ARTICLE IN PRESS

Microelectronics Journal 40 (2009) 644– 649

Contents lists available at ScienceDirect

Microelectronics Journal

0026-26

doi:10.1

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Univers

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journal homepage: www.elsevier.com/locate/mejo

Applications of colloidal quantum dots

Ke Sun a, Milana Vasudev b, Hye-Son Jung a, Jianyong Yang a, Ayan Kar a, Yang Li a, Kitt Reinhardt c,Preston Snee d, Michael A. Stroscio a,b,e,�, Mitra Dutta a,e

a Electrical and Computer Engineering Department, University of Illinois at Chicago (UIC), 851 S. Morgan Street, Chicago, IL 60607, USAb Bioengineering Department, University of Illinois at Chicago, 851 S. Morgan Street, Chicago, IL 60607, USAc Physics and Electronics Directorate, Air Force Office of Scientific Research, Suite 325, 875 N. Randolph Street, Arlington, VA 22203, USAd Chemistry Department, University of Illinois at Chicago, 845 W. Taylor Street, Chicago, IL 60607, USAe Physics Department, University of Illinois at Chicago, 845 W. Taylor Street, Chicago, IL 60607, USA

a r t i c l e i n f o

Available online 8 August 2008

Keywords:

Colloidal quantum dots

Nanostructures

Nanoelectronics

Nanobiotechnology

Conductive polymers

Nanostructure–biomolecule complexes

92/$ - see front matter & 2008 Elsevier Ltd. A

016/j.mejo.2008.06.033

esponding author at: Electrical and Compu

ity of Illinois at Chicago (UIC), 851 S. Morgan S

ail address: stroscio@uic.edu (M.A. Stroscio).

a b s t r a c t

This paper addresses a number of major trends underlying the continuing effort to realize practical

optoelectronic, electronic, and information-processing devices based on ensembles of quantum dots

assembled in a variety of matrix materials. The great diversity of such structures makes it possible to

fabricate numerous ensemble-based devices for applications underlying photoluminescent devices,

light-emitting diodes, displays, photodetectors, photovoltaic devices, and solar cells. In addition, the

application of colloidal quantum dots to allied technologies such as nanobiotechnology is considered for

the case of monitoring conformational changes in biomolecules using luminescent quantum dots.

& 2008 Elsevier Ltd. All rights reserved.

1. Introduction

This paper focuses on the applications of colloidal quantumdots (QDs) and places emphasis on ensembles of such colloidalQDs. Colloidal QDs were studied in the pioneering work ofMichael Faraday, but the last two decades have been marked byenormous progress in the use of QDs for applications. First, QDsself-assembled during growth on a two-dimensional semicon-ductor surface have been initially studied over the last twodecades by international semiconductor device community; seeas examples Refs. [1–8]. Second, over the last 10 years thesemiconductor device community has focused increasingly onthe study of applications of colloidal semiconductor QDs. In thispaper, applications of colloidal QD ensembles to optoelectronicdevices by exploiting advances, including: (a) advances in the useof molecular linkers/spacers; (b) the ability to assemble QDs inensembles; (c) the ability to disperse QDs in conductive polymers;and (d) modeling efforts that have predicted the advantagesof using such QD ensembles in device applications have beenconsidered. In addition to optoelectronic devices based onensembles of colloidal QDs, this paper considers applications ofsuch colloidal QDs in the study of biological systems, includingbiomolecules.

Many different optoelectronic devices—including photodetec-tors, solar cells, and photon sources—are potentially realizable

ll rights reserved.

ter Engineering Department,

treet, Chicago, IL 60607, USA.

based on the variety of QDs with a range of band energies, asillustrated in Fig. 1, and the wide range of lowest unoccupiedmolecular orbital (LUMO) and highest occupied molecular orbital(HOMO) energies of conductive polymers, as illustrated in Fig. 2.By embedding colloidal QDs in conductive polymers, it is possibleto collect electrons with selected conductive polymers and holeswith other conductive polymers [9]. Moreover, for QD ensembleswith reasonably uniform dot-to-dot spacings, it is predicted thatminibands may be formed [10,11]. Moreover, innovative techni-ques for using nanostructure–biomolecule complexes in colloidalQD applications are discussed in this paper. Refs. [12–15] providebackground information on the conductive polymers used inthese studies.

