Fahlman B.D. Materials Chemistry

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It should be noted that an additional variable may be required to describe the full

mechanism for CNT growth.

[224]

Recent studies suggest that there is likely an

important role for metal oxide catalysts, or oxygenated co-reactants, in the overall

growth mechanism of SWNTs. For example, the use of water vapor as a co-reactant

for CVD results in the growth of a dense forest of vertically aligned SWNTs, with

lengths up to 2.5 mm after only 10 min of reaction time (Figure 6.86a).

[225]

The

catalytic enhancement of water vapor is evidence d by the SWNT/catalyst weight

ratio of 50,000% – more than 100 times greater than the HiPCO process. The effect

of an oxidizing species such as water vapor seems logical from the standpoint of

reducing the amount of amorphous carbon deposits, and controlling the degree

of carbon precipitation (thus preventing MWNT growth). However, its role may

be even more signiﬁcant – perhaps even providing a new mode of nucleation.

In particular, it is proposed that SWNT growth may be initiated from oxygen-etched

graphitic shells that encapsulate the catalyst – a route rece ntly termed “nucleati on

via etched carbon shells” (NECS), Figure 6.86b .

[226]

Catalyst particle

after recalescence with

graphitic coating

Embroyonic

cap

SWCNT

growth

Stable

oxidized

ring

Step 1: Step 2: Step 3:

Step 4:

A solidified catalyst particle

with a graphite shell heads

toward a molten catalyst

particle.

The solidified catalyst

particle collides with the

molten catalyst particle.

Oxygen etches the cap and

prevents MWCNT growth by

oxidising precipitating

carbon.

A stable oxidized ring forms

(c.f. preferential burning of

SWCNT caps). This forms the

embryonic cap for SWCNT

growth. The solid catalyst

particle enters the molten

particle.

Carbon subtitutes O at the

oxidized ring interface and

SWCNT growth occurs.

Large molten

catalyst

particle

Oxygen

etching

Cap

Formation

Caddition

substitution

Figure 6.86. (a) Photo of a cm-scale SWNT forest, shown next to a matchstick for comparative purposes.

Reproduced with permission from Hata, K.; Futaba, D. N.; Mizuno, K.; Namai, T.; Yumura, M.; Iijima, S.

SWNTs via metal oxide nanocluster catalysts. Reproduced with permission from Rummeli, M. H.;

Borowiak-Palen, E.; Gemming, T.; Pichler, T.; Knupfer, M.; Kalbac, M.; Dunsch, L.; Jost, O.; Silva, S.

558 6 Nanomaterials

The formation of nanotubes is not limited to carbonaceous materials. In fact, a

number of inorganic-based nanotubes have been synthesized in recent years – such as

metals, oxides, carbides, borides, etc.

[227]

There are many applications for these

materials, such as FETs, LEDs, photodetectors, photovoltaic devices, catalysts,

advanced ceramics, and chemical sensors.

[228]

Themostcommonmethodtosynthesize

these structures is by coating an anodized alumina or CNT substrate with a thin ﬁlm of

the desired material, followed by the sacriﬁcial removal of the template.

[229]

Hydro-

thermal techniques using organic templates have also been widely reported.

[230]

As we

saw earlier for nanocluster growth, templating routes offer a convenient mode of

nanostructural synthesis, as well as surface alignment of the nanoarchitectures on a

substrate surface. In addition, self-assembly of molecular precursors in solution,

analogous to sol-gel growth of thin ﬁlms, may also be used to synthesize single-walled

oxide nanotubes such as aluminosilicate or aluminogermate varieties.

[231]

One interesting application for metal oxide nanotubes is for quantum dot sensi-

tized solar cells (QDSSCs). Recall from Chapter 4 the concept of dye-sensitized

solar cells; in this perturbation, quantum dots are used to gather light that is then

transported to the collection electrode via a suitable medium such as a mesoporous

TiO

ﬁlm.

