In the present work, we report fabrication and electrical characterization of a back gated graphene field effect transistor (GFET). We have focused our study on the interfacial effect (graphene/SiO2) on the performance of the device. Hysteresis was observed in the drain conductance when measured with respect to dual gate sweep voltage, which increases with increasing sweeping voltage range. The conductance was observed to increase with increase in temperature but there was no reduction in the hysteresis. This proved that temperature annealing could improve the channel conductivity but not the interfacial effects. Further, a metal oxide semiconductor (MOS) device was fabricated with SLG inserted in between the metal and oxide layer and its capacitance-voltage (C-V) characteristics were studied. A small series capacitance (2.1 nF) was observed to be existing in series with the oxide capacitance (4.5 nF) which was attributed to the trap states at the interface of graphene and SiO2 layer. Also, the flat band voltage was not affected by the incorporation of graphene layer in the MOS device indicating no change in the work function of the metal gate (Cr/Au). This is an advantageous situation where graphene does not alter its work function also being impermeable, restricts the diffusion of metal particles through the SiO2.

The effect of incorporation of graphene nanosheets on the efficiency of poly [2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) based light emitting diodes (LED) has been examined by varying the graphene concentration from 0 to 0.1wt%. It was observed that graphene doping enhances the photoluminescence (PL) emission from the PPV layer by ~6 times at the blending concentration of 0.005wt%. This is attributed to the isolation of individual polymer chains that quenches the inter-chain relaxations and boosts the intra-chain transitions. The improvement in device luminance is also found to be ~6 times as compared to that with MEH-PPV only LED at 0.005wt% graphene concentration. This is due to the high charge carrier mobility in graphene nanostructure that assists in balancing the charge carrier concentration in the emissive layer. Also due to its low LUMO level, graphene improves electron injection from the cathode. Both these effects lead to enhancement in the device luminescence. Employment of graphene in this manner also leads to lowering of turn-on voltage of the device. This is attributed to the ability of graphene sheets to establish an interconnected conducting network in the polymer matrix that lowers the device resistance. However, at higher graphene concentration, this property short circuits the device structure, which greatly deteriorates its performance. The graphene concentration, therefore, should be kept below the percolation threshold level to develop high efficiency devices.

CuInS 2 (CIS) quantum dots (QDs) with different diameters were prepared and their optical properties were studied. The optical band gap of QDs, as estimated by absorption spectrum, was found to decrease with increase in size. The stokes shift between absorption and photoluminescence peaks was observed to be larger (>100 meV) in all the three samples. This shows that the defect states available in the forbidden gap dominates the recombination mechanism. The variation in the emission peak with QD size, however, indicates that the relaxation dynamics in CIS QDs involves both excitonic level as well as the defect states.

An ethanol sensor based on graphene and functionalized-multiwalled carbon nanotubes (f -MWCNTs) composite has been fabricated using sodium dodecyl sulphate (SDS) as surfactant and characterized by Raman Spectroscopy and Keithley Source meter. The dispersion of f -MWCNT and graphene is found to be homogeneous. f -MWCNT forms a bridge network between graphene sheets which led to an enhancement in the sensitivity of ethanol.

Single layer graphene (SLG) grown by chemical vapor deposition (CVD) has been investigated for its prospective application as horizontal interconnects in very large scale integrated circuits. However, the major bottleneck for its successful application is its degraded electronic transport properties due to the resist residual trapped in the grain boundaries and on the surface of the polycrystalline CVD graphene during multi-step lithographic processes, leading to increase in its sheet resistance up to 5 MΩ/sq. To overcome this problem, current induced annealing has been employed, which helps to bring down the sheet resistance to 10 kΩ/sq (of the order of its initial value). Moreover, the maximum current density of ∼1.2 × 107 A/cm2 has been obtained for SLG (1 × 2.5 μm2) on SiO2/Si substrate, which is about an order higher than that of conventionally used copper interconnects.

In the present work, the effect of incorporation of graphene on the poly(3-hexylthiophene) (P3HT):CuInS2 quantum dot (CIS QD)-based solar cell has been studied. For this purpose, the concentration of graphene is varied from 0 to 0.01% w/w in P3HT-CIS (1: 0.5) film. It is found that graphene does not deteriorate the absorption of the composite film. It assists in dissociating the photogenerated excitons (both in P3HT and QDs) owing to its two-dimensional structure and high electron affinity as is evident by photoluminescence (PL) quenching. At 0.01% w/w concentration of graphene about similar to 95% of PL is quenched. The electrical characteristics show that the incorporation of graphene enhances the efficiency of the device by establishing interconnected conducting pathways in the volume of polymer matrix. The maximum efficiency is observed to be 1.5% at 0.005% w/w content of graphene. However, at higher concentration, i.e., 0.01% w/w, the device starts deteriorating. (c) 2014 Society of Photo-Optical Instrumentation Engineers (SPIE)

Transport behavior of single layer graphene (SLG) grown by chemical vapor deposition technique on copper foil and transferred to SiO2/Si substrate has been studied by measuring the dc conductivity and Hall mobility in the temperature range 2–460K. The samples of size 1×1cm2 have been found to be polycrystalline in nature. Raman spectrum has been studied at various locations of the sample and the formation of SLG has been confirmed. From dc conductivity and mobility measurements it has been concluded that the one dimensional grain boundary defects are mainly responsible for the deterioration of mobility and conductivity of charge carriers in the polycrystalline samples.

