Research Articles (Physics)

Permanent URI for this collectionhttp://hdl.handle.net/2263/1725

A collection containing some of the full text peer-reviewed/ refereed articles published by researchers from
the Department of Physics

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Now showing 1 - 20 of 750

Progress toward stable organic solar cells
(Wiley, 2025-02) Tegegne, Newayemedhin A.; Nchinda, Leonato Tambua; Kruger, T.P.J. (Tjaart); tjaart.kruger@up.ac.za
Organic solar cells (OSCs) are suitable candidates for next-generation renewable energy sources due to their low cost of production and flexibility. Their power conversion efficiency has improved significantly to about 20% in both single- and multi-junction devices due to the tremendous work in optimizing the synthesis of novel active-layer materials while improving device fabrication. Despite a few reports predicting a 20-year lifetime for OSC devices, their stability currently lags behind their commercialization. This Review discusses the issues that impair OSC stability and how to mitigate them. While emphasizing the importance of the International Summit on Organic Photovoltaic Stability (ISOS) protocols, an overview of recent advancements in OSC power conversion efficiency (PCE) and lifetime is provided. Finally, fundamental challenges to developing high-performance and stable OSCs are discussed along with general recommendations for improving the stability of OSCs.
Comparative study of CO adsorption on Au, Cu, MoO2 and MoS2 2D nanoparticles
(Elsevier, 2024-11) Ngom, N.; Etindele, A.J.; Andriambelaza, Noeliarinala Felana; Chetty, Nithaya; Wakata, A.S.; Kenmoe, S.
This study focuses on a comparative analysis of the electronic properties of triangular, irregular hexagonal, and octagonal 2D nanoparticles containing 10, 12, and 14 motifs, made of Au, Cu, MoO2, and MoS2. The investigation was carried out using density functional theory. The formation energies and vibrational frequencies demonstrate that the 2D nanostructure configurations can exist as stable structures. Edge atoms with lower coordination numbers than central atoms, are the preferred sites for CO adsorption. Using orbital-weighting dual descriptors calculated from Fukui functions enabled the identification of a majority nucleophilic attack sites in Au and Cu nanoparticles, while MoO2 and MoS2-based nanoparticles present almost as many electrophilic sites as nucleophilic sites. The charge transferred between the nanostructures and the CO molecule and the redistribution of the projected density of states were used to assess the strength of interfacial bonds and the nature of the fundamental interaction involved in the bonding.
Quantum plasmonics model of refractive index sensing using photon correlations
(American Physical Society, 2024-10-10) Ugwuoke, Luke C.; Kruger, T.P.J. (Tjaart); Tame, Mark S.
The interaction between the electric dipole moments of a quantum emitter and a metal nanoparticle gives rise to unique optical properties, such as interference-induced photon correlations, that could be useful for enhanced intensity-based sensing. Using the quantum theory of photodetection, we propose a nanosensor system comprising a quantum emitter and a metal nanoparticle that explores the possibility of utilizing higher-order photon correlations for refractive index sensing. Both the refractive index sensitivity and resolution of the nanosensor, whose scattering spectrum lies within the visible region, are predicted. The sensor is supported by a substrate and driven weakly by a coherent field. By calculating the mean photocount and its second factorial moment resulting from the scattered field of the system, the sensing performance of the intensity and intensity-intensity correlation g(2)(0) are compared at optimal driving wavelengths. The mean photocount was found to be inherently low, inhibiting the role of interference-induced photon antibunching in minimizing the sensor’s intensity shot noise. However, a regime in which the noise could be reduced below the shot noise limit is identified, leading to a quantum enhancement in the sensing performance.
