Ultra-Thin Films with Eu3+ Ions for Probing Effects of Local Environment

Abstract

Ultra-thin films of Eu(TTA)3(L1a) have been fabricated by Langmuir-Blodgett technique on plasmonic and dielectric substrates and used to probe the effect of local environment on magnetic dipole emission and charge transfer processes.

Full text

Ultra-thin Films with Eu3+ Ions for Probing Effects of Local
Environment
A. Bullock, M. Clemmons, M. Shahabuddin, S. Mashhadi, C. Yang, C.E Bonner, N. Noginova
Norfolk State University, 700 Park Ave, Norfolk, Virginia 23504, USA
a.d.bullock18904@spartans.nsu.edu
!
Abstract: Ultra-thin films of Eu(TTA)3(L1a) are fabricated by Langmuir-Blodgett technique on
plasmonic and dielectric substrates and used to probe the effect of local environment on magnetic
dipole emission and charge transfer processes.
OCIS codes: (300.2140) Emission; (160.4236) Nanomaterials
A strong modification in the photonic mode density in or near plasmonic systems and metamaterials can significantly
alter various quantum processes, such as spontaneous emission, energy transfer or charge transfer processes. Rare
earth ions are convenient spectroscopic tools to study such modifications. Yb and Er ions were used to study plasmon
enhanced energy transfer [1]. Eu3+ ions having both electric and magnetic dipole transitions in the optical range were
used to probe and map optical magnetic fields in metal nanoholes with magnetic resonance at the range of Eu emission
[2]. Since these effects are expected in a very close proximity to plasmonic nanostructures, the emitting layer should
be of controllable thickness and have an efficient luminescence.
The lanthanide complex Eu(TTA)3(L1a), (Fig 1 (a), where TTA is thenoyltrifluoroacetonate and L1a is 2-(N, N-
diethylanilino-4-yl)-4,6-bis(3,5-dimethylprrazol-1-yl)-1,3,5-triazine, is one of the brightest luminescent materials.
This material can be excited in the ultraviolet to blue range, and exhibits red luminescence at Eu3+ ttransitions
sensitized via the efficient siglet energy transfer pathway from ligand L1a to Eu3+ (Fig 2 (b, c)) [3]. In the excitation
spectrum, features observed in the range of 360 -450 are associated with inter-ligand and ligand-metal charge transfer
processes [3, 4]. The emission spectrum shows the magnetic dipole transition 5D0 → 7F1 at 590 nm which is spectrally
close to the brightest electric dipole transition 5D0 → 7F2, [3]. Thus, several features in the spectroscopy of
Eu(TTA)3(L1a) can be sensitive to the effects of optical environment, which makes this material promising for use in
metamaterials and plasmonics studies.
The Langmuir-Blodgett (LB) technique allows one to prepare single layer or multilayer films with a controllable
thickness, and high density of closely packed and similarly oriented molecules which is importnat in molecular
electronics, non-linear optics and conducting thin film technologies [5]. In our work, we demonstrate a possibility to
employ LB technique for fabrication of ultra-thin films of Eu(TTA)3(L1a), and study modifications in the emission
and excitation spectra in the films deposited onto different substrates.
Eu(TTA)3(L1a) is synthesized in-house using the method described in [3]. The solution of Eu(TTA)3(L1a) in
chloroform is spreaded on the water surface. After evaporation of chloroform, the surface layer is compressed. The
area-pressure isotherm is shown in Fig 2 (a), featuring a “solid” phase formation with the decreased area. Then, the
obtained monolayer is transferred from the water surface onto different substrates, including glass, gold, silver and
nanostructured gold. For comparison purposes, we also prepare a thick Eu(TTA)3(L1a) film with random orientation
of molecules by placing a drop of the same solution on a glass substrate and subsequent drying.
Fig.1.!(a) Schematics of Eu(TTA)3(L1a) molecule; (b) eEnergy levels; (c) Excitation (Trace 1) and emission (Trace 2)
spectra of Eu(TTA)3(L1a) (thick fi lm).
Eu3+
Ligand
S0
S1
T15D0
5D1
7F0
7F2
7F6
!
!"#
!"$
!"%
!"&
'
#(! )(! $(! ((! %(!
*+,-+./,012345"6"7
839-: -+;, <121+= 71
'
#
>3? >5? >@?

