Controlling how fast graphene cools down

Collaborative paper published in ACS Nano.

An international study, published in ACS Nano, has demonstrated an unprecedented level of control of the optical properties of graphene. The work has promising applications in different technological fields ranging from photonics to telecommunications.

Graphene is the thinnest material ever produced, with the thickness of a single atomic layer, thinner than a billionth of a meter, it is able to efficiently absorb light from the visible to the infrared through the photoexcitation of its charge carriers. After light absorption, its photoexcited charge carriers cool down to the initial equilibrium state in a few picoseconds, corresponding to a millionth of a millionth of a second. The remarkable speed of this relaxation process makes graphene particularly promising for a number of technological applications, including light detectors, sources and modulators.

A recent study published in ACS Nano has shown that the relaxation time of graphene charge carriers can be significantly modified by applying an external electrical field. The research was conceived within an international collaboration between the CNR-IFN, Politecnico di Milano, the University of Pisa, the Graphene Center of Cambridge (UK) and the Catalan Institute of Nanoscience and Nanotechnology (ICN2, Barcelona).

The change in the relaxation time of charge carriers in graphene that we have observed, demonstrates an unprecedented level of control on the optical response of a crystal and allows to obtain a large variety of behaviors using a single material,” says Dr Eva Pogna, researched from CNR-IFN, first author of the work. This study paves the way to the development of devices that exploit the control of the relaxation time of charge carriers to support novel functionalities. For example, if graphene is used as saturable absorber in a laser cavity to generate ultrashort light pulses, by changing the relaxation time of the charge carriers, we can control the duration of the output pulses.

The specific device that we have used to study graphene, proved to be crucial to observe the strong tunability of its optical properties with the external electric field, allowing to change the number of charge carriers over a broad range by exploiting ionic liquid gating, which is a state-of-the-art technology introduced to study superconductors,” explains Prof. Andrea Ferrari, director of the Graphene Center in Cambridge.

The graphene-based device has been studied by ultrafast spectroscopy, which allowed to monitor the variation of the relaxation time of the charge carriers. “This work represents the latest step of a long-standing research collaboration devoted to the study of the ultrafast carrier dynamics in graphene, aimed at exploring the great potential of this fascinating material,” added Dr Klaas-Jan Tielrooij, leader of the Ultrafast Dynamics in Nanoscale Systems Group at ICN2.

This discovery is of large interest for a number of technological applications, ranging from photonics, for pulsed laser sources or optical limiters that prevent optical components damaging, to telecommunication, for ultrafast detectors and modulators,” concludes Prof. Giulio Cerullo, professor at the Physics Department of Politecnico di Milano.

Image: Graphene charge carriers lying on different energetic levels represented by the Dirac cones, which, depending on the number of charge carriers, are occupied up to the neutrality point (blue level on the left cone) or well into the conduction band (blue level on the right cone). In the two cases, the photoexcited charge carriers relax with faster (left side) or slower (right side) dynamics.

Thin is cool: thermal dissipation of layered semiconductors down to the monolayer

New work published in Adv. Mater.

A study published in “Advanced Materials” reveals the thermal transport properties of ultrathin crystals of molybdenum diselenide, a two-dimensional material of the transition metal dichalcogenide (TMD) family. Outperforming silicon, TMD materials prove to be outstanding candidates for electronic and optoelectronic applications, such as flexible and wearable devices. This research, which involved researchers belonging to four ICN2 groups and from ICFO (Barcelona), Utrecht University (the Netherlands), the University of Liège (Belgium) and the Weizmann Institute of Science (Israel), was coordinated by ICN2 group leader Dr Klaas-Jan Tielrooij.

The increasing demand for extremely small components and devices has led scientists to search for new materials that could best meet these needs. Two-dimensional layered materials (2D materials) – which can be as thin as one or a few atomic layers and are strongly bonded only in the in-plane direction – have attracted the attention of both academia and industry, and do not cease to amaze with their peculiar and remarkable properties. Among them, transition metal dichalcogenides (TMDs) are promising for a variety of electronic, optoelectronic and photonic applications.

When it comes to the integration and miniaturization of devices, a key aspect to take into account is the thermal transport properties of materials: in most applications overheating is a crucial factor limiting performance and lifetime. Therefore, in order to take advantage of the electronic and optical properties of TMDs, a deep understanding and control of heat flow in these materials is required. In particular, comprehending the effects of crystal thickness – down to just one layer – and the environment on thermal transport are key to applications.

