VOLUME 82, NUMBER 19 P H Y S I C A L R E V I E W L E T T E R S 10 MAY 1999

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Direct Evidence for Photoionization in CdTe Nanocrystals Embeddedin Trioctylphosphine Oxide

M. Y. Shen, M. Oda, and T. GotoDepartment of Physics, Tohoku University, Sendai 980-8578, Japan

(Received 17 July 1998)

The existence of photoionization processes in nanocrystals has been experimentally proved fofirst time by measuring the temperature dependences of the photocurrent and the photoluminesof CdTe nanocrystals embedded in trioctylphosphine oxide (TOPO). The phase transition ofmatrix TOPO from amorphous solid to liquid makes the diffusion length of the emitted carrieoutside the nanocrystals increase abruptly, resulting in a steep rise in both the photocurrent anphotoluminescence. The photoionization has turned out to be fundamentally important for the opproperties of nanocrystals. [S0031-9007(99)09094-8]

PACS numbers: 78.55.Et, 61.46.+w

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The on/off behavior and the spectral diffusion of photoluminescence (PL) in a single CdSe nanocrystal [1–3and the persistent hole burning of CdSe, CuCl, and Cunanocrystals [4] have been recently observed and dcussed. These new phenomena are of great importato both fundamental interest and application. The on/obehavior and the spectral diffusion have been qualitativeexplained as due to the Stark effect on size quantized eltronic levels. The Stark effect is believed to be causeby a possible photoionization process that may alsoa cause of photodarkening [5] in nanocrystals. The pesistent hole burning is also explained on the basis of tpossible photoionization. Thus, the possible photoioniztion process is fundamentally important for the opticaproperties of nanocrystals. However, no direct evidenof such photoionization has yet been reported. After thphotoionization occurs, a carrier (an electron or a hole)generated outside the nanocrystal. This photogeneracarrier in the matrix outside the nanocrystal can be drectly detected with a photocurrent (PC) signal. In thpresent work, we experimentally prove the existence ofphotoionization process in nanocrystals for the first timby measuring the temperature dependence of the PCwell as that of the PL of CdTe nanocrystals in the troctylphosphine oxide (TOPO) matrix. It is found thaa phase transition of the matrix TOPO from amorphousolid to liquid makes the diffusion length of the photogenerated carriers outside the nanocrystal increase abrupresulting in a steep rise of both the PC and the PL. Tphotoionization effect is also clearly shown in the measurement of the temperature dependence of the photodaening of PL. Thus, the photoionization is an importanprocess for the optical properties of nanocrystals.

A TOPO capped CdTe nanocrystal powder was preparaccording to the method of Murrayet al. [6], i.e., byinjecting dimethylcadmium and trioctylphosphine (TOPtellurium into a hot bath of TOP and TOPO. The powdeof TOPO capped CdTe nanocrystals was then embeddinto the TOPO matrix with a density of nanocrystals lowenough to have no contact among nanocrystals. The

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of the sample was measured by an experimental seshown in Fig. 1. The sample, with a thickness of abo1 mm, was sandwiched between a copper plate and a gplate coated with an indium tin oxide (ITO) transparenconductive layer. A 476.5 nm laser light was focuseonto the sample through the ITO glass plate. Across tsample, a voltage of 20 V was applied between the ITglass plate and the copper plate, and the PC was dete

FIG. 1. Temperature dependence of the PC. The trianglesexperimental data, and the two straight lines between 319 a330 K are drawn to help see the changes of the experimendata. Inset: Experimental setup. The sample is a TOPO fiabout 1 mm thick containing CdTe nanocrystals sandwichbetween a copper plate and an ITO glass plate. A 476.5light with an intensity of about4 Wycm2 from a cw Ar1 laser isincident on the sample through the ITO glass plate. A voltaof 20 V is applied across the sample between the ITO andcopper electrodes.

© 1999 The American Physical Society 3915

VOLUME 82, NUMBER 19 P H Y S I C A L R E V I E W L E T T E R S 10 MAY 1999

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by a picoammeter. The sample temperature with tprecision of higher than 0.5 K was controlled by heatinthe copper plate. The PC was approximately proportionto the voltage across the sample. This means thatTOPO film was in Ohmic contact with the ITO and coppeelectrodes within the measured range of the PC. In themeasurement, the sample (a TOPO film containing Cdnanocrystals) was mounted in a cryostat, where the samtemperature was controlled between 300 and 360 K. Tsample was excited by a 476.5 nm light from a cw Ar1

laser. The luminescence spectra were measured by usa monochromator with a spectral resolution of 1 nm andcharge coupled device camera.

