Two different interfaces were the focus of study: 1) the interface between disordered amorphous carbon and inorganic materials (metal nanostructures and silicon), and 2) the interface between partially ordered graphene (graphene oxide) and synthetic polymer matrix. Specifically, the uniqueness of this study can be summarized through the following novel findings, fabrication processes, and characterization techniques: • A simple and efficient process for faster, greener, less-expensive, and highly localized transformation of amorphous carbon nanostructures into graphitic nanostructures using low temperature heat and light treatments was developed for the fabrication of low-resistance interfaces between carbon nanomaterials and inorganic metal surfaces. • A new protocol for high resolution mapping the charge distribution and electronic properties of nanoscale chemically heterogeneous domains on non-homogeneous surfaces such as graphene oxide was established. • High strength laminated mechanical nanocomposites based on high interfacial stress transfer between polymer matrices and large area, flat, and non-wrinkled graphene oxide sheets were suggested and demonstrated. • Scanning Thermal Twist Microscopy – a thermal microscopy based technique was developed and demonstrated for characterizing the thermal properties of homogeneous and heterogeneous interfaces with nanoscale spatial resolution and high thermal sensitivity unachievable using traditional techniques.

Thesis (M.Tech.: Mechanical Engineering)-Dept. of Mechanical Engineering, Durban University of Technology, 2007 xiii, 101 leaves / Polymer nanocomposites may be defined as structures that are formed by infusing layered-silicate clay into a thermosetting orthermoplastic polymer matrix. The nanocomposites are normally particle-filled polymers for which at least one dimension of the dispersed particles is in nanoscale. These clay-polymer nanocomposites have thus attracted great interest in industry and academia due to their exhibition of remarkable enhancements in material properties when compared to the virgin polymer or conventional micro and macro-composites. The present work describes the synthesis, mechanical properties and morphology of nano-phased polypropylene structures. The structures were manufactured by melt- blending low weight percentages of montmorillonite (MMT) nanoclays (0.5, 1, 2, 3, 5 wt. %) and polypropylene (PP) thermoplastic. Both virgin and infused polypropylene structures were then subjected to quasi-static tensile tests, flexural tests, micro-hardness tests, impact testing, compression testing, fracture toughness analysis, dynamic mechanical analysis, tribological testing. Scanning electron microscopy studies were then conducted to analyse the fracture surfaces of pristine PP and PP nanocomposite. X-ray diffraction studies were performed on closite 15A clay and polypropylene composites containing 0.5, 1, 2, 3 and 5 wt. % closite 15A nanoclay to confirm the formation of nanocomposites on the addition of organo clays. Transmission electron miscopy studies were then performed on the PP nanocomposites to determine the formation of intercalated, exfoliated or agglomerated nanoclay structures. Analysis of test data show that the mechanical properties increase with an increase in nanoclay loading up to a threshold of 2 wt. %, thereafter the material properties degrade. At low weight nanoclay loadings the enhancement of properties is attributed to the lower percolation points created by the high aspect ratio nanoclays. The increase in properties may also be attributed to the formation of intercalated and exfoliated nanocomposite structures formed at these loadings of clay. At higher weight loading, degradation in mechanical properties may be attributed to the formation of agglomerated clay tactoids. Results of XRD, transmission electron microscopy studies and scanning electron microscopy studies of the fractured surface of tensile specimens verify these hypotheses.

