Co-supervisor – QuMat.1-TUD-4.1C
Optical characterization of exciton-photon conversion and hybridization
Brief research summary over last 5 years / academic profile
Vanmaekelbergh‘s research deals with the opto-electronic properties of low-dimensional semiconductors and superlattices with Dirac electrons, and eventually topologically protected edge states. His group employs a “from start to finish” approach to quantum materials: from chemical synthesis to structural characterization, opto-electrical spectroscopy, and theoretical understanding, with feedback loops to optimize material properties (e.g. ACSNano 2018).
Vanmaekelbergh discovered that nanocrystals at liquid interfaces align their atomic structure and attach to each other with specific facets, to form 2D single crystals with a specific nanoscale geometry, not reachable by lithography (Science 2014). The mechanism of this amazing process was revealed with a “nearly impossible experiment”, using synchrotron radiation reflecting from the liquid/nanocrystal/air interface (Nature Materials 2016, with cover “watching nanocrystals assemble”, see also ScienceDaily 2016 and Volkskrant 2016). Molecular dynamic simulations showed that the alignment is caused by the interactions of the nanocrystals with the interface and with each other and that the crystal geometry and facets are important (PRX 2019).
Vanmaekelbergh surmised that nanostructured 2D semiconductors may have electronic properties that deviate from the “normal”. His collaboration with theoretical physicists, Morais Smith and Delerue, showed a result that was even more spectacular: semiconductors with a honeycomb nanogeometry show a “normal” semiconductor band gap but with Dirac-type valence- and conduction bands (similar to graphene), tailorable with the nanogeometry. Moreover, relativistic effects result in a topological flat band, with prospects for novel electronic phases. These features can be addressed one-by-one by control of the Fermi-level in the semiconductor (PRX 2014, Nature Comm. 2015). This work thus led to a whole new class of materials. Vanmaekelbergh and several other groups are working on further development, following both the nanocrystal- and lithography routes (Chemistry of Materials 2018, Nanotechnology 2019).
Together with Dr. Swart, Vanmaekelbergh built a state-of-the-art cryogenic scanning probe lab with tunneling microscopy, spectroscopy, and atomic force microscopy, to study the atomic structure and electronic properties of 2D systems. The power of the lab was proven with investigations on graphene nanostructures, for which the atomic backbone and energy-resolved wavefunctions could be measured simultaneously, and bond defects could be created and studied (ACSNano 2012, Nature Comm. 2013). This state-of-the-art infrastructure has allowed the group to prepare artificial lattices, atom-by-atom, as analogues of the nanostructured real materials discussed above. This way, Vanmaekelbergh’s team prepared lattices with a honeycomb geometry providing strong evidence for the predicted Dirac-type electronic band structure and flat band (ACSNano 2020). Artificial lattices with specific geometry hold promise for far-reaching physics and discoveries: recently, the lab created the Lieb geometry that also results in Dirac electrons (Nature Physics 2017, PRX 2019), a kagome lattice with topological corner states (Nature Materials 2019), and electronic fractals (Nature Physics 2019, with cover “electrons in a fractal”).
International visibility, activities, prizes, scholarships etc.
In 2002, Vanmaekelbergh formed his group on semiconductor nanocrystalline quantum dots, with his signature “from start-to-finish approach”. He achieved this by forging collaborations with numerous groups in the Netherlands and abroad, leading to breakthroughs that could never be reached by a single group. These include for instance impeded light emission of a quantum dot in a photonic structure (Nature 2004), revealing the dynamics of electron-hole interactions in a quantum dot (PRL 2006). At Utrecht University, Vanmaekelbergh built a state-of-the-art scanning tunneling microscopy lab, to image individual semiconductor nanocrystals and measure the electronic states, charge-and electron-phonon interactions (Chemical Reviews 2016). The combination of chemical synthesis, scanning probe microscopy, and extensive collaborations has resulted in over 260 publications, with many in broad-interest journals such as Science, Nature family, PRL and PRX.
Vanmaekelbergh’s open view and leadership skills helped him to initiate and coordinate large collaborative networks, e.g. a European ITN program on semiconductor quantum dots as single-photon sources in devices (2008), an NWO Physics program (FOM, 2013) on 2D semiconductor superlattices, an NWO Chemistry Toppunt program on nanocrystal self-assembly (2015), and an ERC advanced grant on semiconductors with Dirac-type electronic carriers (2016). With the arrival of Dr. Ingmar Swart, the scanning probe methods were extended with advanced atomic force microscopy and atomic manipulation. Together with Morais Smith (UU), they presented artificial lattices, built up atom-by-atom, as analogue quantum simulations of real materials. The work published in Nature Physics and Nature Materials received immediate attention by scientists and laymen. It was discussed in popular journals such as Physics Today, the new Scientist, and newspapers such as the Dutch NRC. Artificial analogues are deemed as powerful examples for real quantum materials with topological states.
