Self-organization on Nanoparticle Surfaces for Plasmonic and Nonlinear Optical Applications Kai Chen Dissertation submitted to the faculty of the Virginia Polytechnic Institute and State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy In Department of physics Hans Robinson, Chairman Giti Khodaparast James R. Heflin Richey M. Davis December 2nd, 2009 Blacksburg, VA, 24060 Keywords: localized surface plasmon resonance, ionic self-assembly multilayer (ISAM) film, convective self-assembly, surface functionalization, dithiocarbamate
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Self-organization on Nanoparticle Surfaces for
Plasmonic and Nonlinear Optical Applications
Kai Chen
Dissertation submitted to the faculty of the Virginia Polytechnic Institute and State University in partial fulfillment of the requirements for the degree of
Doctor of Philosophy
In Department of physics
Hans Robinson, Chairman Giti Khodaparast James R. Heflin Richey M. Davis
Chapter 3 Interface Effects in Plasmon Enhanced Second Harmonic Generation from Self-Assembled Multilayer Films ............................................................................................42
Fig. 1.1 Finite element simulation of the enhancement g on the top surface of a triangular prism with side length of 200 nm………………………………………………………………………....4
Fig. 1.2 Schematics of bonding of (a) thiol molecules and (b) dithiocarbamate molecules to Au surfaces......………………………………………………………………………………..……..11
Fig. 1.3 Plamonically enhanced SHG from combination of Ag nanoparticles and NLO ISAM films.……………...………………………………………………………………………….......12
Fig. 1.4 Schematic of restricted meniscus convective self-assembly.……………………....…….13
Fig. 1.5 Schematic of synthesis of dithiocarbamate grafted PAH polymers and subsequent adsorption on Au nanoparticle..…….………………………………………………………….....14
Fig. 2.1 Plamonically enhanced SHG from combination of Ag nanoparticles and NLO ISAM films..………………………………….…………………………………………………….…...16
Fig. 2.2 The molecular structures of PAH (cation) and PCBS (anion) polymers used in ISAM NLO film fabrication…………………………………………………………………………………...18
Fig. 2.3 The absorption spectra of PAH/PCBS ISAM films with different number of bilayers.…......................................................................................................................................19
Fig. 2.4 Schematics of nanosphere lithography and a corresponding SEM image of the Ag nanoparticle.…..…………………………………..……………………………………….....…..20
Fig. 2.5 SEM images of latex nanosphere layers. (a) Boundary area between monolayer and bilayer and (b) Image of a monolayer……..…………..………………….……………………....22
Fig. 2.6 SEM image of typical array of silver nanoparticles with different magnification ((a) 10 KX; (b) 145 KX; (c) 99 KX)……………………………………………………………………....…..23
Fig. 2.7 Extinction spectra of NSL-fabricated Ag nanoparticle arrays on a glass slide....………...25
Fig. 2.8 Dependence of plasmon resonance wavelength on Ag nanoparticle thickness...………...26
Fig. 2.9 Resonance wavelengths of nanoparticles arrays deposited on ISAM films of varying thickness………………………………………………………………………………………….27
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Fig. 2.10 Schematics of the experimental setup of the SHG measurements……..………...…......28
Fig. 2.11 Transmission spectra of 5 bilayer PAH/PCBS film before (a, b, c) and after (d, e, f) ultrasonication in DCM…………………………………………………………………………..29
Fig. 2.12 SHG angle scan measurements of 5 bilayer PAH/PCBS film before (black rectangle) and after (red rectangle) the ultrasonication bath…………………………………………………......30
Fig. 2.13 Transmission spectra of a 20-bilayer PAH/PB film before (black) and after (red) one-minute ultrosonication in DCM.…………………………………………………………......31
Fig. 2.14 SHG intensity of a 20-bilayer PAH/PB film before (black rectangle) and after (red rectangle) one-minute ultrasonication in DCM..............................................................................31
Fig. 2.15 SHG angle scan measurements of a hybrid ISAM-nanoparticle sample…………...…...32
Fig. 2.16 SHG signals obtained from a conventional 40 bilayer PAH/PCBS film………..…........35
