Theses and Dissertations (Physics)https://hdl.handle.net/10500/30572024-03-29T09:34:41Z2024-03-29T09:34:41ZRear-earth ions doped mesoporous TiO2 nanostructures and the caffeine booster in MAPbl3 perovskite materialTsotetsi, Dieketseng Glarahttps://hdl.handle.net/10500/305452023-10-02T10:19:05Z2023-05-01T00:00:00ZRear-earth ions doped mesoporous TiO2 nanostructures and the caffeine booster in MAPbl3 perovskite material
Tsotetsi, Dieketseng Glara
Titanium dioxide (TiO2) nanostructures were synthesized by sol-gel method and co polymers polyethylene glycol (PEG), polyvinylpyrrolidone (PVP) and pluronic F127
were used as pore forming agents. The samples were then calcined at 550 °C for 4
hours in a pre-heated muffle furnace. The synthesized TiO2 showed porous
morphology with spherical nanoparticles. Nitrogen adsorption-desorption isotherm
studies revealed that the samples were mesoporous in nature. BET further indicated
that the specific surface areas of TiO2@F127, TiO2@PVP and TiO2@PEG were ~
69.82, 37.80 and 57.08 m2
/g, respectively. The pore size was found to be ~13.01,
10.10 and 8.53 nm For TiO2@F127, TiO2@PVP, and TiO2@PEG.
Mesoporous TiO2 (mp-TiO2) was then doped with rare-earth ions (Ho3+, Er3+ and Tm3+)
and incorporated into methylammonium lead iodide (MAPbI3). Thereafter, photon
absorption and electrical conductivity were evaluated. The photon absorbance
showed a slight redshift in the 400–550 nm range with tiny absorption bands appearing
at ~645–653 nm for TiO2:Ho3+:MAPbI3, ~684–693 nm for TiO2:Er3+:MAPbI3 and ~684–
793 nm for TiO2:Tm3+:MAPbI3. Current – Voltage (I-V) characteristics showed an
improved electrical conductivity and low sheet resistance from TiO2:Er3+:MAPbI3 and
TiO2:Tm3+:MAPbI3 samples, implying that these samples are suitable as active photon
absorption layers in the application of perovskite solar cells.
The thermal stability and crystallinity of MAPbI3 were investigated by introducing
different concentrations of 1,3,7-trimethylxanthine (caffeine) (2%, 4%, 6% and 8%)
into the system. TGA analysis revealed that MAPbI3 was stable at lower temperatures
(0-91 oC) then upon adding caffeine into MAPbI3 the thermal stability was improved in higher temperature range (196-242 oC). XRD analysis showed that the samples
exhibited intense diffraction peaks at ~2θ = 19.66o and 39.81
o
, respectively assigned to
the (112) and (224) planes of tetragonal MAPbI3 structure which indicates that the desired
MAPbI3 film was formed. MAPbI3@Caffeine (8%) and MAPbI3@Caffeine (6%)
exhibited high PL quenching which indicates their high charge carrier extraction
efficacy of which can be attributed to the higher crystallinity and fewer defect states of
the film compared to MAPbI3@Caffeine (2%) and MAPbI3@Caffeine (4%) samples.
