Nanoscale Phenomena
Mildred Dresselhaus
Introduction
The advance of nanoscience has been strongly connected with research carried out on nanoscale materials
with sizes far smaller than can be seen by the human eye or with an optical microscope. At the nanoscale,
which is below 1 µm (or 10-6 m), materials exhibit different properties than the same materials exhibit in
bulk form. This is in part due to the much larger surface-to-volume ratio of nanoscale materials, which for
example has had a great impact on the development of catalysts. Tiny catalytic particles a few nm in size
have a huge impact nowadays in the chemical industry in the production of ever-increasingly novel
nanomaterials which are designed to improve our quality of life. Beyond this, the new physical
phenomena that are discovered at the nanoscale, year by year and at an accelerated rate, are a central
focus of this paper.
The first part of the paper discusses my 50-year adventure with sp2-carbon-based nanomaterials. This
research area has attracted increasing attention in the last two decades, in part because carbon-based
materials have become prototypes for the study of nanoscale materials properties. The second part of the
paper discusses the impact of nanoscale materials on the conversion of thermal energy to electrical
energy, as occurs when waste heat is converted to electricity which can be used directly by consumers.
Whereas the carbon studies of nanostructures were inspired by curiosity, the studies on thermal energy
conversion were inspired by societal need.
Nanoscale Carbon Research
The active study of nanoscale carbons started with the 1947 paper of Wallace1 on a single layer of
graphite, which is known today as graphene. The novelty of his work was the linear dispersion relation
E±(k) = ±vFk for the energy of an electron in graphene as a function of wavevector k and velocity v at the
Fermi level. In 1960, when I started my independent research career, I chose to study the electronic
structure of three-dimensional graphite, highly inspired by the work of Wallace on the linear E(k) relation
in two-dimensional graphene, largely because of its novelty. Although we often used sticky tape in the
1960s to generate a beautiful shiny surface on our graphite samples, we never had an idea to try to study
the single-layer graphene left behind on the sticky tape.
In 1960 the electronic structure of three-dimensional graphene was just in a discovery phase, and I had an
opportunity to work on early experimental studies of this fascinating materials system. Theoretical models
of the electronic band structure for graphite had been developed by Slonczewski and Weiss in 19562 and
by McClure in 19573 in zero magnetic field, and in 1960 McClure4 extended his calculations to a
magnetic field. My research addressed experimental studies of the electronic band structure of graphite in
a magnetic field with the intention of getting a far more complete picture of the electronic band structure
than was then available. This research was important in evaluating the various parameters of the
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Slonczewski-Weiss-McClure band structure, and the study using circularly polarized light yielded the
proper location of electrons and holes in the graphite Brillouin zone.5 As can be seen from Figure 1, the
publication of papers on sp2 carbons during the 1960s was sparse, although the research by H. P. Boehm
on the 1962 observation of monolayer graphene,6 though very important historically, was not widely
known to those working in the sp2 carbon field at that time. In my case, I first met Boehm in 1977 at the
first international meeting of sp2 carbon researchers, which was held in La Napoule, France.7 This
conference was the first international conference on graphite intercalation compounds. At that time,
international travel and international conferences were not yet common, and the most common
communications medium was through the postal system. Intercalation compounds (see Figure 2) allowed
study of graphene monolayers, bilayers, trilayers, etc., sandwiched between the intercalant layers, but also
as somewhat modified by the interaction with these intercalant layers between adjacent graphene layers.
Methods for generating clusters and for studying nanoscale
carbon cluster properties became available in the early 1980s and
the clusters produced by laser ablation that we were studying
turned out to be larger than 50 carbon atoms in size. Such large
clusters were unexpected at that time.
In 1984 I visited the Exxon Research Labs to discuss the large
carbon clusters that we were observing with international experts
on large carbon clusters. Soon after this visit, the Exxon group
reported their results on large carbon clusters8 (see Figure 3),
showing carbon clusters up to C100 but with only even numbers of
Figure 1. Number of physics-related
carbon atoms per cluster above about C24. These results were also
publications on nano-carbons in the last
fifty years
observed independently by Richard Smalley at Rice University9,
who interpreted the large observed intensities for the C60 and C70 species to be connected with the early
observation of all-carbon fullerene molecules. As can be seen in Figure 1, the discovery of fullerenes
attracted much interest from the science community.
