Graphene is an attractive material for optoelectronics and photodetection applications because it offers a broad spectral bandwidth and fast response times. However, weak light absorption and the ...absence of a gain mechanism that can generate multiple charge carriers from one incident photon have limited the responsivity of graphene-based photodetectors to ∼10(-2) A W(-1). Here, we demonstrate a gain of ∼10(8) electrons per photon and a responsivity of ∼10(7) A W(-1) in a hybrid photodetector that consists of monolayer or bilayer graphene covered with a thin film of colloidal quantum dots. Strong and tunable light absorption in the quantum-dot layer creates electric charges that are transferred to the graphene, where they recirculate many times due to the high charge mobility of graphene and long trapped-charge lifetimes in the quantum-dot layer. The device, with a specific detectivity of 7 × 10(13) Jones, benefits from gate-tunable sensitivity and speed, spectral selectivity from the short-wavelength infrared to the visible, and compatibility with current circuit technologies.
The ability to manipulate optical fields and the energy flow of light is central to modern information and communication technologies, as well as quantum information processing schemes. However, ...because photons do not possess charge, a way of controlling them efficiently by electrical means has so far proved elusive. A promising way to achieve electric control of light could be through plasmon polaritons—coupled excitations of photons and charge carriers—in graphene. In this two-dimensional sheet of carbon atoms, it is expected that plasmon polaritons and their associated optical fields can readily be tuned electrically by varying the graphene carrier density. Although evidence of optical graphene plasmon resonances has recently been obtained spectroscopically, no experiments so far have directly resolved propagating plasmons in real space. Here we launch and detect propagating optical plasmons in tapered graphene nanostructures using near-field scattering microscopy with infrared excitation light. We provide real-space images of plasmon fields, and find that the extracted plasmon wavelength is very short—more than 40 times smaller than the wavelength of illumination. We exploit this strong optical field confinement to turn a graphene nanostructure into a tunable resonant plasmonic cavity with extremely small mode volume. The cavity resonance is controlled in situ by gating the graphene, and in particular, complete switching on and off of the plasmon modes is demonstrated, thus paving the way towards graphene-based optical transistors. This successful alliance between nanoelectronics and nano-optics enables the development of active subwavelength-scale optics and a plethora of nano-optoelectronic devices and functionalities, such as tunable metamaterials, nanoscale optical processing, and strongly enhanced light–matter interactions for quantum devices and biosensing applications.
The photoresponse of graphene at mid-infrared frequencies is of high technological interest and is governed by fundamentally different underlying physics than the photoresponse at visible ...frequencies, as the energy of the photons and substrate phonons involved have comparable energies. Here, we perform a spectrally resolved study of the graphene photoresponse for mid-infrared light by measuring spatially resolved photocurrent over a broad frequency range (1000–1600 cm–1). We unveil the different mechanisms that give rise to photocurrent generation in graphene on a polar substrate. In particular, we find an enhancement of the photoresponse when the light excites bulk or surface phonons of the SiO2 substrate. This work paves the way for the development of graphene-based mid-infrared thermal sensing technology.
Cell membranes are intrinsically heterogeneous, as the local protein and lipid distribution is critical to physiological processes. Even in template systems embedding a single protein type, like ...purple membranes, there can be a different local response to external stimuli or environmental factors, resulting in heterogeneous conformational changes. Despite the dramatic advances of microspectroscopy techniques, the identification of the conformation heterogeneity is still a challenging task. Tip‐enhanced infrared nanospectroscopy is here used to identify conformational changes connected to the hydration state of the transmembrane proteins contained in a 50 nm diameter cell membrane area, without the need for fluorescent labels. In dried purple membrane monolayers, areas with fully hydrated proteins are found among large numbers of molecules with randomly distributed hydration states. Infrared nanospectroscopy results are compared to the spectra obtained with diffraction‐limited infrared techniques based on the use of synchrotron radiation, in which the diffraction limit still prevents the observation of nanoscale heterogeneity.
Cell membranes are intrinsically heterogeneous, as the local protein and lipid distribution is critical to physiological processes. Tip‐enhanced infrared nanospectroscopy enables an unprecedented small number of molecules in the mid‐infrared to be probed, on a deepsubwavelength scale. Infrared nanospectroscopy can identify heterogeneous conformational changes connected to hydration of transmembrane proteins contained in membrane monolayers.
Infrared (IR) nanospectroscopy by photothermal induced resonance (PTIR) is a novel experimental technique that combines the nanoscale resolution granted by atomic force microscopy (AFM) and the ...chemical labelling made possible by IR absorption spectroscopy. While the technique has developed enormously over the last decade from an experimental point of view, the theoretical modelling of the signal still varies significantly throughout the literature and misses a solid benchmark. Here, we report an analysis focused on the electromagnetic and thermal simulations of a PTIR experiment. Thanks to a control experiment where the signal is acquired as a function of the thickness of a polymer film and for different tip geometries, we find clear evidence that the interface thermal resistances play a key role in the determination of the measured signal and should therefore always be accounted for by any quantitative modelling.