Data-driven cold starting of good reservoirs
(Elsevier, 2024-12) Grigoryeva, Lyudmila; Grigoryeva, Lyudmila; Kemeth, Felix P.; Kevrekidis, Yannis; Manjunath, Gandhi; Ortega, Juan-Pablo; Steynberg, Matthys J.; manjunath.gandhi@up.ac.za
Using short histories of observations from a dynamical system, a workflow for the post-training initialization of reservoir computing systems is described. This strategy is called cold-starting, and it is based on a map called the starting map, which is determined by an appropriately short history of observations that maps to a unique initial condition in the reservoir space. The time series generated by the reservoir system using that initial state can be used to run the system in autonomous mode in order to produce accurate forecasts of the time series under consideration immediately. By utilizing this map, the lengthy ‘‘washouts’’ that are necessary to initialize reservoir systems can be eliminated, enabling the generation of forecasts using any selection of appropriately short histories of the observations.
Selenium migration and SiC structural evolution post helium and selenium Co-implantation and annealing
(Elsevier, 2025-08) Mthalane, S.; Abdelbagi, H.A.A.; Mtshali, C.B.; Li, B.S.; Skuratov, V.A.; Ntshangase, S.S.; Hlatshwayo, Thulani Thokozani
Please read abstract in the article.
Effect of exposing Se pre-implanted polycrystalline SiC to maximum electronic energy loss of 33.7 keV/nm and annealing
(Elsevier, 2025-05) Mabelane, T.S.; Abdalla, Zaki Adam Yousif; Skuratov, V.A.; Ntshangase, S.S.; Masikane, S.C.; Hlatshwayo, Thulani Thokozani; u22019937@tuks.co.za
Please read abstract in the article.
Electrical activity of aluminum, boron, and n-type impurities defect-complexes in germanium : implications for enhanced Ge-based devices
(Elsevier, 2025-08) Igumbor, Emmanuel; Mapasha, Refilwe Edwin; Raji, Abdulrafiu Tunde; Omotoso, Ezekiel
Please read abstract in the article.
Density functional studies of protonated and alkali metal (Li, Na and K) incorporated T-doped 2D zeolite model (T = B, Ga)
(IOP Publishing, 2025-06) Andriambelaza, Noeliarinala Felana; Perry, C.; Mugo, J.; Sarwar, M.; Jones, G.; Chetty, N.
Ab initio calculations based on density functional theory (DFT) have been performed to investigate the role of trivalent atoms substituting silicon atom in the 2D zeolite model. The effects of the B and Ga atoms on the stability, structural and electronic properties of the 2D zeolite model are explored. Our DFT calculations reveal that the introduction of B atom is exothermic whereas that one of Ga atom is endothermic. The structural analysis shows that the incorporation of B and Ga atoms affects the bond lengths of the system, however it does not lead to a significant deformation of the structure. The Fermi level of the doped systems is shifted towards the valence band, indicating that the incorporation of these trivalent atoms leads to p_ type materials. The second purpose of this study is to find the suitable charge compensations among hydrogen and alkali metals as well as their site preference (either on the surface or in the cages of the silica bilayer). The calculated formation energy values are similar, suggesting both configurations could co-exist. Hydrogen has the lowest formation energy and the proton affinity analysis predicts low acid strength of H-B- compared to H-Ga-doped 2D zeolite, a similar trend to that of bulk zeolite. Among the alkali elements, we found that Na and K atoms are the most stable ones. The density of states analysis shows that the Fermi level is lying within the gap, and defect states are observed near the band edges narrowing the band gap of the system. This work provides detailed and valuable information about the atomic-level properties of the relatively recent 2D zeolite model, which is beneficial for its industrial applications.
