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Title: Long-Range Order in Nanocrystal Assemblies Determines Charge Transport of Films

Abstract

Self-assembly of semiconductor nanocrystals (NCs) into two-dimensional patterns or three-dimensional (2- 3D) superstructures has emerged as a promising low-cost route to generate thin-film transistors and solar cells with superior charge transport because of enhanced electronic coupling between the NCs. Here, we show that lead sulfide (PbS) NCs solids featuring either short-range (disordered glassy solids, GSs) or long-range (superlattices, SLs) packing order are obtained solely by controlling deposition conditions of colloidal solution of NCs. In this study, we demonstrate the use of the evaporation-driven self-assembly method results in PbS NC SL structures that are observed over an area of 1 mm × 100 μm, with long-range translational order of up to 100 nm. A number of ordered domains appear to have nucleated simultaneously and grown together over the whole area, imparting a polycrystalline texture to the 3D SL films. By contrast, a conventional, optimized spin-coating deposition method results in PbS NC glassy films with no translational symmetry and much shorter-range packing order in agreement with state-of-the-art reports. Further, we investigate the electronic properties of both SL and GS films, using a field-effect transistor configuration as a test platform. The long-range ordering of the PbS NCs into SLs leads to semiconducting NC-basedmore » solids, the mobility (μ) of which is 3 orders of magnitude higher than that of the disordered GSs. Furthemore, although spin-cast GSs of PbS NCs have weak ambipolar behavior with limited gate tunability, SLs of PbS NCs show a clear p-type behavior with significantly higher conductivities.« less

Authors:
ORCiD logo [1];  [2];  [3];  [2];  [2]; ORCiD logo [4]; ORCiD logo [2]
  1. Univ. di Pisa, Pisa (Italy); Univ. of Illinois at Chicago, Chicago, IL (United States)
  2. Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
  3. Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States); NYU Tandon School of Engineering, Brooklyn, NY (United States)
  4. Univ. di Pisa, Pisa (Italy)
Publication Date:
Research Org.:
Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States). Molecular Foundry
Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
OSTI Identifier:
1371598
Grant/Contract Number:
AC02-05CH11231
Resource Type:
Journal Article: Published Article
Journal Name:
ACS Omega
Additional Journal Information:
Journal Volume: 2; Journal Issue: 7; Journal ID: ISSN 2470-1343
Publisher:
American Chemical Society (ACS)
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; 37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; crystal structure; electric transport processes and properties; microstructure; nanocrystals; order; phase; quantum dots; self-assembly; solid state electrochemistry; thin films

Citation Formats

Sainato, Michela, Shevitski, Brian, Sahu, Ayaskanta, Forster, Jason D., Aloni, Shaul, Barillaro, Giuseppe, and Urban, Jeffrey J. Long-Range Order in Nanocrystal Assemblies Determines Charge Transport of Films. United States: N. p., 2017. Web. doi:10.1021/acsomega.7b00433.
Sainato, Michela, Shevitski, Brian, Sahu, Ayaskanta, Forster, Jason D., Aloni, Shaul, Barillaro, Giuseppe, & Urban, Jeffrey J. Long-Range Order in Nanocrystal Assemblies Determines Charge Transport of Films. United States. doi:10.1021/acsomega.7b00433.
Sainato, Michela, Shevitski, Brian, Sahu, Ayaskanta, Forster, Jason D., Aloni, Shaul, Barillaro, Giuseppe, and Urban, Jeffrey J. 2017. "Long-Range Order in Nanocrystal Assemblies Determines Charge Transport of Films". United States. doi:10.1021/acsomega.7b00433.
@article{osti_1371598,
title = {Long-Range Order in Nanocrystal Assemblies Determines Charge Transport of Films},
author = {Sainato, Michela and Shevitski, Brian and Sahu, Ayaskanta and Forster, Jason D. and Aloni, Shaul and Barillaro, Giuseppe and Urban, Jeffrey J.},
abstractNote = {Self-assembly of semiconductor nanocrystals (NCs) into two-dimensional patterns or three-dimensional (2- 3D) superstructures has emerged as a promising low-cost route to generate thin-film transistors and solar cells with superior charge transport because of enhanced electronic coupling between the NCs. Here, we show that lead sulfide (PbS) NCs solids featuring either short-range (disordered glassy solids, GSs) or long-range (superlattices, SLs) packing order are obtained solely by controlling deposition conditions of colloidal solution of NCs. In this study, we demonstrate the use of the evaporation-driven self-assembly method results in PbS NC SL structures that are observed over an area of 1 mm × 100 μm, with long-range translational order of up to 100 nm. A number of ordered domains appear to have nucleated simultaneously and grown together over the whole area, imparting a polycrystalline texture to the 3D SL films. By contrast, a conventional, optimized spin-coating deposition method results in PbS NC glassy films with no translational symmetry and much shorter-range packing order in agreement with state-of-the-art reports. Further, we investigate the electronic properties of both SL and GS films, using a field-effect transistor configuration as a test platform. The long-range ordering of the PbS NCs into SLs leads to semiconducting NC-based solids, the mobility (μ) of which is 3 orders of magnitude higher than that of the disordered GSs. Furthemore, although spin-cast GSs of PbS NCs have weak ambipolar behavior with limited gate tunability, SLs of PbS NCs show a clear p-type behavior with significantly higher conductivities.},
doi = {10.1021/acsomega.7b00433},
journal = {ACS Omega},
number = 7,
volume = 2,
place = {United States},
year = 2017,
month = 7
}

