Not all 2D materials are as strong as graphene: Transition Metal Dichalcogenides get weaker when thickness decreases

Paper published in Adv. Mater.

A new study recently published in “Advanced Materials” reveals that MoSe2, a prominent material of the transition metal dichalcogenides (TMDs) family, loses relative stiffness when its thickness is reduced. This work was carried out by researchers from the Adam Mickiewicz University (AMU) in Poznan (Poland) and the ICN2, under the coordination of Dr Bartlomiej Graczykowski and Dr Klaas-Jan Tielrooij, respectively.

Since the discovery of graphene, a wonder material exhibiting remarkable properties due to its being as thin as a single layer of atoms, a wide variety of new 2D materials have been fabricated and studied. The general expectation is that, as for graphene, such materials’ mechanical properties are superior to their bulk counterparts. However, this is not the case for molybdenum diselenide (MoSe2), one of the most attractive members of the transition metal dichalcogenides (TMD) family, which on the contrary becomes increasingly softer when made thinner.

These results, which contradict the common assumption that relative mechanical strength increases at the nanoscale, were reported in a paper recently published in the journal Advanced Materials. The study was coordinated by Dr Bartlomiej Graczykowski, from the Adam Mickiewicz University (AMU) in Poznan (Poland), and Dr Klaas-Jan Tielrooij, leader of the ICN2 Ultrafast Dynamics in Nanoscale Systems group. “Our findings are extraordinary since they clearly show a progressive softening of MoSe2 while reducing its thickness from bulk to three molecular layers,” explains Visnja Babacic, Ph.D. student at AMU and first author of the paper.

The research team was able to study the elastic properties of various samples of MoSe2, of progressively thinner dimensions, by means of a technique called micro-Brillouin light scattering. This contactless and non-destructive analysis method uses the interaction of light with vibrations in the material (acoustic waves in the gigahertz regime) to extract information about its mechanical characteristics. “It is a more reliable and perhaps more useful technique than traditional contact methods,” comments Dr Bartolomej Graczykowski, leader of the project at AMU, “since it can provide both mechanical information and thickness values of the membranes.” The same approach could also be used to study other van der Waals (vdW) materials.

This elastic softening of the material when decreasing the thickness of the sample, called the elastic size effect, has profound implications for the design and development of nanodevices –such as nanomechanical resonators for sensors–, where mechanical properties are essential for their durability and robust performance. “The results of our study are also highly relevant for related research fields, such as nanoscale thermal transport, electronics, or resonators employing vdW materials,” highlights Dr Klaas-Jan Tielrooij, leader of the project at the ICN2.

Enhanced Perovskite Solar Cells: new insights into defects’ impact on stability

Collaborative paper published in Joule.

Title: Decoupling the effects of defects on efficiency and stability through phosphonates in stable halide perovskite solar cells

Abstract:

Understanding defects is of paramount importance for the development of stable halide perovskite solar cells (PSCs). However, isolating their distinctive effects on device efficiency and stability is currently a challenge. We report that adding the organic molecule 3-phosphonopropionic acid (H3pp) to the halide perovskite results in unchanged overall optoelectronic performance while having a tremendous effect on device stability. We obtained PSCs with ∼21% efficiency that retain ∼100% of the initial efficiency after 1,000 h at the maximum power point under simulated AM1.5G illumination. The strong interaction between the perovskite and the H3pp molecule through two types of hydrogen bonds (HI and OH) leads to shallow point defect passivation that has a significant effect on device stability but not on the non-radiative recombination and device efficiency. We expect that our work will have important implications for the current understanding and advancement of operational PSCs.

Featured among “Nanoscale Emerging Investigators 2021”

Review article on Hot Carriers in Graphene published in Nanoscale collection by promising early-career researchers.

Nanoscale Emerging Investigators 2021 collection with invited contribution from ICN2 Junior Group Leader Klaas-Jan Tielrooij

The journal Nanoscale is currently presenting its themed issue Emerging Investigators 2021 “highlighting 2021’s rising stars of nanoscience and nanotechnology research. This issue gathers the very best work from researchers in the early stages of their independent career. Each contributor was recommended by experts in their fields for carrying out work with the potential to influence future directions in nanoscience and nanotechnology.”

