a solution of 4%polyvinyl alcohol. In several respects this long-chainpolymer is like a micelle solution. The polymer hasseveral -OH units on it from the alcohol monomer. Thisportion of the alcohol polymer is soluble in aqueoussolutions. The remaining port ion is basical ly ahydrocarbon.

The 4% solution consists of these long chainstangled around small volumes of water. Since thepolyvinyl alcohol has no cross links, the chains moveover each other making it relatively easy to formsolutions in the occluded water. When the aqueouss olution contains a s mall clus ter of water insoluble material,a quantum dot is f or med. Thes e have very similarproperties to those formed in the s urfactant solution.

Students who have time may want to compare thetwo methods by making the CdS in polyvinyl alcohol

C C

O

H

H H

H

... ...

water side

organic side

C C

O

H

H H

H

C C

O

H

H H

H

C C

O

H

H H

H

C C

O

H

H H

H

C C

O

H

H H

H

Fig. 6: Polyvinyl Alcohol Showing Alignment of Alcohol -OHGroups on Aqueous Side and Hydrocarbon Chain on Organic Side.

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solution or the PbS in the surfactant solution. Commenton your results if you choose to do this.

The Cluster Material:

You will work with both CdS and PbS. Both ofthese materials are semiconductors in the bulk. Theband gap of bulk CdS is 2.6 eV. The band edge for bulkCdS is therefore 476 nm, absorption occurs in the violetand the bulk solid appears the complementary color, or

yellow-orange. In keeping with quantum confinement,as the size of these particles decrease, the band gapincreases. Since the CdS band gap is nearly out of thevisible to begin with, the color change is not dramatic.How will you be able to detect formation of quantumdots rather than bulk solid? Answer: using lightscattering. Since quantum particles are smaller than thewavelength of light in the visible region of the spectrum,they do not scatter light as the bulk solid, which hasmuch larger dimensions, does. Thus, a solutioncontaining the bulk solid appears turbid due to scatteringof light. In contrast, a solution of the quantum sizeparticles is clear. If your solution is turbid, think abouthow the confinement system is created and recheck yourtechnique.

In agreement with the general principal that bandgap decreases as size of atoms in a semiconductorincreases, the band gap in PbS is 0.41 eV. This is on theopposite end of the visible spectrum compared withCdS, in the infrared. As a result PbS absorbs throughout

the visible and appears black to dark brown. A solutionof quantum dot PbS is clear and reddish-brown due to

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transmission of the red end of the spectrum as the bandgap moves into the visible.

Pre Lab Questions:1. PbS and CdS are referred to as II-VI semiconductors.

Suggest a reason for this terminology.

2. Based on reactivity and stability, suggest a reason forthe exclusion of O2from the synthesis apparatus.

3. Look up or predict the band gap in CdSe and CdTe.(Predict the size of the gap relative to CdS and PbS.)

4. Could CdSe or CdTe be used to form quantum-size,semiconductor particles? Why or why not?

5. Could PbSe or PbTe be used? Why or why not?

6. Could the well known semiconductors Ge and Si be

used to form quantum size semiconductors? Wouldthe synthetic procedure differ substantially from thatused in this exercise? Why or why not?

7. Think about the role of surfactant , water, andheptane in this exercise. Which concentrations arecritically important and why?

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Procedure:1. Obtain the following from the store.

Glassware: two 125-mL round bottom flaskstwo 125-mL Erlenmeyer flasks

four 250_L syringestwo 150-mL beakertwo transfer pipettesone 100-mL graduated cylinder

Chemicals: 50-mL heptane/surfactant solution10-mL 4% polyvinyl alcohol solution

Other: one two-hole stopper and two solid stoppers

Surfactant Method, CdS:2. The apparatus is shown in Fig. 7. Add 50 mL

surfactant/heptane solution to a scrupulously cleanand dry round-bottom flask. Insert the pipetteattached to a N2purge, purge and stir vigorously for10 minutes.

3. Add 250_L H2O. Again, stir vigorously and purgefor at least 10 minutes or until clear.

4. Add 25 _L of Cd(NO3)2 to the solution whilecontinuing to stir for another 5 minutes

5. Slowlyadd 25 _L Na2S to the solution while stirring.Continue to stir for another 5 min.

6. Stop stirring. Remove the purge tube and stoppertightly. Wait 15 minutes.

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7. Remove an aliquot of the solution and put into acuvette for the HP diode array spectrometer andrecord the spectrum.

8. Repeat steps 2-6 using 50_L of Cd(NO3)2 and 50_LNa2S in steps 4 and 5. Compare your solutions.

Polymer Method, PbS:9. Combine 10 mL polyvinyl alcohol solution and 88

mL water in scrupulously clean and dry round-bottom flask. Insert the pipe tte at tached to a N2

purge, purge and stir vigorously for 10 minutes.

