1
Self-Assembled Monolayers used to optimize the forming of gold
particles
for
enhancement
in
Raman
Spectroscopy.
13-02-2017
Report Bachelor Project Physics and Astronomy – Anoek Stofberg
[ABSTRACT] Research on plasmonic particles on the nanoscale has grown exponentially over the past 30 years due to their applications in medicine and sustainable energy. Local temperature is an important aspect in some of these applications, but currently there are no methods to measure.
This thesis is part of a bigger project, the purpose of which is to measure the temperature of plasmonic particles on the nanoscale. To investigate the local temperature distribution of plasmonic particles on the nanoscale, we fabricated gold islands and tried surface treatments to control morphology. Two kinds of self-assembled monolayers were compared. Cleaned glass substrates were provided with a hydrophobic monolayer. Using thermal deposition, gold particles were positioned on the substrates.
The absorption and Raman spectra were measured to investigate which substrates lead to the highest enhancement. The findings of this research project indicate that the glass without the hydrophobic monolayers leads to the highest enhancement.
STUDENTNUMBER UVA 10677836 STUDENTNUMBER VU 2526116
SUPERVISOR DR. ELIZABETH VON HAUFF 2ND EXAMINATOR DR. RINKE WIJNGAARDEN
CONDUCTED BETWEEN SEPTEMBER 2017 AND FEBRUARY 2018
SIZE 15 EC
FACULTY PHYSICS AND ASTRONOMY SPECIALIZATION PHYSICS OF ENERGY
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SAMENVATTING
Nanodeeltjes zijn deeltjes met een diameter tussen de 1 en 100 nm. Onderzoek naar nanotechnologie is exponentieel gegroeid in de afgelopen 30 jaar, maar de techniek is al honderden jaren oud. In de middeleeuwen werd nanotechnologie al gebruikt om kleur te geven aan glas in lood ramen. Dit deden ze met gouden nanodeeltjes.
Omdat nanodeeltjes zo klein zijn, hebben ze een groot oppervlakte ten opzichte van het volume. Dit zorgt ervoor dat nanodeeltjes bijzondere eigenschappen hebben. Als een nano-edelmetaaldeeltje, zoals een nanogouddeeltje, wordt beschenen met een laser, duwt het electromagnetische veld van de laser de valentie-elektronen (de elektronen in de buitenste schil) van het nanometaaldeeltje naar één kant van het deeltje, deze kant wordt dan negatief geladen. Aan de andere kant van het deeltje is dan een positieve lading. De laser zorgt voor een trilling van de negatieve ladingen. Als de frequentie van de laser en de frequentie van de trilling van het deeltje overeenkomen, zorgt dit voor lokale opwarming en een versterking van het elektromagnetische veld (resonantie). Bij gouddeeltjes ligt deze frequentie in het zichtbare deel van het lichtspectrum.
De versterking van het elektromagnetische veld kan worden gebruikt om zonnecellen efficiënter te maken. De lokale opwarming kan worden gebruikt in de medische wereld, om bijvoorbeeld kankercellen te vernietigen. Het nanodeeltje wordt dan gekoppeld aan de kankercellen en dan wordt er een laser op geschenen. Het nanodeeltje wordt dan zo warm dat de kankercellen kapot gaan.
Om deze methode daadwerkelijk te kunnen gebruiken is er meer inzicht nodig over de lokale opwarming van nanodeeltjes. Een mogelijke manier om de temperatuur van een nanodeeltje te meten is door het gebruik van Ramanspectroscopie.
Ramanspectroscopie is een techniek om de verschillen in energieniveau’s van de vibraties van moleculen te meten. Een molecuul wordt beschenen met een laser en er wordt gekeken met welke golflengtes de moleculen licht verstrooien. Als de verstrooide golflengte even groot is als de golflengte van de laser heet het Rayleighverstrooiing. Als de verstrooide golflengte anders is dan de golflengte van de laser heet dit Ramanverstrooiing. Bij Ramanverstrooiing zijn er twee mogelijkheden; de verstrooide golflengte is groter dan de golflengte van de laser, dit heet een Stokesverschuiving, of de verstrooide golflengte is kleiner dan de golflengte van de laser, dit heet een anti-Stokesverschuiving.
Bij Ramanspectroscopie wordt een molecuul beschenen met een laser en worden alle verschuivingen bijgehouden. Een Raman-spectrum laat pieken zien bij de energieniveau’s van verschillende vibraties. De verhouding tussen de Stokes- en Anti-Stokesverschuivingen wordt gebruikt om de temperatuur te bepalen.
De nanogouddeeltjes zorgen ervoor dat de pieken van een Ramanspectrum worden versterkt. Om te zien hoeveel de nanogouddeeltjes het spectrum versterken, wordt goud en koperftalocyanine (CuPc) verdampt op glasplaatjes. CuPc wordt gebruikt als medium, omdat het veel onderzocht is en het Ramanspectrum bekend is. Vervolgens wordt een Ramanspectrum van CuPc met een Ramanspectrum van CuPc met goud vergeleken, om te kijken hoe erg de nanogouddeeltjes het signaal hebben versterkt. Bij het verdampen van goud op de glasplaatjes komt een probleem kijken. Glas staat er om bekend slecht te hechten met edelmetalen. Hiervoor is in deze scriptie geprobeerd een oplossing te vinden. Het glas is voorbehandeld om te zorgen dat het goud beter kan hechten. Het glas wordt ondergedompeld in twee verschillende oplossingen van moleculen met silicum. Silicum reageert goed met glas, en zorgt voor een hydrofobe monolaag op het glas. Dit zorgt ervoor dat de gouddeeltjes beter kunnen hechten.