[232]

A key beneﬁt of this technology is the ability to absorb a broad range

of electromagnet ic wavelengths by simply varying the size of QDs; in contrast, dyes

exhibit a relatively narrow spectral absorption window. A recent strategy entails the

surface-functionalization of TiO

or CNT nanotubes with varying sizes of QDs

in order to harvest a wide range of available electromagnetic wavelengths.

[233]

Whereas noncovalent attac hment offers no real control over the distribution of

the QDs along the length of the nanotube, one may covalently attach the nano-

crystals as long as compatible chemical handles (e.g., —COOH, —OH, —NH

, etc.)

are available on both the QD entraining agent and nanotube surface.

6.3.3. Two-Dimensional Nanostructures: The “Graphene Frontier”

In Chapter 4, we discussed an important 2-D nanoarchitecture – thin ﬁlms. As long

as the thickne ss is <100 nm, these are properly classiﬁed within the nanomaterials

umbrella. Though spin-coating may be able to afford thin ﬁlms of this overall

thickness, these types of coatings are generally deposited using vapor deposition

techniques (PVD, CVD, and ALD). The current buzz related to 2-D nanomaterials is

related to the synthesis, characterization, and functionalization of the theoretical

sub-unit of carbon nanotubes – graphene sheets.

Though the term ‘graphene’ has been used since the late 1980s to describe

nanotube and graphite intercalation compounds, the earliest synthesis of graphene

consisted of simply exfoliating graphite with scotch tape (!),

[234]

scratching highly-

oriented pyrolytic graphite (HOPG) with a solid surface,

[235]

or heteroepitaxial

layering of graphene onto single-crystalline SiC.

[236]

The exfoliation of graph ite

via strong mechanical shearing (stirring/sonication) at elevated temperatures will

produce expanded graphite, which is primarily comprised of few-layer graphene

sheets.

[237]

Graphite oxide sheets

[238]

that contain hydroxyl/epoxide groups on basal

6.3. Nanoscale Building Blocks and Applications 559

planes and carbonyl/carboxyl groups on sheet edges,

[239]

may also be reduced to

pristine graphene using hydrazine solutions.

[240]

More recently, chem ical vapor

deposition has been used to grow high-yield, large area graphene ﬁlms that may

be post-transferred to a variety of substrates.

[241]

Since these early discoveries, there has been a slurry of activity throughout the

world to theoretically and empirically study its properties for applica tions in micro-

electronics and energy storage.

[242]

In particular, graphene exhibits the quantum

Hall effect

[243]

at room temperature, extremely high electron/hole mobilities

(>200,000 cm

/Vs at room temperature

[244]

), and high thermal conductivity.

In addition, graphene exhibits a similar mechanical strength as carbon nanotubes,

with a speciﬁc surface area much greater than ﬁnely-divided activated carbon

currently used for water puriﬁcation applications.

[245]

Not unlike carbon nanotubes , solubilization and/or surface functionalization of

graphene is essential to control its reactivity/self-assembly for fut ure applications.

Whereas graphene oxide (GO) is soluble in polar solvents, reduced GO is insoluble.

Thus far, strategies have included both non-covalent (p-stacking) interactions with

aromatic molecules,

[246]

and covalent interactions with amphiphilic polymers

[247]

well as surface ﬂuorination.

[248]

Electronic properties of graphene

The band diagram for graphene sheets features p/p

-bands formed from the overlap

of p

atomic orbitals on neighboring C atoms, with an electronic energy dispersion

described by Eq. 25. However, the graphene band structure is distinct from the

conventional parabolic bands exhibited by metals or n-/p-type semiconductors

discussed in Chapter 4. In particular, the E-k relationship of graphene is linear

around the Dirac point, which directly results in relativistic behavior of the electrons

(zero effective mass, referred to as Dirac fermions) at low energies (Figure 6.87).

Further, the Fermi surface of neutral graphene reduces to a single point (Dirac

point), which deﬁnes the material as a zero-bandg ap semiconductor.

[249]

Due to

their structural relationships, the unique band structure of graphene directly inﬂu-

ences the distinct electronic properties of CNTs.

