Charge Separation on Localized Surface Plasmon and Hot Carrier Transfer to Semiconductors

Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 2003

YOCEFU HATTORI

ACTA UNIVERSITATIS UPSALIENSIS ISSN 1651-6214 ISBN 978-91-513-1111-1 UPPSALA urn:nbn:se:uu:diva-430177 2021 Dissertation presented at Uppsala University to be publicly examined in Polhemsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, Friday, 26 February 2021 at 15:00 for the degree of Doctor of Philosophy. The examination will be conducted in English. Faculty examiner: Professor Stephan Link (Rice University, Houston, Texas).

Abstract Hattori, Y. 2021. Charge Separation on Localized Surface Plasmon and Hot Carrier Transfer to Semiconductors. Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 2003. 76 pp. Uppsala: Acta Universitatis Upsaliensis. ISBN 978-91-513-1111-1.

The relatively recent discovery that plasmonic nanoparticles generate energetic electron-hole pairs known as hot carriers has been the source of interest from many scientific groups. The capability to extract these short-lived hot carriers from metal nanoparticles (NPs) might potentially lead to applications in solar cells, photodetection, and photocatalysis. However, a better understanding of the hot carrier dynamics, starting from the formation process, is required. This thesis seeks to elucidate some aspects of charge formation, extraction, and hot carriers' recombination in plasmonic composite systems. First, two systems based on Ag and Au NPs were designed and studied to elucidate charge carriers' dynamics. The studies revealed that electrons and holes were effectively extracted and injected into suitable acceptors. Additionally, the electron injection and back transfer on TiO2 was significantly affected by the interface's status. The result motivated the following study that consisted of Au plasmonic NPs supported on different metal oxides, namely TiO2, ZnO, SnO2, and Al-ZnO (AZO). The electron dynamics on these systems were widely different. They could not be attributed solely to differences in the Schottky barrier height values, which suggested that interface status, electron bulk mobility, and oxide conduction band density of states are relevant factors to explain electron dynamics. The insertion of an insulator layer between the Au NPs and the metal oxides improved charge separation, which could be further explored to improve device efficiencies. In situ measurements on Au NPs/TiO2 samples were performed to investigate the effect of an increase of temperature in the range expected for device applications. This increase resulted in a higher number of electrons injected, which was attributed to the enhancement of plasmon decay by phonons. The last chapter investigates the change in the electron-phonon relaxation upon electron and hole injection, separately. Ab initio methods allowed theoretical investigation of this process and were used to predict the hole injection efficiency.

Keywords: Plasmonics, hot carrier, metal nanoparticles, semiconductors, ultrafast transient absorption spectroscopy

Yocefu Hattori, Department of Chemistry - Ångström, Physical Chemistry, Box 523, Uppsala University, SE-75120 Uppsala, Sweden.

ISSN 1651-6214 ISBN 978-91-513-1111-1 urn:nbn:se:uu:diva-430177 (http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-430177) To my mother, Miyako.

List of papers

This thesis is based on the following papers, which are referred to in the text by their Roman numerals.

I Light-induced Ultrafast Proton-coupled Electron Transfer Responsible for H2 Evolution on Silver Plasmonics Yocefu Hattori, Mohamed Abdellah, Igor Rocha, Mariia V. Pavliuk, Daniel L.A. Fernandes, Jacinto Sá* Materials Today, 2018, 21, 590-593.

II Simultaneous Hot Electron and Hole Injection upon Excitation of Gold Surface Plasmon Yocefu Hattori, Mohamed Abdellah, Jie Meng, Kaibo Zheng, Jacinto Sá* J. Phys. Chem. Lett., 2019, 10, 3140-3146.

III Role of the Metal Oxide Electron Acceptor on Au-plasmon Hot Carrier Dynamics and its Implication to Photocatalysis and Photovoltaics Yocefu Hattori, Sol A. Gutierrez, Jie Meng, Kaibo Zheng, Jacinto Sá* Manuscript submitted

IV Phonon-assisted Hot Electron Generation in Plasmonic Semiconductor Systems Yocefu Hattori, Jie Meng, Kaibo Zheng, Ageo Meier de Andrade, Jolla Kullgren, Peter Broqvist, Peter Nordlander, Jacinto Sá* Accepted Manuscript - Nano Lett., 2021.

V Ultrafast Hot-hole Injection Modiﬁes Hot-electron Dynamics in Au/p-GaN Heterostructures Giulia Tagliabue, Joseph S. DuChene, Mohamed Abdellah, Adela Habib, David J. Gosztola, Yocefu Hattori, Wen-Hui Cheng, Kaibo Zheng, Sophie E. Canton, Ravishankar Sundararaman, Jacinto Sá*, and Harry A. Atwater* Nature Materials, 2020, 19, 1312-1318.

Reprints were made with permission from the publishers. Papers not included in this thesis:

VI Nano-hybrid Plasmonic Photocatalyst for Hydrogen Production at 20% Efﬁciency Mariia V. Pavliuk , Arthur B. Fernandes , Mohamed Abdellah, Daniel L. Fernandes, Caroline O. Machado, Igor Rocha, Yocefu Hattori, Cristina Paun, Erick L. Bastos, Jacinto Sá* Scientifc Reports 2017, 7, 8670.

VII Hydrated Electron Generation by Excitation of Copper Localized Surface Plasmon Resonance Mariia Pavliuk, Sol Gutierrez, Yocefu Hattori, Maria E. Messing, Joanna Czapla-Masztaﬁak, Jakub Szlachetko, Jose L. Silva, Carlos Moyses Araujo, Daniel L. A. Fernandes*, Li Lu, Christopher J. Kiely, Mohamed Abdel- lah*, Peter Nordlander, and Jacinto Sá* J. Phys. Chem. Lett. 2019, 10, 8, 1743-1749.

VIII Direct Observation of a Plasmon-Induced Hot Electron Flow in a Multimetallic Nanostructure Lars van Turnhout, Yocefu Hattori, Jie Meng, Kaibo Zheng, and Jacinto Sá* Nano Lett. 2020, 20, 11, 8220-8228. Contribution Report:

I - Prepared and carried out most of the characterization of the samples; performed the transient absorption measurements along with Mohamed Abdellah; supported in the revising process.

II - Prepared and carried out most of the characterization of the samples; performed all the transient absorption measurements; analyzed and interpreted the results; wrote the manuscript with the support from Jacinto Sa.

III - Planned the work; prepared and carried out most of the characterization of the samples; performed all the transient absorption measurements; analyzed and interpreted the results; wrote the manuscript.

IV - Planned the work along with Jacinto Sa; prepared all the samples and mount the setup for the in situ measurements; performed all the transient absorption measurements; analyzed and interpreted the experimental results; wrote the manuscript with the support from Jacinto Sa and co- authors.

V - Performed the transient absorption measurements along with Mohamed Abdellah.

Contents

1 Introduction ...... 11 1.1 Plasmonic Hot Carriers ...... 12 1.2 Challenges ...... 13 1.3 Aims and Scope ...... 14

2 Theory ...... 16 2.1 Permittivity ...... 18 2.2 Light-matter Interaction ...... 19 2.3 Bulk Plasmons and the Dielectric Function of metals ...... 20 2.3.1 The Damping Factor (γ) ...... 22 2.4 Localized Surface Plasmon Resonance ...... 24 2.5 Hot Carrier Generation and Relaxation Dynamics ...... 26 2.6 Schottky Barrier ...... 29

3 Materials and Methods ...... 31 3.1 Synthesis of Metal Nanoparticles ...... 31 3.1.1 Bottom-up Method ...... 31 3.1.2 Top-down Method ...... 32 3.2 Semiconductors ...... 32 3.3 Transient (NUV-NIR/mid-IR) Absorption Spectroscopy ...... 34 3.3.1 TAS on Plasmonic NPs ...... 35 3.3.2 TIRAS on Plasmonic NPs / Semiconductor ...... 36

4 Hot Carriers Injection (Papers I and II) ...... 38 4.1 Introduction ...... 38 4.2 AgNP-pABA-TiO2 in IPA (Paper I) ...... 38 4.2.1 The Effect of the Molecular Linker and Capping Ligand ...... 40 4.3 PEDOT:PSS / Au NPs / TiO2 (Paper II) ...... 41 4.4 Conclusions ...... 43 5 Au NPs / Semiconductor Composites: a Comparative Study (Paper III) ...... 44 5.1 Introduction ...... 44 5.2 Results ...... 44 5.2.1 TIRAS: Rise Component ...... 45 5.2.2 TIRAS: Decay Dynamics ...... 47 5.3 Conclusions ...... 48 6 The Effect of Temperature on Hot Carrier Transfer (Paper IV) ...... 49 6.1 Introduction ...... 49 6.2 Results ...... 49 6.2.1 TIRAS ...... 51 6.3 Conclusions ...... 53

7 Electron-phonon Dynamics (Papers III and V) ...... 54 7.1 Results ...... 54 7.1.1 Electron-phonon Dynamics Upon Hot Electron Injection ...... 54 7.1.2 Electron-phonon Dynamics Upon Hot Hole Injection .. 56 7.2 Conclusions ...... 59

8 Concluding Remarks ...... 60 8.0.1 Outlook ...... 61

Popular Science Summary ...... 63

Svensk Sammanfattning ...... 65

Acknowledgments ...... 67

References ...... 69 1. Introduction

Long time before scientists have started studying the optical properties of metal nanoparticles, artists were using gold and silver nanoparticles to make red-colored glasses. The first milestone in the history of gold ruby glass is a Roman opaque glass cup dated to the fourth century, the Lycurgus cup, which is exhibited at the British Museum in Lon- don. The carved decoration depicts a mythological scene that is the tri- Figure 1.1. Lycurgus cup, fourth century umph of Dionysus over Lycurgus, a CE, illuminated from inside (left) and out- king of the Thracians (ca. 800 BCE). side (right). Later studies on the Lycurgus cup revealed the presence of silver-gold alloy nanoparticles of 50-100 nm in diameter, which gives the green coloration when shining light from the outside and red when illuminated from inside the cup. Despite the long history of application, although only applied for artistic purposes, the field of plasmonics only emerged in 1990, becoming a promising domain in science and technology. The research in plasmonics stems from exploiting the functionalities of certain metal nanostructures that can concentrate incoming light flux to volumes much smaller than the diffraction limit. This outstanding phenomenon is a consequence of partially coherent oscillations of free electrons, denominated as surface plasmons, in a metal nanoparticle driven by the external electromagnetic waves commonly referred to as localized surface plasmon resonance (LSPR) or localized surface plasmon polariton (SPP) resonance. The excitation of surface plasmons results in a strong enhancement of the electric field in the nanostructure vicinity, which also is sensitive to the structure morphology and properties of the local environment. The local strong enhancement of the electric field is one of the key points in plasmonics that led to the discovery of surface-enhanced Raman spectroscopy (SERS) technique in 1973 by Mar- tin Fleischmann [1], which allows molecules to undergo much higher scattering efficiencies when adsorbed on metal colloidal nanoparticles or rough metal surfaces. Later, similar techniques that exploit this plasmonic property were also developed, such as surface-enhanced infrared spectroscopy [2], surface-enhanced fluorescence [3] and surface-enhanced hyper Raman scattering (SEHRS) [4]. Conversely, the sensitivity of the LSPR spectral peak profile

11 and position with the local environment resulted in using metallic nanostructures as optical sensors, also known as plasmon-enhanced optical sensors [5]. For instance, functionalized Au nanoparticles have been used in colorimetric detection of heavy metals, biological small molecules and biomacromolecules [6–9]. Another important property is related to the process following the surface plasmon excitation in metal nanoparticles, in which collective oscillation of electrons eventually dephases, thermalize and transfer their energy to the lattice, thus generating local heat [10]. This inevitable process on plasmonics is being applied on photothermal cancer therapy which involves the intra- venous or intratumoral injection to introduce gold nanoparticles to cancerous cells and the subsequent exposure to heat-generating near-infrared light [11]. These examples already illustrate the broad range of applications provided by the surface plasmons which extends even further in non-linear optics [12], photodetection [13] and solar energy harvesting [14]. Even more exciting is the discovery of novel phenomena in quantum plasmonics [15, 16].

1.1 Plasmonic Hot Carriers The rapid expansion of the applications provided by plasmonics eventually reached the domain of dielectric- or semiconductor-based optics and photonic technologies [17]. However, it did not take long until the expected revolution in communication components, such as plasmonic waveguides, resonators and other functional circuit elements, became dampened by the hard reality of fast decay and energy dissipation of SPP. For instance, the fast energy losses reduce the signal propagation in plasmonic waveguides and lead to the distortion of ultrafast pulses [18]. While research aimed at suppressing loss mechanisms is still pursued [19], another research direction emerged that stem from har- nessing rather than fighting material dissipative losses. In other words, losses in plasmonics also provide unique opportunities. A relevant one, which is related to the main content of this thesis, is the utilization of highly energetic charges (electrons and holes) that are generated when SPP decays. Following light excitation, the SPP decay transferring the energy to form energetic electron-hole pairs in the femtosecond timescale known as hot carriers, which can have enough energy to be collected by semiconductors in contact or transferred to adsorbed molecules. The significant experimental effort in plasmonic hot-driven processes and devices has been the focus of several reviews [20, 21]. Indeed, the recent discovery that metal nanoparticles can also generate hot carriers upon light excitation is seen as a breakthrough in the field of plasmonics due to their well-known extraordinary optical properties. Nevertheless, despite all the excitement, there are still several challenges that hamper the theoretical understanding of the microscopic mechanisms underlying the process of hot carrier generation and their utilization.

