RESEARCH NEWS

February 20036

Deciphering optical signatures

A new type of light-emitting diode,

which combines the advantages of

inorganic nanocrystals with organic

materials, could be good news for

displays [Nature (2002) 420, 800]. The

quantum dot-organic light emitting

diode (QD-OLED) created by

researchers at Massachusetts Institute

of Technology shows a 25-fold increase

in efficiency over previous devices.

The idea of combining the robust

fabrication and high performance of

OLEDs with the excellent luminescent

properties of nanocrystals is not a new

one. But, although electroluminescence

has been observed in a mixture of

inorganic crystals and π-conjugated

polymers, emission efficiency was far

lower than conventional polymer LEDs.

The efforts of the researchers, led by

Moungi G. Bawendi and Vladimir

Bulovic, has yielded a new structure

with greatly improved properties. Their

QD-OLED consists of a monolayer of

CdSe QDs – much thinner than

previously used – sandwiched between

two organic thin films. The QD

monolayer forms the emissive layer,

simply prepared using spin-casting.

Results indicate that electrons and

holes are captured directly at the

surface of the CdSe nanocrystals to

produce recombination luminescence.

High recombination efficiencies can be

expected from such an arrangement –

much higher than in conventional

fluorescent materials, where statistical

mechanics limits the number of

electron-hole recombinations resulting

in light emission to less than half.

Combine this fact with the wavelength

tunability afforded by varying the

diameter of the QDs, and 100%

quantum efficiency at any visible

wavelength may be possible.

The researchers have demonstrated

organized assemblies over 1 cm2,

which could, in principle, be used to

make bigger components. “One of the

goals,” says Bulovic, “is to demonstrate

a display that is stable, simple to

produce, flat, high-resolution, and that

uses minimal power.” But the QD-OLED

could have other applications, he adds,

to “miniaturize scientific equipment in

ways we haven't yet imagined.”

Screen test for quantum dot-OLED

A team of chemists from Rice University may have found asolution to one of the key challenges facing researchers workingwith carbon nanotubes.Most preparation methods yield a mixture of single-walledcarbon nanotubes (SWNTs) with different tube diameters andchiral angles. These differences in structure produce variations

in the electronic and optical properties of the nanotubes, whichhamper their investigation and application. Recently, however,researchers at Rice led by Richard E. Smalley and colleague R. Bruce Weisman noticed that semiconducting nanotubes inaqueous micelle-like suspensions fluoresce [Science (2002) 229977,593]. Now the researchers have taken a step forward byidentifying distinct electronic absorption and emissiontransitions for more than 30 different semiconducting nanotube‘species’ [Science (2002) 229988, 2361]. By combining these spectrofluorimetric measurements withresonance Raman data, the researchers can map each opticaltransition to a specific nanotube structure. These opticalsignatures can, therefore, provide a rapid method to determinethe composition of bulk SWNT samples. “Optical nanotubespectroscopy is an important enabling tool for nanotechnologyresearch because it reveals the composition of nanotubesamples through simple measurements,” explains Weisman. Inchemistry and biochemistry, optical instruments are alreadyused to characterize samples in a matter of seconds. Weismanbelieves that similar methodologies could soon be possible fornanotube analysis. The possibility of such rapid analysis could beinvaluable to guide nanotube production processes and sort theresults by structure.The researchers have also come up with simple formulas thatcan predict transitions of other nanotube species, includinglarger diameter tubes produced by other growth processes. Butthey also find that the dependence of optical transitionfrequencies on diameter deviates from a simple relation morethan expected. Quantitative theoretical analysis of the spectraldata may reveal the effect of other phenomena, such as strongexciton effects, say the researchers.

OPTOELECTRONICS

Three-dimensional plot of light-emission intensity of CNTs, as a function of excitation andemission wavelength, showing a peak for each ‘species’ of light-emitting nanotube. Theunique optical signatures are produced by slight differences in nanotube structure anddiameter. (Credit: R. Bruce Weisman, Rice University.)

