508 research outputs found
Ultrafast Optical Spectroscopy Techniques applied to colloidal nanocrystals
In this thesis I will describe my experimental work based on ultrafast optical spectroscopy techniques applied to colloidal nanocrystals.
Colloidal semiconductor nanocrystals in recent years have attracted a lot of attention in particular in optoelectronic applications, because they present
unique optical, electronic and charge carrier transport properties that can be easily modified via colloidal chemical synthesis.
In the first chapter of this thesis, I will introduce the basic concepts of nanocrystals and their optical properties, I will consider some simple models
to explain the physical properties of semiconductor nanocrystals. I will briefly describe the colloidal chemical synthesis of these nanocrystals and how we
can change the optical properties by simply acting on the colloidal chemical synthesis.
In the second chapter I will describe the basic concepts of ultra fast optical spectroscopy techniques used in my experimental work. I will describe the basic principles and the experimental set-up of the two main techniques that I
used: time-resolved photoluminescence spectroscopy (TR-PL) and transient absorption spectroscopy or also called Pump Probe. These two techniques allow to investigate in a very precise way the main optical properties and dynamics of charge carriers in nanocrystals.
In the third chapter I will describe my experimental work based on timeresolved photoluminescence spectroscopy applied to CdSe=CdS core=shell nanocrystals. These nanocrystals are a reference nanocrystals and their optical
properties are extensively investigated, however, some aspects are not fully understood, for example, the instability or blinking of the light emission
under constant illumination in these nanocrystals is still an unsolved problem that limits the real applications. The particular spectroscopic technique that I have used variable pulse rate photoluminescence spectroscopy revealed
the causes of this issue, and will give us a solution to resolve it.
In the fourth chapter I will discuss novel nanostructures of CdSe=CdS multi branched shape, in this case octapod shaped nanocrystals consist of eight arms made of CdS grown on a CdSe core, these present a large crosssection for light absorption and efficient charge separation ideally suited for applications of photocatalysis. The optical spectroscopy technique used to investigate the properties of these nanocrystals are the transient absorption
spectroscopy.
In the fifth chapter I will discuss experimental work on Bi2S3 semiconductor nanocrystals and I will describe related optical techniques used to study the optical and electronic properties. This nanocrystal has excellent
properties of optical absorption of solar radiation and can be used for the realization of solar cells. Another important property of Bi2S3 is its nontoxicity
that will allow in the future to achieve efficient solar cells and easy disposal and simultaneously not damaging to the environment
Light-Induced Charged and Trap States in Colloidal Nanocrystals Detected by Variable Pulse Rate Photoluminescence Spectroscopy
<p>Intensity instabilities are a common trademark of the photoluminescence of nanoemitters. This general behavior is commonly attributed to random fluctuations of free charges and activation of charge traps reducing the emission yield intermittently. However, the actual physical origin of this phenomenon is actively debated. Here we devise an experiment, variable pulse rate photoluminescence, to control the accumulation of charges and the activation of charge traps. The dynamics of these states is studied, with pulse repetition frequencies from the single-pulse to the megahertz regime, by monitoring photoluminescence spectrograms with picosecond temporal resolution. We find that both photocharging and charge trapping contribute to photoluminescence quenching, and both processes can be reversibly induced by light. Our spectroscopic technique demonstrates that charge accumulation and trap formation are strongly sensitive to the environment, showing different dynamics when nanocrystals are dispersed in solution or deposited as a film.</p>
Colloidal synthesis and characterization of Bi2S3 nanoparticles for photovoltaic applications
Bismuth sulfide is a promising n-type semiconductor for solar energy conversion. We have explored the colloidal synthesis of Bi2S3 nanocrystals, with the aim of employing them in the fabrication of solution-processable solar cells and to replace toxic heavy metals chalcogenides like PbS or CdS, that are currently employed in such devices. We compare different methods to obtain Bi2S3 colloidal quantum dots, including the use of environmentally benign reactants, through organometallic synthesis. Different sizes and shapes were obtained according to the synthesis parameters and the growth process has been rationalized by comparing the predicted morphology with systematic physical-chemistry characterization of nanocrystals by X-ray diffraction, FT-IR spectroscopy, Transmission Electron Microscopy (TEM)
Intrinsic time resolution of 3D-trench silicon pixels for charged particle detection