2023-05-01T00:00:00ZWhy has physics failed to completely explain the universe: a philosophical approach to a final theoryThompson, John Frederickhttps://hdl.handle.net/10500/305132023-09-19T07:21:20Z2023-07-25T00:00:00ZWhy has physics failed to completely explain the universe: a philosophical approach to a final theory
Thompson, John Frederick
This thesis investigates why there is still no ‘final’ physical theory of the universe despite the enormous resources involved. Current physical and philosophical methodologies are examined leading to a new strategy. The history of knowledge from mythology to the present day is traced to establish the general nature and psychology of human sapience and group dynamics. This reflects strongly on human common sense, education and entrenchment caused by peer pressure of mathematical and physical ideas. Arguments consider physical and philosophical standpoints of empirical versus rational and mathematical versus non-mathematical deduction. The former is decided by introducing a special foundational philosophy; the latter by arguing the universe has no need of mathematics in any form to exist. Criticizing current ideas is useless unless they can be replaced by a better theory. As a paradigm must be better than that which it replaces, it must stand up to testing against observation. Using the concept of time with a clear definition, possibly the first such definition, shows how a universe must causally develop. The human concept of space, together with a reason for its 3-dimensionality, automatically arises to answer, ‘if a universe is to be created, into what is it placed?’ The conundrum of existence is also explained. The reason for contemporary physics’ failure is its reliance on observation, which is governed by unreliable human perception, in particular its lack of definitions for time, length, mass, electric charge, energy, work, wave function from which its ‘laws’ are deduced. Doubt is raised on physics’ main theories, quantum mechanics and relativistic field theories which deny a fundamental cause for the universe. Mathematics suffers from overconfidence in its efficacy and accuracy. There also exist processes that the foundational theory shows are completely hidden from current physical and astrophysical experiments. The conclusion to be drawn is that mathematical physics cannot produce a final theory whereas non-mathematical reasoning can. Foundational philosophy then becomes the means of attaining a final theory with physics the method of determining philosophy’s accuracy. As no such pointer has been considered in the literature it has to be a testable primary assumption. Lines for further research to produce a complete theory of the universe are given.; Hierdie tesis ondersoek hoekom daar steeds geen 'finale' fisiese teorie van die heelal is ten spyte van
die enorme hulpbronne wat betrokke is. Huidige fisiese en filosofiese metodologieë word ondersoek
wat lei tot 'n nuwe strategie. Die geskiedenis van kennis vanaf mitologie tot vandag word nagespeur
om die algemene aard en sielkunde van menslike weelde en groepdinamika vas te stel. Dit reflekteer
sterk op menslike gesonde verstand, opvoeding en verskansing wat veroorsaak word deur groepsdruk
van wiskundige en fisiese idees. Argumente oorweeg fisiese en filosofiese standpunte van empiriese
teenoor rasionele en wiskundige versus nie-wiskundige afleiding. Eersgenoemde word besluit deur 'n
spesiale grondliggende filosofie in te voer; laasgenoemde deur te redeneer die heelal het geen
behoefte aan wiskunde in enige vorm om te bestaan nie. Om huidige idees te kritiseer is nutteloos,
tensy dit deur 'n beter teorie vervang kan word. Aangesien 'n paradigma beter moet wees as dit wat dit
vervang, moet dit standhou tot toetsing teen waarneming. Die gebruik van die konsep van tyd met 'n
duidelike definisie, moontlik die eerste so 'n definisie, wys hoe 'n heelal oorsaaklik moet ontwikkel.
Die menslike konsep van ruimte, tesame met 'n rede vir sy 3-dimensionaliteit, ontstaan outomaties om
te antwoord 'as 'n heelal geskep moet word, waarin word dit geplaas?' Die raaisel van bestaan word
ook verduidelik. Die rede vir kontemporêre fisika se mislukking is sy vertroue op waarneming wat
beheer word deur onbetroubare menslike persepsie, veral sy gebrek aan definisies vir tyd, lengte,
massa, elektriese lading, energie, werk, golffunksie waaruit sy 'wette' afgelei word. Twyfel word
geopper oor fisika se hoofteorieë, kwantummeganika en relativistiese veldteorieë wat 'n fundamentele
oorsaak vir die heelal ontken. Wiskunde ly aan oorvertroue in die doeltreffendheid en akkuraatheid
daarvan. Daar bestaan ook prosesse wat die grondliggende teorie toon heeltemal verborge is van
huidige fisiese en astrofisiese eksperimente. Die gevolgtrekking wat gemaak moet word, is dat
wiskundige fisika nie 'n finale teorie kan produseer nie, terwyl nie-wiskundige redenering wel kan.
