Towards Infrared Reconfigurable

Plasmonics and Metamaterials

A dissertation presented

by

Kan Yao

to

the Department of Electrical and Computer Engineering

in partial fulfillment of the requirements

for the degree of

Doctor of Philosophy

in the field of

Electrical Engineering

Northeastern University

Boston, Massachusetts

July 2017

ii

© Copyright 2017

Kan Yao

All Rights Reserved

iii

Abstract

Metamaterials are rationally engineered composite artificial structures with exotic

properties not easily obtained or altogether unavailable in nature. Since the proof-of-

concept experiments in the microwave regime in the early 2000s, the operating

wavelength of metamaterials has been quickly pushed from terahertz (THz) to mid-/near-

infrared and down to visible, where metamaterials started to interfere with another

emerging field of plasmonics. The dynamic interplay between them has enabled various

innovative concepts and applications, which are impossible with either area alone.

Despite the continuous blending of metamaterials and plasmonics, there are still

several sub-fields not fully explored and limitations not yet resolved. For instance, metals

do not support pronounced plasmonic responses at mid-infrared and far-infrared

frequencies. On the other hand, infrared light has found widespread applications such as

biomedical sensing, spectroscopy, and surveillance. The lack of suitable platforms

supporting strong resonances at infrared wavelengths, as a consequence, naturally leads

to the quest of better alternatives for infrared plasmonics and metamaterials.

Another restriction is tunability. So far, most metamaterials and plasmonic devices are

in solid phase and have fixed functionalities once they are fabricated. Since the optical

properties of metamaterials and plasmonic structures rely on both the constituents and the

way in which they are shaped and organized, introducing variables into the system, either

changeable material properties or reconfigurable structures, will open up new

opportunities for dynamically tunable metamaterials and plasmonic structures.

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In this work, aiming at solving both problems mentioned above, I will present several

solutions to achieve infrared reconfigurable plasmonics and metamaterials. Simulations

and experiments are conducted respectively for the design and realization of infrared

optical devices, covering different aspects of this topic. I will show plasmonic core/shell

particles and their self-assembled clusters as infrared resonators, ultra-compact

nanoantennas with switchable and unidirectional radiation properties, and tunable

graphene metasurfaces for infrared light beam shaping. Future work and preliminary

results on programmable plasmonic metamolecules via magnetic self-assembly will also

be discussed.

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TABLE OF CONTENTS

Table of Contents

List of Figures

Acknowledgements

Chapter 1 Introduction ............................................................................................. 1

Chapter 2 Plasmonic Core-Shell Particles by Chemical Synthesis .............................. 10

2.1 Introduction ........................................................................................................... 10

2.2 Synthesis of Plasmonic Core-shell Particles ........................................................... 13

2.3 Assembly of Clusters .............................................................................................. 23

Chapter 3 Self-Assembled Plasmonic Core-Shell Particles as Infrared Resonators..... 28

3.1 Bright-Field Reflection Measurements .................................................................. 28

3.2 Simulation of Single Particle Scattering on a Gold Substrate ................................ 35

3.3 Simulation of Single Cluster Scattering on a Gold Substrate ................................. 40

3.4 Summary and Outlook ........................................................................................... 44

Chapter 4 Designer Plasmonic Core-Shell Particles by Microfabrication ..................... 47

4.1 Introduction ........................................................................................................... 47

4.2 Self-Assembly of Polystyrene Particle Monolayers ............................................... 49

4.3 Core-Shell Particles by Microfabrication ................................................................ 52

4.4 Conclusions ............................................................................................................ 56

Chapter 5 Ultracompact Nanoantennas with Switchable Directionality ..................... 57

5.1 Introduction ........................................................................................................... 57

5.2 Design of Ultracompact Unidirectional Nanoantennas ......................................... 59

5.3 Analysis and Discussions ........................................................................................ 70

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5.4 Conclusions ............................................................................................................ 73

Chapter 6 Tunable Graphene Metasurfaces.................................................................. 75

6.1 Introduction ........................................................................................................... 75

6.2 Infrared Light Beam Shaping by Graphene Metasurfaces ..................................... 78

6.2.1 Anomalous Reflection .................................................................................... 82

6.2.2 Reflective Focusing Lens................................................................................. 84

6.2.3 Non-diffracting Airy Beams ............................................................................ 86

6.3 Infrared Circular Dichroism by Graphene Metamirrors ......................................... 88

6.4 Conclusions ............................................................................................................ 90

Chapter 7 Summary and Future work ........................................................................... 92

7.1 Summary ................................................................................................................ 92

7.2 Future Work ................................................................................................... 93

6.2.1 Programmable Plasmonic Metamolecules via Magnetic Self-Assembly ....... 93

6.2.2 Core-Shell Particles for Sensing Applications ................................................. 98

References .............................................................................................................. 100

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LIST OF FIGURES

Figure 1.1: Schematics of some representative metamaterials. (a) A lattice of metallic

wires to achieve negative permittivity at low-frequency. (b) Metallic split ring

resonators (SRRs) to produce artificial magnetism. (c) Metallic helices as building

blocks for chiral metamaterials. ....................................................................................1 Figure 1.2: (a) Illustration of SPPs and oscillation of surface charges at a dielectric/metal

interface. (b) The typical electromagnetic field profile of SPPs, reflecting their

bound, non-radiant nature. ............................................................................................2

Figure 1.3: Reconfigurable metamaterials and metadevices by material property changes.

(a) Functional devices can be written in or erased from a chalcogenide glass (GST)

phase-change film by trains of femtosecond laser pulses at different conditions. (b)

The optical image of two Fresnel zone patterns written by light. (c) The second

Fresnel zone pattern on the right is erased by light. (d) A hybrid metamaterial

consisting of a vanadium oxide (VO2) film sandwiched by gold meshes and a ground

plane. Applying electrical currents will induce the insulator-to-metal transition of

VO2 and change the reflectance. (e) Top left: Microscopic image of the metamaterial.

Letters are bare VO2 areas without gold meshes. Rest panels in clockwise order:

Infrared images at 2.67 μm wavelength with 0 A, 2.03 A, and 2.20 A current,

respectively. (f) An infrared light beam excites tunable plasmon resonances across

the graphene nanoribbons biased at varying gate voltages. (g) Plasmon-enhanced

light-matter interactions improve detection sensitivity of molecules adsorbed on

graphene. (a-c) are adopted from Ref. [13], (d, e) are adopted from Ref. [14], (f) is

adopted from Ref. [17], and (g) is adopted from Ref. [18], respectively. ....................4

Figure 1.4: Reconfigurable metamaterials and metamolecules by finite structural changes.

(a) Split ring resonator (SRR)-bar on a stretchable film. Mechanical deformation of

the elastomeric substrate changes the gap size and thus the coupling, which causes

noticeable resonance shift in (b). (c) A metamaterial consisting of “meander near the

wire” gold nanostructures on silicon nitride strings. (d) Applying voltages to the

neighboring strings induces an attractive electrostatic force in the plane, which

reduces the gap size, enabling modulation of transmission and reflection. (e, f) Two

gold nanorods are hosted on a DNA template to form a chiral metamolecule

exhibiting tunable chiroptical properties as in (g). (e) The relative angle between the

nanorods can be actively controlled by adding certain DNA strands to pair the locks

on the sides of the template. (f) Alternatively, the nanorods are assembled

perpendicular to each other, and the DNA locks function as binding sites that allow

the rod on the top (yellow) to “walk” stepwise between the two ends of the template.

