Optical imaging of strain in two-dimensional crystals

Strain engineering is widely used in material science to tune the (opto-)electronic properties of materials and enhance the performance of devices. Two-dimensional atomic crystals are a versatile playground to study the influence of strain, as they can sustain very large deformations without breaking. Various optical techniques have been employed to probe strain in two-dimensional materials, including micro-Raman and photoluminescence spectroscopy. Here we demonstrate that optical second harmonic generation constitutes an even more powerful technique, as it allows to extract the full strain tensor with a spatial resolution below the optical diffraction limit. Our method is based on the strain-induced modification of the nonlinear susceptibility tensor due to a photoelastic effect. Using a two-point bending technique, we determine the photoelastic tensor elements of molybdenum disulfide. Once identified, these parameters allow us to spatially image the two-dimensional strain field in an inhomogeneously strained sample.Comment: 13 pages, 4 figure

CMOS-compatible graphene photodetector covering all optical communication bands

Optical interconnects are becoming attractive alternatives to electrical wiring in intra- and inter-chip communication links. Particularly, the integration with silicon complementary metal-oxide-semiconductor (CMOS) technology has received considerable interest due to the ability of cost-effective integration of electronics and optics on a single chip. While silicon enables the realization of optical waveguides and passive components, the integration of another, optically absorbing, material is required for photodetection. Germanium or compound semiconductors are traditionally used for this purpose; their integration with silicon technology, however, faces major challenges. Recently, graphene has emerged as a viable alternative for optoelectronic applications, including photodetection. Here, we demonstrate an ultra-wideband CMOS-compatible photodetector based on graphene. We achieve multi-gigahertz operation over all fiber-optic telecommunication bands, beyond the wavelength range of strained germanium photodetectors, whose responsivity is limited by their bandgap. Our work complements the recent demonstration of a CMOS-integrated graphene electro-optical modulator, paving the way for carbon-based optical interconnects.Comment: 18 pages, 4 figures. Nature Photonics, 201

Photovoltaic effect in an electrically tunable van der Waals heterojunction

Semiconductor heterostructures form the cornerstone of many electronic and optoelectronic devices and are traditionally fabricated using epitaxial growth techniques. More recently, heterostructures have also been obtained by vertical stacking of two-dimensional crystals, such as graphene and related two- dimensional materials. These layered designer materials are held together by van der Waals forces and contain atomically sharp interfaces. Here, we report on a type- II van der Waals heterojunction made of molybdenum disulfide and tungsten diselenide monolayers. The junction is electrically tunable and under appropriate gate bias, an atomically thin diode is realized. Upon optical illumination, charge transfer occurs across the planar interface and the device exhibits a photovoltaic effect. Advances in large-scale production of two-dimensional crystals could thus lead to a new photovoltaic solar technology.Comment: 26 pages, 14 figures, Nano Letters 201

Device physics of van der Waals heterojunction solar cells

Heterostructures based on atomically thin semiconductors are considered a promising emerging technology for the realization of ultrathin and ultralight photovoltaic solar cells on flexible substrates. Much progress has been made in recent years on a technological level, but a clear picture of the physical processes that govern the photovoltaic response remains elusive. Here, we present a device model that is able to fully reproduce the current-voltage characteristics of type-II van der Waals heterojunctions under optical illumination, including some peculiar behaviors such as exceedingly high ideality factors or bias-dependent photocurrents. While we find the spatial charge transfer across the junction to be very efficient, we also find a considerable accumulation of photogenerated carriers in the active device region due to poor electrical transport properties, giving rise to significant carrier recombination losses. Our results are important to optimize future device architectures and increase power conversion efficiencies of atomically thin solar cells.Comment: 20 pages, 5 figure