To date, 31 PhDs and 6 postdocs have studied in Vanmaekelbergh’s lab, many of them now hold prestigious positions in academia (5 are full professor) and industry. Vanmaekelbergh’s work has been cited more than 23000 times; he has given numerous lectures on invitation (10/year on average) and plenary lectures in the Falling Walls Conference in Berlin, the MRS spring meeting in San Francisco, and for the Israel Chemical Society, at which occasion he became honorary member. Vanmaekelbergh won the Descartes-Huygens prize in 2018, for his scientific excellence and collaboration with research groups in France. He was member of the editorial board of Europhysics Letters and Nanotechnology, and currently he is co-editor of the review-journal Physics Reports. Vanmaekelbergh is an enthusiastic teacher, his courses range from physical chemistry to quantum mechanics. He is director of the Debye Institute for Nanomaterials Science (Utrecht) and the international ESQSEC association, the latter provides chemistry and physics education for students in photovoltaics.
5 key output/publications
Long-range orientation and atomic attachment of nanocrystals in 2D honeycomb superlattices.
M.P. Boneschanscher, W.H. Evers, J.J. Geuchies, T. Altantzis, B. Goris, F.T. Rabouw, S.A.P. Van Rossum, H.S.J. van der Zant, L.D.A. Siebbeles, G. Van Tendeloo, I. Swart, J. Hilhorst, A.V. Petukhov, S. Bals, D. Vanmaekelbergh, Science 344, 1377-1380 (2014).
This work details the discovery of atomically coherent honeycomb lattices prepared from PbSe nanocrystals and their transformation into CdSe honeycombs. The structures were analyzed with advanced transmission electron microscopy and scanning tunneling microscopy.
Dirac cones, topological edge states, and nontrivial flat bands in two-dimensional semiconductors with a honeycomb nanogeometry.
E. Kalesaki, C. Delerue, C. Morais Smith, W. Beugeling, G. Allan, D. Vanmaekelbergh, Physical Review X 4, 011010 (2014).
Atomistic calculations and analytical theory showed that 2D semiconductors with a honeycomb geometry have electronic bands that differ from conventional semiconductors. The electrons behave in a similar way as in graphene, while the optical properties of the semiconductor remain preserved. Several types of quantum spin Hall states are predicted.
In situ study of the formation mechanism of two-dimensional superlattices from PbSe nanocrystals.
J.J. Geuchies, C. Van Overbeek, W.H. Evers, B. Goris, A. De Backer, A.P. Gantapara, F.T. Rabouw, J. Hilhorst, J.L. Peters, O. Konovalov, A.V. Petukhov, M. Dijkstra, L.D.A. Siebbeles, S. Van Aert, S. Bals, and D. Vanmaekelbergh, Nature Materials 15, 1248-1254 (2016). (cover)
This work used synchrotron X-radiation to monitor the dynamics of nanocrystals at a liquid air interface. This revealed a sequence of intriguing new phases and phase transitions in the superlattice formation by nanocrystals.
Understanding the formation of PbSe honeycomb superstructures by dynamics simulations.
G. Soligno and D. Vanmaekelbergh, Physical Review X 9, 021015 (2019).
This work and further simulations showed that nanocrystals interact with the liquid/air interface and with each other. This results in nanocrystal alignment and, finally, the formation of nanocrystal superlattices with a square or honeycomb geometry.
p-Orbital flat band and Dirac cones in the electronic honeycomb lattice.
T.S. Gardenier, J.J. Broeke, J.R. Moes, I. Swart, C. Delerue, M.R. Slot, C. Morais Smith, D. Vanmaekelbergh, ACS nano 10, 13638-13644 (2020).
Artificial lattices with a honeycomb nanogeometry were built on a Cu surface by manipulating CO molecules. The electronic bands were measured by scanning tunneling microscopy and reveal Dirac bands and a flat band; the band structure can be tailored by the nanogeometry. Meanwhile, evidence for a p-type flat band has been observed in 2D semiconductors with a honeycomb nanogeometry.