IIIa). Images were produced in the height mode using silicon cantilevers (Nano-sensors, 120 mm, tip radius 5±10 nm) at a scan speed of 1 Hz. A 10 mm ´ 10 mmscanner was used for imaging. The monolayers deposited on Au(111) were ana-lyzed by scanning tunneling microscopy and spectroscopy (Pico SPM, MolecularImaging). Images were obtained in the constant current mode under ambient con-ditions at a scan rate of 10±20 Hz per image [sample bias = +200 mV, mechani-cally cut Pt/Ir (Ir = 10 %) tip]. I±V plots were obtained fixing the tip at the centerof the image and at constant conditions (bias voltage = 200 mV, current = 80 nA).A linear voltage profile between ±0.8 V and +0.8 V was applied and the currentresponse was recorded. The cycle was repeated 64 times within a time slot of36 ms and the individual responses were used to compute an averaged I±V curve.

Received: March 13, 2000Final version: May 9, 2000

±[1] a) M. Schulz, Nature 1999, 399, 729. b) D. A. Muller, T. Sorsch, S. Moccio,

F. H. Baumann, K. Evans-Lutterodt, G. Timp, Nature 1999, 399, 758.[2] D. Goldhaber-Gordon, M. S. Montemerlo, J. C. Love, G. J. Opiteck, J. C.

Ellebogen, Proc. IEEE 1997, 85, 521.[3] M. C. T. Fyfe, J. F. Stoddart, Acc. Chem. Res. 1997, 30, 393.[4] Molecular Electronics: Science and Technology (Eds: A. Aviram, M. A.

Ratner), New York Academy of Sciences, New York 1998.[5] a) R. M. Metzger, J. Mater. Chem. 1999, 9, 2027. b) R. M. Metzger, Acc.

Chem. Res. 1999, 32, 950.[6] a) V. Balzani, M. Gómez-López, J. F. Stoddart, Acc. Chem. Res. 1998, 31,

405. b) J.-P. Sauvage, Acc. Chem. Res. 1998, 31, 611. c) P. L. Boulas, M.

Gómez-Kaifer, L. Echegoyen, Angew. Chem. Int. Ed. 1998, 37, 216. d) A.Niemz, V. M. Rotello, Acc. Chem. Res. 1999, 32, 42. e) A. E. Kaifer, Acc.Chem. Res. 1999, 32, 62. f) L. Fabbrizzi, M. Licchelli, P. Pallavicini, Acc.Chem. Res. 1999, 32, 846. g) V. Balzani, A. Credi, F. M. Raymo, J. F. Stod-dart, Angew. Chem. Int. Ed., in press.

[7] a) A. P. de Silva, H. Q. N. Gunaratne, C. P. McCoy, Nature 1993, 364, 42.b) A. Credi, V. Balzani, S. J. Langford, J. F. Stoddart, J. Am. Chem. Soc.1997, 119, 2679. c) M. Asakawa, P. R. Ashton, V. Balzani, A. Credi, G.Mattersteig, O. A. Matthews, M. Montalti, N. Spencer, J. F. Stoddart, M.Venturi, Chem. Eur. J. 1997, 3, 1992.

[8] a) R. M. Metzger, B. Chen, U. Höpfner, M. V. Lakshmikantham, D. Vuil-laume, T. Kawai, X. Wu, H. Tachibana, T. V. Hughes, H. Sakurai, J. W.Baldwin, C. Hosch, M. P. Cava, L. Brehmer, G. J. Ashwell, J. Am. Chem.Soc. 1997, 119, 10 455. b) M. A. Reed, C. Zhou, C. J. Muller, T. P. Burgin,J. M. Tour, Science 1997, 278, 252. c) C. P. Collier, E. W. Wong, M. Beloh-radsky, F. M. Raymo, J. F. Stoddart, P. J. Kuekes, R. S. Williams, J. R.Heath, Science 1999, 285, 391. d) J. Chen, M. A. Reed, A. M. Rawlett, J. M.Tour, Science 1999, 286, 1550. e) E. W. Wong, C. P. Collier, M. Belohradsky,F. M. Raymo, J. F. Stoddart, J. R. Heath, J. Am. Chem. Soc. 2000, 122, 5831.

