Exciton-Plasmon Coupling in Two-Dimensional

Crystals

SENG LENG KIAT

A0094681W

PC4199 Honours Project in Physics In Partial Fulfilment of the Requirements for the

Degree of Bachelor of Science (Honours)

Department of Physics National University of Singapore

AY2015/2016

Acknowledgements I would like to express my gratitude to the following people: Prof. Eda Goki and Dr. Zhao Wei Jie for their advice and guidance. Dr. Ivan Verzhbitskiy, for the help in performing the AFM and SEM scans. Eric Linardy for the synthesis of MoS2 through CVD. Wang Ziying for synthesising the gold colloid solution. And members from the Nanomaterials & Device Group for their kind understanding for my hogging of certain lab equipment.

“With great (instrumental) flexibility

comes great (time consuming)

recalibration”

- paraphrasing the quote from Spider-Man (2002)

Contents

1 Introduction 7

2 Background 72.1 Plasmons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.1.1 Bulk Plasmons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82.1.2 Localised Surface Plasmons . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92.1.3 Optical Properties of Localised Surface Plasmons . . . . . . . . . . . . . . . . . 102.1.4 Field Enhancement by Localised Surface Plasmons . . . . . . . . . . . . . . . . 142.1.5 Beyond Isolated Particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

2.2 Excitons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162.2.1 Excitons in 3D systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172.2.2 Excitons in 2D systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

2.3 Enhancement of Emissive Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182.3.1 Photoluminescence Enhancement . . . . . . . . . . . . . . . . . . . . . . . . . 182.3.2 Raman Enhancement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

2.4 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

3 Experimental 213.1 Characterisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

3.1.1 Microscope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213.1.2 Photoluminescence measurement . . . . . . . . . . . . . . . . . . . . . . . . . 213.1.3 Raman Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223.1.4 Differential Reflectance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233.1.5 Atomic Force Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243.1.6 Scanning Electron Microscope . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

3.2 Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253.2.1 Chemical Vapor Deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253.2.2 Citrate Reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

4 Results and Discussion 264.1 MoS2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

4.1.1 Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264.1.2 Characterisation of MoS2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

4.2 Gold Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274.2.1 Characterisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

4.3 Gold Nanoparticles on Exfoliated MoS2 . . . . . . . . . . . . . . . . . . . . . . . . . . 314.3.1 Issues with Drop Cast with thin MoS2 flakes . . . . . . . . . . . . . . . . . . . 31

5

4.4 Gold Nanoparticles on CVD grown MoS2 . . . . . . . . . . . . . . . . . . . . . . . . . 334.4.1 Optical Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 334.4.2 Anomalous PL Spectrum After SEM imaging . . . . . . . . . . . . . . . . . . . 364.4.3 Dynamics of Particle Deposition . . . . . . . . . . . . . . . . . . . . . . . . . . 364.4.4 PL and Raman Enhancement . . . . . . . . . . . . . . . . . . . . . . . . . . . . 384.4.5 Differential Reflectance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

5 Conclusion 43

References 43

Appendix 48

6

Abstract

Most plasmonic structures on MoS2 are fabricated using electron lithography techniques,which poses its own technical challenges. In this work, the possibility of using solvent basedfabrication methods to create plasmonic gold nanoparticles and then subsequently depositingthe plasmonic particles on MoS2 is explored. Processes that occur after the solution is depos-ited on MoS2 containing substrates were inferred from AFM and SEM imaging, allowing forbetter understanding and control of deposition conditions in the future. Photoluminescenceenhancement was also reported for a single layer of gold nanoparticle coverage on MoS2 .

1 Introduction

The recent decade saw advances in fabrication of ultrathin layered materials down to unit cell thickness[1], thus allowing for the study of a class of materials known as two dimensional (2D) materials. As oneof the emerging 2D semiconductors, molybdenum disulphide (MoS2), a transition-metal dichalcogenidewhich has layered structure with van der Waals forces holding the layers together, allow for the isolationof single layers through the use of mechanical exfoliation for study. A single layer of MoS2 is composedof S-Mo-S sandwiched sheets which are covalently bonded, and is shown to exhibit photoluminescence[2, 3]. However, due to its sub-nanometre thickness, it cannot effectively absorb light and thus have lowquantum yields [4].

On the other hand, plasmonic nanoparticles support localised surface plasmon resonances which cancouple with electromagnetic fields emitted by other particles nearby to modify the radiative and non-radiative properties of these emitters [5]. These nanoparticles are able to confine light to subwavelengthvolumes, leading to the amplification of local electric fields and boosting emissions from nearby emit-ters [5]. This project seeks to boost the photoluminescence of MoS2 sheets through the use of localisedsurface plasmons from gold nanoparticles.

2 Background

2.1 Plasmons

From a solid state physics standpoint, a plasmon is the collective excitation of electrons [6]. When elec-trons are displaced from their equilibrium position, the resulting core of positive ions provides a restoringforce to sustain the oscillation. Plasmons can be further classified into bulk plasmons and surface plas-mons.

The term surface plasmon is used both for polarisation oscillation of metallic nanoparticles (called loc-alised surface plasmons, or LSPs) and for waves propagating along a plane interface and exponentially

7

decaying away from the interface (called surface plasmon polaritons, or SPPs ). The adjective "surface"used in such plasmons stems from the fact that although all electrons are oscillating with respect to apositive-ion background, the main effect producing the restoring forces is from the surface polarisation.

Ordinary plane wave electromagnetic radiation cannot directly couple to the plasmon excitation of aflat, semi-infinite metal surface (SPPs) since energy and momentum cannot be simultaneously conservedin the excitation of these SPPs [7]. However, in nano-clusters, there is no such momentum conservationrequirement and LSPs can be excited by ordinary light.

In this section, the plasmon mode is derived for both bulk and then for a spherical particle. Next, usingthe quasi-static approximation, we will relate the resonance condition to the extinction cross section, andto that of the plasmon modes, and to local enhanced electric fields. Finally, the effects of the clusteringof nanoparticles will be discussed.

2.1.1 Bulk Plasmons

Consider free electron gas oscillating in a background of positive ion cores. The dielectric function ε atfrequency ω is defined as [6]

ε(ω)≡ D(ω)

ε0E(ω)= 1+

P(ω)

ε0E(ω)(1)

where D is the displacement field, E is the electric field, ε0 is the permittivity of free space and P is thepolarisation, defined as dipole moment per unit volume and is related to the displacement field D by thedefinition D = ε0E +P. To elucidate the nature of polarisation P, we consider the situation of a electrondisplaced from its equilibrium position by displacement x, and so a electric field E exists between thepositive ion cores and the electron. Invoking Newton’s equation of motion yields:

md2xdt2 =−eE (2)

where m is the mass of the electron and e is the elementary charge. Since we are looking for oscillatorytype behaviour, x and E have a time dependence of e−iωt , so the above expression becomes:

−ω2mx =−eE; x =

eEmω2 . (3)

The electric dipole p of one electron is p =−ex. Combining this expression with Equation 3 polarisationP then appears as:

P = np =−nex =− ne2

mω2 E (4)

8

where n is the number of electrons per unit volume. Substituting this expression for P into Equation 1yields:

ε(ω) = 1− ne2

ε0mω2 = 1−ω2

p

ω2 ; ω2p =

ne2

ε0m(5)

where ωp is the plasma frequency. If positive ion core background has a dielectric constant ε∞, thenequation 5 becomes

ε(ω) = ε∞−ne2

ε0mω2 = ε∞(1−ω2

p

ε∞ω2 ) = ε∞(1−ω2

p

ω2 ); ω2p =

ne2

ε∞ε0m(6)

In general, the dielectric function can better describe a material if we include the damping term. This isdone through the introduction of a complex dielectric function as [5]

ε(ω) = ε∞−ω2

p

ω(ω + ıΓ)= ε

′+ ıε ′′ (7)

where ı is the imaginary number√−1 and Γ is the collision frequency, reflecting the damping experi-

enced by the electrons when moving within the material.

2.1.2 Localised Surface Plasmons

Conductive nanostructures are able to support collective surface oscillations of electron gas, driven byincident radiation. Due to the non-propagation of these surface plasmons in nanostructures, the term loc-alised surface plasmons (LSP) is used to distinguish from other propagating modes of surface plasmons.

To understand the nature of LSPs, we first consider the case for collective oscillation in a sphericalnanoparticle, using Newton’s equation of motion and electrostatics. For simplicity, the wavelength ofthe incident field is assumed to be much larger than the particles involved (quasi-static condition). For asphere in an electric field, the depolarisation field in the sphere is [6]

E =− 13ε0

P (8)

where polarisation P is defined in Equation 4. From Equations 2, 4, and 8 the equation of motion forplasma oscillation in spherical particles is

md2xdt2 =−ne2

3ε0xE (9)

If x and E have a time dependence of e−iωt , then

ω2 =

13

ne2

ε0mE =

ω2p

3(10)

9

The result indicates that for spherical geometry, the plasmon frequency is 1/√

3 times smaller than that inthe bulk.

We can obtain Equation 10 using another approach. For a spherical particle, matching the boundaryconditions, we have a sphere of dipole moment p expressed as [8]

p = 4πεdε0R3 ε− εm

ε +2εmE0 (11)

where εd is the dielectric constant of the medium. If we introduce the polarisability α , defined as p =

εdε0αE0, we haveα =

pε0E

= 4πR3 ε− εd

ε +2εd(12)

In general, the dielectric function of the metal ε is complex (Equation 7) and the expression divergeswhen Re[ε] =−2εd (Frohlich condition [8, 7]). If the particle is small then ε∞ ≈ εd and setting ε =−2εd

and in Equation 6 yields:

ω2 =

ω2p

3εd(13)

which is reminiscent of Equation 10 if we set εd to 1. For this reason, plasmonic effects in metal nano-particles are usually termed localised surface plasmon resonances (LSPRs) [5]. Equation 13 says that theLSPR condition red shifts as the dielectric constant the particles are immersed in increases, in agreementwith experiments [9]. In general, the condition for LSPR, and can be tuned by size, shape, morphologyas well as by the surrounding dielectric medium [10] .

