Organic bistable devices based on core/shell CdSe/ZnS nanoparticles embedded in a conducting poly …N -vinylcarbazole …polymer layerFushan Li,Dong-Ik Son,Seung-Mi Seo,Han-Moe Cha,Hyuk-Ju Kim,Bong-Jun Kim,Jae Hun Jung,and Tae Whan Kim a ͒Advanced Semiconductor Research Center,Division of Electronics and Computer Engineering,Hanyang University,17Haengdang-dong,Seongdong-gu,Seoul 133-791,Korea͑Received 16July 2007;accepted 16August 2007;published online 21September 2007͒Current-voltage measurements on the Al/͓CdSe/ZnS nanoparticles embedded in a hole-transporting poly ͑N -vinylcarbazole ͒͑PVK ͒layer ͔/indium tin oxide ͑ITO ͒/glass structures at 300K showed a nonvolatile electrical bistability behavior.Capacitance-voltage ͑C -V ͒measurements on the Al/͓CdSe/ZnS nanoparticles embedded in a PVK layer ͔/ITO/glass structures at 300K showed a metal-insulator-semiconductor behavior with a flatband voltage shift due to the existence of the CdSe/ZnS nanoparticles,indicative of trapping,storing,and emission of charges in the electronic states of the CdSe nanoparticles.Operating mechanisms for the Al/͓CdSe/ZnS nanoparticles embedded in the PVK layer ͔/ITO/glass devices are described on the basis of the C -V results.©2007American Institute of Physics .͓DOI:10.1063/1.2783189͔Organic structures containing inorganic nanoparticles have been particularly attractive due to interest in their prom-ising applications in electronic and optoelectronic devices 1–7because of their unique advantages of low-power consump-tion,high mechanical flexibility,and chemical structural ver-satility.Such hybrid organic/inorganic devices are also excel-lent candidates for potential applications in next-generation transistor and memory devices.8,9Potential applications of memory devices utilizing nanoparticles embedded in organic layers have driven extensive effort to form various kinds of nanoparticles.10,11Even though some studies concerning the formation of metal nanoparticles embedded in an organic layer for applications such as nonvolatile organic bistable devices ͑OBDs ͒have been conducted,almost all of the devices were fabricated by using strin-gent high-vacuum evaporation method.12–14The memory effects of core/shell-type cadmium selenium ͑CdSe ͒nano-particles embedded in a conducting poly ͓2-methoxy-5-͑2-ethylhexyloxy ͒-1,4-phenylene-vinylene ͔͑MEH-PPV ͒poly-mer fabricated by using a simple spin-coating technique were reported.15Because the narrow band gap of MEH-PPV leads to a low charge capturing efficiency,resulting in the realization of memory effect at a high bias voltage of 10V,a hole transport poly ͑N -vinylcarbazole ͒͑PVK ͒matrix can be introduced here to obtain the memory effects in CdSe/PVK nanocomposites under an applied bias voltage as small as 2V.Furthermore,studies on the memory effects and their operating mechanisms for OBDs made of semiconductor nanoparticles embedded in a conducting polymer are very important for improving the efficiencies of nonvolatile flash memories.This letter reports data for the bistability and the operat-ing mechanisms of the memory effects of OBDs fabricated utilizing CdSe semiconductor nanoparticles embedded in a PVK polymer layer.Core/shell-type CdSe nanoparticles have become particularly attractive because of their promising ap-plications in next-generation nonvolatile flash memory de-vices with low-power and ultrahigh-density elements.16,17Current-voltage ͑I -V ͒measurements were carried out to in-vestigate the electrical bistable properties of the fabricated OBDs containing CdSe/ZnS nanoparticles embedded in the PVK layer.Capacitance-voltage ͑C -V ͒measurements were carried