IRG 4 Intellectual Focus:
Nanomaterials and molecular assemblies have emerged as new architectures for designing more efficient light harvesting devices. We are developing a comprehensive approach to light harvesting that includes: a) the synthesis and integration of novel materials for light absorption, electron-hole separation, and charge transport; and b) fundamental theoretical modeling studies that will help to guide the experimental research and provide fundamental understanding to the corresponding experimental results.Multiferroics are promising materials for energy harvesting due to possessing ferroelectric properties. Moreover, these are noteworthy for their unique and strong coupling among electric, magnetic, and structural order parameters, giving rise and control of simultaneous ferroelectricity, ferromagnetism, and ferroelasticity. In particular, the ferroics possess magnetization and dielectric polarization, which can be modulated and activated by magnetic and an electric fields respectively. The coexistence of several order parameters brings about novel physical phenomena and offers possibilities for new device functions due to possible coupling among them. The interplay between charge, structure, and strain gives rise to interesting cross-coupled properties in polar oxides. Multiferroics are without a center of symmetry and due to this characteristics; the steady-state photocurrent can exist in a homogeneous medium under uniform illumination. Owing to the abrupt change of polarization at the domain walls, the divergence of the polarization is non-zero, and hence an imbalance of charge develops at the domain walls. The charges induce an electrostatic potential drop across the domain wall. The most outstanding advantage of ferroelectrics, therefore, is its high output voltage. Meanwhile, we target to develop large-area high-mobility graphene to be employed as the transparent conductive electrode of the light harvesting devices.
Faculty Participants: Zhongfang Chena, Peter X. Fenga, Luis F. Fonsecaa, Maxime Guinela, Yasuyuki Ishikawaa, Ram S. Katiyara, Junqiang Lub, Vladimir Makarova, Carlos Marinb, Gerardo Morella (IRG leader), Ratnakar Palaia, Raphael Raptisa, Luis Rosac, James F. Scotta, Maharaj Tomarb, Josee Vedrinec, Julian Veleva, Natalya Zimbovskayac
(aUPR Rio Piedras, bUPR Mayaguez, cUPR Humacao)
Strategic Partners: Dr. Evgeny Tsymbal, Dr. Peter Dowben, Dr. Alex Sinitskii and Dr. Barry Cheung, University of Nebraska at Lincoln
Postdoctoral Fellows: Yafei Li (Z. Chen), Tanaji Pralhad Gujar (R.S. Katiyar, till 9/2012), Nora Patricia Ortega (R.S. Katiyar, since 9/2012)
Graduate Students: Deepak Varshney, Ricardo Martínez, Rajesh Katiyar, Frank Mendoza, Kenneth Perez, Daniel Valencia, Juan Beltrán, Khaled Habiba, José Hernández, José López, Mohamad Sajjad, Rafael Velazquez, Luis Valentin, Jennifer Carpena, Jesuán Betancourt, Omar Vega, Kety Jiménez, Freddy Wong, Fengyu Li
Undergraduate Students: Samuel Escobar, Abelardo Colón, Emmanuel Febus, Abelardo Colón, Christopher A. Boocheciamp, Jennifer Gil, Laura Méndez, Mariel Jiménez, Sergio Méndez
Collaborations with Strategic Partners
Peter A. Dowben, University of Nebraska – Lincoln
Luis Rosa, UPR Humacao
Project: Weak screening of a large dipolar molecule adsorbed on graphene [Carbon, 50, 1981, 2012]
Summary: We compared the electronic structure of a quinonoid zwitterionic type molecule adsorbed on both gold and graphene on copper substrates. This (6Z)-4-(butylamino)-6-(butyliminio)-3-oxocyclohexa-1,4-dien-1-olate, C6H2(NHR)2(O)2 where R = n-C4H9, film is made of small molecules with a large intrinsic dipole of 10 Debyes. We find that the photoemission and inverse photoemission final states are well screened for these dipolar molecules on gold. This is not observed when they are adsorbed on graphene on copper. This weaker screening results in a larger highest occupied molecular orbital to lowest unoccupied molecular orbital gap for the molecules on graphene.
Alex Sinitskii, University of Nebraska – Lincoln
Gerardo Morell, UPR Río Piedras
Project: Synthesis of large area graphene for transparent electrodes
Summary: A doctoral student (F. Mendoza) did an internship at UNL to learn the technique for large area graphene in Sinitskii’s Lab. The student successfully synthesized large area graphene films that he brought back to UPR to be used as transparent electrodes in PV devices. Moreover, he was assigned the project to improve the transfer process of graphene from the substrate onto a polymer, which is a critical step in the device fabrication process. He was able to refine the graphene transfer technique to the advantage of Sinitskii’s research and to our own research. After coming back to UPR, the Mendoza was able to successfully adapt the graphene recipe to our graphene synthesis setup. We are currently manufacturing CVD-graphene monolayers in areas of approximately 6 cm2 suitable for device applications. The enhanced graphene transfer protocol was also implemented in our lab. This enables us to use the material as a transparent electrode for different applications.