EðpÞ¼n

h

ﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ

þ k



;ð25Þ

where: n

is the Fermi velocity (ca. 1  10

m.s

1

) and p is the carrier momentum .

An important consequence of the graphene band structure is its ambipolar ﬁeld

effect, in which both electron/hole carriers may exist by simply varying the applied

voltage (Figure 6.88).

[250]

Though a single graphene sheet is a zero-bandgap semicon-

ductor and cannot be used directly in applications such as ﬁeld-effect transistors

(FETs), adding another graphene layer results in a widely-tunable bandgap.

[251]

Accordingly, there is much current interest in graphene bilayers being used for FET

applications, which have the potential to outperform both Si- and GaAs-based devices

based on their greater carrier mobilities of electrons and holes, respectively.

[252]

560 6 Nanomaterials

Figure 6.87. Top: the energy dispersion relations for the p and p* bands in 2-D graphite are shown

throughout the whole region of the Brillouin zone. The inset shows the energy dispersion along the high

symmetry directions of the 2-D Brillouin zone. Reproduced with permission from Phys. Rev. B 2000, 61,

2981 and Eur. Phys. J. B 2009, 72, 1. Bottom: comparison of band structures for a semiconductor, metal,

Scientiﬁc American, Inc.

−60 −30

0 T

(V)

(kΩ)

1 K

30 60

Figure 6.88. Band structure and the ambipolar ﬁeld effect in graphene. Conduction and valence bands

meet at the Dirac point without an external ﬁeld; in the presence of a gate bias, the Fermi level moves

above/below the Dirac point to generate free carriers. Reproduced with permission from Nature Mater.

6.3. Nanoscale Building Blocks and Applications 561

Graphene nanoribbons (GNRs)

Another method used to overcome the zero-bandgap limit ation of bulk graphene

sheets for electronic applications is to cleave graphene into nanoribbons in order to

induce further quantum conﬁnement. Depending on how the graphene nanoribbons

(GNRs) are fabricated, armchair and/or zigzag edges will be present along its

sidewalls (Figure 6.89). Analogous to carbon nanotubes, the bandgap and resultant

electronic transport properties of GNRs are expected to strongly depend on the width

and crystallographic orientation; however , the presence of armchair/zigzag edge

conﬁgurations is also a major governing variable. Both armchair and zigzag GNRs

are semiconductors with direct bandgaps that decrease with increasing ribbon

widths.

[253]

However, zigz ag GNRs are dominated by edge states, which have

been demonstrated to be spin-polarized under a large electric ﬁeld. This has impor-

tant implications for future spintronic devices, which promise to be smaller, faster,

and more versatile than current technologies.

[254]

The use of nanoribbons for

energy-storage applications (e.g., anode materials for Li-ion batteries) is also

under investigation.

[255]

For instance, theoretical studies have shown that Li atoms

bind much more strongly to zigzag GNRs than armchair varieties, which exhibit

similar Li-adsorption properties as bulk graphene.

[256]

The ﬁrst reports of “top-down” syntheses of graphene nanoribbons (GNRs) via

‘unzipping’ carbon nanotubes were reported in mid-2009. Tour et al. report an acid-

catalyzed permanganate oxidation route (Figure 6.90 – top),

[257]

whereas Dai et al.

report a plasma-based unzipping of polymer-immobilized nanotubes (Figure 6.90 –

bottom).

[258]

Alternatively, lithography

[259]

and chemical vapor deposition

[260]

methods have been used to afford small amounts of GNRs. The ﬁrst “bottom-up”

approach was recently reported by Mullen and coworkers using the controlled

polymerization of aromatic monomers (Figure 6.91 ).

[261]

Figure 6.89. Graphene nanoribbons with zig-zag (red) or armchair (green) edges, relative to a bulk 2-D

graphene sheet.

562 6 Nanomaterials

Figure 6.90. Illustration of top-down graphene nanoribbon formation from unzipping multi-walled

carbon nanotubes via a solution-based permanganate oxidation (top), and Ar plasma (bottom). Reproduced

with permission from Nature 2009, 458, 872, and Nature 2009, 458, 877, respectively.

