Communications

ADVANCED

MATERIALS

be explained by the fragmentation pattern. Typically, the [MI@peak repre-

sents the basis peak (loo%), which additionally confirms the cyclic structure

(related open chain compounds give only a low intens ity of the [MI' peak).

Also the [M HI' and the

[M

Na]@peaks in the electrospray-MS corre-

spond to the cyclic structures.

N o

additional peaks could he detected in the

scanned ranges up to d z = 2000. Therefore, the next possible macrocycle

with larger size is not present and it is reasonable to assume that other

macrocycles with larger sizes are also absent.

Thc

purity was additionally

checked by HP LC and was found to be Y9 %.

Spectroscopic data arc given for compound

: 'H-NMR (500

MHz,

CDCI;, Me4%)

6 3.66-3.72 (m, 12H; CH20). 3.73-3.76 (m, 4H; CH,O),

3.8k3.83 (m, 8H: CH,O), 3.88-3.93 (m, 4H: ar-OCH,), 4.19 (t, 4H, J 9 Hz;

ar-OCHZ),6.84 (d, I2

Hz, 2H: napht.

H), 6.93 (d, .I 9 Hz, 4H; ph. H), 6.97

(dd, J Y Hz,

J 2 Hz, 2H;

napht. H). 7.40 (d,JY

Hz, 2H: napht. H), 7.62 (d,

J

9 Hz. 4 H ph. H): I3C-NMR SO MH7, CDC13, Mc4Si)

6 67.34, 67.83, 69.52,

69.85,70.68, 70.96 (0-CH2),106.76(naph. C-H),

115.20

(naph. C-H), 119.04

(ph. quart. C),

122.73 (ph. C-H), 122.73 (ph. quart.

C), 128.08 (ph. C-H),

129.18(naph. C-H), 129.62 (naph. quart. C), 155.25 (naph. C-0), 161.06 (ph.

C-0) , 166.96 (thiadiazole): EIMS (70eV) m/z

(relative intensity): 746 (100,

M e . 714 ( 2 ) ,627 ( 5 ) . 561 (13), 534 (6) 441 5), 415 5), 402 (9), 297

5 ) ,

212

(23); ESMS ndz: 747.1 [M

HI', 768.8 [M

Na]'.

Receivcd: Septembe r 10,1996

Final version: November 6, 1996

(11 V. Percec, M. Kawasumi, P. M. Rinaldi, V. E. Litman, Macromolecules

1992, 25, 3851; V.

Pcrcec, A. D. Asandei, P Chu,

Macromolecules

1996,29. 3736.

[Z]

B. L. Allwood, N. Spencer.

H.

Shahriari-Zavareh,

J. F.

Stoddart, D. J.

Williams,J. Chem.Soc. Chem. Comn~lm.

987, 1064.

[ 3 ] F? R. Ashton, D. Joachimi.

N.

Spencer, J.

F.

Stoddart,

C.

Tschierske,

Proc.

22. Freiburger

Arbeitaagung Fliissigkristalle 1993,P22.

[4] P. R. Ashton. D. Joachimi. N. Spenccr, J. F.

Stoddart, C. Tschierske,

A F? White, D. J. Williams, K. Zab. Angew Chem., Inf. Engl, 1994,

3.3, 1503.

[S] D. Joachimi. P.

K. Ashton, C. Sauer. N. Spencer, C. Tschierske, K.

Zab, Liq. Crysf. 1996,20 , 337.

[6] P. L. Anelli. P. R. Ashton, R. Ballardini, V. Balzani, M. Delgado, M. T.

Gandolfi, T. T. Goodnow, A. E. Kailer,

D.

I'hilp, M. Pietraszkiewicz,

L. Prodi.

M. V.

Reddington.

A .

M. Z . Slawin, N. Spencer, J. F. Stodd-

art. C. Vicent. D.

J.

Williams. J.

Am

Chem. Soc. 1992,114 193:

D.

B.

Amabilino, J. E Stoddart,D. J.

Williams. Chem. Mafer . 1994, 6, 1159.

[7] I

R .

Ashton.

J.

Huff, S. Menzer, I. W. Parsons, J. A

Preece, J. F.

Frascr. M. S. Tolley, A. J. P. White, D. J Williams, Chem. Eur. J , 1996,

2. 31: M . J. Gunter, D. C. R . Hockless. M. R. Johnston, B. W. Skelton,

A.

