Light-emitting Diodes Constructed By One-dimensional Nanostructural ZnO And Its Composite Materials

Light-emitting diodes constructed by one-dimensionalnanostructural ZnO and its composite materialsZinc oxide(ZnO),as an important wide-band-gap semiconductor,hasbeen widely investigated for applications in light-emitting diodes(LEDs),photovoltaic cells and lasers,because of its excellent photoelectricproperties.Since the first report of ultraviolet lasing from ZnOnanowires,one-dimensional ZnO nanostructures such as nanorods,nanotubes,nanobelts etc.,have been especially focused on for theirpotential uses in nano devices and optoelectronics.LEDs have the potential to become the primary lighting method,replacing conventional light sources(incandescent light bulbs andfluorescent lamp) due to their low energy consumption and high efficiency.ZnO is the most promising optoelectronic material to be commercializedafter GaN.How to exploit one-dimensional nanostructural ZnO better forLED applications has become a popular topic.This dissertation focuses on improving the luminous efficiency of ZnOLED and tuning the electroluminescent(EL) spectrum of one-dimensionalnanostructural ZnO via electrochemical approaches.The work can bedivided into two parts:(1)Electrochemical preparation andcharacterization of ZnO nanorods,ZnO nanorods/poly(3-methylthiophene)(PMT) composite film,ZnO nanorods/CuSCN composite film.Fabrication ofZnO nanorod LED,ZnO nanorod/PMT LED,ZnO nanorod/CuSCN LED.Their ELmechanisms are also studied.(2)Electrochemical preparation andcharacterization of ZnO nanotubes.Fabrication of ZnO nanotube LEDs.Boththe growth and white light EL mechanisms of ZnO nanotubes are discussed.The following text is the details:(A)Electrochemical preparation of ZnO nanorods and fabrication of ZnOnanorod LED1.We electrodeposited ZnO nanorods galvanostatically on ITO substrates.During the electrodeposition process,the crystal structuralcharacteristics of ZnO play an important part in the formation of ZnOnanorod.The hexagonal ZnO nanorods can cover the entire ITO substrateuniformly with an average diameter of about 130 nm.The atomic ratioof zinc to oxygen of as-grown ZnO nanorods is about l:l.The ZnOnanorods are single crystals of wurtzite structure and orientedpreferably along c axis.When exposed to UV illumination at 340 nm undera O V bias(vs SCE),ZnO nanorods exhibit anode photocurrent,indicating that ZnO nanorods are n type semiconductor.Thephotoluminescence(PL) spectrum of ZnO nanorods is made up of two parts,strong UV peak and weak visible emission.The UV emission is centeredat 382 nm,related to exciton combination.The weak visible broad-bandluminescence over 500-600 nm region is ascribed to defect-relatedtransitions.2.We fabricated ITO/ZnO/Al LED by electrochemical approaches.Theturn-on voltage is about 8 V.The EL spectrum of ZnO nanorods showsnearly identical features with their PL spectrum.(B) Electrochemical preparation of ZnO nanorods/PMT composite film andfabrication of ZnO nanorod/PMT LED1.We electropolymerized p-PMT film galvanostatically on the surface ofZnO nanorods.Both photocurrent and PL spectra indicate that ZnO nanorods in composite films contain more defects after the growth ofPMT.2.We fabricated ITO/ZnO/PMT/A1 LED by electrochemical approaches.TheI-V curve of ITO/ZnO/PMT/Al LED shows a good rectificationcharacteristic of p-n heterojunction.Compared with ITO/ZnO/Al LED,ITO/ZnO/PMT/A1 LED has a lower turn-on voltage of 7 V.Its EL spectrumshows similar features with ITO/ZnO/M LED.However,the emissionintensity is greatly enhanced and the UV emission of ITO/ZnO/PMT/AlLED is about three times stronger than ITO/ZnO/Al LED.When increasingthe deposition time of PMT,the visible emission can be improved morethan the UV peak.That is because the growth process of PMT can generatemore defects of ZnO nanorods.The EL enhancement phenomenon can beexplained by the quicker growth rate of forward current and thepresence of PMT layer.The PMT layer can lower the barrier of holeinjection and balance the electrons and holes injection rates.