Fabrication and study of optical properties of multilayer metal insulator metal nanocups

1.2 Localized Surface Plasmon Resonance LSPRChapter 2: Experimental procedures 2.1 Fabrication substrate monolayer Polystyrene nanoparticles on a glass substrate 2.2 Measurement Chapter

VIETNAM NATIONAL UNIVERSITY, HANOI

VIETNAM JAPAN UNIVERSITY

VIETNAM NATIONAL UNIVERSITY, HANOI

VIETNAM JAPAN UNIVERSITY

RESEARCH SUPERVISORS:

DR PHAM TIEN THANH PROF DR NGUYEN HOANG LUONG

First of all, I would like to express my sincere thank to my supervisors: Dr.Pham Tien Thanh, and Prof Nguyen Hoang Luong, for their guidance,encouragement to completed this thesis

Second of all, I would also like to thank Mr Nguyen Van Tien, and Mrs.Nghiem Ha Lien, for their support in fabricating Polystyrene nanoparticles

Third of all, I would like to express my sincere thank to my classmate Mr.Pham Dinh Dat Thank to you, I got basic knowledge of FDTD method Thank youfor your willing to help

Forth of all, I would like to thank Vietnam Japan University and staffworking here for their necessary supports

Last but not least, I also would like to express my sincere thank to my family,for their fully support

1.2 Localized Surface Plasmon Resonance (LSPR)

Chapter 2: Experimental procedures

2.1 Fabrication

substrate

monolayer Polystyrene nanoparticles on a glass substrate

2.2 Measurement

Chapter 3 Results and Discussion

3.1 SEM images

3.2.1.2 Transmittance properties of samples on substrate

3.2.1.3 Transmittance properties of substrate after separate particle

3.2.2 Optical properties of MIM nanocups in water

Conclusion

References

Fig 2.6 Procedure of making Multilayer Metal – Insulator – Metal

nanocups structure on glass substrate and in water

monolayer PS nanoparticles on glass substrate

substrate after separated particles, (b): MIM nanocups inwater

(b), Metal – Insulator nanocups; (c), Metal – Insulator – Metalnanocups

200nm diameter

nm diameter

particle

particle

PS200MI (green straight curve), and PS200MIM (blackstraight curve) on glass substrate

PS500MI (green straight curve), and PS500MIM (blackstraight curve) on glass substrate

nanoparticles for the two different polarizations

separate particle (red dashed curve), after separate particle (redstraight curve); and MIM structure (green straight curve)

Fig 3.16 Transmittance properties of sample PS200MIM: before

separate particle (black dashed curve), after separate particle(black straight curve); and MIM structure (green straightcurve)

particle (black straight curve), sample PS500MIM afterseparate particle (red straight curve) and MIM structure (greenstraight curve)

water

property of sample PS200MIM particles in water

particle in air

Attenuated Total Reflection

Cetyl trimethyl ammonium bromide

Potassium Persulfate

Localized Surface Plasmon Resonance

Metal – Insulator – Metal

Poly(methyl methacrylate)

Polystyrene

Physical Vapor Deposition

Sodium Dodecyl Sulfate

Scanning Electron Microscope

Surface Plasmon Polariton

Surface Plasmon Resonance

The Metal – Insulator – Metal (MIM) nanocups structure on glass substrateand in water were fabricated by chemical method and sputtering, studied bytransmittance properties The morphology of the sample was studied by ScanningElectron Microscope, and transmittance properties were studies by UV-Vis-NIR,and simulated by Finite Difference Time Domain (FDTD) method The resultshowed that MIM nanocups have surface plasmon resonance (SPR) and localizesurface plasmon resonance (LSPR) phenomena, that depend on size of Polystyrene(PS) and thickness of metal and insulator layers

