Excitonic Engineering in 2D-SiC through Organic Donor–Acceptor Adsorption

Abstract

Controlling electronic excitations at molecule-semiconductor interfaces is key to designing hybrid optoelectronics. Using many-body perturbation theory, we study monolayer and bilayer SiC and their interfaces with four organic molecules bearing donor and/or acceptor substituents. Pristine monolayer SiC hosts strongly bound excitons with optical activity in the ultraviolet. Upon stable physisorption, new molecule-derived excitons emerge in the visible range. Through a detailed quantitative analysis of these excitons and the corresponding free-standing molecular excitations, we show that donor–acceptor molecules with predominant intramolecular charge-transfer excitation largely retain their character upon adsorption and undergo only modest optical redshifts in response to the substrate-induced screening. In contrast, donor-only or acceptor-only molecules with predominantly localized excitations exhibit significantly larger redshifts after adsorption. Bilayer SiC with stronger screening amplifies these shifts. The magnitude of the optical redshift reflects the competition between quasiparticle gap renormalization and the simultaneous reduction in exciton binding energy. This balance is controlled by the spatial distribution of the frontier orbitals at the interface and by the extent of electron-hole overlap within the exciton. Our results highlight the interplay between intrinsic molecular polarizability, excitonic characteristics, and substrate-induced screening effects as complementary knobs for tuning interfacial level alignment and optical functionality in molecule-semiconductor heterostructures.

Introduction

Two-dimensional silicon carbide (2D-SiC) is an emerging wide-gap semiconductor with attractive electronic, optical, and thermal properties, making it a promising platform for future optoelectronics. − Bulk SiC has been extensively studied for decades, with its common polytypes exhibiting wide band gaps of 2.3–3.3 eV, enabling its use in high-power electronics, high-frequency devices, and extreme-environment applications. − Recently, experimental breakthroughs have reported that SiC films can also be realized. − In particular, monolayer (ML) honeycomb SiC as well as multilayer SiC sheets have been synthesized through various routes, including growth on liquid Cu substrates by chemical vapor deposition (CVD), controlled CVD with boron additives, liquid-phase exfoliation from bulk SiC, and bottom-up epitaxial growth on ultrathin carbide/SiC templates. These advances establish ML- and multilayer-SiC as experimentally accessible 2D materials, motivating detailed and advanced theoretical investigations of their electronic structure and excitonic optical properties. ,−

Beyond fundamental interest in pristine SiC thin films, their chemical inertness, wide band gaps, and high thermal and mechanical stability hold emerging potential for optoelectronics and quantum technologies. ,,,,− Nonetheless, reduced surface reactivity, relatively large work function, and strongly hybridized Si–C orbitals limit control over charge distribution and optical response, so the optoelectronic properties of the pristine SiC layers are nearly locked in, and mostly responsive to ultraviolet (UV) light. ,, In this study, we employ many-body perturbation theory (MBPT) to investigate the quasiparticle (QP) electronic structure and excitonic properties of ML and bilayer (BL) SiC, examining how their intrinsic limitations can be overcome by adsorbing organic molecules. , We identify a set of organic molecules that physisorb on SiC layer(s), yet induce partial charge transfer (CT), slightly modulate the local potential, and neutralize local surface states. , These effects open optoelectronic functionalities that are not accessible in pristine 2D-SiC; particularly, we show that molecular adsorption enables adsorbate-derived optical features in the visible energy range. Furthermore, our results indicate that sufficient interfacial binding energy in these molecule/2D-SiC systems suppresses molecular diffusion on the surface. Importantly, the hybrid systems nearly preserve the optical excitations of the adsorbates with weak perturbation of their optoelectronics, so that they are particularly promising for controlled real-time visualization of ultrafast charge dynamics in adsorbed organic molecules, an ideal scenario in, e.g., attochemistry studies on solid-state surfaces. −

Although there are a few MBPT studies for pristine 2D-SiC, ,,, previous work on molecule/SiC interfaces has been limited to estimations of ground-state properties based on the density functional theory (DFT). , To provide a quantitatively reliable description, we use a screened hybrid functional for the ground state, suppressing overestimated hybridization and improving interfacial alignment compared to semi-local DFT, and compute QP corrections and absorption spectra through the GW approximation , and the Bethe–Salpeter equation (BSE) formalism. This approach explicitly captures dynamical screening and bound electron-hole interactions, which are essential to accurately estimate level alignment and optical fingerprints at molecule/2D interfaces. ,,,, We first benchmark the electronic structure of pristine ML- and BL-SiC, revealing absorption spectra with bright excitons and large electron-hole binding energies (E _b). These results particularly confirm that enhanced screening in BL reduces both the QP gap (E _g) and the optical onset relative to ML, indicating that the layer number is an effective knob to tune excitonic properties of pristine 2D-SiC. ,,

