Conselho Nacional de Desenvolvimento CientÃfico e TecnolÃgico / O sistema de elÃtrons na superfÃcie de hÃlio lÃquido à considerado um dos melhores objetos na investigaÃÃo de prÃncipios fundamentais da FÃsica de baixa dimensionalidade, uma vez que estes nÃo apresentam heterogeneidades e impurezas geralmente encontradas em heteroestruturas semicondutoras. AlÃm disso, espera-se que estes sistemas tenham uma aplicaÃÃo tecnolÃgica futura como extit{bits} quÃnticos, que sÃo fundamentais na construÃÃo de blocos dos futuros computadores quÃnticos. Geralmente, as estruturas de confinamento de baixa dimensionalidade sugeridas para elÃtrons na superfÃcie de hÃlio lÃquido na literatura sÃo baseadas em superfÃcies planas, onde o confinamento lateral à induzido por um potencial externo controlado por eletrodos. No presente trabalho, à sugerida uma forma alternativa de produzir o confinamento lateral em superfÃcies de hÃlio lÃquido, isto Ã, à demonstrado que a forma da superfÃcie pode ser projetada para produzir anÃis, fios e pontos quÃnticos apenas variando a forma da cavidade do substrato no qual o hÃlio lÃquido se encontra suspenso. A superfÃcie foi calculada para quatro formas diferentes de cavidade: i) uma cavidade cilÃndrica, gerando um ponto quÃntico simples; ii) uma cavidade em forma de anel circular, gerando um anel quÃntico; iii) duas cavidades em forma de canais que se interceptam perpendicularmente, formando um ponto quÃntico simples no ponto de interceptaÃÃo; e iv) duas cavidades retangulares conectadas por um canal estreito, gerando um ponto duplo. O elÃtron à entÃo depositado sobre a superfÃcie e confinado a se mover em cada superfÃcie devido à aÃÃo de um campo elÃtrico externo. Os resultados apresentados aqui mostram que os nÃveis de energia para estes sistemas podem ser alterados atravÃs da variaÃÃo do campo elÃtrico e do banho de hÃlio, que sÃo facilmente ajustÃveis. O efeito de um campo magnÃtico externo à tambÃm investigado em um destes sistemas. / The system of electrons on liquid helium (EoH) is one of the most ideal objects for investigating the fundamental principles of the physics of low dimensionality, since they do not have the inhomogeneities and impurities generally found on semiconductors. Besisdes, these systems are expected to have future technological applications as quantum bits, which are of fundamental importance as building blocks of future quantum computers. Usually, the low-dimensional confinements structures for EoH suggested in the literature are based on a planar surface, where the lateral confinement is induced by an external potential controlled by electrodes. In this work, we suggest an alternative way to produce lateral confinement in liquid helium surfaces, namely, we demonstrate that the shape of the surface can be designed to produce single and double quantum dots by adjusting the shape of a cavity in the substrate. The surface was calculated for four different shapes of substrate cavity: i) a cilyndrical cavity, generating a single quantum dot; ii) a ring-shaped cavity, generating a quantum ring; iii) two cubic cavities connected by a channel, creating a coupled double-dot structure; and iv) two channel-shaped cavities that intersect perpendicularly to each other, where a single dot is formed in the intersection point. The electron is then deposited and confined to move on each surface by an external electric field. Our results show that the electron energy levels in these systems can be tuned by varying the electric field and the bulk level, which are easily adjustably. The effect of an external magnetic field on the energy spectrum in one of these systems is also investigated.

In this conceptual report the idea to establish an European Positron Source for Applied Research ("EPOS") based on new LINAC facilities in Germany (ELBE/Rossendorf or TTF-DESY/Hamburg) is considered. The report contains not only the outline of obvious applications in atomic physics, materials science and surface physics, but also several new methodical developments which are only possible with an intense positron beam. This opportunity will also allow the use and further development of imaging techniques being of special interest for industrial applications.

No description available.