Fig. 2.17 SHG signals from Ag nanoparticles deposited directly on a glass substrate………....…36
Fig. 2.18 SHG signals from the combination of Ag nanoparticles and PAH/PCBS ISAM film….36
Fig. 2.19 SHG intensity of 30 nm thick Ag nanoparticles………………...…………..…….….....38
Fig. 2.20 SHG intensity of 40 bilayer PAH/PCBS ISAM film collected at 46.4°…………..….....38
Fig. 2.21 Comparison of the SHG efficiencies in conventional and hybrid ISAM films of varying thickness………………………………………………………………………………………….40
Fig. 3.1 Illustration of the structures of (a) ISAM films, where cationic and anionic polyelectrolytes are arranged in a layered structure, and (b) HCISAM films, where the cationic layer consists of monomers (in this case Procion Brown) that are covalently bound to the underlying polycation (PAH)…………………………………………………………….….…...43
Fig. 3.2 Absorption spectra of NP-ISAM samples with increasing number of bilayers up to 40....48
Fig. 3.3 Absorption spectra of NP-HCISAM films with increasing number of bilayers up to 40...48
Fig. 3.4 Plasmon resonance wavelength modulations in the NP-ISAM (red rectangle) and NP-HCISAM (blue rectangle) samples with varying number of bilayers......................................49
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Fig. 3.5 SHG intensity as a function of film thickness for conventional PAH/PCBS and PAH/PB films (crossed circles and empty diamonds, respectively), and for the same films as modified by Ag nanoparticles (solid circles and crossed diamonds)……………..…………...…...………..…50
Fig. 3.6 Square root of the SHG intensity from PAH/PCBS ISAM films as function of thickness with and without ten NLO-inactive PAH/PAA buffers………………….………………….……53
Fig. 3.7 Schemes of the four different configurations: (a) film only (b) film-buffer (c) buffer-film (d) buffer-film-buffer….………………………………………………………………………....54
Fig. 3.8 Linear optical properties (absorption) of films with or without buffer layers in the four different film/buffer combinations……....………...…………………………………………......55
Fig. 3.9 Nonlinear optical properties (SHG) of films with or without buffer layers in the four different film/buffer combinations ……………………………………………………………....55
Fig. 3.10 Nonlinear optical properties (square root of normalized SHG) of films with or without buffer layers in the four different film/buffer combinations………….………………………..…56
Fig. 3.11 Comparison of SHG signal from freshly made PAH/PB HCISAM film with the signal from films that are several months old.……………………………………………….…………..61
Fig. 4.1 Illustration of (a) conventional convective self-assembly; (b) restricted meniscus convective self-assembly; (c) the drying zone of the colloidal crystal; (d) The far edge of the wetting film, which marks the boundary between the wet and dry portions of the colloidal crystal, is likely fairly abrupt, as illustrated by the figure.………………………………………...………66
Fig. 4.2 Microscopic view of nanosphere colloidal suspension trapped between two plates…......69
Fig. 4.3 A simplified illustration of the geometry of the trapped colloidal suspension (hatched area)………………………………………………………………………………………………74
Fig. 4.4 Plot of )1(cv , the withdrawal speed for generating uniform closepacked monolayer crystals
with CSA as a function of relative humidity for conventional CSA and RMCSA………........…..77
Fig. 4.5 Micrographs of colloidal crystal films fabricated with withdrawal speeds near )1(cv .……78
Fig. 4.6 Plots of )1(cv versus relative humidity for RMCSA using (a) nanoparticle suspensions with
additives designed to lower capillary pressure (Triton-X 100) and the electrostatic portion of the disjoining pressure (NaCl). (b) substrates with different degrees of wettability.......................…..80
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Fig. 4.7 Photographs of colloidal crystals (a) of high quality, (b) containing bare stripes……......81
Fig. 4.8 Effects of surfactant TX-100 on surface tensions of colloid suspensions measured by pendent drop technique………………………………………………………………...………...82
Fig. 4.9 Plots of )1(cv versus (a) the height H of the angled plate above the substrate and (b) the
