Revisiting the Valence and Conduction Band Size Dependence of ...
Revisiting the Valence and Conduction Band Size Dependence of PbS Quantum Dot Thin Films Elisa M. Miller,1> Daniel M. Kroupa,1,2> Jianbing Zhang,1^ Philip Schulz,3* Ashley R. Marshall,1,2 Antoine Kahn,3 Stephan Lany,1 Joseph M. Luther,1 Matthew C. Beard,1 Craig L. Perkins,1 and Jao van de Lagemaat1# 1
Chemical and Materials Sciences Center, National Renewable Energy Laboratory, Golden, CO 80401, USA 2
3
Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309, USA Department of Electrical Engineering, Princeton University, Princeton, NJ 08544, USA
#
Corresponding author: [email protected]
^ Current address: School of Optical and Electronic Information, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan, China *Current address: Chemical and Materials Sciences Center, National Renewable Energy Laboratory, Golden, CO 80401, USA >
Equally contributing authors
SI Contents: Fig. S1: XPS of bulk PbS and EDT-exchanged PbS QD films fit with a parabolic band model Fig. S2: Extracted VBM for EDT-exchanged PbS QD films and general XPS/UPS correction parameters for PbS QD films Fig. S3: FET results for EDT-exchanged PbS QD films Fig. S4: Seebeck coefficient measurements of EDT-exchanged PbS QD films Fig. S5a: XPS core level shifts of EDT-exchanged PbS QD films before and after air exposure Fig. S5b: VB region and secondary electron cutoff shifts of EDT-exchanged PbS QD films before and after air exposure Fig. S5c: VBM and workfunction summary scheme of EDT-exchanged PbS QD films before and after air exposure
1
Fig. S1. XPS of bulk PbS and EDT-exchanged PbS QD films (blue traces) fit to a DOS parabolic model (green traces). The fit energy range is limited to include only the low energy L and Σ bands. For the bulk PbS fit, the EVBM (0.25 eV see Fig. 3 of main text) and L-Σ difference (0.4 eV) are held constant while the variables A and b are allowed to float. For the PbS QD fits, the b variable (2.5 determined from bulk PbS fit) and L-Σ difference (determined from kp theory, see Fig. 5 main text) are held constant, while the EVBM and A variable are allowed to float. Fitting Details: Fit = (A(DOSL + bDOSΣ))g A = overall scaling factor to match experimental data b = scaling factor between L and Σ DOS parabola g = convoluted with 350 meV Gaussian line width DOS = (2me*)3/2 (E – EVBM)1/2 me* = electron effective mass = 0.10 for L and 0.45 for Σ E = Electron Binding Energy EVBM is the VBM energy and is either known (bulk film) or used as a fit parameter (QD film), where the EVBM energy difference between the L and Σ energies are held at a constant
2
Fig. S2. The correction to the XPS VB spectra of PbS Films. The difference (blue crosses) between the measured EF − Eonset (red crosses) and the “EF − EVBM from model” (green crosses) is plotted as a function of bandgap. The correction is fit to a line (black line: y = 0.382 – 0.226Eg) and is used to determine the ”EF – EVBM from correction” (listed in Table 1 of main text). This linear fit is used to correct both the XPS and UPS data of PbS films.
Fig. S3. n-type FET output characteristics for EDT-exchanged PbS QD films, Eg = 0.71 – 0.98 eV. The key in panel a) remains consistent for panels b) through d). Corresponding electron mobilities are calculated from the transfer characteristics in both linear and saturated regimes (not 3
shown here), with gate voltages ranging from 0 – 60 V and source-drain voltages ranging from 0 –30 V.
Fig. S4. Seebeck coefficient measurements of Eg = 0.66 eV (panels a and b) and Eg = 1.45 eV (panel c) EDT-exchanged PbS QD films. The air exposed Eg = 0.66 eV films show p-type carrier transport as denoted by the direct relationship between voltage and ΔT, where the slope magnitude is equal to the Seebeck coefficient and slope sign indicates carrier type. Over two subsequent measurements, we measure an average Seebeck coefficient of 365.45 +/- 6.32
!" !
for
the Eg = 0.66 eV films. We were unable to determine a conductivity type for the air exposed Eg = 1.45 eV film, which suggests that the carrier concentration of the film is too low.
4
Fig. S5a. XPS core levels before (red trace) and after (blue) air exposure for a Eg = 0.66 eV film (left side) and Eg = 1.45 eV film (right side). The core level XPS spectra for O, C, S, and Pb are shown before and after air exposure. The C, S, and Pb features that are due to the EDTexchanged PbS QD film are shifted towards 0 eV when exposed to air, which indicates the Fermi level moving towards the VBM. Also, additional features grow in with air exposure, which can be attributed to new oxygen environments. For example in the S2p spectra, the features between 165 – 170 eV are due to SOx environments. In the Pb4f spectra, the peaks shift and develop a shoulder to higher binding energy when the samples are exposed to air. The shoulder is consistent with a Pb-O environment.
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Fig. S5b. XPS VB and secondary electron cutoff regions before (red trace) and after (blue trace) air exposure for a Eg = 0.66 eV film (left side) and Eg = 1.45 eV film (right side). The VB region for both sizes of PbS QD films shift towards 0 eV with air exposure, which means the Fermi level is closer to the VBM. The workfunction (Φ) for each film increases with air exposure (Φ = 21.218 eV – SEC (SEC = secondary electron cutoff)).
