Andreas Pospischil

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

The limitations of the bulk semiconductors currently used in electronic devices-rigidity, heavy weight and high costs--have recently shifted the research efforts to two-dimensional atomic crystals such as graphene and atomically thin transition-metal dichalcogenides. These materials have the potential to be produced at low cost and in large areas, while maintaining high material quality. These properties, as well as their flexibility, make two-dimensional atomic crystals attractive for applications such as solar cells or display panels. The basic building blocks of optoelectronic devices are p-n junction diodes, but they have not yet been demonstrated in a two-dimensional material. Here, we report a p-n junction diode based on an electrostatically doped tungsten diselenide (WSe2) monolayer. We present applications as a photovoltaic solar cell, a photodiode and a light-emitting diode, and obtain light-power conversion and electroluminescence efficiencies of ∼ 0.5% and ∼ 0.1%, respectively. Given recent advances in the large-scale production of two-dimensional crystals, we expect them to profoundly impact future developments in solar, lighting and display technologies.

Microcavity-Integrated Graphene Photodetector

There is an increasing interest in using graphene1,2 for optoelectronic applications.3−19 However, because graphene is an inherently weak optical absorber (only ≈2.3% absorption), novel concepts need to be developed to increase the absorption and take full advantage of its unique optical properties. We demonstrate that by monolithically integrating graphene with a Fabry-Pérot microcavity, the optical absorption is 26-fold enhanced, reaching values >60%. We present a graphene-based microcavity photodetector with 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.

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.

Mechanisms of Photoconductivity in Atomically Thin MoS2

Atomically thin transition metal dichalcogenides have emerged as promising candidates for sensitive photodetection. Here, we report a photoconductivity study of biased mono- and bilayer molybdenum disulfide field-effect transistors. We identify photovoltaic and photoconductive effects, which both show strong photogain. The photovoltaic effect is described as a shift in transistor threshold voltage due to charge transfer from the channel to nearby molecules, including SiO2 surface-bound water. The photoconductive effect is attributed to the trapping of carriers in band tail states in the molybdenum disulfide itself. A simple model is presented that reproduces our experimental observations, such as the dependence on incident optical power and gate voltage. Our findings offer design and engineering strategies for atomically thin molybdenum disulfide photodetectors, and we anticipate that the results are generalizable to other transition metal dichalcogenides as well.

Black Phosphorus Mid-Infrared Photodetectors with High Gain

Recently, black phosphorus (BP) has joined the two-dimensional material family as a promising candidate for photonic applications due to its moderate bandgap, high carrier mobility, and compatibility with a diverse range of substrates. Photodetectors are probably the most explored BP photonic devices, however, their unique potential compared with other layered materials in the mid-infrared wavelength range has not been revealed. Here, we demonstrate BP mid-infrared detectors at 3.39 μm with high internal gain, resulting in an external responsivity of 82 A/W. Noise measurements show that such BP photodetectors are capable of sensing mid-infrared light in the picowatt range. Moreover, the high photoresponse remains effective at kilohertz modulation frequencies, because of the fast carrier dynamics arising from BPs moderate bandgap. The high photoresponse at mid-infrared wavelengths and the large dynamic bandwidth, together with its unique polarization dependent response induced by low crystalline symmetry, can be coalesced to promise photonic applications such as chip-scale mid-infrared sensing and imaging at low light levels.

Silver nanoisland enhanced Raman interaction in graphene

Graphene shows great potential for optoelectronic applications but suffers from rather weak interaction with light due its single-atomic thickness. Here, we report the enhanced interaction of graphene and light for Raman transitions using localized surface plasmons. The plasmons are generated in silver nanoislands that we fabricate by simple means of metal deposition on top of graphene. Despite the broad size distribution of the nanoislands, we find a 100-fold enhancement of the Raman signal. We provide an analytical model for the description of the optical properties and obtain the scattering cross section as well as enhancement factors for the Raman transitions. In addition, we investigate, both optically and electrically, the doping that is introduced by the nanoislands.

Photovoltaics in Van der Waals Heterostructures

The peculiar nature of light-matter interaction in atomically thin transition metal dichalcogenides is recently under examination for application in novel optoelectronic devices. Here, we show that heterostructures composed of two or more such layers can be used for solar energy harvesting. The strong absorption in these atomically thin layers makes it possible to achieve an efficient power conversion with a minimal amount of active material. We describe in detail two different fabrication techniques that allow to realize heterostructures with clean, atomically sharp interfaces. The observed electrical and photovoltaic properties are analyzed. 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.

Graphene mode-locked Cr:ZnS chirped-pulse oscillator

We report the first to our knowledge high-energy graphene mode-locked solid-state laser operating in the positive dispersion regime. Pulses with 15.5 nJ energy and 42 nm spectral bandwidth with 0.87 ps duration were obtained at 2.4 μm wavelength. The output can be compressed down to 189 fs. The graphene absorber damage threshold was established at fluence approaching 1 mJ/cm².

Electric field modulation of thermovoltage in single-layer MoS2

We study electric field modulation of the thermovoltage in single-layer MoS2. The Seebeck coefficient generally increases for a diminishing free carrier concentration, and in the case of single-layer MoS2 reaches considerable large values of about S = −5160 μV/K at a resistivity of 490 Ω m. Further, we observe time dependent degradation of the conductivity in single layer MoS2, resulting in variations of the Seebeck coefficient. The degradation is attributable to adsorbates from ambient air, acting as p-dopants and additional Coulomb potentials, resulting in carrier scattering increase, and thus decrease of the electron mobility. The corresponding power factors remain at moderate levels, due to the low conductivity of single layer MoS2. However, as single-layer MoS2 has a short intrinsic phonon mean free path, resulting in low thermal conductivity, MoS2 holds great promise as high-performance 2D thermoelectric material.

Graphene Mode-locked Cr:ZnS Laser with 44 fs Pulse Duration

The first Cr:ZnS laser mode-locked by graphene-based saturable absorber mirror generates the shortest reported so far mid-IR pulses of only 5.5 optical cycles (44 fs) at 2.35 μm with 139 nm spectral bandwidth.