Enhanced Modulation Bandwidth by Integrating 2D Semiconductor and Quantum Dots for Visible Light Communication

Light‐emitting devices present a tremendous potential for visible light communication (VLC) due to their dual functionality as both communication and lighting devices. Herein this study, the significant enhancement in VLC modulation bandwidth by integrating two‐dimensional (2D) semiconductor and quantum dots (QDs) emitter is reported. Generally, the modulation bandwidth of CdSe‐based QDs is limited to only less than 25 MHz; however, with the proposed hybrid emitter, a maximum modulation bandwidth of 130 MHz for CdSe/ZnS QDs emitter is able to be achieved. The WSe2 monolayer is integrated into an Au–nanorod–decorated CdSe/ZnS QDs emitter to achieve high modulation performance. The modulation bandwidth of the hybrid QD–Au–WSe2 emitter (130 MHz) is found to be higher than those of the pristine QDs and QD–WSe2 heterostructure without Au nanorods (79 and 91 MHz, respectively). A significant increase is observed in the transition rate of QDs excitons when they are integrated with Au nanorods and WSe2 monolayer, which is substantiated by a reduction in average carrier lifetime from time‐resolved photoluminescence analysis. This approach and the findings open an opportunity to apply 2D semiconductors into the next‐generation miniature VLC devices, for high‐speed optical communications.


Introduction
The implementation of light-emitting devices in visible light communication (VLC) technology has received considerable attention in recent years because these devices can be used for both lighting and communication simultaneously. [1][2][3][4][5] This has encouraged research into their modulation response because a stronger modulation response corresponds to a higher switching speed, which is desirable for optical communication. White-light-emitting devices are the leading candidates for VLC light sources and can be obtained by combining light-emitting diodes (LEDs) or laser diodes with color-conversion materials such as phosphors and quantum dots (QDs). [2,3,6,7] However, the modulation bandwidth of such devices is limited by the long response time of the color-conversion materials. The modulation response of light emitters is critical when they are being adopted for communication purposes. A higher communication efficiency may result from a faster modulation response or a higher modulation bandwidth. Additionally, when implementing modulation techniques by using color-conversion materials, factors such as flickering and dimming must be considered. The modulation bandwidth of white-light-emitting devices developed from such color-conversion materials is characterized by the intensity fractions of the various contributing colors. Although the modulation bandwidth of single-wavelength laser diodes has been successfully increased, white light systems derived from these devices still have bandwidth of only a few megahertz. [2,3,7] Light-emitting devices present a tremendous potential for visible light communication (VLC) due to their dual functionality as both communication and lighting devices. Herein this study, the significant enhancement in VLC modulation bandwidth by integrating two-dimensional (2D) semiconductor and quantum dots (QDs) emitter is reported. Generally, the modulation bandwidth of CdSebased QDs is limited to only less than 25 MHz; however, with the proposed hybrid emitter, a maximum modulation bandwidth of 130 MHz for CdSe/ZnS QDs emitter is able to be achieved. The WSe 2 monolayer is integrated into an Aunanorod-decorated CdSe/ZnS QDs emitter to achieve high modulation performance. The modulation bandwidth of the hybrid QD-Au-WSe 2 emitter (130 MHz) is found to be higher than those of the pristine QDs and QD-WSe 2 heterostructure without Au nanorods (79 and 91 MHz, respectively). A significant increase is observed in the transition rate of QDs excitons when they are integrated with Au nanorods and WSe 2 monolayer, which is substantiated by a reduction in average carrier lifetime from time-resolved photoluminescence analysis. This approach and the findings open an opportunity to apply 2D semiconductors into the nextgeneration miniature VLC devices, for high-speed optical communications.
The frequency response of the individual color components must be improved if a large-bandwidth white light system is to be developed. In particular, more attention should be paid to the enhancement of the modulation response of the converted light because this response is slow in the conventional colorconversion process.
