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The development of an electron gun for performing ultrafast electron diffraction experimentsErasmus, Nicolas 12 1900 (has links)
Thesis (MSc (Physics))--Stellenbosch University, 2009. / ENGLISH ABSTRACT: This thesis aims to comprehensively discuss ultrafast electron di raction and
its role in temporally resolving ultrafast dynamics on the molecular level. Theory
on electron pulses and electron pulse propagation will be covered, but the
main focus will be on the method, equipment and experimental setup required
to generate sub-picosecond electron pulses, which are needed to perform time
resolved experiments. The design and construction of an electron gun needed
to produce the electron pulses will be shown in detail, while preliminary pulse
characterization experiments will also be illustrated. An introduction into the
theory of electron diffraction patterns and how to interpret these diffraction
patterns will conclude the thesis. / AFRIKAANSE OPSOMMING: Hierdie tesis het ten doel om ultravinnige elektrondi raksie deeglik te bespreek
asook die rol wat dit speel om ultravinnige tyd-dinamika op 'n molekulêre vlak
op te los. Die teorie van elektonpulse en die voortplanting van elektronpulse sal
gedek word, maar die fokus sal op die metode, gereedskap en eksperimentele opstelling
wees wat benodig is om sub-pikosekonde elektronpulse te genereer. Die
ontwerp en konstruksie van 'n elektrongeweer, wat benodig word om elektronpulse
te produseer, sal in detail bespreek word, terwyl aanvanklike pulskarakterisasie
eksperimente ook illustreer sal word. 'n Inleiding tot die teorie van
elektrondi raksie patrone en hoe om hulle te interpreteer sal die tesis afsluit.
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A convergent beam electron diffraction study of some rare-earth perovskite oxidesJones, Daniel M. January 2008 (has links)
This work describes detailed convergent beam electron diffraction (CBED) studies of GdAlO3 and LaAlO3 perovskites. CBED patterns tilted away from major zone axes have been found to have high sensitivity to the presence of mirror or glide mirror symmetry. Such patterns confirm to high accuracy that the space group of GdAlO3 is orthorhombic, Pnma. Tilted patterns from this well characterised structure also serve as benchmarks against which similar patterns may be compared. In the case of LaAlO3, tilted patterns enable the space group to be confirmed as rhombohedral R3c, previously claimed to be cubic (Fm3c) by CBED. Furthermore, no evidence for the low symmetry (I2/a or F1) phases proposed for LaAlO3 has been observed. The LaAlO3 study also gives a careful assessment of the influence of tilted specimen surfaces on the CBED data. Within the qualitative scope of these experiments, no symmetry degrading effects could be observed. Some preliminary Quantitative CBED (QCBED) data from LaAlO3 is also presented. This shows it will be possible to make a detailed study of the bonding charge density (Δρ) in this material when combined with X-ray diffraction data. Also included is a brief CBED study of LaFeO3, a material that is isostructural with GdAlO3. Although this is restricted to exact zone axis patterns, it is noted that tilted patterns have significant potential to improve the quality of the symmetry determination.
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Introducing organic molecular crystals into ultrafast electron diffractionRohwer, Andrea Berenike 12 1900 (has links)
Thesis (MSc)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: Organic molecular salts have a wide range of physical properties which can be chemically tailored
by minor variations of their substituents. These characteristics include high degrees of anisotropy,
electrical conductivity ranging from superconducting to insulating, and structural changes in the
crystal lattice during first order phase transitions brought about by minimal changes in temperature,
effective pressure, and in some cases even light. Hence, these materials are particularly interesting
for the development of molecular electronics and also as study materials in solid state physics.
