Featured paper. Nanometric Origami: organized strain of two-dimensional materials

Paper: Crystal-oriented wrinkles with origami-type junctions in few-layer hexagonal boron nitride. Oliveira, Camilla K.; Gomes, Egleidson F. A.; Prado, Mariana C.; Alencar, Thonimar V.; Nascimento, Regiane; Malard, Leandro M.; Batista, Ronaldo J. C.; de Oliveira, Alan B.; Chacham, Helio; de Paula, Ana M.; Neves, Bernardo R. A. Nano Research. 2015, 8(5): 1680–1688. DOI: 10.1007/s12274-014-0665-y.

Camilla Oliveira at the atomic force microscope.

Camilla Oliveira was at the Federal University of Minas Gerais (UFMG), in Brazil, studying samples of hexagonal boron nitrite (hBN) with an atomic force microscope (AFM) within the framework of her doctoral studies in Physics, when one particular aspect of the control samples caught her attention and that of her advisor, Professor Bernardo Neves. After undergoing a heat treatment (annealing), the hBN had gained nanometric wrinkles, arranged in a geometric pattern that seemed to follow some sort of organization.

The researchers decided to study these wrinkles in more detail. They had an important question to answer: was there any relation between the arrangement of the wrinkles and the hBN crystal structure? In other words, did these wrinkles have a crystallographic orientation?  Until that moment, there were no records in scientific literature of crystallographically-oriented wrinkles in two-dimensional materials, but this property could be useful.

The two-dimensional hBN crystal lattice (1 atom high).

Camilla and her advisor joined other scientists from UFMG and the neighbor Federal University of Ouro Preto (UFOP) in order to carry out that research. The team produced samples composed of a few layers of hBN anchored on a silicon substrate, they heated them at 1,000 degrees Celsius and then cooled them. During this process, the silicon and the boron nitride displayed opposite strain behaviors. Due to the heating, the hBN contracts itself, while the silicon expands, shrinking the hBN. On the other hand, the cooling expands the hBN and shrinks the silicon, folding the boron nitride as origami paper.

After much experimental work using several techniques and approaches, and various simulations, the scientists were able to confirm that the wrinkles were forming in well-defined directions inside the crystal lattice. Analyzing the folding pattern in details, the scientists noticed the triangular-shaped joints by which the wrinkles (usually three of them) met.

AFM images of a 10nm thick hBN flake after the heat treatment, displaying a crystallographically-oriented pattern of wrinkles (left); details of a typical joint (right). The average height of the wrinkles is 10nm.

Detail: as proven by the Brazilian scientists, for the crystallographically-oriented folding patterns to be formed, the heat treatment must consist of rapid heating, followed by slow cooling (for example, citing the rates used in the research, 50 °C per minute to heat, and 8 °C per minute to cool). The wrinkles produced with faster cooling rates are arranged in a disorderly manner, with no crystallographic orientation.

The researchers have also concluded that this type of organized strain could happen, not only to hBN, but to other two-dimensional materials as well, such as graphene, and that it could lead to interesting applications in straintronics – the field of knowledge that studies and explores the capacity of some materials to have their properties deeply changed due to strain processes.

The results of the research were recently published on the scientific journal Nano Research.

“In my opinion, the main contribution of the paper is to present a property that may be shared by many two-dimensional materials: the organized strain, i.e., strain in well-defined crystallographic directions, of a material at the nanoscale”, says Professor Neves, who is the corresponding author of the paper.

The research was funded by the Brazilian agencies Capes, CNPq and Fapemig, and by INCT-Nanocarbono.

SBPMat’ s community people: interview with Helio Chacham.

During his childhood and adolescence in Belo Horizonte, in the 1960s and 1970s, Helio Chacham had many incentives to become interested in science. After that, in higher education phase, Chacham first started Electrical Engineering but ended up choosing Physics. And that was the field he chose for his undergraduate, masters and doctoral degrees at the Federal University of Minas Gerais (UFMG).

Shortly after completing his doctorate program, he joined the UFMG as Associate Professor, and afterwards he left for the United States to engage in a nearly two-year postdoctoral stage at the University of California in Berkeley. Back in Minas Gerais, between 1995 and 1997, he coordinated the graduate program in Physics at UFMG. From 1999 to 2000, he returned to the United States to engage in a second postdoctoral stage at the University of Texas in Austin. In 2004, he became a Full Professor at UFMG.

Over 30 years of scientific activity, professor Chacham has studied various materials with theoretical research based on the intensive use of computations, although in many opportunities he has worked in collaboration with experimental research groups. Early in his career, Chacham made important contributions to the study of properties of materials under ultra-high pressure. Since the mid-1990s, the researcher has dedicated himself, together with his group and collaborators, to predicting, verifying and explaining phenomena occurring in nanomaterials and two-dimensional materials, also making significant contributions on the same subject.

Currently aged 55, Helio Chacham is a level 1A productivity fellow (the highest level) at the Brazilian National Research Foundation – CNPq. He is the author of around 100 papers published in international peer review journals, which have over 1,800 citations. Chacham is the sub-coordinator of the National Institute of Science and Technology (INCT) of Carbon Nanomaterials. In December 2014, he was elected member of the Brazilian Science Academy (ABC).

