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Synthesis of 2-D semiconductors with honeycomb nanogeometry, and study of their Dirac-type band structure and opto-electronic properties

Periodic Reporting for period 3 - FIRSTSTEP (Synthesis of 2-D semiconductors with honeycomb nanogeometry, and study of their Dirac-type band structure and opto-electronic properties)

Reporting period: 2019-12-01 to 2021-05-31

The scientific context and framework, problems addressed in this research:
• 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.
• 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:
• 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 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.
• 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.
• 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 great interest for the transfer of our society to a state “beyond Si CMOS”, boosted by the new quantum revolution that has recently taken off. This thus not mean that the very useful technology on Si should be abandoned, but merely that computing and information processing, that is faster and less energy-consuming than the current state-of-the-art, is highly desired to complement devices based on silicon. New ways of computing, e.g. computing based on dissipationless charge and spin transport and quantum computing, and new ways to transfer information in a safe and rapid way are currently under investigation. The central research of this ERC project should be regarded in this respect; honeycomb semiconductors are predicted to possess dissipationless spin and charge currents at their edges that could be very valuable. Moreover, certain quantum states might be topological protected, which makes them of interest as quantum bits.
The ERC project “First Step” 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 “outside the ERC”). 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.

- 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 incorporated in devices. The electronic characterization is under way.
- Triangular nanoperforation and band engineering of InGaAs quantum wells: a lithographic route toward Dirac cones in III-V semiconductors. Nanotechnology 2019

New STM apparatus (P. van de Belt, T. Gardenier, J. Moes, outside ERC, Prof. I. Swart, outside ERC,):
With the ERC Advanced grant, a state of the art cryogenic STM/AFM apparatus with a magnetic field up to 5 Tesla has been acquired, and optimized for the research on the honeycomb superlattices. At this moment the first promising results have been obtained on InAs/GaAs 2-D quantum wells and honeycomb superlattices.

The understanding of superlattice formation by molecular dynamics (M. van der Sluijs, Dr. G. Soligno, post-doc, F. Montanarella (outside ERC):
The formation of honeycomb superlattices by nanocrystal assembly reflects and intricate process occurring at an interface. With gracing incidence synchrotron radiation, we were able to show that this process occurs at the glycol/NC dispersion /air interface (Nature Materials 2016). We are currently investigating the mechanism of this process in all detail as a better mechanistic understanding is crucial for further improvement of the bottom-up synthesis of honeycomb superlattices (and other geometries) from colloidal semiconductor nanocrystals. (i) We obtained additional beam time in the synchrotron Soleil for in-situ X-ray scattering experiments to study the different stages in the process of nanocrystal assembly at the toluene interface followed by oriented attachment (project van der Sluijs). A specialist in molecular dynamics (Dr. G. Soligno) has been hired to simulate the consecutive processes of superlattice formation, i.e. interfacial nanocrystal adsorption, nanocrystal self-assembly and re-ordering, and oriented attachment.
- Understanding the Formation of PbSe Honeycomb Superstructures by Dynamics Simulations Physical Review X 9 (2), 021015 (2019)
- Crystallization of Nanocrystals in Spherical Confinement Probed by in Situ X-ray Scattering. Nano letters 18 (6), 3675-3681 (2018)

Optical and Electronic characterization of superlattices prepared from colloidal nanocrystals (M. van der Sluijs, Pierre Capiod, M. Alimoradi Jazi, outside ERC, C. Post, outside ERC, A. Brodu, outside ERC, and F. Montanarella, outside ERC):