QDs have recently been attracting attention as a means of color conversion in VLC systems because of their shorter carrier lifetime than phosphors. [3,4,[8][9][10][11][12] Perovskite quantum dots have been extensively utilized in the development of high-speed VLC systems because of their short carrier lifetime and large modulation bandwidth. [2,3,[13][14][15] However, the modulation response of such white light systems is limited by the red component, which is derived from CdSe-based QDs with a modulation bandwidth of less than 25 MHz. [3,16] Consequently, enhancing the frequency response of CdSe-based QDs is essential to achieving a larger modulation bandwidth for white light systems. The reason for the smaller modulation bandwidth in CdSe-based QDs is the longer carrier decay lifetime, which results in a lower spontaneous emission rate; this rate is one of the factors driving VLC applications because they require a high switching speed. The decay lifetime of the light emitted under modulated electrical excitation regulates the modulation response of a light emitter. So, the current research focuses on the reduction of the fall time of the light emitters so as to increase their modulation bandwidth.
The optical emissions from QDs are strongly affected by the surroundings of the QDs, and the carrier lifetimes of QDs can be optimized in various ways, affecting the carrier recombination mechanism. Plasmonic nanostructures have received considerable attention in recent years due to their potential to markedly improve light-matter interactions, especially for quantum light emitters. [17][18][19][20][21][22][23] The surface plasmon coupling of these nanostructures can increase the photonic density of states, which is responsible for controlled enhancement of the spontaneous emission rate. [24][25][26][27][28][29] The coupling effect results in reduction of the spontaneous emission lifetime and thus an increase in the modulation bandwidth, which is critical for high-speed applications. If the emission peak of light emitters matches to the localized surface plasmon resonance (LSPR) wavelength range, the rate of exciton decay in emitters is considerably enhanced due to the Purcell effect, resulting in a shorter carrier lifetime. A high density of states for the surface plasmon can also enhance the rate of coupling between the excitons for quantum light sources (e.g., QDs) and the surface plasmon, enhancing the carrier recombination rate. [25,28] As a result, the modulation characteristics of QDs can be enhanced considerably, which is beneficial for VLC applications. Furthermore, efficient transfer of energy from QD exciton recombination processes to electron vibrations of surface plasmons can result in a major increase in the emission rate. [25] The nonradiative transfer of energy, especially between two light emitters (donor and acceptor), has been demonstrated to accelerate the recombination of carriers in various heterostructures. [17,20,21,30] Nonradiative energy transfer (NRET) is an energy-transfer process that involves the transfer of excitation energy between two fluorophores separated by a nanoscale distance. The process strongly depends on the refractive index of the medium, the distance between the donor and acceptor, and the spectral overlap between donor emission and acceptor absorption. [31][32][33][34][35][36][37] It can considerably enhance the efficiency of color conversion and the exciton decay rate of the donor material simultaneously. Therefore, the carrier recombination of QDs can be controlled through the energy-transfer process by integrating QDs with the other emitters. The 2D semiconductors such as transition metal dichalcogenides (TMDs) have attracted wide attention among researchers investigating hybrid nanostructures because they can be easily coupled with various nanomaterials through NRET due to their tightly bound excitons, which are stable at room temperature. [38][39][40][41][42][43] Furthermore, the dielectric characteristics of TMDs influence the coupling mechanism because a weaker dielectric screening effect results in higher energy transfer in thin-layered TMDs, particularly monolayer TMDs. [39,43] The energy transfer between two quantum light emitters can also be modulated by the presence of metal nanoparticles. The surface plasmon dipole fields of the nanoparticles facilitate the modification of the exciton dipole moments of the donor and acceptor, resulting in a more rapid energy-transfer process. [17,[19][20][21] In this work, the influence of surface plasmons and NRET on the modulation bandwidth of CdSe/ZnS QDs was demonstrated by integrating the QDs with a tungsten diselenide (WSe 2 ) monolayer decorated with gold (Au) nanorods. We discovered considerable enhancement in the transition rate of QDs excitons when integrated with Au nanorods and WSe 2 monolayer, which was supported by the decrease of average carrier lifetime from time-resolved photoluminescence (TRPL) analysis. These findings serve as a foundation for exploiting surface plasmons and energy transfer to enhance the modulation bandwidth of CdSe/ZnS QDs, and such enhancement is beneficial for QD-based VLC applications. In addition, the developed heterostructure can be integrated into the miniature VLC systems to facilitate color conversion in future applications.

