Session C16: Transport in Nanostructures -- Quantum dots, nanocrystals, and nanowires

Three-dimensional imaging of material functionality through nanoscale tracking of energy flow

The ability of energy carriers to move within and between atoms and molecules underlies virtually all biochemical and material function. Understanding and controlling energy flow, however, requires observing it on ultrasmall and ultrafast spatiotemporal scales, where energetic and structural roadblocks dictate the fate of energy carriers. We therefore developed a universal, non-invasive optical scheme that leverages interferometric scattering to track tiny changes in material polarizability created by energy carriers. Our approach enables mapping energy transport trajectories in four dimensions of spacetime with few-nanometer precision and directly correlating them to material morphology. We visualize exciton, charge, and heat transport in polyacene, silicon and perovskite semiconductors and elucidate, in particular, how grain boundaries impact energy flow through their lateral- and depth-dependent resistivities. We reveal new strategies to interpret energy transport in disordered environments that will direct the design of defect-tolerant materials for the semiconductor industry of tomorrow.   

Levy statistics in quasi one-dimensional percolation paths in nano-patterned quantum dot solids

Quantum dot (QD) solids hold promise as a tunable platform for applications such as quantum computation and spintronics, and for exploring many-body physics. However, the predicted, novel electronic properties in QD solids have been obscured by disorder caused by structural defects, variability in electronic energy levels of the QDs and charge traps on the QD surfaces. We nano-pattern QD solids with dimensions ~10−100 nm that are free of structural defects found in many larger superlattices. The QDs are strongly coupled (≤1 nm inter-dot spacing), which offsets inhomogeneities in the electronic energy levels in the QD. We find current noise that increases linearly with the average current at high temperatures, and we find random telegraph noise at low temperatures. We show that charge is transmitted along quasi-one-dimensional channels, which open and close at a rate given by Levy statistics. The trapping and release of charge in the QD matrix is the likely cause for these fluctuations. These nano-patterned QD solids enable measurement of the charge transport mechanism intrinsic to the QDs; and, they offer a direct measurement of the influence of charge trapping on the charge transport mechanism.   

Triplet excitons on the interface of Colloidal Quantum Dots and organic molecules

Colloidal Quantum Dots (QDs) have been long-standing candidates for optoelectronic devices and applications, such as solar photovoltaic, light emitting diodes, detectors, and biological sensing. Theyare composed of semi-conductor materials, and are synthesized from solution. Their optical and electronic properties change with their size, shape, composition, and surface. In the up-conversion (UC) process two low frequency photons are converted in to one high energy photon, this can broaden the absorption spectrum and increase the efficiency of optoelectronic devices. One of the steps in the UC process is the transfer of high energy triplet exciton across the interface of colloidal QD/organic molecule. Using electronic structure calculations combined with experimental observations, we study and try to characterize triplet excitons on various QD structures ligated with organic molecules to get a better understanding of the transfer mechanism. We also use Molecular Dynamics simulations to study the morphology of these interfaces.   

Layer-Dependence of Charge Transfer Kinetics in Hybrids of 2D MoS2 and PbS/CdS Quantum Dots

Two-dimensional (2D) layered transition metal dichalcogenides (TMDs) have attracted tremendous attention owing to their unique optical and physical properties. However, weak photon absorption due to their atomically thin thickness prohibits their spectral sensitivity. Here, 2D-MoS₂ with different layers are combined with core/shell PbS/CdS quantum dots (QDs) to produce hybrids with improved photoresponsivity by interfacial charge transfer. The charge transfer kinetics in the hybrids of MoS₂ and PbS/CdS QDs has been revealed by time-resolved photoluminescence (PL) microscopy, showing that the charge transfer rate increases with the layer of MoS₂ increased due to the increased driving force in between conduction band edge of MoS₂ and QDs. We have evaluated the results with the theoretical model of Marcus theory, which is in good agreement with our observation. Understanding the interfacial charge transfer kinetics between QDs and 2D materials is crucial and useful for improving photon-to-current conversion efficiency in next-generation optoelectronics.   

Charge Kondo effects in a quadruple quantum dot in spinless and spinful regimes.