The family of copper-dimethyl-dicyanoquinone-diimine (Cu(DMe-DCNQI)2) salts forms part of the
radical anion salt subclass of organic molecular crystals and is of particular interest due to its extraordinarily
high conductivity compared to other quasi one-dimensional organic conductors. Its
metal-to-insulator phase transition is characterised by conductivity jumps across several orders of
magnitude within a few kelvin. Over the past three decades the metallic and insulating phases,
as well as the transition behaviour have been investigated extensively utilising a broad spectrum
of methods amongst others electrical conductivity, electron spin resonance, and re
ectivity measurements,
x-ray photoelectron and infrared spectroscopy, x-ray diffraction, and dilatometry. Fast
light-switching between phases has been observed in partially deuterated forms of Cu(DCNQI)2 on
sub-100-ps time scales. Furthermore, the phase transition is believed to be induced by a deformation
of the crystalline lattice and a charge density wave formation which are detectable in diffraction
images. Therefore we want to investigate this metal-to-insulator phase transition structurally and
temporally via ultrafast electron diffraction. The technique of ultrafast electron diffraction employs
the fundamentals of pump-probe spectroscopy: One of the two femtosecond pulsed laser beams
excites the thin, crystalline sample, while the other - after being converted into a pulsed electron
beam via the photoelectric effect - forms a diffraction image of the sample's lattice structure. The
arrival time of the two pulses at the sample can be varied by a few femtoseconds with respect to
each other enabling the resolution of ultrafast structural dynamics of the crystal's atomic lattice via electron diffraction. During the work presented in this thesis the sample preparation and characterisation
leading to a successful introduction of Cu(DCNQI)2 into our ultrafast electron diffraction
setup is presented. A diffraction pattern of comparable quality to that of a commercially available
transmission electron microscope was recorded of the metallic state of partially deuterated d6
Cu(DCNQI)2. Subsequent analysis of the obtained diffraction data and further studies of the low temperature state { including simulations as well as experiments { have narrowed down the factors
still making the diffraction pattern
evasive. Possible solutions to experimental challenges are proposed to make the documentation of
structural ultrafast dynamics in these organic molecular salts an attainable goal in the future. / AFRIKAANSE OPSOMMING: Organiese molekulêre soute het `n wye verskeidenheid van fisiese eienskappe wat chemies verander
kan word deur geringe variasie in die samestelling van die sout. Hierdie eienskappe sluit in `n hoë
graad van anisotropie, elektriese geleidingsvermoë wat strek van supergeleiding tot elektriese isolasie,
en strukturele veranderinge in die kristalstruktuur tydens eerste orde fase-oorgange wat veroorsaak
word deur geringe veranderinge in temperature, effektiewe druk en in sommige gevalle selfs lig.
Gevolglik is hierdie material besonder interessant vir die ontwikkeling van molekulêre elektronika
en ook as studiemateriaal in vastetoestandfisika. Die familie van koperdimetieldisianokinoondiimien
(Cu(DMe-DCNQI)2) soute vorm `n deel van die radikaal-anioon-sout subklas van organiese
molekulêre kristalle en is van besondere belang as gevolg van hulle buitengewone hoë elektriese
geleidingsvermoë in vergelyking met ander kwasi-eendimensionele organiese geleiers. Die metaal-na-isolator fase-oorgang van hierdie kristal word gekenmerk deur die verandering van die geleidingsvermoë met verskeie ordegroottes binne `n paar kelvin. Gedurende die laaste drie dekades is
die metaal en isolator fases, asook die oorgangsgedrag deeglik ondersoek met behulp van `n wye
verskeidenheid van metodes wat onder andere elektriese geleidingsvermoë, elektron-spin resonans
en reeksiemetings, x-straal fotoelektron en infrarooi spektroskopie, x-straal diffraksie en dilatometrie
insluit. Vinnige skakeling tussen fases is waargeneem in gedeeltelik gedeuteerde vorms van
Cu(DCNQI)2 op `n sub-100-ps tydskaal. Daar word verder geglo dat die fase-oorgang geïnduseer
word deur `n deformasie van die kristalstruktuur en die vorming van `n ladingsdigtheidgolf wat
meetbaar is in elektrondiffraksiebeelde. Om hierdie rede wil ons die metaal-na-isolator fase-oorgang
se struktuur- en tydafhanklikheid ondersoek deur gebruik te maak van ultra-vinnige elektron diffraksie.
Die tegniek van ultra-vinnige elektron diffraksie maak gebruik van die beginsels van pomp-toets
spektroskopie: Een van die twee femtosekonde laserpulse wek die dun kristallyne monster op, terwyl
die ander na omskakeling in `n elektronpuls via die foto-elektriese effek `n diffraksiebeeld van die
monster se kristalstruktuur vorm. Die aankomtyd van die twee pulse by die monster kan met `n
paar femtosekondes ten opsigte van mekaar verander word om die tydresolusie van die ultra-vinnige
strukturele dinamika van die kristal se atoomstruktuur deur elektrondiffraksie moontlik te maak. In
hierdie tesis word die monstervoorbereiding en karakterisering wat gelei het tot suksesvolle eksperimente
op Cu(DCNQI)2 in ons ultra-vinnige elektron diffraksie opstelling behandel. `n Diffraksie
patroon waarvan die kwaliteit vergelykbaar is met die van `n kommersiëel beskikbare transmissie
elektron mikroskoop is gemeet vir die metaalfase van gedeeltelik gedeuteerde d6 Cu(DCNQI)2.
