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SELF-ASSEMBLED, FUNCTIONAL PARTICLE
MONOLAYERS ON POLYELECTROLYTE MULTILAYERS
FOR OPTICAL COATINGS AND DIFFUSE REFLECTORS

presented by

Jin Soo Ahn

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SELF-ASSEMBLED, FUNCTIONAL PARTICLE MONOLAYERS ON
POLYELECTROLYTE MULTILAYERS FOR OPTICAL COATINGS AND DIFFUSE
REFLECTORS
By

Jin Soo Ahn

A THESIS

Submitted to
Michigan State University
in Partial Fulfillment of the Requirements
for the Degree of

MASTER OF SCIENCE
Department of Chemical Engineering and Materials Science

2004

ABSTRACT
SELF-ASSMBLED, FUNCTIONAL PARTICLE MONOLAYERS ON
POLYELECTROLYTE MULTILAYERS FOR OPTICAL COATINGS AND DIFFUSE
REF LECTORS
By
J in 800 Ahn

Monolayers of charged polystyrene latex particles ranging in size from 100 nm to
10 um were deposited onto oppositely charged polyelectrolyte multilayers (FEMS) by
electrostatic interactions. Ultrathin PEMs (~30 nm) formed on a glass slide provided an
excellent underlying adhesive layer. As the sample surface was being dried, strong
capillary forces between particles resulted in a unique pattern of 2-D particle monolayers.
The resulting topographically structured coatings strongly influenced visible light
transmission through the slides, resulting in three different characteristics as a function of
particle size: (1) anti-reflection, when the particle diameter (Dpamcle) is around a quarter of
the wavelength of the incident light (Dpanicle~A/4), (2) diffraction when meiclpk, and (3)
diffusive scattering when Dpamcle>k Functional groups present in these novel coatings
allow further customization via chemical modification. The particle coated samples
were further modified using electroless nickel plating technique. Created surfaces are
diffusive metal reflectors with controlled roughness. A UV-VIS spectrometer with fiber
optics was employed to characterize the optical properties of the reflectors. Optical
measurements showed that the proposed method could control the ratio of specular and
diffuse reflection among total reflected light by changing particle size only. This novel

method is simple, cost-effective and appropriate for mass production because the process

consists of simple immersion steps without vacuum technology or special devices.

ACKNOWLEDGMENTS

I would like to first thank my wife for her love and care. I wish to thank my
academic advisor, Dr. Ilsoon Lee, without whom none of this work would have been
finished, for his support and insightful guidance through two years of study. I also
would like to thank the other members of my graduate committee, Dr. Andre Lee and Dr.
Patrick Kwon. I also wish to thank my friend, Tim Wong, for giving me his time and
advice in the calculation of the light distribution. I would like to thank the colleagues
with whom I spent most of graduate works, Troy Hendricks, Srivatsan Kidambi, Neeraj
Kohli and Brian Hassler. Finally, I would like to thank my parents for all of their
support and love throughout the years.

The analytical support is provided through the Surface Characterization Facility
in the Composite Materials and Structures Center at Michigan State University is
gratefully acknowledged. This work was funded by the MSU Start-Up Funds and the

Seed Research Fund by the Center for Fundamental Materials Research at MSU.

TABLE OF CONTENTS

LIST OF TABLES .................................................................................... v
LIST OF FIGURES ................................................................................. vi
1. BACKGROUND ........................................................................... l
2. PARTICLE MONOLAYERS AS OPTICAL COATINGS ............................ 4
2. 1 Introduction .................................................................................. 4
2.2 Experimental Procedures ................................................................... 4
2.3 Results and Discussion ..................................................................... 8
2.3.1 Formation of Particle Monolayers on PEMs ......................................... 8
2.3.2 Structure of Self-Assembled Particle Monolayers on PEMs ..................... 11
2.3.3 Light Transmittance of Particle Coated Samples ................................... 13

2.4 Conclusions ................................................................................ 2O

3. PARTICLE MONOLAYERS AS DIFFUSE REFLECTORS ....................... 22
3. 1 Introduction ................................................................................ 22
3.2 Experimental Procedures ................................................................. 24
3.2.1 Electroless Plating of Particle Monolayers .......................................... 24
3.2.2 Optical Characterizations of Diffuse Reflectors .................................... 24

3.3 Results and Discussion ................................................................... 27
3.3.1 Formation of Nickel Plated Particle Monolayers ................................... 27
3.3.2 Optical Characterizations of Diffuse Reflectors .................................... 29

3.4 Conclusions ................................................................................ 32

4. SUlVflVIARY AND CONCLUSIONS ................................................... 34
5. BIBLIOGRAPHY ........................................................................ 36

LIST OF TABLES

Table 1: Details of Particles Used and Their Monolayers ...................................... 6

Table 2: Ratio of Specular and Diffuse Reflection ............................................. 31

LIST OF FIGURES

Figure 1: Measurement Schematics for the Total and Specular Transmittance ............... 8
Figure 2: SEM Image of Self-Assembled Particle (140 nm) Monolayer ..................... 9
Figure 3: SEM Image of Self-Assembled Particle (0.5 um) Monolayer ...................... 9
Figure 4: Microscopic Image of Self-Assembled Particle (4 pm) Monolayer .............. 10
Figure 5: Total Transmittance ..................................................................... 14
Figure 6: Specular Transmittance ................................................................. 15
Figure 7: Antireflective Coating .................................................................. 16
Figure 8: Diffractive Coating ...................................................................... 18
Figure 9: Diffusive Scattering Coating ........................................................... 20
Figure 10: Reflectivity of Metals .................................................................. 23
Figure 11: Measurement Schematic for the Specular and Diffuse Reflectance ............. 25
Figure 12: Measurement Schematics for the Angular Dependent Reflectance ............. 26
Figure 13: SEM Image of a Nickel-Plated 140 nm Particle Monolayer ..................... 27
Figure 14: SEM Image of a Nickel-Plated 0.5 um Particle Monolayer ...................... 28
Figure 15: Optical Microscopic Image of Nickel-plated 3 urn Particle Monolayer ........ 29
Figure 16: Specular and Diffuse Reflectance ................................................... 30
Figure 17: Distribution of Reflected Light ...................................................... 31
Figure 18: Macroscopic View of Reflectors at Different Angles ............................. 32

vi

1. BACKGROUND

Studies of colloidal particle assembly at surfaces have provided fundamental
insight into colloidal behavior and aggregation, and suggested potential applications such
as optical band gap materials and coatings. Ordered, three-dimensional (3-D) colloidal
multilayers have been investigated as optical filters and photonic crystals”. Two-
dimensional, (2-D) colloidal assemblies have been investigated for applications including
anti-reflection (AR) coatingsg, gratings"), interferometers“, photolithographic masks'z'”,
and optical coatings”. Recent developments in 2-D particle assemblies include the

