Jadab Sharma

Applications of self-assembled monolayers in materials chemistry

Self-assembly provides a simple route to organise suitable organic molecules on noble metal and selected nanocluster surfaces by using monolayers of long chain organic molecules with various functionalities like -SH,-COOH,-NH2, silanes etc. These surfaces can be effectively used to build-up interesting nano level architectures. Flexibility with respect to the terminal functionalities of the organic molecules allows the control of the hydrophobicity or hydrophilicity of metal surface, while the selection of length scale can be used to tune the distant-dependent electron transfer behaviour. Organo-inorganic materials tailored in this fashion are extremely important in nanotechnology to construct nanoelctronic devices, sensor arrays, supercapacitors, catalysts, rechargeable power sources etc. by virtue of their size and shape-dependent electrical, optical or magnetic properties. The interesting applications of monolayers and monolayer-protected clusters in materials chemistry are discussed using recent examples of size and shape control of the properties of several metallic and semiconducting nanoparticles. The potential benefits of using these nanostructured systems for molecular electronic components are illustrated using Au and Ag nanoclusters with suitable bifunctional SAMs.

Size dependent redox behavior of monolayer protected silver nanoparticles (2-7 nm) in aqueous medium

Monolayer protected nanoclusters are of current interest due to their ease of synthesis, high stability and possibility to precisely control their aspect ratio by preparation procedures, so that they can be tuned for a wide range of applications. Since these nanostructured metallic particles show fascinating size dependent optical, electronic, catalytic and magnetic properties, it is important to modulate their size, shape and intercluster spacing during their synthesis. These size dependent phenomena suggest that the electrochemistry of nanometer scale metal particles should be different from that of their bulk analogues. In the present study, we report a systematic variation in the redox behaviour of dodecanethiol protected silver nanoparticles with size (2–7 nm). Cyclic voltammograms in 0.1 M aqueous KCl solution show irreversible nature and the redox behaviour is indeed affected by the size as in agreement with the theoretical calculations of the Kubo gap. More specifically, the separation between oxidation and reduction peaks (ΔEp) increases with an increase in size reaching a maximum (3.5–6 nm) followed by a decline, whereas the E1/2 seems to be almost constant throughout this size regime. As the kinetic parameters are directly related to the ΔEp value, the electron transfer facility should decrease with an increase in size in a similar manner. All the silver nanoclusters were characterized by their surface plasmon peak position, which was found to decrease with increase in size with a concomitant broadening. The particle size calculated from TEM reveals a fairly monodispersed nature whereas selected area electron diffraction (SAED) results confirm the presence of fcc structure for all the Ag clusters.

Improving the efficiency of an organic solar cell by a polymer additive to optimize the charge carriers mobility

We investigate the effect of a high hole mobility triarylamine-based conjugated polymer on a bulk hetero-junction organic solar cell. We employed a polymer blend consisting of poly(3-hexylthiophene) (P3HT), [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), and poly(N-(4u2009-(9,9-dioctyl-fluoren-2-yl)phenyl)-N,N′,N′-triphenyl-l,4-phenylenediamine) (PFLAM) as active materials. The hole mobility of PFLAM is ∼10−3 cm2 V−1 s−1, which is similar to the electron mobility of PCBM. Addition of PFLAM improves the hole mobility of the photovoltaic cell augmenting the charge balance of the system. The overall efficiency gain for such a device is 34%.

Nickel oxide nanotubes: synthesis and electrochemical performance for use in lithium ion batteries

Understanding the electron transfer behavior of nanometer sized, both metallic and semiconducting particles and wires is important due to the fundamental interest in size and shape dependent electronic properties and also because of its applications in nano-electronic devices like single electron transistors and molecular switches. Monolayer protected nanoclusters enable one simple and elegant method of synthesis of these types of metallic and semiconducting materials using interfacial chemistry as has been successfully used in several applications ranging from catalysis to molecular electronics. The success of this type of nanostructured materials is due in part to the well known protecting/stabilizing action of the ligands (also known as surface passivating/capping agents), which facilitate the synthesis and processing of these hydrophobic colloids in solution form. The present article discusses the electron transfer behavior of silver nanowires and nanoparticles with varied sizes. In particular, we have investigated the electrochemical properties of silver nanowires (diameter 70 nm, length several micrometers) and compared with the behavior of similar relatively larger sized nanoparticles (size 40 nm). A critical analysis of the redox behavior of silver nanowires and nanoparticles is presented in aqueous medium under various electrolytic conditions along with a comparison of analogous properties of smaller sized (2-7 nm) silver and gold nanoclusters.