The purpose of this thesis is:

nanoparticles

structure on substrate and in water

CHAPTER 1: INTRODUCTION

Research background

Surface plasmon resonance (SPR) phenomenon describes the oscillation offree electron cloud on the surface under electromagnetic wave of light For opticalbiosensors, some researchers have developed biosensor based on SPR and LSPRphenomena Surface Plasmon Resonance occurs when the frequency of theexcitation light coincides with the frequency of the surface electron oscillation.When SPR occurs, the spectral response at the wavelength and angle of occurrence

of the SPR are sensitive to the change in refractive index at the surrounding surface

To enhance the sensitivity and simplify the working condition of biosensor, Syahirgroup [16] developed biosensor base on metal – insulator – metal (MIM) structurewith the thickness of each size is 10 – 55nm When the incident light comes into theMIM structure, the SPR phenomenon will occur at the appropriate light Thereflection and absorption of the MIM structure at the wavelength are also highlysensitive to slight changes in the refractive indices at the surface of the structure

Some researchers study plasmons resonance phenomenon in sphere and semi– shell nanoparticle, which has both SPR and LSPR resonance Fujimura groupstudied plasmonic properties of gold nanocup The extinction curve had two peaks

at around 570nm and 750nm In this thesis, we will study SPR and LSPRphenomena in MIM nanocups structure

1.1 Surface plasmon resonance (SPR)

1.1.1 Theory [11]

Surface Plasmon Resonance (SPR) is an optical phenomenon involvingexcitation of the free oscillating metal electron It occurs when incident light (p –polarized) propagate in thin film metal under total internal reflection

Surface Plasmon Polariton (SPP) is electromagnetic excitation propagating atthe interface between a dielectric and conductor, evanescently confined in theperpendicular direction The electromagnetic surface waves arise via coupling ofelectromagnetic fields to the oscillation of the conductor’s electron plasma.

The most simple geometry sustaining SPP is that of a single, flat interface between dielectric, non – absorbing half – space (z > 0) with positive real dielectric constant 2 and conducting half – space (z < 0), describe via dielectric function 1 ( ) (Fig 1.1) The requirement of metallic character imply if Re[ε 1 ] < 0.

Figure 1.1 Geometry for SPPs propagation at a single interface between metal and dielectric

First, about transverse magnetic (TM) solution, we have:

Those function (1.1a), (1.1b), (1.1c), (1.2a), (1.2b), (1.3c) are the components of the wave vector perpendicular

to the interface in the two media Its reciprocal ẑ = 1/|k z |, define the evanescent decay length of the fields perpendicular

to the interface, which quantifies the confinement of wave Continuity Hk2 ε2 y and ε i E z at interface require that A 1 = A 2 , and

= − (1.3)

between materials with opposite signs of the real part of their dielectric

further has to fulfill wave equation, yielding:

For transverse electric (TE) surface mode, for z > 0, the expression for field component are:

Since confinement to the surface require Re[k1] > 0, and Re[k2] > 0 which is onlyfulfil when A1 = 0, also A1 = A2 = 0, therefore no TE mode surface for TEpolarization Thus, SPPs only exist for TM polarization.

The properties of SPPs will be examined by taking a closer look at their dispersion relation Radiation into metal occurs in the transparency regime ω >

4

A frequency gap region with purely imaginary β prohibiting propagation existbetween the regime of the bound and radiative modes.

For small wave vector corresponding to low (mid – infrared or lower)

waves extend many wavelengths into the dielectric space In this regime, SPPstherefore, acquire the natural of grazing – incidence field, and are also known asSommerfeld – Zenneck wave

In the opposite regime of large wave vectors, the SPPs frequency approachesthe characteristic surface plasmon frequency

ω = (1.9)

√1+ε2

as can be shown by inserting free – electron dielectric function into (1.5) In the limit of negligible damping of the conduction electron

φ(z) = A 2 eiβxe−k2z (1.10a)

for z > 0, and

φ(z) = A 1 eiβxek1z (1.10b)

metal is equal Continuity of φ and ε ensure continuity of the tangential field components and the normal field components of

