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Tim Völzer, Alina Schubert, Erik von der Oelsnitz, Julian Schröer, Ingo Barke, Rico Schwartz, [Kenji Watanabe](https://orcid.org/0000-0003-3701-8119), [Takashi Taniguchi](https://orcid.org/0000-0002-1467-3105), Sylvia Speller, Tobias Korn, Stefan Lochbrunner

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[Strong quenching of dye fluorescence in monomeric perylene orange/TMDC hybrid structures](https://mdr.nims.go.jp/datasets/c50eab6b-a0ab-4f25-9ca8-291fb8007a88)

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Strong quenching of dye fluorescence in monomeric perylene orange/TMDC hybrid structuresNanoscaleAdvancesPAPEROpen Access Article. Published on 22 May 2023. Downloaded on 6/14/2023 2:11:17 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article OnlineView Journal  | View IssueStrong quenchinaInstitute of Physics, University of RostockGermany. E-mail: stefan.lochbrunner@uni-rbDepartment “Life, Light and Matter”, Univ18059 Rostock, GermanycResearch Center for Electronic and Optical MScience, 1-1 Namiki, Tsukuba 305-0044, JapdResearch Center for Materials NanoarchiteScience, 1-1 Namiki, Tsukuba 305-0044, Jap† Electronic supplementary informationpreparation techniques, photodegradatioresults at 10 K, additional FLIM resultssamples and without the use ofhttps://doi.org/10.1039/d3na00276dCite this: Nanoscale Adv., 2023, 5,3348Received 25th April 2023Accepted 22nd May 2023DOI: 10.1039/d3na00276drsc.li/nanoscale-advances3348 | Nanoscale Adv., 2023, 5, 334g of dye fluorescence inmonomeric perylene orange/TMDC hybridstructures†Tim Völzer, ab Alina Schubert, ab Erik von der Oelsnitz,ab Julian Schröer, abIngo Barke,ab Rico Schwartz,a Kenji Watanabe, c Takashi Taniguchi,d Sylvia Speller,abTobias Korn ab and Stefan Lochbrunner *abHybrid structures with an interface between two different materials with properly aligned energy levelsfacilitate photo-induced charge separation to be exploited in optoelectronic applications. Particularly,the combination of 2D transition metal dichalcogenides (TMDCs) and dye molecules offers strong light–matter interaction, tailorable band level alignments, and high fluorescence quantum yields. In this work,we aim at the charge or energy transfer-related quenching of the fluorescence of the dye peryleneorange (PO) when isolated molecules are brought onto monolayer TMDCs via thermal vapor deposition.Here, micro-photoluminescence spectroscopy revealed a strong intensity drop of the PO fluorescence.For the TMDC emission, in contrast, we observed a relative growth of the trion versus excitoncontribution. In addition, fluorescence imaging lifetime microscopy quantified the intensity quenching toa factor of about 103 and demonstrated a drastic lifetime reduction from 3 ns to values much shorterthan the 100 ps width of the instrument response function. From the ratio of the intensity quenchingthat is attributed to hole or energy transfer from dye to semiconductor, we deduce a time constant ofseveral picoseconds at most, pointing to an efficient charge separation suitable for optoelectronic devices.1. IntroductionIn the past decade, transition metal dichalcogenides (TMDCs)have emerged as 2D semiconductors. The key features of thesematerials are the transition from indirect to direct semi-conductors, which is accompanied by the emergence of theirphotoluminescence,1,2 as well as a drastic rise of the excitonbinding energy when thinned down from bulk to a monolayer(1L).3 This shis the recombination dynamics from the regimeof Auger scattering of free carriers to being governed by thediffusion of excitons.4,5 In combination with the strong light–matter interaction, these monolayer properties promise poten-tial applications in photonics and optoelectronics ranging from, Albert-Einstein-Str. 23, 18059 Rostock,ostock.deersity of Rostock, Albert-Einstein-Str. 25,aterials, National Institute for Materialsanctonics, National Institute for Materialsan(ESI) available: Parametrization ofn of dye molecules, AFM scans, PLfor the hBN, MoS2, MoSe2, and WS2the band pass lter. See DOI:8–3356LEDs to photodetectors.6 In the latter case, charge separationand transfer following optical excitation can be facilitated by thefabrication of heterostructures, where 1L-TMDCs are combinedwith one another,7,8 other 2D semiconductors,9 graphene,10 orhybrid structures with 0D objects such as small molecules orquantum dots, 1D nanostructures, as well as 3D bulk mate-rials.11,12 As an already established research eld, TMDC heter-ostructures represent the rst choice among the above named.Here, ultrafast charge transfer times of 50 fs and less have beenfound.13,14 Yet, these heterostructures remain limited regardingtheir performance and costs.11Beyond such heterostructures, the combination of 2Dsemiconductors with dye molecules brings several benets onits own, which touch different aspects of the systems. First, onthe fundamental side, hybrid structures allow the combinationof