Controllable Synthesis of Transferable Ultrathin Bi2Ge(Si)O5 Dielectric Alloys with Composition-Tunable High-κ Properties.

Two-dimensional (2D) alloys hold great promise to serve as important components of 2D transistors, since their properties allow continuous regulation by varying their compositions. However, previous studies are mainly limited to the metallic/semiconducting ones as contact/channel materials, but very few are related to the insulating dielectrics. Here, we use a facile one-step chemical vapor deposition (CVD) method to synthesize ultrathin Bi2SixGe1-xO5 dielectric alloys, whose composition is tunable over the full range of x just by changing the relative ratios of the GeO2/SiO2 precursors. Moreover, their dielectric properties are highly composition-tunable, showing a record-high dielectric constant of >40 among CVD-grown 2D insulators. The vertically grown nature of Bi2GeO5 and Bi2SixGe1-xO5 enables polymer-free transfer and subsequent clean van der Waals integration as the high-κ encapsulation layer to enhance the mobility of 2D semiconductors. Besides, the MoS2 transistors using Bi2SixGe1-xO5 alloy as gate dielectrics exhibit a large Ion/Ioff (>108), ideal subthreshold swing of ∼61 mV/decade, and a small gate hysteresis (∼5 mV). Our work not only gives very few examples on controlled CVD growth of insulating dielectric alloys but also expands the family of 2D single-crystalline high-κ dielectrics.

Selective confinement of potassium, rubidium, or caesium ions in a non-covalent hydroxyproline octamer cage stabilized by cis-hydroxyl locks.

While numerous studies have focused on the impact of chirality on some magic amino acid clusters, this article investigates the effects of steric isomerization using 4-hydroxyproline octamers as a model system. Through mass spectrometry, infrared photodissociation spectroscopy, and theoretical calculation, it was demonstrated that the cis-4-hydroxy-L-proline octamer can selectively cage potassium, rubidium, or caesium ions through stable cis-hydroxyl locks, while the trans-form cannot. The results highlight the importance of hydroxyl group orientation in designing biocompatible membrane transporters with high ion-selectivity.

Structural Diversity of Protonated Citric Acid-Ammonia Clusters and Its Atmospheric Implication.

Various acid-base molecular clusters involving organic species can serve as precursors that play important roles in the formation of an atmospheric aerosol. Due to its structural flexibility and its ability to form multiple hydrogen bonds, citric acid acts as a key species in forming clusters that are critical in the nucleation of related aerosol precursors. Thus, it is provoking to characterize the structures of these clusters at the molecular level. In this paper, protonated citric acid-ammonia clusters of various sizes were generated by electrospray ionization and studied by tandem mass spectrometry. The structures of [(CA)2+NH4]1+ and [(CA)4+NH4]1+ were further characterized by the method of infrared photodissociation (IRPD) spectroscopy. Combined with theoretical calculations, it is found that the most stable structures of the dimeric and tetrameric isomers show the shapes of an ingot and a lantern, respectively. It has been revealed that the temperature has a great effect on the contributions of different isomers for both dimers and tetramers. The dominat isomers are found to have more open structures at higher temperatures, facilitating the growth of clusters through new hydrogen bonds.

Structural Characterization of the Metalized Radical Cations of Adenosine ([Ade+Li-H]•+ and [Ade+Na-H]•+) by Infrared Multiphoton Dissociation Spectroscopy and Theoretical Studies.

Nucleoside radicals are key intermediates in the process of DNA damage, and alkali metal ions are a common group of ions in living organisms. However, so far, there has been a significant lack of research on the structural effects of alkali metal ions on nucleoside free radicals. In this study, we report a new method for generating metalized nucleoside radical cations in the gas phase. The radical cations [Ade+M-H]•+ (M = Li, Na) are generated by the 280 nm ultraviolet photodissociation (UVPD) of the precursor ions of lithiated and sodiated ions of 2-iodoadenine in a Fourier transform ion cyclotron resonance (FT ICR) cell. Further infrared multiphoton dissociation (IRMPD) spectra of both radical cations were recorded in the region of 2750-3750 cm-1. By combining these results with theoretical calculations, the most stable isomers of both radicals can be identified, which share the common characteristics of triple coordination patterns of the metal ions. For both radical species, the lowest-energy isomers undergo hydrogen transfer. Although the sugar ring in the most stable isomer of [Ade+Li-H]•+ is in a (South, syn) conformation similar to that of [Ado+Na]+, [Ade+Na-H]•+ is distinguished by the unexpected opening of the sugar ring. Their theoretical spectra are in good agreement with experimental spectra. However, due to the flexibility of the structures and the complexity of their potential energy surfaces, the hydrogen transfer pathways still need to be further studied. Considering that the free radicals formed directly after C-I cleavage have some similar spectral characteristics, the existence of these corresponding isomers cannot be ruled out. The findings imply that the structures of nucleoside radicals may be significantly influenced by the attached alkali metal ions. More detailed experiments and theoretical calculations are still crucial.

Bi3O2.5Se2: a two-dimensional high-mobility polar semiconductor with large interlayer and interfacial charge transfer.

