2007 Jaworek Electrospray Droplet sources for thin film deposition

Abstract Electrospraying utilises electrical forces for liquid atomisation.Droplets obtained by this method are highly charged to a fraction of the Rayleigh limit.The advantage of electrospraying is that the droplets can be extremely small,down to the order of 10’s nanometres,and the charge and size of the droplets can be controlled to some extent be electrical means.Motion of the charged droplets can be controlled by electric ?eld.The deposition ef?ciency of the charged spray on an object is usually higher than that for uncharged droplets.Electrospray is,or potentially can be applied to many processes in industry and in scienti?c instruments manufacturing.The paper reviews electro-spray methods and devices,including liquid metal ion sources,used for thin ?lm deposition.This technique is applied in modern material technologies,microelec-tronics,micromachining,and nanotechnology.

Introduction

Electrospraying is a method of liquid atomisation by

electrical forces.The atomiser nozzle is usually made in the form of metal capillary,which is biased by a high voltage.The shear stress on the liquid surface,due to the established electric ?eld,causes elongation of a jet and its disintegration into droplets.The droplets obtained by this method can be extremely small,in

special cases down to nanometers.The advantage of the electrospray is that droplets are highly charged,up to a fraction of the Rayleigh limit.The Rayleigh limit [1]is the magnitude of charge on a drop,which over-comes the surface tension force that leads to the drop ?ssion.This charge is given by the following equation:Q R ?2p e16r l e 0r 3T1=2

e1T

in which r l is the liquid surface tension,e 0is the electric permittivity of the free space,and r is the droplet ra-dius.

The charge and size of the droplets can be con-trolled to some extent by adjusting the liquid ?ow rate and the voltage applied to the nozzle.Charged droplets are self-dispersing in the space due to mutual Coulomb repulsion,that results in the absence of droplet’s agglomeration.The expansion of a spherical cloud of charged droplets is given by the Eq.[2]:à1c d d c d d t ?2C c e 0c d E 2ds g g q l ?c d s exp

e2T

in which c d is the initial mass concentration of the droplets,g g is the gas dynamic viscosity,q l is the liquid density,and C c is the Cunningham slip correction factor.

E ds is the electric ?eld on a single droplet surface charged to the magnitude Q d :E ds ?

Q d 4pe 0r 2

e3T

In Eq.(2)s exp is time constant of the expansion of the cloud:

A.Jaworek (&)

Institute of Fluid Flow Machinery,

Polish Academy of Sciences,Fiszera 14,

Gdan

′sk 80-952,Poland e-mail:jaworek@imp.gda.pl

J Mater Sci (2007)42:266–297DOI 10.1007/s10853-006-0842-9

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Electrospray droplet sources for thin ?lm deposition

A.Jaworek

Received:23August 2004/Accepted:17October 2005/Published online:28November 2006óSpringer Science+Business Media,LLC 2006

s exp?

g g q l

2C c e0E

ds

e4T

The mass concentration of the droplets due to the cloud expansion decreases reciprocally with time:

c d?c d01à

t s exp

e5T

The motion of the charged droplets can be easily controlled(including de?ection or focusing)by an electric?eld.The deposition of a charged spray or solid particles on an object can be more effective than for un-charged one[3].An apparatus for electrospraying is very simple and cheap.Main shortcoming of elec-trospraying,limiting its widespread use in industry,is its low throughput.To overcome this problem,the multi-nozzle or slit-nozzle systems[4–11]were pro-posed.Mechanical spraying by rotary[12–14]or pneumatic atomisers[15–17]with grounded nozzle and high voltage induction electrode can also be used for production of large amount of charged spray.How-ever,the charge of the droplets produced by this method is one order of magnitude lower than the Rayleigh limit.

One of the most important features characterising any spray system is the mode of spraying.There were many spraying modes discovered and discussed in the literature[18–25],but they can be categorised into two main groups:

?The?rst group,which is characteristic in that only fragments of liquid are ejected directly from the meniscus at the capillary outlet.These fragments can be in the form of regular large drops,?ne droplets,or elongated spindles at the moment of their detachment.

?In the second group,the liquid is elongated into a ?ne jet,which disintegrates into droplets due to its instability.It was observed that the jet could be smooth and stable or could move in a regular way: rotate around the capillary axis or oscillate in its plane[22].Sometimes a few jets on the circumfer-ence of the capillary can be formed.

Charged sprays found application in many?elds, including painting or thin?lm deposition.The physical and chemical methods for thin and thick?lms deposi-tion from gaseous,solution,molten or solid state were reviewed by Altenburg et al.[26]but only minor attention was devoted to the electrospraying.Recently, Choy[27]reviewed of the current and potential development of chemical vapour deposition processes and their application to?lm deposition,with only brief presentation of electrospraying.Therefore,there is a need for presentation of electrospray applications in the thin?lm technology with a summary of the bene?ts it offers in this?eld.

The purpose of this paper is to outline electrospray devices,including liquid metal ion sources,used for thin solid?lm deposition.The advantages that elec-trospray has over other methods of metal or ceramic ?lm production are also pointed out.All the relevant details of the available technical data regarding fun-damental experiments and laboratory demonstrations on the electrostatic method of thin?lm deposition are summarised in the tables.

Thin solid?lm deposition

Introduction

Thin solid?lms are used to improve surface properties of mechanical elements or in scienti?c or measuring instruments,and in electronic devices.Electrostatic deposition is the process of depositing a material on a substrate by electrical forces.Initially,electrospray was used to produce thin layers of radioactive materials, such as a-or b particle sources,or targets prepared for activation in particle accelerators or nuclear reactors. Recently,electrospray was used for thin?lm deposi-tion in nanotechnology and nanoelectronics.

There are several methods used for thin layer deposition on a substrate:

1.casting of a solution or colloid suspension on a

substrate,followed by solvent evaporation,

2.cathode spraying,applicable for metal layers

preparation,

3.condensation of vapours of a material on the

substrate,

4.radio-frequency sputtering,

618a4d2eed630b1c59eeb560ser ablation,

6.chemical vapour deposition,

7.physical vapour deposition,

8.microwave plasma coating,

9.?ame-assisted vapour deposition,

10.electrodeposition of the layer by electrolysis,used

for metals deposition,

11.electrospraying.

Large amount of material is lost to the chamber walls when cathode spraying,chemical vapour depo-sition,or vapour condensation is used.When a solution or suspension of a material to be deposited is sprayed

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by mechanical atomisers,or simply poured onto a substrate,the layer is not suf?ciently homogeneous and of the same thickness on the entire surface.

The quality of thin ?lm formed on a substrate strongly depends on the size of particles or droplets forming the layer,their monodispersity,and their uniform distribu-tion on the surface.Smaller particles,of narrow size distribution should be generated in order to reduce the number and size of voids,?aws and cracks in the ?lm.The droplets ought to be uniformly dispersed over the substrate to ensure the layer to be even and of the same thickness.The electrospray is a promising tool for pro-duction of high quality layers and ?lms because it ful?ls all these requirements.The electrostatic technique allows generating ?ne droplets in micro-and submi-crometer size range,with narrow size distribution.Electrostatic forces disperse the droplets homoge-neously in the space between the nozzle and the sub-strate.The electrospray process is also easy to control by adjusting liquid ?ow rate and the voltage applied to the nozzle,and it is less expensive in production of thin ?lms than chemical or physical vapour deposition,or plasma spraying requiring high vacuum installations.The ?lm thickness can be simple controlled by varying the con-centration and ?ow rate of the precursor solution.

There are two main spray systems used for thin ?lm deposition:1.

simple nozzle facing directly the substrate;high voltage is applied either to the nozzle or the sub-strate,while the counter electrode is grounded (Fig.1a),

2.

nozzle–extractor system,which operates indepen-dently of the substrate;the nozzle is at high potential while the extractor and the substrate are grounded (Fig.1b).

A disadvantage of the ?rst system is that the sub-strate is one of the active electrodes,and should be conducting.If an electric current to the substrate is not

allowed,because the substrate could be damaged,the second spray system,with an extractor electrode,is used.However,a disadvantage of this system is that a fraction of the total number of droplets is deposited on the extractor electrode,and special precautions are needed to prevent these droplets to fall onto the sub-strate.Reverse con?guration,with the substrate placed above the nozzle outlet,which now is facing upwards,or systems with horizontal nozzle were therefore used by van Zomeren et al.[28],Chen et al.[29–31],Lap-ham et al.[32],Taniguchi et al.[33,34],and Perednis et al.[35].In these systems,only suf?ciently ?ne droplets are accelerated by the electric ?eld and reach the substrate.

In order to improve operational properties of elec-trospray devices also other modi?cations were pro-posed.A guard electrode was used to obtain more uniform electric ?eld in the interelectrode space by Jaworek and Krupa [22,23](Fig.2a).With the guard plate,the electrospray is more stable,the spray plume angle is narrower,and the system operates in wider voltage range [36].Kim et al.[37,38]applied a pyrex guide tube placed between the nozzle and the substrate,which focused the droplets on the substrate surface (Fig.2b).A heated sheath gas was pumped through this tube for solvent evaporation.The ?lm obtained was more uniform when using this device.Sorensen [39]designed a ‘trumpet-ended’nozzle made of stainless steel capillary with central sharp electrode (Fig.3).The spray plume was wider because the liquid was sprayed both from the electrode tip and the edge of the ‘trum-pet’.Tilted nozzles (ended like a hypodermic needle)were also tested as a source of droplets.Chen [31]argued that such nozzle allow the cone-jet mode to operate in wider voltage and ?ow rate ranges.Recently,Li [40]proposed a nozzle with a concentric dielectric ?bre protruding from the nozzle at some distance.Such insulating ?bre is not an ion injector,and does not affect the spraying by electrical but only by capillary

forces.

Fig.1Thin ?lm deposition by liquid atomisation,(a)simple nozzle for direct

spraying,(b)nozzle–extractor system

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Depositing droplets on an insulating surface pre-sents serious problems due to charge accumulation,repelling on-coming droplets.Kessick et al.[41],solved this problem by using ac potential for electrospraying.Ac potential was capable of producing high quality coverage due to reduction of the net charge on the surface.

Thin ?lm deposition technique using electrospray for thin ?lm deposition is frequently called ‘electro-static spray deposition’(ESD),‘electrostatic spray assisted vapour deposition’(ESAVD)[42],or ‘elec-trostatic spray pyrolysis’(ESP)when the process pro-ceeds at high temperatures.The spray systems usually operate in the cone-jet mode but,sometimes,the multi-jet mode also is used [37,38].The multi-jet mode made it possible to obtain simultaneously a large number of emission cones and droplets smaller than from a single cone.The details of the experiments with

thin ?lm deposition by electrospraying are brie?y summarised in Table 1.

The process of depositing a layer from a liquid phase of an on-demand structure or pattern is called ‘direct writing’.The advantage of such technology is that the patterns are printed directly onto a substrate under computer control without a need of using photolitho-graphic masks,and that the layer needs not to be patterned after deposition.Electrospray technique was also tested for these processes.Direct write techniques are capable of producing structures with feature sizes smaller than 100l m and in near future it is expected to decrease this limit to 10’s l m [129].

Usually,the material to be deposited is sprayed directly onto the substrate.However,the layer can also be obtained from other compounds,known as precur-sors.The precursors can be decomposed at a high temperature or react with other compounds sprayed simultaneously.The decomposition and reactions can be carried out on the heated substrate,or in a heated gas,in the way of a droplet to the substrate.In the latest case,only the reaction products are deposited on the surface.