out to investigate the possibility of fabricating memory effects involving the CdSe/ZnS nanoparticles em-bedded in the PVK layer.Furthermore,the dependence of the memory effects on the thickness of the PVK layer containing CdSe/ZnS nanoparticles was also investigated.The CdSe/ZnS nanoparticles with a diameter of about 6nm were purchased commercially,and a schematic dia-gram of the core/shell-type CdSe/ZnS nanoparticles is shown in Fig.1͑a ͒.The device with a structure shown in Fig.1͑b ͒was fabricated through the following process:At first,the indium tin oxide ͑ITO ͒coated glass acting as a hole-injection layer in the OBDs was alternately cleaned with a chemical cleaning procedure by using trichloroehylene,ac-etone,and methanol solutions.Then,the PVK layer contain-ing the CdSe/ZnS nanoparticles was formed by spin coating a chloroform solution of 1.3%by weight PVK and 0.5%by weight CdSe/ZnS nanoparticles.Finally,a top Al electrode layer with a thickness of about 800nm was deposited by thermal evaporation.The I -V and C -V measurements were performed by using an HP 4284precision LCR meter at room temperature.Figure 2shows I -V curves for the Al/͑CdSe/ZnS nano-particles embedded in the PVK layer ͒/ITO/glass OBD struc-a ͒Author to whom correspondence should be addressed;electronic mail:twk@hanyang.ac.kr FIG.1.Schematic diagrams of the CdSe/ZnS nanoparticles and the fabri-cated device studied in this study.APPLIED PHYSICS LETTERS 91,122111͑2007͒0003-6951/2007/91͑12͒/122111/3/$23.00©2007American Institute of Physics 91,122111-1Downloaded 22 Oct 2007 to 166.104.58.178. Redistribution subject to AIP license or copyright, see /apl/copyright.jspture with a 500-nm-thick active layer.The I -V curve under a forward bias voltage,denoted by the empty rectangles in the lower curve of the Fig.2,shows a dramatic increase in the injection current at about 1V,indicative of a bistable tran-sition of the device from a low-conductivity state ͑off state ͒to a high-conductivity state ͑on state ͒.The bistable transition from the off state to the on state is equivalent to the “writing”process in a digital memory cell.18After that transition is finished,the on state remains in the device even after turning off the power,which is shown in the reverse bias voltage denoted by the filled rectangles in the upper curve of Fig.2.Figure 2clearly shows an electrical hysteresis behavior,which is an essential feature for bistable devices,and reveals the nonvolatile nature of the memory effect.19While the cur-rent difference between the on and off states for the PVK only device is negligible,the bistability in the Al/͑CdSe/ZnS nanoparticles embedded in the PVK layer ͒/ITO/glass device might be attributed to the screening of the applied electric field due to the existence of the internal electric field gener-ated by the captured charge carriers in the CdSe nanopar-ticles.The off state can be recovered by applying a reverse bias voltage.This is equivalent to the “erasing”process of a digital memory cell resulting from the discharge of CdSe nanoparticles.The solid line in Fig.2corresponds to the I -V curve of the device after the application of a −2V bias and is almost identical to the I -V curve denoted by the empty square.The observation that the OBD device main-tains an on state at a reverse applied voltage is similar to those reported for an Al/2-amino-4,5-imidazoledicarbonitrile ͑AIDCN ͒organic/Al layer/AIDCN organic/Al system 18and an Al/AIDCN organic/Al nanoclusters/AIDCN organic/Al system.19The C -V curves measured at 1MHz for the Al/͑CdSe/ZnS nanoparticles embedded in the PVK layer ͒/ITO/glass OBD structure are shown