Results and Discussion
A schematic of the hybrid QD-Au-WSe 2 emitter is presented in Figure 1a and shows the different layers and interfaces between them. After the WSe 2 monolayer film was transferred to a silica substrate, an ultrathin polymethylmethacrylate (PMMA) layer was deposited to facilitate energy transfer, and Au nanorods were then patterned on the surface, as illustrated in Figure 1c. An optical microscopic image of the transferred WSe 2 monolayer is displayed in Figure 1b and shows that the monolayer had a random distribution. Figure 1d shows the surface plasmon absorption spectrum for the Au nanorods, the photoluminescence (PL) emission spectrum for the CdSe/ZnS QDs, and the absorption spectrum of the WSe 2 monolayer. The spectrum of the nanorods exhibited two LSPR peaks at 614 (longitudinal LSPR) and 529 nm (transverse LSPR). The CdSe/ZnS QD spectrum exhibited a PL emission wavelength peak at 620 nm under excitation by a blue laser. The localized LSPR spectrum clearly overlaps between the absorption spectra and the emission spectra of the QDs. Consequently, the exciton decay rates of the QDs were considerably enhanced due to the Purcell effect. The absorption spectrum of the WSe 2 monolayer also exhibited an overlap with the PL emission spectrum of the QDs, indicating energy transfer from the QDs to the WSe 2 monolayer. In addition, the spectral overlap between Au nanorods, QD emission, and WSe 2 absorption confirms that color conversion between QDs and WSe 2 was enhanced by the plasmonic effect. The energy-transfer phenomenon in the QD-WSe 2 heterostructure decorated with Au nanorods is further explained in a later section. Figure 1e shows the Raman spectrum of the QD-WSe 2 monolayer heterostructure decorated with Au nanorods. The Raman peaks at 247 (E 1 2g ) and 260 cm À1 [2LA (M)] are due to the WSe 2 monolayer corresponding to the E 1 2g Raman mode as a result of the Brillouin zone-center and zone-edge phonons, and the Raman peak at 260 cm À1 [2LA (M)] corresponds to longitudinal acoustic phonons at the M-point in the Brillouin zone; these findings confirm the formation of a WSe 2 monolayer. [44][45][46][47] In addition, the absence of a B 1 2g peak at 304 cm À1 confirms the monolayer thickness of the WSe 2 in the heterostructure. [46,47] The peaks at 148 and 208 cm À1 correspond to the transverse and longitudinal optical phonon mode for the CdSe/ZnS QDs. [48,49] When the QDs were decorated with Au nanorods, the spectrum shows peaks that confirm the formation of a QD-WSe 2 monolayer heterostructure.
