Optically switchable thin films: a review

ELSEVIER

Thin Solid

Films, 251 ( 1994) 81-93

Invited Review

Optically switchable thin films: a review Charles B. Greenberg PPGIndustries,

Inc., Glass Technology Center, Pittsburgh, PA 15238, USA

Received 22 February 1994; accepted II May 1994

Abstract Reversible transitions in molecular structure, symmetry and energy banding are very common in solids and liquids. Such transitions are known to occur both gradually and discontinuously, but they are of greatest interest when they occur relatively abruptly. The transitions are induced electrically, thermally, with pressure or by exposure to UV-visible radiation. Materials that respond to these excitations with spectral changes, notably coloration in the visible range, are electrochromic, thermochromic, piezochromic or photochromic respectively. Some of the materials in these classes have been prepared as thin films, but they are relatively few in number. Thin films that have been shown to switch dramatically in the visible range, as well as the near-IR, and their properties, are reviewed here. Spectral switching of these films has been of interest to us because of the potential to control solar loading dynamically through transparencies in buildings and vehicles. Keywords:

Coatings; Optical coatings; Optical properties; Tungsten oxide

1. Introduction In all, there are literally thousands of switchable chemical compounds known in the categories of electrochromism, thermochromism, piezochromism and photochromism, in the three states of matter [l-3]. These switchable materials exhibit reversible spectral coloration in the visible region, when they are subjected to charge injection-extraction, heat, pressurization or W-visible radiation respectively. They are known as optically switching or chromogenic materials or, in the rarer coated form, as chromogenic thin films. As a group, chromogenic films have been deposited on substrates by most of the well-established methods known, both physical and chemical. In many instances the research has been done with the purpose of developing advanced technologies for displays [4-71, optical storage [8-lo], mirrors [ll-141, windows [15-171 or sunglasses [ 12- 141; however, to a great extent, switchable films continue to be research curiosities. They have yet to fulfill some oft-spoken grand dreams. Nevertheless, chromogenic films represent a very exciting group of materials for scientific research, and it is still to be expected that reasonable commercial opportunities may Elsevier Science S.A. SSDI 0040-6090(94)06203-W

evolve. Although all four types of switchable film have been known for more than 20 years, judging by published literature, the study of them is still in a youthful stage and rich with opportunity for learning. In the following sections I shall review the important thin solid films in each subcategory of this field. The spectral switching for some films will be shown, and there will be a description of the most accepted mechanisms believed to be associated with the effect. There will be somewhat less emphasis on the method of film preparation, as that generally is not crucial to observation of significant chromogenism. Often, there are multiple ways to grow the film of interest. The four sections to follow are taken, approximately, in descending order of current activity in the literature.

2. Electrochromic thin films Electrochromism is a reversible and visible change in transmittance and/or reflectance that is associated with electrochemically induced oxidation-reduction reactions at low voltages, typically on the order + 1 V d.c. The most straightforward way to demonstrate the effect

C. B. Greenberg/ Thin Solid Films, 251 (1994) 81-93

82

with an electrochromic thin film is to deposit it on a conductive and transparent electrode, such as SnO,:F or indium-tin-oxide coated glass. The electrode is then immersed in an appropriate electrolyte solution containing H+, an alkali ion or, in the case of some organic films, a suitable anion. One class of inorganic oxide films exhibits cathodic coloration by the reversible double insertion of electrons and monovalent charge-compensating ions, according to M,O;yH,O (clear)

+xe-

+xJ+

c, J,M,O;yH,O

(1)

(dark)

where M is a multivalent cation of the film with valence 2m/n. Both m and n are integers nominally. The ion being inserted while forming the color center is J+. Typically, 0 < x < 0.5 and, as x -+ 0.5, the film changes reversibly from a clear transmitting state in normal white lighting to an increasingly darker shade of the same hue. The film does not change from one hue to another. The y mol of HZ0 indicate that films exhibiting good electrochromism are hydrated to some degree, depending on preparation technique [6, 18-201. Film porosity has, likewise, been closely associated with rapid response. Table 1 includes the most actively studied and most responsive cathodic films, and typical liquid electrolytes that have been used. Coloration efficiencies CE(1) are also given by GE(l)

=

7

The AOD(1) is the change in single-pass transmitted optical density at the wavelength 1 of interest due to a transfer of charge q (C cm-‘). It is determined in liquid electrolyte by spectroelectrochemistry, using a cathodeanode pair, one member of which is sampled by the beam. Adherence to Lambert’s law is assumed. While that may not be strictly appropriate for charge injection Table 1 Some cathodically

and anodically

Film

a-WO,

c-wo, c-MO, a-WO, (240-660 nm) IrO, (90- 180 nm) NiO, Prussian blue Polyaniline Polypyrrole Polythiophene Phthalocyanines

coloring

electrochromic

at a thin-film-coated surface, the linearity of Eq. (2) seems to be retained up to moderate values of q for many inorganic films [22, 451. The coloration efficiencies for W03 in two forms can be compared at 800 nm from the data in Table 1. The amorphous or poorly crystallized film has a higher CE at that i than does the crystallized film. The latter is easily grown or formed above 300 “C and is characterized, in the darkened state, by high reflectivity at longer 1 in the IR, associated with free-electron banding [46]. Over the whole visible and near-IR portions of the spectrum, however, the comparatively high absorption by amorphous W03 (a-WO,) results in a higher overall CE(A) for solar attenuation [21]. Over this same range, crystallized MO3 (c-MO,) is of intermediate value. Films of a-WO, have been widely researched since the initial observation of the electrochromic effect in WO, [47-491, leading to the well-accepted ion insertion model suggested by Eq. (1) [6]. A typical example of optical switching over the solar range is shown in Fig. 1 [50]. These data were obtained for a resistively evaporated a-WO, film deposited on conductive glass and laminated to a second conductive glass with poly-2-acrylamido-2-methylpropanesulfonic acid (PAAMPS) polymer-electrolyte, 0.8 mm thick [4, 511. As formed, this water-based polymer electrolyte is a clear transparent solid but soft. The charge-balancing electrochemical reaction at the second conductive surface during film darkening, at - 2.7 V for 4 min, was the irreversible electrolysis of water: 2H,O (polymer) +0,(g)