Solar-energy conversion and light emission in an atomic monolayer p-n diode

Two-dimensional (2D) atomic crystals, such as graphene and atomically thin transition metal dichalcogenides (TMDCs), are currently receiving a lot of attention. They are crystalline, and thus of high material quality, even so, they can be produced in large areas and are bendable, thus providing opportunities for novel applications. Here, we report a truly 2D p-n junction diode, based on an electrostatically doped tungsten diselenide (WSe2) monolayer. As p-n diodes are the basic building block in a wide variety of optoelectronic devices, our demonstration constitutes an important advance towards 2D optoelectronics. We present applications as (i) photovoltaic solar cell, (ii) photodiode, and (iii) light emitting diode. Light power conversion and electroluminescence efficiencies are ca. 0.5 % and 0.1 %, respectively. Given the recent advances in large-scale production of 2D crystals, we expect them to profoundly impact future developments in solar, lighting, and display technologies.Comment: 23 pages, 7 figures. Nature Nanotechnology (2014

Microcavity-integrated graphene photodetector

The monolithic integration of novel nanomaterials with mature and established technologies has considerably widened the scope and potential of nanophotonics. For example, the integration of single semiconductor quantum dots into photonic crystals has enabled highly efficient single-photon sources. Recently, there has also been an increasing interest in using graphene - a single atomic layer of carbon - for optoelectronic devices. However, being an inherently weak optical absorber (only 2.3 % absorption), graphene has to be incorporated into a high-performance optical resonator or waveguide to increase the absorption and take full advantage of its unique optical properties. Here, we demonstrate that by monolithically integrating graphene with a Fabry-Perot microcavity, the optical absorption is 26-fold enhanced, reaching values >60 %. We present a graphene-based microcavity photodetector with record responsivity of 21 mA/W. Our approach can be applied to a variety of other graphene devices, such as electro-absorption modulators, variable optical attenuators, or light emitters, and provides a new route to graphene photonics with the potential for applications in communications, security, sensing and spectroscopy.Comment: 19 pages, 4 figure

