Charge transport characterization of P3HT thin-film organic semiconductorMatija Filipčič
, 2021, research project (high school)
Abstract: The focus of this diploma seminar is to describe different types of charge transport theory and
models used for finding mobility in semiconductors, more specifically focused on P3HT thin-film
organic semiconductor. It also describes the time-of-flight method, which was used to measure the
experimental data for P3HT with time dependent current I(t) curve. Another data set was obtained
by performing Kinetic Monte Carlo (KMC) simulations using Miller-Abrahams hopping formalism.
KMC was used to determine the material disorder, simulate I(t) curve and transit time for every
charge carrier. Simulated data, different transit times and mobilities were then finally compared
with experimental, in order to find agreements between the two.
Keywords: charge transport, P3HT, thin-film organic semiconductor
Published: 15.09.2021; Views: 1292; Downloads: 0
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Vibrational spectra of am-Al 2 O 3 : tuning a parametric model.Kurtović Jasmin
, 2021, research project (high school)
Abstract: The present diploma seminar work has been dedicated first to the implementation of a procedure
to calculate the vibrational density of states of two structural models of vitreous SiO 2 (v-
SiO 2 ) and amorphous Al 2 O 3 (am-Al 2 O 3 ) models, and next to tune a parametric model for the
calculation of the infrared (IR) spectra of am-Al 2 O 3 , in particular of the imaginary part of
the dielectric function. The ground state of both structural models is obtained by relaxing
the atomic structure by using the conjugate gradient method as implemented in the LAMMPS
code. Vibrational frequencies and modes are obtained, in the harmonic approximation, by
diagonalizing the dynamical matrices calculated for the given v-SiO 2 and am-Al 2 O 3 structural
models. Dynamical matrices are obtained through a finite differences approach and vibrational
density of states are plotted by applying Gaussian broadening. The calculation of the dielectric
function requires the knowledge of the vibrational frequencies as well as the knowledge of the
dynamical (or Born) charge tensors related to the atoms of the am-Al 2 O 3 structural model.
For the latter model, a parametrization of the ab-initio Born charge tensors has been carried
out with the purpose to allow for the fast calculation of the IR spectrum of any other am-
Al 2 O 3 model without the need to calculate for it the Born charge tensors using expensive ab-
initio methods. The parametrization of Born charge tensors takes into account, for aluminium
atoms, only of the isotropic charge which depends on coordination number and average Al-
O bond length of aluminium atoms, while for oxygen atoms coordinated to three Al atoms
(75%), dynamical charges are parametrized also by the area bounded by aluminium atoms
nearest neighbours of the 3-coordinated oxygen atom. Moreover for analyzing the dynamical
charge tensors of 3-coordinated oxygen atoms a decomposition in terms of the representations
of the spatial rotations was used. The IR spectrum obtained by means of the above described
parametrization provides a good approximation to the IR spectrum obtained by using the ab-
initio calculated dynamical charges, as it differs from it, on average, by around 5.1% which
is much better (12%) than using a average isotropic charge model (i.e. diagonal Born charge
tensors where each diagonal element is one third of the average isotropic charge).
Keywords: Vibrational spectra, am-Al 2 O 3, parametric model
Published: 15.09.2021; Views: 1131; Downloads: 0
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Space weather research with the Pierre Auger ObservatoryMiha Živec
, 2019, master's thesis
Abstract: Space weather refers to environmental conditions in the interplanetary space and Earth’s
magnetosphere, ionosphere and exosphere and can influence the performance and reliability
of electronics based technological systems. The major role in space weather
changes plays the solar wind, a stream of charged particles (mostly electrons and protons)
with energies of approximately 1 keV, that can cause geomagnetic storms and auroras.
During their entry into the atmosphere, high energy cosmic rays collide with atomic
nuclei of atmospheric gasses. When scattering occurs extensive air showers are created.
