Photoswitchable optoelectronic properties of 2D MoSe2/diarylethene hybrid structures

The ability to modulate optical and electrical properties of two-dimensional (2D) semiconductors has sparked considerable interest in transition metal dichalcogenides (TMDs). Herein, we introduce a facile strategy for modulating optoelectronic properties of monolayer MoSe2 with external light. Photochromic diarylethene (DAE) molecules formed a 2-nm-thick uniform layer on MoSe2, switching between its closed- and open-form isomers under UV and visible irradiation, respectively. We have discovered that the closed DAE conformation under UV has its lowest unoccupied molecular orbital energy level lower than the conduction band minimum of MoSe2, which facilitates photoinduced charge separation at the hybrid interface and quenches photoluminescence (PL) from monolayer flakes. In contrast, open isomers under visible light prevent photoexcited electron transfer from MoSe2 to DAE, thus retaining PL emission properties. Alternating UV and visible light repeatedly show a dynamic modulation of optoelectronic signatures of MoSe2. Conductive atomic force microscopy and Kelvin probe force microscopy also reveal an increase in conductivity and work function of MoSe2/DAE with photoswitched closed-form DAE. These results may open new opportunities for designing new phototransistors and other 2D optoelectronic devices.

adaptability, scalability, and noninvasive/reversible functionalization, making it a flexible and cost-effective choice for a wide range of applications 20,21 .
Diarylethenes (DAEs) [22][23][24] are photoswitchable organic molecules that, alongside azobenzenes 25,26 and spiropyrans 27 , are the most popular photochromic family.These materials respond to light stimuli; they undergo dynamic structural transformation between two or more isomeric forms upon irradiation of specific wavelengths 28,29 .Several studies have explored interactions between photochromic molecules and TMD flakes.Given that the photoswitching behavior of DAE, FETs with light-controlled on/off behaviors were demonstrated using few-layer WSe 2 /DAE 30 , by modifying charge transport at the hybrid interfaces.Similarly, azobenzenes and spiropyrans were exploited to modulate electrical signals of TMDs [31][32][33] .Despite these efforts, the optical characteristics in photoswitchable TMD devices and underlying theoretical principles remain largely unexplored.
This work elucidates dynamic and reversible photo-modulations of monolayer MoSe 2 /DAE hybrid structures both experimentally and theoretically.Density functional theory (DFT) calculations are used to design and understand the interfaces and photo-processes.DAE undergoes unique transformations when exposed to UV or visible light, switching between two isomers, closed and open forms.We demonstrate that its highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels shift during the transformation and that the photoswitch can be exploited for modulating optoelectronic properties of monolayer MoSe 2 .We also show that PL intensities are modulated drastically by alternating UV and Vis light irradiation, which may be cycled repeatedly.Conductive atomic force microscopy (C-AFM) and Kelvin probe force microscopy (KPFM) confirm the light-controlled charge transfer at the hybrid interface.Our findings will provide critical insights into fundamental properties in 2D semiconductors and offer new opportunities for photoswitchable hybrid devices.

Preparation of MoSe 2
P-doped SiO 2 /Si substrates were used for the sample preparation.These substrates underwent sequential ultrasonication in the following order: acetone, methanol, and deionized water (DI), with each solvent treatment lasting 15 min.Subsequently, the substrates were subjected to forced air drying to remove residual fluids and then placed on a hot plate at 110 °C for 5 min.The mechanical exfoliation from bulk crystals (2D semiconductor) was applied to obtain MoSe 2 flakes, as described previously 17,34 .Briefly, Scotch magic tape was used to affix the exfoliated flakes for 10 min on a hot plate at 50 °C, and they were then compressed for an additional 10 min.Similarly, samples were also prepared on indium tin oxide glass (ITO, MTI KJ Group) substrates with the hot plate treatment extended to 30 min.After tape removal, each sample was subjected to annealing under Argon gas for an hour to improve interfacial contact and eliminate contaminants 19,35 .