De glasplaatjes met goud, CuPc en de verschillende hydrofobe monolaag en het glasplaatje met goud, CuPc en zonder hydrofobe monolaag worden onder de Ramanspectroscoop gehouden en de uitkomsten worden met elkaar vergeleken om te kijken welke van de drie zorgt voor de hoogste pieken in het Ramanspectrum. Daar kwam uit dat de monolagen niet zorgen voor hogere pieken in het Ramanspectrum.
In de toekomst zou een Ramanspectroscoop met verschillende lasers van verschillende golflengten gebruikt moeten worden, omdat de monolagen misschien bij beschijnen met licht van een andere golflengte wel zorgen voor hogere pieken.
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CONTENTS
1 INTRODUCTION ... 4 2 THEORETICAL BACKGROUND ... 5 2.1 PLASMONICS ... 5 2.1.1 NANOPARTICLES ... 52.1.2 LOCALIZED SURFACE PLASMON RESONANCE ... 5
2.2 RAMAN SPECTROSCOPY ... 7
2.2.1 FUNDAMENTALS ... 7
2.2.2 TEMPERATURE EFFECTS ON SERS ... 9
2.2.2.1 ENHANCEMENT FACTOR ... 9
2.3 SURROUNDING MEDIUM: COPPER PHTHALOCYANINE ... 10
2.4 SELF-ASSEMBLED MONOLAYERS ... 11
3 EXPERIMENTAL SETUP ... 13
3.1 MATERIALS & EQUIPMENT ... 13
3.2 SAMPLES ... 13
3.2.1 PREPARING SAMPLES ... 13
3.2.2 THE SAM FABRICATION ... 13
3.2.3 THERMAL DEPOSITION ... 14 3.3 ABSORPTION MEASUREMENT ... 15 3.4 RAMAN MEASUREMENT ... 15 4 RESULTS ... 16 4.1 ABSORPTION SPECTRA ... 16 4.1.1 CUPC ... 16 4.1.2 AU ... 17 4.1.3 AU + CUPC ... 18 4.1.4 SELF-ASSEMBLED MONOLAYERS ... 19 4.1.5 AU 2.5NM + CUPC 30NM... 20 4.2 RAMAN SPECTRA ... 21 4.2.1 CUPC ... 21 4.2.2 AU + CUPC ... 22 4.3 ENHANCEMENT ... 24
5 DISCUSSION AND OUTLOOK ... 26
6 CONCLUSION ... 26
7 REFERENCES ... 27
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1 INTRODUCTION
Although nanoparticles have a long history, for example in the Roman times when metallic nanoparticles were used to give color to stained glass [1], we are now in the beginning of the Nano Age [2]. Research on nanoparticles has grown exponentially over the past 30 years. Nanoparticles give rise to great scientific interest because of the change in physical properties of materials from the macroscopic scale to the nanoscale.
The field of plasmonics is based on the interaction of electromagnetic waves and conduction electrons. The interesting optical properties of plasmonic nanoparticles have inspired new opto-electronic architectures and novel approaches in optical spectroscopy, to increase light absorption and emission.
Plasmonic materials demonstrate very high absorption cross sections in a narrow spectral range, leading to near-field enhancement, light scattering and heat dissipation [3].
The heat dissipation can have medical applications, for example it is used for the destruction of cancer cells [1]. The nanoparticle is coupled to the cancer cells and then excited with light, which causes the nanoparticle to release thermal energy that destroys the cancer cells. To put this to daily use, more information is needed about the local temperature distribution. When a nanoparticle is excited with light and it releases thermal energy, at what rate does it dissipate heat to its surroundings?
Nanothermometry is an exciting field. Research is generally based on applying optical spectroscopy to measure heat at the nanoscale. But to date, these approaches have not been validated and there are many open questions. Pozzi et al. [4] attempted to measure temperature at the nanoscale using Surface Enhanced Raman Spectroscopy (SERS) [4]. This motivated us, the Hybrid Solar Energy Conversion group of the physics department of the VU University in Amsterdam, to investigate whether it is an easy and reliable way to measure temperature on the nanoscale, using the Stokes/Anti-Stokes ratio of a Raman spectrum. To get a clear Raman spectrum, gold (Au) nanoparticles are used. Au nanoparticles are noble, metal particles that are appropriate for our purposes because they are very stable and absorb light in the visible region.
Au thin films completely fill in the dimples and are continuous, this way they form nice plasmonic particles [5]. Au thin films are an easy way to make SERS substrates because they facilitate molecular sensing (Raman scattering) [5].
I combined the Au particles with a well-known organic semiconductor; Copper Phthalocyanine (CuPc). I studied changes to the Raman spectrum of the CuPc matrix as a function of the properties of the Au nanoparticles. The Au nanoparticle enhances the absorbance and the intensity of the Raman peaks through near-field enhancement.
To control the size and distribution of Au nanoparticles, we need to find a substrate to which the Au nanoparticles attach well. The purpose of this thesis is to find an appropriate substrate to optimize the Au particles.
Two organic self-assembled monolayers will be compared. Research has shown that monolayers increase the forming of Au nanoparticle islands [6]. To investigate whether monolayers contribute to a higher intensity of Raman peaks, for each organic monolayer the substrates with Au and/or CuPc will be prepared, the absorbance of the Au substrates and the CuPc substrates will be measured, and the Raman spectrum of the CuPc substrates and CuPc with Au substrates will be obtained. After this, it can be seen which substrate caused the most enhancement of the peaks, which can be used at a later stage to determine the temperature of the Au nanoparticles.
