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No description available.

Understanding long range electron transfer (ET) in proteins is of fundamental interest to elucidate the complex nature of many biological processes. The mechanistic discussion is highly debated in the literature and the factors that control this process are still not clear. Because of the structural complexity and dynamic nature, it is very difficult to correctly evaluate long range ET in proteins. The study of simple model peptides having specific secondary structures is useful for a systematic and accurate evaluation. The polypeptide matrix in the photosynthetic reaction centre is rich in helices and this particular structural motif is believed to play an important role in ET in nature. In this thesis, ET study through some synthetic α-helical model peptides is described. The model peptides studied herein contain the redox-active ferrocene at one end and the thiol-functionalised Cys residue at the other. Films of these peptides were formed on the surface of gold electrodes via the Au-S bond, and by employing cyclic voltammetry, the rate of ET between the pendant ferrocene and the gold electrode through the peptide spacer has been evaluated. My study indicates that ET in α-helical peptides is a function of molecular dynamics and occurs via a tunnelling mechanism. These findings are significant and expected to offer new directions in the highly controversial discussion on ET in proteins.<p>This thesis also describes investigations in two important areas of applications of the α-helices. The first is photocurrent generation upon laser excitation of light-harvesting chromophore-functionalised peptides which mimics the natural photosynthetic centre. This important area of research can promote development of nano-scaled photovoltaic devices. Surprisingly, following the conventional experimental protocols, a photocurrent was observed in the absence of a chromophore and even by the irradiation of a bare gold electrode with laser light. It is suggested that an important consequence of laser irradiation has been overlooked in several publications and the so-called photocurrent phenomenon may be a consequence of laser heating. <p>Peptide-protected nanoparticles is another area of research receiving significant attention these days due to its potential relevance in biomedical applications. However, peptides are highly flexible and their structure can change depending on the nature of the environment. Since the reactivity of a peptide is related to its secondary structure, any conformational change could seriously alter the overall activity of the peptide-protected nanoparticles. In this thesis, the structural investigation of an α-helical peptide was carried out and it was found that the radius of curvature of nanoparticles has a profound effect on the structure of the adsorbate peptides and thereby, may affect the overall activity of the peptide-protected nanoparticles.

Understanding long range electron transfer (ET) in proteins is of fundamental interest to elucidate the complex nature of many biological processes. The mechanistic discussion is highly debated in the literature and the factors that control this process are still not clear. Because of the structural complexity and dynamic nature, it is very difficult to correctly evaluate long range ET in proteins. The study of simple model peptides having specific secondary structures is useful for a systematic and accurate evaluation. The polypeptide matrix in the photosynthetic reaction centre is rich in helices and this particular structural motif is believed to play an important role in ET in nature. In this thesis, ET study through some synthetic α-helical model peptides is described. The model peptides studied herein contain the redox-active ferrocene at one end and the thiol-functionalised Cys residue at the other. Films of these peptides were formed on the surface of gold electrodes via the Au-S bond, and by employing cyclic voltammetry, the rate of ET between the pendant ferrocene and the gold electrode through the peptide spacer has been evaluated. My study indicates that ET in α-helical peptides is a function of molecular dynamics and occurs via a tunnelling mechanism. These findings are significant and expected to offer new directions in the highly controversial discussion on ET in proteins.<p>This thesis also describes investigations in two important areas of applications of the α-helices. The first is photocurrent generation upon laser excitation of light-harvesting chromophore-functionalised peptides which mimics the natural photosynthetic centre. This important area of research can promote development of nano-scaled photovoltaic devices. Surprisingly, following the conventional experimental protocols, a photocurrent was observed in the absence of a chromophore and even by the irradiation of a bare gold electrode with laser light. It is suggested that an important consequence of laser irradiation has been overlooked in several publications and the so-called photocurrent phenomenon may be a consequence of laser heating. <p>Peptide-protected nanoparticles is another area of research receiving significant attention these days due to its potential relevance in biomedical applications. However, peptides are highly flexible and their structure can change depending on the nature of the environment. Since the reactivity of a peptide is related to its secondary structure, any conformational change could seriously alter the overall activity of the peptide-protected nanoparticles. In this thesis, the structural investigation of an α-helical peptide was carried out and it was found that the radius of curvature of nanoparticles has a profound effect on the structure of the adsorbate peptides and thereby, may affect the overall activity of the peptide-protected nanoparticles.

In this thesis, we have setup the measurement systems for photocurrent and electro-absorption (Da) spectroscopy, and have investigated the optical characteristics of semiconductor quantum well structures in the long wavelength regime. The measured samples are of three epi-structures including a p-i-n laser structure of the symmetric multiple quantum wells (SMQWs), a p-i-n laser structure of the asymmetric multiple quantum wells (AMQWs), and n-i-n BRAQWETS structures. The samples are fabricated in mesa type photodiode structures for the measurements. From the Da spectrum of the n-i-n BRAQWETS structures, we observe a blue shift ~ 10nm of Da peak caused by band filling effect at +5V bias. Besides, a red shift ~ 2nm has been obtained at ¡V5V bias caused by the quantum-confined Stark effect. The photocurrent spectrum of the SMQWs shows an e1-hh1 absorption peak at hn=0.813eV (l=1.525mm) which matches the photoluminesce spectrum. The e1-hh1 transition has a red-shift ~ 38nm at ¡V5V bias for the SMQWs. For the AMQWs consisting of 5, 10, 15nm wells, we observe the e1-hh1 absorption peaks at 0.758eV (l=1.64mm), 0.772eV (l=1.6mm) and 0.797eV (l=1.55mm), respectively. As the AMQWs biased at ¡V5V, a red-shift ~ 25nm is obtained for the e1-hh1 transition corresponding to the 15nm-wide wells.