Functionalization of MoSe 2 with DAE
DAE solutions were prepared in chloroform at approximately 0.02 mg/mL.Exfoliated MoSe 2 samples on silicon wafer and ITO glass substrates, were immersed in the DAE solution for 12 h at 4 °C, forming a uniform layer of DAE on MoSe 2 30 .Afterward, the excess DAE solution on the substrate was removed using an air gun, and the samples were soft-baked on a hot plate at 50 °C for 30 min.The deposited DAE molecules demonstrate a uniform film on MoSe 2 flakes, while their exact orientation may be affected by several factors including steric interaction.To prevent unintended light exposure, the samples were stored in a dark room before UV and visible light irradiation.To investigate the photoswitching behavior, MoSe 2 /DAE samples were exposed to UV for 2 min and visible light for 10 min (see SI Sect.S2 and Fig. S3) 30 .UV light was generated using a Spectroline™ TE lamp (312 nm, 8W), while visible light was filtered through a 530 nm long-pass filter on a Xe-arc lamp with a power intensity of ~ 7.7 mW/cm 2 , monitored using a Newport power meter.

DFT calculation
Vienna Ab initio Simulation Package (VASP) was employed for DFT calculations, utilizing the projector augmented wave (PAW) method 38,39 .Electron exchange and correlation were treated with the generalized gradient approximation in the form of Perdew-Burke-Ernzerhof (PBE) functional 40 .The electronic minimization was performed with a tolerance level of 10 -5 eV.To prepare DAE-MoSe 2 hybrid systems, a DAE isomer was placed above the (5 × 5) MoSe 2 monolayer slab (see Fig. S5) and a vacuum of 14 Å was added.The corresponding areal density is 4.2 × 10 13 molecule/cm 2 .To properly describe van der Waals interactions between the MoSe 2 monolayer and DAE, a dispersion correction was added using the DFT-D2 method of Grimme 41 .All ionic positions were relaxed until Hellmann-Feynman forces were less than 10 mV/Å with a 2 × 2 × 1 Γ-centered k-point mesh for the DAE-(5 × 5) MoSe 2 hybrid systems.After the ionic relaxation, a finer k-point mesh of 4 × 4 × 1 Γ-centered mesh was used for generating accurate density of states.

Optical characterization
The Raman and optical measurements were conducted using a Renishaw inVia Raman confocal microscope at room temperature.Flake inspection was conducted with a charge-coupled device camera (CCD) to find monolayer MoSe 2 .The experiments were conducted using 10 ×, 20 ×, 50 ×, and 100 × objective lenses to identify the locations of flakes on the substrates.PL measurements for the flakes were carried out using a 785 nm laser (~ 0.1 mW) and a 100 × objective lens, thus avoiding the absorbance range of DAE while inducing photo-processes in MoSe 2 .Note that the PL distribution does not follow a Gaussian distribution, especially at high energies (or under 790 nm).This is because the MoSe 2 emission is close to the excitation energy (785 nm) which is removed using a notch filter.However, the measured emission peak at ~ 795 nm is consistent with other studies 42,43 , suggesting that this effect may be negligible in characterizing photo-modulations.

AFM, C-AFM, and KPFM characterization
The exfoliated flakes and DAE-functionalized samples were examined with Bruker Dimension Icon AFM.A SCANASYST-AIR probe was employed in topological imaging to determine the flake thickness, while a SCM-PIT-V2 probe coated with Pt-Ir was used for C-AFM and KPFM measurements.For the C-AFM and KPFM measurements, samples were prepared on ITO glass substrates.In C-AFM, a current range was established within ± 12 nA with a sensitivity of 1 nA V −1 to prevent potential damage to both the tip and the sample while applying a bias voltage from − 2 to 2 V 17,19 .