(a, b) are adopted from Ref. [20], (c, d) are adopted from Ref. [21], (e) is adopted

from Ref. [23], and (f, g) are adopted from Ref. [24]. ..................................................6

Figure 2.1: Optical resonators at different wavelengths. (a) A high-index dielectric (lead

telluride (PbTe)) particle can support a series of Mie resonance modes in the mid

infrared (MIR) region. (b) High-index dielectric (silicon) particle clusters exhibit

limited spectral tunability at visible frequencies. (c) Plasmonic core-shell

nanoparticle clusters show complex spectral features in the near infrared (NIR)

region. Figures are adopted from Refs. [31], [35], and [39], respectively. ................12

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Figure 2.2: Flow chart of the synthesis of SiO2/Au core-shell particles. (a) Synthesis of

gold nanoparticle seeds (i) and modification of SiO2 particles (ii). (b) Preparation of

precursor particles, where gold nanoparticles are attached to the surface of the

modified silica beads. (c) Continuous reduction of gold ions in the planting solution

causes the nanoparticles to increase in size, coalesce, and eventually form complete

shells covering the silica cores. ..................................................................................14

Figure 2.3: Typical ultraviolet-visible (UV-Vis) extinction spectra of the THPC gold

nanoparticle solution. Different curves are collected from different batches of

samples. All of them exhibit a vague bump close to 500 nm wavelength, indicating

the sizes of the synthesized nanoparticles are 1-3 nm. Inset shows the image of an

aged solution, which is dark brown in color. ..............................................................16

Figure 2.4: Scanning Electron Microscope (SEM) images of a 1 μm (a) and a 2 μm (b)

core-shell particle (monomer) on a gold-coated substrate. The shell thicknesses are

estimated to be around 30 and 10 nm, respectively. ...................................................19

Figure 2.5: Zoom-in SEM images for shell inspection. (a) The surface of a 2 μm particle

coated by a complete and smooth gold shell. Occasional cracks such as pinhole

defects and excess gold nanoparticles are smaller than 40 nm and have negligible

impact on the infrared optical properties of the core-shell particles. (b) Perspective

view (~70°) of a 2 μm core-shell particle at a rectangular cut on its surface. The cut

is made by focused ion beam (FIB). Measurements at different locations on the

cutting edges suggest a shell thickness of ~10 nm. ....................................................20

Figure 2.6: (a,c) SEM images of a 1 μm (a) and a 2 μm (c) particle after 5 cycles of gold

reduction with a small amount of planting solution. The gold nanoparticles increase

in size (~10 nm) but are isolated. (b) Zoom-in image of a 1 μm particle after 12

cycles of reduction. The grain size increases to 20 nm and larger, whereas the

nanoparticles still do not coalesce into a complete shell. (d) Zoom-in images of a 2

μm particle after 5 cycles of reduction with a small amount of planting solution and

an additional cycle with a large amount (5 times in volume) of planting solution. A

complete shell is formed whereas the smoothness degrades slightly. ........................22

Figure 2.7: SEM image of a 1 μm particle after a single cycle of reduction with a

condensed planting solution. The attached seed nanoparticles increase significantly

in size but are clearly isolated. ....................................................................................23

Figure 2.8: Self-assembly of core-shell particles. A droplet of diluted particle suspension

is pipetted on a gold-coated substrate. During total evaporation, the core-shell

particles are assembled by the capillary force into different planar structures. ..........24

Figure 2.9: Self-assembled 1 μm core-shell clusters. Representative geometries include

but are not limited to: (a) dimers, (b-c) trimers, and (d) chains. The shell thicknesses

are estimated to be about 30 nm. Scale bars are 200 nm. ...........................................24

Figure 2.10: Self-assembled 2 μm core-shell clusters. Representative configurations

include but are not limited to: (a) dimers, (b) trimers, (c) quadrumers, (d) chains, and

(e) pentamers. The shell thicknesses are estimated to be about 10 nm. Scale bars are

500 nm. .......................................................................................................................25

Figure 2.11: Two clusters consisting of particles with incomplete coating. Some of the

attached nanoparticles drop off during the movement of the particles, forming a trial

on the substrate. The scale bars are 500 nm in (a) and 1 μm in (b). ...........................26

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Figure 3.1: Schematic of the Fourier transform infrared (FTIR) single cluster

spectroscopy. Infrared light is generated by a globar source. After passing through a

Michelson interferometer, the incident beam is focused by a 36x Cassegrain

objective lens on the sample. The reflected light is collected by the same lens and

directed to an MCT detector. A knife-edge aperture is applied in the conjugate image

plane to define a small area containing exclusively the individual clusters of interest,

represented by the rectangle bounded by the red dashed lines. ..................................30

Figure 3.2: Infrared optical properties of different clusters obtained by single cluster

FTIR spectroscopy. (a) Reflectance spectra of uncoated 2 μm silica clusters on a gold

substrate. Measured clusters include a monomer (black), a dimer (red), a symmetric

trimer (blue), and a symmetric heptamer (olive). (b) Reflectance spectra of 2 μm

core-shell SiO2/Au clusters on a gold substrate. The shell thickness is about 10 nm.

Measured clusters include a monomer (black), a dimer (red), a symmetric trimer

(blue), a chain of three (cyan), an asymmetric quadrumer (purple), an asymmetric

pentamer (orange), and a chain of five (green). Spectra are shifted vertically for

clarity except for monomers. The numbers given in the figure legend denote the

number of particles in the cluster. ...............................................................................32

Figure 3.3: (a) Infrared optical properties of 2 μm clusters consisting of particles with a

defective shell. The sizes of the defects on the shell are greater than 100 nm. The

existence of large defects enable direct illumination of light on the silica core,

evidenced by the reflection drop between 8 and 9 μm and the valley between 9 and

10 μm (arising from the absorption of silica). Meanwhile, the line shape at shorter

wavelengths also deviates from the measurement results in Figure 2.13(b), indicating

undesired failure of resonant modes excitation. (b, c) SEM images of defective

clusters measured in (a). .............................................................................................33

Figure 3.4: Infrared optical properties of 1 μm core-shell particle clusters on a gold

substrate. The shell thickness is about 30 nm. Measured configurations include a

monomer (black), a dimer (red), a symmetric trimer (blue), an asymmetric

quadrumer (purple), a chain of four (dark cyan), and an asymmetric pentamer

(orange). Spectra are shifted vertically for clarity except for monomer. All the

measurements were based on single cluster FTIR spectroscopy. ...............................34

Figure 3.5: (a) Comparison of measured and simulated reflectance spectra of an uncoated

silica monomer and an uncoated silica trimer on a gold substrate. The very good

agreement between measurements and simulations confirms the validity of the

simulation settings and the material properties. Slight shifts between simulations and

measurements may come from the simplified normal incidence. Because of the

absence of the gold shell and relatively low index of the particles, no resonance can

be excited, as indicated by the field intensity maps at (b) 2.5 μm and (c) 3.3 μm at

two cross sections cutting through a silica particle on the symmetry planes. ............36

Figure 3.6: Optical properties of a 2 μm core-shell monomer on a gold substrate. The

shell thickness is about 10 nm. (a) Comparison of measured (red) and simulated

(black) reflectance. In the simulation, a y-polarized plane wave is incident from the

top. Representative modes are denoted by the dashed lines. (b) Field intensity maps

of the cavity plasmon modes and their correspondence to the spectral features. Only

one quarter of the particle is shown because of symmetry. The electric (a11) and

magnetic (b11) dipole modes show typical ring-like magnetic and electric field

x

distributions, respectively, whereas the electric (a21) and magnetic (b21) quadrupoles

are indicated by the two intensity lobes along the perimeter. (c) The electric field

intensity distribution at 5.3 μm. The extremely weak field inside the particle and

large field intensity outside the particle in the y-z plane (right panel) suggest the

occurrence of the fundamental electric dipole resonance in response to the y-

polarized incident field. (d) Surface charge density at 5.3 μm wavelength, which also

manifests the excitation of the electric dipole resonance of the core-shell particle. ..38

Figure 3.7: Comparison of simulated reflectance spectra for 2 μm (a) and 1 μm (b)

monomers with different shell thicknesses. ................................................................40

Figure 3.8: Optical properties of a 2 μm core-shell trimer on a gold substrate. The shell

thickness is about 10 nm. (a) Comparison of measured (red) and simulated (black)

reflectance. In the simulation, a y-polarized plane wave is incident from the top.

Representative modes are marked by the dashed lines. (b) Field intensity maps of the

cavity plasmon modes and their correspondence to the spectral features. The colored

dashed lines in the central panels indicate the position of the cutting planes for

observing different mode profiles. Only half of the trimer is shown because of

symmetry. Compared with Figure 3.6, the electric dipole (a11) and magnetic

quadrupole (b21) are still conspicuous and almost unshifted. The electric quadrupole

(a21) becomes visible due to the interparticle coupling, while the magnetic dipole (b11)

diminishes. (c) The electric field distribution at 7 μm wavelength. The intense field

in the gap between two particles (left panel) suggests strong coupling between the

electric dipole modes of the two particles. In contrast, the third particle (right panel)

is off-resonance. (d) Surface charge density at 7 μm seen from the dimer side (left

panel) and from the top (right panel). Both confirm the strong dipole resonances in

the dimer. ....................................................................................................................42

Figure 3.9: Comparison of simulated reflectance spectra for 2 μm (a) and 1 μm (b)

trimers with different shell thicknesses. .....................................................................43

Figure 3.10: (a) NIR reflectance spectrum of a 2 μm core-shell trimer on a gold substrate.

Data is obtained by using a tungsten halogen lamp as the light source in combination

with a quartz beam splitter and a MCT detector. (b) Field intensity profiles of the

mode b31. Some of the cavity modes cannot be excited efficiently due to small shell

thickness and symmetry requirement. ........................................................................45

Figure 4.1: Flow chart of the fabrication of PS/Au core-shell particles. ..........................48

Figure 4.2: Schematic (a) and experimental setup (b) for self-assembly of monolayers.

