Molecular and Biomolecular Electronics - ACS Publications

16 Colloid Chemical Approach to Band-Gap Engineering and Downloaded by UNIV MASSACHUSETTS AMHERST on June 18, 2013 | http://pubs.acs.org Publication Date: May 5, 1994 | doi: 10.1021/ba-1994-0240.ch016

Quantum-Tailored Devices Janos H. Fendler Department of Chemistry, Syracuse University, Syracuse, NY 13244-4100

Silver-magnetic and semiconductor particulate films have been generated in situ at monolayer- and bilayer-lipid-membrane interfaces. The chemical method of preparation involved the attachment of one of the precursors to the membrane and the infusion of the other precursor across the membrane from the opposite side. Silver particulate films have also been generated by the electrochemical reduction of silver ions at monolayer interfaces. Evolution of the particulate film involved the initial formation of well-separated, 20-30-Å diameter and 5-6-Å high nanoclusters that grew in height to longer and more densely packed conical particles, which ultimately formed the interconnected arrays of the porous particulate film. Preparation, characterization, and potential applications of these systems are discussed.

CENT ADVANCES IN MOLECULARB -EAM EPITAXY

have permitted the atom-by-atom generation of heterostruetures. Such band-gap engineering has led to the formation of quantum-confined semiconductor particulates and particulate films that have enormous potential applications in memory storage and optical switching devices. This type of molecularbeam epitaxy requires ultrahigh-vacuum technologies, ultrapure facilities, and unique and expensive instrumentation. The approach has been limited to very few selected semiconductors, predominantly involving the GaAs-AlAs system. Furthermore, the approach primarily involved physicists. Realizing the opportunities of semiconductors we initiated a colloid- and surface-chemical approach to advanced materials in our laboratories. The approach was inspired by the organizational capability of the biological membrane, and the approach utilized monomolecular0065-2393/94/0240-0413$09.08/0 © 1994 American Chemical Society

In Molecular and Biomolecular Electronics; Birge, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1994.

414

MOLECULAR AND BIOMOLECULAR ELECTRONICS

Downloaded by UNIV MASSACHUSETTS AMHERST on June 18, 2013 | http://pubs.acs.org Publication Date: May 5, 1994 | doi: 10.1021/ba-1994-0240.ch016

and bimoleeular-layer surfactants as active matrices for the generation of metallic, magnetic, and semiconductor particulate films. The relative ease of preparation has permitted the detailed characterization of this system, both in situ and ex situ. Our initial results on the generation, characterization, and utilization of silver, magnetic, and semiconductor particulate films are discussed in this chapter.

Generation of Metallic, Magnetic, and Semiconductor Particulate Films at Bilayer Lipid Membranes Bimolecular, thick, bilayer or black lipid membranes (BLMs) provide the thinnest human-made semipermeable barrier that separates two compartments containing aqueous solutions {1,2). Advantage has been taken of BLMs to model the functioning of the biological membrane by incorporating synthetic and natural ion carriers (3,4). Sensitive electrical measurements, voltage clamping, and single-channel recording have formed the bases of our current understanding of biological-transport mechanisms (4,5). BLMs have been used in our laboratories as matrices for the in situ generation of metallic (6), magnetic (7), and semiconducting particulate films (8-11). BLMs were made by "painting" the BLM-forming solution across a 0.80-mm-diameter Teflon hole separating two compartments that contained 0.10 M aqueous KC1. Thinning of the films to 50 ± 5-Â-thick BLMs was monitored by observation of the reflected light and by capacitance measurements (9). Silver particles were prepared in situ from A g N 0 on B L M surfaces (6). Aqueous A g N 0 was injected into one side of the B L M and was thoroughly stirred. Silver ions, attracted to the B L M surface, were reduced either by photolysis (illuminating one side of the BLM) or by the addition of 5 uh of 50% aqueous hydrazine to the opposite side of the B L M . A few seconds after the hydrazine injection, small greenish-yellow dots became visible, through the microscope, on the B L M surface. With time, these dots grew in number and size. Merging of the islands resulted in the appearance of a nonuniform silver film. No continuous smooth silver mirror could be formed on the B L M surface. Some control of silver deposition could be accomplished by the prompt exchange of the aqueous solution surrounding the B L M . This also resulted in the removal of all excess ions and particles that were not attached to the B L M . Preformed, positively stabilized, ultrasmall (70-À mean diameter) magnetic particles could be attached to the negatively charged B L M surface (6). Semiconductor particulate films were formed on the B L M by introducing freshly prepared stock solutions (0.10 M) of C d C l , Z n C l , C u C l , or InCl (typically 10-15 uh) to the cis side of the B L M (containing 1.0 mL solution of buffered KCl). Subsequent to 10- to 15min incubation, H S gas (20-25 uh) was injected into the trans side of 3

