[Paper Review] Size-dependent melting of spherical copper nanoparticles

Oleg A. Yeshchenko, Igor Dmytruk+2 more|arXiv (Cornell University)|Jan 12, 2007

nanoparticles nucleation surface interactions1 references3 citations

TL;DR

This study investigates size-dependent melting behavior of spherical copper nanoparticles embedded in a silica matrix using in situ absorption spectroscopy. By analyzing temperature-dependent surface plasmon resonance (SPR) energy and bandwidth, the authors identify two distinct melting regimes: for particles <20 nm, gradual, non-first-order melting or superheating occurs; for particles >20 nm, a jump-like bandwidth increase and non-monotonic SPR energy shift indicate first-order melting akin to bulk copper, with a critical size threshold near 20 nm.

ABSTRACT

We report size-dependent melting of spherical copper nanoparticles embedded into silica matrix. Based on the temperature dependence of the surface plasmon resonance energy and its width we observe two distinct melting regimes. For particles smaller than 20 nm the absorption spectrum changes monotonically with the temperature, and this allows us to assume the gradual solid-liquid phase transition (melting) of the nanoparticles or existence of superheated solid nanoparticles. In contrast, for nanoparticles larger than 20 nm, we observe a jump-like increase of the bandwidth and non-monotonic dependence of surface plasmon energy at the temperatures below the bulk melting point. This indicates that the melting of large nanoparticles is a first-order phase transition similar to the melting of bulk copper.

Motivation & Objective

• To investigate the size-dependent melting behavior of spherical copper nanoparticles in a silica matrix.
• To determine whether the melting transition of small nanoparticles differs fundamentally from that of bulk copper.
• To identify the critical particle size at which the melting behavior transitions from gradual to first-order.
• To use optical spectroscopy as a non-invasive, temperature-controlled method to probe phase transitions in nanomaterials.
• To correlate structural morphology (crystalline vs. non-crystalline) with melting characteristics in copper nanoparticles.

Proposed method

• Employed in situ absorption spectroscopy to monitor temperature-dependent surface plasmon resonance (SPR) energy and bandwidth.
• Measured SPR response across a size range of 5–65 nm for Cu nanoparticles embedded in a SiO2 matrix.
• Used Mie theory to interpret SPR peak positions and validate experimental data.
• Applied the $1/r$ damping law (Eq. 1) to fit the size dependence of SPR bandwidth at room temperature.
• Analyzed temperature-dependent SPR bandwidth and energy shifts to distinguish between first-order and gradual phase transitions.
• Correlated observed transitions with theoretical models of nanoparticle melting, including molecular dynamics simulations and thermodynamic stability criteria.

Experimental results

Research questions

• RQ1 How does the melting behavior of spherical copper nanoparticles vary with size below and above 20 nm?
• RQ2 Does the melting transition of small copper nanoparticles (d < 20 nm) exhibit characteristics of a first-order phase transition like bulk copper?
• RQ3 What is the origin of the observed non-monotonic temperature dependence of SPR energy and bandwidth in larger nanoparticles?
• RQ4 Can optical spectroscopy reliably detect the onset of melting in embedded nanoparticles with real sample temperature control?
• RQ5 What role does nanoparticle morphology (e.g., fcc vs. Ih/Dh structures) play in determining the melting pathway?

Key findings

• For copper nanoparticles smaller than 20 nm, the SPR bandwidth increases gradually with temperature and the SPR energy decreases monotonically, indicating a non-first-order melting process or the existence of superheated solid phases.
• For nanoparticles larger than 20 nm, a jump-like increase in SPR bandwidth and non-monotonic temperature dependence of SPR energy were observed, signaling a first-order phase transition similar to bulk copper melting.
• The critical size threshold for the transition between melting regimes was identified at approximately 20 nm, coinciding with a morphological transition from non-crystalline (Ih/Dh) to fcc crystalline structures.
• The size-dependent SPR bandwidth at 293 K was well described by the $1/r$ damping law, yielding $\Gamma_{\infty,293} = 0.087$ eV and $A_{293} = 0.107$.
• The observed behavior in small particles is consistent with theoretical predictions of unstable or metastable solid-liquid coexistence and potential superheating.
• The silica matrix does not qualitatively alter the melting transition type, though it may shift critical temperatures and sizes, preserving the fundamental distinction between small and large particle melting.