NANOTUBES

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RESEARCH NEWS

Geneva-based semiconductor

manufacturer, STMicroelectronics,

claims to have made a breakthrough in

Si-based light emitters. The new

technology allows Si-based devices to

match the efficiency of traditional light-

emitting compound semiconductors,

such as GaAs.

Developed by researchers based at

the company’s technology R&D

organization in Catania, Sicily, the

technology uses an innovative structure

to achieve significant improvements in

light efficiency. Ions of rare earth

metals such as Er or Ce are implanted

into a layer of Si-rich oxide (SRO) –

SiO2 enriched with Si nanocrystals

1-2 nm in diameter. The frequency of

the emitted light depends upon the

dopant used.

“The quantum efficiencies achieved are

about 100 times better than has

previously been possible with Si and

are, for the first time, comparable to

those obtained from GaAs and other

compound semiconductors traditionally

used to make light-emitting diodes,”

says Salvo Coffa, manager of the

STMicroelectronics research team.

“The ability to combine optical and

electronic processing on the same chip

presents enormous opportunities,”

says STMicroelectronics’ director of

technology R&D, Gian Guido Rizzotto.

The company also claims to have

solved a number of the outstanding

manufacturing issues so that

production of new devices can begin.

One such application is a power control

device in which control circuitry is

electrically isolated from power

switching transistors. The company

has already patented a novel structure

– with actual sample devices to follow

shortly – to do just this, with the

isolated parts of the device

communicating via integrated Si light

emitters and detectors.

Light-emittingbreakthrough?

Yoel Fink and coworkers at MassachusettsInstitute of Technology have created a newlow-loss optical fiber that can transmit highpower laser light [Nature (2002) 442200, 650].The photonic bandgap fiber consists of ahollow optical fiber core lined with theomnidirectional dielectric mirror developedpreviously by Fink [Materials Today (2002) 55(7/8), 6; and 55 (9), 32]. The ‘perfect mirror’consists of layers of materials with verydifferent refractive indices, but which soften ata similar temperature. When the layeredpreform is drawn into a fiber, the thickness ofthe layers are reduced to submicrondimensions. A wide range of wavelengths canbe targeted because the layer thicknessdetermines the transmission window. “Due tothe efficient confinement of light in the hollowcore, enabled by the mirror surface, we areable to utilize materials that would normally bedamaged under such intense illuminationconditions,” explains Fink. As an example of the capabilities of thisapproach, Fink and his team focused on the10.6 µm wavelength, because althoughpowerful, low-cost lasers exist, such as CO2,there are no good fibers. Having successfullydrawn tens of meters of photonic bandgapfibers for transmitting CO2 laser light, the

researchers found transmission losses of lessthan 1.0 dBm-1. The losses are orders ofmagnitude lower than those of the intrinsicfiber materials and can be further improvedupon, say the researchers. Used with suchCO2 lasers, the fibers could enable applicationsin medical treatment or materials processing.“The significance of this work,” says colleagueJohn D. Joannopoulos, “is that it clearlydemonstrates a key attribute of photonicbandgap fibers, namely the ability to achievelower losses than their index-guidedcounterparts.”

Hollow fiber shows less lossPHOTONICS

Member of the Fink group at MIT uses a CO2 laser transmitted

through the new photonic bandgap fiber to draw a smiley face onone of the constituent materials of the fiber. (Credit: Ken Kurikiand Burak Temelkuran, MIT.)

MICROELECTRONICS

Colorful array of nanotubesPHOTONICS

Much recent research on carbon nanotubes hasfocused on the growth, characterization, andproperties of aligned carbon nanotubes (CNTs)synthesized by plasma-enhanced chemical vapordeposition. A team of US-German researchers fromBoston College, Natick Soldier Center, NanoLabInc., University of Massachusetts, and Hahn-Meitner-Institute have developed a self-assemblynanosphere lithography to produce large areaperiodic arrays of well-aligned CNTs [Nano Lett.(2002) 10.1021/nl0258271]. The inexpensive technique uses a monodispersesuspension of polystyrene nanospheres to form amask for the deposition of a Fe, Co, or Ni catalyst.A monolayer of the nanospheres is formed on thesurface of water and then transferred to anysufficiently flat surface, such as a Si substrate.Electron-beam evaporation deposits the catalyst,and the polystyrene particles can then be chemicallyremoved. The resulting honeycomb pattern of

