Letter to the Editor

Simulation of the sublimation process in the preparation ofphotochromic WO3 ®lm by laser microprobe mass

spectrometry

Y.A. Yang a, Y. Ma a, J.N. Yao a,*, B.H. Loo b

a Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100101, People's Republic of Chinab Department of Chemistry, University of Alabama, Huntsville, AL 35899, USA

Received 15 October 1999; received in revised form 31 January 2000

Abstract

It is known that electrophotochromism of a WO3 ®lm dramatically depends on its microstructure. To probe the

in¯uence of the microcomposition and the microstructure on the electrophotochromism of the VE-WO3 ®lm, we

employed a method based on laser microprobe mass spectrometry to simulate the sublimation process of WO3 with

conditions mimicking those in vacuum evaporation. Both positive and negative ion clusters containing no more than six

tungsten atoms were generated from direct laser vaporization of WO3 powder. The fragment valence calculation on the

ion clusters showed that a valence of six was the ideal oxidation state for tungsten. The X-ray photoelectron spec-

troscopy results also con®rmed the formation of stoichiometric units in the deposited ®lm. Based on experimental

results, (WO3)x, especially (WO3)3 and (WO3)4, were found to be the major component units in the vacuum evaporated

(VE) ®lm. Ó 2000 Elsevier Science B.V. All rights reserved.

Among the transition metal oxides withphotochromism and electrochromism which canbe potentially applied in information display andlight modulation devices, WO3 is one of the moststudied. It is generally fabricated into WO3 thin®lms by a number of methods, such as vacuumevaporation [1], chemical vapor deposition [2], sol±gel [3], radio-frequency sputtering [4], etc. Thevacuum evaporated (VE) thin ®lms exhibit highpurity and good photochromic/electrochromicproperties. Hence, this method is most popular forthe preparation of WO3 ®lms. Nevertheless, the

coloration e�ciency of the VE-WO3 ®lms dependson the speci®c preparation conditions and furtheron their microscopic structure and components,and unfortunately the relationship among themhas not been known until now. This present workstems from an attempt to approach this problem.

Vacuum sublimation of WO3 powder wasstudied previously by mass spectrometry [5±7], butthe studies focused primarily on the gaseous re-actions of WO3 powder as a catalyst with the ex-perimental conditions quite di�erent from thoseused in the preparation of VE-WO3 ®lms. In thiswork, we designed a method based on laser mi-croprobe mass spectrometry (LMMS) to simulatethe sublimation process of WO3 with condi-tions mimicking those in vacuum evaporation.The gas-phase chemistry of the clusters provides

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Journal of Non-Crystalline Solids 272 (2000) 71±74

* Corresponding author. Tel.: +86-10 64888154; fax: +86-10

62029375.

E-mail address: jnyao@ipc.ac.cn (J.N. Yao).

0022-3093/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved.

PII: S 0 0 2 2 - 3 0 9 3 ( 0 0 ) 0 0 2 2 6 - X

information on the microstructures of the VE-WO3 ®lms and their photochromic and electro-chromic performance characteristics.

The simulation experiments were done on atandem time-of-¯ight mass spectrometer (TOF-MS) [8], with the experimental setup given inFig. 1. The second harmonic (532 nm) of aNd:YAG laser was used to ablate the WO3 samplewith an energy density of about 108 W/cm2, as-suming an ablated area of 0.2 mm2. The laser wasoperated from 5 to 20 mJ per pulse with 10 nsduration and a 10 Hz repetition rate. The samplewas ablated solely by heating since the laserwavelength used (532 nm) was much longer thanthe excitation wavelength (380 nm) for WO3. Themass resolution was m=Dm � 300.

The conditions for the simulation experimentswere made as close as possible to those employedin the preparation of VE-WO3 ®lm as follows: (1)the distance between the pellet sample and thecenter of the ®rst accelerating region was about10±12 cm, approximately equal to the distance ofthe evaporation source and the substrate targetduring VE process; (2) the TOF-MS chamber wasmaintained at 10ÿ5±10ÿ6 Torr pressure and roomtemperature, the same conditions as in the VEprocess.