Diluted PS particle suspension is slowly injected to the air-water interface to form a

closely packed monolayer, which is picked up by a substrate after draining. ............50

Figure 4.3: Optical images of the fabricated large-scale PS particle monolayers. Each

chip is around 1.5 cm × 1.5 cm. The samples exhibit different vivid colors when

observed from different perspectives, indicating the particles are in a closely packed

pattern. The sizes of the PS particles are 1 μm (left), 2.1 μm (middle), and 2.8 μm

(right), respectively. ....................................................................................................51

Figure 4.4: Reflectance spectra of 500 nm (a) and 1 μm (b) closely packed PS particle

monolayers. Colored solid curves are FTIR measurements, while the black dashed

lines are simulation results..........................................................................................51

Figure 4.5: Reflectance spectra of 2.1 μm closely packed PS particle monolayers after

different O2 etching periods. The power is fixed at 60 watts. ....................................53

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Figure 4.6: PS/Au core-shell particles at different fabrication stages. (a-b) Top view of a

monolayer of 1 μm (a) and 2.1 μm (b) PS beads treated by 30s reactive ion etching.

(c-d) After gold deposition, the particles are flipped over by PDMS stamping to have

their uncoated side facing up. (c) Top view of the transferred 1 μm monolayer. (d) A

second cycle of gold deposition leads to fully coated core-shell particles. A

perspective view at 45° shows the non-spherical shape caused by anisotropic etching.54

Figure 4.7: Individual 2.1 μm (a) and 2.8 μm (b) PS/Au core-shell monomers on a gold

substrate. The orientations are random after releasing. ..............................................55

Figure 5.1: Schematic of the nanoantenna consisting of a gold nanoparticle trimer. The

third particle (diameter D2 = 120 nm) and the dimer (diameter D1 = 100 nm) are

slightly different in size, forming an isosceles triangle. An electric dipole (ED)

emitter is positioned at the center of the dimer and is oriented along the y-axis. The

dimer gap size is fixed to be g1 = 5 nm, while the larger particle can be displaced

along the x-axis. The displacement h is counted relative to the equilibrium position

(top), where the monomer-dimer gaps (g2) are also 5 nm. By moving the larger

particle closer to the dimer (h0,

bottom), the radiation direction is switched to −x or backward direction. .................60

Figure 5.2: Results for the forward-radiation configuration. The larger particle is moved

4 nm towards (+x direction, h = −4 nm) the dimer from the equilibrium position. (a)

Spectra of the total radiated power (green) and of the ED (blue) and MD (red)

amplitude, respectively. (b, c) Surface charge density (left) and electric field

distribution in vector format (right) at (b) 845 nm and (c) 720 nm wavelengths,

where the magnetic and electric resonances take place respectively. The minimum

side lobe level of −22 dB for forward radiation is observed at 857 nm, where the ED

and MD have approximately equal amplitude and are nearly in phase. (d) 3D and (e)

2D radiation patterns at 857 nm. .................................................................................63

Figure 5.3: Evolution of the amplitude (a) and phase (b) of the dipole modes. The

numbers in the legend denote the displacement h of the larger particle. ....................64

Figure 5.4: Results for the backward-radiation configuration. The larger particle is

moved 4 nm away (−x direction) from the dimer counting from the equilibrium

position. (a) Spectra of the radiated power (green) and of the ED (blue) and MD (red)

amplitude, respectively. (b) Surface charge density (left) and electric field

distribution (right) at 710 nm, where the ED and MD mode are both on resonance. (c)

Same as (b) but at 780 nm, where the minimum side lobe level of −4.2 dB for

backward radiation is achieved. (d) 3D and (e) 2D radiation patterns at 780 nm. .....66

Figure 5.5: Minimum side lobe level that can be achieved with different trimer

configurations (given by displacement h). Circled dots correspond to the results

reported in Figures 5.2 and 5.4. Note that for different configurations, the best

directionality occurs at different wavelengths. ...........................................................67

Figure 5.6: Switching of forward-/backward radiation at the same wavelength with a

reconfigurable nanoantenna (a) or at different wavelengths with a fixed structure (b).68

Figure 5.7: Spectra of the Purcell factor Fp (red), emission enhancement f (blue), and

quantum yield η (green) for (a) forward-radiation configuration when h = −4 nm, and

(b) backward-radiation configuration when h = 4 nm, respectively. ..........................69

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Figure 5.8: Radiation patterns reconstructed from the ED-MD interference model. (a)

Forward radiation at 857 nm (left, cf. Figure 5.2(e)). (b) Backward radiation at 780

nm (right, cf. Figure 5.4(e)). .......................................................................................71

Figure 5.9: Comparison of normalized radiation patterns under different excitation

conditions. In non-ideal cases, the emitter is either shifted or tilted. (a) Forward

radiation at 857 nm wavelength with h = −4 nm. (b) Backward radiation at 780 nm

wavelength with h = 4 nm. .........................................................................................72

Figure 6.1: (a) Schematic of a metasurface consisting of subwavelength plasmonic

resonators, which function as an array of local sources with relative phase and

polarizability difference. Manipulation of wavefront, polarization and intensity of the

reflected/transmitted light can be achieved by designing the optical response and

arrangement of the resonators. (b) Generation of orbital angular momentum by a

metasurface. The left panel shows the SEM image of the metasurface, which uses

plasmonic nanoantennas to realize 2π phase distribution in the azimuthal angle

direction. The right panel is the measured spiral patterns created by the interference

of the vortex beam and a co-propagating Gaussian beam. (b) is adopted from Ref.

[97]. .............................................................................................................................76

Figure 6.2: Schematic of the reflective graphene metasurface consisting of

subwavelength graphene ribbons on a dielectric/metal substrate. (a) By properly

arranging the ribbon widths and choosing the material and thickness of the spacing

layer, the resultant plasmonic resonance in each ribbon varies and the reflected IR

light can be controlled in a prescribed manner. (b) The graphene metasurface forms

an asymmetric Fabry-Perot resonator, which substantially enhances the interaction

between light and graphene nanostructures on the top. ..............................................78

Figure 6.3: (a) Reflectivity and (b) relative phase of the reflected light as the functions of

ribbon width (w) and dielectric layer thickness (s). (c) Reflectivity and phase of the

reflected light with varying ribbon widths, while the thickness of the dielectric layer

is fixed to be s = 4.37 μm. The inset shows in cross-section the field distribution of

the plasmonic resonance at a graphene ribbon when w = 1.035 μm. Hereafter unless

specified, the frequency of the incident light is 12.32 THz, the periodicity is 3 μm,

the refractive index of the dielectric layer is 1.4, and the Fermi energy of graphene is

0.64 eV. .......................................................................................................................81

Figure 6.4: Demonstration of anomalous reflection with (a) normal incidence, and (b)

oblique incidence at an angle of 15.0°. Hereafter in this chapter, unless specified, the

incident field is subtracted from the total field for clarity, and the amplitude of the

reflected field is normalized to that of the incidence. Top: Diagrams of rays

indicating the incident and reflected light. Bottom: Electric field distributions. ........83

Figure 6.5: Schematic of achieving reflective focusing in (a) bulk optics and (b) by a

metasurface. ................................................................................................................84

Figure 6.6: Demonstration of a reflective focusing lens at three different frequencies: (a)

12.32 THz, (b) 11.82 THz, and (c) 12.82 THz. ..........................................................85

Figure 6.7: A dynamically tunable reflective focusing lens by varying the Fermi energy

of graphene. (a) Ef = 0.56 eV and (b) Ef = 0.48 eV. ...................................................85

Figure 6.8: Generation of Airy beams. (a) Ideal phase and amplitude profile at the

emitting plane to generate an Airy beam. (b) Electric filed intensity map of an ideal

xiii

Airy beam resulting from (a). (c) Simplified Airy function. (d) An Airy beam

generated by a graphene metasurface following the profile in (c). ............................87

Figure 6.9: Schematic of a graphene metamirror that strongly reflects right circularly

polarized (RCP) light with preserved helicity but weakly reflects left circularly

polarized (LCP) light, giving rise to large differential absorption or circular

dichroism (CD). The inset illustrates the unit cell consisting of a pair of “LI”-shaped

graphene patches. The periodicities in the x and y direction are 250 and 120 nm,

respectively. Other geometric parameters are: Lx = 65 nm, Ly = 100 nm, w = 50 nm,

and g = 50 nm. The CaF2 spacing layer thickness t = 1550 nm. ................................89

Figure 6.10: Helicity-selective reflection of the graphene metamirror with the carrier

mobility μ = 10000 cm2/V/s (a) and μ = 7500 cm

2/V/s (b). The Fermi energy is fixed

to be 0.8 eV. At the resonance frequency, RCP light is largely reflected with

preserved helicity (green curve), while LCP light is strongly absorbed showing much

lower reflection (blue curve). The cross-polarization conversion is weak and

identical for both polarizations (overlapping red solid and black dashed lines). .......90

Figure 7.1: Magnetic field assisted assembly of linear chains consisting of 2 μm

polystyrene particles in a ferrofluid. (Images taken by Rasam Soheilian in the DAPS

Lab) .............................................................................................................................95

Figure 7.2: Schematic of the magnetic-field assisted self-assembly of programmable

metamolecules. (a) In bulk fluid, core-shell microparticles are assembled into certain

geometries by changing the external magnetic fields. (b) By using predefined

magnetic templates on a substrate, core-shell particles can be settled by external

magnetic fields with a global order. ...........................................................................95

Figure 7.3: SEM images of fabricated nanorod pair array by electron beam lithography,

aimed at testing the fabrication limit for potential templates preparation. .................96

Figure 7.4: (a) Fully assembled cell for FTIR spectroscopy in aqueous environment.