The electrostatic spray deposition processes are carried out at lower temperatures than conventional solid-state reactions.The optimum temperature of the substrate is critical for this process,for layer uniformity and porosity.The porosity is a result of fast evapora-tion of the solvent,and increases with the substrate temperature.Changing the composition of the solvent by addition of a liquid of higher boiling temperature can help in control the evaporation process.When,for example,butyl carbitol was added to ethanol,used as the solvent,the ?lm structure was more uniform [61].Surface morphology,chemistry,and crystal struc-ture are usually studied using scanning electron micros-copy (SEM),energy-dispersion X-ray

microanalysis

Fig.2Spray systems (a)with guard plate,(b )with pyrex guide tube and sheath

gas

Fig.3Schematic diagram of the ‘trumpet ended’spray nozzle for thin ?lm deposition [cf.39]

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T a b l e 1T h i n ?l m d e p o s i t i o n –C o m p a r i s o n o f m e t h o d s a n d r e s u l t s

A u t h o r s

A p p l i c a t i o n (l a y e r //s u b s t r a t e )

S u b s t a n c e o r p r e c u r s o r s p r a y e d S o l v e n t N o z z l e V o l t a g e (e l e c t r o d e d i s t a n c e )F l o w r a t e (g r o w t h r a t e o r d e p o s i t i o n t i m e )

S p r a y c u r r e n t (s p e c i ?c c h a r g e )F i l m t h i c k n e s s (p l u m e b a s e d i a m e t e r )

C a r s w e l l a n d M i l s t e d [43]a -s o u r c e s (n u c l e a r c r o s s s e c t i o n s t u d i e s )

N i t r a t e s o f A m 241,C m 242,U 233,P u 238

A c e t o n e

G l a s s c a p i l l a r y 0.2m m o .d .,c o n c e n t r i c w i r e 4–6k V (10–20m m )30l l /h 0.1–0.2l A 40l g /c m 2

G o r o d i n s k y [44]

S o u r c e o f r a d i o a c t i v e r a d i a t i o n R u t h e n i u m s a l t s o r u r a n i u m n i t r a t e E t h a n o l +w a t e r m i x t u r e G l a s s c a p i l l a r y 0.1–0.3m m ,P t w i r e 0.05m m 7–8k V (10–50m m )0.1–0.5l A

1–2l g /c m 2(20–30m m )

B r u n i n x a n d R u d s t a m [45]

S a m p l e s f o r b c o u n t i n g N b 95+C s I ,C s 137+C s I ,C s 137+Z n B r ,C s 137+M e t h y l e n e b l u e ,R u 106+C s I A c e t o n e ,e t h y l e t h e r ,e t h y l a c e t a t e ,e t h a n o l ,m e t h a n o l ,n -b u t y l a l c o h o l G l a s s c a p i l l a r y 0.1–0.5m m ,s t a i n l e s s s t e e l o r P t w i r e 1–10k V (7m m )(500l g )

0.1–0.5l A 50–450l g /c m 2L a u e r a n d V e r d i n g h [46]

N u c l e a r c r o s s s e c t i o n m e a s u r e m e n t s ,n e u t r o n d e t e c t o r s U r a n i u m a c e t a t e ,p l u t o n i u m a c e t a t e ,b o r i c a c i d (H 3B O 3)M e t h y l a l c o h o l S t a i n l e s s s t e e l c a p i l l a r y 0.05–0.08m m i .d .<13k V <10m g /c m 2

M i c h e l s o n [47],S h o r e y a n d M i c h e l s o n [48]

N u c l e a r s p e c t r o s c o p y ,n e u t r o n c r o s s -s e c t i o n s t u d i e s N a C l E t h a n o l ,m e t h a n o l ,i s o p r o p a n o l ,i s o b u t a n o l ,n -b u t a n o l ,a m y l a l c o h o l ,f o r m i c a c i d P y r e x t u b e 3–8k V (10m m )0.3–0.6l A 0.2l m c r y s t a l l i t e s

T e e r a n d D o l e [49]

T h i n ?l m ,(p o l y s t y r e n e l a t e x //A l f o i l )P o l y s t y r e n e l a t e x T o l u e n e ,a c e t o n e +b e n z e n e ,a c e t o n e +c y c l o h e x a n e S t a i n l e s s s t e e l c a p i l l a r y 0.2m m i .d .,0.41m m o .d .9–24k V (220m m )4.8m L /h

M a h o n e y a n d P e r e l [50]S o l a r c e l l s M o l t e n S i (1500°C )10k V

500l A

P a n g e t a l .[51]S o l a r c e l l s M o l t e n S i (1420°C )

10k V 500l A H a l l a n d H e m m i n g [52]P h o t o s e n s i t i v e r e s i s t s C e n t r i f u g a l s p r a y i n g ,

i n d u c t i o n c h a r g i n g 20–30l m

T h u n d a t e t a l .[53]S a m p l e s f o r s c a n n i n g t u n n e l l i n g m i c r o s c o p y

D N A

G l a s s c a p i l l a r y 0.8m m o .d .,p u l l e d t o l m s i z e ,P t w i r e 1.5k V (5m m )(2–6s )(5m m )

v a n Z o m e r e n

e t a l .[28]C a t h o d e s

f o r l i t h i u m b a t t e r i e s

M n (C H 3C O O )2

?4H 2O +L i C l E t h a n o l

S t a i n l e s s s t e e l c a p i l l a r y 0.8m m i .d .8–20k V (n e g .)(5–30m m )

(1–5l m /h g r o w t h r a t e )<50l A 300n m c r y s t a l l i t e s ,(4l m d r o p l e t s )R y u a n d K i m [54]E l e c t r o n i c d e v i c e s (Z n O //S i )(340–400°C )Z i n c t r i ?u o r o a c e t a t e

M e t h a n o l

G l a s s c a p i l l a r y w i t h t u n g s t e n n e e d l e <20k V

20n m c r y s t a l l i t e s

C h e n e t a l .[29,30,55,56]L i t h i u m b a t t e r i e s (L i C o O 2//I T O g l a s s o r s t a i n l e s s s t e e l o r A l )(230–450°C )0.04M C o (N O 3)2?6H 2O +0.04M L i (C H 3C O O )?2H 2O E t h a n o l o r e t h a n o l (15%v o l .)+b u t y l c a r b i t o l (85%)

S t a i n l e s s s t e e l c a p i l l a r y

8–12.5k V (20–60m m )2m L /h

(120m i n )C h e n e t a l .[57]S o l i d e l e c t r o l y t e f o r f u e l c e l l s (250–430°C )Z i c o n i a s t a b i l i s e d b y y t t r i a e t h a n o l +b u t y l c a r b i t o l

8–10k V (30m m )

15l m

D e n i s y u k [58]O p t i c a l l a y e r s

P l a s t i c m a t e r i a l s

D i m e t h y l f o r m a m i d e ,t o l u e n e ,x y l e n e ,b u t y l a c e t a t e ,e t h y l a c e t a t e

P l a s t i c c a p i l l a r y 0.5m m i .d .w i t h m e t a l n e e d l e 20k V 1–10l A 1.5–5l m 270

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T a b l e 1c o n t i n u e d

A u t h o r s

A p p l i c a t i o n (l a y e r //s u b s t r a t e )

S u b s t a n c e o r p r e c u r s o r s p r a y e d S o l v e n t N o z z l e V o l t a g e (e l e c t r o d e d i s t a n c e )F l o w r a t e (g r o w t h r a t e o r d e p o s i t i o n t i m e )

S p r a y c u r r e n t (s p e c i ?c c h a r g e )F i l m t h i c k n e s s (p l u m e b a s e d i a m e t e r )

H o y e r e t a l .[59]

M e m b r a n e s f o r a n a l y t i c a l c h e m i s t r y

C e l l u l o s e a c e t a t e A c e t o n e w i t h m a g n e s i u m p e r c h l o r a t e a d d i t i v e S t a i n l e s s s t e e l c a p i l l a r y 0.14m m i .d .0.3m m o .d .14k V (35m m )1.08m L /h 1–11l A

300n m (3.8m m )

S t e l z e r a n d S c h o o n m a n [60]F u e l c e l l s (t e r b i a -d o p e d y t t r i a -s t a b i l i s e d z i r c o n i a //n i c k e l ,o r s t a i n l e s s s t e e l )(250–500°C )Y t t r i u m -a n d z i r c o n i u m -a c e t y l a c e t o n a t e a n d t e r b i u m -a c e t a t e h y d r a t e E t h a n o l +b u t y l c a r b i t o l (50:50%v o l .,o r 20:80%v o l .)

C h e n e t a l .[61]L i t h i u m b a t t e r i e s ,(L i x M n 2O 4–a s c a t h o d e a n d L i x B P O 4–a s s o l i d e l e c t r o l y t e /A l o r s t a i n l e s s s t e e l )(250°C )0.005M L i (C H 3C O O )?2H 2O +M n (C H 3C O O )2?4H 2O +H 3B O 4+P 2O 5

e t h a n o l o r e t h a n o l +b u t y l c a r b i t o l (+a c e t i c a c i d )

8–12k V (20–30m m )

S o b o t a a n d S o r e n s e n [62];S o r e n s e n [39]

S o l i d l u b r i c a t i n g ?l m s (M o S 2//s i l i c o n )M o S 2(120·1000l m p l a t e l e t s )I s o p r o p a n o l ,A c e t o n e ,a l c o h o l o r t o l u e n e

A n n u l a r -s l i t n o z z l e 10l m g a p ,s t a i n l e s s s t e e l c a p i l l a r y 0.9m m i .d .1.2m m o .d .3–20k V (180m m t o s u b s t r a t e )2.4m L /h (25m i n )(0.5–2.5m A /c m 2)0.28–1l m

T e n g e t a l .[63]

C e r a m i c ?l m s ,m e m b r a n e s ,(Z r O 2//s i l i c o n e r e l e a s e p a p e r )Z r O 2,(0.2l m s u s p e n s i o n )B u t y l a c e t a t e +e t h a n o l S t a i n l e s s s t e e l c a p i l l a r y 0.26m m i .d .0.51m m o .d .3–10k V (8m m t o e x t r a c t o r r i n g o f 10m m d i a .)0.6–45m L /h (0.2g /h d e p o s i t i o n r a t e )<10l m d r o p l e t s :(2–15l m d o n u t -l i k e r e l i c s )

C h e n e t a l .[64]

O p t o e l e c t r o n i c d e v i c e s (T i O 2),l i t h i u m b a t t e r i e s (L i M n 2O 4//A l ,P t o r I T O g l a s s )(250°C )0.005M T i (i -C 3H 7O )4,0.005M L i (C H 3C O O )?2H 2O +0.01M M n (C H 3C O O )2?4H 2O E t h a n o l o r e t h a n o l +b u t y l c a r b i t o l (+a c e t i c a c i d )10k V 1.2m L /h 1l m p a r t i c l e s

C h o y e t a l .[65]

F u e l c e l l s ,m u l t i l a y e r (L a (S r )M n O 3//Z r (Y )O x //N i -Z r O 2)(400–550°C )1.2m L /h (60–300l m /h g r o w t h r a t e )

5–20l m

C i c h e t a l .[66]

P l a s m a d i s p l a y p a n e l s o f (Z n ,M n )2S i O 2//S i (111))

M n (C H 3C O O )2+Z n (C H 3C O O )2+t e t r a e t h y l o r t h o s i l i c a t e E t h a n o l 11–17k V 0.75–4.5m L /h

0.75–1l m

G o u r a r i e t a l .[67,68]H 2g a s s e n s o r s ,(S n O 2o r S n O 2:M n 2O 3//A l o r a l u m i n a p e l l e t s )(400°C )0.01–0.1M S n C l 4?5H 2O +M n (C H 3C O O )2?4H 2O E t h a n o l