Building on the optoelectronic features of ML- and BL-SiC, we then extend our analysis to the functionalized interface by investigating four molecular adsorbates (all commercially available) that share a common benzenoid core but differ in their substituents: para-nitroaniline (PNA) and 4-cyanoaniline (CNA, also known as 4-aminobenzonitrile) containing both donor (D) and acceptor (A) substituents with typical push–pull effect; 4-methoxyaniline (MXA, also known as p-anisidine) with two D moieties; and para-nitrobenzonitrile (PNBN) with two A substituents. In isolated PNA and CNA, spatially separated D and A units localize the highest occupied molecular orbital (HOMO) on the D and the lowest unoccupied molecular orbital (LUMO) on the A, favoring intramolecular CT excitations (D → A). − In contrast, MXA and PNBN molecules with the HOMO and LUMO across the conjugated framework exhibit predominantly localized excitation (LE), associated with the π → π* transitions. , Upon adsorption on SiC, GW calculations show that surface screening substantially renormalizes molecular energy levels, , shifting frontier states into or near the host band gap and introducing distinct optical excitations well below the UV. Compared to the isolated molecule, our results indicate that adsorbates with only D or A substituents (MXA, PNBN) undergo a stronger reduction of the optical gap upon adsorption than molecules with both D and A moieties (i.e., PNA, CNA). − Our detailed analyses demonstrate that the renormalization of the optical gap in a molecular adsorbate depends sensitively on the intrinsic molecular electronic structure and excitation characteristics (CT vs LE), strength of interfacial interactions, and level alignments in the hybrid molecule-surface system.

Altogether, this work establishes practical design rules to tune the functionality and optoelectronic properties of the 2D-SiC substrate by varying the number of layers and by adsorption of organic molecules with different excitonic character due to their D and/or A substituents. This provides experimentally accessible routes for engineering state-of-the-art molecule/2D optoelectronics, detectors, modulators, and light-harvesting interfaces, where substrate screening and molecular character are used as complementary tuning knobs.

Results and Discussion

Pristine ML- and BL-SiC

The ML-SiC structure was initially extracted from the hexagonal 6H–SiC polytype, identified as the preferred stacking sequence for 2D growth. After full relaxation at the hybrid Heyd-Scuseria-Ernzerhof (HSE) level, the pristine ML-SiC adopts a planar, graphene-like configuration as shown in Figure a, with in-plane lattice parameters of a = b = 3.079 Å and Si–C bond length of 1.78 Å, consistent with previous studies. ,− , The planarity of the ML arises from the removal of the bulk interlayer interactions and the resulting balance of covalent sp²-like bonding in 2D, in contrast to the buckled and tetrahedral sp³ bonding of the bulk. For BL-SiC, we examined three different stacking configurations and fully HSE-relaxed their geometries (see Figure S1). Among these, the stacking order in Figure b, where the top-layer C atom sits above the Si atom of the bottom layer, is found to be the most energetically favorable. In this configuration, the interlayer spacing is 3.23 Å, while the Si–C bond length remains unchanged from the ML. −

1.

Honeycomb models of (a) ML- and (b) BL-SiC. For the BL, a side view is shown at the bottom to highlight the stacking sequence. Panels (c–f) show the studied molecules in the isolated phase and in their optimized adsorption geometries on ML-SiC: (c) PNA, (d) CNA, (e) MXA, and (f) PNBN. Black and brown sticks represent C and Si atoms in the substrate, while black, red, blue, and pink circles denote C, O, N, and H atoms in the molecules, respectively. The shaded gray and pink area indicates the presence of A and/or D moieties in the molecule.

The electronic band structures of the pristine ML-SiC, calculated at the HSE level and with QP corrections on top of HSE orbitals (GW@HSE), are shown in Figure a. For comparison, results obtained at the PBE and GW@PBE calculations are also provided in Supporting Information, Note S1. While all calculations suggest an indirect K → M gap, , E_g increases from 2.54 eV at DFT-PBE to 4.84 eV at the GW@HSE level (Table S1 lists direct and indirect gaps). Notably, the direct gap at K is slightly larger than the indirect one, suggesting that moderate strain or dielectric screening could render ML-SiC optically direct. The projected density of states (PDOS) indicates that C _p orbitals dominate the valence-band maximum (VBM), whereas the conduction-band minimum (CBM) is mainly composed of Si _p states. The corresponding absorption spectrum, obtained by solving BSE on top of the GW@HSE energies, is presented in Figure c, showing several discrete peaks (due to quantum confinement effects) with the lowest bright exciton at 3.59 eV (345 nm) corresponding to π → π* transitions at K-point. Compared to the corresponding direct gap of 4.93 eV, therefore, this near-UV exciton corresponds to a large E _b = 1.34 eV, as a hallmark of weakly screened 2D semiconductors. At higher energies, the largest oscillator strengths occur at 4.14 (π → π*) and 5.53 eV (σ → σ*) with electron-hole states located mainly at K/M and Γ-point, respectively (Figure S2). For comparison, absorption spectra computed by the time-dependent HSE (TDHSE) and the BSE-GW@PBE are given in Figure S3.

2.

