Probing the Melting Transitions in Phase-Change Superlattices via Thin Film Nanocalorimetry.

Phase-change superlattices with nanometer thin sublayers are promising for low-power phase-change memory (PCM) on rigid and flexible platforms. However, the thermodynamics of the phase transition in such nanoscale superlattices remain unexplored, especially at ultrafast scanning rates, which is crucial for our fundamental understanding of superlattice-based PCM. Here, we probe the phase transition of Sb2Te3 (ST)/Ge2Sb2Te5 (GST) superlattices using nanocalorimetry with a monolayer sensitivity (∼1 Å) and a fast scanning rate (105 K/s). For a 2/1.8 nm/nm Sb2Te3/GST superlattice, we observe an endothermic melting transition with an ∼240 °C decrease in temperature and an ∼8-fold decrease in enthalpy compared to those for the melting of GST, providing key thermodynamic insights into the low-power switching of superlattice-based PCM. Nanocalorimetry measurements for Sb2Te3 alone demonstrate an intrinsic premelting similar to the unique phase transition of superlattices, thus revealing a critical role of the Sb2Te3 sublayer within our superlattices. These results advance our understanding of superlattices for energy-efficient data storage and computing.

Size effect and odd-even alternation in the melting of single and stacked AgSCn layers: synthesis and nanocalorimetry measurements.

We report a systematic study of melting of layered lamella of silver alkanethiolates (AgSCn). A new synthesis method allows us to independently change the thickness of the crystal in two ways-by modulating chain length (n = 7-18) and by stacking these crystals to a specific layer number (m = 1-10). This method produces magic size lamella, having a well-spaced discrete melting point, Tm, distribution. Nanocalorimetry shows stepwise increases in Tm, as the lamella thickness increases by integer increments of chain length. The relationship between Tm and the inverse thickness follows the linear scaling law of Gibbs-Thomson effect. Layer stacking dramatically changes the degree and nature of size-effect melting. There is odd/even effect in stacked 2, 3, and 4 layers. Tm values of single-layer and multilayer samples do not show noticeable odd/even alternation. We develop a phenomenological model of size effect based on the cumulative excess free energy, G(excess), contributions of four spatially separate regions of the crystal: surface, Ag-S central plane, substrate interface, and interlayer interface. The selective appearance of the odd/even effect is due to the significant stabilization (1.4 kJ/mol) of interlamellae interfaces of odd-chain samples, possibly due to registration/packing. Stabilization occurs only for the mobile lamellae situated close to the free surface, and thus 2-layer samples show the highest degree of stabilization. X-ray diffraction shows that the chains are tilted 18° with respect to the basal plane normal but that the van der Waals gap is 0.3 Å smaller for crystals with odd chains.

Synthesis and characterization of single-layer silver-decanethiolate lamellar crystals.

We report the synthesis of silver-decanethiolate (AgSC10) lamellar crystals. Nanometer-sized Ag clusters grown on inert substrates react with decanethiol vapor to form multilayer AgSC10 lamellar crystals with both layer-by-layer and in-plane ordering. The crystals have strong (010) texture with the layers parallel to the substrates. The synthesis method allows for a precise control of the number of layers. The thickness of the lamellae can be manipulated and systematically reduced to a single layer by decreasing the amount of Ag and lowering the annealing temperature. The single-layer AgSC10 lamellae are two-dimensional crystals and have uniform thickness and in-plane ordering. These samples were characterized with nanocalorimetry, atomic force microscopy (AFM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray reflectivity (XRR), Fourier transform infrared spectroscopy (FTIR), and Rutherford backscattering spectroscopy (RBS).

Approaching the size limit of organometallic layers: synthesis and characterization of highly ordered silver-thiolate lamellae with ultra-short chain lengths.

Approaching the ultimate limits of material sizes provides a route for designing new functional materials with extraordinary properties. We report the first systematic synthesis and characterization study of a wide range of highly ordered silver alkanethiolate (AgSCnH2n+1 or AgSCn, n = 1-16) aliphatic lamellae. Single crystalline multilayer AgSCn are synthesized by a modified solution reaction method. Hot toluene recrystallization or Ostwald ripening enhances the structural ordering of the lamellar crystals. This work approaches the chain length limit of aliphatic lamellae by synthesizing highly ordered AgSCn (n = 1-3) with extremely short chains. All lamellae form single crystals with well-registered interlayer interfaces, similar to other alkyl-based lamellae but different from polyethylene lamellae. AgSC2 with a layer thickness of 1.08 nm is the thinnest organometallic layer ever reported. The composition, morphology, decomposition and structure of the lamellae are comprehensively studied. A new method quantifies the composition of the residual Ag and Ag2S contents after the decomposition of the AgSCn: all of the Ag, none of the C and a fraction of the S remain in the residue. The structural orderings of the AgSCn crystals, which are probed by electron diffraction for the first time, are characterized in terms of chain conformation, interlayer lamellar ordering and intralayer lattice ordering. All AgSCn (n = 2-16) layers, except AgSC1, possess a common lattice packing and the same inorganic network structure.

Self-assembly and ripening of polymeric silver-alkanethiolate crystals on inert surfaces.

We characterize and compare the reaction of alkanethiol with Ag continuous planar thin films and Ag islands on inert substrates. Ag islands generate a significantly larger (3-fold) amount of alkanethiolate than continuous Ag films at comparable conditions. The reaction with planar Ag thin films produces alkanethiol self-assembled monolayers (SAMs), whereas the reaction with Ag islands yields two dissimilar products depending on the size of the islands. Small Ag islands are more likely to be converted into multilayer silver-alkanethiolate (AgSR) crystals, while larger Ag islands form monolayer-protected clusters (MPCs). The AgSR lamellar crystals are initially small having only a few layers. However, during thermal annealing, ripening occurs that generates large AgSR lamellae having diameters of 1 microm and thickness up to 30 layers. Atomic force microscopy shows the single-layer step-heights of individual crystals which match the layer thickness obtained via X-ray diffraction analysis. The crystals have facets and flat terraces with extended area, and have a strong preferred orientation (010) normal to the substrate surface. The MPCs move laterally upon annealing and reorganize into a single-layer network with their separation distance approximately equal to the length of an extended alkyl chain.

Glass transition in ultrathin polymer films: calorimetric study.

The ultrasensitive differential scanning calorimetry is used to observe the glass transition in thin (1-400 nm) spin-cast films of polystyrene, poly (2-vinyl pyridine) and poly (methyl methacrylate) on a platinum surface. A pronounced glass transition is observed even at a thickness as small as 1-3 nm. Using the high heating (20-200 K/ms) and cooling (1-2 K/ms in glass transition region) rates which are typical for this technique, we do not observe appreciable dependence of the glass transition temperature over the thickness range from hundreds of nanometers down to 3 nm thick films. The evolution of calorimetric data with film thickness is discussed in terms of broadening of transition dynamics and loss of transition contrast.