2.1.3 Optical Properties of Localised Surface Plasmons

Experimentally, we detect the scattering or absorption of light. It is thus useful to derive a theoreticalframework to base our predictions on. Consider a light passing through a material. The light can eitherbe absorbed, scattered, or transmitted, that is for beam of light passing through a homogeneous material,the intensity I varies as

I(x) = I0e−ax (14)

where I0 is the incident intensity, x is the path the light has travelled in the material and a is the extinctioncoefficient. The extinction coefficient is sometimes also known as the attenuation coefficient. To accountfor the effect of packing density N has on the attenuation of light, we normalise the extinction coefficienta by the packing density N to obtain the extinction cross section

σext =aN

(15)

10

and we note that the extinction cross section is a combination of two processes, absorption and scattering,to wit

σext = σabs +σsca (16)

where σabs and σsca are the absorption and scattering cross section, respectively. To obtain these expres-sions theoretically we first need to be able to describe the electric field after scattering. When electro-magnetic radiation is incident on a small scatterer, the fields induce dipole moments p and these dipolesradiate energy [8]. We can express the electric field of a dipole characterised by its dipole moment locatedat the origin as [11]

Edip(r) =1

4πεd

eikr

r[k2(n×p)×n+

1r(1r− ik)(3n(n ·p)−p)]e−iωt (17)

where r is the distance from the scatterer, n is the unit vector in the direction of observation, k and ω

are the wavevector and frequency of the radiated field, respectively. The first term in the parenthesis isrelated to the far-field field (kr 1), while the second term in the parenthesis relates to the near fieldelectric field of the dipole. p is related to the polarisability of the sphere, p = αE0. In particular, at thefar-field region, the detected radiation is the difference between the incident radiation and the far fielddipole radiation [8]

Efar(r) = E0eikze0−E01

4πεd

eikr

rk2(n×αe0)×n (18)

where e0 is the polarisation of the incident field E0. This expression is reminiscent of the expressionobtained when the first born approximation is applied to the Lippmann-Schwinger equation in quantummechanics. We identify the scattering amplitude f(n,n0) as

f(n,n0) =1

4πεdk2(n×αe0)×n (19)

To relate the extinction cross section σext with the scattered electric field we can invoke the opticaltheorem [8]

σext =4π√

εdkIme0 · f(n = n0,n0) (20)

where σext is the extinction cross section, n0 is the unit vector of the incident wave, and f(n,n0) is thescattering amplitude. Substituting the expression for scattering amplitude (Equation 19) we obtain

σext?= k√

εdImα (21)

This suggests that for nanoparticle scatterers without any damping in their dielectric function (Imα=0) like perfect dielectrics, the extinction cross section σext = 0, that is they do not scatter or absorb light,which goes against physical intuition. For dissipative scatterers (Imα 6= 0) like metals, there is noissue as the optical theorem yields a non-zero answer for the extinction cross section σext . To verifythe expression we just obtained, we now explore another way to obtain the scattering cross section σsca:

11

through the method of direct integration. We first obtain the differential scattering cross section [8]

dσsca

dΩ(n,e;n0,e0) =

r2 |e ·Esca|2

|e0 ·Einc|2=

k4

(4πεdε0E0)2 |e ·p| (22)

where e is the polarisation of the scattered light and e0 is the polarisation of the incident light. Since fromEquation 11, p = 4πεdε0R3 ε−εd

ε+2εdE0e0 and if we account for the polarisation between incoming light e0

and scattered light e, it can be shown that |e · e0| averages to 12(1+ cos2(θ)). Integrating the resulting

differential cross section equation yields the total scattering cross section [11, 8]

σsca =

ˆdσsca

dΩdΩ =

8π

3k4a6

∣∣∣∣ ε− εd

ε +2εd

∣∣∣∣2 = k4

6π|α|2 (23)

This result contradicts with the expression obtained by the use of the optical theorem in Equation 21.The contradictions are caused by the different orders of approximations required to obtain consistencybetween the two sides of the optical theorem [8]. In the long wavelength limit, it is necessary to evaluatethe forward scattering amplitude to higher powers of k to obtain the scattering cross section contributionin the extinction cross section σext by the use of the optical theorem [8]. Therefore, we conclude that

σabs = k√

εdImα (24)

σsca =k4

6π|α|2 (25)

which are related to the extinction cross section σext by Equation 16. Through the polarisation α (Equa-tion 12), the absorption scales with R3 while scattering scales with R6. Therefore, for smaller particles,the extinction is dominated by absorption, while for large particles scattering dominates. This is exem-plified in simulations done by Jain et al. [12] and shown in Figure 1. In Figure 1, the cross section σ

is scaled with the physical cross section to obtain a dimensionless constant called efficiency Q. FromFigure 1, we can see that for small particles, the extinction curve is fully described by the absorptioncurve and the scattering term is negligible. However, as the particles increase in size the scattering termstart to dominate. This effect has affects how we see these particles, in particular, small particles absorbgreen and blue and thus appear red; large particles scatter green and thus appear greenish.

We notice here that the condition for the absorption cross section to diverge is the same as for the po-larisation α in Equation 12. That is, at the LSPR condition, huge scattering and absorption is expected.This allows us to measure the plasmon frequencies using absorption methods like UV-vis spectroscopyand view metal nanoparticles under dark field microscopy [13].

So far, we have seen that nanoparticles are able to scatter and absorb light. The absorption of light wasintroduced via the complex dielectric function in Equation 7. As expressed in Figure 2, the absorption of

12

Figure 1: Calculated spectra of the eciency of absorption Qabs (red dashed), scattering Qsca (black dotted),and extinction Qext (green solid) for gold nanospheres of (a) D=20 nm, (b) D=40 nm, (c) D=80 nm, andpolystyrene nanospheres (d) D=300 nm. Image from [12].

Figure 2: Decay of localised surface plasmon. These can be radiative (left) or non-radiative (right). Theuorescence of gold particle is negligible [14]. Image from [15].

13

light can be expressed as two decay channels. The first mechanism for non-radiative decay is the Landau-damping, where the plasmon decays non-radiatively by creating electron-hole pairs, which then continueto lose energy non-radiatively through collisions with other electrons [16]. Another decay channel is theelastic scattering of the oscillating electrons with each other, photons or defects [15]. This decay mode iscalled dephasing.

2.1.4 Field Enhancement by Localised Surface Plasmons

Using the electrostatic approximation, we can also see that the electric field is enhanced on the surface ofthe nanospheres. The electric field on the surface of the nanospheres is the sum of the incident radiationand the dipole field due to the plasmons. The field due to dipole p is [17]

Edip =−1

3ε0P =− ε− εd

ε +2εdE0 (26)

since p = 4πR3P and the expression for p is given in equation (11). Thus, the sum of the 2 yields [17]

E =− ε− εd

ε +2εdE0 +E0 =

3εd

ε +2εdE0 (27)

which diverges with the same condition as before, when ε = −2εd . Therefore, in addition to strongabsorption and scattering at the LSPR condition, the electric field may be greatly enhanced near thesurface of the metal nanoparticle. In this project, it is these properties which will be exploited to improveexisting properties, in particular to improve the light emission from photoluminescent 1L MoS2.

2.1.5 Beyond Isolated Particles

Thus far, the particle spheres considered to be well separated and are behaving independently of eachother. However, as two spheres approach one another, forming a dimer, the field in the junction betweenthe two spheres is not a sum of the fields from the individual particles [17]. Therefore, agglomeration, theprocess of formation of clusters of nanoparticles held together by van der Waals forces, can change theoptical properties significantly. In particular, agglomerates have “hot spots” at junctions between nano-particles, where there is significant enhancement of local electric field compared to the incident electricfield. Figure 3 shows the rise of field hot spots between the nanospheres for different configurations.

Heuristically, these “hot spots” arises from the near field of one particle inducing quadrupole or higherorder moments on another particle that is nearby. This then introduces a mutual induction and producesan infinite sum of coherently oscillating higher order multipole moments [17]. The near fields of all thesemoments coherently add to produce maximum external field enhancement between the particles involvedin the junction [17]. The field enhancement between the particles increases with decreasing distance [17].The emergence of such hot spots is shown in Figure 3.

14

Figure 3: Normalised electric eld magnitude maps for dierent congurations outside of 23 nm gold nanosphereson silicon substrate in a dielectric layer (ε = 2.47) evaluated at a distance of 11.5 nm from the substratecomputing using nite-dierence time-domain (FDTD). The particles are placed 2 nm apart and are excited at633nm. Electric eld propagates into the gure with polarisation shown on the gure. (a) Dimer. (b) Lineartrimer. (c) Quadrumer. (d-e) Triangular trimers. (f) Hexagonal-closed pack conguration. Image from [18].

15

2.2 Excitons

In this section, a description of excitons is given for 3D systems followed by 2D systems, in an attemptto show a theoretical basis for the observation of stable excitons in 2D systems.

In an absorption process across the band gap, an electron is excited from a state in the valence bandinto another state in the conduction band. Colloquially, the process is described by the creation of anelectron-hole pair: an electron in the conduction band and a hole in the conduction band have been super-posed on the ground state of a filled valence band and an empty conduction band [19]. Since an electronis negatively charged and a hole is positively charged, the coulombic attraction between these two op-positely charged species can form a bound state analogous to a hydrogen atom. These bound states arecalled excitons and have lower total energy than the free electron-hole pairs and show up as peaks justbelow the absorption spectra (Figure 4). With the simple approximation of parabolic valence bands, theSchrodinger equation for a exciton takes the form

[(Ec−h2

2m0m∗e∇

2e)− (Ev−

h2

2m0m∗h∇

2h)−V (Re−Rh)]ψ(Re,Rh) = Eψ(Re,Rh) (28)

where Ec is the energy of the conduction band, Ev is the energy of the valence band, m0 is the rest massof the electron, V (Re−Rh) is the coulombic potential between the electron-hole pair, m∗ is the effectivemass, and R denotes the coordinate. The subscript e and h are used to denote electron and hole, respect-ively. The first term in the Hamiltonian describes the electron in the conduction band, while the secondterm describes the hole in the valence band, and the third term describes the attractive potential betweenthe electron hole pair.

The motion can be seperated into that of the centre of mass RCM and the relative coordinate R of theelectron and hole. These are defined by:

RCM =m∗eRe +m∗hRh

M, M = m∗e +m∗h (29)

and the relative coordinate R and reduced mass m∗eh are:

R = Re−Rh,1

m∗eh=

1m∗e

+1

m∗h(30)

and the Schrodinger equation is transformed into coordinates of the centre of mass RCM and relativecoordinate R:

[(h2

2m0M∇

2CM)+(Eg−

h2

2m0m∗eh∇

2(R)−V (R))]ψ(RCM,R) = Eψ(RCM,R) (31)

16

where Eg = Ec− Ev is the band gap of the crystal and V (R) is the coulombic potential between theelectron-hole pair. Since the operators of RCM and R dependence appear additively, the wavefunctioncan be factored into two independent functions ψ(RCM,R) = ξ (RCM)φ(R). The transformation yields aHamiltonian with a centre of mass term that describes the exciton of mass M that is free move throughoutthe crystal and an interaction term, which depends only on the difference on the coordinates of theelectron and hole.

