@article {42586, title = {Electrical bistability in a xanthene class molecule: conduction mechanisms}, journal = {Applied Physics Letters}, volume = {89}, year = {2006}, month = {OCT}, pages = {Article No: 142110}, abstract = {

The author study conduction mechanism in two conducting states of a bistable device at range. They find that in the electrical bistable devices, electrical switching is associated with a change in the conduction mechanism. Device current in the low-conducting state follows an injection-limited mechanism. The current in the high-conducting state conforms a bulk-dominated mechanism, namely, space-charge limited conduction with an exponential distribution of traps. The bistability has an associated memory phenomenon. The devices exhibit read-only and random-access memory applications for several hours.

}, doi = {10.1063/1.2358946}, author = {Rath, Arup Kumar and Sahu, Satyajit and Pal, Amlan J.} } @article {42585, title = {Conductance switching in an organic material: from bulk to monolayer}, journal = {Langmuir}, volume = {23}, year = {2007}, month = {SEP}, pages = {9831-9835}, type = {Article}, abstract = {

Fluorescein sodium, which does not exhibit electrical bistability in thin films, can be switched to a high conducting state by the introduction of carbon nanotubes as channels for carrier transport. Thin films based on fluorescein sodium/carbon nanotubes display memory switching phenomenon among a low conducting state and several high conducting states. Read-only and random-access memory applications between the states resulted in multilevel memory in these systems. Results in thin films and in a monolayer (deposited via layer-by-layer assembly) show that instead of different molecular conformers, multilevel conducting states arise from the different density of high conducting fluorescein molecules.

}, doi = {10.1021/la701132f}, author = {Rath, Arup Kumar and Pal, Amlan J.} } @article {42584, title = {Nanowires of metal-organic complex by photocrystallization: a system to achieve addressable electrically bistable devices and memory elements}, journal = {Langmuir}, volume = {24}, year = {2008}, month = {MAY}, pages = {5937{\textendash}5941}, abstract = {

A new method has been achieved to form a Cu:benzoquinone derivative (DDQ) charge-transfer complex by the photoexcitation of [Cu(DDQ)2(CH3COO)2] (1) that has been synthesized by the reaction of DDQ and hydrated cupric acetate in acetonitrile. Photoexcitation of coordinated complex 1 leads to the formation of charge-transfer complex Cu2+(DDQ{\textbullet}-)2 (2). The charge transfer complex 2, when spun on solid substrates, forms nanowires. Sandwich structures of 2 exhibit electrical bistability associated with memory phenomenon. Read-only and random-access memory phenomena are evidenced in nanowires of 2 providing a route to attend the issues pertaining to the addressibility of organic memory devices.

}, doi = {10.1021/la703871g}, author = {Rath, Arup Kumar and Dhara, Koushik and Banerjee, Pradyot and Pal, Amlan J.} } @article {42583, title = {Resistive switching in rose bengal and other xanthene molecules is a molecular phenomenon}, journal = {Organic Electronics}, volume = {9}, year = {2008}, month = {FEB}, pages = {495{\textendash}500}, abstract = {

There has been a debate on the mechanism of resistive switching in Rose Bengal and other Xanthene class molecules. While some authors proposed that the switching was due to an oxide layer at the Rose Bengal/Aluminum interface, some inferred the switching as an extrinsic effect like filament formation. We show results from Rose Bengal and other Xanthene class molecules on doped Si. Conductance switching in such monolayers induced by Pt/Ir tip of a scanning tunneling microscope (STM) in a non-contact mode shows that resistive switching in these molecules, initially reported by us in 2003 (in thin films), is indeed a molecular phenomenon.

}, doi = {10.1016/j.orgel.2008.02.013}, author = {Rath, Arup Kumar and Pal, Amlan J.} } @article {42582, title = {Hybrid core-shell nanoparticles: photoinduced electron-transfer for charge separation and solar cell application}, journal = {Chemistry of Materials}, volume = {21}, year = {2009}, month = {OCT}, pages = {5292{\textendash}5299}, abstract = {

We report growth and formation of hybrid core-shell nanoparticle systems, where photoinduced electron-transfer takes place from the II-VI semiconducting core to an organic shell. With the hybrid core-shell nanoparticles, we fabricate devices so that the photoinduced electron-transfer can finally yield photocurrent and result photovoltaic solar cells. Formation of an organic shell-layer on CdSe nanoparticles is supported by electronic absorption spectroscopy. Electron-transfer from the nanoparticle in the core to a number of organic molecules in the shell is established from quenching of photoluminescence intensity of CdSe nanoparticles as well as from a change in the lifetime of photoluminescence emission. Devices based on the hybrid core-shell nanoparticles in a suitable hole-transporting layer with two dissimilar metal electrodes show efficient photovoltaic performance. Here, following the electron-transfer, electrons flow through the organic molecules and holes, left in the nanoparticles, move through the hole-transporting polymer to the opposite electrodes to yield photovoltaic short-circuit current. The role of CdSe nanoparticles in light-harvesting and charge-generation has been substantiated by control experiments with ZnS nanoparticles in the core. In ZnS-based hybrid core-shell systems, photovoltaic performance is low since photoinduced electron-transfer does not occur from ZnS to the dye.