12 1.2 Challenges In this section, I would like to highlight and comment my personal opinions on the main current challenges that hinder the understanding and development of plasmonic hot carrier based devices. It is important to mention that the points listed below might be incomplete and matter of debate. Nevertheless, it can hopefully shed some light on the situation of the current stage of this ﬁeld.

Time scale. SPP decay happen in few femtoseconds and hot carrier life time is commensurate with the decay event. The ultrafast nature of these events put a big obstacle for experimentalists, since typical laser pump-probe spectroscopy techniques have temporal resolution longer than 10 fs. In the future, an attosecond or single-cycle probing pulse could reveal the plasmons excitation and de-excitation process. In addition, the energy distribution of the initial hot carriers for different excitation energies might be ﬁnally quantiﬁed.

Plasmons are fundamentally quantum mechanical. The optical response of metal nanoparticles can be well described by classical electromagnetic theory. However, the dynamics triggered by light excitation of plasmons need to be treated in the quantum framework. In semiconductors, the properties can be readily predicted using ab initio methods since it only requires the calculation of the structure unit-cell under periodic boundary condition. For nanoparticles, the calculations becomes computationally very expensive. To put in perspective, the simulation of a nanoparticle with 4 nm diameter contain around 1500 atoms which would require the computation of more than 16000 electrons, which is unprecedentedly large. Nevertheless simulation of few nanometers particle is becoming feasible and can explicitly account for the effects of nanoparticle shape with speciﬁc facets and surface states on the optical response and carrier generation [22, 23].

Nanoparticle Shape. With the advances of ion-beam litography, which of- fers high resolution patterning, the fabrication of nanostructures with different shapes became possible. Moreover, there is an extensive list of bottom-up methods in the literature that takes the advantages of speciﬁc surface stabilizers to promote or suppress growth in speciﬁc crystal facets, allowing synthesis of nanoparticles with different shapes in a controlled way. The sharp edges of metal nanoparticles are favored to give rise to hot spots, which can enhance the generation of hot carriers due to increase in the Landau damping. Unfor- tunatelly, the instability of nanoparticles increases with asymmetry due to the higher surface energy and reactivity. Therefore, asymmetrical particles are always prone to change their shape to quasi-spherical shape with time since it

13 possess the lower surface energy between all particle shapes. This process can be even further accelerated by light excitation and charge transfer process.

Beyond noble metals. Gold and silver are almost exclusively employed in hot carrier plasmonic devices due to their chemical stability and well studied properties. But due to cost, they are not considered suitable for wide applications. Nevertheless, copper and aluminum are alternative much cheaper materials with plasmonic behavior that have been pursued over the last years [24, 25]. In addition, certain nonmetallic materials, such as transition metal nitrides, transition metal carbides, and metal oxides have shown to display dielectric functions that are requisite for plasmonic behavior. Although research on nonmetallic materials for hot carrier generation is at an early stage, recent progress have shown that nonmetallic materials can be used for plasmonic photoelectric and photothermal conversions [26].

Hot carriers or just heating? In plasmon-assisted photocatalysis, it is assumed that hot carriers tunnel out of the metal into orbitals of the surrounding molecules and then catalyse the chemical reaction, where thermal effects are considered negligible. This picture has been contested by the work of Dubi et al [27] published in 2020, where is argued that what appears to be photocatalysis is much more likely thermo-catalysis. In their previous paper [28], they have developed a theory that takes into account all channels of energy ﬂow in the electronic system and revisited the main papers in the ﬁeld, showing that it can be used to explain the experimental data observed in those publications. This debate highlights the complexity of events that are triggered by plasmon excitation and might make it prone to different mechanistical interpretations.

1.3 Aims and Scope The field of plasmonics is relatively young, and even more so is the recent interest in plasmonic hot carriers. As such, several open questions has yet to be clarified, which are mainly related to the challenges aforementioned. The absence of a band gap restricts the lifetime of the electron-hole pair generated through plasmon decay to only about a few femtoseconds, which is at least one million times shorter than electron-hole pairs in semiconductors like silicon. This known hard fact along with the complex and incomplete understanding of the microscopic mechanisms underlying different plasmon dephasings make the prediction of the prospects and limitations of plasmonic hot carriers devices difficult. This thesis attempts to address and elucidate the process of generation and extraction of the hot carriers. Chapter 2 introduces the classical theory to describe optical properties of localized surface plasmon along with a conceptual description of plasmon dynamics processes. Chapter 3 briefly describes the

14 sample preparation and characterization methods used. The following chap- ters are dedicated to the results and discussions related to the papers attached to this thesis. In chapter 4, the process of both electron and hole injection from silver and gold nanoparticles is investigated using different hole accepting materials and in different physical states (liquid and solid). The next chapter explores the interface properties that dictate injection efﬁciency and electron recombination by using different metal oxides. Despite the existence of a potential barrier (Schottky barrier) between the metal and the semiconductor, the recombination process was shown to depend on other properties, of which the electron bulk mobility was suggested to also play an important role. Chapter 6 was focused on the indirect investigation of plasmon decays by enhancing one of these mechanisms by increasing the temperature. Thus, the rate of plasmon decay through electron-phonon scattering is also increased and the effect on hot electron injection was investigated. This study relevance also stems from the fact that heat generation in plasmonics is an inevitable event and might be naturally part of plasmonic device conditions. In the last chapter, the electron-phonon process that predominantly occurs following the hot electron thermalization, is brought up to discuss its dynamics change upon electron and hole injection. Moreover, this was revelead to be a potential methodology to theoretically obtain the charge injection efﬁciency values.

15 2. Theory

The majority of materials that possess plasmonic properties are metals and they are characterized by their quasi-free electrons, i.e., weakly interacting electrons with the nucleus that can move through the crystalline structure of the solid. These free electrons are also called electron gas and they are responsible for the main properties of metals: high conductivity and reﬂectivity. This is the opposite of insulating materials where electrons can only slightly shift from their average equilibrium position. In 1953, Pines and Bohm [29] published a paper about their studies involving the collective behavior of electrons in a dense electron gas to explain the energy losses of electrons passing through metal foils. In their theoretical work, it was found that the electron gas displays both individual particle and collective aspects. The latter component includes the effect of the long-range Coulomb force, which leads to the simultaneous interaction of many particles, resulting in an organized oscillation of the system as a whole denominated the plasma oscillation. The quantization of the plasma oscillation is referred to as plasmon or bulk plasmon, in the same way phonons are described as the quantum of a collective mechanical vibration arising in a solid lattice. Rufus Ritchie [30] extended the work by Pines and Bohm to include the interaction of plasma oscillations at the surface of metals where the term surface plasmon was ﬁrst used. In other words, when a bulk metal is terminated by a surface, new plasmons arise that are strongly localized to the surface. When an electromagnetic wave travels along with a metal-dielectric interface a surface plasmon polariton (SPP) is formed, where the term polariton is used to indicate that a plasmon is coupled with the electromagnetic wave. The main subject of this thesis involves the investigation of metal nanoparticles that can be categorized in the third subset of plasmons, known as localized surface plasmon. If a macroscopic metal particle is subject to light no unique physical phenomena occur. However, if the piece of metal is reduced to the nanoscale dimensions, the resulting metallic nanoparticles can start resonating with the electromagnetic wave becoming a powerful source of optical material in the nanoscale dimension. This striking effect gives rise to a drastic alteration in the incident radiation, increasing their optical cross-section by few orders of magnitude in respect to the nanoparticle size. The resonating property of metal nanoparticles with light is commonly referred to as localized surface plasmon resonance (LSPR) or localized SPP resonance. The key aspect of LSPR, as the name suggests, is the resonating property. It is widely known experimentally that gold and silver nanoparticles exhibit

16 this behavior in the visible light range, which are the result of two conditions being simultaneously satisfied: I The permittivity (ε) of the material is negative. II The electromagnetic wavelength (λ) is large in comparison with the nanoparticle dimensions (d), i.e., λ d. The permittivity is a measure of the electric polarizability of the medium. Therefore, the higher its value, the larger will be the induced electric dipole. Light can propagate in materials with positive permittivity, albeit the electric field is decreased. However, macroscopic materials with negative permittivity do not allow the electromagnetic waves to travel deep from the surface and are scattered, i.e., they are absorbed and reemitted back. This is the case for silver, gold, and some other metals where the negative permittivity extends from the ultraviolet to the infrared frequency and is what gives their known reflective property. The second condition of the smallness of particle dimensions compared to the incident light wavelength allows all the electrons to move with the same phase (figure 2.1). If the particle size is commensurate with the wavelength of light, some of the electrons will move in opposite directions, and the collective behavior would be lost. In addition, this condition permits the existence of an almost uniform electric field inside the particle for λ in the visible frequency since d would be lower than the penetration depth of the incident electromagnetic wave. This is directly related to the ability of metal nanoparticles to absorb light.

Figure 2.1. Illustration of a nanoparticle interacting with an electromagnetic wave where λ d. Once an electromagnetic wave impinges on a particle that fulﬁlls these two requirements, the electrons will start oscillating collectively. A maximum amplitude can be achieved for a speciﬁc wavelength referred to as resonance frequency, which occurs in the range where the permittivity of certain metals has negative values.

17 The description mentioned above, although simplistic, grasp fundamental aspects of resonance in plasmonic materials that results in electric ﬁeld enhancement in the nanoparticle vicinity. The following sections in this chapter will be dedicated to explaining the development of the theories that allowed the mathematical description and understanding of plasmonic nanoparticles’ optical response. Section 2.5 will discuss the different mechanisms that can lead to the formation of hot carriers and the dynamics that are triggered upon light excitation. In the last one, a basic theory of metal-semiconductor interface will be introduced.

2.1 Permittivity Dielectric constant, dielectric function, relative permittivity and permittivity are terms that are often seen when studying the optical response of materials but they can easily lead to confusions and misuses. This section has the aim to clarify these concepts since they will be used on the following ones. It was previously stated that the permittivity is a measure of the ability of a material to be polarized by an electric ﬁeld, which is represented by the greek letter ε and the unit is given by F·m−1 (farads per meter). Some textbooks also use the term absolute permittivity or dielectric permittivity but are often just called permittivity. Nevertheless, permittivity is not a quantity but a function that depends on the frequency. Naturally, it also depends on the region of the material, direction and intensity of the incident ﬁeld, and other parameters, but here the simple linear, homogeneous and isotropic case is assumed. Besides, the permittivity is usually represented by the relative permittivity (εr) which is the ratio between the permittivity of the material or medium (ε) and the −12 −1 vacuum permittivity (ε0 ≈ 8.85 × 10 F·m ): ε(ω) εr(ω)= (2.1) ε0 The relative permittivity is also referred to as dielectric function, perhaps to emphasize the dependence with frequency. For εr(0), which is the electrostatic case, the value is denominated dielectric constant or static relative permittivity. Di- electric constant and static relative permittivity are often used terms in the study and design of capacitors since they operate in the low-frequency regime Figure 2.2. Conceptual illustration of (ω → 0). intraband and interband transition in a In the high frequency or optical fre- solid which contribute to ε2. quency regime, the permittivity is repre-

18 sented by a complex function: εr(ω)=ε1(ω)+iε2(ω). The imaginary part (ε2) is related to the ability of a material to absorb electromagnetic energy. In the case of solids, the imaginary part (ε2) is proportional to the probability that a photon can be absorbed to promote an electron to higher energy by intraband or interband transition as is illustrated in the ﬁgure 2.2. The permittivity function or dielectric function of a solid is intimately connected to the band structure and hence, is of extreme importance to describe its optical properties.

2.2 Light-matter Interaction In the work published by James Clerk Maxwell in 1864 "A Dynamical The- ory of the Electromagnetic Field" [31], he stated the following: "The agreement of the results seems to show that light and magnetism are affections of the same substance, and that light is an electromagnetic disturbance propagated through the field according to electromagnetic laws". Back then, Maxwell was following the path to connect all the known electromagnetic laws in a set of twenty equations. It was Oliver Heaviside, an autodidactic engineer, mathe- matician and physicist who borrowed vector calculus notation from fluid me- chanics and condensed the twenty equations in four partial differential equations. This turning point in the classical physics that allowed to describe the interaction between electromagnetic fields and materials which is given by the following equations:

∇ ·D = ρ ∇ ·B = 0 ∂B ∇ ×E = − (2.2) ∂t ∂D ∇ × H = + J ∂t The set of four equations connect the macroscopic fields, i.e., dielectric displacement D, electric field E, magnetic field H and magnetic induction B with the free charge density ρ and current density J. There are additionally two relations that describe how the electromagnetic field interact with matter, denominated as constitutive relations. For dielectric materials the expression is:

D = ε0εr(ω)E (2.3) where εr is the relative permittivity function or dielectric function as was discussed in the previous section. The applied electric ﬁeld (E) induces charge polarization (P) in the medium, which is expressed by P = ε0χE E, where χE

19 is the electron susceptibility and is related to εr through εr(ω)=1 + χE (ω). This allows us to rewrite equation 2.3 as: D = ε0E + P (2.4) One of the main breakthroughs of Maxwell was to show that electromagnetic ﬁelds could propagate as traveling waves by assembling all the four equations listed above. The incident monochromatic plane wave propagating in a solid with the speed of light (c) is:

ω2 ∇2E + ε (ω)E = 0 (2.5) c2 r The solution of the above equation assuming the wave propagating in the z- direction is given by: ε (ω) E(z,t)=E exp iω r z −t (2.6) 0 c where a negative εr leads to an imaginary quantity. This is mathematically facilitated by writingn ˜(ω)=n(ω)+iκ(ω) and deﬁning the following:

2 2 εr(ω)=n˜(ω) → ε1(ω)+iε2(ω)=(n(ω)+iκ(ω)) (2.7) yielding the important relations:

ε (ω)=n(ω)2 − κ(ω)2 1 (2.8) ε2(ω)=2n(ω)κ(ω) The n is the well-known refractive index and κ accounts for attenuation of the wave inside the solid. The intensity of the electric field being proportional to |E|2, an exponential decay of the intensity inside the medium is obtained I(z)=I0exp(−αz), with α = 2ωκ/c its absorption coefficient. It is interesting to point out that indeed is only the imaginary part (κ) that leads to a decrease in the light intensity. The parameters n and κ are physical observables which are carried by the electromagnetic waves. This implies that they can be obtained through re- flectivity measurements, and hence be used to calculate the dielectric function through the equation 2.8. This was relevant for the development of theoretical models describing the electronic structure of a solid, since confirmation through experimental results became possible.