Those cascades of secondary particles create flashes of light due to the Cherenkov
effect as well as excite molecules of nitrogen gas in atmosphere, which then glow in
fluorescent light. In order to observe the light created by air showers, it has to be
collected with telescopes. The particles from the cascades that reach ground can be
detected with surface detectors. The Pierre Auger Observatory is the largest observatory
for cosmic ray measurements. It is located in Argentinian pampas covering an area of
3000 km2. It consists of 1660 surface detectors and 27 fluorescence telescopes. For
cosmic rays with energies above few 1017 eV, a precise reconstruction of energy and
direction of primary particle is achievable. Observatory also allows measurement of
flux of incoming particles down to primary energies in ca. 10 GeV - 10 TeV interval, with a median energy ca. 80-90 GeV. This measurement capability is called "scaler"
mode, since the corresponding data consist of scaler counted cascade particles with
deposited energy between 15 and 100 MeV, at the average rate of 2 kHz per individual
For the purpose of this master thesis I compared the data from scaler mode measurements
with measurements of neutron monitors, which are commonly used for space weather
observations. With the correlation received from the comparison, I showed that scaler
mode operation of Pierre Auger observatory can be used to monitor space weather events
such as solar cycle and the decreases in the observed galactic cosmic ray intensity due
to solar wind (Forbush decrease).
Keywords: Pierre Auger Observatory, cosmic rays, space weather, Forbush decrease
Published: 17.09.2019; Views: 3878; Downloads: 166
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Flares from the centers of galaxies with Gaia and OGLE surveysNada Ihanec
, 2018, master's thesis
Abstract: Modern wide-field-of-view and all-sky satellites (e.g. Gaia) and ground based surveys (e.g. OGLE) repeatedly cover a large part of the sky and are detecting new, transient astrophysical sources on daily basis.
In this thesis I analyzed the data from Gaia and OGLE transient surveys, with special focus on transients located near the centres of galaxies to detect possible Tidal Disruption Events. These occur when a star gets too close to a Super-Massive Black Hole, which lurks in the centres of most galaxies, and gets disrupted due to the black hole's gravitational tidal forces.
The goal of my research was to detect possible Tidal Disruption Events and eliminate false candidates, such as supernovae. The work involved daily inspection of new alerts, identified with Gaia and OGLE Transient Detection System. I searched for potential transients in galactic nuclei and in case there was such a transient detected, follow-up spectroscopic observations were initiated in order to help classify the object.
During the course of my work I analyzed spectra obtained with the largest telescopes in the world (SALT, VLT) and performed the spectral template matching, recognition of spectral features related to known classes of transients, determination of redshift etc.
Keywords: Flares, transients, Gaia, OGLE, supernovae, tidal disruption events, nuclear transients
Published: 14.05.2019; Views: 3145; Downloads: 116
Fulltext (8,11 MB)
ZAČETNE FAZE RASTI ORGANSKIH POLPREVODNIKOV NA SiO2Alen Oršulić
, 2018, master's thesis
Abstract: Pričujoče delo obravnava začetne faze rasti organskega polprevodnika N, N´-1H, 1H- perfluorobutil diciano perilen karboksamid (PDIF-CN2) na površini silicijevega dioksida (SiO2). PDIF-CN2 je bil na površino SiO2 nanešen z rotacijskim nanašalcem pod različnimi pogoji. Glede na pogoje nanosa se je spreminjala morfologija narejenih vzorcev. Morfologije so bile analizirane z mikroskopom na atomsko silo. S slik mikroskopa na atomsko silo smo z višinsko porazdelitvijo in funkcijo spektralne gostote moči pridobili podatke o prekritosti vzorcev z otoki molekul PDIF-CN2, višini otokov na površini vzorca, povprečni hrapavosti vzorca, korelacijski dolžini med otoki ter spektralni dolžini in indeksu. Z višinsko porazdelitvijo je bilo ugotovljeno, da znaša povprečna višina otokov na površini vzorcev približno 1,1 nm in da se prekritost vzorcev z molekulami povečuje s povečevanjem koncentracije raztopine in zmanjševanjem kotne hitrosti pri nanosu z rotacijskim nanašalcem. Analiza funkcije spektralne gostote moči površin je pokazala, da je pri večini vzorcev korelacijska dolžina čez celotno preiskano območje, da se povprečna hrapavost povečuje s povečevanjem koncentracije raztopine in kotne hitrosti ter da pospešek pri rotacijskem nanašanju skupaj s kotno hitrostjo vpliva na obliko fraktalov na površini vzorcev.
Keywords: PDIF-CN2, mikroskop na atomsko silo, rotacijsko nanašanje, Gwyddion, avtokorelacijska funkcija, funkcija spektralne gostote moči.