3

2

2

2

3

2


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Colloid Chemical Approaches

415

the B L M . Sizes of mieroerystalline semiconductors formed on the cis side of the B L M depended on the rate and amount of H S injection. 2

Generation of Metallic and Semiconductor Particulate Films at Monolayers C h e m i c a l Generation. Semiconductor particles were gener ated in situ at the monolayer headgroup-aqueous subphase interface (12-20). An aqueous metal-ion solution (1.0 Χ 10" M C d C l , Z n C l , or C u S 0 ) constituted the subphase. Either a commercial Lauda model Ρ Langmuir film balance or a simple circular trough was used for monolayer formation and the subsequent generation of semiconductor particles (15). The Lauda film balance was enclosed in a Plexiglass hood and placed on a Micro-g optical-isolation table. The water surface was cleaned several times by sweeping with a Teflon barrier prior to monolayer formation/The subphase was deemed to be clean when the surface-pressure increase was less than 0.2 dyn/cm upon compression to one-twentieth of the original area and when this surface-pressure increase remained the same subsequent to aging for several hours. An appropriate amount of chloroform solution of the chosen surfactant (8 Χ 10 molecules per mL) was carefully injected onto the clean, thermostated (25.0 °C), aqueous surface. The surfactants were com pressed at a rate of 2-5 Χ 10" Â per molecule per second. Subsequent to 5-15 min of incubation at the desired surface pressure [25 mN/m for arachidic acid (AA) and 50 mN/m for bovine-brain phosphatidylserine (PS)], dioctadecyldimethylammonium bromide (DODAB), n-hexadecylll-(vinylbenzamide)undecyl hydrogen phosphate (1), and bis(2-nhexadecanoyloxyethyl)methyl(p-vinylbenzyl)ammonium chloride (2)], 200-250 uL H S was slowly injected into the nitrogen-filled Plexiglass hood covering the film balance. The semiconductor particulates were transferred onto solid support where their composition and crystal structure were determined by X-ray and electron-diffraction measurements (16).


3

2

2

4

17

3

2

2

The schematics of the circular, 4.0 cm deep, Pyrex trough (7.0 c m surface area) used for in situ semiconductor particle generation are shown in Figure 1 (15). The trough was placed on a clean (chromic acid and copious amounts of water) flat glass plate and covered by a circular (7.5-cm high and 14.5-cm diameter) glass jar whose flat frosted bottom provided a gas-tight contact. The water surface was cleaned by sweeping it through a water aspirator. A n appropriate amount of the spreading solution (1.5 X 10~ M surfactant in CHC1 ) was carefully injected onto the clean subphase to give a coverage of 20 Â per molecule of A A and 40 Â per molecule of PS, D O D A B , 1 and 2 . Subsequent to 20 min of incubation, a Hamilton syringe containing 2 0 0 - 5 0 0 - M L H S was intro2