2.3 Bulk Plasmons and the Dielectric Function of metals The Drude model is a simple but powerful model to describe the dielectric response of a metal. In this model, the conduction electrons behave as a gas of free, non-interacting electrons, which relax through electron-electron

20 and electron-ion scattering being introduced in a phenomenological way as a scattering with characteristic time (τ, where typically τ ≈ 10-14 s at room temperature), or its counterpart γ = 1/τ known as the damping factor. De- spite its simplicity, the Drude model corresponds well with the experimental results, particularly at lower frequencies. The model consist in describing the motion of electrons with an effective mass (m∗) subject to an electric ﬁeld E(t)=Eoexp(−iωt) and is written by using Newton’s second law:

dr2 dr m∗ = −γm∗ − eE(t) (2.9) dt2 dt with the solution: e r(t)= E(t) (2.10) m∗(ω2 + iγω) The displacement of the electrons (r(t)) induced by the electric ﬁeld E generates a macroscopic polarization P = −ner for a given electron density n. Inserting the equation 2.10 in the polarization expression (P) and substituting in the equation 2.4 one obtains:

ω2 D = ε 1 − p E(t) 0 ω2 + iγω ω2 ω2τ2 =⇒ εD = 1 − p = 1 − p (2.11) ω2 + iγω ω2τ2 + iωτ 2 ∗ with ωp = ne /(ε0m ) being the bulk plasma frequency. Assuming ω γ (valid at the optical frequency range) and isolating the real and imaginary part allows to obtain the following expressions:

ω2 ω2 εD(ω)=1 − p ≈ 1 − p (2.12) 1 ω2 + γ2 ω2 ω2γ ω2 εD(ω)= p ≈ p γ (2.13) 2 ω(ω2 + γ2) ω3

The expression 2.12 and 2.13 correspond to the real and imaginary part of the dielectric function obtained from the Drude model, respectively. Note that εD ω < ω 1 becomes negative when p, where the electromagnetic wave is rapidly screened and reﬂected. The wave penetration distance is described by the skin depth, which is the distance a wave travels until the initial amplitude decays by factor e−1. In the case of light with λ = 500 nm impinging on a gold surface, the skin depth is approximately 20 nm. This implies that metals can be used as mirrors in the frequency range below the bulk plasma frequency (ωp)ifthe

21 Figure 2.3. a) Imaginary and b) real part of the dielectric function ﬁtted using the Drude model, where the interband transition are not accounted.

ω > ω εD thickness is much greater than the skin depth. In contrast, for p the 1 is positive and the metal becomes transparent. In most metals, the bulk plasma frequency (ωp) is in the ultraviolet regime with energies within 5-15 eV, depending on the metal band structure. The figure 2.3 shows the example for Au, where the experimental data [32] was fitted using the equations 2.12 and 2.13. From the fitting, the values for the bulk plasma frequency and the damping factor can be obtained, where h¯ωp = 8.97 eV and h¯γ = 66 meV. Naturally, the Drude model does not account for interband transitions, which cause the well-known yellow color in gold materials. 2.3.1 The Damping Factor (γ) The photon absorption by the conduction electrons are described by the imaginary part of εD that is proportional to the damping factor (γ)asisshown in the equation 2.13. Any intraband transition of the conduction electrons require change in momentum. Since the photon momentum in the optical frequency range is negligibly small, the absorption of a photon by an electron has to be assisted by a third particle (such as an auxiliary electron, phonons, defects) in order to satisfy energy and momentum conservation. This is determined by the factor γ, which can be splitted in mainly three contributions if electron-defect scattering is neglected:

γ(ω,T)=γe−ph(ω,T)+γe−e(ω,T)+γe−S(d,vF ) (2.14) In the expression above ω is the photon frequency and T is the temperature. A detailed description and deduction of the theoretical model that resulted in the expression for the terms γe−ph and γe−e are beyond the scope of this thesis. Nevertheless, it is worth to emphasize that they are expressions validated through experimental studies [33, 34]. The relevant point here is to explicitly present their equations and understand their dependence with temperature and frequency.

22 The expression for the temperature dependence of the collision process between electrons (γe−e) is obtained by employing Born approximation and the Thomas-Fermi screening of Coulomb interaction [35, 36]:

πΔΓ 2 2 hc γe−e = (kBT) + (2.15) 12hEF λ where Γ = 0.55, Δ = 0.77 and EF = 5.5 eV are the average scattering probability over the Fermi surface, the fractional Umklapp scattering and the Fermi energy of free electrons. Holstein [37, 38] derived the expression for electron- phonon (e-ph) scattering by assuming free electrons without Umklapp collisions and single Debye model phonon spectrum:

5 θ 4 2 4T T z γ − = Γ + dz (2.16) e ph 0 θ 5 z − 5 D 0 e 1

For Au, θD = 170 K is the Debye’s temperature and Γ0 = 0.07 eV, which can obtained by ﬁtting Au bulk permittivity at frequencies below the interband transition onset. The ﬁgure 2.4 depicts the γe−e and γe−ph in function of temperature. It is clear that while γe−e is frequency dependent, its change with temperature is negligible at optical region, i.e., near UV to near IR range. The scattering rate plays a fundamental role in the direct current (DC) regime (ω = 0), but it is about three order of magnitude lower than the frequency dependent counterpart. In the other hand, γe−ph can increase the rate by three times from the room temperature to 750 K (pre-melting point of Au).

Figure 2.4. Temperature dependence of the rate for electron-electron (γe−e) and electron-phonon (γe−ph) scatterings.

23 The last term in the equation 2.14 arises from the conﬁnement effect that is present when particles are smaller than the electron mean free path ( below 30 nm for noble metals) and thus depend on the particle size. The phenomenological expression was initially considered by Kreibg and Vollmer [39] and for spherical nanoparticle has the following relation:

vF γ − (d,v ) ∝ (2.17) e S F d where d is the diameter and vF is the Fermi velocity. Classically, this term describes the absorption of a photon by conduction electrons assisted by electron- surface collisions in order to ensure momentum conservation. In the quantum picture, this process is known as Landau damping and, naturaly, is described very differently. This phenomenum is a process of energy transfer between free electrons and electromagnetic waves occurring when their velocities are matched within a certain range. This implies that the wavevector of the electromagnetic field has to be much larger than the free space wavevector. While propagating SPP cannot have such a large wavevector along its direction of propagation, the lateral confinment allows the presence of large wavectors. In short, Landau damping effectively represents a plasmon-electron scattering process, in which a plasmon wave loses a single quantum to generate an electron-hole pair that scales with the confinement length. The last process leading to photon absorption, that is not present in the equation 2.14, is the interband transition from low energy bands (d bands) to bands of higher energy that is accounted by adding the term εib in the dielectric function. Rosei et al. derived a model in order to calculate the interband εib component of the imaginary dielectric function 2 for incoming photon with energy close to the interband transition threshold for Ag [40] and Au [41]. The derived expression contain factors that represents the possible transition prob- abilities, e.g. d → p, p→s, that are empirically obtained by simply fitting it to the experimentally obtained dielectric functions. The Ag interband transition threshold is ca. 3.95 eV and for Au is ca 2.4 eV [32].

2.4 Localized Surface Plasmon Resonance In the beginning of this chapter, a qualitative discussion about the physical behavior of LSPR was brieﬂy introduced. Naturally, for a further understanding it is necessary to treat mathematically the interaction of metal nanoparticles with an electromagnetic wave in order to arrive at the resonance condition. The Mie theory [42] is a theory that describes the absorption and scattering of plane electromagnetic waves by spherical particles of any size which are in a uniform and isotropic dielectric medium. This theory was named after the german physicist Gustav Mie and was developed in order to understand the

24 color of colloidal Au NPs in a solution. This theory expands the electromagnetic fields in multipole contributions and the expansion coefficients are found by applying the boundary conditions for the fields at the interface between the particle and the surroundings. The mathematical derivation of the fields can become extraordinarily long for big particles ( λ d ), however, for small particles ( λ d ) it is sufficient to consider the first term of the multipole expansion, which is dipolar. This is known as the dipolar approximation, quasistatic or Rayleigh limit. In this regime, the electronic polarization is in phase with the excitation field, so that one can calculate the spatial field distribution by assuming the simplified problem of a particle in an electrostatic field. In other words, one needs to find the Laplace equation’s solution for the potential ∇2Φ = 0, which can allow the calculation of the electric field E = −∇Φ. The optical response of the metal nanoparticle can thus be approximated as a simple dipole in the quasistatic approximation:

p(ω)=ε0εmα(ω)E(ω) (2.18) where εm is the static relative permittivity or dielectric constant of the medium surrounding the metal and α is the linear polarizability, which its expression for a particle with volume VNP is given by [39]:

εr(ω) − εm α = 3VNP (2.19) εr(ω)+2εm

It is apparent that α experiences a resonance enhacement when |εr(ω)+2εm| is a minimum, which for the case of small or slowly varying ε2 (valid for noble metals) around the resonance simpliﬁes to:

ε1(ω)=−2εm (2.20) This condition is the Fröhlich condition that leads to the following expression for surface plasmon frequency (ωsp):

ω2 ω = p − γ2 sp ib1 (2.21) 1 + 2εm + ε where εib1 is the real part of the contribution of interband transitions. From α, the scattering and extinction cross-sections (σext = σabs + σsca) can be calculated [39]:

ω 4 ε2 ω4 2 ε2 ε (ω) − ε σ = m |α(ω)|2 = 3 VNP m r m sca 4 (2.22) c 6π 2πc εr(ω)+2εm

25 ω √ / ω ε (ω) σ = ε Im[α(ω)] = 9V ε3 2 2 (2.23) ext m NP m [ε (ω)+ ε ]2 + ε2(ω) c c 1 2 m 2 Two important points can be noticed from the previous equations. First, the scattering cross-section (σsca) depends quadratically with the volume while σext has a linear dependency. The ratio between both leads to: σ d 3 sca ∝ (2.24) σext λ Therefore, as the particle size decreases considerably in respect to the incoming photon (d λ), the scattering component becomes negligible and the extinction cross-section is dominated by the absorption component (σext ≈ σabs, ﬁgure 2.5a). Second, σext and σsca depend on εm, implying that the light absorption cross-section can be enhanced when the nanoparticles are immersed in a medium with high dielectric constant or refractive index (ﬁgure 2.5b). Moreover, it also shifts the resonance frequency of the surface plasmon to lower frequency according to equation 2.21.

Figure 2.5. a) σsca /σext in function of spherical nanoparticle diameter for different incident wavelengths. b) Theoretical σext of Au nanoparticles (Au NPs) in different environments.

2.5 Hot Carrier Generation and Relaxation Dynamics Light excitation with wavelength near the LSPR peak can result in the cre- ation of electron-hole pairs. This process starts with the plasmon excitation −1 which decays within its lifetime τSPP ≈ γ , where γ contains the contributions from three different processes: e-ph scattering, e-e scattering and surface collision-assisted decay (or Landau damping) as was discussed in the section 2.3.1. Naturally, interband transitions also can take place which is taken into

26 Figure 2.6. Mechanisms of electron-hole pair generation in metals. a) Direct vertical interband transition. b) Phonon (or impurity) assisted transition. c) e-e Umklapp scattering assisted transition. d) Landau damping or surface collision assisted transition. account by adding the εib term in the dielectric function. The description of these four mechanisms is illustrated in the figure 2.6. Note that the bands shown in this figure does not represent real band structures of Ag or Au, but they can be found in other publications [43, 44]. The interband transition is a vertical transition of the electron from one band to another (figure 2.6a). In the cases of Au and Ag, it is dominated by the d → p transitions (not shown) that results in a hole with high effective mass (low mobility) and electron with low energy with respect to the Fermi level. All other mechanisms are intraband transitions, i.e., they involve absorption between two states with different wavevectors (momentum) within the same band that needs to be somehow compensated. In the mechanism illustrated in the figure 2.6b, the momentum mismatch is provided by phonons with momentum q. The third mechanism (figure 2.6c), involves electrons undergoing scattering where two electrons and two holes share the energy of the decayed SPP, i.e., E1 +E2 + h¯ω =E1’ +E2’. Since this is a elastic scattering the momentum of electrons is also conserved k1 + k2 = k1’+k2’+G, where G is the reciprocal lattice vector. The last mechanism (figure 2.6d) is classically referred to surface collision- assisted decay. This happens when an electron collides with the surface and the momentum is transferred with the entire metal lattice. In the quantum

27 pictures this process is known as Landau damping and for particles smaller than the mean free path is the most favorable mechanism of carrier generation for their ejection from the metal [45].

Figure 2.7. Conventionally assumed picture of hot carrier generation and relaxation in metal nanoparticles. a) Plasmon is excited while the carriers are distributed according to the Fermi-Dirac statistics in equilibrium with the lattice at temperature TL0.b) Plasmon decay leads to the formation of electron-hole pairs with energies ranging from EF -hω to EF +hω. c) The electrons thermalize after several e-e scatterings reaching once again a Fermi-Dirac distribution but with Te > TL0. d) After the electron- phonon relaxation time (τe−ph), the electron and the lattice are at equilibrium with a new lattice temperature TL1 > TL0. This temperature will eventually decrease by heating the nanoparticle environment.

