Optical Properties of Ultraviolet Quantum Dot Light-Emitting Devices Using ZnO-Cores
With a MgO-Shell
Wen-Jian Kuang,Qing Li,Jiang-Yong Pan,Xiang Liu,Dong-Ping Li,Jing Chen,and Harm Tolner
Abstract—Colloidal quantum dots(QDs)with a ZnO core were synthesized with subquent surface passivation by MgO. The photo-enhancement effect of the near band-edge photolu-minescence(PL)in ZnO nanocrystals,disperd in ethanol,is investigated by obrving changes in the optical properties after ultraviolet(UV)exposure.Finally,the operation as a light-emit-ting device(LED)was demonstrated experimentally for the ZnO-bad quantum dots(QDs).The results indicate that this type of QDs might enable the development of electroluminescent UV sources in the near future.
Index Terms—Electroluminescent,MgO,nanocrystals,photolu-minescence(PL),quantum dots(QDs),ultraviolet(UV),ZnO.
I.I NTRODUCTION
C OLLOIDAL quantum dots(QDs)are solution-procesd
miconducting nanocrystals,and have promising appli-cations in optoelectronic and biomedical technologies[1]–[3]. The QDs with unique size-dependent optical properties have motivated increasingly active rearch aimed for the next gener-ation of light-emitting devices(LEDs).There are some factors that limit the performance of QD-LEDs,such as nonradiative re-combination(including Auger recombination)and spatial p-aration between electrons and holes.By coating the QDs by a thin shell of an inorganic miconductor with a wider bandgap, it has been proven that the photoluminescence(PL)efficiency (photons emitted/absorbed)and photostability of QDs can be in-cread dramatically.The shell-coating process passivates non-radiative recombination sites at the surface more effectively
Manuscript received November06,2014;revid January03,2015;accepted January27,2015.Date of publication January28,2015;date of current version April27,2015.This work was supported in part by the National Basic Rearch Program of China under Grant2013CB328803and Grant2013CB328804,in part by the National High Technology Rearch and Development Program of China under Grant2012AA03A302and Grant2013AA011004,and in part by the National Natural Science Foundation of China under Grant61271053, 91333118and51202028.(Corresponding author:Qing Li.)
The authors are with the Display R&D Center,School of Electronic Sci-ence and Engineering,2Sipailou,Southeast University,Nanjing210096,China (e-mail:e-mail:liqing@u.edu;;220131137@u.edu; ;chenjing@u.).
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Digital Object Identifier10.1109/JDT.2015.2397472than organic ligands alone away from surface trap states.[4], [5]Auger recombination is responsible for efficiency roll-off (also known as droop)at high-driving currents in both conven-tional LEDs and QD-LEDs[6]–[8].The addition of a shell with a miconductor that has a higher conduction-band level can im-prove the charge balance within the QDs by impeding electron injection,which leads to higher efficiency and better roll-off be-havior in devices[9].
The emerging ZnO–MgO core-shell material is one of the promising active nanostructures,thanks to
the ultraviolet (UV)light-emission at the near-band-edge(NBE)emission, while the stability and performance of ZnO nanocrystals are improved by the MgO passivation[10]–[12].The large value of the exciton binding energy of MgO(80meV)and small radius(1.5nm)is advantageous for an efficient transport of excitons[13]–[15].The bottom of the conduction-band of MgO crystals is located around the vacuum energy(depending on the references,the electron affinity is stated as either positive or negative)[16]–[19].This is suppod to balance the injection of electrons and holes by impeding electron injection[9]. One-and two-dimensional ZnO materials,such as nano-wires,[20]needles,[21]and epitaxial layers[22],[23] have shown clear NBE dominated electroluminescence(EL). For the QD-LEDs,UV emissive devices have not yet been studied well,as they are much more difficult to realize than visible light devices.There are only a few articles describing LEDs that u ZnO QDs as the emitting material,but no distinct NBE was obrved[24]–[26].UV-illumination of ZnO QDs has shown to result in the quenching of the deep-trap pho-toluminescence(DPL)and an increa of the NBE intensity, [27],[28]by adding the MgO shell[12].The incread NBE emission was considered to be related to either the abnce of adsorbed oxygen on the surface of the ZnO particles,[27]or to the reduction of surface non-radiative recombination paths [28].
In this paper,we perform a detailed investigation on the op-tical properties of ZnO-MgO core-shell Q
Ds under photo-exci-tation above the ZnO bandgap.The aim of this work is to study the near band-edge luminescence of ZnO-bad QDs,in order to develop UV QD-LEDs and UV detectors with a bandgap that is tunable by changing the QD particle size.This type of QDs is expected to enable in the near future the development of UV QD-LEDs with an emission wavelength of365nm or shorter.A prototype has been constructed that is bad on ZnO QDs using a QD-LED structure.