+ 4H+ + 4e-

(3)

During initial cycling, the generation of 0, does not evidence itself as bubbles, so that it was possible to collect the data of Fig. 1 without obstruction of the optical beam. Useful solid-state electrochromic cells, however, employ either a reversibly oxidizable bulk counter electrode or a second thin film that provides

films Reference

Coloring class

Deposition conditions”

Electrolyte/ dopant

GE(I) (cm* C-‘)

&m)

Cathodic Cathodic Cathodic Cathodic Anodic Anodic Anodic Variable Variable Variable Variable

Sputtering Sputtering (3 10 “C) Sputtering (351 “C) Vacuum evaporation Potential cycling or sputtering Galvanostatic electrodeposition Galvanostatic electrodeposition Electropolymerization Electropolymerization Electropolymerization Vacuum evaporation or sputtering

LiCIO,/PC LiClO,/PC LiCIO,/PC

132 72 93 38-45 15-18 Z50 68

800 800 800 633 633 -440 (peak) 633

H,SO, H2 5%

NaOH/KOH KCF,SO,/PC H+, BrClO,-, BF,-, CIO,-, BF,KC], ClO,-

Li+

-

-

1211 ]2]1 1211 ]221 ]7, 231 ]24, 251

1261 [27-311 [32-381 [33,38-411 [42-441

“Film deposition is by sputtering, vacuum evaporation, potential cycling, galvanostatic electrodeposition or electropolymerization. Required substrate heating is indicated in parentheses for the growth of c-WO, and c-MO,. Otherwise, ambient conditions prevail or are assumed for all but an amorphous copper phthalocyanine case; for the latter, the substrate was cooled to - 130 “C [44].

C. B. Greenberg/ Thin Solid Films, 251 (1994) 81-93

83

lOO

9o 0.8

m

E t-

0.S

O U3 U3

•

80 7O

cyok,:,/, I

2

/1

=

6O

/

so c

¢= =2_

~. o.4

\1

c

,r\ 0.2

40

~.

\\

/

/

\

20 10

o

, 350

"--~ ....

550

~ ....

750

550

0 1

i- . . . . 1150

, , - - - _ : _ -_ S_ " _ ' - : ' ~

1350

1550

1750

,

Fig. 1. Bleached ( ) and darkened ( - - - ) normal transmittance over the solar range for the case of an electrochromic a-WO3 film, 370 + 50 nm thick, resistively evaporated beginning at 5.3 x 10 -4 Pa on 25 fl/[S] conductive Nesa ~ glass, 3 m m thick and laminated with a like conductive glass. Lamination was done by casting in situ an aqueous AAMPS solution and thermally curing to a soft polymer at 70 °C for 1 h. The casting solution contained 75 g o f AAMPS, 22.5 cm 3 o f dimethyl formamide, 0.8 g o f succinic acid peroxide initiator and 0.016 g of hydroquinone in 52 cm 3 of distilled water. The solution was contained by butyl seals and capped after curing. The data were obtained for the fresh sample with a Beckman 5270 spectrophotometer. ( F r o m Ref. [50].)

gas-free complementary charge balancing by ion insertion-extraction during frequent cycling. Bulk counter electrodes of opaque carbon paper [4] and a highly transmitting Cu grid [17, 52] are two of the more successful examples known. A schematic configuration with the latter is shown in Fig. 2, and successful repetitive cycling to more than 10 000 cycles is illustrated by Fig. 3. After 10002 cycles, there was little loss in uniformity with this transparent cell, and no bubbles generated. The small loss in bleached-state transmittance may have been due to some transfer of Cu metal to the WO3 film. The reason for the faster response time after extended cycling is unknown. The reversible counter electrode reactions for the two bulk counter electrode materials are H + + e-

(4)

Cu*-*Cu + + e-*-*Cu z+ + 2e

I

2

1950

Wavelength(rim)

C-OHsurface~--~C---Osurface +

[

1

(5)

Roat Glass

I

I

wo~.xH=O

Film

I I

I

I

I

I

Polymer-Electrolyte

I

~ul~ve

I

BUS Bar

Fig. 2. Schematic diagram o f a laminated solid-state electrochromic device that employs a Cu grid counter electrode. ( F r o m Refs. [17, 52,

53].)