We theoretically study charge Kondo effects in a quadruple quantum dot, whose shape is such that three dots A,B,C are located at the three corners of a triangle respectively and the other dot D is at the center of the triangle. Here, the dots A and B are coupled to reservoirs. We consider a spinless regime under an external magnetic field and a spinful regime in the presence of electron tunneling between the dots A and B, when the system has two-fold degenerate ground-state charge configurations (n_A=1,n_B=1,n_C=1,n_D=0) and (0,0,0,1). In the spinless regime, a single-channel charge Kondo effect occurs. In this case, the two charge configurations act as the pseudospin states of the Kondo effect. In the spinful regime, electrons of the dots A and B in the charge configuration (1,1,1,0) form a spin singlet state, and the system is described by an anisotropic two-channel Kondo Hamiltonian. Because the channel anisotropy is large, the quadruple dot shows a single-channel charge Kondo effect. We compute electron transport through the quadruple dot for the two regimes.   

Metal-insulator transition in a semiconductor nanocrystal network

In bulk semiconductors, the metal-insulator transition is described by the well-known Mott criterion. A recent theory proposes a more stringent criterion condition in nanocrystal (NC) networks, dependent on the electron density n and the inter-NC facet radius ρ: nρ³ ≈ 0.3. Here we use plasma-synthesized ZnO NCs coated with Al₂O₃ via ALD to study the electronic properties of a NC network as a function of n and ρ. Through a xenon-flashlamp annealing process, we selectively sinter/dope the NC network by flashing the film before/after the ALD infill, allowing independent control of ρ and n. While we observe large changes in the mobility and n, we do not see finite conductivity (σ) as T→0K. To cross the transition, prior to the Al₂O₃ ALD infill we coat the NCs with 8 cycles of ZnO ALD. This changes ρ from ~1.5 nm to ~2.8 nm and increases the packing density from 33% to 47%. By then tuning n we achieve a transition from the semiconducting to metallic state, with finite σ as T→0K. At the transition we see power law conductivity of the unusual form σ(T) ∝ T^1/5, and observe critical scaling behavior when scaling σ by the nρ³ criterion. This is the first conclusive evidence for metallic behavior in a NC network.   

Manipulation of non-linear heat currents in dissipative quantum dot systems

The anomalous behavior of phonon transport due to finite electron-phonon interaction is investigated using an Anderson-Holstein based dissipative quantum dot setup in two relevant cases: (a) electron flow stimulated by the voltage bias in the absence of an electronic temperature gradient and (b) electron flow driven by the electronic temperature gradient at zero voltage. We explain the observation of the cumulative effect of voltage and electronic temperature gradient on the non-linear phonon current with the aid of a new transport coefficient called electron induced phonon thermal conductivity. It is demonstrated that under suitable operating conditions in Case (a), the dot can pump in phonons into the hotter phonon reservoir and in Case (b), the dot can extract phonons out of the colder phonon reservoirs. Finally, we elaborate how the non-linear electronic heat current can be stimulated and controlled by engineering the temperatures of the phonon reservoirs.

References:
[1] The non-linear phonon Peltier effect in dissipative quantum dot systems, B. De and B. Muralidharan, Scientific Reports, 8, 5185, (2018).
[2] Thermoelectric study of dissipative quantum dot heat engines, B. De and B. Muralidharan, Phys. Rev. B, 94, 165416, (2016).   

Measurement of the Kondo cloud length via a quantum dot coupled to a 1d interferometer: Theory (1/2)

The Kondo effect, known as the archetype of many-body correlations, arises from the interaction between a localized magnetic moment and surrounding conducting electrons. Advances of nanotechnology allow us to study the Kondo effect for a single magnetic impurity confined in a semiconductor quantum dot in contact with electron reservoirs. This development also makes it possible to embed a Kondo correlated state into an interferometer. However, the special extent of the Kondo cloud has yet to be measured.
Here, we demonstrate that the Kondo cloud length ξK can be measured in a system of a Kondo correlated quantum coupled to a 1D Fabry–Perot (F-P) interferometer. Using numerical renormalization group and poor man’s scaling methods in parallel we characterize the oscillations in Kondo temperature with respect to resonances of the F-P interferometer. We show that the amplitude of the temperature oscillations can be described by the interferometer length as well as the coupling strength. Thus, we demonstrate a system where an experimentally defined length can be used to infer the Kondo length.   

