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II 6I; Anno 20

, June 2002

! Published by. TEKNOSCIENZE ..-

LAJOS BALOGH I *CHUNXIN ZHANG ISTEPHEN O'BRIEN 2NICHOLAS J. TURRO 3LOUIS BRUS 3

INTRODUCTION

Center for Biologic.

Nanotechnology,University of MichiganAnn Arbor, MI 48109, USA

2. Depanment of Applied Physics,Columbia University

New York. NY 10027. USA ~45

3. Depanment of Cbemisb'y.Columbia UniversityNew York, NY 10027, USA

Nanomaterials andnanostructures haveorganizational featuresintermediate on a scale betweensingle molecules and about 100nm. Novel properties occur on thisscale. compared with properties of

bulk materials (1). As a result of

intermolecular Coulomb, Van der

Waals and London forces, aparticle is always surrounded withother molecules adsorbed on itssurface from the environment.However, this simple fact has

much more important implications

when we consider nanoparticles

that have extremely large

surface/volume ratio.

Fi~t, let us take a look at the

results of a simple calculation

(Table I). When we compareuniform nanoparticles to a

macroscopic object (assumingequal total volume) we find that

the surface of these CdSe quantum

dots is 2.63 million times largerthan of the bulk object. This hugesurface results in enonnous surface

energies, increased physical

activity and chemical reactivity

which cannot be neglected, unlike

Corresponding author:The University of Michigan Centerfor Biologic Nanotechnology40 1 0 Kresge Research Building n200 Zina Pitcher PloccAnn Arbor. Ml48I09-O533. USAFax +1-734-615-0621

baloghl @umich.edu

Every solid particle hasadsorbed molecules on its

surface IInder atmosphericconditions. This interface is

particularly important when asystem is composed of

nanoparticles thus change ofoptical properties can directly

be observed.

in the case of bulk materials.Present technological interest in

nanocrystals stems from the

prospect of creating novelmaterials with distinct physicalproperties. It is clear, that asignificant control of particleproperties is needed (size,distribution, interactions) tosuccessfully and reproduciblyassemble nanostructures withrequired properties. One of themajor inherent problems is ageneral one, namely that in aclosed system, all intensiveparameters (pressure, mass,temperature) equalize, while allextensive system parameters(linear characteristic size, volume,surface area. number of particles)sum up. Consequently,manipulation of unifonn particleswould require a perfect process topreserve the narrow distribution ofsize related properties.Surprisingly, there are such

processes that are close to ideal(2,3). Recent improvements in thesynthesis of CdSe nanoparticlesorganically passivated with

trioctylphosphines, allow one to

obtain very low polydispersity

nanocrystals with only a 5-10%

standard deviation in size and with photoluminescence(PL) quantum yields of 10-15%. We will illustrate theimportance of surface conditions from the point ofview of the surface chemistry of these inorganicsemiconductor nanocrystals. The interest in this fieldkeeps increasing due to various potential applicationsof QDs, such as in optical applications (4,5)optoelectronic devices (6-8) and multi-colorbiosensing (9-11) as they are more stable againstphotobleaching than functionalized organic dyes.Understanding and controlling factors that govern

these properties will benefit everyone in society, asthese materials and structures open up the waytowards smaller, faster, bright, and low-cost devices.

SEMICONDUCTOR NANOCRYSTALS

processes in nanocrystals, such as spectral diffusionand the random intermittency of the fluorescence(19-21). It was observed that single CdSe

nanocrystals embedded in a thin polymer film do

individually "blink" (the luminescence turns on/oft)on a several second time scale (17). In previous

experiments this "blinking" was missed, as it

averages to zero when many nanocrystals are

observed simultaneously. Fluorescence intensityversus time traces taken at low and at high intensityshow that the "on" period duration scales inverselywith excitation intensity, while the "off" periodappears to be intensity independent. This resultsuggests that the nonemissive state is created via thenanocrystal excited state. The bright state is howeverrecreated by a spontaneous thermal process from thenonemissive state.