The relaxation dynamics of photoexcited metals (films and nanoparticles) has been extensively studied using time-resolved spectroscopy techniques in the last decades and can be roughly represented as is in the figure 2.7. The time for plasmon decay depends on the mechanism involved, nevertheless, γ is typically ≈ 10−14 s−1 which implies that the plasmon decoherence takes place in few fs that results in the formation of hot carriers (fig.2.7b). The highly energetic electrons quickly relaxes through e-e scattering (≈ 10 fs [46]) estab- lishing a Fermi-Dirac distribution with temperature Te TL0 (fig. 2.7c) within few hundred of fs. Then, electrons transfer their energy to the lattice through e-ph scattering with a characteristic time τe−ph ≈ 1 ps that is a couple of orders of magnitude longer than τe−e. The following process involves phonon- phonon scattering within the metal until it reaches a new lattice temperature TL1 in the ps to ns time scale. This temperature will eventually decrease back to the equilibrium lattice temperature TL0 as it releases thermal energy to the environment.

28 2.6 Schottky Barrier The Schottky barrier refers to the potential barrier height formed when a metal and a semiconductor with different Fermi energy levels (EF) are put in contact.

Figure 2.8. Energy diagram of metal and n-type semiconductor before contact (a) and after contact for a n-type semiconductor (b) and p-type semiconductor (c).

The figure 2.8a shows the energy band diagram for the case where the metal EF is lower than the n-type semiconductor. The work function is defined as the energy difference between the vacuum level Evac and EF, while the electron affinity (χ) is the change in energy when moving an electron from Evac to the conduction band. When both materials with different ΦM and Φsc are put in contact, charge transfer between metal and semiconductor will occur until EF is aligned across the interface, resulting in an electric field at the interface. For instance, electrons will flow from the semiconductor to the metal if ΦM > Φsc (figure 2.8b) simply because available energy states for electrons in the metal are of lower energy. The electrons will move not only from the semiconductor’s surface but also from a specific depth called the depletion region. This charge separation creates an electrostatic field pointing from the semiconductor to the metal, leading to a positive potential in the semiconductor’s surface region, and thus the band will bend upwards. The ΦM of the metal remains the same due to the high free electron density that can effectively screen the electrostatic field. The same reasoning can be applied for the case when ΦM < Φsc (figure 2.8c). The resulting surface energy barrier (ΦB) formed in the conduction band of the semiconductor is given by:

29 ΦB = ΦM − χ , for n-type (2.25)

E Φ = g + χ − Φ , for p-type (2.26) B q M

Naturally, an ohmic contact is formed when ΦM < Φsc for a n-type semiconductor and ΦM > Φsc for a p-type semiconductor. In the work presented in this thesis, the metals used were Ag and Au with ΦM higher than the Φsc of the n-type semiconductors (TiO2,SnO2,ZnO and Al doped ZnO(AZO)) and ΦM lower than the Φsc of the p-type semiconductors (GaN and PEDOT:PSS). The ΦM of Ag is 4.26-4.74 eV [47], while for Au is 5.1 eV [48]. Nevertheless, it is important to emphasize that these are values representing ﬁlms, therefore they might be overestimated values compared to small nanoparticles. The table 2.1 below contains the Schottky barrier values ΦB for different metal/metal oxides junctions obtained from the literature.

Table 2.1. ΦB of different metal/semiconductor composites obtained from other works. Ag / n-TiO2 Au / n-TiO2 Au / n-ZnO Au / n-SnO2 Au / p-GaN ΦB (eV) 1.0(1)[49] 0.9-1.2 [49–52] 0.62 [53], 0.67 [54] < 0.33 [55] 1.1 [56]

The above values were obtained from metal ﬁlms with thickness varying from 2-60 nm, except for the work done by Hwang et al. [54] where the Au was in the nanoparticle form. The ΦB obtained for Ag / TiO2 is unexpectedly high when considering the ΦM of Ag and χ ≈ 4eVofTiO2. However, the above equations are only valid for an ideal metal-semiconductor contact. It is well known that Ag surface can form a oxide layer in the surface, in contrast to the more inert Au surface, that can lead to discrepancies in the expected theoretical Schottky barrier value.

30 3. Materials and Methods

In this chapter the sample preparation methods will be brieﬂy presented together with the main techniques used to investigate the photomechanism of plasmonic systems. Further information about the synthesis procedure can be found in the supporting information of the papers presented in this thesis. Ultrafast transient absorption spectroscopies were the main techniques used and are of great relevance to understand plasmonics, since most of the main processes happen in the femtosecond to picosecond time scale.

3.1 Synthesis of Metal Nanoparticles 3.1.1 Bottom-up Method The synthesis of metal NPs (Ag and Au) is a relatively simple procedure that generally are synthesized via the reduction of metal precursors in aqueous or organic media with the presence of surface stabilizers, commonly referred as capping ligands. However, if the production of NPs with narrow size distribution and specific size is desired, it is important to find the appropri- ate synthetic procedure that is vastly provided in the literature. For example, the synthesis of sub-10 nm Au NPs is based on strong reducing agents (e.g. NaBH4), in the presence of strong capping ligands that quench particle growth. Ag NPs was synthesized by slightly modifying Ajitha et al. [57] proto- col where polyvinylpyrrolidone (PVP) was used as the capping ligand and NaBH4 as reducing agent. The only modification done in their procedure was using 10% ethylene glycol (EG) in water due to its stabilizing properties, thus improving in the growth confinement and preventing agglomeration. The obtained Ag NPs have size distribution of d = 19±7 nm. The great advantage of using PVP as the capping ligand is that the Ag NPs are stable in different organic solvents. Furthermore, they can be precipitated by adding acetone which facilitates the removal of excess of PVP in the solution. The versatility of using PVP became relevant for the experiments carried in the Paper I since it required using the solvent isopropanol free of PVP, which can be mostly removed by adding acetone followed by centrifugation several times. Au NPs were synthesized according to the modified Turkevich method reported by Piella et al. [58]. The Turkevich method [59] is based on the single- phase aqueous reduction of tetracholoauric acid (HAuCl4) by sodium citrate at 100oC. All other similar citrate-based methods lead to the formation of fairly

31 monodisperse quasi-spherical particles larger than 10 nm by varying the synthetic parameters such as pH, reducing agent and solvent. The synthesis of sub-10 nm AuNPs was achieved by Piella et al. with the addition of traces of tannic acid, which can lead to fast production of narrowly dispersed3-10nm NPs. This procedure was applied to synthesize the Au NPs in the Paper III and Paper IV leading to d = 4-5 nm and 7 nm, respectively.

3.1.2 Top-down Method An alternative simple method for the preparation of metal NPs involves the evaporation in a vacuum chamber at ca. 10−3 Torr of few nanometers of Ag or Au followed by annealing at high temperatures. The energy provided by the heat source allows the transition from film to an ensemble of quasi-spherical nanoparticles since it possess the lower surface energy of all particle shapes. This method was applied in the Paper II by depositing 2 nm of Au on top of a film of sintered TiO2 and ZrO2 nanoparticles (figure 3.1), resulting in ca. 6 nm Au NPs. The size of the NPs can be controlled by varying the evaporated film thickness.

Figure 3.1. TEM images and size distribution of AuNPs attached to TiO2 (a,c) and ZrO2 (b,d). e) Illustration of the steps involved in the top-down approach to prepare metal NPs. Figure reprinted from paper II.

3.2 Semiconductors

The main semiconductor used in this thesis is the ubiquitous TiO2 in the metastable anatase form. It was only in the Paper III that a comparative investigation was performed using other metal-oxides, namely, aluminum doped zinc oxide (AZO), zinc oxide (ZnO) and stannic oxide (SnO2). All the aforementioned metal oxides are intrinsic n-type and possess high direct band gap.

32 TiO2, ZnO and AZO have similar values of ca. 3.2 eV [60, 61], while SnO2 displays band gap higher than 3.6 eV [62]. Nevertheless, TiO2 has the highest 2 effective mass of about 5-10 me [63] and bulk mobility of ca. 1 cm /(V·s) [64], while ZnO and SnO2 have much lower effective mass of 0.3 me [65, 66] and thus high bulk mobility of 205 and 200 cm2/(V·s) [67, 68], respectively. These properties are related to different density of states in the conduction band (CB) region, which is two order of magnitude higher for TiO2 than for ZnO and SnO2 [69]. The high density of states in the CB of TiO2 is due to the presence of 3d-orbitals, whereas the CB of other metal oxides are mainly derived from s- and sp-orbitals of metal atoms. An schematic diagram illus- trating the energy levels of the CB and valence band (VB) of different metal oxides, along with the Fermi level of Ag and Au is presented in ﬁgure 3.2 below:

Figure 3.2. Energy diagram of the CB and VB energy levels of TiO2,ZnO, SnO2 and p-GaN taken from other works [56, 60, 70–72] along with the bulk Fermi level of Ag and Au.

TiO2 samples were prepared in two different ways. The ﬁrst was using the commercial anatase TiO2 purchased from Solaronix (15-20 nm NPs) which was diluted with ethanol and spin-coated in the substrate followed by annealing at 475oC for 15 minutes. The other method was based on spray-pyrolysis technique using titanium (IV) isopropoxide and acetyl acetonate as complex agent dissolved in t-butanol. ZnO and AZO NPs solutions were purchased from Sigma-Aldrich and SnO2 from Alfa Aesar. The purchased solutions were also spin-coated and annealed at 200oC and 500oC, respectively for 30 minutes.

33 3.3 Transient (NUV-NIR/mid-IR) Absorption Spectroscopy Transient absorption spectroscopy is a powerful tool for the investigation of the dynamics of of ultrafast photophysical and photochemical phenomena in picosecond-nanosecond time range. In this technique two femtosecond pulses are incident and spatially overlapped in the sample. One consists of an intense quasi-monochromatic pulse that is used to excite (pump) the sample. For each incoming pump pulse the relative change in absorption (eq. 3.1) is recorded by monitoring the variation of the second weaker probe pulse.

ΔA(λ,t)=Ap(λ,t) − Aup(λ,t) (3.1)

Ap and Aup are the absorption of the sample upon excitation (p) and without excitation (up). Aup can be measured by using a chopper with half of the repetition rate of the pulses, therefore removing every other incoming pump pulse. The time between the pulses are controlled by using a mechanical delay stage that can be varied up to few nanoseconds, thereby the temporal evolution is obtained by measuring the absorption at different delay times in respect to the ﬁrst pump pulse recorded through an spectrometer (Newport MS260i spectrograph). A schematic illustration of this technique is shown in the ﬁgure 3.3.

Figure 3.3. Illustration of the main components involved during transient absorption measurement. OPA stands for Optical Parametric Ampliﬁcation.

The probe consist of a broad in spectrum pulsed light. For the probing in the visible region, the fundamental beam (795 nm, FWHM ≈ 40 fs, generated by the laser from Libra Ultrafast Ampliﬁer System designed by Coherent) passes through the delay stage and is focused on Sapphire or CaF2 crystals for white light continuum generation. In order to probe in the mid-IR region, an

34 optical parametric amplification (TOPAS-prime, Light Conversion) is necessary to convert the fundamental pulse in a broad IR one along with an another spectrometer (Horiba iHR 320). In this thesis, the acronym TAS will be used to denominate measurements probing in the near-UV to near-IR region, while TIRAS for measurements probing in the mid-IR region. The kinetic traces are fitted with a sum of convoluted exponentials: − − = − t t0 2 ∗ − t t0 , = IRF S(t) exp ∑Aiexp τ tp (3.2) tp i 2ln2 where IRF is the width of the instrument response function, t0 is the time-zero, Ai and τi are the amplitude and decay times, respectively; * is the convolution operator. The IRF is the convolution of the pump and probe pulses that is well approximated by a gaussian function. This value was obtained by measuring the transient signal of an silicon film and performing band-gap excitation on ZnO film (figure 3.4), which resulted in IRF values of 95 and 91 fs, respectively. The former higher value was used in the papers III and IV.

Figure 3.4. Transient signal from a silicon and ZnO ﬁlms where the rise component signal corresponded to a gaussian function with FWHM of 95 and 91 fs, respectively. Figure reprinted from paper IV.

3.3.1 TAS on Plasmonic NPs The figure 3.5a shows the transient signal of Au NPs. The "bleach" of the signal is located at LSPR peak along with two positive signals at opposite sides of the spectrum. This signal shape is caused by the selective perturbation of the electrons that modifies the resonance property, resulting in a broadening of the initial absorbance spectrum. The information of the transient dynamics can be obtained by following the kinetics of this signal. The relevant component of the decay (that can be measured with our equipment) is the one related to the electron-phonon relaxation (or coupling) time (τe−ph) as is shown in the figure 3.5b. This decay follow an monoexponential model and can be calculated

35 Figure 3.5. a) Representative transient spectra of Au NPs probed at different time delays. b) Kinetics extracted at 550 nm. Inset: Exponential decay related to e-ph relaxation time (τe−ph). by using the two-temperature model [73] which describes the rate of energy transfer between the thermalized electrons and the lattice. It will be discussed in the last chapter one important information that can be provided by analysing the dependence of τe−ph with the excitation ﬂuence.

3.3.2 TIRAS on Plasmonic NPs / Semiconductor

Figure 3.6. Representative transient IR spectrum of Au NPs/TiO2 sample upon excitation at LSPR peak. Figure reprinted from paper II.

