Molecular Electronics Using LangmuirâBlodgett Films - Advances in

4 Molecular Electronics Using Langmuir-Blodgett Films Downloaded by UNIV OF LEEDS on May 21, 2015 | http://pubs.acs.org Publication Date: October 1, 1988 | doi: 10.1021/ba-1988-0218.ch004

G . G . Roberts Department of Engineering Science, University of Oxford, Oxford OX1 3PJ England

Langmuir-Blodgett (LB) films are monomolecular assemblies deposited one molecular layer at a time onto the surface of a suitable solid substrate. The combination of a carefully engineered molecule and a good Langmuir trough can result in high quality films displaying a high degree of structural order. Following a brief introduction to the preparation of LB films, this review describes their importance as model systems in basic research. However, the main emphasis is placed on their potential applications in the field of electronics, particularly those relying on the highly nonlinear properties of supermolecular assemblies.

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R E L A T I V E L Y I N E X P E N S I V E E Q U I P M E N T R E Q U I R E M E N T S associated w i t h

the L a n g m u i r - t r o u g h t e c h n i q u e c o u p l e d w i t h the elegance o f the f u n d a m e n t a l science i n v o l v e d w i t h L a n g m u i r - B l o d g e t t ( L B ) films a n d t h e i r i n teresting a p p l i e d prospects have generated considerable activity i n the electronics field. T h e noticeable increase i n the n u m b e r s o f participants at relevant conferences a n d papers p u b l i s h e d i n the l e a r n e d j o u r n a l s testifies to the i m p o r t a n c e o f the topic, whose pioneers w o r k e d i n the G e n e r a l E l e c t r i c R e s e a r c h L a b o r a t o r i e s b e t w e e n the two w o r l d wars. T h e first d e t a i l e d d e s c r i p t i o n o f s e q u e n t i a l organic m o n o l a y e r transfer was g i v e n b y L a n g m u i r a n d B l o d g e t t ( I , 2) i n the mid-1930s a n d l e d to several s i m p l e applications of L B films i n c l u d i n g step-thickness gauges, a n tireflection coatings, a n d soft X - r a y gratings. T h e majority of the early papers concentrated o n a w e l l - d e f i n e d series o f fatty acids a n d t h e i r salts. T h e subject r e c e i v e d an i m p e t u s i n the early 1960s b y the w o r k of K u h n et a l . (3), w h o s h o w e d h o w m o n o m o l e c u l a r assemblies c o u l d b e u s e d as matrices to s u p p o r t 0065-2393/88/0218-0225$12.25/0 © 1988 American Chemical Society

In Electronic and Photonic Applications of Polymers; Bowden, M., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.

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ELECTRONIC & PHOTONIC APPLICATIONS O F POLYMERS

Downloaded by UNIV OF LEEDS on May 21, 2015 | http://pubs.acs.org Publication Date: October 1, 1988 | doi: 10.1021/ba-1988-0218.ch004

various d y e molecules a n d as spacers to separate d o n o r a n d acceptor species. T h e i r elegant, artificial systems w e r e d e s i g n e d i n g e n i o u s l y to differentiate b e t w e e n the different interactions that c o u l d occur v i a p h o t o n , e l e c t r o n , a n d p r o t o n transfer. T h e p r e s e n t phase of intense activity started about 15 years ago w i t h the synthesis o f a w i d e r range o f m o r e stable materials a n d a g e n u i n e appreciation o f the a p p l i e d prospects for L B films (4). M a n y o f the basic p r i n c i p l e s u n d e r l y i n g the t e c h n i q u e can b e f o u n d i n the text b y G a i n e s (5). T h e first two international conferences o n the subject (6, 7) p r o v i d e a d e t a i l e d account o f r e c e n t d e v e l o p m e n t s . T h i s c h a p t e r p r o v i d e s a b r i e f i n t r o d u c t i o n to the p r e p a r a t i o n o f m u l t i layer films; h o w e v e r , the m a i n focus is o n t h e i r p o t e n t i a l applications i n the field o f electronics, especially i n those areas that capitalize o n the n o n l i n e a r p r o p e r t i e s o f L B layers.

4.1 Preparation of Langmuir-Blodgett Films C h a r t 4.1 shows stearic a c i d , a m o l e c u l e i n w h i c h 16 C H groups f o r m a l o n g h y d r o p h o b i c c h a i n . T h e o t h e r e n d o f the m o l e c u l e terminates i n a h y d r o p h i l i c carboxylic a c i d g r o u p . W h e n d i s s o l v e d i n a suitable solvent a n d spread o n the surface o f w a t e r , molecules m a y be c o m p r e s s e d w i t h the a i d of a b a r r i e r . F i g u r e 4.1 shows a plot o f the surface pressure (differential surface tension) versus area o c c u p i e d p e r m o l e c u l e for stearic a c i d . T h e m o n o l a y e r undergoes a n u m b e r o f phase transformations d u r i n g c o m p r e s sion; the w e l l - d e f i n e d sequence can b e v i e w e d as the t w o - d i m e n s i o n a l a n alogue o f the classical transitions o b s e r v e d w i t h p r e s s u r e - v o l u m e isotherms. 2

A n L B film is f o r m e d b y transferring a floating m o n o l a y e r onto a s o l i d substrate; the surface pressure at w h i c h " d i p p i n g " occurs is established b y the t y p e o f i s o t h e r m s h o w n i n F i g u r e 4.1. T h e subphase is n o r m a l l y u l t r a p u r e w a t e r because it is r e a d i l y available a n d has an exceptionally h i g h v a l u e of surface t e n s i o n . T h e c o m p o s i t i o n of the subphase i n c l u d i n g its p u r i t y , p H , a n d i o n i c strength can have a p r o f o u n d influence o n factors s u c h as the s o l u b i l i t y o f the m o n o l a y e r a n d segregation effects r e s u l t i n g i n m o l e c u l a r aggregates o r d o m a i n s . B y u s i n g c o n v e n t i o n a l L B film technology, the substrate is r a i s e d a n d l o w e r e d v e r t i c a l l y t h r o u g h a compact floating m o n o l a y e r ; the surface p r e s sure at w h i c h this occurs is n o r m a l l y j u s t above the " k n e e " i n the l i q u i d - t o solid transition section of the i s o t h e r m , i n d i c a t i n g l o w c o m p r e s s i b i l i t y i n the monolayer. A t this stage, i f conditions are carefully c o n t r o l l e d a n d a p p r o priate molecules are u s e d , one m o n o l a y e r is transferred d u r i n g each e x c u r sion t h r o u g h the subphase surface. T h e most c o m m o n d e p o s i t i o n m o d e (Ytype) is illustrated i n F i g u r e 4.2, w h e r e the molecules can be seen to stack i n a head-to-head a n d tail-to-tail configuration. T h e floating molecules o n a l i q u i d surface are s h o w n i n F i g u r e 4.2a. W i t h a h y d r o p h i l i c substrate, no p i c k u p occurs d u r i n g the first i m m e r s i o n , a n d the first m o n o l a y e r is therefore


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substituted phthalocyanine (d) Chart 4.1. A selection of molecules used to form LB films: (a) a fatty acid (stearic acid), (b) w-tricosenoic acid (o>-TA), (c) a substituted anthracene (9butyl-10-anthrylpropionic acid), and (d) a substituted phthalocyanine (tetra4-tert-butylphthalocyaninatosilicon dichloride).




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Substrate Y-depositlon Figure 4.2. Langmuir-Blodgett film deposition (Y-type) on a hydrophilic substrate: (a) monolayer on the surface of water, (b) first layer on withdrawal, (c) second layer (second insertion), and (d) substrate with three layers (after second removal).



230


d e p o s i t e d d u r i n g the first w i t h d r a w a l as s h o w n i n F i g u r e 4.2b. T h e surface is n o w h y d r o p h o b i c a n d d e p o s i t i o n occurs d u r i n g the next i m m e r s i o n i n t o the water. T h u s , one m o n o l a y e r coverage is o b t a i n e d w i t h each traversal t h r o u g h the l i q u i d . W i t h a h y d r o p h o b i c surface such as freshly e t c h e d s i l i c o n , p i c k u p occurs d u r i n g the first i n s e r t i o n . S o m e t i m e s , the c o m m o n Y - t y p e d e p o s i t i o n m o d e illustrated i n F i g u r e 4 . 2 d is not f o l l o w e d , a n d one of the o t h e r t w o possible configurations, X - o r Z - t y p e , is o b s e r v e d , w h e r e transfer occurs o n l y d u r i n g i m m e r s i o n or w i t h d r a w a l , respectively. T h e surface q u a l ity a n d c h e m i c a l c o m p o s i t i o n o f the substrate affects the nature o f the d e p o s i t e d layer. P r o v i d e d the surface is v e r y h y d r o p h i l i c o r v e r y h y d r o p h o b i c , good adhesion is n o r m a l l y o b t a i n e d ; satisfactory cohesion b e t w e e n adjacent monolayers is h a r d e r to achieve.

