CHAPTER 6 NANOMATERIALS - Springer

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CHAPTER6
NANOMATERIALS
Imaginehowmuchcontroloverresultantpropertiesyouwouldhaveifyouwereable
to deposit and maneuver individual atoms into predefined arrangements, en route
toward a new material. This is fast becoming a reality, and is the realization of the
ultimate in “bottom-up” materials design. Thus far, one is able to easily fabricate
materials comprised of a small number of atoms, with features on the nanometer
scale (10−9m) – one-billionth of a meter. To put this into perspective, think of a
materialwithdimensionsapproximately1,000timessmallerthanthediameterofa
humanhairfollicle!Aswewillsee,itisnowevenpossibletopushindividualatoms
aroundasurfaceusingspecializedtechniques.
We are at the crossroads of unprecedented applications that will only be possi-
ble using nanoscale building blocks. More effective devices will be constructed to
removepollutantsfromtheenvironmentanddetect/deactivatechemicalandbiolog-
icalwarfareagents.Integratedcircuitrywiththecapabilitiesofcurrentworkstations
will be the size of a grain of sand and will be able to operate for decades with the
equivalentofasinglewristwatchbattery.Roboticspacecraftsthatweighonlyafew
poundswillbesentouttoexplorethesolarsystem,andwidespreadspacetravelwill
bepossibleforthemasses.Oh,yes–onethatisneartousall–inexpensivealterna-
tiveenergysourceswillpowerourvehicles,ratherthandependingondwindlingoil
reservesandthedailyfluctuationsof(soaring)gasprices![1]
Inordertogainrapidprogresstowardtheseintriguinggoals,thelevelofgovern-
ment and private funding in the nanosciences continues to soar. This has spawned
anumberofinstitutesinrecentyears,focusedonresearch,development,andcom-
mercializationofnanoscalediscoveries,aswellaspubliceducation/outreach.Some
recentexamplesare[2]:
– National Nanotechnology Initiative (Federal R&D Program, Washington, DC);
http://www.nano.gov
– RichardE.SmalleyInstituteforNanoscaleScienceandTechnology(RiceUniver-
sity,Houston,TX);http://cohesion.rice.edu/centersandinst/cnst/index.cfm
– Institute for Nanotechnology (Northwestern University, Evanston, IL); http://
www.nanotechnology.northwestern.edu/index.html
– Nano Science and Technology Institute (Cambridge, MA); http://www.nsti.org/
about/
275
276 6 Nanomaterials
– National Cancer Institute Alliance for Nanotechnology in Cancer (Bethesda,
MD);http://nano.cancer.gov/
– ASME Nanotechnology Institute (New York, NY); http://www.nanotechnology-
institute.org/about.html
– Nanotechnology Institute (Philadelphia, PA); http://www.nanotechinstitute.org/
nti/index.jsp)
– Center for Nanoscale Chemical–Electrical–Mechanical Manufacturing Systems
(Urbana,IL);http://www.nano-cemms.uiuc.edu/
– Nano/Bio Interface Center (University of Pennsylvania); http://www.nanotech.
upenn.edu/
– Center on Nanotechnology and Society (Chicago-Kent College of Law, Illinois
InstituteofTechnology);http://www.nano-and-society.org/
– TheNanoTechnologyGroup,Inc.(Nanoscienceeducationaloutreach,Wells,TX);
http://www.thenanotechnologygroup.org/
– TheForesightInstitute(PaloAlto,CA);http://www.foresight.org
– The Institute for Soldier Nanotechnologies (Massachusetts Institute of Techno-
logy);http://web.mit.edu/isn/aboutisn/index.html
Thefirstnationalnetworkfocusedonthedesign/fabrication/testingofnanomate-
rialswasinstitutedin2004throughfundingfromtheNationalScienceFoundation.
The National Nanotechnology Infrastructure Network (NNIN) consists of a con-
glomerateof13sitesacrossthecountry(Figure6.1)thatarefocusedonallaspectsof
nanomaterials.Since“nanotechnology”issuchaninterdisciplinaryfield,manymore
nanorelatedresearchcenterswilllikelybeinstitutedinthenearfuture.However,as
withallscientificdisciplines,amajorroadblocktowardresearchprogressintheUS
is domestic student recruitment. There are a declining number of degrees awarded
in the sciences within recent years (from B.S. to Ph.D. levels) in the US, relative
to other foreign countries (e.g., China, India). This represents an ominous forecast
Figure 6.1.The 13 sites of the National Nanotechnology Infrastructure Network (FY2004-FY2009).
