Exploration of organic-inorganic hybrid perovskites for surface-enhanced infrared spectroscopy of small molecules

Article abstract of DOI:10.1039/c7cc02782f

The organic-inorganic hybrid perovskites efficiently enhance the infrared absorption of small molecules. It is suggested that the quantum wells of perovskites enable the electrons of the perovskites to be excited by light in the infrared region. The exploration has opened a new path for chemical sensing through infrared spectroscopy.

Full text of DOI:10.1039/c7cc02782f

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Exploration of organic-inorganic hybrid perovskites for surface-
enhanced infrared spectroscopy of small molecules
Jia Chen,a Zhi-Hong Mo,*ab Xiao Yang,a Hai-Ling Zhou,a Qin Gaoa
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
with lower carrier concentrations such as the doped materials,12,13
metallic nitrides14 and graphene15,16 to realize the mid-IR plasmonic
The organic-inorganic hybrid perovskites allow the infrared
absorption of small molecules to be efficiently enhanced. It is
suggested that the quantum wells of perovskites enable their resonances.17,18 Recently, the organic-inorganic perovskites have
electrons to be excited by the light in the infrared region. The
exploration has opened a new path for chemical sensing through
infrared spectroscopy.
spectacularly emerged as the next leading generation of semi-
conductor materials with superior electronic and optical functions.
This promising class of materials enable the integration of useful
organic and inorganic characteristics within
a single complex
Infrared spectroscopy caused by molecular vibrations is extremely
powerful for identifying the presence of specific structures, charge
states, and orientations of organic molecules.1,2 However, its
applications have been usually restricted to large sample amounts
because of the poor infrared absorption inherent to the vibrational
polarization. In 1980, Hartstein et al.3 first found that the infrared
absorption of molecules on metal films can be enhanced by a factor
of 20. This effect is called surface-enhanced infrared absorption
(SEIRA). Meanwhile, the surface-enhanced Raman scattering (SERS)
has dramatic enhancements achieving single molecule detection
with metallic nano-islands and nanoparticles as the substrates.4 The
reason is that a tremendous electric field around the nanostructure
is produced from the excitation of free electrons in the metal by the
incident photons in the visible region, which is referred to as
localized surface plasmon resonance (LSPR).5-7
molecule, providing unique properties such as the excellent light
harvesting and the high carrier mobility.19-21 Up to date, the hybrid
perovskites have been applied for various optoelectronic devices
including photovoltaic cells, light-emitting diodes and photo-
detectors.22-24 Interestedly, incorporation of metal nanoparticles
into hybrid perovskites has been explored to improve the
performance of perovskite solar cells.25,26 And the electron transfer
between metal nanomaterials and perovskites has also been
demonstrated.27
Here, we have for the first time discovered that the organic-
inorganic halide perovskites can significantly induce the SEIRA of
adsorbed organic molecules. As a special attention focused on
detection of explosives, four nitroaromatic compounds, including
trinitrophenol (TNP), p-nitrophenol (PNP), dinitrotoluene (DNT),
paranitrotoluene (PNT), were used to demonstrate the enhance-
ment capability of the hybrid perovskites for infrared spectroscopy
of small molecules. All of the IR spectra were measured via
transmission modes under the same detection condition. It was
certified that the CH3NH3PbI3 perovskite films resulted in great
enhancements of the IR absorption bands of surface-adsorbed
nitroaromatic molecules.
A similar mechanism of the enhancement effect might be
considered in SEIRA. However, the LSPR frequencies of metals are
generally located in the visible range due to their high carrier
concentrations, and has been ultimately extended to the near-IR
range by the rational design of the nanostructures.8-11 Thus, the
excitation of plasmonic fields across the broad mid-IR region
needed for SEIRA is very hard to be achieved on metallic substrates.
As a consequence, scientists have to exploit alternative materials
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The X-ray diffraction patterns and the scanning electron micro-
scopic image of the as-prepared CH3NH3PbI3 perovskite (Fig. S1 and
S2, ESI†) indicated that the material was of the perovskite type with
well crystallization. Compared to the bare perovskite, the use of
PMMA as a cooperative additive for the perovskite growth could
enhances the morphological control over the polycrystalline film
and exhibit increased moisture stability.28 Here, the CH3NH3PbI3-
PMMA films prepared by annealing the mixture of the precursors of
the perovskite with PMMA were optimised for the content of
PMMA according to the IR absorption of the perovskite with higher
intensity and better reproducibility (Fig. S3, ESI†). In addition, the
positions of the characteristic IR peaks assigned to CH3NH3PbI3 were
not changed by the fabrication process, implying unchange of the
perovskite structure. It was also demonstrated that the existence of
PMMA in the CH3NH3PbI3 perovskite film supplied a good stability
lasting for 15 days under a humid environment (75% humidity, Fig.