In the absence of defects, internal or surface, ananocrystal should e~hibit near unity fluorescencequantum yield, and partial protection from

quenching. This represents a significant advantageover organic chromophores.

STABILIZATION OF CdSe QDs BY INORGANICOVERCOATlNG: SHELL NANOCRYSTALS

46

Luminescent properties of semiconductor nanocrystalshave attracted a lot of attention recently because atroom temperature nanocrystals can be betterphotoemitters than bulk semiconductors.Semiconductor nanocrystals possess unique propertiesthat change with the particle size due to sizequantization effect on the nanometer scale (12-15).These properties are equally interesting for physicists.material scientists and chemists.Semiconductor nanocrystals. (sometimes are called

quantum dots. QDs). exhibit these unique optical andelectronic properties due to confinement of theelectronic excitations. These nanocrystals act likemolecules as they interact with light via theirelectronic transition dipoles (16). The importantdifference is that the radiation less internal conversion(recombination of an electron-hole' pair byunimolecular decay converting electronic energy into

heat) is extremely slow in these nanocrystals and

excited states decay radiatively in a defect-free. directgap semiconductor such as CdSe (17). These simplefacts have important practical consequences.Semiconductor light-emitting diodes have narrowemission bands and can show near 30% efficiency inconverting electrical power into light. Semiconductorlasers and diodes also show excellent long-termstability against photochemical and current-induceddegradation. when compared with many organic

materials. Their photoluminescence (PL) emission can

be tuned across the visible spectrum by simply

changing the particle radius for CdSe nanocrystals( 18).

NOTE: The strong size dependence of nanocrystaloptical properties. while an asset in their practical use.represents a problem in scientific experiments. Eventhe best samples may have distributions of size. shape.and surface chemistry. and thus have slightly varyingoptical properties.

SINGLE NANOCRYSTAL LUMINESCENCE

After a decade of research, recent single nanocrystalluminescence studies still have revealed new

While the "band edge" emission is intrinsic in origin,the surface still provides sites for nonradiative

recombination. As synthesized, the TOPOffOPSe

passivated CdSe nanocrystals exhibit room-

temperature quantum yields between 5 and 10%.

Substitution of the TOPOffOPSe surface cappinggroups results in variations in the luminescence yieldand a suppression/increase in the "deep trap"emission (22,23).To increase the quantum yield the elimination of

midgap surface states is necessary with appropriatesurface passivation. This is usually done either byinorganic overcoating or by absorbing protectiveorganic molecules onto the surface.The beneficial effect of inorganic overcoating was

understood in terms of a higher band gap material(ZnS) passivating a lower band gap material (CdS)(24). Near epitaxial monolayer of ZnS on CdSenanocrystals (25,26) exhibit quantum yields of up to50% at room temperature. However, the 12%

difference in bond lengths between CdSe and ZnS

results in significant lattice mismatch and increasingstrain with ZnS thickness. Recently CdS-overcoatedCdSe nanocrystals with only 3.9% of lattice

mismatch have also been synthesized (27).Encapsulated nanocrystals exhibit improved

photostability. However, a strong decrease in thephotoluminescence (PL) intensities was observedwith time even for highly luminescent (CdSe)ZnS

core-shell nanocrystals in quantum dot composites,due to degradation. Experiments carried out at highlaser fluences show that the PL intensity decay withtime depends both on the size of the nanocrystals andthe nature of the surrounding matrix (28).Based on theoretical considerations, the influence of

- - ~ --~-~ ~I

external charge~ on the photolumine~cence of aCdSe/Toro have recently been calculated forCds34Se~17' and for CdK,Ses3 nanocry~tallite~ (29). It

has been concluded that when a negative trappedcharge i~ on the surface near a top Cd atom. it isenough to cause a significant seventy.fold reductionon the radiative decay rate by pulling the electron andhole apart.

In short, nanocry'stals hav~ the pot~ntial to ,fen'e a.f;d~al chromophores if th~;r surface chemi,ftry' can btunderstood and controll~d.