Evgeny Tsymbal, University of Nebraska – Lincoln
Julian Velev, UPR Río Piedras
Project: Ferroelectric control of magnetocrystalline anisotropy at cobalt/poly(vinylidene fluoride) interfaces [ACS Nano 6, 9745-9750, 2012]
Summary: Electric field control of magnetization is one of the promising avenues for achieving high-density energy-efficient magnetic data storage. Ferroelectric materials can be especially useful for that purpose as a source of very large switchable electric fields when interfaced with a ferromagnet. Organic ferroelectrics, such as poly(vinylidene fluoride) (PVDF), have an additional advantage of being weakly bonded to the ferromagnet, thus minimizing undesirable effects such as interface chemical modification and/or strain coupling. In this work we use first-principles density functional calculations of Co/PVDF heterostructures to demonstrate the effect of ferroelectric polarization of PVDF on the interface magnetocrystalline anisotropy that controls the magnetization orientation. We show that switching of the polarization direction alters the magnetocrystalline anisotropy energy of the adjacent Co layer by about 50%, driven by the modification of the screening charge induced by ferroelectric polarization. The effect is reduced with Co oxidation at the interface due to quenching the interface magnetization. Our results provide a new insight into the mechanism of the magnetoelectric coupling at organic ferroelectric/ferromagnet interfaces and suggest ways to achieve the desired functionality in practice.
Barry Cheung, University of Nebraska – Lincoln
Luis Fonseca, UPR Río Piedras
Project: Thermoelectric Nanowires
Summary: Sm2S3 nanowires were received from Prof. Barry Cheung (University of Nebraska at Lincoln) for their thermoelectric characterization. This report includes details of the measurements done on 3 representative nanowires (NW1, NW2, and NW3). Each nanowire was placed on the device by micromanipulation. To ensure a good thermal and electrical contact between the device and the nanostructure, platinum-carbon patterns were deposited onto each of the four nanowire-Pt electrode contact regions by focused ion beam (FIB) deposition (FEI STRATA at National Center for Electron Microscopy at Lawrence Berkeley Nat. Lab.). The integrated nanowires are shown in the SEM images in Figures 1 a-c. The thermoelectric measurements were conducted following the four-probe measurement procedure reported in (Mavrokefalos et al. 2007, Zhou et al. 2007). In this method a temperature gradient is formed between the ends of the tested nanowire by Joule heating one side of the device. Two Seebeck voltages were directly measured: one between the external electrodes and other between the internal ones. These values are used to determine the thermal contact resistance to finally obtain the corrected thermal conductivity (k) and the Seebeck coefficient (S) of the nanowire. The electrical conductivity () was obtained by four point measurements. All measurements were done at different environment temperatures. After the thermoelectric characterization, each sample was moved from the cryostat to a TEM for crystal structure characterization. A through-substrate hole under the suspended device allowed HRTEM of the nanowire assembled between the two suspended membranes. Figures 1 a-c (right side) show HRTEM images and SAED patterns (inset) of each NW. NW2 and NW3 have good crystalline quality with no significant defects or dislocations and NW1 is polycrystalline in nature. Figure 2a shows the temperature dependence of the Seebeck coefficientas a function of temperature for each NW. All samples exhibit negative S that increases with T suggesting n-type doping. The Seebeck coefficients of NW1, NW2 and NW3 at room temperature were -22.57 μV/K, -30.54 μV/K and -39.58 μV/K respectively. Fig. 2b shows the electrical conductivity σ as a function of T for the three samples. In all cases, σ increases with T. Samples 2 and 3 have similar σ, which is about 5 times higher than the values for Sample 1. This observation can be attributed to the higher crystal quality of NW2 and NW3 as compared with NW1. Moreover, the electrical conductivity of these two nanowires is 3 times higher than previously reported bulk values (Golubkov et al. 2003).
The thermal conductivities plotted in Fig 2c show that NW2 and NW3 have κ-values in the range of 3.2-3.4 W/m-K at room temperature, which are four times greater than the thermal conductivity (0.84 W/m-K) for bulk Sm2S3 crystals (Gulubkov et al. 2003). The thermal conductivity of these samples decreases when temperature increases. Both observations suggest that phonon scattering is the dominant mechanism limiting the thermal transport and the boundary scattering is not dominant. As for NW1, the thermal conductivity increases as temperature increases, suggesting that other thermal transport mechanism such as dislocations are dominant as expected from the polycrystalline nature confirmed in the HRTEM image and the SAED pattern.
Fig 2d shows the temperature dependence of the figure of merit (ZT) for NW1, NW2 and NW3 respectively. All samples show increasing ZT with temperature, with a maximum value of 1.09 ´10-6 for NW1, 6.82 x10-6 for NW2 and 1.69 ´10-5 for NW3 at 370K. ZT values for NW3 are higher than for NW1, mainly due to the difference in their electrical conductivities but the relative large values observed in NW2 relative to NW3 are due to the higher Seebeck coefficients of the former.