2. Optoelectronic devices based on ensembles of colloidalquantum dots embedded in conductive polymers

Ensembles of colloidal QD electro- and photoluminescencedevices fabricated of CdSe/ZnS core–shell QDs functionalized withorganic ligands and incorporated into multilayered light-emittingdiodes have been considered by Zhao et al. [16]. In this paper, theapproach of Lazarenkova and Balandin [10], where three-dimen-sional minibands are formed by regimented organization of QDs,is modeled via the use of the envelop function approximation forfinite one-dimensional arrays of QDs embedded in conductivepolymers. As in the case of Lazarenkova and Balandin [10],miniband formation is predicted. For some QD–conductive-polymer systems, the transmission coefficients are close to unity

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0

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Evac (eV

CdS

Ga

SnOFe2O3

TiO2ZnCdS

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WO3

Ec

Ev

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Fig. 1. Conduction band energies, Ec, of various semiconductors are denoted with a

dashed line, and the valence band energies, Ev, with a solid line. All energies are

measured relative to the vacuum level in electron volts (eV).

0

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HOMO

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Fig. 2. The lowest unoccupied molecular orbitals (LUMOs) and the highest

occupied molecular orbitals (HOMOs) are depicted here for a selection of

conducting polymers. Poly(2-methoxy-5(2-ethyl) hexoxy-phenylene-vinylene

(MEH-PPV), peroxydisulfate (PDS), poly(3,4-dicyanothiophene) (PDCTh), poly-

(phenylenevinylene) (PPV), poly-3-hexylthiophene (P3HT), polyvinylpyrrolidone

(PVPy), N,N0-diphenyl-N,N0bis(1-naphthyl)-1-10biphenyl-4,400amine (a-NPD), cop-

per-phthalocyanine (CuPc), and 3,4,9,10-perylenetetracarboxylic dianhydride

(PTCDA) have been selected for inclusion in this summary as a result of the

suitability of their LUMO and HOMO energies for device applications. The LUMO

and HOMO energies are measure relative to the vacuum energy, Evac, in electron

volts (eV).

0 500 1000 1500 2000 2500 3000 3500 400010-200

10-150

10-100

10-50

100

Energy (meV)

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Coe

ffici

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Transmission Coefficient - Energy

Fig. 4. Hole minibands calculated using the envelop function approach of Sun et

al. [11] are illustrated for 5-nm-diameter InP quantum dots embedded in CuPC

conductive polymers with 1-nm dot-to-dot spacings.

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Fig. 3. Electron minibands calculated using the envelop function approach of Sun

et al. [11] are illustrated for 5-nm-diameter InP quantum dots embedded in PTCDA

conductive polymers with 1-nm dot-to-dot spacings.

K. Sun et al. / Microelectronics Journal 40 (2009) 644–649 645

and the miniband structure is relatively insensitive to 10–20%variations in the QD placement within the arrays. For such arrays,Sun et al. [11], Vasudev et al. [17], and Yamanaka et al. [18] havemodeled the transmission coefficients of carriers in CdSe, GaN,and TiO2 QD arrays in a variety of conductive-polymer matrices,for both regular and irregular interdot spacings. Such structureshave been fabricated by a growing number of authors, including,as examples, Ouyang and Awschalon [19], who use bifunctionalchemical linkers as a way of providing control over the spacingsbetween adjacent QDs, and Stroscio et al. [20], who have usedbiomolecular peptides to chemically self-assemble alternatinglayers of CdSe–ZnS and CdS on a Au substrate; in the latter work,chemically assembled colloidal QDs of CdS and ZnS-coated CdSeformed an emsemble of dense (41017 cm�3) QDs linked togetherwith the biomolecular peptide of three glycines and one cysteine,GGGC.

In Figs. 3–8, the minibands calculated using the envelopfunction approach of Sun et al. [11] are illustrated for 5-nm-diameter InP QDs embedded in PTCDA, CuPC, and a-NPDconductive polymers with 1-nm dot-to-dot spacings.