H .

White, J. Am Chem. Soc. 1994,28.4810.

[ X I V. Percec, M . Kawasumi.Chem.Marer.

1993,5, 826.

[9] Indeed, it was recently reported that the melting transitions of homo-

chiral macrocyclcs are higher than those of racemic dcrivatives (see V.

Percec.

M.

Kawasumi, Macro?nolecules 1993,26, 3917). Furthermore,

a constitutional uniform macrocyclic trimer is

a

high melting solid and

no liquid crystalline properties have been detected (see S.

S.

Keast,

M . E. Neuhert, presented at the 16th Int. Liq. Crysf. Conf

Kent, OH

Abstract D3P.57).

[ l o ] H. RingsdorL R.Wustefeld, E. Zerta, M. Ebert,

J. H. Wenndorff,

Angrw . C h m i . 1989. 101,934; Angcw. Chem., Int. Ed. Engl. 1989,28,

914; M. Ebert. G. Frick, C. Raehr, J. H. Wendorff,

R .

Wustefeld, H.

Ringsdorf. / i q . Cry.sr. 1992,

11

293; H. Bengs, M. Ebert,

0

Karthaus,

B. Kohne, K. Praelcke, H. Ringsdorf,

J. H. Wendorff, and R.

Wiistefeld, Adv Matrr. 1990,2, 41; K. Praefcke, D. Singer,

B.

Kohne,

M. Ebert, A. Liebmann. J. H. Wendorff, Liq. Crysf. 1991,10, 147, K.

Praefcke, D. Singer,Mol. Muter. 1994.3,265.

1111

I. Letko,

S.

Diele. G. Pclzl,

W.

Weissflog,

Liq. Cryst. 1995, 19, 643; I.

Lctko, S. Diele,

Ci

PeIz1, W. Weissflog. Mol . Cryst. Liq. Crysf . 1995,

260. 171.

1121 N.

K.

Sharma, G. Pelzl,

D.

Demus, W. Weissflog,Z. Phys. Chem.1980,

261, 579; N. Homura, Y. Matsunaga, M. Suzuki, Mol . Crysf Liq. Crysf .

1985, 131.273.

[I31 W. H. de Jeu, L. Longa, D. Demus.1

Phys. Chem. 1986,84,6410.

114) C. Pugh. V. Percec, Polynz. BuU 1990,23 , 177.

[IS] The maximum of the stability is shifted away from

50mol-%.

This

behavior is often observed. especially

if

one of the compounds shows

no inesophasr in an accessible temperatu re region (see [13]).

1161 J. F. W. McOmie, D.E. West, Org. Synfh.

Coll. V d . K 1973,412.

1171 C. Tschierskc. D. Girdziunaite. J. Prakt. Chem. 1991,3.7.7, 135.

[I81

J. Aiidersch. C. Tschierske, Liq. Crys t . 1996,21,51.

Structure and O ptical Property Changes of

Sol-Gel Derived VO2 Thin Films

By Songwei Lu,*

Lisong

Hou, and Fuxi Gun

It is known that vanadium dioxide undergoes a remark-

able, thermally induced, reversible semiconductor-to-metal

phase transition around 67 C, resulting in great changes in

its optical, electrical, and magnetic properties.['-'] This kind

of phase transition makes it possible to fabricate electro-

and photochromic and optical data storage

disks['] by using VOZ. hin films. By means of doping with

other ions, such as W60,[9, 1 he transition temperature Tt

can be successfully decreased to room temperature, sug-

gesting possible applications as energy efficient windows,

for example.r81Although there are various methods of fab-

ricating these films,[12-'61he sol-gel method, involving hy-

drolysis of vanadium alkoxide, dip-coating, and post-heat-

treatment, has proved to be one of the most convenient

routes to synthesize V 0 2 thin films. In the work presented

here, V02 thin films were synthesized using the sol-gel

method and their structure and optical properties were

characterized. A change in transmittance of more than

57YO in the near-infrared region was observed during heat-

ing and cooling, and the transmittance- and reflectivity-

temperature hysteresis phenomena were recorded. The

structural change of V02 thin films was investigated by

electron diffraction. Based on coordinative field theory, a

phase-change mechanism is proposed. The crystal structur-

al conversion from monoclinic at low temperature to tetra-

gonal rutile at high temperature is the essential feature of

the phenomena.