Investigating the effectiveness of borophene on anchoring and influence on kinetics of sodium superoxide in sodium-oxygen batteries
(Elsevier, 2024-04) Fwalo, Chewe; Kochaev, A.; Mapasha, Refilwe Edwin
Owing to the increasing cost and limited resources of lithium metal, there is a growing interest in developing energy storage systems that are more affordable, environmentally friendly, and have high energy densities. This is also driven by the need to significantly reduce carbon dioxide emissions resulting from the use of fossil fuels. The sodium–oxygen battery has emerged as a potential alternative, as the materials used for both of its electrodes are among the most abundant and inexpensive elements on Earth. However, despite these advantages, there are technical challenges in implementing it, such as the insulation of the cathode electrode and failure to prevent discharge products from desorbing into electrolytes during discharging. In this study, using density functional theory based on first principles, we investigated the electronic properties of freestanding 12 and 3-borophenes after adsorption of sodium superoxide (NaO2). Our findings showed that the adsorption energy of sodium superoxide on 12 and 3 borophene are −3.85 eV and −3.24 eV, respectively. The large negative values of adsorption energy suggest that sodium superoxide is anchored spontaneously, which is significant as it can be prevented from migrating to the negative electrode via the electrolyte during the discharging process. Furthermore, the 12 and 3 structures showed moderate diffusion energy barriers of 0.89 eV and 1.37 eV and decomposition energies of 0.73 eV and 0.52 eV, respectively. The latter demonstrates the catalytic effects of nanosheets on the decomposition of the sodium superoxide into separated sodium (Na+) and oxygen (O2). Moreover, the decomposition energies being lower than sodium superoxide formation energy (3.90 eV) in a vacuum, suggests nanosheets effects during the charging process. Most importantly, the metallic characteristics of both crystal structures were preserved after the adsorption of sodium superoxide, and the electronic conductivities were enhanced. This is significant to improve the cycle life of the battery, as the materials can charge back the decomposed species during the charging process, thereby preventing the formation of dendrites at the surface of the cathode. Ultimately, the predicted electronic properties for both structures demonstrate their potential as cathode electrode materials for enhancing the electrochemical processes of sodium–oxygen batteries.
Properties of nanostructured hematite prepared by various coating techniques for potential PEC water splitting applications
(Elsevier, 2024-07) Holtzhausen, Adiel; Nyarige, Justine Sageka; Kyesmen, Pannan Isa; Diale, M.; mmantsae.diale@up.ac.za
Please read abstract in the article.
Characterization of methylammonium tin iodide thin films prepared by sequential physical vapour deposition
(Elsevier, 2024-12) Ligavo, Margdaline Musanga; Sembito, Alex; Sibiya, Sizwe; Thubane, Sandile; Waita, Sebastian; Nyongesa, Francis Wanjala; Erasmus, Rudolph M.; Diale, M.; mmantsae.diale@up.ac.za
Methylammonium tin triiodide (MASnI3) films were grown through Sequential Physical Vapour Deposition (SPVD) without breaking the vacuum and optimized by varying MAI thickness and annealing time while keeping SnI2 thickness constant. The film's crystallinity increased with MAI thickness and annealing time. Optimal bandgap was attained for the film with 500 nm MAI annealed for 20 & 40 min. FE-SEM revealed densely packed, large grains, increasing in size with MAI thickness and on annealing from 0 to 40 min and decreasing at 80 min. The film with 300 nm MAI thickness annealed for 40 min showed the strongest PL intensity suggesting reduced carrier recombination losses. Trap densities reduced with annealing time and MAI thickness due to improvements in films' crystallinity, grain sizes and reduced grain boundaries which act as carrier trapping sites. Hence, films prepared through SPVD, exhibit excellent structural, optical, and morphological properties, suitable for photovoltaic applications.
Understanding diffusion behavior of multiple Li and Na-ions on a β12-borophene electrode : a first-principles study
(Elsevier, 2025-05) Fwalo, Chewe; Kochaev, A.; Mapasha, Refilwe Edwin
Please read abstract in the article.
DLTS characterisation of 107 MeV krypton ion-irradiated nitrogen-doped 4H-silicon carbide
(Springer, 2025-01) Omotoso, Ezekiel; Igumbor, Emmanuel; Meyer, Walter Ernst
Please read abstract in the article.