Conductive domain walls (DWs) in lithium niobate (LiNbO3, LNO) are promising constituents for potential applications in nanoelectronics, due to their high conductance, as compared to the surrounding bulk material, their high local confinement at the nanometer scale, and the ability to be created quasi-on-will through dedicated high-voltage poling. However, electrically contacting the DWs unavoidably leads to the formation of a potential barrier between the DW itself and the electrode material. Thus, the focus of this work is the investigation of the various factors influencing the electronic transport across that barrier, namely, the type of electrode material, the quality of the LNO surface (atomically-smooth versus mirror polished), the quality of the crystal lattice (i.e., the presence of higher concentrations of lithium and oxygen vacancies VLi and VO), and the magnitude of the applied voltages during the domain-wall conductivity (DWC) enhancement procedure. It is found that all the above-mentioned factors have a significant impact on the current-voltage characteristics of the DW-electrode system. For example, the metal electrodes deposited onto the surface of the LNO crystal, once, impede the DW motion, while, secondly, stabilizing the DWs inclination across the LNO crystal. Another important finding is the major role played by large negative voltages in the DWC-enhancement procedure that strongly influences the near-surface structure of the DW, and hence the qualitative characteristics of the formed potential barrier, such as characteristic voltage and saturation current. The application of moderate voltages from –50 V to –100 V is also found to influence the structure of the near-surface DW. The creation of a variety of vacancy defects inside the bulk LNO that accompanies the formation of an atomically-smooth surface, is found to have far more influence on the DW charge transport than the quality of the surface, due to the formation and repulsive interaction of a multitude of spike domains stemming from these defects. In summary, the results demonstrate the importance of providing known and reproducible sample surface conditions and identifying promising directions for implementing reproducible domain wall conductivity.

In this work, physical phenomena related to the growth and phase formation of alumina, Al2O3, are investigated by experiments and computer calculations. Alumina finds applications in a wide variety of areas, due to many beneficial properties and several existing crystalline phases. For example, the α and κ phases are widely used as wear-resistant coatings due to their hardness and thermal stability, while, e.g., the metastable γ and θ phases find applications as catalysts or catalyst supports, since their surface energies are low and, hence, they have large surface areas available for catalytic reactions. The metastable phases are involved in transition sequences, which all irreversibly end in the transformation to the stable α phase at about 1050 °C. As a consequence, the metastable aluminas, which can be grown at low temperatures, cannot be used in high temperature applications, since they are destroyed by the transformation into α. In contrast, α-alumina, which is the only thermodynamically stable phase, typically require high growth temperatures (~1000 °C), prohibiting the use of temperature sensitive substrates. Thus, there is a need for increasing the thermal stability of metastable alumina and decreasing the growth temperature of the α phase. In the experimental part of this work, hard and single-phased α-alumina thin films were grown by magnetron sputtering at temperatures down to 280 °C. This dramaticdecrease in growth temperature was achieved by two main factors. Firstly, the nucleation stage of growth was controlled by pre-depositing a chromia “template” layer, which is demonstrated to promote nucleation of α-alumina. Secondly, it is shown that energetic bombardment was needed to sustain growth of the α phase. Energy-resolved mass spectrometry measurements demonstrate that the likely source of energetic bombardment, in the present case, was oxygen ions/atoms originating from the target surface. Overall, these results demonstrate that low-temperature α-alumina growth is possible by controlling both the nucleation step of growth as well as the energetic bombardment of the growing film. In addition, the mass spectrometry studies showed that a large fraction of the deposition flux consisted of AlO molecules, which were sputtered from the target. Since the film is formed by chemical bonding between the depositing species, this observation is important for the fundamental understanding of alumina thin film growth. In the computational part of the work, the effect of additives on the phase stability of α- and θ-alumina was investigated by density functional theory calculations. A systematic study was performed of a large number of substitutional dopants in the alumina lattices. Most tested dopants tended to reverse the stability between α- and θ-alumina; so that, e.g., Modoping made the θ phase energetically favored. Thus, it is possible to stabilize the metastable phases by additives. An important reason for this is the physical size of the dopant ions with respect to the space available within the alumina lattices. For example, large ions induced θ stabilization, while ions only slightly larger than Al, e.g., Co and Cu, gave a slight increase in the relative stability of the α phase. We also studied the stability of some of these compounds with respect to pure alumina and other phases, containing the dopants, with the result that phase separations are energetically favored and will most likely occur at elevated temperatures.

