Session L18: Focus Session: Electron, Ion, and Exciton Transport in Nanostructures - Quantum Dots and Related Structures

The Role of Surface Ligands in Electronic Charge Transport in Semiconductor Nanocrystal Arrays

The long, insulating ligands commonly used in the synthesis of colloidal semiconductor nanocrystals (NCs) inhibit strong interparticle coupling and charge transport once NCs are assembled in the solid state into NC arrays. We introduce ammonium thiocyanate (NH$_{4}$SCN) and its derivatives, ammonium selenocyanate and selenourea to exchange the long, insulating ligands commonly used in the synthesis of colloidal semiconductor NCs. NCs may be exchanged with the new ligand in solution to form dispersions from which NC arrays are deposited or NC arrays with the long, insulating ligands may be exchanged in the solid state with the new ligands. The new compact ligands enhance interparticle coupling and charge transport in thin film, NC arrays as seen by red-shifts in the optical absorption and concomitant increases in carrier mobilities. Thiocyanate-capped CdSe thin film, NC arrays form sensitive photodetectors and n-type field-effect transistors with electron mobilities of $\sim $10 cm$^{2}$/Vs and current modulation of $>$10$^{6}$, while preserving NC quantum confinement. Temperature-dependent transport measurements reveal band-like transport in NC arrays, overcoming carrier hopping that has typified transport in NC arrays until recently. The non-caustic, chemically benign nature of the ammonium thiocyanate treatment enables the fabrication of NC thin film devices and circuits on flexible plastics.   

n and p Type HgTe Colloidal Quantum Dot Film

HgTe colloidal quantum dots (CQDs) are a new system that can be stably charged n- and p-type by electrochemistry and it exhibits carrier-dependent photoresponse and magneto-resistance. For both electron or hole injection, interband bleach and intraband absorption confirm that the charges can be injected in the delocalized quantum states. Both carrier types lead to conductivity and show similar mobilities ($\sim$ 0.1 cm$^2$/Vs). However p-type films show good photoresponse and long photocurrent lifetime of hundreds of microseconds, while n-type films show 100 times shorter lifetimes and little or no photoresponse. With these samples, p-type will therefore be better for photovoltaic response. The magneto-resistance of n-type films is also found to be large while that of p-type films is negligible. This is consistent with the heavier hole mass compared to electrons in HgTe. The HgTe CQDs share similar features with the II-VI CdSe but with the advantage of allowing both n- and p-type charging. While this property is shared with the narrow gap IV-VI PbSe, HgTe has a simpler electronic structure and better stability in ambient conditions. The stability of the HgTe CQDs for n- and p-type charging opens many further investigations.   

Tailoring Quantum Dot Assemblies to Extend Exciton Coherence Times and Improve Exciton Transport

The motion of excitons through nanostructured assemblies plays a central role in a wide range of physical phenomena including quantum computing, molecular electronics, photosynthetic processes, excitonic transistors and light emitting diodes. All of these technologies are severely handicapped, though, by quasi-particle lifetimes on the order of a nanosecond. The movement of excitons must therefore be as efficient as possible in order to move excitons meaningful distances. This is problematic for assemblies of small Si quantum dots (QDs), where excitons quickly localize and entangle with dot phonon modes. Ensuing exciton transport is then characterized by a classical random walk reduced to very short distances because of efficient recombination. We use a combination of master equation (Haken-Strobl) formalism and density functional theory to estimate the rate of decoherence in Si QD assemblies and its impact on exciton mobility. Exciton-phonon coupling and Coulomb interactions are calculated as a function of dot size, spacing and termination to minimize the rate of intra-dot phonon entanglement. This extends the time over which more efficient exciton transport, characterized by partial coherence, can be maintained.   

