Determination of the electronic energy levels of colloidal nanocrystals using field-effect transistors and Ab-initio calculations

Single PbS colloidal quantum dot transistors

Colloidal quantum dots are sub-10 nm semiconductors treated with liquid processes, rendering them attractive candidates for single-electron transistors operating at high temperatures. However, there have been few reports on single-electron transistors using colloidal quantum dots due to the difficulty in fabrication. In this work, we fabricated single-electron transistors using single oleic acid-capped PbS quantum dot coupled to nanogap metal electrodes and measured single-electron tunneling. We observed dot size-dependent carrier transport, orbital-dependent electron charging energy and conductance, electric field modulation of the electron confinement potential, and the Kondo effect, which provide nanoscopic insights into carrier transport through single colloidal quantum dots. Moreover, the large charging energy in small quantum dots enables single-electron transistor operation even at room temperature. These findings, as well as the commercial availability and high stability, make PbS quantum dots promising for the development of quantum information and optoelectronic devices, particularly room-temperature single-electron transistors with excellent optical properties.

Enabling metallic behaviour in two-dimensional superlattice of semiconductor colloidal quantum dots

Semiconducting colloidal quantum dots and their assemblies exhibit superior optical properties owing to the quantum confinement effect. Thus, they are attracting tremendous interest from fundamental research to commercial applications. However, the electrical conducting properties remain detrimental predominantly due to the orientational disorder of quantum dots in the assembly. Here we report high conductivity and the consequent metallic behaviour of semiconducting colloidal quantum dots of lead sulphide. Precise facet orientation control to forming highly-ordered quasi-2-dimensional epitaxially-connected quantum dot superlattices is vital for high conductivity. The intrinsically high mobility over 10 cm² V^-1 s^-1 and temperature-independent behaviour proved the high potential of semiconductor quantum dots for electrical conducting properties. Furthermore, the continuously tunable subband filling will enable quantum dot superlattices to be a future platform for emerging physical properties investigations, such as strongly correlated and topological states, as demonstrated in the moiré superlattices of twisted bilayer graphene.

Evidence of band filling in PbS colloidal quantum dot square superstructures

PbS square superstructures are formed by the oriented assembly of PbS quantum dots (QDs), reflecting the facet structures of each QD. In the square assembly, the quantum dots are highly oriented, in sharp contrast to the conventional hexagonal QD assemblies, in which the orientation of QDs is highly disordered, and each QD is connected through ligand molecules. Here, we measured the transport properties of the oriented assembly of PbS square superstructures. The combined electrochemical doping studies by electric double layer transistor (EDLT) and spectroelectrochemistry showed that more than fourteen electrons per quantum dot are introduced. Furthermore, we proved that the lowest conduction band is formed by the quasi-fourth degenerate quantized (1S_e) level in the PbS QD square superstructures.

All-Solution-Processed Quantum Dot Electrical Double-Layer Transistors Enhanced by Surface Charges of Ti3C2Tx MXene Contacts

Fully solution-processed, large-area, electrical double-layer transistors (EDLTs) are presented by employing lead sulfide (PbS) colloidal quantum dots (CQDs) as active channels and Ti₃C₂T_x MXene as electrical contacts (including gate, source, and drain). The MXene contacts are successfully patterned by standard photolithography and plasma-etch techniques and integrated with CQD films. The large surface area of CQD film channels is effectively gated by ionic gel, resulting in high performance EDLT devices. A large electron saturation mobility of 3.32 cm² V^-1 s^-1 and current modulation of 1.87 × 10⁴ operating at low driving gate voltage range of 1.25 V with negligible hysteresis are achieved. The relatively low work function of Ti₃C₂T_x MXene (4.42 eV) compared to vacuum-evaporated noble metals such as Au and Pt makes them a suitable contact material for n-type transport in iodide-capped PbS CQD films with a LUMO level of ∼4.14 eV. Moreover, we demonstrate that the negative surface charges of MXene enhance the accumulation of cations at lower gate bias, achieving a threshold voltage as low as 0.36 V. The current results suggest a promising potential of MXene electrical contacts by exploiting their negative surface charges.

Exclusive Electron Transport in Core@Shell PbTe@PbS Colloidal Semiconductor Nanocrystal Assemblies

Assemblies of colloidal semiconductor nanocrystals (NCs) in the form of thin solid films leverage the size-dependent quantum confinement properties and the wet chemical methods vital for the development of the emerging solution-processable electronics, photonics, and optoelectronics technologies. The ability to control the charge carrier transport in the colloidal NC assemblies is fundamental for altering their electronic and optical properties for the desired applications. Here we demonstrate a strategy to render the solids of narrow-bandgap NC assemblies exclusively electron-transporting by creating a type-II heterojunction via shelling. Electronic transport of molecularly cross-linked PbTe@PbS core@shell NC assemblies is measured using both a conventional solid gate transistor and an electric-double-layer transistor, as well as compared with those of core-only PbTe NCs. In contrast to the ambipolar characteristics demonstrated by many narrow-bandgap NCs, the core@shell NCs exhibit exclusive n-type transport, i.e., drastically suppressed contribution of holes to the overall transport. The PbS shell that forms a type-II heterojunction assists the selective carrier transport by heavy doping of electrons into the PbTe-core conduction level and simultaneously strongly localizes the holes within the NC core valence level. This strongly enhanced n-type transport makes these core@shell NCs suitable for applications where ambipolar characteristics should be actively suppressed, in particular, for thermoelectric and electron-transporting layers in photovoltaic devices.