Published: 24.09.2018; Views: 3795; Downloads: 155
Fulltext (2,26 MB)
Characterization of Mg low-index surfaces by first-principlesMiha Gunde
, 2017, master's thesis
Abstract: In this work, three surfaces, namely, Mg(0001), Mg(10 ̄1 0) and Mg(11 ̄2 0) are investigated by means of Density Functional Theory and Generalized Gradient Approximation first-principle calculations. The surfaces have been characterized in terms of their interlayer distances, surface energies, Projected Density of States and Surface Core Level Shifts. Contrary to what was found in previous studies based on Density Functional Theory-Local Density Approximation, the use of Generalized Gradient Approximation exchange correlation functional unravels the oscillatory behavior of both interlayer distances and Surface Core Level Shifts of Mg(10 ̄1 0). Moreover,
the Projected Density Of States of un-reconstructed Mg(11 ̄2 0) exhibit the typical quantisation of electronic levels associated to a 2D-confinement of a 3D nearly-free electron gas, as observed in nano-wires, and a Peierls instability along the unconfined direction. Different possible reconstructions on Mg(11 ̄2 0) have then been investigated. In particular, it is found that the
surface energy of (2X1) and (3X1) reconstructions is almost degenerate and lower than the surface energy of the unreconstructed Mg(11 ̄2 0) surface. Because of the quasi-degeneracy of surface energies, Mg(11 ̄2 0) surface should be composed by a combination of all the quasi degenerate phases.
Keywords: Density Functional Theory, first-principle calculations, Mg surface, Surface Core-Level Shift, surface relaxation, surface reconstruction
Published: 04.07.2017; Views: 5155; Downloads: 2085
Fulltext (1,77 MB)
SiPM as light detector in Cherenkov telescopes cameras at CTAAleš Bogovič
, 2017, master's thesis
Abstract: This master thesis describes characterization of Silicon photomultipliers (SiPM) for the upgrade of large size telescope (LST) camera at Cherenkov telescope array (CTA). Upon the construction, CTA will be the largest and most sensitive facility for detection of very-high energy (VHE, > 30 GeV) gamma-rays ever. It will use three different sizes of telescopes, where small and medium telescopes camera will be SiPM based from the beginning, but LST camera is planned to get SiPM based camera with frst upgrade. Camera upgrade is among others also being developed at University of Padovas Department of Physics and Astronomy (INFN). Due to limitations of their laboratory characterization of prototypes is done at Institute Jooef Stefan in Ljubljana, where I measured breakdown voltage of and determined photon detection effciency depending on angle of photon arrival. This information is important for determining which SiPM is best for CTA telescopes.
Keywords: Silicon photomultiplier, SiPM, CTA
Published: 03.05.2017; Views: 5269; Downloads: 150
Fulltext (14,05 MB)
Measurements and modeling of air mass motion in the troposphereMiha Živec
, 2016, undergraduate thesis
Abstract: Throughout the history human race depended on weather, so one of the priorities for its survival was to understand weather patterns and to be able to forecast weather. With the development of powerful computers, atmospheric numerical methods and precision instruments for atmospheric monitoring, it is possible to predict weather with greater accuracy and for a longer period of time ahead. At the same time, we are able to gain improved understanding of physical processes that occur in the atmosphere and represent one of most important features in our world. This diploma thesis focuses on the lowest part of the atmosphere - troposphere only, as all weather occurs in the troposphere. Weather is a complete collection of momentary thermodynamic states in the atmosphere and is defined with thermodynamic variables and relations between them.
The goal of this thesis is development and presentation of a new way to determine the direction and speed of air mass movement, based on the combination of passive and active remote sensing techniques. A lidar is being used to determine the range to an object, in our case a cloud, that can be used as a tracer in the air current. Simultaneously with lidar ranging of clouds that same clouds are being visually monitored in a series of optical photographs. Selecting and following the temporal evolution of distinct cloud features and their range allows us to calculate the speed of clouds.
The performance of this method was tested on four cases in Feb. and Mar. 2016. Measurements were performed in Ajdovščina in different weather conditions. Along with remote sensing (infra-red lidar and optical cameras), ground measurements of wind at Ajdovščina were performed. Wind speeds and directions obtained from remote sensing were compared to atmospheric sounding data from Ljubljana and Udine at similar heights and performed within as small as possible time window.
In all four cases remote sensing results for wind speeds and directions agree relatively well with atmospheric sounding. Deviations are expected to be primarily due to spatial and temporal mismatch between sounding and remote sensing measurements. Another source of uncertainties are the limitations of the present remote sensing method in the determination of the actual direction of the wind, however, theses limitations could be eliminated in the future by using an all-sky camera and vertical lidar configuration.
Keywords: remote sensing, wind, atmosphere
Published: 13.10.2016; Views: 6781; Downloads: 184
Fulltext (9,48 MB)