Current-voltage characteristics of Pd,Si/p-Si(lIl) Schottky barrier diodes studied over a wide temperature range (60-201 K) are shown to follow a thermionic emission-diffusion mechanism under both the forward and the reverse bias conditions. The barrier parameters as evaluated from the forward I-V data reveal a decrease of zero-bias barrier height (q4bo) but an increase of ideality factor (q) and series resistance (R,) with decrease in temperature. Moreover, the changes in bbO,? and R, become quite significant below -100 K. An ln(ls/T2) versus 1 / T plot IS found to fit well with two straight lines in different temperatures regimes giving an activation energy of 0.33 eV (201-107 K) and 0.24 eV (below 107 K ) and an effective Richardson constant of 33Acm-'K-'.However, the activation energy of 0.33 eV corresponds to the zero-bias barrier height at absolute zero. An ln(t,/P) versus 1/qT plot is suggested to obtain the flat-band barrier height and the effective Richardson constant: the corresponding values obtained are 0.401 eV and 32.2Acm-'K-' respectively. it is shown that the 'To effect' cannot account for t h e apparent increase in ideality factor and decrease of barrier height at low temperatures. Finally, the decrease of barrier height with voltage under the reverse bias condition is attributed mainly to interfacial layer effects with a mail contribution due to image force lowering.

The current-voltage (I -V ) characteristics of palladium silicide-based Schottky diodes on n-type silicon have been measured over a wide temperature range (66-300 K). Their analysis on the basis of the thermionic emission-diffusion (TED) mechanism reveals an abnormal decrease of zero-bias barrier height and increase of ideality factor with decrease in temperature (T ) and nonlinearity in the activation energy plot. Such behaviour is attributed to barrier inhomogeneities by assuming a Gaussian distribution of barrier heights at the silicide/silicon interface. Evidence is given for the existence of a double Gaussian distribution having mean barrier heights of 0.79 V and 0.64 V and standard deviations of 0.081 V and 0.057 V with ideality factors 1.064 and 1.363, and remain effective in the temperature range 134-300 K and 66-120 K respectively. Further, the effect of forward bias on the distribution parameters is discussed. A simple method, involving the use of a zero-bias barrier height versus inverse temperature plot, is suggested to deduce the presence of single/multiple distribution(s) of barrier heights and to determine the respective parameters.

Canonical purine-pyrimidine base pairs, the key to the complementary hydrogen bonding in nucleic acids, are fundamental molecular recognition motifs crucial for the formation and stability of double-helical DNA. Consequently, focused study and modeling of nucleobase hydrogen-bonding schemes have spawned a vast array of chemical and biophysical investigations. The Watson-Crick, reverse Watson-Crick, Hoogsteen, and reverse Hoogsteen hydrogen-bonding schemes stabilize various nucleic acid structures. As a result, numerous modified bases have been designed to maximize such interactions, addressing specific problems related to base pairing and giving rise to supramolecular ensembles in solution or in the solid state. It is also important to realize that suitably predisposed imino nitrogens and other functional groups present in heterocyclic nucleobases present a versatile molecular framework for the construction of coordination architectures, which may be harnessed to mimic base polyads and higher order nucleic acid structures. Adenine, a purine nucleobase, is an important naturally occurring nitrogen heterocycle present in nucleic acids. It is notable that the adenine unit is also frequently encountered as an inextricable part of enzyme cofactors and second messenger systems, such as NAD(+), FADH(2), and cAMP, which are essential for certain catalytic reactions and biochemical processes. In addition, a crucial catalytic role of the adenine moiety is also observed in group II intron catalysis and at the ribosomal peptidyltransferase center. Such versatile functional roles of the adenine framework serve as an inspiration for addressing research problems, ranging from classical coordination chemistry to the development of new materials. In this Account, we begin by describing the emerging use of adenine nucleobase for the design of metal-nucleobase frameworks. The coordination of metal ions affords a variety of oligomeric and polymeric species; we focus on silver- and copper-based structures and also discuss ferrocenylated adenine tetrads. We then consider the use of supramolecular adenine coordination complexes for transferring molecular properties onto surfaces. This technique is particularly useful for transferring noncovalent interactions, such as van der Waals forces, electrostatic interactions, and hydrogen bonding, to designed architectures in nanoscale applications. Finally, we explore the issue of adenine-based catalytic entities. Here, adenine moieties are first fixed in a polymeric matrix, followed by metalation of the matrix. These metalated adenine-containing polymers are then assayed for catalytic assistance in various chemical and biochemical reactions. Taken together, the versatile coordination abilities and hydrogen-bonding capacity of adenine offer a novel entry point for a natural ligand into materials synthesis.