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II.E XPERIMENTAL D ETAILS
A.Synthesis of ZnO–MgO QDs
ZnO cores were prepared in air,using a well-known process, [29]that was slightly modified.Zinc acetate dihydrate(5 mmol,1.10g)was added to aflask containing50ml ethanol at70C,while LiOH(6.9mmol,0.29g)was ultrasonically disperd in50ml ethanol at0 C.A reaction between the two solutions was initiated by pouring the hydroxide solution into the zinc solution,under vigorous stir
ring,and keeping it at room temperature for different times,to grow ZnO cores with slightly incread sizes(ripening process).For the subquent overcoating of the MgO shell,5mmol(1.07g)of magnesium acetate tetrahydrate was added into the synthesized ZnO cores solution and dissolved ultrasonically.The resulting solution was stored at room temperature for about3days.Three times (3)the volume in heptane was added to the resulting solution andfinally the resulting ZnO-MgO core-shell QDs were pa-rated by centrifugation at10000rpm and purified by repeated precipitation with ethanol/heptane(1:4volume ratio)to remove the remaining acetate and Li ions;finally,it was re-disperd in ethanol.The PL and absorption spectra of ZnO–MgO QD samples did not change after5months of storage,which is coincident with the results in references[10]–[12].
B.Fabrication of the QD-LEDs
A patterned ITO glass substrate was cleaned quentially with water,acetone,iso-propanol,andfinally UV-ozone treated for30min.First,a hole injection layer(HIL)of PEDOT:PSS(Baytron P VP AI4083)was spin-coated at3000 rpm for60s,and then baked at C for30min in a ni-trogen-filled glovebox.Thereafter a hole transport layer(HTL) of Poly(N-vinylcarbazole)(PVK,10mg/mL in chlorobenzene) was deposited on top of HIL with the same spinning and baking conditions as above.The ZnO-MgO QDs(0.2mol/L in ethanol) emitter layer(EML)was deposited by spin-coating at1
400rpm for60s,followed by baking at80C for30min.The thickness of the EML is about60nm.An electron transport layer(ETL) consisting of TiO nanoparticles was formed from a precursor [DuPont Tyzor BTP](5%wt in butanol),and this was then spin-coated(3000rpm,60s),followed by baking at150C for15min.Finally,a100nm thick Al cathode was thermally evaporated on top of the ETL,thus completing the multilayered structure of the QD-LEDs.
C.Characterization
Transmission electron microscopy(TEM)images of the ZnO-MgO core-shell QDs were obtained using a JEOL JEM-2100electron microscope.Absorption spectra were recorded by UV/Vis absorption spectroscopy(Shimadzu, UV-2450).PL spectra were measured by a Shimadzu UV-VIS-NIR UV-3600spectrophotometer.UV-exposure treatment was achieved by a commercial UV-fluorescent lamp (Philips TUV TL Mini)with an intensity of about6mW cm, while the PL excitation was done by a280nm UV LED with intensity of about0.5mW cm.UV-lamp-illumination PL,and EL spectra
were taken using an Ocean Optics Maya Pro2000UV/Vis spectrometer.The current density and voltage Fig. 1.(a)Low-magnification(scale-bar10nm)and high-magnification (scale-bar1nm)TEM images of Sample A type ZnO-MgO QDs.(b)XRD pattern of sample A type ZnO-MgO QDs,compared with the peak positions of standard ZnO crystals(dashed lines).
TABLE I
C HARACTERISICS OF T HREE
D IFFERENT Z N O C OR
E S AMPLES. characteristics of the UV QD-LEDs were recorded at ambient
conditions by a Keithley2400m.
III.R ESULTS AND D ISCUSSION
A.ZnO QD-Core Growth With Aging Time
Before coating the ZnO cores with MgO,the samples were continued to ripen during different times,in order to achieve QDs with slightly incread sizes.The ripening times were4 hours(sample A),24hours(sample B)and50hours(sample C),respectively.In Fig.1the X-ray diffraction(XRD)pattern of the QD sample A is compared with that of bulk ZnO crys-tals;it clearly shows that ZnO nano-crystals have been formed. The averaged particle diameter can be estimated by applying Scherrer's formula for the spherical particles[30]
(1) where is the X-ray wavelength,is full-width at half-max-imum(FWHM),and is the Bragg diffraction angle.We then find(Table I)that the diameter of the ZnO cores ranges from4.4–4.8nm.This means that the particle radii are clo to that of the bulk ZnO exciton Bohr radius of2.34nm[31].
