Kamran Rasool

Charge conduction and relaxation in MoS2 nanoflakes synthesized by simple solid state reaction

We present the synthesis of crystalline MoS2 nanoflakes through self-exfoliation in a simple solid state reaction at temperature ∼650 °C. X-ray diffraction and Transmission Electron Microscope analysis indicate the formation of pure hexagonal phase MoS2 nanoflakes. Impedance and modulus plane plots from 20 Hz to 2 MHz show two relaxations associated with bulk and interface phases at temperatures from 180 K to 280 K. The conductivity obeys Motts 2D variable-range hopping phenomenon and density of localized states ∼3.42 × 1019 eV-1 cm−3 is extracted.

TiO2 nanoparticles and silicon nanowires hybrid device: Role of interface on electrical, dielectric, and photodetection properties

We report ∼12, 5, 12, 100, and 70 times enhancement of external quantum efficiency, detectivity, responsivity, AC conductivity, and overall dielectric constant (e′), respectively of hybrid silicon nanowires (SiNWs) and titania (TiO2) nanoparticles (NPs) device as compared to SiNWs only device. Devices show persistent photoconductivity. Metal assisted chemical etching and co-precipitation method were used to prepare SiNWs (length ∼40 μm, diameter ∼30–400 nm) and TiO2 NPs (diameter ∼50 nm), respectively. Formation of acceptor like states at NPs and SiNWs interface improves electrical properties. Presence of low refractive index TiO2 around SiNWs causing funneling of photon energy into SiNWs improves photodetection.We report ∼12, 5, 12, 100, and 70 times enhancement of external quantum efficiency, detectivity, responsivity, AC conductivity, and overall dielectric constant (e′), respectively of hybrid silicon nanowires (SiNWs) and titania (TiO2) nanoparticles (NPs) device as compared to SiNWs only device. Devices show persistent photoconductivity. Metal assisted chemical etching and co-precipitation method were used to prepare SiNWs (length ∼40 μm, diameter ∼30–400 nm) and TiO2 NPs (diameter ∼50 nm), respectively. Formation of acceptor like states at NPs and SiNWs interface improves electrical properties. Presence of low refractive index TiO2 around SiNWs causing funneling of photon energy into SiNWs improves photodetection.

Enhanced electrical and dielectric properties of polymer covered silicon nanowire arrays

We investigate DC and AC electrical characteristics of ∼40 μm long p-type silicon nanowire (SiNW) arrays, prepared by metal assisted chemical etching and filled with polyacrylic acid (PAA). PAA diluted to different concentrations with deionized (DI) water is incorporated into the arrays in three steps, yielding polymer filling to the bottom of the arrays. DC and AC electrical measurements show that PAA reduces the array resistance. The AC conductivity increases up to ∼1000 times, and the real part of the overall dielectric constant (ɛ′) by ∼100. We attribute these effects to the formation of acceptor-like states at nanowire/polymer interface.

Dielectric and transport properties of bismuth sulfide prepared by solid state reaction method

We report synthesis of bismuth sulfide (Bi2S3) via conventional solid state reaction method at low temperature ∼150 °C and ambient pressure. X-ray diffraction analysis confirmed the orthorhombic phase of prepared material. Transmission electron microscope images revealed the formation of nanorods having diameter ∼20 nm and length ∼100 nm to ∼150 nm. Impedance and modulus plane plots from 20 Hz to 2 MHz show presence of bulk and grain boundary phases in Bi2S3 at each measurement temperature from 310 K to 400 K. An equivalent circuit model comprised of two resistance–R and constant phase element-Q (RQ) loops in series explains the electrical parameters (resistance and capacitance) and relaxation processes coupled with grains and grain boundaries. The conduction in Bi2S3 obeyed adiabatic small polaron hopping model. High and temperature dependent dielectric constant was observed in Bi2S3 suggesting it as an efficient material to be used in capacitive energy storage devices.

Enhanced and persistent photoconductivity in vertical silicon nanowires and ZnS nanoparticles hybrid devices

We observe persistent photoconductivity, enhanced detectivity, responsivity, and external quantum efficiency (EQE) in hybrid ZnS nanoparticles (NPs) and vertical silicon nanowire (Si NW) devices. ZnS NPs (diameter ∼30 nm) were prepared by co-precipitation method. Si NWs (length ∼30 μm, diameter ∼30-400 nm) were prepared by electroless chemical etching. Hybrid device shows ∼10, 3, and 10 times enhancement of EQE, detectivity, and responsivity, respectively, as compared with the Si NWs only devices. The enhancement is attributed to presence of low refractive index ZnS around Si NWs causing funneling of photon energy into Si NWs.

Investigation of change in surface area and grain size of cadmium titanate nanofibers upon annealing and their effect on oxygen sensing.