6.3. Nanoscale Building Blocks and Applications 563

IMPORTANT MATERIALS APPLICATIONS V:

NANOELECTROMECHANICAL SYSTEMS (NEMS)

The past decade or so has brought about tremendous growth in the ﬁeld of micro-

electromechanical systems (MEMS) – devices such as sensors, computers, electron-

ics, and machines at the microscale. The design of these devices utilizes a

combination of traditional semiconductor processing and mechanical engineering.

Typically, the output of an elect romechanical device is the movement of the

mechanical component; a transducer is used to convert the mechanical energy into

electrical/optical signals, or vice versa. A sampling of some current MEMS applica-

tions include:

(i) Automotive (e.g ., air ﬂow and tire pressure sensors, “smart” suspension, head-

light leveling, navigation, vehicle security, automatic seatbelt restraint, etc.)

(ii) Micro-nozzles for inkjet printers

(iii) Micro-tweezers

(iv) Aerospace navigationa l gyroscopes

(v) Disposable blood pressure transducers

(vi) Portable skin analysis sensors for cosmetics applications

The extension of MEMS to the nanor egime is referred to as NEMS – representing

the ultimate in future devices – with beneﬁts such as lower power dissipation and

ultra-sensitive and localized responses. Further, due to the size of NEMS, it will be

Figure 6.91. Bottom-up synthesis of graphene nanoribbons. Reagents and conditions:

(a) 4-bromophenylboronic acid, Pd(PPh

)

, toluene, 80



C, 24 h, 93%. (b) (1) n-BuLi, THF,

78



C, 1 h; (2) 2-isopropoxy-4,4,5,5-tetramethyl[1,3,2]dioxaborolane, rt, 2 h, 82%. (c) compound 1,Pd

(PPh

)

, toluene/H

O, reﬂux, 72 h, 75%. (d) FeCl

,CH

/CH

,25



C, 48 h, 65%.

Chemical Society.

564 6 Nanomaterials

possible to directly incorporate a number of auxiliary functionalities alongside

transistors within a single chip. Indeed, the applications for NEMS will span the

ﬁelds of senso rs, electronics, biotech nology, affecting virtually every aspect of our

lives. Though the technology is in place to fabricate NEMS, there are three primary

challenges that must be overcome prior to the realization of widespread commercial

applications:

(i) How to communicate signals from the nanoscale to the macroscopic world

(ii) Understanding and controlling mechanical responses at the nanoscale

(iii) Developing methods for low-cost, high-volume, and reproducible n ano-

fabrication

One recent NEMS device is a mass sensor with a resolution at the zeptogram level (1

zg ¼ 10

21

g).

[262]

Thus far, the lowest detection limit for this device is 7 zg, or ca.

30 Xe atoms. Incredibly, it is suggested that this technology may be combined with

nanoﬂuidics for the genetic analysis of the DNA present within a single cell!

In comparison, current methods utilize PCR ampliﬁcation, whereby small samples

of DNA are repeatedly replicated in order to facilitate detection. Such analyses will

be of use for the detection of genetic markers for cancer or other diseases such as

HIV. The ability to identify proteins will be essential for future diagnostic and

forensics applications.

Another area of NEMS that is receiving tremendous attention is the mimicry of

biological systems, aptly referred to as biomimetics. For instance, in the develop-

ment of linear molecular muscles that undergo contraction and extension move-

ments. Initial work in this ﬁeld utilized transition metal complexes containing

rotaxanes and catenanes, due to the nondestructive redox processes occurring on

the metal centers.

[263]

Though these complexes were actuated by a chemical reac-

tion, the movement was in a noncoherent manner. In order to better mimic skeletal

muscle movement, one has to look at the mode of motion within the most efﬁcient

molecular machines – in our human bodies.