[9] The term co-conformation has been advocated by us (M. C. T. Fyfe, P. T.Glink, S. Menzer, J. F. Stoddart, A. J. P. White, D. J. Williams, Angew.Chem. Int. Ed. Engl. 1997, 36, 2068) to describe the three-dimensionalspatial arrangements of the components of mechanically interlocked mol-ecules such as catenanes.

[10] a) M. Asakawa, P. R. Ashton, V. Balzani, A. Credi, C. Hamers, G. Matter-steig, M. Montalti, A. N. Shipway, N. Spencer, J. F. Stoddart, M. S. Tolley,M. Venturi, A. J. P. White, D. J. Williams, Angew. Chem. Int. Ed. 1998, 37,333. b) V. Balzani, A. Credi, G. Mattersteig, O. A. Matthews, F. M. Ray-mo, J. F. Stoddart, M. Venturi, A. J. P. White, D. J. Williams, J. Org. Chem.2000, 65, 1924.

[11] a) R. C. Ahuja, P.-L. Caruso, D. Möbius, G. Wildburg, H. Ringsdorf, D.Philp, J. A. Preece, J. F. Stoddart, Langmuir 1993, 9, 1534. b) C. L. Brown,U. Jonas, J. A. Preece, H. Ringsdorf, M. Seitz, J. F. Stoddart, Langmuir2000, 16, 1924.

[12] The shapes of the tetracations 14+, 24+, and 34+ can be approximated to arectangle with dimensions of 7 � ´ 11 � for 14+ and squares with the di-mensions of 11 � ´ 11 � in the cases of 24+ and 34+.

[13] D. B. Amabilino, M. Asakawa, P. R. Ashton, R. Ballardini, V. Balzani, M.Belohradksy, A. Credi, M. Higuchi, F. M. Raymo, T. Shimizu, J. F. Stod-dart, M. Venturi, K. Yase, New J. Chem. 1998, 22, 959.

[14] For examples of asymmetric I±V curves determined by STS, see: a) M. Po-merantz, A. Aviram, R. A. Corkle, L. Li, A. G. Schrott, Science 1992, 255,1115. b) A. Stabel, P. Herwing, K. Müllen, J. P. Rabe, Angew. Chem. Int.Ed. Engl. 1994, 34, 1609. c) A. Dhirani, P.-H. Lin, P. Guyot-Sionnest, W.Zehner, L. R. Sita, J. Chem. Phys. 1997, 106, 5249.

[15] For examples of resonant tunneling through the LUMOs of bipyridinium-containing compounds in Al½Al2O3½Langmuir±Blodgett monolayer½-Ti½Al sandwiches, see [8c] and [8e].

[16] For a recent example of partial circumrotation of interlocked molecularrings in a [2]catenane in the solid state, see: T. Gase, D. Grando, P. A.Chollet, F. Kajzar, A. Murphy, D. A. Leigh, Adv. Mater. 1999, 11, 1303.

Full Color Emission from II±VI SemiconductorQuantum Dot±Polymer Composites**

By Jinwook Lee*, Vikram C. Sundar*, Jason R. Heine,Moungi G. Bawendi, and Klavs F. Jensen

The development of full color emitting devices is one of themain challenges in optical displays. While III±V semiconduc-tor-based light-emitting diodes (LEDs) are commercially

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Fig. 5. Current (I)±voltage (V) plots for monolayers of the DMPA± salts of 14+,24+, 26+, 34+, and 36+ deposited on Au(111) determined by STS.

±[*] J. Lee, V. C. Sundar, J. R. Heine, Prof. M. G. Bawendi, Prof. K. F. Jensen

Massachusetts Institute of TechnologyCambridge, MA 02139 (USA)

[**] We thank Drs. R. Moon and J. Miller of Agilent Technologies for dicus-sions and support. We also thank F. Frankel for help with design andphotography for Figure 2. K. Shimizu provided help with Figure 3. Thiswork was funded in part by the NSF-Materials Research Sciecne and En-gineering Center program (DMR-98-08941) and NSF (DMR-98-71996).J. R. Heine acknowledges a DoD NDSG Fellowship.

