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The research described in this thesis is aimed at constructing a quantum interface between a single electron spin and a photon, using a nanowire quantum dot. Such a quantum interface enables information transfer from a local electron spin to the polarization of a photon for long distance readout. Important aspects for the realization of such a device are the optical quality of the quantum dot and access to the optical polarization. In this thesis, we demonstrate high quality nanowire quantum dots and observe spin memory. By embedding them in a vertical nanowire device, we are able to isolate a single electron on the dot, while maintaining access to the intrinsic polarization properties of the nanowire quantum dot. These results demonstrate that quantum dots in vertical nanowire device are promising as an optical interface for single electron spins.

In this Master thesis I report results on a route to find Majorana fermions in indium antimonide nanowires in contact with a superconductor. Theoretically Majorana fermions appear in one-dimensional nanowires with strong spin-orbit coupling, in proximity with a superconductor and an external magnetic field applied parallel to the nanowire. The nanowires are deposited by a deterministic method, in this way the external magnetic field is perfect aligned with the nanowires up to a few degrees. Results we observed are a possible magnetic field tunable pi-junction, measurements of an induced gap in the nanowire and a robust zero-bias peak that persist in both gate and magnetic field scans. This zero-bias peak can be split and recombine with varying the applied magnetic field and the local gate potential.

An important goal for nanoscale opto-electronics is the transfer of single electron spin states into single photon polarization states (and vice versa), thereby interfacing quantum transport and quantum optics. Such an interface enables new experiments in the field of quantum information processing. Single and entangled photon-pair generation can be used for quantum cryptography. Furthermore, photons can be used in the readout of a quantum computer based on electron spins. Semiconducting nanowires are a suitable electron (hole) channel, as they combine confinement of electrons (holes) in two dimensions with carrier transport in the third dimension. In addition, the small nanowire diameter allows for the combination of semiconductors with different lattice constants. Such heterostructures can be used to locally confine electrons and holes along the nanowire, creating an optically active quantum dot. Nanowire quantum dots are therefore a zero dimensional opto-electrical element embedded in a one dimensional electrical transport channel, which is ideal for quantum opto-electronics. In this thesis, we report a number of steps towards an electron spin to photon polarization interface based on nanowire quantum dots. First we develop single InAs0.25P0.75 quantum dots embedded in InP nanowires. We show that the nanowire quantum dots have optical emission linewidths as narrow as about 30 microeV. Due to the narrow emission lines, we are able to resolve individual spin states at magnetic fields of the order of 1 Tesla. We can prepare a given spin state by tuning the excitation polarization or excitation energy. To realize an electron-photon interface in a functional opto-electrical device, we contact the nanowires to obtain InP nanowire photodetectors with a single InAsP quantum dot as light absorbing element. For photon energies above the InP band gap, the nanowire photodetectors have a quantum efficiency of 4 %. Under resonant excitation of the quantum dot, the photocurrent amplitude depends on the polarization of the incident light. The photocurrent is enhanced (suppressed) for a linear polarization parallel (perpendicular) to the axis of the nanowire (contrast 0.83). The active detection volume under resonant excitation is 7 10^(-3) nm^(-3). These results show the promising features of quantum dots embedded in nanowire devices for electrical detection of light with a high spatial resolution. Next, we apply an electric field to induce single electron charging effects in the nanowire quantum dot. We perform optical experiments of a charge tunable, single nanowire quantum dot, in which the charge state is tuned with two independent voltages. First, we control tunneling events through an applied electric field along the nanowire growth direction. Second, we modify the electrochemical potential in the nanowire with a back-gate. We combine these two field-effects to isolate a single electron and independently tune the tunnel coupling of the quantum dot with the contacts. Such charge control is a requirement for opto-electrical experiments involving a single electron spin in a nanowire quantum dot. We successively develop lateral gates next to the optically active nanowire quantum dots. By applying a positive potential to both lateral gates, we observe energy modifications of the emission when one and two electrons are residing in the quantum dot. The energy shifts are explained by a reduction of the electron-electron Coulomb and s-p exchange interactions. In addition, we present large biexciton emission energy control when a lateral electric field is applied to the quantum dot. Here, the emission energy of the biexciton can be tuned to the same energy as the exciton emission energy, a key result for entangled photon pair generation. The coupling of the lateral gates to the negatively charged exciton is promising for future electron spin manipulation experiments in optically active nanowire quantum dots. To move towards on-chip excitation of the quantum dot, we present reproducible fabrication of InP-InAsP nanowire light emitting diodes in which electron-hole recombination is restricted to the quantum-dot-sized InAsP section. The nanowire geometry naturally self-aligns the InAsP section with the n-InP and p-InP ends of the wire, making these devices promising candidates for electrically-driven quantum optics experiments. We have investigated the operation of these nano-LEDs with a consistent series of experiments at room temperature and at 10 K, demonstrating the potential of this system for on-chip sources of single photons. Finally, we present the method to scale up the nanowire quantum dot synthesis in a regular array. We show single-photon and cascaded photon pair emission in the infrared, originating from a single InAsP quantum dot embedded in a standing InP nanowire. To perform electron spin manipulation and optical read-out, it is necessary to reduce the optical emission linewidth of the contacted nanowire quantum dots.