Results and discussion
Figure 1a presents the isomerization scheme for the DAE derivative used in this study.The open isomer absorbs exclusively in the UV region, whereas the closed isomer displays absorption also in the visible spectrum (Fig. S3).
We hypothesize that the photoswitching behavior of DAE molecules would be retained within deposited layers on top of TMD flakes.This is a reasonable assumption given previous reports that included several photochromic molecules interfacing TMDs as discussed above.
In this work, the photoswitchability of DAE molecules is designed to engineer charge transfer mechanisms in MoSe 2 /DAE and thereby modulate the optoelectronic properties of the TMD monolayers.UV and visible irradiation will thus control the exciton formation and behavior.Our scheme is illustrated in Fig. 1b, where the valence band maximum (VBM) and conduction band minimum (CBM) of monolayer MoSe 2 are plotted against the two sets of HOMO and LUMO energy levels corresponding to the open and the closed DAE photoisomers.Both MoSe 2 and DAE energies were obtained from DFT calculations (vide infra) which are in excellent agreement with previous report 44 .The CBM and VBM energies against the vacuum level are approximately − 3.85 and − 5.35 eV, respectively.The energy levels are consistent with previously reported data 45,46  As a result, UV irradiation will reduce the radiative recombination of photoexcited electron-hole pairs, resulting in a PL quenching 16,47 .Visible illumination will isomerize the DAE into the open-form and recover the emission properties.This mechanism should be reversible and fast, occurring on the picosecond timescale 22,48 .
The energies of monolayer MoSe 2 , DAE, and hybrid MoSe 2 /DAE were obtained from DFT calculations using VASP 38,39 .Figure 2a presents the electronic states of DAE isomers with the vacuum level set as zero.The transformation from the closed isomer to the open raises the LUMO energy level and lowers the HOMO, resulting in an increase in the HOMO-LUMO gap by about 1.1 eV.The density of states (DOS) of the DAE/MoSe 2 monolayer slab is shown in Fig. 2b.The VBM of monolayer MoSe 2 is set as zero for comparison purposes.With the open DAE isomer, both HOMO and LUMO stay outside the bandgap of MoSe 2 .When the DAE isomerizes to the closed-form, the LUMO shifts into the bandgap, providing available states for the electrons in MoSe 2 to transfer to.The HOMO remains outside the bandgap.Thus, there would be no hole transfer.The DFT results support our photo-modulation scheme.
To examine our strategy, we first investigated the PL quenching effect.MoSe 2 flakes were prepared on a SiO 2 /Si substrate via mechanical exfoliation and annealed at 250 °C for an hour.This procedure enhances the PL intensity of the n-type MoSe 2 by depleting excess electrons 49,50 .The samples were then immersed in a DAE chloroform solution, forming a layer of DAE molecules on MoSe 2 (see the "Materials and methods" section).Figure 3a shows an optical image of MoSe 2 , with dotted lines marking the monolayer flake's boundary.To examine the surface and height of the MoSe 2 /DAE hybrid before and after functionalization, we conducted AFM measurements, as depicted in Fig. 3b,c.Figure 3d presents the height information derived from the AFM images, revealing the thickness of monolayer MoSe 2 of ~ 0.8 nm and the DAE layer of ~ 2 nm.DAE has an absorption range in the visible spectrum from 450 to 650 nm (Fig. S3).To avoid the absorbance range and excite MoSe 2 exclusively, we selected a 785 nm laser for excitation.To study the interaction of MoSe 2 with closed-and open-form DAE, we irradiated the hybrid MoSe 2 /DAE sample with UV (~ 312 nm for 30 s) and visible light (530-nm long-pass filter for 10 min), respectively.
Monolayer MoSe 2 /DAE exhibited strong PL signature peaking at ~ 795 nm (Fig. 3e), consistent with previous reports 17,42 .A sharp feature at ~ 818 nm (equivalent to ~ 520 cm −1 ) is a Raman signature from the underlying silicon wafer.After UV irradiation, the closed-form isomers promoted the photoexcited electron transfer from MoSe 2 to DAE, which resulted in a drastic PL quenching by ~ 80% in intensity compared to the open-form under visible light.Note that the intensity of the Raman signature did not change significantly during the PL quenching.The energetically favorable charge transfer mechanism is attributed to the closed DAE's LUMO energy being lower than the CBM of MoSe 2 , as designed in our scheme.In contrast, the LUMO level of the open-form is higher than the CBM.Therefore, electron transfer is prohibited, and PL quenching is not observed.To test the reversibility and consistency of this photoswitching behavior, we repeatedly exposed the sample to visible and UV light for 10 cycles.The result in Fig. 3f shows remarkable stability of MoSe 2 /DAE photoswitch with a steady quenching ratio, ranging between 70 and 85%.During the cycle repetition, there is a gradual decrease in PL intensity difference between visible and UV irradiation.For example, it gradually drops to about 90% of the initial PL difference after the 10th cycle.This degradation, termed fatigue, results from the irreversible formation of byproducts during repeated switching, which may be slowed down by optimizing the ligands or functional groups 36 .
UV irradiation to yield the ring-closed isomers will not only facilitate photoinduced electron transfer, suppressing excitonic recombination and leading to PL quenching, but also increase the conductivity in MoSe 2 .To probe this effect, we prepared DAE-coated MoSe 2 flakes onto ITO glasses and examined them under C-AFM for current-voltage measurements.The samples were placed between the ITO and Pt-Ir probe as depicted in Fig. 4a   www.nature.com/scientificreports/light) at the same applied potential.For example, the current after UV irradiation was greater than 12 nA in magnitude at ± 1 V, while it is nearly 0 nA under visible light.This increase in current is attributed to the UVinduced promotion of charge separation and transport due to the LUMO energy lowered by ~ 0.5 eV and below the CBM of MoSe 2 .In contrast, the LUMO energy of the open DAE is higher than the CBM and does not facilitate current generation, unlike the closed DAE.Since the VBM of MoSe 2 is at approximately − 5.35 eV, it is lower than the work function of ITO (about − 4.75 eV) 51 .Therefore, a Schottky contact is established between MoSe 2 and ITO, developing the Schottky barrier and yielding nonlinear current behaviors.When photoisomerization of DAE occurs, it alters the work function of MoSe 2 /DAE due to interlayer charge transfer between MoSe 2 and DAE.We investigated the surface potential differences using KPFM to measure the change in work function.In the MoSe 2 /DAE structure, the energy level shift caused by UV exposure results in electron transfer to DAE, balancing the energy gap.The work function was calculated as follows 52,53 : where CPD stands for contact potential difference, and Φ S and Φ tip are the work functions of the sample and Pt-Ir probe (~ 5.05 eV) 19,54 , respectively.Figure 5

