Science Highlights from 2010

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Structural Order-Disorder Transitions and Phonon Conductivity of Partially Filled Skutterudites

Hyoungchul Kim, Massoud Kaviany, John C. Thomas, Anton van der Ven, Ctirad Uher, and Baoling Huang

Physical Review Letters, 105, 265901 (2010)

Filled skutterudites are among the most promising of novel thermoelectric materials for power conversion applications. Their effectiveness can be further improved by finding ways to reduce their thermal conductivity. While the filler species strongly interact with normal phonon modes and thus reduce transport of heat, how exactly the filler atoms are distributed on the intrinsic void sites of the structure and what order/disorder configurations are possible has not yet been considered. Using ab initio calculations and cluster expansion methods, we calculated the phase diagram for Ba-filled CoSb3 and carried out molecular dynamics simulations of the heat transport that indicated a dramatic reduction in the thermal conductivity in the two phase mixture regime of the phase diagram. This pathbreaking work points out new opportunities for reducing heat transport and thus improving the figure of merit on skutterudites with the prospect of developing even more efficient thermoelectric materials.


Coherent Phonon Optical Spectroscopy Studies of Femtosecond-Laser Modified Sb2Te3 Films

Yuwei Li, Vladimir Stoica, Lynn Endicott, Guoyu Wang, Ctirad Uher, and Roy Clarke

Applied Physics Letters, 97, 171908 (2010)

Sb2Te3 is a key material for thermoelectric energy conversion technology. We have found that the crystal structure of Sb2Te3 thin films can be modified using high intensity laser light. Such changes may provide a new route to enhancing the thermoelectric properties of this important class of chalcogenide thermoelectrics. In a recent publication in the journal Applied Physics Letters, we have reported on time-resolved measurements of the lattice modifications using ultrafast pump-probe spectroscopy. In this work we monitor changes in the coherent optical phonon vibrational spectrum under femtosecond laser irradiation. We found that a phonon mode at 3.64 THz appears after high-fluence laser irradiation, in addition to the phonon modes of Sb2Te3. We determined that the additional mode is due to Te segregation resulting from laser-induced decomposition of the Sb2Te3 film. This experiment clearly illustrates the irreversible effects of femtosecond laser irradiation during the measurement of coherent optical phonon dynamics in Sb2Te3. Future studies include a more detailed investigation of the laser modifications to the structure and its effects on the thermal and electrical transport properties.


Surface Plasmon Mediated Energy Transfer of Electrically-pumped Excitons

Kwang Hyup An, Max Shtein, and Kevin P. Pipe

Optics Express, 18, 4041 (2010)

In this manuscript we report for the first time on strong surface plasmon polariton-mediated transfer of energy between electrically pumped molecular excitons, across the metallic cathode of an organic heterostructure generating the donor excitons at room temperature. The donor molecular excitons at the organic heterojunction resonantly excite surface-plasmon modes on both sides of the optically thick metal electrode, while the modes evanescently couple to dye molecules near the electrode’s exterior surface. Dye fluorescence in the capping layer on the exterior of the device shows a seven-fold increase in intensity due to this effect. We model the process in detail, showing how the coupling efficiency can be controlled via the geometry of the device. Our successful demonstration of this energy transfer mechanism for electrically-pumped excitons suggests new sensing and imaging applications with high signal to noise ratio, and new routes for performance improvement in organic LEDs, energy harvesting devices, and plasmonic devices.


Filler-reduced Phonon Conductivity of Thermoelectric Skutterudites: Ab initio Calculations and Molecular Dynamics Simulations

Baoling Huang and Massoud Kaviany

Acta Materialia, 58, 4516 (2010)

The phonon conductivities of CoSb3 and its Ba-filled structure Bax(CoSb3)4 are investigated using first-principle calculations and molecular dynamics (MD) simulations, along with the Green–Kubo theory. The effects of fillers on the reduction of the phonon conductivity of filled skutterudites are then explored. It is found that the coupling between filler and host is strong, with minor anharmonicity. The phonon density of states and its dispersion are significantly influenced by filler-induced softening of the host bonds (especially the short Sb–Sb bonds). Lattice dynamics and MD simulations show that, without a change in the host interatomic potentials, the filler–host bonding alone cannot lead to significant alteration of acoustic phonons or lowering of phonon conductivity. The observed smaller phonon conductivity of partially filled skutterudites is explained by treating it as a solid solution of the empty and fully filled structures.