Daaropvolgende analiese van die gemete diffraksiedata en verdere studies van die lae temperatuur
toestand wat simulasies sowel as eksperimente insluit het `n klein aantal faktore uitgewys wat
steeds die deteksie van die isolatorfase se ladingsdigtheidgolf se kenmerkende diffraksiepatroon verhoed.
Moontlike oplossings tot eksperimentele uitdagings word voorgestel om die dokumentering
van strukturele ultra-vinnige dinamika in hierdie organiese molekulêre soute `n haalbare toekomstige
doelwit te maak.
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Structural dynamics of 1T-TiSe2 using femtosecond electron diffractionSuleiman, Aminat Oyiza 12 1900 (has links)
Thesis (MSc)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: Trilayered transition metal dichalcogenides such as our sample (1T-TiSe2) have been studied
for many years as systems with strong electron-electron and electron-phonon correlations.
The main attraction to this family of compound is its potential to exhibit ground state phenomena
known as charge density waves whose detailed physical origin has been controversially
determined.
In this study, we have used an ultrafast femtosecond laser based on a pump-probe technique,
namely ultrafast electron diffraction, to investigate these exotic features associated with the
crystal. A pump laser pulse photo-excites the crystal from its ground state and the probe
pulse (ultrashort electron pulse) takes the snapshot of the evolution of the lattice generating
an electron diffraction pattern of the crystal. Hence the dynamical structural behaviour can
be observed in time with a subpicosecond temporal resolution. As a hexagonal close-packed
structure, its signature is expected to be seen in the diffraction pattern in both a steady-state
and electron time-resolved femtosecond electron diffraction.
In addition, simulations of electron diffractions pattern for room and low temperature structural
data via a software called Simulation and Analysis of Electron diffraction (SAED) have
been carried out. Clear signatures of charge density waves were seen at low temperature. / AFRIKAANSE OPSOMMING: Drie-laag oorgangsmetaal dikhalkogeniedes soos ons voorbeeld (1T-TiSe2), word reeds vir
baie jare bestudeer as sisteme met sterk elektron-elektron en elektron-fonon korrelasies. Die
hoof aantrekkingskrag van hierdie sisteme is die verskynsel van ladingdigtheidsgolwe in
die grondtoestand. Die fisiese oorsprong van hierdie ladingdigtheidsgolwe was bepaal te
midde van verskeie teenstrydighede.
In hierdie studie, maak ons gebruik van die ultravinnige femtosekonde laser gebaseerde
aktiveer-interogeer tegniek, genaamd ultravinnige elektron diffraksie (UED) om unicke eienskappe
wat met die kristal geassosieer is te bestudeer. In UED wek ’n ultravinnige laserpuls
(aktivering) die kristal op vanaf die grondtoestand waartydens n ultravinnige elektronpuls
(interogering) ’n foto neem van die evolusie van die elektron diffraksiepatroon wat deur
die kristalrooster gegenereer word. Hierdie wisselwerking van die interogerings elektronpuls
en die sisteem kan gevolglik teen verskeie vasgetelde tye toegelaat word. Dus kan die
dinamiese strukturele gedrag waargeneem word met ’n tydresolusie in die orde van die
elektronpuls (sub-pikosekondes). Siende dat die kristal ’n diggepakte-heksagonale struktuur
vorm, behoort die kenmerkende diffraksiepatroon daarvan waarneembaar te wees in
beide die bestendige diffraksie en femtosekonde elektron diffraksie tegnieke. In hierdie konteks
was duidelike tekens van ladingdigtheidsgolwe waargeneem.
Benewens was daar ook simulasies uitgevoer om die elektron diffraksiepatrone asook die
strukturele data by kamer en lae temperature vas te pen. Die sagteware wat hiervoor gebruik
word is genaamd Simulasie en Ontleding van Elektronendiffraksie (SAED) - Simulation and
Analysis of Electron Diffraction (SAED).