two contrary regimes of exciton mobility, namely the discreteFörster transfer between essentially 0D molecules15,16 asopposed to the continuous exciton diffusion in the 2DTMDCs.4,5 Second, from a preparational point of view, manymethods for molecule deposition on surfaces can be scaled,offering the opportunity of preparing large-area hybrid struc-tures, starting from full-coverage chemical vapor deposition(CVD-)grown monolayers.17,18 Third, the band levels of mole-cules such as perylene diimides (PDIs) can easily be tailored byexchanging their organic substituents,19 while their opticalspectra exhibit characteristic shapes that signicantly change© 2023 The Author(s). Published by the Royal Society of Chemistryhttp://crossmark.crossref.org/dialog/?doi=10.1039/d3na00276d&domain=pdf&date_stamp=2023-06-12http://orcid.org/0000-0002-1085-9594http://orcid.org/0009-0003-4810-3551http://orcid.org/0000-0002-2990-2302http://orcid.org/0000-0003-3701-8119http://orcid.org/0000-0003-4808-391Xhttp://orcid.org/0000-0001-9729-8277https://doi.org/10.1039/d3na00276dhttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d3na00276dhttps://pubs.rsc.org/en/journals/journal/NAhttps://pubs.rsc.org/en/journals/journal/NA?issueid=NA005012Paper Nanoscale AdvancesOpen Access Article. Published on 22 May 2023. Downloaded on 6/14/2023 2:11:17 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlineupon aggregation20 or charge transfer.21 Finally, in view ofspectroscopic investigations, the near-unity uorescencequantum yield of these dyes ensures strong signals,19 incontrast to typical values below 1% for pristine 1L-TMDCs.1,22Several studies on hybridmolecule/1L-TMDC structures havebeen conducted with their focus lying on changes of the pho-toluminescence from the TMDC monolayer.23–26 To that end,comparably thick dye layers of several 10 nm have beendeposited. In this case, the molecules function as an essentiallyinnite charge trap, i.e. draining excited electrons or holes fromthe absorbing TMDC layer on a timescale of picoseconds.27–30In the inverse situation, where molecular layers (ofseveral nm thickness) on a 2D material are excited, the chargetransfer takes place towards the TMDC and proceeds signi-cantly faster, with reported upper limits for the time constantsof a few 100 fs down to 40 fs. The charge transfer is typicallyfollowed by thermal relaxation of the hot charge carriers orinterlayer excitons, even spin ips and the formation of tripletexcitons may occur.29–32While these studies mostly focus on nanometer lms ofelectron-donating metal phtalocyanine molecules30–33 on MoS2,investigations with purely organic dyes such as the electron-accepting PDIs were conducted in wet media.21,34 In this work,however, we create hybrid structures by vapor depositingmonomeric dye molecules of the PDI N,N′-bis(2,6-diisopropylphenyl)-3,4,9,10-perylenetetracarboxylic diimide,commonly referred to as perylene orange (PO). In contrast toprevious studies, we chose a molecular coverage well belowa monolayer, creating essentially 0D regimes as opposed to 2Dcontinuous or even 3D bulk-like dye lms. As the inorganiccounterpart in the hybrid structures and deposition target, weprepared mechanically exfoliated monolayer akes of the fourmost common TMDCs (MX2 with M = Mo, W and X = S, Se) asFig. 1 Electronic energy levels of various 2D materials versus peryleneorange. The vacuum level is set to zero. The horizontal lines show theHOMO and LUMO energies of the isolated dye molecule, whosemolecular structure is depicted in the inset.35 Pale and dark barsindicate the value range for VBM and CBM energies extracted fromvarious publications reporting on density functional theory calcula-tions (hBN;36 MoS2;23,27,37–39 MoSe2;23,38–40 WS2;28,37–39,41 WSe2 (ref. 23,38 and 39)). In these calculations, excitonic effects were not consid-ered. While the hBN band levels lie far from the molecular orbitals, foreach TMDC, at least one of the band extrema falls into theHOMO–LUMO-gap of PO.© 2023 The Author(s). Published by the Royal Society of Chemistrywell as multilayer hexagonal boron nitride (hBN) as a reference.Fig. 1 shows their relevant energy levels, that is the valence band(VB) maximum and conduction band (CB) minimum for the 2Dmaterials and the highest occupied and lowest unoccupiedmolecular orbital (HOMO and LUMO, respectively) of the dye.When a photon is absorbed by the PO (TMDC), an electron fromthe HOMO (VB) will be raised to the LUMO (CB), leaving a holein the HOMO (VB) with whom it forms an exciton. As the hBNband gap completely comprises the relevant energy levels of PO,neither charge nor energy transfer is expected from the dye intothe hBN. For each of the PO/TMDC hybrids, in contrast, bothprocesses are possible, offering a rapid nonradiative decaychannel. Consequently, we expect the uorescence lifetime toshorten drastically and the intensity to decrease strongly, as themolecular exciton will less likely recombine radiatively. Thus,we investigate the hybrids via