4.2 Modern Developments 4.2.1 Organic Synthesis G e n e r a l l y speaking, the synthesis of suitable molecules for examination w i t h a L a n g m u i r t r o u g h has b e e n m a d e b y an ad hoc p r o c e d u r e a n d has r e l i e d o n the modification o f k n o w n materials. F o r example, the a l k y l g r o u p of fatty acids m a y b e r e p l a c e d b y chains c o n t a i n i n g one or m o r e d o u b l e bonds. T h e (o-tricosenoic a c i d (-TA) m o l e c u l e (8) s h o w n i n C h a r t 4 . 1 , w h i c h is s i m i l a r to stearic a c i d b u t contains a t e r m i n a l d o u b l e b o n d , displays a l l the essential film-forming qualities i n c l u d i n g s o l u b i l i t y i n c o n v e n i e n t organic solvents, stability at the surface of water, shear resistance, stability against collapse, a n d suitable orientation features. T h e d i a c e t y l e n i c acids (9,10) have also b e e n w i d e l y investigated because of t h e i r p o l y m e r i z a b i l i t y . H e r e the i n t e r e s t i n g d i a c e t y l e n i c e n t i t y is n o r m a l l y i n c o r p o r a t e d into the structure o f an alkanoic a c i d to give a c o m p o u n d such as C H ( C H ) C = C - f e C ( C H ) C O O H . T h e t o p o c h e m i c a l p o l y m e r i z a t i o n proceeds w i t h i n the L B layer b u t results i n an array o f t w o - d i m e n s i o n a l domains whose size is i n f l u e n c e d b y m a t e r i a l p u r i t y . D i a c e t y l e n e s have also b e e n i n c o r p o r a t e d i n t o l i p i d l i k e molecules a n d p o l y m e r i z e d as m o d e l m e m branes (11, 12). Because of the structural flexibility o f L B films, t h e r e is l i k e l y to be c o n t i n u e d interest i n p o l y m e r L B films research, some of w h i c h w i l l i n v o l v e p r e f o r m e d p o l y m e r s (13). H o w e v e r , the rigidity o f m a n y p o l y m e r films prevents this approach b e i n g u s e d generally. 3

2

1 0

2

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T o attach l o n g a l i p h a t i c chains to a m o l e c u l e a n d spread a m o n o l a y e r is relatively straightforward. H o w e v e r , this p r o c e d u r e m a y w e l l d i l u t e the desirable properties o f the basic m o l e c u l e . M o r e o v e r , for stability reasons, the presence of l o n g side groups w i l l severely restrict t h e i r practical a p p l i cability. T h e r e f o r e , the scope of the L a n g m u i r - t r o u g h t e c h n i q u e w o u l d b e considerably e n h a n c e d i f i n t e r e s t i n g materials c o n t a i n i n g o n l y short, stable side groups c o u l d b e f o r m e d i n t o L B films. A good example is p r o v i d e d b y the anthracene d e r i v a t i v e (14) s h o w n i n C h a r t 4.1; m u l t i l a y e r s of excellent


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q u a l i t y can b e o b t a i n e d , e v e n t h o u g h the a l k y l group contains o n l y four aliphatic carbons a n d the h y d r o p h o b i c group is attached to the r i n g structure v i a o n l y two m e t h y l e n e groups. E x t r e m e l y robust m o n o l a y e r assemblies can be constructed b y u s i n g dye molecules s u c h as the p o r p h y r i n s a n d phthalocyanines. I n general, t h e i r q u a l i t y is r e l a t i v e l y imperfect c o m p a r e d w i t h those of the classic

film-forming

materials, b u t t h e i r significant advantages l i e i n t h e i r t h e r m a l a n d m e c h a n i c a l stabilities. A n example of a s u b s t i t u t e d phthalocyanine m o l e c u l e (15), w h i c h can b e d e p o s i t e d i n m o n o l a y e r f o r m , is s h o w n i n C h a r t 4.1. T h e molecules s h o w n i n C h a r t 4.1 represent o n l y a few of the materials


that have b e e n s t u d i e d i n L B film f o r m . N o n e t h e l e s s , a great d e a l m o r e needs to b e d o n e to tap the vast w e a l t h of opportunities available w i t h organic systems. I n e v i t a b l y , s h o r t - t e r m o p p o r t u n i s t i c attempts w i l l b e a i m e d at d i s c o v e r i n g molecules for specific devices. H o w e v e r , a m o r e p r e s s i n g n e e d is for a systematic approach that w i l l y i e l d rules g o v e r n i n g s t r u c t u r e - p r o p e r t y correlations so as to enable scientists to confidently p r e d i c t the m o l e c u l a r architecture of m o n o l a y e r assemblies.

4.2.2 Langmuir Troughs T h e upsurge i n interest i n L B films has l e d to greater attention b e i n g p l a c e d o n t r o u g h design a n d c o n t r o l systems to m e e t the stringent r e q u i r e m e n t s of scientists a n d engineers. T h e r e f o r e , m o d e r n i n s t r u m e n t s are r e l a t i v e l y sophisticated, a n d , for d e v i c e - r e l a t e d w o r k , n e e d to b e situated o n a n t i v i b r a t i o n tables i n clean e n v i r o n m e n t s . A l t h o u g h it is possible to automate most features, the p r i m a r y benefit at the present t i m e lies i n efficient data c o l l e c t i o n a n d the ease w i t h w h i c h data can be m a n i p u l a t e d . F o r e x a m p l e , phase transitions are m o r e apparent w h e n the differential of the p r e s s u r e - a r e a i s o t h e r m is p l o t t e d . A l s o , the deposition process can b e p r o g r a m m e d a n d m o n i t o r e d i n a straightforward m a n n e r . I n the example s h o w n i n F i g u r e 4.3, the substrate is m o v e d b e t w e e n different v e r t i c a l l i m i t s to p r o v i d e a staircaselike m u l t i l a y e r structure c o m p r i s i n g 4, 8, 12, a n d 16 layers. I n o r d e r to m a i n t a i n a constant surface pressure d u r i n g the Y - t y p e d i p p i n g cycle, the film area enclosed b y the b a r r i e r is r e d u c e d systematically. A recent d e v e l o p m e n t i n t r o u g h design c o u l d have i m p o r t a n t c o m m e r c i a l significance. T h i s d e v e l o p m e n t has arisen because of the n e e d to p r o d u c e n o n c e n t r o s y m m e t r i c structures that display i n t e r e s t i n g n o n l i n e a r p h y s i c a l effects. T h e c o n v e n t i o n a l Y - t y p e films are s y m m e t r i c a l i n character, and experience has s h o w n that X - a n d Z - t y p e layers, although n o n s y m m e t r i c a l , are u s u a l l y imperfect. T h e r e f o r e , an alternative approach to p r o d u c i n g n o n c e n t r o s y m m e t r i c structures is to use alternate layers of two different materials w h e r e the contributions of adjacent molecules do not cancel ( F i g u r e 4.4). T h e additions of a fixed b e a m a n d a r e v o l v i n g center section to an automated c o n s t a n t - p e r i m e t e r - b a r r i e r L a n g m u i r t r o u g h enables the formation of an a l t e r n a t i n g Y - t y p e structure of two different molecules


In Electronic and Photonic Applications of Polymers; Bowden, M., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988. I :

12

1

2

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TROUGH AREA (cm )

8

1 16

r

Figure 4.3. To maintain a constant surface pressure during the deposition process, the area enclosed by the barrier is reduced systematically. This schematic Y-type dipping sequence for a diagram shows a staircase structure of 4, 8, 12, and 16 monolayers. (Courtesy of Joyce-Loebl Company, Gateshead, England.)

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A

*

233

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4

4

Figure 4.4. Top: A combination of two molecules A and B enables a Y-type film to be produced of noncentrosymmetric character. Bottom: A rotating substrate is used to alternately pick up different molecules from separate areas of the trough and thus form an organic superhttice. spread i n the two distinct areas of the subphase. T h e structural qualities of the L B films p r e p a r e d i n this w a y can be of h i g h q u a l i t y (16). A n o t h e r advantage o f the rotating substrate arrangement, w h i c h is c o n d u c i v e to fast d i p p i n g , is that the meniscus, u n l i k e that i n the v e r t i c a l d i p p i n g m e t h o d , is always i n the same d i r e c t i o n .

4.3 Characterization of Langmuir-Blodgett Films M a n y different e x p e r i m e n t a l techniques indicate that carefully

prepared

films of a p p r o p r i a t e l y substituted molecules possess a h i g h degree of strucIn Electronic and Photonic Applications of Polymers; Bowden, M., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.

