---
title: Cadmium Phosphorus Trisulfide (CdPS3)
url: https://www.emergentmind.com/topics/cadmium-phosphorus-trisulfide-cdps3
type: topic
---

# Cadmium Phosphorus Trisulfide (CdPS3)

Cadmium phosphorus trisulfide (CdPS₃) is a layered van der Waals semiconductor of current interest for ultraviolet–visible (UV–vis) nanophotonics and catalytic heterostructures. It is distinguished among the MPX₃ family by an exceptional combination of a wide indirect bandgap ($E_g \sim 3.4$ eV) and a near-UV in-plane refractive index approaching $n = 3$, breaking the prevailing empirical trade-off described by Moss’s law. This article surveys the crystallographic, electronic, and optical properties of CdPS₃, together with its behavior under external stimuli, phase/defect tunability by advanced synthesis, and key implications for device applications.

## 1. Crystal Structure and Polymorphism

CdPS₃ exhibits a strongly 2D crystal structure, with electronically decoupled layers held together by van der Waals (vdW) interactions [2511.14269, 2010.11109, 2512.09073]. At room temperature, the stable phase is monoclinic (space group C2/m), while a low-temperature polymorph is trigonal (space group R3). Lattice parameters determined experimentally and via first-principles optimization agree to within 1%. Typical monoclinic values are
\[
a = 6.218 \,\text{Å}, \;\; b = 10.763 \,\text{Å}, \;\; c = 6.867 \,\text{Å}, \;\; \beta = 107.58^\circ.
\]
The layer structure consists of hexagonally packed CdS₆ octahedra sharing edges, separated and interconnected by P₂S₆⁴⁻ "ethane-like" bipyramidal units. The resulting 2D motif is a honeycomb network (Figure 1a in Povolotskiy et al. [2511.14269]), with each repeating layer approximately 6.8 Å thick. Interlayer spacing is measured as ~3.16 Å for C2/m and ~3.12 Å for R3 [2010.11109]. Each layer is overall charge-neutral as [Cd²⁺(P₂S₆)²⁻], yielding a stoichiometry Cd:P:S = 1:1:3.

Relevant bond lengths at zero pressure are summarized below:

| Bond        | Length (C2/m) [Å] | Length (R3) [Å] |
|-------------|------------------|-----------------|
| Cd–S        | 2.68–2.60        | 2.69            |
| P–S         | 2.07–2.10        | 2.07            |
| P–P         | 2.24             | 2.24            |
| Cd–P        | 3.73             | 3.75            |

High-purity single crystals are obtained via melt growth, and atomically thin flakes down to two monolayers can be prepared by mechanical exfoliation for device studies [2511.14269].

## 2. Electronic Band Structure and Density of States

CdPS₃ is an indirect-gap semiconductor in both polymorphs. First-principles calculations using DFT (PBE+$G_0W_0$), as well as hybrid meta-GGA M06 LCAO methods, consistently yield indirect bandgaps $E_g^{(\text{ind})} = 3.4$ eV (C2/m, room temperature) and $E_g^{(\text{ind})} = 3.3$ eV (R3, low temperature) [2010.11109, 2511.14269]. The VBM is located near the $\Gamma$ point, while the CBM is slightly displaced along the $\Gamma$–Y direction. The onset of direct transitions is only ~0.1 eV above the indirect gap, which matches optical absorption spectra [2511.14269].

Valence band states are principally S 3p character with minor admixtures of Cd 4d and P 3p, while conduction band manifold is derived from hybridized P 3s/3p, S 3p, and Cd 5s/5p orbitals [2010.11109]. Application of hydrostatic pressure modulates the gap in a non-monotonic fashion, with $E_g$ peaking at 3.6 eV for C2/m at 8 GPa, and at 4.0 eV for R3 at 30 GPa, indicating a strong structure–property–pressure correlation.

## 3. Linear Optical Properties and Anomalous Index–Gap Relation

The most distinctive optical property of CdPS₃ is its unusually high in-plane refractive index, which approaches $n_{ab} \approx 3.0$ at $\lambda = 350$ nm and maintains $n_{ab} \approx 2.7$ at 450 nm, with low absorption (extinction $k_{ab} < 10^{-2}$ for $\lambda > 360$ nm) [2511.14269]. The Sellmeier dispersion in the transparent regime follows
\[
n^2(\lambda) = 1 + \sum_{i=1}^2 \frac{B_i \lambda^2}{\lambda^2 - \lambda_i^2}
\]
with $B_1 = 1.12$, $\lambda_1 = 150$ nm; $B_2 = 0.85$, $\lambda_2 = 275$ nm.

The out-of-plane index $n_c$ is lower (e.g., $n_c \approx 2.1$ at 500 nm), with modester anisotropy $\Delta n \approx 0.5$. This refractive index–bandgap combination violates the empirical Moss’s law ($E_g n^4 \approx 95$ eV), with $E_g n^4 \approx 275$ eV for CdPS₃, nearly three times higher than conventional semiconductors. This indicates a capacity for simultaneously high transparency and strong optical confinement in the UV–vis, unattainable for other layered dielectrics.