10k V 1.2–9.3m L /h 1–10l m g r a i n s

H e i n e e t a l .[69]Q u a n t u m d o t s f o r p h o t o -l u m i n e s c e n c e (Z n S c o a t e d C d S e //g l a s s )(1–4m g /C d S e /m L )(100–300°C )

C d S e (i n Z n S m a t r i x )A c e t o n i t r i l e +p y r i d i n e (2:1)

3.7–4k V 0.6–1.8m L /h

0.5–1l m (20–40n m g r a i n s w i t h 2.8n m C d S e n a n o c r y s t a l s )

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123

T a b l e 1c o n t i n u e d

A u t h o r s

A p p l i c a t i o n (l a y e r //s u b s t r a t e )

S u b s t a n c e o r p r e c u r s o r s p r a y e d S o l v e n t N o z z l e V o l t a g e (e l e c t r o d e d i s t a n c e )F l o w r a t e (g r o w t h r a t e o r d e p o s i t i o n t i m e )

S p r a y c u r r e n t (s p e c i ?c c h a r g e )F i l m t h i c k n e s s (p l u m e b a s e d i a m e t e r )

N i s h i z a w a e t a l .[70]

L i t h i u m b a t t e r i e s (L i M n 2O 4//P t ?l m //S i O 2//S i )(400°C )0.025M L i N O 3+0.05M M n (N O 3)2E t h a n o l S t a i n l e s s s t e e l c a p i l l a r y 0.8m m i .d

12k V (25m m )2m L /h (30m i n )

0.5l m

C h e n e t a l .[31]

C e r a m i c ?l m s (Z n O ,Z r O 2,A l 2O 3,C o O //A l )(100–250°C )

0.05M C o (N O 3)2?6H 2O ,o r S n C l 4?5H 20+Z n (C H 3C O O )2?2H 2O E t h a n o l +b u t y l c a r b i t o l o r e t h a n o l S t a i n l e s s s t e e l c a p i l l a r y 0.4m m i .d .0.6m m o .d .o r 0.6m m i .d .0.8m m o .d .(?a t o r t i l t e d )8.8k V (20m m )0.8m L /h o r 1.5m L /h (60m i n )

100n m (p a r t i c l e s i z e )

C h e n e t a l .[71]

S o l a r c e l l s (T i O 2//A l ,P t o r I T O g l a s s )(100–220°C )0.01M T i (O C H (C H 3)2)4E t h a n o l

8–15k V (30m m )

0.08m L /h

1l m p a r t i c l e s (20–30n m c r y s t a l l i t e s )C h o y a n d S u [42]

O p t o e l e c t r o n i c d e v i c e s ,s o l a r c e l l s (T i O 2//S n O 2c o a t e d g l a s s )(350–550°C )T i t a n i u m d i i s o p r o p o x i d e b i s (2,4-p e n t a m e d i o n a t e 2-p r o p a n o l (20m i n )

0.5l m (10–60n m c r y s t a l l i t e s )

M i a o e t a l .[72–75],B a l a c h a n d r a n e t a l .[76]

C e r a m i c ?l m s ,m e m b r a n e s ,(Z r O 2,S i C //S i o r C r F e a l l o y )

Z r O 2,S i C (s u s p e n s i o n )E t h a n o l +w a t e r m i x t u r e (1:1b y v o l .)S t a i n l e s s s t e e l c a p i l l a r y 0.23m m i .d .0.51m m o .d .4–8k V (8m m t o e x t r a c t o r r i n g o f 15m m d i a .)

0.4–1.8m L /h (2m i n )0.2l A (Z r O 2)0.12l A (S i C )0.61C /k g (Z r O 2)0.31C /k g (S i C )<10l m d r o p l e t s :(4–8l m Z r O 2)(3.5–5.5l m S i C )M o e r m a n e t a l .[77]C h e m i c a l a n a l y s i s b y ?u o r e s c e n c e d e t e c t i o n R h o d a m i n e G l y c o l +w a t e r m i x t u r e (0.7/0.3b y v o l u m e )0.1m m i .d .0.2m m o .d .1.2k V (0.3–0.4m m )

0.072n l /h –1.08m L /h (200l m d o t s )

S u a n d C h o y [78]

(S i O 2//g l a s s o r S i w a f e r )(80–120°C )S i l i c a s o l i n H 2O (0.2–1g S i O 2/l )13l m p a r t i c l e s E t h a n o l (3–8k V /c m )5–10m L /h

T u r e t s k y [79]

S e m i c o n d u c t o r ?l m s (C d S ,Z n S )

C d S ,Z n S W a t e r -e t h a n o l m i x t u r e 3–8k V

1–100l m (m o n o d i s p e r s e d r o p l e t s )Y a m a d a e t a l .[80]L i t h i u m b a t t e r i e s ,(L i N i O 2,o r L i C o 0.5N i 0.5O 2,o r L i A l 0.25N i 0.75O 2//A u )(450°C )

0.025M L i (C H 3C O O )?2H 2O +N i (C H 3C O O )2?4H 2O +{C o (N O 3)2?6H 2O o r A l (N O 3)3?9H 2O }S t a i n l e s s s t e e l c a p i l l a r y 0.3m m i .d 0.5m m o .d 11k V (55m m )3.9m L /h (60–360m i n )(1.6l m /h g r o w t h r a t e )

1.6–10.3l m (L i N i O 2)

K i m e t a l .[37,38]P l a s m a d i s p l a y s ,(M g O //g l a s s o r S i O 2/S i )(400–500°C )

M g (C 11H 19O 2)2

E t h a n o l (90%)+a c e t i c a c i d (10%)o r t e t r a h y d r o f u r a n +1-b u t y l a l c o h o l

S t a i n l e s s s t e e l c a p i l l a r y 0.4m m i .d .0.7m m o .d .13k V (g l a s s )14.5k V (S i O 2/S i )(55m m t o r i n g e x t r a c t o r )7.6m L /h (g l a s s )16m L /h (S i O 2/S i )(0.2–0.53l m /h g r o w t h r a t e )(60m i n )

0.5l m (0.1–0.5l m p a r t i c l e s i z e )

R e i f a r t h e t a l .[81]N e u t r o n d e t e c t o r s (34S //A l )

34

S (94.3%p u r i t y )

T o l u e n e 9k V (10m m )

0.5–2.2m g /c m 2

272

J Mater Sci (2007)42:266–297

123

T a b l e 1c o n t i n u e d

A u t h o r s

A p p l i c a t i o n (l a y e r //s u b s t r a t e )

S u b s t a n c e o r p r e c u r s o r s p r a y e d S o l v e n t N o z z l e V o l t a g e (e l e c t r o d e d i s t a n c e )F l o w r a t e (g r o w t h r a t e o r d e p o s i t i o n t i m e )S p r a y c u r r e n t (s p e c i ?c c h a r g e )F i l m t h i c k n e s s (p l u m e b a s e d i a m e t e r )

S u a n d C h o y [82,83];C h o y a n d S u [84];S u e t a l .[85]O p t o e l e c t r o n i c d e v i c e s ,s o l a r c e l l s ,(C d S e o r C d S //I T O c o a t e d g l a s s )(200–450°C )0.005M C d C l 2+(N H 2)2C S e o r 0.005M C d C l 2+(N H 2)2C S

E t h a n o l +w a t e r U l t r a s o n i c n e b u l i s e r (1.7M H z )5–10k V (C d S e )4–20k V (C d S )10–30m L /h (2–5m i n C d S e )(10–60m i n C d S )(3l m /h g r o w t h r a t e )10–35n m (C d S

c r y s t a l l i t e s s i z e )(100–200n m C

d S

e g r a i n s i z e )(80–200n m C d S g r a i n s i z e )

Z a o u k e t a l .[86,87]

O p t o e l e c t r o n i c d e v i c e s ,(S n O 2:F //g l a s s )(500–550°C )0.25M S n C l 4

?5H 2O +0.2M H F

E t h a n o l

10.5k V (60m m )13.2m L /h (1.44l m /h g r o w t h r a t e )C h a n d r a s e k h a r a n d C h o y [88];R a j a n d C h o y [89]I T O ?l m s ,(S n :I n 2O 3//g l a s s )(200–550°C )

0.05M t i n t e t r a c h l o r i d e +i n d i u m c h l o r i d e

E t h a n o l

5–20k V (5–45m i n )

0.5l m (15–55n m c r y s t a l l i t e s ,250–600n m g r a i n s )C h a n d r a s e k h a r a n d C h o y [90]

O p t o e l e c t r o n i c d e v i c e s ,s o l a r c e l l s (S n O 2:F //g l a s s )(350–600°C )0.05M S n (C H 3C O O )+h y d r o ?u o r i c a c i d M e t h a n o l 5–15k V (20–50m m )

20–40m L /h (60m i n )

0.5l m (250–400n m g r a i n s i z e )C h o y [91];W e i a n d C h o y [92]

L i g h t e m i t t i n g d i o d e s (Z n S //g l a s s o r S i (100))(450–550°C )

0.01M Z n C l 2+(N H 2)2C S w a t e r 4–25k V

10–30m L /h (6l m /h d e p o s i t i o n r a t e )

(20n m c r y s t a l l i t e s a t 450°C );(80–200n m c r y s t a l l i t e s a t 500°C )D i a g n e a n d L u m b r e r a s [93]C O 2g a s s e n s o r s ,(S n O 2+L a O C l //S i w a f e r o r A l p e l l e t s )(400°C )0.1M S n C l 4?5H 2O +L a C l 3?6H 2O e t h a n o l

10k V (30m m )

0.7m L /h

10–25l m g r a i n s

L a p h a m e t a l .[32]E l e c t r o c a t a l y s t ,(N i C o 2O 4//a l u m i n a p a r t i c l e s )(350–400°C )

0.05M N i (N O 3)2?6H 2O +C o (N O 3)2?6H 2O

E t h a n o l (20%v o l .)+d i (e t h y l e n e g l y c o l )b u t y l e t h e r (80%)(30m m )

0.55m L /h (120m i n )62l m 0.5l m (p a r t i c l e s )

M o e r m a n e t a l .[94,95]B i o l o g i c a l l y a c t i v e m i c r o m e t e r s p o t s (50n m s i l i c o n e n i t r i d e //S i w a f e r )E n z y m e s ,a n t i b o d i e s ,r h o d a m i n e

W a t e r ,e t h y l e n e g l y c o l +w a t e r m i x t u r e

S t a i n l e s s s t e e l c a p i l l a r y 0.06m m i .d .,0.16m m o .d .

0.95–1.8k V (0.22–0.4m m )

0.36–4l l /h 0.5l A

(130–315l m s p o t s )

M o h a m e d i e t a l .[96]A n o d e s f o r l i t h i u m b a t t e r i e s ,(S n O 2//N i )(400°C )0.05M S n C l 4?5H 2O

E t h a n o l

S t a i n l e s s s t e e l c a p i l l a r y 0.8m m i .d .

12k V (25o r 40m m )2m L /h (30m i n )

1l m (p a r t i c l e s )

M o h a m e d i e t a l .[97–99]

L i t h i u m b a t t e r i e s ,(L i M n 2O 4//A u )(400°C )0.025M L i N O 3+0.05M M n (N O 3)2

E t h a n o l

S t a i n l e s s s t e e l c a p i l l a r y 0.8m m i .d .

