WS₂/WSe₂ Hetero-Bilayer: Tunable 2D Platform

• WS₂/WSe₂ hetero-bilayers are two-dimensional structures formed by stacking monolayers via van der Waals interactions, exhibiting unique electronic and excitonic characteristics.
• The stacking configuration and twist angle modulate interlayer hybridization and band offsets, leading to tunable band gaps, moiré effects, and flat-band phenomena.
• These hetero-bilayers enable innovative applications in optoelectronics, quantum photonics, and valleytronics through gate-tunable interlayer excitons and confined carrier dynamics.

WS$_2$/WSe$_2$ hetero-bilayers consist of monolayers of tungsten disulfide (WS$_2$) and tungsten diselenide (WSe$_2$) stacked via van der Waals interactions. These two-dimensional systems offer tunable electronic, excitonic, and many-body physics owing to their close lattice match, sizable interlayer coupling, and rich moiré superlattice effects. The interplay between stacking, twist angle, and interlayer hybridization gives rise to novel phenomena such as gate-tunable interlayer excitons, moiré-trapped multi-excitonic states, and correlated flat-band physics with significant implications for optoelectronic, quantum photonics, and valleytronic applications.

1. Electronic Structure: Stacking, Band Offset, and Hybridization

WS$_2$/WSe$_2$ hetero-bilayers typically exhibit type-II band alignment, with the conduction band minimum (CBM) dominantly in WS$_2$ and the valence band maximum (VBM) in WSe$_2$. This spatial separation of band edges is robust across stacking configurations and can be quantified by the difference in band edge energies, $$\Delta E_C = E_{CB}^{\mathrm{WSe2}} - E_{CB}^{\mathrm{WS2}}$$ and $$\Delta E_V = E_{VB}^{\mathrm{WSe2}} - E_{VB}^{\mathrm{WS2}}$$, typically yielding an offset in the range of 0.1–0.2 eV (Zhang et al., 2015).

First-principles DFT and GW calculations reveal substantial interlayer hybridization. The mixing of metal $d$-orbitals and chalcogen $p$-orbitals is stacking-dependent, for example, enhanced in AA/AA$'$ stacking regions and modulated by a threefold ($C_3$) symmetry selection rule (Ruiz-Tijerina et al., 2018). The interlayer coupling manifests as anti-crossings in the band structure near the valence band edges and enables hybridized states with both strong dipolar and permanent electric dipole moments. The strength of hybridization can be tuned by crystal symmetry (e.g., Janus WSSe layers induce intrinsic electric polarization and modulate band offset and hopping amplitude directly, without the need for an external field) (Wang et al., 2020).

In laterally fabricated bilayer–monolayer heterojunctions or bilayer lateral heterostructures, sharp interfaces (typified by zigzag step edges) generate quantum wire-like confined states with narrower gaps (e.g., $\sim$0.8 eV confinement at the interface) and well-defined band offsets, supporting robust carrier confinement suitable for device channels (Zhang et al., 2015, Sahoo et al., 2019).

2. Moiré Superlattice: Twist Angle Effects and Flat Bands

Twisting the layers introduces a long-period moiré superlattice whose periodic potential modulates the electronic states. For low twist angles (below $4^\circ$), the moiré unit cell grows and atomic relaxations become prominent, causing local stacking registries (AB/BA) to dominate over AA regions (Vitale et al., 2021). The periodic potential generates miniband formation and can produce nearly flat bands, especially for $\Gamma$-point derived valence states with significant interlayer coupling. In WS$_2$/WSe$_2$, however, the hierarchy of orbital energies positions the K/K$'$-derived valence states at the band top, so the highest valence bands remain dispersive even at small twist angles, while the flat $\Gamma$-derived bands lie lower (Vitale et al., 2021).

Critical ("magic") twist angles (e.g., $17.9^\circ$, $42.1^\circ$) yield particularly symmetric moiré patterns and substantial band gap transitions—from indirect to direct bands—enabling flexible engineering of optoelectronic properties (Lin et al., 9 May 2024). The tunable band gap $E_g(\theta)$ spans from 1.55 to 2.10 eV (WSe$_2$), modulated by stacking and twist angle.

3. Exciton Physics: Interlayer, Hybridized, and Moiré Excitons

Interlayer Excitons and Gate Tunability

The type-II alignment results in spatially indirect interlayer excitons, with electrons in WS$_2$ and holes in WSe$_2$. Bethe–Salpeter equation (BSE) calculations reveal binding energies typically only $\sim$20% lower than intralayer counterparts, counterintuitively strong due to reduced out-of-plane screening (Torun et al., 2018). The lowest energy exciton state thus possesses interlayer character, featuring long lifetimes and diminished oscillator strength (dark in absorption but dominant in low-temperature photoluminescence).

Interlayer coupling enables gate-tunable exciton energies and dipole oscillator strengths. Under vertical electric fields, band offsets and exciton dipole strengths can be tuned over an order of magnitude, modulating radiative lifetime up to two orders (Gao et al., 2017). Simple two- and four-level models recapitulate the essential physics, informed by GW+BSE results, with the off-diagonal hybridization parameter $t$ dictating anti-crossing behavior and excitonic mixing.