Reproducedfromhttp://www.nnin.org.
6 Nanomaterials 277
thatouradvancedtechnologyandwarfarecapabilitieswillgreatlylagbehindother
countries, threatening our everyday way of life and the “superpower” status long
enjoyedbytheUS.
Asexcitingasthefuturisticapplicationsmaysound,isitpossiblethatsuchtechno-
logicalgrowthmaybeassociatedwithdiresocietalconsequences?InEricDrexler’s
book“EnginesofCreation,”[3] itwasforecastedthatself-replicatingnanomachines
would take over all life on Earth! Although that notion is far from reality, there
maybemoreseriousissuesthatarisethroughintroductionofnanomaterialsintothe
biosphere. History has revealed that chemistry is a two-edged sword, with benefits
thatgreatlyimproveourlivesbutalsonegativeconsequencesforhumanhealthand
our environment. For example, think of the chlorofluorocarbons (CFCs) that were
once used for refrigerants. Their discovery was heralded as one of the greatest tri-
umphs of modern science. Alas, many years after their worldwide adoption, it was
realizedthattheycontributedtotheozoneholeandlikelyincreaseintheincidences
ofskincancer.Likewise,emissionsfromfactoriesandautomobileswerenotconsid-
eredasproblematicmanyyearsago,butwearenowawareofthedestructiveconse-
quencesofthesesources(e.g.,acidrainandglobalwarming–theleadingcauseof
catastrophicchangesinourclimate/weatherpatterns).
What will arise from the widescale introduction of nanoscale materials into our
world? Are we on the verge of upsetting the natural balance in ways that cannot
be overturned? These are serious questions that may only be answered through
far-reaching research endeavors, many of which are currently being investigated.
Inparticular,wemustfirstdeterminethetoxicological/environmentalconsequences
of nanoscale materials with comparisons to known contaminants such as colloids,
aerosols/smoke particulates, and asbestos. Some important questions that must be
addressed[4]:
(i) Donanomaterialsbioaccumulate?
(ii) What is the danger of exposure via skin absorption, ingestion, or inhalation
routes?
(iii) Whatisthefate,transport,andtransformationofnanosizedmaterialsafterthey
entertheenvironment?
Inadditiontothesetoxicological-basedquestions,anumberofethicalconsiderations
mustalsobeaddressed[5]:
(i) Equityissues–willnanotechnologybeutilizedtosolvethird-worldproblems,
or will it primarily be used to increase the prowess of industrially advanced
countries?
(ii) Privacyissues–imagineaworldwhereyouhaveinvisiblesensors/microphones
–needwesaymore...
(iii) Security – how will our country and others defend itself against invisible
nanoweaponry?
(iv) Human–machine interactions – there are many religious and philosophical
issuesassociatedwithembeddednanodeviceswithinthehumanbody.
The first news report on the potential damaging effects of nanoscale materials
surfaced about a decade ago, when TiO /ZnO nanoparticles from sunscreen were
2
found to cause free radicals in skin cells, damaging DNA. Since then, there have
278 6.1. Whatis“Nanotechnology”?
been an increasing number of such reports suggesting that nanostructures are able
totraverseacrossmembranesinthebody,withanincreasingtoxicitywithdecreas-
ingnanoparticulatedimensions.Perhapsthemostwidelyreportedstudysurfacedin
mid-2004,whereitwasshownthatfullerenes,ananoscaleallotropeofcarbon,cause
brain damage in aquatic species. Many additional studies are needed to determine
thefullimpactofnanomaterialsbeforetheirfullworldwideadoption.
Theintroductionofanewarchitecturesuchasnanomaterialsnecessitatestheneed
for new terminology and methods of classification and characterization. We must
also understand the mechanisms by which individual nanostructures may assemble
intolargermaterials,asthiswillgreatlyaffectthepropertiesofthebulkdevicefor
a particular application. This chapter will focus on all of these important issues,
with an introduction to the various types of nanomaterials, laboratory techniques
used for their synthesis, and (perhaps most importantly) their role in current/future
applications.
6.1. WHATIS“NANOTECHNOLOGY”?
Althoughthereismuchcurrentexcitementaboutnanomaterials,thereisreallynoth-
ingnewaboutnanoscience.Infact,theearliestcivilizationsusednanoscalematerials
foravarietyofapplications.Forexample,theMayansusedamagnesiumaluminum
silicateclaycalledpalygorskite,whichcontainednanosizedchannelsthatwerefilled
withwater.TheMesopotamiancivilizationsusedcoloredglassfordecorativeappli-
cationsthatcontainedembeddedmetallicnanoparticles.