available with nearly pure colors, mixed colors such as whiterequire combining at least two pure colors. Two differentLEDs then need to be fabricated, each on a different sub-strate, leading to complex fabrication processes. Hybrid de-vices, therefore, are used to produce the mixed colors, inwhich semiconductor diodes are used to excite inorganic[1] ororganic phosphors (organic dye or p-conjugated polymer).[2,3]

However, the inherent instability and narrow absorption pro-files of the organic phosphors and the broad emission spectraof the inorganic often makes it difficult to precisely access de-sired mixed colors. Recently, GaN quantum dots (QDs)grown on Si (111) by molecular-beam epitaxy have demon-strated visible light emission including white light.[4] A widerange of mixed colors and purer colors, however, requiresmuch narrower size distribution of the GaN QDs. In this com-munication, we demonstrate nearly full color emission usingsemiconductor nanocrystals QD±polymer composites. Thecomposites are fabricated by stabilizing chemically synthe-sized II±VI semiconductor QDs into polylaurylmethacrylate(PLMA) matrices in the presence of tri-n-octylphosphine(TOP). The fluorescence of the resulting composites, opticallyexcited by ultraviolet or blue light sources, spans the entirevisible range with narrow emission profiles and high photolu-minescence (PL) quantum yields. Moreover, mixed colors areeasily produced by controlling the mixing ratio of differentsized QDs.

The wide color tunability and stability of our compositesproceed directly from the unique properties of the constituentsemiconductor QDs. Chemically synthesized II±VI semicon-ductor QDs exhibit a nearly monodisperse size distributionand controlled optical properties as a function of their size;their narrow PL spectra are blue-shifted from the bulk PLdue to strong quantum confinement effects.[5] Recent suc-cesses in the surface passivation of these QDs with higher-bandgap semiconductors have yielded highly luminescentcore/shell QDs.[6±8] Specific advantages of these QDs over or-ganic phosphors include their range of emission frequencies,their greater stability and their high density of absorbingstates; ensembles of QDs have nearly continuous absorptionspectra from their bandgap into ultraviolet (UV). This lastfeature permits the simultaneous excitation of all differentsized QDs with a single light source and consequently facili-tates mixed color emission by controlling the mixing ratio ofQDs. Moreover, their narrow emission spectra (full width halfmaximum (FWHM) ~30 nm)[5] compared to those of inorgan-ic phosphors (FWHM = 50~100 nm)[1] make QDs outstandingsources of nearly pure color emission. Judicious choice of theexact II±VI semiconductor allows us to access different re-gions of the visible spectrum. In this communication, ZnS-overcoated CdSe QDs (hereafter, (CdSe)ZnS QDs) are usedto generate bluish green to red colors, and (CdS)ZnS QDs areused to generate violet to blue colors. Their synthesis is basedon previously published methods.[5,6] Figure 1a depicts repre-sentative, normalized emission spectra of QDs in dilute solu-tion, in a range of deep blue to red at room temperature.

In order to apply these QDs to light-emitting devices, it isnecessary to stabilize them in an appropriate matrix with re-tention of their initial PL efficiency. The incorporation of theQDs into a thin film such as a ZnS film[9±11] and a solvent-based polymer thin film[12,13] has been developed with poten-tial applications for flat panel displays. However, a wide appli-cation of the QDs to three-dimensional optical display has notbeen achieved due to aggregation and subsequent lumines-cence quenching of the QDs in a matrix. Therefore, ourchoice of monomer was guided by the need to obtain opticallyclear QD±polymer composites in which the QDs retain theiroriginal emission efficiency. Attempts to obtain such clearcomposites with monomers such as styrene and methylmeth-acrylate showed limited success; during the radical polymer-ization of the initially clear monomer±QD solution, phaseseparation of the QDs from the matrix led to luminescencequenching. Conversely, even though monomers like vinylpyri-dine allowed the formation of clear QD±polymer composites,a precipitous reduction in fluorescence was nonetheless ob-served. We finally settled upon stabilizing the QDs by using a

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Fig. 1. a) PL spectra of dilute solutions of QDs in hexane: from left, (CdS)ZnSQDs emission peaked at 455 nm (core radius ~30 �), (CdSe)ZnS QDs withemission peaked at 514 nm, 594 nm, 626 nm (13 �, 23 �, 28 �) with FWHMsmeasuring 39, 30, 29, 32 nm, respectively. b) PL spectra of the above QDs inQD±polymer composite rods excited by a UV Hg-lamp (kem = 365 nm) withFWHMs measuring 38, 28, 22, 29 nm, respectively. The emission light is col-lected from the top of each rod. The inset shows that the PL spectrum of the23 � core radius CdSe(ZnS) QDs in a polymer matrix is red-shifted from thatin dilute hexane solution (dotted line).