Moiré Excitons, Wannier, and Charge-Transfer States

The moiré superlattice causes spatial modulation of excitonic states, leading to distinct moiré excitons: modulated Wannier excitons (strongly localized at high-symmetry stacking sites), intralayer charge-transfer excitons (with electron and hole separated by nanometers within the same layer), and intermediate mixed character states. The character of these excitons is captured by a superposition of valence–conduction transitions across the moiré Brillouin zone,

$$\chi(\mathbf{r}_e,\mathbf{r}_h) = \sum_{k_m} A_{k_m} \psi^{e}_{k_m}(\mathbf{r}_e) (\psi^{h}_{k_m})^*(\mathbf{r}_h)$$

revealing spatial charge correlations modulated by the moiré potential (Naik et al., 2022).

Experimentally, reflection spectra resolve multiple excitonic peaks corresponding to distinct moiré exciton types. Doping response and magnetic-field Zeeman splitting confirm their spatial structure: Wannier-type excitons show minimal response under carrier injection due to co-location, while charge-transfer excitons exhibit red-shifts due to Coulomb interactions with nearby doping charges.

4. Exciton–Exciton Interaction and Moiré-Biexciton States

Recent work reveals direct evidence of moiré-trapped biexcitons—bound states of two interlayer excitons within the same moiré pocket—stabilized by exchange interaction overcoming typical dipole-dipole repulsion (Chhaperwal et al., 31 Aug 2025). In a flat bilayer, interlayer excitons are repulsive (owing to layer polarization), but when spatially confined to a few-nanometer pocket, the exchange interaction in a spatially antisymmetric (triplet) configuration converts the net interaction to attractive, yielding negative biexciton binding energy:

$$V_\mathrm{int} = V(\mathbf{r}_{e1} - \mathbf{r}_{e2}) + V(\mathbf{r}_{h1} - \mathbf{r}_{h2}) - V^{(1)}(\mathbf{r}_{e1} - \mathbf{r}_{h2}) - V^{(1)}(\mathbf{r}_{e2} - \mathbf{r}_{h1}),$$

where $V(r) = e^2/(\epsilon r)$ and $V^{(1)}(r) = e^2/\epsilon\sqrt{r^2 + d^2}$ (Chhaperwal et al., 31 Aug 2025). Nanopillar-induced strain funnels excitons to pillar tops, enabling detection of sharp biexciton features: photoluminescence full-width at half-maximum narrows from $\sim$34 meV to $\sim$6 meV, alongside a superlinear power-law scaling of PL intensity indicative of interaction-driven occupation statistics.

5. Exciton Transport: Moiré Potential and Many-body Effects

WS$_2$/WSe$_2$ hetero-bilayers exhibit twist-angle-dependent transport behavior for interlayer excitons, governed by the interplay of moiré potential depth (100–150 meV) and many-body repulsion. Time-resolved transient absorption microscopy shows that exciton population dynamics are dictated by the energy difference $\Delta E_{KQ}$ between energetically favorable K–Q and higher-energy K–K excitons: $\Delta E_{KQ} \approx 88$ meV (0$^\circ$ twist) and $62$ meV (60$^\circ$ twist), controlling intervalley scattering and equilibrium populations (Yuan et al., 2019).

Exciton diffusion deviates from normal (linear) behavior owing to collective density-dependent screening; the effective diffusivity $D$ follows

$$D = D_0 \exp\left(-\frac{U}{k_B T + u_0 n(x,t)}\right),$$

with $U$ the local moiré trapping depth, $u_0$ the many-body interaction scale, and $n(x,t)$ the exciton density. At high density, many-body repulsion screens trapping and enables longer-range transport; at low density or low temperature, excitons localize strongly in moiré minima. This tunability is crucial for applications such as excitonic circuits, quantum communication, and quantum emitter arrays.

6. Synthesis, Stability, and Device Engineering

Controlled synthesis of WS$_2$/WSe$_2$ hetero-bilayers is achieved via water-assisted chemical vapor deposition (CVD) enabling sharp, atomically-defined lateral interfaces and stacking control (Sahoo et al., 2019). DFT studies show that chalcogen-terminated (c-edge) flakes are energetically favored, yielding semiconducting behavior and robust interlayer orbital overlap; metal-terminated (m-edge) flakes are unstable and prone to metallic states due to unsatisfied dangling bonds (Ghatak et al., 2019).

Bilayer lateral heterostructures offer enhanced optoelectronic response: rectification currents up to %%%%49$d$50%%%% larger than monolayers, photovoltaic short-circuit currents %%%%51$d$52%%%% times larger, marked photoluminescence from direct K–K$'$ transitions, and room-temperature electroluminescence. The thicker bilayer structure increases photon absorption and reduces environmental susceptibility, boosting device robustness for flexible, low-power applications.

7. Outlook and Applications

WS$_2$/WSe$_2$ hetero-bilayers are at the forefront of next-generation twistronic, optoelectronic, and quantum photonic platforms. Their band structure and excitonic landscape can be tuned precisely by twist angle, stacking, and external fields, enabling direct–indirect band gap transitions, flat-band formation, high carrier mobility, and tunable many-body interactions. Moiré modulation supports highly correlated states, biexciton physics, and excitonic condensates, while robust device engineering via lateral epitaxy and stacking control promises high-efficiency, flexible electronic and photonic devices. These hetero-bilayers thus provide a versatile platform for exploring correlated, valleytronic, and excitonic physics in 2D quantum materials.