Emission and excitainon spectra are measured with the fluorometer. In Fig 2 (c), the magnetic dipole transition
5D0 → 7F1 (at 590 nm) is shown for the monolayer films deposited onto silicon and gold, and the thick film. All the
spectra are normalized on the maximum value corresponding to the electric transition 5D0 →7F1 (at 613 nm). As one
can see, the magnetic line is the strongest in vicinity of silicon, which can be ascribed to the different boundary
conditions for electric and magnetic fields at the reflection from a high-index dielectric material. In the film on gold,
however, the magnetic line is relatively small, which is due to the effect of plasmonic modes as is discussed in detail
in [6].
The most interesting finding is the strong modification in the excitation spectrum observed in the monolayers,
Fig 2 (d, f). First, the spectrum of the monolayers is completely different from that of the thick film. Instead of the
highly asymmetric broad line peaked at 450 nm observed in the thick film, two well-resolved peaks with a Gaussian
shape and the width of ~ 28 nm are observed centered at 360 nm and 420 nm. The relative height of the peaks depends
on the particular substrate. They are of approximately the same height in the films on glass, and strongly different in
the films on metal substrates. According to [3], the 420 nm line can be ascribed to the ligand-metal charge transfer
transition. The relative increase of this peak in the film on the gold nanomesh substrate is consistent with the recently
observed acceleration of the charge transfer process in switching of electrochromic polymer observed in the similar
systems [7].
In conclusion, we demonstrate a possibility to fabricate ultra-thin films with Eu3+ emitters using Langmuir-
Blodgett technique. The spectroscopic properties of the films are defined by energy transfer, charge transfer processes,
and electric and megnatic dipole transitions, and are found to be very sensitive to the local environment.
The work is supported by NSF PREM #1205457 and RISE #1646789 grants.!!
References
[1] D. Lu et al, “Experimental demonstration of plasmon enhanced energy transfer rate in NaYF4:Yb3+,Er3+ upconversion nanoparticles,” Sci.
Rep. 6, 18894 (2016).
[2] R. Hussain et al, “Enhancing Eu3+ magnetic dipole emission by resonant plasmonic nanostructures,” Opt. Lett. 40, 1659-1662 (2015).
[3] C. Yang et al, Angew. Chem. Int. Ed., 43, 5010-5013 (2004); Angew. Chem. 116, 5120-5123 (2004).
[4] L. M. Fu et al. “Role of ligand-to-metal charge transfer state in nontriplet photosensitization of luminescent europium complex,” J. Phys. Chem.
A 114, 4494-500 (2010).
[5] S. A. Hussain, “Langmuir–Blodgett films and molecular electronics, “Mod. Phys. Lett. B 23, 3437 (2009).
[6] R. Hussain, D. Keene, N. Noginova, M. Durach, “Spontaneous emission of electric and magnetic dipoles in the vicinity of thin and thick metal,”
Opt. Express 22, 7744-7755 (2014).
[7] M. Shahabuddin, T. McDowell and N. Noginova, “Exploring Plasmonic Environment with Electrochromic Polymer,” MRS Fall Meeting,
Boston, MA, 2017, # EM03.18.02.
!
!"#
!"$
!"%
!"&
'
'"#
#(! )(! $(! ((!
*+,-+./,012345"6"7
839-:- +;, <121+=7 1
!
!"#
!"$
!"%
!"&
'
#!! (!! $!! )!! %!!
*+,-+./,012345"6"7
839-:- +;,<121 +=71
-10
0
10
20
30
40
50
0300 600 900
Barrier&position&[mm]
(a) (b)
(d) (e) (f)
Thick
>?@;
>?;:3..
M/glass
M/Au
M/nano
!
!"!#
!"!$
!"%&
!"%'
!"&
($! '!! '&! '#!
)*+,*-. +/012,3"4"5
678,3,* 9+:010*;5
>?A/
B<CD
>?@6
(c)
Fig.2.!(a) Surface-pressure isotherm; (b) Luminescent film on the water surface in LB trough under UV illumination;
(c) Magnetic dipole transition of a thick film (Thick) and monolayers deposited on Si (M/Si) and gold (M/Au); (d)
Excitation spectra of a thick film (Thick) and monolayers on glass (M/glass) and silver (M/Ag); (e) SEM image of
gold nanomesh fabricated by depositing gold on the porous alumina membrane; (f) Excitation spectra of monolayes
deposted on glass (M/glass), gold (M/Au) and nanomesh (M/nano).