The VE-WO3 ®lm was prepared according to aprevious report [1]. The valence state of the tung-sten atom in the VE ®lm was determined with anX-ray photoelectron spectrometer. The Ramanspectrum of the ®lm was acquired on a Ramanmicroscope with an Ar� excitation laser operatingat 514.5 nm.

When the WO3 sample was ablated, a largevariety of clusters consisting of positive ions,

negative ions and neutral molecules were generat-ed. However, both the positive and negative ionswere detected during the ablation process of theWO3 pellet. Fig. 2 shows the respective abundancedistribution of positive (A) and negative (B) ions.No cluster signals above 1500 mass units weredetected. In Fig. 2(A), all clusters can be classi®edinto four types, WxO

�3xÿ3, WxO

�3xÿ2, WxO

�3xÿ1,

WxO�3x, x � 1; 2; 3; . . . Among them, clusters with

the formula of WxO�3xÿ1 have the strongest peaks,

and their abundance was found to be independentof the laser power from 5 to 20 mJ. The signals forthe WxO

�3xÿ3 clusters were weak, whereas that for

the W� ion was not detected at all. This ®nding isconsistent with those observed by others [6]. Incontrast to other metal oxides [9,10], the signalintensity for the positive ions did not show anodd±even alternation (Fig. 2(A)), although thesignals for the di-tungsten clusters were muchsmaller than those for the mono- and tri-tungstenclusters.

The observed negative ion clusters could beclassi®ed into two types: WxO

ÿ3x and WxO

ÿ3x�1,

x � 1±6 (see Fig. 2(B)), with very small amounts ofW4Oÿ11 and W5Oÿ14. However, the clusters ofWxO

ÿ3x, x � 2±4, were the most abundant. As just

stated above, the ablated conditions in the LMMSexperiment were close to those used in the prepa-ration of the VE-WO3 ®lm; therefore the clustersin the gas phase may be used to provide clues onthe compositional units of the VE ®lm.

To correlate ion clusters, we employed a uni-form parameter known as fragment valence K

Fig. 1. A schematic of the LMMS experimental setup.

Fig. 2. Mass spectra of the positive (A) and negative (B) clus-

ters formed from the laser-ablated WO3 powder.

72 Y.A. Yang et al. / Journal of Non-Crystalline Solids 272 (2000) 71±74

which was ®rst used by Plog et al. [11] in secondaryion mass spectrometry. Subsequently, Michielsand Gijbels [6] applied it in LMMS. The fragmentvalence K is de®ned as the formal valence numberof the metal atom in an emitted cluster MmOn, andis equal to

K � �q� 2n�=m; �1�where q is the total charge of the cluster ion (+1 forpositive or )1 for negative). According to the Plogvalence model, positive or negative ion intensity ofthe MmOn clusters belongs to the same group. Thatis, the intensity of the clusters having the samenumber of metal atoms �m� but di�erent numberof oxygen atoms �n� is described by a Gaussianfunction of the fragment valence K,

I�K� � Imax expfÿ�K ÿ G�2=2r2g; �2�where Imax is the maximum intensity of the ®ttedcurve, G the K value corresponding to Imax, and r2

is the variance of the distribution.

The fragment valence K of the four types ofpositive ion clusters in Fig. 2(A) are given by thefollowing formula, calculated by taking m � x,n � 3xÿ z, and q � 1 in Eq. (1):

K � 6ÿ �2zÿ 1�=x for WxO�3xÿz �3�

with z � 0; 1; 2; 3.Likewise, the K values for the two types of

negative ions, WxOÿ3x, and WxO

ÿ3x�1, are 6ÿ 1=x

and 6� 1=x, respectively.Fig. 3 shows the plots of relative intensities vs

fragment valence for positive cluster ions obvi-ously ®tting the Gaussian distributions (solid lines)described as Eq. (2), which supports the appli-cability of using the K model to describe theexperimental results. The XPS data of the VE-WO3 ®lm also showed W 4f7=2 peak at 36.1 eVand W 4f5=2 at 38.6 eV, indicating that W had anoxidation state of +6. Of all the ions, the frag-ment valences of WxO

�3xÿ2, WxO

�3xÿ1, WxO

ÿ3x, and

WxOÿ3x�1 are very near the ideal valence of 6.