capillary pressure in the suspension as controlled by the volume of liquid.………………….…...84
Fig. 5.1 Chemical structures of (a) citric acid; (b) 4-(N-Boc-amino)piperidine; (c) trifluoroacetic acid (TFA); (d) linear poly(ethylenimine); and (e) poly(acrylic acid)…..…………...…………...89
Fig. 5.2 Representative absorption spectra of PAH-DTC…………….…………………………..92
Fig. 5.3 Representative absorption spectra of PEI-DTC….………………………..…...…….…..92
Fig. 5.4 PAH-DTC (red line-rectangle) and PEI-DTC (blue line-circle) formation kinetics plotting absorptions of the 290 nm peak..……..……………………………………………..………...….93
Fig. 5.5 Stability test of the PAH-DTC……………………………………………..………….....95
Fig. 5.6 Stability test of the PEI-DTC………………………………………………...……..……96
Fig. 5.7 The degradation of PAH-DTC after addition of 15 mM citric acid buffer……..……..….97
Fig. 5.8 Sensitivity of 4NBaP-DTC to presence of TFA………………………………….……...99
Fig. 5.9 Absorption spectra of (a) Au nanosphere stock suspensions; (b) PAH-coated and (c) PAH-DTC coated Au nanospheres (20 nm in diameter) in ISAM film deposition process…......103
Fig. 5.10 Zeta potential of ISAM film coated Au nanospheres (20 nm in diameter) as a function of layer number…………………………………………………………………………………….105
Fig. 5.11 Autotitration results (zeta potential (filled symbols) and size measurements (empty symbols) of Au nanospheres (20 nm in diameter): PAH-DTC monolayer coated Au NP (squares); PAH monolayer coated Au NP (circles) and unmodified Au NP stock suspension (triangles).…108
Fig. 5.12 Autotitration results (zeta potential and size measurements) of Au nanospheres (100 nm in diameter): PAH-DTC monolayer coated Au NP (squares); PAH monolayer coated Au NP (circles) and unmodified Au NP stock suspension (triangles)…….………………...………......110
Fig. 5.13 Chemical structures of strong polyelectrolytes: (a) PSS and (b) PDDA…………........112
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LIST OF TABLES
Table 1 Deposition conditions for PAH/PCBS and PAH/PB ISAM films………………………46
Table 2 Effective χ(2) values for PAH/PCBS and PAH/PB films as derived by least-squared fit from the data compared to bulk χ(2) values from a single ISAM bilayer………………..………...59
1
Chapter 1 Introduction
Fabrication and functionalization of metal nanoparticles are of great importance to their wide
applications in photonics and optoelectronics. Their applications in nonlinear optics could yield
high performance optical components such as electro-optic modulators and optical switches,
which are essential components for the global telecommunication networks. However, the
implementation is not trivial and requires delicate design and diligent efforts to achieve optimum
performance. For example, 2nd order nonlinear optical effects require material structure has to be
globally non-centrosymmetric a criterion most structures do not satisfy. In addition, it is of great
significance to fabricate novel nanoparticles or nanostructures and subsequently modify their
surfaces with functional molecules, making them suitable for nonlinear applications. For this
reason, I devote this dissertation to addressing issues of integration and functionalization of
nanoparticles for nonlinear optical and other applications.
Accordingly, this dissertation is focused on studies of noble metal nanoparticles and their
applications in nonlinear optics, especially second harmonic generation (SHG). First, silver
nanoparticles were combined with second-order nonlinear optical (NLO) materials consisting of
sodium salt) (PCBS) purchased from Aldrich was used as polyanionic chromophore. Purified
Procion Brown (PB) was used as the reactive monomeric chromophore in the HCISAM films.
Poly(acrylic acid) (PAA) was used as polyanion in the NLO inactive buffer layers. The films were
deposited on glass microscope slides that were cleaned using the RCA cleaning procedure159 and
stored in de-ionized (DI) water until used. Silver pellets (99.99% pure) were purchased from Kurt
J. Lesker Company. Carboxyl functionalized polystyrene latex nanospheres (Invitrogen) with
diameter of 780 nm were used in the NSL by which the Ag NPs were fabricated172.