spectrally, spatially and tempo-rally resolved emission spectroscopy.2. ExperimentalHybrid structures were prepared by thermal vapor deposition(TVD) of monomer PO lms onto mechanically exfoliated 2Dakes transferred to Si/SiO2 wafers.42 PO powder (Exciton, Exa-lite 578) is heated to temperatures from 430 K to 450 K andevaporated in vacuum (#1 × 10−2 mbar) in order for themolecules to condense onto the target substrate kept at aboutroom temperature. The total process duration amounted to40 min, including roughly 15 min of heating up the powderreservoir from room to the evaporation temperature. Thespecic evaporation temperature was adjusted to the targetcoverage by coating glass slides as references in advance. Here,we aimed at a coverage of about 2.4 × 10−2 nm−2, corre-sponding to roughly130of a monolayer and ensuring themonomeric behavior of the deposited molecules (see ESI,Sections 1.1 and 1.2†). As an alternative coating technique,stamping was also tested in Section 1.3 of the ESI.† This offersa low-threshold method similar to the deterministic transfer of2D materials. However, it yields more inhomogeneous molec-ular lms.For micro-photoluminescence (m-PL) spectroscopy, a 532 nm(RLTMGL-532-100-3, Roithner) or 633 nm (OBIS 633LXSF,Coherent) continuous wave laser is focused onto the sample.The emitted radiation is collected by a microscope objective,ltered from the scattered excitation light using a long-passplus a spatial lter with a 50 mm pinhole, and analyzed bya grating spectrometer (Acton 2300i). In our experiments, theexcitation power was about 4.5 mW for the green and 9 mW forthe red laser, with a focus diameter of about 1.0 mm to 1.5 mm.The sample was scanned beneath the xed optical beam path toobtain a 2D raster of the relevant area, with one spectrumacquired at each position within 1 s. To prevent photooxidationof the dye molecules,43 the samples were kept in vacuum duringthe experiment. An investigation of photodegradation underambient conditions is presented in the ESI, Section 2.† If notmentioned otherwise, all measurements were conducted atroom temperature and employing the green laser. The coolingNanoscale Adv., 2023, 5, 3348–3356 | 3349http://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d3na00276dNanoscale Advances PaperOpen Access Article. Published on 22 May 2023. Downloaded on 6/14/2023 2:11:17 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinefor low-temperature scans was provided by liquid nitrogen for80 K and by liquid helium for 10 K.Fluorescence lifetime imaging microscopy (FLIM) measure-ments were performed using the MicroTime 200 system byPicoQuant. In these experiments, the sample is excited by442 nm pulses with an energy of 0.3 pJ at a repetition rate of40 MHz (equivalent to 12.5 mW), focused down to a diameter of0.9 mm. The emitted light is detected via time-correlated singlephoton counting, yielding a high sensitivity as well as a timeresolution of about 100 ps. Analogously to the m-PL spectros-copy, the sample area is scanned to obtain a 2D map, witha dwell time of 50 ms per pixel. This short exposure time iscrucial to minimize the photodegradation of the dye in thosemeasurements, as they have to be conducted under ambientconditions. Another difference of this setup compared to them-PL spectroscopy lies in the lack of spectral resolution.Consequently, we are not able to separate the PO uorescencefrom the contributions of the monolayer photoluminescence(see ESI, Section 4.2†). To solve this problem, we placed a 50 nmband pass lter with a central wavelength of 525 nm in theprobe beam right in front of the detector, in addition to thestandard lters for blocking the excitation light. We tested thelter performance using 1L-WS2, which – among the fourinvestigated TMDCs – has the strongest PL (in that particularsetup) that is also the closest to the lter transmission range.Here, we demonstrated an intensity suppression of the PL bya factor of about 104. As a positive side-effect, the selectedspectral range roughly matches the 0–0-peak of the dye uo-rescence. When comparing this to the spectra of any agglom-erated molecular species (see ESI, Sections 1 and 3†), we ndthat those contributions are essentially excluded from thesignal. Accordingly, the measurements with the band pass lterare mostly sensitive to the monomer PO. The instrumentresponse function (IRF) was determined as the detected timetrace of the back-reected excitation light. It was measuredusing a blank wafer sample and omitting both lters. Here, weextracted an IRF duration of about 150 ps (FWHM) with anasymmetry in terms of a prolonged tail at positive times.3. Spectral emission landscape ofhybrid structuresTo characterize the deposited molecular lms and their inter-action with the substrate or the underlying akes, we conductedoptical experiments on TVD-fabricated 1L-TMDC/PO andhBN/PO hybrid structures. First, m-PL spectroscopy was per-formed to elaborate any spectral changes that would indicatea coupling between the 2D