Hydrogen atom scattering from physisorbed overlayers: II. A different kind of resonance crossing

Measurements and calculations of resonances in the scattering of H atoms from a Xe monolayer on graphite are presented. At the crossing of two resonances, we observe a splitting which is qualitatively different from those previously known and from the predictions of first-order theories. Its behaviour is explained when coupling through continuum states is included in a second-order theory. Because of this effect, data taken along a symmetry direction do not, in some cases, show resonance features expected from the commonly used zeroth- or first-order theories and therefore contain less information about the bound states than, for example, azimuthal scans.     

Nanografting of alkanethiols by tapping mode atomic force microscopy

Nanografting, an atomic force microscopy (AFM) based nanolithography technique, is becoming a popular method for patterning self-assembled monolayers (SAMs). In this technique, a nanoscale patch of a thiol-on-gold SAM is exchanged with a different thiol by the action of an AFM tip operated in contact mode at high load. The results are then imaged in topographic or lateral force microscopy again at low values of the load. One of the problems of contact mode nanografting is that monolayers of large molecules such as proteins are likely to be deformed, damaged, or even removed from the surface by contact mode imaging even when small loads are used. Furthermore, we need to note that the stiffness of the cantilevers used in contact mode is different than that of the cantilevers used in tapping mode and that tip changing in the course of an experiment can be quite inconvenient. Here, we show that a monolayer on a gold substrate can be nanografted using tapping mode AFM (also referred to as amplitude modulation AFM) rather than the commonly used contact mode. While the grafting parameters are somewhat trickier to choose, the results demonstrate that nanografting in tapping mode can make patches of the same quality as those made by contact mode, therefore allowing for gentle imaging of the grafted molecules and the whole SAM without changing the microscope tip.     

On the sub-optimality cost of immediate annuitization in DC pension funds

We consider the position of a member of a defined contribution (DC) pension scheme having the possibility of taking programmed withdrawals at retirement. According to this option, she can defer annuitization of her fund to a propitious future time, that can be found to be optimal according to some criteria. This option, that adds remarkable flexibility in the choice of pension benefits, is not available in many countries, where immediate annuitization is compulsory at retirement. In this paper, we address and try to answer the questions: “Is immediate annuitization optimal? If it is not, what is the cost to be paid by the retiree obliged to annuitize at retirement?”. In order to do this, we consider the model by Gerrard et?al. in Quant Finance, (2011) and extend it in two different ways. In the first extension, we prove a theorem that provides necessary and sufficient conditions for immediate annuitization being always optimal. The not surprising result is that compulsory immediate annuitization turns out to be sub-optimal. We then quantify the extent of sub-optimality, by defining the sub-optimality cost as the loss of expected present value of consumption from retirement to death and measuring it in many typical situations. We find that it varies in relative terms between 6 and 40%, depending on the risk aversion of the member. In the second extension, we make extensive numerical investigations of the model and seek the optimal annuitization time. We find that the optimal annuitization time depends on personal factors such as the retiree’s risk aversion and her subjective perception of remaining lifetime. It also depends on the financial market, via the Sharpe ratio of the risky asset. Optimal annuitization should occur a few years after retirement with high risk aversion, low Sharpe ratio and/or short remaining lifetime, and many years after retirement with low risk aversion, high Sharpe ratio and/or long remaining lifetime. This paper supports the availability of programmed withdrawals as an option to retirees of DC pension schemes, by giving insight into the extent of loss in wealth suffered by a retiree who cannot choose programmed withdrawals, but is obliged to annuitize immediately on retirement.     

A new green synthesis method of CuInS<Subscript>2</Subscript> and CuInSe<Subscript>2</Subscript> nanoparticles and their integration into thin films

A new preparation method for CuInS₂ and CuInSe₂ nanoparticles synthesis is described without using any organic solvent. Heating Cu, In, and S/Se precursors dissolved in water for 30 min in a microwave oven in the presence of mercapto-acetic acid leads to monodispersed chalcopyrite nanoparticles. No precipitation of these nanoparticles is observed after several months at room temperature. These new materials have been thoroughly characterized to confirm their compositions, sizes, and structure without any filtration. Transmission electron microscopy (TEM) confirmed particle sizes below 5 nm. Energy dispersive X-ray analysis (EDXA) confirmed the chemical composition of these samples. X-ray diffraction (XRD) showed a chalcopyrite-type structure with crystallite size of about 2 nm. No difference has been observed between batch and continuous synthesis processes. Cu _x InS₂ and Cu _x InSe₂ nanoparticles, with x < 1, have been also synthesized and identified. Simulation using a commercial software confirmed the difference between copper poor (Cu _x InS₂) and copper rich (CuInS₂) chalcopyrite structures. Conventional spray deposition techniques have been used to form relatively thin films on solid substrates.     

