Tejashree M. Bhave

Experimental imaging of silicon nanotubes

Transmission electron microscopy (TEM), electron energy loss near edge structures (EELNES) and scanning tunneling microscopy (STM) were used to distinguish silicon nanotubes (SiNT) among the reaction products of a gas phase condensation synthesis. TEM images exhibit the tubular nature with a well-defined wall. The EELNES spectra performed on each single nanotube show that they are constituted by nonoxidized silicon atoms. STM images show that they have diameter ranging from 2 to 35 nm, have an atomic arrangement compatible with a puckered structure and different chiralities. Moreover, the I-V curves showed that SiNT can be semiconducting as well as metallic in character.

Swift heavy ion induced growth of nanocrystalline silicon in silicon oxide

Recystallization of nanocrystalline silicon in silicon oxide has been initiated with swift heavy ion irradiation. 100 MeV Ni ions from pelletron were used for irradiating the thin films of silicon oxide (SiOx) at fluences varying from 1×1012 to 5×1013 ions/cm2. Phase separation between silicon and silicon oxide is seen to be responsible for the photoluminescence spectrum peaking around 350 and 610 nm. This spectral nature is understood on the basis of defects and interface states in SiOx matrix and silicon nanocrystals, respectively. The formation of silicon nanocrystals resulting from the phase separation has been confirmed from the complimentary evidence of change in the refractive index, Fourier transform infrared spectroscopy, and energy despersive x-ray analysis. High electronic loss associated with the 100 MeV Ni ions is thought to be responsible for the recrystallization, and rearrangement of silicon.

Photoluminescence and I-V characteristics of a CdS-nanoparticles-porous-silicon heterojunction

Chemically capped CdS nanoparticles are embedded in porous silicon (PS) by a dip coating method. Atomic force microscopy measurements reveal that the PS surface is covered with CdS nanoparticles forming well-defined rectangular blocks of nearly uniform size (200×200 nm2). Photoelectron spectroscopy and energy dispersive x-ray analysis confirm the presence of CdS in PS. Optical and electrical properties of the heterojunctions so-formed are investigated. Junction characteristics show that the composite so-formed exhibits very high forward current density (145 mA cm-2) and high reverse breakdown voltage (15 V).

Improvement in the photoluminescence efficiency of porous silicon using high-energy silicon ion irradiation

An order of magnitude enhancement in the intensity of visible room temperature photoluminescence (PL) from porous silicon (PS) was observed when irradiated with 10 MeV silicon ions from a pelletron source. The effect was associated with a blue shift of 70 nm. The stability of PL with respect to ambients was seen to be remarkably improved. The energy band gap determined from photoluminescence and photoreflectance measurements indicated a shift from 1.67 eV to 1.83 eV. Subsequently partial restructuring of Si - O - Si and Si - H type species into Si - OH was confirmed by infrared measurements recorded before and after irradiation. The effects have been correlated to a reduction in the extent of non radiative recombination centres as a consequence of chemical restructuring of the surface. By assuming that the restructured surface is sufficiently thick to reduce the crystallite size, the blue shift can be accounted for. Formation of Si - OH bonds at the surface was also observed when PS was intentionally exposed to low-energy (10 - 30 eV) ions from an electron cyclotron resonance (ECR) plasma; associated with this, the PL intensity was enhanced.

FTIR studies of swift silicon and oxygen ion irradiated porous silicon

Fourier Transform Infrared Spectroscopy has been used to study the bond restructuring in silicon and oxygen irradiated porous silicon. Boron doped p-type (111) porous silicon was irradiated with 10 MeV silicon and a 14 MeV oxygen ions at different doses ranging between 1012 and 1014 ions cm−2. The yield of PL in porous silicon irradiated samples was observed to increase considerably while in oxygen irradiated samples it was seen to improve only by a small extent for lower doses whereas it decreased for higher doses. The results were interpreted in view of the relative intensities of the absorption peaks associated with O–Si–H and Si–H stretch bonds.

Radiation induced recrystallisation and enhancement in photoluminescence from porous silicon

Abstract Porous Silicon (PS) samples were irradiated with different ionizing radiations which included 1 and 6 MeV electrons, 60 Co gamma rays and 10 MeV silicon ions. Improvement in the efficiency of photoluminescence (PL) and its stability with time were invariably observed in all the irradiated PS samples. Improvement in the luminescent properties was best for samples irradiated with 10 MeV silicon ions. Partial restructuring of SiOSi and SiH type species into SiOH was confirmed from the infrared spectra of pre- and post- irradiated samples. Grazing angle X-ray diffraction (XRD) analysis revealed that preferential recrystallisation occurs in the irradiated region. The virgin PS sample exhibited only the (1 1 1) peak in the XRD pattern; whereas the irradiated PS sample showed a (3 1 1) peak along with the (1 1 1) peak. The average size of the microcrystallites was calculated from the diffraction peak broadening, using Scherrers formula. Depth profile studies, corresponding to the average sizes of the microcrystallites confirmed the existence of (3 1 1) planes, and revealed that the degree of recrystallisation is maximum at the end of the trajectories of silicon ions.