中译日翻译器KUANG et al.:OPTICAL PROPERTIES OF ULTRA VIOLET QD LEDs
怎样挽留女友
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Fig.2.(a)Absorption and(b)PL spectra of ZnO–MgO QDs samples A(4h ZnO-core growing time),B(24h growing),and C(50h growing).ZnO mmol/L.Note:the dashed lines indicate the shift in the band gap.
The bandgap is estimatedfirst by using a linear plot of the ab-sorption of the QD suspension versus wavelength.As discusd in detail by Klingshirn,[32]the resulting value is however only an indication of the bandgap value,as it neglects exciton ab-sorption just below the bandgap completely.But it can be ud to monitor the shift of this value as a function of the growing time.
A more accurate method to derive the bandgap is the u of the short wavelength emission peak values,when the samples are excited by photons that have an energy above the bandgap value of ZnO.The emitted spectra are given in Fig.2,and show two major emission bands,with peak wavelengths of and respectively,which also are indicated in Table I.The EPL is the excitonic photoluminescence near the band edge, that originates from the radiative recombination of electrons and holes,after the generation of excitons,bound to donor and ac-ceptor levels.[32]The DPL band around550nm,is generally considered to be caud by deep-trap levels related to oxygen vacancies[33],[34].This is the familiar green phosphorescence from ZnO.
For the largest crystals wefind that the EPL peak wavelength is at365nm,so eV.Assuming an exciton binding energy of60meV(valid for bulk ZnO),this then would mean that the bandgap value is3.46eV.The dominant EPL emission band however is not only caud by free exciton emission,but originates also from the bound exciton complex and from exci-tons trapped at the surface.In commer
cial ZnO powders this peak emission is located around3.30eV[32].The value of Eg eV is higher than the known value of Eg eV for bulk ZnO at room temperature.Also there is a blue shift of the EPL wavelength for smaller ZnO crystals.Both results in-dicate that quantum confinement effects play a significant role in our4nm MgO coated ZnO QDs.
B.Enhanced Band-Edge PL by UV Photo-Excitations
By applying UV illumination the DPL band can be almost fully quenched,while the EPL band is enhanced[12],[28].We find however that the DPL band fully recovers after about1000 s.Wefirst applied254nm(4.9eV)UV illumination from a UV-fluorescent lamp during50min to sample A solution.The UV exposure was carried out in optical quartz cuvettes with in-ternal dimension of21030mm.The sample A solution was diluted to2,4,6,and8
mmol/L,respectively.Fig.3shows the initial spectra,before(“initial”),and after(“0s”)the50min Fig.3.PL spectra of the ZnO-MgO QDs,at different times after stopping the UV-lamp-illumination.All of the samples are of type A in ethanol,with the concentration at:(a)2mmol/L;(b)4mmol/L;(c)6mmol/L;and(d)8mmol/L, respectively.The“initial”spectrum means that the PL from280nm excitation was obtained before applying the
UV-lamp-illumination.The green DPL band restores gradually with time to its initial intensity during about1020s.
Fig.4.(a)ZnO–MgO QDs EPL peak intensity and(b)the DPL peak amplitude. (Int:the photos of initial luminous QD solutions.)Note:open circles,solid circles,open squares and solid squares correspond to2,4,6,and8mmol/L, respectively.
UV exposure,and also the change in the spectra,after the dif-ferent waiting times.After1020s the EPL peak has been re-duced back to its original value before the UV exposure,while the DPL band has been fully recovered.
The changes in peak intensity recovering time,both for the EPL and DPL band are shown on Fig.4.After an initial expo-nential change,with time constant of the order of50s,the in-tensity changes are much slower.The rates of decay of the EPL and the DPL recovery however are different,and appear not to be related to each other.
The DPL reduction can be interpreted as a result of elec-tron–hole generation inside the ZnO cores.When ZnO cores are excited by light with an energy above the bandgap,a part of the photo-generated holes moves to MgO shells,while the electrons tend to stay in the cores.The recovery times of the PL intensities clearly depend on the solvent concentration ud,indicating that the process depends on the number of hole scavengers(ethanol solvent)[35].
contrast是什么意思
Fig.5shows the two major emission mechanisms of the ZnO–MgO QDs.The conduction band has discrete energy levels,due the quantum confinement effect.Electrons injected
464JOURNAL OF DISPLAY TECHNOLOGY ,VOL.11,NO.5,MAY
2015
Fig.5.Propod ZnO–MgO QDs emitting mechanisms in ethanol
solvent.