1

f

7

8

0

i

I

1

2

3

Neither bulk material contributes directly to coloration. Preferably therefore the counter electrode is a film that colors anodically when a - W O 3 colors cathodically. The important inorganic anodic films are given in Table I. Two of the ways that the complementary films may be paired, thin film stacking and lamination, are illustrated by the schematic diagrams in Fig. 4. There are working examples in the literature of both types of pairings using inorganic films from Table 1 [12, 13, 26, 54-56]. In addition, an excellent descriptive model has been developed and applied to the WO3-IrO2 complementary pair [57]. However, because of the relatively high coloration efficiency of Prussian blue (PB) in the visible, shown at 633 nm in Table 1, it is, perhaps, the most interesting film to pair with a - W O 3 [26, 54]. Also, among these anodically coloring inorganic films, ion insertion-extraction is best understood for PB. Electrochromic PB films have been grown by electroless deposition, by the sacrificial anode method and by electrodeposition [58-64]. Since the early work of Neff [58], two different equations have been given for electrochromic reduction from the blue form of PB to the transparent form: JFe3+[Fe2+(CN)6] (dark)

+

e-

+ J+*--*J2Fe2+[Fe2+(CN)6]

(6)

(clear)

F e 4 3 + [ F e 2 + ( C N ) 6 1 3 + 4 e - + 4 J + ,---~J4Fe42+[Fe2+(CN)613 (dark)

Glass

I

Fig. 3. Electrochromic cycling at room temperature with an active area of 14 cm × 28 cm, based on the configuration described in Fig. 2. The a-WO 3 film was prepared as in Fig. I. The thicknesses of conductive glass, cover glass and polymer-electrolyte are 5 mm, 3 mm and 0.4 mm respectively. The polymer-electrolyte in this case is a solid and firm UV-cured PAAMPS sheet, with an ion conductivity 10 -3 t"2-t cm -~ (similar preparation to that in [54]). The cell was assembled by air autoclaving for 1 h at 80°C and 1501bin -2. Cycling was done with - 0 . 5 0 V to darken and 0.60 V to bleach, and optical data were obtained with a Cary 14 spectrophotometer. (From Refs. [52, 53].)

I 4-Sided •Butyl Seal

!

3 4 5 6 Powering Time (rain)

(clear)

(7)

Very often J+ = K +. Both KFe3+[Fe2+(CN)6], the socalled wate r-soluble comp 0 und, and F e 4 3+ [ Fe 2+( C N ) 6 ]3, the insoluble form, have been called PB. Actually,

C. B. Greenberg/ Thin Solid Films, 251 (1994) 81-93

84 counter electrode film

3.0

lOO~

ITO

2.5 electrolyte film 2.0 bus bar

1.5

u

glass

t~..... ° ° ' °

=4>

~ 40

(a)

v

m 60

tungsten oxide film

conductive ~lass

a~

i 1.0

~ eo

0.5

unter electrode film

0 O

' bus bar

(b)

I

I

I

I

4

8

12

16

C y c l e s ( x 10 3)

~':,~\\'q K'N~N,\\\\\\\\'q

I

conductive glass

'

tungsten oxide film

polymer-electrolyte

Fig, 4. Schematic diagrams of two solid-state versions of an electrochromic cell with complementary thin films: (a) thin film stack; (b) laminate with a polymer-electrolyte. One or the other electrode is pre-charged, for cycling, with the insertion-extraction ion J+ in Eq. (1).

only the water-soluble form peptizes easily. The reduced compounds, K2Fe2+[Fe2+(CN)6] and K4Fe42+ [Fe2+(CN)6]3, are known as Everitt's salt and Prussian white respectively. Some of the confusion about which reaction, Eq. (6) or (7), occurs is reconciled by the commonly observed incompleteness of J+ extraction, after insertion, during cycling [65, 66]. The approach to ideal reversibility is a function of ion size. Reversibility, although imperfect, is best for K +, Rb ÷, Cs ÷ and NH4 +, all with hydrated ion radii in the range 0.118-0.125 nm [611. The highly hydrated proton in solution normally exhibits very limited reversibility in PB and is often dismissed entirely as a candidate for sustained cycling. However, cycling is substantially improved with a true solid proton-based polymer-electrolyte. Fig. 5 therefore gives evidence for the more successful cycling of a complementary thin film pair of a-WO3-PB with a PAAMPS polymer-electrolyte [54]. Good cycling resuits are shown to 20 000 cycles. Fig. 6 shows the spectra for the bleached and darkened states. All these data are for laminated samples that are l l.5cm x 15.5 cm in active area. The overall CE(2) is the sum of coloration efficiencies for the cathodically coloring (indicated by subscript c) and anodically coloring (indicated by subscript a) electrodes, according to CE(2) = CE(2), + CE(2),

-- (AOD('~').)c --[-(-AOD(/~'))a

0 2O

(8)

Fig. 5. Bleached ( © ) and darkened-state ( Q ) normal transmittances at 550 nm, and charge passed ([2), as functions o f cycling at + 1.2 V and - 0 . 6 V for an a-WO3/PAAMPS/PB laminate. Each glass member is conductive transparent NESA glass, 5 mm thick. The optical data were obtained with a Cary 14 spectrophotometer. (From Ref. 54].)

IBS,@

=

68,8iB

@,88

@.88 8,e8 3~

550

758

%e 1158 l~e

1558 1751] 1%0

2158

Wavelength (nm) Fig. 6. Normal transmittance curves in the bleached (curve A) and darkened (curve B) states for a fresh duplicate sample to that in Fig. 5. The data were obtained with the same voltages as in Fig. 5, using a Perkin-Elmer Lambda 9 spectrophotometer. ( F r o m Ref. [54].)