Fig. S5c. Summary scheme for the EF - EVBM, EF - ECBM, and workfunction (Φ) XPS results. The arrow indicates the energetic shift with respect to the Fermi energy following air exposure. The shaded line is the result before air exposure, and the solid line is the result following air exposure.
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Chemical and Materials Sciences Center, National Renewable Energy Laboratory, Golden, CO 80401, USA 2
3
Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309, USA Department of Electrical Engineering, Princeton University, Princeton, NJ 08544, USA
#
Corresponding author: [email protected]
^ Current address: School of Optical and Electronic Information, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan, China *Current address: Chemical and Materials Sciences Center, National Renewable Energy Laboratory, Golden, CO 80401, USA >
Equally contributing authors
SI Contents: Fig. S1: XPS of bulk PbS and EDT-exchanged PbS QD films fit with a parabolic band model Fig. S2: Extracted VBM for EDT-exchanged PbS QD films and general XPS/UPS correction parameters for PbS QD films Fig. S3: FET results for EDT-exchanged PbS QD films Fig. S4: Seebeck coefficient measurements of EDT-exchanged PbS QD films Fig. S5a: XPS core level shifts of EDT-exchanged PbS QD films before and after air exposure Fig. S5b: VB region and secondary electron cutoff shifts of EDT-exchanged PbS QD films before and after air exposure Fig. S5c: VBM and workfunction summary scheme of EDT-exchanged PbS QD films before and after air exposure
1
Fig. S1. XPS of bulk PbS and EDT-exchanged PbS QD films (blue traces) fit to a DOS parabolic model (green traces). The fit energy range is limited to include only the low energy L and Σ bands. For the bulk PbS fit, the EVBM (0.25 eV see Fig. 3 of main text) and L-Σ difference (0.4 eV) are held constant while the variables A and b are allowed to float. For the PbS QD fits, the b variable (2.5 determined from bulk PbS fit) and L-Σ difference (determined from kp theory, see Fig. 5 main text) are held constant, while the EVBM and A variable are allowed to float. Fitting Details: Fit = (A(DOSL + bDOSΣ))g A = overall scaling factor to match experimental data b = scaling factor between L and Σ DOS parabola g = convoluted with 350 meV Gaussian line width DOS = (2me*)3/2 (E – EVBM)1/2 me* = electron effective mass = 0.10 for L and 0.45 for Σ E = Electron Binding Energy EVBM is the VBM energy and is either known (bulk film) or used as a fit parameter (QD film), where the EVBM energy difference between the L and Σ energies are held at a constant
2
Fig. S2. The correction to the XPS VB spectra of PbS Films. The difference (blue crosses) between the measured EF − Eonset (red crosses) and the “EF − EVBM from model” (green crosses) is plotted as a function of bandgap. The correction is fit to a line (black line: y = 0.382 – 0.226Eg) and is used to determine the ”EF – EVBM from correction” (listed in Table 1 of main text). This linear fit is used to correct both the XPS and UPS data of PbS films.
Fig. S3. n-type FET output characteristics for EDT-exchanged PbS QD films, Eg = 0.71 – 0.98 eV. The key in panel a) remains consistent for panels b) through d). Corresponding electron mobilities are calculated from the transfer characteristics in both linear and saturated regimes (not 3
shown here), with gate voltages ranging from 0 – 60 V and source-drain voltages ranging from 0 –30 V.
Fig. S4. Seebeck coefficient measurements of Eg = 0.66 eV (panels a and b) and Eg = 1.45 eV (panel c) EDT-exchanged PbS QD films. The air exposed Eg = 0.66 eV films show p-type carrier transport as denoted by the direct relationship between voltage and ΔT, where the slope magnitude is equal to the Seebeck coefficient and slope sign indicates carrier type. Over two subsequent measurements, we measure an average Seebeck coefficient of 365.45 +/- 6.32
!" !
for
the Eg = 0.66 eV films. We were unable to determine a conductivity type for the air exposed Eg = 1.45 eV film, which suggests that the carrier concentration of the film is too low.
4
Fig. S5a. XPS core levels before (red trace) and after (blue) air exposure for a Eg = 0.66 eV film (left side) and Eg = 1.45 eV film (right side). The core level XPS spectra for O, C, S, and Pb are shown before and after air exposure. The C, S, and Pb features that are due to the EDTexchanged PbS QD film are shifted towards 0 eV when exposed to air, which indicates the Fermi level moving towards the VBM. Also, additional features grow in with air exposure, which can be attributed to new oxygen environments. For example in the S2p spectra, the features between 165 – 170 eV are due to SOx environments. In the Pb4f spectra, the peaks shift and develop a shoulder to higher binding energy when the samples are exposed to air. The shoulder is consistent with a Pb-O environment.
5
Fig. S5b. XPS VB and secondary electron cutoff regions before (red trace) and after (blue trace) air exposure for a Eg = 0.66 eV film (left side) and Eg = 1.45 eV film (right side). The VB region for both sizes of PbS QD films shift towards 0 eV with air exposure, which means the Fermi level is closer to the VBM. The workfunction (Φ) for each film increases with air exposure (Φ = 21.218 eV – SEC (SEC = secondary electron cutoff)).
Fig. S5c. Summary scheme for the EF - EVBM, EF - ECBM, and workfunction (Φ) XPS results. The arrow indicates the energetic shift with respect to the Fermi energy following air exposure. The shaded line is the result before air exposure, and the solid line is the result following air exposure.
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