Electrolum inescence from Exc iplex on the In terface Between TPD and La (PM IP) 3 (Bipy) 3

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1 CH EM. R ES. CH IN ESE U. 2004, 20 (6), Electrolum inescence from Exc iplex on the In terface Between TPD and La (PM IP) 3 (Bipy) 3 GAO D e2qing 1, B IAN Zu2qiang 1, HUAN G Yan2yi 1, HUAN G Chun2hu i 13 3 IBRA H IM K. 2 and L IU Feng2qin 2 1. S ta te K ey L abora tory of R a re E a rth M a teria ls Chem istry and A pp lica tions, P ek ing U n iversity, B eij ing , P. R. Ch ina; 2. P hotoem ission S p ectroscopy S ta tion, Institu te of H ig h E nergy P hy sics, Ch inese A cad em y of S ciences, B eij ing , P. R. C h ina R eceived D ec. 17, 2003 T he bilayer o rganic ligh t2em itting diode (OL ED ) w ith a blue fluo rescent lanthanum comp lex, tris (12 phenyl232m ethyl242isobutyryl252pyrazo lone) 2(2, 2 2dipyridyl) lanthanum [L a (PM IP ) 3 (B ipy) ], as a ligh t em itting m aterial and N, N 2diphenyl2N, N 2bis (32m ethylphenyl) 21, 1 2biphenyl24, 4 2diam ine (T PD ) as a ho le transpo rting m aterial em its brigh t green ligh t instead of blue ligh ṫ T he data of the abso rp tion, the pho2 to lum inescence (PL ) and the pho to lum inescence excitation (PL E) spectra of T PD, L a (PM IP) 3 (B ipy) and the m ixture of T PD and L a (PM IP) 3 (B ipy) (mo lar ratio 1 1) p rove that the electro lum inescent em ission o rigi2 nates from the excip lex on the interface betw een T PD and L a (PM IP) 3 (B ipy). By imp roving device configura2 tion w ith tris (82hydroxyquino line) alum inum (ALQ ) as an electron transpo rting m aterial, a m axim um lum i2 nance of 800 cdgm 2 w as obtained. Ke yw o rds E lectro lum inescence, Pho to lum inescence, L anthanum comp lex A rtic le ID (2004) In troduction Since T ang and V an Slyke [1 ] repo rted the first doub le2layer g reen th in2film o rgan ic elect ro lu2 m inescen t device, fluo rescen t m a teria ls, ca rrier2 tran spo rting m aterials and electrode m aterials have been w idely studied [2_ 5 ]. O n the o ther hand, dif2 feren t k inds of m u ltilayer o rgan ic ligh t em itting diodes have been m ade in o rder to imp rove electro2 lum inescence efficiency and o ther function s [6_ 8 ]. Fo r in stance, in o rder to avo id exciton quench ing at m etal electrodes, the em itting region can be sepa2 rated from the electrodes by adding a carrier2tran s2 po rting layer. T he flow of the po sitive and negative po laron s is in jected in to the em itting region, then a better balance w as ach ieved, so a h igher efficiency is ob tained [9, 10 ]. In layered o rgan ic ligh t2em itting diodes, the excip lex fo rm ation m ay take p lace on the in terface betw een the charge2tran spo rting layer and the em itting layer fo r certain com b ination s of em itting and charge2tran spo rting m aterials [11_ 15 ]. T he excip lex is a tran sien t dono r2accep to r comp lex that som etim es em its a strong fluo rescence at longer w avelength s than the dono r o r the accep to ṙ T he recen t studies have suggested the po ssib ility of u t ilizing enhanced excip lex em ission in L ED de2 vices [16, 17 ]. H ere is repo rted the electro lum inescence from b ilayer o rgan ic devices in w h ich the green e2 m ission peak ing at 522 nm o riginates from an exci2 p lex fo rm ed on the in terface of a lan thanum com 2 p lex and T PD. T he lan thanum terna ry com p lex, [L a (PM IP) 3 (B ipy) ], w as syn thesized and u sed as a ligh t em itting m aterial. L a 3+ has no 4f elec2 tron s and does no t em it visib le ligh t [18 ] ; how ever, the title comp lex em its b lue pho to lum inescence (PL ) peak ing at 460 nm and belongs to the ligand em itting comp lex, w h ich is ju st like comp lex tris ( 82hydroxyqu ino line ) alum inum (ALQ ). T he de2 vices p repared in th is study are listed as fo llow s: D evice 1: ITO gt PD ( 40 nm ) gl a ( PM IP ) 32 (B ipy) (80 nm ) gm g A g (200 nm ) ga g (100 nm ). D evice 2: ITO gt PD ( 40 nm ) gl a ( PM IP ) 32 (B ipy) (40 nm ) galq (40 nm ) gm g A g (200 nm ) g A g (100 nm ). Exper im en ta l Comp lex L a (PM IP ) 3 (B ipy) (F ig. 1) w as syn2 thesized as described p reviou sly [19 ]. E lem en tal anal 3 Suppo rted by the N ational N atural Science Foundation of Ch ina (N o s , , ), State 863 P rogram (N o. 2022AA ) and the Docto rial P rogram Foundation of H igh Education (N o ). 