PLMA matrix in the presence of extra TOP organic ligands.The long alkyl branches of the PLMA polymer prevents theTOP covered QDs from phase separating from the polymermatrix during polymerization.

The PL quantum yield of the QDs in a polymer matrix is al-most as high as the initial efficiency of the QDs in diluted so-lution. The quantum yields of a range of CdSe(ZnS) QDs in0.1 mm-thick QDs±PLMA films, sandwiched between two op-tical flats, range from 22 % to 40 % (Table 1). The slight dropsfrom the initial quantum yields in diluted solution are likely tobe caused by variations in contact between films and opticalflats after polymerization. These variations in contact increasescattering and wave-guiding of the output emission, therebyreducing the number of photons reaching the detector.

Table 1. PL quantum yields of a range of (CdSe)ZnS QDs in dilute hexane solu-tions and in thin polymer matrices (thickness ~0.1 mm), respectively.

Core radius [a] In dilute solution[%] [b]

In polymer matrix[%] [c]

28 � 27 22

23 � 30 24

13 � 49 40

10 � 36 30

[a] The emission peaks of the QDs corresponds to 626 nm, 594 nm, 514 nm,and 495 nm, respectivley, in dilute hexane solution. [b] The quantum yields ofQDs in dilute solutions are compared to those of dilute laser dye solutions in a1 cm UV cuvette. [c] The quantum yields of QDs in polymer matrices are com-pared to those of dilute laser dye solutions in optical flats.

QD±polymer composite rods were fabricated in 60 mm ´5 mm (length ´ diameter) cylindrical glass molds. The fluores-cent image of these composite rods under a UV lamp (Fig. 2a)shows nearly pure color emissions, a result of their narrowemission spectra (Fig. 1b, measured from the top of the rods).The red-shift of emission peaks compared to those of the QDsin dilute solution (the inset of Fig. 1b) is attributed to light re-absorption by the larger QDs in the distribution as theemitted light from the smaller QDs travels along the rod.

We demonstrate a range of color control possible with theseQD±polymer composites. Figure 2b shows a top view of the

PL of composite rods excited by an UV lamp from below therods(kem = 365 nm). The photographic images of the lumines-cent rod tops are positioned on the CIE 1964 chromaticitydiagram.[14] Each position is determined based on the calcu-lated color-coordinates[14] from the PL spectrum of each rod.The rods on the edge show nearly pure color emissions fromsingle sized QDs. The bluish green and green rods on thecurved edge, however, are placed slightly away from the edgedue to increased size distribution effect; even a small broaden-ing in distribution, e.g., 20 nm ~ 30 nm FWHM, contributes todeviation of the color coordinate of single sized QDs from theedge. Interior points, such as white and purple, representmixed color emissions of different sized QDs. The mixing ra-tio was determined by the lever rule based on the emission in-tensity of each single sized QDs.

Device design and overall quantum yield considerations ledus to choose a ªlayeredº geometry over a randomly ªmixedºgeometry when making mixed color emissions of differentsized QDs. In the case of the randomly mixed structure, emis-sion from smaller dots with higher energy than the bandgap ofthe larger dots is likely to be reabsorbed by the larger dots. Incontrast, for the layered structure, which is prepared by se-quential polymerization of the smaller QD±monomer mixtureon top of the larger QD±monomer mixture, the undesirablereabsorption can be avoided in order to access any desiredmixed color in the chromaticity diagram. For example, whiteand purple colors of Figure 2b are easily produced from thelayered structure of blue/yellow (top/bottom) and blue/redQD±polymer composites, respectively, excited by a UV lampfrom the bottom. Furthermore, greenish-yellow and yellow-ish-green mixed colors along the tie line between bluish greenand yellow demonstrate that this layered structure can pre-cisely control any mixed color.