Efficient Exciton Transport Between Strongly Quantum-Confined Silicon Quantum Dots

First-order perturbation theory and many-body Green function analysis are used to quantify the influence of size, surface reconstruction and surface treatment on exciton transport between small silicon quantum dots. Competing radiative processes are also considered in order to determine how exciton transport efficiency is influenced. The analysis shows that quantum confinement causes small ($\sim$ 1 nm) Si quantum dots to exhibit exciton transport efficiencies far exceeding that of their larger counterparts. We also find that surface reconstruction significantly influences the absorption cross-section and leads to a large reduction in both transport rate and efficiency. Exciton transport efficiency is higher for hydrogen-passivated dots as compared with those terminated with more electronegative ligands. This is because such ligands delocalize electron wave functions towards the surface and result in a lower dipole moment. This work [1] is a first step in the development of a framework for the design of quantum dot assemblies with improved exciton transfer efficiency. \vskip 2mm \noindent [1] Z. Lin, A. Franceschetti and M. T. Lusk, arXiv:1110.6456v1 [cond-mat.mes-hall]   

Finite Element Modeling of Current-Induced Filaments in Nanocrystalline Silicon

Rapid, heat induced phase transitions in a mixed phase semiconductor may lead to current percolation along a highly conductive preferred path, or filament. In this study, we use 2-D, finite element simulations to model the time dependent evolution of current-induced filaments in nanocrystalline silicon (nc-Si) wires. Nc-Si wires are modeled as isolated crystalline silicon (c-Si) circles randomly distributed in an amorphous silicon (a-Si) wire 500 nm in length and 75 nm in width. Simulations include temperature dependent material parameters for electrical conductivity, thermal conductivity, heat capacity, and account for latent heat of fusion during phase transition from solid to liquid silicon. Field dependent material parameters are neglected to improve simulation convergence. Voltage pulses of amplitude 300 V and rise time less than 1 ns produce molten filaments $\sim $ 5 nm in width extending the length of the wire. The resistance of each wire decreases by four orders of magnitude during formation and filament current density exceeds 500 MA/cm$^{2}$.   

Ultrashort channel length quantum-dot photodetectors

We present an efficient photodetector based on a one-dimensional array of parallel contacted PbSe quantum dots. In this device-architecture, the electrodes act as optical nano-antennae which concentrate incident light into the nanoparticle junction where they are converted into electron-hole pairs. The excitons are extracted with high efficiency due to the fact that the quantum dots are in direct contact with both source and drain electrodes, in contrast to previous studies which employed assemblies of quantum dots.   

The Influence of Phonons and Surface Termination on Optical Transitions in Small Silicon Quantum Dots and Implications for Exciton Transport

Bulk silicon is an indirect-gap semiconductor, and radiative recombination can proceed only through phonon assistance. Recent experiments suggest, though, that fast, zero-phonon, pseudo-direct transitions occur in silicon quantum dots (Si QDs) as a result of quantum confinement. On the other hand, previous theoretical studies based on tight-binding and effective mass methodologies yield contradictory conclusions on the degree to which phonons play a role in radiative recombination within Si QDs. The resolution of this issue has important repercussions for ways in which Si QDs can be incorporated into future photovoltatic designs. This also has implications as to how QD size, phonons, and surface ligands collectively influence exciton transport. Density functional theory, in concert with a phonon-corrected version of Fermi's Golden Rule, is used to investigate the degree to which phonons influence the rate of optical transitions in Si QDs and to elucidate how the role of phonons changes with dot size and surface termination (H, CH3, and OH). The results are compared with available experimental data and those of previous calculations. In addition, the implications for phonon-assisted exciton transport dynamics within Si QD assemblies will be discussed.   

Dual MOSFET Charge Sensing in PbS Nanocrystal Quantum Dots

We use nanoscale metal-oxide-semiconductor field-effect transistors (MOSFETs) as charge sensors for measuring transport in a nearby nanocrystal array. While our technique enables a high resistance measurement, and enables us to probe a wide range of conductance, the main limitations of using single MOSFET charge sensors is the step-like switching of the current caused by electrons tunneling into and out of traps, presumably in the oxide. This makes it difficult to distinguish events that originate in the film from those that originate in the MOSFETs. We use two MOSFETs as simultaneous charge sensors to perform a correlation analysis and distinguish these events. We pattern a 80 nm wide ordered array of PbS nanocrystals, approximately 50 nm away from each sensor, to maximize the signal in the MOSFETs from charge fluctuations in the nanocrystal film. This configuration then enables us to probe electron transport in the nanocrystal array.   