One of the articles in the themed collection is a contribution from ICN2 Junior Group Leader Klaas-Jan Tielrooij, with a review article on “Hot Carriers in Graphene – Fundamentals and Applications”. The review is written together with three colleagues from Canada, the UK, and Germany. Hot carriers in graphene exhibit fascinating physical phenomena and offer great promise for exciting optoelectronic and photonic applications. In particular, hot-carrier-enabled processes that give rise to the emission, conversion, and detection of light can have a disruptive impact, for example in the field of data communication, high-frequency electronics, and industrial quality control. The review describes the current status of the field, and is aimed at a broad audience, including prospective students as well as scientists, engineers and technologists from both academia and industry. 

News & Views: Hot plasmons make graphene shine

News & Views piece published in Nature Materials

A new significant research on ultrafast excitation and relaxation mechanisms in graphene, published in the current issue of “Nature Materials”, is featured in the “News & Views” section of the journal in an introductive article by ICN2 group leader Dr Klaas-Jan Tielrooij and ICFO group leader Prof. Frank Koppens.

Dr Klaas-Jan Tielrooij, leader of the ICN2 Ultrafast Dynamics in Nanoscale Systems group, and Prof. Frank Koppens, group leader at the Institute of Photonic Sciences (ICFO), published a News & Views article in Nature Materials as an introduction to highlight a paper published in the same issue of the journal.  The work they summarize is a remarkable study on the ultrafast carrier dynamics of graphene, carried out by Dr Laura Kim –from the Massachusetts Institute of Technology– and her colleagues, which provides the first experimental evidence of hot plasmon emission in graphene as a consequence of carrier excitation by ultrashort laser pulses.  

When graphene carriers are excited, carrier-carrier interactions take place on a femtosecond timescale. A cooling phase follows, which can occur through different processes, chiefly via phonon emission. According to this research, an additional relaxation (and cooling) channel exists, which involves ultrafast energy flow from excited carriers into graphene plasmons. This process leads to the emission of mid-infrared light, which increases its brightness when gold nanodisks are added.

As highlighted by Dr Tielrooij and Prof. Koppens, this study is not only noteworthy in its own rights, but also for the opportunities of new practical applications that the identified mechanism provides.

Image credit: Matteo Ceccanti

Electrical tunability of terahertz nonlinearity in graphene

Collaborative paper published in Science Advances.

Abstract:

Graphene is conceivably the most nonlinear optoelectronic material we know. Its nonlinear optical coefficients in the terahertz frequency range surpass those of other materials by many orders of magnitude. Here, we show that the terahertz nonlinearity of graphene, both for ultrashort single-cycle and quasi-monochromatic multicycle input terahertz signals, can be efficiently controlled using electrical gating, with gating voltages as low as a few volts. For example, optimal electrical gating enhances the power conversion efficiency in terahertz third-harmonic generation in graphene by about two orders of magnitude. Our experimental results are in quantitative agreement with a physical model of the graphene nonlinearity, describing the time-dependent thermodynamic balance maintained within the electronic population of graphene during interaction with ultrafast electric fields. Our results can serve as a basis for straightforward and accurate design of devices and applications for efficient electronic signal processing in graphene at ultrahigh frequencies

Defects to the rescue in graphene/WS2 heterostructures

Collaborative paper published in Science Advances.

Title: Long-lived charge separation following pump-wavelength-dependent ultrafast charge transfer in graphene/WS2 heterostructures

Abstract:

Van der Waals heterostructures consisting of graphene and transition metal dichalcogenides have shown great promise for optoelectronic applications. However, an in-depth understanding of the critical processes for device operation, namely, interfacial charge transfer (CT) and recombination, has so far remained elusive. Here, we investigate these processes in graphene-WS2 heterostructures by complementarily probing the ultrafast terahertz photoconductivity in graphene and the transient absorption dynamics in WSfollowing photoexcitation. We observe that separated charges in the heterostructure following CT live extremely long: beyond 1 ns, in contrast to ~1 ps charge separation reported in previous studies. This leads to efficient photogating of graphene. Furthermore, for the CT process across graphene-WSinterfaces, we find that it occurs via photo-thermionic emission for sub-A-exciton excitations and direct hole transfer from WSto the valence band of graphene for above-A-exciton excitations. These findings provide insights to further optimize the performance of optoelectronic devices, in particular photodetection.

