news & views

One promise of polaritonics is the Single-walled carbon nanotubes support required for lasing. Applying vertical
achievement of low-threshold lasing, tightly-bound, strongly light-emitting fields can polarize the excitons, producing
which does not rely on inversion of the excitons at room temperature and around ‘dipolaritons’ (a superposition of a photon
excited state population but on the bosonic telecommunications wavelengths, but and a direct and an indirect exciton) with
nature of the lower polariton state. Instead because their energies depend on how the much larger Coulombic scattering 12 — of
of needing to scale down the device size nanotubes roll up, the narrow emission great interest in stacked TMD systems,
(so that inverting all emitters costs less lines required can only be obtained by and also potentially of interest in low
energy), this offers a different paradigm size-selection. Laying down mats of these dimensional systems such as nanotubes.
for coherent light emission, using the nanotubes and electrically contacting from Another promising approach just emerging
stimulated scattering of polaritons into each side allows both electrons and holes is the use of plasmonic resonators, which
their lowest state where they condense to be injected from opposite ends (Fig. 1d). confine light millions of times more tightly
into a macroscopic condensate. While a When these recombine in the central zone than conventional microcavities while
number of polariton lasers have worked of a nanotube they form excitons that emit retaining polariton splittings exceeding
with optical pumping, this is unsatisfactory light. A simple planar cavity formed of thermal energies6. This can strongly
for real devices and a key goal has been mirrors above and below the nanotubes enhance the Coulombic scattering and
incorporating electrical injection into reflects the light, giving exciton–photon potentially open the way to realistic
such microcavities. strong coupling. The one metallic mirror can nonlinear optical devices for optical and
The combination of high oscillator elegantly be used as a voltage gate that shifts quantum processing. ❐
strength excitons in small optical cavities the lateral position of the polariton emission
creates problems for wiring up such devices within the microcavity, as well as controlling Jeremy J. Baumberg is at the NanoPhotonics Centre,
(Fig. 1). If the electrons overlap the cavity the polariton energies. Cavendish Laboratory, University of Cambridge,
photons, the extra free-carrier absorption While strong polariton emission is Cambridge CB3 0HE, UK.
often ruins the strong coupling, so that seen in these devices, condensation e-mail: jjb12@cam.ac.uk
conventional LED structures have to is not yet observed at high injection
incorporate trade-offs that yield polaritons currents. A key aspect is the formation References
but no condensation7 (Fig. 1a). On the of the runaway population of the lower 1. Graf, A. et al. Nat. Mater. 16, 911–917 (2017).
2. Christopoulos, S. et al. Phys. Rev. Lett. 98, 126405 (2007).
other hand, sideways injection of the polariton by stimulated scattering from 3. Kéna-Cohen, S. & Forrest, S. R. Nat. Photon. 4, 371–375 (2010).
electrons has required complex fabrication the injected reservoir of hot excitons. Both 4. Plumhof, J. D., Stöferle, T., Mai, L., Scherf, U. & Mahrt, R. F.
and highly compromised coupling 8,9 optical phonons and Coulomb-induced Nat. Mater. 13, 247–252 (2014).
5. Liu, X. et al. Nat. Photon. 9, 30–34 (2014).
(Fig. 1b). Even electrical control of the exciton–exciton scatterings contribute, 6. Chikkaraddy, R. et al. Nature 535, 127–130 (2016).
polaritons has been challenging — for and enhancing the latter is critical. In 7. Tsintzos, S. I., Pelekanos, N. T., Konstantinidis, G., Hatzopoulos, Z.
instance, in TMD open cavities10 (Fig. 1c) carbon nanotubes (and other systems with & Savvidis, P. G. Nature 453, 372–375 (2008).
8. Schneider, C. et al. Nature 497, 348–352 (2013).
— though now advanced to the stage where high exciton binding energy), the Bohr 9. Bhattacharya, P. et al. Phys. Rev. Lett. 112, 236802 (2014).
ultra-low-energy switching is possible at radius of excitons is on the nanoscale, 10. Sidler, M. et al. Nat. Phys. 13, 255–261 (2017).
low temperatures11. This is where Zaumseil with centre of mass delocalization below 11. Dreismann, A. et al. Nat. Mater. 15, 1074–1078 (2016).
12. Cristofolini, P. et al. Science 336, 704–707 (2012).
and co-authors show significant progress, 100 nm, which reduces the exciton overlap
using a carbon-based material system not and thus the Coulombic in-scattering.
previously considered for polaritonics. This suggests that high densities will be Published online: 17 July 2017

RAMAN SPECTROSCOPY

Enhanced by organic surfaces

Nanostructured films of organic semiconductors are now shown to enhance the Raman signal of probe molecules,
paving the way to the realization of substrates for Raman spectroscopy with molecular selectivity.