The work presented in this thesis deals with experimental and theoretical studies related to the growth of crystalline alumina thin films. Alumina, Al2O3, is a polymorphic material utilized in a variety of applications, e.g., in the form of thin films. Many of the possibilities of alumina, and the problems associated with thin film synthesis of the material, are due to the existence of a range of different crystalline phases. Controlling the formation of the desired phase and the transformations between the polymorphs is often difficult. In the experimental part of this work, it was shown that the thermodynamically stable alpha phase, which normally is synthesized at substrate temperatures of around 1000 °C, can be grown using reactive sputtering at a substrate temperature of 500 °C by controlling the nucleation surface. This was done by predepositing a Cr2O3 nucleation layer. Moreover, it was found that an additional requirement for the formation of the α phase is that the depositions are carried out at low enough total pressure and high enough oxygen partial pressure. Based on these observations, it was concluded that energetic bombardment, plausibly originating from energetic oxygen, is necessary for the formation of α alumina (in addition to the effect of the chromia nucleation layer). Further, the effects of impurities, especially residual water, on the growth of crystalline films were investigated by varying the partial pressure of water in the ultra high vacuum (UHV) chamber. Films deposited onto chromia nucleation layers exhibited a columnar structure and consisted of crystalline α-alumina if deposited under UHV conditions. However, as water to a partial pressure of 1x10-5 Torr was introduced, the columnar growth was interrupted. Instead, a microstructure consisting of small, equiaxed grains was formed, and the gamma-alumina content was found to increase with increasing film thickness. When gamma-alumina was formed under UHV conditions, no effects of residual water on the phase formation was observed. Moreover, the H content was found to be low (< 1 at. %) in all films. Consequently, this shows that effects of residual gases during sputter deposition of oxides can be considerable, also in cases where the impurity incorporation in the films is found to be low. In the modeling part of the thesis, density functional theory based computational studies of adsorption of Al, O, AlO, and O2 on different alpha-alumina (0001) surfaces have been performed. The results give possible reasons for the difficulties in growing the α phase at low temperatures through the identification of several metastable adsorption sites, and also provide insights related to the effects of hydrogen on alumina growth. / Report code: LiU-TEK-LIC-2007:1.

Biosensors are devices that detect biological elements and then transmit a readable signal. Biosensors can automatize diagnostics that would otherwise have to be performed by a physician or perhaps not be possible to perform at all. Current biosensors are however either limited to particular diseases or prohibitively expensive. In order to further the field, sensors capable of many parallel measurements at a lower cost need to be developed. Field effect transistor (FET) based sensors are possible candidates for delivering this, mainly by allowing miniaturization. Smaller sensors could be cheaper, and enable parallel measurements. Graphene is an interesting material to use as the channel of FET-sensors. The low electrochemical reactivity of its plane makes it possible to have graphene in direct contact with the sample liquid, which enhances the signal from impedance changes. Graphene-FET based impedance sensors should be able to sense almost all possible analytes and allow for scaling without losing sensitivity. In this work the steps needed to make graphene based biosensors are presented. An improved graphene transfer is described which by using low pressure to dry the graphene removes most contamination. A method to measure the contamination of graphene by surface enhanced Raman scattering is presented. Methods to produce double gated and electrolyte gated graphene transistors on a large scale in an entirely photolithographic process are detailed. The deposition of 1-pyrenebutyric acid (PBA) on graphene is studied. It is shown that at high surface concentrations the PBA stands up on graphene and forms a dense self-assembled monolayer. A new process of using Raman spectroscopy data to quantify adsorbents was developed in order to quantify the molecule adsorption. Biosensing has been performed in two different ways. Graphene FETs have been used to read the signal generated by a streaming potential setup. Using FETs in this context enables a more sensitive readout than what would be possible without them. Graphene FETs have been used to directly sense antibodies in high ionic strength. This sensing was done by measuring the impedance of the interface between the FET and the electrolyte.