3 3 To w hom co rrespondence should be addressed. E2m ail: chhuang@pku. edu. cn

2 N o. 6 GAO D e2qing et al. 791 found (% ) : C 61137, N 11102, H 5116; ca lcd. (% ) fo r C 52 H 53 N 8O 6L a: C 60194, N 10193, H O rgan ic m aterials w ere sequen tially depo sit2 ed by h igh vacuum (10-4 Pa) therm al evapo ration on to a p recleaned indium tin ox ide ( ITO ) glass sub2 strate w ith a sheet resistance of 15 8 g supp lied by Ch ina Shenzhen W elligh t Conduct ive Coa t ing Co., L td. A shadow m ask w ith 5 mm diam eter open ings w as u sed to define the cathode of a layer of M g and A g alloy (10 1, the th ickness w as 200 nm ),w ith a A g cap of 100 nm. Fo r the PL studies, the m ix tu re of T PD and L a (PM IP ) 3 (B ipy) (mo lar ratio of 1 1 by u sing a dich lo roethane so lu tion of them ) w as sp in2coated on to a quartz sub strate. T he u ltravio let2visib le (UV 2V is) ab so rp tion spectra of the film s w ere m easu red by u sing a Sh im adzu F ig. 1 The absorption spectra of NaPM IP, Bipy and both sp in coated and evaporated La (PM IP) 3- (Bipy) in f ilm s(about 60 nm ) (A), the PL spectra of La ( PM IP ) 3 ( Bipy ) ( Κex = 270 nm ), NaPM IP (Κex= 240 nm ) and Bipy (Κex= 360 nm ) in f ilm s(b) and the the chem ical structural formula of La (PM IP) 3 (Bipy) (C). UV UV gv is spectrom eter. T he PL and eletro lum inescence (EL ) w ere m easu red w ith a H i2 tach i F24500 fluo rescence spectropho tom eter. T he layer th ickness w a s con t ro lled in vacuum w ith shu tters and an IL 2100 quartz crystal mon ito r. T he b righ tness w as m easu red by an ST 286LA spo t pho2 tom eter and a clo se2up len s w ith a focal spo t of 5 mm in diam eter. T he synch ro tron radiation pho2 ton2electron spectrum (SR PES) w as m easu red w ith a pho ton2electron spectrom eter (vs. w Scien tific In2 strum en ts, L td. ) th rough synch ro tron radiation. U nder u ltra2h igh vacuum ( Pa ), the pho ton2electron spectrum w as reco rded at 1817 ev synch ro tron radiation ligh t, from w h ich the first ion ization energy (I p), the HOM O energy lev2 el, w as derived. Results and D iscuss ion L a (PM IP ) 3 (B ipy) film depo sited by vacuum sub lim ation is tran sparen t and homogeneou s. F ig. 1 (A ) show s the ab so rp tion spectra of the sp in2 coated and evapo rated L a ( PM IP ) 3 (B ipy ) film ṡ T he spectra of bo th the film s are almo st the sam e, suggest ing tha t the com pound can be evapo ra ted w ithou t decompo sition. It can also be seen that the ab so rp tion spectrum of L a (PM IP ) 3 (B ipy ) is at2 tribu ted to the co2ab so rp tion of PM IP - and B ipy. But the PL spectrum of L a (PM IP) 3 (B ipy) is differ2 en t from tho se of N apm IP and B ipy. F ig. 1 (B ) show s the PL spectrum peak ing at 460 nm fo r L a (PM IP) 3 (B ipy) and tho se fo r N apm IP and B ipy peak ing at 398 nm and 410 nm, respectively. T hat m ean s the PL spectra of PM IP - and B ipy are red2 sh ifted due to the cen tral L a 3+ ion. T he m easu re2 m en ts of the ab so rp tion, PL and pho to lum i2 nescence excitation (PL E) spectra w ere m ade by u s2 ing the sp in2coated film s on the quartz sub strates. F ig. 2 (A ) show s the PL spectra of T PD excited at 310 nm, L a (PM IP) 3 (B ipy) excited at 270 nm, and the m ix tu re of T PD and L a (PM IP ) 3 (B ipy) (mo lar ratio = 1 1) excited at 280 and 380 nm, respec2 tively. T he PL spectra have em ission peak s at 406 and 422 nm fo r T PD, and at 460 nm fo r L a (PM IP ) 3 (B ipy). In terestingly w e found that the m ix tu re of T PD and L a (PM IP) 3 (B ipy) (mo lar ratio = 1 1) show s a b road PL band peak ing at 522 nm in stead of at 406, 422 o r 460 nm and that the PL spectra of it excited at 280 and 380 nm are the sam e excep t the d ifference in em ission in ten sity. T he disappearance of em ission s from the spectra of bo th T PD and L a (PM IP) 3 (B ipy) and the appearance of