Device applications of self-assembled monolayers and monolayer-protected nanoclusters

Abstract The present status of self-assembled monolayers (SAMs) on different surfaces (2D systems) as well as monolayer formation on metallic and semiconducting cluster surfaces (3D SAM) to form monolayer-protected nanoclusters (MPCs) and their assemblies is reviewed briefly. Attention is focused mainly on the potential electronic and photonic applications of SAMs, MPCs and their 2D and 3D structures fabricated using covalent and hydrophobic interactions in contrast to the usual electrostatic assemblies. These examples illustrate the rational use of organic molecules and nanoclusters using the concept of self-assembly, where subtle systems of double tunnel junctions, hetero junctions and single-electron transition devices could be developed based on the structure and chemistry of multifunctional molecules. The tailoring of cluster size and cluster–cluster spacing to reveal interesting transitions in electronic properties is also demonstrated using the low temperature behavior of the 3D network of nanoclusters as an example. These devices are believed to play an important role in the coming years as the chip functions and clock frequencies reach orders of magnitude beyond those extrapolated from Moore’s law.

Tuning the aspect ratio of silver nanostructures: the effect of solvent mole fraction and 4-aminothiophenol concentration

In this report, we study the role of solvent on controlling the aspect ratio of silver nanostructures during their growth. More specifically, a single-step preparation of different aspect ratio silver nanostructures (R, 1–100) is demonstrated in aqueous acetonitrile using 4-aminothiophenol (ATP) as a reducing as well as surface passivating agent, where the variation of the mole fraction of acetonitrile has a dramatic effect on the morphology. The combined effect of ATP concentration and solvent mole fraction on aspect ratio is investigated by UV-Visible Spectroscopy (UV-Vis), Transmission Electron Microscopy (TEM), Fourier Transform Infra-red Spectroscopy (FTIR) and X-ray Diffraction analysis (XRD). At lower values of mole fraction (i.e. 0.4), high aspect ratio silver nanorods are formed, whereas a mole fraction close to 1 gives no such nanostructures. In comparison, only spherical nanoparticles are formed when the mole fraction is close to 0. High aspect ratio silver nanorods are also favored by higher ATP concentration.

Highly resolved quantized double-layer charging of relatively larger dodecanethiol-passivated gold quantum dots

Monolayer-protected quantum dots (Q-dots) show multivalent redox property, popularly known as the quantized double-layer (QDL) charging phenomena. In this report, we demonstrate the QDL behavior of the larger-sized Au Q-dots (ca.3.72nm) protected with dodecanethiol using differential pulse voltammetry (DPV) and cyclic voltammetry (CV). The voltammetric results show that the QDL property is evident even for these larger-sized Q-dots as reflected by a large population of well-resolved charging events in a narrow potential range with an almost equidistant voltage (ΔV) spacing. The theoretical calculation of the variation of charging energy with size using the well-known concentric sphere capacitance model facilitates the understanding of electrochemical behavior of these sidelined larger-sized Au Q-dots. The calculated capacitance value is in well agreement with the experimentally obtained value of 1.6aF.

Directed organization of gold nanoclusters on silver nanowires: a step forward in heterostructure assembly

We investigate the directed assembly of tridecylamine protected gold nanoclusters of 4–5nm size on functionalized silver nanowires of 55–60nm diameter and the electron transfer behavior of this integrated structure using transmission electron microscopy, non-contact atomic force microscopy, and scanning tunneling microscopy/spectroscopy. Linear I-V for bare silver nanowire suggests metallic behavior but high tunnel resistance indicates presence of insulating layer on the surface. Identical I-Vs obtained for isolated gold nanoparticle and heterostructure suggests that electron transport across nanowires in the latter is governed by gold nanoparticles in contrast to expected ballistic or diffusive transport along their length.