Electronic band structures and optical absorption spectra of the primitive (a,c) ML and (b,d) BL-SiC. Band structures are obtained using HSE and GW@HSE approaches, and VBMs are set to zero energy. The absorption spectra are normalized to one with a Gaussian broadening of 0.1 eV.

As illustrated in Figure b, interlayer interactions due to weak van der Waals (vdW) forces and electrostatic coupling, along with the reduced symmetry in the BL-SiC, give rise to broken band degeneracies and induce a clear splitting of the electronic states (e.g., see the separation of the π bands). Compared to ML-SiC, therefore, E_g in BL-SiC is reduced across all levels of theory (Table S1). At the mean-field level, the BL gap decreases by 0.5 eV relative to the ML one, − while QP corrections yield a larger reduction of about 0.9 eV. This stronger reduction highlights the impact of interlayer screening on the self-energy correction. Consequently, the optical absorption of the BL-SiC in Figure d features a broadened and red-shifted spectrum compared to that of the ML, consistent with the typical evolution of optical spectra in layered 2D semiconductors, with an onset of 3.09 eV (401 nm) in the violet subrange of the visible window. Notably, BSE-GW@HSE for BL suggests not only a smaller onset, due to the reduction of E _g, but also a relatively smaller E _b = 0.9 eV (direct gap is 3.99 eV; Table S1). This indicates a weaker electron-hole Coulomb attraction due to the enhanced dielectric screening in the BL. Analysis of the electron-hole states confirms that optical transitions below 5 eV are dominated by π → π* transitions localized near the K and M points of the Brillouin zone, while σ-band states appear at significantly higher energies, giving rise to a peak in the far-UV region at 5.34 eV (Figure S2).

Molecular Adsorption on ML-SiC Surface

Structural Configuration and the Electronic Structure

The adsorption of four representative molecules, with D and/or A substituents as illustrated in Figure c–f, onto the ML-SiC substrate was investigated. To model the substrate, we employed a 5 × 5 supercell that ensures sufficient lateral separation among periodic images of the molecular adsorbate. For each molecule/SiC system, several adsorption sites along the zigzag and armchair directions of the underlying ML-SiC were examined, and all configurations were relaxed at the HSE level. The most energetically favorable structure was then selected for further analysis. For instance, in the case of PNA/ML-SiC, the lowest-energy configuration corresponds to PNA along the armchair direction with its aromatic backbone centered above a C atom of ML-SiC (see Figure S4).

Upon HSE optimization, all studied molecules adsorbed on ML-SiC remain nearly planar, with average adsorption distances of 2.9–3.1 Å. As shown in Figures and S5, however, the underlying SiC layer exhibits slight indentations below the adsorbates. The structural rigidity of the molecules arises mainly from their aromatic π-conjugated backbones and para-substitution patterns, which favor planarity. In contrast, the Si atoms in the underlying SiC layer, in the absence of bulk restoring forces, can relax more flexibly under adsorbate-induced electrostatic and vdW interactions. As a result, the adsorption energies (E _ads) summarized in Table S2 show comparable binding strengths across the studied systems. The strongest binding is obtained for MXA, with E _ads = −0.98 eV, suggesting a relatively stronger interaction between the amino group and the underlying Si atom, yet remaining within the physisorption regime. PNA and CNA exhibit weaker adsorption energies of ≈ −0.75 eV, while PNBN acceptor shows an intermediate strength (E _ads = −0.84 eV). Compared with the E _ads values reported for gaseous molecules adsorbed on SiC, which are significantly lower, the DA substituents of a molecular adsorbate can enhance interfacial binding. Additionally, we performed nudged elastic band (NEB) calculations for the representative MXA molecule translated along a primitive in-plane lattice vector of ML-SiC. The NEB images and the corresponding total energies in Figure S6 indicate that intermediate configurations lie above the adsorption minimum, yielding a finite migration barrier of 0.4 eV. Besides the relatively large E _ads, this barrier suggests that the most favorable adsorption site is associated with hindered surface diffusion.

Supporting Information Table S2 lists the Bader charge analysis and changes in the work function (Φ) after molecular adsorption. Depending on their DA character, molecular adsorbates exhibit either an excess or a deficiency of charge. At the same time, all adsorbates slightly increase Φ by 0.15–0.2 eV (Φ = 5.36 for the pristine ML-SiC due to the strong Si–C bonds). This shift indicates charge redistribution at the interface creates an interfacial dipole, associated with an upward shift of the vacuum level (Figure S7). The largest shift in Φ is obtained by adsorption of PNBN (5.56 eV), which is associated with a partial charge withdrawal of 0.22 $|e|$ from the substrate by the two acceptor –CN and –NO₂ moieties. PNA and CNA with mixed DA substituents exhibit a smaller charge gain of 0.14 $|e|$ and a weaker shift in Φ. In contrast, MXA donates 0.15 $|e|$ to the SiC substrate due to its only donor character. Despite this electron donation, MXA adsorption still increases Φ, which can be attributed to the formation of the interfacial dipole, as reported by previous studies. ,