Fig. 6.(a)QD-LED device structure;(b)energy levels of the different QD-LED layers.(c)EL spectrum at 6.4V and Gaussian peak fit [int:absorption and PL spectra of PVK](d)422nm (circles),484nm (squares)and 637nm (triangles)peak intensity and current density versus voltage.不来梅大学
into (EL-mode)or excited (PL-mode)in the ZnO core occupy the lowest energy levels of the conduction band.The trap level at 2.45eV above the valence band is most probably caud by an oxygen vacancy VO [32].When electrons and holes recombine,then this is expected to result in NBE UV emission around 3.4eV (365nm),and 2.45eV (550nm)green emission [28].
C.Device Design and Performance
Fig.6(a)prents the device structure,while Fig.6(b)shows the energy-levels in the different layers.The energy diagram demonstrates that the HTL materials needs to have the highest occupied molecular orbital (HOMO)level that is aligned with the corresponding HOMO level of the ZnO QDs in order to assure the hole flow into the emissive QDs with minimal barrier for injection,whereas the HTL lowest unoccupied molecular orbital (LUMO)has to be sufficiently high to prevent electron leakage from the QDs into the HTL.
By applying an external voltage,the light emitted through the ITO coated plate,is detected.The resulti
ng EL emission spec-trum [Fig.6(c)]of the device can be matched by three Gaussian emission bands,with peaks at about 422,484,and 637nm.The 422nm emission is not the result of NBE from ZnO.Instead,it appears to be either the EL emission from PVK,or the fluores-cence from PVK,excited by the expected 365nm from the ZnO QDs.The int of Fig.6(c)shows the measured absorption and photoluminescence spectra of the PVK layer.The figure indi-cates that strong absorption occurs at 365nm,while producing fluorescent emission at about 422nm,detected at the outside of the QD-LED device.
The additional broad band emission band at 484nm is as-sumed to be related to the so-called “green emission band”,[32]obrved in the thin film ZnO QDs at 513nm (e Fig.S1in Supplementary Materials).The energy level is slightly lower than that of 2.45eV ,corresponding to pure oxygen va-cancies [32].The red emission band at 637nm cannot clearly be assigned to specific energy levels,but a similar emission band is also obrved in well annealed epitaxial ZnO films [32].Fig.6(d)shows the current-voltage characteristics of the device,and the peak intensities of the EL spectra for a driving voltage varying from 3.0to 6.8V.英语歌
IV .C ONCLUSION
MgO-coated ZnO–QDs with a particle sizes around 4nm have been made.The PL and EL properties have been inves-tigated.The PL results clearly show near-band-edge emission at around 365nm,and also the well known green emission re-sulting from oxygen vacancies.The green band can be fully sup-presd by 50min exposure to UV radiation,while this enhances the 365nm emission.The recovery times are dependent on the ethanol solvent concentration ud.An LED structure by using the MgO coated ZnO-QDs,shows promising results for future UV QD-LEDs.
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2001.
Wen-Jian Kuang received the B.S.degree in
information optics from Jiangnan University,China,
in2008.He is currently working toward the Ph.D.
degree from Southeast University,Nanjing,China.
His rearch at the Display Center of Southeast
ui是什么University primarily involves the analysis of ZnO
and MgO materials,and the development of UV
quantum-dot LEDs as well asflexible displays and
lighting.
Qing Li received the B.S.and M.S.degrees from
the University of Science and Technology,Nanjing,
China,and the Ph.D.degree from Southeast Univer-
sity,Nanjing,China.
Since1995,she has been with the School of
Electronic Science and Engineering,Southeast
University,Nanjing,China.She worked in the
Electronic Devices Institute in Nanjing,China,from
1990to1995.Her studyfield coversflat display
technology and opto-electronic devices rearch.
Her main rearch work focus on PDPs with the correlative material in plasma devices,on LCDs as well as optical devices using liquid crystal,and on quantum dot materials and their application in LEDs.Currently,she has authored or co-authored more than50publications and30patents..
Dr.Li is a nior member of the Society for Information Display.
Jiang-Yong Pan,photograph and biography not available at time of publication. Xiang Liu,photograph
and biography not available at time of publication. Dong-Ping Li,photograph and biography not available at time of publication. Jing Chen,photograph and biography not available at time of publication. Harm Tolner,photograph and biography not available at time of publication.
All in-text references underlined in blue are linked to publications on RearchGate, letting you access and read them immediately.