For these InP-based colloidal QD-based ensembles, the trans-mission coefficients are seen to be close to unity for many of theminibands in Figs. 3–5. In connection with these two-polymercollection models, it is noted that the use of two differentpolymers for carrier extraction has been demonstrated by Maet al. [9], where post-device-fabrication annealing is shown toenhance carrier collection. Moreover, the electron and holeminiband structure for the InP-based system of Figs. 3–5 has aminiband energy structure that is appropriate for the biexcitonexcitation scheme of photovoltaic energy conversion consideredby Schaller et al. [21] and Nozik et al. (see Ref. [22]). Thesedevelopments are especially promising in light of the great varietyof possible nanostructured devices, including solar cells; however,in the case of solar cells a number of known difficulties occur.

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Fig. 5. Hole minibands calculated using the envelop function approach of Sun

et al. [11] are illustrated for 5-nm-diameter InP quantum dots embedded in a-NPD

conductive polymers with 1-nm dot-to-dot spacings.

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Fig. 6. Electron minibands calculated using the envelop function approach of

Sun et al. [11] with an applied bias of 0.2 V are illustrated for 5-nm-diameter

InP quantum dots embedded in PTCDA conductive polymers with 1-nm dot-to-dot

spacings.

Transmission Coefficient - Energy

10-200

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10-100

10-50

100

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0 500 1000 1500 2000 2500 3000 3500 4000Energy (meV)

Fig. 7. Hole minibands calculated using the envelop function approach of Sun et al.

[11] with an applied bias of 0.2 V are illustrated for 5-nm-diameter InP quantum

dots embedded in CuPC conductive polymers with 1-nm dot-to-dot spacings.

0 500 1000 1500 2000 2500 3000 3500 400010-60

10-50

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Fig. 8. Hole minibands calculated using the envelop function approach of Sun

et al. [11] with an applied bias of 0.2 V are illustrated for 5-nm-diameter InP

quantum dots embedded in a-NPD conductive polymers with 1-nm dot-to-dot

spacings.

K. Sun et al. / Microelectronics Journal 40 (2009) 644–649646

As has been highlighted previously [12] these difficulties forquantum-well-based solar cells include: (a) the polarization-sensitive nature of the absorption reducing absorption efficiencyand necessitating the use of elaborate incident optics, (b) thevoltage drop across complex structures leading to reduced poweroutput, and (c) the localization of carriers increasing the difficultyof carrier collection, which has motivated carrier collectionthrough complex means such as hot-carrier transport or minibandtransport. In the case of QD-based nanostructured solar cells,the difficulty associated with polarization dependence is absent.Moreover, the miniband engineering concepts discussed pre-viously in this paper offer a yet-to-be-tested approach of thehighly efficient carrier collection, except for that associated withthe severely restrictive polarization dependence of quantum-well-based devices. Nevertheless, there are ongoing efforts aimedat realizing QD-based solar cells. As an example illustrating the

difficulty of achieving high efficiencies, The work of Ruangdetet al. [23] on performance of structures based on multi-stackedhigh-density InAs QDs demonstrates the difficulty of realizinghigh efficiencies. In particular, Ruangdet et al. [23] used a thin-capping-and-regrowth molecular-beam epitaxy (MBE) process tofabricate multiple layers of InAs-based QDs and show by electricalcharacterization of homojunction p–n solar cells with one layerof QDs and five layers of QDs that a short-circuit current of14.4 mA/cm2 results for the five-layer case as compared to9.6 mA/cm2 for the one-layer case. The efficiency for the five-layer case is only 5.1%. In related studies, Luque and Marti [24],Marti et al. [25], and Luque and Marti [26] have analyzed QD-based solar cells utilizing intermediate bands. Luque and Marti[26] presented an analysis under ideal conditions of a solar cellwith an impurity level in the semiconductor band gap. For theseideal conditions, an efficiency of 63.1% was considered possible

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instead of the Shockley and Queisser model limit of 40.7%; seeWerner et al. [27]. In attempts to implement this scheme, theresults of Luque and Marti [24] indicate that the absorptionof light in a ten-layer system is low and there was a consequentneed to increase the number of layers while avoiding materialdefects potentially associated with the use of thicker layers.Finally, regarding QD-based solar cell concepts, Nozik [28] hasestimated that QD-based solar cells have the potential to increasethe maximum attainable thermodynamic conversion efficiencyof solar photon conversion up to 66%; these assessments take intoaccount the potential exploitation of hot photogenerated carriersto produce higher photovoltages or higher photocurrents.