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Gas-phase electron-diffraction investigations of, I. WF���, ReF���, OsF���, IrF���, PtF���, O������PtF������, II. 3-aminoacrolein, III. 3-chloro-1-propanol, IV. 2,2',5,5'-tetramethyl-1,1'-distibacerroceneRichardson, Alan D. 10 December 1996 (has links)
Graduation date: 1997
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RF compression of electron bunches applied to ultrafast electron diffractionChatelain, Robert P., 1982- January 2008 (has links)
The dynamics of atomic scale structures during structural change can be studied by Ultrafast Electron Diffraction (UED). The time resolution needed to reveal the fastest dynamics is 100 fs. Sub-angstrom structural resolution becomes possible with 1-1000 pC of charge necessary for diffraction pattern analysis during subtle structural changes. This combination of requirements cannot currently be realized due to the space-charge temporal broadening inherent to bunches of electrons of high fluence and short temporal duration. Simulations show that the incorporation of a specially designed Radio-Frequncy (RF) cavity into the UED apparatus removes this technical limitation. The RF cavity reverses the near linear position-momentum distribution of the temporally broadened electron bunch, causing the bunch to recompress itself as it propagates. It is found that our proposed method allows for sub-100 fs bunches with maximum charge of 0.6 pC, almost 3 orders of magnitude improvement over today's state of the art.
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RF compression of electron bunches applied to ultrafast electron diffractionChatelain, Robert P., 1982- January 2008 (has links)
No description available.
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Phase transitions in transition metal dichalcogenides studied by femtosecond electron diffractionHaupt, Kerstin Anna 12 1900 (has links)
Thesis (PhD)--Stellenbosch University, 2013. / ENGLISH ABSTRACT: Low-dimensional materials are known to undergo phase transitions to differently or-
dered states, when cooled to lower temperatures. These phases often show a periodic
modulation of the charge density (called a charge density wave – CDW) coupled with
a periodic perturbation of the crystal lattice (called a periodic lattice distortion – PLD).
Although many experiments have been performed and much has been learnt about
CDW phases in low-dimensional materials, the reasons for their existence are still
not fully understood yet. Many processes, involving either strong electron–electron or
electron–lattice coupling, have been observed which all might play a role in explaining
the formation of different phases under different conditions.
With the availability of femtosecond lasers it has become possible to study materials
under highly nonequilibrium conditions. By suddenly introducing a known amount
of energy into the system, the equilibrium state is disturbed and the subsequent relax-
ation processes are then observed on timescales of structural and electronic responses.
These experiments can deliver valuable information about the complex interactions
between the different constituents of condensed matter, which would be inaccessible
under equilibrium conditions.
We use time resolved electron diffraction to investigate the behaviour of a CDW
system perturbed by a short laser pulse. From the observed changes in the diffraction
patterns we can directly deduce changes in the lattice structure of our sample.
A femtosecond electron diffraction setup was developed at the Laser Research In-
stitute in Stellenbosch, South Africa. Short laser pulses produce photo electrons which
are accelerated to an energy of 30 keV. Despite space charge broadening effects, elec-
tron pulses shorter than 500 fs at sample position can be achieved. Technical details
of this system and its characterisation as well as sample preparation techniques and
analysis methods are described in detail in this work.
Measurements on two members of the quasi-two-dimensional transition metal di-
chalcogenides, namely 4Hb-TaSe2 and 1T-TaS2, are shown and discussed. Both show
fast (subpicosecond) changes due to the suppression of the PLD and a rapid heating
of the lattice. When the induced temperature rise heats the sample above a phase tran-
sition temperature, a complete transformation into the new phase was observed. For
4Hb-TaSe2 we found that the recovery to the original state is significantly slower if
the PLD was completely suppressed compared to only disturbing it. On 1T-TaS2 we
could not only study the suppression of the original phase but also the formation of
the higher energetic CDW phase. Long (100 ps) time constants were found for the tran-
sition between the two phases. These suggest the presence of an energy barrier which
has to be overcome in order to change the CDW phase. Pinning of the CDW by de-
fects in the crystal structure result in such an energy barrier and consequently lead to
a phase of domain growth which is considerably slower than pure electron or lattice
dynamics. / AFRIKAANSE OPSOMMING: Dit is bekend dat lae-dimensionele materie fase oorgange ondergaan na anders ge-
ori¨enteerde toestande wanneer afgekoel word tot laer temperature. Hierdie fases toon
dikwels ’n periodiese modulasie van die elektron digtheid (genoem ’n “charge density
wave” – CDW), tesame met ’n periodiese effek op die kristalrooster (genoem ’n “peri-
odic lattice distortion” – PLD). Alhoewel baie eksperimente al uitgevoer is en al baie
geleer is oor hierdie CDW fase, is die redes vir hul bestaan nog steeds nie ten volle
verstaan nie. Baie prosesse, wat of sterk elektron–elektron of elektron–fonon interaksie
toon, is al waargeneem en kan ’n rol speel in die verduideliking van die vorming van
die verskillende fases onder verskillende omstandighede.