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2 THEORETICAL BACKGROUND
2.1 PLASMONICS
2.1.1 NANOPARTICLES
Nanoparticles are particles with diameters ranging between 1 and 100 nanometers [7]. The interfacial layer of nanoparticles typically consists of ions and (in)organic molecules. One distinguishing characteristic of nanoparticles is that they typically have a high percentage of atoms at the surface. The volume of an object (V ∝ l3) decreases more quickly than its surface area (A ∝ l2) as the size diminishes ( 𝐴𝑉 ∝ l-1) [8], where lhas an atomic or a molecular dimension. Nanoparticles have a high surface-to-volume ratio, meaning that the surface properties of the particles have an important impact on the nanoparticle’s properties.
If a nanoparticle is excited with light, the energy of the light beam can be absorbed and/or scattered. Whether the energy gets absorbed and/or scattered depends on the type of particle, its size and shape, and the wavelength of the light [7].
There are different types of nanoparticles, each with their own properties. One type is semiconducting nanoparticles; these nanoparticles are called quantum dots. When light or electricity is applied to a semiconducting nanoparticle, the quantum dot will emit light of a specific frequency. The frequency can be precisely tuned by the dots size, its shape, and its material [10]. In quantum dots, the excitons are spatially confined, leading to quantum confinement, therefore semiconducting nanoparticles are promising for photonic device applications [2].
Another kind of nanoparticle is the metallic nanoparticle. Metal nanoparticles can alter the absorption or emission of light [11, 12] and are therefore considered to be ideal candidates for a variety of electronic and electrical applications [2]. When light is incident on a metal nanoparticle, its electric field pushes the electrons in the particle toward one side of the particle, leaving behind a positive charge on the opposite side. This is known as surface plasmons [13, 14]. The negative and positive charges attract each other. If the frequency of the incident light matches the natural resonance frequency, it will produce large oscillations of all of the free electrons in the metal.
2.1.2 LOCALIZED SURFACE PLASMON RESONANCE
As mentioned above, in some spherical metallic nanoparticles, which are smaller than the wavelength of the incident light, the valence electrons come into resonance with a specific frequency of the incident light [15]. The outer electrons coherently oscillate with the same frequency as the incoming light. This is known as Localized Surface Plasmon Resonance (SERS) and it is visualized in Figure 1. In gold and silver particles, the resonance frequency lies in the visible range of the electromagnetic spectrum [16].
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Figure 1: The localized surface plasmon resonance. The electron cloud is in resonance with the applied magnetic field [17].
The oscillation of the electron cloud causes a large resonant enhancement of the local electromagnetic field inside and near the nanoparticle. In other words, the nanoparticle behaves as a nano-antenna. The resulting electric field around the particle depends on the polarization of the incoming light and on the shape and the size of the nanoparticle. This effect can be exploited in spectroscopy.
To understand the interaction between the nanoparticle and the applied magnetic field the quasi-static approximation was used [18]. The quasi-static approximation only holds when the nanoparticle’s diameter d ≪ λ (the wavelength of the incident light). This is called the Rayleigh limit [7].
In the Rayleigh limit the electric field around the nanoparticle is uniform. Therefore, the Maxwell equations for the electromagnetic field can be replaced by the Laplace’s equations [18]. The electromagnetic fields and the potentials are given by
Ein = -∇ϕin
E0 = -∇ϕ0 (1)
∇ 2ϕin = 0
∇ 2ϕ0 = 0
Where E0 and ϕ0 are the applied electric field and potential outside the particle, and Ein and
ϕin are the electric field and potential inside the particle. The condition ϕin = ϕ0, at the
boundary, and the fact that the electrical displacement is constant, result in 𝜖𝑖𝑛𝛿𝜙𝑖𝑛
𝛿𝑟 = 𝜖𝑜𝑢𝑡
𝛿𝜙0
𝛿𝑟 (2)
Where 𝜖𝑖𝑛 is the dielectric function of the metal, 𝜖𝑜𝑢𝑡 is the dielectric constant of the medium, and r is the distance measured from the center of the particle. The potential outside the nanoparticle is given by adding the potential of a dipole, with the poles separated by a distance 2R, and the potential of the applied electric field, resulting in
ϕout = - E0r𝑐𝑜𝑠 𝜃 + 𝜖𝜖𝑖𝑛− 𝜖𝑜𝑢𝑡
𝑖𝑛+ 2 𝜖𝑜𝑢𝑡 𝑅 3E
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Combining equation 1 and equation 3, and converting it into Cartesian coordinates, gives the resulting near-field enhancement
Eout (x,y,z) = E0 𝒙̂ – α 𝑅3E0 [ 𝒙̂ (𝑥2+𝑦2+𝑧2)32 – 3𝑥 (𝑥2+𝑦2+𝑧2)52 (x 𝒙̂ + y 𝒚̂ + z 𝒛̂)] (4) α = 𝜖𝜖𝑖𝑛− 𝜖𝑜𝑢𝑡 𝑖𝑛+ 2 𝜖𝑜𝑢𝑡 (5)
where E0 is the applied electric field, polarized in the x direction. Maximal enhancement occurs when the denominator of equation 2 approaches zero. This occurs when ϵin = −2 ϵout.
2.2 RAMAN SPECTROSCOPY
2.2.1 FUNDAMENTALS
When light interacts with a sample, the light can be transmitted, reflected, absorbed or scattered. Two types of scattering can occur [19]. The first type is Rayleigh scattering. Rayleigh scattering is an elastic scattering process. The second type is Raman scattering. Raman scattering is an inelastic scattering process. When a molecule gets excited with a photon, this excitation can set the electron into a virtual energy level; this is depicted in Figure 2. Inelastic scattering means that the energy of the emitted photon is of either lower or higher energy than the incident photon.
