Resolving Crystallization Kinetics of GeTe Phase-Change Nanoparticles by Ultrafast Calorimetry

Abstract

Chalcogenide-based phase change materials (PCMs) are promising candidates for the active element in novel electrical nonvolatile memories and have been applied successfully in rewritable optical disks. Nanostructured PCMs are considered as the next generation building blocks for their low power consumption, high storage density, and fast switching speed. Yet their crystallization kinetics at high temperature, the rate-limiting property upon switching, faces great challenges due to the short time and length scales involved. Here we present a facile method to synthesize highly controlled, ligand-free GeTe nanoparticles, an important PCM, with an average diameter under 10 nm. Subsequent crystallization by slow and ultrafast rates allows unravelling of the crystallization kinetics, demonstrating the breakdown of Arrhenius behavior for the crystallization rate and a fragile-to-strong transition in the viscosity as well as the overall crystal growth rate for the as-deposited GeTe nanoparticles. The obtained results pave the way for further development of phase-change memory based on GeTe with sub-lithographic sizes.

Correlating ultrafast calorimetry, viscosity, and structural measurements in liquid GeTe and Ge15Te85

“…Taking the experimental U max ≈ 3.3 m s −1 at 0.85T m for as-deposited thinfilm GeTe (d = 30 nm), by Santala et al [30] using dynamical transmission electron microscopy (TEM), the theoretical fragility can be estimated from the empirical relation presented by Orava and Greer [31] to be m ≈ 50-70, i.e., between AIST (m ≈ 37) and GST (m ≈ 90) at T g . Chen et al [16] suggested the occurrence of a crossover in GeTe nanoparticles, with an average diameter of 10 nm, and they derived m = 78 by using a generalized-MYEGA model [32] which is surprisingly similar to the present value. It should be noted that viscosity of GST nanoparticles also shows a crossover with m = 57-62 [33], which is lower than that for the thin film (m ≈ 90) [6].…”

“…Although the liquid Ge 15 Te 85 is not of interest as an active material for PCM because of the sluggish crystallization kinetics, the existence of a crossover similar to that for AIST, in contrast to PC liquid GeTe and Ge 2 Sb 2 Te 5 , is of fundamental interest. The existence of a crossover means that fast crystallization can be maintained at high temperatures of the supercooled liquid, and longer incubation times and lower growth rates at larger T [2,16], due to lower mobility, could help to improve the resistance against spontaneous crystallization. In the presence of a crossover there is no, or negligible, role of the decoupling between U kin and η around T g [9,14].…”

Colloidal Phase-Change Materials: Synthesis of Monodisperse GeTe Nanoparticles and Quantification of Their Size-Dependent Crystallization

“…Presented models describe a trend for ultrasmall GeTe nanoparticles ( Figure S10 ), 12 while slightly higher crystallization temperature is observed for larger GeTe sizes by Arachchige et al and Chen et al 13 , 32 The latter discrepancy can be explained by the nonstoichiometric effects—when the composition of GeTe deviates from Ge:Te 1:1 ratio, the crystallization temperature increases. 7…”

Magnetron-sputtered copper nanoparticles: lost in gas aggregation and found by in situ X-ray scattering

“…The method has acquired particular popularity as is evidenced by an increasing number of papers that have been published in recent years. Single-metal, [4][5][6][7][8][9][10][11][12][13][14][15] alloyed [16][17][18][19][20][21][22][23][24][25][26] and heterostructured NPs [27][28][29][30][31][32][33][34][35][36][37][38][39][40] have been successfully synthesized.…”