Head of the Group
Paulina Płochocka-Maude
office: 357, A-1
phone: +48 71 320 37 40
Paulina Plochocka holds a permanent research position of “directeur de recherche” in CNRS, she is also a full Professor in Faculty of Fundamental Problems of Technology, Wroclaw University of Science and Technology. She obtained her PhD cum-laude in 2004 at the University of Warsaw working on the dynamics of many-body interactions between carriers in doped semimagnetic quantum wells (QW). During her first post doc at Weizmann Institute of Science, she started working on the electronic properties of a high mobility 2D electron gas in the fractional and integer quantum Hall effect regime. She continued this topic during a second post doc in LNCMI Grenoble, where she was holding individual Marie Curie scholarship. At the same time, she expanded her interest in 2D materials towards graphene and other layered materials as transition metal dichalcogenides or black phosphorus. In 2012, she obtained permanent position in LNCMI Toulouse, where she created the Quantum Electronics group, which investigates the electronic and optical properties of emerging materials under extreme conditions of high magnetic field and low temperatures. Examples include semiconducting layer materials such as transition metal dichalcogenides, GaAs/AlAs core shell nanowires and organic inorganic hybrid perovskites.
Activities:
Experimental capabalities:
Transition metal dichalcogenides
The field of 2D materials has experienced unwavering interest since many years. For instance, semiconducting TMDs, especially these from Group 6, have non-negligible contribution to the global “boom” of 2D materials. The huge interest in TMDs originates from their unique electronic properties, especially in the monolayer (1L) form, which differ strongly from their bulk counterparts. 1L-TMDs, such as MoS2, MoSe2, WS2, and WSe2, crystallize in a honeycomb lattice that lacks the inversion center. The direct bandgap is located at the nonequivalent K and K’ points of the hexagonal Brillouin zone and are characterized by contrasting magnetic moments and spin orientations. This is reflected in the optical selection rules and valley Hall effect, opening the way for the valleytronic concepts. Together with enhanced excitonic effects, leading to strong light-matter interaction, TMDs are very interesting for new generation nano(opto)electronic devices. Below we present short description of our most important investigation in this field.
In spite of the overall similar band structure, W and Mo based TMDs exhibit different behavior as far as the valley polarization is concerned. In W based TMDs (WX2), valley polarization is very robust, and shows a relatively weak dependence on the energy of the excitation beam. In the case of Mo-based TMDs, a high degree of polarization of the PL is much harder to achieve, even in the case of quasi-resonant excitation, as shown by our measurements of Fig. 1(a–c). We have proposed a model which identifies electron-hole exchange interaction is the main intervalley scattering channel. The efficiency of this scattering mechanism depends on the momentum of the excitonic center of mass, which provides a key to understand the dependence of the degree of circular polarization on the excitation energy, particularly pronounced in MoX2. In the case of MoX2, the efficiency of the dark-bright intraband scattering is reduced, due to the band alignment (the bright state is the lowest lying state). This leads to the exchange interaction as the dominant scattering process, making valley polarization harder to achieve in MoSe2. These results, summarized in Fig. 1(d), can be found in Baranowski, et al., 2D Materials 4, 025016 (2017).
CVD-grown TMDs are known to exhibit poor optical qualities, as demonstrated by the low temperature micro-photoluminescence (μPL) spectra shown in Fig. 2(a,b). In as-grown samples, the free exciton peak is barely visible on the high energy side of a broad emission, as in the case of as-grown MoS2 [see Fig. 2(a)] or completely absent, as in the case of as-grown MoSe2 [see Fig. 2(b)]. In Surrente et al., Nano Letters 17, 4130 (2017), we demonstrated that the optical quality of CVD-grown MoSe2 can be dramatically improved by sandwiching it between two MoS2 monolayers. This defect healing process results in μPL spectra of MoSe2 composed by sharp, well-resolved peaks, identified with the free and charged exciton of MoSe2, as well as in the suppression of emission related to the bound excitons, as suggested by the absence of a broad peak on the low energy side of the MoSe2 charged exciton.