In the last years, high-resolution time tagging has emerged as the tool to
tackle the problem of high-track density in the detectors of the next
generation of experiments at particle colliders. Time resolutions below 50ps
and event average repetition rates of tens of MHz on sensor pixels having a
pitch of 50m are typical minimum requirements. This poses an important
scientific and technological challenge on the development of particle sensors
and processing electronics. The TIMESPOT initiative (which stands for TIME and
SPace real-time Operating Tracker) aims at the development of a full prototype
detection system suitable for the particle trackers of the next-to-come
particle physics experiments. This paper describes the results obtained on the
first batch of TIMESPOT silicon sensors, based on a novel 3D MEMS (micro
electro-mechanical systems) design. Following this approach, the performance of
other ongoing silicon sensor developments has been matched and overcome, while
using a technology which is known to be robust against radiation degradation. A
time resolution of the order of 20ps has been measured at room temperature
suggesting also possible improvements after further optimisations of the
front-end electronics processing stage.Comment: This version was accepted to be published on JINST on 21/07/202
Digital Pixel Test Structures implemented in a 65 nm CMOS process
The ALICE ITS3 (Inner Tracking System 3) upgrade project and the CERN EP R&D
on monolithic pixel sensors are investigating the feasibility of the Tower
Partners Semiconductor Co. 65 nm process for use in the next generation of
vertex detectors. The ITS3 aims to employ wafer-scale Monolithic Active Pixel
Sensors thinned down to 20 to 40 um and bent to form truly cylindrical half
barrels. Among the first critical steps towards the realisation of this
detector is to validate the sensor technology through extensive
characterisation both in the laboratory and with in-beam measurements. The
Digital Pixel Test Structure (DPTS) is one of the prototypes produced in the
first sensor submission in this technology and has undergone a systematic
measurement campaign whose details are presented in this article.
The results confirm the goals of detection efficiency and non-ionising and
ionising radiation hardness up to the expected levels for ALICE ITS3 and also
demonstrate operation at +20 C and a detection efficiency of 99% for a DPTS
irradiated with a dose of 1 MeV ncm.
Furthermore, spatial, timing and energy resolutions were measured at various
settings and irradiation levels.Comment: Updated threshold calibration method. Implemented colorblind friendly
color palette in all figures. Updated reference
f0(980) production in inelastic pp collisions at s = 5.02 TeV
The measurement of the production of f0(980) in inelastic pp collisions at sqrt(s) = 5.02 TeV is presented. This is the first reported measurement of inclusive f0(980) yield at LHC energies. The production is measured at midrapidity, |y| pi+pi- hadronic decay channel using the ALICE detector. The pT-differential yields are compared to those of pions, protons and ϕ mesons as well as to predictions from the HERWIG 7.2 QCD-inspired Monte Carlo event generator and calculations from a coalescence model that uses the AMPT model as an input. The ratio of the pT-integrated yield of f0(980) relative to pions is compared to measurements in e+e- and pp collisions at lower energies and predictions from statistical hadronisation models and HERWIG 7.2. A mild collision energy dependence of the f0(980) to pion production is observed in pp collisions from SPS to LHC energies. All considered models underpredict the pT-integrated 2f0(980)/(pi+ + pi-) ratio. The prediction from the canonical statistical hadronisation model assuming a zero total strangeness content of f0(980) is consistent with the data within 1.9σ and is the closest to the data. The results provide an essential reference for future measurements of the particle yield and nuclear modification in p–Pb and Pb–Pb collisions, which have been proposed to be instrumental to probe the elusive nature and quark composition of the f0(980) scalar meson
Light (anti)nuclei production in Pb-Pb collisions at sNN=5.02 TeV
The measurement of the production of deuterons, tritons and 3 He and their antiparticles in Pb-Pb collisions
at √s NN = 5.02 TeV is presented in this article. The measurements are carried out at midrapidity (|y| <
0.5) as a function of collision centrality using the ALICE detector. The pT -integrated yields, the coalescence
parameters and the ratios to protons and antiprotons are reported and compared with nucleosynthesis models. The
comparison of these results in different collision systems at different center-of-mass collision energies reveals a
suppression of nucleus production in small systems. In the Statistical Hadronisation Model framework, this can
be explained by a small correlation volume where the baryon number is conserved, as already shown in previous
fluctuation analyses. However, a different size of the correlation volume is required to describe the proton yields