234



tural order. T h e surface q u a l i t y a n d c h e m i c a l c o m p o s i t i o n of the substrate can have a p r o f o u n d effect o n the nature of the d e p o s i t e d layers. H o w e v e r , as i n the case of the spread m o n o l a y e r , researchers have f r e q u e n t l y i g n o r e d the u n d e r l y i n g science a n d have a r r i v e d at e m p i r i c a l procedures for p r e p a r i n g specimens. N a t u r a l l y , the q u a l i t y o f the floating l a y e r is i m p o r t a n t a n d needs to b e c h a r a c t e r i z e d , as does the interface b e t w e e n the first d e p o s i t e d layer a n d the substrate. A p o p u l a r objective i n studies of floating monolayers is to m o n i t o r a p h y s i c a l parameter d u r i n g compression of the f i l m a n d thus correlate phase transitions i n the p r e s s u r e - a r e a i s o t h e r m w i t h effects s u c h as the formation of aggregates. F o r example, R a m a n spectroscopy (17) can p r o v i d e e v i d e n c e of m o l e c u l a r o r i e n t a t i o n , a n d fluorescence m i croscopy (18) can b e u s e d to illustrate the change i n texture of a m o n o l a y e r d u e to d e n d r i t e formation. E l e c t r o n diffraction was first u s e d to study L B films o v e r 50 years ago, b u t o n l y recently have the p a c k i n g arrangements of molecules i n m u l t i l a y e r films b e e n investigated i n d e t a i l . F o r e x a m p l e , transmission e l e c t r o n diffraction has p r o v i d e d e v i d e n c e that epitaxial g r o w t h occurs u n d e r c e r t a i n conditions w h e r e the m u l t i l a y e r f i l m assumes the s t r u c t u r a l o r d e r p r e s e n t i n the first layer. V e r y few e x p e r i m e n t s have b e e n c o n c e r n e d w i t h films c o n s t i t u t i n g fewer t h a n 10 monolayers, a n d little attention has b e e n p a i d to the structure o f the i n i t i a l few monolayers deposited onto a substrate. I n these situations, reflection h i g h - e n e r g y e l e c t r o n diffraction ( R H E E D ) is p a r t i c u l a r l y c o n v e n i e n t because it is nondestructive a n d p e r m i t s a r a p i d i d e n t i f i c a t i o n o f t h e f i l m s t r u c t u r e to b e a c h i e v e d w i t h m i n i m a l s a m p l e preparation. A R H E E D p a t t e r n f r o m a m o l e c u l e specially d e s i g n e d for n o n linear optics applications (19) is i l l u s t r a t e d i n F i g u r e 4.5. T h e e l e c t r o n m i croscope can also b e u s e d i n transmission; the first d i r e c t image of an L B film was r e p o r t e d b y F r y e r et a l . (20) for a single m o n o l a y e r of a s u b s t i t u t e d p h t h a l o c y a n i n e . T h e structure e x h i b i t e d i n F i g u r e 4.6 is for a single m o n olayer of tricosenoic a c i d (21). X - r a y diffraction techniques have b e e n u s e d extensively to d e t e r m i n e the m o n o l a y e r thicknesses of L B films. Because the scattering of X - r a y s from carbon a n d h y d r o g e n atoms can b e assumed to be v e r y s m a l l c o m p a r e d to that from h e a v i e r m e t a l ions i n c o r p o r a t e d i n a film, the lattice spacing n o r m a l to the film corresponds to the distance b e t w e e n adjacent planes c o n t a i n i n g m e t a l ions. S o m e w o r k e r s have n o t e d X - r a y d spacings that are significantly less t h a n those expected from consideration of the m o l e c u l a r l e n g t h ; such e v i d e n c e n o r m a l l y points to a tilt i n the h y d r o c a r b o n c h a i n . F o r e x a m p l e , the long-axis m o l e c u l a r tilt angle i n films of the anthracene c o m p o u n d s h o w n i n C h a r t 4.1 is 60°, a n d the molecules i n consecutive layers appear to i n terpenetrate (14). N e u t r o n diffraction a n d reflection measurements have also b e e n r e p o r t e d (22, 23). T h e precise r e g u l a r i t y o f the layer spacing a n d the finite n u m b e r of layers enables a precise quantitative fit to be o b t a i n e d . O p t i c a l techniques w e r e a p p l i e d b y B l o d g e t t a n d L a n g m u i r (24) to d e t e r m i n e


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4.

Figure 4.5. An 80-keV reflection high-energy electron-diffraction (RHEED) pattern from a single layer of the nitrostilbene molecule shown in Structure 4.1b. The substrate is {111} silicon.



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Figure 4.6. A 100-keV transmission-electron-diffraction pattern of a single monolayer of tricosenoic acid. (See reference 21.)

the o p t i c a l constants of fatty a c i d salt monolayers; t h e y s h o w e d h o w the refractive i n d e x c o u l d be m o d i f i e d b y s k e l e t o n i z i n g the films a n d adjusting the s a l t - a c i d ratio. M o d e r n methods a l l o w thickness d e t e r m i n a t i o n s to b e m e a s u r e d r o u t i n e l y b y u s i n g e l l i p s o m e t r y , a n d , b y u s i n g i n t e n s i t y variations, the lateral heterogeneity from domains larger than the w a v e l e n g t h of l i g h t can b e established. V e r y t h i c k films can b e used to d e t e r m i n e b o t h the thickness a n d the refractive i n d e x b y s t u d y i n g m u l t i m o d e propagation i n a w a v e g u i d e configuration. F o r example, W a l p i t a a n d P i t t (25) m e a s u r e d the w a v e g u i d i n g characteristics of m o r e than 200 layers of c a d m i u m stearate a n d s i m i l a r data have b e e n r e p o r t e d for a diacetylene p o l y m e r b y C h e n et a l . (26). Surface analytical t e c h n i q u e s such as A u g e r electron spectroscopy (27), X - r a y p h o t o e l e c t r o n spectroscopy (28), a n d secondary-ion mass s p e c t r o m e t r y (29) have b e e n u s e d to study L B films. S y n c h r o t r o n radiation is a p a r t i c u l a r l y p o w e r f u l p r o b e e n a b l i n g X - r a y near-edge structure a n d e x t e n d e d X - r a y a b sorption fine structure to b e m e a s u r e d . A n g l e - r e s o l v e d p h o t o e m i s s i o n s t u d ies (30) c o n f i r m e d the existence of a o n e - d i m e n s i o n a l energy b a n d a l o n g the ( C H ) c h a i n i n a fatty a c i d salt film. 2

X

Infrared spectroscopy is a p o w e r f u l tool to study L B films. W i t h m o d e r n i n s t r u m e n t s s u c h as F o u r i e r transform spectrometers, d e t a i l e d i n f o r m a t i o n can b e o b t a i n e d about f u n c t i o n a l groups a n d t h e i r o r i e n t a t i o n . F o r e x a m p l e , I R spectroscopy can b e u s e d (I) to show that the s i m p l e a c i d salts are i n c l i n e d at o n l y a few degrees from the n o r m a l to the substrate surface (31) a n d to


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study p o l y m e r i z a t i o n processes (32). R a m a n spectroscopy can also p r o v i d e s i m i l a r data, b u t , b e i n g an i n h e r e n t l y weak process, s e n s i t i v i t y is l i m i t e d . M o d e r n e n h a n c e m e n t t e c h n i q u e s enable the signal-to-noise ratio to b e increased b y interactions w i t h surface plasmons (33) o r surface effects (34). Inelastic t u n n e l i n g spectroscopy (35), B r i l l o u i n scattering (36), a n d p h o t o acoustic spectroscopy (37) have also b e e n u s e d to explore t h e v i b r a t i o n a l modes i n L B films.


D u r i n g the next few years, scientists w i l l devise n e w a n d i m p r o v e d methods o f c h a r a c t e r i z i n g m o n o m o l e c u l a r assemblies. A great d e a l needs to be l e a r n e d about the o r d e r i n g o f monolayers, a n d techniques s u c h as scann i n g t u n n e l i n g m i c r o s c o p y m a y p r o v i d e some of the answers. T h e four separate diagrams i n F i g u r e 4.7 a l l describe results for fatty

Figure 4.7. These diagrams are designed to emphasize the reproducibility of various physical parameters in monolayer assemblies of different thicknesses: (a) reciprocal capacitance per unit area versus number of monolayers of cadmium arachidate on an aluminum substrate (see reference 38), (b) absorption intensity versus number of monolayers for the symmetric carhoxylate stretching mode of cadmium arachidate at 1432 cm' (see reference 39), (c) count rate of C rays versus number of layers of barium stearate labeled with C (see reference 40), and (d) X-ray photoelectron signal (XPS) intensity versus number of layers of cadmium dimethylarachidate on silver (see reference 28). 1

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acids or t h e i r derivatives a n d are d e s i g n e d to emphasize the r e p r o d u c i b i l i t y o f various p h y s i c a l parameters from one m o n o l a y e r to the next. F i g u r e 4.7a shows the capacitance (C) as a f u n c t i o n of film thickness for c a d m i u m a r achidate d e p o s i t e d onto a l u m i n u m (38). T h e l i n e a r d e p e n d e n c e o f C versus the n u m b e r o f monolayers demonstrates clearly the repeatability of the d i e l e c t r i c thickness o f each monolayer. I n F i g u r e 4.7b, a b a n d i n the I R reflection s p e c t r u m o f the same m a t e r i a l has b e e n u s e d to demonstrate the u n i f o r m i t y o f successive monolayers (39) . F i g u r e 4.7c is based o n e x p e r i ments u s i n g b a r i u m stearate as the absorber for L - s h e l l A u g e r electrons (40). B y l a b e l i n g the molecules i n these overlays w i t h C a n d e x a m i n i n g t h e i r autoradiographs, the u n i f o r m i t y o f the d e p o s i t i o n process may b e c o n f i r m e d b y p l o t t i n g the count rate o f C rays versus the n u m b e r o f monolayers. T h e final d i a g r a m i n the set ( F i g u r e 4.7d) illustrates another p o w e r f u l tool for investigating organic coatings o n metals. I n this case, different thicknesses o f c a d m i u m d i m e t h y l a r a c h i d a t e have b e e n u s e d to attenuate the substrate X - r a y p h o t o e m i s s i o n signal (28). _ 1

1 4


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4.4 Langmuir-Blodgett Films as Model Systems in Basic Research O n e o f the p r i n c i p a l v i r t u e s o f L B films is t h e i r usefulness i n f u n d a m e n t a l research. M a n y areas of science can benefit from investigations o f m o d e l systems based o n m o n o m o l e c u l a r assemblies. T h e s e areas i n c l u d e • energy transfer i n c o m p l e x monolayers, • biological membranes, and • metal ion incorporation.

