679 research outputs found
Effects of laser wavelength and fluence on the growth of ZnO thin films by pulsed laser deposition
Transparent, electrically conductive and c-axis oriented ZnO thin films have been grown by the pulsed laser deposition (PLD) technique on silicon and Corning glass substrates employing either a KrF excimer laser (¿ = 248 nm) or a frequency-doubled Nd:YAG laser (¿ = 532 nm). The crystalline structure, surface morphology, optical and electrical properties of the deposited films were found to depend not only on the substrate temperature and oxygen partial pressure, but also on the irradiation conditions. The quality of the ZnO layers grown by the shorter wavelength laser was always better than that of the layers grown by the longer wavelength, under otherwise identical deposition conditions. This behaviour was qualitatively accounted for by the results of the numerical solution of a one-dimensional heat diffusion equation which indicated a strong superheating effect of the melted target material for the case of frequency-doubled Nd:YAG laser irradiations. By optimizing the deposition conditions we have grown, employing the KrF laser, very smooth c-axis oriented ZnO films having a full-width at half-maximum value of the (002) X-ray diffraction value less than 0.16° and optical transmittance around 85% in the visible region of the spectrum at a substrate temperature of only 300°C
Correlation between molecular orbitals and doping dependence of the electrical conductivity in electron-doped Metal-Phthalocyanine compounds
We have performed a comparative study of the electronic properties of six
different electron-doped metal phthalocyanine (MPc) compounds (ZnPc, CuPc,
NiPc, CoPc, FePc, and MnPc), in which the electron density is controlled by
means of potassium intercalation. In spite of the complexity of these systems,
we find that the nature of the underlying molecular orbitals produce observable
effects in the doping dependence of the electrical conductivity of the
materials. For all the MPc's in which the added electrons are expected to
occupy orbitals centered on the ligands (ZnPc, CuPc, and NiPc), the doping
dependence of the conductivity has an essentially identical shape. This shape
is different from that observed in MPc materials in which electrons are also
added to orbitals centered on the metal atom (CoPc, FePc, and MnPc). The
observed relation between the macroscopic electronic properties of the MPc
compounds and the properties of the molecular orbitals of the constituent
molecules, clearly indicates the richness of the alkali-doped
metal-phthalocyanines as a model class of compounds for the investigation of
the electronic properties of molecular systems
Tuning the electronic transport properties of graphene through functionalisation with fluorine
Engineering the electronic properties of graphene has triggered great
interest for potential applications in electronics and opto-electronics. Here
we demonstrate the possibility to tune the electronic transport properties of
graphene monolayers and multilayers by functionalisation with fluorine. We show
that by adjusting the fluorine content different electronic transport regimes
can be accessed. For monolayer samples, with increasing the fluorine content,
we observe a transition from electronic transport through Mott variable range
hopping in two dimensions to Efros - Shklovskii variable range hopping.
Multilayer fluorinated graphene with high concentration of fluorine show
two-dimensional Mott variable range hopping transport, whereas CF0.28
multilayer flakes have a band gap of 0.25eV and exhibit thermally activated
transport. Our experimental findings demonstrate that the ability to control
the degree of functionalisation of graphene is instrumental to engineer
different electronic properties in graphene materials.Comment: 6 pages, 5 figure
Direct observation of a gate tunable band-gap in electrical transport in ABC-trilayer graphene
Few layer graphene systems such as Bernal stacked bilayer and rhombohedral
(ABC-) stacked trilayer offer the unique possibility to open an electric field
tunable energy gap. To date, this energy gap has been experimentally confirmed
in optical spectroscopy. Here we report the first direct observation of the
electric field tunable energy gap in electronic transport experiments on doubly
gated suspended ABC-trilayer graphene. From a systematic study of the
non-linearities in current \textit{versus} voltage characteristics and the
temperature dependence of the conductivity we demonstrate that thermally
activated transport over the energy-gap dominates the electrical response of
these transistors. The estimated values for energy gap from the temperature
dependence and from the current voltage characteristics follow the
theoretically expected electric field dependence with critical exponent .
These experiments indicate that high quality few-layer graphene are suitable
candidates for exploring novel tunable THz light sources and detectors.Comment: Nano Letters, 2015 just accepted, DOI: 10.1021/acs.nanolett.5b0077
Double-gated graphene-based devices
We discuss transport through double gated single and few layer graphene
devices. This kind of device configuration has been used to investigate the
modulation of the energy band structure through the application of an external
perpendicular electric field, a unique property of few layer graphene systems.
Here we discuss technological details that are important for the fabrication of
top gated structures, based on electron-gun evaporation of SiO. We perform
a statistical study that demonstrates how --contrary to expectations-- the
breakdown field of electron-gun evaporated thin SiO films is comparable to
that of thermally grown oxide layers. We find that a high breakdown field can
be achieved in evaporated SiO only if the oxide deposition is directly
followed by the metallization of the top electrodes, without exposure to air of
the SiO layer.Comment: Replaced with revised version. To appear on New Journal of Physic
Electronic transport properties of few-layer graphene materials
Since the discovery of graphene -a single layer of carbon atoms arranged in a
honeycomb lattice - it was clear that this truly is a unique material system
with an unprecedented combination of physical properties. Graphene is the
thinnest membrane present in nature -just one atom thick- it is the strongest
material, it is transparent and it is a very good conductor with room
temperature charge mobilities larger than the typical mobilities found in
silicon. The significance played by this new material system is even more
apparent when considering that graphene is the thinnest member of a larger
family: the few-layer graphene materials. Even though several physical
properties are shared between graphene and its few-layers, recent theoretical
and experimental advances demonstrate that each specific thickness of few-layer
graphene is a material with unique physical properties.Comment: 26 pages, 8 figure
Contact resistance in graphene-based devices
We report a systematic study of the contact resistance present at the
interface between a metal (Ti) and graphene layers of different, known
thickness. By comparing devices fabricated on 11 graphene flakes we demonstrate
that the contact resistance is quantitatively the same for single-, bi-, and
tri-layer graphene (), and is in all cases
independent of gate voltage and temperature. We argue that the observed
behavior is due to charge transfer from the metal, causing the Fermi level in
the graphene region under the contacts to shift far away from the charge
neutrality point
Unforeseen high temperature and humidity stability of FeCl intercalated few layer graphene
We present the first systematic study of the stability of the structure and
electrical properties of FeCl intercalated few-layer graphene to high
levels of humidity and high temperature. Complementary experimental techniques
such as electrical transport, high resolution transmission electron microscopy
and Raman spectroscopy conclusively demonstrate the unforeseen stability of
this transparent conductor to a relative humidity up to at room
temperature for 25 days, to a temperature up to 150\,^\circC in atmosphere
and up to a temperature as high as 620\,^\circC in vacuum, that is more than
twice higher than the temperature at which the intercalation is conducted. The
stability of FeCl intercalated few-layer graphene together with its unique
values of low square resistance and high optical transparency, makes this
material an attractive transparent conductor in future flexible electronic
applications.Comment: Scientific Reports, volume 5, article no. 760
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