catalyst dots can then be used to grow arrays ofaligned CNTs. The technique is eminently scalable,say the researchers, and could be used to producelarge area arrays.After growth, the researchers note that the CNTarrays appear colorful – indicating the highmetallicity of the nanotubes and the high degree ofordering. But it also indicates, say the researchers,that the arrays of CNTs should act as two-dimensional photonic bandgap crystals. They areconfident that photonic bandgaps in the visiblerange will be possible, and are currently working onexperimental demonstrations of their predictions.One problem could be the imperfections in thestraightness of the CNTs, which would tend toreduce the photonic bandgap size. But bettercontrol of the growth technique could remedy this.The nanotubes could also be coated to vary thephotonic crystal properties or used as structuraltemplates for nonmetallic photonic arrays.

February 2003 7

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RESEARCH NEWS

February 200310

Paul Alivisatos of the University of

California, Berkeley and the Lawrence

Berkeley National Laboratory,

presented his group’s latest results on

controlling the synthesis of inorganic

nanorods at the MRS Fall meeting.

Alivisatos’ group fabricates nanorods

by injecting organometallic precursors

into hot surfactants, where the

surfactants direct growth along one

axis. The group has also produced

CdTe ‘tetrapods’, which form when

CdTe nucleates first with a zinc blende

crystal structure and then grows in a

wurtzite structure. This gives four

arms radiating at a tetrahedral angle

from a central particle.

The switching between growth in zinc

blende and wurtzite structures is

under kinetic and, therefore,

temperature control. If it was

possible to switch at will, branched

nanostructures could be formed. This

would require changing temperature on

a timescale of 10-60 seconds, says

Alivisatos.

His group is now working on the

development of a microfluidics system

that controls temperature during

synthesis as the reactants flow

through a ‘lab-on-a-chip’. A fluorimeter

at the end characterizes the growth of

the nanorods through the changing

fluorescence. Preliminary studies show

that varying the flow rate can be used

to specify the emission wavelength of

CdSe nanorods.

Alivisatos is hopeful that it will be

possible to control temperature on a

timescale allowing switching between

different crystal forms during

nanocrystal growth. This may enable

the growth of samples that are not

possible in a standard flask.

One application Alivisatos is

investigating is the improved efficiency

of solar cells made from polymer-

nanocrystal blends.

ControllinggrowthNANOCRYSTALS

Large area displays fabricated from

semiconducting organic polymers may

be a step closer according to Beng

Ong of Xerox Research Center,

Canada. Semiconducting polymers

could offer low cost, lightweight,

flexible, mechanically robust displays

and circuits for many devices.

At the MRS Fall Meeting, Ong

reported a newly-designed polymer. In

collaboration with Motorola Labs and

Dow Chemical, the polymer has been

used to fabricate organic field effect

transistors (FETs) with mobilities of

10-1 cm2/V.s using solution processes

in an ambient environment.

Current organic semiconductors do not

have the necessary properties to

enable low cost transistor fabrication,

says Ong. Electron-rich π-conjugated

materials are susceptible to oxidation,

which results in low performance and

expensive processing.

In designing the new polymer, Ong

perturbed the π-conjugation by

including greater rotational freedom in

the polymer backbone, while retaining

long side chains for solubility and

structural regularity to enable the

molecules to self-organize. Importantly,

the perturbation gives stability in air

and devices can be fabricated and

characterized in ambient conditions.

While he is not able to disclose exact

details about the polymer at the

moment, the material is currently

named XPT for Xerox polythiophene

and is liquid crystalline, Ong reveals.

X-ray spectroscopy shows polymer

chains interdigitating to form sheets,

which stack against each other 3.8 Å

apart.

Using this material and direct-write

ink-jet printing techniques, Xerox is

aiming to produce a prototype

240 × 240 thin film transistor array

this year. Ong believes products could

be on the market in three years.