If there exists the photogenerated electron or hoemitted outside a nanocrystal as suggested by soauthors [1–5], then we can expect the matrix to becomconductive. Accordingly, if our sample is excited bya light with the energy higher than the lowest energdifference between the size quantized electron and henergy levels, the nanocrystals may be photoionizthrough some processes, for example, an Auger ionizatto emit an electron or a hole outside the nanocrystaand then the matrix can become conductive. The PCthe TOPO containing CdTe nanocrystals can be causby two possibilities: one is the PC of TOPO itselfanother is the photoionization of CdTe nanocrystals. TTOPO has no optical absorption of the laser light usein the experiment, thus no photogenerated carriers froTOPO contribute to the PC. Hence the PC in thTOPO containing CdTe nanocrystals directly proves thphotoionization process.

Figure 1 shows the PC density as a function otemperature when the sample is excited with a 476.5 nlight from an Ar1 laser. By this exciting light, the sizequantized electron and hole are generated in the Cdnanocrystal. The PC is very small below 318 K, whicmeans that the diffusion length of the emitted electronnegligibly small. The PC, however, suddenly increasabove 318 K that is just below the melting point of thTOPO, 328 K. Such a sudden increase of the PCinterpreted as follows. The emitted electron from thnanocrystal can be trapped at a deep center in the TOmatrix below 318 K. The number of trapped electronin the matrix decreases drastically just under the meltinpoint of the matrix because the atoms surrounding ttrapping center may become quite mobile when the matchanges from the amorphous solid phase to the liquphase. As a result, the diffusion length of the emitteelectron increases dramatically when the matrix changfrom the amorphous solid phase to the liquid phase, whiresults in increment of the PC. Hence, the PC provdirectly the existence of a photoionization process of thnanocrystals. The decrease of the PC above 322 K mbe due to a polarization effect of ions. Namely, thincreases of an ionic conductivity of the TOPO matricause the decrease of the internal electric field. Th

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explanation is supported by our observation that the dcurrent increases with the temperature rise.

At the temperatures of 300 and 320 K, we found an aproximate linear relation between the PC and the pulight power. This linear relation may be closely tied tthe on/off behavior mentioned in Ref. [2]. The averaon time is inversely proportional to the intensity of the ecitation light, while the average off time is independentthe excitation light intensity. The phenomenon has beinterpreted as a two-step process, in which photogeneraelectrons are first accidentally trapped in a stable statethe nanocrystal and then Auger ionized [7,8]. Thus tnumber of the emitted electrons outside the nanocrystaproportional to the excitation light intensity. Such a linerelation causes the ionization to occur easily, which hasignificant effect on the optical properties of nanocrysta

In the PL spectra of the same sample under the excitaof a 476.5 nm light at different temperatures, onlysingle PL band with the peak at about 690 nm are fouThe surface of the CdTe microcrystallites is stabilizedTOPO which leads to a strong band-edge luminesceas well as to nonluminescence in the infrared region dto a deep trap. This PL band is known to be associawith a radiative annihilation of the size quantized electrand hole [9]. We also found that the PL at about 324is abnormally stronger than that at about 305 K. Ttemperature dependence of the PL band intensity is shoin Fig. 2. The measurement was carried out whentemperature changes from low to high. Probably duenonradiative annihilation of the size quantized electronhole, the PL intensity decreases with the temperature risbelow about 300 K (not shown here). The PL intens

FIG. 2. Temperature dependence of the integrated intensitthe PL band of the sample which is excited by a 476.5 nm ligfrom a cw Ar1 laser.

VOLUME 82, NUMBER 19 P H Y S I C A L R E V I E W L E T T E R S 10 MAY 1999

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gradually increases, however, as the temperature rises fr305 to 320 K, and steeply increases between 320 a324 K, after which it again decreases. The temperaturange in which the PL intensity abruptly increasesclose to the melting point of TOPO, 328 K. When thmeasurements are made from high to low temperature,temperature dependence of the PL intensity between 3and 300 K is complicated and no reliable data are obtaineThis is due to the nature of the phase transition, becauthe solidifying process is not simply reversible.

