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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - FIRSTSTEP (Synthesis of 2-D semiconductors with honeycomb nanogeometry, and study of their Dirac-type band structure and opto-electronic properties)

Teaser

Summary

The scientific context and framework, problems addressed in this research:
â€¢ Scientific context: Graphene redirected the pathways of solid-state physics with a revival of electronic 2-D materials. Of special interest are solid state electronic materials that, due to their honeycomb nanogeometry, obtain a Dirac-type electronic band structure, i.e. with mass-less charge carriers with the kinetic energy proportional to their momentum. These charge carriers are fundamentally different from those in conventional (semiconductor) electronic systems for which the kinetic energy is proportional to the momentum squared. A genuinely new class of materials will emerge provided that classic semiconductor compounds can be molded in the nanoscale honeycomb geometry: The Dirac-type band structure is then combined with the beneficial properties of semiconductors, e.g. a band gap, optical and electrical switching, and strong spin-orbit coupling. The PI recently prepared atomically coherent 2-D PbSe and CdSe semiconductors by nanocrystal assembly and epitaxial attachment. Moreover, he showed theoretically that these systems combine a semiconductor gap with Dirac-type valence and conduction bands, while the strong spin-orbit coupling results in the quantum spin Hall effect.
â€¢ Problems addressed in this research: (i) The PI wishes to design and develop a new class of two-dimensional semiconductors in which the band structure depends on the superimposed nano-geometry; especially honeycomb type semiconductors with electronic Dirac bands. Dirac physics in solid-state honeycomb systems should result in novel and very useful electronic properties, related to the quantum anomalous Hall effect and the quantum spin Hall effect. The PI will develop a robust bottom-up synthesis platform for 2-D metal-chalcogenide semiconductor compounds with honeycomb nanoscale geometry, based on the current state-of-the-art. (ii) The PI will study their band structure and opto-electronic properties using several types of scanning tunnelling micro-spectroscopy and optical spectroscopy. The Fermi- level will be controlled with an electrolyte-gated transistor in order to measure the carrier transport properties. The results will be compared directly with those obtained on the same 2-D semiconductors without honeycomb geometry, hence showing the conventional band structure. This should unambiguously reveal the Dirac features of honeycomb semiconductors: valence band and conduction band Dirac cones, non-trivial band openings at the K-points that may host the quantum spin Hall effect, and non-trivial flat bands. 2-D semiconductors with massless holes and electrons open new opportunities in opto-electronic devices and spintronics.
Objectives of the research:
â€¢ Develop a robust bottom-up synthesis platform for 2-D metal-chalcogenide semiconductors with honeycomb nanogeometry by nanocrystal self-assembly, oriented attachment, and cation exchange. In a parallel project (outside the ERC) honeycomb systems will be prepared top-down.it will be very useful to compare both methods in terms of lattice vectors, quality and disorder, and feasibility. In a third initiative, artificial honeycomb lattices will be prepared atom-by-atom, using atomic scatterers on a CU(111) surface state.
â€¢ Study the theoretically predicted Dirac-type valence- and conduction bands and non-trivial band openings at the K-points by measuring the density of states, the optical inter- and intra-band transitions, the electron- and hole occupation of the bands, and the band-specific transport properties.
â€¢ Compare the opto-electronic properties of the honeycomb semiconductors with their counterparts with the same atomic lattice and thickness but without nanostructuring, or with a square periodicity. In this way, the unique electronic features of the honeycomb geometry will be revealed in an unambiguous way.
Importance for Society: Materials with novel, unique and tailorable electronic properties are of

Work performed

The ERC project â€œFirst Stepâ€ started on Dec 2016, and results obtained up to 31 may 2019 will be presented. This project is worked out by the PI, an ERC technician (half position), three ERC PhD student and two ERC postdocs, all working together on the overall objectives of the project, see above. Furthermore, the ERC work has resulted in a strong collaboration and interdisciplinary projects with other researchers in my research group and outside it. In the text below, the PI clearly distinguishes the researchers in the ERC advanced grant [(M. van der Sluijs (PhD), Thomas Gardenier (PhD), Jette van den Broeke (PhD), Pierre Capiod (postdoc), Giuseppe Soligno (postdoc), Peter van der Belt (technician)], and other researchers in the group of the PI (indicated by â€œoutside the ERCâ€). Below, the PI summarizes the work on the synthesis of the honeycomb superlattices, the understanding of their formation by molecular dynamics, the acquisition and use of a new STM apparatus, the optical and electronic characterization of the superlattices, and finally, the design and electronic characterization of artificial superlattices. The PI is involved in all projects.
Synthesis of superlattices (M. van der Sluijs, Dr. J. Peters (outside ERC) and C. Post (outside ERC):

Building blocks: The building blocks for the formation of superlattices are PbSe (PbTe, PbS) nanocrystals. The organo-metallic synthesis of these building blocks is currently optimized, and combined with studies of the surface chemistry.
Published: Sizing Curve, Absorption Coefficient, Surface Chemistry, and Aliphatic Chain Structure of PbTe Nanocrystals. Chemistry of Materials 31 (5), 1672-1680 (2019).