material and the dye molecules oreven within the molecular layer.Fig. 2(a) and (d) show two investigated hybrid structures of POon hBN and WSe2, respectively, with the resulting m-PL spectrabeing depicted in (c) for PO on the wafer substrate, on hBN, andon 1L-WSe2. Despite the use of a long-pass lter, a sharp peakfrom the excitation light remains around 2.33 eV. Although thelong-pass lter weakens the 0–0 band of the molecular uores-cence band, PO contributes the characteristic multi-peak3350 | Nanoscale Adv., 2023, 5, 3348–3356Franck–Condon structure of its emission.19 This indicates dyemonomers as the dominant species (Spectral signatures of otherspecies are discussed in the ESI, Sections 1.1 and 3†). Addi-tionally, the monolayer WSe2 ake features a strong, sharp, anddistinct photoluminescence peak at around 1.65 eV. This allowsa spectral separation of the two emission bands, facilitatinga separate mapping of the integrated dye uorescence (1.8 eV to2.325 eV) and 1L-PL intensity (1.6 eV to 1.7 eV), as depicted inFig. 2(b), (e) and (f), respectively. The so-obtained integratedintensity maps reproduce the crystal topography, as can be seencomparing Fig. 2(a) and (d) with Fig. 2(b) and (e). Whenregarding the spectral range of the semiconductor PL (Fig. 2(f)),the monolayer regions clearly stand out of their darksurroundings, as expected by the strong luminescenceenhancement due to the transition from direct to indirect bandgaps in TMDCmonolayers compared to thicker akes.1,2 The dyeuorescence, on the other hand, appears weaker on the hBNakes compared to the wafer substrate and vanishes almostcompletely in the PO/WSe2 hybrid, as evident in the PL spectra aswell. For an ideal PO/hBN hybrid structure, we would not expectany modulation or reduction of the uorescence intensity, giventhe band level alignment. In reality, the moderate intensity dropon the hBN can be caused by adsorbed species acting as chargetraps.44 Alternatively, it could be that during TVD, evaporatedmolecules less likely deposit on hBN compared to the SiO2surface of the wafer, resulting in a lower PO coverage andcorrespondingly lower signal. Nevertheless, sufficiently many POmolecules reside on top of the ake, since the dye uorescence isclearly detected from the hBN area. Due to the similarity of thehBN and TMDC surfaces, we assume a similarmolecule coverageon the WSe2 ake. We conclude that in this case, the PO emis-sion is unambiguously quenched as a consequence of charge orenergy transfer into the TMDC.With the suppression of the dye uorescence below theexperimental sensitivity, we turn to investigate the WSe2 PLcounterpart in more detail to track changes introduced by thedeposited molecules. Under this scope, low-temperaturemeasurements offer the distinction of different emissionfeatures like neutral excitons and charged ones, i.e. trions,45whose balance reacts to changes in doping or defect avail-ability.46 PL spectra acquired at liquid nitrogen temperatureshow a clear separation of the exciton (X0) and negative trion (X−,identied via measurements at 10 K, see ESI Section 4.1†)emission bands (Fig. 3). Upon deposition of PO, the latter growsrelatively to the former. On rst sight, this appears as a sign forcharge transfer.33 In our case, however, this interpretation wouldcontradict the band alignment of PO and 1L-WSe2, as depicted inFig. 1, since electrons would have to migrate energetically uphilltowards the WSe2 CB or holes downhill into the PO HOMO. Inother words, the observed change in the trion emission is ofopposite sign as the charge transfer expected from the bandlevels. Furthermore, the spectral shape is independent of theexcitation wavelength and thus unaffected by whether themolecules are excited or not. Ergo, the major effect originatesalready from the presence of molecules in their electronicground state. A possible explanation for the enhanced trion PLcould arise from changes in the nonradiative recombination© 2023 The Author(s). Published by the Royal Society of Chemistryhttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d3na00276dFig. 2 m-PL spectroscopy after excitation of TVD-coated hBN and WSe2 at 532 nm. (a and d) Micrographs of the sample flakes and theirenvironment. The dashed square in (a) marks the area depicted in (b), while (d)–(f) show identical regions. (b and e) Integrated PO fluorescenceintensitymaps. The intensity appears slightly lower on the thin hBN and almost zero on theWSe2 flake compared to the surrounding substrate. (c)PL spectra of PO on different 2D crystals vs. substrate reference. The sharp peak at 2.33 eV is assigned to stray light from the excitation, thethree-peak structure matches the PO emission, and the single low-energy contribution corresponds to the monolayer WSe2 PL. (f) Integrated1L-WSe2 PL intensity map. The monolayer part stands out brightly from the surrounding thicker crystals and the substrate.Fig. 3 Normalized PL spectra of blank WSe2 (solid lines) and PO/WSe2hybrid structure (dashed lines) at