Covalently bound organic monolayers on hydrogen-terminated silicon surfaces = covalent gebonden organische monolagen op waterstof-getermineerde siliciumoppervlakken

Monolayers of 1-alkenes and 1-alkynes can be prepared on hydrogen-terminated Si(100) and Si(111) surfaces by a reaction between the organic compound and the Si surface. This reaction, which is schematically depicted below, results in the formation of densely packed, covalently bound, and well-ordered monolayers of the organic compound on the hydrogen-terminated Si surface, that inhibit the oxidation of the underlying Si surface.In this research, several aspects of this surface modification have been investigated. The scope of the reaction has been explored by using a variety of functionalized and nonfunctionalized alkenes. The results show that many functional groups can be used, provided that: a) the functional group is properly protected, and b) the formation of well-ordered monolayers is not disturbed by too much steric hindrance between these functional groups, once they are in the monolayer. A very interesting property of the resulting functionalized monolayers is that the functional groups can be deprotected and/or further modified, without damaging the monolayer or the underlying Si substrate. This gives access to functionalized monolayers that so far could not be prepared by other methods.The method for the preparation of the monolayers has been improved by showing that the reaction can also be done using solutions of the 1-alkenes and 1-alkynes in aromatic solvents. The best solvent was found to be mesitylene (1,3,5-trimethylbenzene). In this solvent monolayers are formed that are at least as good as those prepared using neat 1-alkenes/1-alkynes, even at concentrations as low as 0.1 M. This is an important improvement, as it considerably reduces the amount of 1-alkene/alkyne needed in the surface modification.In the case of the hydrogen-terminated Si(100) surface, there are two hydrogen atoms on each Si surface atom. Upon reaction of this surface with a 1-alkyne, not just one, but two covalent Si-C bonds are formed per organic molecule, as had been demonstrated by a combination of IR spectroscopy, X-ray reflectivity measurements, and quantum chemical calculations. This type of reactivity has so far not been observed for 1-alkynes on other H-terminated (crystalline) Si surfaces.To get more insight in the structure of the monolayers on a molecular level, they have been investigated by molecular modeling simulations. Large modified Si surfaces, with &gt;30 alkyl chains attached to the Si surface, were investigated, using the approach of two-dimensionally repeating boxes. Calculations without this repeating box approach failed completely, as did calculations using small boxes (&lt;30 alkyl chains). The results show that: a) there is a good correlation between the structure as observed in the simulations and the structure as deduced from a combination of experimental data, and b) that the currently obtained substitution percentage of the Si-H for Si-alkyl groups is close to the maximum substitution percentage that can be reached. This latter conclusion shows that the obtained monolayers are (almost) as densely packed as possible, as desired.The possibility to use the monolayers for silicon surface passivation has been investigated, determining the effective lifetimes of the minority charge carriers in p-type Si wafers modified with 1-alkenes. The passivating properties of the monolayers are found to be comparable to those of HF and iodine/ethanol solutions, two methods commonly used in semiconductor technology, but the monolayer-modified surfaces are far more stable than these two systems. This shows that these monolayers provide an interesting alternative for Si surface passivation.</p

Photovoltaic performance of an ultrasmall band gap polymer

A conjugated polymer (PBTTQ) that consists of alternating electron-rich bithiophene and electron-deficient thiadiazoloquinoxaline units was synthesized via Yamamoto polymerization with Ni(cod)(2) and provides a band gap of 0.94 eV. This represents one of the smallest band gaps obtained for a soluble conjugated polymer. When applied in a bulk heterojunction solar cell together with [84]PCBM as the electron acceptor, the polymer affords a response up to 1.3 mu m

Soft X-ray induced oxidation on acrylic acid grafted luminescent silicon quantum dots in ultrahigh vacuum