Influence of crystal thickness on thermal dissipation properties

A combined experimental and theoretical study recently published in Advanced Materials investigates the thermal conductivity of molybdenum diselenide (MoSe2), which is an archetypal TMD material. David Saleta Reig, PhD student and first author of the work explains: “We performed a systematic study of the effects of crystal thickness and surrounding environment on heat flow. This fills an important gap in the scientific literature about 2D materials.” Indeed, performing either reliable experimental studies or computer simulations of thermal transport over a broad range of thicknesses from bulk down to a single molecular monolayer is not an easy task. The authors of this research were able to overcome these challenges and produce protocols and results that are valid not only for the case study, MoSe2, but also for a broader range of 2D materials.

Ultrathin MoSe2 transports heat faster than ultrathin silicon

The experimental measurements, in combination with numerical simulations, led to a remarkable result: “We found that the in-plane thermal conductivity of the samples decreases only marginally when reducing the thickness of the crystal all the way to a monolayer with sub-nanometer thickness,” explains Sebin Varghese, PhD student and second author of the study. This behaviour originates from the layered nature of MoSe2 and sets TMD materials apart from non-layered semiconductors, such as the industry standard, silicon. In the latter, the thermal conductivity decreases dramatically when the thickness approaches the nanometer, due to increased scattering at the surface. This effect is much less significant in layered materials, such as MoSe2.

First principles thermal transport simulations reproduced the experimental results in an excellent way, and led to another surprising result: “For the thinnest films, the heat is carried by different phonon modes than for thicker ones,” says Dr Roberta Farris, postdoctoral researcher who developed and carried out the ab initio simulations. Finally, this study also clarifies the influence of the material’s environment on heat dissipation, demonstrating that ultrathin MoSe2 is able to dissipate heat very efficiently to surrounding air molecules.

Dr Klaas-Jan Tielrooij, who coordinated the work, comments: “This work shows that TMD crystals with (sub)nanometer thickness have the potential to outperform silicon films both in terms of electrical and thermal conductivity in this ultrathin limit”. These results thus demonstrate the excellent prospects of TMDs for applications that require thicknesses on the order of a few nanometers or less, for example in the case of flexible and wearable devices and nanoscale electronic components. “Of course it remains to be seen if TMDs will live up to their promises”, concludes Dr Tielrooij, “as there are many hurdles to overcome before these materials will be applied on an industrial scale. At least we now know that their thermal properties are – in principle – not a show-stopper.”

About the study

The authors of this study used the Raman thermometry technique to measure the thermal conductivity of a large set of suspended, crystalline, and clean MoSe2 crystals with systematically varied thickness, taking care to identify and suppress possible thickness-dependent artifacts. They compared the experimental results with ab initio simulations –based on density functional theory and Boltzmann transport theory— performed with the SIESTA method and software, which is particularly suitable for atomistic simulations with a large number of atoms.

This research, coordinated by Dr Klaas-Jan Tielrooij, leader of the ICN2 Ultrafast Dynamics in Nanoscale Systems Group, involved several ICN2 researchers and group leaders belonging to: the Phononic and Photonic Nanostructures Group, led by ICREA Prof. Clivia Sotomayor Torres, the Theory and Simulation Group, led by Prof. Pablo Ordejón, and the Physics and Engineering of Nanodevices Group, led by ICREA Prof. Sergio Valenzuela. Researchers ICFO (Barcelona), Utrecht University (the Netherlands) –with Prof. Zeila Zanolli, former member of Prof. Ordejón’s group—, the University of Liége (Belgium) –with Prof. Matthieu Verstraete, former visiting scientist at ICN2— and the Weizmann Institute of Science (Israel) were also involved.

Terahertz signatures of ultrafast Dirac fermion relaxation at the surface of topological insulators

Collaborative paper published in npj Quantum Mater.

Abstract:

Topologically protected surface states present rich physics and promising spintronic, optoelectronic, and photonic applications that require a proper understanding of their ultrafast carrier dynamics. Here, we investigate these dynamics in topological insulators (TIs) of the bismuth and antimony chalcogenide family, where we isolate the response of Dirac fermions at the surface from the response of bulk carriers by combining photoexcitation with below-bandgap terahertz (THz) photons and TI samples with varying Fermi level, including one sample with the Fermi level located within the bandgap. We identify distinctly faster relaxation of charge carriers in the topologically protected Dirac surface states (few hundred femtoseconds), compared to bulk carriers (few picoseconds). In agreement with such fast cooling dynamics, we observe THz harmonic generation without any saturation effects for increasing incident fields, unlike graphene which exhibits strong saturation. This opens up promising avenues for increased THz nonlinear conversion efficiencies, and high-bandwidth optoelectronic and spintronic information and communication applications.