The ISAM films were deposited on glass slide substrates with a StratoSequence Mark VI
robotic deposition system (nanoStrata Inc.) (Akhilesh Garg made these films.). Table 1 displays
the deposition parameters for ISAM (PAH/PCBS) and HCISAM (PAH/PB) films. PB molecules
were deposited at pH ~ 10.5 because at this pH higher than the pKa value of PAH (~ 8.7160), the
amine groups of PAH polymers are totally unprotonated to ensure the covalent bonding between
the dichlorotriazine moiety in PB and the amino moiety in PAH. Then the deposition of PAH is
performed at pH of 7 which is lower than its pKa value. This enables electrostatic attraction
between the positively charged amine groups in PAH and negatively charged sulfonate groups in
PB. Therefore, all the PB dye molecules are positioned with the dichlorotriazine ends towards the
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substrates, producing a high degree of polar order and resulting in high χ(2) values for the PAH/PB
films.
It is known that increasing salt concentration and hence the ionic strength in solutions leads to
a decrease of the Debye screening length. Therefore, more charged polyelectrolytes can be
deposited onto the finite surface area, resulting in a higher chromophore density in each bilayer. It
was found that the presence of 0.5M NaCl in PB solutions gave rise to the highest χ(2) values of the
PAH/PB films156 and is therefore employed in our experiments.
Table 1 Deposition conditions for PAH/PCBS and PAH/PB ISAM films
PAH/PCBS Film PAH/PB Film
PAH PCBS PAH PB
concentration (monomer) 10mM 10mM 10mM 5mg/ml
pH 7 7 7 10.5
deposition time (minutes) 2 2 2 5
salts (NaCl) 0 0 0 0.5M
The Ag nanoparticles were deposited on top of the ISAM and HCISAM films using NSL with
the thickness (or height) of the nanoparticles at 50 nm. Convective self-assembly119 was used to
generate uniform coverage of nanospheres over large areas. The details of this technique are
thoroughly discussed in Chapter 4. A Varian Cary 5000 spectrometer was used to measure the
absorption spectra of the samples at normal incidence.
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The SHG measurements were made using a Spectra-Physics Q-switched 10 Hz Nd:YAG laser
with a fundamental wavelength of 1064 nm and pulse width of 11 ns. All the SHG data were taken
with a p-polarized incident beam and collected by a Hamamatsu R1924 photomultiplier tube
(PMT) in transmission geometry. SHG measurements were made at incident angles between 40°
and 50°. The SHG exhibits a relatively weak angular dependence172 for the NP-ISAM films while
regular fringes are observed from the ISAM and HCISAM films. The explanations of the choice of
incident angle and the SHG intensity angular dependence are detailed in Chapter 2.
3.3 Nanoparticle Resonance
To compare the effects of Ag NPs on ISAM and HCISAM films, four sets of samples were
fabricated. Each set consists of ten to twelve films where the thickness is varied between one and
forty bilayers. Two of the sets were fabricated by conventional LbL growth using PAH and PCBS
as the constituents. The other two sets consist of PAH/PB HCISAM films. In each material system,
one set of films was decorated with Ag nanoparticles formed by nanosphere lithography, while the
other was left unmodified.
Fig. 3.2 shows the absorption spectra of the NP-ISAM samples, consisting of PAH/PCBS
ISAM films with varying number of bilayers and 50 nm thick Ag nanoparticles. The absorption
spectra of the NP-HCISAM samples are shown in Fig. 3.3. Like the NP-ISAM samples, the
NP-HCISAM samples are made up of PAH/PB films and 50 nm thick Ag nanoparticles. Three
plasmon modes are excited in the nanoparticles in both sets of samples as evidenced by the three
major peaks in the spectra. As I stated in Chapter 2, the dipole-like mode is the dominant resonance
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Fig. 3.2 Absorption spectra of NP-ISAM samples with increasing number of bilayers up to 40.
The noise on the traces around 800 nm is due to the changes of detectors and gratings
inside the spectrometer. The numbers the right side of the figure indicate the number of
bilayers in the ISAM films.
Fig. 3.3 Absorption spectra of NP-HCISAM films with increasing number of bilayers up to 40.
The noise on the traces around 800 nm is due to the changes of detectors and gratings
inside the spectrometer. The numbers the right side of the figure indicate the number of
bilayers in the ISAM films.