The overall coloration efficiency at 550 nm for the fresh WO3/PAAMPS/PB laminate is 75 cm 2 C-i. This closely equals the sum of individually measured coloration efficiencies at 550 nm: 38-40 cm 2 C -1 for a WO3/ PAAMPS/grid laminate [17] and 35cm2C -1 for PB cycled in PAAMPS [54]. A similar consistency was demonstrated for paired films such as these at 633 nm, when the electrolyte was poly(ethylene oxide)-based polyurethane containing K ÷ as the insertion-extraction ion [26]. These results clearly demonstrate the formation of electrochromic coloring centers in two films with relatively high coloration efficiencies, and how the films may be coupled in a complementary way. Table 1 also gives data for three electrochromic polymeric films that may be better known for their high metal-like conductivities. Dopants are indicated for charge balancing, but often it is not completely clear

C. B. Greenberg/ Thin Solid Films, 251 (1994) 81-93

which ions are being inserted-extracted in the film structure. Also summarized are the phthalocyanines of various cations, including Lu 3+ and other rare earths, Ni 2+, Cu 2÷ and Zn 2+. In all the organic cases shown in Table 1 the reversible coloring is indicated as variable because, unlike films of WO3 and PB, color rather than any clear state is typical for both oxidation and reduction. Often the colors are of different hues, which accounts for interest in these films for display applications. This twofold coloration differentiates the organic films from the so-called cathodically or anodically coloring inorganic films. Polyaniline films are amongst the most interesting of the polymer class. As a typical case, they have been prepared on conductive glass to 10-100 nm thickness by anodic oxidation in 1.5 M aniline-3 M HCI solution [29, 30]. The films have been described, qualitatively, as changing color in trasmission from light yellow in the most reduced state, to green and then to blue with oxidation. In the reduced and more oxidized states, polyaniline is insulating while, in the green form, it is a good e- conductor. One suggestion for the structural transformations is given in Fig. 7. It is proposed that the conductive state is a radical cation. A more complete analysis has been given for protonation and oxidation-reduction at different pH values and as a function of applied potential [29, 30]. These observations with polyaniline films were made in liquid electrolyte, and that has been the pattern, for the most part, with the organic electrochromic films. An exception is the complementary pairing of polyaniline and derivative films with WO3 using various solid, gel or glassy polymer-electrolytes [67, 68]. There are other examples of solid-state configurations with organic films, such as those using lutetium and erbium diph-

OX[1

n

red

n ox

I l red

n Fig. 7. Suggested structural transformations for polyaniline films for the case of electrolyte pH = 5. (From Ref. [29].)

85

thalocyanine [69, 70], although these particular examples are apparently not charge balanced in the complementary way highlighted herein by WO3-PB.

3. Thermochromic thin films

The numerous examples of thermochromism in the bulk solid state and in liquid require, perhaps, as many phenomological explanations as there are examples. Still, unifying models are to be found. Changes in visible absorption spectra are often found in 3d transition metal compounds in association with change in the surrounding molecular environment of the metal ion. Crystal field theory, although it does not include charge transfer coloration, has proven to be qualitatively adequate for giving insight into many of the thermally driven changes to absorption band shape and spectral peak dominance. This follows from the fact that the 3d states are not well shielded from ligand fields. For 3d transition metal compounds, crystal field splittings are on the order of 10 000 cm -~, while the better-shielded lanthanides have crystal field splittings of only about 100 cm -~ [71]. So, it is not surprising to find plentiful examples of reversible absorption color switching among complexes and chelates, and solutions thereof, for the 3d transition metal ions. Although rarer, there are also examples of oxides exhibiting thermally induced absorption changes in the solid state. Oxides are particularly interesting because they are relatively easy to grow in thin film form. However, the few well-known oxide transitions do not occur sharply, and sometimes not at a transition temp¢rature close to room temperature. Red ruby, A12_xCrxO3, x < 0.16, is a single-crystal example of this; at low doping levels it requires heating to high temperatures to promote a gradual optical effect [72-74]. It exhibits absorption band thermochromism as a function of Cr 3+ concentration, and the ligand field strength, which is in turn dependent on lattice expansion-contraction. In some oxides and sulfides, on the other hand, switching is associated with a reversible non-metal-tometal energy band transition, usually first order, which manifests itself as an abrupt decrease in resistivity with heating [75]. Interesting magnetic switches have been widely discussed as well. Often, the abrupt transition is accompanied by a change in crystal symmetry. Because the higher temperature state is a metal, although usually a poor metal, one can hope for interesting switching in reflecting properties. That contributes to why, of the very few inorganic thermochromic thin film preparations to be found in the open literature, the examples are from this group. Some of the more actively studied bulk thermochromic oxides and sulfides exhibiting resistivity and magnetic switching, as well as three rare films from the literature, are summarized in Table 2.

86

C. B. Greenberg/ Thin Solid Films, 251 (1994) 81-93

Table 2 Some bulk solids and films exhibiting resistivity transitions Compound

Type of transition a

Form of compound

T¢b

Best

(°C)

prim~pro ¢

VO 2

(Monoclinic/tetragonal)

V203

(Monoclinic/rhombohedral)

Fe304 Ag2S

(Orthorhombic/cubic) (Monoclinic/cubic)

FeS NiS(hexagonal) Sml _ x Lnx 3+S

(Tetragonal/hexagonal) (Antiferromagnetic/paramagnetic) Sm2+/Sm3+

Single crystal Film Single crystal and polycrystal Film Single crystal and polycrystal Polycrystal Film ( <2000 nm) Single crystal Single crystal and polycrystal Well-crystallized bulk

68 68 -123 ~ - 134 -154 180 178 157 -9 d

104 104 107 106 102 102-103 102-103 103 < 102 __

References

[76-78] [78-84, 16] [85, 86, 76] [87] [88-91] [92, 93] [94] [95, 91] [96-98] [99-- 101]

aExpressed as the states at a low temperature/high temperature. bTransition temperature. CRatio of the resistivity P,m of the non-metallic state to the resistivity Pm of the metallic state. dDependent on the particular lanthanide and concentration.