5

1.1.2 SPR in metal – insulator – metal (MIM) structure

MIM structure is a structure have an insulator layer sandwich between twometals layer (Fig 1.2) When incident light goes into the structure, the light energy

II

Insulator

Glass substrate

Figure 1.2 Metal – Insulator – Metal structure

trap inside the insulator layer, therefore, enhance surface plasmon resonance signal Set ε 2 = ε 2 (ω) as dielectric function of metal and ɛ 1 as dielectric constant of insulator in equation:

−k2ε1tanh k1a = (1.12a)

−k 1 ε 2 tanh k1a = (1.12b)

Where 2a is thickness of insulator layer Function (1.12a) describe mode of odd

are odd) [9] From an energy confinement point of view, the most interesting mode

is the fundamental odd mode of the system, which does not exhibit a cut – off forvanishing I layer thickness [12] For example, Fig 1.3 illustrate dispersion relation

of this mode for a Silver/Air/Silver heterostructure [11] In this time, the dielectric

Figure 1.3 Dispersion relation of the fundamental coupled SPP modes of a

silver/air/silver multilayer geometry for an air core of size 100 nm (broken graycurve), 50 nm (broken black curve), and 25 nm (continuous black curve) Alsoshown is the dispersion of a SPP at a single silver/air interface (gray curve) andthe air light line (gray line)

function ε(ω) was taken as a complex fit with data of Silver obtain by Johnson and

Christy [8] Therefore, β does not go to infinite when surface plasmon frequency is

approached, but folds back and eventually cross the light line, as SPPs propagating

at single interface

It is apparent that large propagating constants β can be achieved even for excitation lower that , provide width of dielectric layer is chosen sufficient small Theability to access such large wave vectors and thus small penetration lengths into themetallic layers by adjusting the geometry indicates that localization effect that for a single interface can only be sustain at excitation near , can for MIM structures also beattained for excitation out in the infrared

Surface Plasmon Polaritons on a flat metal/dielectric interface cannot beexcited directly by light beam since β > k, where k is wave vector of light on thedielectric side of interface Therefore, to occur SPR on flat metal/dielectricinterface, a prism is needed However, in MIM structure Prism is not needed MIMstructure also have another benefit, for example, easy to make, and enhance thesignal of SPR which is explained as when incident light goes into MIM structure,the electromagnetic wave is trap inside insulator layer (Fig 1.5), thus increase theenergy that loses in structure

1.2 Localized Surface Plasmon Resonance (LSPR)

1.2.1 Mie theory [11]

The theory of scattering and absorption of radiation by a small sphere predicts

circumstances, the nanoparticle acts as an electric dipole, resonantly absorbing andscattering electromagnetic fields This theory of the dipole particle plasmonresonance is strictly valid only for vanishingly small particles However, in reality,the calculations outlined above provide a reasonably good approximation forspherical or ellipsoidal particles with dimensions below 100 nm illuminated withvisible or near-infrared radiation

ε+2εm

However, for a particle with large dimension, where quasi – approximation isnot justified due to significant phase – changes of the driving field over particlevolume The simplest theoretical approach available for modeling the opticalproperties of nanoparticles is the Mie theory estimation of the extinction of ametallic sphere in the long wavelength, electrostatic dipole limit [15] In thefollowing equation [9]:

E(λ) =

Where E(λ) is the extinction, which is equal to the sum of absorption and Rayleighscattering, NA is the areal density of nanoparticles, a is radius of metallic

nanosphere (assumed to be positive, real number and wavelength independent), λ isthe wavelength of absorbing radiation, ɛi is the imaginary portion of metallicnanosphere’s dielectric function, and ɛr is the real portion of the metallicnanosphere’s dielectric function The LSPR condition is met when the resonance

LSPR excitation results in wavelength selective absorption with extremelylarge molar extinction coefficient, resonance Rayleigh scattering with an efficiencyequivalent and the enhanced local electromagnetic fields near the surface of thenanoparticle