Since its discovery in 2004, graphene, a one-atom-thick layer of carbon atoms arranged in a hexagonal lattice, has attracted huge interest from the scientific community due to its extraordinary ...electronic, mechanical, and optical properties. While most of the earliest studies focused on electronic transport, in recent years the fields of graphene photonics and optoelectronics have thriven.
The goal of this thesis is to explore the use of graphene for novel optoelectronic devices, adopting different approaches to enhance the electrically tunable graphene-light interaction in a broad spectral range, from the visible to the mid-infrared. This includes investigating the sub-wavelength interaction and energy transfer between a dipole and a graphene sheet, as well as working on efficient photodetection schemes.
Indeed graphene high electronic mobility, broadband absorption, flexibility and tunable optoelectronic properties (described in Chapter 1) make it extremely appealing for the development of optoelectronic applications with new functionalities.
Concerning the devices, the starting point of the experiments presented in the thesis are graphene field effect transistors of different geometries, whose fabrication and characterization techniques are described in Chapter 2. The tunability of the optoelectronic properties via control over the Fermi energy is an essential feature of the fabricated devices. The change in the Fermi level is achieved applying a voltage to a back-gate or a polymer electrolyte top-gate.
We address both aspects at the core of optoelectronics, i.e. the control of optical properties with electric fields and the modification of electrical quantities, such as current, with light. Therefore the first part of the thesis (comprising Chapter 3, 4 and 5) is devoted to graphene nanophotonics and plasmonics, while the second part deals with graphene-based photodetection (Chapter 6, 7, 8 and 9).
In Chapter 3, the main concepts at the basis of graphene nanophotonics are presented, such as the electrical tunability and the strong field confinement of the 2D plasmons, as well as the coupling of an optical emitter to graphene plasmons or electron-hole pair excitations. Then we present two experiments showing the control of light by means of static electric fields. In Chapter 4 we show the electrical control of the relaxation pathways of erbium ions in close proximity to a graphene sheet: the energy flow from the emitters is tuned to electron-hole pairs in graphene, to free space photons and to plasmons by changing the graphene Fermi level. In Chapter 5 we present the real-space imaging and tuning of highly confined graphene plasmons in the mid-infrared, launched by the dipole of a metallized s-SNOM tip (Chapter 5). In this case modifying the graphene Fermi level leads to a change in the plasmon wavelength.
In Chapter 6 we review existing schemes for graphene photodetectors and the main mechanisms enabling photodetection with graphene, with particular emphasis toward the photothermoelectric effect. Then we present three cases where graphene photoresponse is enhanced exploiting the interaction with surrounding materials. A hybrid graphene-quantum dot photodetector in the visible and near-infrared is reported in Chapter 7: a photogating effect after light absorption in the quantum dots leads to extremely high responsivities (over one million A/W). In Chapter 8 we demonstrate how the excitation of bulk phonons of a polar substrate enhances the mid-infrared photocurrent via a photothermoelectric effect. Also substrate surface phonons, launched by illuminating a metal edge with light polarized perpendicularly to it, lead to an increase in the photoresponse, as described in Chapter 9.
The results presented in this thesis open new avenues in the field of graphene-based optoelectronics for active nano-photonics and sensing.
Desde su descubrimiento en 2004, el grafeno, una sola capa átomos de carbono en un retículo hexagonal, ha atraído un gran interés de la comunidad científica debido a sus propiedades electrónicas, mecánicas y ópticas extraordinarias. Los primeros estudios se centraron en el transporte electrónico, pero en los últimos años estudios en el campo de la fotónica y de las propiedades optoelectrónicas del grafeno han suscitado un mayor interés. El objetivo de esta tesis es explorar el uso del grafeno para nuevos dispositivos optoelectrónicos, adoptando diferentes enfoques para mejorar la interacción del grafeno con la luz en un amplio rango espectral, desde el rango visible hasta el infrarrojo medio. Esto incluye la investigación de la interacción y la transferencia de energía entre un dipolo y una monocapa de grafeno cercana, así como trabajar en esquemas de fotodetección eficientes.
La alta movilidad electrónica, la absorción de banda ancha, la flexibilidad y las propiedades optoelectrónicas sintonizables (véase Capítulo 1) hacen que el grafeno sea extremadamente atractivo para el desarrollo de aplicaciones optoelectrónicas con nuevas propiedades funcionalidades.
En cuanto a los dispositivos, el punto de partida de los experimentos presentados en esta tesis son transistores de efecto de campo con diferentes geometrías, cuya fabricación y técnicas de caracterización se describen en el Capítulo 2. La capacidad de ajuste de las propiedades optoelectrónicas a través del control de la energía de Fermi es una característica esencial de los dispositivos, y se logra con la aplicación de un voltaje de puerta.
Nos dirigimos a ambos aspectos a la base de la optoelectrónica, es decir, el control de las propiedades ópticas con campos eléctricos y la modificación de magnitudes eléctricas, como la corriente con la luz incidente. Por tanto, la primera parte de la tesis (Capítulos 3, 4 y 5) se dedica al estudio de la nanofotónica y plasmónica del grafeno, mientras que la segunda parte se ocupa de fotodetección basada en grafeno (Capítulos 6, 7, 8 y 9).