Grating-graphene metamaterial for THz nonlinear photonics

Collaborative paper published in ACS Nano.

Researchers from various institutes in Barcelona and Germany have demonstrated that a hybrid material consisting of a monolayer of graphene and a metallic grating structure is an excellent candidate for relevant commercial applications in which efficient nonlinear conversion of (invisible) terahertz light is required. This work has just been published in ACSNano, in a paper with Dr Klaas-Jan Tielrooij, leader of the ICN2 Ultrafast Dynamics in Nanoscale Systems group, as last author. A metal grating combined with graphene opens up the road to terahertz nonlinear photonic applications. 

Nonlinear optical conversion – i.e. the process by which an incident light beam of a certain wavelength turns into rays of different wavelengths, due to its interaction with the material it passes through – is relevant to for many current and future technologies, such as imaging, information storage and processing, telecommunication, quantum technologies, and other fields.

The ideal material for these applications should provide, first of all, a very high conversion efficiency, which means that a significant fraction of the incoming beam has to be converted into light of the desired wavelengths. It is also required to have a small footprint (that is, as little material as possible has to be used); to be compatible with standard CMOS technology used in most electronic devices; and to be able to operate at room temperature. Particularly needed is an optimal solution for applications in which the incident light is in the terahertz (THz) region of the electromagnetic spectrum. This light cannot be seen by the human eye, yet it is commonly used for applications ranging from airport security to product inspection, and can play an important role in future communication technologies.

Two-dimensional materials are highly interesting for light conversion. They consist of a single layer (or a couple of them) of atoms and thus have almost zero-thickness (this is why they are called 2D materials). This characteristic ensures a small material footprint. In addition, since they are thinner than the wavelength of light, the optical waves propagating in these materials remain in phase. Among them, graphene, a by-now-well-known material made of a monolayer of carbon atoms arranged in a honeycomb structure, is particularly promising. This is because it exhibits very large nonlinear conversion coefficients, especially in the THz range. On the other hand, though, its extremely reduced thickness affects its conversion efficiency, due to the small quantity of matter the light can interact with. To overcome this problem, the authors decided to combine graphene with another material system that enhances this interaction.

A team of researchers from the Catalan Institute of Nanoscience and Nanotechnology (ICN2, Spain), the Helmholtz-Zentrum Dresden-Rossendorf  (HZDR, Germany), the Institute of Photonic Sciences (ICFO, Spain), the Max Planck Institute of Polymer Research, Mainz (Germany), the University of Bielefeld (Germany) and the Technical University of Berlin (TUB, Germany) have combined graphene with a metallic structure that provides field-enhancement, leading to a hybrid material characterized by very high nonlinear light conversion efficiency. As explained in a scientific article recently published in ACSNano, this structure (grating-graphene) produces outcoming light at the new wavelength having an intensity more than 1000 times higher than the one obtained using just graphene.

Dr Klaas-Jan Tielrooij, leader of the ICN2 Ultrafast Dynamics in Nanoscale Systems group and last author of the paper explains: “The combination of graphene and a metal grating leads to highly efficient conversion of terahertz light, reaching up to 1% (in field) for rather weak incident light.” Dr Jan-Christoph Deinert, from HZDR, first author of the work, adds: “This hybrid material made it possible for us to observe light that oscillates three times, five times, seven times, and even nine times faster than the incoming light.

The outstanding conversion efficiency of this hybrid material guarantees low power consumption in the conversion process, while the compatibility of graphene with CMOS technology allows for integration in devices based on such technology. Overall, this grating-graphene structure presents itself as an excellent candidate for commercially viable applications requiring nonlinear conversion in the terahertz regime, chip-integration, room temperature operation and low power consumption.

Image credit and copyright: HZDR / Werkstatt X

Plasmonic antenna coupling to hyperbolic phonon-polaritons for sensitive and fast mid-infrared photodetection with graphene

Collaborative paper published in Nature Communications.