Physics Nobel Laureate Richard Feynman gave the first lecture regarding the
applications for nanoscale materials. His talk, entitled “There’s Plenty of Room at
theBottom,”wasdeliveredon29December1959attheannualAmericanPhysical
Society meeting on the campus of Caltech. Appendix 2 contains a transcript of his
entiretalk,whichcontainsreferencestoafutureworldthatwasneverbeforeimagi-
ned.Feynmanpointedoutthatdesigningmaterialsatom-by-atomisarealpossibility,
asitwouldnotviolateanyphysicallaws.Healsopredictedsuchsci-fiaccomplish-
mentsaswriting24volumesoftheEncyclopediaBrittanicaontheheadofapin,and
evenmoreamazingly,thecompletereproductionofeverybookeverproducedtofit
withinasmallhandheldpamphletoflessthan40pages!Toputthesepropheticstate-
mentsintocontext,atthetimehedeliveredthisspeech,computerssuchasUNIVAC
1filledanentireroom(Figure6.2)andcarriedapricetagofover$1million.
Thefirstuseoftheterm“nanotechnology”wasbyNorioTaniguchiin1974atthe
International Conference on Precision Engineering (ICPE). His definition referred
to“productiontechnologytogetextrahighaccuracyandultrafinedimensions,i.e.,
theprecisenessandfinenessontheorderof1nm(nanometer),10−9minlength.”[6]
Althoughmanydefinitionsfornanotechnologyhavebeensuggested,NASArecently
suggestedthemostthoroughdescription:
Thecreationoffunctionalmaterials,devicesandsystemsthroughcontrolof
matteronthenanometerlengthscale(1–100nm),andexploitationofnovel
phenomena and properties (physical, chemical, biological) at that length
scale.[7]
6 Nanomaterials 279
Figure6.2.Photooftheroom-sizedUNIVAC1computersystemthatwasintroducedinthelate1950s.[8]
Figure6.3.ScanningtunnelingmicroscopeimageoftheplacementofindividualXeatomsonaNi(110)
surface–nosurprise,byresearchersatIBM.ReproducedwithpermissionfromEigler,D.M.;Schweizer,
E.K.Nature1990,344,524.Copyright1990MacmillanPublishersLtd.
Although Feynman and Drexler certainly popularized nanotechnology, their influ-
ence did not directly lead to the design of nanoscale materials. Rapid progress
in nanotechnology could only take place after the arrival of sophisticated instru-
mentation, capable of viewing and manipulating materials on the nanoscale. In the
1980s, scanning probe microscopy (SPM) was developed which allowed scientists
tofulfillFeynman’svisionofpushingindividualatomsaroundasurface(Figure6.3).
280 6.2. NanoscaleBuildingBlocksandApplications
This technique was co-invented by Calvin Quate and Hemantha Kumar
Wickramasinghe. Interestingly, when Quate and Binnig first submitted their work
to the peer-reviewed journal Physical Review Letters, it was rejected due to such
far-fetched claims as being able to measure forces on individual atoms. However,
theseresultswereeventuallypublished,directlyinfluencingthefutureofmolecular
nanotechnology.The1986NobelPrizeinPhysicswasawardedtoGerdBinnigand
HeinrichRohrertohonortheirdesignofthescanningtunnelingmicroscope(STM).
TheysharedthePrizewithErnstRuska,theinventorofthefirstelectronmicroscope,
anotheressentialtoolforthemodernnanomaterialsscientist.Infact,theresolution
of modern electron microscopes are now high enough to provide images of indi-
vidualatoms,andareoftenfittedwithdetectorsthatarecapableofdeterminingthe
chemical composition and/or oxidation state of the surface atoms. Chapter 7 will
describe these and other instruments that are commonly used for materials-related
researchanddevelopment.