3

3

2

2

2



416

MOLECULAR A N D BIOMOLECULAR

ELECTRONICS

Figure 1. Schematics of the experimental arrangements (not drawn to scale) used for the generation of semiconductor particles at the negatively charged, surfactant headgroup-aqueous (1.0X 10~ MMCl ) subphase interface and that used for the in situ monitoring of reflectivities. Ρ is polarizer, D is detector, and R is chart recorder. 3

2

duced into the atmosphere covering the monolayer via a rubber septum (Figure 1). The barrel of the syringe was kept at the same position. This ensured the extremely slow (several hours) diffusion of H S into the chamber and, hence, into the monolayer-water interface. Formation of semiconductor particles was monitored by reflectivity measurements. The semiconductor particulate films formed were transferred onto solid substrates by horizontal lifting for subsequent characterization (16). Silver particles were formed analogously at negatively charged monolayers floating on aqueous A g N 0 upon exposure to formaldehyde (see Figure 2) (21). 2

3

Figure 2. Schematics (not drawn to scale) illustrating the chemical gen eration of silver particles.



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Electrochemical Generation. Electrochemical reduction of sil ver ions at the interfaces of monolayers prepared from positively charged surfactants resulted in the two-dimensional (2D) formation of uniform films that consisted of interconnecting, roughened silver particles (22). The schematics of the experimental setup are shown in Figure 3. A 1.0mm diameter, 3-cm-long silver electrode was immersed into the subphase. Electrical connection was made through a 20-Mm-diameter plat inum electrode that was floated (subsequent to monolayer formation) on the water surface at the middle of the trough. The surface of the aqueous 1.0 X 10~ M A g N 0 solution was cleaned with a water aspirator just prior to monolayer spreading. A n appropriate amount of the spreading solution (1.0 Χ 10~ M surfactant in CHC1 ) was carefully injected into the cleaned aqueous surface to form 20 À molecular monolayers from negatively charged surfactants. Ten to 20 min subsequent to monolayer formation, a potential of 1.8-1.9 V was applied across the electrodes (keeping Pt to be negative) by means of a direct current (dc) power supply. 3

3

3

3

2

The silver-particulate films formed at the surfactant-water interface were transferred to solid substrates by horizontal lifting (22). With time, silver particles grew concentrically, forming larger and larger circles at the monolayer-water interface. The rate of this 2D growth was 1-2 cm /h. Importantly, no silver particles could be observed upon applying the same potential to the water surface in the absence of surfactants or to monolayers prepared from positively charged surfactants. Negatively charged monolayers are essential for the electrochemical generation of silver particles. These monolayers provide binding sites for silver ions that are reduced at the cathodic surface. The initially formed silver particles extend the cathode and continue to reduce silver counterions at the monolayer surface. The absorption spectrum of silver particles on quartz is unique in that it shows an interband transition at 321 nm, in addition to an absorption maximum at 387 nm (Figure 4). Interband transitions characteristically appear in the reflection spectrum of rough surfaces (23-25). 2

Figure 3. Schematics of the circular trough (not drawn to scale) used for the electrochemical generation of silver particulate films at monolayer interfaces.


418

MOLECULAR A N D BIOMOLECULAR ELECTRONICS


12 .0

Γ

02 .0 00 .0 I 200

1

1

1

1

1

1

300 400 500 600 700 800 WAVELENGTH, nm

Figure 4. Absorption spectrum of a silver particulate film prior, a, and subsequent to 5 min, b and 15 min, c annealing at 300 °C. The silver-particulatefilm was formed electrochemically at the interface of a monolayer prepared from a dialkyl-polymerizable phosphate surfactant. The subphase contained 1.0 X 10 M AgN0 . 3

