Interparticle Spacing and Structural Ordering in Superlattice PbS Nanocrystal Solids Undergoing Ligand Exchange

Abstract

Controlling the interparticle spacing in quantum dot (QD) thin films is the most readily accessible way to control transport rates between neighboring QDs and a critical component of device optimization. Here, we use X-ray scattering measurements to accurately measure the interparticle spacing in films of highly monodisperse lead sulfide (PbS) QDs that have undergone a variety of device-relevant ligand exchanges. We tabulate these values for use in simulations and data analysis. We find that monothiol and dithiol ligand species typically result in interparticle spacing values that are equal to the length of a single monothiol or dithiol ligand. Additionally, we find that spin-coating a thick film of QDs followed by a long-duration ligand exchange results in a more complete ligand exchange than spin-coating many thin layers with short-duration ligand exchanges in between. The former method also preserves a remarkable degree of the long-range ordering that was present in the film prior to ligand exchange. These results shed light on ways to produce highly-ordered QD solids with compact and functional ligands, which could lead to enhanced interdot coupling and transport phenomena.

Structure, transport and photoconductance of PbS quantum dot monolayers functionalized with a copper phthalocyanine derivative

“…[27]- [32] This is a clear advantage of the liquid/air assembly procedure. In contrast to most other phthalocyanines, tetraaminophthalocyanines are soluble in organic solvents, which allows us to apply Cu4APc directly as part of the liquid substrate.…”

Quantifying Angular Correlations between the Atomic Lattice and the Superlattice of Nanocrystals Assembled with Directional Linking

“…1,2 Colloidal NCs stabilized by organic surfactants have been shown to pose excellent building blocks for the design of synthetic MCs with tailored structural properties which are conveniently obtained by self-assembly of NCs from solution on a solid or liquid substrate by exploiting ligand-ligand 3 interactions. [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] Typically, the utilized ligands consist of wide-gap, bulky hydrocarbons which render the MCs insulating. [26][27][28][29][30][31][32][33] MCs obtained in this way exhibit average grain sizes of ~150 µm 2 , which enables a detailed characterization by electron and/or X-ray microscopy.…”

Morphology of Passivating Organic Ligands around a Nanocrystal

“…In solar cells and LEDs, quantum dot size is used to tune band gaps, and this is commonly exploited to produce varied spectral properties [7][8][9][10][11].These optoelectronic devices function through electronic processes that are often strongly dependent on distance [5,[12][13][14][15]. For example, conductivity in a quantum dot array is mediated by Marcus-type charge transfer events between dots [9,13,[16][17][18][19][20][21][22]. As the dot-todot distance increases, the charge transfer rate decays exponentially, making the conductivity extremely sensitive to the dot-to-dot distance [16,17,23,24].…”

“…Excitonic energy transfer, relevant in solar cells and light emitters, usually occurs through Forster resonant energy transfer (FRET) [1,21,[25][26][27][28][29][30] or Dexter processes [31][32][33]; these are also dependent on distance. Since the organic ligand shell is usually composed of insulating alkane chains, they behave as a spacer layer that can determine that closest approach distance [13,[19][20][21]34].Ligand exchange reactions [35][36][37][38] give us in situ synthetic access to the ligand shell, and using this design space it is possible to achieve fine control of the aforementioned electronic processes.Recently, the ability to control the energy gap and energy transfer has been exploited for novel optoelectronic devices [8][9][10], allowing for down-conversion of a high energy UV photon into two lower energy photons [26,32,33], and up-conversion of two low energy IR photons into one higher energy photon [39][40][41]. The ability to up-, and down-convert photon energy can allow solar cells to capture more of the solar spectrum, thereby circumventing the Shockley-Queisser limit [42][43][44].…”