Structure and stability of small H clusters on graphene

The structure and stability of small hydrogen clusters adsorbed on graphene is studied by means of Density Functional Theory (DFT) calculations. Clusters containing up to six H atoms are investigated systematically -- the clusters having either all H atoms on one side of the graphene sheet (\textit{cis}-clusters) or having the H atoms on both sides in an alternating manner (\textit{trans}-cluster). The most stable cis-clusters found have H atoms in ortho- and para-positions with respect to each other (two H's on neighboring or diagonally opposite carbon positions within one carbon hexagon) while the most stable trans-clusters found have H atoms in ortho-trans-positions with respect to each other (two H's on neighboring carbon positions, but on opposite sides of the graphene). Very stable trans-clusters with 13-22 H atoms were identified by optimizing the number of H atoms in ortho-trans-positions and thereby the number of closed, H-covered carbon hexagons. For the cis-clusters, the associative H$_2$ desorption was investigated. Generally, the desorption with the lowest activation energy proceeds via para-cis-dimer states, i.e.\ involving somewhere in the H clusters two H atoms that are positioned on opposite sites within one carbon hexagon. H$_2$ desorption from clusters lacking such H pairs is calculated to occur via hydrogen diffusion causing the formation of para-cis-dimer states. Studying the diffusion events showed a strong dependence of the diffusion energy barriers on the reaction energies and a general odd-even dependence on the number of H atoms in the cis-clusters.Comment: 11 pages, 11 figures, to appear in Phys. Rev.

Theoretical study of kinks on screw dislocation in silicon

Theoretical calculations of the structure, formation and migration of kinks on a non-dissociated screw dislocation in silicon have been carried out using density functional theory calculations as well as calculations based on interatomic potential functions. The results show that the structure of a single kink is characterized by a narrow core and highly stretched bonds between some of the atoms. The formation energy of a single kink ranges from 0.9 to 1.36 eV, and is of the same order as that for kinks on partial dislocations. However, the kinks migrate almost freely along the line of an undissociated dislocation unlike what is found for partial dislocations. The effect of stress has also been investigated in order to compare with previous silicon deformation experiments which have been carried out at low temperature and high stress. The energy barrier associated with the formation of a stable kink pair becomes as low as 0.65 eV for an applied stress on the order of 1 GPa, indicating that displacements of screw dislocations likely occur via thermally activated formation of kink pairs at room temperature

Gildun á mælistikum : hvaða orðgildi er best að nota á svarmöguleika mælikvarða?

Neðst á síðunni er hægt að nálgast greinina í heild sinni með því að smella á hlekkinn View/OpenGildunarannsókn var gerð þar sem þátttakendur voru beðnir um að meta merkingu íslenskra orðagilda sem hægt er að nota á mælistiku matskvarða. Úrtak 598 Íslendinga var tekið úr 10.000 manna netpanel sem valinn var með tilviljunaraðferð úr þjóðskrá. Alls tóku 398 þeirra þátt og var svarhlutfall því 65,9%. Þátttakendur, sem voru á aldrinum 17 til 73 ára, mátu 96 orð og orðasambönd. Erlendar rannsóknir á merkingu orðagilda voru hafðar til hliðsjónar þegar orðagildi voru valin en einnig var leitað fanga í íslenskum orðabókum. Á grundvelli niðurstaðna er hægt að velja orðagildi sem hafa nákvæma merkingu og spanna þá viðhorfavídd sem verið er að mæla með jöfnu millibili

Long time scale simulation of a grain boundary in copper

doi:10.1088/1367-2630/11/7/073034 Abstract. A general, twisted and tilted, grain boundary in copper has been simulated using the adaptive kinetic Monte Carlo method to study the atomistic structure of the non-crystalline region and the mechanism of annealing events that occur at low temperature. The simulated time interval spanned 67µs at 135 K. Similar final configurations were obtained starting from different initial structures: (i) by bringing the two grains into contact without any intermediate layer, and (ii) by inserting an amorphous region between the grains. The results obtained were analyzed with a radial distribution function and a common neighbor analysis. Annealing events leading to lowering of the energy typically involved concerted displacement of several atoms—even as many as 10 atoms displaced by more than half an Ångström. Increased local icosahedral ordering is observed in the boundary layer, but local HCP coordination was also observed. In the final low-energy configurations, the thickness of the region separating the crystalline grains corresponds to just one atomic layer, in good agreemen