2.2.1 Excitons in 3D systems

In bulk crystals, the attractive coulombic potential for a radial hydrogenic atom is:

V (R) =− e2

4πεε0R(32)

where e is the electron charge, ε0 is the permittivity of free space ε is the relative permittivity of thematerial (also called dielectric constant) and R is the distance between the electron and hole. With thispotential, solving the eigenvalue Equation 31 will give the energy of the excitonic state E = Eg− Reh

n2 .The binding energy Ebind of the bound states of excitons are given by:

E3Dbind =−Reh

n2 , Reh =m0m∗eh

2(

e2

4πεε0h)2 = 13.6eV ×

m∗ehε2 (33)

where n is the quantum number and takes on integer values larger or equal to 1 and Reh here is theRydberg energy scaled by the dielectric constant of the crystal and the reduced mass of the electron-holepair. While the equation predicts a series of absorption lines corresponding to different bound stateslabelled by n, in practice usually the n = 1 state is observed since the weaker binding energy associatedwith higher n numbers means that the excitons can be rapidly ionized at room temperature by phononswhose energy is much greater than Reh [19].

2.2.2 Excitons in 2D systems

In 2D crystals, the coulombic potential appears the same as before, but now that the wavefunction isassumed to be zero along the z-direction and solving the eigenvalue Equation 31 for the “two-dimensionalhydrogen atom” yields energy levels of E = Eg− Reh

(n− 12 )

2 , and thus the binding energy of 2D exciton isgiven by

E2Dbind =− Reh

(n− 12)

2(34)

with Reh defined as before in Equation 33. For the lowest state (n = 1), the binding energy is 4Reh, fourtimes larger than the corresponding 3D result. The coulomb interactions are enhanced in low dimensionalsystems as a result of spatial confinement and reduced coulomb screening. The larger binding energymeans that excitons in 2D crystals are more stable against ionization at room temperature by phonons.

17

Figure 4: Schematic diagram of the excitons in (a) bulk, (b) 2D material and the associated absorption bandsin (b) bulk, (d) 2D material. The binding energy for 2D excitons is greater than that in bulk, and thus weexpect an absorption peak associated with 2D excitons to be further away from the band edge.

This indicates that the phenomenon of excitons should be more significant in 2D crystals than in 3Dcrystals. Indeed, MoS2 excitons are strictly orientated in plane [20] and are strongly bound [21, 22].

2.3 Enhancement of Emissive Processes

2.3.1 Photoluminescence Enhancement

To understand the how enhancement of photoluminescence, we first review the mechanism which governsdipole transition. A description of photoluminescence is deferred to Section 3.1.2. According to Fermi’sgolden rule, the dipole transition rate is:

γ =2π

h2

∣∣〈 f | p · E |i〉∣∣2 ρ(ω) (35)

where γ is the transition rate from the initial state |i〉 to the final state | f 〉, p is the electric dipole oper-ator of the quantum emitter, E is the electric field operator located at the emitter, and ρ(ω) is the finalphotonic density of states. Since the dipole is usually defined by the material, increase in the radiativetransition rate can be achieved by increasing the local electric field E at the emitter, or by increasing thedensity of states ρ(ω) the emitter can decay into.

Einstein developed a phenomenological description of spontaneous and emission by atoms based onblack-body radiation, with the so called “Einstein A and B coefficients”, where spontaneous emission

18

is induced by the coupling of a the emitter with the vacuum fluctuations of the electromagnetic fields,introduced through the use of Planck’s black-body radiation law, which lead to the assumption that theemission properties are determined only by the emitter [5]. However, in 1946, Purcell showed that therate of spontaneous emission is not solely dependent on the emitter, but also depends on the environmentsurrounding the emitter. This enhancement of local photonic density of states is called the Purcell effectand the magnitude of the enhancement is given by the Purcell factor. Note that if the Purcell factor is lessthan one, emissions are inhibited [5].

Placing a plasmon near the emitter might increase the transition rate (Equation 35) by both enhancing thelocal electric field Eloc and the photonic density of state ρ(ω). In particular, the final photonic density ofstates is expected to have a peaked maximum at the plasmon resonance wavelength, as it provides a newand strong decay channel for the emitter [5].

A useful quantity, the quantum yield QY , can be expressed as:

QY =γrad

γrad +∑γnonrad(36)

where γrad is the radiative transition decay rate and γnonrad is the non-radiation transition decay rate.Equation 36 is a fraction between the radiative transition rate and the sum of the transition rates of allpossible transition pathways the excited state may undergo to relax back into the ground state, that is, thequantum yield (QY) for emission is determined by the competition between radiative and non-radiativerates. Non-radiative relaxations include emission of phonons (heat) or through non-radiative relaxationassisted by defects. To have a large quantum yield, we can increase the electric field E and final photonicdensity of states ρ(ω) to increase γrad as previously mentioned, but in order to achieve a large quantumyield enhancement, the radiative rates must be increased while keeping the the non-radiative rates low.

The observed emission intensity of emitters are determined by several factors such as collection effi-ciency of the optical setup η , excitation rate kex, as well as the quantum yield QY. Since the experiment isperformed under identical conditions, the collection efficiency is considered the same [23]. In a previousstudy [20], the excitons of MoS2 effectively couple to the vertical incident laser whose polarisation is alsoin-plane, and so the excitation rate (kex) is also considered to be linearly to |E|2. Thus, the enhancementfactor (ratio of measured emission intensity) is [23, 24]

EF =η ′

η· k′ex

kex· QY ′

QY≈∣∣∣∣E ′E

∣∣∣∣2 · QY ′

QY(37)

where the symbols with and without the prime represents the parameters with and without the plasmons,respectively.

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2.3.2 Raman Enhancement

The scattering probability for conventional Raman spectroscopy are intrinsically weak [25]. However,surface plasmons are able to increase Raman scattering as well, an effect commonly known as SurfaceEnhanced Raman Scattering (SERS). A description of Raman process will be deferred to Section 3.1.3and we first focus on how the enhancement occurs. Consider an incident electric field incident on aplasmonic material. The local electric field Eloc is enhanced by

Eloc = gE0 (38)

where E0 is the amplitude of the incident electric field, and g is the magnitude of the local field enhance-ment factor near the surface of metal particle. The molecules near the plasmonic material will therefore beexcited by a field with a magnitude of Eloc and the Raman scattered field strength would be ER ∝ αREloc,where αR is the polarisability of the molecule [25]. The Raman-scattered fields can be further enhancedby the metal particle in the same manner as the incident field, and we denote the enhancement as g′,where the prime is used to indicate that the field enhancement at the Raman shifted wavelength will bein general different from the value at the incident wavelength. Therefore, the surfaced enhanced Ramanfield ESERS would be [25]

ESERS ∝ g′ER ∝ g′αREloc ∝ g′αRgE0 (39)

and the intensity of the scattered field is simply the modulus square of the enhanced Raman field ESERS

ISERS ∝ |ESERS| ∝ |αR|2∣∣gg′

∣∣2 I0

Since the Raman shift ∆ν = νR−νS between the incoming and scattered photon is generally much smallerthan the line width of the localised surface plasmons mode, |g| ≈ |g′| , which brings us to the importantresult that is electromagnetic contribution to the total SERS enhancement R is proportional to the fourthpower of the field enhancement factor [25]

R ∝ |g|4 ∝|Eloc|4

|E0|4(40)

where the last equality follows from the use of equation (38). The physical basis of the electromagneticenhancement consists of two main contributions - the enhancement due to the resonant excitation of theLSP and the lightning rod effect, where the former is shows strong frequency dependence while the latteris purely a geometric phenomenon of field crowding accompanying enhancement near sharp metallicfeatures [25, 7]. The enhancement factor for a spherical nanoparticle can be obtained from Equation 27,but in general the enhancement factors have to be calculated numerically [7].

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2.4 Motivation

After having given an introduction of the processes that will be involved, it is time to restate the motiva-tion of the project. 1L MoS2 displays strong luminescence originating from excitonic decay [2, 3]. Fromthe derivation from Section 2.2.2, we saw that the excitons have stronger binding energy and thus willhave excitons which are stable against ionisation by thermal excitation, making them a practical choicefor optical devices operating at room temperatures. However, due to the limited optical absorption lengthconferred by the ultrathin film (<1 nm for single layer), the light matter interactions are weak in absoluteterms [4]. To solve this, plasmons, which are able to enhance the local electric field as we saw in Section2.1.4 are utilised. From Fermi’s golden rule (Equation 35), we expect an increase the the absorption ofphotons due to an increased electric field, increasing the population of excitons which can then decayradiatively. Further, the plasmons provide additional optical states (Purcell effect) for the excitons to de-cay into, allowing for increased emission. The plasmons used in this project will be gold nanoparticles.Thus, the motivation of the project is to further the understanding of the processes leading up to the thefabrication of plasmons on 1L MoS2 hybrid structures, as well as to investigate how the optical propertiesof 1L MoS2 are affected.

3 Experimental

This section highlights some of the experimental techniques used in the fabrication and characterisationof the samples.

3.1 Characterisation

3.1.1 Microscope

Microscope images were captured using the Olympus BX51 microscope with an attached Olympus XC10Color Camera. Thin films of MoS2 flakes appear as faint contrast against the background under brightfield microscopy mode [26, 27] while the gold nanoparticles appear as bright spots in both bright anddark field microscopy mode due to the strong scattering of LSPs.

3.1.2 Photoluminescence measurement

Photoluminescence (PL) describes a light emission process following the absorption of electromagneticradiation. In a PL measurement, electromagnetic radiation with energy greater than the band gap of thesample (hω > Eg) is incident on the sample, which excites some of the electrons from the valance to theconduction band. These electrons may lose energy though vibrational relaxation or other non-radiativemeans, and become trapped at the bottom of the valance band or slightly below the valance band (ex-citonic states). These electrons then de-excite radiatively back to the conduction band, and the photoncount is recorded as a function of energy.

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Figure 5: Schematic of the light path for photoluminescence and Raman spectroscopy measurement. The greenarrow traces out the path to the sample, while the red arrow traces the path to the spectrometer. (a) Forone-shot Raman/PL. (b) For Raman/PL mapping.

PL measurements were performed using the NTEGRA Spectra (NT-MDT) spectrometer. A 100x ob-jective lens with N.A. 0.9 was used in measurements. All PL measurements were calibrated to the MoS2

Raman peak. Figure 5 shows the light path for the laser and collected light to and from the microscopeassembly.

3.1.3 Raman Spectroscopy

Raman spectroscopy is an invaluable method used to study the rotational and vibrational modes in mo-lecules. Since rotational modes carry angular momentum and the vibrational modes behave as quantumoscillators, the energy levels of these modes are quantised. As symmetry, chemical nature of bonds andmasses of atoms involved differ for different material, the energy associated with these modes in a ma-terial can be used as a fingerprint to identify the species of the material. However, for most crystals,rotational modes are not possible as it would involve the rotation of the whole crystal and thus only vi-brational modes are probed. In crystals, these vibrational modes are called lattice vibrations or describedas phonons.