A new type Rayleigh scattering of Mössbauer radiation (RSMR) spectrometer was developed, which due to favorable time scale of scattering process is suitable for the studies of complicated dynamics of biomolecules (not containing mössbauer nucleus) Densely packed iron nanowire arrays were created using template method of aluminum anodic oxide (AAO). Pore size and density of AAO template can be easily controlled by adjusting anodizing conditions. Shape and diameter of nanowires appear to be exact replicas of AAO pores. the effectiveness of different aqueous electrolytes used in nanowire deposition was investigated. Growth rate in different electrolytes was determined to be 52±2 nm/min. and 11,8±0,6 nm/min. This study has demonstrated the effects of aging of iron nanowires embedded within the pores of the AAO template. the influence of annealing parameters on the chemical composition of iron nanowires was investigated. CEMS studies have revealed. / Panaudojus puslaidininkinį γ-kvantų detektorių sukurtas naujo tipo Mesbauerio spinduliuotės Relėjaus sklaidos spektrometras, kuris dėl tinkamo γ-kvantų sklaidos laiko gali būti taikomas konformacinių judesių biologiniuose objektuose ir polimeruose, neturinčiuose mesbauerinių branduolių, tyrimams. Paruošti geležies nanosiūlų kolonijos, esančias aliuminio anodinio oksido (AAO) matricoje. Parenkant anodavimo sąlygas galima kontroliuoti, susidariusio tvarkingo porėto AAO, porų dydį ir tankį. Pagamintos AAO matricos užpildytos geležimi, taip susiformuoja žinomo dydžio nanosiūlai, kurių diametras atitinka porų diametrą. Palygintas vandeninių elektrolitų, skirtų užpildyti AAO poras, efektyvumas. Augimo greitis skirtinguose elektrolituose yra 52 ± 2 nm/min. ir 11,8 ± 0,6 nm/min. Nustatyti geležies nanosiūlų cheminės sudėties pokyčiai susidarymo ir senėjimo metu. Eksperimentiniais tyrimais įrodyta, kad geležies nanosiūlai AAO matricoje oksiduojasi ne visu tūriu, bet nuo atvirojo galo. Pakaitinus (daugiau nei 200 °C) metalinius nanosiūlus įkalintus AAO matricose, šie reaguoja su anodinio aliuminio oksidu ir sudaro špinelines struktūras.

One-dimensional ZnO nanostructures have great potential applications in the fields of optoelectronic and sensor devices.  Therefore, it is very important to realize the controllable growth of one-dimensional ZnO nanostructures and investigate their properties. The main points for this thesis are not only to successfully realize the controllable growth of ZnO nanorods (ZNRs), ZnO nanotubes (ZNTs) and ZnMgO/ZnO heterostructures, but also investigate the structure and optical properties in detail by means of scanning electron microscope (SEM), transmission electron microscope (TEM), resonant Raman spectroscopy (RRS), photoluminescence (PL), time resolved PL (TRPL), X-ray photoelectron spectroscopy (XPS) and Secondary ion mass spectrometry (SIMS). For ZNRs, on one hand, ZNRs have been successfully synthesized by a two-step chemical bath deposition method on Si substrates. The diameter of ZNRs can be well controlled from 150 nm to 40 nm through adjusting the diameter and density of the ZnO nanoparticles pretreated on the Si substrates. The experimental results indicated that both diameter and density of ZnO nanoparticles on the substrates determined the diameter of ZNRs. But when the density is higher than the critical value of 2.3×108cm-2, the density will become the dominant factor to determine the diameter of ZNRs. One the other hand, the surface recombination of ZNRs has been investigated in detail. Raman, RRS and PL results help us reveal that the surface defects play a significant role in the as-grown sample. It is the first time to the best of our knowledge that the Raman measurements can be used to monitor the change of surface defects and deep level defects in the CBD grown ZNRs. Then we utilized TRPL technique, for the first time, to investigate the CBD grown ZNRs with different diameters. The results show that the decay time of the excitons in ZNRs strongly depends on the diameter. The altered decay time is mainly due to the surface recombination process. A thermal treatment under 500°C can strongly suppress the surface recombination channel. A simple carrier and exciton diffusion equation is also used to determine the surface recombination velocity, which results in a value between 1.5 and 4.5 nm/ps. Subsequently, we