Displays withmobility ORGANIC ELECTRONICS

Berend T. Jonker of the Naval Research Laboratory, Washington DCpresented his latest results on spin injection in semiconductorheterostructures at the MRS Fall Meeting.Spin injection from semimagnetic semiconductor contacts into a GaAsquantum well gives a spin-LED where the polarization of the emittedlight from recombination events can be related to electron spin. Spinpolarizations of 85% have been produced. This device allows theinvestigation of spin scattering at interfaces. The density of planarstacking faults at the interface, generic to devices of this sort, is foundto be inversely proportional to spin injection. Assuming this is due toenhanced spin-flip scattering at the defects shows good agreementwith experiment. Jonker’s group is now investigating spin injection atFe/AlGaAs interfaces for room temperature devices.

Spin injectionSPINTRONICS

Wiring up new devicesNANOTECHNOLOGY

Charles M. Lieber of Harvard University gave the standoutpresentation at the Materials Research Society (MRS) Fallmeeting at the end of last year. Awarded with the MRS Medal,his address to a packed hall raced through an astonishingnumber of results from his lab. He outlined the control of structure, composition, and size ofnanowires to give ideal building blocks for assembly into newdevice architectures. “This has tremendous power in principle,”says Lieber. “There is the opportunity to revolutionize virtuallyany technology by this pathway.” Demonstrating simple devicesand explaining his vision for complex systems in many fields,from biotechnology and medicine to photonics and computing, itis possible to believe his claim is justified.Lieber has shown the use of nanocluster catalysts to direct theaxial growth of one-dimensional nanowires. Here, growth occursby addition at the catalyst site and the size of the catalystparticle controls the diameter of the wire. By modifying thereactants during synthesis, it is also possible to create axialheterojunctions, for example repeated GaAs/GaP junctions.Lieber’s group has also demonstrated control over radialcomposition in work that has now been published [Nature(2002) 442200, 57-61]. By altering conditions, growth byhomogenous deposition on the nanowire surface is favored overaddition at the catalyst. This adds a shell to the nanowire.Multiple shell structures can then be formed. Lieber hasdemonstrated this in the preparation of Si, Ge, and alloyoverlayers on Si and Ge cores.This ability to synthesize diverse nanowire heterostructures isone thing, but can they be assembled into devices? Lieber islooking to assemble nanowires into arrays using methods thatcan be scaled-up. He has been able to use fluid flow to orientnanowires for deposition in arrays on patterned surfaces andhas even been able to address single array cross-points.Lieber’s vision is to be able to create a universal computingarchitecture based on nanowire arrays assembled in ahierarchical structure. With his control over the building blocks,this may not be so far-fetched after all.

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RESEARCH NEWS

February 200312

Using non-uniform AC electric fields to

assemble micro- and nanoscale

particles into ordered structures may

provide a general method of fabricating

nanodevices, say researchers at the

University at Buffalo, New York. They

have demonstrated the controlled

organization of colloidal microparticles

into one-, two-, and three-dimensional

structures over length scales much

bigger than the particle size

[Electrophoresis (2002) 23, 2174].

Non-uniform electric fields can be used

to manipulate micron-sized polarizable

objects. The motion of the objects

(dielectrophoresis) depends on a

number of factors including electrode

configuration, voltage and AC

frequency, particle size and

concentration, and the difference in

dielectric properties between the

particles and the surrounding medium.

Aristides Docoslis and Paschalis

Alexandridis show that this motion can

be predicted for different electrode,

particle, and media combinations, and

used to direct assembly of structures.

“This process enables you to guide

particles to where you want them to

go and then scale them up into

ordered structures with desired

electrical, optical, or mechanical

properties,” explains Alexandridis.

Once the structure has been

assembled between the electrodes, it

can be fixed permanently by cross-

linking the particles, or through non-

covalent interactions.

The great advantage of this method is

its general applicability to almost any

particle. “Because of this flexibility,

there’s no limit to the applications of

this process,” says Alexandridis. He is

working to extend the technique to

nanoscale particles, which could

benefit the manufacture of sensors and

photonic devices.