The staggered band alignment of the MoS2/MoSe2 heterostructure results in the formation of interlayer excitons with long lifetimes and robust valley polarization. We have observed long lived interlayer exciton emission in this heterostructure, identified with the low energy peak in the power dependent PL spectra of Fig. 3(a). Their interlayer character has been confirmed by the sublinear increase of the integrated intensity as a function of the excitation power [see Fig. 3(b)], and by the slow recombination dynamics, with PL life times of the order of 10 to 100 ns, see Fig. 3(c). Under circularly polarized excitation, the interlayer exciton emission is intriguingly counter polarized [Fig. 3(d)], with the emitted light which has the opposite helicity as compared to the excitation. Such an effect has never been observed previously. We consistently detected counterpolarized PL of the interlayer exciton for all the excitation energies we adopted, with an increase of the degree of circular polarization at energies corresponding to the resonances of A and B excitons of MoS2 and MoSe2. More details can be found in Baranowski, et al., Nano Letters 17, 6360 (2017).
We have also investigated the magnetophotoluminescence (magnetoPL) of the interlayer exciton in this heterostructure. In Fig. 4(a), we show typical magnetoPL spectra of the interlayer exciton up to 24 T. We observed a large splitting of the interalyer exciton peak, due to valley Zeeman effect, and a significant dependence of the PL intensities of the Zeeman split branches as a function of the magnetic field. By applying magnetic field, we gradually build up an unbalanced interlayer exciton population of the two valleys, which leads to a gradually increasing degree of circular polarization Pc in the system up to Pc ∼ 1 at 28 T, summarized in Fig. 4(b,c) for linear and circular polarization excitation. The dependence of Pc on the magnetic field results from the interplay between intervalley scattering via a variety of channels and the attempt of the exciton population to establish a thermal distribution between the Zeeman split intervalley exciton states. These results are summarized in Surrente, et al., Nano Letters 18, 3994 (2018).
We studied the influence of a prolonged laser exposure on the PL spectrum of intra- and interlayer excitons in the MoS2/MoSe2/MoS2 heterostructure. Photodoping occurs here following the dynamic photoionization of impurities or defects. More details can be found in Zhang, Surrente, et al., Applied Physics Letters 113, 062107 (2018).
A moiré pattern can be formed in TMD heterostructures, due to the lattice mismatch and/or nonzero (but small) stacking angle. According to theoretical predictions, the moiré pattern should result in a periodically modulated potential with minima for both intra- and interlayer excitons, which should lead to an energy splitting of both the intra- and interlayer transitions. In a hBN encapsulated MoSe2 /MoS2 heterostructure, we systematically observed the presence of an additional high energy peak in the optical spectra of intralayer excitons whenever the excitation spot was placed on a heterostructure, as shown in Fig. 5(a) for both PL and reflectivity contrast spectra. As expected, we observed no splitting when the excitation spot was on monolayer areas, off the heterostructure, which is shown in Fig. 5(b). The correlation between the observation of the moiré doublet with the presence of the heterostructure is confirmed by the binary map of Fig. 5(c). Additional results can be found in Zhang, Surrente (equal contributors), et al., Nano Letters 18, 7651 (2018).
We took part in the optimization of the growth of MoSe2 on GaAs (111)B substrates by molecular beam epitaxy. Our measurements focused on polarization resolved second harmonic generation and Raman spectroscopy to demonstrate the single layer character of the grown film and to analyze the orientation of the grown flake. These measurements are reported in Chen, et al., ACS Nano 11, 6355 (2017).
TMD/2D perovskite heterostructures
In our investigations we have also put attention on photophysics of recently emerged van der Waals heterostructures consisting of 2DPs and mono and few-layer TMDs. Joining these two types of materials can potentially reduce their drawbacks and enhance their advantages, providing an exceptional flexibility in the heterostructure design.