utilized XPS technique to investigate the surface composition of as-grown and annealed ZNRs so that we can identify the surface recombination centers. The experimental results indicated that the OH and H bonds play the dominant role in facilitating surface recombination but specific chemisorbed oxygen also likely affect the surface recombination. Finally, on the basis of results above, we explored an effective way, i.e. sealing the beaker during the growth process, to effectively suppress the surface recombination of ZNRs and the suppression effect is even better than a 500oC post-thermal treatment. For ZNTs, the structural and optical properties have been studied in detail. ZNTs have been successfully evolved from ZNRs by a simple chemical etching process. Both temperature-dependent PL and TRPL results not only further testify the coexistence of spatially indirect and direct transitions due to the surface band bending, but also reveal that less nonradiative contribution to the emission process in ZNTs finally causes their strong enhancement of luminescence intensity. For ZnMgO/ZnO heterostructures, the Zn0.94Mg0.06O/ZnO heterostructures have been deposited on 2 inch sapphire wafer by metal organic chemical vapor deposition (MOCVD) equipment. PL mapping demonstrates that Mg distribution in the entire wafer is quite uniform with average concentration of ~6%. The annealing effects on the Mg diffusion behaviors in Zn0.94Mg0.06O/ZnO heterostructures have been investigated by SIMS in detail. All the SIMS depth profiles of Mg element have been fitted by three Gaussian distribution functions. The Mg diffusion coefficient in the as-grown Zn0.94Mg0.06O layer deposited at 700 oC is two orders of magnitude lower than that of annealing samples, which clearly testifies that the deposited temperature of 700 oC is much more beneficial to grow ZnMgO/ZnO heterostructures or quantum wells. This thesis not only provides the effective way to fabricate ZNRs, ZNTs and ZnMgO/ZnO heterostructures, but also obtains some beneficial results in aspects of their optical properties, which builds theoretical and experimental foundation for much better understanding fundamental physics and broader applications of low-dimensional ZnO and related structures. / Endimensionella nanostrukturer av ZnO har stora potentiella tillämpningar för optoelektroniska komponenter och sensorer. Huvudresultaten för denna avhandling är inte bara att vi framgångsrikt har realiserat med en kontrollerbar metod ZnO nanotrådar (ZNRs), ZnO nanotuber (ZNTs) och ZnMgO/ZnO heterostrukturer, utan vi har också undersökt deras struktur och optiska egenskaper i detalj. För ZNRs har diametern blivit välkontrollerad från 150 nm  ner till 40 nm. Den storlekskontrollerande mekanismen är i huvudsak relaterad till tätheten av ZnO partiklarna som är fördeponerade på substratet. De optiska mätningarna ger upplysning om att ytrekombinationsprocessen spelar en betydande roll för tillväxten av ZNR. En värmebehandling i efterhand  vid 500 grader Celsius eller användande av en förseglad glasbägare under tillväxtprocessen kan starkt hålla nere kanalerna för ytrekombinationen.För ZNT, dokumenterar vi inte bara samexistensen av rumsliga indirekta och direkta  övergångar på grund av bandböjning, men vi konstaterar också att vi har mindre icke-strålande bidrag till den optiska emissionsprocessen i ZNT. För ZnMgO/ZnO heterostrukturer konstaterar vi med hjälp av analys av Mg diffusionen i den växta och den i efterhand uppvärmda Zn(0.94)Mg(0.06)O filmen, att en tillväxt vid 700 grader Celsius är den mest lämpliga för att växa ZnMgO/ZnO heterostrukturer eller kvantbrunnar.   Denna avhandling ger en teoretisk och experimentell grund för bättre förståelse av grundläggande fysik och för tillämpningar av lågdimensionella strukturer. / SSF, VR

