- Email: [email protected]
Growth of thin films by solution-gas interface: A new technique
hl~tcrials
(I 984) 46 1 419
C‘llcn?i.strj~ urrtl PIlJ,sics, 1 I
461
GROWT:-IOF TIHIN FILVS :.‘: SOLUTIO?j-GAS iVTE {FACE : A X&W TEC!-NIJ!_iE -_-___________________. .._.._ _ _ .- __ --_ _-- ..-- . _-.- _ -... - _ _._ _ l_l--__ ___.-..
En--. ,y C, :?versioi? Shivaji University,
Lj’:lc?ratory Kolhapur
( 3epartnent 416
cf (India)
004
Physics,
ABSTRACT sol uti on-gas
The unifor-?
and
i.i!iS
pap”“.
tion
are
i.hz
area
The
growth
i!iscussed
solutian.
The
Inf laenc~z
films
interface
large
is
thin
of
mechanism
ton tPe
optical of
techniql;e films
basis
used
and Itiilctics of
ti;e
and c!ec+,rical
the
fcr
prepdrative
t! e formation
c1-alc)geni.des
is of
surf ace
the
film
tension
of
properties
parameters
are
on the
.>f
described
in
formathy
studied
growth
and
cf
th;n
discusstd.
I NT ~ODUCTION Thin
chalcogenide
films
fabrication
of
area
these
by vacuum
fi_lms
and chemical qrowth cted
of due
bath large
to
Therefore, uniform,
area
films
of
the of
ducting
film
Bi 2 S 3, progress
are
consists
at the cations
interface and the reaction
the
interface.
The
As2S3
PbS and
Z,?O With
growth
of
been of
thin
films
gas
at
however, source
restri-
mater<-al.
interface
technique
formation
of
thin
.and gas.
anions; required
thin
method
grow
of
solution
the
the
The
to
contains
growth
[l-4].
developed
the
forms this
is,
the of
the
pyrolysis
known
and the
by a solution-gas
method
spray
well
tecilniques
subs trate
a simple
the
0254-0584/84/$3.00
t.he
by
at
Sb&, in
films
the
contains -which
by these
for
Th e d e p (7s 1 t i o .n o f
and dry,
techniques of
int.erost
arrays. dip
new me+.hod has
s~bst~~nce
soliltio:1
product
particular
photo-diode
fi!.ms
geometry
a relatively large
of
evaporation,
deposition area
the
Basically
[5,5]. The
J.arge
are
films is
interface
0 Elsevier Sequoia/Printed
t’r,e se-niconof
studied. has
CdS, Recent
brclught
in The Netherlands
462
us closer to understanding the mfcroscopic basis of surface science. The microscopic view poi.nt of solid state physics has injected new intellectual vigo r into electrochemistry, sv..rface chemistry and the study of heterogeneous systems. The develop:ments in moleculer orientation at the Interfaces of solid-c~as and solution-gas have turned chemical kinetics into one of the most active fields of electrochemistry, Special attention has been paid to the mechanism of surface reacticns of sclutlon-gas interface films.
Theoretical and experimental results on the proper-
ties of solution-gas interfaces such as coTposition of solution, temperature, surface tension, pH and pressure of exposing gas to the solution surface are given. brief account of
phJse
Work
or!
the
The present article gives a solut~ion-gas interface and the
growth of thin films. TYPES OF INTERFACES Recent efforts have stressed the surface properties of compound semiconducting thin fiJ.ms and their interfaces.
Both chemists and physicists have intensively studied the surface properties. The molecular orientation contributes individual properties to the intecfacial regions. In order to increase interest in the sl_trface, interface neons
and thin
systems
to
films,
exp1ai.n
we have considered the
inter-relation
a :rtodel of between
the
hetsrogeinterfaces
(Fig.1) such as the solution-gas, the solid-gas and the solid-solution interface. The solution-gas interface is studied in detail. SGLUTIOti-GAS INTERFACE A gas molecule feels an attracting potential upon approaching the oppositely charged solution surface. The strength of this attracting potential determines the strong affinity of gas atoms Frequently -the ran.ge of attraction of oppo-
towards metal ions.
sitely charged ions depends on the distance between the solution If the range of the attracting surface and the gas molecules. force is short then the intraction of positive and negative ions takes place Snstantaneously due to the strong affinity of anionic species towards cationic species. If the distance of the gas source from the solution surface is increased, then the solution surface experiences 13weak intraction with the gas molecules. The electrostatic potential energy of the surface molecules of the solution involve a direct-charge transfer with colliding gas
463
-r:ol."cclPs ; sill.‘-
3 iiiarge-tr%nsfer-iq+ :;cticn is ctf !,cnJ __~,-'I:~+ 2nd
leads to lo;-!la-tic>n fof <3new ::i:~.L+ct~le \rlL?!! :!!Iltl:zliz 2ti3n of :;'r 4.rges. The
Y32Ctloq
u~321:
the
mec'i ‘XT
:f !Idture
&?d
t11e
ii\rd~::Q?Z
of
the
StllELd;,
gas
wlljli
3
xeclj..dpj
is
dc:pet?:l,~nt
the ac5dlty or slkality c!f t3e
solvent,
zlectrc r. affinity of t!ieoppositely cbar~ed ;cns.
HcrrlLck [.;;stludiedthe effect of hydrooen sl~lfide gas on the surface te:7si*ln. ~,fso!..~tior:s and a!.so sttibied the effect of iemperature on surface tenslon. Ions en the surface of a soluLion are of ions ervironment marked: y diffeJ,e?l.fron, +he wviron~-ent in iit? sc‘^‘face>f 2 SOILI t ‘3’1 in the 5ulk of the solution. The ions
is
surr3:Jnded 3~
by
fewer
neighbors
thar:
sre
the bt~J.kj~c'ns.Tiiere
anisotrzpia distrlbutio~ 05 these neighbors near the surface:
The compu+:atior in (genera! is carried cut in two parts; first the surface is pcodcced by breaking bonds, and by removing atoms adjacent to the newly created surface ions. A number of lhin fjl-Qs of chslcogenide compounds have been studied by this technique[5,6,8,9]+ Gubbins and Thompson [lo] studied the molec,Jlar orientation at the solution-gas interface and the der,sity orientation profile,
The
rnclecular arrangement at the surface has been given by Tokahashi [.lii,At the equilibrium ions tend to form tte preferred orientation at the surface of the solution. However, the sitklation is different in the bulk of the solution.
Regular arrangement of the
ions takes place at higher surface tension. Solution-Gas Interface ----7Non-rev'
i Physj.cal or Van der Waals adsorption (Reversible)
Reacting gas
t Chemisorption (Irreversible)
Solution-gas interface with a non-reacting gas : Gases like E2, Co, C02, N2, cH3, etc., do not react with the solid or'liquid surface.
These gases are anly adsorbed on the
surface of +.he solids. To interpret the adsorption phenomena at the solution-gas interface it is important to establish the connection between (r) the excess of an adsorbed substance in the surface layer (C) the concentration of surfactant in the solution (C) and the surface tension at the solution gas interface (6)for
dilute solutions.
464
Liquid-Liquid \
Solid -Solid
Fig. 1. Types of Interfaces.
(1) AccoT-ding to Gibbs the quantity db -777
- G = surface activity
Therefore
c m
- G .-
(2)
Electrostatic forces undoubtedly affect the surface tension of the sslzltion in many instances,
Consider an aqueous solution of bis-
muth citrate in which the citrate anion is a hydrocarbon chain and ai3+ is a cationic species; this compcund is highly surface active and it is certain that it is a mixture of anions arid cations which lowers 6
and consequently accumulates in the surface layer. Howe-
ver the anio;-,sare charyed and their preferential transfer towards the surface means a separation of charges takes place.