Figure compares the HSE band structures of pristine ML-SiC with those of ML-SiC after adsorption of PNA, CNA, MXA, and PNBN. For pristine ML-SiC, the 5 × 5 supercell results in a folding of the primitive-cell band structure, while retaining the indirect K → M band gap (E _g = 3.36 eV) and the dispersion. For molecule/ML-SiC interfaces, the HSE band structures in Figure b indicate the host-derived bands are almost unchanged upon adsorption, confirming vdW interactions and weak hybridization at the interfaces. As a result, frontier orbitals of the adsorbates introduce narrow-bandwidth localized states at or near band edges, as highlighted by colored dots (see also their spatial distribution in Figure b). For PNA and CNA, both include the same D (–NH₂) moiety, HOMO is found at ≈ 1.2 eV below the VBM, while the LUMO lies below the host CBM. Notably, the stronger electron-accepting nature of –NO₂ in PNA stabilizes the LUMO by about 0.8 eV with respect to CNA, creating an in-gap state. For MXA, the HOMO derived from the central benzenoid core and D substituents appears slightly deeper than that of PNA and CNA, while its LUMO and LUMO+1 contribute slightly above the host CBM. Adsorption of the only acceptor PNBN introduces a distinct midgap state at ≈ 1.6 eV, while its HOMO lies about 2 eV deeper within the valence manifold compared to the other adsorbates.

3.

(a) HSE band structures of the pristine 5 × 5 ML-SiC surface and (b) of the ML-SiC with adsorbed PNA, CNA, MXA, and PNBN. Projected bands on molecular states are indicated by colored circles whose radii represent the contribution strength. The VBM is set to zero energy.

4.

(a) QP-DOS of ML-SiC with adsorbed PNA, CNA, MXA, and PNBN obtained via the GW@HSE approach. Molecular PDOS traces are shown in color, with the total DOS represented by the shaded gray area. The VBM is set to zero energy. (b) Spatial distribution of selected orbitals in the hybrid systems with contributions from molecular frontier orbitals. (c) QP spectra of the free-standing molecules, together with the spatial distributions of the HOMO and LUMO, and the corresponding GW@HSE energy gaps. All vertical axes are in arbitrary units.

An energetic comparison between the molecular frontier energy levels in the band structures of Figure b and those of the corresponding free-standing molecules in Table S4, both computed via the DFT-HSE, shows that the molecular energy levels and particularly their HOMO–LUMO gap are only slightly changed after adsorption on ML-SiC. For instance, the HOMO and LUMO of PNA adsorbed on ML-SiC appear at −1.2 and 2.4 eV, respectively, while the HOMO–LUMO gap of isolated PNA at the same level of theory is 3.88 eV. This nearly surface-independent character of the adsorbate energy levels clearly highlights the HSE limitation in appropriately capturing surface-induced screening (image charge) effects that often renormalize molecular energy levels by orders of magnitude. ,,,,, To capture these effects and obtain a reliable level alignment in our hybrid molecule/surface systems, we computed the perturbative GW correction (GW@HSE). ,,,, The resulting QP density of states (QP-DOS) is presented in Figure a, with the molecular PDOS traces in color. A direct comparison with the QP energies of the free-standing molecules at the same GW@HSE level (Figure c) highlights the significant impact of ML-SiC screening on the molecular energy levels, resulting in a multi-eV renormalization of the HOMO–LUMO gap upon adsorption. Additionally, the QP corrections to the molecular energies on ML-SiC shift the in-gap states of PNA and PNBN upward, placing the molecular LUMO closer to the host CBM. Such shallower trap states may facilitate thermally activated CT to the substrate and are generally less detrimental than deeper in-gap states, which strongly localize carriers and promote nonradiative recombination.

Optical Properties and Adsorbate-Derived Excitons

With a reliable QP level alignment for the molecule/ML-SiC interfaces, we computed the BSE absorption spectra for the four hybrid systems. Spectra are shown in Figure a, alongside the spectrum of the pristine 5 × 5 ML-SiC surface as a gray reference. Comparing the 5 × 5 ML-SiC spectrum with that of the primitive cell (Figure c), the low-energy spectral shape below 5 eV is largely retained, although both π → π* peaks are red-shifted by approximately 0.25 eV. Together with the extensive band folding inherent to the enlarged supercell, we attribute this shift to the limited k-point sampling and the reduced transition manifold in the BSE kernel, which includes only about 10% of the transitions expected from a strict scaling of the primitive cell (see Methods). Importantly, since the same computational setup is applied consistently to all molecule/surface calculations, this numerical shift does not affect our analysis of the adsorbate-derived excitons.

5.

(a) Optical absorption spectra of pristine ML-SiC (shaded gray) and ML-SiC with adsorbed PNA, CNA, MXA, and PNBN (colored curves), calculated at the BSE-GW@HSE level. The spectra are broadened with a Gaussian factor of 0.08 eV and normalized to their respective maximum intensity. The inset highlights the lowest exciton energy with considerable adsorbate weight. (b) Corresponding hole and electron density isosurfaces for the molecular-derived excitons. (c) BSE-GW@HSE optical absorption lines of the four isolated molecules, along with the transition densities of the lowest excitation.

