Plexcitonic interactions in spherical and bi-pyramidical Au nanoparticles with monolayer WSe₂

Gold nanoparticles (AuNPs) have been the subject of growing research interest among the optoelectronics and materials communities, owing to their unique optical properties, including evidence of surface plasmons. Incoming light at an optimal frequency can result in free electrons within the AuNPs to oscillate resonantly, which is the source for strong light scattering and enhancement of local electric fields leading to optical absorption bands.^1,2 This resonant phenomenon, collectively referred to as surface plasmon resonance (SPR), has an unprecedented capacity to confine light to subwavelength volumes. Researchers from diverse fields have, thus, harnessed the use of AuNPs for various sensing modalities, including for catalysis, biosensing, and photonics.^3–5 Furthermore, the characteristic features of SPRs in AuNPs are intimately tied to their size, shape, surface chemistry, aggregation state, and the surrounding dielectric environment.^6,7 Thus, substantial opportunities exist to tune the SPR response through mere geometrical and structural considerations of the AuNPs themselves, in order to tailor the response toward a specific application.

Many of the two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDCs)^8,9 exhibit large exciton binding energies,¹⁰ where the excitons are long-lived and remain stable at elevated temperatures,¹¹ even beyond room temperature, opening possibilities for their use in quantum-information science as quantum emitters and optoelectronic devices.^12–14 While isolated TMDCs are rightfully of great scientific interest on their own, heterojunctions and heterostructures involving TMDCs with other dissimilar material systems^9,15 provide added degrees of freedom to tune physical properties through synergistic interactions which arise at the interfaces.¹⁶ One such hybrid system with TMDCs includes a lower dimensionality, i.e., a zero-dimensional (0D) system,¹⁷ such as the aforementioned AuNPs.¹⁸ The inclusion of AuNPs allows for the assembly of a broad variety of van der Waals (vdWs) material stacks with the 0D systems, in including from our own prior work on 0D–2D hybrids,^19,20 to engineer properties desirable toward optoelectronics and electronics.^17,21,22 Particularly, tuning the optical response of TMDCs through the interactions SPRs in metallic NPs have with excitons in TMDCs is an intriguing consideration. Such interactions have been explored in the past where a strong coupling between the photogenerated excitons in the 2D TMDCs and plasmons in the 0D NPs is observed.^23,24 Hence, the interactions of plasmons with excitons, referred from here-on as “plexcitons,” are a burgeoning field and provide exciting prospects to enable highly sensitive photoabsorbers toward applications in quantum photonics.²⁵ 

In this work, we have constructed 0D–2D vdWs heterostructures utilizing two different geometries of AuNPs, namely, spherical (referred to as Au-Sp) and bi-pyramidical (referred to as Au-BP) on halide-assisted-low-pressure chemical-vapor-deposition (HA-LPCVD) grown monolayer (1L) WSe₂ crystallites. The AuNP's geometry-dependent plexciton interactions in Au-Sp/WSe₂ and Au-BP/WSe₂ vdWs hybrids are evaluated using temperature-dependent and wavelength-dependent optoelectronic transport measurements. Our data reveal the ability of the geometry-dependent SPR in AuNPs to regulate the wavelength-dependent optoelectronic response of the 1L WSe₂. The strong light–matter plexcitonic interactions evident in our AuNP/WSe₂ hybrids through the geometry-dependent studies we have conducted will serve as an important guide to modulate the performance of photoabsorbers based on TMDCs for quantum photonics in the future.

The WSe₂ nanosheets employed in this work were nucleated on ∼270 nm SiO₂/Si substrates using the HA-LPCVD route, where the incorporation of the salt is known to facilitate the reduction of the WO_2.9 precursor to lower synthesis temperatures below 1000 °C.^22,26 The details of the samples and alumina (Al₂O₃) crucible cleaning prior to the CVD growth process as well as the precursors used are reported in the supplementary material (Secs. S1.1–S1.3). The precursors in the Al₂O₃ crucibles were then placed in a quartz tube and arranged spatially as illustrated in Fig. 1(a)-left. Figure 1(a)-right illustrates the temperature-time profile segmented into regions I–VII, where the temperature was taken at the nominal center of the heating zone. Region III in the temperature-time profile represents the onset of nucleation and the growth of WSe₂ crystallites; for the significance of the remaining regions in this profile, see subsequent discussion in S1.4 of the supplementary material. Through the growth procedures outlined, Fig. 1(b) shows an optical micrograph of a typical WSe₂ crystallite synthesized using our adapted HA-LPCVD process.