- --- ---

Figure I - Differences in photoluminescence betweenchloroform solutions of otherwlw identical CdSe quantumdots with different organic surfaces as observed in front ofa UV lamp (365 nm)

ORGANIC SURFACE LAYERS ON THE CDSESURFACE capped with organic molecules as a function of their

binding functionalities. In our approach. we haveused constant amount of nanoparticles and preset anidentical amount of air present in each vial. We alsoused chloroform as a solvent. which does not favorthe absorption of moisture. Therefore. we assume thatshort-term changes are dominated by the exchange ofligands and their donacity. but later data areinformative of the protection of nanoparticles by thevarious dendrimer caps. We followed the optical

properties of the CdSe/PAMAM systems for over two

months.

47

Now it is generally accepted that the origin of thenarrow, band-edge photoluminescence of QD bandedge emission is intrinsic (30-32), while the surfaceprovides nonradiative recombination sites thatinfluences quantum yield (17). Organic ligands weremainly used only to provide QDs with solubility andstability. Recently. several studies have reponedresults emphasizing the importance of organic surfacelayers (33). For example, enhancedphotoluminescence was observed using simple amine(e.g. triethylamine) caps for CdS or Cd;lAs2 (34)

thioglycolic acid for CdTe (35). and hexadecylaminefor CdSe (36).

The PL enhancements .in the above cases were

explained by binding at the defect Cd site to decrease

nonradiative recombination. formation of cadmium

thioglycolic acid complexes. and a redistribution ofsurface electronic density. respectively. The ad!iorbedmolecules not only determine the stability of thenanopanicles, but also influence their opticalproperties and control their compatibility with theactual physical environment.

Apparent/.", or,~clnic .\"urface /igands p/a.¥ a very'

important role in cha,rging the PL propertit's of QD.\".¥.\"tt'ms. ho"oe"el: tlri.\" i.\".\""t' i.\" not comp/ett'/.¥

undt'rstcJOd so fa I:

Systematically varying the binding moieties "'efound that different binding functionalities led todramatically different PL properties for the CdSeQDs (Figure I).We started with CdSe nanocrystals capped with

TOPrfOPO (trioctylphosphine and

prioctyplphosphine oxide). Based on NMR (371 and

XPS (38) analysis ofCdSe nanoparticle surface. it is

believed that TOPO coordinates with the Cd sites on

the surface. (Notice that although Cd atoms are

usually passivated by the TOPO molecules. for the

unpassivated Cd dangling bonds. they are the

trapping sites for electrons.)The TOPrfOPO were first exchanged with pyridine.

which was then exchanged with the new caps ~aringamine. acid and thiol functionalities - lauric acid.dodecanethiol. and octadecylamine (Scheme I). Thequantitative cap exchange ~'a~ always checked by

NMR.

Equal aliquots of the ~tock ~olutions ~ere stort'd inCHCI.1 in ~eparate po~itive sealing glass ,'ials "ith

-iJin.

Mechanism of "capping" nanoparticles with organicmolecules appear to be similar to metal ioncomplexation. i.e., a donor-acceptor type interactionbetween the NP surface and the ligand site of theorganic molecule. If surface caps werebound covalently, dry nanoparticles mayremain soluble when solvent is added.

However. in ca.~ of a dynamic ' " '~

equilibrium between the absorbed and f -I'll"

the "free caps". an excess of organic , ~r:11 ~ 'cmolecules must be present in the ! .11 I

solution and dilution of a sample may -;( ;F

lead to precipitation.

In an effort to elucidate the influence ofthe different functional groups. westudicd the optical behavior and stability ~

~

(j ""I

Schenle 1 . Cap ~~chan~e prf)Ccdurl

were made to remove the TOPOrrOp and/or quantifythe amount of residual phosphorus caps. As a result.what have been studied are usually a mixture of newcaps and TOPOrrOp of unknown amounts and

proportions.

Teflon-lined screw caps. Each vial contained 0.50 mL

solution and 1.5 mL air. The vials were also sealed

with parafilm and kept in a refrigerator in the dark.Only one set of samples was removed each time.when the solutions were transferred into closedquartz cuvettes for PL and UV-vis absorptionmeasurements. Optical measurements were conducted

on Day 1.4. 12.26.33.42 and 70. (Details arereported in a separate publication (39).)