room temperature and 80 K (brightand dark colors, respectively) after excitation at 532 nm (green) and633 nm (red). The latter are pairwise essentially identical. At lowtemperature, the 1L-PL blue-shifts and splits up spectrally into exciton(X0) and negative trion (X−) emission. Upon deposition of the dyemolecules, the trion contribution grows relative to the excitonic one.Additionally, a weak signal appears at 750 nm (marked as P).Paper Nanoscale AdvancesOpen Access Article. Published on 22 May 2023. Downloaded on 6/14/2023 2:11:17 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinechannels in 1L-WSe2. Deposited molecules could screen defects,thereby reducing the rate for trapping at those sites24,47 that takesplace within several 10 ps at room temperature.4 This would© 2023 The Author(s). Published by the Royal Society of Chemistrylargely benet the trionic emission that occurs on similar timescales while the comparatively short-living excitons remainrelatively unaffected.48,49 Consequently, the peak ratio mightevolve in favor of the negative trions without an actual change ofthe doping. In principle, this effect could even conceal a minorcharge transfer in the (expected) opposite direction. As an alter-native explanation, the deposition of PO molecules could elimi-nate other adsorbates that initially suppressed the formation ofor the emission from negative trions. Either way, the dye mole-cules apparently do not drain enough electrons from or – ifexcited – inject enough holes into the 1L-WSe2 to signicantlyalter the charge balance there. This appears reasonable, given thelow dye coverage of 2.4× 1012 cm−2, i.e. onemolecule per roughly40 nm2 (see ESI, Section 1.2†) compared to the areal size of the1L-WSe2 unit cell of 0.1 nm2 (h1015 cm−2).39 Hence, in thiscoverage regime, molecular functionalization does not affect thedoping level of 2D semiconductors signicantly. Indeed, studiesthat observed a pronounced charge transfer-related change ofthe exciton–trion balance employed much thicker molecularlayers of several nm.33 In addition to the excitonic and trionicpeak, we observe a weak emission band (P) around 750 nm thatemerges in the hybrid structure at low temperatures. Whilea detailed investigation of this feature exceeds the scope of thiswork, several possible physical origins are conceivable. First, theNanoscale Adv., 2023, 5, 3348–3356 | 3351http://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d3na00276dNanoscale Advances PaperOpen Access Article. Published on 22 May 2023. Downloaded on 6/14/2023 2:11:17 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlineemission could stem from the triplet state T1 of PO. For perylenederivatives, these states usually lie energetically low, frequentlydown to half of the singlet S1 energy.50 However, this phospho-rescence would be expected to exhibit a spectral width compa-rable to the uorescence peaks, which does not hold for the Pcontribution.51 Second, excitons and trions could combine toform (charged) biexcitons (see ESI Section 4.1†) or bind todefects, creating localized, yet luminescent excitons.52,53 Thisprocess could be enabled by the deposited dye molecules eithervia the modied Fermi level or the screening of defects. Finally,the emission peak could be a sign of interlayer exciton forma-tion, where the electron resides in the PO layer and the hole inthe WSe2.28,54,55Summing up the m-PL measurements, we found a strongindication for uorescence intensity quenching as well asa relatively enhanced trion PL when combining PO with WSe2 ina hybrid structure. Yet, we cannot distinguish unambiguouslybetween charge and energy transfer as the dominant contribu-tion to the uorescence quenching. Nevertheless, as chargetransfer is generally observed in type II band alignments,27–33 wefavor this interpretation. Even if energy transfer would outcom-pete hole transfer from the molecular into the semiconductorlayer, a consecutive back-transfer of electrons is expected inPO/WSe2 hybrids, resulting in charge separation aer all.30Regardless of the specic mechanism responsible for thequenching, any fast nonradiative decay channel does not onlyreduce the uorescence intensity but shortens its lifetime as well.Aiming at the quantication of this phenomenon, we turn totemporally resolved measurements.4. Rapid fluorescence quenching inTMDC hybridsTo be capable of time-resolved detection of even the weakuorescence from various PO/TMDC hybrids, we performedFig. 