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March 13, 2012

C 2012 American Chemical Society

Two-Dimensional Nanomembranes:Can They Outperform LowerDimensional Nanocrystals?Babak Nikoobakht†,* and Xiuling Li‡

†Surface and Microanalysis Science Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States and ‡Department ofElectrical and Computer Engineering, University of Illinois at Urbana�Champaign, Urbana, Illinois 61801, United States

Since the discovery of nanoparticlesand quantum dots (QDs), surfactantshave often been their companions

because of their indispensable roles in sta-bility, shape, and size control. Shape trans-formation from 0D to 1D nanocrystals (i.e.,quantum rods and nanorods) has beenachieved using surfactant molecules suchas hexyl-phosphonic acid, hexadecyltri-methyl ammonium bromide, or oleic acidin the growth of CdSe,1 Au,2 or TiO2

nanorods.3 Surfactants such as linearamines have also been used to increasethe dimensionality of cobalt nanoparticlesto two-dimensional (2D) structures4 and toform zirconium disulfide nanodisks;5 similarmolecules such as stearic acid, arachidicacid, or octadecyl amine have also beenused for directing 2D growth in ironnanostructures.6 In general, the presenceof surface adsorbed surfactants modulatesthe growth rate of different nanocrystalfacets and promotes an anisotropic crystalgrowth.7,8

One fascinating aspect of 0D and 1Dnanoparticles has been the manifestationof size-dependent properties and quantumeffects; however, so far, realization and uti-lization of such effects in technological ap-plications have yielded results far belowexpectations. One key reason is likely thedifficulty in accessing individual particlesdue to their small size using current fabrica-tion techniques. Discovery of the self-assembly of nanoparticles into 2D net-works9,10 shaped one of the premises ofnanotechnology, namely, bottom-up synthe-ses of materials by design, and offered theuse of the collective properties of nano-crystals in their ensemble. The latter wasperhaps also an evolutionary remedyto make commercial integration of nano-crystals more facile using conventional

approaches, which, indeed, remains some-thing to be seen. However, because theseassembly processes are self-driven, theirhierarchical order and size have beenshown to be difficult to control and oftenlimited in scale. As a result of the challengesin the controlled assembly of 0D nanocryst-als, directed assembly with an intense focuson nanorods and nanowires brought newadvances to large-scale manipulation andintegration of nanowire-based devices, forexample, in thin-film electronics,11,12 electro-optical devices such as sensors,13 light-emit-tingdiodes,14 andhighmobility transistors.15 Itis interesting tonote that the key innovation inall of these platforms has been the use of 2Dassembliesofnanowires;adistinct character-istic that can also be achieved using inorganicnanomembranes, which have been a lessknown member of this family until recently.Researchers have knownof inorganic nano-

membranes for at least two decades,16,17

* Address correspondence tobabak.nikoobakht@nist.gov.

Published online10.1021/nn300893x

ABSTRACT Inorganic nanomembranes, ana-

logues to graphene, are expected to impact a wide

range of device concepts including thin-film or

flexible platforms. Size-dependent properties and

high surface area;two key characteristics of

zero- (0D) and one-dimensional (1D) nanocryst-

als;are still present in most nanomembranes,

rendering their use more probable in practical

applications. These advantages make nanomem-

branes strong contenders for outpacing 0D and 1D nanocrystals, which are often difficult to

integrate into commercial device technologies. This Perspective highlights important progress

made by Wang et al. (doi: 10.1021/nn2050906) in large-scale fabrication of free-standing

nanomembranes by using a solution-based technique, as reported in this issue of ACS Nano.

The simplicity of this new approach and the elimination of typical delamination processes used

in top-down nanomembrane fabrications are among the strengths of this technique. Areas for

improvement along with an overview of other related work are also discussed.

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but these nanomembranes haverecently recaptured significant at-tention. From a basic science view-point, this increased attention isdue to the rich science of nano-membranes, with size-dependentproperties including optical pro-perties,18 thermal conductivity,and electrical conductivity. From adevice-integration perspective,nanomembranes are well suitedfor incorporation into other materi-als and offer the unique opportunityof utilizing their size-dependentproperties in commercially viableproducts, something that has notbeen realized for lower dimensionnanocrystals. Nanomembranes be-come even more attractive becausethe existing knowledge on 0D and1D nanoparticles, such as compo-sition control, doping, heterostruc-ture growth, and nanomanipulation,canbe extended to nanomembranesas well.