Figure 2: A schematic view of the Rayleigh and Raman scattering with Stokes and anti-Stokes shifts [19].
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If the energy of the emitted photon is of lower energy, it is called a Stokes shift [19]. The phonon gets from the ground state into the excited state. If the energy of the emitted photon is of higher energy, it is called an Anti-Stokes shift [19]. The phonon goes from the excited state into the ground state. Because most phonons are in the ground state, Stokes shift occurs more, and so the Raman peaks are more intense than the peaks of the Anti-Stokes shift. A schematic example of a spectrum with both the Rayleigh scattering and the Stokes/Anti-Stokes (Raman) shift is depicted in Figure 3. The probability of Raman scattering is very low, for approximately ten million photons Rayleigh scattered by a molecule, only one photon is Raman scattered [20], which means that Raman signals are inherently very weak [21].
Figure 3: A schematic view of a spectrum with Rayleigh and Raman (Stokes + Anti-Stokes) scattering. It can be seen that the intensity of the Rayleigh scattering is much higher than the intensity of the Stokes/Anti-Stokes
peaks.
However, Raman scattering gives a good insight into the chemical nature of the molecule. Raman spectroscopy is a technique used to examine the vibrational and rotational modes of molecules [22]. A Raman spectrum will show peaks corresponding to the different vibrational energy levels of the probed molecule. This way, it serves as a structural fingerprint.
The magnitude of the Raman Effect correlates with the anisotropy polarizability of the bonds in a molecule. When the polarizability of the molecule changes, the incident light and the molecule can exchange energy. For the total energy of the system to remain constant, the change in energy is exactly the same as the change in virtual energy levels. This is called the Raman shift. The Raman shift is defined as
ΔEe = Ei – Es (6)
where Ei is the energy of the incident photon and Es is the energy of the scattered photon. The Raman shift is often converted to wavenumbers by
Ramanshift [cm-1] = ( 1 λi [nm] - 1 λs [nm]) 107[nm] [cm] (7)
where λi is the wavelength of the incident photon and λs is the wavelength of the scattered photon. If the final energy level is higher in energy than the initial state it is called a Stokes shift.
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2.2.2 TEMPERATURE EFFECTS ON SERS
As mentioned above, the anti-Stokes shift is less intense than the Stokes shift. This is because for an anti-Stokes shift the system already has to be in a higher vibrational energy level. Due to thermal excitement, more molecules will be in a higher energy level and the probability of finding a molecule in a higher energy level will increase. Therefore a higher temperature leads to a higher ratio (ρ) of anti-Stoke (aS) and Stokes (S) intensities (I). Because the populations of the initial states are described by the Boltzmann distributions, Raman spectroscopy allows for temperature measurements. The contributions to ρ are governed by equation 8 [4].
ρ =
𝐼𝑎𝑆 𝐼𝑆= A [
𝜏 𝜎𝑆 ′𝐼𝐿 ℎ 𝑣𝐿+ 𝑒
−ℎ 𝑣𝑚 𝑘𝐵 𝑇]
(8)where 𝑣𝑚 denotes the Raman mode shift frequency, 𝜏 represents the vibrational excited state lifetime, 𝜎′ denotes the Raman cross section, T is the local temperature, and the subscript L denotes the laser excitation. The asymmetry factor A is given by equation 9 [4].
A = 𝜂𝜂𝑎𝑆 𝑆 𝜎𝑎𝑆′ 𝜎𝑆′
|
𝐸𝐿𝐸𝑎𝑆 𝐸𝐿𝐸𝑠|
2 (9)where 𝜂 denotes the wavelength-dependent detection efficiency of unpolarized light [4] and E represents the local wavelength-dependent electrical field strength. Combining equation 8 and 9 gives the wavelength dependence of ρ, equation 10.
ρ ∝ |
𝐸𝐸𝑎𝑆𝑠
|
2 (10)
2.2.2.1 ENHANCEMENT FACTOR
In Raman spectroscopy the scattered intensity is linear with the incident field intensity |𝐸𝑜𝑢𝑡|2 [15]. LSPR results in enhancement of the electromagnetic field, and thus also in enhancement in Raman spectroscopy. Keeping in mind the field is enhanced at the surface, where r = R, manipulating Equation 4 gives
IRAMAN ∝ |𝐸𝑜𝑢𝑡|2 = 𝐸0 2 [ |1-𝛼|2 + 3cos2𝜃(2Re(𝛼) + |𝛼|2) ] (11)
where 𝜃 is the angle between the incident field vector and the vector of the molecule on the surface. Equation 8 gives the general expression for the enhancement of the electrical field. When 𝛼 is large, the maximum intensity becomes
10 If cos2𝜃is at maximum, the intensity is 4|𝐸
0|2|𝛼|2, when cos2𝜃is at minimum, the intensity is |𝐸0|2|𝛼|2. The Radially averaged intensity is given by
|𝐸̅𝑜𝑢𝑡|2 = 2|𝐸0|2|𝛼|2 (13)
The enhancement of the peaks in Raman spectroscopy depend not only on the wavelength, but also on the size and shape of the nanoparticle. The equation for the enhanced emission intensity (EF) of a SERS system is described by [4, 15]
EF = 𝐼𝑆𝐸𝑅𝑆𝐼𝑁𝑅𝑆/𝑁/𝑁𝑠𝑢𝑟𝑓
𝑣𝑜𝑙 ∝
𝐼𝑆𝐸𝑅𝑆
𝐼𝑁𝑅𝑆 (14)
Where ISERS is the surface-enhanced Raman intensity, INRS is the normal Raman intensity,
Nsurf is the number of molecules excited by the surface plasmon, and Nvol is the number of
molecules in the excited volume. Because it’s very difficult to estimate the values of Nsurf and
Nvol, in this research the enhancement is calculated by only comparing the relative peak
intensities and thus can be too optimistic.
