M. A. Rafiq

Charge injection and trapping in silicon nanocrystals

The temperature dependence of the conduction mechanism in thin films of ∼8nm diameter silicon nanocrystals is investigated using Al∕Sinanocrystal∕p‐Si∕Al diodes. A film thickness of 300 nm is used. From 300 to 200 K, space charge limited current, in the presence of an exponential distribution of trapping states, dominates the conduction mechanism. Using this model, a trap density Nt=2.3×1017cm−3 and a characteristic trap temperature Tt=1670K can be extracted. The trap density is within an order of magnitude of the nanocrystal number density, suggesting that most nanocrystals trap single or a few carriers at most.

Hopping conduction in size-controlled Si nanocrystals

We investigate the temperature dependence of conduction in size-controlled silicon nanocrystals. The nanocrystals are ∼8nm in diameter, covered by ∼1.5nm thick SiO2 shells. In 300nm thick films for temperatures T from 30to200K, the conductivity σ follows a ln(σ) vs 1∕T1∕2 dependence. This may be associated with either percolation-hopping conductance or Efros-Shklovskii variable range hopping. Assuming hopping sites only on the nanocrystals, the data agree well with the percolation model.

Enhancement in the multiferroic properties of BiFeO3 by charge compensated aliovalent substitution of Ba and Nb

Polycrystalline ceramics, Bi1-2xBa2xFe1-x Nb xO3 (x = 0.00–0.15), were synthesized by solid state reactions method. X-ray diffraction data have revealed elimination of impurity phases and an increase in unit cell volume with Ba and Nb substitution. Diffraction peak splitting is found to be suppressed which indicates a decrease in octahedral distortion. The Mossbauer spectra demonstrate the suppression of spiral spin modulation of the magnetic moments resulting in enhanced ferromagnetism with increasing dopant concentration. The leakage current density of the sample with x = 0.10 is found to be greatly reduced up to six orders of magnitude as compared to the undoped sample. Ohmic conduction is found to be dominant mechanism in all the samples, however, undoped sample showed space charge limited conduction in high electric filed region, while the sample with x = 0.15 exhibited grain boundary limited conduction in low electric field region.

Influence of nanocrystal size on the transport properties of Si nanocrystals

In this study, we have investigated the carrier transport mechanism across silicon nanocrystals with the Al/p-Si/Si nanocrystals/Al structure. Sizes of silicon nanocrystals were controlled at diameters of ∼6, ∼8, and ∼11 nm. It is shown that the conductivity σ of silicon nanocrystals, both as-grown and annealed, exhibits σ∝exp[−(T0/T)]1/2 behavior under low electrical fields and over a wide temperature range. The phenomenon of material constant T0 increasing with the decrease of nanocrystal size has been observed. Considering nanocrystal size effect, experimental results can be explained by the hopping-percolation model. The influence of nanocrystal size on transport properties has been discussed. Based on this model, changes in T0 after annealing treatment are attributed to an increase in effective decay length.

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.

Room temperature single electron charging in single silicon nanochains

Single-electron charging effects are observed at room temperature in single Si nanochains. The nanochains, grown by thermal evaporation of SiO solid sources, consist of a series of Si nanocrystals ∼10nm in diameter, separated by SiO2 regions. Multiple step Coulomb staircase current-voltage characteristics are observed at 300K in devices using single, selected, nanochains. The characteristics are investigated using a model where the nanochain forms a multiple tunnel junction. The single-electron charging energy for a nanocrystal within the multiple-tunnel junction is EC=e2∕2Ceff∼0.32eV, ∼12kBT at 300K.

Temperature induced delocalization of charge carriers and metallic phase in Co0.6Sn0.4Fe2O4 nanoparticles

Thermally induced semiconductor to metal transition has been investigated for tin doped cobalt ferrite nanoparticles using impedance spectroscopy in a wide frequency range (100 Hz-2 MHz) from 300 K to 400 K. In addition dc measurements are carried out in temperature range from 285 K to 410 K. Temperature dependence of impedance spectroscopy and dc resistivity reveal semiconductor to metal transition around 360 K. Metallic nature of the system above 360 K has been attributed to dominancy of delocalized charge carriers Fe3+–Fe2+/Co3+–Co2+ interactions over localized charge carriers Fe3–O2−–Fe3+/Co2+–O2−–Co2+ interactions. Interesting temperature dependent electrical behavior of the grain boundaries is reported and has been discussed in term of depletion space-charge layer in the vicinity of grain boundaries.

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.