In Raman Spectroscopy, a monochromatic laser is incident on the sample, but the majority of photonswill be elastically scattered (Rayleigh scattering) and would have the same energy as the incident laserlight. As these elastically scattered light is highly intense and not useful for the determination of vibra-tional modes, it is usually filtered away using a notch or edge filter to prevent damage to the detector.

However, there exists a small probability that the incident photon from the laser will be inelasticallyscattered (Raman scattering) and can either gain the energy of a vibrational mode, destroying a phononin the process (anti-Stokes process) or lose energy associated with a vibrational mode, creating a phononin the process (Stokes process). As these vibrational modes are mostly material specific, by measuringthe energy difference between the incident and inelastically scattered photons, information about the en-

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Figure 6: Schematic of the light path for dierential reectance measurement. The green arrow traces out thepath to the sample, while the red arrow traces the path to the spectrometer.

ergy needed to excite the vibrational mode in the material is obtained. This energy loss is usually reportedas Raman shift and the units are in wavenumbers (cm-1). Since Raman transitions can have very sharplines, it enables a detailed analysis of the molecule under study. Thus, by collecting the spectrum ofRaman shifts, the identity of a material can be established. As the quantised modes are not dependent onthe excitation light source, Raman shifts should be independent of laser source used. In addition, not allvibration modes are Raman active, that is not all vibration modes show up in a Raman spectra. This isbecause for a vibrational mode to be active, the vibrational mode must oscillate in such a way that thereis a change in polarisation of molecule. The number of Raman modes can be quickly determined byidentifying the point group symmetry of the material and looking up the character tables for the material.

Raman measurements were performed using the NTEGRA Spectra (NT-MDT) spectrometer. This set-upis also used for PL measurements. A 100x objective lens with N.A. 0.9 was used in measurements. AllRaman measurements were calibrated to the silicon peak located at 520 cm−1. Figure 5 shows the lightpath for the laser and collected light to and from the microscope assembly.

3.1.4 Differential Reflectance

Differential reflectance measures the difference in light reflected off a surface with and without a layerof thin film. It can be shown that the differential reflectance of thin films is directly proportional to theabsorbance of the sample [28]:

∆RRsubstrate

=R f ilm−Rsubstrate

Rsubstrate=

4n f ilm

1−n2substrate

α f ilmd f ilm ∝ α f ilmd f ilm

where n f ilm is the refractive index of the film, nsubstrate is the refractive index of the substrate, R f ilm isthe reflectance of the film on substrate, Rsubstrate is the reflectance of the substrate, α f ilm is the absorptioncoefficient of the film, and d f ilm is the thickness of the film. Differential reflectance can be used to probethe excitonic absorption of thin film MoS2 [29] as well as the plasmon frequency of gold nanoparticlesdeposited on substrates [13].

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Differential reflectance is performed using the same spectrometer used in Raman spectroscopy as well asin PL measurement. However, the excitation light is from a tungsten halogen light source instead of laseras well as a additional pinhole to restrict the area of light that is excited (Figure 6).

3.1.5 Atomic Force Microscopy

Atomic Force Microscopy allows for the study of the morphology of a material surface. In AFM, thelocalised force between the sharp probe on a cantilever and the surface is monitored and regulated togenerate the morphology of the surface. In AFM, a laser beam is reflected off the back of the canti-lever into a quadruple photodiode array and the relative intensities between the photodiodes allow for thecharacterisation of forces acting on the probe at a particular location. A feedback electronic loop andpiezoelectric scanner is used to maintain a constant tip-sample interaction during imaging. The AFM canoperate in contact mode, where the probe is directly touching the surface of the sample, or tapping mode,where the tip is made to oscillate away from the surface of the sample.

For tapping mode AFM, the cantilever beam is activated by the piezoelectric driver to oscillate at itsnatural frequency. When tip is close to the sample, the tip-sample interaction damps the oscillation andshifts the resonant frequency, which is detected by the optical-electronic system. The advantage of tap-ping mode is that it reduces the possibility of tip/sample damage, and enhanced the applicability of AFMto soft materials, such as bio-polymers.

AFM measurements was performed using the Bruker Dimension FastScan AFM. In this project, AFMis used to identify the height of MoS2 flakes as well as to determine the coverage height of gold nano-particles on a sample.

3.1.6 Scanning Electron Microscope

The scanning electron microscopy uses electrons to probe the sample. It consists of an electron gun,which produces electrons which are accelerated using high voltages and are focused by the Lorentz forcethrough the use of magnetic lenses before impinging on the sample. The accelerating voltages are of afew thousand volts, which causes the de Broglie wavelength associated with the electron to be smallerthan that of visible light, allowing for better resolving power. Upon reaching the sample, the high energyelectrons can interact with the sample and generate many useful information, however for our purpose,we will only continue the discussion on one such interaction: Secondary Electrons (SE).

During the collision of the electrons with the sample, these high energy electrons can knock electrons outof their usual orbits, and these electrons that are knocked out are known as SE [30]. A single incidentelectron will typically generate a shower of thousands of SE as it passes deeper into the sample. Since SE

24

have low energy, the detected SE are from a region close to the sample surface as SE generated deeperin the sample are unable to travel to the surface and leave the sample [30]. Thus, SE are used to producetopographical information of the sample.

The SEM used for measurements was the FEI Verios 460 FESEM. The accelerating voltage used is 2keV with a current of 50 pA. SEM is used primarily for the imaging of the gold nanoparticles that aredeposited.

3.2 Synthesis

This section covers the growth methods for both CVD grown MoS2 as well as for the gold nanoparticles.

3.2.1 Chemical Vapor Deposition

MoS2 on SiO2/Si substrates were grown using the Chemical Vapor Deposition (CVD) method. Thismethod is based on vaporising the precursors and allowing them to react on the substrate. Silicon sub-strates with 90 nm of SiO2 film were used as growth substrates. These substrates were first cleaned withacetone and isopropyl alcohol and then rinsed with deionised water. These substrates were then placedupside down on an alumina crucible containing 15 mg of MoO3 (99.99 % purity, Alfa Aesar). Anothercrucible containing 120 mg of Sulphur (99.5 % purity, Sigma Aldrich) was prepared and placed at theupstream position about 1 cm away from the outer edge of the furnace. The MoO3 powder and siliconsubstrate containing crucible is placed about 17 cm downstream from the sulphur containing crucible.The CVD growth was performed at atmospheric pressure in a 1-inch horizontal tube furnace (Carbolite).The tube is initially pumped down to a base pressure of 2×10−4 mbar and purged with argon gas for 10mins to remove oxygen contamination. The growth conditions are as follows: (1) Temperature ramp at30 °C/min to 650 °C with 50 sccm of Ar; (2) Maintain the temperature at 650 °C and dwell 5 min; (3)Open furnace for rapid cooling.

3.2.2 Citrate Reduction

The gold nanoparticles (Au NPs) were synthesised using the so called Turkevich-Frens method or “clas-sical citrate reduction method” [31, 32]. This method is based on the reduction of Au3+ precursor withsodium citrate in a boiling solution of sodium citrate. The citrate ions initially acts as a reducing agent toconvert Au3+ into Au and subsequently as a capping agent to stabilise the Au NPs. To create the Au NPsof the desired size, the method described by Bastus et al. [33] was used. Briefly, the reaction is separatedinto two steps. The first step is to grow Au NP seeds, which are ≈10 nm in size, and subsequently moreHAuCl4 are added to successively grow spherical particles with larger sizes. The method are as follows:(1) 1 ml of 25 mM aqueous HAuCl4 was injected to a boiling solution of 150 ml of 2.2 mM of sodiumcitrate. The reaction was allowed to run until it reached a red-wine colour; (2) Decrease the temperature

25

of the solution from 100°C to 90°C to inhibit the new nucleations of Au NPs; (3) Add of 1ml of 25 mMaqueous HAuCl4 and allowing the reaction run for 30 min. (4) Repeat the addition of 1ml of 25 mMaqueous HAuCl4 and allowing 30 mins the reaction to complete was repeated until the desired Au NPsize is reached.

4 Results and Discussion

4.1 MoS2

4.1.1 Preparation

Initially for this project, MoS2 (SPI supplies) was exfoliated with blue Nittol tape and transferred ontoa 10×5×0.5 mm epi polished quartz substrate (Latech) and a great deal of time was spent on locatingmonolayers from exfoliated MoS2 on quartz substrate using optical microscopy. The monolayers thatwere identified were often less than 5 µm2 in size, making them hard to work with due to the natureof the project (see Section 4.3.1). All exfoliated MoS2 referred to in this report is deposited on quartzsubstrate.

Towards the end of the project deadline, CVD grown MoS2 on 90 nm SiO2/Si substrates were usedinstead.

4.1.2 Characterisation of MoS2

Few layer MoS2 were first identified using optical microscopy. Through the optical contrast, we canidentify different thin layers of MoS2. Work by [26, 27] inspired a side project which involved usingstatistical techniques to identify few layered MoS2 (see Appendix A.2).

As layered MoS2 are thinned from few layers to single layer (1L MoS2), the material transitions frombeing a indirect bandgap material to a direct bandgap material. This indirect-to-direct bangap transitionresults in a significant enhancement in PL quantum yield, a feature which can be utilised to distinguishbetween monolayer and multilayer MoS2 counterparts [2, 3]. Further, in addition to the significant dif-ference in the PL QY, a broad feature (peak I), appearing below emission energy associated with theA exciton peak, appears in the PL spectrum for few-layered MoS2 and systematically red shifts as thenumber of layers increase [3, 34]. The PL peak I was attributed to indirect-gap luminescence [3].

Crystalline 2H-MoS2 belong to the D6h point group, whose character table predict a total of 4 Ramanactive modes: one A1g, one E1g, and two E2g modes. For bulk MoS2, the modes are assigned as: E1g (286cm−1), E1

2g (383 cm−1), A1g (408 cm−1), E22g (32 cm−1) [35, 36]. In particular, the progressive decrease

in the Raman shift (cm−1) of the A1g mode and the progressive increase in Raman shift of the E12g mode

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Figure 7: (a) PL spectrum of exfoliated 1L to bulk MoS2. Inset: Microscope image of exfoliated MoS2 akes.(b) Raman Intensity of exfoliated 1L to few layer MoS2. Raman spectra are displaced for clarity.

as the crystal is thinned from six layers down to the monolayer was observed [35, 37], and a systematicstudy concluded that the monotonous variation below 6 layers of these two modes can function as a toolfor identifying ultrathin films of MoS2 [35].

Exfoliated MoS2 Figure 7a shows the PL spectrum for different layers of exfoliated MoS2. Apartfrom the decrease in the photoluminescence feature of the A exciton peak centred at ≈660nm, this peakalso red shifts as the layers increases as well. Meanwhile, the B exciton peak centered at ≈ 610nm onlydecreases in intensity as the layers increases. Further, an indirect peak I red shifts as the number of layersincreases. All these trends are in agreement with [3, 34, 2]. Figure 7b shows the Raman spectrum fordifferent layers of exfoliated MoS2. The difference in Raman shift between the two modes in 1L MoS2

is 20.1 cm−1, in agreement with [35].