In this thesis, luminescence properties of ZnO nanostructures (nanorods, nanotubes, nanowalls and nanoflowers) are investigated by different approaches for possible future application of these nanostructures as white light emitting diodes. ZnO nanostructures were grown by different growth techniques on different p-type substrates. Still it is a challenge for the researchers to produce a stable and reproducible high quality p-type ZnO and this seriously hinders the progress of ZnO homojunction LEDs. Therefore the excellent properties of ZnO can be utilized by constructing heterojunction with other p-type materials. The first part of the thesis includes paper I-IV. In this part, the luminescence properties of ZnO nanorods grown on different p-type substrates (GaN, 4H-SiC) and different ZnO nanostructures (nanorods, nanotubes, nanoflowers, and nanowalls) grown on the same substrate were investigated. The effect of the post-growth annealing of ZnO nanorods and nanotubes on the deep level emissions and color rendering properties were also investigated. In paper I, ZnO nanorods were grown on p-type GaN and 4H-SiC substrates by low temperature aqueous chemical growth (ACG) method. The luminescence properties of the fabricated LEDs were investigated at room temperature by electroluminescence (EL) and photoluminescence (PL) measurements and consistency was found between both the measurements. The LEDs showed very bright emission that was a combination of three emission peaks in the violet-blue, green and orange-red regions in the visible spectrum. In paper II, different ZnO nanostructures (nanorods, nanotubes, nanoflowers, and nanowalls) were grown on p-GaN and the luminescence properties of these nanostructures based LEDs were comparatively investigated by EL and PL measurements. The nanowalls structures were found to be emitting the highest emission in the visible region, while the nanorods have the highest emissions in the UV region due to its good crystal quality. It was also estimated that the ZnO nanowalls structures have strong white light with the highest color rendering index (CRI) of 95 with correlated color temperature (CCT) of 6518 K. In paper III, we have investigated the origin of the red emissions in ZnO by using post-growth annealing. The ZnO nanotubes were achieved on p-GaN and then annealed in different ambients (argon, air, oxygen and nitrogen) at 600 oC for 30 min. By comparative investigations of EL spectra of the LEDs it was found that more than one deep level defects are involved in the red emission from ZnO nanotubes/p-GaN LEDs. It was concluded that the red emission in ZnO can be attributed to oxygen interstitials (Oi) and oxygen vacancies (Vo) in the range of 620 nm (1.99 eV) to 690 nm (1.79 eV) and 690 nm (1.79 eV) to 750 nm (1.65 eV), respectively. In paper IV, we have investigated the effect of post-growth annealing on the color rendering properties of ZnO nanorods based LEDs. ZnO nanorods were grown on p-GaN by using ACG method. The as grown nanorods were annealed in nitrogen, oxygen, argon, and air ambients at 600 oC for 30 min. The color rendering indices (CRIs) and correlated color temperatures (CCTs) were estimated from the spectra emitted by the LEDs. It was found that the annealing ambients especially air, oxygen, and nitrogen were found to be very effective. The LEDs based on nanorods annealed in nitrogen ambient, have excellent color rendering properties with CRIs and CCTs of 97 and 2363 K in the forward bias and 98 and 3157 K in the reverse bias. In the 2nd part of the thesis, the junction temperature of n-ZnO nanorods based LEDs at the built-in potential was modeled and experiments were performed to validate the model. The LEDs were fabricated by ZnO nanorods grown on different p-type substrates (4H-SiC, GaN, and Si) by the ACG method. The model and experimental values of the temperature coefficient of the forward voltage near the built-in potential (~Vo) were compared. It was found that the series resistance has the main contribution in the junction temperature of the fabricated devices. In the 3rd part of the thesis, the influence of helium (He+) ion irradiation bombardment on luminescence properties of ZnO nanorods based LEDs were investigated. ZnO nanorods were grown by the vapor-liquid-solid (VLS) growth method. The fabricated LEDs were irradiated by using 2 MeV He+ ions with fluencies of ~ 2×1013 ions/cm2 and ~ 4×1013 ions/cm2. It was observed that the He+ ions irradiation affects the near band edge emissions as well as the deep level emissions in ZnO. A blue shift about 0.0347 eV and 0.082 eV was observed in the PL spectra in the near band emission and green emission, respectively. EL measurements also showed a blue shift of 0.125 eV in the broad green emission after irradiation. He+ ion irradiation affects the color rendering properties and decreases the color rendering indices from 92 to 89.

