Ultrathin ferroic HfO2-ZrO2 superlattice gate stack for advanced transistors.

With the scaling of lateral dimensions in advanced transistors, an increased gate capacitance is desirable both to retain the control of the gate electrode over the channel and to reduce the operating voltage1. This led to a fundamental change in the gate stack in 2008, the incorporation of high-dielectric-constant HfO2 (ref. 2), which remains the material of choice to date. Here we report HfO2-ZrO2 superlattice heterostructures as a gate stack, stabilized with mixed ferroelectric-antiferroelectric order, directly integrated onto Si transistors, and scaled down to approximately 20 ångströms, the same gate oxide thickness required for high-performance transistors. The overall equivalent oxide thickness in metal-oxide-semiconductor capacitors is equivalent to an effective SiO2 thickness of approximately 6.5 ångströms. Such a low effective oxide thickness and the resulting large capacitance cannot be achieved in conventional HfO2-based high-dielectric-constant gate stacks without scavenging the interfacial SiO2, which has adverse effects on the electron transport and gate leakage current3. Accordingly, our gate stacks, which do not require such scavenging, provide substantially lower leakage current and no mobility degradation. This work demonstrates that ultrathin ferroic HfO2-ZrO2 multilayers, stabilized with competing ferroelectric-antiferroelectric order in the two-nanometre-thickness regime, provide a path towards advanced gate oxide stacks in electronic devices beyond conventional HfO2-based high-dielectric-constant materials.

Design of Chiral ß-Double Helices from γ-Peptide Foldamers.

Chirality is ubiquitous in nature, and homochirality is manifested in many biomolecules. Although ß-double helices are rare in peptides and proteins, they consist of alternating L- and D-amino acids. No peptide double helices with homochiral amino acids have been observed. Here, we report chiral ß-double helices constructed from γ-peptides consisting of alternating achiral (E)-α,ß-unsaturated 4,4-dimethyl γ-amino acids and chiral (E)-α,ß-unsaturated γ-amino acids in both single crystals and in solution. The two independent strands of the same peptide intertwine to form a ß-double helix structure, and it is stabilized by inter-strand hydrogen bonds. The peptides with chiral (E)-α,ß-unsaturated γ-amino acids derived from α-L-amino acids adopt a (P)-ß-double helix, whereas peptides consisting of (E)-α,ß-unsaturated γ-amino acids derived from α-D-amino acids adopt an (M)-ß-double helix conformation. The circular dichroism (CD) signature of the (P) and (M)-ß-double helices and the stability of these peptides at higher temperatures were examined. Furthermore, ion transport studies suggested that these peptides transport ions across membranes. Even though the structural analogy suggests that these new ß-double helices are structurally different from those of the α-peptide ß-double helices, they retain ion transport activity. The results reported here may open new avenues in the design of functional foldamers.

Exploring Macroscopic Dipoles of Designed Cyclic Peptide Ordered Assemblies to Harvest Piezoelectric Properties.

Crystalline materials exhibiting non-centrosymmetry and possessing substantial surface dipole moments play a critical role in piezoelectricity. Designing biocompatible self-assembled materials with these attributes is particularly challenging when compared to inorganic materials and ceramics. In this study, we elucidate the crystal conformations of novel cyclic peptides that exhibit self-assembly into tubular structures characterized by unidirectional hydrogen bonding and piezoelectric properties. Unlike cyclic peptides derived from alternating L- and D-amino acids, those derived from new δ-amino acids demonstrate the formation of self-assembled tubes with unidirectional hydrogen bonds. Further, the tightly packed tubular assemblies and higher macrodipole moments result in superior piezoelectric coefficients compared to peptides with lower macrodipole moments. Our findings underscore the potential for designing cyclic peptides with unidirectional hydrogen bonds, thereby paving the way for their application in design of biocompatible piezo- and ferroelectric materials.

Foldamer Nanotubes Mediated Label-Free Detection of Protein-Small Molecule Interactions.