CVD grown MoS2 Figure 8 shows the AFM image, step height, PL spectrum and Raman spectrum fora CVD on 90nm SiO2/Si substrate. The step height was found to be ≈0.6 nm which corresponds to asingle layer of MoS2. The Raman peaks for the CVD sample are in agreement with [38].

4.2 Gold Nanoparticles

Gold nanoparticles (Au NPs) were prepared using the citrate reduction method as described in Section3.2.2.

4.2.1 Characterisation

The prepared solution appears red (Figure 9b, inset) and using UV-vis spectroscopy (Shimadzu UV-2600), a strong absorbance peak was located at 532 nm (Figure 9a). A narrow peak at 532 nm suggests

27

Figure 8: (a) AFM image of the step edge of a CVD grown 1L MoS2 ake, (b) Step height of the edge in (a),inset shows the bright eld micrograph of a CVD grown 1L MoS2, (c) PL spectrum of a CVD grown 1L MoS2,(d) Raman spectrum of a CVD grown MoS2.

Figure 9: (a) UV-vis spectrum of the gold nanoparticles, inset shows the dark eld micrograph of the goldnanoparticles on quartz substrate. (b) Size distribution of 196 gold nanoparticles imaged using SEM, insetshows the gold nanoparticle colloidal solution.

28

Figure 10: Dierent agglomeration structures of gold nanoparticles imaged using SEM. (a) Isolated, (b) dimers,(c-d) larger circular clusters, (e) lines, (f) highly packed single layer, (g) fractals.

that the size distribution of the Au NPs are rather uniform in size. Using a formula developed by Haisset al. that relates the observed absorbance peak of the Au NPs with the diameter of the Au NPs [39], thediameter of the Au NPs was estimated to be ≈50 nm.

To determine the size distribution of the Au NPs, 1 µm of the Au NP colloidal solution was extrac-ted using a micro pipette and drop cast onto a transparent quartz substrate and left to dry in in laboratoryambient. The particles were then subsequently imaged using the scanning electron microscope (SEM)and the particles sizes were tracked using imageJ [40]. The Au NPs were determined to be 54±8 nm indiameter, in good agreement with the estimate obtained from using the expression developed by Haiss etal [39]. Figure 9b shows the size distribution of the 196 Au NPs that was used to obtain the diameter.

From dark field micrographs of the Au NPs drop cast on a quartz substrate, it was observed that the goldparticles tend to agglomerate to form structures. Earlier, from the narrow peak of the UV-vis spectrum(Figure 9) of the Au NP solution, we concluded that the size distribution of the Au NPs is rather uniform.Thus, the agglomerations events occur after the Au NPs are drop cast onto the substrate. Figure 10 showsthe SEM images of some of the agglomerations that form on the substrate. From the SEM images, weattribute the larger bright spots in the dark field micrograph (Figure 9a inset) to Au NPs agglomeratingto form a larger circular cluster. Au NPs may form chains as well as fractal like patterns, or form orderlypacked assemblies (Figure 10).

29

Figure 11: (left) Dierent gold nanoparticle dispersion on the substrate and their (right) dierential spectrum.

To investigate the effect of agglomeration on the absorption spectra, quartz substrates with differentnumber of solution drops were prepared. Subsequent drops were only added after the previous drop hasdried. To measure the plasmon frequency of Au NPs on quartz substrate, differential reflectance can beemployed. However, as the measurement of a small area of the sample restricted by the pinhole proveddifficult to obtain a meaningful spectra for Au NPs clusters, the pinhole (Figure 6) was removed to allowto increase the number of Au NPs being analysed by increasing the area of incident light and reflectedlight collected. However, the now larger area of reflected light saturates the CCD detector, even at thesmallest allowed integration time of 0.1s. To solve this another beam splitter was placed before the firstbeam splitter to reduce the light intensity. In addition, from the SEM images, we know that the Au NPshas agglomerated on the substrate, and therefore we expect higher order modes to appear. Further, thedistribution of the nanoparticles was not uniform. Therefore, Figure 11 only shows the differential re-flectance of various uniform large area differential reflectance spectra. More spectrum are included in theAppendix A.3, especially for other agglomerations, but they were not easy to interpret.

From the spectra, a few important can still be extracted. A red shift of the LSPR peak originally centredabout 532 nm in solution can be observed when the Au NPs are deposited on the quartz substrate. Thisred shift is in agreement with previous studies and is attributed to be caused by the increase of effectivereflective index [9]; on the quartz substrate, the effective medium in which the Au NPs reside can beconsidered to be a combination of air and quartz. The trend of plasmon resonance red shifting when therefractive index increases can be seen from Equation 13. The peak that appears at about 750 nm is attrib-uted to dipolar longitudinal modes [41]. We note that this peak increases in peak height as the number

30

Figure 12: (a) Coalescence of gold nanoparticles, where the particles merge together to form a commonboundary. (b) Onset of a longitudial dipolar mode as agglomeration increases. Image from [41].

density of larger Au NPs appear (Figure 11). The trend of the absorption spectra here agrees well withan earlier study done by Jiang et al. with 30 nm Au NPs in solution, with a 528 nm absorption peak, andan onset of another peak at about 700 nm as agglomeration increases (Figure 12b) [41].

4.3 Gold Nanoparticles on Exfoliated MoS2

After characterising the individual components, we shift our attention to depositing Au NPs on 1L MoS2

flakes. The Au NP containing solution was initially drop cast on mechanically exfoliated MoS2 samples.

4.3.1 Issues with Drop Cast with thin MoS2 flakes

As mentioned, the Au NPs were deposited on a substrate with MoS2 exfoliated flakes using the dropcast technique as described earlier. It was observed that this simple drop cast technique could potentiallycrush monolayer MoS2 flakes (Figure 13a,b). Moreover, it may displace thin flakes as well (Figure 13c).To this end a few approaches were taken in an attempt to prevent the destruction of the thin flakes.

We can understand the process where the flake was crushed with the aid of Figure 14b. Initially, theadvancing surface of the solvent drop encounters the single MoS2 layer. Due to stronger interaction sur-face interaction between the water-quartz interface (contact angle ≈0°) as compared to the water-MoS2

interface (contact angle ≈ 83° [42]), the thin layer is easily lifted at the front of the droplet and as thedroplet wave front advances, more sections of thin MoS2 lifted and eventually pile near another obstacle(Figure 14b).

It was also observed that if the drop was done with bulk layers facing the direction where the drop was

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Figure 13: MoS2 nanosheets crushed by capillary forces. Exfoliated MoS2 (a) before, (b) after drop cast. (c)Movement of exfoliated MoS2 after drop cast. CVD grown MoS2 (d) before, (e) after drop cast.

Figure 14: (a) Proposed mechanism for which the MoS2 layer can be protected and, (b) mechanism for thecrushing of MoS2 layer.

32

done, the single layers would survive the drop cast. This can be visualised as Figure (14a). The incomingwave front of the droplet meets the a bulk layer obstacle but does not lift the layers, and subsequentlythe solvent rolls over the bulk layer obstacle. Therefore, if the thin flake resides adjacent to a bulk MoS2

flake, by carefully selecting where the drop is placed the thin flake can be protected against crushing bythe drop cast process. In addition, by stamping a thin BN layer on top of a MoS2 thin flake, the BN flakeis able to stay in its position after the drop cast, protecting the MoS2 thin flake in the process. However,the thin BN layers that were used for testing are generally rather thick, and due to the difficulty in obtain-ing atomically thin sections of BN through exfoliation to increase plasmon-exciton distance, this idea isnot as useful. Third, a corral of BN layer around the single layer can help prevent the monolayer MoS2

flake from collapsing (see Appendix A.6). The corral was built using the dry transfer method as per [43].

With low success and yield, a decision was eventually made to switch over from exfoliated MoS2 flakesto CVD grown MoS2 flakes.

4.4 Gold Nanoparticles on CVD grown MoS2

The use of CVD samples supplied much larger area coverage of MoS2 thin film as compared to thatobtained by mechanical exfoliation. Deploying drop cast technique again, Au NPs were deposited on thesample. As seen from Figure 13e, plenty of nanoflakes ended up being crushed. However, there weresome Au NPs deposited on MoS2.

4.4.1 Optical Measurements

Next, PL mapping was employed to identify enhancement hotspots, which will allow us to visually loc-ate areas with higher PL intensities on a heatmap. However, form Figure 15b, we can see that the edgesare blurred, and more complicated geometries are not captured by the PL map. This is likely due to thelarger size of the laser spot for the mapping set up as compared to the confocal system, causing the PLobserved to be a convolution of the profile of the laser spot and the sample (see Appendix A.4). Smallenhancements like those observed by Gao et al. for single silver nanoparticle [24] would be spread overa larger area and become indistinguishable from the background signal. Further, it would not be possibleto identify the particular formations that produces particular hot spot other than concluding that thereis a hotspot in the vicinity. Nevertheless, rather uniform enhancements were observed indeed observedfor larger agglomerations, in particular for a uniform layer of Au NP coverage was identified to havedisplayed strong enhancements to the PL spectrum (Figure 15c).

From the PL map, we can also determine the peak position. We see that from Figure 15d that thereis a blue shift of the peak position of the MoS2 A exciton peak corresponding to areas where there is acoverage of Au NPs. In particular, the observed A exciton peak at 667 nm (1.86eV) is blue shifted to 657nm (1.89eV). This shift cannot be explained from the natural variability of peak position of CVD MoS2

33

Figure 15: Gold nanoparticles on CVD 1L MoS2. (a) SEM image, (b) AFM image, (c) PL peak map, (d) PLpeak position. The PL maps are curve tted using a Lorentzian prole.

34

Figure 16: (a) Comparison of PL spectra between Au NPs on MoS2that have been imaged using the SEM andthose that have not been imaged. (b) Dierential reectance spectra of SEM imaged MoS2 and normal MoS2.The graphs are displaced for clarity. (c) Bright eld micrograph of an SEM imaged spot. (d) Fluorescenceimage of a SEM imaged spot. (e) SEM image of the Au NP on MoS2 ake.

(see Appendix A.7) nor from the increase in electric field that could be incident MoS2 on due to fieldenhancement (see Appendix A.5). Blue shifts and enhancement of the A exciton peak have been reportedby Li et al. when an ionic liquid (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) wasdeposited on 1L MoS2 flakes [44]. In their work, the shifts (1.85eV in air to 1.88eV in ionic liquid) andare ascribed to the passivation of surface states on the MoS2 flake [44]. In the synthesis of the Au NPs,chlorine ions are produced and chlorine is known to passivate sulphur vacancies in MoS2 flakes as well[45]. Therefore, we ascribe the blue shifts to be due to the passivation of defect states by the chlorine leftover from the synthesis of the Au NPs.