3

Characterization of Particulate Films Growth of particles at B L M and monolayer surfaces was monitored by reflectivity measurements (15). A typical behavior of the reflected-light intensity as a function of F e 0 particle deposition onto the glyceryl monooleate (GMO)-BLM is illustrated in Figure 5 (7). The perpendic ularly polarized 6328-À laser line was allowed to impinge upon the middle of the cis side of the B L M at an incident angle of 45° during the introduction of the magnetic F e 0 particles into the trans side of the bathing solution. Point A in Figure 5 represents the introduction of ca. 0.5 mL of 4.0 Χ Ι Ο " M solution of stabilized F e 0 into the bottom of the B L M bathing solution by a Teflon syringe. Parallel with the devel opment of a brown color in the bathing solution, a thin gray layer ap peared on the B L M . With time, the intensity of the reflected light in creased exponentially to a plateau value. At this point (point Β in Figure 5), the aqueous bathing solution, surrounding both sides of the B L M , was carefully exchanged with dust-free pure water by pumping. Water exchange continued until no measurable absorbance (A < 10~ at 300 nm in a 1.00-cm cell) was detected in the bathing solution. Subsequent to this water exchange, all of the colloidal F e 0 particles remained firmly attached to the very much stabilized B L M , where they formed a particulate thin film. This particulate thin magnetic film could not be pulled away from the B L M by a magnet even as strong as 400 Oersted. 3

4

3

4

3

3

4

4

3

4



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419

TIME (MINUTES)

Figure 5. Intensity of the light reflected from the middle of the cis side of a GMO BLM as a function of time prior and subsequent to the deposition of Fe 0 particles into the trans side. The perpendicularly polarized 6328-Â laser line was allowed to impinge upon the cis side at a 45° incident angle. Point A represents the introduction of the magnetic particles. Point Β represents the interchange of the solution bathing the BLM. 3

4

Apparently, strong forces maintain the magnetic particles in the matrix of the B L M . These Fe 0 -particle-coated BLMs remained stable for sev eral days. The steady value obtained for the reflected-light intensity was found to be independent of the concentration of the F e 0 particles injected but depended somewhat on their deposition rate. Similar results were obtained on introducing the ferromagnetic particles into both sides of the BLMs. Reflectivity and capacitance measurements have provided the basis for constructing a model for the GMO-BLM-deposited thin particulate magnetic film (7). Prior to the deposition of magnetic particles, the optical thickness of the G M O - B L M , d\>, was determined to be 62 ± 2 À. This value is the sum of the thicknesses of the hydrocarbon bilayer (d = 48 À), and the polar headgroups (d = 6-8 Â) and agree well with that reported in the literature (26). Capacitances measured across the G M O - B L M were found to change less than 1% during the entire course of formation of thin F e 0 films. This small percentage of change is interpreted to imply that the magnetic particulate thin film does not penetrate beyond the headgroup region into the B L M . The density of the magnetic particles per unit area in the G M O - B L M was assessed, therefore, by the effective medium theory, 3

4

3

h

4

p

3


4

420


fruitfully employed previously in treating island films and other composite structures (27). A model of the thin magnetic F e 0 film on the G M O - B L M , and the parameters used in the films assessment are shown in Figure 6. The calculated diameter of the magnetic particle on the B L M is smaller than that measured for the isolated particles, but it agrees well with that determined by transmission electron microscopy for a similarly deposited particulate F e 0 layer on a phospholipid monolayer (59 ± 2 A) (28). Introduction of the magnetic particles into the bathing solution results in their attraction to the surface of the G M O - B L M . This attraction is strong enough to overcome electrostatic repulsions between neighboring particles, and it permits the coverage of the B L M to the extent that it is tantamount to the formation of a monolayer of particulate film. Figure 7 represents the artist's impression of such an F e 0 particulate film. Electrostatic forces provide a long-term stability. The integrity of the individual, tightly packed particles remain intact for extended periods on the B L M . The observed optical data are not compatible with the presence of a smooth-surfaced, solid-state thin film (29). Conversely, the data are compatible with rough surfaces (30). Once the B L M is coated by a monoparticulate layer of magnetite, the forces of the membrane are insufficient to allow the deposition of a second layer of F e 0 particles. Reflectivity and Brewster-angle microscopy (31,32) provide the most convenient methods for characterizing semiconductor particulate films 3

3


ELECTRONICS

4

4

3

4

3

magnetite particles

BLM

Figure 6. Schematics of the particulate thin Fe 0 film on one side of the GMO-BLM. 3

4


4


16.