16-21

positioning of particles on desired surface spots and incorporation of molecular and

colloidal materials into spatial pattemsus’zz.

2-D particle monolayers assembled on surfaces can be categorized as either
densely close-packed or randomly adsorbed. Densely close-packed particle monolayers
have been fabricated using spin coating”, Langmuir Blodgett (LE-techniques“, thin
laminar flow liquid film”, monolayer transfer”, and electrophoretic deposition”.
Although these methods have successfully produced hexagonally packed particle arrays,
they were weakly adsorbed on the substrate, and tedious to perform or required specific
devices; in addition, long-range ordering has been a problem for the practical applications.
Evaporation methods are simple and widely used in making densely packed particle
monolayers”, but the size of ordered regions produced has been limited. To achieve
close packing over a larger area, additional control is required (e.g., tilting samples in the

. 4
evaporation processl .).

On the other hand, fabrication of randomly dispersed monolayers via

electrostatic attractive forces between the particles and the substrate is relatively easy and

does not require special devicesg’zg'33 .

The properties of randomly dispersed monolayers
depend on particle dispersion and clustering tendencies. Lateral capillary forces help
form ordered regions by convective flow during solvent evaporation but the particles are

. 4
repulsrve to each other27’3 ’35.

When the capillary force is greater than the repulsive
force, monolayers exhibit random-close-packed (RCP) particle clusters that contain
branch-like structures of interconnected and/or hexagonally packed particles. Among
various random deposition methods, electrostatic deposition using polyelectrolyte
multilayers (PEMS) as an ultrathin adhesive layer has advantages of being simple to
perform, cost-effective, and environmentally friendly. PEMs are formed in the layer-by-
layer assembly process, which consists of the sequential alternating immersion of a
substrate into polycation and polyanion solutions to construct a nanostructured ultrathin
film. Charged particles can then be deposited via electrostatic interactions to form
monolayers atop the PEM.

Layer-by-layer electrostatic assembly of PEM, originally developed by
Decher36’37, has been widely studied because of its versatility and potential applications.
Fabrication of nanostructured PEM films is simple and can be successfully achieved

38,39

without the need for a clean-lab facility The PEMs can be formed on flat, curved,

and even spherical surfaces (e.g., a colloidal particle)7’40’4l.

Additionally, PEMs contain
multiple functional groups that can serve as molecular templates for further
customization. Dendrimers, proteins, inorganics, and micro-/nano-particles have been
co-assembled into PEM42'57.

Inspired by the report that specifically sized particle monolayer on adhesive layer

of PEM provided simple and cost-effective AR coatingg, we expanded the study of light
transmittance by colloidal monolayers using variously sized colloidal particles. Details
of transmitting properties of colloidal monolayers and corresponding optical coating
applications will be discussed in Chapter 2.

One of good advantage in using this approach for optical coatings is the coatings
are still available for further modifications for various applications. In this work the
coatings were further modified with nickel by a two-step electroless plating using
palladium catalysts. Resulting surfaces are rough metal controlled by particle sizes
suggesting a novel method of creating a diffusive metal reflector application. For the
characterization of optical properties a UV-VIS spectrometer with fiber optics was
employed. Optical fibers permit versatile and precise measurements of specular and
diffuse reflectance and angular dependent reflectance. Also, we demonstrate how to
estimate the distribution of reflected light from the surface and the ratio of specular and
diffuse reflection among the total reflected light using angular dependent reflectance.
Optical measurements showed that this approach could control the portion of the diffuse
reflection from 8.25 to 59.97 %. Proposed method is simple, cost-effective and apt for
mass production because the process consists of simple immersion steps without vacuum

technology or special devices.

2. PARTICLE MONOLAYERS AS OPTICAL COATINGS

2.1 Introduction

Hideshi demonstrated that monolayers of silica and polymeric nanoparticles
(Dpamcle~l/4A) yield an anti-reflective AR coating effectg, while many others have

investigated 2-D or 3-D microparticle monolayers or multilayers (Dlmmcle ~ A) for

photonic and band gap materialsss’”.

We have formed polystyrene (PS) particle
monolayers via electrostatic interactions between charged particles and oppositely
charged PEM surfaces. The structure and optical properties of these coatings are
controlled by electrostatic interactions and capillary forcesls’”.

While the optical properties of colloidal multilayers and close-packed
monolayers have been extensively studied, those of randomly adsorbed or RCP
monolayers have not. In this study, we systematically investigated RCP produced by
electrostatically adsorbing PS particles between 100 nm and 10 pm in diameter onto
PEM-coated glass slides. Surface coverage and fractal dimensions of the RCP
monolayers were measured using image analysis, and the optical properties were studied
using both total and specular transmittance. The possibility of modeling RCP

monolayers based on the 2-D coverage and fractal dimension analysis using a box-count

method was also considered“).

2.2 Experimental Procedures

Poly(diallydimethylammonium chloride) (PDAC) and sulfated polystyrene (SPS)

were purchased from Aldrich. Average molecular weights of PDAC and SP8 were

~100,000 — 200,000 and 70,000, respectively. Microscope glass slides were ordered
from Coming and used as transparent substrates for PEMs and particle coatings. All
aqueous solutions in the process were prepared using deionized (DI) water (>18.1 MO)
supplied by a Barnstead Nanopure Diamond-UV purification unit equipped with a UV
source and final 0.2 um filter.

