Featured by SPBMat

Our last article has been featured by the Brazilian Material Research Society: See their post “Engenharia de estrutura de banda usando a geometria de tubos enrolados.” Thanks for this.

 

New article in ACS Applied Nano Materials

After seeing the effect the first time 10 years ago in old samples, we finally figured out what was happening. See out latest results in ACS Appl. Nano Mater. with the title “Rolled-Up Quantum Wells Composed of Nanolayered InGaAs/GaAs Heterostructures as Optical Materials for Quantum Information Technology”: 

Strain-based band structure engineering is a powerful tool to tune the optical and electronic properties of semiconductor nanostructures. We show that we can tune the band structure of InGaAs semiconductor quantum wells and modify the helicity of the emitted light by integrating them into rolled-up heterostructures and changing their geometrical configuration. Experimental results from photoluminescence and photoluminescence excitation spectroscopy demonstrate a strong energy shift of the valence-band states in comparison to flat structures, as a consequence of an inversion of the heavy-hole with the light-hole states in a rolled-up InGaAs quantum well. The inversion and mixing of the band states lead to a strong change in the optical selection rules for the rolled-up quantum wells, which show vanishing spin polarization in the conduction band even under near-resonant excitation conditions. Band structure calculations are carried out to understand the changes in the electronic transitions and to predict the emission and absorption spectra for a given geometrical configuration. Comparison between experiment and theory shows an excellent agreement. These observed profound changes in the fundamental properties can be applied as a strategic route to develop novel optical devices for quantum information technology.

Leonarde N. Rodrigues, Diego Scolfaro, Lucas da Conceição, Angelo Malachias, Odilon D. D. Couto, Jr, Fernando Iikawa, and Christoph Deneke

ACS Appl. Nano Mater., 4 (3), 3140–3147 (2021)
DOI: 10.1021/acsanm.1c00354

Research was supported by FAPESP and CNPq.

As featured by the SBPMat.

 

 

Nice book for people interested in Science

I finished reading a very good philosophy book trying to figure out, what makes science in its way to acquire knowledge different from other approaches. “The Scientific Attitude: Defending Science from Denial, Fraud, and Pseudoscience” by Lee McIntyre is interesting making the point the difference is not the method, but the attitude how evidence is gathered and used to progress the knowledge of the world. He tells us that this attitude separates scientist from non-scientiest and lead to the strength of the scientific obtained understanding of the world making it special against all other knowledge.

Whereas I like the idea to separate the scientific endeavor by its attitude and values involved rather then by a scientific method, I have to admit, his science idea is to much an empirical one influenced by logical positivism, Popper and Kuhn. I personally see a strong component that acquiring of  knowledge and therefor also science has its base in empirical phenomena, but also a synthesis part as pointed out by Kant. In some way, McIntyre might agree with this as he points out that science not only reports the empirical finding, but the knowledge must come with a warrant.

Also it is interesting to see, how Galileo Galilei is mentioned to be a victim of the science believe of his time. There should be mentioned: No one believed in this time that the earth is flat – the solar system just was assumed to be heliocentric; the catholic church is not really the science community – Galileo’s science colleagues of the time like Copernicus (who suggested the heliocentric defended by Galileo Galilei), Johannes Kepler  and Tycho Brahe  already gave up on the geocentric world idea – it were people outside the sciences that did not want to believe this.

Anyway, the book is very interesting and I enjoyed reading it. I can just recommend it for people doing science as well as people not doing science. It is an important contribution in these times, where scientific knowledge is seen as a opinion among others  and forgotten that generations of scientist worked to arrive on the current point of knowledge. Therefore, the current state of scientific knowledge contains the experience of many tries (and failures) to understand the world. If the knowledge is gathered with the right, scientific attitude, it is our best chance to be the correct one.

P.S.: Peer review is not as good as hoped in the book and the important fact checking is done afterwards by the community; and scientific progress does not happened in Paradigmatic shifts, but in a hard creeping way of small continuous steps (sometimes with some impressive jumps from people at the right time and place, which would not possible, if the previous generations of scientist had not collected all the data).

