Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session S15: Focus Session: Transport in Ensemble of Nanocrystals |
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Sponsoring Units: FIAP Chair: V. Klimov, LANL Room: LACC 405 |
Wednesday, March 23, 2005 2:30PM - 2:42PM |
S15.00001: Charge transport properties of nanocrystals studied by electrostatic force microscopy Zonghai Hu, Michael Fischbein, Hugo Romero, Marija Drndic Charge transport in semiconductor and metal nanocrystal multilayers between two electrodes is probed by electrostatic force microscopy. The in-plane charge diffusion coefficients are deduced from the charge distribution imaged in real time. Temperature dependence of the transport properties and effects of photoionization and oxidation are also investigated. Implications of these results on the transport mechanisms will be discussed. This work was supported by the ONR Young Investigator Award N000140410489, the American Chemical Society (ACS) PRF award {\#} 41256-G10, and the startup funds at the University of Pennsylvania. MF acknowledges funding from the NSF IGERT program (Grant {\#}DGE-0221664) and SENS. [Preview Abstract] |
Wednesday, March 23, 2005 2:42PM - 2:54PM |
S15.00002: Memory Effects in CdSe Nanocrystal Quantum Dots Michael Fischbein, Arif Shiliwala, Marija Drndic Memory effects in the charge transport in arrays of CdSe nanocrystals have been observed and characterized. These semiconducting colloidal quantum dots have previously been shown to demonstrate a non-steady state current transient response to the application of a constant negative source-drain voltage bias. In this study we have shown that CdSe nanocrystals display memory of the voltage pulses applied to them. In particular, for a sequence of two negative voltage pulses, the nanocrystals' response to the second pulse will be dependent on the value and duration of the first pulse. We define the first voltage pulse as the ``write'' step and the second voltage pulse as the ``read'' step. To probe the programmability of the nanocrystals, a range of different write steps were performed and the current transients generated by the read steps were characterized. We have demonstrated the ability to undo the effect of the write steps by either shining band gap light on the nanocrystals or by applying a positive voltage bias; such events are naturally defined as ``erase'' steps. The full write-read-erase cycle demonstrates the potential for the application of CdSe nanocrystals to memory technology and offers new information on the charge transport. * This work is supported by the ONR Young Investigator Award {\#} N000140410489, the American Chemical Society PRF award, and the startup funds at Penn. MF acknowledges funding from the NSF IGERT Program. [Preview Abstract] |
Wednesday, March 23, 2005 2:54PM - 3:06PM |
S15.00003: Photoconductivity Studies of Treated CdSe Quantum Dot Films Exhibiting Increased Exciton Ionization Efficiency Venda Porter, Mirna Jarosz, Brent Fisher, Marc Kastner, Moungi Bawendi We present a photocurrent study of CdSe quantum dot films exhibiting unity internal quantum efficiency as a result of post-deposition treatments. While the photocurrent of untreated films is highly voltage dependent at all voltages, the treated films depend strongly on voltage at low voltage, linearly with voltage above a voltage threshold, and finally saturate at high voltage. The voltage dependence of the treated films can be reproduced with a model assuming blocking contacts and a field dependent exciton ionization efficiency that saturates to unity. The increase in exciton ionization efficiency is a result of increased surface passivation and decreased QD spacing. [Preview Abstract] |
Wednesday, March 23, 2005 3:06PM - 3:42PM |
S15.00004: Charging of Colloidal Quantum Dots. Spectroscopy and Transport Invited Speaker: Colloidal inorganic semiconductor nanocrystals promise optimized electronic, optical and magnetic properties using size, shape and composition control. It is also interesting to look at the effect of a few charges in such small structures, particularly when placed in the quantum dot states. We observed that charging the colloidal quantum dots leads to electrochromic response covering the visible and IR spectral ranges via changes in the available interband and intraband transitions. In particular, charging the dots leads to reduced absorption at the band edge and this affords a lowered threshold for stimulated emission. Charging also a-priori saturates traps that otherwise impede transport and charged films of quantum dots are indeed observed to be conducting. Shell to shell ( S or P) transport has been observed and, to date, the films conduct via a variable range hopping mechanism in the Coulomb-gap regime. These findings will possibly impact the use of colloidal quantum dots in their opto-electrical applications and they more generally provide support to the growing effort to generate novel materials based on self-assembled organic/inorganic nanostructures. [Preview Abstract] |