In TIRAS, the temporal evolution of the difference in IR absorption is measured and the positive signal is assigned to free carriers (electrons and/or holes) in the accepting bands of the materials. Free carriers have a strong absorption in the IR spectrum by exhibiting a broad featureless signal that increases in the entire mid-IR region. This absorption is typically an indirect phonon-assisted

36 (intraband) transition and it can be also described using the Drude model [74]. TIRAS was one of the main techniques used in the works presented in this thesis as it can provide the dynamics of hot carriers injected in the femtosecond time scale into the accepting materials upon excitation of plasmonic NPs. The ﬁgure 3.6 shows a representative three-dimensional plot of electrons injected into the conduction band of TiO2 from Au NPs.

37 4. Hot Carriers Injection (Papers I and II)

4.1 Introduction The excitation of plasmons in metal nanoparticles quickly lead to the dephasing of the collective movement of the electrons in few femtoseconds (≈ 10 fs), leaving behind a continuum of non-thermal electrons and holes, some with several electron volts above and below the fermi level. These carriers are referred as hot carriers and they are going to be the main subject of this chapter. Eventually, the hot carriers will thermalize on the femtosecond time scale through several electron-electron scattering and then release the excess of energy by heating the metal lattice on the picosecond to nanosecond time scale as was discussed in the section 2.4. The challenge of utilizing hot carriers in plasmonics stands on the sub-picosecond lifetime of the electron-hole pairs, in contrast to photoexcited carriers in a semiconductor like silicon that exist for at least one million times longer before recombination takes place. The energy of these hot carriers are commensurate with the photon energy and may have energy sufficient to overcome the binding forces that keep them within the metal and carry charge into the adjacent material. The photocurrent measurement in a device designed using plasmonic metal NPs as light absorbers and electron-accepting semiconductors was first demonstrated by the pioneering work done by Tatsuma group in 2004 and 2005 [75, 76]. Later in 2007, Furube et al. [77] measured the signal of electrons transferred from Au NPs into the CB of TiO2 using TIRAS technique. In this chapter, the investigation of hot carrier injection process was further explored using the same TIRAS technique. Herein, two systems composed of plasmonic Ag and Au NPs will be presented with spectroscopic evidences that electron and hole are injected upon light pulse excitation. One novelty has been the use of a molecular linker that allowed higher attachment of PVP- capped Ag NPs on TiO2 film. Moreover, the hot hole injection process was taken place in materials with different phases, i.e. liquid phase and solid phase.

4.2 AgNP-pABA-TiO2 in IPA (Paper I) The first system consists a film of self-assembled nano-hybrid architecture composed of PVP-stabilized Ag NPs (d = 19 ± 7 nm) molecularly linked to TiO2 anatase with para-aminobenzoic acid (pABA) immersed in isopropanol (IPA). A conceptual illustration of the Ag nano-hybrid system and the pho- toinduced processes is shown in figure 4.1.

38 The ﬁgure 4.2a shows the temporal evolution of the TIRAS signal upon excitation at 430 nm. The transient signal is dominated by a broad absorption ascribed to free electrons in the TiO2 CB and two bleach peaks at 1160 and 1128 cm−1. The bleach signals appear to overlap with the peaks attributed to the C- Figure 4.1. Conceptual represen- O stretching of IPA [78], suggestive of C- tation of the system with the pho- OH oxidation (IPA oxidation potential onset toinduced processes. on Pt electrode is 0.25 V vs SCE both in acid [79] and alkaline [80] conditions). Figure 4.2b shows the kinetic traces for the −1 electrons in the TiO2 CB (average signal between 1222 and 1227 cm ) and IPA C-O stretching bleach (average signal between 1157 and 1161−1). The signal related to electrons was subtracted from the kinetic trace at the bleach position, resulting in a decay behavior. The IPA bleach is attributed to one- electron donation to Ag to quench its reactive hole. One possible scenario is that proton-coupled electron transfer (PCET) is taking place, where the proton is released to a separate base upon IPA oxidation, in this case OH− and/or water present in the solvent. Further support comes from process reversibility, which is only consistent with PCET.

Figure 4.2. a) Temporal evolution of the transient infrared absorption spectra upon excitation at 430 nm. Side banner shows the Fourier-transformed infrared spectrum of neat IPA. b) Kinetic traces and ﬁttings for (top panel) electrons in TiO2 CB and (bottom panel) IPA bleach due to oxidation after subtraction of the electron signal.

The same TIRAS measurements were performed in a dry film (absence of IPA) and with the solvent n-hexane that has no reactivity (figure 4.3). It is clear that in dry films and in n-hexane, electrons in TiO2 CB decay significantly faster than in the presence of IPA. This observation is consistent with PCET, which promotes charge separation stabilization in the system.

39 Figure 4.3. a) Decay kinetics at related to electrons in TiO2 CB for dry ﬁlm and in IPA. b) Decay kinetics at related to electrons in TiO2 CB in IPA and hexane.

4.2.1 The Effect of the Molecular Linker and Capping Ligand A similar sample but Ag NPs without capping ligand and in direct contact with TiO2 was prepared by evaporating 4 nm of Ag followed by annealing at 300oC under continuous argon ﬂow. These two samples were measured using TIRAS technique with excitation and probing wavelengths at 430 nm and 8300 nm, respectively.

Figure 4.4. Electron dynamics of two systems where Ag NPs/pABA/TiO2 is PVP- capped Ag NPs linked through pABA on TiO2 surface while Ag NPs/TiO2 have bare Ag NPs in direct contact with TiO2.

The ﬁgure 4.4 above clearly depicts the difference in the electron dynamics when Ag NPs are directly in contact and when linked through pABA molecule. In the former case, almost all electrons are recombined back to Ag NPs in ca. 200 ps. In contrast, the presence of the linker and capping-ligand suppressed 30% of electron recombination in our longest delay time of ca. 5 ns. This result indicates that the formation of a Schottky barrier is not sufﬁcient to

40 outplay the recombination process that happens due to continuous accepting states existent on metals above their Fermi level. The short lifetime of the electrons in the TiO2 CB was also observed for Au NPs attached directly to TiO2 (Papers II-IV) and the possible factors dictating the electron dynamics will be discussed in the next chapter. In respect to the electron injection mechanism, Tan et al. [81] reported that for Ag NPs in direct contact with TiO2, the plasmon (polariton) can be de-excited by directly scattering the oscillating electrons into a semiconductor in less than 10 fs. This process is in contrast to the common charge injection picture that consist of three steps (excitation, transport and transfer), which indicates to take place in the system where the capping-ligand and the linker is present. Naturally, electron injection process through tunneling cannot be ruled out.

4.3 PEDOT:PSS / Au NPs / TiO2 (Paper II) The second system investigated in which electron and hole injection process occurs upon light excitation is the system composed of a compact layer of TiO2 decorated with Au NPs under a thin film layer of poly(3,4- ethylenedioxythiophene):poly(styrenesulfonic acid (PEDOT:PSS). PEDOT is a semiconductor, which is degenerately doped by PSS to achieve a p-type conductive mechanism [82]. ZrO2 was used as a reference material since it does not accept any hot Figure 4.5. Structure of p- carriers from Au NPs and have similar morpholog- type polymer semiconductor PEDOT:PSS. ical structure of the TiO2 film. The system was investigated once again using TIRAS technique, where the probe interval was λprobe= 7600-8200 nm and the excitation wavelength was set to λexc = 530 nm (near the Au LSPR peak). Initially, the system composed of only Au NPs on TiO2 was measured. The time zero, i.e., the overlap in time of the pump and probe pulse, was shifted for plotting conve- nience. The insertion of the PEDOT:PSS thin film over Au NPs/TiO2 causes a dramatic change on the transient IR signal as it is compared in the figure 4.6a. In order to clarify this change in the signal, the effect of PEDOT:PSS was investigated by replacing TiO2 for ZrO2. The strong negative signal appearing at early times is caused by the heating of PEDOT:PSS due to the perturbation caused by the laser pulse, which causes changes in the absorption of IR light [83]. By comparing both curves in figure 4.6b (dashed blue circle), it is possible to observe a faster decay when the plasmonic NPs are present. Previous studies of fully undoped PEDOT indicates a band gap > 1.5 eV [84]. Notwith- λ standing, irradiation of PEDOT:PSS/ZrO2 with exc = 530 nm (2.34 eV) leads to the same power law decay behavior (1/tβ ) with β = 0.9±0.1 observed by

41 Figure 4.6. a) Kinetics at 7730 nm extracted from Au NPs/ TiO2 (red) and PE- DOT:PSS/Au NPs/ TiO2 (black). b). Kinetics at 7730 nm of PEDOT:PSS/ZrO2 with (black) and without (green) Au NPs. The inset shows the kinetics in log-log presenta- tion. Power-law decay ﬁtting yielded β= 0.9±0.1.

Meskers et al. [83] (βlit = 0.8±0.1) when pump/probed at 1440 nm (0.86 eV). Therefore, photoconduction from photo-generated charge in the thin film of PEDOT:PSS is unlikely. Moreover, a deviation from the power law decay behavior in the initial transient decay can only be observed when Au NPs is present (insert of figure 4.6b). This leads to conclude that the divergence from power decay law behavior is caused by hole injection upon Au plasmon excitation and not from PEDOT:PSS optical excitation. In addition, the negative signal after 0.5 ps is significantly more intense for the sample without Au NPs, suggesting that a positive signal offsets the trace consistent with holes populating PEDOT:PSS valence band. The kinetics of the hole signal is obtained by removing the transient signal from the heat contribution caused by the laser perturbation, i.e. subtracting the signal from PEDOT:PSS/ZrO2 to the one containing Au NPs as is shown in the figure 4.7a, which presents an exponential decay behavior. Conversely, in the system with Au NPs on TiO2, there is an increase in the signal by a factor of ca. 2.5 when PEDOT:PSS is present. This substantial change could be related to the effect of hot holes/electrons injection process favoring the formation of hot electrons/holes in the following plasmon decays within the pulse duration. Nevertheless, an accurate assess- ment requires higher temporal resolution in the attosecond to femtosecond time scale that would allow monitoring the change in the plasmon dephasing dynamics. The kinetics of PEDOT:PSS on ZrO2 at longer time delays (> 3 ps) shows a negative signal which was attributed to heat dissipation from Au NPs. The comparison with the Au NPs dynamics measured on TAS indicates that this feature start appearing at the ending of electron-phonon relaxation component, where is commensurate with the time for lattice heating (figure 4.7b). Hartland et al. [85] have studied the energy dissipation of Au NPs to their surroundings with pump-probe spectroscopy and verified that, for very small particles (ca. 4 nm), significant energy loss occurs before electron and phonons reach ther-

42 mal equilibrium. This result further conﬁrms that PEDOT:PSS decreases IR absorption upon heating, as previously mentioned.

Figure 4.7. a) Kinetics of holes in the conducting polymer extracted from the subtraction of PEDOT:PSS/Au NPs/ZrO2 by PEDOT:PSS/TiO2 transient signals (the green and black lines are just connecting the data points). b) TIRAS and TAS kinetics of PEDOT:PSS/Au NPs/ZrO2 probed at 7730 and 700 nm, respectively.

4.4 Conclusions In the two pieces of work presented in this chapter, it was shown spectroscopic evidence of hot electron and hole injection processes of plasmonic Ag and Au NPs upon light excitation. The use of a molecular linker pABA revealed to be an alternative strategy to increase the attachment of PVP-capped Ag NPs on TiO2, which substantially suppressed electron recombination. The charge separation was further extended by using IPA as hole acceptor, which was tentatively attributed to a PCET process. Electron and hole injection was also observed in the system composed of PEDOT:PSS/Au NPs/TiO2. In particular, the hole injection process was clariﬁed by understanding the PEDOT:PSS optical behavior upon light excitation.

43 5. Au NPs / Semiconductor Composites: a Comparative Study (Paper III)

5.1 Introduction In the work published so far related to hot carriers extraction from plasmonic nanoparticles into semiconductors, there has been a lack of discussion about the initial charge dynamics on the accepting materials. The efficiencies of devices based on hot carriers harnessed from plasmonic are typically very low (few per cent) [86–88], which contrasts the high hot carrier injection efficiency reported to be in the range of 25-40% [77, 89] and hole injection up to 85% (Paper V). This suggest that the majority of charges transferred are recombining back to the metal, as is observed to happen already in the first 100 ps. In the work presented in this chapter, the factors affecting the initial electron dynamics were investigated by performing a comparative study using different semiconductor metal oxides (MOs), namely titanium dioxide (TiO2), zinc oxide (ZnO), stannic oxide (SnO2) and aluminum doped zinc oxide (AZO) attached to Au NPs. Despite the existence of a Schottky barrier, the obtained results showed here indicate that this parameter cannot be exclusively account for the differences in electron dynamics. One of the main differences between TiO2 and the other mentioned metal oxides lies in the electron bulk mobility value, which is much smaller compared to other MOs, that also might influence the recombination process in the early timescale. An interesting feature observed for the samples except for TiO2/Au NPs, was the presence of rise component related to free carriers in the CB that was slower than the instrument response function (IRF), which is incompatible with the known rapid formation of hot carriers. This apparent delayed behavior is similar to what is observed from dyes chemically linked to these metal oxides and will be also discussed in this chapter along with the influence of 1 nm of Al2O3 insulating layer between Au NPs and the MOs.

5.2 Results The samples were initially consisted of bare Au NPs of ca. 4 nm attached to a thin glass substrate. The different metal oxides were deposited on top by spray-pyrolysis (TiO2) and by spin-coating followed by annealing of commercial nanoparticles solutions (ZnO, SnO2 and AZO). The total transmission and reﬂection of the samples were measured using an integrating sphere in order

44 to determine their absorption (ﬁgure 5.1b), allowing determination of the absorbed energy per laser pulse or absorbed ﬂuence given in J/cm−2. All the initial samples containing only Au NPs have the same absorption and therefore the distinct absorption peak wavelength and intensity observed is solely due to the different dielectric constants of the MOs (eq. 2.21 and 2.23).

