Revealing unexplored dopants in semiconducting materials

The advancement of transparent electronics, one of the most anticipated technological developments for the future, is currently inhibited by a shortage of high-performance p-type conductors.
In materials, doping is the intentional introduction of impurities into an intrinsic semiconductor for the purpose of modulating its electrical and optical and structural properties. Recent demonstration of tin monoxide as a successful transparent p-type thin-film transistor and the discovery of its potential for  ambipolar doping, suggests that tin monoxide—an environmentally-friendly earth-abundant material—could offer a solution to this challenge. However, only a handful of elements have been investigated as candidate dopants for this material.
Identifying suitable elements for doping requires an enormous amount of experimentation, that involves expensive trial-and-error approaches and can take years before a suitable substitutional element is found. With the aim of accelerating this process and finding a path for successfully enhancing the electronic properties of SnO, an extensive computational search for useful dopant elements was performed using “Piz Daint” at the CSCS. Employing computer simulations researchers at the University of Basel have discovered that substitutional doping with the family of alkali metals provides an as off yet unexplored possibility to increase the concentration of acceptors (i.e. increase p-type conductivity) without being detrimental to the transparency of SnO.
Over ten shallow donors, which, to the best of our knowledge, have not been previously contemplated, were identified in this accelerated discovery thanks to Piz Daint.  The work presents a detailed analysis of the most promising n-/p -type dopants—offering new insights into the design of an ambipolar semiconductor. If synthesized successfully, such a doped ambipolar oxide could open new avenues for many transparent technologies.
Reference: Miglė Graužinytė, Stefan Goedecker, and José A. Flores-Livas. “Towards bipolar tin monoxide: Revealing unexplored dopants”,  Phys. Rev. Materials 2, 104604 – October 2018.

Emergence of Perovskite (MAPI) phases upon little compression

Perovskites are among the most promising and versatile class of candidate compounds for new or improved materials in energy applications, including photovoltaics, superconductivity, and lasing. With the general formula ABX3, the perovskite structure consists of corner-sharing BX6 octahedra forming a three-dimensional (3D) framework that provides room for the A units in the resulting cuboctahedral cavities.

In our latest work we use classical interatomic potential and coupled with minima hopping method to screen the potential energy surface using simulation cells containing 2, 4, and 8 units (i.e., 24, 48, and 96 atoms), on were more than 140,000 structures were generated.

We also investigated how pressure affects the enthalpy and volume of the experimental and predicted phases of MAPI with respect to the orthorhombic phase (see figure). Based on our careful analysis of the available literature work, none of the experimental high-pressure experiments used the low-temperature orthorhombic phase as the starting material. Instead, our simulated XRD patterns show that the tetragonal phases is present in the samples of experimental work. Based on our calculations, we therefore suggest that either the delta or double delta phases could be synthesized by compressing precursor samples in the orthorhombic (Pnma) phase in low-temperature compression experiments. Since the orthorhombic phase is strongly destabilized upon compression, a transition towards the double-delta or delta phases will be rapidly favored with increasing pressure. Our work is now published in “Phys. Rev. Materials 2, 085201 2018″.

Not all materials are metallic under pressure: tuning the gap !

Highly stable materials are usually wide-gap insulatorswhere covalency dominates the ionic exchange, such as in carbon (diamond), MgO, and LiH, to name a few.  In our latest work,  in collaboration with experimental teams in USA,  UK, and Japan we studied the enhanced stability of Sn3N4 to applied pressure and temperature. Our predicted phase transitions were confirmed by state-of-the-art Synchrotron X-ray diffraction and EXAFS measurements upon different annealing procedures. 

The unique stability of the spinel (Sn3N4) phase  is attributed to its dominant ionic character. This work is now published in Angewandte Chemie (International Edition).  We also demonstrated that the application of pressure allowed us a systematic tuning of the charge density, cleanly, that is, without changes to the chemical composition via dopants.
More importantly we demonstrated the mechanism responsible for the opening of the electronic band upon compression.  A continuous opening of the optical band gap was observed from 1.3 eV to 3.0 eV over a range of 100 GPa, a 540 nm blue‐shift spanning the entire visible spectrum. The pressure‐mediated band gap opening is general to this material across numerous high‐density polymorphs, implicating the predominant ionic bonding in the material as the cause. The rate of decompression to ambient conditions permits access to recoverable metastable states with varying band gaps energies, opening the possibility of pressure‐tuneable electronic properties for future applications.

