<i>
                     <b>In situ</b>
                  </i> small-angle X-ray scattering environment for studying nanocrystal self-assembly upon controlled solvent evaporation

Koof

Walther

et al. 2019

Small

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involve a variety of interactions and driving forces between inorganic hard core, organic soft surface-coating ligands, and surrounding solvent molecules. [2,[7][8][9] As a consequence, a thorough understanding of a solvent-mediated assembly process is required to produce NC solids with programmable properties.Here, we have chosen lead sulfide (PbS) nanocrystals as a model system for the investigation of the in situ self-assembly due to the possibility to produce monodisperse particles in a colloidal solution with a precisely controlled size. Additionally, the unique electronic properties of semiconductor PbS NCs with tunable bandgap make them attractive for many potential technological applications, including solar cells, light-emitting diodes, transistors, photodetectors, etc. [10][11][12][13] The structure and degree of order in NC superlattices depend on several factors, such as NC size and shape, ligand length, grafting density, ligand-solvent interactions, NC concentration, solvent evaporation rate (in case of evaporative assembly), and cell geometry or substrate where the assembly takes place. Thus, recent studies on lead chalcogenide NC assemblies revealed the formation of face-centered cubic (fcc) or body-centered cubic (bcc) superstructures as well as lattice distorted facecentered tetragonal (fct) and body-centered tetragonal (bct) superlattices. [14][15][16][17][18] However, their assembling mechanism remains largely unresolved mainly due to the challenge in measuring intermediate transitions between the initial colloidal state and the final superlattice state, especially in real time.Recently, in situ scattering methods have emerged as a powerful characterization tool to study the nucleation and growth of nanoparticle superlattices. [19][20][21][22] Small-angle X-ray scattering (SAXS) allows for monitoring the self-assembly dynamics with subsecond temporal resolution over large sample volumes under controlled conditions (e.g., controlled solvent vapor environment and temperature) and provides realtime information on long range order in the assembled structures. Often, the grazing-incidence small-angle X-ray scattering (GISAXS) technique is applied for in situ measurements, where a drop of nanocrystal suspension is placed on a substrate and evaporated in a controlled manner. [16,20,23,24] However, at small incident angles of the X-ray beam GISAXS is able to probe only up to several monolayers and is thus limited to the surface regime. For bulk measurements transmission SAXS is Self-assembled nanocrystal superlattices have attracted large scientific attention due to their potential technological applications. However, the nucleation and growth mechanisms of superlattice assemblies remain largely unresolved due to experimental difficulties to monitor intermediate states. Here, the self-assembly of colloidal PbS nanocrystals is studied in real time by a combination of controlled solvent evaporation from the bulk solution and in situ small-angle X-ray scattering (SAXS) in transmission geometry. For the fi...

Section: Resultsmentioning

confidence: 99%

“…The real‐time self‐assembly of PbS nanocrystals was monitored during controlled solvent evaporation in a custom‐built chamber . Briefly, the solvent vapor chamber used for in situ SAXS measurements was equipped with two Kapton windows for the incident and scattered X‐ray beam.…”

Section: Methodsmentioning

confidence: 99%

Monitoring Nanocrystal Self‐Assembly in Real Time Using In Situ Small‐Angle X‐Ray Scattering

Koof

Walther

et al. 2019

Small

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“…[11][12][13][14][15]19 However, the GISAXS geometry is limited to the surface characterization due to small incident angles of the X-ray beam and cannot deliver information about bulk superlattice structure. In our previous studies, 21,22 we demonstrated in situ small-angle X-ray scattering (SAXS) measurements in transmission mode of the evaporative self-organization of colloidal NCs into superstructures with long range order. In this technique, the structural information on a thick nanocrystal solid can be monitored both in the solvated and dried states during the real-time in situ NC assembly.…”

mentioning

confidence: 99%

“…11,12,15,[23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38] In this study, we use a recently designed custom-made sample environment for in situ SAXS measurements in transmission mode described in detail in Ref. 21. In this chamber, the solvent in the colloidal suspension evaporates slowly (e.g., the evaporation rate is < 0.3 µl/min for heptane and < 0.2 µl/min for toluene) and the nanocrystals form highly ordered superlattices along the cell windows (Figure 1a).…”

mentioning

confidence: 99%

“…The start of the superlattice formation can be observed in SAXS patterns when the evaporation front line reaches the corresponding vertical position on the cell windows (Figure 1a). 21 Representative transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images of PbS NCs used in this study are shown in Figures 1b and c, respectively. The HRTEM images (Figure 1c) confirm that the NCs exhibit pronounced faceting and can be approximated by a cuboctahedral shape in accordance with previous observations for similar sized lead chalcogenide nanocrystals.…”

mentioning

confidence: 99%

See 1 more Smart Citation

Coexistence of hcp and bct Phases during In Situ Superlattice Assembly from Faceted Colloidal Nanocrystals