Fundamentele filosofie word dan die middel om 'n finale teorie te bereik met fisika die metode om
filosofie se akkuraatheid te bepaal. Lyne vir verdere navorsing om 'n volledige teorie van die heelal te
produseer, word gegee; Le thisisi iphanda ukuba kutheni kungekabikho ithiyori yendalo iphela nangona zizininzi izixhobo
ezisetyenziswayo. Iindlela zangoku zokwemvelo nezefilosofi ziyavavanywa ukuze kufikelelwe
kwisicwangciso-qhinga esitsha.
Imbali yolwazi ukusuka kwizifundo ngeentsomi ukuza kuthi ga ngoku iyalandelwa ukuze kusekwe
imeko yendalo ngokubanzi nemeko yobulumko bengqondo bomntu kunye nenkqubo yokuziphatha
kunye neenkqubo zengqondo ezenzeka ngaphakathi kweqela. Oku kubonakalisa
ngamandla/ngokukuko kwindlela yokucinga komntu, imfundo kunye nokuzinza okubangelwa
luxinzelelo loontanga kwiingcamango zemathematika kunye nezendalo.
Iingxoxo zithathela ingqalelo iimbono zendalo nezefilosofi zamava achasene nengqiqo kunye
nemethamatika ngokuchasene nokunciphisa ekungeyoyamethametika. Eyokuqala igqiba ngokwazisa
ifilosofi eyodwa esisiseko; ze engeyokugqibela igqibe ngokuxoxa ukuba indalo iphela ayifuni
imathematika nangaluphi na uhlobo ukuze ibekho.Ukugxeka iingcamango zangoku akuncedi nto
ngaphandle kokuba zinokuthatyathelw’ indawo yithiyori engcono kunazo.Njengoko iphatheni/
imodeli(paradigm) kufuneka ibengcono kunaleyo inqwenela ukuba ithathe indawo yayo, kufuneka
ikumele ukuvavanywa ngokwemigqaliselo ngokokuqwalasela. Inkcazo ecacileyo yengcamango
yexesha, ekunokwenzeka yingcaciso yokuqala enjalo, ibonisa indlela indalo ekhula ngayo
ngokuzenzekelayo. Ingcamango yomntu yesithuba/ indawo ejikeleze ihlabathi, kunye nesizathu
sobukhulu bayo ngokobuthathu, iwuphendula ngokuzenzekelayo umbuzo othi ‘ukuba indalo iphela
iza kuyilwa, ibekwe kwintoni?’ Uqashi qashi wobukho ucacisiwe. Isizathu sokungaphumeleli
kwefiziksi yanamhlanje ukuchaza indalo kukuxhomekeka kwayo ekuqwalaseleni, okulawulwa
yimbono yabantu engathembekanga kwaye, ngokukodwa, ukusilela kwayo kwiinkcazo zexesha,
ubude, ubunzima, ubungakanani bombane, amandla, umsebenzi, kunye nomsebenzi wamaza apho
'imithetho' isekelwe/ ithathwa khona. Amathandabuzo abekwa kwiithiyori eziphambili zefiziksi,
ubungakanani bobuxhakaxhaka obufunekayo kunye neethiyori eziphikisa ukuba kukho unobangela
osisiseko wendalo iphela. IMathematika inengxaki yokuzithemba ngokugqithisileyo kumandla ayo
okusebenza nangokuchaneka kwayo. Ithiyori yesiseko ibonisa ukuba iinkqubo ezithile zangoku
zifihlwe ngokupheleleyo kwimifuniselo ngokobunzululwazi bezemvelo nokwakheka
kweenkwenkwezi.
Isigqibo sesokuba ifiziksi (ubunzululwazi ngezinto zemvelo ezingaphiliyo) yemathematika
ayinakwakha /ayinakuyila ithiyori egqibeleleyyo ngelixa ukuqiqa okungengokwemathematika
v
kunokukwenza oko. Ifilosofi esisiseko ngoko iba yindlela yokuseka ithiyori yokugqibela ze ifiziksi
ibe yindlela yokumisela ukuchaneka kwefilosofi. Ekugqibeleni, kucetyiswa imikrwelo/izihloko
zophando zokuqhubela phambili uphando ngenjongo yokuvelisa ingcamango/ ithiyori epheleleyo
ngendalo iphela.