Possible superconductivity in hydrogenated carbon nanostructures

In this work we present an application of density-functional theory for superconductors (SCDFT) to superconductivity in hydrogenated carbon nanotubes and fullerane (hydrogenated fullerene). We show that these systems are chemically similar to graphane (hydrogenated graphene) and like graphane, upon hole doping, develop a strong electron phonon coupling. This could lead to superconducting states with critical temperatures approaching 100 K. However this possibility depends crucially on if and how metallization is achieved.

(Left) Real space anomalous potential ∆ (R, s) (left) and order parameter χ (R, s) (center and right) as a function of the Cooper pair center of mass R. On the top (a–c) an xy cut of the tube and on the bottom (d–f) a vertical cut of the tube. The doping level (rigid shift) is indicated on the top. The value of the functions is given according to the colorscale in the center (left scale refers to the left plot and the two right scales to the two right plots. A white dashed line in the a–c plots indicates the cut shown on the (d–f) plots.

This work is dedicated to Hardy Gross who, not only jointly invented SCDFT, but also devoted a large effort to develop the theoretical framework into a fully functioning method, investigating functionals, extensions  and transforming it into a useful and predictive tool in material science.

Doping@HP the case of polyethylene: superconductivity!

High pressure is an exciting field that has evolved incredibly far since the pioneering work of Cailletet, Amagat and Bridgman. A substantial amount of research in the field of high pressure (post-Bridgman era) was triggered by the tantalizing idea of metalizing hydrogen (Wigner and Huntington transition) which dates back to the mid 30’s. The metalization of hydrogen is seen as the holy-grail of high pressure research, it has been a compelling subject of great interest for many scientists ranging from experimental chemists and physicists to theoreticians,  including Prof. Gross (my former boss in Max-Planck, Halle).

It is well understood, that compression of molecular systems at high pressure increases the electron-orbital overlap between neighboring atoms resulting in an increase of the band dispersions consequently closing the electronic band gap. Chemical pre-compression is certainly one promising route to reduce the metalization pressure on insulating elements, but not the only one! Another method to reach metalization is chemical doping under pressure –a path previously used at ambient pressures to render standard insulators superconducting.–  We demonstrated theoretically this approach for H2O.

In our latest article, we investigated the structural stability of polyethylene (H2C)n under pressure. The questions we want to address in the work were: is there a stable polyethylene phase under pressure that can be doped? if yes, is it superconducting? Finally, we dedicated this article to Prof. Hardy Gross, for his 65th birthday. This research article would appear in the Topical Issue “Special issue in honor of Hardy Gross” edited by C.A. Ullrich, F.M.S. Nogueira, A. Rubio, and M.A.L. Marques.

“Cold-Passivation” of Defects in Tin-Based Oxides

Our latest results arising from a collaborative work with experimental groups in École Polytechnique Fédérale de Lausanne (EPFL), Neuchâtel,  University of California Berkeley, National Institute of Advanced Industrial Science and Technology (AIST Japan) and the Institute for Nanotechnology, University of Twente, is published now in Journal of Physical Chemistry C  (link).

In this work we studied transparent conductive oxides (TCOs), which are essential in technologies coupling light and electricity. For Sn-based TCOs, oxygen deficiencies and under-coordinated Sn atoms result in an extended density of states below the conduction band edge. While shallow states provide free carriers necessary for electrical conductivity, deeper states inside the bandgap are detrimental to transparency. In zinc tin oxide (ZTO), the overall optoelectronic properties can be improved by defect passivation via annealing at high temperatures. Yet, the high thermal budget associated with such treatment is incompatible with many applications. In our most recent work, we demonstrate an alternative, low-temperature passivation method, which relies on co-sputtering Sn-based TCOs with silicon dioxide (SiO2). Using amorphous ZTO and amorphous/polycrystalline tin dioxide (SnO2) as representative cases, we demonstrate through optoelectronic characterization and density functional theory simulations that the SiO2 contribution is two-fold. First, oxygen from SiO2 passivates the oxygen deficiencies that form deep defects in SnO2 and ZTO. Secondly, the ionization energy of the remaining deep defect centers is lowered by the presence of silicon atoms. Remarkably, we find that these ionized states do not contribute to subgap absorptance. This simple passivation scheme significantly improves the optical properties without affecting the electrical conductivity, hence overcoming the known transparency-conductivity trade-off in Sn-based TCOs.