Development of miniaturized lab-on-chip devices for the detection of rapid and specific small molecule-protein binding interactions at very low concentrations holds significant importance in drug discovery and biomedical applications. Here, the label-free detection of small molecule-protein interactions is reported on the surface functionalizable nanotubes of α,γ-hybrid peptide helical foldamers using nanoscale capacitance and impedance spectroscopy. The 12-helix conformation of the α,γ-hybrid peptide observed in the single crystals, self-assembled into nanotubes in an aqueous environment with exposed cysteine thiols for small molecule conjugation. The binding of streptavidin to the covalently linked biotin on the surface of nanotubes was detected at the picomolar concentrations. No change in the capacitance and impedance were observed in the absence of either immobilized biotin or protein streptavidin. The functionalizable hybrid peptide nanotubes reported here pave the way for the label-free detection of various small molecule protein interactions at very low concentrations.

Anion Tuned Structural Modulation and Nonlinear Optical Effects of Metal-Ion Directed 310 -Helix Networks.

Metals play an important role in the structure and functions of various proteins. The combination of metal ions and peptides have been emerging as an attractive field to create advanced structures and biomaterials. Here, we are reporting the anion-influenced, silver ion coordinated diverse networks of designed short tripeptide 310 -helices with terminal pyridyl groups. The short peptides adopted classical right-handed, left-handed and 310 EL -helical conformations in the presence of different silver salts. The peptides have displayed conformational flexibility to accommodate different sizes and interactions of anions to yield a variety of metal-coordinated networks. The complexes of metal ions and peptides have shown different porous networks, right- and left-handed helical polymers, transformation of helix into superhelix and 2 : 2 metal-peptide macrocycles. Further, the metal-peptide crystals with inherent dipoles of helical peptides gave striking second harmonic generation response. The optical energy upconversion from NIR to red and green light is demonstrated. Overall, we have shown the utilization of short 310 -helices for the construction of diverse metal-coordinated helical networks and notable non-linear optical effects.

Accurate Threshold Voltage Reliability Evaluation of Thin Al2O3 Top-Gated Dielectric Black Phosphorous FETs Using Ultrafast Measurement Pulses.

Few-layer black phosphorus (BP) has attracted significant interest in recent years due to electrical and photonic properties that are far superior to those of other two-dimensional layered semiconductors. The study of long term electrical stability and reliability of black phosphorus field effect transistors (BP-FETs) with technologically relevant thin, and device-selective, gate dielectrics, stressed under realistic (closer to operation) bias and measured using state-of-the-art ultrafast reliability characterization techniques, is essential for their qualification and use in different applications. In this work, air-stable BP-FETs with a thin top-gated dielectric (15 nm Al2O3, SiO2 equivalent thickness of 5 nm) were fabricated and comprehensively characterized for threshold voltage ( Vth) instability under negative gate bias stress at various measurement delays ( tm), stress biases ( VGSTR), temperatures ( T), and stress times ( tstr) for the first time. Thin top-gated oxide enables low VGSTR that is closer to the operating condition and ultrafast Vth measurements with low delay ( tm = 10 µs, due to high drain current) that ensure minimal recovery. The resultant time kinetics of Vth degradation (Δ Vth) shows fast saturation at longer stress times and low-temperature activation energy. Vth instability in these top-gated devices is suggested to be dominated by hole trapping, which is modeled using first-order equations at different VGSTR and T. It is shown that measurements using larger tm show lower degradation magnitude that do not saturate due to recovery artifacts and give inaccurate estimation of hole trap densities. Conventional, thick, and global back-gated oxide BP-FETs were also fabricated and characterized for varying tm (1 ms being the lowest due to a low drain current level for thick oxide), VGSTR, and T to benchmark our top-gated results. Nonsaturating Δ Vth in the back-gated devices is shown to result from recovery artifacts due to the large tm (1 ms and greater) values. Finally, using a VGSTR and T-dependent first-order model, we show that the top-gated Al2O3 BP-FETs with scaled gate oxide thickness can match state-of-the-art Si reliability specifications at operating voltage and room/elevated temperature.