Emergent Electronic and Dielectric Properties of Interacting Nanoparticles at Finite Temperature

Lead chalcogenide nanoparticle solids have been successfully integrated into certified solar cells and represent promising platforms for the design of novel photoabsorbers for photoelectrochemical cells. While much attention has been drawn to improving efficiency and device performance through altering the character of the individual nanoparticles, the role of interactions between nanoparticles is not yet well-understood. Using first-principles molecular dynamics and electronic structure calculations, we investigated the combined effect of temperature and interaction on functionalized lead chalcogenide nanoparticles (NPs). Here, we show that at finite temperature, interacting NPs are dynamical dipolar systems, with the average values of dipole moments and polarizabilities substantially increased with respect to those of the isolated building blocks. In addition, we show that the interacting NPs exhibit slightly smaller fundamental gaps that decrease as a function of temperature and that the radiative lifetimes of both the isolated NPs and the solids are greatly reduced at finite temperature compared to T = 0. Finally, we present a critical discussion of various results reported in the literature for the values of dipole moments of nanoparticles.

Stoichiometric control of the density of states in PbS colloidal quantum dot solids

Colloidal quantum dots, and nanostructured semiconductors in general, carry the promise of overcoming the limitations of classical materials in chemical and physical properties and in processability. However, sufficient control of electronic properties, such as carrier concentration and carrier mobility, has not been achieved until now, limiting their application. In bulk semiconductors, modifications of electronic properties are obtained by alloying or doping, an approach that is not viable for structures in which the surface is dominant. The electronic properties of PbS colloidal quantum dot films are fine-tuned by adjusting their stoichiometry, using the large surface area of the nanoscale building blocks. We achieve an improvement of more than two orders of magnitude in the hole mobility, from below 10^-3 to above 0.1 cm²/V⋅s, by substituting the iodide ligands with sulfide while keeping the electron mobility stable (~1 cm²/V⋅s). This approach is not possible in bulk semiconductors, and the developed method will likely contribute to the improvement of solar cell efficiencies through better carrier extraction and to the realization of complex (opto)electronic devices.

Endeavor of Iontronics: From Fundamentals to Applications of Ion-Controlled Electronics

Iontronics is a newly emerging interdisciplinary concept which bridges electronics and ionics, covering electrochemistry, solid-state physics, electronic engineering, and biological sciences. The recent developments of electronic devices are highlighted, based on electric double layers formed at the interface between ionic conductors (but electronically insulators) and various electronic conductors including organics and inorganics (oxides, chalcogenide, and carbon-based materials). Particular attention is devoted to electric-double-layer transistors (EDLTs), which are producing a significant impact, particularly in electrical control of phase transitions, including superconductivity, which has been difficult or impossible in conventional all-solid-state electronic devices. Besides that, the current state of the art and the future challenges of iontronics are also reviewed for many applications, including flexible electronics, healthcare-related devices, and energy harvesting.

Revisiting the Valence and Conduction Band Size Dependence of PbS Quantum Dot Thin Films

We use a high signal-to-noise X-ray photoelectron spectrum of bulk PbS, GW calculations, and a model assuming parabolic bands to unravel the various X-ray and ultraviolet photoelectron spectral features of bulk PbS as well as determine how to best analyze the valence band region of PbS quantum dot (QD) films. X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) are commonly used to probe the difference between the Fermi level and valence band maximum (VBM) for crystalline and thin-film semiconductors. However, we find that when the standard XPS/UPS analysis is used for PbS, the results are often unrealistic due to the low density of states at the VBM. Instead, a parabolic band model is used to determine the VBM for the PbS QD films, which is based on the bulk PbS experimental spectrum and bulk GW calculations. Our analysis highlights the breakdown of the Brillioun zone representation of the band diagram for large band gap, highly quantum confined PbS QDs. We have also determined that in 1,2-ethanedithiol-treated PbS QD films the Fermi level position is dependent on the QD size; specifically, the smallest band gap QD films have the Fermi level near the conduction band minimum and the Fermi level moves away from the conduction band for larger band gap PbS QD films. This change in the Fermi level within the QD band gap could be due to changes in the Pb:S ratio. In addition, we use inverse photoelectron spectroscopy to measure the conduction band region, which has similar challenges in the analysis of PbS QD films due to a low density of states near the conduction band minimum.

Schmitt trigger using a self-healing ionic liquid gated transistor

Electrical double layer transistors using ionic liquids as the gate and ZnO as the semiconductor exhibit stable operation in the presence of redox active additives. The characteristics of the device enable single components with the response of a Schmitt trigger.

High mobility and low density of trap states in dual-solid-gated PbS nanocrystal field-effect transistors

Dual-gated PbS nanocrystal field-effect transistors employing SiO2 and Cytop as gate dielectrics are fabricated. The obtained electron mobility (0.2 cm(2) V(-1) s(-1) ) and the high on/off ratio (10(5) -10(6) ), show that the controlled nanocrystal assembly (obtained with self-assembled monolayers), as well as the trap density reduction (using Cytop as dielectric), are crucial steps for the future application of nanocrystals.