rus
Issue #7/2023
A. O. Savostianov, I. Yu. Eremchev, A. V. Naumov
Luminescence Nanothermometry by Single Organic Molecules: Manifestation of Electron-Phonon Interaction
Luminescence Nanothermometry by Single Organic Molecules: Manifestation of Electron-Phonon Interaction
DOI: 10.22184/1993-7296.FRos.2023.17.7.508.514
Luminescent thermometry is a rapidly growing scientific method based on the dependence of the luminescent and spectral characteristics of nano-sized emitters on temperature. The accuracy of this method depends significantly on the theoretical models used to describe the temperature behavior of the spectra. In this paper, we provide a brief overview of our recent results related to new approaches to interpreting the temperature broadening of the spectral lines of single organic molecules in a polymer matrix as a result of electron-phonon interaction. We believe that the approach under consideration can be successfully applied to a variety of promising emitters used in luminescent thermometry.
Luminescent thermometry is a rapidly growing scientific method based on the dependence of the luminescent and spectral characteristics of nano-sized emitters on temperature. The accuracy of this method depends significantly on the theoretical models used to describe the temperature behavior of the spectra. In this paper, we provide a brief overview of our recent results related to new approaches to interpreting the temperature broadening of the spectral lines of single organic molecules in a polymer matrix as a result of electron-phonon interaction. We believe that the approach under consideration can be successfully applied to a variety of promising emitters used in luminescent thermometry.
Теги: electron-phonon interactionr luminescence polymers thermometry люминесценция полимеры термометрия электрон-фононное взаимодействие
Luminescence Nanothermometry by Single Organic Molecules: Manifestation of Electron-Phonon Interaction
A. O. Savostianov 1, I. Yu. Eremchev 2, 3, A. V. Naumov 1, 3
Lebedev Physical Institute of the Russian Academy of Sciences, Troitsk Branch, Troitsk, Moscow, Russia
Institute of spectroscopy RAS, Troitsk, Moscow, Russia
Moscow Pedagogical State University (MPGU), Moscow, Russia
Luminescent thermometry is a rapidly growing scientific method based on the dependence of the luminescent and spectral characteristics of nano-sized emitters on temperature. The accuracy of this method depends significantly on the theoretical models used to describe the temperature behavior of the spectra. In this paper, we provide a brief overview of our recent results related to new approaches to interpreting the temperature broadening of the spectral lines of single organic molecules in a polymer matrix as a result of electron-phonon interaction. We believe that the approach under consideration can be successfully applied to a variety of promising emitters used in luminescent thermometry.
Keywords: luminescence, thermometry, polymers, electron-phonon interactionr
Article received:05.10.2023
Article accepted: 31.10.2023
Luminescent thermometry today is a rapidly developing scientific field, covering a wide range of areas from cell biology [1] and terranostics [2] to catalytic chemistry [3] and microelectronics [4]. In fact, we are talking about a type of nanosensing: luminescent thermometry uses nano-sized fluorescent labels, which can be used in tasks where the use of traditional methods of temperature measurement is ineffective or completely impossible. The most widely used fluorescent labels are organic molecules in shells of biocompatible polymers [5, 6], nanoparticles containing lanthanide ions [7] and transition metal ions [8], color centers in diamonds and other crystals [9, 10], semiconductor [11] and carbon quantum dots [12].
Among the various methods for measuring temperature using luminescence [13], which include monitoring the intensity of a bright peak in the spectrum, ratiometry (changes in the relative intensity of different peaks in the spectrum), as well as measuring the fluorescence lifetime or its polarization, approaches related to measuring spectral characteristics of a zero-phonon line (ZPL) [14] (its width and spectral shift) are widespread. The temperature behavior of ZPLs is largely determined by the interaction of the emitter with phonons [15], which makes it extremely important to search for theoretical models capable of predicting the temperature behavior of ZPLs with high accuracy.