49
with a peak around 1000-1100nm, which can be tuned by changing the thickness of the Ag
nanoparticle162, 165. By overlapping the plasmon resonance peak with the excitation wavelength, I
ensure the maximum enhancement in SHG efficiency. In this case, 50 nm is chosen to let the
dipole resonance overlap with the 1064 nm laser wavelength used in SHG measurements. The
spectral position of the dipole resonance peak varies from sample to sample, but always lies in the
980 nm ~ 1080 nm wavelength range, which provides a good overlap with the fundamental
excitation wavelength. Fig. 3.4 shows the oscillation of the dipole resonance peak with the
increasing number of bilayers in the films. This oscillation resembles to what I observed in
Chapter 2 and further investigation is needed. The overall redshift of the resonance peak with
film thickness is likely due to the higher refractive index of the polymer films.
Fig. 3.4 Plasmon resonance wavelength modulations in the NP-ISAM (red rectangle) and
NP-HCISAM (blue rectangle) samples with varying number of bilayers.
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3.4 SHG from Conventional ISAM/HCISAM Films and NP-ISAM/NP-HCISAM Films
SHG data from all four sets of samples are plotted in Fig. 3.5. The SHG intensity from Ag
nanoparticles deposited directly on glass is also shown as a reference. There are several points
worthy of note in these data:
First, for the NP-PCBS films, an SHG efficiency enhancement of ~1400 times for a 3-bilayer
sample is obtained, consistent with the previous study in Chapter 2172.
Fig. 3.5 SHG intensity as a function of film thickness for conventional PAH/PCBS and PAH/PB
films (crossed circles and empty diamonds, respectively), and for the same films as
modified by Ag nanoparticles (solid circles and crossed diamonds). The dash-dotted
line indicates SHG intensity from Ag nanoparticles in the absence of an ISAM film.
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Second, one would generally expect the SHG signal from an ISAM film to increase as the
square of the film thickness, but this is not seen in these data. For the unmodified films, a quadratic
SHG intensity dependence does eventually obtain, but only for films thicker than 10 bilayers. Thin
PAH/PCBS films show an SHG signal which grows with thickness, but much slower than for
thicker films, while the SHG signal from thin PAH/PB films actually decreases with thickness for
the first 10 bilayers. These effects are due to the presence of significant SHG from the films’
interfaces, as will be discussed below. The amount of interface SHG does not depend on the film
thickness, so it can be neglected for thick ISAM films, even though it dominates for thin films. The
differences in behavior between the two material systems can be explained by differences in the
relative phase between the interface and bulk SHG. As shown below, in the PAH/PCBS films the
two are in phase, while in PAH/PB films they are opposite in phase. This leads to a near
cancellation of the SHG signal in PAH/PB films that are between 7 and 10 bilayers thick, but not in
the PAH/PCBS films.
For the nanoparticle decorated films, it is evident that the SHG signal is roughly constant for
films thicker than two bilayers. This is consistent with the results from Ag NP-PAH/PCBS films in
Chapter 2 and is owing to the same fact that the decay length of the plasmon resonance away from
the NPs is only a few nanometers, and consequently only the top few bilayers of the films benefit
from plasmonic enhancement. Therefore, increasing film thickness does not necessarily lead to an
increase in SHG intensity. Since the interface SHG is dominant in thin ISAM and HCISAM films,
I can conclude that plasmonic enhancement of the NLO response is primarily an interface effect,
52
and bulk χ(2) is not necessarily a good indicator of how a film will respond when combined with
metal NPs.
Finally, the plasmonic enhancement for the 3-bilayer NP-PAH/PB film is only ~200 times, and
the film is actually a less efficient emitter of SHG radiation than the corresponding
NP-PAH/PCBS film despite the fact that at large bilayer numbers for the unmodified films, the
SHG intensity is much larger for PAH/PB than for PAH/PCBS due to its larger bulk χ(2) value.
Since the plasmon resonances primarily enhance the interface χ(2) of the films, we must consider
the differences in interface properties between the PAH/PB and PAH/PCBS films in order to
explain these observations. As explained in more detail below, it is likely that the PAH/PB-air
interface is more susceptible to disruption than the PAH/PCBS-air interface. The Ag nanoparticles
are deposited by electron-beam evaporation, which is a fairly severe process. The organic films are
impacted by hot, fast-moving metal atoms, easily capable of rearranging the chromophores at the
surface.