CaF2 slides and the spacer are sandwiched between the aluminum nest plates. Core-

shell particle suspension is injected from one of the two ports. The IR transparent

window (orange box) allows both observation and measurement. (b) Comparison of

transmission through the cell filled by water (red) and by ferrofluid EMG 705 (blue),

respectively. ................................................................................................................97

xiv

Acknowledgements

At this point of approaching the very end of my graduate study, a lot of memorable

moments come out in my mind. During the past four and a half years, I have received so

much invaluable help and support from my colleagues, friends, and family.

First of all, I would like to thank my advisor, Prof. Yongmin Liu, for choosing me as

his first student—this really saved me from a hard time deviating from my dream

career—and for his patient guidance and unreserved support throughout all these years.

The privilege of being the only audience of his lectures on the fundamentals of

plasmonics and metamaterials could never happen again to the later group members, and

either will I forget the wonderful discussions on the latest literatures. The most important

thing I have learned from him is the ambition to keep chasing tough but sweeping

research topics, even if they appear impossible at the very beginning. The lessons on time

management and on establishing collaborations also influenced me a lot.

I would also like to express my grateful thanks to all the committee members: Prof.

Matteo Rinaldi, Prof. Randall Erb, and Prof. Nian. X. Sun for their helpful guidance,

inspiring advice, and kind support. Before starting the PhD I had no prior knowledge on

nanofabrication or chemistry. It was Matteo’s course that opened the door of

nanofabrication to me and introduced me how everything works out. In the collaborative

work on reconfigurable metamaterials, where I even needed to start from learning using a

pipette, I received numerous suggestions from Randy on assembly, surface chemistry,

magnetism and so forth. When there were any unplanned requests on metal deposition,

Nian always kindly gave me permission to use the facilities in his lab, even they already

xv

had a tight schedule. Without the support from any of you, it would be impossible for me

to present most results in this dissertation.

Practicing research in the multi-disciplinary field is highly collaborative. Here I

would give some special thanks to the colleagues who had selflessly shared their time,

thoughts, and expertise to promote my research. I acknowledge Yu Hui and Fangze Liu

for their supervision on SEM and ELB, and thank Zheng Liu for spending so much time

with me polishing these skills. I greatly appreciate Liao Chen, who helped me out with

almost all the troubles in the chemical synthesis, and Shuai Chen, who assisted me to

overcome the initial hesitation in performing chemistry. It was a great pleasure to work

with Rasam Soheilian on the project of magnetic self-assembly. I have bothered him too

much while he was busy. Hope this collaboration could become fruitful in the near future

and the friendship lasts a lifetime. I am also very grateful to Qing Hu for her help on

fabrication, to Xuefei Tan for her training on FTIR, and to Scott McNamara, David

McKee, and Sivasubramanian Somu for their continuous support on using the facilities.

I thank the rest of the Liu group, including but not limited to Zubin Li, Hui Jia, Zuojia

Wang, Feng Cheng, Anna Nardella, Xiaokang Bai, Wei Ma, Lin Li, Zhong Huang, and

Han Gao. And there is even a longer list of external colleagues and collaborators, such as

Ji Hao, Xinjun Wang, Zhenyun Qian, Ryan Sungho Kang, Ismail Ebraheim, William

Fowle, Wentao Liang, Abdelkrim Benabbas, etc.

Last and most importantly, my deepest appreciation goes to my parents. Their love

and understanding supported me not only these years but all my life so far. This work is

dedicated to them.

1

Chapter 1

Introduction

Metamaterials are rationally engineered composite materials with exotic properties

not easily obtained or altogether unavailable in nature [1]. Since the early 2000s,

metamaterials have emerged as a new frontier of science involving physics, material

science, optics, engineering, nanotechnology and so forth, mainly because they provide

unprecedented opportunities to control light-matter interactions and light propagation in

unusual ways. The exceptional properties of metamaterials arise from the elegant design

of subwavelength resonators or “meta-atoms” as well as judicious arrangement of these

building blocks (see Figure 1.1). This is distinctly different from natural materials, whose

properties are primarily determined by the chemical constituents and bonds [2].

Figure 1.1: Schematics of some representative metamaterials. (a) A lattice of metallic

wires to achieve negative permittivity at low-frequency. (b) Metallic split ring resonators

(SRRs) to produce artificial magnetism. (c) Metallic helices as building blocks for chiral

metamaterials.

Early proof-of-concept experiments of metamaterial were mainly done in the

microwave regime [3]. The operating frequency was quickly pushed higher from

2

terahertz (THz) to infrared (IR) and then to visible [4]. Because most metamaterials are

composed of metals, the plasmonic effect of metals plays an important role in optical

metamaterials.

Plasmonics focuses on the unique properties and applications of surface plasmon

polaritons (SPPs), the quasiparticles arising from the strong interaction between light and

free electrons in metals [5-7]. At the interface between a semi-infinite metal and a semi-

infinite dielectric, SPPs behave as a surface wave propagating along the interface while

exponentially decaying into both sides, as illustrated in Figure 1.2.

Figure 1.2: (a) Illustration of SPPs and oscillation of surface charges at a dielectric/metal

interface. (b) The typical electromagnetic field profile of SPPs, reflecting their bound,

non-radiant nature.

The interplay between metamaterials and plasmonics has led to tremendous efforts

devoted to the invention and applications of plasmonic metamaterials in the optical

region [8, 9]. However, a few areas under this research topic are not fully explored, and

some restrictions limiting its further development are yet to be overcome. For instance,

infrared light has found widespread applications such as biomedical sensing,

spectroscopy, and surveillance [10]; while on the other hand, metals do not support

pronounced plasmonic resonances at mid-infrared (MIR) and far-infrared (FIR)

frequencies as in the visible and near-infrared (NIR) region. The lack of suitable

3

platforms that can provide strong light-matter interactions at infrared wavelengths, as a

consequence, naturally leads to the quest of better alternatives for infrared plasmonics

and metamaterials.

Another restriction is the tunability. Up to date, most metamaterials and plasmonic

devices are in solid phase and have fixed functionalities once they are fabricated. Since

the optical properties of metamaterials and plasmonic nanostructures rely on both the

constituents and the way in which they are shaped and arranged, introducing variables

into the system, either changeable material properties or reconfigurable architectures, will

open up new opportunities for dynamically tunable metamaterials and plasmonics [11].

The recent progress towards this target is exciting. Figure 1.3 summarizes representative

attempts that employ materials with tunable dielectric properties. To this end, phase-

change materials are among the most promising candidates [12, 13]. It has been shown

that the refractive-index-changing phase transition of chalcogenide glass (GST) can be

induced optically by trains of femtosecond laser pulses [13]. Hence, despite the

achievable refractive index is limited, it is possible to write arbitrary patterns on the GST

film to perform functionality such as lensing or erase them, both with light (Figure 1.3(a-

c)). Vanadium oxide (VO2) is also popular as a phase-change material [14, 15]. Figure

1.3(d) and (e) shows a design of metamaterial absorber, which utilizes electrical currents

to induce Joule heating and in turn the phase transition of the VO2 film [14]. Because the

infrared reflection of the devices is sensitive to the index change, applying different

currents allows turn-on/off and continuous contrast tuning of the infrared image. Another

important class of materials that has tunable dielectric properties is the semiconductor. In

particular, two-dimensional (2D) materials such as graphene can be dynamically tuned on

4

Figure 1.3: Reconfigurable metamaterials and metadevices by material property changes.