Bismuth
citrate has a permanent dipole moment and orientation in the surface layer is such that the dipoles are parallel to each other; thus electrostatic repulsion takes place.
However, in aqueous
medium the surface activity increases with an increase in the length of the hydrocarbon chain.
It is found that on addition of
hydrocarbon radicals the surface activity of the aqueous solution increases on average 3.2 times per CH2 group [12].
According to
Langmuri's equation at low concentration of surface active substances the surface tensicn of the solution decreases in direct proportion to the concentration A=6-,-6= where A
C
as
kc = decrease in surface tension, 6, = surface tension of pure solvent and k = constant
(3)
465
Slurf ace
adsorption
Adsorption
is
tration
of
example
between
are
between
them
the surface
forces
surface
and the
Adsorption by the
is
or
pt,iion
equilibriam.
a process
amount
of
surface
small
particles
adsornot
tk,e layer
plane
field
ions
the
w‘rlich
that
a surface
force
attract
of
arises
3r molecules
of
the
i.e.
adsorbant
of
these
it
two
mutually This
the
accompanied
, t,he passage surface
equilibrium. of
is into
of
the
opposite is
known
adsoroticsn
adsor-
solutj.on. processes as
adsor-
equilibrium
factors.
ioils
are
adsorbed
adsorbed
of
the
and large
on the is
surface
and adsorption
surface
directly
areas
so
of
t,he adsorbent.
proportional
Amorphous
adsorbant.
crystalline
hand,
because
desorption;
dynamic
substance
areas
the
substance so
and
at
are
and in the
a free
can
The position
area
other
surface
of
or
total on the
of from
various
Substances
substance
process,
occurance
to
The
part.icles
called
the In
balanced.
the
is
directions;
Therefore,
process
leads
upon
into
film)
a substarnce
within
all
For
solid
concentrated
solid of
concen-
[12:.
molecules
Tl-ie simultaneous
depends
mutually
a reversible
oppcsite
ions
the
in
phases.
thin
usually
as particles
balanced.
substances
is
in
two
or
surface
neiyhbors
changes
between
of
on the
are
the
(precipitate
surface
directed
are
dissolved
bed
The
their
interface
substance
same conditions
the
the
the
1.0 describe
phase
molecules
with
only at
or
t’he bound
fcrces
at
a sol.id
a solid.
Ions
under
used
The dissolved of
bdnt.
term
substances
a solution. surface
the
adsorption
precipitates
to
precipitates
have
is
the have
great;
much smaller
smaller.
is
STRJCTURE OF SOLUTION SURFACES The surface
structure tension
dynamics
of
Surface and it in one
spite
of is
explained
surface
tension.
tension
has of
a solution
been the
has
succeeded
transition
layer
is
by
a well-known
studied
and the
a hybrid
in
creating
explaining
cause
has for of
mechanism
approach
phenomenon
by many scientis-ts
many ways in
surface
for been
using
therno-
common to
all
hundreds
of
investigated,
a surface surface
of
with
tension.
liquids years: no
a
466
It is reoorted fll: that if the liquid is subdivided into a droplets of less than a certain critical size, the liquid becomes a super-liquid with a different structure and increased entrony. The conditions at the outermost surface of a liquid, where the ambient gas and the normal liquid are in contact, allow the existence of a super-liquid. In order to understand the structure of a liquid surface, Takshashi .rll; has made the following assumptions : . i) The whole surface is covered with the surface active layer or a super-liquid, ii) The Gibb's Helmholtz equation showing the surface holds :
energy W=rand
Tddl
dT
(4)
S = - dd / dT
where
U
and
S
(5) represent
the surface energy and entropy,
respectively due to a change in unit area of the surface. iii} The entropy sum is zero at the surface : s1 + s*
= 0
(6)
where Sl and S2 are the differences of the entropies of the super-liquid and the underlying layer from that of the normal 1iqui.d respectively. As Sl is positive, S2 is made negative by eq.(6), so it is understood conceptually that the underlying layer is ordered, and it is called the tension layer. Fig.2 shows the scheme of the proposed surface active ions consisting of two layers of different entropy. Thickness of the super liquid -_ If the change in entropy represented by Sl and S2, due to the change in unit area of the surface is As1+AS2
=
0
(7)
we obtain TASl + TAS2 = 0
(8)
467
This relation shows that if one iayer generates heat, the other layer absorbs the same amount of heat, now TAS2
is
given by
eqn (5) as, - Tddj
(9)
dT
TAS1 is given by hoH
where
h = thickness of super liquid Q,= density of super liquid H = heat of normal-to-super transformaiion is given in cm by the equation :
Thus
h
h=_
T(db/dT) x 0.24 x 1G-7i~H
(10)
The value of h calculated from eqn (10) for H20 is 660 .&and for NaCl solution is 1060 A". The vcilue of Q, was regarded as the same as that of the normal liquid. The value of Ho were caiculated by Takahashi ill] it is seen that, HQ
=
0.134 cdl/cm2
(li)
wzs GAS super Wuicl ( Surfaceactive Layer 1 K
\Under\ying
Layer
52 Tension Norma\
Layer Soiut ion
Fig. 2. Model of surface structure of liquid consisting layer of surfactive ions and an underlying layer of solution, with respective entropy differences S1 and S2 from a normal salt solution.
Thickness of the tension layer -.-As indicated, the entropy of the tension layer is lower than that of the normal liquid, so this layer is regarded as a kind of thin solid film. The thickness of the layer is calculated as, t = hH/H'
(12) where H' is the heat of fusion of the normal solid. The thickness of the tension layer can be calculated using the above equation.
468
,,.‘l,,,,,,,f__...
._..- -
.----_-
_._-___--.-_
_. .___
-7 The solution surface is slowly exposed to the fJrmation of a thin solid film.
'The gas is bubbled through the _.* oulk of. liquid giving Precipitation - ._._. ._~____
Xhen a reactinq gas is bubbled through the bulk of the solution, precipitation occurs; the precipitation depends upon various parameters - solubility product, temperature, concentration, solvent medium, etc.
There are three types of precipitate, namely,
crystalline, curdy and gelationous. particle size.
These differ only in ti-leir
According to the Van '2eimertnrule [13], slow
precipitation gives larger particles; i.e. slow crystallization gives larger crystals, because ions or molecules have time to rearrange themselves in an orderly manner upon tile faces of the crystals.
In very rapid crystallization, ions or molecules
cannot build on the existing crystals fast enough ano so they form new nuclei and very small crystals. The speed of precipitation is greater when the solution concentration is high and it depends upon the solubility of the precipitate, i.e. Q-S Speed of precipitation = K -7where
(13)
Q L: momentary concentration of solution, S = solubility and
K = proportionality constant.
The numerator Q - S expresses the force favoring precipitation and the denominator S expresses the force opposing the precipitation. The formation of a solid phase from a solution invclves two processes, one of which is nucleation and other is particle growth. The size of the particles of a solid phase is dependent upon the relative rates at which these competing processes take place. Nucleation For a precipitate there is a minimum number of ions or molecules required to produce a stable second phase in contact with a solution; unless a number of ions or molecules collects together, a solid phase having a finite life time will not exist in the solution.
469
The minimum sized stable particle is called a nucleus. nA+ + nB
T-L
(Ad),
(14)
where n is the minimum number of A+ and B+ ions that must combine in order to yield the stable particle (As),: . Tarticle growth l__ll..^~.l_._ _-. The second process that can occur during precipitation is the growth of a particle already present in the solution.