3 7 92 CH EM. R ES. CH IN ESE U. V o l. 20 F ig. 2 The PL spectra of TPD ( Κex = 310 nm ), La (PM IP) 3 (Bipy) (Κex= 270 nm ) and the m ixture of TPD and La (PM IP) 3- (Bipy) (molar ratio= 1 1) (Κex= 280 and 380 nm ) in the f ilm s on quartz substrates (A ), the PL spectra of TPD and the m ixture of TPD and La (PM IP) 3 (Bipy) (Κex= 280 nm ) in the f ilm s with differen t molar ratios {n (TPD ) n [La (PM IP) 3- (Bipy) ]= 100 1, 10 1, 1 1} (B). the new em ission imp ly that a new species w as gen2 erated in the m ix tu re. T h is em ission is attribu ted to the excip lex o rig ina t ing from the m ix tu re of T PD and L a (PM IP) 3 (B ipy) (mo lar ratio = 1 1). T he PL spectra of the m ix tu re of T PD and L a ( PM IP ) 3 ( B ipy ) w ith differen t mo lar ratio s (100 1, 10 1, 1 1) in the sp in2coated film s on the quartz sub strates w ere also m easu red. A s show n in F ig. 2 (B ), w hen the mo lar ratio of T PD to L a (PM IP) 3 (B ipy) is equal to 100 1, tw o strong em ission bands can be found in the PL spec2 trum, w h ich peak at 406 and 522 nm, respectively; the fo rm er is attribu ted to T PD em ission, w h ile the o ther one o riginates from the excip lex em ission. If the con ten t L a ( PM IP ) 3 (B ipy ) w as fu rther in2 creased, the relative in ten sity of the em ission from T PD con tinuou sly decreases. A s the mo lar ratio reached 1 1, the T PD em ission to tally disappears in the PL sp ect rum of the m ix tu re. How ever, no excip lex w as found in the CHC l3 so lu tion of T PD CHC l3 and L a (PM IP) 3 (B ipy) w ith the mo lar ratio of 1 1 (the concen tration is abou t mo lgl ). T he reason m ay be that the so lven t mo lecu les su r2 round the mo lecu les of T PD and L a ( PM IP ) 32 (B ipy), w h ich greatly w eaken s the in teraction of T PD and L a (PM IP) 3 (B ipy). F ig. 3 (A ) show s the ab so rp tion spectra of T PD, L a (PM IP ) 3 (B ipy) and the m ix tu re of T PD and L a (PM IP) 3 (B ipy) (mo lar ratio = 1 1). W hen the ab so rbance of the m ix tu re of T PD and L a (PM IP) 3 (B ipy) (mo lar ratio= 1 1) is compared w ith the sum of the T PD layer ab so rbance and the L a ( PM IP ) 3 (B ipy ) layer ab so rbance, it can be found that there is no new ab so rp tion fo r the m ix2 tu re of T PD and L a (PM IP) 3 (B ipy) (mo lar ratio = 1 1), imp lying that the new species is no t directly fo rm ed in the g round sta te, g iving suppo rt to the ex istence of an excip lex [20 ]. F ig. 3 (B ) show s the excitation spectra of T PD, L a (PM IP ) 3 (B ipy) and the m ix2tu re of T PD and L a (PM IP) 3 (B ipy) (mo lar ratio = 1 1) reco rded at their m ax im um em ission w avelength s 406, 460 and 522 nm, respectively. T he PL E in ten sity of the m ix tu re of T PD and L a (PM IP) 3 (B ipy) (mo lar ratio= 1 1) is also ap2 p rox im ately con sisten t w ith the sum of the PL E in2 ten sities of T PD and L a (PM IP ) 3 (B ipy), fu rther confirm ing the conclu sion m ade above. F ig. 3 The absorption spectra (A ) and the photolum i- nescence exc itation ( PL E ) spectra of TPD, La (PM IP) 3 (Bipy), the sum of spectra of TPD and La (PM IP) 3 (Bipy), the m ixture of TPD and La (PM IP) 3 (Bipy) (molar ratio 1 1) on quartz substrates(b). The PL Es w ere reco rded at their m axim um em ission w avelengths 406, 460 and 522 nm, respectively.