(a) Functional devices can be written in or erased from a chalcogenide glass (GST)

phase-change film by trains of femtosecond laser pulses at different conditions. (b) The

optical image of two Fresnel zone patterns written by light. (c) The second Fresnel zone

pattern on the right is erased by light. (d) A hybrid metamaterial consisting of a vanadium

oxide (VO2) film sandwiched by gold meshes and a ground plane. Applying electrical

currents will induce the insulator-to-metal transition of VO2 and change the reflectance.

(e) Top left: Microscopic image of the metamaterial. Letters are bare VO2 areas without

gold meshes. Rest panels in clockwise order: Infrared images at 2.67 μm wavelength with

0 A, 2.03 A, and 2.20 A current, respectively. (f) An infrared light beam excites tunable

plasmon resonances across the graphene nanoribbons biased at varying gate voltages. (g)

Plasmon-enhanced light-matter interactions improve detection sensitivity of molecules

adsorbed on graphene. (a-c) are adopted from Ref. [13], (d, e) are adopted from Ref. [14],

(f) is adopted from Ref. [17], and (g) is adopted from Ref. [18], respectively.

5

their dielectric properties via electrostatic gating [16]. Figure 1.3(f) and (g) shows such

an example, in which graphene is patterned into nanoribbons to provide a platform for

plasmon-enhanced MIR biosensing [17-19].

The other side of the solution to reconfigurable metamaterials is altering the structure.

As the coupling between the constituent parts is critical in determining the global optical

properties, introducing finite structural changes is sufficient to induce notable variations

in the spectra and field distributions. In Figure 1.4(a)-(d), two prototypes of gap-tuning

metamaterials are presented. The first one is fabricated on an elastomeric stamp; thus

stretching the substrate will enlarge the gap size and shift the resonance [20]. In the

second example, similar in-plane mechanical deformation can be induced by applying

electrical voltages to the supporting strings [21]. The associated attractive forces are in

the range of a few nanonewtons, providing a very fine control on the delicate structures.

It will not be too ambitious to imagine that the ultimate reconfigurable metamaterials

or metamolecules should possess the capability to actively transform between distinct

structures under a regulated control. This reorganization of architecture will significantly

extend the bounds of the material properties available so far. The path to the Holy Grail is

rather rocky, and the attempts with self-assembly techniques appear to be most promising.

Figure 1.4(e) and (f) illustrates two types of reconfigurable chiral metamolecules enabled

by the DNA stapling [22], where upon the addition of specifically designed DNA strands,

the relative position of the paired nanorods varies under control [23, 24]. The resulting

states of the metamolecules correspond to different chirality, which can be readily

distinguished in the spectra in Figure 1.4(g). Although the structural changes are still

finite, topologically the metamolecules at each state are different.

6

Figure 1.4: Reconfigurable metamaterials and metamolecules by finite structural changes.

(a) Split ring resonator (SRR)-bar on a stretchable film. Mechanical deformation of the

elastomeric substrate changes the gap size and thus the coupling, which causes noticeable

resonance shift in (b). (c) A metamaterial consisting of “meander near the wire” gold

nanostructures on silicon nitride strings. (d) Applying voltages to the neighboring strings

induces an attractive electrostatic force in the plane, which reduces the gap size, enabling

modulation of transmission and reflection. (e, f) Two gold nanorods are hosted on a DNA

template to form a chiral metamolecule exhibiting tunable chiroptical properties as in (g).

(e) The relative angle between the nanorods can be actively controlled by adding certain

DNA strands to pair the locks on the sides of the template. (f) Alternatively, the nanorods

are assembled perpendicular to each other, and the DNA locks function as binding sites

that allow the rod on the top (yellow) to “walk” stepwise between the two ends of the

template. (a, b) are adopted from Ref. [20], (c, d) are adopted from Ref. [21], (e) is

adopted from Ref. [23], and (f, g) are adopted from Ref. [24].

7

In this work, I explore the possible solutions for achieving infrared reconfigurable

plasmonics and metamaterials. Simulations and experiments are both conducted to study

the different aspects of this topic, from fundamentals to applications, from near-infrared

to far-infrared, and from static material property tuning to finite structure altering and

eventually towards the ultimate architecture programming. The contents of this work are

organized in the following order:

The first two consecutive chapters, namely Chapters 2 and 3, explore in depth the

optical properties of SiO2/Au core-shell particles and their self-assembled clusters for the

application of mid-infrared resonators. In Chapter 2, based on wet chemistry, high quality

core-shell particles are synthesized and the conditions leading to different surface

morphologies are discussed. Self-assembly is then carried out to obtain clusters of varied

configurations.

The optical characterization of the core-shell particles and associated numerical study

are presented in Chapter 3. Infrared spectroscopy is performed to measure the optical

properties of the particles at the single cluster level. It is found that depending on the

number and configuration of the constituent particles, each cluster exhibits a unique

spectrum line shape at the infrared, especially mid-infrared wavelengths. Full-wave

simulations are conducted to reproduce the measured spectra. In addition to the excellent

agreement, clear correspondence is established between the spectral features and various

resonance modes. The systematic study in this chapter undoubtedly shows that plasmonic

core-shell particle can be tailored to achieve tunable resonances.

Chapter 4 discusses an alternative method to make core-shell particles. Based on

microfabrication techniques, polystyrene/Au core-shell particles are produced with a high

8

yield. Self-assembled monolayers (SAMs) of polystyrene (PS) particles are realized first

as a template. Plasma etching is applied to reshape the particles. Mediated by a transfer

process, the dielectric cores are coated by two cycles of deposition, resulting in another

type of designer infrared resonators.

In Chapter 5, an ultra-compact plasmonic nanoantenna with switchable directionality

is proposed and systematically studied by simulations and analyses. The antenna

comprises an asymmetric gold nanoparticle trimer and is fed by a nano emitter in the gap.

When the so-called “Kerker conditions” are satisfied, the antenna shows superior

radiation directionality in the near-infrared region. Moreover, by displacing one of the

three particles for less than 10 nm, the radiated light beam can be switched to the

opposite direction. Analyses based on a dipole interference mdoel are performed to

understand the evolution of the radiation patterns. Techniques for potential experimental

demonstrations are also discussed.

In Chapter 6, several designs of tunable flat lenses based on graphene metasurfaces

are proposed to steer far-infrared light. Practical functionalities, such as anomalous

reflection, reflective focusing, generation of non-diffracting beams and strong chiroptical

effects are demonstrated by full-wave simulations. Because the dielectric properties of

graphene can be tuned via chemical doping or electrostatic gating dynamically, the

possibility of tuning the focal length of a reflective metalens is also numerically proven

by changing the Fermi energy. These results expand the applications of graphene-based

devices towards complete beam shaping in the infrared region.

9

Finally, following a brief summary, an outlook and some preliminary results on the

ongoing study of programmable plasmonic metamolecules via magnetic self-assembly

and their sensing applications are presented in Chapter 7.

10

Chapter 2

Plasmonic Core-Shell Particles by Chemical

Synthesis

2.1 Introduction

The use of subwavelength structures to achieve exotic control of electromagnetic

fields has stimulated the blossom of metamaterials, plasmonics, and nanophotonics over

the past decades [9]. The cornerstone of these fields lies on the fact that micro- or nano-

scale building blocks can resonantly interact with light waves in a predefined manner by

programmatically designing their structures and arrangements. Metallic structures play a

crucial role in building resonators operating in the visible and NIR region, predominantly

because of their capability to support SPPs. The collective oscillation of free electrons in

metals can couple efficiently with light, which ensures strong light-matter interactions

necessary for further construction and engineering of metamaterials. However, using

metallic elements as building blocks for metamaterials is accompanied by several

limitations, among which the most influential one is the loss. Ohmic losses in metals exist

across the entire visible spectrum and remain considerable in the infrared region, whereas

the excitation efficiency of surface plasmons decreases rapidly as the wavelengths

deviating from the visible. As a consequence, metals are not ideal platforms for NIR

plasmonic devices and are seldom used to implement resonators at MIR wavelengths.

11

With the urgent quest of infrared optical applications, suitable building blocks for

infrared optics are in a pressing need [25, 26].