12k V (25m m )2m L /h (3–30m i n )

0.1–1l m

N g u y e n a n d D j u r a d o [100]

F u e l c e l l s ,(Z r O 2:Y 2O 3//s t a i n l e s s s t e e l )(300–400°C )

0.05M z i r c o n i u m a c e t y l a c e t o n a t e +0.05M y t t r i u m a c e t y l a c e t o n a t e

E t h a n o l (14%v o l .)+b u t y l c a r b i t o l (56%)+a c e t i c a c i d (30%)

14–20k V (30m m )

2–3.9m L /h (6–120m i n )

0.5l m (a m o r p h o u s )(7.8n m c r y s t a l l i t e s f o r 3.9m L /h )

J Mater Sci (2007)42:266–297

273

123

T a b l e 1c o n t i n u e d

A u t h o r s

A p p l i c a t i o n (l a y e r //s u b s t r a t e )

S u b s t a n c e o r p r e c u r s o r s p r a y e d S o l v e n t N o z z l e V o l t a g e (e l e c t r o d e d i s t a n c e )F l o w r a t e (g r o w t h r a t e o r d e p o s i t i o n t i m e )

S p r a y c u r r e n t (s p e c i ?c c h a r g e )F i l m t h i c k n e s s (p l u m e b a s e d i a m e t e r )

R h e e e t a l .[101]

B u f f e r l a y e r f o r s u p e r c o n d u c t o r s o r f e r r o e l e c t r i c ?l m s (M g O //S i )(265–400o

C )0.1o r 0.2M M g (C H 3C O O )2E t h a n o l 10–17k V (50m m )

0.36–3.6m L /h (4l m /h g r o w t h r a t e )(1/3o f R a y l e i g h l i m i t )0.3–1l m (5–20n m r o u g h n e s s )

Y o o n e t a l .[102]

L i t h i u m b a t t e r i e s ,(L i C o O 2//P t c o a t e d a l u m i n a )(300°C )

0.04M L i N O 3+0.04M C o (N O 3)2?6H 2O E t h a n o l 10–15k V (40m m )

2m L /h (15–120m i n )(0.67l m /h g r o w t h r a t e )0.1–0.7m g /c m 2

C a o a n d P r a k a s h [103]

L i t h i u m b a t t e r i e s ,(L i M n 2O 3//A l o r S n O 2

c o a t e

d g l a s s )(300°C )0.006M M g (C H 3C O O )2?4H 2O +0.003M L i (C H 3C O O )2?2H 2O 10k V (30m m )

2m L /h 10l m

J a y a s i n g h e e t a l .[104,105];J a y a s i n g h e a n d E d i r i s i n g h e [106–108]C e r a m i c ?l m s (r e p r o g r a p h i c t e c h n i q u e f o r m i c r o e n g i n e e r i n g )c e r a m i c f o a m A l u m i n a (20%b y v o l )(0.5l m p a r t i c l e s )E t h a n o l S t a i n l e s s s t e e l c a p i l l a r y 0.2m m i .d .,0.48m m o .d .5–12k V (6o r 8m m t o e x t r a c t o r )0.3o r 6m L /h (1h s p r a y t i m e f o r f o a m p r o d u c t i o n )0.053–0.059l A (30–60l m d r o p l e t s r e l i c s )

K o b a y a s h i e t a l .[109]L i t h i u m b a t t e r i e s (L i M n 2O 4//(L i ,L a )T i O 3)(400°C )0.03M L i N O 3+0.05M M n (N O 3)2?6H 2O E t h a n o l S t a i n l e s s s t e e l c a p i l l a r y ,0.5m m i .d .

9k V (25m m )

2m L /h

(40m m )

D o k k o e t a l .[110,111]L i t h i u m b a t t e r i e s ,(L i C o x M n 2-x O 4//A u )0.025M L i N O 3+0.05M {C o (N O 3)2+M n (N O 3)2}

E t h a n o l 12k V

(15m i n )0.5l m

H u a n g e t a l .[112]D y n a m i c R A M ,p i e z o e l e c t r o n i c d e v i c e s (P b T i O 3//n -S i )(150o C )P b (O A c )2?3H 2O +[C H 3(C H 2)3O ]4T i 2-e t h o x y e t h a n o l Q u a r t z t u b e 0.4m m 12k V (25m m )2.7m L /h (20–40m i n )0.35l m (45n m g r a i n s )

K i m e t a l .[113]L i t h i u m b a t t e r i e s (V 2O 5//P t )(200o C )0.05M [(C H 3)2C H O ]3V O E t h a n o l (40m m )

2m L /h (60m i n )L u e t a l .[114]P i e z o e l e c t r i c m i c r o a c t u a t o r f o r M E M S (P b (Z r ,T i )O 3//S i O 2//S i )(25o r 100o C )Z r (C 3H 7O )4+T i ((C H 3)2C H O )4+P (C H 3C O O )2

4.5k V 0.6m L /h

2l m (150–200n m c r y s t a l l i t e s )

T a n i g u c h i e t a l .[33,34]F u e l c e l l s (G d x C e 1-x O 2o r L a 1-x S r x C o 1-y F e y O 3//s t a i n l e s s s t e e l o r G d C e O p e l l e t s )(230–350o C )G d (N O 3)3?6H 2O +C e (N O 3)3?6H 2O o r L a (N O 3)3?6H 2O +G a (N O 3)2?5H 2O +S r C l 2?6H 2O +M g (N O 3)2?6H 2O E t h a n o l +b u t y l c a r b i t o l (0.2/0.8o r 0.33/0.67b y v o l u m e )S t a i n l e s s s t e e l c a p i l l a r y 0.4m m i .d .0.6m m o .d .4.1–5.5k V o r 5.9–6.5k V (15m m )0.5–2.5m L /h (15–240m i n )(14.9m m )

S h u e t a l .[115,116];C h u n g e t a l .[117]L i t h i u m b a t t e r i e s (L i M n 2O 4//P t c o a t e d q u a r t z )0.025M L i (C H 3C O O )?2H 2O +0.05M M n (N O 3)2?4H 2O

E t h a n o l S t a i n l e s s s t e e l c a p i l l a r y 2m L /h (60m i n )11.9l g /c m 2;0.7–1l m

274

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123

T a b l e 1c o n t i n u e d

A u t h o r s

A p p l i c a t i o n (l a y e r //s u b s t r a t e )

S u b s t a n c e o r p r e c u r s o r s p r a y e d S o l v e n t N o z z l e V o l t a g e (e l e c t r o d e d i s t a n c e )F l o w r a t e (g r o w t h r a t e o r d e p o s i t i o n t i m e )S p r a y c u r r e n t (s p e c i ?c c h a r g e )F i l m t h i c k n e s s (p l u m e b a s e d i a m e t e r )

H o w a n d C h o y [118]A n t i c o r r o s i o n ?l m ,(T i O 2//g l a s s ,o r s t a i n l e s s s t e e l )(300–600°C )0.05–0.5M t i t a n i u m d i i s o p r o p o x i d e b i s (2,4-p e n t a m e d i o n a t e 2-p r o p a n o l 4l m (200–500n m g r a i n s )

J a y a s i n g h e e t a l .[119]C e r a m i c ?l m s ,e l e c t r o s t a t i c p r i n t i n g (S i O 2//q u a r t z g l a s s )

S i l i c a (17%b y v o l .)(20n m p a r t i c l e s )E t h y l e n e g l y c o l S t a i n l e s s s t e e l c a p i l l a r y 0.2m m i .d .,0.5m m o .d .

10k V (10m m t o r i n g e x t r a c t o r )36m L /h

(0.5–20l m d r o p l e t s ,1–80l m d r o p l e t s r e l i c s )

K e s s i c k e t a l .[41](C a r b o x y m e t h y l c e l l u l o s e //S i o r p o l y c a r b o n a t e )

C a r b o x y m e t h y l -c e l l u l o s e (0.01%s o l u t i o n )M e O H +

w a t e r (0.5/0.5b y v o l .)G l a s s c a p i l l a r y 0.02m m i .d .5–7.5k V a c /60H z (30m m )0.12m L /h

K i m e t a l .[120]

C a p a c i t o r s (R u O //P t //S i o r R u O //P t //q u a r t z )(200°C )R u C l 3?x H 2O

E t h a n o l +b u t y l c a r b i t o l (0.2/0.8b y v o l u m e )13l g /c m 2

M a t s u s h i m a e t a l .[121]G a s s e n s o r s (S n O 2//p y r e x g l a s s s u b s t r a t e )

S n C l 2(500o C )E t h a n o l

10k V (50m m )

1m L /h (30–180m i n )(0.4l m /h g r o w t h r a t e )0.2–1l m (30–160n m p a r t i c l e s i z e )M o r o t a e t a l .[122](P o l y (e t h y l e n e o x i d e )//A l )(25°C )P o l y (e t h y l e n e o x i d e )50–70g /L

W a t e r +(e t h a n o l o r m e t h a n o l o r 1-p r o p a n o l )+h e x a n o l +C a C l 2

G l a s s c a p i l l a r y 0.05m m t i p ,P t 1.15m m i o n i n j e c t o r

3–7k V (70m m )

0.017–0.63l l /h (1m i n )

0.3–0.4m A S a f e t a l .[123]

E l e c t r o s t a t i c p r i n t i n g (p o l y m e r //I T O //g l a s s )O l i g o (m e t h y l -m e t h a c r i l a t e )n =9

p o l y m e r i s a t i o n d e g r e e A c e t o n e (94%b y v o l .)+T H F (5%)+w a t e r (1%)

S t a i n l e s s s t e e l c a p i l l a r y

2.7k V (t o e x t r a c t o r c y l i n d e r ),5.1k V (t o g r o u n d e d s u b s t r a t e )

0.18m L /h (0.08–0.15l g /m i n g r o w t h r a t e )30–130n m (1.3n m r o u g h n e s s ),0.1m m s p o t S a n d e r s e t a l .[124]F u e l c e l l s (N a ?o n //P T F E )N a ?o n (5%b y w t .)A l i p h a t i c a l c o h o l +w a t e r 20k V (70m m )2m L /h

50–60l m

S i e b e r s e t a l .[125];L e e u w e n b o u r g h e t a l .[126]E n d o s s e o u s i m p l a n t s (C a P c o a t i n g //T i A l 6V 4a l l o y )(300o C )0.0031M C a (N O 3)2?4H 2O +0.0019M H 3P O 4E t h a n o l o r b u t y l c a r b i t o l

S t a i n l e s s s t e e l c a p i l l a r y 1.0m m i .d 1.4m m o .d .

5.5–8k V (15o r 20m m )1o r 2m L /h (60m i n )

0.5–2l m

U e m a t s u e t a l .[127]B i o t e c h n o l o g y (a -l a c t a l b u m i n a //A l //

p o l y (e t h y l e n e t e r e p h t h a l a t e ))(25°C )a -l a c t a l b u m i n a (0.4–1.8l g /m L )W a t e r G l a s s c a p i l l a r y 0.05m m t i p ,P t 1.15m m i o n i n j e c t o r 3.5k V (30–40m m )(i o n c o l l i m a t o r a t 100V t o s u b s t r a t e )

(10m i n )300–700l m

J a y a s i n g h e a n d E d i r i s i n g h e [128]E l e c t r o s t a t i c p r i n t i n g (a l u m i n a //p o l y e s t e r )A l u m i n a (21%b y v o l .)(500n m p a r t i c l e s )E t h a n o l

S t a i n l e s s s t e e l c a p i l l a r y 0.2m m i .d .,0.48m m o .d .5–13k V (6m m t o c o u n t e r n e e d l e )

0.0036–0.036m L /h

(1–35l m r e l i c s ,4–8l m m e a n )

P e r e d n i s e t a l .[35]F u e l c e l l s (Z r O 2:Y 2O 3//N i 72C r 16F e 8–s u b s t r a t e d i s k 35m m d i a .,75l m t h i c k )(185–350°C )0.17o r 0.085M Z r (C 5H 7O 2)4o r Z r (C 3H 7O )4o r Z r O (N O 3)2?x H 2O o r Z r C l 4+0.03o r 0.015M Y C l 3?6H 2O o r Y (N O 3)3?6H 2O

E t h a n o l +b u t y l c a r b i t o l (1:1v o l .)o r e t h a n o l +1-m e t h o x y -2-p r o p a n o l

0.6m m i .d .