6.2. NANOSCALEBUILDINGBLOCKSANDAPPLICATIONS
The first question almost everyone new to the nanoregime asks is “why are nano-
materials so special?” The leading advantage of this size regime is the large sur-
face area/volume ratio exhibited by nanomaterials (Figure 6.4). Accordingly, this
translates to a very high surface reactivity with the surrounding surface, ideal for
catalysis or sensor applications. Further, since biological systems feature the sys-
tematic organization of nanoscale materials (e.g., proteins are 1–20nm in size, the
diameterofDNAisca.2.5nm),beingabletofabricatematerialsinthissizeregime
holdspromiseforartificialcomponentswithincells(thathaveca.10,000–20,000nm
Figure 6.4.Comparison of the surface area/volume ratio of macroscopic particles (marbles) and
nanoscopicaluminumoxideparticles.Sincenanoparticulescontainaproportionatelylargenumberof
surfaceatoms,thereareasignificantlygreaternumberofadsorption/reactionsitesthatareavailableto
interactwiththesurroundingenvironment.Further,whereasbendingofabulkmetaloccursviamove-
mentofgrainsinthe>100nmsizeregime,metallicnanostructureswillhaveextremehardness,with
significantlydifferentmalleability/ductilityrelativetothebulkmaterial.
6 Nanomaterials 281
Figure6.5.Decreaseinthemeltingpointofgoldnanoparticleswithdecreasingdiameter.Itshouldbe
notedthatthemeltingpointofbulkgoldis1,064◦C!AdaptedwithpermissionfromUnruh,K.M.etal.
“MeltingBehaviorinGranularMetalThinFilms,”MaterialsResearchSocietySymposiumProceedings,
vol.195.MaterialsResearchSociety,Apr16–20,1990.Copyright1990MaterialsResearchSociety.
diameters) to diagnose/fight diseases, ilnesses, viruses, and other superficial weak-
nesses(e.g.,artificialmuscles).
Another key benefit for nanomaterials is the ability of varying their funda-
mental properties (e.g., magnetization,[9] optical properties (color), melting point
(Figure6.5),hardness,etc.),relativetobulkmaterialswithoutachangeinchemical
composition. Although bulk properties such as melting point and hardness are re-
lated to the enhanced surface interactions among nanoparticulates, the size-tunable
electronicpropertiesareduetoquantumconfinementeffects,asdiscussedinlaterin
thischapter.
Sinceweliveinamacroscopicworld,thenextgenerationofmaterialswillbeof
similarphysicaldimensionsastoday’sconsumerproducts.Thatis,wehaveshrunk
downthesizeofcellphonesandcomputerstoalmosttheirusefullimits–anyfurther,
and one would inconveniently need to use a sharp stylus to dial a phone number!
However, as articulated in Chapter 4, although the size of electronic devices will
remain somewhat constant, the speed and computational ability of these devices
mustcontinuetoincrease.Thistranslatestomaterialsthatarebuiltfromtheground
up, one nanoscale building block at a time. However, it is synthetically too expen-
sive (and not industrially scaleable) to arrange such small units into their desired
positionsbyhand.Consequently,materialschemistsarelargelyfocusedon“bottom-
up” techniques that afford the self-assembly of nanoscale species. As we will see
later in this chapter, parallel efforts in “top-down” processing are being developed
282 6.2. NanoscaleBuildingBlocksandApplications
Figure6.6.Comparisonofthe“top-down”and“bottom-up”approachtonanomaterialssynthesis.
by materials engineers to yield nanoscale building blocks and devices through ad-
vanced lithographic, ablation, and etching techniques (Figure 6.6). In this respect,
one can consider a nanoscale object as being “mesomolecular” or “mesoatomic” –
anaggregateofsmallermolecular/atomicsubunits.
There are two primary types of nanoscale building blocks that may be used for
furtherdevicefabricationandapplications:
(i) 0D(e.g.,nanoparticles,nanoclusters,nanocrystals)
(ii) 1D(e.g.,nanotubes,nanofibers,nanowires)
Thedirectincorporationofthesenanoarchitecturesinexistingmaterialstoimprove
theirpropertiesisoftenreferredtoasincrementalnanotechnology.However,aswe
will see later in this chapter, the self-assembly of these nanosized building blocks
into 2D and 3D architectures may yield entirely new devices and functionalities –
referredtoasevolutionarynanotechnology.
6.2.1. Zero-DimensionalNanomaterials
Analogoustotheperiodinthissentence,a“zero-dimensional”structureisthesim-
plestbuildingblockthatmaybeusedfornanomaterialsdesign.Thesematerialshave
6 Nanomaterials 283
diameters<100nm,andaredenotedbynanoparticles,nanoclusters,ornanocrystals,
which are used synonymously in the literature. However, in order to continue our
rapid nanoscience developments, the active participants must share a common lan-
guage. Since there has been no broad adoption of such terminology, a goal of this
chapteristoprovideexplicitdefinitionsandexamples(Figure6.7),inordertoavoid
thecurrentnomenclatureambiguities.