This growth
can only begin when nuclei particles are present in the solution. In the case of an ionic solid, the process involves deposition of cations and anions
on aoorooriate sites in the orecipitate.
(AB), + A+ + B-
+
(.@) n+l
+ A+ + B-
(15.3)
(AB),+l
+==* (AB),+~
(15b)
Growth of thin films at the interface A typical experimental set up [53 designed and fabricated by us for the formation of thin film at a solution-gas interface is shown in Fig.3.
It consists of mainly three compartments, namely,
gas chamber, uniform gas exposing unit and the solution container. Purified, dry H2S gas is placed in the glass chamber to the desired gas pressure.
The growth of a thin film at the interface
mostly depends on the gas exposing unit, the details of which are shown in F1 g.3.
It consists of mainly uniformly distributed
lumps of glass wool sandwiched between nylon cloth, plastic wire gauze and a metallic sheet with a circular hole.
The solution is
taken into the glass ccntainer which is modified to enable
the
solution to be removed from its bottom without disturbing the surface of the solution.
For this a vibrationally free mechanical
arranqement is provided to move the solution up and down. In computing the interaction energy between the surface and gas, the surface may be treated as a single conductor system. A gas atom is assumed to interact with the surface as a whole instead of with individual surface atoms. The whole solution surface comes under reaction at the same time; the surface tension of the solution decreases as the pressure of the hydrogen sulfide gas increases.
When the surface tension of the solution was lowered, the
film formed on the surface was non-uniform and ruptured in places.
470
Ex erimental set-up for solution-gas interface technique: Fi 3 1 z>s*cham ! er; 2 Uniform gas exposing unit, M=metallic sheet, w = plastic guaze, N = nylon cloth, G = glass wool. 3,Shutter; 4. Solution container; 5. Purified H2S gas; 0. Gear wheel; 7. Axial wheel.
Th.n films of a number of chalcogenide compounds were prepared by the solution-gas interface technique, such as Bi2S3, Sb2S3, As2S3, CdS, PbS, MoS2 and ZnO. We have discussed the growth of a bismuth sulfide film [5]. The bismuth sulfide thin films are grown at the interface between the hydrogen sulfide gas and the bismuth nitrate solution.
In the bismuth nitrate solution
(pH = 3.5) the solvated BiO+ ions are weakly bound with unsolvawhen the molecule of hydrogen sulfide comes into ted N03-ions. contact with the surface of the solution, then the bonding between H' and HS- ions of the H2S molecule is broken:
471
H+ +
H2S
-.---)
(16)
HS-
Hydrogen sulfide gas is a weak acid dissociating as follows : H$ K
_
1-
K1, -
i-H20
H30
(;130+) (HS-) _.-_.--
+
+
ES-
= 5.7 x 10-8 mol/l .
22s)
5
HS- + H2O 7
H30+
-I-s2- ,
(H,o+)(S--) __......__~____= 1.2 x 10-15 mol/l. K2 = ..I_ ( HS-) Eqs.(l7) and (19) combine to give an impression for the overall dissociation of hydrogen sulfide gas into sulfide ion : H2S + 2H20
<===
2H30+
+
S2-
(20)
(H30+)2 (S2-) __-I.---.-.-__.-. (H,S)
K1' K2 =
= 6.8
x
(21)
io-23mol/l.
The constant for this relation is simply the product of K1 and K2. The solubility of H2S in water is about 0.1 F; for practical purposes, we may assume that
[H2S] = 0.1 mol/l.
Substituting this value in eq.(21) we get,
p30+i2 [S2_]
---
--
= 6.8 x 1O'23 mol/l.
(22)
0.1 Therefore
mol/l.
(23)
In general, the gas molecule and solution follow the reaction path thus : if molecules of gas A and gas B react over a solution surface S such that we get the product C : k+B
-,.
A + B --+ A- S+B.*
C (gas phase reaction) A-S c+s
(24)
Here the solvent serves as a catalyst, which is free after the reaction; that means that A and B (H+ and S2-) react with the solution surface S (with Bi 3') to give the product C (Bi2S3). For example, when the metallic salt solution is exposed to hydrogen sulfide gas, breaking of bonds takes place due to the strong
3
2
1
0
1.0
LUG CONC. Fig.4. Variation of thickness with concentration for Bi2S3 thin films.
aff$i-ity of oppositely charged ions.
The triply charged bismuth
(Bi ) ion has a relatively small size and exerts a large polarizing force to attract the large sulfide ion to form the bismuth sulfide molecule.
The sequence of reactions taking place is
Bi(N03)3 + HN03 + H20
----t(BiO+
----- N03-) + 3HN03
(25a)
BiO+ --- N03- + H+ - SH- ----?- BiO+ -.-- HS- + HN03
(25b)
2BiO+ ----- f-S- + H+ ---- HS-
(25~)
-tBi2S3
+ 2H20
Reactant = Intermediate = Product. 2Bi(N03)3 + HN03 + 2H2C + 3H2S --+2(BiO+----
N03-) +
+ 5HN03 + 3(H+ ---- SH-) -----+Bi2S3 + 7HN03 + 2H20
(25d)
The overall reaction is 2Bi(N03)3 + 3H2S
-+Bi2S3
+ 6HN03
(25)
with a bond breaking and making process taking place due to the strong affinity of oppositely charged ions.
The electron affinity
of a sulfide ion towards a bismuth ion will be stronger than other ions present at the solution surface. Bismuth nitrate solution reacts with hydrogen sulfide gas as shown in eqn (25).
473
Fig. 5. Constant temperature assembly for solution-gas interface film growth : 1) Solution container; 2) Glass substrates; 3) Glass stand: 4) Aluminum bath: 5) Solution outlet: 6) Water inlet; 7) Water outlet.
Effect of preparative parameters _-_._...._.j_ _---_-_This technique is relatively much simpler and more convenient than the other techniques, and does not require any sophisticated or delicate equipment.
However, the growth of the thin chalcoge-
nide films is influenced by the various preparative parameters, namely, composition of the solution, temperature, pH, H2S gas exposure time, H2S gas pressure, and surface tension [14].
Composition of the solution The concentration of the salt solution was varied from 0.001 to 0.5M to grow the films.
At low concentrations the film formed was
not uniform, as the required number of cationic species may not have been available on the surface of the solution.
However, as
the concentration increased the quality and uniformity of the film increased.
The variation of the thickness with the solution con-
centration for Bi2S3 film is shown in Fig.4.
The film thickness increased up to 0.1M concentration and attained a steady state
value. Effect of temoerature A constant temperature assembly for solution-gas interface film growth is shown in Fig.5.
The temperature of the solution
474
0.35
0.25 ^i % 1
0.15
z
/ P
0.05
(.
0
10
20
30
40
50
TEMP (Co) Fig.6. Variation of thickness with temperature for Bi2S3 thin film. varied-from 5 to 5OoC and films were formed. thickness with temperature is shown in Fiq.6.
The variation of the The film thickness
increased initially with a rise in temperature and attained a steady state value beyond 40°C.
However, hign temperatures can
cause the film to expand and may ultimately result in thermal forces overcoming the rather weak van der Waal's binding forces which hold the molecules together.