Figure 5.1. a) Schematic illustration of the samples (top) and TEM images of bare Au NPs (bottom). b) Transmission (top), reﬂection (middle) and absorption (bottom) of MOs/Au NPs and PMMA/Au NPs.

5.2.1 TIRAS: Rise Component The dynamics of electrons injected into the semiconductor upon light excitation of Au NPs was investigated by using TIRAS technique. The broad probe light was centered at λ = 4800 nm and the excitation wavelength was set to the respective LSPR peaks of the MOs/Au NPs samples. Figure 5.2a shows the kinetics of all MOs/Au NPs samples in the first two picoseconds. While for the sample with TiO2 the expected ultrafast injection is observed, the sample with other MOs showed an injection time higher than the IRF of ca. 95 fs. The rising component of each system was fitted through a convoluted fitting that is shown in the table 5.1. The obtained values are at least one order of magnitude longer than the dephasing time of plasmons polaritons and cannot be related to its dynamics, even if 2nd and 3rd generation of hot carriers [45] are considered since the scattering of the most energetic electrons takes place within few femtoseconds [90]. Cushing et al. [91] also observed a slow rise component which they associated to the formation of electron-hole pair through resonant energy transfer (RET) on the Cu2O shell surrounding core Au NPs with a 5 nm SiO2 intermediate layer. However, steady-state and time-resolved emission measurements of the samples exciting at the plasmonic peak showed no signal, which would otherwise indicate the presence of RET

45 mechanism. Moreover, the spectra of Au NPs and MOs do not possess the overlap requirement for this process to happen according to their formalism.

Figure 5.2. a) Kinetic traces extracted at 4800 nm of Au NPs (a) and Z907 dye (b) attached to MOs. c) Kinetics traces extracted at 4800 nm of MOs / Al2O3 (1 nm) / Au NPs. d) Band edge bleach kinetics of samples with AZO (top) and ZnO (middle) excited at 330 nm and at LSPR peak wavelength. Kinetics at 4800 nm of the sample with TiO2 excited at LSPR peak and at 330 nm (bottom). The narrow positive peak is ascribed to cross-phase modulation in the sample.

Table 5.1. Rise component values obtained for each MOs/Au NPs sample.

AZO/Au NPs ZnO/Au NPs TiO2/Au NPs SnO2/Au NPs τrise (fs) 389 (105) 368 (43) < IRF 124 (20)

The same MOs were sensitized with a Z907 dye depicting the similar injection time trend, but in a longer time scale (figure 5.2b). This trend has also been observed in other work [92–95] involving dye with these metal oxides and has mostly been attributed to the formation of intermediate states at the interface and/or to the difference in the density of states in the CB region, which is significantly higher for TiO2. The insertion of 1 nm Al2O3 between the Au NPs and the MOs still resulted in the appearance of a slow rise component, as is showed in figure 5.2c. This result suggests that this behavior is mainly independent of the electronic coupling at the interface of MOs and Au, and is related to the intrinsic surface

46 electronic properties of the MOs, such as the density of states at the CB region and/or surface trap states [96]. This was further supported by comparing the different initial kinetics at the band edge bleach using TAS and TIRAS technique (ﬁgure 5.2d) when excited at 330 nm and at the LSPR peaks.

5.2.2 TIRAS: Decay Dynamics Following the stage of maximum signal amplitude in TIRAS, each sample has distinct electron dynamics in their respective materials (ﬁgure 5.3a). Clearly, ZnO/Au NPs and AZO/Au NPs have the slowest recombination rate followed by SnO2/Au NPs and TiO2/Au NPs. One important factor that affects the recombination rate is the metal-semiconductor Schottky barrier, which values were taken from the literature and are listed in the table 5.2. It is evident that this parameter alone cannot account for the results, otherwise the sample containing TiO2 should have the slowest recombination rate. In this regard, 2/( · ) ZnO and SnO2 have electron bulk mobility of 205 and 200 cm V s re- 2/( · ) spectively, while for TiO2 it is only 1 cm V s . The electron bulk mobility depends on the effective mass of electrons in the CB, which is ≈ 0.3 me for ZnO and SnO2 and 5-10 me for TiO2. This is a consequence of the presence of 3d-orbital in the CB, while ZnO and SnO2 CB are mainly derived from empty s- and sp-orbital of the metal atoms. Therefore, one possible explana- tion is that the electron bulk mobility can inﬂuence the escape velocity from the interface further into the bulk, affecting the tunneling probability back to the metal, which in turn depends on the Schottky barrier height. Although the aforementioned MOs have similar electron bulk mobility, the faster electron recombination observed for SnO2 can be attributed to the lower Schottky barrier height.

Figure 5.3. Kinetic traces extracted at 4800 nm of Au NPs attached to MOs without (a) and with (b) 1 nm of Al2O3.

The insertion of 1 nm of Al2O3 decreased the recombination rate, especially for the samples with AZO and ZnO, followed by SnO2 (ﬁgure 5.3b).

47 Table 5.2. Schottky barrier heigh values obtained from other works. Au / n-TiO2 Au / n-ZnO Au / n-SnO2 ΦB (eV) 0.9-1.2 [49–52] 0.62 [53], 0.67 [54] < 0.33 [55]

Interestingly, the sample with TiO2 exhibited almost no difference. Through the information provided by the excitation power dependence measurements of the samples without 1 nm Al2O3 and with the absorption data of the samples with Al2O3 insulating layer, it was possible to calculate the relative injection efﬁciency (RIE) that is listed in the table 5.3. The normalized signal at 400 ps was also compared with the samples with 1 nm of Al2O3 by multiplying with the RIE for each samples. Curiously, the sample with TiO2 is the one with lowest RIE that also did not show prolongation in the charge separation.

Table 5.3. Relative injection efﬁciency (RIE) and comparison of ΔOD signal at 400 ps with and without 1 nm of Al2O3 AZO ZnO TiO2 SnO2 Relative Injection Efﬁciency (RIE) (%) 65.6 69.9 24.7 80.0 Δ OD400ps w/o1nmAl2O3 (%) 20.2 23.2 3.4 2.8 Δ · OD400ps w/1nmAl2O3 RIE (%) 41.0 38.4 1.2 8.2

5.3 Conclusions In this chapter, the electron dynamics of MOs/Au NPs were investigated using TIRAS technique. Except for TiO2, all other samples exhibited a rise component that is slower than the IRF of ca. 95 fs. The observed behavior and trend was shown to be similar to what is observed with dyes molecules. The time-resolved investigation of the electron injection dynamics with the insertion of 1 nm Al2O3 indicated that the delayed behavior is mainly independent of the electronic coupling between Au NPs and the MOs. It is suggested that the interplay between Schottky barrier height and electron bulk mobility are playing the main roles in determining the electron dynamics at this short time scale. The insertion of the insulating layer also showed that it can greatly suppress electron recombination. However, the sample TiO2/1nm Al2O3/Au NPs showed to be an exception. This could be due to the highest Schottky barrier value reported for TiO2/Au NPs that leads to a wider depletion layer, which supresses electron recombination similarly to the insulating layer.

48 6. The Effect of Temperature on Hot Carrier Transfer (Paper IV)

6.1 Introduction In the context of the recent interest in utilizing hot carriers generated from plasmonic nanostructures, it is relevant to understand what is the effect of the temperature in the range expected for electronic device applications. In particular, hot carriers that are created and not extracted will eventually lead to heat generation. This implies that, unless unit efﬁciency charge injection is achieved, the increase in temperature will always be a by-product of plasmonic based systems which further highlights the importance of understanding the temperature effect on the plasmon dynamics. In the section 2.4 of this thesis, it was discussed that after the plasmon excitation, the decay will occur within its lifetime of τ ≈ γ−1, where γ = γe−e+γe−ph+γLD. The γe−ph is related to the decay through electron-phonon scattering, which is the only term that has a strong dependence with temperature (see ﬁgure 2.2). In theory, the increase in the phonon population induced by the increase in the plasmonic NPs temperature should enhance plasmon dephasing through electron-phonon scattering, while others mechanisms remain approximately unchanged. In order to investigate this effect on the hot electrons generation, in situ measurement by TIRAS of Au NPs/TiO2 sample was carried out. As it is presented in the next section of this chapter, the increase in temperature resulted in a higher number of electrons injected from Au NPs into the CB of TiO2 upon plasmon excitation. The band gap of TiO2 was shown to only change by 0.2% and ab initio molecular dynamic calculations indicated that the Schottky barrier height is expected to increase with temperature, consistent with the experimental work of Mahato and Puigdollers [97]. These results strongly suggest that phonon-assisted plasmon decay enhancement is the main responsible for the higher electron injection observed.

6.2 Results Au NPs with d = 7 ± 3 nm (figure 6.1a) were synthesized and attached to a mesoporous film of anatase TiO2 or ZrO2 by adding small amounts of HCl to drive the NPs adsorption through electrostatic attraction. Samples with different concentrations were prepared by varying the immersion time of the films in the solution as is shown in the figure 6.1b. The films were then heated

49 to 450 oC to remove the capping-ligand, resulting in the direct attachment of bare Au NPs on the metal oxides. The last important step consisted in spin- coating a solution of polymethyl methacrylate (PMMA) on top of the ﬁlm to diminish contact with air, and thus mitigating changes in surface moisture and hydroxylation.

Figure 6.1. a) TEM image of Au NPs with average size and histogram as insert re- vealing size distribution. b) Absorbance spectra of Au NPs on TiO2 at three different concentrations. Figure 6.2 shows the steady-state absorbance measurements of the samples at different temperatures where a small broadening and a red-shift of the LSPR peak can be observed. Yeshchenko et al. [98] performed a theoretical analysis based on the Drude model where they have considered the increasing in γe−ph factor and the thermal expansion of the NPs in the expression of the surface plasmon frequency (equation 2.19). Their theoretical and experimental results showed that the LSPR red-shift is mainly caused by the thermal expansion, while the broadening is the result of plasmon damping enhanced due to increase in γe−ph. The change in the dielectric constant of the environment εm with temperature was also considered but it was revealed to be negligible.

Figure 6.2. Absorbance spectra of Au NPs attached to TiO2 (a) and ZrO2 (b) in two representative temperatures (inset: difference in absorbance in respect to the one in room temperature).

50 6.2.1 TIRAS All the samples were excited at the same wavelength of 580 nm since this wavelength provides the balance between the reduction of interband transition and variations in Au NPs absorbance change upon external heating (less than 0.3% variation, inset on figure 6.2). In addition, laser pump-power dependence analysis was carried out to ensure all the measurements to take place within the linear response in respect to the signal (figure 6.3a). The figure 6.3b depicts the transient signal extracted at 4800 nm of Au NPs/TiO2 with different concentrations excited with the same laser intensity. The kinetics of samples Au NPs/TiO2 #1,#2 and #3 reveals faster electron recombination with increasing concentration of Au NPs, as the electrons in the TiO2 can easier recombine with the neighbors NPs.

Figure 6.3. a) ΔODmax in function of excitation ﬂuence for samples with different concentrations of Au NPs on TiO2 ﬁlm. b) TIRAS signal extracted at 4800 nm for Au NPs/TiO2 and excited at 580 nm.

The figure 6.4a-c shows the transient signal in function of temperature at different time delays of Au NPs/TiO2 with different concentrations #1 (highest concentration), #2 and #3 (lowest concentration) excited and probed in the same wavelength mentioned previously. A linear fitting was performed, where the slope (α) values as a function of time is ploted in the figure 6.4d. A positive α implies a higher number of electrons injected into the CB of TiO2 with temperature, and vice-versa. The sample #1 has only positive α values before 100 fs which is within the IRF. In contrast, the sample with lowest concentration (#3) exhibit positive α for all the time delays investigated. This concentration dependent behavior can be attributed to increase in the conductivity of semiconductors with temperature, which leads to faster recombination depending on the amount of neighbor Au NPs. Tauc plot of only TiO2 at the temperature range used in the TIRAS measurements, showed that the optical band gap is only affected by 0.2%. In addition, theoretical simulations on the effect of phonons on the Schottky barrier was also performed through ab initio molecular dynamics calculations for Δ an Au/TiO2 anatase model. The Vavg, i.e. the change in the Schottky bar-

51 Figure 6.4. Signal in function of temperature at different time delays in respect to time-zero of the sample a) Au NPs/TiO2 #1 (highest loading) b) Au NPs/TiO2 #2 α c) Au NPs/TiO2 #3 (lowest loading). d) Compilation of the slope obtained from each sample in function of time. Positive values of α implies an increase of electrons injected into CB of TiO2 with temperature, and vice-versa. rier height with temperature was computed and an increase of ca. 0.12 eV is predicted, which is similar to experimental values reported by Mahoto and Puigdollers for other Au/Transition Metal interfaces [97] (Supporting information of paper IV). The experimental evidence that the changes in the band gapofTiO2 is negligible along with theoretical investigation predicting the increase in the Schottky barrier height with temperature, only reinforces that the enhancement in the electron injection observed is ascribed to phonon-assisted plasmon decay. Alternatively stated, the number of electrons with sufﬁcient energy to surpass the Schottky barrier is increased by favoring this mechanism between others, i.e., plasmon decay by electron-electron scattering (γe−e) and by Landau damping (γLD). The excitation wavelength (580 nm) is before the d→p transition onset, therefore this contribution can be considered minor. This result is in agreement with the theoretical description given by Khurgin, in which is stated that plasmon decay by phonon generates electrons with more energy than by plasmon decaying through electron-electron scattering [99]. In the former case, one electron-pair is generated by receiving momentum from phonon(s), while in the latter, two electron-hole pairs with shared momentum are generated but with lower energies.

