Investigations of graphene with the Free Electron Laser at HZDR.
with international colleagues, scientists from the Helmholtz-Zentrum
Dresden-Rossendorf (HZDR) have added another important component towards
understanding the material graphene; a material that is currently
receiving a lot of attention: They have determined the lifetime of
electrons in graphene in lower energy ranges. This is of great relevance
for the future development of fast electronic and optoelectronic
components. The results were published just recently in the online
edition of the journal Physical Review Letters.
the discovery of graphene had been awarded the Nobel Prize in Physics
last year, many research teams around the globe have been seeking to
better understand the material’s fundamental properties to permit such
promising electronic and optoelectronic applications as transistors and
rapid detectors for optical data transmission. Graphene—a single carbon
layer that has its atoms arranged in a hexagon like a honeycomb—is also
very interesting as a transparent electrode material for flat screens
and solar cells. According to the HZDR researcher Dr. Stephan Winnerl,
graphene might replace the scarce high tech metal indium in this field.
subsidies from the German Research Foundation’s Priority Program
“Graphene” and funds from the European Union, Stephan Winnerl and his
colleagues at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) together
with scientists from the Technische Universität (TU) Berlin, the
Grenoble High Magnetic Field Laboratory, and the Georgia Institute of
Technology managed to determine the “lifetime” of electrons in
graphene in lower energy ranges which had not been researched before.
characteristic behavior of electrons in specific energy ranges
typically found in solids is one of the many physical properties in
which graphene is fundamentally different from most other materials:
Normally, electrons can only adopt specific energy levels (these are
referred to as energy bands), but not others (these are referred to as
energy gaps). This principle is used, for example, for such
optoelectronic components as light emitting diodes which emit light at
very specific wavelengths: This releases energy which the electrons set
free while “skipping over” energy gaps.
graphene’s behavior differs from other semiconductors: The energy bands
touch each other without the appearance of any gaps. Instead of
emitting light, graphene is capable of absorbing the radiation of lower
energies below the visible spectrum, such as terahertz and infrared
light; thus, making it a superb material for detectors.
be able to develop rapid electronic and optoelectronic components based
on graphene, one has to know precisely how long electrons linger at
specific energy levels. The examination of such processes, which occur
in the picosecond range, i.e. the time scale of one millionth of a
millionth second, requires extremely rapid observation methods. The
unique feature of the experiments conducted at the Helmholtz-Zentrum in
Dresden is the exposure of the graphene samples to light that had longer
wavelengths than ever before. This was made possible through the short
radiation pulses of the HZDR’s Free Electron Laser (FEL). The
researchers were, thus, able to study the lifetime of electrons near the
contact point of the energy bands which is the unique physical property
characteristic of graphene.
FEL excited the graphene samples with light that had different
wavelengths in the infrared range. The researchers discovered that the
energy of the light particles exciting the electrons as well as the
oscillations of the atomic lattice influence the lifetime of the
electrons: If the energy of the light particles is greater than the
energy of the lattice oscillations, then the electrons will alter their
energy state more rapidly and have a shorter lifetime. Conversely, the
electrons will linger longer at a specific energy level if the
excitation energy is lower than the energy of the lattice oscillations.
insights gained from the experiments are substantiated by model
calculations from the TU Berlin. These calculations permit a clear
assignment of the experimental data to the physical mechanisms in
graphene. The researchers have, thus, made a valuable contribution
towards a better understanding of the electronic and optical properties