Jaco Geuchies

Figure 8.2: The plethora of X-ray scattering techniques used during the experiments presented in this thesis. In the forward scattering direction, the GISAXS signal is collected which gives information on for example the NC-NC distance. Closer to the sample, and under a wider angle, the atomic diffraction is collected on a GIWAXS detector. This can be used to determine the crystallographic orientation of the NCs with respect to the liquid-air interface. Finally, we performed specular X-ray reflectivity measurements, which gives the density profile of the NC monolayer in the direction perpendicular to the liquid-air interface.

In Chapter 4 we study in-situ and time-resolved the formation mechanism of superlattices with a square nanogeometry. To this end, we use synchrotron based small-angle (and wide-angle) X-ray scattering (GISAXS/GIWAXS). Photons which are scattered under small angles will provide geometric information on the NC length scale (e.g. the interparticle distance), whereas photons which are scattered under wider angles will provide information on the crystallographic orientation and atomic attachment of the NCs. The X-ray beam is aligned such that the photons glance the substrate at a grazing-angle around the critical angle for total external reflection for PbSe. This means the photons have a very low penetration depth (<20 nm) into the liquid and we are able to probe the NC crystallization at the liquid-air interface. We show that the NCs adsorb at the liquid-air interface and form a hexagonal monolayer. As the capping ligands are gradually desorbed from the {100} facets into the ethylene glycol subphase, the interparticle distance is reduced and the lattice is transformed via a pseudo-hexagonal phase into a square superlattice. During this process the NCs align atomically and connect via the in-plane {100} facets. The resulting structures are analyzed with high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) to show the residual disorder on the atomic length scale. Furthermore the experiments are corroborated with Monte Carlo simulations, which give further insight into the inter-NC interactions during the self-assembly. Chapter 5 presents a study of the adsorption geometry of PbSe NCs with different sizes. We add specular X-ray reflectivity (XRR) measurements to GISAXS and GIWAXS measurements. Fitting of the XRR data allows us to obtain a density profile in the direction perpendicular to the ethylene glycol-air interface and, combined with GISAXS and GIWAXS, gives us a full three-dimensional description of the NC monolayer at the interface. We show that the larger PbSe NCs align crystallographically with a [001] direction pointing upwards and prove that the NCs all float on top of the ethylene glycol-air interface. Figure 8.3: Schematic of Chapters 3, 4 and 5. In Chapter 3 we studied the formation and structure of a PbSe honeycomb nanocrystal superlattice. The superlattice turned out to be buckled, with the constituent NCs occupying a different plane of height. The NC connect atomically via three out of six {100} facets. In Chapter 4 we studied the mechanism of formation of PbSe superlattices with a square nanogeometry. We resolved the process using a combination of X-ray scattering techniques, advanced electron microscopy and Monte Carlo simulations. In Chapter 5 we studied adsorption geometry of PbSe NCs with different sizes at the ethylene glycol-air interface. We showed that large NCs (> 5.5 nm) align crystallographically with a [001] axis perpendicular to the ethylene-glycol air interface. We combine GISAXS and GIWAXS with XRR measurements to obtain a full three-dimensional description of the adsorption geometry of these PbSe NCs.

Future research should be divided into two directions: (1) structural investigations regarding the self-assembly process and (2) studies regarding the electronic structure of the superlattices. Regarding the first direction, there are still a lot of unanswered questions. First of all, we do not fully understand the formation mechanism of the honeycombs superlattice, and how it is different from the formation of the square superlattice. Initial GISAXS and GIWAXS experiments have been performed, but the data is inconclusive. Furthermore the adsorption behaviour of PbSe NCs at the toluene-air interface should be studied. This is significantly more challenging than the experiments presented in Chapter 5, as the toluene evaporation has to be halted. It requires the design of a new liquid cell which is dedicated to these experiments, for which inspiration can be drawn from work done by Calzolari and coworkers1.

Naturally, different NC systems which self-organize at liquid-air interfaces should also be considered in future experiments, e.g. iron oxide NCs under the influence of a magnetic field. The electronic structure characterization should be focussed on measuring whether or not the charge carriers behave relativistically. For the moment, this is on-going: scanning tunneling microscopy and spectroscopy (STM/STS) data has been collected, but is also not conclusive yet. The difficulty here lies in the fact that the samples are relatively ‘dirty’ and measurements are hindered by ligand molecules desorbing from the samples and adsorbing to the STM tip, which hampers stable measurements. Different types of surface passivation techniques can be attempted, either replacing the oleate ligands with smaller carboxylic chains or passivating the superlattice with inorganic shells. Angle-resolved photo-emission (ARPES) can be used to directly measure a materials band structure (i.e. the E,k relationship). For now, the angular resolution with this technique is the limiting factor for characterizing the superlattices obtained in this thesis. Other experimental techniques that can be considered and are being utilized are transport measurements (either by electrochemical gating or in transistor geometries2) and ultrafast optical spectroscopy (transient-absorption and THz-spectroscopy3).

