Discovery of Designed Nanostructures for Energy Conversion and Optoelectronic Devices

Dr. Mu's and his nanomaterials research team have directed their research on “Discovery of Designed Nanostructures for Energy Conversion and Optoelectronic Devices” as a part of the CREST center. The project consists of three research subareas:

• Surface plasmon – exciton and surface plasmon – rare earth (RE) ion interactions;
• Growth and functionalization of ZnO nanostructures for energy application; and
• Exploring the possibility of using pulsed electron-beam deposition (PED) and pulsed laser deposition (PLD) to directly deposit QDs onto nanostructured surfaces, CdTe QDs on ZnO NWs, for example, to substantially change and control electronic, magnetic, and optical (EMO) properties.

Surface Plasmon – Exciton or RE Ion Interactions in Semiconductors and Glasses

Research Objectives

• Study energy transfer and dynamics between luminescent emitters: excitons in ZnO and localized surface plasmons (LSPs) and surface plasmon polaritons (SPPs) in nanostructured materials.
• Determine how metal nanoparticles embedded in transparent dielectrics (which exhibit a localized surface plasmon resonance (LSPR)) respond to visible light.
• Characterize the fundamental mechanisms that result in optical enhancement and quenching of luminescent emitters (excitons for ZnO, Tb ions in glasses) when interacting with visible light.

  The Study of ZnO/Ag and ZnO/MgO/Ag Structures

  • Research Achievements:Through use of variable thickness MgO spacers within a ZnO/MgO/Au multilayer structure, successful decoupling of the enhancement and quenching mechanisms was achieved. The band edge PL enhancement is a result of hot electron transfer from metal nanoparticles to the ZnO conduction band edge. The enhancement of the visible luminescence occurs through dipole-dipole scattering with LSPs supported on metal islands and asperities on metal films. The Purcell enhancement of the ZnO band-edge luminescence is observed on 30 nm Ag films. The scattering of SPPs to photons occurs in structures where the evanescent waves have decayed sufficiently not to cause Purcell enhancement of the band-edge emission. The slow decay rate of the pump-probe signal for bare ZnO films dominates the fast rate as the MgO thickness increases.
  • 4 research publications: Optics Express 17 2565-2572 (2009); Thin Solid Films, 518, 4637 (2010); physica status solidi (c), 8, 159 – 162 (2011);Optical Lett (Accepted, 2012).
  • Education Achievements:
    • 1 PhD Degree awarded: Dr. Benjamin John Lawrie, Postdoc @ ORNL (Masters and PhD Degree in 5 years)
    • 2 Master Degree Awarded: Mr. Ben Lawrie and Mr. Daniel Mayo (Accepted to PhD program in Prof. Haglund group at Vanderbilt)
    • 2 for 2 from MA-to-PhD. Reason for success: Effective research alignment and seamless research collaboration.

  The Study of RE ion (Er3+ and Tb3+) Doped Glasses

  • Research Achievements: We have studied Er-doped Ge-Ga-S (GGS) glass containing silver NPs and Tb-doped 28Li2O-11LaF2-6Al2O3-55SiO2 (LLAS) glass containing Ag NPs to examine visible and infrared to visible up conversion luminescence of Er3+ in comparison with samples without silver co-doping. In the Tb3+ co-doped system, we examined the possible mechanisms for plasmonic enhancement and quenching effects on Tb3+ luminescence. For the Er co-doped system, the largest enhancement was observed for an excitation wavelength of 488 nm, which is near the peak of the surface plasmon resonance (SPR) of silver NPs in GGS glass and in resonance with Er3+ ions. The observed enhancement of Er3+ emission is mainly attributed to the local field enhancement on the excitation and emission of Er3+ ions provided by the SPR excitation. In the case of Tb co-doped in LLAS glass, both optical enhancement and quenching of Tb3+ luminescence were observed for glass containing Ag NPs at certain excitation wavelength regions, dependent on the Ag doping concentration and annealing time. The luminescence enhancement in the presence of Ag NPs is attributed to local field effects due to SPR of Ag NPs. Ag NPs in the glass may also participate in an observed luminescence quenching for excitation wavelengths from 345 to 380 nm and 484 nm, particularly at the four resonance wavelengths of Tb3+ ions.
  • 3 Journal Publications: J. Nanophotonics, Vol. 4, 043522 (1 December 2010); J. Non-Cryst Solids 356, 1097 (2010); SPIE Optics+Photonics Conference Proceedings 7757 DOI: 10.1117/12.863061 (2010).
  • Education Achievements:
    • 2 Master Degrees awarded in Dec 2011: Mr. Patryk Piasecki and Ms. Ashley Piasecki
    • 2 Undergrad REU students participated and were trained.

  Growth of ZnO Nanostructures

  The team has established a solid foundation for ZnO nanostructure vapor-liquid-solid growth with a primary focus on size, shape, and density control. Considerable progress has been made in growing ZnO nanostructures with our custom designed two-zone furnace. We can now routinely grow ZnO nanostructures with good control of the shape and orientation. More work is needed under the PREM to control nanowire size, nanowire spacing (density), and length over tens of nanometer scales.