S4, ESI†). Based on those results, the CH3NH3PbI3-PMMA film was
subsequent-ly fabricated by using a solution containing 0.2 mol/L
precursors and 10 mg/mL PMMA.
The IR detection of nitroaromatic analytes was performed as
follows. A toluene solution of the analyte was dropcast onto and
spread over the whole surface of the perovskite film, which was
wettable by the solvent. And then a fast-heating process was
applied to evaporate the solvent in a few minutes, avoiding the
aggregation of analyte molecules. After that, no changes in the
characteristic IR signals of the films were observed, indicating no
disturbance to the perovskite film by the operation. And the same
processes were performed on the blank substrate to ensure that
the amount of analyte is the same for the perovskite surface and
the blank substrate.
Fig. 2 IR detection of nitroaromatic molecules with a lower amount (0.25
nmol/mm2). IR spectra (left) and peak heights of characteristic modes (right)
of TNP (a,b), PNP (c,d), DNT (e,f) and PNT (g,h) on the blank substrate (black)
and the perovskite-PMMA film (red), respectively.
composite, respectively. In order to avoid the interference of the
infrared absorption of the perovskite and PMMA, we selected some
characteristic peaks of the analytes for analysis which could be
distinguished from the peaks of the substrate (marked in Fig.1a and
2a, also listed in Table S1, ESI†). Compared to the weak infrared
absorptions of the TNP molecules on the blank substrate, it was
clear to see that both the CH3NH3PbI3 film and the CH3NH3PbI3-
PMMA film significantly enhanced the infrared signals of the TNP
molecules. Hence, the CH3NH3PbI3 perovskite was believed to
function as an amplifier in infrared spectroscopy of small molecules.
To most characteristic vibrational modes of the TNP molecule, the
enhancement capability of the CH3NH3PbI3-PMMA film was a little
larger than that of the CH3NH3PbI3 film, which indicated the
existence of PMMA in the film would not reduce the surface
enhancement. It might be contributed from the polymeric structure
of PMMA, which would not hinder the contact of the analyte with
the perovskite. The peak height of the infrared absorption, defined
as the variation between the peak top and the lower adjacent
bottom, was measured to show the enhancing effect more clearly.
As shown in Fig. 1b, the results further proved that the two
substrates with the CH3NH3PbI3 film and the CH3NH3PbI3-PMMA
film, respectively, assisted in much higher absorption peaks than
the blank substrate. The enhancement of absorption was from
several times up to hundreds of times (for 3100 cm-1). It should be
noticed that the signal peaks at different wavenumbers were
enhanced with different extents. It might be arisen from the
orientation of the molecules adsorbed on the perovskite surface
Fig. 1a shows the infrared spectra of the TNP molecule on the
blank substrate (without the hybrid perovskite films), and on the
film substrates of the CH3NH3PbI3 alone and the CH3NH3PbI3-PMMA
2 | Chem. Commun., 2017, 00, 1-3
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nitro groups (Fig. S7, ESI†). It indicated that there existed not
just the physical adsorption to the surface but also the chemical
Fig. 3 Analysis of the energy-levels of the hybrid perovskite. a) Schematic
representation of layered perovskites. b) The quantum wells resulted from
the alternative arrangement of the inorganic sheets and the organic layers
of the hybrid perovskite.
Fig. 4 Analysis of the electron transitions of the hybrid perovskite. a) Photo-
luminescence spectrum of the perovskite (λex = 330 nm). b) Schematic
illustration of the energies of bandgaps and the electron transitions of the
perovskite.
(also see below), making different functional groups have different
directions toward the surface and different distances from the
surface.
The infrared spectra of a less amount of the four nitroaromatic
analytes (0.25 nmol/mm2) on the blank and the CH3NH3PbI3-PMMA
film substrates, respectively, were shown in Fig. 2. On the blank
subs-trate, the IR signals of the analyte were too weak to be
observed. However, the signals of all four molecules were obviously
presented on the film substrate, indicating the nearly infinite
enhancement of infrared absorption. The amount of the
nitroaromatic analyte really contributed to the optical absorption
was probably less than 1.1 nmol, which is estimated from the areal
density of the TNP molecule and the spot area of the light beam
(4.5 mm2). It means that the IR detection limit of small molecules
using the new SEIRA substrate could be greatly lowered down.