Met die beskikbaarheid van femtosekonde lasers is dit nou moontlik om materie
onder hoogs nie-ewewig voorwaardes te bestudeer. Deur skielik ’n bekende hoeveel-
heid energie in die stelsel in te voer, word die ewewigstaat versteur en word die daar-
opvolgende ontspanning prosesse waargeneem op die tydskaal van atomies struktu-
rele en elektroniese bewiging. Hierdie eksperimente kan waardevolle inligting lewer
oor die komplekse interaksies tussen die verskillende atomiese komponente van ge-
kondenseerde materie, wat ontoeganklik sou wees onder ewewig voorwaardes.
Ons gebruik elektrondiffraksie met tyd resolusie van onder ’n pikosekonde om die
gedrag van ’n CDW stelsel te ondersoek nadat dit versteur is deur ’n kort laser puls.
Van die waargenome veranderinge in die diffraksie patrone kan ons direk aflei watse
veranderinge die kristalstruktuur van ons monster ondergaan.
’n Femtosekonde elektronendiffraksie opstelling is ontwikkel by die Lasernavors-
ingsinstituut in Stellenbosch, Suid-Afrika. Kort laser pulse produseer foto-elektrone
wat dan na ’n energie van 30 keV versnel word. Ten spyte van Coulomb afstoting ef-
fekte, kan elektron pulse korter as 500 fs by die monster posisie bereik word. Tegniese
besonderhede van hierdie opstelling, tegnieke van die voorbereiding van monsters
asook analise metodes word volledig in hierdie tesis beskryf.
Metings op twee voorbeelde van kwasi-tweedimensionele semi-metale, naamlik
4Hb-TaSe2 en 1T-TaS2, word gewys en bespreek. Beide wys ’n vinnige (subpikosekon-
de) verandering as gevolg van die versteuring van die PLD en ’n vinnige verhitting van
die kristalrooster. Wanneer die ge¨ınduseerde temperatuur bo die fase oorgang tempe-
ratuur styg, is ’n volledige transformasie na die nuwe fase waargeneem. Vir 4Hb-TaSe2
het ons gevind dat die herstelling na die oorspronklike toestand aansienlik stadiger is
as die PLD heeltemal viernietig is in vergelyking met as die PLD net versteur is. Met
1T-TaS2 kon ons nie net alleenlik die vernietiging van die oorspronklike fase sien nie,
maar ook die vorming van ’n ho¨er energie CDW fase. Lang (100 ps) tydkonstante is
gevind vir die oorgang tussen die twee fases. Hierdie dui op die teenwoordigheid van
’n energie-versperring wat eers oorkom moet word om die CDW fase voledig te ver-
ander. Vaspenning van die CDW deur defekte in die kristalstruktuur veroorsaak so’n
energie versperring en gevolglik lei dit tot ’n fase van groeiende CDW gebiede wat
heelwat stadiger as pure elektron of kritalrooster dinamika is.
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Ultrafast electron diffraction : source development, diffractometer design and pulse characterisationKassier, Gunther Horst 12 1900 (has links)
Thesis (PhD (Physics))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: Ultrafast Electron Diffraction (UED) is a rapidly maturing field which allows investigation of the
evolution of atomic arrangement in solids on timescales comparable to the vibrational period of
their constituent atoms (~10-13 s). The technique is an amalgamation of conventional high energy
electron diffraction methods and pump-probe spectroscopy with femtosecond (1 fs = 10-15 s) laser
pulses. Ultrafast pulsed electron sources generally suffer from limitations on the attainable electron
number per pulse (brightness) due to Coulomb repulsion among the electrons. In this dissertation,
the design and construction of a compact UED source capable of delivering sub-300 fs electron
pulses suitable for diffraction experiments and containing about 5000 electrons per shot is
described. The setup has been characterised by measurement of the transverse beam size and
angular spread, and through recording and analyzing an electron diffraction pattern from a titanium
foil. Measurement of the temporal duration of fs electron pulses is not trivial, and a specialised
compact streak camera operating in accumulation mode has been developed as part of this study. A
sub-200 fs temporal resolution has been achieved, and the dependence of temporal duration on
electron number per pulse was investigated for the current UED source. The observed trends
correlate well with detailed electron bunch simulations. In order to investigate ultrafast processes on
samples that cannot be probed repeatedly, it becomes necessary to significantly increase the
brightness of current state of the art compact sources such as the one constructed in the present
study. UED sources employing electron pulse compression techniques offer this possibility.