At room temperature films of
definite thicknesses can be prepared without any contrast difficulty. Effect of pH .-I-___-__ The growth of thin films by the present method is very much dependent on the pH value of the solution. An increase in pH causes an increase in the relative molecular area. However, as a result of this, the moleculer cohessive binding forces between the mofecules are expected to decrease, causing reduced stability of the film on the solution surface i.lS]. With the increase in pH the solubility of hydrogen sulfide gas increases relatively and may result in hydroxide precipitation. The deleterious effects can be avoided by optimizing the pH value, SO that the desired quality of the chalcogenide films can be achieved. In the case of a low pt-l value, the metal hydroxide will not precipitate and
1
1.4
1.4
2.2 hv(eV)
2.4
3.0
Fig. 7. Variation of absorption coefficient (4) with energy for Bi2S3 film. reaction of the film with the solution will be comparatively small,
The films deposited from low pH value solutions have
been observed to exhibit minimum defects,
The optimum pH value
is thus determined as one in vihich both uniformity and adhesion are satisfactory and there are minimum defects. pH value (((7) is
A relatively low
desirable to produce thin chalcogenide films.
Effect,_o;t_iaS_gaspress= It. is in fact the case for most chemical reactions that the equilibrium is disturbed by pressure.
In a gaseous equilibrium,
the equilibrium position is found to be influenced by pressure, if there is change in volume (i.e. the number of moles of the gaseous reactants) during the reaction.
A rise in pressure tends
to push the reaction to a smaller volume, and lowering of the pressure tends to push the reaction in the forward direction,, However, at low pressure the molecules are loosely packed in a large area and may be separated when the gas reacts with the exposed solution surface, thus ensuriny that a small change in pressure does not cause the film to break up or fold over.
As a result, the uniform large area films are grown only at low pressures. Furthermore, higher pressures of the H2S gas cause the rupture of the thin film. This is in general accordance with the
476
40
*
4 0
30
.
/
/ /
0
0 30
-
/ 0P
10
.
/ &? 1
o-0 0
/OH ,’
L
I’
1.5
.
1.75
2.0
2.25
hv(eV) Fig. 8.
Plot of (dhzlf2.._-versus
(h*j)for Bi2S3 thin film,
principle expounded by Henry Le Chatelier [16].
This principle
will be applied with some caution to the behavior of chemical equilibria in solution-gas interface reactions.
When the molecu-
les of the hydrogen sulfide gas come in contact with the solution surface, the bonding between IiS and SH- ions is broken : H2S (g)
--+
H+
+ SH-
(26)
Effect of gas exposure time ----_-_A long exposure of the hydrogen sulfide gas over the solution surface results in precipitate formation by rupturing the film. Therefore, a specific time is required for completion of the reaction. In a surface chemical reaction the bond breaking and making processes involve a definite time interval.
In the present
study the gas exposure time was varied from 15 to 180 sec. It was found that uniform films were obtained in between 60 to 90 sec. Effect of surface tension The growth mechanism and kinetics of film formation are related to surface tension.
This is the result of the existence of an
internal pressure; i-e. a force which draws molecules into solution and which is perpendicular to the surface. The greater the polarity of the substance the higher will be the internal pressure, as it is caused by molecular forces. By virtue of the surface
JO’
5.0
1.6
2.0
?.5
3.0
+G3(12jl) Fig,
9.
Plot
of
tension
effect
tighUy
and
introduced the
the
idea
structure
Surface
surface
of
and tension
activity. with
Optical
and
T!!e films
of
of the
the
formed
Bi&
thin
solution,
the
films.
ions
are
bound
structure.
Solution, layer,
solution
to
which ff
the
may exhibit
tension
depends
an increase
electrical ------_.--
the
for
Takahashi [ll) describe the Structure
of
thickness of the super-liquid and underbeen calculated using the model of the
a tension
Surface
Sharply
x lo3
a Super-liquid The have
layer
Super-liquid
l/T
a two-dimensional
surf ace.
(tension)
Surface
R versus
the
on
form
solution
lying
log
in
consists
of
solution
a layer
has
considerable
of
a
a high surface
an concentration,
and
drops
concentration.
characterization were
Supported
on a glass
substrate,
conduct-
stainless steel subt;tcate, or The optical absorption of the chromium plated stainl.ssS steel. film w:is mcasJred as a funct2 on of energy in the range 1.3 to 3 ef! Tfle curve absorption to obtain the band gay, of the material, coefficie4 (a) ~trsus photon energy (t’v)is plotted in Fis.7. The tng
glass
absorp-Lion which
is
mirror-grade
coefii::ient cbaractcrist~_c
poJ ished
of of
the
film
direct
is
of
transitions.
*he
order
0:
li’ccm
-I
,
CONCLUSIONS The solution-gas interface technique has the advantage of being capable oi producing pure sulfide films for a number of compounds.
The technique is mainly adopted for producing uniform
large-area films of definite thickness.
Thin films.of a number of
chalcogenide compounds can be prepared by chemical as well as physica
methods, but some contrast difficulties arise duz5n7
deposition, i.,e_.cost of the material, instrumentation, fairly large area, reproducibility, surface area of deposition, and temperature are the disadvantages of the ohysical methods. On the other hand,
low
cost
per surface area films of distinct advantages
can be prepared by this technique.
The films which are prepared
by this method show properties which are comparable with those formed by other conventicnal methods.
This work was supported by the Department of Non-Conventional Energy Sources, New Delhi, under Contract No.r(l)i3/8l-NES/4~8. Assistance of the ECL group during preparation of this manuscript Is gratefully acknowledged. REFERENCES 1
C.Ghosh and B.P.Verma, Thin Solid Films, 60 (1979) 61.
2
B.B.Nayak, H.N.Acharya, T. K. Chaudhuri and G.%.Mitra, Thin Solid Films, 92 (1982) 309.
3
Y.Ma.Yale and R.H.Bube, J.Electrochem,Soc. 124 (1977) 1430. __--.
479
a
N.R.Pavaskar, C.A.Menzes and A.P.B.Sinha, .._._ J.Electrochem,Soc., _I-_..- _.__
5
s.H.Pa;~ar, P.N.Bhosale, M.D.Uplane and S.P.Tamhankar,
124 (1977) 743.
Thin Solid Films 110 (1983) 165. _-___-_____.__ __..._!.._--_ 4
S.H.Pawar, S.P.Tamhankar, P.N.Bhosale and M.D.'Jplane, Ind.J.Pure ..--_._~_ Appl.Phys., 21 (1983) 665. .__.__-_..
7 8 9 i0
C.S.Herrick and G.L.Gaines, .__-.'!-_ J.Phys.Chem 77 (1973) 2703. S.D.Sathya and A.P.B.Sinha, Thin Solid Films 37 (1976) 15. pp___-S.D.Sathya and A.P.B.Sinha, _-----I--_ Thin Solid Films 44 (1977) 57. K.E.Gubbins and S.M.Thompson, Proc. Faraday_*.Chem.Soc. ----(G.B.), 16, (1981) 59.
11
R. Takahashi, Jpn.J.Pure Appl.Phys., 22 (1983) 17.
12
S.H.Pawar and P.N.Bhosale, National Seminar on Emerqinq _.__._-_-_--______ Trends in SlJrfaCes_,_interfaces and Thin Films 16-18, --_---_-__-_ .,-) --.--
13
D.A.Skoog and n.M.'West, --__ Ftindamentals of Analytical.-.__-__ Chemistg-, Holt Rinehart and Ninston, Inc., New York. 2nd Ea.(1969) p.164.
14
2nd All India Conference on Thin S.H.Pawar and P.N.Bhosale, -_.".____ Film State Phenomena I.I.T.Madras, India l-4 Feb. 1984. ,_~_..._-_--._---'
15
C.W.Pitt and L.M.Walxita, Thin --- Solid-
16
G.F.Liptrot,
Oct. 1983, PUNE (India).
68 (1980) 101.
Modern Inorganic Chemistry. The E.L.B.S. and
Mills and Boon, 2nd Ed.,London, 1974, p.240-300. 17
P.Pramanik and R.N.Bhattacharya, J.Electrochem.Soc.,_-127 (1980) 2087.