Bringingparticles intolineCOLLOIDS

The development, and acceptance, of molecularelectronics has been limited by the inability to testthe properties of single molecules in a reliable,defined, and simple manner. It remains an openquestion as to whether the reported measurementsfrom test devices represent the true electronicproperties of a single molecule or are the result ofcollective intermolecular characteristics. Anadditional problem is that the current method ofevaporating a metal contact onto molecular devicescan result in asymmetries of the metal/moleculeinterface, which could skew measurements.Now, however, researchers from Motorola Labs andArizona State University claim to have come up withan answer. The team has developed anonlithographic method for the electrical testing ofsingle or small bundles of molecules [Appl. Phys.

Lett. (2002) 81 (16), 3043]. The new approachuses an Au-coated tip of a conducting atomic forcemicroscope (cAFM) to contact an Au nanoparticle,which is attached to the molecule(s) in question.The Au nanoparticle is attached to the unreactedtop thiol terminus of a dithiolated molecule, whichhas been by chemically inserted into the naturaldefect sites of an insulating, self-assembledmonolayer formed on top of an epitaxial Ausubstrate. To demonstrate the capabilities of the newapproach, the researchers made transportmeasurements on two test molecules. The resultsshow qualitative agreement with previously reporteddata for similar thousand-molecule arrays. This alsoindicates, say the researchers, that such data is amolecular and not intermolecular phenomenon.

Single molecule testingMOLECULAR ELECTRONICS

Nanoparticles by accidentNANOTECHNOLOGY

A simple method for depositing precious metalnanoparticles on a surface “came about byaccident,” said Lon A. Porter of PurdueUniversity at the MRS Fall meeting.Porter has observed the growth of Au, Pd,

and Pt nanoparticles on the surface of Ge andGaAs wafers while simply bathing the wafersin dilute aqueous solutions of the metal saltsHAuCl4, Na2PdCl4, and Na2PtCl4. Thedeposited particles show excellent adhesion,and are not removed by sonication or Scotch®tape. X-ray photoelectron spectroscopy showsthe metal is in the zero oxidation state.Au, Pd, and Pt are indispensable to electronicsdevice fabrication, catalysis, supportsubstrates, and sensor elements. This newelectroless deposition method, in contrast tocomplex and expensive vacuum depositionmethods, is a simple technique for theinterfacing of metal nanostructures withsemiconductor surfaces.Control of particle size is possible throughvarying the soaking time, temperature, andsolution concentration. Particles grow by anisland mechanism, and after an hour form acontinuous film. Porter has also been able topattern the nanoparticle deposition usingphotolithography, microcontact printing, anddip-pen nanolithography.Porter is confident that this simple control ofa very straightforward technique has thepotential to be developed for applications andadded functionality. He is investigating thechemisorption of thiols to the very highsurface area given by films of Aunanoparticles. He hopes it will be possible todevelop this process for biosensor arrays.

Nanoparticles forming on a Ge surface over a 500 minute period.(Courtesy of Lon A. Porter.)

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RESEARCH NEWS

February 200314

Quantum dots (QDs) are potentially

ideal fluorescent markers for

bioimaging. The tunable and efficient

emission from these robust

nanoparticles could provide distinct

advantages over the markers used

currently. Such fluorophores and

fluorescent proteins have a limited

range of colors and photobleaching

restricts their use. However, a number

of problems with QDs must still be

overcome, including aggregation and

nonspecific adsorption, and they must

be shown to have no toxicity.

Benoit Dubertret and colleagues from

Rockefeller University, NEC Research

Institute, and the University of

Minnesota may have found a solution

to these problems by encapsulating

single nanocrystals in phospholipid

block-copolymer micelles [Science

(2002) 298, 1759-1762].

The group encased ZnS-overcoated

CdSe QDs in micelles with a

hydrophobic lipid core and a

poly(ethylene glycol) (PEG) outer

surface. In both in vitro and in vivo

experiments, the QD-micelles show

stability, resistance to photobleaching,

and provide efficient fluorescence.

