Optical Multilevel Spin Bit Device Using Chiral Quantum Dots.

The technological advancement of data storage is reliant upon the continuous development of faster and denser memory with low power consumption. Recent progress in flash memory has focused on increasing the number of bits per cell to increase information density. In this work an optical multilevel spin bit, based on the chiral induced spin selectivity (CISS) effect, is developed using nanometer sized chiral quantum dots. A double quantum dot architecture is adsorbed on the active area of a Ni based Hall sensor and a nine-state readout is achieved.

Chiral Molecules and the Spin Selectivity Effect.

This Perspective discusses recent experiments that bear on the chiral induced spin selectivity (CISS) mechanism and its manifestation in electronic and magnetic properties of chiral molecules and materials. Although the discussion emphasizes newer experiments, such as the magnetization dependence of chiral molecule interactions with ferromagnetic surfaces, early experiments, which reveal the nonlinear scaling of the spin filtering with applied potential, are described also. In many of the theoretical studies, one has had to invoke unusually large spin-orbit couplings in order to reproduce the large spin filtering observed in experiments. Experiments imply that exchange interactions and Pauli exclusion constraints are an important aspect of CISS. They also demonstrate the spin-dependent charge flow between a ferromagnetic substrate and chiral molecules. With these insights in mind, a simplified model is described in which the chiral molecule's spin polarization is enhanced by a spin blockade effect to generate large spin filtering.

The relationship between interfacial bonding and radiation damage in adsorbed DNA.

We have performed a comparison of the radiation damage occurring in DNA adsorbed on gold in two different configurations, when the DNA is thiolated and bound covalently to the substrate and when it is unthiolated and interacts with the substrate through the bases. Both molecules were found to organize so as to protrude from the surface at ~45 degrees. Changes in the time-dependent C 1s and O 1s X-ray photoelectron (XP) spectra resulting from irradiation were interpreted to arise from cleavage of the phosphodiester bond and possibly COH desorption. By fitting the time-dependent XP spectra to a simple kinetic model, time constants were extracted, which were converted to cross sections and quantum yields for the damage reaction. The radiation induced damage is significantly higher for the thiolated DNA. N 1s X-ray absorption spectrum revealed the N-C=N LUMO is more populated in the unthiolated molecule, which is due to a higher degree of charge transfer from the substrate to this LUMO in the unthiolated case. Since the N-C=N LUMO of the thiolated molecule is comparatively less populated, it is more effective in capturing low energy electrons resulting in a higher degree of damage.

Chiral-Induced Spin Selectivity Effect.

The chiral-induced spin selectivity (CISS) effect was recently established experimentally and theoretically. Here, we review some of the new findings and discuss applications that can result from special properties of this effect, like the reduction of the elastic backscattering in electron transfer through chiral molecules. The CISS effect opens the possibility of using chiral molecules in spintronics applications and for providing a deeper understanding of spin-selective processes in biology.

Spin selectivity in electron transmission through self-assembled monolayers of double-stranded DNA.

In electron-transfer processes, spin effects normally are seen either in magnetic materials or in systems containing heavy atoms that facilitate spin-orbit coupling. We report spin-selective transmission of electrons through self-assembled monolayers of double-stranded DNA on gold. By directly measuring the spin of the transmitted electrons with a Mott polarimeter, we found spin polarizations exceeding 60% at room temperature. The spin-polarized photoelectrons were observed even when the photoelectrons were generated with unpolarized light. The observed spin selectivity at room temperature was extremely high as compared with other known spin filters. The spin filtration efficiency depended on the length of the DNA in the monolayer and its organization.

Collective effects in charge transfer within a hybrid organic-inorganic system.

A collective electron transfer (ET) process was discovered by studying the current noise in a field effect transistor with light-sensitive gate formed by nanocrystals linked by organic molecules to its surface. Fluctuations in the ET through the organic linker are reflected in the fluctuations of the transistor conductivity. The current noise has an avalanche character. Critical exponents obtained from the noise power spectra, avalanche distributions, and the dependence of the average avalanche size on avalanche duration are consistent with each other. A plausible model is proposed for this phenomenon.