Note that 3d transition metal ions dominate, as would be the case for a wider selection of similar thermochromic materials. By far, VO2 has been the most widely studied film of the three in Table 2, since the early demonstration of resistivity switching in bulk by Morin [76]. The near proximity of the transition temperature to room temperature, at 68 °C, is an important reason. Two other considerations have driven researchers to focus on the film specifically rather than crystals: to survive stress during repetitive cycling at the transition and to conform to practical requirements for optical applications. The VO2 film has been grown in various ways and on various substrates. For example, it has been grown on glass by post-oxidation of resistively evaporated vanadium [79], on glass and single-crystal sapphire and rutile by d.c. reactive sputtering of vanadium [80], on fused quartz, sapphire a n d carbon by r.f. sputtering from a vanadium cathode [81] and by thermal oxidation of polished vanadium [10, 82]. It has been grown on sapphire and glazed ceramic by chemical vapor deposition (CVD) from V ( C 5 H T O 2 ) 4 [83]. It has been grown by post-reduction of V205 in CO-COz, after either CVD growth of the film from VOC13 on singlecrystal sapphire or oxidation of a sputtered vanadium film [84]. The VO2 film has also been formed on SnO2primed soda-lime-silica float glass by CVD from V O ( O C 3 H 7 ) 3 , and on unprimed float and Vycor glasses by thermal post-reduction of V205 in aromatic hydrocarbon vapors [16]. Both the magnitude of the resistivity transition and its temperature location depend on stoichiometry [81] and doping [102-105]; yet, there is no indication in all this literature generally that the spectral range of VO2 switching varies appreciably. Switching occurs most remarkably in the IR as the free-electron carrier conductivity increases abruptly with the transition to tetragonal symmetry. However,

anomalous color switching has been reported for VO2 films deposited on sapphire or fused quartz by reactive evaporation in 02, the substrate being heated above 550 °C during deposition or post-heated at 575-590 °C [106, 107]. A blue-red shift was observed by transmission during subsequent switching. The films were characterized as coarse grained and possibly more stoichiometric than usual. The original demonstrations of transition temperature depression by doping were done with powders and single crystals [102-105]. This was extended to films [16], and the example of a suppressed transition temperature is shown in Fig. 8 for a molybdenum-doped film 180 nm thick. The film was grown on soda-limesilica glass by post-reduction of a sol-gel precursor film. The hysteresis, which is typically observed whether doped or not, is entirely below 68 °C, the center of the transition when undoped. The magnitude of resistance switching is on the order of tenfold, not on the order of 104-fold, the best given in Table 2 for VO2 films. This is a consequence of the doping apparently, and not the use of a non-crystalline substrate. A 102-fold to 103-fold magnitude of resistivity switching has often been observed on amorphous substrates without doping [16]. Despite the smaller magnitude of resistivity switching, spectral switching in the near-IR is substantial, as shown in Fig. 9. Because of the now lower transition temperature and in-beam heating in the spectrophotometer, there is some uncertainty about whether Fig. 9 gives the full magnitude of transmittance switching. However, the near-IR data shown here are not greatly different from data for undoped VO2 films. There is only a relatively small thermochromic effect in the visible for this sample. With reference again to Table 2, V203 gives the largest resistivity transition of all, up to 10T-fold in magnitude in bulk. The thermally driven V203 symme-

C. B. Greenberg[ Thin Solid Films, 251 (1994) 81-93

30

-

-

O

=

~, z0

re.

10

0

I 20

I

30

I

I

40 50 6o TEMPERATURE (°C)

I ,o

• oo

Fig. 8. Thermally driven resistance switching ('or a Vl - x M o x O 2 thin film 180 nm thick on the top side of float glass; x = 0.018 nominally. The data were obtained by a two-probe method. (From Ret'. [16].)

50 - -

40 ,~ AT ROOM TEMPERATURE

i z

20

k-

IO

o 0.8

HEATED

I ,.o

I ,.2

I ,.4

I ,.s

I ,.o

I 2.0

I 2.2

WAVELENGTH (MICRONS)

Fig. 9. Thermochromic switching of the film in Fig. 8, at temperatures just below the transition hysteresis and just above it. The data were obtained using a Cary 14 spectrophotometer. The curve in the IR for the monoclinic state was obtained in 100-200rim steps, cooling to about 10 °C between steps to prevent excessive heating from the beam. In the tetragonal state the sample was heated in a Teflon insulating holder. The actual sample temperatures above and below the transition are not well known. (From Ref. [16].)

87

try transition gives a value only about an order of magnitude less for the film shown. The thin film was prepared on single-crystal sapphire and Pyrex glass by annealing V205 in wet H~ [87]; optical data were not given. V203 films have also been prepared on Vycor glass by CVD [16] and on both Vycor glass and polycrystalline alumina by thermal reduction of a precursor oxide in H2 [108]. In these instances also, no effort was made to study the optical transition at about - 123 °C, but the tail of the resistivity switch was apparently observed [ 108]. Some spectral data have been given and discussed for a rhombohedral V203 crystal [109]. In the case of Ag2S films, the resistivity transition from monoclinic symmetry, 13-Ag2S, to cubic, ct-Ag2S, is at 178 °C. Films were prepared by flzlsh evaporation onto polished single-crystal NaCI [94]. The magnitude of the resistivity transition in Ag2S films, 102-fold to 103-fold, is equivalent to that of a typical VO2 film. Very good transmittance switching was reported at 10.6 ~m for a film of 1830 nm thickness, and generally good spectral switching was shown in the far-IR to millimeter wavelength range above the plasma edge. The effect is attributed, as with VO2, to free-carrier-like conductivity and high reflection above the transition temperature. The observations are similar to those for polished polycrystalline Ag2S [93]. Hexagonal or a-NiS gives the rare case of a thermally driven resistivity transition, at - 9 °C, without a change in crystallographic symmetry. The hexagonal symmetry of stoichiometric NiS is not the thermodynamically stable form below about 379 °C [110, 111], but it, as well as its unstable non-stoichiometric forms, can be quenched in at room temperature and below. In fact, the ease with which this occurs has been a well-recognized problem in flat glassmaking historically [I10]. Microscopic nickel sulfide defects, arising from tramp nickel sources under reducing conditions in the molten glass, are often present in a very low concentration in the final product. If the glass is reheated and quenched, as it is for thermal tempering, hexagonal Ni~_xS can persist at room temperature. If an ~-Ni~ _xS defect in the tensile zone later reverts to the stable 13or rhombohedral millerite form, when the glass warms in the sun, the increase in volume, up to 4%, may cause spontaneous fracture [110]. It is less well known to glassmakers that the transition at - 9 °C, for stoichiometric a-NiS, is accompanied by a lattice contraction up to 2% with heating [96, 112]. If persistent, hexagonal nickel sulfide is impure, the antiferromagnetic-to-paramagnetic transition temperature may be higher than - 9 °C [ 113]. In that event, fracture may be initiated by thermal cycling under ordinary wintertime conditions, without involvement of miUerite. In our laboratory, we have succeeded in demonstrating the antiferromagnetic-to-paramagnetic hexagonal nickel sulfide transition in a thin film and report it here