Ultrasonic Assisted Nano-Dimensional Self-Assembly of Poly-3-hexylthiophene for Organic Photovoltaic Cells

Bong-Gi Kim, Myung-Su Kim, and Jinsang Kim

ACS Nano, 4, 2160 (2010)

We have devised the sonication-assisted self-assembly of P3HT together with PC61BM in a co-solvent system containing acetonitrile, a polar solvent, and made P3HT nanowires having a diameter of 40 – 50 nm. There are several factors which affect the self-assembly of P3HT, such as solvent polarity, ultrasonic irradiation, the illumination condition, and the regio-regularity of P3HT. As the regio-regularity increased, more effective nanowire formation was observed, as evident in the UV-Vis spectrophotometry and x-ray diffraction analysis. Thermal annealing enhanced further the packing of the nanofibers. Photovoltaic cells having a ITO/PEDOT-PSS/P3HT-PC61BM/Al structure were fabricated from the self-assembled P3HT NW/PC61BM and a homogeneous P3HT/PC61BM solution and their device performance was compared and analyzed. Devices made of the self-assembled P3HT nanowires/PC61BM showed better Jsc, series resistance, fill factor, and power conversion efficiency. Thermally annealed photovoltaic cells having the self-assembled 98% regio-regular P3HT nanowires/PC61BM by the sonication-assisted self-assembly achieved 4.09% power conversion efficiency. The devised sonication-assisted self-assembly method of P3HT is a promising tool to prepare well-defined P3HT nanowires together with other additives and is useful for various organic electronic applications and suitable for roll-to-roll mass-production because it can obtain fully assembled P3HT without long-time annealing process.


Nanoscale Thermometry Using Point Contact Thermocouples

Seid Sadat, Aaron Tan, Yi Jie Chua, and Pramod Reddy

Nano Letters, 10, 2613 (2010)

In this work, we demonstrate an atomic force microscope (AFM)-based technique capable of mapping temperature fields in metallic films with 10 mK temperature resolution and <100 nm spatial resolution. A platinum-coated AFM cantilever placed in soft mechanical contact with a metallic (gold) surface is used to sequentially create point contact thermocouples on a grid. The local temperature at each point contact is obtained by measuring the thermoelectric voltage of the platinum−gold point contact and relating it to the local temperature. These results demonstrate a direct measurement of the temperature field of a metallic surface without using specially fabricated scanning temperature-probes. We believe that this technique will enable a variety of studies aimed at understanding energy dissipation at the nanoscale which is essential to develop novel “nano-engineered” energy conversion devices that are efficient and inexpensive. Almost all energy dissipation processes have an associated thermal signature, therefore, the ability to probe temperature fields of nanometer-sized devices is critical to understand energy dissipation at the nanoscale. It is expected that this approach will enable us to obtain important insights into energy dissipation at the nanoscale.


Probing Thermoelectric Properties of Self-Assembled Monolayers

Aaron Tan, Seid Sadat, and Pramod Reddy

Applied Physics Letters, 96, 013110 (2010)

It is widely believed that understanding the transport of charge and energy at the nanometer length scale holds the key to the creation of novel energy conversion devices. In order to probe the electronic structure and thermoelectric properties of metal-molecule-metal junctions, we have developed a novel scanning probe technique: thermoelectric atomic force microscopy (ThAFM). Using this technique we concurrently measure the Seebeck coefficient and the current-voltage characteristics of a metal-molecule-metal junction to determine the identity and energetic separation of the molecular orbital closest to the electrodes’ Fermi level. Junctions created by contacting a gold-coated atomic force microscope tip with a monolayer of molecules assembled on a gold substrate were found to have a Seebeck coefficient of (+16.9 ± 1.4) µV/K. This positive value unambiguously shows that the highest occupied molecular orbital (HOMO) dominates charge transport. Further, by analyzing the current-voltage characteristics, the HOMO level is estimated to be ~0.69 eV with respect to the Fermi level. This technique, capable of identifying orbital alignment and measuring the thermoelectric properties of molecular junctions will be used to obtain valuable insights into charge transport in metal-molecule-metal junctions. We believe that such insights will enable us in the long-term to create novel nanostructured materials that will make possible a new generation of efficient and inexpensive thermoelectric energy conversion devices.


ZnTe:O for Intermediate Band Solar Cells

Weiming Wang, Albert S. Lin, Jamie D. Phillips, and Wyatt K. Metzger

Applied Physics Letters, 95, 261107 (2009)

Carrier generation and recombination processes of ZnTeO thin films are studied by time-resolved photoluminescence, where carrier lifetimes at oxygen states and the conduction band are inferred to be ≺1 μs and ≻100 ps, respectively. The radiative recombination coefficient for optical transitions from oxygen states to the valence band is extracted to be 1.2x10−10 cm3 sec−1 based on the excitation dependence of decay time constants. Rate equation analysis further suggests an increase in electron lifetime at the conduction band as oxygen states occupation is critical in achieving high conversion efficiency for solar cells based on multiphoton processes in these materials.