(C) Electrochemical preparation of ZnO nanorods/CuSCN composite film andfabrication of ZnO nanorod/CuSCN LED1.We electrodeposited a compact and uniform layer of p typeβ-CuSCNon the surface of ZnO nanorods.In the composite film,the contentof ZnO is about 7.9 times of CuSCN and the molar ratio of Cu to SCNis about 1.0.The transient photocurrents of ZnO nanorods/CuSCNcomposite film exibit sharp peaks,indicating that ZnO nanorods incomposite films contain more surface states after the growth of CuSCN,which is also certified by their PL spectrum.2.We fabricated ITO/ZnO/CuSCN/Au LED by electrochemical approaches.TheI-V curve of the ITO/ZnO/CuSCN/Au LED shows a good rectificationcharacteristic of p-n heterojunction.When applied a forward direct voltage of 7 V,the EL signal of ITO/ZnO/CuSCN/Au LED becomesdetectable.The EL spectrum consists of a weak UV peak centered at400 nm and a strong visible broad-band emission centered at 530 nm,which shows quite different features from the ITO/ZnO/Au LED.Thatis because the growth process of CuSCN can generate more suface statesof ZnO nanorods.The EL enhancement phenomenon can be explained bythe quicker growth rate of forward current and the presence of CuSCNlayer.As a good hole transport material,the CuSCN layer can balancethe electrons and holes injection rates and increase the probabilityof their efficient recombination.(D) Electrochemical preparation of ZnO nanotubes and fabrication of ZnOnanotube LEDs1.The ZnO nanorods were selectively dissolved to form nanotubes byapplying a constant cathodic current density.In our experiments,wechosel.35μA/cm~2,1.65μA/cm~2,2.00μA/cm~2 as the etching cathodiccurrents.The structural characteristics of ZnO play an important partin the formation of ZnO nanotube.Although the tube wall is partiallydissolved,the protective function of the EDA molecules on the lateralsurfaces and the metastability of the(O001) plane make the etchingrate in the [0001] direction much faster than at the lateral surfaces,resulting in the formation of the hollow-structured ZnO nanotubes.2.As-prepared ZnO nanotubes show the hexagonally faceted morphology,maily 200-300 nm in diameter.The wall thickness of ZnO nanotubes isabout 10-20 nm and the length is about 1-1.5μm.The ZnO nanotubescontain sections that are tubular and sections that maintain their rodform.The raman spectra,absorption spectra,transient photocurrentsand PL spectra indicate that ZnO nanotubes contain more surface states and bulk defects,which are generated during the etching process.Compared with the ZnO nanorods,the excitonic emission of ZnO nanotubesis much weaker and centered at 400 nm.Meanwhile,the broad-bandvisible PL of ZnO nanotubes in the range of 500-600 nm,which originatesfrom defects,becomes much stronger.3.We fabricated ITO/ZnO nanotube/Au LEDs by electrochemical approaches.Their turn-on voltages is about 7 V.At low bias values,the EL spectraconsist of an excitonic emission centered at 400 nm and a broadluminescence from 450 to 600 nm.The characteristics of the EL spectraare nearly identical to those of the PL spectra.Further increasingthe bias causes the EL intensity to increase drastically,and thevisible emission region also broadens gradually.A white light ELemission is visible by the naked eyes above a voltage of ca.12 V.Ata high bias value of 20 V,the EL spectrum almost covers the entirevisible spectrum from 400 to 600 nm.4.ZnO nanotube LEDs show a much higher current growth rate than ZnOnanorod LEDs.Due to the large surface area,ZnO nanotube LEDs offera superior total electric current with the same forward bias,whichmight also account for the greatly enhanced EL.The high defect densityof ZnOnanotubes is generated during the etching process.In addition,the generation of surface states is favored by the large specificsurface of ZnO nanotubes.The energy levels of the defects may bedistributed over a relatively wide range.By increasing the appliedvoltage,more radiative recombination centers can be activated,resulting in a white light emission from the ZnO nanotubes.