Effects of hole self-trapping by polarons on transport and negative bias illumination stress in amorphous-IGZO

The effects of hole injection in amorphous-IGZO is analyzed by means of first-principles calculations. The injection of holes in the valence band tail states leads to their capture as a polaron, with high self-trapping energies (from 0.44 to 1.15 eV). Once formed, they mediate the formation of peroxides and remain localized close to the hole injection source due to the presence of a large diffusion energy barrier (of at least 0.6eV). Their diffusion mechanism can be mediated by the presence of hydrogen. The capture of these holes is correlated with the low off-current observed for a-IGZO transistors, as well as, with the difficulty to obtain a p-type conductivity. The results further support the formation of peroxides as being the root cause of Negative bias illumination stress (NBIS). The strong self-trapping substantially reduces the injection of holes from the contact and limits the creation of peroxides from a direct hole injection. In presence of light, the concentration of holes substantially rises and mediates the creation of peroxides, responsible for NBIS.Comment: 8 pages, 8 figures, to be published in Journal of Applied Physic

Ideal, Defective, and Gold--Promoted Rutile TiO2(110) Surfaces: Structures, Energies, Dynamics, and Thermodynamics from PBE+U

Extensive first principles calculations are carried out to investigate gold-promoted TiO2(110) surfaces in terms of structure optimizations, electronic structure analyses, ab initio thermodynamics calculations of surface phase diagrams, and ab initio molecular dynamics simulations. All computations rely on density functional theory in the generalized gradient approximation (PBE) and account for on-site Coulomb interactions via inclusion of a Hubbard correction, PBE+U, where U is computed from linear response theory. This approach is validated by investigating the interaction between TiO2(110) surfaces and typical probe species (H, H2O, CO). Relaxed structures and binding energies are compared to both data from the literature and plain PBE results. The main focus of the study is on the properties of gold-promoted titania surfaces and their interactions with CO. Both PBE+U and PBE optimized structures of Au adatoms adsorbed on stoichiometric and reduced TiO2 surfaces are computed, along with their electronic structure. The charge rearrangement induced by the adsorbates at the metal/oxide contact are also analyzed and discussed. By performing PBE+U ab initio molecular dynamics simulations, it is demonstrated that the diffusion of Au adatoms on the stoichiometric surface is highly anisotropic. The metal atoms migrate either along the top of the bridging oxygen rows, or around the area between these rows, from one bridging position to the next along the [001] direction. Approximate ab initio thermodynamics predicts that under O-rich conditions, structures obtained by substituting a Ti5c atom with an Au atom are thermodynamically stable over a wide range of temperatures and pressures.Comment: 20 pages, 12 figures, accepted for publication in Phys. Rev.

Equilibrium shape and dislocation nucleation in strained epitaxial nanoislands

We study numerically the equilibrium shapes, shape transitions and dislocation nucleation of small strained epitaxial islands with a two-dimensional atomistic model, using simple interatomic pair potentials. We first map out the phase diagram for the equilibrium island shapes as a function of island size (up to N = 105 atoms) and lattice misfit with the substrate and show that nanoscopic islands have four generic equilibrium shapes, in contrast with predictions from the continuum theory of elasticity. For increasing substrate-adsorbate attraction, we find islands that form on top of a finite wetting layer as observed in Stranski-Krastanow growth. We also investigate energy barriers and transition paths for transitions between different shapes of the islands and for dislocation nucleation in initially coherent islands. In particular, we find that dislocations nucleate spontaneously at the edges of the adsorbate-substrate interface above a critical size or lattice misfit.Comment: 4 pages, 3 figures, uses wrapfig.sty and epsfig.st

Fitness for work after vocational rehabilitation organized by the State Social Security Institute of Iceland

