Bimetallic Janus Nanoparticles

Updated 11 October 2025

Bimetallic Janus nanoparticles are dual-domain nanoscale objects with distinct metal compositions, offering anisotropic optical, catalytic, and transport properties.
Their morphology and self-assembly are tuned by strain, capillarity, and phase evolution, enabling precise design for catalysis and nanoscale energy conversion.
Field-directed techniques, including magnetic and optical manipulation, combined with advanced simulations, guide controlled assembly and multifunctional application development.

Bimetallic Janus nanoparticles are nanoscale objects comprising two distinct metallic domains or hemispheres, each with different elemental composition, crystalline structure, or ligand chemistry. This anisotropic arrangement imparts unique physicochemical, optical, catalytic, and transport properties not attainable in homogeneous or conventional core–shell nanoparticles. Bimetallic Janus nanoparticles can manifest as perfect two-sided (Janus-type) structures, quasi-Janus architectures resulting from strain-driven instabilities, or as more complex morphologies governed by kinetic and thermodynamic factors. Their bifunctional surfaces facilitate asymmetric interactions with external fields, interfaces, or reactants and enable applications in catalysis, plasmonics, nanomedicine, and nanoscale energy conversion.

1. Geometric Structures, Morphological Instabilities, and Phase Evolution

The geometric arrangement and atomic ordering of bimetallic Janus nanoparticles are sensitively dictated by competition between intrinsic strain, capillarity, and chemical miscibility. In icosahedral nanoparticles, global-optimization studies demonstrate that a centered core can initially be accommodated, but exceeding a critical core size $k_c$ induces a morphological instability: the minority species (core) shifts asymmetrically toward the particle surface, resulting in a quasi-Janus morphology (Bochicchio et al., 2013). The critical atom number in a $k$ -shell icosahedron follows

$N = \frac{10k^3 - 15k^2 + 11k - 3}{3}$

A measurable displacement, $d_C$ , quantifies the core’s off-center shift. This asymmetry is mechanistically analogous to the Stranski–Krastanov instability in thin-film growth, where accumulated lattice mismatch strain triggers a structural transformation from symmetric (layered/centrosymmetric) to asymmetric (quasi-Janus/islanded) morphology.

Spinodal decomposition in embedded nanoparticles offers a general framework for understanding Janus formation (P et al., 2021, Pankaj et al., 2022). Surface-directed composition waves nucleate concentric modulated rings, which break up via coarsening and lateral diffusion into Janus or core–shell morphologies depending on contact angle $\theta$ , particle size, and capillary effects. The phase field evolution is governed by the Cahn–Hilliard equation: $\frac{\partial c}{\partial t} = \nabla \cdot M(\mathbf{r}) \nabla \left(\frac{\partial f}{\partial c} - 2\kappa_c \nabla^2 c\right)$ where $c(\mathbf{r},t)$ is the composition field and $M(\mathbf{r})$ the spatially varying mobility. Smaller nanoparticles and higher contact angles favor the development of metastable and stable Janus morphologies.

A comprehensive morphologic phase map arises from embedded-domain phase field models, showing that the combination of effective bulk driving force $\Delta \tilde{f}$ and contact angle $\theta$ selects among core–shell, Janus, and inverse core–shell architectures (Pankaj et al., 2022). For Ag–Cu and similar systems, this $\theta$ – $\Delta \tilde{f}$ trajectory can be mapped as a function of temperature and used to predict morphological transitions.

2. Dynamics, Interfacial Behavior, and Diffusion

Bimetallic Janus nanoparticles exhibit highly anisotropic translational and rotational dynamics at interfaces, as revealed by molecular dynamics simulations (Rezvantalab et al., 2014). When adsorbed at liquid–liquid interfaces, such particles experience:

In-plane translational diffusion (with a coefficient $D_t$ ) that slows as amphiphilicity (differential wettability of each face) increases, due to dense fluid layering and enhanced drag near the preferred hemisphere.
Suppressed rotational diffusion, especially for in-plane (about interface normal) motion, as surface amphiphilicity increases and local fluid order constrains motion.
Diffusion coefficients scaling more rapidly with particle size than the Stokes–Einstein ( $D_t \sim 1/R$ ) or Stokes–Einstein–Debye ( $D_r \sim 1/R^3$ ) relations predict, due to the increased interaction with the structuring fluid at interfaces.