Glass slides were first cleaned twice in an ultrasonic unit, first with a
commercially available detergent (Alconox, Alconox Inc.) and then without. Slides
were dried under a N2 gas stream and then treated with oxygen plasma for 10 minutes at
150 millitorr vacuum to activate negative surface charges on the glass.

Aqueous polyelectolyte solutions were prepared containing either 20 mM PDAC
or l0 mM SPS in 0.1 M NaCl. PDAC/SPS bilayers were then deposited by sequential
immersion of the glass slides into the two solutions using a Microm DS 50 Slide Stainer
purchased from Richard-Allan Scientific. PDAC was deposited first, because its
positively charged amine groups bind to the negatively charged hydroxyl groups of glass.
In each step, the slides were immersed in a polyelectrolyte solution for 20 minutes,
followed by two 5-minute rinses in DI water. After each PDAC/SPS bilayer was
deposited, the slides were immersed in an ultrasonic bath for 1 minute to remove loosely
attached polyelectrolyte. This sequence was repeated until (PDAC/SPS)10,5 bilayers
were formed. (PDAC/SPS)10_5 bilayers have a positive PDAC top layer, which is
needed to electrostatically bind the negatively charged PS particles. Substrates were
finally dried under a N2 gas stream and stored for particle coating.

The sulfate-functionalized and carboxylic-acid-functionalized PS particles were

obtained from Interfacial Dynamics Corp, and Polysciences, Inc., respectively. Size-

distribution and surface-charge density information obtained from the manufacturers is
shown in Table 1.

Table 1. Details of Particles Used and Their Monolayers

 

Size Surface Monolayer

 

 

Particle Distribution S ace Charge Coverage by Fractal
Size . Functional . . DimenSIOn
( ) DIameter i SD Grou DenSIty partlcles i SD D
um (um) p (peg/g) (%) ’ f
0.14 0.14:t0.003 Sulfate 3.4 32.60i1.803 1.778
0.2 0. l 94i0.009 C arboxylate 100-200 N/A N/A

0.5 0.477i0.01 Carboxylate 100-200 40.38i9.928 1.799

1 1.0a0.047 Sulfate 83.9 39.40i0.809 1.804
2 1.96a006 Carboxylate 100-200 39.36zh1.377 1.800
3 3.00d: 144 Carboxylate 100-200 54.2011 .684 1.828
4 4.0a0.14 Sulfate 0.8 59.363c2.853 1.848
5 4.9a0.275 Sulfate 0.5 47.44al:6.307 1.846
8 7.9:t0.845 Sulfate 0.4 67.38i18.709 1.826
10 9.610710 Sulfate 12 65.14zt19.656 1.835

 

To bind PS particles to the PEM, a 0.5 wt % colloidal solutions was gently
dropped on the PEM surface. After a 1-hour incubation, the particle-coated substrates
were washed carefully with deionized water and dried under a N2 stream. Microscopic
images of the coatings were obtained using a Nikon Eclipse ME600 optical microscope
and a JEOL 6400V scanning electron microscope (SEM) with a LaB6 emitter. The
degree of surface coverage was determined by counting particles in a given area.

Coverage measurements were repeated several times in both particle-rich and particle-

poor regions. Data were reported as average standard deviation values.

Microscopic images were further investigated by fractal analysis using a 2-D
box-counting method“). All monolayer images were scaled to have the same particle
sizes, and each particle was considered as a Single point. The maximum number of
boxes was 4096. Boxes were counted manually and plotted on logarithmic-scale graph.
Fractal dimensions were calculated using a least squares regression method. A
UV/VIS/NIR Spectrometer Lambda 900 (Perkin-Elmer) was used to study the optical
properties of the samples. Both photomultiplier tube (PMT) and integrating sphere (IS)
detectors were used for Specular and total transmittance characterization, respectively.
All spectroscopic spectra of the samples were referenced against air, without a substrate.

Figure 1 illustrates the difference in measurement principle between the PMT
and IS detectors. The IS detector collects all light passing through the substrate
regardless of the back scattering angle (Total Transmittance), while the PMT detector
only captures light transmitted in the same direction as the incident light (Specular
Transmittance). Both detectors scanned transmittance by 1 nm wavelength scale.
Transmittance spectra from both detectors were averaged at 400~800 nm wavelength
range to compare the Size effects on scattering (averaged total and specular
transmittances). The total transmittance Spectrum of every particle monolayer is not
reported, because there was little change in light intensity with wavelength. However,
all specular transmittance spectra are presented and categorized into 3 groups, depending

on the optical behavior.

SAPM Coated Glass Substrate SAPM Coated Glass Substrate

     
      

 

 

 

 

N \.
Diffuse‘Trargmittance k .-, .1
Incident Light A Incident Light PMT DC’L’CIOI’
......:Spccular Transmlfiance . ‘.-': "Specular TransmTttance
A Diffuse Trafismittance B ‘. . m
\l. [S Detector \L

 

 

 

 

Figure 1: Measurement Schematics for the Total and Specular Transmittance

2.3 Results and Discussion

2.3.1 Formation of Particle Monolayers on PEMs

Figure 2, 3, and 4 Show the SEM and optical microscope images of the particle
monolayers assembled on the (PDAC/SPS)10,5-coated glass slides. Particles forming
multilayers on top of the first adsorbed particle layer were easily washed away during the
rinsing steps because of the repulsive forces with the first layer of identically charged
particles. The layer of particles is strongly adhered to the surface by electrostatic
attraction forces between negatively charged colloidal particles and the positively

charged PDAC top surface of the PEMs.