To characterise these spots, AFM and SEM were used. The use of SEM was found to be particularlyuseful in providing information about whether a MoS2 film resided underneath a coverage of Au NP byproviding a sharp contrast between areas covered by MoS2 flakes and just the bare substrate (Figure 16e).This allows us to determine if PL enhancement is expected to occur at a particular spot. However, theissue is that the SEM itself might damage the sample.

35

4.4.2 Anomalous PL Spectrum After SEM imaging

After characterising the height profiles and surface coverage of the sample with AFM and SEM scans,more in depth optical studies were planned. However, it was observed that the PL spectra of these spotswere drastically modified (Figure 16a). In particular, we observe a huge broad peak centred around 750nm and spanning into beyond the 900 nm region where only MoS2 peaks were expected. These modific-ations of PL emissions were observed for both MoS2 and Au NPs on MoS2, thereby suggesting that theMoS2 itself was damaged during SEM.

Exciting the sample with green emission from a filtered Mercury lamp light, a fluorescence image wasobtained from a microscope. From the fluorescence micrograph image (Figure 16d), we can observe afaint rectangular area around one of the MoS2 flakes which does not correspond to any other MoS2 flake.Comparing this with the images obtained from the SEM of the same sample (Figure 16e), we see that thisrectangular area corresponds with the edges of the SEM image. Further, the MoS2 flake that was imagedwith the SEM appeared darker than its neighbors. This further suggests that by impinging the samplewith 2 keV electrons during the immersion mode FESEM scan, defects were introduced to the MoS2

flake and modified the emission of MoS2. However, the PL modification is not consistent throughout allsamples. In some cases, like those shown in Figure 16a, the samples display both the A exciton peakas well as a broad ≈700 nm peak, while other samples have spectrum that show only a broad ≈700 nmpeak, and suggesting the role electron impingement dose plays on emission modification of MoS2 whichmight be useful for further tuning of emission properties of MoS2 nanosheets for other purposes.

To further investigate this modification, differential reflectance was used to locate the excitonic statesof MoS2. The differential reflectance profile did not yield any sharp excitonic features, as compared tothe normal 1L MoS2 (Figure 16b). Raman spectroscopy was also employed, and shows that the MoS2

Raman signatures remained relatively unaltered (see Appendix A.8). The reason for this is unclear andpermits for further investigation.

4.4.3 Dynamics of Particle Deposition

From the SEM and AFM images, we can infer the sequence of events that occurred before the charac-terisation techniques mentioned were performed. From the SEM and AFM images, we observe creaseson the MoS2 flake that originate from dots on the MoS2 flake. From their height profile (Figure 17), itis likely that these are Au NPs and not from defects (see Appendix A.10). This creases are thought tohave originated from a process similar to when a piece of paper is placed over bottle cap. When the paperis stretched flat or pressed down around side of the obstacle, creases like the ones observed will appear.Therefore we attribute that these dots are Au NPs that were deposited under the MoS2 flake.

But how would these particles end up there? To do so we consider an earlier observation that the flakes

36

Figure 17: (a) AFM obtained height prole of possible Au NPs that may be under a MoS2 ake. (b) AFMshowing creases that form around these suspected Au NPs, which are ascribed to be particles residing underneaththe MoS2 ake.

are able to move when the solvent was drop cast on the sample (Figure 13c). These events mean thatthe MoS2 flake is lifted and may be shifted before landing back on the substrate. During this time, AuNPs could have landed on the SiO2 surface. Subsequently, when the MoS2 flakes land on the substratesurface, these Au NPs act as disruptions and create creases in the MoS2 flake as it tries to flatten out tomaximize SiO2-MoS2 surface interaction. As there are more Au NPs deposited above the MoS2 flakethan below, we can speculate that most of the Au NPs are deposited sequentially after the MoS2 flakelands on the substrate after the initial lift-off.

Next, from the SEM images (Figure 16e,18b), we can observe the sharp contrast between areas that arecovered by the MoS2 flake and areas that do not. From these images we can easily distinguish areas whichhave high density Au NP coverage against areas that only have Au NP coverage as well. We observe thatthe MoS2 flakes generally have cracks in them, presumably due to the drop cast. Focusing on areas withhigh Au NP coverage above MoS2 flakes, we observe that the cracks in the Au NP layer corresponds tothe cracks along the MoS2 flakes as well. We observe this trend in the AFM images as well (Figure 18b).This correlation can be attributed to the tearing of MoS2 flakes occurring sequentially after that of thehigh density Au NP coverage deposition, presumably due to the process of solvent evaporation due tostress induced by the change of environment.

Putting it altogether, the sequence of events can be described as follows. Right after the Au NP con-taining solution is dropped cast on the substrate, some MoS2 flakes are lifted. During this time, Au NPsstart to deposit onto the substrate. When a MoS2 flakes lands on the substrate with Au NPs deposited,creases form in the MoS2. Sequentially, the majority of Au NPs that land on the substrate arrive after theMoS2 flake lands on the substrate. As the solvent dries, more and more Au NPs are forced on land oneither the MoS2 or SiO2 surface. Finally, when the solvent dries, the MoS2 flake is stressed again, and

37

Figure 18: (a) SEM, (b) AFM images that show the cracks that form apply to both MoS2 and Au NPs,suggesting that these cracks appear after the Au NPs have been deposited on the MoS2 sheets.

cracks form.

By understanding the events that occurred during the deposition of Au NPs, we can further improveour experimental design. Namely, if we wish to have better quality MoS2 flakes on the surface, weshould consider processes that improve MoS2 flake adhesion on the surface of the substrate, such as heat-ing the substrate or using a less polar solvent. If we wish to have better Au NP coverage on MoS2, wecan consider slower processes that allow for Au NPs to assemble before landing on the 1L MoS2. Onepotential candidate for the slow process would be the use of dodecanethiol ligated Au NPs, which allowsfor the Au NPs to assemble as a single layer on the surface of the drop cast water droplet, which wouldthen uniformly cover the substrate when the solvent dries [46].

The lifting of MoS2 suggests that we could potentially have a fast process which allow for Au NPsto quickly assemble on the substrate before the MoS2 flake lands. In particular, a solution of Au NPscontaining both a combination of fast and slow processes would allow for the fabrication of a MoS2

layer sandwiched by 2 plasmonic layers in a single step. This sandwich structure has one particular ad-vantage, namely the field enhancement “hot spots” pass directly through the MoS2 sheet, and thus theenhancement is expected to be greater.

4.4.4 PL and Raman Enhancement

Identifying new sample spots, as shown in Figure 19, Raman enhancement was explored. In a bid toaccount for variability that might occur within each spot, three measurements from Spot A, Spot B andthe bare MoS2 patch between Spot A and B were taken and averaged. Their resultant spectrum is plottedin Figure 19d. Figure 19d shows the Raman spectrum that was obtained on a 1L coverage of Au NPs and

38

Figure 19: (a) Bright eld, (b) Fluorescence, (c) AFM images of a 1L Au NP coverage on a 1L MoS2 ake.(d) Raman spectra of the MoS2 on the 1L Au NPs as labeled in (a) and the Raman spectrum for a 1L MoS2that is not covered by Au NPs using 532 nm laser.

39

a regular 1L MoS2 flake. We see that the silicon peak at 520 cm−1 was weakened and there was a generalincrease in the background signal, presumably due to the Purcell effect. Using a Lorentzian functionto fit the MoS2 Raman peaks [47] after substracting the background, the enhancement was found to be≈0.6 for the E1

2g mode and ≈ 1.3 for the A1g mode. The decrease in the intensity of the E12g Raman mode

could be due to the increased scattering provided by the Au NPs, which is not collected by the set-up used.

Next, PL enhancement was explored. In a bid to account for variability that might occur within eachspot, three measurements from Spot A, Spot B and the bare MoS2 patch between Spot A and B weretaken and averaged. Their resultant spectrum is plotted in Figure 20. Since the emission peak of MoS2

is centered away from the plasmonic modes of the Au NPs, we do not expect the Purcell effect to havemuch effect on the excitonic emission of MoS2. The emission enhancement through Purcell effect fromthe Au NPs can be observed at ≈550 nm and ≈750 nm (Figure 20) and the Purcell factor is expected tobe the same throughout all different excitation wavelengths as the sample used is the same.

At 473nm (Figure 20e,f), the excitation wavelength has small overlap with the LSPR peak centred at≈550 nm, and weakly satisfies the LSPR condition, and a enhancement of electric field occurs, albeitweaker than the case using 532 nm laser. This enhancement of electric field increases the absorption ofphotons as the transition rate between ground state to excited state increases. This process increases thepopulation of excited electrons and thus translates to an increase in photoluminescence as observed asthese electrons de-excite.

At 532 nm (Figure 20c,d), the excitation wavelength overlaps with the LSPR peak centred at ≈550nm.The local electric field is enhanced and thus an increased absorption of photons occurs as the transitionrate from the lower to upper state increases due to the increased electric field. The now larger populationof excited electrons eventually de-excites and an thus there is an increase in PL intensity. The weakoverlap between the exciton emission peaks and and longitudinal dipolar peak of Au NPs at ≈750 nmsuggests that the enhancement would be mainly due to the enhancement fo electric field.

At 633 nm (Figure 20a,b), the excitation wavelength does not overlap with the transversal dipole peak(≈550nm) or the longitudinal (≈750 nm) peak of the Au NP. As a result, the field enhancement is weak(relative to other excitation wavelengths). However, the excitation wavelength is near the resonant en-ergy of the MoS2 excitonic state, and thus even though the field enhancement is expected to be weak, thephotons are expected to be readily absorbed. This then translates to a higher emission observed.

Table 1 summarises the peak intensity ratios. The first two columns reports the highest intensity ratio asreported in Figure 20. Note that the values for the 633 nm laser excitation have been fitted with a Lorent-zian curve and then have the ratios of the resultant curves reported. This is to remove the presence of theRaman peak and obtain a better estimate of the enhancement. Furthermore, since there is an observed

40

Figure 20: PL spectra and intensity ratio of the samples labelled in Figure 19a. The excitation wavelengthused is (a-b) 633 nm, (c-d) 532 nm, (e-f) 473 nm. From the intensity ratio plot (grey), the enhancement ofMoS2 PL peak is between two other broad peak features, which corresponds to the plasmon frequencies of theagglomerated Au NPs observed from the dierential reectance spectra (Figure 11), and thus attributed as thecontribution of radiative decay channels from the Au NP LSP through the Purcell eect.