A cross-sectional view of the hybrid QD-Au-WSe 2 emitter is provided in Figure 2a, along with a transmission electron microscope (TEM) image illustrating the spin-coated Au nanorods and CdSe/ZnS QDs on the top of WSe 2 monolayer. The QDs had an average diameter of 7 nm, and the spacer between the QDs and WSe 2 created an average separation distance of 3 nm. The PMMA spacer prevents charge transfer between the donor and acceptor, so that NRET is the dominant phenomenon in the heterostructure. The separation distance of 3 nm will facilitate in maximum energy transfer between the QDs donor atoms and the WSe 2 monolayer acceptor. [50,51] The TEM images confirm the formation of a QD-Au-WSe 2 heterostructure with Au nanorods and QDs covered by the PMMA matrix. Figure 2b presents a bright-field scanning transmission electron microscopy image of the WSe 2 monolayer, whereas Figure 2c shows a higher-magnification TEM image of the area within the dotted blue line in Figure 2b. The cross-sectional TEM image of WSe 2 presents that the thickness of the WSe 2 monolayer is approximately 0.6 nm, which perfectly matches the previously reported value. Moreover, the magnified TEM image displays the explicit atomic configuration of the WSe 2 monolayer. Energy-dispersive X-ray spectroscopy images for the elements W and Se are displayed in Figure 2d,e, and the spatial distributions in these images confirm the formation of a WSe 2 monolayer. Figure 3 presents the PL emission spectra of the pristine QDs and WSe 2 monolayer and the QD-WSe 2 heterostructure with and without Au nanorods. Figure 3a shows the PL spectra of the pristine QDs with and without Au nanorods and it is clear that the PL intensity is found to be increased with the introduction of Au nanorods. This PL enhancement is attributed to the Purcell effect induced by an enhancement in the spontaneous emission rate of QD excitons caused by exciton-plasmon coupling between QDs and Au nanorods. The enhancement of spontaneous emission rate will be further explained using TRPL analysis in the following section. However, we observed a PL quenching of the QDs when they were integrated with the WSe 2 monolayer owing to energy transfer from the QDs to the WSe 2 as shown in Figure 3b. In addition, the WSe 2 PL intensity was higher than the original PL signal, indicating the existence of energy transfer and the conversion of energy from QD emission into WSe 2 emission in the heterostructure. Figure 3c shows further enhancement of the PL intensity of the WSe 2 www.advancedsciencenews.com www.adpr-journal.com monolayer when the heterostructure was decorated with Au nanorods. The amount of PL quenched for the QDs and the PL enhancement of the WSe 2 monolayer were greater when the heterostructure was decorated with Au nanorods, indicating a stronger NRET and color-conversion process. These results indicate that the LSPR of Au nanorods facilitates the transfer of energy from QDs to WSe 2 through exciton-plasmon coupling, resulting in a faster energy-transfer process. Surface plasmons of Au nanorods are thus critical to energy transfer between the donor and the acceptor. The efficiency of energy transfer from QDs to the WSe 2 monolayer was determined using TRPL analysis through the calculation of the decay rate and energy-transfer rate. It is worth to note that the exciton recombination rate will have more impact on the modulation bandwidth compared to PL quenching for VLC application. [52] We obtained TRPL decay curves for the QDs, QD-Au nanorods and QD-WSe 2 heterostructure to understand the energy transfer and plasmonic coupling phenomenon. In the heterostructure, the CdSe/ZnS QDs behave as a donor because of the wide energy bandgap, which involves exciton transition and energy-transfer processes to the WSe 2 monolayer as the acceptor. Hence, analyzing the PL decay characteristics of the QDs in all heterostructures would reveal the plasmonic coupling effect, energy-transfer behavior, and both tow effects. Figure 4a displays the TRPL decay curves for the QD-Au and the QD-WSe 2 heterostructure while Figure 4b illustrates the TRPL decays curves for the pristine QDs and QD-Au-WSe 2 heterostructure. The decay curves are fitted to biexponential decay described by the following equation where I(t) represents the PL intensity, and A 1 and A 2 are constants. The PL decay components of the pristine QDs, τ 1 and τ 2 , can be attributed to the fast decay component from neutral excitons and the slow component from nonradiative components corresponding to deep-levels traps, respectively. [53][54][55] The details of TRPL analysis from all heterostructures are shown in Supporting Information S1 and the corresponding PL decay lifetimes for the donor atom in the pristine QD, QD-Au, QD-WSe 2 , and QD-Au-WSe 2 heterostructure are listed in Table S1, Supplementary Information, together with their corresponding fitting parameters. The average PL carrier lifetime for the QDs in the pristine QDs, QD-Au, and QD-WSe 2 heterostructure are 15.09, 11.07, and 10.65, respectively.