421


FENDLER

Figure 7. Artist's rendering of the GMO-BLM coated on both sides by monoparticulate Fe 0 thin films. The arrows represent magnetization vectors in a section of the assumed 400-Â Néel wall and indicate the in-plane rotation. The transmembrane wall-to-wall interaction is purely speculative. 3

4

both at monolayer interfaces and in the solid state. The effective refraction indices, n , and volume fractions, F, of arachidate-monolayersupported semiconductor particulate films are collected in Table I. The semiconductor particulate films subsequent to their transfer onto substrates can be characterized by a myriad of different techniques. Absorption spectroscopy has proven to be the most convenient method for estimating the sizes of the semiconductor particles in semiconductor particulate films (16). Typical absorption spectra of in situ-generated semiconductor particulate films at different stages of their growth are shown in Figure 8. The absorption spectra of semiconductor particulate films thicker than 30-40 À showed long tails near their absorption edges. These long tails could be due to defect states, indirect transitions, and particle size s

Table I. Effective Refractive Indices and Volume Fractions of Semiconductor Particulate Films at Monolayers and on Solid Supports MonolayerSupported n

F (%)

n/

F ' (%)

2.14 1.84

75.5 55.4

2.25 1.88

79.5 58.3

s

CdS ZnS

Solid-Supported

Bulk Semiconductor n b

2.50 2.37

N O T E S : n is the effective refractive index of the monolayer-supported semiconductorparticulate film; n ' is the effective refractive index of the substrate-supported semiconductor-particulate film; F, is the volume fraction of the monolayer-supported semiconductor-particulate film; and F' is the volume fraction of the substrate-supported semiconductor-particulate film. s

s


422


CdS

Ο*!


ELECTRONICS

200

300

400

WAVELENGTH, Urn

Figure 8. Absorption spectra of8,35,218,298, and 328 Â thick (indicated on the curves) CdS-particulatefilmson quartz supports. The arrow indicates the absorption spectrum of a 328 Â thick, quart-supported CdS-particulate film after it was heated at 400 °Cfor 20 min. Particulatefilmswere prepared by the infusion of H S onto a cadmium-arachidate monolayer. 2

distribution. Theoretical calculations for semiconductor crystallites established that the square-of-absorption coefficients times energy, (σΐιω) , increases gradually above the band gap ( £ transition) with increasing energy (33). This increase can be approximated by 2

0

(ahw) = (hw - E )c 2

(1)

g

where /ιω is the photon energy, E is the direct band gap, and c is a constant. Reflectivity measurements provided values for the thickness (d ) of the semiconductor particulate films, which permitted the cal culation of σ (σ = A/as, where A is the measured absorbance) and, upon substitution into equation 1, led to the plot of (σΗω) against ho). Thus, absorption spectra provided band-gap values for semiconductor partic ulate films. Evolution of semiconductor particulate films from their earliest stages of growth can be convincingly seen in scanning tunneling micro scopic (STM) images (12). The initially formed (typically 2-6 min of H S exposure), well-separated, 2 0 - 3 0 - Â diameter, and 5-6-Â-high nanoclusters grew quickly in height to longer and more densely packed, conical particles. Longer exposure (>20 min) to H S resulted in increased lateral growth of the nanocrystallites and their clumping into intercong