48

Absorption spectra of CdSe with different capsWe have attempted the reconstruction of the original

starting material from the CdSe/pyridine and we

found that it took about 10 times of the 'removed'TOPO to bring the CdSe back to solution. An

insufficient amount of TOPO simply led to insoluble

CdSe aggregates.

Shown in Figure 2 are the UV-vis spectra of thelauric acid, dodecanethiol and octadecylamine cappedidentical CdSe right after the new caps have beenintroduced and the pyridine completely removed.Compared with the original CdSe/TOPOrrOp,CdSe/pyridine intermediate shows a blue shift of

-20nm in its absorption edge. However, when it wasreplaced with dodecanethiol, the original positionwas fully restored (with lauric acid or

octadecylamine, it was only partially restored). The

immediate changes either in PL or in absorption

wavelength and intensity cannot be related to sizechange of the CdSe particles. Apparently, the above-observed shifts were caused by the interactionsbetween the CdSe surface and ligands, similar to theredistribution of electronic density in thesemiconductor under the influence of passivatinggroups.

What we have learned from the CdSe surfaceexchange proceduresWe have found that CdSe/pyridine is an essentialintermediate in this process because a direct capexchange from TOprrOPO to the new caps isimpossible. Pyridine capped and vacuum-dried

CdSe/pyridine particles were insoluble in anysolvents including chloroform and toluene. and theywere only soluble in pyridine and piridazine (40).However. surface bound pyridine in the solution ofCdSe/pyridine system. can be replaced readily bynew functional caps and the resultant QDs weresoluble in common organic solvents.

It must be noted that even after seven cycles ofpyridine exchange there are still some residual andpresently unknown phosphorous compounds

remaining. the amount of which can not be reducedany further (41.32).We performed a quantitative surface derivatization

of CdSerrOporrOP using P(OMe)3 as an internal

standard in IH- and 31P-NMR analysis. we were ableto quantify the remaining phosphorous compounds(expressed as TOPO). These compounds werereproducibly 10-20% of the weight of CdSe fordifferent batches of CdSerrOPOrrop. However. withthe new caps introduced. the phosphorus residue onlyaccounts for -I mol% of the total amount of caps inour experiments. so the change of optical propertiescan be safely attributed to the new caps.

NOTE: in most of the previous work on surfacederivatization of CdSerrOPOrrOP. limited efforts

-Figure 2 . l'V.vls absorption of CdSe nanocrystals takenright after the new caps were introduced

-~ -'--' -

0.08

I0.04

0.02

Photoluminescence

While the UV-vis absorption of the acid, amine andthiol capped CdSe showed blue shift and decrease inintensity. the PL of these systems also showed blueshifts, but the intensity increased over time. Shown in

Figure 2 are the PL spectra of amine. thiol and acid

capped CdSe on Day I, in comparison with those on

Day 10. In the case of lauric acid capped CdSe. the

PL intensity exhibited an order of magnitude increase

during the two months, and the final PL intensity ""as

about three times of the original CdSerrOPOfTOP(Figure 3). The observation that placement ofcarboxyl groups that carry negative charge over theCdSe nanocrystals binding sites leads to increa~e in

PL intensity, seems to support nicely the theoretical

calculations above.

Different surface ligands also led to different decaypatterns over time. Among the three functionalligand~ studied. lauric acid greatly enhanced thephotoluminescence, and led to a three-fold increase inintensity compared to the original CdSefTOPfTOPOnanoparticles. Other ligands resulted in lower activityand faster decay.Gradual blue shifts of the absorption and emission

maxima were observed over time, probably due toslow oxidation of the nanoparticle surface. PL

intensity of the resultant CdSe nanoparticles showed

an initial increase. and then decreased over time.This "a~in~" effects can be attributed to a

1200000

1000000

800000

I

i800000

400000

200000

QI .00 500 600 I

L[. Yo8veIength (nm)

I FiauR J . PL spectra or CdSe nanocrystab taken rlabtalter die .w caps were Introduced

!

