Modulação acústica de éxcitons em semicondutores bidimensionais

Neste seminário, apresentado para o Departamento de Física da Universidade Federal do Ceará, mostramos alguns de nossos resultados recentes em experimentos de modulação acústica em semicondutores bidimensionais.

 

Highlights of Fapesp´s Research News website

Our recently published manuscript (“Acoustically driven Stark effect in transition metal dichalcogenide monolayers“) has received one of the highlights of Fapesp´s Research News website in December 2021. 

To read the article click HERE.

Surface acoustic waves for exciton manipulation in 2D materials

We employed Surface Acoustic Waves (SAWs) to manipulate excitonic states in MoSe2 and MoS2 monolayers (MLs), as illustrated in the figure below. By transferring the MLs to high dielectric constant LiNbO3 substrates, we have shown how the SAW strain and piezoelectric field modifies the light emission of neutral and negatively charged excitonic states (trions). As the SAW in-plane piezoelectric field increases, excitons and trions are efficiently dissociated, leading to a strong photoluminescence quenching.   In particular, we have observed for the first time the red-shift of the exciton emission lines induced by the SAW piezoelectric field, the so-called Stark effect (see the figure below). The experimental results were complemented by finite element simulations, which allowed us to experimentally determine the in-plane polarizability of neutral and negatively charged excitons in MoSe2 MLs.

This work has been published in ACS Nano under the title “Acoustically driven Stark effect in transition metal dichalcogenide monolayers“. To view the whole manuscript click HERE.

 

Studying the optical properties of lanthanide-doped multi-shell nanoparticles

In collaboration with Prof. Fernando Sigoli (from the Institute of Chemistry) and other researchers, we contributed to the study of the emission properties of core-shell nanoparticles. We investigated nanoparticles with a cubic core containing DyIII ions and hexagonal shells where ErIII, YbIII and NdIII ions were distributed. We observed that these systems consistently show emission in three different biological windows and evaluated their applicability as temperatures sensors in different temperature ranges.  In particular, we probed the dependence of their optical emission on the applied magnetic field, as shown in the figure below. These measurements allowed us to describe the energy transfer processes in this kind of mulit-shell nanoparticle system.

This type of nanostructure is very interesting for multi-functional materials fabrication since they can be employed in magnetic field and temperature sensing applications. Our work was published in Nanoscale. To read article HERE.

 

Unpolarized light sources from rolled-up quantum wells (Destaque da SBPMat)

Our work, in collaboration with Prof. Christoph Deneke (IFGW),  Ângelo Malachias (UFMG), and Leonarde Nascimento (UFV), is one latest highlights of the Brazilian Materials Research Society (SBPMat).  To see the press release click HERE.

We demonstrate how the strain state of a rolled-up InGaAs/GaAs quantum well can be employed to induce strong valence band mixing and produce rolled-up tubes which emit unpolarized light. We show that the rolling-up process induces a strong modification in the optical selection rules of the system which, even under quasi-resonant excitation with the upper valence band states, does not allow us to excite a measurable spin polarization in the  conduction band of the system.

The conclusions are reached after a careful collaborative research which involved optical spectroscopy (circularly polarized PL and PLE), X-Ray measurements, and simulations. The article was published in ACS Applied Nano Materials. To read it, click HERE.

 

 

 

 

Efficient carrier injection in strain-free mesoscopic GaAs structures

The fabrication of strain-free mesoscopic GaAs structures (MGS) by Ga-assisted droplet etching (see sketch below) has attracted attention of the scientific community due to the excellent optical emission characteristics with sharp emission lines associated to different excitonic states. One way to expand the MGS application perspectives is to explore band structure engineering. In this sense, type-II band-alignment offers an additional degree of freedom in order to modify the light emitting properties of this type of semiconductor nanostructure. However, the induced spatial separation between electron and hole wavefunctions has been demonstrated to lead to considerable quenching of photoluminescence (PL) emission in quantum rings grown by droplet epitaxy.

 In this work, we show that, by changing the Al concentration, we can modify the intensity and the energy range of the optical emission of the MGS. We show that, in comparison to previous works, MGS emission can be enhanced by up two orders of magnitude by controlling the coupling of the MGS with a neighboring quantum well. We present Al-rich structures with high emission intensity and long/tunable carrier lifetimes which can be interesting for future applications in solid-state optoelectronic devices and solar cells.

This work was done in collaboration with Prof. Christoph Deneke from IFGW and Prof. Leonarde Rodrigues from the University of Viçosa (UFV).

For the full text, click HERE.

Self-calibrated molecular thermometer

Heat generation on larger circuit boards is well-understood; however, at the nanoscale, the relationship
between heat and electricity remains unanswered due to lack of appropriate thermometers capable of thermal monitoring without perturbing the system. Unfortunately, temperature measurement at the submicrometer spatial range is not possible with conventional contact thermometers. A way to overcome this impasse is building microdevices with materials capable of also acting as in situ thermometers having submicrometric spatial resolution.

In collaboration with Prof. Fernando Sigoli from Unicamp and Muralee  Murugesu from the University Ottawa, we studied the optical properties of self-calibrated molecular thermometers based on single magnet molecules.  We demonstrate the feasibility of stable micrometer-scale thermometers operating in the range of 5 to 398 K.  Stability is achieved up to 7 T applied magnetic fields.

For full manuscript click HERE.

Nanostructured membranes for flexible optoelectronics

In collaboration with Prof. Christoph Deneke, we studied the optical emission of molecular beam epitaxy grown semiconductor systems on free and flexible membranes. We obtain optically active nanostructures (quantum wells) regrown on released GaAs/InGaAs/GaAs membranes used as virtual substrates. Together with our demonstrated ability to transfer thin semiconductor membranes before overgrowth to a new host substrate,  this work establishes III–V semiconductor membranes as a compliant substrate for epitaxy, allowing strain engineering of
III–V heterostructures for optical and electrical applications.

For the complete manuscript click HERE.

For the press release HERE.

 

Revealing the optical properties of two-dimensional materials

In collaboration with the Federal University of São Carlos, the University of Brasília, the University of Southampton, and the University of Nottingham, we investigated the properties of charged excitons in large-area WS2, a transition metal dichalcogenide. We investigate in details the recombination dynamics of the different excitonic complexes of the system. We show that, in particular, the neutral and singly-charged (trion) exciton dynamics are influenced by charge injection from SiO2 substrates, which contribute to a redistribution of the exciton complexes along time. The Figure below shows the time evolution of the neutral (X), trions (T1 and T2), bound (XB), and localized ecitons (LS) as the pumping (laser) power conditions are modified, demonstrating irreversible processes in the exciton emission after long exposure times and pumping power. This work contributes to the understanding of the excitonic dynamics in this new class of materials, which may be important for future optoelectronic devices in graphene-like materials.

To access to the full text, click HERE.

You can also see the web release of this work in our institute webpage.

Bimolecular recombination reviewed

Carrier recombination processes are demonstrated to be tuned by tunneling effects in a quantum well coupled to a quantum dot ensemble. Carrier density is demonstrated to affect recombination rates in a dramatic way. The understanding of the experimental results is performed by an examination of the bimolecular recombination dynamics of the system. To get the full discussion click HERE.