Bottom-up formation of superlattices by nanocrystal self-assembly: The self-assembly method has been improved to provide honeycomb superlattices with maximum domain size. The surface passivation of these lattices has been developed successfully. The defects in the honeycomb superlattices have been characterized. Current tasks: (i) Understanding the nanocrystal assembly by in-situ microscopy, using X-ray synchrotron radiation methods, (ii) understanding nanocrystal assembly by using molecular dynamics, (iii) formation of nanocrystal superlattices with a minimum of structural disorder, (IV) developing ion-exchange on the level of the superlattices to transform superlattices with a PbSe (S, Te) composition into CdSe(S, Te) and HgSe(S,Te) composition. The scientific interest derives from the fact that, due to their strong intrinsic spin-orbit coupling, CdSe and HgSe honeycomb superlattices show a strong quantum spin Hall effect, with the emergence of dissipationless, helical edge modes in which the spin and the momentum are locked together. These quantum modes are the next step in electronic topology, after the famous quantum Hall effect. Besides for the huge scientific interest, the quantum spin Hall effect is technological promising for spintronic devices and quantum computing.

Published:
- Mono-and multilayer silicene-type honeycomb lattices by oriented attachment of PbSe nanocrystals: synthesis, structural characterization, and analysis of the disorder Chemistry of Materials 30 (14), 4831-4837 (2018)
- Interfacial self-assembly and oriented attachment in the family of PbX (X= S, Se, Te) nanocrystals. The Journal of Physical Chemistry C 122, 12464-12473 (2018)

Top-down fabrication of honeycomb semiconductors by nanolithography. The ERC project of the PI with its strong scientific and applied interest was the inspiration for a complementary program at the CNRS in France on top-down fabrication of honeycomb semiconductors at CNRS and IEMN - Lille in France, initiated by Dr. Delerue (ISEN, Lile). In this project, honeycomb semiconductors of III-V materials are top-down fabricated by electron-beam lithography, and inc

Final results

Progress in bottom-up and top-down fabrication of semiconductor honeycomb superlattices.

On this moment, july 2019, the synthesis of large (up to 50 micrometers) domains of honeycomb superlattices has become possible. The fabrication is not yet entirely reproducible. In this field, the PI is recognized world-wide for his achievements. However, the PI is also happy to see progress in other groups on the synthesis, defect analysis and annealing of NC superlattices (Prof. Vlieg, Radboud-Nijmegen, Prof. Manhart, Cornell University, Prof. Alivisatos, Berkeley). The extensive international attention and efforts will push this field forward. We expect that we can make progress towards a more reproducible fabrication of less disordered honeycomb semiconductors, both by learning-by-practice and from the strong and clear results of molecular dynamic simulations (see above).

We need and expect progress in (i) the synthesis of atomically coherent superlattices, and (ii) defect annealing, (iii) the transformation of PbSe superlattices into CdSe and HgSe superlattices by cation exchange. On the other hand, the French CNRS initiative has resulted in an extensive collaboration with the group of the PI on lithographically-prepared honeycomb superlattices of III-V materials. These materials have a strong technological interest already, and are prepared with an extreme crystalline quality. The PI expects that the comparison of the properties of these technological advanced III-V materials with the chemically prepared superlattice materials in Utrecht will provide much insight on the promises and limitations of wet colloidal chemistry for advanced electronic materials, and on all effects related to the chemical composition (II-VI vs. III-V superlattices).

Obtaining honeycomb superlattices with Dirac-type electronic bands and a topological quantum spin Hall edge state.

We have recently made enormous progress on the preparation and characterization of artificial lattices (with CO on the Cu(111) surface) having the same geometry as the real materials. As said above, artificial lattices are prepared atom-by-atom in an STM, and can only exist at cryogenic temperatures. They form nearly ideal analogue quantum simulations for our real honeycomb materials and allow to test the validity of the theories existing in this field. Our recent (unpublished) work on artificial honeycomb lattices has unambiguously demonstrated that a Dirac band structure beyond graphene can be realized! Moreover, with other artificial lattices we have shown that theoretically anticipated topological properties, which were so far observed in optical lattices of cold atoms only, can be achieved in technological relevant electronic solid state systems. Within only two years, artificial lattice quantum simulations have become a forefront scientific topic; the team of the PI with prof. Swart is recognized worldwide as the initiator and leading group.

We expect much progress in this field by: (i) studying the quantum (anomalous) Hall effect in our real and artificial honeycomb lattices, (ii) introducing Coulomb interactions and correlations in these systems by using substrates different from the Cu(111) surface, with a lower electron density; such correlations can result in new topological phases, (iii) using substrates and electron scatters with a strong intrinsic spin-orbit coupling, enabling to observe the long-sought quantum spin Hall effect in electronic honeycomb superlattices.