C. B. Greenberg/ Thin Solid Films, 251 (1994) 81-93

88

for the first time. The film was prepared using a similar procedure to that used previously for bulk growth [98]. A Ni film was prepared on a 2.5 cm x 5 cm x 0.5 cm Vycor glass substrate by thermal evaporation of Ni powder, 99.999% pure, from an alumina-coated molybdenum boat at an initial pressure ~<5.3 mPa. The final normal luminous transmittance was ~ 10% (illuminant C). The substrate was ion cleaned in vacuum with argon immediately preceding evaporation. This substrate, together with Ni and sulfur powders, each 99.999% pure, in a 1.1:1 molar ratio, was sealed into a Pyrex glass tube at approximately 670 Pa. The sealed tube with contents was thermally ramped over several hours up to 550 °C, at which temperature it was held for 5 days and then cooled to 400 °C overnight. At discharge from the furnace, the Pyrex glass and contents were quenched with air from a spray gun at 345 kPa. The switching data in Fig. 10 confirm an ~-Ni~_xS film. The position of the hysteresis well below - 9 °C suggests the Ni deficiency [98], despite having used excess Ni in the preparation. Also, very weak X-ray diffraction peaks have been noted, with a Cu K~ source, at lattice spacings of 0.298, 0.260 and 0.199 nm; these fit the compound NiS~.o3 [114]. Over the range of 400-2600 nm, however, there was no evidence of significant switching of transmittance when comparing spectral data at room temperature and -118 °C. The transmittance equals 8-10% at all wavelengths. Data were obtained while the sample was maintained in an evacuated cryostat with heated windows and in the path of the beam of a Cary 14 spectrophotometer. The quality of the evacuation was unquantified. For single1500 1250 1OOO

.~

75G

50n

crystal "NiS", with Tc in the range from - 4 3 to - 7 3 °C, some change in reflectance has clearly been observed above 2 ~ 1 lam from room temperature to - 193 °C [97].

Sm l_xLnx3+S compounds undergo dramatic color change with cooling through To, qualitatively from metallic yellow to black. The transition is associated with lattice expansion, without change in the cubic structure. Thermochromism in the substituted compounds is a derivative of the remarkable piezochromism observed for SmS, both in the bulk and as a thin film. This interesting example is discussed in the section to follow.

4. A piezoehromic thin film

The data in Table 3 show a sampling of materials that undergo discontinuous or first-order transitions as a result of pressurization. In all these cases, an abrupt volume change has been observed at the transition pressure by obtaining lattice parameters by X-ray diffraction [115]. In two cases, EuO and SmS, semiconductor-metal transitions occur isostructurally, at ~30 GPa and 0.65 GPa respectively, with collapse of an energy gap between the localized 4f level and conduction band. Compression results in an effective valence change from Ln 2÷ towards Ln 3÷. In the other cases shown in Table 3, there is simply a change from the collapsed NaC1 structure to a CsCl-type structure at the transition without apparent valence change. Generally, it is only in cases of 4f electron delocalization into the conduction band that a reflected color change is observed in the course of a transition, suggestive of a semiconductor-to-metal switch. This has been observed with both discontinuous and continuous electronic collapse [I 15]. Because of the relatively low pressure at which SmS exhibits its sharp optical transition, it has been the material of greatest interest. Measurements of reflectance below and above the pressure transition have been made, and the spectacular and Table 3 Pressure-induced transitions in bulk monochalcogenides (from Refs.

[lO0, 115])

250

77777

. . . . .

TEMPERATURE DEG. C Fig. 10. Thermally driven resistance switching of an ct-Ni~ _xS thin film on Vycor glass. Cycling o f the surface temperature began with cooling from room temperature, while the glass chamber containing the sample was flushed with Matheson extra dry N 2 (99.9% m i n i m u m purity). The resistance was measured with a two-probe method [108]. Care was taken to avoid marring the soft film. This is the second run for this sample.

Substance

Type of transition a

Transition pressure (GPa)

EuTe EuSe EuS EuO

NaCI/CsCI NaCI/CsCI NaCI/CsCI Electronic collapse NaCI/CsCI Electronic collapse

~ ~ ~ ~ ~

SmS

11 14.5 21.5 30 40 0.65

"Expressed as structural type at a low pressure/high pressure if a structural transition has occurred.