This indicates that tungsten atoms in the VE ®lmexist in the +6 valence. Clusters with the frag-ment valences of 6ÿ 1=x and 6� 1=x can bethought of as another form of the stoichiometricmolecule since they cannot show an exact oxi-dation state of 6 in the charged clusters. It isappropriate to treat them as neutral molecules�WO3�x, since the ionic clusters will becomeneutral molecules during the ®lm formationprocess. For example, W3O�8 , W3O�9 , W3Oÿ9 andW3Oÿ10 can be all regarded as the molecule W3O9.According to this treatment, all clusters, bothpositive and negative ions, can be sorted intogroups with normalized relative integrated in-tensity (NRII) given in Table 1.

It is seen from this table that four types ofclusters with NRII over 10% make up over 80% ofall the clusters produced, especially W3O9 andW4O12 comprising nearly 60%. This ®nding issimilar to Arnoldussen's [12] results that W3O9

was the most abundant component unit in the

Fig. 3. The plots of relative intensities vs fragment valence for

the positive ion clusters.

Table 1

Normalized relative integrated intensity of clusters

Cluster WO3 W2O6 W3O�7 W3O9 W4O�10 W4O12 W5O�13 W5O15 W6O18 Others

NRII (%) 5.75 � 0.08 12.8 � 0.4 2.54 � 0.07 30.6 � 0.8 3.06 � 0.09 27.4 � 0.7 2.55 � 0.08 11.3 � 0.3 2.08 � 0.07 1.84 � 0.06

Y.A. Yang et al. / Journal of Non-Crystalline Solids 272 (2000) 71±74 73

WO3 ®lm. Fig. 2 suggests that the cluster groupsdi�er in multiples of the WO3 unit. Using colli-sional activation dissociation method, Malekniaet al. [5] studied the dissociation behavior of(WO3)x, x � 3±7, and found that W2O6 and W3O9

were the most predominant lost units. Our resultsshowing high abundance of W2O6 and W3O9 areessentially in agreement with theirs. In view ofMaleknia's results [5], the relative high abundanceof W4O12 and W5O15 clusters may be explained byregarding W4O12 as the combination of two W2O6

clusters, and W5O15 as the combination of W2O6

and W3O9 clusters. Hence, the most stable gaseouscomponents in the sublimation of WO3 powdercan thus be written as (WO3)x, x � 2±5.

Stable gas-phase clusters detected in TOF-MScould represent the component units in the VE-WO3 ®lm, since the detection region was designedto be located at the same distance from the ablatedsample as the deposition region (substrate) fromthe sublimation source in the vacuum evaporation.Knowing the microstructure of the VE ®lm andthe gas-phase chemistry of the clusters is helpfulfor ®nding the way to improve its coloration e�-ciency and stability. In our experiment, (WO3)x,x � 2±5 are the most likely component units in theVE-WO3 ®lm, which suggests that the VE ®lmonly has short-range order. This conclusion is inagreement with the Raman results on the VE-WO3

®lm, which show the ®lm to be amorphous. Twobroad bands at about 750 and 951 cmÿ1 show upin the Raman spectrum of VE-WO3 ®lm, in con-trast to two sharp peaks at 714 and 805 cmÿ1

observed in the WO3 powder. The 951 cmÿ1 bandwas attributed to the newly formed W¸O doublebonds [13].

The sublimation process in the preparation ofVE-WO3 ®lm was mimicked and studied by usingLMMS. (WO)x clusters, x � 2±5 dominated thecomponent units of VE-WO3 ®lm, especially W3O9

and W4O12. The valence model and XPS showedthat the VE-WO3 ®lm ideally formed stoichio-metric and that the oxidation state for the tungstenatoms was +6. Though the cause is still unclear,the clusters of W3O9 and W4O12 form with manyW¸O double bonds which can be reduced and actas cages to easily capture the incorporated reduc-tive substances, resulting in photochromism and/or electrochromism.

Acknowledgements

This work was supported by the NationalNatural Science Foundation of China and theChinese Academy of Sciences.

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