It is found that the average PL carrier lifetime for the pristine QDs and QDs decorated with Au nanorods is decreased from 15.09 to 11.07 ns in the presence of donor QDs, indicating the enhancement of transition rate for QDs. The TRPL decay process and the reduction of the QDs carrier lifetime after the integration of Au nanorods are found to be consistent with the previously reported results. [17,[56][57][58] The integration of Au nanorods is to act as the surface plasmon that can facilitate the recombination rate improvement of QDs through surface plasmon coupling and plamson-enhanced NRET from the QDs to WSe 2 monolayer. A stronger coupling between the QDs emissions and the localized plasmonic resonances could be achieved by optimizing the plasmonic nanostructures in the future study. [59] As a result, the exciton recombination rate    www.advancedsciencenews.com www.adpr-journal.com of PL decay lifetime of the acceptor atom. It can be seen that the energy-transfer method is more advantageous to increase the transition rate of QDs excitons.
In the presence of Au nanorod LSPR, we observed a further decrease in donor average carrier lifetime up to 8.94 ns, indicating a stronger energy-transfer phenomenon. The possible excitonic pathways in the heterostructure, including radiative and nonradiative pathways, are depicted in Figure S2a, supporting Information. LSPR enhances the transfer of energy between the QDs and WSe 2 , thereby reducing the average carrier lifetime for the donor. The much faster PL dynamics observed in the heterostructure than in the pristine QDs suggest that an additional efficiently quick decaying process occurs in this system, which is critical for light modulation phenomenon. The exciton-plasmon coupling in the hybrid QD-Au-WSe 2 emitter not only results in enhanced energy transfer through a plasmon-coupled resonance energy-transfer process but also improves the PL decay rate for the QDs. The total PL decay rate for the QD-WSe 2 heterostructure will be contributed by the QD exciton decay rate and energytransfer rate, as shown in Figure S2a, Supporting Information.
In general, the PL decay rate is related to the carrier decay lifetime as follows The PL decay rates of the donor atoms in the pristine QDs, QD-Au, and QD-WSe 2 heterostructure, and QD-Au-WSe 2 heterostructure, estimated using Equation (2), are 0.15, 0.20, 0.22, and 0.31 ns À1 . The PL decay rate is highest for the QD-Au-WSe 2 heterostructure due to the presence of surface plasmons. The faster decay rate corresponds to a higher rate of recombination, which is highly significant for VLC applications. In addition to the exciton decay rate, the energy-transfer rate and energytransfer efficiency are found to be increased when the QDs is integrated with Au nanorods and WSe 2 monolayer. The estimation of energy-transfer rate and energy-transfer efficiency is given in Supporting Information S2. The energy-transfer mechanism in the QD-WSe 2 heterostructure can be considered to be dipoledipole coupling, meaning that excitation energy of photoexcited states is transferred from the donor QDs to the acceptor WSe 2 monolayer. However, when the heterostructure is decorated with Au nanorods, the energy-transfer process is a combination of dipole-dipole coupling and exciton-plasmon coupling, which serve as the enhancement factor. The exciton recombination and average PL decay lifetime are clearly enhanced due to excitonplasmon coupling between the QDs and Au nanorods. Consequently, the spontaneous emission rate of the donor QDs in the heterostructure is enhanced, which is one factor responsible for the enhancement of the modulation bandwidth for the QDs. In this manner, high-speed and efficient light emission can be achieved from the QD atoms through exciton-plasmon coupling and NRET, which is critical for VLC applications.
