Session P34: Nanoscale Charge Transport

Carrier, ion, and phonon mediated phase transitions in mixed halide perovskite nanostructures via low-exposure cathodoluminescence imaging

Mixed halide perovskite nanostructures support a wide variety of intriguing transport properties, be they electronic, ionic, or thermal. These transport properties are largely influenced by the structural phase and by the ionic composition. Interestingly, the local composition and structure of these materials can also be greatly affected by the transport of carriers, ions, or even heat. I will describe low-exposure, high-resolution, dynamic cathodoluminescence imaging that highlights the myriad relationships between various forms of non-equilibrium states and structures of these materials. They range from photo-induced halide demixing phase transitions in thin films and single-crystal nanoplates induced via polaronic fields to visualization and statistical characterization of structural temperature-induced phase transitions in a series of halide perovskite nanowires.   

Laser-Induced Conductance Enhancement in Single-Molecule Junctions

Recent studies have demonstrated light-induced current enhancement in nano-scale junctions via photon-assisted transport and hot-electron transport. We use a non-equilibrium Green's function model to show that these two mechanisms have identical current-voltage characteristics and argue that hot-electron transport accounts for the majority of photocurrent for nanoscopic junctions operating in the visible and near-infrared spectrum. Using 4,4'-bipyridine bound to Au electrodes as a prototypical single-molecule junction, we report up to 60\% enhancement in conductance by illuminating single-molecule junctions with 980 nm wavelength continuous wave laser. Furthermore, we probe the subtle effects of the transmission function on light-induced current and show that discrete variations in the binding geometry result in a significant change in conductance enhancement. This work provides a robust experimental framework for studying light-induced transport mechanisms in single-molecule junctions, which could lead to improved designs for organic optoelectronic devices.   

Observation of noise mediated interaction between coherent conductors

Transport in quantum conductors is affected by their electromagnetic environment, a phenomenon known as Dynamical Coulomb Blockade. This can be understood as a feedback of the noise generated by the sample on itself, and has been thoroughly studied for various macroscopic environmental impedances such as an RC circuit or a resonator. Here, we have devised a sample where two quantum coherent conductors (normal-metal--insulator--normal-metal tunnel junctions) are capacitively coupled so that each of them plays the role of the environment for the other one. The two junctions can be dc biased independently, but interact via their finite frequency noise. We have characterized this interaction by measuring the differential resistance of each junction as well as their trans-resistance, which measures how the current in one junction affects the voltage on the other one.   

Semiconductor type dependent role of metal nanoparticle in metal and semiconductor nanostructured junction.

Among hybrid nanostructures, semiconductor with metal nanomaterial has been more exploited because metal and semiconductor have different properties that, in combination, result in unique electrical and optical properties. Localized surface plasmon resonance (LSPR), which is one of novel properties of metal nanoparticles (NPs), has been used as a good strategy for increasing an opto-electric performance in semiconductors. In this presentation, improvement of the opto-electronic properties of non-single crystallized nanowire (NW) devices with space charges generated by LSPR is demonstrated. The photocurrent and spectral response of single polypyrrole (PPy) NW devices are increased by electrostatically attached Ag NPs. In particular, it is also proved the space charge generation by LSPR of Ag NPs by means of characterizing current-voltage (J-V) dependence and finite differential time domain (FDTD) simulation on the NW devices. Moreover, semiconductor type dependent role of metal NP in metal NPs decorated semiconductor NW is demonstrated by using light irradiated Kevin probe force microscopy.   

Electron and heat transport in graphene-based single-molecule devices

Graphene nano-electrodes provide a versatile platform for contacting individual molecules. Unlike metal electrodes, graphene is atomically stable at room temperature and screening of the gate electric field is strongly reduced by the two-dimensional nature of the electrodes. Molecules can be anchored to the graphene via $\pi $-$\pi $ stacking bonds. We will present single electron transport measurements of single pyrene-functionalised C60 molecules. Strong electron-phonon coupling in these molecules leads to the observation of Franck-Condon blockade. In addition to spectroscopic transport features arising from the electronic and mechanical degrees of freedom of the fullerene molecule, we observe the effect of quantum interference in the graphene leads. Density-of-states fluctuations due to multi-mode Fabry-Perot interference in graphene result in energy dependent coupling between the graphene leads and the molecule. Finally, we will present thermoelectric measurements of our graphene-based nanostructures, and show the energy dependent Seebeck coefficient both in the sequential electron tunnelling and quantum interference regime. Our experiments demonstrate the capability of graphene-based molecular junctions for studying transport in single molecules, and highlight spectroscopic features that cannot readily be observed in metal-molecule junctions.   