Fabrication and characterization of large-area suspended MoSe2 crystals down to the monolayer

Paper published in Emerging Leaders 2021 collection of J. Phys. Mater.

Abstract:

Many layered materials, such as graphene and transition metal dichalcogenides, can be exfoliated down to atomic or molecular monolayers. These materials exhibit exciting material properties that can be exploited for several promising device concepts. Thinner materials lead to an increased surface-to-volume ratio, with mono- and bi-layers being basically pure surfaces. Thin crystals containing more than two layers also often behave as an all-surface material, depending on the physical property of interest. As a result, flakes of layered materials are typically highly sensitive to their environment, which is undesirable for a broad range of studies and potential devices. Material systems based on suspended flakes overcome this issue, yet often require complex fabrication procedures. Here, we demonstrate the relatively straightforward fabrication of exfoliated MoSe2 flakes down to the monolayer, suspended over unprecedentedly large holes with a diameter of 15 µm. We describe our fabrication methods in detail, present characterization measurements of the fabricated structures, and, finally, exploit these suspended flakes for accurate optical absorption measurements.

Ultra-efficient heat dissipation thanks to graphene electrons

Collaborative ICFO-ICN2-BIST paper published in Nature Nanotechnology.

The continuous miniaturization and integration of electronic circuits in mobile phones, computers etc. leads to increasingly strong demands for thermal management: Dissipating excess heat is required to guarantee the correct functioning of the devices. Research published today in Nature Nanotechnology reveals that graphene enables ultra-efficient heat dissipation, spreading heat over micrometers during a time as short as a few hundred femtoseconds – less than a millionth of a millionth of a second. This could make it possible to efficiently cool down local circuit elements, such as transistors, of (opto)electronic devices.

Graphene is known to possess one of the highest thermal conductivities: a few thousand W/m2K – even higher than diamond. This is due to heat transported by lattice vibrations, while the contribution of charge carriers – electrons and holes – is typically small. However, graphene had something up its sleeve: an unconventional transport regime called the hydrodynamic regimewhere an even higher thermal conductivity, with heat carried by charges, is possible. In this hydrodynamic regime, charges interact strongly with each other, and follow similar laws as those that apply to classical fluid transport. 

Within this regime – under certain conditions – the quantum-critical Dirac fluid regime exists, where the system no longer behaves similarly to a classical fluid. What is special about this regime is that if the electronic system is heated up, hot electrons and hot holes coexist and move in the same direction under a thermal gradient, conserving total momentum. This enables heat spreading that is much more efficient than in other transport regimes. Until now, attempts to observe this at room temperature had not been successful

The authors of this work were able to follow heat transport in graphene (specifically, in a graphene device encapsulated by hexagonal boron nitride) at room temperature by means of a technique called ultrafast spatiotemporal thermoelectric microscopy. “The breakthrough that enabled us to observe electronic heat spreading in graphene at room temperature was using ultrashort light pulses, offset in space and time with nanometer and femtosecond accuracy, respectively, while measuring the generated thermoelectric current”, explains Dr Alexander Block. The researchers first demonstrated that their technique gave the expected heat spreading when examined in the common diffusive regime. Then they studied the hydrodynamic regime, where they observed strongly enhanced heat spreading, corresponding to a giant thermal diffusivity and an electronic thermal conductivity exceeding the record-high lattice thermal conductivity.  

The scientists moreover demonstrated the ability to control the amount of heat spreading by tuning the system in and out of the Dirac fluid regime. For this, they varied independently the laser power –which translates to a modification of the electron temperature –  and the voltages applied to the device through electrical gates – which act on another important graphene parameter, called the Fermi energyThis controllable transition between different heat spreading regimes at room temperature and the giant thermal diffusivity had not been observed before. “It is amazing to see that hydrodynamic phenomena that until a few years ago were experimentally inaccessible, are now reachable at room temperature, using standard encapsulated graphene, and even potentially useful for real-life applications in thermal management,” concludes Dr Klaas-Jan Tielrooij, coordinator of the work and Junior Group Leader at ICN2. These applications in thermal management would exploit the ultra-efficient heat spreading observed in graphene to extract heat from local hot spots in (opto)electronic circuitry in a broad range of devices used for information and communications technology. 