The cellular unit that is active toward the contraction of skele tal muscles,

known as the sarcomere, is comprised of alternatively stacked ﬁlaments of the

proteins actin and myosin. During muscle contraction, the protein ﬁlaments slide

past each other as a result of a rowing action of the surface myosin heads

(Figure 6.92a).

[264]

Hence, a n effective biomimetic approach would entail the

design of a linear architecture that features sliding components that will respond

to a che mical stimulus. This approach has recently been demonstrated with the

design of a rotaxane molecule that exhibits redox-controlled contraction and

extension of the molecular architecture, in response to a chemical or electrochem-

ical stimulus (Figure 6.92b).

[265]

The controlled movement in this system is controlled by the interaction among

redox-active units at the redox-active tetrathiafulvalene (TTF) units. In the neutral,

unperturbed state, the tetracationic cyclophane units (cyclobis(paraquat-para-

phenylene), CBPQT

) are most stable coordinated to the TTF moieties, due to

electron donation and p-stacking interactions. However, upon oxidation of the TTF

units, the CBPQT

rings become electrostatically repelled, migrating to the

6.3. Nanoscale Building Blocks and Applications 565

naphthalene component (Figure 6.92b). Hence, the “rowing” action exhibited by

skeletal muscles is emulated by the Coulombic repulsion and p-donation of the

naphthalene rings.

[266]

By attaching disulﬁde tethers to the CBPQT

rings, the

attachment of the linear muscle may be fashioned to a gold surface en route toward

a biomimetic NEMS device.

[267]

In the next few years, there will be many exciting developments related to NEMS

devices. In particular, replacing the current chemically driven molecular machines

with those stimulated by optical or electrical pulses – dramatically extending the

range of applications. With so many possible nanobuilding blocks at our disposal,

the scope of devices and resultant applications is limited only by our imaginations –

an exciting area of discovery awaits!

Actin Filament Myosin Filament Z Disc

Contraction

Extension

TTF

CBPQT

Naphalene

4.2 nm

OOO OOO OOO OOO

OOO OOO

OOO

Extension

(TTF/CBPQT

couple preferred)

Contrction

(Naphalene/CBPQT

couple preferred)

−4e +4e

1.4 nm 1.4 nm 1.4 nm

PPR

12+

PPR

Figure 6.92. A biomimetic approach toward skeletal muscle movement. Shown is (a) the stacked protein

ﬁlaments of the sarcomere and (b) a redox-controlled molecular analogue. Adapted with permission from

Liu, Y.; Flood, A. H.; Bonvallet, P. A.; Vignon, S. A.; Northrop, B. H.; Tseng, H.-R.; Jeppesen, J. O.;

Huang, T. J.; Brough, B.; Baller, M.; Magonov, S.; Solares, S. D.; Goddard, W. A.; Ho, C.-M.; Stoddart,

566 6 Nanomaterials

References and Notes

Note: a sampling of some intriguing applications that are already possible using nanomaterials

include: self-cleaning fabrics (via TiO

nanoparticles), automobile clearcoats that prevent scratches

(PPG nanoparticle-based coatings), car wash solutions that prevent dirt from adhering to a painted

surface, bandages that kill bacteria, drug-release agents and time-release biocidal coatings, and tennis

balls that bounce twice as long as conventional balls.

Only U.S.-based institutes/centers are listed here; for a more comprehensive list of worldwide

nanotechnology efforts, see http://sunsite.nus.sg/MEMEX/nanolink.html, a comprehensive listing

of nanorelated websites hosted by the University of Singapore.

Now available online at: http://e-drexler.com/p/06/00/EOC_Cover.html

By 2014, commercial products that incorporate nanomaterials will be worth an estimated $6 trillion,

with an estimated annual global market of $2 billion. For instance, see: (a) Holman, M. W.; Lackner,

D. I. The Nanotech Report, 4th ed., Lux Research: New Yorki, 2006. (b) Thayer, A. M. Chem. Eng.

News 2007, 85, 29.