52 6.3 Conclusions

Steady-state absorbance measurements of Au NPs on TiO2 and ZrO2 at temperatures ranging from the room-temperature to ≈ 360 K have shown the broadening and also a red-shift of the Au NPs LSPR peak. Thermal expansion of Au NPs and the increase in γe−ph are the responsibles for the absorbance change, which can be calculated by accounting it in the Drude model. TIRAS measurement of Au NPs/TiO2 with three different NPs concentrations showed an increase in electron injection with temperature. However, semiconductor conductivity is also increased with temperature, and hence, promotes faster recombination with higher amount of Au NPs neighbors. The observed enhancement in the electron injection efﬁciency is attributed to increase in the phonon-assisted plasmon decay mechanism over others. A direct proof of the main mechanism underlying the observed trend would require higher temporal resolution. For instance, atto-second XUV pump- probe experiment, which will not require the presence of an acceptor material, might reveal fundamental informations about hot carrier formation through plasmon decay. Nevertheless, the study performed at operating temperatures in the range expected for electronic device applications may be useful for the development of new strategies to engineer more efﬁcient devices. Apart from the temperature effect, the study involving different Au NPs concentration showed this to be a relevant factor to be considered, since metal NPs possess a continuum of states above the Fermi level that can act as charge recombination centers.

53 7. Electron-phonon Dynamics (Papers III and V)

In the section 2.4 the relaxation dynamics of plasmons upon excitation was briefily discussed. One important process following the formation of hot carriers is the electron-phonon scattering process that happens with a characteristic time of τe−ph ≈ 1 ps, which is related to the time it takes for the electron gas with temperature Te to reach thermal equilibrium with the lattice through in- elastic collisions. The fundamental understanding of this scattering process is significant to many phenomena in solid state physics such as superconductiv- ity [100], electronic and thermal transport, spin caloritronics [101] and laser induced phase transitions [102]. In the scope of plasmon-induced hot carriers, the analysis of the electron-phonon scattering process also becomes a valu- able tool to understand and quantify the transfer of hot carriers to accepting materials, as will be presented in section 7.1.2. This is related to the fact that the electron-phonon relaxation time (τe−ph) depends on the initial electronic temperature (Te), which is reached after the hot electrons redistribute their energy through several electron-electron scattering events. Hot electron or hole injection is anticipated to modify the electron-phonon dynamics because they alter the electronic temperature (Te). These changes can be measured using the TAS technique, allowing their comparison with predicted τe−ph values that are attained using time-dependent ab initio method. Herein, TAS technique was applied to investigate how hot electron and hole injection processes from Au NPs modifies the electron-phonon relaxation time (τe−ph). For this purpose, Au NPs in contact with electron and hole accepting semiconductors (n-type and p-type) were fabricated and characterized. For Au NPs, the LSPR absorption peak overlaps with the interband transition from the d-bands, which gives an additional electronic transition channel besides the sp-bands. This implies that hole injection process is more sensitive to the excitation wavelength and will be also discussed in this chapter.

7.1 Results 7.1.1 Electron-phonon Dynamics Upon Hot Electron Injection Herein, TAS studies of the same samples presented in the chapter 5 will be discussed, namely Au NPs covered by TiO2, ZnO, SnO2 and PMMA. Speciﬁ- cally, the kinetics of Au NPs is obtained and ﬁtted with a monoexponential

54 function for each absorbed fluence at their respective LSPR bleach wavelengths. Figure 7.1a depicts a representative case (Au NPs/TiO2) of the kinetics up to 3.5 ps with absorbed fluence values. The same procedure was performed for all samples where different τe−ph values were fitted with a linear function (figure 7.1b). The linear behavior is predicted from the well studied two-temperature model that describes the flow of energy between the electron bath with temperature Te and the lattice with temperature Tl. Hot electron injection process is commensurate with electron-electron scattering time, thereby it affects the following electron-phonon dynamics. Therefore, the smaller slope values of MOs/Au NPs in respect to PMMA/Au NPs is attributed to the removal of hot electrons from Au NPs by MOs, which reduces Te and, consequently, the electron-phonon relaxation time. This implies that the smaller the slope, the higher is the amount of hot electrons extracted [89]. In particular, the sample with TiO2 showed the shallower slope as expected, followed by ZnO, SnO2 and AZO. Nevertheless, since it is unknown whether the Au NPs coverage are equal for all systems, general statements on the relative electron injection efficiencies would require other techniques such as atomic layer deposition to ensure same surface contact between Au NPs and the MOs. Moreover, the MOs/Au NPs LSPR peak absorption (except TiO2) overlaps with the interband transition contribution that might reduce the formation of hot electrons.

Figure 7.1. a) Representative transient kinetics up to 3.5 ps of Au NPs/TiO2 excited with different absorbed ﬂuences. b) Electron-phonon relaxation time (τe−ph) in function of the absorbed ﬂuence of different MOs/Au NPs. The sample PMMA/Au NPs is used as reference since hot electron injection does not occurs.

Table 7.1. Slope obtained from τe−ph values for each MO/Au NPs sample excited at their respective LSPR absorption peak. The hot electron injection efﬁciency is in- versely proportional to the slope value.

AZO/Au NPs ZnO/Au NPs TiO2/Au NPs SnO2/Au NPs PMMA/Au NPs Slope (ps · cm2/μJ) 0.135(31) 0.113(16) 0.075(5) 0.126(7) 0.158(17)

55 7.1.2 Electron-phonon Dynamics Upon Hot Hole Injection Most of the plasmonic-semiconductor composites have been designed to extract hot electrons and only few systems were dedicated to the study of hot holes [56, 103]. The presence of other electronic bands (mainly d and p) below the Fermi level lead to the formation of injectable hot holes in different bands depending on the excitation wavelength, which is anticipated to af- fect the electron-phonon dynamics. In this work, a system composed of Au ± NPs with 7 3 nm dispersed on top of Figure 7.2. Transient IR signal related p-GaN (Eg = 3.4 eV) was prepared. The to holes injected into p-GaN upon Au Fermi level position of p-GaN is lower NPs excitation at 530 nm. than for the Au NPs, thus resulting in the formation of a Schottky barrier of ΦB = 1.1 eV at the valence band. The hot hole injection into p-GaN was confirmed by TIRAS measurements, with excitation wavelength at 530 nm and probe wavelength centered at 4850 nm (figure 7.2). Excitation with the same wavelength at bare p-GaN showed no signal indicating that Au NPs are responsible for hole injection. Moreover, measurements carried on a sample with Au NPs attached to ZrO2, which has the CB level below of p-GaN, verified that only hot holes, and not hot electrons, are transferred upon plasmon excitation. The figure 7.3a and d depicts τe−ph in function of the absorbed energy den- −3 sity Uabs (Jm ) excited at 530 nm and 730 nm. The photon energy of both wavelengths used are higher than the Schottky barrier of Au NPs/p-GaN (ΦB = 1.1 eV), which allows hole injection to occur, while the other samples with Al2O3 and SiO2 instead of p-GaN were used as reference since they do not allow charge transfer to happen due to their CB minimum and VB maximum position. A linear fitting was performed for each sample, where the slope obtained for Au NPs/p-GaN presented a much shallower one (table 7.2). No- tably, relative to Au NPs/Al2O3 the relaxation dynamics on Au NPs/p-GaN were not as noticeably altered when excited at 730 nm as compared to 530 nm. This difference will be later discussed. The nearly identical slope of the insulators used confirms that the thermal conductivity of the environment does not influence τe−ph. Therefore, this substantial difference observed in the electron-phonon dynamics for Au NPs/p-GaN is attributed to ultrafast hole injection process, which does not occur in the other two insulator materials. The electronic heat capacity (Ce(Te,Ne)) is proportional to both the electronic temperature (Te) and the electron density (Ne). The hole injection pro-

56 Figure 7.3. Influence of ultrafast hot hole collection on the dynamics of hot electrons in Au NPs. a) Experimentally derived values of electron-phonon relaxation −3 times (τe−ph) as a function of absorbed energy density Uabs (Jm ) for Au NPs/p- GaN (black squares), Au NPs/Al2O3 (blue circles) and Au NPs/SiO2 (blue triangles). b) Ab initio calculation showing the temporal evolution of the electronic temperature × 8 −3 (Te) after laser excitation at 530 nm (Uabs = 5.3 10 Jm ) in Au NPs/Al2O3 (blue curve) and Au NPs/p-GaN (grey curves). For Au NPs/p-GaN, the influence of removing hot holes 1 eV below the Au Fermi level is shown (light-grey curve: 25% → black curve: 100% hot hole collection). c) Ab initio calculation of τe−ph as a function of absorbed energy density (Uabs) to show the influence of removing hot holes 1 eV below the Au Fermi level in Au NPs/p-GaN (white squares: 0% → black squares: 100% hot hole collection). The shaded region denotes the upper and lower bounds of computed τe−ph values. The slope of the experimental data points from panel a is also plotted (dashed black line) to aid comparison between theory and experiment. d) Experimentally derived values of τe−ph as a function of Uabs for Au NPs/p-GaN (open black squares) and Au NPs/Al2O3 (open blue circles) at a pump wavelength of 730 nm. τ Table 7.2. Slope obtained from e−ph values for Au NPs/p-GaN and Au NPs/Al2O3 excited at 530 nm and 730 nm. Au NPs/p-GaN Au NPs/Al2O3 −9 −3 −1 Slope530 nm (10 ps m J ) 1.09 6.42 −9 −3 −1 Slope730 nm (10 ps m J ) 1.77 6.42

cess raises Ne before electron-phonon interaction becomes dominant, thereby increasing Ce(Te,Ne) of Au NPs. Consequently, this modiﬁes the electron-

57 electron scattering process and limits the peak of electronic temperature Te that is manifested in the electron-phonon relaxation time. Ab initio electronic structure calculations of carrier excitation by the excitation pulse, electron thermalization by electron-electron scattering and electron-phonon scattering were performed to predict the evolution of Te. Figure 7.3b depicts the theoretical evolution of Te for different hole injection efficiencies following laser 8 −3 excitation at 530 nm (Uabs = 5.3×10 Jm ), which is shown to be consistent with the observed trend in figure 7.3a and d. The comparison between the predicted and experimental values of τe−ph (figure 7.3c) indicates that the majority (≈ 88 ± 8%) of hot holes are able to inject into the valence band of p-GaN upon light excitation at 530 nm.

Inﬂuence of Excitation Wavelength

Figure 7.4. Inﬂuence of incident pump wavelength on hot hole injection at the Au NPs/p-GaN interface. a) Ab initio calculation of the energy distributions of hot electrons (positive energies) and hot holes (negative energies) produced on the Au NPs at a pump wavelength of 530 nm (hν = 2.34 eV, solid lines) and 730 nm (hν = 1.70 eV, dashed lines). The vertical dashed line denotes the Schottky barrier (ΦB = 1.1 eV) at the Au NPs/p-GaN interface. b) Ab initio calculation of the velocity distributions of hot electrons and hot holes produced on the Au NPs at a pump wavelength of 530 nm (solid lines) and 730 nm (dashed lines). Negative velocities of hot holes used only as a visual aid to distinguish them from the hot electrons.

58 Light excitation of Au NPs at 530 nm produces preferentially hot holes in the d-bands [104] but hot holes distributed within the sp-band is also present [105]. Conversely, excitation at 730 nm leads to hot holes only in the sp-bands. The shallower slope obtained when exciting at 530 nm suggest that hot hole injection from the d-bands exert a greater inﬂuence over the electron dynamics. In order to explain this difference, ab-initio calculations were carried out where the hot carriers energy and velocity distribution is shown in the ﬁgure 7.4 for laser excitation at 530 nm and 730 nm. It is clear from the graph that for only intraband transition along sp-band, the excitation of Au NPs results in a nearly uniform distribution of carriers with higher velocity, while interband transition leads to a narrower and higher energy distribution but with lower carrier velocity. This theoretical result suggest that the shallower slope observed when exciting at 530 nm, despite the formation of less mobile d-bands holes, is an indication that they have higher probability of injecting into the VB of p-GaN than for holes from sp-bands because the Schottky barrier limit their injection probability.

7.2 Conclusions Investigation of the Au NPs electron-phonon dynamics on two type of materials, namely n-type and p-type semiconductors showed that both hot electron and hot hole injection processes decrease the electron-phonon relaxation time (τe−ph). In the former case, the extraction of hot electrons lowers the thermalized electron temperature (Te) that reduces τe−ph, as is predicted from the two-temperature model. The same effect is observed for hot hole injection process, since the higher number of electrons in the Au NPs increases the electron heat capacity, thereby also limiting Te. This trend was predicted theoretically, which also showed that majority of hot holes from d-bands are able to inject into the valence band of p-GaN. Light excitation of Au NPs/p-GaN at 530 and 730 nm resulted in different electron-phonon dynamics that is associated to formation of hot holes in different bands (d and sp, respectively), which possess different energies and velocities as was showed by ab initio calculations. Despite the lower mobility of d-band holes, their higher energies than the sp-band ones makes them more probable to surpass the Schottky barrier. It must be pointed out that this approach of predicting hot carrier injection efﬁ- ciency through investigation of changes in the electron-phonon dynamics has not been done before. Therefore, more studies on this aspect is necessary until this methodology becomes well estabilished.