Wednesday, March 23, 2005 3:42PM - 3:54PM |
S15.00005: Electron transport of n-type semiconductor nanocrystal thin films Dong Yu, Congjun Wang, Brian Wehrenberg, Philippe Guyot-Sionnest The conductivity of thin films of n-type colloidal CdSe nanocrystals increases by up to 12 orders of magnitude as the occupation of the first two electronic shells 1Se and 1Pe increases, either by potassium or electrochemical doping. \textit{[D. Yu, C. Wang, P. Guyot-Sionnest, Science 300, 1277 (2003)}] In the low electrical field regime, the conductivity follows $\sigma \sim \exp (-(T^\ast /T)^{1/2})$ in the temperature range 10K$<$T$<$120K. At higher electrical field, the conductivity becomes strongly field dependent. At 4K, the conductance increases by eight orders of magnitude over one decade of bias. At extremely high field conductivity becomes temperature independent, where $\sigma \sim \exp (-(E^\ast /E)^{1/2})$. The conduction behavior follows Efros {\&} Shklovskii's variable range hopping model with Coulomb gap very well and the parameters determined by experiment agree well with the theoretical prediction. \textit{[D. Yu, C. Wang, B. Wehrenberg, P. Guyot-Sionnest, Phys. Rev. Lett. 92, 216802 (2004)]} Our current interest is to dope magnetic impurities like Mn$^{2+}$ inside the NCs. The doped magnetic spins provide strong local magnetic field and large magnetoresistive effect is expected. [Preview Abstract] |
Wednesday, March 23, 2005 3:54PM - 4:06PM |
S15.00006: Probing electronic transport in SiO2 films containing Si nanocrystals by conductive atomic force microscopy Tao Feng, Harry Atwater Understanding the mechanisms of charge injection into Si nancrystals embedded in SiO2 is the central issue governing device performance in Si nanocrystal nonvolatile memories. We use conductive atomic force microscopy (c-AFM) to study transport in partially etched SiO2 films containing Si nanocrystals fabricated by Si ion-implantation into a 15 nm SiO2 layer on p-Si (001). The 2$\sim $3 nm size nanocrystals were identified by scanning tunneling microscopy. In the c-AFM experiments, correlation between morphologies and tunneling current images shows dependence of tunneling currents on SiO2 thickness fluctuation. Highly localized tunneling paths were recorded and attributed to localized state-assisted tunneling through nanocrystals and/or defects. The electron tunneling current changes much more rapidly with SiO2 thickness than the hole tunneling current, a result explained by the combination of Fowler-Nordheim tunneling and inversion layer formation in the channel for electron tunneling. Current-voltage spectroscopy data showing resonant tunneling and itinerant single electron storage will be presented and discussed. [Preview Abstract] |
Wednesday, March 23, 2005 4:06PM - 4:18PM |
S15.00007: Controlling the Assembly of Nanorods Inside Electronic Devices Dong Tran, Hugo Romero, Zonghai Hu, Michael Fischbein, Marija Drndic Semiconductor nanorods are versatile nanostructures with exceptional electrical and optical properties that can be exploited for their applications as functional nanoscale devices. The manipulation and assembly of nanorods inside electronic devices are crucial for the study and fabrication of nanoelectronics. Here we present a simple technique to align colloidal suspensions of CdSe nanorods across lithographically pre-patterned metal electrodes on silicon nitrite substrates by an ac electric field. We synthesized CdSe nanorods with diameters of a few nanometers by a conventional chemical technique and the assembly is characterized by AFM and TEM. We probed the nanorod assembly at different frequencies of the applied ac E-field. [Preview Abstract] |
Wednesday, March 23, 2005 4:18PM - 4:30PM |
S15.00008: Electrical Transport Properties of CdSe Nanorod Solids Hugo Romero, Marija Drndic Semiconductor nanorods are of great interest for fundamental research because they allow us to study how the electronic and optical properties of semiconductor nanocrystals depend on their shape. Nanorods have also attracted much attention because of their potential applications in light-emitting diodes, in low-cost photovoltaic devices, and their propensity to form liquid crystalline phases. So far, most of the studies have focused on the electrical transport in ``spherical'' nanocrystals, where transport mechanisms from variable-range hopping to Coulomb-glass-like behavior have been reported. Compared with nanocrystals, nanorods are expected to exhibit interesting anisotropic effects. We have chemically synthesized the CdSe nanorods and integrated them into electronic devices. Here, we report on our experimental studies of the charge transport in these CdSe nanorod solids. [Preview Abstract] |