John R. Lombardi

S
urface-enhanced Raman spectroscopy technique is its use in ultra-sensitive possibility of semiconductor substrates.
(SERS) is a phenomenon in which qualitative analytic tools for the detection Writing in Nature Materials, Mehmet Yilmaz
the normally weak Raman signal of a and identification of trace quantities of a and colleagues5 now demonstrate that
molecule is strongly enhanced by proximity molecular species. In order for this to be organic semiconductors, either pristine or
to a nanostructured surface. Most of the realized, a SERS substrate must be found in combination with thin metal coatings,
early work on this topic utilized silver as that is inexpensive, reproducible, stable, represent a previously unexplored platform
a substrate, as this metal resulted in the robust and, if possible, reusable. Substrates for the realization of high-performing SERS-
largest observed enhancement factors1,2 — constructed from silver and other coinage based sensors.
up to 1012 — enabling the detection of a metals have not been found to fit all these Crucial to the Raman enhancement
single molecule3,4. In fact, one of the most criteria, so there have been attempts to on metal substrates is the existence of a
promising applications of this spectroscopic find alternative substrates, including the plasmon resonance. Plasmons are generated

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 news & views

when an electromagnetic wave excites the a b

electrons in the conduction band of the
metal to collective and in-phase motion,
oscillating at the optical frequency of the
exciting laser. This results in a very high Ec , –1.97
electric field on the nanoparticle surface,
which leads to a strongly augmented LUMO, –3.45
Raman signal from the adsorbed molecule. Eg
µCT
In addition, other resonances in the µmol
molecule–metal system also contribute to
Ev , –5.15
the enhancement; namely, a charge-transfer HOMO, –5.3
resonance between the metal Fermi level
2 µm
and an empty orbital on the molecule, or the MB
reverse transition between a filled orbital in DFH–4T
the molecule to the Fermi level of the metal.
Further enhancements are observed when
a molecular resonance is in the vicinity of Figure 1 | Mechanisms for the enhancement of the Raman signal due to the interaction of a molecule
the laser frequency. All these resonances are with a semiconductor substrate. a, Scanning electron microscopy image of the nanostructured organic
combined in a unified manner, contributing films used by Yilmaz and colleagues5. b, Enhancement of MB Raman signal in proximity of an organic
multiplicatively, and coupled by a vibronic semiconductor substrate (DFH-4T). The charge-transfer transition μCT between the edge of the valence
expression6 that enables the prediction of the band EV of DFH-4T and the LUMO level of MB is marked with a red arrow, and the molecular transition
changes in the observed Raman spectrum μmol between the HOMO and LUMO levels of MB is marked with a green arrow. These transitions are
due to the interaction of the molecule with close to each other spectroscopically and coupled to produce the Raman enhancement. The exciton
the surface. The mechanism of enhancement transition Eg between EV and the edge of the conduction band EC in the semiconductor, shown in blue, lies
can then be attributed to the increase of the too far away to contribute significantly to the enhancement. Numbers represent the energy values in eV
(normally weak) charge-transfer intensity as measured from the vacuum (0.0 eV). Panel a reproduced from ref. 5, Macmillan Publishers Ltd.
due to intensity ‘borrowing’ from a nearby
allowed optical transition7.
When semiconducting nanostructured adding the molecule, enhancement factors specific molecule or class of molecules.
films or nanoparticles are used instead of of up to 1010 were obtained, illustrating the The coinage metals are more of a universal
metals as substrates, lower enhancement possible contribution of a plasmon resonance. substrate, effective with almost any molecule
factors are expected due to lack of plasmon The experimental observations, further for which the Fermi level of the metal lies
resonances in the conduction band of supported by density functional calculations, between the highest occupied molecular
the semiconductor. However, additional showed that the enhancement of the pristine orbital (HOMO) and the lowest unoccupied
resonances could be harnessed due to organic film is due to a charge-transfer molecular orbital (LUMO). This facilitates
Mie scattering resonances that depend on transition between the molecule and the charge transfer that, coupled to the plasmon