  Understanding of Pulsed Electron-beam Ablation Processes

  Our previous study with pulsed electron beam deposition shows it to be a valuable technique for high performance nanostructure synthesis, and it will be applied to such studies in the present PREM proposal. We will also develop other techniques, which are applicable to high quality nanomaterials.

  Understanding of PED Ablation/Deposition Mechanisms

  • Deposition (PED) – focusing on plume dynamics and film uniformity:To avoid complexity in data analysis, pure Zn has been used as a target for this study. We found the profiles to be asymmetric along one axis due to interaction with the charged source. Along the other orthogonal axis we found profiles to be described well by distributions commonly used in laser ablated plasma expansion. The profiles were broader than those typically found in laser depositions, reflecting the increased penetration depth of the electron pulse as compared to an optical pulse. Parameters that control profile broadening were found to decrease with increasing target-substrate distances, indicating interaction with the ambient backing gas. Optimal growth rates were found to decrease linearly versus distance for target-substrate distances less than 5 cm, due to the virtual one-dimensional expansion caused by the aspect ratio of the initial ablated spot.
  • Deposition – focusing on stoichiometry: Based on the understanding of plume dynamics from Zn studies, we then began investigations of binary systems of CdTe and PbTe? Preservation of stoichiometry is not a primary concern since the mass ratio of Cd/Te is close to 1. In the case of PbTe, however, the mass ratio of Pb/Te is 1.63. This can result in a nonstoichiometric distribution on a substrate. Therefore, it was critical to investigate both how stoichiometry changes as a function of target-to-substrate distance as well as how deposition efficiency varies so that it becomes feasible to produce materials for device fabrication.
  • We found that spatial and compositional variations in pulsed electron-beam deposited films occur on the order of centimeters. The films have an elliptical distribution in both deposition thickness and composition. Variations are attributed to initial anisotropic velocities that arise from non-circular ablation spots and varying target masses that ultimately define the stoichiometry and intensity of the angular particle fluxes. We analyzed the deposition per pulse as a function of distance from the center of the film, displaying a clear linear dependence over short target-to-substrate distances in contrast to previous work. Finally, we have shown from a manufacturing standpoint that it is within the ability of the pulsed electron-beam deposition technique to deposit uniform films over large area.
  • 3 Research Publications: J. Vac. Sci. Technol. A 26, 513 (2008); J. Vac. Sci. Technol. B 26, 1001 (2008); Nanotechnology 20 465204 (2009).
  • Education Achievements:
    • 1 MA Degree awarded: Mr. Shawn Eastmond
    • 3 REU students trained.
    • 1 Postdoc Trained: Dr. Akira Ueda.

  Functionalization of ZnO Nanostructures for Solar Cells

  • Functionalization of ZnO Nanowires with CdTe Quantum Dots with PED CdTe QDs have been successfully deposited on ZnO-NWs via PED, resulting in an enhanced photosensitization effect. This effect is due to photon-induced charge transfer from CdTe to the ZnO and results in increases in efficiency in ZnO nanowire-based PVs. While this preliminary work shows the potential value of this system, more research is needed to understand the nature of the enhancement effects, to characterize optical absorption and photoluminescence and photoluminescence quenching. Time resolved studies are critical to developing a complete picture of energy and charge transfer between dot and wire. Additional study to understand the pulsed electron-beam ablation physics, which also is an important step to fully explore the potential of the PED technique, is also key. This area of research will be one of the central tasks in the proposed PREM.
  • Photocurrent Enhancement due to the Insertion of CdTe Nanolayer in OPV Cells The insertion of nc-CdTe: 1) improves the diode behavior of the devices; 2) yields better photovoltaic response; and 3) promotes better charge transport as exhibited by the fast photoresponse of the short circuit current. The advantage of inserting nc-CdTe as an interlayer is its contribution to the short circuit current. Due to its lower band gap energy (Eg = 1.45 eV), it scavenges photons that are not absorbed by the nanocomposite active layer. The external quantum efficiency (EQE) of PV devices is considerably increased at longer wavelengths with nc-CdTe. Without the nc-CdTe (reference) the EQE drops rapidly for wavelengths greater than around 650 nm, coinciding nicely with the drop in absorbance. The results suggest the possibility of broadening the wavelength sensitivity of polymeric-inorganic nanostructured solar cells via insertion of a CdTe nanolayer.
  • 2 Journal Publications: Nanotechnology 20 465204 (2009); J Phys. Condens Matter 20, 385206 (2009);
  • 1 Provisional patent filing on “Optical Cavity Enhanced Photovoltaic Device” (2010)
  • Education Achievements:
    • 4 MA Degrees awarded: Ms. Darlene Gunther and Mr. Anthony Mayo
    • 1 Postdoc Trained: Dr. Roberto Aga

  Summary of Boarder Impacts in Nanomaterials Research Teams alone with Prior Support

  From the establishment of the NSF CREST center in 2004, 19 master degrees have been awarded at Fisk and 14 of them have moved into PhD programs. The success rate is 74% for Masters to PhD transition. However, 7 MA awarded since 2009 and 8 moved on to PhD and one is working at Army Corp Engineer. Further, two PhDs have been awarded, which were co-advised by the PI and Professors Tolk and Haglund at Vanderbilt University. Two postdoctoral associates have been trained and have moved on to research labs and academia.