4 . 4 . 1 Energy Transfer in Complex Monolayers T h e L a n g m u i r - t r o u g h t e c h n i q u e provides a m e t h o d o f c o n s t r u c t i n g s i m p l e artificial systems of cooperating molecules o n a substrate. K u h n has b e e n the p i o n e e r is this field. T h e elegance of his w o r k a n d that of his colleagues is e v i d e n t i n t h e i r reviews o f the subject. T h e s e reviews describe the use of L B films to investigate i n t e r m o l e c u l a r interactions a n d various p h o t o p h y s i c a l a n d p h o t o c h e m i c a l processes. A n example o f this research (41), d e s i g n e d to investigate the F o r s t e r t y p e o f energy transfer from a s e n s i t i z i n g m o l e c u l e , S, to an acceptor m o l e c u l e , A , is g i v e n i n F i g u r e s 4.8 a n d 4.9. I f S is a c o m p o u n d that absorbs i n the U V part of the s p e c t r u m a n d fluoresces i n the b l u e , a n d A absorbs i n the b l u e a n d fluoresces i n the y e l l o w , t h e n i n t e r e s t i n g effects are o b s e r v e d w h e n the system is irradiated w i t h U V light. I f there is a sufficient distance b e t w e e n S a n d A , as i n F i g u r e 4.8a, the fluorescence o f S appears because A does not absorb U V radiation. H o w e v e r ,


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Ultra-violet

Absorbs In U.V.

Absorbs In BLUE

Fluoresces In BLUE

Fluoresces In YELLOW

(a)

(b)

Figure 4.8. Schematic diagram showing basis of experiments designed to investigate energy transfer from a sensitizing molecule (S) to an acceptor molecule (A). The number of monolayers separating the two species governs the spectral response of the fluorescence spectrum, (a) The acceptor molecule does not absorb the UV radiation, (b) The separation distance is sufficiently small for the excitation energy of S to be transferred to A.

b e l o w a c e r t a i n t h r e s h o l d distance, as i n F i g u r e 4.8b, the excitation e n e r g y of S is transferred to A , a n d the y e l l o w fluorescence of A is expected. S i m i l a r e x p e r i m e n t s based o n fluorescence q u e n c h i n g indicate that the rate constant o f the e l e c t r o n transfer decreases exponentially w i t h i n c r e a s i n g b a r r i e r t h i c k ness separating a d o n o r c h r o m o p h o r e a n d an e l e c t r o n acceptor. I n the exa m p l e s h o w n i n F i g u r e 4.9, N , N ' - d i o c t a d e c y l t h i a c y a n i n e has b e e n u s e d i n c o n j u n c t i o n w i t h a v i o l o g e n acceptor layer to observe the steady fluorescence intensities o f the c y a n i n e d y e m o n o l a y e r i n the absence (I ) a n d i n the presence (I) of the acceptor layer. T h e q u a n t i t y , (I II) - 1, is p r o p o r t i o n a l to the rate constant of the e l e c t r o n transfer. Its l i n e a r d e p e n d e n c e w i t h d, the distance b e t w e e n the c h r o m o p h o r e s , is e v i d e n c e o f e l e c t r o n t u n n e l i n g . I n a s i m i l a r series o f e x p e r i m e n t s , the energy-transfer m e c h a n i s m r e s p o n sible for spectral sensitization has b e e n investigated. 0

0


In Electronic and Photonic Applications of Polymers; Bowden, M., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988. 2.0

0

2.2

2.4

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2.6

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Figure 4.9. The fluorescence intensity (I ) of a donor dye is reduced to a value I in the presence of an acceptor dye. The logarithm o / ( I / I ) — 1 is shown as a function of d, which is the spacing between the donor and acceptor planes (see reference 41).

0

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4.4.2 Biological Membranes


T h e p h y s i c a l structure a n d c h e m i c a l nature of classical L B films gives t h e m a close resemblance to naturally o c c u r r i n g biological m e m b r a n e s . F o r exa m p l e , because the two ends o f a l i p i d m o l e c u l e have i n c o m p a t i b l e s o l u b i l i t i e s , they spontaneously organize i n the f o r m of a b i l a y e r , o r essentially a t w o - l a y e r L B film. Scientists have suggested that t h e y m i g h t p r o v i d e a suitable m o d e l of the l i p i d m e m b r a n e for p r o b i n g the cooperative i n t e r a c tions b e t w e e n its constituents. H o w e v e r , caution must b e exercised i n assessing the biological relevance of this type of w o r k a n d associated studies a i m e d at i n c o r p o r a t i n g ionophores i n t o p h o s p h o l i p i d layers. M u c h of this research is targeted at n o v e l integrated solid-state devices i n c o r p o r a t i n g biological molecules s u c h as e n z y m e s , b u t i n some cases, L B films have b e e n u s e d to facilitate p h y s i c a l studies of biological molecules (e.g., to measure the i o n i c p e r m e a b i l i t y of reconstituted membranes). S u p e r m o l e c u l a r structures have also b e e n d e s i g n e d to m i m i c the p r i m a r y p r o cess i n photosynthesis a n d for a c h i e v i n g a n efficient p h o t o i n d u c e d charge separation b y appropriate m o d e l i n g of p o t e n t i a l profiles. C h l o r o p h y l l has b e e n s t u d i e d i n this context. S o m e of the results m a y have relevance to solar p h o t o c h e m i c a l c o n v e r s i o n devices (42).

4.4.3 Metal Ion Incorporation T h e a d d i t i o n of d i v a l e n t ions i n t o the l i q u i d subphase i n a L a n g m u i r t r o u g h can increase the shear resistance a n d cohesion of the monolayer. F o r this reason, studies of fatty a c i d salts are m o r e c o m m o n than o n t h e i r acids. B y adjusting the p H of the subphase, m u l t i l a y e r s c o n t a i n i n g m e t a l ions separated b y the w i d t h o f an i n t e g r a l n u m b e r of monolayers (for Y - t y p e d e p o sition, two monolayers) can b e assembled. T h e a b i l i t y to d o this has b e e n c a p i t a l i z e d u p o n i n several f u n d a m e n t a l investigations, t h r e e o f w h i c h w i l l be m e n t i o n e d h e r e . T h e first of these relates to t w o - d i m e n s i o n a l magnetic monolayers i n v o l v i n g i r o n or manganese ions. U s i n g e l e c t r o n s p i n resonance, P o m e r a n t z (43) d e m o n s t r a t e d that at temperatures near 2 K , the resonance field a n d l i n e shapes are affected. T h i s result signals the r a p i d d e v e l o p m e n t of a large i n t e r n a l magnetic field. Pomerantz's results have b e e n i n t e r p r e t e d i n terms of a p r e d o m i n a n t l y antiferromagnetic state b u t w i t h a weak f e r r o m agnetic c o m p o n e n t . F u r t h e r experiments are r e q u i r e d to clarify magnetic o r d e r i n g i n t w o - d i m e n s i o n a l space. I n the area of surface science, X - r a y p h o t o e m i s s i o n is n o w u s e d e x t e n sively to study organic materials o n surfaces. I n such e x p e r i m e n t s , establ i s h i n g e l e c t r o n m e a n free p a t h lengths as a f u n c t i o n of k i n e t i c e n e r g y is i m p o r t a n t . A n example of this t y p e of investigation i n L B films is i l l u s t r a t e d i n F i g u r e 4.7d. G e n e r a l l y , the m e a n free paths for o r d e r e d m u l t i l a y e r s are significantly longer t h a n those for c o n v e n t i o n a l l y p r o d u c e d p o l y m e r s (28). A t h i r d example of the usefulness of the L a n g m u i r - t r o u g h t e c h n i q u e to


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p r o v i d e matrices c o n t a i n i n g r e g u l a r l y spaced m e t a l ions lies i n radioactivity. M o r i et a l . (40) u s e d radioactive stearate monolayers, i n w h i c h some o f the h y d r o g e n atoms h a d b e e n r e p l a c e d b y n u c l i d e s , such as C r , ^ M n , ^ F e , C o , ^ Z n , and C d , to p r o d u c e d i l u t e a n d standard radioactive sources. B y l a b e l i n g the molecules w i t h C a n d e x a m i n i n g autoradiographs, t h e y w e r e able to c o n f i r m the u n i f o r m i t y of the deposition process as s h o w n i n F i g u r e 4.7c. U s i n g c o n v e n t i o n a l monolayers w i t h w e l l - c o n t r o l l e d d i m e n s i o n s as overlays, they w e r e also able to demonstrate that A u g e r electrons f r o m the L s h e l l w i t h an e n e r g y of approximately 0.5 k e V are almost c o m p l e t e l y absorbed b y 15 monolayers of b a r i u m stearate. E x p e r i m e n t s of this k i n d are of i m p o r t a n c e i n fields s u c h as m e d i c a l physics a n d u p p e r atmosphere s c i ence. 5 1

