Zinc oxide nanorods as electron transport layer in MEH-PPV/ZnO organic light emitting diodes / Shafinaz Sobihana Shariffudin

This thesis discusses on the development of zinc oxide (ZnO) nanorods for the application as electron transport layer in MEH-PPV/ZnO Organic Light Emitting Diode (OLED). In general, the mobility of holes is higher than electrons in most semiconducting organic material. Therefore, it is essential to develop an efficient electron transport layer in order to balance out the number of carriers, hence increases the efficiency of the device. Furthermore, combining inorganic material with organic material in a device can take advantages of both materials. The thesis work involved three main parts for the fabrication of MEH-PPV/ZnO OLEDs. The first part was the deposition of novel layer-by-layer ZnO seeded catalyst. High (0 0 2) oaxis ZnO seed catalyst was prepared using simple and low cost method of sol-gel spin coating technique. Layer-by-layer method was applied in the deposition of the ZnO seed catalyst layer, which is defined by annealing of every layer of the thin film. The second part involved the growth of ZnO nanorods using thermal chemical vapour deposition (TCVD) method by employing vapour-solid (VS) mechanism, without using pump vacuum to control the pressure in the tube. Using single furnace TCVD, substrate positions with a distance of 3 cm from the zinc powder, at an oxygen flow rate of 5 seem and temperature of 825°C were found to be the optimum conditions for the growth of the ZnO nanorods in this study. The characterization revealed that the nanorods had low resistivity of 5 x 10⁻² Ω-cm at highest (0 0 2) crystalline structure. This strongly indicates that the ZnO nanorods had good electrical behaviour which can transport electrons more efficiently in the device. Third part involved the study of MEH-PPV thin films dissolved in non-aromatic solvent such as 1,2 dichlorobenzene and toluene to produce p-type conducting emissive layer. With an optimum solution concentration of 5 mg/ml, highest photoluminescence (PL) spectrum was produced with an emission centred at 590 nm. From the optimized thin films, OLEDs were fabricated. As a conclusion, by combining the ZnO nanorods with MEH-PPV in the OLEDs had improved the electrical characteristics of the device compared to single layer MEH-PPV based device. The fabricated device worked as a diode; which showed a rectifying current. The lowest turn on voltage was found to be at 0.4V using the ZnO nanorods grown at temperature of 825°C. At this deposition temperature, the Schottky barrier height was 0.59 eV, with an ideality factor of 12.91. The OLEDs were also tested using electroluminescence (EL) spectroscopy to prove the devices' functionality. The EL emission spectra were found to match with the PL emission for both ZnO and MEH-PPV thin films

Signal-to-noise ratio study on pipelined fast fourier transform processor

Fast Fourier transform (FFT) processor is a prevailing tool in converting signal in time domain to frequency domain. This paper provides signal-to-noise ratio (SNR) study on 16-point pipelined FFT processor implemented on field-programable gate array (FPGA). This processor can be used in vast digital signal applications such as wireless sensor network, digital video broadcasting and many more. These applications require accuracy in their data communication part, that is why SNR is an important analysis. SNR is a measure of signal strength relative to noise. The measurement is usually in decibles (dB). Previously, SNR studies have been carried out in software simulation, for example in Matlab. However, in this paper, pipelined FFT and SNR modules are developed in hardware form. SNR module is designed in Modelsim using Verilog code before implemented on FPGA board. The SNR module is connected directly to the output of the pipelined FFT module. Three different pipelined FFT with different architectures were studied. The result shows that SNR for radix-8 and R4SDC FFT architecture design are above 40dB, which represent a very excellent signal. SNR module on the FPGA and the SNR results of different pipelined FFT architecture can be consider as the novelty of this paper

Effect of Deposition Temperature on Self-Catalyzed ZnO Nanorods via Chemical Vapour Deposition Method

The morphological, structural, optical and electrical properties of ZnO nanorods are investigated as a function of deposition temperature. The ZnO nanorods were grown on ZnO seed catalyst layer at temperatures between 750oC – 825oC using thermal chemical vapour deposition method.  Sample deposited at 825oC showed the highest crystalline orientation. The FE-SEM micrographs and the intense peak along (002) direction in the XRD spectra of this sample implied that the nanorods possess c-axis orientation. PL spectra showed two common ZnO peaks which centered at 380 nm and 540 nm. Two-point probe I-V measurement revealed ohmic behaviour with the gold metal contact, whereby the current increase with the deposition temperature

Effect of metal catalysts type and annealing time on the growth of zinc oxide nanostructures by thermal vapor deposition method

This paper reports the results of zinc oxide (ZnO) nanostructure growth on different types of metal catalysts, namely gold and platinum, and also the effect of annealing time of the metal catalysts prior to the deposition of ZnO nanostructures. The metal catalysts layers with 15 nm thickness were deposited on glass substrates by sputter coater and then annealed in air ambient for 15 and 30 min at 500 ˚C. ZnO nanostructure was then deposited on the metal catalysts by thermal chemical vapour deposition (TCVD) method. We found that the Au catalyst morphologies varied with the annealing time, and the growth morphology of the ZnO followed the morphology of the Au catalyst. The morphology of the metal catalysts and ZnO nanostructures were characterized using field emission scanning electron microscopy (FESEM). The grown ZnO nanostructures were tested for their ability for extended gate field effect transistor (EGFET) sensor application. The samples were attached to the gate of an NFET and were dipped in acid and alkali buffer solutions while the gate voltage was measured. We found that the extended gate gave different voltage in buffer solutions with different pH which indicated that the samples can act as the extended gate of an EGFET sensor

Effects of Annealing Environments on the Solution-Grown, Aligned Aluminium-Doped Zinc Oxide Nanorod-Array-Based Ultraviolet Photoconductive Sensor

We have fabricated metal-semiconductor-metal- (MSM-) type ultraviolet (UV) photoconductive sensors using aluminium- (Al-) doped zinc oxide (ZnO) nanorod arrays that were annealed in different environments: air, oxygen, or a vacuum. The Al-doped ZnO nanorods had an average diameter of 60 nm with a thickness of approximately 600 nm that included the seed layer (with thickness ~200 nm). Our results show that the vacuum-annealed nanorod-array-based UV photoconductive sensor has the highest photocurrent value of   2.43  ×  10-4 A. The high photocurrent is due to the high concentration of zinc (Zn) interstitials in the vacuum-annealed nanorod arrays. In contrast, the oxygen-annealing process applied to the Al-doped ZnO nanorod arrays produced highly sensitive UV photoconductive sensors, in which the sensitivity reached 55.6, due to the surface properties of the oxygen-annealed nanorods, which have a higher affinity for oxygen adsorption than the other samples and were thereby capable of reducing the sensor’s dark current. In addition, the sensor fabricated using the oxygen-annealed nanorod arrays had the lowest rise and decay time constants. Our result shows that the annealing environment greatly affects the surface condition and properties of the Al-doped ZnO nanorod arrays, which influences the performance of the UV photoconductive sensors