Our involvement in fullerene research was mostly in carrying out detailed symmetry-based infrared and
Raman spectroscopy studies in collaboration with Peter Eklund who was at the University of Kentucky at
that time; this collaboration and resulting extensive research program was reported in a large book on
fullerene spectroscopy published by Academic Press.10 Fullerenes were a manifestation of nanodot
nanocarbons, in comparison with graphite intercalation compounds which had a two-dimensionality
within large planes of few-layer graphene and a one-dimensionality in the direction of the superlattice
perpendicular to these graphene planes.
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Through the intercalation
research program, I
attended the Second
International Conference on
Graphite Intercalation
Compounds. It was there in
1980 that I met Professor
Morinobu Endo of Shinshu
University and heard him
speak about carbon fibers.
This talk gave me the idea
of intercalating carbon
fibers, and started a longterm collaborative program
on nanocarbon research.
Figure 2. Graphite
Figure 3. The ion signal for the number of
My heavy involvement in
intercalation compounds
nanocarbon clusters in a given sample as a
showing a superlattice of a
function of the number of carbon atoms per
the field of intercalated
stage 1 compound with a
cluster, showing a large peak at C60 and small
carbon fibers led to my
unit cell containing a
peak at C70
monolayer graphene layer
giving a review talk on
and a single intercalant layer
carbon fibers11 at a
Nanocarbon Meeting in Washington, DC, where Richard Smalley gave a review talk on fullerenes. When
Professor Smalley and I were both asked about the connection between carbon fullerenes and carbon
fibers, he responded that we could perhaps think of a C60 molecule elongating to form a C70 fullerene and
then elongating to C80 and eventually reaching a tube-like shape (see Figure 4). I could see that this
concept excited the audience.
Carbon Nanotube Studies
Figure 4. The elongation of fullerenes starting with
C60, C80, . . . , to reach a single wall carbon
nanotube on the right
The idea of fullerenes reaching a long tube-like shape
suggested to me the idea in September 1991, when
Matsutaka Fujita and Riichiro Saito, two very young
physics faculty members from Japan, came to MIT for
a year’s sabbatical, that I ask them if they would like
to calculate the electronic structure of what is now
called a single-wall carbon nanotube to see whether, if
we could ever make such a nanocarbon material
experimentally, such a structure might have interesting
properties (see Figure 5).
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Their calculation of the
rolling up of a monolayer
graphene sheet to form a
(a)
single wall carbon
nanotube (SWCNT)
(Figure 5) showed that
the linear E(k) relations
for graphene electrons
would be subjected to the
boundary conditions of
the cutting lines on the
Dirac cone for both the
Figure 5. a) Rolling up a graphene sheet seamlessly to form a single wall carbon
nanotube. b) The 1D van Hove singularities give a high density of electronic states
valence and conduction
corresponding to the cutting lines of the Dirac cone on the left
bands, as shown in
Figure 5. If the cutting lines would go through the apex of the cones, then a metallic nanotube would be
formed, but if the cutting lines did not go through the apex, then a semiconducting nanotube with a
bandgap would be formed.12 The prediction that nanotubes could be either semiconducting or metallic,
depending on the orientation of the hexagons of carbon atoms relative to the nanotube axis became very
controversial between 1992 and 1998. This controversy remained until experiments in 1998 directly
identified semiconducting and metallic nanotubes.13,14,15 Once researchers became successful in
synthesizing carbon nanotubes, many experimental and theoretical researchers entered the carbon
nanostructure field (see Figure 1) and found many challenging topics related to the overall behavior of
electrons, phonons, and the electron-phonon interaction between them.
(b)
Figure 6a. The Raman effect (top) and Raman
spectroscopy (bottom)
Figure 6b. Raman spectra for a bundle of carbon nanotubes (left)
and from individual (n,m) carbon nanotubes (right)
Our group became heavily involved with Raman spectroscopy studies explained in Figure 6a, and results
on Raman spectra from bundled16 and individual SWCNTs17 shown in Figure 6b. The reason for the entry
of large numbers of researchers into the nanotube field can be understood when considering the
complexity of carbon nanotubes, largely due to their chirality, which denotes the (n,m) index pair that is
used to describe the orientation of the nanotube hexagons relative to the nanotube axis (see Figure 5).