Below we provide a brief overview of our recent results [16, 17] related to the search for adequate microscopic models of electron-phonon (EP) interaction for impurity molecules in polymer matrices. To do this, we firstly measured the temperature dependences of the ZPL width in the fluorescence excitation spectra of single tetra-tret-butylterrylene (TBT) molecules (SMs) encased in the polymer matrix of polyisobutylene (PIB) in the temperature range of 15–70 K, where the EP interaction is significant. Experimental measurements were carried out on a unique cryogenic epi-luminescent microscope-spectrometer (see details in [18]), which allows to simultaneously detect fluorescent images of all SMs located in the CCD camera field of view. During the experiment, synchronous detuning of the tunable laser wavelength in the region of absorption of impurity SM and detection of fluorescent images was carried out. Using computer analysis, individual SMs were recognized in the fluorescent images, after which the dependences of the integral luminescence intensity of the SM on the excitation wavelength were plotted. In this way, SM fluorescence excitation spectra were obtained, where the accuracy of the ZPL width determining depends on the parameters of the laser (laser linewidth, detuning step) and the signal-to-noise ratio in the experiment.
An example of temperature-dependent spectra for SM TBT in a PIB matrix is shown in Fig. 1. We approximated the data using either a Lorentz contour or the sum of two contours. The latter was due to the detection of the phonon sideband in the spectra at temperatures >40 K [14]. Since the true ZPL shape corresponds to the Voigt profile, we used computer simulation of the spectrum to determine the phonon contribution to the broadening γph(T). The contour of the spectral line I(ωlaser, T) measured in the experiment was modeled as a convolution of the Gaussian laser profile and the Lorentzian ZPL shape, where integration was replaced by summation to simplify calculations:
(1),
where ωlaser corresponds to the laser frequency, γlaser is the laser line width, γ(T) is the desired ZPL width, ωZPL is its spectral position. Using the previously known values of γlaser and ωZPL in the calculations, we simulated spectral lines, which, like the experimentally measured ZPLs, were approximated by the Lorentz function. The value of γ(T) when the widths of two Lorentzian profiles corresponding to the actually measured and model spectra coincided, was taken as the desired one.
The accuracy of temperature measurements using fluorescent thermometry is determined by the theoretical approaches used to describe the temperature behavior of the spectra. For the case of EP interaction, a traditional approach is usually used: It considers the temperature broadening of the ZPL as a result of weak interaction of the emitter either with acoustic phonons (the well-known broadening law γph(T) ~ T2)) [19] or with individual quasi-localized modes [20], when the quasi-exponential law of the broadening is predicted. In our opinion, this approach cannot be considered completely comprehensive. Firstly, it is not at all obvious that it is the weak EP interaction that takes place; secondly, consideration of the density of vibrational states (VDOS) both within the framework of the Debye model and for the case of a single mode of the Lorentzian shape seems overly simplified and does not corresponds to the real solid-state emitters.
To unravel the observed temperature broadening for SMs of TBT in a matrix of PIB, we proposed another explanation, based on the general theory of EP interaction developed by I. S. Osadko [21]. This approach allows one to consider EP coupling of an arbitrary strength based on VDOS of arbitrary shape. By means this theory, we were able to give an acceptable explanation of all the experimental data obtained only by considering the so-called resonant vibrational modes [22]. In fact, such vibrations result from the hybridization of the eigenmodes of the impurity molecule and the normal modes of the matrix. We have shown that, taking into account the VDOS for solids under study, it is possible to explain various types of broadening γph(T) using only two fitting parameters.
An example of a successful description is shown in Fig. 2. To calculate, the following system of equations was used:
(2)
Here Γ(0,1,PIB)(ω) are spectral phonon functions corresponding to the spectrum of resonant modes for the ground (0) and excited (1) electronic states of the SM, as well as the spectrum of normal modes of impurity-free PIB (PIB). DPIB(ω) – VDOS of pure PIB, U0,1 – quadratic coupling constants. From the system of Eqs. 2 follows that using DPIB(ω) and vary U0,1 we can calculate γph(T).
Although our results directly relate to organic molecules in a polymer matrix, we suppose that they can be generalized to a wide range of luminescent materials. So, for example, with U0,1 → 0 and D(ω) ~ ω2, the model under consideration will be reduced to the well-known law γph(T) ~ T2. Of much greater interest, however, are cases where D(ω) can be obtained from the first-principles calculations. Recent publications devoted to the analysis of EP interactions for color centers in diamonds [23] and defects in hexagonal boron nitride [24] have shown that such a task is fundamentally feasible.