EF = 𝐼𝐼𝑁𝑅𝑆𝑆𝐸𝑅𝑆 (15)
2.3 SURROUNDING MEDIUM: COPPER PHTHALOCYANINE
In order to see what the local heat transportation of the Au nanoparticles does to its surroundings, a well-known organic matrix is used. Copper phthalocyanine (CuPc) is chosen because it is a very well-studied organic semi-conductor, and there are many reports on the optical properties including absorption and Raman spectra[22, 23]. The molecular structure of CuPc can be seen in Figure 4.
Figure 4: The molecular structure of CuPc [25].
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The Raman spectrum is well documented, and the Raman lines are identified. The Raman spectrum of CuPc can be seen in Figure 5. The vibrational states that correspond with the peaks relevant to this research can be seen in Table 1.
Wavenumber [cm-1] Assignments of bands 1528 1452 1341 1143 954 680 596
Benzene and pyrrole C-C stretching Isoindole deformation Pyrrole deformation C-H deformation C-H out of plane Macrocycle breathing Macrocycle breathing
Table 1: The assignments of bands of CuPc with the corresponding wavelengths [16].
2.4 SELF-ASSEMBLED MONOLAYERS
Noble metals are known to exhibit poor adhesion to glass[6]. Self-assembled monolayers (SAM) can be applied to improve the adhesion of the Au particles to the glass. The SAM has a nanometer scale thickness [8, 9]. SAMs are molecular assemblies formed on surfaces by adsorption. The molecules possess a functional head group, and a tail group. This can be seen in Figure 6. The head group chemisorbs, often in the presence of a catalyst, with the substrate to form a well-ordered monolayer, followed by a slow organization of the tail group [26].
Figure 6: A schematic view of the head group, the tail group and the functional group of s SAM.
For this research, the molecules used for the SAMs are trichloro(octadecyl)silane (OTS) and (3-mercaptopropyl)trimethoxysilane (MPTS). OTS and MPTS are amphiphilic molecules; they have a hydrophilic head and a hydrophobic tail. In these molecules the head group consists of silane. Silane will react with metal oxides, including SiO2 (glass).
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Figure 7: A schematic view of the contact angle. In a) there is no hydrophobic monolayer, so the contact angle θ < 90. In b) there is a hydrophobic monolayer, so the contact angle is θ > 90.
The hydrophobic tail provides the substrate with a hydrophobic monolayer. To check whether the hydrophobic monolayer assembled well, the contact angle can be used. This is depicted in Figure 7. When there is no hydrophobic monolayer and there is a water droplet dropped on the glass sample, the water droplet and the glass sample will have a contact angle of θ < 90° (Figure 7a) [27]. When the glass sample is provided with a hydrophobic monolayer, the water droplet and the glass sample will have a contact angle of θ > 90° (Figure 7b) [27]. The OTS SAM and the MPTS SAM change the surface energy of the glass substrate, and provide a more suitable surface for Au nanoparticle formation [28]. The binding mechanism of OTS and the MPTS on the substrate, and of the Au particles to the OTS and MPTS, is depicted in Figure 8.
Figure 8: A schematic view of a) the glass substrate and the OTS, b) the OTS SAM on the glass, c) the Au nanoparticles on top of the OTS SAM, d) the glass substrate and the MPTS, e) the MPTS SAM on the glass, and f) the Au nanoparticles on top of the MPTS SAM.
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3 EXPERIMENTAL SETUP
3.1 MATERIALS & EQUIPMENT
MATERIALS
Pure H2O, Ethanol, Acetone, 2-Propanol, OTS, MPTS, Toluene, Au, and CuPc were all purchased from Sigma-Aldrich.
EQUIPMENT
Ultrasonic Bath, UV Ozone Oven, MBraun Nitrogen Chamber, Siemens Simatic HM1, Inficon SGC-310C, PerkinElmer Instruments Lambda 900 UV/VIS/NIR Spectrometer, Renishaw Raman Microscope.
3.2 SAMPLES
3.2.1 PREPARING SAMPLES
To prepare the samples, 27 glass substrates were cut into the size of 24 x 26 mm. These substrates were cleaned 8 at a time. The cleaning occurred by putting 8 substrates in a glass container, and then filling the container with 4 different solutions, one after the other. Then, the glass container with the substrates was put inside an ultrasonic bath. The cleaning protocol, including solvent and cleaning time is summarized in Table 2. Afterwards, the substrates were dried with a nitrogen gun and brought to a UV-ozone oven to dry for 20 minutes.
Step Duration Solution
1 2 3 4 5 minutes 5 minutes 5 minutes 5 minutes Pure H2O Ethanol Acetone 2-Propanol
Table 2: The order and solutions in which the substrates were cleaned.
3.2.2 THE SAM FABRICATION
After the substrates were cleaned, 9 samples were dipped in a solution of OTS and 9 samples were dipped in a solution of MPTS. Both solutions were a 10mM solution in toluene. The samples were inside the solution for 15 minutes and were heated up to 60 °C. This was done inside of the MBraun nitrogen filled chamber. The samples were dried with a nitrogen gun. The clean samples of glass, OTS and MPTS were then brought to the Siemens Simantic HM1 evaporation chamber.
According to the contact angle (Figure 7), both the OTS and the MPTS formed a hydrophobic monolayer. Pictures of an OTS sample and a MPTS sample with a water droplet on top can be seen in Figure 9. The water droplet does not attach to the glass well because the glass now has a hydrophobic layer.