As just emphasized, the ability to apply a bias potential acrossthe QD-based device structure is essential. In the case of InPembedded in PTCDA the miniband structure is seen to be welldefined for applied voltages of 0.2 V.

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Fig. 9. Electron minibands calculated using the envelop function approach of Sun

et al. [11] for 5-nm-diameter CdSe quantum dots embedded in MEH-PPV

conductive polymers with 1-nm dot-to-dot spacings.

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Fig. 10. Electron minibands calculated using the envelop function approach of Sun

et al. [11] for 5-nm-diameter CdSe quantum dots embedded in MEH-PPV

conductive polymers with 0.7-nm dot-to-dot spacings.

For the case of CdSe embedded in MEH-PPV, the minibandtransmissions are found to be substantially less than unity for5-nm QDs and 1-nm dot-to-dot spacings as illustrated in Fig. 9.

However, for the case of CdSe embedded in MEH-PPV, theminiband transmissions are found to be close to unity for 5-nmQDs and 0.7-nm dot-to-dot spacings as illustrated in Fig. 10.

Such 0.7-nm spacings are achievable using short biomoleculessuch a peptides as was demonstrated in Ref. [20]. These theoreticalresults and the great diversity of possible QD–conductive-polymerstructures imply that miniband engineering is a promising approachfor optimizing the performance of QD-based optoelectronicdevices, including: photoluminescent devices, light-emitting diodes,displays, photodetectors, photovoltaic devices, and solar cells.

3. Novel applications of the optoelectronic properties ofcolloidal quantum dots to nanobiotechnology

Luminescent QDs find a novel application in the study of theconformational states of deoxyribonucleic acid (DNA). DNA is thefundamental building block of the genome and the expression ofgenetic information involves interacting chemically as well aselectronic and ionic interactions. The use of DNA as a molecularwire in electrical circuits has been considered and there areongoing studies of DNA’s conductivity [29]. Its various properties,including self-assembly, replication, modification by usingenzymes, and resilience to heat, make DNA a robust choice as amolecular nanowire. Lattices made of DNA may potentially beused as scaffolds for the nanoelectronic circuits [30]. Here, weattempt to design a nanomechanical switch based on theconformational transformation property of DNA; i.e., DNA innature can exist in A, B, and Z forms as its native form. The mostcommonly known conformation is the B form, which is a right-handed double helix, whereas the Z form is a left-handed doublehelix [31]. DNA can switch between one conformation to the otherwhen there are controlled conditions, including the change in saltconcentrations such as hexamine cobalt chloride and having analternating stretch of adenine and guanine bases in the DNAstructure [30]. Such a DNA structure can be used to change therelative location of a luminescent QD and a nearby quencher asshown in Fig. 11. This change in distance produces a change in theintensity of the luminescence due to the change in the efficiencyof the resonant energy transfer between the QD and the quencher;ideally, the QD–quencher complex behaves as a switch betweenthe quenched and the non-quenched states as it switches from Bform (native form) to Z form, causing the central GCG–CGCsegment of the DNA to rotate. The DNA strands that form the stemwere obtained with d(mC) [deoxycytosine with methylation]molecules, which implies that the 5-position of the cytosine baseis methylated, which is a modification known to increase theability of DNA to undergo the B–Z transition. The transitions thatoccur are mainly a rotary motion in the amount of 1281 for eachdinucleotide (CG). Fig. 12 shows the control case with no CGC

Fig. 11. Representation of molecular switch made of DNA designed with a

quantum dot and a black hole quencher. The methylated cytosine bases are shown

here.

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K. Sun et al. / Microelectronics Journal 40 (2009) 644–649648

bases and no methylation of the cytosine bases, which can rotatefor the transitions to form the Z state.