The effect of ion implantation and annealing temperatures on the migration behavior of ruthenium in glassy carbon
(Elsevier, 2024-12) Jafer, T.A.O.; Odutemowo, Opeyemi Shakirah; Abdelbagi, Hesham Abdelbagi Ali; Thabethe, Thabsile Theodora
Nuclear waste storage materials are inevitable in nuclear industry for preventing the release of radioactive waste products. Glassy carbon has been considered being beneficial to be used in the dry cask needed for nuclear waste storage. Thus, we studied the migration of ruthenium implanted in glassy carbon upon annealing. Our investigations show that ruthenium implantation caused defects in the glassy carbon structure, with more defects observed in the room temperature as-implanted samples compared to those implanted at 200 ◦C. Annealing the as-implanted samples from 500 to 800 ◦C showed no significant change in the ruthenium depth profiles, indicating the non-diffusivity of ruthenium in glassy carbon at these temperatures. However, annealing at higher temperatures (from 900 and 1300 ◦C) resulted in an increase in the maximum depth profile peaks, accompanied by a shift towards the surface, and a decrease in the full-width at half-maximum. These changes indicate the aggregation of ruthenium atoms in the near-surface region. Additionally, more ruthenium aggregation was observed in room temperature implanted samples compared to those implanted at 200 ◦C. This difference is attributed to the higher concentration of defects in room temperature implanted samples, which promotes ruthenium aggregation. Moreover, the migration and aggregation of ruthenium in the near-surface region contributed to an increase in the surface roughness of the glassy carbon.
Unveiling the thermal stability of diketopyrrolopyrrole-based terpolymers : a key element for enhanced efficiency and stability of organic solar cells
(Royal Society of Chemistry, 2024-06) Nchinda, Leonato Tambua; Genene, Zewdneh; Mammo, Wendimagegne; Tegegne, Newayemedhin A.; Kruger, T.P.J. (Tjaart); tjaart.kruger@up.ac.za
With the advent of novel polymers, organic solar cell (OSC) research has evolved significantly over the past decade. The molecular engineering of terpolymers has allowed for simple morphological control in binary devices over ternary blends, with the highest power conversion efficiencies (PCEs) exceeding 18%. However, research on the stability of OSCs is still lagging behind. In this regard, we examined the thermal stability of a series of terpolymers comprising one electron donor (thienyl-substituted benzodithiophene, BDTT) and two types of electron acceptors namely fluorobenzotriazole (FTAZ) and thienothiophene-capped diketopyrrolopyrrole (TTDPP) and their blends with PC71BM. The terpolymers demonstrated broad absorbance ranging from below 350 nm to 900 nm. The thermal stability of the terpolymers was investigated as pristine thin films and as bulk heterojunction (BHJ) films of the terpolymers blended with PC71BM by heating at 85 1C. We observed that thermal degradation had no sizeable effect on the properties of the pristine terpolymers while the blended films demonstrated significant changes in their morphology due to the inclusion and aggregation of PC71BM. After thermal annealing at 85 1C, the width of the symmetric CQC stretching Raman mode and the CQC/C–C intensity ratio of pristine terpolymers and terpolymer:PC71BM thin films revealed that incorporation of the FTAZ acceptor improves the thermal stability of the BHJ active layers. Furthermore, prolonged thermal annealing times (43 hours) resulted in the development of PC71BM aggregates and terpolymer decomposition with no evident changes in the molecular and chemical structure of the terpolymers. Our findings indicate that by gradually annealing the blended films using an appropriate annealing time, the diffusion of PC71BM molecules to form aggregates can be carefully regulated, resulting in a nanostructure critical to the efficiency of organic solar cells.
