Self-Organized Phase-Composite Nanocrystal Solids with Superior Charge Transport

Interparticle electronic coupling is essential for self-assembled colloidal nanocrystal (NC) solid semiconductors to fulfill their wide-tunable electrical and optoelectrical properties, but it has been limited by disorders. Here, a disorder-tolerant coupling approach is presented by synthesizing self-organized NC solids based on amorphous/nanocrystalline phase-composites. The ZnO amorphous matrix, which infills the space between the less regularly ordered ZnO NCs, enables robust electronic coupling between neighboring NCs via the resonant wave function overlap, leading to a disorder-tolerant resonant conducting state. Field-effect transistors based on phase-composite semiconductors show delocalized band-like transport with superior field-effect mobility values (∼75 cm2 V-1 s-1), compared to amorphous or polycrystalline ZnO semiconductors. Furthermore, the broad amorphous matrix can mitigate interfacial defects between crystalline regions through atomic relaxation, in contrast to narrow grain boundaries in polycrystalline films, resulting in a significantly low interface trap density for phase-composite NC solids. Density function theory calculations and quantum transport simulations using the nonequilibrium Green's function formalism elucidate the origins of superior and highly disorder-tolerant electron transport in phase-composite NC solids. Our report introduces a new class of NC solids complementary to the colloidal counterpart and will be applicable to CMOS-compatible emerging device technologies.

Ligand Crosslinking Boosts Thermal Transport in Colloidal Nanocrystal Solids

The ongoing interest in colloidal nanocrystal solids for electronic and photonic devices necessitates that their thermal-transport properties be well understood because heat dissipation frequently limits performance in these devices. Unfortunately, colloidal nanocrystal solids generally possess very low thermal conductivities. This very low thermal conductivity primarily results from the weak van der Waals interaction between the ligands of adjacent nanocrystals. We overcome this thermal-transport bottleneck by crosslinking the ligands to exchange a weak van der Waals interaction with a strong covalent bond. We obtain thermal conductivities of up to 1.7 Wm^-1  K^-1 that exceed prior reported values by a factor of 4. This improvement is significant because the entire range of prior reported values themselves only span a factor of 4 (i.e., 0.1-0.4 Wm^-1  K^-1 ). We complement our thermal-conductivity measurements with mechanical nanoindentation measurements that demonstrate ligand crosslinking increases Young's modulus and sound velocity. This increase in sound velocity is a key bridge between mechanical and thermal properties because sound velocity and thermal conductivity are linearly proportional according to kinetic theory. Control experiments with non-crosslinkable ligands, as well as transport modeling, further confirm that ligand crosslinking boosts thermal transport.

Setting Carriers Free: Healing Faulty Interfaces Promotes Delocalization and Transport in Nanocrystal Solids

Superlattices of epitaxially connected nanocrystals (NCs) are model systems to study electronic and optical properties of NC arrays. Using elemental analysis and structural analysis by in situ X-ray fluorescence and grazing-incidence small-angle scattering, respectively, we show that epitaxial superlattices of PbSe NCs keep their structural integrity up to temperatures of 300 °C; an ideal starting point to assess the effect of gentle thermal annealing on the superlattice properties. We find that annealing such superlattices between 75 and 150 °C induces a marked red shift of the NC band-edge transition. In fact, the post-annealing band-edge reflects theoretical predictions on the impact of charge carrier delocalization in these epitaxial superlattices. In addition, we observe a pronounced enhancement of the charge carrier mobility and a reduction of the hopping activation energy after mild annealing. While the superstructure remains intact at these temperatures, structural defect studies through X-ray diffraction indicate that annealing markedly decreases the density of point defects and edge dislocations. This indicates that the connections between NCs in as-synthesized superlattices still form a major source of grain boundaries and defects, which prevent carrier delocalization over multiple NCs and hamper NC-to-NC transport. Overcoming the limitations imposed by interfacial defects is therefore an essential next step in the development of high-quality optoelectronic devices based on NC solids.

Charge Transport Modulation in PbSe Nanocrystal Solids by Au xAg1- x Nanoparticle Doping

Nanocrystal (NC) solids are an exciting class of materials, whose physical properties are tunable by choice of the NCs as well as the strength of the interparticle coupling. One can consider these NCs as "artificial atoms" in analogy to the formation of condensed matter from atoms. Akin to atomic doping, the doping of a semiconducting NC solid with impurity NCs can drastically alter its electronic properties. A high degree of complexity is possible in these artificial structures by adjusting the size, shape, and composition of the building blocks, which enables "designer" materials with targeted properties. Here, we present the doping of the PbSe NC solids with a series of Au _xAg_{1- x} alloy nanoparticles (NPs). A combination of temperature-dependent electrical conductance and Seebeck coefficient measurements and room-temperature Hall effect measurements demonstrates that the incorporation of metal NPs both modifies the charge carrier density of the NC solids and introduces energy barriers for charge transport. These studies point to charge carrier injection from the metal NPs into the PbSe NC matrix. The charge carrier density and charge transport dynamics in the doped NC solids are adjustable in a wide range by employing the Au _xAg_{1- x} NP with different Au:Ag ratio as dopants. This doping strategy could be of great interest for thermoelectric applications taking advantage of the energy filtering effect introduced by the metal NPs.

Hole Mobility in Nanocrystal Solids as a Function of Constituent Nanocrystal Size

Solids of semiconductor nanocrystals (NCs) are semiconductors in which the band gap can be controlled by changing the size of the constituent NCs. To date, nontrivial dependencies of the carrier mobility on the NC size have been reported. We use the time-of-flight (TOF) technique to measure the carrier mobility as a function of the NC size and find that the hole mobility of the NC solid increases dramatically with decreasing NC radius. We show that this result is in agreement with an analytic model for carrier mobility in NC solids. We further implement Monte Carlo simulations to aid in understanding the transient measurements in the context of models of dispersive transport. This work highlights that changing NC size in a device has important implications for charge transport.

Importance of Polaronic Effects for Charge Transport in CdSe Quantum Dot Solids

We developed an accurate model accounting for electron-phonon interaction in colloidal quantum dot supercrystals that allowed us to identify the nature of charge carriers and the electrical transport regime. We find that in experimentally analyzed CdSe nanocrystal solids, the electron-phonon interaction is sufficiently strong that small polarons localized to single dots are formed. Charge-carrier transport occurs by small polaron hopping between the dots, with mobility that decreases with increasing temperature. While such a temperature dependence of mobility is usually considered as a proof of band transport, we show that the same type of dependence occurs in the system where transport is dominated by small polaron hopping.