DNA strands used in these experiments were obtained fromIDT DNA Technologies and all the chemicals were obtained fromSigma. Four independent DNA strands were obtained with oneof the strands containing an amino modification for attachingQDs, which are used as a fluorophore in these experiments. Thequencher used for this structure is an organic one known as ablack hole quencher 1 (BHQ1; Biosearch Technology), which has ahigh quenching efficiency and a quenching range of 480–580 nms.The carboxyl QDs are attached to the DNA strand using standardconjugation procedures. The carboxyl groups allow crosslinkingwith biological materials such as proteins, peptides, and DNA inthe presence of crosslinkers such as 1-ethyl-3-[3-dimethylamino-propyl] carbodiimide hydrochloride (EDC) and sulfo-NHS. In thiscase, the amine groups on the thymine attach to the carboxylgroup on the QDs. Following this conjugation, excess salts areremoved using the Millipore centrifugal filter (MWCO 10 K) withcentrifugation at 5000g. Following the conjugation of the DNAstrand with the QDs, the four strands of DNA are annealedtogether to form the I-structure. The four strands of DNA aredesigned to be partially complementary to each other andassumed to form the I-structure on annealing. The oligos are

Fig. 12. Representation of control molecular switch made of DNA designed with a

quantum dot and a black hole quencher. The cytosine bases are not methylated.

Fig. 13. The switching of the B form to the Z form of DNA induced in the presence

of salts, shown in the quenched state.

dissolved in TE buffer (10 mM Tris pH 8.0, 1 mM EDTA with 50 mMsodium chloride—NaCl). The presence of some salt is necessaryfor the oligos to hybridize. Each oligo is dissolved at theconcentration of 20mM for all the experiments. All the DNAstrands are mixed together in equimolar volumes. Then thestrands are heated at 941C and gradually cooled to roomtemperature by using a temperature block and unplugging themachine after 6 min. The resulting product will be in stable,double-stranded form and can be stored at 41C or frozen.

The transitions occur in the solution of 10 mM cacodylate buffer(pH 7.5) with 2.5mM hexamine cobalt chloride (CO(NH3)6Cl3), 1 mMmagnesium chloride (MgCl2), and 10 mM NaCl. The DNA in B form isin the same buffer with 1 mM MgCl2 and 10 mM NaCl. The controlstrands are obtained with no modifications to the cytosine base.

The moderate level of quenching of the 525-nm QDs inFig. 13 indicates that some degree of conformation change inthe DNA–QD–quencher complex is being sensed by the changingluminescence of the QD as a function of the conformational stateof the DNA and consequently as a function of the QD–quencherseparation distance. The moderate level of partial quenching maybe explained by the relative masses of the QDs, quenchers, andDNA molecules, and may reflect that the final conformationalstate is a statistical admixture of different conformations of theDNA molecules functionalized with QDs and quenchers. Thecontrol sample of Fig. 12 is unaffected by the change in pH as isillustrated by the photoluminescent signals of Fig. 14. Theseresults indicate that QDs may be used to monitor biologicalprocesses on the scale of a single molecule.

4. Summary

The application of colloidal quantum dot (QD) ensembles tooptoelectronic devices leverages a number of advantages including:(a) advances in the use of molecular linkers/spacers; (b) the abilityto assemble QDs in ensembles; (c) the ability to disperse QDs inconductive polymers; and (d) modeling efforts that have predictedthe advantages of using such QD ensembles in device applications.The great diversity of such structures has opened the possibility ofnumerous device applications related to photoluminescent devices,

Fig. 14. Control experiment results showing no change in intensity following the

switching induced by the high salt concentration.

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light-emitting diodes, displays, photodetectors, photovoltaic de-vices, and solar cells. As highlighted in this paper, minibandengineering is among the promising avenues open to optimize theperformance of these devices. Finally, this paper has presentedthe means of exploiting the optoelectronic properties of QDs fornovel applications in the study of biological systems, includingbiomolecules.

Acknowledgments

Two of us, M.V. and M.A.S., thank Prof. N. Seeman of NYU andProf. C. Mao of Purdue University for many helpful discussions on theconformational changes of DNA between the B and Z forms of DNA.

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