5 7

1 0 9


1 4

4.5 Promising Applied Research Areas for Langmuir-Blodgett Films D e s p i t e the increased activity d u r i n g the past 10 years, no i m p o r t a n t c o m m e r c i a l v e n t u r e based o n L B films has b e e n started. H o w e v e r , the l e v e l of financial i n v e s t m e n t , e v e n i n the general field of organic m o l e c u l a r solids, is still a s m a l l fraction of the s u m d e v o t e d to inorganic materials. W h e n an i n s t i t u t i o n decides to invest i n an expensive i t e m s u c h as m o l e c u l a r b e a m epitaxial-growth e q u i p m e n t , it i n v a r i a b l y ensures that a reasonable n u m b e r of w o r k e r s are associated w i t h the project. L a n g m u i r troughs are r e l a t i v e l y i n e x p e n s i v e , a n d n o r m a l l y m u c h smaller size teams are i n v o l v e d . T h i s factor, c o u p l e d w i t h the difficulty of o r g a n i z i n g i n t e r d i s c i p l i n a r y teams, p r o b a b l y accounts for the c u r r e n t situation. A c o m p a r i s o n can b e made b e t w e e n the present situation for L B films a n d that w h i c h existed 15 years ago for l i q u i d crystals. A t that t i m e , w h e n the display application was clearly a p p r e c i a t e d , a large a m o u n t of b a c k g r o u n d k n o w l e d g e o f s t r u c t u r e - p r o p e r t y relationships was available. T h i s k n o w l e d g e base e n a b l e d a p p l i e d scientists to replace deficient materials b y m o r e desirable, t a i l o r e d structures that h a d already b e e n t h o r o u g h l y investigated b y chemists engaged i n p u r e research p r o grams. T h e situation is totally different for L B films; h e n c e , t e c h n i c a l progress s h o u l d not b e e x p e c t e d to take place at the same dramatic rate as o c c u r r e d for l i q u i d crystals. U n t i l r e c e n t l y , i n d u s t r y has h a d a suspicious v i e w of organic solids o n account of t h e i r i n h e r e n t stability c o m p a r e d w i t h t h e i r inorganic c o u n t e r parts. H o w e v e r , spectacular advances i n l i q u i d crystal displays, organic p h o toconductors, and piezoelectric polymers have given industry more confidence i n such materials. N e v e r t h e l e s s , the average industrialist w i l l seek an order-of-magnitude i m p r o v e m e n t i n d e v i c e performance i f a n e w t e c h n i q u e s u c h as the L a n g m u i r t r o u g h is to be i n t r o d u c e d . T h u s , for c o m m e r c i a l applications, the L B films w i l l n e e d to b e an essential, i n t e g r a l part of the d e v i c e . T h a t is, one m u s t capitalize o n t h e i r u n i q u e features such as


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the degree o f c o n t r o l over the m o l e c u l a r a r c h i t e c t u r e , t h e i r thinness, o r the selective w a y i n w h i c h they m i g h t react w i t h the e n v i r o n m e n t . N o w that a c o m m i t m e n t has b e e n m a d e to the subject, w i t h h i g h e r levels o f f u n d i n g a n d p e r s o n n e l b e i n g m a d e available, at least one i n n o v a t i v e application is l i k e l y to emerge. W h e n this happens, a n e e d w i l l arise to


p r o d u c e a specially d e s i g n e d t r o u g h capable, for example, o f coating a m o v i n g b e l t or m u l t i p l e wafers of silicon. N o difficulties are envisaged i n c o n s t r u c t i n g continuous fabrication e q u i p m e n t , a n d substantial progress has already b e e n m a d e i n this d i r e c t i o n . L a n g m u i r - B l o d g e t t films m a y have v a l u e i n m a n y a p p l i e d areas of t r a d i t i o n a l interest to the i n d u s t r i a l c h e m i s t , such as adhesion, encapsulation, a n d catalysis. T h e p e r m e a b i l i t y characteristics of m o n o l a y e r assemblies m a y also find a p p l i c a t i o n as synthetic m e m b r a n e s for ultrafine filtration, gas separation, a n d reverse osmosis. F o r example, A l b r e c h t et a l . (44) p r o v e d the efficiency o f p o l y m e r i c diacetylene monolayers o n s e m i p e r m e a b l e supports i n r e d u c i n g the flow of C H . O n e i n t e r e s t i n g p o s s i b i l i t y lies i n u s i n g L B monolayers as l u b r i c a n t s i n magnetic tape technology. U n p u b l i s h e d reports have i n d i c a t e d that frictional coefficients can b e r e d u c e d m a r k e d l y w h e n the tape is coated w i t h a few monolayers. I n applications such as those l i s t e d p r e v i o u s l y , difficulties m a y w e l l be e n c o u n t e r e d w i t h the m e c h a n i c a l stability of the films. T o date, r e l a t i v e l y little research has b e e n c a r r i e d out i n this area. 4

T h e l o n g - t e r m interest, as far as the a p p l i e d physicist is c o n c e r n e d , lies i n the possible uses o f s u p e r m o l e c u l a r assemblies for m e m o r y storage, m o l e c u l a r s w i t c h i n g , a n d s u p e r c o n d u c t i n g devices. H o w e v e r , at the p r e s e n t t i m e , p o t e n t i a l i m p r o v e m e n t areas are w h e r e m o n o m o l e c u l a r films show most p r o m i s e a n d w h e r e the prospects o f c o m m e r c i a l exploitation seem reasonable i n the m e d i u m t e r m . A few o f these areas are d e s c r i b e d i n this section; most of the illustrations are based o n w o r k c a r r i e d out i n m y research laboratories. T h e s e p o t e n t i a l i m p r o v e m e n t areas are g r o u p e d into t h r e e categories, b u t most of the emphasis is p l a c e d o n u t i l i z i n g the n o n l i n e a r properties of L B films.

4.5.1 N o n l i n e a r Physics M a n y organic m o l e c u l e s possess v e r y h i g h n o n l i n e a r coefficients. T h e r e f o r e , i f L B films w i t h the r e q u i r e d architecture can be f o r m e d , these c o u l d f o r m the basis of n o v e l devices. T o a v o i d the s y m m e t r y i n h e r e n t w i t h c o n v e n t i o n a l Y - t y p e d e p o s i t i o n , X - a n d Z - t y p e films have b e e n s t u d i e d . S o m e o f these films d i s p l a y e d a p e r m a n e n t p o l a r i z a t i o n w i t h a strong c o m p o n e n t i n a d i r e c t i o n p e r p e n d i c u l a r to the substrate. H o w e v e r , films p r o d u c e d i n this w a y , w i t h t h e i r dipoles supposedly a l i g n e d i n a c o m m o n d i r e c t i o n , are i n v a r i a b l y of p o o r e r q u a l i t y t h a n Y - t y p e layers. A possible m e t h o d o f i m p r o v i n g the structure is to use electric o r magnetic fields to h e l p align the m o l e c u l e s ,


244



b u t efforts to o r i e n t films o n the subphase a n d substrate have m e t w i t h o n l y l i m i t e d success. T h e p r o b l e m can be o v e r c o m e b y u s i n g organic superlattices based o n alternating layers o f t w o different materials ( F i g u r e 4.4). A good e x a m p l e is g i v e n i n F i g u r e 4.10, w h i c h shows a superlattice c o m p r i s i n g a c i d a n d a m i n e molecules whose d i p o l e m o m e n t s are i n opposite senses, b u t w h e n d e p o s i t e d i n Y - t y p e L B film f o r m , are a l i g n e d i n the same d i r e c t i o n . T w o areas of p a r t i c u l a r interest that w o u l d capitalize o n this feature o f organic superlattices are p y r o e l e c t r i c i t y a n d optoelectronics. E a c h o f these areas w i l l n o w b e c o n s i d e r e d . T h e acoustoelectric o p p o r t u n i t i e s are m e n t i o n e d i n the section d e v o t e d to sensor-type applications. 4.5.1.1. P Y R O E L E C T R I C D E V I C E S

W h e n the centers o f positive a n d negative charge i n a c r y s t a l l i n e m a t e r i a l do not c o i n c i d e , t h e n a spontaneous p o l a r i z a t i o n exists across i t . I f this

Figure 4.10. Left: An organic superlattice with a unique polar axis. The two types of molecules involved could be a fatty acid and a fatty amine. The insert is designed to show that these two materials have dipole moments in opposite senses with respect to the hydrophobic chain. Thus, the Y-type film has a resultant dipole moment.



4.