Controlling two-photon photoemission using polarization pulse shaping.

Our understanding of processes involved in two-photon photoemission (2PPE) from surfaces can be tested when we try to exercise control over the electron emission. In the past, coherently controlled 2PPE has been demonstrated using very short pulses and single crystal surfaces. Here we show that by applying polarization pulse shaping on surfaces, it is possible to vary both the angular distribution of the emitted photoelectrons and the total photoemission yield. The presented 2PPE experimental setup introduces pulse shaping in the visible range, which is a unique property that allows control of polarization. We relate the ability to use polarization as a means of control to the surface corrugation.

Chiral imprinting of palladium with cinchona alkaloids.

In the search for new materials and concepts in materials science, metallo-organic hybrids are attractive candidates; they can combine the rich diversity of organic molecules with the advantages of metals. Transition metals such as palladium are widely applied in catalysis, and small organic molecules such as those in the cinchona alkaloid family can control the stereochemistry of a number of organic reactions. Here, we show that reducing a metal salt in the presence of a cinchona alkaloid dopant gives a chirally imprinted metallo-organic hybrid material that is catalytically active and shows moderate enantioselectivity in hydrogenation. Furthermore, using photoelectron emission spectroscopy, we show that the metal retains some chiral character even after extraction of the dopant. This simple and effective methodology opens exciting opportunities for developing a variety of chiral composite materials.

Three-dimensional surface patterning by DNA-modifying enzymes.

Self-assembled patterned multilayers may be fabricated using DNA monolayers and the orchestrated reactions of DNA-modifying enzymes. To demonstrate this approach, DNA monolayers were formed on silicon and cleaved quantitatively with a restriction enzyme. Subsequently, fluorescently labeled nucleotides were covalently incorporated to the cleaved DNA. Nucleotide addition was shown to be highly selective according to the sequence at the cleavage site, and no nonspecific adsorption to the surface was observed. The dual action of the DNA-modifying enzymes was quantitative and could be utilized in the fabrication of multilayered structures. Other DNA-modifying enzymes can be exploited in this manner to enrich the repertoire of self-assembly supramolecular structure fabrication.

Controlling the reactivity of adsorbed DNA on template surfaces.

The ability to place DNA on surfaces with increased and controllable reactivity is of fundamental importance in the development of next-generation DNA and protein biochips. The present work demonstrates the ability to control both the localization of the DNA on a surface and its reactivity by a self-assembly approach that is dependent on two variables: DNA structure and surface environment. Here we utilize a two-step adsorption scheme to control the adsorption and reactivity of DNA embedded within two types of alkyl thiol monolayers (either methyl-terminated or hydroxyl-terminated). In addition, by changing the structure of the chemisorbed DNA from fully single stranded to a 50% double stranded at its side adjacent to the surface, we were able to observe a clear dependence of DNA reactivity on both the DNA structure and the type of alkyl thiol monolayer covering the surface. The adsorption and the reactivity yield of the DNA were monitored either by its ability to hybridize to a complementary target molecule or by an enzymatic reaction involving DNA phosphorylation catalyzed by the enzyme T4 polynucleotide kinase.

The effect of self-assembled monolayers on polarization-dependent two-photon photoemission and on the angular distribution of the photoelectrons.

The effect of a self-assembled organized organic monolayer on the two-photon photoemission from semiconductor substrates was investigated. It has been found that the monolayer affects the relative yield of photoelectrons emitted by p-polarized versus s-polarized light. In addition, the monolayer affects the angular distribution of the ejected electrons. The effect on the photoelectron yield is attributed to the monolayer "smoothing" the electronic potential on the surface by eliminating surface states and dangling bonds. The effect on the angular distribution is attributed to a post-ejection interaction between the photoelectrons and the adsorbed molecules.

Sequence dependence of charge transport properties of DNA.