4 (a and b) Intensity maps of TVD-coated hBN1 and WSe2, respectivmatch the micrographs from Fig. 2(a) and (d), with the crystal topograpsubstrate to flake is drastically stronger on WSe2 than on hBN. (c) FLIMenvironment. The bars cover the uncertainty interval of each value. W1L-TMDCs exhibit relative intensities in the order of 10−3. As discussed in trelative intensity of PO on hBN. Note that the value for WS2 is overestimatpass filter (see ESI, Sections 4.3 and 4.4†).3352 | Nanoscale Adv., 2023, 5, 3348–3356FLIM on these samples, achieving a higher sensitivity than inthe m-PL spectroscopy and tracking the temporal evolution ofthe emission signal. For the analysis, spatial and temporalinformation were regarded separately. First, we take a look atthe time-integrated FLIM data for each pixel. Here, in analogy tothe m-PL, the integration yields 2D intensity maps of the scan-ned sample areas, as illustrated in Fig. 4(a) and (b) for the hBNand WSe2 akes shown in Fig. 2(a) and (d), respectively. ForhBN, the ake topography is resembled by the intensity of thePO uorescence varying between different regions of the crystal.Yet, no systematic dependence on the thickness is found andthe variation of the overall intensity within the hBN area as wellas versus the substrate reaches one order of magnitude at most.The PO-coated WSe2 akes, in contrast, exhibit an enormousdiscrepancy compared to the environment, with the intensitiesdropping by three orders of magnitude. To properly quantifythis effect, we determined both the minimum and maximumabsolute intensity values for the substrate area and the thinnestake regions for two different hBN samples as well as for thefour 1L-TMDCs. We calculated the minimum relative uores-cence intensity as the ratio of the minimum value on the akedivided by the maximum on the substrate and vice versa for themaximum relative intensity. This way, the comparison with thesubstrate should eliminate differences in dye molecule coveragebetween the samples. The resulting relative intensity ranges arepresented in Fig. 4(c), while the original data for the remainingakes in terms of 2Dmapsmay be looked up in the ESI, Fig. S9.†Regarding the relative intensities, we clearly observea systematic difference between the TMDC monolayers and thehBN references. Surprisingly, the hBN1 and hBN2 samples –albeit prepared in an identical manner – yield clearly differentvalues. However, experiments on a third sample with a slightlyhigher coverage (hBN3) demonstrated that the uorescencesignal is affected by the formation of PO agglomerates on thehBN surface (see ESI, Section 4.2†). Such agglomerates mayform due to diffusion of the molecules on the surface, whichely, as obtained by time-integrating the FLIM data. The depicted areashy being clearly reproduced in the FLIM map. The intensity drop fromintensity on different 2D flakes relative to their respective substratehile the three hBN samples range between 0.2 and near-unity, thehe ESI, Section 4.2,† the hBN3 sample represents a lower bound for theed, as there is still some amount of 1L-PL transmitted through the band© 2023 The Author(s). Published by the Royal Society of Chemistryhttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d3na00276dFig. 5 Normalized fluorescence decay of PO films on differentmaterials. The curves result from integrating the FLIM data over therespective areas of interest. Solid lines represent the data, dashed linesshow the fits, which are in good agreement with the data, and the filledarea marks the instrument response function. The TMDC curvesessentially follow the IRF – despite a small long-living component –while the curves for substrate and hBN show a much slower decreaseor at least a long-term tail.Table 1 Time constants and amplitude ratios from biexponential timetrace fitting. All underlying fluorescence time traces were measuredemploying the 500–550 nm band pass filter. Here, s1 for the PO/TMDChybrids is very close to the time constants for the IRF, indicating thatthe underlying dynamics could not be separated from the instrumentresponse. The SiO2 entries represent a summary of the values for thecoated wafer environment of all hBN and TMDC samples. The timetraces for hBN3 are depicted in the ESI, Fig. S8(b)Substrate for PO layer s1 [ns] s2 [ns]A2A1SiO2 0.5–1.0 2.2–3.2 0.60–1.3IRF 0.065 0.14 0.53hBN1 0.12 2.9 0.13hBN2 0.22 3.1 0.97hBN3 (homogeneous) 0.46 3.5 3.3hBN3 (agglomerates) 0.10 2.2 0.0451L-MoS2 0.073 1.4 0.00561L-MoSe2 0.087 0.92 0.0481L-WS2 0.071 0.92 0.0151L-WSe2 0.085 3.1 0.050Paper Nanoscale AdvancesOpen Access Article. Published on 22 May 2023. Downloaded on 6/14/2023 2:11:17 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehas been monitored by AFM scans (see ESI, Section 3†). Asa result, small differences in PO coverage together with varyingsurface characteristics of the hBN1 and hBN2 akes can lead tostrong distinctions between those samples regarding agglom-erate formation and correspondingly differing uorescenceintensities. To exclude this effect, the white bar for hBN3 inFig. 