Enhancement of the electrochemical properties of vanadium dioxide via nitrogen-doped reduced graphene oxide for high-performance supercapacitor applications
(Royal Society of Chemistry, 2024-08) Sarr, Samba; Bakhoum, Daba T.; Sylla, Ndeye Fatou; Ndiaye, Ndeye M.; Tarimo, Delvina Japhet; Maphiri, Vusani M.; Ngom, Balla D.; Manyala, Ncholu I.; ncholu.manyala@up.ac.za
A one-step solvothermal approach was used to integrate nitrogen-doped reduced graphene oxide into vanadium dioxide (VO2) to prepare a V@XN-G (where X = 24, 48 and 96 and is the mass of urea as the nitrogen (N) source) nanocomposite material. After optimization, V@48N-G showed better performance with a specific capacitance of 197.4 F g 1 at 0.5 A g 1 in a wide working potential window of 0.0–0.8 V vs. silver/silver chloride and was adopted as a positive electrode. Thus, cocoa waste-based activated carbon (ACC) was investigated and considered a negative electrode in assembling an asymmetric full-cell V@48NG// ACC. The device exhibited good specific energy and power of 28.8 W h kg 1 and 425.1 W kg 1, respectively, at a voltage window and specific current of 1.7 V and 0.5 A g 1, respectively. Its cycling stability resulted in a Coulombic efficiency (CE) and capacitance retention (CR) of 99.8% and 73%, respectively, over 10000 galvanostatic charge/discharge cycles at 10 A g 1. Therefore, the V@48N-G//ACC device shows excellent electrochemical performance and is suitable for energy storage application technology.
The VLBA CANDELS GOODS-North Survey -I. Survey design, processing, data products, and source counts
(Oxford University Press, 2024-04) Deane, Roger; Radcliffe, Jack F.; Njeri, Ann; Akoto-Danso, Alexander; Bernardi, Gianni; Smirnov, Oleg M.; Beswick, Rob; Garrett, Michael A.; Jarvis, Matt; Whittam, Imogen H.; Bourke, Stephen; Paragi, Zsolt
The past decade has seen significant advances in wide-field cm-wave very long baseline interferometry (VLBI), which is timely given the wide-area, synoptic survey-driven strategy of major facilities across the electromagnetic spectrum. While wide-field VLBI poses significant post-processing challenges that can severely curtail its potential scientific yield, many developments in the km-scale connected-element interferometer sphere are directly applicable to addressing these. Here we present the design, processing, data products, and source counts from a deep (11 μJy beam −1 ), quasi-uniform sensitivity, contiguous wide-field (160 arcmin 2 ) 1.6 GHz VLBI survey of the CANDELS GOODS-North field. This is one of the best-studied extragalactic fields at milli-arcsecond resolution and, therefore, is well-suited as a comparative study for our Tera-pixel VLBI image. The derived VLBI source counts show consistency with those measured in the COSMOS field, which broadly traces the AGN population detected in arcsecond-scale radio surveys. Ho we ver, there is a distinctive flattening in the S 1.4GHz ∼100–500 μJy flux density range, which suggests a transition in the population of compact faint radio sources, qualitatively consistent with the excess source counts at 15 GHz that is argued to be an unmodelled population of radio cores. This survey approach will assist in deriving robust VLBI source counts and broadening the disco v ery space for future wide-field VLBI surveys, including VLBI with the Square Kilometre Array, which will include new large field-of-view antennas on the African continent at 1000 km baselines. In addition, it may be useful in the design of both monitoring and/or rapidly triggered VLBI transient programmes.
MIGHTEE- H I : deep spectral line observations of the COSMOS field
(Oxford University Press, 2024-09) Heywood, I.; Ponomareva, A.A.; Maddox, N.; Jarvis, M.J.; Frank, B.S.; Adams, E.A.K.; Baes, M.; Bianchetti, A.; Collier, J.D.; Deane, Roger; Glowacki, M.; Jung, S.L.; Pan, H.; Rajohnson, S.H.A.; Rodighiero, G.; Ruffa, I.; Santos, M.G.; Sinigaglia, F.; Vaccari, M.