7–30k V (60m m )

1.4–5.6m L /h (60–120m i n )

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(EDX),X-ray diffractometry(XRD),X-ray photo-electron spectroscopy(XPS),Fourier-transform infra-red spectroscopy(FTIS),atomic force microscopy (AFM),transmission electron microscopy(TEM),or wavelength dispersion scanning(WDS).

Nuclear instruments

Initially,in1950s and1960s,the electrospray was used to produce thin?lms for nuclear-physics instruments. Carswell and Milsted[43]prepared layers for nuclear collision cross-section studies.The material to be deposited(U233,Pu238,Am241or Cm242nitrates)was dissolved in acetone and electrosprayed using a glass capillary with an ion injector made of a thin wire, which was connected to a high voltage dc source.The droplets evaporated without coalescence when they ?owed to the substrate,and the material was deposited as a thin?lm.The layer was?at and of sharp edges.

This method was adopted for radioactive instru-ments production by Gorodinsky et al.[44].The base of the spray plume was controlled by the variation of the distance between the capillary tip and the sub-strate.The differences in the layer thickness between the centre and the circumference of the deposit were lower than20%.Bruninx and Rudstam[45]used electrospray to deposit thin uniform?lms for b counting.They tested over40substances dissolved in acetone,ethyl ether,ethyl acetate,ethanol,methanol or n-buthyl alcohol.The grain size was lower than 1l m,and the samples were more even than those prepared by other methods.Michelson and Richardson [130]also prepared a b-source by this 618a4d2eed630b1c59eeb560uer and Verdingh[46],and Reifarth et al.[81]produced neutron detectors using electrospray.The layer was perfectly stable,adherent to the substrate,and of homogeneity of the order of1%[46].

Michelson[47],and Shorey and Michelson[48] produced neutron emitters from Uranium,Plutonium or Boron by using electrospray.Distilled water,formic acid,ethanol,methanol,isobutanol,n-butanol,ethyl-ene dichloride,amyl alcohol,n-propanol,and isopro-panol were tested as solvents.The?lm prepared by electrospraying consisted of?ne(200nm)uniformly sized,randomly oriented crystals of structure better than that obtained by vacuum evaporation method [130].This technique was particularly attractive be-cause the loss of the solute was small and the substrate was not overheated.Shorey and Michelson[48]noted that the spray system usually operated in the cone-jet mode for all the liquids tested.However,for liquids of dielectric constant lower than about10,the multijet mode with two or three cones was observed.

A main advantage of preparation of thin radioactive sources by electrospraying is that the substance is not damaged,as it could be when chemical vapour or plasma spraying deposition would be used.The?lm thickness could be adjusted between1l g/cm2and 10mg/cm2(cf.Table1).

Solar cells

Since1980s,the electrospray was tested as a new tool for preparation of thin?lms for solar cells.Mahoney and Perel[50],and Pang et al.[51]made the solar cells from silicone melted at a temperature of1500°C,in a graphite vessel to avoid material contamination.The advantage of the electrospray was that the?lm was of lower porosity than that obtained by another method.

Since late1990s the electrospray was used for production of solar cells from TiO2[42,71],CdS[82, 84,85],CdSe[83],or SnO2[90].Films produced by electrospray were homogeneous and composed of agglomerates built of particles smaller than1l m, which were the dry powder particles used for preparing a suspension.The structure and optical properties of the CdSe?lms depend on the temperature and depo-sition method.For the electrospraying,the CdSe star-ted to crystallise at the substrate temperature of300°C and the number of crystals increased rapidly above 400°C[82,131].For the?lms deposited at a temper-ature lower than350°C the CdSe grain size was below 100nm,while for those deposited above400°C,was larger than200nm[83].The CdS?lm deposited at a temperature of250°C was crystalline of hexagonal structure[84]but at300°C,the?lm was less crystal-lised with some pinholes on the surface.The?lm be-came smooth and well crystallised as the deposition temperature increased but at450°C large particles could be formed on the smooth surface.The ZnS?lms deposited at450°C contained crystallites with very small grain size of20nm,but at the substrate tem-perature of500°C the grain size increased rapidly to about80–200nm[91].

The CdS and ZnS?lms were highly oriented in the electric?eld,but no preferred orientation was observed for the CdSe?lms.Due to the grain orien-tation of CdS and ZnS?lms in the electric?eld during precursors transformation to nanocrystals,they had good optical and opto-electrical properties for solar cell applications[85].

Fuel cells

Interest in production of electrodes and solid electro-lytes for fuel cells by the electrospray deposition has

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grown in recent years.The materials used for fuel cells fabrication were reviewed by Jiang and Chan[132]. Different thin-?lm deposition methods,such as CVD, PVD,RF sputtering,laser deposition,and casting,used for fuel cells production were presented by Will et al. [133].However,the electrospray method has not been mentioned in these papers.

Electrostatic spray deposition was used to produce solid electrolyte for fuel cells from yttria-stabilised zir-conia[35,57,65,134],terbia-doped yttria-stabilised zirconia[60],barium cerate(BaCeO3)[134],or gad-olinia-doped ceria[34].The details of the electrospray methods are cited in Table1.The electrospray systems used for thin?lm deposition usually operated in a con?guration with capillary facing upwards.The obtained?lms were amorphous or composed of nano-crystallites but could be annealed at higher tempera-tures to obtain the perovskite structure.For example, yttria-stabilised zirconia can be produced directly in the process of spraying,whereas barium cerate layer needs to be annealed[134].Choy et al.[65]found that for the substrate temperatures lower than500°C,the electro-spray-produced?lm of yttria-stabilised zirconia was amorphous or nanocrystalline,but after2h annealing at800°C the structure was changed to perovskite. However,at temperatures higher than650°C the perovskite could be obtained in one step that is an important advantage of the ESD.Nguyen and Djurado [100]observed that?lms of ZrO2+Y2O3(yttria-doped-zirconia)were cracked with reticular outer layer when deposited at lower temperatures(300°C).When the temperature was increased to350°C,cracks were formed in lower layer,and agglomerated particles have grown on its top.The cracks were attributed to the thermal stresses during the drying process.At375°C the cracks disappeared,and agglomerates were growing on the top of a dense layer.Taniguchi et al.[33,34] concluded that the quality of the electrosprayed?lm for fuel cells was comparable or better than that obtained by laser-assisted deposition,magnetron sputtering, CVD,microwave plasma coating,or?ame assisted vapour deposition.

Perednis et al.[35]compared the ESD with pres-surised spray deposition technique.The morphology of the layers produced by ESD were dependent on the substrate temperature,precursor concentration and deposition time,whereas those prepared by the pres-surised spray deposition not.The dependence of the ESD process on the substrate temperature was attrib-uted to the solvent evaporation from the droplets approaching the 618a4d2eed630b1c59eeb560ing an infrared pyrome-ter,the authors discovered that the gas temperature increased rapidly at the distance of10mm from the substrate,but for pressurised spray deposition this distance was only5mm due to the cooling effect of the nebulising gas.Additionally,the electrospray droplets were smaller than those produced by pressurised neb-uliser that made evaporation faster.They also noticed that for higher precursor?ow rates a porous?lm could be obtained when ESD technique is used.The?lm morphology in the pressurised spray deposition was less sensitive to the?ow rate.The pressurised spray deposition could,therefore,be used for large area coating and for aqueous solution spraying but for precise coating of small areas the ESD is better.

Proton conducting membranes made of a polymer for fuel cells applications were fabricated by Sanders et al.[124].The?lm was formed on a rotating PTFE-coated mandrel by coalescence of deposited droplets. The?lm thickness was approximately50–60l m.Naf-ion membranes produced by electrospray absorbed 15%by weight more water and exhibit diffusion coef?cients higher by30–70%compared to commer-cial membranes.

Lithium batteries

Van Zomeren et al.[28]were probably the?rst who used electrospray to produce electrodes for alkaline batteries.The spray system operated in the cone-jet mode,generating droplets of mean diameter of4l m. The?lm prepared by this method was of low porosity, with crystallites of average size of0.3l m.Interest in production of electrodes for lithium microbatteries by ESD deposition has grown in recent years.The demand for production of microbatteries is an effect of miniaturisation of electronic devices and a reduction of their power consumption.

Deposition of thin?lm of LiMn2O4as a cathode for lithium ion batteries is usually accomplished by spraying a mixture of lithium acetate or nitrate,and manganese nitrate,as the precursors,which are dis-solved in ethanol[70,97–99,109,115–117].For the production of LiCoO2cathodes,Chen et al.[29,30,55, 56],and Yoon et al.[102]used lithium acetate and cobalt nitrate dissolved in ethanol or ethanol and butyl carbitol mixture.The process took place at the ambient atmosphere and at a temperature in the range between 250and400°C.The?lm deposited by the ESD showed very stable charging/discharging characteristics.Dokko et al.[110,111]produced LiCo x Mn2–x O4cathodes from lithium,cobalt,and manganese nitrates dissolved in ethanol.Yamada et al.[80]tested LiNiO2,LiCo0.5-Ni0.5O2,and LiAl0.25Ni0.75O2cathodes produced from lithium and nickel acetates,and cobalt and aluminium nitrates dissolved in ethanol.The cyclic voltammetry

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and charge-discharge tests indicated that the?lms produced by ESD were electrochemically active for lithium ion extraction/insertion,similarly to conven-tionally fabricated cathodes.The substrate tempera-ture for these processes is usually in the range of 250–500°C.

Cao and Prakash[103]used the ESD to fabricate LiMn2O4spinel thin?lms as a cathode for lithium batteries.The?lms showed similar electrochemical cycling pro?le as porous laminate?lms that could indicate structural similarity between them.The authors noticed that an organic binder and a carbon conductive additive,which could be found in a lami-nate?lm,were not present within the?lm prepared by ESD.These inclusions negatively affect the charging/ discharging processes of a battery.Additionally,the size of the particles forming the layer was lower when the electrostatic deposition was used.The authors observed that the time constant of the diffusion processes is one order of magnitude larger for the?lm deposited by the electrostatic method than for the porous laminate?lm.

Two-layer structure comprising a cathode(Li x M-n2O4)and solid electrolyte(Li x BPO4)was prepared from lithium acetate and manganese acetate precursors by Chen et al.[61].Kim et al.[113]tested vanadium pentoxide(V2O5)cathodes prepared by spraying of triisopropoxyvanadium oxide precursor dissolved in ethanol.The?lm exhibited a stable charge/discharge capacity during at least25cycles.Porous morphology of the?lm was bene?cial for the lithium intercalation/ de-intercalation from the positive electrode.The V2O5?lm prepared by ESD method became crystalline at a temperature of225°C.This temperature was lower than that required by rf sputtering,pulsed laser depo-sition or electron beam evaporation processes,carried out at a temperature above300°C[113].

Anodes for rechargeable lithium-ion batteries made of SnO2were proposed and tested by Mohamedi et al.

[96].The?lm was prepared by the ESD at a temper-ature of400°C,followed by the layer annealing at 500°C.The ESD deposited?lm was cycled electro-chemically versus lithium and showed very good reversibility even at high current densities.