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Figure 2: SEM Image of Self-Assembled Particle (140 nm) Monolayer

 

Figure 3: SEM Image of Self-Assembled Particle (0.5 urn) Monolayer

Functional nanospheres (140 nm) are well dispersed and deposited independently
on the surface (Figure 2); whereas the larger microspheres (0.5 pm ~ 10 um) also formed
two-dimensional monolayers but have clusters of close packed particles connected with
each other, forming fractal-like structures (Figures 3 and 4). The formation of the
fractal-like particle monolayers (i.e., RCP monolayer) is obvious for particles bigger than
140 nm. 200 nm-sized particles did not form monolayers even with several trials;
instead they formed 2 or 3 layered aggregates. The reason for the formation of the
layered aggregates is not clear, but the optical properties were compared with other

particle monolayers.

 

Figure 4: Microscopic Image of Self-Assembled Particle (4 pm) Monolayer

In the beginning of functional particle depositions in aqueous solution, the
negatively charged particles were attracted to the positively charged PEM surfaces until

the available PEM surfaces were fully occupied. Then, during the sample drying step

10

when the water level became comparable to the size of adsorbed particles, a
rearrangement of adsorbed particles started to occur, resulting in uniquely self-assembled
monolayer patterns on the PEM-coated glass slides, as shown in Figure 3 and 4. During
the rearrangement of adsorbed particles near or on the PEM-substrate, two conflicting
forces, repulsive and attractive forces, compete to determine the pattern of the self-
assembled particle monolayers. The repulsive forces between particles adsorbed on
PEM surfaces are due to the same particle charges and the attractive forces are because of
capillary forces stimulated from the menisci of water formed around the particles '7.
Repulsive forces hinder the packing of two-dimensional particles while attractive forces
help the packing of particles by the convective transport of particles toward the close
packed region. We believe that the attractive capillary forces were smaller than the
repulsive forces in the case of 140 nm nanospheres, these results in particles that are
relatively isolated from each other“. This conclusion is supported by the fact that
particle closed packing can be obtained by reducing electrostatic repulsive forces
between particles using surfactant charge screening“. For particles larger than 0.5 um,
the capillary forces were dominant at the last stage of drying, resulting in the RCP
particle monolayers.
2.3.2 Structure of Self-Assembled Particle Monolayers on PEMs

Table 1 Shows physical data of the particles used in this work and monolayer
coverage characterized by direct particle counting. The fifth column of Table 1 shows
the calculated particle coverage from the microscopic images. Particle coverage was
formed to be independent of the two different types of charged groups on the particles’

surface. Sulfate groups make the surface of the particle hydrophobic, and carboxylate

ll

groups hydrophilic. Both cases Showed attractive forces among particles. Particle
monolayer coverage was affected more by the size of the particles. The coverage of the
particle monolayers generally increased with increase in the size of the particles. This is
because the particle clustering, due to the increased capillary forces among particles,
increased with the increase in particle Size. The surface coverage did not reach a
theoretical maximum because monolayers created by this method cannot form a close-
packed monolayer. The calculated maximum surface coverage by Spherical objects is
90.69% when they are hexagonally packed.

When particles are strongly attached to the substrate, the diffusion process or
surface migration of particles is hindered and there is a limit for coverage known as the
jamming limit of random sequential adsorption (RSA)63’64. 54.7% is in good agreement
for the maximum coverage by this model63’64. However, RCP monolayers with particles
greater than 2 pm have coverages greater than this limit (Table 1). This means there is
movement and diffusion of particles during deposition and evaporation which allow the
particles to surpass the jamming limit. The structure of colloidal aggregates formed in
this process is a fingerprint of the kinetics and mechanism of aggregation. Simple
aggregation models and real aggregation processes frequently lead to fractal structures
that can be described in terms of fractal geometry concepts“. Fractal geometry, also
known as the fractal dimension, enables us to relate certain aggregates to a proper kinetic
and growth mechanism model“. In two-dimensional analysis, the fractal dimension has
a value between one and two. When fractal geometry completely fills a certain area of
the surface, the fractal dimension reaches the maximum value of 2.

We calculated the fractal dimensions of our samples in an effort to find a proper

l2

model to explain the uniquely self-arranged particle monolayers on PEMs. The last
column of Table 1 presents the calculated 2-D fractal dimensions, Df, of each sample.
Fractal dimensions of RCP monolayers range from 1.778 to 1.848. The aggregation
Shape of the RCP model closely resembles that of cluster-cluster aggregation by primary
clusters. There are two different mechanisms for this type of aggregation, ‘reaction-
limited cluster-cluster aggregation’ (RLCCA) and ‘diffusion-limited cluster-cluster
aggregation’ (DLCCA)67. When there is a diffusion process during the aggregation and
the particle adsorption is instant compared with the diffusion time, the ‘diffusion-limited-
aggregation’ (DLA) model can explain the process. On the other hand, an aggregation
process with residual repulsive forces between particles can be explained by the
‘reaction-limited-aggregation’ (RLA) model“. It is known that the fractal dimension of
RLCCA is higher than that of DLCCA67. A high fractal dimension of RCP monolayer
indicates its close relationship with the RLCCA model. However, relating the aggregate
structure to a specific growth mechanism is not fully understood yet. AS is the problem
with modeling all colloidal aggregations, modeling RCP monolayers using a Specific
model is very challenging and needs further studies. However, one thing that remains
clear is that the capillary forces that occur during the sample drying step play a very
important role in the resulting RCP monolayers on PEMs which are very Similar to the
description by the RLCCA model.
2.3.3 Light Transmittance of Particle Coated Samples

Figure 5 shows the average total transmittance of the samples at the visible light
range (400~800 nm), which was measured using an UV/Vis Spectrometer equipped with

an IS detector. No Significant reflection or absorption by the coated polystyrene particle

13

monolayer has been found in this work. The average total transmittances, ranging from
90.06 % to 94.12 %, remained very high and are comparable to the values of a pure glass
Slide. The average transmittance of a pure glass slide measured by the same IS detector
was 91.86 % at the same wavelength range. This implies that the particle monolayer
coatings did not affect the total transmittance of the glass Slides. Also, there is no

significant particle Size effect on the observed total transmittance.