Authors acknowledge support from Ministry of education of Russia (state task MPGU AAAA-A20-120061890084-9) and Project for leading scientific school of Russia (NSh‑776.2022.1.2).
AUTHORS
A. O. Savostianov, Lebedev Physical Institute of the Russian Academy of Sciences, Troitsk Branch, Moscow, Troitsk 108840, Russia
ORCID: 0000-0001-8815-8440
I. Yu. Eremchev, Institute of spectroscopy RAS, Moscow, Troitsk; Moscow Pedagogical State University, Moscow, Russia
ORCID: 0000-0002-2239-5176
A. V. Naumov, Lebedev Physical Institute of the Russian Academy of Sciences, Troitsk Branch, Moscow, Troitsk; Moscow Pedagogical State University, Moscow, Russia
ORCID: 0000-0001-7938-9802
A. O. Savostianov 1, I. Yu. Eremchev 2, 3, A. V. Naumov 1, 3
Lebedev Physical Institute of the Russian Academy of Sciences, Troitsk Branch, Troitsk, Moscow, Russia
Institute of spectroscopy RAS, Troitsk, Moscow, Russia
Moscow Pedagogical State University (MPGU), Moscow, Russia
Luminescent thermometry is a rapidly growing scientific method based on the dependence of the luminescent and spectral characteristics of nano-sized emitters on temperature. The accuracy of this method depends significantly on the theoretical models used to describe the temperature behavior of the spectra. In this paper, we provide a brief overview of our recent results related to new approaches to interpreting the temperature broadening of the spectral lines of single organic molecules in a polymer matrix as a result of electron-phonon interaction. We believe that the approach under consideration can be successfully applied to a variety of promising emitters used in luminescent thermometry.
Keywords: luminescence, thermometry, polymers, electron-phonon interactionr
Article received:05.10.2023
Article accepted: 31.10.2023
Luminescent thermometry today is a rapidly developing scientific field, covering a wide range of areas from cell biology [1] and terranostics [2] to catalytic chemistry [3] and microelectronics [4]. In fact, we are talking about a type of nanosensing: luminescent thermometry uses nano-sized fluorescent labels, which can be used in tasks where the use of traditional methods of temperature measurement is ineffective or completely impossible. The most widely used fluorescent labels are organic molecules in shells of biocompatible polymers [5, 6], nanoparticles containing lanthanide ions [7] and transition metal ions [8], color centers in diamonds and other crystals [9, 10], semiconductor [11] and carbon quantum dots [12].
Among the various methods for measuring temperature using luminescence [13], which include monitoring the intensity of a bright peak in the spectrum, ratiometry (changes in the relative intensity of different peaks in the spectrum), as well as measuring the fluorescence lifetime or its polarization, approaches related to measuring spectral characteristics of a zero-phonon line (ZPL) [14] (its width and spectral shift) are widespread. The temperature behavior of ZPLs is largely determined by the interaction of the emitter with phonons [15], which makes it extremely important to search for theoretical models capable of predicting the temperature behavior of ZPLs with high accuracy.
Below we provide a brief overview of our recent results [16, 17] related to the search for adequate microscopic models of electron-phonon (EP) interaction for impurity molecules in polymer matrices. To do this, we firstly measured the temperature dependences of the ZPL width in the fluorescence excitation spectra of single tetra-tret-butylterrylene (TBT) molecules (SMs) encased in the polymer matrix of polyisobutylene (PIB) in the temperature range of 15–70 K, where the EP interaction is significant. Experimental measurements were carried out on a unique cryogenic epi-luminescent microscope-spectrometer (see details in [18]), which allows to simultaneously detect fluorescent images of all SMs located in the CCD camera field of view. During the experiment, synchronous detuning of the tunable laser wavelength in the region of absorption of impurity SM and detection of fluorescent images was carried out. Using computer analysis, individual SMs were recognized in the fluorescent images, after which the dependences of the integral luminescence intensity of the SM on the excitation wavelength were plotted. In this way, SM fluorescence excitation spectra were obtained, where the accuracy of the ZPL width determining depends on the parameters of the laser (laser linewidth, detuning step) and the signal-to-noise ratio in the experiment.