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Figure 9: Pictures of the samples, held by a tweezer, with a water droplet on top. In a) you see a picture of a sample with a hydrophobic monolayer provided by dipping the sample in a solution of OTS. In b) you see a picture of a sample with a hydrophobic monolayer provided by dipping the sample in a solution of MPTS.
3.2.3 THERMAL DEPOSITION
Samples were prepared so that only half of the sample was coated in Au before depositing CuPc on the full sample. This was to facilitate the comparison between optical spectra taken on CuPc and spectra taken on CuPc/Au with samples prepared under the same conditions. Each thermal deposition was done in vacuum with a pressure lower than 1∙ 10−6 mbar to ensure purity.
For thermal deposition, material is deposited in a boat. A current heats the boat and the material sublimates. A calibrated quartz crystal with a resonance frequency of 5 MHz is used to monitor the thickness of the film. Once the desired thickness is reached, the substrate shutter closes to prevent further deposition.
The evaporation chamber is connected to an Inficon SQC-310C to monitor the process. A program was used to control the deposition rate and shutter during the thermal deposition. The used Au is 99.99% pure, has a density of 19.3 cmg3 and has Z-factor of 0.381. The thicknesses of the Au were 0.8 nm and 2.5 nm.
The deposition parameters for CuPc are more complex, and the thermal deposition is controlled by hand to while the rate of deposition is monitored. To prevent a chemical reaction between the organic semiconductor and the boat, a ceramic crucible is used for the deposition of CuPc. It was heated up manually to 330 °C, with 20 °C per minute. Once the desired thickness was reached, the substrate shutter was closed manually. The thicknesses of the CuPc were 30 nm and 50 nm. Additionally, a picture of a sample can be seen in Figure 10.
Figure 10: A photograph of a sample. The light blue (upside) is the half with only CuPc. The darker blue (downside) is the CuPc with Au. Every sample has a half of CuPc and a half of CuPc with Au.
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On each of the 3 different substrates the following combination of Au and CuPc thicknesses were evaporated:
Au thickness (nm) CuPc thickness (nm)
0 0 0 0.8 0.8 0.8 2.5 2.5 2.5 0 30 50 0 30 50 0 30 50
Table 3: All combinations of Au and CuPc thicknesses evaporated on each of the 3 different substrates.
3.3 ABSORPTION MEASUREMENT
The absorption measurement of the samples was done using a PerkinElmer Instruments Lambda 900 UV/VIS/NIR spectrometer. The absorption was measured from 280nm to 850nm.
3.4 RAMAN MEASUREMENT
For the Raman measurement the Renishaw Raman microscope at the Laser Lab of the VU University was used. The Renishaw Raman microscope is equipped with two lasers, one with an excitation wavelength of 532nm and one with an excitation wavelength of 785nm. The best laser to use is the one that excites in the Au absorption band[22, 28]. The laser was used with 1% intensity (this corresponds to a power of 3mW), 10 seconds exposure time, 3 acquisitions and a 50x objective lens.
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4 RESULTS
4.1 ABSORPTION SPECTRA
The absorption spectra are measured to see which laser excitement we should use to check Au enhancement. This decision is made by looking at the absorption peak of the different substrates with different thicknesses. The Raman enhancement is highest when the laser excitation is as close as possible to the absorption peak [21]. This is because higher absorption leads to more enhanced resonance in LSPR, which leads to more enhancement in Raman spectroscopy. For chemical substrates the 785nm excitation is most suitable and most used [29, 30], so preferably we find that the absorption of the Au is higher with the 785nm excitation than with the 532nm excitation. For every combination of thicknesses the absorption is measured to see which combination has the highest absorption.
4.1.1 CUPC
The absorption spectrum of CuPc 30nm and CuPc 50nm is depicted in Figure 11. The absorption has two peaks. The first peak occurs at 626nm and the second peak occurs at 694nm.
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4.1.2 AU
The absorption spectra of Au 0.8nm and Au 2.5nm are depicted in Figure 12. When a nanoparticle’s size gets bigger, the wavelength of the absorption peak shifts to a higher wavelength [1]. The results are in accordance to this theory. The Au 0.8nm has a peak at 554nm and the Au 2.5nm has a peak at 621nm.
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4.1.3 AU + CUPC
Figure 13 shows the Raman spectra of Au (0.8nm and 2.5nm) and CuPc (30nm and 50nm) in all combinations. The peaks occur at 626nm and 700nm.
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4.1.4 SELF-ASSEMBLED MONOLAYERS
The absorption spectra of all the combination of thicknesses of Au and CuPc on glass, OTS, and MPTS are depicted in Figure 14. The laser lines of the Raman microscope are indicated. The absorption of the Au nanoparticles is not resonant with the available laser lines (532, 785nm). It is easy to see that the sample with Au 2.5nm + CuPc 30nm has the highest absorption at 785nm for all the different substrates, so these samples are expected to have the most enhancement.
Figure 14: The absorption spectra of all the different combination thicknesses on a) glass, b) OTS, and c) MPTS. On each of the substrates the Au 2.5nm + CuPc 30nm has the highest absorption at 785nm.
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4.1.5 AU 2.5NM + CUPC 30NM
Figure 15 shows again the absorption spectra of the 2.5nm Au + 30nm CuPc sample of glass, OTS, and MPTS, to see with what substrate we expect the most enhancement. According to Figure 15 we expect the most enhancement on the glass sample, follow by MPTS, and the least enhancement on the OTS sample.
Figure 15: The absorption lines of Au 2.5nm + CuPc 30nm on glass, OTS, and MPTS.