(I 984) 46 1 419
C‘llcn?i.strj~ urrtl PIlJ,sics, 1 I
461
GROWT:-IOF TIHIN FILVS :.‘: SOLUTIO?j-GAS iVTE {FACE : A X&W TEC!-NIJ!_iE -_-___________________. .._.._ _ _ .- __ --_ _-- ..-- . _-.- _ -... - _ _._ _ l_l--__ ___.-..
En--. ,y C, :?versioi? Shivaji University,
Lj’:lc?ratory Kolhapur
( 3epartnent 416
cf (India)
004
Physics,
ABSTRACT sol uti on-gas
The unifor-?
and
i.i!iS
pap”“.
tion
are
i.hz
area
The
growth
i!iscussed
solutian.
The
Inf laenc~z
films
interface
large
is
thin
of
mechanism
ton tPe
optical of
techniql;e films
basis
used
and Itiilctics of
ti;e
and c!ec+,rical
the
fcr
prepdrative
t! e formation
c1-alc)geni.des
is of
surf ace
the
film
tension
of
properties
parameters
are
on the
.>f
described
in
formathy
studied
growth
and
cf
th;n
discusstd.
I NT ~ODUCTION Thin
chalcogenide
films
fabrication
of
area
these
by vacuum
fi_lms
and chemical qrowth cted
of due
bath large
to
Therefore, uniform,
area
films
of
the of
ducting
film
Bi 2 S 3, progress
are
consists
at the cations
interface and the reaction
the
interface.
The
As2S3
PbS and
Z,?O With
growth
of
been of
thin
films
gas
at
however, source
restri-
mater<-al.
interface
technique
formation
of
thin
.and gas.
anions; required
thin
method
grow
of
solution
the
the
The
to
contains
growth
[l-4].
developed
the
forms this
is,
the of
the
pyrolysis
known
and the
by a solution-gas
method
spray
well
tecilniques
subs trate
a simple
the
0254-0584/84/$3.00
t.he
by
at
Sb&, in
films
the
contains -which
by these
for
Th e d e p (7s 1 t i o .n o f
and dry,
techniques of
int.erost
arrays. dip
new me+.hod has
s~bst~~nce
soliltio:1
product
particular
photo-diode
fi!.ms
geometry
a relatively large
of
evaporation,
deposition area
the
Basically
[5,5]. The
J.arge
are
films is
interface
0 Elsevier Sequoia/Printed
t’r,e se-niconof
studied. has
CdS, Recent
brclught
in The Netherlands
462
us closer to understanding the mfcroscopic basis of surface science. The microscopic view poi.nt of solid state physics has injected new intellectual vigo r into electrochemistry, sv..rface chemistry and the study of heterogeneous systems. The develop:ments in moleculer orientation at the Interfaces of solid-c~as and solution-gas have turned chemical kinetics into one of the most active fields of electrochemistry, Special attention has been paid to the mechanism of surface reacticns of sclutlon-gas interface films.
Theoretical and experimental results on the proper-
ties of solution-gas interfaces such as coTposition of solution, temperature, surface tension, pH and pressure of exposing gas to the solution surface are given. brief account of
phJse
Work
or!
the
The present article gives a solut~ion-gas interface and the
growth of thin films. TYPES OF INTERFACES Recent efforts have stressed the surface properties of compound semiconducting thin fiJ.ms and their interfaces.
Both chemists and physicists have intensively studied the surface properties. The molecular orientation contributes individual properties to the intecfacial regions. In order to increase interest in the sl_trface, interface neons
and thin
systems
to
films,
exp1ai.n
we have considered the
inter-relation
a :rtodel of between
the
hetsrogeinterfaces
(Fig.1) such as the solution-gas, the solid-gas and the solid-solution interface. The solution-gas interface is studied in detail. SGLUTIOti-GAS INTERFACE A gas molecule feels an attracting potential upon approaching the oppositely charged solution surface. The strength of this attracting potential determines the strong affinity of gas atoms Frequently -the ran.ge of attraction of oppo-
towards metal ions.
sitely charged ions depends on the distance between the solution If the range of the attracting surface and the gas molecules. force is short then the intraction of positive and negative ions takes place Snstantaneously due to the strong affinity of anionic species towards cationic species. If the distance of the gas source from the solution surface is increased, then the solution surface experiences 13weak intraction with the gas molecules. The electrostatic potential energy of the surface molecules of the solution involve a direct-charge transfer with colliding gas
463
-r:ol."cclPs ; sill.‘-
3 iiiarge-tr%nsfer-iq+ :;cticn is ctf !,cnJ __~,-'I:~+ 2nd
leads to lo;-!la-tic>n fof <3new ::i:~.L+ct~le \rlL?!! :!!Iltl:zliz 2ti3n of :;'r 4.rges. The
Y32Ctloq
u~321:
the
mec'i ‘XT
:f !Idture
&?d
t11e
ii\rd~::Q?Z
of
the
StllELd;,
gas
wlljli
3
xeclj..dpj
is
dc:pet?:l,~nt
the ac5dlty or slkality c!f t3e
solvent,
zlectrc r. affinity of t!ieoppositely cbar~ed ;cns.
HcrrlLck [.;;stludiedthe effect of hydrooen sl~lfide gas on the surface te:7si*ln. ~,fso!..~tior:s and a!.so sttibied the effect of iemperature on surface tenslon. Ions en the surface of a soluLion are of ions ervironment marked: y diffeJ,e?l.fron, +he wviron~-ent in iit? sc‘^‘face>f 2 SOILI t ‘3’1 in the 5ulk of the solution. The ions
is
surr3:Jnded 3~
by
fewer
neighbors
thar:
sre
the bt~J.kj~c'ns.Tiiere
anisotrzpia distrlbutio~ 05 these neighbors near the surface:
The compu+:atior in (genera! is carried cut in two parts; first the surface is pcodcced by breaking bonds, and by removing atoms adjacent to the newly created surface ions. A number of lhin fjl-Qs of chslcogenide compounds have been studied by this technique[5,6,8,9]+ Gubbins and Thompson [lo] studied the molec,Jlar orientation at the solution-gas interface and the der,sity orientation profile,
The
rnclecular arrangement at the surface has been given by Tokahashi [.lii,At the equilibrium ions tend to form tte preferred orientation at the surface of the solution. However, the sitklation is different in the bulk of the solution.
Regular arrangement of the
ions takes place at higher surface tension. Solution-Gas Interface ----7Non-rev'
i Physj.cal or Van der Waals adsorption (Reversible)
Reacting gas
t Chemisorption (Irreversible)
Solution-gas interface with a non-reacting gas : Gases like E2, Co, C02, N2, cH3, etc., do not react with the solid or'liquid surface.
These gases are anly adsorbed on the
surface of +.he solids. To interpret the adsorption phenomena at the solution-gas interface it is important to establish the connection between (r) the excess of an adsorbed substance in the surface layer (C) the concentration of surfactant in the solution (C) and the surface tension at the solution gas interface (6)for
dilute solutions.
464
Liquid-Liquid \
Solid -Solid
Fig. 1. Types of Interfaces.
(1) AccoT-ding to Gibbs the quantity db -777
- G = surface activity
Therefore
c m
- G .-
(2)
Electrostatic forces undoubtedly affect the surface tension of the sslzltion in many instances,
Consider an aqueous solution of bis-
muth citrate in which the citrate anion is a hydrocarbon chain and ai3+ is a cationic species; this compcund is highly surface active and it is certain that it is a mixture of anions arid cations which lowers 6
and consequently accumulates in the surface layer. Howe-
ver the anio;-,sare charyed and their preferential transfer towards the surface means a separation of charges takes place.