One dimensional (1-D), zinc oxide (ZnO) and copper (II) oxide (CuO), nanostructures have great potential for applications in the fields of optoelectronic and sensor devices. Research on nanostructures is a fascinating field that has evolved during the last few years especially after the utilization of the hydrothermal growth method. Using this method variety of nanostructures can be grown from solutions, it is a cheap, easy, and environment friendly approach. These nanostructures can be synthesized on various conventional and nonconventional substrates such as silicon, plastic, fabrics and paper etc. The primary purpose of the work presented in this thesis is to realize controllable growth of ZnO, CuO and nanohybrid ZnO/CuO nanostructures and to process and develop white light emitting diodes and sensor devices from the corresponding nanostructures. The first part of the thesis deals with ZnO nanostructures grown under different hydrothermal conditions in order to gain a better understanding of the growth. Possible parameters affecting the growth such as the pH, the growth temperature, the growth time, and the precursors  concentration which can alter the morphology of the nanostructures were investigated (paper 1). Utilizing the advantage of the low temperature for growth we synthesized ZnO nanostructures on different substrates, specifically on flexible substrates, which are likely to be integrated with flexible organic substrates for future foldable and disposable electronics (paper 2, 3). In the second part of the thesis, using the results and findings from the growth of ZnO nanostructures, it was possible to successfully implement ZnO nanostructures for white light emitting diodes (LEDs) on different flexible substrates (paper 4, 5). In paper 4 we realized a ZnO/polymer LED grown on a paper substrate. In paper 5 we extended the idea to print the ZnO nanorods/polymer hybrid LEDs with potential application to large area flexible displays. In the last part of the thesis, CuO and nanohybrid ZnO/CuO nanostructures were utilized to fabricate Ag+ detection and humidity sensors. In paper 6 we reported Ag+ selective electrochemical sensor based on the use of functionalized CuO nanopetals. To combine the advantages of both oxides nanostructures and to improve the performance we fabricated a pn-heterojuction using intrinsic n-ZnO nanorods and p-CuO nanostructures which were then utilized as an efficient humidity sensor (paper 7).

Research on nanomaterials has become increasingly popular because of their unique physical, chemical, optical and catalytic properties compared to their bulk counterparts. Therefore, many efforts have been made to synthesize multidimensional nanostructures for new and efficient nanodevices. Among those materials, zinc oxide (ZnO), has gained substantial attention owing to many outstanding properties. ZnO besides its wide bandgap of 3.34 eV exhibits a relatively large exciton binding energy (60 meV) at room temperature which is attractive for optoelectronic applications. Likewise, cupric oxide (CuO), having a narrow band gap of 1.2 eV and a variety of chemo-physical properties that are attractive in many fields. Moreover, composite nanostructures of these two oxides (CuO/ZnO) may pave the way for various new applications. This thesis can be divided into three parts concerning the synthesis, characterization and applications of ZnO, CuO and their composite nanostructures. In the first part the synthesis, characterization and the fabrication of ZnO nanorods based hybrid light emitting diodes (LEDs) are discussed. The low temperature chemical growth method was used to synthesize ZnO nanorods on different substrates, specifically on flexible non-crystalline substrates. Hybrid LEDs based on ZnO nanorods combined with p-type polymers were fabricated at low temperature to examine the advantage of both materials. A single and blended light emissive polymers layer was studied for controlling the quality of the emitted white light. The second part deals with the synthesis of CuO nanostructures (NSs) which were then used to fabricate pH sensors and exploit these NSs as a catalyst for degradation of organic dyes. The fabricated pH sensor exhibited a linear response and good potential stability. Furthermore, the catalytic properties of petals and flowers like CuO NSs in the degradation of organic dyes were studied. The results showed that the catalytic reactivity of the CuO is strongly depending on its shape. In the third part, an attempt to combine the advantages of both ZnO and CuO NSs was performed by developing a two-step chemical growth method to synthesize the composite NSs. The synthesized CuO/ZnO composite NSs revealed an extended light absorption and enhanced defect related visible emission.