in Fig.3.The C -V curve shows a metal-insulator-semiconductor ͑MIS ͒behavior with charge trap regions,and the C -V behavior is similar to that of MIS memories with floating gates containing Si nanocrystals.20,21A clockwise hysteresis is clearly observedin the C -V characteristics,indicative of the existence of sites occupied by charges.The presence of such sites is attributed to carrier charging and discharging in the CdSe/ZnS nano-particles.The flatband voltage shift of the C -V curve for the OBDs with a relatively thinner hybrid layer of 500nm,which originates from charge accumulation and depletion due to variations in the applied voltage,is approximately 3V,as shown in Fig.2͑a ͒,which is enough to capture car-riers inside the nanoparticles.The C -V curve for samples without CdSe nanoparticles under identical measurement conditions showed no hysteresis.Therefore,the hysteresis appearing in Fig.2may be attributed to carriers trapped in the embedded CdSe/ZnS nanoparticles,a clear indication of the memory effect.21The flatband voltage shift of the C -V curve decreases to approximately 1.5V with increasing thickness of the hybrid active layer to about 900nm,as shown in Fig.2͑b ͒.These results indicate that the flatband voltage shift of the C -V curve related to the magnitude of the memory is significantly affected by the thickness of the hy-brid active layer.When a positive voltage is applied,after the injection of holes from the ITO into the highest occupied molecular or-bital ͑HOMO ͒level occurs through the Fowler-Nordheim tunneling process,the holes existing at the HOMO level are transported along the direction of the applied voltage through the hopping mechanism among the PVK molecules.22AnFIG. 2.Current-voltage curves for the Al/͑CdSe/ZnS nanoparticles embedded in a PVK layer ͒/ITO/glass device.The scanning step of the applied voltage is 0.01V.Empty and filled rectangles represent the current-voltage curves of the forward and the reverse applied bias volt-ages,respectively.The solid line indicates the current-voltage curve after application of a reverse voltage pulse of −2V.FIG. 3.Capacitance-voltage curves for the Al/͑CdSe/ZnS nanoparticles embedded in PVK layer ͒/ITO/glass devices with the active layer thicknesses of ͑a ͒500nm and ͑b ͒900nm.Downloaded 22 Oct 2007 to 166.104.58.178. Redistribution subject to AIP license or copyright, see /apl/copyright.jspintuitive proposal is that the holes actually encounter CdSe/ZnS nanoparticles which are traversing the sample,and with increasing electrical field to a certain value,holes can tunnel through the ZnS shell into the valence band of the CdSe nanoparticles,as shown in Fig.4,resulting in the for-mation of an internal electric field along the direction of the applied voltage.Because the capacitance of the device ex-hibits a larger decrease in comparison with the capacitance in the depletion layer formed in the ITO substrate under a posi-tive bias voltage,the C -V curve of the device shifts to the left,as shown in Fig.2.When a negative voltage is applied to the electrode,because the holes captured in the valence band of the CdSe nanoparticles under the negative electric field are released into the PVK matrix and then transported to the ITO substrate,the erasing process is performed.Since the generated internal electric field disappears due to the re-lease of the holes captured in the CdSe/ZnS nanoparticles,the C -V curve of the device shifts to the right,as shown in Fig.3.Because the flatband voltage shift for the Al/͑CdSe/ZnS nanoparticles embedded in the PVK layer ͒/ITO/glass device is significantly affected by the thickness of the