The ordering of surrounding fluid—manifested in pronounced radial distribution function (g(r)) peaks—leads to increased effective drag, further reducing both translational and rotational mobility. These dynamics, compared to homogeneous (non-Janus) nanoparticles, signify reduced rates but increased anisotropy in interfacial environments, directly impacting residence times at biological membranes or catalytic supports.

3. Self-Assembly, Confined Behavior, and Interparticle Interactions

Self-assembly and collective dynamics of bimetallic Janus nanoparticles are critically influenced by their anisotropic surfaces, interaction potentials, and environmental constraints.

In block copolymer matrices, Janus nanoparticles orient with respect to the microphase-separated interfaces, minimizing the system’s free energy via anisotropic interactions. This orientational degree of freedom—the “Janus axis”—stabilizes particle arrangements at interfaces with minimal bridging across domains, promoting robust lamellar morphologies and hierarchical organization (Diaz et al., 2019).
In confined geometries, such as between parallel plates, Janus nanoparticles modeled as dimers with distinct interaction potentials exhibit new aggregation motifs (micelles, lamellae), reentrant fluid phases, and thermodynamic anomalies (shifted temperature of maximum density, nonmonotonic diffusion behavior) (Bordin et al., 2016).
When functionalized with oppositely charged ligands (e.g., zwitterionic monolayer-protected clusters), bimetallic Janus nanoparticles can form chain-like or cyclic superstructures, governed primarily by electrostatic attraction between complementary hemispheres. The strength and directionality of these interactions can be tuned via ligand selection, solvent properties, or environmental parameters (Bhattacharya et al., 2023).

These collective assembly phenomena are key for templating advanced materials, creating switchable nanoscale architectures, and engineering structures with predetermined optical or catalytic response.

4. Optical, Plasmonic, and Hot-Carrier Phenomena

The anisotropic architecture of bimetallic Janus nanoparticles enables unique optical responses through the hybridization of plasmonic resonances and interfacial coupling.

In spherical Janus nanoparticles composed of Au, Ag, or Cu, incident light induces localized surface plasmon resonances (LSPRs) specific to each hemisphere. The hot-carrier generation spectrum shows distinct peaks—e.g., Ag LSPR near 3.4 eV (intraband) and Au LSPR near 2.4 eV (interband)—with the spatial and energetic distribution of hot electrons and holes governed by the material composition and geometry (Jin et al., 9 Oct 2025).
Dumbbell-shaped Janus nanoparticles exhibit enhanced hot-carrier generation as the neck connecting the hemispheres increases in size. The electric field in the neck region is dramatically amplified (up to >3000 enhancement in $|E|^2$ ), resulting in pronounced local transition rates in carrier generation.
Light polarization is critical: fields perpendicular to the metal–metal interface maximize dipole coupling and hot-carrier rates, while parallel polarization leads to weaker interfacial enhancement.

These properties are pivotal for applications in photocatalysis, photovoltaics, and sensing. By combining macroscopic Maxwell equation solutions with tight-binding quantum models, it is possible to predict the detailed spatial, energetic, and polarization-dependent features of hot-carrier generation and design Janus nanoparticles for optimal efficiency (Jin et al., 9 Oct 2025).

Geometry	Plasmon Resonance Zones	Field Enhancement	Hot-Carrier Effect
Spherical	Hemispheric (Au/Ag/Cu)	Moderate	Localized, spectral hybridized
Dumbbell	Neck/interface region	Very high	Strongly concentrated, tunable

5. Manipulation, Trapping, and Field-Directed Assembly

Bimetallic Janus nanoparticles can be selectively manipulated and assembled via external fields, exploiting their intrinsic anisotropy:

Magnetic Janus particles at fluid–fluid droplet interfaces respond to applied magnetic fields by aligning and migrating to field-directed positions, forming tunable assemblies (hexagonal, ring-like, or anisotropic) as determined by the orientation and magnitude of the field (Xie et al., 2019). The interface energy model,

$\Delta E = 4\phi a^2\gamma_{12}\sin\beta$

captures the dependence of migration energy on amphiphilicity ( $\beta$ ), particle radius ( $a$ ), and angular misalignment ( $\phi$ ).