Neðst á síðunni er hægt að nálgast greinina í heild sinni með því að smella á hlekkinn View/OpenObjective: The evaluation of a vocational rehabilitation programme initiated by the State Social Security Institute in Iceland (SSSI) in 1999 with the aim of reducing disability. Material and methods: New disability claimants who had been unable to work because of illness for a few months at least were referred by SSSI physicians to a multidisciplinary team for assessment of rehabilitation potentials and an advice on the appropriate type of rehabilitation. The study group included all the 109 individuals who were referred to the team in the year 2000. Data on marital status, number of children and level of education was compared with information about the Icelandic population obtained in a national survey. The outcome of the rehabilitation was assessed in a telephone survey, carried out by the Social Science Research Institute, University of Iceland, 1-2 years after the assessment and by information obtained from the disability register at SSSI. The effectiveness of the rehabilitation programme was evaluated by comparing the study group with a comparable group that had started to receive rehabilitation pension before the SSSI could offer vocational rehabilitation. Their progress was assessed a year and half after they had contacted the SSSI, the same length of time as the study group had been in the in the rehabilitation programme. Results: In the study group there were about twice as many women as men. The mean age was 35 years (range 18-57 years). The main medical reasons for referral to the team were musculosceletal and psychiatric disorders. Those evaluated were more likely to be unmarried or divorced, had more children and a lower educational level than the general Icelandic population. After evaluation 40 individuals were referred to vocational rehabilitation for approximately 2 months in a rehabilitation clinic; 19 were referred to a 6 week personal computer training at a vocational rehabilitation centre and 15 to a longer (usually 18 months) rehabilitation program in the same centre. In all, 46 individuals received other treatment or education. Almost three quarters (72%) of the participants in the telephone survey said that their fitness for work had increased after rehabilitation, but only 47% had returned to work. At the time of the reserach, 23% were students and it is likely that a part of them will return to work when their studies are completed. Between one and two years after the evaluation by the multidisciplinary team 44 out of 109 (40.4%) in the study group received disability pension and a equal number received no social insurance benefits at all. In the comparison group 97 out of 119 (81.5%) received disability pension and 21 (17.7%) received no social insurance benefits at all. Conclusions: This study shows that vocational rehabilitation organized by the SSSI is effective and can prevent disability. The results of this study are similar to the results of two Swedish studies on the same topic.Tilgangur: Markmiðið með rannsókninni var að meta árangur starfsendurhæfingar á vegum Tryggingastofnunar ríkisins (TR) og hvort hún geti komið í veg fyrir örorku. Efniviður og aðferðir: Læknar TR vísuðu einstaklingum sem verið höfðu óvinnufærir í nokkra mánuði vegna sjúkdóms til þverfaglegs teymis, til mats á möguleikum á endurhæfingu og væri mælt með henni, hvernig henni yrði þá best háttað. Aflað var upplýsinga úr skýrslum endurhæfingarmatsteymisins um sjúkdómsástand, kyn, aldur, hjúskaparstöðu, fjölda barna á framfæri og menntunarstig þeirra einstaklinga sem metnir voru á árinu 2000. Upplýsingarnar um hjúskaparstöðu, fjölda barna á framfæri og menntunarstig voru bornar saman við upplýsingar um þjóðina frá sama ári (svör við Þjóðmálakönnun Félagsvísindastofnunar Háskóla Íslands um hjúskaparstöðu og barnafjölda og upplýsingar frá Hagstofu Íslands um menntunarstig). Upplýsingar um starfshæfni og vinnu þátttakenda eftir endurhæfingu voru fengnar með símakönnun sem Félagsvísindastofnun framkvæmdi. Fengnar voru upplýsingar úr gagnasafni TR um hvort þeir sem metnir höfðu verið af endurhæfingarmatsteymi á árinu 2000 voru á bótum frá TR einu til tveimur árum eftir matið. Til samanburðar var skoðuð bótastaða einstaklinga sem fengið höfðu endurhæfingarlífeyri áður en starfsendurhæfing á vegum TR hófst. Niðurstöður: Af þeim 109 sem metnir voru af endurhæfingarmatsteymi á árinu 2000 voru konur í talsverðum meirihluta (um helmingi fleiri en karlar). Meðalaldur matsþega var 35 ár. Læknisfræðilegar forsendur tilvísana til matsteymis voru fyrst og fremst stoðkerfisraskanir og geðraskanir. Matsþegar voru í meiri mæli ógiftir eða fráskildir, höfðu fleiri börn á framfæri og lægra menntunarstig en Íslendingar almennt. Fjörutíu matsþegum var vísað í atvinnulega endurhæfingu á Reykjalundi, 19 á tölvunámskeið hjá Hringsjá (starfsþjálfun fatlaðra) og 15 í fullt starfsnám hjá Hringsjá, en 46 fóru í aðra meðferð eða nám. Tæplega þrír fjórðu þátttakenda í símakönnun Félagsvísindastofnunar töldu starfshæfni sína hafa aukist, en innan við helmingur þátttakenda taldi sig þó vinnufæran (43,6%) og hafði stundað einhverja vinnu (46,9%). Auk þess var nær fjórðungur (22,8%) þátttakenda í námi og er líklegt að hluti þeirra skili sér út á vinnumarkaðinn að því loknu. Einu til tæplega tveimur árum eftir að mati lauk fengu 44 af 109 (40,4%) örorkubætur (örorkulífeyri eða örorkustyrk) frá TR og jafnmargir fengu engar bætur frá TR, en í samanburðarhópi fólks sem fengið hafði endurhæfingarlífeyri áður en starfsendurhæfing á vegum TR hófst fengu 97 af 119 (81,5%) örorkubætur, en 21 (17,7%) engar bætur frá TR hálfu öðru ári eftir upphaf lífeyrisgreiðslu. Ályktanir: Árangur af starfsendurhæfingu á vegum TR er svipaður og af starfsendurhæfingu á vegum Tryggingastofnana í Svíþjóð. Árangurinn sýnir að væntingar um að starfsendurhæfing geti komið í veg fyrir örorku hafa staðist