Comparison between pristine ML-SiC and molecule/ML-SiC in Figure a reveals clear adsorbate signatures by either enhanced intensity or the appearance of new optically active states associated with hybrid or molecule-localized excitons. To elucidate the adsorbate-induced features in the optical absorption spectra, we computed the real-space electron and hole densities as ${\rho }_{e/h}(\mathbf{r})=\sum _{vc}{|w_{vc}|}^2{|{\psi }_{c/v}(\mathbf{r})|}^2$ , where w _vc denotes the normalized weight of the transition from valence (v) to conduction (c) in the BSE eigenvector, and ψ _c/v (r) are the corresponding HSE eigenfunctions (see isosurfaces in Figure b). Given these densities, we extracted quantitative indicators such as the electron and hole centroids ⟨r _e/h ⟩ = ∫ r ρ _e/h (r)d r, their centroid separation $⟨r_{eh}⟩=|⟨{\mathbf{r}}_e⟩-⟨{\mathbf{r}}_h⟩|$ , and the probability of finding the electron or hole on the molecular adsorbate P _e/h = ∫_Ωmol ρ_e/h(r)d r. To characterize the spatial extent of an exciton, moreover, we evaluated the root-mean-square (RMS) electron-hole separation $d_{\mathrm{R}\mathrm{M}\mathrm{S}}=\sqrt{⟨{|{\mathbf{r}}_e-{\mathbf{r}}_h|}^2⟩}\approx \sqrt{⟨r_e^2⟩+⟨r_h^2⟩-2⟨{\mathbf{r}}_e⟩·⟨{\mathbf{r}}_h⟩}$ , where the second equality follows from constructing the electron and hole densities independently and $⟨r_{e/h}^2⟩=\int r^2{\rho }_{e/h}(\mathbf{r})\mathrm{d}\mathbf{r}$ . Since the magnitude of the substrate-induced screening is related to the degree of electron-hole overlap (localization or delocalization) rather than to the centroid displacement, d _RMS provides a more reliable indicator of an exciton’s sensitivity to environmental screening. Within the definition of d _RMS, the dot term (overlap) reduces its value when the electron and hole densities strongly overlap. Consequently, the strong electron-hole overlap in an LE diminishes the squared terms and yields a smaller d _RMS, whereas a CT exciton with spatially separated electron and hole densities (weaker overlap) features a larger d _RMS.

The absorption lines of the free-standing PNA molecule (Figure c) exhibit its lowest bright excitation at 3.82 eV with pronounced intramolecular CT transition from HOMO to LUMO, in agreement with previous reports. , Upon adsorption on ML-SiC, the absorption spectrum shows a substrate-dominated onset at 3.16 eV. The lowest predominantly molecule-derived exciton appears at 3.56 eV (Figure b). For this near-UV state and the subsequent one at 3.63 eV, ρ _e/h (r) reveals considerable weight from PNA frontier orbitals: we obtain P _h /P _e = 0.13/0.37 and 0.15/0.29, with centroid separations ⟨r _eh ⟩ = 1.52 and 0.97 Å, respectively. For the free-standing CNA molecule, the absorption lines in Figure c show bright excitations at 4.18 and 4.80 eV with mixed CT-LE character, as detailed in Table S4. The signatures of these excitations are retained in the CNA/ML-SiC spectrum (Figure a), with enhanced intensity between 3.4 and 3.7 eV and around 4.2 eV. Analysis of ρ _e/h (r) for these excitons at 3.42 eV (Figure b) and 4.17 eV yields centroid separations of ⟨r _eh ⟩ = 1.05 and 0.79 Å, respectively. For these states, ρ _h (r) and ρ _e (r) assign approximately 10% of the hole density and 20% of the electron density to the CNA frontier orbitals.

Figure quantitatively illustrates how adsorption on ML-SiC surface modifies the HOMO–LUMO gaps and E _b of the molecular adsorbates relative to their free-standing gas phase (gray bars). The QP gap of the free-standing PNA is renormalized from 7.72 to 5.53 eV as adsorbed on ML-SiC (ΔE _g = −2.19 eV). At the same time, the optical onset is red-shifted from 3.82 to 3.56 eV (ΔE _opt = −0.26 eV); this corresponds to a decrease of the E _b from 3.90 to 1.97 eV (ΔE _b = E _b -E _b = −1.93 eV). These results indicate that the modest 0.26 eV redshift of the optical gap upon adsorption arises from the near cancellation between the renormalization of the QP gap driven by substrate screening (monopole contribution) and the simultaneous reduction of E _b due to screened electron-hole interactions (dipolar contribution). ,, For CNA, the QP gap is relatively stronger renormalized upon adsorption (8.67 → 5.59 eV, ΔE _g = −3.08 eV) and the optical onset shifts from 4.18 to 3.42 eV (ΔE _b = −2.32 eV). Thus, it is clear that CNA experiences a larger renormalization in both gap and E _b than PNA (see Figure a); the stronger QP renormalization |ΔE _g| for CNA indicates that its frontier orbitals couple relatively stronger to the ML-SiC screening, while the larger |ΔE _b| reflects the relatively stronger electron-hole overlap within CNA, as expected due to the mixed LE-CT character. By contrast, PNA with more separated electron-hole due to its stronger DA and dominant CT (computed d _RMS = 8.29 Å for PNA versus 7.94 Å for CNA), features less sensitivity of its electron-hole interaction to the surface-induced screening and yields a smaller net optical redshift.