100

 

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0.14 0.2 0.5 1 2 3 4 5 8 1O
Particle Size (um)

Figure 5: Total Transmittance

Figure 6 shows the averaged specular transmittance measured using a PMT
detector. The Specular transmittance varies with the Size of particle. A decrease in the
specular transmittance compared to the observed total transmittance of the same sample
indicated that some portion of incident light did not reach the detecting point because of a

‘Scattering event’. Monolayers of particles bigger than 0.5 pm yielded an ‘apparent

l4

scattering’ event when incident visible light passed through the particle coated glass
Slides. Apparent scattering happens when the size of the scattering particle is equal to or
greater than the wavelength of incident radiation“). Particle coated glass slides with 140
and 200 nm Sized particles did not exhibit significant scattering events. The samples
looked either transparent without scattering events or translucent with those scattering

events, depending on the size of particles.

 

 

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0.14 0.2 0.5 1 2 3 4 5 8 1o
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Figure 6: Specular Transmittance

Figure 7, 8, and 9 Shows the specular transmittance Spectra from a uv/vis-
Spectroscopy experiment using a PMT detector, which was not averaged. There are
three main observed behaviors which are dependent on the particle size: Anti-reflection
behavior for the particles ranging in size from 0.14 to 0.2 micrometers (Figure 7),
diffraction for the particles ranging in Size from 0.5 to 4 micrometers (Figure 8), and

diffusive scattering for the particles ranging in Size from 5 to 10 micrometers (Figure 9).

15

 

100
(b) Double side

95 — (a) 0.14 pm _ w ‘ A

 

 

 

 

 

 

 

 

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5..
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2
D.
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80
75 I l l 1 l l I
400 450 500 550 600 650 700 750 800
Wavelength (nm)

Figure 7: Antireflective Coating

Figure 7 shows the transmittance spectra using a PMT detector in the visible
light range, 400 ~ 800 nm. Particles with diameters of 140 and 200 nm have an AR
behavior and exhibit an enhanced transmittance that is greater than that of a pure glass.
Glass Slides coated with particles on both sides have a higher transmittance than those
coated on a single Side. The maximum value of transmittance for this particle size was
measured to be 97.52 % at a wavelength of 635 nm, which is much higher than that of the
pure glass slide (ca, 92 %).

To be an AR coating, the designed film should satisfy two conditions”. The
thickness of the AR coating must be a quarter of the incident wavelength and 11C = (nans)m,
where nc, na and 11s are the refractive indices of the coating, air, and substrate, respectively.

Anti-reflective behavior is due to the destructive interference between reflected lights

16

from the top surface of AR coating and from the interface between the AR coating and
substrate. Also, nc must have a value between n, and n, so that the change in refractive
indices of the media is gradual and reduces the amount of light reflecting backwards from
the substrate.

The particle coatings produced with 140 and 200 nanometers Sized particles
satisfy both conditions for an AR coating. The thickness of these monolayers is directly
decided by the diameters of the particles, which is around a quarter of the visible light
wavelengths. Refractive indices of air, glass, and polystyrene particles are generally 1,
1.5, and 1.59 (data from Interfacial Dynamics Corp.), respectively. Due to the random
deposition of particles, the monolayer has many pores between polystyrene particles that
are filled with air. The monolayer has a refractive index somewhere in between that of
air and polystyrene particles. As illustrated by Hideshig, this approach to forming an AR
coating does not require a high vacuum that is common in many AR coating processes.
This process is more cost-effective when compared to phase separation or selective
dissolution approaches“ and Simpler than vapor deposition or sputtering approachesn’73 .

Figure 8 shows the oscillating specular transmittance spectra from particles with
0.5, 1, 2, 3 and 4 um. In the late sixties O’Neill, et al.69, demonstrated that
monodispersed latex particles exhibited diffraction in both colloidal solution and dry
particle layers. We believe this diffraction event can be categorized into Fraunhofer
diffraction, which occurs when light propagates through an assembly of apertures or
translucent screens”. This is supported by the fact that there is a typical airy disc

created by a circular aperture when the laser from a laser pointer is shined through the

samples. However, this airy disc pattern is unclear because the self-assembled particle

17

monolayers on PEMs are RCP structures. Maxima and minima in the spectra represent

the change in intensity with the change in the wavelength of incident monochromatic

 

 

 

 

light.
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,x- 30 '
3" i
E i ((1)3 um I
320 -
m J (041m
/
10 I \
1 (a)0.5 tun '
O 1 L l 1 1 l 1
400 450 500 550 600 650 700 750 800

Wavelength (nm)

Figure 8: Diffractive Coating

Interesting results from the diffracted transmittance spectra are obtained for
monolayers made from 0.5 and 1 urn sized particles. A 0.5 um sized particle monolayer
completely scatters blue light but transmits about 58% of incident red light. To the
contrary, a 1 um Sized particle monolayer scatters almost all incident red lights but passes
42 % of incident blue light. By simply changing the Size of the particles a particle
monolayer can selectively block a specific wavelength.

Current studies on the selective transmission of incident light by diffraction have

18

“I

 

 

 

focused on 3-D colloidal crystals. In this case, sharp peaks at specific wavelengths are
exhibited due to the Bragg diffraction through the ordered arrays. Selectivity in the
transmission of light in narrow wavelength regions is useful in diffractive components of
optical filters or in grating applicationsl. Different from ordered aggregations, RCP
monolayers produce broad peaks that are unique in their diffraction through 2-D or 3-D
multilayers.

While the details of this diffraction mechanism need to be further studied, we
expect that there may be other potential applications in which this broad selective
transmission can be very useful. We can further tune the optical selectivity using
commercially available fluorescent particles”. Additionally, we can control these broad
peaks using mixed or polydispersed particle monolayers.