Solar cell consisting of low band gap polymer poly[N-900-hepta-decanyl-2,7-carbazole-alt-5,5-(40,70-di-2-thienyl-20,10,30-benzothiadiazole)] (PCDTBT) as donor and cadmium selenide/zinc sulphide (CdSe/ZnS) core shell quantum dots (QDs) as an acceptor has been developed. The absorption measurements show that the absorption coefficient increases in bulk heterojunction (BHJ) structure covering broad absorption spectrum (200nm-700nm). Also, the photoluminescence (PL) of the PCDTBT:QDs film is found to decrease by an order of magnitude showing a significant transfer of electrons to the QDs. With this approach and under broadband white light with an irradiance of 8.19 mW/cm(2), we have been able to achieve a power conversion efficiency (PCE) of 3.1 % with fill factor 0.42 for our typical solar cell.

Hybrid solar cells consisting of a composite of poly (3-hexylthiophene) (P3HT), single walled carbon nanotube (SWCNT), and cadmium selenide/zinc sulphide (CdSe/ZnS) coreshell quantum dots (QDs) have been fabricated in the present work. The bulk hetrojunction has been formed from the bilayer of P3HT:SWCNT composite and QDs using inter-diffusion process. Due to low percolation limit and high conductivity of SWCNT, the photo-generated electrons are collected at the electrode very fast (within few femto-seconds) enhancing the efficiency of the solar cell. The absorption measurements on the composite film show that the addition of SWCNT in the hybrid structure increases the absorption coefficient in the near infrared region and also makes the spectrum wider as compared to that of P3HT. The photoluminescence (PL) measurements show that the PL of hybrid P3HT, SWCNT, and QDs is quenched about ∼15 times as compared to that of P3HT film. This shows that a significant charge transfer of electrons occurs through SWCNT to the electrode. The morphology of P3HT:SWCNT:CdSe/ZnS was observed using atomic force microscopy. With this approach, we have been able to achieve power conversion efficiency of 5.4% using a standard solar simulator with an irradiance of 100 mW/cm2.

Absorption spectrum of the graphene grown by chemical vapor deposition technique on the Nickel substrate has been studied in the energy range 1.55 to 6.53 eV at room temperature. The Raman studies proved our samples to be turbostratic multilayer polycrystalline graphene with crystallite size of 761 nm. In high energy optical range, two prominent peaks are observed in the absorption spectrum. The first peak at 5.35 eV is the combined result of the inter-band transition between pi and pi* bands and inter-layer plasmonic interactions. Whereas, unusually high energy resonant exciton leads a second peak at 6.35 eV

Bulk heterojunction (BHJ) solar cells consisting of poly(3-hexylthiophene) (P3HT) as donor and cadmium selenide/zinc sulphide (CdSe/ZnS) core shell quantum dots (QDs) as acceptor have been developed. Starting from the bilayer of P3HT/QD structure a BHJ is induced using the process of thermal inter-diffusion. The absorption measurements on the bilayer structure show that the absorption coefficient increases and the absorption spectrum becomes broader in the annealed device. Also, the photoluminescence of the annealed device is found to decrease by an order of magnitude showing a significant transfer of electrons to the QDs. With this approach and under broadband white light with an irradiance of 8.19 mW/cm2, we have been able to achieve a power conversion efficiency of 5.1% and fill factor 0.45 for this solar cell.

CdSexS1-xquantum dots (QDs) have been grown in borosilicate glass matrix using two step annealing technique and are characterized by AFM, XRD, UV-Vis absorption, HRTEM and Raman spectroscopy measurements. AFM studies show that nearly spherical clusters of about 5–10 nanocrystals were grown. From XRD measurements, it is found that the nanocrystals are grown in hexagonal phase. From HRTEM measurements, it is observed that the size of QDs increases from 5.1 to 9.5 nm with increasing annealing duration from 3 to 10 hrs. Blue shift in absorption spectrum indicates that quantum confinement increases with decreasing annealing durations. Raman spectrum of CdSexS1-xQDs shows two mode type of behavior i.e., CdSe like longitudinal optical phonon mode (LO1) and CdS like longitudinal optical phonon mode (LO2). It has been observed that with increasing the size of QDs from 5.1 to 9.5 nm, LO1shifts from 205 to 210 cm-1while LO2shifts from 278 to 291 cm-1. The shift in the LO1and LO2Raman peaks towards high frequencies has been interpreted due to increase in the lattice compressive strain with the increase in QDs size. The effect of phonon negative dispersion on the shift of LO1and LO2is found to be comparatively insignificant. It is also found that the ratio of intensity of overtones to the fundamental mode increases from 0.34 to 0.48 with decreasing QD size. This indicates that the electron–phonon coupling increases with the decrease in size of QD due to increasing overlap of electron and hole wave function in small size QDs. It is further observed that the frequency of the surface optical phonon mode remains unchanged as the size of QD changes but the intensity decreases with the increase in the size of QDs finally merging in LO mode Raman spectrum.