Optical manipulation leverages the plasmonic properties of Janus particles: evanescent fields from nanofibers or nanoapertures generate substantial gradient and scattering forces on the metallic hemisphere, facilitating propulsion, trapping, and precise localization with strongly enhanced speeds and confinement compared to dielectric particles (Esporlas et al., 2021, Koya et al., 2023). Dual-laser resonant excitation combines on-resonance plasmonic heating (e.g., at 532 nm) and off-resonant trapping (1064 nm), yielding up to threefold enhancements in trapping force.

Such field-driven approaches enable remote, low-power control and patterning of Janus nanoparticles for applications in nanorobotics, targeted drug delivery, and the fabrication of complex mesoscale assemblies.

6. Directional Thermal Transport and the "Thermal Janus Effect"

Janus architectures offer unprecedented control over heat dissipation at the nanoscale. The combination of dissimilar interfacial thermal resistances in each hemisphere leads to highly directional temperature fields.

Analytical and finite-difference modeling of Janus nanoparticles illuminated by laser or magnetic fields demonstrates the emergence of strong temperature contrasts between the two hemispheres (Xie et al., 2023). The magnitude of directional heating is quantified by

$\zeta = \frac{\Delta T_1}{\Delta T_2}$

where $\Delta T_{1,2}$ are the temperature rises on the respective faces. Small particles and sharply defined polar angles optimize $\zeta$ , with transient (pulsed) heating yielding even larger contrasts due to thermal confinement.

Advanced photothermal "J-Nanojet" designs integrate a plasmonic doughnut-shaped metallic core, a low-conductivity (PDMS) base, and a high-conductivity (diamond) cap, achieving up to 91% of the generated heat flux directed onto the target substrate while suppressing backward dissipation (González-Colsa et al., 2021). This thermal rectification enables precise control in photothermal hyperthermia for cancer therapy, improving efficiency and minimizing collateral heating.

These insights provide a framework for engineering Janus nanoparticles with tailored energetic landscapes for programmable heat flow, nanomedicine, and active matter applications.

7. Structural Complexity: Dislocations, Amorphization, and Defect Engineering

At atomic and mesoscopic scales, Janus nanoparticles can accommodate complex strain and geometric frustration:

Atomic-resolution studies of icosahedral Janus nanoparticles reveal that perfect fivefold symmetry is disrupted on one hemisphere (the "C5'" side) by edge dislocations, bond distortions, and the emergence of amorphous domains (Sun et al., 2023). This structural bifurcation relieves the angular deficiency intrinsic to multiply twinned particles and creates two-sided distributions in bond length, packing efficiency, and local strain tensor.
The resulting duality—one well-ordered (C5) domain and one defect-rich (C5') domain—has implications for catalysis, plasmon response, and mechanical properties. Engineering synthesis conditions (e.g., by tuning cooling rates) enables control over the prevalence of these features, opening pathways for functional property modulation.

8. Advanced Material Systems: Janus Nanotubes and Heterostructures

Janus architectures have been extended into 1D TMDC nanotubes using coaxial van der Waals heterostructures. For example, MoSSe Janus nanotubes synthesized via H $_2$ plasma treatment on SWCNT–BNNT templates exhibit unique "Se–Mo–S" layer ordering, broken structural symmetry, and energetically preferred states for diameters exceeding 40 Å (Yang et al., 21 Jul 2024). These systems enable tunable Rashba splitting, excitonic properties, spintronic effects, and potentially enhanced functionality for nanoelectronic devices.

Bimetallic Janus nanoparticles therefore represent a versatile and architecturally tunable class of nanomaterials. Their formation is governed by an interplay of strain, capillarity, interfacial energy, and kinetics. Their physicochemical duality enables unique functionalities in catalysis, photonics, thermal transport, self-assembly, and nanomedicine. The ability to predict and control their morphology—using phase field models, atomistic simulation, or tailored synthesis—provides a foundation for the rational design of advanced hybrid nanomaterials, with further innovation expected in interface engineering, field-driven assembly, and anisotropic energy management.