“Discussion draft: Addressing nanomaterials as an issue of global concern”, May 2009, Center for

International Environmental Law (CIEL). Found online at: http://www.ciel.org/Publications/CIEL_

NanoStudy_May09.pdf

(a) De Jong, W. H.; Borm, P. J. A. Int. J. Nanomed. 2008, 3, 133. (b) Donaldson K, Stone V, Tran CL,

Kreyling W and Borm PJA. Nanotoxicology. Occup Environ Med 2004, 61, 727–728.

For details on the biological effects of CNTs, see: Liu, Z.; Cai, W.; He, L.; Nakayama, N.; Chen, K.;

Sun, X.; Chen, X.; Dai, H. Nat. Nanotechnol. 2007, 2, 47, and references therein. The biological

effects of dendritic polymers is described in Boas, U.; Heegaard, P. M. H. Chem. Soc. Rev. 2004, 33,

43, and references therein. Some websites that have information regarding the toxicological effects of

nanostructures include: (a) http://orise.orau.gov/ihos/Nanotechnology/nanotech_OSHrisks.html; (b)

http://www.cdc.gov/niosh/topics/nanotech; (c) http://www.bnl.gov/cfn; (d) http://www.ciel.org/

Publications/CIEL_NanoStudy_May09.pdf; (e) http://www.nsf.gov/pubs/2007/nsf07590/nsf07590.htm

One of the most recent sources of nanotoxicity data is from the European Union Scientiﬁc Committee

on Emerging and Newly Identiﬁed Health Risks (SCENIHR), Risk Assessment of Products of

Nanotechnologies, Jan. 19, 2009. May be accessed online at: http://ec.europa.eu/health/ph_risk/

committees/04_scenihr/docs/scenihr_o_023.pdf. The main website of the SCENIHR, that houses

other reports, such as the dangers associated with nanomaterials for cosmetics applications, may be

found at: http://ec.europa.eu/health/ph_risk/committees/04_scenihr/04_scenihr_en.htm

Cross, S. E.; Innes, B.; Roberts, M. S.; Tsuzuki, T.; Robertson, T. A.; McCormich, P. Skin Pharma-

col. Physiol. 2007, 20, 148.

Zhu, S.; Oberdorster, E.; Haasch, M. L. Marine Environ. Res. 2006, 62, S5, and references therein.

Tinkle, S. S, Antonini, J. M., Rich, B. A., Roberts, J. R., Salmen, R., DePree, K., Adkijns, E. J.

Environ. Health Perspect. 2003, 111, 1202.

Holsapple, M. P.; Farland, W. H.; Landry, T. D.; Monteiro-Riviere, N. A.; Carter, J. M.; Walker, N.

J.; Thomas, K. V. Toxicol. Sci. 2005, 88, 12.

For instance, see: Yang, X.; Fan, C.; HS, Z. Toxicol. In Vitro 2002, 16, 41, and references therein.

Sayes, C. M.; Fortner, J. D.; Guo, W.; Lyon, D.; Boyd, A. M.; Ausman, K. D.; Tao, Y. J.; Sitharaman,

B.; Wilson, L. J.; Hughes, J. B.; West, J. L.; Colvin, V. L. Nano Lett. 2004, 4, 1881, and references

therein.

(a) Rouse, J. G.; Yang, J.; Ryman-Rasmussen, J. P.; Barron, A. R.; Monteiro-Riviere, N. A. Nano

Lett. 2007, 7, 155. (b) Rouse, J. G.; Yang, J.; Barron, A. R.; Monteiro-Riviere, N. A. Toxicol. In Vitro

2006, 20, 1313. (c) Yang, J.; Barron, A. R. Chem. Commun. 2004, 2884 (Article describing the

synthesis of amino-acid functionalized fullerenes).

(a) Blundell, G.; Henderson, W. J.; Price, E. W. Ann. Trop. Med. Parasitol. 1989, 83, 381–385. (b)

Corachan, M.; Tura, J. M.; Campo, E.; Soley, M.; Traveria, A. Trop. Geogr. Med. 1988, 40, 359–364.

EnViron. Health Perspect. 2003, 111, 1202–1208.

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