The electrical conduction through three short oligomers (26 base pairs, 8 nm long) with differing numbers of GC base pairs was measured. One strand is poly(A)-poly(T), which is entirely devoid of GC base pairs. Of the two additional strands, one contains 8 and the other 14 GC base pairs. The oligomers were adsorbed on a gold substrate on one side and to a gold nanoparticle on the other side. Conducting atomic force microscope was used for obtaining the current versus voltage curves. We found that in all cases the DNA behaves as a wide band-gap semiconductor, with width depending on the number of GC base pairs. As this number increases, the band-gap narrows. For applied voltages exceeding the band-gap, the current density rises dramatically. The rise becomes sharper with increasing number of GC base pairs, reaching more than 1 nA/nm2 for the oligomer containing 14 GC pairs.

Molecular chirality and charge transfer through self-assembled scaffold monolayers.

The effect of molecular chirality on electron transmission is explored by photoelectrochemistry. Thiol-terminated chiral scaffold molecules containing a porphyrin chromophore were self-assembled on gold surfaces to form a monolayer. Incorporation of the SAM-coated gold into an electrochemical cell and illumination with visible light generated a cathodic photocurrent. When using circularly polarized light, the photocurrent displayed an asymmetry (different magnitude of photocurrent for right versus left polarization) that changed with the molecular chirality (left- or right-handedness of the scaffold). A symmetry constraint on the electronic coupling between the porphyrin and the organic scaffold is proposed as a possible mechanism for the photocurrent asymmetry.

Chirality-induced spin-selective properties of self-assembled monolayers of DNA on gold.

Here we show that self-assembled monolayers on gold of double-stranded DNA oligomers interact with polarized electrons similarly to a strong and oriented magnetic field. The direction of the field for right-handed DNA is away from the substrate. Moreover, the layer shows very high paramagnetic susceptibility. Interestingly, thiolated single-stranded DNA oligomers on gold do not show this effect. The new findings are rationalized based on recent results in which high paramagnetism was measured for diamagnetic films adsorbed on diamagnetic substrates.

Adsorption of phenylphosphonic acid on GaAs (100) surfaces.

The adsorption of phenylphosphonic acid (PPA) on GaAs (100) surfaces from solutions in acetonitrile/water mixtures was studied using Fourier transform infrared spectroscopy in attenuated total reflection in multiple internal reflections (ATR/MIR), X-ray photoelectron spectroscopy (XPS), high-resolution electron energy loss spectroscopy (HREELS), and atomic force microscopy (AFM). ATR/MIR in situ showed that the accumulation of PPA molecules near the GaAs surface increased with the water concentration in the solution. For water contents lower than 4%, ATR/MIR and XPS results are consistent with the formation of a low-density monolayer. A mechanism is proposed for H2O percentages lower than 4% involving the creation of interfacial bonds through a Brønsted acid-base reaction, which involves the surface hydroxyl groups most probably bound to Ga. It was found that the morphology of the final layer depended strongly on the water concentration in the adsorbing solution. For water concentrations equal to or higher than 5%, the amount of adsorbed molecules drastically increased and was accompanied by modifications in the infrared spectral region corresponding to P-O and P=O. This sudden change indicates a deprotonation of the acid. XPS studies revealed the presence of extra oxygen atoms as well as gallium species in the layer, leading to the conclusion that phosphonate and hydrogenophosphonate ions are present in the PPA layer intercalated with H3O+ and Ga3+ ions. This mechanism enables the formation of layers approximately 10 times thicker than those obtained with lower H2O percentages. HREELS indicated that the surface is composed of regions covered by PPA layers and uncovered regions, but the uncovered regions disappeared for water contents equal to or higher than 5%. XPS results are interpreted using a model consisting of a monolayer partially covering the surface and a thick layer. This model is consistent with AFM images revealing roughness on the order of 7 nm for the thick layer and 0.2-0.5 nm for the thin layer. Sonication proves to be an effective method for reducing layer thickness.

Organization-induced charge redistribution in self-assembled organic monolayers on gold.

The charge redistribution that occurs within dipolar molecules as they self-assemble into organized organic monolayer films has been studied. The extent of charge transfer is probed by work function measurements, using low-energy photoelectron spectroscopy (LEPS), contact potential difference (CPD), and X-ray photoelectron spectroscopy (XPS), with the latter providing fine details about the internal charge distribution along the molecule. In addition, two-photon photoelectron spectroscopy is applied to investigate the electronic structure of the adsorbed layers. We show that charge transfer acts to reduce the dipole-dipole interaction between the molecules but may either decrease or increase the molecule-to-surface dipole moment.