Micro-and nanoelectronic devices

In late1990s,many authors have demonstrated the feasibility of atomising ceramic suspensions by elec-trostatic methods to produce thin isolation or semi-conductor?lms,mainly for the purposes of electronic devices manufacturing.Metal oxide?lms,were made of,for example,TiO2[42,64,71,91,118,135],ZrO2[31,63,72–75,135],ZnO[31,54],MgO[37,38,101], SnO2[67,68,86,87,90,96,121],CoO[31],PbTiO3 [112],BaZrO3[136],or alumina[31,104–108,112,119] (cf.Table1).SiO2and ITO coatings of glass were made by Su and Choy[78],Chandrasekhar and Choy [88,90],and Raj and Choy[89].Semiconductor?lms made of CdSe[83,131],SiC[72,73,75,76,137],CdS or ZnS[79,82,84,91,131,138]were also prepared by electrospraying(cf.Table1).Silicon and glass were most frequently used as the substrate in these experi-ments.The oxide layers were usually produced from ?ne metal-oxide powders,which after its deposition on a substrate were sintered to form a thin?lm.

TiO2is used for electronic devices manufacturing, for example,optoelectronic devices,solar cells,inte-grated circuits,as a dielectric in capacitors or?eld-effect transistors,gas sensors;and in materials engi-neering as a catalyst[42,71,91].ZrO2?lms are applied as dielectrics in nanoelectronics due to a high dielectric constant of the material(e r=23)and relatively high band gap,of about5.8eV.ZrO2is also used for fuel cells production[100],and for corrosion and thermal protection layers[74–76].ZnO exhibits semiconduct-ing,photoconductive,and piezoelectric properties, and,therefore,founded many applications in various ?elds of electronics,for transducers or sensors manu-facturing[54].MgO is used in plasma displays as a surface protection layer,as a buffer layer for high-temperature superconductors deposition,and as perovskite-type ferroelectric?lms[37,38,101].PbTiO3 [112]and BaZrO3[136]are deposited on a substrate as piezoelectric materials in sensors and microactuators.

Ryu and Kim[54]reported that the ZnO?lm deposited on silicon wafer at a temperature ranging from230to450°C was composed of crystallites of mean size of about20nm,uniform,densely packed, pinhole-free,and predominantly c-axis oriented.

SnO2is used as an active layer in gas sensors,for example,for?ammable and toxic gases[67,68],CO2 [93],and H2[67,121].It was also applied in electro-chromic,photoluminescent phosphor,and liquid crys-tal displays,solar cells and heat mirrors[86,87,90]. However,Matsushima et al.[121]found that the sen-sitivity of a SnO2gas sensor produced by electrospray is lower than for those fabricated by an ion-beam sputtering method.This lower sensitivity was attrib-uted to differences in the structure of primary particles formed in both processes,because the ESP process was carried out at relatively low temperature(500°C). They also proved that the same effect took place when the crystallites were too large.The optimum size of the crystallites was found to be of about10nm.The ESP should,therefore,be optimised by decreasing the

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precursor concentration,and improved by raising the substrate temperature.This requires using a quartz glass substrate,which is high-temperature resistant.It was noticed by Gourari et al.[67]that the sensitivity of SnO2gas sensor deposited on an alumina substrate to H2can be increased almost six times with an addition of manganese oxide,with the ratio of SnO2:M-n2O3=10:1.

Recently,Kim et al.[120]using the ESD method, have produced an anhydrous and crystalline ruthenium oxide thin?lm electrodes with high speci?c capacitance (‘supercapacitor’).The speci?c capacitance of anhy-drous ruthenium oxide thin?lm was500F/g.

The electrospray was also used to fabricate more complex structures.Bi-layer ceramic?lms composed of ZrO2and SiC were produced by Miao et al.[75],and multilayer?lms of different materials were deposited on a substrate by Choy et al.[65],and Choy[139].Such ?lms can be easily deposited in consecutive processes by simple changing the precursor solution.The feasi-bility of production of mixed layers composed of crystallites of different materials,like,for example, SnO2+LaOCl[93]or ZrO2+Y2O3[100,139]by using the ESD was also demonstrated.The Y2O3:Eu?lms synthesised by Choy[91,139]were amorphous when deposited at400°C,or of cubic form at550°C.

Fast development of polymer optoelectronic devices requires new,?exible and cheap technologies for their production.Denisyuk[58],and Cich et al.[66]tested the electrospray as a tool for the deposition of optically active layers on a?exible plastic material.The layer made by electrospraying was reproducible and the process was very ef?cient because at least80%of the solution sprayed was deposited onto the substrate.The solution was sprayed from a dielectric nozzle in the axis of which an ion injector(a sharp needle)was placed. The thickness of the layer was between1.5and5l m with nonuniformity lower than3%[58].Cich et al.[66] produced a plasma display active layer deposited on Si(111)substrate.A precursor composed of zinc ace-tate,manganese acetate,and tetraethylorthosilicate dissolved in ethanol was electrosprayed from an upwards-facing glass capillary with a tungsten ion injector.After heating of the deposited?lm,an active layer of manganese doped zinc silicate(ZnMg)SiO4 was obtained.The spraying process took place in an atmosphere of nitrogen and ethanol.The morphology of the layer and photoluminescence intensity were found to be dependent on the deposition conditions such as voltage and?ow rate,and on the droplets’charge.

Heine et al.[69]used electrospray combined with CVD to incorporate photoluminescent CdSe quantum dots into ZnS matrix on a glass substrate.The quantum dots were encapsulated within a ZnS?lm of100nm thickness prepared by CVD method.The CdSe cores and ZnS coating were deposited simultaneously. Additionally,approximately100nm ZnS layer was grown using the electrospray technique,before and after the process of quantum dots forming,in order to encapsulate the CdSe dots in the ZnS matrix.By varying the size of the CdSe nanocrystals,the light could be tuned from blue to red.Films grown at100°C exhibited a hexagonal crystal structure(wurtzite)while those grown at higher temperatures have a cubic structure(zinc blend).At temperatures greater than 200°C,?lms with a larger grain size were produced. Light intensity emitted by?lms grown at250°C was brighter than those deposited at100°C. Biotechnology

In biotechnology,the electrospray was used,for example,to deposit biomolecules on a substrate for further studying them under a microscope or for medical diagnostics.For example,Thundat et al.[53] applied electrospray to the DNA molecules deposition onto a gold substrate for studies under a scanning tunnelling microscope.The electrospray,in compari-son with standard electrodeposition or drop evapora-tion technique,offers more uniform sample distribution,isolated strands,and reduced number of molecular aggregates.A disadvantage of the method is that some of the strands can be distorted that masks intrinsic features of DNA.

The membranes for electroanalytical chemistry were prepared by Hoyer et al.[59].The cellulose acetate phase inversion membranes on glassy carbon electrodes are used for electrodes protection in elec-troanalytical chemistry from fouling by macromolec-ular particles.Pro?lometric measurements showed that thin(300nm)membranes were more uniform than those formed by solvent casting[59].The ratio of the maximum to minimum thickness of the membrane ranged from1.2to1.4,whereas for the membranes obtained by a conventional method the ratio of the thickness in the centre to that near the edge was up to ?ve.

Moerman et al.[77,94,95]produced arrays of identical spots of130–350l m in diameter consisting of biologically active substances,such as enzymes or antibodies,for the purpose of their use in medical diagnosis,environmental research,or combinatorial chemistry.The substances remained biologically active after electrospraying,provided that the current is lower than500nA.The electrospraying in the stable cone-jet

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mode allowed an accurate and reproducible dispersion of ultrasmall volume of liquid onto an array of spots without splashing,that is unobtainable by other depo-sition techniques,such as piezodispensing or contact-printing.The authors noticed that liquid droplets of a volume of200pL evaporate within1s at room tem-perature upon landing onto substrate.Thus,a dry reagent dot is obtained that preserves protein stability. The enzymes sprayed on-chip(silicon nitride) remained stable for a time of1.5–2months at a storage temperature of–20°C[95].

Uematsu et al.[127]produced biologically active protein thin?lms for protein-based biomaterials,bio-sensors and biochips.The?lm was produced from a-lactalbumina.A?ne porous(reticular)structure was obtained with pores in the size range of40–600nm. Tests indicated that the electrospraying had no effect on biological activity of the proteins.

The electrostatic spray deposition method was also used to produce ceramic calcium phosphate coatings on endosseous titanium implants[125,126,140].No signi?cant differences between the behaviours of the cells cultured on ESD and RF magnetron sputtered coatings were observed,although the two coatings differed in roughness,Ca/P ratio and molecular com-position.The coatings were amorphous,irrespective of the initial Ca/P solution ratio,but they crystallised into different crystalline phases after annealing at650°C.

Other applications

Since1990s the electrospray thin?lm deposition was used in many industrial applications such as materials technology and nanotechnology,for the production of solid lubricating?lms or photoresists.The electrosp-raying was also tested as a tool for electroprinting on solid surfaces.

Solid lubricating?lms of MoS2were obtained by electrospraying of MoS2dissolved in isopropanol, acetone,alcohol or toluene[39,62].In this process,the solvent evaporated when the droplets?owed to a grounded substrate,and only solid nanoparticles were deposited onto it.The?lm was composed of?ne?at particles about120nm thick and with diameter up to 1000nm.The?lm thickness was between0.28and 1l m.The friction coef?cient for stainless steel was in the range of0.01–0.02in a dry nitrogen atmosphere.

The production of thermal barrier coatings for pro-tection of the components exposed to high temperature in gas turbines was demonstrated by Choy[139].The layer consisted of yttria stabilised zirconia (Y2O3+ZrO2)doped with1mol.%of Eu.The coating was deposited on Ni based alloy substrate using the ESAVD.The author produced a single-layered and a double-layered coatings consisting of a thin Eu doped ?lm and undoped?lm.The thermographic properties of the layer enabled measurement of the surface tem-perature up to1100K.

Recently,Lu et al.[104]produced piezoelectric microactuators made of zirconium n-propoxide, Zr(C3H7O)4,titanium tetraisopropoxide,Ti((CH3)2-CHO)4,and lead acetate,Pb(CH3COO)2for MEMS. The?lm deposited by electrospray had the crystallites of the size of about150–200nm.The?lm deposited at 100°C consisted of particle agglomerates with spheri-cal grains.At25°C,the?lm had dense microstructure and smoother surface morphology,and the grains were rhomboid with sharp boundaries.The difference in morphology was probably due to a longer time of interaction between the precursor droplet and the substrate that could take place at lower substrate temperatures.The permeation of the precursor solu-tion through the earlier formed porous structure was deeper before the solvent evaporation.

Photosensitive resist(thin?lm deposited on a sub-strate and used as a mask,for example,in microelec-tronics or MEMSs)were prepared by electrospraying by Hall and Hemming[52].The aerosol was sprayed by a centrifugal atomiser and charged by induction.Re-sults showed that the layer was of suf?ciently high quality,free of air entrapments,and its thickness was between20and30l m.Although this method was not purely electrohydrodynamic,it is expected that similar effects could be achieved with electrospray.

The cobaltite(NiCo2O4)electrocatalyst for oxygen reduction was fabricated by electrospraying from nickel and cobalt nitrates by Lapham et al.[32].The size of the particles produced from the electrosprayed aerosol was smaller than the size of the particles gen-erated by spray pyrolysis.As a result,the?lm has a larger contact surface area.Because the spraying pro-cess was accomplished at lower temperatures(350–400°C)the spinel structure,required for electrocatal-ysis,was not damaged.