the nanostructures’ shape and/or exciton organic substrate (Fig. 1b). This interpretation resonance, produces high enhancements. In
(band gap) resonances in semiconductor is consistent with the resonance description contrast, charge transfer between a molecule
nanoparticles. These resonances can also of SERS in that the charge-transfer resonance and a semiconductor substrate involves the
couple vibronically with charge-transfer and (around 1.58 eV) between molecule and semiconductor band edges, rather than its
molecular resonances8 to produce larger substrate is vibronically coupled to the Fermi level. This provides an advantage over
SERS enhancements. Compared to metal nearby molecular transition and borrows metallic substrates, because the band edge
enhancements, these were rather weak at first, intensity from it. In MB, the molecular of a semiconductor can be easily tailored to
typically on the order of 103 or 104. However, transition is at 1.85 eV (668 nm), close optimize the charge transfer to a particular
by taking advantage of improvements in enough to the experimental charge- molecule8. Since many real-life samples come
the synthesis of inorganic semiconductor transfer transition to provide additional as mixtures, the appropriate semiconductor
nanoparticles, higher enhancements enhancement through intensity borrowing. can act as a selective detector for a
(105–106) have been obtained9–11. On the contrary, changing the substrate to specific molecule in that mixture. Organic
Yilmaz and co-workers now extend DH-4T (unfluorinated substrate) moves semiconductors can further this selectivity,
SERS to an entirely new realm of the charge-transfer resonance farther away since we often have chemical control of the
organic semiconductor substrates. The from the excitation energy, severely lowering exact location of the band edges. For example,
researchers utilized a π-conjugated the observed enhancement. Changing the by adjusting the number of thiophene rings
organic substrate composed of analyte to R6G has a similar effect. These in the central chain of DFH-4T (for instance,
α,ω-diperfluorohexylquaterthiophene observations also emphasize the importance to DFH-5T), or by replacing the fluorines
(DFH-4T) to form a thin nanostructured film of structural features of the substrate in with hydrogen (such as with DH-4T), we
(Fig. 1a). This consisted of vertically aligned explaining the observed effect. can finely tune the location of the band
two-dimensional nanoplates connected Why should we be so encouraged by the edges, providing more precise control over
to a cylindrical central spine. The dyes promise of organic semiconductors for SERS the location of charge-transfer transitions to
methylene blue (MB) and rhodamine 6G substrates? In this application, it has been and from a specific molecule to be detected.
(R6G) were chosen for the probe molecules, shown that nanostructured semiconductor The results of Yilmaz and colleagues have
and the excitation energy was 1.58 eV films are generally more stable and the potential to open up a whole new area of
(wavelength 785 nm). For MB, they found an reproducible than their metallic counterparts. research. Considerable work has been carried
enhancement factor of over 103. By depositing Another advantage is the ability to tailor out on the optical properties of many other
a thin layer of Au over the DFH-4T before the properties of the semiconductor to a organic semiconductor systems, which, with

NATURE MATERIALS | VOL 16 | SEPTEMBER 2017 | www.nature.com/naturematerials 879

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news & views

proper integration with theory, can vastly References 7. Albrecht, A. C. J. Chem. Phys. 34, 1476–1484 (1961).
expand the applicability of SERS to the field 1. Fleischmann, M., Hendra, P. J. & McQuillan, A. J. Chem. Phys. 8. Lombardi, J. R. & Birke, R. L. J. Phys. Chem. C
Lett. 26, 163–166 (1974). 118, 11120–11130 (2014).
of molecular sensors. ❐ 2. Jeanmaire, D. L. L. & Van Duyne, R. P. J. Electroanal. Chem. 9. Maznichenko, D., Venkatakrishnan, K. & Tan, B. J. Phys. Chem. C
Interfacial Electrochem. 84, 1−20 (1977). 117, 578−583 (2013).
John R. Lombardi is in the Department of 3. Nie, S. & Emory, S. R. Science 275, 1102–1106 (1997). 10. Li, W. et al. J. Am. Chem. Soc. 135, 7098−7101 (2013).
4. Kneipp, K. et al. Phys. Rev. Lett. 78, 1667–1670 (1996). 11. Islam, S. K., Tamargo, M., Moug, R. & Lombardi, J. R. J. Chem.
Chemistry, City College of New York, 138th Street at Phys. C 117, 23372 (2013).
5. Yilmaz, M. et al. Nat. Mater. 16, 919–924 (2017).
Convent Avenue New York, New York 10031, USA. 6. Lombardi, J. R. & Birke, R. L. J. Phys. Chem. C
e-mail: jlombardi@ccny.cuny.edu 112, 5605−5617 (2008). Published online: 7 August