41

Highest Intensity Ratio Fitted Peak Intensity RatioSpot A Spot B Spot A Spot B

633 nm 7.8* 7.4* 7.0 7.6532 nm 11.4 10.6 7.4 7.5473 nm 9.5 6.4 5.0 4.2

Table 1: Enhancement ratio of 1L Au NPs on MoS2 for dierent excitation wavelengths. Highest intensityratio is read o from the graph in Figure 20. The tted peak intensity ratio is obtained by comparing the ratioof peak height obtained from a Lorentzian curve t. *For 633 nm, the results are obtained from tting thespectrum with a Lorentzian prole to remove the Raman peak.

blue shift of the PL peak, to compensate for the peak shift, the last two columns are included, which onlyconsiders the ratios of the peak height of the Lorentzian fitted PL curves. More data about the peak fitscan be found in Appendix A.9.

From the discussion that followed, we conclude that 532 nm will have the highest enhancement, dueto the effective enhancement of electric field due to the near resonant excitation of the plasmonic modes,followed by the 633 nm laser as it is quite close to the excitonic absorption of MoS2, and then finally the473 nm laser, which is expected to have the least enhancement.

Raman enhancement is expected to scale as∣∣∣Eloc

E0

∣∣∣4 (Equation 40) while PL enhancement should scale

as∣∣∣Eloc

E0

∣∣∣2 (Equation 37). From the experimental results, it the expected field enhancement is modest atbest. This could be due to a few reasons. First, we note that the field enhancement “hot spots” occurbetween particles [17, 18]. However, due to the geometry of spherical particles, the junction betweenany two gold particles will about 25 nm above the MoS2. Second, from SEM images it suggests that theparticles might have coalesced, that is they have merged to form become a single particle (Figure 12a).This process would lead to the disappearance of the “hot spots” of field enhancement [41]. Nevertheless,a 1L Au NP coverage shows appreciable enhancement of radiative properties, as can be seen from afluorescence image taken using green light from a mercury burner lamp (Figure 19b).

4.4.5 Differential Reflectance

This project initially began to investigate the presence of strong coupling between plasmons in Au NPsand excitons in MoS2. However, as observed from the differential reflectance measurements (Figure 11),the synthesized Au NPs have its plasmonic peaks (≈550nm and ≈750nm) with little overlap with theexciton emission peaks, the energy transfer is one way and thus no strong coupling [48] is expected. Ifstrong coupling between excitons and plasmons, we expect a shift in the excitonic emission energies [23].Figure 21a shows the differential reflectance the samples on 90nm SiO2/Si substrate. Note that for MoS2

on SiO2/Si substrates, the MoS2 absorption appears as dips in the differential reflectance spectra. Here,we observe that the Au NP on 1L MoS2 differential reflectance profile is approximately a combination

42

Figure 21: Dierential Reectance spectrum for (a) 1L Au NP on CVD MoS2, (b) High density Au NP onexfoliated MoS2. Inset: Micrograph of Au NP on MoS2 on quartz substrate. The MoS2 layer was protectedfrom crushing by the solvent by building a corral of BN (see Appendix A.6).

of the Au NP and 1L MoS2 and no shifts in the Au MoS2 dips are observed, suggesting that there isno strong coupling interaction. Further, we note that for as grown 1L MoS2 structure, the A exciton dipappears ≈659nm, in good agreement with PL peak centred ≈657 nm for the case of Au NP on MoS2,further suggesting that passivation of defects might have occurred.

As it is difficult to observe clearly the plasmonic contributions on a SiO2/Si substrate due to interference,attempts were made to investigate to prepare similar samples on MoS2 on quartz substrate. However, thecoverage by the Au NPs were not as uniform. Figure (21b) shows the differential reflectance of an AuNP on MoS2 structure on quartz substrate. As with the hybrid structure on SiO2/Si substrate, no strongcoupling is observed.

5 Conclusion

Using 54±8 nm gold nanoparticles (Au NPs) prepared in a solution using citrate reduction and transfer-ring the Au NPs onto CVD grown 1L MoS2 substrates by drop cast, 1L MoS2 flakes with 1L coverage ofAu NPs were identified and the PL enhancement of 1L coverage of Au NPs on 1L MoS2 was investigatedat and off the plasmon resonance frequency. An enhancement factor of≈10 was observed for the PL peakintensity for the hybrid structure when it was excited with 532 nm laser, which is close to the plasmonresonance frequency. Furthermore, the mechanisms involved the deposition of Au NPs on MoS2 areinvestigated by deducing the events that occurred from characterisation performed after the solvent hasdried, which furthers our understanding on how to better optimise the processes for depositing solutiongrown plasmonic nanoparticles onto 2D materials.

43

References

[1] K S Novoselov, A K Geim, S V Morozov, D Jiang, Y Zhang, S V Dubonos, I V Grigorieva,and A A Firsov. Electric field effect in atomically thin carbon films. Science (New York, N.Y.),306(5696):666–9, 2004.

[2] Kin Fai Mak, Changgu Lee, James Hone, Jie Shan, and Tony F. Heinz. Atomically thin MoS2: anew direct-gap semiconductor. Physical review letters, 105(13):136805, sep 2010.

[3] Andrea Splendiani, Liang Sun, Yuanbo Zhang, Tianshu Li, Jonghwan Kim, Chi Yung Chim, Gi-ulia Galli, and Feng Wang. Emerging photoluminescence in monolayer MoS2. Nano Letters,10(4):1271–1275, 2010.

[4] Goki Eda and Stefan A. Maier. Two-dimensional crystals: Managing light for optoelectronics. ACS

Nano, 7(7):5660–5665, jul 2013.

[5] Vincenzo Giannini, Antonio I. Fernandez-Dominguez, Susannah C. Heck, and Stefan A. Maier.Plasmonic Nanoantennas: Fundamentals and Their Use in Controlling the Radiative Properties ofNanoemitters. Chemical Reviews, 111(6):3888–3912, jun 2011.

[6] C Kittel. Introduction to Solid State Physics. John Wiley and Sons, Inc., New York, 8th edition,2004.

[7] Stefan A. Maier. Plasmonics: Fundamentals and applications. Springer, 2007.

[8] John David Jackson. Classical Electrodynamics Third Edition. John Wiley & Sons, Inc., 3rd edition,1999.

[9] Satoshi Kawata and Motoichi Ohtsu. Nano-Optics. Springer, Germany, 2002.

[10] Sujit Kumar Ghosh and Tarasankar Pal. Interparticle Coupling Effect on the Surface PlasmonResonance of Gold Nanoparticles: From Theory to Applications. Chemical Review, 107(11):4797–4862, 2007.

[11] Stefan Enoch and Nicolas Bonod. Plasmonics, volume 167 of Springer Series in Optical Sciences.Springer Berlin Heidelberg, Berlin, Heidelberg, 2012.

[12] Prashant K. Jain, Kyeong Seok Lee, Ivan H. El-Sayed, and Mostafa A. El-Sayed. Calculated absorp-tion and scattering properties of gold nanoparticles of different size, shape, and composition: Ap-plications in biological imaging and biomedicine. Journal of Physical Chemistry B, 110(14):7238–7248, 2006.

[13] Christopher J. Orendorff, Tapan K. Sau, and Catherine J. Murphy. Shape-dependent plasmon-resonant gold nanoparticles. Small, 2(5):636–639, 2006.

44

[14] E. Dulkeith, T. Niedereichholz, T. A. Klar, J. Feldmann, G. Von Plessen, D. I. Gittins, K. S. Mayya,and F. Caruso. Plasmon emission in photoexcited gold nanoparticles. Physical Review B - Con-

densed Matter and Materials Physics, 70(20), 2004.

[15] Alexander Urban. Optothermal manipulation of phospholipid membranes with gold nanoparticles.PhD thesis, lmu, 2010.

[16] C D Geddes and J R Lakowicz. Radiative Decay Engineering. Topics in Fluorescence Spectroscopy.Springer US, 2007.

[17] Jiang, Ken Bosnick, Mathieu Maillard, and Louis Brus. Single Molecule Raman Spectroscopy atthe Junctions of Large Ag Nanocrystals. The Journal of Physical Chemistry B, 107(37):9964–9972,sep 2003.

[18] Salvatore Campione, Sarah M Adams, Regina Ragan, and Filippo Capolino. Comparison of electricfield enhancements: linear and triangular oligomers versus hexagonal arrays of plasmonic nano-spheres. Optics Express, 21(7):7957–73, 2013.

[19] John H. Davis. The Physics of Low-Dimensional Semiconductors: An Introduction. CambridgeUniversity Press, 1998.

[20] Jon a Schuller, Sinan Karaveli, Theanne Schiros, Keliang He, Shyuan Yang, Ioannis Kymissis,Jie Shan, and Rashid Zia. Orientation of luminescent excitons in layered nanomaterials. Nature

nanotechnology, 8(4):271–6, 2013.

[21] Diana Y. Qiu, Felipe H. Da Jornada, and Steven G. Louie. Optical spectrum of MoS2: Many-bodyeffects and diversity of exciton states. Physical Review Letters, 111(21):1–5, 2013.

[22] Tawinan Cheiwchanchamnangij and Walter R L Lambrecht. Quasiparticle band structure calcula-tion of monolayer, bilayer, and bulk MoS2. Physical Review B - Condensed Matter and Materials

Physics, 85:205302, 2012.

[23] Weijie Zhao, Shunfeng Wang, Bo Liu, Ivan Verzhbitskiy, Shisheng Li, Francesco Giustiniano, Dai-chi Kozawa, Kian Ping Loh, Kazunari Matsuda, Koichi Okamoto, Rupert F Oulton, and Goki Eda.Exciton-Plasmon Coupling and Electromagnetically Induced Transparency in Monolayer Semicon-ductors Hybridized with Ag Nanoparticles. Advanced Materials, feb 2016.

[24] Wei Gao, Yih Hong Lee, Ruibin Jiang, Jianfang Wang, Tianxi Liu, and Xing Yi Ling. Localizedand Continuous Tuning of Monolayer MoS 2 Photoluminescence Using a Single Shape-ControlledAg Nanoantenna. Advanced Materials, 28(4):701–706, jan 2016.

[25] Katrin Kneipp, Martin Moskovits, and Harald Kneipp. Surface-enhanced raman scattering physics

and applications, volume 103. 2006.

45

[26] Y Y Wang, R X Gao, Z H Ni, H He, S P Guo, H P Yang, C X Cong, and T Yu. Thicknessidentification of two-dimensional materials by optical imaging. Nanotechnology, 23(49):495713,2012.

[27] Hai Li, Gang Lu, Zongyou Yin, Qiyuan He, Hong Li, Qing Zhang, and Hua Zhang. Optical identi-fication of single- and few-layer MoS 2 sheets. Small, 8(5):682–686, 2012.

[28] J.D.E. McIntyre and D.E. Aspnes. Differential reflection spectroscopy of very thin surface films.Surface Science, 24(2):417–434, feb 1971.