Abstract:

Integrating and manipulating the nano-optoelectronic properties of Van der Waals heterostructures can enable unprecedented platforms for photodetection and sensing. The main challenge of infrared photodetectors is to funnel the light into a small nanoscale active area and efficiently convert it into an electrical signal. Here, we overcome all of those challenges in one device, by efficient coupling of a plasmonic antenna to hyperbolic phonon-polaritons in hexagonal-BN to highly concentrate mid-infrared light into a graphene pn-junction. We balance the interplay of the absorption, electrical and thermal conductivity of graphene via the device geometry. This approach yields remarkable device performance featuring room temperature high sensitivity (NEP of 82 pW/√Hz) and fast rise time of 17 nanoseconds (setup-limited), among others, hence achieving a combination currently not present in the state-of-the-art graphene and commercial mid-infrared detectors. We also develop a multiphysics model that shows very good quantitative agreement with our experimental results and reveals the different contributions to our photoresponse, thus paving the way for further improvement of these types of photodetectors even beyond mid-infrared range.

Fast electrical modulation of erbium emitter decay rates using graphene

Collaborative paper published in Nature Communications.

Crystals doped with rare-earth ions, such as erbium (Er3+), play an important role for existing and future technologies. For example, erbium-doped fibers are crucial for optical communications systems carrying information as optical signals over long distances. Rare-earth ions furthermore have several properties that make them highly promising and quantum technologies, such as long lifetimes and long decoherence times. Indeed, systems based on rare-earth ions are gaining increasing attention for serving, for example, as quantum bits and quantum memories. 

Scientists at ICFO (Spain), CNRS (France) and ICN2 (Spain) have now made several breakthroughs regarding novel quantum and plasmonic applications based on nanoscale rare-earth-ion systems. In particular, they have managed to create erbium emitter layers of nanoscale thickness (~10 nm) with emission properties that are similar to those of bulk emitter materials. Owing to the very small layer thickness, the emitters can efficiently interact with their environment. By exploiting this property, the researchers have created nanophotonic devices where the environment of the nanolayer of emitters is formed by electrically tunable monolayer graphene. These hybrid erbium-graphene devices allow for extremely strong emitter-environment interactions via dipole-dipole coupling. This interaction is so strong that for a large fraction of the erbium emitters the decay is enhanced by a factor of more than 1000. This means that more than 99.9% of the energy flows from excited emitters to graphene. 

Besides providing a platform for very strong emitter-environment interactions, the hybrid devices allow for tuning the Fermi energy of graphene in a dynamical fashion using a small electrical voltage. As a result, the interaction strength can be controlled with a high modulation frequency – up to 300 kHz. Remarkably, this is more than three orders of magnitude faster than the natural decay rate of excited erbium ions. Furthermore, the energy flow pathway can be controlled with this high modulation frequency, where excited ions lead to mainly electron-hole pair creation or to plasmon launching in graphene. Thus, the device allows for temporal control of plasmon launching, as well as creating controlled waveforms, with applications in plasmonic and quantum technologies. 

The results have been recently published in Nature Communications under the study “Fast electrical modulation of strong near-field interactions between erbium emitters and graphene.” The work was carried out within the framework of the Horizon2020 FET Open project “NanOQTech” (2016-2019) that aimed to establish nanoscale rare-earth systems as a novel material platform for various technological applications. 

Kinetic Ionic Permeation and Interfacial Doping of Supported Graphene

Collaborative paper published in Nano Letters.

Abstract:

Due to its outstanding electrical properties and chemical stability, graphene finds widespread use in various electrochemical applications. Although the presence of electrolytes strongly affects its electrical conductivity, the underlying mechanism has remained elusive. Here, we employ terahertz spectroscopy as a contact-free means to investigate the impact of ubiquitous cations (Li+, Na+, K+, and Ca2+) in aqueous solution on the electronic properties of SiO2-supported graphene. We find that, without applying any external potential, cations can shift the Fermi energy of initially hole-doped graphene by ∼200 meV up to the Dirac point, thus counteracting the initial substrate-induced hole doping. Remarkably, the cation concentration and cation hydration complex size determine the kinetics and magnitude of this shift in the Fermi level. Combined with theoretical calculations, we show that the ion-induced Fermi level shift of graphene involves cationic permeation through graphene. The interfacial cations located between graphene and SiO2 electrostatically counteract the substrate-induced hole doping effect in graphene. These insights are crucial for graphene device processing and further developing graphene as an ion-sensing material.