Because the synthesis of metal clusters with multiple ligand types results in a distribution of ligands, high-resolution separation of each unique cluster from the mixture is required for precise control of the ligand composition. Reverse-phase high-performance liquid chromatography combined with appropriate transitioning of the mobile phase composition is an extremely effective means of separating ligand combinations when working with metal clusters protected by two different types of thiolates. We report herein advanced use of this method. The studies involving Au₂₄Pd(SR₁)₁₈-x(SR₂)x and Au₂₄Pd(SR₁)₁₈-x(SeR₂)x (SR₁, SR₂ = thiolate, SeR₂ = selenolate) revealed the following. (1) In general, an increase in the difference between the polarities of the functional groups incorporated in the two types of ligands improves the separation resolution. A suitable ligand combination for separation can be predicted from the retention times of Au₂₄Pd(SR₁)₁₈ and Au₂₄Pd(SR₂)₁₈, which cause the terminal peaks in a series of peaks. (2) The use of a step-gradient program during the mobile phase substitution results in improved resolution compared to that achievable with the linear gradients applied in prior work. (3) This technique is also useful for the evaluation of the chemical compositions of metal clusters protected by two different types of ligands with similar molecular weights. These findings will provide clear design guidelines for the functionalization of metal clusters via control of the ligand composition, and will also improve our understanding of the high-resolution isolation of metal clusters.

Thiolate (RS)-protected gold clusters (Aun(SR)m) have attracted much attention as building blocks of functional nanomaterials. Our group has been studying the high-resolution separation of Aun(SR)m clusters using reversed-phase high-performance liquid chromatography. In this perspective, we summarize our recent results on the separation of Aun(SR)m clusters and their doped clusters according to the core size, charge state, ligand composition, and coordination isomers. Additionally, this perspective describes new findings obtained by using high-resolution separation and future prospects for the separation of such types of metal clusters. We believe that the techniques and knowledge gained in this study would contribute to the creation of Aun(SR)m clusters with the desired functions and associated functional nanomaterials.

Recent studies have demonstrated that reversed-phase high-performance liquid chromatography (RP-HPLC) is a very effective means of separating thiolate (SR)-protected gold clusters (Au n (SR) m ). In the present study, we applied RP-HPLC to the separation of hydrophilic glutathionate (SG)-protected Au n (SG) m clusters. To achieve such separation, an ion pair reagent was dissolved in the solution to form ion pairs with the functional groups on the cluster surfaces, improving the interaction between the cluster surfaces and the hydrophobic stationary phase, representing ion pair chromatography. This technique resulted in the high-resolution separation of Au n (SG) m clusters via RP-HPLC. Experiments at different solution pH values led to slight improvements in the resolution, showing that pH adjustment, which modifies the efficiency of the ion pair formation process, is a useful technique for improving this type of separation.

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The authors present an Ag-GeTe-based conductive bridge memory that allows radio frequency (RF) input and transmission. The RF input modifies the resistance switching characteristics and the memory device can switch the order of harmonics of the transmitted RF wave. The transmittance for the RF input above 500 MHz was higher than 0.5 for both the low-resistance state (LRS) and the high-resistance state (HRS). Below 500 MHz the transmittance decreased with decreasing frequency and depended on the resistance state (LRS or HRS). At 20 MHz, the transmission for the HRS was 0.02, while that for the LRS was 0.2. An RF input of 20 MHz at 10 dBm modified the SET voltage from 0.19 to 0.05 V, and the RESET voltage from −0.17 to −0.05 V. The RF wave transmitted through the LRS consisted almost entirely of fundamental waves, while that in the HRS provided harmonics or frequency multiplication up to the fifth order at 150 MHz at 10 dBm. Heat cycle measurements with RF input revealed the difference between the non-linear resistance producing the frequency multiplication and the DC resistance.