Measurement of the Kondo cloud length via a quantum dot coupled to a 1d interferometer: Experiment (2/2)

The Kondo effect, known as the archetype of many-body correlations, arises from the interaction between a localized magnetic moment and surrounding conducting electrons. Advances of nanotechnology allow us to study the Kondo effect for a single magnetic impurity confined in a semiconductor quantum dot in contact with electron reservoirs. This development also makes it possible to embed a Kondo correlated state into an interferometer. However, the special extent of the Kondo cloud has yet to be measured.
In this work, we perform direct experimental measurement of the Kondo cloud length ξK by coupling a Kondo correlated quantum dot to ballistic, multi-channel quantum wires, one of which couples more strongly to the dot and works as a 1D Fabry–Perot (F-P) interferometer defined on a GaAs/AlGaAs heterostructure. We demonstrate that the measured Kondo temperature undergoes resonances with respect to the F-P fluctuations. By looking at the strength of the resonances versus the interferometer lengths, we are able to access the length of the Kondo cloud. We back up our results by comparing the behavior of the interferometer in the Kondo as well as the Coulomb Blockade Regimes.   

Phonon backscatter, trapping, bottlenecking, and misalignment effects on thermal conductivity of Si Nanostructures

Nanostructured systems offer the ability to reduce thermal conductivity which, for instance, may be used to improve the thermoelectric efficiency. The internal surfaces of these nanostructures significantly curtail the ballistic free flight of phonons, and thus resist thermal transport. In this talk, we present a recent investigation of a simple one-parameter geometry that simultaneously modulates backscattering and trapping effects to enable directed study of controlling phonons. The geometry is a simple sequence of chambers offset from one another by a defined distance. We use the geometry to study the effects of phonon backscatter, trapping, bottlenecking, and corner-turning on the thermal conductance in Si nanowires (NWs). By creating a geometry that maximizes backscatter, a roughly 8-fold reduction in thermal conductance below the Casimir limit can be achieved at room temperature which is a factor of four smaller than the nearest reported value in the literature. The geometry is also useful for systematic investigation of other means of controlling phonons and affecting thermal transport. Specifically, we investigate the induced misalignment between the phonon flow and thermal flux due to the shape of the geometry as well as phonon filter-like behavior of the geometry.   

Band Structure and Carrier Thermalization in InGaAs and InGaAs/InP Nanowires Probed by Transient Rayleigh Scattering and Photoluminescence

We use transient Rayleigh scattering (TRS) and photoluminescence (PL) to characterize the band structure and carrier thermalization of Wurtzite InGaAs and InGaAs-InP core-shell nanowires (NW). TRS measurements use an ultrafast (~150 fs) near-infrared pump (1.51 eV) and probe (0.79 - 1.16 eV) pulses. TRS spectra of a single NW exhibit a band-to-band transition substantially blue-shifted relative to the equivalent Zincblende band edge. PL measurements (with 1.51 eV excitation) of NW clusters confirm this transition as the fundamental Wurtzite band edge. Polarization analysis of the PL and TRS spectra indicate a smaller A-B splitting than observed in WZ GaAs. PL emission is enhanced dramatically in core-shell wires compared to core-only wires. Likewise, TRS measurements show longer carrier lifetime for core-shell wires, due to passivation by the InP shell. Thermalization modeling based on these measurements suggests that the core-only wires cool rapidly via optical phonon emission, whereas the core-shell wires cool by a slower process with early optical and later acoustic phonon emission.   

All-solution processed micro/nano-wires as transparent conducting electrodes

We have developed an all-solution processed transparent conductive electrode with sheet resistance an order of magnitude smaller than conventional nanowire-based transparent conductors. This is achieved by integrating all-solution-produced microwires¹ with nanowires and electroplate welding. As a result, the advantages of these transparent conductors are indium-free, vacuum-free, lithographic-facility-free, metallic-mask-free, with small domain size, high specific conductivity and mechanical flexibility, thus making them an excellent replacement for ITO.

1. C. Yang, J. M. Merlo, J. Kong, Z. Xian, B. Han, G. Zhou, J. Gao, M. J. Burns, K. Kempa, M. J. Naughton, All-Solution-Processed, Scalable, Self-Cracking Ag Network Transparent Conductor, Phys. Status Solidi A 215, 1700504 (2017)   

Formation of Interfacial Dipole Formations during Contact Electrification

This study introduces a plausible origin of the driving force for elec- tron transfer in contact electrification and triboelectrification for dielectric materials. As two material approaches each other, surface lattices of both materials form a weak interaction, the perturbed states of each surface lattice can be approximated a dipole. These surface dipoles induce a potential field in the proximity and provides a driving force for electron transfer. A tribopair of barium titanate and magnesia are investigated as an example. The simulation. results show that such tribopair can generate up to 104 V/cm², which is comparable with the experimental measurements in published literature.