O estudo de sistemas híbridos compostos por interfaces orgânico/inorgânico, ou sólido/líquido, tem apresentado crescente interesse nas áreas de eletroquímica e nanotecnologia. Além de objetos de pesquisas básicas, estes sistemas apresentam um potencial para inúmeras aplicações, dentre elas: línguas eletrônicas, \\emph{self-assembled monolayers} (SAMs), dispositivos fotovoltáicos, baterias alternativas, carreadores de drogas, entre outras. Em especial, o uso de superfícies de ouro em pesquisas ocorre principalmente por causa da natureza inerte deste material, permitindo explorar uma ampla quantidade de potenciais eletrostáticos que induzem a eletrólise ou reações eletroquímicas em outras superfícies. O estudo de voltametria cíclica em SAMs formadas por cadeias polipeptídicas funcionalizadas com ferroceno vem sendo realizado durante as duas últimas décadas. Recentemente, iniciou-se uma controvertida discussão a respeito do mecanismo de transporte eletrônico entre o centro oxidativo e o eletrodo destes sistemas. Alguns grupos argumentam a favor de tunelamento eletrônico do centro oxidativo, enquanto outros grupos têm proposto uma interação de pares \\emph{elétron-buraco} dos grupos amida dos peptídeos. Além disto, interfaces com sistemas primitivos, como por exemplo a água, ainda são temas de pesquisa correntes de diversos grupos de pesquisa, devido à complexidade dos resultados experimentais reportados. De forma a contribuir com estas discussões correntes na literatura, selecionamos dois problemas distintos utilizando a superfície Au(111) como base comum: i) estudo do mecanismo de transferência de carga de um peptídeo funcionalizado com ferroceno; ii) estudo das propriedades eletrônicas e estruturais da água interagindo com NaCl. Para realizarmos a investigação das propriedades eletrônicas, empregamos a Teoria do Funcional da Densidade no esquema de Kohn-Sham (KS). Para analisar as propriedades dinâmicas e estruturais, foi utilizada também a técnica de dinâmica molecular clássica (MD). A partir de diversos modelos da interação do peptídeo sobre Au(111), investigamos as densidades de estados, cargas de Löwdin e funções de onda de KS. Notamos a presença de estados eletrônicos localizados tanto sobre o ferroceno quanto sobre o ouro, ambos sempre próximos da região da energia de Fermi, em todos os casos propostos. Estes resultados sugerem um tunelamento eletrônico entre sítio do ferroceno e a superfície Au(111) como o mecanismo de transferência eletrônica. Para o caso do sal dissociado em água, investigamos e discutimos a estrutura eletrônica em diversas situações e configurações. Além disto, realizamos um estudo MD, onde observamos que o ordenamento das moléculas de água é bastante sensível à presença da superfície Au(111). Os resultados obtidos apresentam uma visão ampla dos comportamentos eletrônicos e dinâmicos de sistemas envolvendo a superficie Au(111) que discutem questões correntes na literatura. / The study of hybrid interfaces, e.g. organic/inorganic or solid/liquid, have been showing an increasing interest in electrochemistry and nanotechnology. Within this subject, there are basic and applied studies, such as electronic tongues, self-assembled monolayers (SAMs), photovoltaic devices, alternative batteries, drug carriers and others. In special, the preference for gold surface occurs due to its inert nature, allowing the exploration of a wide range of electrostatic potentials which induces electrolysis and chemical reactions in other surfaces. The Cyclic Voltammetry study in Peptide-SAMs modified by ferrocene has been investigated in the literature. In recent years, a controversy on the charge transfer mechanism in biological materials started: at one hand, there are arguments in favor of an electronic tunneling process from the oxidative center to the eletrode; on the other hand, some authors suggest electronic hopping from the amide groups of the peptides, generating an electron-hole pair that ``walks\'\' from the ferrocene to the eletrode. Furthermore, systems with primitive interfaces, such as water, are also subject of current research due to the complexity of the experimental results reported in the literature. Within this scenario, we selected two distinct problems using the surface Au(111): i) the study of charge transfer mechanism using a peptide modified by ferrocene; ii) the study of electronic and structural properties of water interacting with NaCl. In order to obtain the electronic properties, we employed the Density Functional Theory in the Kohn-Sham (KS) scheme. For the structural and dynamics properties, we also used classical molecular dynamics (MD). Based on different models for the ferrocene-peptide/Au(111) interface, we investigate the density of states, Löwdin charges and KS wavefunctions. We notice the presence of localized electronic states on the ferrocene and gold which are close to the Fermi energy in all studied cases. These results suggest an electronic tunneling from the ferrocene site to the surface Au(111) as the mechanism for the charge transfer. In the case of salt dissociated in water, we investigated the electronic properties in several different configurations. Furthermore, in a MD perspective, the orientation of the water molecules presented a high sensitivity for the Au(111) interface. These results represent a wide view of the electronic and dynamic behavior of systems using the surface Au(111) as a common subject.