ROBERTS

Langmuir-Blodgett

Films

245

polarization is t e m p e r a t u r e d e p e n d e n t , the crystal develops an e l e c t r i c charge o n its surface a n d exhibits the p y r o e l e c t r i c effect. D e v i c e s based o n p y r o e l e c t r i c i t y thus r e s p o n d to a rate of change of t e m p e r a t u r e rather t h a n to changes of t e m p e r a t u r e as i n a s e m i c o n d u c t o r type of t h e r m a l detector. T h i s response gives t h e m i n h e r e n t advantages, b u t t h e i r f u l l p o t e n t i a l has yet to b e r e a l i z e d . F o r applications w h e r e b o t h h i g h speed a n d sensitivity are r e q u i r e d , c o n v e n t i o n a l materials have b e e n unsuccessful. T h i s failure can b e a t t r i b u t e d p a r t l y to the fact that c o n v e n t i o n a l materials have not b e e n available i n v e r y t h i n films. F o r d e v i c e applications, a t h i n - f i l m geometry is p r e f e r r e d because one can t h e n f o r m large area i m a g i n g devices d i r e c t l y o n m i c r o e l e c t r o n i c a m p l i f y i n g c i r c u i t s . T h e p y r o e l e c t r i c properties of inorganic materials such as the titanates t e n d to be offset b y t h e i r h i g h relative p e r m i t t i v i t i e s , a n d p y r o e l e c t r i c organic single crystals such as t r i g l y c i n e sulfate cannot b e p r o d u c e d as t h i n films. T h u s , the future d e v e l o p m e n t of r e l a t i v e l y cheap t h e r m a l i m a g i n g systems w i t h reasonable performance a n d a n o p t i m u m thickness o f approximately 0.5 p,m r e q u i r e s a materials b r e a k t h r o u g h . S o m e research effort is b e i n g g i v e n to ferroelectric l i q u i d crystals (45), b u t difficulties w i t h encapsulating such materials i n l o w t h e r m a l capacity s t r u c tures are l i k e l y to h i n d e r such d e v e l o p m e n t s . O n the other h a n d , the o p p o r t u n i t i e s for d e p o s i t i n g h i g h l y anisotropic p y r o e l e c t r i c L B films onto r e t i c u l a t e d s i l i c o n substrates seem good. T h e s e assemblies w o u l d b e i n h e r e n t l y polar a n d w o u l d not r e q u i r e " p o l i n g " , as do c o n v e n t i o n a l p y r o e l e c t r i c p o l y m e r s such as p o l y v i n y l difluoride) ( P V D F ) . T h e first r e p o r t of p y r o e l e c t r i c b e h a v i o r i n L B films was b y B l i n o v et al. (46). E l e c t r o a b s o r p t i o n studies w e r e m a d e to c o n f i r m the o r i e n t a t i o n of the a z o c o m p o u n d monolayers u s e d i n t h e i r w o r k . N o d e t a i l e d results are p r e s e n t e d i n t h e i r p a p e r , b u t p y r o e l e c t r i c coefficients approximately one o r d e r of m a g n i t u d e less than those for P V D F are r e p o r t e d for X - a n d Z type structures. T h e p y r o e l e c t r i c coefficient (p) is a useful p a r a m e t e r w i t h w h i c h to compare different materials. I f the t h i n film acts as a d i e l e c t r i c i n a capacitor, a n d an external resistance is connected b e t w e e n the electrodes, t h e n a p y r o e l e c t r i c c u r r e n t (I) flows i n the c i r c u i t . T h i s situation can b e expressed as

I = pA-z

(4.1)

w h e r e dT/dt is the rate of change o f t e m p e r a t u r e , a n d A is the cross sectional area of the d e v i c e . T h e p y r o e l e c t r i c coefficient is a measure o f the c u r r e n t generated b y a specific rate o f change o f t e m p e r a t u r e . H o w e v e r , the i n d u c e d voltage (V) is a m o r e useful p a r a m e t e r i n an I R detection system. F o r an


246


i n c r e m e n t a l change i n t e m p e r a t u r e , dT generates a charge dQ T h e r e f o r e , w h e n dQ

=

=

pAdT.

CdV,


w h e r e € is the p e r m i t t i v i t y a n d d is the p y r o e l e c t r i c m a t e r i a l thickness. T h e q u a n t i t y (p/e) is a useful figure of m e r i t for a p y r o e l e c t r i c m a t e r i a l . T a b l e 4.1 lists this p a r a m e t e r for a n u m b e r of different materials. T h e p o t e n t i a l across a film is clearly d e p e n d e n t o n the a m o u n t of surface charge. T h u s , i f the p o l a r i z a t i o n of the film is constant t h r o u g h o u t the s a m p l e , the m a g n i t u d e of V s h o u l d increase i n p r o p o r t i o n to the n u m b e r of layers d e p o s i t e d . M o r e o v e r , the sign of this increase i n surface p o t e n t i a l s h o u l d b e r e v e r s e d b y i n v e r t i n g the polar axis. T h i s result occurs w h e n organic superlattices are p r e p a r e d b y u s i n g the alternate-layer t r o u g h d e s c r i b e d p r e v i o u s l y . F i g u r e 4.11 shows the surface p o t e n t i a l as a function of n u m b e r of layers for two c o m p l e m e n t a r y orientations o f the polar axis of the m o l e c u l a r structure i l l u s t r a t e d i n F i g u r e 4.10. T h e top h a l f corresponds to the case w h e r e the a m i n e l a y e r is d e p o s i t e d first. T h e surface p o t e n t i a l steps are of different sign w h e n the a c i d layer is d e p o s i t e d first. T h e s e e x p e r i m e n t s c o n f i r m that a t h i n film w i t h a u n i q u e polar axis has b e e n assembled. U s i n g d y n a m i c a n d static detection t e c h n i q u e s , C h r i s t i e et a l . (47) s h o w e d that this c o m b i n a t i o n of s i m p l e molecules is p y r o e l e c t r i c w i t h p ~ 1 C c m " K . M o r e recent results, i n v o l v i n g a system w h e r e p r o t o n transfer occurs from the a c i d to the a m i n e , have y i e l d e d h i g h e r values. T h e r e f o r e , e v e n w h e n standard materials are u s e d , figures of m e r i t comparable w i t h those of the best alternatives can b e a c h i e v e d . 2

- 1

H o w e v e r , the exploitation of p y r o e l e c t r i c L B films d e p e n d s not o n l y o n the figures of m e r i t l i s t e d i n T a b l e 4.1, b u t also o n the ability to deposit such layers successfully o n surfaces w i t h l o w t h e r m a l mass. A substrate imposes two constraints o n the p y r o e l e c t r i c response of a film. N o t o n l y does it p r o v i d e an u n d e s i r a b l e heat sink a n d r e d u c e the excess t e m p e r a t u r e of Table 4.1. Pyroelectric Coefficients and Figures of M e r i t for a Selection of Different Materials

Material

Form

Lithium tantalate Triglyeine sulfate Strontium barium niobium oxide P o l y v i n y l difluoride) Phenylbenzoate ester Acid/amine superlattice a

Bulk crystal Bulk crystal Ceramic Polymer film Liquid crystal L B film

Pyroelectric Relative Coefficient (nC cm K- ) Permittivity 19.0 30.0 85.0

46.0 50.0 607.0

p/e (nC cm K- ) 0.41 0.60 0.14

3.0 0.7 1.0

10.0 4.0 2.5

0.30 0.18° 0.40

2

1

This value is 5 °C from the Curie point.


2

1

4.

ROBERTS

1000

Langmuir-Blodgett

F«-0»|«

1

Films

H«

247

3

»|« o

????

5

H

1000

-o

///////

600 2 2

600 OOP0°^ ^ ^ ^

CM


•S

1

Amine

i

Acid

CO

2 200

- 200

C0

.s

I -200

-200

o

Oh

8

t m

3 CO

-600

777777?

-600

Number of Monolayers Figure 4.11. Surface potential of to-TA-docosylamine alternate layers as a function of the number of layers for two complementary orientations of the polar axis. (Reproduced with permission from reference 47. Copyright 1986 American Institute of Physics.) the p y r o e l e c t r i c l a y e r (thermal clamping), b u t i t also l i m i t s a secondary p y r o e l e c t r i c c o n t r i b u t i o n d u e to t e m p e r a t u r e - d e p e n d e n t v o l u m e changes (mechanical clamping). R e t i c u l a t e d substrates w i t h appropriate t h e r m a l e x p a n s i o n coefficients w i l l b e r e q u i r e d to fully exploit t h e advantages o f u s i n g o p t i m u m - t h i c k n e s s ( ~ 0 . 5 p,m) p y r o e l e c t r i c s u p e r m o l e c u l a r a s s e m b l i e s . N o i s e considerations also dictate that the p y r o e l e c t r i c t h i n f i l m m a t e r i a l m u s t e x h i b i t l o w d i e l e c t r i c loss. 4.5.1.2 O P T O E L E C T R O N I C D E V I C E S

A l t h o u g h m a n y o f the p o t e n t i a l optical applications o f L B films are i n transm i s s i o n optics, e m p l o y i n g t h e l i n e a r response properties o f m o l e c u l e s , t h e


248



most e x c i t i n g applications are p e r c e i v e d i n the area of n o n l i n e a r optics. F u n c t i o n s s u c h as second h a r m o n i c generation a n d parametric amplification can be o b t a i n e d b y u s i n g inorganic single-crystal materials such as l i t h i u m niobate, b u t r e c e n t l y , organic crystals s u c h as 3 - m e t h y l - 4 - n i t r o a n i l i n e have b e e n s h o w n to possess exceptionally large second-order electrooptic coefficients (48). S i m i l a r l y , c u b i c effects s u c h as four-wave m i x i n g , phase conjugation, a n d o p t i c a l b i s t a b i l i t y f o r m the basis of o p t i c a l functions o f strategic i m p o r tance. I n this case, materials such as polyacetylene c o u l d , i n p r i n c i p l e , c o m p e t e effectively w i t h inorganic solids s u c h as g a l l i u m arsenide. T h e macroscopic p o l a r i z a t i o n (P) of a s o l i d c o m p r i s i n g m a n y i n d i v i d u a l molecules m a y b e expressed as P = P„ + x i ( E ) +

X

2(E ) + X (E ) + * • • 2

3

3

(4-3)

w h e r e P is a constant, £ is the electric field, a n d \n * the n t h o r d e r susceptibility tensor. I n this e q u a t i o n , X2 & X3 responsible for the second a n d t h i r d h a r m o n i c generation, respectively. W h e n a n i n d i v i d u a l m o l e c u l e is i n v o l v e d , the p o l a r i z i n g effect can be expressed i n terms of an i n d u c e d d i p o l e m o m e n t (p,) a n d a series of polarizabilities. T h a t is, s