On the capturing of low-energy electrons by DNA.

Many of the mutagenic or lethal effects of ionization radiation can be attributed to damage caused to the DNA by low-energy electrons. To gain insight on the parameters affecting this process, we measured the low-energy electron (<2 eV) transmission yield through self-assembled monolayers of short DNA oligomers. The electrons that are not transmitted are captured by the layer. Hence, the transmission reflects the capturing efficiency of the electrons by the layer. The dependence of the capturing probability on the base sequence was studied, as was the state of the captured electrons. It is found that the capturing probability scales with the number of G bases in the single-stranded oligomers and depends on their clustering level. Using two-photon photoelectron spectroscopy, we find that, once captured, the electrons do not reside on the bases. Rather, the state of the captured electrons is insensitive to the sequence of the oligomer. Double-stranded DNA does not capture electrons as efficiently as single-stranded oligomers; however, once captured, the electrons are bound more strongly than to the single strands.

Effects of a short-term calcium and vitamin D treatment on serum cytokines, bone markers, insulin and lipid concentrations in healthy post-menopausal women.

In vitro studies have shown that 1,25 dihydroxyvitamin D3 [1,25(OH)2D3] decreases cytokine production by monocytes and lymphocytes. In addition, intravenous or oral pulse calcitriol treatment suppresses interleukin 6 (IL6) and interleukin1beta (IL1beta) in hemodialysis patients. We studied the effect of a daily 12-week course of 1000 mg calcium and 800 U cholecalciferol on circulating 25 hydroxyvitamin D [25(OH)D], PTH, cytokines, osteoprotegerin (OPG), C-reactive protein (CRP), bone markers, lipid parameters and insulin levels in 47 healthy post-menopausal women. Thirty-nine women completed the study. A significant increase in 25(OH)D and a significant decrease in PTH were observed (p=0.0043 and p<0.0001, respectively). In addition, alkaline phosphatase, osteocalcin and, to a lesser extent, urinary free deoxypiridinoline (DPD) decreased significantly (p<0.0001, p=0.0002 and p=0.026, respectively). No change in circulating IL6, tumor necrosis factor alpha (TNFalpha), CRP, OPG, triglycerides, LDL- and HDL-cholesterol, and insulin levels was observed. Correlation studies in the 47 women enrolled in the study revealed inverse significant correlations between OPG on one side and body mass index, LDL-cholesterol, IL6, CRP and insulin levels on the other (p=0.002, p=0.002, p=0.004, p=0.023 and p=0.0001). Also, IL6 was significantly correlated with insulin levels (p=0.0005). In a multivariate model, both insulin and LDL-cholesterol were independently associated with OPG, while only insulin was independently associated with IL6. Our results showed no effect of a short-term calcium-vitamin D treatment on circulating cytokines, CRP, insulin levels and lipid parameters. This could be related to the low circulating cytokine concentrations in healthy subjects or to the short duration of treatment. The interesting association we found between OPG and some cardiovascular risk markers deserves further investigation.

Electron transmission through organized organic thin films.

Low-energy electron photoemission spectroscopy (LEPS) allows the study of the electronic properties of organized organic thin films (OOTF) adsorbed on conducting surfaces by monitoring the energy and angular distribution of electrons emitted from the substrate and transmitted through the film. The transmission properties are explained by electronic band structure in the organic film. This band is an example of an electron resonance that is delocalized in the layer. It results from the two-dimensional nature of the layer. Other resonances in the transmission spectra are also discussed, as well as their experimental manifestation. The temperature dependence of the electron transmission efficiency is explained in terms of dependence of the transmission probability on the initial momentum of the electron and on the relative orientation of the electron velocity and the molecules in the film. Hence, despite the fact that the molecules in the OOTF are weakly interacting, when not charged, the electron transmission through the film is governed by cooperative effects. These effects must be taken into account when considering electronic properties of adsorbed layers.