In vitro, QD-micelles with covalently

attached DNA molecules were shown

to bind specifically to complementary

DNA molecules attached to beads. The

low nonspecific adsorption of the

particles gave a fluorescence signal to

background ratio >150. This can be

compared to ~4 for silica-coated QDs.

The researchers injected single cells in

early-stage Xenopus embryos with the

QD-micelles to evaluate their use in

vivo. As the embryos grew, QD

fluorescence was observed in cells

derived from the initially injected cell

with no detectable toxicity. It may be

possible in future to use these

encapsulated QDs to visualize embryo

development with single cell resolution.

Encapsulatedquantum dotsNANOBIOTECHNOLOGY

By engineering a protein molecule to give atemplate, researchers at NASA Ames ResearchCenter and the SETI Institute, California, andArgonne National Laboratory have formed orderedarrays of quantum dots [Nature Materials (2002)1, 247]. This demonstrates the possibility of usingself-assembling biomolecules as an alternative tolithography in the nanoscale fabrication of newelectronic or photonic devices.The group, led by Jonathan D. Trent, uses a proteinthat assembles into a structure consisting of twostacked rings. This assembly can then form two-dimensional arrays. By mutating a specific aminoacid in the protein, rings of reactive thiol groupscan be introduced at the top and bottom of thedouble ring structure.

Both Au and semiconductor quantum dots (QDs)can bind at the thiol groups on the surface of theprotein arrays, giving ordered QD regions. Byengineering the protein at different positions,creating 3 nm or 9 nm rings of thiols, it is possibleto order different sizes of Au nanoparticles (5 nmand 10 nm respectively).“Much of the success of today’s electronics industrycomes from knowing how to arrange materials in anorganized fashion on a Si substrate, and theprospects of using proteins on a nanometer scale isencouraging,” says Trent. Not only have the groupshown that proteins can be engineered to givetailored templates, but alternative binding sites canalso be introduced into the protein to enable the QDarrays to be wired into functional devices.

Proteins order quantum dotsNANOBIOTECHNOLOGY

Polymer instabilities LITHOGRAPHY

Building smaller chemical analysis and biochipdevices is likely to require soft lithographytechniques for patterning polymer materials. Asimple process for forming features in polymerlayers accurate to 100 nm has now beendeveloped by Ullrich Steiner and coworkersfrom the University of Groningen, Netherlands,the Max Planck Institute of Molecular CellBiology and Genetics, Germany, and theUniversity of Massachusetts at Amherst[Nature Materials (2003) 22, 48].The lithography technique makes use of, andcontrols, instabilities between two thinpolymer layers. An electric field has adestabilizing effect on the interface between

two dielectric materials with differentpolarizabilities. In combination with dewettingforces, these electrostatic forces acting at thepolymer-polymer-air interfaces lead to aredistribution of both polymers.The researchers mounted a Si wafer above apolystyrene (PS)/poly(methyl-methacrylate)(PMMA) bilayer on another Si wafer, leavingan air gap. An electric field was applied, andthe assembly placed in an oven for 24 hours.Polymer columns formed between the two Siwafers. Atomic force microscopy shows anouter rim on each column, which after removalof PS, is revealed to be PMMA. Thesestructures form sequentially, first asinstabilities result in the upper PS layerforming columns, then PMMA is drawn up thePS columns. The group was able to use apatterned upper Si wafer to produce a replicain the polymer. The polymer bilayer is drawn tothe features extending from the wafer. Again,by removing the PS, PMMA structures withsmaller dimensions are left. These features,now smaller than those on the master wafer,can be controlled by the initial thickness of thePMMA layer and the annealing time.It is the exploitation of instabilities in thesecond polymer layer that creates features of~100 nm. The other advantage of this simplemethod is that two materials are patterned inone step. The authors claim it will be possibleto extend their technique to hierarchicalstructures consisting of larger numbers ofdifferent materials.

Columns form due to instabilities in a PS/PMMA bilayer in anelectric field. (Reprinted with permission. ©2003 Nature Materials.)

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