Exciton Transport in 2D Perovskites: Visualising the Flow of Energy in a Soft Semiconductor


Exciton Transport in 2D Perovskites: Visualising the Flow of Energy in a Soft SemiconductorArticle: published in Nature Communications by Nerea Alcázar-Cano, Marc Meléndez and Rafael Delgado-Buscalioni members of the Theoretical Condensed Matter Physics Department.

Researchers from the Universidad Autonoma de Madrid have for the first time visualised the unique physics of the transport of optical excitations (or excitons) in two-dimensional perovskites. Using transient fluorescence microscopy, they show how small changes in the softness of these materials can result in dramatic differences in their excitonic energy transport characteristics. This discovery is crucial in the development of high efficiency solution-processed photovoltaic and light emitting technologies – a field in which metal-halide perovskites are playing an increasingly central role.

Within a decade after their first reports, solar cells based on metal-halide perovskites have advanced to the top of the list of most efficient single cell junctions, rivalling even crystalline silicon. However, one key missing ingredient in perovskites is chemical stability, posing a challenge for commercialisation. A promising route to tackle the stability issue is the use of lower dimensional perovskites, such as two-dimensional (2D) perovskites, which show a better resistance towards degradation than the conventional 3D phases. Indeed, recent solar cells based using 2D perovskite show dramatically improved stability while maintaining high performance. Reduced dimensionality, however, significantly alters the photophysical properties of the perovskites. Importantly, rather than acting as free carriers, electrons and holes are bound to each other and travel through the material as neutral pairs called excitons. The exciton-binding energy of these materials is in some cases so high that excitons can only be split up into electrons and holes at charge separation sites. To guarantee that every absorbed photon can also contribute to the energy generation in a solar cell, excitons need to diffuse far enough to reach such sites. This makes charge separation and energy extraction much more challenging and it is somewhat of a surprise that 2D perovskite based solar cells work as well as they do.

Researchers from the Condensed Matter Physics Centre (IFIMAC) at the Universidad Autonoma de Madrid have now directly visualised exciton transport in 2D perovskites using transient photoluminescence microscopy (TPLM). By detecting the photoluminescent signal from a population of excitons as a function of time and position, TPLM can create little movies of the broadening of this population with a spatial resolution of just a few tens of nanometers and at a sub-nanosecond frame duration. The results show that excitons undergo an initial fast diffusion through the crystalline plane, followed by a slower subdiffusive regime as excitons get trapped. Interestingly, the regime of slow trap state-limited transport disappears at higher illumination intensities. Using theoretical modelling, the authors find that this can be explained by a trap filling behaviour. This suggests that under typical illumination by the sun in a photovoltaic device, the role of traps in these materials may actually be minimal. The most important finding of this study though, is that the early intrinsic diffusivity depends sensitively on the choice of organic spacer that is used to separate the inorganic sheets in 2D perovskites. Between commonly used organic spacers (phenethylammonium and butylammonium), diffusivities and diffusion lengths differ by as much as an order of magnitude. Across a wide range of different spacers, it is shown that these changes are closely correlated with variations in the softness of the lattice, suggesting a dominant role for exciton–phonon coupling and exciton–polaron formation in the spatial dynamics of excitons in these materials. Crucially, long exciton diffusion lengths in light emitting applications such as LEDs can reduce device performance, as it increases the possibility of encountering a trapping site. Meanwhile, for light harvesting applications, long diffusion lengths are essential for the successful extraction of excitons. The insights from this study therefore provide a clear design strategy to further improve the performance of 2D perovskite solar cells and light emitting devices. [Full article]

References

  1. Exciton diffusion in two-dimensional metal-halide perovskites. Michael Seitz, Alvaro J. Magdaleno, Nerea Alcázar-Cano, Marc Meléndez, Tim J. Lubbers, Sanne W. Walraven, Sahar Pakdel, Elsa Prada, Rafael Delgado-Buscalioni & Ferry Prins, Nature Communications volume 11, Article number: 2035, (2020). [URL]



Dynamical Coulomb Blockade as a Local Probe for Quantum Transport


Dynamical Coulomb blockade revealed by an STM.

Dynamical Coulomb blockade revealed by an STM.

Article: published in Physical Review Letters by Alfredo Levy Yeyati and Juan Carlos Cuevas, IFIMAC researchers and members of the Theoretical Condensed Matter Physics Department.