Our QD±polymer composites can also be used to make aªQD±polymer composite down-conversion LED.º This devicewas fabricated by combining our composite with a commer-cially available blue GaN LED. Figure 3 demonstrates an am-ber emission from a QD±polymer-capped blue LED, whichwas prepared by dipping a LED (Hewlett±Packard blue LED,kem = 425 nm) into the QD±monomer solution followed by

radical polymerization. By using thelayered structure, other mixed colors can bemade analogously to the result in Fig-ure 2b.

In conclusion, we have demonstrated anovel strategy to produce full color emis-sion using semiconductor QD±polymercomposites. These composites were pro-duced by stabilizing the QDs in long chainPLMA matrices in the presence of TOP li-gands. The choice of II±VI semiconductorsand their size-controlled growth enabled usto access a range of nearly pure colors. Inaddition, bright mixed colors were easilyachieved through engineered layered struc-tures of different sized QDs, showing the

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Fig. 2. a) Color fluorescence image of QD±polymer composite rods excited by a UV Hg-lamp (core ra-dii of (CdSe)ZnS QDs = 10 �, 13 �, 23 �, 28 �). b) End-on photographs of QD±polymer compositerods excited by a UV lamp from below. These rods are positioned on the CIE chromaticity diagram ac-cording to the computed coordinates (photographs by F. Frankel).

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ability to tune nearly all the colors within the CIE chromatic-ity diagram and demonstrating the potential application ofour QD±polymer composites to full color display.

Experimental

The synthesis of (CdSe)ZnS and (CdS)ZnS QDs was based on publishedtechniques [5,6]. The synthesis of CdS QDs paralleled that of the CdSe QDs ex-cept that hexadecylamine was used as a coordinating solvent instead of tri-n-oc-tylphophine oxide (TOPO). The synthesized QDs were precipitated from thegrowth solution and then redispersed into laurylmethacrylate monomer withTOP (5 % v/v). Then, ethyleneglycol dimethacrylate crosslinker was added tothe QD±monomer solution with 1:4 volume ratio of cross-linker to monomer.After azobisisobutyronitrile (AIBN) radical initiator (<1 % (w/w)) was added,the final solution was transferred to a 60 mm ´ 5 mm (length ´ diameter) glasstube and polymerized in an oven at 70~75 �C for 2 h. A high-clarity QD±poly-mer composite rod was then removed from the glass mold. The PL quantumyield of the QDs in dilute hexane solution was measured with an optical densityof 0.1 in 1 mm-quartz cuvette. For the PL quantum yield measurement of theQD±polymer composite, a QD±monomer solution was polymerized betweentwo optical flats to form a composite film (thickness ~0.1 mm). All PL spectrawere acquired in a front-face configuration on a SPEX Fluorolog-2 spectrome-ter and the quantum yields were determined by comparison of the integratedemissions of reference laser dyes: Rhodamine 590, 610, and 640. UV-vis absorp-tion spectra were acquired on a Hewlett±Packard 8452 diode array.

Received: April 7, 2000

±[1] Y. Sato, N. Takahashi, S. Sato, Jpn. J. Appl. Phys. Part 2 1996, 35, L838.[2] S. Guha, R. A. Haight, N. A. Bojarczuk, D. W. Kisker, J. Appl. Phys.

1997, 82, 4126.[3] F. Hide, P. Kozodoy, S. P. DenBaars, A. J. Heeger, Appl. Phys. Lett. 1997,

70, 2664.[4] B. Damilano, N. Grandjean, F. Semond, J. Massies, M. Leroux, Appl. Phys.

Lett. 1999, 75, 962.[5] C. B. Murray, D. J. Noris, M. G. Bawendi, J. Am. Chem. Soc. 1993, 115, 8706.[6] B. O. Dabbousi, J. Rodriguez-Viejo, F. V. Mikulec, J. R. Heine, H. Mattousi,

R. Rober, K. F. Jensen, M. G. Bawendi, J. Phys. Chem. 1997, 101, 9463.[7] M. A. Hines, P. Guyot-Sionnest, J. Phys. Chem. 1996, 100, 468.[8] X. Peng, M. C. Schlamp, A. V. Kadavanich, A. P. Alivisatos, J. Am. Chem.

