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NANoPhotoNIcS
Making the most of photons
The performance of photodetectors based on quantum dots may be improved by exploiting a process called
multiple exciton generation.
Arthur J. Nozik
t
he absorption of photons by a solar cell
or photodetector excites electrons to
a higher energy level, leaving behind
positive charges, called holes, at a lower
level. The holes and/or electrons then leave
the device to provide power, in the case
of a photovoltaic cell, or to be detected, in
the case of a photodetector. However, the
semiconducting materials used inside these
devices can only absorb photons whose
energy is equal to or greater than their
bandgap energy. When the photon energy is
greater than the bandgap, the excess energy
is usually lost from the system as heat, which
in the case of photovoltaic cells reduces the
device efficiency. Such losses can be partially
avoided by capturing different colours of
light with different materials, each with its
own bandgap, but at the expense of added
cost and complexity. It is also possible to
reduce these losses by creating two or more
electron–hole pairs from a single absorbed
photon. In quantum dots the electron–hole
pairs are bound together and are called
excitons, and the multiplication process is
called multiple exciton generation or MEG,
depicted in Fig. 1 (refs. 1,2). (Some authors
label this process carrier multiplication3,4.)
Although the detection and
characterization of MEG has been the
subject of much recent research, it has not
yet been shown to improve the efficiency
of a photovoltaic device5. Now in Science,
Edward Sargent and colleagues at the
University of Toronto have reported a
different approach to measuring MEG in
quantum dots6. Their data lends support to
previous spectroscopic measurements, while
using a simpler experimental system.
Quantum dots have drawn attention
because they show MEG at lower photon
energies relative to the bandgap than bulk
materials. To satisfy energy conservation,
MEG requires that the photon energy be
at least twice the quantum-dot bandgap
Eg. The threshold for electron–hole pair
multiplication in bulk semiconductors is
even higher, typically 4–6 times the bulk
bandgap, as a result of extra conservation
rules related to crystal momentum, and high
carrier-cooling rates7–9. In quantum dots,
on the other hand, the need for momentum
conservation is relaxed, and quantum
548
e–
e–
Eph
Eg
e–
MEG
O
h+
O
h+
Figure 1 | Multiple exciton generation (MEg)
in a quantum dot. A photon with energy Eph
excites an electron e− to a higher energy level
(red arrow), leaving behind a positively-charged
hole (h+). This electron and hole form an exciton
that then relaxes to a lower energy level, and at
the same time a second electron is excited to a
higher energy level (green arrows). As a result,
a single photon creates two excitons. Eg is the
quantum-dot bandgap; horizontal blue lines
represent quantum dot energy levels; and black
lines represent potential barriers.
confinement reduces the cooling rate relative
to MEG. This has led to the prediction of
increased efficiency and a reduced energy
threshold for MEG in quantum dots8,9. A
threshold of 2.7–3.0 Eg has been reported for
MEG in lead sulphide, selenide and telluride
quantum dots based on time-resolved
spectroscopy measurements1–4.
Sargent and colleagues measure MEG in a
photodetector configuration consisting of an
array of lead sulphide quantum dots spaced
closely enough to produce appreciable interdot electronic coupling, carrier mobility and
conductivity. An electric field was applied
to break the excitons apart and extract
charges to an external circuit. The gain
of the device — defined as the number of
electrons per second flowing in the external
circuit divided by the number of photons
per second absorbed by the quantum
dots — was found to be constant for incident
photon energies less than 2.7 Eg. However,
the gain started to grow as the photon
energy rose above 2.7 Eg, and increased by a
factor of 4.4 ± 0.5 to a value of over 100, at a
photon energy of 4.2 Eg. The consistency of
the measured threshold value (2.7 Eg) with
previous spectroscopic measurements1–4
supports the claim that MEG is occurring.
Sargent and colleagues suggest that
this enhanced gain results from MEG,
followed by the trapping of electrons at the
quantum-dot surfaces. The trapping process
leaves behind long-lived holes that are able
to circulate through the photoconductor
and external circuit multiple times before
finally recombining with their negative
partners. They propose that the electron
trapping in their system occurs through a
process called Auger-assisted ionization,
which creates excited electrons and is
known to be efficient in the presence of
multiple excitons. The possibility that the
electron trapping occurs directly from
initial photoexcited excitons, independently
of MEG, was discounted by examining the
dependence of gain on quantum-dot size10.
However, the measured size dependence
was not dramatic, and the interpretation
could have been strengthened by studying a
wider range of dot sizes. Other ambiguities
in the interpretation include the absence of
information on the role of reflection losses,
possible errors in the peak absorbance
measurement, the origin of significant gain
(about 20) below the MEG threshold, and
the lack of data for larger quantum-dot sizes
and lower photon energies.
The detection of MEG through a
photoconductivity measurement is
important because, so far, all robust
demonstrations of this process in quantum
dots have been based on exciton relaxation
dynamics measured by ultrafast timeresolved spectroscopy 1–4. Such experiments
typically measure the ‘quantum yield’ of
the MEG process, which is defined as the
number of excitons produced per absorbed
photon. Time-resolved spectroscopy has
the potential disadvantage of exaggerating
the MEG quantum yield in the presence
of unrelated phenomena, such as direct
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© 2009 Macmillan Publishers Limited. All rights reserved
news & views
photoionization and trapping 11. This has
contributed to large variations in reported
MEG quantum yields, which, together with
the lack of a photovoltaic device with a
quantum yield greater than unity, has led
to some scepticism in the community. By
presenting data based on photocurrent gain
rather than time-resolved spectroscopy, the
work by Sargent and colleagues supports
the importance of MEG for optoelectronics.
The Toronto team also exploit the process to
demonstrate photodetectors with both a high
responsivity and fast response time, with
potential applications in low-light imaging.