1.2.2 LSPR in nanocups structure

Nanocups structure have hopefully application in verify field, from drugdelivery to free – label sensor which is applied LSPR phenomenon [5] R Fujimura

et al studied plasmons properties of Gold nanocups through both DDASCAT [3].

fabrication of semi – shell is randomly oriented in water, optical response for three

different conditions was calculated (i) (k‖ , ⏊ ), (ii) (k⏊e z , E || e x ), and (iii) (k⏊e z , E⏊e z ); where k is wave vector, E is electric field vector of incident wave, and e z is rotation axis of target semi – shell Optical density and extinction were showed in Fig 1.8.

semi-shell, (b) deposition type semi-shell without metal migration, (c) with

metal migration (droplet-type semi-shell)

Figure 1.5 two dipolar plasmon mode in semi – shell, the

transverse mode; b the axis mode [13]

Pol Van Dorpe and Jian Ye also showed model of nanocups [18] Because the

structure of nanocup is anisotropy, therefore there are two distinct bonding dipoleresonances in an asymmetric nanoshell: one is axial mode and another is transverse

mode Fig 1.8 In transverse mode, the electric field component of the light excitesthe loop current in metallic shell, leading to strong magnetic component of theplasmon resonance The hybridization and the large field confinement near the edge

of the rim lead to strong red shifts of the nanocups transverse plasmon resonance

1.3 Application of SPR and LSPR phenomena

1.3.1 Application of SPR phenomenon

SPR can be applied in many fields, such as electronic field [20][21]; biosensors

[15][16][17], gas detection [12] by using sensitive characteristics of SPR Syahir et al

[16] fabricated Au – PMMA – Au by using PVD method for Au layers and spin –coating method for PMMA layer for free – label biosensor application (Fig

for biosensing application Layer 1 is considered to be a detected biolayer

An insulator film (layer 3) is sandwiched with two gold media (b) Opticalsetup for reflectivity measurement Reflectivity in the absence of layer 1, R0,

is taken as reference [15]

1.6) By changing the thickness of insulator sandwich between two metal layers, onecan control surface plasmons excited in MIM waveguides The MIM surface plasmonscan be excited with propagating light without using attenuated total reflection (ATR)geometry [16] Unlike SPR phenomenon in monolayer have to have prism to occur[11], SPR phenomenon can be achieved at a normal incident without prism Theresonance wave of MIM structure is independent of incident angle [14] MIM structurealso helps enhance, and make SPR signal clear, lead to easier to detect

Figure 1.7 Molecular interactions (biotin-avidin) detected using (a)

MIM (0, 0), (b) MIM (10, 30), (c) MIM (10, 40), and (d) MIM (10,

55) substrates (Syahir et al).

Syahir et al using MIM structure to detect Biotin and Avidin [16] (Fig 1.7) For the

signal of sensing Biotin and Avidin using just only monolayer Gold, the signal is verysmall and unclear Remain thickness of Au layer on top (10nm) and changing thethickness of PMMA layer: 30nm; 40nm; 55nm As reflectance properties showed inFig 1.2, the sensing signal increase and clearer with increasing thickness of PMMA

1.3.2 Application of LSPR phenomenon

There are also many application fields for LSPR phenomenon, such as biosensor[15] Recently, many researchers are interested in nanocups structure and thatapplication, for example free – label sensing [5], or enhancing the charge transfer of thecounter electrode in dye – sensitized solar cells [7], and due to semi – shell structure, it

can be used in drug delivery and release R Fujimura et al studied Au nanocups with

core is PS by deposited PS onto glass substrate and vacuum evaporated

Figure 1.8 (Color online) Spectra of the semi-shells obtained bysimulation and experiment Optical density spectrum was obtained

by experiment (solid line); extinction spectra obtained by

calculation with k || e z , E z (dashed line), k ⏊ e z , E || e z (long

dashed line), k || ez, E || ez (dot-dashed line), and their average(dotted line) [3]

