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Title: Band Edge Energetics of Heterostructured Nanorods: Photoemission Spectroscopy and Waveguide Spectroelectrochemistry of Au-Tipped CdSe Nanorod Monolayers

Abstract

Conduction and valence band energies ( E CB, E VB) for CdSe nanorods (NRs) functionalized with Au nanoparticle (NP) tips are reported here, referenced to the vacuum scale. We use (a) UV photoemission spectroscopy (UPS) to measure E VB for NR films, utilizing advanced approaches to secondary electron background correction, satellite removal to enhance spectral contrast, and correction for shifts in local vacuum levels; and (b) waveguide-based spectroelectrochemistry to measure E CB from onset potentials for electron injection into NR films tethered to ITO. For untipped CdSe NRs, both approaches show E VB = 5.9–6.1 eV and E CB = 4.1–4.3 eV. Addition of Au tips alters the NR band edge energies and introduces midgap states, in ways that are predicted to influence the efficiency of these nanomaterials as photoelectrocatalysts. UPS results show that Au tipping shifts E VB closer to vacuum by up to 0.4 eV, shifts the apparent Fermi energy toward the middle of the band gap, and introduces additional states above E VB. Spectroelectrochemical results determine these trends: Au tipping shifts E CB closer to vacuum, by 0.4–0.6 eV, and introduces midgap states below E CB, which are assigned as metal–semiconductor interface (MSI) states. Characterization of thesemore » band edge energies and understanding the origins of MSI states is needed to design energy conversion systems with proper band alignment between the light absorbing NR, the NP catalyst, and solution electron donors and acceptors. Furthermore, the complementary characterization protocols presented here should be applicable to a wide variety of thin films of heterogeneous photoactive nanomaterials, aiding in the identification of the most promising material combinations for photoelectrochemical energy conversion.« less

Authors:
 [1];  [1];  [1];  [1];  [1];  [2];  [1]
  1. Univ. of Arizona, Tucson, AZ (United States)
  2. Univ. of Arizona, Tucson, AZ (United States); Seoul National Univ. (Korea, Republic of)
Publication Date:
Research Org.:
Energy Frontier Research Centers (EFRC) (United States). Center for Interface Science: Solar Electric Materials (CISSEM)
Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES); National Science Foundation (NSF)
OSTI Identifier:
1369928
Grant/Contract Number:  
SC0001084; FG03-02ER15753; DMR-130792
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
ACS Nano
Additional Journal Information:
Journal Volume: 9; Journal Issue: 9; Related Information: CISSEM partners with the University of Arizona (lead); Georgia Institute of Technology; National Renewable Energy Laboratory; Princeton University; University of Washington; Journal ID: ISSN 1936-0851
Publisher:
American Chemical Society (ACS)
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; semiconductor nanorods; metal nanoparticle; UV photoemission; spectroelectrochemistry; electron transfer; band edge energies

Citation Formats

Ehamparam, Ramanan, Pavlopoulos, Nicholas G., Liao, Michael W., Hill, Lawrence J., Armstrong, Neal R., Pyun, Jeffrey, and Saavedra, S. Scott. Band Edge Energetics of Heterostructured Nanorods: Photoemission Spectroscopy and Waveguide Spectroelectrochemistry of Au-Tipped CdSe Nanorod Monolayers. United States: N. p., 2015. Web. doi:10.1021/acsnano.5b01720.
Ehamparam, Ramanan, Pavlopoulos, Nicholas G., Liao, Michael W., Hill, Lawrence J., Armstrong, Neal R., Pyun, Jeffrey, & Saavedra, S. Scott. Band Edge Energetics of Heterostructured Nanorods: Photoemission Spectroscopy and Waveguide Spectroelectrochemistry of Au-Tipped CdSe Nanorod Monolayers. United States. https://doi.org/10.1021/acsnano.5b01720
Ehamparam, Ramanan, Pavlopoulos, Nicholas G., Liao, Michael W., Hill, Lawrence J., Armstrong, Neal R., Pyun, Jeffrey, and Saavedra, S. Scott. Thu . "Band Edge Energetics of Heterostructured Nanorods: Photoemission Spectroscopy and Waveguide Spectroelectrochemistry of Au-Tipped CdSe Nanorod Monolayers". United States. https://doi.org/10.1021/acsnano.5b01720. https://www.osti.gov/servlets/purl/1369928.
@article{osti_1369928,
title = {Band Edge Energetics of Heterostructured Nanorods: Photoemission Spectroscopy and Waveguide Spectroelectrochemistry of Au-Tipped CdSe Nanorod Monolayers},
author = {Ehamparam, Ramanan and Pavlopoulos, Nicholas G. and Liao, Michael W. and Hill, Lawrence J. and Armstrong, Neal R. and Pyun, Jeffrey and Saavedra, S. Scott},
abstractNote = {Conduction and valence band energies (ECB, EVB) for CdSe nanorods (NRs) functionalized with Au nanoparticle (NP) tips are reported here, referenced to the vacuum scale. We use (a) UV photoemission spectroscopy (UPS) to measure EVB for NR films, utilizing advanced approaches to secondary electron background correction, satellite removal to enhance spectral contrast, and correction for shifts in local vacuum levels; and (b) waveguide-based spectroelectrochemistry to measure ECB from onset potentials for electron injection into NR films tethered to ITO. For untipped CdSe NRs, both approaches show EVB = 5.9–6.1 eV and ECB = 4.1–4.3 eV. Addition of Au tips alters the NR band edge energies and introduces midgap states, in ways that are predicted to influence the efficiency of these nanomaterials as photoelectrocatalysts. UPS results show that Au tipping shifts EVB closer to vacuum by up to 0.4 eV, shifts the apparent Fermi energy toward the middle of the band gap, and introduces additional states above EVB. Spectroelectrochemical results determine these trends: Au tipping shifts ECB closer to vacuum, by 0.4–0.6 eV, and introduces midgap states below ECB, which are assigned as metal–semiconductor interface (MSI) states. Characterization of these band edge energies and understanding the origins of MSI states is needed to design energy conversion systems with proper band alignment between the light absorbing NR, the NP catalyst, and solution electron donors and acceptors. Furthermore, the complementary characterization protocols presented here should be applicable to a wide variety of thin films of heterogeneous photoactive nanomaterials, aiding in the identification of the most promising material combinations for photoelectrochemical energy conversion.},
doi = {10.1021/acsnano.5b01720},
url = {https://www.osti.gov/biblio/1369928}, journal = {ACS Nano},
issn = {1936-0851},
number = 9,
volume = 9,
place = {United States},
year = {2015},
month = {8}
}

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