Jayasinghe et al.[119]studied the electrospraying of silica suspension in ethylene glycol,and Jayasinghe and Edirisinghe[128]alumina suspension in ethanol for the purpose of electrostatic printing on solid surfaces.The silica layer was deposited on a quartz glass,and alu-mina on a polymer substrate.The authors noticed that silica relics with diameter1–80l m were obtained from 0.5to20l m droplets.The alumina relics were in the size range of1–35l m with mean diameter of4–8l m. Jayasinghe and Edirisinghe[106,128]developed an electrostatic droplet concentrator consisting of a sharp needle placed behind the dielectric substrate,just in

280J Mater Sci(2007)42:266–297 123

the axis of the capillary nozzle.The grounded elec-trode helped to converge the spray in the target point on the substrate.However,the authors observed that ?ner droplets were dif?cult to converge and they were scattered around the target point.The?aw of the process was that silica relics were not homogeneous but contained an outer ring of the solvent(ethylene glycol)and small dense inner region of silica nano-particles(20nm in diameter).

Similar procedure of electrostatic printing of an organic material on ITO covered glass was devel-oped by Saf et al.[123]for its application in molecular electronics.Oligo(methylmethacrilate)of degree of polymerisation n=9was dissolved in acetone and electrosprayed from a stainless steel capillary in N2gas at atmospheric pressure.The particles were dried on their way to the substrate, forming a?lm with a thickness between30and 130nm,and roughness smaller than 1.3nm.The novelty was that,using the electrostatic lenses,the spray was focused on the substrate surface,forming a spot of diameter100l 618a4d2eed630b1c59eeb560rger areas were cov-ered by moving the substrate on an x–y table. Computer simulations indicated that the kinetic energy of the ions sprayed at atmospheric pressure is only a few electron-volts,which is suf?ciently low to remain the substrate undamaged.

Fundamental studies

Electrospray experiments in thin?lm deposition can be categorised as either for the production of solid thin layers,or for fundamental studies of electrosprayed-?lm growth mechanisms,or a combination of both.The fundamental studies are aimed at better understanding of the layer formation processes,its morphology,and electrospray optimisation.

The layer formation is a complex and multi-step process,and,therefore,numerous explanations can be met in the literature.The morphology of the layer depends,in general,on the gas and substrate temper-ature,the solvent used for spraying,and the time of solvent evaporation from the layer.Chen et al.[141], and Schoonman[142]distinguished and drawn sche-matically four types of layer morphologies.But the research by Chen et al.[29–31,55,56,61]and SEM photographs presented by,for example,Cich et al.[66], Gourari et al.[67],Diagne and Lumbreras[93],Yoon et al.[102],Miao et al.[75],Nguyen and Djurado[100], and Taniguchi et al.[33,34],have indicated that other structures should be incorporated into this classi?ca-tion.The layer morphologies can be categorised into two main groups:dense and porous.The dense layer can be amorphous,crystalline(of different structures) or amorphous with incorporated particles(intrusions). The porous layer can be reticular,grainy,or fractal-like.These structures are schematically shown in Fig.4.The morphology can change with the layer depth.Changing the physical properties of the liquid to be sprayed allows tailoring up to some extent the?lm 618a4d2eed630b1c59eeb560yer annealing at elevated tempera-tures,for1h or longer,can also modify the morphol-ogy,that was investigated by Chen et al.[30],and Yoon et al.[102].

The SEM images taken by Balachandran et al.[76] revealed that the ZrO2?lm was composed of agglom-erates of the size of primary droplets,which in turn were built of powder particles,0.5l m in diameter, used for preparation of the suspension.The authors concluded that an advantage of spraying suspensions, instead of chemically prepared solutions,is the lack of chemical by-products,which could contaminate the ?lm.

The substrate temperature is a critical parameter for ?lm morphology.Nguyen and Djurado[100],and Huang et al.[112]observed that?lms prepared at too high temperatures consist of agglomerates,or are porous.On the other hand,Chen et al.[29],Kim et al. [37,38],Zaouk et al.[86,87],Rhee et al.[101] observed that?lms deposited at high temperatures become polycrystalline whereas those deposited at mediate substrate temperatures are amorphous.Chen et al.[30,61],for example,noticed that at temperatures of340°C,in the case of LiCoO2,the crystallites are formed.When the substrate temperature is extremely high(700°C)only porous?lms can be obtained.The critical temperature depends on the material to be deposited.Chen et al.[31,135]explained that when the ambient temperature is too high,the solvent evaporates too fast from the droplets,and only dry

or Fig.4Film structure morphologies:dense(a),porous(b)

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semi-dry particles arrive at the substrate,forming a porous,fractal-like 618a4d2eed630b1c59eeb560ing a solvent of low evaporation rate(for example,a mixture of ethanol and butyl carbitol)makes evaporation slower,and the droplets can easily be spread on the surface,forming a dense amorphous?lm.Stelzer and Schoonman[60] reported that?lms with low porosity were produced when using a solvent with a high boiling point.They also noticed that the porosity of the substrate also could in?uence the?lm morphology.Raj and Choy [89]discovered that ITO surfaces consisted of crystal-line and amorphous phases,and with an increase in the temperature(from300to550°C)the number of crystallites increased.At higher temperatures,the?lm was homogenous and had fairly uniform spherical grains of the size ranging from250to850nm.

Films of PbTiO3prepared by Huang et al.[107]at a temperature of150°C,which is slightly higher than the boiling point of the solvent(2-ethoxy ethanol),were smooth and dense but at higher temperatures (>150°C)powder-like agglomerates composed of tiny particles were formed.Huang et al.[112]concluded that at high gas temperatures,the?lm became porous because mainly dry powder fell onto the substrate due to solvent evaporation from the droplets.Similar mechanism of porous?lm formation was also consid-ered by Perednis et al.[35].When solubility of a pre-cursor is low,the precursor can precipitate within the droplet,and after impingement of such droplet onto the substrate,these?ne crystallites could not be spread over the surface before ultimate drying.

The explanation of?lm formation mechanisms in the case of electrostatic-assisted CVD was proposed by Choy and Su[84].At lower temperatures(300°C for CdS)the charged aerosol is directly sprayed onto a substrate,followed by solvent evaporation and decomposition of the precursor.The?lm becomes porous and amorphous.At high substrate temperatures (450°C),the solvent vaporises and chemical precursors are decomposed before the droplet could impact on the heated surface.Then,only the solid particles are deposited on the substrate.The resultant?lm is porous, of powder-like structure.At intermediate tempera-tures both processes occur simultaneously,and small grains could be found at an amorphous?lm.The optimum temperature is,therefore,that at which the solvent evaporates close to the surface,and the decomposition and chemical reactions take only place at the substrate.

The layers can be distorted by cracks,voids or pin-holes.The crack formation mechanism was proposed by Chen et al.[30].At too low substrate temperatures, the solvent evaporation is slow and too much of it remains within the?lm,but after its evaporation the cracks are formed due to mechanical stresses.At higher temperatures the cracks disappear and the?lm be-comes porous.Nguyen and Djurado[100],and Huang et al.[112]also noticed that too low temperatures cause the?lm cracking due to solvent accumulation on the surface.Huang et al.[112]proposed that the optimal deposition conditions are when the solvent evaporation rate is equal to the liquid deposition rate.Longer deposition times,with lower?ow rates give,therefore, the layer more uniform,free of cracks and pores[33, 34].Slow evaporation of the solvent helps to form a relatively dense?lm[103].Choy et al.[65]noticed that multiple deposition,with subsequent sintering at an elevated temperature(1000°C),for example,for2h, can also produce dense and crack-free?lms.

Recently,Perednis et al.[35]also investigated the effect of substrate temperature on?lm morphology. At too low temperatures(<200°C)the solvent evaporated slowly and a thin wet layer remained at the surface.At too high temperatures(>350°C)the droplets were almost dry when falling onto a sub-strate.Spreading of such droplets over the surface is dif?cult,and therefore,discrete,separated particles formed a porous or rough?lm.The authors found that an optimal temperature for dense?lm forming is in the range of280±50°C.They also noticed that too fast drying of the?lm caused cracks.The results presented by Perednis et al.[35]are in opposite to those proposed by Chen et al.[141].Chen and co-workers reported that solvent with a high boiling point has to be used in order to deposit a dense?lm. Perednis and co-workers concluded that the boiling point of the solvent has only its effect on the lowest possible deposition temperature required to obtain a dense,crack-free?lm.Their results indicated that the optimal temperature for the crack-free?lm deposition is slightly above the boiling point of the solvent.

Choy[91]observed that?lm morphology prepared by ESAVD depends not only on the substrate tem-perature but also on the precursor concentration.At low precursor concentrations(0.05M of titanium di-isopropoxide bis2,4-pentanedionate in2-proponol), the TiO2(anatase)?lms deposited at400°C were amorphous,but at higher concentrations,crystalline anatase?lms were formed.For the substrate temper-ature of450°C,an anatase?lm with crystallites of the size of about10nm was obtained.At higher pre-cursor concentrations(0.2M),the?lm was formed from crystal aggregates,with crystals of the size of 50–60nm.The?lms composed of10nm particles were transparent and smooth whereas those build of crystal aggregates were opaque.

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The problem of liquid evaporation during elec-trospraying was also considered theoretically and investigated experimentally by Grigoriev and Ediri-singhe[143].For the cone-jet mode,the liquid evapo-ration,heat lost in the needle and ambient air,and heat consumed by the emitted liquid causing an increase of its temperature were considered in the heat balance equations for the liquid meniscus.It was assumed that the Joule heating is the only heat source and that there lack of liquid bubbling.The problem of liquid evapo-ration is a vital one in the case of spraying of easily evaporating liquids,mainly hydrocarbons.For exam-ple,for toluene at the?ow rate lower than10–12m3/s, more than10%of the liquid can evaporate from the meniscus.The authors also concluded that an increase in liquid conductivity results in higher thermal?ux and rate of evaporation.

Chen et al.[135]have noticed that?lm morphology could be manipulated to some extent by adding some additives to the liquid.With the addition of acetic acid, routinely used for increasing liquid conductivity,to the ethanol solution,the structure of the layer was changed from severely cracked to crack-free reticular.This effect was attributed to the smaller droplets generation with acetic acid added.When ethanol is added to,for example,an aqueous solution,the solvent easier evaporates before the droplets are deposited on the substrate.Chandrasekhar and Choy[88]observed that a mixture of water and methanol used as a solvent for SnO2:F thin?lm precursor,made the?lm rough and porous because of high surface tension of the solvent and its ability to dispersion into a large number of droplets in an electrospray system.A pure methanol was,therefore,recommended for a dense?lm pro-duction.Perednis et al.[35]noticed that an addition of 1%of polyethylene glycol to water solvent changed the yttria-stabilised zirconia?lm morphology from cracked to a crack-free.This was attributed to the binding properties of the polymer.The advantages and disad-vantages of different precursors and solvents used for thin?lm deposition were discussed by Perednis et al.

[35].

Morota et al.[122]have studied the effect of liquid physical properties such as viscosity,surface tension, conductivity,and molecular weight on nanostructured thin?lms morphology.The?lm was prepared by ESD from poly(ethylene oxide)in aqueous solution. By changing these properties and the applied voltage, the particles forming the layer can vary from nano-spheres through nanospindles to nano?bers.The authors found that the use of alcohol as an additive to the polymer solution changed the viscosity,surface tension,and conductivity of the liquid,and helped to form the?brous structure.An addition of hexanol reduced the surface tension of the solution without changing its viscosity and conductivity,but no effect of the surface tension on the?lm structure was ob-served.The effect of liquid viscosity was studied by changing the solution concentration within the range of5and70g/L that resulted in viscosity between0.4 and1410mPa s.With an increase in polymer vis-cosity,the?lm structure changed from grainy com-posed of?ne beads,to grainy composed of elongated spindles,and?nally to a porous composed of?bres. For viscosity higher than80mPa s only polymer?-bres were produced.The effect of solution viscosity on the?lm structure was also investigated by Perednis et al.[35].With increasing viscosity,the spreading rate of the solution over the surface be-came lower,and,as a result,the?lm became rough. The conclusion was that the concentration of the precursor should not be too high when smooth,dense ?lms are required.