Bismuth
citrate has a permanent dipole moment and orientation in the surface layer is such that the dipoles are parallel to each other; thus electrostatic repulsion takes place.
However, in aqueous
medium the surface activity increases with an increase in the length of the hydrocarbon chain.
It is found that on addition of
hydrocarbon radicals the surface activity of the aqueous solution increases on average 3.2 times per CH2 group [12].
According to
Langmuri's equation at low concentration of surface active substances the surface tensicn of the solution decreases in direct proportion to the concentration A=6-,-6= where A
C
as
kc = decrease in surface tension, 6, = surface tension of pure solvent and k = constant
(3)
465
Slurf ace
adsorption
Adsorption
is
tration
of
example
between
are
between
them
the surface
forces
surface
and the
Adsorption by the
is
or
pt,iion
equilibriam.
a process
amount
of
surface
small
particles
adsornot
tk,e layer
plane
field
ions
the
w‘rlich
that
a surface
force
attract
of
arises
3r molecules
of
the
i.e.
adsorbant
of
these
it
two
mutually This
the
accompanied
, t,he passage surface
equilibrium. of
is into
of
the
opposite is
known
adsoroticsn
adsor-
solutj.on. processes as
adsor-
equilibrium
factors.
ioils
are
adsorbed
adsorbed
of
the
and large
on the is
surface
and adsorption
surface
directly
areas
so
of
t,he adsorbent.
proportional
Amorphous
adsorbant.
crystalline
hand,
because
desorption;
dynamic
substance
areas
the
substance so
and
at
are
and in the
a free
can
The position
area
other
surface
of
or
total on the
of from
various
Substances
substance
process,
occurance
to
The
part.icles
called
the In
balanced.
the
is
directions;
Therefore,
process
leads
upon
into
film)
a substarnce
within
all
For
solid
concentrated
solid of
concen-
[12:.
molecules
Tl-ie simultaneous
depends
mutually
a reversible
oppcsite
ions
the
in
phases.
thin
usually
as particles
balanced.
substances
is
in
two
or
surface
neiyhbors
changes
between
of
on the
are
the
(precipitate
surface
directed
are
dissolved
bed
The
their
interface
substance
same conditions
the
the
the
1.0 describe
phase
molecules
with
only at
or
t’he bound
fcrces
at
a sol.id
a solid.
Ions
under
used
The dissolved of
bdnt.
term
substances
a solution. surface
the
adsorption
precipitates
to
precipitates
have
is
the have
great;
much smaller
smaller.
is
STRJCTURE OF SOLUTION SURFACES The surface
structure tension
dynamics
of
Surface and it in one
spite
of is
explained
surface
tension.
tension
has of
a solution
been the
has
succeeded
transition
layer
is
by
a well-known
studied
and the
a hybrid
in
creating
explaining
cause
has for of
mechanism
approach
phenomenon
by many scientis-ts
many ways in
surface
for been
using
therno-
common to
all
hundreds
of
investigated,
a surface surface
of
with
tension.
liquids years: no
a
466
It is reoorted fll: that if the liquid is subdivided into a droplets of less than a certain critical size, the liquid becomes a super-liquid with a different structure and increased entrony. The conditions at the outermost surface of a liquid, where the ambient gas and the normal liquid are in contact, allow the existence of a super-liquid. In order to understand the structure of a liquid surface, Takshashi .rll; has made the following assumptions : . i) The whole surface is covered with the surface active layer or a super-liquid, ii) The Gibb's Helmholtz equation showing the surface holds :
energy W=rand
Tddl
dT
(4)
S = - dd / dT
where
U
and
S
(5) represent
the surface energy and entropy,
respectively due to a change in unit area of the surface. iii} The entropy sum is zero at the surface : s1 + s*
= 0
(6)
where Sl and S2 are the differences of the entropies of the super-liquid and the underlying layer from that of the normal 1iqui.d respectively. As Sl is positive, S2 is made negative by eq.(6), so it is understood conceptually that the underlying layer is ordered, and it is called the tension layer. Fig.2 shows the scheme of the proposed surface active ions consisting of two layers of different entropy. Thickness of the super liquid -_ If the change in entropy represented by Sl and S2, due to the change in unit area of the surface is As1+AS2
=
0
(7)
we obtain TASl + TAS2 = 0
(8)
467
This relation shows that if one iayer generates heat, the other layer absorbs the same amount of heat, now TAS2
is
given by
eqn (5) as, - Tddj
(9)
dT
TAS1 is given by hoH
where
h = thickness of super liquid Q,= density of super liquid H = heat of normal-to-super transformaiion is given in cm by the equation :
Thus
h
h=_
T(db/dT) x 0.24 x 1G-7i~H
(10)
The value of h calculated from eqn (10) for H20 is 660 .&and for NaCl solution is 1060 A". The vcilue of Q, was regarded as the same as that of the normal liquid. The value of Ho were caiculated by Takahashi ill] it is seen that, HQ
=
0.134 cdl/cm2
(li)
wzs GAS super Wuicl ( Surfaceactive Layer 1 K
\Under\ying
Layer
52 Tension Norma\
Layer Soiut ion
Fig. 2. Model of surface structure of liquid consisting layer of surfactive ions and an underlying layer of solution, with respective entropy differences S1 and S2 from a normal salt solution.
Thickness of the tension layer -.-As indicated, the entropy of the tension layer is lower than that of the normal liquid, so this layer is regarded as a kind of thin solid film. The thickness of the layer is calculated as, t = hH/H'
(12) where H' is the heat of fusion of the normal solid. The thickness of the tension layer can be calculated using the above equation.
468
,,.‘l,,,,,,,f__...
._..- -
.----_-
_._-___--.-_
_. .___
-7 The solution surface is slowly exposed to the fJrmation of a thin solid film.
'The gas is bubbled through the _.* oulk of. liquid giving Precipitation - ._._. ._~____
Xhen a reactinq gas is bubbled through the bulk of the solution, precipitation occurs; the precipitation depends upon various parameters - solubility product, temperature, concentration, solvent medium, etc.
There are three types of precipitate, namely,
crystalline, curdy and gelationous. particle size.
These differ only in ti-leir
According to the Van '2eimertnrule [13], slow
precipitation gives larger particles; i.e. slow crystallization gives larger crystals, because ions or molecules have time to rearrange themselves in an orderly manner upon tile faces of the crystals.
In very rapid crystallization, ions or molecules
cannot build on the existing crystals fast enough ano so they form new nuclei and very small crystals. The speed of precipitation is greater when the solution concentration is high and it depends upon the solubility of the precipitate, i.e. Q-S Speed of precipitation = K -7where
(13)
Q L: momentary concentration of solution, S = solubility and
K = proportionality constant.
The numerator Q - S expresses the force favoring precipitation and the denominator S expresses the force opposing the precipitation. The formation of a solid phase from a solution invclves two processes, one of which is nucleation and other is particle growth. The size of the particles of a solid phase is dependent upon the relative rates at which these competing processes take place. Nucleation For a precipitate there is a minimum number of ions or molecules required to produce a stable second phase in contact with a solution; unless a number of ions or molecules collects together, a solid phase having a finite life time will not exist in the solution.
469
The minimum sized stable particle is called a nucleus. nA+ + nB
T-L
(Ad),
(14)
where n is the minimum number of A+ and B+ ions that must combine in order to yield the stable particle (As),: . Tarticle growth l__ll..^~.l_._ _-. The second process that can occur during precipitation is the growth of a particle already present in the solution.