The combined effect of a salmon deoxyribonucleic acid (DNA)-based electron blocking layer and a single walled carbon nanotube (SWCNT) composite-based electron transport layer on the performance of a poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) polymer light emitting diode (PLED) has been examined. The SWCNT network in the composite layer improves electron injection from cathode and the DNA blocks these high mobility electrons at the electron blocking layer–polymer interface, leading to high luminance from the device. The luminous efficiency of the PLED is increased ∼20 times compared to that of a PLED using only MEH-PPV.

An improved strategy has been developed for detection of DNA sequence by using water-soluble cationic conjugated polymer (PFP)/single-strand (ss) DNA and peptide nucleic acid labeled with fluorescent dye (PNAC*), where an anionic surfactant (sodium dodecyl sulphate, SDS) system has been used to improve the sensitivity of the sensor. The method of detection is simple to use, fast and cost-effective. This method uses the phenomenon of Forester Resonance Energy Transfer (FRET). The detection sensitivity of the biosensor has been improved by about ten times by using the anionic surfactant. It is observed that the effect of surfactant is to increase the photoluminescence (PL) intensity of the PNAC* when the sequence of the DNA is complementary (to that of PNA probe). On the other hand when the two sequences are non-complementary, the PL intensity of the PNAC* is further reduced as compared to the case when surfactant was absent.

Zinc oxide (ZnO) nanorods grown by the electrochemical technique have been used to enhance the luminance of poly[2-methoxy-5-(2'-ethylhexoxy)-1,4-phenylenevinylene] (MEH-PPV)-based polymer light-emitting diodes. The luminance of the device with ZnO nanorods is found to increase by more than two times as compared with the device without ZnO nanorods. The diameter of the nanorods used in device fabrication was similar to 145 nm. The size of the nanorods was estimated from field emission scanning electron microscope images. Optical and structural characterizations of the nanorods were also performed by using absorption, photoluminescence and x-ray diffraction, confirming the formation of ZnO nanorods.

CdS quantum dots (QDs) in a polyvinyl alcohol (PVA) matrix have been grown by a chemical method and are characterized by transmission electron microscopy (TEM), UV-vis absorption, photoluminescence (PL) and energy dispersive x-ray diffraction (EDX). TEM studies of CdS films show that a nearly spherical cluster of CdS QDs with an average radius of 10-15 nm is formed. From absorption measurements, it is observed that with increasing the PVA concentration from 5 to 8 wt.%, the absorption edge shifts from 3.1 to 3.6 eV, which is attributed to an increase in quantum confinement with decreasing the QD size. PL studies in an energy range of 1.8-3.3 eV show two distinct peaks. The higher-energy peak corresponds to band edge emission, whereas the lower-energy peak corresponds to defect emission. EDX results revealed that the atomic concentration of cadmium is much lower than that of sulfur, indicating that cadmium vacancies are predominant. It was concluded that cadmium vacancies are mainly responsible for defect emission in the PL spectrum.

An efficient white polymer light-emitting diode (WPLED) with stable Commission Internationale de l’éclairage (CIE) coordinates is fabricated. A blue electroluminescence (EL)-emitting conducting polymer [poly(9,9-di-n-hexyl-fluorenyl-2,7-diyl)] is used as a host for red [Bis(1-phenyl-isoquinoline)(acetylacetonate)iridium(III)] and green [iridium(III)tris(2-(4-tolyl)pyridinato-N,C2)] phosphorescent dyes. Although efficient triplet energy transfer is not possible in the green phosphorescent dye, the self-trapping mechanism is utilized for the emission of EL in the green region while an efficient triplet exciton energy transfer from the host to the red dye is utilized for EL in the red wavelength region. Concentrations of the three constituents are optimized to obtain pure white light of appropriate CIE coordinates. An efficient electron-blocking layer based on a biomaterial (salmon-DNA) is also incorporated in the WPLED to improve the device performance. The WPLED shows three distinguished peaks for the primary colors and achieved a maximum luminance and luminous efficiency of 350 cd/m2 and 0.86 cd/A, respectively.

An efficient electron transporting layer (ETL) based on single walled carbon nanotube (SWCNT) composites has been developed for poly [2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) based orange polymer light emitting diodes (PLEDs) and its effect on the performance of PLEDs has been examined. It is observed that with increase in SWCNT concentration, in ETL, the luminance and luminous efficiency of the PLEDs increase (about 5 times increase in luminance is observed at 5%w/w SWCNT concentration). The SWCNTs present in the MEH-PPV ETL boost the mobility of electrons injected from the cathode towards the emissive layer by establishing highly conducting percolation paths. This balances the concentration of holes and electrons in the emissive layer, which leads to enhanced emission from the PLEDs.