Traditional pulse compression schemes based on RF cavities, while simple in principle, pose
significant technical challenges in their realisation. The current thesis describes two novel UED
pulse compression methods developed by the author: achromatic reflectron compression and pulsed
cavity compression. Both concepts are expected to be easier to realise than conventional RF
compression. Detailed simulations predict that such sources can attain a brightness improvement of
more than one order of magnitude over compact sources that do not employ compression
techniques. In addition, such sources show much promise for the attainment of pulse durations in
the sub-100 fs range. / AFRIKAANSE OPSOMMING: Ultra vinnige elektron diffraksie is ‘n meettegniek wat tans in die proses is om vinnige ontwikkeling
te ondergaan. Die tegniek het ten doel om strukturele omsettingsprosesse op ‘n lengteskaal van
atoombindings en ‘n tydskaal van die vibrasie periode van atome in materie, ongeveer 10-13 s, te
ondersoek. Dit word bewerkstellig deur die spasieresolusievermoë van gewone hoë energie elektron
diffraksie met die tydresolusievermoë van femtosekonde (1 fs = 10-15 s) laserspektroskopie te
kombineer. Die aantal elektrone per puls (intensiteit) van ultravinnige gepulsde elektronbronne
word beperk deur die Coulomb afstootingskragte tussen die elektrone. Hierdie dissertasie beskryf
die ontwerp en konstruksie van ‘n kompakte ultravinnige elektron bron. Die elektronpulse wat
geproduseer word bevat tot 5000 elektrone per puls met ‘n tyd durasie van minder as 300 fs, en is
geskik vir diffraksie eksperimente. Die aparaat is gekarakteriseer deur die volgende metings:
elektronpulsdiameter, straaldivergensie, en ‘n titaan foelie se statiese diffraksie patroon. Dit is nie
triviaal om die durasie van femtosekonde elektronpulse te meet nie, en n spesiale kompakte
akkumulerende “streak camera” is vir die doeleindes van hierdie projek onwikkel. Die tydresolusie
van hierdie “streak camera” is beter as 200 fs, en die afhanklikheid van die pulsdurasie wat deur die
ultravinnige elektron bron geproduseer word as n funksie van die elektrongetal per puls is met
behulp van hierdie toestel bepaal. Die resultate klop redelik goed met gedetaileerde simulasies van
die elektron puls dinamika. Die karakterisasie van monsters wat nie herhaaldelik gemeet kan word
nie vereis verkieslik ‘n nog hoër pulsintensiteit as wat met huidige bronne bereik kan word. ‘N
verdere doelstelling is dus om ultravinnige elektron bronne te ontwikkel wat pulse met meer
elektrone per puls kan genereer. Dit kan bewerkstellig word deur bronne wat van elektron puls
kompressie tegnieke gebruik maak. Die tradisionele manier waarop dít gedoen word is deur middel
van n kontinu gedrewe radio frekwensie holte. Hierdie metode gaan egter gepaard met aanmerklik
hoë tegniese uitdagings. Om hierdie rede het die outeur twee alternatiewe puls kompressie konsepte
ontwikkel: akromatiese reflektron kompressie and gepulsde holte kompressie. Albei konsepte sal
waarskeinlik makliker wees om te realiseer as die tradisionele radio frekwensie kompressie, en is
deur middel van gedetaileerde simulasies geverifiseer. Hierdie simulasies voorspel dat die
intensiteit van genoemde bronne met ten minste n grooteorde meer kan wees as wat tans met
kompakte ultravinnige elektron bronne moontlik is. Verder blyk dit dat sulke bronne n pulsdurasie
van minder as 100 fs kan bereik.