Journal Article:
Free Publicly Available Full Text
Publisher's Version of Record at 10.1021/acsomega.7b00433

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  • Self-assembly of semiconductor nanocrystals (NCs) into two-dimensional patterns or three-dimensional (2- 3D) superstructures has emerged as a promising low-cost route to generate thin-film transistors and solar cells with superior charge transport because of enhanced electronic coupling between the NCs. Here, we show that lead sulfide (PbS) NCs solids featuring either short-range (disordered glassy solids, GSs) or long-range (superlattices, SLs) packing order are obtained solely by controlling deposition conditions of colloidal solution of NCs. In this study, we demonstrate the use of the evaporation-driven self-assembly method results in PbS NC SL structures that are observed over an area of 1 mmmore » × 100 μm, with long-range translational order of up to 100 nm. A number of ordered domains appear to have nucleated simultaneously and grown together over the whole area, imparting a polycrystalline texture to the 3D SL films. By contrast, a conventional, optimized spin-coating deposition method results in PbS NC glassy films with no translational symmetry and much shorter-range packing order in agreement with state-of-the-art reports. Further, we investigate the electronic properties of both SL and GS films, using a field-effect transistor configuration as a test platform. The long-range ordering of the PbS NCs into SLs leads to semiconducting NC-based solids, the mobility (μ) of which is 3 orders of magnitude higher than that of the disordered GSs. Furthemore, although spin-cast GSs of PbS NCs have weak ambipolar behavior with limited gate tunability, SLs of PbS NCs show a clear p-type behavior with significantly higher conductivities.« less
  • Colloidal semiconductor nanocrystals have attracted significant interest for applications in solution-processable devices such as light-emitting diodes and solar cells. However, a poor understanding of charge transport in nanocrystal assemblies, specifically the relation between electrical conductance in dark and under light illumination, hinders their technological applicability. Here we simultaneously address the issues of 'dark' transport and photoconductivity in films of PbS nanocrystals, by incorporating them into optical field-effect transistors in which the channel conductance is controlled by both gate voltage and incident radiation. Spectrally resolved photoresponses of these devices reveal a weakly conductive mid-gap band that is responsible for charge transportmore » in dark. The mechanism for conductance, however, changes under illumination when it becomes dominated by band-edge quantized states. In this case, the mid-gap band still has an important role as its occupancy (tuned by the gate voltage) controls the dynamics of band-edge charges.« less
  • We have measured x-ray absorption spectra (XAS) at the oxygen K edge for hafnium oxide (HfO2) films grown by chemical vapor deposition (CVD) and atomic layer deposition (ALD), as well as for hafnium silicate (HfSiO) films grown by CVD. The XAS results are compared to x-ray diffraction (XRD) and spectroscopic ellipsometry (SE) data from the same films. Features characteristic of crystalline HfO2 are observed in the XAS spectra from all CVD-grown HfO2 films, even for a thickness of 5 nm where XRD is not sensitive. XAS and XRD spectra from the ALD-grown HfO2 films exhibit the signature of crystallinity onlymore » for films that are 20 nm or thicker. These characteristic XAS features are absent in all HfSiO films measured, which is consistent with their being amorphous. The appearance of these peaks in XAS and XRD is correlated with sub-band-gap absorption in the SE spectra, which appears to be intrinsic to crystalline HfO2 in the monoclinic phase.« less