Thetermnanoparticleisgenerallyusedtoencompassall0Dnanosizedbuilding
blocks (regardless of size and morphology), or those that are amorphous and pos-
sessarelativelyirregularshape.Herein,wewilldefinenanoparticlesasamorphous
or semicrystalline0D nanostructures withdimensions larger than 10nm, and arel-
ativelylarge(≥15%)sizedispersion.Foramorphous/semicrystallinenanostructures
smaller in size (i.e., 1–10nm), with a narrow size distribution, the term nanoclus-
ter ismoreappropriate.Thisdistinctionisasimpleextensionoftheterm“cluster,”
whichistypicallyusedininorganic/organometallicchemistrytoindicatesmallmole-
cularcagesoffixedsizes(Figure6.7).Analogoustobulkmaterials,theagglomera-
tion of noncrystalline nanostructural subunits should best be termed a nanopowder
(Figure6.7).
Itisalsoimportantheretonotethedifferencebetweennanoparticles/nanoclusters
and traditional colloids, which date back to the early 1860s (Table 6.1). We are
all familiar with the term colloid, which is used to describe solid/liquid and
solid/gas suspensions such as milk, paints, butter, smoke, and smog. Although
both types of materials have sizes within the nanoregime, the leading difference is
the control one has over composition and morphology. As we will see shortly, in
order to stabilize metal nanostructures, a stabilizing agent must be used to prevent
agglomeration into a larger powder. This is also the case for colloids, which gen-
erally employ polydispersed organic polymers and other ionic species that may
adsorb to the colloid surface. Such a variation in the nature of the encapsulat-
ing environment leads to a large dispersity in overall morphology and properties
of colloids. By contrast, in order for nanomaterials to be used for “bottom-up”
design, their synthesis and resultant properties must be reproducible. This is easily
accomplishedthroughtheuseofstabilizingagentswithwell-definedstructuresthat
do not react with/surface deactivate the entrained nanostructures (e.g., dendrimers,
polyoxoanions,etc.).
Thusfar,wehavedefinednomenclatureforamorphous0Dnanostructures.Anal-
ogous to bulk materials, any nanomaterial that is crystalline should be referred to
as a nanocrystal. This term should be reserved for those materials that are single-
crystalline; if a particle exhibits only regions of crystallinity, it is better termed
a nanoparticle or nanocluster depending on its dimensions. Transmission electron
microscopy, especially in tandem with electron diffraction is most useful in deter-
miningthecrystallinityofanynanostructure(Figure6.8).
Aspecialcaseofnanocrystalthatiscomprisedofasemiconductorisknownasa
quantum dot.[12] Typically, the dimensions of these nanostructures lie in the range
1–30nm,basedonitscomposition(seebelow).Quantumdotscurrentlyfindappli-
cationsassensors,lasers,andLEDs.Infact,newhigh-densitydisks(e.g.,HD-DVD
andBlu-rayhigh-definitionDVDformats)mayonlybereadviabluelasers,which
284 6.2. NanoscaleBuildingBlocksandApplications
Clusters
m
n
1
< a) b) c)
<
Nanoclusters Nanocrystals
5 nm
m
0 n 500 nm
10 5 nm
1 -
Nanoparticles Nanopowder
100 nm
50 nm
m Sub-Micron Particles
n
0
0
0
1
m -
n
0
0
1
100 nm
Bulk Powder
1 - 100 µm: “microstructures”
> 100 µm: “particulates”
Figure6.7.0Dnanostructurenomenclature.Shownarethewell-definedcagesizesofmolecularclusters
(Os5(CO)16),(Os6(CO)18),[Os8(CO)22]2−.[13]Comparatively,nanoclustersshouldbeusedtodescribe
0Dnanostructuresofahomogeneoussizedistribution.[14] Bycontrast,nanoparticlesexhibitagreater
rangeofsizes/shapes.[15]Nanocrystalsarecharacterizedbythepresenceofanorderedlatticearrayofthe
constituentsubunits,asillustratedbyasinglenanocrystalofCdSe.[16]Instarkcontrasttoananocrystal,
anexampleofananopowderisshownthatconsistsofmicroscopicgrains,eachcomprisedofnanoscale
amorphousunits.[17] Thesizeregimethatisintermediatebetweenthenano-andmicroregimesisbest
referredtoassubmicron.[18]Thebulkpowderscalebaris200µm.