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Spatially resolved and operando characterization of cathode degradation in Li-ion batteriesHestenes, Julia Carmen January 2024 (has links)
The global energy transition, involving the widespread adoption of electric vehicles and grid-scale energy storage, demands energy storage devices made up of abundant, inexpensive minerals. For this to be achieved, the large Co content in conventional Li-ion battery cathodes (e.g., LiCoO₂) must be replaced while also maintaining or improving the energy density of the battery. Alternative low-Co and Co-free materials (e.g., layered LiNixMnyCozO₂, spinel LiNi₀.₅Mn₁.₅O₄, and olivine LiFePO₄) are promising alternatives due to their theoretically higher energy densities or improved safety properties from the industry standards. However, in practice, these materials exhibit both bulk and interfacial instabilities that limit their practical energy density and cycle lifetime. It is well known that reactions between the delithiated (charged) cathode surface with the electrolyte generates electrolyte decomposition species that form an interphase layer called the cathode electrolyte interphase (CEI), where such reactions are concomitant with a crystallographic reconstruction of the surface of the bulk material. The CEI is air sensitive, disordered, nanometers thick and evolves as a function of state of charge and cycle number, making it difficult to fully understand its composition and effect on device performance.
The dynamic nature of the CEI necessitates development of chemical characterization tools that can analyze surface reactivity during battery operation. Commercial cathode films are also composites including not just the electrochemically active material but also conductive carbon additive and polymer binder, meaning we need spatially resolved tools to study CEI composition across the film to isolate reactivity by film component. In this thesis, we have developed and applied spatially resolved and operando characterization tools to study the CEI of low-Co and Co-free cathode materials and use these data to pinpoint the degradation reactions at play during battery operation. In the first chapter, we introduce the three most prevalent types of cathode materials (layered, spinels, and olivines) used in Li-ion batteries. We then highlight recent progress in the analytical characterization tools that have been developed to elucidate CEI composition, spatial arrangement, and formation pathways during battery operation while discussing the difference in surface reactivity between each cathode active material as revealed by these techniques. Major findings from my own thesis work, detailed in following chapters, are discussed in parallel within this broader context. Finally, equipped with a deeper understanding of the CEI and the processes that lead to its formation, we discuss what remains to be discovered and enabled by optimizing these complex interfaces.
The second chapter investigates the composition of the CEI formed by the Li-rich layered cathode material, Li₂RuO₃, to better understand performance decline in this class of materials. To bridge this gap in understanding, we use solid-state NMR (SSNMR) and surface-sensitive dynamic nuclear polarization (DNP) NMR to achieve high resolution compositional assignment of the CEI. We show that the CEI that forms on Li₂RuO₃, when cycled in carbonate-containing electrolytes, is similar to the solid electrolyte interphase (SEI) that has been observed on anode materials, containing components such as polyethylene oxide (PEO) structures, Li acetate, carbonates, and LiF. The CEI composition deposited on the cathode surface on charge is chemically distinct from that observed upon discharge, supporting the notion of crosstalk between the SEI and the CEI, with Li+-coordinating species leaving the CEI during delithiation. We use electrochemical impedance spectroscopy (EIS) to assess the impedance of the CEI on Li₂RuO₃ as a function of state of charge in connection with the migration of CEI species as identified with NMR. Migration of the outer CEI combined with the accumulation of poor ionic conducting components on the static inner CEI may contribute to the loss of performance over time in Li-excess cathode materials. This work demonstrates the utility of SSNMR for studying electrolyte decomposition at the cathode-electrolyte interface which is then applied in the following chapter to more commercially relevant materials.
In the third chapter, we study the CEI and surface reactivity of the Ni-rich layered material LiNi₀.₈Mn₀.₁Co₀.₁O₂ (NMC811). The high specific capacities of Ni-rich transition-metal oxides have garnered immense interest for improving the energy density of Li-ion batteries. However, Ni-rich cathodes suffer from interfacial instabilities that lead to formation of electrochemically inactive phases at the cathode particle surface as well as the formation of a CEI layer on the composite surface during electrochemical cycling. We use a combination of ex situ SSNMR spectroscopy and X-ray photoemission electron microscopy (XPEEM) to provide chemical and spatial information, on the nanometer length scale, on the CEI deposited on NMC811 composite cathode films. XPEEM elemental maps offer insight into the lateral arrangement of the electrolyte decomposition products that comprise the CEI and paramagnetic interactions (assessed with electron paramagnetic resonance (EPR) and relaxation measurements) in 13C SSNMR provide information on the radial arrangement of the CEI from the NMC811 particles outward. Using this approach, we find that LiF, Li₂CO₃, and carboxy-containing structures are directly appended to NMC811 active particles, whereas soluble species detected during in situ 1H and 19F solution NMR experiments (e.g., alkyl carbonates, HF, and vinyl compounds) are randomly deposited on the composite surface. We show that the combined approach of ex situ SSNMR and XPEEM, in conjunction with in situ solution NMR, allows spatially resolved, molecular-level characterization of paramagnetic surfaces and new insights into electrolyte oxidation mechanisms in porous electrode films. The in situ solution NMR cell developed here is one of the first of its kind developed specifically for studying electrolyte decomposition products during or directly after battery operation, which is further developed in the next chapter.