8.3 - Perovskite CsPbBr3 nanocrystals

Perovskite NCs made of CsPbX3 (X = Cl, Br, I) have received a tremendous amount of attention since their first discovery in 2015 by the group of Kovalenko at the ETH, Zürich. Their emission spectrum can be tuned over the complete visible spectrum by changing the constituent halides of the anion sublattice. Since they have a simple cubic atomic lattice, the NCs themselves attain a well defined cubic shape. Chapters 5 and 6 deal with cation-exchange reactions and the self-assembly of these NCs respectively.

In Chapter 6 we present a novel method to perform cation-exchange on CsPbBr3 NCs. As the NCs are stabilized by the cation sublattice instead of the anion sublattice (in contrast with many II-VI and VI-VI semiconductor NCs), this is not straightforward. We exchange Pb2+ for other divalent metal ions such as Cd2+, Zn2+ and Sn2+ and show that upon doping the NCs a blueshift of the photoluminescence (PL) spectrum is observed, while retaining the narrow PL linewidth and high PL quantum yield. We argue that due to the smaller size of the incorporated ions, the atomic lattice contracts, which leads to an increase in ligand field strength and hence a blueshift of the PL. This is shown through a combination of electron diffraction and high-resolution HAADF-STEM measurements. Up to date, this is the only known system in which sequential cation- and anion-exchange reactions can be combined.

The results presented in Chapter 7 show that we can induce the self-assembly of CsPbBr3 NCs into cuboidal supraparticles. Inside the supraparticles the NCs are aligned crystallographically, but are not epitaxially connected, and stack into a simple cubic lattice. We observe the formation of localized NC vacancies in the bulk and on the surface of the supraparticles, which hints towards attractive interactions between the NCs during the self-assembly process. The clustering of the NCs into a supraparticle is induced by addition of an anti-solvent and, using X-ray scattering techniques, is proven to happen in solution. Furthermore we study the optical properties of the supraparticles and show that the PL is redshifted by 30 meV, which is most likely due to energy transfer inside the supraparticle.

Figure 8.4: Schematic of Chapters 6 and 7. In Chapter 6 we demonstrated that we can perform cation exchange on the CsPbBr3 NCs, removing Pb ions and incorporating Sn, Cd or Zn ions. The PL is blueshifted with respect to the undoped NCs, which is caused by an atomic lattice contraction which leads to an increase in ligand field in the emitting PbBr6 octahedra inside the NCs. In Chapter 7 we show that we can self-assemble the CsPbBr3 nanocubes into cuboidal supraparticles consisting out of hundreds of NCs. The NCs inside such a supraparticle are crystallographically aligned, but not connected into a single crystal. We observed localized NC vacancies on both the surface and in the bulk of the supraparticles, which hint towards attractive interactions between the NCs. The PL spectrum of the supraparticles is slightly redshifted compared to the individual NCs in solution.

Several different directions can be explored in future experiments. Based on Chapter 6, there are a number of relevant follow-up experiments possible. First of all it would be interesting to see the extent of the observed blueshift of the PL spectrum with decreasing lattice constants, by performing diffraction experiments at elevated pressures. Not only will this, after full Rietveld refinement of the data, give the full unit cell structure, it can be combined with simultaneous optical spectroscopy. This will ultimately link the unit cell structure (and deviations of the ideal unit cell structure) to the observed changes in optical properties4. Also, a variety of spectroelectrochemical experiments can be performed. Recently, it has been shown that the main non-radiative channel in pure CsPbBr3 NCs is the capture of photogenerated holes in structural defects5. Similar experiments on the doped NCs can shed light on the reduction in quantum yield that is observed during the cation-exchange experiments.

The experiments presented in Chapter 7 provide a pathway towards more optical experiments. Due to the high refractive index contrast between the CsPbBr3 supraparticle and its surrounding, the PL can be confined inside and give rise to whispering gallery modes6. These supraparticles would be, owing to the high PL quantum yields of the constituent NCs, ideal candidates for lasing cavities. Not only optical experiments can be explored, also more structural characterization would be interesting. One could try to create monolayers of the CsPbBr3 NCs and fuse them into single crystalline materials. It has already been shown that it is possible to directly pattern lead structures into these perovskite materials through electron- or X-ray lithographic techniques7-9. This leaves behind a metallic Pb structure in the sample and opens up a route to ‘write’ a Pb lattice with any type of geometry into the perovskite thin film. Of particular interest here is again the honeycomb lattice, which, combined with the Pb lattice, could give rise to superconductivity in a material with large spin-orbit coupling and a nanoscale honeycomb geometry.