[29] Weijie Zhao, Zohreh Ghorannevis, Leiqiang Chu, Minglin Toh, Christian Kloc, Ping-Heng Tan, andGoki Eda. Evolution of Electronic Structure in Atomically Thin Sheets of WS 2 and WSe 2. ACS

Nano, 7(1):791–797, jan 2013.

[30] C W Shong, C H Sow, and A T S Wee. Science at the Nanoscale: An Introductory Textbook. PanStanford, 2010.

[31] John Turkevich, Peter Cooper Stevenson, and James Hillier. a Study of the Nucleation and GrowthProcesses I N the Synthesis of Colloidal Gold. Discuss. Faraday Soc., 55(c):55–75, 1951.

[32] G. Frens. Controlled Nucleation for the Regulation of the Particle Size in Monodisperse GoldSuspensions. Nature Physical Science, 241(105):20–22, 1973.

[33] Neus G. Bastus, Joan Comenge, and Victor Puntes. Kinetically Controlled Seeded Growth Synthesisof Citrate-Stabilized Gold Nanoparticles of up to 200 nm: Size Focusing versus Ostwald Ripening.Langmuir, 27(17):11098–11105, sep 2011.

[34] Krishna P Dhakal, Dinh Loc Duong, Jubok Lee, Honggi Nam, Minsu Kim, Min Kan, Young HeeLee, and Jeongyong Kim. Confocal absorption spectral imaging of MoS 2 : optical transitions de-pending on the atomic thickness of intrinsic and chemically doped MoS 2. Nanoscale, 6(21):13028–13035, sep 2014.

[35] Hong Li, Qing Zhang, Chin Chong Ray Yap, Beng Kang Tay, Teo Hang Tong Edwin, AurelienOlivier, and Dominique Baillargeat. From bulk to monolayer MoS 2: Evolution of Raman scatter-ing. Advanced Functional Materials, 22(7):1385–1390, 2012.

[36] Z M Wang. MoS2: Materials, Physics, and Devices. Lecture Notes in Nanoscale Science andTechnology. Springer International Publishing, 2013.

[37] A. Molina-Sánchez and L. Wirtz. Phonons in single-layer and few-layer MoS2 and WS2. Physical

Review B - Condensed Matter and Materials Physics, 84(15), 2011.

46

[38] Hennrik Schmidt, Shunfeng Wang, Leiqiang Chu, Minglin Toh, Rajeev Kumar, Weijie Zhao, A. H.Castro Neto, Jens Martin, Shaffique Adam, Barbaros Özyilmaz, and Goki Eda. Transport propertiesof monolayer MoS2 grown by chemical vapor deposition. Nano Letters, 14(4):1909–1913, 2014.

[39] Wolfgang Haiss, Nguyen T K Thanh, Jenny Aveyard, and David G. Fernig. Determination of sizeand concentration of gold nanoparticles from UV-Vis spectra. Analytical Chemistry, 79(11):4215–4221, 2007.

[40] Caroline a Schneider, Wayne S Rasband, and Kevin W Eliceiri. NIH Image to ImageJ: 25 years ofimage analysis. Nature Methods, 9(7):671–675, 2012.

[41] Wei Jiang, D Brynn Hibbert, Grainne Moran, Jan Herrmann, Asa K Jamting, and Victoria a Cole-man. Characterisation of gold agglomerates: size distribution, shape and optical properties. RSC

Advances, 3(20):7367–7374, 2013.

[42] Philippe K. Chow, Eklavya Singh, Bartolomeu Cruz Viana, Jian Gao, Jian Luo, Jing Li, ZhongLin, Ana L. Elías, Yunfeng Shi, Zuankai Wang, Mauricio Terrones, and Nikhil Koratkar. Wet-ting of mono and few-layered WS2 and MoS2 films supported on Si/SiO2 substrates. ACS Nano,9(3):3023–3031, 2015.

[43] Andres Castellanos-Gomez, Michele Buscema, Rianda Molenaar, Vibhor Singh, Laurens Janssen,Herre S J van der Zant, and Gary a Steele. Deterministic transfer of two-dimensional materials byall-dry viscoelastic stamping. 2D Materials, 1(1):011002, 2014.

[44] Zhen Li, Shun Wen Chang, Chun Chung Chen, and Stephen B. Cronin. Enhanced photocurrentand photoluminescence spectra in MoS2 under ionic liquid gating. Nano Research, 7(7):973–980,2014.

[45] M. Amani, D.-H. Lien, D. Kiriya, J. Xiao, A. Azcatl, J. Noh, S. R. Madhvapathy, R. Addou, S. KC,M. Dubey, K. Cho, R. M. Wallace, S.-C. Lee, J.-H. He, J. W. Ager, X. Zhang, E. Yablonovitch, andA. Javey. Near-unity photoluminescence quantum yield in MoS2. Science, 350(6264):1065–1068,2015.

[46] Terry P Bigioni, Xiao-Min Lin, Toan T Nguyen, Eric I Corwin, Thomas a Witten, and Heinrich MJaeger. Kinetically driven self assembly of highly ordered nanoparticle monolayers. Nature mater-

ials, 5(4):265–270, 2006.

[47] Robert J Meier. On art and science in curve-fitting vibrational spectra. Vibrational Spectroscopy,39(2):266–269, 2005.

[48] P. Törmä and W. L. Barnes. Strong coupling between surface plasmon polaritons and emitters.arXiv, 013901:1405.1661, 2014.

47

48

Appendix

A.1: Attempting to calculate the LSPRs MEEP [1] is a free finite-difference time-domain (FDTD) simulation software package developed at

MIT to model electromagnetic systems. Modifying code released by Bala Krishna Juluri [2] and

dielectric parameters computed by Aaron Webster [3], attempts to compute the plasmon

frequencies were attempted. However, the computation cost required to perform higher resolution

simulations quickly went beyond the average home computer. However, the above figure shows an

attempt to calculate the plasmon frequency.

Figure A1: 50 nm gold nanoparticle in vacuum using MEEP.

A.2: 1 L finder using R

Figure A2: (left) Image of thin layer MoS2 taken using optical microscope. (right) False colour image

to identify layers by using k-means clustering.

Inspired by the work of Li et al. [4], a script was developed using R and through the use of k-means

clustering to identify the layers purely by the RGB channels. However, k-means is an optimisation

algorithm; thus if two datasets are close together k-means might not be able to separate them (i.e.

dirt and monolayer). Nevertheless, it might be useful to be used as a tool to quickly narrow down

the images that a person needs to look through if a mapping microscopy setup is used to look

through the substrate for possible monolayers.

I have not included the codes here since it will take up space, but if you need to take a look please

contact me.

A.3: Differential Spectra for different number of drops Using the drop cast technique, it was found that the particles preferred to be deposited at the sides.

In addition, the particle distribution within the centre of the drop is not uniform either.

Nevertheless, by performing a reflectance spectrum over a large area, the plasmon peaks can be

identified. Here are some morphologies of agglomerations and the taken. Note that the drop

number does not mean that the structure only occurs at that number of drops, most likely the

spectrum is recorded due to its small size or that the structure has already been captured.

Figure A3: Differential Reflectance for different drop number and micrograph of the associated

structures (inset).

A.4 Size of laser spot during mapping

Figure A4: Laser spot and scale bar for comparison

Using ImageJ [5], the radius of the laser spot for the mapping setup as seem form the computer

screen captured by the CCD camera is estimated to be 3.6 μm. The actual laser profile may be

smaller.

A.5 Dependence of peak shift on incident radiant power

Figure A5: PL spectra for various ND filter, excited using 532 nm laser. (Left) Small peak wavelength

redshift on increasing laser power. (Right) No peak wavelength shift on increasing laser power.

ND value 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5

Incident Power (μW)

57.7 42.4 31.8 23 18.2 14 10.9 8.5 6.9

Table A1: Incident 532 nm laser power for various ND filter values. Measured using Thorlabs S130C

detector.

A.6: Corral Structure to block advancing wave front

Figure A6: BN corral to damp capillary forces.

BN layers

1 L MoS2

A.7: Variations in CVD MoS2 peak position

Figure A7: (Left) Before curve fitting (right) After curve fitting.

A.8: Raman Spectrum after SEM scan

Figure A8: Comparison for a sample that was scanned in the SEM with a typical CVD 1 L Raman

spectrum.

A.9: Peak fit parameters used in Section 4.4.4

MoS2 E2

g MoS2 A1

g Diff btw E

2g and

A1

g (cm-1

)

Si Peak Bkg

Amp FWHM (cm

-1)

Position (cm

-1) EF Amp

FWHM (cm

-1)

Position (cm

-1) EF Amp FWHM Position Amp

730.9 -2.8 385.9

631.8 -4.6 404.7

18.8 915.6 -12.9 521.4 154.2

408.2 -4.2 385.7 0.6 815.6 4.0 405.8 1.3 20.1 - - - 308.4

457.7 -4.2 385.4 0.6 907.4 4.1 405.8 1.4 20.4 - - - 369.2

Table A2: For Raman peaks using 532 nm laser.

633 nm Amp Pos(nm) FWHM(nm) EF

B 94138.69 659.99 23.38 7.60

A 86570.99 659.18 23.42 6.99

MoS2 12385.46 661.71 31.21 -

Table A3: For PL peaks using 633 nm laser.

532 nm Amp Pos(nm) FWHM(nm) EF

B 222695.57 658.77 -25.16 7.52

A 220759.53 657.75 -25.19 7.45

MoS2 29616.38 666.34 -34.98 -

Table A4: For PL peaks using 532 nm laser.

473 nm Amp Pos(nm) FWHM(nm) EF

B 80339.63 657.45 -23.76 4.16

A 96427.19 656.65 23.73 4.99

MoS2 19305.19 664.21 -30.47 -

Table A5: For PL peaks using 473 nm laser.

A.10: Defects from CVD growth

Figure A9: Defects from CVD growth are generally irregular and <20nm in height.

Bibliography

[1] “Meep,” 10 8 2015. [Online]. Available: http://ab-initio.mit.edu/wiki/index.php/Meep. [Accessed

2016 4 1].

[2] B. K. Juluri, “Electric field at localized plasmon resonance using MEEP,” 22 1 2013. [Online].

Available: juluribk.com/2013/01/22/electric-field-at-localized-plasmon-resonance-using-meep/.

[Accessed 1 4 2016].

[3] A. Webster, “Notes on Metals in Meep,” 3 11 2011. [Online]. Available:

http://falsecolour.com/aw/meep_metals/meep-metals.pdf. [Accessed 1 4 2016].

[4] G. L. Z. Y. Q. H. H. L. Q. Z. a. H. Z. Hai Li, “Optical Identification of Single- and Few-Layer MoS2

Sheets,” small, vol. 8, no. 5, pp. 682-686, 2012.

[5] “ImageJ,” [Online]. Available: https://imagej.nih.gov/ij/.