  The list below gives the PhD programs, post-doctoral appointments, and industrial or educational positions these students and post docs have moved into following their time at Fisk:

  • PhD Degrees Awarded (2)
    • Benjamin Lawrie (2011 PhD@VU, Postdoc ORNL, MA & PhD in 5 years)
    • Andrew Steigerwald (2010 PhD@VU, Postdoc VU, MA & PhD in 4 years)
  • Master Degree Awarded (7)
    • Jennifer Jones (2011, VU, PhD, IMS)
    • Daniel Mayo (2011, VU, PhD, IMS)
    • Patryk Piasecki (2011, PhD UDayton)
    • Ashley Piasecki (2011, @Army Corp Engineer)
    • Shawn Eastmond (2010, Howard U, PhD, Mat Sci)
    • Joseph Sturgess (2009, Case PhD, Mat Sci)
    • Darlene Gunther (2009, VU, PhD, Chem Eng)
  • Postdocs Supervised and Trained:
    • Dr. Akira Ueda, Research Associate Professor at Fisk University
    • Dr. Roberto Aga, Research Staff at Wright Patterson Research Lab

  Future Direction

  1. Surface Plasmon – Exciton Interactions
     • To go beyond recent experiments in several groups on self-assembled metal nanoparticles and rough metal films by fabricating ZnO-metamaterial heterostructures that can support either localized surface plasmons or surface plasmon polaritons, then use these structures to elucidate the energetics and dynamics of band-edge and DAP luminescence in the presence of the metallic substrates.
     • To measure the effects of the ZnO exciton-plasmon coupling on excited-state lifetimes for both the band-edge emission and the DAP luminescence, and to investigate the effects of impurities in the ZnO and the distance between the ZnO and the plasmonic material on the dynamics and quantum efficiencies of the coupling between emitter and plasmon.
     • To demonstrate light-induced modulation — real-time control — of the exciton-plasmon coupling in two ways, first, by direct electrical modulation of the optical excitation of the exciton or other emitting species, and second by taking advantage of the dynamical control achievable through proper design of metamaterials structures built on ZnO substrates and making use of ZnO optical gain.
  2. ZnO Nanowire based PV program:
     • Development of polymeric nanocomposite based cost effective, flexible, and high solar cells with specific research focus on:
     • Enhancing solar energy absorption quantum efficiency and minimize the active layer thickness via inclusion of inorganic nanomaterials, such as semiconductor quantum dots and metal nanparticles
     • Improving electron-hole separation efficiency through creating large materials interface
     • Increase charge transport efficiency by doping high mobility inorganic nanofibers, such as ZnO nanowires
     • Development of “inverted” solar cell structures where the top electrode is transparent conducting oxide. The inverted solar cells will allow us to fabricate self-powered, monolithic optoelectronic devices and sensors.
     • Establishment of basic and comprehensive materials synthesis, characterization, and device fabrication and evaluation infrastructure so that the program becomes an effective incubator to train a broad range of students with STEM background.
  3. Development of New Theme of Quantum Cutting Strategy Based on Rare Earth Ion Doped Glass Matrix (With Steve Morgan Group)
     • Fabrication of rare-earth doped glass and nano-crystals embedded transparent glass-ceramics (TGCs) The team will fabricate rare-earth doped oxyfluoride glass and glass-ceramics containing fluoride nano-crystals, which host the rare-earth ions. The growth of the fluoride nano-crystals in the glass is controlled by appropriate heat treatment. The obtained oxyfluoride TGCs will have very low levels of the scattering losses [12]. Based on our previous work, we will fabricate alumino-silicate based TGCs system containing LaF3/CdF3/PbF2 nano-crystals. Further development will be dependent on the results for improving the performance.
     • Down-conversion quantum cutting (QC) investigation: Based on previous work, we will first investigate QC on Tb3+ and Yb3+ co-doped SiO2-Al2O3-Li2O-LaF3 system, with absorption in 480 – 488 nm range and QC emission in 940 – 1020 nm range. Further choice of doping ions and host materials is for combining a strong absorption in 350 – 550 nm range and an efficient QC process to emit light in 700 – 1700 nm range.
     • Frequency up-conversion investigation: Based on previous work, we will first investigate on Er3+ doped SiO2-Al2O3-Li2O-LaF3 system, with absorption at ~ 800, ~ 980, ~ 1500 nm and emission at ~ 410, 530, 550, 660, 810, 980 nm and Yb3+ and Tb3+ or Tm3+ co-doped SiO2-Al2O3-Li2O-LaF3 system, with absorption at ~ 980 nm and emission in 375 – 675 nm range. Further choice of doping ions and host materials is for combining a strong absorption in 800 – 2500 nm range and efficient upconversion process to emit light in 400 – 1100 nm range.