Novel route for the synthesis of surfactant-assisted MoBi2(Se0.5Te0.5)5 thin films for solar cell applications

We have successfully synthesized MoBi2(Se0.5Te0.5)5 thin films by using the self-organised arrested precipitation technique (APT) on ultrasonically cleaned glass and FTO-coated glass substrates. In the present investigation, we have synthesized MoBi2(Se0.5Te0.5)5 thin films and studied the effects of the surfactants sodium dodecyl sulphate (SDS) and tri-n-octyl phosphine oxide (TOPO). The effects of the different surfactants on the opto-structural, morphological, compositional and electrical properties of the MoBi2(Se0.5Te0.5)5 thin films were investigated by using UV-visible-NIR spectrophotometry, X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), electrical conductivity (EC) and thermoelectric power (TEP) measurements. Optical study revealed that the as-deposited, SDS-assisted and TOPO-assisted thin films exhibited band gap energies of 1.46, 1.51 and 1.62 eV, respectively. XRD study confirmed that the surfactant-assisted thin films have broad and intense diffraction peaks compared to the as-synthesized thin film, indicating a more crystalline nature. SEM images confirmed that all the thin films are well adherent without containing any cracks, and that the different surfactants play a vital role in engineering of the surface morphology. The AFM study clearly showed that the surface roughness decreases for the surfactant-assisted thin films. The TEM and SAED patterns demonstrated that all samples are nanosized with porous surface morphology. The EDS and XPS measurements confirmed nearly stoichiometric thin film formation. The electrical study showed that all samples are semiconducting in nature with n-type conductivity. Finally these prepared electrodes were tested for their photoelectrochemical (PEC) performance in polysulphide electrolyte and promising results were obtained.

Low temperature and controlled synthesis of Bi2(S1−xSex)3 thin films using a simple chemical route: effect of bath composition

Nanostructured bismuth sulphoselenide (Bi2(S1−xSex)3) thin films have been synthesized using a simple, cost-effective chemical bath deposition (CBD) method at room temperature (300 K). Structural, compositional, morphological and optical characterization and photoelectrochemical performance testing of these Bi2(S1−xSex)3 thin films has been carried out. The X-ray diffraction (XRD) study demonstrates that these thin films are nanocrystalline in nature with pure orthorhombic crystal structures. X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray spectroscopy (EDS) show that the deposited thin films are nearly stoichiometric in nature. Field emission scanning electron microscopy (FESEM) reveals different morphologies for the Bi2(S1−xSex)3 thin films. The linear nature of the plots seen in the UV-Vis-NIR absorption study confirms the direct allowed type of transition. J–V measurements with a solar simulator were carried out for all samples and the highest photoconversion efficiency, 0.3845%, has been recorded for the Bi2Se3 thin film. The significant boost in photoelectrochemical (PEC) performance might be due to the larger surface area with lower dislocation density and microstrain with a lower level of grain boundary resistance of Bi2Se3 thin films.

A Novel Optical Fiber Humidity Sensor Coated with Superhydrophilic Silica Nanoparticles

This work presents an optical fiber based relative humidity sensor. Highly mono‐dispersed and stable silica nanoparticles were synthesized by colloidal route. Layer by layer technique has been utilized to create ultrathin films of silica nanoparticles on the optical fiber end. This fabrication technique is a simple method based on the alternative immersion in oppositely charged colloidal solutions. The coated thin film of silica nanoparticles acts as nanometer‐scale interferometric cavity. Sensor shows a high sensitivity and fast response towards relative humidity change.

Photon assisted conducting atomic force microscopy study of nanostructured additives in P3HT:PCBM

One of the ways of improving the efficiency of polymer solar cells is to increase the conductive paths in the photoactive layer. The present work focuses on the study of the effect of additives (silver nanoparticles (Ag NPs) and graphene (Gr)) in photoactive poly-3-hexyl thiophene:phenyl-C61-butyric acid methyl ester (P3HT:PCBM). The morphology and localised photocurrent obtained using Photon Assisted Conducting Atomic Force Microscopy reflect the role of these additives in the photocurrent produced by the active layer. The study depicts that the morphology of a P3HT:PCBM film changes completely with Gr additives, whereas a very small change occurs with the addition of Ag NPs. A localised photocurrent measurement exhibits that the space charge limited conduction (SCLC) phenomenon could be the dominant process of charge conduction in the P3HT:PCBM film with additives. The study also demonstrates that the carrier mobility is enhanced by more than an order of magnitude with Gr as the additive. This is a significant change for achieving efficient charge separation and transportation in polymer solar cell application.