The MIGHTEE survey utilizes the South African MeerKAT radio telescope to observe four extragalactic deep fields, with the aim of advancing our understanding of the formation and evolution of galaxies across cosmic time. MIGHTEE’s frequency coverage encompasses the H I line to a redshift of z 0.58, and OH megamasers to z 0.9. We present the MIGHTEE- H I imaging products for the COSMOS field, using a total of 94.2 h on-target and a close-packed mosaic of 15 individual pointings. The spectral imaging covers two broad, relatively interference-free regions (960–1150 and 1290–1520 MHz) within MeerKAT’s L -band, with up to 26 kHz spectral resolution (5.5 km s −1 at z = 0). The median noise in the highest spectral resolution data is 74 μJy beam −1 , corresponding to a 5 σH I mass limit of 10 8 . 5 M for a 300 km s −1 line at z = 0.07. The mosaics co v er > 4 deg 2 , provided at multiple angular resolution / sensitivity pairings, with an angular resolution for H I at z = 0 of 12 arcsec. We describe the spectral line processing workflow that will be the basis for future MIGHTEE- H I products, and validation of, and some early results from, the spectral imaging of the COSMOS field. We find no evidence for line emission at the position of the z = 0.376 H I line reported from the CHILES survey at a > 94 per cent confidence level, placing a 3 σupper limit of 8.1 ×10 9 M on M HI for this galaxy. A public data release accompanies this article.
On the implausible physical implications of a claimed lensed neutral hydrogen detection at redshift z = 1.3
(Oxford University Press, 2024-09) Deane, Roger
The Square Kilometre Array mid-frequency array will enable high-redshift detections of neutral hydrogen ( H I ) emission in galaxies, providing important constraints on the evolution of cold gas in galaxies over cosmic time. Strong gravitational lensing will push back the H I emission frontier towards cosmic noon ( z ∼2), as has been done for all prominent spectral lines in the interstellar medium of galaxies. Chakraborty & Roy report a z = 1 . 3 H I emission detection towards the well-modelled, galaxy-scale gravitational lens, SDSS J0826 + 5630. We carry out H I source modelling of the system and find that their claimed H I magnification, μH I = 29 ±6, requires an H I disc radius of 1 . 5 kpc, which implies an implausible mean H I surface mass density in excess of H I > 2000 M pc −2 . This is several orders of magnitude abo v e the highest measured peak values ( H I 10 M pc −2 ), above which H I is converted into molecular hydrogen. Our re-analysis requires this to be the highest H I mass galaxy known ( M H I 10 11 M ), as well as strongly lensed, the latter having a typical probability of the order of 1 in 10 3 –10 4 . We conclude that the claimed detection is spurious.
A novel study on chronoamperometry electrodeposition of hexagonal boron nitride with Eucalyptus grandis-derived activated carbon for supercapacitor applications
(Elsevier, 2025-01) Mutua, Hellen Ngunya; Tarimo, Delvina Japhet; Rutavi, Gift; Maphiri, Vusani M.; Mwabora, Julius; Musembi, Robinson J.; Manyala, Ncholu I.; ncholu.manyala@up.ac.za
Activated carbon (AC) and hexagonal boron nitride (hBN) electrodes were successfully synthesized from Eucalyptus grandis tree bark and a mixture of boric acid and urea in the ratio of 2:3, respectively. The optimal composite hBN/AC-1:0.05 electrode was prepared by chronoamperometry electrodeposition at an optimal potential of 0.45 V for 60 min. The prepared electrodes were analyzed using Raman spectroscopy, Fourier Transform Infrared (FTIR), X-ray diffraction (XRD), Scanning Electron Microscope (SEM), and Energy Dispersive X-ray spectroscopy (EDS). Half- and full-cell setups were used to investigate the as-prepared AC, hBN, and hBN/AC composite electrodes in 3 M KOH for electrochemical characterization. A supercapacitor was assembled using hBN/AC-1:0.05//AC. The device displayed a specific energy of 24.1 Wh/kg and a corresponding specific power of 1025 W/kg at 1 A/g in a potential window of 1.6 V. Furthermore, the developed device demonstrated a coulombic efficiency of 99.4 % and a capacity retention of 76.3 % after 10,000 GCD cycles at 5 A/g. The results of hBN/AC-1:0.05//AC contribute to the scientific community by highlighting the importance of biomass recycling (Eucalyptus grandis tree bark) and providing a simple approach to producing a composite material for energy storage.

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