To investigate the modulation bandwidth of our hybrid QD-Au-WSe 2 emitter for VLC applications, we measured the frequency response for every sample by using the experimental setup shown in Figure 5a. A vector analyzer (Keysight E5071C)  was used to generate alternating current, which was coupled to direct current through a bias tee used to drive the laser diode. The modulation signal was fed to a 450 nm laser diode to pump the QD-WSe 2 heterostructure decorated with Au nanorods. The emitted light was then collected by a photodetector (SPA-3, DC À 2 GHz) by using an optical fiber, which later converted the optical signal into an electrical signal. The network analyzer employed the electrical signal to generate the frequency response of the measured sample. The modulation bandwidth for the laser diode was measured by directly using a photodetector to collect the blue light. However, for the QD-Au-WSe 2 heterostructure, the emitted light was detected by the photodetector after it had passed through an optical filter, so that any remnant blue light from the laser diode would be filtered out. The frequency response measurements for the pristine QDs and QD-WSe 2 and QD-Au-WSe 2 heterostructure are presented in Figure 5b having modulation bandwidths of 79, 91, and 130 MHz, respectively. The considerable enhancement of the modulation bandwidth by integrating the WSe 2 monolayer and Au nanorods is in agreement with the TRPL measurement indicating that the excitons in the QDs efficiently couple with the surface plasmons, leading to a higher recombination rate. In addition, the plasmoncoupled resonance energy transfer from the QDs to the WSe 2 monolayer results in a higher carrier decay rate and higher recombination efficiency. The 3 dB modulation bandwidth is related to the differential carrier lifetime as follows where τ diff is the differential carrier lifetime. However, the lifetime value obtained from the TRPL fitting data corresponds to the carrier lifetime whose value is approximately 2-3 times shorter than the differential carrier lifetime. Moreover, the overall modulation bandwidth of a light emitting hybrid device is inversely proportional to the carrier lifetime of the active lightemitting materials. Thus, the large bandwidth of the hybrid QD-Au-WSe 2 emitter is attributed to the short carrier lifetime and high spontaneous emission rate of the donor atoms in the heterostructure. The modulation bandwidth of CdSe-based QDs has significantly increased after the introduction of Au nanorods and WSe 2 monolayer as compared to the previously reported values, which are limited to less than 25 MHz. [3,16,60] With the proposed heterostructure, we were able to boost the modulation bandwidth of CdSe/ZnS QDs to reach a maximum value of 130 MHz. Figure 5c illustrates the frequency response for the laser diode and QD-Au-WSe 2 heterostructure without an optical filter; the modulation bandwidth is 959 and 774 MHz, respectively, revealing that the high-speed system is suitable for VLC. Table 1 presents a comprehensive summary of the current research on the modulation bandwidth of CdSe-based QDs for VLC applications. The table displays the modulation bandwidth, exciton decay lifetime, and the driving conditions for each study, providing a valuable reference for researchers and practitioners in the field. The table clearly demonstrates that by integrating with Au nanorods and WSe 2 monolayer, the modulation bandwidth of QDs can be significantly increased up to 130 MHz. A benchmark of modulation for CdSe-based QDs is given in Figure 6a. This finding is particularly significant as it provides a promising approach for enhancing the performance of QDbased devices, which have gained widespread attention in recent years for their unique optical and electronic properties.
In VLC, light-emitting devices are turned on or off depending on the data bit, where the on and off states represent bits "1" and "0," respectively. The shorter the fluorescence lifetime, the more quickly the luminous intensity drops to a characteristic point in the "0" state. The time needed to switch between "0" and "1" should be as short as possible to ensure clear and rapid data representation, and the luminous intensities of the "0" and "1" states should differ considerably; if these requirements are met, more data can be accurately reproduced in a unit of time. Faster modulation of light from the devices is implied by a higher switching speed, which depends on the carrier recombination lifetime of the light emitters. Figure 6b shows the light modulation representation in pristine CdSe/ZnS QDs and QD-Au-WSe 2 heterostructure, respectively. With the introduction of Au nanorods and a WSe 2 monolayer, the carrier recombination lifetime of QDs decreases due to exciton-plasmon coupling and NRET. The PL decay rate of QDs in the heterostructure has been estimated from the TRPL data, which indicates that it is higher than that of the pristine QDs. The higher recombination rate implies that the speed of switching between the on and off states is faster, which in turn means that more data can be transmitted in a certain amount of time, as depicted in Figure 6b. Consequently, the modulation bandwidth of QDs in the QD-Au-WSe 2 heterostructure is higher than that of pristine QDs. The proposed heterostructure exhibiting high modulation bandwidth can be integrated with micro-LEDs to fabricate a full-color display for high speed VLC.