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This complexity generated a very large literature devoted to the study of the chirality dependence of the
many interesting carbon nanotube properties that have been demonstrated in the literature.18
Recent studies on carbon nanotubes have included more in-depth chirality studies,18 investigations of the
electronic Raman effect in carbon nanotube’s,19 combination modes and overtones in the Raman spectra20,
and electrochemical studies of the Raman effect.21 In general, major advances have occurred in the
general control of the chirality of carbon nanotubes in the last two or three years. Control of the synthesis
process has been achieved both for producing only metallic nanotubes,22 only semiconducting
nanotubes,22 and specific (n,m) nanotubes to reasonable purities,23 with further work ongoing mostly in
industry and government laboratories to achieve ever-higher purity levels, and likewise for the
characterization of nanotube samples for their metallicity (S,M) and chirality (n,m). Photoluminescence
techniques for the identification of the (n,m) of nanotubes have also advanced greatly and are now
commercialized.24 The observation of coherent phonon relaxation processes has greatly advanced both
experimentally25 and theoretically.26 Optical processes in nanotubes have been shown to be highly
sensitive to both excitonic excitation27 and dielectric screening effects;28 these effects need to be
characterized in specific samples and controlled where these effects are at issue. Recent nanotube studies
have been extended to double wall29 and triple wall30 nanotubes, with studies also done on individual
(n,m) nanotubes and bundles of nanotubes with mixed or with similar chiralities.
Graphene Studies
The discovery of graphene in 2004 by Geim and Novoselov31 as a stable single layer of carbon atoms
with sp2 bonding had a profound effect on nanocarbon science, transforming the Wallace 1947 conceptual
picture1 of monolayer graphene into a physical reality. The demonstration of a unique quantum Hall effect
in graphene, never seen before for any materials system, aroused great interest in the physics
community32. This was soon followed by the demonstration of a series of remarkable properties, such as
very high current densities, very high thermal conductivity, very similar behavior of the hole and electron
carriers regarding band parameters, and very high carrier mobilities. My involvement with graphene has
been largely with exploiting the relative simplicity of the electronic structure, allowing study of
symmetry-breaking phenomena that had previously been somewhat neglected in condensed matter
systems such as substrate effects, and edge effects.
One powerful tool that the lattice vibrations provide is the ability to use isotopes to distinguish surface
and environmental effects from bulk material effects. Since the 12C and 13C isotopes have masses that
differ by almost 10 per cent, the phonon vibrations will occur at different frequencies according to the
relation
(0-)/0 = 1 + [(12+C0)/(12+C)]1/2
where 0 and  are, respectively, the frequencies of the 12C graphene G-band carbon-carbon vibrational
frequency and that of a 13C-enriched carbon sample, C0 = 0.0107 is the natural abundance of 13C, while C
= 0.99 is the concentration of 13C in an enriched sample. Since the 12C and 13C isotopes have the same
electronic structure, use of isotopes allows the separation of the phonon-related effects from electronic
structure phenomena, since phonon-related phenomena are sensitive to the different isotopes while the
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electronic structure is not. Thus if a trilayer graphene sample is prepared such that the 13C-enriched layer
is placed on a substrate, a 12C/13C layer of mixed 12C and 13C composition is placed in the middle (where
the 12C/13C ratio could be varied from one sample to another), and a 12C layer is placed on the top, then we
can distinguish the environments of each of the three layers independently. Figure 7 shows the Raman
spectrum for the 12C top layer taken at a laser excitation energy of 2.33 eV which is similar to other traces
in the literature showing a strong intensity for the second-order G′ feature and a much weaker intensity
for the first-order G-band vibration due to the optical phonon modes of monolayer graphene. The
anomalously large intensity of the second-order feature in Figure 7 has been explained by the double
resonance process33 in sp2 carbon systems, but the IG′/IG intensity ratio for the G′ and G band features in
monolayer graphene is especially large because every step in the double resonance process is in
resonance.
(b)
Raman intensity, a.u.
(a)
3-LG
12
C 1-LG
12
13
C+ C 1-LG
13
1500
C 1-LG
2000
-12500
Raman shift, cm
3000
Figure 7. a) Electrochemical Raman spectra for graphene from monolayer 12C, 13C, 12C+13C, and for these three
layers stacked on top of one another. b) Raman frequency of the G-band feature for the top layer in contact with the
external environment, middle layer, and bottom layer vs. electrode potential from –1.5V to +1.5V
Figure 8. Arrangement of the electrochemical experiment shown in inset together with Raman spectra for the G band
and for the G′ band taken at 0.1V intervals in the –1.5V to +1.5V applied potential range
Electrochemistry allows us to put a controlled voltage across the layers using both negative and positive
voltages, which is equivalent to varying the Fermi level. These results (Figure 7b) show that the middle
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layer is protected by the two surrounding graphene layers, so that it is possible to clearly see features in
Figure 7b in the G band peak frequency for both positive and negative potentials at electrode potentials
corresponding to the G-band vibrational frequency, but this effect is not well seen for the bottom layer or
for the top layer in the triple layer sample in Figure 7b.