4(c) represents a largely agglomerate-free region, i.e. witha homogeneous, almost pure monomer coating. Consideringthat the agglomerates form by draining molecules from theirenvironment, this homogeneous area is probably characterizedby a lower molecule coverage than the ake in total as well asthe surrounding substrate. Consequently, the hBN3 value canbe regarded as a lower bound for the relative intensity of theuorescence of monomer PO on hBN, leaving a range fromabout 0.3 to unity. The PO/TMDC hybrids, in contrast, reachvalues of about 10−3, close to the noise level. Merely PO/1L-WS2scores 1%. However, in this case, the monolayer PL is strongenough to still contribute some photon counts despite theusage of the band pass lter. Taking this into account, weconclude a uorescence intensity reduction by a factor ofroughly 103 on the TMDC monolayers compared to the hBN.This stands out from other hybrid structures with thickermolecular layers where the uorescence intensity on the1L-TMDC vs. substrate differ by a factor of ten at most.21,31,32,41 Inour case, the planar shape of the molecule probably leads toa face-down orientation on the TMDC monolayers, which inturn promotes efficient charge transfer. Another reason for thedrastic effect in our case supposedly lies in the monomericcoating, while in lms of several nanometer thickness, excitonshave to diffuse towards the interface before charge transfer cantake place. This leaves more time for radiative recombinationand therefore increases the residual uorescence intensity.Although these ndings strongly point to the occurrence ofsome quenching mechanism, a nal conclusion still requiresinformation on the time evolution of the uorescence. In orderto analyze the emission decay dynamics, we extracted the time-resolved intensity by integrating the FLIM data over therespective regions of interest, namely the substrate, the thinnesthBN areas, and the TMDC monolayers. The obtained normal-ized time traces are depicted in Fig. 5 as bright, solid lines. Forthe substrate and the hBN akes, we observe a fast signalreduction on the sub-nanosecond scale followed by a long-termcontribution decaying over several ns. The time traces of thePO/TMDC hybrids, on the contrary, essentially follow the courseof the IRF, indicating a decay time constant well below theexperimental time resolution.For a detailed analysis, we tted the time traces by twoexponentials, yielding a good agreement with the data, as canbe seen from the dark, dashed lines in Fig. 5. In this process,the signal rise was modeled by an error function. The resultingparameters are summarized in Table 1. For the shorter expo-nential time constant s1 on hBN, we obtain values of a few100 ps, varying between the different samples and regions. Thelonger time constant s2, in contrast, consistently yields about3 ns. We assign this to the monomer PO molecules, as it iscomparable to the uorescence lifetime of almost 4 ns insolutions of similar monomers.56,57 The origin of the shorter© 2023 The Author(s). Published by the Royal Society of Chemistrylifetime can again be elucidated with the help of the hBN3sample. Here, we were able to separate regions dominated byagglomerates from areas largely governed by homogeneouslydeposited monomer PO (see ESI Section 4.2†). While for theformer regions, the fast decay accounts for most of theamplitude and the slow component almost vanishes, anopposite behavior is observed in the latter. Thus, we concludethat s1 can be assigned to energy transfer from excited POmonomers into agglomerate-associated multi-particle states,e.g. excimers, which do not emit in the spectral range of thebandpass lter.20 Furthermore, when comparing the tparameters for the different hBN akes and regions, we ndNanoscale Adv., 2023, 5, 3348–3356 | 3353http://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d3na00276dNanoscale Advances PaperOpen Access Article. Published on 22 May 2023. Downloaded on 6/14/2023 2:11:17 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinethat s1 takes smaller values, the higher the amplitude of thefast decay compared to the slow one. This could point toa mechanism where the presence of more or bigger agglom-erates actually speeds up the decay, e.g. in a diffusion-mediatedexciton trapping model. Alternatively, this behavior may resultas a tting artifact from the superposition with the asymmetricIRF. Indeed, the latter can also be tted biexponentially withs1 = 65 ps and s2 = 140 ps.The PO/TMDC hybrids, as already expected from themeasured time traces, essentially follow the IRF, manifesting inthe values of s1 from 70 ps to 90 ps. Note that these only indicatethe resolution limit as determined by the IRF. They are not to beconfused with the actual time constants of the underlyingphysical processes, which lie well below the time resolution ofthis setup. In addition to the fast decay, the ts reveal different,yet in any case weak long-term contributions to the time traces.Following the interpretation of charge or energy transfer-induced quenching, this tail could stem from imperfectcontact between the dye and the TMDC layer, leaving some POmolecules only weakly or even unquenched. They would emitresidual long-living uorescence, thereby leading to an over-estimation of the relative intensity on the PO/TMDC hybridstructures in Fig. 4(c) as well.With this information and taking into account the intensityratios discussed above, we can estimate the time constant of