As the dimensions of a conductor are reduced, quantum mechanics starts to play a significant role in its electronic transport properties. Atomic-size contacts, as those produced using STM techniques, constitute the ultimate limit in the miniaturization of electronic devices. In this extreme limit, electrical conduction is mainly determined by the quantum mechanical transmission probability of electrons through the junction. However, deviations from this simple picture can occur at very low temperatures due to the effect of quantum fluctuations in the applied voltage, leading to a phenomenon called dynamical Coulomb blockade.

Now, a novel insight into dynamical Coulomb blockade (DCB) at the atomic scale has been reported in a work published in Physical Review Letters by a collaboration between groups of the Max Planck Institute for Solid State Research (Stuttgart), the Okinawa Institute of Science and Technology, the University of Ulm, the University of Konstanz, and the IFIMAC researchers Alfredo Levy Yeyati and Juan Carlos Cuevas. In this work, these researchers used an ultra-low temperature STM to form few-atom junctions with an exquisite control and they revealed the influence of DCB in measurements of the electrical current. More importantly, they demonstrated that these measurements can be used to determine the transmission coefficients of these atomic-scale junctions. Such a determination was possible thanks to an excellent agreement with a microscopic theory of DCB and the nature of the conduction channels was elucidated with the help of ab initio DFT transport calculations. Thus, they concluded that probing the DCB by STM provides a complementary technique for locally resolving quantum transport characteristics. [Full article]

References

  1. Dynamical Coulomb Blockade as a Local Probe for Quantum Transport, Jacob Senkpiel, Jan C. Klöckner, Markus Etzkorn, Simon Dambach, Björn Kubala, Wolfgang Belzig, Alfredo Levy Yeyati, Juan Carlos Cuevas, Fabian Pauly, Joachim Ankerhold, Christian R. Ast, and Klaus Kern, Phys. Rev. Lett. 124, 156803, (2020). [URL]



Polaritonic Molecular Clock: Listening to Molecules


Polaritonic Molecular Clock: Listening to Molecules

Article: published in Nature Communications by R. E. F. Silva, Francisco J. García-Vidal and Johannes Feist, IFIMAC researchers and members of the Theoretical Condensed Matter Physics Department.

The motion of atoms in a molecule typically occurs on the femtosecond timescale (10-15 s). The standard technique used to study chemical reactions at these timescales is pump-probe spectroscopy, where first an ultrashort pump laser pulse initiates the dynamics in the molecule and, some time later, an ultrashort probe laser pulse is used to obtain information about the current state of the molecule. However, the information obtained in these approaches is always somewhat indirect. If you imagine the molecule as a musical instrument and the laser pulses as short sound pulses, these approaches correspond to monitoring the change in the sound frequencies that the instrument responds to when it has been struck by a first sound already (this is known as transient absorption spectroscopy), or even to using such a strong second pulse that it blows apart the instrument, and trying to understand its motion before falling apart by measuring the pieces that fly away. These experiments are then repeated many times with different time delays between the pulses, to construct something akin to a movie of the molecular motion after it has been excited. One important reason that these approaches are used is that simply “listening” to the molecule, i.e., measuring the light that it emits after the first pulse, normally does not provide the required information. The fundamental reason for this is that a molecule only couples quite weakly to light, which has two main consequences: First, the amount of light that is emitted is typically quite low (making it hard to “hear” the molecule), and second, it takes a long time for the light to be emitted so that even if you can measure it, it is produced at times long after the interesting motion has taken place.

A theoretical proposal by a group of researchers from the Departamento de Física Teórica de la Materia Condensada and the Condensed Matter Physics Center (IFIMAC) at the Universidad Autónoma de Madrid now provides a way to resolve both of these problems at the same time and make it possible to listen to the molecule directly in those first few crucial tens of femtoseconds. The basic idea of this work is to place a molecule in a nanoplasmonic cavity, where the strong coupling between the molecule and the electromagnetic modes of the cavity transforms the molecule into a “loudspeaker”. The nanoplasmonic cavity in this case acts both as a resonator that changes the “sound” of the molecule and as an amplifier that makes it possible to “hear” the molecule. In the article, the authors show that ultrafast molecular dynamics can be traced with femtosecond resolution by looking at the photon emission by the cavity, which only happens when the molecular vibrational wavepacket is close to a position where the molecule and the cavity are in resonance with each other. In this way, the setup acts like a clock that measures how long the molecule takes to reach a given position. In particular, the authors show that this idea works in the regime of strong light-matter coupling, where the molecule and the cavity interact so strongly that their excitations become hybrid light-matter states known as polaritons. In this regime, this novel technique could provide direct insight into the modification of molecular motion induced by polariton formation. This approach could open the window for experimental and theoretical advances in the way we explore ultrafast chemical dynamics, paving a new way to combine femtochemistry and molecular polaritonics using plasmonic nanocavities. [Full article]