6.

(a) Stacked-bar decomposition of the HOMO–LUMO gap into the optical gap (lower segment) and E _b (upper segment) for PNA, CNA, MXA, and PNBN in the gas phase (gray) and as adsorbed on ML-SiC (blue) and BL-SiC (orange). (b) Correlation between ΔE _g and ΔE _b upon adsorption. Circles and squares denote adsorption on ML- and BL-SiC. The dashed and dotted lines highlight the trend followed by CNA, MXA, and PNBN upon absorption on ML- and BL-SiC, respectively, whereas PNA, with pronounced CT character, deviates from this correlation.

For the donor-only MXA molecule, the lowest neutral excitation appears at 4.11 eV, followed by a stronger absorption line at 5.14 eV. The former is a HOMO → LUMO transition whose density is predominantly localized on the aromatic ring (Figure c). Upon adsorption of MXA on ML-SiC, the BSE spectrum exhibits noticeable broadening and a redshift of 0.2 eV in the main peak compared to the pristine surface, followed by enhanced intensity at about 3.6 eV. The onset of this spectrum occurs at 2.96 eV with a hybrid character (P _h /P _e = 0.16/0.21, ⟨r _eh ⟩ = 0.59 Å, and d _RMS = 7.42 Å), indicating increased adsorbate-substrate coupling with a partial interfacial CT admixture, consistent with the donor characteristics of MXA and its relatively large E _ads (Figure b). Given this interfacial interaction and the corresponding level alignment in MXA/ML-SiC, the surface-induced screening renormalizes the QP gap and weakens the initially large gas-phase E _b, producing the pronounced net redshift in the MXA-derived exciton (Figure ).

BSE absorption lines for the isolated PNBN molecule in Figure c indicate the lowest excitation at 4.27 eV (localized on the aromatic ring), followed by a far stronger transition at 4.69 eV (Table S4). Within the absorption spectrum of PNBN/ML-SiC in Figure a, a new set of low-energy excitons emerges around 2.7 eV, accompanied by enhanced intensity in the 3.4–3.8 eV range. These excitons exhibit pronounced hybrid character: while P _e remains significant, the probability of finding the hole on the PNBN increases from 3% at 2.7 eV to 10% at 3.4 eV (see ρ _e/h (r) in Figure b and metrics in Table S5). This indeed indicates progressive involvement of the PNBN states and the formation of hybrid excitons with partial surface-to-molecule CT character, consistent with the electron-accepting nature of PNBN. Importantly, despite the renormalization of the molecular gap from 8.60 to 6.89 eV (ΔE _g ≈ −1.71 eV; Figure a) and the redshift of the optical transition from 4.27 to 2.72 eV (ΔE _opt ≈ −1.55 eV), the magnitude of E _b remains almost unchanged upon adsorption on ML-SiC (ΔE _b ≈ −0.06 eV). This indeed indicates that the electron-hole attraction for the relevant PNBN-like exciton is only slightly reduced by the substrate screening. Thus, the large 1.55 eV redshift of the optical onset upon adsorption originates mainly from QP renormalization rather than from a reduction in E _b, consistent with previous studies on acceptor adsorbates. , Notably, the larger optical redshift of PNBN compared to MXA upon adsorption, despite their similar character and transition dipoles in the gas phase, arises from their different level alignment with respect to the ML-SiC band edges. In PNBN, the LUMO lies below the CBM of the host, suppressing an effective hybridization, and therefore the optical redshift closely follows the QP gap renormalization, with little change in E _b. In contrast, the LUMO of MXA is nearly resonant with the host CBM, promoting hybridization (Figure b). This improves the screening impact on the electron-hole interaction, resulting in a more reduction of E _b and a partial cancellation of ΔE _g, and therefore a relatively smaller optical redshift upon adsorption.