f

s

2

2

2



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Approaches

423

nected arrays. Quite often, individual nanoerystallites of chained or diskshaped semiconductor particulate assemblies are visible in the STM images. Large clusters were also demonstrated to be composed of much smaller (15-40-Â diameter) nanoparticles by STM. Prolonged exposure (>30 min) ultimately lead to the formation of a "first layer" of particulate semiconductor film. Inspection of STM images at this stage revealed the presence of 3 0 - 4 0 - À thick, 3 0 - 8 0 - À diameter, disk-shaped semiconductor particles. Transmission electron microscopy has been used to confirm the sizes and size distributions of semiconductor particulate films. Electrical and photoelectrical measurements were carried out on semiconductor particulate films deposited on glass substrates or on Teflon sheets. The resistivity (p) of a semiconductor particulate film, measured between two parallel copper electrodes, is given by

(2)

a

where R is the measured resistivity in Ω, L is the length of the copper electrodes, a is the distance between them, and d ' is the thickness of the semiconductor particulate film. For example, resistivities of 200300-À thick, CdS particulate films were 3-6 Χ ΙΟ Ω cm. This range represents measurements of 10 samples of different thicknesses and is caused partly by the presence of different amounts of water in the films. The ρ values determined for CdS particulate films are some six orders of magnitude higher than those observed for materials having intrinsic conductivity. The dark resistance of CdS particulate films decreases with increas ing temperature exponentially. Illumination decreased the resistivity (i.e., increased the conductivity) of CdS particulate films by some two orders of magnitude and matched the absorption spectrum of the cor responding CdS particulate film nicely. Photoconductivity originates, therefore, in the production of conduction-band electrons, e B~, and valence-band holes, / I V B , in band-gap irradiation of CdS: s

7

C

+

hv CdS

eB C

+

hvB

+

(3)

Electrochemistry and photoelectrochemistry on semiconductor partic ulate films have also been carried out at nanometer resolution by STM. The structures of semiconductor particulate films have been elucidated by X-ray and electron diffractometry. Demonstration of size quantization in semiconductor particulate films is the most significant result of our work. Confinement of the elec-



424


tron and the hole in a particle that is smaller than the exciton diameter (i.e., the DeBroglie wavelength) of the bulk semiconductor results in the quantization of the energy levels (33-38). This behavior is in contrast to a bulk semiconductor in which the conduction bands constitute virtual continua. The length of the exciton diameter depends on the extent of electron derealization and on the effective mass of the charge carrier. In the crystal lattice of semiconductors (e.g., CdS, ZnS, and GaAs), the effective mass of the charge carrier is substantially smaller than that in free space and, hence, the exciton diameter can be quite long (e.g., 6 0 80 Â for CdS) (38). Consequently, size quantization becomes observable in CdS particles whose diameters are equal to or smaller than 60-80 Â. It manifests in the transformation of the continuous bands of the bulk semiconductor into a series of discrete-energy levels and in the shifting of the lowest allowed absorptions to higher energy.

Electronic Behavior of Particulate Films Electron Transfer (ET) and Junctions. In the absence of additives or adventitious impurities, the B L M is an electrical insulator. Current flow, on the order of only 10~ A, was detected in the range of —0.10 to +0.10 V (Figure 9a). The determined resistance and capacitance of a 1.00-mm diameter G M O - B L M bathed in 0.10 M K C l , 3-5 X ΙΟ Ω and 2.0-2.2 nF, agreed well with those reported previously (3 X ΙΟ Ω, 0.380 uF/mm ) (9, 39). In situ semiconductor formation on the B L M surface resulted in marked changes in the electrical response. De pending on the system, the current flow was found to increase asym metrically and the B L M became very much more stable and longer lived. E T across biological membranes and their artificial analogs have been rationalized in terms of three different mechanisms (40). The first model is the electronic conductance by direct electron tunneling. In the second model, E T is considered to occur by electron hopping via impurity states. This is sometimes referred to as resonance tunneling. It is assumed in the third model that charge is carried by chemical species (i.e., the conductance is electrolytic). Three different systems have been investigated (Figure 10). A single composition of particulate semiconductor deposited only on one side of the B L M constituted system A. Two different compositions of particulate semiconductors sequentially deposited on the same side of the B L M represent system B. Finally, two different compositions of particulate semiconductors deposited on the opposite sides of the B L M made up system C. Single-composition microcrystalline-semiconductor particles incor porated onto only one side of the B L M represent the most straightfor ward system, already investigated in some detail by other methods (89

8

8

2


16.