thenonradiative decay. For hBN, the relative intensity takes valuesof 0.3–1, although in the case of the hBN2 sample with a pre-vailing monomer contribution in the decay, it approaches unity.In contrast, the TMDCs achieve values of about 10−3. Assuminga comparable PO coverage due to the similar surface, we obtaina quenching factor of nearly 103. As this also resembles the ratioof the nonradiative vs. the radiative time constant of 3 ns, weend up with a charge or energy transfer time from dye tosemiconductor of several picoseconds. Yet, we have to considerthat even the hBN2 area shows agglomerate features in the timetrace. This results in a lower uorescence than for a perfectmonomer coating and thus an underestimation of thequenching in the PO/TMDC hybrids. The same holds for thepossibly imperfect contact of themolecules on the respective 2Dmaterial that – in the case of the TMDCs – would inhibit chargeor energy transfer for a fraction of the molecules and thusreduce the observed quenching. All taken together, this is inagreement with charge transfer time constants of 40 up toseveral 100 fs for metal phtalocyanine/TMDC hybridsystems.29–325. ConclusionIn this study, we investigated PO/1L-TMDC hybrid structureswith a molecular sub-monolayer by means of m-PL spectroscopyand FLIM to gather evidence for charge or energy transfer aeroptical excitation. We observe a drastic reduction of the dyeuorescence intensity on all TMDCs, as opposed to the PO/hBNreferences. Simultaneously, the lifetime of the emission signalis strongly reduced from about 3 ns to values well below theFLIM time resolution. This veries the occurrence of a strongquenching mechanism induced by the TMDC monolayers. Vice3354 | Nanoscale Adv., 2023, 5, 3348–3356versa, the deposition of PO molecules enhances the trionemission of WSe2 at low temperatures. This cannot beaccounted for by charge transfer, so it may result from the POscreening defects of the monolayer. Based on the quenchingratio and the radiative lifetime of the unquenched dye mono-mers, we deduced an upper limit for the corresponding hole orenergy transfer time constant in the order of several picosec-onds. These results are in line with previous research on chargetransfer in metal phtalocyanines and pave the way for itsexploitation in optoelectronic devices.Author contributionsT. V., S. L., T. K., I. B., and S. S. conceptualized and planned theproject as well as contributed to writing the manuscript. T. V.,A. S. and E. O. tested and established the coating techniques.K. W. and T. T. provided the hBN bulk crystals. R. S. set up them-PL spectroscopy. T. V. and A. S. performed the preparationand optical characterization of the hybrid structures at roomtemperature. T. V. and J. S. executed the low-temperatureexperiments. I. B. conducted the AFM measurements. S. L.,T. K., and S. S. supervised the project.Conflicts of interestThere are no conicts to declare.AcknowledgementsThis work was funded by the Deutsche For-schungsgemeinscha (DFG, German Research Foundation) –SFB 1477 “Light–Matter Interactions at Interfaces”, projectnumber 441234705. T. V. expresses his gratitude to theUniversity of Rostock for its nancial support via the PhDScholarship Program. T. K. gratefully acknowledges funding bythe DFG via grant no. KO 3612/7-1, project number 467549803.K. W. and T. T. acknowledge support from the JSPS KAKENHI(grant numbers 20H00354 and 23H02052). We thank ReginaLange for assisting the AFM measurements.References1 K. F. Mak, C. Lee, J. Hone, J. Shan and T. F. Heinz, Phys. 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Published by the Royal Society of Chemistryhttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d3na00276d Strong quenching of dye fluorescence in monomeric perylene orange/TMDC hybrid structuresElectronic supplementary information (ESI) available:... Strong quenching of dye fluorescence in monomeric perylene orange/TMDC hybrid structuresElectronic supplementary information (ESI) available:... Strong quenching of dye fluorescence in monomeric perylene orange/TMDC hybrid structuresElectronic supplementary information (ESI) available:... Strong quenching of dye fluorescence in monomeric perylene orange/TMDC hybrid structuresElectronic supplementary information (ESI) available:... Strong quenching of dye fluorescence in monomeric perylene orange/TMDC hybrid structuresElectronic supplementary information (ESI) available:... Strong quenching of dye fluorescence in monomeric perylene orange/TMDC hybrid structuresElectronic supplementary information (ESI) available:... Strong quenching of dye fluorescence in monomeric perylene orange/TMDC hybrid structuresElectronic supplementary information (ESI) available:... Strong quenching of dye fluorescence in monomeric perylene orange/TMDC hybrid structuresElectronic supplementary information (ESI) available:... Strong quenching of dye fluorescence in monomeric perylene orange/TMDC hybrid structuresElectronic supplementary information (ESI) available:...