Tittle of the article: “New Route for “Cold-Passivation” of Defects in Tin-Based Oxides

Crystal defects for qubits

We had the opportunity to attend the CECAM-Workshop “Crystal defects for qubits, single photon emitters and nanosensors” in Bremen (Germany), an interesting and forefront workshop in crystal defects for quibits.

Since we all know, the leading contender is the nitrogen-vacancy center in diamond which may be considered as a robust quantum tool. Several quantum algorithms and protocols for sensing have been already demonstrated by this center.  However, researchers face many materials science problems in order to maintain the favorable intrinsic properties of this color center that can be perturbed by other defects either in bulk or at the surface of diamond that is difficult to resolve because of its chemical hardness and the concurrent stability of carbon  allotropes.

Theory-driven search for alternative materials could identify other quantum bit candidates in technologically mature wide band gap semiconductors, particularly silicon carbide, that have been recently demonstrated in experiments. However, the knowledge about these color centers is scarce.

World-leaders, experts in the filed presented the state of the art in this research topic,  invited among others:
David Awschalom (University of Chicago, Illinous) Adám Gali (Hungarian Academy of Science, Wigner Research Centre for Physics, Budapest) Audrius Alkauskas (Center for Physical Sciences and Technology, Vilnius)Sophia Economou (Virginia Polytechnic Institute and State University Blacksburg, Virginia) Ronald Hanson (Delft University of Technology) Marcus Doherty (Australian National University, Canberra) Arne Laucht (University of New South Wales, Sydney) Fedor Jelezko (Ulm University).

During this meeting we could exchange ideas of methods to use, and listen to discussions of new developments between scientists working on different aspects of diamond, silicon carbide and related materials. The interdisciplinary character of this workshop was a unique opportunity for me to learn  and to have a better idea on which problems I should focus.

Stay tuned! this research topic is very promising and will lead to nice discoveries!


SuperMUC will build a high pressure materials database.

Our computing project has just been accepted to run in the SuperMUC Petascale System.

With more than 2.3 millions of computing cores at our disposition the grant in its first phase will serve to construct a database for materials under pressure. The project will runs until 2020 including a second phase with possible extension of the requested computing allocation.

The supercomputer is SuperMUC ranked top 4 as the fastest supercomputer in Germany. It is composed of more than 147,000 computing cores.  SuperMUC (the suffix ‘MUC’ alludes to the IATA code of Munich’s airport) is operated by the Leibniz Supercomputing Centre, a European centre for supercomputing.

Stay tuned! coming months surely we will have very exciting news !

Structures of exohedrally decorated C60-Fullerenes

In our most recent publication we studied the exohedrally metal decorated carbon-fullerenes. These systems are a promising material for its good hydrogen adsorption (high concentrations and with optimal binding energies) properties. Since their geometry and type of coverage play a key role in determining the H2 adsorption mechanism, in this paper just accepted in Carbon Journal, we studied in a fully  ab-initio, unbiased structure fashion the configurational space of decorated C60 fullerenes.

Many of the hitherto postulated ground state structures are not ground states. We could determine the energetically lowest configurations for decorations with a varying number of decorating atoms for alkali metals, alkaline-earth metals as well as some other important elements and find that the dense uniform distribution of the decorating atoms over the surface of the C60, desired for hydrogen storage, can be obtained only for a few elements.  An understanding of the behavior of the decorating atoms can be obtained
by analyzing their bonding characteristics.

Searching hole-electron substitutional dopants for TCO technologies

The combination of optical transparency and high electrical conductivity enables transparent conductive oxide (TCO) materials to be used for a wide range of applications -from simple smart window coatings to OLEDs and futuristic see-through displays.  Doped tin-dioxide (SnO2) is an important semiconductor that is already used for these applications. However, in order to uncover the entire potential of this material in more advanced applications of  optoelectronics further improvements in electrical properties are necessary.

We conducted an extensive search for useful substitutional dopants of SnOfor which a novel and well-converged protocol was used. The entire periodic chart was scanned for stable charges and hole-electron dopants.  Our finding are in excellent agreement with current known dopants, besides we predicted other possible substitutional dopants that have been not experimentally examined to date.

This work has been just accepted (link here) and appeared in Chemistry of Materials.