Figure 9 shows the specular transmittance Spectra using a PMT detector from
particles with diameters of 5, 8 and 10 pm. This group exhibits uniform diffusive
scattering of incident light. The transmitted light reaching the PMT detector shows a
uniform intensity along the wavelengths. The specular transmittance was decreased
when compared to the total transmittance. This means that there happened to be a
scattering event that changed the direction of the incident light. Optical coatings of the
particle monolayers in this group transmitted diffraction free and uniform light. Once
again, it is noted that the total transmittance measured using a IS detector remained
almost the same regardless of the particle Size. The transmitted light, through the self-
assembled monolayers on PEM-coated glass slides, were scattered or diffracted
differently depending on the topological surface structures controlled by the Size of

particles.

19

 

4:.
O

(C) 10m
(b)3um

U) DJ
O M
l

IQ

H
Vi
I

O—l
C)
I

Specular T ransmittance
I")
0 VI
g

 

 

0 .L A l 1 4L

400 450 500 550 600 650 700 750 800
Wavelength (nrn)

 

Figure 9: Diffusive Scattering Coating

2.4 Conclusions

We have studied the functional PS particle size effects on the formations,
structures, and optical properties of the uniquely self-assembled particle monolayers on
PEM-coated glass slides. The particles formed RCP monolayers that deposited on the
substrate by electrostatic attraction forces and then rearranged by capillary forces among
the attached monolayer particles. Compared to other particle deposition approaches, the
overall procedure used in this work was very simple, cost-effective, and environment
friendly. The formation and structure of the self-assembled particle monolayers (i.e.,
RCP particle monolayers) was analyzed using surface particle coverage and fractal
dimension analysis. The optical properties of the self-assembled particle monolayers
measured using two different detectors Showed very interesting particle size

dependencies. Even though the apparent changes dependent on particle Size could be

20

identified with the bare eye, the total transmittance, measured using an IS detector,
remained unchanged and conversely the Specular transmittance, measured using a PMT
detector, changed considerably. No Significant reflection or absorption by the coated
monolayer has been found. This indicates that the incident visible light optically
interacts with the particle monolayers through optical interference, diffraction, and/or
scattering events. Three main optical characteristics as a ratio of particle diameter vs.
wavelength of incident beam (D/A) are the following: (a) Anti-reflection (when
D/A~0.25), (b) diffraction (when D/A~1), and (c) diffusive scattering (when, D/A>l).
These functional and topographically structured surfaces can be further used as templates
for selective and non-selective metal plating”, cell adhesion“, and quantum dot

deposition7.

21

3. PARTICLE MONOLAYERS AS DIFFUSE REFLECTORS

3.1 Introduction

Difiuse reflectors are widely used in back-light unit of liquid crystal display

(LCD) panels”79 and solar cell devicesso’gl.

A perfect diffuse reflector refers to a matter
that reflects incident energy unifome to all directions and is observable at all viewing
angles. The Similar characteristics for a practical diffuse reflector can be simply
achieved by having topographical roughness on a highly reflective surface, such as metal.
A uniformly controlled rough surface reflects incident light in a diffusive manner
reducing specular reflection due to the random scattering of light to all directionssz'ss.
The most important aspect in the design of a diffuse reflector is the control of the ratio of
specular and diffuse reflection for the designated purpose of use.

Metals easily satisfy the general requirements for diffuse reflectors with high
reflectivity and low absorption. When it is flat and smooth, a metal surface reflects
most of the incident light specularly with low diffuse reflection. Manipulation of a
diffuse-reflective metal surface by a sputtering method has been reported“. One of
good advantage of vacuum deposition is that a roughened surface can be directly created
in the process. However, the process requires a high vacuum that is costly to procedure
and not suitable for mass production.

Wet plating is a low cost and suitable for mass production of diffuse reflectors if
the surface roughness of deposited metal can be controlled. It has been Shown that PEM
can be metallized to form metal nanoparticle/polymer composites or metal/polymer

22,44,75

multilayered nanocomposites by electroless plating Our group previously

22

demonstrated the selective or non-selective electroless plating of self-assembled particle
monolayers for opto-electronics applications”. Given by the fact that a non-selectively
plated particle monolayer is uniformly rough over large areas due to the particles, we
have investigated the possibility of using the surface as a diffuse reflector. Nickel is one
of the most common electro-formed metals with moderate reflectivity along many
wavelengths (Figure 10)”. When the reflectivity of nickel is less than what is expected,
over-coating onto nickel surface by wet plating or a vacuum technique can provide better
optical properties. For the higher industrial requirements of most applications today,
metal reflectors are demanded to be heterogeneous or multilayered with further

88,89

customizations. Further modifications include a brightening agent , multilaying or

91,92

over-coating90, and protective coating The nickel reflector is more cost-effective

and efficient basis when a heterogeneous reflector is required.

 

........................
l

 

\__fl

 

Reflectivity

 

 

 

 

 

 

0.2 r
0.1 r
0 I I 1 I l 1 I l l
35) (I) 450 5D 55) 61) $0 700 750 81) $0

Warm (nm)

Figure 10: Reflectivity of Metals

23

To characterize the optical properties of the prepared diffuse reflectors, a phase
angle optical microscope and a UV-VIS spectrometer with fiber optics and a scanning
electron microscope are employed. Optical fibers permit versatile measurements for

diffuse and specular reflectance of the sample.