This research is the result of a collaboration between the Institute of Photonic Sciences (ICFO) and the Catalan Institute of Nanoscience and Nanotechnology (ICN2), both part of the Barcelona Institute of Science and Technology in Spain. The work was  coordinated by Dr Klaas-Jan Tielrooij, group leader at ICN2, in collaboration with ICREA Profs. Niek van Hulst and Frank Koppens at ICFOand Stephan Roche at ICN2. The first author of the paper is Dr Alexander Block who is affiliated to both ICFO and ICN2. This work also involved Alessandro Principi from the University of Manchester (UK) and researchers from the National Institute for Materials Science of Tsukuba (Japan).

Congratulations to Xiaoyu!

PhD defended at UvA (NL)

Dr. Xiaoyu Jia has just successfully defended his PhD at the University of Amsterdam (NL). His project was a collaboration between ICN2 and the Max Planck Institute of Polymer Research (MPIP) in Mainz (DE), supervised by Mischa Bonn and Hai I. Wang at the MPIP and Klaas-Jan Tielrooij at ICN2. Congrats!

New insights into charge carrier dynamics in graphene-based materials for optoelectronic applications

Collaborative paper published in ACS Nano.

A study published in “ACSNano” investigates the mechanism governing the cooling dynamics of photo-excited charge carriers in graphene-based structures, which are interesting candidates for future optoelectronic devices. This research was led by Dr Klaas-Jan Tielrooij, leader of the ICN2 Ultrafast Dynamics in Nanoscale Systems Group.

Various emerging technologies for optoelectronic applications, in particular in the field of data communication, take advantage of the ultrafast dynamics of photo-excited charge carriers in graphene. After reaching an excited state by photon absorption, these carriers undergo a multi-step relaxation phase, in which they release the extra energy they acquired and return to their ground state. First, scattering between carriers occurs, which leads to a state with an elevated electronic temperature. Then, these hot charge carriers cool down to room temperature via various processes. Large research efforts have been devoted to the identification of the physical mechanisms that drive this cooling phase, which depends on how graphene is prepared and by the materials that surround it. The cooling processes include interactions with graphene optical and acoustic phonons, and with substrate phonons in nearby materials. A deep understanding of the phenomena governing this phase is key to graphene’s use in optoelectronic devices.

In a paper recently published in ACSNano, the cooling dynamics of hot carriers are studied in two technologically relevant material systems based on high-quality graphene. This work, carried out by an international team of researchers from various institutes in Spain, Italy, Germany, UK, Belgium and China, and coordinated by Dr Klaas-Jan Tielrooij, leader of the ICN2 Ultrafast Dynamics in Nanoscale Systems Group, revealed the crucial role played by a cooling process in particular, which is always present in graphene, thus providing the intrinsic limit of the lifetime of its electronic excitations.

Using three different time-resolved optical measurement techniques, the researchers analysed the behaviour of photo-induced hot carriers in WSe2-encapsulated graphene, where graphene is sandwiched between two layers of tungsten diselenide, and in suspended graphene, grown by chemical vapour deposition. Both systems are meant for applications requiring very high quality of graphene, which implies large charge mobility. In these materials, a cooling timescale of tens of picoseconds was expected, because the so-called “disorder-assisted supercollision cooling” is relatively inefficient in high-mobility graphene, and because, in the two cases under study, graphene interacts weakly with phonons in its environment. The experimental results, though, showed that in both samples the relaxation of hot carriers occurs in a few (2-4) picoseconds, a much shorter time than expected.

To explain these outcomes, the authors of the study suggest that another cooling mechanism intrinsic to graphene, involving optical phonon emission, is at work. Specifically, hot carriers initially decay by emitting optical phonons, and, in turn, these optical phonons couple to acoustic phonons, producing a slower decay.  In the meanwhile, the electronic system undergoes a continuous rethermalization (a process that leads to thermal equilibrium). This enable constant emission of optical phonons, which therefore provides a continuous heat sink. The researchers also developed an analytical modelof this cooling pathway, which fits well with the experimental results and indicates what are the key parameters to play with for slowing down or accelerating the charge carriers relaxation.

This study provides fundamental insights into the hot-carrier cooling dynamics in high-quality graphene, which will be extremely useful for the future development of graphene-based optoelectronic devices.

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.