Roles of Nitrogen Functionalities in Enhancingthe ... · Roles of Nitrogen Functionalities in Enhancingthe Excitation-Independent Green-Color Photoluminescence of Graphene Oxide Dots.
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Electronic Supplementary Information
Roles of Nitrogen Functionalities in Enhancing the
Excitation-Independent Green-Color Photoluminescence of
(9) UPS analysis for valence band maxima of the GO-based dots
(10) Application of A-GODs as a phosphor for white-light emission
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1. Color and quantum yield of graphene oxide dots Table S1 Summary of the color/wavelength (λ) and quantum yield (QY) of photoluminescence emissions from graphene oxide dots synthesized through top-down and bottom-up routes.
8. The Mott-Schottky equation for the conductivity-type determination
The GODs were deposited on a glassy carbon substrate. The conductivity types and Fermi
level (EF) potentials of the GOD films were then determined via electrochemical impedance
spectroscopic analysis based on the Mott-Schottky equation,46,47 i.e.
)(21F
02 e
kTEENeC D
−−=εε
for n-type conductivity
)(21F
02 e
kTEENeC A
++−−=εε
for p-type conductivity
where C represents the capacitance of the space−charge region, ε0 is the vacuum permittivity,
ε is dielectric constant of semiconductors, e is the electron charge, E is applied potential, EF is
the Fermi level potential, k is the Boltzmann constant, T is the absolute temperature, and NA
(ND) is the acceptor (donor) density. Nota that the temperature term is generally small and can
be neglected. The capacitance values of the space−charge region were obtained at various
applied potentials. According to the Mott-Schottky equation, 1/C2 and E are linearly related,
with a negative slope indicating p-type conductivity and a positive slope indicating n-type
conductivity.
Fig. S7 presents the variation of the capacitance in the space-charge region of the GODs
and A-GODs with the applied potential in compliance with the Mott–Schottky equation.
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Fig. S7 Variation of capacitance (C) with applied potential in 2 M H2SO4 presented in the
Mott-Shottky relationship for electrodes deposited with (a) GODs and (b) A-GODs. The
capacitance was determined by electrochemical impedance spectroscopy and the negative and
positive slopes correspond to p- and n-type conductivities, respectively.
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9. UPS analysis for valence band maxima of the GO-based dots
To identify the valence band maxima (i.e., the n-state, denoted as Ev), the GODs were
deposited on the silicon substrate and the value of Ev was determined using UPS with He I
light (21.2 eV) irradiation. The UPS analysis was performed in accordance with:
EB + Ek + ϕ = 21.2 (1)
where EB is the binding energy measured from the Fermi level, Ek is the kinetic energy of
electrons, ϕ is the work function of the GODs, and 21.2 eV is the excitation energy of the He
I light.
Ev was then calculated as:
Ev = 21.2 − (EB2 − EB1) (2)
where EB2 is the secondary cutoff binding energy in the UPS spectra, in which the Ek of the
excited electrons is equal to 0 and the EB1 is the difference between the Fermi edges and the
valence band edges. Fig. S8 shows the UPS spectra of the GODs and A-GODs. Note that EB1
can be determined from the intercepts of the extrapolated straight lines on the abscissa at low
binding energy. Similarly, EB2 can be estimated using the secondary cutoff values (Ek = 0 eV)
in the UPS spectra, which are obtained from the intercepts of the extrapolated straight lines
on the abscissa at high binding energy. The UPS widths is obtained directly as the difference
between EB2 and EB1. Finally, Ev is obtained by subtracting these UPS widths from the
excitation energy (21.2 eV).
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Fig. S8 UPS spectra of the samples: (a) GODs and (b) A-GODs. The VBM levels with
respect to the Fermi levels were determined from the intercepts of the extrapolated straight
lines (blue dashed line) on the abscissa at low binding energy. The intersections of the tangent
(red dashed line) with the abscissa at high binding energy give the secondary electron onset
binding energy. The UPS widths (black lines) can be determined by these two intercept
binding energies, and the VBM levels can be calculated by subtracting these widths from the
excitation energy (21.2 eV).
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10. Application of A-GODs as a phosphor for white-light emission
We mixed aqueous solutions of A-GOD (0.5 g L-1) and poly(vinyl alcohol) (PVA) (10
wt%) to form the precursor mixture of the A-GODs-embedded PVA film. For the fabrication
of white light emitting diode (LED), the mixture was dispensed on a violet (365 nm)-LED
chip and thermally dried at 60 °C for 24 h. The combination of the A-GODs-embedded PVA
film and violet-LED chip provides white light emissions, which are tunable through adjusting
the concentration of A-GODs in the PVA film. The device was characterized in a N2-filled
glove box with oxygen and water contents less than 1 ppm. The Commission International
d’Eclairage color coordinate of the light emission from the device was measured using a
Keithley 2400 source meter and a PR650 colorimeter.
Fig. S9 A-GODs as a phosphor for white-light emission. (a) A device consisting of an
A-GODs-embedded PVA film covering a violet (365 nm)-light emitting diode (LED) chip. (b)
White light emission from the device when the LED turned on. (c) The Commission
International d’Eclairage color coordinate for the white light emission shown in panel (b).
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