4 N o. 6 GAO D e2qing et al. 793 By u sing T PD as a ho le tran spo rting layer, de2 vice 1 w ith the structu re of ITO gt PD ( 40 nm ) g L a (PM IP) 3 (B ipy) ( 80 nm ) gm g A g ( 200 nm ) g A g (100 nm ) w as fab ricated w ithou t b reak ing vacu2 um. A s show n in the in set of F ig. 4, the EL spec2 trum w ith a peak at 517 nm and a shou lder peak at 420 nm resu lted from T PD, is app rox im a tely iden tical to the PL spectrum of the m ix tu re of T PD and L a (PM IP) 3 (B ipy) (mo lar ratio = 1 1). It is attribu ted to the excip lex fo rm ed on the in terface betw een T PD and L a (PM IP) 3 (B ipy). U nder a fo r2 w ard b ias, a m ax im um lum inance of 30 cdgm 2 at 20 V w as ob served. T he cu rren t den sity2vo ltage and lum inance2vo ltage cha racterist ics of device 1 a re show n in F ig. 4. T he device has a peak pow er effi2 ciency of 0101 lm gw w ith a cu rren t den sity of 0141 ma gcm 2 at 7 V and the tu rn2on vo ltage of 6 V. F ig. 4 The curren t den sity-voltage and lum inance-voltage character istics of dev ice 1. Inset: a. PL spectra ( right axis) of the m ixture of TPD and L a (PM IP ) 3 (B ipy) (mo lar ratio = 1 1) (Κex = 280 nm ) ; b. EL (left axis) of device 1 at 14 V ; c. device A at 10 V ; d. device B at 10 V. In layered o rgan ic ligh t em itting devices, exci2 p lex fo rm ation often takes p lace on the in terface betw een the carrier tran spo rting layer and the ligh t em itting layer [11 ]. T h is can be confirm ed by design2 ing the fo llow ing tw o devices: ITO gt PD (40 nm ) g Y (PM IP ) 3 (T PPO ) 2 (80 nm ) gm g A g (200 nm ) g A g (100 nm ) (device A ) and ITO gt PD ( 40 nm ) g Y (PM IP) 3 (T PPO ) 2 (20 nm ) gl a (PM IP) 3 (B ipy) (40 nm ) gm g A g (200 nm ) ga g (100 nm ) (device B ). It is know n that the peak of the PL spectrum of the com p lex t ris (12p henyl232m ethyl242isobu tyryl252 pyrazo lone) 2b is ( triphenyl pho sph ine ox ide) yttri2 um [Y (PM IP) 3 (T PPO ) 2 ] is at 430 nm, and device A show s the peak of the EL spectrum at 425 nm (in the in set of F ig. 4), giving suppo rt to the fact that no excip lex w as fo rm ed under electrical excitation. W hen a layer of Y (PM IP ) 3 (T PPO ) 2 (20 nm ) w as in serted betw een the T PD and L a (PM IP ) 3 (B ipy) layers in device 1 to fo rm device B, it is found that device B exh ib its the EL peak at 425 nm w h ile the peak at 517 nm ob served in device 1 disappears ( in the in set of F ig. 4 ). T h is fact strongly demon2 strates that the peak at 517 nm is due to the exci2 p lex fo rm ed on the in terface betw een the T PD and L a (PM IP) 3 (B ipy) layers. T he exp lanation is based on the relation sh ip of the energy levels of the m aterials in F ig. 5 (A ). T he I p of 615 ev fo r L a (PM IP ) 3 (B ipy) w as ob tained from SR PES. T he elect ron affin ity ( E A ) fo r L a (PM IP) 3 (B ipy), the LUM O energy level, w as estim ated to be 214 ev w ith reference