3.5 Interface Effects in Conventional ISAM and HCISAM Films
Since interface effects are critical to the NLO responses of plasmonically enhanced ISAM and
HCISAM films, a set of samples was fabricated with the purpose of separating the contributions of
each interface and the bulk to the total SHG signal (This work was done by Charles Brands). This
is accomplished by fabricating NLO-inactive buffer layer(s) above and/or below the NLO-active
film. Each buffer consists of 10 PAH/PAA bilayers and has similar dielectric and other properties
as the NLO-active films. The buffer films are considered NLO-inactive since they do not possess
53
any of the conjugated bonds responsible for large nonlinearities in organic molecules. The buffer
films remove the majority of the interface dipoles, create a less sharp surface and thereby virtually
eliminate the SHG signals from the adjacent interface. This is demonstrated by the data in Fig. 3.6,
which plots the square root of the SHG intensity generated by a series of PAH/PCBS films,
fabricated with and without ten buffer layers, as a function of the number of NLO active bilayers.
The slopes of the curves are constant and identical in both cases, stemming from the bulk
contribution to SHG. The y-intercept of the series without buffer layers indicates the contribution
of the interfaces to SHG, and when buffer layers are added, this contribution goes to zero.
Fig. 3.6 Square root of the SHG intensity from PAH/PCBS ISAM films as function of thickness
with and without ten NLO-inactive PAH/PAA buffers. The addition of the buffers
eliminates the interface contribution (y-intercept) to the SHG signal. The lines are a
guide to the eye.
54
Four combinations of buffers and NLO-active films were fabricated: film only, buffer-film (i.e.
the buffer is located below the film), film-buffer, and buffer-film-buffer, as is indicated in Fig. 3.7.
(Cemil Durak made these films and did the measurements.) The NLO active film was either one
or three bilayers thick, and consisted either of a PAH/PCBS ISAM film or a PAH/PB HCISAM
film, for a total of 16 different samples. Peak optical absorbance (at 365 nm for PCBS ISAM films
and 460 nm for PB HCISAM films) and SHG efficiency was measured for each sample, and the
data are shown in Fig. 3.8-3.10. In these cases, the SHG exhibits Maker-like fringes due to
interference between the films on opposite sides of the substrate170, and the peak intensity near an
incident angle of 45° is reported in the figure.
substrate
(a)
film
buffer
substrate
film
(b)
substrate
buffer
buffer film
(c)
substrate
buffer
film
(d)
Fig. 3.7 Schemes of the four different configurations: (a) film only (b) film-buffer (c) buffer-film
(d) buffer-film-buffer. The films are NLO-active 1-bilayer or 3-bilayer PAH/PCBS or
PAH/PB films, while each buffer consists of a 10 bilayers of NLO-inactive PAH/PAA.
55
Fig. 3.9 Nonlinear optical properties (SHG) of films with or without buffer layers in the four
different film/buffer combinations.
Fig. 3.8 Linear optical properties (absorption) of films with or without buffer layers in the four
different film/buffer combinations. The NLO-inactive buffer layer consists of a
10-bilayer PAH/PAA ISAM film.
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As before, the SHG is used as a measure of the second order NLO properties (the square root of
SHG intensity is proportional to the χ(2) of the film, and is plotted in Fig. 3.9). The absorbance (Fig.
3.8) is proportional to the surface density of the NLO chromophore, which varies from sample to
sample. The square root of the SHG intensity divided by the absorbance is a figure of merit for the
films, indicating the NLO efficiency of the film per chromophore, and is plotted in Fig. 3.10.
Considering first the absorption data, I first note that the 3-bilayer films contain, not
surprisingly, more chromophores than the 1-bilayer films. More interestingly, the films that have a
buffer at the bottom (buffer-film and buffer-film-buffer) consistently have larger absorption than
those where the NLO active layers are fabricated directly on the glass substrate. This is true for all
samples, but particularly noticeable in the 1-bilayer PAH/PB film. This is likely so because the
Fig. 3.10 Nonlinear optical properties (square root of normalized SHG) of films with or without
buffer layers in the four different film/buffer combinations. Square root of the SHG
intensity divided by absorption is used as a figure of merit of film performance.