An example of temperature-dependent spectra for SM TBT in a PIB matrix is shown in Fig. 1. We approximated the data using either a Lorentz contour or the sum of two contours. The latter was due to the detection of the phonon sideband in the spectra at temperatures >40 K [14]. Since the true ZPL shape corresponds to the Voigt profile, we used computer simulation of the spectrum to determine the phonon contribution to the broadening γph(T). The contour of the spectral line I(ωlaser, T) measured in the experiment was modeled as a convolution of the Gaussian laser profile and the Lorentzian ZPL shape, where integration was replaced by summation to simplify calculations:
(1),
where ωlaser corresponds to the laser frequency, γlaser is the laser line width, γ(T) is the desired ZPL width, ωZPL is its spectral position. Using the previously known values of γlaser and ωZPL in the calculations, we simulated spectral lines, which, like the experimentally measured ZPLs, were approximated by the Lorentz function. The value of γ(T) when the widths of two Lorentzian profiles corresponding to the actually measured and model spectra coincided, was taken as the desired one.
The accuracy of temperature measurements using fluorescent thermometry is determined by the theoretical approaches used to describe the temperature behavior of the spectra. For the case of EP interaction, a traditional approach is usually used: It considers the temperature broadening of the ZPL as a result of weak interaction of the emitter either with acoustic phonons (the well-known broadening law γph(T) ~ T2)) [19] or with individual quasi-localized modes [20], when the quasi-exponential law of the broadening is predicted. In our opinion, this approach cannot be considered completely comprehensive. Firstly, it is not at all obvious that it is the weak EP interaction that takes place; secondly, consideration of the density of vibrational states (VDOS) both within the framework of the Debye model and for the case of a single mode of the Lorentzian shape seems overly simplified and does not corresponds to the real solid-state emitters.
To unravel the observed temperature broadening for SMs of TBT in a matrix of PIB, we proposed another explanation, based on the general theory of EP interaction developed by I. S. Osadko [21]. This approach allows one to consider EP coupling of an arbitrary strength based on VDOS of arbitrary shape. By means this theory, we were able to give an acceptable explanation of all the experimental data obtained only by considering the so-called resonant vibrational modes [22]. In fact, such vibrations result from the hybridization of the eigenmodes of the impurity molecule and the normal modes of the matrix. We have shown that, taking into account the VDOS for solids under study, it is possible to explain various types of broadening γph(T) using only two fitting parameters.
An example of a successful description is shown in Fig. 2. To calculate, the following system of equations was used:
(2)
Here Γ(0,1,PIB)(ω) are spectral phonon functions corresponding to the spectrum of resonant modes for the ground (0) and excited (1) electronic states of the SM, as well as the spectrum of normal modes of impurity-free PIB (PIB). DPIB(ω) – VDOS of pure PIB, U0,1 – quadratic coupling constants. From the system of Eqs. 2 follows that using DPIB(ω) and vary U0,1 we can calculate γph(T).
Although our results directly relate to organic molecules in a polymer matrix, we suppose that they can be generalized to a wide range of luminescent materials. So, for example, with U0,1 → 0 and D(ω) ~ ω2, the model under consideration will be reduced to the well-known law γph(T) ~ T2. Of much greater interest, however, are cases where D(ω) can be obtained from the first-principles calculations. Recent publications devoted to the analysis of EP interactions for color centers in diamonds [23] and defects in hexagonal boron nitride [24] have shown that such a task is fundamentally feasible.
Authors acknowledge support from Ministry of education of Russia (state task MPGU AAAA-A20-120061890084-9) and Project for leading scientific school of Russia (NSh‑776.2022.1.2).
AUTHORS
A. O. Savostianov, Lebedev Physical Institute of the Russian Academy of Sciences, Troitsk Branch, Moscow, Troitsk 108840, Russia
ORCID: 0000-0001-8815-8440
I. Yu. Eremchev, Institute of spectroscopy RAS, Moscow, Troitsk; Moscow Pedagogical State University, Moscow, Russia
ORCID: 0000-0002-2239-5176
A. V. Naumov, Lebedev Physical Institute of the Russian Academy of Sciences, Troitsk Branch, Moscow, Troitsk; Moscow Pedagogical State University, Moscow, Russia
ORCID: 0000-0001-7938-9802
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