## 4. Waveguiding, Confinement, and Near-field Optical Characterization

Scattering-type scanning near-field optical microscopy (s-SNOM) has been used to directly visualize tightly confined waveguide modes in mechanically exfoliated CdPS₃ flakes [2511.14269]. For a 254 nm-thick flake at 700 nm, effective indices are measured as $n_{\text{eff}}^{(\text{TE}_0)} \approx 2.75$ and $n_{\text{eff}}^{(\text{TM}_0)} \approx 2.15$, reducing to 1.85 and 1.60 at 1600 nm, respectively. These results agree with transfer-matrix modeling based on experimentally retrieved $(n, k)$ values, confirming that CdPS₃ guides extreme-subwavelength optical modes.

The high $n$ enables channel waveguides and metasurfaces with dimensions (width $w \approx 94$ nm at 375 nm wavelength for a decay constant $\alpha = 2.49$ µm⁻¹ in SiO₂) that are significantly reduced relative to TiO₂, Si, or MoS₂. Simulations show two 94 nm-wide CdPS₃ guides at 50 nm separation maintain crosstalk lengths on the order of 10 µm, supporting ultra-dense routing architectures. Propagation lengths in the blue/green exceed 100 µm, with mode areas as small as $A_{\text{mode}} \sim (0.1\lambda)^2$.

## 5. Phase Engineering and Photocatalytic Nanocomposites

CdPS₃ can be processed into nanostructures by femtosecond pulsed laser ablation in liquid (fs-PLAL) [2512.09073]. The choice of solvent establishes a route to control phase and defect population. In deionized water, the laser-induced plasma condenses into stoichiometric CdPS₃ nanocrystals preserving the C2/m lattice. In contrast, ablation in isopropanol or acetonitrile induces reductive chemistry, removing P atoms to yield a mixture of CdS quantum dots (QDs) and metallic Cd clusters, registered as mixed-phase or binary/ternary nanocomposites according to Table 1 in [2512.09073]:

| Solvent    | CdPS₃ Content (mol %) | CdS Content (mol %) |
|------------|-----------------------|---------------------|
| DI H₂O     | 88                    | 12                  |
| Acetonitrile | 52.6                | 47.4                |
| IPA        | 11.3                  | 88.7                |

Raman spectroscopy, TEM/SAED, and EDX confirm the phase assignments. Band alignment measurements show type-II heterojunctions between CdPS₃ and CdS, as well as Schottky barriers to metallic Cd ($\Phi_B \sim 0.1$ eV). These facilitate spatial charge separation and enhanced photocatalytic activity.

## 6. Optoelectronic Functionality and Photocatalytic Performance

Optical band positions with respect to the normal hydrogen electrode are $E_{\text{CBM}} \approx -0.8$ V, $E_{\text{VBM}} \approx +2.2$ V for CdPS₃, and $E_{\text{CBM}} \approx -1.0$ V, $E_{\text{VBM}} \approx +1.56$ V for CdS QDs. The hybrid systems, especially CdPS₃/CdS nanocolloids produced via fs-PLAL in isopropanol, exhibit superior charge separation due to the engineered junctions: electrons flow from CdS to metallic Cd, while holes remain in the CdPS₃ valence band.

Photocatalytic degradation of Methylene Blue under 532 nm irradiation demonstrates a kinetic constant $k_{\text{IPA}} \approx 0.077$ min⁻¹, yielding 90% degradation in 30 minutes for CdPS₃/CdS hybrids in IPA, compared to negligible activity in pure water-prepared CdPS₃ [2512.09073]. A plausible implication is the critical dependence of visible-light photocatalytic activity on engineered phase composition and defect chemistry, which extends the functionality of wide-bandgap vdW crystals beyond their native UV absorption.

## 7. Prospects for Ultraviolet–Visible Nanophotonics and Beyond

Owing to its record-high in-plane refractive index and wide gap, CdPS₃ is identified as a benchmark vdW dielectric for deep-UV and visible nanophotonics, metasurfaces, and integrated photonic circuits [2511.14269]. Material dispersion, transparency, and experimentally validated wave confinement far surpass the limitations imposed by conventional index–gap trade-offs. Sub-100 nm waveguides, deep-UV metasurfaces, and dense on-chip routing are accessible using this material platform.

Solvent-assisted fs-PLAL further offers tunable phase and defect engineering for application to hybrid photochemistry, where precise modulation of optoelectronic structure is enabled by rational solvent choice and pulse protocol [2512.09073]. The approach is generalizable to other MPX₃- and ternary-layered systems, but open questions persist regarding quantitative control of defect densities, in situ monitoring of plasma dynamics, long-term operational stability, and integration into functional UV/vis photonic and catalytic devices.

In summary, CdPS₃ is emerging as a multifaceted semiconductor, combining foundational advances in photonic materials science with avenues for tunable optoelectronics and catalysis [2511.14269, 2010.11109, 2512.09073].

Source: https://www.emergentmind.com/topics/cadmium-phosphorus-trisulfide-cdps3