En el Capítulo 3, se explican los principales conceptos del campo de la nanofotónica de grafeno, como la capacidad de ajuste eléctrico y el fuerte confinamiento de los plasmones 2D, así como el acoplamiento de un emisor óptico con los plasmones o pares electrón-hueco. Luego se presentan dos experimentos que muestran el control de la luz por medio de campos eléctricos estáticos. En el Capítulo 4 se muestra el control eléctrico de las vías de relajación de iones de erbio en las proximidades de una monocapa de grafeno: el flujo de energía a partir de los emisores se puede dirigir a pares electrón-hueco en el grafeno, a fotones y a plasmones cambiando el nivel de Fermi del grafeno. En el Capítulo 5 se presenta la excitación y el ajuste de plasmones de grafeno altamente confinados en el infrarrojo medio, activado mediante el dipolo de una punta de microscopia de campo cercano (Capítulo 5).
En el Capítulo 6 se revisan los fotodetectores de grafeno existentes y los principales mecanismos que permitan fotodetección con grafeno. A continuación se presentan tres casos donde la fotorrespuesta del grafeno se mejora con la explotación de la interacción con los materiales circundantes. Un fototransistor híbrido de grafeno y puntos cuánticos (véase Capitulo 7) llega a responsividad extremadamente alta en el visible y infrarrojo cercano (más de un millón de A/W). En el Capítulo 8 se demuestra cómo la excitación de fonones de bulk de un sustrato polar aumenta la fotocorriente en el infrarrojo medio a través de un efecto
fototermoeléctrico. También fonones superficie del sustrato, lanzados por la iluminación de un borde de metal con luz polarizada perpendicularmente, conducen a un aumento en la fotorrespuesta (Capítulo 9).
Los resultados presentados en esta tesis abren nuevos caminos en el campo de la optoelectrónica basada en el grafeno en el campo de la nano-fotónica activa y de los sensores
The photoresponse of graphene at mid-infrared frequencies is of high technological interest and is governed by fundamentally different underlying physics than the photoresponse at visible ...frequencies, as the energy of the photons and substrate phonons involved have comparable energies. Here we perform a spectrally resolved study of the graphene photoresponse for mid-infrared light by measuring spatially resolved photocurrent over a broad frequency range (1000-1600 cm\(^{-1}\)). We unveil the different mechanisms that give rise to photocurrent generation in graphene on a polar substrate. In particular, we find an enhancement of the photoresponse when the light excites bulk or surface phonons of the SiO\(_2\) substrate. This work paves the way for the development of graphene-based mid-infrared thermal sensing technology.
Graphene has emerged as a novel platform for opto-electronic applications and photodetector, but the inefficient conversion from light to current has so far been an important roadblock. The main ...challenge has been to increase the light absorption efficiency and to provide a gain mechanism where multiple charge carriers are created from one incident photon. Here, we take advantage of the strong light absorption in quantum dots and the two-dimensionality and high mobility of graphene to merge these materials into a hybrid system for photodetection with extremely high sensitivity. Exploiting charge transfer between the two materials, we realize for the first time, graphene-based phototransistors that show ultrahigh gain of 10^8 and ten orders of magnitude larger responsivity compared to pristine graphene photodetectors. These hybrid graphene-quantum dot phototransistors exhibit gate-tunable sensitivity, spectral selectivity from the shortwave infrared to the visible, and can be integrated with current circuit technologies.
The ability to manipulate optical fields and the energy flow of light is central to modern information and communication technologies, as well as quantum information processing schemes. However, as ...photons do not possess charge, controlling them efficiently by electrical means has so far proved elusive. A promising way to achieve electric control of light could be through plasmon polaritons - coupled excitations of photons and charge carriers - in graphene. In this two-dimensional sheet of carbon atoms, it is expected that plasmon polaritons and their associated optical fields can be readily tuned electrically by varying the graphene carrier density. While optical graphene plasmon resonances have recently been investigated spectroscopically, no experiments so far have directly resolved propagating plasmons in real space. Here, we launch and detect propagating optical plasmons in tapered graphene nanostructures using near-field scattering microscopy with infrared excitation light. We provide real-space images of plasmonic field profiles and find that the extracted plasmon wavelength is remarkably short - over 40 times smaller than the wavelength of illumination. We exploit this strong optical field confinement to turn a graphene nanostructure into a tunable resonant plasmonic cavity with extremely small mode volume. The cavity resonance is controlled in-situ by gating the graphene, and in particular, complete switching on and off of the plasmon modes is demonstrated, thus paving the way towards graphene-based optical transistors. This successful alliance between nanoelectronics and nano-optics enables the development of unprecedented active subwavelength-scale optics and a plethora of novel nano-optoelectronic devices and functionalities, such as tunable metamaterials, nanoscale optical processing and strongly enhanced light-matter interactions for quantum devices and (bio)sensors.
PDF
