Room-Temperature Electron Transport in Self-Assembled Sheets of PbSe Nanocrystals with a Honeycomb Nanogeometry

Abstract

It has been shown recently that atomically coherent superstructures of a nanocrystal monolayer in thickness can be prepared by self-assembly of monodisperse PbSe nanocrystals, followed by oriented attachment. Superstructures with a honeycomb nanogeometry are of special interest, as theory has shown that they are regular 2-D semiconductors, but with the highest valence and lowest conduction bands being Dirac-type, that is, with a linear energy-momentum relation around the K-points in the zone. Experimental validation will require cryogenic measurements on single sheets of these nanocrystal monolayer superstructures. Here, we show that we can incorporate these fragile superstructures into a transistor device with electrolyte gating, control the electron density, and measure the electron transport characteristics at room temperature. The electron mobility is 1.5 ± 0.5 cm2 V–1 s–1, similar to the mobility observed with terahertz spectroscopy on freestanding superstructures. The terahertz spectroscopic data point to pronounced carrier scattering on crystallographic imperfections in the superstructure, explaining the limited mobility.

Oriented Attachment: From Natural Crystal Growth to a Materials Engineering Tool

“…So far, experimental evidence for these compelling predictions has not been found, as the electronic mobilities are relatively low (1–10 cm 2 /(V s)), being limited by imperfect necks and geometrical disorder. 48 − 50 However, the big message here is that the electronic band structure of 2D crystals can be engineered by the lateral nanogeometry of the system. 51 Specific geometries can be achieved by NC assembly followed by oriented attachment.…”

“…Honeycomb semiconductors show a conventional band gap, but the electron conduction bands (hole valence bands) show Dirac-type bands, similar to those of graphene! , This brings the exciting opportunity to combine typical semiconductor properties with electron and hole excitations that are “massless” as in graphene. So far, experimental evidence for these compelling predictions has not been found, as the electronic mobilities are relatively low (1–10 cm 2 /(V s)), being limited by imperfect necks and geometrical disorder. − However, the big message here is that the electronic band structure of 2D crystals can be engineered by the lateral nanogeometry of the system . Specific geometries can be achieved by NC assembly followed by oriented attachment.…”

Assembly, Properties, and Application of Ordered Group II–VI and IV–VI Colloidal Semiconductor Nanoparticle Films

“…[ 65 ] On the other hand, crystallographic imperfections, such as nonideal orientation of QD facets, impede charge transport. [ 190 ] “Hot” charge carriers, which are generated at high pump photon energies, also cooled down up to two times as fast in well‐ordered PbS QD films (obtained via Langmuir–Blodgett techniques) compared to randomly oriented ones, which Cao et al attributed to increased dipolar coupling. [ 222 ] With the help of transient absorption microscopy, Yoon et al determined the exciton diffusion constant to be an order of magnitude higher in ordered CdSe QD superlattices compared to unordered ones.…”

PbI2 Passivation of Three Dimensional PbS Quantum Dot Superlattices Toward Optoelectronic Metamaterials

“…Highly ordered superlattices of the CQDs can be obtained by assembling them on a liquid–liquid or liquid–vapor interface and subsequently removing the ligands to induce epitaxial necking of specific facets. − Most studies have focused on 2D SLs that usually have a square geometry. With the usage of an ionic liquid gating in a field-effect transistor (FET), electron mobilities up to 15 cm 2 V –1 s –1 have been recently reported for PbS CQDs, values which are on par with analogous 2D PbSe SLs (13–24 cm 2 V –1 s –1 ). ,,, We recently demonstrated electron mobilities up to 270 cm 2 V –1 s –1 in 3D PbSe CQDs SLs with analogous ionic gel-gated FETs, owing to a significantly improved ordering with respect to the 2D systems. This fact motivated us to study 3D superlattices composed of PbS CQDs, given the higher interest and versatility for technological applications.…”

“…With the usage of an ionic liquid gating in a field-effect transistor (FET), electron mobilities up to 15 cm 2 V –1 s –1 have been recently reported for PbS CQDs, values which are on par with analogous 2D PbSe SLs (13–24 cm 2 V –1 s –1 ). 22 , 29 , 42 , 43 We recently demonstrated electron mobilities up to 270 cm 2 V –1 s –1 in 3D PbSe CQDs SLs with analogous ionic gel-gated FETs, owing to a significantly improved ordering with respect to the 2D systems. This fact motivated us to study 3D superlattices composed of PbS CQDs, given the higher interest and versatility for technological applications.…”