0

m

\L

= aE

+ PE + 7E 2

a r e

3

+ • ••

(4.4)

w h e r e the coefficients a , (3, a n d 7 are tensor quantities. T h e basic m o l e c u l a r properties that give rise to h i g h values of the h y p e r p o l a r i z a b i l i t i e s , s u c h as P a n d 7, are reasonably w e l l u n d e r s t o o d i n organic solids. F o r quadratic m o l e c u l a r effects, conjugation a n d i n t r a m o l e cular charge transfer are i m p o r t a n t . C u b i c - n o n l i n e a r i t y - r e l a t e d effects are e n h a n c e d i n o n e - d i m e n s i o n a l conjugated structures, b u t no charge-transferi n d u c e d a s y m m e t r y is r e q u i r e d . T h u s , i n b o t h cases, the m o l e c u l a r u n i t can be " t a i l o r e d " to m e e t a specific r e q u i r e m e n t . A second c r u c i a l step i n e n g i n e e r i n g a m o l e c u l a r structure for n o n l i n e a r applications is to o p t i m i z e the crystal structure. F o r seco n d - o r d e r effects, a n o n c e n t r o s y m m e t r i c a l geometry is essential. A n i s o t r o p i c features, s u c h as p a r a l l e l conjugated chains, are also useful for t h i r d - o r d e r effects. A n i m p o r t a n t factor i n the o p t i m i z a t i o n process is to shape the m a terial for a specific d e v i c e so as to enhance the n o n l i n e a r efficiency of a g i v e n structure. A t h i n - f i l m g e o m e t r y is n o r m a l l y p r e f e r r e d because n o n l i n e a r interactions, l i n e a r filtering, a n d transmission functions can be i n t e g r a t e d into one precise m o n o l i t h i c structure. I n a l l stages of the process of d e s i g n i n g n o n l i n e a r organic integrated devices, L B film techniques appear to offer considerable advantages. S e v e r a l researchers are c u r r e n t l y s u b s t i t u t i n g organic molecules w i t h appropriate side groups to enable L B film deposition to occur. I n m a n y cases, n o n l i n e a r coefficients comparable to those of inorganic materials have b e e n a c h i e v e d .


4.

ROBERTS

Langmuir-Blodgett

Films

249


H o w e v e r , o t h e r considerations are also i n v o l v e d i f L B films are to c o m p e t e effectively w i t h materials s u c h as l i t h i u m niobate a n d g a l l i u m arsenide. M o lecular e n g i n e e r i n g w i l l also b e r e q u i r e d to cater to needs such as phase m a t c h i n g , suitable spectral response a n d refractive i n d e x , good o p t i c a l d a m age t h r e s h o l d , l o w scattering coefficients, a n d m e c h a n i c a l stability before practical objectives can b e a c c o m p l i s h e d . T o illustrate the p o t e n t i a l o f L B films i n n o n l i n e a r optics, two examples w i l l b e g i v e n to illustrate b o t h q u a d r a t i c - a n d c u b i c - o r d e r effects. T h e m o tivation for the first example came from early accounts o f n o n l i n e a r i t y i n a specific m e r o c y a n i n e d y e m o l e c u l e (49). N o good crystals o f this m a t e r i a l are available, b u t an e x t r e m e l y large value of (3 was p r e d i c t e d o n the basis of measurements w i t h p o w d e r e d samples. T h e r e l i a b i l i t y of such data is not h i g h because p o w d e r efficiency is a f u n c t i o n o f particle size d i s t r i b u t i o n s . T h e i n i t i a l e x p e r i m e n t s w e r e w i t h a s i m p l e m e r o c y a n i n e d y e alternated w i t h (o-TA. H o w e v e r , the best results (50) w e r e o b t a i n e d b y u s i n g organic s u perlattices based o n the two molecules s h o w n b y Structures 4. l a a n d 4. l b .

NO2

Structures 4.1a and 4.1b. These two molecules, one (a) a hemicyanine and the other (b) a nitrostilbene dye, can be used to form an organic superlattice displaying a high coefficient for second harmonic generation (see reference 50).


250


A s i n the case of the a c i d - a m i n e structure d e s c r i b e d p r e v i o u s l y , the two m o l e c u l e s , w h e n d e p o s i t e d i n alternate-layer f o r m , have t h e i r dipoles m u tually a l i g n e d a n d p r o d u c e a h i g h l y n o n c e n t r o s y m m e t r i c structure i d e a l for second h a r m o n i c generation. T h e insert i n F i g u r e 4.12 shows that the n o n l i n e a r response of a h e m i c y a n i n e - n i t r o s t i l b e n e layer is greater t h a n that


expected from the s i m p l e a d d i t i o n of contributions a r i s i n g from the i n d i v i d u a l (separated) monolayers. T h e coefficient for second h a r m o n i c generation, (3, of the alternate l a y e r structure is approximately 5 times the average value of the same p a r a m e t e r m e a s u r e d for h e m i c y a n i n e a n d n i t r o s t i l b e n e . T h i s

2000 LB FILM

1800 1600

|8(x10" m V" ) 27

NITROSTILBENE

2

2

21

HEMICYANINE

116

SUPERLATTICE

316

1400 UJ 1200 DC

1000 CO Q

800

111

5

600

• o z UJ

o iZ U.

tu UJ

> -I UJ

DC

4

8

12

16

20

24

28

NUMBER O F M O N O L A Y E R S

Figure 4.14. Top (not to scale): An Au-LB film-GaP structure. The semiconductor-organic film interface is common to all four devices. Bottom: A plot of the electroluminescent efficiency versus the number of monolayers of substituted phthalocyanine (see reference 55).


4.

ROBERTS

Langmuir-Blodgett

255

Films

T h e L B thickness r e q u i r e d to o p t i m i z e the e l e c t r o l u m i n e s c e n c e efficiency is f o u n d to be a p p r o x i m a t e l y 21 n m . T h i s value is d e t e r m i n e d b y the a b i l i t y of m i n o r i t y carriers to cross the s e m i - i n s u l a t i n g p h t h a l o c y a n i n e film. S i m i l a r results have r e c e n t l y b e e n a c h i e v e d b y u s i n g z i n c selenide layers g r o w n w i t h M O C V D (57). B l u e e l e c t r o l u m i n e s c e n c e is o b s e r v e d , p r o v i d e d the


organic film is present. A surface layer on a s e m i c o n d u c t o r can be p r o d u c e d b y m a n y methods. H o w e v e r , experience has s h o w n that w h e n an energetic process s u c h as evaporation, s p u t t e r i n g , or g r o w t h from a p l a s m a is u s e d to deposit a t h i n film onto a semiconductor, a surface-damaged l a y e r is p r o d u c e d that i n v a r i a b l y dominates the e l e c t r i c a l characteristics of the j u n c t i o n s so f o r m e d . H o w e v e r , the L a n g m u i r - t r o u g h t e c h n i q u e , b e i n g a l o w - t e m p e r a t u r e d e p osition process, provides a means of c i r c u m v e n t i n g this p a r t i c u l a r difficulty. O n the other h a n d , h o w the substrate is p r e p a r e d before d i p p i n g is of considerable i m p o r t a n c e i n d e t e r m i n i n g the q u a l i t y of the interface p r o d u c e d . T h a t is, the nascent " o x i d e " layer f o r m e d d u r i n g the e t c h i n g p r o c e d u r e remains r e l a t i v e l y u n d i s t u r b e d , a n d this layer can play a v i t a l role e v e n after it has b e e n coated w i t h an L B film. F o r this reason, a systematic study of the surface c h e m i s t r y of the semiconductor substrates s h o u l d b e c a r r i e d out first.

4.5.2.3 T U N N E L I N G D E V I C E S

F i g u r e s 4.8 a n d 4.9 show h o w the fine c o n t r o l of thickness available w i t h m o n o m o l e c u l a r films has b e e n u t i l i z e d to great advantage i n f u n d a m e n t a l research, e v e n to d i m e n s i o n s as l o w as 1 n m . S o m e researchers have h i g h l i g h t e d the benefits of the t r o u g h t e c h n i q u e to p r o d u c e t a i l o r e d materials that m i g h t c o n t r o l the critical c u r r e n t , s w i t c h i n g speeds, a n d energy gap parameters i n l o w - t e m p e r a t u r e devices such as S Q U I D S (superconducting q u a n t u m interference devices). C e r t a i n semiconductor devices (e.g., a floating-gate transistor or a s w i t c h i n g structure) also r e q u i r e a u n i f o r m , u l t r a t h i n i n s u l a t i n g film of less than 1 0 - n m thickness. T h e result s h o w n i n F i g u r e 4.15 is for a bistable s w i t c h based o n a m e t a l - t h i n i n s u l a t o r - n - p + structure. P o t e n t i a l applications of these M I S S (metal insulator d o u b l e semiconductor) devices, i f a r e p r o d u c i b l y t h i n , h i g h q u a l i t y s e m i - i n s u l a t i n g layer can be found, i n c l u d e m e m o r i e s a n d shift r e g isters. W i t h s i l i c o n , oxide layers approximately 3 n m thick can b e u s e d , although t h e y are difficult to g r o w u n i f o r m l y . F o r h i g h - m o b i l i t y group I I I - V c o m p o u n d s , the difficulties are m o r e severe o w i n g to the lack of a suitable native oxide. H o w e v e r , the c u r r e n t - v o l t a g e characteristics s h o w n have b e e n o b t a i n e d b y u s i n g G a A s as the semiconductor a n d the (o-TA m o l e c u l e s h o w n i n C h a r t 4.1 as the insulator (59). O p e r a t i o n for 1 0 cycles at 100 H z does not degrade the s w i t c h i n g characteristics, w h i c h are insensitive to i l l u m i nation a n d t e m p e r a t u r e . 7


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5 mA/div Figure 4.15. Top: Cross section of a MISS diode. The device can be regarded as a reverse-biased metal-insulator-semiconductor diode in series with a forward-biased n-p junction. It then exhibits two stable states separated by an unstable negative resistance region. Bottom: Current-voltage characteristics for a GaAs-m-TA MISS device. The LB film thickness is approximately 9 nm (see reference 59). +

4.5.3 Sensors and Transducers T h e feet that organic c o m p o u n d s n o r m a l l y r e s p o n d m o r e p o s i t i v e l y than inorganic materials to e x t e r n a l s t i m u l i s u c h as pressure, t e m p e r a t u r e , o r radiation p r o v i d e s a means o f m a k i n g sensitive transducers. M o r e o v e r , b y c o n t r o l l i n g the a r c h i t e c t u r e o f the L B film, the interactions can b e d e s i g n e d to b e o f a l o c k - k e y t y p e ; t h u s , the selectivity o f the d e v i c e can b e e n h a n c e d .