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PAH/PAA buffer layer has a larger surface charge density than the bare glass surface and therefore
attracts a larger quantity of PAH for the first layer of the NLO-active film than the bare glass. The
differences in chromophore density are accounted for by normalizing the SHG data with
absorbance in order to compare the NLO characteristics of the different buffer/film configurations.
The 3-bilayer films exhibit stronger SHG signal than the 1-bilayer films and the PAH/PB films
exhibit stronger SHG than the PAH/PCBS films, as expected. Most of the remainder of this
discussion is based on Fig. 3.10.
First, interface SHG is indeed dominant in these films, as is evidenced by the much smaller
signal from the buffer-film-buffer samples compared to the film only samples. The reduction of
the square root of the SHG intensity is 62% and 47% in the 1- and 3-bilayer PAH/PCBS films, and
78% and 80% in the 1- and 3-bilayer PAH/PB films, respectively. It is interesting to note that the
reduction is greatest in the PB-based films, demonstrating that the interface is even more important
in these films than in conventional ISAM films.
Second, we can quantify the importance of the interface between film and air by comparing the
SHG signal from the film only samples to the corresponding film-buffer samples and also by
comparing the signal from the buffer-film samples to the buffer-film-buffer samples. In all cases,
removing the air interface results in a substantial reduction in the normalized SHG signal, ranging
from 23% to 86%. It appears that the air interface is the dominant contribution to the SHG signal in
most if not all cases studied here.
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Third, the NLO properties of the glass-film interface are responsible for the differences
between the film only and buffer-film samples and between the film-buffer and buffer-film-buffer
samples. In the PAH/PCBS films, these differences are small, and we conclude that the glass
interface does not contribute significantly to the NLO properties of these samples. The situation is
more complex for the PAH/PB films. There is a significant drop-off in signal from the film-buffer
to the buffer-film-buffer configurations, consistent with an important contribution to the SHG
from the glass interface. By contrast, the signal is actually larger in the buffer-film sample than in
the film only sample. This means that the SHG fields from the two interfaces must have opposite
phase, leading to a partial cancellation when they are both present. The bulk SHG is too weak for
us to be able to assign it a phase based on the results shown in Fig. 3.10, but the minimum in the
SHG intensity from the unmodified PAH/PB plot in Fig. 3.5 indicates that the bulk SHG must be
out of phase with the dominant interface SHG contribution from the air interface.
Based on the above discussion, we can extract approximate quantitative values for
interface-χ(2) for both kinds of films. The effective χ(2) value of each film can be modeled as
blageff γχ ++=)2( (1)
where g and a are the interface χ(2) values of the glass and air interfaces, γ is the absorptivity of the
films at 365 nm for PCBS films and 460 nm for PB films, and b is a materials parameter chosen
such that γb is the bulk χ(2) of the film. l is the thickness of the film, taken to be 1.17 nm per bilayer
for the PAH/PCBS films170, and 0.57 nm per bilayer for the PAH/PB films169. The values for γ are
read from the data in Fig. 3.8, while the )2(effχ values are proportional to the square root of the SHG
59
intensities given in Fig. 3.9. For each material system, the parameters a, g and b are found with a
least square fit to the data. All parameters are assumed positive, expect for the PAH/PB films
where the air interface is unbuffered, where both a and )2(effχ are taken to be negative.
Since systematic errors are likely to be fairly large, I use +100%/-50% as a normative error
rather than the smaller error produced by the least square fit. By comparing the data to the SHG
signal from a known reference, a 68 bilayer PS-119/PAH ISAM film that has maintained a
constant value of thickness and second order susceptibility over the past 10 years 154, the results of
the fit can be scaled to yield the values for interface and bulk χ(2) that are listed in Table 2. Note
that interface and bulk χ(2) have different dimensions, so the values given for each are not directly
comparable to each other.