Conducting polymers are attractive chemical sensing materials due to their outstanding characteristics including low cost, room-temperature operations, easy device fabrication, high sensitivity and short response time. The new nanowires architecture, with high surface-to-volume ratio, makes possible the conducting polymers an ultra fast detection of chemical at low concentrations. Polymer-coated nanowires are thus the potential cost effective solution for the new generation gas sensors. As a sensing material, the molecular design of the conducting polymer is utterly important. The conductive polymers can be tailored to fulfill the sensing requirement by its modifying functional groups in accordance to the applications. Molecular modeling which predicts the material properties of conductive polymers helps in the design of the sensor material. In this thesis, I present a molecular modeling approach to design and evaluate conducting polymer as chemical sensing material for polymer nanowire or polymer-coated nanowire carbon dioxide (CO2) sensors in greenhouse application. In order to provide an overview of the rapid progress in the application of chemical sensing materials with nanowire architecture, literature study on nanowire gas sensors has been presented in the Chapter 2. A comparison between the two basic approaches (top-down and bottom-up) in the nanowire synthesis is given. The sensing principles and configurations of nanowire gas sensors with their relevant assembly technologies are summarized. Based on the review work, a polyaniline-coated nanowire field-effect transistor (NanoFET) is proposed for CO2 sensing system in greenhouse. This sensor set combines the advantages of nanowire architecture, FET sensor configuration and conducting polymers. A crucial part of any molecular simulation study is the choice of forcefields. In Chapter 3, we evaluate the validity of COMPASS and PCFF forcefields in predicting the physical and thermophysical properties of amorphous polymer emeraldine based polyaniline (EB–PANI). A combination of molecular mechanics (MM) and molecular dynamics (MD) analysis is employed to determine the polymer’s properties, including density (ρ) and solubility parameter (δ). The temperature dependence of specific volume (υ), non-bond energy (Enon-bond) and solubility parameters are used to estimate the glass transition temperature (Tg). Comparing the simulation results with experimental data, the accuracy of forcefields (COMPASS and PCFF) is elucidated. The COMPASS forcefield has been demonstrated as a better forcefield which provides a closer agreement with experiment data than the PCFF. Thus, the molecular modeling design of PANI for CO2 sensing is conducted by using the COMPASS forcefield. For effective sensing, the dissolution of an analyte, as quantified as the solubility parameter δ, in the sensing materials is crucial. Understanding of the temperature dependence of solubility parameter can provide adequate information for the sensitivity issue induced as the temperature changes. In Chapter 4, I have developed a compact model to describe the solubility parameter change due to the temperature impacts. It is showing that in the working temperature range of greenhouse, the temperature impact on the solubility parameter is limited and can be neglected. To verify the accuracy of our calculation, two kind of analysis has been are performed: (i) the δ value at 298 K for EB–PANI is predicted and compared with the literature reported data; (ii) the Tg of the polymer is determined from the δ–T curve and compared with the experimental value. The temperature dependence of solubility parameter of the EB-PANI has been determined by molecular modeling approach. The sensing mechanism of the PANI for CO2 materials is based on protonic acid doping. Molecular modeling of the sensing mechanism can offer useful information for the sensitivity and the selectivity of PANI. In Chapter 5, a compact model has been developed to describe the protonic acid doping of PANI with reasonable accuracy. The atomistic model is developed by using a statistical thermodynamic analysis method. The molecular modeling method is comprised of three key steps: (i) developing the atomistic models; (ii) defining the doing criteria; and (iii) simulating the protonic acid doping. By using the molecular model, the relationships including pKa/pH and doping percentage/pH are established. The computed results compare favorably with the reported experimental data. The change of charge carrier density causes the changes in the conductivity of the gas-sensitive conducting polymers. Thus, the relationship between macroscopic conductivity and charge carrier density is very useful in the design and evaluation of PANI as chemical sensing materials. In Chapter 6, by using the molecular model derived from Chapter 5, the relationships include the charge carrier density/pH and the conductivity/charge carrier density of EB-PANI are established properly. It is to find that the conductivity has an exponential function relationship with the charge carrier density [σ = (A*n)a] in PANI. Using the computing relationship of conductivity/charge carrier density, the sensitivity of EB-PANI and its derivative K-SPANI for the detection of HCl is evaluated. The finding shows that by introducing function groups (–SO3K), the sensitivity of K-SPANI is greatly improved by two times. Thus the conducting polymer K-SPANI is a good candidate for acidic gas sensing, such as HCl, H2S, or CO2 in high humidity conditions. With the fundamental knowledge established in Chapters 3-6, the molecular design of PANI for greenhouse CO2 gas sensing can be achieved. Chapter 7 investigates the effect of functional group on the working range of polyaniline sensors for CO2 in agriculture industry. The humidity, temprature and the concentration of CO2 in the tightly clad greenhouses have been considered in the molecular model. The work compares the response of the pure EB, the polymer mixture of EB-PANI and undoped sodium sulfonated polyaniline (NaSPANI) with sulfur to nitrogen ratio (S/N) of 0.6, 0.5 and 0.4 to CO2. Under the working condition in a greenhouse, the working range of NaSPANI has been estimated as ~ [102- 104] ppm which demonstrates it is a good candidate for CO2 detection in agricultural industry. In considering the synthetic difficulty, I propose the conducting polymer NaSPANI (S/N = 0.5) is a good candidate for agricultural CO2 sensing. In summary, a molecular modeling method which helps in the design and evaluation of conductive polymers for carbon dioxide sensing in greenhouses has been established. This thesis work contributes at use of computational approaches in designing and optimizing chemical sensing materials for various applications.

