A Chemical Route to Monolithic Integration of Crystalline Oxides on Semiconductors

This work demonstrates the growth of crystalline SrTiO₃ (STO) directly on germanium via a chemical method. After thermal deoxidation, the Ge substrate is transferred in vacuo to the deposition chamber where a thin film of STO (2 nm) is deposited by atomic layer deposition (ALD) at 225 °C. Following post‐deposition annealing at 650 °C for 5 min, the STO film becomes crystalline with epitaxial registry to the underlying Ge (001) substrate. Thicker STO films (up to 15 nm) are then grown on the crystalline STO seed layer. The crystalline structure and orientation are confirmed via reflection high‐energy electron diffraction, X‐ray diffraction, and transmission electron microscopy. Electrical measurements of a 15‐nm thick epitaxial STO film on Ge show a large dielectric constant (k ≈ 90), but relatively high leakage current of ≈10 A/cm² for an applied field of 0.7 MV/cm. To suppress the leakage current, an aluminum precursor is cycled during ALD growth to grow crystalline Al‐doped STO (SrTi_1‐x­Al_xO_3‐δ) films. With sufficient Al doping (≈13%), the leakage current decreases by two orders of magnitude for an 8‐nm thick film. The current work demonstrates the potential of ALD‐grown crystalline oxides to be explored for advanced electronic applications, including high‐mobility Ge‐based transistors.     

Band‐Gap Engineering at a Semiconductor–Crystalline Oxide Interface

The epitaxial growth of crystalline oxides on semiconductors provides a pathway to introduce new functionalities to semiconductor devices. Key to electrically coupling crystalline oxides with semiconductors to realize functional behavior is to control the manner in which their bands align at interfaces. Here, principles of band‐gap engineering traditionally used at heterojunctions between conventional semiconductors are applied to control the band offset between a single crystalline oxide and a semiconductor. Reactive molecular beam epitaxy is used to realize atomically abrupt and structurally coherent interfaces between SrZr_xTi_1−xO₃ and Ge, in which the band‐gap of the former is enhanced with Zr content x. Structural and electrical characterization of SrZr_xTi_1−xO₃‐Ge heterojunctions for x = 0.2 to 0.75 are presented and it is demonstrated that the band offset can be tuned from type‐II to type‐I, with the latter being verified using photoemission measurements. The type‐I band offset provides a platform to integrate the dielectric, ferroelectric, and ferromagnetic functionalities of oxides with semiconducting devices.     

Atomic‐Scale Fabrication of In‐Plane Heterojunctions of Few‐Layer MoS2 via In Situ Scanning Transmission Electron Microscopy

Layered MoS₂ is a prospective candidate for use in energy harvesting, valleytronics, and nanoelectronics. Its properties strongly related to its stacking configuration and the number of layers. Due to its atomically thin nature, understanding the atomic‐level and structural modifications of 2D transition metal dichalcogenides is still underdeveloped, particularly the spatial control and selective precision. Therefore, the development of nanofabrication techniques is essential. Here, an atomic‐scale approach used to sculpt 2D few‐layer MoS₂ into lateral heterojunctions via in situ scanning/transmission electron microscopy (STEM/TEM) is developed. The dynamic evolution is tracked using ultrafast and high‐resolution filming equipment. The assembly behaviors inherent to few‐layer 2D‐materials are observed during the process and included the following: scrolling, folding, etching, and restructuring. Atomic resolution STEM is employed to identify the layer variation and stacking sequence for this new 2D‐architecture. Subsequent energy‐dispersive X‐ray spectroscopy and electron energy loss spectroscopy analyses are performed to corroborate the elemental distribution. This sculpting technique that is established allows for the formation of sub‐10 nm features, produces diverse nanostructures, and preserves the crystallinity of the material. The lateral heterointerfaces created in this study also pave the way for the design of quantum‐relevant geometries, flexible optoelectronics, and energy storage devices.     

Atomic‐Scale Structure Evolution in a Quasi‐Equilibrated Electrochemical Process of Electrode Materials for Rechargeable Batteries

Lithium‐ion batteries have proven to be extremely attractive candidates for applications in portable electronics, electric vehicles, and smart grid in terms of energy density, power density, and service life. Further performance optimization to satisfy ever‐increasing demands on energy storage of such applications is highly desired. In most of cases, the kinetics and stability of electrode materials are strongly correlated to the transport and storage behaviors of lithium ions in the lattice of the host. Therefore, information about structural evolution of electrode materials at an atomic scale is always helpful to explain the electrochemical performances of batteries at a macroscale. The annular‐bright‐field (ABF) imaging in aberration‐corrected scanning transmission electron microscopy (STEM) allows simultaneous imaging of light and heavy elements, providing an unprecedented opportunity to probe the nearly equilibrated local structure of electrode materials after electrochemical cycling at atomic resolution. Recent progress toward unraveling the atomic‐scale structure of selected electrode materials with different charge and/or discharge state to extend the current understanding of electrochemical reaction mechanism with the ABF and high angle annular dark field STEM imaging is presented here. Future research on the relationship between atomic‐level structure evolution and microscopic reaction mechanisms of electrode materials for rechargeable batteries is envisaged.     