}, doi = {10.1021/cm902404s}, author = {Guchhait, Asim and Rath, Arup Kumar and Pal, Amlan J.} } @article {42581, title = {Induce negative differential resistance in organic devices through a ferroelectric polymer}, journal = {Organic Electronics}, volume = {10}, year = {2009}, month = {JUN}, pages = {1116{\textendash}1119}, abstract = {

We report how ferroelectric materials induce negative differential resistance (NDR) in organic devices. Fluorescein, which exhibits semiconducting current{\textendash}voltage characteristics, shows NDR effect in a ferroelectric matrix. Here, we vary the concentration of fluorescein in the ferroelectric matrix to study its effect on NDR. We also show how the degree of polarization controls NDR. We infer that under a suitable bias, the ferroelectric polymer becomes polarized to facilitate electron-injection in the device followed by a double-reduction of fluorescein molecules. From the capacitance{\textendash}voltage measurements, we substantiate the role of polarization in inducing NDR effect in organic molecules.

}, doi = {10.1016/j.orgel.2009.05.027}, author = {Rath, Arup Kumar and Pal, Amlan J.} } @article {42580, title = {Organic memory and electrical bistability in a quinone {\textendash}based charge transfer complex}, journal = {Proceedings of the IEEE}, volume = {97}, year = {2009}, month = {AUG}, abstract = {

We present an overview of the issues of organic memory devices and discuss the mechanisms involved in conductance switching. To make the memory elements addressable, we introduce nanostructures of a quinone-based charge-transfer complex. The devices based on charge-transfer complexes exhibit electrical bistability. Apart from characterizing complex formation, we study characteristics of memory devices based on the complexes. The mechanism of bistability has been discussed in terms of electroreduction of the quinone derivative with the formation of a percolating network of conducting molecules or channels across the device. Depending on the device architecture, a device may exhibit memoryswitching or threshold-switching phenomenon. The former system has displayed read-only and random-access memory applications.

}, doi = {10.1109/JPROC.2009.2021962}, author = {Rath, Arup Kumar and Pal, Amlan J.} } @article {42579, title = {Mn-doped nanocrystals in light-emitting diodes: energy-transfer to obtain electroluminescence from quantum dots}, journal = {Applied Physics Letters}, volume = {97}, year = {2010}, month = {SEP}, pages = {Article number: 113502}, abstract = {

We fabricatelight-emitting diodes(LEDs) based on Mn-doped ZnSnanocrystals along with hole-transporting bis(3-methylphenyl)--diphenyl-benzidine (TPD). With Mn-doping, ZnSnanostructures exhibit a strong photoluminescence. The LEDs exhibit electroluminescence(EL) from Mn-doped ZnSquantum dots and TPD. In order to open up channels for energy-transfer from TPD to quantum dots and to achieve EL from only the nanoparticles, we grow core-shell nanoparticles with Mn-doped ZnS in the core and CdS as the shell layer. Excitons formed in TPD can now transfer their energy directly to the shell-layer to yield EL from only the nanoparticles.

}, doi = {10.1063/1.3489099}, author = {Rath, Arup Kumar and Bhaumik, Saikat and Pal, Amlan J.} } @article {42578, title = {Near-IR activity of hybrid solar cells: enhancement of efficiency by dissociating excitons generated in PbS nanoparticles}, journal = {Applied Physics Letters}, volume = {96}, year = {2010}, month = {FEB}, pages = {Article no: 073505}, abstract = {

Photovoltaic devices based on PbSnanoparticles remained inactive in the near-IR region due to a not-so-favorable energy band-diagram that does not allow dissociation of excitons generated in PbS. In this work, with the introduction of nanostructures in the PbS-based hybrid system, we show an enhancement of photovoltaic performance in both visible and near-IR regions. The addition of increases the power conversion efficiency from 0.006\% to 0.12\%. With the aid of energy band-diagram, we show that excitons generated in PbS even in the near-IR range can now become dissociated to yield photocurrent in the external circuit.

}, doi = {10.1063/1.3292183}, author = {Guchhait, Asim and Rath, Arup Kumar and Pal, Amlan J.} } @article {42577, title = {To make polymer: quantum dot hybrid solar cells NIR-active by increasing diameter of PbS nanoparticles}, journal = {Solar Energy Materials and Solar Cells}, volume = {95}, year = {2011}, month = {OCT}, pages = {651{\textendash}656}, abstract = {

We fabricate NIR-active solar cells based on PbS quantum dots and a conventional conjugated polymer. These devices act as solar cells under exclusively NIR wavelengths above 650 nm. Here PbS nanoparticles absorb photons in the NIR range that in turn generate excitons. We show that with an assistance from a strong electron-acceptor (TiO2), these excitons can be dissociated to electrons and holes to yield a photocurrent in the external circuit. We then aim to extend the spectral window of the solar cells to higher wavelength region by increasing the diameter of PbS nanoparticles to make the cells further NIR-active. We observe that the short-circuit current (JSC) shows a peak when the diameter of PbS nanoparticles increases. Here, the spectral window can be extended till conduction band-edge of PbS quantum dots falls below that of TiO2 nanostructures cutting off the electron-transfer pathway. The NIR-active photovoltaic solar cells yield a short-circuit current (JSC) of 1.0 mA/cm2, open-circuit voltage (VOC) of 0.42 V, and power conversion efficiency (η) of 0.16\% and remain operative till 1200 nm.

}, doi = {10.1016/j.solmat.2010.09.034}, author = {Guchhait, Asim and Rath, Arup Kumar and Pal, Amlan J.} }