For small magnetic memories, the ultimate limit is a single magnetic particle, a single electron spin. A lot of research is put in developing devices that can utilize such a single spin memory. At the same time, there is also interest from a more fundamental point of view: on these small scales, Quantum Mechanics starts to play a role. Devices of this size have unique possibilities that have no parallel in the classical world around us. In this work we study a single electron spin that is captured in a small box, a quantum dot. This is all made inside a semiconducting nanowire. By measuring the current we can determine how the spin orients in a magnetic field. We can also see interactions of the spin with its environment. We study how the semiconductor crystal influences the spin, via spin-orbit interaction and via its nuclei. We also determine how this affects the interaction between two such spins.

This thesis describes the experimental study of devices based on single carbon nanotubes in the context of (bio)sensing in aqueous solutions. Carbon nanotubes are cylindrical molecules of sp2- carbon, about one nanometer in diameter and typically several micrometers long, which have semiconducting or metallic electronic properties. Nanotube devices can interact both electrostatically and electrochemically with the solution and the (bio)molecules dissolved in it. We study these interactions electronically with the aim to learn how carbon nanotube devices interact with their environment and how they can be used as the active elements in highly sensitive nanoscale (bio)sensors. First, we study the electrochemical interaction of redox molecules with carbon nanotube devices. An applied potential difference over the interface between a carbon nanotube and the solution can drive the electrochemical transfer of electrons from dissolved redox molecules to the nanotube and vice versa. We demonstrate that individual carbon nanotubes, both metallic and semiconducting, can be used as nanoelectrodes for electrochemistry. Due to the small diameter of nanotubes, the relative influx of electrochemically active molecules is so high that the kinetics of charge transfer become rate limiting. We provide a theoretical description of electrochemical charge transfer at nanotube and graphene electrodes. We find that, although the distinct electronic structure of nanotubes does play a role in the charge transfer process, metallic and semiconducting nanotubes cannot readily be distinguished. Even when a semiconducting nanotube is switched OFF, charge transfer can still take place at high rates. Next we explore carbon nanotubes employed as liquid-gated field-effect transistors. Although the literature contains an increasing amount of studies that use nanotubes for sensing purposes, a thorough fundamental understanding of how exactly these transistors interact with their environment is lacking. We elucidate and demonstrate several physical mechanisms that allow nanotubes to act as nanoscale electrostatic sensors. We show that the sensor response can be affected by an artifact related to the reference electrode. By eliminating this artifact we can study the effect of biomolecule adsorption near nanotube sensors unambiguously. Then we describe a method to identify the different mechanisms that can lead to a sensor response. We find that the origin of sensor response to biomolecule adsorption is a combination of a change in surface potential, and alterations to the tunnel barrier at the nanotube-metal contact. Contact effects make sensing unreliable, but these can be suppressed by covering up the contact regions. Finally, we show that carbon nanotube and graphene transistors are sensitive to changes in the ionic strength, the pH, and even the type of ions of the electrolyte. Changes in these electrolyte properties lead to a sensor response by changing the surface charge and the spatial distribution of ions, and thus the surface potential. We proceed by studying the signal-to-noise ratio for biosensing with liquid-gated carbon nanotube transistors. We show that the low-frequency noise is consistent with the fluctuation of nearby charges that gate the nanotube through a field-effect. The power of the noise is inversely proportional to the length of the nanotube. Surprisingly, the signal-to-noise ratio is highest in the sub-threshold regime. The decrease of the signal-to-noise ratio in ON state is related to additional noise sources and depends on device architecture. In specific cases the back gate can enhance the signal-to-noise ratio. Finally, we report our exploratory studies of carbon nanotube sensors as probes to study living cells. Although our results are suggestive that we can successfully detect cellular activity, the transistor stability and electrochemical sensitivity need to be improved. We show that the electrochemical sensitivity can be improved by coating nanotubes with catalytic nanoparticles. In conclusion, we have studied carbon nanotube devices in aqueous solution. The work presented in this thesis elucidates a number of different physical mechanisms, both electrochemical and electrostatic, through which carbon nanotube devices can interact with their environment. In addition, many of the concepts developed and studied here may be extended to other nanoscale sensors, such as nanowires and graphene. This knowledge can be used to further exploit the unique properties of carbon nanotubes, and pursue the ultimate goals of single-molecule detection and single-cell probing.