The material to be sprayed can,at higher voltages, be damaged.Teer and Dole[49]studied the electro-spray technique for the purpose of deposition of polystyrene latex particles on an aluminium foil. Electron microscope examination of the layer indi-cated that about10%of the polystyrene beads was degraded at voltages higher than–24kV.Spraying at positive voltage up to+20kV,or in SF6atmosphere eliminated this degradation.The degradation was probably caused by bombardment of the droplets by electrons produced in the glow discharge in nitrogen, but the SF6gas quenched the discharge.

The?lm deposited by electrospraying can be con-taminated due to solvent or precursor decomposition. Rutherford backscattering investigations of an MgO ?lm,carried out by Rhee et al.[101],revealed that carbon impurities could be found in the?lm.These impurities were attributed to the precursor (Mg(CH3COO)2)decomposition.The content of car-bon impurities decreased with an increase in the sub-strate temperature that allowed contaminants evaporation.Mohamedi et al.[96]observed some Cl impurities in a SnO2?lm when it was prepared from SnCl4precursor.X-ray photoelectron spectroscopy results obtained by Su and Choy[82]showed that a small amount of C,O and Cl impurities existed in the CdS?lm produced by ESAVD method as a result of the precursor decomposition.Additionally,the pres-ence of C and O were attributed to the adsorption of CO2from the air.These impurities were removed by ?lm sputtering with Ar atoms for3min.

It was proved by many authors that the quality of the thin?lm strongly depends on the size of the

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particles or droplets forming the layer,their monodis-persity,and their uniform distribution on the substrate. Smaller particles,having narrow size distribution,re-duce the number and size of voids,?aws and cracks in the?lm.Uniform spatial dispersion of the droplets causes that the layer becomes more even and of the same thickness.All these properties affect the mechanical and electrical properties of the layer.

It can be concluded that electrospray allows pro-duction of extremely thin layers,which can be crack-free and more homogeneous than those obtained by other methods.The process is simple,cheap,?exible, and is easily 618a4d2eed630b1c59eeb560pared to other methods like CVD or PVD,its main advantage is that the growth rate of the layer is relatively high.The process can be carried out in an ambient atmosphere,in air or other gas,and at low temperature,without the need for a complex reactor and vacuum system.The deposited material is not damaged in this process that is impor-tant in the case of radioactive or biological substances. The ESD can produce highly pure materials with structural control at the nanometre scale.The crystal-linity,texture,?lm thickness,and deposition rate can be controlled by adjusting voltage,?ow rate,and the substrate temperature.Microscope inspections con-?rmed that the electrospray deposited layer is even, without micro-?ssures and structural dislocations.The electrospray is a very ef?cient process because at least 80–90%of the solution can be deposited onto the substrate.

Many authors have optimised the electrospray pro-cess with regard to its application for thin?lm depo-sition.They made the observation that to obtain a dense,uniform?lm the deposition rate should be nearly equal to the solvent evaporation rate.For too low substrate temperature,cracks can be formed be-cause the solvent remains on the substrate.For too high temperatures the droplets evaporate,and are deposited as solid particles forming a porous layer.

Although each research group reports the successful ?lm deposition and fabricated device operation,most of them suggest that further development is required. Despite many experimental data,the de?nitive expla-nation of the electrospray mechanisms and?lm for-mation processes is not yet available.

Liquid metal ion and droplets sources

Introduction

Liquid Metal Ion Source(LMIS)is a device,which is used to produce a beam of metal ions,charged clusters,or charged nonodroplets from a molten metal.LMISs were introduced in1970s,allowing development of fo-cused ion beams(FIB)technology.LMISs have had signi?cant impact on the semiconductor industry.Metal beams are used in micro-and nanoelectronics for the production of thin metal?lms,ion deposition and implantation,maskless doping,and direct writing. LMIS is also used for maskless etching of semiconduc-tors,sputtering,scattering,micromachining,microli-thography,patterning,and production of ultra?ne metal powders[144].LMISs are used in atomic physics,in secondary ions mass spectrometry,scanning ion beam microscopy[145,146],and for surface coating[147].

Applications of LMISs in semiconductor industry, microelectronics and material technology were reviewed by Melngailis[148],Jeynes[149],Mair[150], Orloff[151],Stevie et al.[152],and Reyntjens and Puers[153].Focused ion beam technology,including LMIS bibliography until1990was presented by Mac-kenzie and Smith[154].Mitterauer[155]reviewed the designs and applications of microstructured multinoz-zle liquid metal ion and electron sources,and Gomer [156]the physical mechanisms of electron and ion generation from Taylor cones.

The metal ions are emitted from the tip of the Taylor cone with small jet-like protrusion at its apex formed at the outlet of a capillary nozzle,like in the electrospray devices,or from the apex of a solid needle covered with a liquid metal?lm.The difference between a LMIS and the electrospray of semicon-ducting liquids lies in that the meniscus and jet in the LMIS are primarily driven by Maxwell normal stresses whereas in the electrospray the tangential stress on the interphase surface is dominant.The voltage drop along the jet of the LMIS is lower than for other liquids,that is an effect of higher conductivity of metals[157,158]. Three modes of droplet formation were,therefore, distinguished:detachment of the whole liquid cusp, detachment of droplets from the tip of the Taylor cone, and detachment of?ne droplets from the shank of the Taylor cone and/or the needle or capillary emitter[cf. 158].The third mode is only speci?c to the metal electrospray.

The volume?ow rate is a parameter of minor importance in the LMISs than in other electrospray devices.The performances of a LMIS are characterised by the emission current,or the beam energy or its brightness.The brightness of LIMS is de?ned by the current I emitted by a source of an area A into a solid angle W[148]:

B?

I

A X

e6T

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A typical LMIS consists of a nozzle made of tungsten or glass capillary of 0.02to 0.2mm i.d.through which the liquid metal ?ows,and an extractor electrode producing an electric ?eld.A metal (usually tungsten)needle can be placed co-axially at the cap-illary outlet.A schematic diagram of the LMIS is shown in Fig.5a.A schematic diagram of an ion emitter with a needle protruding from the capillary is presented in Fig.5b.The emitted ions or droplets are accelerated by electric ?eld of the extractor,and fo-cused by electrostatic lens.Ions of different q/m can be separated by an E ·

B ?lter (cf.Fig.5a),which combines electric and magnetic ?elds to select the ions of required magnitude of q/m.Only the ions of proper q/m can pass through the aperture in the mass separator,and other collide with the separator plate.Next,the ion beam can be de?ected and focused onto the substrate.

Taylor cone is formed at the capillary outlet when the liquid metal is subjected to suf?ciently high electric ?eld.The Taylor cone is more stable for the nozzle with protruding needle than for a capillary with free meniscus.Van Es et al.[159]noticed that the liquid cone is the most stable when the cone angle of the needle is nearly equal to the angle of the Taylor cone,i.e.,about 98o .In the needle ion emitter,a sharp re-gime of spraying conditions required for the stable Taylor cone formation is not necessary because the liquid is sprayed from the tip of the needle.A tungsten needle is usually used as such emitter because it is inert to most of metals and alloys.However,for the Sn emission,a nickel needle was recommended by Bisc-hoff et al.[160].

Needles with rough surface have improved wetting properties and their operation is more stable than pol-ished emitters.This effect was observed by Wagner and Hall [161],and Bell and Swanson [162].Van Es et al.[159]have modi?ed the emitter surface by making rough microchannels having length of several hundreds of micrometres and width of about 10l m.These channels allowed supply of liquid metal along the needle cone to its apex by capillary action.This pre-vented off-axis ion emission and multiple Taylor cones forming,characteristic for the polished tips.The dis-advantage of such microchannels is that they can be contaminated by the material of the extractor electrode sputtered due to ion bombardment.These contami-nants can cause the blockade of ?ow of liquid metal.Emitters with needle protruding from the nozzle were used as ion sources,for example,for scanning ion probe by Seliger et al.[163],for maskless ion implantation by Cheng and Steckl [164],for material surface studies by Prewett and Jefferies [165],and for micromachining,microlithography and metal implantation purposes by Benassayag et al.[166,167].In the early stage of LMIS development,the spot size was about 10l m.The spot size of the Ga ion source used by Seliger et al.[163]and Prewett and Jefferies [165]was 100–500nm,by the ion current density of about 1.5A/cm 2.In recent years there is a growing interest in nanotechnology in designing LMISs producing ion beam of the spot size smaller than 10nm [153,159,168,

169].

Fig.5Liquid metal ion sources:capillary

nozzle—ions are focused and separated with electrostatic lenses and E ·B ?lter (a ),capillary nozzle with a needle emitter (b )

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Many modi?cations to the devices based on the principle of spraying from a needle tip were proposed in the literature.Vladimirov et al.[170]used a steel needle of tip radius of 10l m protruding from a graphite container.The liquid metal was evaporated due to additional sputtering by electron bombardment.Longitudinal groves on the needle surface were etched in order to decrease the ?ow resistance of the metal.Purcell et al.[171]covered a tungsten W(111)needle of 50nm tip radius with the metal to be sprayed (Au or Ag).The metal was deposited on the emitter by evaporation from a circular loop surrounding the emitter tip.The emission took place due to ?eld desorption of metal atoms from the cone apex.This source could operate at least 10h with a stable emis-sion current in the range of 1–3.5pA.This ion source was designed for ion implantation or nanowiring in nanotechnology.

In order to avoid generating large droplets,which can be produced while spraying from a capillary facing downwards,Clampitt and Jefferies [172]developed a liquid metal ion source with a needle placed in a vessel and protruding above a molten metal surface (Fig.6).The ions were extracted from the needle tip wetted with the molten metal,by the electric ?eld produced by the extractor electrode.The tip of the needle should be suf?ciently sharp in order to a single cusp to be formed at the needle tip.The device has an advantage of not requiring a capillary for liquid metal supplying,but the shortcoming is that this ion source can operate only in one position.Liquid Cs,Ga and Hg were tested as ion sources.This type of metal ion source is characterised by high purity,low power consumption,and long life-time.

Hairpin emitter was designed by Wagner and Hall [161]for emission small amounts of liquid metal in vacuum conditions.This type of emitter is made in the form of tungsten hairpin with a tungsten sharp needle at its tip (Fig.7).The molten metal is held at the

hairpin free-end due to the surface tension force.To increase the volume of such reservoir a small coil was mounted beneath the needle or the end of the wire is twisted to a double-coil [160].The liquid metal ?ows to the needle tip due to capillary forces,and ions are extracted by an electric ?eld.The hairpin is usually resistively heated to melt the metal in the reservoir but can also be melted due to electron bombardment [173,174].

To obtain high emission currents for metal ?lm deposition on large areas,a multiple emitter can be used [175].However,Mitterauer [155]applied a por-ous material as ion emitter (Fig.8).The nozzle

was

Fig.6Liquid metal ion source;molten metal sprayed from a cone-needle [cf.

172]Fig.7Hairpin-type liquid metal ion

source

Fig.8Liquid metal ion source with porous metal emitter [cf.155]

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