Photodetektion und Photovoltaik mit atomar dünnen Kristallen

Atomar dünne Kristalle sind neuartige Materialien mit außergewöhnlichen optischen und elektronischen Eigenschaften. Diese sind zu einem großen Teil auf die stark eingeschränkte Bewegungsfreiheit der Ladungsträger aufgrund der geringen Schichtdicken zurückzuführen. Der Fokus dieser Dissertation liegt auf der Erforschung der optoelektronischen Eigenschaften zweidimensionaler Halbleiter sowie daraus hergestellter Heterostrukturen. Übergangsmetall-Dichalkogenide treten häufig in geschichteter Kristallstruktur auf. Die schwachen Van der Waals Bindungskräfte zwischen den einzelnen Ebenen ermöglichen die Synthese von Kristallen mit atomarer Dicke. Werden elektronische oder optoelektronische Bauelemente aus solchen Materialien hergestellt, zeigt sich eine starke Abhängigkeit der Bauteilseigenschaften von den Umgebungsparametern. Diese entsteht durch das extreme Verhältnis von aktivem zu umgebenden Material. Anhand von MoS2-Feldeffekttransistoren konnten wir nachweisen, dass deren Lichtempfindlichkeit maßgeblich durch das Einfangen von Ladungsträgern beeinflusst wird. Dabei gelang es, die zwei dominanten Komponenten zu identifizieren und isolieren. Es wurde nachgewiesen, dass ein Teil des delektierten Signals auf das Einfangen von Ladungsträgern durch bandkantennahe Zustände zurückzuführen ist. Dies hat eine Erhöhung der effektiven Ladungsträgerlebensdauer, und damit einhergehend eine höhere Leitfähigkeit des Kanals, zufolge. Da die Anlagerungsniveaus ihren Ursprung in Gitterfehlern haben, hängt diese Komponente direkt von den Materialeigenschaften des Kanals ab. Im Gegensatz dazu, basiert die zweite Komponente auf dem Einfangen von Ladungsträgern in direkter Umgebung zum Kanal. Die eingefangenen Ladungsträger agieren als lokale Gate-Elektroden und führen zu einer Veränderung der Schwellspannung. Durch einen Austausch der gasförmigen Umgebung des Kanals konnte daher die Lichtempfindlichkeit des Transistors maßgeblich beeinflusst werden. Die experimentellen Ergebnisse konnten anhand eines einfachen Modells vollständig reproduziert werden. Darüber hinaus gelang es, den Grund für die weite Streuung der bisher publizierten Ergebnisse zu erklären. Thermoplastische Polymere können verwendet werden, um einzelne, atomar dünne Kristalle von einem Trägersubstrat aufzunehmen und übereinander zu stapeln. Es gelang uns zu zeigen, dass die so erzeugten Heterostrukturen aus zwei bzw. drei unterschiedlichen halbleitenden Übergangsmetall-Dichalkogeniden verwendet werden können, um optische Energie in elektrische umzuwandeln. Durch die außerordentlich starke Lichtabsorption in atomar dünnen Schichten konnte dies mit nur wenigen Atomlagen dicken Zellen erzielt werden. Die gemessenen Von den gemessenen elektrischen und photovoltaischen Kennlinien konnte der Umwandlungsprozess erklärt werden: Die Typ-II Heterostruktur führt zu einer Dissoziation der optisch generierten Exzitonen. Da sie darüber hinaus als diskriminierende Barriere für den Transport der Löcher und Elektronen fungiert, kommt es zu einer Ladungstrennung und somit zu dem beobachteten Photovoltaischen Effekt. Im Zuge dieser Arbeit wurden zwei Polymer-basierte Fabrikationsprozesse optimiert und weiterentwickelt. Diese ermöglichten die Herstellung von Heterostrukturen mit sauberen, abrupten Materialübergängen. In Betracht der rapiden Weiterentwicklung von Wachstumsprozessen für atomar dünne Übergangsmetall-Dichalkogenide, suggerieren unsere Ergebnisse, dass Van der Waals Heterostrukturen die Grundlage für eine neue Generation von exzitonischen Solarzellen bilden könnten.Atomically thin crystals are an emerging class of materials with outstanding optical and electronic properties. Many of these arise from the confinement of charge carriers to a quasi-two-dimensional plane. This thesis focuses on the fundamental optoelectronic characteristics of two-dimensional semiconductors and heterostructures made therefrom. Within the area of solid state physics, single layers of transition metal dichalcogenides have become subject of intensive studies. The extreme ratio of surrounding-to-active material in electronic and optoelectronic devices made from mono- and few-layer transition metal dichalcogenides strongly enhances the impact of the environment on the device properties. We demonstrated, by the example of molybdenum disulfide mono- and bilayer field effect transistors, that the photoresponse observed under non-zero bias strongly depends on the trapping of charge carriers. The two dominating components were successfully identified and separated. The first one is intrinsic and arises from trapping of photogenerated carriers in band tail trap states. This results in an increase of the effective carrier lifetime, and thus in a reduction of the channel resistance. The second, extrinsic one, was found to stem from trapping in the surroundings of the channel. It was shown that the trapped charge carriers act as local gates that are the origin of a transistor threshold voltage shift. Hence, it was possible to modulate the conductance by modifying the gaseous environment of the device. We not only presented a simple model that reproduced our experimental findings, but were also able to elucidate the origin of the strong variation found in previously reported values. Van der Waals bound heterostructures can be easily fabricated by thermoplastic polymer based stacking techniques. By using two and more atomically thin semiconducting layers we could demonstrate that the so formed staggered heterostructures are able to harvest solar energy. Because of the extraordinarily strong absorption of light in atomically thin layers, it was possible to achieve an efficient power conversion with a minimal amount of active material. By carefully analyzing the observed electrical and photovoltaic properties we could explain their origin: the band-structure of the fabricated stacks efficiently dissociates the photogenerated excitons and forms a discriminating barrier for the transport of the so generated electrons and holes. During this work, two fabrication methods that allow the realization of heterostructures with clean, atomically sharp interfaces were optimized. Our findings suggest that, accompanied by the advances in large area fabrication of atomically thin transition metal dichalcogenides, van der Waals heterostructures are promising candidates for a new generation of excitonic solar cells