Below is an interview with the scientist.

SBPMat newsletter: – How did you become interested in science? What led you to become a scientist and to work in Condensed Matter Physics?

Helio Chacham – My childhood was during the 60s and 70s, a time when there was great interest in science and technology – in part due to the space race and the man going to the moon. As a child and teenager, I always had access to science books (I remember “The Universe” by Isaac Asimov) and also science fiction books (also several by Asimov). At this time I also collected science experiment kits that were sold at newsstands – they were great kits with materials and instructions for experiments, also including small texts on scientists associated with the experiments. The schools I attended as from the 5th grade (both linked to the Federal University of Minas Gerais, UFMG) had good laboratories and good science teachers, which also encouraged me in that direction.

Upon my entry at the University (UFMG), I started as an Electrical Engineering student, but after the first year, I found that my biggest interest was in the fields of Physics and Computer Science. So I switched to the Physics course and meanwhile, for some time, I performed research in Computer Science. Then I was accepted in master´s courses in both – Physics and Computer Science – and ended up choosing the former. Since then, I have devoted myself to research in Condensed Matter Physics, perhaps because it is somehow related to my previous interests (Engineering and Computer Science).

SBPMat newsletter: – In your own assessment, what are your main contributions to the field of Materials?

Helio Chacham – In the 90s I devoted myself primarily to theoretical investigation of properties of materials under ultra-high pressure. These properties are relevant, on the one hand, under the academic point of view, because they allow investigating conditions similar to those of planetary interiors. In addition, these properties determine the limits of hardness of materials, such as the diamond. My largest contributions in this field were the determination of the pressure above which hydrogen becomes a metal – which occurs within Jupiter – and the theoretical determination of one of the diamond hardness measurements, the optimum shear strength.

Since the mid-90s, I started a research line on nanomaterials. This has been one of the most active areas of research in materials since the discovery of fullerenes and carbon nanotubes. My first contributions in the area, in collaboration with students, were predicting morphologies of boron nitride fullerenes and predicting the transformation of electronic properties of carbon nanotube – from insulating into metallic – when subjected to compression. The latter phenomenon was only experimentally demonstrated several years later, in a collaborative work with experimental researchers in my own department – the UFMG Physics Department. These theoretical/experimental collaborations have had a fruitful continuing so far, which has allowed us to predict, verify and explain various new phenomena in carbon nanotubes, graphene and two-dimensional materials, phenomena such as: the negative dynamics compressibility in graphene; wrinkle crystallization in boron nitride; and talc exfoliation up to the single layer boundary, similar to that of the graphene, and determination of properties of this new two-dimensional material.

During all of these projects I was always concerned in training masters´ and doctors, whose theses dealt with electronic and structural properties of nanotubes, fullerenes, DNA, nanoparticles, nanowires, graphene and other two-dimensional materials. These former students are now professors and researchers at UFMG and other universities, and have carried out several projects, mainly in the nanomaterials field.

SBPMat newsletter: – Last year you were elected member of the Brazilian Science Academy (ABC). What that means to you? How do you see your role within the ABC?

Helio Chacham – I deeply appreciate the support of my colleagues of the Academy in the election. I will take office in May, and then, will be able to seek ways to contribute with ABC, whether by participating in committees or in specific projects of the Academy, or by collaborating with Science Academies in other countries, one of which I have already participated (Brazil/India) before joining as a member. As I have been providing service to the community, whether, for example, as a member of the advisory committee of the CNPq or by coordinating projects in nanomaterials, I believe that my election will allow me to continue to contribute with the research community in many ways.

SBPMat newsletter: – Leave a message for our readers who are starting their careers as scientists.

Helio Chacham: – Based on my professional experience so far, I may be able to give some advice – which can be useful or not depending on the personality of each person, of course:

a) Work on what you really enjoy – the researcher’s career is one of the few that allow you to do so.

b) Search research areas with many issues to be solved, or new materials being produced, and which are consistent with item (a) above. For that matter, it is important to always keep up with scientific literature.

c) Master the methods you use as deeply as possible. That will allow you to attack difficult and important issues.

d) Always be willing to study and learn new methods. That will give you the flexibility and the ability to search for new issues and research areas, as well as to collaborate with researchers using other methodologies. Science changes continuously and constantly.

Featured paper: Accurate engineering in spin valves manufacturing.

The scientific paper by members of the Brazilian community on Materials research featured this month is:

T. E. P. Bueno, D. E. Parreiras, G. F. M. Gomes, S. Michea, R. L. Rodríguez-Suárez, M. S. Araújo Filho, W. A. A. Macedo, K. Krambrock and R. Paniago.Noncollinear ferromagnetic easy axes in Py/Ru/FeCo/IrMn spin valves induced by oblique deposition. Appl. Phys. Lett. 104, 242404 (2014). DOI: 10.1063/1.4883886.