This growth
can only begin when nuclei particles are present in the solution. In the case of an ionic solid, the process involves deposition of cations and anions
on aoorooriate sites in the orecipitate.
(AB), + A+ + B-
+
(.@) n+l
+ A+ + B-
(15.3)
(AB),+l
+==* (AB),+~
(15b)
Growth of thin films at the interface A typical experimental set up [53 designed and fabricated by us for the formation of thin film at a solution-gas interface is shown in Fig.3.
It consists of mainly three compartments, namely,
gas chamber, uniform gas exposing unit and the solution container. Purified, dry H2S gas is placed in the glass chamber to the desired gas pressure.
The growth of a thin film at the interface
mostly depends on the gas exposing unit, the details of which are shown in F1 g.3.
It consists of mainly uniformly distributed
lumps of glass wool sandwiched between nylon cloth, plastic wire gauze and a metallic sheet with a circular hole.
The solution is
taken into the glass ccntainer which is modified to enable
the
solution to be removed from its bottom without disturbing the surface of the solution.
For this a vibrationally free mechanical
arranqement is provided to move the solution up and down. In computing the interaction energy between the surface and gas, the surface may be treated as a single conductor system. A gas atom is assumed to interact with the surface as a whole instead of with individual surface atoms. The whole solution surface comes under reaction at the same time; the surface tension of the solution decreases as the pressure of the hydrogen sulfide gas increases.
When the surface tension of the solution was lowered, the
film formed on the surface was non-uniform and ruptured in places.
470
Ex erimental set-up for solution-gas interface technique: Fi 3 1 z>s*cham ! er; 2 Uniform gas exposing unit, M=metallic sheet, w = plastic guaze, N = nylon cloth, G = glass wool. 3,Shutter; 4. Solution container; 5. Purified H2S gas; 0. Gear wheel; 7. Axial wheel.
Th.n films of a number of chalcogenide compounds were prepared by the solution-gas interface technique, such as Bi2S3, Sb2S3, As2S3, CdS, PbS, MoS2 and ZnO. We have discussed the growth of a bismuth sulfide film [5]. The bismuth sulfide thin films are grown at the interface between the hydrogen sulfide gas and the bismuth nitrate solution.
In the bismuth nitrate solution
(pH = 3.5) the solvated BiO+ ions are weakly bound with unsolvawhen the molecule of hydrogen sulfide comes into ted N03-ions. contact with the surface of the solution, then the bonding between H' and HS- ions of the H2S molecule is broken:
471
H+ +
H2S
-.---)
(16)
HS-
Hydrogen sulfide gas is a weak acid dissociating as follows : H$ K
_
1-
K1, -
i-H20
H30
(;130+) (HS-) _.-_.--
+
+
ES-
= 5.7 x 10-8 mol/l .
22s)
5
HS- + H2O 7
H30+
-I-s2- ,
(H,o+)(S--) __......__~____= 1.2 x 10-15 mol/l. K2 = ..I_ ( HS-) Eqs.(l7) and (19) combine to give an impression for the overall dissociation of hydrogen sulfide gas into sulfide ion : H2S + 2H20
<===
2H30+
+
S2-
(20)
(H30+)2 (S2-) __-I.---.-.-__.-. (H,S)
K1' K2 =
= 6.8
x
(21)
io-23mol/l.
The constant for this relation is simply the product of K1 and K2. The solubility of H2S in water is about 0.1 F; for practical purposes, we may assume that
[H2S] = 0.1 mol/l.
Substituting this value in eq.(21) we get,
p30+i2 [S2_]
---
--
= 6.8 x 1O'23 mol/l.
(22)
0.1 Therefore
mol/l.
(23)
In general, the gas molecule and solution follow the reaction path thus : if molecules of gas A and gas B react over a solution surface S such that we get the product C : k+B
-,.
A + B --+ A- S+B.*
C (gas phase reaction) A-S c+s
(24)
Here the solvent serves as a catalyst, which is free after the reaction; that means that A and B (H+ and S2-) react with the solution surface S (with Bi 3') to give the product C (Bi2S3). For example, when the metallic salt solution is exposed to hydrogen sulfide gas, breaking of bonds takes place due to the strong
3
2
1
0
1.0
LUG CONC. Fig.4. Variation of thickness with concentration for Bi2S3 thin films.
aff$i-ity of oppositely charged ions.
The triply charged bismuth
(Bi ) ion has a relatively small size and exerts a large polarizing force to attract the large sulfide ion to form the bismuth sulfide molecule.
The sequence of reactions taking place is
Bi(N03)3 + HN03 + H20
----t(BiO+
----- N03-) + 3HN03
(25a)
BiO+ --- N03- + H+ - SH- ----?- BiO+ -.-- HS- + HN03
(25b)
2BiO+ ----- f-S- + H+ ---- HS-
(25~)
-tBi2S3
+ 2H20
Reactant = Intermediate = Product. 2Bi(N03)3 + HN03 + 2H2C + 3H2S --+2(BiO+----
N03-) +
+ 5HN03 + 3(H+ ---- SH-) -----+Bi2S3 + 7HN03 + 2H20
(25d)
The overall reaction is 2Bi(N03)3 + 3H2S
-+Bi2S3
+ 6HN03
(25)
with a bond breaking and making process taking place due to the strong affinity of oppositely charged ions.
The electron affinity
of a sulfide ion towards a bismuth ion will be stronger than other ions present at the solution surface. Bismuth nitrate solution reacts with hydrogen sulfide gas as shown in eqn (25).
473
Fig. 5. Constant temperature assembly for solution-gas interface film growth : 1) Solution container; 2) Glass substrates; 3) Glass stand: 4) Aluminum bath: 5) Solution outlet: 6) Water inlet; 7) Water outlet.
Effect of preparative parameters _-_._...._.j_ _---_-_This technique is relatively much simpler and more convenient than the other techniques, and does not require any sophisticated or delicate equipment.
However, the growth of the thin chalcoge-
nide films is influenced by the various preparative parameters, namely, composition of the solution, temperature, pH, H2S gas exposure time, H2S gas pressure, and surface tension [14].
Composition of the solution The concentration of the salt solution was varied from 0.001 to 0.5M to grow the films.
At low concentrations the film formed was
not uniform, as the required number of cationic species may not have been available on the surface of the solution.
However, as
the concentration increased the quality and uniformity of the film increased.
The variation of the thickness with the solution con-
centration for Bi2S3 film is shown in Fig.4.
The film thickness increased up to 0.1M concentration and attained a steady state
value. Effect of temoerature A constant temperature assembly for solution-gas interface film growth is shown in Fig.5.
The temperature of the solution
474
0.35
0.25 ^i % 1
0.15
z
/ P
0.05
(.
0
10
20
30
40
50
TEMP (Co) Fig.6. Variation of thickness with temperature for Bi2S3 thin film. varied-from 5 to 5OoC and films were formed. thickness with temperature is shown in Fiq.6.
The variation of the The film thickness
increased initially with a rise in temperature and attained a steady state value beyond 40°C.
However, hign temperatures can
cause the film to expand and may ultimately result in thermal forces overcoming the rather weak van der Waal's binding forces which hold the molecules together.
