Monte Carlo simulations for phonon transport in silicon nanomaterials

In nanostructures phonon transport behaviour is distinctly different to transport in bulk materials such that materials with ultra low thermal conductivities and enhanced thermoelectric performance can be realized. Low thermal conductivities have been achieved in nanocrystalline materials that include hierarchical sizes of inclusions and pores. Nanoporous structures present a promising set of material properties and structures which allow for ultra-low thermal conductivity, even below the amorphous limit. In this paper we outline a semiclassical Monte Carlo code for the study of phonon transport and present an investigation of the thermal conductivity in nanoporous and nanocrystalline silicon. Different disordered geometry configurations are incorporated to investigate the effects of pores and grain boundaries on the phonon flux and the thermal conductivity, including the effects of boundary roughness, pore position and pore diameter. At constant porosity, thermal conductivity reduction is maximized by having a large number of smaller diameter pores as compared to a small number of larger diameter pores. Furthermore, we show that porosity has a greater impact on thermal conductivity than the degree of boundary roughness. Our simulator is validated across multiple simulation and experimental works for both pristine silicon channels and nanoporous structures.Comment: 10 pages, 8 figure

Sarajevo Incident

The Sarajevo incident refers to the events surrounding the assassination of Archduke Franz Ferdinand, heir to the Austro-Hungarian throne, and his wife Archduchess Sophie during a state visit to Sarajevo on 28 June 1914. It is traditionally regarded as the immediate catalyst for the First World War

Propaganda at Home and in Exile (South East Europe)

This article explores the nature of propaganda in those South East European states and territories that participated in the First World War, specifically Bulgaria, Greece, Montenegro, Romania, Serbia, along with Yugoslav separatists from the Austria-Hungary’s South Slavonic territories. During this period, propaganda essentially represented a continuation of pre-existing ideological narratives, often centred on vague, patriotic shared notions of ethno-national unity through territorial aggrandizement or secession. However, the widely differing war aims among regional parties resulted in these narratives becoming increasingly dominated by the war’s more immediate political contexts or specific domestic concerns. This growing divergence was accentuated by the diversity of wartime experiences, such as foreign occupation or internal division, among the belligerents. Nevertheless, a number of thematic similarities existed around narratives such as honour, sacrifice, and national defence

Corfu Declaration

The Corfu Declaration was a formal agreement between the government-in-exile of the kingdom of Serbia and the Yugoslav Committee (anti-Habsburg South Slav émigrés) that pledged to unify Serbia with Austria-Hungary’s South Slav territories in a post-war Yugoslavian state. It was signed on 20 July 1917 on the island of Corfu

Trumbić, Ante

Ante Trumbić was a Croat politician who played an instrumental role in the formation of the Kingdom of Serbs, Croats and Slovenes (Yugoslavia) in December 1918. Between 1905 and 1934, he served in a range of influential posts including as mayor of Split; chairman of the anti-Habsburg “Yugoslav Committee”; and as the kingdom’s first foreign minister

Reviving the Völkerabfälle:The South Slavonic Left, Balkan Federalism and the Creation of the First Yugoslavia

This article explores how the Southern Slavs, decried as Völkerabfälle by Engels in 1849, managed nevertheless to develop a distinctively social­ist movement and culture of their own, particularly from 1903 to 1914, capable of both challenging and shaping politics in the Balkans. Although heavily influenced by Marxist theoretical currents and external ideas such as Austro-Marxism, the formation of this South Slavonic Left was rooted in the social and historic contexts of its adherents’ respective homelands. Limited industrialisation, coupled with the rise of rival political movements such as nationalism and peasant agrarianism, prompted many on the Left to turn to the region’s early socialist heritage, specifically the philosopher Svetozar Markovic’s concept of Balkan Federalism. As well as providing a means by which the region could begin to modernise through closer economic and political cooperation, the perceived threat of Austro-Hungarian and Italian expansionist ambitions legitimised the left-wing belief that a Balkan Federation was now essential to the future preservation of regional identity and political freedoms. Consequently, the creation of the first Yugoslavian state in December 1918 was welcomed as the first step to fulfilling these goals

British Medical Volunteers and the Balkan Front 1914-1918: The Case of Dr Katherine Stuart MacPhail

This paper examines the British presence on the First World War’s Balkan Front in the British popular imagination with a particular focus on the lesser-known woman humanitarian Doctor Katherine Stuart MacPhail. Allied military inertia from 1915 to 1918 led to the British presence in the Balkan theatre being mainly associated through the large number of medical aid volunteers, the majority of whom were women, defining it through a civilian rather than military paradigm. Having been unable to secure a career within the medical establishment of her native Glasgow, Dr MacPhail served as a volunteer doctor in both the Balkans and France during the war. Her experiences of living and working amongst the peasants of Serbia and Macedonia inspired her to remain in the region following the armistice in the newly created Kingdom of Yugoslavia. Amongst her achievements was the establishment of Serbia’s first children’s hospital in 1919. Whilst the example of MacPhail was not typical of the average medical volunteer, it represented a broader trend of record. Unlike many of her contemporaries she did not leave any precise record detailing her own views or ambitions. In Britain her name and mention of her work tended to reach wider attention only through the accounts of her more high-profile contemporaries. However it serves as a prime illustration of a unique historical episode in Anglo-Balkan cultural contact in which issues of gender, the changing role of humanitarianism in war and wider public investment within an Allied campaign primarily viewed as pointless, coalesced. It also reflected the limitations such an extraordinary yet brief historical context imposed on this equally extraordinary situation illustrated by the sudden change in MacPhail’s own fortunes following the war’s conclusion

