Topics Covered
Background
SWNT Photoluminescence and the
NanoLog®
Photoluminescence
of Quantum Dots
Photoluminescent Analysis of
OLEDs
Spex® Instruments
for Quantum Dots and OLEDs
Conclusions
Background
This article describes some applications of fluorescence instruments from HORIBA
Scientific to nanophotonics, e.g., singlewalled carbon nanotubes (SWNTs),
quantum dots, and organic light-emitting diodes (OLEDs).
Quantum confinement affects nanomaterials’ photoluminescence: when the
semiconducting nanoparticle is smaller than the bulk material’s Bohr-exciton
radius, the bandgap energy is inversely proportional to the nanoparticle size.
Smaller nanoparticles usually have higher energy absorbance and emission
properties than larger nanoparticles of the same material.
SWNT Photoluminescence
and the NanoLog®
Fig. 1 sketches the process of semiconducting SWNT photoluminescence. The
decreasing absorption and emission energies of individual SWNT species correlate
directly with diameters from analysis of radial breathing modes from Raman
spectroscopy. Certain (n,m) values of semiconducting SWNTs match
predicted bandgaps between valence and conductance bands. (Metallic and
semi-metallic SWNTs with continuous valence- and conductance-bands show little
or no photoluminescence.)
A NANOLOG® (double-grating excitation monochromator, imaging
emission spectrograph with a selectable-grating turret, and multichannel
liquid-N2-cooled InGaAs-array detector) has optimal excitation optics
for SWNT research or any solid sample in right-angle or front-face mirror
configurations. The emission spectrometer has selectable gratings in a turret
mount for rapid, easy acquisition of near-IR spectra. One grating has
single-shot coverage of > 500 nm with a detector sensitive from 800–1700
nm.
Figure 1.
Semiconducting-SWNT photoluminescence absorption and emission. Conduction bands
are red; valence bands are blue. Electrons are yellow; holes are white circles.
Small black arrows are radiative or nonradiative transitions of e–s or holes
between different band-levels. Vx and cx are specific valence or conductance
bands.
Corrected emission spectra provide EEMs for a range of excitation
wavelengths; acquisitions take only minutes. Excitation bandpass ranges from
0–14 nm; spectrometer slits vary from 0–16 mm with a 1200 groove/mm grating. An
order-sorting filter prevents visible light from entering the spectrometer.
EEMs are compiled (Figs. 2A and B) by our exclusive NANOSIZER® software to
determine SWNT composition (Fig. 2C). A double convolution algorithm (U.S. Pat.
Pending) in the NANOSIZER® simultaneously computes excitation and emission
wavelength coordinate line shapes for each species; contributions from all
spectral bands in a region of interest are found. EEM data (Fig. 2, solid lines)
and simulations (contour maps) from two SWNT suspensions of different
manufacturing processes distinguished by different size and helical
distributions are given: high-pressure carbon-monoxide method (HiPCO, Fig. 2A);
cobalt-molybdenum catalytic method (CoMoCAT, 2B). Fig. 2A identifies five main
HiPCO species; Fig. 2B indentifies four main CoMoCAT species. Fig. 2C, a helical
map of species found in Figs. 2A and B, plots helical angle versus SWNT diameter
against intensity of emission (symbol size). Note that HiPCO tubes have a larger
mean diameter than CoMoCAT. The simulation gives precise analysis of SWNT
composition on an IBM-compatible PC in minutes.
Figure 2.
Quantum excitation-emission (A and B) and helical (C) maps of HiPCO and Co-MoCAT
SWNT suspensions, using a NANOLOG®. Solid lines (A and B) are data; color contours are
simulations. Symbol sizes (C) show relative amplitudes for HiPCO (circles) and
CoMOCAT (squares), each normalized to 1. R2 values for the
simulations are 0.997 (HiPCO) and 0.999 (CoMoCAT).
Photoluminescence of Quantum
Dots
Quantum dots’ absorption bands have broad spectral features and precise
tunability of their emission bands. Their absorption spectra stem from many
overlapping bands increasing at higher energies. Each absorption band
corresponds to an energy-transition between discrete electron-hole (exciton)
energy-levels; smaller dots give a first exciton peak at shorter
wavelengths.
A photon is emitted when an electron crosses from conduction-band edge to
valence band. Photon energy is proportional to bandgap, determined by the bulk
material’s Bohr-exciton radius and the quantum dot’s size (Fig. 3).
Figure 3.
Quantum confinement for quantum dots. Valence and conductance bands are blue and
red, respectively. Composition of dots A and B is identical; only the bulk
radius varies relative to the fixed Bohr-exciton radius.