Koof

Walther

et al. 2019

J. Phys. Chem. Lett.

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We study the in situ self-assembly of faceted PbS nanocrystals from colloidal suspensions upon controlled solvent evaporation using time-resolved small-angle X-ray scattering and X-ray crosscorrelation analysis. In our bulk-sensitive experiment in transmission geometry, the superlattice crystallization is observed in real time revealing a hexagonal closed-packed (hcp) structure followed by formation of a body-centered cubic (bcc) superlattice. The bcc superlattice undergoes continuous tetragonal distortion in the solvated state shortly after its formation, resulting in the body-centered tetragonal (bct) structure. Upon solvent evaporation, the bct superstructure becomes more pronounced with the still coexisting hcp phase. These findings corroborate the existing simulations of assembling cuboctahedral-shaped particles and illustrate that we observed the predicted equilibrium states. This work is essential for a deeper understanding of the fundamental forces that direct nanocrystal assembly including nanocrystal shape and ligand coverage. TOC GRAPHICSKEYWORDS. Small angle X-ray scattering (SAXS), X-ray cross correlation analysis (XCCA), evaporative induced self-assembly, lead sulfide (PbS), time-resolved superlattice crystallization.

Recent Notable Approaches to Study Self‐Assembly of Nanoparticles with X‐Ray Scattering and Electron Microscopy

Schulz

Part & Part Syst Charact

Parak

et al. 2021

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several issues far from being resolved: for many systems long-range periodic order is not routinely achieved, reproducibility is often not optimal and truly emergent and new properties arising from the supercrystal formation could be demonstrated for just a few systems. [3][4][5][6][7] From the materials science perspective there is no established figure of merit or common set of parameters allowing to quantify "quality" of NP superlattices and supercrystals. In lieu of such standard procedures, terms as well-ordered, crystalline order, quasicrystalline, etc., are usually used in literature. However, there exist no commonly accepted definitions of what qualifies, e.g., as "well-ordered."In this review, we want to highlight some recent experimental developments and new approaches to study self-assembly of NPs or structure formation of NP supercrystals and their final structure. We focus on NP, the broad field of photonic materials prepared by self-assembly of larger colloidal particles is not covered herein. [8][9][10] The introduction of the basic concepts of NP self-assembly will be kept short and should be considered as an overview, for more background and details excellent comprehensive reviews are available. [2,11] We will then briefly address and discuss the expectations associated with self-assembled materials, especially the proposed emergence of new properties directly related to order. In the second part of the review, we will present recent innovative approaches for the investigation of self-assembly and self-assembled structures that go beyond the standard characterization. We focus mainly on structure and structure formation and less on properties, so most of the studies are based on scattering and electron microscopy. Figure 1 provides an overview roughly following this outline. Self-Assembly of NanoparticlesSelf-assembly is a broad term and occurs on all scales from atoms, ions, and molecules to galaxies, as pointed out by Whitesides and Grzybowski. [12] It can be defined as the autonomous organization of multiple discrete components into ordered structures, driven by energetics or entropy or both. [2,13] In nature, countless examples of highly complex and functional structures resulting from self-assembly can be found, Self-assembly of nanoparticles (NPs) has evolved into a powerful tool for the synthesis of superstructures with tailored properties. The quality, diversity, and complexity of synthesized structures are continuously improving and fascinating new collective properties are demonstrated. At the same time, the rapid development of electron microscopy and synchrotron sources for X-rays has enabled new exciting experimental approaches to study structure and structure formation in the context of NP self-assembly. In this review, some recent studies and what can be learned from them are highlighted and discussed. It is started with a general introduction covering important concepts, experimental approaches, commonly obtained structures, the ideas of artificial atoms, and emerging properties a...