Soc. 1997, 119, 7019.[9] M. Danek, K. F. Jensen, C. B. Murray, M. G. Bawendi, J. Cryst. Growth

1994, 145, 714.[10] J. Rodriguez-Viejo, K. F. Jensen, H. Mattousi, J. Michel, B. O. Dabbousi,

C. B. Murray, M. G. Bawendi, Appl. Phys. Lett. 1997, 70, 2132.[11] J. R. Heine, J. Rodriguez-Viejo, M. G. Bawendi, K. F. Jensen, J. Cryst.

Growth 1998, 195, 564.[12] D. E. Fogg, L. H. Radzilowski, R. Blanski, R. R. Schrock, E. L. Thomas,

Macromolecules 1997, 30, 417.

[13] D. E. Fogg, L. H. Radzilowski, B. O. Dabbousi, R. R. Schrock, E. L. Tho-mas, M. G. Bawendi, Macromolecules 1997, 26, 8433.

[14] P. Keller, Proc. SID 1983, 24, 317.

Thermo-Reversible Self-Assembly ofNanoparticles Derived from Elastin-MimeticPolypeptides**

By Terrence A. T. Lee, Alan Cooper, Robert P. Apkarian, andVincent P. Conticello*

Polymer nanoparticles are under increasing scrutiny as vehi-cles for the encapsulation and sustained release of small mole-cules and proteins for medical applications.[1] These materialsare usually prepared from self-assembly of amphiphilic di-block (AB) and triblock (ABA) copolymers. Selective segre-gation of the lipophilic block occurs in aqueous solutions af-fording micelle-like aggregates in which the corona is derivedfrom the hydrophilic block A and the core from the lipophilicblock B.[2] Genetic engineering of polypeptides enables thecreation of synthetic protein polymers composed of complexblock sequences in which the individual blocks within the poly-peptides have significantly different conformational, chemical,mechanical, and biological properties.[3] The uniform architec-ture of these biosynthetic protein polymers facilitates the anal-ysis of structure±function relationships, and, ultimately, thecontrol of materials properties. We describe herein the bio-synthesis of an amphiphilic block copolymer 1 derived fromelastin-mimetic peptide sequences,[4] which undergoes revers-ible, temperature-dependent segregation of the hydrophobicblock in aqueous solution affording potentially biocompatiblenanoparticles under physiologically relevant conditions.

Elastin-mimetic protein polymers [(Val/Ile)±Pro±Gly±Xaa±Gly]n display temperature-dependent phase behavior in aque-ous solution,[5] in which spontaneous phase separation of thepolypeptides occurs above a lower critical solution tempera-ture Tt.

[6] The relative position of the phase transition in thewater window depends on the polarity of the unique aminoacid residues (Xaa) within the repeat sequence.[7] In the de-sign of block copolymer 1 (see Scheme 1) twenty-five amino

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Fig. 3. A QD±polymer down-conversion light-emitting device emitting at~590 nm using a GaN LED emitting at 425 nm as the excitation source.

±[*] Prof. V. P. Conticello, Dr. T. A. T. Lee

Department of Chemistry, Emory UniversityAtlanta, GA 30322 (USA)E-mail: vcontic@emory.edu

Prof. A. CooperChemistry Department, University of GlasgowGlasgow G12 8QQ, Scotland (UK)

Dr. R. P. ApkarianIntegrated Microscopy & Microanalytical FacilityEmory UniversityAtlanta, GA 30322 (USA)

[**] The authors acknowledge Professor Fred Menger and Dr. Jason Keiperfor use of the Coulter N4 Plus instrument and assistance in the measure-ments, and Kevin Caran and Elizabeth Wright for technical assistancewith the EM measurements. V.P.C. thanks the Molecular Design Instituteof the Georgia Institute of Technology for financial support of this project.The UK Biotechnology and Biological Sciences (BBSRC) and Engineer-ing and Physical Sciences (EPSRC) Research Councils jointly fund theBiological Microcalorimetry Facility at Glasgow University.

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