Table 4 shows the absorbance at both the laser excitations and the absorbance of the CuPc peaks to show that the 785nm laser excitation is the better of the two available lasers. It also shows that the 785nm laser excitation is not the ultimate wavelength and that it would be even better to have another laser excitation around 620nm or 700nm.
Absorbance at 532nm Absorbance at 785nm Absorbance Peak 1 Wavelength Absorbance Peak 1 Absorbance Peak 2 Wavelength Absorbance Peak 2 Glass OTS MPTS 28.44 % 22.36 % 22.82 % 46.16 % 23.45 % 41.65 % 46.88 % 35.64 % 49.09 % 622 nm 618 nm 622 nm 49.04 % 32.16 % 48.56 % 710 nm 694 nm 700 nm Table 4: The absorbance at the two laser excitations and the absorbance of the peaks of the CuPc on glass, OTS, and MPTS.
500 600 700 800 0 5 10 15 20 25 30 35 40 45 50 55 60 Absor ptio n [ %] Wavelength [nm] Glass OTS MPTS 785 532
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4.2 RAMAN SPECTRA
4.2.1 CUPC
Figure 16 shows a CuPc Raman spectrum on glass. This spectrum is as we expect from the literature (Figure 5). The peaks we would expect (Table 1) are indicated.
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4.2.2 AU + CUPC
The Raman spectra of the samples with Au 2.5nm + CuPc 30nm show the most enhancement on all substrates. All the other Raman spectra, which have less enhancement, can be seen in Appendix 1.
Figure 17 shows the Raman spectrum of the glass sample with Au 2.5nm + CuPc 30nm compared to the Raman lines of CuPc 30nm without Au. It also shows the graphs normalized, here it can be seen that the enhancement is wavelength dependent because the enhancement is not the same at every wavelength. The Raman peaks of Au 2.5nm + CuPc 30nm are much stronger, as expected, due to the plasmonic effects of the Au nanoparticles.
Figure 17: Above, the Raman spectrum of the glass sample with Au 2.5nm + CuPc (red) compared to CuPc 30nm (black) and below the normalized Raman spectrum of Au 2.5nm + CuPc compared to CuPc 30nm.
Figure 18 and Figure 19 show the Raman spectrum of respectively an OTS sample and a MPTS sample with 2.5nm Au + 30nm CuPc compared to a glass sample of CuPc 30nm without Au. It also shows the peaks normalized to see that here the enhancement is also wavelength dependent. 500 1000 1500 0 20000 40000 60000 80000 100000 500 1000 1500 0.0 0.2 0.4 0.6 0.8 1.0 Rama n I nte nsity Ramanshift [cm-1]
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Figure 18: Above, the Raman spectrum of the OTS sample with Au 2.5nm + CuPc (red) compared to CuPc 30nm (black) and below the normalized Raman spectrum of Au 2.5nm + CuPc compared to CuPc 30nm.
Figure 19: Above, the Raman spectrum of the MPTS sample with Au 2.5nm + CuPc (red) compared to CuPc 30nm (black) and below the normalized Raman spectrum of Au 2.5nm + CuPc compared to CuPc 30nm.
500 1000 1500 0 5000 10000 15000 20000 25000 30000 500 1000 1500 0.0 0.2 0.4 0.6 0.8 1.0 OTS Rama n I nte nsity Ramanshift [cm-1] 500 1000 1500 0 20000 40000 60000 80000 100000 500 1000 1500 0.0 0.2 0.4 0.6 0.8 1.0 Rama n I nte nsity MPTS Ramanshift [cm-1]
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4.3 ENHANCEMENT
The Raman spectra of the Au 2.5nm + CuPc 30nm on glass, OTS, and MPTS together can be seen in Figure 20 (above). The normalized peaks are also in Figure 18 (below). It can be seen that both the monolayers have less enhancement than the glass. How much every peak on each substrate is enhanced can be seen in Table 5. This is calculated by comparing the Raman intensities of both peaks (Equation 15) on every Ramanshift (Equation 7). The last two columns show the enhancement of the monolayers compared to the enhancement of the glass. The red numbers indicate a negative increase (less enhancement). The enhancement of the other samples is less, the enhancement table of these samples can be seen in Appendix 2.
Figure 20: The Raman spectra of Au 2.5nm + CuPc 30nm on glass (yellow), OTS (red), and MPTS (blue) all together in one graph. Above you can see the enhanced peaks and below the peaks are normalized.
Wavenumber [cm-1] Wavelength [nm] Glass Enhancement OTS Enhancement MPTS Enhancement
OTS vs Glass MPTS vs Glass
1528 1451 1341 1143 954 680 596 893 886 877 862 848 829 823 55.28 x 25.28 x 66.53 x 78.76 x 77.61 x 154.34 x 84.20 x 18.86 x 8.59 x 24.08 x 30.48 x 30.17 x 65.27 x 33.22 x 58.51 x 26.70 x 67.39 x 76.70 x 69.49 x 137.59 x 67.69 x 0.34 0.34 0.36 0.39 0.39 0.42 0.39 1.06 1.06 1.01 0.97 0.90 0.89 0.80
Table 5: The enhancement factors of the Glass, OTS, and MPTS on each peak of table 1.
600 800 1000 1200 1400 1600 0 20000 40000 60000 80000 100000 600 800 1000 1200 1400 1600 0.0 0.2 0.4 0.6 0.8 1.0 Rama n I nte nsity Glass OTS MPTS Ramanshift [cm-1]
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The Raman lines do not match the absorption of the Au nanoparticles; this can be seen in Figure 21. Since the Raman lines do not match the absorption of the Au nanoparticles, the enhancement is quite surprising. If the Raman lines would match the absorption of the Au nanoparticles, the enhancement is expected to be even more.