Merging 2DPs with TMDs can potentially favour the emergence of a new family of van der Waals materials with unprecedented flexibility for their functional design. The organic spacer -- inherently part of the 2DPs -- naturally provides a separator between the octahedral slab and the TMD layer, where the exciton states are mainly localized. At the same time, TMD layers are highly stable at ambient conditions and can be used to encapsulate fragile 2DP layers. This combination of materials can unlock band alignments that otherwise would be inaccessible with only hBN and TMDs. Taking into account the large number of available organic spacers together with many other tuning mechanisms, this novel approach of heterostructure design can provide a higher flexibility for the band alignment and the excitation transfer mechanism engineering, which is a crucial factor, especially for energy harvesting and detection applications. For example, an efficient ET can be used to sensitize the photoresponse of a material part of a heterostructure by the other material without the reduction of the overall photoresponse yield, characteristic for the interlayer charge transfer.
However, despite the potential advantages of TMD/2DP heterostructures, very little is known about their fundamental properties and if all the available knobs can be indeed utilized for deterministic functionality design, control over excitation transfer or band alignment. At this early stage of the research on these heterostructures, there is even lack of consensus about how the excitation is transferred between TMD and 2DP layers.
2D perovskites
2D perovskites have attracted attention as a significantly more stable alternative to their 3D counterparts while maintaining good performance in photovoltaic and light-emitting applications. Currently they are experiencing a renaissance not only due to their enhanced stability, but also because of the supreme tunability of their optoelectronic properties.
2D perovskites are often regarded as natural, quantum wells consisting of octahedral metal-halide slabs separated by large organic spacers, which are not plagued by interface roughness characteristic of epitaxially grown quantum wells. The structure of 2D perovskites is periodic in the out of plane direction usually they exhibit I band alignment where carriers are located in the octahedral layer. Confinement is provided by the organic spacers, which in addition provide dielectric confinement leading to a huge enhancement of the exciton binding energy, significantly larger in fully inorganic quantum-wells (for example GaAs/AlGaAs), reaching values of a few hundred meV. This strong electron-hole interaction dominates the optical properties of these materials, which are also often affected by exciton-phonon coupling.
A unique feature of 2D perovskites is the immense tunability of their opto-electronic properties in many different ways. As in classic perovskite semiconductors, the bandgap can be tuned by chemical composition and alloying. An additional degree of freedom is provided by the thickness of the inorganic slabs which strongly affect exciton binding energies and bandgaps. Moreover, the properties of 2D perovskites can be significantly modified by an appropriate choice of the building blocks from a plethora of different organic spacers. By varying the organic spacer, a number of properties can be tuned, namely the distortion of the octahedral cages and the dielectric environment, which significantly impact the exciton physics and, potential occurrence of a structural phase-transitions.
In the field of 2D perovskites our investigation focuses on the understanding how all available tuning knobs affects the fundamental properties in this material system. With the use of magnetic field we show how the octahedral distortion control the carriers effective mass [ACS Energy Lett. 2020, 5, 11, 3609–3616, J. Phys. Chem. Lett. 2021, 12, 6, 1638–1643, ACS Energy Lett. 2019, 4, 10, 2386–2392] and put some light on exciton-phono coupling in 2D perovskites [J. Phys. Chem. Lett. 2020, 11, 15, 5830–5835]. Very recently for the first time we directly reveal the position of dark exciton states brightening it by extreme magnetic field [Sci. Adv. 7, eabk0904 (2021)].
Metal-halide perovskites
Metal-halide perovskites are an emerging class of materials, proposed for use as active layers in multiple optoelectronic devices, but brought to the spotlight mainly be the role of light absorbers in a new generation of photovoltaic solar cells. The hybrid perovskites combine excellent absorption properties with large diffusion lengths and long lifetime of the carriers, resulting in photon conversion efficiencies exceeding 25%, or near 30% in perovskite-silicon tandem. Another advantage is the low cost and simplicity of solution-based fabrication process. Therefore, resulting from the rapid development, the perovskite photovoltaics has perspectives to outperform the well-established silicon technology.