Two-dimensional structures withnanoscale thickness have beencommonly used in the epitaxial

crystal growth world and playedan essential role in semiconductorlasers, and photodetectors.19 Morerecently, such layered structureshave been transferred to a varietyof substrates using top-down ap-proaches.20�22 To release a thin slabof epitaxial 2D structures from amechanical support, it is necessaryto place a sacrificial layer betweenthe substrate and the to-be-released membrane. In epitaxiallygrown layers, there needs to be amatch between the sacrificial layerand the overlayer, which has beenreadily accomplished in many ma-terials systems (e.g., lattice-matchedAlAs or InGaP as the sacrificial layerfor GaAs- and InGaAs-based sys-tems and lattice-matched InGaAsas the sacrificial layer for InP-basedsystems). One advantage here isthat superlattices composed of al-ternating layers of heterogeneousmaterials with multiple embeddedsacrificial layers can be used duringthe epitaxial growth process, thusenabling the release of multiplestacks of membrane either simulta-neously or sequentially. Membranesformed by epitaxial methods canalso be engineered with specificoptical and electronic properties,such as the use of potential barriersfor electron or light confinement,which are suitable for photovoltaicapplications.23 Another advantageis that the released membranes;semiconductor, dielectric, or me-tal;can also bend, twist, and rollto form three-dimensional (3D)membranes, depending on the em-bedded strain and aspect ratio ofthe membrane.22,24�27 Althoughthis is a dexterous methodology,potential limitations may includethe need for single-crystal sub-strates for the formation of crystal-line membranes, or extra stepsmay be necessary in the formationand release of stacks of mem-branes, which could add to theoverall cost.Bottom-up chemical techniques

have also been used for the growthof large-area nanomembranes. Oneof the advantages of such methods

Figure 1. (a) Large-scale formation of zinc hydroxydodecylsulfate thin membraneat the water�air interface and the role of sodiumdodecylsulfate are schematicallyillustrated. (b) An optical microscope image of part of a zinc hydroxydodecylsul-fate nanomembrane. The inset image represents the top view of the as-synthe-sized nanomembrane on the water surface in a glass dish. Reprinted from ref 30.Copyright 2012 American Chemical Society.

Nanomembranes are

well suited for

incorporation into

other materials and

offer the unique

opportunity of utilizing

their size-dependent

properties in

commercially viable

products, something

that has not been

realized for lower

dimension

nanocrystals.

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is the potential for creating diversenanostructures that either cannotbe made by other techniques orare difficult to make. The richscience developed around lowerdimension nanocrystals can belinked and used for creating 2Dstructures as well. In this regard, ithas been shown that 2D networksof PbS QDs with cubic structurescan be converted to 2-nm-thicknanosheets with lateral dimensionsof a few hundred nanometers; inthis collective shape transformation,more active facets (110) of neigh-boring QDs disappear in a processdriven by the use of a surfactant(oleic acid) as the template and sur-face energy is minimized amongtheQDs.28Wet chemical techniquescan also be used to break down less-ordered materials into more-or-dered structures by meddling withmolecular forces such as van derWaals interactions. Solvent exfolia-tion, as an example, has been usedto produce single- or multilayercrystalline sheets of some transitionmetal oxides or dichalcogenidessuch as MoS2, WS2, MoTe2, andBe2Te3.

29 Thesematerials, like graph-ite, are naturally layered structures;by using this technique and appro-priate solvents, these materials canbe dispersed as thin sheets andeventually as thin-film compositeswith modified electronic, thermal,or mechanical properties.Diversifying growth methods and

controlling the single-crystalline stru-cture of formed nanomembranes are

among the existing challenges. Inthis issue of ACS Nano, Wang et al.

extend the growth of nanomem-branes using surfactant-directed sur-face assembly at the water�airinterface and produce wafer-scale-thin layers of zinc hydroxy dode-cylsulfate.30 The formed nanosheetscan be transferred to a variety ofsubstrates and surfaces and con-verted to ZnO 2D membranes byprocesses such as thermal annealing.In their approach, negatively chargeddodecylsulfate ligands are used in aconcentration regime where surfac-tants can form close-packed assem-blies at the water�air interface withthe hydrophobic side of the ligandsfacing the air (Figure 1a). The polarhead groups of the surfactants hostthe Zn cations in a sandwich format.In surfactant-assisted nanocrystalgrowth, cations and anions undergoa rapid reaction to form nanocrystalsthat instantaneously bind to the sur-rounding surfactants to reduce theirsurface energy. If surfactant ligandsform a certain template in solutionthen the nanocrystal growth couldbe guided; however, often the tem-platemorphology is disturbed by thepresence of cations and anions. Themain cause is a change in the head-group size of surfactant ligands and,thus, a change in the electrostaticforces among the head groups, lead-ing to variations in the overall sizeand shape of the surfactant assem-blies. Therefore, growth strategiesmust be developed to mitigate thenegative impact on the surfactant