Nevertheless, the photoconductivity
measurement remains an indirect one
because it probes the effect of MEG in
the presence of an electric field. This
means that interpretation of the data
requires an understanding of how the
electric field affects the MEG process,
and how it causes gain to occur below the
MEG threshold.
The most compelling demonstration
of MEG would come from a photovoltaic
cell that does not have an electric field
applied to it, and that shows a quantum
yield for photocurrent greater than
unity. But, by attempting to relate MEG
to photocurrent rather than just optical
spectroscopy, the photoconductivity
measurements discussed here are a step in
the right direction.
❐
Arthur J. Nozik is a Senior Research Fellow at
the US Department of Energy (DOE) National
Renewable Energy Laboratory (NREL), 1671 Cole
Boulevard, Golden, Colorado 80401, USA and
Professor Adjoint in the Department of Chemistry
and Biochemistry at the University of Colorado,
Boulder, Colorado 80309, USA.
e‑mail: [email protected]
References
Ellingson, R. J. et al. Nano Lett. 5, 865–871 (2005).
Nozik, A. J. Chem. Phys. Lett. 457, 3–11 (2008).
Schaller, R. D. & Klimov, V. I. Phys. Rev. Lett. 92, 186601 (2004).
Schaller, R. D., Agranovich, V. M. & Klimov, V. I. Nature Phys.
1, 189–194 (2005).
5. Luther, J. M. et al. Nano Lett. 8, 3488–3492 (2008).
6. Sukhovatkin, V., Hinds, S., Brzozowski, L. & Sargent, E. H. Science
324, 1542–1544 (2009).
7. Werner, J. H., Kolodinski, S., Queisser, H. J. Phys. Rev. Lett.
72, 3851–3854 (1994).
8. Nozik, A. J. Ann. Rev. Phys. Chem. 52, 193–231 (2001).
9. Nozik, A. J. Physica E 14, 115–120 (2002).
10. Poles, E., Selmarten, D. C., Mićić, O. I. & Nozik, A. J. Appl. Phys.
Lett. 75, 971–973 (1999).
11. McGuire, J. A., Joo, J., Pietryga, J. M., Schaller, R. D. & Klimov, V. I.
Acc. Chem. Res. 41, 1810–1819 (2008).
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gRAPheNe
Surfing ripples towards new devices
The electronic properties of graphene can be changed by exploiting its unusual thermal properties to introduce
periodic ripples with given wavelengths and amplitudes.
Rodolfo Miranda and Amadeo l. Vázquez de Parga
h
opes for a new generation of strainbased electronic devices made from
graphene will rest on our ability to
control the electromechanical properties
of these two-dimensional sheets of carbon
atoms. These hopes have now moved a step
closer to reality following demonstrations
on page 562 of this issue by Chun Ning Lau
and colleagues at the University of California
at Riverside that periodic ripples can be
introduced in a controlled way to suspended
samples of few-layer graphene1. This new
approach takes advantage of the thermal
strains produced by the negative thermal
expansion coefficient of graphene.
Ripples are difficult to avoid in twodimensional materials and have been seen
in graphene before. The first ripples to
be observed were randomly distributed
across the surface2, but periodic ripples
were subsequently created by depositing
single-layer graphene on single-crystal
metal surfaces3,4. The ripples resulted
from the surface and the graphene having
different lattice parameters. For example,
when graphene was deposited on iridium
or ruthenium surfaces, ripples with a
wavelength of 2.4–3.0 nm and an amplitude
of 0.02–0.05 nm were observed. These
ripples have also been shown to be associated
with periodic variations in the electronic
properties of the graphene1,3.
Figure 1 | Artistic impression of periodic ripples in
single-layer graphene. The ripples lead to a small
distortion of the carbon–carbon bonds, which
leads to changes in the amplitude of the electron
hopping between atoms and to the simultaneous
appearance of inhomogeneities in the electronic
structure. The whiter atoms indicate that the
density of occupied electronic states at the Fermi
level is higher at the peaks of the ripples.
In general, ripples in graphene (Fig. 1) are
expected to strongly influence its electronic
properties by introducing spatially varying
potentials5 or effective magnetic fields (also
known as gauge fields)6. And if the ripples
are periodic, one can, in principle, open up a
bandgap between the valence and conduction
bands occupied by the electrons. Graphene
nature nanotechnology | VOL 4 | SEPTEMBER 2009 | www.nature.com/naturenanotechnology
© 2009 Macmillan Publishers Limited. All rights reserved
does not have an intrinsic bandgap, which is
why it is known as a zero-gap semiconductor
or a semi-metal. Moreover, one could arrange
for the bandgap and the energy of the Fermi
surface (which separates occupied and
unoccupied electron energy levels at absolute
zero temperature) to coincide by doping or
applying a suitable bias voltage. Having a gap
at the Fermi level is vital for applications in
digital electronics because it is essential that
transistors should not permit any current to
flow in the ‘off ’ state – this cannot be achieved
with zero-gap semiconductors.
Lau and co-workers place their graphene
samples across pre-defined trenches on
a silicon/silicon dioxide substrate and
observe the ripples with a scanning electron
microscope and an atomic force microscope.
The crests of the ripples are usually
perpendicular to the edges of the trench,
with amplitudes between 0.7 and 30 nm,
and wavelengths in the range 370–5,000 nm.
The ripples disappear when the samples
are annealed or heated to 700 K, but new
ripples with larger amplitudes and longer
wavelengths appear when the graphene
is cooled again, although the graphene
sometime buckles on cooling instead.
The Riverside team explains this behaviour
in terms of the unusual thermal properties
of graphene. Unlike most materials, apart
from water in a small range of temperatures,
549