700

reproducibly from generation to generation.Physical properties of dendrimers, such assolubility, and interactions with thesurrounding environment, can be altered bychanging the characteristics of terminal groups(42-44).

We have selected hydrophobically modified

poly(amidoamine) PAMAM dendrimers to

carry multiple electron donating amine ligands(45,46).Using dendrimers as surface ligands, we wishto develop a universal method for adjusting thenanoparticle surface in a controlled manner.Dendritic molecules, with exact number offunctional groups, should provide excellentmodel compounds to study surface pas~ivation

I of NPs. These multifunctional dendrimer capspossess "heads" that are able to bind to the NPsurface, while other parts of the molecule may

800 increase the solubility and may provide

stability against aggregation and crystalgrowth. In our case the binding is providedthrough the fourteen lone electron pairs per

molecule located on the tertiary nitrogens that serveas branching sites in the dendrimer structure.When generation 2.0 PAMAM with primary amine

terminal groups was added directly to the,CdSerrOprrOPO system or to the CdSe/pyridinesolution, (d=5.5 nm CdSe was used in theseexperiments) a solid precipitate formedinstantaneously, which proved to be insoluble in anysolvents. This precipitation is most likely induced bythe strong interactions between the primary aminogroups on the multifunctional dendrimer surface andthe CdSe surface. To decrease the inter-particleinteraction through terminal amine groups and avoidphysical cross-linking, we have modified the surfaceproperties of the full generation ethylenediamine

(EDA) core PAMAM dendrimers by reacting it with1.2-epoxyhexane (47). In order to tailor theamphiphilic properties of PAMAMs we varied the1.2-epoxyhexane/dendrimer ratio. Starting with 8. 16.

24. and 32 equivalents of 1,2-epoxyhexane. we

obtained PAMAMs with 8. 16. 24, aad 28 respective

side arms attached on average. designated as

PAMAM_E2Rs. PAMAM_E2RI6' PAMAM_E2R2~.

and PAMAM_E2R2S'. The previously described

standard cap-exchange procedure was used(Scheme 2).

The flexible network of the PAMAM_E2.Rn

molecules undergo a conformational change upon

49

the QD size or surface characteristics or of both.

TEM analysis of the aged QDs did not indicate a

significant change in size. Thus. we believe that theslow and irreversible blue shifts in absorptionindicate the shrinkage of the effective particle size,creating CdSe particles surrounded with oxidespecies. However, if "shrinkage" is the origin of theobserved effect. only a very small change is requiredto produce the effect.With the blue shift of absorption edges, absorption

intensity of CdSe systems showed a gradual decrease

in which suggest the slow degeneration! disassembly

of nanoparticles especially by a primary amine evenin a chlorofonn solution.

The size of the above CdSe nanoparticles was

studied under TEM, but no significant change in size

was observed. Based on the absorption and emission

changes observed in (he CdSe systems, we think that

the effective size of the CdSe particles may get

smaller over time due to surface oxidation. This

explanation is also supported by the PLcharacteristics. as indicated by the blue shift of thePL emission maxima.

AMPHIPHILIC DENDRIMERS AS CAPSScheme 2 . Surface modincalion of CdSenanopa"lcles by amphiphilic PA\'AM dendrimers

~.

P' ridilM:. .~Dk"e

n)I~Jl()l'

C 0o~o ~\I.UI

00

..~""

p~ndi~

Ba~ed on analogy with metal ion chelation. we

as~ume that multi-ligand polymer~ should form more

stable ~urface caps than molecules with a singleligand if bound by non-covalent interactions.Thus. in addition to single molecular agents. we also

placed dendrimer caps on preformed CdSenanocrystals. These molecules have pre-determinedsize with narrow molecular weight distribution~ and

. .. ... . ..~:, I , . . ., ,.:...11. .," ~

,1/" 1 1/-I' .. ..-'u "." "),'"

71~" «*'.Iof

the slight differences in chemical structure, e.g. thenumber and nature of amine groups of the modifiedPAMAMs involved in the capping.Our observations suggest that primary amines (just

like ethylenediamine) probably slowly react with thesurface of the CdSe nanoparticles gradually makingthem physically smaller. Following the absorption

and PL change over a two months period (Figure 5),

we have found that the PAMAM_E2.R!8 provided the

most effective protection of the NPs against oxidation

(about 50% reduction in PL intensity in dilute CHCl)

solution under air in 65 days).These simultaneous actions of passlvation,

stabilization, and compatibilization are due to theadsorption of the amphiphilic dendrimers on thesurface of the nanoparticles.