head groups. Taking this into consid-eration,Wang et al.30 increase the pHof the growth solution by thermaldecomposition of hexamethylenete-tramine, which also acts as the bufferagent. The release of ammonia as adecomposition product gradually in-creases the concentration of hydrox-yl groups, causing crystallization ofzinc hydroxyl species on the dode-cylsulfate polar surface; it also allowsfor a controlled precipitation reac-tion, avoiding rapid release of ZnOfrom solution. A similar strategy hasbeen used in the growth of mono-disperse CdS QDs where thioureawas used to release sulfide ionshomogeneously and slowly in aqu-eous cadmium solutions.31

In the manipulation of nano-membranes for device applications,it is important to control the num-ber of stacked layers on a givensurface. Top-down approaches en-able great control over this param-eter but solution-based techniquesdo not offer such mechanisms.This is evident, for instance, inthe solvent-exfoliation technique29

in which dispersed layered materi-als form multilayers once depositedon a surface. This stacking tendencyis typically due to the attractionforces between the single sheets.The approach reported by Wanget al.30 may offer a new opportunity,as a supersize single layer of zinchydroxydodecylsulfate forms at theair�solution interface and can be

Figure 2. Nanomembranes or 2D nanostructures are of great interest in both basicand applied sciences. This illustration depicts nanomembranes as versatile vehiclesfor integration to flexible or soft surfaces with a wide range of applications, andsubstrates for epitaxial growth of lower dimensional nanocrystals.

In this issue of ACS

Nano, Wang et al.

extend the growth of

nanomembranes using

surfactant-directed

surface assembly at the

water�air interface

and produce wafer-

scale-thin layers of zinc

hydroxydodecylsulfate.

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transferred to a solid surface(Figure 1b). Wang et al.30 also ex-amined some of the electrical prop-erties of such 2D structures in wetand dried states by fabricating field-effect transistors. They noted lowerelectron mobilities compared totypical ZnO films. This was attribu-ted to the existence of water in thestructure which should improvethrough further dehydration. Theresults of this report can reinvigo-rate the use of surfactants in solu-tion growth of a variety of novelsemiconductors and large-scalemetal nanosheets.

Outlook and Future Challenges.Synthesis and fabrication of nano-membranes is expected to growand encompass a diverse range oftechnologically important materialssuch as II�VI, III�V, and IV semicon-ductors. As schematically shownin Figure 2, considering their thingeometries and advances in micro-and nanofabrication techniques, awhole host of designs and materialcombinations are envisaged forrealization of smarter materials, forexample, in thin-film photovoltaics,light energy storage, and thermo-electricity. To explore the potentialsof this class of nanocrystals fully,new synthetic routes are needed,for instance, for their heterojunc-tions, bandgapengineering, full dop-ing, and partial doping. In deviceintegration, understanding the con-tribution of surface states and theeffects ofmembrane shape or struc-tural strain on light, charge trans-port, or energy transport is key foroptimizing the mobility or charge-separation efficiencies.32 Maintain-ing pristine surfaces of the mem-branes is critically important formany types of devices includingMOSFETs, lasers, and solar cells, inorder to avoid surface recombinationor scattering of carriers. In the case ofsolution-based approaches, the stateof the surface and the contributionsof surfactants or their residues onelectro-optical properties of themembranes need to be understood.Other challenges related to control-ling nanomembranes' morphology

and assembly include controllingthe homogeneity in their dimensionsand developing methods for theirtransfer and positioning on sur-faces.33 Among the less-exploredareas is the formation of heterojunc-tions or interfaces on nanomem-branes with other materials includ-ing molecules, solid surfaces, QDs, ornanowires, which are consideredgateways to important areas suchas sensing, electronics, and tissueengineering. Nanomembranes havebeen shown topreservemost of theirproperties in out-of-plane modes,34

which has also made them excellentcandidates for implementation in 3Darchitectures in ordinary productssuch as clothing, automobile coat-ings, and glasses. They can also beintegrated in more advanced prod-ucts such as airplanes and un-manned vehicles, with applicationsin thin-film batteries, coolingdevices,micropower generators, powerfulsensors, and analyzers. Regardlessof the dimensionality of nanocrystals,it is clear that harnessing their excit-ing and unique properties will con-tinue to drive innovation. In search-ing for new materials with betterproperties and performance, thequestion to pose is, what new tech-nology horizon can be reached withcombination of our imagination andmaterials at the nanoscale?

Conflict of Interest: The authors de-clare no competing financial interest.

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