 

 

 

New article: Band structure engineering in strain-free GaAs mesoscopic structures

We investigate the optical properties of strain-free mesoscopic GaAs/Al(x)Ga(1-x)As structures (MGS) coupled to thin GaAs/A(x)Ga(1-x)As quantum wells (QWs) with varying Al content (x). We demonstrate that quenching the QW emission by controlling the band crossover between AlGaAs X-point and GaAs Gamma-point gives rise to long carrier lifetimes and enhanced optical emission from the MGS. For x = 0.33, QW and MGS show typical type-I band alignment with strong QW photoluminescence emission and much weaker sharp recombination lines from the MGS localized exciton states. For x >= 0.50, the QW emission is considerably quenched due to the change from type-I to type-II structure while the MGS emission is enhanced due to carrier injection from the QW. For x >= 0.70, we observe PL quenching from the MGS higher energy states also due to the crossover of X and Gamma bands, demonstrating spectral filtering of the MGS emission. Time-resolved measurements reveal two recombination processes in the MGS emission dynamics. The fast component depends mainly on the X-Gamma mixing of the MGS states and can be increased from 0.3 to 2.5~ns by changing the Al content. The slower component, however, depends on the X-Gamma mixing of the QW states and is associated to the carrier injection rate from the QW reservoir into the MGS structure. In this way, the independent tuning of X-Gamma mixing in QW and MGS states allows us to manipulate recombination rates in the MGS as well as to make carrier injection and light extraction more efficient.

Vanessa Ors Gordo, Leonarde Nascimento Rodrigues, Floris Knopper, Ailton J Garcia, Fernando Iikawa, Odilon D. D. Couto Jr. and Christoph Deneke

Nanotechnology 31, 255202 (2020)

 

New article: Scanning Tunneling Measurements in Membrane-Based Nanostructures

Our new article: “Scanning Tunneling Measurements in Membrane-Based Nanostructures: Spatially- Resolved Quantum State Analysis in Postprocessed Epitaxial Systems for Optoelectronic Applications” is out in ACS Appl. Nano Mater.

Nanoscale heterostructure engineering is the main target for the development of optoelectronic devices. In this sense, a precise knowledge of local electronic response after materials processing is required to envisage technological applications. A number of local probe techniques that address single nanostructure signals were satisfactorily employed in semiconductor epitaxial systems. In this work we show that the use of chemically etched semiconductor nanomembranes allows carrying out scanning tunneling spectroscopy (STS) measurements in a postprocessed system which was otherwise studied mainly under in situ conditions that differ from the operational regime. We were able to acquire STS spectra with energy level resolved response on InAs quantum dots grown within a 15 nm-thick GaAs single-crystalline film transferred to an Au(111) surface. The presence of a native oxide layer does not affect the result, keeping the reliability of the usual ultra high vacuum (UHV) procedures. The use of nanomembranes also opens up the possibility of tailoring properties via additional variables such as nanomembrane thickness and surface charge depletion. Our method is applicable to a broad class of postprocessed layers extracted in nanomembrane format from epitaxial systems that are potential candidates for optoelectronic applications.

Authors: Barbara L. T. Rosa, Carlos A. Parra-Murillo, Thais Chagas, Ailton J. Garcia Junior, Paulo S. S. Guimarães, Paulo S. S. Guimarães, Ch. Deneke, Rogerio Magalhães-Paniago and Angelo Malachias

ACS Appl. Nano Mater. 2, 4655-4664,(2019)
DOI: 10.1021/acsanm.9b01124

As boas universidades no Brasil custam pouco, caro mesmo é não saber criar conhecimento

I would like to share out an interesting article (in Portuguese) thinking a bit about the cost and befits of the public founded universities of Brazil.

It is worth reading “Excelência acadêmica requer custeio público“, an articled found at the webpages of Unicamp.

 

 

After two years of refereeing: Nanoletter finally out

That must have been the longest submission in my scientific life.  After two years, with 14 referee reports from the first submission to Nature Photonics and three more rounds with Nanoletters our work: “Anisotropic Flow Control and Gate Modulation of Hybrid Phonon-Polaritons” is out.