The optoelectronic properties of metal-halide perovskites can be easily tuned depending which building blocks are incorporated into the generic perovskite chemical formula ABX3. Most often, A is a large organic cation (Methylammonium, Formamidinium) or inorganic Cs+ cation, B stands for a divalent heavy metal (Pb2+, Sn2+) and X represents halide anions (Br-, Cl- or I-, or alloyed combination of these). However, at the early stage of development in the field, demonstration of various new materials left behind the understanding of their fundamental properties. One of main controversies was related to the value of exciton binding energy, a parameter critical for the operation of photovoltaics devices. Through low-temperature magnetospectroscopy in pulsed magnetic field, we directly determined values of exciton binding energy and reduced mass for various high- and low band gap metal-halide perovskites (ACS Energy Lett. 2019, 4, 3, 615–621, Fig. 1,2). Our results show that the exciton binding energy in low band gap, “photovoltaic” iodides is lower than 16 meV, revealing that the dominant photogenerated species at typical operational conditions of solar cells are free charges rather than excitons. The reduced mass increases approximately proportionally to the band gap, and the mass values can be described with a two-band k·p perturbation model extended across the broad band gap range of 1.2–2.4 eV (Fig. 1). Our findings can be generalized to predict values for the effective mass and binding energy for other members of this family of materials.
An emergence of high optical quality single crystal hybrid perovskites enabled us to investigate exciton fine structure splitting, which reflects the underlying symmetry of the crystal and quantum confinement. Because the latter factor strongly enhances the exchange interaction, most work has focused on nanostructures. We reported on the first observation of the bright exciton fine structure splitting in a bulk semiconductor crystal, where the impact of quantum confinement can be specifically excluded, giving access to the intrinsic properties of the material (Nano Lett. 2019, 19, 10, 7054–7061). Detailed investigation of the exciton photoluminescence and reflection spectra of a bulk methylammonium lead tribromide (MAPbBr3) single crystal revealed a zero magnetic field splitting as large as ∼200 μeV (Fig. 3). This result provides an important starting point for the discussion of the origin of the large bright exciton fine structure splitting observed in perovskite nanocrystals.
Recently, so-called dobuble perovskites (A2B’BX6), compounds containing alternating mono- and trivalent B-site cations (such as Cs2AgBiBr6) have been proposed as interesting alternative to conventional bulk perovskite system. Improved structural stability and elimination of Pb is accompanied here by enhancement of the ionic character of the structure, thus carriers are expected to couple to lattice vibrations even stronger than in ABX3 counterparts. In our studies of in Cs2AgBiBr6 we demonstrated that the photoluminescence (PL) emission is strongly influenced by the strong electron–phonon coupling (J. Mater. Chem. C, 2019,7, 8350-8356). Combining photoluminescence excitation (PLE) and Raman spectroscopy we showed that the PL emission is related to a color center rather than a band-to-band transition (Fig. 4). The broadening and the Stokes shift of the PL emission from Cs2AgBiBr6 is well explained using a Franck–Condon model with a Huang–Rhys factor of S = 11.7, indicating a strong electron–phonon interaction in this material.
Steady-state optical measurements
The optical spectroscopy setup (Fig. 1) is capable of performing the typical characterization experiments including reflectivity, transmission and photoluminescence. Different types of semiconducting materials can be measured (thin films, single crystals). The sample is mounted on the cold finger of a liquid helium cooled cryostat allowing for measurements between 2.2 and 300 K.
The broadband light/laser beam is focused on the sample by a 50x microscope objective with a numerical aperture of 0.55, resulting in the spot size of 1 μm. The signal from the sample is collected by the same objective, directed to the 0.5 m long monochromator and detected by liquid nitrogen cooled CCD camera.
For reflectivity measurements the excitation source is tungsten or xenon lamp. For photoluminescence (PL) measurements a series of continuous wave lasers is available with excitation wavelengths of 405, 532 and 633 nm. For PL, the laser beam was filtered out by the appropriate long pass edge filter.
Spatial PL and reflectivity maps with the step of 1 μm along the X and Y axes are possible with the use of an automated XY translation stages on which the cryostat was mounted.
