INFLUENCE OF REACTION TEMPERATURE ON OPTICAL ...

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Advances in Optics, Photonics, Spectroscopy & Applications VI ISSN 1859 - 4271theCdSeTPssynthesizedathigherreactiontemperatureshavelargersize,thisbaseonthestrongershiftofabsorptionandPLspectratothelongwavelength;(i)FortheCdSeTPssynthesizedathigherreactiontemperatures,theappearancethedouble-peakstructureinPLspectraoccurearly,andthelongwavelengthPLpeakwasdisappearedquickly.Therefore,thePLcharacteristicsofCdSeTPsdependstronglyontheirsize.TheincreaseofthesizeofCdSeTPsleadstheappearancethedouble-peakstructureinPLspectraandthelongwavelengthPLpeakintensitydecreasesgradualyandthendisappears.BasingontheslowtemporalevolutionofabsorptionandPLspectraofCdSeTPssynthesizedat160 o C,thetransitionstagesfromthesingle-peakstructuretothedouble-peakstructureintheopticalcharacteristicscouldbeobservedindetail.Thepicture4presentsTEM imagesofCdSeTPssynthesizedat160 o Cwithdiferentreactiontimes.ItcanbeseenthattheshapeofCdSeTPsbecomesmoreclearlyandthesizeofthem increasewithreactiontime.TEM imagesindicatedtherelationbetweenthevariationofopticalcharacteristicsofCdSeTPsandthesizeevolutionofthem.(a) (b) (c)20nm20nm20nmFig. 4. TEM images of CdSe TPs synthesized at 160 o C with different time: (a) 3 min;(b) 60 min and (c) 140 min.260 (a)Temperature( o C)2402202001800 60 120 180Reactiontime(min)Fig. 5. Scheme of temperature – time and TEM images of CdSe TPssynthesized at 0.25 min of reaction time.Thus,itisposiblethattheappearancethehighenergyPLpeakandtheincreaseoftheirintensitywithreactiontimeisrelatedtothegrowthofthearmsofTPs.However,thestagebeginstoappearthehighenergyPLpeakwasn’tobservedclearlyinfig.3,duetotheoverlapofitandthelowenergyPLpeak.Therefore,twoPLpeaksmustbeseparatedbythesynthesisCdSeTPswithdiameterofcorewasrelativelylargerthanthatofarms.Figure5(a)presentsthe468
Những tiến bộ trong Quang học, Quang phổ và Ứng dụng VI ISSN 1859 - 4271schemesynthesistemperature-timewasappliedtoperformthisidea.Byusingthetemperatureofnucleationprocesat260 o Candgrowthprocesat200 o Candsuitablereactionparameters,wehavesynthesizedtheCdSeTPswithdiameterofcorewasrelativelylargerthanthatofarmsascanbeseeninFig.5(b).Thereactiontemperatureandmonomerconcentrationarelowerinthesecondstageofthereactionproces,thiscausetheslowgrowthofCdSeTPs.Thus,thechangeofopticalcharacteristicsofCdSeTPscouldbeobservedindetail.Figure6(a)showstheabsorptionandPLspectraofCdSeTPsatdiferentreactiontimes.ThePLspectraofthesamplesintherangefrom 0.25to80minofreactiontimearecomparedcarefulyintheFigure6(b).TheseparationoftwoPLpeakoccurat0.25minofreactiontime,thusthechangeoftheopticalcharacteristicscanbeobservedclearlyinthegrowthofTPsforthissamples.NormalizedIntensity(a)min1801206020123(b)min80(b)0,250,25400 480 560 640 720500 550 600 650 700Wavelength(nm)Wavelength(nm)Fig. 6. The temporal evolution of absorption and PL in the range from 0.25 to 180 min (a)and PL spectra from 0.25 to 80 min (b).Figure6(b)indicatedthegradualincreaseofhighenergyPLpeakandgradualdecreaseoflowenergyPLpeakwithreactiontime.At180minofreactiontime(fig.6(a),thelowenergyPLpeakisdisappearedcompletely.Simultaneously,thespacing betweenthetwoPL peaksgradualyincreaseswithincreasingreactiontime.NormalizedIntensityIntensity(a.u.)500 550 600 650 700Wavelength (nm)Fig. 7. The absorption spectrum of a TP CdSe sample and its second differential line (dot line).469
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INFLUENCE OF REACTION TEMPERATURE ON OPTICAL ...

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