Recently, high-refractive-index semiconductors such as silicon, germanium, and lead

telluride are proposed as alternative platforms for infrared metamaterials and photonic

devices [27-31]. As shown in Figure 2.1(a), due to the large refractive index and low-loss

properties, individual subwavelength particles can interact with infrared light strongly via

a series of Mie resonances. In addition, the opportunity of tuning the dielectric constants

by doping [30] or by thermal effects [31] makes these materials very attractive compared

with conventional dielectrics. However, the fabrication of regular and uniform micro- or

nano-particles is challenging for high index semiconductors. Laser ablation can create

small amounts of particles from various substrates but lacks control of the particle size

and roundedness (see insets of Figure 2.1(a) and (b)) [31-33]. Chemically synthesized

amorphous silicon particles show improved size uniformity, but they are still far from

being identical, hindering the acquisition of strictly symmetric clusters [34]. Furthermore,

compared with their metallic counterparts, dielectric particles generate less intense local

fields and possess weaker coupling when they are tightly packed [35]. The former issue

limits the light-matter interaction strength, which is critical in numerous applications such

as sensing and spectroscopy; the latter restricts the spectral tunability of the cluster,

because introducing additional particles into an existing cluster will not cause significant

changes to its optical properties [33]. In Figure 2.1(b), although the number, size, and

configuration of the particles vary a lot, the reflection curves only show limited changes

in the line shape. Therefore, the task of seeking suitable building blocks for infrared

12

plasmonics and metamaterials with notable spectral tunability is still unsolved by using

dielectrics alone.

Figure 2.1: Optical resonators at different wavelengths. (a) A high-index dielectric (lead

telluride (PbTe)) particle can support a series of Mie resonance modes in the mid infrared

(MIR) region. (b) High-index dielectric (silicon) particle clusters exhibit limited spectral

tunability at visible frequencies. (c) Plasmonic core-shell nanoparticle clusters show

complex spectral features in the near infrared (NIR) region. Figures are adopted from

Refs. [31], [35], and [39], respectively.

Core-shell particles consisting of a dielectric core and a metallic shell offer a

promising solution. Such core-shell structures can support sophisticated, hybridized

plasmonic modes as well as cavity modes [36-38]. When they are closely packed into

clusters, strong interparticle coupling arises, resulting in complex near-field distribution

and far-field scattering patterns [39, 40]. It has been demonstrated that magnetic

resonance and Fano resonance can be excited in core-shell nanoparticle clusters at large

incidence angles (Figure 2.1(c)). However, the sizes of nanoparticles determine that these

resonances occur only in the NIR region, and cavity modes can hardly be excited or

13

utilized. In this chapter, the chemical synthesis of micron-sized plasmonic core-shell

SiO2/Au particles (SiO2/Au denotes silica core with gold shell) is discussed in detail. The

resulting particles are uniformly coated and almost identical, enabling the preparation of

highly symmetric core-shell clusters by subsequent self-assembly process. The infrared

optical characterization of individual clusters and the theoretical study on their optical

properties will be discussed in the next chapter.

2.2 Synthesis of Plasmonic Core-Shell Particles

The chemical synthesis of SiO2/Au core-shell particles involves a series of steps. The

general flow is illustrated in Figure 2.2. For all the experiments, the chemicals are

purchased from Sigma-Aldrich without further modification. Although silica nano- and

micro-spheres can also be synthesized in the lab following the standard procedure

developed by Stöber et al. [41], commercially available products are used for better size

and shape control. Because the main focus of this work is to explore the optical properties

of plasmonic core-shell particles as infrared resonators, the associated silica core particles

are chosen to be 1 and 2 μm in size. Smaller particles resonate in the visible region rather

than in the infrared, whereas larger particles may not fulfill the requirement for potential

applications for infrared metamaterials. Other necessary chemicals include: chloroauric

acid (gold(III) chloride or HAuCl4·3H2O, 99%), tetrakis(hydroxymethyl)phosphonium

chloride (THPC, 80% aqueous solution), sodium hydroxide (NaOH), potassium

carbonate (K2CO3), sodium chloride (NaCl), 3-aminopropylthiethoxysilane (APTES,

98%), formaldehyde (H2CO, 37%), and 200 proof ethanol. This protocol is adapted from

an earlier work by Brinson et al. [42, 43]. Mainly caused by the large sizes of the

particles used in this study, the amount of reactants in many key steps needs to be

14

modified, which nevertheless does not follow the scaling law simply. Meanwhile, how

the way of introducing gold ions and reducing agent will affect the shell completeness

and surface morphology is also investigated, which appears less sensitive for small

particles.

Figure 2.2: Flow chart of the synthesis of SiO2/Au core-shell particles. (a) Synthesis of

gold nanoparticle seeds (i) and modification of SiO2 particles (ii). (b) Preparation of

precursor particles, where gold nanoparticles are attached to the surface of the modified

silica beads. (c) Continuous reduction of gold ions in the planting solution causes the

nanoparticles to increase in size, coalesce, and eventually form complete shells covering

the silica cores.

During a single synthesis cycle, aging takes quite a large portion of time. Therefore,

despite several sections and associated preparations are parallel in the flow chart, careful

considerations in arranging the sequence of experiments can be very helpful to shorten

the synthesis period. In the following, the experimental details will be introduced in an

optimized order by assuming all the chemicals are in the as-arrived state.

First, 1 g of gold(III) chloride powder is dissolved in deionized water (referred to DI

water or simply water herein) to produce a 1% (by weight) chloroauric acid solution. The

15

dissolving and subsequent storage should both be in a glass container, and the final

solution is stored in the dark for at least 48 hours before use.

Second, the silica particles are functionalized with APTES. After diluting the as-

purchased suspension to 1 wt%, the treatment starts by washing the particles thoroughly

three times via centrifugation for 30 minutes, complete removal of the supernatant, and

redispersion in water under strong sonication. These operations are intended to detach

undesired stabilization groups on the particle surface. Following the last washing cycle,

the particles are redispersed in ethanol and washed for additional three times. After that,

the particles are resuspended in ethanol, placed in a polypropylene bottle and stirred

rapidly. The volume of ethanol is equal to the diluted 1 wt% aqueous suspension before

washing (4 mL in my experiments). The modification is carried out in a glovebox filled

with nitrogen. Fresh APTES is added to the suspension with continuous stirring for 10

minutes. The amount of APTES can be calculated by:

30[μL] / 4[mL] 126[nm]/ ,APTESV V D (2.1)

where V is the volume of suspension and D is the diameter of the particles. The amount

of APTES from Equation (2.1) is about five times the amount needed to form a silane

monolayer (~0.7 nm thick) covalently attached to the silica surface. The actual thickness

of the silane film varies from 1 to 3 nm, corresponding to several monolayers. However,

it is also noticed that in my experiments, successful synthesis of high quality core-shells

is achieved even with wild excess of APTES. The impact of the silane film thickness on

the properties of core-shell particles may need further study. The suspension is sealed and

stirred slowly overnight. Subsequent operations can be done in the open lab environment.

The suspension is boiled in a borosilicate container for 2 hours. Total evaporation is

16

prevented by continuous addition of ethanol. Care should be taken to prevent bumping.

The particles are then washed three times in ethanol by centrifugation, followed by

redispersion in a proper volume of ethanol (2 mL in my experiments). The final

suspension is stored at 4 °C and has a long shelf life of one year or longer. After

functionalization, the surface of silica particles is coated by amino groups, which allows

attachment of gold nanoparticles to be prepared in the next step.

Gold nanoparticles are synthesized following the procedures developed by Duff et al.

[44]. 1.2 mL of THPC is diluted to 100 mL, and 1 M NaOH solution is prepared by

dissolving a NaOH pellet in a certain amount of water. Under rapid stirring to a strong

vortex, 1.2 mL of the 1 M NaOH solution is added into 180 mL of water in a glass beaker,

Figure 2.3: Typical ultraviolet-visible (UV-Vis) extinction spectra of the THPC gold

nanoparticle solution. Different curves are collected from different batches of samples.

All of them exhibit a vague bump close to 500 nm wavelength, indicating the sizes of the

synthesized nanoparticles are 1-3 nm. Inset shows the image of an aged solution, which is

dark brown in color.

followed by addition of 4 mL of diluted THPC. After 5 minutes of continuous stirring,

6.795 mL of 1% HAuCl4 solution is poured into the beaker in a quick motion. Within a

second, the solution turns from colorless to medium brown, indicating preliminary

17

success of the reaction. After 10 minutes of stirring, the resulting solution is aged at 4 °C

in the dark for 14 days, during which its color changes to dark brown gradually. Because

the optical extinction is very sensitive to the nanoparticle size, any deviation of color

from above may signify potential failure, which will cause quick settlement of the

precursor particles in the later step. Figure 2.3 compares the ultraviolet-visible (UV-Vis)

extinction spectra from three batches of samples. As can be seen, although the curves

offset slightly from each other, they follow the same trend consistently and exhibit a

weak plasmon feature: a broad and vague bump close to 500 nm wavelength. According

to transmission electron microscopy (TEM) inspection, the mean size of the gold

nanoparticles is between 1 and 3 nm. Larger particles giving stronger plasmonic

responses will cause a red-shifted and more distinct extinction peak, rendering the

solution purpler in color. The THPC gold solution has a shelf life of several months and

can be used reliably at least four months after synthesis according to my experiments.