3.2 Experimental Procedures

Self-assembled particle monolayers were prepared by the same procedure
described in Section, 2.2. Particles used in this study range from 140 nm to 5 urn.
3.2.1 Electroless Plating of Particle Monolayers

Two Pd based catalyst solutions were prepared using Pd(NH4)4Cl2 (catalysts 1)
and Na2PdCl4 (catalyst 2), each with a 5 mM concentration in deionized (DI) water.
The samples of 2-D colloidal monolayers were immersed into two catalysts solutions
consecutively for 10 sec (step 1). After washing with DI water followed by N2 drying,
the pretreated samples were dipped into a Ni bath (step 2) for around 30 minutes. The
electroless nickel bath contains nickel sulfate (Ni source, 4 g), sodium citrate
(complexant, 2 g), lactic acid (buffer, complexant, lg), and DMAB (reductant, 0.2g) in
100 ml of DI water. The pH of the Ni bath was adjusted to be 6.5 (i 0.1) using
ammonium hydroxide. For a particle free nickel surface, the same electroless plating
method was applied to a polyelectrolyte coated substrate without particle monolayers
atop. In this case, only the negative catalyst has been used and preloaded because
topmost surface was positively charged PDAC.
3.2.2 Optical Characterizations of Diffuse Reflectors

For microscopic images of the particle monolayers on surfaces, an optical

24

microscope (Nikon Eclipse ME600) and a Scanning Electron Microscope (JEOL 6400V
with a LaB6 emitter) were used. To study the optical properties of the samples, a
USB2000 fiber optic spectrometer from Ocean Optics was employed. All specular and
diffuse reflectance spectra of the samples were referenced against high specular and
diffuse reflectance Standards purchased from Ocean Optics.

Figure 11 shows the measurement schematic for the specular and diffuse
reflectance. To measure the specular reflectance the light source and the detecting probe
are placed on opposite sides. Both the incident and reflecting angles were the same, 45°.
For the diffuse reflectance the incident angle was 75° and the reflecting angle was 45°.
Reflectance spectra were then averaged over the visible light range (400~500nm) to get
averaged single values.

szooo-UV-VISSpec.
0 ean Optics, Inc.

    
 

UVNIs Light Source

Diffuse Reflectance

 
 
 
 

Specular Reflectance

Nickel Coated Particle Menolayerrr

Figure 11: Measurement Schematic for Specular and Diffuse Reflectance

Figure 12 shows the scheme of measuring a distribution of light reflected from
the sample surface. Incident angle was 5° and the mobile detecting probe was placed at

different reflection angle (5°~90°) to acquire the angular dependent reflectance. All

25

reflectance intensities were measured at different angles, and averaged across visible light
range. Average values were divided by the detector area resulting in flux density (i.e.,
number of photons per unit time and area). The flux densities at each angle were
multiplied by the area calculated from the integration of an imaginary sphere at a given
angle range. Once multiplied by the area, the flux densities were converted to flux (i.e.,
number of photons per unit time) and integration assumed that the distribution of light
was hemispherical because the incident angle was close to 0°. The ratio of specularly
and diffusively reflected light was derived from the distribution of reflected light. It was
assumed that the specularly reflected light from the surface was populated in the region
between 0 ~ 3° reflecting angle when incident light impinged the surface at an angle of 5°.
The area of region in between the reflecting angle 0~3 on the imaginary hemisphere was
equivalent to that of the detector.

Specular - Original Axis, 5'
Reflectance

  
 
  
   

        
  
  

New Axis, 0"

Diffuse """""""""
efleetanee

 

          
    

   

«fin» "Mt”! )3; {31:1 {Agni}:

' give» 9' .
akin? Mtg; K '
«1‘11"» .‘ 53$: «3;; flabby“)? etyww.

‘ «83.315 :33:me oi

   

Figure 12: Measurement Schematics for Angular Dependent Reflectance

To compare the brightness from macroscopic view of the reflectors, a Nikon
digital camera was employed. The reflectors were placed on a desk and photographed
by varying the viewing angle to capture both the specularly and the diffusively reflected

lights from a fluorescent lamp.

26

3.3 Results and Discussion

3.3.1 Formation of Nickel Plated Particle Monolayers

Figure 13 shows the SEM image of a nickel-plated colloidal monolayer using
140 nm particles. As it was explained in Section 2.3.1, nanospheres (~100 nm) are
adsorbed independently due to the same charge repulsion of the particles before the
plating. During electroless plating the thick nickel metal layer growing on the surface
of covered spherical particles on the substrate results in the development of hemispherical
and irregular shapes. The thickness of nickel was comparable to the diameter of
nanospheres used. Bigger particles appeared to be a merger or aggregates of two or

more particles grown from a necking.

 

Figure 13: SEM Image of a Nickel-Plated 140 nm Particle Monolayer

Figure 14 shows the nickel-plated colloidal monolayer using 0.5 pm sized

particles. Different from the nanospheres (meicte~100 nm), microspheres (meicte>0.5

27

um) developed branch-like structures of clusters (see Figure 14). This image shows that
spherical particles kept their shape but there were heavy neckings between particles due

to the grth of nickel on the surface of the particles.

 

Figure 14: SEM Image of a Nickel-Plated 0.5 pm Particle Monolayer

Figure 15 shows the optical microscopic image of the monolayer using 3 pm
particles. As shown in the Figure 15, long electroless plating develops crazes on the
particle. As reported elsewhere, this is due to the internal stress build-up during

electroless plating, which has been a great limitation of the process93’94.

For optical
reflector applications, there should be little transmittance of light through the
electroformed reflectors. However, plating nickel over 30 minutes on the particle
adsorbed surface results in the delamination of whole metal layer. Sometimes the

crazed surface of nickel scatters more light in a diffuse reflection manner. However,

when a thicker metal layer or composite multilayers are required the plating rate,

28

temperature, pH of bath solution, and selection of a different substrate for Ni plating

93’94. It is notable that the electroformed

should be optimized to lower internal stress
nickel on a rough surface can bear greater internal stress than on a smooth surface at the
same plating conditions. So we believe that particles incorporated into the nickel
plating enhance the strength between nickel and the substrate, as demonstrated by Dr.

Mackay, et £11.95.

This opens an interesting future work focusing on strengthening
mechanical properties by the incorporation of nanoparticles or dendrimers during the

plating process.

 

Figure 15: Optical Microscopic Image of Nickel-Plated 3 pm Particle Monolayer

3.3.2 Optical Characterizations of Diffuse Reflectors
Figure 16 shows the average specular and diffuse reflectances as a function of
particle size. The average specular reflectance decreases and the average diffuses

reflectance increases along with the increase in particle size. Since the measured

29

reflectances are referenced against the high specular standard, the specular reflectance

38.09 % of 0.14 pm sample is the highest. The samples made of particles greater than 3

pm have values of the specular reflectance less than 0.6 %.