10 .Add 25 _L of Pb(NO3)2 to the solution whilecontinuing to stir vigorously. Stir for another 5minutes.

11. Slowlyadd 25 _L Na2S to the solution while stirring.Continue to stir for another 5 min.

12. Stop stirring. Remove the purge tube and stopper

tightly. Wait 15 minutes.

13. Remove an aliquot of the solution and put into acuvette for the HP diode array spectrometer andrecord the spectrum.

Bulk Solutions:

14. To compare quantum dots to a bulk solid, add 50 mL

of water, 25_L Cd(NO3)2and 25_L Na2S andswirl. Record your observation and compare it with

your quantum dots.

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15. Repeat 14 with Pb(NO3)2and Na2S.

Hints for Success: Keep stirring as vigorously as possible without the

stir bar going crazy.

Make sure all glassware is very clean anddry.

M eas ure all quantities ver y pr ecis ely. This exper imentdeals with relatively small quantities and is sensitive

to even minor changes.

If a precipitate is found on the bottom of the flaskthen it is possible no quantum dots formed since thisis the bulk product. (What might have gone wrong?Review your technique and ask your instructor forassistance!)

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magnetic stir plate

N2purgeVent andinjection

port

Fi g. 7: Apparat us for Synt hesi s of CdS and PbS Nanopart i cl es.

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P ost L ab Report:

1. Report any obs ervations that you made while performingthe s ynthesis . Include your s pectra. Compare yours pectra w ith your colleagues in l ab. Ar e there anydifferences? Dis cus s the effect of w ater on quantumdots .

2. Could you make quantum particles w ith any of th efollow ing: K

2S , CaS , CoS, S nS. Why or why not?

N ote: If you have not yet dis cus s ed solubility in clas s,then ask your ins tructor w hich of thes e ar e s oluble inw ater and think about the solubility of the cadmium andlead s alts used in this experiment to ans w er this ques tion.

3. Could quantum par ticles be formed with CdO , P bO,CaO? Why or w hy not? (S ee note in 2.) What color arethe bulk materials ? What color w ould the quantumparticles be?

4. S uppose that one wanted a quantum dot w hich abs orbsin the blue region of the spectrum. What color w ould itappear? What material w ould you choos e to make itw ith?

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A ppen dix A :Suggestions for further work as problem of semester:A. Work on stabilizing the micelles, perhaps with a

polymer so that they do not fuse into the bulk onremoval of the solvent.

B. Investigate the feasibility of controlling the bandedge using different cations and anions.

Appendix B:THEORY OF BAND GAP IN QUANTUM DOTS:Extension of the Bohr Model: The energy of the bandgap in quantum particle consists of a term due toconfinement of the electron and hole to the sphere plus acontribution from the Coulombic attraction of thenegatively charged electron for the positively chargedhole:

E R m m

e

Rg e h= +

2 2

2

2

2

1 1 18.

Here Eg is the change in energy of the band gap, Ris

the particle radius,_is the permittivity of free space,is Planks constant divided by 2_, eis the charge on theelectron and me (mh) is the effective mass of theelectron (hole). In PbS, these are me=0.19m a n dmh=0.8mwhere m is the mass of the electron. SeeNedeljkovic,J. Chem. Ed,70, 342 (1993).

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For large sizes, the 1/Rterm dominates and the bandgap is given by the Coulombic term. AsRdecreases,the first term grows in importance and the confinementterm dominates as the particle size gets into the 40nmand smaller range.

Modification of Band Theory: In the molecular orbitalview, electronic bands arise due to interaction of a largenumber of individual orbitals resulting in states so closein energy that they overlap into a continuous band of

allowed states bounded by a band of non-allowed states.The total number of states for the solid is equal to thenumber of atomic orbitals which interact. For everyatom removed from a bulk solid, a correspond ingnumber of extended orbitals is removed from theallowed bands of states. Due to the extremely largenumber of atoms in a typical solid (on the order of 1020

even for a small sample) this removal of states is notdetectable.

However, when the total number of atoms in thesolid is a much smaller number, e.g. on the order of 103,the total number of states is also correspondinglysmaller. Some of this large number of missing statescome from near the top of the valence band while otherscome from the bottom of the conduction band. Since theband-gap energy is the energy difference between thebottom of the conduction band and the top of the valenceband, it increases when states are missing from theseregions.

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needed

For each group:50 mL surfactant solution (25.6 g AOT per liter heptane,AOT is dioctyl sulfosuccinate, sodium salt)10-mL, 4% polyvinyl alcohol solution, polyvinyl alcoholis 124,000-186,000 molecular weight. Heat water to

~80C, stir vigorously, add PVA slowly. Continue tomaintain 80C with stirring for a couple of hours.Solution should be nearly clear.