to the I p and the UV 2V is op tical ab so rp tion on set (Κ= 300 nm, 411 ev ). F rom the relation sh ip of the energy lev2 els, electron s in jected from the cathode m ay recom 2 b ine w ith the ho les in the T PD layer o r the L a (PM IP) 3 (B ipy) layer. By comparison, there is no barrier fo r electron in jection from L a (PM IP ) 3 (B ipy) to T PD, w h ile the ba rrier fo r ho le in ject ion from T PD to L a (PM IP) 3 (B ipy) is 1 ev. It w as likely that T PD w as first excited. T h is suppo sition w as demon2 strated by the fo llow ing experim en ṫ W e selected 380 nm as the excitation w avelength. A s show n in F ig. 3 (B ), by u sing th is excitation ligh t, on ly T PD can be excited in the tw o componen ts of the m ix2 tu re of T PD and L a (PM IP) 3 (B ipy) (mo lar ratio = 1 1) since th is ligh t can be ab so rbed by T PD on2 ly. A s an exp ected resu lt of excita t ion, the PL spectrum show s the sam e cu rve as that ob tained by being excited at 280 nm w avelength ligh t. It m ean s that p rocess 1 is the m ain one among the th ree p ro2 cesses, tha t is, the excited sing le2sta te T PD m ay in teract w ith L a ( PM IP ) 3 (B ipy ) in the ground state, fo rm ing the excip lex. T he fo rm ation of excip lexes is easy in the sin2 glet excited state bu t is difficu lt in the trip let excit2 ed state [21, 22 ]. W ith the sam e ligands as tho se of L a (PM IP) 32 (B ipy), the gado lin ium ternary comp lex, tris ( 12 phenyl232m ethyl242isobu tyryl252pyrazo lone) 2(2, 2 2 dipyridyl ) gado lin ium [Gd (PM IP) 3 (B ipy) ] w as syn thesized. Gd 3+ has a half2fu ll 4f 7 electron ic con2 figu ra t ion and does no t em it in visib le ligh t range [18 ] ; how ever, the above comp lex em its green PL p eak ing a t 516 nm and belong s to the ligand em itting comp lex, ju st like L a (PM IP) 3 (B ipy). W ith Gd (PM IP) 3 (B ipy) as a ligh t em itting m ateri2 al, the device, ITO gt PD ( 40 nm ) ggd ( PM IP ) 3

5 7 94 CH EM. R ES. CH IN ESE U. V o l. 20 w as demon strated by the b ilayer device ITO g L a (PM IP) 3 (B ipy) (40 nm ) galq (40 nm ) gm g A g ( 200 nm ) ga g ( 100 nm ), w h ich exh ib its a m ax i2 m um lum inance of 380 cdgm 2 at 16 V. Con sidering the elect ron conduct ivity in device 1, w e can con2 clude that the ligh t em itting m aterial L a (PM IP ) 32 (B ipy) can tran spo rt bo th electron s and ho les. F ig. 5 The relative energy levels of mater ials in dev ice 1 ( A ) and the PL (L ef t ax is) of TPD and Gd ( PM IP) 3 (Bipy) ( Κex = 280 nm ) and EL (r ight ax is) of dev ice ITOgTPD (40 nm ) ggd (PM IP) 3- (Bipy) (80 nm ) gm g Ag (200 nm ) gag (100 nm ) at 10 V (B). (B ipy) (80 nm ) gm g A g (200 nm ) ga g (100 nm ), w as fab ricated. A s show n in F ig. 5 (B ), the EL spectrum w ith a peak at 516 nm, and a shou lder peak at 420 nm resu lted from T PD is iden tical to the PL spectrum of Gd (PM IP) 3 (B ipy). A s w as re2 po rted [23 ], the sp in2o rb it pertu rbation of the levels of the ligand elect ron ic sta tes, p redom inan t ly in2 duced by the param agnetic Gd 3+ ion, cau ses the longer lifet im e of the excited sta te ( ca. 1 Λs) of Gd (PM IP) 3 (B ipy). But no excip lex w as found on the in terface betw een T PD and Gd (PM IP) 3 (B ipy). How ever, T PD and L a (PM IP) 3 (B ipy) have the sho rter lifetim e of the excited states ( 1 n s), and the excip lex at 522 nm can be fo rm ed on the in ter2 face betw een T PD and L a (PM IP) 3 (B ipy). W ith ALQ as an electron2tran spo rting layer, device 2 w as fab ricated. U nder a fo rw ard b ias, the device em it s g reen ligh t w ith a lum inance of 800 cdgm 2 at 16 V, resu lted from the co2em ission of the excip lex and ALQ. T h is fact suppo rt s tha t L a (PM IP) 3 (B ipy) is ab le to tran spo rt ho les. T h is Re fe re nce s [ 1 ] T ang C. W., V anslyke A., A pp l. P hy ṡ L etṫ, 1987, 51, 913 [ 2 ] Burroughes J. H., B radley D. D. C., B row n A. R., et al., N ature, 1990, 347, 539 [ 3 ] B ian Z. Q., Gao D. Q., Sun C. Y., et al, Chem. R eṡ Ch i2 nese U niversities, 2002, 18 (4), 466 [ 4 ] Baldo M. A., O B rien D. F., You Y., et al., N atu re, 1998, 395, 151 [ 5 ] A dach i C., Baldo M. A., Fo rrest S. R., et al., A pp l. P hy ṡ L etṫ, 2001, 78, 1622 [ 6 ] T ang J. Y., Zhang W. G., Chem. J. Ch inese U niversities, 2003, 24 (3), 541 [ 7 ] H u B., Yang Z., Karasz F. E., J. A pp l. P hy ṡ, 1994, 76, 2419 [ 8 ] Berggren M., Granstro m M., Ingana β s O., et al., A d v. M ater., 1995, 7, 900 [ 9 ] B row n A. R., Greenham N. C., Burroughes J. H., et al., Chem. P hy ṡ L etṫ, 1992, 200, 46 [ 10 ] Capecch i S., Renault O., M oon D. G., et al., A d v. M ater., 2000, 12, 1591 [ 11 ] Itano K., O gaw a H., Sh iro ta Y., A pp l. P hy ṡ L etṫ, 1998, 72, 636 [ 12 ] W ang J. F., Kaw abe Y., Shaheen S. E., et al., A d v. M ater., 1998, 10, 230 [ 13 ] Gebler D. D., W ang Y. Z., Fu D. K., et al., J. Chem. P hy ṡ, 1998, 108, 7842 [ 14 ] A dach i C., T sutsui T., Saito S., A pp l. P hy ṡ L etṫ, 1990, 56, 799 [ 15 ] Chandro ss E. A., L ongwo rth J. W., V isco R. E., J. A m. Chem. S oc., 1965, 87, 3259 [ 16 ] Jenekhe S. A., O saheni J. A., S cience, 1994, 265, 765 [ 17 ] Jenekhe S. A., A d v. M ater., 1995, 7, 309 [ 18 ] H uang C. H., Coord ination Chem istry of R are E arths, S ci2 ence P ress, B eij ing, 1997, 363 [ 19 ] L i Q., Zhou D. J., H uang C. H., et al., A cta Ch im ica S inica, 1998, 56, 52 [ 20 ] Granlund T., Pettersson A. A., A ndersson M., et al., J. A pp l. P hy ṡ, 1997, 81, 8097 [ 21 ] H uang C. H., L i F. Y., H uang Y. Y., U ltrath in F ilm s f or Op tics and E lectronics, Peking U niversity P ress, Beijing, 2001 [ 22 ] M ich l J., Bonacic2Koutecky V., E lectronic A sp ects of O r2 ganic P hotochem istry, John W iley and Sons, Inc., N ew Yo rk, 1990 [ 23 ] Zhang X. M., Sun R. G., Zheng Q. B., et al., A pp l. P hy ṡ L etṫ, 1997, 71, 2596