The electronic properties of the nanocrystal superlattices are studied with (i) scanning tunneling spectroscopy and (ii) transistor-transport measurements. With scanning tunneling spectroscopy, the electronic density of states is measured in a local way. The major challenge is that extremely clean samples are required. Nanocrystal self-assembly is a wet-chemical synthesis with the result that capping ligands, and toluene and glycol solvent molecules are adsorbed as impurities on the sample. They have to be removed before tunneling spectroscopy in an STM is feasible. We have put much effort in cleaning techniques and recently acquired the first trustful spectroscopic results. A publication on this important work is under way. (postdoc Dr. Pierre Capiod). In addition, scanning tunneling microscopy studies have also been performed on the dop-down fabricated superlattices, in a collaboration with the France National CNRS project of Prof. Delerue (C. Post, outside but attached to the ERC grant). (ii) Superlattices, consisting of a nanocrystal monolayer, have been synthesized and incorporated in a transistor with electrolyte gating. The occupation of the electronic bands in PbSe and CdSe superlattices, and the carrier mobility at room temperature, have been investigated in detail (M. Alimoradi Jazi, project outside but attached to the ERC grant). At present, measurements on the temperature dependence of the carrier mobility are under way (V. Janssen, Project outside but attached to the ERC grant).

For optical characterization, superlattices with a square and honeycomb geometry and consisting of PbSe and CdSe nanocrystal monolayers, have been put on a quartz substrate and thoroughly investigated with optical spectroscopy. It was observed that the screening of the electromagnetic field in these superlattices is much reduced compared to the situation with isolated nanocrystals; this results in a 5-10 times stronger light absorption. Furthermore, the opto-electronic properties of other types of self-assembled nanocrystal solids have been studied in detail, in projects outside but complementary to the ERC project. (ITN projects of F. Montanarella and A. Brodu).

- Transport properties of a two-dimensional PbSe square superstructure in an electrolyte-gated transistor. Nano letters 17 (9), 5238-5243 (2017).
- Room-temperature electron transport in self-assembled sheets of PbSe nanocrystals with a honeycomb nanogeometry. The Journal of Physical Chemistry C 123, 14058 (2019)
- Lasing Supraparticles Self-Assembled from Nanocrystals. ACS nano 12, 12788-12794 (2018)
- Reversible Charge-Carrier Trapping Slows Förster Energy Transfer in CdSe/CdS Quantum-Dot Solids. Nano letters 18 (9), 5867-5874 (2018)
- Exciton fine structure and lattice dynamics in InP/ZnSe Core/Shell quantum dots. ACS photonics 5 (8), 3353-3362 (2018)

Design and characterization of artificial superlattices (Thomas Gardenier, Jette van den Broeke, Prof. I. Swart, outside ERC, C. de Morais Smith, outside ERC, Dr. Marlou Slot, outside ERC, S. Kempkes, outside ERC, J. Moes, outside ERC):

The study of Dirac-type electronic properties in honeycomb semiconductors is feasible but difficult due the intricate fabrication of the superlattices, and due to the presence of intrinsic disorder. This study will remain the main challenge of this ERC project. Meanwhile, we discovered that we can prepare ultra-clean and perfectly ordered superlattices with the same honeycomb geometry (and other geometries) by creating these superlattices atom-by-atom on an atomically flat metallic surface. For this, we use the cryogenic STM, and position CO molecules on a Cu(111) surface by atomic manipulation with the STM tip. The CO molecules, precisely positioned on the Cu(111) surface, act as electron scatterers that repel the electrons from the Cu surface state. By this, the electrons are forced to live in artificial atomic sites. We are able to engineer atomic sites that - all together - form artificial lattices. We can design any thinkable periodic lattice geometry. We have designed and studied artificial lattices with a Honeycomb geometry, a Lieb geometry, a squared lattice with SSH topology, and a Sierpinski fractal. The methodology of quantum simulation with such artificial lattices (see Nobel laureate R. P. Feynman, ”There is plenty of room at the bottom”, 1959) is so powerful that it became possible to test and correct lattice theories and study the topological states of the lattices.

- Design and characterization of electrons in a fractal geometry. Nature physics 15 (2), 127 (2019)
- p-Band Engineering in Artificial Electronic Lattices. Physical Review X 9 (1), 011009 (2019)
- Experimental realization and characterization of an electronic Lieb lattice. Nature physics 13 (7), 672 (2017).
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.