This research work is aimed at understanding the electronic properties of Bi(110) nanostructures. This study chiefly uses Scanning Tunneling Microscopy (STM), Scanning Tunneling Spectroscopy (STS) and Non Contact Atomic Force Microscope (NCAFM) to investigate the geometric and electronic structure of Bi(110) islands on highly oriented pyrolytic graphite (HOPG) substrate. STM measurements are the primary focus of the thesis which involves imaging the bismuth islands and study of its atomic structure. STM images of the Bi(110) islands reveal a ‘wedding cake’ profile of the bismuth islands that show paired layers on top of a base. I(V) (Current vs voltage) data was acquired via STS techniques and its first derivative was compared to DFT calculations. The comparison implied the presence of a dead wetting layer which was present only underneath the bismuth islands. We observed bilayer damped oscillations in the surface energy that were responsible for the stability of paired layers in Bi(110) islands. Interesting Moiré pattern arising out of misorientation between the substrate and the overlayer are also observed in STM images on some bismuth islands. Bright features pertaining to enhanced LDOS (local density of states) were observed on the perimeter of the bismuth islands and stripes in the STM images and STS dI/dV maps which appear at energies around the Fermi level. The bright features which we termed as ‘bright beaches (BB)’ are also observed on grain boundaries and defects that suggest that they are related to termination of the chain of bismuth atoms. The Bi(110) islands and stripes were observed to form preferred widths with a well defined periodicity. This peculiar phenomenon was attributed to a lateral quantum size effect (QSE) that results from a Fermi wave vector with appropriate shifts in Fermi energy. The widths of the islands prefer to adjust themselves at the nodes of this in-plane Fermi wavelength. NaCl deposited on a HOPG substrate forms cross shaped islands which were used as spacers to limit the interaction between the bismuth films and the underlying HOPG substrate. The NaCl islands are transparent to the flow of tunneling current and allow STS measurements. The LDOS of Bi/HOPG was very similar to the LDOS of Bi deposited on NaCl/HOPG which suggests that the wetting layer underneath the bismuth islands plays an important role in decoupling the film from the underlying substrate.

Cubic-phase silicon carbide (andamp;acirc;-SiC) nanostructures were successfully synthesized by the reaction of silicon monoxide (SiO) powder with multi-walled carbon nanotubes (MWCNTs) at high temperatures. Experiments were conducted under vacuum or in the presence of argon gas in a high-temperature furnace and the fabrication parameters of temperature (1300 -1500andamp;deg;C), time, and reactant material mass were varied to optimize the material. The resulting samples were then physically characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). XRD analysis revealed the presence of dominant andamp;acirc;-silicon carbide phases. SEM images depicted morphologies similar to the starting MWCNTs, having relatively larger diameter sizes, shorter lengths and reduced curvature. TEM observations showed the presence of solid and hollow nanostructures with both crystalline and amorphous regions. Additional experiments were performed to investigate de-aggregation and dispersion procedures for the andamp;acirc;-SiC nanostructures fabricated. Optimum results for these experiments were achieved by ultrasonication of 0.01 wt.% andamp;acirc;-SiC in N,N dimethyl formamide (DMF) and dispersion using a spin coater. A methodology for electrical testing of andamp;acirc;-SiC nanostructures was developed using the de-aggregation and dispersion process established. SEM observations revealed that the random nature of the dispersion procedure used was not efficient in forming contacts regions that would allow electrical measurements of andamp;acirc;-SiC nanostructures on the pre-patterned silicon substrate.

Dans ce travail, nous avons étudié les modifications du taux d'émission spontanée de sources internes utilisées comme sondes des variations de densités locales d'états photoniques. Des matériaux aussi divers que des milieux désordonnés polymères ou inorganiques, des cristaux photoniques et des nanostructures plasmoniques ont été caractérisés. Ainsi, nous discutons successivement i) les changements des taux d'émission spontanée de sources internes dus aux variations locales de densité du milieu polymère environnant, en dessous et au dessus de la température de transition vitreuse; ii) les changements des taux d'émission spontanée consécutives aux changements de conformation des molécules fluorescentes dus aux mouvements des chaînes environnantes; iii) l'étude de l'inhibition/exaltation de l'émission de fluorescence dans les cristaux photonique et iv) l'émission dans les nanostructures plasmoniques. Finalement, nous décrivons quelques perspectives de recherche telles que i) le développement de matériaux extrêmement désordonnés afin d'étudier les phénomènes de localisation de la lumière et de lasers aléatoires; ii) le developpement des nanostructures plasmoniques combinant plasmons de surface localisés et propagatifs afin de générer des facteurs de Purcell élevés dans une gamme spectrale soit très large soit très étroite et iii) le développement des structures plasmoniques sensibles à l'effet Kerr optique afin de réaliser des bistables optiques.