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In less than a decade of research, graphene has earned a long list of superlatives to its name and is expected to have applications in various fields such as electronics, photonics, optoelectronics, materials, biology and chemistry. Graphene has also attracted a lot of attention because its properties can be engineered either via intrinsic changes or by modification of its environment. Raman spectroscopy has become an ideal characterization method to obtain qualitative and quantitative information on these changes. This thesis investigates the possibility to change, supplement and monitor the electonic and optical properties as well as the chemical reactivity of graphene. It is achieved by i) substrate effect, ii) introduction of defects in the structure of graphene and iii) the combination of graphene with other two- dimensional crystals such as hexagonal boron nitride (h-BN) and transition metal dichacolgenides. In particular, the experimental work presented here describes: I - The influence of the type of substrate on the Raman intensity of graphene. This work leads to the calculation of the Raman scattering efficiency of graphene after CaF2 is found to be a suitable substrate for this kind of study in contrast to Si/SiOx that strongly modulates the Raman intensities. The G peak scattering efficiency is found to be about 200 x 10-5 m-1 Sr-1 at 2.4 eV while that of the 2D peak is one order of magnitude higher, confirming the resonant nature of the 2D peak Raman scattering process. II - An attractive method to produce large (up to several hundreds of microns across) and high quality graphene by anodic bonding. This cheap, fast and solvent-free method also allows introduction of vacancy like defects in the samples in a relatively controllable way. III - The Raman signatures of several types of defect such as sp3 sites, vacancies and substitutional atoms. For low defect concentration (stage 1) the intensitiy ratio I(D)/I(D') is constant and is 13 for sp3 sites, 9 for substitutional atoms and 7 for vacancies. This signature is explained using the local activation model recently proposed to model the amorphization trajectory of graphene with containing vacancy-like defects. IV - Controlled modification of graphene through mild oxygen plasma. The influence of sp3 sites on monolayer and bilayer graphene's electrical properties are discussed. In the case of bilayer under controlled conditions, it is possible to modify only the top layer. This may lead to decoupling between the two layers, which could explain the good mobility measured for this system. The possiblity to use such system as a sensor is discussed. V - The characteristic Raman signature of aligned graphene/h-BN superlattices. The Raman spectrum shows strong changes in perfectly aligned superlattices, which could be attributed to the reconstruction of the Dirac spectrum. VI - A prototype photovoltaic cell made of a graphene and tungsten disulphide (WS2) heterostructure with an external quantum efficiency of about 30%. The beneficial combination of an excellent absorption in WS2 atomically thin films due to the presence of van Hove singularities and graphene used as a transparent, flexible and conductive electrode is demonstrated.

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Photodetectors are semiconductor devices capable of converting optical signals into electrical signals. There is a wide range of applications for photodetectors such as fiber optics communication, infrared heat camera sensors, as well as in medical and military equipment.Nanowires are thin needle-shaped structures consisting of semiconductor materials such as gallium arsenide (GaAs), indium phosphide (InP) or silicon (Si). They are ideally suited for sensitive photodetectors with low noise due to their small size, well-controlled crystal structure, and composition tunability, as well as the possibility to fabricate them monolithically on silicon.In this thesis, Fourier Transform Infrared (FTIR) Spectroscopy was used to investigate the optical characteristics of InP nanowire-based n+-i-n+ photodetectors with 20 embedded InAsP quantum discs in each InP nanowire. The spectrally resolved photocurrent was measured and analyzed at different angles of incidence. Also, detailed current-voltage characteristics in dark and under illumination were recorded and analyzed.Summarized, the samples showed very good I-V performance with low dark leakage currents. The photocurrent scales with the numbers of nanowires, from which we conclude that most of the photocurrent is generated in the nanowires. Spectrally resolved photocurrent data, recorded at room-temperature, shows strong absorption in the near-infrared region with interesting peaks that reveal, the underlying optical processes in the substrate and nanowires.

Graphene, a one-atom-thick sheet of hexagonally packed carbon atoms, is a novel material with high electron mobility due to its unique linear and gapless electronic band structure. Its broadband absorption and unusual doping properties, along with superb mechanical flexibility make graphene of promising application in optoeletronic devices such as solar cell, ultrafast photodetectors, and terahertz modulators. How- ever, the current performance of graphene-based devices is quite unacceptable owning to serious limitations by its inherently small absorption cross section and low quan- tum efficiency. Fortunately, nanoscale optical antennas, consisting of closely spaced, coupled metallic nanoparticles, have fascinating optical response since the collective oscillation of electrons in them, namely surface plasmons, can concentrate light into a subwavelength regime close to the antennas and enhance the corresponding field considerably. Given that optical antenna have been applied in various areas such as subwavelength optics, surface enhanced spectroscopies, and sensing, they are also able to assist graphene to harvest visible and near-infrared light with high efficiency. Moreover, the efficient production of hot electrons due to the decay of the surface plasmons can be further implemented to modulate the properties of graphene. Here we choose plasmonic oligomers to serve as optical antenna since they pos- sess tunable Fano resonances, consisting of a transparency window where scattering is strongly suppressed but absorption is greatly enhanced. By placing them in di- rect contact with graphene sheet, we find the internal quantum efficiency of hybrid antenna-graphene devices achieves up to 20%. Meanwhile, doping effect due to hot electron is also observed in this device, which can be used to optically tune the elec- tronic properties of graphene.