Conclusions
In this study, we demonstrated the influence of LSPR and NRET on the modulation bandwidth of CdSe/ZnS QDs by integrating them with a WSe 2 monolayer decorated with Au nanorods. Through this integration, the exciton lifetime of the QDs decreased from 15.09 to 8.94 ns, indicating enhanced energy transfer between the QDs and the WSe 2 monolayer near the Au nanorods because of exciton-dipole coupling. In addition, the spontaneous emission rate of the CdSe/ZnS QDs was considerably enhanced by exciton-plasmon coupling, leading to a larger modulation bandwidth. Due to the considerable increase in the exciton-transition rate, the hybrid QD-Au-WSe 2 emitter is able to www.advancedsciencenews.com www.adpr-journal.com achieve a maximum modulation bandwidth of 130 MHz. These results provide a foundation for utilizing surface plasmons and energy transfer to increase the modulation bandwidth of CdSe/ZnS QDs, which is desirable for QD-based VLC applications.

Experimental Section
Growth and Transfer Process of WSe 2 Monolayer: Chemical vapor deposition (CVD) was used to grow a WSe 2 monolayer on a sapphire substrate by using a three-zone furnace. We placed Se powder, WO 3 powder, and a sapphire substrate in the first zone upstream, third zone, and downstream zone of the furnace chamber and then heated the precursors until they turned to vapor form. The monolayer growth on the sapphire substrate was facilitated by Ar/H 2 flowing gas, and the monolayer was grown at the low pressure of 30 Torr at a temperature of 800°C, after which it was cooled down to room temperature.
The WSe 2 monolayer was transferred from the sapphire substrate to the target substrate through liquid-phase exfoliation. First, the sapphire substrate with the WSe 2 monolayer was spin-coated with PMMA A5 and baked at 100°C for 30 min. Each side of the polymer was removed, which facilitated the subsequent etching. The PMMA-coated substrate was immersed in buffered oxide etch (BOE) at a temperature of 100°C for 3 h to establish a narrow gap between the PMMA/WSe 2 layer and sapphire substrate. The sample was then immersed in deionized water to remove BOE, and the PMMA/WSe 2 layer was separated from the sapphire substrate and floated on the water-air interface. Our target silica substrate was then used to pick up the floating PMMA/WSe 2 layer, which was subsequently baked obliquely on a hot plate at 100°C for 8 h. Finally, the sample was placed in acetone for 3 h to remove the PMMA coating, after which the sample was baked again at 100°C to complete the transfer.
Spin-Coating of Au Nanorods: Before the top of the pre-transferred WSe 2 on the silica substrate was coated with nanorods, a thin layer of PMMA A3 was spin-coated and served as a dielectric spacer between the WSe 2 and QDs to facilitate the energy transfer between them. The Au nanorods have an average diameter of 25 nm, an average length of 57 nm, a peak LSPR wavelength of 580 nm, and a weak localized LSPR wavelength of 514 nm. The nanorods were in toluene solution and then spin-coated on the sample at a spinning rate of 1000 rpm for 10 s, followed by heat treatment to ensure favorable adhesion of the Au nanorods to the sample.
Aerosol-Jet Printing of QDs: CdSe/ZnS QD solution was obtained from Taiwan Nanocrystals Inc. The QDs had an emission wavelength of approximately 620 nm, a concentration of 5 mg mL À1 in toluene, and an average diameter of 7 nm. Aerosol-jet (AJ) printing equipment was employed to aerosolize QD droplets according to aerodynamic principles and without the use of additional masks. The density of the QDs could be precisely controlled by optimizing the flow concentration, and a pattern could be made through computer network operation. The atomizer pressure, power, and flow were maintained by their respective controllers at 0.61 psi, 40 V, and 1 ccm, respectively. The sample was then baked in an oven for 5 min to evaporate toluene from the QD solution. Finally, the sample was ready for characterization. In this study, we prepared two samples using silica substrates for the characteristics analysis. The first sample was specifically designed to explore the characteristics of QDs both with and without gold nanorods while the second sample was intended for investigating the properties of the hybrid structure. The QDs solution was printed on both substrates using identical printing conditions, and the gold nanorods in toluene solution were also coated onto the substrates using the same revolutions per minute (rpm) to ensure uniformity across both samples.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.