C. B. Greenberg/ Thin Solid Films, 251 (1994) 81-93 ENERGY IN eV

3.1 6.02 90

I i

2.06 i

1.24

0.62 i

0.88

0.51 1

0.44

I

80

I ,o

(METALttC)

I-

~ 50 k-

~ 40

.~ 30 z 2o (SEMICONOUCTING)

v

o

0.2 I

0.6

1.0

I

1.4

1.8

WAVELENGTH IN MICRONS

22

2.6

P

Fig. 11. Near-normal reflectance o f the semiconducting and metallic states of single-crystal piezochromic SmS as a function o f wavelength. The metallic state was under 0.8 GPa pressure. The arrows E 1 to E5 show the peak positions from thin film absorption and photoemission data. (From Ref. [116].)

89

These were polished to permit measurements of reflectance and transmittance in both states of SmS [120]. Optical switching was confirmed despite the poor quality of these films. Strictly speaking, the need to heat for reversal means the rubbing effect in the SmS film is not purely piezochromic. On the other hand, the reversible thermochromic effect indicated for Sml _xLnx3÷S in Table 2 has been demonstrated in bulk when Ln 3+= Ce 3+, Pr 3+, Nd 3+, Gd 3+, Tb 3+, Dy 3+, Ho 3+, Er 3+ or Tm 3+ [99-101]. At critical levels of doping, x = 0.155 in the much discussed case of Gd 3+, the golden yellow metallic state is stabilized at room temperature and atmospheric pressure. Sample preparation included various thermal steps in vacuum, but not pressurization. Subsequently, upon cooling at atmospheric pressure with liquid N2, there is a reversible transformation to the black semiconducting state. The NaCl-type structure, in all cases, is retained; this represents undoing of electron delocalization. It is accompanied by a discontinuous volume increase of the lattice. Sample cracking or explosive disintegration has been observed at the critical temperature; this may be a limitation for demonstrating the effect spectrophotometrically in bulk or with films. Nevertheless, the excellent optical effect in Fig. 11 suggests the value of further research on induced thermochromism in doped piezochromic films.

5. Photochromic thin films

reversible result for a single crystal is shown in Fig. 11 [116]. Visually, the color has been reported, qualitatively, to go from black to a golden yellow with increasing pressure. This is an especially good example of piezochromism in the bulk. Good films of rare earth compounds, with controlled stoichiometry, can be grown by co-evaporation of the elements [117, 118]. This method offers better control of stoichiometry than preparing films directly from the bulk material. Nevertheless, films 200 nm thick have been grown directly from SmS, on fused quartz, by electron beam evaporation and then converted, as is well known for SmS crystals, to the metallic state by rubbing or polishing [119]. It is interesting that the semiconductor-metal transition could not be uniaxially pressure induced in such a film up to 0.8 GPa, apparently because of the dominating compressibility and Poisson's number of the substrate. The semiconductormetal conversion for rubbed films was accompanied by the reflected color change to golden yellow, and by metallic reflectance in close agreement with that shown in Fig. 11. The color change with rubbing has been reported qualitatively by others, as has reversal of it by heating. For example, SmS films contaminated with carbon and oxygen were deposited on glass and fused quartz by reactive evaporation of samarium in H2S.

Some films that color visibly to actinic radiation are given in Table 4. All but one of the films in Table 4 are reversed in some way, for at least one full cycle of darkening-bleaching. Such films are often called photochromic in the literature, although the definition can be limited to materials that bleach spontaneously upon withdrawal of actinic radiation. The A g - C u halide films are in the latter category [121, 122]. They are thermally self-fading at room temperature. The actinic stimulus for darkening, usually including UV radiation, is turned off, and thermal depopulation of color centers becomes dominant. That also was observed with a-WO3 when it was only 10nm thick [124]. As shown in Table 4, however, a WO3 film 340 nm thick did not self-fade, following UV darkening, after several hours at room temperature. As this is a more usual thickness for WO3 films, it is not surprising that stability of coloration is also commonly seen after electrochromic darkening. The photoinduced color centers in WO3 and those formed electrochromically have been said to be the same [124, 129]. A spectral curve has already been shown in Fig. 1 for relatively rapid electrochemical coloration of a-WO3. In other work with a-WO 3 and a-MoO 3 films, it was suggested that bleaching, after either coloration stimulus, is not effected by light of the frequency of the

C. B. Greenberg/ Thin Solid Films, 251 (1994) 81-93

90

Table 4 Some actinically darkened and reversible thin films Thin films (thickness)

Deposition method

Darkening stimulusa

Bleaching stimulus a

Number of cycles

AgCI-CuCI (150-2500 nm)

Vacuum evaporation on glass

UV

AgBr-CuBr ( 170- 540 nm) Ag3VO4, Ag~SiO 4 and Ag3PO , a-WO3 (10 and 340 nm)

Vacuum evaporation on glass Anodic oxidation Vacuum evaporation on Ag

Xe lamp Visible 2 < 450 nm

a-MoO3 (108 and 243 nm)

Vacuum evaporation on fused quartz

UV

MoO 3 ( 1000 nm)

Vacuum evaporation on NESA ® glass

UV

Self-fading; enhanced many by heat and light, 2 > 550 nm Self-fading /> 35 Heat, 150-250 °C >3 10 nm, self-fading; />1 340 nm, no self-fading Not optically induced; I fading at 300 °C in O2 Very slow self-fading; /> 2 electrolytically induced

References

[121]

[ 122] [123] [124] [ 125] [ 126-128]