Opinions of clients on vocational rehabilitation organized by the State Social Security Institute of Iceland

Neðst á síðunni er hægt að nálgast greinina í heild sinni með því að smella á hlekkinn View/OpenObjectives: Evaluation of opinions of those evaluated by a multidisciplinary team on the evaluation, vocational rehabilitation they participated in and on the effect of the process on their self-confidence and self-reliance. Material and method: The Institute of Social Sciences carried out a telephone survey in October 2001, where it was attempted to contact the 109 individuals evaluated for rehabilitation potential by a multidisciplinary team in the year 2000. The data was analysed using descriptive statistics. Results: 83 (76.1%) replied. After the evaluation 40 individuals were referred to vocational rehabilitation for approximately 2 months in a rehabilitation clinic; 19 were referred to a 6 week personal computer training at a vocational rehabilitation centre and 15 to a longer (usually 18 months) rehabilitation program in the same centre. Approximately 80% were content with the vocational rehabilitation offered. 54% of those evaluated by the multidisciplinary team said that it had been useful for them to meet the members of the team and 59% said that the team had informed them on resources they had not been aware of. Approximately half of the participants said that their self-confidence and self-reliance had increased. Conclusion: Approximately 80% of the participants were content with the vocational rehabilitation offered and approximately 50% had gained more self-confidence and self-reliance.Tilgangur: Að kanna viðhorf matsþega til endurhæfingarmats og starfsendurhæfingarúrræða á vegum Tryggingastofnunar ríkisins (TR) og áhrif þessa ferlis á sjálfstraust og sjálfsbjargarviðleitni þeirra. Efniviður og aðferðir: Félagsvísindastofnun Háskóla Íslands kannaði viðhorf matsþega fyrir TR í október 2001. Reynt var að finna þá 109 einstaklinga sem metnir höfðu verið af matsteymi á árinu 2000 og tekið viðtal við þá í síma. Við úrvinnslu var notuð lýsandi tölfræði. Niðurstöður: Svör fengust frá 83 (76,1%) af matsþegum. Fjörutíu (48,2%) hafði að tillögu matsteymis verið vísað í atvinnulega endurhæfingu á Reykjalundi, 19 (22,9%) á tölvunámskeið í Hringsjá og 15 (18,1%) í fullt starfsnám í Hringsjá. Um fjórir af hverjum fimm reyndust ánægðir með þá starfsendurhæfingu sem TR hafði boðið þeim uppá á Reykjalundi eða í Hringsjá. Rúmlega helmingur þeirra sem metnir voru af matsteyminu töldu það hafa verið gagnlegt fyrir sig að hitta teymið og 59% matsþega töldu teymið hafa vísað á úrræði sem þeir vissu ekki um fyrir. Um helmingur þátttakenda sagði sjálfstraust sitt og sjálfsbjargarviðleitni hafa aukist frá því mat teymis fór fram. Ályktun: Um 80% þeirra sem vísað var í starfsendurhæfingu á vegum TR voru ánægðir með hana og um helmingur hafði öðlast aukið sjálfstraust og aukna sjálfsbjargarviðleitni við endurhæfinguna