Raman spectra taken as a function of applied potential, ranging from –1.5Vto +1.5V, are highly
informative in showing the effect of chemical doping on the sample itself as well as asymmetries in the
valence and conduction bands. These effects are shown in Figure 8 for both the G band and the G′ band.
The G band reflects intraband transitions in a first order process, while the G′ band is highly sensitive to
interband transitions. The G′ band is a second order process, which is sensitive to the wave vectors of the
phonons involved in the transition.21
My involvement with graphene edges started in
1996 when a visiting student, Nakada, worked
with me and her Japanese supervisor, Fujita, on
armchair and zigzag edges (Figure 9). They had
shown theoretically that a zigzag edge had a high
density of electron states at the Fermi level but
armchair ribbons did not have a similar
singularity in the electronic density of states.
Armchair ribbons could instead be either metallic
or semiconducting depending on the ribbon
width34 as shown in Figure 10. Here we can see a
correspondence between one-dimensional
Figure 9. Armchair and zigzag graphene ribbons
armchair graphene ribbons and carbon nanotubes insofar as armchair ribbons can be either
semiconducting or metallic depending on their width.34 Differences between the Raman spectra of zigzag
and armchair edges in the Raman spectra were demonstrated in terms of the D-band Raman intensity35
which was also confirmed by theory. Studying edges experimentally is difficult because of difficulties in
separating edge roughness artifacts from intrinsic edge effects. We have more recently shown that joule
heating within a transmission electron microscope of graphitic ribbons (Figure 11) can yield atomically
sharp edges, as can be seen in the high resolution TEM image in Figure 11.36,37
Graphene has also provided a vehicle for identifying the physical origin of the weak features in the
Raman spectra such as the weak G* mode in the Raman spectra. In this case by varying both the gate
voltage (VG) and the laser excitation energy (Elaser) we have been able to identify specific low-intensity
features in the Raman spectrum of monolayer graphene with specific harmonics and phonon combination
modes as in molecular spectroscopy. The same approach appears to be promising more generally. One
benefit of studying such features in graphene is that related spectral features appear in the Raman spectra
of sp2 nanocarbons generally, but they are complicated by metallicity issues and chirality phenomena that
are not present in graphene. In this sense, graphene provides a useful metrology tool for studying a whole
range of related sp2 nanocarbons, and this is another reason people care about graphene.
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Figure 10. All zigzag graphene ribbons are metallic with a
high density of electron states but armchair ribbons can
be either metallic or semiconducting
Figure 11. High resolution transmission electron
microscopy of graphene ribbons and edges. a) SEM
image of the graphene ribbon used in the experiments.
b) high magnification TEM image of (a). c) Higher
resolution TEM image of (b)
Nanoscience of Energy Conversion Technologies
There is an abundance of sources of thermal energy that could be used to improve our quality of life
which instead is often considered to be waste heat and a pollutant to our environment. Thermoelectric
processes offer opportunities to convert some of the waste heat from automobiles and commercial
industrial processes into useful electrical energy. Such conversion could become more attractive if the
efficiency of the energy conversion process could be enhanced. It was proposed starting in 1992, under
encouragement by the US and French navies, that the use of nanostructures could significantly enhance
the efficiency of the thermoelectric conversion process. This proposal came after a 30-year history from
1960-1990 when thermoelectric research was mostly inactive. More specifically, the 1992 suggestion to
the Navy was that nanomaterials in the form of thin films and quantum wires could show better
performance than the same materials in bulk form.38 The merit of the proposal was discussed within the
research community in the 1990s and strategies were next developed for testing of this concept. The
following few years involved proof-of-principle studies, followed by implementation at the pilot scale
study level.39 Once commercial implementation of nanothermoelectric materials started in the last five
years, new concepts for further enhancing the thermoelectric performance came to the fore. This 20-year
development of nanothermoelectricity is briefly summarized in this review.
Thermoelectric energy conversion depends on the temperature difference between a hot junction and a
cold junction as shown in Figure 12, thereby creating a voltage between them because of the difference in
carrier concentration between the two ends of the rod. This voltage difference results in a Seebeck effect
with a Seebeck coefficient S for thermoelectric materials which we try to make as large as possible for
improved thermoelectric performance. It is important for the thermal conductivity  to be as low as
possible to maintain the temperature difference between the hot and cold junctions, but at the same time
we need electrons to carry electricity so that a large carrier density and a large electrical conductivity 
are used. The thermoelectric figure of merit Z = S2/ is the quantity which controls the performance of
thermoelectric devices, and is normally reported as a dimensionless quantity ZT = S2T/. Since S is a
decreasing function of carrier density and  increases with carrier density, as shown in Figure 12, and
8/13
since  increases when  increases because of the Wiedemann-Franz law,30 it had been difficult in bulk
materials to increase ZT beyond the value of 1 using bulk thermoelectric materials.