MICROPOROUS POLYMERS

Ultrapermeable membranes
Microporous membranes were designed from the loose packing of two-dimensional polymer chains — a
breakthrough giving both ultrahigh permeability and good selectivity for gas separations.

Yan Yin and Michael D. Guiver

M
embrane technology is displacing In polymeric membranes, gas Condensable gases such as CO2 achieve
established molecular-separation transport occurs by the solution–diffusion additionally higher permeability through
processes by avoiding energy- mechanism, whereby gases sorb and then gas sorption interactions with the polymer.
intensive phase changes, achieving higher permeate by pressure differential through Higher gas permeability often comes with
efficiency at lower cost. Industrial membrane voids (fractional free volume, FFV) created lower selectivity, and this performance
gas separation began in the 1980s and the by intermolecular gaps between polymer trade-off was initially defined in 1991 by
dominant separations are currently N2 chains. Highly rigid polymer chains allow the Robeson upper-bound plots (log
production from air, CO2 removal from selective permeation of gases with smaller permeability of faster gas versus log
natural gas, and H2 recovery from various kinetic diameters (for example, H2) over selectivity of specified gas pair)2. The
industrial process streams1. larger kinetic diameters (for example, N2). development of polymers of intrinsic
microporosity (PIMs)3 in 2008 pushed the
upper bounds higher4. Now, writing in
a b c CO2 Nature Materials, Ian Rose and colleagues5
report a PIM membrane with both
Solution casting ultrapermeability and superior selectivity,
launching a new chapter in the design of
PIMs for high-performance membrane
gas separation.
Microporous materials are materials
with interconnected pores less than
2 nm in diameter, comparable to
molecular diameters6. In the membrane
field, poly(trimethylsilyl-1-propyne)
Membrane (PTMSP) has long been established as an
O
ultrapermeable microporous polymer,
CN
O
O
with the highest known gas permeability 7.
This ultrapermeability derives from
O
n
O
O
O CN
N2
very large FFV and restricted chain
CN O
CH4 mobility, attributable to a rigid alternating
NC n
double-bond backbone substituted with
PIM–TMN-SBI PIM–TMN-Trip
trimethylsilyl groups. For example, CO2
permeability is 20,000–30,000 barrer, several
Figure 1 | Representation of the PIM-TMN-Trip membrane, and comparison of chemical structure and orders of magnitude higher than other
polymer packing of 2D and 3D PIMS. a, The 3D chain packing of PIM-TMN-SBI, a contorted polymer with polymeric membranes. However, very low
primarily smaller micropores, thus giving lower gas permeability than the PIM-TMN-Trip membrane. gas selectivity and the propensity for free-
b, The chemical structure of ribbon-like 2D polymer PIM-TMN-Trip, and energy-minimized chain packing. volume collapse hinders applications.
The polymer generates 3D amorphous molecular sieve-like solids with additional intrinsic microporosity, PIMs were invented by Peter Budd and
arising from the coexistence of larger micropores (giving ultrapermeability) and smaller micropores Neil McKeown3, and are characterized
(which enhance selectivity). c, Schematic of PIM-TMN-Trip membrane, which allows unprecedented as having rigid, ladder-like chains
ultrapermeability of gases such as CO2 through the membrane, while simultaneously maintaining good connected to contortion sites, giving
selectivity against gases with larger kinetic diameter such as N2 and CH4. twisted macromolecular structures with

880 NATURE MATERIALS | VOL 16 | SEPTEMBER 2017 | www.nature.com/naturematerials

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