25_L 0.6 M Cd(NO3)2 4H2O25_L 0.6 M Pb(NO3)250_L 0.6 M Na2Stwo 125-mL Round-Bottom Flaskstwo 125-mL Erlenmeyer FlasksN2for purge2 stoppers for round-bottom flasks1 two hole stopper1 cuvette for HP spectrophotometer250-_L syringe for

Cd(NO3)2

, Pb(NO3

)2, and Na

2S solutions

2 transfer pipettes

Magnetic stir plate and stir barN2tank with regulator for purge

For the Lab: one HP diode array spectrometer.

Instructors notes:Bulk CdS band gap = 2.53 eVPdS = 0.41 eV

bulk PbS is nearly black due to absorption of all colorsquantum size PbS is wine-red to brown - tea colored

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Answers to Pre Lab Questions:

1 . P b S a n d C d S a r e r e f e r r e d t o a s I I - V Isemiconductor s . Sugges t a r eason for th i sterminology.

Answer: The common ionization states for Pb and Cdis +2, while that of S is +6: hence II-VI.

2 . Based on reactivity and stability, suggest a reasonfor the exclusion of O2from the synthesis apparatus.

Answer: Since oxygen is more reactive than sulfur, it

may interfere with the synthesis of a sulfide by formingthe oxide or a mixed oxide/sulfide.

3. Look up or predict the band gap in CdSe and CdTe.(Predict the size of the gap relative to CdS and PbS.)

Answer: The band gap in CdSe and CdTe are smallerthan that in CdS, and CdTe is smaller than CdSe. BothCdSe and CdTe have larger gaps than PbS, although thisprediction is not so straight forward. (It has to do withthe very large size of Pb2+).

4. Could CdSe or CdTe be used to form quantum-size,semiconductor particles? Why or why not?

Answer: Both CdSe and CdTe could form quantumdots utilizing a procedure similar to the one is thelaboratory. Both CdSe and CdTe are insoluble, andNa2Se and Na2Te are soluble. Both have been made, butthe need to rigorously exclude oxygen is even strongerfor these materials.

5. Could PbSe or PbTe be used? Why or why not?

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Answer: PbSe and PbTe also form quantum dots.They could be used, but again oxygen would have to beexcluded. In these cases, the dots would be black sincethe band gap is even smaller than that of PbS which isalready in the infrared. (PbS is reddish because thequantum size particle transmits a bit of the red due to itssmall size.)

6. Could the well known semiconductors Ge and Si beused to form quantum size semiconductors? Would

the synthetic procedure differ substantially from thatused in this exercise? Why or why not?

Answer: Both Ge and Si could be used. The syntheticmethod would have to be different since these are notbinary solids.

7 . Think about the role of surfactant, water, andheptane in this exercise. Which concentrations arecritically important and why?

Answer: Since heptane is in great excess (as the

solvent) its concentration is not critical. Water andSurfactant are both critical since it is the water/surfactantratio that determined the pool size and therefore the dotsize.

An swers to Pos t Lab Q u es tion s :

1. Report any obs ervations that you made while performingthe s ynthesis . Include your s pectra. Compare yours pectra w ith your colleagues in l ab. Ar e there any

differences? Dis cus s the effect of w ater on quantumdots .

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A n swer: Report r esults and obs ervations. The effect ofw ater is ver y important. If the ves s els are not completelydry, the pools or reaction ves sels ins ide the micelles are largeand precipitate is formed rather than quantum dots .

2. Could you make quantum particles w ith any of th efollow ing: K2S , CaS , CoS, S nS. Why or why not?N ote: If you have not yet dis cus s ed solubility in clas s,then ask your ins tructor w hich of thes e ar e s oluble inw ater and think about the solubility of the cadmium and

lead s alts used in this experiment to ans w er this ques tion.A n swer: K2S and CaS are both quite s oluble, so themethod w e us ed w ould not w ork. CoS is r easonablyinsoluble and SnS is very ins oluble, hence s hould work.

3. Could quantum particles be formed with CdO , P bO,CaO? Why or w hy not? (S ee note in 2.) What color arethe bulk materials ? What color w ould the quantumparticles be?

A n swer: CdO and P bO are insoluble, s o could be us ed.

CaO is pr obably too s oluble. CdO is brow n and P bO isyellow . Both would make colorful quantum dots , though theP bO would be pale.

4. S uppose that one wanted a quantum dot w hich abs orbsin the blue region of the spectrum. What color w ould itappear? What material w ould you choos e to make itw ith?

Answer: A quantum dot that absorbs in the blue regionof the spectrum would appear orange. To absorb in the

blue region, the band gap would have to be somewhatlarger than that of CdS which absorbs in the violet

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region. CdSe is a candidate. The bulk is red, so aquantum dot might well be orange. A second candidateis HgS, also red in the bulk.

Quant Dot

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