Water soluble acrylic acid grafted luminescent silicon quantum dots (Si-QDs) were prepared by a simplified method. The resulting Si-QDs dissolved in water and showed stable strong luminescence with peaks at 436 and 604?nm. X-ray photoelectron spectroscopy (XPS) was employed to examine the surface electronic states after the synthesis. The co-existence of the Si2p and C1s core levels infers that the acrylic acid has been successfully grafted on the surface of silicon quantum dots. To fit the Si2p spectrum, four components were needed at 99.45, 100.28, 102.21 and 103.24?eV. The first component at 99.45?eV (I) was assigned to Si–Si within the silicon core of the Si-QDs. The second component at 100.28?eV (II) was from Si–C. The third at 102.21?eV (III) was a sub-oxide state and the fourth at 103.24?eV (IV) was from SiO2 at Si-QDs surface. With an increase in exposure to soft X-ray photons, the intensity ratio of the two peaks within the Si2p region A and B increased from 0.5 to 1.4 while the peak A intensity decreased, and eventually a steady state was reached. This observation is explained in terms of photon-induced oxidation taking place within the surface dangling bonds. As the PL profile for Si-QDs is influenced by the degree of oxidation within the nanocrystal structure, the inducement of oxidation by soft X-rays will play a role in the range of potential applications where such materials could be used – especially within biomedical labelling

Impact of Marginal Exciton-Charge-Transfer State Offset on Charge Generation and Recombination in Polymer:Fullerene Solar Cells

The energetic offset between the initial photoexcited state and charge-transfer (CT) state in organic heterojunction solar cells influences both charge generation and open-circuit voltage (Voc). Here, we use time-resolved spectroscopy and voltage loss measurements to analyze the effect of the exciton–CT state offset on charge transfer, separation, and recombination processes in blends of a low-band-gap polymer (INDT-S) with fullerene derivatives of different electron affinity (PCBM and KL). For the lower exciton–CT state offset blend (INDT-S:PCBM), both photocurrent generation and nonradiative voltage losses are lower. The INDT-S:PCBM blend shows different excited-state dynamics depending on whether the donor or acceptor is photoexcited. Surprisingly, the charge recombination dynamics in INDT-S:PCBM are distinctly faster than those in INDT-S:KL upon excitation of the donor. We reconcile these observations using a kinetic model and by considering hybridization between the lowest excitonic and CT states. The modeling results show that this hybridization can significantly reduce Voc losses while still allowing reasonable charge generation efficiency

Проблематика научных исследований И.Г. Спасского

У статті розглянуто широке коло наукових інтересів відомого радянського вченого І.Г. Спасського у російській нумізматиці – від староруських монет Х ст. й українських дукачів, якими він почав займатися ще в студентські роки, до російських і радянських монет першої половини ХХ ст. та західноєвропейських єфимків з російським надкарбуванням періоду Олексія Михайловича.В статье рассмотрен широкий круг научных интересов крупнейшего советского ученого И.Г.Спасского в русской нумизматике – от древнерусских монет Х в. и украинских дукачей, которыми он начал заниматься еще в студенческие годы, до русских и советских монет первой половины ХХ в. и западноевропейских ефимков с русскими надчеканками времени Алексея Михайловича.The wide circle of scientific interests of I.G. Spassky – the famous soviet scientist in Russian numismatics is considered in the article – from the old-russian chinks of 10 century and Ukrainian dukach, which he began to be engaged in as early as student years, to the Russian and soviet chinks of the first half of 20 century and West European yefimks with the Russian supercoinage of period of zar Alexei Michailivich

Grafted block complex coacervate core micelles and their effect on protein adsorption on silica and polystyrene