At room temperature films of
definite thicknesses can be prepared without any contrast difficulty. Effect of pH .-I-___-__ The growth of thin films by the present method is very much dependent on the pH value of the solution. An increase in pH causes an increase in the relative molecular area. However, as a result of this, the moleculer cohessive binding forces between the mofecules are expected to decrease, causing reduced stability of the film on the solution surface i.lS]. With the increase in pH the solubility of hydrogen sulfide gas increases relatively and may result in hydroxide precipitation. The deleterious effects can be avoided by optimizing the pH value, SO that the desired quality of the chalcogenide films can be achieved. In the case of a low pt-l value, the metal hydroxide will not precipitate and
1
1.4
1.4
2.2 hv(eV)
2.4
3.0
Fig. 7. Variation of absorption coefficient (4) with energy for Bi2S3 film. reaction of the film with the solution will be comparatively small,
The films deposited from low pH value solutions have
been observed to exhibit minimum defects,
The optimum pH value
is thus determined as one in vihich both uniformity and adhesion are satisfactory and there are minimum defects. pH value (((7) is
A relatively low
desirable to produce thin chalcogenide films.
Effect,_o;t_iaS_gaspress= It. is in fact the case for most chemical reactions that the equilibrium is disturbed by pressure.
In a gaseous equilibrium,
the equilibrium position is found to be influenced by pressure, if there is change in volume (i.e. the number of moles of the gaseous reactants) during the reaction.
A rise in pressure tends
to push the reaction to a smaller volume, and lowering of the pressure tends to push the reaction in the forward direction,, However, at low pressure the molecules are loosely packed in a large area and may be separated when the gas reacts with the exposed solution surface, thus ensuriny that a small change in pressure does not cause the film to break up or fold over.
As a result, the uniform large area films are grown only at low pressures. Furthermore, higher pressures of the H2S gas cause the rupture of the thin film. This is in general accordance with the
476
40
*
4 0
30
.
/
/ /
0
0 30
-
/ 0P
10
.
/ &? 1
o-0 0
/OH ,’
L
I’
1.5
.
1.75
2.0
2.25
hv(eV) Fig. 8.
Plot of (dhzlf2.._-versus
(h*j)for Bi2S3 thin film,
principle expounded by Henry Le Chatelier [16].
This principle
will be applied with some caution to the behavior of chemical equilibria in solution-gas interface reactions.
When the molecu-
les of the hydrogen sulfide gas come in contact with the solution surface, the bonding between IiS and SH- ions is broken : H2S (g)
--+
H+
+ SH-
(26)
Effect of gas exposure time ----_-_A long exposure of the hydrogen sulfide gas over the solution surface results in precipitate formation by rupturing the film. Therefore, a specific time is required for completion of the reaction. In a surface chemical reaction the bond breaking and making processes involve a definite time interval.
In the present
study the gas exposure time was varied from 15 to 180 sec. It was found that uniform films were obtained in between 60 to 90 sec. Effect of surface tension The growth mechanism and kinetics of film formation are related to surface tension.
This is the result of the existence of an
internal pressure; i-e. a force which draws molecules into solution and which is perpendicular to the surface. The greater the polarity of the substance the higher will be the internal pressure, as it is caused by molecular forces. By virtue of the surface
JO’
5.0
1.6
2.0
?.5
3.0
+G3(12jl) Fig,
9.
Plot
of
tension
effect
tighUy
and
introduced the
the
idea
structure
Surface
surface
of
and tension
activity. with
Optical
and
T!!e films
of
of the
the
formed
Bi&
thin
solution,
the
films.
ions
are
bound
structure.
Solution, layer,
solution
to
which ff
the
may exhibit
tension
depends
an increase
electrical ------_.--
the
for
Takahashi [ll) describe the Structure
of
thickness of the super-liquid and underbeen calculated using the model of the
a tension
Surface
Sharply
x lo3
a Super-liquid The have
layer
Super-liquid
l/T
a two-dimensional
surf ace.
(tension)
Surface
R versus
the
on
form
solution
lying
log
in
consists
of
solution
a layer
has
considerable
of
a
a high surface
an concentration,
and
drops
concentration.
characterization were
Supported
on a glass
substrate,
conduct-
stainless steel subt;tcate, or The optical absorption of the chromium plated stainl.ssS steel. film w:is mcasJred as a funct2 on of energy in the range 1.3 to 3 ef! Tfle curve absorption to obtain the band gay, of the material, coefficie4 (a) ~trsus photon energy (t’v)is plotted in Fis.7. The tng
glass
absorp-Lion which
is
mirror-grade
coefii::ient cbaractcrist~_c
poJ ished
of of
the
film
direct
is
of
transitions.
*he
order
0:
li’ccm
-I
,
CONCLUSIONS The solution-gas interface technique has the advantage of being capable oi producing pure sulfide films for a number of compounds.
The technique is mainly adopted for producing uniform
large-area films of definite thickness.
Thin films.of a number of
chalcogenide compounds can be prepared by chemical as well as physica
methods, but some contrast difficulties arise duz5n7
deposition, i.,e_.cost of the material, instrumentation, fairly large area, reproducibility, surface area of deposition, and temperature are the disadvantages of the ohysical methods. On the other hand,
low
cost
per surface area films of distinct advantages
can be prepared by this technique.
The films which are prepared
by this method show properties which are comparable with those formed by other conventicnal methods.
This work was supported by the Department of Non-Conventional Energy Sources, New Delhi, under Contract No.r(l)i3/8l-NES/4~8. Assistance of the ECL group during preparation of this manuscript Is gratefully acknowledged. REFERENCES 1
C.Ghosh and B.P.Verma, Thin Solid Films, 60 (1979) 61.
2
B.B.Nayak, H.N.Acharya, T. K. Chaudhuri and G.%.Mitra, Thin Solid Films, 92 (1982) 309.
3
Y.Ma.Yale and R.H.Bube, J.Electrochem,Soc. 124 (1977) 1430. __--.
479
a
N.R.Pavaskar, C.A.Menzes and A.P.B.Sinha, .._._ J.Electrochem,Soc., _I-_..- _.__
5
s.H.Pa;~ar, P.N.Bhosale, M.D.Uplane and S.P.Tamhankar,
124 (1977) 743.
Thin Solid Films 110 (1983) 165. _-___-_____.__ __..._!.._--_ 4
S.H.Pawar, S.P.Tamhankar, P.N.Bhosale and M.D.'Jplane, Ind.J.Pure ..--_._~_ Appl.Phys., 21 (1983) 665. .__.__-_..
7 8 9 i0
C.S.Herrick and G.L.Gaines, .__-.'!-_ J.Phys.Chem 77 (1973) 2703. S.D.Sathya and A.P.B.Sinha, Thin Solid Films 37 (1976) 15. pp___-S.D.Sathya and A.P.B.Sinha, _-----I--_ Thin Solid Films 44 (1977) 57. K.E.Gubbins and S.M.Thompson, Proc. Faraday_*.Chem.Soc. ----(G.B.), 16, (1981) 59.
11
R. Takahashi, Jpn.J.Pure Appl.Phys., 22 (1983) 17.
12
S.H.Pawar and P.N.Bhosale, National Seminar on Emerqinq _.__._-_-_--______ Trends in SlJrfaCes_,_interfaces and Thin Films 16-18, --_---_-__-_ .,-) --.--
13
D.A.Skoog and n.M.'West, --__ Ftindamentals of Analytical.-.__-__ Chemistg-, Holt Rinehart and Ninston, Inc., New York. 2nd Ea.(1969) p.164.
14
2nd All India Conference on Thin S.H.Pawar and P.N.Bhosale, -_.".____ Film State Phenomena I.I.T.Madras, India l-4 Feb. 1984. ,_~_..._-_--._---'
15
C.W.Pitt and L.M.Walxita, Thin --- Solid-
16
G.F.Liptrot,
Oct. 1983, PUNE (India).
68 (1980) 101.
Modern Inorganic Chemistry. The E.L.B.S. and
Mills and Boon, 2nd Ed.,London, 1974, p.240-300. 17
P.Pramanik and R.N.Bhattacharya, J.Electrochem.Soc.,_-127 (1980) 2087.