However, the
introduction of
composite nanothermoelectric
materials (Figure 13)
increased the
performance of many
thermoelectric
materials in the past 15
Figure 12. a) Schematic of the Seebeck effect.
Figure 13. Nanothermoelectrics
b) The dependece on the carrier density of the
materials can be either asemblies of
years. In Figure 13 both
Seebeck coefficient S, the electrical
nanoparticles, superlattice
2
the bulk nano
assemplies of thin layers of different
conductivity , and the product S , which is
thermoelectric materials such as
called the power factor
composite material and
Bi2Te3 and Sb2T3, or they can be
nanostructural
nanoparticle inclusions formed in a
thermolectric material such as PbTe,
inclusions-based
which has constituents of
material are shown. To
AgPbmSbTe2m
make three-dimensional
(3D) bulk material with useful heat-carrying capacity,
it is necessary to assemble large amounts of
nanomaterials into some kind of composite. This
consolidation process produces, with some
optimization, bulk thermoelectric materials with
enhanced ZT, as obtained from measurement of the
constituent thermoelectric quantities S, ,  (Figure
14).40. The most important scattering mechanism here
is the boundary scattering at the boundaries of the
nanoparticles contained in the composite material,
Figure 14. Temperature dependence of the
which in practice implies that the particles should be
nanocomposite of Bi2Te3 shows an increase of the
a) electronic conductivity only, b) the Seebeck
~10 nm in size. The PbTe-based material containing
coefficient, as well as a concurrent decrease in the
nanostructural precipitates (Figure 13) also is
thermal conductivity leading ot a significant increase
in ZT
effective as a bulk nanostructural thermoelectric
material. This boundary scattering approach had been the main driving force during the 1990s and into
the following decade for improving the performance of thermoelectric materials.
More recently four other research strategies have been implemented to enhance ZT. First came the
concept of using resonant states in the energy dependence of the Seebeck coefficient by Heremans.41
Although first demonstrated as a Tl dopant in PbTe, this effect is expected to be quite general and can be
extended to other dopants and to other host materials. The concept of modulation doping, taken originally
from the doping of III-V semiconductors such as GaAs,42 was recently successfully extended to
thermoelectrics by Zebarjadi.43 According to this concept, dopants are introduced in one special region of
a sample (see Figure 15) and the carriers from this special region are transported across a very thin spacer
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region to the active channel that is doped. In modulation-doped thermoelectric materials, the host material
is a 3D bulk composite, and the dopant is located inside tiny nanoparticles which donate carriers to the
host semiconductor to enhance its conductivity with highly mobile carriers. These carriers maintain high
mobility because the carrier pathway is predominantly through the high-mobility material with only a
small amount of scattering by the tiny nanoparticles containing dopants. The measured ZT enhancement
of a Si80Ge20 composite material is shown in Figure 16 using this concept. Other novel methods for ZT
enhancement have been proposed and these are also in an early demonstration stage, such as energy
filtering, nanoparticle doping,44 and the spin Hall effect.45
Figure 15. Modulation doping to enhance thermoelectric
performance, based on the fundamental concept taken from
semiconductor physics (left) using the band alignment shown
in the center to prepare a thermoelectric modulation doped
material. On the right is a schematic for the 3D bulk
thermoelectric modulation doped material where the yellow
background denotes the high mobility host material which
contains dopant nanoparticles embedded in large nanoscale
particles shown in green.
Figure 16. Plots of the temperature dependence of
the power factor, thermal conductivity, and ZT for a
two-phase composite modulation doped sample
showing that the modulation doped sample exceeds
the thermoelectric performance of the best Si30Ge20
material prepared previously
Since nanostructures have different properties from their bulk counterparts we can expect a continuing
stream of novel concepts for various energy conversion strategies both within the thermal energy to
electrical energy conversion initiative and more generally, for thermal energy, solar energy, and perhaps
even wind energy conversion as independent technologies. Our planet has a large untapped renewable
energy resource available from both waste heat and solar energy. The potential for the utilization of
nanoscience for both enhanced renewable energy recovery and for the control of global warming is large,
and is mostly untapped, offering many opportunities for the younger generation to contribute strongly to
these very active current research areas.
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