Sols containing vanadium alkoxide were synthesized

using tri-isopropoxyvanadyl [ V O ~ O C ~ H S ) ~ ;RI Chemical

Laboratory, Japan, 99.999YO purity] as the raw material. By

hydrolysis of the alkoxide with an H20:VO('OC2H5)3mo-

lar ratio of 2.0, 50 mL sols were carefully prepared with

vigorous stirring at room temperature in a brown flask

sealed from the ambient atmosphere. The as-prepared sols

were transparent orange in color, and are stable for months

without any precipitation. Thin films were dip-coated at a

rate of 11.6 cmmin-I on slide glass and silica glass sub-

strates.

The

as-dipped coatings were bright orange in color

and of high surface quality. After the films had been cured

under low vacuum (6.7 Pa) at 400-500 C for 2 h,

V5@

ons

were found to be reduced to V40 ions, and gray-black V02

films were formed.

The structure and optical properties of the thin films

were characterized by transmission electron microscopy

and spectrometry, respectively. The transmittance spectra

were taken using a Perkin-Elmer Lambda ultraviolet-

[*]

S. Lu,

Prof. L. Hou, Prof.

F.

Can

Shanghai Institute of Optics and Fine Mechanics

Academia Sinica

P 0.Box 800-211,Shanghai 201800

(PR China)

244

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visible-near-infrared spectrometer with a heating cell and a

temperature controller and referenced to uncoated silica

glasses. The transmittance- and reflectivity-temperature

hysteresis curves were recorded at

1.51

pm by a 44 W in-

frared spectrometer with a heatable sample holder. Film

thickness was determined by means of a Dektak stylus in-

strument. Electron diffraction patterns were taken by

JEM-6A electron transmission microscope. Coatings were

immersed into dilute HF aqueous solutions for 10 min.

After being separated from the substrate, the free-standing

films were then washed in distilled water for

5

min, and

afterwards transferred to a copper grid coated with carbon

film. The sample holder can be turned in three dimensions

and heated to more than 100 G.

Figure l a presents the transmittance spectra of

V02

films on silica glasses at 25 C and 100 C. On heating, the

transmittance decreases considerably in the long wave-

Wavelength (nm)

Fig. 1. a) Transmittance spectra of

85

nm thick VOz films on silica glass at

25 C and 100 C. h )

The transmittance- and reflectivity-temperature hys-

teresis of 170 nm thick V02 ilms on silica glass.

length region. The transmittance difference in the near-

infrared region is larger than that in the visible region. The

transmittance change at 2.5 pm is more than 57 %.

The changes in transmittance and reflectivity are revers-

ible on heating and cooling. The hysteresis curves of V 0 2

films at 1.51 pm are shown in Figure lb . The transmittance

decreases on heating and increases on cooling. However,

the reflectivity increases during heating and vice versa. The

transmittance change is larger than the reflectivity change.

The transition temperature Tt can be deduced as 67°C

from the hysteresis curves.

Electron diffraction patterns of

VOz

coatings are demon-

strated in Figure 2. The structural change of VO, crystals

can be explained from these patterns. According to the

mechanism of electron diffraction and Bragg's law, Equa-

tion

l

holds, where R is the distance between each diffrac-

tion spot and the electron diffraction pattern center,

d

is

the spacing between planes of the crystal, L is the electron

diffraction camera length, I is the incident electron wave-

length, and K is the electron transmission microscope cam-

era parameter. For a certain electron transmission

microscope, L and are constants, that is, K is also a con-

stant: K = 2.336 mm nm for the JEM-6A electron transmis-

sion microscope. Therefore, d can be deduced from R

(Eq. 2).

Fig.

2.

Electron diffraction patterns

af VOz films at 25

C (a) and

100 C

(h).

The two electron diffraction patterns were taken from the same spot on the

sample at different temperatures.

Rd

=

LA= K (1)

The calculated results fit very well with the standard val-

ues of V0 2 crystals. At low temperature, the VO, crystals

have calculated d values of 0.331, 0.321, 0.269, and

0,186 nm. They are in good agreement with monoclinic

V 0 2

crystals with planes (hk l )

= ( i l l )

(d = 0.331 nm),

(011) (d = 0.320 nm), (702) (d 0.268 nm), and (302) (d =

0.1874 nm), respectively. At high temperature, the calcu-

lated d data are 0.433,0.340, 0.234, and

0.194

nm. They are

satisfied with tetragonal rutile V02 structure with planes

(hkl) =

(110)

d = 0.43.5 nm), (120) d = 0.339 nm), (040) d

= 0.2353 nm) and (230) (d = 0.1933 nm), respectively.