Next, for the fabrication of the AuNPs-WSe₂ hybrid devices, a colloidal solution of 15 nm Au-Sp [Sigma-Aldrich, part number: 777 099, 1 optical density (O.D.), stabilized in 0.1 mM phosphate buffered saline (PBS) solution] and 30 nm Au-BP (nanopartz, part number: A1B-30–780-CIT-PBS, 1 O.D. stabilized in 0.1 mM PBS) were used. Figure 1(c) depicts the process scheme used to produce 16 O.D. AuNPs from the as received 1 O.D. AuNPs. The details of the AuNP preparation process, preparation of AuNP-WSe₂ hybrids, and the ensuing atomic force microscopy (AFM) characterization of AuNPs spin coated at various O.D.s on SiO₂/Si substrates are presented in Sec. S2 of the supplementary material.

Next, electron (⁠$e$⁠)-beam lithography was used to electrically contact the WSe₂ membranes. The details of e-beam lithography (EBL) and the atomic force microscopy (AFM) characterization to electrically contact the WSe₂ membranes before AuNPs decoration are presented in Sec. S3 of the supplementary material. The AFM scans of WSe₂ membranes after Au-Sp and Au-BP decoration are presented in Figs. 1(d) and 1(e), respectively. The insets in Figs. 1(d) and 1(e) refer to individual Au-Sp and Au-BP NPs, measured using tapping mode AFM at a scan rate of 0.1 Hz. The AFM micrographs reveal the spherical symmetry of the Au-Sp NPs, while, in contrast, the structural asymmetry in the Au-BP NPs is evident in the inset of Fig. 1(e). The WSe₂ and its associated Au-Sp/WSe₂ and Au-BP/WSe₂ were next characterized using the Horiba LabRAM Raman spectrometer equipped with a 532 nm excitation laser. After AuNPs decoration, the Raman data presented in Figs. S4(a) and S4(b) reveal a blueshift in the in-plane $E2g1$ phonon mode and the out-of-plane $A1g$ phonon mode of WSe₂, possibly indicating the transfer of electrons from WSe₂ to AuNPs, as discussed in Sec. S4 of the supplementary material.

Furthermore, UV-Vis optical absorption spectroscopy enabled us to measure the spectral absorbance [in arbitrary units (A.U.)] of Au-Sp and Au-BP, as shown by the data in Figs. 2(a) and 2(b), respectively. The absorbance spectra of the 15 nm diameter d Au-Sp depicted schematically in the inset of Fig. 2(a) resulted in an SPR peak located at ∼520 nm, in agreement with prior repors.²⁷ On the other hand, the absorbance spectra of Au-BP result in two SPR peaks due to its geometrical anisotropy, where a primary SPR peak ascribed to the longitudinal axis (along $L$⁠), as shown through its geometric rendition in the inset of Fig. 2(b), occurs at ∼790 nm, while a relatively low-intensity secondary SPR peak arises at ∼545 nm, attributed to the transverse axis (along $W$⁠). Incidentally, the positions of the primary and secondary SPR peaks for Au-BP are geometry-dependent and related to the $L/W$ aspect ratio.²⁸ In our case, the Au-BP NPs had a nominal $L ∼ 100 nm$ and $W ∼ 30 nm$⁠, and, thus, an $L/W$ ratio of $∼ 3$⁠. $33¯$⁠.