Light–matter interaction in two-dimensional photonic or phononic materials allows for the confinement and manipulation of free-space radiation at sub-wavelength scales. Most notably, the van der Waals heterostructure composed of graphene (G) and hexagonal boron nitride (hBN) provides for gate-tunable hybrid hyperbolic plasmon phonon-polaritons (HP3). Here, we present the anisotropic flow control and gate-voltage modulation of HP3 modes in G-hBN on an air–Au microstructured substrate. Using broadband infrared synchrotron radiation coupled to a scattering-type near-field optical microscope, we launch HP3 waves in both hBN Reststrahlen bands and observe directional propagation across in-plane heterointerfaces created at the air–Au junction. The HP3 hybridization is modulated by varying the gate voltage between graphene and Au. This modifies the coupling of continuum graphene plasmons with the discrete hBN hyperbolic phonon polaritons, which is described by an extended Fano model. This work represents the first demonstration of the control of polariton propagation, introducing a theoretical approach to describe the breaking of the reflection and transmission symmetry for HP3 modes. Our findings augment the degree of control of polaritons in G-hBN and related hyperbolic metamaterial nanostructures, bringing new opportunities for on-chip nano-optics communication and computing.

Authors: Francisco C. B. Maia, Brian T. O’Callahan, Alisson R. Cadore , Ingrid D. Barcelos , Leonardo C. Campos, Kenji Watanabe , Takashi Taniguchi, Christoph Deneke , Alexey Belyanin, Markus B. Raschke , and Raul O. Freitas

Nano Lett.  19, 2, 708-715 ( 2019)
DOI: 10.1021/acs.nanolett.8b03732

Destaque em Física (SBF)

Article accepted in Nanoscale

In-place bonded semiconductor membranes as compliant substrates for IIIV compound devices

Overcoming the critical thickness limit in pseudomorphic growth of lattice mismatched heterostructures is a fundamental challenge in heteroepitaxy. On-demand transfer of light-emitting structures to arbitrary host substrates is an important technological path for optoelectronics and photonics devices implementation.  The use of freestanding membranes as compliant substrates is a promising approach to address both issues.

In this work, the feasibility of using released GaAs/InGaAs/GaAs membranes as virtual substrates to thin films of InGaAs alloy is investigated as a function of the Indium content in the films. Growth of flat epitaxial films is demonstrated with critical thickness beyond typical values regarding growth on bulk substrates.  Optically active structures are also grown on these membranes with strong photoluminescence signal and clear red shift for an InAlGaAs/InGaAs/InAlGaAs quantum well. The red shift is ascribed to strain reduction in the quantum well due to the use of completely relaxed membrane as substrate. Our results demonstrate that such membranes constitute a virtual substrate allowing further heterostructure strain engineering not possible by other post-growth methods.

Authors:  Ailton Jose Garcia Junior,  Leonarde N. Rodrigues,  Saimon Filipe Covre da Silva,  Sergio L.  Morelhao,  Odilon D. D. Couto Jr.,  Fernando Iikawa  and  Christoph Deneke  

Journal: Nanoscale 11, 3748-3756 (2019)

As featured on the page of the IFGW

Conference right before 1st semester 2019

I will be at the EuroMBE 2019 in February taking the chance to see, what’s going on in the European (and International) community. The conference will be in the week before the 1st semester starts from February, 17th to 20th, 2019. 

I will also present our work over membrane overgrowth in a poster session on Monday.

CU there.

 

Why the State of São Paulo is good in science and why it is important

The is a very nice article in Nature  discussing, why the state of SP is so good in science. Indeed, the state alone is responsible for more than 50% of Brazilian science production and  the leading place in South America.

That this is of great importance for the country and the society is shown on the example of the Zika virus. As we have an excellent existing infra-structure, the scientist here were one of the leading groups figuring out, what was going on and how to fight it!

The foundation for this is laid by the excellent founding conditions – mainly FAPESP – and with this allows to run two of the best universities of South America (Unicamp and USP), which provide highly qualified people for industry, governmental research institutes like the CNPEM and independent research institutes in the private sector.

We should always remember that all solutions for our society problems are based on a good and rigorous  understanding enabled by organized and well founded academic research – independent if done in academic institutions or outside in the private sector.