Verification of biochemical activity for proteins nanografted on gold surfaces

We demonstrate that the Atomic Force Microscope (AFM) can be used to immobilize a dicysteine-terminated protein (Maltose Binding Protein, MBP-cys-cys for short) at well-defined locations directly on gold substrates via nanografting and characterize the in situ bioactivity of these proteins within the fabricated nanopatterns. This method exploits the high spatial and orientational control of the protein monolayer assembly allowed by nanografting, combined with the high sensitivity of the AFM for detecting ligand-binding events. The maltose-mediated conformational changes within the MBP have been found to change the AFM-tip-protein interaction, therefore causing the frictional signal to change. Our measurements show that the protein ligand-binding function is maintained upon the immobilization process and is not affected by (a) the addition of the cysteine dipeptide, (b) the spatial confinement associated with nanografting, and (c) the interaction between the protein and the Au substrate. These surface-confined proteins can also be regenerated, and their frictional response is reproducible through several maltose exposure/washing cycles. By measuring the change in the frictional force above the protein nanopatterns as a function of maltose concentration, we determined the dissociation constant for the MBP-cys-cys/maltose system to be kd = (1 +/- 0.04) microM. Our results show that the MBP-cys-cys system provides a very sensitive surface-based, protein nanobiosensor for maltose detection at the attogram level (approximately 100 nM concentration). The implications of our study for the fabrication of molecular-scale biological sensors are discussed at the end of the paper.     

Nanografting de novo proteins onto gold surfaces

The immobilization of novel proteins onto addressable locations on a flat surface has potential applications in a range of biotechnologies. Here we describe the nanopatterning of a de novo protein onto a gold surface. Patterning was achieved using a technique called nanografting, in which the tip of an atomic force microscope is used to disrupt a preexisting monolayer of alkanethiol molecules on a gold surface, thereby facilitating exchange with alternative thiol-linked molecules from the surrounding solution. The protein used for these studies was chosen from a designed combinatorial library of de novo sequences expressed in E. coli and was engineered to have a glycine-glycine-cysteine tag at its C-terminus, thereby enabling attachment to the gold surface through a single cysteine thiol. The average height of the grafted protein patterns was found to be somewhat higher than expected from the known NMR structure of the protein. Compression of the nanografted patches by an external force (below 10 nN) was reversible but showed some hysteresis. Interestingly, both the energy required to deform the immobilized protein patterns and the energy defined by the hysteresis loop were found to be of the same order as the energy required to unfold the monomeric protein in solution. These studies demonstrate the possibility of preparing nanometer scale protein arrays, lowering significantly the volume requirements of the protein samples necessary to fabricate protein-based biosensor arrays and thereby providing a base for increasing their sensitivity.     

The preparation of 3,6‐bis(3‐hexylthien‐2‐yl)‐s‐tetrazine and its conjugated polymers

We demonstrated, for the first time, that 3,6‐bis(3‐hexylthien‐2‐yl)‐s‐tetrazine (TTz) with hexyl group at the 3‐position of thiophene rings can be prepared using a modified sulfur‐assisted Pinner synthesis. Although the hexyl group creates large steric hindrance to the tetrazine ring formation reaction, and the reaction under a traditional condition only produces trace amount of the target product, the yield of this reaction under a modified reaction condition using anhydrous hydrazine at 68 °C can reach 65%. Two new copolymers of the resulting TTz and hexyl‐ or 2‐ethylhexyl‐substituted cyclepentadithiophene have been prepared. The polymers show a broader light absorption in film than in solution attributing to the large distribution of effective conjugation length of polymer chain due to the existence of both cis‐ and trans‐orientations of the 3‐hexylthiophene units in the planar polymer chain in solid state. Although the polymers show a narrow band gap and a deep HOMO level, which are desirable for generating an efficient light absorption and a larger open circuit voltage (V_oc) of the resulting solar cell devices, the device performance is not as good as expected. It is attributed to the random distribution of the cis‐ and trans‐conformations along the polymer chain. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011.     

A simple but reliable method for the prediction of intermolecular potentials

A simple but reliable method for the prediction of intermolecular potentials is presented and tested for the case of the lighter noble gases and some of their mixtures.