We have studied the formation and the stability of grafted block complex coacervate core micelles (C3Ms) in solution and the influence of grafted block C3M coatings on the adsorption of the proteins β-lactoglobulin, bovine serum albumin, and lysozyme. The C3Ms consist of a grafted block copolymer PAA21-b-PAPEO14 (poly(acrylic acid)-b-poly(acrylate methoxy poly(ethylene oxide)), with a negatively charged PAA block and a neutral PAPEO block and a positively charged homopolymer P2MVPI (poly(N-methyl 2-vinyl pyridinium iodide). In solution, these C3Ms partly disintegrate at salt concentrations between 50 and 100 mM NaCl. Adsorption of C3Ms and proteins has been studied with fixed-angle optical reflectometry, at salt concentrations ranging from 1 to 100 mM NaCl. In comparison with the adsorption of PAA21-b-PAPEO14 alone adsorption of C3Ms significantly increases the amount of PAA21-b-PAPEO14 on the surface. This results in a higher surface density of PEO chains. The stability of the C3M coatings and their influence on protein adsorption are determined by the composition and the stability of the C3Ms in solution. A C3M-PAPEO14/P2MVPI43 coating strongly suppresses the adsorption of all proteins on silica and polystyrene. The reduction of protein adsorption is the highest at 100 mM NaCl (>90%). The adsorbed C3M-PAPEO14/P2MVPI43 layer is partly removed from the surface upon exposure to an excess of β-lactoglobulin solution, due to formation of soluble aggregates consisting of β-lactoglobulin and P2MVPI43. In contrast, C3M-PAPEO14/P2MVPI228 which has a fivefold longer cationic block enhances adsorption of the negatively charged proteins on both surfaces at salt concentrations above 1 mM NaCl. A single PAA21-b-PAPEO14 layer causes only a moderate reduction of protein adsorption

The Relative Importance of Topography and RGD Ligand Density for Endothelial Cell Adhesion

The morphology and function of endothelial cells depends on the physical and chemical characteristics of the extracellular environment. Here, we designed silicon surfaces on which topographical features and surface densities of the integrin binding peptide arginine-glycine-aspartic acid (RGD) could be independently controlled. We used these surfaces to investigate the relative importance of the surface chemistry of ligand presentation versus surface topography in endothelial cell adhesion. We compared cell adhesion, spreading and migration on surfaces with nano- to micro-scaled pyramids and average densities of 6×102–6×1011 RGD/mm2. We found that fewer cells adhered onto rough than flat surfaces and that the optimal average RGD density for cell adhesion was 6×105 RGD/mm2 on flat surfaces and substrata with nano-scaled roughness. Only on surfaces with micro-scaled pyramids did the topography hinder cell migration and a lower average RGD density was optimal for adhesion. In contrast, cell spreading was greatest on surfaces with 6×108 RGD/mm2 irrespectively of presence of feature and their size. In summary, our data suggest that the size of pyramids predominately control the number of endothelial cells that adhere to the substratum but the average RGD density governs the degree of cell spreading and length of focal adhesion within adherent cells. The data points towards a two-step model of cell adhesion: the initial contact of cells with a substratum may be guided by the topography while the engagement of cell surface receptors is predominately controlled by the surface chemistry

Graphene Schottky diodes: an experimental review of the rectifying graphene/semiconductor heterojunction

In the past decade graphene has been one of the most studied material for several unique and excellent properties. Due to its two dimensional nature, physical and chemical properties and ease of manipulation, graphene offers the possibility of integration with the exiting semiconductor technology for next-generation electronic and sensing devices. In this context, the understanding of the graphene/semiconductor interface is of great importance since it can constitute a versatile standalone device as well as the building-block of more advanced electronic systems. Since graphene was brought to the attention of the scientific community in 2004, the device research has been focused on the more complex graphene transistors, while the graphene/semiconductor junction, despite its importance, has started to be the subject of systematic investigation only recently. As a result, a thorough understanding of the physics and the potentialities of this device is still missing. The studies of the past few years have demonstrated that graphene can form junctions with 3D or 2D semiconducting materials which have rectifying characteristics and behave as excellent Schottky diodes. The main novelty of these devices is the tunable Schottky barrier height, a feature which makes the graphene/semiconductor junction a great platform for the study of interface transport mechanisms as well as for applications in photo-detection, high-speed communications, solar cells, chemical and biological sensing, etc. In this paper, we review the state-of-the art of the research on graphene/semiconductor junctions, the attempts towards a modeling and the most promising applications.Comment: 85 pages. Review articl