The steep increase of the PL intensity with the temperature rise between 320 and 324 K cannot be explainby the mechanism [10] of a thermal up-conversion ofsize quantized hole from a stable level [7] to the radiative level which is dozens of meV above the stable leveThe PL intensity increase is too abrupt to be interpreteby the thermal excitation of the hole. The steep risethe PL is explained as follows. As stated before, whethe temperature becomes close to the melting point of tTOPO matrix, the number of the trapping centers abrupdecreases, and the electron at the trapping center inTOPO matrix is abruptly released. If the released eletron is recaptured by the ionized nanocrystal and recobines with the trapped hole in the ionized nanocrystal, telectric field in the nanocrystal disappears. The Stark efect of the electric field created by the emitted electroaround the ionized nanocrystal will also disappear, athe PL efficiency will steeply increase [1–3]. The following photodarkening measurement [11] shows the effectphotoionization more clearly.

The samples used show an obvious photodarkeniproperty at temperatures lower than 318 K. In Fig. 3the PL intensity is shown as a function of time at 30and 322 K when the sample is continuously excited ba 476.5 nm cw light with an intensity of about 1 o5 Wycm2 in the time ranges oft0 tA, tB tC, tD tE ,and tF tG. In the time earlier thant0, the sample hasnever been excited by light. At 300 K, the PL intensitdecreases after the laser light is irradiated att0. At tA,the laser light is extinguished and then reirradiated attB.The PL intensity attB slightly recovers and then decreasewith the passage of time untiltC. From tC the samplewas heated to 322 K untiltD. In the case of the lightintensity of1 Wycm2, when the laser light is irradiated attD , the PL intensity attD is similar to that att0, and thena very small part of the PL intensity suddenly disappeabut the rest remains unchanged untiltE . On the otherhand, in the case of the light intensity of5 Wycm2, whenthe laser light is irradiated attD, the PL intensity attD

is also similar to that att0, and then about 30% of thePL intensity suddenly disappears but the rest also remaunchanged untiltE . At tF after the nonexcitation periodtE tF , almost all the PL intensity recovers for both thexcitation intensities. This photodarkening process resufrom the photoionization mechanism as suggestedRef. [5]. After an electron is emitted from a nanocrysta

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FIG. 3. Excitation time profiles of the PL intensity at 300 an322 K. The sample is continuously excited by a 476.5 ncw light with an intensity of about 1 or5 Wycm2 in the timeranges oft0 tA, tB tC , tD tE , and tF tG . In the other timeranges, the laser light is extinguished. In the time earlier tht0 the sample has never been excited by light. From timetCthe sample is heated to 322 K untiltD .

to the matrix by an ionization process, the PL efficiencdue to annihilation of an electron-hole pair inside thnanocrystal may be greatly diminished by the electric fiecreated by the emitted electron around the nanocrysas suggested in Refs. [1] and [12]. When the emittelectron returns to the nanocrystal, the influence of telectric field on the nanocrystal disappears, and theefficiency is restored. From Fig. 3, we see that the Pefficiency decreases due to the electric field inducedthe emitted electron outside the ionized nanocrystal300 K, but not at 322 K when the light excitation intensitis weak. In the5 Wycm2 excitation intensity at 322 K,a part of the PL intensity suddenly disappears attD andtF and then the rest of the intensity remains unchanguntil the excitation light is extinguished. This means ththe emitting and the returning processes can quickly reaa balance at 322 K, which is much different from thaat 300 K. At tF after the nonexcitation periodtE tF ,almost all the PL intensity recovers at 322 K, while a300 K a very small part of the PL intensity recoverat tB after the nonexcitation periodtA tB for both theexcitations. In the experiments, we also found that, if thnonexcitation periodtA tB is longer than 1 h, almost allthe PL intensity recovers even at 300 K, but this recovetime is much longer than that at 322 K. These facts me

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that the emitted electron can quickly return back to thionized nanocrystal at 322 K, but cannot quickly returback at 300 K. The steep rise of the PL between 3and 324 K, as shown in Fig. 2, is closely associated wthis restoring process. Therefore, the critical temperatuTc may be defined as 320 K. The PL intensity in Fig.below the Tc was measured with the sample conditiosimilar to those attB andtC in Fig. 3, i.e., the sample wasphotodarkened, because the laser light was irradiateda long time before the measurement. Hence, the amoof the increment in the PL intensity aboveTc dependson the time integrated incident photon numbers when tsample temperature is belowTc. This fact means that thesteep rise of the PL is a light induced phenomenon. Tphenomena above are independent of the nanocrystalin the range from 4 to 8 nm in diameter, and independeof the excitation light wavelength in the range from 45to 510 nm. The photodarkening effect at temperaturlower than 320 K is similar to that at 300 K, and the effebetween 320 and 326 K is similar to that at 322 K. Thphenomenon at temperatures higher than 327 K, howevis complex because the sample is in a liquid phase andatoms are always moving.

