Nanoparticle Solids Research Articles

Electron transport mechanism in colloidal SnO2 nanoparticle films and its implications for quantum-dot light-emitting diodes

Charge transport behavior in SnO2 nanoparticle (NP) films is rather crucial to the optoelectronic devices. Temperature-dependent electrical results show that the electron transport in SnO2 NP films is dominated by the Mott variable-range hopping processes, i.e. the electrons are transported between different NPs through surface states rather than the conduction band of the nanocrystals, which is identical to the commonly used ZnO NP solids. Compared with ZnO, SnO2 films exhibit similar electron mobility but lower density of states (DOS). Therefore, we deduce that the low DOS in the SnO2 NP films should be the key factor limiting the device performance in compared with the ZnO as reported in most of the quantum-dot light-emitting diodes (QLEDs). Our work sheds light on optimizing SnO2 NP films for QLEDs. Moreover, we believe that the SnO2 remains a desirable candidate as the electron transport material for the QLEDs due to its excellent physicochemical stability.

Hierarchical carrier transport simulator for defected nanoparticle solids

The efficiency of nanoparticle (NP) solar cells has grown impressively in recent years, exceeding 16%. However, the carrier mobility in NP solar cells, and in other optoelectronic applications remains low, thus critically limiting their performance. Therefore, carrier transport in NP solids needs to be better understood to further improve the overall efficiency of NP solar cell technology. However, it is technically challenging to simulate experimental scale samples, as physical processes from atomic to mesoscopic scales all crucially impact transport. To rise to this challenge, here we report the development of TRIDENS: the Transport in Defected Nanoparticle Solids Simulator, that adds three more hierarchical layers to our previously developed HINTS code for nanoparticle solar cells. In TRIDENS, we first introduced planar defects, such as twin planes and grain boundaries into individual NP SLs superlattices (SLs) that comprised the order of 103 NPs. Then we used HINTS to simulate the transport across tens of thousands of defected NP SLs, and constructed the distribution of the NP SL mobilities with planar defects. Second, the defected NP SLs were assembled into a resistor network with more than 104 NP SLs, thus representing about 107 individual NPs. Finally, the TRIDENS results were analyzed by finite size scaling to explore whether the percolation transition, separating the phase where the low mobility defected NP SLs percolate, from the phase where the high mobility undefected NP SLs percolate drives a low-mobility-to-highmobility transport crossover that can be extrapolated to genuinely macroscopic length scales. For the theoretical description, we adapted the Efros-Shklovskii bimodal mobility distribution percolation model. We demonstrated that the ES bimodal theory’s two-variable scaling function is an effective tool to quantitatively characterize this low-mobility-to-high-mobility transport crossover.

Disordered Mott-Hubbard Physics in Nanoparticle Solids: Transitions Driven by Disorder, Interactions, and Their Interplay.

We show that adapting the knowledge developed for the disordered Mott-Hubbard model to nanoparticle (NP) solids can deliver many very helpful new insights. We developed a hierarchical nanoparticle transport simulator (HINTS), which builds from localized states to describe the disorder-localized and Mott-localized phases of NP solids and the transitions out of these localized phases. We also studied the interplay between correlations and disorder in the corresponding multiorbital Hubbard model at and away from integer filling by dynamical mean field theory. This DMFT approach is complementary to HINTS, as it builds from the metallic phase of the NP solid. The mobility scenarios produced by the two methods are strikingly similar and account for the mobilities measured in NP solids. We conclude this work by constructing the comprehensive phase diagram of PbSe NP solids on the disorder-filling plane.

Commensuration effects in layered nanoparticle solids

We have developed HiNTS, the {\bf Hi}erarchical {\bf N}anoparticle {\bf T}ransport {\bf S}imulator, and adapted it to study commensuration effects in two classes of Nanoparticle (NP) solids: (1) a bilayer NP solid (BNS) with an energy offset, and (2) a BNS as part of a Field-Effect Transistor (FET). HiNTS integrates the ab initio characterization of single NPs with the phonon-assisted tunneling transition model of the NP-NP transitions into a Kinetic Monte Carlo based simulation of the charge transport in NP solids. First, we studied a BNS with an inter-layer energy offset $\Delta$, possibly caused by a fixed electric field. Our results include the following. (1) In the independent energy-offset model, we observed the emergence of commensuration effects when scanning the electron filling factor $FF$ across integer values. These commensuration effects were profound as they reduced the mobility by several orders of magnitude. We analyzed these commensuration effects in a five dimensional parameter space, as a function of the on-site charging energy $E_C$, energy offset $\Delta$, the disorder $D$, the electron filling factor $FF$, and the temperature $k_{B}T$. We demonstrated the complexity of our model by showing that at integer filling factors $FF$ commensuration effects are present in some regions of the parameter space, while they vanish in other regions, thus defining distinct dynamical phases of the model. We determined the phase boundaries between these dynamical phases. (2) Using these results as a foundation, we shifted our focus to the experimentally much-studied NP-FETs. NP-FETs are also characterized by an inter-layer energy offset $\Delta$, which, in contrast to our first model, is set by the gate voltage $V_G$ and thereby related to the electron filling $FF$. We demonstrated the emergence of commensuration effects and distinct dynamical phases in these NP-FETs.

Nanoparticle assemblies in supramolecular nanocomposite thin films: concentration dependence.

The phase behavior of supramolecular nanocomposite thin films was systematically investigated as a function of nanoparticle (NP) loading from 1 to >50 wt %. The coassembly of NP and supramolecule can be divided into five regimes, from a supramolecule-guided assembly to a NP governing assembly process, depending on the energetic contributions from the surface energy, NP-supramolecule interaction, and the kinetic pathway of the assembly process. A range of morphologies such as 1D NP chains, 2D sheets, 3D NP assemblies, and NP solids can be readily obtained, providing opportunities to meet structural control in nanocomposites for a wide range of applications.

Synthesis of Nanoporous Network Materials with High Surface Areas from the Cooperative Assemblage of Alkyl-Chain-Capped Metal/Metal Oxide Nanoparticles

A simple route for the synthesis of a new class of porous metal/metal oxide nanoparticle (NP) materials such as Cu−TiO2, Au−ZrO2, Cu−ZrO2, and Au−TiO2 NPs through the cooperative assembly of presynthesis hydrophobic oleic acid (OA)-capped metal and metal oxide NPs is reported. In such a way, the synergistic interaction between metal and metal oxide NPs has gained significant interest owing to new properties that arise at the metal−metal oxide interface. Various technique including XRD, N2 adsorption/desorption isotherms, FTIR, TEM, XPS, and catalytic test were used to monitor the physicochemical and catalytic properties of these materials. The results revealed that these porous materials exhibit homogeneous dispersion in between metal and metal oxide NPs, high surface area, and narrow interparticle pore size distribution. The catalytic properties of these metal/metal oxide NP solids (even Cu oxide NP catalysts, e.g., no noble metal catalyst) in the CO oxidation reaction are better than those of commercial noble metal catalyst (Pt/Al2O3) and the conventional metal oxide-supported Cu catalysts. Based on this synthesis approach, a variety of nanoporous multicomponent solids of both metal and metal oxide with desired proportions can be synthesized.