arXiv subjects

Subhradeep Chatterjee

Publications and source records attributed to Subhradeep Chatterjee.

4 recordsLinked to original sources

arXiv2026

Reaction-Diffusion Driven Patterns in Immiscible Alloy Thin Films

Controlling the microstructure of thin films is of critical importance for various applications. We demonstrate a methodology for tuning the local microstructure through film-substrate interactions using Ag-Cu as a model system. Metastable single-phase Ag-Cu thin films are deposited on Si substrates pre-patterned by FIB milling. During post-deposition annealing, localized film-substrate reaction around the milled patterns produces a distinct microstructure termed as the 'halo'. It consists of copper silicide and almost pure Ag, while the far-field film forms a random mixture of Cu and Ag-rich domains through phase separation. We show that the extent of the halo can be controlled by varying the temperature and duration of annealing. We present a semi-analytical kinetic model of product and halo growth that incorporates species balance, diffusional transport and a modified Stefan condition. Predictions from the model reveal two distinct growth regimes of the product with power law indices of 1/2 and 2/7 and experimental data fall into the latter regime. These regimes originate from the dimensionality of growth (2d or 3d) compared to that of solute transport (2d), which in turn depend on film thickness and species diffusivity. Using an inverse optimization procedure, we also estimate the diffusivity, which suggests grain boundary diffusion to be the dominant transport mechanism. This study provides an avenue and framework for microstructural engineering of alloy thin films through interfacial reaction.

cond-mat.mtrl-sci↗

arXiv2023

Substrate interaction mediated control of phase separation in FIB milled Ag-Cu thin films

Nanofabrication is an integral part of realization of advanced functional devices ranging from optical displays to memory devices. Focused ion beam (FIB) milling is one of the widely used nanofabrication methods. Conventionally, FIB milling has been carried out for patterning single-phase stable thin films. However, the influence of FIB milling on phase separation of metastable alloy films during subsequent treatments has not been reported. Here, we show how FIB milling of Ag-Cu thin films influences the separation process and microstructure formation during post-milling annealing. Phase-separated microstructure of the film consists of fine, randomly distributed Ag-rich and Cu-rich domains, whereas adjacent to milled apertures (cylindrical holes), we observe two distinctly coarser rings. A combination of imaging and analysis techniques reveals Cu-rich islands dispersed in Ag-rich domains in the first ring next to the aperture, while the second ring constitutes mostly of Ag-rich grains. Copper silicide is observed to form in and around apertures through reaction with the Si-substrate. This substrate interaction, in addition to known variables like composition, temperature, and capillarity, appears to be a key element in drastically changing the local microstructure around apertures. This current study introduces new avenues to locally modulate the composition and microstructure through an appropriate choice of the film-substrate system. Such an ability can be exploited further to tune device functionalities with possible applications in plasmonics, catalysis, microelectronics and magnetics.

cond-mat.mtrl-sci↗

arXiv2022

Surface-directed and bulk spinodal decomposition compete to decide the morphology of bimetallic nanoparticles

An embedded-domain phase-field formalism is used for studying phase transformation pathways in bimetallic nanoparticles (BNPs). Competition of bulk and surface-directed spinodal decomposition processes and their interplay with capillarity are identified as the main determinants of BNP morphology. The former is characterized by an effective bulk driving force $\Delta\tilde{f}$ which increases with decreasing temperature, while the latter manifests itself through a balance of interfacial energies captured by the contact angle $\theta$. The simulated morphologies, namely, core-shell, Janus and inverse core-shell, cluster into distinct regions of the $\Delta\tilde{f}$-$\theta$ space. Variation of $\theta$ with $\Delta\tilde{f}$ in the Ag-Cu alloy system is computed as a function of temperature using a CALPHAD approach in which surface energies are estimated from a modified Butler equation. This $\theta-\Delta\tilde{f}$ trajectory for Ag-Cu, when superimposed on the morphology map, enables the prediction of different morphological transitions as a function of temperature. Therefore, the study establishes a unique thermodynamic framework coupled with phase-field simulations for predicting and tailoring nanoparticle morphology through a variation of processing temperature.

cond-mat.mtrl-sci↗

arXiv2021

Competition of Core-Shell and Janus Morphology in Alloy Nanoparticles: Insights From a Phase-Field Model

Bimetallic nanoparticles (BNPs) exhibit diverse morphologies such as core-shell, Janus, onion-like, quasi-Janus, and homogeneous structures. Although extensive effort has been directed towards understanding the equilibrium configurations of BNPs, kinetic mechanisms involved in their development have not been explored systematically. Since these systems often contain a miscibility gap, experimental studies have alluded to spinodal decomposition (SD) as a likely mechanism for the formation of such structures. We present a novel phase-field model for confined (embedded)systems to study SD-induced morphological evolution within a BNP. It initiates with the formation of compositionally modulated rings as a result of surface-directed SD and eventually develops into core-shell or Janus structures due to coarsening/breakdown of the rings. The final configuration depends crucially on contact angle and particle size -Janus is favored at smaller sizes and higher contact angles. Our simulations also illustrate the formation of metastable, kinetically trapped structures as a result of competition between capillarity and diffusion.

cond-mat.mtrl-sci↗