Wednesday, March 23, 2005 4:30PM - 4:42PM |
S15.00009: DC Electrical Transport Properties Of PbSe Nanocrystal Quantum Dot Solids Hugo Romero, Marija Drndic We have studied temperature-dependent electronic charge transport in three-dimensional, closed-packed arrays of PbSe colloidal nanocrystals in the form of thin disordered films. PbSe nanocrystal quantum dots offer unique access to the regime of extreme quantum confinement because of the large Bohr radii of electrons and holes. These materials are expected to have significantly different physical properties from those of the better-known II-VI (CdSe) nanocrystals. Current-voltage characteristics of PbSe nanocrystal arrays show a variety of phenomena, which can be well described using the framework established in the context of transport measurements in metallic quantum dots. [Preview Abstract] |
Wednesday, March 23, 2005 4:42PM - 4:54PM |
S15.00010: Integrated colloidal nanocrystal and epitaxical quantum nanostructures Atul Konkar, Siyuan Lu, Yi Zhang, Anupam Madhukar Colloidal semiconductor nanocrystals (NCs) and epitaxically grown semiconductor quantum wells, wires, and dots represent two classes of quantum nanostructures which currently serve complementary purposes, the former being well suited for biological applications the latter for electronic and optoelectronic systems as applied to non-hazardous environments. In this work we report on the integration of InAs NCs with InGaAs/GaAs epitaxy based nanostructures through overgrowth on the NCs. We report on the cleaning conditions needed to remove the chemical contamination arising from the solvent during deposition of the NCs on GaAs, the reduction of the as-deposited NC sizes due to the Kelvin effect during thermal treatment, and the GaAs molecular beam epitaxical overgrowth. High-resolution transmission electron microscopy and photoluminescence spectroscopy examinations reveal high quality overgrowth, thus opening the study of a new class of integrated quantum nanostructures that can provide unprecedented functionalities not to be found in either component. Work supported by DARPA/AFOSR under the DURINT program. [Preview Abstract] |
Wednesday, March 23, 2005 4:54PM - 5:06PM |
S15.00011: Thermopower Enhancement in PbTe with Pb Precipitates Joseph Heremans, Christopher Thrush, Donald Morelli The thermoelectric power of polycrystalline PbTe samples containing nanometer-sized precipitates of Pb metal is enhanced over that of bulk PbTe. Samples of PbTe containing excess Pb and Ag were prepared using conventional metallurgical heat treatments. These samples are shown, by X-ray diffraction, by microscopy, and by the presence of a superconductive transition, to contain Pb precipitates with sizes on the order of 30 to 40 nm. The thermopower enhancement is related to an increase in the energy-dependence of the relaxation time, as evidenced by a complete set of measurements of thermoelectric and thermomagnetic transport coefficients. [Preview Abstract] |
Wednesday, March 23, 2005 5:06PM - 5:18PM |
S15.00012: Fully Ordered and Nano-Structured Inorganic-Organic Hybrid Semiconductors Yong Zhang, G. M. Dalpian, B. Fluegel, S.-H. Wei, A. Mascarenhas, X.-Y. Huang, J. Li A family of novel inorganic-organic hybrid nanostructures based on II-VI semiconductors has been synthesized, including the first monolayer inorganic/organic superlattices with all covelent bonds (3D structures) and the smallest quantum wires (1D), the chains being formed of single II-VI atomic bonds [1]. These materials are atomistically reassembled crystals without the structural fluctuation typically found in other nanostrutures, and exhibit a number of remarkable properties (e.g., a giant bandgap tunability of 1-2 eV [1,2]). As a prototype system, a 3D structure $\beta $-ZnTe(en)$_{0.5}$ shows a strongly enhanced free exciton absorption (a few times of that in the II-VI binary), Raman lines as sharp as any binary semiconductor, band edge free exciton emission, and more than 10 times enhancement in the exciton binding energy. First-principles density function band structure calculations have been performed to obtain the band gap shift, dispersion relations (effective masses), and dielectric constants of the hybrid material, and the relevant band offsets. [1] X.-Y. Huang, J. Li, Y. Zhang, and A. Mascarenahs, JACS 125, 7049 (2003). [2] B. Fluegel, Y. Zhang, A. Mascarenahs, X.-Y. Huang, and J. Li, PRB 70, 205308 (2004). [Preview Abstract] |
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