Additionally, photoluminescence (PL) measurements shown in Figs. 2(c) and 2(d) yield a symmetric exciton peak $X0$ for WSe₂ centered at ∼767.0 nm (∼1.62 eV) and ∼768.3 nm (∼1.61 eV), respectively, where the PL intensity decreases following decoration of WSe₂ with AuNPs. Interestingly, the peak position of the $X0$ peak did not change neither significantly nor the full-width-half-maximum (FWHM) value. Specifically, a modest $Δλ+$ of ∼1.3 nm (i.e., 768.3 − 767.0 nm) after Au-Sp decoration and ∼2 nm (i.e., 770.3 − 768.3 nm) after Au-BP decoration was observed, where the PL data pertaining to the latter are shown in Fig. 2(d). It should be noted that as a result of interfacial charge transfer between 0D and 2D hybrids, a shift in shape and PL peak position was found in some of the published literature.²⁹ Furthermore, the trion-to-exciton ratio also shifted as a result of the interfacial charge transfer between the decorated NPs and the underlying TMDC.³⁰ One possible explanation for this behavior may have to do with the distribution density of AuNPs on the surface of WSe₂, since there appears to be little or no pronounced shift in trion-to-exciton ratio, given the minimal shift in the PL peak position or its shape. A future investigation would be interesting to assess the effect of AuNPs distribution density on the PL peak behavior of AuNPs/WSe₂ hybrids as well as the effect of PL excitation wavelength used for the measurements. Next, electronic transport measurements were conducted at 300 K using a Lakeshore CRX-4K probe station interfaced to a Keysight B1500A semiconductor parameter analyzer. The source–drain current $Isd$ was measured as a function of applied source–drain bias $Vsd$ in the dark (lights OFF) and under broadband illumination using an optical power $P$ of ∼3.32 mW/cm² (lights ON) to activate the AuNP's plasmons; a schematic of our device architecture is shown in Fig. 2(e). The details of the electronic transport data measured in the lights OFF condition are presented in Fig. S5 of the supplementary material. In the lights OFF condition, no stark contrast in $Isd$–$Vsd$ trend was observed when compared to the bare WSe₂ and its associated hybrids, whereas a more pronounced difference was observed in the $Isd$–$Vsd$ behavior of WSe₂ and its associated hybrids in the lights ON condition. Figures 2(e) and 2(f) show the difference in $Isd$–$Vsd$ of WSe₂ and its associated hybrids with lights ON. For WSe₂ before Au-Sp decoration, an $Isd$ ∼0.24 nA was measured at a $Vsd$ = +40 V; after Au-Sp decoration, at the same $Vsd$⁠, a modest ∼2× increase in $Isd$ ∼ 0.48 nA was noted for Au-Sp/WSe₂ with lights ON, as shown in Fig. 2(e). Similarly, in the case of WSe₂ before Au-BP decoration, an $Isd$ ∼ 0.29 nA was measured at a $Vsd$ = +40 V, and after Au-BP decoration, at the same $Vsd$⁠, a pronounced ∼180× increase in $Isd$ ∼ 52.59 nA was evident under illumination, as shown in Fig. 2(f). We believe this increase in $Isd$ in Au-BP/WSe₂ arises from an enhanced photoexcited plexcitonic coupling, which we delve into more detail shortly.

Optoelectronic transport measurements were initiated as a function of temperature T subsequent to the above procedures, in order to decipher the extent to which Au-NP's geometry plays a role in governing its optoelectronic response. The T-dependent photocurrent $Iph$ was computed between 4 and 350 K at $Vsd$ = +40 V, where $Iph=Ilight−Idark$⁠; $Ilight$ and $Idark$ are the currents under illumination and in the dark, respectively. In Fig. 3(a), the T-dependent $Iph$ for bare WSe₂ under illumination is shown at $λ$ = 750 nm and laser power $P$ = 200 μW. Evidently, the $Iph$ for WSe₂ increases with T due to excitation of carriers, a well-rooted trait of semiconducting materials, where a steeply rising region is seen for T > 200 K. To better understand the wavelength $λ$-dependent behavior of $Iph$⁠, we normalized the $Iph$ at each $T$⁠, where 1 represents the highest $λ$-dependent $Iph$ at a given $T$⁠. Shown in Fig. S6(a) are the normalized $Iph$ values for WSe₂ as a function of both $T$ and $λ$⁠, while the data in Fig. S6(b) depict the absolute value of $Iph$ as a function of these two parameters. For better visualization of the T- and $λ$-dependent behavior of the normalized $Iph$⁠, a contour plot was tabulated, as illustrated in Fig. 3(b). A distinct pattern is evident in the contour plot, where for $λ$ < $X0$ peak position, the normalized $Iph$ values increase and approach 1, denoted by the red regions within the contour plot, across the entire T-range considered.