Measuring electron transport in nano-patterned films of PbS nanocrystals using a charge sensor

The ability to form nanoscale patterns of semiconductor nanocrystal films and to align those patterns to a substrate with nanoscale precision opens the possibility of novel electronic measurements and optoelectronic devices. We demonstrate a novel method for patterning nanoscopic films of semiconductor nanocrystals with electron-beam lithography. The resulting films are ordered and do not suffer from the cracking that arises when annealing or exchanging the capping ligand of the film. The patterning method is effective for a wide range of nanocrystal materials and capping ligands. We pattern a film of PbS nanocrystals approximately 80 nm wide, and position it within 100 nm of charge sensors made from nanoscale metal-oxide-semiconductor field-effect transistors (MOSFETs). The charge sensors, which can measure the fluctuations of individual electrons in the nearby electrostatic environment, are used to measure electron transport in the films as a function of temperature and applied field.   

Photoinduced modification of surface states in nanoporous InP observed by terahertz spectroscopy

A precise control of the surface properties of semiconductor nanomaterials is vital for their functionality and use in many opto-electronic applications. Terahertz time-domain spectroscopy allows the non-contact investigation of electron transport in semiconductor nanomaterials, without the complication of contact fabrication. The technique allows the photoconductivity to be determined on picosecond timescales, under the assumption that the material's properties are not permanently altered by photoexcitation. Here we demonstrate that this assumption is not always valid. We report an investigation of nanoporous honeycombs of n-type InP using terahertz time-domain and X-ray photoemission spectroscopies. After photoexcitation the dark conductivity was found to increase quasi-irreversibly, recovering only after several hours in air. The calculated electron density for different surface pinning energies suggests that photoexcitation may reduce the density of surface states. The photoinduced modification of porous semiconductors may be useful in material processing as it is a clean, dry, and area-selective method to increase the conductivity.   

Delayed photoluminescence in three-dimensional silicon/silicon germanium nanostructures

In three-dimensional (3D), SiGe nano-island multi-layers separated by nanometer-thick Si layers with the enhanced local strain field visualized by transmission electron microscopy (TEM), we find unusual low temperature photoluminescence (PL) dynamics. The PL detected at 1350 nm rises practically instantly and decays with a lifetime faster than 20 nanoseconds. In contrast, the PL detected at 1550 nm has a rise time of longer than 3-4 microseconds, and it decays with a characteristic lifetime which changes from 10 microseconds to milliseconds. The proposed model considers recombination of excitons bound to SiGe/Si interface as a mechanism responsible for the fast PL at 1350 nm. The observed slow rising PL with a peak near 1550 nm is associated with Auger ionization of SiGe clusters and separation of electrons and holes followed by carrier/exciton diffusion within Si layers toward a longer wavelength luminescence sites. These sites are associated with SiGe cluster areas containing a higher Ge concentration, and they are detected by analytical TEM.   

Transport of exciton-polariton condensation in semiconductor microcavity at BEC-BCS crossover

We study the transport properties of exciton-polariton condensation in the microcavity at the BEC-BCS crossover. Exciton-polariton (EP) is the quasiparticle of exciton and photon that can condense at a temperature as high as room temperature due to its extreme lightness. So far intense studies of this intriguing condensation have been limited to the photoluminescence (PL) measurement, mostly at the lower density BEC regime. When increasing density, the condensate enters the EP BCS regime in which the fermionic nature of electron and hole inside of exciton becomes important. This electron-hole nature is more prominent in the transport than in the PL measurements. We propose a transport measurement scheme of creating indirect-excitons in semiconductor bilayer which is embedded in the planar microcavity filled with photons. Leads are attached to the two sides of the bilayers to perform transport measurements. We present the different features of tunneling conductance signature for EP condensation in the BCS and in the BEC regime.