A comparison across the four studied systems highlights a clear correlation between exciton spatial character and the substrate-induced screening effects. Particularly, panel b of Figure depicts a quantitative trend for which molecular adsorbates with a larger QP gap renormalization |ΔE _g| feel a larger reduction of binding energy |ΔE _b|. Although PNA slightly departs from this trend: near cancellation between two many-body corrections (ΔE _g = −2.19 eV and ΔE _b = −1.93 eV) leads to only a small net optical redshift of 0.26 eV. This is related to the pronounced intramolecular CT of the PNA, showing the largest d _RMS as an indicator of the reduced electron-hole overlap, which makes its E _b less sensitive to the surface screening. On the other hand, CNA with a mixed LE-CT character and relatively stronger electron-hole overlap leads to a larger renormalization of the gap and a larger reduction in E _b, and therefore a net redshift of 0.76 in the optical gap. For only-donor or only-acceptor molecules with predominantly LE character in the gas phase, namely MXA and PNBN, both exhibit a relatively larger interfacial interaction and partial CT character across the interface, resulting in hybrid excitons with small ⟨r _eh ⟩ and d _RMS. These features lead to intermediate screening sensitivity in MXA, whereas PNBN, with relatively weaker hybridization, shows an almost unchanged E _b upon adsorption and consequently the largest redshift of the lowest exciton energy. Finite-field calculations of the ground-state molecular polarizability (Supporting Information, Note S2) corroborate the gas-phase trend that larger molecular polarizability can provide the smaller gas-phase E _b, with PNA’s intramolecular CT as the notable exception.

Adsorption of Molecules on BL-SiC

On BL-SiC, the molecular adsorption trends remain qualitatively similar to those of the ML. As listed in Table S2, shifts in Φ and Bader charges follow the same ordering as in the ML: PNBN, PNA, and CNA adsorbates extract charge and raise the Φ, whereas MXA donates charge yet still shows a slight increase in Φ via interfacial dipoles. , Despite this, E _ads of the molecules on BL-SiC shows an increase which can be attributed to the stronger charge stabilization in the thicker slab and increased long-range dispersion. Overall, BL improves the binding strength while preserving the electronic trends observed in ML-SiC.

The HSE band structures of the 5 × 5 BL-SiC surface with and without adsorbed molecules are shown in Figure . Pristine BL-SiC exhibits a narrower band gap compared to ML-SiC, which aligns with the primitive-cell results in Figure . , In molecule/BL-SiC, molecular orbitals are also slightly downshifted (by about 0.2 eV) relative to molecule/ML-SiC, suggesting additional stabilization of the LUMO. Despite this, the interfacial bonding character remains qualitatively similar to that of ML-SiC, with only a modest increase in hybridization (see orbitals in Figure S8). By applying the GW corrections, QP-DOS in Figure shows an increased host band gap compared to HSE, along with substantial renormalization of molecular energy levels due to the increased substrate-induced screening. In particular, the QP gap of the molecular adsorbates undergoes a multi-eV reduction compared to freestanding molecules (Figure a).

7.

HSE band structures of (a) pristine 5 × 5 BL-SiC surface and (b) BL-SiC with adsorbed PNA, CNA, MXA, and PNBN molecules. Colored circles highlight contributions from the adsorbate states.

8.

(a) QP-DOS of PNA, CNA, MXA, and PNBN adsorbed on BL-SiC. Color curves show the PDOS on molecular states. (b) Absorption of 5 × 5 BL-SiC without (shaded gray) and with adsorbed PNA, CNA, MXA, and PNBN (colored curves) obtained by BSE-GW@HSE. Spectra are renormalized and shown in arbitrary units when the relative intensities are consistent across all. (c) Hole and electron densities of the lowest molecule-derived excitons with energies indicated in the zoom-in of panel (b).

Figure also includes the optical absorption spectra of pristine BL-SiC (gray shaded area) and of BL-SiC with the four molecular adsorbates (colored curves). In all cases, adsorption introduces distinct spectral features in the visible region, which underscores the potential of these hybrid systems as tunable platforms for optoelectronic and photonic applications. As already noted, the enhanced substrate-induced screening in molecule/BL-SiC compared to molecule/ML-SiC leads to a larger |ΔE _g| in the molecular HOMO–LUMO gap. At the same time, the stronger screening further weakens the electron-hole interaction, resulting in a larger reduction of the binding energy, |ΔE _b|. Given the competition between |ΔE _g| and |ΔE _b| in determining the optical redshift upon adsorption, we found a trend similar to that of the molecule/ML-SiC systems (Figure ). Notably, the ratio |ΔE _g|/|ΔE _b| is enhanced for all molecule/BL-SiC systems compared to the molecule/ML-SiC counterparts. This indeed indicates that the stronger optical redshift in molecule/BL-SiC is primarily driven by the more dominant |ΔE _g| contribution. Consequently, only donor- or acceptor-type adsorbates with predominantly LE character in the gas phase experience the largest optical redshifts (MXA: ΔE _opt = −1.52 eV; PNBN: ΔE _opt = −1.99 eV), whereas PNA with pronounced intramolecular CT character shows the smallest redshift of 0.46 eV upon adsorption on BL-SiC relative to the isolated molecule. CNA, with mixed LE-CT character and stronger coupling of its frontier orbitals to substrate screening, undergoes a larger |ΔE _g| than PNA, while its |ΔE _b| is slightly more reduced. As a result, CNA adsorbate features an intermediate optical redshift of ΔE _opt = −1.07 eV on BL-SiC. Qualitatively, the lowest adsorbate-derived excitons in all molecule/BL-SiC systems, illustrated in Figure c, exhibit slightly greater spatial delocalization in ρ _e/h (r) compared to the corresponding exciton in molecule/ML-SiC, which aligns with the larger ⟨r _eh ⟩ and d _RMS for these excitons (see Table S5).