Accurate engineering in spin valves manufacturing

The production and characterization of spin valves is the theme of  a collaborative work between Brazil and Chile, whose results were published recently in the prestigious journal Applied Physics Letters (APL).

Spin valves are devices consisting of three or more layers of nanometric thickness composing a sandwich of magnetic and non-magnetic materials. Sensors consisting of such structures fulfill a fundamental role in reading the information written on the hard disc drives, among other applications.

The operation of spin valves is based on an effect called “giant magnetoresistance”, which was the reason behind the Nobel Prize in Physics in 2007. The giant magnetoresistance of spin valve consists of a large change in the electrical resistance in response to the action of a magnetic field. This resistance depends on the relative orientation among the magnetization of the magnetic material layers.

The magnetization of a magnetic material is determined by the orientation of the spins of its electrons. Electrons have two intrinsic features: electric charge and magnetic moment, the latter known as spin. Explore the degree of freedom of the electron spin in addition to its charge led to the emergence of a new field of research called spintronics.

Then, on giant magnetoresistance of spin valves, when the layers of magnetic material have the same direction of magnetization, the device reduces its electrical resistance and becomes a better conductor of electricity. When the magnetic layers acquire opposite directions of magnetization, a significant increase of electrical resistance occurs.

For better understand this effect and, later, the results presented in the article of APL, it is important to remember that the magnetization is a vector physical quantity and that, therefore, besides having an intensity, it has a direction (parallel, perpendicular) and an orientation (indicated by the arrowhead representing the vector). Usually, metallic multilayers composed of magnetic materials separated by a non-magnetic layer, as spin valves, have the magnetization of ferromagnetic layers coupled, says Thiago Bueno, first author of the APL article and PhD student in Physics at the Brazilian Federal University of Minas Gerais (UFMG), supervised by professor Roberto Magalhães Padilla. This coupling can result in parallel magnetization (called “collinear”) with same or opposite orientations, and also in non-collinear magnetization.

Ferromagnetic layers “making a sandwich” with a non-magnetic layer of ruthenium. The red and green arrows represent the direction and the way of magnetization of layers composed by Py and FeCo, respectively. (a) Parallel magnetizations with equal orientation; (b) Parallel magnetizations with opposite orientation; (c) Perpendicular magnetizations.

However, to magnetize the magnetic layers of the spin valve does not occur homogeneously in all directions; they feature the so-called magnetic anisotropy. “The magnetic anisotropy is an important magnetic property, because it establishes an easy direction of magnetization,” says Thiago Bueno. “This property is determined by a number of factors, including the types of materials, the thickness of layers, and the details of the method of sample manufacturing”.

On the work that originated the APL article, the team of scientists has made some adjustments to the method of spin valves manufacturing, obtaining interesting results on the properties of these devices.

Controlling the direction of magnetization

“This work was only possible due to the great collaboration between the parties along the preparation of samples of excellent quality, accurate experimental measures, interpretation of the data, until the publication of the results,” says Thiago Bueno.

Initially, at the Brazilian Center for Development of Nuclear Technology (CDTN in Portuguese) the team has made thin films composed of multi-layers with thickness of a few tens of nanometers. The films were obtained through the technique known as magnetron sputtering, in which argon ions are accelerated against the targets that contain the materials to be deposited, ripping off its atoms. With the aid of magnetrons, these atoms are deposited on a substrate, forming the layers of films. “Through this technique it is possible to obtain films with well-determined chemical composition, thickness and structural morphology,” says Thiago Bueno.

Oblique deposition scheme with 5 sputtering sources (magnetrons) producing an angle of 72 between them. The (β) angle between the direction of deposition and normal direction of the film is estimated at 38° for all sources.

In this study, the scientists set up an oblique deposition scheme by putting the magnetrons making an angle of 72o between them and inclined towards the sample. Using the oblique deposition scheme, scientists made spin valves with ferromagnetic layers up to 10nm-thickness, composed of metallic alloys (Py and FeCo), and separated by a non-magnetic layer of ruthenium (Ru) of thickness between 1nm and 3.5nm. The devices were characterized in the Physics Department at UFMG using ferromagnetic resonance (FMR), an extremely sensitive technique that provides relevant information on the magnetization of materials.

After the interpretation of experimental results, which involved researchers from the Pontifical Catholic University of Chile, the scientists concluded that the oblique deposition induced non-parallel magnetization directions (non-collinear) on ferromagnetic layers of manufactured spin valves.  “The angle between the easy axes, approximately equal to the angle between the magnetrons, was determined by the manufacturing geometry”, reinforces the author, Bueno. “One of the main contributions of our work is the demonstration that it is possible to manufacture spin valves where the axes of easy magnetization of ferromagnetic layers (Py and FeCo) are non-collinear,” he sums up.

According to the doctoral student, at the beginning of the work the authors already knew the oblique deposition effects in ferromagnetic/anti-ferromagnetic bilayers. With this study, the team took a step further and has investigated these effects in a more complex structure, the spin valve.

“We believe that our work will compel other researchers into manufacturing these devices, seeking new magnetic configurations between layers of the spin valve “, says Bueno.