Gold planting solution is prepared next by adding 300 μL of 1 wt% HAuCl4 solution

into 20 mL of 1.8 mM K2CO3 aqueous solution. The latter is prepared by dissolving 50

mg of K2CO3 powders in 200 mL of water. The final planting solution has a pale yellow

color and should be aged at 4 °C in the dark for 2-3 days before use, while its shelf life

has not been tested.

Once the THPC gold solution is ready for use (sufficiently aged), the preparation of

precursor particles can be performed. This is the last step before the final core-shell

formation and is the trickiest one. The precursor particles consist of a functionalized

silica core and uniformly distributed gold nanoparticles attached on its surface. The

attachment is naturally driven by the electrostatic force, since the amino groups on silica

18

are positively charged whereas the gold nanoparticles are negatively charged. On the

other hand, the Coulombic repulsion between nanoparticles will limit the achievable

surface coverage. Radloff compared two methods to increase the ionic strength of the

solution and thereby decrease the repulsive force for better coverage [45]. I adopt the

more convenient one in my experiments, which requires only addition of an inert salt

(NaCl) solution. To do this, 1 M NaCl aqueous solution is prepared by dissolving 0.584 g

NaCl powder in 10 mL of water. Then, 40 mL of aged THPC gold solution is sonicated

for 1 minute, followed by addition of 4 mL of the NaCl solution. 300 μL of the

functionalized silica particle ethanol suspension is added immediately after that. The final

solution is stored in the dark after additional sonication for 2-3 minutes. Two findings

different from or not mentioned in previous literatures are found in my experiments. First,

although highly concentrated NaCl can effectively increase the ionic strength of the

solution, a threshold seems to exist for a certain size of the silica beads. According to my

trials, 2 M NaCl already has a high risk to cause aggregation and settlement of the 2 μm

particles in a few hours. Therefore, 1 M concentration is used for all my experiments.

Second, the reliable period for the solution to reach equilibrium is about 72 hours, much

longer than the reported values (12-20 hours). This is probably because of the larger

particle sizes. The overall surface coverage of the silica particles is estimated to be

around 25-30% according to literature. However, direct evidence from TEM is not

practical due to the large particle size. The resulting precursor particles are washed 2-3

times in water and then redispersed in 5 mL of water. The solution is stored at 4 °C and

has a shelf life of about 3 months. Beyond this period aggregation may occur.

19

Figure 2.4: Scanning Electron Microscope (SEM) images of a 1 μm (a) and a 2 μm (b)

core-shell particle (monomer) on a gold-coated substrate. The shell thicknesses are

estimated to be around 30 and 10 nm, respectively.

The final step of synthesis is the core-shell formation. With the help of a reducing

agent, the attached THPC gold nanoparticles function as “seeds” or nucleation sites, upon

which the gold ions in the planting solution are continuously reduced. Depending on the

ratio of gold ion to precursor particle, core-shells with different surface morphologies can

be readily synthesized under control. The first example is plasmonic core-shell particles

with complete coating, as shown in Figure 2.4. Typically, 200 μL of precursor particle

suspension is placed in a glass vial, followed by addition of 1 mL of planting solution

under rapid stirring. 2.2 μL of formaldehyde is then added into the mixture in a quick

motion. With continuous stirring, the color of the solution turns from light brown

gradually to light purple or pink within 10-15 minutes. The ultimate color does not

correspond to the plasmonic response of the micron-sized core-shell particles but is

determined by that of the excess THPC gold nanoparticles, which are unavoidable in the

precursor particle solution. Upon successful synthesis, the silica particles are uniformly

coated by a gold shell, while the surface is quite smooth, showing only occasional cracks

such as pinhole defects and attached gold nanoparticles. From high magnification

20

scanning electron microscope (SEM) images (e.g. Figure 2.5(a)), it is clearly seen that

these imperfections are not larger than 40 nm, much smaller than the particles and the

infrared wavelengths. Therefore, their influence on the optical properties will be

negligible.

Figure 2.5: Zoom-in SEM images for shell inspection. (a) The surface of a 2 μm particle

coated by a complete and smooth gold shell. Occasional cracks such as pinhole defects

and excess gold nanoparticles are smaller than 40 nm and have negligible impact on the

infrared optical properties of the core-shell particles. (b) Perspective view (~70°) of a 2

μm core-shell particle at a rectangular cut on its surface. The cut is made by focused ion

beam (FIB). Measurements at different locations on the cutting edges suggest a shell

thickness of ~10 nm.

21

The determination of the exact shell thickness, however, becomes difficult. Typically

for nanoparticles that allow high energy electrons to penetrate through, transmission

electron microscopy (TEM) is a standard technique for shell inspection. In the present

study, the large particle size impedes the use of TEM while the tiny pinhole defects can

only provide a rough estimate of the shell thickness. In Figure 2.5(b), by making a

through cut on the surface by focused ion beam (FIB) milling, the shell thickness is

measured at different locations of the cutting edge. Although the poor conductivity of the

silica core hinders a sharp imaging contrast at the gold-silica interface, the measurements

give direct evidence of the shell thickness, which is about 10 nm.

The way how gold ions are introduced into the reaction is critical in determining the

surface morphology. As the second example, it is found that if the planting solution is

added by a small amount at one time and for multiple times, the attached THPC gold

nanoparticles will only increase in size but cannot coalesce into a complete shell. Figure

2.6(a) and (c) shows the SEM images of a 1 μm and a 2 μm particle, respectively.

Compared with the protocol used to obtain core-shells in Figure 2.4, the volumes of the

precursor particle suspension and of the planting solution are reversed. The reaction is

conducted for 5 cycles, and particles are washed carefully after each cycle to remove

excess formaldehyde. From the SEM images, it can be seen that in both cases, the growth

of gold nanoparticles are obvious. However, despite the high surface coverage, the grains

are isolated and do not form a continuous monolayer. When the same process is repeated

for 12 cycles, interestingly, the grain size increases significantly, as shown in Figure

2.6(c), but the shell is still not compete, exhibiting widespread discontinuities and large

roughness. Up to 20 cycles of reduction have been tested. Nevertheless, the particles

22

aggregate easily during later cycles of reaction, frustrating further attempts on shell

formation. In comparison, in Figure 2.6(d), when a large amount of planting solution is

added to react with the particles shown in Figure 2.6(c), a complete shell is obtained,

although the surface smoothness degrades slightly. Therefore, it can be concluded that the

amount of gold ions introduced to a single reaction is critical to the shell formation.

Figure 2.6: (a,c) SEM images of a 1 μm (a) and a 2 μm (c) particle after 5 cycles of gold

reduction with a small amount of planting solution. The gold nanoparticles increase in

size (~10 nm) but are isolated. (b) Zoom-in image of a 1 μm particle after 12 cycles of

reduction. The grain size increases to 20 nm and larger, whereas the nanoparticles still do

not coalesce into a complete shell. (d) Zoom-in images of a 2 μm particle after 5 cycles of

reduction with a small amount of planting solution and an additional cycle with a large

amount (5 times in volume) of planting solution. A complete shell is formed whereas the

smoothness degrades slightly.

The concentration of gold ions also plays an important role. In a separate trial, a

condensed planting solution is used. To make a fair comparison to the result in Figure

2.6(a), the concentration is increased by 5 times while the volume of planting solution is

23

unchanged (200 μL), and the reduction is conducted for only one cycle. The image of

resulting particles is shown in Figure 2.7, where the morphology further differs from

previous results. The increase of gold ion concentration causes abrupt growth of the seed

nanoparticles but does not promote the coalescence. The grains are clearly isolated,

forming satellite nanostructures on the silica surface.

Figure 2.7: SEM image of a 1 μm particle after a single cycle of reduction with a

condensed planting solution. The attached seed nanoparticles increase significantly in

size but are clearly isolated.

In this section, the synthesis of SiO2/Au core-shell particles is discussed in detail.

Three different morphologies can be obtained by changing the ratio of the reactants and

by varying the concentration of the planting solution. Silica particles coated by a

complete shell (Figure 2.4), a defective shell (Figure 2.6(d)), a discontinuous shell

(Figure 2.6(a-c)), and satellite nanostructures (Figure 2.7) have different optical

properties and applications, which will be addressed in later chapters.