~At or near ambient conditions, unless otherwise indicated for bleaching. 1.00~

induced absorption [129]. For MOO3, the similarity of spectra for photo-induced and electrochromic darkening-bleaching has also been pointed out, but the color centers do not behave identically in all respects [127, 128]. Also, like the non-halogen silver compounds in Table 4 [123], MoO3 required heat to effect fading, if it was not done electrolytically [ 125-127]. To sort out various observations in the literature, more photochromic data for WO3 and MoO 3 films, having different thicknesses and structural properties, will be needed. On the other hand, it is clear that more consistent knowledge about A g - C u halide films has evolved from a very sustained body of work in bulk. This is because of the extensive efforts that have gone into the development of photochromic glasses [130132]. Some of these exhibit proven reversibility on a commercially acceptable scale. The base glass is usually an alkali borosilicate, in which are precipitated 8 15 nm silver halide microcrystals, typically at less than 0.7wt.%Ag +. Spectral excitation is tuned by using mixed halides; excitation is extended into the visible by silver bromide and iodide. A trace amount of cuprous ion is incorporated into the silver halide crystals as a sensitizer, such that the key reactions for coloration of AgC1 are believed to be Ag + + e-

hv

, Ag o

(9)

n Ag o ~ Ag,, o

(10)

C1- h', C l ° + e-

(11)

C1° + Cu + ---*C1- + Cu 2+

(12)

Slow atomic diffusion of C1° in relatively inert glass contributes to reversibility, whereas chlorine reacts irreversibly with photographic emulsions. Outward diffusion of C1° as well as inward diffusion of atmospheric reactants are to be expected with unprotected thin silver halide films. Nevertheless, monolithic photochromic films, such as indicated in Table 4, have been made.

~z

o.ao'

/ LONG WAVELENGTHUV 0.60 ~ o

ROOMTEMPERATURE,ILLUMINANT C 20

40

Go

ao

~o0

:~ UV'P~-R.T.,'LL. C +GUN'~°'----~ -I R.T.,OARK"J tzo

,40

tGO

~GO t~,.R~

ri.E (.n~urEs)

Fig. 12. Spectral switching of a freshly prepared photochromic A g Cu-CI thin film on float glass 3 mm thick. The sample's luminous transmittance (illuminant C) at normal incidence was monitored successively under ambient conditions with a Gardner hazemeter: after 15 min exposure to UV radiation with a peak efficiency at 366 nm; while removed from the UV source and in the beam of the hazemeter; after a second UV exposure; during a second time removed from the UV source and in the beam of the hazemeter; after exposure to direct summer sunlight; after a relatively long rest period at room temperature in the dark.

An example of optical switching in a photochromic A g - C u - C I film is shown in Fig. 12. The film was prepared by flash thermal evaporation of a mechanical mixture of reagent grades of AgC1 and CuC1, in equal amounts by weight; each constituent was probably contaminated to some small degree from atmospheric alteration. The float glass substrate was not heated intentionally, nor was the film given a post-heating to promote crystal growth, as is necessary for bulk glasses. As neither film thickness nor illuminant intensities were quantified, the data only indicate semiquantitatively that the film was photochromic. Clearly it was responsive to UV and solar radiation for darkening, and it did bleach thermally in the dark at room temperature. At the end of this experiment the film was no longer as

C. B. Greenberg[ Thin Solid Films, 251 (1994) 81-93

responsive to UV, an indication of irreversible reactions. There is nothing in the data in Fig. 12 that is inconsistent with the cited literature [ 121] that gave rise to the attempt to grow this film. The result supports the efforts by others to encapsulate photochromic films to simulate photochromic glass more closely [133].

6. Concluding remarks Looking back to the optically switchable bulk and thin film materials of the preceding sections, there are examples of materials exhibiting more than one kind of chromogenism and examples of hybrid cycles. Both WO3 and M o O 3 are reversibly electrochromic, and they also darken when exposed to actinic U V radiation. As indicated in Table 4, the bleaching of actinic darkening has been accomplished in several ways, at least for 1 or 2 cycles, including electrolytically. The reversible A g Cu halide films, like the bulk photochromic glasses on which basis they are modeled, darken actinically to UV and self-bleach thermally at r o o m temperature. Piezochromic SmS in bulk undergoes a spectacular s e m i c o n d u c t o r - m e t a l spectral switch with a pressure hysteresis. The band edge of the metallic state extends into the visible and contributes to a lustrous golden yellow appearance. The s e m i c o n d u c t o r - m e t a l and color transition that accompanies pressurization has been mimicked in thin films by rubbing, and reversed by heating. It has been transposed into a b l a c k - y e l l o w thermochromism in bulk at atmospheric pressure by partial cation substitution with other rare earths. Adding to these the other more straightforward examples of chromogenism, the subject of this review, there is a very rich variety indeed. Although that variety has not necessarily led yet to wonderful practical applications for thin films, there are implications that this will happen, although the applications m a y not be so traditional as imagined by some. M o r e importantly, the often dramatic spectral effects have presented opportunity for understanding materials at the molecular level. That opportunity is still great. There are m a n y lures for expanding the currently small pool o f chromogenic thin films out of a relatively much larger pool of switchable bulk materials.

Acknowledgment For the m a n y people at our laboratory who have given support to me, worked with me or provided analytical data for me over the years, I am deeply grateful. For data, samples and literature used herein, I am indebted to K.-C. Ho, T. G. Rukavina, D. E. Singleton, J. B. McCandless, J. B. Slobodnik, D. LBackfisch, I. S. Tanowitz, P. A. Fraino, M. E. Helzel,

91

F. H. Gillery, J. O. Bookmyer, P. C. Edge and any others that I m a y have missed. Thanks are due to Professor K. Vedam and The American Physical Society for approval for the use o f Fig. 11.

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