User Experience Design for Presence-Aware Spaces and Technologies

User experience design is a diverse field of study that is constantly changing as unique technologies and modes of interaction are developed. Metaphors are a critical aspect of UX design, serving to acclimate users to new technologies by comparing them to existing objects and ideas. As newer technologies become increasingly distant from real-world objects, developers are quick to look to existing technology for metaphors. This results in a lack of experience-unique metaphors that would create a more immersive experience. This thesis focused on identifying potential real-world metaphors through the use of emerging technologies in an interactive art installation. Based on observations and participant responses, it was clear that the installation was successful at establishing an engaging user experience. However, findings exposed that this experience was facilitated not by metaphor, but by stimulation more along the lines of mimicry. Though different than the initial objective, this discovery was profound due to the implications it holds for developing presence-aware technologies and spaces in the future

On the influence of physical and chemical structure on charge transport in disordered semiconducting materials and devices

Achieving fast charge carrier transport in disordered organic semiconductors is of great importance for the development of organic electronic devices. Disordered organic materials generally show low charge carrier mobilities due to their inherent energetic and configurational disorder, and the presence of chemical and physical defects. Efforts to improve mobility typically involve chemical design and materials processing to control macromolecular conformation and/or induce greater crystalline or liquid crystalline order. Whilst in many cases fruitful, these approaches have not always translated into higher bulk mobilities in devices. Addressing the adverse effect on mobility of specific types of disorder or specific defects has proven difficult due to problems distinguishing the many such features spectroscopically and controlling their formation in isolation. In the three experimental Chapters following, we attempt to make clear links between the charge carrier mobility and the presence of specific structural defects or sources of energetic or configurational disorder. In the first experimental study, we investigate hole transport in a family of polyfluorenes based on poly(9,9-dioctylfluorene) (PFO). By controlling the phase formation of the materials through processing and by virtue of their chemical design, we examine the effect on transport of distinct material phases. Remarkably, we are able to isolate the effect of the single chain conformation of PFO known as the beta-phase and show that when embedded in a glassy PFO matrix it acts as a strong hole trap, reducing the mobility of the bulk material by over two orders of magnitude. By fabricating a device with negligible beta-phase, we demonstrate the highest time-of-flight mobility in PFO to date, at over 3 10-2 cm2/Vs. This study provides the first clear and unambiguous example of the effect on transport of a distinct conformational defect in a conjugated polymer. We also demonstrate the adverse effect on mobility of crystallinity in the polyfluorenes. We suggest that our findings may generalise to other systems in the sense that the mobility may be limited by a minority population of structural traps, which may include highly ordered, crystalline regions. Significant mobility improvements may then be more easily achieved by removing the minority ordered phases than by increasing their concentration. We believe that this approach offers an alternative paradigm by which higher mobilities may be obtained in general, and in particular in systems where crystallinity is undesirable. In the second experimental study, we study charge transport in the fullerene derivatives [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), bis-PCBM and tris-PCBM. The fullerene multi-adducts bis-PCBM and tris-PCBM are of interest as alternative OPV acceptor materials with the potential to increase open-circuit voltage. However, most OPV blends employing the multi-adducts have failed to improve upon those employing PCBM. This is thought to be a result of the inferior electron transport properties of the multi-adducts, due to either (i) higher energetic disorder in the multiadducts due to the presence of isomers with varying LUMO energies or (ii) higher con gurational disorder due to a lower degree of order in molecular packing in the multi-adducts than in PCBM. We distinguish the e ects of energetic and con gurational disorder using temperature-dependent ToF and FET measurements. We find that differences in configurational disorder appear negligible, and that the reduced mobility in the multi-adducts is due predominantly to the energetic disorder resulting from the presence of a mixture of isomers with varying LUMO energies. In the third and final experimental study, we examine the charge transport properties of polymer: PCBM blends for OPV, focusing on the PTB7:PCBM and P3HT:PCBM systems. In particular, we address the question of why state-of-the-art OPV systems such as PTB7:PCBM perform so much worse at large active layer thicknesses than P3HT:PCBM. We find that low electron mobility is the main cause of this di erence. The electron mobility in PTB7:PCBM blends, at 10-5 { 10-4 cm2/Vs, is 1-2 orders of magnitude lower than the electron mobility in annealed P3HT:PCBM, at over 10-3 cm2/Vs. The hole mobility, in contrast, is the same to within a factor of approximately three. We hypothesise that the low tendency of PTB7 to order leads to a low degree of phase separation in the blend and to a poorly connected, disordered PCBM phase. We find that increasing the PCBM fraction is very effective in improving electron transport and electrical Fill Factor, but strongly reduces absorption. We suggest that a key challenge for OPV researchers is thus to achieve better connectivity and ordering in the fullerene phase in blends without relying on either (i) a large excess of fullerene or (ii) strong crystallisation of the polymer