Dmitry K. Polyushkin

Novel highly conductive and transparent graphene-based conductors

Transparent conductors based on few-layer graphene (FLG) intercalated with ferric chloride (FeCl(3)) have an outstandingly low sheet resistance and high optical transparency. FeCl(3)-FLGs outperform the current limit of transparent conductors such as indium tin oxide, carbon-nanotube films, and doped graphene materials. This makes FeCl(3)-FLG materials the best transparent conductor for optoelectronic devices.

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.

THz generation from plasmonic nanoparticle arrays.

We investigate the generation of THz pulses when arrays of silver nanoparticles are irradiated by femtosecond laser pulses, providing the first reproducible experimental evidence in support of recent theoretical predictions of such an effect. We assess our results in the context of a model where photoelectrons are produced by plasmon-mediated multiphoton excitation, and THz radiation is generated via the acceleration of the ejected electrons by ponderomotive forces arising from the inhomogeneous plasmon field. By exploring the dependence of the THz emission on the femtosecond pulse intensity and as a function of metal nanoparticle morphology, and by comparing measurements to numerical modeling, we are able to verify the role of the particle plasmon mode in this process.

A microprocessor based on a two-dimensional semiconductor

The advent of microcomputers in the 1970s has dramatically changed our society. Since then, microprocessors have been made almost exclusively from silicon, but the ever-increasing demand for higher integration density and speed, lower power consumption and better integrability with everyday goods has prompted the search for alternatives. Germanium and III–V compound semiconductors are being considered promising candidates for future high-performance processor generations and chips based on thin-film plastic technology or carbon nanotubes could allow for embedding electronic intelligence into arbitrary objects for the Internet-of-Things. Here, we present a 1-bit implementation of a microprocessor using a two-dimensional semiconductor—molybdenum disulfide. The device can execute user-defined programs stored in an external memory, perform logical operations and communicate with its periphery. Our 1-bit design is readily scalable to multi-bit data. The device consists of 115 transistors and constitutes the most complex circuitry so far made from a two-dimensional material.

Device physics of van der Waals heterojunction solar cells

Two-dimensional group-VI transition metal dichalcogenide semiconductors, such as MoS2, WSe2 and others, exhibit strong light-matter coupling and possess direct band gaps in the infrared and visible spectral regimes, making them potentially interesting candidates for various applications in optics and optoelectronics. Here, we review their optical and optoelectronic properties with emphasis on exciton physics and devices. As excitons are tightly bound in these materials and dominate the optical response even at room-temperature, their properties are examined in depth in the first part of this article. We discuss the remarkably versatile excitonic landscape, including bright, dark, localized and interlayer excitons. In the second part, we provide an overview on the progress in optoelectronic device applications, such as electrically driven light emitters, photovoltaic solar cells, photodetectors and opto-valleytronic devices, again bearing in mind the prominent role of excitonic effects. We conclude with a brief discussion on challenges that remain to be addressed to exploit the full potential of transition metal dichalcogenide semiconductors in possible exciton-based applications.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.Device physics: photovoltaic response of van der Waals heterojunctionsThe photophysics of van der Waals heterojunctions can be captured by a transport equation fully reproducing their photovoltaic response. A team led by Thomas Mueller at Vienna University of Technology presented an experimental study of a MoS2/WSe2 van der Waals heterostructure along with a device model capable of fully reproducing the current-voltage characteristics of type-II van der Waals heterojunctions under optical illumination. The model is able to capture some peculiar behaviors occurring in heterostructures of atomically thin transition metal dichalcogenides, including their high ideality factors and bias-dependent photocurrents. As a result, a modified transport equation should be used to describe their photovoltaic response, instead of the commonly used Shockley equation. These results pave the way to the optimization of future device architectures for photovoltaic applications.

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 extraction of 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.Strain is an effective tool to tune the optoelectronic properties of two-dimensional materials. Here, the authors demonstrate that second harmonic generation can be used to extract the full strain tensor of MoS2 and to spatially image its two-dimensional strain field.

Nanophotonics with two-dimensional atomic crystals

Electronic and photonic devices based on two-dimensional (2D) atomic crystals, such as graphene and layered transition-metal dichalcogenides (TMDCs), are perceived as potential candidates to complement, or even replace, conventional semiconductor devices in various applications. 2D crystals are of high material quality and stability, even so, they can be produced with large-area dimensions and at low cost. Moreover, the possibility of stacking different atomically-thin 2D layers on top of each other provides the opportunity of creating “artificial” designer materials, so-called van der Waals heterostructures. In this paper, optoelectronic devices based on 2D materials will be presented. We will discuss photodetection, light emission and photovoltaic energy conversion in 2D monolayers and van der Waals heterojunctions.

Plasmonics and THz frequency generation

We report the generation of THz pulses from arrays of silver nanoparticles when irradiated by femtosecond laser pulses. We suggest that this effect arises from the emission of photoelectrons by multi-photon excitation and subsequent acceleration of these emitted electrons by ponderomotive forces associated with the optical fields of the plasmons in the metallic nanostructures.

Plasmonics for THz frequency applications

In this contribution we concentrate on two aspects of THz science related to surface and particle plasmons. Firstly, we report on the generation of THz pulses via irradiation of arrays of silver nanoparticles by femtosecond laser pulses. We propose that this effect arises from the emission of photoelectrons by multi-photon excitation and subsequent acceleration of these emitted electrons by ponderomotive forces associated with the optical fields of the plasmons in the metallic nanostructures. Secondly, we demonstrate that semiconductors supports strongly confined surface plasmons in the THz frequency range. We show that these SPs can be utilized to enhance the light-matter interaction with dielectric layers above the semiconductor surface, thereby allowing us to detect the presence of layers around one thousand times thinner than the free space wavelength of the THz light. We discuss the viability of using semiconductor SPs for the purposes of THz sensing and spectroscopy.