Figure 21: A Raman spectrum converted to wavelength, together with the absorption of 2.5nm Au to see that the Raman spectrum does not match the maximum absorption of the Au nanoparticles.
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5 DISCUSSION AND OUTLOOK
The purpose of thesis was to answer the question whether the use of the self-assembled monolayers OTS and MPTS contribute to higher enhancement in Raman spectroscopy. The results of this research project are different from what was expected based on the literature. According to the previous theory, the monolayers would increase the enhancement of the gold [6]. Over all, the enhancement on OTS and MPTS was not found to be significantly higher than the enhancement on glass.
The first test to see if the monolayers formed well on the cleaned glass was successful; both the monolayers formed a hydrophobic layer on the glass samples (Figure 9). Following this, the Au and CuPc were evaporated on the samples. During the evaporation of the CuPc, the Inficon monitors the rate of the evaporation. At some times during the evaporation, the rate became negative. When this happened, the substrate shutter was closed and opened when the rate was stable again. Nevertheless, this may have resulted in a different thickness than was expected. However, although the thickness may differ from the expected values, the thicknesses were the same on each substrate.
Subsequent to the thermal deposition, the absorption was measured. The absorption of the OTS samples was less than the glass samples. The absorption of the MPTS samples was mostly less than the absorption of the glass samples, except in the wavelength range of 600nm – 642nm. Looking at the absorption at the laser excitation of 785nm, the absorption of the glass is highest, followed by MPTS, and the least with OTS. The enhancement in Raman spectroscopy follows the same order, as expected. The highest enhancement was found on glass, followed by MPTS, and the least on OTS.
According to previous research, the enhancement is in order of 106, while the enhancement found in the current research is in order of 102. A probable explanation for this result is that the wavelength of the laser and the maximum absorption do not correspond.
In the wavelength range of 600nm – 642nm, the absorption has a peak on all the substrates and the MPTS absorption in this range is higher than the absorption of glass. A Raman spectroscope with a laser excitation in this range, and the possibility to measure the Anti-Stokes shift should be used to do another Raman measurement. It is expected that then the Raman enhancement is higher on every substrate, and the highest on the MPTS. Then this information can be used to use the enhancement to do temperature measurements, which later can be used in the research of optimizing solar cells and the destroying of cancer cells.
6 CONCLUSION
The findings of this thesis indicate that the Raman peaks are strongly enhanced in the presence of Au. When normalizing the peaks, it can be seen that the enhancement is wavelength dependent, as expected.
The OTS does not contribute to higher enhancement. On each peak the enhancement is less than the enhancement of the glass. The enhancement is 0.34 – 0.42 times the enhancement of the glass. The MPTS leads to higher enhancement (1.01 – 1.06 x) on three peaks (1528 cm -1, 1451 cm-1, and 1341 cm-1) and to less enhancement (0.8 – 0.97) on the other 4 peaks.
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8 APPENDIX
8.1 RAMAN
8.1.1 GLASS
Figure 21: The Raman spectra of glass with Au 0.8nm and CuPc 30nm.
Figure 22: The Raman spectra of glass with CuPc 50nm with 0/0.8/2.5nm Au.
500 1000 1500 0 500 1000 1500 2000 2500 3000 Rama n I nte nsity Ramanshift [cm-1] CuPc 30nm Au 0.8nm + CuPc 30nm 600 800 1000 1200 1400 1600 0 20000 40000 60000 80000 Rama n I nte nsity Ramanshift [cm-1] CuPc 50 nm Au 0.8nm + CuPc 50nm Au 2.5nm + CuPc 50nm
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8.1.2 OTS
Figure 23: The Raman spectra of OTS with Au 0.8nm and CuPc 30nm.
Figure 24: The Raman spectra of OTS with CuPc 50nm with 0/0.8/2.5nm Au.
500 1000 1500 0 500 1000 1500 2000 2500 3000 Rama n I nte nsity Ramanshift [cm-1] CuPc 30nm Au 0.8nm + CuPc 30nm 500 1000 1500 0 5000 10000 15000 20000 Rama n I nte nsity Ramanshift [cm-1] CuPc 50 nm Au 0.8nm + CuPc 50nm Au 2.5nm + CuPc 50nm
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8.1.3 MPTS
Figure 25: The Raman spectra of MPTS with Au 0.8nm and CuPc 30nm.
Figure 26: The Raman spectra of MPTS with CuPc 50nm with 0/0.8/2.5nm Au.
500 1000 1500 0 500 1000 1500 2000 2500 3000 Rama n I nte nsity Ramanshift [cm-1] CuPc 30nm Au 0.8nm + CuPc 30nm 500 1000 1500 0 20000 40000 60000 80000 100000 Rama n I nte nsity Ramanshift [cm-1] CuPc 50 nm Au 0.8nm + CuPc 50nm Au 2.5nm + CuPc 50nm
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8.2 ENHANCEMENT
8.2.1 CUPC 30NM
Wavenumber [cm-1] Glass Au 0.8nm + CuPc 30nm OTS Au 0.8nm + CuPc 30nm MPTS Au 0.8nm + CuPc 30nm 1528 1451 1341 1143 954 680 596 2.8 x 3.2 x 2.7 x 2.6 x 2.2 x 2.0 x 2.8 x 3.2 x 2.4 x 2.8 x 3.3 x 1.6 x 2.0 x 2.4 x 2.3 x 1.9 x 2.1 x 2.0 x 1.6 x 1.4 x 1.5 xTable 6: The enhancement of the samples with Au 0.8nm + CuPc 30nm.