Quantum dots’ advantages compared to standard organic fluorophores are: A
single source can excite multiple dots emitting over a broad range, giving
selective exclusion of excitation light from the measured emission. Quantum dots
have high fluorescent and strong two- photon absorption yields, so they are up
to 1000 times brighter, for better imaging resolution. Their tunable bandgaps
offer applications such as white-light LEDs and other displays.
Most quantum dots are made of toxic elements (e.g., Pb, Cd, Se, and Te).
Their photoluminescence may be sensitive to biological interactions, so most
biological applications of quantum dots require a coating (usually a triblock
copolymer), rendering the dots non-toxic but also helping to conjugate the dots
to molecular probes, and protecting the dots from biomolecular agents. Antibody
conjugate imaging of these dots may be useful for diagnosis and treatment of
cancer. Near-IR quantum dots may aid deeper tissue-imaging, for near-IR light
penetrates tissue deeper than visible light. Quantum dots’ excited state
lifetimes (2~10 ns) increase their worth for time-resolved fluorescence
instruments. Many conjugation choices and excited-state properties of dots make
them useful for biosensors based on fluorescence resonance energy-transfer.
Photoluminescent Analysis of
OLEDs
Based on thin-films, OLEDs offer advantages over LCDs: no backlighting,
emission of light only from active pixels for lower power, higher contrast and
color-fidelity, brighter emission, wider viewing-angles, faster temporal
response, better temperature-stability, and deposition on flexible or
transparent substrates.
A voltage applied across an OLED circuit drives the electrons (Fig. 4A) and
holes (Fig. 4B) into the organic layer where recombination occurs to emit
photons (Fig. 4C). Here, photons from blue, green, and red emitters yield white
light. Composition, thickness, and relation between the various layers regulate
OLED luminescence.
Figure 4. OLED
operation in 3 stages, A to C. White arrows show flow of e–s (yellow) and holes
(white) from electrodes. Starbursts in C are electron-hole recombination in the
organic layer followed by photon emission.
Fig. 5 shows the phosphorescent decay of a Universal Display emitter with a
lifetime of > 1 µs, recorded on a TCSPC-FLUOROLOG®.
Figure 5. Phosphorescent
decay of an organic emitter from a PHOLED, using a TCSPC-FLUOROLOG ® in front-face mode (for solid samples), resolving
from <100 ps to > 200 µs. ëexc = 335 nm NanoLED (800 ps pulses); ëem =
520 nm. R2 for the tail-fit = 0.995.
Spex® Instruments for
Quantum Dots and OLEDs
The modular FLUOROLOG® spectrofluorometer is equipped for UV to near-IR
steady-state and time-resolved measurements (from <100 ps) for
photoluminescence research. The instrument can do steady-state and time resolved
anisotropy for molecular motions and shapes, with two TCSPC detectors: our
TBX-05 (300–850 nm, 180 ps), and the Hamamatsu 9170-75 (900–1700 nm, 300 ps).
Monochromators and gratings blazed for UV-visible or near-IR in the T-format can
optimize the system. A switchable adapter for xenon lamp and NanoLED converts
between steady-state and time-resolved modes. NanoLEDs are pulsed TCSPC
light-sources (~1 ns to = 200 ps, 10 kHz–1 MHz repetition rates), including
deep-UV (265, 280, and 295 nm), and are interchangeable with SpectraLEDs (500 ns
pulses to CW) for phosphorescence studies.
Cell-imaging, biosensing, and nanophotonic circuit-analysis require
microscopic resolution, broad spectral sensitivity, and wide dynamic and kinetic
ranges, provided by our modular DYNAMIC™ confocal microscope with steady-state
ps-to-ns time resolution, and mapping to 1 µm.
The system can be coupled to the FLUOROLOG®.
Fig. 6 shows CdS quantum dots in a semiconductor wafer, along with the dots’
spec- tral contribution (ëexc = 350 nm).
Figure 6.
Spectral and spatial mapping of CdSe quantum dots in a solid state matrix of a
semiconductor wafer. A is the bright-field image; color-coded spots for spectral
regions of interest are emission spectra (B).
Conclusions
For researchers of SWNTs, we offer the NANOLOG® and
NANOSIZER®. Researchers of quantum dots can use our FLUOROLOG®.
For maturing OLED technology, we have TCSPC instruments to resolve fluorescent
lifetimes. Biological applications will find the DYNAMIC ™ important. HORIBA
Scientific has the optimum spectrofluorometer for nanotechnology research in
these areas.
Source: SPEX® Fluorescence Group Application Note F-28
“Nanophotonics with Fluorescence Instruments from HORIBA Scientific”
For more information on this source please visit HORIBA
Scientific