Light in the Tunnel


Light in the Tunnel

Article: published in Nature Communications by Antonio I. Fernández-Domínguez, Rodolfo Miranda, Francisco J. García-Vidal, Roberto Otero, IFIMAC researchers and members of the Condensed Matter Physics Department and Theoretical Condensed Matter Physics Department.

Researchers from the Universidad Autónoma de MadridIMDEA Nanociencia and IFIMAC have developed a new method for the fabrication and characterization of atomic-sized photonic cavities, by exploiting the mechano-quantum tunnel effect. This discovery may be fundamental for the understanding and design of new, nanometric size, opto-electronic devices which will be key for the development of new technologies based on quantum properties, such as sensors or quantum computers.

Photonic cavities are an essential part of many modern optical devices, from a laser pointer to a microwave oven. Just as we can store water in a tank and create standing waves on the surface of the water, we can confine light in a photonic resonator whose walls are strongly reflective. Just as water surface waves depend on the geometry of the tank (shape, depth), specific optical modes can be created in a photonic cavity whose properties (color and spatial distribution of intensity) can be tuned by changing the dimensions of the cavity. When the size of the cavity is very small, much smaller than the wavelength of the light confining it (nano-cavity in the case of visible light), an intensification effect of the light is produced that is so strong that it influences the electrons on the walls of the cavity. A mixture between photons and electrons is then produced, giving rise to hybrid modes between light and matter known as plasmons. Plasmons in optical nano-cavities are extremely important for many applications such as chemical sensors that allow the detection of individual molecules or the manufacture of nanolasers that could operate with hardly any electrical current consumption. However, the characterization of these plasmonic modes is generally very complex, because the tiny size of the cavities makes it extremely difficult to access them by external signals.

On the other hand, the tunnel effect is one of the most characteristic, mysterious and best documented effects of Quantum Mechanics. In a tunnel process, a particle (e.g. an electron) can pass through a narrow barrier (the space that separates two metals at nanometric distances) despite not having enough energy to overcome it. It is as if we could pass from one side to the other of the Great Wall of China without having to jump over it. Incredible as it may seem, particles from the quantum world can do this under certain conditions. In most of these processes, the energy of the particle before and after the process is the same. However, in a small fraction of these events, the particle can give up some of its energy, for example, by generating light, which is known as the inelastic tunnel process. Although it is well known that the properties of the light emitted in the inelastic tunnel process between two metals depend on the plasmonic modes that exist in the cavity, it also depends strongly on the energy distribution of the particles performing the tunnel process. Until now, it had been impossible to distinguish unequivocally between these two effects and therefore extract the information on the plasmonic modes from the analysis of the light emitted by the tunnel effect.

In an article published this week in the prestigious journal Nature Communications, researchers from the Universidad Autónoma de Madrid, IMDEA Nanociencia and IFIMAC have developed a method to solve this problem by simultaneously determining the energy distribution of the tunneling electrons and the light emitted in a Scanning Tunnel Microscope. This effort has allowed them to exploit the tunneling effect to create optical resonators of atomic dimensions and to study their optical properties, unravelling for the first time the contributions due to the energy of the tunneling particles from the effects originated by the plasmonic modes in the cavity. This work proposes a novel methodology for the characterization of light-matter interaction at atomic size and may have important technological implications for the development of chemical sensors of single molecules, new sources of single or interlaced photons or nanolasers that are active at extremely low pumping powers. [Full article]

References

  1. Martín-Jiménez, A., Fernández-Domínguez, A.I., Lauwaet, K. et al. Unveiling the radiative local density of optical states of a plasmonic nanocavity by STM. Nat Commun 11, 1021 (2020). [URL]



Non-equilibrium Autonomous Maxwell (or not) Demons


Non-equilibrium Autonomous Maxwell (or not) Demons

Article: published in Physical Review Letters by Rafael Sánchez, IFIMAC researcher and member of the Theoretical Condensed Matter Physics Department.