Conclusions
In summary, we have demonstrated a drastic photo-modulation of the optoelectronic properties of TMD monolayers using photochromic DAE molecules.Photoisomerization of DAE within an ultrathin layer on MoSe 2 flakes regulates photoinduced charge transfer at the hybrid interface.PL studies, conductive AFM, and KPFM measurements all confirm the proposed photoswitch scheme.Our findings prove the viability of charge transport modulation on 2D semiconductors by simple external light illumination.Given the extensive library of photochromic molecules, this approach may be applied to other 2D materials matching their electronic band structures.This will allow for novel designs of optoelectronic devices, offering simplicity in both preparation and device operation.We envision that this work will open new possibilities in 2D hybrid TMD systems. https://doi.org/10.1038/s41598-024-57479-zwww.nature.com/scientificreports/ Figure1apresents the isomerization scheme for the DAE derivative used in this study.The open isomer absorbs exclusively in the UV region, whereas the closed isomer displays absorption also in the visible spectrum (Fig.S3).We hypothesize that the photoswitching behavior of DAE molecules would be retained within deposited layers on top of TMD flakes.This is a reasonable assumption given previous reports that included several photochromic molecules interfacing TMDs as discussed above.In this work, the photoswitchability of DAE molecules is designed to engineer charge transfer mechanisms in MoSe 2 /DAE and thereby modulate the optoelectronic properties of the TMD monolayers.UV and visible irradiation will thus control the exciton formation and behavior.Our scheme is illustrated in Fig.1b, where the valence band maximum (VBM) and conduction band minimum (CBM) of monolayer MoSe 2 are plotted against the two sets of HOMO and LUMO energy levels corresponding to the open and the closed DAE photoisomers.Both MoSe 2 and DAE energies were obtained from DFT calculations (vide infra) which are in excellent agreement with previous report44 .The CBM and VBM energies against the vacuum level are approximately − 3.85 and − 5.35 eV, respectively.The energy levels are consistent with previously reported data45,46 .The LUMO and HOMO levels of open-form DAE are approximately − 3.80 eV and − 6.32 eV, while − 4.31 eV and − 5.75 eV for closed-form isomers.Notably, the LUMO level of closed-form DAE is below the CBM of monolayer MoSe 2 , whereas that of the open-form is above the CBM.Both isomers have their HOMO energies below the VBM.This suggests the feasibility of the following scenario.The photoexcitation in a MoSe 2 flake will create excitons, excited electrons in the conduction band and holes in the valence band.The electrons may be transferred to the LUMO of the closedform DAE, but not the open-form.The hole transfer to either form is energetically not favorable.Therefore, it is possible to facilitate the photoexcited electron transfer by irradiating UV (to form closed DAE) or suppress it with visible light (to form open-DAE).As a result, UV irradiation will reduce the radiative recombination of photoexcited electron-hole pairs, resulting in a PL quenching16,47 .Visible illumination will isomerize the DAE