Subsequently, more quantitative analyses were performed, and it
was shown that the infrared absor-bance exhibited a good linear
relationship with the areal density of the analyte (Fig. S6 and Table
S2, ESI†). And thereof the lower limit of detection (LOD) was
estimated to be less than 0.16 nmol/mm2. In addition, the
maximum relative standard deviation of the SEIRA detection of
nitroaromatic molecules was about 4.5 % (for five repeats, 0.25
nmol/mm2), demonstrating a good reproducibility of the assay. It
was worth mentioned that the SEIRA substrate we used was of a
non-optimized prototype, and great improvements can be achieved
after further optimization such as the composition and morphology
of hybrid perovskites, as well as the use of the analyte
concentrating effect due to the wettability of various substrates.29
According to the surface selection rule, the vibrational modes
that have dipole moment derivative components perpendicular to
the surface are preferentially enhanced.30,31 The orientation of the
four nitroaromatic molecules adsorbed on the perovskite film has
been analyzed by the infrared spectroscopy. The peaks around 1063
cm−1 attributed to vibrations of C-H in-plane and bending from the
benzene ring are enhanced more than other vibrational modes (Fig.
1 for TNP and Fig. S5 for PNP, DNT and PNT, ESI†). It could be drawn
that the nitroaromatic molecules were almost flat-lying on the
surface. In addition, there were changes in IR peak positions
corresponding to stretching vibrations of the aromatic C-H and the
interaction of the small molecules with the CH3NH3PbI3 perovskite,
which was also been suggested in a previous report.32
The mechanism of the SEIRA using the CH3NH3PbI3 perovskite
was also analysed through the energy levels of the organic–
inorganic perovskite. The basic layered perovskite structure is
schematically depicted in Fig. 3a. Each of the inorganic sheets
consists of corner-sharing metal halide octahedrons. Fig. 3b shows
that the common arrangement of the energy bands in the material,
where the conduction band (CB) of the inorganic sheets is
substantially below that of the organic layers, and the valence band
(VB) of the inorganic sheets is rationally above that of the organic
layers. Therefore, the alternation of the inorganic sheets and the
organic layers generate the quantum wells for both electrons and
holes, resulting in very narrow bandgaps. Consequently, the
electrons could be excited by the photons at longer wavelengths
from the ground state of the quantum well (organic VB or inorganic
CB) to the excited state (inorganic VB or organic CB). To prove this,
photoluminescence (PL) measurements were performed, as shown
in Fig. 4a. The PL peak at the longer wavelength of 770 nm in the
CH3NH3PbI3 was attributed to the electron transition from the VB
maximum to the CB minimum of the inorganic sheets with a lower
energy (Einorg. = 1.61 eV). The peak located at 721 nm corresponds
mainly to a direct transition between the organic layers, giving a
higher bandgap energy (Eorg. = 1.72 eV). According to Equation (1),
the bandgap energy of the quantum well (Eqw) was less than
0.055eV calculated on the basis of the PL spectra.
Eorg-Einorg
Eqw
≤
= 0.055 eV
(1)
2
It meant that electrons of CH3NH3PbI3 can be excited by
photons in the mid-IR region (provide energy from 0.05 eV to 0.50
eV, see Fig. 4b), generating electron-hole pairs. The charge
separation occurred in the hybrid perovskite was expected to
induce electromagnetic changes and efficiently increase the dipolar
polarization of the small molecules adjacent to the perovskite,
which might be contributed to the enhancement of the infrared
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In conclusion, we demonstrated for the first time that the
organic-inorganic perovskite films enable the acquisition of SEIRA
spectra of small molecules. The new semiconducting substrate
greatly enhanced most of the infrared absorptions of molecular
vibrations, organic molecules, allowing the much more sensitive
detection of small molecules with infrared spectroscopy. This
enhancement effect was probably attributed to the
electromagnetic coupling originated from the electron transitions
across the much narrower bandgap of the quantum wells in the
hybrid perovskite, which could be excited by the mid-IR light. To go
a step further, more efforts would be made to deeply unravel the
complex mechanisms involved in the surface-enhanced
spectroscopic process using the organic-inorganic perovskite films.
Besides, additional polymers pro-tecting the perovskite film while
giving minimal infrared absorption peaks, such as the simplest
polymer polyethylene, could be explored to create a background as
clean as possible, such that the spectral window for analyte
detection is clear and unambiguous. Our exploration of the hybrid
perovskites not only opens up a new direction for the search the
holy grail of SEIRA similar to SERS, but also help to build one big
new brick of the promising applications of the superior material.
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Acknowledgements
We acknowledge the support from the National Defense Pre-
research Foundation of China (Grant No. JG2014073).
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