The second law of thermodynamics dictates that a heat engine can generate power provided that it absorbs heat from its environment. Soon after its formulation, Maxwell protested against this claim by arguing that a microscopic “demon” could produce the same effect by selectively controlling the system based on the detailed knowledge of its state. The system hence produces work without changing neither its energy nor the number of its particles.

In a paper published in Physical Review Letters we show that a non-equilibrium distribution generates a paradoxical effect similar to a “Maxwell demon”: it raises the apparent paradox of reducing another system’s entropy at no cost, thereby suggesting that perpetual motion is possible. We call this a “N-demon” (with the “N” for non-equilibrium). Bennett showed that the paradox of the Maxwell demon was resolved by treating information as a thermodynamic resource like heat or work. Similarly, we resolve the paradox of the N-demon by treating “non-equilibrium” as a thermodynamic resource, which is used up as it reduces another system’s entropy.  This forbids the building of a perpetual motion machine, but does allow us to propose devices that use such resources (in particular non-equilibrium distributions of electrons or photons) to generate more useful energy than is conventionally believed possible. Non-equilibrium distributions of states are all around us, and are often generated as an unwanted by-product of some physical process, so it is very appealing to think that we might be able to take such a distribution as a resource, and recycle it into useful energy.

In another work, published in Physical Review Research, we consider an autonomous implementation of Maxwell’s demon based on quantum dots. Via capacitive couplings, two quantum dots are able to measure and perform feedback on the system conductor where electric power is generated. This setup allows for comparing different descriptions based on information flows to a more conventional thermoelectric approach. It further allows us to investigate the entropic cost of breaking detailed balance as well as fluctuation theorems describing information to work conversion. In particular, we derive a fluctuation relation using a novel kind of time-reversal on the single-particle level for the system alone. [Full article – PRL][Full article – PRR]




Francisco J. Garcia-Vidal included in Clarivate 2019 Compilation of Most Influential Authors


Prof. Francisco J. García-Vidal – IFIMAC Director.

Prof. Francisco J. García-Vidal – IFIMAC Director.

Highly Cited Researchers from Clarivate Analytics is an annual list recognizing leading researchers in the sciences and social sciences from around the world. Researchers are selected for their exceptional performance in one or more of 21 fields (those used in Clarivate Analytics Essential Science Indicators, or ESI) or across several fields. Approximately 6,000 researchers are named Highly Cited Researchers in 2019 – some 4,000 in specific fields and about 2,000 for cross-field performance. This is the first year that researchers with cross-field impact are identified. The number of researchers selected in each field is based on the square root of the population of authors listed on the field’s highly cited papers. The number of those with cross-field influence is determined by finding those who have influence equivalent to those identified in the 21 fields.

In the list of Physics, in which Prof. Garcia-Vidal has been selected, only four researchers working in Spanish institutions have been included. Clarivate 2019 compilation of researchers list.

We congratulate Prof. Francisco J. Garcia-Vidal.




Experimental Realization of a Quantum Dot Energy Harvester


Experimental Realization of a Quantum Dot Energy Harvester

Article: published in Physical Review Letters by Rafael Sánchez, IFIMAC researcher and member of Department of Theoretical Condensed Matter Physics.

We demonstrate experimentally an autonomous nanoscale energy harvester that utilizes the physics of resonant tunneling quantum dots. Gate-defined quantum dots on GaAs/AlGaAs high-electron-mobility transistors are placed on either side of a hot-electron reservoir. The discrete energy levels of the quantum dots are tuned to be aligned with low energy electrons on one side and high energy electrons on the other side of the hot reservoir. The quantum dots thus act as energy filters and allow for the conversion of heat from the cavity into electrical power. Our energy harvester, measured at an estimated base temperature of 75 mK in a He3/He4 dilution refrigerator, can generate a thermal power of 0.13 fW for a temperature difference across each dot of about 67 mK. [Full article]




Reversible Thermal Diode and Energy Harvester with a Superconducting Quantum Interference Single-electron Transistor


Reversible Thermal Diode and Energy Harvester with a Superconducting Quantum Interference Single-electron Transistor

Articles: published in Applied Physics Letters by Rafael Sánchez, IFIMAC researcher and member of Department of Theoretical Condensed Matter Physics.