Conclusion

Using many-body perturbation theory, we investigated the quasiparticle electronic structure and excitonic optical response of SiC layer(s) and their interfaces with four donor and/or acceptor organic molecules. GW quasiparticle corrections (performed on top of hybrid HSE solutions) combined with the solution of the BSE provide a consistent description of substrate-renormalized level alignment and excitonic effects in these hybrid organic/2D heterostructures. For pristine mono- and bilayer SiC, we discussed the quasiparticle band structures and the strongly bound excitons that define the intrinsic optically active states in the UV. Upon molecular physisorption (vdW-dominated but energetically stable), the host band dispersion is preserved while localized molecular states appear near the band edges; these states produce clear signatures in the absorption, either by the emergence of new optically active states or through intensity enhancement.

We demonstrated that substrate-induced dielectric screening renormalizes molecular quasiparticle energies and simultaneously reduces exciton binding energies. The competition between these two many-body corrections determines the net optical redshift of a molecule-derived exciton upon adsorption relative to the isolated molecule: a near cancellation yields only a minor redshift, whereas dominant quasiparticle gap renormalization produces a larger redshift. Molecular adsorbates with localized transition density (donor-only MXA and acceptor-only PNBN) undergo substantial quasiparticle renormalization, while their exciton binding energies are less strongly reduced. By contrast, a donor–acceptor molecule with pronounced intramolecular charge-transfer character and spatially separated electron and hole densities (PNA in our study) experiences a near balance between the two substrate-induced renormalizations, yielding only a small net optical shift. This trend persists for both monolayer and bilayer SiC, where increasing substrate screening (ML → BL) enhances both renormalization contributions. These results provide a practical design principle: by tuning exciton confinement and donor–acceptor character, one can modulate optical redshifts and hybrid exciton behavior, offering a molecular-level handle to engineer tunable light-matter interactions at molecule/2D-SiC interfaces.

Overall, the GW-BSE framework provides a reliable and physically transparent description of quasiparticle renormalization and excitonic phenomena in molecule/SiC heterostructures. Our results establish general principles for how the character of a molecular adsorbate and the substrate-induced screening jointly control interfacial level alignment and optical response of a hybrid molecule/surface system. The demonstrated broadening and redistribution of absorption features across the visible spectrum highlight that such molecule/SiC hybrids are promising systems for next-generation optoelectronics, including tunable photodetectors, modulators, and light-harvesting interfaces.

Method

For isolated molecules, GW@HSE calculations were performed with the all-electron Molgw code, using a Gaussian cc-pVQζ basis set. The corresponding absorption spectra, averaged over the three Cartesian polarizations, were obtained within the full diagonalization of the BSE.

Calculations for the solid-state systems were performed using the plane-wave Vasp 5.4 package and the projector augmented-wave method, optimized for MBPT to ensure an accurate description of high-lying unoccupied states. Optimizations and ground-state simulations carried out with a plane-wave cutoff energy of 600 eV and the screened hybrid HSE functional, incorporating 25% exact exchange and a screening parameter of 0.2 Å^–1. The convergence criteria set to 10^–2 eV/Å for forces and 10^–5 eV for total energy. A vacuum of at least 15 Å was introduced to suppress periodic coupling. Long-range vdW interactions were included through DFT-D3. Dipole corrections were applied along the out-of-plane direction to remove spurious electrostatic interactions.

For primitive ML- and BL-SiC, QP corrections were computed using a Γ-centered 15 × 15 × 1 k-point mesh, 1120 bands, and a dielectric response cutoff of 450 eV, which ensured convergence of the gap (see convergence in Note S4). The GW band structures were interpolated along high-symmetry paths using maximally localized Wannier functions. The BSE-resolved spectra included 6(12) valence and 12(24) conduction bands for the primitive ML­(BL) cell, sufficient to capture the near-Fermi transitions. TDHSE spectra were computed within linear response, using a consistent number of occupied/unoccupied states. For 5 × 5 supercells with and without adsorbates, the same workflow was adopted with a reduced 2 × 2 × 1 k-mesh sampling alongside 1100 empty bands (Figure S10). In absorption spectra of molecule/2D systems, the BSE Hamiltonian included 35 valence and 15 conduction states, providing coverage of molecular frontier orbitals on either side of the Fermi level. Analysis of the BSE eigenvalue spectrum confirmed that the lowest excitonic state across all systems is optically active, with finite oscillator strength, and no lower-lying dark exciton was found. Exciton postprocessing analyses and visualization of isosurfaces were performed using home-built scripts and Vesta codes.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/prechem.5c00407.

• Additional details on the electronic structure and optical properties at different levels of theory, geometry optimization, isolated molecule spectra, and convergence tests (PDF)

The authors declare no competing financial interest.