<p>Thermistor material can be used to fabricate un-cooled IR detectors their figure of merit is the Temperature Coefficient of Resistance (TCR). Ge dots in Si can act as a thermistor material and they have a theoretical TCR higher than for SiGe layers but they suffer from intermixing of Si into the Ge dots. Ge dots were grown on unstrained or strained Si layers and relaxed or strained SiGe layers at temperatures of 550 and 600°C by reduced pressure chemical vapor deposition (RPCVD). Both single and multilayer structures where grown and characterized. To achieve a strong signal in a thermal detector a uniform shape and size distribution of the dots is desired. In this thesis work, an endeavor has been to grow uniform Ge dots with small standard deviation of their size. Scanning electron microscopy (SEM) and Atomic force microscopy (AFM) have been used to characterize the size and shape distribution of Ge dots. Ge contents measured with Raman spectroscopy are higher at lower growth temperatures. Simulation of TCR for the most uniform sample grown at 600°C give 4.43%/K compared to 3.85%/K for samples grown at 650°C in a previous thesis work.</p><p>Strained surfaces increases dot sizes and make dots align in crosshatched pattern resulting in smaller density, this effect increases with increasing strain.</p><p>Strain from buried layers of Ge dots in a multilayer structure make dots align vertically. This alignment of Ge dots was very sensitive to the thickness of the Si barrier layer. The diameter of dots increase for each period in a multilayer structure. When dots are capped by a Si layer at the temperature of 600°C intermixing of Si into the Ge dot occurs and the dot height decrease.</p>

<p>The work presented in this thesis deals with experimental and theoretical studies related to the growth of crystalline alumina thin films. Alumina, Al₂O₃, is a polymorphic material utilized in a variety of applications, e.g., in the form of thin films. Many of the possibilities of alumina, and the problems associated with thin film synthesis of the material, are due to the existence of a range of different crystalline phases. Controlling the formation of the desired phase and the transformations between the polymorphs is often difficult.</p><p>In the experimental part of this work, it was shown that the thermodynamically stable alpha phase, which normally is synthesized at substrate temperatures of around 1000 °C, can be grown using reactive sputtering at a substrate temperature of 500 °C by controlling the nucleation surface. This was done by predepositing a Cr₂O₃ nucleation layer. Moreover, it was found that an additional requirement for the formation of the <em>α</em> phase is that the depositions are carried out at low enough total pressure and high enough oxygen partial pressure. Based on these observations, it was concluded that energetic bombardment, plausibly originating from energetic oxygen, is necessary for the formation of <em>α</em> alumina (in addition to the effect of the chromia nucleation layer). Further, the effects of impurities, especially residual water, on the growth of crystalline films were investigated by varying the partial pressure of water in the ultra high vacuum (UHV) chamber. Films deposited onto chromia nucleation layers exhibited a columnar structure and consisted of crystalline <em>α</em>-alumina if deposited under UHV conditions. However, as water to a partial pressure of 1x10^-5 Torr was introduced, the columnar growth was interrupted. Instead, a microstructure consisting of small, equiaxed grains was formed, and the gamma-alumina content was found to increase with increasing film thickness. When gamma-alumina was formed under UHV conditions, no effects of residual water on the phase formation was observed. Moreover, the H content was found to be low (< 1 at. %) in all films. Consequently, this shows that effects of residual gases during sputter deposition of oxides can be considerable, also in cases where the impurity incorporation in the films is found to be low.</p><p>In the modeling part of the thesis, density functional theory based computational studies of adsorption of Al, O, AlO, and O2 on different alpha-alumina (0001) surfaces have been performed. The results give possible reasons for the difficulties in growing the <em>α</em> phase at low temperatures through the identification of several metastable adsorption sites, and also provide insights related to the effects of hydrogen on alumina growth.</p> / Report code: LiU-TEK-LIC-2007:1.