The density of states of proximitized normal nanowires interrupting superconducting rings can be tuned by the magnetic flux piercing the loop. Using these as the contacts of a single-electron transistor allows us to control the energetic mirror asymmetry of the conductor, thus introducing rectification properties. In particular, we show that the system works as a diode that rectifies both charge and heat currents and whose polarity can be reversed by the magnetic field and a gate voltage. We emphasize the role of dissipation at the island. The coupling to substrate phonons enhances the effect and furthermore introduces a channel for phase tunable conversion of heat exchanged with the environment into electrical current.

We thank discussions and comments from A. Levy YeyatiC. Urbina, and F. Giazotto. This work was supported by the Spanish Ministerio de Economía, Industria y Competitividad (MINECO) via the Ramón y Cajal Program No. RYC-2016-20778 and the “María de Maeztu” Programme for Units of Excellence in R&D (No. MDM-2014-0377). We also acknowledge the Université Paris-Saclay international grants, the EU Erasmus program. [Full article]




A Protein-based Junction Serves as a Current Switch


A Protein-based Junction Serves as a Current Switch

Articles: published in Angewandte Chemie by Juan Carlos Cuevas and Linda A. Zotti, IFIMAC researchers and members of Department of Theoretical Condensed Matter Physics.

Proteins are key biological molecules that are responsible for numerous energy conversion processes such as photosynthesis or respiration. In recent years, proteins have been investigated in a new setting, namely in solid-state electronic junctions, with the goal of understanding the charge transfer mechanisms in these biomolecules, but also with the hope of developing a new generation of bio-inspired nanoscale electronic devices. Now, a new step towards this goal has been reported in a piece of work published in Angewandte Chemie by a collaboration between the group of David Cahen in the Weizmann Institute of Science (Israel) and the IFIMAC researchers Carlos Romero-MuñizJuan Carlos Cuevas, and Linda A. Zotti. In this work, these researchers show that a redox protein, cytochrome C, can behave as an electrically driven switch when incorporated in a solid-state junction with gold electrodes. By changing the external bias voltage in the junction, it was shown that the relevant molecular orbitals of the protein can be brought in and out of resonance with the chemical potential of the electrodes, which leads to the current-switch behavior. Showing transition from off- to on- resonance can be very challenging and this is the first time it has been achieved for proteins within the same working junction. Extensive ab initio DFT calculations revealed that the charge transport proceeds through the heme unit in these proteins and that the coupling between the protein’s frontier orbitals and the electrodes is sufficiently weak to prevent Fermi level pinning. The on-off change in the electrical current was shown to persist up to room temperature, demonstrating reversible, bias-controlled switching of a protein ensemble, which provides a realistic path to protein-based bioelectronics. [Angewandte Chemie – full article]

References

  1. A Solid-State Protein Junction Serves as a Bias-Induced Current Switch, Jerry A. Fereiro, Ben Kayser, Carlos Romero-Muñiz, Ayelet Vilan, Dmitry A. Dolgikh, Rita V. Chertkova, Juan Carlos Cuevas, Linda A. Zotti, Israel Pecht, Mordechai Sheves, David Cahen. Published in Angewandte Chemie International Edition, Volume 58, Issue34, Pages 11852-11859, August 19 (2019). [URL]



Promotional video: Theoretical Condensed Matter Physics (UAM)


Promotional video: Theoretical Condensed Matter Physics (UAM)

In the Department of Theoretical Condensed Matter Physics at the Universidad Autónoma de Madrid, we focus on understanding and predicting the behaviour of condensed systems, which are ubiquitous in the world around us.

We are interested in problems in areas such as nanotechnology, biophysics, nanophotonics or material science. We employ a wide range of theoretical approaches to gain insight into diverse physical systems, from living matter to the atom itself. We work in optics, quantum mechanics, biophysics, fluid dynamics or material physics.

We carry out creative research, which requires imagination and creativity. We work with fundamental equations, we study them, analyse them in different contexts, we take them to places they have never been and return with new and surprising information. Our findings reveal how simple rules can give rise to complex phenomena, which is helping us to understand and develop new material platforms for the implementation of the technology of the future.

Fundamentally, this is research with which we need your help.

Long version.

Short version.