Therefore, the structure of VOz is monoclinic at low tem-

perature and tetragonal rutile a t high temperature.

The reason why the

V 0 2

structure changes with tempera-

ture is explained based on coordinative field theory. At low

temperature, i.e. below the transition temperature T,, V

and

0

atoms are located in an octahedral field (Fig. 3). The

3d orbits of V ions are split into two energy levels of dZ2

and dxlY7, and dxy,dyr, and dxz n accordance with the V4@

Ad v Muter 1997 ,

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Distorted octahedron

ctahedton

a

_...--

--

....

..

312 Fig. 3. Schem atic representa tion of the

[VO,] octahedron and its distortion, the

VO, -tctragonal rutile structure and the

Tclragonal rutile structure Split 3d orbits Resplit 3d orbils in Monoclinic monoc linic struc ture [3], th e split 3d orbi ts

P4,lmnm in octahddral field distorted octahedral field P2,ic in the octahed ral field and the resplit 3d or-

bits in the distorted octahedral field. Empty

circles arc 02 ions and filled circles are

v40 ons.

. v

0 0

Low temperature ( T

igh temperature

( T ; , )

electronic structure of [Ar]3d1, the only electron in the 3d

orbit can be in only one of the energy levels d dyz,or d,,

(for example dxy).Therefore, other 3d orbits of the V4@on

are still empty and the 3d electron cloud is unsymmetrical.

On coordinating with 0 2@ons, the unsymmetrical electron

cloud of the

V4@

on is distorted further. Two

02

ions

on

the Z axis approach the V40 ion slightly due to the weak

repelling force, for there is

no

electron on the Z axis in the

unsymmetrical electron cloud of the V4@ on. However,

four 2@ons in the X - Y plane move slightly away from

the octahedron center due to the repulsion of the only elec-

tron in the d,. orbit. Therefore, the distances between V

and

0

are different ] ( V - 0 distances are 0.176, 0.186,

0.187, 0.201, 0.203, and 0.205 nm and V-V distances are

0.265 and 0.312 nm), and the octahedron becomes dis-

torted. As a result of the distortion, the d,? and d,:+ and

d,xy,

d,,, and

d,:

orbits are split further (Fig. 3). Then the

dxyorbit has the lowest energy, and the V4@ on is more

stable because it obtains an extra energy of (3/2)S2. This

kind of compressed [VO,] group reduces the crystalline

symmetry: vanadium dioxide has monoclinic P21/c struc-

ture at low temperature.

At

high temperature, i.e., above the transition tempera-

ture Tt, the weak distortion effect is disturbed by the ther-

mal movement of the atoms. The six 0 ions return to the

original positions of the octahedron (Fig. 3). The distorted

[VO,]

group becomcs a regular octahedron.

As

a result,

VO2 has the tetragonal rutile P42/rnnm structure. The dis-

tances of V-0 and V-V are also changed?] d v - 0 becomes

0.194

nm

and dv. becomes 0.287 nm.

The change in the optical properties of V02 coatings is

related to the change in structure. Due to the distortion at

low temperature, polarization occurs when the lengths of

some of the shorter V-0 bonds are less than the sum of the

radii of the

V4@

and 2 ons

(Rv-0 = rvo

role =

0.200 nm). At high temperature, the V-0 distance is

0.194 nm, very close to Rv-o. Therefore, the polarizability

of monoclinic V 0 2 is larger than that of tetragonal rutile

V02, for the greater the difference between dv-o and

Rv-o, the greater the polarization and the larger the polar-

izability. As a result, the refractive index of

V 0 2

decreases

when VOZ changes from monoclinic at low temperature to

the tetragonal rutile structure at high temperature, because

the refractive index increases with increasing ion polariz-

ability.

In conclusion, the interesting phase transition of V02

materials has been confirmed as the monoclinic-to-tetrago-

nal rutile transition explained by coordinative field theory.

The optical properties of V 0 2 exhibit remarkable revers-

ible changes due to the polarizability conversion resulting

from the structure transition.

Received. Sep temb er 23, 1996

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246

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Adv . Mater. 1997, 9, No. 3