Photoluminescence excitation
Photoluminescence excitation (PLE) experiments are conducted using the same setup as the PL measurements. The excitation source is a supercontinuum laser. The excitation signal is filtered by a monochromator before entering the microscopic objective. The PLE is a useful technique to probe the charge (and energy) transfer in various heterostructures. Fig. 2 shows the exemplary PLE result for the TMDs monolayer (WS2, MoSe2) and hybrid 2D perovskite (PEPI, BAPI). One can notice that in all panels of Fig. 2 the PLE signal (scatter plot) peaks at the energy corresponding to the B exciton of the respective TMD monolayer as well as at the energy of the 1 s exciton of respective 2D perovskite. Such a result is a fingerprint of hybridization between the two constituent materials of the heterostructure.
Time-resolved measurements
The time-resolved PL signal is spectrally filtered with a monochromator and detected with an Excelitas SPCM-AQRH-15 avalanche photo-diode coupled with a single photon counting system. The PL is excited with a supercontinuum laser or a Hamamatsu C8898 pulsed laser. The time resolution of the setup is 100 ps.
High magnetic field
The group possess a longstanding expertise in the spectroscopic measurements in high magnetic fields, performed with the unique setup developed in National High Magnetic Field Laboratory (LNCMI) in Toulouse, France. This facility offers pulsed magnetic fields with amplitudes up to 91 T with a full width at half maximum of about 100 ms. The LNCMI-Toulouse laboratory possess all the required instrumentation and knowledge to perform the spectrally-resolved optical studies within the timescale of the magnetic field impulse. The general overview of the experimental setup is presented in Fig. 1. The sample under investigation is mounted at the tip of the experimental probe (available probes discussed below). The excitation and detection beams are provided by optical fibres. The probe is inserted into the helium-bath cryostat. The narrow end of the helium cryostat enters the bore of the pulse magnet. Both helium cryostat and magnet are submerged in liquid nitrogen. The magnet is connected to the generator. The magnetic field is generated on demand when the CCD camera is triggered.
The following experimental techniques are well established by the members of the Quantum Electronics group:
I) magneto-transmission
The transmission probe allows to measure the semi-transparent samples (thin films) in both the Faraday (B ‖ k) and Voigt (B ꓕ k) geometries. The broadband light from tungsten/Xe lamp is provided by a 200 μm input fibre and the transmitted signal is collected by 400 μm output optical fibre (see Fig. 2a). The probe’s dimensions (see Fig. 2b) allow for optical measurements up to 70 T at 2.2 K and up to 91 T at 77 K. The evolution of an exemplary transmission spectra with increasing strength of magnetic field is presented in Fig. 2c (2D perovskite (PEA)2PbI4). Clearly at high magnetic field the Zeeman splitting is observed. For more information, please see [1]–[5].
II) magneto-reflectivity
For optical measurements of non-transmitting samples (single crystals) the reflectivity probe was developed. The probe head is presented in Fig. 3a, with a probe tip enlarged in Fig. 3b. The broadband light from tungsten/Xe lamp is provided by a centre fibre (Fig. 3c) and the reflected signal is collected by six outer optical fibres. Due to reduced dimensions (4 mm diameter) this probe allows for optical measurements up to 91 T at T = 2.2 K. [6], [7]
III) magneto-photoluminescence
Photoluminescence measurements in the high magnetic field are realized by both transmission and reflectivity probes, depending on the sample requirements. For both measurement probes the Zeeman-split branches can be measured. Fig. 4a shows typical PL spectra measured at B = 65 T for both left- and right-handed circularly polarized light together with spectrum measured without magnetic field. Additionally, in case of transmission probe measurements in Voigt geometry (B ꓕ k) are possible. Fig. 4b shows the result of such measurement for 2D perovskite (PEA)2PbI4. Clearly, with increasing strength of magnetic field the main emission is redshifting, which is a fingerprint of dark exciton. Fig. 4c shows the low magnetic field regime, where the transfer from the bright (BX) to dark (DX) exciton emission is visible.