CdSe/PAMAM_E2.R280.2

0.18.

0.18

0.14

0.12

0.1...

O.~'

O~.

. I

004.

0.02

o.

400

~!~~

50500 700J -I

-Fi~~~5~. c~:~rls~n or L'V~Vlslble 5~~~~~~-1-~~~~~~~~unctlon or time !

75t1450 55OaeeoW8vei8ngII (rwn)

Filure 4b . Comparison or VV.vis absorption and PLspectra or 1M CdSe/PAMAM_E2.Rn.series tak~n on Day I

--

In summary, there seems to be two simultaneousmechanisms for the changes in thephotoluminescence of the CdSe/organic cap systems.One is a reversible process. which is probably relatedto the direct physicochemical interaction between the

CdSe and the surface ligands. and an irreversible one.likely related to the shrinkage of effective particlesize and other chemical reaction. such as oxidation ofsurface layers without losing the atoms of the surfacelayers themselves to the solvent. The reversible shiftoccurs when new caps are introduced. and theirreversible one occurs during storage in ambientatmosphere for a longer time. A plausible explanationfor the irreversible changes. is the slow surfaceoxidation of the particles. similar to the CdSe/ZnSsystem (23).

FUTURE RESEARCH

In our present research we focus on the betterunderstanding of nanoscale phenomenon using avariety of particles and organic molecules. To gain abetter insight into the mechanism of the change of the

_C'dSe/PAMAMs. a more d~t:lil~d -:tllrlV Ilnrl..r "tri"tlv

adsorption. and the outer carbon chains form ahydrophobic halo around the CdSe nanoparticlemaking it soluble in apolar solvents. The absorptionoccurs as the interaction of the electron donatingamines with the electron deficient Cd sites.

(Calculated number of dendrimers that may be

directly absorbed is approximately between 7 and

30.) The resultant CdSe/PAMAM systems were well

soluble in common organic solvents.Amine surface ligands. as hole acceptors. should

quench the photoluminescence of NPs througheliminating the surface trapped electron-holerecombination via binding to the hole trapping

emission sites on the surface (36). This quenching

effect was also observed in our current dendritic

PAMAM modified CdSe systems. compared with the

original TOPO passivated NP.

Despite of their very similar structure theseamphiphilic PAM AM dendrimers provided different

~~n'n~';n~.. f~~ .h.. PoJp,. W.. thin\- rh"r rhi~ j.. cill" tn

controlled conditions is in order. We are presentlyinvestigating the optical properties of CdSenanoparticles modified by various surface ligandsthat are bound to linear and dendritic molecules.These results will be reported in a forthcoming paper.In another similar study, higher generation and hencemore rigid PAMAM dendrimers are to be used toprovide further insight into the utilization ofamphiphilic dendritic ligands as nanoparticle surfacemodification agents. In the long run. nanoparticles

that are stable in air will be essential for many

different applications. To achieve this stabilization is

an existing challenge for both chemists and materialscientists.The key to the success of our future research lies

certainly in better understanding of the underlyingscience in these structures. Better understanding ofthese nanomaterials will lead ultimately to bettermaterials and construction of novel structures.Patterned placement of uniform nanomaterials can

generate new nanostructures with useful, and

consequently many innovative applications will open

up in the near future.

ACKNOWLEDGMENT

This work has been funded in whole with Federalfunds from the National Science Foundation by Grant

# DMR 9809687 and DMR 9809687 NO 2 as part of

the MRSEC Program at the Columbia University.

New York, NY. :51

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