in the same data sets. The coalescence model can describe this suppression by the fact that the wave functions
of the nuclei are large and the fireball size starts to become comparable and even much smaller than the actual
nucleus at low multiplicities
Inclusive and multiplicity dependent production of electrons from heavy-flavour hadron decays in pp and p-Pb collisions
Measurements of the production of electrons from heavy-flavour hadron decays in pp collisions at root s = 13 TeV at midrapidity with the ALICE detector are presented down to a transverse momentum (p(T)) of 0.2 GeV/c and up to p(T) = 35 GeV/c, which is the largest momentum range probed for inclusive electron measurements in ALICE. In p-Pb collisions, the production cross section and the nuclear modification factor of electrons from heavy-flavour hadron decays are measured in the p(T) range 0.5 < p(T) < 26 GeV/c at root s(NN) = 8.16 TeV. The nuclear modification factor is found to be consistent with unity within the statistical and systematic uncertainties. In both collision systems, first measurements of the yields of electrons from heavy-flavour hadron decays in different multiplicity intervals normalised to the multiplicity-integrated yield (self-normalised yield) at midrapidity are reported as a function of the self-normalised charged-particle multiplicity estimated at midrapidity. The self-normalised yields in pp and p-Pb collisions grow faster than linear with the self-normalised multiplicity. A strong p(T) dependence is observed in pp collisions, where the yield of high-p(T) electrons increases faster as a function of multiplicity than the one of low-p(T) electrons. The measurement in p-Pb collisions shows no p(T) dependence within uncertainties. The self-normalised yields in pp and p-Pb collisions are compared with measurements of other heavy-flavour, light-flavour, and strange particles, and with Monte Carlo simulations
Production of pions, kaons, and protons as a function of the relative transverse activity classifier in pp collisions at = 13 TeV
Abstract: The production of π±, K±, and ( p )p is measured in pp collisions at √s = 13 TeV
in different topological regions of the events. Particle transverse momentum (pT) spectra are
measured in the “toward”, “transverse”, and “away” angular regions defined with respect
to the direction of the leading particle in the event. While the toward and away regions
contain the fragmentation products of the near-side and away-side jets, respectively, the
transverse region is dominated by particles from the Underlying Event (UE). The relative
transverse activity classifier, RT = NT/〈NT〉, is used to group events according to their UE
activity, where NT is the measured charged-particle multiplicity per event in the transverse
region and 〈NT〉 is the mean value over all the analysed events. The first measurements
of identified particle pT spectra as a function of RT in the three topological regions are
reported. It is found that the yield of high transverse momentum particles relative to the
RT-integrated measurement decreases with increasing RT in both the toward and the away
regions, indicating that the softer UE dominates particle production as RT increases and
validating that RT can be used to control the magnitude of the UE. Conversely, the spectral
shapes in the transverse region harden significantly with increasing RT. This hardening
follows a mass ordering, being more significant for heavier particles. Finally, it is observed
that the pT-differential particle ratios (p + p )/(π+ + π−) and (K+ + K−)/(π+ + π−) in
the low UE limit (RT → 0) approach expectations from Monte Carlo generators such as
PYTHIA 8 with Monash 2013 tune and EPOS LHC, where the jet-fragmentation models
have been tuned to reproduce e+e− results
Investigation of K+K- interactions via femtoscopy in Pb-Pb collisions at sNN =2.76 TeV at the CERN Large Hadron Collider
Femtoscopic correlations of nonidentical charged kaons (K+K-) are studied in Pb-Pb collisions at a center-of-mass energy per nucleon-nucleon collision sNN=2.76 TeV by ALICE at the CERN Large Hadron Collider. One-dimensional K+K- correlation functions are analyzed in three centrality classes and eight intervals of particle-pair transverse momentum. The Lednický and Luboshitz interaction model used in the K+K- analysis includes the final-state Coulomb interactions between kaons and the final-state interaction through a0(980) and f0(980) resonances. The mass of f0(980) and coupling were extracted from the fit to K+K- correlation functions using the femtoscopic technique. The measured mass and width of the f0(980) resonance are consistent with other published measurements. The height of the φ(1020) meson peak present in the K+K- correlation function rapidly decreases with increasing source radius, qualitatively in agreement with an inverse volume dependence. A phenomenological fit to this trend suggests that the φ(1020) meson yield is dominated by particles produced directly from the hadronization of the system. The small fraction subsequently produced by final-state interactions could not be precisely quantified with data presented in this paper and will be assessed in future work
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