Experimental Signatures for Bose-Einstein Condensation of Semiconductor Excitons in a Trap

Semiconductor excitons, i.e. electron-hole pairs bound by Coulomb attraction, have been studied for long in the quest for Bose-Einstein condensation (BEC). Unfortunately, this had not led to clear success so far despite the expected critical temperature of about 1K. In 2007, M. Combescot et al. showed that the ground exciton state is optically dark so that the condensate is made of a macroscopic population of dark excitons. Quantum signatures can only be detected directly above a density threshold, when fermion exchanges between excitons can introduce coherently a small fraction of bright excitons to the dark BEC. The latter then becomes "grey" and is possibly studied through a weak and coherent optical signal.\\ Here we report experimental evidences for a grey BEC: We confine long-lived excitons in a trap where we probe a homogeneously broadened gas at controlled density and temperature. We show that the photoluminescence (PL) emitted from the trap anomalously decreases while excitons are cooled to the sub-Kelvin regime. The darkening marks the quantum condensation in the lowest energy dark states. We also reveal that the weak PL radiated from the trap exhibits both quantum spatial coherence and increased temporal coherence below 1K.   

Computational investigation of electrode effects in molecular electronics

The field of molecular electronics offers an avenue to reach the next generation of electronic devices due to the high tunability and relatively inexpensive manufacturing costs of molecules. The field has advanced to the point where molecular junctions can be routinely fabricated and their conductance measured. In terms of computational studies, the state-of-the-art is the non-equilibrium Green's function technique combined with density functional theory (NEGF-DFT) approach that provides the transmission function, $T(E)$, from which the conductance characteristics of the molecular junction can be obtained. A typical system involves a molecule with some anchoring groups (e.g. thiol, amine) connected to Au electrodes. However, using a different metal (Cu, Pt, Al) for the electrodes results in a different alignment between the molecular orbitals and the Fermi level of the electrodes, thus yielding different transport properties. Additionally, the exact atomic configuration at the electrode/molecule interface can play a dramatic role. In this presentation, the effects on molecular conductance of different metal electrodes, their atomic orientation and configuration will be discussed, and some insights into how to obtain control over these parameters will be offered.   

Local Cooling Engineered by Asymmetric Electrodes in Nanoscale Junctions.

Searching for a solution to prevent electronic components from overheating has been a constant concern for the electronics industry. Local heating is caused by inelastic electron-phonon scattering. The mechanism corresponds to the electron incidence from the right or left electrode that relaxes (cools) or excites (heats) the energy level of normal--mode vibrations in the device region. Much research has been conducted on local heating. However, quite few studies have discussed local-cooling phenomena. Local cooling refers to the decrease in local temperature with increased applied voltage. Such an unusual phenomenon is possible when the rate of energy in cooling exceeds that in heating. Here, we propose an engineer-able local-cooling mechanism which utilizes asymmetric electrodes, e.g., one electrode is made of metal, whereas the other is made of bad metal. Substantial local cooling can be achieved at room temperature when the bandwidth of the bad-metal electrode is comparable to the energy of the phonon in inelastic e-p scatterings. The local cooling is caused by the narrowed bandwidth which obstructs certain inelastic heating scattering processes, i.e., the Pauli exclusion principle prohibits certain heating processes, where electrons heat up the device, lose energy via inelastic scattering, and being scattered to the forbidden region below the bottom of the narrowed band. Local-cooling phenomenon is meritorious to stability and performance of electronic devices.   

The evolution of molecule-electrode coupling at biased interfaces: An ab initio approach

Coupling of discrete molecular states to a metallic continuum at an interface, together with level alignment between frontier molecular orbital energies and the Fermi level, determine its transport and spectroscopic properties. In addition, phenomenological coupling parameters between the discrete molecular states and the continuum are necessary for understanding I-V characteristics and constructing models of charge dynamics at the interfaces. In this work, we compute such coupling parameters based on a non-equilibrium Green's function approach, and analyze the bias-induced change of such coupling parameters and their effect in transport properties and I-V characteristics. We study and compare a series of model interfacial systems, molecular junctions including bipyridine and its derivatives (both symmetric and asymmetric), under bias. Our study provides new understanding of finite bias transport properties in terms of molecular orbitals for such junctions, and new insight in interpreting experimental measurement of I-V characteristics.   

Schottky diode model for non-parabolic dispersion in narrow-gap semiconductor and few-layer graphene

Despite the fact that the energy dispersions are highly non-parabolic in many Schottky interfaces made up of 2D material, experimental results are often interpreted using the conventional Schottky diode equation which, contradictorily, assumes a parabolic energy dispersion. In this work, the Schottky diode equation is derived for narrow-gap semiconductor and few-layer graphene where the energy dispersions are highly non-parabolic. Based on Kane's non-parabolic band model, we obtained a more general Kane-Schottky scaling relation of $J\propto\left(T^2+\gamma k_BT^3\right)$ which connects the contrasting $J\propto T^2$ in the conventional Schottky interface and the $J\propto T^3$ scaling in graphene-based Schottky interface via a non-parabolicity parameter, $\gamma$. For $N$-layer graphene of $ABC$-stacking and of $ABA$-stacking, the scaling relation follows $J\propto T^{2/N+1}$ and $J\propto T^3$ respectively. Intriguingly, the Richardson constant extracted from the experimental data using an incorrect scaling can differ with the actual value by more than two orders of magnitude. Our results highlights the importance of using the correct scaling relation in order to accurately extract important physical properties, such as the Richardson constant and the Schottky barrier’s height.