2023-07-25T00:00:00ZGround states of triton and alpha nucleiChinyanga, Kumbirai Theotimushttps://hdl.handle.net/10500/302662023-07-13T07:25:51Z2022-11-01T00:00:00ZGround states of triton and alpha nuclei
Chinyanga, Kumbirai Theotimus
The few-body integrodifferential equations are approximations to the few-body Schr¨odinger
equation in hyperspherical coordinates. The few-body integrodifferential equations for
identical particles reduce to a single integrodifferential equation in two variables only.
The structure of the integrodifferential equation does not dependent of the number of
particles in the system. The integrodifferential equation has previously been solved
using the adiabatic approximation method, the projection method and the perturba tion method. In this dissertation we solve the few-body integrodifferential equations
directly using the Lagrange mesh method. We apply the equations to Triton and alpha
nuclei which are three-body and four-body systems, respectively. We treat the nucleons
as interacting through spin-dependent nucleon-nucleon potentials. The few-body inte-grodifferential equations develop into a system coupled integrodifferential equations in
the case of spin-dependent potentials. All our results are compared to those found in
literature.
2022-11-01T00:00:00ZElectrical conduction and resistive switching in polymer and biodegradable nanocompositesDlamini, Zolile Wisemanhttps://hdl.handle.net/10500/300812023-06-08T09:22:22Z2022-10-01T00:00:00ZElectrical conduction and resistive switching in polymer and biodegradable nanocomposites
Dlamini, Zolile Wiseman
Modern memory devices such as static random-access memory (SRAM),
dynamic random-access (DRAM), and Flash memories demonstrated inevitable
limitations, i.e., large cell size (50 − 120 F2) of SRAM, accompanied
by current leakage; high operating voltages of 3 V and up to 6 V
for DRAM and NOR Flash, respectively; DRAM capacity should sustain
enough charges (there is a limit to how small the DRAM capacitor can be)
and Flash need a novel array structure. Additionally, these current memory
devices contribute significantly to the world’s earth pollution. These memories
still use heavy metals such as Pb, which are harmful to humans. There
is a demand for a next-generation random-access memory (RAMs) having
fast read and write operations as the SRAM, high density and cost-benefit
as the DRAM, and nonvolatility as the Flash. Furthermore, new memory
device must be compatible with on-chip computing. Resistive switching
memories (ReRAMs) are an emerging memory technology with prospects
of combined benefit found in all current memories. Furthermore, ReRAMs
can be fabricated using any material, including organic polymers and biological
materials. This gives ReRAM environmentally friendly properties
and compatibility with futuristic electronics, where special mechanical properties
such as transparency and flexibility are important. In this study, we
conducted intense research on electrical conduction and resistive switching
in biodegradable polymers such as chitosan and polyvinylpyrrolidone, and in
the process, we discovered, for the first time, resistive switching in raw cow
milk. First resistive switching and conduction mechanisms in spin-coated devices
consisting ofcadmium telluride/cadmium selenide (CdTe/CdSe) coreshell
quantum dots embedded in a chitosan active layer sandwiched between
(1) aluminium (Al) and silver (Ag) and (2) indium-doped tin oxide (ITO) and Ag electrodes were studied. Here, both devices exhibited bipolar memory
behavior at low (+0.70 V ) voltage, enabling both devices to be operated
at low powers. The devices displayed different switching mechanisms,
i.e., conductive bridge mechanism in the Al-based device and space-chargelimited
driven conduction filament attributed in the ITO device. Additionally,
the Al-based device showed long retention (> 103 s) and a reasonable
large (> 103) ON/OFF ratio. We also observed a sweeping cycle-induced reversal
of the voltage polarity of the VSET and VRESET in the Al-based device,
which is a new observation. Using the same composite but changing the
film deposition method, i.e., now using the drop-casting method. All devices
consisting of 0.96 wt%, 0.48 wt%, 0.32 wt% and 0.24 wt% CdTe/CdSe
QDs to chitosan showed ‘O-type’ memory behavior with OFF-state current
conduction mechanism attributed to the hopping mechanism. However, the
ON-state current in each device followed a unique mechanism, such that
Ohmic behavior was observed for the device with 0.96 wt%, while linear
then hopping, space-charge limited, and lastly, hopping conduction mechanisms
were attributed to devices with 0.48 wt%, 0.32 wt% and 0.24 wt%,
respectively. Proving that memory behavior and conduction in these devices
can be exploited by controlling the amount of CdTe/CdSe.