Patterning: Strategies for Patterning Biomolecules with Dip‐Pen Nanolithography (Small 8/2011)

Dip‐pen nanolithography (DPN) is an atomic force microscopy (AFM)‐based lithography technique, which has the ability to fabricate patterns with a feature size down to approximately 15 nm using both top‐down and bottom‐up approaches. DPN utilizes the water meniscus formed between an AFM tip and a substrate to transfer ink molecules onto surfaces. A major application of this technique is the fabrication of micro‐ and nano‐arrays of patterned biomolecules. To achieve this goal, a variety of chemical approaches has been used. This review concisely describes the development of DPN in the past decade and presents the related chemical strategies that have been reported to fabricate biomolecular paterns with DPN at micrometer and nanometer scale, classified into direct‐ and indirect DPN methodologies, discussing tip‐functionalization strategies as well.     

A Generalizable Top‐Down Nanostructuring Method of Bulk Oxides: Sequential Oxygen–Nitrogen Exchange Reaction

A thermal reaction route that induces grain fracture instead of grain growth is devised and developed as a top‐down approach to prepare nanostructured oxides from bulk solids. This novel synthesis approach, referred to as the sequential oxygen–nitrogen exchange (SONE) reaction, exploits the reversible anion exchange between oxygen and nitrogen in oxides that is driven by a simple two‐step thermal treatment in ammonia and air. Internal stress developed by significant structural rearrangement via the formation of (oxy)nitride and the creation of oxygen vacancies and their subsequent combination into nanopores transforms bulk solid oxides into nanostructured oxides. The SONE reaction can be applicable to most transition metal oxides, and when utilized in a lithium‐ion battery, the produced nanostructured materials are superior to their bulk counterparts and even comparable to those produced by conventional bottom‐up approaches. Given its simplicity and scalability, this synthesis method could open a new avenue to the development of high‐performance nanostructured electrode materials that can meet the industrial demand of cost‐effectiveness for mass production.     

Niobium‐Based Oxides Toward Advanced Electrochemical Energy Storage: Recent Advances and Challenges

Niobium‐based oxides including Nb₂O₅, TiNb_xO_2+2.5x compounds, M–Nb–O (M = Cr, Ga, Fe, Zr, Mg, etc.) family, etc., as the unique structural merit (e.g., quasi‐2D network for Li‐ion incorporation, open and stable Wadsley– Roth shear crystal structure), are of great interest for applications in energy storage systems such as Li/Na‐ion batteries and hybrid supercapacitors. Most of these Nb‐based oxides show high operating voltage (>1.0 V vs Li⁺/Li) that can suppress the formation of solid electrolyte interface film and lithium dendrites, ensuring the safety of working batteries. Outstanding rate capability is impressive, which can be derived from their fast intercalation pseudocapacitive kinetics. However, the intrinsic poor electrical conductivity hinders their energy storage applications. Various strategies including structure optimization, surface engineering, and carbon modification are effectively used to overcome the issues. This review provides a comprehensive summary on the latest progress of Nb‐based oxides for advanced electrochemical energy storage applications. Major impactful work is outlined, promising research directions, and various performance‐optimizing strategies, as well as the energy storage mechanisms investigated by combining theoretical calculations and various electrochemical characterization techniques. In addition, challenges and perspectives for future research and commercial applications are also presented.     

Semiconductor Nanowire Light‐Emitting Diodes Grown on Metal: A Direction Toward Large‐Scale Fabrication of Nanowire Devices

Bottom‐up nanowires are attractive for realizing semiconductor devices with extreme heterostructures because strain relaxation through the nanowire sidewalls allows the combination of highly lattice mismatched materials without creating dislocations. The resulting nanowires are used to fabricate light‐emitting diodes (LEDs), lasers, solar cells, and sensors. However, expensive single crystalline substrates are commonly used as substrates for nanowire heterostructures as well as for epitaxial devices, which limits the manufacturability of nanowire devices. Here, nanowire LEDs directly grown and electrically integrated on metal are demonstrated. Optical and structural measurements reveal high‐quality, vertically aligned GaN nanowires on molybdenum and titanium films. Transmission electron microscopy confirms the composition variation in the polarization‐graded AlGaN nanowire LEDs. Blue to green electroluminescence is observed from InGaN quantum well active regions, while GaN active regions exhibit ultraviolet emission. These results demonstrate a pathway for large‐scale fabrication of solid state lighting and optoelectronics on metal foils or sheets.