The current–voltage characteristics of Pd2Si based Schottky diodes on both n- and p-type silicon measured over a wide temperature range (52–295 K) have been interpreted on the basis of thermionic emission-diffusion mechanism and the assumption of a Gaussian distribution of barrier heights. It is shown that while the occurrence of a distribution of barrier heights is responsible for the apparent decrease of the zero-bias barrier height (φb0) and nonlinearity in the activation energy plot, the voltage dependence of the standard deviation causes the unusual increase of ideality factor (η) at low temperatures. Also, it is demonstrated that the forward bias shifts the mean barrier height towards the higher side and causes narrowing of the distribution as well. A simple method, involving the use of φb0 vs 1/T data, is suggested to gather evidence for the occurrence of a Gaussian distribution of barrier heights and obtain values of mean barrier height and standard deviation. The experimental results correspond to a mean barrier height of 0.80 V, standard deviation 0.05 V, and ideality factor 1.21 for Pd2Si based Schottky barriers on n-type silicon; these values for p-type silicon are 0.38 V, 0.03 V, and 1.07, respectively.

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The current-voltage characteristics of a Schottky diode are simulated numerically using the thermionic emission-diffusion mechanism and considering a Gaussian distribution of barrier heights, with a linear bias dependence of both the mean and standard deviation. The resulting data are analyzed to get insight into the effects of distribution parameters on the barrier height, activation energy plots and the ideality factor over a temperature range 50-300 K. It is shown that with a Gaussian distribution of the barrier heights the system continues to behave like a single Schottky diode of apparently low zero-bias barrier height and a high ideality factor. Its barrier height decreases, activation energy plot becomes non-linear and ideality factor increases with a decrease in temperature. While the distribution parameters are responsible for the abnormal decrease of barrier height, their bias dependences account for the higher ideality factor at low temperatures. Also, the pivotal role played by series resistance in influencing the linearity of the ln(I)-V plots of Schottky diodes with a Gaussian distribution of barrier heights is discussed.

We report the synthesis and characterization of adenine-single-walled carbon nanotube (SWCNT) hybrid materials, where for the first time nucleobases are covalently attached to the exosurface of SWCNTs. The structural properties of all hybrids have been characterized using usual spectroscopic and microscopic techniques. The degree of functional groups for functionalized SWCNTs (f-SWCNTs) 2a and 2b is one adenine group for each 26 and 37 carbon atoms, respectively. Solid-state magic angle spinning (13)C NMR spectroscopy (MAS NMR) and electrochemistry have been also applied for the characterization of these f-SWCNTs. AFM images of f-SWCNT 2b showed an interesting feature of horizontally aligned nanotubes along the surface when deposited on highly oriented pyrolytic graphite surface. Furthermore, we evaluated the coordinating ability of these hybrid materials toward silver ions, and interestingly, we found a pattern of silver nanoparticles localized over the surface of the carbon nanotube network. The presence of aligned and randomly oriented CNTs and their ability to coordinate with metal ions make this class of materials very interesting for applications in the development of novel electronic devices and as new supports for different catalytic transformations.

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