We have investigated the phase changes in CdTiO3 nanofibers as the annealing temperature of nanofibers was increased from 600 to 1200 °C. The nanofibers annealed at 600 °C were ilmenite with a very small amount of CdO. Upon annealing at 950 °C, CdO was completely removed. Annealing at 1000 °C yielded pure perovskite nanofibers, and at temperatures above 1100 °C rutile TiO2 nanofibers were obtained. Brunauer-Emmett-Teller (BET) analysis showed that with increase in annealing temperature the surface area of nanofibers was decreased. The nanofibers annealed at 600 °C have the higher surface area of ∼9.41 m(2)/g. Then oxygen sensors using CdTiO3 nanofibers annealed at 600 °C (ilmenite) and 1000 °C (perovskite) were fabricated. The sensitivity of the ilmenite nanofibers sensor was 2 times than that of the perovskite nanofibers sensor. The response and recovery times were 120 and 23 s, respectively, for the ilmenite nanofibers sensor, whereas response and recovery times were 156 and 50 s, respectively, for the perovskite nanofibers sensor. Better oxygen characteristics of ilmenite nanofibers are attributed to their large surface area and porosity. Therefore, we believe that ilmenite CdTiO3 nanofibers are potential candidates to develop practical oxygen sensors.

The role of surface states in modification of carrier transport in silicon nanowires

We investigate transport properties of polyacrylic acid (PAA) capped n and p-type silicon nanowire (SiNW) arrays. PAA diluted with deionized water at different concentrations was spun directly on vertically grown SiNW arrays prepared by metal assisted electroless chemical etching. PAA provides mechanical support to electrical contacts and acts as a source of interface doping by creating acceptor like states (holes) on SiNWs surface. PAA capping results in increase in current in p-type SiNWs and decrease in current in n-type SiNWs. Schottky emission model fits current voltage (IV) characteristics of p-type SiNWs/PAA device. Ohmic like conduction at lower voltages followed by space charge limited current (SCLC) with and without traps is observed in p-type SiNWs, n-type SiNWs, and n-type SiNWs/PAA devices. Using SCLC model with exponential distribution of traps, the extracted trap density was 7.20 × 1011/cm3 and 6.0 × 1011/cm3 for p-type SiNWs and n-type SiNWs devices, respectively. Our findings also demonst...

Photodetection and transport properties of surface capped silicon nanowires arrays with polyacrylic acid

Efficient hybrid photodetector consisting of silicon nanowires (SiNWs) (∼40 μm) capped with Polyacrylic Acid (PAA) is demonstrated. Highly diluted PAA with deionized (DI) water was spun directly on vertical SiNW arrays prepared by metal assisted electroless chemical etching (MACE) technique. We have observed ∼9, 4 and 9 times enhancement in responsivity, detectivity and external quantum efficiency in SiNWs/PAA hybrid device in comparison to SiNWs only device. Higher electrical current and photodetection may be due to the increment of hydrophilic content (acceptor like states) on SiNWs interface. The higher photosensitivity can also be attributed to the presence of low refractive index PAA around SiNWs which causes funneling of photon energy into SiNWs. Surface roughness of SiNWs leads to immobilization of charge carriers and hence shows persistent photoconductivity.

Comparison of different phases of bismuth silicate nanofibers for photodegradation of organic dyes

Two different phases of bismuth silicate nanofibers [Bi2SiO5 and Bi4(SiO4)3] were synthesized using electrospinning technique. BS nanofibers were tested for the photocatalytic degradation of methyl orange and safranin O dyes. Different phases of BS affect the photodegradation efficiency of nanofibers. Impressive enhancement in photocatalytic efficiency and BET surface area of Bi4(SiO4)3 was observed over Bi2SiO5. A speedy reduction in dyes concentration was attributed to the rapid formation of oxygenated radicals by the capture of electrons and holes, generated in the BS nanofiber by UV irradiation. Therefore, the photocatalytic mechanism was elucidated using impedance spectroscopy at room temperature. The lower impedance value of Bi4(SiO4)3 nanofibers had improved high-efficiency charge transfer capability. The cycling efficiency (30 times) and recovery characteristics pointed out that Bi4(SiO4)3 nanofibers photocatalysts had high constancy, resilience, and regeneration ability.

Effect of different electrodes on the transport properties of ZnO nanofibers under humid environment

Relative humidity (RH) sensing properties of zinc oxide nanofibers (ZNF), synthesized using electrospinning technique, were studied by impedance spectroscopy. RH sensors were fabricated with two different electrodes (Au and Ni) using lithography on top of the nanofibers deposited on Si/SiO2 substrate. Compare with the Ni electrode sensor, Au electrode sensor exhibits larger sensitivity and quicker response/recovery. Capacitance, electrical conductivity and electrical modulus were studied at 40%-90% RH as a function of the frequency of the applied AC signal in the frequency range of 10−2-106 Hz. The corresponding response and recovery times are 3s and 5s for Au, and 6s and 10s for Ni electrode sensor, respectively. The sensors exhibited a reversible response with small hysteresis of less than 4% and 12% for Au and Ni electrodes respectively. Stability of the sensor device with Au electrode was confirmed by testing the device for 13 days. The excellent sensing characteristics and comparison of sensors with ...