In conclusion, it is strongly suggested that as thtemperature rises from 319 to 322 K, the emitted electroin the matrix migrate more and more easily and retumore and more quickly back to the ionized nanocrystaThis causes the steep increases of the PC in the frsample and of the PL intensity in the photodarkenesample as shown in Figs. 1 and 2, respectively, andconsistent with the result that the photodarkening effebetween 318 and 325 K is not so obvious as that at 300

In summary, the existence of the photoionization prcesses in nanocrystals has been experimentally provedthe first time by measuring the temperature dependencethe PC and the PL of the CdTe nanocrystals embeddedTOPO. The PC directly shows the migration of the emted electrons from nanocrystals in the matrix. The phatransition of the matrix of TOPO from amorphous solid tliquid makes the diffusion length of the emitted electroincrease steeply, which results in the abnormal steepcreases of the PC and the PL. The photoionization effis also clearly shown in the measurement of photodaening of the CdTe nanocrystals. And the photoionizatioturns out to be fundamentally important for the opticaproperties of nanocrystals [13].

This work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Sciencand Culture of Japan. We are grateful to Mr. T. Yokouchi, Mr. S. Koyama, and Mr. M. Saito of Tohoku University for technical assistance.

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[1] S. A. Empedocles, D. J. Norris, and M. G. Bawendi, PhyRev. Lett.77, 3873 (1996).

[2] M. Nirmal, B. O. Dabbousi, M. G. Bawendi, J. J. MacklinJ. K. Trauman, T. D. Harris, and L. E. Brus, Natur(London)383, 802 (1996).

[3] M. Y. Shen, T. Goto, E. Kurtz, Z. Zhu, and T. Yao,J. Phys. Condens. Matter10, L171 (1998).

[4] Y. Masumoto, S. Okamoto, T. Yamamoto, andT. Kawazoe, Phys. Status Solidi (b)188, 209 (1995).

[5] D. I. Chepic, Al. L. Efros, A. I. Ekimov, M. G. Ivanov,V. A. Kharchenko, I. A. Kudriavtsev, and T. V. YazevaJ. Lumin.47, 113 (1990).

[6] C. B. Murray, D. J. Norris, and M. G. Bawendi, J. AmChem. Soc.115, 8706 (1993).

[7] A long-lived surface state or an inactive state predictedT. Richard, P. Lefebvre, H. Mathieu, and J. Allegre, PhyRev. B53, 7287 (1996).

[8] It is suggested from our PL decay measurement at 300that a state with a long lifetime of about 1ms exists atroom temperature. This lifetime, however, is not lonenough to support the two-step Auger ionization procein our weak power light excitation. There may existmuch more stable state that supports the two-step Auprocess.

[9] M.-Y. Shen, M. Saito, T. Goto, F. Sato, and M. TanakaJ. Phys. Condens. Matter6, 8479 (1994).

[10] S. Koyama, T. Yokouchi, M. Y. Shen, and T. Gotoin Proceedings of the II ASIA Symposium on Condensed Matter Photophysics, Nara, Japan, 1996,edited byT. Hayashi, H. Nishimura, and A. H. Matsui (Konan University Copy Center, Kobe, 1996), p. 313, where evewhen the TOPO capped CdTe nanocrystals are embded in a polymethylmethacrylate (PMMA) polymer, thetemperature dependence of the PL is similar to thin this work. No photocurrent could be measured ithe sample because PMMA is a good insulator, anhence the emitted electrons are trapped at TOPO srounding the CdTe nanocrystals. This fact suggeststhe same time that the PC increment in Fig. 1 is ninfluenced by any change in carriers inside the CdTnanocrystals.

[11] The photodarkening process may include two processone is of a permanent change where some chemireactions may happen which result in the structure chanof the nanocrystal; another one is of a recoverable chanwhere some electrical processes may happen. We pattention only to the latter because the recoverable procis more obvious than the process of the permanent chain the present work.

[12] A. Sacra, D. J. Norris, C. B. Murray, and M. G. BawendJ. Chem. Phys.103, 5236 (1995).

[13] The photoionization processes in other materials, fexample, II-VI compound nanocrystals embedded in glasare also noticed with a femtosecond pulsed laser ligexcitation in our experiment.