Two-dimensional semiconductors with large intrinsic polarity are highly attractive for applications in high-speed electronics, ultrafast and highly sensitive photodetectors and photocatalysis. However, previous studies mainly focus on neutral layered polar 2D materials with limited vertical dipoles and electrostatic potential difference (typically <1.5 eV). Here, using the first-principles calculations, we systematically investigated the polarity of few-layer Bi3O2.5Se2 semiconductors with ultrahigh predicted room-temperature carrier mobility (1790 cm2 V-1 s-1 for the monolayer). Thanks to its unique non-neutral layered structure, few-layer Bi3O2.5Se2 contributes to a substantial interlayer charge transfer (>0.5 e-) and almost the highest electrostatic potential difference (ΔΦ) of ∼4 eV among the experimentally attainable 2D layered materials. More importantly, positioning graphene on different charged layers ([Bi2O2.5]+ or [BiSe2]-) switches the charge transfer direction, inducing selective n-doping or p-doping. Furthermore, we can use polar Bi3O2.5Se2 as an exemplary assisted gate to gain additional holes or electrons except for the external electric field, thus breaking the traditional limitations of gate tunability (∼1014 cm-2) observed in experimental settings. Our work not only expands the family of polar 2D semiconductors, but also makes a conceptual advance on using them as an assisted gate in transistors.

Structures and growth pathways of AunCln+3- (n ≤ 7) cluster anions.

Gold chloride clusters play an important role in catalysis and materials chemistry. Due to the diversity of their species and isomers, there is still a dearth of structural studies at the molecular level. In this work, anions of AunCln+3 - and AunCln+5 - (n = 2-4) clusters were obtained by laser desorption/ionization mass spectrometry (LDI MS), and the most stable isomers of AunCln+3 - were determined after a thorough search and optimization at the TPSSh/aug-cc-pVTZ/ECP60MDF level. The results indicate that all isomers with the lowest energy have a planar zigzag skeleton. In each species, there is one Au(III) atom at the edge connected with four Cl atoms, which sets it from the other Au(I) atoms. Four growth pathways for AunCln+3 - (n = 2-7) clusters are proposed (labelled R1, R2, R3 and R4). They are all associated with an aurophilic contact and are exothermic. The binding energies tend to stabilize at ∼ -41 kcal/mol when the size of the cluster increases in all pathways. The pathway R1, which connects all the most stable isomers of the respective clusters, is characterized by cluster growth due to aurophilic interactions at the terminal atom of Au(I) in the zigzag chains. In the pathway of R4 involving Au-Au bonding in its initial structures (n ≤ 3), the distance between intermediate gold atoms grows with cluster size, ultimately resulting in the transfer of the intermediate Au-Au bonding into aurophilic interaction. The size effect on the structure and aurophilic interactions of these clusters will be better understood based on these discoveries, potentially providing new insights into the active but elusive chemical species involved in the corresponding catalytic reactions or nanoparticle synthesis processes.

K+-Selectivity Due to Coordination with a D4d-Symmetric Homochiral Proline Octamer Verified by Mass Spectrometry and Infrared Photodissociation Spectroscopy.

Both phenomena of homochirality and sodium-potassium ion selectivity in cells have been regarded as important issues in the process of the origin of life. However, whether K+/Na+ selectivity was involved in homochirogenesis has never been considered. Herein, we report that a homochiral proline octamer shows high K+-selectivity. Coordination of K+ results in formation of a stable, noncovalent, D4d-symmetric complex, as demonstrated by mass spectrometry, infrared photodissociation spectroscopy, and calculations. A cooperative relationship between an eight-coordinated metal cation and a homochirality-restricted topological hydrogen-bonded proline network is the key for the K+/Na+ selectivity. As the complex comprises merely the basic chiral amino acid, it provides a possible linkage between K+/Na+ selectivity and the origin of chirality on the prebiotic Earth.

Structure and bonding properties of the platinum-mediated tetrametallic endohedral fullerene La3Pt@C98.

Nowadays, focus is on encapsulating a greater variety and amount of metal species into fullerene cages due to their diverse structures and fascinating properties. Nevertheless, the encapsulation of more positively charged metal atoms inside one cage means more Coulomb repulsion, which makes the formation of such endohedral metallofullerenes (EMFs) difficult. In general, non-metallic atoms such as N and O should be introduced as mediators for the formation of trimetallic or tetrametallic endohedral fullerenes. However, it is still unknown whether metal atoms can serve as mediators themselves to form such EMFs. In this paper, the endohedral tetrametallic fullerene La3Pt@C98 with the platinum atom as a metallic mediator is reported. The EMFs of La3Pt@C2n (2n = 98-300) were generated by the method of laser ablation in the gas phase and verified by mass spectrometry. Among them, the EMF of La3Pt@C98 was selected and studied by theoretical calculations. Results show that the two most stable isomers are La3Pt@C2(231010)-C98 and La3Pt@C1(231005)-C98. For both of them, the inner La3Pt metallic cluster appears in a pyramidal shape, different from the planar triangular pattern of La3N clusters previously reported. Further calculations prove the existence of encaged La-Pt bonds in the La3Pt cluster. It was also revealed that the negatively charged Pt atom is situated near the center of the four-center two-electron (4c-2e) metal bond with the highest occupancy number. The platinum-mediated cluster stabilizes the EMFs greatly, promising the possibility of synthesizing new species of Pt-containing EMFs.