Similar to the aforementioned approach, Fig. 3(c) presents the T-dependent $Iph$ for the Au-Sp/WSe₂ at $λ$ = 750 nm. In Fig. S6(c), the normalized $Iph$ of Au-Sp/WSe₂ as a function of $T$ and $λ$ is shown, while the equivalent contour plot is captured in Fig. S6(d). The absolute magnitude of $Iph$ for Au-Sp/WSe₂ is shown in Fig. S6(d), where again, a steeper increase in $Iph$ is occurring for T >200 K. Interestingly, for the Au-Sp/WSe₂ hybrid system, a shift in the location of the maximal normalized $Iph$ now appears to the left of 550 nm, indicated by the white dashed line in Fig. 3(d). Few pockets of elevated values of normalized $Iph$ still appear close to WSe₂'s $X0$ peak position of ∼767.0 nm, as corroborated through the PL data acquired in Fig. 2(c). We hypothesize that the plexcitonic interactions between Au-Sp and WSe₂ result in the modulation of the $λ$-dependent photo-response, which we discuss in detail shortly through our postulated model.

Moving to the second hybrid case study for the Au-BP/WSe₂ system, Fig. 3(e) presents the $T$-dependent $Iph$ under illumination at $λ$ = 750 nm. Distinct from the WSe₂ and the Au-Sp/WSe₂ system, the $Iph$ for Au-BP/WSe₂ steadily increased from T > 50 K. Interestingly, the magnitude of $Iph$ was larger in the Au-BP/WSe₂ system, as shown in Fig. S6(f), when compared to the Au-Sp/WSe₂ hybrid. From the contour plot in Fig. 3(f) for the Au-BP/WSe₂, a clear shift in the location of the maximum normalized $Iph$ is seen across the entire T-range to the left of the white dashed line located at ∼650 nm, which is close to the secondary SPR peak of the Au-BP NPs. Just as for the Au-Sp/WSe₂ hybrid system, the T-dependent pockets of maximal normalized $Iph$ still appeared close to WSe₂'s $X0$ peak position of ∼767.0 nm. We believe that the increase in cut off wavelength for the increase in $Iph$ in the Au-BP/WSe₂ hybrid system comes under the influence of the secondary SPR peak, as a consequence of the plexciton coupling between Au-BP and WSe₂, discussed in more detail next. Furthermore, there appear to be regions near the electrical interconnect lines where two prominent Au islands are visible (above both of the interconnect lines), given their higher brightness viewed through the AFM maps of Fig. S3(b) (before AuNPs dispersion) and Fig. 1(d) (after AuNPs dispersion). These two larger Au islands in Fig. 1(d) are likely to have resulted from the Au metal liftoff process during the e-beam lithography (EBL) of our device fabrication process to form the interconnect lines. Comparing Fig. S3(b) AFM of Au contacted WSe₂ before AuNP dispersion and Fig. 1(d), the traces of Au metal islands from the liftoff process are seen in both cases and, thus, are not a result of clustering of our Au-NPs, which is done subsequent to the interconnect formation and e-beam lithography step. Despite their presence, since the comparison of the transport is done for the same device, before and after the Au-NPs, the effect of these two metal regions would be nulled out, and only the relative difference is analyzed here.

To better illustrate the governing mechanisms of the optoelectronic response observed in our two geometrically unique AuNP systems, we postulate a model here based on plexcitonic interactions in order to interpret our results. First, for Sp/WSe₂, in Fig. 4(a)-left, clearly, the center of the Au-Sp SPR peak is blue shifted from the $X0$ location of WSe₂ by 770 meV, which likely results in injection of high-energy “hot” charge carriers from Au-Sp to WSe₂. These carrier transitions are shown in the context of the energy band diagram depicted in Fig. 4(a)-right, which likely alters the optoelectronic response of the Au-Sp/WSe₂ hybrid system, compared to bare WSe₂. Evidence of such injection of plasmon-mediated hot charge carriers overcoming the bandgap of the underlying semiconductor has previously been studied using transient optical absorption spectroscopy in Au nano-disk/bi-layer-WSe₂,³¹ Au-Sp/few-layer-MoSe₂,³² and Au nano-disk/monolayer-WS₂.³³ Additionally, we believe that these hot charge carriers experience increased columbic scattering, as shown schematically in Fig. 4(a)-right, that only results in a modest increase in $Iph$ compared to the bare WSe₂.