PVK layer containing CdSe nanoparticles,the magnitude of the memory effects can be moderately adjusted by varying the thickness of the active layer in the devices,which might be very im-portant for practical applications in memory devices.A simple mechanism for the dependence of the memory effects on the thickness of the PVK layer in which the CdSe/ZnS nanoparticles are embedded can be proposed.The probability of the charges being trapped by the CdSe/ZnS nanoparticles embedded in the PVK layer through the tunneling process is typically determined by the electric field at the organic/inorganic interface.23A smaller number of the holes in the OBDs will be captured by the CdSe/ZnS nanoparticles due to the smaller electric field resulting from an increase in thickness.Therefore,a smaller flatband voltage shift is attrib-uted to a decrease in the internal electric field generated by the trapped charges.In summary,the bistability and the operating mecha-nisms of an organic/inorganic hybrid device consisting of CdSe/ZnS nanoparticles and a PVK composite were inves-tigated.The I -V curves at ambient temperature for the Al/͑CdSe/ZnS nanoparticles embedded in the PVK layer ͒/ITO/glass devices exhibited a nonvolatile electrical bistable be-havior.The C -V curve at room temperature for the Al/͑CdSe/ZnS nanoparticles embedded in the PVK layer ͒/ITO/glass capacitors showed a MIS behavior with a large flatband voltage shift due to the existence of the CdSe/ZnS nanopar-ticles,indicative of trapping,storing,and emission of holes in the electronic states of the CdSe nanoparticles.The mag-nitude of the flatband voltage shift was significantly affected by the thickness of the PVK layer containing CdSe/ZnS nanoparticles,and a possible operating mechanism corre-sponding to the writing and erasing processes of memory devices was described on the basis of the C -V results.These results indicate that OBDs fabricated utilizing CdSe/ZnS nanoparticles embedded in PVK layers hold promise for po-tential applications in next-generation nonvolatile memories.This work was supported by the Korea Science and En-gineering Foundation ͑KOSEF ͒grant funded by the Korea government ͑MOST ͒͑No.R0A-2007-000-20044-0͒.1H.Sirringhaus,N.Tessler,and R.H.Friend,Science 280,1741͑1998͒.2C.D.Dimitrakopoulos and P.R.L.Malefant,Adv.Mater.͑Weinheim,Ger.͒14,99͑2002͒.3C.D.Muller and Aurelie Falcou,Nature ͑London ͒421,829͑2003͒.4A.C.Mayer,D.J.Herman,T.G.Kasen,and G.G.Malliaras,Appl.Phys.Lett.85,6272͑2004͒.5S.Yoo,B.Domercq,and B.Kippelen,Appl.Phys.Lett.85,5427͑2004͒.6F.Li,Z.Chen,C.Liu,and Q.Gong,Chem.Phys.Lett.412,331͑2005͒.7R.Könenkamp,R.C.Word,and M.Godinez,Nano Lett.5,2005͑2005͒.8L.Bakueva,S.Musikhin,M.A.Hines,T.-W.F.Chang,M.Tzolov,G.D.Scholes,and E.H.Sargent,Appl.Phys.Lett.82,2895͑2003͒.9J.H.Jung,J.H.Kim,T.W.Kim,C.S.Yoon,Y .-H.Kim,and S.Jin,Appl.Phys.Lett.89,022112͑2006͒.10X.Li,Y .Wu,D.Steel,D.Gammon,T.H.Stievater,D.S.Katzer,D.Park,C.Piermarocchi,and L.J.Sham,Science 301,809͑2003͒.11J.Heitmann,F.Müller,M.Zacharias,and U.Gösele,Adv.Mater.͑Wein-heim,Ger.͒17,795͑2005͒.12S.Moller,C.Perlov,W.Jackson,C.Taussig,and S.R.Forrest,Nature ͑London ͒426,166͑2003͒.13D.Ma,M.Aguiar,J.A.Freire,and I.A.Hummelgen,Adv.Mater.͑Wein-heim,Ger.͒12,1063͑2000͒.14J.H.Kim,J.Y .Jin,J.H.Jung,I.Lee,T.W.Kim,S.K.Lim,C.S.Yoon,and Y .-H.Kim,Appl.Phys.Lett.86,032904͑2005͒.15Fushan Li,Dong-Ick Son,Han-Moe Cha,Seung-Mi Seo,Bong-Jun Kim,Hyuk-Ju Kim,Jae-Hun Jung,and Tae-Whan Kim,Appl.Phys.Lett.90,222109͑2007͒.16E.Kapetanakis,P.Normand,D.Tsoukalas,and K.Beltsios,Appl.Phys.Lett.80,2794͑2002͒.17M.Perego,S.Ferrari,M.Fanciulli,G. 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