Next, we investigated the effect of molybdenum(IV) sulfide (MoS2) on
both conduction and memory behavior in polyvinylpyrrolidone (PVP) by
fabricating various ReRAM devices using (1) plain MoS2 (device A), (2)
plain PVP (device B), (3) PVP and MoS2 bilayer (device C), and (4) PVP
+MoS2 nanocomposites with 10 wt% (device D), 20 wt% (device E), 30 wt%
(device F) and 40 wt% (device G) MoS2 fabricated with Al and Ag as
bottom and top electrodes, respectively. We did not observe switching in
devices A and B. Device C showed a combination of bipolar and threshold
switching at 0.40 V . Device G portrayed bipolar switching at 0.56 V . In
Device C, space charge-limited conduction while Ohmic behavior followed by trapping of charge before switching was noticed in device G. Both devices
C and G showed reasonably (≥ 102) ON/OFF ratio. In the nanocomposite
devices, we observed that an increase in MoS2 content increased electrical
conductivity in the Ohmic region, leading to threshold switching at 30 wt%
(device F) and ultimately bipolar switching at 40 wt% (device G). These
studies showed that both switching and conduction mechanisms are sensitive
to the type and composition of the active layer in the devices studied.
Next, we investigated resistive switching in chitosan/PVP composite as
the active layers sandwiched between Al and Ag electrodes. ReRAMs with
active layers consisting of 1 : 3, 1 : 1, and 3 : 1 chitosan to PVP ratios were
studied. Asymmetric threshold switching with only the negative voltage bias
was obtained for the device with a chitosan to PVP ratio of 1 : 3. The 1 : 1
chitosan to PVP ratio device showed optimal memory behavior with bipolar
switching with low (0.28 V ) switching voltage in the first cycle, followed by
asymmetric threshold switching during the second cycle and back to bipolar
switching. We did not observe memory behavior in the 3 : 1 chitosan to
PVP-based device. Electrochemical conduction metalization was attributed
to the switching mechanism in the device with a 1 : 1 ratio of chitosan
to PVP. Our results reveal the applicability of chitosan and PVP blend in
memory device fabrication and that both the memory and switching can be
exploited by varying the ratio of chitosan to PVP in the composite.
Lastly, we fabricated the first resistive switching memory devices that
use raw organic cow milk as active layers. Our devices comprised fat-free,
medium cream, and full cream raw cow milk active layers sandwiched between
ITO and Ag. All devices showed low switching voltages, with the
medium fat milk-based device showing the lowest VSET = +0.45 V and
VRESET = −0.25 V . Additionally, the medium fat-based device showed
an ‘S-type’ memory mode attributed to the space-charge-limited conduction mechanism. Alternatively, fat-free and fill-cream-based devices both
showed ‘O-type’ memory behavior attributed to hopping conduction. EDS
analysis of all active layers revealed a relatively higher weight percentage
of metallic ions in the medium fat milk film than in fat-free and full-cream
milk films, which explains the different behaviors. These devices combine
biodegradability and low power characteristics that are important for green
computing.
2022-10-01T00:00:00Z