Moving to the proposed model for the Au-BP/WSe₂ system, Fig. 4(b)-left illustrates the plexcitonic interaction, where the Au-BP high intensity primary SPR peak mostly coincides with the location of the $X0$ peak for WSe₂, while the low intensity secondary SPR peak is blue shifted from the $X0$ peak of WSe₂ by ∼640 meV. Upon Au-BP photoexcitation, we believe that the primary SPR peak excites charge carriers in WSe₂ with energies just above its $X0$⁠, where the increased flux of this primary SPR-mediated transition is schematically represented by a thicker red arrow in Fig. 4(b)-right. On the other hand, the secondary SPR peak injects hot charge carriers well-above the $X0$ of WSe₂, but given that the intensity of the secondary SPR peak is lower, it injects a lower density of such hot charge carriers into WSe₂ upon photoexcitation. This charge carrier transition is schematically represented by a thinner teal colored arrow in Fig. 4(b)-right, compared to the more dominant red arrow attributed to the transitions mediated via the primary SPR peak. As a result, we expect the probability for columbic scattering to be lower here when compared to the Au-Sp/WSe₂ system, which is a possible explanation for the significant increase in the $λ$-dependent $Iph$ rise in our Au-BP/WSe₂ system, mediated via the plexcitonic interaction. The proposed hypothetical plexcitonic interaction model in Fig. 4 is solely based on empirical evidence derived from our $T$- and $λ$-dependent photocurrent measurements, as seen in Fig. 3. There appear to be limited theoretical studies conducted previously that have examined the localized electric field arising from Au-Sp and Au-BP NPs on the surface of WSe₂ and its influence on the photocurrent response. Such theoretical studies conducted in the future on this specific system would, nonetheless, be very insightful to determine the localized electric fields between AuNPs and WSe₂ and their influence on the plexcitonic interactions. Furthermore, the hypothetical plexcitonic interaction model that is proposed here can be further verified using finite-difference time-domain (FDTD) simulations to evaluate the localized electric fields and the interactions between AuNPs and WSe₂. Such FDTD simulations have been used by Chen et al.³⁴ to examine localized electric fields in AuNPs/TMDC nanosheets, which has shown the localization of electric fields between adjacent NPs and between NPs and the underlying TMDC nanosheets. Hence, FDTD simulations would be ideally suited to further validate our proposed plexcitonic interaction model shown in Fig. 4, between Au-Sp and Au-BP on the surface of WSe₂, which will be interesting to consider in a future study. Also, we note that the number distribution and density of AuNPs as well as their influence on plexcitonic interactions may also have an effect on the spectral photoresponse of the underlying WSe₂, which was not investigated in this work but could be an exciting prospective study in the future.

In conclusion, we have demonstrated the synthesis of 1L WSe₂ crystallites grown using a HA-LPCVD process. The $Iph$ resulting from the WSe₂, Au-Sp/WSe₂ as well as the Au-BP/WSe₂ were presented, where the contour plots indicated plexcitonic interactions, shifting the spectral response of 1L WSe₂ toward the SPR peak positions of Au-Sp and Au-BP. A hypothetical model was proposed to describe the plexcitonic interactions in the Au-Sp/WSe₂ and Au-BP/WSe₂ systems, where the $Iph$ trend was taken into account based on the energy and relative flux of the charge carriers injected from the SPR peaks associated with Au-Sp and Au-BP. The results presented here demonstrate a unique approach for tailoring the spectral response of WSe₂ through the consideration of AuNP's structural and geometrical attributes, as they decorate the surface of WSe₂. This study will also help guide the investigation of plexcitonic interactions in other material systems to widen the possible selection of 0D–2D ensembles for high performance optoelectronic and quantum photonics applications in the future.

See the supplementary material for further details on the growth of the HA-LPCVD WSe₂ crystallites and their characterization with the AuNPs, device fabrication and transport measurement details, alongside Raman analysis before and after the AuNPs deposition.

We greatly appreciate the support received from the Air Force Office of Scientific Research (Grant No. FA9550-21-1-0404) that enabled us to pursue this work. A.B.K. is also grateful to the support received from the PACCAR Technology Institute at the University of North Texas.