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A n o t h e r advantage o f u l t r a t h i n organic films is t h e i r fast response a n d r e c o v e r y t i m e s because so little m a t e r i a l is present. T h e a m p l i f y i n g features o f an e l e c t r o n i c d e v i c e can b e c o m b i n e d w i t h t h e attributes o f an L B film to f o r m sophisticated microsensors. H o w e v e r , optical a n d acoustic devices frequently show i n t e r e s t i n g t h r e s h o l d or resonance effects that can also f o r m the basis o f useful sensors. I n this section, d e t a i l e d results are p r e s e n t e d o n l y for acoustoelectric devices.


4.5.3.1 S E M I C O N D U C T O R D E V I C E S

T h e good i n s u l a t i n g p r o p e r t i e s o f L B films suggest t h e i r possible use i n fieldeffect devices, not so m u c h to c o m p e t e w i t h e x i s t i n g s e m i c o n d u c t o r t e c h nology, b u t to capitalize o n the advantages of b e i n g able to incorporate an organic layer w i t h i n a s e m i c o n d u c t o r structure. F i g u r e 4.16 shows schematic

Figure 4.16. Schematic diagrams (not to scale) showing (top) a metalinsulator-semiconductor (MIS) that forms an integral part of the field-effect transistor (bottom).



258


diagrams of b o t h a field-effect transistor ( F E T ) a n d the " h e a r t " of this d e v i c e , w h i c h is the m e t a l - i n s u l a t o r - s e m i c o n d u c t o r ( M I S ) diode. C o n v e n t i o n a l l y , these devices are m a d e b y u s i n g inorganic materials; s i l i c o n holds a p r e e m i n e n t p o s i t i o n , m a i n l y because o f the i n s u l a t i n g qualities o f its native oxide. T h e first transistor i n c o r p o r a t i n g L B monolayers as the insulator was o b t a i n e d several years ago (60) w i t h the t y p e o f t h r e e - t e r m i n a l d e v i c e ( F i g u r e 4.16) based o n i n d i u m p h o s p h i d e a n d c a d m i u m stearate. Roberts et al. (60) s h o w e d that the c h a n n e l c o n d u c t i v i t y b e t w e e n the source a n d the d r a i n c o u l d b e m o d u l a t e d b y the action of a gate electrode. S u b s e q u e n t l y , other s e m i c o n ductors have b e e n u s e d , a n d results have c o n f i r m e d the ease w i t h w h i c h a range o f single-crystal surfaces can be a c c u m u l a t e d , d e p l e t e d , or i n v e r t e d w i t h an a p p l i e d voltage. I n a l l cases, the L B film is d e p o s i t e d o n top o f a nascent " o x i d e " l a y e r , a n d the i n s u l a t i o n is p r o v i d e d essentially b y a d o u b l e d i e l e c t r i c structure. F o l l o w i n g the n o n l i n e a r physics w o r k d e s c r i b e d e a r l i e r , p y r o - or p i e z o F E T s based o n i n s u l a t i n g L B films w i t h an i n - b u i l t p o l a r i z a t i o n , o r fieldeffect devices i n c o r p o r a t i n g biological m e m b r a n e s , can b e envisaged. F u r t h e r m o r e , the discussion o f m i c r o e l e c t r o n i c L B film based o n sensors does not have to be c o n f i n e d to M I S or F E T structures. F o r e x a m p l e , the s w i t c h i n g voltage of the bistable s w i t c h d e s c r i b e d i n F i g u r e 4.15 or the characteristics of a gate-controlled d i o d e c o u l d b e made v e r y sensitive to a change i n a m b i e n t conditions (61). 4.5.3.2 O P T I C A L S E N S O R S

O n e area that is r e c e i v i n g p a r t i c u l a r l y strong attention is that o f surface p l a s m o n resonance (SPR) (62). T h e p r i n c i p l e of this optical detection m e t h o d is illustrated i n F i g u r e 4.17. A surface p l a s m o n is a surface charge d e n s i t y wave at a m e t a l surface. I f the m e t a l is s a n d w i c h e d b e t w e e n two materials o f different d i e l e c t r i c constants, t h e n resonances can occur. T h i s p h e n o m e n o n is o b s e r v e d as a v e r y sharp m i n i m u m of the l i g h t reflectance w h e n the angle of i n c i d e n c e (9) is v a r i e d . T h e resonance angle is ultrasensitive to variations i n the refractive i n d e x o f the m e d i u m adjacent to the m e t a l film. F o r e x a m p l e , the s m a l l change i n an organic m a t e r i a l d u e to gas absorption can easily b e m o n i t o r e d e v e n for concentrations i n the p a r t s - p e r - b i l l i o n range. I n a practical situation, one n o r m a l l y selects an angle o f i n c i d e n c e a p p r o x i m a t e l y halfway d o w n the reflectance m i n i m u m c u r v e w h e n no special gas is present; the change i n i n t e n s i t y of the reflected l i g h t is t h e n m o n i t o r e d at a constant angle. W i d e s p r e a d interest has arisen i n the p o t e n t i a l o f L B films as biosensors because m a n y b e l i e v e that the i n c o r p o r a t i o n o f biological molecules s u c h as e n z y m e s w i l l l e a d to n o v e l devices. Some are e x p l o r i n g the d e p o s i t i o n o f biologically active molecules onto the gate electrodes or oxides of field-effect transistors, b u t o p t i c a l sensors, p r o b a b l y based o n fiber optics, are the most favored t e c h n i q u e . I n a l l cases, the a i m is to c o u p l e the specificity o f i n t e r -



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Figure 4.17. Schematic diagrams illustrating the basis of the surface plasmon resonance technique. Top: A beam of radiation striking the back surface of a glass prism coated with a metal film (usually evaporated silver). The reflected intensity and angle of reflection are extremely sensitive to variations in the dielectric on the metal surface. Bottom: Shift in the R vs. 0 plot when the organic film is exposed to a gas. action o f c h e m i c a l o r b i o c h e m i c a l s w i t h proteins or e n z y m e s (e.g., the change i n t h e i r m o l e c u l a r conformation) w i t h the sensitivity a n d signal t r a n s d u c t i o n properties of the d e v i c e . Stability a n d l i f e t i m e m a y be p r o b l e m areas, a n d , for this reason, c r o s s - l i n k e d p o l y m e r s are b e i n g e x p l o r e d as the hosts for the active species. 4.5.3.3 A C O U S T O E L E C T R I C D E V I C E S

W h e n materials are a d d e d or r e m o v e d from a v i b r a t i n g b o d y , its resonant frequency is changed. T h i s p h e n o m e n o n has b e e n used for mass d e t e r m i -


260


nation, n o r m a l l y w i t h a resonator m a d e o f p i e z o e l e c t r i c q u a r t z cut w i t h a specific crystallographic o r i e n t a t i o n . A m o r e sophisticated acoustoelectric m i c r o g r a v i m e t r i c sensor is a surface acoustic wave (SAW) structure. S u c h devices h a d a great i m p a c t i n the field o f signal processing, especially as filters a n d delay l i n e s . T h e absorption o f a gas introduces m i n u t e changes i n the mass o f a sensing layer. T h u s , b o t h b u l k q u a r t z oscillators a n d S A W devices can f o r m the basis o f gas detectors.


4.5.3.4 Q U A R T Z O S C I L L A T O R

T r a d i t i o n a l l y , the most c o m m o n t e c h n i q u e to check the r e p r o d u c i b i l i t y o f L B film m o n o l a y e r d e p o s i t i o n has b e e n the capacitance m e t h o d , w h e r e the a i m is to p r o d u c e good l i n e a r plots o f inverse capacitance versus n u m b e r o f layers ( F i g u r e 4.7a). H o w e v e r , this t e c h n i q u e involves evaporating m e t a l contacts onto the organic film a n d possibly d a m a g i n g the surface r e g i o n . T h e use o f s i m p l e q u a r t z oscillators c i r c u m v e n t s this difficulty (63). T y p i c a l results for p i e z o e l e c t r i c crystals l o a d e d w i t h L B films o f

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