Figure 1 .
Figure 1.Schematic and energy diagram of a hybrid structure from monolayer MoSe 2 and DAE.(a) Irradiation of UV and visible (Vis) light switches the molecular structure of DAE, transitioning between π-conjugated closed and cross-conjugated open forms.(b) Alignment of the VBM and CBM of monolayer MoSe 2 against the HOMO and LUMO energy levels of open-(Vis) and closed-form (UV) isomers of DAE.The HOMO and LUMO levels of open-form DAE are lower and higher than the VBM and CBM of MoSe 2 , respectively.In contrast, the closed-form has its LUMO level lower than the CBM, while its HOMO level remains lower than the VBM.As a result, photoexcited electron transfer may be energetically favored from MoSe 2 to closed-form DAE, whereas it will be forbidden between MoSe 2 and open-form DAE.Overall, photoswitching can regulate charge transfer between MoSe 2 and DAE.
, while applying a DC bias voltage of ± 2 V and limiting the current to ± 12 nA to protect the samples and the tip from localized charges.The results for open-and closed-ring isomers are shown in Fig. 4b.The UV-irradiated closed DAE sample exhibited a significantly higher current than the MoSe 2 with open-ring DAE (under visible

Figure 2 .Figure 3 .
Figure 2. DFT-calculated density of electronic states (DOS) of (a) a DAE molecule and (b) a DAE-MoSe 2 hybrid.(a) DOS of closed-and open-forms of DAE, highlighting a shift of the LUMO and HOMO energies through photoisomerization.The vacuum level is set to zero.The HOMO and LUMO levels of the closed-form (under UV irradiation) are approximately − 5.75 eV and − 4.31 eV, respectively; for the open-form (irradiated by visible light), they are about − 6.32 eV and − 3.80 eV.(b) In the hybrid DAE-MoSe 2 monolayer, the LUMO level of the closed isomer becomes lower than the MoSe 2 CBM, while it is higher with the open-form.This photoswitchability of DAE provides available states within the bandgap for photoinduced electron transfer from MoSe 2 to closed-form DAE (but not to open-form) and allows for a photo-modulation of optoelectronic properties of monolayer MoSe 2 .

Figure 4 .
Figure 4. Conductive AFM of hybrid MoSe 2 /DAE after irradiation with visible and UV light.(a) Schematic of C-AFM where a TMD flake is exfoliated onto an ITO substrate and then coated with the DAE layer.(b) Current vs bias voltage plots measured by sweeping DC voltage from − 2 to 2 V.Note 12 nA are the maximum in the measured current window.After UV irradiation, the electron transfer between MoSe 2 and DAE enhances current considerably.

Figure 5 .
Figure 5. KPFM measurement of a MoSe 2 -DAE hybrid and a schematic of related band energy modulation.(a) Optical microscope image of MoSe 2 exfoliated on an ITO glass substrate and measured surface potential of MoSe 2 /DAE following an exposure to (b) visible and (c) UV light.The average surface potentials are approximately 0.47 eV (Vis) and 0.22 eV (UV), from which we estimate work functions for MoSe 2 /DAE of about − 4.58 eV and − 4.83 eV, respectively.Band energy diagram (d) before contact between MoSe 2 and closed-ring DAE, (e) shortly after establishing a hybrid interface, and (f) after charge transfer from MoSe 2 to the DAE LUMO.Charge transfer at the hybrid interface leads to a band bending and downward shifts in the Fermi level of MoSe 2 .