+1.0X

FENDLER


icr i

+2.0 Χ 10~

7

- 1 . 0 Χ 10"

425 5

-1.2 Χ 10"

7

4

+1.0

+2.0X 10~

-1.0

+2.0X10" Ι 7

6

+1.0

•

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3

Ο

-2.0 Χ 1 Ό -

-1.2 Χ 1 0 '

6

-1.0

6

-0.6

+1.0

+0.6

VOLTS

Figure 9. Cyclic voltammograms of a GMO-BLM in the absence, a, and in the presence ofZnS, b, CdS, c, and In S , d, particles on its surface (system A). Scan rate, 100 mV/s. 2

3

Figure 10. Schematic representation of the different semiconductor-coated BLMs. A single composition of particulate semiconductor deposited only on one side of the BLM constituted system A, Two different compositions of particulate semiconductors sequentially deposited on the same side of the BLM represent system B. Finally, two different compositions of particulates deposited on the opposite sides of the BLM made up system C.


426


JO). The composition of the electrochemical cell used in investigating system A is explicitly shown in equation 4: Ag/AgCl 0.1 M KC1, E 5.0 χ 10"" M 0 W

4

ΙΟ" M M 3

2

Ag/AgCl

0.1 M K C l , 5.0 χ 10~

M 0

4

2

2

ca. 10~ M II S ("SID 3

+

(4)

2

-trans side-

-cis side


M = Cd, Zn, C u , In

where E , E , and E indicate the working, the counter, and the ref erence electrodes, respectively. Presumably, systems Β and C are also porous structures, explicitly shown respectively in equations 5 and 6. w

c

R

Ag/AgCl 0.1 M K C l , E 5.0 χ ΙΟ" Μ Ο W

0.1 M K C l , 5.0 χ 10"" M 0 4

4

ΙΟ" M M 3

10"

2

M M'

3

3

0.1 M K C l , 5.0 χ 10~ M 0 4

10

3

-

M M

J

2+

2

s ** 0

(

\*

M

ca. 1 0 " M H S f S H ) 3

cis

R

(5)

2

2 +

Ag/AgCl w

C

2

ca. 10' M H S CSH)

+

cis side

E

Ag/AgCl E , E

2

S

"trans side-

)

Ag/AgCl

0.1 M K C l ,

B

l

5.0 χ 10*" M

0

4

10~

M

ca. 1 0 "

3

3

M M'

2

E ,

Ee

c

2

(6)

+

M H S fSH) 2

trans side

side

Equations 4, 5, and 6, as well as Figure 10, are gross oversimplifications and should in no way be considered to convey any structural information beyond the location of the semiconductor particles and their penetration into the B L M . Typical current-voltage (I-V) curves of ZnS, CdS, In S , and C u - ( + y ) S are illustrated in Figures 9 and 11, respectively. The shapes and characteristics of the I-V curves remained essentially independent of the rate of scanning from 10 to 10 mV/s. Independence of cyclic voltammetric behavior on the frequency of scanning is used as a criterion to support electronic (as opposed to electrolytic) charge-transfer mech anisms (41-43). Uncertainties in and nonlinearities of the resistances of BLM-incorporated semiconductor systems do not allow an unambiguous use of this criterion. Different semiconductor particles penetrate to dif ferent extents into the B L M . Although the membrane remained intact (as seen by the blackness of the reflected light), formation of microscopic defects cannot be excluded. Such pinholes would facilitate electrolytic 2

2

x

6


3

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427



16.