 

 

 

 

 

 

 

 

 

40 40
A35» -35
0930r ‘30v
0
c 8
%25~ ‘255
a: ‘6
$20 ”4’03
0‘ 6‘2
515* 450
3 (I)
§10- 10g

5
(05. .5
0 0
0.14 0.2 0.5 1 3 5
Pa‘ticIeSizeQJn)

Figure 16: Specular and Diffuse Reflectance

Figure 17 Shows the distribution of lights reflected from the sample surfaces,
converted from the integration of angular dependent reflectance. When the incident
angle is 5°, specularly reflected light is in between 0 ~3° on the graph. It turns out that
the reflectors using this method don’t produce a focused reflection. Instead, the

reflectors reflect lights randomly in all directions.

30

 

 

 

 

 

Log Intensity (Counts/Sec)

 

 

 

 

0—10 10—20 20—30 30-40 40~50 50-60 60-70 70~80 60-90
Angle (°)

Figure 17: Distribution of Reflected Light

From the reflected light distribution study, the ratio of the Specalar and diffuse
reflection at the incident angle of 5° was derived and tabulated in Table 2. The result
Shows that the samples can control the amount of specular reflection from ~50 to 90% by
only changing the size of the particles. Table 2 provides a guideline to select a Size of

the particle used for a required amount of diffuse reflectance for the purpose.

Table 2: Ratio of Sepcular and Diffuse Reflection

 

Particle Size

 

0.14 0.2 0.5 1 3 5
(11m)
Specular
Reflection(%) 91.75 57.66 45.38 43.52 40.03 40.79
Diffuse
Reflection(%) 8.25 42.34 54.62 56.48 59.97 59.21

 

Figure 18 shows two macroscopic images of reflectors taken by a digital camera

31

at different angles. The size of particles is increasing from left to right and the
brightness of the specular (upper) and the diffuse reflections (lower) is inversely
changing along with the increase in particle size. Particle free or nanosphere
monolayers (left) are brighter in specular reflection and darker in diffuse reflection than
bigger particle monolayers (right). It is because they (lefi) are reflecting most of the
lights in a specular reflection as calculated in Table 1. On the other hand, samples with
microspheres (right) are darker under the specular reflection condition and brighter under
the diffuse reflection condition because they reflects about half of incident lights

specularly and the other half diffusively.

.‘.‘

" ~ . ‘ ti u}
“shuffle“:

Nanospheres Particle Size Increases Microspheres

    

 

Brighter
" at " “T“F’fj.
' e. "i" t .. ~
a . M , .. .
4:. .._L #-

 

 

Diffuse Reflectance

Figure 18: Macroscopic View of Reflectors at Different Angles

3.4 Conclusions

We have developed a novel fabrication method of diffuse reflectors. Tuning of

32

the specular and diffuse reflectance has been easily achieved by changing the Size of
particles. The new process is cost-effective and convenient for mass production due to
the simple immersion steps without the need of a high vacuum. Depending on the
required Optical property, this method can be combined with the additional over-coatings
of different metals such as aluminum or silver by wet plating or a vacuum technique”.
However, the vacuum deposition technique will decrease the production yield and
increase the processing cost as well.

The portion of specularly reflected light from the total reflected light ranged from
50 to 90% as a function of particle Size. To decrease Specular reflection or to increase
diffuse reflectance, further modifications are required. In summary, this new simple
process consists of the highly concentrated self-assembled monolayer and the creation of
rough metal coatings on top.

However, it must be noted that PEM binder between a substrate and a metal layer
may not be strong enough to hold a heavy load of metal due to the internal stress build-up.

”’94 or silane

When a strong binder is required, annealing of electroformed metals
modified surfaces as a covalently adhesive layer can be considered”. The stability or

durability of the prepared metal-polymer reflectors remains as a future investigation.

33

4. Summary and Conclusions

We have studied the functional PS particle Size effects on the formations,
structures, and optical properties on PEM-coated glass Slides. The particles formed
RCP or dispersed particle monolayers that deposited on the substrate by electrostatic
attraction forces and then rearranged by capillary forces among the attached monolayer
particles. Compared to other particle deposition approaches, the overall procedure used
in this work was very Simple, cost-effective, and environmentally fi'iendly. The total
transmittance, measured using an IS detector, remained unchanged along with the particle
sizes and conversely the specular transmittance, measured using a PMT detector, changed
considerably. No Significant reflection or absorption by the coated monolayer has been
found. This indicates that the incident visible light optically interacts with the particle
monolayers through optical interference, diffraction, and/or scattering events. Three
main optical characteristics as a ratio of particle diameter vs. wavelength of incident
beam (D/A) are the following: (a) Anti—reflection (when D/A~0.25), (b) diffraction (when
D/A~l), and (c) diffusive scattering (when, D/A>l). We believe this study is important
in that the work includes fundamental colloidal behavior and provides functional optical
coatings available for the further modifications.

The coatings were further plated with nickel by a two-step electroless plating
technique with two palladium catalysts. Using these demonstrated new steps we have
developed a novel method of creating difiusive metal reflectors. This newly proposed
method can remove the vacuum procedures and provide a low cost process without

Special equipment. Optical characterizations revealed that the reflector can reduce the

34

Specular reflectance and enhance the diffuse reflectance by only changing the Size of the
particles used. Angular dependent measurements and integrations step successfully
demonstrated how to derive the distribution of reflected light and the ratio of the specular
and diffuse reflections from the total reflected lights. This method is also available for
further customizations including changing adhesive layer, incorporation of other
molecules, over-coating of other metals, and heat treatment to enhance the optical
performance. We believe this study opens a way to fabricate more complex systems and
to perform various future works, and can be further used as templates for selective and

non-selective metal plating”, cell adhesion“, and quantum dot deposition7.

35

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