The values obtain through the fit are consistent with the qualitative discussion above. For
example, g << a for the PAH/PCBS film, but not for the PAH/PB film. Bulk χ(2) is an order of
magnitude larger for PAH/PB compared to PAH/PCBS, while the total interface χ(2) is only about
three times as big.
Table 2 Effective χ(2) values for PAH/PCBS and PAH/PB films as derived by least-squared fit from the data compared to bulk χ(2) values from a single ISAM bilayer.
PAH/PCBS Film PAH/PB Film
Air (a) Glass (g) Bulk (γb) Air (a) Glass (g) Bulk (γb)
spectroscopy (SERS)98, fabrication of “patchy” nanoparticles184, and nanosphere lithography185.
They can be fabricated through a number of different methods186-188, including sedimentation189-192,
slow evaporation114, 177, spin or drop casting90, 107-109, microfluidic packing181, 193, electrostatic
assembly194, covalent attachment195, and Langmuir-Blodgett methods196. In Chapter 2, I applied
nanosphere lithography to the fabrication of triangular Ag nanoparticles, using drop casting to
obtain close-packed monolayers of latex nanospheres. In my experiments, the quality and size of
the monolayer regions are essential for the fabrication of Ag nanoparticles, whose shapes and
orientations directly determine the plasmonic enhancements in the materials. Large-sized
monolayers are preferred even though they are polycrystalline. Convective assembly, also known
as evaporation-induced self-assembly1, 119, 197-200, is probably the fastest and most convenient
technique to implement for my purpose. I used this technique in Chapter 3 to fabricate Ag
nanoparticles. In this chapter, I give more details about the technique.
In one standard version of this technique, illustrated in Fig. 4.1(a), a plate is placed at an acute
angle immediately above the substrate, and a small volume of nanoparticle suspension is placed in
the corner formed by the plate and substrate. The plate is then withdrawn at a velocity vw, dragging
65
the suspension and a thin wetting film attached to the suspension with it. Evaporation from the film
induces a flow Jw of solvent toward its edge. Particles are pulled along with the flow, which drives
the growth of a thin colloidal crystal with one or more layers. Uniform films can be deposited on
multiple square centimeters in a few minutes with this technique.
In our version of the technique, illustrated in Fig. 4.1(b), the meniscus of the solvent is
restricted by placing a straight-edge above the substrate just before the drying zone of the film.
This can be accomplished simply by running the setup just described in reverse so that the upper
contact line of the fluid meniscus from which the film grows is attached along the bottom edge of
the angled plate. I will call this growth mode Restricted Meniscus Convective Self-Assembly
(RMCSA) to distinguish it from the conventional configuration, where the upper contact line is
free to attach anywhere along the flat side of the plate. Since both modes can be accommodated
with the same apparatus run in opposite directions, I will use negative withdrawal speeds (vw) to
denote RMCSA and positive for conventional CSA.
66
Fig. 4.1 Captions (see next page)
67
Fig. 4.1 (previous page) Illustration of
(a) Conventional convective self-assembly. The colloidal crystal forms from a meniscus where the
upper contact line is free to move along the flat surface of the angled plate. I denote this with
positive withdrawal velocities vw.
(b) Restricted meniscus convective self-assembly. The colloidal crystal grows from a meniscus
where the upper contact line is fixed at the lowest corner of the angled plate. I denote this mode
with negative withdrawal velocities. This mode grows crystals at roughly twice the rate of the
conventional approach.
(c) The drying zone of the colloidal crystal. A wetting film from the suspension extends into the
crystal from x = 0 to x = l. Water evaporates from the film at the rate je(x), which in steady state
is replaced by a flow JW from the suspension, pulling nanospheres toward the edge of the crystal,
causing it to grow.
(d) The far edge of the wetting film, which marks the boundary between the wet and dry portions of the colloidal crystal, is likely fairly abrupt, as illustrated by the figure. The boundary may move in discrete steps as it gets depinned from the last wet row of nanospheres in a step-wise fashion. Some fluid may be left behind on the nominally dry side of the boundary. If either the conditions required to initiate a depinning event or the amount of fluid left behind after depinning depend on the withdrawal velocity vw, this could change the scaling of the drying rate from the results expressed by equations 8 and 9.
4.2 Experimental
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