VOLTS

Figure 11. Cyclic voltammograms of the initial formation of semimetallic Cu - S, a, and its subsequent reduction to Cu -( + )S, b, on the cis surface of a GMO-BLM (system A). Scan rate is 100 mVI s from +1.0 to -1.0 V. 2 x

2

x

y


428


charge transport. Similarly, presence of adventitious and deliberately added (H S, 0 ) dopants in the B L M matrix may well mediate resonance electron tunneling. Accordingly, electron transfer across a semiconduc tor-containing B L M may be governed either by electronic or by elec trolytic conductance or indeed by some combination of both of these mechanisms. Regardless of the mechanism involved, the role of the semiconductor particles is crucial. Different semiconductor particles have elicited substantially different ί - V behavior (see Figures 9 and 11) and the observed photovoltage action spectra corresponded to the ab sorption spectra of semiconductor particles deposited on the BLMs (9). Pigmented (44) and protein-containing BLMs (42, 43, 45-47) behaved analogously. Their electrochemical responses also depended on the given pigment (or protein) incorporated into the B L M and on the potential of the redox species surrounding it. To a first approximation, the B L M can be considered to behave like a parallel-plate capacitor immersed in a conducting electrolyte solution (44, 48). The equivalent circuit describing the working (E ), the reference (E ), and the counter (E ) electrodes; the resistance (R ) and the capacitance (C ) of the B L M ; the resistance (R ) and capaci tance (C ) of the Helmholtz electrical double layer surrounding the B L M ; as well as the resistance of the electrolyte solution (R i) are shown in Figure 12a. Deposition of a particulate semiconductor on the cis side of the B L M (system A) alters the equivalent circuit to that shown in Figure 12b, where Rf and Cf are the resistance and capacitance due to the particulate semiconductor film; R ' and CJ are the resistance and ca pacitance of the parts of the B L M that remained unaltered by the in corporation of the semiconductor particles; R and C are the spacecharge resistance and capacitance at the semiconductor particle-BLM interface; and R and C are the resistance and capacitance due to surface state on the semiconductor particles in the B L M . Electrolytes short cir cuit the porous semiconductor particles (Rf = R i =1.4 kil) and their contribution, along with that due to the Helmholtz layer, can be ne glected. This allows the simplification of the equivalent circuit to that shown in Figure 12c. As seen, the working electrode is connected (via ions) to the semiconductor particulate film. Band models (49, 50) of n- and p-type semiconductor-containing BLM-ES junctions (system A) are drawn in Figure 13. Charge injection into the conduction band of the η-type semiconductor by a sufficiently active surface donor or by an applied voltage (making the trans side positive relative to the cis side) results in the accumulation of the majority carriers at the space-charge region (cis) surface of the semiconductor particles (Figure 13a). The variation of potential with distance accom2


ELECTRONICS

2

w

R

c

m

m

H

H

so

m

sc

ss

s c

ss

so


16.

FENDLER

ο


-0

Wvv


429

sol

E

R

sol

v^vO

vws8

cis

R

"W

,

BLM ,

side

trans

side

H

-5»-

o—vw' sol

K

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Figure 12. Proposed equivalent circuits for an "empty", a, and a semicon ductor-particle-coated, b, BLM. Porous structure of the semiconductor par ticles allowed the simplification of the equivalent circuit to that shown in c. R m , Rf/> and B. i are resistances due to the membrane, to the Helmholtz electrical double layer, and to the electrolyte solutions, while C and C are the corresponding capacitances; R/and C/are the resistance and capac itance due to the particulate semiconductorfilm;R ' and CJ are the resistance and capacitance of the parts of the BLM that remained unaltered by the incorporation of the semiconductor particles; R and C are the space charge resistance and capacitance at the semiconductor particle-BLM interface; and B. and C are the resistance and capacitance due to surface state on the semiconductor particles in the BLM. so

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Molecular and Biomolecular Electronics - ACS Publications

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