Au on them In shell thickness is 32 nm, optical density of the sample has two peaks

at around 570nm and 750nm (Fig 1.8) with optical density is over 0.8 and above1.0 respectively

J Chen group studied optical fiber biosensor base on Silver Nanoparticles(AgNPs) [1] AgNPs was fabricated by using chemical method, as well as the sensorprobe The results showed that when the refractive index increase due to increase ofconcentration of Sucrose, the reflective properties decrease, and LSPR wavelengthincreased with the increase of refactive index

C Li studied LSPR sensing molecular biothiols based on noncoupled goldnanorod [10] Gold nanorod was fabricated in aqueous solution, by a typical – seed

1

surfactant, then the rod was grown by add HAuCl4 and AgNO3, using ascorbic acid

as reductant The mPEG – Au nanorod and the FITC – PEG – Au nanorod wereprepared by chemical method The result showed that there is a red shift of peak ofthe LSPR band with addition of GSH, compared with peak of LSPR of mPEG – Aunanorod

Because in MIM structure, SPR phenomenon can be occurred without prism,and sphere structure have LSPR phenomenon, therefore, MIM nanocups structurepromise a large field of application, for example biosensor [4] Therefore, thepurpose of this research is fabricated and study optical properties of MIM nanocupsstructure, hoping that this structure will have both SPR phenomenon in MIMstructure and LSPR in sphere structure

1.4 Finite Difference Time Domain (FDTD) approach

1.4.1 Theory

The Finite Difference Time Domain (FDTD) approach is based on a directnumerical of the time – dependent Maxwell’s curl equation In 3D simulation, spaceand time step is defined as below:

∆x is the size in real unit of a space step along the X direction

∆y is the size in real unit of a space step along the Y direction

∆z is the size in real unit of a space step along the Z direction

∆t is the size in real unit of a time step

Each field components represented for a 3D array: Ex(i, j, k); Ey(i, j,

k); Ez(i, j, k); Hx(i, j, k); Hy(i, j, k); Hz(i, j,k) The field components position

is showed in Fig 1.9 This arrangement is called a Yee Cell E and H field

components are interleaved at intervals of ½ cell in both space and time This

way it is possible to solve sequentially all E fields and H fields in the

simulation domain using a leapfrog algorithm

The time domain vectorial Maxwell’s equation are given in differential formby

For H components:

15

]

The fundamental constraint of the FDTD method is the step size for both time andspace Space and time steps relate to the accuracy, numerical dispersion, and thestability of the FDTD method To keep result the results as accurate as possible, withlow numerical dispersion, the mesh size usually quoted is 10 cells per wavelength,

18

meaning the size of each cell should be smaller or equal to one – tenth of theshortest wavelength to simulate.

FDTD is a volumetric computational method so that if some portioncomputational space is filled by penetrable material, the maximum cell size must bedetermined by using wavelength in the material Equation (1.16) shows adetermined method for suitable mesh size for simulator

max(∆x, ∆y, ∆z) ≤

Where nm is maximum refractive index value in the computational domain When the cellsize is determined, the maximum size for the time step ∆t immediately follows the Courant– Friedrichs – Levy (CFL) condition For the 3D simulation, the CFL condition is:

∆t ≤

1.4.2 Calculation transmittance model of MIM nanocups structure

Calculation model of MIM nanocups structure is described in Fig 1.10 Inthis thesis, the core is PS nanoparticle with diameter of 200 nm, the first layer is Au

third layer is Au with thickness of 19 nm The size of each grid is 1.5 × 1.5 ×1.5, and it took about a week to done calculation

For sample with core is PS nanoparticle with diameter is 500 nm, the timeneed to calculation is too long (around 2 – 3 weeks), therefore, the simulation ofMIM nanocups structure with core is PS nanoparticles with diameter is 500 nm wasnot done

19nm Au

PS

21 nm Au

20