Studies On Synthesis, Characterization, Optical And Magnetic Properties Of Transition Metal Ions Doped CdS Nanostructures

Among II-VI semiconductors, CdS is one of a remarkable semiconductor material due toits unique properties and extensive use in many fields including solar cell, light-emittingdiodes, nanolasers, field-effect transistors, photodetectors, gas sensors, optical switches,etc. Although CdS has shown its potential as dilute magnetic semiconductors (DMS), butstill doping effect of transition metals (TM) is needed to be explored. In this researchwork, we aim to investigate the synthesis, morphologies, growth mechanism, optical andmagnetic properties of TM ion (Mn(II), Ni(II),&Co(II))-doped CdS nanowires/nanobeltsvia condition-controlled thermal evaporation methods. The main research findings areshown as follows:1-1-D Mn(II)-doped CdS nanobelts with controlled Mn ions concentration are synthesizedthrough a Chemical Vapor Deposition (CVD) technique. Surprisingly, these belts revealtunable emissions from570nm to850nm and ferromagnetic responses simultaneously,with Mn aggregation number. The aggregation Phenomenon strongly depends on thedoping concentration, the temperature and reaction time at which the samples weresynthesized. These aggregates with ferromagnetism and shifted luminescence haverelation to the excitonic magnetic polaron (EMP) and localized EMP formations,subsequently verified by ab-initio calculations. The correlation between optical emissionsand ferromagnetic responses are investigated in detail. These ferromagnetic properties ofDMS can be used to fabricate the spin related photonic devices in future.2-High quality Ni(II)-doped CdS nanoribbons have shown novel near-infrared (IR)emission with ferromagnetic character. The near-IR emission corresponds to two highlevel of d electronic state of Ni with strong p-d hybridization between Ni ions and S,which seldom observed in these DMS with Mn doping. The doping of Ni as a transitionmetal in CdS is further confirmed by energy dispersive X-ray (EDX) spectroscopy andnon-uniform peak shifting of Raman spectrum. The p-d hybridization is verified throughfirst-principles electronic band structure calculations of Ni-doped CdS wires andphotoluminescence (PL) lifetime measurements. This material can be used as nanoscalelight source tuned by heavier TM doping, possibly in solar cell applications.3-Later on, to verify experimentally the correlation between EMP formation andferromagnetic response, we have also conducted the studied on Co(II) doped CdSnanobelts prepared by the same technique. Co doping is confirmed by EDX, XPS and Raman spectroscopy. Room temperature PL spectrum shows that the individual Co(II)-doped CdS nanobelt have a strong green emission centered at513.6nm and an orangeemission at606.5nm. The former emission is related to CdS with EMP transition state,while the later one to the d-d transition4T1(P)â†'4A2(F) within Co ion, similar to Ni ion. Itis also a kind of intermediate d-d transition state. On increasing the doping concentration,redshift of green emission band may correspond to EMP formation and d-bandlocalization in orange-red band, associated with the doped Co cations and ferromagneticcoupling, named LEMP. Their PL intensities up to373K have shown the sametemperature dependence as its ferromagnetic response. The successful doping of Co ion inCdS nanobelts and formation of EMP has been reported for the first time, which can beused for spintronics devices.4-At the end, under appropriate conditions, novel morphologies such as Cd metal semi-spheres and Sea-Urchin-like CdS microstructures have been synthesized by using CVDmethod.(1) Novel Cd metal semi-spheres were synthesized in bulk quantity throughthermal evaporation method and EDX and XPS analysis are sufficient to prove the highpurity of metallic microstructures. Room temperature PL spectrum has shown intenseemission band at around591nm. This is the first report about such a strong emission inCd metallic microsphere. This phenomenon can be attributed to radiative recombination ofelectrons in the s, p conduction band near the Fermi surface and the holes at the d bandsdue to plasmonic band at local microstructures. Its emission wavelength can be tuned byadjusting the microsphere’s size. Surface enhanced Raman scattering (SERS) resultsconfirm that our Cd semi-spheres are as strong as the silver metal particles do for SERSagents. We might infer from these results that our Cd spheres may be more cheaper forapplications in LEDs and other optoelectronic device applications, for example, theplasmonic enhanced solar cells.(2) Novel morphology of CdS sea-urchin-like wurtzitethree dimensional microstructures prepared by CVD method are of high purity and well-crystallized. Room temperature PL measurements of individual sea-urchin-likemicrostructures have shown strong green emission, even lasing occur at extremely lowthreshold intensity of9.07μJ cm-2. These results are important in the design of futuregreen luminescence and display devices.