2.3 Assembly of Clusters

The self-assembly process is then conducted in a relatively straightforward manner

[39, 46]. A droplet of diluted core-shell particle suspension is pipetted on a gold-coated

24

substrate and dried slowly at room temperature. During total evaporation, the particles are

assembled by the capillary force randomly into small clusters of different planar

geometries. Figure 2.8 illustrates the schematic. Although any insoluble substrate can be

used, gold-coated high reflective mirrors are favored in the subsequent infrared optical

characterization. Polished silicon substrate, TEM grids, infrared transparent windows

made of Calcium Fluoride (CaF2) are also possible options.

Figure 2.8: Self-assembly of core-shell particles. A droplet of diluted particle suspension

is pipetted on a gold-coated substrate. During total evaporation, the core-shell particles

are assembled by the capillary force into different planar structures.

Figure 2.9: Self-assembled 1 μm core-shell clusters. Representative geometries include

but are not limited to: (a) dimers, (b-c) trimers, and (d) chains. The shell thicknesses are

estimated to be about 30 nm. Scale bars are 200 nm.

25

Figure 2.10: Self-assembled 2 μm core-shell clusters. Representative configurations

include but are not limited to: (a) dimers, (b) trimers, (c) quadrumers, (d) chains, and (e)

pentamers. The shell thicknesses are estimated to be about 10 nm. Scale bars are 500 nm.

Figures 2.9 and 2.10 show the SEM images of representative clusters consisting of 1

and 2 μm core-shell particles, respectively. In addition to single particles (monomers),

other regular configurations include dimers, trimers, quadrumers, chains of different

lengths, and by chance pentamers, heptamers or larger ensembles. The interparticle

spacing within a cluster is tiny compared with the particle sizes, not larger than 30 nm

estimated from the SEM images. On the other hand, the assembled clusters are relatively

isolated from each other. The typical distance between neighboring clusters ranges from

10 to some hundreds micrometers. As a result, the scattering from one cluster will not be

affected much by nearby particles, which allows us to characterize the optical properties

of individual clusters.

During the assembly process there are two phenomena worthy of attention. First, the

probability of forming simple and symmetric structures, such as dimers, trimers, and

26

quadrumers, are much higher than that of forming large and complex structures.

Meanwhile, it is found that large clusters tend to appear near the edge of the droplet,

accompanied by more excess nanoparticles, as shown in Figure 2.10(d) and (e). This is a

consequence of the kinetics of evaporation. To make the clusters cleaner, either thorough

washing after synthesis or moving the droplet during evaporation could be helpful.

Figure 2.11: Two clusters consisting of particles with incomplete coating. Some of the

attached nanoparticles drop off during the movement of the particles, forming a trial on

the substrate. The scale bars are 500 nm in (a) and 1 μm in (b).

The second phenomenon is about the bonding strength between the silica core and the

gold shell. In Figure 2.6(a), (b) and (d), several defects sized over 100 nm can be seen.

However, it is not very clear if they are caused by incomplete functionalization—this

cannot be fixed in later synthesis steps—or by centrifugation. The reason is distinguished

27

by a revised assembly process, during which the droplet is gently blown to move a short

distance on the substrate. Figure 2.11 shows the resulting clusters formed by 2 μm

particles with incomplete coating, which are synthesized by using small amounts of

plating solution as discussed above. From the images, unexpectedly, one can see dropped

nanoparticles from the silica surface. Moreover, the distribution of these nanoparticles is

not random but follows clear trials, indicating that the exfoliation occurs during the

movement of the silica cores. In an extreme case in Figure 2.11(b), numerous

nanoparticles drop off from a particle in a heptamer, likely during rolling, leaving a belt-

like uncoated area on the silica surface. Similar defects are only observed for particles

with incomplete coating or from multi-step reduction, whereas for complete shells, no

visible damage is found after centrifugation, sonication, and assembly. Defective shells

can diminish spectral features of the particles and clusters and thus have negative impacts

on their optical properties. These results provide useful information to avoid defects in

the synthesis and assembly processes.

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Chapter 3

Self-Assembled Plasmonic Core-Shell Particles

as Infrared Resonators

The successful synthesis of high quality core-shell particles and assembly of

symmetric clusters provide a set of targets that is ideal for optical characterization.

Following the fabrication processes above, in this chapter, the infrared optical properties

of individual core-shell particles and self-assembled clusters are systematically studied by

both experiments and simulations. It is shown that core-shell particle clusters exhibit

unique optical properties in the infrared, especially MIR region. Depending on the

number and configuration of the constituent particles, the reflection spectra from

individual ensembles show distinct line shapes. In other words, the optical properties of

the clusters are sensitive to structural and geometric changes, indicating a promising

toolbox for infrared plasmonics, metamaterials and associated applications.

3.1 Bright-Field Reflection Measurements

Fourier transform infrared (FTIR) spectroscopy is employed to characterize the

optical properties of individual clusters with different numbers of particles and varying

configurations. The measurements are conducted using the Bruker spectrometer Vertex

70 combined with a microscope Hyperion 1000. Figure 3.1 illustrates the schematic of

the setup for reflection measurement. Infrared light is generated by a globar source (for

MIR) or a tungsten halogen lamp (for NIR). After passing through a Michelson

29

interferometer, the light beam is focused on the sample surface by a 36x Cassegrain

objective (NA = 0.5), which shapes the incident beam to be donut-like in cross section,

corresponding to incident angles ranging from ~18 to 30°. In the present study, the

reflected light is collected by the same objective and directed to a Mercury-Cadmium-

Telluride (MCT) detector. In the case of transmission measurement, two identical

Cassegrain objectives are needed. The incident light is launched by one lens below the

sample stage, and the transmitted light is collected by the other on the top. In order to

measure the signal from a single cluster, a knife-edge aperture in the conjugate image

plane is carefully adjusted to define a rectangular area containing exclusively the cluster

of interest. Depending on the signal-to-noise ratio (SNR) as well as the number and

configuration of the particles in the clusters, the size, shape, and orientation of the

aperture varies, roughly from 100 to 400 μm2. Focusing should be carefully tuned to

maximize the signal intensity. For each measurement at MIR (NIR) wavelengths, the

sample is scanned for at least 2500 times with a 4 cm-1

(8 cm-1

) spectral resolution and a

proper amplification. This scheme can effectively suppress noises and improve SNR. A

reference measurement in a nearby sample-free area is carried out immediately after each

sample measurement, while all the settings on focusing, knife-edge aperture size, number

of scans and spectral resolution remain unchanged. Reflectance is then calculated by

normalizing the sample spectrum to the reference spectrum. All the measurements are

based on unpolarized illumination.

Uncoated silica particles and particles with incomplete coating (see Figures 2.6(c) and

2.11) are first measured. Because bare silica particles will aggregate into a monolayer

easily on the substrate, extremely low concentration is used to obtain isolated monomers.

30

Figure 3.1: Schematic of the Fourier transform infrared (FTIR) single cluster

spectroscopy. Infrared light is generated by a globar source. After passing through a

Michelson interferometer, the incident beam is focused by a 36x Cassegrain objective

lens on the sample. The reflected light is collected by the same lens and directed to an

MCT detector. A knife-edge aperture is applied in the conjugate image plane to define a

small area containing exclusively the individual clusters of interest, represented by the

rectangle bounded by the red dashed lines.

Surprisingly, it is found that the infrared optical properties of an uncoated silica particle

and of a particle with incomplete coating are nearly identical. Although the gold

nanoparticles cover about half of the silica surface, their sizes are around 10 nm, more

than 150 times smaller than the infrared wavelengths (1.5 μm and longer). Therefore, the

existence of the non-coalesced gold nanoparticles on silica beads will only change the

optical properties in the visible region but will not affect the infrared optical response.

This allows one to investigate the infrared optical properties of uncoated silica particle

clusters by measuring their counterparts with incomplete coating, which are easy to

assemble. In the following, these two types of particles will not be distinguished, and

both of them will be referred to uncoated silica particles.

31

Figure 3.2(a) compares the reflectance spectra of four different 2 μm silica bead

clusters on a gold substrate. With the number of constituent particles increasing from 1

up to 7, the measured clusters include a monomer, a dimer, a symmetric trimer, and a

symmetric heptamer. As one can see, in addition to the absorption from carbon dioxide

(CO2) in air at 4.3 μm and from silica at 8-10 μm, a broad reflection dip occurs at 1.5-4

μm, which is caused mainly by light scattering. However, instead of exhibiting any

noticeable shifts, all the spectral features (peaks and dips) line up very well in the vertical

direction and simply become more evident with the increasing number of particles. This

is because the refractive index of silica is relatively low, less than 1.5 below 8 μm and not

significantly greater than the surrounding medium (air). When the particle size is

comparable to the wavelengths, no