The fourth chapter focuses on studying the surface reactivity of the high-voltage LiNi₀.₅Mn₁.₅O₄ (LNMO) spinel cathode material. Unfortunately, LNMO-containing batteries suffer from poor cycling performance because of the intrinsically coupled processes of electrolyte oxidation and transition metal dissolution that occurs at high voltage. In this work, we use operando EPR and NMR spectroscopies to study these high voltage reactions, applying the in situ cell design from the previous chapter to operando conditions (characterization during battery charging). We demonstrate that transition metal dissolution in LNMO is tightly coupled to HF formation (and thus, electrolyte oxidation reactions as detected with operando and in situ solution NMR), indicative of an acid-driven disproportionation reaction that occurs during delithiation (battery charging). Leveraging the temporal resolution (s-min) of magnetic resonance, we find that the LNMO particles accelerate the rate of LiPF6 decomposition and subsequent Mn²⁺ dissolution, possibly due to the acidic nature of terminal Mn-OH groups and protic species generated upon oxidizing the solvents. X-ray photoemission electron microscopy (XPEEM) provides surface-sensitive and localized X-ray absorption spectroscopy (XAS) measurements, in addition to X-ray photoelectron spectroscopy (XPS), that indicate disproportionation is enabled by surface reconstruction upon charging, which leads to surface Mn³⁺ sites on the LNMO particle surface that can disproportionate into Mn²⁺(dissolved) and Mn⁴⁺(s). During discharge of the battery, we observe high quantities of metal fluorides (in particular, MnF₂) in the cathode electrolyte interphase (CEI) on LNMO as well as the conductive carbon additives in the composite. Electronic conductivity measurements indicate that the MnF₂ decreases film conductivity by threefold compared to LiF, suggesting that this CEI component may impede both the ionic and electronic properties of the cathode. Ultimately, to prevent transition metal dissolution and the associated side reactions in spinel-type cathodes (particularly those that operate at high voltages like LNMO), the use of electrolytes that offer improved anodic stability and prevent acid byproducts will likely be necessary.
In the fifth chapter, we conduct an in situ X-ray spectroscopy, electron microscopy, and electron diffraction experiment to study the oxidation of the surface of Li metal, which is of critical importance for next generation Li metal batteries. Elemental Li is one of the most promising anode materials for high energy density Li batteries if it can replace graphite because it increases the specific capacity by an order of magnitude. However, Li metal is extremely reactive and is easily oxidized by air and moisture, even under inert conditions (e.g., in argon-filled gloveboxes, ultrahigh vacuum chambers). The industrial production of Li metal anodes, their surface evolution upon contact with the electrolyte, and electrodeposition behavior upon battery cycling all rely on the initial oxidative processes that take place prior to cell assembly. To better understand Li metal oxidation, we deposit pure Li on a Cu substrate and dose the Li deposit with various amounts of oxygen gas. During this experiment, we monitor the surface composition in situ using low-energy electron microscopy (LEEM), low-energy electron diffraction (LEED), and XPS measurements. We show that by evaporating Li onto Cu substrates, we can bypass long sputtering times needed to study commercial Li foils that usually exhibit alkali metal impurities and thick contamination layers from their external environment. Combined insights from LEED, LEEM and DFT calculations indicate that upon oxygen dosing of this ultrapure Li film, oxygen adsorbs to Li, forming a disordered layer, followed by (111) oriented polycrystalline Li₂O growth. DFT was particularly instrumental in elucidating the precise work function of the surface for the intermediate oxide phases (timescale of seconds) to correlate with trends observed via in situ LEEM imaging experiments.
To conclude, we reflect on the overarching insight on cathode degradation that we have learned from these studies and discuss remaining knowledge gaps in the field. We highlight promising future avenues to study for stabilizing the cathode-electrolyte interface of these materials, such as adapting the characterization methods developed here for more high throughput study of next generation electrolyte formulations.
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