59 8. Concluding Remarks

The unique property of plasmonic nanoparticles to absorb light that exceeds their cross-section by orders of magnitude along with the ability to form energetic electron-hole pairs are the key points that push the interest towards plasmonic hot carrier devices. This domain of plasmonics is in an early stage and the prospects rely on understanding the limitations on the generation and utilization of hot carriers. The work presented in this thesis are a mere attempt to elucidate these processes by inserting plasmonic nanoparticles in different systems and analyzing their behaviors through spectroscopic techniques. The fact that the initial hot carrier energy distribution is yet unknown along with their extremely short life time motivated the work done in papers I and II where it was confirmed that both hot electrons and holes can be injected into respective accepting materials upon light excitation. For the first paper involving Ag NPs, prolongation of charge separation in the system was achieved as expected. In the latter, enhancement of charge injection was observed when both materials (TiO2 and PEDOT:PSS) are put together when compared to using them separately. This result indicates that might exist an interplay between electron and hole injection processes on the energy of the charge carriers. The interface of plasmonic nanoparticles with semiconductors was studied using a variety of metal oxides in paper III, by comparing the electron dynamics in these materials. The different Schottky barrier values were not sufficient to explain the electron dynamics in the conduction band of the metal oxides and the relative injection efficiencies, where it was suggested that electron bulk mobility might be playing an important role in the recombination process. This preliminary study can hopefully assist in designing more suitable plasmonic systems based on metal nanoparticles/semiconductors. The relative quantification of different plasmon decays mechanisms and their role in the formation of hot carriers has not been investigated yet. In paper IV the effect of promoting the plasmon decay through phonons is enhanced by increasing the sample temperature in the range that would be expected for device applications, which lead to a higher number of electrons injected into TiO2. This result was attributed to greater yield of hot electrons and not to changes in the interface since theoretical calculations predicted a small increase in the Schottky barrier height with temperature. The last paper, together with the results from paper III, demonstrated that both electron and hole injection from plasmonic nanoparticles decrease the electron-phonon relaxation time (τe−ph). The complementary investigation using ab initio methods showed that the analysis on the change in this parameter (τe−ph) upon charge injection can be evaluated to predict injection

60 efficiencies. For Au NPs, the hot holes energy and velocity distribution was shown to be more sensitive to the incident wavelength due to the existence of interband transitions into the deeper d-bands and intraband transitions across the sp-band, which dictates the injection efficiency. The previous statements comprise the main findings of the works presented in this thesis. Not surprisingly, one of the main limitations of the spectroscopic techniques used in these work has been the time resolution and probing energy range. For instance, time-resolved spectroscopy with attosecond-femtosecond resolution would allow to directly quantify the effect of enhancing plasmon decay through phonons (paper IV). It is hoped that with the advances of time- resolved spectroscopic techniques towards higher timer resolution and devel- opments in achieving a unified quantum mechanical framework using first- principles calculations, many other open questions regarding the plasmon decay and the formation of hot carriers can be further clarified.

8.0.1 Outlook As was aforementioned, plasmonic nanoparticles’ ability to work as light antennas is a known and well-studied property, and resulted in many applications in different fields. However, for hot carrier-based applications, the prospects rely on efficiently using the plasmons’ absorbed electromagnetic energy before it is dissipated into heat. There has been experimental evidence that hot carriers can be extracted into accepting materials with efficiencies suitable for applications. However, these results cannot be generally expected since plasmons can decay through different mechanisms that depend on nanoparticle size, shape, material, temperature, interface, and other parameters. Understanding the mechanisms involving plasmon decay relies strongly on experiments with a high temporal resolution that can record the nonequilibrium electronic processes, which might be obtained by advanced techniques such as X-ray free-electron lasers (XFELs) and two-photon photoemission spectroscopy. One important example is the finding of Tan et al. [81] where it was showed that the dephasing of the plasmon field at the Ag NPs and TiO2 interface directly creates hot electrons in the conduction band of the semiconductor in less than ten femtoseconds, which is in contrast to the less efficient process where hot electrons are generated within the metal and then transferred to the semiconductor. Their studies highlight the importance of tun- ing interface states to quench the plasmon into a semiconductor or adsorbed molecule effectively. These points to promising directions in plasmonics research and development, of course. Nevertheless, other plasmon decay mechanisms might be detrimental for the extraction of hot carriers. The mechanistic insights of these processes may allow the control of one over the others, leading to designing plasmonic systems suitable for the application. One may ask,

61 however, whether only experimental approaches would be able to disentangle these different mechanisms. Fortunately, complementary theoretical and computational approaches have been developed that possess the great advantage of scrutinizing the relevant microscopic processes. Note that the modelling should be beyond the jellium model, and hence, the electronic and atomic structure must be accounted for. The effort necessary to clarify these fundamental questions is substantial, but so are the rewards beyond the domain of plasmonic hot carriers. In the context of solar energy harvesting, the brighter prospects on plasmonics so far are pointing out the applications that are focused on mainly exploiting the optical properties of plasmonic nanoparticles, which surface plasmons are used to assist on light-trapping rather than acting as a photosensitizer or a material with photovoltaic effect.

62 Popular Science Summary

Metals are materials that we come across almost all the time and are ubiquitous in any electronic device due to their well-known property of high conductivity, in other words, the electrons are allowed to flow freely. If light hits a piece of metal, the metal free electrons reflect the light giving its shine. A unique property emerges when the metal is reduced to the nanoscale dimension (1000 times smaller than a human hair), and the electrons become confined - they can start resonating with the incident light. The result is an enhancement of light absorption area by orders of magnitude regarding their size where the metal nanoparticle starts acting as nanoantennas. This unique phenomenon is a consequence of collective oscillations of electrons within the nanoparticle interacting with the light known as surface plasmons, hence the name "plasmonic nanoparticles". It is pretty much like a well-synchronized dance between electrons and the light. Besides, the nanoparticles’ light absorption can be tuned by changing their structure, shape, and size. Not only do they behave as antennas, but they also convert the light absorbed into localized heat. The discovery of these effects led to an array of applications ranging from photothermal cancer therapy to invisible cloaks. Some of them are still transitioning from the laboratory to the market, while others are already a commercial reality. During this ongoing transition in plasmonic technologies, it was found out that the surface plasmons can do even more: transfer energy to create high- energy electrons known as hot electrons. This discovery gathered scientists’ attention due to the potential of combining their unique antenna effect with the capability to produce electricity or drive chemical reactions. Using the sun’s energy to create electricity and solar fuels demand technologies in the face of the environmental crisis we are currently facing. Every day, the sun sends out an enormous amount of energy in the form of light. Only a small portion of the light that travels space can strike the Earth: one part in two billion. However, the corresponding energy accumulated in an hour is still enough to power the entire European continent for a year. This remarkable fact has been pushing many scientists worldwide to pursue renewable energy production based on solar power to decrease the effects caused by global warming. The field of plasmonics is now becoming part of this endeavor. However, there is still plenty to explore and understand how these so-called hot electrons are created and how we can efficiently use them. This is a big challenge in the field of plasmonics because the hot electrons are formed in few femtoseconds, that is one-millionth of one billionth of a second or thou- sand million times shorter than a blink of our eyes.

63 Figure 8.1. Plasmonic nanoparticles can enhance light absorption and use the absorbed energy to create hot electrons or generate local heat.

Herein, silver and gold nanoparticles were fabricated, and their behavior was recording by sending femtosecond light pulses. Semiconductors, which are materials with the capability to store electrons, were put in contact with the metal nanoparticles to extract the hot electrons created before they lose energy. Based on this design, some strategies were applied to gain insights into the plasmonic behaviour and improve the extraction and transport of these hot electrons. As is common in any technology advance, theory and experiment must work together. The combination of both allowed us to understand the complex and extremely fast events that happen after the plasmon is hit by light in more detail. Further answers are hidden in a time shorter than femtosecond timescale. Whether the plasmonic community will ever unravel the new events will depend on advancing other technologies and the powerful and necessary combination of experiment and theoretical investigation.

64 Svensk Sammanfattning

Metaller är material som vi stöter på nästan hela tiden och de är allmänt förekommande i alla elektroniska enheter på grund av deras välkända egenskap med hög ledningsförmåga, som med andra ord får elektronerna att flyta fritt. Om ljus träffar en bit metall så reflekterar de metallfria elektronerna ljuset som skapar glans. En unik egenskap uppstår när metallen reduceras till nanoskala-dimensionen (1000 gånger mindre än ett människohår) och elektronerna blir begränsade - de kan då börja producera något med det infallande ljuset. Resultatet är en för- bättring av ljusabsorptionsarean med deras storlek som gör att metallnanopar- tiklarna börjar fungera som nanoantenner. Detta unika fenomen är en följd av svängningar av sammanförda elektroner i nanopartiklarna som interagerar med ljuset som kallas ytplasmoner, därav namnet "plasmoniska nanopartiklar". Det fungerar ungefär som en väl synkroniserad dans mellan elektroner och ljuset. Dessutom kan nanopartiklarnas ljusabsorption justeras genom att ändra deras struktur, form och storlek. De beter sig inte bara som antenner utan de omvandlar också det absorberade ljuset till direkt värme. Upptäckten av dessa effekter ledde till en rad applikationer, allt från fototermal cancert- erapi till osynliga mantlar. Vissa av de håller fortfarande på att övergå från laboratoriet till marknaden, medan andra redan är i en kommersiell verklighet. Under denna pågående övergång inom plasmonteknik så upptäcktes det att ytplasmonerna kan göra ännu mer: de kan även överföra energi för att skapa högenergielektroner som kallas heta elektroner. Denna upptäckt sam- lade forskarnas uppmärksamhet på grund av den potential som hittade kring att kombinera deras unika antenneeffekt med förmågan att producera el eller driva kemiska reaktioner. Att använda solens energi för att skapa el och sol- bränslen kräver tekniker inför den miljökris som vi för närvarande står inför. Varje dag skickar solen ut en enorm mängd energi i form av ljus. Endast en liten del av ljuset som reser runt i rymden kan nå jorden: en del på två miljarder. Motsvarande energi som ackumulerats på en timme räcker dock fortfarande för att driva hela den europeiska kontinenten under ett år. Detta anmärkningsvärda faktum har drivit många forskare världen över för att driva produktion av förnybar energi baserad på solenergi för att minska effekterna av den globala uppvärmningen. Området av plasmonisk blir nu en del av denna strävan. Det finns dock fortfarande mycket att utforska och förstå kring hur de så kallade heta elektroner skapas och hur vi effektivt kan använda dem. Detta är en stor utmaning med plasmon eftersom de heta elektronerna bildas på

65 bara några få femtosekunder, det vill säga en miljondel av en miljarddel av en sekund eller tusen miljoner gånger snabbare än ett ögonblick.

Figure 8.2. Plasmoniska nanopartiklar kan förbättra ljusabsorptionen och använda den absorberade energin för att skapa heta elektroner eller generera lokal värme.

Häri tillverkades silver- och guldnanopartiklar, och deras beteende registr- erades genom att skicka femtosekundens ljuspulser. Halvledare, som är material med en förmåga att lagra elektroner, sattes i kontakt med metallnanopartik- larna för att extrahera de heta elektroner som skapats innan de tappar energi. Baserat på denna design tillämpades några strategier för att få mer insikt i plasmatiskt beteende och förbättra extraktion och transport av deras heta elektroner. Som vanligt i alla tekniska framsteg så måste teori och experiment ar- beta tillsammans. Kombinationen av båda gjorde det möjligt för oss att förstå de komplexa och extremt snabba händelserna som inträffar efter att plasmon träffas av ljus mer detaljerat. Ytterligare svar döljs på en tid som är kortare än femtosekund. Huruvida den plasmatiska gemenskapen någonsin kommer att avslöja de nya händelserna beror på hur vi kan utveckla andra tekniker och den kraftfulla och nödvändiga kombinationen av experiment och teoretisk undersökning.

66 Acknowledgements

During my PhD I had the great opportunity to meet people who helped me in many aspects within and beyond the workplace. I want to dedicate this page to brieﬂy thank all the support I received in this short journey in Sweden.

First of all, I would like to thank my supervisor Jacinto Sa for giving me the opportunity to do my PhD in Sweden. Since the beginning, the support and trust you gave me were crucial for my academic development and delivery of this work. Thank you for introducing me to the beautiful and complex ﬁeld of plasmonics, which I have decided to continue pursuing at least for a few years more.

Daniel Fernandes, your support at the beginning of my studies was essential for my work adaptation. Your incredible capability of basically doing anything and voracious curiosity was and has been a great source of inspiration.

Mohamed Abdellah, for introducing me to the fs-laser and for your collab- oration in many works included in this thesis. Mariia Pavliuk, thank you for welcoming me to the group and for your warm and positive energy. The cakes were all delicious!

Robert, Vitor, and Xinjian, it was great to ﬁnally have PhD colleagues to discuss and work together in the same research ﬁeld. The work at the lab was way too fun with you guys.

Leif Haggman, Prof. Marina Freitag, Hannes Michael, and all the other professors and students at H6K1, thank you for allowing and helping me to use your laboratory equipment for all these years.

Vincent Wang, it was a unique experience to meet someone obnoxious and warm-hearted like you :-). I hope I can keep asking the "important questions" as you kept religiously repeating.

Belinda and Jens, I’m grateful for the innumerous times we discussed about the fs-laser system and the assistance you both gave me during the troubleshoot- ing.

Prof. Leif Hammarström and all the other seniors, for leading the meetings and creating this open environment for discussions. The Physical Chemistry

67 division has a great working atmosphere, and I have learned a lot during the weekly group meetings.

People from the Wine club: Ben, Hemlata, Lei, Martin, Nora, Robing, and others, for making my Friday nights more fun! Hongwei, I will miss playing basket with you. You are a great mid-range shooter.

Sven Johansson, thank you for being this warm and super helpful presence in the department.

Ageo, Jose, and Rodrigo, thank you for bringing a little bit of Brazil to my life here in Sweden.

Aijie, Joao Rodrigues and Nidhi, arigato for all for the support and for the amazing friendship. I will carry with me all the moments we have shared, especially the daily late evening talks at Ångström.

68 References

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